Mechanistic modeling of SEI-induced capacity fade in cell under external loads

Abstract

With the advancement of battery technology, batteries are gradually evolving from purely energy storage systems to integrated structures that combine both load-bearing and energy storage functions. This trend is particularly evident in the cell-to-chassis technology used in EV, which requires batteries to withstand mechanical loads. Numerous experimental studies have shown that batteries under certain pressures can suppress capacity degradation, but the analysis models for the degradation mechanisms remain incomplete. Considering that battery capacity degradation is mainly caused by the loss of reversible lithium due to the formation and growth of the SEI film, this study attributes the changes in the battery under external loads to modifications in the battery configuration and the redistribution of stress in the active particles, which in turn causes changes in the multi-physical field relationships. Based on the redistribution of stress on the active particles, a crack propagation model is used to calculate the new surface area generated by cracks in the active particles. The currents of electrochemical reactions, including side reactions, occurring on the active particle surface are then corrected based on this new surface area. Furthermore, this study also develops an SEI growth model that takes stress effects into account and modifies the electrode conductivity and diffusion coefficient under configuration changes. By combining configuration changes and stress redistribution, a multi-physics heterogeneous battery unit model is constructed to analyze the SEI formation and growth process under external loading. The results show that configuration changes alter the transport capabilities of ions and electrons, leading to a decrease in ion concentration for surface reactions. Additionally, stress redistribution reduces the side reaction current on the active particle surface and crack propagation, thereby suppressing SEI formation and growth, and reducing the consumption of reversible lithium. This model provides a theoretical basis for capacity loss under external loading and offers new guidance for the design of novel multifunctional energy storage structures.