Precise Potential Tuning for Polymer-Mediated Aqueous Redox Flow Battery with Lithium Iron Phosphate as Target Cathode

A highly hydrophilic ferrocene-containing polymer with an ammonium group was synthesized as a polymer mediator for redox targeting flow batteries (RTFB) by using LiFePO4 as a charge storage material. Based on the Nernst equation, suitable conditions for the single mediator to be quantitatively charged and discharged was established through precise tuning of the redox potential by changing the electrolyte concentration. In fact, the mediation behavior changed significantly depending on the difference in the redox potential. The mediator acted for charging of LiFePO4 under conditions where the potential of the mediator was more positive than that of LiFePO4 and for its discharging under conditions where the potential was more negative. When the redox potentials of the two components were almost equal, the mediation effect was clearly shown for both charging and discharging and the charge/discharge capacity and Coulombic efficiency reached the most balanced and desirable, resulting in excellent charge–discharge cyclability. These results demonstrated the importance of potential tuning between the charge storage material and the mediator in RTFB and showed that controlling the Nernst potential by the electrolyte concentration was a simple and effective means of accurately tuning the performance of the mediator.