Enhanced Electrochemical Stability and Moisture Reactivity of Al ₂ S ₃ Doped Argyrodite Solid Electrolyte

All-solid-state lithium-ion batteries are considered a promising next-generation lithium-ion batteries for their safety and high energy density. Among solid electrolytes, sulfide-based solid electrolytes have received attention from academia for their good contact properties such as mechanical deformability and high ionic conductivity. Specifically, argyrodite, which has Li 6 PS 5 X (X = Cl, Br, I) structure, shows a high lithium ionic conductivity (>10 −3 S cm −1 ) at room temperature. However, sulfide-based solid electrolytes are chemically unstable in reaction to lithium metal and moisture because of the high reactivity of sulfur. Also, sulfide-based solid electrolytes react with active materials on the interface in composite cathodes. Moreover, this insufficient electrochemical stability of sulfide-based solid electrolytes can deteriorate the cell performance such as discharge capacity in prolonged cycles. We investigated the effects of partial substitution of Al 2 S 3 for P 2 S 5 to form the Al-S bonding instead of the P-S bonding in the PS 4 3− main unit of the Li 6 PS 5 X structure. The Al-S bonding is electrochemically more stable than the P-S bonding due to a larger electronegativity difference between the two atoms. We show that the Al 2 S 3 substitution in an argyrodite structure used for the composite cathode significantly improves the electrochemical stability.