Vinylidene fluoride copolymer for gel-form solid electrolyte...

Details Vinylidene fluoride copolymer for gel-form solid electrolyte... Vinylidene fluoride copolymer for gel-form solid electrolyte...

2000-01-24

2002-04-16

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S317000, C429S217000, C429S303000

Reexamination Certificate

active

06372388

ABSTRACT:

This application is a 371 of PCT/JP98/03292 filed Jul. 23, 1998.
TECHNICAL FIELD
The present invention relates to a vinylidene fluoride copolymer providing a polymer matrix for forming a gel-form solid electrolyte suitable for forming a non-aqueous battery, particularly a lithium ion battery, and a gel-form solid electrolyte formed of the vinylidene fluoride copolymer and a non-aqueous battery comprising the solid electrolyte.
BACKGROUND ART
The development of electronic technology in recent years is remarkable, and various apparatus and devices have been reduced in size and weight. Accompanying the reduction in size and weight of such electronic apparatus and devices, there has been a remarkably increasing demand for reduction in size and weight of a battery as a power supply for such electronic apparatus and devices. In order to generate a larger energy from a battery of small volume and weight, it is desirable to generate a higher voltage from one battery. From this viewpoint, much attention has been called to a battery using a negative electrode substance comprising, e.g., lithium or a carbonaceous material capable of being doped with lithium ions, and a positive electrode active substance comprising, e.g., a lithium-cobalt oxide.
However, in case where an aqueous electrolytic solution is used, it is easily decomposed in contact with lithium, a carbonaceous material doped with lithium ions or a lithium aluminum alloy, so that a non-aqueous electrolytic solution formed by dissolving a lithium salt in an organic solvent has been used as the electrolytic solution. As the electrolyte for such a non-aqueous electrolytic solution, there are known LiPF
6
, LiAsF
6
, LiClO
4
, LiBF
4
, LiCH
3
SO
3
, LiCF
3
SO
3
, LiN(CF
3
SO
2
)
2
, LiC(CF
3
SO
2
)
3
, LiCl, LiBr, etc. Further, as the organic solvent for the electrolyte, there is principally used a solvent mixture of a solvent having a high dielectric constant and well dissolving the electrolyte, such as propylene carbonate, ethylene carbonate or &ggr;-butyrolactone, and a low-boiling point solvent, such as 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate or ethyl propionate. The solvent having a high dielectric constant generally has a high boiling point of ca. 200° C. or higher and a low vapor pressure at ordinary temperature, whereas most low viscosity solvents generally have a boiling point around ca. 100° C. and a high vapor pressure at ordinary temperature.
On the other hand, in case where such a non-aqueous secondary battery filled with an organic electrolytic solution is exposed to a high temperature causing a very high vapor pressure of the electrolytic solution inside thereof or excessively charged to generate a decomposition gas of the electrolytic solution, a dangerous state of causing an increase in battery internal pressure possibly leading to an explosion is expected. For this reason, currently commercially available non-aqueous secondary batteries are equipped with a rupture plate for releasing an excessively high pressure before explosion of the battery per se. The operation of the rupture plate results in leakage of a readily ignitable organic electrolytic solution outside the battery. Such leakage of the electrolyte may presumably be also caused by a deterioration with time of a packing between the can body and the cap or a deformation of the packing due to careless handling of the battery. Accordingly, a battery using a non-aqueous electrolyte involves a potential risk of a fire in case of leakage of the non-aqueous electrolytic solution outside the battery by any chance due to a high pressure and ready ignitability of the electrolytic solution.
Non-aqueous lithium-based secondary batteries have been heretofore principally used as power sources for home-use small-capacity electronic appliances, such as portable telephone sets, personal computers and video camera-covers. Heretofore, no fire accident has been caused at all on the market in ordinary environments of use, and general understanding has been attained regarding the safeness of secondary batteries. Accordingly, based on such actual results of safety, the development of secondary batteries as large electricity sources, such as those for electromotive vehicles and load leveling for effective utilization of night electricity, has recently become earnest. As the batteries become larger, the risk of an accidental fire becomes larger to an extent beyond comparison with that in the case of small-capacity batteries.
The present inventors have studied for improvement of problems regarding the safeness of a secondary battery, while noting that such problems are attributable to the use of an organic solvent, particularly a low-viscosity solvent having a high vapor pressure at low temperatures and the structure wherein the organic electrolytic solution is readily leaked out on an occasion of mal-function of the packing of the battery caused by any chance. Accordingly, it has been considered essential to use solid polymer electrolytes, inclusive of, e.g., one formed by dispersing a lithium electrolyte, such as LiClO
4
or LiPF
6
in a gel-form substance composed of polyethylene oxide as a polymer and propylene carbonate as a highly dielectric solvent, developed since 1970's. Several solid polymer electrolytes have been reportedly developed, and actually primary batteries using them have been commercialized. However, no secondary batteries having a cycle characteristic of more than several hundred cycles, have been realized. One cause thereof may be the reduction of the polymer matrix substance used for the solid electrolyte at the boundary with a negative electrode of lithium metal or doped with lithium resulting in a growth of a passive state film showing a poor conductivity for lithium ions. Another cause may be the use of a solid polymer electrolyte showing a lower conductivity for lithium ions than a conventional electrolytic solution using an organic solvent, thus resulting in a battery having a high internal resistance, whereby the utilization of a full capacity of the electrode active substance is liable to cause excessive charging and excessive discharging, thus leading to a deterioration of the electrode active substance in a short period.
By the way, vinylidene fluoride polymer is currently extensively used as a binder for binding an electrode active substance in small-capacity lithium ion secondary batteries using non-aqueous electrolytic solutions. This is because the vinylidene fluoride polymer is not at all reduced in a reducing atmosphere on a negative electrode where tetrafluoroethylene polymer is readily reduced, or is not at all oxidized in an oxidizing atmosphere on a positive electrode where most organic electrolytic solutions are oxidized, so that it is electrochemically stable over a wide potential window.
Further, vinylidene fluoride monomer has two hydrogen atoms functioning as electron donors, and two fluorine atoms functioning as electron acceptors, and therefore has a high polarization as a monomer unit so that it functions as a medium capable of well dissolving therein polar substance, such as an electrolyte.
As has been clarified in Japanese Patent Publication (JP-B) 54-044220, it is known that even a macromolecule such as an organic dye molecule can be migrated at a high speed within a polymer at room temperature if the polymer has a low glass transition temperature. Vinylidene fluoride polymer has a glass transition temperature as low as −45° C., which means that room temperature is higher than its glass transition temperature by more than 50° C., so that the molecular movement at an amorphous portion thereof is sufficiently active and it is considered to exhibit a capability of transporting an electrolyte contained therein at a high speed.
For the above-mentioned reasons in combination, vinylidene fluoride polymer is considered to be extensively used as a binder which is required to satisfy mutually contradictory properties that it encloses an electro

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