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 1,2dimethoxyethane (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 NaPF 6 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 NaPF 6 . 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 −2 for 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 NaPF 6 . The results herein demonstrate that NaB(hfip) 4 •3DME has the physical and electrochemical properties required for a next-generation Na-electrolyte material.
Rechargeable zinc batteries (RZBs) are of immense interest as low-cost and sustainable energy storage devices. However, formation of Zn dendrites, Zn corrosion, and undesired side reactions in aqueous electrolytes as well as the use of costly fluorinated salts in organic electrolytes, have hindered the commercialization of RZBs. In this work, a cost-efficient and environmentally friendly, non-aqueous electrolyte comprised of zinc dicyanamide (Zn(dca) 2 ) in dimethyl sulfoxide (DMSO) is shown to support the electrochemical cycling of zinc. Fouriertransform infrared (FT-IR) spectroscopy complemented with theoretical studies suggest that the solvation of Zn 2 + is stabilized with both [dca] À anions and DMSO molecules at high concentrations (� 1.0 M) of the zinc salt content. Stable charge/ discharge cycles in zinc symmetrical cells with low overpotentials (0.05 V) were especially observed for 1.0 M Zn(dca) 2 / DMSO over 90 cycles at 1.0 mA cm À 2 with scanning electron microscopy (SEM) images confirming the formation of a dense and smooth zinc morphology on metal anode surface postcycling. X-ray photoelectron spectroscopy (XPS) also shows that the presence of zinc nitride (Zn 3 N 2 ) helps form a stable SEI layer in the presence of 1.0 M and 2.5 M systems, making [dca]-based electrolytes highly promising candidates in rechargeable zinc batteries.
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