Phase reconfiguration of heterogeneous CoFeS/CoNiS nanoparticles for superior battery-type supercapacitors

Developing advanced battery-type materials with abundant active sites, high conductivity, versatile morphologies, and hierarchically porous structures is crucial for realizing high-quality hybrid supercapacitors. Herein, heterogeneous FeS@NiS is synthesized by cationic Co doping via surface-structure engineering. The density functional theory (DFT) theoretical calculations are firstly performed to predict the advantages of Co dopant by improving the OH⁻ adsorption properties and adjusting electronic structure, benefiting ions/electron transfer. The dynamic surface evolution is further explored which demonstrates that CoFeS@CoNiS could be quickly reconstructed to Ni(Co)Fe₂O₄ during the charging process, while the unstable structure of the amorphous Ni(Co)Fe₂O₄ results in partial conversion to Ni/Co/FeOOH at high potentials, which contributes to the more reactive active site and good structural stability. Thus, the free-standing electrode reveals excellent electrochemical performance with a superior capacity (335.6 mA h g⁻¹, 2684 F g⁻¹) at 3 A g⁻¹. Furthermore, the as-fabricated device shows a quality energy density of 78.1 W h kg⁻¹ at a power density of 750 W kg⁻¹ and excellent cycle life of 92.1% capacitance retention after 5000 cycles. This work offers a facile strategy to construct versatile morphological structures using electrochemical activation and holds promising applications in energy-related fields.