Halide doping effect on solvent-synthesized lithium argyrodites Li6PS5X (X= Cl, Br, I) superionic conductors

Halide-doped lithium argyrodites (Li6PS5X, X = Cl, Br or I) are one class of the most promising solid electrolyte candidates for solid-state Li batteries. However, similar to other superionic conductors, Li6PS5X argyrodites are mostly synthezied through high temperature solid-state reactions. Herein, we employ a solvent-based method to synthesize Li6PS5X argyrodites and study the halide doping (type and concentration) effect on their structure and properties. Using ethanol as the solvent medium, Li6PS5X and Li6PS5Cl·xLiCl (0 ≤ x ≤ 2) materials with precise composition control were achieved. The liquid-synthesized Li6PS5Cl showed the best ionic conductivity of 0.34 mS cm−1 at room temperature, followed by Li6PS5Br, with Li6PS5I being the worst. Interestingly, excess Cl content further increased the ionic conductivity of Li6PS5Cl·LiCl up to 0.53 mS cm−1 at room temperature and 5 mS cm−1 at 90 °C. In addition, the Li4Ti5O12/Li cell with Li6PS5Cl·LiCl solid electrolyte exhibited higher specific capacity (135 mAh g−1) than that of Li6PS5Cl-based cell (110 mAh g−1) at 0.2C. These results highlight a solvent-based synthesis method with precise composition control to study the halogen ion's effect on the structure and properties of lithium argyrodites, which would promote the development of lithium argyrodite solid electrolytes for applications in all-solid-state Li batteries.