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 mum (NG-25 and NG40) have been investigated in 1 mol dm(-3) LiClO4-ethylene carbonate (EC)/diethyl carbonate (DEC) at 25 degreesC. 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 degreesC. 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 similar to 10 and similar to 13% for 25 and 40 pra graphite samples, respectively. Nevertheless, the first Coulombic efficiencies are the same as those of original graphites for the samples fluorinated between 150 and 300 degreesC. The increase in the capacities was discussed on the basis of surface compositions and structures of fluorinated graphite samples. (C) 2001 Elsevier Science B.V All rights reserved.