Vapor-grown carbon fiber anode for cylindrical lithium ion rechargeable batteries
Introduction
New generation high energy, Li-ion rechargeable batteries have attracted a great deal of attention. Until now, many types of carbon materials have been investigated as the negative electrode for these batteries. Vapor-Grown Carbon Fiber (VGCF) is a soft, fibrous carbon material. The crystallinity of this material after graphitization is close to that of single crystal graphite. Further, VGCF consists of concentric layers of basal plane graphite with fiber diameter, length, and aspect ratio that can be changed by process control. At the 7th International Meeting on Lithium Batteries (Boston, 1994), effects of VGCF preparing method on electrochemical performance were discussed, and electrochemical behaviors with various electrolytes were presented 1, 2. VGCF showed high capacity (>360 mAh/g) and high cyclability. Moreover, the coulombic efficiency of the first cycle can be improved by controlling the fiber length of VGCF [3]. Therefore, VGCF is regarded with keen interest as a promising candidate carbon anode for Li-ion rechargeable batteries.
We have developed a new, improved Li-ion rechargeable battery using VGCF as anode material. VGCF/4V Li-ion batteries (A-size, diameter=16 mm, height=50 mm) were tested. Recently, 8 A h cells were constructed in order to enter the electric-bicycle market. In this paper the key materials for this cell and construction of cells are discussed. The properties of this cell, such as charge/discharge characteristics, temperature dependence on discharge capacity, self-discharge, cyclability, and comparison with commercial Li-ion batteries are also covered. In the conclusion, the characteristics of Nikkiso's lithium-ion rechargeable batteries are summarized.
Section snippets
Experimental
VGCF (Grasker™, produced by Nikkiso) heat-treated at 2800°C was used as the anode material. SEM image of VGCF is shown in Fig. 1a. The VGCF consists of short-fiber graphite as seen in Fig. 1a. The VGCF cross-section (as grown), shown in Fig. 1b, displays the concentric layers of basal plane graphite of the material.
The VGCFs were prepared from hydrocarbons by a vapor-growth method and chopped to ca. 10 μm in length by a hybridizer. The chopping process snapped the VGCFs repeatedly by rapidly
Li-ion cylindrical (A-size)
Fig. 3 shows the discharge profiles of LiCoO2/VGCF at 0.8 C (21°C). Batteries of the types LiMn2O4/VGCF and LiNiO2/VGCF were also prepared to compare their charge/discharge characteristics with those of the LiCoO2/VGCF system. As shown in Fig. 3, lithium intercalation/deintercalation into/from these oxides takes place at a potential of about 4 V. With LiCoO2, the polarization is smaller and the capacity larger, when compared with other transition metal oxides [5]. The average discharge voltage
Conclusion
Cylindrical batteries using VGCF as anode material were constructed. The performance of the LiCoO2/VGCF battery at 21°C are summarized in Table 1. In this battery system, high capacity (763 mAh) and high energy density (107 W h/kg or 258 W h/L) were obtained. Further, high cyclability was also obtained at all temperatures and current densities investigated.
At low temperatures (−20 to 0°C), Nikkiso's Li-ion batteries show a good performance. This technology is thus well suited for use in space
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