Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-12-19
2004-12-28
Ryan, Patrick (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C429S231400, C429S231800, C423S448000, C423S449300
Reexamination Certificate
active
06835215
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a carbon material which intercalates into or deintercalates from lithium, and to a method for manufacturing the same. In particular, the present invention relates to a lithium secondary battery, which uses carbon material as a negative electrode active material, having a high energy density and a long life. The lithium battery is suitable for use in portable apparatus, electric automobiles, power storage, etc.
The lithium secondary battery using lithium metal for the negative electrode has some problems relating to safety. For example, lithium easily deposits like dendrite on the lithium metal negative electrode during repeated charging and discharging of the battery, and if the dendritic lithium grows to a positive electrode, an internal short circuit will be caused between the positive electrode and the negative electrode.
Therefore, a carbon material has been proposed as the negative electrode active material in place of lithium metal. Charge and discharge reactions involve lithium ion intercalation into the carbon material and deintercalation from the carbon material, and so lithium is hardly deposited like dendrite. As for the carbon material, graphite is disclosed in JP-B-62-23433 (1987).
The graphite disclosed in JP-B-62-23433 (1987) forms an intercalation compound with lithium, because of intercalation or deintercalation of lithium. Thus, graphite is used as a material for the negative electrode of the lithium secondary battery. In order to use graphite as the negative active material, it is necessary to pulverize the graphite to increase the surface area of the active material, which constitutes a charge and discharge reaction field, so as to allow the charging and discharging reactions to proceed smoothly. Desirably, it is necessary to pulverize the graphite to powder having a particle diameter equal to or less than 100 &mgr;m. However, as is apparent from the fact that graphite is used as a lubricating material, the graphite easily transfers its layers. Therefore, its crystal structure is changed by the pulverizing process, and formation of the lithium intercalated compound might be influenced by undesirable effects of the pulverizing process. Accordingly, the graphite after the pulverizing process has a great deal of crystalline structural defects. In a case when graphite is used as an active material for the negative electrode of the lithium secondary battery, a disadvantage results in that a large capacity can not be obtained. Furthermore, preferable performances of rapid charging and discharging are not obtained because the lithium intercalation-deintercalation reaction is disturbed by the above defects.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the above problems, to provide a carbon material having a large lithium intercalation-deintercalation capacity and a method for manufacturing the same, and to provide a non-aqueous secondary battery which has a large capacity and is superior in its rapid charging and discharging characteristics using the above-mentioned materials.
The crystalline structure of the graphite powder relating to the present invention has a feature that an existing fraction of the rhombohedral structure in the crystalline structure of the graphite is small (equal to or less than 20%). Another feature is that an existing fraction of the hexagonal structure is great (at least 80%). The above existing fractions of the rhombohedral structure and the hexagonal structure can be determined by analyzing the intensity ratio of the peaks in X-ray diffraction of the material.
The graphite powder relating to the present invention is manufactured by a method comprising the steps of graphitizing treatment (heating at least 2000° C.) of raw material such as oil cokes and coal cokes, pulverizing the graphitized raw material to powder, sieving the powder for obtaining the maximum particle diameter equal to or less than 100 &mgr;m, heating the powder to at least 900° C. as a heat treatment, and further heating the powder to at least 2700° C. for eliminating impurities such as Si. For instance, when the powder is heated to at least 2700° C., Si, which is a main component of the impurities, can be reduced to less than 10 ppm. The heat treatment of the powder for eliminating impurities can be omitted depending on the content of the impurities in the raw material. In the pulverizing process, various conventional pulverizers can be used. However, a jet mill is preferable, because pulverization with the jet mill generates the minimum destruction of the graphite crystalline structure in the raw material.
Furthermore, the graphite powder relating to the present invention can be obtained by immersing into an acidic solution containing at least one compound selected from a group consisting of sulfuric acid, nitric acid, perchloric acid, phosphoric acid, and fluoric acid as an immersing treatment, after pulverizing the raw graphite to obtain graphite powder having a particle diameter equal to or less than 100 &mgr;m, subsequently washing with water, neutralizing, and drying.
The non-aqueous secondary battery for achieving the object of the present invention can be manufactured by using the graphite powder relating to the present invention as the negative electrode active material, and the positive electrode is desirably composed of a material comprising a compound expressed by a chemical formula of Li
x
MO
2
(where; X is in a range from zero to 1, and M is at least any one of chemical elements selected from a group of Co, Ni, Mn and Fe), or LiMn
2
O
4
, that is a lithium transient metal complex oxide.
The active materials for the battery are generally used in the form of a powder in order to facilitate the charging and discharging reaction by increasing the surface area of the active material, which constitutes a reaction field of the charging and discharging reaction. Therefore, the smaller the particle size of the powder is, the more will performance of the battery be improved. Furthermore, when the electrode is manufactured by applying an agent mixed with the active material and a binding agent to a current collector, the particle diameter of the active material is desirably equal to or less than 100 &mgr;m in view of applicability and maintaining preciseness of thickness of the electrode.
As for the negative electrode active material for the lithium secondary battery, natural graphite, artificial graphite, and others are disclosed. However, for the above described reason, it is necessary to pulverize these materials. Therefore, in the pulverizing process, various graphite powders having a is diameter equal to or less than 100 &mgr;m were prepared with various pulverizing methods using a ball mill, a jet mill, a colloidal mill and other apparatus, for various times. And, the lithium intercalation-deintercalation capacity of the various graphite powders were determined for determining a superior material for the negative electrode material of the lithium secondary battery.
However, the graphite powder obtained by the above method had lithium intercalation-deintercalation amounts per weight in a range of 200-250 mAh/g, and their capacities as the material for the negative electrode of the lithium secondary battery were not enough.
In order to investigate the reason for the small capacity, crystalline structures of the above various graphite samples were determined by an X-ray diffraction method.
FIG. 1
indicates an example of the results. Four peaks can be observed in a range of the diffraction angle (2&thgr;, &thgr;: Bragg angle) from 40 degrees to 50 degrees in the X-ray diffraction pattern. The peaks at approximately 42.3 degrees and 44.4 degrees are diffraction patterns of the (
100
) plane and the (
101
) plane of hexagonal structure of the graphite, respectively. The peaks at approximately 43.3 degrees and 46.0 degrees are diffraction patterns of the (
101
) plane and the (
102
) plane of the rhombohedral structure of the graphite, respectively. As explained above, it wa
Honbo Hidetoshi
Horiba Tatsuo
Ishii Yoshito
Momose Hideto
Muranaka Yasushi
Hitachi , Ltd.
Parsons Thomas H.
Ryan Patrick
LandOfFree
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