Coupled influence of state-of-charge and storage temperature on calendar aging and subsequent cycle degradation in LiFePO₄/graphite pouch cells

Understanding the full-lifecycle degradation behavior of lithium-ion batteries requires simultaneous attention to calendar aging and cycle aging, which differ in mechanisms but are inherently interconnected. This study systematically investigates how high-temperature storage under different state-of-charge (SOC) conditions influences subsequent cycling performance in LiFePO₄/graphite pouch cells. Under 55 °C storage, batteries aged at high SOC exhibit more severe capacity fade, lithium inventory loss, and interfacial degradation compared to those stored at lower SOC. Multi-scale and multi-dimensional analyses reveal that calendar aging at elevated SOC accelerates side reactions, promotes SEI thickening, induces interfacial inhomogeneity, and triggers structural disorder. These chemical and mechanical deteriorations do not terminate with storage but persist and evolve under subsequent cycling, leading to increased resistance, reduced phase reversibility, and long-term performance decline. In contrast, low-SOC storage preserves structural and interfacial stability, enabling better cycling durability. The findings demonstrate that calendar aging functions as a critical precursor that shapes the trajectory and severity of ensuing cycle aging. Aging behavior in storage stages sets the foundation for electrochemical failure under dynamic conditions. This study highlights the need to address both aging types within an integrated framework, offering mechanistic insights for predictive modeling and practical guidance for optimizing storage and usage protocols in electric vehicle and stationary energy storage systems.