A green solution to improve the performance of zinc-polyiodide flow batteries

Zinc-polyiodide flow batteries (FBs) offer a promising alternative to conventional vanadium-based systems, combining lower cost with the potential for higher energy density. In this work, we introduce an environmentally benign additive, consisting of a mixture of acetic acid and sodium acetate ligands, providing both buffering and metal complexing functions, to enhance reversibility and improve the morphology of zinc deposits at the anode. The additive operates via a dual mechanism: (i) modulating Zn2+-I-complexation to lower the energy barrier for zinc plating/stripping; and (ii) stabilizing the pH at the electrode-electrolyte interface to suppress side reactions leading to inactive byproducts. Single-cell tests demonstrate notable performance improvements over state-of-the-art systems, with coulombic efficiency exceeding 98.2 % and energy efficiency reaching 85 % after 100 cycles. Spectroscopic and morphological analyses link these gains to the suppression of less active [ZnIx(H2O)4-x]2−xspecies (for 2 ≤ x ≤ 4) and the inhibition of ZnO formation on the electrode surface during cycling. These findings highlight metal coordination by acetate ligand forming [ZnIxAcy(H2O)z]2-(x+y)species in solution as a simple, non-toxic, and effective strategy to improve the long-term performance of Zn-I2FBs, paving the way for the development of more durable and energy-dense aqueous flow batteries.