Failure mechanism for high voltage graphite/LiNi0.5Mn1.5O4 (LNMO) Li-ion cells stored at elevated temperature

Graphite/LNMO cells show severe capacity fade after being stored for one week at 55 °C in the fully discharged state. The failure mechanism of the cell has been investigated by electrochemical methods and physical analysis techniques (XRD, SEM, FTIR and ICP-OES). Independent electrochemical analysis of anode and cathode extracted from the ET-stored cell suggests that both electrodes have significant capacity loss. It was observed that capacity loss of the aged cathode can be recovered by charging at a constant high potential (4.9 V vs. Li/Li+), while that of the aged anode cannot be recovered with a constant potential charge (10 mV vs. Li/Li+). Capacity fade for the LNMO cathode is attributed to sluggish kinetics of Li+ intercalation/deintercalation during cycling. Ex-situ surface analysis of the electrode reveals that the sluggish electrochemical kinetics is related to the formation of inactive surface layer on the cathode. Failure of the graphite electrode may result from Mn deposition and subsequent dissolution of the SEI layer on the anode.