Preparation of highly conductive metal doped/substituted Li7P2S8Br(1-x)Ix type lithium superionic conductor for all-solid-state lithium battery applications

Sulfide-based lithium superionic conducting solid electrolytes for all-solid-state lithium batteries have received much attention due to their high safety, ionic conductivity, and compatibility. However, high-temperature treatment over a long time is needed to achieve high ionic conductivity. In this work, we have prepared the dual-halide-based Li7P2S8X type lithium superionic conductors by dry ball mill process and the high ionic conducting phase was achieved by low temperature (200celcius) heat-treatment process. High ionic conducting halogen-halogen composition was arrived at through a series of optimization processes. Among the compositions tested, the Li7P2S8Br0.25I0.75 solid electrolyte exhibited a high ionic conductivity value of 6.16 mS cm-1, at room temperature. The ionic radius of the halogen element played an important role in the formation of the high ionic conducting phase. Further, the ionic conductivity of Li7P2S8I0.75Br0.25 solid electrolyte was improved by metal doping at P-site. The Sn-doped Li7P2S8I0.75Br0.25 solid electrolyte exhibited a high ionic conductivity value of 7.78 mS cm- 1, at room temperature. The addition of large-sized Sn atoms extended the crystal lattice parameters and increased the Li-ions transport path. The partial substitution of Sn atoms at P-site was confirmed by NMR and Laser Raman analysis. The electrochemical stability of the prepared solid electrolyte was studied by cyclic voltammetry and DC charge-discharge analysis. The addition of a metal atom to the Li7P2S8Br0.25I0.75 solid electrolyte decreased the side reaction with the Li metals. Thus, low-temperature treated metal-doped Li7P2S8Br0.25I0.75 solid electrolytes are highly favourable for the fabrication of high-performance all-solid-state batteries. The fabricated all-solid-state battery using Sn doped Li7P2S8Br0.25I0.75 solid electrolyte exhibited the high specific capacity value of 170 mAh g-1 (0.1C), at room temperature.