Intrinsic energy-storage mechanism of low crystallinity nickel-cobalt sulfide as anode material for supercapacitors

NiCo2S4 (NCS) is a ternary spinel sulfide widely applied in supercapacitors and hybrid supercapacitors. Several electrochemical reaction formulas have so far been proposed in last decade. However, the energy storage mechanism and electrochemical behavior of NCS in alkaline electrolytes are still not fully understood. In this study, the interactions between hydroxide and NCS with low crystallinity are investigated through electrochemical testing and both surface and structure characterization. Compared to previous reports, electrochemical desulfurization is first introduced in electrochemical charge storage to reveal the intrinsic energy-storage mechanism of NCS in alkaline electrolyte. The highest specific capacitance of NCS reaches 666.27 F g(-1), at 5 A g(-1), originating from the irreversible phase transition during the activation process. The long cycle stability retains 65.29% of the highest specific capacitance after 10 000 cycles, attributed to the transformation product (nickel-cobalt dihydroxide) without further phase change after the activation process. In sum, clarification of the intrinsic energy-storage mechanism of NCS with low crystallinity can be helpful for future preparation of anode materials for applications in alkaline supercapacitors, batteries, and hybrid devices.