Coordination Chemistry toward Advanced Zn–I₂ Batteries with Four-Electron I^–/I⁰/I⁺ Conversion

Aqueous zinc-iodine (Zn-I₂) batteries with four-electron (4e) I^-/I⁰/I⁺ conversion (4eZIBs) offer high energy density but face significant challenges for application, including the polyiodide shuttle effect and I⁺ hydrolysis for the I₂ cathodes and poor reversibility for the Zn anodes. Here, we report a coordination chemistry strategy to address these issues simultaneously by introducing hexamethylenetetramine (HMTA) as an electrolyte additive. In aqueous electrolytes, HMTA undergoes protonation to form positively charged nitrogen moieties that effectively precipitate the polyiodides and I⁺ species (ICl₂^-) to mitigate the polyiodides shuttle and I⁺ hydrolysis. This strategy enables 4eZIBs to achieve a near-theoretical specific capacity of 425 mA h g^-1 (based on the mass of iodine) and a Coulombic efficiency (CE) exceeding 99%. On the Zn anode, HMTA preferentially adsorbs onto its surface, inhibiting competitive water adsorption to suppress both Zn dendrite formation and hydrogen evolution. As a result, for the first time, we achieve durable 4eZIB performance in pouch-cell configurations with limited Zn supply. A 0.5 A h pouch cell with 15% Zn utilization exhibits a high energy density of 113.0 W h kg^-1 (based on the mass of cathodes and anodes) and excellent cycling stability for over 1400 cycles, highlighting the potential of 4eZIBs for next-generation energy storage systems.