Tailoring Ion Transport in Li_3‐3yHo_1+yCl_6‐xBr_x via Transition‐Metal Free Structural Planes and Charge Carrier Distribution

Abstract Localized atomistic disorder in halide‐based solid electrolytes (SEs) can be leveraged to boost Li⁺ mobility. In this study, Li⁺ transport in structurally modified Li₃HoCl₆, via Br⁻ introduction and Li⁺ deficiency, is explored. The optimized Li_3‐3yHo_1+yCl_6‐xBr_x achieves an ionic conductivity of 3.8 mS cm⁻¹ at 25 °C, the highest reported for holmium halide materials. ^6,7Li nuclear magnetic resonance and relaxometry investigations unveil enhanced ion dynamics with bromination, attaining a Li⁺ motional rate neighboring 116 MHz. X‐ray diffraction analyses reveal mixed‐anion‐induced phase transitions with disproportionate octahedral expansions and distortions, creating Ho‐free planes with favorable energetics for Li⁺ migration. Bond valence site energy analysis highlights preferred Li⁺ transport pathways, particularly in structural planes devoid of Ho³⁺ blocking effects. Molecular dynamics simulations corroborate enhanced Li⁺ diffusion with Br⁻ introduction into Li₃HoCl₆. Li‐Ho electrostatic repulsions in the (001) plane presumably drive Li⁺ diffusion into the Ho‐free (002) layer, enabling rapid intraplanar Li⁺ motion and exchange between the 2d and 4h sites. Li_3‐3yHo_1+yCl_6‐xBr_x also demonstrates good battery cycling stability. These findings offer valuable insights into the intricate correlations between structure and ion transport and will help guide the design of high‐performance fast ion conductors for all‐solid‐state batteries.