Rational design of self-supported Ni₃S₂ nanosheets array for advanced asymmetric supercapacitor with a superior energy density

We report a rationally designed two-step method to fabricate self-supported Ni₃S₂ nanosheet arrays. We first used 2-methylimidazole (2-MI), an organic molecule commonly served as organic linkers in metal-organic frameworks (MOFs), to synthesize an a-Ni(OH)₂ nanosheet array as a precursor, followed by its hydrothermal sulfidization into Ni₃S₂. The resulting Ni₃S₂ nanosheet array demonstrated superior supercapacitance properties, with a very high capacitance of about 1,000 F g^-1 being delivered at a high current density of 50 A g^-1 for 20,000 charge-discharge cycles. This performance is unparalleled by other reported nickel sulfide-based supercapacitors and is also advantageous compared to other nickelbased materials such as NiO and Ni(OH)2. An asymmetric supercapacitor was then established, exhibiting a very stable capacitance of about 200 F g^-1 at a high current density of 10 A g^-1 for 10,000 cycles and a surprisingly high energy density of 202 W h kg^-1. This value is comparable to that of the lithium-ion batteries, i.e., 180 W h kg^-1. The potential of the material for practical applications was evaluated by building a quasi-solid-state asymmetric supercapacitor which showed good flexibility and power output, and two of these devices connected in series were able to power up 18 green light-emitting diodes.