Synthesis of nickel-cobalt sulfide electrode materials for high-performance asymmetric supercapacitors

Transition metal sulfide is a promising energy material with high conductivity as well as abundant electrochemical activity. However, their poor rate performance and cyclic stability, aggregation problem, low utilization rate of active materials, large volume change and severe structural exfoliation from conductive substrate have become the bottlenecks restricting their large-scale application. The selection of sulfur source and sulfurization time during sulfurization process will seriously affect the electrochemical properties of nickel-cobalt sulfide. However, there are few systematic reports on the effects of different sulfur sources on electrode materials. Therefore, the effects of different organic sulfur sources (thioacetamide and thiourea) and inorganic sulfur source (sodium sulfide), and sulfurization time on the properties of nickel-cobalt sulfide were systematically studied in this paper. The spherical NiCo2S4 nanoparticles (NiCo2S4-TAA-6) vulcanized by thioacetamide (TAA) have the best specific capacitance (1180 F g- 1 at 1 A g- 1 ) and an admirable cycle stability (80.3 % after 5000 cycles). In addition, N-doped biomass porous carbon (NPC) is prepared by two-step calcination method with diaphragma juglandis fructus as the precursor. The asymmetric supercapacitor NiCo2S4-TAA-6//NPC device also has good electrochemical properties and satisfactory energy density (28.6 Wh kg- 1 at 804.3 W kg- 1 ). More importantly, the capacitor retention rate is as high as 98.3 % after 10,000 cycles, which means that it has excellent cycling stability. Meanwhile, the assembled device achieved a high potential window of 0-1.6 V and could also light up the red LED, which indicates that the NiCo2S4-TAA-6 nanomaterials have great advantages and potential in practical supercapacitor applications.