Design and synthesis of a weakly solvated electrolyte for high-performance fluoride-ion batteries

Abstract

The energy densities of fluoride-ion batteries (FIBs) are up to 10 times higher than those of lithium-ion batteries, with improved safety features, positioning them as promising future energy storage devices. However, the practical application of FIBs is limited by the narrow electrochemical stability window (ESW) and low ionic conductivity of conventional electrolytes. In this study, a weakly solvated electrolyte was designed to mitigate N⁺–F⁻ interactions and address these limitations. By incorporating a β-H-free neopentyl chain and a benzene ring into a resonance-stabilized imidazolium cation, we reduced cation rigidity and improved F⁻ tolerance. Consequently, the electrolyte exhibited an ionic conductivity of ∼8 mS cm⁻¹ and extended the ESW to 4.98 V at room temperature. Furthermore, introducing the –OCH₃ at the ortho and para positions on the benzene ring decreased N⁺–F⁻ binding energy and improved the thermal stability to 235 °C. The optimized electrolyte suppressed HF₂⁻ formation and reduced the desolvation energy from 0.69 to 0.43 eV, significantly enhancing F⁻ transport. When paired with a dihydrophenazine-based polymer‖LiF–CeF₃ full cell, the resulting battery achieved an open-circuit voltage of 3.1 V and retained 73.5% of its capacity after 500 cycles, positioning FIBs as promising energy storage devices.