A Structurally Flexible Halide Solid Electrolyte with High Ionic Conductivity and Air Processability

Abstract 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.