Diagnosis of Electrochemical Degradation in a Large-Format NCM Pouch Cell under Over-Discharge Using EIS and Acoustic Emission

Over-discharge of large-format lithium-ion cells can cause electrochemical and mechanical degradation, leading to safety concerns. This study investigated a 67 Ah NCM pouch cell using electrochemical impedance spectroscopy (EIS), acoustic emission (AE), and X-ray computed tomography (X-ray CT). Repeated low-rate discharge under 0% SOC changed the EIS spectrum from two arcs to one dominant arc, suggesting a possible overlapping of SEI-related relaxation and charge transfer processes. During forced over-discharge, burst-type AE signals were detected irregularly, particularly near the voltage reversal region. Post-test X-ray CT showed pouch swelling and displacement or separation of the internal electrode stack. The results demonstrate that EIS and AE/X-ray CT have the potential to serve as complementary diagnostic indicators for estimating gradual electrochemical degradation and abrupt structural changes during severe over-discharge. Further repeated tests are required to improve the reliability of AE-based diagnosis.