Effect of Radical–Solvent Interaction on Battery Performance in Benzophenone-Based Charge Storage Systems

Abstract

The reversibility and redox potential of a benzophenone negative redox couple are enhanced by utilizing a supporting electrolyte based on a low-dielectric and less polar solvent for non-aqueous flow batteries. The benzophenone redox couple generates a ketyl radical after reduction, and the solvent–benzophenone radical interaction considerably affected the reversibility and redox potential of benzophenone. The major factor causing these desirable changes in the electrochemical characteristics is the polarity of the solvents, which is directly correlated with the solvent–redox couple interaction strength. The solvent affects the lowest unoccupied molecular orbital energy level of benzophenone, which is higher in less polar solvents than in high-dielectric solvents. This leads to a negative shift in the redox potential of benzophenone. Furthermore, reducing the polarity of the solvent enhances the chemical reversibility of the redox couple and the Coulombic efficiency of the cell. Computational analysis reveals that the less polar solvent undergoes less chemical degradation after reduction because the kinetic barrier for binding the ketyl radical to the solvent molecule increases.