Rethinking glyme electrolytes in sodium batteries: from passive solvents to active reaction media.

Abstract

Sodium-based batteries are emerging as promising alternatives to lithium technologies owing to the abundance and low cost of sodium. However, their performance is strongly governed by electrolyte chemistry. This perspective examines the role of glyme-based electrolytes in sodium batteries, with particular emphasis on Na-O2 systems, where the electrolyte actively participates in electrochemical reactions. The effect of glyme chain length, sodium salt identity and salt concentration on the shape of Na⁺ solvation structures, ion pairing, mass transport, superoxide solubility and solid-electrolyte interphase formation are discussed. These interrelated effects control oxygen reduction and evolution kinetics, discharge product formation and long-term reversibility, highlighting the limitations of classical electrolyte descriptors based solely on conductivity or viscosity. The analysis shows that optimal electrolyte performance in Na-O2 batteries arises from a balance between solvation stability, transport properties and interfacial chemistry rather than from maximizing isolated bulk parameters. Comparisons with sodium-ion batteries further emphasize the broader relevance of coordination chemistry and desolvation processes. Finally, this perspective argues for a redefinition of what constitutes a 'good solvent' in reactive battery systems, integrating molecular-level solvation, interfacial reactivity, safety and sustainability considerations. Such an approach is essential for advancing efficient, durable and environmentally responsible sodium-based energy storage technologies. This article is part of the theme issue 'Electrolytes within the domain of electrochemistry and electrochemical energy storage'.