Growth of yolk-shell CuCo2S4 on NiO nanosheets for high-performance flexible supercapacitors

A poor electrical conductivity and short cycle life limits the wide application of transition metal oxides in energy storage applications. In this study, yolk-shell structured CuCo2S4 was grown on NiO nanosheets ([email protected]2S4) via a hydrothermal method and vulcanization, which combined the synergistic interactions between Ni, Co, and Cu. The effect of varying the amount of the vulcanizing agent (thiourea) on the electrochemical performance of [email protected]2S4 was also investigated. Moreover, the amount of thiourea not only tuned the morphology of the composite, but significantly influenced its electrochemical performance. When the Cu2+: Co2+: thiourea molar ratio in the precursor solution was 1:2:6, the obtained [email protected]2S4 exhibited the best electrochemical performance of the various systems examined, with a specific capacitance of 1658 F g−1 being achieved at 1 A g−1. Density functional theory calculations further confirmed the excellent synergistic effect between NiO and CuCo2S4. In addition, the asymmetric supercapacitor composed of a [email protected]2S4 positive electrode reached an ultrahigh energy density of 73 Wh kg−1 at a power density of 802 W kg−1, as well as excellent cycling stability (i.e., 91% capacitance retention after 5000 cycles). These results suggest that [email protected]2S4 is a promising candidate for use in energy storage applications.