Sodium-vanadium-phosphate-based materials have garnered significant interest as cathodes for high-rate sodium-ion batteries, owing to their stable framework, minimal volume change, thermodynamic stability, and excellent sodium storage capacity with fast ion transport kinetics¹. Furthermore, as these materials consist of both alkali and transition metal (TM) ions, which can exist in various oxidation states (V⁴⁺, V⁵⁺), these systems can exhibit the mixed ionic-polaronic conduction mechanism. Such feature has proven to be highly effective in facilitating the intercalation and deintercalation of alkali ions². Another crucial property of cathode materials is thermal stability which can be significantly enhanced by incorporating metal oxides such as Nb₂O₅³. Based on this premise, the current study focuses on investigating the electrical properties of glasses within the Na₂O-V₂O₅-Nb₂O₅-P₂O₅ system. The P₂O₅ component is gradually replaced by Nb₂O₅ while maintaining constant Na₂O and V₂O₅ content. By varying the concentration of V₂O₅ (10 and 25 mol%), the influence of its content on the electrical transport mechanism is examined, enabling the evaluation of its possible polaronic contribution. Solid-state impedance spectroscopy (SS-IS) is employed to examine electrical transport across a wide frequency (0.01 Hz to 1 MHz) and temperature (–90 °C to 240 °C) range and the conductivity spectra are studied in detail using two model-free scaling procedures, namely Summerfield and Sidebottom scaling. The successful construction of conductivity master curves for all glasses with lower V₂O₅ content (10 mol%) validates the time-temperature superposition (TTS) and confirms a purely ionic conduction mechanism, indicating that V₂O₅ does not contribute to electrical conductivity via a polaronic mechanism. However, master curves cannot be obtained for glasses with higher V₂O₅ (25 mol%) and low Nb₂O₅ content (0 and 5 mol%), suggesting the presence of mixed ionic-polaronic conductivity with a dominant polaronic contribution. Furthermore, with the addition of Nb₂O₅ above 10 mol%, the ionic conductivity mechanism prevails. The findings of this study provide valuable insights into the mixed-conductive glass system and role of V₂O₅ and/or Nb₂O₅, and demonstrate the ability to tune the mechanism of electrical conductivity by adjusting the content of oxide glass and its ratio.

ACKNOWLEDGMENTS
This work is supported by the CSF under the projects IP-2018-01-5425 and DOK-2021-02-9665.

1. Wang, C. et al. A multiphase sodium vanadium phosphate cathode material for high-rate sodium-ion batteries. J. Mater. Sci. Technol. 66, 121–127 (2021).
2. Wang, C. & Hong, J. Ionic/electronic conducting characteristics of LiFePO4 cathode materials. Electrochem. Solid-State Lett. 10, A65 (2007).
3. Getachew, B., Ramesh, K. P. & Honnavar, G. V. Nickel ferrite doped lithium substituted zinc and niobo vanadate glasses: thermal, physical, and electrical characterization. Mater. Res. Express 7, 095202 (2020).