KTaCl6: High-voltage stable potassium-ion conducting chloride solid electrolyte

The exceptional electrochemical oxidative stabilities of halide solid electrolytes (SEs) have led to extensive research on Li and Na all-solid-state batteries. In this study, we report a new K⁺ SE, cubic KTaCl₆, with a remarkable K⁺ conductivity of 1.0 × 10⁻⁵ S cm⁻¹, synthesized via a mechanochemical method. This value represents a 1000-fold enhancement over that of samples prepared through heat treatment, which is remarkable among halide K⁺ SEs reported to date. Through structural characterization via X-ray diffraction, Rietveld analysis, and bond valence energy landscape calculations, we reveal three-dimensional K⁺ migration pathways facilitated by face-sharing KCl₁₂¹¹⁻ cuboctahedra. This configuration is in contrast to that of the monoclinic KTaCl₆ produced through annealing, which features discontinuous K⁺ migration pathways. These pathways are formed by the edge- or corner-sharing of KCl₁₂¹¹⁻ anti-cuboctahedra, resulting in a significantly reduced K⁺ conductivity. Cyclic voltammetry measurements employing three-electrode cells indicate high electrochemical stability up to ≈3.7 V (vs. K/K⁺).