Inorganic-Organic Hybrid Ionic Liquid Electrolytes for Na Secondary Batteries

Sodium secondary batteries are attracting more and more attraction as energy storage devices for stationary use and electric vehicles due to high abundance and low cost of sodium resources as well as the low redox potential of Na⁺/Na. Ionic liquid electrolytes are attractive to improve the safety issues of sodium secondary batteries, owing to their unique properties such as low flammability and low vapor pressure [1-3]. Our recent works suggest FSA^- (FSA^- = bis(fluorosulfonyl)amide anion) based ionic liquids, such as Na[FSA]-K[FSA] and Na[FSA]-[C₃C₁pyrr][FSA] (C₃C₁pyrr⁺ = N-propyl-N-methylpyrrolidinium cation) systems, exhibit good physical and electrochemical properties as electrolytes for Na secondary batteries [4-6]. This study will present a new series of inorganic-organic hybrid ionic liquids as electrolytes for Na secondary batteries.

 Four FSA salts of organic cations, N₆₁₁₁⁺ (hexyltrimethylammonium), N₄₄₂₂⁺ (dibutyldimethylammonium), AS[4.5]⁺ (azoniaspiro[4.5]nonane), and C₂C₁im⁺ (1-ethyl-3-methylimidazolium), are mixed with Na[FSA]. In all the cases, a wide liquid temperature range around room temperature was observed in a certain x(Na[FSA]) range. Ionic conductivity and viscosity increase and decrease with decreasing x(Na[FSA]), respectively. The highest ionic conductivity was observed for the Na[FSA]-[C₂C₁im][FSA] system. Electrochemical windows are around 5 V and Na metal deposition and dissolution are observed at the cathode limit. The efficiency of Na metal deposition and dissolution is improved by increasing temperature due to suppression of dendritic Na metal deposition.

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