Na3–xEr1–xZrxCl₆—A Halide-Based Fast Sodium-Ion Conductor with Vacancy-Driven Ionic Transport

Driven by the rising demand for consumer electronics, the field of all solid-state batteries employing solid electrolytes as the ion-conducting separator has attracted enormous attention in the last years. Recently, the lithium-conducting rare-earth halides A3MX6 (A = Li, M = Y, Er, X = Cl, Br, I) and Li3InX6 have been rediscovered as potential solid electrolytes, showing a good overall electrochemical performance, while the corresponding sodium-based compounds have been mostly overlooked yet. Here, we report the ionic transport properties of the Na-ion conducting rare-earth halide solid electrolyte Na3–xEr1–xZrxCl6. Na3–xEr1–xZrxCl6 shows a conductivity enhancement from 10–9 S·cm–1 up to of ∼0.04 mS·cm–1 for Na2.4Er0.4Zr0.6Cl6, alongside interesting local structural rearrangements of the polyhedral motifs along the series of solid solutions. This series of halide-based sodium-ion conductors sheds light on promising compositions in search for superionic materials.