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

Abstract

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.