Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-07-27
2004-06-29
Wu, David W. (Department: 1713)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C526S319000, C429S231950, C252S182100
Reexamination Certificate
active
06756153
ABSTRACT:
TECHNICAL FIELD
This invention relates to a binder composition for an electrode of a lithium ion secondary battery, and utilization of the binder.
BACKGROUND ART
In recent years, portable electronic appliances such as a notebook-sized personal computer, a portable telephone and a personal digital assistant have spread wide. Among secondary batteries used as a power source of the portable electronic appliances, lithium ion secondary batteries are widely used.
Requirements for rendering portable electronic appliances small in size, thin in thickness, light in weight and high in performance are becoming severe. Thus the same requirements are also becoming severe for lithium ion secondary batteries (hereinafter abbreviated to “batteries” when appropriate). In addition, requirement for cost reduction is eagerly desired.
As a binder for an electrode of a lithium ion secondary battery, polyvinylidene fluoride (hereinafter abbreviated to “PVDF”) is widely used. However, secondary batteries having desired performances are difficult to fabricate with PVDF, because it is presumed that PVDF has a poor binding power.
Now various binders are being suited for providing secondary batteries having high performances. Polymers having structural units derived from a polar group-having monomer such as an ethylenically unsaturated carboxylic acid ester monomer attract widespread attention because these polymers give high adhesion between an active material and a collector. For example, polymer binders having structural units derived from an ethylenic hydrocarbon monomer was proposed, which include a copolymer of an ethylenically unsaturated carboxylic acid ester monomer, an ethylenic hydrocarbon monomer and an ethylenically unsaturated dicarboxylic acid anhydride monomer, which copolymer contains at least 40% by weight of structural units derived from the ethylenic hydrocarbon monomer (Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] No. H6-223833), and a copolymer of an ethylenically unsaturated carboxylic acid ester monomer, an ethylenic hydrocarbon monomer and an ethylenically unsaturated dicarboxylic acid (or ester) monomer, which copolymer contains at least 40% by weight of structural units derived from the ethylenic hydrocarbon monomer (JP-A H6-325766). Further, a copolymer of at least one acrylic acid ester or methacrylic acid ester monomer, an acrylonitrile monomer and a vinyl monomer having an acid ingredient (JP-A H8-287915) and a methyl methacrylate-butadiene copolymer rubber (JP-A H4-255670) were proposed. A secondary battery having a positive electrode or negative electrode made by using these copolymer binders, exhibits good battery performances, especially good characteristics at repetition of charge-discharge cycles and a larger capacity. This is because these copolymer binders exhibits stronger force for binding an active material to each other and to a collector, than that obtained with PVDF binder. The amount of the binders can be reduced and thus the weight of a secondary battery can be reduced, as compared with the case where PVDF binder is used. Further, these copolymer binders are relatively inexpensive and thus the fabrication cost of a battery can be reduced. Thus, these copolymers are expected to be good binders.
With spread and development of portable electronic appliances, they are occasionally used and stored under various conditions, especially at a high temperature of 50° C. or higher. A lithium ion secondary battery with an electrode made using the conventional PVDF binder exhibits good characteristics at repetition of charge-discharge cycles at room temperature of 20 to 25° C., but poor characteristics at repetition of charge-discharge cycles at a high temperature of 50° C. or higher. Thus, researches are being made for improving materials used for a battery, a method for fabricating a battery and structure of a battery to obtaining good characteristics of a battery at a high temperature, but satisfactory results have not been obtained. There is still an increasing need for improvement of a binder used for an electrode.
The present inventors found that a secondary battery having an electrode made by using the above-specified copolymer binders instead of the PVDF binder exhibits good characteristics at repetition of charge-discharge cycles at room temperature, but the characteristics at repetition of charge-discharge cycles are still reduced at a high temperature of 50° C. or higher, which is prominent with copolymer binders containing structural units derived from an ethylenic hydrocarbon monomer or having a salient amount of a nitrile group.
A binder composition in the form of a polymer dispersion which comprises a polymer having a gel content of at least 50% as calculated as a toluene insoluble matter, and an organic dispersion medium having a boiling point of at least 80° C. at normal pressure, has been proposed in WO 98/14519. This binder composition in a polymer dispersion form exhibits an enhanced strong force for binding an active material to each other and to a collector, and a battery with an electrode made by using the binder composition exhibits good characteristic at repetition of charge-discharge cycles, as compared with a binder composition in the form of a polymer solution in an organic solvent.
Further researches of the present inventors have revealed that a secondary battery having an electrode made by using a binder composition, specifically described in WO 98/14519, exhibits good characteristics at repetition of charge-discharge cycles at room temperature, but the characteristics at repetition of charge-discharge cycles are reduced at a high temperature of 50° C. or higher. It is presumed that the reduction of the characteristics at repetition of charge-discharge cycles at a high temperature is due to the fact that the specific binder composition contains structural units derived from an aromatic hydrocarbon monomer.
DISCLOSURE OF THE INVENTION
As extensive researches of a lithium ion secondary battery exhibiting good characteristics at repetition of charge-discharge cycles at a high temperature, the present inventors have found that, when a polymer binder comprising a dispersion of a specific polymer dispersed in a specific organic liquid dispersion is used for the preparation of an electrode, a secondary battery with the electrode exhibits enhanced characteristics at repetition of charge-discharge cycles at a high temperature. The present invention has been completed on the basis of this finding.
Thus, in a first aspect of the present invention, there is provided a binder composition for an electrode of a lithium ion secondary battery, which comprises particles of a polymer and an organic liquid dispersion medium having a boiling point in the range of 80° C. to 350° C. at normal pressure; the polymer particles being dispersed in the organic liquid dispersion medium, and said polymer comprising:
(a) structural units derived from a monoethylenically unsaturated carboxylic acid ester monomer, and
at least one kind of structural units selected from (b) structural units derived from a monoethylenically unsaturated carboxylic acid monomer, and (c) structural units derived from a conjugated diene monomer;
the ratio of structural units (a)/[structural units (b) plus structural units (c)] being in the range of 99/1 to 60/40 by weight, provided that the sum of structural units (a), structural units (b) and structural units (c) is 100;
the sum of structural units (a), structural units (b) and structural units (c) being at least 80% by weight based on the weight of the total structural units of the polymer; and
said polymer being substantially free from structural units derived from a monoethylenically aromatic hydrocarbon monomer.
In a second aspect of the present invention there is provided a slurry for an electrode of a lithium ion secondary battery (which slurry is hereinafter referred to as “slurry” when appropriate), which comprises the above-mentioned binder composition and an active material.
In a third
Nakayama Akira
Yamamoto Haruhisa
Armstrong Kratz Quintos Hanson & Brooks, LLP
Hu Henry S
Wu David W.
Zeon Corporation
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