Calendar Aging of LiFePO ₄ /Graphite Batteries: Quantitative Analysis of Synergistic Effects of SEI Evolution and Electrolyte Decomposition

Quantifying calendar aging mechanisms is critical for developing long-life lithium-ion batteries in large-scale energy storage systems and electric vehicles. This study investigates the temperature-dependent calendar aging behavior of LiFePO4/graphite cells through integrated mass spectrometry titration (MST), nuclear magnetic resonance (NMR), and gas chromatography–mass spectrometry (GC-MS) analyses. The results reveal a synergistic degradation mechanism involving accelerated solid electrolyte interphase (SEI) evolution and electrolyte decomposition. The loss of lithium inventory (LLI) is the predominant degradation mechanism during calendar aging. Elevated storage temperatures significantly accelerate the decomposition of vinylene carbonate (VC), PF6–, and FSI–, leading to excessive SEI growth and a deterioration of interfacial kinetics. The coupling effects of electrolyte decomposition, SEI accumulation, and kinetic degradation collectively drive the rapid capacity fade during high-temperature storage. This work provides a comprehensive and quantitative understanding of temperature-induced calendar aging mechanisms, offering valuable insights for extending the operational lifetime of lithium-ion batteries.