Reassessment of Electrical and Dielectric Properties in the Borophosphate Glass System: A Promising Solid Electrolyte for High-Temperature Batteries

Abstract

This study investigates the conduction mechanism of ternary sodium borophosphate glass 30Na₂O-(70 - <i>x</i>)B₂O₃-<i>x</i>P₂O₅ with 0 ≤ <i>x</i> ≤ 35 mol % from a different perspective, focusing on previously unreported high-temperature electrical and dielectric properties for potential solid electrolytes in high-temperature batteries. The glass composition with B₂O₃/P₂O₅ = 1 exhibits a conductivity of approximately 10^-4 S/cm at 250 °C. Dielectric analysis supports this improved conduction, showing higher dielectric values and minimal energy dissipation during storage, indicating promising conductivity and favorable dielectric properties. This enhancement is attributed to the large-polaron (QMT) model, deduced from the power law exponent, due to the creation and spreading of lattice distortion of a long-range order with interconnected B⁴-O-P¹ and B⁴-O-P² linkages. Contrary to previous results, the glass transition temperature does not vary coherently with the conductivity and activation energy, displaying a discontinuity at 14 mol %. This discontinuity is caused by the initial extreme depolymerization of P₂O₅, leading to an increase in nonbridging oxygens (NBOs) within the glass network and forming B⁴-O-P⁰ linkages. Despite this, the ionic mobility of Na⁺ is continuously enhanced, correlated with the increase in the molar volume. This new perspective highlights the significant impact of both free volume expansion and reduced Coulombic effects on conduction improvement.