A hydroxylated zwitterion enables dual-modal synergy for stable zinc–iodine batteries

Abstract

Aqueous zinc–iodine (Zn–I₂) batteries face a universal challenge: soluble polyiodides formed at the cathode migrate to the zinc anode, causing severe active-material loss and Zn corrosion. Here, we report a hydroxylated betaine-type zwitterion that enables dual-modal interfacial control at both electrodes in Zn–I₂ batteries. At the zinc anode, the synergistic sulfonate–hydroxyl pair disrupts the Zn²⁺ hydration shell, decouples Zn²⁺–SO₄²⁻ interactions, increases the Zn²⁺ transference number, and promotes preferential (002) deposition below a compact, zwitterion-derived solid–electrolyte interphase. At the iodine cathode, the ammonium–hydroxyl pair captures polyiodides to suppress shuttling while maintaining an electrochemically beneficial polyiodide concentration. Owing to this adaptive regulation, Zn||Zn symmetric cells and Zn||Cu half-cells exhibit exceptional cycling stability (>8000 h) at 1 mA cm⁻²; Zn–I₂ full cells deliver 50 000 cycles at 20 A g⁻¹ with an ultralow per-cycle capacity fade of 0.00012%; and pouch cells (37.9 mg cm⁻² of I₂) show only 0.0003% capacity loss per cycle. This work establishes a paradigm of adaptive interfacial control that reconciles the distinct electrode processes in Zn–I₂ batteries.