Graphical Abstract: Amide cosolvents, organophosphate base: In this article we re-examine the use of amides (principally N-methyl pyrrolidone, N,N-dimethyl acetamide and N,N-dimethyl formamide; NMP, DMAc and DMF) as cosolvents for flame resistant sodium ion battery electrolyte composed of sodium bis(oxalato)borate (NaBOB) in triethyl phosphate. Without sacrificing flame resistance, the use of cosolvents all allowed for a modest increase of the concentration of NaBOB and near doubling of the ionic conductivity of the resultant electrolyte solutions. Electrolyte formulations are tested in high mass-loading full cells using Prussian white cathodes and hard carbon anodes.
Abstract: In selecting electrolytes for Na-ion batteries, simply importing the analogue of common lithium-ion battery electrolytes to sodium-ion batteries does not address safety concerns like toxicity and flammability. Electrolytes based on sodium bis(oxalato)borate (NaBOB) in organophosphates like triethyl phosphate (TEP) largely alleviate these specific safety concerns. However, it may be beneficial to obtain solutions with higher ionic conductivities than NaBOB in TEP, and compare the performance in Na-ion batteries with high mass loading electrodes. Here, we have shown that N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc), and N-methyl pyrrolidone (NMP) cosolvents almost double the ionic conductivity (from ~3.5 mS/cm to ~7 mS/cm) without sacrificing the flame-retarding properties of the base NaBOB in TEP electrolyte. The physical properties of these cosolvent electrolyte mixtures are investigated, along with the electrochemical performance of these electrolytes full-cells based on hard carbon anodes and Prussian white cathodes with near-commercial areal capacity (~2 mAh/cm2).
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