A weakly solvating solvent-based quasi-solid electrolyte for sodium metal batteries

Abstract

Sodium-ion batteries represent a more sustainable and, potentially, cost-effective alternative to lithium-ion technology, with sodium–metal anodes showing promise for achieving high-energy densities. However, the strong reactivity between sodium–metal and conventional liquid electrolytes leads to unstable solid electrolyte interphases, undermining battery performance and safety. To address this challenge, this work introduces a novel weakly solvating quasi-solid electrolyte. This electrolyte is fabricated via in situ polymerization of polyethylene glycol diacrylate with sodium bis(fluorosulfonyl)imide in a mixed solvent system of 2-methyltetrahydrofuran and cyclopentyl methyl ether, which enables targeted manipulation of the solvation of the sodium cation. Computational and spectroscopic analyses reveal that this design promotes anion-dominated solvation, facilitates the formation of a robust anion-derived solid electrolyte interphase, suppresses dendrite formation, and enhances stability and cell performance. Batteries using this weakly solvating solvent-based quasi-solid electrolyte achieve an average coulombic efficiency of 98.4% over 400 cycles (at 0.5 mA cm⁻², 0.5 mAh cm⁻² in half-cell tests) and retain a capacity of 1077 mAh g⁻¹ (based on sulfur content) over 250 cycles when paired with sulfurized polyacrylonitrile cathodes. These findings establish a new paradigm for developing practical, high-performance sodium–metal batteries.