In this work, a structurally revivable, chloride-ion conducting solid electrolyte (SE), CsSn$_{0.9}$In$_{0.067}$Cl$_3$, with a high ionic conductivity of 3.45 × 10$^{−4}$ S cm$^{−1}$ at 25 °C is investigated. The impedance spectroscopy, density functional theory, solid-state $^{35}$Cl NMR, and electron paramagnetic resonance studies collectively reveal that the high Cl$^−$ ionic mobility originates in the flexibility of the structural building blocks, Sn/InCl$_6$ octahedra. The vacancy-dominated Cl$^−$ ion diffusion encompasses co-ordinated Sn/In(Cl) site displacements that depend on the exact stoichiometry, and are accompanied by changes in the local magnetic moments. Owing to these promising properties, the suitability of the CsSn$_{0.9}$In$_{0.067}$Cl$_3$, as an electrolyte is demonstrated by designing all-solid-state batteries, with different anodes and cathodes. The comparative investigation of interphases with Li, Li–In, Mg, and Ca anodes reveals different levels of reactivity and interphase formation. The CsSn$_{0.9}$In$_{0.067}$Cl$_3$ demonstrates an excellent humidity tolerance (up to 50% relative humidity) in ambient air, maintaining high structural integrity without compromises in ionic conductivity, which stands in contrast to commercial halide-based lithium conductors. The discovery of a halide perovskite conductor, with air processability and structure revival ability paves the way for the development of advanced air processable SEs, for next-generation batteries.
