The capacity and energy density of the current rechargeable batteries are not sufficient to meet thefuture energy storage demands. One of the strategies to solve this issue is to replace the existing electrode material with high performance materials. The commercial electrodes are composites which consist of active material particles (that are responsible for energy storage), a polymer binder with conductive additives which holds all the particles together and provides electrical network. Both the negative (i.e., anode) and positive (i.e., cathode) electrodes are composites. Graphite is the conventional active material in the anode, and the high performance materials such as Si, Sn, Ge are being considered as a replacement for graphite due to their energy density. For example, Si offers nearly 10 times more capacity compared to graphite (i.e., 3579 mAh/g compared to 372 mAh/g). However, these high-performance materials exhibit poor cyclic performance and undergo significant capacity fade, i.e., reduction of usable capacity with cycling. To address these issues, the dissertation has two broad goals: 1) to develop novel experimental methods for interface fracture characterization in batteries and 2) to develop a comprehensive multiphysics model for rechargeable batteries.Capacity fade occurs in batteries mainly due to two different mechanisms: chemical andmechanical processes. The chemical process involves loss of active ions due to irreversible reaction resulting in the formation of a passivation layer called the solid electrolyte interphase (SEI). The mechanical process involves fracture of active material particles or failure of the interface between binder and active material particles. In this work, the focus is on the mechanical process and especially the failure of binder/active material interfaces in the negative electrode (i.e., anode). Although the binder/active material interfaces exist in both anode and cathode, large volume changes of anode materials make the interface failure a critical issue for anodes. For example, the most promising next generation anode material Si undergoes nearly 270% volume change during electrochemical cycling. This level of volume change causes interface failure and loss of electrical network in the electrode resulting in capacity fade. In spite of its importance, there is a lack of understanding on the interface failure in rechargeable batteries. In this study we developed a novel experimental method to characterize the interface failure behavior in lithium-ion battery system. Specifically, PVdF polymer was used as a binder and Si as active material in this model system. Samples for fracture characterization were prepared by depositing PVdF on Si substrate followed by a series of nanofabrication processes. The blister test samples fabricated in this process were tested in a novel electrochemical cell in conjunction with an in-house optical system based on Michelson interferometer principle. The samples were pressurized until the PVdF film delaminated from Si substrate. The mechanical response of the pressurized film was measured, and the PVdF/ Si interface fracture was characterized in terms of critical energy release rate Gc. The effect of thermal oxide (i.e., SiO2) on the interface failure behavior was investigated. Further, the same setup was used to determine the effect of galvanostatic electrochemical cycling of Si on the interface failure behavior.The significant volume change behavior of the next generation high-performance materials during electrochemical cycling can generate stresses as high as 1 GPa. These high stresses in high-performance material undergoing large deformation affects the diffusion of ions in active material particle, affects the voltage of a battery, and also affects the electrochemical kinetics at the electrode/electrolyte interface. Theoretical models are necessary to develop high energy density and durable batteries for future energy storage demands. The current battery models account for the stress-potential coupling but assume steady state electrochemical kinetics. However, transient electrochemical kinetics are required to capture rate dependent electrochemical behavior usually observed in batteries during operation, i.e., when current is drawn at various rates during discharge process of a battery. Also, the existing models were developed based Li-ion batteries, and there is a need to extend the models to other battery systems (i.e., other chemistries such as Na-ion). Therefore, we have developed a theory for Li-ion andNa-ion electrode active materials. Adiffusiondeformation model with transient electrochemical kinetics was developed and implemented in a finite element package.By combining the experimental and modeling tasks outline above, this dissertation successfully characterized and simulated the failure behavior of binder/active material interface and attempted to predict the capacity fade behavior in rechargeable batteries.
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