Non-Flammable Sodium Asymmetric Imide Salt-Based Deep Eutectic Solvent for Supercapacitor Applications

This study reports two deep eutectic solvents (DESs) based on alkaline imide salts with asymmetric anions as functional electrolytes for supercapacitor (SC) application. The eutectic mixture of sodium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (NaFTFSI) or sodium cyano-trifluoromethanesulfonyl imide (NaTFSICN) with ethylene carbonate (EC) delivers a non-flammable and stable liquid. The eutectic diagrams of the electrolytes directed to an optimal composition (w_salt =0.25), hinging to that of conventional carbonate-based electrolytes, i. e., 1 mol L^-1 . The volumetric properties of the DESs revealed a "stacking" effect, reflecting a strong coordination bond between the imide and EC anions without solvating the Na⁺ cations. The DES transport properties (i. e., viscosity, conductivity, and ionicity) and temperature variations designate a high organization, similar to ionic liquids. The DESs, when coupled with activated carbon electrodes in a two-electrode symmetric configuration, yield specific capacities of 150 F g^-1 at a normalized current density of 0.5 A g^-1 (and 120 F g^-1 at 2 A g^-1 ). The SC maintained 80 % of its initial capacity beyond 100 h of floating at an operating voltage of 2.4 V and showed a 150 mV per hour potential loss under self-discharge. The devised eutectic mixtures offer a promising new pathway for simple, safe, and effective electrolytes for SC applications.