Tailored Ion Transport in Li_3-3yHo_1+yCl_6-xBr_x via Anion-Mixing and Lithium Vacancies

The growing demand for dependable energy storage solutions emphasizes the need for advancements in the field. All-solid-state lithium-ion batteries (ASSLBs) offer promising advantages such as higher energy density, lower costs, and improved safety. Halide solid electrolytes are noteworthy for their broad electrochemical stability window and compatibility with high-voltage cathodes. This study reports the halide series Li_3-3<i>y</i>Ho_1+<i>y</i>Cl_6-<i>x</i>Br<i>_x</i>(0 ≤ <i>x </i>≤ 3; 0 ≤ <i>y</i> ≤ 0.09), synthesized using a co-melting method, achieving a high ionic conductivity of~3.79 mS/cm at 25 ^oC, with a low activation energy of ~ 0.32 eV. High-resolution powder x-ray diffraction analyses reveal mixed-anion-induced phase transitions accompanied by enlarged bottlenecks for ion transport, increased vacancies, and favorable redistribution of Li⁺ ions, facilitating the creation of new energy-efficient migration pathways. Solid-state ^6,7Li nuclear magnetic resonance and relaxometry investigations unveil enhanced ion dynamics with bromination, achieving a Li⁺ motional rate neighboring 116 MHz. Bond valence site energy analysis sheds light on preferred Li⁺-ion transport pathways, particularly in structural planes devoid of Ho³⁺ blocking effects. These findings offer valuable insights into the intricate correlations between structure and ion transport, laying the foundation for designing high-performance fast ion conductors for ASSBs and diverse applications in energy storage, separation, and actuation.