Fast Ion Conduction and Its Origin in Li6–xPS5–xBr1+x

High ionic conductivity of solid electrolytes is key to achieving high-power all-solid-state rechargeable batteries. The superionic argyrodite family is among the most conductive Li-ion conductors. However, their potential in ionic conductivity and stability is far from being reached, especially with Li6PS5Br. Here, we synthesized Li6–xPS5–xBr1+x with increased site mixing of Br–/S2–. An ionic conductivity of 11 mS cm–1 at 25 °C is achieved with a low activation energy of 0.18 eV for Li5.3PS4.3Br1.7. The influence of Br–/S2– mixing on ion conduction is systematically investigated with multinuclear solid-state NMR coupled with X-ray diffraction and impedance spectroscopy. A statistically random distribution of Br– and S2– at 4d sites is observed with 31P NMR. The resulting local structures regulate the jump rates of their neighboring Li ions and Li redistribution. As a result, the increased Li+ occupancy at 24g sites promotes fast ion conduction, and the role of Li (24g) in ion conduction has been elucidated with tracer-exchange NMR. Experimental evidence combined with density functional theory calculations has revealed that the particular arrangement of 1S3Br at 4d sites near Li maximizes overall Li+ conduction. This insight applies to other argyrodites and will be useful to the design of new fast ion conductors.