Preparation of Metal-Oxide-Doped Li₇P₂S₈Br_0.25I_0.75 Solid Electrolytes for All-Solid-State Lithium Batteries

The all-solid-state lithium battery (ASSB) has received great attention due to its greater safety than the conventional lithium-ion battery (LIB). Sulfide-based inorganic solid electrolytes are an important component to fabricate the ASSB. But to attain a better performance, the ionic conductivity and electrochemical stability of the sulfide-based solid electrolytes need to be improved. In this work, we prepared the metal-oxide-doped/mixed Li₇P₂S₈I_0.75Br_0.25 lithium superionic conductors by a dry ball-milling process. The high ionic conductivity was achieved by a low-temperature (200 °C) heat-treatment process. The metal-oxide-doped Li₇P₂S₈I_0.75Br_0.25 solid electrolyte exhibited a higher ionic conductivity value of 7.3 mS cm^-1 at room temperature than the bare Li₇P₂S₈I_0.75Br_0.25 solid electrolyte. The structural characteristics of the prepared solid electrolytes were studied by solid 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 metal oxide increased the electrochemical stability and dry-air stability of the Li₇P₂S₈I_0.75Br_0.25 solid electrolyte. The Ta₂O₅-doped Li₇P₂S₈I_0.75Br_0.25 solid electrolyte was stable even after 300 charge-discharge DC cycles and also 100 h of dry-air exposure. Further, the Ta₂O₅-doped Li₇P₂S₈I_0.75Br_0.25 solid electrolyte-based ASSB exhibited a high discharge capacity value of 184 mA h g^-1 at 0.1 C rate with 66% initial cycle Coulombic efficiency.