Mechanochemical synthesis of air-stable hexagonal Li 4 SnS 4 -based solid electrolytes containing LiI and Li 3 PS 4 .

Sulfide solid electrolytes with high ionic conductivity and high air stability must be developed for manufacturing sulfide all-solid-state batteries. Li 10 GeP 2 S 12 -type and argyrodite-type solid electrolytes exhibit a high ionic conductivity of ∼10 -2 S cm -1 at room temperature, while emitting toxic H 2 S gas when exposed to air. We focused on hexagonal Li 4 SnS 4 prepared by mechanochemical treatment because it comprises air-stable SnS 4 tetrahedra and shows higher ionic conductivity than orthorhombic Li 4 SnS 4 prepared by solid-phase synthesis. Herein, to enhance the ionic conductivity of hexagonal Li 4 SnS 4 , LiI was added to Li 4 SnS 4 by mechanochemical treatment. The ionic conductivity of 0.43LiI·0.57Li 4 SnS 4 increased by 3.6 times compared with that of Li 4 SnS 4 . XRD patterns of Li 4 SnS 4 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, Li 3 PS 4 , which is the most air-stable in the Li 2 S-P 2 S 5 system and has higher ionic conductivity than Li 4 SnS 4 , was added to the LiI-Li 4 SnS 4 system. We found that 0.37LiI·0.25Li 3 PS 4 ·0.38Li 4 SnS 4 sintered at 200 °C showed the highest ionic conductivity of 5.5 × 10 -4 S cm -1 at 30 °C in the hexagonal Li 4 SnS 4 -based solid electrolytes. The rate performance of an all-solid-state battery using 0.37LiI·0.25Li 3 PS 4 ·0.38Li 4 SnS 4 heated at 200 °C was higher than those obtained using Li 4 SnS 4 and 0.43LiI·0.57Li 4 SnS 4 . In addition, it exhibited similar air stability to Li 4 SnS 4 by formation of LiI·3H 2 O in air. Therefore, addition of LiI and Li 3 PS 4 to hexagonal Li 4 SnS 4 by mechanochemical treatment is an effective way to enhance ionic conductivity without decreasing the air stability of Li 4 SnS 4 .