Exploring Tetra-/Penta-/Hexavalent Ion Substitution in Yttrium-Based Halide Solid-State Electrolytes

Although aliovalent ion substitution is an important strategy for enhancing ionic conductivity in halide electrolytes, the choice of doping ions is often restricted to tetravalent ions, and investigations into the intrinsic origin of the doping mechanism are lacking. In this work, we investigated the effects of Zr⁴⁺, Ta⁵⁺ and W⁶⁺ doping on the crystal structure and ionic conductivity of yttrium-based rare-earth halides. Only Zr⁴⁺ achieves fast ion diffusion in both the (001) and (002) crystal planes by affecting the volume of the octahedron and the tetrahedral interstitial space, whereas Ta⁵⁺ significantly enhances the ion diffusion rate in the (001) crystal plane while suppressing it in the (002) plane, and W⁶⁺ does the opposite. As a result, an optimal ionic conductivity (0.437 mS cm^-1) is obtained for Zr⁴⁺ substitution, while the corresponding full battery also exhibits excellent capacity, cycling and rate performance.