Rational construction of phosphate layer to optimize Cu-regulated Fe3O4 as anode material with promoted energy storage performance for rechargeable Ni-Fe batteries

Flexible aqueous energy storage devices with high security and flexibility are crucial for the progress of wearable energy storage. Particularly, aqueous rechargeable Ni-Fe batteries owning a large theoretical capacity, low cost and outstanding safety characteristics have emerged as a promising candidate for flexible aqueous energy storage devices. Herein, Cu-doped Fe3O4 (CFO) with 3D coral structure was prepared by doping Cu2+ based on Fe3O4 nanosheets (FO). Furthermore, the Fe-based anode material (CFPO) grown on carbon fibers was obtained by reconstructing the surface of CFO to form a low-crystallization shell which can enhance the ion transport. Excitingly, the newly developed CFPO electrode as an innovative anode material further exhibited a high capacity of 117.5 mAh g−1 (or 423 F g−1) at 1 A g−1. Then, the assembled aqueous Ni-Fe batteries with a high cell-voltage output of 1.6 V deliver a high capacity of 49.02 mAh g−1 at 1 A g−1 and retention ratio of 96.8% for capacitance after 10 000 continuous cycles. What's more, the aqueous quasi-solid-state batteries present a remarkable maximal energy density of 45.6 Wh kg−1 and a power density of 12 kW kg−1. This work provides an innovative and feasible way and optimization idea for the design of high-performance Fe-based anodes, and may promote the development of a new generation of flexible aqueous Ni-Fe batteries.