Spatially heterogeneous degradation in LiFePO₄//graphite pouch batteries under temperature accelerated aging process

The accelerated degradation of lithium-ion batteries under elevated temperature has emerged as a critical challenge for large-format energy storage systems. While thermal stress is known to intensify performance decline, its spatial influence within individual batteries remains underexplored. This study investigates the position-dependent aging behavior of LiFePO₄//graphite (LFP//Gr) pouch batteries subjected to long-term cycling at 25 °C and 45 °C. A spatially resolved, multi-scale analysis framework is employed to differentiate degradation characteristics between inner and outer electrode regions. The results reveal that temperature-accelerated aging leads to pronounced spatial heterogeneity in electrochemical performance, structural evolution, and interfacial stability, with inner regions showing more severe deterioration. Irreversible lithium inventory loss emerges as the dominant degradation mechanism, primarily driven by intensified interfacial reactions and uneven side-product accumulation. These findings demonstrate that battery aging under high-temperature conditions is not spatially uniform and that internal position significantly influences degradation severity. The study highlights the limitations of homogeneous evaluation methods and underscores that spatial heterogeneity must be explicitly considered when analyzing and modeling temperature-accelerated aging processes. This work provides mechanistic insight essential for developing thermally robust and spatially optimized lithium-ion batteries for demanding applications such as electric vehicles and stationary energy storage.