Carbonization and graphitization of pitch applied for anode materials of high power lithium ion batteries

The artificial graphite materials were prepared by carbonizing coal tar pitch using two methods, namely, one- and two-step processes, and all sintered samples were graphitized at 2800 degrees C. Effects of different heat treatments on the performance of the samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction, Brunauer-Emmett-Teller, electrochemical impedance spectroscopy (EIS), particle size analysis, polarized light microscopy, and charge-discharge measurements. All samples show a typical graphite crystalline structure; moreover, the degree of graphitization (g factor) and crystallite size along the c-axis (L-c) were calculated from (002) peak. The polarized light microscopy indicates that the coke with carbonization at 700 degrees C has an obvious wide domain (D) optical structure, while that with two-step sintering at 400 and 700 degrees C has a mixed optical structures of wide D, flow domains, and mosaics. TEM analysis revealed a number of irregular graphene layer images which are caused by the defects of graphite. EIS shows that the sample carbonized by two-step has a larger diffusion coefficient than the sample carbonized at 700 degrees C by one step. Higher carbonization temperature leads to better cycle performance as the temperature increasing from 500 to 700 degrees C in the one-step route. Specifically, the charge (Li+ extraction) capacity at the 50th cycle increases from 318 mAhg(-1) to 357 mAhg(-1). The results show that the rate performance of the artificial graphite is improved with the addition of the presintering at 400 degrees C.