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.