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.