Toward Achieving a High Ionic Conducting Halide Solid Electrolyte through Low-Cost Metal (Zr and Fe) and F Substitution and Their Admirable Performance in All-Solid-State Batteries

Recently, the halide solid electrolyte (SE) system has been widely used in lithium solid-state batteries due to their specific properties, such as the high electrochemical stability window that prevents any side reaction with the electrode/electrolyte interface. Conspicuously, the halide SE possesses very low ionic conductivity values in the range (0.2-0.5) mS cm^-1. In this work, we enhance the ionic conductivity of Li₃YCl₆ SE by the substitution of low-cost Fe and Zr elements on the Y-site and F on the Cl site, in which the electrolyte is prepared through high-energy ball milling without a heat treatment process. The structural analysis reveals that the prepared halide SEs showed the pure phase of the Li₃YCl₆ tetragonal crystal structure and were free from impurity phases. In the prepared composition, the Li_2.4Y_0.4Zr_0.6Cl₆ and Li_2.4Y_0.4Zr_0.6Cl_5.85F_0.15 electrolyte exhibited a higher ionic conductivity of 2.05 and 1.45 mS cm^-1, respectively, than Li₃YCl₆ (0.26 mS cm^-1). Interestingly, the Li_2.4Y_0.4Zr_0.6Cl_5.85F_0.15 electrolyte possesses a better electrochemical stability window of 1.29-3.9 V than Li_2.4Y_0.4Zr_0.6Cl₆ (2.1-3.79 V). Moreover, the electrochemical results revealed that the assembled solid-state battery using Li_2.4Y_0.4Zr_0.6Cl₆ and Li_2.4Y_0.4Zr_0.6Cl_5.85F_0.15 electrolyte demonstrated the higher initial Coulombic efficiency of 84.7 and 87%, respectively, than Li₃YCl₆ of 82.6%. We consider Li_2.4Y_0.4Zr_0.6Cl_5.85F_0.15 to be an important electrolyte candidate in all-solid-state batteries.