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 40μm (NG-25 and NG-40) have been investigated in 1moldm^-3 LiClO₄-ethylene carbonate (EC)/diethyl carbonate (DEC) at 25°C. The fluorine contents obtained by elemental analysis are 0.2-0.4at.% and the surface fluorine concentrations obtained by X-ray photoelectron spectroscopy (XPS) are 4-12at.% 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 2-3nm 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, 372mAhg^-1. The increments of the capacities are ~10 and ~13% for 25 and 40μm 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.