Utilizing reactive polysulfides flux Na2S for the synthesis of sulfide solid electrolytes for all-solid-state sodium batteries

Sulfide solid electrolytes are attractive for the development of all-solid-state sodium batteries owing to their high ionic conductivities and formability. While the performance related to ionic conductivity and additional functions is strongly influenced by composition and crystal polymorphs, the use of delicate and less stable sulfide starting materials limits their variability. In this study, we propose a universal and convenient synthesis method for sulfide solid electrolytes utilizing Na2Sx polysulfide flux as a stoichiometric reactant. These polysulfides mitigate synthetic constraints by eliminating the requirement for unstable sulfide starting materials. They efficiently react with raw materials, including single elements, under ambient pressure, without the necessity for a sealed reaction vessel. Na3BS3 glass, and Na2.88Sb0.88W0.12S4 solid electrolytes was prepared by using Na2Sx working as lowly volatile flux to prepare. Na2S and S immediately reacted with each other without sulfur vaporization, and stoichiometric Na2Sx liquid was formed at temperatures above 500 °C. The Na2Sx flux itself oxidized B, Sb, and W uniformly at moderate temperatures, and stoichiometric electrolytes were obtained in a one-step process without sealed vessels and a vacuum condition. In addition, Na3BS3 glass was produced by direct quenching at 700ºC. The obtained Na2.88Sb0.88W0.12S demonstrated the extremely high ionic conductivity of 125 mS cm−1 at 25ºC, due to the stoichiometric substitution of WS42−.