A four-electron Zn-I2 aqueous battery enabled by reversible I−/I2/I+ conversion

Electrochemically reversible redox couples that embrace more electron transfer at a higher potential are the eternal target for energy storage batteries. Here, we report a four-electron aqueous zinc-iodine battery by activating the highly reversible I2/I+ couple (1.83 V vs. Zn/Zn2+) in addition to the typical I−/I2 couple (1.29 V). This is achieved by intensive solvation of the aqueous electrolyte to yield ICl inter-halogens and to suspend its hydrolysis. Experimental characterization and modelling reveal that limited water activity and sufficient free chloride ions in the electrolyte are crucial for the four-electron process. The merits of the electrolyte also afford to stabilize Zn anode, leading to a reliable Zn-I2 aqueous battery of 6000 cycles. Owing to high operational voltage and capacity, energy density up to 750 Wh kg−1 based on iodine mass was achieved (15–20 wt% iodine in electrode). It pushes the Zn-I2 battery to a superior level among these available aqueous batteries.