Electrochemical Nature of the Cathode Interface for a Solid-State Lithium-Ion Battery: Interface between LiCoO₂ and Garnet-Li₇La₃Zr₂O₁₂

Garnet-structured solid electrolytes have been extensively studied for a solid-state lithium rechargeable battery. Previous works have been mostly focused on the materials' development and basic electrochemical properties but not the cathode/electrolyte interface. Understanding the cathode interface is critical to enhance chemical stability and electrochemical performance of a solid-state battery cell. In this work, we studied thoroughly the cathode/electrolyte interface between LiCoO₂ and Li₇La₃Zr₂O₁₂ (LLZO). It was found that the high-temperature process to fuse LiCoO₂ and LLZO induced cross-diffusion of elements and formation of the tetragonal LLZO phase at the interface. These degradations affected electrochemical performance, especially the initial Coulombic efficiency and cycle life. In a clean cathode interface without the thermal process, an irreversible electrochemical decomposition at > ∼ 3.0 V vs Li⁺/Li was identified. The decomposition was able to be avoided by a surface modification of LLZO (e.g., Co-diffused surface layer and/or presence of an interlayer, Li₃BO₃), and the surface modification was equally important to suppress a reaction during air storage. In a LiCoO₂/LLZO interface, it is important to separate direct contacts between LiCoO₂ and pure LLZO.