A Green Method of Synthesizing Battery-Grade Lithium Sulfide: Hydrogen Reduction of Lithium Sulfate

Lithium sulfide (Li 2 S) is a critical material for two systems of next-generation advanced lithium batteries. However, its practical applications are seriously impeded by its expensive price due to its troublesome storage and problematic production. Herein we report the synthesis of Li 2 S by thermally reducing lithium sulfate with hydrogen. Compared with the industrial approach of carbothermal reduction, this new method using a gaseous reductant is advantageous because of emitting zero amount of carbon oxides, having no solid byproducts or precursor residuals, generating only one environmentally benign and recyclable byproduct of water, and not requiring postsynthesis purification. Those features make this method cost-effective and easy for material storage. Moreover, the actual synthesis involves many parasitic reactions, which surprisingly do not cause real troubles for producing high-purity Li 2 S. Furthermore, compared with the commercial Li 2 S, the as-synthesized Li 2 S demonstrates superior performance when used as the cathode material in lithium–sulfur batteries and as the precursor to make sulfide solid electrolyte Li 6 PS 5 Cl for all-solid-state lithium batteries. Therefore, this green method enables paving the way for practical sustainable applications of Li 2 S.