Synthesis of metal sulfide compounds for solid-state electrolytes using metathesis reactions

Abstract

Solid-state batteries hold promise for improved energy density and safety for short-term storage or electric vehicles compared to conventional lithium-ion batteries. The most promising class of inorganic solid electrolytes are the sulfide-based materials due to their high lithium-ion conductivity and ease of processing. However, the cost of the requisite metal-sulfide precursors constrains the large-scale production of sulfide-based solid electrolytes. In this work, scalable approaches to synthesize precursors – in particular Li2S and SiS2 – from metathesis reactions of Na2S and metal salts are developed and applied to the synthesis of sulfide solid-state electrolytes. First, the production of the Na2S reagent was developed. It was found that anhydrous Na2S can be produced from purification of technical-grade Na2S hydrate flakes or synthesized directly. Low cost Na2S hydrate was purified by drying, then reacting with H2 gas at elevated temperatures. Alternatively, Na2S was formed by reacting H2S gas with a sodium methoxide solution. The H2S reagent was completely abated, and H2 was generated during the methoxide preparation from Na metal and methanol. The Na2S was recovered from solution by solvent evaporation. The resulting Na2S was characterized with a complementary suite of techniques showing purity similar, if not superior, to commercially obtained anhydrous Na2S. Next, Li2S metathesis was developed. In this process Na2S is reacted with LiCl to form a solution of Li2S and solid NaCl byproduct, with ethanol being the preferred solvent. Removal of the NaCl precipitate and evaporation of the supernatant yields Li2S that retains significant levels of solvent-related impurities. A slow, step-wise annealing process was devised to remove or decompose these impurities resulting in Li2S that was significantly purified but still retained residual levels of oxygenated impurities such as Li3OCl, Li2CO3, LiOH, and Li2O. The resulting Li2S was used to synthesize the argyrodite Li6PS5Cl, the prototype sulfide solid-state electrolyte. While Li2S impurities manifested as side-products in the final electrolyte, ionic conductivity was still similar to or better than electrolyte synthesized from commercially-available battery-grade Li2S, with room-temperature conductivities over 4 mS cm-1. Next, the mechanism of impurity formation in metathesis-derived Li2S was investigated. It was discovered that the impurities likely result from the thermal decomposition of ethoxide compounds that form as a result of the reaction of Na2S with ethanol, which proceeds in parallel with the intended metathesis reaction. With this mechanism in mind, several approaches to iv purify the metathesis Li2S were formulated. The optimal approach was to dry the Li2S material at 80 °C under an H2S environment, which resulted in removal of solvent impurities and retention of the desired nanocrystalline morphology which is lost at elevated annealing temperatures. Li6PS5Cl argyrodites synthesized from this approach exhibited phase-purity with state-of-the-art ionic and electronic conductivity (3.1 and 6.4•10-6 mS cm-1, respectively). Finally, the concept of cascaded metathesis was proposed and demonstrated. Li2S is a powerful metathesis reagent that can be used to synthesize nearly any metal sulfide from the appropriate metal chloride, including those that are unstable in protic solvents. When coupled to the first metathesis reaction, LiCl and the solvents are recycled and reused, resulting in metal sulfide synthesis from low cost Na2S and metal chloride salts. Cascaded metathesis was demonstrated through the first solution-based synthesis of SiS2. Li2S was reacted with SiCl4 in ethyl acetate to generate SiS2 and a LiCl byproduct, which precipitates from solution. The latter was then used to regenerate the Li2S reagent, and it was shown that over 90% of the lithium could be recovered and recycled along with the solvents employed to repeat a second reaction cycle. The metathesis-derived Li2S and SiS2 were then used to synthesize the glassy solid electrolyte 60Li2S•40SiS2, which exhibited an ionic conductivity of 0.11 mS cm-1 in good agreement with literature reports. In principle, cascaded metathesis could be used for the synthesis of nearly any metal sulfide, which are employed in numerous applications including energy storage/conversion, catalysis, opto-electronics, and solid-state lubricants.