Material Search for a Li10GeP2S12-Type Solid Electrolyte in the Li–P–S–X (X = Br, I) System via Clarification of the Composition–Structure–Property Relationships

All-solid-state Li-metal batteries require fast Li-ion conductors that are compatible with Li-metal electrodes. Herein, we aim to obtain such Li-ion conductors in Li2S–P2S5–LiX (X = Br, I) systems, where new tetragonal phases with P42/nmc symmetry were formed at compositions of Li10P3S12Br (LPSBr) and Li10.25P3S12.25I0.75 (LPSI). Rietveld refinement analyses indicated that both materials were structural analogues of a renowned superionic conductor, Li10GeP2S12 (LGPS), with additional anions at 2a or 4c sites within LPSBr or LPSI, respectively. The LPSBr and LPSI phases exhibited ionic conductivities of 5.8(1) and 9.1(2) mS cm–1 at 300 K, respectively, with the latter having the highest conductivity among the reported Li–P–S–X (X = Br or I) systems. All-solid-state Li metal cells were prepared using LPSBr or LPSI as the separator to compare their charge–discharge cycle performances with those previously reported for Li cells using separator electrolytes with various chemical compositions. The most stable cyclability was observed for the Li cell using LPSBr, which exhibited a high ionic conductivity and phase purity, indicating that these two properties of the solid electrolyte are important for ensuring extended cycling of all-solid-state Li-metal batteries.