Activating the I⁰/I⁺ redox couple in an aqueous I₂–Zn battery to achieve a high voltage plateau

Rechargeable iodine conversion batteries possess promising prospects for portable energy storage with complete electron transfer and rich valence supply. However, the reaction is limited to the single I⁻/I⁰ redox at a potential of only 0.54 V vs. the standard hydrogen electrode (SHE), leading to a low voltage plateau at 1.30 V when Zn is employed as the anode. Herein, we show how to activate the desired reversible I⁰/I⁺ redox behavior at a potential of 0.99 V vs. SHE by electrolyte tailoring via F⁻ and Cl⁻ ion-containing salts. The electronegative F⁻ and Cl⁻ ions can stabilize the I⁺ during charging. In an aqueous Zn ion battery based on an optimized ZnCl₂ + KCl electrolyte with abundant Cl⁻, the I-terminated halogenated Ti₃C₂I₂ MXene cathode delivered two well-defined discharge plateaus at 1.65 V and 1.30 V, superior to all reported aqueous I₂–metal (Zn, Fe, Cu) counterparts. Together with the 108% capacity enhancement, the high voltage output resulted in a significant 231% energy density enhancement. Metallic Ti₃C₂I₂ benefits the redox kinetics and confines the interior I species, leading to exceptional cyclic durability and rate capability. In situ Raman and ex situ multiple spectral characterizations clarify the efficient activation and stabilization effects of Cl⁻ (F⁻) ions on reversible I⁰/I⁺ redox. Our work is believed to provide new insight into designing advanced I₂–metal batteries based on the newly discovered I⁻/I⁰/I⁺ chemistry to achieve both high voltage and enhanced capacity.