Ethylene Carbonate-based Concentrated Electrolyte Composed of Asymmetric Amide Anion for Rechargeable Sodium-ion Batteries

The phase behavior, Na+ coordination structure, Na+ conduction and transport properties, and battery performance of liquid electrolytes comprising sodium (fluorosulfonyl) (trifluoromethylsulfonyl) amide (NaFTA) and ethylene carbonate (EC) are investigated. A highly concentrated electrolyte with a molar ratio of EC/NaFTA = 1.5 is shown to exhibit a stable liquid state at room temperature. It is revealed that Na+ in the concentrated electrolyte coordinates—not only with EC but also with anions—to satisfy stable coordination numbers. From this specific coordination structure, dynamic ligand exchange of Na+ is shown to be enhanced, and [Na | NaFTA+1.5EC | Na0.44MnO2] cells are shown to achieve stable cycling performance by constant charge-discharge tests at 303 K.