Synthesis and transport properties of the brannerite-type oxides Na_1-xV_1-xMo_1+xO₆

Widespread commercialization of sodium-ion batteries (SIB) is limited by the shortcomings of existing electrode materials, so the search and testing of various sodium compounds suitable for SIB are relevant. This paper presents the results of a study of the sodium diffusion mechanisms in quasi-layered oxides Na_1-xV_1-xMo_1+xO₆, which are potentially promising for applications for SIB. A simple synthesis procedure has been developed, which makes it possible to obtain compounds in a wide range of compositions up to x = 0.2. To elucidate the mechanisms of sodium diffusion, we applied a comprehensive approach that combines material characterization at the “macro” (XRD, impedance spectroscopy) and “atomic-scale” levels (NMR, ab-initio calculations). Our results reveal rather fast sodium dynamics: Ionic conductivity reaches the values of 10^–3 S/cm at T > 730 K. It has been found moreover that the diffusion mechanism changes with increasing temperature. At T < 625 K, sodium motion occurs mainly along the crystallographic b axis due to atomic jumps with the shortest jump length ≈ 3.6 Å and activation energy E_a ~ 1 eV. With increasing temperature, another type of jumps along a axis (in the ab plane) with a jump length of ≈ 5 Å and a barrier value of 2 eV is also activated.