WO2021015269A1 - Microporous polyolefin membrane and separator for nonaqueous electrolyte secondary batteries - Google Patents

Abstract

The present invention relates to a microporous polyolefin membrane which has a shrinkage ratio of less than 10% in the MD direction at 105°C/8h and a fibril diameter of from 10 nm to 50 nm, wherein if a 11.7 μm × 9.4 μm rectangular field of view of a surface SEM observation image as obtained by the observation with a scanning electron microscope (SEM) is divided into 0.5 μm × 0.5 μm pieces, the sum of resin areas in the pieces satisfying R2 ≥ R1, namely (non-opening parts a in the surface) is 20 μm2 or less in the 11.7 μm × 9.4 μm area of the surface SEM observation image.

Description

Polyolefin microporous membrane and separator for non-aqueous electrolyte secondary battery

The present invention is a polyolefin microporous membrane widely used as a separation membrane used for separation of substances, selective permeation, etc., and a separating material for an electrochemical reaction device such as an alkaline battery, a lithium secondary battery, a fuel cell, and a capacitor. Also referred to as a porous polyolefin film). In particular, the present invention is a polyolefin microporous film preferably used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion battery, which exhibits excellent battery characteristics and is high as compared with a conventional polyolefin microporous film. Used as a safe separator.

Polyolefin microporous membranes are used as filters, fuel cell separators, condenser separators, etc. In particular, it is suitably used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion battery widely used in notebook personal computers, mobile phones, digital cameras and the like. The reason is that the polyolefin microporous membrane has excellent mechanical strength, shutdown characteristics, and ion permeation performance.

In particular, in recent years, lithium-ion secondary batteries have been used for in-vehicle applications, so it is necessary to shorten the charging time and improve acceleration, and the required characteristics of batteries include quick charging (large current charging) and increased power consumption (large current). Discharge) is required. Along with this, the improvement of output characteristics has become even higher as a requirement for separators.

In addition, as the cruising range of automobiles increases, the capacity of batteries is increasing, and there is a further demand for thinner separators. However, thinning the separator reduces its strength, so short circuits due to electrodes and foreign matter (foreign matter resistance) and film rupture (decrease in impact resistance) when the battery is impacted are likely to occur, and the safety of the battery is reduced. descend. Therefore, higher strength than before is required. In addition, in a battery with high energy, the temperature inside the battery continues to rise even if the progress of the electrochemical reaction is stopped by the shutdown function of the separator, and as a result, the separator heat shrinks and ruptures, and both electrodes. Has a problem of short-circuiting. Therefore, the separator is required to have high strength, low shrinkage, and high output characteristics.

As a method for improving the output characteristics, Patent Document 1 describes a method for improving ionic conductivity by blending polyethylene (PE) and polypropylene (PP) to increase the porosity and controlling the number of pores, the pore diameter and the curvature ratio. It is disclosed.

Patent Document 2 discloses a method of controlling the pore structure on the surface and the inside, improving the impregnation property of the electrolytic solution, and improving the output characteristics.

Patent Document 3 discloses a method for improving output characteristics and cycle characteristics by controlling the volatility of the porosity per unit volume and forming a uniform pore structure.

Patent Document 4 discloses a method of reducing the fiber diameter (fibril diameter) of the microporous membrane to improve the output characteristics.

In addition, as a method for increasing the strength, a method such as increasing the draw ratio, increasing the resin concentration, or using a high molecular weight raw material is usually taken. In Patent Document 5, as a method of achieving both strength and shrinkage characteristics, polyethylene having a molecular weight of 280,000 and polyethylene having a molecular weight of 2 million are blended, and under the condition of a resin concentration of 32 wt%, 7.0 times in the MD direction and 6. in the TD direction. A method of stretching 4 times, washing and drying, and then stretching 1.2 times (area magnification 54 times) is described.

Further, in Patent Documents 6 and 7, high molecular weight polyethylene is stretched to 6.4 × 6.0 times (area magnification 38.4 times) to control the ratio of crystal portions contained in the fiber structure (fibril structure). Discloses a method of arranging ion permeability, strength and shutdown characteristics side by side.

Japanese Patent Application Laid-Open No. 2017-140840 Japanese Patent Application Laid-Open No. 2012-48987 Japanese Patent Application Laid-Open No. 2016-191069 Japanese Patent Application Laid-Open No. 2014-15697 Japanese Patent Application Laid-Open No. 2006-124652 Japanese Patent Application Laid-Open No. 2018-147858 Japanese Patent Application Laid-Open No. 2018-147690

However, in the conventional microporous membranes as described in Patent Documents 1 to 3, it is necessary to control the pore structure such as increasing the pore ratio and increasing the number of pores and through holes in order to improve the output characteristics. Therefore, sufficient strength could not be obtained, and it was insufficient to achieve both high output characteristics and strength required for in-vehicle applications and the like.

Even in the method of miniaturizing the fibril diameter described in Patent Document 4, the pore ratio is as high as 70%, and it is considered that there is a problem in achieving both strength and strength.

Patent Documents 5 to 7 describe methods for improving the balance between strength and shrinkage rate and strength and output characteristic battery. However, the thin film of the separator, high strength, low shrinkage rate, and the like as the capacity of the battery increases. It was not sufficient from the viewpoint of parallelizing high output characteristics.

In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane having better output characteristics, strength, and shrinkage rate than before.

Therefore, the present inventors have described the lumpy portion of a separator having a fibrous network structure composed of a resin component (hereinafter referred to as an unopened portion, which is not fibrillated by stretching, or the fibrils are fused in a heat treatment step. We focused on the relationship between the resistance value and the capacity retention rate during high-power discharge. As a result, they have found that by reducing the area of the unopened portion of the separator, a polyolefin microporous membrane having excellent output characteristics, strength, and shrinkage rate can be obtained as compared with the prior art, and the present invention has been completed. That is, the present invention adopts the following configuration.

The surface SEM observation image of the polyolefin microporous film according to the embodiment of the present invention, which has a shrinkage rate of less than 10% in the MD direction at 105 ° C./8 h and is obtained by observation using a scanning electron microscope (SEM). A 7 μm × 9.4 μm rectangular field is divided into 0.5 μm images in each of the vertical and horizontal directions, and the total resin area (unopened hole portion a on the surface) in the section satisfying the relationship of R2 ≧ R1 is the surface SEM observation image. It is 20 μm ² or less at 11.7 μm × 9.4 μm.
It is characterized by having a fibril diameter of 10 to 50 nm.
Here, the unopened hole portion a on the surface is the area (F1) and the hole area of the fibrils in which the total length of the fibrils in one divided section is 0.875 μm, and the total length of the fibrils is multiplied by the fibril diameter calculated by the palm porometer. The ratio (F1 / P1) of (P1) is R1,
The ratio (F2 / P2) of the resin area (F2) and the hole area (P2) in each of the divided sections is defined as R2.
It refers to the total area of the resin in each section that satisfies the relationship of R2 ≧ R1.

In the embodiment of the present invention, by using a polyolefin microporous membrane having few unopened pores, the ion resistance, strength, and shrinkage rate are excellent as compared with the conventional polyolefin microporous membrane, so that the capacity retention rate at high output A microporous polyolefin membrane with excellent properties and high safety can be obtained.

The polyolefin microporous membrane having few unopened pores according to the embodiment of the present invention is excellent in ion resistance, strength, and shrinkage rate, and is therefore useful as a separator for batteries, and has excellent safety and output characteristics. doing. The present invention can be realized by satisfying the fibril structure (the fibril diameter and the area of the unopened hole portion) described later.
That is, the present invention is included in the polyolefin microporous membrane as a result of studying the problem of achieving both the output characteristics and safety of the battery when the polyolefin microporous membrane having a mesh-like fibril structure is used as the battery separator. By finding out that the unopened part hinders the permeation of ions and increases the resistance, which leads to a decrease in strength and an increase in the shrinkage rate, and by using a polyolefin microporous membrane having a uniform fine fibril structure with a small area of the unopened part. , It was found that it leads to both the output characteristics and safety of the battery, which has been in a trade-off relationship in the past.

Hereinafter, the present invention will be described in more detail.
[1] Polyporous Microporous Membrane The microporous polyolefin membrane according to the embodiment of the present invention has a shrinkage rate of less than 10% in the MD direction at 105 ° C./8 h, and can be obtained by observation using a scanning electron microscope (SEM). Surface SEM observation image 11.7 μm × 9.4 μm rectangular field of view is divided into 0.5 μm images in each of the vertical and horizontal directions, and the total resin area in the section satisfying the relationship of R2 ≧ R1 (unopened hole portion a on the surface). However, in the surface SEM observation image 11.7 μm × 9.4 μm, it is 20 μm ² or less, and the fibril diameter is 10 to 50 nm.
Here, the unopened hole portion a on the surface is the area (F1) and the hole area of the fibrils in which the total length of the fibrils in one divided section is 0.875 μm, and the total length of the fibrils is multiplied by the fibril diameter calculated by the palm porometer. The ratio (F1 / P1) of (P1) is R1,
The ratio (F2 / P2) of the resin area (F2) and the hole area (P2) in each of the divided sections is defined as R2.
It refers to the total area of the resin in each section that satisfies the relationship of R2 ≧ R1.

The polyolefin microporous membrane according to the embodiment of the present invention is characterized in that the area of the surface unopened portion measured by the method described later is 20 μm ^{2 /} 11.7 × 9.4 μm ² or less. When the area of the unopened portion is 20 μm ² or less, when used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion battery, there is no structure that blocks the flow of ions, and good ion permeability can be obtained. As a result, an increase in ion resistance can be suppressed. Therefore, it is preferable that the area of the unopened portion is small. Area unopened hole is preferably not more 18 [mu] m ² or less, more preferably 17 .mu.m ² or less, more preferably 16 [mu] m ² or less.

The surface unopened portion of the polyolefin microporous membrane has a structure in which fibrils are fused in an unstretched portion that has not been fibrillated by stretching or in a heat treatment step, and the unstretched portion is not fibrillated, so that the strength is reduced. .. In addition, since the structure formed by melting and fusion of fibrils has an unstable crystal structure, it is easy to relax at high temperatures, leading to deterioration of the shrinkage rate. Therefore, if the area of the unopened portion observed by the SEM is larger than 20 μm ^2, it becomes difficult to reduce the ion resistance, maintain a high capacity in a high output discharge, maintain a high strength, and achieve a low shrinkage rate, which are the objects of the present invention.

From the viewpoint of ion permeability, the smaller the area of the surface unopened portion is, the more preferable it is. However, the unopened portion 0 μm ² has a fibrous structure and does not have a network structure preferably used as a battery separator. Therefore, in order to obtain sufficient shutdown property, the lower limit of the area of the unopened portion is preferably 0.1 μm ² or more, and more preferably 1.0 μm ² or more.

The polyolefin microporous film according to another embodiment of the present invention has a shrinkage rate of less than 10% in the MD direction at 105 ° C./8 h, and is a surface SEM observation image obtained by observation using a scanning electron microscope (SEM). The area ratio of the unopened hole portion b on the surface to the total area of the field of view is 25% or less, and the fibril diameter is 10 to 50 nm.
Here, the unopened portion b on the surface is an opening in which the ratio of the resin portion is calculated from the total area of the surface SEM observation image in each division in which the surface SEM observation image is divided into sections of 0.5 μm each in length and width. It is a resin part of a section having a pore ratio (%) -2 (%) or more.
The area ratio of the surface unopened portion b is the ratio of the total area of the surface unopened portion to the total area of the visual field of the surface SEM observation image.

In the present invention, the average diameter of the fibril diameter measured by the method described later is 10 to 50 nm. When the fibril diameter is in the range of 10 to 50 nm, since it has a high surface area, good electrolyte retention is obtained and the battery characteristics are improved. Further, by making the fibril diameter fine and uniform, it is possible to suppress the damming of ions due to the thick fibril structure and suppress the increase in resistance, and the number of fibrils per unit volume is increased, so that high strength can be obtained. Therefore, the fibril diameter is preferably 30 nm or less, more preferably 19 nm or less, further preferably 17 nm or less, further preferably 16 nm or less, and particularly preferably 15 nm or less. Usually, in a polyolefin microporous film, the crystallinity is increased by orienting a crystalline polyolefin such as polyethylene or polypropylene by stretching, and fine fibrils are formed to increase the strength. Therefore, the residual stress of the fibril structure of polyolefin increases, and the shrinkage rate increases as the strength increases. From the viewpoint of shrinkage, the fibril diameter is preferably 10 nm or more, more preferably 12 nm or more.

In the present invention, the average pore size in the surface SEM observation image of the polyolefin microporous film measured by the method described later is preferably 30 to 55 nm. When the average pore diameter is 55 nm or less, short circuits due to the growth of dendrites can be suppressed, a good number of pores can be obtained, and the ion transmission path increases, so that excellent battery characteristics can be obtained. The average pore diameter is preferably 53 nm or less, more preferably 50 nm or less. When the average pore diameter is 30 nm or more, a good aperture ratio can be obtained and high ion permeability can be obtained. Therefore, the average pore diameter is preferably 30 nm or more, more preferably 35 nm or more, further preferably 40 nm or more, and even more preferably 45 nm or more.

In the present invention, the number of holes in the surface SEM observation image measured by the method described later is preferably 75 holes / μm ² or more. Normally, in the process of charging and discharging a battery, the electrolytic solution is electrochemically decomposed inside the battery to generate solid products such as organic substances and gases such as ethylene and hydrogen. These products block the pores in the microporous polyolefin membrane, degrading battery characteristics. In addition, the decomposition reaction is likely to proceed due to the bias in the ion permeation path. Therefore, from these viewpoints, it is preferable that the number of ion permeation paths is large. Therefore, the number of holes is preferably 80 / μm ² or more, more preferably 85 / μm ² or more, and even more preferably 90 / μm ² or more. Moreover, since an increase in the number of holes leads to an increase of thinning or stoma diameter, tortuosity of the fibrils diameter, number of holes is preferably 200 pieces / [mu] m ² or less, more preferably 150 pieces / [mu] m ² or less, 100 cells / It is more preferably μm ² or less.

The microporous polyolefin membrane according to the embodiment of the present invention has an unopened cross section with respect to a rectangular viewing area of 11.7 μm × 9.4 μm in cross section SEM observation image obtained by observation using a scanning electron microscope (SEM). The area ratio of the holes is preferably 22% or less.
Here, the unopened portion of the cross section is a resin portion in a section in which the cross-sectional SEM observation image is divided into sections of 0.3 μm in length and width, and the resin ratio contained in the divided sections is 80% or more.
The area ratio of the unopened portion of the cross section is the ratio of the total area of the unopened portion of the cross section to the total area of the visual field of the cross section SEM observation image.
When the area ratio of the non-perforated portion in the cross section is 22% or less, when used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion battery, the structure that blocks the flow of ions disappears and good ion permeability is obtained. Be done. As a result, an increase in ion resistance can be suppressed. Therefore, it is preferable that the area of the unopened portion is small. The area ratio of the unperforated portion in the cross section is preferably 22% or less, more preferably 20% or less, further preferably 19% or less, still more preferably 18% or less, and particularly preferably 6% or less. Is.

The unopened portion of the polyolefin microporous membrane has a structure in which the unstretched portion is not fibrillated by stretching and the fibrils are fused in the heat treatment step, etc., and the unstretched portion is not fibrillated, so that the strength is lowered. In addition, since the structure formed by melting and fusion of fibrils has an unstable crystal structure, it is easy to relax at high temperatures, leading to deterioration of the shrinkage rate. Therefore, by setting the area ratio of the unopened cross-section observed by SEM to 22% or less, it is possible to reduce the ion resistance, maintain high capacity in high output discharge, maintain high strength, and have a low shrinkage rate, which are the objects of the present invention. It will be easier to achieve.

In the present invention, the average pore size in the cross-sectional SEM observation image of the polyolefin microporous film measured by the method described later is preferably 20 to 85 nm. When the average cross-sectional pore diameter is 85 nm or less, short circuits due to the growth of dendrites can be suppressed, a good number of pores can be obtained, and the ion transmission path increases, so that excellent battery characteristics can be obtained. The average cross-sectional pore diameter is preferably 85 nm or less, more preferably 80 nm or less, further preferably 78 nm or less, further preferably 76 nm or less, and particularly preferably 75 nm or less. When the average cross-sectional pore diameter is 20 nm or more, a good aperture ratio can be obtained and high ion permeability can be obtained. The average pore diameter is preferably 20 nm or more, more preferably 50 nm or more, and even more preferably 70 nm or more.

In the present invention, the number of holes in the cross-sectional direction measured by the method described later is preferably 60 holes / μm ² or more. Normally, in the process of charging and discharging a battery, the electrolytic solution is electrochemically decomposed inside the battery to generate solid products such as organic substances and gases such as ethylene and hydrogen. These products block the pores in the microporous polyolefin membrane, degrading battery characteristics. In addition, the decomposition reaction is likely to proceed due to the bias in the ion permeation path. Therefore, from these viewpoints, it is preferable that the number of ion permeation paths is large. Therefore, the number of holes is preferably 65 / μm ² or more, and more preferably 68 / μm ² or more. Further, since an increase in the number of holes leads to a reduction in the diameter of the fibril, a reduction in the diameter of the fibrils, and the formation of voids, the number of holes is preferably 100 / μm ² or less, and more preferably 70 / μm ² or less.

The porosity of the microporous polyolefin membrane according to the embodiment of the present invention is preferably 35% or more, more preferably 40% or more, still more preferably 45, from the viewpoint of permeation performance and electrolyte content. % Or more. When the porosity is 35% or more, the balance between permeability, strength and electric field liquid content is improved, and the non-uniformity of the battery reaction is eliminated. As a result, the generation of dendrites is suppressed and the battery can be used without impairing the performance of the conventional battery, and can be suitably used as a separator for a secondary battery. Further, although good output characteristics can be obtained by increasing the pore ratio, the safety of the battery is lowered due to a decrease in puncture strength and an increase in shrinkage rate. Therefore, the porosity is preferably 50% or less, more preferably 48% or less. The porosity is expressed by the following formula and can be measured by the method described in Examples.
Pore ratio = [(volume-mass / polymer density) / volume] x 100

The higher the puncture strength of the polyolefin microporous membrane according to the embodiment of the present invention is, the more preferable it is because it affects safety such as suppression of short circuit due to foreign matter in the battery. In addition, the number of holes affects the ion permeability in the battery reaction, and as the number of holes increases, the ion transmission path increases, so that the resistance decreases and charging / discharging at high output becomes possible. However, the piercing strength is an index showing the strength in the depth direction of the film, and when the number of holes per unit volume increases, the ratio of voids increases and the amount of resin decreases, so that the piercing strength decreases. Therefore, in order to achieve both piercing strength and battery characteristics at high output, the number of unopened holes is reduced to eliminate the damming structure that hinders the permeation of ions, and a uniform fine fibril structure is formed to increase the number of fibrils per unit volume. A method of increasing the puncture strength is effective.

From the viewpoint of preventing film rupture due to the electrode active material or the like, the puncture strength of the polyolefin microporous film in which the film thickness is converted to 7 μm is preferably 3.0 N or more, more preferably 3.5 N or more, still more preferably 4.0 N or more. 4.3N or more is even more preferable, and 5.0N or more is particularly preferable. When the puncture strength is 3.0 N or more, a short circuit due to a foreign substance or the like in the battery is suppressed, and good battery safety can be obtained.

In order to increase the puncture strength, it is advisable to use ultra-high molecular weight polyolefin and increase the number of Thai molecules connecting the lamella crystals from the viewpoint of the crystal structure of fibril. From the viewpoint of reducing the unstretched portion, a uniform stress load on the resin layer due to increased entanglement of molecular chains in the stretching step is preferable, and in order to suppress the unopened portion structure caused by melting and fusion of fibrils in the heat fixing step and the like. Is preferably a high molecular weight polyolefin having a small molecular weight component and a sharp molecular weight distribution. Further, by applying the raw material, the resin concentration, and the draw ratio described later, the unopened portion is reduced and good piercing strength can be obtained.

The puncture strength when the film thickness is converted to 7 μm is determined by the formula: L2 = (L1 × 7) / T1 when the puncture strength is L1 (N) in the polyolefin microporous film having a film thickness of T1 (μm). It refers to the calculated puncture strength L2 (N). In the following, unless otherwise specified, the term "piercing strength" is used to mean "piercing strength when the film thickness is converted to 7 μm".

Batteries are tensioned in the MD direction, so if the shrinkage rate in the MD direction is high, the film will break and a short circuit will occur. Therefore, the microporous polyolefin membrane according to the embodiment of the present invention has a shrinkage rate of less than 10% in the MD direction at 105 ° C./8 h. Further, it is preferably 3.0% or more. When the shrinkage rate in the MD direction is in this range, the balance between the shape stability of the polyolefin microporous film and the shutdown property is improved. If the shrinkage rate exceeds 10%, the shape stability at high temperature deteriorates, and an internal short circuit occurs when abnormal heat is generated locally, which lowers safety. Further, when the shrinkage rate is 3.0% or more, deterioration of the fluidity of polyolefin can be suppressed, and deterioration of shutdown characteristics can be prevented. The shrinkage rate in the MD direction is preferably 9% or less, more preferably 8% or less, still more preferably 7% or less. Further, 5% or more is more preferable, and 6% or more is further preferable.

Further, in the polyolefin microporous membrane according to the embodiment of the present invention, the shrinkage rate in the TD direction at 105 ° C./8 h is preferably 1.0 to 15% from the viewpoint of heat resistance such as stress relaxation characteristics. When the shrinkage rate is in this range, the balance between the shape stability and the shutdown property of the polyolefin microporous membrane is improved. When the shrinkage rate is 1.0% or more, deterioration of the fluidity of polyolefin can be suppressed, and deterioration of shutdown characteristics can be prevented. Further, when the shrinkage rate is 15% or less, deterioration of shape stability at high temperature can be suppressed, internal short circuit can be prevented from occurring when abnormal heat is locally generated, and safety can be maintained. The shrinkage rate in the TD direction is preferably 10% or less, more preferably 7% or less, still more preferably 5% or less. Further, 3% or more is more preferable, and 4% or more is further preferable.

The shrinkage rate in the MD direction and the TD direction at 105 ° C./8 h can be measured by the method described in Examples.

Further, since the polyolefin microporous membrane obtained by the present invention has a uniform fine fibril structure with a small area of unopened pores, it shrinks uniformly. Therefore, even if abnormal heat is generated locally, the expansion of the internal short circuit can be prevented and the influence can be minimized, and high safety can be obtained.

The tensile (breaking) strength in the MD direction and the TD direction (hereinafter, also simply referred to as "MD strength" and "TD strength") is preferably 150 MPa or more, more preferably 180 MPa or more, further preferably 200 MPa or more, and 220 MPa or more. Even more preferable. When the MD strength and the TD strength are 150 MPa or more, it is possible to prevent a short circuit due to a foreign substance or the like in the battery and maintain the safety of the battery. From the viewpoint of improving safety, the higher the tensile strength, the more preferable, but since there are many cases where there is a trade-off between the tensile strength and the shrinkage rate, 330 MPa or less is preferable, and 280 MPa or less is more preferable. In order to set the tensile strength in the above range, it is preferable that the raw material composition of the polyolefin microporous film is in the range described later, and the stretching conditions at the time of forming the polyolefin microporous film are in the range described later.

Further, the tensile (breaking) elongation in the MD direction and the TD direction (hereinafter, also simply referred to as “MD elongation” and “TD elongation”) is preferably 30% or more, more preferably 40% or more, and more preferably 50%. The above is more preferable, and 60% or more is even more preferable. .. When the MD elongation and the TD elongation are 30% or more, short circuits due to foreign matter in the battery or the like during winding are suppressed, and good safety can be obtained, which is preferable. Further, both MD elongation and TD elongation are preferably 150% or less, more preferably 80% or less, still more preferably 70% or less. It is preferable that the MD elongation and the TD elongation are 150% or less because both strength and elongation can be achieved at the same time.

The tensile strength and tensile elongation in the MD direction and the TD direction can be measured by a tensile test defined by ASTM D882 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting) as described in Examples.

In the present invention, the direction parallel to the film forming direction of the polyolefin microporous film is referred to as the film forming direction, the longitudinal direction or the MD direction, and the direction orthogonal to the film forming direction within the polyolefin microporous film surface is the width direction. Alternatively, it is referred to as the TD direction.

In the polyolefin microporous membrane according to the embodiment of the present invention, the air permeability refers to a value measured in accordance with JIS P 8117 (2009). Unless otherwise specified, the term "air permeability" is used in the present specification to mean "air permeability when the film thickness is 7 μm". When the air permeability measured for a polyolefin microporous membrane having a film thickness of T1 (μm) is p1 (sec / 100 cm ³ ), the air permeability p2 (p2) calculated by the formula: p2 = (p1 × 7) / T1. sec / 100 cm ³ ) is the air permeability when the film thickness is 7 μm.

Preferably the air permeability (Gurley value) is not more than 100 sec / 100 cm ^3, more preferably of 80 sec / 100 cm ³ or less, and more preferably 60 sec / 100 cm ³ or less. When the air permeability is 100 sec / 100 cm ³ or less, good ion permeability can be obtained and the electric resistance can be reduced.

The resistance of the microporous polyolefin membrane according to the embodiment of the present invention increases as the film thickness increases, and the output characteristics of the battery deteriorate. From the viewpoint of the output characteristics of the battery, the film thickness is preferably 12 μm or less, more preferably 10 μm or less, further preferably 8 μm or less, and even more preferably 7 μm or less. Further, the thinner the film thickness, the lower the strength and the lower the safety. From the viewpoint of safety, it is preferably 3 μm or more, and more preferably 4 μm or more.

[2] Polyolefin Resin The resin raw material in the polyolefin microporous film according to the embodiment of the present invention may have a single composition, may be a composition in which a main raw material and an auxiliary raw material are combined, and may be composed of two or more kinds of polyolefin resins. It may be a polyolefin resin mixture (polyolefin resin composition). The raw material form of the polyolefin microporous membrane is preferably a polyolefin resin, and examples of the polyolefin resin include polyethylene and polypropylene, and a single composition is preferable.

The polyolefin resin is preferably a homopolymer such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and particularly preferably an ethylene homopolymer (polyethylene). Polyethylene may be a copolymer containing a homopolymer of ethylene and another α-olefin.

Other α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, or alkene having a carbon number of more than that, vinyl acetate, methyl methacrylate, styrene, and the like. Can be mentioned.

Polyethylene is preferable as the type of polyolefin resin, high density polyethylene such as density exceeding 0.94 g / cm ^3, density polyethylene in the range density of 0.93 ~ 0.94g / cm ^3, density of 0 used. Examples thereof include low density polyethylene lower than 93 g / cm ^{3 and} linear low density polyethylene.

Further, as the polyolefin resin, from the viewpoint of the number of unopened pores, the number of pores, and the film strength, it is preferable to use an ultra-high molecular weight polyolefin having a weight average molecular weight (Mw) of 900,000 or more alone.

Ultra-high molecular weight polyolefin has a weight average molecular weight is preferably preferably 0.9 × 10 ⁶ or more and 1.0 × 10 ⁷ or less.
When the weight-average molecular weight of 0.9 × 10 ⁶ or more, since the increase in the stretching temperature and the heat treatment temperature relaxation time is not too short is suppressed, a fine fibrils are melted, some savage holes to fuse increases At the same time, it is possible to prevent the number of holes from decreasing and the output and characteristics from deteriorating.
Further, since the weight average molecular weight is used 0.9 × 10 ⁶ or more polyolefins, for entanglement density of the amorphous region increases uniformly stress the polyolefin resin layer in the stretching process is loaded, unopened hole Can be reduced and a fine fibril structure can be formed. Therefore, good output characteristics can be obtained.
Moreover, Thailand number of molecules is increased by a weight-average molecular weight is used 0.9 × 10 ⁶ or more polyolefin resins, high strength can be obtained. In addition, since ultra-high molecular weight polyethylene has a long relaxation time, melting and fusion of fine fibrils during stretching and heat fixation at high temperature are suppressed, and the balance between shrinkage rate and unopened portion is improved. Therefore, the trade-offs of ion resistance, strength, and shrinkage are improved, and both battery safety and output characteristics can be achieved at the same time.

The weight average molecular weight of the order ultrahigh molecular weight polyolefin is preferably 1.0 × 10 ⁶ or more, more preferably 1.5 × 10 ⁶ or more, more preferably 2.0 × 10 ⁶ or more, particularly preferably 3. It is 0 × ¹⁰⁶ or more. The weight average molecular weight is preferably 1.0 × 10 ⁷ or less, more preferably 8.0 × 10 ⁶ or less, more preferably 6.0 × 10 ⁶ or less, still more preferably 5.0 × 10 ⁶ or less , Particularly preferably 4.0 × ¹⁰⁶ or less.

The molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) of the ultrahigh molecular weight polyolefin is preferably in the range of 3.0 to 100. The narrower the molecular weight distribution, the more unified the system is and the more uniform micropores can be obtained. Therefore, the narrower the molecular weight distribution is, the more preferable, but the narrower the distribution, the lower the molding processability. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or more, more preferably 5.0 or more, and further preferably 6.0 or more. As the molecular weight distribution increases, the low molecular weight components increase, which tends to reduce the strength and melt / fuse fine fibrils during stretching / heat fixation. Therefore, the upper limit is preferably 80 or less, more preferably 50 or less, still more preferable. Is 20 or less, particularly preferably 10 or less. Within the above range, good molding processability can be obtained and uniform micropores can be obtained because the system is unified.

The ultra-high molecular weight polyolefin, inter alia, primitive holes, film strength, moldability from the viewpoint, the weight average molecular weight (Mw) of 0.9 × 10 ⁶ or more 1.5 × 10 ⁶ or less ultra high molecular weight polyethylene Is particularly preferable.

In the process for producing a microporous polyolefin membrane according to the embodiment of the present invention, it is preferable to add a plasticizer for the purpose of improving molding processability. The blending ratio of the polyolefin resin and the plasticizer may be appropriately adjusted as long as the molding processability is not impaired, but the ratio of the polyolefin resin is 10 to 50% by mass, where the total of the polyolefin resin and the plasticizer is 100% by mass. It is preferable to have. When the proportion of polyolefin resin is 10% by mass or more (the proportion of plasticizer is 90% by mass or less), swell and neck-in can be suppressed at the outlet of the base when molding into a sheet, and the formability and film forming property of the sheet can be suppressed. Is improved. On the other hand, when the proportion of the polyolefin resin is 50% by mass or less (the proportion of the plasticizer is 50% by mass or more), the pressure increase in the film forming process can be suppressed and good molding processability can be obtained.

When the total of the polyolefin resin and the plasticizer is 100% by mass, the proportion of the polyolefin resin is preferably 10% by mass or more, more preferably 16% by mass or more, further preferably 17% by mass or more, and particularly preferably 20% by mass or more. preferable. By increasing the proportion of the polyolefin resin, the overlapping density of the molecular chains in the presence of the plasticizer is increased, the uniformity of stretching is improved, and the unopened portions can be reduced. Further, by increasing the proportion of the polyolefin resin, it is possible to suppress an increase in the draw ratio and "crushing of holes" in the washing and drying process, leading to higher strength and reduction of unopened holes.

When a polyolefin resin having an Mw of 900,000 or more is used, the proportion of the polyolefin resin is preferably 30% by mass or less, with the total of the polyolefin resin and the plasticizer being 100% by mass, from the viewpoint of pressure and stretching stress in the film forming process. Less than 28.5% by mass is more preferable, and less than 25% by mass is further preferable.

Further, a polyolefin resin having an Mw of less than 900,000 is more likely to have a lower melting point due to a plasticizing effect than a polyolefin resin having an Mw of 900,000 or more, and the diffusion rate of the polyolefin is faster. Therefore, melting and fusion in the stretching / heat fixing process Is likely to occur, and the number of unopened holes increases. Therefore, a proportion of a polyolefin resin having an Mw of 900,000 or more of 20% by mass or more and a plasticizer of less than 80% is particularly preferable.

In addition, the polyolefin microporous membrane according to the embodiment of the present invention includes antioxidants, heat stabilizers and antistatic agents, ultraviolet absorbers, and further blocking inhibitors and fillers as long as the effects of the present invention are not impaired. Various additives such as the above may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyolefin resin.

Examples of the antioxidant include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris (3,5-tris). Di-t-butyl-4-hydroxybenzyl) benzene (eg, BASF's "Irganox"® 1330: molecular weight 775.2), tetrakis [methylene-3 (3,5-di-t-butyl-4) -Hydroxyphenyl) propionate] It is preferable to use one or more selected from methane (for example, "Irganox" (registered trademark) 1010: molecular weight 1177.7) manufactured by BASF.

Appropriate selection of the type and amount of antioxidant and heat stabilizer is important for adjusting or enhancing the characteristics of the polyolefin microporous membrane.

Further, the layer structure of the polyolefin microporous membrane according to the embodiment of the present invention may be a single layer or a laminated layer, and the laminated layer is preferable from the viewpoint of physical property balance. When the above formulations are laminated and used as a high output characteristic layer, it is preferable that the high output characteristic layer contains 50% by mass or more in the total film thickness.

[3] Method for Producing Polyolefin Microporous Membrane Next, the method for producing the polyolefin microporous membrane according to the embodiment of the present invention will be specifically described. The method for producing a microporous polyolefin membrane according to the embodiment of the present invention preferably has the following steps (a) to (e).

(A) Step of melt-kneading a polymer material containing one or more kinds of polyolefin resins and, if necessary, a solvent to prepare a polyolefin resin solution (b) Extruding the solution and molding it into a sheet. Step of cooling and solidifying (c) Step of stretching the obtained sheet by a roll method or a tenter method (d) Then, a step of extracting a plasticizer from the obtained stretched film and drying the film (e) Heat treatment / re-stretching Process to do

Hereinafter, each step will be described.
(A) Step of preparing a polyolefin resin solution The above polymer material is heated and dissolved in a plasticizer to prepare a polyolefin resin solution. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving the polyolefin resin, but the solvent is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification.

Solvents include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil distillates having corresponding boiling points, and dibutylphthalates. Examples thereof include phthalates that are liquid at room temperature, such as dioctyl phthalates.

In order to obtain a gel-like sheet with a stable liquid solvent content, 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, but at room temperature, a solid solvent may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax and the like. However, if only a solid solvent is used, uneven stretching may occur.

The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40 ° C. When the viscosity at 40 ° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin resin solution from the die is unlikely to become non-uniform. On the other hand, if the viscosity at 40 ° C. is 200 cSt or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is the viscosity measured at 40 ° C. using an Ubbelohde viscometer.

(B) Formation of Extruded Product and Formation of Gel-like Sheet The method for uniformly melt-kneading the polyolefin resin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin resin solution, it is preferably performed in a twin-screw extruder. .. If necessary, metal soaps such as calcium stearate, ultraviolet absorbers, light stabilizers, antistatic agents and other known additives are also added as long as the effects of the present invention are not impaired without impairing the film-forming property. You may. In particular, it is preferable to add an antioxidant in order to prevent oxidation of the polyolefin resin.

In the extruder, the polyolefin resin solution is uniformly mixed at a temperature at which the polyolefin resin completely melts. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (melting point of the polyolefin resin + 10 ° C.) to (melting point of the polyolefin resin + 120 ° C.). More preferably, it is (melting point of polyolefin resin + 20 ° C.) to (melting point of polyolefin resin + 100 ° C.).

Here, the melting point means a value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987) (hereinafter, the same applies). For example, when the polyolefin-based resin is polyethylene, the melt-kneading temperature of the polyethylene-based resin is preferably in the range of 140 to 250 ° C. More preferably, it is 160 to 230 ° C, and most preferably 170 to 200 ° C. Specifically, since the polyethylene composition has a melting point of about 130 to 140 ° C., the melt-kneading temperature is preferably 140 to 250 ° C., most preferably 180 to 230 ° C.

From the viewpoint of suppressing deterioration of the polyolefin resin, it is preferable that the melt-kneading temperature is low, but if it is lower than the above-mentioned temperature, unmelted material is generated in the extruded product extruded from the die, and film rupture or the like is caused in the subsequent stretching step. It may cause it. On the other hand, if the temperature is higher than the above temperature, the thermal decomposition of the polyolefin resin becomes severe, and the physical properties of the obtained polyolefin microporous film, for example, strength and porosity may deteriorate. In addition, the decomposed product precipitates on a chill roll, a roll in the stretching process, or the like and adheres to the sheet, which leads to deterioration of the appearance. Therefore, it is preferable that the melt-kneading temperature is within the above range.

Next, a gel-like sheet is obtained by cooling the obtained extruded product, and the microphase of the polyolefin resin separated by the solvent can be immobilized by cooling. In the cooling step, it is preferable to cool the gel sheet to 10 to 50 ° C. This is because the final cooling temperature is set to be equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching can be easily performed in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30 ° C./min or higher at least up to the gelation temperature or lower.

Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse, and the gel structure that forms it also becomes large. On the other hand, when the cooling rate is high, small and uniform crystals are formed, so that the higher-order structure of the gel-like sheet becomes dense, which leads to uniform stretching and reduction of unopened holes.

As a cooling method, there are a method of directly contacting with cold air, cooling water, and other cooling media, a method of contacting with a roll cooled with a refrigerant, a method of using a casting drum, and the like.

Although the case where the polyolefin microporous membrane is a single layer has been described so far, the polyolefin microporous membrane according to the embodiment of the present invention is not limited to a single layer, and may be a laminated body. The number of layers is not particularly limited, and may be two layers or three or more layers. As described above, the laminated portion may contain a desired resin in addition to the polyolefin resin to the extent that the effects of the present invention are not impaired.

A conventional method can be used as a method for forming a polyolefin microporous film as a laminate. For example, desired resins are prepared as needed, these resins are separately fed to an extruder to melt at the desired temperature and merge in a polymer tube or die to achieve the desired thickness of each laminate. There is a method of forming a laminated body by extruding from a slit-shaped die.

(C) Stretching Step The obtained gel-like (including laminated sheet) sheet is stretched. The stretching methods used include rolling and uniaxial stretching in the sheet transport direction (MD direction) by a roll stretching machine, uniaxial stretching in the sheet width direction (TD direction) by a tenter, roll stretching machine and tenter, or tenter and tenter. Examples include sequential biaxial stretching by the combination of the above and simultaneous biaxial stretching by the simultaneous biaxial tenter.

The stretching ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of film thickness, but it is preferable to stretch 7 times or more in any direction.

The area magnification is preferably 45 times or more, more preferably 60 times or more, further preferably 80 times or more, and particularly preferably 100 times or more. If the area magnification is less than 45 times, the stretching is insufficient and unopened holes are likely to be formed. The area magnification is preferably 150 times or less. When the area magnification is large, tearing is likely to occur frequently during the production of the polyolefin microporous film, and the productivity is lowered.

From the viewpoint of improving the stretching uniformity in the stretching step, a preferable form of the stretching ratio and the raw material composition is to wet-stretch a raw material having an Mw of 900,000 or more by 60 times or more, more preferably 80 times or more, still more preferably 100. The above is the wet stretching. A particularly preferable form is to stretch a raw material having a Mw of 1 million or more in a wet manner 60 times or more, more preferably 70 times or more, and most preferably 90 times or more.

The stretching temperature is preferably in the range of (melting point of the gel-like sheet + 10 ° C. or lower) to (crystal dispersion temperature of polyolefin resin Tcd) to (melting point of the gel-like sheet + 5 ° C.). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 110 ° C., the stretching temperature is preferably 100 to 130 ° C., more preferably 115 to 125 ° C., and further preferably 117. It is 5 to 125 ° C. The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012).

If the stretching temperature is less than 100 ° C., the pores are insufficiently opened due to the low temperature stretching, it is difficult to obtain the uniformity of the film thickness, and the pore ratio is also low. If the stretching temperature is higher than 130 ° C., the sheet melts and the pores are likely to be closed. In particular, when polyethylene having a weight average molecular weight of less than 900,000 is stretched at a temperature of 115 ° C. or higher, fibrils are melted and fused, and unopened portions are likely to be formed. On the other hand, when polyethylene having a weight average molecular weight of 900,000 or more is used, the relaxation time is longer than that of polyethylene having a weight average molecular weight of less than 900,000 and it is difficult to be deformed by stretching. Therefore, the temperature is higher than when polyethylene having a weight average molecular weight of less than 900,000 is used. It is preferable to form a film at a temperature of 117.5 ° C. or higher because the unopened portion can be reduced even if the film is stretched with.

Due to the above stretching, the higher-order structure of the gel sheet is cleaved, the crystal phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Since the mechanical strength is improved by stretching and pores are formed, the polyolefin microporous membrane according to the embodiment of the present invention is suitable for a battery separator. Since the polyolefin microporous film of the present application has a uniform fine structure, bleed-out of plasticizers and the like is small even when stretched at a higher stretching temperature than in the prior art, and process stability is improved.

Further, by stretching before removing the plasticizer, the polyolefin resin is in a state of being sufficiently plasticized and softened, so that the higher-order structure can be cleaved smoothly and the crystal phase can be uniformly refined. it can. In addition, since the higher-order structure is easily cleaved by stretching before removing the plasticizer, strain during stretching is less likely to remain, and the shrinkage rate can be lowered as compared with the case of stretching after removing the plasticizer. it can.

(D) Plasticizer extraction (cleaning) / drying step Next, the plasticizer (solvent) remaining in the gel sheet is removed using a cleaning solvent. Since the polyolefin resin phase and the solvent phase are separated, a polyolefin microporous membrane can be obtained by removing the solvent.

Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and ethane trifluoride. Chain fluorocarbon and the like can be mentioned.

These cleaning solvents have low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a cleaning solvent with a low surface tension, in a network structure that forms microporous, shrinkage due to surface tension at the gas-liquid interface is suppressed during drying after cleaning, and polyolefin microporous with excellent porosity and permeability. A film is obtained. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

Examples of the cleaning method include a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the gel-like sheet with the cleaning solvent, or a method of combining these. The amount of the cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more with respect to 100 parts by mass of the gel sheet.

The cleaning temperature may be 15 to 30 ° C, and if necessary, heat to 80 ° C or less. At this time, from the viewpoint of enhancing the cleaning effect of the cleaning solvent, from the viewpoint of preventing the physical properties of the obtained polyolefin microporous film (for example, the physical properties in the TD direction and / or the MD direction) from becoming non-uniform, the mechanical property of the polyolefin microporous film From the viewpoint of improving the physical and electrical characteristics, the longer the gel sheet is immersed in the cleaning solvent, the better.

The above-mentioned cleaning is preferably performed until the residual solvent in the gel-like sheet after cleaning, that is, the polyolefin microporous membrane becomes less than 1% by mass.

After that, the solvent in the polyolefin microporous membrane is dried and removed in the drying step. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40 to 100 ° C, more preferably 40 to 80 ° C. If the drying is insufficient, the porosity of the polyolefin microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

(E) Heat Treatment / Re-stretching Step The dried polyolefin microporous film may be stretched (re-stretched) at least in the uniaxial direction. The re-stretching can be performed by the tenter method or the like in the same manner as the above-mentioned stretching while heating the microporous polyolefin membrane. The re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, simultaneous biaxial or sequential stretching is performed in combination.

The re-stretching temperature is preferably equal to or lower than the melting point of the polyolefin resin composition, and more preferably within the range of (Tcd-20 ° C. of the polyolefin resin composition) to the melting point of the polyolefin resin composition. Specifically, in the case of the polyethylene composition, the re-stretching temperature is preferably 70 to 135 ° C., more preferably 110 to 135 ° C., further preferably 125 to 135 ° C., and even more preferably 130 to 135 ° C.

Especially the weight-average molecular weight is less than 0.9 × 10 ^6, the relaxation time is subjected to heat treatment temperature of above 130 ° C. for a short, pores with the fine fibrils are melted, savage holes is increased to fuse The rate and the number of holes decrease, leading to a decrease in output characteristics. Against it, it is possible to suppress melting and fusion of the weight average molecular weight is also fine to implement the stretching and heat setting at 0.9 × 10 ⁶ or more polyethylene relaxation time is long for 130 ° C. or higher temperatures fibrils, high Heat can be fixed by temperature. Therefore, the balance between the shrinkage rate and the unopened portion is improved. It is preferable to heat-set at 130 ° C. or higher when the weight average molecular weight is used 0.9 × 10 ⁶ or more polyethylene.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 2.0 times, particularly preferably 1.1 to 1.6 times, and more preferably 1.2 to 1.4 times in the TD direction. .. When biaxial stretching is performed, it is preferable to stretch 1.01 to 2.0 times in the MD direction and the TD direction, respectively. The re-stretching magnification may be different in the MD direction and the TD direction. By re-stretching within the above range, the porosity and permeability are increased, the aggregation of fibrils due to the neck can be suppressed, and the unopened portion of the polyolefin microporous membrane is reduced.

From the viewpoint of shrinkage rate and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 20% or less, more preferably 10% or less, still more preferably 5% or less. When the relaxation rate is 20% or less, a uniform fibril structure can be obtained.

(F) Other Steps Further, depending on other uses, the polyolefin microporous membrane can be hydrophilized. The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer graft is preferably carried out after the cross-linking treatment.

It is preferable that the polyolefin microporous film is crosslinked by irradiation with ionizing radiation such as α-ray, β-ray, γ-ray, and electron beam. 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 cross-linking treatment raises the meltdown temperature of the microporous polyolefin membrane.

In the case of surfactant treatment, any of nonionic surfactant, cationic surfactant, anionic surfactant or amphoteric surfactant can be used, but nonionic surfactant is preferable. The polyolefin microporous membrane is immersed in water or a solution prepared by dissolving a surfactant in a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyolefin microporous membrane by the doctor blade method.

The polyolefin microporous film according to the embodiment of the present invention is a fluororesin porous body such as polyvinylene sulfide fluoride or polytetrafluoroethylene for the purpose of improving meltdown characteristics and heat resistance when used as a battery separator. , Polyimide, polyphenylene sulfide, and other porous materials may be surface-coated, and ceramics and other inorganic coatings may be applied. In particular, since the porous polyolefin film obtained by the present invention has finely and uniformly controlled pore diameter and fibril structure, clogging due to the coating material can be suppressed, and the coating suitability is excellent.

The microporous polyolefin membrane obtained as described above can be used for various purposes such as filters, separators for fuel cells, separators for capacitors, etc., but is particularly excellent in safety and output characteristics when used as a separator for batteries. .. Therefore, the separator can be preferably used as a separator for a non-aqueous electrolytic solution secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like.

The present invention will be described in more detail by way of examples, but the embodiments of the present invention are not limited to these examples.
The evaluation method and analysis method used in the examples are as follows.

(1) Weight average molecular weight (Mw)
The weight average molecular weights of ultra high molecular weight polyethylene (UHPE) and high density polyethylene (HDPE) were determined by gel permeation chromatography (GPC) under the following conditions.
-Measuring device: GPC-150C manufactured by WATERS CORPORATION
-Column: SHODEX UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
-Solvent (mobile phase): O-dichlorobenzene-Solvent flow rate: 1.0 mL / min-Sample concentration: 0.1 wt% (dissolution condition: 135 ° C / 1H)
-Injection amount: 500 μL
-Detector: WATERS CORPORATION differential refractometer (RI detector)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

(2) Film thickness (μm)
The film thickness at 5 points was measured with a contact thickness meter (Lightmatic Co., Ltd., manufactured by Mitutoyo Co., Ltd.) at arbitrary randomly selected points within the range of 95 mm × 95 mm of the polyolefin microporous film, and the average value of the film thickness at 5 points. Asked.

(3) Area ratio (%) of unopened holes on the surface
The microporous membrane obtained in the examples was Pt-deposited, and the surface of the microporous membrane was secondary to the surface of the microporous membrane with a field of view of 11.7 μm × 9.4 μm at a magnification of 10000 times at an acceleration voltage of 2 kV using a scanning electron microscope (SEM). It was observed with an electron image. The photographed surface SEM observation image was divided into squares having a side of 0.5 μm and binarized using HALCON 13 manufactured by MVTec Software. As the image before the binarization process, an image having an acceleration voltage of 2 kV, a magnification of 10000 times, 11.7 μm × 9.4 μm (1280 pixels × 1024 pixels), and 8 bits (256 gradations) gray scale was used. In the binarization process, noise is removed from the surface SEM observation image by averaging 3 pixels x 3 pixels, and then the image obtained by averaging 21 pixels x 21 pixels is dynamically binarized with -30 gradations as a threshold value. By performing the binarization process, the dark part was extracted (image processing) and the binarization process was performed. With the dark part as the opening part, the ratio of the dark part to the total area of the visual field was defined as the surface average aperture ratio (aperture ratio calculated from the total area of the surface SEM observation image) (%). Also for each partition of each square divided, Plot size (0.5μm × 0.5μm) - calculating the area of the resin portion from the area ([mu] m ²⁾ of the openings (dark portion) ([mu] m ^2), in each compartment The ratio of the resin part was calculated. The total area of the resin part in the compartment where the ratio of the resin part is the surface average aperture ratio (%) -2 (%) or more is defined as the area of the surface SEM unopened part, and the total area of the field of view (11.7 μm × 9. The area ratio of the unopened portion at 4 μm) was calculated.

(5) Number of holes on the surface In the surface SEM observation image image-processed in (3) above, the number of independent dark areas existing per 1 μm ² was counted as the number of surface holes to obtain the number of surface holes (pieces / μm ² ).

(5) Average pore diameter of the surface The average pore diameter (nm) of the surface SEM was calculated from the total area of the opened portions (dark portions) and the number of surface pores in the surface SEM observation image processed in (3) above.
Average hole diameter on the surface = (total area of holes ÷ number of surface holes ÷ 3.14) ^0.5 × 2

(6) Fibril diameter Using a palm polo meter (manufactured by PMI, CFP-1500A), the maximum pore diameter and the average pore diameter were measured in the order of Dry-up and Wet-up. For Wet-up, pressure is applied to a polyolefin microporous membrane sufficiently immersed with PMI's Galwick (trade name) having a surface tension of 15.6 dynes / cm, and the maximum pore diameter converted from the pressure at which air begins to penetrate is the maximum pore diameter. And said.

The average pore diameter was converted from the pressure at the intersection of the pressure and the half slope of the flow rate curve in the Dry-up measurement and the pressure at the intersection of the Wet-up measurement curve. The following formula was used to convert the pressure and the average pore size.

d = C · γ / P
(In the above formula, "d (μm)" is the average pore size of the polyolefin microporous membrane, "γ (mN / m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is a constant. .)

Next, assuming that the air flow in the separator follows the Knudsen flow, the specific surface area (Sv) was calculated from the pressure and air permeability obtained by the poromometer measurement using the Kozeny-Carman equation (1).

Q: Air flow rate at 2000 kPa measured by Dry-up l: Film thickness (μm)
δPa: 2000 kPa pressure measured by Dry-up a: Cross-sectional area (mm ² )
ε: Pore ratio (%)
k (constant): 5 (Kozeny coefficient)
Sv: Specific surface area (m ² / g)
η (air viscosity at 20 ° C): 1.8 × ^10-5 Pas

Then, assuming that the fibril is a cylinder, the fibril diameter was calculated using the formula (2) based on the specific surface area.
R = 4 × (1-ε) / Sv / d ・ ・ ・ Equation (2)
R: Fibril diameter (nm)
Sv: Specific surface area (m ² / g)
d: Volumetric density (g / m ³ )
ε: Pore ratio (%)

(7) Calculation of the area of unopened holes on the surface (μm ² )
Of the surface SEM observation images image-processed in the unopened portion of the surface (3), the area of the dark portion in each divided square section is calculated and used as the hole area of each section, and the area of each section (0.5 μm × 0.5 μm) -The resin area (F2) in each section was calculated from the hole area (dark area: P2) in each section, and the ratio R2 of the hole area (P2) and the resin area (F2) in each section was obtained. (R2 = F2 / F2).
Subsequently, the total length of the fibrils in one section is set to 0.875 μm, and the area of the fibrils (F1) obtained by multiplying the total length of the fibrils by the fibril diameter calculated by the palm poromometer and the hole area (P1) calculated by surface SEM observation. The ratio R1 was determined (R1 = F1 / P1). R2 and R1 in each of the above compartments are compared, and the resin portion in the compartment satisfying the relationship of R2 ≧ R1 is defined as the unopened portion, and the total area of the resin in the unopened compartment is defined as the unopened portion area. The area of the unopened hole in 7.7 × 9.4 μm ² was calculated.

(8) Area ratio of unopened portion of cross section Pt-deposited microporous film obtained in Example, using a scanning electron microscope (SEM) at an acceleration voltage of 2 kV, magnification of 10000 times, 11.7 μm × 9.4 μm The cross section of the microporous film was observed with a secondary electron image in the field of view. The photographed cross-sectional SEM observation image was divided into squares having a side of 0.3 μm, and binarized using HALCON 13 manufactured by MVTec Software. As the image before the binarization process, an image having an acceleration voltage of 2 kV, a magnification of 10000 times, 11.7 μm × 9.4 μm (1280 pixels × 1024 pixels), and 8 bits (256 gradations) gray scale was used. The binarization process is performed by removing noise from the cross-sectional SEM observation image with an average of 3 pixels × 3 pixels, and then dynamically binarizing the image obtained by averaging 21 pixels × 21 pixels with a threshold value of -20 gradations. By performing the binarization process, the dark part was extracted (image processing) and the binarization process was performed. The dark part was defined as the opening, and the ratio of the dark part to the total area of the visual field was defined as the cross-sectional average aperture ratio (%). Also for each partition of each square divided, Plot size (0.3μm × 0.3μm) - calculating the area of the resin portion from the area ([mu] m ²⁾ of the openings (dark portion) ([mu] m ^2), in each compartment The ratio of the resin part was calculated. The area ratio of the unopened portion in the total area of the visual field (11.7 μm × 9.4 μm) was calculated by using the total area of the resin portion of the section where the ratio of the resin portion is 80% or more as the area of the cross-sectional SEM unopened portion. ..

(9) Number of holes in cross section In the cross-section SEM observation image image-processed in (8) above, the number of independent dark areas existing per 1 μm ² was counted as the number of surface holes to obtain the number of holes in cross section (pieces / μm ² ).

(10) Average hole diameter of cross section The average hole diameter of cross section SEM was calculated from the total area of the opened portion (dark portion) and the number of surface holes in the cross-section SEM observation image processed in (8) above.

(11) Air permeability (sec / 100 cm ³ )
In the time required to permeate 100 cm ³ of air with an air permeability meter (made by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117 for a polyolefin microporous membrane with a film thickness of T1 (μm). A certain air permeability p1 (sec / cm ³ ) was measured, and the air permeability p2 (sec / cm ³ ) when the film thickness was 7 μm was calculated by the formula: P2 = (P1 × 7) / T1.

(12) Pore ratio (%)
A 5 cm square sample was cut from a microporous polyolefin membrane, and its volume (cm ³ ) and mass (g) were determined, and the volume (cm ³ ) and mass (g) were calculated from them and the polymer density of 0.99 (g / cm ³ ) using the following formula. The above measurement was performed at 3 points at different random sampling points in the same polyolefin microporous membrane, and the average value of the porosity (%) at the 3 points was obtained.
Pore ratio = [(volume-mass / polymer density) / volume] x 100

(13) Puncture strength (N)
When a needle with a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm is used to pierce a polyolefin microporous film with a film thickness of T1 (μm) at a speed of 2 mm / sec using a piercing meter manufactured by MARUBISHI. The maximum load was defined as the piercing strength (N).

Further, the measured value L1 (N) of the maximum load was converted into the maximum load L2 when the film thickness was 7 μm by the formula: L2 = L1 / T1 × 7, and the piercing strength was converted into the film thickness 7 μm.

(14) Tensile breaking strength (MPa), tensile breaking elongation (%)
MD is calculated by scoring 3 points from the central part of the polyolefin microporous film in the width direction and calculating the average value of the measurement results measured under the conditions of a chuck distance of 20 mm and a strain rate of 100 mm / min for each of the strip-shaped test pieces having a width of 10 mm. The tensile breaking strength (MPa) and the tensile breaking elongation (%) of each of the direction and the TD direction were determined.

(15) Shrinkage rate (%) at 105 ° C. for 8 hours
Three test pieces cut into 50 mm squares were scored from the central portion of the polyolefin microporous membrane in the width direction, and the shrinkage rate (heat shrinkage rate) in the MD direction when each was held at 105 ° C. for 8 hours was measured. The average value was taken as the shrinkage rate (%) in the MD direction. Further, the same measurement was performed in the TD direction, and the shrinkage rate in the TD direction was determined.

(16) Capacity retention rate at 10C In order to evaluate the rate characteristics of the polyolefin microporous membrane, the polyolefin microporous membrane is incorporated as a separator into a non-aqueous electrolyte secondary battery composed of a positive electrode, a negative electrode, a separator and an electrolyte, and is charged and discharged. The test was performed.

NMC532 (Lithium Nickel Manganese Cobalt Composite Oxide (Li _1.05 Ni _0.50 Mn _0.29 Co _0.21) at 9.5 mg / cm ² on an aluminum foil with a width of 38 mm, a length of 33 mm and a thickness of 20 μm. O ₂ )) is laminated on the cathode, and natural graphite with a density of 1.45 g / cm ³ is laminated on a copper foil having a width of 40 mm, a length of 35 mm, and a thickness of 10 μm at a unit area mass of 5.5 mg / cm ^2. Was used. The positive electrode and the negative electrode were dried and used in a vacuum oven at 120 ° C.

As the separator, a microporous polyolefin membrane having a length of 50 mm and a width of 50 mm was dried in a vacuum oven at room temperature and used. The electrolytic solution is a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (30/35/35, volume ratio) in which vinylene carbonate (VC) and LiPF ₆ are dissolved, and the VC concentration is 0.5% by mass, LiPF _6. A solution of concentration: 1 mol / L was prepared.

The positive electrode, the separator, and the negative electrode are stacked, the obtained laminate is placed in the laminate pouch, the electrolytic solution is injected into the laminate pouch, and the laminate pouch is vacuum-sealed to form a non-aqueous electrolyte secondary battery. Made.

The prepared non-aqueous electrolyte secondary battery was charged for 10 to 15% at a temperature of 35 ° C. and 0.1 C, and left at 35 ° C. overnight (12 hours or more) for degassing. Next, CC-CV charging with a temperature of 35 ° C., a voltage range of 2.75 to 4.2 V, and a charging current value of 0.1 C (constant current constant voltage charging (termination current condition 0.02 C)), discharge current value 0. 1C CC discharge (constant current discharge) was carried out. Next, CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) with temperature 35 ° C., voltage range; 2.75 to 4.2V, charging current value 0.2C, discharge current value 0. The time when 2C CC discharge (constant current charging) was performed for 3 cycles was defined as the initial stage of the non-aqueous electrolyte secondary battery.

Next, CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) at a temperature of 35 ° C., a voltage range; 2.75 to 4.2V, and a charging current value of 0.2C is performed, and then at 15 ° C. A 0.2 C CC discharge (constant current discharge) was performed, and the discharge capacity at that time was set to a 0.2 C capacity. Next, after CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) at a temperature of 35 ° C., a voltage range of 2.75 to 4.2V, and a charging current value of 0.5C, at 15 ° C. A rate test was performed on a non-aqueous electrolyte secondary battery at 10 C (306 mA, 24.48 mA / cm ² ). From this result, the ratio of 10C capacity to 0.2C capacity {(10C capacity / 0.2C capacity) × 100} (%) was defined as the capacity retention rate (%).

[Example 1]
Mw as the starting material was used an ultra-high molecular weight polyethylene of 10 × 10 ^5. 83 parts by mass of liquid paraffin is added to 17 parts by mass of ultra-high molecular weight polyethylene, and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 mass by mass based on the mass of ultra-high molecular weight polyethylene. A polyethylene resin solution was prepared by adding tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane as an antioxidant and mixing them. The obtained polyethylene resin solution was put into a twin-screw extruder, kneaded at 180 ° C., supplied to a T-die, and the extruded product was cooled with a cooling roll controlled at 15 ° C. to form a gel-like sheet. The obtained gel-like sheet was sequentially biaxially stretched 10 times in the longitudinal direction and the width direction at 117.5 ° C. by a tenter stretching machine, and the sheet width was fixed in the tenter stretching machine as it was, and the temperature was 117.5 ° C. Was held for 10 seconds. Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank, and after removing liquid paraffin, it was dried to obtain a polyolefin microporous membrane. Finally, using an oven, heat fixation was carried out at a temperature of 130 ° C. for 10 minutes without stretching. Table 1 shows the raw material formulation and film forming conditions of the polyolefin microporous film, and Table 3 shows the evaluation results of the polyolefin microporous film.

[Examples 2 to 10, 12, 14, Comparative Examples 1 to 9]
A polyolefin microporous film was prepared in the same manner as in Example 1 except that the raw material formulation and the film forming conditions were changed as shown in Tables 1 and 2.

[Example 11]
Mw as the starting material was used an ultra-high molecular weight polyethylene of 9.0 × 10 ^5. 75 parts by mass of liquid paraffin is added to 25 parts by mass of ultra-high molecular weight polyethylene, and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 mass by mass based on the mass of ultra-high molecular weight polyethylene. A polyethylene resin solution was prepared by adding tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane as an antioxidant and mixing them. The obtained polyethylene resin solution was put into a twin-screw extruder, kneaded at 180 ° C., supplied to a T-die, and the extruded product was cooled with a cooling roll controlled at 15 ° C. to form a gel-like sheet. The obtained gel-like sheet was sequentially biaxially stretched 9.0 times in the longitudinal direction and 6.7 times in the width direction at 118 ° C. by a tenter stretching machine, and the sheet width was fixed as it was in the tenter stretching machine at 118 ° C. It was held at temperature for 10 seconds. Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank, and after removing liquid paraffin, it was dried to obtain a polyolefin microporous membrane. After removing the liquid paraffin, the dried film was stretched 1.4 times in the width direction at 135.5 ° C. and heat-fixed for 3 minutes.

[Example 13, Comparative Examples 10 and 11]
A polyolefin microporous film was prepared in the same manner as in Example 11 except that the raw material formulation and the film forming conditions were changed as shown in Tables 1 and 2.

In Tables 1 and 2, "UHPE" means ultra-high molecular weight polyethylene, and "HDPE" means high-density polyethylene.
The evaluation results of the obtained polyolefin microporous membrane are as shown in Tables 3 and 4.

In the embodiment of the present invention, by using a polyolefin microporous membrane having few unopened pores, the ion resistance, strength, and shrinkage rate are excellent as compared with the conventional polyolefin microporous membrane, so that the capacity retention rate at high output A microporous polyolefin membrane with excellent properties and high safety can be obtained.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on July 25, 2019 (Japanese Patent Application No. 2019-137308) and a Japanese patent application filed on July 25, 2019 (Japanese Patent Application No. 2019-137309). Is taken here as a reference.

Claims (11)

1. The shrinkage rate in the MD direction at 105 ° C./8 h is less than 10%, and the surface SEM observation image obtained by observation using a scanning electron microscope (SEM) is a rectangular field of 11.7 μm × 9.4 μm in each of the vertical and horizontal directions. The total resin area (unopened hole a on the surface) in the section that is divided into 0.5 μm images and satisfies the relationship of R2 ≧ R1 is 20 μm ² or less in the surface SEM observation image 11.7 μm × 9.4 μm. ,
   A microporous polyolefin membrane having a fibril diameter of 10 to 50 nm.
   Here, the unopened hole portion a on the surface is the area (F1) and the hole area of the fibrils in which the total length of the fibrils in one divided section is 0.875 μm, and the total length of the fibrils is multiplied by the fibril diameter calculated by the palm porometer. The ratio (F1 / P1) of (P1) is R1,
   The ratio (F2 / P2) of the resin area (F2) and the hole area (P2) in each of the divided sections is defined as R2.
   It refers to the total area of the resin in each section that satisfies the relationship of R2 ≧ R1.
2. The shrinkage rate in the MD direction at 105 ° C./8 h is less than 10%, and the unopened portion b on the surface with respect to the total field of view of the surface SEM observation image obtained by observation using a scanning electron microscope (SEM). A polyolefin microporous film having an area ratio of 25% or less and a fibril diameter of 10 to 50 nm.
   Here, the unopened portion b on the surface is an opening in which the ratio of the resin portion is calculated from the total area of the surface SEM observation image in each division in which the surface SEM observation image is divided into sections of 0.5 μm each in length and width. It is a resin part of a section having a pore ratio (%) -2 (%) or more.
   The area ratio of the unopened hole portion b on the surface is the ratio of the total area of the unopened hole portion b on the surface to the total area of the visual field of the surface SEM observation image.
3. Claim that the area ratio of the unopened portion of the cross section is 22% or less with respect to the rectangular viewing area of 11.7 μm × 9.4 μm of the cross section SEM observation image obtained by observation using a scanning electron microscope (SEM). The polyolefin microporous film according to 1 or 2.
   Here, the unopened portion of the cross section is a resin portion in a section in which the cross-sectional SEM observation image is divided into sections of 0.3 μm in length and width, and the resin ratio contained in the divided sections is 80% or more.
   The area ratio of the unopened portion of the cross section is the ratio of the total area of the unopened portion of the cross section to the total area of the visual field of the cross section SEM observation image.
4. The polyolefin microporous film according to any one of claims 1 to 3, wherein the average pore size in the surface SEM observation image is 30 to 55 nm.
5. The polyolefin microporous film according to any one of claims 1 to 4, wherein the number of pores in the surface SEM observation image is 90 / μm ² or more.
6. The polyolefin microporous membrane according to any one of claims 3 to 5, wherein the average pore size in the cross-sectional SEM observation image is 20 to 85 nm.
7. The polyolefin microporous membrane according to any one of claims 3 to 6, wherein the number of pores is 60 / μm ² or more in the cross-sectional SEM observation image.
8. The microporous polyolefin membrane according to any one of claims 1 to 7, wherein the puncture strength when the film thickness is converted to 7 μm is 3.0 N or more.
9. The polyolefin microporous membrane according to any one of claims 1 to 8, wherein the porosity is 50% or less.
10. The polyolefin microporous membrane according to any one of claims 1 to 9, wherein the film thickness is 12 μm or less.
11. A separator for a non-aqueous electrolytic solution secondary battery made of the polyolefin microporous membrane according to any one of claims 1 to 10.