Caging Polyhalide Anions in Polycyclodextrin for Long-Lasting Aqueous Zinc-Iodine Batteries

Confining polyiodide species at the cathode is pivotal to the stability and long life of aqueous zinc-iodine batteries (AZIBs). Current approaches using porous inorganic materials to physically trap polyiodide species or organic additives to electrostatically bind them remain ineffective due to size mismatches or weak ionic interactions. Here, we introduce a host-guest strategy to cage polyhalides while enabling their transport between niches within a cross-linked polycyclodextrin (polyCD) network. Experimental and computational results indicate that the polymer derived from β-CD exhibits a moderate binding affinity to polyhalide anions, attributable to its appropriate inner diameter and hydrophobic cavity. For our poly(β-CD)/KI₃ cathode, the addition of Br^- results in more effective binding to I⁺ and inhibition of its hydrolysis compared to Cl^-. As such, AZIBs with poly(β-CD)/KI₃ can achieve 2e storage at 205 mAh/g or 4e storage at 365 mAh/g. The specific inclusion chemistry between polyhalide anions and poly(β-CD) enables batteries to operate for more than 60,000 cycles, with a capacity fade of 0.0001%-0.0003% per cycle for 2e and 4e storage, respectively. This work offers a new approach to mitigate polyhalide shuttling by using polymers derived from inexpensive, biodegradable oligosaccharides, thereby enabling sustainable, long-lasting AZIBs.