Exploring the effect of anion substitution on the solid ionic conductor NaTaCl6

Isovalent anion substitution has been shown to have a tremendous impact over the transport properties in lithium halide solid ionic conductors. Although sodium-ion solid state batteries based on chloride ionic conductors have recently gathered significant attention, investigations on anion substitution in sodium containing chlorides remain scarce. Here, we investigate the role of Br− isoelectronic anion substitution in the perovskite-related compound with nominal composition of NaTaCl6. The presence of a large amount of ordered cation vacancies in the A- and B-sites is suggested to favor the Br− accommodation allowing the complete solid solution in a sodium halide ionic conductor for the first time. Rietveld refinements against X-ray diffraction data show increasing unit cell volume, while nuclear magnetic resonance reveals the gradual changes in the Na-coordination with increasing Br− substitution. Analysis of impedance spectroscopy shows decreasing room-temperature ionic conductivities with Br− substitution revealing an opposite trend with respect the lithium chlorides solid ionic conductors. Structural factors are proposed to be behind the different Na+ transport evolution as a function of Br− content suggesting that special attention should be paid when well-explored principles from the Li+ chemistry are transferred to the design of Na+ halide solid electrolytes for Na+ solid-state batteries.