RFBs, which use soluble redox species dissolved in electrolytes, show great promise for large-scale energy storage systems (ESSs) because of their scalability, cost-effectiveness, and design flexibility that can decouple power and energy for diverse purposes; however, their low energy density has limited their widespread application. Most conventional ways to increase the energy density to date have relied on either exploiting high-concentration electrolytes that contain large amounts of redox materials per volume or developing redox materials with higher solubility. Although this approach of exploiting high-concentration electrolytes is simple and effective, it results in unfavorable trade-offs, including increased electrolyte viscosity and high cell resistance, and often leads to the precipitation of salts, which causes the failure of the system. Furthermore, non-aqueous RFBs can be more vulnerable to these trade-offs because of the intrinsically lower ionic conductivity of non-aqueous electrolytes compared with that of their aqueous counterparts, which typically results in inferior power capability of the system. Here, we demonstrate that the use of multi-electron molecules as redox materials can offer an alternative route to overcome these issues and achieve high-energy-density RFBs. Unlike doubling the electrolyte concentration, for instance, which would easily result in the solubility limit being exceeded or significant reduction of the kinetics, the utilization of the double-redox activity from a single-redox material would approximately double the energy density without negatively affecting the electrolyte properties. Nevertheless, the use of a second-electron redox reaction in most redox-active materials in RFBs has often been shown to be irreversible or to promote severe degradation because of the poor chemical stability of the redox materials at highly oxidized states in the solution.

In this study, we demonstrate, for the first time, an all organic flow system using a multi-electron redox-active organic material with the superior chemical stability, which is a phenazine derivative. In addition, an analysis of the electrochemical kinetic properties of redox couple on both diffusion- and charge transfer-controlled kinetic is conducted through rotating disk electrode test; The redox couple displays outstanding kinetic characteristics. Moreover, the redox couple presents the highest theoretical and practical energy density among reported redox couples for both aqueous and non-aqueous RFB at the same concentration. The redox mechanism and the chemical stability of redox couple are investigated through raman spectroscopy and density functional theory calculations. These findings on this multi-electron redox material provide a potential new pathway for the design of high-energy-density RFBs and practically feasible ESSs.