Structural Distribution of Formed Oxysulfide Polyanion Units in Na ₃ PS _{4− <i>x</i>} O _<i>x</i> Glasses via Polysulfide Flux

All-solid-state sodium batteries have attracted considerable attention as next-generation batteries due to their safety and high energy density. Glassy solid electrolytes with Na 3 PS 4 −Na 3 PO 4 pseudo-binary compositions have been reported to combine the advantages of both sulfide and oxide materials. However, most previous studies have relied on mechanochemical processes for synthesis due to the difficulty of glass formation at high alkali content. In this work, we report the synthesis of Na 3 PS 4− x O x (0 ≦ x ≦ 1.0) by melt-quenching via Na 2 S x flux. By utilizing the flux, sulfur in PS 4 3– units was substituted by oxygen while maintaining the tetrahedral units, and the resulting increase in local structural complexity enhanced the glass-forming ability. The melt-quenched Na 3 PS 3 O glass, composed of PS 4− x O x 3– units, exhibited lower ionic conductivity and interfacial stability than the Na 3 PS 3 O glass synthesized via the mechanochemical process. Analysis of differences in a local structure arising from the synthesis methods revealed that the ball-milled samples exhibit sulfide-phase segregation, forming an ion-conduction pathway, whereas oxide addition enhances interfacial stability. These findings revealed that the synthesis process determines the structural distribution, including the ratios of polyanion units (PS 4 3–, PS 3 O 3–, PS 2 O 2 3–, and PSO 3 3– ), as well as arrangement in oxysulfide glass. The preferential distribution significantly affects ionic conductivity and interfacial stability.