Synthesis of high-purity Li2S nanocrystals via metathesis for solid-state electrolyte applications

Abstract

Li₂S is the key precursor for synthesizing thio-LISICON electrolytes employed in solid state batteries. However, conventional synthesis techniques such as carbothermal reduction of Li₂SO₄ aren't suitable for the generation of low-cost, high-purity Li₂S. Metathesis, in which LiCl is reacted with Na₂S in ethanol, is a scalable synthesis method conducted at ambient conditions. The NaCl byproduct is separated from the resulting Li₂S solution, and the solvent is removed by evaporation and thermal annealing. However, the annealing process reveals the presence of oxygenated impurities in metathesis Li₂S that are not usually observed when recovering Li₂S from ethanol. In this work we investigate the underlying mechanism of impurity formation, finding that they likely derive from the decomposition of alkoxide species that originate from the alcoholysis of the Na₂S reagent. With this mechanism in mind, several strategies to improve Li₂S purity are explored. In particular, drying the metathesis Li₂S under H₂S at low temperature was most effective, resulting in high-purity Li₂S while retaining a beneficial nanocrystal morphology (∼10 nm). Argyrodite electrolytes synthesized from this material exhibited essentially identical phase purity, ionic conductivity (3.1 mS cm⁻¹), activation energy (0.19 eV), and electronic conductivity (6.4 × 10⁻⁶ mS cm⁻¹) as that synthesized from commercially available battery-grade Li₂S.