Rescue of dead MnO 2 for stable electrolytic Zn-Mn redox-flow battery: a metric of mediated and catalytic kinetics.

The virtues of electrolytic MnO 2 aqueous batteries are high theoretical energy density, affordability and safety. However, the continuous dead MnO 2 and unstable Mn 2+ /MnO 2 electrolysis pose challenges to the practical output energy and lifespan. Herein, we demonstrate bifunctional cationic redox mediation and catalysis kinetics metrics to rescue dead MnO 2 and construct a stable and fast electrolytic Zn-Mn redox-flow battery (eZMRFB). Spectroscopic characterizations and electrochemical evaluation reveal the superior mediation kinetics of a cationic Fe 2+ redox mediator compared with the anionic ones (e.g. I - and Br - ), thus eliminating dead MnO 2 effectively. With intensified oxygen vacancies, density functional theory simulations of the reaction pathways further verify the concomitant Fe-catalysed Mn 2+ /MnO 2 electrolysis kinetics via charge delocalization and activated O 2p electron states, boosting its rate capability. As a result, the elaborated eZMRFB achieves a coulombic efficiency of nearly 100%, ultra-high areal capacity of 80 mAh cm -2 , rate capability of 20 C and a long lifespan of 2500 cycles. This work may advance high-energy aqueous batteries to next-generation scalable energy storage.