N-doped carbon coated hollow Ni_xCo_9-xS₈ urchins for a high performance supercapacitor

N-doped carbon coated Ni_xCo_9-xS₈ (Ni_xCo_9-xS₈@C) hollow urchins have been synthesized via a two-step solvothermal synthesis and an in situ polymerization in dopamine together with a post-annealing process. The characterization indicated that Ni_xCo_9-xS₈@C hollow urchins have urchin-like morphology and a uniform size distribution. Furthermore, there is a complete phrase transformation from the as obtained NiCo₂S₄/Ni_xCo_9-xS₈ hybrid to Ni_xCo_9-xS₈ during the thermal annealing process. More importantly, as electrochemical materials, Ni_xCo_9-xS₈@C has a high specific capacitance (1404.0 F g^-1 at 2.0 A g^-1) and excellent cycling performance (95.8% capacitance retention of the highest value after 2000 cycles). These results can be attributed to the coating of N-doped carbon, which gives the composite good conductivity. Additionally, the phase transformation from NiCo₂S₄/Ni_xCo_9-xS₈ to Ni_xCo_9-xS₈ during the thermal annealing greatly enhanced the redox reaction of the Co and Ni species.