Hierarchically porous hexagonal microsheets constructed by well-interwoven MCo2S4 (M = Ni, Fe, Zn) nanotube networks via two-step anion-exchange for high-performance asymmetric supercapacitors

We report a significant advance in the design and fabrication of MCo2S4 (M = Ni, Fe, Zn) complex hierarchical structures with well-defined morphologies by achieving novel hierarchically porous hexagonal microsheets constructed by well-interwoven nanotube networks using a controllable two-step anion-exchange technique. Uniform and smooth hexagonal sheets are initially achieved for the first anion-exchange leading to nanowire-woven hexagons, followed by transformation of each nanowire to rough MCo2S4 nanotube via the second anion-exchange. The involved mechanism of this general top-down method allowing fine nanostructure control is clarified based on our proposed new insights into ion-induced anisotropic growth and time-dependent anion-exchange reaction kinetics. The merits of both maximized porosity and low resistance facilitate fast electron transfer/ion diffusion, thus NiCo2S4 electrode material exhibits a higher specific capacitance of 1780 F g−1 and superior rate capability than most reported NiCo2S4 nanostructures with different morphologies as well as excellent stability (92.4% capacity retention after 10,000 cycles at 10 A g−1). Furthermore, an asymmetric solid-state supercapacitor using such NiCo2S4 as positive and N-doped graphene film as negative electrodes achieves outstanding cycle ability (92.1% retention over 5000 cycles at 20 A g−1) and higher energy density of 67.2 W h kg−1 (at 900 W kg−1) than that of similar devices. Such MCo2S4 electrode materials are promising for the future generation of high performance supercapacitors.