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 degrees 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 P-31 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.