Li3.6In7S11.8Cl: 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, Li3.6In7S11.8Cl, which exhibits high structural stability in the presence of water and air, is synthesized. At 25 °C, Li3.6In7S11.8Cl 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 Li3.6In7S11.8Cl 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 Li3.6In7S11.8Cl make it a preferable electrolyte candidate for high-performance ASSBs.