Sustainable Binder-Driven Four-Electron I ^– /I ⁰ /I ⁺ Conversion in Metal–Iodine Batteries

Achieving four-electron transfer (I – /I 0 /I + ) in iodine cathodes is crucial for realizing high energy density in metal batteries, but faces limited conversion efficiency and instability of I + species. Here, we present a binder-centered approach that leverages nucleophilic carboxyl groups in polymer binders to stabilize four-electron iodine redox chemistry confined within the electrode. This design decouples iodine redox chemistry from the electrolyte environment, enabling universal applicability across diverse electrolyte systems. As a result, aqueous Zn–I 2 batteries deliver a high specific capacity of 411 mAh g –1 and retain 88% of their capacity after 10,000 cycles at 10 C, while organic Li–I 2 batteries achieve a capacity of 400 mAh g –1 and a discharge platform (I + /I 0 ) at 3.5 V, leading to a record high energy density of 1344 Wh kg –1 based on I 2 . This work offers a simple, scalable, and sustainable halogen-free approach for enabling stable multielectron iodine conversion in both aqueous and organic systems.