Assessing the Impact of $Li^+$ Concentration and Stacking Faults in the Aliovalent-Substituted Ionic Conductor $Li_3ScCl_6$

Abstract

Halide electrolytes have gained interest due to their decent conductivities in the $mS·cm^{–1}$ range and wide electrochemical stability windows. The ionic transport can be influenced by changing the $Li^+$ concentration in the structure. Due to the high cost of the rare-earth elements in the halide electrolytes, the substitution of lower-cost elements is favored. Based on the idea of changing the $Li^+$ concentration and substituting with low-cost elements, the two substitution series $Li_{3–x}Sc_{1–x}Zr_xCl_6$ and $Li_{3–x}Sc_{1–x}Mg_xCl_6$ (0 ≤ $x$ ≤ 0.3) are investigated in this work. Structural information was obtained by X-ray and neutron diffraction and combined with transport properties obtained by impedance spectroscopy. Two main transport influencing factors were found: The $Li^+$ concentration and the c/a lattice parameter. The occupation of the $Li^+$-only layers seems to affect the lattice parameter in the c-direction. However, the structural refinement was not straightforward as stacking faults appear in layered halide materials that complicate the refinements, and the substitution with $Mg^{2+}$ seems to influence the extent of stacking fault formation. Overall, this work highlights the need to consider several factors in halide materials to correlate the structure–transport processes.