Unraveling the Dominance of Structural Vacancies in Sodium Ion Conductivity in Na₃SO₄F

Solid electrolytes are important materials for energystorage andconversion applications, and the coexistence of the paddle-wheel effectand vacancy diffusion mechanism is commonly observed in many solidelectrolytes. However, the mechanism that significantly contributesto this remains unknown. To address this issue, we assess the phasestability and conduction properties of Na3SO4F (NSOF) and magnesium-doped NSOF (Na2.98Mg0.01SO4F, NMSOF). Our results reveal that incorporating Navacancies in NSOF (i.e., NMSOF) leads to a significant increase inionic conductivity, with a 2 order of magnitude difference comparedto NSOF. The phase transition temperature of NMSOF is also significantlylower than that of NSOF, demonstrating the role of vacancies in enhancingthe mobility of Na cations. Furthermore, Raman spectroscopy confirmsthat the polyanion SO4 (2-) rotation hasa minor effect on the sodium conduction mechanism. Our study providesa fundamental understanding of the sodium conduction mechanism ofpolyanion-based sodium superionic conductors, including the impactof vacancies on Na conductivity.