Fabrication of Fully Fluorinated Graphene Nanosheets Towards High‐Performance Lithium Storage

liation of F-graphite (50 at%) in 2-isopropanol (IPA) solvent. The resulting F-graphene nanosheets exhibit two-dimensional structure, amorphous-like microstructure and high fl uorine content. Such unique features render abundant F active sites for lithium storage and facilitate the rapid diffusion of lithium ions during charging and discharging processes. As a consequence, the resulting F-graphene nanosheets exhibit high reversible capacity of 780 mAh g −1 at 50 mA g −1 and excellent cycle performance. F-graphene nanosheets were fabricated by a simple liquid exfoliation procedure. In a typical experiment, 50 mg F-graphite powders (fl uorine content, 50 at%) were suspended in 100 ml IPA solvent and then sonicated at room temperature. After sonication for 6 h, a uniform dispersion of F-graphene with the same milk-like color to that of original F-graphite in IPA was achieved (see Supporting Information, Figure S1). To investigate the effect of solvents on the exfoliation of F-graphite, Nmethyl-pyrrolidone (NMP) and N,N-dimethylformamide (DMF) were also studied for comparison. However, when NMP and DMF solvents were used as solvents, most of the F-graphite could not be exfoliated after the same sonication time (see Supporting Information, Figure S2). Furthermore, with extending the exfoliation time to 24 h, the color of F-graphene/NMP (or F-graphene/DMF) dispersion signifi cantly changes from milklike to black (see Supporting Information, Figure S1) because of the fl uorine removal from F-graphene nanosheets (see Supporting Information, Figure S3). The morphology and structure of F-graphene nanosheets were initially investigated via scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis and transmission electron microscopy (TEM). As shown in Figure 1 a, many two-dimensional nanosheets with the sizes from 500 nm to several micrometers can be clearly observed. The high-resolution TEM (HRTEM) image discloses the amorphous-like structure on the resulting nanosheets, possibly resulted from the high fl uorination which can cause the breakage of the hexagonal net-works and parallel basal planes of graphene. EDS analysis reveals the high fl uorine content of 49.7 at% (Figure 1 b), corresponding to the atomic ratio of ∼1:1 between F and C. It suggests that each fl uorine atom attaches one carbon, forming C-F bonds, and the direction of C-F bond obeys the upwarddownward sequence along the C-C hexagonal net-work as illustrated in Figure 1 d. The atomic force microscopy (AFM) analysis (Figure 1 e and 1 f) further displays the two-dimensional features of F-graphene nanosheets with a thickness of about 10 nm. A high Brunauer– Emmett–Teller (BET) surface area of up to 125 m 2 g −1 is achieved for F-graphene nanosheets based on their nitrogen adsorption/desorption analysis as shown in Figure S4. Thermogravimetric analysis (TGA) of F-graphene nanosheets (see the Supporting Information, Figure S5) revealed that they are thermally stable under the temperature of 500 °C. Dr. L. Zhan, S. Yang Department of Mechanical Engineering & Materials Science Rice University Houston , Texas 77005 , USA E-mail: sy13@rice.edu Dr. L. Zhan, Y. Wang, Y. L. Wang, L. C. Ling State Key Laboratory of Chemical Engineering East China University of Science and Technology Key Laboratory of Specially Functional Polymeric Materials and Related Technology (Ministry of Education) Shanghai, 200237 , China Prof. X. L. Feng Max Planck Institute for Polymer Research Ackermannweg 10, Mainz D-55128 , Germany E-mail: feng@mpip-mainz.mpg. de