Wencheng Li

Structure and Lithium Insertion Properties of Carbon Nanotubes

Carbon nanotubes were obtained by pyrolysis of acetylene or ethylene catalyzed by iron or iron oxide nanoparticles. The morphology, microstructure, and lithium insertion properties of these carbon nanotubes were investigated by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and electrochemical measurements, respectively. The results showed that the structures of the carbon nanotubes play major roles in both specific capacity and cycle life. Slightly graphitized carbon nanotubes showed a specific capacity of 640 mAh/g during the first charge, whereas well-graphitized carbon nanotubes showed a specific capacity of 282 mAh/g during the first charge. After 20 charge/discharge cycles the charge capacity of the slightly graphitized samples degraded to 65.3% of their original charge capacities, but the well-graphitized samples maintained 91.5% of their original charge capacities. The effects of charge-discharge rates and cycling temperature on lithium insertion properties of carbon nanotubes with different extents of graphitization are discussed.

Lithium Insertion in Carbon‐Silicon Composite Materials Produced by Mechanical Milling

Carbons containing nanosize silicon particles were prepared by mechanical milling of graphite and silicon mixtures with different atomic ratios. The microstructure, morphology, and electrochemical performance of ballmilled (x = 0, 0.1, 0.2, 0.25) were analyzed by X‐ray diffraction, Raman, high‐resolution and transmission electron microscopy, and electrochemical methods. After ballmilling, the crystal size of graphite increased but the size of silicon decreased with increasing content of silicon. Ballmilled materials reacted reversibly with lithium, and the reversible specific capacity increased from 437 mAh/g in the ballmilled pure graphite to 1039 mAh/g in ballmilled materials. The excess capacity due to the Li extraction from silicon appeared at a potential about 0.4 V vs. Li metal. After 20 charge/discharge cycles the reversible capacity of was 794 mAh/g. This behavior is a result of nanosize silicon particles decreasing the crumbling rate during Li insertion and extraction. These materials appear to be promising candidates for negative electrodes in lithium‐ion rechargeable batteries.

New method of carbon onion growth by radio-frequency plasma-enhanced chemical vapor deposition

Large quantities of carbon onions with high purity were synthesized by radio-frequency plasma-enhanced chemical vapor deposition. The produced carbon onions are solid, clean and can be separated easily from the catalytic particles. The formation of onions is based on the formation of many cages in successive stages from the core to the surface. Around the edge of a carbon onion, discontinuous curved lines are shown in the high-resolution TEM image, reflecting the wavy behavior of carbon onions.

Generation of curved or closed-shell carbon nanostructures by ball-milling of graphite

Abstract Curved or closed-shell carbon nanostructures were produced by ball-milling of graphite. A high resolution indicates that the ball-milling not only produces bend of graphite sheets, forming carbon nanoarches, but also produces closed-shell carbon nanostructures, nearly carbon ‘onions’. The possible formation mechanism is proposed.

Lithium insertion in ball-milled graphite

Abstract The effects of mechanical milling on the microstructure, morphology and electrochemical performance of graphite powders with respect to lithium insertion are studied. After 150 h of ball-milling, the well-graphitized graphite has been pulverized into small particles with a size of about 50 nm, in which there are a lot of excess vacancies, microcavities and metastable carbon interstitial phases with sizes around 13 A. Due to the large surface energy, the merging of single particles is favoured and results in the formation of agglomerates with average size about 1 μm. Voids are formed among the agglomerated particles. The ball-milled graphite shows reversible specific capacity for lithium of 700 mA h g −1 (L 1.88 C 6 ) with large hysteresis. The large reversible capacity is due mainly to Li doping at vacancies, microcavities (or at the edges of the metastable carbon interstitial phase) and voids. The bonding change between the interstitial carbon and the carbon in the aromatic plane that is induced by insertion of Li atoms leads to hysteresis. During charge–discharge cycles, the reversible capacity above 1 V decreases rapidly, which may be due to some vacancies and microcavities being annihilated by moveable and some bound interstitial carbon and to electrolyte penetrating gradually into voids formed by agglomerated particles during the Li insertion and desertion process.

Branching carbon nanotubes deposited in HFCVD system

Abstract Branching carbon nanotubes were deposited as a by-product of diamond thick films in a hot filament chemical vapor deposition (HFCVD) system using acetone and hydrogen. The branching nanotubes were observed to be uniformly ‘Y’ shaped. Electron diffraction pattern (EDP) and Raman scattering analysis showed that the nanotubes were well graphitized. A broadened graphite (0002) diffraction spot in EDP from the nanotube ‘Y’ junction was also observed as a result of negative curvature surfaces. Energy dispersive X-ray (EDX) analysis indicated that the nanotubes contained a small amount of Cu, which originated from the parts near the hot filaments. The Cu is believed to have acted as a catalyst for the large production of the branching carbon nanotubes.

Preparation, morphology and microstructure of segmented graphite nanofibers

Abstract Vapor-grown of segmented graphite nanofibers were prepared by pyrolysis of acetylene at 600–800°C using a foam Ni catalyst. Through transmission electron microscopy observation, it was found that this kind of nanofiber looks like a line of lens-like segments with nearly equal separation, which consist of well-ordered graphite platelets intermittently stacked perpendicularly to the filament axis. It was suggested that the catalyst particles adopting a thin slice shape and the flow rate of acetylene are critical for forming segmented graphite nanofibers. The formation mechanism of segmented graphite nanofibers has also been discussed.

Growth of well-crystallized segmented graphite nanofibers by catalytic chemical vapor deposition

Abstract Well-crystallized segmented graphite nanofibers were prepared by pyrolysis of acetylene at 600–800°C using foam Ni catalyst. Transmission electron microscopy indicates that this kind of nanofiber looks like a linear array of lens-like segments with nearly equal separation, which consist of well-ordered graphite platelets intermittently stacked perpendicularly to the filament axis. It was found that the catalyst particles adopting thin slice shape and the flow rate of acetylene are critical for forming segmented graphite nanofiber. The formation mechanism of segmented graphite nanofiber is also discussed.