Cathode Design via Iron-Coordinated Covalent Organic Frameworks Facilitating Four-Electron Transfer to Achieve High-Capacity Aqueous zinc-iodine Batteries.

The variable valence states of iodine(I) render Zn-I batteries an intriguing area of research. However, current Zn-I batteries are mostly based on I/I redox chemistry. Effective strategies for activating the high-voltage I/I redox couple in iodine-based cathode materials remain relatively scarce. Herein, an iron (Fe)-coordinated porphyrin bipyridine covalent organic framework (PPBY-Fe-COF) is designed as a host material featuring Fe and conjugated C═N active sites to enable consecutive I/I/I redox chemistry. I migrate to cationic Fe sites for oxidation to I, followed by its immobilization on anionic C═N groups. Assisted by OTF, the formation of N-I-O bonds suppresses the I hydrolysis tendency, enabling reversible redox reactions. Consequently, the four-electron transfer Zn||I@PPBY-Fe-COF battery exhibited a specific capacity of 240 mAh g (based on iodine loading) at 1 A g and a capacity retention of 90.9% after 8000 cycles. This work presents an effective methodology for developing high-energy-density aqueous Zn-I battery systems.