Confinement of Polyiodides by Dual-Functional Tetrazine Cathodes in Zn-I₂ Batteries

Zinc-iodine batteries offer great potential for energy storage due to their long-term cycle stability, flat voltage plateau, inherent safety, and cost-effectiveness. However, their performance is limited by capacity fading and low Coulombic efficiency (CE) caused by the I₃ ^- shuttle effect. In this work, we propose a molecularly engineered tetrazine derivative, 3,6-bis(2-morpholinoethyl)-1,2,4,5-tetrazine (BMT) as a multifunctional cathode to address these challenges. BMT exhibits a reversible two-electron redox process, boosting charge storage capacity, and forms stable precipitation with I₃ ^- ions at a 1:2 stoichiometric ratio, effectively inhibiting the shuttle of polyiodide by covalent-electrostatic synergistic confinement. As expected, the BMT-based cathode exhibits a CE of 99.6% at 2 A g^-1, a high specific discharge capacity of 207 mAh g^-1 at 0.5 A g^-1 as well as ∼100% capacity retention over 33 000 cycles at 2 A g^-1, achieving a record iodine anchoring efficiency. Furthermore, the Zn-I₂ pouch cell with high iodine mass loading (15.5 mg cm^-2) delivers a practical cathode energy density of 145.2 Wh kg^-1 and maintains 76.2% of its capacity after 800 cycles at 2 A g^-1. This work presents a mechanism-driven cathode design strategy that integrates redox activity and polyiodide confinement, providing a blueprint for the development of stable iodine-based batteries.