Vanadium-Mediated High Areal Capacity Zinc–Manganese Redox Flow Battery

Aqueous manganese redox flow batteries (AMRFBs) that rely on the two-electron transfer reaction of Mn2+/MnO2 have garnered significant interest because of their affordability, high voltage, and excellent safety features. Nevertheless, the deposited MnO2 tends to partially dissolve during discharge, impeding the sustained operation of AMRFBs, particularly at high areal capacities (>5 mA h cm–2). Herein, VO2+/VO2+ was introduced into the catholyte as a redox mediator (RM) to facilitate the efficient dissolution/deposition process of MnO2. In situ UV–vis spectroscopy and kinetic analysis elucidated the swift reaction mechanism between MnO2 and VO2+, which allows for efficient rejuvenation of "dead" MnO2 with facile amounts of mediator, further achieving a highly reversible MnO2 cathode. The assembled zinc–manganese redox flow battery with RM demonstrates a high Coulombic efficiency of 99% at 20 mA h cm–2 over 50 cycles. The areal capacity is further increased to 50 mA h cm–2, achieving an exceptional areal energy density exceeding 100 mW h cm–2, surpassing most reported areal capacity in the AMRFB. This work introduces a novel RM to address "dead" MnO2 and sheds valuable insights into the reaction mechanism between MnO2 and RM, which will promote the development of other deposition-type batteries.