Iron‐Free Anode Boosting High Energy Efficiency Aqueous Full Iron‐Ion Batteries

Abstract Aqueous iron‐ion batteries with reversible storage of Fe 2+ have undergone rapid development in recent years. Consistently throughout these studies, metallic iron is selected as the anode material. However, the large overpotential (250 mV) associated with the plating/stripping process of iron in aqueous solutions leads to unsatisfactory energy efficiency of the battery, although high capacity and Coulomb efficiency can be achieved. Herein, an iron‐free anode material, 9,10‐anthraquinone (AQ) is proposed in aqueous iron‐ion batteries, which shows a low reaction potential and minimal polarization during storing iron ions. The organic anode exhibits favorable specific capacity of 106 mAh g −1 at 0.5 A g −1 and excellent cycling stability (92.6% retention after 500 cycles). In addition, an aqueous full iron‐ion battery is constructed using AQ as the anode and 9,10‐phenanthraquinone (PQ) as the cathode. The full battery demonstrates an enhanced energy efficiency of 72%, which is 206% higher than that of metal iron anode, and shows excellent cycling stability and Coulombic efficiency. This work provides a viable route to overcome the high polarization of metallic iron anode and promote the development of aqueous iron‐ion batteries.