Synthesis of Deliquescent Lithium Sulfide in Air

In the field of lithium-sulfur batteries (LSBs) and all-solid-state batteries, lithium sulfide (Li₂S) is a critical raw material. However, its practical application is greatly hindered by its high price due to its deliquescent property and production at high temperatures (above 700 °C) with carbon emission. Hereby, we report a new method of preparing Li₂S, in air and at low temperatures (∼200 °C), which presents enriched and surprising chemistry. The synthesis relies on the solid-state reaction between inexpensive and air-stable raw materials of lithium hydroxide (LiOH) and sulfur (S), where lithium sulfite (Li₂SO₃), lithium thiosulfate (Li₂S₂O₃), and water are three major byproducts. About 57% of lithium from LiOH is converted into Li₂S, corresponding to a material cost of ∼$64.9/kg_Li₂S, less than 10% of the commercial price. The success of conducting this water-producing reaction in air lies in three-fold: (1) Li₂S is stable with oxygen below 220 °C; (2) the use of excess S can prevent Li₂S from water attack, by forming lithium polysulfides (Li₂S); and (3) the byproduct water can be expelled out of the reaction system by the carrier gas and also absorbed by LiOH to form LiOH·H₂O. Two interesting and beneficial phenomena, i.e., the anti-hydrolysis of Li₂S and the decomposition of Li₂S₂O₃ to recover Li₂S, are explained with density functional theory computations. Furthermore, our homemade Li₂S (h-Li₂S) is at least comparable with the commercial Li₂S (c-Li₂S), when being tested as cathode materials for LSBs.