Storage life at high state of charge is a key factor affecting the application of Li ion battery, but the related mechanism research is insufficient. In this study, the storage life of LiNi_0.8Co_0.15Al_0.05O₂ (NCA)/graphite battery was evaluated via accelerating experiment at elevated temperature, and the electrochemistry performances and interface properties of fresh and aged active materials were analyzed. Storage experiment shows that the storage life of NCA/graphite battery at high state of charge under 55 ℃ is about 90 d. Electrochemical test shows that the capacities of NCA and graphite decrease when the batteries reach the end of life, but that is not the main cause inducing capacity degradation. Interface analysis illustrates that the solid electrolyte interface (SEI) film on graphite anode significantly grows when batteries reach the end of life, while the positive solid electrolyte interface (PEI) film on NCA cathode keeps basically unchanged. The growth of SEI film on graphite owing to the decomposition of carbonates in electrolyte results in Li loss in anode, which contributes to the capacity loss of NCA/graphite battery during the storage aging at high state of charge.