Impurities in Na <sub>2</sub> S Precursor and Their Effect on the Synthesis of W‐Substituted Na <sub>3</sub> PS <sub>4</sub> : Enabling 20 mS cm <sup>−1</sup> Thiophosphate Electrolytes for Sodium Solid‐State Batteries
Felix Schnaubelt, Arpita Panda, Daniel Wagner, Maya Ziegler, Hoang Anh Dang, Wolfgang G. Zeier, Anja Bielefeld, Jürgen Janek
Abstract
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.