Highly Concentrated Catholyte Based on Solvate Ionic Liquid for Flow Battery
In this study, we report an innovative strategy to prepare the liquid active materials in order to increase their energy density. Our technology focuses on the melting of pure redox compounds to maximize their concentration instead of dissolving them in solvents. This simple yet challenging approach has been successfully realized using supercooled liquid technology by the combination of a specific plasticizing salt and a low melting point redox active compound. Such a supercooled liquid can be obtained by mixing of 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-methoxy-TEMPO, MT) and Li bis(trifluoromethanesulfonyl) imide (LiTFSI, LT) and stabilizing the mixture further below their melting points. The redox-active supercooled liquid exists in a highly stable liquid state over a wide temperature range that extends to below its normal melting points. The special properties of this supercooled liquid are due to the scientifically unique and rare solvation state known as a ‘solvate ionic liquid’, which is typified by a liquefied ion-pair stabilized by a neutral third compound. As a catholyte, the addition of an appropriate amount of solvent such as water helps to enhance the electrochemical advantage while maintaining the liquid’s supercooled nature, and a battery with a Li+ conducting architecture exhibits 93% electrochemical activity and 99% coulombic efficiency in the 1st cycle with an average discharge voltage of 3.6 V (Fig. 1). The catholyte also performed an energy density of 200 Wh/l, which is one of the highest values for catholytes reported to date with an organic redox compound.