Functionalized graphene sheets (FGSs) comprise a unique member of the carbon family, demonstrating excellent electrical conductivity and mechanical strength. However, the detailed chemical composition of this material is still unclear. Herein, we take advantage of the fluorination process to semiquantitatively probe the defects and functional groups on graphene surface. Functionalized graphene sheets are used as substrate for low-temperature (<150 °C) direct fluorination. The fluorine content has been modified to investigate the formation mechanism of different functional groups such as C=F, CF2, O=CF2 and (C=O)F during fluorination. The detailed structure and chemical bonds are simulated by density functional theory (DFT) and quantified experimentally by nuclear magnetic resonance (NMR). The electrochemical properties of fluorinated graphene are also discussed extending the use of graphene from fundamental research to practical applications. Energetics of defects on graphene: The chemical composition and structural evolution of graphene during fluorination is investigated by a combination of experiments and theoretical simulations, which provides a novel method to study defect chemistry. In a model electrode, the energy/power ratio of as-prepared fluorinated graphene is tunable by modifying the C/F ratio, demonstrating both an informative fundamental phenomenon as well as a promising practical use for energy storage © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.