Raman spectroscopy and electron microscopy of heat-treated petroleum cokes for lithium-intercalation electrodes

Raman spectroscopy, coupled with high-resolution transmission electron microscopy (HRTEM) and x-ray diffraction analysis, were used to characterize the physical properties of carbonaceous materials obtained by heat-treatment of petroleum coke at 1800, 2100, and 2350 degrees C. The effects of heat-treatment and air milling process (to obtain an average particle size of 10 mu m) on the physical and microstructural properties of the carbon particles were examined. The Raman intensities of the D and G bands were used to estimate the crystallite size, L-a, and x-ray diffraction was used to obtain L-c and the d(002) spacing of the petroleum cokes. Heat-treatment of the petroleum coke at temperatures above 2100 degrees C produces a L-a value of about 100 Angstrom, interplanar distance, L-c, of >600 Angstrom, and d(002) spacing of 3.358 Angstrom, close to that of graphite. HRTEM showed that a distinct ordering of the layer planes occurs with heat-treatment, and a perceptible difference in the surface morphology is evident with petroleum coke that is heat-treated at 2350 degrees C and then air milled. The electrochemical results for lithium intercalation/deintercalation of the petroleum cokes in 0.5 M LiN(CF3SO2)(2)/ethylene carbonate:dimethyl carbonate electrolyte revealed that heat-treatment at 2350 degrees C improves the reversible Li storage capacity of the petroleum coke, and that air milling after heat-treatment produces a petroleum coke with high reversible capacity, equivalent to Li0.93C6.