Nitrogen-doped graphene encapsulated cobalt iron sulfide as an advanced electrode for high-performance asymmetric supercapacitors

To develop supercapacitors (SCs) with high energy and power densities, a newtype of electrodematerial with unique properties is strongly required. Metal sulfides (MSs) have been recently studied as promising electrode materials for high-performance SCs because they have excellent redox properties, outstanding electrical conductivity, exceptional catalytic activity, and ultra-high specific capacity. However, their large volume changes and poor rate performances seriously hinder their commercial applications. Herein, a novel nitrogen-doped graphene encapsulated cobalt iron sulfide core-shell (Co8FeS8@NG) hybrid was successfully synthesized through a facile, scalable, and single-step in situ hydrothermal technique. The hierarchical core-shell Co8FeS8@NG hybrid was employed as an advanced electrode material for high-performance solid-state asymmetric SCs (ASCs). The Co8FeS8@NG electrode delivers an ultra-high specific capacitance of -1374 F g(-1) at a current density of 2 A g(-1), with tremendous rate capability (-71.69% of capacitance retention at 40 A g(-1)) and excellent cycling stability (similar to 96.1% of capacitance retention after 10 000 cycles). Furthermore, the assembled Co8FeS8@NG//FeS@NG ASC device exhibits an ultra-high energy density of similar to 70.4 W h kg(-1) at a power density of 0.598 kW kg(-1), exceptional power density (similar to 22.55 kW h kg(-1) at 49.5 W h kg(-1)), and outstanding cycling stability (similar to 93.7% of initial capacitance after 10 000 cycles). Therefore, these results demonstrate a simple and cost-effective route for the development of new types of NG encapsulated ternary MSs for future electronics.