High Efficiency Alkaline Iodine Batteries with Multi-Electron Transfer Enabled by Bi/BiO Redox Mediator.

The multi-electron transfer I/IO redox couple is attractive for high energy aqueous batteries. Shifting from an acidic to an alkaline electrolyte significantly enhances the IO 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 IO 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/IO couple in alkaline electrolytes for the first time and propose that Bi/BiO acts as a redox mediator (RM) to "catalyze" the reduction of IO . 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/BiO RM cathode, the full battery with I/IO redox mechanism achieves high areal capacity of 12 mAh cm and stable operation at 5 mAh cm for over 400 cycles.