Air Sensitivity and Degradation Evolution of Halide Solid State Electrolytes upon Exposure

Abstract Halide solid state electrolytes (SSEs) have attracted the attention of researchers as a new family of SSEs due to simple synthesis, high ionic conductivity, and good softness. However, until now, most of the reported works are focused on promotion of ionic conductivity, and little attention is paid to their air stability and degradation mechanism upon exposure. Herein, the degradation evolution of typical halide SSEs upon moisture is investigated in detail. It is found both Li 3 InCl 6 and Li 3 YCl 6 halide SSEs are easy to absorb water and deteriorate, and the air sensitivity is closely related to the contact area with air. In comparison, the water absorption rate of Li 3 InCl 6 is faster than that of Li 3 YCl 6 , while the amount of water absorption of Li 3 YCl 6 is larger than that of Li 3 InCl 6 , due to the higher solubility of InCl 3 compared to YCl 3 . Along with water absorption, Li 3 InCl 6 first forms a crystalline hydrate, then partially decomposes to InCl 3 and LiCl, and InCl 3 further hydrolyzes and produces acid which is corrosive; finally In 2 O 3 impurities are formed. Coating the surface of Li 3 InCl 6 with Al 2 O 3 can effectively improve the air stability. This work can help to understand the degradation mechanism of halide SSEs and provide guidance for the future design of new halide SSEs.