Li_3.6In₇S_11.8Cl: an air- and moisture-stable superionic conductor

All-solid-state batteries (ASSBs) have drawn significant attention as future energy storage technologies. Sulfide-based solid electrolytes are promising due to their high ionic conductivity and favorable mechanical properties. However, their reactivity with moisture, leading to decomposition and release of toxic gases such as H₂S, poses health and safety risks. In this study, a superionic conductor, Li_3.6In₇S_11.8Cl, which exhibits high structural stability in the presence of water and air, is synthesized. At 25 °C, Li_3.6In₇S_11.8Cl delivers an ionic conductivity of 1.1 mS cm^-1, reaching 4.2 mS cm^-1 post-exposure to humid air. Multimodal investigations reveal that trapped water inside the Li_3.6In₇S_11.8Cl pellet facilitates ion conduction, which can be reversibly removed without compromising the structural integrity. The structure features a cubic-closed-packed anion sublattice with Li⁺ ions diffusing via a three-dimensional isotropic network, confirmed by ab initio molecular dynamics simulations. ⁶Li NMR and relaxometry identify the Wyckoff 16c and 8a as active Li⁺ sites for ion conduction. The high ionic conductivity, long-term stable cycling performance, and moisture stability of Li_3.6In₇S_11.8Cl make it a preferable electrolyte candidate for high-performance ASSBs.