JP4138188B2 - Porous film - Google Patents

Description

【００４９】
以上の結果より、実施例１、２の多孔質フィルムは、圧縮条件下での破膜温度、膜面方向に対する張力条件下での破断温度がともに高く、非常に優れた耐熱性を有しているのに対し、熱可塑性ポリエステルエーテルエラストマーが含有されていない比較例１の多孔質フィルムは破膜温度及び破断温度が低く、耐熱性に欠けていることが分かる。
【００５０】
【発明の効果】
本発明により、リチウムイオン二次電池用セパレーターとして使用し、高温・高圧状態にさらされた場合においても、その形状を維持し、破断・破膜による電極間の短絡を防止することができる、より安全な高耐熱性の多孔質フィルムを提供することが可能となった。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a porous film. More specifically, the present invention relates to a porous film that is preferably used as a battery separator or the like that is disposed between positive and negative electrodes of a battery to isolate them.
[0002]
[Prior art]
The porous film is used for battery separators, electrolytic capacitor diaphragms, moisture-permeable waterproof materials, various filters, and the like. In particular, battery separators are attracting attention as important components for lithium secondary batteries that are lightweight, have high electromotive force and high energy, and have little self-discharge. Is also expected.
[0003]
Conventionally, several proposals have been made for improving the heat resistance of a separator for a lithium secondary battery, focusing on a porous film made of a polyolefin resin. For example, in Japanese Patent Application Laid-Open No. 10-12111, a glass fiber non-woven fabric is laminated on a polyolefin microporous film. In addition, as in JP-A-3-245457 and JP-A-3-193125, many polymers using a polymer crosslinked by a technique such as polymerization after film formation are used. Further, not only in terms of composition, but also in terms of the structure in the battery, such as bonding the protruding portion of the separator from the electrode plate, as disclosed in Japanese Patent Laid-Open No. 5-74444, has been proposed.
[0004]
The porous film is used as a battery separator, and the resin that forms the film melts and closes the pores of the film, losing ion permeability during abnormal heating of the battery, and further heating with current. Has a shutdown (SD) function. However, when an abnormality occurs in which a short-circuit current flows in the battery due to an external short circuit of the battery, the temperature rise is extremely rapid, so that the SD function does not sufficiently develop, and the temperature is higher than the SD temperature of the porous film. There is a high risk of reaching much higher temperatures. At that time, when the porous film is in a molten state due to high temperature, particularly when foreign matter or precipitated lithium dendrite is present, the porous film easily breaks and breaks, and the positive electrode and the negative electrode are in direct contact with each other, Risk of short circuit. In particular, in an overcharged state or the like, the internal pressure in the battery is increased, and the risk of short circuit due to high pressure in the film thickness direction is also increased.
[0005]
[Problems to be solved by the invention]
The object of the present invention is to be used as a separator for a lithium ion secondary battery and maintain its shape even when exposed to high temperature and high pressure, and prevent short-circuiting between electrodes due to breakage or film breakage. An object of the present invention is to provide a safer high heat resistant porous film.
[0006]
[Means for Solving the Problems]
The gist of the present invention relates to a porous film comprising 50 to 95% by weight of a polyolefin resin and 5 to 50% by weight of a thermoplastic polyester ether elastomer.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The porous film of the present invention has a great feature in that it contains a thermoplastic polyester ether elastomer in addition to the polyolefin resin as a matrix. In the present invention, in order to achieve both high strength and moldability, when the thermoplastic polyester ether elastomer has a high melting point and high elasticity, in addition to the strength of the ultra high molecular weight polyolefin resin, the ultra high molecular weight polyolefin Since it is thought that shape maintenance property and mechanical strength in a high temperature region exceeding the melting point of the resin are maintained, the thermoplastic polyester ether elastomer preferably has the following melting point and bending elastic modulus.
[0008]
That is, the melting point of the thermoplastic polyester ether elastomer is 160 ° C. or higher in order to improve the heat resistant shape maintaining property, and 240 ° C. or lower in order to perform the melt-kneading step in producing the porous film under an appropriate temperature condition. Is more preferable, and it is 180-220 degreeC. The melting point of the thermoplastic polyester ether elastomer can be measured using a differential scanning calorimeter.
[0009]
In addition, the flexural modulus of the thermoplastic polyester ether elastomer is preferably 3 × 10 ⁷ Pa or more, and more preferably 5 × 10 ⁷ Pa or more in order to maintain sufficient strength against a load in the compression direction at a high temperature range. In addition, the bending elastic modulus of a thermoplastic polyester ether elastomer can be measured based on ASTM D790.
[0010]
In the porous film of the present invention, as a polyolefin resin as a matrix, when the porous film is used as a battery separator, if the strength is low, the positive electrode and the negative electrode come into contact with each other due to breakage, causing an internal short circuit and functioning as a battery. Since it is eliminated, the one having as high a film strength as possible is preferable. In this respect, the polyolefin resin preferably contains an ultrahigh molecular weight polyolefin resin. Examples of the ultrahigh molecular weight polyolefin resin include homopolymers and copolymers of olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene, and mixtures thereof. Then, from the viewpoint of increasing the strength of the resulting porous film, an ultrahigh molecular weight polyethylene resin is preferred.
[0011]
The weight average molecular weight of the ultrahigh molecular weight polyolefin resin is 5 × 10 ⁵ or more, preferably 5 × 10 ^{5 to} 20 × 10 ⁶ , more preferably 1 × 10 ⁶ to 15 × 10 ⁶ .
[0012]
The ultra high molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more is 1% by weight or more, preferably 5% in order to obtain sufficient entanglement of the polymer chain of the polyolefin resin and the thermoplastic polyester ether elastomer in the polyolefin resin. It is desirable that it is contained in an amount of not less than wt%, more preferably 10 to 100 wt%.
[0013]
Examples of the resin that may be contained in the polyolefin resin in addition to the ultrahigh molecular weight polyolefin resin include the same olefin homopolymers, copolymers, and mixtures thereof as described above. Among these, high density polyethylene Resins are preferred. The weight average molecular weight of these resins is preferably less than 5 × 10 ⁵ , more preferably 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , and particularly preferably 1 × 10 ^{4 to} 3 × 10 ⁵ .
[0014]
The compounding composition of the polyolefin resin and the thermoplastic polyester ether elastomer is that the polyolefin resin is 50 to 95% by weight, preferably 60 to 90% by weight, and the thermoplastic polyester ether elastomer is 5 to 50% by weight, preferably 10 to 40% by weight. %. The polyolefin resin is preferably 50% by weight or more from the viewpoint of film strength, and preferably 95% by weight or less from the viewpoint of shape maintenance at high temperatures.
[0015]
The thickness of the porous film of the present invention is preferably 10 to 50 μm, more preferably 15 to 40 μm. The porosity is preferably 30% or more for preventing clogging, and 70% or less for preventing short circuit of the battery due to lithium dendrite precipitation in an overcharged state when used as a battery separator or the like.
[0016]
Further, the air permeability is preferably 10 to 1000 seconds / 100 cc, more preferably 100 to 800 seconds / 100 cc, from the viewpoint of the permeability and liquid retention of the electrolyte when used as a battery separator or the like, and the electrical resistance. Particularly preferred is 150 to 750 seconds / 100 cc. The needle puncture strength is preferably 3.0 N / 25 μm or more, and more preferably 4.0 N / 25 μm or more, in order to prevent film breakage.
[0017]
The porous film of the present invention is obtained by, for example, melt-kneading a resin composition containing a polyolefin resin, a thermoplastic polyester ether elastomer and a solvent, and molding the obtained melt-kneaded product into a sheet shape. After the product is rolled, it is obtained by stretching and solvent removal.
[0018]
The solvent used for the preparation of the resin composition is not particularly limited as long as it is excellent in the solubility of the polyolefin resin and the thermoplastic polyester ether elastomer, but those having a freezing point of −10 ° C. or lower are preferably used. Preferable specific examples of such solvents include, for example, nonane, decane, decalin, undecane, aliphatic or alicyclic hydrocarbons such as liquid paraffin, and mineral oil fractions having boiling points corresponding to these, among others. A non-volatile solvent such as liquid paraffin is preferable, and a non-volatile solvent having a freezing point of −45 to −10 ° C. and a kinematic viscosity at 40 ° C. of 65 cst or less is more preferable.
[0019]
The mixing ratio of the polyolefin resin, thermoplastic polyester ether elastomer and solvent varies depending on the type of resin, solubility, kneading temperature, etc., so it cannot be determined unconditionally, but the resulting slurry-like resin composition is melt-kneaded and sheeted The total amount of the polyolefin resin and the thermoplastic polyester ether elastomer is usually 5 to 50% by weight, preferably 10 to 50% by weight of the resin composition.
[0020]
It should be noted that additives such as an antioxidant, an ultraviolet absorber, a dye, a nucleating agent, a pigment, and an antistatic agent are added to the resin composition as necessary within a range that does not impair the object of the present invention. be able to.
[0021]
The melt kneading of the resin composition is preferably performed by applying a sufficient shearing force to the resin composition in order to obtain sufficient entanglement of the polymer chains of the polyolefin resin and the thermoplastic polyester ether elastomer. Therefore, for the melt-kneading of the resin composition, a kneader or a twin screw extruder that can give a strong shearing force to the mixture is usually preferably used.
[0022]
The temperature at the time of melt-kneading the resin composition may be an appropriate temperature condition and is not particularly limited, but is preferably 115 to 230 ° C. The temperature at the time of melt-kneading is preferably 115 ° C. or higher in order to sufficiently knead the resin composition and obtain sufficient entanglement of the polymer chain of the polyolefin resin and the thermoplastic polyester ether elastomer, with a suitable viscosity, and the resin. In order to apply a sufficient shearing force to the composition, 230 ° C. or lower is preferable.
[0023]
The method of forming the melt-kneaded material into a sheet is not particularly limited. For example, the melt-kneaded material may be rapidly cooled by being sandwiched between metal plates cooled with dry ice and rapidly cooled to crystallize, and a T-die or the like is attached. After forming into a sheet shape using an extruder or the like, it may be cooled and crystallized.
[0024]
The thickness of the sheet-like molded product is usually preferably 1 to 20 mm, and more preferably 2 to 10 mm.
[0025]
The sheet-like molded product thus obtained is stretched by melt-kneading, and the intertwined fibril fibers return to a hair-globule to form thick fibers and large through-holes are formed in the sheet-like molded product. In order to prevent this, it is preferable that the crystal structure of the polyolefin resin and the thermoplastic polyester ether elastomer is maintained by immediately subjecting it to a rolling treatment described later, or storing it at a temperature below the freezing point of the solvent used.
[0026]
The rolling process can be performed using, for example, a heat press. By rolling, the thickness of the sheet-like molded product can be made uniform, and a porous film having higher strength can be obtained. 0.1-3 mm is preferable and, as for the thickness of the sheet-like molding after a rolling process, 0.2-0.5 mm is more preferable.
[0027]
The stretching method is not particularly limited, and may be a normal tenter method, roll method, inflation method, or a combination of these methods. In addition, any method such as uniaxial stretching or biaxial stretching can be applied. In the case of biaxial stretching, either longitudinal or transverse simultaneous stretching or sequential stretching may be used, but longitudinal and transverse simultaneous stretching is preferable.
[0028]
The stretch ratio of the rolled sheet-like molded product is preferably 2 to 20 times in one direction, and the surface stretch ratio is preferably 4 to 400 times.
[0029]
The temperature during the rolling treatment and the stretching treatment is preferably a temperature of the melting point of the polyolefin resin of −30 ° C. or more and a melting point of the polyolefin resin of + 5 ° C. or less, and a temperature of the melting point of the polyolefin resin of −25 ° C. or more and the melting point of the polyolefin resin or less. More preferred. In the present specification, the melting point of the polyolefin resin refers to an onset temperature in DSC measurement. As other rolling treatment and stretching treatment conditions, commonly used known conditions can be adopted.
[0030]
The solvent removal treatment is a step of removing the solvent from the sheet-like molded product to form a porous structure, and can be performed, for example, by washing the sheet-like molded product with a solvent to remove the remaining solvent. The solvent can be appropriately selected according to the solvent used for the preparation of the resin composition. Specifically, hydrocarbons such as pentane, hexane, heptane and decane, chlorination such as methylene chloride and carbon tetrachloride, etc. Easily volatile solvents such as hydrocarbons, ethers such as diethyl ether, dioxane and the like can be mentioned, and these can be used alone or in admixture of two or more. The method of desolvation treatment using such a solvent is not particularly limited, and examples thereof include a method of extracting a solvent by immersing a sheet-like molded product in a solvent, a method of showering the solvent on the sheet-like molded product, and the like. .
[0031]
In the present invention, for example, the sheet-shaped molded product may be subjected to a solvent removal treatment and then subjected to a stretching treatment, or the sheet-shaped molded material may be stretched as it is, and then the solvent removal treatment may be performed. Alternatively, a mode in which the solvent removal treatment is performed before the stretching treatment, and the solvent removal treatment is performed again after the stretching treatment to remove the residual solvent may be employed.
[0032]
The porous film of the present invention is obtained by subjecting the porous film thus obtained to a heat-setting treatment for preventing thermal contraction of the film, if necessary, and fixing the shape. There may be.
[0033]
The porous film of the present invention maintains its shape even when exposed to high temperature and high pressure conditions, and can prevent short-circuiting between electrodes due to breakage and film breakage. Can be suitably used as a separator for a conductive lithium ion secondary battery.
[0034]
【Example】
EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Various characteristics are measured as follows.
[0035]
[Melting point]
A differential scanning calorimeter “DSC-200” manufactured by Seiko Denshi Kogyo Co., Ltd. is used and measured at a rate of temperature increase of 10 ° C./min, and the onset temperature is defined as the melting point.
[0036]
[Film thickness]
Using a 1/10000 mm thickness gauge, the thickness of the porous film is arbitrarily measured at 9 points in the range from the end to the center, and the average value is obtained.
[0037]
[Porosity]
The weight is calculated from the weight W (g) per area S (cm ² ) of the porous film, the film thickness t (μm), and the resin density d (g / cm ³ ) by the following equation.
[0038]
Porosity (%) = (1-10 ⁴ × W / S / t / d) × 100
[0039]
[Air permeability]
Measured according to JIS P8117.
[0040]
[Needle puncture strength]
It is measured using a compression tester “KES-G5” manufactured by Kato Tech Co., Ltd., and the maximum load is read from the load displacement curve obtained by the measurement to obtain the needle puncture strength. A needle having a diameter of 0.75 mm and a radius of curvature of the tip of 0.5 mm is used, and the indentation speed is 2 cm / sec. All values are converted to 25 μm.
[0041]
Example 1
Ultra high molecular weight polyethylene resin (weight average molecular weight: 3 × 10 ⁶ , melting point: 136 ° C.) 12 parts by weight, thermoplastic polyester ether elastomer “Hytrel 4047” (manufactured by Toray DuPont, melting point: 182 ° C., flexural modulus: 7. 2 × 10 ⁷ Pa) 3 parts by weight and liquid paraffin 85 parts by weight were uniformly mixed in a slurry state, and melt kneaded for about 60 minutes at a temperature of 200 ° C. using a small kneader. The obtained melt-kneaded product was sandwiched between metal plates cooled to 0 ° C. and rapidly cooled into a sheet shape. These rapidly crystallized sheet-like molded products were heat-pressed at a temperature of about 115 ° C. until the sheet thickness became 0.4 to 0.6 mm. Next, it is biaxially stretched 3.5 times to 3.5 times in length and width at the same time at a temperature of about 115 ° C., desolvated using heptane, and heated sequentially at 85 ° C. for 20 minutes and at 134 ° C. for 20 minutes. A set treatment was performed to obtain a porous film having a thickness of 25 μm and a porosity of 50%.
[0042]
Example 2
Using 15 parts by weight of ultrahigh molecular weight polyethylene resin (weight average molecular weight: 3 × 10 ⁶ , melting point: 136 ° C.), 1 part by weight of thermoplastic polyester ether elastomer “Hytrel 4047” (manufactured by Toray DuPont) and 85 parts by weight of liquid paraffin A porous film having a thickness of 23 μm and a porosity of 45% was obtained in the same manner as in Example 1 except that the slurry was uniformly mixed.
[0043]
Comparative Example 1
Except that the ultra-high molecular weight polyethylene resin (weight average molecular weight: 3 × 10 ⁶ , melting point: 136 ° C.) and 15 parts by weight of liquid paraffin were uniformly mixed into a slurry and the melt-kneading step was performed at 160 ° C. In the same manner as in Example 1, a porous film having a thickness of 24 μm and a porosity of 44% was obtained.
[0044]
Table 1 shows the film thickness, porosity, air permeability, and needle puncture strength of the porous films obtained in Examples 1 and 2 and Comparative Example 1.
[0045]
Test example 1
Lithium cobalt oxide (LiCoO ₂ ) is added with phosphorous graphite as a conductive additive at a weight ratio of 90: 5 and mixed, and this mixture is mixed with a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone to form a slurry. Got. The obtained slurry was passed through a 70-mesh net to remove a large one, and then uniformly applied to both sides of a positive electrode current collector made of 20 μm thick aluminum foil, dried, and then compressed by a roller press. After forming, it was cut and the lead body was welded to produce a strip-like positive electrode. Next, a carbon material having an average particle diameter of 10 μm was mixed with a solution in which vinylidene fluoride was dissolved in N-methylpyrrolidone to form a slurry. The obtained slurry was passed through a 70-mesh net to remove a large one, and then uniformly applied to both sides of a negative electrode current collector made of a strip-shaped copper foil having a thickness of 18 μm and dried. After compression molding and cutting, the lead body was welded to produce a strip-shaped negative electrode.
[0046]
The electrode plate thus obtained was punched to 14 mmφ and 16 mmφ, and a porous film punched to 20 mmφ was inserted therebetween, and set in a SUS cell. The cell was fixed using a torque screw, and the melt fracture temperature when a surface pressure (2 × 10 ⁶ Pa) was applied in the film thickness direction was measured. That is, the resistance value between both electrodes is measured while raising the temperature in a dryer, and the temperature at which the resistance value suddenly drops from 1 × 10 ⁶ Ω or more (range over) to 1 × 10 ⁴ Ω or less is melted. The temperature (breaking temperature under compression conditions) was measured using a thermocouple attached to the electrode surface. The results are shown in Table 1.
[0047]
Test example 2
Using a dynamic viscoelasticity measuring device DMS-210 (manufactured by Seiko Denshi Kogyo Co., Ltd.), predetermined measurement conditions (sample width: 8 mm, sample length: 20 mm, heating rate: 5 ° C./min., Frequency) : 10 Hz, tension: 1.5 Pa) The temperature at which the porous film breaks under the above conditions is the melt fracture temperature with respect to the tension in the film surface direction (1.5 Pa) (breakage under the tension condition in the film surface direction) Temperature). The results are shown in Table 1.
[0048]
[Table 1]

[0049]
From the above results, the porous films of Examples 1 and 2 have both a high film breaking temperature under compression conditions and a high breaking temperature under tension conditions with respect to the film surface direction, and very excellent heat resistance. On the other hand, it can be seen that the porous film of Comparative Example 1 containing no thermoplastic polyester ether elastomer has low film breaking temperature and breaking temperature, and lacks heat resistance.
[0050]
【The invention's effect】
According to the present invention, it can be used as a separator for a lithium ion secondary battery, and can maintain its shape even when exposed to high temperature and high pressure, and can prevent short-circuiting between electrodes due to breakage and film breakage. It has become possible to provide a safe and highly heat-resistant porous film.

Claims (4)

  A porous film comprising 50 to 95% by weight of a polyolefin resin and 5 to 50% by weight of a thermoplastic polyester ether elastomer. The porous film according to claim 1, wherein the thermoplastic polyester ether elastomer has a melting point of 160 to 240 ° C. and a flexural modulus of 3 × 10 ⁷ Pa or more. The porous film according to claim 1 or 2, wherein the polyolefin resin contains 1% by weight or more of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more.   The porous film according to claim 3, wherein the ultrahigh molecular weight polyolefin resin is an ultrahigh molecular weight polyethylene resin.