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 - x Nb 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.