Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-02-22
2004-10-12
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S213000, C252S502000, C423S414000, C029S623500
Reexamination Certificate
active
06803150
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a high-performance nonaqueous electrolyte secondary battery using an organic solvent as an electrolyte solution, and more particularly, it relates to a negative electrode material for a lithium ion secondary battery.
(ii) Description of the Related Art
In recent years, as a power supply of a portable electronic device, demands for a secondary battery having a high energy density and excellent charge and discharge cycle properties are increasing. In this regard, the nonaqueous electrolyte secondary battery, especially a lithium ion secondary battery is very expected as a battery having a high voltage and a high energy density.
In particular, much attention has been nowadays paid to a battery system using a lithium-containing transition metal oxide as a positive electrode active material and a carbonaceous material as a negative electrode. For both positive and negative electrodes of this battery, there is utilized a mechanism of lithium ion intercalation, deintercalation, lithium ion doping or de-doping with respect to each active material, and hence, no metal lithium dendrite is formed even if the charge and discharge cycles are repeated, in contrast to a battery using metal lithium. This battery is, therefore, expected to exert the excellent charge and discharge cycle properties and safety.
At present, a carbon material is widely used as a negative electrode material for such a nonaqueous electrolyte secondary battery. As a proposal of using the carbon material as the negative electrode material, each of Japanese Patent Applications Laid-Open Nos. 208079/1982, 102464/1983, 192266/1983, 143280/1984 and 54181/1985 discloses the employment of, e.g., graphite as the negative electrode material. However, the crystallite of graphite is in the extremely developed state, and so, in the nonaqueous electrolyte secondary battery using such a negative electrode, the decomposition of the electrolyte solution is apt to occur as a side reaction at the hexagonal mesh surface ends of graphite crystals during charge, so that there is a problem that a charge and discharge efficiency and the charge and discharge cycle properties are poor.
In order to solve such a problem, there has been proposed the use of a carbon material which has a low graphitization degree and whose crystallite is not in the extremely developed state. Specifically, it has been suggested that the graphitization degree can be controlled by a calcination temperature, and a method using, as the negative electrode, a calcined organic material obtained at a calcination temperature of not more than 1500° C. is disclosed in Japanese Patent Applications Laid-Open Nos. 93176/1993 and 235372/1985. In such a carbon material having the low graphitization degree, the decomposition of the electrolyte solution during charge can be more suppressed as compared with a carbon material calcined at a temperature not less than 2800° C. and having the high graphitization degree.
The carbon material having the low graphitization degree has, however, lower charge and discharge efficiency than the material having the high graphitization degree, and its true density is also low. Therefore, the obtained battery energy density becomes low, which is insufficient as a battery capacity.
Accordingly, each of Japanese Patent Applications Laid-Open Nos. 059703/1998, 343196/1996, 368778/1992 and 66404/1992 discloses an attempt to suppress the side reaction such as decomposition of the electrolyte solution to thereby improve the battery properties by coating the surface of the carbon material with amorphous carbon or carbonaceous decomposition components to reduce the surface area of the carbon material or covering the hexagonal mesh surface end of the active graphite crystal.
Further, Japanese Patent Application Laid-Open No. 2428905/1993 discloses an attempt to suppress the side reaction such as decomposition of the electrolyte solution to thereby improve the battery properties by restricting a ratio of particles which have a small particle diameter to reduce the surface area of the graphite material.
When using the graphite material to a negative electrode of the nonaqueous electrolyte secondary battery, since a main cause of the irreversible capacity of the battery is the decomposition reaction of the electrolyte solution which occurs at the edge surface of the graphite crystal during charge, reducing the surface area of the negative electrode material or covering the material surface with the electrolyte solution and the inactive coating film are effective for improving the charge and discharge efficiency.
In general, as a technique for working the carbon particles into an electrode, there is widely used a method which comprises mixing the carbon particles with a binder, dispersing the resultant mixture in an aqueous or organic solvent to obtain a slurry, applying this slurry on a current collector, and then drying the same. The negative electrode manufactured by this method has, however, a high void ratio and small filling density as it is, and hence the energy density of the battery can not be sufficiently increased.
In particular, since the material from which the carbon particles having a small particle diameter are removed for reducing the specific surface area has inferior filling properties, it is hard to increase the electrode density. Therefore, the electrode manufactured by the above-described method is usually compressed by using a roll press machine or a uniaxial press machine in order to increase the filling density.
In case of manufacturing the electrode by using the conventional carbon material, however, when the electrode is tried to be compressed in the manufacturing process, the improvement effect is decreased and the charge and discharge halo efficiency becomes lower. Thus, the battery in which such a carbon material is used for the electrode can not be compressed to sufficiently increase the filling density of the electrode, and the energy density of the battery becomes lower, which leads to the insufficient battery capacity.
The present inventor has discovered that the reason for the fact that the charge and discharge efficiency lowers by pressure compression of the electrode is due to the increase in the specific surface area of the electrode. Since compressing the electrode decreases the void ratio of the electrode, the surface area of the electrode is seemingly reduced. However, when the electrode is compressed to, the specific surface area of the electrode is surprisingly greatly increased.
That is because compressing the electrode causes the carbon particles to be fragmentized. When the carbon particles are fragmentized, the specific surface area of the electrode is increased, which readily provokes decomposition reaction of the electrolyte solution, thereby lowering the charge and discharge efficiency. Further, in case of the graphite material having the surface covered with amorphous carbon, when the carbon particles are fragmentized, the active carbon hexagonal mesh surface edge which is not covered with amorphous carbon is exposed, and the effect obtained by coating is thus reduced. Therefore, deterioration of the charge and discharge efficiency due to compression becomes prominent.
Moreover, Japanese Patent application Laid-Open No. 214615/1998 discloses that before the adhesion of amorphous carbon onto the surfaces of the graphite particles, the graphite particles are treated with potassium permanganate, which permits amorphous carbon to further firmly adhere thereto. It was, however, difficult to increase the filling density without deteriorating the charge and discharge effect by this method.
Japanese Patent Application Laid-Open No. 27316/1997 discloses that the graphite-based carbon and amorphous carbon are mixed to be used as a negative electrode active material. Additionally, Japanese Patent Application Laid-Open No. 153514/1996 discloses that a multilayer film having a graphite layer and an amorphous carbon layer or a film formed
Iriyama Jiro
Nakahashi Tamaki
Ohta Tomoyuki
Mercado Julian
NEC Corporation
Ryan Patrick
LandOfFree
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