Capacity Fading Mechanism of the Commercial 18650 LiFePO₄-Based Lithium-Ion Batteries: An in Situ Time-Resolved...

In situ high-energy synchrotron XRD studies were carried out on commercial 18650 LiFePO₄ cells at different cycles to track and investigate the dynamic, chemical, and structural changes in the course of long-term cycling to elucidate the capacity fading mechanism. The results indicate that the crystalline structural deterioration of the LiFePO₄ cathode and the graphite anode is unlikely to happen before capacity fades below 80% of the initial capacity. Rather, the loss of the active lithium source is the primary cause for the capacity fade, which leads to the appearance of inactive FePO₄ that is proportional to the absence of the lithium source. Our in situ HESXRD studies further show that the lithium-ion insertion and deinsertion behavior of LiFePO₄ continuously changed with cycling. For a fresh cell, the LiFePO₄ experienced a dual-phase solid-solution behavior, whereas with increasing cycle numbers, the dynamic change, which is characteristic of the continuous decay of solid solution behavior, is obvious. The unpredicted dynamic change may result from the morphology evolution of LiFePO₄ particles and the loss of the lithium source, which may be the cause of the decreased rate capability of LiFePO₄ cells after long-term cycling.