A multiscale X-ray CT study of degradation and safety behaviour of different Li-ion battery positive electrode morphologies.

While battery thermal failures are rare, improving our understanding of battery state-of-safety is essential to further reduce failure rates and support the energy transition. Although batteries undergo rigorous certification, the evolution of safety characteristics with cell ageing remains poorly understood. Here, electrochemical testing accelerating rate calorimetry (ARC), and multiscale X-ray computed tomography (CT) are combined to investigate how state-of-safety of lithium-ion pouch cells containing single-crystal or polycrystalline NMC cathode evolves with cycling. Comparable pouch cells were assessed before and after cycling to 80% capacity retention. The single-crystal containing cells gave an approximate 50% improvement in cycle life compared with polycrystalline cells. In the pristine state, ARC showed that the single-crystal cells had a lower self-heating onset temperature but also a lower peak temperature than polycrystalline cells, indicating lower thermal stability but less destructive failure. After cycling, the single-crystal cells showed higher self-heating onset temperature and lower peak temperatures. In contrast, the polycrystalline cells showed reduced self-heating onset temperatures and thermal runaway initiation temperatures indicating a loss of thermal stability. Multiscale X-ray CT provided mechanistic insight into these changes by linking safety behaviour to degradation across multiple length scales. Whole-cell imaging revealed greater gas generation, electrode displacement, and loss of layer contact in the polycrystalline cells, while high-resolution CT identified extensive particle cracking and fragmentation that was largely absent in the single-crystal material. These observations suggest that ageing-induced particle fracture increases the reactive cathode-electrolyte interfacial area, accelerating exothermic reactions during thermal abuse and reducing thermal stability. This work demonstrates how multiscale X-ray CT can reveal the mechanisms that influence the changes in of battery state-of-safety with cycling and highlights the potential safety advantages of single-crystal cathode architectures in aged lithium-ion batteries.