β-Cyclodextrin supramolecular hydrogel enables dual regulation of iodine species for low-cost zinc-iodine single flow batteries

A polysaccharide-functionalized hydrogel strategy is proposed for ZISFB cathodes. • A “capture–immobilization” mechanism enables dual regulation of iodine species. • The assembled battery delivers an ultrahigh energy density of 179 Wh L −1 . • Compatible with low-cost microporous membranes, scalable for commercialization. Zinc‑iodine single flow batteries (ZISFBs) possess an exceptionally high theoretical energy density; however, the static cathode configuration imposes severe challenges, including interfacial passivation and pronounced shuttle effects. To address these issues, we propose a novel poly(vinyl alcohol) (PVA)/β-cyclodextrin (β-CD) composite hydrogel with supramolecular functionalization. Benefiting from the host–guest inclusion provided by β-CD and the physical confinement dominated by the three-dimensional hydrophilic PVA network, the hydrogel effectively mitigates I₂-induced passivation on the electrode surface while suppressing the shuttling of polyiodide species (I₃ − /I₅ − ). Building on this, we impregnated the PVA/β-CD hydrogel into the pores of cathodic carbon felt in situ, leveraging the framework's high specific surface area and three-dimensional conductive pathways. This integration markedly shortens the charge-transport length, overcomes the intrinsic ionic-transport limitations of the gel medium, and mitigates polarization. Under highly concentrated 6 M KI electrolyte and deep I − /I₂ conversion conditions, the assembled ZISFBs deliver an ultrahigh discharge energy density of 179 Wh L −1 , an energy efficiency of 82.56%, and stable cycling for over 340 h. Importantly, this material is compatible with lower-cost microporous membranes, reducing the membrane cost by ∼96% compared with Nafion-based systems, demonstrating outstanding potential for commercial scale-up. Zinc–iodine single-flow batteries (ZISFBs) are promising for large-scale energy storage due to their intrinsic safety and high theoretical energy density. However, their practical operation is still hindered by two key issues: (i) severe cathode passivation caused by insulating iodine (I₂) deposition during deep charging, and (ii) pronounced polyiodide (e.g., I₃ − /I₅ − ) shuttling across separators, which triggers self-discharge and low coulombic efficiency—especially when low-cost microporous membranes are used. To address these challenges, we propose an in situ constructed PVA/β-cyclodextrin (β-CD) supramolecular hydrogel (CD–P) impregnated into porous carbon felt. Benefiting from β-CD host–guest inclusion and the three-dimensional hydrogel confinement, CD–P establishes a “capture–immobilization” mechanism that synchronously stabilizes iodine species and suppresses polyiodide crossover, thereby realizing high-performance and highly stable zinc–iodine single flow batteries. As a result, the assembled ZISFBs deliver an ultrahigh discharge energy density of 179 Wh L −1 (approaching the theoretical limit of ∼220 Wh L −1 in 6 M KI), a high energy efficiency of up to 82.56%, and stable operation for over 340 h. Importantly, our strategy enables the replacement of expensive Nafion membranes with inexpensive microporous membranes, substantially reducing membrane costs and improving system-level economic competitiveness. Download: Download high-res image (185KB) Download: Download full-size image