Controlled synthesis of unique Co9S8 nanostructures with carbon coating as advanced electrode for solid-state asymmetric supercapacitors.

Stimulated by both the rich electrochemical properties and unique structural merits, much attention is directed to construct novel cobalt sulfides nanoarchitectures for energy storage devices. Here, carbon-coated cobalt sulfide nanostructures are fabricated with the assistance of polyvinylpyrrolidone during hydrothermal process. The morphology of carbon-coated cobalt sulfide can evolve from well-defined uniform nanooctahedrons, nanoflowers to nanospheres by simply controlling the polyvinylpyrrolidone amount. Remarkably, the unique carbon-coated cobalt sulfide configuration takes full advantage of the synergistic contributions from both the homogeneous incorporated carbon layer and the ideal hierarchical flowerlike structures, thereby making it to be a suitable electrode for supercapacitors. By virtue of the intriguing structural features of carbon-coated cobalt sulfide, a solid asymmetric supercapacitor based on carbon-coated cobalt sulfide nanoflowers as the positive electrode and active carbon as the negative electrode achieves remarkable cycling stability (around 86% capacitance retention even up to 10,000 times), and outstanding energy density of 40 Wh kg-1 at power density of 850 W kg-1. The exceptionably electrochemical performance of the asymmetric supercapacitor is related to the enhanced electrical conductivity and increased electrode/electrolyte contact area of carbon-coated cobalt sulfide. Thus, the as-prepared carbon-coated cobalt sulfide nanoflowers is demonstrated to be a promising electrode for asymmetric supercapacitors.