Exploring the Effect of V(2)O(5) and Nb(2)O(5) 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, V(2)O(5) and Nb(2)O(5), on the thermal, (micro)structural, and electrical properties of the 35Na(2)O-25V(2)O(5)-(40 - x)P(2)O(5) - xNb(2)O(5) system is examined. The starting glass shows the highest values of DC conductivity, σ(DC), reaching 1.45 × 10(-8) Ω(-1) cm(-1) at 303 K, along with a glass transition temperature, T(g), of 371 °C. The incorporation of Nb(2)O(5) influences both σ(DC) and T(g), resulting in non-linear trends, with the lowest values observed for the glass with x = 20 mol%. Electron paramagnetic resonance measurements and vibrational spectroscopy results suggest that the observed non-monotonic trend in σ(DC) arises from a diminishing contribution of polaronic conductivity due to the decrease in the relative number of V(4+) ions and the introduction of Nb(2)O(5), 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 x < 10 mol%, whereas for x ≥ 10 mol%, the ionic conductivity mechanism becomes prominent. To assess the impact of the V(2)O(5) content on the electrical transport mechanism, a comparative analysis of two analogue series with varying V(2)O(5) content (10 and 25 mol%) is conducted to evaluate the extent of its polaronic contribution.