Millerite Core–Nitrogen-Doped Carbon Hollow Shell Structure for Electrochemical Energy Storage

Nickel sulfides have drawn much attention with the benefits of a high redox activity, high electrical conductivity, low cost, and fabrication ease; however, these metal sulfides are susceptible to mechanical degradation regarding their cycling performance. Conversely, hollow carbon shells exhibit a substantial electrochemical steadiness in energy storage applications. Here, the design and development of a novel millerite core–nitrogen-doped carbon hollow shell (NiS–NC HS) structure for electrochemical energy storage is presented. The nitrogen-doped carbon hollow shell (NC HS) protects against the degradation and the millerite-core aggregation, giving rise to an excellent rate capability and stability during the electrochemical charging–discharging processes, in addition to improving the NiS–NC HS conductivity. The NiS–NC HS/18h supercapacitor electrode displays an outstanding specific capacitance of 1170.72 F g⁻¹ (at 0.5 A g⁻¹) and maintains 90.71% (at 6 A g⁻¹) of its initial capacitance after 4000 charge–discharge cycles, owing to the unique core–shell structure. An asymmetric-supercapacitor device using NiS–NC HS and activated-carbon electrodes exhibits a high power and energy density with a remarkable cycling stability, maintaining 89.2% of its initial capacitance after 5000 cycles.