Metal Halide Double Perovskite Fast Lithium Ion Conductors with a Unique Octahedral B-Site Vacancy Migration Mechanism

Although oxide- and sulfide-type fast Li-ion conductors employing various crystal structures have been extensively investigated over the past decades, the intrinsic disadvantages including the poor mechanical deformability of oxides and the narrow electrochemical window of sulfides limited their applications in all-solid-state lithium metal batteries. Metal halide-type fast Li-ion conductors have recently attracted intensive attention due to their prominent ductility, excellent interface compatibility, and wide electrochemical window. Here, we report a metal halide perovskite (HP)-type Li-ion conductor, Cs2LiYCl6, which crystallizes in a double perovskite structure, with Cs+ residing in the cuboctahedral spaces formed within the LiCl65– and YCl63– octahedron-connected frameworks. We unravel that this metal halide fast Li-ion conductor has a unique octahedral B-site vacancy Li-ion migration pathway, exhibiting an ionic conductivity of 0.015 mS cm–1 and a low Li-ion migration energy barrier of ∼0.40 eV. Our reported Cs2LiYCl6 lithium-ion conductor represents the first metal HP-type solid electrolyte, and the proposed B-site vacancy migration mechanism in this double perovskite structure will open a promising avenue for the exploration of metal HP-type Li-ion conductors.