Cation-Mediated Water Activity Balance Enables High-Rate Four-Electron Zn-I2 Batteries.

High-rate 4e- aqueous zinc-iodine (Zn-I2) batteries are promising for large-scale storage, yet they face a fundamental trade-off: high water activity triggers hydrolysis, while conventional water-suppression strategies severely impede kinetics. To solve this, we selected CHA-HBr and HA-HBr as additives to investigate the structural modulation effects on the Zn-I2 chemistry. Compared to its flexible linear counterpart, the rigid cyclic framework in CHA-HBr not only enriches local electron density at the amino group via a pronounced electron-donating inductive effect, but also imposes superior steric hindrance with restricted conformational degrees of freedom. Comparative testing and theoretical calculations prove that these synergistic structural and electronic features of CHA-HBr facilitate a water-depleted solvation sheath, diminishing the nucleophilic reactivity of iodine species and inhibiting the I+ hydrolysis while simultaneously ensuring superior high-rate kinetics by significantly lowering desolvation energy barriers. Consequently, the Zn-I2 (CHA-HBr) battery exhibits a high capacity of 250 mAh g-1 at 20 A g-1, maintaining stability over 20 000 cycles with a 99.8% coulombic efficiency. Furthermore, a pouch cell with a high mass loading of 14.0 mg cm-2 delivers a capacity of 0.56 Ah, validating the potential for constructing highly stable 4e- Zn-I2 batteries.