Unraveling the Origin of Moisture Stability of Halide Solid-State Electrolytes by In Situ and Operando Synchrotron X-ray Analytical Techniques

Recently, halide solid-state electrolytes (SSEs) have been reported to exhibit high ionic conductivity and good compatibility with cathode materials. However, the air stability of halide-based electrolytes is one important factor related to ionic conductivity upon exposure to air for practical applications. The instability mechanism of Li 3 InCl 6 toward air is not clearly understood. Herein, we for the first time report the application of operando optical microscopy, Raman spectroscopy, synchrotron-based X-ray powder diffraction, and in situ X-ray absorption near-edge structure for the study of halide electrolyte air stability. Using these methods, we have been able to track the degradation process of Li 3 InCl 6 exposed to air. It is for the first time found that Li 3 InCl 6 is hydrophilic in character, leading to the absorption of moisture from the air, and a portion of Li 3 InCl 6 reacts with absorbed H 2 O to form In 2 O 3, LiCl, and HCl. Moreover, the remaining electrolyte absorbs H 2 O to form a hydrate, Li 3 InCl 6 · x H 2 O. The reaction results in a decrease of ionic conductivity. Additionally, the influence of air stability on the practical application of Li 3 InCl 6 has been explored. Li 3 InCl 6 shows much better stability against air with low humidity (3%) and in battery dry rooms, making it a promising SSE for application in the commercial lithium-ion manufacturing industry.