Investigation of Structure, Ionic Conductivity, and Electrochemical Stability of Halogen Substitution in Solid-State Ion Conductor Li ₃ YBr _<i>x</i> Cl _{6– <i>x</i>}

Abstract

Li₃YX₆ (X = Cl, Br) materials are Li-ion conductors that can be used as solid electrolytes in all solid-state batteries. Solid electrolytes ideally have high ionic conductivity and (electro)chemical compatibility with the electrodes. It was proven that introducing Br to Li₃YCl₆ increases ionic conductivity but, according to thermodynamic calculations, should also reduce oxidative stability. In this paper, the trade-off between ionic conductivity and electrochemical stability in Li₃YBr _<i>x</i> Cl_6-<i>x</i> halogen-substituted compounds is investigated. The compositions of Li₃YBr_1.5Cl_4.5 and Li₃YBr_4.5Cl_1.5 are reported for the first time, along with a consistent analysis of the whole Li₃YBr _<i>x</i> Cl_6-<i>x</i> (<i>x</i> = 0-6) tie-line. The results show that, while Br-rich materials are more conductive (5.36 × 10^-3 S/cm at 30 °C for <i>x</i> = 4.5), the oxidative stability is lower (∼3 V compared to ∼3.5 V). Small Br content (<i>x</i> = 1.5) does not affect oxidative stability but substantially increases ionic conductivity compared to pristine Li₃YCl₆ (2.1 compared to 0.049 × 10^-3 S/cm at 30 °C). This work highlights that optimization of substitutions in the anion framework provide prolific and rational avenues for tailoring the properties of solid electrolytes.