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).