Electrochemical characteristics and structures of surface-fluorinated graphites with different particle sizes for lithium ion secondary batteries

Electrochemical characteristics of surface-fluorinated graphite samples with average particle diameters of 25 and 40 μm (NG-25 and NG-40) have been investigated in 1 mol dm−3 LiClO4–ethylene carbonate (EC)/diethyl carbonate (DEC) at 25 °C. The fluorine contents obtained by elemental analysis are 0.2–0.4 at.% and the surface fluorine concentrations obtained by X-ray photoelectron spectroscopy (XPS) are 4–12 at.% for the samples fluorinated between 150 and 300 °C. Raman spectra and transmission electron micrographs indicate the increase in the surface disordering of graphite. The surface areas of graphite samples are increased by 64–77% and the mesopores with diameter of 2–3 nm are also increased by surface fluorination. The charge capacities of surface-fluorinated samples are larger than those of original graphites and even the theoretical capacity of graphite, 372 mAh g−1. The increments of the capacities are ∼10 and ∼13% for 25 and 40 μm graphite samples, respectively. Nevertheless, the first Coulombic efficiencies are the same as those of original graphites for the samples fluorinated between 150 and 300 °C. The increase in the capacities was discussed on the basis of surface compositions and structures of fluorinated graphite samples.