Fluoride ion-mediated morphology control of fluorine-doped CoFe2O4/graphene sheet composites for hybrid supercapacitors with enhanced performance

Abstract A one-pot facile hydrothermal synthesis of fluorine-doped CoFe 2 O 4 sphere (FS-CoFe 2 O 4 )/graphene sheet (GS) composites, based on the Ostwald ripening mechanism, is presented in this work. Incorporation of fluorine in CoFe 2 O 4 /GS improves ion diffusion by forming a highly porous structure that serves as an electrolyte reservoir during the redox reactions and facilitates charge transfer in the electrode material. The influence of the fluorine concentration on the relationship between structure and electrochemical performance of CoFe 2 O 4 /GS is also studied. The FS-CoFe 2 O 4 /GS composite exhibits a high specific capacity of 288 C g −1 at a current density of 1 A g −1 , along with an excellent rate capability of 80% (230 C g −1 at a current density of 10 A g −1 ). Moreover, the FS-CoFe 2 O 4 /GS composite is used to assemble a hybrid supercapacitor, which exhibits a high operating voltage of 1.6 V and delivers a high energy density of 28.4 Wh kg −1 at a power density of 0.9 kW kg −1 , as well as an outstanding cycling performance (90% capacity retention after 20,000 cycles, in which the GSs support FS-CoFe 2 O 4 for the whole duration of the cycling tests). The present study can open up a new avenue for exploiting fluorine as a dopant in high-performance hybrid supercapacitors.