Enhancement of Electrical Conductivity in Sodium Phosphate-based Glasses through Modifications with Nb2O5 and Al2O3

In pursuit of novel and cost-effective materials for the development of solid-state batteries as energy storage systems characterized by high safety, high energy density, and long cycling, sodium phosphate-based glasses have emerged as potential candidates for use as solid-state electrolytes1. However, simple binary sodium phosphate glass is limited by its poor chemical durability and low conductivity, nonetheless, these limitations can be easily overcome by changing its composition, e.g. by incorporating an intermediate oxide (Al2O3) and/or another (conditional) glass-forming oxide (Nb2O5) that strengthen the glass network by cross-linking phosphate groups via mixed P–O–M (M = Al, Nb) bonds2. What's more, the formation of a mixed network can have a beneficial effect on sodium ion transport, thereby increasing ionic conductivity, which is referred to as the mixed-glass forming effect (MGFE)3. The aim of this study is to investigate the effects of structural changes, brought by stepwise substitution of P2O5 by Nb2O5, on the electrical properties of two series of glasses from system Na2O–(Al2O3)–P2O5–Nb2O5. The glass-forming region (GFR) is determined by the conventional melt- quenching technique, and the amorphous character of the obtained glasses is confirmed by PXRD analysis. Partially crystallized samples formed at the edge of the GFR due to spontaneous crystallization at high Nb2O5 content are additionally qualitatively analysed. The thermal behaviour of prepared glass-(ceramics) is investigated by DTA, while their (micro)structural properties are evaluated by SEM-EDS, Raman, and IR-ATR spectroscopy. Solid-state impedance spectroscopy (SS-IS) over a broad frequency range (0.01 Hz to 1 MHz) and temperature range (–90 °C to 240 °C) is employed for detailed study of electrical transport. Findings of this research show a significant improvement in ionic conductivity, reaching a maximum at 20 mol% Nb2O5 in both glass series confirming the MGFE. The influence of structural changes induced by introduction of Nb2O5 and Al2O3 on the physical and chemical properties of these glass-(ceramics) is comprehensively studied and explained. This work is supported by the Croatian Science Foundation under the project IP-2018-01-5425. 1. S. Gandi et al., J. Power Sources 521 (2022) 230930. 2. Z. Teixeira et al., J. Am. Ceram. Soc. 90 (2007) 256–263. 3. S. Renka et al., Phys. Chem. Chem. Phys. 23 (2021) 9761–9772.