Sustainable Production of Metal Sulfide Nanocrystals for Electrochemical Storage Applications via Cascaded Metathesis

Sulfide-based solid-state batteries are among the leading candidates to displace conventional lithium and sodium ion battery technologies. Metal sulfides (Li₂S, Na₂S, SiS₂, TiS₂, etc.) are critical precursors for both the solid state electrolytes and the conversion type cathodes employed in these devices. The high cost of these materials hinders development and deployment of these technologies. Their high cost reflects the high temperature carbothermal reduction and solid state synthesis techniques currently employed. Herein we describe the production of these materials employing room temperature, solution-based metathesis. In the first step Li₂S is produced through metathesis of two low-cost precursors – LiCl and Na₂S – at ambient temperature in an alcohol solution, with salt as a byproduct. Trace impurities in metathesis Li₂S introduced by side reactions with the solvent can be removed by a low temperature purification step that retains the desired nanocrystalline morphology. The quality of Li₂S was validated through the production of argyrodite (Li₆PS₅Cl) solid state electrolytes with state-of-the-art conductivity (&gt; 4 mS/cm). In the 1970s researchers at Exxon showed that Li₂S was a powerful reagent that can be used for the metathesis of transition metal sulfides (MoS₂, TiS₂ etc.), but practical implementation of this approach was limited by exorbitant cost and quality of the Li₂S precursor. These reactions are typically conducted in an aprotic solvent and driven by the precipitation of LiCl. Herein we couple these two reactions in a process we’ve named cascaded metathesis. The net reaction is <i>n</i>Na₂S + MCl_n → MS_n + <i>2n</i>NaCl. Both the lithium chloride intermediate and the solvents are recovered and recycled in a circular economy. We demonstrate this approach through the cascaded metathesis of SiS₂, which to our knowledge is the first time metathesis has been used to synthesize this compound. It is shown that ethyl acetate is the preferred solvent for conducting this reaction and that the reactivity of metathesis derived Li₂S is superior to its commercial counterpart. Finally, the metathesis-derived SiS₂ and Li₂S were employed as precursors in the synthesis of a glassy electrolyte that exhibited a conductivity of 0.11 mS cm^-1 at 30 °C with a low activation energy of 20.2 kJ mol^-1, validating the promise of this method to enable the synthesis of silicon-based sulfide electrolytes for solid-state lithium-ion batteries.