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