Soft acid Bi³⁺ doping in Li₂ZrCl₆ to enhance ionic conductivity and electrochemical stability

All-solid-state lithium batteries (ASSLBs) are garnering increasing attention as a state-of-the-art energy storage technology. Halide solid electrolytes have emerged as a focal point of research in recent years, owing to their straightforward preparation methods and high ionic conductivity at room temperature. In this study, Bi-doped Li₂ZrCl₆ was synthesized using a high-energy ball milling method. The Bi³⁺ ion effectively replaces some of the Zr⁴⁺ ions. Due to its lower valence state and larger ionic radius, the concentration of Li⁺ in the electrolyte increases, and the lattice volume increases, which facilitates Li⁺ migration. Consequently, the ionic conductivity of modified Li_2.15Zr_0.85Bi_0.15Cl₆ at room temperature is 4.9 × 10^-4 S cm^-1, more than double that of Li₂ZrCl₆. This enhancement was further validated through cyclic voltammetry and wet air stability tests, which confirmed that Bi³⁺ doping not only significantly improves the electrochemical high-pressure oxidation stability of Li₂ZrCl₆ but also enhances its stability in humid air. Additionally, charge/discharge tests on ASSLB using Li_2.15Zr_0.85Bi_0.15Cl₆ as the electrolyte demonstrated that Bi doping enhances the electrochemical performance of Li₂ZrCl₆, leading to improvements in discharge specific capacity and cycle life.