Structural and Chemical Compatibilities of Li1−xNi0.5Co0.2Mn0.3O2 Cathode Material with Garnet‐Type Solid Electrolyte for All‐Solid‐State Batteries

All-solid-state batteries (ASSBs) based on ceramic materials are considered a key technology for automobiles and energy storage systems owing to their high safety and stability. However, contact issues between the electrode and solid-electrolyte materials and undesired chemical reaction occurring at interfaces have hindered their development. Herein, the chemical compatibility and structural stability of composite mixtures of the layered cathode materials Li_{1- x} Ni_0.5 Co_0.2 Mn_0.3 O₂ (NCM523) with the garnet-type solid electrolyte Li_6.25 Ga_0.25 La₃ Zr₂ O₁₂ (LLZO-Ga) during high-temperature co-sintering under various gas flowing conditions are investigated. In situ high-temperature X-ray diffraction analysis of the composite materials reveals that Li diffusion from LLZO-Ga to NCM523 occurs at high temperature under synthetic air atmosphere, resulting in the decomposition of LLZO-Ga into La₂ Zr₂ O₇ and the recovery of charged NCM523 to the as-prepared state. The structural stability of the composite mixture at high temperature is further investigated under N₂ atmosphere, revealing that Li diffuses toward the opposite direction and involves the phase transition of LLZO-Ga from a cubic to tetragonal structure and the reduction of the NCM523 cathode to Ni metal. These findings provide insight into the structural stability of layered cathode and garnet-type solid-electrolyte composite materials and the design of stable interfaces between them via co-sintering for ASSBs.