A highly concentrated catholyte based on a solvate ionic liquid for rechargeable flow batteries.

A redox-active supercooled liquid is obtained by forming a "solvate ionic liquid" from a mixture of a stabilized organic radical and a specific Li salt and stabilizing the mixture further below their melting points. As a catholyte, the addition of an appropriate amount of water helps to enhance the electrochemical advantage while maintaining its supercooled nature and the liquid shows a high energy density of 200 W h L(-1) with reversible charge/discharge.