CN113024881A

CN113024881A - Polyolefin microporous membrane, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Info

Publication number: CN113024881A
Application number: CN202110235715.5A
Authority: CN
Inventors: 松下胜彦; 春本亘祐
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2017-03-31
Filing date: 2018-03-19
Publication date: 2021-06-25
Also published as: JP6665966B2; KR102126212B1; JPWO2018180714A1; WO2018180714A1; KR20190127663A; TWI700851B; CN110431176B; CN113024882A; CN110431176A; TW201838224A

Abstract

The present invention addresses the problem of providing a polyolefin microporous membrane that has excellent impact resistance and battery characteristics when incorporated as a separator in a battery. The present invention is a polyolefin microporous membrane having the following characteristics (1) to (4). (1) The tensile strength (MPa) and tensile elongation (%) in the MD and TD directions satisfy the following relational expression (I) [ (tensile strength in MD x tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2The formula (I) is more than or equal to 300 … …; (2) tensile strength in the MD direction and TD direction is 196MPa or more; (3) the porosity is more than 40%; (4) the ratio of the tensile elongation in the MD direction and the tensile elongation in the TD direction, i.e., the tensile elongation in the MD direction/the tensile elongation in the TD direction, is 0.75 to 1.25.

Description

Polyolefin microporous membrane, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

The present application is a divisional application of the invention having an application date of 2018, 3/19, an application number of 201880020119.1, and an application name of "polyolefin microporous membrane, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery".

Technical Field

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

Background

The microporous membrane can be used in various fields such as filters such as filtration membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among them, a polyolefin microporous membrane using polyolefin as a resin material has been widely used as a battery separator in recent years because it is excellent in chemical resistance, insulation properties, mechanical strength, and the like, and has shutdown characteristics.

Secondary batteries, for example, lithium ion secondary batteries, have been widely used as batteries for personal computers, cellular phones, and the like because of their high energy density. In addition, the secondary battery is also used as a power source for driving an engine of an electric vehicle or a hybrid vehicle.

In particular, in the case of large-sized high-capacity lithium ion batteries, battery characteristics and higher reliability are important, and even separators used in these batteries are required to have high impact resistance from the viewpoint of safety.

In order to improve the impact resistance of the separator, high film strength and high elongation are required. However, the strength and the elongation of the polyolefin microporous membrane are inversely related, and it is difficult to increase the strength of the membrane while maintaining the elongation. To date, there have been some reports on polyolefin microporous films having improved film strength or elongation.

For example, patent document 1 describes a microporous polyolefin membrane having a porosity of 10% or more and less than 55%, a tensile strength in MD and TD of 50 to 300MPa, a total of the tensile strength in MD and the tensile strength in TD of 100 to 600MPa, a tensile elongation in MD and TD of 10 to 200%, and a total of the tensile elongation in MD and the tensile elongation in TD of 20 to 250%. According to patent document 1, the polyolefin microporous membrane is not easily deformed and is excellent in the membrane rupture resistance and the stress relaxation property.

Patent document 2 describes a polyolefin microporous membrane having a ratio of a tensile strength in a longitudinal direction to a tensile strength in a width direction of 0.75 to 1.25, and a heat shrinkage rate in the width direction at 120 ℃ of less than 10%. According to patent document 2, the polyolefin microporous membrane has good resistance to foreign substances and the like.

Patent document 3 describes a polyolefin microporous membrane containing polypropylene, and having a transverse tensile breaking strength of 100 to 230MPa, a transverse tensile breaking elongation of 10 to 110%, and a longitudinal tensile breaking strength of 0.8 to 1.3 with respect to the transverse tensile breaking strength.

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

Patent document 1 Japanese patent laid-open No. 2006-124652

Patent document 2 International publication No. 2010/070930

Patent document 3 International publication No. 2009/123015

Patent document 4 Japanese patent laid-open publication No. 2013-234263

Disclosure of Invention

Patent documents 1 to 4 describe a polyolefin microporous membrane having improved tensile strength and tensile elongation while maintaining battery characteristics, but further improvement in impact resistance is required in accordance with recent improvement in battery performance. Further, it is more difficult to achieve battery performance such as strength, elongation, output characteristics, and cycle characteristics, and a separator having battery characteristics such as impact resistance and output characteristics is desired.

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

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 and TD directions satisfy the following relational expression (I).

[ (tensile Strength in MD X tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2Not less than 300 … … formula (I)

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

(3) The maximum pore diameter measured by a Palm Porometer is 60nm or less.

(4) The average flow pore diameter measured by a Palm Porometer was 40nm or less.

(5) The porosity is 40% or more.

The polyolefin microporous membrane of the present invention may have the following characteristics (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 to 1.2.

The polyolefin microporous membrane of the present invention may have the following characteristics (7).

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

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

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

The puncture strength of the polyolefin microporous membrane may be 5N or more in terms of a film thickness of 12 μm. The polyolefin microporous membrane may have tensile strength (MPa) and tensile elongation (%) in the MD and TD directions satisfying the following relational expression (II).

[ (tensile Strength in MD X tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2≥350……(II)。

The present invention also provides a separator for a nonaqueous electrolyte secondary battery, which is obtained by using the polyolefin microporous membrane of the present invention.

The present invention also provides a nonaqueous electrolyte secondary battery comprising the separator for a nonaqueous electrolyte secondary battery of the present invention.

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

Detailed Description

The present embodiment of the present invention will be explained below. The present invention is not limited to the embodiments described below.

1. Polyolefin microporous membrane

In the present specification, the polyolefin microporous membrane refers to a microporous membrane containing polyolefin as a main component, and for example, refers to a microporous membrane containing polyolefin in an amount of 90 mass% or more based on the total amount of the microporous membrane. The physical properties of the polyolefin microporous membrane of the present embodiment will be described below.

[ relationship between tensile Strength and tensile elongation ]

When a polyolefin microporous membrane has only high tensile strength or high tensile elongation, impact resistance may be insufficient. The present inventors have found that in order to obtain a polyolefin microporous membrane having higher impact resistance, it is important to have high tensile strength and high tensile elongation (good isotropy) in a good balance between the MD direction (mechanical direction, longitudinal direction) and the TD direction (direction perpendicular to the MD direction: width direction, transverse direction when the polyolefin microporous membrane is viewed in plan view). The present inventors have also found that a polyolefin microporous membrane having very excellent impact resistance is obtained when the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction have a specific relationship.

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

[ (tensile Strength in MD X tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2Not less than 300 … … formula (I).

In addition, from the viewpoint of further improving the impact resistance, the relationship between the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction preferably satisfies the following formula (II), and more preferably satisfies the following formula (III).

[ (tensile Strength in MD X tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2Not less than 330 … … type (II)

[ (tensile Strength in MD X tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2350 to 350 … ….

Note here that the above [ (tensile strength in MD direction. times. tensile elongation in MD direction/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2The upper limit of the value of (A) is not particularly limited, and from the viewpoint of shrinkage characteristics,for example, 1000 or less, preferably 800 or less, and more preferably 600 or less.

[ tensile Strength ]

The polyolefin microporous membrane of the present embodiment has tensile strengths in the MD direction and the TD direction of 196MPa or more, preferably 200MPa or more, and more preferably 230MPa or more, respectively. When the tensile strength is in 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 film breakage due to foreign matter, impact, or the like in the battery can be suppressed. From the viewpoint of shrinkage resistance, the upper limit of the tensile strength in the MD direction and the TD direction is preferably 500MPa or less, more preferably 450MPa or less, and still more preferably 400MPa or less. The tensile strength can be measured by a method according to ASTM D882 using a long test piece having a width of 10 mm.

[ tensile elongation ]

The polyolefin microporous 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, the occurrence of film rupture and short circuit (short circuit) of the separator is suppressed by the flexibility of the separator when an impact is applied to the inside of the battery. The upper limit of the tensile elongation in the MD and TD 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 separator is not deformed by elongation at the time of winding the electrode, and the winding property is good. The tensile elongation can be measured by a method according to ASTM D-882A.

[ tensile Strength in MD/tensile Strength in TD ]

The ratio of the tensile strength in the MD direction and the TD direction (tensile strength in the MD direction/tensile strength in the TD direction) of the microporous polyolefin membrane of the present embodiment is preferably 0.8 to 1.2. When the ratio of the tensile strength is in the above range, the impact resistance is improved and stabilized to suppress film breakage and short circuit (short circuit) because the force is applied more uniformly against the impact in all directions.

[ tensile elongation in MD/tensile elongation in TD ]

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) of the microporous polyolefin membrane of the present embodiment is preferably 0.75 to 1.25. When the ratio of tensile elongation is in the above range, the impact resistance is improved and stabilized to suppress film breakage and short circuit (short circuit) because the force is applied more uniformly against the impact in all directions.

From the viewpoint of more stabilizing the impact in all directions and suppressing film breakage, it is preferable that the ratio between the tensile strength and the tensile elongation is close to 1. Further, when the tensile strength in the MD direction is too high, tearing in the MD direction may occur. If the TD tensile strength is too high, TD tearing or separation of the electrode tab bonded portions may occur, which may cause short-circuiting.

[ puncture Strength ]

The puncture strength of the polyolefin microporous membrane is preferably 5N or more, more preferably 5.2N or more, and still more preferably 6N or more, when the puncture strength is 12 μm in terms of the membrane thickness. The upper limit of the puncture strength is not particularly limited, and is, for example, 10N 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 has excellent resistance to unevenness of an electrode, impact, and the like, and can suppress occurrence of short circuit of the electrode and the like.

The piercing strength was measured by piercing the film thickness T at a rate of 2 mm/sec with a needle having a diameter of 1mm and a spherical tip (radius of curvature R: 0.5mm)₁The maximum load (N) at the time of the polyolefin microporous membrane (μm) was measured. In addition, regarding the film thickness T₁The puncture strength (N/12 μm) of the microporous polyolefin membrane was calculated as 12 μm in film thickness according to the following equation.

Formula (II): puncture strength (converted to 12 μm) measured as puncture strength (N) × 12(μm)/film thickness T₁(μm)

[ film thickness ]

The upper limit of the thickness of the polyolefin microporous membrane is not particularly limited, and is, for example, 20 μm or less, preferably 17 μm or less, and more preferably 13 μm or less. When the film thickness is in the above range, the permeability and the film resistance are more excellent, and the battery capacity can be improved by making the film thin. 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.

[ void fraction ]

The porosity of the polyolefin microporous membrane is preferably 40% or more, more preferably 40% or more and 70% or less, when used as a battery separator. From the viewpoint of film formability, mechanical strength, and insulation properties, the upper limit of the porosity is more preferably 60% or less, and still more preferably 55% or less. When the porosity is in the above range, the amount of electrolyte to be held can be increased, high ion permeability can be ensured, and the output characteristics are excellent. When the porosity is low, when the porous material is used as a battery separator, the output characteristics may be poor due to an increase in fibrils that inhibit ion permeation and a decrease in the content of an electrolyte solution, and the cycle characteristics may be rapidly deteriorated due to an increase in clogging caused by-products generated during a battery reaction. The porosity can be adjusted to the above range by adjusting the composition, the draw ratio, and the like of the polyolefin resin during the production process.

Weight w to microporous membrane₁And the weight w of the void-free polymer equivalent thereto₂The void ratio can be measured by comparing (polymers having the same width, length, and composition) and using the following formula (1).

Void ratio (%) - (w)₂-w₁)/w₂×100……(1)。

[ average flow pore diameter ]

The polyolefin microporous membrane has an average pore diameter (average flow pore diameter) of 40nm or less, preferably 10nm or more and 40nm or less. When the average pore diameter is within the above range, the balance between strength and permeability is excellent, and self-discharge originating from coarse pores can be suppressed. When the average pore diameter exceeds 40nm, the ion-permeable flow path selectively concentrates in large pores, which may result in an increase in resistance or a partial blockage of the electrolyte decomposition by-product, which may result in deterioration of cycle characteristics. The average pore diameter is a value determined by a method (semi-dry method) according to ASTM E1294-89. A Palm Porometer (model: CFP-1500A) manufactured by PMI was used as the measuring instrument, and Galwick (15.9dyn/cm) was used as the measuring solution.

[ maximum pore diameter (bubble point diameter) ]

The maximum pore diameter (bubble point diameter: BP diameter) is preferably 60nm or less, more preferably 30nm or more and 60nm or less. When the maximum pore diameter exceeds 60nm, the positive electrode and the negative electrode may come into contact with each other (minute short circuit) or be broken by lithium dendrite (dendrite) to cause short circuit. On the other hand, when the maximum pore diameter is too small, the resistance of the battery may increase, the cycle performance may become insufficient, and the capacity retention rate at the time of high-rate discharge may decrease.

[ air resistance ]

The upper limit of the gas barrier property of the polyolefin microporous membrane in terms of the thickness of the membrane of 12 μm is not particularly limited, but is, for example, 300 seconds/100 cm³Air/12 μm or less, preferably 200 sec/100cm³Air/12 μm or less. The lower limit of the air resistance is, for example, 50 seconds/100 cm³Air or above. When the gas resistance is in the above range, the ion permeability is excellent when the separator is used as a battery separator, and the resistance of a secondary battery incorporating the separator is reduced, and the output characteristics or rate characteristics are improved. The gas barrier can be adjusted to the above range by adjusting the stretching conditions and the like in the production of the polyolefin microporous membrane.

The air resistance is a value P which can be measured by an air permeability meter (EGO-1T, manufactured by Asahi Seiki Kasei Kogyo Co., Ltd.) according to JIS P-8117 Wangshan tester method₁(seconds/100 cm)³Air). In addition, regarding the film thickness T₁(μm) air resistance P of the microporous membrane in terms of film thickness of 12 μm₂(seconds/100 cm)³Air/12 μm)) is a value which can be obtained by the following formula.

Formula (II): p₂＝P₁(seconds/100 cm)³Air). times.12 (μm)/film thickness T₁(μm)

2. Process for producing polyolefin microporous membrane

The method for producing the polyolefin microporous membrane is not particularly limited as long as the polyolefin microporous membrane having the above-described characteristics can be obtained. A known method for producing a polyolefin microporous membrane can be used. The method for producing the microporous polyolefin membrane of the present embodiment is preferably a wet film-forming method from the viewpoint of ease of control of the structure and physical properties of the membrane. As a wet film forming method, for example, the methods described in japanese patent No. 2132327 and japanese patent No. 3347835, international publication No. 2006/137540, and the like can be used.

An example of a method for producing a polyolefin microporous membrane (wet film-forming method) will be described below. The following description is an example of a manufacturing method, and is not limited to this method.

(1) Preparation of polyolefin solutions

First, a polyolefin resin used as a raw material and a film-forming solvent are melt-kneaded to prepare a polyolefin solution. As a 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 known, the description thereof is omitted.

(polyolefin resin)

As the polyolefin resin used 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, and for example, there can be used: ultra-high molecular weight polyethylene (UHMwPE), High Density Polyethylene (HDPE), medium density polyethylene, branched low density polyethylene, linear low density polyethylene, and the like. The polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another α -olefin. As the α -olefin, there may be mentioned: propylene, 1-butene, 1-hexene, 1-pentene, 4-methyl-1-pentene, octene, vinyl acetate, methyl methacrylate, styrene, etc.

Preferably the polyolefin resin comprises ultra high molecular weight polyethylene (UHMwPE). When the ultra-high molecular weight polyethylene is contained, the film strength of the obtained polyolefin microporous film can be improved. Further, the fibrils of the polyolefin microporous membrane can be made fine (densified), and a membrane having a small pore diameter can be uniformly developed as a whole. The ultra-high-molecular-weight polyethylene may be used alone or in combination of two or more, and for example, two or more types of ultra-high-molecular-weight polyethylene having different Mw may be mixed with each other and used.

The weight-average molecular weight (Mw) of the ultrahigh-molecular-weight polyethylene is preferably 1X 10⁶More than (10 ten thousand or more), preferably 2X 10⁶Above and less than 4 × 10⁶. When Mw is in the above range, film formability is good. When the Mw of the ultra-high molecular weight polyethylene is 4X 10⁶In the above case, the viscosity of the melt may become too high to extrude the resin from the nozzle (die), which may cause a problem in the film-forming step. 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, and more preferably 20% by mass or more, based on 100% by mass of the entire polyolefin resin. The upper limit of the content of the ultrahigh-molecular weight polyethylene is not particularly limited, and is, for example, 50% by mass or less. When the content of the ultrahigh molecular weight polyethylene is in the above range, the film strength and the gas barrier property can be both obtained at a high level by adjusting the stretching conditions and the like described later.

The polyolefin resin may contain high-density polyethylene (HDPE, density: 0.942 g/cm)³Above). In addition, it is preferable that the polyolefin resin contains ultra-high molecular weight polyethylene and high density polyethylene. When the high-density polyethylene is contained, the melt extrusion property is excellent, and the uniform drawing property is excellent. As the high-density polyethylene, a polyethylene having a weight average molecular weight (Mw) of 1X 10 can be exemplified⁴Above and less than 1 × 10⁶The high density polyethylene of (1). Mw is a value measured by Gel Permeation Chromatography (GPC). The content of the high-density polyethylene is preferably 50 mass% or more and 90 mass% or less, and more preferably 50 mass% or more and 80 mass% or less, with respect to 100 mass% of the entire polyolefin resin.

The polyolefin resin may further contain polypropylene. The polypropylene is not particularly limited, and a homopolymer of propylene, a copolymer of propylene with other α -olefin and/or diene (propylene copolymer), or a mixture thereof can be used. The content of the polypropylene is, for example, 0 mass% or more and less than 10 mass%, preferably 0 mass% or more and 5 mass% or less, with respect to 100 mass% of the entire polyolefin resin. In addition, when polypropylene is contained, the pore diameter of the obtained polyolefin microporous membrane tends to be large.

The polyolefin resin may contain other resin components than polyethylene and polypropylene, as required. As the other resin component, for example, a heat-resistant resin or the like can be used. The polyolefin microporous membrane may further contain various additives such as an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an anti-blocking agent, a filler, a crystal nucleating agent, and a crystal retarder, as long as the effects of the present invention are not impaired.

(solvent for film formation)

The film-forming solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin resin. The film-forming solvent is preferably a solvent that is liquid at room temperature in order to enable stretching at a high magnification. Examples of the film-forming solvent include: aliphatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin, cyclic aliphatic hydrocarbons or aromatic hydrocarbons, 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 them, a nonvolatile liquid solvent such as liquid paraffin is preferably used. The solvent may be a solvent that is solid at room temperature and is mixed with the film-forming solvent. Examples of such solid solvents include: stearyl alcohol, hexacosanol, paraffin wax (paraffin wax), and the like.

(polyolefin solution)

The blending ratio of the polyolefin resin and the film-forming solvent in the polyolefin solution is not particularly limited, and the polyolefin resin is preferably 20 to 35 parts by mass per 100 parts by mass of the polyolefin resin solution. When the proportion of the polyolefin resin is within the above range, expansion or contraction at the die exit can be prevented when the polyolefin solution is extruded, and the moldability and self-supporting property of the extruded molded article (gel-like molded article) can be improved.

(2) Formation of gel-like sheet

Next, a gel-like sheet was formed by feeding the polyolefin solution prepared in the above to a die from an extruder, extruding in a sheet form, and cooling the resulting extrusion-molded body. The cooling is preferably performed to 90 ℃ or lower, which is the crystal dispersion temperature (Tcd) of the polyolefin resin, more preferably to 50 ℃ or lower, and still more preferably to 40 ℃ or lower. Upon cooling, the microphase of the polyolefin separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallinity 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 refrigerant such as cold air or cooling water, a method of contacting with a cooling roller, or the like can be used, and it is preferable to cool by contacting with a roller cooled with a refrigerant. It should be noted that a plurality of polyolefin solutions having the same or different compositions may be fed from a plurality of extruders to a die, laminated in layers, and extruded in a sheet form. As a method for forming the gel-like sheet, for example, methods disclosed in japanese patent No. 2132327 and japanese patent No. 3347835 can be used.

(3) Extension of

Next, the gel-like sheet is stretched at least in a uniaxial direction. The extension of the gel-like sheet is also referred to as wet extension. The stretching may be uniaxial stretching or biaxial stretching, and preferably biaxial stretching. In the case of biaxial stretching, the stretching may be any of simultaneous biaxial stretching, sequential stretching and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching), preferably sequential stretching, and preferably stretching in the MD direction (machine direction, longitudinal direction) and then in the TD direction (width direction, transverse direction). When the stretching is performed in the MD direction and the TD direction, it is considered that the molecular orientation is facilitated by applying stretching tension only in each direction during the stretching. The TD direction is a direction perpendicular to the MD direction when the microporous membrane is viewed in plan.

The final area draw ratio (area ratio) in the drawing step is required to be 30 times or more and 150 times or less. When the area magnification is within the above range, the film forming property is good, and the proportion of unoriented free molecules is reduced, so that a polyolefin microporous film having high strength can be obtained. The area draw ratio is preferably 35 times or more and 120 times or less. Further, the stretching ratio in both MD and TD is preferably more than 5 times.

It is necessary to set the ratio of the MD stretch ratios to the TD stretch ratios (MD stretch ratio/TD stretch ratio) to 0.7 or more and 1.0 or less. When the ratio of the draw ratio is in the above range, the balance between the MD direction and the TD direction becomes good with respect to the tensile strength and the tensile elongation of the obtained polyolefin microporous membrane, and the membrane strength can be further improved to improve the impact resistance. From the viewpoint of further improving the film strength, the TD-direction draw ratio is preferably larger than the MD-direction draw ratio. The reason for this is not particularly limited, and it is considered that when stretching in the TD direction after stretching in the MD direction, molecular orientation in the MD direction once becomes difficult to orient in the TD direction by stretching in the MD direction, and therefore, stretching in the TD direction with a larger magnification makes it possible to more uniformly orient molecules in both directions. The draw ratio in this step means: the stretching ratio of the gel-like sheet immediately before the present step was defined as the stretching ratio of the gel-like sheet immediately before the next step. The ratio of the MD directional stretch ratios (MD directional stretch ratio/TD directional stretch ratio) is preferably 0.75 to 1.0.

The stretching temperature is preferably in the range of not lower than the crystal dispersion temperature (Tcd) of the polyolefin resin but not higher than the melting point of the polyolefin resin. Here, the melting point of the polyolefin resin means the melting point of the polyolefin resin in the gel-like 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 is suppressed, and the molecular chains can be efficiently oriented by stretching. When the stretching temperature is not less than the crystal dispersion temperature (Tcd) of the polyolefin resin, the polyolefin resin in the gel-like sheet can be sufficiently softened to reduce the stretching tension, and therefore, the film formability is improved, film breakage during stretching can be suppressed, and stretching at a high magnification can be performed. The stretching temperature may be set to, for example, 100 ℃ or higher and 127 ℃ or lower. Here, the stretching temperature is a temperature of the gel sheet, and is a thickness direction center temperature when there is a difference in surface and back temperatures such as roll stretching.

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. As a result, the stretching in the MD direction makes it difficult to orient the molecules in the TD direction once, and therefore, the stretching in the TD direction at a higher temperature makes it possible to more uniformly orient the molecules in both directions. The stretching temperature in the MD is 100 ℃ to 110 ℃, preferably 103 ℃ to 110 ℃. The stretching temperature in the TD direction is 115 ℃ to 127 ℃, preferably 115 ℃ to 125 ℃. When the stretching temperatures in the MD direction and the TD direction are in the above ranges, the film forming property is good, the film strength of the obtained polyolefin microporous film can be improved, and the pore diameter can be controlled to an appropriate range.

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

Then, the film-forming solvent is removed from the stretched gel-like sheet to obtain a microporous film. The solvent is removed by washing with a washing solvent. Since the polyolefin phase and the film-forming solvent phase are phase-separated, when the film-forming solvent is removed, a porous film containing fibrils forming a fine three-dimensional mesh structure and having pores (voids) irregularly connected in three dimensions can be obtained. Since a cleaning solvent and a method for removing a film-forming solvent using the same are known, a description thereof will be omitted. For example, the method disclosed in the specification of Japanese patent No. 2132327 or Japanese patent laid-open publication No. 2002-256099 can be used.

(5) Drying

Then, 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 not more than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably not less than 5 ℃ lower than Tcd. The microporous membrane is preferably dried to a residual cleaning solvent of 5% by mass or less, more preferably dried to a residual cleaning solvent of 3% by mass or less, based on 100% by mass (dry weight) of the microporous membrane. When the residual cleaning solvent is within the above range, the porosity of the polyolefin microporous membrane can be maintained, and deterioration of permeability can be suppressed.

(6) Others

Further, the dried microporous membrane may be subjected to heat treatment. As the heat treatment method, a heat setting treatment and/or a heat relaxation treatment may be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the TD dimension of the film constant. The thermal relaxation treatment refers to a treatment of 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 japanese patent application laid-open No. 2002-256099 is given. The heat treatment temperature is preferably within the range of Tcd to Tm of the polyolefin resin, more preferably within the range of ± 5 ℃ of the second stretching temperature of the microporous membrane, and particularly preferably within the range of ± 3 ℃ of the second stretching temperature of the microporous membrane.

The dried microporous membrane may be re-stretched at a predetermined area stretch ratio at least in a uniaxial direction. The stretching of the dried microporous membrane is also referred to as dry stretching.

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

The polyolefin microporous membrane may be a single layer, or a layer containing the polyolefin microporous membrane may be laminated. The multilayer polyolefin microporous membrane may be provided as two or more layers. In the case of a multilayer polyolefin microporous film, the polyolefin resins constituting the respective layers may have the same composition or different compositions.

The polyolefin microporous membrane may be formed by laminating a porous layer other than the polyolefin resin on at least one surface thereof to form a laminated polyolefin porous membrane. The other porous layer is not particularly limited, and 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, and for example, acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, 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, and for example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, and the like can be used. The laminated polyolefin porous film may be a film obtained by laminating the binder resin, which is formed into a porous state, on at least one surface of the polyolefin microporous film.

By appropriately adjusting the final area draw ratio (area ratio), the ratio of the MD and TD draw ratios (MD draw ratio/TD draw ratio), and the MD and TD draw temperatures in the drawing step described earlier in the present invention, it is possible to provide a polyolefin microporous membrane having excellent impact resistance and battery characteristics (output characteristics, dendrite resistance, etc.) at a high level when used as a battery separator.

Examples

The present invention will be described in further detail below with reference to examples. The present invention is not limited to these examples.

1. Measurement method and evaluation method

[ film thickness ]

The thickness of the microporous membrane at 5 points in the range of 95mm × 95mm was measured by a contact thickness meter (Lightmatic manufactured by Mitutoyo Co., Ltd.) to obtain an average value.

[ void fraction ]

Weight w to microporous membrane₁And the weight w of the void-free polymer equivalent thereto₂(width, length, composition of the same polymer) by comparison, through the following formula for determination.

Void ratio (%) - (w)₂-w₁)/w₂×100

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

The measurement was carried out in the order of Dry-up and Wet-up using a Palm Porometer (trade name, model: CFP-1500A) of PMI. In Wet-up, a microporous membrane sufficiently impregnated with Galwick (trade name) having a known surface tension was subjected to pressure, and the pore diameter as converted from the pressure at which air starts to penetrate was defined as the bubble point pore diameter (maximum pore diameter). For the mean flow pore diameter, the pore diameter is scaled according to the pressure at the point where the curve representing the 1/2 slope of the pressure and flow curves in the Dry-up measurement intersects the curve for the Wet-up measurement. The following equation is used for conversion of the pressure and the pore diameter.

d＝C·γ/P

In the formula, "d (μm)" represents the pore diameter of the microporous membrane, "γ (mN/m)" represents the surface tension of the liquid, "p (pa)" represents the pressure, "C" represents a constant.

[ puncture Strength ]

A needle having a diameter of 1mm and a spherical tip (radius of curvature R: 0.5mm) was pierced through the film thickness T at a rate of 2 mm/sec₁(μm) maximum load L in microporous membrane₁(N) measurement was carried out. In addition, the measured value L of the maximum load₁By the formula: l is₂＝(L₁×12)/T₁The maximum load L was calculated for a film thickness of 12 μm₂(converted to 12 μm) (N/12 μm).

[ air resistance ]

For film thickness T₁The microporous membrane (μm) was measured for gas barrier property P measured with a gas permeability meter (EGO-1T, manufactured by Asahi Seiki Kasei K.K.) according to JIS P-8117 Wang's research method₁(sec/100cm³Air). In addition, by the formula: p₂＝(P₁×12)/T₁The air resistance P was calculated with the film thickness being 12 μm₂(converted to 12 μm) (sec/100 cm)³Air/12μm)。

[ weight average molecular weight (Mw) ]

The weight average molecular weight (Mw) of the polyolefin microporous membrane was determined by a Gel Permeation Chromatography (GPC) method under the following conditions.

The measurement device: GPC-150C manufactured by Waters Corporation

Column chromatography: shodex UT806M manufactured by Showa Denko K.K

Column temperature: 135 deg.C

Solvent (mobile phase): ortho-dichlorobenzene

Solvent flow rate: 1.0 ml/min

Sample concentration: 0.1 wt% (dissolution conditions: 135 ℃/1h)

Sample size: 500. mu.l

The detector: differential refractometer (RI detector) manufactured by Waters Corporation

Standard curve: from the calibration curve obtained using the monodisperse polystyrene standard sample, a polyethylene conversion constant (0.46) was used.

[ tensile Strength ]

The MD tensile strength and TD tensile strength were measured by a method in accordance with ASTM D882 using a long test piece having a width of 10 mm.

[ tensile elongation ]

Measured by a method according to ASTM D-882A.

[ impact resistance test ]

The cylindrical battery was fabricated according to the following procedure, and an impact test was performed.

< production of Positive electrode >

92.2% by mass of LiCoO, a lithium cobalt composite oxide as an active material₂The slurry was prepared by dispersing 2.3 mass% of flake graphite and acetylene black as conductive agents and 3.2 mass% of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). The slurry was applied by a die coater so that the active material application amount was 250g/m²Active material volume density of 3.00g/cm³The coating was applied to one surface of an aluminum foil having a thickness of 20 μm to be a positive electrode current collector. Then, the sheet was dried at 130 ℃ for 3 minutes, compression-molded by a roll press, and cut into a strip having a width of about 57mm to prepare a tape.

< production of negative electrode >

A slurry was prepared by dispersing 96.9 mass% of artificial graphite as an active material, 1.4 mass% of an ammonium salt of carboxymethyl cellulose as a binder, and 1.7 mass% of a styrene-butadiene copolymer latex in pure water. The slurry was applied by a die coater so that the active material application amount was 106g/m²The active material volume density is 1.55g/cm³Is applied to one surface of a copper foil having a thickness of 12 μm to be a negative electrode current collector. Then, the sheet was dried at 120 ℃ for 3 minutes, compression-molded by a roll press, and cut into a tape having a width of about 58 mm.

< preparation of nonaqueous electrolyte solution >

LiPF as a solute was added so that the concentration became 1.0mol/l₆The mixture was dissolved in 1/2 (volume ratio) to prepare a solution.

< diaphragm >

The separators described in examples and comparative examples were cut into 60mm strips.

< Battery Assembly >

The strip-shaped negative electrode, the separator, the strip-shaped positive electrode, and the separator were stacked in this order, and wound in a spiral shape a plurality of times with a winding tension of 250gf, thereby producing an electrode plate laminate. The electrode plate laminate was housed in a stainless steel container having an outer diameter of 18mm and a height of 65mm, and an aluminum tab derived from the positive electrode current collector was welded to the container lid terminal portion, and a nickel tab derived from the negative electrode current collector was welded to the container wall. Then, the nonaqueous electrolytic solution was dried at 80 ℃ for 12 hours under vacuum, and then the container was filled with the nonaqueous electrolytic solution in an argon box, and the container was sealed.

< impact resistance test >

First, the assembled batteries were charged at a constant current of 500mA, and after the battery voltages reached 4.20V, the batteries were charged at constant voltages until the current values reached 10mA or less, thereby obtaining fully charged batteries. Next, the fully charged cylindrical battery was set with the long side in the lateral direction, and a rod having a diameter of 15.8mm and a mass of 9.1kg was dropped from a height of 61cm onto the central flat surface of the battery, thereby applying an impact to each battery. The battery was evaluated as "x" in 1 out of 3 tests in which ignition was caused by the impact, as "Δ" in 3 tests in which ignition was not caused but smoke was caused in 1 out of 3 tests, and as "o" in 3 tests in which neither ignition nor smoke was caused in 1 out of 3 tests.

[ dendrite resistance characteristics ]

The maximum pore diameter was defined as ≈ 60nm, and the other cases were defined as ×. If the maximum pore diameter is large, exceeding 60nm, lithium dendrites (dendrites) generated by the precipitation of Li metal peculiar to lithium ion secondary batteries tend to easily enter the pores. The separator is thus easily damaged, resulting in a small short circuit due to the design of the battery.

[ film resistance (impedance) ]

From the porous film, 5 circular shape measurement samples having a diameter of 19mm and 20 circular shape measurement samples having a diameter of 16mm were cut. Further, members (a case, a PP gasket, a separator (diameter 16mm, thickness 1mm), a gasket, and a cap) of a CR2032 type coin cell were prepared (manufactured by baoquan corporation).

First, in a drying chamber in which the dew point temperature was-35 ℃ or lower, a sample for measurement (diameter 19mm) × 1 was placed on a case, a spacer was placed to fix the sample, and a plurality of the samples for measurement (diameter 16mm) ×, a spacer, and a wave washer were sequentially placed thereon. The number of measurement samples having a diameter of 16mm was set to 2, 3, and 4, and batteries each having the above-described number of measurement samples were prepared. Then, the electrolyte solution was injected into LiPF in the cell provided with the wave washer₆A mixed solvent containing Ethylene Carbonate (EC) and Ethyl Methyl Carbonate (EMC) (EC/EMC: 4:6[ volume ratio ]]) The electrolyte solution (KISHIDA CHEMICAL co., ltd., manufactured) at a concentration of 1M. After the injection, the battery was allowed to stand under a pressure of about-50 kPa for 10 minutes to impregnate the measurement sample with the electrolyte. Then, a cover is covered on the batteryThe sample cell was sealed with a button cell riveting device (manufactured by baoquan corporation).

The obtained sample cell was left to stand in a thermostatic bath at 25 ℃ for 3 hours, and then the resistance of the cell was measured at an amplitude of 20mV by an AC impedance measuring apparatus (manufactured by Nichikoku corporation). The measured value of the resistance component of the cell (the value of the real number when the imaginary axis is 0) is plotted against the number of porous membranes arranged in the cell, and the slope is determined by linear approximation of the plot. Multiplying the slope by the area of the diaphragm (2.01 cm)²(＝(1.6cm/2)²X π) as the value of the membrane resistance of the porous film (Ω · cm)²)。

The value of the membrane resistance (Ω · cm) of the porous membrane²) Is 1.4 omega cm²The value of the membrane resistance of the porous membrane (. omega. cm) was determined to be good when the value was not more than 10 μm²) Over 1.4. omega. cm²The case of 10 μm was X (bad).

If the membrane resistance (impedance) is 1.4. omega. cm²When the particle diameter is 10 μm or less, the output characteristics of the battery can be expected to be good when the particle diameter is used as a battery separator in a secondary battery.

[ cycle life ]

< production of Battery for test >

The wound body was prepared using a positive electrode (manufactured by yatsuka corporation), a negative electrode (manufactured by yatsuka corporation) and each microporous membrane with a tab. Next, the wound body was placed in an aluminum laminate bag, and 750. mu.L of an electrolyte (1.1mol/L, LiPF) was added dropwise₆A solution of 0.5 wt% of vinylene carbonate and 2 wt% of fluorine-containing ethylene carbonate was added to 3/5/2 (volume ratio) of ethylene carbonate/ethylmethyl carbonate/diethylene dicarbonate, and the mixture was sealed in a vacuum laminator. This was used as a 300mAh test cell.

< test of cycle Performance >

The above test cell was used to carry out a cycle performance test under the following charge and discharge conditions.

Charging: constant current, constant voltage charging, Cutoff (Cutoff) current 0.05C at 1C, 4.35V

Discharging: 1C, 3V constant current discharge

Measuring temperature: 25 deg.C

The test was performed using 3 test cells, and an average value of the capacity retention rate, which is a ratio of the charge capacity at the 200 th time based on the 1 st time 1C charge capacity, was derived as an index of the cycle performance. The case where the average value of the capacity retention rate was 85% or more was regarded as "good", and the case where the average value of the capacity retention rate was less than 85% was regarded as "poor".

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

(examples 1 to 5)

Using a twin-screw extruder, the Mw of the polyolefin resin contained in the blend ratios (mass%) shown in Table 1 was 2.5X 10⁶And an Mw of 2.8X 10⁵A polyolefin resin of High Density Polyethylene (HDPE), liquid paraffin (film forming solvent), and tetrakis [ methylene-3- (3, 5-di-tert-butyl-4-hydroxyphenyl) -propionate as an antioxidant]Methane (0.2 parts by mass per 100 parts by mass of the polyolefin resin) was melt-kneaded to prepare a polyolefin solution. The polyolefin resin concentration in the polyolefin solution is shown in table 1 with respect to 100 parts by mass of the total of the polyolefin resin and the film-forming solvent. The polyolefin solution was fed from the twin-screw extruder to a T-die and extruded. The extruded molded article was cooled while being pulled by a cooling roll to form a gel-like sheet. The gel-like sheet was wet-stretched in the MD direction and the TD direction under the conditions shown in table 1. After removing liquid paraffin from the wet-stretched gel-like sheet with methylene chloride and drying, the obtained polyolefin microporous membrane was re-stretched in the TD direction at the temperature and magnification shown in table 1 using a tenter stretcher, and then subjected to a thermal relaxation treatment at the same temperature. Regarding the relaxation rate (%) as the amount of the thermal relaxation treatment, the TD direction film width before the thermal relaxation treatment was defined as L₁And the TD-direction film width after the thermal relaxation treatment is defined as L₂The calculation is performed by the formula shown below.

Formula (II): relaxation rate (%) - (L)₁-L₂)/L₁×100

The evaluation results and the like of the obtained polyolefin microporous membrane are shown in table 1.

Comparative examples 1 to 13

A polyolefin microporous membrane was produced under the same conditions as in examples, 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).

[ Table 1]

[ Table 2]

Industrial applicability

The polyolefin microporous membrane of the present embodiment is very excellent in impact resistance when incorporated as a separator into a secondary battery. The polyolefin microporous membrane of the present embodiment can be preferably used as a separator for a nonaqueous electrolyte secondary battery because it can have both impact resistance and battery characteristics.

Claims

1. A polyolefin microporous membrane which contains 50% by mass or more of a polyethylene resin per 100% by mass of the entire polyolefin resin, has a membrane thickness of 13 [ mu ] m or less, and has the following characteristics (1) to (4):

(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 X tensile elongation in MD/100)²+ (tensile Strength in TD X tensile elongation in TD/100)²]^1/2The formula (I) is more than or equal to 300 … …;

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

(3) the porosity is more than 40%;

(4) the ratio of the tensile elongation in the MD direction and the tensile elongation in the TD direction, i.e., the tensile elongation in the MD direction/the tensile elongation in the TD direction, is 0.75 to 1.25.

2. The polyolefin microporous membrane according to claim 1, which has the following characteristics (5):

(5) the ratio of the tensile strength in the MD direction and the tensile strength in the TD direction, i.e., the tensile strength in the MD direction/the tensile strength in the TD direction, is 0.8 to 1.2.

3. The polyolefin microporous membrane according to claim 1 or 2, which has the following characteristics (6):

(6) the tensile elongation in the MD direction and the TD direction is 90% or more, respectively.

4. The polyolefin microporous membrane according to claim 1 or 2, having a puncture strength of 5N or more in terms of a membrane thickness of 12 μm.

5. The microporous polyolefin membrane according to claim 1 or 2, wherein the tensile strength (MPa) and the tensile elongation (%) in the MD direction and the TD direction satisfy the following relational formula (II),

6. A separator for a nonaqueous electrolyte secondary battery, which is obtained by using the polyolefin microporous membrane according to any one of claims 1 to 5.

7. The separator for a nonaqueous electrolyte secondary battery according to claim 6, wherein a coating layer containing inorganic particles and a binder resin is provided on at least one surface.

8. A nonaqueous electrolyte secondary battery comprising the separator for nonaqueous electrolyte secondary batteries according to claim 6 or 7.