Experimental and Theoretical Investigations on WO3-Substituted Na3PS4 Thiophosphate Solid Electrolyte with Enhanced Ionic Conductivity and Air Stability for All-Solid-State Sodium-Ion Batteries

All-solid-state sodium-ion batteries (ASSBs) have emerged as promising next-generation energy storage systems owing to their enhanced safety and thermal stability compared with conventional liquid-electrolyte batteries. Among thiophosphate-based solid electrolytes, Na3PS4 has attracted considerable attention because of its cold-press processability and relatively higher ionic conductivity than oxide-based electrolytes. However, its practical application remains limited by moderate ionic conductivity and poor air stability. In this study, pristine Na3PS4 (NPS) and WO3-substituted Na3PS4 (Na2.9P0.9W0.1S3.7O0.3; NPS-WO-10) solid electrolytes were synthesized through a combination of mechanical milling and low-temperature heat treatment. The substitution of W6+ at the P5+ sites induces sodium vacancies within the cubic Na3PS4 framework, thereby facilitating Na+ ion transport by reducing the migration barrier. The optimized composition, NPS-WO-10 (x = 0.10), exhibited an ionic conductivity of 0.94 mS cm–1 at 30 °C, which is approximately 4.7 times higher than that of pristine Na3PS4 (0.20 mS cm–1). Furthermore, the electronic conductivity remained in the range of 10–6–10–7 S cm–1, indicating its suitability as a solid electrolyte. Moisture stability tests conducted under dry-air conditions (15% relative humidity for 3 h) revealed that WO3 substitution significantly suppressed H2S gas evolution from 16 ppm for pristine NPS to 6 ppm, demonstrating enhanced air stability. An all-solid-state sodium-ion battery employing the NPS-WO-10 electrolyte, Na15Sn4 alloy anode, and NaNi1/3Fe1/3Mn1/3O2 composite cathode delivered an initial discharge capacity of 118 mAh g–1 within the voltage range of 2.3–4.25 V, with an initial Coulombic efficiency of 86.7%. These findings demonstrate that WO3 substitution is an effective strategy for simultaneously enhancing the ionic conductivity and moisture stability of Na3PS4-based solid electrolytes, highlighting their potential for practical ASSB applications.