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

ABSTRACT 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–I 2 /I 3 − cathode couple, which stays stable up to the pH of 9.5, is applied due to its facile kinetics over I − –IO 3 − . Meanwhile, considering the decreasing redox potential of Zn anode in the alkaline region, the Zn–Zn(NH 3 ) 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.