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.