Elsevier

Journal of Power Sources

Volume 77, Issue 2, February 1999, Pages 110-115
Journal of Power Sources

Vapor-grown carbon fiber anode for cylindrical lithium ion rechargeable batteries

https://doi.org/10.1016/S0378-7753(98)00158-XGet rights and content

Abstract

A lithium ion rechargeable battery based on carbon anode that is a viable replacement for lithium metal anode has been developed. In this investigation, the Vapor-Grown Carbon Fiber was used as the anode material of a cylindrical battery. The charge/discharge experiments were carried under various temperatures and current densities. Excellent cyclability was obtained at 21°C at a charge/discharge of 0.8 C with three cathode materials (LiCoO2, LiMn2O4, and LiNiO2). High discharge capacity was obtained at low temperature (0°C). Good cyclability was also obtained at high temperature (40°C). At the charge/discharge rate of 4.0 C, energy density did not decay significantly. Good cyclability was obtained for rates ranging from 0.8 C to 4.0 C. Self-discharge was investigated at 3 temperatures (21, 40 and 60°C). The measured self-discharge was 8, 15 and 31% per month at 21, 40 and 60°C, respectively.

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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