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.