A simple approach through reduction of Na2SO4 to prepare high-purity Na2S for sulfide electrolytes toward all-solid-state sodium batteries

Development of sulfide solid-state electrolytes (SSEs) with high ionic conductivity plays a leading role in achieving all-solid-state sodium batteries (ASSSBs). As an important raw material for synthesizing sulfide SSEs, the purity of Na2S significantly affects the ionic conductivity and electrochemical properties of sulfide SSEs. In order to obtain high-purity and low-cost Na2S, a hydrogen reduction of industrial grade Na2SO4 followed by ethanol purification has been successfully employed to synthesize and purify Na2S, which possesses a high purity of 98 %. Na3SbS4 has been synthesized by as-synthesized Na2S (A-Na2S) and commercial Na2S (C–Na2S) by liquid-phase synthesis and solid-state reaction methods, respectively. The electrochemical properties indicate that Na3SbS4 samples synthesized by A-Na2S possess higher ionic conductivity and lower Ea, and the assembled ASSSBs exhibit higher specific capacities, superior cycle and rate performance. At 0.5C, the assembled TiS2/Na3SbS4/NaSn ASSSB exhibits a high initial capacity of 211.4 mAh g−1 with capacity retention of 96.2 % after 360 cycles at room temperature. This work provides a simple approach for synthesizing high-purity Na2S at low cost, enabling the large-scale application of sulfide SSEs for ASSSBs.