A non-fluorinated, weakly solvating electrolyte for efficient sodium–sulfurized polyacrylonitrile batteries

Abstract

Room-temperature sodium–sulfur (RT/Na–S) batteries are gaining increasing attention due to their high energy density (1274 W h kg⁻¹) and the abundance of sodium and sulfur. However, traditional carbonate- and glyme-based electrolytes exhibit significant drawbacks, including capacity loss from nucleophilic reactions and polysulfide solubility, respectively. In this study, we propose a novel weakly solvating electrolyte (WSE) utilizing tetrahydropyran (THP) as an eco-friendly solvent. The WSE, composed of 2 M sodium bisfluorosulfonyl imide in THP, forms an anion-rich solvation sheath that enhances the stability of both the solid-electrolyte and cathode-electrolyte interfaces. This design promotes uniform sodium deposition and reversible sulfur redox reactions. The WSE achieves a remarkable Na plating/stripping coulombic efficiency of 99.1% over 1000 cycles at 1.0 mA cm⁻². Coupled with sulfurized polyacrylonitrile (SPAN) cathodes, the Na‖SPAN full cells deliver a reversible capacity of 365 mA h g⁻¹ at 1 A g⁻¹ (based on the mass of SPAN), sustaining 90% of their initial capacity over 500 cycles. This WSE represents a promising avenue for developing high-performance and environmentally friendly electrolytes for long-life Na–S batteries.