Composition controlled nickel cobalt sulfide core-shell structures as high capacity and good rate-capability electrodes for hybrid supercapacitors

Herein, we report a type of core-shell structured NixCo3-xS4 (x = 1, 1.5 and 2) through tuning the ratios of Ni to Co by a simple two-step hydrothermal method. Scanning electron microscopy and transmission electron microscopy images of all the Ni-Co sulfides show high porosity and uniform spherical morphology. Moreover, this study demonstrates that controlling the appropriate ratio of Ni to Co in the core-shell structure is crucial for improving the electrochemical performance in a hybrid supercapacitor. Among them, NiCo2S4 with core-shell structure possesses the highest capacity of 155 mA h g(-1), whereas as Ni1.5Co1.5S4 shows the excellent rate-capability of up to 80% from 1 to 20 A g(-1) and cycling stability, which retains 74% after 2000 cycles. Therefore, tuning the metal element contents provides a feasible approach to achieving the desired high-rate performance and cycling stability of electrode materials in polynary metal sulfides for hybrid supercapacitors.