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.