Detection of Thermally Induced Damage in Large-Format Lithium-Ion Batteries Using Electrochemical Impedance Spectroscopy

Thermally induced damage in lithium-ion batteries (LIBs) is a critical safety concern, especially when approaching thermal runaway (TR) conditions. In this study, electrochemical impedance spectroscopy (EIS) was used to detect thermally induced damage in large-format prismatic LIBs under sub-critical TR conditions. Thermal abuse tests were first conducted to determine the TR trigger temperature of the investigated cells. A sub-critical target temperature of 170 °C was then selected below the TR trigger temperature for the subsequent thermal load tests. EIS measurements were performed at a constant reference temperature to isolate damage-related impedance changes from temperature effects. Structural changes were analyzed using X-ray computed tomography (CT) and an established temperature estimation method was applied to estimate the cell’s internal temperature. Thermal exposure to 170 °C resulted in reproducible impedance increases, particularly in the low- to mid-frequency region, indicating degradation of transport and interfacial processes. In contrast, elevated temperature caused a pronounced impedance decrease, highlighting the importance of separating temperature effects from damage-related changes. CT analysis revealed significant cell expansion and internal gap formation, providing evidence of structural changes consistent with thermally induced gas generation, which may contribute to the observed electrochemical changes. The combined results demonstrate that temperature-controlled EIS can detect impedance changes associated with thermally induced structural changes under the investigated conditions prior to the TR. The findings highlight the potential of temperature-controlled EIS as a non-destructive diagnostic approach for future battery safety monitoring and early-warning applications.