Biredox ionic liquids with solid-like redox density in the liquid state for high-energy supercapacitors

• Published in: Nature materials Vol. 16; no. 4; pp. 446 - 453
• Main Authors: Mourad, Eléonore, Coustan, Laura, Lannelongue, Pierre, Zigah, Dodzi, Mehdi, Ahmad, Vioux, André, Freunberger, Stefan A., Favier, Frédéric, Fontaine, Olivier
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.04.2017; Nature Publishing Group
• Subjects: 639/301/299/161/891; 639/638/161/891; 639/638/298/923/614; Biomaterials; Bulk density; Chemical reactions; Condensed Matter Physics; Electrochemistry; Electrodes; Energy storage; Ionic liquids; Ions; Kinetics; Leakage current; Materials Science; Nanotechnology; Optical and Electronic Materials; Pores; Reaction kinetics; Redox reactions; Solid state; Solvents; Storage batteries; Supercapacitors
• ISSN: 1476-1122; 1476-4660
• DOI: 10.1038/nmat4808

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. Electrochemical storage devices in the liquid state are promising but they suffer from low redox species density. An approach based on biredox ionic liquids now demonstrates bulk-like redox density compatible with supercapacitor applications.