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

Abstract

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 Na3SO4F (NSOF) and magnesium-doped NSOF (Na2.98Mg0.01SO4F, 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 SO4 2– 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.

Effects of Sodium Vacancies and Concentrations in Na3SO4F Solid Electrolyte

“…Moreover, Na 3 SO 4 F undergoes a phase transition between low and high temperatures. After calculating the energy barriers of the Na 3 SO 4 F distinct phases using the bond valence site energy (BVSE) method, we discovered that the ion migration energy barrier in the high-temperature phase is higher than that in the low-temperature phase, so ions are not prone to movement . In order to further confirm the fact that vacancies are more important for increasing ionic conductivity, in situ XRD experiments were conducted on Na 3 SO 4 F and Na 2.96 Mg 0.02 SO 4 F within a temperature range of 30–300 °C, with a heating rate of 5 °C/min (Figure ).…”

“…Afterward, we evaluated NSOF and Na 2.96 Mg 0.02 SO 4 F using this in situ Raman spectroscopy technique to determine whether the polyanion SO 4 2– rotates at high temperatures (Figure S8). The v 1 mode (symmetric stretching vibration) and v 2 mode (bending vibration) are located at wavelengths of 997 and 467 cm –1 , respectively, while the v 4 mode (bending vibration) is located in the wavelength range of 622–646 cm –1 .…”

“…Codoping with Mg 2+ and Cl – has been identified as a promising strategy for increasing the ionic conductivity of NSOF according to the research . In a prior study, our research group assessed the phase stability and conduction properties of Na 3 SO 4 F and Na 2.98 Mg 0.01 SO 4 F through experimental analyses, BVSE calculations, and Raman spectroscopy results . We found that SO 4 2 – does not contribute to the paddle-wheel effect of Na + migration in NSOF and Na 2.98 Mg 0.01 SO 4 F, underscoring the significance of introducing vacancies.…”