Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-11-19
2001-09-11
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231950
Reexamination Certificate
active
06287729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to graphite materials which are well suited to negative electrode materials for use in a lithium secondary battery, and which are capable of enhancing the performance of the battery. More particularly, the present invention pertains to graphite materials for a negative electrode used in a lithium secondary battery which materials are capable of enhancing the performance of the battery by a technique in which milled graphite fibers and specific graphite materials (natural graphite, artificial graphite or a mixture thereof) are mixed with each other at a an appropriate proportion so as to regulate the specific surface area of the mixture within a specific range.
2. Description of the Related Art
There has been widely investigated and put into practice the use of mesophase pitch-based milled graphite fibers as a negative electrode for a lithium secondary battery, as disclosed for example, in Japanese Patent Application Laid-Open Nos. 325967/1993 (Heisei-5), 36802/1994 (Heisei-6), and 90725/1995 (Heisei-7).
The mesophase pitch-based milled graphite fibers are excellent in that the fibers have high efficiency of initial charge and discharge, good load characteristics and further favorable adhesivity to electricity collectors. However, when compared with natural graphite and artificial graphite which have a high degree of graphitization, the aforesaid fibers are somewhat inferior with respect to initial capacity and low rate capability presumably because of their lower degree of graphitization. Accordingly, it is considered to be some room for their further improvement.
On the one hand, there has been investigated and put into practice in part, the use of natural graphite and artificial graphite which have a high degree of graphitization as negative electrode materials for a lithium secondary battery. Nevertheless, there is recognized the tendency that these graphite has low efficiency of initial charge and discharge, poor load characteristics, further unfavorable adhesivity to electricity collectors and the like as compared with the mesophase pitch-based milled graphite fibers.
In addition, for the purpose of complementing the items to be improved in a single kind of material, researches have been made on the mixed use of various materials.
In regard to the mixing system, the scope of researches covers wide fields including a variety of combinations of the materials to be mixed, such as graphite base/graphite base, carbon base/carbon base, graphite base/carbon base, graphite base/carbon base that are incorporated with an electroconductive material and a mixed system of at least three kinds of materials.
With respect to the mixing of graphite base/graphite base, for example, there is disclosed that improvements are recognized in the capacity and cycle characteristics on the discharge per hour by the mixing of spheroidal graphitized carbon particles and 5 to 30% by weight of graphitized carbon short fiber in Japanese Patent Application Laid-Open No.111818/1994 (Hesei-6).
There are also disclosed for the same purpose, the mixing of graphite materials comprising spheroidal particles (MCMB) and not more than 20% by weight of graphite fine particles (natural graphite, artificial graphite or the like) having a particle diameter smaller than that of MCMB in Japanese Patent Application Laid-Open No. 37618/1995 (Heisei-7); the mixing of graphite materials comprising spheroidal particles (MCMB) or the like and 5 to 20% by weight of fibrous graphite in Japanese Patent Application Laid-Open No. 287952/1996 (Heisei-8).
There are also disclosed for the same purpose, the mixing of mesophase pitch-based milled carbon fibers and 5 to 20% by weight of natural or artificial graphite in Japanese Patent Application Laid-Open No. 83608/1996 (Heisei-8); and the mixing of graphite material and graphite material different in bulk density from each other in Japanese Patent Application Laid-Open No. 180873/1996 (Heisei-8).
SUMMARY OF THE INVENTION
On the basis of the understanding of the prior arts as described hereinbefore, a general object of the present invention is to provide graphite materials for a negative electrode used in a lithium secondary battery which materials make use of mesophase pitch-based milled graphite fibers, and are capable of enhancing the performances such as the capacity of the battery, the efficiency of charge and discharge, low rate characteristics and adhesivity to electricity collectors.
Other objects of the present invention will be obvious from the text of the specification hereinafter disclosed.
In the light of the above-mentioned object, research and investigation were made by the present inventors on the mixing of mesophase pitch-based milled graphite fibers as the base with a variety of graphite materials, paying attention to the mixing of graphite materials/graphite materials. As a result, it has been found that excellent performances as a negative electrode for a lithium secondary battery are manifested through the use of a mixed graphite materials that are formed by mixing mesophase pitch-based milled graphite fibers which have specific properties with natural graphite or artificial graphite which has specific properties and a high degree of graphitization within a specific proportion so as to regulate the specific surface area of the resultant mixture within a specific range. The present invention has been accomplished on the basis of the above-mentioned findings and information.
That is to say, the present invention provides graphite materials for a negative electrode used in a lithium secondary battery which graphite materials have an average specific surface area in the range of 1.5 m
2
/gram to 5 m
2
/gram and comprise (A) milled graphite fibers which are produced by milling and graphitizing mesophase pitch-based carbon fibers, and which have an interplanar spacing of the graphite layer d
002
of at most 0.337 nm and a specific surface area in the range of 0.8 m
2
/gram to 1.3 m
2
/gram; and (B) graphite materials which have an interplanar spacing of the graphite layer d
002
of at most 0.336 nm, a degree of graphitization (P
101
,/P
100
) of at least 2.0 and a specific surface area in the range of 2 m
2
/gram to 15 m
2
/gram so that the ratio (A)/(B) by weight is set in the range of 90/10 to 50/50.
In what follows, the present invention will be described in more detail.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[I] Preparation and Properties of Mesophase Pitch-Based Milled Graphite Fibers
In the following, some description will be given of the preparation process for mesophase pitch-based milled graphite fibers that are well suited for the present invention.
<Outline of Preparation Process>
The mesophase pitch-based milled carbon fibers according to the present invention are produced usually in accordance with a conventional process, by spinning a mesophase pitch as a starting material, infusibilizing the resultant spun fibers, carbonizing the resultant infusibilized fibers at a relatively low temperature to form carbon fibers, thereafter milling the resultant carbon fibers, regulating the milled fibers to proper particle sizes, and then subjecting said fibers to graphitizing treatment at a relatively high temperature.
In the following, detailed description will be given of each of the production steps.
<Pitch as the Starting Material>
A pitch as the starting raw material according to the present invention is not specifically limited, provided that it is spinnable, and it is exemplified by a petroleum base, coal base and synthesis base. In particular, there is used an easily graphitizable pitch, preferably a mesophase pitch having a mesophase content of 100%.
The softening point of a pitch as the starting material is not specifically limited. It is advantageous, however, to employ a pitch having a low softening point from the aspect of spinning temperature, and also a pitch having a high infusibilization reaction rate from the viewpoint of production cost and t
Ishigami Kazunori
Tamaki Toshio
Brouillette Gabrielle
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Petoca Ltd.
Wills M.
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