Reduced CoNi2S4 nanosheets decorated by sulfur vacancies with enhanced electrochemical performance for asymmetric supercapacitors

Nowadays, it is a matter of great concern to design electrode materials with excellent electrochemical performance for supercapacitors by a safe, efficient and simple method. And these characteristics are usually related to the vacancies and impurities in the electrode. To investigate the effect of the vacancies on the electrochemical properties of the supercapacitor cathode material, the uniform reduced CoNi2S4 (r-CoNi2S4) nanosheets with sulfur vacancies have been successfully prepared by a one-step hydrothermal method. And the formation of sulfur vacancies are characterized by Raman, X-ray photoelectron spectroscopy and other means. As the electrode for supercapacitor, the r-CoNi2S4 nanosheet electrode delivers a high capacity of 1918.9 F g−1 at a current density of 1 A g−1, superior rate capability (87.9% retention at a current density of 20 A g−1) and extraordinary cycling stability. Compared with the original CoNi2S4 nanosheet electrode (1226 F g−1 at current density of 1 A g−1), the r-CoNi2S4 nanosheet electrode shows a great improvement. The asymmetric supercapacitor based on the r-CoNi2S4 positive electrode and activated carbon negative electrode exhibits a high energy density of 30.3 W h kg−1 at a power density of 802.1 W kg−1, as well as excellent long-term cycling stability. The feasibility and great potential of the device in practical applications have been successfully proved by lightening the light emitting diodes of three different colors.