Preparation Of Fluorine-Doped Graphene Materials And Their Electrochemical Properties

In this paper,the fluorine-doped graphene materials with different morphology and structures were synthesized through solvothermal reaction and applied as the electrode materials for supercapacitors,lithium ion batteries and sodioum ion batteries.We prepared 3D fluorine-doped graphene hydrogel?FGH?in the GO aqueous dispersion through the one-step hydrothermal process,with hydrofluoric acid?HF?as the fluorine source.Fluorine atoms incorporate into the graphene framework when the hydrothermal temperature between 90 and 150?.The fluorine contents and the C-F bond configurations are easily adjusted by the hydrothermal temperature.Both covalent C-F bond and semi-ionic C-F bond were existed in FGH.The doping of fluorine could increase the crumpled and wrinkled surface of graphene,generate more defects in the graphene basal plane and enlarge the layer distance.Fluorination mechanisms were also studied.Graphene oxide paper?GOP?was prepared by the cast drying method,and then undergoes a solvothermal treatment to fabricate F and N co-doped graphene paper?FNGP?.The existents of semi-ionic C-F bonds increase the electric conductivity of paper.The tensile strength and Young's modulus of FNGP-120 reaches 43.5 MPa and9.5 GPa.The prepared FNGP-120 shows excellent flexibility and could fold/unfold several times without apparent damage.F-doped graphene nanosheets?F-r GO?was synthesized by utilizing reduced graphene nanosheets?rGO?as a precursor and hydrofluoric acid as a fluorinated reagent.During the solvothermal process,the residual carbonyl and ether/phenol groups in rGO react with HF molecules,forming-CF₂ components and-CF components according to the fluorination mechanisms.The F-r GO was tested as a negative electrode for LIBs.The expanded interlayer spacing and heteroatomic defects result in ultrafast Li⁺diffusion and electron transport,thus F-rGO exhibits high reversible capacity and excellent rate performance.The existence of high stable and electrochemically inert-CF₂ components provides the long-term stability.The electrochemical performances of FGHs were tested as the additive and binder-free electrodes for supercapacitors.The relatively high fluorine content and the abundance of semi-ionic C-F bond in FGH-150 endowed the high electrical conductivity,superb hydrophilicity and pseudo capacitance,so FGH-150 shows improved specific capacitance and excellent rate capability.The mass specific capacitance of FGH-150 is 227 F g^-1 at the current density of 1 A g^-1.The maximum power density of FGH-150 reaches to 50.05 kW kg^-1 at the current density of 50 A g^-1.The high power density accompanied with the medium energy density can be realized in the case of FGH-150 at high current densities load.The prepared FNGPs were used as free-standing anodes in SIBs without any binder or additive.The sodium storage mechanism in FNGP-120 is reversible Na⁺adsorption/insertion on/between graphene layers.FNGP-120 exhibited improved Na⁺storage performance,high reversible capacity,satisfactory rate capability and outstanding cycling stability,which are associated with enlarged layer distance and increased active sites.FNGP-120 delivers a reversible capacity of 203 m Ah g^-1 after100 cycles at 50 mA g^-1.In addition,the specific capacity of FNGP-120 is maintained at as high as 56.3 mAh g^-1 over 5000 cycles at a current density of 1000 mA g^-1.FNGP-120 shows outstanding stability as a free-standing anode for flexible SIBs and after 100 cycles,FNGP-120 still maintains original shape and structural integrity,indicating an excellent structural stability.