Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2002-01-17
2004-10-26
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S245000, C429S232000, C429S231100
Reexamination Certificate
active
06808846
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a negative electrode for lithium secondary battery and a manufacturing method thereof, and to a lithium secondary battery.
2. Related Art
Conventionally, the following negative electrodes for lithium secondary battery have been proposed in Japanese Patent Publication No. 2948205 and Japanese Patent Laid Open No. 11-339777. The former negative electrode has been manufactured by sintering silicon or silicon/carbon compound applied on a metallic substrate. The latter negative electrode has been manufactured by sintering a complex of silicon and conductive carbon or conductive metal together with a conductive metallic foil. According to the above conventional methods, it is possible to manufacture a negative electrode which is excellent in conductivity due to a reduction of a contact resistance between a sintered body containing an active material and a substrate.
However, an active material containing silicon has an extremely high expansion and shrinkage coefficient by charge and discharge. Further, when charge-discharge cycle is repeated, the active material separates from a current collector, and current collection is reduced, therefore sufficient cycle characteristics can not be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a negative electrode for lithium secondary battery, which is excellent in charge-discharge cycle characteristics, and a manufacturing method thereof, and to provide a lithium secondary battery.
A negative electrode for lithium secondary battery according to the present invention is an electrode obtained by sintering a mixture of an active material alloy and a binder arranged on a current collector made of metallic foil, or sintering a mixture of the active material alloy, conductive metal powder and a binder arranged on a current collector made of metallic foil, and is characterized in that the active material alloy after sintering is substantially amorphous.
In the negative electrode for lithium secondary battery of the present invention, the active material alloy is sintered; therefore, a binding strength between the active material alloy particles is large. Further, the active material alloy is substantially amorphous; therefore, it is possible to absorb and release lithium without causing a great change of crystal structure during charge and discharge. Accordingly, in the negative electrode for lithium secondary battery of the present invention, the active material particles are hardly pulverized and detached from the current collector when the active material is expanded and shrunk by absorption and release of lithium on charge and discharge. Therefore, it is possible to improve charge-discharge cycle characteristics.
The manufacturing method of a negative electrode for lithium secondary battery of the present invention is characterized in that a mixture of an active material alloy which is substantially amorphous and a binder, or a mixture of the active material alloy, conductive metal powder and a binder is arranged on a current collector, and then sintered under a condition such that the active material alloy after sintering is substantially amorphous.
In the manufacturing method of the present invention, the mixture is sintered under condition such that the active material alloy after sintered is substantially amorphous. More specifically, for example, heat treatment is carried out at a temperature lower than the crystallization temperature of the active material alloy, and thereby, the active material alloy can be sintered in a substantially amorphous state. The crystallization temperature of the active material alloy can be measured by, for example, DSC (differential scanning calorimeter).
Further, it is preferable to carry out heat treatment for sintering in a non-oxidizing atmosphere. The heat treatment in the non-oxidizing atmosphere can be carried out in a vacuum or in an inert gas atmosphere such as argon. Further, the heat treatment can be carried out in a reducing atmosphere such as hydrogen atmosphere. A discharge plasma sintering method and hot press method may be employed as the sintering method.
According to the present invention, in order to arrange the mixture of the active material alloy and the binder, or the mixture of the active material alloy, the conductive metal powder and the binder, on the current collector, the slurry of these mixtures can be applied on the current collector and then dried. More specifically, a slurry is prepared by mixing the active material alloy or the active material alloy and the conductive metal powder with a solution of the binder, and the obtained slurry is applied onto the current collector and then dried.
Further, after application and drying, it is preferable that the mixture layer is rolled together with the current collector before sintering. Because of such rolling, it is possible to improve a packing density of the mixture layer, and to improve an adhesion between active material particles and an adhesion of the active material particles to the current collector.
The negative electrode for the lithium secondary battery of the present invention is not limited to the electrode manufactured by the above manufacturing method of the present invention.
In the present invention, the words “substantially amorphous” means that existence of a halo portion in X-ray diffraction profile is observed, and a degree of non-crystallinity defined by the following equation is 0.3 or more.
Degree of non-crystallinity=maximum peak strength of halo portion profile/maximum peak strength of entire profile
FIG. 2
is a diagram to explain the maximum peak strengths of entire profile and halo portion profile in X-ray diffraction profile. As shown in
FIG. 2
, the maximum peak strength of entire profile is determined from the height of the highest peak of the entire profile from the base line. On the other hand, the maximum peak strength of halo portion is determined from the height of the highest peak of the halo portion from the base line.
In the manufacturing method of the present invention, the active material alloy which is substantially amorphous is used. The substantially amorphous active material alloy is prepared by liquid quenching method, vacuum evaporation method, ion plating method, mechanical alloying method or the like. In these methods, the liquid quenching method is preferable for preparing a large amount of amorphous alloy at a low cost. The liquid quenching method is a rapid solidification method including; single roll and twin roll methods of making an alloy molten and injecting the molten alloy onto a copper roll rotating at high speed; a gas atomization method of spraying the molten alloy using an inert gas; a water atomization method of spraying the molten alloy using water; and a gas-spraying water atomization method of spraying molten alloy using gas and then cooling it using water.
The active material alloy used in the present invention contains preferably Si, further preferably Al, Si and transition metal. The alloy containing Al, Si and transition metal is readily prepared as an amorphous alloy by the above-mentioned liquid quenching method. Examples of the transition metal are chromium, manganese, iron, cobalt, nickel and the like.
Preferably, a metallic foil having a surface roughness Ra of 0.2 &mgr;m or more is used as the current collector in the present invention. The surface roughness Ra in the present invention is a value before sintering. The metallic foil having the above surface roughness Ra provides a larger contact area between the active material particles and the surface of the metallic foil, which improves current collection. Further, the larger contact area provides effective sintering and greatly improved adhesion between the current collector and the active material particles. The above surface roughness Ra is determined by Japanese Industrial Standard (JIS B 0601-1994), and is measured by, for example, a surface roughness meter.
The upper limit
Fujitani Shin
Fukui Atsushi
Hashimoto Takuya
Nakamura Hiroshi
Kubovcik & Kubovcik
Sanyo Electric Co,. Ltd.
Weiner Laura
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