Storage Fading Mechanism of a Commercial 18650 Cell Comprised with Composite NMC/LMO Cathode and Graphite Anode

In this study, the capacities and electrochemical impedances at various states of charge (SOC) are measured after the screened commercial 18650 lithium ion cells assembled with composite cathode of layered mixed oxide (NMC) and spinel lithium manganite (LMO) and graphite anode were stored at various states of depth-of-discharge (DOD) at various temperatures for various durations. The results manifest that the capacity fading is strongly affected by the storage temperature and becomes prominent at temperatures higher than 45^oC. Incremental capacity analyses (ICA) for the charge/discharge curves before and after storage were performed for the qualitative mechanism study. It demonstrates that the capacity fade of the cell stored at 60^oC and 0% DOD can be attributed to the loss of active materials, and the loss of graphite anode is higher than that of composite cathode. The capacity fade of the cell stored at 60^oC and 25% DOD can be caused by the lithium inventory loss, the loss of NMC, and the loss due to the morphology change of LMO. Cell stored at 50% DOD manifests similar fading behavior as that of the cell stored at 0% DOD, however, the ICA plots demonstrate that the capacity loss of the cell comes from losses of lithium inventory and NMC caused by the dissolution of transition metals with additional losses of LMO and graphite anode occur after 10 months of accumulated storage. For cell stored at 75% DOD, most the capacity loss can be due to the loss of NMC with minor contribution of lithium inventory loss, while cell stored at 100% DOD shows almost no change after storage for 12 accumulated months. These results revealed from the ICA study are consistent with those determined from the post mortem study of the electrodes obtained from cells before and after storage.