Made to Measure Squaramide COF Cathode for Zinc Dual‐Ion Battery with Enriched Storage via Redox Electrolyte

Abstract Aqueous rechargeable batteries are promising grid‐scale energy storage devices because of their affordability, operational safety, and environmental benignity. Among these, Zn‐ion batteries (ZIBs) have unfolded new horizons. Designing superior cathodes for ZIBs is crucial. Covalent organic frameworks (COFs) can be made redox active with a high storage surface. Here, for the first time, a chelating COF with redox‐active ZnI 2 in a ZnSO 4 (aq) electrolyte is combined. Including iodide harvests an approximately threefold enhancement in capacity from 208 to 690 mAh g −1 at 1.5 A g −1 , the highest among all the COF‐derived ZIBs. Remarkably, a charge–discharge curve at 1.3 V exhibits very limited dropout voltage and super‐flat platform, with a remarkable capacity of 600 mAh g −1 at 5 A g −1 stable up to 6000 cycles, confirming that the polyiodide generation and storage are sustainable. The COF's dual‐ion storage (Zn 2+ and polyidode) delivers a ZIB with the highest energy density. Spectro‐electrochemical measurements coupled with X‐ray photoelectron spectroscopy unambiguously unveil the existence of multiple polyiodide species, with I 3 − and IO 3 − ions as the prominent species. The latter gets reduced at the COF electrode under an applied potential, leaving I 3 − as the major species stored on the COF. The prospect of COF‐polyiodide (aq) is a windfall for metal‐ion batteries.