Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-07-26
2003-12-02
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
06656633
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a binder for an electrode for a lithium ion secondary battery, and utilization of the binder.
(2) Description of the Related 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. The lithium ion secondary batteries include, for example, a secondary battery comprising a lithium ion-containing electrolyte solution, a lithium polymer secondary battery comprising a gel-like electrolyte, a solid-type lithium secondary battery comprising a solid electrolyte, and a non-aqueous electric double layer capacitor using movement of a lithium ion.
Requirements for rendering portable electronic appliances small in size, thin in thickness, light in weight and high in performances are becoming severe. Thus the same requirements are also becoming severe for lithium ion secondary batteries. In addition, requirement for cost reduction is eagerly desired.
An electrode most widely used for a lithium ion secondary battery has a structure such that an active material is supported on a collector by a binder. As the binder for an electrode, polyvinylidene fluoride (PVDF) is widely used. However, PVDF does not have a binding power of a high level nowadays desired for an electrode collector, and thus, secondary batteries having desired performances are difficult to fabricate with PVDF binder.
Now various binders are being studied 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. As a specific example of such polymers, there can be mentioned a copolymer of at least one acrylic acid ester or methacrylic acid ester monomer, an acrylonitrile monomer and an acid ingredient-containing vinyl monomer (Japanese Unexamined Patent Publication No. H8-287915). A secondary battery having a positive electrode or negative electrode made by using this copolymer binder, exhibits good battery performances, especially good characteristics at repetition of charge-discharge cycles and a larger capacity. This is because this copolymer binder exhibits stronger force for binding an active material to each other and to a collector, than that obtained with PVDF binder. Thus, a high adhesion is manifested between an active material and a collector by using a small amount of this copolymer binder derived from a polar group-having monomer, and hence, the weight of a secondary battery can be reduced, as compared with the case where PVDF binder is used. Further, this copolymer binder is relatively inexpensive and thus the fabrication cost of a battery is reduced,
However, the present inventors found that a secondary battery having an electrode made by using the above-specified copolymer binder exhibits good characteristics at repetition of charge-discharge cycles at room temperature of 20° C. to 25° C., but, especially when the copolymer binder is used for a positive electrode, the characteristics at repetition of charge-discharge cycles are drastically reduced at a high temperature of 60° C. or higher, and further that this is due to the fact that the copolymer binder contains structural units derived from an acrylonitrile monomer. It is presumed that the reduction of the characteristics at repetition of charge-discharge cycles at a high temperature is due to high electrochemical reactivity.
SUMMARY OF THE INVENTION
In view of the foregoing prior art, a primary object of the present invention is to provide a binder exhibiting good resistance to an electrolyte solution and good electrochemical stability, and giving an enhanced adhesion between an active material and a collector, which binder is suitable for the preparation of an electrode for a lithium ion secondary battery.
Another object of the present invention is to provide a binder composition comprising the above-mentioned binder and a liquid medium.
A still another object of the present invention is to provide a slurry of the above-mentioned binder, which is used for the preparation of an electrode for a lithium ion secondary battery.
A further object of the present invention is to provide an electrode for a lithium ion secondary battery exhibiting improved charge-discharge cycling characteristics at a high temperature as well as at room temperature.
A further object of the present invention is to provide a lithium ion secondary battery exhibiting improved charge-discharge cycling characteristics at a temperature of 60° C. or higher as well as at room temperature.
The present inventors have found that, when a polymer binder exhibiting a low electrochemical reactivity as measured by the cyclic voltammetry 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 temperature of 60° C. or higher. The present invention has been completed on the basis of this finding.
Thus, in one aspect of the present invention, there is provided a binder for an electrode of a lithium ion secondary battery, which is comprised of a polymer comprising:
(a) structural units derived from a monofunctional ethylenically unsaturated carboxylic acid ester monomer,
(b) structural units derived from an ethylenically unsaturated carboxylic acid monomer, and
(c) structural units derived from a methacrylonitrile monomer;
the ratio of [structural units (a) plus structural units (c)]/structural units (b) being in the range of 99.9 to 1.5 by weight;
the sum of structural units (a), structural units (b) and structural units (c) being at least 70% 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 an ethylenically hydrocarbon monomer and structural units derived from a diene monomer.
In another aspect of the present invention, there is provided a binder composition for an electrode for a lithium ion secondary battery, which comprises the above-mentioned polymer for binder and a liquid medium having a boiling point in the range of 80° C. to 350° C. at normal pressure; said polymer being in the form of particles dispersed in the liquid medium.
In still another aspect of the present invention, there is provided a slurry for an electrode for a lithium ion secondary battery, which comprises the above-mentioned binder and an active material.
In a further aspect of the present invention, there is provided an electrode for a lithium ion secondary battery, which comprises a collector and an active material layer supported on the collector; said active material layer comprising the above-mentioned binder and an active material.
In a further aspect of the present invention, there is provided a lithium ion secondary battery which has the above-mentioned electrode comprising a collector and an active material layer supported on the collector.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(1) Binder
The binder of the present invention is comprised of a polymer comprising (a) structural units derived from a monofunctional ethylenically unsaturated carboxylic acid ester monomer (which units are hereinafter abbreviated to “structural units (a) when appropriate), (b) structural units derived from an ethylenically unsaturated carboxylic acid monomer (which units are hereinafter abbreviated to “structural units (b) when appropriate), and (c) structural units derived from a methacrylonitrile monomer (which units are hereinafter abbreviated to “structural units (c) when appropriate).
The ratio of [structural units (a) plus structural units(c)]/structural units (b) is in the range of 99.9 to 1.5 by weight, preferably 99.5 to 2 by we
Suzuki Takao
Yamakawa Masahiro
Yamamoto Haruhisa
Armstrong Kratz Quintos Hanson & Brooks, LLP
Hu Henry S.
Wu David W.
Zeon Corporation
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