Bromine Assisted MnO2 Dissolution Chemistry: Toward a Hybrid Flow Battery with Energy Density of over 300 Wh L−1

Mn2+ /Mn3+ redox pair has been considered as a promising cathode for high energy density batteries, due to its attractive features of high redox potential, solubility and outstanding kinetics. However, the disproportionation side reaction of Mn3+ , which results in accumulation of "dead" MnO2 limits its reversibility and further energy density. Herein, a novel catholyte based on mixture of Mn2+ and Br- was proposed for flow batteries with high energy density and long cycle life. In the design, the "dead" MnO2 can be fully discharged via Br- by a chemical-electrochemical reaction. Coupled with Cd/Cd2+ as anode, the assembled Bromine-Manganese flow battery (BMFB) demonstrates a high energy efficiency of 76 % at 80 mA cm-2 with energy density of 360 Wh L-1 . The battery assembled with silicotungstic acid as anode could continuously run for over 2000 cycles at 80 mA cm-2 . With high power density, energy density and durability, the BMFB shows great potential for large-scale energy storage.