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

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.