Double-layered yolk-shell microspheres with NiCo2S4-Ni9S8-C hetero-interfaces as advanced battery-type electrode for hybrid supercapacitors

NiCo2S4-Ni9S8-C double-layered yolk-shell microspheres were synthesized by using bimetallic MOF as precursor. With the aid of NiCo2S4-Ni9S8 hetero-interface and unique double-layered yolk-shell structure, the special composite as battery-type electrode material shows excellent specific capacity of 293.6 mAh g−1 at 1 A g−1 and improved rate ability of 81.1% from 1 A g−1 to 20 A g−1. The electrochemical energy storage performance of this electrode material is superior to many recently reported cobalt-based and nickel-based electrodes materials. [Display omitted] •Unique MOF pyrolysis and sulfuration method was used to prepare electrode material.•The NiCo2S4-Ni9S8-C electrode shows a high capacity of 293.6 mAh g−1 at 1 A g−1.•The hetero-interfaces in NiCo2S4-Ni9S8-C endow high electrochemical activity.•The double-layered yolk-shell structure reduces diffusion resistance of electrolyte. It requires excellent conductivity, rapid diffusion of electrolyte and high active specific area of active materials to achieve efficient supercapacitor. Herein, the novel NiCo2S4-Ni9S8-C double-layered yolk-shell microspheres (NiCo2S4-Ni9S8-C DYMs) were synthesized by using bimetallic metal-organic framework (MOF) as self-template. The microspheres are composed of numerous tiny heterogeneous NiCo2S4-Ni9S8 nanoparticles (~10 nm in size) decorated in amorphous carbon. As expected, the sample exhibits high specific capacity of 293.6 mAh g−1 at 1 A g−1, excellent rate capacity (81.1% from 1 A g−1 to 20 A g−1) and good cycling stability (capacity retention of 87.3% over 5000 cycles). The hybrid supercapacitor assembled by NiCo2S4-Ni9S8-C DYMs and grapheme hydrogel, shows an energy density of 51 Wh kg−1 at a power density of 1399.4 W kg−1 and even can retain 32.5 Wh kg−1 at 8004.4 W kg−1. The density functional theory (DFT) calculation show the hetero-interfaces of NiCo2S4-Ni9S8 can optimize the electronic distribution, coupled with the excellent electroconductivity of dispersed carbon within microspheres, which boost the electrochemical performance. This work provides an approach to fabricate heterogeneous microspheres by MOF route for developing advanced battery-type electrode materials.