From Calendar Aging to Cycle Degradation: SOC-Dependent Structural and Interfacial Evolution in LiFePO₄/Graphite Batteries

This study investigates the impact of calendar aging conditions on the subsequent cycling performance of lithium-ion batteries. The experiments utilized LiFePO₄/graphite (LFP/Gr) pouch cells, which were charged to two states of charge (SOC): 50% and 100%. After storage at 45 °C for 100 d, cycling aging tests were conducted. The results indicate that the pre-storage conditions significantly affect cycling stability: batteries stored at high SOC exhibited more severe capacity degradation and mechanical deterioration, whereas those stored at low SOC maintained better electrochemical reversibility and mechanical stability. Through a multiscale investigation, it was found that high SOC calendar aging induces side reactions at the electrode interface and promotes uneven formation of the solid electrolyte interphase (SEI) on the anode. The structural and chemical damages incurred during the storage process become potential failure sources and manifest during the cycling aging process. This research establishes a statistical correlation framework between calendar aging damage and cycling failure, suggesting that the performance degradation of lithium-ion batteries is not solely attributed to long-term cycling but is also significantly influenced by prior storage conditions. The findings provide important insights for optimizing SOC management and storage strategies to enhance battery lifespan and reliability.