Tailoring Li6PS5Br ionic conductivity and understanding of its role in cathode mixtures for high performance all-solid-state Li–S batteries

Abstract

The ultrafast ionic conductivity of Li₆PS₅Br, 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 Li₆PS₅Br 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 Li₆PS₅Br 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–Li₆PS₅Br/Li₆PS₅Br/Li–In battery arises from both the active S–C composite and the Li₆PS₅Br in the cathode mixture. The contribution of the latter is verified from all-solid-state batteries using Li₆PS₅Br and its analogues as active materials. Ex situ XRD and electrochemical performance results show that the contribution of capacity from Li₆PS₅Br in the cathode mixture may be associated with the decomposition product Li₂S, while the Li₆PS₅Br in the bulk solid electrolyte layer is stable during cycling.