Study of LiCoO₂/Li₇La₃Zr₂O₁₂:Ta Interface Degradation in All-Solid-State Lithium Batteries

The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) ceramic solid electrolyte combines high Li-ion conductivity at room temperature with high chemical stability. Several all-solid-state Li batteries featuring the LLZO electrolyte and the LiCoO₂ (LCO) or LiCoO₂-LLZO composite cathode were demonstrated. However, all batteries exhibit rapid capacity fading during cycling, which is often attributed to the formation of cracks due to volume expansion and the contraction of LCO. Excluding the possibility of mechanical failure due to crack formation between the LiCoO₂/LLZO interface, a detailed investigation of the LiCoO₂/LLZO interface before and after cycling clearly demonstrated cation diffusion between LiCoO₂ and the LLZO. This electrochemically driven cation diffusion during cycling causes the formation of an amorphous secondary phase interlayer with high impedance, leading to the observed capacity fading. Furthermore, thermodynamic analysis using density functional theory confirms the possibility of low- or non-conducting secondary phases forming during cycling and offers an additional explanation for the observed capacity fading. Understanding the presented degradation paves the way to increase the cycling stability of garnet-based all-solid-state Li batteries.