JP2001118558A - Partially coated separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To develop a high-performance separator for lithium ion secondary battery that is high both in adhesive strength between separator electrodes and in ion conducting property between positive and negative electrodes. SOLUTION: A separator for a lithium ion secondary battery is characterized in that a polymer layer of 5 μm or less thick exists with 50% or equal or less of a coating ratio of the powder in one side or both side of a film surface of a polyolefine fine porous film. A polymer constituting a polymer layer is at least one kind selected from among poly vinylidene fluoride, polyethylene, oxide, polyacrylonitrile, polymethyl methacrylate and copolymer therewith.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a novel separator for a high performance lithium ion secondary battery.

[0002]

2. Description of the Related Art Intense competition for the development of lithium ion secondary batteries has been taking place in the trend of reducing the size and weight of portable electronic devices. Even among the same lithium ion secondary batteries, the required characteristics of the batteries are different due to different uses depending on the shape of the batteries. In the case of prismatic batteries, competition for thinning for mobile phones is fierce. In response to this trend, in addition to the study to make the conventional rigid outer can as thin as possible, the development of a sheet-shaped battery in which the outer can is eliminated by using a laminated film is also becoming active. In such a thin battery, the force to press the electrode coil inside the battery from outside is weaker than the conventional square battery, so a gap is created between the electrode and the separator, and the battery capacity is reduced due to the inhibition of ion conductivity. And a problem that a short circuit between electrodes easily occurs at a high temperature in a safety test. On the other hand, it is pointed out that the improvement is achieved by improving the adhesion between the separator and the electrode.

[0003] In cylindrical batteries, power supplies for personal computers are mainly used, and competition for high capacity is intense in response to high performance and miniaturization of personal computers. In order to increase the capacity, studies are being made to improve the electrode active material and pack the active material in the battery can as much as possible. However, if the active material is packed too much, the volume change of the electrode at the time of charging and discharging of the battery becomes large, and there is a problem that the cycle performance is reduced. It is said that the improvement of the adhesion between the separator and the electrode is also good against this phenomenon.

Conventionally, as a method for improving the adhesive strength between the electrode and the separator, Japanese Patent Application Laid-Open No. 10-177865,
JP-A-10-189054, JP-A-10-275633
It has been proposed to provide an adhesive resin layer or a pressure-sensitive adhesive layer on a separator as disclosed in JP-A-11-213981.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-open No. Hei 10-1
77865, JP-A-10-189054, JP-A-10-905
No. 275633 proposes providing an ion-conductive and adhesive resin layer on the entire upper and lower sides of the separator. However, in a so-called lithium polymer battery using a polymer gel having ion conductivity for a separator, low ion conductivity is the biggest drawback, and in a battery having polymer gel layers on both the upper and lower surfaces of the separator as in the above proposal, It is necessary to make the thickness of the resin layer as small as possible in order not to lower the ion conductivity. If the resin layer becomes thinner, the adhesive strength decreases, so that it is difficult to achieve both ion conductivity and adhesive strength.

Japanese Patent Application Laid-Open No. 11-213981 proposes a separator in which a porous adhesive layer is formed on both upper and lower surfaces of a film by discontinuous coating. In this case, the composition of the adhesive and the method of forming the pores are not described, and details are unknown,
When the thickness of the porous pressure-sensitive adhesive layer formed only by coating on the film is 5 μm or less, the adhesive strength is weak, so that the thickness is 10 μm.
A thickness of about m (5 to 15 μm) is required. According to the description, `` the thickness of the adhesive used in the present invention is 5 to 1
5 μm is preferred. If the thickness is less than 5 μm, the adhesive strength
After laminating by g / 20 mm and pasting together, rubbing occurs. And a thickness of 5 to 15 μm is required. When an adhesive layer having a thickness of about 10 μm is provided on both sides of the film, there is a problem that the film thickness becomes too large as compared with a 25 μm-thick separator normally used for a lithium ion secondary battery.

An object of the present invention is to develop a high-performance separator for a lithium ion secondary battery capable of realizing both a bonding strength between a separator and an electrode and ionic conductivity between a positive electrode and a negative electrode at a practically high level.

[0008]

Means for Solving the Problems As a result of intensive studies on the above problems, the present inventors have selected a polymer which swells moderately with an electrolytic solution, exhibits ionic conductivity and has adhesive strength, and selects a polymer. It has been found that, by dispersing and partially presenting a minimum necessary polymer layer on the surface of the polymer, both ion conductivity and adhesiveness to the electrode can be achieved.
That is, the present invention provides: (1) a film thickness of 5 to 50 μm and a porosity of 20 to 80%
A separator for lithium ion secondary batteries, characterized in that a polymer layer having a thickness of less than 5 μm is interspersed with a surface coverage of 50% or less on one or both surfaces of the microporous polyolefin membrane of (1). (2) The polymer constituting the polymer layer contains at least one kind of polymer that swells with an organic electrolyte solution solvent for a lithium ion secondary battery and generates ionic conductivity and has adhesiveness to an electrode. ). (3) The above-mentioned (1) or (2), wherein the polymer species constituting the polymer layer is at least one selected from polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polymethyl methacrylate and a copolymer thereof. )).

Here, the material of the microporous polyolefin membrane serving as the base material may be any polymer that can be used for a lithium ion secondary battery, but polyethylene, polypropylene, a copolymer thereof, or a combination thereof is used. preferable. There are no particular restrictions on the method for producing the microporous membrane, for example,
It can be obtained by means of adding and mixing a plasticizer and, if necessary, inorganic fine powder or the like using the above-mentioned raw materials, followed by extraction, drying and stretching after molding.

The thickness of the polyolefin film as the substrate is preferably at least 5 μm, more preferably at least 10 μm, in order to ensure the reliability of the separation between the electrodes. However, for higher capacity and thinner, a thin film is preferable, and the maximum thickness is 50 μm.
More preferably, it is 40 μm or less. As for the porosity, the higher porosity has better ionic conductivity and requires at least 20% or more, and more preferably 30% or more. However, if it is too high, the film strength will be weak, so that it is preferably 80% or less. It is more preferably at most 60%.

[0011] The thicker the polymer layer is, the higher the adhesiveness to the electrode is, but the thicker the polymer layer is, the less practical it is as a separator. The surface coverage of the polymer layer is preferably large from the viewpoint of adhesiveness, but is preferably dispersed and dispersed as many as possible from the viewpoint of ion conductivity, and the coverage is preferably 50% or less, more preferably 40% or less. It is. The shape to be dispersed and scattered is not particularly limited, but is preferably a shape in which small circular polymer layers are regularly arranged, such as a polka dot pattern, or a shape in which the polymer layers intersect linearly, such as a lattice pattern.

It is necessary that the polymer constituting the polymer layer does not cause a decomposition reaction or dissolution in the lithium ion secondary battery. However, at least one kind of polymer which swells in the electrolytic solution used and exhibits ionic conductivity is used. In addition to containing, it is necessary to have adhesiveness to the electrode. As these polymers, a fluorinated polymer or a polymer containing an ethylene oxide group is preferable, and in particular, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile,
It is preferably at least one selected from polymethyl methacrylate and respective copolymers. Conversely, in the case of a polymer that does not have a functional group and is not fluorine-based, such as polystyrene, it is possible to select an appropriate solvent and apply it in a uniform solution, but it is not sufficient in adhesiveness and has high peel strength. It is difficult to obtain. Examples of the solvent for dissolving the polymer include N-methylpyrrolidone (hereinafter abbreviated as NMP), dimethylformamide (DMF), and dimethylsulfoxide (DMSO).

(Preparation of Microporous Membrane of Substrate) Although not particularly limited to this method, a polyolefin and a plasticizer are added, mixed with a Henschel mixer or the like, and melt-kneaded and extruded with a twin-screw extruder or the like. Form and obtain a sheet. This is 2
The film is simultaneously or sequentially biaxially stretched by an axial stretching machine, and further extracted and removed with a solvent that dissolves only the plasticizer. Thereafter, the solvent is dried and removed. Further, stretching and heat treatment can be performed as necessary. In this way, a microporous polyolefin membrane serving as a substrate can be obtained. (Coating of a polymer solution) A gravure printing plate for coating a polymer solution is a grid in which a straight line having a width of about 0.5 mm intersects a side surface of a printing roll at regular intervals in a width direction and a circumferential direction. There is a pattern. Further, each straight line portion is formed by engraving, for example, a cell of 100 mesh and a depth of 50 μm. After applying the coating using such a gravure printing plate and drying, a polymer layer is formed on the upper and lower surfaces of the microporous film of the base material using the polymer solution. Thus, a separator partially covered with the polymer layer can be obtained.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on embodiments, but the present invention is not limited thereto. In addition, the test method in an Example is as follows. (1) Film thickness dial gauge (Ozaki Seisakusho: PEACOCK No.
25). The thickness of the polymer layer is calculated from the difference between the thickness of the base microporous polyolefin membrane and the thickness of the polymer layer. (2) Porosity A 20 cm square sample was prepared, the sample volume (cm ³ ) and weight (g) were measured, and the porosity (%) was calculated from the obtained result using the following equation.

Porosity = {1- (weight / resin density) / sample volume} × 100 (3) Surface coverage The ratio of the area occupied by the polymer layer to the area of the upper surface of the microporous polyolefin membrane of the substrate is calculated. (4) Air permeability Measured with a Gurley air permeability meter according to JIS P-8117. (5) Peel strength Measured by T-shaped peel test according to JIS K-6854. (6) Electric resistance Electric resistance device (LCR meter AG-431 manufactured by Ando Electric)
Measured at 1 V, 1 kHz AC using 1).

[0016]

Example 1 (Preparation of Microporous Membrane of Substrate) 99.7 parts by weight of high-density polyethylene having a weight average molecular weight (Mw) of 250,000 (density 0.956, comonomer unit content 0.0%) and 2,2 6-di-t-butyl-p-cresol
3 parts by weight were dry-blended using a Henschel mixer and charged into a 35 mm twin-screw extruder. Further, liquid paraffin (a kinematic viscosity at 37.78 ° C. of 7.59) was added to the extruder.
× 10 ⁻⁵ m ² / sec), melt-kneaded at 200 ° C., and extruded through a coat hanger die onto a cooling roll controlled at a surface temperature of 40 ° C. to form a 1.8 mm thick sheet. Obtained. Here, the composition ratio was adjusted so that liquid paraffin was 55 parts by weight with respect to 45 parts by weight of the polyethylene mixture. Using a simultaneous biaxial tenter stretching machine, the obtained sheet was stretched to 7 × at a stretching temperature of 122 ° C.
The film was stretched 7 times. Subsequently, it was immersed in methylene chloride to extract and remove the liquid paraffin, and then the attached methylene chloride was dried and removed. Further, the film was stretched 1.4 times in the width direction using a tenter stretching machine, and then heat-treated while relaxing in the width direction. Physical properties of the obtained microporous film were a film thickness of 25 μm, a porosity of 41%, and an air permeability of 510 seconds.

(Preparation of Polymer Solution) 10 parts by weight of a copolymer of polyvinylidene fluoride and hexafluoropropylene (KYNAR2801 manufactured by Elphatochem Co., Ltd.) having a melt viscosity at 230 ° C. of about 2500 Pa · s and a number average molecular weight (Mn) A mixture of 10,000 parts by weight of polyethylene glycol (manufactured by Aldrich) and 88 parts by weight of NMP was mixed and sufficiently stirred to form a uniform solution to prepare a high-viscosity polymer solution. (Coating of polymer solution) 0.5 mm width on the side surface of a roll having a diameter of 16 cm and a length of 30 cm at intervals of 4.5 mm in the width direction.
A grid pattern is created by intersecting a straight line of 0.5 mm in width at intervals of 9.5 mm in the circumferential direction. Further, a cell having a depth of 50 μm and a depth of 100 mesh is carved on each straight line portion. Using such a plate for gravure printing, the above-mentioned polymer solution was gravure-coated on both the upper and lower surfaces of the microporous film of the substrate using a coating machine with a drying unit.

In this way, a partially coated separator having a 2 μm-thick polymer layer in a grid pattern on 15% of the surface of the polyethylene microporous membrane substrate was obtained.

[0019]

Example 2 A polymer layer having a thickness of 2 μm was formed in a grid pattern on 15% of the surface of the microporous polyethylene film of the same base material as in Example 1 except that the preparation of the polymer solution in Example 1 was changed as follows. An existing partially coated separator was obtained. (Preparation of polymer solution) Weight average molecular weight (Mw) is 30
8 parts by weight of polyethylene oxide of 000 (manufactured by Aldrich) and a melt viscosity of 230 ° C. of about 2000 Pa ·
s polyvinylidene fluoride (KYNAR741) was mixed at a composition ratio of 4 parts by weight and NMP at 88 parts by weight, and sufficiently stirred to form a uniform solution to prepare a high-viscosity polymer solution.

[0020]

Example 3 A polymer layer having a thickness of 2 μm was formed in a grid pattern on 15% of the surface of the polyethylene microporous membrane of the same base material as in Example 1 except that only the preparation of the polymer solution of Example 1 was changed as follows. An existing partially coated separator was obtained. (Preparation of polymer solution) Weight average molecular weight (Mw) is 8
8,200 parts by weight of polyacrylonitrile (manufactured by Aldrich) and a melt viscosity at 230 ° C of about 2000 Pa ·
s polyvinylidene fluoride (KYNAR741) was mixed at a composition ratio of 4 parts by weight and NMP at 88 parts by weight, and sufficiently stirred to form a uniform solution to prepare a high-viscosity polymer solution.

[0021]

Example 4 A polymer layer having a thickness of 2 μm was formed in a grid pattern on 15% of the surface of a polyethylene microporous membrane of the same base material as in Example 1 except that the preparation of the polymer solution in Example 1 was changed as follows. An existing partially coated separator was obtained. (Adjustment of polymer solution) Weight average molecular weight (Mw) is 35
000 polymethyl methacrylate (Aldric)
h) 8 parts by weight, melt viscosity at 230 degree is about 2000P
Polyvinylidene fluoride (KYNAR741) of a.s was mixed at a composition ratio of 4 parts by weight and NMP at 88 parts by weight, and sufficiently stirred so as to form a uniform solution to prepare a high-viscosity polymer solution.

[0022]

COMPARATIVE EXAMPLE 1 The coating of the polymer solution of Example 1 was changed as follows to obtain a full-surface coated separator in which the entire surface of the base polyethylene microporous membrane was coated with a polymer layer having a thickness of 2 μm. Was. (Coating of polymer solution) A gravure printing plate in which a cell having a depth of 50 μm was engraved with 100 mesh on the entire surface of a roll having a diameter of 16 cm and a length of 30 cm was prepared, and the above polymer solution was coated using a coating machine with a drying unit. Was gravure coated on both the upper and lower surfaces of the microporous membrane of the substrate.

The separators obtained in Examples 1 to 4 and Comparative Example 1 were evaluated using a lithium ion secondary battery manufactured as follows. (Preparation of positive electrode) 80 parts by weight of LiCoO ₂ (manufactured by Nippon Heavy Industries, Ltd.), 6 parts by weight of carbon black, and 14 parts by weight of polyvinylidene fluoride (KYNAR741) were NMP
The paste dispersed in the above was coated on an aluminum foil having a thickness of 30 μm, dried and rolled to prepare a sheet-like positive electrode having an active material layer having a thickness of 100 μm on one surface.

(Preparation of negative electrode) 85 parts of mesophase microbeads (manufactured by Osaka Gas Co., Ltd.), polyvinylidene fluoride (K
YNAR741) A paste in which 15 parts by weight of the paste was dispersed was applied to a copper foil having a thickness of 20 μm, and dried and rolled to prepare a sheet-shaped negative electrode having an active material layer having a thickness of 100 μm on one surface. (Preparation of Battery) A battery laminate having a size of 40 mm × 50 mm was prepared using the separator, the sheet-shaped positive electrode, and the sheet-shaped negative electrode with the separator interposed between the positive electrode and the negative electrode. After placing this laminate in an aluminum laminate film, LiPF ₆ was added to a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 by weight) in an amount of 1.0 mol.
An electrolyte dissolved at a concentration of 1 / liter was injected at room temperature. After impregnation for 12 hours, the battery laminate was pressed from the outside of the aluminum laminate film with a roll, and the opening was heat-sealed and sealed to obtain a lithium ion secondary battery. (Evaluation Results) The electric resistance of the battery cell and the peel strength of the laminate (positive electrode / separator / negative electrode) taken out of the battery were measured. Table 1 shows the results.

[0025]

[Table 1]

[0026]

The partially coated separator of the present invention simultaneously satisfies a low electrical resistance and a high peel strength, which cannot be achieved by conventional separators. It is useful as a separator for a rechargeable battery.

Claims (3)

[Claims] 1. A microporous polyolefin membrane having a thickness of 5 to 50 μm and a porosity of 20 to 80% is coated on one or both sides with a polymer layer having a thickness of 5 μm or less at a surface coverage of 50% or less. A separator for a lithium ion secondary battery, wherein the separator is present. 2. The polymer constituting the polymer layer contains one or more polymers which swell with an organic electrolyte solution solvent for a lithium ion secondary battery and generate ionic conductivity, and have an adhesive property to an electrode. Claim 1
The separator as described. 3. The method according to claim 1, wherein the polymer species constituting the polymer layer is at least one selected from polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polymethyl methacrylate and their respective copolymers. The separator as described.