Kunio Nishimura

Recent development of carbon materials for Li ion batteries

Abstract Lithium ion secondary batteries are currently the best portable energy storage device for the consumer electronics market. The recent development of the lithium ion secondary batteries has been achieved by the use of selected carbon and graphite materials as an anode. The performance of lithium ion secondary batteries, such as the charge/discharge capacity, voltage profile and cyclic stability, depend strongly on the microstructure of the anode materials made of carbon and graphite. Due to the contribution of the carbon materials used in the anode in last five years, the capacity of the typical Li ion battery has been improved 1.7 times. However, there are still active investigations to identify the key parameters of carbons that provide the improved anode properties, as carbon and graphite materials have large varieties in the microstructure, texture, crystallinity and morphology, depending on their preparation processes and precursor materials, as well as various forms such as powder, fibers and spherule. In the present article, we describe the correlation between the microstructural parameters and electrochemical properties of conventional and novel types of carbon materials for Li ion batteries, namely, graphitizable carbons such as milled mesophase pitch-based carbon fibers, polyparaphenylene-based carbon heat-treated at low temperatures and boron-doped graphitized materials, by connecting with the market demand and the trends in Li ion secondary batteries. The basic scientific theory can contribute to further developments of the Li ion batteries such as polymer batteries for consumer electronics, multimedia technology and future hybrid and electric vehicles.

Vapor-grown carbon fibers (VGCFs): Basic properties and their battery applications

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.

Raman spectroscopic characterization of submicron vapor-grown carbon fibers and carbon nanofibers obtained by pyrolyzing hydrocarbons

Variations of the properties of submicron vapor-grown carbon fibers (VGCFs) and nanofibers, with diameters around 0.1–0.2 μm and 80–100 nm, respectively, are observed by Raman spectroscopy as a function of heat-treatment temperature. The microstructural evolution strongly depends on the original properties of the material, such that the main transition temperatures associated with the onset for establishing two-dimensional graphene ordering are defined below 1500 °C for the nanofibers and 2000 °C for the submicron VGCFs, respectively. The relative intensities ( I D / I G ) of the as-grown phase for submicron VGCFs and nanofibers are 3.44 and 1.35, while those for the corresponding graphitized samples are 0.393 and 0.497, respectively.

Structural Characterization of Boron-doped Submicron Vapor-grown Carbon Fibers and Their Anode Performance

Structural evolution of undoped and boron-doped submicron vapor-grown carbon fibers (S-VGCFs) was monitored as a function of heat-treatment temperature (HTT). Based on x-ray and Raman data, over the range of HTT from 1800 to 2600 °C, it was found that boron atoms act as catalysts to promote graphitization due to boron’s higher diffusivity. For the range of HTT from 2600 to 2800 °C, the process of boron out-diffusion from the host material induces defects, such as tilt boundaries; this process would be related with the improved capacity and Coulombic efficiency of boron-doped S-VGCFs. When 10 wt% S-VGCFs was used as an additive to synthetic graphite, the cyclic efficiency of the capacities was improved to almost 100%.

Resonant Raman study of polyparaphenylene-based carbons

A resonant Raman study of polyparaphenylene (PPP) prepared by the Kovacic and the Yamamoto methods and heat-treated at temperatures T HT between 650 and 750°C has been performed using different laser excitation energies E laser between 1.92 and 3.05 eV. For samples treated at low T HT , the Raman spectra change with E laser , and this behavior is ascribed to the coexistence of two forms of the PPP polymer (benzenoid and quinoid) as well as a disordered carbon material. For higher T HT samples, only a dispersion of the position of the Raman band as a function of E laser is observed, and this is explained as due to the carbonization of the original polymer. The transition temperature between these two regions of resonance behavior is lower for the Yamamoto-PPP samples than for the Kovacic-PPP samples.

Graphitization behaviors of vapor-grown carbon fibers with different diameters as studied by Raman spectroscopy

Abstract VGCFs have been used as the anode materials as well as conductive additives in Li-ion batteries due to these special chemical and physical properties. Three types of VGCFs with different diameter sizes from 10μm to 30nm were characterized by Raman spectroscopy to monitor the microstructure based on the size effect.

Effects of boron doping for the structural evolution of vapor-grown carbon fibers studied by Raman spectroscopy

The structural deviation of boron-doped vapor-grown carbon fibers (VGCF s ) with diameters around 10 μm relative to their undoped counterparts was investigated by polarized microprobe Raman spectroscopy and field-omission scanning electron microscopy as a function of heat-treatment temperature (HTT). Boron doping induces the formation of dislocation loops in the surface, which combine into larger loops with increasing HTT. The depolarization ratio, D p , of the E 2g2 mode for VGCFs increases gradually with increasing HTT, and finally approaches the value of highly oriented pyrolytic graphite, which is consistent with the asymmetric shape of the peak at ∼2725 cm −1 in the second-order Raman spectra. On the other hand, the D p ratios of the E 2g2 mode for boron-doped VGCFs show no deviations up to an HTT of 2100 °C, as compared to that of VGCFs, and decrease with increasing HTT, whereas the D p ratios of the D peak show a maximum value at 2100 °C, and decrease gradually with increasing HTT. Consistent with these Raman results, boron atoms in the graphite lattice introduce a decreased d 002 spacing (accelerating graphitization), but also hinder two-dimensional structural development and increase the amount of disorder. This is done by introducing tilt boundaries and vacancies, which make the D p ratio of the E 2g2 mode lower than the value for polycrystalline graphite, even though the fibers are heat treated at 2800 °C.

Preparation and Structure of Carbon Fibres and Carbon Nanotubes from the Vapour Phase

Over the past two decades, the carbon fibre industry has experienced dramatic growth, particularly with respect to technological developments. Carbon fibres can be grouped into three major categories; namely, 1) polyacrylonitrile (pAN)-based, 2) isotropic and mesophase pitch-based, and 3) vapour-grown fibres. Commercial PAN- and mesophase pitch-based carbon fibres are typically available as a form of continuous yarn, whereas so called vapour-grown carbon fibres (VGCFs) [1,2] are typically discontinuous.

Structure and anode performance of pristine and B-doped graphites for Li-ion batteries

Abstract The microstructure and electrochemical properties of pristine graphitized and boron-doped materials have been comparatively analyzed by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and electrochemical measurements. The electrochemical properties in a Li ion secondary battery of boron-doped graphitized materials depends strongly on the structural geometry and chemical composition of the pristine host materials.