Performance Degradation of Lithium‐Ion Batteries with LiNi0.33Co0.33Mn0.33O2 Cathodes during Long‐Term, High‐Temperature Storage: Behaviors and Mechanism

Abstract The behaviors and mechanism for the different performance degradation trends of 18650 cylindrical lithium‐ion batteries (LIBs) with LiNi0.33Co0.33Mn0.33O2 cathodes under long‐term storage at a high temperature (HT) of 80 °C are investigated to gain insight on the safe use of LIBs. It is indicated that the direct current resistance (DCR) increases sharply within 240 h and then slows down gradually, whereas the open‐circuit voltage (OCV) and the recoverable capacity (RC) fading are relatively stable in the initial 240 h, but then speeds up from 240 h to 720 h. With the aid of analyses of the anode, cathode, and separator materials in batteries stored at 80 °C for 0 h, 240 h, 480 h, and 720 h, the mechanism of the performance degradation of batteries is elucidated. It is pointed out that the DCR rise is mainly caused by the anode materials peeling off from the copper foil, as is the subsequent interface resistance increase. The RC fading is attributed to the partial break of the cathode structure, owing to the dissolution of transition metal ions (Ni2+, Co3+, and Mn4+), and the dissolution becomes more severe as the time extends. In addition, the separator suffers a combination of stretching (machine direction), squeezing (anode side), oxidation (cathode side), and destruction, resulting in internal short circuit and sharp OCV drop, as well as the other potential safety risks of LIBs. These results are valuable to establish an accelerated testing method to evaluate batteries in a short time range to advance the safe use of LIBs under high‐temperature storage.