Sulfide Solid-State Electrolytes with Li₂S Synthesized via Room Temperature Metathesis

All-solid-state lithium-ion batteries (ASSLIBs) are poised to play a critical role in the wider adoption of electric vehicles (EVs). Among the solid-state electrolyte (SSE) materials used in ASSLIBs, sulfide based ceramics and glasses provide high lithium-ion conductivity and superior ductility. However, a major drawback of sulfide SSEs is their high cost, which is dependent on the high cost of the key precursor lithium sulfide (Li₂S, ~$1,000 kg^-1). The high cost of Li₂S is due, in part, to the unfavorable synthesis methods used to produce it – most commonly carbothermal reduction of Li₂SO₄ or sulfurization of LiOH with H₂S gas. The carbothermal reduction process is energy-intensive, requiring temperatures on the order of 800-1,000 °C and directly emits CO₂. The sulfurization process requires the direct handling of extremely hazardous H₂S, and the reaction releases steam, which poses a corrosion hazard when combined with H₂S. Lithium metal has been a favorite Li₂S synthesis precursor in research labs, reacting easily with either elemental sulfur or H₂S, but the high cost and reactivity of lithium metal makes it difficult to scale-up these synthetic routes. Metathesis, or counter-ion exchange reactions, have been used to synthesize a variety of metal-sulfide compounds, but have not yet been reported for the synthesis of Li₂S. Herein, we report the production of Li₂S via metathesis of two low-cost precursors – LiCl and Na₂S – at ambient temperature in an alcohol solution. Reactive precipitation of sparingly soluble NaCl provides favorable energetics and facilitates product separation. The Li₂S is recovered from the solution using solvent evaporation and purified by subsequent annealing at temperatures <400 °C and with no handling of H₂S. A complementary suite of characterization techniques are employed to demonstrate the effect of processing on the impurity profile and particle morphology. Finally, metathesis Li₂S is employed as a reagent for synthesis of a leading SSE material, and its performance is compared to that of an SSE synthesized from commercial Li₂S.