Effects of Halogen and Sulfur Mixing on Lithium-Ion Conductivity in Li7–x–y(PS4)(S2–x–yClxBry) Argyrodite and the Mechanism for Enhanced Lithium Conduction

All-solid-state lithium-ion batteries containing phosphorus sulfides are promising next-generation batteries because they have higher energy densities than their liquid-electrolyte counterparts. Halogen-rich argyrodites, Li7−α(PS4)(S2−αXα) (X = Cl, Br, I; α > 1), exhibit higher ionic conductivities than other phosphorus sulfides; varying the content of a single anion enables halogen substitution. However, there is no appropriate model for the ion-conducting path in these argyrodites. Herein, we discuss a range of composition maps to explain the lithium-ion conductivity of Li7–x–y(PS4)(S2–x–yClxBry) (x + y > 1). The ionic conductivities (∼11.6 mS/cm) of Li5.4(PS4)(S0.4Cl1.0Br0.6) and Li5.4(PS4)(S0.4Cl0.6Br1.0) are among the highest reported. Bond valence sum mapping and maximum entropy methods for Li7–x–y(PS4)(S2–x–yClxBry) (x + y > 1) were used. The results showed that these argyrodites have a lithium-cage structure surrounding the halide or sulfide ions at the 4d sites and a lithium-ion-conduction path between the cages via an interstitial site (16e). The degree of disorder of chloride, bromide, and sulfide ions on the 4a and 4d sites controls the conduction path length between the cages; a short lithium-ion-conduction path between the cages enhances lithium-ion conductivity. These results could inspire the development of halogen-rich compositions of Li7−α(PS4)(S2−αXα) (α > 1) with enhanced ionic conductivities.