Kinetics of electrochemical reactions are several orders of magnitude slower in solids than in liquids as a result of the much lower ion diffusivity. Yet, the solid state maximizes the density of redox species, which is at least two orders of magnitude lower in liquids because of solubility limitations. With regard to electrochemical energy storage devices, this leads to high-energy batteries with limited power and high-power supercapacitors with a well-known energy deficiency. For such devices the ideal system should endow the liquid state with a density of redox species close to the solid state. Here we report an approach based on biredox ionic liquids to achieve bulk-like redox density at liquid-like fast kinetics. The cation and anion of these biredox ionic liquids bear moieties that undergo very fast reversible redox reactions. As a first demonstration of their potential for high-capacity/high-rate charge storage, we used them in redox supercapacitors. These ionic liquids are able to decouple charge storage from an ion-accessible electrode surface, by storing significant charge in the pores of the electrodes, to minimize self-discharge and leakage current as a result of retaining the redox species in the pores, and to raise working voltage due to their wide electrochemical window.

We report here electrochemical capacitors using an aqueous electrolyte based on the concept of "water-in-salt" with the aim to improve the energy density by increasing the voltage of the cell. A "water-in-salt" consists of a highly concentrated aqueous LiTFSI solution in which both volume and mass of LiTFSI are greater than those of water. With activated carbon supercapacitor electrodes (PICA) and 31 m "water-in-salt" electrolytes (m stands for molality), we were able to reach a cell voltage of 2.4 V whereas it is difficult to exceed 1.6 V in conventional aqueous devices because of water splitting. Moreover, it was observed that the specific capacitance of the cell is improved using "water-in-salt" electrolytes. In these conditions, an energy density of 30 Wh kg −1 was obtained which is at least three times greater than for conventional aqueous devices and in the same order of magnitude than for redox enhanced capacitors. Interestingly, fair stability, over 2000 cycles, was obtained for the 7 m electrolyte. Up to 90 sec chargingdischarging rate, this latter electrolyte offers the best compromise between voltage, power and energy densities and stability. This study demonstrates the feasibility of water-in-salt as an electrolyte for supercapacitors and points out the most suited compositions for these electrolytes.

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