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 Li3InCl6 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 Li3InCl6 exposed to air. It is for the first time found that Li3InCl6 is hydrophilic in character, leading to the absorption of moisture from the air, and a portion of Li3InCl6 reacts with absorbed H2O to form In2O3, LiCl, and HCl. Moreover, the remaining electrolyte absorbs H2O to form a hydrate, Li3InCl6·xH2O. The reaction results in a decrease of ionic conductivity. Additionally, the influence of air stability on the practical application of Li3InCl6 has been explored. Li3InCl6 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.