Na_3–xEr_1–xZr_xCl₆—A Halide-Based Fast Sodium-Ion Conductor with Vacancy-Driven Ionic Transport

Abstract

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 A 3 MX 6 (A = Li, M = Y, Er, X = Cl, Br, I) and Li 3 InX 6 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 Na 3−x Er 1−x Zr x Cl 6 . Na 3−x Er 1−x Zr x Cl 6 shows a conductivity enhancement from 10 −9 S•cm −1 up to of ∼0.04 mS•cm −1 for Na 2.4 Er 0.4 Zr 0.6 Cl 6 , 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.

Exploring Aliovalent Substitutions in the Lithium Halide Superionic Conductor Li3–xIn1–xZrxCl6 (0 ≤ x ≤ 0.5)

“…The latter can lead to unexpected behavior and certain decoupling of polyhedral volumes from each other, as recently found in other alkaline rare-earth halides, Na3ErCl6, Li3ErCl6 and Li3YBr6. 29,48 These distortions within the structure can be rationalized based on the polyhedral changes (see Figure 5a). The height of the distorted MCl6 octahedra, representing the distance of the apical chloride positions (see inset Figure 5a), of the M2/Li4 site decreases, while the octahedron height of M1 site stays almost constant.…”

Exploring Aliovalent Substitutions in the Lithium Halide Superionic Conductor Li3-xIn1-xZrxCl6 (0 ≤ X ≤ 0.5)

“…xZrxCl6, Li3-xY1-xZrxCl6, LixScCl3+x, Li2.25Zr0.75Fe0.25Cl6 and sodium analogue Na3-xEr1-xZrxCl6. 26,27,33,48 However, here the enhancement of the conductivity may not only arise from the decrease of lithium ion concentration but also from the changes to the lithium substructure that includes a cation-site disorder on M2/Li4 site, leading to more Li + on this site further simplifying a three-dimensional diffusion.…”

On the underestimated influence of synthetic conditions in solid ionic conductors

“…Even in other compositions in these halide conductors, mechanochemical syntheses affect the ionic transport signicantly. 28,195,196 3.5 Solvent inuence on crystallization, phase formation and ionic conductivity in Li-P-S phases materials), solvent molecules strongly complex to form Li 3 PS 4 -$nsolv; the number of solvent molecules per unit varies but is typically 2-3. 56,86,197 The stability of these units seem to be such that, even when additional Li 2 S or LiX (X ¼ Cl, Br, I) are present, the predominant initial product contains the crystalline solvent complex, and stoichiometric products such as Li 7 P 3 S 11 or Li 6 PS 5 X are only achieved aer higher-temperature heat treatments in the solid-state.…”