Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2001-06-15
2004-02-24
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S316000, C429S317000
Reexamination Certificate
active
06696204
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polymer battery having a high degree of safety, and to a method for its manufacture.
2. Prior Art
Advances over the past few years in electronics equipment have led to smaller sizes, lighter weights and higher energy densities, and also to a desire in the industry for the development of secondary batteries which can be recharged many times. Lithium secondary cells and lithium ion secondary cells in which the electrolyte is a non-aqueous solution rather than an aqueous solution have attracted particular interest.
In solution-type lithium secondary cells where lithium metal or a lithium alloy serves as the negative electrode, thread-like bodies of lithium crystal known as dendrites form on the negative electrode with repeated charging and discharging, resulting in undesirable effects such as short-circuiting of the electrodes. Hence, a need has been felt for a solid polymer electrolyte that inhibits dendrite deposition and also has the properties of a separator.
Lithium ion secondary cells were developed to resolve the problem of dendrite formation in lithium secondary cells. Yet, because the separator used in lithium ion secondary cells to prevent short-circuiting between the electrodes is incapable of adequately retaining the electrolyte, leakage of the electrolyte solution tends to arise, making it necessary to use a metal can as the outer enclosure. This increases production costs for the battery and prevents a sufficient reduction in battery weight from being achieved. Therefore, to eliminate electrolyte leakage and at the same time reduce the weight of the cell, a need has similarly arisen in lithium ion secondary cells for a very safe polymer electrolyte which also functions as a separator.
Vigorous efforts are thus underway to develop polymer electrolytes prepared with fluoropolymer materials.
Examples include physically crosslinked gels arrived at using such fluoropolymers as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-chlorotrifluoroethylene (CTFE) copolymers (P(VDF-CTFE)), vinylidene fluoride-hexafluoropropylene fluororubbers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene fluororubbers and vinylidene fluoride-tetrafluoroethylene-perfluoro(alkyl vinyl ether) fluororubbers.
Such fluoropolymers are known to have good chemical stability to the electrolytes and ions in the solutions used in batteries. For example, U.S. Pat. No. 5,296,318 and U.S. Pat. No. 5,418,091 describe both a gelled electrolyte containing a lithium salt dissolved in a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), abbreviated hereinafter as “P(VDF-HFP),” and also a lithium intercalation cell using the gelled electrolyte. These cells have a better ionic conductivity and discharge characteristics, and in particular a better rate capability, than cells made using earlier gelled electrolytes. That is, increasing the discharge current does not lower to any great degree the discharge capacity.
Yet, although gelled electrolytes made with PVDF-based copolymers such as P(VDF-HFP) copolymers have excellent properties, they also have a number of serious drawbacks.
The copolymerization involved in formation of the PVDF copolymer lowers the crystallinity of the polymer, making it subject to swelling by the electrolyte. Hence, in spite of the good electrical properties achieved, PVDF copolymers are more prone to deformation and have a lower physical strength than PVDF homopolymers. This appears to be attributable to the essential nature of the material. As a result, a film thickness of at least 60 &mgr;m is required for practical use.
Such a large thickness is clearly a drawback when compared with the normal film thickness of 25 &mgr;m in separators currently used in conventional solution-type lithium ion cells. The inability to achieve a lower film thickness in lithium ion secondary cells that use a solid electrolyte has until now made it impossible to exploit the considerable practical advantages of such cells.
Another problem with such PVDF-based copolymers is that, because they are polymerized as copolymers, they have a structure in which crystallization has been inhibited to a great degree, and thus melt at a lower temperature. For example, PVDF homopolymer has a melting point of 170° C., whereas P(VDF-HFP) copolymer has a melting point of 140° C.
Furthermore, in the gelled state containing a large amount of electrolyte solution, the gel film distortion temperature is lower than the melting point of the polymer by itself. In fact, heat distortion occurs at 130° C. in a gel film made with PVDF homopolymer, whereas it occurs at about 90° C. in a gel film made with P(VDF-HFP) copolymer.
Because the heat distortion temperature in the gelled state is low, at elevated temperatures, the separator has a lower strength and is softer than at room temperature, making it more likely for short circuits to occur between the positive and negative electrodes. For example, in cases where expanded metal is used as the current collector, the electrodes cut into the expanded metal. Local thinning occurs in corresponding portions of the PVDF-based copolymer electrolyte, increasing the likelihood of shorting between the positive and negative electrodes. This is a major obstacle to battery production.
Also, the use of a fluoropolymer electrolyte in electrochemical devices such as lithium ion secondary cells and electrical double-layer capacitors often leads to problems with adhesion of the electrolyte (separator) to the electrodes and current collectors. Inadequate adhesion can result in poor battery storage properties. Storage of the battery at room temperature or at an elevated temperature (e.g., 40° C., 60° C., 80° C., 100° C.) results in a deterioration in the capacity and frequent internal shorting. Moreover, lowering the melting point places limits on use of the battery at high temperatures and, as noted above, compromises the high-temperature storage properties.
Because fluoropolymers have an inherently low surface energy and thus do not adhere well to many substances, sufficient adhesion to the positive and negative electrodes cannot be achieved when a fluoropolymer electrolyte is disposed as an electrolyte film between the electrodes.
Quoting directly from JP-A 11-312535:“Fluoropolymers with a weight-average molecular weight of at least 550,000 exhibit excellent adhesion to the active material layers of positive and negative electrodes. It is therefore possible to bond a solid or gelled polymer electrolyte with an electrode active material layer to a sufficient adhesive strength, thus lowering internal resistance within the electrodes and achieving good charge/discharge cycle properties.” However, the degree of swelling by the fluoropolymer varies depending on the type of electrolyte solution used, and so sufficient adhesive strength is not achieved with all electrolyte solutions.
The heat distortion temperature of a gel is not readily affected by the molecular weight of the polymer. Hence, adhesion within the high temperature region is inadequate even when a fluoropolymer having a sufficiently large molecular weight is used. For this reason and because fluoropolymers have a large heat expansion coefficient, the electrodes and the electrolyte tend to separate with repeated heat cycling between high temperatures and room temperature.
Polymer batteries must also have a high degree of safety. Electrolytes composed of a lithium-based electrolyte salt such as LiPF
6
dissolved in a non-aqueous solvent such as a low-molecular-weight carbonate (e.g., ethylene carbonate, propylene carbonate, diethyl carbonate) have been widely used in prior-art lithium secondary cells because of their relatively high conductivity and stable electric potential.
Yet, in spite of their high performance, lithium secondary cells made with such non-aqueous electrolytes are flammable. For example, if a large current suddenly flows into the cell when a short circuit occurs
Hata Kimiyo
Maruo Tatsuya
Sato Takaya
Birch & Stewart Kolasch & Birch, LLP
Kalafut Stephen
Nisshinbo Industries Inc.
LandOfFree
Polymer battery and method of manufacture does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polymer battery and method of manufacture, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polymer battery and method of manufacture will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3351629