JPWO2018180714A1 - Polyolefin microporous membrane, separator for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery - Google Patents

Abstract

An object of the present invention is to provide a microporous polyolefin membrane having excellent impact resistance and battery characteristics when incorporated into a battery as a separator. The present invention is a microporous polyolefin membrane having the following properties (1) to (5). (1) The 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+ (TD Tensile strength in the direction × tensile elongation in the 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) Using a Palm Porometer (4) The average flow pore size measured using a palm porometer is 40 nm or less. (5) The porosity is 40% or more.

Description

  The present invention relates to a microporous polyolefin 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 membrane using a polyolefin as a resin material is excellent in chemical resistance, insulation, mechanical strength, and the like, and has shutdown characteristics, and thus is widely used as a battery separator in recent years.

  Secondary batteries, for example, lithium ion secondary batteries, have a high energy density and are therefore widely used as batteries for personal computers, mobile phones, and the like. 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, higher reliability is important together with the characteristics of the batteries, and the separators used in these batteries are also required to have high impact resistance from the viewpoint of safety. Have 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. Some reports have been made on polyolefin microporous membranes having improved membrane strength and elongation.

  For example, Patent Document 1 discloses that 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, which has a total MD tensile elongation and TD tensile elongation of 20-250%. According to Patent Literature 1, this microporous polyolefin membrane is hardly deformed, and is excellent in film rupture resistance and stress relaxation characteristics.

  Patent Literature 2 discloses that a ratio of tensile strength in the length direction to tensile strength in the width direction is 0.75 to 1.25, and that the heat shrinkage in the width direction at 120 ° C. is less than 10%. A porous membrane is described. According to Patent Document 2, this microporous polyolefin membrane has good resistance to foreign substances and the like.

  Patent Document 3 contains polypropylene, has a transverse rupture strength of 100 to 230 MPa, a transverse tensile rupture elongation of 10 to 110%, and a longitudinal tensile rupture strength of 0.8 to the transverse tensile rupture strength. A polyolefin microporous membrane of ~ 1.3 is described.

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

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

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

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

The present invention is a microporous polyolefin membrane having the following properties (1) to (5).
(1) The tensile strength (MPa) and the 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 size 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.

Further, the microporous polyolefin membrane of the present invention may have the following property (6).
(6) The ratio of the tensile strength in the MD direction and the tensile strength in 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.

Further, the microporous polyolefin membrane of the present invention may have the following property (7).
(7) 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 0.75 or more and 1.25 or less.

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

In addition, the microporous polyolefin membrane may have a puncture strength of 5 N or more in terms of a film thickness of 12 μm. Further, the microporous polyolefin membrane may have a tensile strength (MPa) and a tensile elongation (%) in the MD and TD directions satisfying 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).

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

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

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

  Hereinafter, the present 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, a 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 microporous polyolefin membrane of the present embodiment will be described.

[Relationship between tensile strength and tensile elongation]
In a polyolefin microporous membrane, impact resistance may not be sufficient if it has only high tensile strength or high tensile elongation. In order to obtain a polyolefin microporous membrane having higher impact resistance, the present inventor has found that in the MD direction (mechanical direction, longitudinal direction, longitudinal direction) and TD direction (when the polyolefin microporous membrane is viewed in a plane, the MD direction). It has been found that it is important to have a high balance between high tensile strength and high tensile elongation (good isotropy) in both directions perpendicular to (width direction and transverse direction). Further, the present inventor has found that when the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction have a specific relationship, a polyolefin microporous film having extremely excellent impact resistance is obtained. .

That is, in the microporous polyolefin membrane of the present embodiment, the relationship between the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction satisfies the following expression (I). When the polyolefin microporous membrane satisfies the following formula (I), the 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).

From the viewpoint of further improving the impact resistance, the microporous polyolefin membrane of the present embodiment has a relation between the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction represented by the following formula (II). ) Is more preferably satisfied, and (III) is more preferably satisfied.
[(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 Expression (III).

The upper limit of 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 ] ^1/2 is particularly limited. Although not limited, it is, for example, 1000 or less, preferably 800 or less, and more preferably 600 or less from the viewpoint of shrinkage characteristics.

[Tensile strength]
The microporous polyolefin membrane of the present embodiment has a tensile strength in the MD and TD directions of 196 MPa or more, preferably 200 MPa or more, and more preferably 230 MPa or more. When the tensile strength is within the above range, the film strength is more excellent, a high tension can be applied when the electrode body is wound in the battery manufacturing process, and the rupture of the film due to foreign matter or impact in the battery is 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 further preferably 400 MPa or less from the viewpoint of shrink resistance. In addition, the tensile strength can be measured by a method according to ASTM D882 using a strip-shaped test piece having a width of 10 mm.

[Tensile elongation]
The microporous polyolefin membrane of the present embodiment preferably has a tensile elongation of 90% or more in each of the MD direction and the TD direction. When the tensile elongation is within the above range, when a shock is applied in the battery, the flexibility of the battery suppresses the film breakage of the separator and the occurrence of a short circuit (short circuit). The upper limit of the tensile elongation in the MD and TD directions is not particularly limited, but is, for example, 400% or less, preferably 300% or less, and more preferably 200% or less. When the tensile elongation is in the above range, the winding property is good without the separator being stretched and deformed when the electrode is wound. 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]
In the microporous polyolefin membrane of the present embodiment, the ratio of the tensile strength in the MD direction and the tensile strength in the TD direction (tensile strength in the MD direction / tensile strength in the TD direction) is preferably 0.8 or more and 1.2 or less. When the ratio of the tensile strength is in the above range, the force is more uniformly applied to the impact in all directions, so that the impact resistance is improved, and the film rupture and short circuit (short circuit) can be more stably suppressed. it can.

[Ratio of MD elongation / TD elongation]
In the microporous polyolefin membrane of the present embodiment, the ratio of the tensile elongation in the MD and TD directions (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 tensile elongation ratio is in the above range, a more uniform force is applied to impacts in all directions, so that impact resistance is improved, and film breakage and short circuit (short circuit) are more stably suppressed. Can be.

  The ratio between the tensile strength and the tensile elongation is preferably closer to 1 from the viewpoint of more stably suppressing the rupture of the membrane against impacts in all directions. Further, if 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 large, a tear in the TD direction or disconnection of the electrode tab bonding portion may occur, and a short circuit may easily occur.

[Puncture strength]
The piercing strength of the microporous polyolefin membrane is preferably 5 N or more, more preferably 5.2 N or more, and still more preferably 6 N or more, when converted to a film thickness of 12 μm. The upper limit of the piercing strength is not particularly limited, but is, for example, 10 N or less. When the puncture strength is in the above range, the membrane strength of the microporous polyolefin membrane is excellent and a good balance of physical properties can be exhibited. Also, a secondary battery using this microporous polyolefin membrane as a separator has excellent resistance to unevenness and impact of the electrode, and suppresses the occurrence of a short circuit in the electrode.

The piercing strength is determined by the maximum load (N) when a microporous polyolefin film having a thickness of T ₁ (μm) is pierced 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 the measured value. The puncture strength (N / 12 μm) in terms of a film thickness of 12 μm for a polyolefin microporous film having a film thickness T ₁ (μm) can be obtained by the following equation.
Formula: Puncture strength (12 μm conversion) = Puncture strength measured (N) × 12 (μm) / Film thickness T ₁ (μm)
[Thickness]
The upper limit of the thickness of the microporous polyolefin membrane is not particularly limited, but 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 film has excellent transparency and film resistance, and the battery capacity can be improved 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 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 microporous polyolefin membrane is preferably 40% or more, more preferably 40% or more and 70% or less. In addition, the upper limit of the porosity is more preferably 60% or less, and further preferably 55% or less, from the viewpoints of film forming properties, mechanical strength, and insulating properties. When the porosity is in the above range, the holding amount of the electrolytic solution can be increased, high ion permeability can be secured, 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 electrolyte content, and by-products generated during the battery reaction Clogging may increase and the cycle characteristics may rapidly deteriorate. The porosity can be set in the above range by adjusting the composition of the polyolefin resin, the stretching ratio, and the like in the production process.

Porosity, the microporous membrane weight w ₁ and its equivalent pore-free weight w ₂ of the polymer _(width, length, same polymer composition) were compared, and by the following equation (1), measured it can.
Porosity (%) = (w ₂ −w ₁ ) / w ₂ × 100 (1).

[Average flow hole diameter]
The average pore size (average flow pore size) of the microporous polyolefin membrane is 40 nm or less, preferably 10 nm or more and 40 nm or less. When the average pore diameter is within the above range, the balance between strength and permeability is excellent, and at the same time, self-discharge originating from the coarse pores is suppressed. When the average pore diameter exceeds 40 nm, the ion permeation flow path is selectively concentrated on the coarse pores to increase the electric resistance, and the cycle characteristics are deteriorated due to local clogging of the electrolyte decomposition by-product. Sometimes. The average pore diameter is a value measured by a method (half dry method) based on ASTM E1294-89. A palm porometer (model number: CFP-1500A) manufactured by PMI can be used as a measuring device, and Galwick (15.9 dyn / cm) can be used as a measuring solution.

[Maximum hole diameter (bubble point diameter)]
The maximum pore diameter (bubble point diameter: BP diameter) is preferably 60 nm or less, 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 come into contact with each other (small short circuit) or may be broken by lithium dendrites (dendrites) to cause a short circuit. On the other hand, when the maximum pore size is too small, the electric resistance of the battery becomes high, the cycle performance becomes insufficient, and the capacity retention at the time of high-speed discharge may become low.

[Air resistance]
The upper limit of the air permeability of the polyolefin microporous membrane in terms of the film thickness of 12 μm is not particularly limited, but 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 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 separator has excellent ion permeability, and a secondary battery incorporating this separator has reduced impedance and improved output characteristics and rate characteristics. The air permeability resistance can be set to the above range by adjusting the stretching conditions and the like when producing the microporous polyolefin membrane.

The air permeability resistance is a value P ₁ (sec / 100 cm ³ ) which can be measured by an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117 Oken type testing machine method. Air). 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 a film thickness of 12 μm can be obtained by the following equation. .
Formula: P ₂ = P ₁ (sec / 100 cm ³ Air) × 12 (μm) / Film thickness T ₁ (μm)
2. Method for Producing a Microporous Polyolefin Membrane The method for producing a microporous polyolefin membrane is not particularly limited as long as a polyolefin microporous membrane having the above properties can be obtained, and a known method for producing a microporous polyolefin membrane can be used. As the method for producing the microporous polyolefin membrane of the present embodiment, a wet membrane production method is preferred from the viewpoint of easy control of the membrane structure and physical properties. As the wet film forming method, for example, the methods described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835, WO 2006/137540 and the like can be used.

  Hereinafter, an example of a method for producing a microporous polyolefin membrane (wet membrane production method) will be described. Note that 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 as a raw material and a solvent for film formation 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. The melt-kneading method is well-known, and therefore the description is omitted.

(Polyolefin resin)
As the polyolefin resin as a raw material, for example, polyethylene, polypropylene, or the like can be used. The polyethylene is not particularly limited, and various polyethylenes can be used. For example, ultra high molecular weight polyethylene (UHMwPE), high density polyethylene (HDPE), medium density polyethylene, branched low density polyethylene, linear low density polyethylene Polyethylene or the like is used. In addition, 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.

  Preferably, the polyolefin resin includes ultra high molecular weight polyethylene (UHMwPE). When ultra-high molecular weight polyethylene is contained, the membrane strength of the resulting microporous polyolefin membrane can be improved. Further, the fibrils of the polyolefin microporous membrane can be miniaturized (densified), and a membrane having a small pore size can be uniformly formed over the entire membrane. 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 may be used as a mixture.

The weight average molecular weight (Mw) of the ultrahigh 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 within the above range, the film-forming properties are 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 a problem may be caused in a film forming process such that a resin cannot be extruded from a die. In addition, Mw is a value measured by gel permeation chromatography (GPC).

  The content of the 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 whole polyolefin resin. The upper limit of the content of the ultrahigh molecular weight polyethylene is not particularly limited, but is, for example, 50% by mass or less. When the content of the ultrahigh molecular weight polyethylene is within the above range, the film strength and the air permeability can be made compatible at a high level by adjusting the stretching conditions described later.

The polyolefin resin can contain high-density polyethylene (HDPE, density: 0.942 g / cm ³ or more). Further, the polyolefin resin preferably contains ultra high molecular weight polyethylene and high density polyethylene. When high-density polyethylene is contained, it is excellent in melt extrusion characteristics and uniform stretching characteristics. 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 . In addition, Mw is a value measured by gel permeation chromatography (GPC). The content of the high-density polyethylene is preferably from 50% by mass to 90% by mass, more preferably from 50% by mass to 80% by mass, based on 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 with another α-olefin and / or diolefin (propylene copolymer), or a mixture thereof can be used. The content of the 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 based on 100% by mass of the total amount of the polyolefin resin. Further, when polypropylene is contained, the pore size of the resulting microporous polyolefin membrane tends to be large.

  In addition, the polyolefin resin can contain other resin components other than polyethylene and polypropylene, if necessary. As the other resin component, 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. And other various additives.

(Solvent for film formation)
As the solvent for film formation, any solvent can be used without particular limitation as long as it can sufficiently dissolve the polyolefin resin. 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 solvent for film formation include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions whose boiling points correspond to these. And room temperature liquid phthalates such as dibutyl phthalate and dioctyl phthalate. Among them, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is miscible with the polyolefin resin. However, a solvent that is solid at room temperature and the above-mentioned solvent for film formation may be mixed and used. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, and paraffin wax.

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

(2) Formation of Gel Sheet Next, the polyolefin solution prepared above is fed from an extruder to a die, extruded into a sheet, and the obtained extruded product is cooled to form a gel sheet. 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 lower, and further preferably 40 ° C. or lower. By cooling, the microphase of the polyolefin separated by the film-forming solvent can be fixed. When the cooling rate is within the above range, the crystallinity is kept 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. A plurality of polyolefin solutions having the same or different compositions may be fed from a plurality of extruders to one die, where they may be laminated in layers and extruded into sheets. 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-like sheet is also referred to as 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 multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used, but sequential stretching is preferred, and stretching is performed in the MD direction (machine direction, longitudinal direction). Then, it is preferable to stretch in the TD direction (width direction, lateral direction). When the stretching in the MD direction and the stretching in the TD direction are performed separately, it is considered that in stretching, a stretching tension is applied only in each direction, and the molecular orientation easily proceeds. The TD direction is a direction orthogonal to the MD direction when the microporous membrane is viewed in a plane.

  In the stretching step, the final area stretching ratio (area ratio) needs to be 30 times or more and 150 times or less. When the areal magnification is in the above range, the film-forming properties are good, the ratio of unoriented free molecules 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. Further, it is preferable that the stretching ratio in both the MD direction and the TD direction exceeds 5 times.

  The ratio of the stretching ratio in the MD direction and the TD direction (the stretching ratio in the MD direction / the stretching ratio in the TD direction) needs to be 0.7 or more and 1.0 or less. When the ratio of the stretching ratio is in the above range, the MD and TD directions of the tensile strength and the tensile elongation of the obtained polyolefin microporous film are well-balanced, and the film strength can be further improved, and the impact resistance can be improved. improves. Further, 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. The reason for this is not particularly limited, but when stretching in the TD direction after stretching in the MD direction, by stretching in the MD direction, the molecular orientation once oriented in the MD direction is less likely to be oriented in the TD direction. Therefore, it is considered that by stretching in the TD direction at a higher magnification, the molecular orientation can be more uniformly promoted in both directions. The stretching ratio in this step refers to the stretching ratio of the gel sheet immediately before being subjected to the next step, based on the gel sheet immediately before this step. The ratio of the stretching ratio in the MD and TD directions (the stretching ratio in the MD direction / the stretching ratio in the TD direction) is preferably 0.75 or more and 1.0 or less.

  The stretching temperature is preferably in the range from the crystal dispersion temperature (Tcd) of the polyolefin resin to 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 equal to or lower than the melting point of the polyolefin resin, melting of the polyolefin resin in the gel-like sheet can be suppressed, and the molecular chains can be efficiently oriented by stretching. Further, 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 reduced, so that the film-forming property becomes good and The film can be prevented from breaking at the time of stretching, and stretching at a high magnification can be performed. The stretching temperature can be, for example, 100 ° C. or more and 127 ° C. or less. Here, the stretching temperature is the temperature of the gel sheet, and when there is a temperature difference between the front and back sides such as roll stretching, it refers to 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. Although the details are unknown, the molecular orientation once oriented in the MD direction is difficult to be oriented in the TD direction by the stretching in the MD direction. It is considered that the molecular orientation can be promoted more uniformly in the direction. The stretching temperature in the MD direction is 100 ° C. or more and 110 ° C. or less, preferably 103 ° C. or more and 110 ° C. or less. The stretching temperature in the TD direction is from 115 ° C to 127 ° C, preferably from 115 ° C to 125 ° C. When the stretching temperature in the MD and TD directions is within the above range, the film-forming properties are good, the film strength of the resulting microporous polyolefin membrane can be improved, and the pore size can be controlled in an appropriate range.

(4) Removal (cleaning) of solvent for film formation
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 solvent phase for film formation, when the solvent for film formation is removed, it is composed of fibrils forming a fine three-dimensional network structure, and pores (voids) communicating irregularly in three dimensions. Is obtained. Since the washing solvent and the method of removing the film-forming solvent using the same are known, their description is omitted. For example, the method disclosed in Japanese Patent No. 2132327 or JP-A-2002-256099 can be used.

(5) Drying Next, the microporous film from which the solvent for film formation 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 lower than Tcd by 5 ° C. or more. The drying is preferably performed until the residual washing solvent becomes 5% by mass or less, more preferably 3% by mass or less, with the microporous membrane being 100% by mass (dry weight). When the remaining washing solvent is within the above range, the porosity of the polyolefin microporous membrane is maintained, and deterioration of permeability is suppressed.

(6) Others A heat treatment may be applied to the dried microporous film. As the heat treatment method, a heat setting treatment and / or a heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which the film is heated while keeping the dimension in the TD direction unchanged. The thermal relaxation treatment is a 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 a thermal relaxation treatment method, a method disclosed in JP-A-2002-256099 can be mentioned. 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 of the microporous membrane ± 5 ° C, and within the range of the second stretching temperature of the microporous membrane ± 3 ° C. Particularly preferred.

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

  Further, the obtained polyolefin microporous membrane may be subjected to a crosslinking treatment and a hydrophilic treatment. For example, a crosslinking treatment is performed by irradiating the microporous polyolefin membrane 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 crosslinking treatment increases the meltdown temperature of the microporous membrane. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer grafting is preferably performed after the crosslinking treatment.

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

  The microporous polyolefin membrane may be a laminated polyolefin porous membrane by laminating a porous layer other than the polyolefin resin on at least one surface thereof. The other porous layer is not particularly limited. For example, a coating layer such as an inorganic particle layer containing a binder and inorganic particles may be laminated. The binder component constituting the inorganic particle layer is not particularly limited, and known components can be used, for example, an acrylic resin, a polyvinylidene fluoride resin, a polyamideimide resin, a polyamide resin, an aromatic polyamide resin, a polyimide resin, and the like. 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, and the like can be used. it can. Further, the laminated polyolefin porous membrane may be one in which the porous binder resin is laminated on at least one surface of a polyolefin microporous membrane.

  In the present invention, the final area stretching ratio (area ratio) in the stretching step described above, the ratio of the MD and TD stretching ratios (MD stretching ratio / TD stretching ratio), MD and TD direction stretching ratios. By appropriately adjusting the stretching temperature, the impact resistance is extremely excellent, and when used as a battery separator, both impact resistance and battery characteristics (output characteristics, dendrite resistance, etc.) are achieved at a high level. It is possible to provide a polyolefin microporous membrane.

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

1. Measurement method and evaluation method [Thickness]
Five thicknesses of the microporous membrane in a range of 95 mm × 95 mm were measured with a contact thickness meter (Lightmatic, manufactured by Mitutoyo Corporation), and the average value was determined.

[Porosity]
The weight w _{1 of the} microporous membrane was compared with the weight w _{2 of a} polymer having no pores equivalent thereto (a polymer having the same width, length, and composition) and measured by the following equation.
Porosity (%) = (w ₂ −w ₁ ) / w ₂ × 100
[Bubble point pore size (maximum pore size) and average flow pore size]
The measurement was performed in the order of Dry-up and Wet-up using a palm porometer (trade name, model number: CFP-1500A) manufactured by PMI. In Wet-up, pressure is applied to a microporous membrane sufficiently immersed in Galwick (trade name) with a known surface tension, and the pore diameter calculated from the pressure at which air starts to penetrate is defined as the bubble point pore diameter (maximum pore diameter). . As for the average flow hole diameter, the hole diameter was converted from the pressure at the point where the curve indicating the half of the pressure and flow curve in the Dry-up measurement and the curve in the Wet-up measurement intersect. The following formula was used for conversion of pressure and pore size.
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) is measured when a microporous membrane having a thickness of T ₁ (μm) is pierced at a speed of 2 mm / sec with a needle having a spherical surface (radius of curvature R: 0.5 mm) and a diameter of 1 mm. did. Further, the maximum load L ₂ (converted to 12 μm) (N / 12 μm) when the film thickness is set to 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 resistance]
Air permeability measured with a gas permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) for a microporous film having a thickness of T ₁ (μm) in accordance with JIS P-8117 Oken type testing machine method. The resistance P ₁ (sec / 100 cm ³ Air) was measured. Further, the air permeability resistance P ₂ (converted to 12 μm) (sec / 100 cm ³ Air / 12 μm) when the film thickness was 12 μm was calculated from the equation: P ₂ = (P ₁ × 12) / T ₁ .

[Weight average molecular weight (Mw)]
The weight average molecular weight (Mw) of the microporous polyolefin membrane was determined by a gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by Waters Corporation
-Column: Shodex UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
-Solvent (mobile phase): o-dichlorobenzene-Solvent flow rate: 1.0 ml / min-Sample concentration: 0.1 wt% (dissolution condition: 135 ° C / 1h)
・ Injection volume: 500 μl
-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]
The MD tensile strength and the TD tensile strength were measured by a method according to ASTM D882 using a rectangular test piece having a width of 10 mm.

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

[Impact test]
A cylindrical battery was manufactured according to the following procedure, and an impact test was performed.

<Preparation of positive electrode>
92.2% by mass of a lithium-cobalt composite oxide LiCoO ₂ as an active material, 2.3% by mass of flaky graphite and acetylene black as a conductive agent, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder was N. -Dispersed in methylpyrrolidone (NMP) to prepare a slurry. The slurry was applied to one surface of a 20 μm-thick aluminum foil serving as a positive electrode current collector with a die coater at an active material application 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, compression-molded by a roll press, and cut into a band of about 57 mm in width.

<Preparation of negative electrode>
96.9% by mass of artificial graphite as an active material, 1.4% by mass of an ammonium salt of carboxymethyl cellulose and 1.7% by mass of a styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. This slurry was coated with a die coater on one surface of a 12 μm thick copper foil serving as a negative electrode current collector at a high packing density of an active material coating amount of 106 g / m ² and an active material bulk density of 1.55 g / cm ^3. Attached. Then, it was dried at 120 ° C. for 3 minutes, compression-molded by a roll press, and then cut into a band of about 58 mm in width.

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

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

<Battery assembly>
The strip-shaped negative electrode, the separator, the strip-shaped positive electrode, and the separator were stacked in this order, and spirally wound with a winding tension of 250 gf to form an electrode plate laminate. The electrode plate laminate was 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 was provided on the container lid terminal portion, and a nickel product derived from the negative electrode current collector was provided. The tub was welded to the vessel wall. After drying at 80 ° C. for 12 hours under vacuum, the above non-aqueous electrolyte was injected into the 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 each constant voltage until the current value becomes 10 mA or less to obtain a fully charged battery. Was. Next, a fully charged cylindrical battery was placed so that the long sides were horizontal, and a rod having a mass of 9.1 kg and a diameter of 15.8 mm was dropped from a height of 61 cm onto the central flat surface of the battery to obtain a battery. Shocked. If the battery ignited due to this impact even once in the test three times, × if the battery did not ignite during the test three times, but did smoke even once. △ If the battery ignited or smoked once during the test three times. Those in which no was confirmed were evaluated as ○.

[Dendrite resistance]
A sample having a maximum pore diameter of less than 60 nm was rated as “○”, and the others were rated as “x”. When the maximum pore diameter is larger than 60 nm, lithium dendrites (dendrites) generated by deposition of Li metal specific to lithium ion secondary batteries tend to easily penetrate into the pores. As a result, the separator is easily broken, which leads to a minute short circuit depending on the battery design.

[Membrane resistance (impedance)]
From the porous film, five circular measurement samples having a diameter of 19 mm and 20 circular measurement samples having a diameter of 16 mm were cut out. In addition, 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, a sample for measurement (diameter 19 mm) × 1 is placed on a case in a dry room where the open-air temperature is −35 ° C. or less, and a gasket is placed so as to fix the sample, and the measurement is performed thereon. A sample (diameter 16 mm) × a plurality of pieces, a spacer, and a wave washer were sequentially installed. The number of measurement samples having a diameter of 16 mm was 2, 3, and 4, and one cell in which each of the measurement samples was arranged was produced. Then, a 1 M concentration 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 provided with a wave washer. (Manufactured by Kishida Chemical Co., Ltd.) was injected. After the injection, the cell was allowed to stand at a pressure of about −50 kPa for 10 minutes, and the sample for measurement was impregnated with the electrolytic solution. Thereafter, 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.

After placing the obtained sample cell in a thermostat at 25 ° C. and allowing it to stand for 3 hours, 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 value of the resistance component of the cell (the value of the real number when the value of the imaginary axis is 0) is plotted against the number of porous films arranged in the cell, and the slope is obtained by linearly approximating this plot. Was. The value obtained by multiplying the slope by the area of the spacer (2.01 cm ² (= (1.6 cm / 2) ² × π)) was defined 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), 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]
<Preparation of test battery>
A wound body was produced using the tabs attached to the positive electrode (manufactured by Yayama Corporation) and the negative electrode (manufactured by Yayama Corporation) and each microporous membrane. Next, the wound body was placed in an aluminum laminate bag, and 0.5 wt. In an electrolytic solution (1.1 mol / L, LiPF ₆ , ethylene carbonate / ethyl methyl carbonate / diethylene carbonate = 3/5/2 (volume ratio)). % Vinylene carbonate and 2% by weight of fluoroethylene carbonate) were added dropwise in an amount of 750 μL and sealed with a vacuum laminator. This was used as a 300 mAh test battery.

<Cycle performance test>
A cycle performance test was performed using the test battery under the following charge / discharge conditions.
Charging: 1C, 4.35V constant current constant voltage charging, cutoff current 0.05C
Discharge: 1C, 3V constant current discharge Measurement temperature: 25 ° C
The test was performed using three test batteries, and the ratio of the 200th charge capacity based on the 1C charge capacity at the first time, that is, the average value of the capacity retention rate was derived and used as an index of cycle performance. When the average value of the capacity retention ratio was 85% or more, it was evaluated as ○ (good), and when it was less than 85%, it was evaluated as x (bad).

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

(Examples 1 to 5)
Ultra-high molecular weight polyethylene (UHMwPE) having Mw of 2.5 × 10 ⁶ and high-density polyethylene (HDPE) having Mw of 2.8 × 10 ⁵ are contained as polyolefin resins at the compounding ratio (% by mass) shown in Table 1, respectively. Polyolefin resin, liquid paraffin (solvent for film formation), and tetrakis [methylene-3- (3,5-ditert-butyl-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. Table 1 shows the polyolefin resin concentration in the polyolefin solution with respect to the total of 100 parts by mass of the polyolefin resin and the solvent for film formation. 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-drawn in the MD and TD directions under the conditions shown in Table 1. Liquid paraffin was removed from the wet-stretched gel 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. Heat relaxation treatment was performed at the same temperature. The relaxation rate (%), which is the amount of the heat relaxation treatment, was calculated by the following formula, assuming that the film width in the TD direction before the heat relaxation treatment was L ₁ and the film width L _{2 in} the TD direction after the heat relaxation treatment.
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 microporous polyolefin membrane was produced under the same conditions as in the example except that the conditions shown in Table 1 or Table 2 were used. The evaluation results and the like of the obtained polyolefin microporous membrane are described in Table 1 (Examples 1 to 5, Comparative Examples 1 to 4) or Table 2 (Comparative Examples 5 to 13).

The microporous polyolefin membrane of the present embodiment is extremely excellent in impact resistance when incorporated in a secondary battery as a separator. Further, the microporous polyolefin membrane of the present embodiment can achieve both impact resistance and battery characteristics, and thus can be suitably used as a separator for a non-aqueous electrolyte secondary battery.

Claims (10)

A polyolefin microporous membrane having the following properties (1) to (5).
(1) The tensile strength (MPa) and the 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 size 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) The ratio of the tensile strength in the MD direction and the tensile strength in 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. The polyolefin microporous membrane according to claim 1 or 2, having the following property (7).
(7) 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 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 and TD directions is 90% or more, respectively.   The polyolefin microporous membrane according to any one of claims 1 to 4, having a thickness of 20 µm or less.   The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the piercing 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 non-aqueous electrolyte secondary battery separator comprising the microporous polyolefin membrane according to claim 1.   9. The non-aqueous electrolyte secondary battery separator 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.