Non-destructive local degradation detection in large format lithium-ion battery cells using reversible strain heterogeneity

Heterogeneous degradation is a key challenge faced in the production of large format lithium-ion battery (LIB) cells, and is difficult to evaluate non-destructively. This study demonstrates that reversible strain heterogeneity has the potential of becoming a useful non-destructive tool for local degradation analysis of large format LIB cells. A commercial 59.5 Ah LIB cell with a Li[Ni0.6Co0.2Mn0.2]O2 (NCM622) cathode and graphite anode was degraded at 1.3C current with and without external constraint. The aged unconstrained cells experienced a sudden capacity drop and abnormal expansion at certain locations during discharging at 87% SOH (state of health), which was not observed for the constrained ones. Detailed post-mortem analysis was carried out to understand the capacity drop mechanism. The abnormal expansion was ascribed to the gas bubbles produced by localized severe side reactions between the graphite particles and electrolyte, and the significant heat in certain regions with high impedance. The quick spread of the defective regions was responsible for the sudden capacity drop. This work confirms that the reversible strain distribution contains useful information inside the battery and can help monitor battery degradation and capacity drop.