Facile Synthesis toward the Optimal Structure-Conductivity Characteristics of the Argyrodite Li 6 PS 5 Cl Solid-State Electrolyte.

The high Li-ion conductivity of the argyrodite Li6PS5Cl makes it a promising solid electrolyte candidate for all-solid-state Li-ion batteries. For future application, it is essential to identify facile synthesis procedures and to relate the synthesis conditions to the solid electrolyte material performance. Here a simple optimized synthesis route is investigated that avoids intensive ball milling, by direct annealing of the mixed precursors at 550 oC for 10 hours, resulting in argyrodite Li6PS5Cl with a high Li-ion conductivity of up to 4.9610-3 Scm-1 at 26.2 oC. Both the temperature-dependent AC impedance conductivities and solid-state NMR spin-lattice relaxation rates demonstrate that the Li6PS5Cl prepared under these conditions results in a higher conductivity and Li-ion mobility compared to materials prepared by the traditional mechanical milling route. The origin of the improved conductivity appears to be a combination of the optimal local Cl structure and its homogeneous distribution in the material. All-solid-state cells consisting of an 80Li2S-20LiI cathode, the optimized Li6PS5Cl electrolyte, and an In anode showed a relatively good electrochemical performance with an initial discharge capacity of 662.6 mAhg-1 when a current density of 0.13 mAcm-2 was used, corresponding to a C-rate of approximately C/20. On direct comparison with a solid-state battery using a solid electrolyte prepared by the mechanical milling route, the battery made with the new material exhibits a higher initial discharge capacity and coulombic efficiency at a higher current density with better cycling stability. Nevertheless, the cycling stability is limited by the electrolyte stability, which is a major concern for these type of solid-state batteries.