Inhomogeneous degradation mechanisms in LiFePO₄/Graphite pouch cells under temperature and over-discharge coupled accelerated aging

Ensuring the long-term durability of LiFePO₄/Graphite (LFP/Gr) pouch cells is essential for their deployment in electric vehicles and stationary energy storage systems. To clarify how multiple external stressors jointly influence failure behavior, this study investigates degradation under coupled high-temperature and over-discharge conditions (45 °C, 1.0 V) in comparison with baseline cycling (25 °C, 2.5 V). A multiscale framework integrating electrochemical diagnostics, structural and interfacial characterization, multimodal imaging, and finite-element modeling was employed to correlate macroscopic performance decay with microscopic failure mechanisms. The coupled condition results in a markedly faster loss of capacity and a nonlinear aging trajectory, in contrast to the nearly linear trend observed under baseline operation. The two stressors show distinct temporal contributions: temperature-driven interfacial breakdown and Fe dissolution appear early and evolve gradually, whereas over-discharge–induced Cu dissolution, graphite disordering, and lithium plating intensify sharply during later stages, establishing a clear sequence of degradation events. Dynamic resistance evolution further confirms staged failure involving SEI reconstruction, lithium inventory depletion, and metal dissolution–related impedance rise. Multimodal imaging reveals pronounced spatial inhomogeneity, including edge-focused lithium accumulation and non-uniform heat and current distribution, highlighting localized regions that are more vulnerable to degradation and safety concerns. Overall, the results provide mechanistic insight into how elevated temperature and over-discharge jointly shape the timing, severity, and spatial distribution of degradation in LFP/Gr pouch cells, and the integrated multiscale analysis framework established here offers a promising basis for extending such coupled-stressor investigations to other chemistries and battery architectures.