Interfacial challenges of all-solid-state Li-ion batteries : multi-scale computational approach

"All-solid-state Li-ion batteries (ASSLB) with solid electrolytes (SEs) have enhanced safety and higher volumetric/gravimetric energy density than conventional Li-ion batteries with liquid electrolytes. However, the applications of ASSLB are still limited by the interfacial issues, such as Li dendrite growth through the SEs and the high SE/electrode interfacial resistance. This thesis developed a multi-scale computational approach, combining Density Functional Theory (DFT) calculation and Finite Element Method (FEM), to investigate the interfacial challenges in ASSLB. The Li dendrite growth through pores in SEs and the resulting short-circuit limit the highest current density in ASSLB. The underlining mechanism of Li dendrite nucleation and growth in SEs is still unclear. A DFT model was developed to evaluate the electronic properties of the bulk and surface structures of different SEs. It was revealed that the reduced bandgap and trapped electrons on the pore and crack surfaces are the main reasons for Li dendrite to form. The DFT computed material properties were compared for different SEs, and it was found that the ranked Li dendrite resistance in these SEs, based on the surface electronic properties instead of mechanical properties, is consistent with a broad range of experimental observations. The DFT results also served as the input to a phase-field model, which predicted the formation of isolated Li dendrite that matched with experimental observations. Furthermore, materials design strategies were proposed based on the critical material properties that can resist Li dendrite growth in SEs.The physically imperfect contact at interfaces is formed during the fabrication process of ASSLB and gets worse during cycling, resulting in high interfacial resistance and damaging to the battery performance. A 1D FEM battery model was constructed to investigate the relationship between the contact area and the discharging performance. Furthermore, the multi-scale Persson's contact theory was applied to predict the necessary pressure to prevent ASSLB capacity degradation due to contact area loss during the cycling of ASSLB. Cracked SE and SE/electrode interfaces also increase the impedance in ASSLB. The mechanical degradation of ASSLB is expected to be more severe than that in traditional Li-ion batteries with liquid electrolytes, as the solid-electrolyte also imposes mechanical constraints on the deformation of electrodes. A coupled electrochemical-mechanical FEM model was developed to evaluate the stress development in ASSLB. Two sources of volume change, namely the expansion/shrinkage of electrodes due to lithium concentration change and the interphase formation at the SE/electrode interface due to the decomposition of SEs, were considered. The favorable SE decomposition reactions and the associated volume change were predicted by DFT calculations. It was found that the SE-decomposition induced stress can be much larger than the electrodes volume changes due to Li concentration change, up to tens of GPa, if there are no voids in ASSLB to release some induced-stress. This model can also be used to design 3D ASSLB architectures to minimize the stress generation in ASSLB."--Pages ii-iii.