Temperature-dependent degradation mechanisms of LiFePO₄/graphite batteries under multi-step fast charging protocols

The development of fast-charging strategies is crucial for advancing lithium-ion battery (LIB) technologies, particularly in applications requiring rapid energy replenishment without compromising long-term durability. This study systematically investigates the temperature-dependent degradation behavior of LiFePO₄/graphite (LFP/Gr) pouch cells under a multi-step fast-charging protocol. A combination of multi-scale non-destructive evaluations and post-mortem structural analyses was employed to elucidate the underlying mechanisms. Results demonstrate that at moderate temperatures (45 °C), the multi-step charging strategy effectively shortens charging time by approximately one-third compared to conventional methods while maintaining stable cycling performance. However, under elevated temperatures (65 °C), despite the improvement in charging speed, significant acceleration of capacity fading and structural deterioration is observed. Mechanistic insights reveal that active lithium inventory loss, rather than active material degradation, predominantly governs the aging process, with thermal effects exacerbating side reactions, interfacial instability, and lattice disorder. Furthermore, the interplay between lithium-ion transport, polarization effects, and mechanical stress under varying thermal conditions critically impacts electrode integrity. These findings highlight that while multi-step fast charging provides considerable efficiency advantages under controlled conditions, it substantially amplifies degradation at higher temperatures, necessitating temperature-sensitive optimization to balance charging speed with long-term battery stability.