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.