Probing the Cause of Capacity Fade in Large‐Format Li‐Ion Pouch Cells through Multimodal Characterization

Understanding the degradation of large format lithium‐ion pouch cells – critical for electric vehicle applications – is vital to extend their lifetime and allow potential second‐life application. Here, the impact on capacity fade and material degradation in two end‐of‐life cells, which were additionally subjected to accelerated aging to mimic extended use in second‐life applications, were examined using powder synchrotron X‐ray diffraction, Raman spectroscopy and electrochemical impedance spectroscopy, complemented by detailed post mortem analyses. The dominant mechanism of capacity loss under these conditions was found to be lithium inventory depletion, driven by processes such as electrolyte decomposition, lithium plating and solid electrolyte interphase growth. Structural changes in the graphite anode, including amorphization and reduced active material, were more pronounced under severe overcharging conditions. The blended cathode showed lithium inventory loss in both phases, but 92–94% capacity recovery was observed on subsequent cycling in half cells vs Li, illustrating its robustness, with little structural degradation observed. The finding that electrolyte degradation/loss in these cells was a more critical contributor to cell degradation toward the knee‐point than electrode active material degradation/loss indicates that increasing – or replenishing – the electrolyte content could be a strategy to extend the usable life of such cells.