Synthesis-Controlled Cation Solubility in Solid Sodium Ion Conductors $Na_{2+x}Zr_{1–x}In_xCl_ 6$

Mechanochemically synthesized sodium halide solid solutions with the general formula Na2+xZr1–xMxCl6, as a class of potential catholytes, show promising ionic transport in comparison to their parental materials such as Na3YCl6. However, the influence of subsequent heat treatment protocols on the structure and transport properties of these materials is still not fully understood. In this work, a series of Na2+xZr1–xInxCl6 solid solutions are prepared by ball milling with subsequent annealing at different temperatures. X-ray diffraction analyses show a full indium solubility in Na2+xZr1–xInxCl6 when synthesized at low temperatures and crystallizing in the P21/n phase. In contrast, at higher heat treatment temperatures, exsolution is observed as the indium-rich Na2+xZr1–xInxCl6 compound tends to partially transform to the trigonal P3̅1c phase. By assessing the ionic conductivity of the differently synthesized Na2+xZr1–xInxCl6 series, we can show the synergistic effect of the Na+/vacancy ratio and crystallinity on sodium ion transport in this class of materials.