Mechanochemical synthesis of air-stable hexagonal Li4SnS4-based solid electrolytes containing LiI and Li3PS4

Abstract

Sulfide solid electrolytes with high ionic conductivity and high air stability must be developed for manufacturing sulfide all-solid-state batteries. Li₁₀GeP₂S₁₂-type and argyrodite-type solid electrolytes exhibit a high ionic conductivity of ∼10⁻² S cm⁻¹ at room temperature, while emitting toxic H₂S gas when exposed to air. We focused on hexagonal Li₄SnS₄ prepared by mechanochemical treatment because it comprises air-stable SnS₄ tetrahedra and shows higher ionic conductivity than orthorhombic Li₄SnS₄ prepared by solid-phase synthesis. Herein, to enhance the ionic conductivity of hexagonal Li₄SnS₄, LiI was added to Li₄SnS₄ by mechanochemical treatment. The ionic conductivity of 0.43LiI·0.57Li₄SnS₄ increased by 3.6 times compared with that of Li₄SnS₄. XRD patterns of Li₄SnS₄ with LiI showed peak-shifting to lower angles, indicating that introduction of I⁻, which has a large ionic radius, expanded the Li conduction paths. Furthermore, Li₃PS₄, which is the most air-stable in the Li₂S–P₂S₅ system and has higher ionic conductivity than Li₄SnS₄, was added to the LiI–Li₄SnS₄ system. We found that 0.37LiI·0.25Li₃PS₄·0.38Li₄SnS₄ sintered at 200 °C showed the highest ionic conductivity of 5.5 × 10⁻⁴ S cm⁻¹ at 30 °C in the hexagonal Li₄SnS₄-based solid electrolytes. The rate performance of an all-solid-state battery using 0.37LiI·0.25Li₃PS₄·0.38Li₄SnS₄ heated at 200 °C was higher than those obtained using Li₄SnS₄ and 0.43LiI·0.57Li₄SnS₄. In addition, it exhibited similar air stability to Li₄SnS₄ by formation of LiI·3H₂O in air. Therefore, addition of LiI and Li₃PS₄ to hexagonal Li₄SnS₄ by mechanochemical treatment is an effective way to enhance ionic conductivity without decreasing the air stability of Li₄SnS₄.