Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
2001-10-09
2004-08-17
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S217000, C429S303000, C429S316000, C429S317000, C029S623100
Reexamination Certificate
active
06777136
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a lithium polymer battery in which a gel electrolyte comprising a liquid organic electrolyte and a host polymer retaining thereof is interposed between the positive electrode and the negative electrode as a separator.
Lithium ion secondary batteries, which include a liquid organic electrolyte, a lithium-containing complex oxide as a positive electrode active material and a carbon material as a negative electrode active material, have a high voltage and high energy density, and exhibit excellent characteristics at low temperatures as compared with secondary batteries having an aqueous electrolyte. These batteries are also excellent in the cycle stability and safety since they do not use a lithium metal in the negative electrode, and they have rapidly been put into practical use. Also, lithium polymer batteries using, as a separator, a gel electrolyte comprising a liquid organic electrolyte and a host polymer retaining thereof have been researched as thin and lightweight novel batteries.
Since the separator of lithium ion batteries are composed of materials which do not readily dissolve or swell in a liquid organic electrolyte, the cycle characteristic of the batteries rarely deteriorate due to the reaction of the separator with the electrolyte. Also, binders contained in the positive electrode and the negative electrode of lithium ion batteries do not relate to the deterioration in the cycle characteristics.
However, since lithium polymer batteries use a gel electrolyte as the separator, chemical stability and reactivity with a liquid organic electrolyte of a host polymer greatly influence the deterioration of the batteries particularly at high temperatures. For example, a liquid organic electrolyte using lithium hexafluorophosphate as a solute reacts with a host polymer such as polyethylene oxide at high temperatures and cuts a network structure formed by the host polymer. As a result, the electrolyte becomes unable to stay in the gel state and the function of bonding the positive electrode and the negative electrode is impaired.
As the host polymer of the gel electrolyte, a variety of polymer materials have been proposed so far. Polymer materials containing an ethylene oxide unit (e.g. Japanese Laid-Open Patent Publication No. Hei 3-171567) have excellent affinity with liquid organic electrolytes, but they have problems concerning thermal stability because they cause sol/gel transition at high temperatures and are easily oxidized.
Materials composed of polyacrylonitrile (e.g. Japanese Laid-Open Patent Publication No. Hei 4-306560) show incombustibility and give a high ion conductivity. However, they have a good affinity with a limited number of liquid organic electrolytes and have problems in the thermal stability of the gel.
Polymer materials containing a vinylidene fluoride unit (e.g. U.S. Pat. No. 5,296,318) have a wide potential range where they are electrochemically stable and have incombustibility because they contain fluorine. However, they have the problem that they have a low affinity with liquid organic electrolytes at high temperatures.
Materials composed of polyacrylate (e.g. Japanese Laid-Open Patent Publication No. Sho 55-35420) are excellent in retention of liquid organic electrolytes, but they are electrochemically unstable.
Also, there are proposed methods of copolymerizing each of the above materials with other monomers, chemically crosslinking the same, or alloying the same with other polymers.
For example, proposed are a mixture of alkylene oxide with a fluorocarbon polymer (Japanese Laid-Open Patent Publication No. Hei 11-35765), and a mixture of polyvinylidene fluoride with a copolymer containing an acrylate unit capable of bonding with metals and an organic compound having a mercapto group (Japanese Laid-Open Patent Publication No. Hei 11-228902). However, there is the problem that a gel electrolyte in the homogenous state cannot be obtained by using these mixtures.
Further, proposed are a copolymer of fluoroolefin with a hydrocarbon having an unsaturated bond (Japanese Laid-Open Patent Publication No. Hei 11-39941), and a copolymer in which acrylic acid is grafted with polyvinylidene fluoride by irradiation of &ggr;-ray (U.S. Pat. No. 6,037,080). However, these copolymers have a low affinity with liquid organic electrolytes and do not readily form a gel electrolyte.
Consequently, lithium polymer batteries having a gel electrolyte are generally inferior in the storage characteristics at high temperatures as compared with lithium ion batteries having no gel electrolyte. For example, when lithium polymer batteries are stored at 80° C. for three days, the capacity obtained by one hour rate discharging could be reduced to 80% or less of the capacity before the storage.
BRIEF SUMMARY OF THE INVENTION
The present invention has an object to render a gel electrolyte homogenous and excellent in the affinity with a liquid organic electrolyte by using a specific copolymer as a host polymer of the gel electrolyte constituting a separator, thereby to improve the stability at high temperatures of the gel electrolyte and to provide a highly reliable lithium polymer battery which is excellent in the storage characteristics at high temperatures.
Specifically, the present invention relates to a lithium polymer battery including: a positive electrode comprising a lithium-containing complex oxide; a negative electrode comprising a material capable of absorbing and desorbing a lithium ion; and a separator comprising a liquid organic electrolyte and a host polymer retaining the liquid organic electrolyte, wherein the host polymer is a crosslinked copolymer, which has a main-chain comprising a vinylidene fluoride unit, and a side-chain comprising an alkylene oxide unit and at least one of an acrylate unit and methacrylate unit.
In the aforementioned copolymer, the content of the side-chain is preferably 1 to 30 wt %.
The aforementioned side-chain is preferably composed of polyethylene glycol diacrylate or polyethylene glycol dimethacrylate wherein an average molecular weight of the diacrylate or dimethacrylate is 300 to 1600.
At least one of the positive electrode and the negative electrode preferably contains a binder comprising a modified polyvinylidene fluoride having an oxygen-containing group.
The positive electrode preferably contains a binder comprising a modified vinylidene fluoride-hexafluoropropylene copolymer having an oxygen-containing group.
The negative electrode preferably contains a binder comprising an ionomer containing at least one of an acrylate unit and methacrylate unit.
The negative electrode preferably contains a binder comprising a particulate rubber containing an acrylonitrile unit, a styrene unit and a butadiene unit.
The present invention also relates to a method for producing a lithium polymer battery comprising:
(1) a step of preparing an electrode assembly by laminating a positive electrode and a negative electrode while interposing therebetween a copolymer having a main-chain comprising a vinylidene fluoride unit and a side-chain comprising an alkylene oxide unit and at least one of an acrylate unit and methacrylate unit;
(2) a step of housing the aforementioned electrode assembly into a battery case, and subsequently introducing a thermal polymerization initiator for the copolymer and a liquid organic electrolyte therein and sealing the battery case; and
(3) a step of forming a separator comprising a gel electrolyte between the positive electrode and the negative electrode by heating the sealed battery to crosslink the copolymer and make the crosslinked copolymer retain the organic electrolyte.
By the above method, since the host polymer is crosslinked by thermal polymerization after the host polymer have contained the liquid organic electrolyte, a gel electrolyte which has a close and chemically stable network structure and is excellent in the stability at high temperatures and in the resistance to oxidization. As a result, the storage characteristics at high temperatures of the po
Morigaki Kenichi
Nanai Norishige
Shibano Yasuyuki
Dove Tracy
Matsushita Electric - Industrial Co., Ltd.
McDermott Will & Emery LLP
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