Akihiro Mabuchi

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.

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.

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.

Charge/discharge characteristics of synthetic carbon anode for lithium secondary battery

Abstract Recent studies on carbon anodes for lithium secondary batteries have revealed that the electrochemical performances of carbon anodes largely depend on the nature of carbon precursors, heat-treatment condition, structural characteristics of carbons, and so on. In order to clarify the relationship between the carbon structures and electrochemical properties, several kinds of model carbons with different structures were synthesized from three types of pure compounds, acenaphthylene, coronene and phenolphthalein, and their electrochemical characteristics were investigated. As a result, all the model carbons carbonized at 800 °C demonstrate higher capacities than the theoretical one (372 Ah kg −1 ). Moreover, the structures of the carbons synthesized from the admixtures of acenaphthylene and phenolphthalein were determined by the dominant component, acenaphthylene or phenolphthalein and their discharge capacities were also determined by the corresponding concentration in the carbon mixture.

Properties of graphite prepared from boron-doped pitch as an anode for a rechargeable Li ion battery

Abstract Boron-doped graphites were derived from a naphthalene-based pitch mixed with para-xylene glycol (PXG) or dimethyl para-xylene glycol (DMPXG) as a cross-linking agent and three types of boron-containing compounds as a graphitization catalyst, and their anode performances were investigated. The structural analysis of the obtained graphites revealed that PXG functioned mainly as a two-dimensional cross-linking agent during the heat treatment process and DMPXG functioned partially as a three-dimensional. The average interlayer spacing decreased and lattice constant, a0, and graphitizability increased with increasing the amount of boron atoms added. The result indicated that the carbon atoms were replaced by boron atoms. The anode performance was improved by the enhancement of graphitizability. The structural parameters and anode performance of boron-doped graphites did not depend on the kind of boron-containing compounds but the amount of boron atoms added in pitch and the kind of cross-linking agent.