Impurities in Na ₂ S Precursor and Their Effect on the Synthesis of W‐Substituted Na ₃ PS ₄ : Enabling 20 mS cm ⁻¹ Thiophosphate Electrolytes for Sodium Solid‐State Batteries

Abstract Sodium solid‐state batteries are intensively researched, expecting a resource‐uncritical alternative to their lithium counterparts. As in the case of lithium, sulfide‐type electrolytes show promising ionic conductivities σ , and Na 3 PS 4 ‐type solid electrolytes are intensively investigated. Aliovalent substitution of P 5+ by W 6+ is shown to achieve sodium ion conductivity σ (Na + ) beyond 10 mS cm −1 , rendering them good candidates for cathode composites. Yet, incorporating WS 4 2− into the crystal lattice of Na 3 PS 4 is deemed challenging, and Na 3− x P 1− x W x S 4 electrolytes suffer from WS 2 residue after synthesis. In this work, impurities in the precursor Na 2 S are identified and the detrimental influence of SO x groups in Na 2 S on the synthesis of Na 3 PS 4 and Na 3− x P 1− x W x S 4 is demonstrated. The behavior of oxygen as impurity during synthesis is pinpointed, and complete incorporation of tungsten up to x ≈ 0.25 in Na 3− x P 1− x W x S 4 by purified Na 2 S, realizing up to σ (Na + ) = 26.4 mS cm −1 at room temperature.