Heterojunction induced activation of iron oxide anode for high-power batteries

Aqueous Ni-Fe batteries show promise for grid level energy storage due to their high safety and low cost. However, high capacities of Fe-based anodes can only be achieved under slow discharging rates. Moreover, an activation process is often required, the mechanism of which has not been fully understood. Herein, we present a facile and controllable method to uniformly deposit Fe3O4 nanoparticles on a 3D graphite substrate. Postmortem analysis demonstrates the partial conversion of Fe3O4 to FeOOH during the subsequent in-situ electrochemical activation process, forming a Fe3O4/FeOOH heterostructure. Density functional theory calculations suggest that a built-in electric field is formed near the Fe3O4/FeOOH interface, which facilitates the charge transfers and lowers the adsorption energy of OH-. The above modification on the active material significantly improves its electrochemical activity. The activated electrode delivers a high capacity of 509 mAh g(-1) at the ultra-high current density of 100 A g(-1). A Ni-Fe cell assembled with activated Fe3O4/FeOOH anode and Ni-Co double hydroxide cathode provides a high energy density of 161.3 Wh kg(-1) and a maximum power density of 43 kW kg(-1), making it a good candidate for high safety, low cost, and environmental friendliness energy storage systems.