Unraveling the Dominance of Structural Vacancies in Sodium Ion Conductivity in NaSOF.

Solid electrolytes are important materials for energy storage and conversion applications, and the coexistence of the paddle-wheel effect and vacancy diffusion mechanism is commonly observed in many solid electrolytes. However, the mechanism that significantly contributes to this remains unknown. To address this issue, we assess the phase stability and conduction properties of NaSOF (NSOF) and magnesium-doped NSOF (NaMgSOF, NMSOF). Our results reveal that incorporating Na vacancies in NSOF (i.e., NMSOF) leads to a significant increase in ionic conductivity, with a 2 order of magnitude difference compared to NSOF. The phase transition temperature of NMSOF is also significantly lower than that of NSOF, demonstrating the role of vacancies in enhancing the mobility of Na cations. Furthermore, Raman spectroscopy confirms that the polyanion SO rotation has a minor effect on the sodium conduction mechanism. Our study provides a fundamental understanding of the sodium conduction mechanism of polyanion-based sodium superionic conductors, including the impact of vacancies on Na conductivity.