Superionic Conductors: Li10+δ[SnySi1–y]1+δP2−δS12 with a Li10GeP2S12-type Structure in the Li3PS4–Li4SnS4–Li4SiS4 Quasi-ternary System

Solid solutions of Sn–Si derivatives with an LGPS (Li10GeP2S12)-type structure are synthesized by a solid-state reaction in the Li3PS4–Li4SnS4–Li4SiS4 quasi-ternary system. The monophasic region of the LGPS-type structure deviates from the tie line between Li10SiP2S12 and Li10SnP2S12, and the composition of the solid solution is determined to be −0.1 ≤ δ ≤ 0.5 and 0 ≤ y ≤ 1.0 in Li10+δ[SnySi1–y]1+δP2−δS12 (0.50 ≤ x ≤ 0.7 and 0 ≤ y ≤ 1.0 in Li4–x[SnySi1–y]1–xPxS4). The solid solution is formed by a double substitution that changes the Sn/Si ratio and the M4+ (Sn4+ and Si4+)/P5+ ratio, which adjusts the sizes of the lithium conduction tunnels and the lithium concentration, and contributes to the optimal conductivity value. The highest ionic conductivity value of 1.1 × 10–2 S cm–1 is achieved for the composition of Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4) at 298 K, which is close to the value for the original LGPS compound (1.2 × 10–2 S cm–1). The Ge-free solid electrolyte could be suitable for practical applications in all-solid-state batteries.