High Efficiency Alkaline Iodine Batteries with Multi‐Electron Transfer Enabled by Bi/Bi2O3 Redox Mediator

The multi‐electron transfer I‐/IO3‐ redox couple is attractive for high energy aqueous batteries. Shifting from an acidic to an alkaline electrolyte significantly enhances the IO3‐ formation kinetics due to the spontaneous disproportionation reaction, while the alkaline environment also offers more favorable Zn anode compatibility. However, sluggish kinetics during the reduction of IO3‐ persists in both acidic and alkaline electrolytes, compromising the energy efficiency of this glorious redox couple. Here, we establish the fundamental redox mechanism of the I‐/IO3‐ couple in alkaline electrolytes for the first time and propose that Bi/Bi2O3 acts as a redox mediator (RM) to “catalyze” the reduction of IO3‐. This mediation significantly reduces the voltage gap between charge/discharge from 1.6 V to 1 V with improved conversion efficiency and rate capability. By pairing the Zn anode and the Bi/Bi2O3 RM cathode, the full battery with I‐/IO3‐ redox mechanism achieves high areal capacity of 12 mAh cm‐2 and stable operation at 5 mAh cm‐2 for over 400 cycles.