Bromine Assisted MnO 2 Dissolution Chemistry: Toward a Hybrid Flow Battery with Energy Density of over 300 Wh L -1 .

Mn 2+ /Mn 3+ 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 Mn 3+ , which results in accumulation of "dead" MnO 2 limits its reversibility and further energy density. Herein, a novel catholyte based on mixture of Mn 2+ and Br - was proposed for flow batteries with high energy density and long cycle life. In the design, the "dead" MnO 2 can be fully discharged via Br - by a chemical-electrochemical reaction. Coupled with Cd/Cd 2+ 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.