Deciphering SN2-Type Nucleophilic Substitution via Halogen-Free Intermediates for High-Energy Zinc–Iodine Batteries

High-valence halogen conversion reactions are promising for realizing high-energy-density aqueous batteries. It has been more challenging to realize a stable and reversible I0/I+ conversion than the facile I-/I0 conversion. While interhalogen chemistry has been employed to realize a reversible I-/I0/I+ (i.e., four-electron) redox couple, the introduction of additional halide species raises corrosion and toxicity issues. Herein, we propose an SN2-type nucleophilic substitution mechanism for the activation of the I0/I+ redox process. A series of halogen-free intermediates containing imide groups is proven effective in promoting four-electron halogen reactions due to conjugation-stabilized nitrogen anions. A structure-kinetics relationship among various imide-containing molecules is elucidated using two descriptors: the hydrogen dissociation energy (Ed) of the imide group and the local nucleophilicity (LN) of the corresponding nitrogen anion. A Zn-I2 battery with a theobromine intermediate exhibits stable cycling (a capacity retention of 93.9% at 5 A g-1 over 1000 cycles). This work provides new insights into the mechanism of the I0/I+ redox process and a new avenue to the development of high-capacity multielectron iodine cathodes.