Production and purification of anhydrous sodium sulfide

Anhydrous sodium sulfide (Na₂S) is a key component in sodium-sulfur batteries as well as an important chemical reagent. However, anhydrous Na₂S is currently prohibitively expensive for applications outside of research labs, and purity is a concern. Herein, we compare the properties of three forms of anhydrous Na₂S: (i) commercially supplied, (ii) Na₂S produced through dehydration and purification of commercial hydrate flakes (Na₂S•xH₂O), and (iii) Na₂S formed by the reaction of hydrogen sulfide with dissolved sodium alkoxide and recovered through solvent evaporation. Vacuum annealing of low-cost Na₂S hydrate at 150°C produced anhydrous Na₂S. This dehydrated material retains impurity signatures attributed to polysulfide (Na₂S_x) and oxysulfur impurities (SO_x) that were also observed in commercially supplied Na₂S. Impurity removal typically requires hydrogen reduction at very high temperature (700–900°C), but it is demonstrated here that this can instead be accomplished at 400°C, preventing auto-oxidation and following kinetics well-described by a shrinking core model. The solution-based approach resulted in the direct synthesis of crystalline Na₂S anhydride at low temperatures (100°C) without need for further purification. Both approaches presented herein are inherently scalable with materials costs that are one to two orders of magnitude lower than the current price of anhydrous Na₂S.