Tailoring manganese coordination environment for a highly reversible zinc-manganese flow battery

Zinc-manganese flow batteries have drawn considerable attentions owing to its advantages of low cost, high energy density and environmental friendliness. On the positive carbon electrode, however, unstable MnO2 depositions can be formed during oxidation through disproportionation reaction of Mn3+, which result in poor reversibility of Mn2+/MnO2 and bring instability to Zn-Mn flow battery limiting its performance and further development. To tackle this issue, in this study, we reported a highly reversible Zn-Mn flow battery by employing EDTA-Mn as the positive electrolyte. In aqueous solutions, EDTA exhibits a strong ligand field for Mn2+ from ab initio calculations, which proves to form bonds with Mn2+ through carboxyl/amino groups and replace bonded waters in the solvation structure of Mn2+. Benefiting from the coordination effect of EDTA, both electrochemical and material characterizations demonstrate a highly reversible Mn2+/Mn3+ redox reaction in EDTA-Mn, which effectively inhibits the disproportionation reaction of Mn3+ without forming any deposited MnO2 on carbon electrode. The constructed Zn-Mn flow cell adopting EDTA-Mn not only demonstrates excellent rate performance with a high CE over 95 % operated at 10-50 mA cm(-2), but also realizes a superior cycling stability over 300 cycles at 20 mA cm(-2) affording 98 % CE and 75 % EE.