Microwave-anion-exchange route to ultrathin cobalt-nickel-sulfide nanosheets for hybrid supercapacitors

The emergence of graphene-like 2D nanostructures has stimulated tremendous attention due to their fascinating physicochemical properties and promising applications in the field of energy conversion and storage. Herein, we report a gram-scale synthesis of non-layered ultrathin CoNi₂S₄ nanosheets via microwave-assisted liquid-phase growth of layered double hydroxide precursor poste-sulfurized at room-temperature, the method is time-saving and scalable for massive production. The CoNi₂S₄ nanosheets exhibit a micron-sized planar area, ultrathin thickness (∼2 nm) and mesoporous feature. When loaded on flexible carbon cloth for electrochemical capacitor, the CoNi₂S₄ nanosheets deliver a high specific capacitance of 247 mAh g⁻¹ at 8 A g⁻¹ with an excellent rate capability of 91% from 1 to 20 A g⁻¹. Remarkably, as a positive electrode material for hybrid supercapacitor, CoNi₂S₄ nanosheets demonstrate excellent electrochemical performances with a high energy density of 67.7 Wh kg⁻¹ at a power density of 0.8 kW kg⁻¹. Even at an extreme power density of 9.6 kW kg⁻¹, the energy density is still as high as 50.6 Wh kg⁻¹ with a robust long-term cycling stability up to 10,000 cycles. Such results are among the best even superior to those reported for nickel-cobalt-sulfide-based materials used as positive electrode for hybrid supercapacitors.