Electron-Rich Azo Sites on Ordered Mesoporous Covalent Organic Frameworks Boosting Redox Kinetics in Aqueous Zn-I2 Batteries

The practical application of aqueous zinc-iodine (Zn-I2) batteries is frequently hindered by the notorious shuttle effect of polyiodides and sluggish redox kinetics. To overcome these challenges, two isostructural mesoporous covalent organic frameworks (COFs) were synthesized via the condensation of 1,3,5-triformylphloroglucinol (TP) with either 4,4'-azodianiline (AZO) or benzidine (BD), yielding AZO-TP-COF and BD-TP-COF, respectively. When employed as cathode hosts in Zn-I2 batteries, the AZO-TP-COF@I2 composite delivered a specific capacity of about 228 mAh g-1 at 0.2 A g-1, along with remarkable rate capability and long-term cycling stability (95% capacity retention after 3000 cycles at 3 A g-1), better than those of BD-TP-COF@I2. This enhanced performance of the AZO-TP-COF@I2 cathode is attributed to the electron-rich azo moieties in AZO-TP-COF, which effectively suppress the polyiodide shuttle effect and facilitate the adsorption and conversion of I-/I2/I3- species. These findings highlight the pivotal role of azo-based engineering in COFs for regulating redox mediators in metal-halogen batteries.