The Extent of Catholyte Gelation as a Critical Safety Factor in NMC‐Based Solid‐State Battery Design

Solid‐state batteries (SSBs) are considered a safer alternative to conventional lithium‐ion batteries due to the replacement of flammable liquid electrolytes with solid electrolytes. However, many practical SSB designs incorporate liquid catholytes to reduce interfacial resistance at the cathode. The presence of liquid components introduces potential safety risks during failure events, yet the influence of catholyte formulation on cell safety remains poorly understood. In particular, the effect of catholyte gelation on thermal runaway behavior has not been experimentally evaluated. Here, we investigate the safety characteristics of QSSBs containing catholytes with varying degrees of gelation using nail penetration testing. Tests were performed on 2 Ah pouch cells at 100% SOC, with the surface temperature and video recorded throughout. Cells containing liquid catholytes underwent violent thermal runaway, reaching temperatures exceeding 400°C. In contrast, cells incorporating fully gelled catholytes showed no thermal runaway, with temperatures remaining below 25°C. Intermediate levels of catholyte gelation did not produce intermediate safety responses, with partial gelation failing to significantly mitigate failure severity. These results demonstrate that catholyte gelation can dramatically alter the thermal runaway behavior of QSSBs under internal short‐circuit conditions and highlight the importance of electrolyte formulation when evaluating the safety of SSB systems.