Oxygen Substitution for Li–Si–P–S–Cl Solid Electrolytes toward Purified Li10GeP2S12-Type Phase with Enhanced Electrochemical Stabilities for All-Solid-State Batteries

Li9.54Si1.74P1.44S11.7Cl0.3 (LSiPSCl), which exhibits a Li10GeP2S12 (LGPS)-type structure, presents the highest reported Li-ion conductivity for solid electrolytes, but the formation of a secondary phase and a limited electrochemical stability restricts its performance in all-solid-state cells. Herein, oxygen atoms were substituted into LSiPSCl, and a monophasic LGPS-type solid solution was obtained (Li9.54Si1.74P1.44S11.7–z Cl0.3O z , LSiPSClO z ; 0 < z ≤ 0.6). Compared with LSiPSCl, the oxygen-substituted sample showed an improved ionic conductivity (7.4 ± 0.2 mS cm–1) for its cold-pressed powder pellet containing both bulk and grain-boundary resistances. This trend is consistent with the bulk conductivities at 298 K (28 ± 3 mS cm–1) estimated from impedance measurements at low temperatures. The electrochemical stabilization effect of oxygen substitution was confirmed by the charge–discharge measurement for an all-solid-state cell using the oxygen-substituted material as a separator electrolyte, which exhibited improved cycling compared to the cell using the nonsubstituted phase. Oxygen substitution in the LGPS-type phase of the Li–Si–P–S–Cl system therefore provides a higher purity and enhances the electrochemical stability of all-solid-state batteries, suggesting that oxygen substitution could lead to stable LGPS-type superionic conductors in the halogen-substituted Li–M–P–S (M = Ge, Si, and Sn) system.