Ultrathin nanosheets-assembled NiCo2S4 nanocages derived from ZIF-67 for high-performance supercapacitors

Abstract

The design and synthesis of new microstructure electrodes with large surface area and high electrochemical response play an essential role in advanced supercapacitors. Herein, NiCo2S4 hollow nanocages assembled by ultrathin nanosheets were synthesized through a facile solid-state chemical sulfuration method derived from the sacrificial template of Co-based zeolitic imidazolate frameworks (ZIF-67). The obtained hollow NiCo2S4 possess inner cavities and largely ultrathin nanosheets for both short ion diffusion distance and rich interfacial active sites which can dynamically speed up faradaic reactions. A specific capacitance of 1232 F g−1 is obtained at a current density of 2.0 A g−1 for the NiCo2S4 electrode. Notably, compared with Co–Ni LDH, NiCo2S4 electrode yields impressive rate capability and excellent cycling stability (80% capacitance retention after 8000 charge–discharge cycles), mainly due to its robust porous architecture and good electronic conductivity. The electrochemical reaction mechanism is studied by in situ XRD, indicating that NiCo2S4 exhibits a single-phase OH− insertion/extraction behavior. The aqueous asymmetric supercapacitor assembled with NiCo2S4 as positive electrode also delivers high energy density of 41.4 Wh kg−1 at power density of 689.5 W kg−1. This work demonstrates that the well-designed NiCo2S4 is promising electrode materials for high-performance supercapacitors.