Insights into the interfacial nanostructuring of NiCo₂S₄ and their electrochemical activity for ultra-high capacity all-solid-state flexible asymmetric supercapacitors

Ternary metal sulfide based nanostructured materials are promising for commercialization of the electrochemical energy storage devices. Herein, three different NiCo₂S₄ nanostructures (nanoflakes, nanosheets, and nanoparticles) were fabricated by electrodeposition. Of these, nanosheets consisting of interconnected nanoparticles formed a highly porous network for supercapacitive energy storage. The electrochemical properties of each electrode were studied in detail and it was observed that the self-supported NiCo₂S₄ nanosheets possess a highest specific capacity of 590 mA h g⁻¹ (2655 F g⁻¹) at 0.25 A g⁻¹ current density and cycling stability of 88.7% after 5000 charge-discharge cycles. This excellent behavior is attributed to several factors of the electrode such as high electrochemical active sites and ability of a nanostructure to withstand under high strain and accommodate large number of electrolyte ions during charge-discharge. The electrochemical storage properties of the NiCo₂S₄ nanosheets were further explored by fabricating battery-like solid-state asymmetric supercapacitor with activated carbon that delivered an ultra-high specific energy and power of 69.7 Wh kg⁻¹ and 8 kW kg⁻¹, respectively. These outcomes indicate that the novel nanostructured NiCo₂S₄ network has great potential for the development of energy storage devices.