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

TL;DR

This paper introduces biredox ionic liquids that combine high redox density with fast kinetics, enabling high-capacity, high-rate supercapacitors with improved energy storage performance.

Contribution

The study presents a novel class of biredox ionic liquids that achieve solid-like redox density in a liquid state, enhancing supercapacitor energy density and charge storage capabilities.

Findings

Biredox ionic liquids store significant charge within electrode pores.

They enable high-voltage operation due to a wide electrochemical window.

The approach decouples charge storage from ion diffusion limitations.

Abstract

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 ILs 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 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.