In this work, a simple low-temperature method has been employed to synthesize carbon nanoballs (CNBs). In a typical procedure, 1 g of petroleum asphalt in an alumina boat was directly heated to 450 ℃ at a rate of 10 ℃/min in air in a conventional box furnace to obtain the CNBs. Subsequently, the synthesized CNBs were treated in tubular furnace at 1500 ℃ under nitrogen atmosphere for 3 h and the obtained products were denoted as H-CNBs. The as-synthesized CNBs and H-CNBs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and infrared spectroscopy. It was found that the diameters of CNBs and H-CNBs were in the range of 50～80 nm, and both of them showed a relatively low degree of graphitization. Compared with the CNBs, the H-CNBs showed a slightly higher degree of graphitization than the CNBs. A proposed formation mechanism of the CNBs is suggested to be as follows. Firstly, petroleum asphalt decomposed and formed a large amount of hydrocarbons at 220～500 ℃, such as benzene, methane, acetylene and so on. Carbon atoms from thermal cracking products of hydrocarbons deposited on the substrate in the absence of O₂. Carbon atoms developed to CNBs as the temperature increased and the reaction time prolonged. The CNBs and H-CNBs were used as anode material in lithium ion batteries. The electrochemical performance was studied by galvanostatic charge/discharge at ambient temperature. The CNBs as anode materials exhibited a high initial capacity of 1260 mAh/g at a current of 1 C, which stabilized at around 500 mAh/g after 100 cycles. In contrast, the H-CNBs showed an initial capacity of 413 mAh/g under the same electrochemical conditions, which stabilized at 200 mAh/g after about 20 cycles with slight increase upon further cycling. Coulombic efficiency of as high as 98% was recorded after initial cycling for 10 cycles and 30 cycles for CNBs and H-CNBs, respectively. After cycling at different currents from 0.1 C, to 1 C, 5 C, and 10 C, the capacity of CNBs was almost fully recovered to the initial capacity at 0.1 C.

Key words: petroleum asphalt, carbon nanoballs, electrode material, specific capacity