Aqueous hybrid iron-ion battery capacitors with ultra-long cycle life

With the over-exploitation of lithium resources, there is a tendency to find a new metal-ion energy storage device to replace lithium-ion batteries. In recent years, Fe-ion energy storage devices are acknowledged for their low cost, abundant resources, and safe characteristics, but their full potential has yet to be widely explored. Herein, we propose a new hybrid iron-ion battery capacitor (H-IIBC) energy storage device. The H-IIBC device prepared with nano flower-like activated carbon (FAC) as the cathode material, high-purity iron sheet as the anode material, and 1 M FeSO4 + NH4Cl aqueous solution as the electrolyte. After atomic molecular dynamics simulations and ex situ characterization analysis, ion adsorption/desorption occurs on the surface of the FAC cathode during the charging/discharging process, accompanied by the redox of iron ions. The stripping/deposition of iron occurs at the high-purity iron anode. With the charging/discharging process, the reaction at the anode favors the interconversion between Fe2+ and Fe3+ which requires less energy, thus ensuring that the high purity iron will not be depleted. Thus, the specific capacitance of prepared H-IIBC could reach 644 mF g−1 (399 mF cm−2) at a current density of 1 mA cm−2 in the voltage window of 0–1 V and shows ultra-high cycling stability at 3,000 cycles test. This economical, long-life and safe H-IIBC is expected to realize great potential in the field of energy storage in the near future.