Template synthesis of NiCo2S4/Co9S8 hollow spheres for high-performance asymmetric supercapacitors

Nickel cobalt sulfides have been widely used as the battery-type electrode materials for asymmetric supercapacitors. However, it is still a challenge that obtains a unique structure to improve their electrochemical characteristics and practical applications. Hollow porous structures with hetero-interfaces are currently promising to endow electroactive materials with excellent properties, such as well-defined interior voids, low density, high pore volume, and surface permeability. Herein, hetero-structure NiCo2S4/Co9S8 hollow spheres were synthesized by using the carbon spheres as the sacrificial template via the solvothermal method and anion exchange process. When evaluated as a promising electrode for supercapacitors, the NiCo2S4/Co9S8 electrode exhibited an excellent specific capacitance of 2180 F g(-1) at the current density of 1 A g(-1), a high capacity retention that the specific capacitance retained 1400 F g(-1) even at 20 A g(-1), and a long stable cycling performance of retaining similar to 85% after 10,000 cycles. In addition, the asymmetric supercapacitors based on these hollow hetero-spheres exhibited a high energy density of 96.5 Wh kg(-1) at a power density of 0.8 kW kg(-1). The energy density can achieve 51.7 Wh kg(-1) even at a higher power density of 20 kW kg(-1), indicating that the NiCo2S4/Co9S8 hollow spheres have superior practical applications for energy storage.