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

Abstract

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 MnO 2 depositions can be formed during oxidation through disproportionation reaction of Mn 3+ , which result in poor reversibility of Mn 2+ /MnO 2 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 Mn 2+ from ab initio calculations, which proves to form bonds with Mn 2+ through carboxyl/amino groups and replace bonded waters in the solvation structure of Mn 2+ . Benefiting from the coordination effect of EDTA, both electrochemical and material characterizations demonstrate a highly reversible Mn 2+ /Mn 3+ redox reaction in EDTA-Mn, which effectively inhibits the disproportionation reaction of Mn 3+ without forming any deposited MnO 2 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. • A highly reversible Zn–Mn RFB is proposed based on EDTA-Mn catholyte. • EDTA proves to form bonds with Mn 2+ through carboxyl and amino groups. • A highly reversible Mn 2+ /Mn 3+ redox reaction is proved in EDTA-Mn. • Disproportionation of Mn 3+ is completely inhibited in EDTA-Mn without forming MnO 2 . • The Zn–Mn cell using EDTA-Mn shows superior rate performance and cycling stability.