Understanding the Degradation Mechanisms in Silicon-Graphite Composite Electrodes during Fast Charging

Silicon-graphite composite anodes (CAs) are promising for energy-dense lithium-ion batteries offering a significant increase in specific capacity through silicon, while leveraging graphite’s cycling stability. However, CAs endure substantial volume expansion potentially reaching up to 300% during lithiation thereby presenting varying modes of chemo-mechanical degradation over long-term cycling. In this study, we develop a microstructure-aware thermo-electrochemical model that incorporates the mechanistic interactions from both active materials thereby quantifying their coupled degradation response under fast charging. We focus on the interplay between three primary degradation modes: volume expansion, lithium plating and solid electrolyte interphase growth to assess the relative degradation signatures arising from both active materials. We reveal the existence of preferential lithiation dynamics in CAs that impact its in-operando volume expansion and lithium plating behavior. Furthermore, the implications of cycle-to-cycle variation in anode microstructure due to volume expansion and operating conditions will be explored. We also deduce the influence of silicon content in CAs that govern the trade-offs between energy density, electrochemical performance and long-term cycling stability. This work presents critical insights into the design of silicon-graphite CAs with superior cycling stability by understanding their intrinsic degradation mechanisms.