Abstract

Solid-state batteries are considered as the next generation of energy storage devices because they have higher potential energy density and better safety than commercial lithium-ion batteries based on organic liquid electrolytes. However, solid-state electrolytes still have many problems to solve before they can be brought to the market. Most solid-state electrolytes are sensitive to air, resulting in complex and expensive cell assembly and vulnerable interfaces. Therefore, solid-state electrolytes are expected to be stable in the atmosphere, which will certainly bring significant benefits to the production of solid-state batteries. This review focuses on Li₄SnS₄, a sulfide electrolyte that is stable in moist air and soluble in water. The reactions between Li₄SnS₄ and H₂O and CO₂ molecules will be analyzed. Based on the understanding of the hydrolysis reaction of Li₄SnS₄, we summarize the solid electrolytes that are stabilized in air by using Li₄SnS₄ in their chemical composition. The HSAB theory indicates that the weak bonding between hard acid cations (e.g., P⁵⁺) and soft base anions (S^2-) in sulfides exacerbates the tendency towards hydrolysis. Introducing softer acid cations (e.g., Sn⁴⁺, Sb⁵⁺) enhances the stability of the internal structural units. Furthermore, optimizing conventional synthesis processes or developing novel synthesis methods significantly reduces material porosity and increases density, thus improving the intrinsic structural stability of SE crystals. However, even very small levels of CO₂ can initiate hydrolysis in sulfite-grouped SEs, which has significant implications for material handling and device fabrication under atmospheric conditions.