A simple approach through reduction of Na 2 SO 4 to prepare high-purity Na 2 S 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 Na 2 S significantly affects the ionic conductivity and electrochemical properties of sulfide SSEs. In order to obtain high-purity and low-cost Na 2 S, a hydrogen reduction of industrial grade Na 2 SO 4 followed by ethanol purification has been successfully employed to synthesize and purify Na 2 S, which possesses a high purity of 98 %. Na 3 SbS 4 has been synthesized by as-synthesized Na 2 S (A-Na 2 S) and commercial Na 2 S (C–Na 2 S) by liquid-phase synthesis and solid-state reaction methods, respectively. The electrochemical properties indicate that Na 3 SbS 4 samples synthesized by A-Na 2 S 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 TiS 2 /Na 3 SbS 4 /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 Na 2 S at low cost, enabling the large-scale application of sulfide SSEs for ASSSBs.