Sodium Metal Oxyhalides NaMOCl₄ (M = Nb, Ta) with High Ionic Conductivities

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 Na M OCl 4 ( 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, NaNbOCl 4 and NaTaOCl 4 exhibit high ionic conductivities of 1.2 and 1.5 mS cm –1, respectively. Although the instability of Na M OCl 4 against Na excludes their use as stand-alone separators in solid-state sodium metal batteries, the successful operation of a solid-state battery employing NaTaOCl 4 as a catholyte at room temperature demonstrates that Na M OCl 4 is a promising catholyte material.