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.