Regulating the electro-deposition behavior of Fe metal anode and the applications in rechargeable aqueous iron-iodine batteries

The iron metal electrode based on the Fe2+/Fe redox reaction is a promising anode candidate for aqueous batteries. It possesses high capacity, large abundance, low price, as well as a little higher redox potential than Zn2+/Zn which corresponds to suppressed hydrogen evolution reaction. However, the reversibility and stability of Fe electrodes require improvement. Herein, we regulate the Fe deposition behavior with Zn doping. The positively charged [ZnI]+ complex is introduced into FeCl2 electrolytes by adding ZnI2, which is adsorbed on the negatively charged surface of Fe metal and realizes the doping of Zn into Fe lattice during deposition. In comparison to the large chunks with sole Fe deposition, the Zn doped Fe forms homogeneous small particles with the (110) plane preferred to grow parallel to the substrate. Enhanced coulombic efficiencies of 98% are achieved for over 1200 cycles. Stable Fe plating/stripping is accomplished for over 2500 h at the current density of 1 mA cm−2 and capacity of 1 mAh cm−2, as well as for over 300 h under ultra-high current density and capacity of 10 mA cm−2 and 10 mAh cm−2. Benefiting from the iodine species in electrolyte, an iron-iodine full cell is demonstrated. The battery exhibits excellent capacity retention of 99.8% with average coulombic efficiency of 96.7% for over 1200 cycles. The work provides a potential path for high-performance aqueous Fe-metal batteries.