Flow-Oriented Synthesis of Li₂S and Li₃PS₄·3THF: Opening Up a Completely Solvent-Based Solid Electrolyte Value Chain

Sulfide-based solid electrolytes are considered as the likeliest enabler of all-solid-state batteries. However, Li 2 S—the indispensable starting material for their preparation—is complicated to synthesize and thus extremely expensive. This work reports an innovative approach to the synthesis of Li 2 S and its simultaneous linking to the transformation to Li 3 PS 4 ·3THF in a sequential one-pot reaction. Lithium and sulfur are reacted in tetrahydrofuran (THF) with cheap naphthalene as an electron transfer agent. Mechanistically, naphthalene is reduced by lithium to its corresponding radical anion, which in turn reduces the sulfur species until S 2– is reached. Without intermediate workup, P 2 S 5 is added to the reaction vessel to form Li 3 PS 4 ·3THF. High loadings of naphthalene allow for a quick completion of the first step in <8 h, while they do not hamper the subsequent formation of Li 3 PS 4 ·3THF. After removing naphthalene, Li 3 PS 4 ·3THF is utilized to synthesize promising sulfide-based electrolytes, such as β-Li 3 PS 4 or Li 6 PS 5 Cl exhibiting typical Li + -ion conductivities of 0.1 and 1.9 mS/cm, respectively. β-Li 3 PS 4 produced this way and β-Li 3 PS 4 synthesized from commercial Li 2 S do not exhibit any difference in their performance in an all-solid-state battery.