WO2018180714A1

WO2018180714A1 - Polyolefin microporous membrane, separator for secondary battery with nonaqueous electrolyte, and secondary battery with nonaqueous electrolyte

Info

Publication number: WO2018180714A1
Application number: PCT/JP2018/010834
Authority: WO
Inventors: 勝彦松下; 亘祐春本
Original assignee: 東レ株式会社
Priority date: 2017-03-31
Filing date: 2018-03-19
Publication date: 2018-10-04
Also published as: CN110431176A; CN113024882A; CN113024881A; JPWO2018180714A1; TW201838224A; CN110431176B; TWI700851B; KR20190127663A; KR102126212B1; JP6665966B2

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, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

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

Microporous membranes are 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 polyolefin microporous film using polyolefin as a resin material is excellent in chemical resistance, insulation, mechanical strength, etc., and has shutdown characteristics, and thus has been widely used as a battery separator in recent years.

Secondary batteries, such as lithium ion secondary batteries, are widely used as batteries for personal computers, mobile phones and the like because of their high energy density. Secondary batteries are also used as motor drive power sources for electric vehicles and hybrid vehicles.

In particular, in the case of large-capacity lithium-ion batteries, higher reliability is important along with the characteristics of the batteries, and separators used in these batteries are also required to have high impact resistance from the viewpoint of safety. It has been.

In order to improve the impact resistance of the separator, high film strength and high elongation are required. However, the strength and elongation of the microporous polyolefin membrane are in a trade-off relationship, and it has been difficult to increase the strength of the membrane while maintaining the elongation. Several polyolefin microporous membranes with improved membrane strength and elongation have been reported so far.

For example, in Patent Document 1, the porosity is 10% or more and less than 55%, the tensile strength of MD and TD is 50 to 300 MPa, the total of MD tensile strength and TD tensile strength is 100 to 600 MPa, and the tensile elongation of MD and TD is 10 A polyolefin microporous membrane is described that is ˜200% and the sum of MD tensile elongation and TD tensile elongation is 20 to 250%. According to Patent Document 1, this polyolefin microporous membrane is said to be difficult to deform and excellent in film resistance and stress relaxation properties.

In Patent Document 2, a ratio of the tensile strength in the length direction to the tensile strength in the width direction is 0.75 to 1.25, and the thermal shrinkage in the width direction at 120 ° C. is less than 10%. A porous membrane is described. According to Patent Document 2, this polyolefin microporous film is said to have good resistance to foreign matters and the like.

Patent Document 3 includes polypropylene, has a transverse breaking strength of 100 to 230 MPa, a transverse tensile breaking elongation of 10 to 110%, and a longitudinal tensile breaking strength with respect to the transverse tensile breaking strength of 0.8. Polyolefin microporous membranes of ~ 1.3 are described.

In Patent Document 4, the bubble point is 500 to 700 kPa, the ratio of length direction (MD) tensile strength / width direction (TD) tensile strength is 1.0 to 5.5, and the shutdown temperature is 130 to 140. A polyolefin microporous membrane at 0 ° C. is described. According to Patent Document 4, this polyolefin microporous membrane is said to achieve both good cycle characteristics and high voltage resistance characteristics.

JP 2006-124652 A International Publication No. 2010/070930 International Publication No. 2009/123015 JP 2013-234263 A

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

In view of the above circumstances, an object of the present invention is to provide a microporous polyolefin membrane that is extremely excellent in impact resistance. Another object of the present invention is to provide a polyolefin microporous membrane that, when used as a battery separator, has both high impact resistance and battery characteristics (output characteristics, dendrite resistance, etc.).

The present invention is a polyolefin microporous membrane having the following characteristics (1) to (5).
(1) The tensile strength (MPa) and tensile elongation (%) in the MD direction and TD direction 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 direction and the TD direction is 196 MPa or more.
(3) The maximum pore diameter measured using a palm porometer is 60 nm or less.
(4) The average flow pore diameter measured using a palm porometer is 40 nm or less.
(5) The porosity is 40% or more.

The polyolefin microporous membrane of the present invention may have the following property (6).
(6) Ratio of tensile strength in MD direction and TD direction (tensile strength in MD direction / tensile strength in TD direction) is 0.8 or more and 1.2 or less.

Moreover, the polyolefin microporous film of the present invention may have the following property (7).
(7) Ratio of tensile elongation in MD direction and TD direction (tensile elongation in MD direction / tensile elongation in TD direction) is 0.75 or more and 1.25 or less.

Moreover, the polyolefin microporous film of the present invention may have the following property (8).
(8) The tensile elongation in the MD direction and the TD direction is 90% or more, respectively.

The polyolefin microporous membrane may have a puncture strength converted to a film thickness of 12 μm of 5N or more. Moreover, the said microporous polyolefin film | membrane may satisfy | fill the following relational expression (II) in the tensile strength (MPa) and tensile elongation (%) of MD direction and TD direction.
[(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 (II).

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

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

The polyolefin microporous membrane of the present invention is extremely excellent in impact resistance and, when used as a battery separator, achieves both impact resistance and battery characteristics (output characteristics, dendrite resistance, cycle characteristics) at a high level. Can do.

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

1. Polyolefin microporous membrane In this specification, the polyolefin microporous membrane refers to a microporous membrane containing polyolefin as a main component, for example, a microporous membrane containing 90% by mass or more of polyolefin with respect to the total amount of the microporous membrane. Hereinafter, the physical properties of the polyolefin microporous membrane of this embodiment will be described.

[Relationship between tensile strength and tensile elongation]
In a microporous polyolefin membrane, impact resistance may not be sufficient only by having high tensile strength or high tensile elongation. In order to obtain a polyolefin microporous membrane having higher impact resistance, the present inventor has the MD direction (machine direction, longitudinal direction, longitudinal direction) and TD direction (when the polyolefin microporous membrane is viewed in a plane, the MD direction). It was found that it is important to have a high balance between high tensile strength and high tensile elongation (isotropy) in both the directions orthogonal to the width direction and the transverse direction. Moreover, this inventor discovered that when the tensile strength (MPa) and tensile elongation (%) of MD direction and TD direction have a specific relationship, it becomes a polyolefin microporous film which is very excellent in impact resistance. .

That is, the polyolefin microporous membrane of this embodiment satisfies the following formula (I) in relation to the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction. When the polyolefin microporous membrane 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 Formula (I).

Further, the polyolefin microporous membrane of the present embodiment has a relationship between the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction, from the viewpoint of further improving the impact resistance. ) Is more preferable, and (III) is more preferable.
[(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 Formula (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 Formula (III).

In addition, the upper limit of the value of [(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 Although not limited, from the viewpoint of shrinkage characteristics, it is, for example, 1000 or less, preferably 800 or less, more preferably 600 or less.

[Tensile strength]
The polyolefin microporous membrane of this embodiment has a tensile strength in the MD direction and TD direction of 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 excellent, a high tension can be applied when the electrode body is wound in the battery manufacturing process, and film breakage due to foreign matter or impact is suppressed in the battery. Further, the upper limit of the tensile strength in the MD direction and the TD direction is preferably 500 MPa or less, more preferably 450 MPa or less, and further preferably 400 MPa or less, from the viewpoint of shrinkage resistance. In addition, about tensile strength, it can measure by the method based on ASTMD882 using a strip-shaped test piece of width 10mm.

[Tensile elongation]
The polyolefin microporous membrane of this embodiment preferably has a tensile elongation in the MD direction and TD direction of 90% or more, respectively. When the tensile elongation is in the above range, when a shock is applied in the battery, the film breakage of the separator and the occurrence of a short circuit (short circuit) are suppressed due to its flexibility. Moreover, although the upper limit of the tensile elongation of MD direction and TD direction is not specifically limited, For example, it is 400% or less, Preferably it is 300% or less, More preferably, it is 200% or less. When the tensile elongation is in the above range, the winding property is good without winding and deforming the separator when the electrode is wound. The tensile elongation can be measured by a method based on ASTM D-882A.

[Ratio of tensile strength in MD direction / tensile strength in TD direction]
The polyolefin microporous membrane of the present embodiment preferably has a ratio of tensile strength in the MD direction and TD direction (tensile strength in the MD direction / tensile strength in the TD direction) of 0.8 or more and 1.2 or less. When the ratio of the tensile strength is within the above range, the force is applied more uniformly to the impact in all directions, so that the impact resistance is improved and the film breakage and the short circuit (short circuit) can be more stably suppressed. it can.

[Ratio of tensile elongation in MD direction / tensile elongation in TD direction]
In the microporous polyolefin membrane of this embodiment, the ratio of the tensile elongation in the MD direction and the TD direction (tensile elongation in the MD direction / tensile elongation in the TD direction) is preferably 0.75 or more and 1.25 or less. . When the ratio of tensile elongation is in the above range, the force is applied more uniformly to impacts in all directions, so impact resistance is improved, and film breakage and short-circuiting (short-circuiting) are suppressed more stably. Can do.

The ratio between the tensile strength and the tensile elongation is preferably close to 1 from the viewpoint of more stably suppressing film breakage against impact in all directions. Further, when the tensile strength in the MD direction is too large, tearing in the MD direction may occur. If the tensile strength in the TD direction is too high, the TD direction tears or the electrode tab bonding part may be disconnected, and short-circuiting may occur easily.

[Puncture strength]
The puncture strength of the polyolefin microporous membrane is preferably 5N or more, more preferably 5.2N or more, and further preferably 6N or more, when converted to a film thickness of 12 μm. Although the upper limit of puncture strength is not specifically limited, For example, it is 10 N or less. When the puncture strength is in the above range, the polyolefin microporous membrane is excellent in membrane strength and can exhibit a good balance of physical properties. In addition, a secondary battery using the polyolefin microporous membrane as a separator is excellent in resistance to unevenness and impact of the electrode, and the occurrence of a short circuit of the electrode is suppressed.

The puncture strength is the maximum load (N when a polyolefin microporous film having a film thickness T ₁ (μm) is punctured at a speed of 2 mm / sec with a needle having a spherical surface (curvature radius R: 0.5 mm) and a diameter of 1 mm. ) Is a measured value. Further, for a polyolefin microporous film having a film thickness T ₁ (μm), the puncture strength (N / 12 μm) in terms of a film thickness of 12 μm can be obtained 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, but is, for example, 20 μm or less, preferably 17 μm or less, more preferably 13 μm or less. When the film thickness is within the above range, the permeability and film resistance are excellent, and the battery capacity can be improved by thinning the film. 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 further preferably 4 μm or more. When the film thickness is in the above range, the film strength is further improved.

[Porosity]
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. Further, the upper limit of the porosity is more preferably 60% or less, and further preferably 55% or less, from the viewpoint of film forming property, mechanical strength, and insulation. When the porosity is within the above range, the amount of electrolyte retained can be increased, high ion permeability can be ensured, and the output characteristics are excellent. When the porosity is low, when used as a battery separator, the output characteristics may be inferior due to an increase in fibrils that impede ion permeation and a decrease in the electrolyte content, and due to by-products generated during the battery reaction. Clogging may increase and cycle characteristics may deteriorate rapidly. The porosity can be in the above range by adjusting the composition of the polyolefin resin, the draw ratio and the like in the production process.

The porosity is measured by the following formula (1), which compares the weight w _{1 of the} microporous membrane with the weight w _{2 of the} polymer without pores equivalent to the weight (a polymer having the same width, length and composition). it can.
Porosity (%) = (w ₂ −w ₁ ) / w ₂ × 100 (1).

[Average flow pore size]
The average pore size (average flow pore size) of the polyolefin microporous membrane is 40 nm or less, 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 permeability is excellent, and at the same time, self-discharge derived from coarse pores is suppressed. In addition, when the average pore diameter exceeds 40 nm, the ion permeation channel selectively concentrates on the coarse pores, which causes an increase in electrical resistance and a deterioration in cycle characteristics due to local clogging of electrolyte decomposition byproducts. Sometimes. The average pore diameter is a value measured by a method (half dry method) based on ASTM E1294-89. A palm porometer manufactured by PMI (model number: CFP-1500A) can be used as a measuring instrument, and Galwick (15.9 dyn / cm) can be used as a measuring liquid.

[Maximum hole 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 may contact each other (minute short circuit), or may be destroyed by lithium dendrites (dendrites), resulting in a short circuit. On the other hand, when the maximum hole diameter is too small, the electric resistance of the battery becomes high, the cycle performance becomes insufficient, and the capacity retention during high-speed discharge may be low.

[Air permeability resistance]
The upper limit of the air permeability resistance in terms of the film thickness of 12 μm of the polyolefin microporous film is not particularly limited, but is, for example, 300 seconds / 100 cm ³ Air / 12 μm or less, preferably 200 seconds / 100 cm ³ Air / 12 μm or less. . The lower limit of the air resistance is, for example, 50 seconds / 100 cm ³ Air or more. When the air permeability resistance is in the above range, when used as a battery separator, the ion permeability is excellent, and a secondary battery incorporating this separator has a reduced impedance and improved output characteristics and rate characteristics. The air permeation resistance can be adjusted to the above range by adjusting the stretching conditions when producing the polyolefin microporous membrane.

The air resistance is a value P ₁ (seconds / 100 cm ³ ) that can be measured with an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117 Oken type testing machine method. Air). In addition, for a microporous film having a film thickness T ₁ (μm), the air resistance P ₂ (second / 100 cm ³ Air / 12 μm) converted to a film thickness of 12 μm is a value that can be obtained by the following equation. .
Formula: P ₂ = P ₁ (second / 100 cm ³ Air) × 12 (μm) / film thickness T ₁ (μm)
2. Production method of polyolefin microporous membrane The production method of the polyolefin microporous membrane is not particularly limited as long as the polyolefin microporous membrane having the above-described characteristics can be obtained, and known production methods of polyolefin microporous membrane can be used. As a manufacturing method of the polyolefin microporous film of this embodiment, a wet film forming method is preferable from the viewpoint of easy control of the structure and physical properties of the film. As a wet film forming method, for example, the methods described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835, International Publication No. 2006/137540, and the like can be used.

Hereinafter, an example of a method for producing a polyolefin microporous membrane (wet film-forming method) will be described. In addition, the following description is an example of a manufacturing method, Comprising: It does not limit to this method.

(1) Preparation of polyolefin solution First, a polyolefin resin as 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 twin-screw extruder described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well-known, description is abbreviate | omitted.

(Polyolefin resin)
Examples of the polyolefin resin used as a raw material include polyethylene and polypropylene. The polyethylene is not particularly limited, and various polyethylenes 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 or the like is used. 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, styrene and the like.

The polyolefin resin preferably contains ultra high molecular weight polyethylene (UHMwPE). When ultra high molecular weight polyethylene is included, the film | membrane intensity | strength of the polyolefin fine porous film obtained can be improved. Further, the fibrils of the polyolefin microporous membrane can be refined (densified), and a membrane having a small pore diameter can be expressed uniformly over the entire membrane. In addition, ultra high molecular weight polyethylene can be used individually by 1 type or in combination of 2 or more types, For example, 2 or more types of ultra high molecular weight polyethylene from which Mw differs may be mixed and used.

The weight average molecular weight (Mw) of the ultrahigh molecular weight polyethylene is 1 × 10 ⁶ or more (100,000 or more), preferably 2 × 10 ⁶ or more and less than 4 × 10 ⁶ . When Mw is in the above range, the film forming property is good. When the Mw of the ultra high molecular weight polyethylene is 4 × 10 ⁶ or more, the viscosity of the melt becomes too high, so that there may be a problem in the film forming process such that the resin cannot be extruded from the die. Mw is a value measured by gel permeation chromatography (GPC).

The content of ultrahigh 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. Although the upper limit of content of ultra high molecular weight polyethylene is not specifically limited, For example, it is 50 mass% or less. When the content of the ultrahigh molecular weight polyethylene is within the above range, the film strength and the air resistance can be made compatible at a high level by adjusting the stretching conditions described later.

Polyolefin resin, high density polyethylene (HDPE, density: 0.942 g / cm ³ or higher) can contain. The polyolefin resin preferably contains ultra high molecular weight polyethylene and high density polyethylene. When high-density polyethylene is included, the melt extrusion characteristics are excellent, and the uniform stretch processing characteristics are excellent. 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 . 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, 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 include polypropylene. The polypropylene is not particularly limited, and a propylene homopolymer, a copolymer of propylene and another α-olefin and / or diolefin (propylene copolymer), or a mixture thereof can be used. The content of polypropylene is, for example, from 0% by mass to less than 10% by mass, and preferably from 0% by mass to 5% by mass with respect to 100% by mass of the total polyolefin resin. Moreover, when polypropylene is included, the pore diameter of the resulting polyolefin microporous membrane tends to increase.

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

(Film forming solvent)
As the solvent for film formation, any solvent that can sufficiently dissolve the polyolefin resin can be used without particular limitation. The solvent for film formation is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification. Examples of the film-forming solvent include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions having boiling points corresponding to these. And phthalic acid esters which are liquid at room temperature such as dibutyl phthalate and dioctyl phthalate. Among these, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is mixed with the polyolefin resin, but at room temperature, a solid solvent and the film-forming solvent may be mixed and used. Examples of such a solid solvent include stearyl alcohol, seryl alcohol, and paraffin wax.

(Polyolefin solution)
The blending ratio of the polyolefin resin and the film-forming solvent in the polyolefin solution is not particularly limited, but is preferably 20 to 35 parts by mass with respect to 100 parts by mass of the polyolefin resin solution. When the ratio of the polyolefin resin is within the above range, swell and neck-in can be prevented at the die exit when extruding the polyolefin solution, and the moldability and self-supporting property of the extruded molded body (gel-shaped molded body) are improved.

(2) Formation of gel-like sheet Next, the polyolefin solution prepared above is fed from an extruder to a die, extruded into a sheet shape, and the obtained extruded product is cooled to form a gel-like sheet. The cooling is preferably performed to 90 ° C., which is lower than the crystal dispersion temperature (Tcd) of the polyolefin resin, more preferably 50 ° C. or less, and even more preferably 40 ° C. or less. By cooling, the polyolefin microphase separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallization degree is maintained in an appropriate range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but it is preferable that the cooling is performed by contacting with a roll cooled with a cooling medium. A plurality of polyolefin solutions having the same or different compositions may be fed from a plurality of extruders to a single die, laminated there in a layer form, and extruded into a sheet form. As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

(3) Stretching Next, the gel-like sheet is stretched in at least a uniaxial direction. The stretching of the gel sheet is also called wet stretching. The stretching may be uniaxial stretching or biaxial stretching, but 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 stretching and sequential stretching) may be used, but sequential stretching is preferable, and stretching in the MD direction (machine direction, longitudinal direction). Then, it is preferable to stretch in the TD direction (width direction, transverse direction). In the case where the MD direction and the TD direction are separately stretched, it is considered that the stretching tension is applied only in each direction during the stretching, and the molecular orientation easily proceeds. The TD direction is a direction orthogonal to the MD direction when the microporous film is viewed in a plane.

The final area draw ratio (surface ratio) in the drawing process needs to be 30 times or more and 150 times or less. When the surface magnification is in the above range, the film-forming property is good, the proportion of play molecules that are not oriented is reduced, and a polyolefin microporous film having high strength can be obtained. The area stretching ratio is preferably 35 times or more and 120 times or less. Moreover, it is preferable that all the draw ratios of MD direction and TD direction exceed 5 times.

The ratio of the draw ratio in the MD direction and the TD direction (the draw ratio in the MD direction / the draw ratio in the TD direction) needs to be 0.7 or more and 1.0 or less. When the ratio of the draw ratio is in the above range, the MD and TD directions have a good balance with respect to the tensile strength and tensile elongation of the resulting polyolefin microporous membrane, the membrane strength can be further improved, and the impact resistance is improved. improves. From the viewpoint of further improving the film strength, it is preferable that the draw ratio in the TD direction is larger than the draw ratio in the MD direction. The reason for this is not particularly limited, but when stretching in the TD direction after stretching in the MD direction, the molecular orientation once oriented in the MD direction is less likely to be oriented in the TD direction due to stretching in the MD direction. Therefore, it is considered that the molecular orientation can be advanced more uniformly in both directions by stretching in the TD direction at a larger magnification. In addition, the draw ratio in this step means the draw ratio of the gel-like sheet immediately before being used for the next step on the basis of the gel-like sheet immediately before this step. The ratio of the draw ratio in the MD direction and the TD direction (the draw ratio in the MD direction / the draw ratio in the TD direction) is preferably 0.75 or more and 1.0 or less.

The stretching temperature is preferably in the range of not less than the crystal dispersion temperature (Tcd) of the polyolefin resin and not more than the melting point of the polyolefin resin. Here, the melting point of the polyolefin resin refers to the melting point of the polyolefin resin in the gel sheet. When the stretching temperature is not higher than the melting point of the polyolefin resin, the melting of the polyolefin resin in the gel-like sheet can be suppressed, and the molecular chain can be efficiently oriented by stretching. In addition, when the stretching temperature is equal to or higher than the crystal 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 lowered, so that the film forming property is improved and the stretching is performed. The film breakage at the time is suppressed, and stretching at a high magnification becomes possible. The stretching temperature can be, for example, 100 ° C. or higher and 127 ° C. or lower. Here, the stretching temperature is the temperature of the gel sheet, and when there is a temperature difference between the front and back surfaces such as roll stretching, it means the center temperature in the thickness direction.

When stretching in the TD direction after stretching in the MD direction, it is important that the stretching temperature in the TD direction is higher than the stretching temperature in the MD direction. Details are unknown, but once oriented in the MD direction due to stretching in the MD direction, it is difficult to orient in the TD direction, so by stretching in the TD direction at a higher temperature, both In this direction, it is considered that the molecular orientation can be promoted more uniformly. The stretching temperature in the MD direction is 100 ° C. or higher and 110 ° C. or lower, preferably 103 ° C. or higher and 110 ° C. or lower. The stretching temperature in the TD direction is 115 ° C. or higher and 127 ° C. or lower, preferably 115 ° C. or higher and 125 ° C. When the stretching temperature in the MD direction and the TD direction is in the above ranges, the film forming property is good, the film strength of the resulting polyolefin microporous film can be improved, and the pore diameter can be controlled in an appropriate range.

(4) Removal of film forming solvent (cleaning)
Next, the film forming solvent is removed from the stretched gel-like sheet to obtain a microporous film. The removal of the solvent is performed using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, removing the film-forming solvent consists of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the following is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the same are known, the description thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(5) Drying Next, the microporous membrane 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 lower than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than Tcd. Drying is preferably carried out until the residual washing solvent is 5% by mass or less, more preferably 3% by mass or less, with the microporous membrane being 100% by mass (dry weight). When the residual cleaning solvent is within the above range, the porosity of the polyolefin microporous membrane is maintained, and deterioration of permeability is suppressed.

(6) Others The microporous membrane after drying may be subjected to heat treatment. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the dimension of the film in the TD direction unchanged. The thermal relaxation treatment is a treatment for thermally contracting the film in the MD direction and / or TD direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as a thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be given. The heat treatment temperature is preferably within the range of Tcd to Tm of the polyolefin resin, more preferably within the range of the second stretching temperature ± 5 ° C of the microporous membrane, and within the range of the second stretching temperature ± 3 ° C of the microporous membrane. Particularly preferred.

Further, the microporous membrane after drying may be re-stretched at a predetermined area stretch ratio in at least uniaxial direction. Stretching of the microporous membrane after drying is also referred to as dry stretching.

Moreover, the obtained polyolefin microporous membrane may be subjected to a crosslinking treatment and a hydrophilic treatment. For example, the crosslinking treatment is performed by irradiating the polyolefin microporous film with ionizing radiation such as α rays, β rays, γ rays, and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The meltdown temperature of the microporous membrane is increased by the crosslinking treatment. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting is preferably performed after the crosslinking treatment.

The polyolefin microporous membrane may be a single layer, or a layer composed of a polyolefin microporous membrane may be laminated. The multilayer polyolefin microporous membrane can be made into 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 or different.

The polyolefin microporous membrane may be a laminated polyolefin porous membrane by laminating a porous layer other than the polyolefin resin on at least one surface thereof. Although it does not specifically limit as another porous layer, For example, you may laminate | stack coating layers, such as an inorganic particle layer containing a binder and an inorganic particle. 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, polyamideimide resin, polyamide resin, aromatic polyamide resin, 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 can be used. it can. Moreover, as a laminated polyolefin porous membrane, the porous binder resin may be laminated on at least one surface of the polyolefin microporous membrane.

In the present invention, the final area stretching ratio (surface ratio) in the stretching process described above, the ratio of the stretching ratio in the MD direction and the TD direction (stretching ratio in the MD direction / stretching ratio in the TD direction), the MD and TD directions By adjusting the stretching temperature as appropriate, it is extremely excellent in impact resistance, and when used as a battery separator, both impact resistance and battery characteristics (output characteristics, dendrite resistance, etc.) are compatible at a high level. A polyolefin microporous membrane can be provided.

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

1. Measurement method and evaluation method [Film thickness]
The film thickness at 5 points in the range of 95 mm × 95 mm of the microporous membrane was measured with a contact thickness meter (Lightmatic manufactured by Mitutoyo Corporation), and the average value was obtained.

[Porosity]
The weight w _{1 of the} microporous membrane was compared with the weight w _{2 of the} polymer without pores equivalent to the weight (a polymer having the same width, length and composition), and the measurement was performed by the following equation.
Porosity (%) = (w ₂ −w ₁ ) / w ₂ × 100
[Bubble point pore size (maximum pore size) and average flow pore size]
Using a palm porometer (trade name, model: CFP-1500A) manufactured by PMI, measurement was performed in the order of Dry-up and Wet-up. For wet-up, pressure is applied to a microporous membrane sufficiently immersed in Galwick (trade name) with known surface tension, and the pore diameter converted from the pressure at which air begins to penetrate is defined as the bubble point pore diameter (maximum pore diameter). . For the average flow pore size, the pore size was converted from the pressure at the point where the curve showing the slope of 1/2 of the pressure / flow rate curve in the Dry-up measurement and the curve of the Wet-up measurement intersect. The following formula was used for conversion of pressure and pore diameter.
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]
Measures the maximum load L ₁ (N) when a microporous membrane with a film thickness T ₁ (μm) is pierced at a speed of 2 mm / sec with a 1 mm diameter needle with a spherical tip (curvature radius R: 0.5 mm). did. Further, the maximum load L ₂ (12 μm conversion) (N / 12 μm) when the film thickness is 12 μm is calculated from the measured value L ₁ of the maximum load by the formula: L ₂ = (L ₁ × 12) / T ₁ did.

[Air permeability resistance]
Permeability measured with a gas permeability meter (AGO Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117 Oken type tester method for a microporous membrane with a film thickness T ₁ (μm) The resistivity P ₁ (sec / 100 cm ³ Air) was measured. Further, the air resistance P ₂ (converted to 12 μm) (sec / 100 cm ³ Air / 12 μm) when the film thickness was 12 μm was calculated by the formula: P ₂ = (P ₁ × 12) / T ₁ .

[Weight average molecular weight (Mw)]
The weight average molecular weight (Mw) of the polyolefin microporous membrane was determined by gel permeation chromatography (GPC) method under the following conditions.
・ Measurement 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 volume: 500μl
・ Detector: Differential refractometer (RI detector) manufactured by Waters Corporation
-Calibration curve: A polyethylene conversion constant (0.46) was used from a calibration curve obtained using a monodisperse polystyrene standard sample.

[Tensile strength]
About MD tensile strength and TD tensile strength, it measured by the method based on ASTMD882 using the strip-shaped test piece of width 10mm.

[Tensile elongation]
It was measured by a method based on ASTM D-882A.

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

<Preparation of positive electrode>
92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as the active material, 2.3% by mass of flake graphite and acetylene black as the conductive agent, and 3.2% by mass of polyvinylidene fluoride (PVDF) as the binder are N -A slurry was prepared by dispersing in methylpyrrolidone (NMP). This slurry was applied to one side of an aluminum foil having a thickness of 20 μm serving as a positive electrode current collector with a die coater at an active material coating amount of 250 g / m ² and an active material bulk density of 3.00 g / cm ³ . And after drying at 130 degreeC for 3 minute (s) and compression-molding with the roll press machine, it cut | disconnected about 57 mm in width | variety, and was made into strip | belt shape.

<Production of negative electrode>
A slurry was prepared by dispersing 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder in purified water. This slurry was coated with a die coater on one side of a 12 μm thick copper foil serving as a negative electrode current collector at a high packing density of 106 g / m ² of active material and a bulk density of 1.55 g / cm ^{3 of} active material. It was attached. And after drying at 120 degreeC for 3 minute (s) and compression-molding with the roll press machine, it cut | disconnected about 58 mm in width | variety, and was made into strip | belt shape.

<Preparation of non-aqueous electrolyte>
It was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 1/2 (volume ratio) to a concentration of 1.0 mol / l.

<Separator>
The separators described in the examples and comparative examples were slit into 60 mm to form a strip shape.

<Battery assembly>
A strip-shaped negative electrode, a separator, a strip-shaped positive electrode, and a separator were stacked in this order, and the electrode plate laminate was fabricated by winding a plurality of times in a spiral shape with a winding tension of 250 gf. The electrode plate laminate is housed in a stainless steel container having an outer diameter of 18 mm and a height of 65 mm, and an aluminum tab derived from the positive electrode current collector is provided on the container lid terminal part, and is made of nickel derived from the negative electrode current collector. The tab was welded to the container wall. And after drying for 12 hours at 80 degreeC under vacuum, the said nonaqueous electrolyte solution was inject | poured in the container in the argon box, and it sealed.

<Impact resistance test>
The assembled battery is first charged with a constant current of 500 mA, and after each battery voltage reaches 4.20 V, the battery is fully charged by charging each constant voltage until the current value becomes 10 mA or less. It was. Next, a fully charged cylindrical battery was installed so that the long side lies sideways, and a rod having a diameter of 15.8 mm having a mass of 9.1 kg from a height of 61 cm was dropped on the central flat surface of the battery. Shocked. What caused the battery to ignite due to this shock even once in the 3 times test × ignited even if it did not ignite during the 3 times test, but caused smoke even once, ignited or smoked once during the 3 times test The case where was not confirmed was evaluated as “Good”.

[Dendrite resistance]
A sample having a maximum pore diameter of less than 60 nm was evaluated as ○, and the others as ×. When the maximum pore diameter is larger than 60 nm, lithium dendritic crystals (dendrites) generated by the precipitation of Li metal peculiar to the lithium ion secondary battery tend to easily enter the pores. This makes it easier to break the separator, which can lead to micro shorts depending on the battery design.

[Membrane resistance (impedance)]
Five circular measurement samples having a diameter of 19 mm and 20 circular measurement samples having a diameter of 16 mm were cut out from the porous film. Further, CR2032-type coin cell members (case, PP gasket, spacer (diameter 16 mm, thickness 1 mm), washer, cap) (manufactured by Hosen Co., Ltd.) were prepared.

First, in a dry room with an outdoor temperature of -35 ° C or less, place a measurement sample (diameter 19 mm) x 1 on the case, place a gasket to fix the sample, and then place the measurement on it. Samples (diameter 16 mm) × multiple sheets, spacers, and wave washers were installed in this order. The number of measurement samples having a diameter of 16 mm was two, three, and four, and one cell was prepared in which each of the measurement samples was arranged. Next, a 1M electrolytic solution in which a mixed solvent (EC / EMC = 4: 6 [volume ratio]) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was mixed with LiPF ₆ in a cell in which a wave washer was installed. (Kishida Chemical Co., Ltd.) was injected. After the injection, the cell was allowed to stand for 10 minutes at a pressure of about −50 kPa, and the measurement sample was impregnated with the electrolyte. Then, the cell was covered with a cap and sealed with a coin cell caulking device (manufactured by Hosen Co., Ltd.) to obtain a sample cell.

The obtained sample cell was put in a thermostatic bath at 25 ° C. and allowed to stand for 3 hours, and then the resistance of the cell was measured at an amplitude of 20 mV using an AC impedance measuring device (manufactured by Hioki Electric Co., Ltd.). The measured resistance component value of the cell (the real value when the imaginary axis value is 0) is plotted against the number of porous films placed in the cell, and the slope is obtained by linear approximation of this plot. It was. A value obtained by multiplying this inclination 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 (Ω · cm ² ) of the porous film.
Those porous membrane resistance value of the film (Ω · cm ²⁾ was the 1.4Ωcm ² / 10μm or less ○ (good), it was × (poor) to in excess of 1.4Ωcm ² / 10μm.

When the film resistance (impedance) is 1.4Ωcm 2 ^/ 10μm or less, when used as a battery separator in a secondary battery, it is expected that the output characteristics of the battery is improved.

[Cycle life]
<Production of test battery>
A wound body was prepared using a tab attached to a positive electrode (manufactured by Yayama Co., Ltd.) and a negative electrode (manufactured by Yayama Co., Ltd.) and each microporous membrane. Next, a wound body was placed in the aluminum laminate bag, and 0.5 weight in electrolyte solution (1.1 mol / L, LiPF ₆ , ethylene carbonate / ethyl methyl carbonate / diethylene carbonate = 3/5/2 (volume ratio)) % Vinylene carbonate, 2% by weight fluoroethylene carbonate) was added dropwise and sealed with a vacuum laminator. This was used as a 300 mAh test battery.

<Cycle performance test>
A cycle performance test was performed under the following charge / discharge conditions using the above test battery.
Charging: 1C, 4.35V constant current constant voltage charging, cut-off current 0.05C
Discharge: 1C, 3V constant current discharge Measurement temperature: 25 ° C
The test was carried out using three test batteries, and the ratio of the 200th charge capacity based on the 1C charge capacity of the first time, that is, the average value of the capacity retention rate, was derived and used as an index of cycle performance. A sample having an average capacity retention rate of 85% or more was evaluated as “good” and a sample having a capacity retention rate of less than 85% was evaluated as “poor”.

When the capacity maintenance rate is 85% or more, it can be determined that the charge capacity can be sufficiently retained even after repeated charge and discharge for a long period of time, and a good battery can be expected.

(Examples 1 to 5)
As the polyolefin resin, ultra high molecular weight polyethylene (UHMwPE) with Mw of 2.5 × 10 ⁶ and high density polyethylene (HDPE) with Mw of 2.8 × 10 ⁵ are included in the blending ratio (mass%) shown in Table 1, respectively. Polyolefin resin, liquid paraffin (film forming solvent), and tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant (0 per 100 parts by mass of polyolefin resin) 2 parts by mass) was melt-kneaded using a twin screw extruder to prepare a polyolefin solution. In addition, Table 1 shows the polyolefin resin concentration with respect to 100 parts by mass in total of the polyolefin resin and the film-forming solvent in the polyolefin solution. The polyolefin solution was fed from a twin screw extruder to a T die and extruded. The extruded product was cooled while being taken up by a cooling roll to form a gel-like sheet. The gel sheet was wet-stretched in the MD direction and the TD direction under the conditions shown in Table 1. Liquid paraffin was removed from the wet-stretched gel-like sheet using methylene chloride, dried, and the resulting polyolefin microporous membrane was re-stretched in the TD direction at the temperature and magnification shown in Table 1 using a tenter stretching machine. Thermal relaxation treatment was performed at the same temperature. The relaxation rate (%), which is the amount to be subjected to thermal relaxation treatment, was calculated according to the following formula, with the TD direction film width before the thermal relaxation treatment as L ₁ and the TD direction film width L ₂ after the thermal relaxation treatment.
Formula: Relaxation rate (%) = (L ₁ −L ₂ ) / L ₁ × 100
The evaluation results and the like of the obtained polyolefin microporous membrane are shown in Table 1.

(Comparative Examples 1 to 13)
Except for the conditions shown in Table 1 or 2, a polyolefin microporous membrane was produced under the same conditions as in the Examples. The evaluation results and the like of the obtained polyolefin microporous membrane are shown in Table 1 (Examples 1 to 5, Comparative Examples 1 to 4) or Table 2 (Comparative Examples 5 to 13).

The polyolefin microporous membrane of this embodiment is extremely excellent in impact resistance when incorporated into a secondary battery as a separator. Moreover, since the polyolefin microporous film of this embodiment can achieve both impact resistance and battery characteristics, it can be suitably used as a separator for non-aqueous electrolyte secondary batteries.

Claims

A polyolefin microporous membrane having the following characteristics (1) to (5):
(1) The tensile strength (MPa) and tensile elongation (%) in the MD direction and TD direction 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 direction and the TD direction is 196 MPa or more.
(3) The maximum pore diameter measured using a palm porometer is 60 nm or less.
(4) The average flow pore diameter measured using a palm porometer is 40 nm or less.
(5) The porosity is 40% or more.
The polyolefin microporous membrane according to claim 1, which has the following property (6).
(6) Ratio of tensile strength in MD direction and TD direction (tensile strength in MD direction / tensile strength in TD direction) is 0.8 or more and 1.2 or less.
The polyolefin microporous membrane according to claim 1 or 2, which has the following property (7).
(7) Ratio of tensile elongation in MD direction and TD direction (tensile elongation in MD direction / tensile elongation in TD direction) 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 property (8).
(8) The tensile elongation in the MD direction and the TD direction is 90% or more, respectively.
The polyolefin microporous film according to any one of claims 1 to 4, wherein the film thickness is 20 µm or less.
6. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the puncture strength converted to a film thickness of 12 μm is 5 N or more.
The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction satisfy the following relational expression (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 (II)
A separator for a non-aqueous electrolyte secondary battery using the polyolefin microporous membrane according to any one of claims 1 to 7.
The separator for a non-aqueous electrolyte secondary battery according to claim 8, wherein a coating layer containing inorganic particles and a binder resin is provided on at least one surface.
A non-aqueous electrolyte secondary battery comprising the non-aqueous electrolyte secondary battery separator according to claim 8.