Hollow nickel-cobalt sulfide nanospheres cathode hybridized with carbon spheres anode for ultrahigh energy density asymmetric supercapacitors

There has been increasing attention to the transition metal sulfides because of their unique property such as high electrical conductivity. Moreover, hollow sphere structures can provide excellent performance in electrochemical energy conversion applications due to their rich holes volume and unique properties. In this study, nickel-cobalt sulfide （Ni-Co-S） hollow spheres are successfully synthesized by two step methods of hard template method and hydrothermal method. Benefiting from the hollow sphere structure with optimum size and mesoporous surface, the Ni-Co-S exhibits an excellent specific capacity of 819.9 C g−1, and has good stability in the charge-discharge cycles. In addition, uniform size and monodisperse carbon sphere (CS) that synthesized by a simple self-assembly method also shows excellent electrochemical properties. Furthermore, the fabricated Ni-Co-S//CS asymmetric supercapacitor shows energy density of 65 Wh kg−1 at a power density of 850 W kg−1 and outstanding cycling stability with a capacity retention of 82.8% after 10,000 cycles, demonstrating that as-prepared Ni-Co-S hollow spheres and carbon spheres possess promising potential as electrode materials for high-performance asymmetric supercapacitors.