Reaction Suppression Between a High‐Ni Cathode Material (NMC622) and Li ₇ La ₃ Zr ₂ O ₁₂ on Co‐Sintering for Manufacturing Bulk‐Type All‐Solid‐State Batteries: A New Method and Its Mechanism

Co-sintering cathode materials with Li₇La₃Zr₂O₁₂ (LLZ) is a promising strategy for fabricating bulk-type all-solid-state batteries (ASSBs). However, preventing reactions between different materials, which is difficult with high-capacity cathode materials such as LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622), is a pre-requisite for applying this strategy. To overcome this issue, Li₁₊ _xNi_0.6Mn_0.2Co_0.2O₂ (x = 0.01-0.2), which intentionally deviates from the stoichiometric NMC622 composition, is synthesized here. The formation of impurity phases in the co-sintering process can be controlled by adjusting the co-sintering temperature and x. Impurity phases are not formed on co-sintering with x = 0.075 at 800 °C because reduced cation mixing in NMC622 and the presence of a self-formed Li₂CO₃ layer on the particle surface, ensured by adjusting x, effectively suppresses reactions. Furthermore, good results are observed at sintering temperatures where the proportions of Ni²⁺ and Co²⁺, which promote cation mixing, are low. This study clarifies relevant reaction mechanisms using various analytical methods (such as temperature-rise X-ray absorption fine structure analysis and scanning transmission electron microscopy-electron energy loss spectroscopy), and confirms the repetitive operation of bulk-type ASSBs assembled using co-sintered Li₁₊ _xNi_0.6Mn_0.2Co_0.2O₂ (x = 0.075)/LLZ electrolyte systems. The method reported herein can be potentially adopted for cost-effective and high-energy-capacity ASSB production.