Conjugation effect-mediated iodine conversion for high-rate and stable aqueous zinc - iodine batteries

Aqueous zinc-iodine batteries are promising candidates for grid scale energy storage due to high energy density, good safety and cost-effectiveness. Nevertheless, polyiodide shuttle effect and susceptibility of I⁺ to hydrolysis severely impede their practical applications. In this work, conjugation effect is coupled into iodine conversion reaction via anion-π interaction to simultaneously immobilize polyiodides/ICl₂^- anions and modulate I^-/I⁰/I⁺ conversion kinetics. Experimental results combined with theoretical calculations reveal that methylene blue (MB) cation with π-acidic conjugated system tightly binds to polyiodide and ICl₂^- anion via strong anion-π interaction, which effectively suppresses polyiodides shuttle effect and I⁺ hydrolysis, hence ensuring reversibility of I^-/I⁰/I⁺ conversion reaction. Moreover, the conjugated MB cation with highly mobile and homogeneous delocalized electron can serve as redox mediator to accelerate the iodine conversion with the aid of anion-π interaction, thus improving electrochemical kinetics. As a consequence, the batteries exhibit a specific capacity of 365.5 mAh g^-1 at 2 A g^-1 (based on the mass of iodine) and achieve ultralong cycling life of 0.10 million cycles with 65.2% capacity retention at a high rate of 40 A g^-1. This study provides valuable insights into the conjugation effect-mediated iodine conversion for high-performance aqueous zinc-iodine batteries.