Novel carbon nanofibers of high graphitization as anodic materials for lithium ion secondary batteries

摘要

Carbon nanofibers (CNFs) of high graphitization degree were prepared by a CVD process at 550-700 deg C. They showed different structures according to catalyst and preparation temperatures. The structure of CNF prepared from CO/H_2 over an iron catalyst was controlled from platelet (P) to tubular (T) by raising the decomposition temperature from 550 to 700 deg C. The CNFs prepared over a copper-nickel catalyst from C_2H_4/H_2 showed the typical herringbone (HB) structure regardless of the reaction temperatures. The CNFs prepared over Fe showed d_(002) of 0.3363-0.3381 nm, similar to that of graphite, indicating very high graphitization degree in spite of the low preparation temperature. Such CNFs of high graphitization degree showed high capacity of 297-431 mAh/g, especially in the low potential region. However, low first cycle coulombic efficiency of approx= 60 percent) is a problem to be solved. The graphitization of the CNF preserved the platelet texture, however, and formed the loops to connect the edges of the graphene sheets. Higher graphitization temperatures made the loop more definite. The graphitized CNF showed high capacity (367 mAh/g); however, its coulombic efficiency was not so large despite its modified edges by graphitization, indicating that the graphene edges were not so influential for the irreversible reaction of Li ion battery.