JP2000030685A - Battery separator and secondary battery with it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery separator having an excellent battery performance and to provide a secondary battery using it. SOLUTION: This battery separator is made of a polyorefin resin porous film containing a stabilizer, the air permeability of the porous film is 10-1000 see/100 cc, and the pin piercing strength is 300 gf/25 μm or above. The total quantity of the extracted stabilizer when the porous film is dipped in propylene carbonate at 80 deg.C for two hr is 2000 ppm or below per porous film weight.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a battery separator comprising a microporous polyolefin resin membrane useful as a battery separator and a secondary battery using the same.

[0002]

2. Description of the Related Art Conventionally, various proposals have been made on useful separators for batteries, especially for secondary batteries such as lithium secondary batteries. Generally, a porous membrane obtained by using a polyolefin-based resin, making a plasticizer porous by extracting or stretching a plasticizer, and performing heat treatment or the like as necessary is used for the separator for a lithium secondary battery. In addition, in the field of resin processing such as sheet and film molding of resin raw materials including the production of the porous membrane, when a resin is introduced into an extruder, a certain amount of a stable material such as an antioxidant or a neutralizing agent is used. It is common to add agents. If such a stabilizer is not added, it is difficult for the resin to exhibit its inherent properties due to, for example, thermal degradation, and there is a possibility that the physical properties such as strength are reduced and the molding stability is impaired.

[0003]

Although it is desirable to add a stabilizer to the resin in view of the above points, on the other hand, even though the stabilizer is used, a battery separator made of a resin containing the stabilizer is used. Since it can be said that the use of a compound adds impurities to the battery, it is expected that the battery performance is reduced by the stabilizer.

[0004]

Means for Solving the Problems In view of the above-mentioned problems, the present inventors have conducted a detailed study on the effect of the stabilizer on the battery performance. In the case of a separator, the adverse effect of the stabilizer was suppressed to a minimum, and it was found that the separator had extremely good performance, and the present invention was completed. That is, the present invention
Consisting of a polyolefin resin porous membrane containing a stabilizer,
The air permeability of the porous membrane is 10 to 1000 sec / 100 cc, the pin puncture strength is 300 gf / 25 μm or more, and
Battery separator characterized in that the total amount of the stabilizer extracted by immersing the porous membrane in propylene carbonate at 80 ° C. for 2 hours is 2,000 ppm or less based on the weight of the porous membrane, and a secondary battery using the same. About.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The porous membrane constituting the battery separator of the present invention is made of a polyolefin resin. The polyolefin resin is not particularly limited, but a polyolefin resin having a viscosity average molecular weight of 300,000 or more and less than 3,000,000 is preferably used. In general, if the molecular weight is less than 300,000, it is difficult to obtain sufficient strength, and if it is more than 3,000,000, the processability of the film tends to decrease, which is not preferable. The viscosity average molecular weight is ASTM D40
Measured according to No. 20. As the polyolefin resin, a polyethylene resin, a polypropylene resin, a polybutene resin, or the like is used. The polyolefin resin is not limited to a single resin, and a blend of a plurality of grades of polyolefin can be used as long as the molecular weight after blending is within the above range. As the polyolefin resin, usually, a polyethylene resin is contained, in particular, a resin occupying a main component of 50% by weight or more,
It is particularly preferable from the viewpoint of the shutdown property, which is one of the performances of the battery separator.

In the present invention, an antioxidant or the like, which is a stabilizer necessary for preventing deterioration during molding, is added to the polyolefin resin in consideration of an amount that can be eluted from the produced polyolefin resin porous membrane. You. As this stability, the total amount extracted by immersing the porous membrane in propylene carbonate at 80 ° C. for 2 hours is 2,000 ppm or less, preferably 1 to 1000 p / w, based on the weight of the porous membrane.
pm to the polyolefin resin. When no additive is added, the polyolefin resin is greatly deteriorated during the molding process, so that not only the physical properties of the film are impaired, but also the stability of continuous molding is likely to be impaired. If the amount of the additive extracted from the polyolefin resin porous membrane after immersion in propylene carbonate at 80 ° C. for 2 hours exceeds 2000 ppm, it is not preferable because deterioration of the battery performance becomes particularly remarkable.

As a method for producing the battery separator of the present invention, a known method for producing a polyolefin resin porous membrane may be applied. For example, it can be manufactured by melt-extruding a mixture of a polyolefin resin and a plasticizer, and then extracting and removing the plasticizer. At that time, stretching, heat treatment, or stretching can be performed before and / or after the extraction and removal of the plasticizer.

The plasticizer used in combination with the polyolefin resin is an organic solvent which has good compatibility with the polyolefin resin, has a lower melting point than the polyolefin resin, has a higher boiling point, and is insoluble in the polyolefin resin. Soluble substances are preferably used. Examples of such a plasticizer include higher aliphatic alcohols such as stearyl alcohol and ceryl alcohol, n-alkanes such as n-decane and n-dodecane, paraffin wax, liquid paraffin, and kerosene. Also, considering the simplicity of raw material handling at the time of extrusion molding, a plasticizer at room temperature is suitably used as the plasticizer.

The proportion of the polyolefin resin and the plasticizer can be adjusted in accordance with the desired porosity when the polyolefin resin porous membrane is used as a battery separator. Is 10 to 50% by weight, and the plasticizer is usually 95 to 4% by weight.
0% by weight, preferably 90 to 50% by weight.

Examples of the stabilizer to be added to the raw material composition which is a mixture of a polyolefin resin and a plasticizer include known antioxidants such as a phenolic antioxidant and a phosphorus antioxidant. For example, as the phenolic antioxidant, those having a very bulky t-Bu group in at least one of the two ortho positions of the hydroxyl group as in the following general formula (1) are particularly preferable.
6-t-butylphenol, 2-t-butyl-4-methoxyphenol, 2,4-dimethyl-6-t-butylphenol, 2,6-di-t-butyl-p-cresol,
2,6-di-t-butyl-4-ethylphenol and the like.

[0011]

Embedded image (Wherein, R1 represents a hydrocarbon group having 4 or more carbon atoms;
2 and R3 represent a hydrogen atom or an electron donating group, and may be different from each other)

The phosphorus antioxidant has a basic phosphite ester skeleton in which phosphorus has three alkoxy groups as shown in the general formula (2), and at least one alkoxy group has 3 carbon atoms. Those having the above bulky groups are preferred. For example, phenyldiisoalkyl (C1-C1
0) phosphite, diphenylisoalkyl (C1-C
10) Phosphite, triphenyl phosphite, tris- (2,4-di-t-butylphenyl) phosphite, etc.

[0013]

Embedded image (Wherein, R4 represents a hydrocarbon group having 3 or more carbon atoms;
5 and R6 represent hydrocarbon groups which may be different from each other)

As described above, the amount of the stabilizer to be extracted after the polyolefin resin porous membrane is immersed in propylene carbonate at 80 ° C. for 2 hours is 2000 pp.
m or less, for example, 10,000 ppm with respect to the raw material composition of the polyolefin resin.
Hereinafter, preferably by 50 to 5000 ppm,
The final extraction amount from the polyolefin resin porous membrane may be adjusted to a desired amount.

The polyolefin resin containing the above stabilizer is usually extruded at a temperature of 140 to 240 ° C.
A known method such as die and inflation molding is used.
It is formed into a film or sheet-like film having a thickness of from 500 to 500 μm, preferably from 10 to 300 μm. In stretching the formed film, any known stretching device such as a roll stretching machine or a tenter can be used. Regarding uniaxial stretching, either longitudinal stretching or transverse stretching can be selected. As for biaxial stretching, sequential biaxial stretching,
Any of simultaneous biaxial stretching is possible. Regarding any stretching, stretching may be performed in one stage or in two or more stages. Further, as a method for removing the plasticizer, there is a known organic solvent method in which the plasticizer in the film is dissolved in an organic solvent such as ethanol, isopropanol, or hexane, and the resulting solution is extracted and removed by solvent replacement.

The polyolefin resin porous membrane obtained as described above has physical properties of air permeability of 10 to 1000 sec / 100 cc, pin piercing strength of 300 gf / 25 μm or more, and is immersed in propylene carbonate at 80 ° C. for 2 hours. Is 2000 ppm or less, and can be suitably used as the battery separator of the present invention.

A secondary battery using the above-mentioned separator for polyolefin resin batteries is represented by a lithium ion secondary battery. The battery comprises at least a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte. As the negative electrode active material, lithium and a lithium alloy may be used, but a carbon material capable of inserting and extracting lithium with higher safety is preferable.
This carbon material is not particularly limited, but graphite and coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, needle coke, pitch coke, phenolic resin / crystalline cellulose, etc. Partially graphitized carbon material, furnace black,
Examples include acetylene black and pitch-based carbon fiber. The positive electrode active material is preferably a metal oxide compound or a chalcogenite compound that can occlude or intercalate lithium. Li _x CoO ₂ , Li _x MnO ₂ , L
_{i x Mn 2 O 4, Li} x V 2 O 5, Li x TiS 2 and the like.

As the negative electrode, a product obtained by applying a slurry of a negative electrode active material and a binder (binder) with a solvent, applying the slurry, and then drying can be used. Further, as the positive electrode, a product obtained by applying a slurry of a positive electrode active material, a binder (binder), and a conductive agent with a solvent, applying the slurry, and then drying can be used. Examples of the binder (binder) of the negative electrode / positive electrode active material include polyvinylidene fluoride, polytetrafluoroethylene, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), and NBR ( Acrylonitrile-butadiene rubber),
Fluororubber and the like are listed. As the conductive agent for the positive electrode, graphite fine particles, carbon black such as acetylene black, and amorphous carbon fine particles such as needle coke are used. Furthermore, when a current collector is used for the negative electrode,
Copper, nickel, stainless steel, nickel-plated steel, or the like is used. When a current collector is used for the positive electrode, aluminum, stainless steel, nickel-plated steel, or the like is used.

As a solvent for forming a slurry, an organic solvent that dissolves a binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N-
Examples include, but are not limited to, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, and the like. In some cases, the active material is slurried with latex such as SBR by adding a dispersant, a thickener, and the like to water.

As the electrolytic solution, a lithium salt is used as an electrolyte.
A solution obtained by dissolving this in an organic solvent is used. electrolytic
The quality is LiClO_Four, LiAsF₆, LiP
F₆, LiBF_Four , LiB (C₆H_Five)_Four, LiCl, Li
Br, CH_ThreeSO_ThreeLi, CF_ThreeSO _Three Li etc. are used
You. Examples of the organic solvent include carbonates and ethers.
, Ketones, sulfolane compounds, lactones,
Tolyls, halogenated hydrocarbons, amines, esters
, Amides, phosphate compounds, etc.
it can. The organic solvent is used to dissociate the electrolyte.
High dielectric constant solution with a relative dielectric constant of about 20 or more at 25 ° C
It is particularly preferred that a medium is included. This high dielectric constant solvent
Are ethylene carbonate, propylene carbonate
And their hydrogen atoms to other elements such as halogen or
Compounds substituted with alkyl groups are exemplified.
The proportion of the compound in the electrolyte is usually 20% by weight or less.
Above, preferably 30% by weight or more.

[0021]

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the present invention. The test method for the porous membrane in the examples is as follows. (1) Air permeability: Measured according to JIS P8117. (2) Pin stab strength: Measured according to Japanese Agricultural Standards Notification No. 1019. (3) Quantification of phenolic stabilizer: A polyolefin resin porous membrane was immersed in propylene carbonate at 80 ° C. for 2 hours, and the amount of the additive extracted into propylene carbonate was measured by gas chromatography (GC) and gel permeation chromatography. Analysis was performed by means of chromatography (GPC) to determine the amount per unit weight of the porous membrane. (4) Quantification of phosphorus-based additive: The same propylene carbonate extract as in (3) above was subjected to fluorescent X-ray analysis to determine the amount per unit weight of the porous membrane.

Example 1 (Porous membrane) 8 parts by weight of polyethylene having a viscosity average molecular weight of 500,000, 16 parts by weight of polyethylene having a viscosity average molecular weight of 1,000,000 and 76 parts by weight of paraffin wax (average molecular weight: 389), tetrakis as a phosphorus-based antioxidant (2,4-di-
A mixture of 0.1 part by weight of (t-butylphenyl) -4,4-biphenylene fosnite and 0.1 part by weight of 2,6-di-t-butyl-p-cresol as a phenolic antioxidant was added to a mixture of 40 mmφ. Was extruded at an extrusion temperature of 170 ° C. and an extrusion rate of 10 kg / h, and a raw film was formed by an inflation method.

[0023] The resulting film was rolled using a roll stretching machine.
After stretching 2.5 times in the machine direction at 0 ° C.,
The film was stretched 8 times in the transverse direction at a temperature of 0 ° C. The obtained stretched film was immersed in isopropanol at 60 ° C. to extract and remove paraffin wax, and then heat-set at 115 ° C. using a roll stretching machine. At the time of heat setting, the roll speed ratio was adjusted so that the longitudinal stretching ratio was 2.0 times. Table 1 shows the physical properties and additive contents of the obtained 25 μm porous membrane.

Next, a coin battery was prepared for measuring battery characteristics. (Negative electrode) Coal needle coke 9 having an average particle size of 10 μm
0 parts (weight ratio; the same applies hereinafter unless otherwise specified)
It was mixed with an N-methylpyrrolidone solution (2% by weight) containing 10 parts of polyvinylidene fluoride to obtain a negative electrode mixture slurry. It was applied to a copper foil having a thickness of 20 μm, dried, evaporated to a solvent, rolled, and punched into a disk to form a negative electrode.

(Positive electrode) 1 mol of lithium carbonate and 2 mol of cobalt carbonate are mixed and pulverized by a ball mill, heat-treated in air at 850 ° C. for 5 hours, mixed and pulverized again by a ball mill, and further air-mixed at 850 ° C. for 5 hours. Heat-treated inside 9
A mixture obtained by adding 5 parts of acetylene black as a conductive agent to 0 parts and mixing the resultant mixture with a solution of 5 parts of polyvinylidene fluoride in N-methylpyrrolidone (2% by weight) was prepared as a positive electrode mixture slurry. The composition was applied to an aluminum foil having a thickness of 25 μm, dried, evaporated to a solvent, rolled, and punched into a disk to form a positive electrode.

(Separator) The above porous membrane was used as a separator. (Electrolytic solution) Lithium perchlorate was dissolved in an equal volume mixed solvent of propylene carbonate, ethylene carbonate and 1,2-dimethoxyethane at a ratio of 1 M (mol / L) to prepare an electrolytic solution.

(Assembly / Evaluation of Battery) A coin-type battery was assembled using the positive and negative electrodes, the electrolyte and the separator. Using this battery, the capacity retention as a battery characteristic was measured. The capacity retention measurement is the percentage of the 40th discharge capacity to the first discharge capacity. The results are shown in Table 1.

Comparative Example 1 A porous membrane was obtained in the same manner as in Example 1 except that no antioxidant was added. Table 1 shows the physical properties of the obtained porous membrane and the characteristics of a battery using the same as a separator.
Shown in Since the antioxidant was not added, the battery characteristics were good, but a decrease in pin puncture strength, which is presumed to be due to thermal degradation during molding, was observed.

Comparative Example 2 A porous membrane was obtained in the same manner as in Example 1 except that the addition amount of both antioxidants was changed to 0.5 part by weight. Table 1 shows the physical properties of the obtained porous membrane and the battery characteristics of a battery using the same as a separator.

Comparative Example 3 A porous membrane was obtained in the same manner as in Example 1 except that the phenolic antioxidant was added in an amount of 1 part by weight. Table 1 shows the physical properties of the obtained porous membrane and the battery characteristics of a battery using the same as a separator.

[0031]

[Table 1]

[0032]

According to the battery separator of the present invention, the adverse effect of the stabilizer in the porous membrane used as the separator is minimized, and the excellent performance as the porous membrane is maintained. Has very good performance.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) C08L 23/04 C08L 23/04 (72) Inventor Kyosuke Watanabe 3-10 Utsudori, Kurashiki-shi, Okayama Prefecture Mitsubishi Chemical Corporation Mizushima Plant (72) Inventor Yasushi Yasumi 3-10 Utsudori, Kurashiki City, Okayama Prefecture Mitsubishi Chemical Corporation Mizushima Plant F-term (reference) 4F074 AA17 AD12 AD16 AG02 AG04 CA03 CB42 DA08 DA10 DA49 4H025 AA15 AA63 AC05 4J002 BB001 BB031 BB121 BB171 EJ026 EW066 FD076 5H021 CC00 EE01 EE04 EE31 HH00 HH01 HH06 5H029 AK03 AL06 AM03 AM05 AM06 DJ04 DJ08 DJ13 EJ12 HJ00 HJ01 HJ14

Claims (6)

[Claims] 1. A porous polyolefin resin film containing a stabilizer, wherein the air permeability of the porous film is 10 to 1000 seconds /
100 cc, the pin piercing strength is 300 gf / 25 μm or more, and the total amount of the stabilizer extracted by immersing the porous membrane in propylene carbonate at 80 ° C. for 2 hours is 2000 ppm or less per weight of the porous membrane. Characteristic battery separator. 2. The total amount of the stabilizer extracted by immersion in propylene carbonate at 80 ° C. for 2 hours is 1 to 100.
2. The battery separator according to claim 1, wherein the amount is 0 ppm. 3. The battery separator according to claim 1, wherein the polyolefin resin comprises a polyethylene resin. 4. The battery separator according to claim 1, wherein the stabilizer is an antioxidant. 5. The antioxidant comprises at least one of a phenolic antioxidant represented by the general formula (1) and a phosphorus antioxidant represented by the general formula (2). Item 4. A battery separator according to Item 4. Embedded image (Wherein, R1 represents a hydrocarbon group having 4 or more carbon atoms;
2 and R3 represent a hydrogen atom or an electron donating group, which may be different from each other. (Wherein, R4 represents a hydrocarbon group having 3 or more carbon atoms;
5 and R6 represent hydrocarbon groups which may be different from each other) 6. A secondary battery using the battery separator according to claim 1.