Cost-effective synthesis of sulfide-based solid electrolytes without using Li2S

Solid electrolytes possess highly attractive potential which can improve safety properties and achieve high energy density when they replace the conventional liquid electrolytes in Li-ion batteries. Sulfide-based solid electrolytes have received considerable attention owing to their high ionic conductivities and good wettability due to their soft nature. These materials are typically synthesized using Li2S as a precursor. However, Li2S is expensive and thus cannot be widely used to prepare sulfide-based solid electrolytes for all-solid-state batteries (ASSBs). Here, we propose a novel approach to synthesize a sulfide-based solid electrolyte without using Li2S as a precursor. In this method, the starting powders of Li2O, Mg, and S were converted into Li2S and MgO by ball milling. The Li2S-MgO composite was then sequentially milled with P2S5 and LiBH4, and a BH4-substituted thiophosphate solid electrolyte was synthesized. The resulting Li3PS4-LiBH4-MgO composite exhibited a high ionic conductivity of 7.2 mS cm–1 at 25 °C, and the ASSB prepared using the solid electrolyte functioned well. The thiophosphate local structure of the prepared samples was examined by Raman and 31P solid-state nuclear magnetic resonance spectroscopies. Although the ionic conductivity slightly decreased upon MgO addition, the thiophosphate local structure was maintained. Moreover, the presence of MgO improved the chemical stability of the electrolyte in air.