Sodium Metal Oxyhalides Na<i>M</i>OCl₄ (<i>M</i> = Nb, Ta) with High Ionic Conductivities

Abstract

Halide-based ionic conductors have attracted growing interest as solid electrolyte candidates because of their suggested electrochemical oxidation stability and deformability. However, most of the discovered sodium metal halides exhibit relatively low ionic conductivities. To address this, a new class of mechanochemically stabilized, low-crystalline sodium metal oxyhalides NaMOCl4 (M = Nb, Ta) is developed. By using the combination of scanning electron microscopy–energy dispersive X-ray spectroscopy, X-ray diffraction, pair distribution function analysis, Raman spectroscopy, and nuclear magnetic resonance spectroscopy, we qualitatively explored the composition and local structure of these oxyhalides. Notably, NaNbOCl4 and NaTaOCl4 exhibit high ionic conductivities of 1.2 and 1.5 mS cm–1, respectively. Although the instability of NaMOCl4 against Na excludes their use as stand-alone separators in solid-state sodium metal batteries, the successful operation of a solid-state battery employing NaTaOCl4 as a catholyte at room temperature demonstrates that NaMOCl4 is a promising catholyte material.