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

Achieving four-electron transfer (I^-/I⁰/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-I₂ 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-I₂ batteries achieve a capacity of 400 mAh g^-1 and a discharge platform (I⁺/I⁰) at 3.5 V, leading to a record high energy density of 1344 Wh kg^-1 based on I₂. This work offers a simple, scalable, and sustainable halogen-free approach for enabling stable multielectron iodine conversion in both aqueous and organic systems.