Exploring the Effect of VO and NbO Content on the Structural, Thermal, and Electrical Characteristics of Sodium Phosphate Glasses and Glass-Ceramics.

Na-V-P-Nb-based materials have gained substantial recognition as cathode materials in high-rate sodium-ion batteries due to their unique properties and compositions, comprising both alkali and transition metal ions, which allow them to exhibit a mixed ionic-polaronic conduction mechanism. In this study, the impact of introducing two transition metal oxides, VO and NbO, on the thermal, (micro)structural, and electrical properties of the 35NaO-25VO-(40 - )PO - NbO system is examined. The starting glass shows the highest values of DC conductivity, , reaching 1.45 × 10 Ω cm at 303 K, along with a glass transition temperature, , of 371 °C. The incorporation of NbO influences both and , resulting in non-linear trends, with the lowest values observed for the glass with = 20 mol%. Electron paramagnetic resonance measurements and vibrational spectroscopy results suggest that the observed non-monotonic trend in arises from a diminishing contribution of polaronic conductivity due to the decrease in the relative number of V ions and the introduction of NbO, which disrupts the predominantly mixed vanadate-phosphate network within the starting glasses, consequently impeding polaronic transport. The mechanism of electrical transport is investigated using the model-free Summerfield scaling procedure, revealing the presence of mixed ionic-polaronic conductivity in glasses where < 10 mol%, whereas for ≥ 10 mol%, the ionic conductivity mechanism becomes prominent. To assess the impact of the VO content on the electrical transport mechanism, a comparative analysis of two analogue series with varying VO content (10 and 25 mol%) is conducted to evaluate the extent of its polaronic contribution.