A flow battery has very unique and significant benefits due to its simple structure and flexible handling of its liquid active materials as ‘electrochemical fuels’, however, the energy density of the regular catholytes and anolytes is still relatively low (&lt;100 Wh/l). A non-aqueous system is an effective method of improving the discharging voltage more than 2 V, and the challenge is how to increase the concentration of redox species over 2 M to achieve high energy density (&gt;400 Wh/l). In this study, we report an innovative strategy to prepare highly-concentrated liquid active materials by directly melting redox compounds to maximize their concentration instead of dissolving them in solvents. The mixture of 4-methoxy-TEMPO and LiTFSI demonstrated 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 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. The catholyte also performed an energy density of 200 Wh/l, and was reported previously. To further boost the energy density, 2-Azaadamantane-N-oxyl(AZADO) and its derivatives are examined by applying the same preparation strategy. Then, we confirmed stable two-electron charge/discharge battery performance and one of the active materials achieved the energy density as high as 431.2 Wh/l(fig. 1). It is, to the best of our knowledge, one of the highest values in the world by far.

Two-electron redox catholyte based on solvate ionic liquid for flow battery

254th National Meeting (2017)

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