Impact of transient charge transfer kinetics on mechanics and electrochemistry of rechargeable batteries

High performance electrode materials such as Si and Ge experience high volume expansion induced stresses and associated mechanical (and possibly chemical) degradation which leads to capacity fade. To develop damage-tolerant electrodes, robust multiphysics models are necessary. Here, a coupled large deformation and diffusion model for next-generation electrodes is presented. The model includes diffusion of solute species (i.e., Li, Na, or other ions) in the solvent (i.e., electrode materials such as Si and Ge). In situ stress and electrochemical measurements were performed on Na–Ge system while the data for Li–Ge system is available from Nadimpalli et al., (2015b). Since the electrode materials undergo significant volume changes, the model considers diffusion in a volume changing domain. In addition to the steady-state Butler–Volmer kinetics which govern charge transfer reactions, the transient charge transfer electrochemical kinetics at the electrode/electrolyte interface are included to capture the dynamics of charge transfer kinetics. The proposed model successfully captured the mechanical and electrochemical behavior of the sodium germanium and lithium germanium electrode systems, demonstrating its robustness, while also enabling comparison with the lithium silicon system. The model clearly demonstrates the errors associated with capacity and electrode stress estimates under realistic service conditions (i.e., frequent changes in current and potential) when transient charge transfer kinetics are ignored. Hence, the inclusion of transient charge transfer kinetics in large deformation models is necessary for accurately capturing the multiphysics behavior of rechargeable battery anodes.