Urea Chelation of I⁺ for High-Voltage Aqueous Zinc-Iodine Batteries

The multielectron conversion electrochemistry of I^-/I⁰/I⁺ enables high specific capacity and voltage in zinc-iodine batteries. Unfortunately, the I⁺ ions are thermodynamically unstable and are highly susceptible to hydrolysis. Current endeavors primarily focus on exploring interhalogen chemistry to activate the I⁰/I⁺ couple. However, the practical working voltage is below the theoretical level. In this study, the I⁰/I⁺ redox couple is fully activated, and I⁺ is efficiently stabilized by a chelation agent of cost-effective urea in the conventional aqueous electrolyte. A record-high plateau voltage of 1.8 V vs Zn/Zn²⁺ has been realized. Theoretical calculations combined with spectroscopy studies and electrochemical tests reveal that the coordination between the electron-deficient I⁺ and the electron-rich O and N atoms in urea molecules is thermodynamically favorable for I⁰/I⁺ conversion and inhibits the self-disproportionation of I⁺, which in turn promotes rapid kinetics and excellent reversibility of I⁰/I⁺. Moreover, urea decreases the water activity in the electrolyte by forming hydrogen bonds to further suppress the hydrolysis of I⁺. Accordingly, a high specific capacity of 419 mAh g^-1 is delivered at 1C, and 147 mAh g^-1 capacity is retained after 10,000 cycles at 5C. This work offers effective insights into formulating halogen-free electrolytes for high-performance aqueous zinc-iodine batteries.