Understanding the capacity fading mechanism in LiNi0.5Mn1.5O4/graphite Li-ion batteries

High voltage LiNi0.5Mn1.5O4 (LNMO) spinel with an operating voltage of 4.7 V is a promising candidate as the positive electrode in future lithium ion batteries for electric vehicle applications. However, LNMO displays a capacity fading problem in LNMO/graphite full-cells. Understanding the capacity fading mechanism of LNMO is important for implementing it in next-generation lithium ion batteries. Performance comparisons between LNMO/Li half-cell cycled and LNMO/graphite full-cell cycled were carried out. Whereas no degradation was observed for half-cells, full-cell usable capacity decreased by >50% after 100 cycles. The performance of LNMO and graphite electrodes that experienced full-cell cycling for >100 cycles were then evaluated in fresh half-cells. Results indicated that there is no degradation of the individual LNMO and graphite electrodes. The voltage profiles and dQ/dV curves of full-cells were compared with those of simulated profiles based on half-cell data. Experimental data were successfully reproduced by simulation based on an assumption that the capacity fading in full-cells was originated from the Li+ loss in LNMO. The amount of Mn deposited on Li-metal in the LNMO/Li half-cells was determined to be ∼0.3% of the total Mn weight in the LNMO electrode after 200 cycles at 30 °C. The capacity fading of the LNMO/graphite can be explained by the impact of Mn dissolution, and active Li+ loss in the full-cell system through continuous SEI formation (electrolyte reduction) prompted by Mn reduced on top of graphite surface.