Mechanism Study on the Interfacial Stability of a Lithium Garnet-Type Oxide Electrolyte against Cathode Materials

All-solid-state lithium-ion battery is considered to be one of the most promising next-generation battery technologies. Understanding the interfacial evolution of a solid electrolyte and a cathode electrode during mixing and sintering is of great importance and can provide guidance to avoid forming unwanted compounds and decrease the interfacial resistance. In this work, chemical compatibilities are investigated between a Ta-doped Li7La3Zr2O12 (LLZO) solid electrolyte and major commercial metal-oxide cathodes LiCoO2 (LCO) and Li(NiCoMn)1/3O2 (NCM) through ball-milling and cosintering processes. As revealed by X-ray absorption spectroscopy and transmission electron microscopy, LLZO spontaneously covers the majority of the large LCO and NCM particles with a thickness of ∼100 nm after ball milling. The thickness of LLZO layer on these cathodes decreases to about 10 nm after cosintering at 873 K, and an interfacial layer of approximately 3 nm is observed for NCM/LLZO. LCO shows a higher thermal stability than NCM. Density functional theory (DFT)-based simulations and electrochemical measurements suggest Ni–La and Ni–Li exchange could happen at the NCM/LLZO interface and Li can diffuse from the interface into NCM to occupy the Ni vacancy at high temperature. The Li depletion layer after diffusion at the interface induces the decomposition of LLZO and the formation of La2Zr2O7 and LaNiO3 interfacial layer.