High-capacity zinc–iodine flow batteries enabled by a polymer–polyiodide complex cathode

Abstract

Consuming one-third of iodide to stabilize the iodine for reversible I⁻/I₃⁻ reactions is the major challenge for zinc–iodine flow batteries (ZIFBs) to realize high volumetric capacity. In this study, we report a polymer–polyiodide complex cathode to boost the iodide capacity of ZIFBs. By adding polyvinylpyrrolidone (PVP) to KI, electrochemical tests show that the formation of a polyiodide complex significantly promotes the oxidation of I⁻ to I₃⁻ by enabling an enhanced solubility of the I₂ film through interactions between PVP and I₂. Materials characterization and theoretical calculation further prove that the lone-pair electrons of oxygen from the carbonyl group of monomer NVP facilitate the formation of an I–O bond and result in the breaking of the I–I bond in I₂, creating an intermediate structure of NVP·2I⁻. Subsequently, the iodide in NVP·2I⁻ can further complex with oxidized I₂ forming soluble NVP–2I₃⁻, which facilitates the dissolution of the I₂ film and effectively frees up 1/3 iodide ions. Benefitting from the polyiodide complex, a ZIFB demonstrates a superior discharge capacity of 115 A h L⁻¹ for up to 6 M I⁻, significantly unlocking the capacity of iodine. For long-term operation, the ZIFB with PVP in 6 M I⁻ can continuously run for 600 cycles, affording a stable energy efficiency of 70%.