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

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.