竹节状纳米碳纤维的制备及嵌锂性能研究

Preparation and Electrochemical Lithium Intercalation Performance of Segmented Carbon Nanofibers

以泡沫镍为催化剂 ,在 6 0 0和 70 0℃下 ,以CVD法热解乙炔气体制备大量的纳米碳纤维 .随着制备温度增加 ,纳米碳纤维直径变小 ,竹节状含量减少 ,d0 0 2 值减小 ,微晶片层平面Lc 和La 值增大 ,碳材料的可逆容量则下降 .分别用透射电镜、X射线衍射和拉曼光谱观察和测定了纳米碳纤维的形貌、微结构 ,发现在不同条件下生长的纳米碳纤维有不同的形貌和结构 .对纳米碳纤维的电化学嵌锂性能的研究表明 ,纳米碳纤维的结构对其电化学嵌锂容量和充放电循环寿命起重要影响 ,制备温度越低 ,纳米碳纤维的石墨化程度越差 。

Segmented carbon nanofibers were prepared by pyrolysis of acetylene on foam Ni at 600 and 700degreesC in a fixed bed flowed-reactor. The morphology, microstructure and lithium insertion properties of these carbon nanofibers were investigated by TEM, XRD, Raman and electrochemical methods. Through TEM observations, it was found that this kind of carbon nanofibers was composed of lens-like segments with nearly equal separation stacking along the nanofiber axis. When the reaction temperature was 600degreesC, segmented carbon nanofibers were the major production. However, when the reaction temperature increased to 700degreesC, the content of segmented carbon filaments decreased and their diameter became smaller. The crystallite size d(002) and L-c were determined by the 002 carbon Bragg peak of XRD patterns using the Bragg and Scherrer formulas. The intensity ratios of the 1350 cm(-1) line and the 1580 cm(-1) line (R =I-D/I-G) was used to evaluate the L-a value, which was inversely proportional to the effective crystallite size in the direction of the graphite plane (L-a). With the reaction temperature increased, the d(002) value decreased, L-a and L-c values increased, which indicated the degree of crystallinity increased. Segmented carbon nanofibers were used as positive electrodes of C/Li cells. The first charge capacities of C/Li cells were 480 and 300 mAh/g for samples produced at 600 and 700degreesC, respectively. The samples at 600degreesC showed capacities higher than the theoretical value of graphite, which was attributed to accommodation of more. lithium at the edge of graphene layers and on the surface of graphene layers according to the mechanisms of lithium insertion in carbons prepared by low-temperature pyrolysis of hydrocarbons. As confirmed by the XRD and Raman spectra, the samples at 700degreesC had larger L-a and L-c, which led to the capacity decreasing.