Polythiophene-Coated FeC₂O₄@FeCO₃ Core-Shell Architecture: An Ultrahigh Capacity Anode for Advanced Ni-Fe Batteries

Aqueous rechargeable Ni-Fe batteries exhibit considerable potential for use in large-scale energy storage systems due to their stable operating voltage, inherent safety, low cost, and high power density. Herein, a core-shell composite was designed for anode applications in which the FeC₂O₄@FeCO₃ polyhedron and the polythiophene (PTh) layer serve as the core and shell, respectively. Owing to its core-shell structure, multicomponent synergistic effect, rough surface morphology, and enhanced electrical conductivity, the as-fabricated binder-free FeC₂O₄@FeCO₃-PTh electrode delivers excellent electrochemical performance, including high specific capacities (10.8 mAh cm^-2 and 490 mAh g^-1 at 5 mA cm^-2), good rate capability (1.76 mAh cm^-2 at 100 mA cm^-2), and stabilized cycling performance (95.5% capacity retention after 2500 cycles). These metrics surpass those of the uncoated FeC₂O₄@FeCO₃ electrode as well as the individual FeC₂O₄ (3.11 mAh cm^-2), FeCO₃ (1.54 mAh cm^-2), and PTh (0.8 mAh cm^-2) electrodes. Notably, the simultaneous achievement of high areal and gravimetric specific capacities in Fe-based electrodes is scarce, further highlighting the value of our FeC₂O₄@FeCO₃-PTh electrode. Moreover, when assembled into a Ni-Fe battery with the optimal FeC₂O₄@FeCO₃-PTh electrode, the NiC₂O₄//FeC₂O₄@FeCO₃-PTh device achieves an energy density of 3.53 mWh cm^-2 (421 Wh kg^-1) at a power density of 2 mW cm^-2 (238.2 W kg^-1), surpassing most previously reported aqueous energy storage devices. This work provides a feasible approach for designing advanced iron-based electrodes and paving the way for the practical application of Ni-Fe battery systems and other iron-based energy storage devices.