Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-09-27
2002-03-19
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C029S623500
Reexamination Certificate
active
06358646
ABSTRACT:
TECHNICAL FIELD
This invention relates to a film-type lithium secondary battery and its manufacturing method, and in details to an improvement in an electrode of the film-type lithium secondary battery and an improving method for the electrode.
BACKGROUND ART
In recent years, portable devices such as a portable telephone, a PHS and a small personal computer etc. are undergoing remarkable development in fabrication into small-size and light-weight with a progress of electronics technology. Further, batteries serving as power supplies for use in these portable devices are also required to be built into small-size and light-weight form.
A lithium battery can be mentioned here as an example of a battery to be expected for use in such purpose. In addition to a lithium primary battery already put in practical use, studies have been made on the lithium secondary battery to be put it in practical use, and to achieve its high capacity and long service life.
Almost all of the foregoing various lithium batteries are of cylindrical type. While, in the lithium primary battery, a film-type form is also put in practical use by a manufacturing method for which a solid electrolyte is used and a printing technology is applied. Utilizing this method, many studies have been made to put the film-type form into practical use in fields of the lithium secondary battery and a lithium ion secondary battery, too.
The cylindrical lithium secondary battery is made up through a process in which an electrode group composed of a positive electrode, a negative electrode and a separator is inserted in a cylindrical container and then a liquid electrolyte is filled in it. On the other hand, the film-type lithium secondary battery is made up through a process in which the positive electrode, the negative electrode and the separator composed of a solid-state or a gel-state electrolyte are made up respectively and then laminated one on the other. However, the film-type lithium secondary battery of this type has included such defects as a poor high-rate charge/discharge property and a short cycle life.
The following two points may be considered as reasons for the above defects.
{circle around (1)} In case of the cylindrical battery, the electrode group composed of the positive electrode, the negative electrode and the separator is inserted in the cylindrical container and the liquid electrolyte is filled in the container, so that it is easy to control electronic isolation of an electrode active material due to swelling of the liquid electrolyte by applying a potential to the electrode group. In case of the film-type battery, however, the positive electrode and the negative electrode are opposed each other through the electrolyte, so that it is difficult to apply a potential to the electrode.
{circle around (2)} Since distribution of the electrolyte in the electrode become not uniform, a degree of transfer of lithium ion in the electrode is small. In addition, since fine corrugations remain on electrode surfaces, a surface resistance between the electrode and the separator is large.
DISCLOSURE OF THE INVENTION
This invention is made in consideration of the above-mentioned problems, and an object of this invention is to provide a film-type lithium secondary battery which includes a small surface resistance of the battery inside and therefore is able to offer a high and stable battery performance, and to provide a manufacturing method by which such a film-type lithium secondary battery can be obtained easily.
A film-type lithium secondary battery of this invention provides a film-type lithium secondary battery in which at least a positive electrode and a negative electrode are installed, the positive electrode and the negative electrode are composed by coating an electrode composite onto a current collector respectively, and the electrode composite contains at least an electrode-active material and a solid-state or a gel-state electrolyte; characterized by that, in at least the positive electrode among the positive and negative electrodes, an electrolyte layer composed only of an electrolyte integrated with the electrolyte in the electrode composite is formed on a surface of the electrode composite, and the positive electrode is opposed against the negative electrode through the electrolyte layer.
In the film-type lithium secondary battery of this invention, since the corrugated surface of the electrode composite is covered with the electrolyte layer, the surface resistance between the electrode and the electrolyte layer is reduced by a large margin. Consequently, the film-type lithium secondary battery of this invention becomes superior in an initial capacity, a high-rate charge/discharge property and a cycle life characteristic etc.
In the film-type lithium secondary battery of this invention, following structures (1) to (5) may be used.
(1) The positive electrode is opposed against the negative electrode only through the electrolyte layer.
In this structure, since the electrolyte of the electrolyte layer is integrated with the electrolyte in the electrode composite, the electrolyte layer has a sufficient mechanical strength. Therefore, the electrolyte layer can serve as a function of the separator so that another separator becomes unnecessary. In addition, the positive electrode becomes in contact with the negative electrode only through their electrolyte layers. As the result, an inside resistance of battery is reduced further. Consequently, the film-type lithium secondary battery of this invention becomes superior in the initial capacity, the high-rate charge/discharge property and the cycle life characteristic etc.
(2) The positive electrode is opposed against the negative electrode through the electrolyte layer and the separator.
In this structure, a contact between the electrode and the separator means a contact between the electrolytes because it means a contact between the electrolyte layer and the separator. Consequently, the film-type lithium secondary battery of this invention becomes superior in the initial capacity, the high-rate charge/discharge property and the cycle life characteristic etc.
(3) A total thickness of the electrolyte layer ranges from 2 &mgr;m to 100 &mgr;m.
According to this structure, an effect gained by forming the electrolyte layer on the surface of electrode composite can be obtained effectively. In other words, when the total thickness of the electrolyte layer is smaller than 2 &mgr;m, the reduction in the surface resistance between the electrode and the electrolyte layer becomes insufficient because there is a possibility that the corrugation of the surface of electrode composite would not be covered completely. When the total thickness of the electrolyte layer is larger than 100 &mgr;m, a bulk resistance of the electrolyte layer becomes large. Therefore, in any case there is a possibility that the high-rate charge/discharge property and the cycle life characteristic would not be improved. Especially, when the positive electrode is opposed against the negative electrode only through the electrolyte layer and the total thickness of the electrolyte layer is smaller than 2 &mgr;m, an inside short-circuiting is apt to occur because there is a possibility that the corrugation of surface of the electrode composite would not be covered completely. Therefore, this structure is not preferable.
(4) In at least the positive electrode among the positive and negative electrodes, the electrode composite contains the binder, and the electrolyte is distributed uniformly in the electrode composite while maintaining a binding ability provided by the binder.
In this structure, a bulk density of the electrode active material in the electrode composite can be improved, because the binder is contained in the electrode composite so as to maintain the binding ability provided by the binder. In addition, the degree of transfer of lithium ion in the positive electrode can be improved because the electrolyte is distributed uniformly in the electrode composite. Consequently, the film-type lithium secondary
Itoh Koji
Itoh Takashi
Izuchi Syuichi
Nakagawa Hiroe
Ochiai Seijiro
Crowell & Moring LLP
Yuasa Corporation
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