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 NaPF₆and 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 (<2 mV) overpotentials observed at 0.1 mA cm^-2for over 100 cycles at 0.1 mAh cm^-2. Cycling is investigated via postmortem analysis of the electrode surfaces, confirming Na deposition and revealing substantially reduced electrolyte decomposition when compared to NaPF₆. The results herein demonstrate that NaB(hfip)₄·3DME has the physical and electrochemical properties required for a next-generation Na-electrolyte material.