Hidden risk: How pre-existing mechanical defects worsen battery safety under slight overcharge

Lithium-ion batteries in real applications often experience combined mechanical and electrical stresses, yet the influence of pre-existing minor mechanical defects on subsequent overcharge-induced aging remains poorly understood. This study investigates how controlled minor spherical indentation interacts with graded overcharge cycling (4.2–4.6 V) in NCM523/graphite pouch cells by combining incremental capacity analysis (ICA), electrochemical impedance spectroscopy (EIS) with Distribution of Relaxation Times (DRT) analysis, and post-mortem characterization. Under normal-voltage cycling at 4.2 V, the indented cell showed electrochemical signatures consistent with localized loss of active material, including reduced IC peak intensity and a relatively stable whole-cell impedance response, supported by SEM evidence of particle cracking and detachment near the indentation site. With increasing overcharge severity, these indentation-related signatures became progressively less distinguishable in cell-level diagnostics, as overcharge-driven degradation increasingly dominated the ICA, EIS, and DRT responses of both pristine and indented cells. Despite this convergence in cell-level indicators at 4.6 V, post-mortem inspection revealed localized plating-like deposits and apparent separator transparency near the indentation region, indicating a possible local vulnerability that was not reflected by conventional capacity-based monitoring. These findings reveal a diagnostic blind spot in which overcharge-driven degradation can mask the cell-level signature of pre-existing mechanical damage, while localized safety-relevant vulnerability may still persist.