Reversible self-discharge and calendar aging of 18650 nickel-rich, silicon-graphite lithium-ion cells

Lithium-ion cells with nickel-rich cathodes and silicon-graphite (SiC) anodes are expected to be deployed in the next generation electric vehicles (EV) due to their high specific energy density and price advantages. While the majority of current research focuses on cycle performance of nickel-rich/SiC cells, there is no general understanding of reversible self-discharge processes and degradation mechanisms during calendar aging. In this study, capacity fade of commercial 18650 nickel-rich/SiC cells after 11 months of storage was analyzed using differential voltage analysis (DVA). Reversible self-discharge losses were determined via capacity measurements before and after the storage and via decay of cell voltages, which were monitored throughout the experiment. The results obtained in this work reveal that capacity fade was mainly caused by the loss of active material (LAM) of the anode, which was linked to the presence of silicon. In the range between 33% and 58% state of charge (SOC), in which anode overhang and active area of the anode exhibited similar potential, both self-discharge measurement methods showed consistent results. Calculated reversible self-discharge current ranged between 2 μA and 4.5 μA at 25°C and was linked to coupled side reactions between the negative and positive electrode.