Nanosheet-assembled NiS hollow structures with double shells and controlled shapes for high-performance supercapacitors

Hollow micro- and nanostructured materials with controllable shapes are appealing materials for electrochemical energy storage devices with high energy and power densities. Herein, double-shelled hollow nickel sulfides with controlled shape (cube-, ellipsoid- and capsule-shaped) were successfully synthesized by using different morphological α-Fe 2 O 3 templates. The as-obtained nickel sulfides have uniform size and well-defined double shells that consist of crossed ultrathin NiS nanosheets. When evaluated as electrode materials for supercapacitors, the capsule-shaped structure exhibits the highest specific capacitance of 1159   F   g −1 at a current density of 2   A   g −1 due to its high specific surface area (100.2   m 2   g −1 ), low equivalent series resistance (0.80   Ω) and ion diffusive resistance. More importantly, the asymmetric capacitor NiS (capsule-shaped)||RGO@Fe 3 O 4 shows a high energy density of 43.7   Wh   kg −1 at a power density of 664   W   kg −1 (21.8   Wh   kg −1 at 8000   W   kg −1 ), and after 5000 charge–discharge cycles, 83.3% of its capacitance is remained at the current density of 5   A   g −1 .