Understanding the mechanism of capacity increase during early cycling of commercial NMC/graphite lithium-ion batteries

A capacity increase is often observed in the early stage of Li-ion battery cycling. This study explores the phenomena involved in the capacity increase from the full cell, electrodes, and materials perspective through a combination of non-destructive diagnostic methods in a full cell and post-mortem analysis in a coin cell. The results show an increase of 1% initial capacity for the battery aged at 100% depth of discharge (DOD) and 45 °C. Furthermore, large DODs or high temperatures accelerate the capacity increase. From the incremental capacity and differential voltage (IC-DV) analysis, we concluded that the increased capacity in a full cell originates from the graphite anode. Furthermore, graphite/Li coin cells show an increased capacity for larger DODs and a decreased capacity for lower DODs, thus in agreement with the full cell results. Post-mortem analysis results show that a larger DOD enlarges the graphite d-space and separates the graphite layer structure, facilitating the Li⁺ diffusion, hence increasing the battery capacity.