Hierarchical design of Cu_1-xNi_xS nanosheets for high-performance asymmetric solid-state supercapacitors

Novel supercapacitor electrodes comprising hierarchical architectures with high specific surface areas, unique porosities, excellent conductivities, and admirable mechanical stabilities are necessary for developing high-performance solid-state supercapacitors. Herein, a novel ultra-thin copper nickel sulfide (Cu_1-xNi_xS) nanosheet array supercapacitor electrode was constructed on a 3D Ni backbone through a powerful anion exchange technique and it demonstrated a unique architecture with a substantial degree of porosity. Accordingly, Cu_1-xNi_xS plays an imperative role in the electrochemical energy storage characteristics of the electrode by accomplishing an ultra-high areal capacitance of 5.88 F cm^-2 and a specific capacitance of 2672 F g^-1 at a current density of 2 mA cm^-2 with an excellent rate capability (71.26% capacitance retention at 20 mA cm^-2) and a superior cycling performance (97.33% capacitance retention after 10 000 cycles). To design asymmetric supercapacitors (ASCs), Cu_1-xNi_xS and N, S co-doped graphene nanosheets (NSGNSs) are employed as positive and negative electrodes, respectively. Remarkably, the fabricated ASC exhibits a potential window of ∼1.8 V, which demonstrates an ultra-high energy density of ∼94.05 W h kg^-1 at 1.09 kW kg^-1 as well as an excellent life cycle (95.86% capacitance retention after 10 000 cycles). Owing to this fact, this investigation offers a simple, scalable, and cost-effective approach for the fabrication of other ternary transition metal sulfides (TMSs), emphasizing great prospects in next-generation energy storage applications.