Transient Self-Discharge after Formation in Lithium-Ion Cells: Impact of State-of-Charge and Anode Overhang

A fast determination of cell quality after formation is challenging due to transient effects in the self-discharge measurement. This work investigated the self-discharge of NMC622/graphite single-layer pouch cells with varying anode dimensions to differentiate between SEI growth and anode overhang equalization processes. The transient self-discharge was measured directly after formation via voltage decay and for 20 weeks of calendar storage at three states-of-charge (SOC), 10%, 30%, and 50%. The transient behavior persisted for the entire measurement duration, even at a low SOC. Still, the low SOC minimized the impact of SEI growth and anode overhang equalization compared to moderate SOCs. Evaluating the coulombic efficiency from cycle aging showed a distinct capacity loss for the first cycle after storage, indicating further SEI growth, which stabilized in subsequent cycles. The aged capacity after cycling showed no significant dependence on the calendar storage, which further promotes fast self-discharge characterization at low SOC.