A High-Voltage Mild Alkaline Zinc-Iodine Battery Enabled by Tailored Redox Couples

Aqueous zinc-iodine batteries are promising energy storage systems. However, the voltage is limited in mild acidic electrolytes, while alkaline electrolytes lead to severe passivation at both electrodes. Herein, we optimize zinc-iodine battery systems by tailoring the redox couples of both electrodes based on the Pourbaix diagrams. The I-I₂/I₃ ^- cathode couple, which stays stable up to the pH of 9.5, is applied due to its facile kinetics over I^--IO₃ ^-. Meanwhile, considering the decreasing redox potential of Zn anode in the alkaline region, the Zn-Zn(NH₃)₄ ²⁺ couple is selected with pH adjusted toward the cathode boundary of 9.3 by ammonium. This reaction pathway also fundamentally avoids ZnO passivation. The resulting optimal mild alkaline system achieves 1.6 V average voltage, exceeding both mild acidic and strong alkaline electrolytes. A quaternary ammonium cation is further employed to inhibit polyiodide dissolution from the cathode and suppress hydrogen evolution reaction at the anode. Consequently, the zinc-iodine battery realizes 2.13 mAh cm^-2 capacity at 3 mA cm^-2 and retains 1.08 mAh cm^-2 capacity at 20 mA cm^-2, together with low overpotentials of 0.06 and 0.21 V, respectively. It also achieves 93.6% capacity retention for 2000 cycles with 99.2% coulombic efficiency and 88.3% energy efficiency.