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

The multielectron conversion electrochemistry of I – /I 0 /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 I 0 /I + couple. However, the practical working voltage is below the theoretical level. In this study, the I 0 /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/Zn 2+ 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 I 0 /I + conversion and inhibits the self-disproportionation of I +, which in turn promotes rapid kinetics and excellent reversibility of I 0 /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.