NANOSTRUCTURED  GRAPHENE  NANOPLATELETS   FOR  ENERGY  STORAGE  APPLICATIONS           By   Anchita  Monga               A  DISSERTATION   Submitted  to   Michigan  State  University   in  partial  fulfillment  of  the  requirements   for  the  degree  of     Chemical  Engineering-­‐Doctor  of  Philosophy   2013     ABSTRACT   NANOSTRUCTURED  GRAPHENE  NANOPLATELETS  FOR  ENERGY  STORAGE  APPLICATIONS   By   Anchita  Monga   There   is   an   increasing   demand   for   high   performance   compact   batteries   for   diverse   applications   ranging   from   portable   electronics   to   electric   automotive   vehicles.   This   need   has   driven   the   direction   of   research   towards   newer   materials,   improved   synthesis   and   architectured   assembly.   This   research   addresses   the   gravimetric   and   volumetric   density   challenges  as  well  as  the  cost  issues  faced  by  energy  storage  devices  by  developing  structured   graphitic   materials,   aiming   at   better   electrochemical   performance,   improved   energy   density   and  reduced  cost.   The   few   layer   graphene   nanoplatelets   (GnP)   used   in   this   study   can   be   produced   from   natural  graphite  in  thicknesses  from  1-­‐10  nm  and  in  widths  from  0.3  to  50  microns  via  an  acid   intercalation/thermal   exfoliation   process.     The   GnP   serves   as   an   inexpensive   alternative   to   carbon   nanotubes   and   single   graphene   sheets.   The   ability   to   nanostructure   GnP   and   tailor   its   inherent   properties   for   lithium   storage   and   electrical   conductivity,   allows   it   to   be   used   for   customized   applications   in   three   different   lithium   ion   battery   components   viz.,   active   anode   material,  current  collector  and  conducting  additive.       Metal  nanoparticle  doped  GnP  in  which  nanosized  metal  particles  are  coated  onto  the   GnP   basal   surface,   have   been   assembled   to   make   a   ‘pillared’   nanostructure   in   which   the   particles  maintain  a  fixed  distance  between  adjacent  GnPs  facilitating  improved  transport  and   enhanced   lithium   storage   capacity,   especially   at   faster   charge   rates.   Graphene   nanoplatelets   synthesized  with  different  sizes  of  metal  nanoparticles  effectively  create  a  nano-­‐architectured   GnP  multilayer  assembly  with  flexible  interlayer  spacing.  The  creation  of  a  lithium  ion  battery   anode   with   controllable   GnP   interlayer   spacing   facilitates   lithium   ion   diffusion   through   the   electrode,  and  this  in  turn  leads  to  improved  transport  and  enhanced  capacity.   Graphene  nanoplatelets  are  also  intrinsically  excellent  electrical  conductors,  which  can   be  assembled  into  continuous  conductive  thin  films  to  replace  metal  foils  as  current  collectors   for   electrochemical   applications.   Self-­‐standing,   binder-­‐free,   flexible   and   porous   paper   is   prepared  by  a  simple  filtration  process  using  an  aqueous  suspension  of  GnP.  This  GnP  paper  is   an   attractive   alternative   current   collector   to   replace   copper   in   lithium   ion   batteries,   because   of   its  lower  areal  density,  desirable  electrical  conductivity  and  good  electrochemical  stability.  The   performance   of   GnP   as   a   current   collector   with   different   electrode   materials   and   its   role   in   reducing  the  overall  battery  cell  weight  has  been  investigated.   The   third   application   combines   the   benefits   of   electrical   conductivity   and   nanostructuring   of   GnP   to   function   as   a   conducting   additive   for   different   electrode   materials.   An  investigation  of  the  use  of  different  sizes  of  GnP  as  a  conducting  additive  for  lithium  titanate   electrodes   and   as   a   conducting   host/matrix   for   lithium   sulfur   batteries   has   demonstrated   its   applicability  here  also.     Copyright  by   ANCHITA  MONGA   2013     Dedicated  to  my  parents  and  to  my  sister     v     ACKNOWLEDGEMENTS   First   of   all,   I   would   like   to   express   my   sincere   gratitude   and   appreciation   to   my   research   advisor,  Prof.  Lawrence  T.  Drzal,  for  his  patience,  guidance  and  support  through  the  course  of   my   Ph.D.   program.   I   am   deeply   thankful   to   him   for   the   encouragement   and   numerous   opportunities  given  to  me,  which  helped  me,  showcase  my  research  and  develop  my  potential.   I   am   very   thankful   to   my   Committee   members   Prof.   Martin   Hawley,   Prof.   Jeff   Sakamoto   and  Prof.  Phillip  Duxbury  for  their  kind  support  and  valuable  suggestions.  I  am  also  grateful  to   Prof.  Jeff  Sakamoto  and  Ezhiyl  Rangasamy  for  helping  me  with  the  experiments  and  technical   nuances  of  this  research  field.     I  would  like  to  extend  a  sincere  thanks  to  all  the  research  staff  at  CMSC,  Mike  Rich,  Brian   Rook,  Per  Askeland,  Hazel  Ann  Hosein  and  Ed  Drown  for  always  being  there  and  for  providing   their   endless   help,   patience,   efforts   and   support   for   my   research.   Also,   I   would   like   to   thank   Hiroyuki   Fukushima,   Inhwan   Do,   Wanjun   Liu,   Hwanman   Park,   Xiaobing   Li,   Saswata   Bose   and   Frederic   Vautard   for   their   valuable   inputs   and   feedback.   Also,   my   deepest   appreciation   goes   to   Karen   Lillis,   Inger   Weitlauf,   Shirley   Wohlfert,   Joann   Peterson,   Lauren   Brown   and   Danielle   Murphy  for  their  assistance  in  making  this  journey  smooth.   A   special   thanks   goes   to   my   colleagues   and   officemates   through   these   years,   Xian,   Jinglei,  Dee,  Yan  ,  Pat  ,  Sanjib,  Huang,  Tao  ,  Deb,  Dana,  Steve,  Eeshwar,  Markus,  Keith,  Albert     vi     and   Isabel   David,   for   providing   me   the   best   workplace   possible.   I   will   always   remember   the   smiles,  conversations,  serious  discussions  and  celebrations,  experienced  in  your  company.   A   special   acknowledgement   goes   to   my   friends   Durga,   Anbu,   Raghav,   Madhumitha,   Abinand,   Ezhiyl,   Rengarajan,   Nikita,   Prachi,   Leo,   Venkat   and   Sudhanwa   for   being   my   support   system   and   my   extended   family.   Thank   you   for   always   being   there   through   all   the   ups   and   downs  and  making  my  stay  at  MSU  a  memorable  one.       Life   in   US,   away   from   home,   would   not   have   been   easy   without   my   family   in   Iowa   and   I   can’t  imagine  taking  this  step  without  the  support  of  my  brother  Amit,  my  sister  in  law  Sonia   and  my  nieces  Soumya  and  Anya.   Finally,  I  would  like  to  thank  my  parents,  Mr.  B  D  Monga  and  Dr.  Kanchan  Monga  and   my   sister,   Samita   for   their   unconditional   love,   support   and   encouragement   at   every   step,   in   making  me  who  I  am  today.  Also,  I  would  like  to  thank  my  brother-­‐in-­‐law  Atin  and  my  adorable   niece  Azalea  for  bringing  endless  joy  to  our  lives.             vii   TABLE  OF  CONTENTS     LIST  OF  TABLES   .................................................................................................  xii   LIST  OF  FIGURES  ..............................................................................................   xiii   1   INTRODUCTION  ..............................................................................................  1   DEMAND  FOR  ENERGY  STORAGE  ........................................................................................  1   LITHIUM  BASED  BATTERIES  .................................................................................................  1   LITHIUM-­‐ION  BATTERIES  .................................................................................................  3   Anodes  ........................................................................................................................  5   Graphite  ..................................................................................................................  5   Transition  Metal  Oxides   ...........................................................................................  6   Metal  Anodes  /  Alloying  Anodes  ..............................................................................  6   Cathodes  .....................................................................................................................  7   Layered  Lithium  Transition  Metal  Oxides  .................................................................  7   Lithium  transition  metal  oxide  Spinels  .....................................................................  7   Lithium  transition  metal  phosphates  Olivines  ..........................................................  8   Electrolytes  .................................................................................................................  8   LITHIUM-­‐SULFUR  BATTERIES  ...........................................................................................  9   Anode  .......................................................................................................................  11   Cathode  ....................................................................................................................  11   Electrolyte  .................................................................................................................  12   LITHIUM-­‐AIR/OXYGEN  BATTERIES  .................................................................................  13   CHALLENGES  ASSOCIATED  WITH  LITHIUM  BATTERIES  ....................................................  14   Performance  .............................................................................................................  14   Ionic  transport  ..........................................................................................................  15   Electrical  Conductivity  ...............................................................................................  15   Volumetric  expansion   ...............................................................................................  15   Side  Reactions  and  Insulating  products  .....................................................................  16   Safety  ........................................................................................................................  17   GOING  THE  NANO-­‐ROUTE  .................................................................................................  17   SIGNIFICANCE  OF  NANOSTRUCTURING   .........................................................................  20   Template-­‐Directed  Materials  .....................................................................................  20   Sol-­‐Gel  ......................................................................................................................  21   Self-­‐Assembly  ............................................................................................................  21   Layer  by  layer  (LBL)  assembly  ....................................................................................  22   CARBON  BASED  NANOMATERIALS  ....................................................................................  23   GRAPHENE  BASED  MATERIALS  ......................................................................................  23   GRAPHENE  USE  IN  LITHIUM  BATTERIES  .........................................................................  25     viii   Anode  Material  .........................................................................................................  25   Graphene  Composites  with  metal/metal  oxides  as  Anode  Material   ..........................  26   Graphene  as  Conducting  Additive  for  Anode  and  Cathodes  .......................................  27   GRAPHENE  NANOPLATELETS  (GNP):  OUR  MATERIAL  OF  INTEREST  ....................................  27   DISSERTATION  OBJECTIVE  .................................................................................................  30   REFERENCES  .....................................................................................................................  32   2   METAL  DOPED  GRAPHENE  NANOPLATELETS  AS  ANODE  MATERIAL  ..............  38   SIGNIFICANCE  ...................................................................................................................  38   APPROACH  .......................................................................................................................  39   EXPERIMENTAL  SECTION   ...................................................................................................  40   SYNTHESIS   ....................................................................................................................  40   Graphene  Nanoplatelets  ...........................................................................................  40   Nickel  doped  GnP  materials  ......................................................................................  41   Microwave  Assisted  Polyol  Method  .......................................................................  42   Solventless  Method  ...............................................................................................  42   MORPHOLOGY  OBSERVATION   .......................................................................................  43   Epoxy  Embedding  and  Polishing  ................................................................................  43   Focused  Ion  Beam  (FIB)  Milling/Sectioning  ................................................................  44   PHYSICAL  CHARACTERIZATION  ......................................................................................  45   ELECTROCHEMICAL  CHARACTERIZATION  .......................................................................  46   RESULTS  &  DISCUSSION  ....................................................................................................  47   MORPHOLOGY  OBSERVATION  OF  NICKEL  DOPED  GNP  ..................................................  47   X-­‐RAY  DIFFRACTION  ANALYSIS  ......................................................................................  55   RAMAN  SPECTROSCOPY  CHARACTERIZATION  ...............................................................  57   ELECTRODE  MORPHOLOGY  ...........................................................................................  60   ELECTROCHEMICAL  CHARACTERIZATION  .......................................................................  62   Galvanostatic  Cycling  ................................................................................................  62   Cyclic  Voltammetry  ...................................................................................................  66   Electrochemical  Impedance  Spectra  ..........................................................................  68   CONCLUSIONS  ..................................................................................................................  70   FUTURE  WORK   ..................................................................................................................  70   NI  NANOPARTICLE  ACTIVATION  ....................................................................................  70   GNP-­‐SILICA  PAPER  .........................................................................................................  71   REFERENCES  .....................................................................................................................  74   3   GRAPHITE  NANOPLATELETS  AS  A  CONDUCTING  ADDITIVE  FOR  LITHIUM   TITANATE  ELECTRODES  ....................................................................................  77   BACKGROUND  ..................................................................................................................  77   SIGNIFICANCE  ...................................................................................................................  80   APPROACH  .......................................................................................................................  81   EXPERIMENTAL  SECTION   ...................................................................................................  81   MATERIALS  ...................................................................................................................  81   ELECTRODE  PREPARATION  ............................................................................................  82     ix   MORPHOLOGY  CHARACTERIZATION  ..............................................................................  82   ELECTROCHEMICAL  CHARACTERIZATION  .......................................................................  82   RESULTS  &  DISCUSSION  ....................................................................................................  83   MORPHOLOGICAL  OBSERVATION  ..................................................................................  83   ELECTROCHEMICAL  CHARACTERIZATION  .......................................................................  88   Influence  of  different  GnP  materials  ..........................................................................  91   Influence  of  concentration  of  additive  .......................................................................  96   CONDUCTION  MECHANISM  ...........................................................................................  99   CONCLUSIONS  .................................................................................................................   102   FUTURE  WORK   .................................................................................................................   102   REFERENCES  ....................................................................................................................   103   4   GRAPHENE  NANOPLATELET  PAPER  AS  CURRENT  COLLECTOR   .....................  106   SIGNIFICANCE  ..................................................................................................................   106   APPROACH  ......................................................................................................................   110   EXPERIMENTAL  SECTION   ..................................................................................................   110   GNP  PAPER  SYNTHESIS  .................................................................................................   110   ELECTRODE  PREPARATION  ...........................................................................................   112   MORPHOLOGY  OBSERVATION   ......................................................................................   112   PROPERTIES  .................................................................................................................   112   ELECTROCHEMICAL  CHARACTERIZATION  ......................................................................   113   RESULTS  &  DISCUSSIONS  .................................................................................................   114   GNP  PAPER  ..................................................................................................................   114   GNP  ELECTRODE  ON  GNP  PAPER  ..................................................................................   115   COMPARISON  OF  GNP  PAPER  AND  COPPER  AS  CURRENT  COLLECTORS   .........................   116   Electrochemical  Impedance  Spectra  .........................................................................   120   LITHIUM  TITANATE  (LTO)  ELECTRODE  ON  GNP  PAPER  ..................................................   122   UNIQUE  GNP  ELECTRODE  ON  C  VEIL   .................................................................................   125   ELECTROCHEMICAL  PERFORMANCE   .............................................................................   128   CONCLUSIONS  .................................................................................................................   131   FUTURE  WORK   .................................................................................................................   131   REFERENCES  ....................................................................................................................   134   5   GRAPHENE  NANOPLATELETS  AS  A  CONDUCTING  TEMPLATE  FOR  LITHIUM-­‐ SULFUR  BATTERIES   .........................................................................................  137   BACKGROUND  .................................................................................................................   137   POLYSULFIDE  SHUTTLE  .................................................................................................   138   SIGNIFICANCE  ..................................................................................................................   139   APPROACH  ......................................................................................................................   140   EXPERIMENTAL  DETAILS  ..................................................................................................   141   MATERIALS  ..................................................................................................................   141   ACTIVE  MATERIAL  PREPARATION  .................................................................................   142   ELECTRODE  PREPARATION  ...........................................................................................   142   PHYSICAL  CHARACTERIZATION  .....................................................................................   143       x   ELECTROCHEMICAL  CHARACTERIZATION  ......................................................................   143   RESULTS  &  DISCUSSION  ...................................................................................................   144   GNP-­‐SULFUR  COMPOSITE  SYNTHESIS  ...........................................................................   144   PHYSICAL  CHARACTERIZATION  .....................................................................................   144   ELECTRODE  MORPHOLOGY  ..........................................................................................   148   ELECTROCHEMICAL  CHARACTERIZATION  ......................................................................   150   CONCLUSIONS  .................................................................................................................   156   FUTURE  WORK   .................................................................................................................   157   REFERENCES  ....................................................................................................................   159   6   CONCLUSIONS  ............................................................................................  163       xi   LIST  OF  TABLES     Table  2-­‐1:  Summary  Table  for  different  nickel  doped  GnP  materials  synthesized   ........................  41   Table  2-­‐2:  Intensity  values  ID,  IG,  ID/IG  and  disorder  parameter  of  GnP_Ni  Materials    ................  60   Table   2-­‐3:   Resistance   values   of   different   components   GnP   and   GnP_Ni   electrodes   obtained   by   impedance  fitting  analysis  ............................................................................................................  69   Table  3-­‐1:  Different  carbon  materials  with  their  physical  properties  ...........................................  84   Table   3-­‐2:   Resistance   values   obtained   by   fitting   experimental   impedance   data   to   the   equivalent   circuit  ............................................................................................................................................  96   Table  4-­‐1:  Resistance  values  obtained  by  parametric  fitting  of  impedance  data   .......................  122   Table  4-­‐2:  Comparison  of  properties  of  copper  foil  and  GnP  paper  as  current  collectors  ..........  131   Table  5-­‐1:  Resistance  values  obtained  by  fitting  impedance  data  using  the  equivalent  circuit  .  155               xii   LIST  OF  FIGURES     2 Figure  1-­‐1:      Gravimetric  and  Volumetric  energy  density  of  different  battery  systems      ..............  2   Figure   1-­‐2   :   Schematic   showing   the   working   of   lithium   ion   battery       (Adapted   from   http://bestar.lbl.gov/venkat/files/batteries-­‐for-­‐vehicles.pdf)   ........................................................  4   15 Figure  1-­‐3:  Schematic  showing  the  working  of  Lithium-­‐Sulfur  Battery      Text  within  figure  is  not   meant  to  be  readable  and  is  for  visual  reference  only.  ................................................................  10   Figure   1-­‐4:   Schematic   showing   the   setup   of   Lithium-­‐air   (O2   )   batteries   (a)   non-­‐aqueous   10 electrolyte  (b)  aqueous  electrolyte   ............................................................................................  14   Figure  1-­‐5  :  Nanostructures  based  on  graphene  (a)  0-­‐D  fullerene  (b)  1-­‐D  carbon  nanotube  (c)  3-­‐D   44 graphite    ....................................................................................................................................  25   Figure  1-­‐6  :  Schematic  showing  synthesis  of  Graphene  nanoplatelets  (GnP)  by  intercalation  and   61 exfoliation  procedure  and  SEM  images  of  the  corresponding  stages    (Scale  bar  :  (a)  300  μm,   (b)  500  μm,  (c)  50  nm)  ..................................................................................................................  29   Figure   2-­‐1:   Schematic   showing   the   potential   of   metal-­‐doped   carbon   as   anodes   for   lithium   ion   batteries  .......................................................................................................................................  39   12 Figure  2-­‐2:  Schematic  of  FIB-­‐SEM  setup   ....................................................................................  45   Figure   2-­‐3:   Schematic   &   Picture   of   Three   electrode   Swagelok   type   T-­‐   cell   for   electrochemical   measurements  ..............................................................................................................................  47   Figure  2-­‐4:  Nickel  nanoparticle  doped  GnP  synthesized  by  Polyol  assisted  microwave  process  ..  48   Figure  2-­‐5:  STEM-­‐EDS  Analysis  of  GnP_Ni-­‐5  nanoparticles.  Text  within  figure  is  not  meant  to  be   readable  and  is  for  visual  reference  only.  .....................................................................................  49   Figure  2-­‐6:  TGA  Analysis  of  GnP_Ni-­‐5  showing  the  concentration  of  metal  nanoparticles  ..........  50   Figure  2-­‐7:  GnP  nanoparticles  doped  with  nickel  NPs  clusters  of  30-­‐40nm  ..................................  51   Figure  2-­‐8:  Thermogravimetric  analysis  of  GnP_Ni-­‐30c  material  .................................................  51   Figure  2-­‐9:  GnP  nanoplatelets  doped  with  nickel  NPs  of  size  60-­‐80  nm  .......................................  52     xiii   Figure  2-­‐10:  Thermogravimetric  analysis  of  GnP_Ni-­‐60  material  ................................................  52   Figure  2-­‐11:  SEM  Images  of  GnP_Ni-­‐60  at  different  concentrations:(a)  5%,(b)  10%,(c)15%   ........  53   Figure  2-­‐12:  Thermogravimetric  Analysis  (TGA)  of  GnP-­‐Ni  materials  showing  the  concentration   of  nickel  nanoparticles  in  the  sample  ...........................................................................................  54   Figure   2-­‐13:   XRD   pattern   of   nickel   doped   materials   in   comparison   with   undoped   GnP     (a)   Full   spectra  (b)  Ni  peaks  ......................................................................................................................  56   Figure  2-­‐14:  Raman  Spectroscopy  Analysis  of  different  GnP_Ni  materials  ..................................  59   Figure  2-­‐15:  Electrode  Morphology  of  top-­‐view  and  cross-­‐section  view  (respectively)  of  Undoped   GnP  [(a),  (b)];  GnP_Ni-­‐5  [(c),  (d)];  GnP_Ni-­‐30c,  GnP_Ni-­‐60  [(e),  (f)]  .............................................  61   Figure  2-­‐16:  Testing  Protocol  for  galvanostatic  analysis  of  electrodes  ........................................  63   Figure  2-­‐17:  Galvanostatic  performance  of  nickel  doped  materials  in  comparison  with  undoped   GnP  (a)  Undoped  GnP  (b)  GnP_Ni-­‐5  (c)  GnP_Ni-­‐30c  (d)  GnP_Ni-­‐60c  ...........................................  64   Figure  2-­‐18:    Cyclic  Voltammogram  of  Ni  doped  materials  ..........................................................  67   Figure  2-­‐19:  (a)  Nyquist  plots  of  undoped  GnP  and  GnP_Ni-­‐60  obtained  after  5  cycles  of  charge   discharge   at   C/5   rate.   The   solid   lines   correspond   to   GnP   and   the   dotted   lines   correspond   to   GnP_Ni-­‐60.  (b)  Equivalent  circuit  used  for  fitting  analysis  ...........................................................  69   Figure  2-­‐20:  Schematic  showing  the  synthesis  procedure  for  GnP-­‐Silica  Nanocomposite  ...........  71   Figure  2-­‐21:  Self-­‐Assembled  silica-­‐graphene  paper:  Top  view  .....................................................  72   Figure  2-­‐22:  Cross-­‐Sectional  View  of  Self-­‐Assembled  silica-­‐graphene  paper  (a)  Whole  electrode   (b)  High  magnification  (c)  Back-­‐scatter  image  of  the  section  shown  in  (b)  ..................................  72   Figure  3-­‐1:  Schematic  showing  battery  setup  with  lithium  titanate  as  anode  .............................  79   4 Figure  3-­‐2:  Crystal  structure  of  (a)  Spinel  Li4Ti5O12  and  (b)  Rock-­‐salt  Li7Ti5O12        .....................  79   Figure  3-­‐3:  Schematic  and  pictures  of  Two-­‐electrode  Coin  cell  ....................................................  83   Figure  3-­‐4:  SEM  Images  of  (a)  GnP-­‐25,  (b)  GnP-­‐5,  (c)  GnP-­‐1  ,  (d)  Super  P,  (e)  LTO    (Scale  bar-­‐  (a):   10 μm;   (b),(c):   1   μm;   (d),(e):   300   nm).The   image   of   LTO   is   taken   from   LTO+GnP   electrode,   because  of  difficulty  in  doing  SEM  of  pure  LTO  powder  due  to  its  poor  conductivity  ...................  85   Figure  3-­‐5:  SEM  Images  of  top  view  of  various  LTO  electrodes  (a)  LTO+GnP-­‐25,  (b)  LTO  +GnP-­‐5,   (c)  LTO  +GnP-­‐1,  (d)  LTO+Super  P  electrodes.    In  all  these  electrodes,  the  concentration  of  carbon   additive  material  is  10%  by  weight.   ..............................................................................................  86     xiv   Figure  3-­‐6:  Low  magnification  (Left)  and  corresponding  High-­‐magnification  (Right)  SEM  Images   of  the  cross-­‐sectional  views  of  the  following  electrodes    (a),  (b)  LTO+GnP-­‐25;  (c),(d)  LTO  +GnP-­‐5;   (e),(f)   LTO   +GnP-­‐1;   (g),(h)   LTO+   Super   P.     In   all   these   electrodes,   the   concentration   of   carbon   additive  material  is  10%  by  weight.   ..............................................................................................  87   Figure   3-­‐7:   Galvanostatic   performance   of   at   different   charge   rates   (C/5,C/2,C   and   2C)   of     (a)   LTO+GnP-­‐25,  (b)  LTO+GnP-­‐5,  (c)  LTO+GnP-­‐1,  (d)  LTO+Super-­‐P  ....................................................  89   Figure  3-­‐8:  Capacity  profiles  of  LTO+GnP-­‐25  Electrode  at  different  charge  rates  ........................  91   Figure   3-­‐9:   Comparison   of   charge-­‐discharge   performance   of   different   LTO   electrodes,   with   10   wt%  conducting  additive  on  different  scales  ................................................................................  94   Figure  3-­‐10:  (a)  Nyquist  plots  of  different  electrodes,  obtained  after  5  cycles  at  C/5  charge  rate   (b)  Equivalent  circuit  used  for  fitting  (c)  Experimental  vs.  fitting  data  for  LTO+GnP-­‐5  .................  95   Figure   3-­‐11:   Cross-­‐sectional   images   of   LTO-­‐GnP-­‐25   electrodes,   with   GnP   concentration     (a)   10wt%,  (b)  5  wt%,  (c)  2  wt%  .........................................................................................................  97   Figure  3-­‐12:Galvanostatic  performance  of  LTO+GnP-­‐25  electrodes  for  different  concentration  of   GnP-­‐25  in  the  electrodes  ..............................................................................................................  98   Figure  3-­‐13:  Nyquist  plots  of  LTO  electrodes  with  varying  concentration  of  GnP-­‐25    ..................  98   Figure  3-­‐14:  Secondary  electron  and  back  scatter  electron  images  of  (a)  LTO  +  Super  P  electrodes   (b)  LTO  +  GnP  electrodes  ............................................................................................................  101   Figure   4-­‐1   :   Weight   and   cost   distribution   of   different   battery   components   for   High   Power   Cell   1 (based  on  data  from  Gaines  et.  al   )  ..........................................................................................  107   Figure  4-­‐2:  Schematic  illustrating  the  synthesis  of  GnP  Paper  ...................................................  111   Figure  4-­‐3:  SEM  Images  of  the  GnP  paper  (as  made)  at  different  magnifications  .....................  114   Figure   4-­‐4:   Cross-­‐sectional   SEM   Images   of   the   electrode   at   different   magnifications   (a)   &   (b),   followed  by  high  mag  images  of  (c)  GnP  paper  (d)  active  material  region  ................................  115   Figure   4-­‐5:   SEM   images   of   X-­‐sectional   view   of   (a)   current   collector   GnP   paper,   GnP   electrode   on   GnP  current  collector  (b)  the  unpressed  and  (c)  pressed  electrode  ............................................  116   Figure  4-­‐6:  SEM  images  showing  cross-­‐sectional  view  of  GnP-­‐15  electrode  on  different  current   collectors:    (a)  Copper  (b)  GnP  paper  ..........................................................................................  117   Figure  4-­‐7:  Galvanostatic  Performance  of  GnP-­‐15  electrode  on  Copper  current  collector  .........  118     xv   Figure   4-­‐8:   Galvanostatic   Performance   of   GnP-­‐15  electrode   on   GnP   paper   current   collector   (a)   Active  material  (b)  Total  weight  including  the  substrate  weight  of  GnP  paper   ..........................  119   Figure   4-­‐9:   Nyquist   plots   of   GnP   electrode   on   different   current   collectors:   Copper   foil   and   GnP   Paper,  after  10  complete  charge  discharge  cycles  .....................................................................  120   Figure   4-­‐10:   (a)   Equivalent   circuit   for   fitting   of   impedance   data;   Comparison   of   experimental   and  fitted  data  for  (a)  GnP  electrode  on  Cu  foil  after  10  cycles  (c)  GnP  electrode  on  GnP  Paper   after  10  cycles  .............................................................................................................................  121   Figure   4-­‐11:   SEM   images   showing   cross-­‐sectional   view   of   LTO   electrodes   on   different   current   collectors  (a)  On  copper  (b)  On  GnP  Paper  .................................................................................  123   Figure   4-­‐12:   Electrochemical   Performance   of   LTO   electrodes   casted   on   different   current   collectors  (a)  On  copper  (b)  On  GnP  Paper  .................................................................................  124   ® Figure  4-­‐13:  Optical  Images  of  the  Optimat  carbon  veil  .........................................................  125   Figure   4-­‐14:   (a)   Picture   and   (b),(c),(d)   SEM   Images   of   unpressed   GnP   electrode   on   C   veil   as   current  collector  .........................................................................................................................  127   Figure   4-­‐15:   (a)   Picture   and   (b)   SEM   Images   of   pressed   GnP   electrode   on   C   veil   as   current   collector  (Scale  bar-­‐  (b):  10  μm)  .................................................................................................  127   Figure  4-­‐16:  Cyclic  Voltammogram  (third  sweep)  of  GnP  on  C  veil  ............................................  128   Figure   4-­‐17:   Galvanostatic   Performance   of   GnP   on   C   veil   electrode   (a)   based   on   active   electrode   material  (b)  based  on  complete  weight  ......................................................................................  129   1 Figure  5-­‐1:  Schematic  showing  the  working  of  Lithium-­‐Sulfur  Battery    ....................................  138   Figure  5-­‐2:  Schematic  depicting  synthesis  process  for  GnP-­‐S  composite   ....................................  144   Figure   5-­‐3:   TGA   Analysis   showing   the   removal   of   sulfur   around   300°C,   giving   an   indication   of   Sulfur  concentration  in  the  composite  ........................................................................................  145   Figure  5-­‐4:  EDS  Analysis  of  GnP-­‐S  composite  material  ...............................................................  146   Figure   5-­‐5:   XRD   Diffraction   of   GnP-­‐S   composite   compared   to   the   as   received   sulfur   and   intermediate  ball  milled  mixture  ................................................................................................  147   Figure  5-­‐6:  Schematic  showing  the  electrode  preparation  of  GnP-­‐S  composite  .........................  148   Figure  5-­‐7:  Top-­‐view  of  GnP-­‐S  electrode  ....................................................................................  149   Figure  5-­‐8:  Secondary  electron  and  Back-­‐scatter  electron  images  of  GnP+S  electrode  .............  149     xvi   Figure  5-­‐9:  Cross-­‐sectional  Image  of  GnP-­‐S  electrode  at  different  magnifications  ....................  150   Figure   5-­‐10:   Galvanostatic   Performance   of   GnP-­‐S   composite   electrode   for   (a)   different   charge   rates  (b)  long  cycling  ..................................................................................................................  153   Figure  5-­‐11:  Charge-­‐Discharge  capacity  profiles  at  different  charge  rates  C,  2C,  5C.  ................  154   Figure  5-­‐12:  Nyquist  plot  of  GnP+S  electrode  before  and  after  10  and  100  cycles  .....................  154   Figure   5-­‐13:   (a)   Equivalent   circuit   used   for   fitting   impedance   data   (b)   Comparison   of   experimental  and  fitted  impedance  data  for  GnP-­‐S  electrode  after  cycling  for  10  cycles  ..........  155   Figure  5-­‐14:  Schematic  depicting  the  synthesis  of  sulfur  impregnated  GnP  Paper  ....................  157     xvii     1 INTRODUCTION   DEMAND  FOR  ENERGY  STORAGE   With   the   finite   availability   of   fossil   fuels   and   the   imbalance   in   demand   and   replenishment   of   coal   and   petroleum   based   reserves,   there   has   been   immense   focus   on   alternative   energy   resources   for   catering   to   the   increasing   power   demands.   The   utilization   of   alternative   energy   sources   such   as   solar,   wind,   water   or   nuclear   based   energy   have   been   critically   evaluated   by   their   availability,   sustainability,   efficiency,   environmental   impact   and   need  for  energy  storage  and  transmission.  The  first  three  are  very  attractive  but  limited  by  their   availability   at   certain   times   and   locations   thus   necessitating   the   additional   need   for   energy   storage   and   transmission.   However   nuclear   energy   requires   a   sophisticated   setup   and   complicated  disposal  due  to  the  hazardous  nature  of  the  reactants  involved  leading  to  a  high   capital  investment.  Moreover,  with  advancements  in  technology,  diverse  high  power  and  high-­‐ energy   requirements   for   portable   applications   has   been   created.   Electrochemical   energy   storage   is   a   potential   solution   that   can   meet   these   requirements   in   an   efficient   and   environmentally   friendly   manner   if   gravimetric,   volumetric   and   economic   metrics   can   be   achieved.   LITHIUM  BASED  BATTERIES   Lithium  based  rechargeable  batteries  have  emerged  as  the  most  promising  solution  to   the   increasing   energy   needs,   ever   since   its   advent   in   1970s.   Firstly   their   use   as   primary     1   batteries,   and   then   their   use   as   rechargeable   batteries   (after   commercialization   by   Sony   in   1990),  lithium  ion  batteries  were  the  preferred  choice  for  diverse  portable  applications.    They   had   significant   advantages   over   other   battery   technologies   because   of   high   energy   density   (Figure   1-­‐1),   lower   weight   and   good   cycle   life.   These   advantages   can   be   attributed   to   high   1 electropositive  nature  and  low  weight  of  lithium .  The  potential  of  these  batteries  extends  over   a  wide  domain  of  mobile  applications  ranging  from  small  portable  electronic  appliances  such  as   cell  phones,  iPods  and  laptops  to  their  use  in  hybrid  electric  vehicles.     2 Figure  1-­‐1:      Gravimetric  and  Volumetric  energy  density  of  different  battery  systems     “For  interpretation  of  the  references  to  the  color  in  this  and  all  other  figures,  the  reader   is  referred  to  the  electronic  version  of  this  dissertation”     2   LITHIUM-­‐ION  BATTERIES   A  commercial  lithium  ion  battery  involves  a  graphitic  anode  with  a  lithium  metal  oxide   (LiMO2)   cathode,   sandwiched   in   a   cell   with   a   porous   non-­‐conducting   separator   and   LiPF6   electrolyte   in   a   mixture   of   organic   solvents   (Ethylene   Carbonate   +   Dimethyl   Carbonate).   The   lithium   ions   shuttle   between   the   two   electrodes   during   charge   and   discharge,   with   a   corresponding   exchange   of   electrons   through   the   external   electrical   circuit.   During   discharge,   the  lithium  ions  travel  from  the  graphite  anode  to  the  cathode  through  the  electrolyte,  and  the   process   is   reversed   in   the   charging   step,   where   lithium   ion   migrate   from   cathode   and   move   towards  the  anode,  and  re-­‐intercalate  between  the  layers  of  graphite.   In   the   first   cycle,   a   thin   film   called   Solid   Electrolyte   Interface   (SEI)   is   formed   on   the   surface  of  electrodes,  due  to  the  high  field  strength  at  the  surface  resulting  in  the  degradation   of   the   electrolyte.   This   passivated   electrically   insulated   film   is   comprised   of   lithium   organic   and   inorganic   compounds   and   consumes   lithium   in   the   first   cycle,   resulting   in   irreversible   loss   of   capacity.   However,   the   SEI   layer   does   not   decompose   on   charging   and   can   be   beneficial   in   3,4 stabilizing   the   electrode-­‐electrolyte   interface   and   avoid   further   electrolyte   degradation .   Though,  for  high  surface  area  electrode  materials,  there  can  be  significant  SEI  film  formation,   which  can  result  in  a  battery  temperature  increase  (exothermic  reaction)  and  hindered  charging   and  discharging  kinetics.  Many  different  electrode  materials  can  be  used  as  cathode,  and  anode   materials   for   lithium   ion   batteries.   Different   cathode   and   anode   materials   corresponding   to   their  potential  and  capacity  valueshave  been  studied  and  summarized  in  detail  by  Tarascon  et.   2 al .  Some  of  the  most  popular  electrode  materials  are  described  briefly  in  the  next  section.     3   e"# - Al Current Collector Cu Current Collector + Li+# Anode Separator Cathode Anode Material : Graphite, LTO, Si, SnO2 Cathode Material : LiFePO4, LiCoO2 Conducting Additive: Carbon black, Acetylene Black Binder: PVDF, PTFE   Figure  1-­‐2  :  Schematic  showing  the  working  of  lithium  ion  battery       (Adapted  from  http://bestar.lbl.gov/venkat/files/batteries-­‐for-­‐vehicles.pdf)   6𝐶 +  𝐿𝑖𝐶𝑜𝑂!   ⇋ 𝐿𝑖! 𝐶! + 𝐿𝑖!!! 𝐶𝑜𝑂!     4   Anodes         Metallic  lithium  could  have  been  the  ideal  choice  for  anode  material,  because  of  its  high   energy   density   and   maximum   potential   difference   with   respect   to   the   cathode,   however   its   usability   has   not   been   realized   because   of   its   high   reactivity,   dendrite   formation   and   low   5 melting   point,   thus   raising   concerns   about   safety .   Some   common   anode   materials   that   are   being  considered  for  these  batteries  include:   Graphite     Graphite   is   the   most   commonly   used   anode   material   for   lithium   ion   batteries   due   to   its   abundance,  good  cycle  life,  reasonable  capacity  and  low  cost.  These  electrodes  operate  by  an   intercalation  mechanism  based  on  stepwise  insertion  of  lithium  ions  in  between  the  graphene   6 layers  (staging).     C6  +  Li+  +  e−  ↔  LiC6.   The  theoretical  storage  capacity  of  graphite  is  372  mAh/g  and  it  has  a  nominal  volume   expansion   of   10   %   in   the   direction   perpendicular   to   the   graphene   plane   after   lithium   7 intercalates  in  between  graphene  layers .     Nano-­‐carbon  materials  such  as  carbon  nanotubes  and  graphene  have  been  a  subject  of   interest   in   recent   years,   and   have   been   known   to   deliver   significantly   higher   capacity   in   comparison  to  graphite  powder.  These  materials  are  discussed  in  detail  in  a  later  section.     5   Transition  Metal  Oxides     Lithium   titanate   is   another   promising   anode   material,   which   operates   by   intercalation   approach.  Despite  of  its  low  storage  capability  170mAh/g  ,  LTO  has  shown  potential  because  of   its  high  rate  performance,  zero  strain  with  intercalation  and  excellent  cycle  life.   Li4Ti5O12  +  3Li+  +  3e−  →  Li7Ti5O12   Moreover,   LTO   operates   at   relatively   higher   potential,   at   which   there   is   no   electrolyte   decomposition  and  hence,  is  safer  and  more  reliable.     Several   other   metal   oxides   based   on   conversion   reactions   such   as   NiO,   MnO2,   Co3O4   5,8 and  Fe2O3,  are  being  explored  as  anode  materials .     Metal  Anodes  /  Alloying  Anodes       Other   anode   materials   such   as   Si,   Sn,   and   Ge   are   based   on   alloying   interaction   with   lithium   and   hence   can   deliver   significantly   higher   capacity   (4200   mAh/g,   990   mAh/g   and   1600mAh/g   respectively) 8,9 .   However   they   undergo   a   significant   volume   change   during   the   alloying-­‐ dealloying   process,   which   can   result   in   material   failure,   loss   of   conductivity   and   electrode   pulverization.   The   current   research   focus   is   on   developing   nanostructured   composite   electrodes   of   these   high   capacity   materials   with   a   flexible   conducting   matrix,   so   as   to   ensure   high   capacity   without   loss   of   electrode   conductivity   and   act   as   a   buffer   for   accommodating   strain  during  volume  expansion.     6     Cathodes      Cathodes  are  the  limiting  factor  so  far  that  are  restricting  any  drastic  improvements  in   the  performance  of  lithium-­‐ion  batteries.  The  selection  of  cathode  materials  is  evaluated  on  the   basis  of  its  storage  capacity,  rate  performance  and  thermal  stability,  which  in  turn  affects  the   10 safety   of   the   batteries .   The   different   categories   of   cathode   materials   currently   being   investigated  are  briefly  described  below.     Layered  Lithium  Transition  Metal  Oxides   LiCoO2   is  the  most  popular  cathode  material  for  lithium  ion  batteries  because  of  good  cycling   stability,   and   easy   synthesis.   Because   of   concerns   about   the   cost,   toxicity   and   availability   of   11 cobalt,  other  layered  systems  such  as  LiMnO2  and  LiNiO2  are  also  being  explored .  The  layered   structure   of   these   materials   is   attractive   for   easy   accessibility   of   lithium   ions.   However   these   materials  suffer  from  the  problems  of  low  tap  density(“amount  of  material  that  can  be  loaded   10 into  a  specific  volume ”),  lower  power  performance  and  capacity  fade  on  cycling.   Lithium  transition  metal  oxide  Spinels       Spinel   LiMn2O4   is   another   potential   candidate   for   cathodes   for   lithium   in   batteries   and   has   attractive  features  such  as  eco-­‐friendly,  high  power  capability,  abundance,  low  cost  and  better   2 2+ safety .  But  it  has  a  high  reactivity  with  non  aqueous  electrolytes,  which  can  cause  the  Mn   ions   to   dissolve   in   the   electrolyte,   and   get   deposited   on   the   anode,   thus   degrading   the     7   10 performance  and  causing  capacity  fade .    This  problem  is  being  addressed  by  nanostructuring,   10 developing  new  materials  and  use  of  electrolyte  additives .   Lithium  transition  metal  phosphates  Olivines       LiFePO4   has   emerged   to   be   a   promising   cathode   material   primarily   because   of   its   low   cost,   thermal   stability   and   good   cycle   performance.   In   the   initial   studies   of   LiFePO4   starting   1997,   several   challenges   such   as   low   electronic   and   ionic   conductivity   and   low   energy   density   were   10 faced,   which   limited   its   potential .   However,   these   drawbacks   were   taken   into   account   by   developing   nanomaterials   for   improved   ionic   mobility   and   conductive   surface   coatings   for   7 enhanced  conductivity .    Recent  studies  have  explored  alternate  transition  metals  such  as  Mn,   Ni,  Co  to  replace  Fe,  which  can  increase  the  operating  potential  of  this  material.   Electrolytes    Conventionally,  Lithium  hexafluorophosphate  (LiPF6)  is  used  as  the  electrolyte  salt  in  a   combination   of   organic   solvents   (Ethylene   carbonate,   Dimethyl   carbonate,   Propylene   carbonate).   These   electrolytes   pose   a   potential   safety   hazard   in   unusual   situations   of   short-­‐ circuits,  leakage,  overcharging  and  thermal  runaway  reactions,  because  of  their  high  reactivity   and  corrosive  nature.    In   addition   to   developing   new   electrode   materials,   there   has   been   a   tremendous   interest   in   exploring   electrolyte   improvements,   to   enable   the   applications   of   lithium   ion   batteries  for  diverse  high  power  and  high  energy  applications.  The  progress  in  this  direction  is     8   directed  towards  solid  polymer  electrolytes,  which  are  advantageous  due  to  their  higher  energy   density,  no  leakage  or  explosion  problems,  ease  in  manufacturing  and  low  cost.  However,  these   materials  are  still  restricted  due  to  low  conductivities  at  room  temperature,  thus  making  them   applicable  only  to  slow  charge  rate  systems 2,10 .  Gel  electrolytes  are  the  intermediate  answer  to   these  two  categories,  having  leakage  free  properties  with  good  ionic  conductivity.     LITHIUM-­‐SULFUR  BATTERIES   Lithium-­‐sulfur  batteries  offer  a  promising  solution  for  high  energy  density  applications.   They   are   of   particular   interest   due   to   low   cost,   non-­‐toxicity,   abundance   and   high   theoretical   capacity  of  sulfur.  These  batteries  are  based  on  the  redox  reaction  mechanism  for  conversion  of   -­‐ Sulfur   (S8   molecule)   to   Li2S.   Theoretically   these   batteries   are   capable   of   delivering   2500   Wh.kg 1   -­‐1 12 and   2800   Wh.l   gravimetric   and   volumetric   energy   density   respectively .   However,   they   suffer  from  serious  limitations,  such  as  poor  conductivity  of  sulfur  and  polysulfides,  dissolution   of   reaction   intermediates   into   the   electrolyte,   thus   compromising   the   efficiency   and   12–14 reversibility  of  these  electrodes .  The  high  order  polysulfides  formed  at  the  cathode,  which   are   soluble   in   the   electrolyte,   can   migrate   to   the   anode,   get   converted   to   the   low-­‐order   polysulfide   and   transport   back   to   the   cathode,   where   they   get   oxidized   and   return   back   to   anode.   This   shuttle   mechanism   leads   to   capacity   fade   and   loss   of   active   electrode   area   when   the  reaction  proceeds  to  form  insoluble  and  insulating  Li2S.      Significant  research  is  being  done   in   the   recent   times,   trying   to   overcome   these   challenges   by   developing   conducting   matrices,     9   which   are   partially   restrictive   to   polysulfides,   and   by   exploring   alternative   electrolytes   which   14 don’t  react  with  polysulfide  compounds .   Why Lithium Sulfur Te 2  Li + 2  𝑒 ! + S ⇌   Li! S   Charge (Li plating) Discharge (Li stripping) Anode (-) Li+ Li+ S Li+ Li S S S Li 0 S S S S S S Li S S S Li+ Li S Li S S Li Li+ Li+ S8 Li+ Li+ S • Cathode (+) Li S Li Li2S6 Li Li S Li Li+ – Li2S8 • S S S Li S Li2S4 Li S Li2S3 Li+ • Li+ Li S Li + S S S S S S Li S Li Li S Li2S S Li Li S Polysulfide Shuttle - Li2S2 S S Li+ Li S S Li S Li Load / Charger Lith the form cath Li Li+ On plat the Hig to L elec ano gro +   15 Figure  1-­‐3:  Schematic  showing  the  working  of  Lithium-­‐Sulfur  Battery       Text  within  figure  is  not  meant  to  be  readable  and  is  for  visual  reference  only.   Theoretical Energy: ~2800Wh/l an   10   Anode   Lithium   metal   is   used   as   the   anode   for   lithium   sulfur   battery   and   accounts   for   their   high   performance,   because   of   the   high   theoretical   specific   energy   of   lithium.   However,   there   is   a   necessity  to  use  excessive  lithium,  since  complete  utilization  of  its  capacity  is  not  achieved.  This   inefficiency  results  from  two  factors,  repeated  formation  of  SEI  layer  in  every  cycle  and  parasitic   consumption   of   Lithium   in   polysulfide   reaction   to   form   Li2S   on   the   anode   (polysulfide   shuttle) 16,17 .  Also,  dendrite  formation  is  another  problem,  that  can  lead  to  short-­‐circuits,  and   hence   the   failure   of   the   batteries.   To   address   these   issues,   different   approaches   such   as   improved  SEI  layer  formation,  electrolyte  additives  for  protective  films  and  polymer  protection   18 layer  for  lithium  anode  protection,  are  being  explored .  These  disadvantages,  safety  concerns   and   limited   amount   of   Lithium   resources   has   prompted   the   need   for   alternative   anode   materials 16,18 .   Alloying   anodes,   such   as   Sn   or   Si   can   be   used   as   anode   options;   however,   their   usage   in   this   system   will   either   involve   prelithiation   before   cell-­‐assembly   or   the   use   of   Li2S   as   the   16,19 cathode .     Cathode   Sulfur   composites   with   carbon   materials   are   used   as   the   cathode   for   this   system.   The   poor   conductivity   of   sulfur   and   the   polysulfide   compounds   makes   the   presence   of   a   conducting   material   indispensable.     Porous   carbon   frameworks,   mesoporous   carbon   materials   and   other     11   nano   carbon   materials   like   carbon   nanotubes,   graphene,   carbon   fibers   etc.   are   evaluated   for   their   role   in   imparting   necessary   conductivity   and   restraining   polysulfide   dissolution 12,13,20 .   The   morphology   and   structure   of   electrode   can   make   a   significant   impact   here,   and   hence   different  nanostructures  such  as  template  structures,  mesoporous  frameworks,  porous  /tubular   21 hosts  and  hierarchical  structures,  are  being  evaluated .  This  approach  is  discussed  in  detail  in   Chapter  5.   Electrolyte   Electrolyte  is  one  of  the  most  crucial  components  for  practical  development  of  efficient   lithium-­‐sulfur  batteries.  The  problem  of  dissolution  of  polysulfides  in  the  liquid  electrolyte  is  a   major   challenge,   which   results   in   parasitic   consumption   of   lithium,   loss   of   sulfur   and   capacity   fade.  Selection  of  appropriate  liquid  electrolytes,  use  of  ionic  liquids  and  additive  materials  such   as   LiNO3,   can   help   in   protecting   lithium   anode   by   forming   a   passivating   layer   and   reducing   polysulfide   dissolution 13,18 .   Various   polymer   electrolytes,   and   gel–electrolytes   and   ceramic   electrolytes   are   being   explored,   owing   to   slow   dissolution   of   polysulfides   and   better   separation   16,17 of  lithium  anode  from  the  cathode .  However,  these  alternatives  are  still  facing  challenges   13 of  low  ionic  conductivity,  electrode-­‐electrolyte  interface  and  reactivity  with  lithium  metal .     12   LITHIUM-­‐AIR/OXYGEN  BATTERIES   Lithium  –air  batteries  are  being  envisioned  as  a  future  alternative  to  high  performance   energy   solution,   since   their   theoretical   and   also   estimated   practical   energy   density   are   significantly   higher   than   all   other   battery   systems 5,10 .   The   technology   has   been   explored   for   primary   batteries   for   a   while,   however   its   application   for   rechargeable   batteries   gained   10,14 attention  after  reports  of  reversibility  were  shown  by  the  use  of  MnO2  as  catalyst .  These   batteries   have   been   investigated   for   aqueous   as   well   as   non-­‐aqueous   electrolytes.   Figure   1-­‐4   below  shows  the  schematics  of  lithium-­‐air  batteries  in  both  setups.  The  anode  here  is  Lithium   + metal,   which   gets   oxidized   on   discharge   and   forms   Li ,   which   is   transferred   through   the   electrolyte.   On   the   cathode,   which   is   a   porous   carbon   network,   oxygen   from   the   atmosphere   dissolves   in   the   electrolyte   and   interacts   with   the   lithium   ion   and   is   reduced.   Different   reaction   products  are  formed  based  on  the  kind  of  electrolyte  used  as  shown  below.     2𝐿𝑖 !   + 2  𝑒 !   + 𝑂! ⇋ Li! 𝑂!   2𝐿𝑖 !   + 2  𝑒 !   + 1 𝑂 +   𝐻! 𝑂 ⇋ 2𝐿𝑖𝑂𝐻   2 ! These   batteries   face   significant   hurdles   before   they   can   be   used   for   commercial   applications.   Some   of   them   involve   the   dendritic   formation   and   safety   issues   associated   with   lithium   metal   anodes.   Other   challenges   involve   use   of   suitable   electrolytes   to   avoid   any     13   2 2 2 voltage of corresponding to a theoretical open electrolyte 2 eÀQ2 LiOH,3.35 V.[139] It should be noted that thecircuit voltage of 3.35 V.[139] involved in noted that the electrolyte solvent, H2O, is It should be the cell reaction and this is solvent, H2O,referred to asin Li–water reaction The aqueous Li– sometimes is involved a the cell battery. and this is sometimes referred to as a Li–water battery. The where the overall air battery can also operate in acidic media, aqueous Li– precipitation,   and   can also operate in materials   allowing   O2   but   restricting   CO2   and   of   cathode   air battery selectivity  Li + 1=2 O2 acidic media, whereH2Ooverall an cell reaction is 2 + 2 AHQ2 ALi + the and with H2 O cell reaction is 2 Li + 1=2 O2 + 2 AHQ2 ALi + H2O and with an 10,14,22 .     Figure 17. Figure 17. Challen mance mance pr properties fading. fading. D Data take Li/1 in pr Li/1 m LiPF6m LiP [149] (a)  (b)     wires). wires).[149 Use of reduces, although reduces, Li dendrites taken Li dendrit Figure  1-­‐4:  Schematic  showing  the  setup  of  Lithium-­‐air  (O2  )  batteries  (a)  non-­‐aqueous   10 try, Figure 16.16. Schematic representation of aqueousnon-aqueous Li– Schematic representation of aqueous and and non-aqueous Li– American Che try, Amer Figure electrolyte  (b)  aqueous  electrolyte   Society.Society. airair cells. cells. 10010 www.angewandte.org LITHIUM  BATTERIES   2012 Wiley-VCH Verlag GmbH GmbHKGaA, Weinheim  & Co. CHALLENGES  ASSOCIATED  WITH   10010 www.angewandte.org  2012 Wiley-VCH Verlag & Co. KGaA, We Performance   To  cater  to  the  increased  energy  and  power  requirements,  new  materials  with  higher  capacity,   better   rate   performance   and   high   coulombic   efficiency   are   needed.   This   need   motivates   the   development  of  new  chemistry,  new  materials  and  new  morphologies  (nano),  which  will  have   inherently   better   storage   capabilities.   In   addition   to   high   storage   capacity,   reversibility,   cell   voltage  and  rate  capability  are  crucial  factors  governing  the  application  of  a  material  for  desired   applications.   However,   complete   utilization   of   theoretical   capacity   can   be   a   challenge.   Such   situations   can   arise   due   to   incomplete   accessibility,   low   conductivity   and   undesirable   side   reactions.  Another  challenge  faced  in  the  performance  is  in  case  of  gravimetric  energy  density     14   of   the   battery.   This   entails   the   use   of   inactive   components   in   the   battery   such   as   conducting   additives,   binders,   current   collectors   and   packaging   materials.   Hence,   there   is   a   focus   on   reducing  the  dead  weight  in  the  cell  by  reducing  the  weight  of  conducting  additives  and  binders   in  the  electrodes.  Also,  self-­‐standing  and  lightweight  current  collectors  are  being  investigated  to   improve  the  gravimetric  energy  density  of  these  electrodes.   Ionic  transport   In  all  different  types  of  active  materials  for  cathode  and  anodes,  lithium  ion  diffusion  is   one  of  the  most  critical  factors  that  impact  the  performance  of  the  electrode.    This  problem  is   mainly   addressed   by   developing   nanosized   material   or   nanostructured   electrode,   which   will   result   in   reduced   diffusion   length   and   hence   shorter   diffusion   time   of   lithium   ions,   especially   during  charge.   Electrical  Conductivity    Many  of  these  electrode  materials  such  as  silicon,  lithium  transition  metal  compounds   for   cathodes   and   anodes   have   inherently   lower   electrical   conductivity,   which   can   affect   its   high   rate   capability   and   high   power   performance.   Such   materials   require   the   addition   of   a   conducting   supplement,   which   can   ensure   good   electrical   contact   through   fast   and   long   term   cycling.   This   can   be   done   by   doping,   conductive   coating,   use   of   conducting   templates   or   by   addition  of  conducting  additives  in  the  electrode.   Volumetric  expansion    Some  of  the  electrode  materials  undergo  significant  expansion  and  contraction  during   the   charge-­‐discharge   process,   which   eventually   has   a   negative   effect   on   the   mechanical     15   integrity  of  the  electrode.  In  particular  the  electrode  materials  based  on  alloying  mechanisms   such  as  Si,  can  increase  in  volume  as  much  as  300%.  Repeated  changes  in  volume  can  exert  high   stresses   on   the   electrode,   thus   causing   cracking,   delamination   or   pulverization.   In   addition,   this   will   cause   discontinuity   in   the   conducting   network   of   the   electrode,   thus   resulting   in   higher   resistance   and   inefficient   utilization   of   the   electrodes.   To   tackle   this   problem,   design   of   electrodes  utilizing  nanomaterials  organized  into  a  more  robust  network  allowing  for  flexibility   without   fracture   is   considered.   Various   nanosize   active   materials   of   different   shapes   and   morphologies   have   shown   good   prospects   in   resolving   this   problem.   Porous   or   hollow   nanostructures   (such   as   nanotubes   or   template   mesoporous   structures),   accommodate   a   volume   change   without   affecting   the   electrode   structure.   Other   approaches   involve   use   of   composite   electrodes   with   porous   conducting   matrices   and   addition   of   high   aspect   ratio   conducting   additives   to   ensure   conductivity.   The   distribution   of   nanoparticles   within   the   structure   does   not   change   the   overall   volumetric   change   but   does   distribute   it   is   such   a   way   as   to  avoid  failure  of  the  overall  electrode  structure.       Side  Reactions  and  Insulating  products     There   are   some   side   reactions   that   happen   during   the   process   of   potential   change   during   cycling,   which   can   pose   a   challenge   to   the   performance   of   electrode   materials.   For   example,  a  passivating  SEI  layer  is  formed  in  the  first  cycle  of  carbon  anodes,  which  is  a  result  of   electrolyte  degradation.  Despite  of  its  benefit  in  stabilizing  the  electrode-­‐electrolyte  interface,   increased   SEI   formation   can   lead   to   low   conductivity,   reduced   kinetics,   and   loss   of   active   3 surface   area   .     Also,   there   are   some   intermediate   reaction   products,   which   are   insulating   in     16   nature,   and   hence   can   inhibiting   the   further   reaction   by   blocking   ion   transport   or   by   loss   of   electrical   contact.   Such   a   phenomenon   is   observed   in   Lithium   sulfur   batteries,   where   insulating   intermediate   polysulfides   products   can   deposit   on   the   electrode   surface,   thus   restricting   the   14 availability  and  efficiency  of  the  active  material .  A  similar  problem  is  experienced  in  lithium-­‐   air  batteries  also,  where  insoluble  Li2O2  can  deposit  on  the  active  electrode  material,  reducing   the  electron  and  ion  transport,  thus  decreasing  the  efficacy  of  active  material 5,14 .   Safety     Safety   is   one   of   the   most   important   concerns   restricting   the   usage   of   lithium   ion   batteries  for  some  applications  such  as  automobiles.  The  electrolyte  currently  being  used  can   cause   serious   problems   such   as   high   vapor   pressure,   high   flammability,   and   environmental   and   10,22 health  hazards,  in  case  of  a  runaway  reaction  or  leakage .  The  use  of  Li  metallic  electrode  is   restricted  due  to  the  risk  of  dendrite  formation  or  lithium  plating.  Other  safety  issues  revolve   around   the   thermal   stability   of   the   electrode   materials   (particularly   lithium   transition   metal   10 cathodes   and   high   surface   area   carbon   electrodes)   and   the   chances   of   short   circuit   in   the   batteries.   GOING  THE  NANO-­‐ROUTE   To   satisfy   the   increasing   performance   requirements   of   the   lithium   based   batteries,   drastic  changes  in  the  existing  technology  are  required.  The  solution  is  moving  to  the    “Nano”   dimension,   i.e.,   developing   nano-­‐scale   materials   or   doing   nano-­‐architectured   assembly.   The   concepts,  efficacy  and  challenges  associated  with  the  development  of  nanomaterials  have  been     17   1,8,11,23–25 well  studied  and  comprehensively  reviewed .  Some  of  the  important  advantages  and   disadvantages  of  nanomaterials  are  described  below.   With   nano-­‐features,   unique   behavior   of   material   is   observed   which   can   be   attributed   to   the   combination   of   bulk   and   surface   properties.   Developing   materials   at   the   nanoscale   can   23 result   in   the   prominence   of   reactions,   which   are   restricted   on   a   macroscopic   scale .   The   increased   surface   area   enhances   electrode-­‐electrolyte   interaction,   thus   improving   the   performance  of  the  reaction.      Another  significant  advantage  of  using  nano-­‐size  particles  is  the  resulting  shorter  path   25 length  between  constituents,  resulting  in  improvements  in  electron  and  lithium  ion  transport .   The  effect  of  enhanced  diffusion  on  nanostructuring  can  be  understood  by  looking  at  the  basic   equation  for  solid  state  diffusion   2 τ=  L /2D,   where     τ   is   the   mean   diffusion   time,   L   is   the   diffusion   length,   D   is   the   diffusion   coefficient.   The   diffusion   length   can   play   a   crucial   role   in   reducing   the   diffusion   time,   which   corresponds   to   fast   transport   of   lithium   ions   in   and   out   of   the   system.   Based   on   diffusion   models,   it   can   be   seen   that   a   change   in   L   from   10   µm   to   100   nm   can   bring   about   a   change   of   4   24 orders   in   magnitude   in   diffusion   time .   The   effect   of   fast   diffusion   of   lithium   ions   translates   into  improved  capacity  at  faster  charge  rates.  In  a  real  system,  various  other  parameters  such     18   as   nature   of   electrolyte,   size   of   active   materials,   tortuous   pathway   and   solid-­‐liquid   interface,   which  will  play  a  role  in  diffusion  of  lithium  ions.  Based  on  a  simplified  estimation,  it  has  been   3 3 3   observed   that   specific   capacity   will   depend   on   volume   ratio,   {r -­‐(r-­‐L) }/r   where   r   is   the   radius   25 of  the  active  particle .  Using  this  parameter,  it  has  been  estimated  that  for  a  charge  discharge   25 cycle  of  1  min,  the  particle  size  of  active  material  should  be  2  nm  in  dia .   In  addition  to  improved  diffusion,  nanostructuring  can  also  play  a  role  in  improving  the   mechanical  integrity  of  the  electrode.  This  is  particularly  advantageous  in  the  case  of  alloying   anodes,   which   can   undergo   a   high   volume   change,   (as   high   as   300%   for   Silicon)   during   the   charge-­‐discharge   process.   Nanomaterials,   even   if   they   undergo   the   same   relative   volume   expansion,  don’t  undergo  a  huge  change  in  the  absolute  size,  and  hence  are  better  capable  of   handling  strain.  Thus,  reducing  the  active  material  to  nanoscale  and  encapsulation  in  a  flexible   conducting   matrix,   can   allow   buffer   for   volume   expansion   without   loss   of   conductivity,   thus   8 holding  the  electrode  together  and  solving  the  problem  of  electrode  pulverization .   However,   there   are   some   critical   disadvantages   of   nanomaterials,   which   can   restrict   26 their  efficacy  for  energy  storage  applications .  First  and  foremost,  the  synthesis  methods  are   complex,   not   well   understood   and   require   sophisticated   techniques,   which   increases   the   difficulty  level  and  cost  involved  in  the  large  scale  synthesis  of  the  material.    Also,  the  attractive   feature   of   a   high   interfacial   area   can   be   disadvantageous   by   providing   more   area   for   undesirable  side  reactions.  Excessive  SEI  layer  formation  can  occur  because  of  high  surface  area   3 availability,  which  can  lead  to  temperature  elevation  and  capacity  degradation .  In  addition,  the     19   poor   compaction   of   smaller   particles   results   in   lower   volumetric   energy   densities.   Hence,   the   development   of   nanomaterials   requires   significant   consideration   of   the   pros   and   cons,   thus   demanding  careful  optimization  of  morphology,  to  achieve  the  desired  performance.     One   way   of   approaching   this   situation   is   to   develop   nano/micro   hierarchial   structures   such   as   self-­‐assembled   nano/micro   materials,   nanostructured   composites,   mesoporous   24 materials,   hierarchical   3-­‐D   mixed   conducting   network .   Such   a   structure   can   encompass   the   benefits  of  easy  synthesis,  good  conductivity  &  mechanical  integrity  of  the  microstructure  and   fast  diffusion  and  better  performance  of  nanostructure.   SIGNIFICANCE  OF  NANOSTRUCTURING   In   conjunction   to   developing   new   materials   with   a   nanodimension,   structuring   of   electrodes   to   facilitate   electron   and   ion   transport   is   the   next   step   towards   achieving   the   performance   targets   in   this   field.   Developing   well-­‐architectured   nanostructures   in   different   ordered  shapes  can  enable  benefits  such  as  high  surface  area,  short  diffusion  length,  ordered   porosity   and   interconnectivity.   These   features   are   crucial   for   getting   good   electrode   electrolyte   contact  area,  thus  resulting  in  more  active  sites  and  hence  improved  performance.  An  ordered,   well-­‐  connected  structure  will  help  maintain  good  cycle  life  by  absorbing  volumetric  strain  and   maintaining  conductivity,  thus  maintaining  electrode  integrity.  Some  common  approaches  used   for  nanostructuring  are  described  below:     Template-­‐Directed  Materials   One  of  the  most  facile  methods  of  developing  controlled  structures  with  desired  functionality  is   by  the  templating  method.  Anodic  aluminum  oxide  (AAO)  and  MCM-­‐silica  are  commonly  used     20   templates,  owing  to  their  availability  in  different  shapes  and  pre  sizes.  The  resulting  structure  is   generally  the  inverse  of  the  template  structure  and  hence  the  choice  of  template  scaffold  can   27 decide   the   morphology   and   dimensions   of   the   resulting   nanostructure .   Nanowires,   nanotubes  or  rod-­‐shaped  structure  can  be  created  using  an  AAO  template  with  isolated  pores,   27 and   3-­‐D   porous   structures   can   be   created   with   templates   with   continuous   pores .   This   technique   is   very   versatile   and   can   be   used   to   synthesize   any   type   of   electrode   material,   ranging   from   carbon   nanotube,   carbon   fibers,   silicon   forest,   to   lithium   transition   metal   compounds  and  sulfur  cathodes.     Sol-­‐Gel    Sol-­‐gel   is   a   well   understood   and   popular   synthesis   technique,   particularly   for   synthesis   of   lithium   transition   metal   compounds   for   cathode   for   lithium   ion   batteries.   This   method   offers   better   control   over   synthesis   procedure   because   of   homogeneous   mixing   and   lower   heating   temperature,  thus  yielding  nanometer  size  uniform  particles.  This  technique  is  widely  used  to   28,29 synthesize  most  of  the  cathode  materials 29  and  some  anode  materials  (  SnO2,  TiO2,  LTO   etc.)   .   It   has   been   demonstrated   that   materials   prepared   by   sol-­‐gel   method   have   shown   28–32 improved  capacity,  cycle  life  and  rate  performance .     Self-­‐Assembly   Self-­‐assembly  of  two  components  in  a  composite  electrode  is  another  technique  being  explored   to  develop  uniform,  well-­‐connected  electrodes  for  batteries  applications  The  assembly  is  driven   by   use   of   appropriate   surfactants,   opposite   charged   polyelectrolytes,   resulting   in   the   formation     21   of  ordered  structure  with  alternating  layers  of  active  components.  This  methodology  is  widely   33,34 used   to   make   graphene-­‐metal   oxide   composite   material   for   anodes ,   and   for   making   conductive  additive  and  lithium  metal  compounds  for  cathodes.  In  addition  to  the  benefits  of   the  two  components  in  the  composite,  such  electrodes  have  demonstrated  a  better  and  faster   rate  performance,  reduced  capacity  fade  and  improved  cycle  life.   Layer  by  layer  (LBL)  assembly   Another   methodology   of   developing   structured   electrodes   is   by   LBL   technique,   in   which   repeated  sequential  immersion  of  substrate  into  polycation  or  polyanion  solution  alternatively   35 can  result  in  an  well-­‐organized  assembled  structure .  This  method  is  advantageous  due  to  its   versatility  with  different  types  of  materials,  environmentally  friendly  and  inexpensive  process.     Carbon   nanotubes   and   graphene   composite   electrodes   with   metal   nanoparticles   can   be   synthesized  effectively  using  this  technique  with  less  aggregation,  no  polymeric  binder  and  high   35 packing   density .   However,   this   process   involves   multiple   steps   and   can   be   time   consuming,   from  the  perspective  of  commercial  applications.    Using   these   techniques,   we   can   synthesize   diverse   nanostructured   architectures,   such   as   nanoparticles,   nanorods,   nanotubes,   core-­‐shell   nanostructures,   3-­‐D   pillared   architectures,   5,36 mesoporous   3-­‐D   structure .   These   approaches   to   tailor-­‐made   structure   have   shown   improved  performance  and  have  opened  the  way  forward  for  addressing  the  challenges  faced   by  the  rechargeable  lithium  battery  scenario.     22     CARBON  BASED  NANOMATERIALS   Nano-­‐scale  materials  such  as  carbon  nanostructures  show  distinct  thermodynamic  and   kinetic   properties   in   comparison   to   their   bulky   counterparts.   This   miniaturization   also   results   in   unique   properties,   which   can   be   suitably   modified   when   required.   The   reduced   length   scale   improves  the  kinetics  by  increasing  the  diffusion  rate  and  decreasing  the  diffusion  length.   Graphitic   materials   are   currently   being   used   as   anode   materials   in   commercial   lithium   ion   batteries   because   of   their   good   performance   and   exceptional   properties   such   as   electrochemical  stability,  cycle  life  and  low  cost.    Graphene,  the  2-­‐D  carbon  nanostructure,  has   been   an   active   material   of   interest   for   energy   applications,   because   of   its   high   surface   area   and   37 excellent   electronic   conductivity   over   its   other   carbon   counterparts .   After   its   discovery   in   2004,  the  potential  for  graphene  as  an  anode  material  in  itself  and  its  composites  with  different   metal  oxides  and  silicon  has  been  a  focus  of  research  in  the  development  of  new  materials.  In   addition   to   individual   graphene   sheets,   other   graphene   nanostructures   such   as   carbon   nanotubes,   graphene   nanoribbons   and   graphene   nanoplatelets   have   been   the   subject   of   interest  and  have  been  evaluated  based  on  their  performance,  safety,  feasibility  and  scalability.     GRAPHENE  BASED  MATERIALS   2 38 Graphene   is   the   single   atom   layer   of   hexagonally   packed   sp   carbon   atoms ,   which   forms  the  building  block  for  many  carbon  materials.  This  two-­‐  dimensional  monolayer  of  carbon   atoms   can   be   wrapped   up   into   0D   fullerenes,   rolled   into   1D   nanotubes   or   stacked   into   3D   graphite   as   shown   in   Figure   1-­‐5.   Graphene,   as   a   stable   material   was   extracted/obtained   for   the     23   39 first  time  by  the  Scotch-­‐tape  method  by  Geim  and  Kostya  in  2004   ,  and  this  brought  about  a   revolution   in   terms   of   nano-­‐graphitic   applications.   Since   then   numerous   studies   have   been   done  to  synthesize  large  quantities  of  graphene  sheets  by  different  methods  viz.,  Scotch  tape,   chemical   vapor   deposition,   oxidation   and   exfoliation,   thermal   exfoliation,   sonication   38,40,41 etc .  However,  the  variation  in  synthesis  procedure  can  result  in  considerable  differences   in  the  physical  properties  of  graphene.   2 Graphene  is  a  really  thin  flexible  material  with  a  large  surface  area  (up  to  2600  m /g),   chemical  inertness,  good  mechanical  strength  and  excellent  electrical  and  thermal  conductivity,   which   makes   it   very   desirable   for   several   electrochemical   applications.   There   are   multiple   literature  reports  showing  the  use  of  graphene  in  lithium  ion  batteries,  fuel  cells,  transparent   40,42,37,43 films  for  solar  cells,  conductive  support  for  platinum  catalyst  for  DSSC   24   .     Figure  1-­‐5  :  Nanostructures  based  on  graphene  (a)  0-­‐D  fullerene  (b)  1-­‐D  carbon  nanotube   44 (c)  3-­‐D  graphite     GRAPHENE  USE  IN  LITHIUM  BATTERIES   Anode  Material   Graphene   sheets   because   of   its   nano   dimension   thickness   and   high   electrical   conductivity  was  expected  to  be  an  excellent  anode  material  for  lithium  ion  batteries.  Lithium   45,46 storage  capability  of  graphene  sheets  was  reported  in  the  range  of  500-­‐1500  mAh/g .  This   was   a   significant   improvement   over   the   theoretical   372   mAh/g   lithium   storage   capacity   of   carbon   materials.   Different   explanations   have   been   proposed   to   explain   the   increase   in   storage   capacity  and  since  the  synthesis  methodology  is  varied,  there  is  no  consensus  on  what  might  be   the  primary  reason  behind  the  improved  performance     Graphene  sheets  are  known  to  have  high  surface  area,  since  both  sides  of  every  sheet   are   available,   hence   lithium   interaction   on   both   sides   has   been   projected   as   a   possible     25   explanation   for   higher   storage   capacity.   Also,   because   of   its   nano-­‐dimensions,   graphene   electrodes  will  have  shorter  diffusion  pathway,  and  improved  transport,  which  can  enhance  its   storage  capability.  Other  hypotheses  revolve  around  increases  in  lithium  insertion  active  sites,   45 due   to   high   surface   area,   defects,   nano   cavities,   edge   sites ,   and   functionality   on   the   46 graphene .  Suitable  modifications  in  graphene  sheets  such  as  pillaring  of  graphene  sheets  with   47 38 38 CNTs  or  fullerenes  or  metal  nanoparticles  to  avoid  agglomeration  or  creating  nanopores   48 and  holes  in  graphene  sheets  to  allow  facile  Li+  ion  diffusion,  has  also  shown  improved  high   rate  capacity  performance.     However,   for   most   of   the   graphene   based   systems,   because   of   large   surface   area   and   more   edge   sites,   increased   SEI   formation   has   been   observed,   which   results   in   high   first   cycle   49 capacity   loss .   In   addition,   poor   rate   performance,   hysteresis,   capacity   fade,   large   scale   production   and   high   synthesis   cost   are   some   of   the   deterrents   towards   the   commercial   application  of  graphene  as  anode  material 47,49 .     Graphene  Composites  with  metal/metal  oxides  as  Anode  Material   Metal  anodes  (Si,  Ge,  Sn)  and  the  metal  oxides  (TiO2,  Co3O4,  etc.)  are  alternate  anode   materials  for  lithium  ion  batteries.  However,  most  of  these  materials  suffer  from  low  electronic   conductivity.  In  addition,  metal  anodes  based  on  an  alloying  mechanism  such  as  Si,  Ge  ,  Sn  are   also  prone  to  huge  volume  changes,  resulting  in  loss  of  conductivity  and  electrode  degradation.   Making   a   composite   of   these   materials   with   graphene   can   impart   the   necessary   electronic     26   49,50 conductivity   coupled   with   high   storage   capacity electrodes,   such   as   graphene–Si 51,52 .   Also,   appropriately   nanostructured     and   graphene–SnO2   53,54,55   composites   can   form   an   expandable  structure,  which  can  adjust  for  the  mechanical  strain  during  alloying  process.     Graphene  as  Conducting  Additive  for  Anode  and  Cathodes   56,57,58 Graphene  has  been  used  as  a  conducting  additive  for  lithium  titanate  electrodes .   It  has  been  shown  that  a  graphene  aided  nanostructure  can  improve  performance  by  reduced   56 58 polarization ,  improved  conductivity  and  reduced  impedance .     Similar  phenomenon  was  observed  for  different  cathode  materials,  such  as  LiCoO2  and   LiFePO4   59,60 ,   where   the   desired   conductivity   can   be   observed   at   a   lower   percolation   in   comparison  to  commercial  Super  P.     GRAPHENE  NANOPLATELETS  (GNP):  OUR  MATERIAL  OF  INTEREST   Graphene   nanoplatelets   (GnP)   are   an   inexpensive   alternative   to   other   carbon   61 nanomaterials   such   as   carbon   nanotubes   and   single   graphene   sheets .     These   nanoplatelets   are  a  few  sheets  of  graphene  stacked  together  and  have  micron  range  diameter,  thus  giving  it   the  beneficial  high  aspect  ratio.     GnP  is  synthesized  from  natural  graphite  by  a  simple  acid  intercalation  and  exfoliation   62 procedure ,   which   is   schematically   represented   in   Figure   1-­‐6.   Graphite   intercalated   compounds   (GICs),   synthesized   by   intercalating   a   combination   of   sulfuric   and   nitric   acid,   are     27   obtained   commercially   from   Asbury   Carbons.   This   acid   intercalated   material   is   subjected   to   microwave   irradiation,   which   results   in   rapid   heating.     This   sudden   increase   in   temperature   vaporizes  the  acid  trapped  in  graphite  galleries,  thus  causing  expansion  and  ultimate  exfoliation   of   graphite,   forming   an   expanded   “worm”   like   structure.   These   worms   are   then   further   subjected   to   a   combination   of   sonication   and   milling   procedures   to   form   graphene   nanoplatelets   of   different   dimensions.   These   nanoplatelets   can   be   produced   with   dimensions   ranging   from   50   microns   to   0.1   micron   in   diameter   and   thicknesses   ranging   from   20   nm   to   3   2 2 nm.   Correspondingly   the   surface   area   can   be   modified   from   20   m /g   to   200   m /g.   The   GnP   platelets   have   a   pristine   basal   plane   with   few   carboxylic   and   amine   functional   groups   on   the   edge.       28     !!   (a)! (b)! (c)!       Figure  1-­‐6  :  Schematic  showing  synthesis  of  Graphene  nanoplatelets  (GnP)  by   61 intercalation  and  exfoliation  procedure  and  SEM  images  of  the  corresponding  stages     (Scale  bar  :  (a)  300  μm,  (b)  500  μm,  (c)  50  nm)       The  high  aspect  ratio  and  nano  thickness  of  these  platelets  impart  excellent  electronic,   mechanical   and   transport   properties   associated   to   these   platelets.   Also,   GnP   platelets   have   good   thermal   stability   and   chemical   inertness,   which   makes   it   an   attractive   candidate   for   myriads  of  applications  ranging  from  polymer  composites  to  electrochemical  energy  storage.       29     Due   to   their   comparable   properties   and   simple,   cost   effective   synthesis   procedure,   these   platelets   are   a   promising   candidate   for   carbon   nanomaterials   for   electrochemical  energy  storage  applications.   DISSERTATION  OBJECTIVE   This  dissertation  is  focused  on  nanostructuring  graphene  nanoplatelets  (GnP)  by  taking   advantage   of   its   inherent   properties   of   lithium   storage   capability   and   electrical   conductivity,   and  tailoring  GnP  for  different  applications  for  lithium-­‐based  secondary  batteries.   In   the   first   part   of   this   dissertation   we   are   evaluating   the   potential   of   GnP   as   anode   active   material   for   developing   nanoarchitectured   electrode   structure,   aiming   at   increase   in   energy  density,  reduction  of  cost  and  improvement  in  performance.    Graphene  platelets  doped   with   different   size   of   nickel   metal   nanoparticles   are   used   to   tailor-­‐made   different   structured   electrodes  with  improved  transport  and  enhanced  capacity.     In   the   next   section,   we   have   investigated   the   role   of   different   sizes   of   GnP   as   a   conducting   additive   in   lithium   titanate   electrodes   for   lithium   ion   batteries.   This   work   is   focused   on   providing   alternative   to   commercial   carbon   black   materials,   focused   on   reducing   the   additive  concentration  without  compromising  performance.       Further   on,   we   are   capitalizing   on   the   high   in-­‐plane   electrical   conductivity   of   these   platelets,   to   form   current   collector   for   electrochemical   applications.   The   underlying   fundamentals   are   to   arrange   these   platelets   in   a   structured   manner,   to   optimize   desirable   properties  of  electrical  conductivity  and  mechanical  integrity.       30   In  the  last  section  we  have  explored  the  effect  of  GnP  as  a  conducting  matrix  for  sulfur   electrodes   for   lithium   sulfur   batteries.   Based   on   the   results   from   the   prior   work,   we   have   developed   GnP-­‐sulfur   electrodes,   targeted   at   achieving   maximum   capacity   with   long   term   cyclability.  Two  different  approaches  of  using  GnP  as  conducting  additive  and  using  GnP  paper   as  conducting  template  are  assessed.     31                         REFERENCES     32   REFERENCES     1.   Kumar,  H.,  Rajan,  S.  &  Shukla,  A.  K.  Development  of  lithium-­‐ion  batteries  from  micro-­‐ structured  to  nanostructured  materials:  its  issues  and  challenges.  Science  Progress  95,   283–314  (2012).   2.   Tarascon,  J.  M.  &  Armand,  M.  Issues  and  challenges  facing  rechargeable  lithium   batteries.  Nature  414,  359–67  (2001).   3.   Liu,  D.  &  Cao,  G.  Engineering  nanostructured  electrodes  and  fabrication  of  film   electrodes  for  efficient  lithium  ion  intercalation.  Energy  &  Environmental  Science  3,   1218–1237  (2010).   4.   Li,  H.,  Wang,  Z.,  Chen,  L.  &  Huang,  X.  Research  on  Advanced  Materials  for  Li-­‐ion  Batteries.   Advanced  Materials  21,  4593–4607  (2009).   5.   Song,  M.,  Park,  S.,  Alamgir,  F.  M.,  Cho,  J.  &  Liu,  M.  Nanostructured  electrodes  for  lithium-­‐ ion  and  lithium-­‐air  batteries :  the  latest  developments  ,  challenges  ,  and  perspectives.   Materials  Science  &  Engineering  72,  203–252  (2011).   6.   Winter,  B.  M.,  Besenhard,  J.  O.,  Spahr,  M.  E.  &  Novuk,  P.  Insertion  Electrode  Materials  for   Rechargeable  Lithium  Batteries  .  Advanced  Materials  10,  725–763  (1998).   7.   Rev,  A.  et  al.  Materials  for  Rechargeable  Lithium-­‐Ion  Batteries.  Annu.  Rev.  Chem.  Biomol.   Eng.  3,  445–471  (2012).   8.   Bruce,  P.  G.,  Scrosati,  B.  &  Tarascon,  J.  Lithium  Batteries  Nanomaterials  for  Rechargeable   Lithium  Batteries.  Angewandte  Chemie  47,  2930–2946  (2008).   9.   Mukherjee,  R.,  Krishnan,  R.,  Lu,  T.-­‐M.  &  Koratkar,  N.  Nanostructured  electrodes  for  high-­‐ power  lithium  ion  batteries.  Nano  Energy  1,  518–533  (2012).   10.   Choi,  N.  et  al.  Challenges  Facing  Lithium  Batteries  and  Electrical  Double-­‐Layer  Capacitors  .   Angewandte  Chemie  51,  9994–10024  (2012).   11.   Alc,  R.,  Lavela,  P.  &  Carlos,  P.  Nanostructured  Electrodes  for  Lithium  Ion  Batteries.  Solid   State  Electrochemistry  II:  Electrodes,  Interfaces  and  Ceramic  Membranees  383–413   (2011).   12.   Evers,  S.  &  Nazar,  L.  F.  New  Approaches  for  High  Energy  Density  Lithium  Sulfur  Battery   Cathodes.  Accounts  of  chemical  research  ,  (2012). DOI:  10.1021/ar3001348     33   13.   Su,  Y.  Challenges  and  Prospects  of  Lithium  Sulfur  Batteries.  Carbon,  (2012).  DOI:   10.1021/ar300179v   14.   Bruce,  P.  G.,  Freunberger,  S.  A.,  Hardwick,  L.  J.  &  Tarascon,  J.  Li  –  O  2  and  Li  –  S  batteries   with  high  energy  storage.  Nature  materials  11,  19–30  (2012).   15.   Mikhaylik  et.  al.,  High  Energy  Rechargeable  Li-­‐S  Cells  for  EV  Application.  Status,   Challenges  and  Solutions.  Available  from   http://sionpower.com/pdf/articles/SionPowerECS.pdf   16.   Bruce,  P.  et  al.,  Li  –  O  2  and  Li  –  S  batteries  with  high  energy  storage.  Nature  Materials   11,  19–30  (2012).   17.   Song,  M.-­‐K.,  Cairns,  E.  J.  &  Zhang,  Y.  Lithium/sulfur  batteries  with  high  specific  energy:   old  challenges  and  new  opportunities.  Nanoscale  5,  2186–204  (2013).   18.   Zhang,  Y.,  Zhao,  Y.,  Sun,  K.  E.  &  Chen,  P.  Development  in  Lithium  /  Sulfur  Secondary   Batteries.  The  Open  Materials  Science  Journal,  215–221  (2011).   19.   Yang,  Y.  et  al.,  New  nanostructured  Li2S/silicon  rechargeable  battery  with  high  specific   energy.  Nano  letters  10,  1486–91  (2010).   20.   Xin,  S.,  Guo,  Y.-­‐G.  &  Wan,  L.-­‐J.  Nanocarbon  networks  for  advanced  rechargeable  lithium   batteries.  Accounts  of  chemical  research  45,  1759–69  (2012).   21.   Evers,  S.  &  Nazar,  L.  F.  New  Approaches  for  High  Energy  Density  Lithium-­‐Sulfur  Battery   Cathodes.  Accounts  of  chemical  research  ,  (2012).  DOI:  10.1021/ar3001348   22.   Scrosati,  B.  &  Garche,  J.  Lithium  batteries :  Status  ,  prospects  and  future.  Journal  of   Power  Sources  195,  2419–2430  (2010).   23.   Bruce,  P.  G.,  Scrosati,  B.  &  Tarascon,  J.-­‐M.  Nanomaterials  for  rechargeable  lithium   batteries.  Angewandte  Chemie    47,  2930–46  (2008).   24.   Guo,  Y.-­‐G.,  Hu,  J.-­‐S.  &  Wan,  L.-­‐J.  Nanostructured  Materials  for  Electrochemical  Energy   Conversion  and  Storage  Devices.  Advanced  Materials  20,  2878–2887  (2008).   25.   Jiang,  C.,  Hosono,  E.  &  Zhou,  H.  Nanomaterials  for  lithium  ion  batteries.  Nano  Today  1,   28–33  (2006).   26.   Aricò,  A.  S.,  Bruce,  P.,  Scrosati,  B.,  Tarascon,  J.-­‐M.  &  Van  Schalkwijk,  W.  Nanostructured   materials  for  advanced  energy  conversion  and  storage  devices.  Nature  materials  4,  366– 77  (2005).     34   27.   Cheng,  F.,  Tao,  Z.,  Liang,  J.  &  Chen,  J.  Template-­‐Directed  Materials  for  Rechargeable   Lithium-­‐Ion.  Chemistry  of  Materials  20,  667–681  (2008).   28.   Liu,  H.,  Wu,  Y.  P.,  Rahm,  E.,  Holze,  R.  &  Wu,  H.  Q.  Cathode  materials  for  lithium  ion   batteries  prepared  by  sol-­‐gel  methods.  Journal  of  Solid  State  Electrochemistry  8,  450–466   (2004).   29.   Fu,  L.  J.  et  al.  Electrode  materials  for  lithium  secondary  batteries  prepared  by  sol  –  gel   methods.  Progress  in  Materials  Science  50,  881–928  (2005).   30.   Hu,  Y.,  Doeff,  M.  M.,  Kostecki,  R.  &  Fiñones,  R.  Electrochemical  Performance  of  Sol-­‐Gel   Synthesized  LiFePO[sub  4]  in  Lithium  Batteries.  Journal  of  The  Electrochemical  Society   151,  A1279  (2004).   31.   Choi,  D.  &  Kumta,  P.  N.  Surfactant  based  sol–gel  approach  to  nanostructured  LiFePO4  for   high  rate  Li-­‐ion  batteries.  Journal  of  Power  Sources  163  (2),  1064–1069  (2007).   32.   Rho,  Y.  H.  et  al.  Preparation  of  Li  4  Ti  5  O  12  and  LiCoO  2  thin  film  electrodes  from   precursors  obtained  by  sol  –  gel  method.  Solid  State  Ionics  151–157  (2002).   33.   Wang,  D.  et  al.  Ternary  self-­‐assembly  of  ordered  metal  oxide-­‐graphene  nanocomposites   for  electrochemical  energy  storage.  ACS  nano  4,  1587–95  (2010).   34.   Cassagneau,  B.  T.  &  Fendler,  J.  H.  High  Density  Rechargeable  Lithium-­‐Ion  Batteries  Self-­‐ Assembled  from  Graphite  Oxide  Nanoplatelets  and  Polyelectrolytes  .  Advanced   Materials,  10  (11),877–881  (1998).   35.   Lee,  S.  W.,  Gallant,  B.  M.,  Byon,  H.  R.,  Hammond,  P.  T.  &  Shao-­‐Horn,  Y.  Nanostructured   carbon-­‐based  electrodes:  bridging  the  gap  between  thin-­‐film  lithium-­‐ion  batteries  and   electrochemical  capacitors.  Energy  &  Environmental  Science  4,  1972–1985  (2011).   36.   Liu,  C.,  Li,  F.,  Ma,  L.-­‐P.  &  Cheng,  H.-­‐M.  Advanced  materials  for  energy  storage.  Advanced   materials  22,  E28–62  (2010).   37.   Brownson,  D.  a.  C.,  Kampouris,  D.  K.  &  Banks,  C.  E.  An  overview  of  graphene  in  energy   production  and  storage  applications.  Journal  of  Power  Sources  196,  4873–4885  (2011).   38.   Pumera,  M.  Electrochemistry  of  graphene:  new  horizons  for  sensing  and  energy  storage.   Chemical  record  9,  211–23  (2009).   39.   Novoselov,  K.  S.  et  al.  Electric  field  effect  in  atomically  thin  carbon  films.  Science  306,   666–9  (2004).     35   40.   Zhu,  Y.  et  al.  Graphene  and  graphene  oxide:  synthesis,  properties,  and  applications.   Advanced  materials  22,  3906–24  (2010).   41.   Choi,  W.,  Lahiri,  I.,  Seelaboyina,  R.  &  Kang,  Y.  S.  Synthesis  of  Graphene  and  Its   Applications:  A  Review.  Critical  Reviews  in  Solid  State  and  Materials  Sciences  35,  52–71   (2010).   42.   Geim,  a  K.  Graphene:  status  and  prospects.  Science  324,  1530–1534  (2009).   43.   Sun,  Y.,  Wu,  Q.  &  Shi,  G.  Graphene  based  new  energy  materials.  Energy  &  Environmental   Science  4,  1113–1132  (2011).   44.   Geim,  a  K.  &  Novoselov,  K.  S.  The  rise  of  graphene.  Nature  materials  6,  183–191  (2007).   45.   Lian,  P.  et  al.  Large  reversible  capacity  of  high  quality  graphene  sheets  as  an  anode   material  for  lithium-­‐ion  batteries.  Electrochimica  Acta  55,  3909–3914  (2010).   46.   Wang,  G.,  Shen,  X.,  Yao,  J.  &  Park,  J.  Graphene  nanosheets  for  enhanced  lithium  storage   in  lithium  ion  batteries.  Carbon  47,  2049–2053  (2009).   47.   Yoo,  E.  et  al.  Large  reversible  Li  storage  of  graphene  nanosheet  families  for  use  in   rechargeable  lithium  ion  batteries.  Nano  letters  8,  2277–2282  (2008).   48.   Zhao,  X.,  Hayner,  C.  M.,  Kung,  M.  C.  &  Kung,  H.  H.  Flexible  holey  graphene  paper   electrodes  with  enhanced  rate  capability  for  energy  storage  applications.  ACS  nano  5,   8739–8749  (2011).   49.   Liang,  M.  &  Zhi,  L.  Graphene-­‐based  electrode  materials  for  rechargeable  lithium   batteries.  Journal  of  Materials  Chemistry  19,  5871-­‐5878  (2009).   50.   Yang,  S.,  Feng,  X.,  Ivanovici,  S.  &  Müllen,  K.  Fabrication  of  graphene-­‐encapsulated  oxide   nanoparticles:  towards  high-­‐performance  anode  materials  for  lithium  storage.   Angewandte  Chemie  49,  8408–8411  (2010).   51.   Zhou,  X.,  Yin,  Y.-­‐X.,  Wan,  L.-­‐J.  &  Guo,  Y.-­‐G.  Facile  synthesis  of  silicon  nanoparticles   inserted  into  graphene  sheets  as  improved  anode  materials  for  lithium-­‐ion  batteries.   Chemical  communications  48,  2198–2200  (2012).   52.   Lee,  J.  K.,  Smith,  K.  B.,  Hayner,  C.  M.  &  Kung,  H.  H.  Silicon  nanoparticles-­‐graphene  paper   composites  for  Li  ion  battery  anodes.  Chemical  communications  46,  2025–2027  (2010).   53.   Du,  Z.  et  al.  In  situ  synthesis  of  SnO2/graphene  nanocomposite  and  their  application  as   anode  material  for  lithium  ion  battery.  Materials  Letters  64,  2076–2079  (2010).     36   54.   Wang,  G.  et  al.  Sn/graphene  nanocomposite  with  3D  architecture  for  enhanced   reversible  lithium  storage  in  lithium  ion  batteries.  Journal  of  Materials  Chemistry  19,   8378-­‐8384  (2009).   55.   Wang,  X.,  Zhou,  X.,  Yao,  K.,  Zhang,  J.  &  Liu,  Z.  A  SnO2/graphene  composite  as  a  high   stability  electrode  for  lithium  ion  batteries.  Carbon  49,  133–139  (2011).   56.   Shi,  Y.,  Wen,  L.,  Li,  F.  &  Cheng,  H.-­‐M.  Nanosized  Li4Ti5O12/graphene  hybrid  materials   with  low  polarization  for  high  rate  lithium  ion  batteries.  Journal  of  Power  Sources  196,   8610–8617  (2011).   57.   Zhu,  N.  et  al.  Graphene  as  a  conductive  additive  to  enhance  the  high-­‐rate  capabilities  of   electrospun  Li4Ti5O12  for  lithium-­‐ion  batteries.  Electrochimica  Acta  55,  5813–5818   (2010).   58.   Zhang,  B.  et  al.  Percolation  threshold  of  graphene  nanosheets  as  conductive  additive  in  Li   4  Ti  5  O  12  anodes  of  Li-­‐ion  batteries.  Nanoscale  5,  2100–2106  (2013).   59.   Su,  F.-­‐Y.  et  al.  Could  graphene  construct  an  effective  conducting  network  in  a  high-­‐power   lithium  ion  battery,  Nano  Energy  1,  429–439  (2012).   60.   Su,  F.-­‐Y.  et  al.  Flexible  and  planar  graphene  conductive  additives  for  lithium-­‐ion  batteries.   Journal  of  Materials  Chemistry  20,  9644-­‐9650  (2010).   61.   Drzal,  L.  T.  Exfoliated  Graphite  Nanoplatelets:  A  Carbon  Nanotube  Alternative.  Available   from  http://www.xgsciences.com/docs/xGnP_tech_overview_web.pdf.   62.   Fukushima,  H.  Graphite  Nanoreinforcements  in  Polymer  Nanocomposites.  Chemical   Engineering  &  Materials  Science  Department,  Michigan  State  University,  East  Lansing   (2003).             37   2 METAL  DOPED  GRAPHENE  NANOPLATELETS  AS  ANODE  MATERIAL   SIGNIFICANCE   Nanographitic   materials   such   as   nanotubes   and   graphene   nanosheets   have   gained   considerable   attention   as   potential   energy   storage   materials,   because   of   their   desirable   properties   such   as   high   surface   area,   good   conductivity,   and   significant   mechanical   and   electrochemical   stability.   Particularly,   anodes   made   of   graphene   sheets   and   their   composites   have   shown   a   significant   increase   in   specific   capacity   (nearly   100%)   by   careful   control   of   1 interlayer  spacing   .   Another   category   of   anode   material,   which   is   of   prime   significance,   is   the   metal   nanoparticles,   which   have   excellent   storage   capacity   but   are   restricted   in   their   use   due   to   poor   mechanical  integrity  and  high  cost.    The  focus  of  this  research  in  developing  anode  materials  for   lithium   ion   batteries   is   to   integrate   the   two   different   materials,   and   develop   a   metal-­‐ nanographite   composite,   thereby   achieving   the   benefits   from   both   constituents.     Such   electrodes   capitalize   on   the   benefit   of   high   lithium   storage   capability   from   metals   and   their   oxides   and   combine   it   with   excellent   conductivity   and   mechanical   robustness   of   the   graphitic   systems 2–6 .     There   are   numerous   reports   on   combining   SnO2,   silicon   and   other   metal   oxides   with   graphene   to   form   a   high   capacity   nanostructures   with   a   highly   conductive   support.     These   metal-­‐based   electrode   materials   interact   with   lithium   via   alloying   mechanism   and   undergo     38   huge   volume   change   during   cycling.   Making   a   composite   structure   with   high   aspect   ratio   graphene   helps   to   sustain   the   strain   due   to   lithium   alloying/dealloying   process   of   the   metal   component   3,7–9  .   APPROACH   Our   approach   is   to   develop   a   3   dimensional   layered   graphene   nanoplatelet   structure   where   the   addition   of   metal   nanoparticles   allows   for   the   construction   of   a   tailorable   nano-­‐ architecture.   The   schematic   illustration   shown   in   Figure   2-­‐1   gives   an   overview   of   the   concept   behind  the  technique   Graphene( Nanoplatelets((GnP)( Metal(doped(GnP( nanostructure( • Nanoscale(thick( graphite(nanoplatelets( show(improved( mechanical,(transport( and(kine:c(proper:es( • Graphite(nanoplatelets( are(separated(by(metal( nanopar:cles,(thus( increasing(interlayer( spacing(and(available( surface(   Figure  2-­‐1:  Schematic  showing  the  potential  of  metal-­‐doped  carbon  as  anodes  for   lithium  ion  batteries   Based   on   the   concepts   discussed   in   the   introduction,   having   a   nanostructured   electrode   will   have   the   benefits   of   fast   Li   ion   diffusion   coupled   with   better   mechanical   robustness.   The     39   addition  of  these  nanoscale  metal  particles  results  in  a  structured  nanographite  assembly  that   can  potentially  have  the  following  advantages:     1. Metal  nanoparticles  will  act  as  spacers,  increasing  interlayer  spacing  and  thus  allowing   easy  diffusion  of  lithium  ions  in  and  out  of  the  electrode  thereby  enhancing  the  reaction   rate.   2. Metal   nanoparticles   will   act   as   conducting   additive,   thereby   reducing   the   electric   resistance  of  the  electrode.   3. Metal  nanoparticle  dopants,  such  as  nickel,  tin,  antimony  and  silicon,  can  contribute  to   the   cell   capacity   because   of   their   inherent   interaction   with   lithium,   thereby   augmenting   the  specific  capacity  of  the  anode   4. Compared  to  monolithic  metal  anodes,  improved  cycle  should  be  achieved  by  restricting   the  extent  of  volume  expansion  in  the  nanographitic  matrix.   EXPERIMENTAL  SECTION   SYNTHESIS   Graphene  Nanoplatelets   Graphene  Nanoplatelets  were  synthesized  by  a  well-­‐established  microwave  exfoliation   10 and   sonication   procedure   from   acid   intercalated   graphite .   Around   1   g   of   acid   intercalated   graphite   material   from   Asbury   (A3772)   placed   in   a   600   ml   beaker   was   heat-­‐treated   in   a   kitchen   microwave  at  1000  W  for  1  min.  During  the  microwave  heating,  the  acid  vaporizes  and  tries  to   escape   the   galleries   of   graphite,   thus   resulting   in   rapid   volumetric   expansion,   exfoliation   and     40   formation   of   GnP   worms.   This   procedure   was   repeated   with   intermittent   stirring   3   times,   to   ensure  complete  exfoliation  and  acid  removal.  The  obtained  GnP  worms  were  then  dispersed  in   isopropanol  and  ultrasonicated  at  100  W  for  2  hours  to  separate  the  GnP  worms  into  individual   nanoplatelets   and   to   reduce   their   overall   lateral   dimensions.   .   The   solution   was   air   dried,   followed   by   vacuum   drying   at   100   C   to   obtain   GnP   platelets   of   ~15   μm   diameter   and   with   a   2 surface  area  of  25-­‐30  m /g.     Nickel  doped  GnP  materials   Three   different   sizes   of   nickel   nanoparticles   supported   on   GnP,   were   synthesized   by   different   procedures.   The   summary   of   the   synthesis   methodology,   conditions   and   concentration   details   are   summarized   below   in   Table   2-­‐1.   The   nomenclature   used   to   denote   different  materials  is  GnP_Ni-­‐x,  where  x  is  indicative  of  the  size  of  nickel  nanoparticles.     Table  2-­‐1:  Summary  Table  for  different  nickel  doped  GnP  materials  synthesized   Material   Methodology   Starting   nanoparticles   Concentration   concentration  of   size   (Nomenclature)   Nickel   of   Nickel   in   Ni   nanoparticles   precursors   Actual   (TGA  Analysis)     GnP_Ni-­‐5   Polyol  Method   5  nm   10%   2.5%   GnP_Ni-­‐30c   Solventless     30  nm  clusters   10%   10  %   GnP_Ni-­‐60   Solventless   60-­‐80   10%   10  %       41     Microwave  Assisted  Polyol  Method   The  GnP  nanoplatelets  were  acid  treated  in  H2SO4  +  HNO3  (1:3)  mixture  to  oxidize  the   nanoplatelet  edges  which  can  aid  in  interaction  with  glycols  and  metal  salts.  1  g  oxidized  GnP   was   dispersed   in   80   ml   triethylene   glycol   with   the   aid   of   ultra   sonication   and   mechanical   stirring.     A   nickel   salt,   nickel   nitrate,   was   dissolved   separately   in   40   ml   triethylene   glycol   at   appropriate  concentration  (10%).  The  two  solutions  were  then  stir  mixed  overnight  to  ensure   good  interaction.  The  mixed  solution  was  heated  to  270  °C  in  a  programmable  Ethos  Microwave   (5   min   ramp).   The   resulting   solution   was   centrifuged   three   times   with   acetone   to   remove   excess  salt  and  triethylene  glycol.  The  final  solution  was  dried  in  air  and  then  heated  at  100  °C   to  evaporate  acetone  and  to  produce  GnP  doped  with  5  nm  nickel  nanoparticles  (denoted  as   GnP_Ni-­‐5).   Solventless  Method   The   synthesis   of   two   other   GnP-­‐Ni   materials   (GnP_Ni-­‐30c,   GnP_Ni-­‐60)   was   done   by   a   simple  procedure  involving  reduction  of  nickel  salts  on  the  graphene  surface  by  heat  treatment   11 in   an   inert   environment .   GnP   and   the   appropriate   concentration   of   nickel   acetate   were   weighed  and  put  in  a  ball  mill  chamber  of  65  ml  size.  The  mixture  was  milled  with  the  aid  of  5  g   PP  balls  for  20  minutes  using  a  SPEX  8000  M  Mixer  Mill.  The  obtained  mixture  was  heated  for  3   hours  in  a  tube  furnace  under  Argon  flow  at  temperatures  of  350  °C  and  600  °C  to  obtain  nickel   nanoparticles  supported  on  GnP,  of  30  nm  clusters  and  60  nm  size  (GnP_Ni-­‐30c  and  GnP_Ni-­‐60)     42   respectively.  GnP_Ni-­‐30c  materials  has  small  nanoparticles  combined  together  to  form  30  nm   clusters,  hence  the  notation  c  for  clusters.   MORPHOLOGY  OBSERVATION   The  characterization  of  GnP_Ni-­‐5  material  was  done  using  a  JEOL  2200  FS  Transmission   Electron   Microscope   (TEM),   at   an   accelerating   voltage   of   200   kV.   The   sample   preparation   involved   creating   a   dilute   suspension   of   GnP_Ni   powder   in   acetone   by   tip   sonication   for   2   minutes,  followed  by  drop  casting  on  a  Cu  TEM  grid.   The   morphology   of   other   Ni   doped   GnP   materials   prepared   from   the   solventless   approach   and   the   electrode   morphologies   were   observed   by   JEOL   JSM   7500-­‐F   Field   Emission   SEM  at  an  accelerating  voltage  of  15  kV.  For  cross-­‐section  observation  of  electrodes,  Cu  foil  was   etched  from  the  electrodes  with  a  mild  acid  treatment  for  2  days.     The   morphology   of   GnP_Si   paper   was   observed   by   Focused   Ion   Beam   (FIB)   Milling   technique  using  the  Carl  Zeiss  Auriga®  CrossBeam  scanning  electron  microscope.  This  technique   helps  in  eliminating  the  artifacts  in  the  morphology  due  to  epoxy  embedding  and  polishing.     The  details  of  the  procedure  of  the  two  sample  preparation  techniques  for  viewing  the   cross-­‐section  are  described  below:   Epoxy  Embedding  and  Polishing   A  small  piece  of  electrode  was  cut  and  mounted  to  a  clip  with  the  electrode  edge  facing   upwards.   The   clip   was   placed   in   a   ring   holder,   and   a   slow   cure   room   temperature   epoxy   was   poured  around  the  sample.  After  curing,  polishing  of  the  epoxy  embedded  sample  was  done  by     43   the  Abramis  polisher,  in  multiple  steps  starting  with  320  grit  sandpaper,  followed  by  600  grit,   1200   grit,   and   finally   4000   grit   sand   paper.   Material   removal   rates   for   the   graphene   are   very   slow  compared  to  the  epoxy.    The  polished  sample  was  then  exposed  to  oxygen  plasma  for  30   minutes  to  etch  residual  epoxy  on  the  surface,  thus  enhancing  the  contrast  between  the  sample   and  epoxy  matrix.  The  samples  were  lightly  gold  coated  prior  to  SEM  observation  to  eliminate   charging.   Focused  Ion  Beam  (FIB)  Milling/Sectioning   The  cross-­‐section  morphology  of  LTO  electrodes  were  studied  using  (FIB)-­‐SEM  (Carl  Zeiss   Auriga®  CrossBeam)  instrument.    The  cross-­‐section  of  the  electrode  is  exposed  for  observation   by  a  systematic  process  of  milling  from  the  surface  through  the  thickness  of  the  electrode,  using   a   focused   gallium   ion   beam.   Figure   2-­‐2   shows   the   layout   of   the   sectioning   setup   in   the   FIB-­‐SEM   column.   As   shown,   the   electron   and   ion   beam   are   inclined   at   an   angle   of   36°   in   the   vacuum   column.  Thus,  the  sample  is  inclined  to  54°  to  ensure  that  the  ion  beam  is  hitting  the  sample  at   90  degrees  thus  exposing  the  true  nature  of  electrode  cross-­‐section.  For  SEM  observation,  the   image   is   tilt   corrected   (by   the   software)   to   account   for   the   36°   angle   between   the   electron   beam   and   the   sample.   The   milling   process   is   a   multiple   step   procedure,   to   make   sure   the   section   is   exposed   well   and   the   sputtered   material   doesn’t   redeposit   back   on   the   sample   surface.     First,   a   large   beam   of   30   kV,   20   nA   is   used   to   cut   out   a   trapezoid.   Two   further   rectangular   fine   sections   with   smaller   beams   (30kV,   4nA   and   30   kV,   2nA)   are   done   on   the   small   parallel   side   of   the   trapezoid,   removing   any   redeposited   material,   and   exposing   the   cross-­‐ sectional  electrode  surface.       44   Electron)Beam) Viewing) )direc8on) ) Ion)Beam) 3D)Stage) Ion)milling) Incident)direc8on) Target)Sample)   12 Figure  2-­‐2:  Schematic  of  FIB-­‐SEM  setup   PHYSICAL  CHARACTERIZATION   The   concentration   of   nickel   nanoparticles   was   evaluated   by   using   a   TA   Instruments   thermogravimetric  analysis  (TGA)  instrument  by  heating  up  to  900C  in  air  at  a  ramp  rate  of  10   C/min.     The   X-­‐ray   diffraction   (XRD)   analysis   of   different   materials   was   obtained   on   a   Philips   Rigaku   Rotaflex   200B   X-­‐ray   powder   diffractometer   using   Cu-­‐Kα   radiation   to   determine   the   composition  of  metal  nanoparticles  in  the  2θ  range  of  10°  to  80°.     The   Raman   spectra   of   the   undoped   and   Ni   doped   GnP   materials   was   obtained   using   the   Lab   RAM   ARAMIS   instrument   from   Horiba.   The   powders   were   dispersed   in   acetone   with   tip   sonication  and  drop  cast  on  a  glass  slide  to  make  a  thick  film.  All  the  materials  were  analyzed   -­‐1. with  the  532  nm  50  mW  DPSS  laser  in  the  range  of  0  -­‐  4000  cm     45   ELECTROCHEMICAL  CHARACTERIZATION   Electrodes  were  prepared  from  the  undoped  GnP,  GnP_Ni-­‐5,  GnP_Ni-­‐30c  and  GnP_Ni-­‐ 60  materials  by  spreading  its  slurry  with  N-­‐Methyl-­‐2-­‐pyrrolidone  and  PVDF  as  the  binder  (5%),   on   the   copper   substrate   using   a   microfilm   applicator.   These   anodes   were   then   characterized   in   a   three   electrode   Swagelok   cell   setup   (Figure   2-­‐3)   with   lithium   foil   as   counter   and   reference   electrode.  The  separator  used  was  a  glass  microfiber  filter.      1M  LiPF6  in  equivolume  of  ethylene   carbonate-­‐dimethyl  carbonate  (EC-­‐DMC)  solution  was  used  as  the  electrolyte.  The  galvanostatic   performance  of  these  electrodes  was  evaluated  using  an  Arbin  BT  2000  Battery  Analyzer  by  the   protocol  adopted  shown  below  in  Figure  2-­‐16.  The  galvanostatic  cycling  was  done  at  different   rates   for   every   5   cycles,   keeping   the   rate   identical   for   both   charge   and   discharge.   Further   electrochemical   characterization,   viz.   cyclic   voltammetry   and   electrochemical   impedance   spectroscopy   was   done   with   a   Versa   STAT   MC   instrument.   The   CV   for   all   the   electrodes   was   collected   from   0-­‐2.0V   at   a   scan   rate   of   0.05mV/s.   AC   impedance   spectra   were   obtained   by   applying  a  sine  wave  of  5  mV  amplitude  in  the  frequency  range  of  100  kHz  to  0.01  Hz.         46   Cu(rods( ( ( ( ( ( ( ( Li(foil( Separator( ( (( GnP(on(Cu(foil( (           Figure  2-­‐3:  Schematic  &  Picture  of  Three  electrode  Swagelok  type  T-­‐  cell  for   electrochemical  measurements   RESULTS  &  DISCUSSION   MORPHOLOGY  OBSERVATION  OF  NICKEL  DOPED  GNP   The  synthesis  of  small  size  metal  doped  GnP  by  the  microwave  assisted  polyol  process       has  the  advantages  of  rapid,  homogeneous  heating  coupled  with  enhanced  reaction  rates,  thus   facilitating   synthesis   of   small   size   uniform   metal   doping.   Figure   2-­‐4   shows   the   TEM   images   of   nickel  nanoparticle  doped  GnP-­‐15  platelets  at  different  magnifications,  from  which  we  can  see   that   the   average   size   of   these   nickel   nanoparticles   is   in   the   range   of   3-­‐4nm.   Despite   the   fact   that   we   start   with   a   high   metal   concentration   in   the   reactants,   the   actual   concentration   of   nickel  nanoparticles  was  found  to  be  2.5%  by  XPS  (as  confirmed  by  the  TGA  (Figure  2-­‐6).  This   might  result  from  insufficient  GnP  surface  area  available  for  nickel  particles  to  attach,  nucleate   and   grow   with   the   excess   nickel   compound   washed   off   during   repeated   centrifugation   separation  with  acetone.     47   (a)   (b)   20#nm# m" m" 10#nm#         (c)             (d)   m" 5"nm"           m" 2"nm"     Figure  2-­‐4:  Nickel  nanoparticle  doped  GnP  synthesized  by  Polyol  assisted  microwave   process     To   confirm   the   identity   of   the   doped   nanoparticles,   Scanning   Transmission   Electron   Microscopy  (STEM)  combined  with  energy  dispersive  x-­‐ray  spectroscopy  (EDS)  was  done.  In  the   results   shown   in   Figure   2-­‐5,   we   see   a   copper   signal   throughout   the   line   scan   analysis   due   to   signal  from  the  grid.  There  is  a  constant  signal  from  carbon,  which  is  the  signal  reflected  from   GnP  platelets.  Nickel  is  clearly  seen  in  the  spectra  as  nanoparticles  on  the  GnP  surface.  In  the   nickel  spectrum,  we  can  notice  the  bump  in  the  line  scan  corresponding  to  the  nanoparticle.     48     Carbon Nickel Oxygen Copper   Figure  2-­‐5:  STEM-­‐EDS  Analysis  of  GnP_Ni-­‐5  nanoparticles.  Text  within  figure  is  not  meant   to  be  readable  and  is  for  visual  reference  only.     49   100" Weight"(%)"" 80" 60" GnP" GnP_Ni>5" 40" 20" 0" 0" 200" 400" 600" Temperature"(oC)" 800" 1000"   Figure  2-­‐6:  TGA  Analysis  of  GnP_Ni-­‐5  showing  the  concentration  of  metal  nanoparticles   The   other   nickel   nanoparticle   doped   GnP   materials   which   were   obtained   by   a   solventless   method   using   solid   mixing   and   inert   heat   treatment   procedure   have   larger   size   nanoparticles   and   were   characterized   using   SEM.   Figure   2-­‐7   and   Figure   2-­‐9   below   shows   the   images   of   nickel   nanoparticle   doped   GnP-­‐15   platelets   with   metal   nanoparticles   of   30   nm   clusters   and   60   nm   respectively.   From   the   SEM   images,   we   can   observe   that   the   metal   nanoparticles  are  uniform  and  are  well  distributed  over  the  GnP  platelets.  The  concentration  of   the  metal  doping  is  verified  by  the  TGA  data  shown  in  Figure  2-­‐8  and  Figure  2-­‐10.       50   (b)   (a)   1"μm" m"       100#nm# m"   Figure  2-­‐7:  GnP  nanoparticles  doped  with  nickel  NPs  clusters  of  30-­‐40nm     100" Weight"(%)"" 80" 60" GnP" GnP_Ni>30c" 40" 20" 0" 0" 200" 400" 600" Temperature"(oC)" 800" 1000" Figure  2-­‐8:  Thermogravimetric  analysis  of  GnP_Ni-­‐30c  material       51     (b)   (a)   1"μm" m"       100#nm# m"   Figure  2-­‐9:  GnP  nanoplatelets  doped  with  nickel  NPs  of  size  60-­‐80  nm     100" Weight"(%)"" 80" 60" GnP" GnP_Ni>60" 40" 20" 0" 0" 200" 400" 600" Temperature"(oC)" 800" 1000" Figure  2-­‐10:  Thermogravimetric  analysis  of  GnP_Ni-­‐60  material         52       The   Ni-­‐GnP   materials   synthesized   by   the   solventless   approach   can   be   produced   in   different   concentrations.   Figure   2-­‐11   below   shows   the   GnP_Ni-­‐60   material   at   different   concentrations  of  Nickel  doping.  The  distribution  of  nanoparticles  is  very  uniform,  however,  we   can   see   that   at   higher   concentration   of   15   wt%   Ni,   there   is   some   evidence   of   agglomeration.   The   concentration   of   metal   nanoparticles   in   the   composite   was   confirmed   by   doing   TGA   analysis  by  heating  in  air  to  900°C  (Figure  2-­‐12).  The  GnP  platelets  decompose  around  800°C,   leaving  behind  the  metal  nanoparticles.  From  the  weight  percentage  of  the  residual  material,   we  can  verify  the  metal  concentration  in  different  GnP_Ni  material  of  different  concentrations.   (a)   (b)   500#nm# m" 500#nm# m"   (c)   500#nm# m"   Figure  2-­‐11:  SEM  Images  of  GnP_Ni-­‐60  at  different  concentrations:(a)  5%,(b)  10%,(c)15%     53   100" Weight"(%)"" 80" 60" GnP" GnP_Ni>60"(5%)" GnP_Ni>60"(10%)" 40" GnP_Ni>60"(15%)" 20" 0" 0" 200" 400" 600" Temperature"(oC)" 800" 1000" Figure  2-­‐12:  Thermogravimetric  Analysis  (TGA)  of  GnP-­‐Ni  materials  showing  the   concentration  of  nickel  nanoparticles  in  the  sample     54       X-­‐RAY  DIFFRACTION  ANALYSIS   XRD   analysis   was   conducted   to   analyze   and   confirm   the   composition   of   metal   nanoparticles.   The   XRD   pattern   for   different   nickel   doped   materials   is   shown   below   in   comparison  with  undoped  GnP.  As  seen  from  Figure  2-­‐13,  the  characteristic  graphitic  peaks  at   26°   and   55°   are   observed   in   all   the   materials.   The   characteristic   peaks   of   Ni   occur   at   2Θ=   44.5°,   51.86°,   76.39°   13   and   for   NiO   at   2Θ:   37.26°,   43.29°,   62.88°,   75.42°,   79.41°   9,14 .   From   Figure   2-­‐13(b),  we  can  see  that  all  the  materials  show  a  mixture  of  Ni/NiO  peaks,  which  indicates  the   possibility   of   partial   oxidation   of   the   nanoparticles.   From   the   analysis,   the   presence   of   metal   nanoparticles   is   very   clearly   distinguishable   with   the   peak   widths   corresponding   to   the   invesrse   relationship  between  peak  width  and  particle  size  as  shown  by  the  Scherrer  equation.     55   (a)   40000" 35000" Intensity"("a.u.)" 30000" 25000" GnP" 20000" GnP_Ni:5" GnP_Ni:30c" 15000" GnP_Ni:60" 10000" 5000" 0" 10" 30" 2000" (b)   50" 2"theta" 70" Ni"peaks" Intensity"("a.u.)" 1500" GnP" GnP_Ni=5" 1000" GnP_Ni=30c" GnP_Ni=60" 500" 0" 35" 45" 55" 2"theta" 65" 75"   Figure  2-­‐13:  XRD  pattern  of  nickel  doped  materials  in  comparison  with  undoped  GnP    (a)   Full  spectra  (b)  Ni  peaks     56   RAMAN  SPECTROSCOPY  CHARACTERIZATION     To   understand   the   effect   of   doping   on   GnP   and   evaluate   the   interaction   of   nickel   nanoparticles   with   GnP,   further   characterization   was   done   using   Raman   spectroscopy.   The   G   -­‐1 band   at   1580   cm ,   known   as   the   graphite   or   tangential   band,   is   indicative   of   the   in-­‐plane   stretching   of   the   C-­‐C   carbon   bond 15,16 .   The   D   band,   known   as   the   defect   or   disorder   band,   around   1350   cm-­‐1   corresponds   to   visible   laser   excitation,   and   can   give   us   useful   information   15,16 about   disorder   in   the   graphitic   materials .   The   peak   at   2700   cm-­‐1   is   referred   to   as   2D   or   G’   peak,   which   is   attributed   to   second   order   two   phonon   process 15,16 .   From   Figure   2-­‐14   (b)   we   can   observe   slight   downshift   in   the   G   peak   for   all   nickel   doped   GnP   materials   which   is   an   indication   of   charge   transfer,   the   possibility   being   the   electrons   are   being   transferred   from   17 metal  dopants  to  GnP  platelets .  These  two  parameters  can  give  us  useful  information  about   the  graphitic  character  of  the  material  viz.,  ID/IG   ratio,  which  is  an  indication  of  the  impurity  in   the  material  and  disorder  parameter:  ID/(ID+IG).       57     Table   2-­‐2   shows   the   values   of   these   two   parameters,   and   we   can   see   that   no   significant   change   was   observed.   This   implies   that   the   nickel   particles   are   not   intercalated   in   between   the   graphene   layers   and   there   isn’t   any   damage   to   the   inherent   structure   of   the   graphene   nanoplatelets.     58   450" 400" 450" 350" 400" 300" 350" 250" 300" 200" 250" 150" 200" 100" 150" 50" 100" 0" 50" 0" Intensity"("a.u.)" Intensity"("a.u.)" (a)   0" 0" 450" 400" 450" 350" 400" 300" 350" 250" 300" 200" 250" 150" 200" 100" 150" 50" 100" 0" 50" 0" 2D  or  G’   GnP_Ni:5" GnP" GnP_Ni:30c" GnP_Ni:5" GnP_Ni:60" GnP_Ni:30c" GnP_Ni:60" 500" 1000" 1500" 2000" 2500" 3000" 3500" 4000" Raman"Shi8"(cm:1)" 500" 1000" 1500" 2000" 2500" 3000" 3500" 4000" Raman"Shi8"(cm:1)"   GnP" GnP" Intensity"("a.u.)" Intensity"("a.u.)" (b)   GnP" GnP_Ni:5" GnP_Ni;5" GnP_Ni:30c" GnP_Ni;30c" GnP_Ni:60" GnP_Ni;60" 500" 1000" 1500" 2000" 2500" 3000" 3500" 4000" Raman"Shi8"(cm:1)" 0" 1250" 1300" 1350" 1400" 1450" 1500" 1550" 1600" 1650" Raman"Shi9"(cm;1)"   Figure  2-­‐14:  Raman  Spectroscopy  Analysis  of  different  GnP_Ni  materials   59     Table  2-­‐2:  Intensity  values  ID,  IG,  ID/IG  and  disorder  parameter  of  GnP_Ni  Materials   ID# IG# ID/IG# Disorder# Parameter# GnP# 147! 302! 0.49! 0.33! GnP_Ni35## 143! 297! 0.48! 0.33! GnP_Ni330c# 168! 340! 0.49! 0.33! GnP_Ni360# 351! 351! 0.46! 0.31! !!   ELECTRODE  MORPHOLOGY   The   morphology   of   the   electrodes   was   observed   both   from   the   top   view   and   cross-­‐ section   by   scanning   electron   microscopy   (Figure   2-­‐15).   The   cross-­‐section   view   of   the   GnP   electrode  shows  that  it  consists  of  the  nanoplatelets  arranged  next  to  each  other  and  formed   into   an   aligned   network.   This   can   be   due   to   the   π-­‐π   interactions   of   the   GnP   basal   plane.   The   interconnected  matrix  of  GnP  platelets  imparts  good  conductivity  and  strength  to  the  electrode   in  the  in-­‐plane  direction.   For   GnP_Ni-­‐5,   we   can   see   that   the   platelets   are   well-­‐connected   but   not   perfectly   aligned.  This  is  possibly  related  to  the  interaction  of  metal  nanoparticles  with  the  π  electrons  of   the   basal   plane.     For   both   the   nickel   doped   GnP   materials   made   by   solventless   approach   (GnP_Ni-­‐30c   and   GnP_Ni-­‐60),   the   electrode   morphology   is   the   same.   The   GnP   platelets   have   experienced   a   morphology   change   and   size   reduction   during   the   ball   milling   process.   The   electrode   cross-­‐section   shows   well-­‐connected   but   a   relatively   random   and   disordered   arrangement   of   the   nanoplatelets.     This   change   on   electrode   structure   can   explain   the   difference  in  electrochemical  performance  of  these  electrodes.     60     (a)     (b)   10#μm# m"       10#μm# m"   (d)   (c)   10#μm# m"       10#μm# m"   (f)   (e)   10#μm# m"       10#μm# m" Figure  2-­‐15:  Electrode  Morphology  of  top-­‐view  and  cross-­‐section  view  (respectively)  of   Undoped  GnP  [(a),  (b)];  GnP_Ni-­‐5  [(c),  (d)];  GnP_Ni-­‐30c,  GnP_Ni-­‐60  [(e),  (f)]             61     ELECTROCHEMICAL  CHARACTERIZATION   Galvanostatic  Cycling     Figure   2-­‐17   (a)   shows   that   the   capacity   values   of   baseline   GnP-­‐15   without   any   metal   nanoparticles   at   different   charge   rates   as   per   the   protocol.   The   capacity   of   GnP   electrode   is   around  330  mAh/g  at  C/5  charge  rate  and  a  decrease  in  the  capacity  values  is  observed  at  faster   charge   rates,   as   expected   in   graphitic   systems.   Also,   the   highly   aligned   structure   of   the   GnP   + electrode   might   hamper   the   diffusion   of   Li   ions   through   the   thickness   of   the   electrodes,   particularly   at   faster   charge   rates.   The   storage   capacity   of   the   GnP   electrode   decreases   significantly  at  faster  charge  rates  of  C  &  2C,  however  it  recovers  back  to  high  capacity  when  we   cycle   the   electrode   at   the   slower   rate.   This   is   a   verification   of   the   fact   that   the   decrease   in   capacity  is  due  to  restricted  kinetics,  and  there  is  no  permanent  degradation  of  the  electrode.     Also,   we   can   see   the   discharge   and   charge   capacity   of   the   electrode   is   very   close,   translating   to   a  high  coulombic  efficiency  of  >  98  %.   For  the  GnP  doped  with  small  size  nickel  nanoparticles  (GnP_Ni-­‐5),  the  performance  is   the   same   as   the   undoped   GnP.   This   suggests   that   there   is   no   role   of   nanoparticles   towards   capacity  contribution,  neither  as  active  material  nor  as  nanostructuring  agent.  The  size  of  nickel   nanoparticles  is  probably  too  small  to  make  an  impact  on  creating  a  nanostructured  electrode   that   can   influence   performance.   For   the   second   set   of   Ni   doped   systems   prepared   by   solventless  approach   mentioned  above,  the   performance   is  shown   in   Figure  2-­‐17   (c)  and   (d)   for   GnP_Ni-­‐60   and   GnP_Ni-­‐30   respectively.   The   capacity   of   the   electrode   was   maintained   at   the   same  value  even  when  the  charge  rate  was  varied  from  C/5  to  C/2,  C  and  2C.  This  demonstrates     62   the  potential  of  the  metal  doped  material  to  perform  very  well  even  at  faster  charge  rates.  This   change  in  performance  can  be  attributed  to  the  connected  but  random  arrangement  of  active   material  in  the  electrode.  Also,  the  metal  doping  will  aid  in  keeping  the  GnP  platelets  apart  and   will   prevent   agglomeration.   These   two   factors   combined   together   promote   fast   diffusion   of   lithium   ions   and   better   access   to   the   graphitic   surface   at   faster   charge   rate,   thus   showing   improved  performance.   Current(A)%% Current(A)% Voltage(V)%% Voltage(V)%% 0.004% 0.003% Current(A)% 0.002% 0.001% 3.0% 2.5% 2.0% 1.5% 0% 1.0% !0.001% !0.002% 0.5% !0.003% 0.0% !0.004% C/2 C/5 !0.005% 0% 100000% C 2C C C/2 200000% 300000% Test%Time(s)% C/5 !0.5% 400000% Figure  2-­‐16:  Testing  Protocol  for  galvanostatic  analysis  of  electrodes     Voltage(V)% 0.005% 63       (a)   Performance:"GnP" 600" Capacity"(mAh/g)" C$ $ C/2$ C/5$ 500" 2C$ C$ C/2$ C/5$ Discharge" 400" Charge" 300" 200" 100" 0" 0" 5" 10"     15" 20" Cycle"Number" 25" 30" 35" Performance:"GnP_NiD5" (b)   600" C/5$ C/5$ Capacity"(mAh/g)" 500" C/2$ C/2$ C$ C$ $$ 2C$ 2C$ C$ C$ C/2$ C/2$ C/5$ C/5$ Discharge" 400" Charge" 300" 200" 100" 0" 0" 5" 10" 15" 20" Cycle"Number" 25" 30" 35"   Figure  2-­‐17:  Galvanostatic  performance  of  nickel  doped  materials  in  comparison  with   undoped  GnP  (a)  Undoped  GnP  (b)  GnP_Ni-­‐5  (c)  GnP_Ni-­‐30c  (d)  GnP_Ni-­‐60c       64   Figure  2-­‐17  (cont’d)   Performance:"GnP_NiF30c" (c)   800" Capacity"(mAh/g)" 700" C/5$ 600" C/2$ C$ $ 2C$ C$ C/2$ C/5$ Discharge" Charge" 500" 400" 300" 200" 100" 0" 0" 5" 10" (d)   15" 20" Cycle"Number" 25" 30" 35"   Performance:"GnP_NiD60" 600" Capacity"(mAh/g)" 500" Discharge" 400" Charge" 300" 200" C/2$ C/5$ 100" C$ $ 2C$ C$ C/2$ C/5$ 0" 0" 5" 10" 15" 20" Cycle"Number"     65     25" 30" 35"     Cyclic  Voltammetry   To  understand  the  mechanism  behind  the  improved  performance  at  faster  charge  rates,   cyclic  voltammograms  at  low  scan  rate  (0.05  mV/s)  were  collected  to  see  the  role  of  Ni  in  the   system.  Figure  2-­‐18  shows  the  CV  of  fresh  anode  (without  cycling),  where  we  can  identify  the   characteristic  peaks  of  lithium  intercalation  in  and  out  of  the  anode.  The  first  cathodic  peak  at   0.6   V   is   attributed   to   SEI   layer   formation,   which   completely   eliminated   in   successive   sweeps.   The  other  cathodic  peaks  between  0  to  0.15V  represented  in  all  sweeps  are  related  to  lithium   insertion   into   carbon.   On   the   anodic   side,   the   peaks   in   the   range   0.2-­‐0.35V   represent   lithium   18 extraction   from   LixC6 .   There   are   no   peaks   in   the   expected   electrochemical   region   corresponding  to  NiO  (cathodic  peaks  at  0.26  and  0.52  V  and  anodic  peaks  at  1.60V  and  2.39  V,   9 corresponding  to  electrochemical  reduction  and  oxidation  reaction  respectively ).   From   the   CV   profiles   it   was   observed   that   intercalation   peaks   were   relatively   more   enhanced  for  doped  material  relative  to  undoped  GnP.  Hence,  we  can  infer  that  metal  doping   of  >30  nm  and  reasonable  concentration  can  help  to  keep  platelets  accessible  for  fast  lithium   ion   diffusion.   However,   for   GnP_Ni   -­‐30c,   on   disassembly   of   cells,   a   red   color   discoloration   of   electrolyte   was   observed,   which   is   probably   due   to   a   side   reaction   between   the   metal   nanoparticles   and   lithium   ions   or   electrolyte   components.   The   undesirable   reaction   products   could   not   be   identified   due   to   hazardous   nature   of   the   material.   The   GnP_Ni-­‐60   material   showed  the  best  capacity  performance  amongst  the  three  materials.         66   Current%(A/g)% 0% 0.0% (a)   0.1% 0.2% 0.3% 0.4% 0.5% Poten7al%(V)% !0.5% GnP% GnP_Ni!5% GnP_Ni!30c% GnP_Ni!60% Current)(A/g)) !1% Scan%rate%:%0.05mV/s% !1.5% 1% 1% Poten&al)(V)) 0.5% 0.5% Current%(A/g)% Current%(A/g)% Scan)rate:)0.05mV/s) (b)   0% 0% 0.0% 0.0% !0.5% !0.5% 0.1% 0.1% 0.2% 0.2% 0.3% 0.5% 0.5% Poten7al%(V)% Poten7al%(V)% GnP% GnP_Ni!5% GnP_Ni:30c% GnP_Ni!30c% GnP_Ni!60% !1% !1% Current)(A/g)) 0.4% Scan%rate%:%0.05mV/s% Scan%rate%:%0.05mV/s% !1.5% !1.5% Poten&al)(V)) Scan)rate:)0.05mV/s)   Figure  2-­‐18:    Cyclic  Voltammogram  of  Ni  doped  materials       67   Electrochemical  Impedance  Spectra   Impedance  spectra  for  the  undoped  GnP  and  GnP_Ni-­‐60  electrode  were  obtained  after   5  galvanostatic  cycles  at  C/5  rate  (Figure  2-­‐19).  The  experimental  data  was  further  analyzed  by   fitting   for   the   equivalent   circuit   shown   in   Figure   2-­‐19   (b)   with   Z-­‐view   software.   Rs   is   the   bulk   or   solution  resistance  which  corresponds  to  the  intercept  on  the  x-­‐axis.  The  semi-­‐circular  loops  at   high  and  medium  frequencies  are  attributed  to  the  SEI  film  and  charge  transfer  resistances.  The   inclined   line   is   an   indication   of   the   diffusion   behavior   of   the   electrode.   Based   on   the   fitting   analysis   (Table   2-­‐3),   the   resistance   contribution   due   to   the   SEI   film   formation   and   charge   transfer  is  lower  for  the  GnP_Ni-­‐60  electrode.    The  decreased  SEI  film  formation  is  possibly  due   to  a  reduction  in  surface  area,  and  hence  fewer  active  sites  for  electrolyte  decomposition.  Also,   the   addition   of   Nickel   nanoparticles   might   have   contributed   to   increasing   the   conductivity   through   the   electrode,   thus   diminishing   charge   transfer   resistance.   However,   these   two   hypotheses  need  further  experimentation  and  detailed  analysis  for  confirmation.   Hence,   from   the   cyclic   voltammogram   and   capacity   profiles,   it   was   observed   that   the   doped   anodes   retain   the   graphitic   profile   and   do   not   show   any   significant   distortions.   This   indicates  that  nickel  plays  an  electrochemically  inactive  role  here,  and  does  not  participate  in   any   direct   conversion   reaction   with   lithium.   The   improved   capacity   values   are   probably   attributable   to   the   faster   diffusion   due   to   more   open   structure   produced   by   the   nickel   spacers,   resulting  in  improved  performance  at  faster  charge-­‐discharge  conditions.                 68   (a)   80" +"Z"imag"(ohms)" 70" 60" 50" 40" GnP" 30" GnP_Ni+60" 20" 10" 0" 0" 10" 20" 30" 40" 50" Z"real"(ohms)" 60" 70" 80"   (b)   Rs Rsei Rct CPE 1 W CPE 2     Element (a)  Nyquist  plots  of  undoped  GValue GnP_Ni-­‐60  obtained  after  5  cycles  oError % Freedom Error Figure  2-­‐19:   nP  and   f   Rs Fixed(X) 3.183 N/A N/A charge  discharge  at  C/5  rate.  The  solid  lines  correspond  to  GnP  and  the  dotted  lines  correspond   to  GnP_Ni-­‐60.  (b)  Equivalent  circuit  used  for  fitting  analysis     Rsei Free(+) 6.374 0.065027 1.0202 CPE 1-T2-­‐3:  Resistance  values  of  different  components  GnP  and  GnP_Ni  electrodes   N/A Fixed(X) 0.0007836 N/A Table   CPE 1-P Fixed(X) 0.65798 N/A N/A obtained  by  impedance  fitting  analysis   Rct Fixed(X) 0.13337 N/A N/A Rs# Rsei# Rct# W-R Fixed(X) 0 N/A N/A Undoped'GnP':'5cyc' 3' 14' 6' W-T Fixed(X) 0 N/A N/A GnP_Ni560':'5cyc' 3' 6' 1' W-P Fixed(X) 0.5 N/A   N/A CPE 2-T Free(+) 2.9607E-7 8.0504E-7 271.91   CPE 2-P Free(+) 1.4 0.22109 15.792 Chi-Squared: Weighted Sum of Squares:   Data File: 0.0061365 69   0.82843 D:\\E6p_Run101_EIS_After cycle 5.z CONCLUSIONS     The   methodology   described   here   resulted   in   a   multilayered   ‘parking   garage’   structure   composed  on  parallel  arrays  graphene  nanoplatelets,  each  of  which  were  decorated  with  nickel   metal  nanoparticles  which  produced  an  open  porous  structure,  thus  allowing  flexibility  for  easy   diffusion  of  lithium  in  and  out  of  anodes,  resulting  in  high  reversible  capacity  for  batteries.     Although   there   are   several   publications   which   have   demonstrated   that   nickel   is   electrochemically  active  and  synthesizing  NiO-­‐graphene  composites  in  a  3-­‐D  arrangement  can   enhance  the  performance  of  the  anode   9,13 ,    in  this  research,  the  nickel  dopants  seems  to  be   passivated   and   has   no   significant   contribution   to   the   capacity   of   the   anode   material.   The   increase   in   performance   at   faster   charge   rate   can   be   inferred   as   due   to   the   effect   of   facile   diffusion  of  ions.   FUTURE  WORK   NI  NANOPARTICLE  ACTIVATION   The   nickel   nanoparticles   produced   in   this   research   did   not   display   the   electrochemical   activity  expected.    Heat  treatment  of  the  GnP-­‐Ni  material  in  selected  gaseous  atmospheres  at   high   temperature   or   with   liquid   reactants   can   be   used   to   remove   either   a   nickel   oxide,   or   a   nickel   organic   compound   that   may   cover   the   nanoparticle   surface   and   restrict   its   interaction         Activation  by  any  of  these  means  should  be  explored  to  see  if  the  electrochemical  performance   would  be  improved.         70   GNP-­‐SILICA  PAPER   Preliminary   work   has   been   conducted   on   an   electrode   material   nanostructured   with   silicon   to   produce   a   silicon-­‐graphene   self-­‐standing   electrode,   with   ordered   porosity.   The   methodology   is   based   on   polyelectrolyte   mediated   self-­‐assembly   of   an   aqueous   dispersion   of   graphene   and   colloidal   silica.   The   synthesis   procedure,   as   shown   in   schematic   Figure   2-­‐20,   shows   two   separate   solutions   of   GnP   with   positive   and   negative   polyelectrolytes,   combined   together   and   filtered   to   obtain   GnP-­‐silica.     The   resulting   product   is   a   self-­‐   standing   film   of   GnP-­‐ silica  material,  with  an  in-­‐plane  conductivity  of  330  S/cm.   Ultrasonica+on, GnP,+PEI,in,Water, Silica+PSS,in,water, GnP,+Silica,solu+ons, Washing,&,, ,,,,,,,,,,,,,,mixed, Heat,Treatment,, at,340,C, Filtra+on, GnP,–Silica,Paper, GnP,+Silica,Paper, (Residual,binder,s+ll,leE), Figure  2-­‐20:  Schematic  showing  the  synthesis  procedure  for  GnP-­‐Silica  Nanocomposite     71     (a)   (b)   1"μm! 1"μm!     Figure  2-­‐21:  Self-­‐Assembled  silica-­‐graphene  paper:  Top  view                                     (a)   (b)   500#nm! (c)   1"μm! 500#nm! Figure  2-­‐22:  Cross-­‐Sectional  View  of  Self-­‐Assembled  silica-­‐graphene  paper  (a)  Whole   electrode  (b)  High  magnification  (c)  Back-­‐scatter  image  of  the  section  shown  in  (b)     72     Morphological   characterization   of   the   paper   observed   from   top-­‐view   (Figure   2-­‐21),   shows   that   silica   nanoparticles   are   distributed   over   the   GnP   platelet   surface   with   some   slight   aggregation  of  silica  nanoparticles.  The  cross-­‐sectional  SEM  images  shown  in  Figure  2-­‐22  verify   the   highly   organized   electrode   structure   with   silica   nanoparticles   interspersed   between   graphene   nanoplatelets,   analogous   to   cars   parked   in   a   parking   garage.   Also,   shown   in   Figure   2-­‐22  (b)  and  (c)  are  the  secondary  electron  and  backscatter  image  of  the  same  section.  In  the   backscatter   image,   we   can   observe   that   the   brighter   region   represent   the   silica   nanoparticles     (due  to  higher  atomic  number).   Further   trials   are   being   done   to   reduce   the   silica   to   active   silicon   by   heat   treatment   under   H2   reducing   conditions.   It   is   expected   that   this   electrode   will   be   able   to   blend   the   advantages  of  capacity  of  both  active  materials  and  the  effect  of  nanostructuring  the  electrode.       73                         REFERENCES     74     REFERENCES     1.   Lian,  P.  et  al.  Large  reversible  capacity  of  high  quality  graphene  sheets  as  an  anode   material  for  lithium-­‐ion  batteries.  Electrochimica  Acta  55,  3909–3914  (2010).   2.   Yang,  S.,  Feng,  X.,  Ivanovici,  S.  &  Müllen,  K.  Fabrication  of  graphene-­‐encapsulated  oxide   nanoparticles:  towards  high-­‐performance  anode  materials  for  lithium  storage.   Angewandte  Chemie  49,  8408–11  (2010).   3.   Wang,  D.  et  al.  Ternary  self-­‐assembly  of  ordered  metal  oxide-­‐graphene  nanocomposites   for  electrochemical  energy  storage.  ACS  nano  4,  1587–95  (2010).   4.   Li,  X.  et  al.  Functionalized  graphene  sheets  as  molecular  templates  for  controlled   nucleation  and  self-­‐assembly  of  metal  oxide-­‐graphene  nanocomposites.  Advanced   materials  24,  5136–41  (2012).   5.   Guo,  Y.-­‐G.,  Hu,  J.-­‐S.  &  Wan,  L.-­‐J.  Nanostructured  Materials  for  Electrochemical  Energy   Conversion  and  Storage  Devices.  Advanced  Materials  20,  2878–2887  (2008).   6.   Sun,  Y.,  Wu,  Q.  &  Shi,  G.  Graphene  based  new  energy  materials.  Energy  &  Environmental   Science  4,  1113–1132  (2011).   7.   Wu,  Z.-­‐S.,  Ren,  W.,  Xu,  L.,  Li,  F.  &  Cheng,  H.-­‐M.  Doped  graphene  sheets  as  anode   materials  with  superhigh  rate  and  large  capacity  for  lithium  ion  batteries.  ACS  nano  5,   5463–71  (2011).   8.   Zhou,  X.,  Yin,  Y.-­‐X.,  Wan,  L.-­‐J.  &  Guo,  Y.-­‐G.  Facile  synthesis  of  silicon  nanoparticles   inserted  into  graphene  sheets  as  improved  anode  materials  for  lithium-­‐ion  batteries.   Chemical  communications  48,  2198–200  (2012).   9.   Tao,  L.  et  al.  3D-­‐hierarchical  NiO–graphene  nanosheet  composites  as  anodes  for  lithium   ion  batteries  with  improved  reversible  capacity  and  cycle  stability.  RSC  Advances  2,   3410–3415  (2012).   10.   Fukushima,  H.  Graphite  Nanoreinforcements  in  Polymer  Nanocomposites.  Chemical   Engineering  &  Materials  Science  Department,  Michigan  State  University,  East  Lansing   (2003).     75   11.   Lin,  Y.  et  al.  Rapid,  solventless,  bulk  preparation  of  metal  nanoparticle-­‐decorated  carbon   nanotubes.  ACS  nano  3,  871–84  (2009).   12.   Fu,  J.  &  Joshi,  S.  Optimization  Based  Geometric  Modeling  of  Nano/Micro  Scale  Ion  Milling   of  Organic  Materials  for  Multidimensional  Bioimaging.  Journal  of  Nanotechnology  in   Engineering  and  Medicine  1,  031003  (2010).   13.   Mai,  Y.  J.,  Tu,  J.  P.,  Gu,  C.  D.  &  Wang,  X.  L.  Graphene  anchored  with  nickel  nanoparticles   as  a  high-­‐performance  anode  material  for  lithium  ion  batteries.  Journal  of  Power  Sources   209,  1–6  (2012).   14.   Kottegoda,  I.  R.  M.,  Idris,  N.  H.,  Lu,  L.,  Wang,  J.-­‐Z.  &  Liu,  H.-­‐K.  Synthesis  and   characterization  of  graphene–nickel  oxide  nanostructures  for  fast  charge–discharge   application.  Electrochimica  Acta  56,  5815–5822  (2011).   15.   Ferrari,  a.  C.  et  al.  Raman  Spectrum  of  Graphene  and  Graphene  Layers.  Physical  Review   Letters  97,  187401  (2006).   16.   Dresselhaus,  M.  S.,  Jorio,  A.,  Hofmann,  M.,  Dresselhaus,  G.  &  Saito,  R.  Perspectives  on   carbon  nanotubes  and  graphene  Raman  spectroscopy.  Nano  letters  10,  751–8  (2010).   17.   Terrones,  M.,  Filho,  A.  G.  S.  &  Rao,  A.  M.  Doped  Carbon  Nanotubes :  Synthesis  ,   Characterization  and  Applications.  566,  531–566  (2008).   18.   Levi,  M.  D.  &  Aurbach,  D.  Simultaneous  Measurements  and  Modeling  of  the   Electrochemical  Impedance  and  the  Cyclic  Voltammetric  Characteristics  of  Graphite   Electrodes  Doped  with  Lithium.  Journal  of  Physical  Chem.  B  5647,  4630–4640  (1997).       76     3 GRAPHITE   NANOPLATELETS   AS   A   CONDUCTING   ADDITIVE   FOR   LITHIUM   TITANATE  ELECTRODES   BACKGROUND   Lithium  titanate  (LTO)  is  one  of  the  prominent  materials  being  considered  for  lithium  ion   batteries   anodes,   with   a   theoretical   lithium   storage   capacity   of   175   mAh/g.   Despite   of   its   significantly   lower   storage   capacity   in   comparison   to   other   anode   materials   such   as   Graphite   (375   mAh/g),   SnO2   (990   mAh/g)   and   Silicon   (4200   mAh/g),   LTO   has   attracted   interest   due   to   its   high   rate   performance,   cyclic   stability,   reversibility   and   no   volume   change   during   the   charge-­‐ discharge   process 1,2 .   In   addition,   since   the   operating   voltage   of   LTO   is   higher   than   that   for   deposition   of   metallic   lithium,   this   material   is   not   prone   to   problems   of   dendritic   growth   and   2 hence,  is  safer  than  other  anode  materials .     Lithium   titanate   can   be   combined   with   a   variety   of   cathode   materials   such   as   LiCoO2,   LiFePO4   and   LiMn2O4   to   form   a   Li   ion   battery   setup   capable   of   delivering   2.5   V   (schematic   shown  in  Figure  3-­‐1).   Spinel   lithium   titanate   structure   acts   as   an   intercalation   host,   allowing   reduction   of   3   4+ Ti ,  and  changing  to  a  stable  Li7Ti5O12  rock  salt  structure  (Figure  3-­‐2).  The  insertion  process   3,4 allows   inclusion   of   3   Li   atoms   per   formula   unit   with   minimal   change   in   cubic   unit   cell   77   .   This   zero-­‐strain  feature  of  LTO  helps  to  maintain  electrode  structure  over  cycle  life,  thus  making  it   an  attractive  anode  material. 𝐿𝑖! 𝑇𝑖! 𝑂!" + 3  𝐿𝑖 !   +  3  𝑒 ! ⇌   𝐿𝑖! 𝑇𝑖! 𝑂!"   However,  lithium  titanate  suffers  from  the  problem  of  low  conductivity  and  thus  offers   + high   resistance   to   electron   and   Li   transport,   resulting   in   low   power   density   of   the   batteries.   The   conductivity   problem   of   lithium   titanate   electrodes   is   addressed   by   two   different   approaches.  One  methodology  is  to  develop  nanoscale  or  nanostructured  particles,  which  will   5,6 have  shorter  lithium  diffusion  paths  and  hence  will  allow  better  ionic  conductivity .  Electrical   7 conductivity  can  be  improved  by  doping  and  conductive  coatings  of  active  materials  or  by  the   2,6 addition  of  conducting  materials  to  the  electrode   78   .     e"# 1$ e"# 1$ +$ +$ Anode$Material$:$Lithium$Titanate$$ Anode$Material$:$Lithium$Titanate$$ Cathode$Material$:$LiFePO4,$LiCoO2$ Li+$ Cathode$Material$:$LiFePO ,$LiCoO 4 2$ Conduc?ng$Addi?ve:$Carbon$black$ Conduc?ng$Addi?ve:$Carbon$black$ Li+$ Binder:$PVDF,$PTFE$ Binder:$PVDF,$PTFE$ Anode$ Separator$ Cathode$ $ Anode$ Separator$ Cathode   Figure  3-­‐1:  Schematic  showing  battery  setup  with  lithium  titanate  as  anode       4 Figure  3-­‐2:  Crystal  structure  of  (a)  Spinel  Li4Ti5O12  and  (b)  Rock-­‐salt  Li7Ti5O12             79   SIGNIFICANCE   Conductivity   is   one   of   the   major   concerns   associated   with   the   performance   of   lithium   titanate  and  several  other  cathode  materials  (LiCoO2,   LiMnO2,  LiFePO4).  The  ionic  conductivity   is  tackled  by  making   the  size   of  active  material  smaller   which   will   reduce   the   diffusion   distance   for   lithium   ion.   For   electrical   conductivity,   suitable   modifications   are   done   by   addition   of   conductive  coatings,  conductive  templates  or  by  addition  of  conductive  materials  such  as  metal   fibers 7,8  and  carbon  materials 6,9,10  in  the  electrode  structure.       Carbon  black  is  the  preferred  choice  of  conducting  additive  for  such  electrodes,  which  is   primarily   due   to   good   conductivity,   good   cycle   life   and   low   cost.   However,   to   achieve   percolation,  high  concentration  10-­‐20%  of  carbon  black  is  needed.  Such  high  concentration  of   additives  can  have  negative  impacts  such  as  lower  gravimetric  performance  and  increased  side   reactions.  Hence,  there  have  been  attempts  to  reduce  the  amount  of  inactive  materials  in  the   electrode  to  increase  its  gravimetric  capacity/energy  density.     11 12 High   aspect   ratio   carbon   nanomaterials,   such   as   CNTs ,   CFs   and   graphene   1,2,13,14 sheets   have   been   investigated   as   potential   additive   materials.   Graphene,   It   has   been   2,14 observed   that   electrical   percolation   can   be   obtained   at   lower   concentrations   of   5   %   or   less .   In   addition   some   results   have   also   shown   that   using   high   aspect   ratio   graphene   sheets   also   reduces   polarization 7,15 .     The   combination   of   carbon   black   with   conductive   high   aspect   ratio   12,16 materials  has  been  used  to  improve  the  performance  of  the  electrode  materials   80   .   APPROACH   Graphene   nanoplatelets   (GnP)   are   two-­‐dimensional,   high   aspect   ratio   nanoplatelets,   which   can   be   synthesized   at   a   low   cost   of   ~$20/lb.,   by   a   simple   intercalation-­‐exfoliation   procedure.  These  platelets,  which  are  a  stack  of  few  graphene  sheets,  have  the  advantage  of   high   aspect   ratio,   nanoscale   thickness   dimension,   and   can   percolate   to   form   an   interconnected   network   at   low   concentrations.   High   chemical   tolerance,   good   thermal   stability   and   good   mechanical   strength   are   additional   benefits,   which   can   help   maintain   electrode   integrity   and   good   cycle   life.   In   this   section,   we   have   investigated   the   effect   of   the   addition   of   different   sizes   of  GnP  on  the  lithium  storage  capacity  of  LTO  and  have  investigated  the  effect  of  reducing  the   concentration  of  conducting  additive  on  the  impedance  and  cycling  performance.     EXPERIMENTAL  SECTION   MATERIALS   Spinel   lithium   titanate   nanopowder   (<100nm   nanopowders),   polyvinylidene   fluoride   (PVDF)   and   N-­‐methyl   pyrrolidone   were   obtained   from   Sigma-­‐Aldrich.   Lithium   foil   used   as   the   counter   electrode   was   obtained   from   Alfa-­‐Aesar.   Different   grades   of   graphene   nanoplatelets   ® viz.   GnP-­‐1,   GnP-­‐5   and   GnP-­‐25   were   obtained   from   XG   Sciences .   Commercial   conducting   additive  Super  P  was  obtained  from  Timcal.  Aluminum  foil  of  15μm thickness,  obtained  from   MTI  Corporation  was  used  as  the  current  collector  for  casting  electrodes.     81   ELECTRODE  PREPARATION   The   electrode   slurry   is   prepared   by   ball   milling   the   appropriate   amounts   of   lithium   titanate  powder,  conducting  additive  (varies  from  2-­‐10%),  and  10%  binder  PVDF,  with  desired   amount   of   N-­‐methyl   pyrrolidone.   The   well-­‐dispersed   viscous   slurry   is   cast   on   the   aluminum   foil   current   collector   using   the   adjustable   film   applicator   on   the   MTI   automatic   film   coater.   The   electrodes  were  dried  under  ambient  condition  for  12  hours,  and  then  heated  in  a  vacuum  oven   at  a  temperature  of  around  120°C  for  8  hours.  Finally  the  electrodes  were  pressed  in  a  Carver   press   at   a   pressure   of   0.1   MPa   to   ensure   good   electrical   contact,   inter-­‐particle   contact   and   lower  volumetric  density.   MORPHOLOGY  CHARACTERIZATION   The   morphology   of   the   LTO,   different   carbon   materials,   electrode   top   view   and   electrode  cross-­‐section  was  observed  using  the  Carl  Zeiss  Auriga®  CrossBeam  scanning  electron   microscope.  The  sample  preparation  for  observing  electrode  cross-­‐section  was  done  by  epoxy   embedding  and  polishing  method,  as  described  previously.   ELECTROCHEMICAL  CHARACTERIZATION   The   electrochemical   performance   of   LTO-­‐GnP   Electrodes   were   evaluated   in   a   two   electrode  coin  cell  configuration  with  lithium  foil  as  the  counter  electrode  and  1M  solution  of   LiPF6   in   1:1   (v/v)   EC/DMC   solution,   as   the   electrolyte.   Figure   3-­‐3   shows   the   schematic   and   pictures   of   the   2-­‐electrode   coin   cell   setup.   The   CR-­‐2032   coin   cells   were   assembled   in   argon   filled  glove  box    (<1  ppm  H2O,  <1  ppm  O2),  and  were  tested  using  an  Arbin  Instruments  BT  2000   Battery  Testing  System.  The  electrodes  were  galvanostatically  cycled  between  1  V  and  2.5  V  at     82   different  charge  rates  of  C/5,  C/2,  C  and  2C.  The  C-­‐rates  are  calculated  based  on  the  mass  of   LTO  in  the  electrode.  Electrochemical  Impedance  Spectroscopy  was  done  using  a  VersaSTAT  MC   Instrument  in  the  frequency  range  of  1  Hz  to  1  MHz  at  amplitude  of  5mV.     !! SS!cap! Wave!ring! !! Spacer!disc! !! !! Li!foil!or!Cathode! !! Separator! GnP!Electrode! !! !! O(ring! SS!base! !! !!   Figure  3-­‐3:  Schematic  and  pictures  of  Two-­‐electrode  Coin  cell     RESULTS  &  DISCUSSION   MORPHOLOGICAL  OBSERVATION   The   morphology   of   Lithium   titanate   nanopowder   (Figure   3-­‐4(e))   and   the   conducting   additives  were  analyzed  by  SEM.  Figure  3-­‐4  (a),  (b)  &  (c)  shows  SEM  images  of  three  different   GnP   materials   being   used   for   this   study.   These   platelets   are   represented   by   GnP-­‐x   nomenclature,  where  x  represents  the  average  diameters  of  these  platelets.  Figure  3-­‐4  (d)  is  the   SEM  image  of  Super  P  particles,  which  are  spherical  in  shape  and  are  of  nano-­‐dimension.  The   properties  of  different  carbon  materials  are  summarized  below  in  Table  3-­‐1.     83   Table  3-­‐1:  Different  carbon  materials  with  their  physical  properties   Material   Particle  size   Dia  (μm)   Thickness   (μm)   Surface  Area   GnP-­‐1   1   <0.01   100   <100   GnP-­‐5   5   <0.01   100   ~500   GnP-­‐25   25   <0.01   120   ~2500   Super  P   <0.1     <0.1   62   1   2 Aspect  Ratio   (m /g)     The   top   view   and   cross-­‐sectional   view   of   all   the   electrodes   SEM   images   are   shown   in   Figure   3-­‐5   and   Figure   3-­‐6.   The   SEM   images   illustrate   uniform   distribution   of   the   conducting   additives   through   the   electrode   matrix   however   their   interaction   with   LTO   are   significantly   different,  which  is  discussed  in  detail  in  later  section.  From  the  cross-­‐sectional  images  of  GnP   electrodes,   we   can   observe   that   the   GnP   platelets   are   dispersed   in   the   electrode   forming   an   interconnected   network.   The   platelets   are   flexible   and   bend   out-­‐of-­‐plane   providing   contact   with   the   LTO   particles.   There   is   overlapping   of   platelets   in   the   in-­‐plane   and   through-­‐plane   direction,   which   ensures   a   continuous   electrical   pathway,   resulting   in   good   electrical   conductivity.  Also,  the  large  dimension  of  platelets  can  provide  good  mechanical  integrity  to  the   electrode  made  of  LTO  nanopowders.       84     (a)   (b)   10"μm! 1"μm! 1"μm! 1"μm! (c)   (d)   300"nm! 1"μm! 1"μm! 1"μm!   60 (e)   300"nm!   Figure  3-­‐4:  SEM  Images  of  (a)  GnP-­‐25,  (b)  GnP-­‐5,  (c)  GnP-­‐1  ,  (d)  Super  P,  (e)  LTO     (Scale  bar-­‐  (a):  10 μm;  (b),(c):  1  μm;  (d),(e):  300  nm).The  image  of  LTO  is  taken  from  LTO+GnP   electrode,  because  of  difficulty  in  doing  SEM  of  pure  LTO  powder  due  to  its  poor  conductivity     85   (a)   (b)   1"μm! 1"μm! (c)   (d)   1"μm! 1"μm! Figure  3-­‐5:  SEM  Images  of  top  view  of  various  LTO  electrodes  (a)  LTO+GnP-­‐25,  (b)  LTO   +GnP-­‐5,  (c)  LTO  +GnP-­‐1,  (d)  LTO+Super  P  electrodes.     In  all  these  electrodes,  the  concentration  of  carbon  additive  material  is  10%  by  weight.     300"nm!   86     (a)   (b)   1#μm! 10#μm! (c)     (d)   (f)   (e)     Figure  3-­‐6:  Low  magnification  (Left)  and  corresponding  High-­‐magnification  (Right)  SEM   Images  of  the  cross-­‐sectional  views  of  the  following  electrodes    (a),  (b)  LTO+GnP-­‐25;  (c),(d)  LTO  +GnP-­‐5;  (e),(f)  LTO  +GnP-­‐1;  (g),(h)  LTO+  Super  P.     In  all  these  electrodes,  the  concentration  of  carbon  additive  material  is  10%  by  weight.         87   Figure  3-­‐6  (cont’d)   1#μm! 10#μm! 48 (h)   (g)   10#μm! 1#μm!   ELECTROCHEMICAL  CHARACTERIZATION   The  performances  of  the  different  LTO  electrodes  were  investigated  in  a  coin  cell  setup   with  lithium  foil  as  counter  electrode.  The  electrodes  were  galvanostatically  cycled  at  different   charge   rates,   for   5   cycles   each.   The   performances,   shown   in   Figure   3-­‐7and   Figure   3-­‐9   represent   the  mean  value  of  the  capacities  obtained,  with  the  error  bars  representing  the  range  of  data.   We   observe   from   Figure   3-­‐7,   which   shows   both   charge   and   discharge   capacity   of   all   the   electrodes   that   the   capacity   values   of   these   electrodes,   is   very   repeatable,   with   low   error   margin  and  high  coulombic  efficiency.  The  capacity  profiles  of  LTO+GnP  electrodes  were  plotted   at   different   charge   rates   (Figure   3-­‐8),   which   shows   typical   plateaus   corresponding   to   charge-­‐ discharge.  We  observe  relatively  flat  plateau  for  C/5  and  C/2  charge  rates,  however  for  faster   charge   rate   of   C   and   2C,   the   profiles   start   having   a   slope   shape,   which   is   indicative   of   increased   15 polarization .     88   (a)   200" 180" Capacity"(mAh/g)" 160" 140" 120" 100" 80" 60" C/2" C/5" 40" C" " 2C" C" C/2" Discharge" Charge" 20" 0" 0" (b)   5" 10" C/5" 15" 20" No."of"cycles" 25" 30" 35" 200" 180" Capacity"(mAh/g)" 160" 140" 120" 100" 80" 60" C/2" C/5" 40" C" " 2C" C" C/2" Discharge" Charge" 20" 0" 0" 5" 10" C/5" 15" 20" No."of"cycles" 25" 30" 35"   Figure  3-­‐7:  Galvanostatic  performance  of  at  different  charge  rates  (C/5,C/2,C  and  2C)  of     (a)  LTO+GnP-­‐25,  (b)  LTO+GnP-­‐5,  (c)  LTO+GnP-­‐1,  (d)  LTO+Super-­‐P       89   Figure  3-­‐7  (cont’d)   (c)   200" 180" Capacity"(mAh/g)" 160" 140" 120" 100" 80" 60" 40" C" " C/2" C/5" 2C" C" C/2" C/5" Discharge" Charge" 20" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35" (d)   200" 180" Capacity"(mAh/g)" 160" 140" 120" 100" 80" 60" C/2" C/5" 40" C" " 2C" C" C/2" Discharge" Charge" 20" 0" 0"   C/5" 5" 10" 15" 20" No."of"cycles" 90   25" 30" 35"   C/2$ 2.60$ 2.40$ 2C$ C$ 2C$ C$ C/5$ Poten/al$(V)$ 2.20$ 2.00$ 1.80$ 1.60$ 1.40$ 1.20$ 1.00$ 0.80$ 0$ 50$ 100$ Capacity$(mAh/g)$ C/2$ 150$ C/5$ 200$   Figure  3-­‐8:  Capacity  profiles  of  LTO+GnP-­‐25  Electrode  at  different  charge  rates   Influence  of  different  GnP  materials   The   comparison   of   capacity   values   of   different   sizes   of   GnP   material   as   conducting   additive  was  evaluated.  Figure  3-­‐9  shows  the  performance  of  LTO  electrodes  with  a  10  weight   %   addition   of   different   conducting   additives.   It   was   observed   that   the   capacity   values   of   LTO   electrodes   with   GnP-­‐25   and   GnP-­‐5   are   slightly   larger   than   the   one   with   Super   P   as   the   conducting   material.   However,   the   performance   of   LTO+GnP-­‐1   was   found   to   be   lower   than   these   electrodes.   These   results   were   in   accordance   with   the   trends   observed   in   the   EIS   data.   Figure   3-­‐10   below   shows   the   Nyquist   plots   of   the   electrodes,   obtained   after   5   complete   galvanostatic   cycles   at   a   rate   of   C/5.   The   intercept   on   x-­‐axis   corresponds   to   the   electrolyte   solution   resistance.   The   depressed   semi-­‐circle   at   high   frequencies   comes   from   two   overlapping   semi-­‐circles  associated  with  surface  film  resistance  and  charge  transfer  resistance.  The  line  at     91   low   frequencies   is   indicative   of   diffusion   behavior.   The   impedance   curves   were   fitted   to   the   1,2 equivalent   circuit   (shown   in   Figure   3-­‐13   (b))   using   Z-­‐view   software.   Figure   3-­‐13   (c)   shows   the   comparison   of   experimental   and   fitted   data,   which   indicates   a   good   fit   with   very   small   error   percentage.     Based   on   the   fitting,   the   values   of   resistance   parameters   are   summarized   in   the   Table  3-­‐2.  The  electrolyte  solution  resistance  was  found  to  be  the  same  for  all  the  electrodes.     There   is   an   increase   in   resistance   due   to   the   surface   film   with   reduced   size   of   GnP   particles,   which   can   be   possibly   translated   to   increased   edges,   which   acts   as   active   sites   for   electrolyte   decomposition   and   surface   film   formation.   However,   the   variation   of   SEI   for   different   GnP   materials  is  not  completely  understood  and  needs  further  experimental  verification.  The  charge   transfer  resistance  values  of  the  electrodes  show  the  following  trend  GnP-­‐5  <  GnP-­‐25  <  Super  P   100),  hence  interacts  with  LTO  particles  via  a  ‘plane  to  point  contact’  mechanism.     This  phenomenon  is  demonstrated  in  the  SEM  images  of  the  electrode  morphologies  shown  in   Figure  3-­‐5  and  Figure  3-­‐6.       99   To   verify   and   demonstrate   the   behavior   of   these   materials   throughout   the   electrode   thickness,  cross-­‐sections  of  the  electrodes  was  obtained  by  FIB  sectioning.  FIB  milling  gives  us   an  advantage  over  the  epoxy  embedding  and  polishing  method,  since  it  allows  us  to  view  the   sections   using   the   backscatter   electron   detector   (BSD),   (in   addition   to   the   usual   secondary   electron  images),  thus  giving  meaningful  information  about  the  interaction  of  the  two  phases.   The  BSD  images  show  brighter  spots  of  LTO  particles  in  comparison  to  carbon  particles,  because   of   their   higher   atomic   number   elements.     These   images   (shown   in   Figure   3-­‐14)   distinctly   highlight   the     ’point   to   point’   and   ‘plane   to   point’   contact   of   LTO-­‐Super   P,   and   LTO-­‐GnP,   respectively.         100   (a)   1"μm!   1"μm! (b)   1"μm!     Figure  3-­‐14:  Secondary  electron  and  back  scatter  electron  images  of  (a)  LTO  +  Super  P   electrodes  (b)  LTO  +  GnP  electrodes     101   CONCLUSIONS     In   summary,   we   have   demonstrated   the   potential   of   GnP   platelets   as   a   conducting   additive  for  low  conductivity  electrode  materials  such  as  transition  metal  compounds.  The  large   size  GnP  materials  such  as  GnP-­‐25  and  GnP-­‐5  have  shown  better  performance  in  comparison  to   the   commercial   Super   P   material.     However,   for   small   size   GnP   platelets   (GnP-­‐1),   a   reduction   in   capacity  was  observed  which  could  be  a  result  of  aggregation,  inhomogeneous  distribution  or   higher  particle  contact  resistance.   FUTURE  WORK     Future   work   should   explore   improving   the   gravimetric   performance   by   reducing   the   percentage   of   conducting   additive   used.   The   following   approaches   are   suggested   to   achieve   electrical  percolation  at  lower  weight  percentage  of  carbon  additives:   § The  dispersion  of  GnP  platelets  can  be  improved  by  a  pre-­‐mixing  step  involving   sonication  of  GnP  in  the  solvent,  before  addition  of  LTO  material.   § Developing   hybrid   electrodes   with   the   combination   of   small   size   carbon   materials   (with   low   aspect   ratio)   and   high   aspect   ratio   platelets,   can   lead   to   improved  capacity  at  certain  additive  concentration.  The  high  surface  area  small   platelets   can   provide   good   interaction   contact   with   LTO   particles,   and   the   high   aspect  ratio  platelets  can  provide  overall  external  conductivity  to  the  electrode.   The   synergistic   effect   of   the   two   types   of   GnP   platelets   can   be   carefully   optimized  to  obtain  improved  performance.     102                       REFERENCES     103     REFERENCES     1.   Zhu,  N.  et  al.  Graphene  as  a  conductive  additive  to  enhance  the  high-­‐rate  capabilities  of   electrospun  Li4Ti5O12  for  lithium-­‐ion  batteries.  Electrochemica  Acta  55,  5813–5818   (2010).   2.   Zhang,  B.  et  al.  Percolation  threshold  of  graphene  nanosheets  as  conductive  additive  in   Li4Ti5O12  anodes  of  Li-­‐ion  batteries.  Nanoscale  (2010).doi:10.1039/b000000x   3.   Pralong,  V.  Lithium  intercalation  into  transition  metal  oxides:  A  route  to  generate  new   ordered  rock  salt  type  structure.  Progress  in  Solid  State  Chemistry  37,  262–277  (2009).   4.   Park,  J.,  Lee,  S.,  Kim,  S.  &  Kim,  J.  Effect  of  Conductive  Additives  on  the  Structural  and   Electrochemical  Properties  of  Li  4  Ti  5  O  12  Spinel.  Bull.  Korean  Chem.  Soc.  33,  4059–4062   (2012).   5.   Bruce,  P.  G.,  Scrosati,  B.  &  Tarascon,  J.-­‐M.  Nanomaterials  for  rechargeable  lithium   batteries.  Angewandte  Chemie  (International  ed.  in  English)  47,  2930–46  (2008).   6.   Rev,  A.  et  al.  Materials  for  Rechargeable  Lithium-­‐Ion  Batteries.  Annu.  Rev.  Chem.  Biomol.   Eng.  3,  445–471  (2012).   7.   Cheng,  L.  et  al.  Carbon-­‐Coated  Li4Ti5O12  as  a  High  Rate  Electrode  Material  for  Li-­‐Ion   Intercalation.  Journal  of  The  Electrochemical  Society  154,  A692  (2007).   8.   Ahn,  S.  High  Capacity,  High  Rate  Lithium-­‐Ion  Battery  Electrodes  Utilizing  Fibrous   Conductive  Additives.  Electrochemical  and  Solid-­‐State  Letters  1,  111  (1999).   9.   Guo,  Y.-­‐G.,  Hu,  J.-­‐S.  &  Wan,  L.-­‐J.  Nanostructured  Materials  for  Electrochemical  Energy   Conversion  and  Storage  Devices.  Advanced  Materials  20,  2878–2887  (2008).   10.   Choi,  N.  et  al.  Challenges  Facing  Lithium  Batteries  and  Electrical  Double-­‐Layer  Capacitors.   Angewandte  Chemie  (International  ed.  in  English)  51,  9994–10024  (2012).   11.   Landi,  B.  J.,  Ganter,  M.  J.,  Cress,  C.  D.,  DiLeo,  R.  a.  &  Raffaelle,  R.  P.  Carbon  nanotubes  for   lithium  ion  batteries.  Energy  &  Environmental  Science  2,  638  (2009).     104   12.   Kang,  X.  et  al.  Effect  of  Conductive  Additives  and  Surface  Fluorination  on  the   Electrochemical  Properties  of  Lithium  Titanate  (Li4/3Ti5/3O4).  Journal  of  The   Electrochemical  Society  157,  A437  (2010).   13.   Su,  F.-­‐Y.  et  al.  Could  graphene  construct  an  effective  conducting  network  in  a  high-­‐power   lithium  ion  battery.  Nano  Energy  1,  429–439  (2012).   14.   Su,  F.-­‐Y.  et  al.  Flexible  and  planar  graphene  conductive  additives  for  lithium-­‐ion  batteries.   Journal  of  Materials  Chemistry  20,  9644  (2010).   15.   Shi,  Y.,  Wen,  L.,  Li,  F.  &  Cheng,  H.-­‐M.  Nanosized  Li4Ti5O12/graphene  hybrid  materials   with  low  polarization  for  high  rate  lithium  ion  batteries.  Journal  of  Power  Sources  196,   8610–8617  (2011).   16.   Wang,  Q.,  Su,  F.,  Tang,  Z.,  Ling,  G.  &  Yang,  Q.  Synergetic  effect  of  conductive  additives  on   the  performance  of  high  power  lithium  ion  batteries.  New  Carbon  Materials  27,  427–432   (2012).         105     4  GRAPHENE  NANOPLATELET  PAPER  AS  CURRENT  COLLECTOR   SIGNIFICANCE   For   lithium   ion   batteries,   copper   is   used   as   current   collector   for   anode   materials.   This   copper   film,   which   is   around   10   µm   in   thickness,   contributes   to   12%   of   the   total   weight   and   5%   1 of   total   cost   of   a   high   power   battery   (Figure   4-­‐1) .     For   high   performance   compact   batteries,   improvements  in  all  components  are  being  sought,  and  this  need  motivates  the  exploration  of   alternate  current  collectors,  which  adds  less  weight  to  the  battery  setup  without  increasing  any   cost.   To  increase  the  energy  density  and  reduce  the  cost  contribution  of  the  current  collector,   various  alternatives  are  being  explored.  The  approach  used  is  generally  to  make  self-­‐supporting   2 films   of   the   anode   material   ,   so   that   there   is   no   need   for   a   current   collector.   But   such   technologies  are  restricted  by  the  morphology  and  properties  of  the  active  electrode  materials.   It’s  either  limited  to  certain  kinds  of  materials  or  requires  addition  of  conductive  matrix,  fillers   3–5 and  binders   .         106     Aluminum& 6& Copper& 13&                 Figure  4-­‐1  :  Weight  and  cost  distribution  of  different  battery  components  for  High  Power   1 Cell  (based  on  data  from  Gaines  et.  al   )     107   Figure  4-­‐1  (cont’d)   Cost%Distribu+on%%for%High%Power%(107A.h%Cell)% Cathode$ Anode$ Electrolyte$ Separator$ Current$Collector$ Other$ 3%$ 28%$ 21%$ 17%$ 23%$ 8%$   Conductive   graphitic   films   because   of   their   low   density,   low   price,   high   electrical   and   thermal  conduction,  good  chemical  and  thermal  stability,  conformability  and  flexibility,  find  its   2,6,7 use   in   numerous   electrochemical   applications   such   as   supercapacitors 5 8 ,   fuel   cells   ,   9 sensors ,   lead   acid   cells   and   DSSC   counter   electrodes .   For   lithium   ion   batteries,   conducting   graphitic   films   have   enormous   potential   for   its   use   as   current   collector   owing   to   its   excellent   electronic   transport,   good   adhesion   to   active   materials,   and   conformability.   Copper   has   a     108   density  of  9  g/cc  while  graphene  has  a  density  of  ~2  g/cc.  In  addition  to  being  lightweight  and   flexible,   graphitic   films   are   known   to   have   comparable   thermal   conductivity   to   copper,   which   will  ensure  fast  heat  dissipation  avoiding  thermal  build  up  during  high  charging  conditions.       Graphitic  current  collectors  are  a  perfect  choice  of  current  collector  for  anode  materials,   due   to   inherent   lithium   storage   capability   of   carbon,   in   addition   to   the   other   properties   mentioned   above.     With   this   concept   in   mind,   the   potential   of   commercial   Grafoil   flexible   10 graphite   sheet   has   already   been   explored   as   anode   material   and   current   collector .   Carbon   nanotube   films   (Bucky   paper   and   others)   are   a   popular   choice   for   self-­‐standing   electrodes,   composite   films   11 7,12,13   and   current   collector   supports .   CNT   films   are   low   density   and   have   7 good  properties  and  have  found  widespread  use  in  capacitors ,  lithium  ion  batteries 7,11,13–15 .   However  the  complex  synthesis  and  high  cost  associated  of  CNTs  can  restrict  their  commercial   viability   and   utilization.   Also,   there   have   been   literature   evidence   showing   that   carbon   fiber   based   papers   have   advantages   over   copper   current   collectors,   particularly   for   tin   based   and   4 silicon   based   anode   systems .   These   systems,   specifically   the   ones   involving   heat   treatment   on   cast  electrodes,  face  a  problem  of  flaking  material  off  of  copper  electrodes  due  to  a  significant   difference  in  coefficient  of  thermal  expansion  between  copper  and  the  coated  material.  Using   carbon   or   graphitic   current   collectors   has   resolved   this   problem   by   capitalizing   on   the   similar   coefficient  of  thermal  expansion  of  the  electrode  material  with  carbon  papers.  Use  of  carbon   16,17   fiber  mats  (by  Toray  and  Pyrograf  CFs)  as  current  collector  owing  to  enhanced  electronic   transport,  thermal  management  and  better  adhesion  has  been  shown.       109   APPROACH   Our  approach  involves  the  use  of  GnP  Paper  synthesized  by  a  simple  method  involving   18 filtration  of  an  aqueous  GnP  suspension .  This  self-­‐standing  film  of  nanostructured  graphene   18 nanoplatelets  (GnP)  is  known  to  have  low  density,  reasonable  mechanical  strength  and  good   18 18,19 electrical   and   thermal   conductivity   ,   thus   making   it   a   promising   candidate   for   the   role   of   current  collector.    In  comparison  to  copper,  it’s  a  lightweight  material,  less  expensive  and  has   good   adhesion   to   most   common   anode   materials   (carbon   based).   Furthermore,   the   technology   proposed  here  offers  more  versatility  and  has  several  advantages  as  listed  below:   • GnP   paper   is   a   self-­‐   sustaining   current   collector   which   has   the   desired   electrical   conductivity  with  significantly  less  areal  density       • GnP   paper   has   inherent   lithium   storage   capability   and   hence   contributes   to   energy   density,  thus  reducing  the  dead  weight   • This   paper   can   also   be   used   as   current   collector   for   other   electrode   materials   and   other   electrochemical  systems     EXPERIMENTAL  SECTION   GNP  PAPER  SYNTHESIS   The   self   standing   flexible   GnP   paper   is   prepared   by   a   simple   filtration   process   starting   with   an   aqueous   suspension   of   GnP   as   shown   in   Figure   4-­‐2   18 .     GnP   was   dispersed   in   an   aqueous  solution  with  the  aid  of  Polyethyleneimine  (PEI)  as  a  dispersing  aid  in  the  weight  ratio   of   GnP:PEI:Water=1:1:1000.   Then   the   dispersed   solution   is   filtered   using   a   Durapore   0.65   μm     110   filter  paper,  and  washed  with  deionized  water  to  remove  excess  PEI.  After  filtration,  the  paper   is   dried   under   ambient   condition,   peeled   off   from   the   filter   paper   and   then   annealing   at   340   °C to   eliminate   residual   PEI   from   the   paper.   The   GnP   paper   can   be   compressed   to   any   level   to   control  porosity  and  thickness.       GnP$in$Water$ Aggrega-on,$Not$dispersed$ Filtra-on$ Ultrasonica-on$ GnP$$ GnP$+PEI$in$Water$ Well$dispersed$ PEI$in$water$ Heat$Treatment$$ at$340$C$ Cold$Pressed$ GnP$Paper$ (Pressed)$$ GnP$Paper$ (No$PEI)$$ GnP$+PEI$Paper$ Figure  4-­‐2:  Schematic  illustrating  the  synthesis  of  GnP  Paper     111     ELECTRODE  PREPARATION   For   evaluating   the   performance   of   GnP   paper   as   current   collector,   electrodes   of   different   active   materials   viz.     GnP-­‐15   and   Lithium   Titanate   (LTO)   were   investigated.   The   electrodes   were   prepared   by   coating   the   slurry   of   active   anode   material   with   N-­‐Methyl-­‐2-­‐ pyrrolidone   and   10   %   PVDF   as   the   binder,   using   a   microfilm   applicator   on   an   automatic   electrode   coating   instrument.   The   electrodes   were   cast   on   two   different   current   collectors,   viz.   the  Cu  foil  and  GnP  Paper.  For  LTO  Electrodes,  Super  P  was  used  as  a  conducting  additive  in  the   proportion  of  10  wt  %.   MORPHOLOGY  OBSERVATION     To  understand  the  interaction  of  electrode  materials  with  the  current  collector  and  to   evaluate   the   quality   of   electrodes,   an   assessment   of   electrode   morphology   was   done   by   observing   the   material   distribution   in   both   the   top-­‐view   and   cross-­‐sectional   view.     The   GnP   paper   and   GnP   electrode   samples   for   cross-­‐   section   observation   were   prepared   by   an   epoxy   mounting  and  polishing  procedure  (described  before).  The  LTO  samples  were  prepared  by  FIB   milling  procedure  using  Carl  Zeiss  Auriga®  CrossBeam)  instrument.       PROPERTIES   The   electrical   conductivity   of   GnP   paper   was   measured   with   a   4-­‐probe   conductivity   method,  using  the  Keithley  2400  Source  Meter.  GnP  paper  was  cut  into  thin  strips  of  4  cm  X  1   cm  and  4  probes  having  1  cm  spacing  were  pressed  into  the  surface  of  the  paper  to  make  the   resistance  measurement.     112   -­‐1 The   tensile   strength   of   the   paper   was   measured   at   0.1   %   min   strain   rate   using   TA   Instruments  Dynamic  Mechanical  Analyzer  (DMA  Q  800)  with  the  film  tension  clamp.   ELECTROCHEMICAL  CHARACTERIZATION     The   anode   material   was   tested   in   a   three-­‐electrode   half-­‐cell   Swagelok   setup   (for   GnP   electrodes)   or   Coin   cell   setup   (for   LTO)   with   ethylene   carbonate-­‐dimethyl   carbonate-­‐lithium   hexafluorophosphate   as   electrolyte   and   lithium   foil   as   counter   and   reference   electrodes.   The   relative   loading   of   active   material   on   the   electrode   was   kept   around   4-­‐5   mg/cm2.   Fundamental   performance  characterization  at  different  charge  rates  was  done  to  assess  the  response  of  this   electrode.   The   protocol   adopted   to   evaluate   the   performance   at   different   charge   rates   is   the   same  as  explained  in  Section  2.3.3.  The  galvanostatic  cycling  was  done  at  different  rates  every  5   cycles,   keeping   the   rate   same   for   both   charge   and   discharge.   Charge   rate   C   corresponds   to   a   charge  or  discharge  rate  equal  to  the  theoretical  capacity  of  a  battery  in  one  hour.     Electrochemical   Impedance   measurements   were   also   done   on   the   GnP   electrode   on   different  substrates  viz.,  GnP  paper  and  copper  as  current  collectors.  For  EIS  studies,  coin  cells   with  lithium  foil  as  counter  electrode  were  used.  This  setup  was  preferred  over  three  electrode   Swagelok  T-­‐cell  setup  to  avoid  any  manual  errors.  AC  impedance  spectroscopy  was  obtained  by   applying  a  sine  wave  of  5mV  amplitude  over  a  frequency  range  of  100  KHz  to  0.01  Hz.     113   RESULTS  &  DISCUSSIONS   GNP  PAPER   GnP  paper  is  a  binder-­‐free  self-­‐standing,  flexible  and  porous  paper  prepared  by  a  simple   18 .   filtration   process   using   aqueous   suspension   of   GnP   . This   paper   can   be   made   in   controlled   porosities   (ranging   from   30-­‐90   %)   and   thickness   (ranging   from   3   microns   to   several   hundred   microns)   using   different   sizes   of   GnP   materials.   This   GnP   paper   has   very   good   electrical   conductivity,  which  ranges  from  500-­‐2000  S/cm  depending  on  the  size  of  platelets,  porosity  and   thickness   of   paper.   Figure   4-­‐3   below   shows   the   cross-­‐sectional   morphology   of   GnP   paper   at   different  magnifications.  As  can  be  seen  from  the  high  magnification  images,  the  paper  is  a  very   ordered  network  of  GnP  platelets  having  a  high  degree  of  in-­‐plane  alignment  with  GnP  particles   connected  together  to  produce  good  electrical  conductivity.   (a)! (b)! 10#μm! 20#μm! Figure  4-­‐3:  SEM  Images  of  the  GnP  paper  (as  made)  at  different  magnifications       114     GNP  ELECTRODE  ON  GNP  PAPER   Figure   4-­‐4   below   shows   the   SEM   images,   at   different   locations   and   magnifications,   of   the  cross-­‐section  of  electrodes  of  graphene  nanoplatelets  of  15  μm  diameter  on  GnP  Paper  as   current  collector  by  the  procedure  described  above.     (a)! (b)! 100#μm! 100#μm! (c)! (d)! 20#μm! 10#μm!   Figure  4-­‐4:  Cross-­‐sectional  SEM  Images  of  the  electrode  at  different  magnifications  (a)  &   (b),  followed  by  high  mag  images  of  (c)  GnP  paper  (d)  active  material  region     From  the  cross-­‐sectional  SEM  images,  we  can  see  that  there  is  good  adhesion  between   the   GnP   paper   and   the   electrode   material   coated   on   top   of   it.   The   actual   junction   between   the   GNP  current  collector  and  the  anode  is  not  easily  differentiated  indicating  the  high  degree  of   compatibility  between  the  two  components.    The  high-­‐resolution  images  shown  in  Figure  3(c)   and  (d)  show  that  the  morphology  of  both  the  paper  and  electrode  active  material  indicating  a     115   highly  porous  well-­‐arranged  network,  although  the  GnP  paper  is  more  compacted  dense  than   the  electrode  coated  above  it,  which  is  the  result  of  the  preparation  procedure.     For   electrochemical   testing,   the   electrodes   were   pressed   at   0.1   MPa   to   produce   the   controlled   porosity.   SEM   images   in   Figure   4-­‐5   below   show   the   changes   that   the   GNP   paper   morphology  undergoes  in  the  process  of  electrode  coating,  and  battery  anode  fabrication.     (a)! 20#μm! (c)! (b)! 20#μm! 20#μm!   Figure  4-­‐5:  SEM  images  of  X-­‐sectional  view  of  (a)  current  collector  GnP  paper,  GnP   electrode  on  GnP  current  collector  (b)  the  unpressed  and  (c)  pressed  electrode     COMPARISON  OF  GNP  PAPER  AND  COPPER  AS  CURRENT  COLLECTORS   Figure  4-­‐6  below  shows  the  comparison  of  electrode  made  by  GnP-­‐15  as  active  material   on   two   different   substrates,   viz.   copper   and   GnP   Paper.   We   can   clearly   observe   that   the   adhesion  of  electrode  material  is  better  to  the  GnP  paper  in  comparison  to  the  copper  foil.   (b)! (a)! 100'μm! 100'μm!   116     Figure  4-­‐6:  SEM  images  showing  cross-­‐sectional  view  of  GnP-­‐15  electrode  on  different   current  collectors:    (a)  Copper  (b)  GnP  paper     Figure  4-­‐7  shows  the  performance  of  GnP  electrode  casted  on  Cu  foil  at  different  charge   rates.  The  results  of  the  electrochemical  performance  for  the  GnP  electrode  on  GnP  paper  as   current   collector   are   shown   below   in   Figure   4-­‐8,   calculated   based   on   the   weight   of   just   the   active   material   coated   on   GnP   paper   (Figure   4-­‐8(a))   and   also   using   the   complete   weight   including  that  of  GnP  paper  (Figure  4-­‐8  (b)).  As  expected,  GnP  paper  itself  contributes  towards   lithium   storage;   hence   calculations   based   on   weight   of   active   material   indicate   exceptional   performance.  However,  in  a  real  system,  considering  the  weight  of  GnP  paper  as  a  contribution   to  electrode  weight,  the  storage  capability  is  similar  to  GnP  electrode  on  Cu  foil.  However,  over   long  cycling,  the  lithium  storage  capacity  of  the  material  is  not  completely  utilized.  This  can  be   attributed   to   incomplete   access   of   material   at   very   high   loading   7-­‐8   mg/cm2.   Also,   there   is   a   higher   irreversible   capacity   loss   in   the   first   cycle   for   electrodes   with   GnP   paper   as   current   collector.  This  can  be  a  consequence  of  increased  SEI  layer  formation  due  to  higher  surface  area   available.   The   performance   at   faster   charge   rates   is   relatively   slower   due   to   restricted   ion   diffusion.     117   600" Capacity"(mAh/g)" C/2" C/5" 500" C" " 2C" C" C/2" C/5" 400" 300" 200" 100" Discharge" Charge" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35"   Figure  4-­‐7:  Galvanostatic  Performance  of  GnP-­‐15  electrode  on  Copper  current  collector       118   (a)   1600" C/2" C/5" 1400" C" " 2C" C" C/2" C/5" Capacity"(mAh/g)" 1200" 1000" Discharge" Charge" 800" 600" 400" 200" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35" (b)   700" C/5" Capacity"(mAh/g)" 600" C/2" C" " 2C" C" C/2" C/5" 500" 400" Discharge" 300" Charge" 200" 100" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35"   Figure  4-­‐8:  Galvanostatic  Performance  of  GnP-­‐15  electrode  on  GnP  paper  current   collector  (a)  Active  material  (b)  Total  weight  including  the  substrate  weight  of  GnP  paper     119   Electrochemical  Impedance  Spectra   Electrochemical   impedance   spectroscopy   (EIS)   was   done   to   evaluate   the   resistances   involved  in  the  system.  Figure  4-­‐9  shows  the  EIS  data  of  the  GnP  electrodes  made  on  Copper   and   GnP   paper   as   current   collector   after   10   complete   charge   discharge   cycles.   The   bulk   resistance   (Rs),   corresponding   to   the   intercept   on   the   x-­‐axis,   is   a   combination   of   electrolyte   ionic   resistance,   intrinsic   resistance   of   electrode   materials   and   the   contact   resistance   of   the   20 active   material   and   current   collector .   The   two   depressed   semi-­‐circles   at   high   and   medium   frequencies   are   associated   with   the   resistances   due   to   SEI   layer   formation   (Rsei)   and   charge   transfer  (Rct).  The  inclined  line  at  low  frequencies  is  attributed  to  the  diffusion  of  lithium  ions  in   the  electrodes.     70" *Z"imaginary"(ohms)" 60" 50" 40" GnP"on"GnP"Paper" 30" GnP"on"Cu"foil" 20" 10" 0" 0" 10" 20" 30" 40" 50" Z"real"(ohms)" 60" 70"   Figure  4-­‐9:  Nyquist  plots  of  GnP  electrode  on  different  current  collectors:  Copper  foil  and   GnP  Paper,  after  10  complete  charge  discharge  cycles     120   Rs Rsei Rct CPE 1 (a)   W CPE 2   (b)   70" Element 60" Rs Rsei 50" CPE 1-T 40" CPE 1-P Rct 30" W-R W-T 20" W-P 10" CPE 2-T 0" CPE 2-P *Z"imaginary"(ohms)" Freedom Value Fixed(X) 3.183 Free(+) 6.374 Fixed(X) 0.0007836 Fixed(X) 0.65798 Fixed(X) 0.13337 Fixed(X) 0 Fixed(X) 0 Fixed(X) 0.5 Free(+) 2.9607E-7 Free(+) 1.4 0" 10" 20" 30" 40" 50" 60" Z"real"(ohms)" Chi-Squared: 0.0061365 0.82843 70" (c)  Weighted Sum of Squares: 60" Data File: Circuit50" Model File: Mode: 40" Maximum Iterations: Optimization Iterations: 30" Type of Fitting: 20" Type of Weighting: 10" Error N/A 0.065027 N/A N/A Experimental" N/A Fi?ng" N/A N/A N/A 8.0504E-7 0.22109 70" Error % N/A 1.0202 N/A N/A N/A N/A N/A N/A 271.91 15.792   *Z"imaginary"(ohms)" D:\\E6p_Run101_EIS_After cycle 5.z D:\current collector Cu_After cycle 5.mdl Run Fitting / Freq. Range (0.01 - 80000) 100 Experimental" 100 Fi?ng" Complex Calc-Modulus 0" 0" 10" 20" 30" 40" 50" Z"real"(ohms)" 60" 70"    Figure  4-­‐10:  (a)  Equivalent  circuit  for  fitting  of  impedance  data;  Comparison  of   experimental  and  fitted  data  for  (a)  GnP  electrode  on  Cu  foil  after  10  cycles  (c)  GnP  electrode  on   GnP  Paper  after  10  cycles     121     Table  4-­‐1:  Resistance  values  obtained  by  parametric  fitting  of  impedance  data   Rs# Rsei# Rct# GnP$on$Cu$foil+10cyc$ 3$ 14$ 8$ GnP$on$GnP$Paper+10cyc$ 3$ 15$ 6$     The   experimental   impedance   data   was   fitted   with   the   Z   view   software   using   the   equivalent   circuit   shown   in     Figure   4-­‐10   (a).   The   data   calculated   with   the   equivalent   circuit   model   fits   the   experimental   data   well   as   can   be   observed   in     Figure   4-­‐10   (b)   and   (c).   The   estimated  resistance  parameters  obtained  from  this  analysis  are  summarized  in  Table  4-­‐1.  We   can   see   that   no   significant   difference   in   the   values   of   resistances   was   observed   in   the   two   electrodes,  which  is  a  clear  indication  that  GnP  paper  as  a  current  collector  has  good  interaction   with  the  active  material  and  hence  demonstrates  comparable  performance  to  copper  foil.     LITHIUM  TITANATE  (LTO)  ELECTRODE  ON  GNP  PAPER   The  use  of  these  GnP  Papers  as  current  collector  is  not  restricted  to  carbon  based  anode   materials,  and   this   was   verified  by  using  it   as  current   collector  for   Lithium  Titanate  (LTO)  anode   material.   The   electrodes   were   cast   by   coating   NMP   based   slurry   by   similar   procedures   as   described  above.  The  cross-­‐sectional  morphology  of  the  electrode  was  compared  by  cutting  the   cross-­‐section  using  FIB  and  was  compared  as  shown  in  Figure  4-­‐11.         122   (a)   (b)   2"μm! 1"μm!   Figure  4-­‐11:  SEM  images  showing  cross-­‐sectional  view  of  LTO  electrodes  on  different   current  collectors  (a)  On  copper  (b)  On  GnP  Paper   The   analysis   below   in   Figure   4-­‐12   compares   the   electrochemical   performance   of   LTO   electrodes   cast   on   copper   foil   vs.   GnP   Paper.   From   this   we   can   observe   that   the   GnP   paper   performs  very  well  with  LTO  as  anode  material,  in  fact  slightly  better  than  the  electrode  made   on  copper.  Such  an  improvement  can  be  a  consequence  of  improved  conduction  due  to  better   adhesion  of  electrode  with  current  collector  thus  providing  good  electronic  conductivity.     123   (a)   300" Capacity"(mAh/g)" C/2" C/5" 250" C" " 2C" C" C/2" C/5" 200" 150" 100" 50" Discharge" Charge" 0" 0" 5" 10" 15" 20" 25" 30" 35" No."of"cycles"   (b)   300" 250" C/2" Capacity"(mAh/g)" C/5" 200" C" " 2C" C" C/2" C/5" 150" 100" 50" Discharge" Charge" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35"   Figure  4-­‐12:  Electrochemical  Performance  of  LTO  electrodes  casted  on  different  current   collectors  (a)  On  copper  (b)  On  GnP  Paper       124   This  analysis  with  a  non-­‐graphitic  anode  material  operates  above  the  potential  range  in   which  carbon  gets  involved  in  any  interaction  with  lithium.  The  results  clearly  demonstrate  the   diversity  in  the  use  of  GnP  Paper  as  current  collector  even  in  situations  where  it  has  no  active   electrochemical  role  to  play.       UNIQUE  GNP  ELECTRODE  ON  C  VEIL   ® A  unique  approach  was  adapted  to  make  a  GnP  electrode  on  an  Optimat  carbon  veil,  a   lightweight   C   mat.   These   carbon   veils   are   140   μm   in   thickness   and   have   an   electrical   conductivity  of  12  S/cm  and  tensile  strength  of  6.5  MPa.  Figure  4-­‐13  below  shows  the  optical   microscope  images  of  the  C  veil,  from  which  we  can  see  that  the  diameter  of  the  fibers  are  <10   μm  and  the  veil  is  highly  porous  with  empty  spaces  of  the  order  of  50-­‐100  μm.  The  electrical   conductivity  of  as  received  C  veil  was  measured  at  different  conditions  to  ensure  that  there  will   not  be  a  degradation  of  C  veil  during  electrode  preparation  procedure.   250$μm! 10$μm!   ® Figure  4-­‐13:  Optical  Images  of  the  Optimat  carbon  veil     The   GnP   electrode   on   C   veil   was   prepared   by   the   same   methodology   as   GnP   paper   synthesis.  The  carbon  veil  was  placed  as  a  second  filter  in  addition  to  the  Durapore  filter  paper,     125   and  an  aqueous  suspension  of  GnP:PEI:Water  was  filtered.  The  GnP  collects  on  the  C  veil  and   forms  the  GnP-­‐veil  electrode.  The  electrode  was  repeatedly  washed  with  water  and  then  heat   treated  at  340  °C  to  remove  PEI  (the  dispersion  agent).  The  electrode  was  further  pressed  in  the   Carver  press  to  achieve  good  compaction.    Figure  4-­‐14  and  Figure  4-­‐15  below  show  the  pictures   and  SEM  images  of  the  unpressed  and  pressed  electrodes,  respectively.  The  SEM  images  show   that  the  GnP  penetrates  through  the  veil  structure  and  forms  a  well-­‐distributed  interconnected   network.  Also,  we  can  observe,  that  on  pressing  the  uncovered  C  veil  area  (the  edges)  degrade,   however,   the   central   portion   stay   intact.   This   is   possibly   due   to   the   mechanical   strength   imparted  by  GnP  to  the  C  veil  structure.       126   (b)   (a)       200#μm!   (d)   (c)   20#μm! 20#μm!    Figure  4-­‐14:  (a)  Picture  and  (b),(c),(d)  SEM  Images  of  unpressed  GnP  electrode  on  C  veil   as  current  collector   (a)   (b)           10#μm!   Figure  4-­‐15:  (a)  Picture  and  (b)  SEM  Images  of  pressed  GnP  electrode  on  C  veil  as  current   collector  (Scale  bar-­‐  (b):  10  μm)     127   ELECTROCHEMICAL  PERFORMANCE   Electrochemical   measurements   of   this   electrode   was   done   in   the   three   electrode   Swagelok  setup  with  Li  foil  as  counter  and  reference  and  1M  LiPF6   in  EC-­‐DMC  (1:1v/v)  as  the   electrolyte.     1.5% Scan%rate:%0.05%mV/s% Current%density%(A/g)% 1.0% 0.5% 0.0% 0.0% 0.5% 1.0% 1.5% !0.5% Based%on%ac9ve%material%loading%:% GnP%amount% !1.0% !1.5% Poten9al%(V)% Figure  4-­‐16:  Cyclic  Voltammogram  (third  sweep)  of  GnP  on  C  veil       2.0% 128     (a)     800" 700" Capacity"(mAh/g)" 600" 500" 400" Discharge" 300" Charge" 200" 100" C" C/2" C/5" 2C" C" C/2" C/5" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35" Figure  4-­‐17:  Galvanostatic  Performance  of  GnP  on  C  veil  electrode  (a)  based  on  active   electrode  material  (b)  based  on  complete  weight                 129       Figure  4-­‐17  (cont’d)   (b)   800" Discharge" 700" Charge" Capacity"(mAh/g)" 600" ~C/8" ~C/3" 500" ~C/1.5" ~1.25C" ~C/1.5" ~C/3" ~C/8" " 400" 300" 200" 100" 0" 0" 5" 10" 15" 20" No."of"cycles" 25" 30" 35"   The   cyclic   voltammogram   done   at   a   scan   rate   of   0.05   mV/s   shows   the   typical   carbon   intercalation   peaks,   as   shown   in   Figure   4-­‐16.   The   galvanostatic   performance   was   done   at   different   charge   rates   (Figure   4-­‐17),   and   the   capacity   based   on   active   material   GnP’s   weight   (excluding   the   C   veil   weight),   was   found   to   be   around   450   mAh/g   at   C/5   charge   rate.   The   additional  contribution  to  lithium  capacity  is  coming  from  the  electrochemical  activity  of  the  C   veil,  which  itself  is  participating  in  the  lithium  intercalation-­‐deintercalation  process.  When  the   capacity  calculations  were  done  with  complete  electrode  weight  in  mind  (GnP+C  veil),  we  see   the  expected  performance  of  ~310  mAh/g  at  C/5  charge  rate     130   In   summary   the   GnP-­‐C   veil   electrode   has   significant   advantages   as   a   lightweight   current   collector,  and  improved  gravimetric  energy  density.  However,  the  C  veil  as  current  collector  has   drawbacks  of  significantly  low  electrical  conductivity  and  may  have  some  electrode  fabrication   and  processing  challenges.     CONCLUSIONS      We   have   demonstrated,   that   the   GnP   paper   has   the   potential   to   replace   copper   as   current   collector   and   can   impart   electrical   conductivity   to   the   anode   material.   Based   on   performance  data,  it  can  be  asserted  that  GnP  Paper  can  match  and  potentially  replace  the  use   of  copper  as  current  collector.  Table  4-­‐2  shows  the  relative  comparison  of  properties  of  the  two   current  collectors.   Table  4-­‐2:  Comparison  of  properties  of  copper  foil  and  GnP  paper  as  current  collectors   Proper&es( Copper(Current( Collector( GnP(Paper(Current( Collector( Areal&density& 8.6&mg/cm2& 4.3&mg/cm2& Electrical& Conduc;vity& 8*10^5&S/cm& 750&S/cm& Tensile&Strength& 70&Mpa&(Yield)& 2.85+/L&0.30&MPa&   FUTURE  WORK     GnP   paper   has   immense   potential   as   a   possible   alternative   to   the   currently   used   copper   current  collector  and  will  increase  the  energy  density  of  the  battery.  The  same  GnP  paper  can   also   be   used   to   eliminate   aluminum   foil   as   a   current   collector   for   cathodes.   Although   it   has   lower   conductivity   in   comparison   to   its   metal   foil   counterparts,   it   appears   to   be   capable   of     131   functioning   equally   well   as   a   current   collector.     In   addition,   the   use   of   a   GnP   based   current   collector   current   can   eliminate   the   corrosion   associated   with   metal   current   collectors,   and   hence  might  allow  the  usage  of  more  aggressive  electrolytes.   GnP   paper   can   be   easily   made   as   a   continuous   product   with   easily   scalable   and   well-­‐ established   techniques   commonly   used   in   paper   industry.   Since   the   cost   of   GnP   itself   as   raw   material  is  very  reasonable,  and  the  economics  of  paper  processing  methods  are  optimized,  the   commercialization  of  GnP  paper  can  be  a  feasible  alternative  to  metal  based  current  collector.   The  thickness  of  GnP  paper  can  be  easily  reduced  to  around  10  microns,  and  that  should   account   for   around   ~75%   reduction   in   weight.   However,   there   are   certain   challenges   that   should  be  addressed,  particularly  in  terms  of  its  mechanical  strength  and  ductility  to  allow  its   versatility   for   industrial   scale   continuous,   roll-­‐to-­‐roll   and   calendaring   processes   commonly   used   in   battery   fabrication.   Also,   improvement   of   conductivity   of   these   papers   and   improved   diffusion  through  the  GnP  paper  can  provide  additional  benefits  for  the  use  of  GnP  papers  as   current  collectors   To   address   some   of   the   challenges   and   further   improve   the   performance,   following   approaches  can  be  adopted:   i. Mechanical  properties  of  the  GnP  paper  can  be  modified  by  selective  use  of  a  polymeric   binder   to   make   the   GnP   paper   robust   enough   to   be   inserted   as   a   replacement   for   copper   foil   in   current   electrode   and   battery   fabrication   processes.   However   the   selection   of   polymeric   materials   will   be   restricted   based   on   their   reactivity   with   the   electrolyte.   Other   methodologies   involve   making   polymer-­‐GnP   paper   composites   by     132   making   laminates   or   cofiltration   of   co-­‐suspended   polymer   binder   powders   or   fibers   followed   by   sintering   without   negatively   affecting   the   lithium   extra   storage   capability   provided  by  the  GnP  papers  itself.     ii. Another   way   of   handling   the   problem   of   mechanical   strength   can   be   to   have   the   GnP   paper  supported  on  another  robust  and  flexible  but  sacrificial  film.  Such  films  will  make   the   processability   of   GnP   paper   more   feasible,   particularly   during   the   electrode   preparation   and   calendaring   process,   and   can   eventually   be   dissolved,   etched   or   peeled   off  before  its  assembled  into  a  battery  setup.  There  is  a  literature  report  verifying  this   7 concept,  in  which  CNT  coating  on  paper  has  resulted  in  flexible  current  collectors   .   iii. Incorporating  GnP  paper  with  additional  carbon  nanomaterials  or  metal  nanoparticles  to   improve   the   conductivity   of   the   paper   aiming   at   improving   its   performance   as   a   current   collector  in  electrochemical  energy  storage  systems.  Also,  GnP  paper  can  be  coated  with   metal  nanoparticles  on  one  side  to  get  extra  conductivity  as  is  done  in  case  of  polymeric   21 films  for  current  collector .   Based   on   our   results   and   the   scope   of   further   improvements,   the   GnP   paper   can   be   customized   to   suit   its   use   with   varied   materials   for   different   applications.   Hence,   we   can   foresee   the   potential   of   GnP   paper   for   diverse   electrochemical   applications   such   as   lithium-­‐ sulfur,  lithium-­‐air  or  electrochemical  capacitors.             133                       REFERENCES       134   REFERENCES   1.   Gaines,  L.  &  Cuenca,  R.  Costs  of  Lithium-­‐Ion  Batteries  for  Vehicles.  Energy  48,  73  (2000).   2.   Futaba,  D.  O.  N.  N.  et  al.  Shape-­‐engineerable  and  highly  densely  packed  single-­‐walled   carbon  nanotubes  and  their  application  as  supercapacitor  electrodes.  Nature  materials  5,   987–994  (2006).   3.   Gowda,  S.  R.  et  al.  Three  dimensionally  engineered  porous  silicon  electrodes  for  Li  ion   battery.  Nanoletters,  12,  6060-­‐6065  (2012).   4.   Guo,  J.,  Sun,  A.  &  Wang,  C.  A  porous  silicon–carbon  anode  with  high  overall  capacity  on   carbon  fiber  current  collector.  Electrochemistry  Communications  12,  981–984  (2010).   5.   Xiao,  F.  et  al.  Growth  of  Metal  –  Metal  Oxide  Nanostructures  on  Freestanding  Graphene   Paper  for  Flexible  Biosensors.  Adv.  Funct.  Mater.  (2012).doi:10.1002/adfm.201200191   6.   Wang,  S.,  Pei,  B.,  Zhao,  X.  &  Dryfe,  R.  a.  W.  Highly  porous  graphene  on  carbon  cloth  as   advanced  electrodes  for  flexible  all-­‐solid-­‐state  supercapacitors.  Nano  Energy    (2013).   doi:10.1016/j.nanoen.2012.12.005   7.   Hu,  L.  et  al.  Highly  conductive  paper  for  energy-­‐storage  devices.  PNAS  106,  (2009).   8.   Zhang,  H.  &  Hsing,  I.-­‐M.  Flexible  graphite-­‐based  integrated  anode  plate  for  direct   methanol  fuel  cells  at  high  methanol  feed  concentration.  Journal  of  Power  Sources  167,   450–454  (2007).   9.   Chen,  J.  et  al.  A  flexible  carbon  counter  electrode  for  dye-­‐sensitized  solar  cells.  Carbon   47,  2704–2708  (2009).   10.   Yazici,  M.,  Krassowski,  D.  &  Prakash,  J.  Flexible  graphite  as  battery  anode  and  current   collector.  Journal  of  Power  Sources  141,  171–176  (2005).   11.   Marschilok,  A.,  Lee,  C.,  Subramanian,  A.,  Takeuchi,  J.  &  Takeuchi,  E.  S.  Carbon  nanotube   substrate  electrodes  for  lightweight  ,  long-­‐life  rechargeable  batteries.  Energy  Environ.  Sci.   4,  2943–2951  (2011).doi:10.1039/c1ee01507a   12.   Hu,  L.,  Wu,  H.,  Mantia,  F.  La,  Yang,  Y.  &  Cui,  Y.  Thin,  Flexible  Secondary  Li-­‐Ion  Paper   Batteries.  ACS  Nano  4,  5843–5848  (2010).     135   13.   Wang,  K.  et  al.  Super-­‐Aligned  Carbon  Nanotube  Films  as  Current  Collectors  for   Lightweight  and  Flexible  Lithium  Ion  Batteries.  Advanced  Functional  Materials  23,  846– 853  (2013).   14.   Lashmore,  D.  S.,  Schauer,  M.  W.,  Gurau,  M.  &  Dev,  B.  CNT  Current  Collectors  for   Advanced  Automotive  Batteries.     Available  from  http://www.speautomotive.com/SPEA_CD/SPEA2012/pdf/NN/NN3.pdf   15.   Hu,  L.  et  al.  Silicon-­‐conductive  nanopaper  for  Li-­‐ion  batteries.  Nano  Energy  2,  138–145   (2013).   16.   Kercher,  a.  K.,  Kiggans,  J.  O.  &  Dudney,  N.  J.  Carbon  Fiber  Paper  Cathodes  for  Lithium  Ion   Batteries.  Journal  of  The  Electrochemical  Society  157,  A1323  (2010).   17.   Martha,  S.  K.,  Kiggans,  J.  O.,  Nanda,  J.  &  Dudney,  N.  J.  Advanced  Lithium  Battery   Cathodes  Using  Dispersed  Carbon  Fibers  as  the  Current  Collector.  Journal  of  The   Electrochemical  Society  158,  A1060  (2011).   18.   Wu,  H.  &  Drzal,  L.  T.  Graphene  nanoplatelet  paper  as  a  light-­‐weight  composite  with   excellent  electrical  and  thermal  conductivity  and  good  gas  barrier  properties.  Carbon  50,   1135–1145  (2012).   19.   Xiang,  J.  &  Drzal,  L.  T.  Thermal  conductivity  of  exfoliated  graphite  nanoplatelet  paper.   Carbon  49,  773–778  (2011).   20.   Gamby,  J.,  Taberna,  P.  .,  Simon,  P.,  Fauvarque,  J.  .  &  Chesneau,  M.  Studies  and   characterisations  of  various  activated  carbons  used  for  carbon/carbon  supercapacitors.   Journal  of  Power  Sources  101,  109–116  (2001).   21.   Yun,  J.  H.  et  al.  Low  Resistance  Flexible  Current  Collector  for  Lithium  Secondary  Battery.   Electrochemical  and  Solid-­‐State  Letters,  14  (8)  116-­‐119    (2011).         136   5  GRAPHENE   NANOPLATELETS   AS   A   CONDUCTING   TEMPLATE   FOR   LITHIUM-­‐ SULFUR  BATTERIES   BACKGROUND   The   current   Lithium   ion   batteries   are   suffering   from   technological   challenges   to   cater   to   the   increasing   high   power   energy   and   power   demands,   for   applications   such   as   electric   vehicles.  Though  there  are  several  anode  materials,  which  can  deliver  high  capacity,  the  main   roadblock   has   been   to   develop   high   performance   cathode   materials.   Lithium-­‐sulfur   batteries   offer   a   promising   solution   due   to   high   theoretical   capacity   of   sulfur   cathode   (1680   mAh/g)   with   additional  benefits  of  low  cost,  non-­‐toxicity  and  ample  availability.  However,  their  applicability   has   been   restricted   due   to   challenges   faced   by   different   components,   which   have   been   discussed  in  detail  in  the  introduction.   The  Lithium-­‐sulfur  battery  typically  consists  of  a  sulfur  cathode,  combined  with  binder   and   conducting   carbon   material,   and   lithium   metal   anode.   The   electrolyte   used   is   generally   a   non-­‐aqueous   system,   and   current   research   focuses   on   use   of   solid   electrolytes.   These   batteries   are   based   on   the   redox   reaction   mechanism   for   conversion   of   Sulfur   (S8   molecule)   to   Li2S,   + 0     delivering  a  potential  of  2.2  V  vs.  Li /Li .   𝑆!   +  16Li   ⇌  8Li! S     137   Discharge# Charge# Cathode# S8###Li2S8###Li2S6###Li2S4###Li2S3###Li2S2###Li2S# Porous# Separator# Polysulfide#diffusion## through#separator# # ShuFle# Insoluble# compounds# S8#####Li2S8####Li2S6###Li2S4###Li2S3# Li+# Polysulfides#ReducBon#on#the#Anode#surface# Li0# Lithium#plaBngDstripping# Anode#   1 Figure  5-­‐1:  Schematic  showing  the  working  of  Lithium-­‐Sulfur  Battery     POLYSULFIDE  SHUTTLE     Polysulfide   shuttle   phenomenon   is   a   concentration   driven   repeated   transfer   of   polysulfides   between   cathode   and   anode,   due   to   solubility   of   high   order   polysulfides   in   the   liquid  electrolyte  (shown  in  Figure  5-­‐1).  This  shuttle  mechanism  leads  to  capacity  fade  and  loss   of  active  electrode  area  when  the  reaction  proceeds  to  form  insoluble  and  insulating  Li2S.  The     138   2 different  reactions  involved  in  polysulfide  shuttling  have  been  explained  in  detail  by  Liu  et.  al,     and  are  summarized  below:   Cathode-­‐  Electrochemical  Oxidation   The  sulfides  and  disulfides  get  oxidized  to  form  high  order  polysulfides,  at  the  cathode.   !! !! 2  𝑆 !! +   𝑆! ⇌ 𝑆!   + 𝑥𝑒 !   Anode-­‐  Chemical  Reduction   The  soluble  high  order  polysulfides,  which  are  soluble  in  the  electrolyte,  diffuse  to  the  anode   and  react  with  lithium  to  form  low-­‐order  polysulfides.   !! !!  h − x 𝑆! +  2𝑥𝐿𝑖 !   ⇌  h𝑆!!! +  2𝑥𝐿𝑖 !   Cathode-­‐  Electrochemical  Oxidation   The  low  order  polysulfides  formed  at  the  anode,  then  shuttle  back  to  the  cathode  and  get   oxidized  to  form  high  order  polysulfides.   !! !!  h𝑆!!!   ⇌  h − x 𝑆!   + 2𝑥𝑒 !   SIGNIFICANCE   Sulfur  as  a  cathode  material  has  high  theoretical  capacity,  but  suffers  from  problems  of   poor   conductivity   of   sulfur   and   polysulfides,   dissolution   of   reaction   intermediates   into   the   3–5 electrolyte,   thus   compromising   the   efficiency   and   reversibility   of   its   electrodes   139   .To   address   these   issues,   extensive   research   has   been   focused   on   coupling   sulfur   with   conductive   materials   6,7 in  a  restricted  electrode  structure .     Carbon   materials   are   the   most   attractive   candidates;   owing   to   their   conductivity,   variable   pore   sizes   and   electrochemical   stability.   All   kinds   of   nanographitic   system   viz.   8–12 graphene fibers 17,18 ,   carbon   nanotubes 13–15 16 ,   activated   carbon,   expanded   graphite   and   carbon   10,14,16,17   are   being   explored   as   conducting   additive   to   sulfur   electrodes .     Diverse   18 techniques  such  as  melting,  sublimation,  sulfur  infusion,  encapsulation  or  different  template   methodologies 14,19,20   are   being   attempted   to   optimize   the   interaction   between   sulfur   and   nanographitic  additive.     Hence,   for   sulfur   cathodes,   a   conductive   yet   restricted   nanostructure   needs   to   be   developed.  This  nanostructure,  should  allow  for  good  conduction,  enhanced  ion  transport  and   volumetric  expansion,  but  on  the  other  hand,  should  have  limited  sulfur-­‐electrolyte  contact  and   restricted  polysulfide  dissolution.     APPROACH   The   carbon   additive   being   investigated   here   is   exfoliated   graphene   nanoplatelets   (GnP),   Because   of   its   two   dimensional   morphology   and   nano-­‐thickness   dimension,   GnP   shows   excellent   electrical   conductivity,   which   is   a   necessary   requirement   of   any   electrochemical   system.       140   Most  of  the  studies  on  carbon-­‐sulfur  materials  follow  a  melting  or  sublimation  approach   by   heat   treatment   around   150°C   and   300   °C.   This   can   result   in   liquid   and   vapor   diffusion   of   sulfur   in   pores   of   the   nanocarbon   materials.   However,   melting   results   in   sulfur   coating   the   external  surface  of  carbons,  which  allows  easy  dissolution  of  polysulfides  in  electrolyte  causing   shuttling   and   degradation   in   performance.   However,   sublimation   conditions   (~300°C)   help   sulfur   to   penetrate   into   micropores   of   carbon   hosts,   which   are   small   enough   to   restrict   polysulfide   dissolution.   Heat   treatment   at   even   higher   temperatures   of   >450°C,   does   beyond   vapor  diffusion  and  can  induce  interactions  between  sulfur  and  carbon.  The  S8   sulfur  converts   to   S6   and   S2   forms   at   such   conditions,   which   are   more   reactive   due   to   a   higher   fraction   of   14 terminal   S   atoms .   There   are   reports   of   sulfur   forming   C-­‐S   bonds   and   intercalating   into   14 graphene  layers  in  such  high  temperature  conditions .   GnP   has   a   non-­‐porous,   platelet   type   morphology,   hence,   higher   temperature   conditions   (>   450°C),   which   are   known   to   create   C-­‐S   interactions   by   bonding   or   intercalation,   are   being   used  in  our  procedure.   EXPERIMENTAL  DETAILS   MATERIALS   Sulfur   powder,   PVDF   binder   and   NMP   were   obtained   from   Sigma-­‐   Aldrich.   Graphene   2 nanoplatelets  of  25 μm  diameter  (xGnP-­‐M-­‐25)  and  surface  area  of  100  m /g,  obtained  from  XG     141   ® Sciences   are   used   as   the   carbon   host   material   for   this   application.   GnP-­‐15   (surface   area   of   25   2 m /g)  made  in  our  lab  is  used  as  the  conducting  additive  for  these  electrodes.   ACTIVE  MATERIAL  PREPARATION   Graphene  Nanoplatelets  and  Sulfur  powder  were  measured  in  desired  weight  ratio  and   were  mixed  by  a  simple  ball  milling  process  in  the  SPEX  8000M  Mixer  Mill.    The  milling  media   used  for  this  process  was  ¼”  Polypropylene  balls.  Further  treatment  of  GnP+S  mixture  was  done   in   a   sealed   pressure   vessel   to   avoid   loss   of   sulfur   at   temperatures   greater   than   sublimation   conditions.  PARR  4740  High  Pressure  Vessel  with  graphite  gaskets,  compatible  for  temperatures   up  to  540  °C,  and  pressure  rating  of  1500  psi  (at  540  °C)  was  used  for  synthesis  of  composites.   The   pressure   vessel   was   sealed   inside   a   glove   box   under   Argon   atmosphere,   to   avoid   any   interference   due   to   ambient   moisture.   The   GnP-­‐Sulfur   material   sealed   in   the   pressure   vessel   was   subjected   to   different   heat   treatment   temperatures   (300   °C,   500   °C)   in   the   box   furnace   for   2  hours.     ELECTRODE  PREPARATION   The  GnP-­‐S  composite  material  composed  of  5  wt%  GnP-­‐15  and  10  wt  %  PVDF  as  binder   was  combined  with  NMP  as  solvent  to  make  a  slurry,  which  was  cast  on  Al  foil  by  procedures   described   elsewhere.   The   addition   of   high   aspect   ratio   GnP-­‐15   as   an   additive   was   done   to   ensure  good  conductivity  throughout  the  electrode.    The  electrodes  were  dried  under  ambient   condition,  followed  by  vacuum  annealing  at  120  °C  for  8  hours.     142   PHYSICAL  CHARACTERIZATION   The  morphology  of  the  sulfur  electrodes  was  observed  from  top  view  and  cross-­‐section   view  using  Auriga  FIB  SEM.  The  sample  preparation  for  cross-­‐sectional  imaging  involved  epoxy   embedding   and   polishing   procedure   (described   earlier).   The   sulfur   concentration   was   also   verified  by  doing  Energy  Dispersive  Spectroscopy,  on  a  pellet  formed  by  compressing  the  GnP-­‐ Sulfur  composite  powder  using  a  Carl-­‐Zeiss  EVO  SEM.     Thermogravimetric  analysis  (TGA)  was  conducted  by  heating  the  material  up  to  900°C  at   a   ramp   rate   of   10°C/min,   in   airflow.   The   sulfur   concentration   is   evaluated   by   loss   of   material   at   300  °C,  which  is  the  sublimation  temperature  of  Sulfur.   The   X-­‐ray   diffraction   analysis   was   done   on   powder   samples   by   a   Bruker   Davinci   Diffractometer,  using  Cu  K  α  radiation  and  λ  =  1.5418  Å.   ELECTROCHEMICAL  CHARACTERIZATION   The   electrodes   were   characterized   in   a   coin   cell   CR   2032   with   Lithium   foil   as   counter   electrode   and   1M   lithium   bis-­‐trifluoromethanesulfonylimide   (LiTFSI)   solution   in   (1:1   v/v)   1,3-­‐ dioxolane  (DO)  and  dimethoxyethane  (DME)  mixture.  The  cells  were  assembled  and  tested  in   Argon  atmosphere  in  Braun  Glove  Box  (<2ppm  O2,  <2ppm  H2O).   The   electrochemical   characterization   of   GnP-­‐S   was   done   using   a   VersaSTAT   MC   Instrument.   The   charge–discharge   performance   of   the   electrodes   was   evaluated   by   cycling   between   1-­‐3.0   V   at   different   charge   rates.   AC   impedance   spectra   were   collected   in   the     143   6 frequency   range   from   1   Hz-­‐10   Hz   with   amplitude   of   5   mV,   before   and   after   cycling   for   10   cycles  at  C  rate.     RESULTS  &  DISCUSSION   GNP-­‐SULFUR  COMPOSITE  SYNTHESIS   GnP-­‐sulfur   nanocomposites   were   synthesized   by   incorporating   sulfur   into   GnP   matrix,   by  a  simple  mixing-­‐melting  procedure  (schematic  illustration  shown  in  Figure  5-­‐2).   S"powder" (as"received)" Sublima450°C),   sulfur   doesn’t   exist   in   S8   form   and   some  interaction  between  carbon  and  sulfur  can  happen  such  as  a  weak  bond  or  intercalation   14 of  sulfur  into  graphite .  However,  in  our  case,  no  signs  of  further  intercalation  are  observed   from  the  XRD  data.    Such  a  change  in  the  initial  C-­‐S  interaction  can  change  the  pathway  of  the     151   reaction   during   galvanostatic   cycling.   These   materials   act   similar   to   Ni-­‐S   alloys   and   follow   a   2-­‐   2-­‐ 2-­‐ reaction  pathway  S4 è  S2 è  S  ,  which  can  explain  the  sloping  profile.       152     (a)   Capacity"(mAh/g)" 1600" 1200" 800" 400" 2C C C 5C Discharge" Charge" 0" 0" (b)   10" 20" 30" No."of"cycles" 40" 50"   Capacity"(mAh/g)" 1600" 1200" 800" 400" Mixed C Discharge" 0" 0" Charge" 10" 20" 30" 40" 50" 60" 70" 80" 90" 100" No."of"cycles"   Figure  5-­‐10:  Galvanostatic  Performance  of  GnP-­‐S  composite  electrode  for  (a)  different   charge  rates  (b)  long  cycling     153   2C$ 5C$$ 3.00$ C$ Poten/al$(V)$ 2.60$ 2.20$ 1.80$ 1.40$ 1.00$ 2C$ 5C$$ C$ 0.60$ 0$ 200$ 400$ 600$ 800$ 1000$ Capacity$(mAh/g)$ 1200$ 1400$   Figure  5-­‐11:  Charge-­‐Discharge  capacity  profiles  at  different  charge  rates  C,  2C,  5C.       20" A5er"10"cycles" &"Z"imag"(ohms)" 15" A5er"100"cycles" 10" 5" 0" 0" 5" 10" Z"real"(ohms)" 15" 20"   Figure  5-­‐12:  Nyquist  plot  of  GnP+S  electrode  before  and  after  10  and  100  cycles     154   Rs (a)   R ct W CPE1   &Z"imaginary"(ohms)" (b)   20" Element Rs R ct 10"W-R W-T 5"W-P CPE1-T CPE1-P Freedom Fixed(X) Fixed(X) Fixed(X) Fixed(X) Fixed(X) Fixed(X) Fixed(X) 15" 0" 0" 5" 10" Z"real"(ohms)" Value 3.469 5.961 217.8 20.15 0.59357 0.00028 0.6416 15" Error N/A N/A N/A Experimental" Fi;ng"N/A N/A N/A N/A 20" Data File:   Circuit Model File: D:\GnP+S-2.mdl  Figure  5-­‐13:  (a)  Equivalent  circuit  used  for  fitting  impedance  data  (b)  Comparison  of   Mode: Run Fitting / Freq. Range (0.12 - 8 experimental  and  fitted  impedance  data  for  GnP-­‐S  electrode  after  cycling  for  10  cycles     Maximum Iterations: btained  by  fitting  impedance  data  using  the  equivalent   100  Table  5-­‐1:  Resistance  values  o circuit   Optimization Iterations: 100 Rs# Complex Rct# Type of Fitting: GnP+S&10cyc+ 3.5+ 5.3+ Type of Weighting: Calc-Modulus GnP+S&100cyc+ 3.8+     155   5.1+   EIS  analysis  was  conducted  to  evaluate  the  presence  of  insulating  polysulfide  products   deposited   on   the   electrode   surface.   Figure   5-­‐12   below   shows   the   Nyquist   plots   of   GnP-­‐S   electrode  after  10  and  100  cycles.  The  intercept  of  the  curve  on  the  Z  real  axis  gives  us  the  bulk   or   solution   resistance   of   the   cell,   which   is   a   combination   of   the   electrolyte   resistance   and   24 intrinsic   resistance   of   the   electrode .   The   charge   transfer   resistance   associated   with   the   kinetics   of   charge   transfer   is   indicated   by   the   semi-­‐circular   part   of   the   curve   at   medium   frequencies 10,24 .  The  inclined  line  at  low  frequencies  corresponds  to  the  Warburg  impedance,   24 which  is  a  measure  of  the  diffusion  of  Li  ions  in  the  electrode .  The  experimental  impedance   12,25 data   obtained   was   fitted   with   Zview   software   using   the   equivalent   circuit   shown   in     Figure   5-­‐13(a).   A   comparison   of   experimental   and   equivalent   circuit   calculated   data   for   GnP-­‐S   electrodes   after   10   cycles   is   shown   in     Figure   5-­‐13(b).   Very   good   fitting   in   the   high   and   medium   frequency  range  was  obtained,  however  there  was  only  reasonable  fitting  with  15-­‐20%  error  for   the  diffusion  region,  which  is  a  complex  process  in  lithium-­‐sulfur  system.  Based  on  the  values  of   the   solution   and   charge   transfer   resistance   (   Table   5-­‐1)   obtained   from   this   analysis,   we   can   observe  that  there  is  no  significant  increase  in  resistance  after  100  cycles.  This  confirms  that  the   interconnected  network  of  GnP  particles  imparts  good  conductivity  to  the  electrode  over  long   cycling.   CONCLUSIONS     From   the   above   results,   we   can   infer   that   GnP   can   act   as   a   good   conducting   host   for   Lithium-­‐sulfur   batteries.   We   have   demonstrated   good   performance   of   900   mAh/g   after   100     156   cycles  at  C  charge  rate.  However,  the  sulfur  loading  in  these  experiments  is  quite  low  and  focus   should   be   on   improving   the   overall   sulfur   percentage   in   the   electrode   for   better   gravimetric   capacity.     FUTURE  WORK   Based   on   our   understanding   of   the   lithium-­‐sulfur   batteries,   we   have   concluded   that   the   potential  sulfur  electrode  should  be  sulfur  encapsulated  in  a  conducting  but  flexible  or  spacious   matrix,   which   will   allow   for   volumetric   expansion,   but   restrict   the   interaction   of   sulfur   with   the   electrolyte.   We   have   demonstrated   that   graphene   nanoplatelets   have   good   electrical   conductivity  and  can  be  arranged  into  a  nanostructure  with  controlled  porosities.  Hence,  from   previous   experiments   and   current   progress   in   the   field,   GnP   Paper   can   be   used   as   a   conducting   template   for   lithium   sulfur   batteries,   capable   of   delivering   high   performance   and   good   cycle   life.  The  process  approach  is  schematically  shown  in  Figure  5-­‐14.   S"powder"" (as"received)" S"dissolved"" in"toluene","carbon" disulfide" GnP"Paper" Unpressed" Melt"Diffusion" SublimaAon" GnP"Paper+"S"in"solvent" Soaking"followed"by" drying"" GnP3S"" composite"" "   Figure  5-­‐14:  Schematic  depicting  the  synthesis  of  sulfur  impregnated  GnP  Paper   This  conducting  template  can  be  altered  in  porosity  and  hence  can  be  made  selectively   permeable   to   lithium   ions,   but   not   to   the   undesirable   polysulfides   reaction   intermediates.   Such   a   structure   can   entrap   the   active   sulfur   material   and   will   prevent   it   from   dissolving   into   the     157   active  electrolyte  during  the  reaction,  in  the  form  of  polysulfides.  Literature  references  show  a   retention  of  80%  of  theoretical  capacity  of  sulfur  by  ensuring  restricted  polysulfide  dissolution   26 by  having  a  polymer  coated  carbon  nanostructure .     Future  work  in  this  application  will  revolve  around  encapsulating  sulfur  in  a  GnP  matrix   for   enhanced   conduction   and   tailoring   the   GnP   paper   to   desired   porosities   for   achieving   maximum  performance  for  lithium  sulfur  cathodes.         158                         REFERENCES     159     REFERENCES     1.   Mikhaylik,  Y.,  Kovalev,  I.,  Schock,  R.  &  Affinito,  J.  High  Energy  Rechargeable  Li-­‐S  Cells  for   EV  Application.  Status,  Challenges  and  Solutions.  Sion  Power  Corporation  .  Available  from   http://sionpower.com/pdf/articles/SionPowerECS.pdf   2.   Liu,  Z.,  Fu,  W.  &  Liang,  C.  Handbook  of  Battery  Materials.  811–840  (2011).   3.   Evers,  S.  &  Nazar,  L.  F.  New  Approaches  for  High  Energy  Density  Lithium  Sulfur  Battery   Cathodes.  Accounts  of  Chemical  Research  ,  (2012).  DOI:  10.1021/ar3001348   4.   Manthiram,  A.,  Fu,  Y.  &  Su,  Y.  Challenges  and  Prospects  of  Lithium-­‐Sulfur  Batteries.   Accounts  of  Chemical  Research,  (2012).  DOI:  10.1021/ar300179v   5.   Bruce,  P.  G.,  Freunberger,  S.  A.,  Hardwick,  L.  J.  &  Tarascon,  J.  Li  –  O  2  and  Li  –  S  batteries   with  high  energy  storage.  Nature  materials  11,  19–30  (2012).   6.   Zhang,  Y.,  Zhao,  Y.,  Sun,  K.  E.  &  Chen,  P.  Development  in  Lithium  /  Sulfur  Secondary   Batteries.  The  Open  Materials  Science  Journal  5,  215–221  (2011).   7.   Song,  M.-­‐K.,  Cairns,  E.  J.  &  Zhang,  Y.  Lithium/sulfur  batteries  with  high  specific  energy:   old  challenges  and  new  opportunities.  Nanoscale  5,  2186–204  (2013).   8.   Cao,  Y.  et  al.  Sandwich-­‐type  functionalized  graphene  sheet-­‐sulfur  nanocomposite  for   rechargeable  lithium  batteries.  Physical  chemistry  chemical  physics :  PCCP  13,  7660–5   (2011).   9.   Chuvilin,  a  et  al.  Self-­‐assembly  of  a  sulphur-­‐terminated  graphene  nanoribbon  within  a   single-­‐walled  carbon  nanotube.  Nature  materials  10,  687–92  (2011).   10.   Wang,  J.-­‐Z.  et  al.  Sulfur-­‐graphene  composite  for  rechargeable  lithium  batteries.  Journal   of  Power  Sources  196,  7030–7034  (2011).   11.   Wang,  H.  et  al.  Graphene-­‐Wrapped  Sulfur  Particles  as  a  Rechargeable  LithiumÀSulfur   Battery  Cathode  Material  with  High  Capacity  and  Cycling  Stability.  Nano  Letters  11,   2644–2647  (2011).     160   12.   Zhang,  F.  et  al.  Preparation  and  performance  of  a  sulfur/graphene  composite  for   rechargeable  lithium-­‐sulfur  battery.  Journal  of  Physics:  Conference  Series  339,  012003   (2012).   13.   Zhou,  G.  et  al.  A  flexible  nanostructured  sulphur–carbon  nanotube  cathode  with  high   rate  performance  for  Li-­‐S  batteries.  Energy  &  Environmental  Science  5,  8901  (2012).   14.   Guo,  J.,  Xu,  Y.  &  Wang,  C.  Sulfur-­‐impregnated  disordered  carbon  nanotubes  cathode  for   lithium-­‐sulfur  batteries.  Nano  letters  11,  4288–94  (2011).   15.   Kim,  H.,  Lee,  J.  T.  &  Yushin,  G.  High  temperature  stabilization  of  lithium–sulfur  cells  with   carbon  nanotube  current  collector.  Journal  of  Power  Sources  226,  256–265  (2013).   16.   Li,  S.,  Xie,  M.,  Liu,  J.,  Wang,  H.  &  Yan,  H.  Layer  Structured  Sulfur/Expanded  Graphite   Composite  as  Cathode  for  Lithium  Battery.  Electrochemical  and  Solid-­‐State  Letters  14,   A105  (2011).   17.   Elazari,  R.,  Salitra,  G.,  Garsuch,  A.,  Panchenko,  A.  &  Aurbach,  D.  Sulfur-­‐impregnated   activated  carbon  fiber  cloth  as  a  binder-­‐free  cathode  for  rechargeable  Li-­‐S  batteries.   Advanced  materials  23,  5641–4  (2011).   18.   Zheng,  G.,  Yang,  Y.,  Cha,  J.  J.,  Hong,  S.  S.  &  Cui,  Y.  Hollow  carbon  nanofiber-­‐encapsulated   sulfur  cathodes  for  high  specific  capacity  rechargeable  lithium  batteries.  Nano  letters  11,   4462–7  (2011).   19.   Ji,  X.,  Evers,  S.,  Black,  R.  &  Nazar,  L.  F.  Stabilizing  lithium-­‐sulphur  cathodes  using   polysulphide  reservoirs.  Nature  communications  2,  325  (2011).   20.   Xin,  S.,  Guo,  Y.-­‐G.  &  Wan,  L.-­‐J.  Nanocarbon  networks  for  advanced  rechargeable  lithium   batteries.  Accounts  of  chemical  research  45,  1759–69  (2012).   21.   Wu,  F.,  Wu,  S.  X.,  Chen,  R.  J.,  Chen,  S.  &  Wang,  G.  Q.  Electrochemical  performance  of   sulfur  composite  cathode  materials  for  rechargeable  lithium  batteries.  Chinese  Chemical   Letters  20,  1255–1258  (2009).   22.   Wang,  J.  L.,  Yang,  J.,  Xie,  J.  Y.,  Xu,  N.  X.  &  Li,  Y.  Sulfur  –  carbon  nano-­‐composite  as  cathode   for  rechargeable  lithium  battery  based  on  gel  electrolyte.  Science  And  Technology  4,   499–502  (2002).   23.   Yu,  X.  et  al.  Stable-­‐cycle  and  high-­‐capacity  conductive  sulfur-­‐containing  cathode   materials  for  rechargeable  lithium  batteries.  Journal  of  Power  Sources  146,  335–339   (2005).     161   24.   Wang,  Y.-­‐X.  et  al.  Facile  synthesis  of  a  interleaved  expanded  graphite-­‐embedded  sulphur   nanocomposite  as  cathode  of  Li–S  batteries  with  excellent  lithium  storage  performance.   Journal  of  Materials  Chemistry  22,  4744  (2012).   25.   Zhang,  B.,  Qin,  X.,  Li,  G.  R.  &  Gao,  X.  P.  Enhancement  of  long  stability  of  sulfur  cathode  by   encapsulating  sulfur  into  micropores  of  carbon  spheres.  Energy  &  Environmental  Science   3,  1531  (2010).   26.   Ji,  X.,  Lee,  K.  T.  &  Nazar,  L.  F.  A  highly  ordered  nanostructured  carbon-­‐sulphur  cathode   for  lithium-­‐sulphur  batteries.  Nature  Materials  8,  500–506  (2009).       162     6  CONCLUSIONS   The  increasing  energy  and  power  requirements  for  portable  devices  have  driven  strong   interest  in  the  development  and  efficient  utilization  of  active  materials  for  all  parts  of  lithium   ion   batteries.     The   direction   of   current   research   is   oriented   towards   development   of   nanomaterials  and  structured  electrodes  for  improved  performance.   Graphene   Nanoplatelets   (GnP)   are   a   promising   nanocarbon   material,   which   can   be   integrated  into  different  lithium  ion  battery  components  and  because  of  its  morphology,  can  be   nano-­‐structured  to  enhance  battery  performance  in  a  variety  of  ways.  We  have  demonstrated   the   potential   of   GnP   as   an   active   anode   material.   Also,   GnP   can   be   combined   with   metal   or   metal   oxide   nanoparticles   (with   inactive   or   inherent   lithium   storage   capacity)   to   form   composite   electrodes   with   high   performance.   Nickel   doped   graphene   nanoplatelets,   with   60   nm   nanoparticle   size   have   shown   improved   capacity,   around   300   mAh/g   at   fast   charge   rates   of   C  and  2C.  This  improvement  has  been  attributed  to  a  tailored  nanostructures  with  benefits  of   short  diffusion  length,  ordered  porosity  and  interconnectivity.     The   role   of   different   sizes   of   GnP   as   a   conducting   additive   has   been   evaluated   for   Lithium   Titanate   electrodes.   LTO-­‐GnP   electrodes   (using   GnP-­‐25   and   GnP-­‐5)   have   shown   up   to   10%   improvement   in   capacity   with   reference   to   the   use   of   commercial   Super   P   carbon   additives.   Based   on   our   results,   we   can   infer   that   GnP,   because   of   its   high   aspect   ratio   and   excellent  conductivity  can  provide  efficient  contact  with  active  materials  and  thus,  is  a  strong   contender  for  conducting  additives  for  different  electrode  materials.       163   GnP  can  be  easily  fabricated  into  free-­‐standing  ‘paper’  films  as  a  current  collector.  The   GnP   current   collector   has   ~50%   the   areal   density   of   copper,   and   hence   can   help   reduce   the   dead  weight  in  the  batteries.  Moreover,  the  current  collector  itself  can  contribute  to  increased   lithium   storage   capacity   in   graphite   based   anode   systems.   The   GnP   current   collector   can   be   extended   to   the   cathode   side   and   replace   the   aluminum   current   collector,   thus   eliminating   any   corrosion   problems   with   metal   current   collectors.   Free   standing   GnP   papers   can   be   customized   for   use   in   various   components   of   battery   systems   and   other   electrochemical   systems   such   as   sensors  and  supercapacitors  as  well.   Lithium   sulfur   batteries   are   being   envisioned   as   the   future   of   portable   energy   storage   because  of  their  low  predicted  cost  and  high  performance.    The  use  of  GnP  as  a  conducting  host   material   for   lithium   sulfur   batteries   enables   this   chemistry   to   be   capable   of   delivering   high   performance  at  fast  charge  rates  because  of  its  nanostructuring  and  conductivity.  High  capacity   of  900  mAh/g  after  100  cycles  at  C  charge  rate  has  been  delivered  by  the  GnP-­‐S  composite.   ACKNOWLEDGEMENT   This   material   is   based   upon   work   supported   in   part   by   the   U.   S.   Army   Research   Laboratory   and   the   U.   S.   Army   Research   Office   under   contract/grant   number   No.   W911NF0910451.   The   support   for   the   A123   Michigan   Center   of   Energy   Excellence   Grant   106979  is  also  greatly  acknowledged.           164