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–I2 batteries an intriguing area of research. However, current Zn–I2 batteries are mostly based on I–/I0 redox chemistry. Effective strategies for activating the high-voltage I0/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–/I0/I+ redox chemistry. I– migrate to cationic Fe sites for oxidation to I0, 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 gI–1 (based on iodine loading) at 1 A g–1 and a capacity retention of 90.9% after 8000 cycles. This work presents an effective methodology for developing high-energy-density aqueous Zn–I2 battery systems.