The ultrafast ionic conductivity of Li ₆ PS ₅ Br, which is higher than 1 mS cm ^-1 at room temperature, makes it an attractive candidate electrolyte for the all-solid-state Li-S battery. A simple synthesis route with an easy scale up process is critical for practical applications. In this work, the highest room temperature ionic conductivity (2.58 × 10 ^-3 S cm ^-1 ) of Li ₆ PS ₅ Br is obtained by an optimal annealing temperature in a simple solid-state reaction method. Neutron diffraction and XRD show that the origin of the highest ionic conductivity is due to the higher purity, smaller mean lithium ion jumps and the optimal Br ordering over 4a and 4c sites. All-solid-state Li-S batteries using a S-C composite cathode in combination with the optimized Li ₆ PS ₅ Br electrolyte and Li-In anode show high (dis)charge capacities. Different cycling modes (charge-discharge and discharge-charge) reveal that the capacity of the S-C-Li ₆ PS ₅ Br/Li ₆ PS ₅ Br/Li-In battery arises from both the active S-C composite and the Li ₆ PS ₅ Br in the cathode mixture. The contribution of the latter is verified from all-solid-state batteries using Li ₆ PS ₅ Br and its analogues as active materials. Ex situ XRD and electrochemical performance results show that the contribution of capacity from Li ₆ PS ₅ Br in the cathode mixture may be associated with the decomposition product Li ₂ S, while the Li ₆ PS ₅ Br in the bulk solid electrolyte layer is stable during cycling.