Unveiling the Graphite Electrolyte Interphase Evolution under Fast Charging Conditions in Commercial Cells

The resurgence of LiFePO₄ lithium-ion batteries as a competitive alternative to nickel-cobalt systems for electric vehicle (EV) applications, driven by their superior thermal stability and cycle life, necessitates a thorough understanding of their degradation modes to develop strategies for performance and safety enhancements. This study investigates cycling-induced degradation in 18650 LiFePO₄/graphite full cells at varying charge rates. We analyze capacity degradation mechanisms through electrochemical performance, surface and bulk morphology, composition, and structure of both the cathode and anode. Our results reveal that irreversible lithium loss, primarily due to solid-electrolyte interphase formation, dominates at lower charging rates. However, above 4C, graphite electrode degradation is distinct and limited by Li-ion intercalation kinetics. Notably, degradation mechanisms vary not only with charging rate but also spatially across the graphite electrode. This work highlights the degradation mechanisms of commercial LiFePO₄/graphite systems under high charge rates, providing insights into critical bottlenecks in lithium-ion battery development for fast-charging applications.