Crystal Phase Engineering of Dual-Halogen Superionic Conductors via Anion Regulation

Abstract

Halide solid-state electrolytes (SSEs) have emerged as promising candidates owing to their excellent chemical oxidation stability, mechanical deformability, and good compatibility with oxide cathode materials. However, studies on the correlation between ionic conductivity and structural characteristics remain limited. Here, we introduce equivalent state halide anions bromide (Br⁻) into the Li₃YbCl₆ matrix through mechanical chemistry and heat treatment, and report the dynamic evolution of the crystal structure of Li₃YbCl_6-xBr_x (0 ≤ x ≤ 6). The significant change in the anion sublattice framework leads to the transformation of the initial hexagonal close-packed arrangement of Li₃YbCl₆ (space group: Pnma) to the cubic close-packed arrangement of Li₃YbCl_6-xBr_x (1 ≤ x ≤ 6) (space group: C2/m). Ab initio molecular dynamics (AIMD) and nudged elastic band (NEB) simulations investigate that within the highly symmetric monoclinic crystal system, the ion transport path along the c-axis changes from the octahedron-octahedron configuration to the octahedron-tetrahedron-octahedron configuration. This transformation effectively mitigates c-axis blockage caused by Li/Yb co-occupation and significantly enhances lithium-ion transport within the lattice. The results of this work highlight the complex relationship between structure and ionic transport mechanism and provide valuable insights into the ionic conduction mechanism of this class of halide electrolytes.