TW201838224A - Polyolefin microporous membrane, separator for secondary battery with nonaqueous electrolyte, and secondary battery with nonaqueous electrolyte - Google Patents

Abstract

The present invention addresses the problem of providing a polyolefin microporous membrane that, when incorporated into a battery as a separator, exhibits excellent impact resistance and contributes to excellent battery characteristics. This polyolefin microporous membrane has the following features (1) to (5). (1) The tensile strength (MPa) and tensile elongation (%) in the MD and TD directions satisfy the following relationship (I): [(tensile strength in MD direction * tensile elongation in MD direction/100)2 + (tensile strength in TD direction * tensile elongation in TD direction/100)2]1/2 ≥ 300. (2) The tensile strength in the MD and TD direction are 196 MPa or more. (3) The maximum pore size determined using a Perm-Porometer is 60 nm or less. (4) The mean flow pore size determined using a Perm-Porometer is 40 nm or less. (5) The porosity is 40% or more.

Description

   Polyolefin microporous membrane, non-aqueous electrolyte-based secondary battery separator, and non-aqueous electrolyte-based secondary battery   

The present invention relates to a polyolefin microporous membrane, a separator for a non-aqueous electrolyte system secondary battery, and a non-aqueous electrolyte system secondary battery.

Microporous membranes are used in various fields, such as filters such as filter membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among them, polyolefin microporous membranes using polyolefin as a resin material have been widely used in recent years as battery separators due to their excellent chemical resistance, insulation, mechanical strength, and the like, and have shutdown characteristics.

Secondary batteries, such as lithium ion secondary batteries, are widely used as batteries used in personal computers, mobile phones, etc. due to their high energy density. The secondary battery is also used as a power source for driving a motor of an electric vehicle or a hybrid vehicle.

In particular, in the case of large high-capacity lithium-ion batteries, battery characteristics and higher reliability are important, and separators used for such batteries require high impact resistance from the viewpoint of safety.

In order to improve the impact resistance of the separator, high film strength and high elongation are required. However, the strength and elongation of polyolefin microporous membranes are in a trade-off relationship. It is difficult to increase the strength of the membrane while maintaining the elongation. At present, there are several reports on polyolefin microporous membranes that increase the strength or elongation of the membrane.

For example, Patent Document 1 describes a polyolefin microporous membrane having a porosity of 10% to less than 55%, a tensile strength of MD and TD of 50 to 300 MPa, and a total of MD tensile strength and TD tensile strength of 100 to 600 MPa, MD and TD tensile elongation is 10 ~ 200%, the total of MD tensile elongation and TD tensile elongation is 20 ~ 250%. According to Patent Document 1, this polyolefin microporous membrane is not easily deformed, and has excellent film breakage resistance and stress relaxation characteristics.

Patent Document 2 describes a polyolefin microporous film having a ratio of the tensile strength in the longitudinal direction to the tensile strength in the width direction of 0.75 to 1.25, and the thermal shrinkage ratio in the width direction at 120 ° C. is less than 10%. According to Patent Document 2, this polyolefin microporous membrane system has good resistance to foreign substances and the like.

Patent Document 3 describes a polyolefin microporous membrane that includes polypropylene, has a transverse breaking strength of 100 to 230 MPa, and a transverse tensile breaking elongation of 10 to 110%. It is longitudinally tensile breaking relative to the transverse tensile breaking strength. The strength is 0.8 ~ 1.3.

Patent Document 4 describes a polyolefin microporous membrane having a bubble point of 500 to 700 kPa, a ratio of tensile strength in the longitudinal direction (MD) to tensile strength in the width direction (TD) of 1.0 to 5.5, and a shutdown temperature of 130 to 140. ℃. According to Patent Document 4, this polyolefin microporous membrane system has both good cycle characteristics and high withstand voltage characteristics.

    [Prior technical literature]         [Patent Literature]    

Patent Document 1: Japanese Patent Application Laid-Open No. 2006-124652

Patent Document 2: International Publication No. 2010/070930

Patent Document 3: International Publication No. 2009/123015

Patent Document 4: Japanese Patent Application Publication No. 2013-234263

In Patent Documents 1 to 4 described above, polyolefin microporous membranes having improved tensile strength or tensile elongation while maintaining battery characteristics are described. However, with the improvement of battery performance in recent years, further improvement in impact resistance is required. In addition, it is more difficult for a battery to have both strength and elongation, output characteristics, and cycle characteristics, and a separator that has both battery characteristics such as impact resistance and output characteristics is required.

In view of the foregoing, the present invention is to provide a polyolefin microporous membrane having excellent impact resistance. It is also an object of the present invention to provide a polyolefin microporous membrane which, when used as a battery separator, has both a high level of impact resistance and battery characteristics (output characteristics, anti-dendritic characteristics, etc.).

The present invention is a polyolefin microporous membrane having the following characteristics (1) to (5).

(1) MD direction and TD direction tensile strength (MPa) and tensile elongation (%) satisfy the following relational expression (I); [(tensile strength in MD direction × tensile elongation in MD direction / 100) ) ² + (tensile strength in TD direction × tensile extension in TD direction / 100) ² ] ^1/2 ≧ 300‧‧‧Formula (I)

(2) The tensile strength in the MD and TD directions is 196 MPa or more;

(3) The maximum pore diameter measured using a Perm-Porometer is 60 nm or less;

(4) The average flow pore diameter measured using a Perm-Porometer is 40 nm or less;

(5) The porosity is 40% or more.

The polyolefin microporous membrane of the present invention may also have the following characteristics (6).

(6) The ratio of the tensile strength in the MD direction and the TD direction (tensile strength in the MD direction / tensile strength in the TD direction) is 0.8 or more and 1.2 or less.

In addition, the polyolefin microporous membrane of the present invention may also have the following characteristic (7).

(7) The ratio of the tensile elongation in the MD direction and the TD direction (the tensile elongation in the MD direction / the tensile elongation in the TD direction) is 0.75 or more and 1.25 or less.

The polyolefin microporous membrane of the present invention may also have the following characteristic (8).

(8) The tensile elongation in the MD and TD directions is 90% or more.

In addition, the puncture strength of the polyolefin microporous film in terms of a film thickness of 12 μm may be 5N or more. In addition, the tensile strength (MPa) and tensile elongation (%) of the polyolefin microporous membrane system in the MD direction and the TD direction can satisfy the following relationship (II); [(tensile strength in the MD direction × MD direction) Tensile elongation / 100) ² + (tensile strength in the TD direction × tensile elongation in the TD direction / 100) ² ] ^1/2 ≧ 350‧‧‧ (II).

The present invention is a separator for a non-aqueous electrolyte system secondary battery, which is formed using the polyolefin microporous membrane of the present invention.

The present invention is a non-aqueous electrolyte-based secondary battery including the separator for a non-aqueous electrolyte-based secondary battery of the present invention.

The polyolefin microporous membrane system of the present invention is very excellent in impact resistance. When used as a battery separator, it can have both high impact resistance and battery characteristics (output characteristics, anti-dendritic characteristics, cycle characteristics).

    [Form of Implementing Invention]    

Hereinafter, this embodiment of the present invention will be described. However, the present invention is not limited to the embodiments described below.

Polyolefin microporous membrane

As used herein, the term "polyolefin microporous membrane" refers to a microporous membrane containing polyolefin as a main component, and for example, refers to a microporous membrane containing 90% by mass or more of polyolefin with respect to the entire amount of the microporous membrane. The physical properties of the polyolefin microporous membrane according to this embodiment will be described below.

[Relationship between tensile strength and tensile elongation]

In a polyolefin microporous membrane, when it has only a high tensile strength or a high tensile elongation, the impact resistance is insufficient. In order to obtain a polyolefin microporous film having higher impact resistance, the present inventors found that the MD direction (mechanical direction, long-side direction, longitudinal direction) and the TD direction (when viewed in plan view of the polyolefin microporous film, are orthogonal to the MD direction) (Direction: width direction, lateral direction), it is important to have a well-balanced high tensile strength and high tensile elongation (good isotropy). In addition, the inventors have found that when the tensile strength (MPa) and tensile elongation (%) in the MD and TD directions have a specific relationship, the polyolefin microporous film is excellent in impact resistance.

That is, the relationship between the tensile strength (MPa) and tensile elongation (%) in the MD direction and the TD direction of the polyolefin microporous membrane system of this embodiment satisfies the following formula (I). When the polyolefin microporous film satisfies the following formula (I), impact resistance can be improved.

[(Tensile strength in MD direction × tensile elongation in MD direction / 100) ² + (tensile strength in TD direction × tensile elongation in TD direction / 100) ² ] ^1/2 ≧ 300‧‧‧ (I).

In addition, from the viewpoint of further improving the impact resistance of the polyolefin microporous membrane of this embodiment, the relationship between the tensile strength (MPa) and tensile elongation (%) in the MD direction and the TD direction is preferably satisfied. The formula (II) is more preferably satisfied (III).

[(Tensile strength in MD direction × tensile elongation in MD direction / 100) ² + (tensile strength in TD direction × tensile elongation in TD direction / 100) ² ] ^1/2 ≧ 330‧‧‧ (II)

[(Tensile strength in MD direction × tensile elongation in MD direction / 100) ² + (tensile strength in TD direction × tensile elongation in TD direction / 100) ² ] ^1/2 ≧ 350‧‧‧ (III).

The value of [(tensile strength in the MD direction × tensile elongation in the MD direction / 100) ² + (tensile strength in the TD direction × tensile elongation in the TD direction / 100) ² ] value of ^1/2 The upper limit is not particularly limited, but from the viewpoint of shrinkage characteristics, it is, for example, 1,000 or less, preferably 800 or less, and more preferably 600 or less.

[Tensile Strength]

The tensile strength of the polyolefin microporous membrane system of this embodiment in the MD direction and the TD direction is 196 MPa or more, preferably 200 MPa or more, and more preferably 230 MPa or more. When the tensile strength is in the above range, the film strength is more excellent. When the electrode body is wound in the battery manufacturing process, a high tension can be applied, and the film breakage caused by foreign matter or impact in the battery can be suppressed. The upper limit of the tensile strength in the MD and TD directions is preferably 500 MPa or less, more preferably 450 MPa or less, and even more preferably 400 MPa or less from the viewpoint of shrinkage resistance. As for the tensile strength, a rectangular test piece having a width of 10 mm can be used and measured by a method according to ASTM D882.

[Stretching Elongation]

The polyolefin microporous membranes of this embodiment each have a tensile elongation of 90% or more in the MD direction and the TD direction. When the tensile elongation is within the above range, when an impact is applied to the battery, the flexibility of the separator can suppress the occurrence of film breakage and short circuit of the separator. The upper limit of the stretch elongation in the MD direction and the TD direction is not particularly limited, and is, for example, 400% or less, preferably 300% or less, and more preferably 200% or less. Stretching When the stretch is in the above range, the separator is not deformed and stretched when the electrode is wound, and the winding property is good. The tensile elongation can be measured by a method according to ASTM D-882A.

[Ratio of tensile strength in MD direction / tensile strength in TD direction]

The ratio of the tensile strength in the MD direction and the TD direction (tensile strength in the MD direction / tensile strength in the TD direction) of the polyolefin microporous membrane of the embodiment is preferably 0.8 or more and 1.2 or less. When the ratio of the tensile strength is in the above range, since the force is applied more uniformly to impacts in all directions, the impact resistance is increased, and film breakage and shorts can be more stably suppressed.

[The ratio of the stretch elongation in the MD direction / the stretch elongation in the TD direction]

The ratio of the tensile elongation in the MD direction and the TD direction (the tensile elongation in the MD direction / the tensile elongation in the TD direction) of the polyolefin microporous film of this embodiment is preferably 0.75 or more and 1.25 or less. When the ratio of the stretch elongation is in the above range, since impact is applied more uniformly to impacts in all directions, the impact resistance is increased, and film breakage and shorts can be more stably suppressed.

From the viewpoint of more stable suppression of film breakage with respect to impact in all directions, the above-mentioned ratio of tensile strength and tensile elongation is preferably close to one. When the tensile strength in the MD direction is too large, cracking in the MD direction may occur. When the tensile strength in the TD direction is excessively large, cracks in the TD direction or the bonding and detachment of the electrode tab adjoining parts may occur, which may cause short circuit.

[Puncture strength]

The puncture strength of the polyolefin microporous membrane converted to a film thickness of 12 m is preferably 5N or more, more preferably 5.2N or more, and even more preferably 6N or more. The upper limit of the puncture strength is not particularly limited, and is, for example, 10 N or less. When the puncture strength is in the above range, the polyolefin microporous membrane has excellent film strength and exhibits good physical property balance. In addition, the secondary battery using the polyolefin microporous film as a separator is excellent in resistance to unevenness, impact, and the like of the electrode, and suppresses occurrence of short-circuiting of the electrode.

The puncture strength was measured using a 1 mm diameter needle with a spherical end (curvature radius R: 0.5 mm) and a polyolefin microporous membrane having a film thickness T ₁ (μm) at a rate of 2 mm / sec. value. The puncture strength (N / 12 μm) in terms of film thickness of 12 μm with respect to a polyolefin microporous film having a film thickness T ₁ (μm) can be determined by the following formula.

Formula: puncture strength (12 μm conversion) = measured puncture strength (N) × 12 (μm) / film thickness T ₁ (μm)

[Film thickness]

The upper limit of the film thickness of the polyolefin microporous membrane is not particularly limited, and is, for example, 20 μm or less, preferably 17 μm or less, and more preferably 13 μm or less. When the film thickness is in the above range, the penetrability or the film resistance is more excellent, and the battery capacity can be increased by thinning. On the other hand, the lower limit of the film thickness is not particularly limited, but is preferably 2 μm or more, more preferably 3 μm or more, and even more preferably 4 μm or more. When the film thickness is in the above range, the film strength is further increased.

[Void ratio]

When used as a battery separator, the porosity of the polyolefin microporous membrane is preferably 40% or more, more preferably 40% or more and 70% or less. From the viewpoints of film-forming properties, mechanical strength, and insulation properties, the upper limit of the porosity is more preferably 60% or less, and even more preferably 55% or less. Since the porosity is in the above range, it is possible to increase the holding amount of the electrolytic solution, ensure high ion permeability, and have excellent output characteristics. When the porosity is low, when used as a battery separator, the output characteristics will be deteriorated due to the increase in fibrils that prevent ion penetration and the decrease in the electrolyte content, and it will be caused by by-products that occur during battery reactions. The clogging system increases, and the cycle characteristics deteriorate drastically. The porosity can be made into the above range by adjusting the composition of the polyolefin resin or the stretching ratio during the manufacturing process.

The porosity can be compared with the weight of the microporous membrane w ₁ and the equivalent weight of the polymer without voids w ₂ (the polymer having the same width, length, and composition) by the following formula (1) Determination.

Porosity (%) = (w ₂ -w ₁ ) / w ₂ × 100‧‧‧ (1).

[Average flow aperture]

The average pore diameter (average flow pore diameter) of the polyolefin microporous membrane is 40 nm or less, and preferably 10 nm or more and 40 nm or less. When the average pore diameter is in the above range, the balance between strength and penetration is excellent, and self-discharge from coarse pores is suppressed. In addition, when the average pore diameter exceeds 40 nm, an increase in resistance due to selective concentration of ions penetrating a flow path to a large pore or deterioration in cycle characteristics due to local blockage of by-products of decomposition of an electrolyte may occur. The average pore diameter is a value measured by a method (semi-dry method) according to ASTM E1294-89. As a measuring device, a Perm-Porometer (model: CFP-1500A) manufactured by PMI was used, and Galwick (15.9 dyn / cm) was used as a measuring solution.

[Maximum pore diameter (bubble point diameter)]

The maximum pore diameter (bubble point diameter: BP diameter) is preferably 60 nm or less, and more preferably 30 nm or more and 60 nm or less. When the maximum pore diameter exceeds 60 nm, the positive electrode and the negative electrode come into contact with each other (micro short circuit) or are damaged by lithium dendrites, and a short circuit may occur. On the other hand, if the maximum pore diameter is too small, the resistance of the battery becomes high, the cycle performance becomes insufficient, and the capacity retention rate during high-speed discharge becomes low.

[Degree of ventilation resistance]

The upper limit of the air permeability resistance in terms of the thickness of the polyolefin microporous membrane in terms of 12 μm is not particularly limited, and is, for example, 300 seconds / 100 cm ³ air / 12 μm or less, and preferably 200 seconds / 100 cm ³ air / 12 μm or less. The lower limit of the airflow resistance is, for example, 50 seconds / 100 cm ³ or more of air. When the air permeability resistance is within the above range, when used as a battery separator, the ion permeability is excellent, and the secondary battery system incorporating the separator has a reduced impedance and an increased output characteristic or rate characteristic. The degree of air permeability resistance can be in the above range by adjusting the elongation conditions and the like when producing a polyolefin microporous film.

The air permeability resistance is a value P1 (sec / 100cm ³ air) that can be measured with an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117 Wang Yan type testing machine method. For a microporous film having a film thickness T ₁ (μm), the air permeability resistance P ₂ (second / 100 cm ³ air / 12 μm) in terms of film thickness can be a value obtained by the following formula.

Formula: P ₂ = P ₁ (sec / 100cm ³ air) × 12 (μm) / film thickness T ₁ (μm)

2. Manufacturing method of polyolefin microporous membrane

The method for producing a polyolefin microporous film is not particularly limited as long as a polyolefin microporous film having the above characteristics can be obtained, and a known method for producing a polyolefin microporous film can be used. As a manufacturing method of the polyolefin microporous membrane of this embodiment, a wet-type film forming method is preferable from a viewpoint of the ease of control of a film structure and physical properties. As the wet-type film forming method, for example, the methods described in the specification of Japanese Invention Patent No. 2132327 and Japanese Invention Patent No. 3347835, International Publication No. 2006/137540, and the like can be used.

Hereinafter, an example of the manufacturing method (wet-type film-forming method) of a polyolefin microporous film is demonstrated. In addition, the following description is an example of a manufacturing method, and is not limited to this method.

(1) Preparation of polyolefin solution

First, a polyolefin resin to be a raw material and a film-forming solvent are melt-kneaded to prepare a polyolefin solution. As the melt-kneading method, for example, a method using a biaxial extruder described in the specifications of Japanese Invention Patent No. 2132327 and Japanese Invention Patent No. 3347835 can be used. Since the melt-kneading method is well known, description is omitted.

(Polyolefin resin)

Examples of the polyolefin resin used as the raw material include polyethylene and polypropylene. The polyethylene is not particularly limited, and various types of polyethylene can be used. For example, ultrahigh molecular weight polyethylene (UHMwPE), high density polyethylene (HDPE), medium density polyethylene, branched low density polyethylene, Linear low-density polyethylene. The polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. .

The polyolefin resin preferably contains ultra-high molecular weight polyethylene (UHMwPE). When ultra-high molecular weight polyethylene is included, the film strength of the obtained polyolefin microporous film can be improved. In addition, the fibril fibers of the polyolefin microporous film can be made finer (densified), and a film having a uniform small pore diameter can be exhibited for the entire film. The ultra-high molecular weight polyethylene can be used alone or in combination of two or more. For example, two or more ultra-high molecular weight polyethylenes having different Mw can be used in combination.

The weight average molecular weight (Mw) of the ultra-high molecular weight polyethylene is 1 × 10 ⁶ or more (100,000 or more), and preferably 2 × 10 ⁶ or more and less than 4 × 10 ⁶ . When Mw is in the above range, the film forming properties are improved. When the Mw of the ultra-high molecular weight polyethylene is 4 × 10 ⁶ or more, since the viscosity of the melt becomes too high, resins and the like cannot be extruded from a nozzle (die), and a problem may occur during the film formation step. The Mw is a value measured by gel permeation chromatography (GPC).

The content of the ultra-high molecular weight polyethylene is preferably 10% by mass or more, more preferably 20% by mass or more, based on 100% by mass of the entire polyolefin resin. The upper limit of the content of the ultra-high molecular weight polyethylene is not particularly limited, and is, for example, 50% by mass or less. When the content of the ultra-high molecular weight polyethylene is in the above-mentioned range, it is possible to achieve both a high level of film strength and a high degree of air permeability resistance by adjusting the stretching conditions and the like described later.

The polyolefin resin may contain high-density polyethylene (HDPE, density: 0.942 g / cm ³ or more). The polyolefin resin preferably contains ultra-high molecular weight polyethylene and high-density polyethylene. When high-density polyethylene is included, it has excellent melt-extrusion properties and excellent uniform stretch processing properties. Examples of the high density polyethylene include those having a weight average molecular weight (Mw) of 1 × 10 ⁴ or more and less than 1 × 10 ⁶ . The Mw is a value measured by gel permeation chromatography (GPC). The content of the high-density polyethylene is preferably 50% by mass or more and 90% by mass or less, and more preferably 50% by mass or more and 80% by mass or less with respect to 100% by mass of the entire polyolefin resin.

The polyolefin resin may also include polypropylene. The polypropylene is not particularly limited, and homopolymers of propylene, copolymers of propylene and other α-olefins and / or diolefins (propylene copolymers), or mixtures thereof can be used. The content of polypropylene is, for example, 0% by mass or more and less than 10% by mass, and preferably 0% by mass or more and 5% by mass or less with respect to 100% by mass of the entire polyolefin resin. Moreover, when polypropylene is included, the pore diameter of the obtained polyolefin microporous membrane tends to become large.

Moreover, the polyolefin resin may contain other resin components other than polyethylene and polypropylene as needed. As other resin components, for example, a heat-resistant resin can be used. Moreover, as long as the effect of the present invention is not impaired, the polyolefin microporous membrane may contain an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorbent, an anti-blocking agent, a filler, a crystal nucleating agent, and crystal Various additives such as chemical retarders.

(Film forming solvent)

As the film-forming solvent, any solvent that can sufficiently dissolve the polyolefin resin can be used without particular limitation. In order to be able to stretch at a relatively high magnification, the solvent for film formation is preferably a solvent which is liquid at room temperature. Examples of the film-forming solvent include aliphatic, cyclic aliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, para-xylene, undecane, dodecane, and flowing paraffin; and The boiling points correspond to these mineral oil fractions, as well as liquid phthalates at room temperature, such as dibutyl phthalate and dioctyl phthalate. Among them, it is preferable to use a nonvolatile liquid solvent such as a flowing paraffin. In addition, it may be mixed with the polyolefin resin in a melt-kneaded state, but a solid solvent and the above-mentioned film-forming solvent are used at room temperature and used. Examples of such a solid solvent include stearyl alcohol, wax alcohol, and paraffin.

(Polyolefin solution)

The blending ratio of the polyolefin resin in the polyolefin solution and the film-forming solvent is not particularly limited. The polyolefin resin is preferably 20 to 35 parts by mass relative to 100 parts by mass of the polyolefin resin solution. When the ratio of the polyolefin resin is within the above range, it is possible to prevent swelling or necking at the die exit when extruding the polyolefin solution, and the moldability and self-support of the extruded molded article (gel-like molded article). Sex becomes better.

(2) Formation of gelatinous flakes

Next, the prepared polyolefin solution is sent from an extruder to a die, extruded into a sheet shape, and the obtained extruded molded body is cooled to form a gel-like sheet. The cooling is preferably performed to 90 ° C. or lower, preferably 50 ° C. or lower, and most preferably 40 ° C. or lower of the crystalline dispersion temperature (Tcd) of the polyolefin resin. The microphase of the polyolefin separated by the solvent for film formation can be fixed by cooling. When the cooling rate is within the above range, the degree of crystallinity is maintained in a suitable range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting a cooling medium such as cold air or cooling water, a method of contacting a cooling roller, and the like can be used, but it is preferable to be cooled by contacting a roller cooled by the refrigerant. In addition, a plurality of polyolefin solutions having the same or different composition can also be sent from a plurality of extruders to a die where they are laminated into a layer and extruded into a sheet form. As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent Application No. 2132327 and Japanese Patent Application No. 3347835 can be used.

(3) Extend

Next, the gel-like sheet is extended at least in a uniaxial direction. The extension of a gelatinous sheet is also called a wet extension. The extension system may be a uniaxial extension or a biaxial extension, but a biaxial extension is preferred. In the case of biaxial extension, it can be any of simultaneous biaxial extension, successive extension, and multi-stage extension (for example, a combination of simultaneous biaxial extension and successive extension), but it is preferable to extend successively, preferably in the MD direction (mechanical direction). , Long side direction), and then extended in the TD direction (width direction, lateral direction). When the MD direction and the TD direction are separately stretched, it is easy to judge the molecular alignment by applying stretching tension only to each direction during the stretching. The TD direction is a direction orthogonal to the MD direction when the microporous membrane is viewed in a plane.

The final area stretching magnification (area magnification) of the stretching step must be 30 times or more and 150 times or less. The surface magnification is in the above range, the film forming property is good, and the proportion of free molecules without alignment is reduced, and a polyolefin microporous film having high strength can be obtained. The area extension ratio is preferably 35 times or more and 120 times or less. Further, it is preferable that the stretching magnifications in the MD direction and the TD direction both exceed 5 times.

The ratio of the stretch magnification in the MD direction and the TD direction (the stretch magnification in the MD direction / the stretch magnification in the TD direction) must be 0.7 or more and 1.0 or less. When the ratio of the stretching ratio is in the above range, regarding the tensile strength or tensile elongation of the obtained polyolefin microporous film, the balance between the MD direction and the TD direction becomes good, the film strength can be further improved, and the impact resistance is increased. From the viewpoint of further improving the film strength, it is preferable that the stretching ratio in the TD direction is larger than the stretching ratio in the MD direction. Although the reason for this is not particularly limited, it is thought that when extending in the MD direction and then extending in the TD direction, by extending in the MD direction, once the molecular alignment has been directed in the MD direction, it becomes difficult to enter the TD direction Alignment can be performed more uniformly in both directions by extending in the TD direction at a higher magnification. Moreover, the so-called stretch magnification in this step refers to the stretch magnification of the gel-like sheet immediately before the next step based on the gel-like sheet immediately before this step. The ratio of the stretching magnification in the MD direction and the TD direction (the stretching magnification in the MD direction / the stretching magnification in the TD direction) is preferably 0.75 or more and 1.0 or less.

The elongation temperature is preferably within a range of the crystalline dispersion temperature (Tcd) of the polyolefin resin and the melting point of the polyolefin resin. The melting point of the polyolefin resin herein means the melting point of the polyolefin resin in the gel-like sheet. When the stretching temperature is equal to or lower than the melting point of the polyolefin resin, the melting of the polyolefin resin in the gel-like sheet is suppressed, and molecular chains can be efficiently aligned by stretching. In addition, when the stretching temperature is equal to or higher than the crystalline dispersion temperature (Tcd) of the polyolefin resin, the polyolefin resin in the gel-like sheet can be sufficiently softened and the stretching tension can be reduced. Therefore, the film forming property is improved, and the The film is broken and can be extended at a high magnification. The elongation temperature can be, for example, 100 ° C or higher and 127 ° C or lower. Here, the so-called elongation temperature refers to the temperature of the gel sheet. When there is a temperature difference between the surface and the surface such as roll extension, it refers to the central temperature in the thickness direction.

After extending in the MD direction, when extending in the TD direction, it is important that the extension temperature in the TD direction is higher than the extension temperature in the MD direction. Although the details are not known, it is believed that by extending to the MD direction, once the molecular orientation that has been oriented in the MD direction becomes difficult to align in the TD direction, it can be extended in both directions by extending in the TD direction at higher temperatures In addition, molecular alignment is performed more uniformly. The stretching temperature in the MD direction is 100 ° C to 110 ° C, and preferably 103 ° C to 110 ° C. The extension temperature in the TD direction is from 115 ° C to 127 ° C, preferably from 115 ° C to 125 ° C. When the stretching temperatures in the MD direction and the TD direction are in the above ranges, the film forming properties are good, the film strength of the obtained polyolefin microporous film can be improved, and the pore diameter can be controlled in an appropriate range.

(4) Removal of solvent for film formation (washing)

Next, the film-forming solvent is removed from the stretched gel-like sheet to obtain a microporous film. The solvent is removed by washing with a washing solvent. The polyolefin phase is separated from the film-forming solvent phase system. If the film-forming solvent is removed, a porous film is obtained, which contains fibrils that can form a fine three-dimensional mesh structure, and has three-dimensional irregular communication. Holes (voids). Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are well known, description thereof will be omitted. For example, the method disclosed in Japanese Patent Application No. 2132327 or Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(5) Drying

Subsequently, the microporous membrane after removing the film-forming solvent is dried by a heat-drying method or an air-drying method. The drying temperature is preferably equal to or lower than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C or higher than Tcd. The drying system uses the microporous membrane as 100% by mass (dry weight), and is preferably performed until the remaining cleaning solvent becomes 5% by mass or less, and more preferably until it becomes 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the polyolefin microporous membrane is maintained, and the deterioration of the permeability is suppressed.

(6) Other

The dried microporous film may be subjected to a heat treatment. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The so-called heat setting treatment is a heat treatment in which the dimensions in the TD direction of the film are not changed while heating. The so-called thermal relaxation treatment is a heat treatment for thermally shrinking the film in the MD direction and / or the TD direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as the thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be mentioned. The heat treatment temperature is preferably within a range of Tcd to Tm of the polyolefin resin, more preferably within a range of the second extension temperature of the microporous membrane ± 5 ° C, and particularly preferably within a range of the second extension temperature of the microporous membrane ± 3 ° C. Inside.

In addition, the microporous membrane after drying may be re-stretched at least in a uniaxial direction at a predetermined area stretch ratio. The extension of the dried microporous membrane is also called dry extension.

The obtained polyolefin microporous membrane may be subjected to a crosslinking treatment and a hydrophilization treatment. For example, a polyolefin microporous film is irradiated with ionizing radiation such as α-rays, β-rays, γ-rays, or electron rays, and is subjected to a crosslinking treatment. When the electron beam is irradiated, the electron beam amount is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV. The meltdown temperature of the microporous membrane is increased by the crosslinking treatment. The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, and the like. The monomer grafting is preferably performed after the crosslinking treatment.

Furthermore, the polyolefin microporous membrane may be a single layer, or a layer including a polyolefin microporous membrane may be laminated. The multilayer polyolefin microporous film can be made of two or more layers. In the case of a multilayer polyolefin microporous membrane, the composition of the polyolefin resin constituting each layer may be the same composition or different compositions.

In addition, a polyolefin microporous film may be laminated with a porous layer other than a polyolefin resin on at least one surface thereof to form a laminated polyolefin porous film. The other porous layer is not particularly limited. For example, a coating layer such as an inorganic particle layer including a binder and inorganic particles can be laminated. The binder component constituting the inorganic particle layer is not particularly limited, and known components can be used. For example, acrylic resin, polyvinylidene fluoride resin, polyimide resin, polyimide resin, and aromatic resin can be used. Polyamine resin, polyimide resin, etc. The inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used, and examples thereof include alumina, gibbsite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon. In addition, as the laminated polyolefin porous film, a porous resin may be laminated on at least one surface of the polyolefin microporous film.

By appropriately adjusting the final area stretch magnification (area magnification), the MD stretch and the TD stretch stretch ratio in the stretch step previously described in the present invention (MD stretch stretch magnification / TD stretch stretch magnification), MD and TD direction elongation temperature and excellent impact resistance. In addition, when used as a battery separator, it can provide polyolefin microporous materials that have both high impact resistance and battery characteristics (output characteristics, anti-dendritic characteristics, etc.). membrane.

    [Example]    

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

1. Measurement method and evaluation method

[Film thickness]

Using a contact thickness meter (Litematic, manufactured by Mitutoyo Co., Ltd.), the thickness of five points in a range of 95 mm × 95 mm of the microporous membrane was measured, and the average value was determined.

[Void ratio]

The weight w _{1 of the} microporous membrane is compared with the equivalent weight w ₂ (a polymer having the same width, length, and composition) of a non-voided polymer, and the weight is measured by the following formula.

Porosity (%) = (w ₂ -w ₁ ) / w ₂ × 100

[Bubble point pore diameter (maximum pore diameter) and average flow pore diameter]

Perm-Porometer (trade name, type: CFP-1500A) from PMI was used to measure in the order of Dry-up and Wet-up. In Wet-up, for a microporous membrane that has a well-known Galwick (trade name) and is fully impregnated with a surface tension, a pressure is applied, and the pore diameter converted from the pressure at which air starts to penetrate is regarded as the bubble point pore diameter (the maximum pore diameter) . Regarding the average flow pore diameter, the pore diameter was converted from the pressure at the point where the pressure in the Dry-up measurement, the curve showing the slope of 1/2 of the flow curve, and the curve measured by Wet-up. The conversion of pressure and pore diameter is based on the following formula.

d = C‧γ / P

In the formula, “d (μm)” is the pore diameter of the microporous membrane, “γ (mN / m)” is the surface tension of the liquid, “P (Pa)” is the pressure, and “C” is a constant.

[Puncture strength]

The maximum load L ₁ (N) was measured when a microporous membrane having a film thickness T ₁ (μm) was punctured at a speed of 2 mm / sec using a needle with a diameter of 1 mm whose tip is a spherical surface (curvature radius R = 0.5 mm). The measured value L _{1 of the} maximum load was calculated by the formula: L ₂ = (L ₁ × 12) / T ₁ to calculate the maximum load L ₂ (12 μm conversion) (N / 12 μm) when the film thickness was 12 μm.

[Degree of ventilation resistance]

For a microporous film with a film thickness T ₁ (μm), the air permeability resistance P ₁ (sec / 100cm ³ air). In addition, by using the formula: P ₂ = (P ₁ × 12) / T ₁ , the degree of air permeability resistance P ₂ (equivalent to 12 μm) (sec / 100 cm ³ air / 12 μm) when the film thickness is 12 μm is calculated.

[Weight average molecular weight (Mw)]

The weight average molecular weight (Mw) of the polyolefin microporous membrane was determined by a gel permeation chromatography (GPC) method under the following conditions.

‧Measuring device: GPC-150C manufactured by Waters Corporation

‧Pipe: Shodex UT806M, manufactured by Showa Denko Corporation

‧Column temperature: 135 ℃

‧Solvent (mobile phase): o-dichlorobenzene

‧Solvent flow rate: 1.0ml / min

‧Concentration of sample: 0.1wt% (dissolution conditions: 135 ° C / 1h)

‧Injection volume: 500μl

‧Detector: Differential refractometer (RI detector) manufactured by Waters Corporation

‧Calibration curve: Polyethylene conversion constant (0.46) is used from the calibration curve obtained using monodisperse polystyrene standard samples.

[Tensile Strength]

The MD tensile strength and the TD tensile strength were measured using a rectangular test piece having a width of 10 mm by a method according to ASTM D882.

[Stretching Elongation]

The measurement was performed by a method according to ASTM D-882A.

[Impact resistance test]

A cylindrical battery was produced according to the following procedure, and an impact test was performed.

<Production of the positive electrode>

92.2% by mass of lithium-cobalt composite oxide LiCoO ₂ as an active material, 2.3% by mass of flaky graphite as a conductive agent and acetylene black, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder were dispersed in N- Methylpyrrolidone (NMP) to prepare a slurry. Using a die coater, this slurry was applied to one side of an aluminum foil having a thickness of 20 μm as a positive electrode current collector with an active material coating amount of 250 g / m ² and an active material bulk density of 3.00 g / cm ³ . Then, it was dried at 130 ° C. for 3 minutes, and after compression-molded with a roll press, it was cut into a width of about 57 mm and formed into a belt shape.

<Production of Negative Electrode>

96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose as a binder, and 1.7% by mass of a styrene-butadiene copolymer latex were dispersed in pure water to prepare a slurry. Using a die coater, this slurry was coated with a high filling density of 106 g / m ² of active material and a bulk density of 1.55 g / cm ³ of active material, and was applied to one side of a copper foil having a thickness of 12 μm as a negative electrode current collector. . Then, it was dried at 120 ° C. for 3 minutes, and after compression-molded with a roll press, it was cut into a width of about 58 mm and formed into a band shape.

<Preparation of non-aqueous electrolyte>

LiPF ₆ as a solute was dissolved in a mixed solvent of ethyl carbonate / ethyl methyl carbonate = 1/2 (volume ratio) to prepare a concentration of 1.0 mol / l.

<Barrier material>

The separators described in the examples and comparative examples were cut into 60 mm and formed into a band shape.

<Battery assembly>

The strip-shaped negative electrode, the separator, the strip-shaped positive electrode, and the separator are stacked in this order, and wound into a spiral shape with a winding tension of 250 gf several times to produce an electrode plate laminate. This electrode plate laminate was housed in a stainless steel container having an outer diameter of 18 mm and a height of 65 mm. The aluminum tabs derived from the positive electrode current collector were welded to the terminal portion of the container cover, and the negative electrode current collector was led out. The nickel fins were welded to the container wall. Then, after drying under vacuum at 80 ° C. for 12 hours, the above-mentioned non-aqueous electrolytic solution was poured into a container in an argon box and sealed.

<Impact resistance test>

First, the assembled battery is charged at a constant current of 500 mA, and after the battery voltage reaches 4.20 V, the battery is charged at the constant voltage until the current value becomes 10 mA or less to obtain a fully charged battery. Next, the fully-charged cylindrical battery was set so that the long side became horizontal. From a height of 61 cm, a rod having a diameter of 15.8 mm with a mass of 9.1 kg was dropped on the center flat surface of the battery to give each battery an impact. One of the three tests was evaluated as a person who ignited the battery as a result of the impact, and the person who did not ignite a fire in one of the three tests was evaluated as △. Those who caught fire or smoke evaluated it as ○.

[Anti-dendritic properties]

Those with a maximum pore diameter of less than 60 nm were regarded as ○, and others were regarded as ×. If it has a large pore diameter exceeding a maximum pore diameter of 60 nm, in lithium ion secondary batteries, there is a tendency that lithium dendrites caused by the precipitation of a unique Li metal tend to enter the pores. Therefore, it is easy to damage the separator and cause a small short circuit depending on the design of the battery.

[Film resistance (impedance)]

From the porous film, 5 pieces of a circular shape measurement sample with a diameter of 19 mm and 20 pieces of a circular shape measurement sample with a diameter of 16 mm were cut out. In addition, the components (case, PP gasket, spacer (16mm in diameter, 1mm in thickness), gasket, and cover) of CR2032 type coin-type battery were prepared (made by Baoquan Co., Ltd.).

First, in a dry room with a dew-point temperature of -35 ° C or lower, a measurement sample (diameter 19 mm) is set on the casing × 1, and a gasket is placed to fix the sample. On this, a measurement sample (diameter 16 mm in diameter) is set in order. ) × Plural pieces, spacers, wave washers. The number of measurement samples with a diameter of 16 mm was two, three, and four, and battery cells were prepared for each measurement sample and each of the foregoing number of cells. Next, in a battery cell provided with a wave washer, a mixed solvent (EC / EMC = 4: 6 [volume ratio) of LiPF ₆ mixed with ethyl carbonate (EC) and ethyl methyl carbonate (EMC) was injected. ]) Electrolyte solution (manufactured by Kishida Chemical Co., Ltd.) at a concentration of 1M. After the liquid injection, the battery cell was allowed to stand at a pressure of about -50 kPa for 10 minutes, and the measurement sample was impregnated with the electrolytic solution. Then, the battery cell was covered with a lid and sealed with a coin-type battery riveter (manufactured by Baoquan Co., Ltd.) to obtain a sample battery cell.

The obtained sample battery cells were placed in a 25 ° C thermostatic bath, and after standing for 3 hours, the resistance of the battery cells was measured at an amplitude of 20 mV using an AC impedance measuring device (manufactured by Nikkiso Electric Co., Ltd.). The measured value of the resistance component of the battery cell (the value of the real number when the value of the virtual axis is 0) is plotted with respect to the number of the porous films arranged on the battery cell, and the drawing is approximated linearly to obtain the slope. The value obtained by multiplying this slope by the area of the spacer (2.01 cm ² (= (1.6 cm / 2) ² × π)) was taken as the value of the membrane resistance of the porous film (Ω · cm ² ).

The value of the film resistance of the porous film (Ω‧cm ²⁾ is deemed ○ 1.4Ωcm ² / 10μm were less (good), more than 1.4Ωcm ² / 10μm were as × (poor).

If the film resistance (impedance) of 1.4Ωcm ² / 10μm or less, in the secondary battery as a battery separator materials used, the expected output characteristics of the battery.

[Cycle life]

<Manufacture of Test Battery>

The positive electrode (manufactured by Yayama Co., Ltd.) and the negative electrode (manufactured by Yashan Co., Ltd.) were used with fins and each microporous membrane to prepare a wound body. Next, a wound body was set in an aluminum laminated bag, and 750 μL of an electrolytic solution (1.1 mol / L, LiPF _{6 was added} , and ethyl carbonate / ethyl methyl carbonate / diethylene carbonate = 3/5/2 ( The volume ratio) was added with 0.5% by weight of ethylene vinyl carbonate and 2% by weight of ethyl fluorocarbonate), and closed with a vacuum laminator. This was regarded as a 300mAh test battery.

<Cycle performance test>

Using the test battery described above, the cycle performance test was performed under the following charge and discharge conditions.

Charging: 1C, 4.35V constant current constant voltage charging, cut-off current 0.05C

Discharge: 1C, 3V constant current discharge

Measurement temperature: 25 ° C

Three test batteries were used to derive the ratio of the 200th charge capacity based on the first 1C charge capacity, that is, the average value of the capacity maintenance rate, as an indicator of cycle performance. Those with an average capacity retention rate of 85% or more were regarded as ○ (good), and those with less than 85% were regarded as × (bad).

If the capacity retention rate is 85% or more, it can be determined that even if the charge and discharge are repeated for a long period of time, the charging capacity can be sufficiently maintained, and a good battery can be expected.

(Examples 1 to 5)

Using a biaxial extruder, the ultra-high molecular weight polyethylene (UHMwPE) with a Mw of 2.5 × 10 ⁶ and the high-density polyethylene (HDPE) with a Mw of 2.8 × 10 ⁵ as polyolefin resins are shown in Table 1 respectively. Blending ratio (mass%) of polyolefin resin, flowing paraffin (solvent for film formation), and antioxidant [methylene-3- (3,5-di-tert-butyl-4-hydroxyl] Phenyl) -propionate] methane (0.2 parts by mass per 100 parts by mass of polyolefin resin) was melt-kneaded to prepare a polyolefin solution. Table 1 shows the polyolefin resin concentration in the polyolefin solution relative to 100 parts by mass of the total of the polyolefin resin and the film-forming solvent. The polyolefin solution was supplied from a biaxial extruder to a T-die, and was extruded. The extruded formed body is cooled while being pulled by a cooling roller to form a gel-like sheet. The gel-like sheet was wet-drawn in the MD direction and the TD direction under the conditions shown in Table 1. From the wet-stretched gelatinous sheet, the mobile paraffin was removed using dichloromethane, dried, and the obtained polyolefin microporous membrane was subjected to TD at the temperature and magnification shown in Table 1 using a tenter stretcher. After extending in the direction, a thermal relaxation treatment was performed at the same temperature. The relaxation rate (%) of the amount of thermal relaxation treatment is calculated by using the following formula as the width of the TD direction film before the thermal relaxation treatment, and the width of the TD direction film after the thermal relaxation treatment as L2.

Formula: Relaxation rate (%) = (L ₁ -L ₂ ) / L ₁ × 100

Table 1 shows the evaluation results and the like of the obtained polyolefin microporous membrane.

(Comparative Examples 1 to 13)

A polyolefin microporous membrane was produced under the same conditions as in the examples except that the conditions shown in Table 1 or Table 2 were satisfied. The evaluation results of the obtained polyolefin microporous membrane described in Table 1 (Examples 1 to 5, Comparative Examples 1 to 4) or Table 2 (Comparative Examples 5 to 13), and the like.

    [Industrial availability]    

When the polyolefin microporous membrane of this embodiment is incorporated into a secondary battery as a separator, it has excellent impact resistance. In addition, the polyolefin microporous membrane of this embodiment can be used as a separator for a non-aqueous electrolyte-based secondary battery because it has both impact resistance and battery characteristics.

Claims (10)

A polyolefin microporous film having the following characteristics (1) to (5): (1) tensile strength (MPa) and tensile elongation (%) in the MD and TD directions satisfy the following relational expression ( I); [(tensile strength in MD direction × tensile elongation in MD direction / 100) ² + (tensile strength in TD direction × tensile elongation in TD direction / 100) ² ] ^1/2 ≧ 300‧ ‧ Formula (I) (2) The tensile strength in the MD and TD directions is 196 MPa or more; (3) The maximum pore diameter measured by a Perm-Porometer (fine pore size distribution measurement device) is 60 nm or less; (4) Perm is used -The average flow pore diameter measured by the Porometer is 40 nm or less; (5) The porosity is 40% or more.     For example, the polyolefin microporous membrane of claim 1 has the following characteristics (6): (6) the ratio of the tensile strength in the MD direction and the TD direction (tensile strength in the MD direction / tensile strength in the TD direction) It is 0.8 or more and 1.2 or less.         For example, the polyolefin microporous membrane of claim 1 or 2 has the following characteristics (7): (7) the ratio of the tensile elongation in the MD direction and the TD direction (the tensile elongation in the MD direction / the TD direction) The tensile elongation) is 0.75 or more and 1.25 or less.         The polyolefin microporous membrane according to any one of claims 1 to 3, which has the following characteristics (8): (8) The tensile elongation in the MD direction and the TD direction are each 90% or more.         The polyolefin microporous membrane according to any one of claims 1 to 4, which has a film thickness of 20 μm or less.         The polyolefin microporous membrane according to any one of claims 1 to 5, which has a puncture strength of 5N or more when converted into a film thickness of 12 μm.     The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the tensile strength (MPa) and tensile elongation (%) in the MD direction and the TD direction satisfy the following relationship (II): [(MD Tensile strength in the direction x tensile elongation in the MD direction / 100) ² + (tensile strength in the TD direction x tensile elongation in the TD direction / 100) ² ] ^1/2 ≧ 350‧‧‧ (II).     A separator for a non-aqueous electrolyte system secondary battery, which is formed by using a polyolefin microporous membrane according to any one of claims 1 to 7.         The separator for a non-aqueous electrolyte based secondary battery according to claim 8 is provided on at least one surface thereof with a coating layer containing inorganic particles and a binder resin.         A non-aqueous electrolyte-based secondary battery including the separator for a non-aqueous electrolyte-based secondary battery according to claim 8 or 9.