Electrochemical behavior of plasma-fluorinated graphite for lithium ion batteries

Electrochemical properties of plasma-fluorinated graphite samples have been investigated in 1 mol dm^-3 LiClO₄ ethylene carbonate (EC)/diethyl carbonate (DEC) solution at 25°C. Fluorine contents in plasma-fluorinated graphite samples were in the range of 0-0.3at.% by elemental analysis and surface fluorine concentrations obtained by X-ray photoelectron spectroscopy (XPS) were in the range of 3-12at.%. Raman spectroscopy revealed that surface disordering of graphite was induced by plasma fluorination. Plasma treatment increased the surface areas of graphite samples by 26-55% and the pore volumes for the mesopores with diameters of 1.5-2 and 2-3nm. Plasma-fluorinated graphites showed capacities higher than those of original graphites and even higher than the theoretical capacity of graphite, 372 mAhg^-1, without any change of the profile of charge-discharge potential curves. The increments in the capacities were approximately 5, 10 and 15% for graphites with average particle diameters, 7, 25 and 40μm, respectively. Furthermore, the coulombic efficiencies in first cycle were nearly the same as those for original graphites or higher by seral percents.