Abstract

Submicron vapor grown carbon fibers (VGCFs) obtained by a floating growth method were evaluated in terms of their microstructural development with heat treatment temperature (HTT), physical properties of a single fiber and of the bulk state, and additional effects, such as the filler in the electrode of a lead–acid battery and a Li-ion battery system. Its desirable properties, such as relatively high mechanical strength and electrical conductivity, both in the single fiber state and in the bulk state, including their very special network-like morphology, improved the performance of the electrodes in lead–acid batteries and Li-ion batteries, especially their cycle characteristics.

Abstract

The commercial lithium ion cells with LiCoO₂ as cathode, artificial graphite as anode and 1 M LiPF₆/EC–DEC–EMC (ethylene carbonate–diethyl carbonate–dimethyl carbonate) (1:1:1, v/v/v) with additives (1 wt.% vinylene carbonate (VC) + 1 wt.% propylene sulfite (PS)) as electrolyte were aged at 60% and 100% state of charge (SOC) for 6 months at room temperature and the corresponding cycle performance was measured. Charge/discharge results showed that the capacity retentions after 100 cycles were in the order of fresh cell >60% SOC > 100% SOC. The composition of SEI on the anode was analyzed by X-ray photoelectron spectroscopy (XPS) and the sulfur atom in PS was used as a tagged atom in XPS analysis. The results suggested that the transformation of organic species to inorganic species and the species containing sulfur atom from the reduction of PS was dissolved for the cells aged at 60% and 100% SOC. The SEM and XPS surface and depth profile analysis showed that the increase of the thickness of SEI layer and the variation of compositions on storage or cycling, is one of the most important reasons that results in the deterioration of the cycle performance of commercial lithium ion cells aged at 60% and 100% SOC at room temperature for 6 months.

Abstract

A carbon nanofiber (CNF)/natural graphite (NG) composite was prepared to improve the rate capability of as-received NG to be used as the anode material in a Li-ion battery. Optimum control over both the amount and shape of the CNFs to enable their growth on NG remarkably improved the cycle performance and rate capability of the as-received NG. The first-cycle discharge capacity of the CNF/NG composite was 95% of that of the as-received NG. The amount of grown CNFs was controlled to be less than 15% of the as-received NG. CNF growth on the surface edges of NG in an ivy leaf shape proved to be most effective in improving the rate capability by controlling the extent of volumetric expansion and shrinkage occurring in the charge and discharge processes. Schematic structural models of the as-received NG and CNF/NG composite were proposed to account for the volumetric stability of the electrode in repeated charge-discharge processes. The improved rate capability is ascribed to the thin solid electrolyte interphase and reduced volumetric change of the anode in the charge/discharge processes, both of which are achieved by growing ivy-like CNFs over the graphite surface.

Abstract

Expanded mesocarbon microbeads (EMCMB) were prepared from graphitized mesocarbon microbeads (MCMB) by intercalation reaction and rapid heating processes. The morphologies and structural characters of MCMB before and after expansion were compared by SEM, BET and XRD measurements. The electrochemical properties of EMCMB electrodes were examined by a variety of electrochemical testing techniques. It was found that, at high current density EMCMB with appropriate porosity (16–18 m² g⁻¹) exhibited high reversible capacity (260 mA h g⁻¹), with almost 100% capacity retention for up to 50th cycle. The kinetic properties of EMCMB electrodes were characterized via AC impedance measurement. It was indicated that the exchange current density i₀ of EMCMB electrodes was 5–6 times larger than that of MCMB electrode, suggesting the electrochemical activity of MCMB was significantly enhanced by the expansion treatment.

The natural graphite was processed with technology of intercalation compounds (GIC), coated with precursor of soft carbon, and heated under inert gas. The interlayer space of the sample was increased, and the room for expanding of the sample was reserved. The improving method of increasing the interlayer space and reserving room for expanding has been found successfully, on condition that the changes of the diameter of carbon hexagonal layer and the particle size were not obvious. The analysis showed that there were a lot of oxygen-containing functional groups on the surface of GIC. In the process of GIC being coated with soft carbon, the recovery of the interlayer space and room for expanding was blocked by the reaction between those functional groups and petroleum pitch, and by the gas emission resulted from GIC pyrogenation. Using this sample as anode material of lithium-ion batteries, the reversible capacity changed little, but the rate discharge performance and cycle performance were improved remarkably. This was because of the increasing of the interlayer space and reserving room for expanding resulted in the enlargement of Li-ion diffusion path and the decrease of the coating layer damage.

利用石墨嵌入化合物(GIC)制备技术处理天然石墨, 然后在其表面包覆一层软炭前驱体, 并在惰性气氛下热处理. 所得样品不但层间距保持了拉大的状态, 而且还在天然石墨内部预留了膨胀空间. 成功地找到了在保持天然石墨粒度和碳六角平面直径不显著改变情况下, 提高石墨层间距, 预留膨胀空间的石墨改性方法. 分析表明, 石墨嵌入化合物表面含有的大量含氧官能团, 在软炭前驱体包覆石墨嵌入化合物的过程中, 含氧官能团与沥青之间的反应和石墨嵌入化合物分解产生气体的溢出阻碍了层间距和预留空间的恢复. 将这种材料用于锂离子电池负极材料, 石墨的可逆储锂容量变化不大, 但是倍率放电性能和循环性能得到明显改善. 这主要是因为加大层间距和预留膨胀空间, 拓宽了锂离子扩散通道, 降低了石墨嵌锂膨胀引起的包覆层破裂.

Abstract

Graphene nanosheets were synthesized in large quantities using a chemical approach. Field emission electron microscope observation revealed that loose graphene nanosheets agglomerated and crumpled naturally into shapes resembling flower-petals. High resolution transmission electron microscope analysis, Raman spectroscopy and ultraviolet–visible spectroscopy measurements confirmed the graphitic crystalline structure of the graphene nanosheets. The nanosheets exhibited an enhanced lithium storage capacity as anodes in lithium-ion cells and good cyclic performance.