Mesoporous design of ultrathin NiO nanosheet-coated vertically aligned hexagonal CoS nanoplate core–shell array for flexible all-solid-state supercapacitors

Supercapacitors are becoming the next-generation high-power and carriable energy storage systems. However, large-scale manufacturing of supercapacitors is limited by their inferior energy storage densities, high costs, and rigidity; thus, a cost-effective binder-free approach is required to assemble advanced electrodes on flexible substrates. Multidimensional core–shell electrodes that are vertically aligned on an inexpensive conductive substrate can facilitate improved electrochemical performance. Herein, a core–shell heterostructure of hexagonal cobalt sulfide (CoS) nanoplates encased with ultrathin nickel oxide (NiO) nanosheets was fabricated on a flexible stainless-steel foil by simple chemical methods. Owing to the synergistic effect between the CoS nanoplates and ultrathin NiO nanosheets, the CoS–NiO core–shell electrode showed improved electrochemical performance with a specific capacitance of 1527 F g⁻¹ at a current density of 1 A g⁻¹, high stability retaining 94% of the capacitance after 5000 charge–discharge cycles, and good capacitive retention during 30-fold enhancement in the current density. Finally, a flexible all-solid-state supercapacitor was fabricated using the CoS–NiO core–shell electrode and dip-coated carbon nanotubes as the positive and negative electrodes, respectively. The fabricated prototype exhibited a maximum energy density of 39 W h kg⁻¹, energy efficiency of 47%, and cycling stability retaining 89.5% of the capacitance after 5000 charge–discharge cycles.