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, Li2S—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 Li2S and its simultaneous linking to the transformation to Li3PS4·3THF 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 S2– is reached. Without intermediate workup, P2S5 is added to the reaction vessel to form Li3PS4·3THF. 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 Li3PS4·3THF. After removing naphthalene, Li3PS4·3THF is utilized to synthesize promising sulfide-based electrolytes, such as β-Li3PS4 or Li6PS5Cl exhibiting typical Li+-ion conductivities of 0.1 and 1.9 mS/cm, respectively. β-Li3PS4 produced this way and β-Li3PS4 synthesized from commercial Li2S do not exhibit any difference in their performance in an all-solid-state battery.