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 Mn 2+ /MnO 2 have garnered significant interest because of their affordability, high voltage, and excellent safety features. Nevertheless, the deposited MnO 2 tends to partially dissolve during discharge, impeding the sustained operation of AMRFBs, particularly at high areal capacities (>5 mA h cm –2 ). Herein, VO 2+ /VO 2 + was introduced into the catholyte as a redox mediator (RM) to facilitate the efficient dissolution/deposition process of MnO 2 . In situ UV–vis spectroscopy and kinetic analysis elucidated the swift reaction mechanism between MnO 2 and VO 2+, which allows for efficient rejuvenation of “dead” MnO 2 with facile amounts of mediator, further achieving a highly reversible MnO 2 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” MnO 2 and sheds valuable insights into the reaction mechanism between MnO 2 and RM, which will promote the development of other deposition-type batteries.