A Dual-Stabilization Strategy for Tubular Zinc–Iodine Flow Batteries

Abstract

Zinc–iodine flow batteries offer a sustainable, aqueous-based solution for grid-scale energy storage, with tubular cell design further offering enhanced power density. However, non-uniform Zn deposition remains a critical barrier to long-term stability. Here, we report a dual-stabilization strategy that combines geometrical electrode modification with electrolyte engineering to stabilize Zn metal deposition. Geometrically, a tri-helical Zn anode architecture redistributes the electric field and homogenizes current density, promoting uniform deposition. Chemically, the NH₄Br additive modulates the zinc-ion solvation structure through NH₄⁺ and captures free iodine species with Br^– to inhibit polyiodide shuttling. Electrochemical impedance spectroscopy confirms reduced interfacial resistance and diffusion impedance, with dense and uniform Zn deposition in the dual-modified system. The approach enables over 350 stable charge–discharge cycles at 10 mA cm^–2 with Coulombic efficiencies exceeding 98% in a tubular cell, and allows high-rate cycling up to 72 mA cm^–2 under full discharge and charge conditions, outperforming cells with only geometrical or chemical modification. This work demonstrates that coupling anode geometry and electrolyte engineering effectively mitigates persistent failure modes in tubular Zn–I₂ systems, offering a viable pathway toward safer and longer-lasting flow batteries for large-scale energy storage.