Influence of Defect-Induced Transition-Metal Valence State Changes on Ion Transport in Li ₃ YbCl ₆ Solid Electrolyte

A unique phenomenon in high-performance solid-state electrolytes of rare-earth halides is the strong dependence of ionic conductivity on the valence state anisotropy of the rare-earth elements, which potentially impedes ion transport. To probe the presence and effects of such anisotropy, the stable crystal structures, electronic structures, ionic conductivities, and Li-migration barriers in Li3YbCl6 ionic conductors with two space groups (Pna21/Pnma) have been investigated using a first-principles approach. The change in valence state is quantified by calculating the density-of-states integral of the transition metal Yb, and ion transport is investigated using first-principles molecular dynamics. Thus, a research framework was established to investigate the relationship between valence state changes and ion conductivity in this work. The results indicate that the Yb ions exhibit anisotropic valence states and are incompletely oxidized in Pnma-Li3YbCl6. In contrast, the Yb ions in Pna21-Li3YbCl6 possess a uniform valence state and are fully oxidized. The ionic conductivity of Pna21-Li3YbCl6 is higher than that of Pnma-Li3YbCl6. This demonstrates that the homogeneity of the transition-metal ion valence state is associated with higher ionic conductivity. To regulate this valence anisotropy, we systematically introduce Li vacancies into the worse-performing Pnma-Li3YbCl6 polymorph, constructing a Pnma-Li2.5YbCl6 structure that achieves a uniform Yb valence distribution. This valence homogenization strategy significantly enhances ionic conductivity by an order of magnitude and reduces the activation energy from 0.3 to 0.1 eV. The uniform valence state mitigates the heterogeneous electronegative blocking effect of Yb during Li+ migration, facilitating more efficient ion transport. These findings highlight the critical role of transition-metal valence uniformity in optimizing the performance of halide solid electrolytes.