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 ZnI2 in a ZnSO4(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 (Zn2+ 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 I3− and IO3− ions as the prominent species. The latter gets reduced at the COF electrode under an applied potential, leaving I3− as the major species stored on the COF. The prospect of COF-polyiodide(aq) is a windfall for metal-ion batteries.