Crack-Mediated Degradation in Low-Temperature Co-Sintered Garnet-Based Bulk All-Solid-State Batteries

Oxide-based all-solid-state batteries (ASSBs) are considered promising next-generation batteries; however, the degradation mechanisms during cycling remain insufficiently understood. This study combined experimental and computational approaches to investigate the degradation of ASSBs fabricated through low-temperature co-sintering of LiCoO₂ and Li 7 La 3 Zr 2 O 12 (LLZ). No clear evidence of elemental interdiffusion or oxygen-vacancy-driven chemical degradation at the LiCoO₂/LLZ interface was detected in the analyzed regions. By contrast, electrochemical, microstructural, and simulation results indicate that crack-induced pathway disruption is the dominant degradation mode. Near the cathode–electrolyte interface, cracks are formed between LLZ particles and LiCoO 2 /LLZ interface, whereas near the current collector, cracks occur between particles within LiCoO₂. This crack formation behavior was corroborated by multiphysics analysis. During cycling, LiCoO₂ expansion places it mainly under compression and LLZ mainly under tension, consistent with the different crack patterns observed after cycling. The simulation indicates that tensile stress is the highest near the cathode–electrolyte interface, where cracks are most likely to start during early cycling. The crack propagates through regions with weak interparticle bonding caused by insufficient amounts of the sintering aid (Li₃BO₃), leading to an increase in the grain boundary resistance of LLZ as well as the interfacial resistance between LiCoO₂ and LLZ. The evaluated battery showed resistance increasing by 106.3 mΩ·cm² after 60 cycles. The increase in grain boundary resistance and interface resistance accounted for 50.2% and 35.6% of the total increase, respectively, making these the dominant factors. Therefore, avoiding the formation and propagation of these cracks is crucial for improving the cycle characteristics.