Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-05-19
2001-09-18
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S309000, C429S310000, C429S314000, C429S231800
Reexamination Certificate
active
06291106
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrolytic-solution-supporting polymer film having high strength and heat resistance feasible for use in lithium and lithium ion secondary batteries and having excellent safety during over-charging, a so-called gel electrolyte film, and a secondary battery comprising the same.
PRIOR ART OF THE INVENTION
With developments in electronic machines and equipment in recent years, it is desired to develop a secondary battery which is decreased in size and weight and has a high energy density and which is rechargeable an increased number of times. As a battery of this type, attention is drawn to lithium and lithium ion batteries using not an aqueous electrolytic solution but an organic electrolytic solution (non-aqueous electrolytic solution).
In a solution type lithium secondary battery using lithium and a lithium alloy as a negative electrode, a filate lithium crystal (dendrite) is formed when recharging and discharging are repeated, which causes a short circuit, etc., and it is therefore desired to develop a solid electrolyte film which prevents the above phenomenon and has properties as a separator.
Further, in a lithium ion secondary battery which has been commercialized by overcoming the dendrite problem of the lithium ion secondary battery, the separator used for the prevention of an electrode short circuit has no sufficient capacity of holding an electrolytic solution and is liable to cause the leakage of an electrolytic solution, so that it is inevitable to use a metal container for sheathing. The sheathing creates a situation in which not only an additional cost is required, but also it is difficult to fully decrease the battery weight. Under the circumstance, it is desired to develop a high-safety electrolyte film having the function as a separator as well, with a view to overcoming the electrolytic solution leakage and decreasing the weight of the lithium ion secondary battery.
Under the circumstances, studies are being energetically made for an electrolytic film system which attains both a high ionic conductivity and high safety. One of the approaches to the above object is found in an attempt to produce a solid electrolyte of a polymer and an electrolyte alone without incorporating a liquid component (solvent or plasticizer) into a polymer, a so-called intrinsic polymer electrolyte. Since the electrolyte of this type contains no liquid component, an electrolyte film having a relatively high strength can be obtained. However, the limit of its ionic conductivity is as low as approximately 10
−5
S/cm, and it is difficult to attain its sufficient junction to an electrode active substance. For these reasons, the above electrolyte has not yet been materialized for practical use although it has been long studied.
On the other hand, a so-called gel electrolyte produced by adding a liquid component (solvent or plasticizer) to an intrinsic polymer electrolyte has been and is energetically studied as a system which is to overcome the low ionic conductivity and the insufficient interfacial junction of the above intrinsic polymer. In this system, the ionic conductivity of the gel electrolyte film depends upon the content of the contained liquid component, and some systems have come to be reported to exhibit an ionic conductivity of 10
−3
S/cm or more which is considered sufficient for practical use. However, most of these systems show a sharp decrease in mechanical properties due to the addition of the liquid component, and the safety function as a separator, which a solid electrolyte is to have inherently, is no longer obtained.
Under the circumstances, U.S. Pat. No. 5,296,318 discloses a system which is described to satisfy both the strength of a gel electrolyte film and an ionic conductivity. The rechargeable battery comprises a gel electrolyte film containing, as a polymer, a copolymer from vinylidene fluoride and hexafluoropropylene and attracts attention as a system which exhibits specially noted mechanical properties as a gel electrolyte film. However, even this system has a puncture strength one-digit lower than that of a generally used separator, which puncture strength is one index for functions of a separator for a secondary battery, and the gel electrolyte film thereof has a mechanical heat resistance temperature (melt flow temperature) of 100° C. or a little higher, which is lower than that of a polyolefin-based separator by approximately 50° C. At present, therefore, the above system is not necessarily satisfactory for securing the safety of a lithium ion secondary battery.
Under the circumstances, there have been proposed various gel electrolyte films which use a support as a reinforcing material in combination for covering the mechanical properties which are said to be insufficient for a gel electrolyte film. For example, JP-A-9-22724 discloses a method of using a polyolefin-based synthetic non-woven fabric as a support for producing a coated polymer gel electrolyte film. A coarse non-woven fabric is required for impregnating it with a high-viscosity polymer solution and for attaining a high ionic conductivity. When a polyolefin-based non-woven fabric is used, however, the polyolefin fiber itself has no sufficient strength, and it is difficult to decrease the film thickness. Further, since the mechanical heat resistance of the obtained electrolyte film depends upon the polyolefin non-woven fabric, it is approximately 160° C. at the highest.
Further, U.S. Pat. No. 5,603,982 discloses a method of producing a thin film solid polymer electrolyte, in which a non-woven fabric of a polyolefin, etc., having a high gas permeability is impregnated with an electrolyte and a polymerizable monomer in a solution state and then the monomer is polymerized to form a solid electrolyte. In this method, the non-woven fabric can be easily impregnated with the solution since the solution has a low viscosity. Since, however, the non-woven fabric has no sufficient capacity to hold the solution, it is required to sandwich the non-woven fabric with flat substrates to allow the non-woven fabric to hold the solution when a film is formed, and it is required to polymerize the monomer in the above state. In this method, not only the production step is complicated, but also the film is insufficient in mechanical strength due to the use of a polyolefin-based non-woven fabric, and it is difficult to form a thin film.
As a system which can be more easily formed as a thin film than the system using a non-woven fabric, there have been proposed some systems using not the polyolefin-based non-woven fabric but a polyolefin-based finely porous film as a support. Unlike the above non-woven fabric, however, it is difficult to impregnate a finely porous film having a submicron or smaller pore size with a high-viscosity dope, and in a present situation, it is not possible to employ a method of coating a polymer solution, which method is considered easy as an industrial step to take. For overcoming this problem, JP-A-7-220761 discloses a method of impregnating a finely porous polyolefin film of a polyolefin with a low-viscosity solution containing an electrolytic solution and an ultraviolet-curable resin and irradiating the resin with ultraviolet light to cure the resin. However, even if a low-viscosity solution suitable for the impregnation is used, it is difficult to impregnate a hydrophobic finely porous film of a polyolefin with the solution. Not only the finely porous film needs to be treated for imparting hydrophilic nature, but also it is required to sandwich the film with glass plates treated with a fluorine resin when the resin is cured, so that its production steps are complicated. It is also pointed out that the above finely porous film impregnated with a gel electrolyte cannot show any sufficient conductivity (Abraham, et al. J. Electrochem. Soc., 142, No. 3, 1995).
As described above, various attempts have been made to develop an electrolyte-supporting polymer film which can satisfy both a high ionic conductivity and the saf
Daido Takahiro
Kawaguchi Takeyuki
Rader Fishman & Grauer
Teijin Limited
Weiner Laura
LandOfFree
Electrolytic-solution-supporting polymer film and secondary... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrolytic-solution-supporting polymer film and secondary..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrolytic-solution-supporting polymer film and secondary... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2524654