Katsuhisa Tokumitsu

Charge‐Discharge Characteristics of the Mesocarbon Miocrobeads Heat‐Treated at Different Temperatures

Mesocarbon microbeads (MCMB) is one of the promising carbon materials as anodes for rechargeable lithium batteries among commercially available carbon materials. have examined the correlation between carbon structures and charge-discharge characteristics of the MCMBs prepared at different heat-treatment temperatures. It was found that the MCMB heat-treated at 700 C possesses a tremendously high charge-discharge capacity of 750 Ah/kg. This suggests that there is another mechanism for the charge-discharge reaction besides a graphite intercalation compound mechanism which is well known. Therefore, the authors propose a cavity mechanism in which intercrystallite spaces in MCMB are capable of storing lithium species.

Preparation of isotropic pitch precursors for general purpose carbon fibers (GPCF) by air blowing—II. Air blowing of coal tar, hydrogenated coal tar, and petroleum pitches

Three representative isotropic pitches, coal tar pitch NP80, its hydrogenated NHP and petroleum pitch A60, were air blown at 330°C in order to study comparatively their oxidation reactivities in the preparation of isotropic pitch precursors with a high softening point. They showed different oxidation behaviors during air blowing. Coal tar pitch with a low softening point of 80°C showed the most rapid rise of softening point but suffered the smallest pitch yield. Air-blown coal tar pitches, NP80-1 and NHP-1, exhibited a higher degree of aromatic condensation and larger QI content than the petroleum one even when their softening points were around 280°C. The structure characterization of the parent and air-blown pitches by FT-IR, FD-mass and solution 1H-, 13C-NMR suggests the mechanism of air blowing to raise the softening point. Unexpectedly slow rises in softening points of hydrogenated coal tar and petroleum pitches appear to be ascribed to their alkyl and naphthenic groups, which may terminate the chain reaction of oxidation.

Irreversible Capacity of Lithium Secondary Battery Using Meso-Carbon Micro Beads as Anode Material

Abstract One of the most important problems in lithium secondary battery using carbon anodes is the difference between the charge and discharge capacity, the so-called ‘retention’. It is caused partly by the reaction of Li ion with functional groups on the surface of the carbon. Especially, carbons heat-treated at lower temperatures than 1000 °C, have many functional groups such as −COOH and −OH on the surface. As these functional groups are very reactive, Li ions might smoothly react with them in the initial charge-reaction process. In order to evaluate these contributions to the irreversible capacity, the n -butyllithium method was applied for meso-carbon micro beads (MCMB) heat-treated at lower temperatures than 1000 °C. As a result, there are some reactive sites such as functional groups and cavities against Li ions except interlayers. However, the irreversible capacity due to the functional groups is a minor factor, and the dominant factor is due to the decomposition of the solvent followed by the film formation on the surface of the carbon electrode or/and the doping of Li species into the reactive sites such as cavities.

Preparation of isotropic pitch precursors for general purpose carbon fibers (GPCF) by air blowing. I: Preparation of spinnable isotropic pitch precursor from coal tar by air blowing

A QI-free coal tar pitch with a softening point around 80°C was air blown in an autoclave at 330°, 360° and 380°C, respectively, for various periods in order to prepare a pitch precursor for GPCF. Air blowing markedly increased the softening point and contents of solvent insoluble fractions such as BI, QI, as well as atomic CH ratio. The pitches blown at 360°C for 2 to 4 h showed excellent spinnability into thin fibers of 12 to 14 μm in diameter at a wide range of 255° to 330°C. Structural characterization by NMR, FT-IR and elemental analysis on the blown pitches showed that distillation of lighter fractions, aromatization and polymerization of the remaining fractions due to oxidative dehydrogenation raised their softening points. Although all blown pitches stayed completely isotropic, further heat treatment at 380°C under N2 flow for 14 to 21 h allowed generation of mesophase spheres in the isotropic matrix. After the carbonization at 600°C, all pitches showed anisotropic texture, suggesting that blown pitches maintain fusibility and reactivity to follow the growth and development of mesophase spheres in the successive carbonization.

Charge‐Discharge Mechanism of Graphitized Mesocarbon Microbeads

The charge-discharge reaction mechanism of the graphitized mesocarbon microbead (MCMB) anode was investigated with cyclic voltammetry and X-ray diffractometry. It is concluded that the charge-discharge reaction of graphitized MCMB involves intercalation of lithium, which is essentially similar to that for graphite. However, the in-plane ordering of the stage 1 and 2 Li-GICs (Graphite Intercalation Compounds) obtained from the graphitized MCMB is not LiC{sub 6} like graphite, but is close to LiC{sub 8}, according to the results of both X-ray diffractometry and cyclic voltammetry.

High capacity carbon anode for Li-ion battery: A theoretical explanation

Abstract Mesocarbon microbeads (MCMBs) heat-treated below 1000°C have discharge higher than the theoretical value of graphite, 372 Ah kg −1 . The high capacity has been explained on the basis of cavity model, which lithium species are not only intercalated in carbon layers but also doped in cavities, derived from structural parameters of MCMB. As for the theoretical values corresponding to the intercalation capacity and total capacity were in good agreement with each capacity. The cavity size and distribution was characterized based on the Fourier analysis of 002 diffraction profiles on a XRD. This analysis revealed that the high capacity MCMBs have cavities with the size of 0.5–1.5 nm.

Effect of crystallite size on the chemical compositions of the stage 1 alkali metal-graphite intercalation compounds

Abstract The relation between the compositions of alkali metal-graphite intercalation compounds and the crystallite size of pristine graphite was mathematically analyzed taking into account the inplane structure and the stacking sequence of the graphite intercalation compounds. It has been found that the atomic ratio of carbon and alkali metal in GICs, C M (M = Li, K, Rb, and Cs) was approximated by a function of the crystallite size (La and Lc) and the interlayer spacing (d002) of the pristine graphite. The derived equation shows good agreement with the experimental results previously reported.

Reduction of the irreversible capacity of a graphite anode by the CVD process

Abstract The irreversible capacity of a graphite anode was reduced by CVD coating of carbon from a 5% ethylene in Ar gas at temperatures from 700 to 1000°C. The amount of carbon coated on the graphite powder, surface area of the resultant powder and its electrochemical characteristics depend on deposition parameters. The surface area was reduced by ∼10% and coulombic efficiency increased by 5 to 92% for the graphite powders coated with carbon at 800°C. The influence of the deposition temperature on the electrode performance has been discussed based on the physical and electrochemical analysis.

New structural parameters for carbon: Comprehensive crystallization index and cavity index

Abstract New structural parameters designated ‘comprehensive crystallization index’ (CCI) and ‘cavity index’ have been mathematically derived from the density, lattice constant and crystallite size of carbon materials by assuming a model of crystallite. These indices calculated for several carbon materials were plotted as a function of the heat-treatment temperature (HTT). Thereby, it is demonstrated that CCI well represents the crystal growth with increasing HTT in the form of a logistic curve.

Recent trends in carbon negative electrode materials

Abstract The use of graphite-type materials as the negative electrodes for rechargeable lithium batteries is increasing. For graphite-type materials, we proposed the intercalation mechanism taking into account the influence of crystallite size and stacking of the graphitic layers. We found graphite-type materials with a reversible capacity of 430 mAh g −1 over a theoretical limit capacity of 372 mAh g −1 . This higher capacity is due to cavities existing in carbon that are capable of storing lithium ions.