We present a facile one-step solvothermal approach for in situ growth of ultradispersed nickel cobalt sulfide (NiCo2S4) nanoparticles on the graphene (GR) sheets without adding any surfactant. When acting as an electrochemical supercapacitor electrode material, the NiCo2S4@GR delivers a highly improved specific capacitance of 1708 F g(-1) at a current density of 1.0 A g(-1), compared with that of free NiCo2S4 (950 F g(-1)). The performances of NiCo2S4@GR electrode material can also be optimized by carefully tailoring the amount of each ingredient NiCo2S4 and GR in the nanocomposites. Asymmetric supercapacitor devices based on these NiCo2S4@GR nanocomposites and aqueous electrolyte showd a long-term cycling ability with a high-energy density (68.5 Wh kg(-1) at 850.0 W kg(-1)) and a high-power density (17.0 kW kg(-1) at 37.7 Wh kg(-1)), implying its great potential application in high-performance energy storage devices. (C) 2015 Elsevier Ltd. All rights reserved.