Reversible two-electron redox conversion enabled by an activated electrode and stabilized inter-halogen electrolyte for high performance zinc–iodine flow batteries

Abstract

Iodine-based flow batteries have been considered as a promising energy storage device for large-scale energy storage. However, a two-electron transfer reaction (I⁻/I₂) coupled with the shuttle behavior of iodine species results in insufficient capacity, a low redox potential (0.536 V vs. SHE), and poor cycle stability. Herein, we implemented a novel strategy to achieve the desired reversible two-electron transfer behavior by utilizing a tailored chloride cathode and modified electrode. Both experimental characterization and theoretical calculations prove that the Cl⁻ is coupled with I⁺ forming inter-halogen species (ICl₂⁻) to stabilize I⁺ leading to a complete multi-electron transfer reaction and high discharge voltage, while the activated electrode served as the confinement host to anchor iodine species, which effectively alleviates the shuttle behavior of free iodine and facilitates reaction kinetics. Compared with the conventional zinc–iodine flow battery with 6 M KI electrolytes (61.06 Ah L⁻¹, 61.28 W h L⁻¹), the designed zinc–iodine flow battery using 2.6 M KI + MgCl₂ electrolyte exhibits a high capacity of 110.56 Ah L⁻¹ at 100 mA cm⁻², while a high energy density of 132.25 W h L⁻¹ is also realized. Moreover, the proposed flow cell further achieves an exceptional cycling durability of 239 cycles at 100 mA cm⁻² with an energy efficiency of 72.90% and only 0.02% capacity decay per cycle.