JP2019102126A - Battery separator and non-aqueous electrolyte secondary battery - Google Patents

Abstract

To provide a battery separator capable of obtaining a secondary battery excellent in self-discharge characteristics, rate characteristics and cycle characteristics from a viewpoint of a tearing phenomenon of a separator in the secondary battery, when a polyolefin microporous membrane is used as the separator of the secondary battery.SOLUTION: In a polyolefin microporous membrane, a ratio (SMmax/STmax) of a maximum load (SMmax) in an MD direction in tear strength measurement (in accordance with JIS K 7128-3) by a right-angled tear method to a maximum load (STmax) in a TD direction is 0.9 or more and 1.4 or less. The maximum load (STmax) in the TD direction is 0.2 N/μm or more.SELECTED DRAWING: None

Description

  The present invention relates to a battery separator and a non-aqueous electrolyte secondary battery.

  The microporous membrane is used in various fields such as filters such as filtration membranes and dialysis membranes, separators for batteries, and separators for electrolytic capacitors. Among these, a microporous film made of polyolefin as a resin material is excellent in chemical resistance, insulating properties, mechanical strength and the like, and has a shutdown characteristic, and thus is widely used as a secondary battery separator in recent years.

  Secondary batteries, such as lithium ion secondary batteries, are widely used as batteries used for personal computers, mobile phones, etc. because of their high energy density. In addition, secondary batteries are also expected as power sources for driving motors of electric vehicles and hybrid vehicles.

  In recent years, with the increase of the volume of the electrode due to the densification of the energy density of the secondary battery, the thinning of the microporous film used as the separator is required. However, due to the thinning of the separator, the self-discharge of the secondary battery may increase as the film strength of the separator decreases.

  Patent Document 1 discloses that it is effective to control the tear strength by the Elmendorf tear method (in accordance with JIS K 7128-2) in order to improve the stability of the secondary battery.

  Further, Patent Document 2 discloses that it is effective to control the tear strength according to the trousers tear method (JIS K 7128-1) in order to improve the productivity of the secondary battery.

Patent No. 605 3903 Patent No. 4734397

  In Patent Documents 1 and 2, when the microporous polyolefin membrane is used as a separator of a secondary battery, assuming the tearing phenomenon of the microporous polyolefin membrane that may occur in the secondary battery, the Elmendorf tearing method (JIS K 7128- 2) It is stipulated that the tear strength is controlled by the method of T. 2) or the Trouser Tear Method (JIS K 7128-1).

  These two tear strengths, when applied to a separator, are tests in which the film surface of the separator is subjected to tearing in the vertical direction.

  However, assuming that tearing of the separator may occur in the secondary battery, it is considered that the force acting on the separator acts in two dimensions parallel to the film surface of the separator, not in the vertical direction. Therefore, the Elmendorf tearing method or the trousers tearing method does not substitute or reproduce the tearing phenomenon of the separator in the secondary battery.

  On the other hand, the tearing strength by the right-angled tearing method (in accordance with JIS K 7128-3) is a test in which the tearing is performed in parallel to the film surface of the separator when applied to the separator. It is believed that the tearing phenomenon can be substituted or reproduced. In Patent Document 1, although the separator is evaluated by such a right-angled tearing method, a separator having sufficient strength for application to a secondary battery is not obtained, and the separator is actually added to the separator in the battery. It is difficult to say that it captures the stress that

  In view of the above-described circumstances, the present invention provides a battery separator capable of obtaining a secondary battery having excellent self-discharge characteristics from the viewpoint of tearing of the separator in the secondary battery, and a non-aqueous electrolysis provided with this battery separator It aims at providing a liquid secondary battery.

The battery separator according to the first aspect of the present invention is a battery separator using a microporous membrane, wherein tear strength in the MD direction of the microporous membrane was measured by the right-angled tearing method according to JIS K 7128-3. The ratio (SMmax / STmax) between the maximum load (SMmax) at the time of measurement and the maximum load (STmax) when the tear strength in the TD direction of the microporous membrane is measured by the right-angled tear method according to JIS K 7128-3 ) Is 0.9 or more and 1.4 or less,
The maximum load (STmax) in the TD direction is 0.2 N / μm or more.

  Here, the sample shape in the tear strength measurement by the right-angled tearing method of the microporous membrane is specified in JIS K 7128-3, and the measurement sample for measuring the tear strength in the MD direction has a long shape in the TD direction The measurement sample for measuring the tear strength in the TD direction has a long shape in the MD direction. Furthermore, the maximum value of the load (stress) obtained while measuring the tear strength in the MD direction and the maximum value of the load obtained while measuring the tear strength in the TD direction are divided by the thickness of the microporous membrane, microporous Values converted to 1 μm of film are respectively set as maximum loads SMmax and STmax.

The microporous membrane preferably further comprises any one of the following [1] to [5] or a combination of two or more of the requirements.
[1] The tensile elongation at break in the TD direction is 100% or more.
[2] The porosity is 30% or more and 60% or less.
[3] The film thickness is 3 μm or more and 10 μm or less.
[4] It is composed mainly of a polyolefin resin and contains 50% by mass or more of high density polyethylene (HDPE) with respect to 100% by mass of the entire polyolefin resin.
[5] The weight average molecular weight is 8 × 10 ⁵ or more.

  In addition, the battery separator may have at least one surface of the microporous membrane provided with one or more coated microporous layers.

  The non-aqueous electrolyte secondary battery of the second aspect of the present invention uses the battery separator.

  According to the present invention, it is possible to obtain a battery separator and a non-aqueous electrolyte secondary battery excellent in self-discharge characteristics.

  Hereinafter, a microporous polyolefin membrane which is a battery separator of the present invention will be described based on a preferred embodiment. In the present specification, the microporous polyolefin membrane refers to a microporous membrane containing polyolefin as a main component. The “main component” refers to containing 80% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass, based on 100% by mass of the microporous polyolefin membrane. The microporous membrane has a large number of micropores inside the membrane, and these micropores are connected to each other, and at least one of gas and liquid can pass from one side to the other side. Say the membrane that has become.

1. Polyolefin microporous membrane The polyolefin microporous membrane of the present invention has the maximum load (SMmax) when the tear strength in the MD direction is measured by the right-angled tearing method according to JIS K 7128-3, and the TD of the microporous membrane The ratio (SMmax / STmax) to the maximum load (STmax) when measured by the right-angled tearing method in accordance with JIS K 7128-3 according to JIS K 7128-3 is preferably 0.9 or more and 1.4 or less. It is 1.0 or more and 1.2 or less, more preferably 1.1 or more and 1.2 or less. In addition, the maximum load (STmax) in the TD direction by the right-angled tearing method is 0.2 N / μm or more, preferably 0.24 N / μm or more, and more preferably 0.28 N / μm or more. The maximum load (STmax) in the TD direction according to the right-angled tearing method is, for example, 1.0 N / μm or less, preferably 0.5 N / μm or less, and more preferably 0.4 N / μm or less. The maximum load (SMmax) in the MD direction by the right-angled tearing method is 0.21 N / μm or more, preferably 0.25 N / μm or more, and more preferably 0.3 N / μm or more. Further, the maximum load (SMmax) in the MD direction by the perpendicular tearing method is 1.0 N / μm or less, preferably 0.5 N / μm or less, and more preferably 0.4 N / μm or less. Here, the maximum load in the MD direction and the TD direction is a load obtained by dividing the load at the time of measurement by the thickness (μm) of the microporous membrane and converting it to a thickness of 1 μm. When the ratio between the maximum load in the MD direction and the maximum load in the TD direction of the microporous polyolefin membrane and the maximum load of tear strength in the TD direction are in the above ranges, a secondary battery using the microporous polyolefin membrane as a separator The self-discharge characteristics of The tear strength may be, for example, at least one of ultra-high molecular weight polyethylene and a nucleating agent, or a weight average molecular weight Mw or a stretching ratio (in particular, wet stretching to be described later). The above range can be obtained by adjusting the magnification and the draw ratio of the film after drying.

  In the tear test defined by JIS, Elmendorf tear method (JIS K 7128-2) specified by Patent Document 1 in addition to tear strength measurement by the perpendicular tear method specified by the present invention (in accordance with JIS K 7128-3) And the tear strength measurement by the Trouser Tear method (JIS K 7128-1) specified by patent documents 2 exists. As described above, since the tearing phenomenon of the separator in the secondary battery is considered to be close to the tearing in two dimensions parallel to the film surface of the separator, the Elmendorf tearing method and the Trouser tearing method are in the secondary battery. It does not substitute or reproduce the tearing phenomenon of the separator.

  On the other hand, in the present invention, attention is paid to tear strength measurement (JIS K 7128-3 compliant) by the right-angled tear method as a tear test which can substitute or reproduce the tear phenomenon of the separator in the secondary battery. The ratio (SMmax / STmax) of the maximum load in the MD direction (SMmax) to the maximum load in the TD direction (STmax) and the maximum load in the TD direction (STmax) are adjusted in a specific range.

(Tensile strength)
The lower limit of the tensile strength (tensile breaking strength) in the MD direction of the microporous polyolefin membrane is, for example, 100 MPa or more, preferably 230 MPa or more, more preferably 250 MPa or more, and still more preferably 280 MPa or more. The upper limit of the tensile strength in the MD direction is not particularly limited, but is, for example, 600 MPa or less. When the tensile strength in the MD direction is in the above range, the film is less likely to be broken even when a high tension is applied, and the film can be used in applications requiring high durability. For example, when the microporous film excellent in strength as described above is used as a separator, short circuit between electrodes at the time of battery preparation or battery use can be suppressed and high tension can be applied to wind the separator. Capacity can be achieved. In addition, when a coating layer is formed on at least one surface of the thinned polyolefin microporous membrane, the flatness of the microporous membrane at the time of coating of the coating layer is required, so the tensile strength in the higher MD direction is higher. Required Therefore, from the viewpoint of improving the coatability of the coating layer, the tensile strength in the MD direction of the microporous polyolefin membrane is preferably 230 MPa or more, more preferably 250 MPa or more, and still more preferably 280 MPa or more. . When the tensile strength in the MD direction is in the above range, the microporous membrane of the present invention can be suitably used as a substrate for coating.

  The lower limit of the tensile strength in the TD direction of the microporous polyolefin membrane is not particularly limited, but is, for example, 100 MPa or more, preferably 150 MPa or more, and more preferably 170 MPa or more. The upper limit of the tensile strength in the TD direction is not particularly limited, but is, for example, 300 MPa or less. In the microporous polyolefin membrane, the ratio of MD tensile strength to TD tensile strength (MD tensile strength / TD tensile strength) is preferably more than 1.0 and more than 1.0 and not more than 1.8. Preferably, it is preferably 1.2 or more and 1.7 or less.

  When at least one of the tensile strength in the TD direction (TD tensile strength) of the microporous polyolefin membrane and the ratio of the tensile strength in the MD direction (MD tensile strength) to the TD tensile strength falls within the above range, the tensile strength is excellent Therefore, it can be suitably used for applications requiring high strength and durability. Further, since the winding direction of the separator is usually the MD direction, the ratio of the MD tensile strength to the TD tensile strength is preferably within the above range.

  In addition, about MD tensile strength and TD tensile strength, it is the value measured by the method based on ASTMD882.

(Tensile elongation)
The tensile elongation (tensile breaking elongation) in the MD direction of the microporous polyolefin membrane is not particularly limited, but is, for example, 50% to 300%, preferably 50% to 100%, and further 70%. It is preferable that it is more than 100%. When the breaking elongation in the MD direction is in the above range, it is difficult to deform even when high tension is applied during coating, and wrinkles are not easily generated, so the occurrence of coating defects is suppressed and the flatness of the coated surface Because it is good.

  The tensile elongation (tensile breaking elongation) in the TD direction of the microporous polyolefin membrane is preferably 100% or more. When the breaking elongation in the TD direction is in the above range, the collision resistance can be evaluated by an impact test etc., and when a microporous polyolefin membrane is used as a separator, electrode irregularities, battery deformation, battery heat generation Is preferable because the separator can follow the occurrence of internal stress and the like due to the above.

  In addition, MD tensile elongation and TD tensile elongation are the values measured by the method based on ASTMD882.

(Stab strength)
The lower limit of the puncture strength of the microporous polyolefin membrane is preferably 2.0 N or more, more preferably 2.5 N or more. The upper limit of the puncture strength is not particularly limited, and is, for example, 7.0 N or less. When the puncture strength is in the above range, the membrane strength of the microporous polyolefin membrane is excellent. In addition, in the secondary battery using this microporous polyolefin membrane as a separator, the occurrence of short circuiting of the electrodes and self discharge can be suppressed. The puncture strength is, for example, at least one of ultrahigh molecular weight polyethylene and a nucleating agent when producing the microporous polyolefin membrane, or the weight average molecular weight (Mw) or stretching ratio of the polyolefin resin constituting the microporous polyolefin membrane In particular, the above-mentioned range can be obtained by adjusting the draw ratio of the film after drying described later.

  The microporous polyolefin film preferably has a puncture strength in a thickness of 5 μm of 2.0 N or more, more preferably 2.3 N or more, and still more preferably 2.5 N or more. The upper limit of the piercing strength in terms of a film thickness of 5 μm is not particularly limited, but is, for example, 4.0 N or less. When the puncture strength is in the above range, the membrane strength is excellent even when the polyolefin microporous membrane is thinned, and the secondary battery using this polyolefin microporous membrane as a separator suppresses the occurrence of short circuiting of the electrode and the self-discharge. .

The pin puncture strength is the maximum load (N) when a polyolefin microporous membrane with a film thickness T ₁ (μm) is punctured at a speed of 2 mm / sec with a needle with a tip of a spherical surface (curvature radius R: 0.5 mm) and a diameter of 1 mm. It is the value which measured). Moreover, the piercing strength (N / 5 μm) in terms of a film thickness of 5 μm is a value which can be obtained by the following equation.
Pin puncture strength (5 μm equivalent) = measured pin puncture strength (N) × 5 (μm) / film thickness T ₁ (μm)
(Air resistance)
The air permeability resistance (Gurley value) of the microporous polyolefin membrane is not particularly limited, and is, for example, 50 seconds / 100 cm ³ or more and 300 seconds / 100 cm ³ or less. When the microporous polyolefin membrane is used as a separator for a secondary battery, it is preferably 250 seconds / 100 cm ³ or less, more preferably 200 seconds / 100 cm ³ or less, and still more preferably 150 seconds / 100 cm. It is ³ or less. When the air resistance is in the above range, when used as a secondary battery separator, the ion permeability is excellent, the impedance of the secondary battery is reduced, and the battery output is improved. The degree of air resistance can be made into the above-mentioned range by adjusting the stretching conditions and the like at the time of producing the microporous polyolefin membrane.

In addition, the air permeability resistance (Gurley value) of the microporous polyolefin membrane is preferably 150 seconds / 100 cm ³ or less when converted to a film thickness of 5 μm.

(Film thickness)
The upper limit of the film thickness of the microporous polyolefin membrane is not particularly limited, and is, for example, 30 μm or less, preferably 15 μm or less, more preferably 10 μm or less, and still more preferably 7 μm or less. The lower limit of the film thickness is not particularly limited, but is, for example, 1 μm or more, preferably 3 μm or more. When the film thickness is in the above range, the battery capacity is improved when the microporous polyolefin film is used as a battery separator. The microporous polyolefin membrane of the present embodiment has high puncture strength and the like, and has high self-discharge characteristics and rate characteristics even when it is thinned.

(Porosity)
The lower limit of the porosity of the microporous polyolefin membrane is not particularly limited, and is, for example, 10% or more, preferably 30% or more, and more preferably 40% or more. The upper limit of the porosity is not particularly limited, but is, for example, 70% or less, preferably 60% or less, and more preferably 50% or less. When the microporous polyolefin membrane is used as a separator for a secondary battery, the porosity of the microporous polyolefin membrane is preferably 30% to 60%, and more preferably 40% to 50%. When the porosity is in the above range, the amount of electrolyte held can be increased, and high ion permeability can be ensured. In addition, when the porosity is in the above range, the rate characteristics are improved. In addition, the porosity is preferably 40% or more from the viewpoint of further enhancing ion permeability and rate characteristics. The porosity can be made into the above-mentioned range by adjusting the blending ratio of the component of the polyolefin resin, the stretching ratio, the heat setting condition and the like in the production process.

(Weight average molecular weight: Mw)
The weight average molecular weight (Mw) of the microporous polyolefin membrane is, for example, 7 × 10 ⁵ or more and less than 1.5 × 10 ⁶ . When the Mw of the polyolefin microporous membrane is in this range, it is excellent in moldability, mechanical strength and the like. In addition, Mw of a polyolefin fine porous film can be made into the said range by adjusting the compounding ratio of the structural component of polyolefin resin, and the conditions of melt-kneading suitably. The Mw of the microporous polyolefin membrane is a value measured in terms of polystyrene by gel permeation chromatography (GPC).

When the polyolefin microporous membrane does not contain a nucleating agent, the Mw of the polyolefin microporous membrane is preferably 8 × 10 ⁵ or more, more preferably 8.2 × 10 ⁵ or more, and still more preferably 8.5 × It is 10 ⁵ or more, particularly preferably 1.0 × 10 ⁶ or more. When the Mw of the polyolefin microporous membrane is in this range, a polyolefin microporous membrane having a uniform and fine crystal structure can be obtained without adding a nucleating agent. And in the manufacturing process of a polyolefin fine porous film, even if it extends | stretches by comparatively high magnification, local stress concentration does not occur and a uniform and fine pore structure can be formed. On the other hand, the upper limit of the Mw of the microporous polyolefin membrane is, for example, 2.5 × 10 ⁶ or less, preferably 2.0 × 10 ⁶ or less, and more preferably 1.8 × 10 ⁶ or less. When the Mw exceeds 2.5 × 10 ⁶ , the viscosity becomes high, which may make it unsuitable for melt extrusion.

(composition)
The microporous polyolefin membrane contains a polyolefin resin as a main component. As polyolefin resin, polyethylene, a polypropylene, etc. can be used, for example. For example, 50 mass% or more of polyethylene can be contained with respect to the resin whole quantity of a polyolefin fine porous film. The polyethylene is not particularly limited, and various polyethylenes can be used. For example, high density polyethylene, medium density polyethylene, branched low density polyethylene, linear low density polyethylene and the like are used. The polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like.

The microporous polyolefin membrane can contain high density polyethylene (HDPE) (density: 0.920 g / m ³ or more and 0.970 g / m ³ or less). When the high density polyethylene is contained, it is excellent in melt extrusion characteristics and excellent in uniform stretching processing characteristics. The weight average molecular weight (Mw) of high density polyethylene used as a raw material is, for example, 1 × 10 ⁴ or more and less than 1 × 10 ⁶ . The content of the high density polyethylene is, for example, 50% by mass or more based on 100% by mass of the entire polyolefin resin. The upper limit of the content of the high density polyethylene is, for example, 100% by mass or less with respect to 100% by mass of the entire polyolefin resin, and is 90% by mass or less when other components are contained.

The microporous polyolefin membrane can also include ultra-high molecular weight polyethylene (UHMwPE). The ultrahigh molecular weight polyethylene used as a raw material has a weight average molecular weight (Mw) of 1 × 10 ⁶ or more (1,000,000 or more), preferably 1 × 10 ⁶ or more and 8 × 10 ⁶ or less. When the Mw of the ultrahigh molecular weight polyethylene is in the above range, the formability is good. The ultrahigh molecular weight polyethylene can be used singly or in combination of two or more. For example, two or more ultrahigh molecular weight polyethylenes having different Mw may be mixed and used.

  The ultrahigh molecular weight polyethylene can be contained, for example, in an amount of 0% by mass to 70% by mass, preferably 10% by mass to 50% by mass, based on 100% by mass of the entire polyolefin resin. When the content of the ultrahigh molecular weight polyethylene is 10% by mass to 50% by mass, the Mw of the resulting microporous polyolefin membrane can be easily controlled to the range described above, and the productivity such as extrusion kneadability is excellent. Tend. Moreover, when ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the microporous polyolefin membrane is thinned.

  The microporous polyolefin membrane may comprise polypropylene. The type of polypropylene is not particularly limited, and may be a homopolymer of propylene, at least one of a copolymer of propylene and another α-olefin and a diolefin (propylene copolymer), or a mixture of these. It is preferable to use a homopolymer of propylene from the viewpoints of mechanical strength and miniaturization of through hole diameter. The content of polypropylene is preferably 2.5% by mass or more based on 100% by mass of the entire polyolefin resin, based on 100% by mass of the entire polyolefin resin, from the viewpoint of heat resistance improvement. Moreover, it is preferable from the point of suppressing that a shutdown temperature becomes high to be 15 mass% or less with respect to 100 mass% of whole polyolefin resin.

  In addition, the microporous polyolefin membrane can contain, if necessary, polyolefins other than polyethylene and polypropylene and other resin components. As the other resin component, for example, a heat resistant resin can be used. In addition, the microporous polyolefin membrane is an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent or a filler, a crystal nucleating agent, a crystallization retarder, as long as the effects of the present invention are not impaired. And the like may be contained.

2. Method for producing a microporous polyolefin membrane A method for producing a microporous polyolefin membrane is, for example, a wet film formation represented by the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835, WO2006 / 137540, etc. In the stretching of the gel-like sheet in the method, simultaneous biaxial stretching is carried out to obtain a microporous polyolefin membrane having the above-mentioned properties.

  Hereinafter, the method for producing the microporous polyolefin membrane (wet film forming method) will be described. The following description is an example of the manufacturing method, and the present invention is not limited to this method.

  First, a polyolefin resin and a film forming solvent (solvent) are melt-kneaded to prepare a resin solution. As the melt-kneading method, for example, a method using a twin-screw extruder described in the specification of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. The melt-kneading method is known and thus the description thereof is omitted.

  The polyolefin resin preferably comprises high density polyethylene. When high density polyethylene is contained, it is excellent in melt extrusion characteristics and excellent in uniform stretching processing characteristics. The polyolefin resin can also include ultra-high molecular weight polyethylene. When the ultra-high molecular weight polyethylene is contained, the Mw of the obtained microporous polyolefin membrane can be easily controlled to the above-described specific range, and there is a tendency to be excellent in productivity such as extrusion kneadability. About the detail of the kind and compounding quantity which can be used as polyolefin resin, since it is the same as that of the above, description is abbreviate | omitted.

  The resin solution may contain components other than the above-mentioned polyolefin resin and film forming solvent (solvent), and may contain, for example, a crystal nucleating agent (nucleating agent), an antioxidant, and the like. The nucleating agent is not particularly limited, and known compound-based and particle-based crystal nucleating agents can be used. The nucleating agent may be a masterbatch in which the nucleating agent is mixed and dispersed in advance in a polyolefin resin.

  When the resin solution does not contain a crystal nucleating agent, the polyolefin resin preferably contains the above-mentioned ultrahigh molecular weight polyethylene and high density polyethylene. The microporous polyolefin membrane may also contain high density polyethylene, ultra high molecular weight polyethylene and a nucleating agent. By including these, the puncture strength of the microporous polyolefin membrane can be further improved.

  The molten resin is then extruded and cooled to form a gel-like sheet. For example, the resin solution prepared above is fed from an extruder to one die and extruded into a sheet to obtain a molded body. The gel-like sheet is formed by cooling the obtained molded body.

  As a method of forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of 50 ° C./min or more at least to the gelation temperature. Cooling is preferably performed to 25 ° C. or less. By cooling, the microphase of the polyolefin separated by the film forming solvent can be immobilized. When the cooling rate is in the above range, the degree of crystallinity is maintained in a suitable range, and a gel-like sheet suitable for stretching can be obtained. As a cooling method, a method of contacting with a refrigerant such as cold air, cooling water or the like, a method of contacting with a cooling roll, or the like can be used, but it is preferable to cool by contacting with a roll cooled with a refrigerant.

  The gel-like sheet is then stretched. Stretching of the gel-like sheet (first stretching) is also referred to as wet stretching. Wet stretching is performed in at least one direction. Since the gel-like sheet contains a solvent, it can be stretched uniformly.

  After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. Although stretching may be uniaxial or biaxial, biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multistage stretching (for example, a combination of simultaneous biaxial and sequential stretching) may be used.

  9 times or more is preferable, 16 times or more is more preferable, and, as for the final area draw ratio (area ratio) in wet extending | stretching, 25 times or more is more preferable. The upper limit of the area stretch ratio of the wet stretching is preferably 49 times or less, more preferably 36 times or less. In addition, the magnification of wet stretching is preferably 3 or more in any of the MD direction and the TD direction, and the draw ratios in the MD direction and the TD direction may be the same or different. When the draw ratio is 5 times or more, improvement in tensile strength and piercing strength can be expected. On the other hand, increasing the draw ratio in wet stretching generally improves tensile strength and puncture strength, but when the draw ratio in at least one of the MD and TD directions exceeds 7 times, tear strength measurement by the perpendicular tear method The ratio (SMmax / STmax) between the maximum load in the MD direction (SMmax) and the maximum load in the TD direction (STmax) in (5) or the maximum load (STmax) in the TD direction can not be defined within a predetermined range, Even when the microporous polyolefin membrane is used as a separator of a secondary battery, a secondary battery having excellent self-discharge characteristics can not be obtained. In addition, the draw ratio in this step means the draw ratio of the gel-like sheet just before being provided to the following step on the basis of the gel-like sheet just before this step. Further, the TD direction is a direction orthogonal (cross) to the MD direction when the microporous film is viewed in a plane.

  The stretching temperature is preferably in the range of the crystal dispersion temperature (Tcd) to (Tcd) + 30 ° C. of the polyolefin resin, and the crystal dispersion temperature (Tcd) + 5 ° C. to the crystal dispersion temperature (Tcd) + 28 ° C. Is more preferable, and (Tcd) + 10 ° C. to (Tcd) + 26 ° C. is particularly preferable. When the stretching temperature is in the above-mentioned range, the film breakage by the polyolefin resin stretching is suppressed, and the stretching at a high magnification can be performed. Here, the crystal dispersion temperature (Tcd) refers to a value determined by measuring temperature characteristics of dynamic viscoelasticity based on ASTM D4065. The above ultra high molecular weight polyethylene, polyethylene other than ultra high molecular weight polyethylene and polyethylene compositions have a crystal dispersion temperature of about 90-100 ° C. Therefore, the stretching temperature in the case of using such a polyethylene composition can be, for example, 90 ° C. or more and 130 ° C. or less.

  The stretching as described above causes cleavage between polyethylene lamellas to refine the polyethylene phase and form a large number of fibrils. The fibrils form a three-dimensionally irregularly linked network structure. Although stretching improves the mechanical strength and enlarges the pores, when stretching is performed under appropriate conditions, it is possible to control the through hole diameter and to have a high porosity even with a thinner film thickness. For this reason, it is suitable for a safer and high performance battery separator.

  Next, the film forming solvent is removed from the stretched gel-like sheet to form a microporous film (film). The film forming solvent is removed by washing with a washing solvent. Since the polyolefin phase is separated from the film forming solvent phase, when the film forming solvent is removed, it is composed of fibrils forming a fine three-dimensional network structure, and three-dimensionally irregularly communicated holes (voids) A porous membrane is obtained. The cleaning solvent and the method for removing the film forming solvent using the same are known and thus the description thereof is omitted. For example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

  Next, the microporous film from which the film-forming solvent has been removed is dried by a heat-drying method or an air-drying method. The drying temperature is preferably equal to or less than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than (Tcd). The drying is preferably carried out until the residual washing solvent is 5% by mass or less, and more preferably 3% by mass or less, with the microporous membrane film as 100% by mass (dry weight). When the remaining cleaning solvent is in the above range, the porosity of the microporous polyolefin membrane is maintained when the subsequent microporous membrane film stretching step and heat treatment step are performed, and the deterioration of the permeability is suppressed.

  Next, the dried microporous membrane is stretched. Stretching of the microporous membrane after drying (second stretching, third stretching) is also referred to as dry stretching. Specifically, the dried microporous membrane film is dry-stretched in at least uniaxial direction. The dry stretching of the microporous film can be carried out by a tenter method or the like while heating while heating. The dry stretching may be uniaxial stretching or biaxial stretching. When dry stretching is biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but sequential stretching is preferred. When dry stretching is sequential stretching, it is preferable to stretch in the MD direction (second stretching) and then continuously stretch in the TD direction (third stretching).

  It is preferable that it is 1.2 times or more, and, as for the surface magnification (area draw ratio) of dry extending | stretching, it is more preferable that they are 1.2 times or more and 9.0 times or less. By setting the area magnification to the above range, the piercing strength and the like can be easily controlled within the desired range. When dry stretching is uniaxial stretching, for example, it is 1.2 times or more, preferably 1.2 times or more and 3.0 times or less in the MD direction or TD direction. When dry stretching is biaxial stretching, it is 1.1 times or more and 3.0 times or less each in the MD direction and the TD direction, and the draw ratio in the MD direction and the TD direction may be the same or different. It is preferable that the draw ratio in the TD direction be substantially the same. In the dry stretching, after stretching (second stretching) in the MD direction by more than 1 time and 3 times or less, it is preferable to continuously stretch in the TD direction by more than 1 time and 3 times or less (third stretching). In addition, the draw ratio in this step means the draw ratio of the microporous membrane just before being provided to the next step on the basis of the microporous film (film) just before this step. The stretching temperature in this step (dry stretching) is not particularly limited, but is usually 90 to 135 ° C.

  The microporous membrane after drying may be subjected to heat treatment. The heat treatment stabilizes the crystals and makes the lamella uniform. As a heat treatment method, at least one of a heat setting treatment and a heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which heating is performed while holding both ends in the TD direction of the film so that the dimension in the TD direction of the film does not change. The heat setting treatment is preferably performed by a tenter method or a roll method. The thermal relaxation treatment is a heat treatment which causes the film to be thermally shrunk in at least one of the MD direction and the TD direction during heating. As a thermal relaxation treatment method, the method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be mentioned. The heat treatment temperature is preferably in the range of (stretching temperature of microporous membrane) ± 5 ° C., and more preferably in the range of (stretching temperature in the second stretching of the microporous membrane) ± 3 ° C.

  For example, heat treatment and thermal relaxation treatment may be performed after the third stretching. In the thermal relaxation treatment, the relaxation temperature is, for example, 80 ° C. or more and 135 ° C. or less, preferably 90 ° C. or more and 133 ° C. or less. When the heat relaxation treatment is performed, the final dry stretching ratio is, for example, 1.0 times or more and 9.0 times or less, preferably 1.2 times or more and 4.0 times or less. The relaxation rate can be 0% or more and 70% or less.

  In addition, the polyolefin microporous membrane after dry stretching may be further subjected to a crosslinking treatment and a hydrophilization treatment. For example, the cross-linking treatment is carried out by irradiating the microporous film with ionizing radiation such as alpha rays, beta rays, gamma rays and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an accelerating voltage of 100 to 300 kV is preferable. The crosslinking treatment raises the meltdown temperature of the microporous membrane. The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting is preferably carried out after the crosslinking treatment.

  The microporous polyolefin membrane may be a single layer, but one or more layers composed of the microporous polyolefin membrane may be laminated. When the polyolefin microporous membrane has a layer composed of two or more polyolefin microporous membranes (hereinafter, also referred to as a multilayer polyolefin microporous membrane), the compositions of the polyolefin resins constituting the respective layers may be the same or different. .

  The microporous polyolefin membrane may form a battery separator as a laminated microporous polyolefin membrane by laminating other porous layers (microporous layers) other than the polyolefin resin. The other porous layer is not particularly limited. For example, a coating layer such as an inorganic particle layer containing a binder and inorganic particles may be laminated. It does not specifically limit as a binder component which comprises an inorganic particle layer, A well-known component can be used, For example, an acrylic resin, polyvinylidene fluoride resin, polyamidoimide resin, a polyamide resin, aromatic polyamide resin, a polyimide resin etc. Can be used. The inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used. For example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon or the like may be used. it can. In addition, as the laminated polyolefin microporous membrane, one or more coated microporous layers, for example, an inorganic particle layer containing the binder and the inorganic particles are laminated on at least one surface of the polyolefin microporous membrane May be

  Hereinafter, the present invention will be described in more detail by way of examples. The present invention is not limited to these examples.

1. Measurement method and evaluation method [film thickness]
The film thickness of five points in the range of 95 mm x 95 mm of the microporous film was measured by a contact thickness meter (Light Matic manufactured by Mitutoyo Co., Ltd.), and the average value was determined.

[Air resistance; Gurley value]
For a microporous film with a film thickness T ₁ (μm), the air resistance P ₁ (sec) measured with an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) according to JIS P-8117 / 100 cm ³ ) was measured. Further, by the _formula of _{P 2 = (P 1 × 5} ) / T 1, to calculate the air resistance of _P 2 when a 5 [mu] m film thickness (5 [mu] m in terms ^{of) (sec / 100cm 3 / 5μm} ).

[Porosity]
It was measured according to the following equation comparing the weight w _{1 of the} microporous membrane with the weight w ₂ (the same polymer of width, length and composition) of the non-void equivalent polymer.

Porosity (%) = (w ₂ −w ₁ ) / w ₂ × 100
[Tensile strength]
The tensile strength in the MD direction and the tensile strength in the TD direction were measured using a strip-shaped test piece with a width of 10 mm according to the method according to ASTM D882.

[Tensile elongation]
The tensile elongation in the MD direction and the tensile elongation in the TD direction were measured by the method according to ASTM D882.

[Stab strength]
The maximum load L ₁ (N) when a microporous membrane with a thickness of T ₁ (μm) is pierced at a speed of 2 mm / sec with a needle with a diameter of 1 mm and a spherical tip (curvature radius R: 0.5 mm) It was measured.

[Weight average molecular weight (Mw)]
The weight average molecular weight (Mw) of the polyolefin resin and the obtained polyolefin microporous membrane was determined by gel permeation chromatography (GPC) under the following conditions.
-Measuring device: GPC-150C manufactured by Waters Corporation
・ Column: Shodex UT806M manufactured by Showa Denko KK
・ Column temperature: 135 ° C
Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 0.1 wt% (dissolution condition: 135 ° C./1 h)
Injection amount: 500 μl
Detector: Differential refractometer (RI detector) manufactured by Waters Corporation
Calibration curve: A calibration curve obtained using a monodispersed polystyrene standard sample was prepared using a polyethylene conversion coefficient (0.46).

[Tear strength measurement]
The tear strength measurement by the right-angled tearing method was carried out by the method according to "JIS K 7128-3 Test method for tearing strength of plastic film and sheet-Part 3: Right-angled tearing method".

The measurement sample shape and size are shape sizes defined in JIS K 7123-3, and were prepared by punching. In addition, the measurement sample of MD tear strength is a long shape in the TD direction, and the measurement sample of TD tear strength is a long shape in the MD direction.
Equipment: Precision universal testing machine AGS-J (manufactured by SHIMADZU)
Measurement conditions: Tension speed 200 mm / min
The thickness (μm) of the test piece is measured with a contact thickness meter (Light Matic manufactured by Mitutoyo Co., Ltd.), and the maximum load (stress) obtained at the time of measurement in the MD direction and the maximum load obtained at the time of measurement in the TD direction The maximum load in the MD direction per 1 μm thickness and the maximum load in the TD direction divided by 1 were taken as SMmax and STmax, respectively.

[Self discharge characteristics]
The self-discharge characteristics were evaluated by the following method. That is, after the test secondary battery assembled according to the following (Method for producing a battery for evaluation) is constant current charged to a battery voltage of 3.85 V at a current value of 0.5 C, 0.05 C at a battery voltage of 3.85 V Constant voltage charging was performed until The open circuit voltage was measured after leaving the battery for 24 hours, and this value was set to V1. The open circuit voltage after leaving this battery for another 24 hours, that is, after leaving for a total of 48 hours after charging, was measured, and this value was set to V2.
From the values of V1 and V2 thus obtained, the K value was calculated by the following equation, and evaluated by the following (evaluation criteria).

Formula: K value = (V1−V2) / 24
(Evaluation criteria)
((Excellent): K value less than 0.03) (good): K value 0.03 or more and less than 0.07 × (not good): K value 0.07 or more.

Rate characteristics
Rate characteristics were evaluated by the following method. The secondary battery for a test produced by the following (production method of the battery for evaluation) was used for the measurement of a rate characteristic. After constant current charging to a battery voltage of 4.2 V at a current value of 1.0 C and then constant voltage charging to a current value of 0.05 C at a battery voltage of 4.2 V, then at a current value of 0.2 C Discharge (constant current discharge) was performed until the battery voltage reached 3.0 V, and the discharge capacity was measured. Subsequently, the battery was charged again to 4.2 V by the above-mentioned procedure, and then discharged (constant current discharge) at a current value of 5 C until the battery voltage became 3.0 V, and the discharge capacity was measured. The discharge capacity ratio was calculated by the following equation, and the rate characteristics were evaluated by the following (evaluation criteria).
Formula: Discharge capacity at discharge capacity ratio = 5 C × discharge capacity at 100 / 0.2 C (evaluation criteria)
((Excellent): 90% or more ((good): 80% or more and less than 90% x (improper): less than 80%.

[Cycle characteristics]
The test secondary battery produced in the above (fabrication of battery) was used for measurement of cycle characteristics. After constant current charging at a current value of 1.0 C to a battery voltage of 4.2 V, constant voltage charging was performed until the current value of 0.05 C was reached at a battery voltage of 4.2 V. After 10 minutes of rest, constant current discharge was performed until the battery voltage became 3.0 V at a current value of 1.0 C, and then rested for 10 minutes. The charge and discharge was repeated 500 times with the charge and discharge as one cycle.
The remaining capacity ratio was calculated by the following equation, and the cycle characteristics were evaluated by the following (evaluation criteria).
Formula: Remaining capacity ratio = 500th cycle discharge capacity × 100 / 1st cycle discharge capacity (evaluation criteria)
○ (good): 80% or more × (not good): less than 80%.

(Production method of evaluation battery)
The battery used for evaluation (battery for evaluation) was a lithium cobalt complex oxide LiCoO ₂ as a positive electrode active material, graphite as a negative electrode active material, and 1 mol / L LiPF ₆ prepared in a mixed solvent of EC / EMC / DMC as an electrolytic solution. A positive electrode, a separator comprising a microporous polyolefin membrane, and a negative electrode are laminated, and then a wound electrode body is produced by a conventional method, inserted into a battery can, impregnated with an electrolytic solution, sealed, and produced. did. Below, the detail of the manufacturing method of the battery for evaluation is demonstrated.

(Production of positive electrode)
A solvent N is mixed with lithium cobalt complex oxide LiCoO ₂ as a positive electrode active material, acetylene black as a conductive material, polyvinylidene fluoride (PVDF) as a binder at a mass ratio of 93.5: 4.0: 2.5. -The slurry was prepared by mixing and dispersing in methyl pyrrolidone (NMP). The slurry was applied to both sides of a 12 μm thick aluminum foil serving as a positive electrode current collector, dried, and rolled by a roll press. The rolled product was slit into a width of 30 mm to form a positive electrode.

(Fabrication of negative electrode)
Artificial slurry as a negative electrode active material, carboxymethyl cellulose as a binder and a styrene-butadiene copolymer latex were mixed and dispersed in purified water so as to have a mass ratio of 98: 1: 1 to prepare a slurry. The slurry was applied to both sides of a 10 μm thick copper foil serving as a negative electrode current collector, dried, and rolled by a roll press. The rolled product was slit to a width of 33 mm to form a negative electrode.

(Non-aqueous electrolyte)
LiPF ₆ was dissolved as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 3: 5: 2 (volume ratio) to a concentration of 1.15 mol / liter. Furthermore, 0.5 mass% of vinylene carbonate was added with respect to 100 mass% of non-aqueous electrolyte, and the non-aqueous electrolyte was prepared.

(Production of battery)
After laminating the positive electrode, the microporous polyolefin membrane and the negative electrode, a flat wound electrode body (height 2.2 mm × width 36 mm × depth 29 mm) was produced. A tab with a sealant was welded to each electrode of this flat wound electrode body to form a positive electrode lead and a negative electrode lead. The flat wound electrode body part is sandwiched by an aluminum laminate film and sealed leaving a part of the opening, and this is dried in a vacuum oven at 80 ° C. for 6 hours, and 0.7 mL of the electrolyte is immediately poured after drying The solution was sealed, sealed with a vacuum sealer, and press molded at 80 ° C. and 1 MPa for 1 hour. Subsequently, charge and discharge were performed. The charge and discharge conditions were 300 mA current value, and after constant current charge to a battery voltage of 4.2 V, constant voltage charge was performed up to 15 mA at a battery voltage of 4.2 V. After 10 minutes of rest, constant current discharge was performed to a battery voltage of 3.0 V at a current value of 300 mA, and then rested for 10 minutes. The above charging / discharging was carried out for 3 cycles, and a test secondary battery with a battery capacity of 300 mAh was produced.

(Examples 1 to 7)
The polyolefin resin and liquid paraffin were melt-kneaded with a twin-screw extruder according to the composition shown in Table 1 to prepare a polyolefin solution. The polyolefin solution was fed from a twin-screw extruder to a T-die and extruded. The extruded body was cooled while being pulled up with a cooling roll to form a gel-like sheet. The gel-like sheet was simultaneously biaxially stretched or sequentially biaxially stretched (first stretching) at a temperature of 105 ° C. or more and 115 ° C. or less at a temperature of 105 ° C. to 115 ° C. both at 5 times in both MD and TD directions. The stretched gel-like sheet was immersed in a methylene chloride bath to remove liquid paraffin and then dried to obtain a dry film. The dried film was dry stretched (second stretching) at a temperature of 115 ° C. in the MD direction according to the magnification described in Table 1 using a batch type stretching machine. Thereafter, dry stretching (third stretching) was performed at a temperature of 130 ° C. in the TD direction at a magnification described in Table 1. Next, the obtained film was relaxed by a tenter method in a temperature range of 125 ° C. or more and 130 ° C. or less with a relaxation rate of 2% or more and 5% or less. The production conditions of the obtained polyolefin microporous membrane, the evaluation results and the like are described in Table 1.

(Comparative Examples 1 to 4)
A polyolefin resin and liquid paraffin were melt-kneaded with a composition shown in Table 1 by a twin-screw extruder, and a microporous polyolefin membrane was produced in the same manner as in Examples under the draw ratio and conditions shown in Table 1. The production conditions of the obtained polyolefin microporous membrane, the evaluation results and the like are described in Table 1.

(Evaluation)
In the microporous polyolefin membranes of Examples 1 to 7, the ratio (SMmax / STmax) of the maximum load (SMmax) in the MD direction to the maximum load (STmax) in the TD direction in tear strength measurement by the right-angled tear method is Since it is 0.9 or more and 1.4 or less, and the maximum load (STmax) in the TD direction is 0.20 N / μm or more, it is excellent in the self-discharge characteristic, and further, the rate characteristic and the cycle characteristic are also excellent. Indicated.

  On the other hand, the microporous polyolefin membranes of Comparative Example 1 and Comparative Example 2 have a puncture strength of 2.0 N or more and an MD tensile strength of 200 MPa or more, but since STmax is less than 0.2 N / μm, self-discharge The characteristics were poor. In addition, the microporous polyolefin membrane of Comparative Example 3 similarly has a puncture strength of 2.0 N or more and an MD tensile strength of 200 MPa or more, but since SMmax / STmax is less than 0.9, the self-discharge characteristics and cycle are The characteristics were poor. Furthermore, Comparative Example 4 had poor self-discharge characteristics and cycle characteristics because STmax was less than 0.2 N / μm.

  From the above, it is clear that the microporous polyolefin membrane satisfying the conditions of the present embodiment has excellent self-discharge characteristics, and also has good rate characteristics and cycle characteristics.

When incorporated in a secondary battery as a separator, the microporous polyolefin membrane of the present invention is excellent in self-discharge characteristics, rate characteristics and cycle characteristics. Therefore, it can be used suitably for the separator for secondary batteries in which thin film formation is required.

Claims (8)

In a battery separator using a microporous membrane,
The maximum load (SMmax) when the tearing strength in the MD direction of the microporous membrane is measured by the right-angled tearing method according to JIS K 7128-3, and the tearing strength in the TD direction of the microporous membrane is JIS K 7128- The ratio (SMmax / STmax) to the maximum load (STmax) when measured by the right-angled tear method according to 3 is 0.9 or more and 1.4 or less,
The battery separator, wherein the maximum load (STmax) in the TD direction is 0.2 N / μm or more.   The battery separator according to claim 1, wherein the microporous membrane has a tensile elongation at break of 100% or more in the TD direction.   The battery separator according to claim 1 or 2, wherein the porosity of the microporous film is 30% or more and 60% or less.   The battery separator according to any one of claims 1 to 3, wherein the microporous film has a thickness of 3 μm to 10 μm. The microporous membrane is composed mainly of a polyolefin resin,
The battery separator according to any one of claims 1 to 4, containing 50% by mass or more of high density polyethylene (HDPE) with respect to 100% by mass of the entire polyolefin resin. The battery separator according to any one of claims 1 to 5, wherein the weight average molecular weight (Mw) of the polyolefin resin forming the microporous film is 8 x 10 ⁵ or more.   The battery separator according to any one of claims 1 to 6, wherein at least one surface of the microporous membrane is provided with one or more coated microporous layers.   A non-aqueous electrolyte secondary battery comprising the battery separator according to any one of claims 1 to 7.