Shizukuni Yata

Structure and properties of deeply Li-doped polyacenic semiconductor materials beyond C6Li stage

Abstract The structure and properties of deeply Li-doped polyacenic semiconductor (PAS) materials have been investigated in association with their utilization as electrodes in rechargeable batteries. The doped lithium was found neither in metallic nor in ionic states even in the most deeply doped state (C2.2Li stage) by X-ray photoelectron spectroscopy (XPS) measurements. It has also been confirmed that the PAS electrode has a large capacity (530 mA h/g) with good stability and reversibility. These results strongly suggest that the PAS can make an excellent anode material for secondary batteries. Finally, we discuss that the PAS doped up to the C2Li stage can exhibit an energy density per volume as high as lithium metal.

Development of a Lithium-Type Advanced Energy Storage Device

We have developed a device using a novel composite material as the negative electrode and ordinary activated carbon as the positive electrode. The composite material was obtained by heat-treatment of activated carbon with pitch. The structure of this composite was shown to be amorphous based on the X-ray diffraction pattern, and the surface area decreased from 2200 m2/g (activated carbon) to 250 m2/g (composite). This device can discharge at an extremely high current density of 10,000 mA/g based on the composite weight. The power density of around 2.2 kW/L was two times higher than EDLCs, and the energy density of around 20 Wh/L was three times higher. The discharge capacity maintained its initial capacity for at least 100,000 cycles. This device also had excellent storage characteristics at 60°C.

A study on the pristine and the doped polyacenic semiconductive materials

Abstract A new material supposed to have a mixed structure of polyacenic skeletons has been prepared from phenol-formaldehyde resin through the pyrolytic process. This material is air-stable, showing a wide variation of electrical conductivity with pyrolytic temperature. Moreover, it has been confirmed that the present material can be doped with an electron acceptor (iodine) and an electron donor (sodium) in a successful manner, giving rise to a maximum increase in the conductivity of more than seven and eleven orders of magnitude, respectively. The results of the electron probe X-ray microanalysis (EPMA) with respect to the doped samples have shown a uniform distribution of the dopants inside these materials.

Characteristics of deeply Li-doped polyacenic semiconductor material and fabrication of a Li secondary battery

Abstract Electrochemical properties of deeply Li-doped polyacenic semiconductor (PAS) material have been investigated. It has been found that a PAS electrode can be doped up to the C 2 Li stage without any Li-metal electrolysis and that it has a reversible capacity of 850 mAh/g. Fabrication of the secondary PAS battery of the cylindrical type has also been attempted. The energy density of this battery has turned out to be 450 Wh/l, which is about twice as large as that of conventional Li-ion batteries.

Electronic structure of polyacenacene. A one-dimensional graphite

Abstract The investigation of the electronic structure of polyacenacene (PAA) has been performed on the basis of the tight-binding SCF-MO (self-consistent field-molecular orbital) method. The predicted skeleton for PAA is of an aromatic structure without the band gap. Hence it can be considered that PAA will show the metallic property. In this connection one could regard PAA as a one-dimensional graphite.

Polymer battery employing polyacenic semiconductor

Abstract Recently, we have succeeded to prepare a new type of polyacenic semiconductor (PAS) which is made in the form of sheet or plate with a bulk density of about 0.4g/cm 3 from phenol-formaldehyde resin through pyrolytic process. A typical PAS has a H/C molar ratio of 0.25 and an electrical conductivity of 10 −5 S/cm. The specific surface area is above 2000m 2 /g and the bending strength is 100–150kg/cm 2 . We constructed two types of storage batteries using a disk formed PAS as electrode and solution of lM LiClO 4 in SL + γ-BL (1:1 in volume) as electrolyte. These batteries displayed excellent reversibility for electrochemical doping-undoping process and very small self-discharge. We believe PAS battery is one of the first application of conductive polymers for practical use.

X-ray diffraction studies of pristine and heavily-doped polyacenic materials

Abstract An X-ray diffraction study was performed for pristine and heavily-doped polyacenic materials prepared by pyrolytic treatment of phenolformaldehyde (PF) resin. Seven kinds of pristine samples prepared under different pyrolysis temperatures were studied, as well as the moulded PF resin itself. The result of the analyses shows that the structural change of the samples in the heavily-doped regime differs in a peculiar manner from what is expected in conventional intercalated graphite. An attempt is made to interpret the obtained results in terms of the amorphous structure of the present polyacenic materials.

A New Carbonaceous Material with Large Capacity and High Efficiency for Rechargeable Li‐Ion Batteries

Polycyclic aromatic hydrocarbons (PAHs) with a discharge capacity of 1017 mAh/g and efficiency of 81.5% were developed for rechargeable Li‐ion batteries using isotropic pitch as the raw material. The PAHs have a very small specific surface area and small micropores. X‐ray diffraction results show that PAHs consist of a few graphene sheets stacked in a parallel fashion and separated by a small distance . The results of solid‐state magnetic resonance suggest that the graphene sheets have a coronene‐like structure, namely, they show a disk‐like shape. Based on its structural characteristics, we proposed a mechanism for Li‐ion storage.

Studies of porous polyacenic semiconductors toward application III. Characteristics of practical batteries employing polyacenic semiconductive materials as electrodes

Abstract The energy density of the practical test-type battery consisting of the porous polyacenic semiconductor (PAS) material reported in our preceding papers I and II ( Synth. Met., 38 (1990) 169–175, 177–184) and lithium as the electrodes is examined in detail. Furthermore, two kinds of coin-type rechargeable (secondary) batteries (20-mm diameter, 2-mm thickness) have been fabricated. Various characteristics of these have been checked. For instance, it has become clear that the coin-type Li/PAS battery shows high energy density (110 W h/l) and high voltage (4.0 V) and that the PAS/PAS battery can drain a large quantity of charge (3.3 mA h during the discharging process from 2.5 to 0 V) with a long-cycle life and a small self-discharging property.

Studies of porous polyacenic semiconductors toward application I. Preparation and structural analysis

Abstract A detailed description of the preparation of porous polyacenic semiconductor (PAS) materials is reported. The structural analyses of these materials by the combination of mercury porosimetry and adsorption isotherm measurements have revealed that there are two kinds of pores distinguished by their radii therein. The smaller pore (micropore) size of the porous PAS sample prepared by specific pyrolysis temperature is estimated as about 5 A being much larger than those of active carbons and, hence, rationalizes the ‘dopability’ of the present sample with bulky dopants already reported.