Revisiting Amides as Cosolvents for Flame Resistant Sodium Bis(oxalato)borate in Triethyl Phosphate Electrolyte

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/cm 2 ).