Exploring Hindered Glymes as Electrolyte Solvents for Sodium-Oxygen Batteries: Impact on Electrochemical Performance and Discharge Product Stability

Abstract

Sodium-oxygen (Na-O2) batteries hold great promise for high-energy storage applications, yet their practical implementation is hindered by electrolyte instability and limited cycling performance. This study explores the impact of hindered glymes—ether-based solvents with bulky tert-butyl groups—on the electrochemical behaviour of Na-O2 batteries. By comparing conventional glymes with their hindered counterparts, discharge capacity, cycling stability, and electrolyte interactions are assessed. Hindered monoglymes show higher discharge and charge overpotentials but outperform conventional glymes in cycle life performance. Post-mortem analysis confirms sodium superoxide (NaO2) as the primary discharge product, though hindered glymes show a greater tendency to form hydrated sodium peroxide (Na2O2·2H2O), likely due to superoxide attack. Molecular dynamics simulations reveal that steric hindrance weakens Na⁺–solvent interactions, facilitating desolvation and improving cycling stability. In hindered glymes, the bulky tert-butyl groups reduce solvent flexibility and increase molecular rigidity, impairing complete Na⁺ solvation and limiting the stabilization of discharge products. This results in higher cell polarization and the formation of smaller NaO2 cubes during discharge. However, the enhanced Na⁺ desolvation in hindered glymes contributes to improved cycling performance, with hindered monoglyme showing a particularly pronounced effect due to its shorter chain, which increases the likelihood of TFSI⁻ entering the Na⁺ coordination environment. These findings highlight hindered glymes as a promising alternative to conventional solvents, optimizing stability, cycle life, and discharge product control in Na-O2 batteries.