Ternary Transition Metal Sulfides Embedded in Graphene Nanosheets as Both the Anode and Cathode for High-Performance Asymmetric Supercapacitors

Owing to their low electronegativity, excellent electrical conductivity, high specific capacitance, and rich electrochemical redox sites, various transition metal sulfides have attracted significant attention as promising pseudocapacitive electrode materials for supercapacitors. However, their relatively poor electrical conductivity and large volume changes seriously hinder their commercial applications. Herein, ternary Co0.33Fe0.67S2 nanoparticles are in situ embedded between graphene nanosheets through a facile one-step hydrothermal approach to form a sandwich-like composite. Because of its unique and robust structure, the graphene nanosheet/Co0.33Fe0.67S2 composite (GCFS-0.33) exhibits a high specific capacitance (310.2 C g(-1) at 2 mV s(-1)) and superb rate capability (61.8% at 200 mV s(-1)) in 3 M KOH aqueous electrolyte. Using transition metal sulfides simultaneously as both positive and negative electrodes, for the first time, an aqueous asymmetric supercapacitor (ASC) was fabricated with the GCFS-0.33 composite as the negative electrode and sulfidized graphene/CoNiAl-layered double hydroxides as the positive electrode with well-separated potential windows. Our fabricated ASC delivered an excellent energy density of 66.8 Wh kg(-1) at a power density of 300.5 W kg(-1) and still retained 13.1 Wh kg(-1) even at a high power density of 29.4 kW kg(-1), which is highly comparable with that of previously reported transition-metal-sulfide-based ASC devices. Moreover, the as-fabricated ASC cell displays impressive long-term cycling stability with a capacitance retention of 102.2% relative to the initial capacitance after 10 000 cycles. This versatile synthetic strategy can be readily extended to synthesize other transition-metal-sulfide-based composites with excellent electrochemical performances.