Unified quasi-solid electrolyte design for coupled stabilization of Zn anode and I₂ cathode in aqueous Zn-I₂...

Aqueous Zn-I₂ batteries are promising candidates for large-scale energy storage owing to their high safety and low cost. However, uncontrolled zinc dendrite growth and severe polyiodide shuttle effects continue to hinder their long-term stability and Coulombic efficiency (CE). Here we report a unified quasi-solid electrolyte that simultaneously stabilizes both the Zn anode and the I₂ cathode. The polymeric matrix, constructed from quaternized chitosan and tannic acid, provides abundant hydroxyl and quaternary ammonium groups that coordinate Zn²⁺ and strongly immobilize polyiodide species through electrostatic and hydrogen-bonding interactions, thereby effectively suppressing the polyiodide shuttle. This dual regulation boosts the Zn²⁺ transference number to 0.55, suppresses interfacial side reactions, and mitigates dendrite growth. Consequently, Zn symmetric cells exhibit ultralong cycling stability over 2000 h at a low current density of 0.25 mA cm^-2, while Zn-I₂ full cells deliver a high average CE of 99.87% and stable cycling for 4000 cycles at 1 A g^-1. This work establishes a molecularly engineered quasi-solid electrolyte framework that bridges anode and cathode regulation, offering a general design principle to overcome the issues of dendrite growth and shuttle effect in aqueous metal-halogen batteries.