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.