High-Ionicity Electrolytes Based on Bulky Fluoroborate Anions for Stable Na-Metal Cycling

Currently available electrolyte materials for sodium battery technology remain unoptimized, despite the success of their lithium counterparts. Herein, highly fluorinated borate ester anions are synthesized and characterized in terms of their physical and electrochemical properties as sodium salts in 1,2-dimethoxyethane (DME) solvent. Walden analysis and nuclear magnetic resonance pulsed field gradient diffusion measurements are utilized to probe the ionicity of the electrolytes, demonstrating that sodium 1,1,1,3,3,3-(tetrakis)­hexafluoroisopropoxy borate (NaB­(hfip)4·3DME) possesses notably high ionicity when compared to NaPF6 and NaFSI at an equivalent Na+ concentration. NaB­(hfip)4/DME is explored electrochemically, with the anion showing excellent oxidative stability and the desired passivation behavior toward Al surfaces, akin to NaPF6. Investigating the electrolyte in Na-metal symmetrical cells shows very impressive cycling behavior with very low (