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

Abstract 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.