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

Consuming one-third of iodide to stabilize the iodine for reversible I-/I-3(-) 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 I-3(-) by enabling an enhanced solubility of the I-2 film through interactions between PVP and I-2. 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 I-2, creating an intermediate structure of NVP center dot 2I(-). Subsequently, the iodide in NVP center dot 2I(-) can further complex with oxidized I-2 forming soluble NVP-2I(3)(-), which facilitates the dissolution of the I-2 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%.

Automated summary

In this study, a polymer-polyiodide complex cathode was developed to enhance the iodide capacity of zinc-iodine flow batteries. By utilizing polyvinylpyrrolidone (PVP) and KI, the formation of the polyiodide complex significantly increased the solubility of the I-2 film, resulting in a superior discharge capacity and stable energy efficiency over 600 cycles.