JP2022155243A - Polyolefin microporous film - Google Patents

Abstract

To provide a polyolefin microporous film that can achieve the long life and high productivity of a battery in a balanced manner and can be used as a battery separator.SOLUTION: A polyolefin microporous film has one side and the other side. The one side and the other side have a surface smoothness of 50,000 sec/10 cm3 or more and 130,000 sec/10 cm3 or less. The polyolefin microporous film has an air permeability of 30 sec/100 cm3 or more and 300 sec/100 cm3 or less.SELECTED DRAWING: None

Description

The present invention relates to polyolefin microporous membranes.

Polyolefin microporous membranes are used in battery separators, capacitor separators, fuel cell materials, microfiltration membranes, etc., and are particularly used as lithium ion secondary battery (LIB) separators or constituent materials thereof. The separator prevents direct contact between the positive and negative electrodes, and also allows ions to permeate through the electrolyte held in its micropores.

In recent years, LIBs have been applied not only to small electronic devices such as mobile phones and laptop computers, but also to electric vehicles such as electric vehicles and small electric motorcycles. In-vehicle LIBs tend to have a larger capacity per unit cell in order to extend the cruising range, so they are being developed to have a larger energy capacity per unit volume. Therefore, various polyolefin microporous membranes have been studied as separators for LIB having a large energy capacity per volume (Patent Documents 1 to 4).

In Patent Document 1, from the viewpoint that thermal runaway can be suppressed when an internal short circuit occurs inside a battery with a high energy density due to the presence of foreign matter, etc., it is possible to measure the melt viscoelasticity of a polyolefin microporous film at 230 ° C. The loss tangent (tan δ) is discussed.

In Patent Document 2, in order to improve the battery characteristics, the surface of the polyolefin microporous membrane in contact with the electrode is made uneven, and specifically, the surface smoothness is adjusted to 50000 seconds or less and the surface porosity is adjusted to 40% or more. By doing so, it is described that the effective cross-sectional area of the electrodes is improved, the amount of electrolyte retained is improved, and the movement speed of lithium ions between electrodes during charging and discharging is improved.

In Patent Document 3, the dynamic friction coefficient of the surface of a microporous membrane containing a mixture of polypropylene and polyethylene is studied from the viewpoint of excellent high-temperature cycle performance, productivity, and battery winding performance when used as a separator.

Patent Document 4 describes a polyolefin microporous membrane having a specific film thickness, tensile strength, and tensile elongation from the viewpoint of thinning of the polyolefin microporous membrane, coatability, and reduction of neck-in under high tension. and lamination of porous layers thereon.

WO2020/179101 JP-A-9-296060 JP 2012-14914 A JP 2020-164791 A

In recent years, the demand for LIBs has increased rapidly with the spread of smart phones and electric vehicles. In particular, in-vehicle LiBs, which are produced in large quantities and used for a long period of time, are required to have high productivity and long battery life.

However, conventional polyolefin microporous membranes proposed as separators for LIB with large energy capacity per volume did not pay attention to high productivity and long battery life, or failed to balance them.

Accordingly, an object of the present invention is to provide a polyolefin microporous membrane that can achieve a good balance between long battery life and high productivity and that can be used as a battery separator.

The present inventors have found that the above problems can be solved by setting the surface smoothness and air permeability of one side and the other side of the polyolefin microporous membrane to specific ranges, and have completed the present invention. rice field. One aspect of the present invention is illustrated below.
[1]
A polyolefin microporous membrane having one side and the other side,
The surface smoothness of one surface and the other surface is 50,000 sec/10 cm ³ or more and 130,000 sec/10 cm ³ or less, and the air permeability is 30 sec/100 cm ³ or more and 300 sec/100 cm ³ or less. A polyolefin microporous membrane.
[2]
The polyolefin microporous membrane according to [1], wherein at least one surface has a surface smoothness of 50,000 sec/10 cm ³ to 100,000 sec/10 cm ³ .
[3]
The polyolefin microporous membrane according to [1] or [2], wherein the smoothness ratio between one surface and the other surface of the polyolefin microporous membrane is 1.0 or more and 2.6 or less.
[4]
One surface of the polyolefin microporous membrane has a smoothness of 50,000 sec/10 cm ³ or more and 100,000 sec/10 cm ³ or less, and the other surface has a smoothness of 60,000 sec/10 cm ³ or more and 130. ,000 sec/10 cm ³ or less, the polyolefin microporous membrane according to [3].
[5]
The polyolefin microporous membrane according to [3] or [4], wherein the smoothness ratio is more than 1.0 and less than or equal to 2.6.
[6]
The polyolefin microporous membrane according to any one of [1] to [5], wherein one or both surfaces of the polyolefin microporous membrane have a dynamic friction coefficient of 0.20 to 0.60.
[7]
The polyolefin microporous membrane according to any one of [1] to [6], wherein one or both surfaces of the polyolefin microporous membrane have a dynamic friction coefficient of 0.25 or more and 0.55 or less.
[8]
The polyolefin microporous membrane according to any one of [1] to [7], wherein the polyolefin microporous membrane has a porosity of 40% or more and 60% or less.
[9]
The polyolefin microporous membrane has a viscosity average molecular weight (Mv) of 400,000 to 1,000,000, and is expressed as a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polyolefin microporous membrane. The polyolefin microporous membrane according to any one of [1] to [8], which has a molecular weight distribution Mw/Mn of 10 to 25.

ADVANTAGE OF THE INVENTION According to this invention, the long life of a battery and high productivity can be achieved in good balance, and the polyolefin microporous membrane which can be used as a battery separator can be provided.

An embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail below for the purpose of illustrating, but the present invention is not limited to the present embodiment. In this specification, the upper limit and lower limit of each numerical range can be combined arbitrarily. In addition, in this specification, unless otherwise specified, the term "to" means to include both numerical values as an upper limit and a lower limit.

<Microporous membrane>
One aspect of the present invention is a polyolefin microporous membrane. A preferred embodiment of the polyolefin microporous membrane has low electronic conductivity, ionic conductivity, high resistance to organic solvents, and fine pore diameters. Further, the polyolefin microporous membrane can be used as a battery separator or its component, particularly a secondary battery separator or its component.

<Relationship between surface smoothness and air permeability>
The polyolefin microporous membrane according to this embodiment has one surface and the other surface, and the surface smoothness of the one surface and the other surface is 50,000 sec/10 cm ³ or more and 130,000 sec/10 cm ³ and an air permeability of 30 sec/100 cm ³ or more and 300 sec/100 cm ³ or less.

If the air permeability of the polyolefin microporous membrane and the surface smoothness of one side and the other side (hereinafter also referred to as "both surfaces") are within the above numerical ranges, the polyolefin microporous membrane is suitable for use in batteries. When used as a separator, it tends to be possible to achieve a good balance between long battery life and high productivity. This tendency is remarkable in lithium ion secondary battery separators, particularly LIB separators.

According to the present invention, the smoothness of both surfaces of the polyolefin microporous membrane, along with the air permeability of the polyolefin microporous membrane, (i) the productivity of the battery or LiB separator, such as line transportability, permeability after coating, and (ii) battery life, such as electrolyte absorption, local ion conduction, rate performance, cycle performance, and the like. Therefore, properties (i) and (ii) can be achieved in good balance by controlling the air permeability of the polyolefin microporous membrane and the surface smoothness of one side and the other side within appropriate numerical ranges.

The air permeability of the polyolefin microporous membrane is 30 sec/100 cm ³ or more and 300 sec/100 cm ³ or less from the viewpoint of balancing the above characteristics (i) and (ii). The air permeability of the polyolefin microporous membrane should be 40 sec/100 cm ³ or more from the viewpoint of suppressing the generation of deposits due to local ion conduction or suppressing self-discharge to further extend the battery life. , more preferably 50 sec/100 cm ³ or more, and even more preferably 60 sec/100 cm ³ or more. The air permeability of the polyolefin microporous membrane is preferably 290 sec/100 cm ^{3 or less, more preferably 250 sec/100 cm 3 or} less, from the viewpoint of improving electrolyte absorption and securing rate characteristics. It is preferably 200 sec/100 cm ³ or less, more preferably 90 sec/100 cm ³ or less. The above air permeability (sec/100 cm ³ ) is measured by the method described in Examples.

The air permeability can be adjusted within the above numerical range by controlling the biaxial stretching ratio, heat setting (HS) temperature, minimum heat setting ratio, etc. in the production of the polyolefin microporous membrane, for example.

The surface smoothness of both surfaces of the polyolefin microporous membrane is 50,000 sec/10 cm ³ or more and 130,000 sec/10 cm ³ or less from the viewpoint of balancing the above characteristics (i) and (ii). The surface smoothness of both surfaces is 53,000 sec from the viewpoint of suppressing the generation of deposits due to local ion conduction, further extending the battery life, and suppressing the increase in air permeability after coating the membrane. /10 cm ³ or more, more preferably 55,000 sec/10 cm ³ or more, and even more preferably 60,000 sec/10 cm ³ or more. The surface smoothness of both surfaces is preferably 95,000 sec/10 cm ³ or less, more preferably 90,000 sec/10 cm ³ or less, and 85,000 sec/ It is more preferably 10 cm ³ or less, and particularly preferably 80,000 sec/10 cm ³ or less. Surface smoothness is measured by the method described in Examples.

The surface smoothness of at least one surface of the polyolefin microporous membrane is preferably 50,000 sec/10 cm ³ to 100,000 sec/10 cm ³ . The surface smoothness of at least one surface is preferably 50,000 sec/10 cm from the viewpoint of suppressing local ion conduction to further extend the battery life and suppressing the increase in air permeability after coating to the membrane. ³ or more, more preferably 60,000 sec/10 cm ³ or more, still more preferably 65,000 sec/10 cm ³ or more, and particularly preferably 70,000 sec/10 cm ³ or more. The surface smoothness of at least one surface is preferably 100,000 sec/10 cm ³ or less, more preferably 98,000 sec/10 cm ³ or less, from the viewpoint of improving electrolyte solution absorption and transportability. It is preferably 94,000 sec/10 cm ³ or less.

The ratio of surface smoothness between one surface and the other surface of the polyolefin microporous membrane (hereinafter also referred to as "surface/back smoothness ratio") is preferably 1.0 or more and 2.6 or less. The front and back smoothness ratio is obtained by dividing the higher smoothness by the lower smoothness for both surfaces of the polyolefin microporous membrane, and is 1.0 when the smoothness of both surfaces is equal. . The front and back smoothness ratio is preferably 1.0 or more and 2.6 or less, and more preferably exceeds 1.0, from the viewpoint of improving the transportability of the polyolefin microporous membrane on the separator or battery production line. and 2.6 or less, more preferably 1.1 or more and 2.5 or less, and particularly preferably 1.2 or more and less than 2.3.

Further, the polyolefin microporous membrane can be configured so as to specify the surface with lower smoothness and the surface with higher smoothness among both surfaces of the polyolefin microporous membrane. In this embodiment, one surface of the polyolefin microporous membrane has a surface smoothness of 50,000 sec/10 cm ³ or more and 100,000 sec/10 cm ³ or less, and the other surface has a surface smoothness of 60,000 sec/10 cm 3 or more. It is preferably 10 cm ³ or more and 130,000 sec/10 cm ³ or less.

While not wishing to be bound by theory, the identification of the less smooth and smoother surfaces of polyolefin microporous membranes is associated with suppression of local fibril structure in the membrane. can be considered. One side of the polyolefin microporous membrane has a surface smoothness of 50,000 sec/10 cm ³ or more, which suppresses local ion conduction due to the local fibril assembly structure, further improving the long life of the battery. It is possible to suppress an increase in air permeability after coating the film, and by adjusting the surface smoothness to 100,000 sec/10 cm ³ or less, the electrolyte solution absorption can be improved. In addition, the other surface of the polyolefin microporous membrane is adjusted to have a surface smoothness of 60,000 sec/10 cm ³ or more to suppress local ion conduction due to the local fibril assembly structure, thereby improving battery performance. It can further improve the long life, suppress the increase in air permeability after coating to the membrane, and improve the electrolyte liquid absorption by adjusting the surface smoothness to 130,000 sec/10 cm ³ or less. be able to.

The surface smoothness of both surfaces of the polyolefin microporous membrane, the surface smoothness of one surface and the surface smoothness of the other surface, and the front and back smoothness ratio are the polyolefin microporous membrane (hereinafter also referred to as "raw sheet"). In production, the molecular weight/molecular weight distribution of the raw material, biaxial stretching temperature, HS minimum magnification, HS temperature, cooling rate until the raw fabric surface crystallizes, etc. can be adjusted within the above numerical range. can.

Components of the polyolefin microporous membrane and preferred embodiments are described below.

[Component]
Examples of polyolefin microporous membranes include porous membranes containing polyolefin resins, and resins such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene. Examples include porous membranes containing polyolefin fibers, nonwoven fabrics of polyolefin fibers, paper, and aggregates of insulating substance particles. Among these, when obtaining a multilayer porous film through a coating process, that is, a separator for a secondary battery, the coating liquid has excellent coating properties, and the thickness of the separator can be made thinner than the conventional separator. A porous film containing a polyolefin resin (hereinafter also referred to as a "polyolefin resin porous film") is preferable from the viewpoint of increasing the capacity per volume by increasing the active material ratio in the electricity storage device.

A polyolefin resin porous membrane will be described.
The polyolefin resin porous membrane is a polyolefin resin composition in which the polyolefin resin accounts for 50% by mass or more and 100% by mass or less of the resin component constituting the porous membrane from the viewpoint of improving the shutdown performance etc. when used as a separator for a secondary battery. It is preferably a porous membrane formed of a substance. The proportion of the polyolefin resin in the polyolefin resin composition is more preferably 60% by mass or more and 100% by mass or less, and even more preferably 70% by mass or more and 100% by mass or less.

The polyolefin resin contained in the polyolefin resin composition is not particularly limited. Examples thereof include homopolymers, copolymers and multistage polymers obtained. Moreover, these polyolefin resins may be used alone or in combination of two or more.
Among them, from the viewpoint of adjusting the surface smoothness and molecular weight of the polyolefin microporous membrane within a predetermined numerical range, the polyolefin resin is preferably a homopolymer, and more preferably does not contain a copolymer. From the standpoint of shutdown characteristics when the porous membrane is used as a secondary battery separator, the polyolefin resin is preferably polyethylene, polypropylene, copolymers thereof, and mixtures thereof.
Specific examples of polyethylene include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), high molecular weight polyethylene (HMWPE), and ultra high molecular weight polyethylene ( UHMWPE) and the like.
Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene.
Specific examples of copolymers include ethylene-propylene random copolymers and ethylene-propylene rubbers.

Moreover, the polyolefin resin is preferably composed mainly of polyethylene having a melting point in the range of 130° C. to 140° C. from the viewpoint of stopping the thermal runaway of the battery at an early stage.

In the present specification, high molecular weight polyethylene means polyethylene having a viscosity average molecular weight (Mv) of 100,000 or more. Mv of polyethylene can be calculated by the following formula by measuring the intrinsic viscosity [η] (dl/g) at 135°C in decalin solvent based on ASTM-D4020.
[η]=6.77×10 ⁻⁴ Mv ^0.67
Generally, the Mv of ultra-high molecular weight polyethylene is 1 million or more, so if such a definition is followed, high molecular weight polyethylene (HMWPE) in the present specification by definition includes UHMWPE. In addition, even polyethylene called "ultra-high molecular weight polyethylene" based on a definition different from such definition may fall under the high molecular weight polyethylene in the present embodiment when Mv is 100,000 or more. be.

As used herein, high density polyethylene refers to polyethylene having a density of 0.942 to 0.970 g/cm ³ . In the present invention, the density of polyethylene refers to a value measured according to D) density gradient tube method described in JIS K7112 (1999).

It is preferable to use polyethylene, particularly high-density polyethylene, as the polyolefin resin from the viewpoint of satisfying the required performance of low melting point and high strength when the polyolefin resin porous membrane is used as a secondary battery separator. Furthermore, from the viewpoint of exhibiting rapid fuse behavior, it is preferable that the main component of the polyolefin resin porous membrane is polyethylene. "The main component of the polyolefin resin porous membrane is polyethylene" means that the polyolefin resin porous membrane contains more than 50% by mass of polyethylene with respect to the total mass. With respect to the total mass of the polyolefin resin porous membrane, polyethylene is preferably 70% by mass or more, more preferably 75% by mass or more, from the viewpoint of adjusting the surface smoothness of the polyolefin microporous membrane within the above numerical range, More preferably 85% by mass or more, still more preferably 90% by mass or more, particularly preferably 95% by mass or more, most preferably 98% by mass or more, and may be 100% by mass.

The viscosity-average molecular weight (hereinafter referred to as Mv) of the polyolefin resin used as a raw material for the polyolefin microporous membrane is preferably 50,000 or more and less than 5 million, more preferably 80,000 or more and less than 2 million, and still more preferably 100,000 or more and less than 1,000,000. It is preferably 400,000 or more and 1,000,000 or less. When the viscosity-average molecular weight is 50,000 or more, uniform melt-kneading is facilitated, and sheet formability, particularly thickness stability tends to be excellent, which is preferable. Furthermore, if the viscosity-average molecular weight is less than 5,000,000 when used as a separator for a secondary battery, the pores tend to be easily blocked when the temperature rises, and a good shutdown function tends to be obtained, which is preferable.

From the viewpoint of adjusting the surface smoothness of the polyolefin microporous membrane within the above numerical range, it is preferable to use a mixture of a plurality of polyolefin raw materials as the raw material for the polyolefin microporous membrane. When a plurality of polyolefin raw materials are mixed and used, among others, it is preferable that a polyethylene having an Mv of less than 500,000 and a polyethylene having an Mv of 500,000 or more and less than 1,000,000 are included. By containing polyethylene having an Mv of less than 500,000 (for example, 100,000 or more and 300,000 or less), the viscosity does not increase excessively during melt kneading, and the deterioration of the molecular weight of the polyolefin can be suppressed, and excessive residual stress is generated during stretching. There is no residue, and thermal shrinkage tends to be small. In addition, the holes tend to be easily closed when the temperature rises, and a good shutdown function tends to be obtained. Furthermore, when the polyolefin microporous membrane melts, it tends to become viscous, so when it melts after the short circuit of the battery, it penetrates into the electrode moderately and easily exhibits an anchor effect, suppressing thermal shrinkage and reducing the short circuit area. It is presumed that it will be easier to suppress the increase. Including polyethylene having an Mv of 500,000 or more and less than 1,000,000 increases the stress during melt-kneading, making it possible to uniformly knead the resin. In addition, since the polyolefin microporous membrane expresses entanglement between the polymers, it tends to have high strength, and when the polyolefin microporous membrane melts and reaches a high temperature near 300 ° C., the viscosity does not decrease too much. It is presumed that the thermal runaway can be easily suppressed because the resin can easily stay in place without flowing out.

The ratio of polyethylene having a Mv of less than 500,000 used as a raw material for the polyolefin microporous membrane is preferably 10% by mass or more and 40% by mass or less, more preferably 12% by mass or more and 38% by mass, based on the total amount of the polyolefin raw material as 100% by mass. Below, more preferably 14% by mass or more and 36% by mass or less, still more preferably 16% by mass or more and 34% by mass or less, most preferably 18% by mass or more and 32% by mass or less (or 18% by mass or more and less than 30% by mass) be. When the proportion of polyethylene having an Mv of less than 500,000 is 10% by mass or more, good shutdown characteristics, an effect of suppressing heat shrinkage, and an effect of suppressing an increase in short-circuit area due to moderate viscosity when reaching a high temperature are obtained. tend to be able to When the proportion of polyethylene having an Mv of less than 500,000 is 40% by mass or less, there is a tendency that entanglement between polymers can be developed during melt-kneading. In addition, when the polyolefin microporous membrane reaches a high temperature, the fluidity of the molten resin does not become too large, and thermal runaway due to exposure of the electrodes due to outflow of the resin tends to be avoided.

The proportion of polyethylene having a Mv of 500,000 or more and less than 1,000,000 used as a raw material for the polyolefin microporous membrane is preferably 40% by mass or more and 90% by mass or less, more preferably 45% by mass or more, based on the total amount of the polyolefin raw material as 100% by mass. It is 85% by mass or less, more preferably 50% by mass or more and 80% by mass or less, even more preferably 55% by mass or more and 75% by mass or less, and most preferably 62% by mass or more and 73% by mass or less. When the ratio of polyethylene having Mv of 500,000 or more and less than 1,000,000 is 40% by mass or more, the effect of increasing the strength of the polyolefin microporous membrane and suppressing thermal runaway without outflow of the resin when reaching a high temperature is achieved. tend to be obtainable. When the proportion of polyethylene having an Mv of 500,000 or more and less than 1,000,000 is 90% by mass or less, excessive residual stress does not remain during stretching, and thermal shrinkage tends to be small. In addition, when the temperature rises, the pores tend to be easily blocked, and a good shutdown function tends to be obtained. As a result, it is assumed that the anchor effect is exhibited, thermal contraction is suppressed, and an increase in the short-circuit area is easily suppressed.

From the viewpoint of adjusting the surface smoothness of the polyolefin microporous membrane within the above numerical range, it is preferable to use polyethylene having an Mv of less than 1,000,000 as the raw material for the polyolefin microporous membrane.

When low-density polyethylene is included as a raw material for the polyolefin microporous membrane, the total amount of the polyolefin raw material is 100% by mass, preferably 10% by mass or less, more preferably 8% by mass or less, and still more preferably 6% by mass or less or 5% by mass. Less than, more preferably less than 4 wt% (or less than 3 wt%, even less than 1 wt%), most preferably no low density polyethylene. When the proportion of low-density polyethylene is 10% by mass or less, the polyolefin microporous membrane does not easily break when it reaches a high temperature of about 150 ° C., and the resin melts when it reaches a high temperature of about 300 ° C. fluidity is not excessively increased, and it tends to be possible to avoid thermal runaway due to exposure of the electrodes due to outflow of the resin. For the same reason, low-molecular-weight polyethylene having an Mv of less than 50,000 may also be included as long as it does not significantly impede the effects of the present invention, and its content is the same as in the case of low-density polyethylene. And it is preferable to use polyethylene with Mv of 50,000 or more.

Moreover, from the viewpoint of improving the heat resistance of the porous membrane, a mixture of polyethylene and polypropylene may be used as the polyolefin resin. The proportion of polypropylene used as a raw material for the polyolefin microporous film is preferably 1% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 10% by mass or less, and still more preferably, based on the total amount of the polyolefin raw material as 100% by mass. It is 4% by mass or more (or more than 4% by mass) and 9% by mass or less, more preferably 5% by mass or more and 8% by mass or less, and most preferably more than 5% by mass and less than 8% by mass.
That is, the proportion of polypropylene is preferably 1% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 10% by mass or less, and still more preferably, with the total amount of polyolefin resin in the resin components constituting the film being 100% by mass. is 4% by mass or more (or more than 4% by mass) and 9% by mass or less, more preferably 5% by mass or more and 8% by mass or less, and most preferably more than 5% by mass and less than 8% by mass.
When the proportion of polypropylene is 1% by mass or more, the polyolefin microporous membrane is less likely to rupture when it reaches a high temperature of around 150°C, and it is less likely that minute pinholes will occur at the initial stage of battery short circuit. . When the proportion of polypropylene is 20% by mass or less, the fluidity of the molten resin does not become too large when reaching a high temperature of nearly 300 ° C., and the heat caused by the exposure of the electrode due to the outflow of the resin or excessive penetration into the electrode. Easier to avoid runaway. Also, when the proportion of polypropylene is within the range of 1 to 20% by mass, the dynamic friction coefficient of one or both surfaces of the polyolefin microporous membrane can be adjusted within an appropriate numerical range as described later.

Mv of polypropylene used as a raw material for the polyolefin microporous membrane is preferably 200,000 or more and 1,000,000 or less, more preferably 250,000 or more and 900,000 or less, and still more preferably 300,000 or more and 800,000 or less. Although not wishing to be bound by theory, when the Mv of polypropylene is 200,000 or more, the entanglement between the polymers becomes strong during melt kneading, so that the polypropylene is uniformly dispersed in the polyethylene, and the heat resistance of the polypropylene is increased. It is speculated that sexuality can be expressed effectively. Moreover, even when the polyolefin microporous membrane reaches a high temperature close to 300° C., the viscosity does not increase too much, which is preferable. When the Mv of the polypropylene is 1,000,000 or less, it becomes easy to suppress deterioration of the molecular weight of the polymer due to excessive entanglement during melt-kneading. Moreover, it becomes easy to suppress the residual stress of a polyolefin microporous film.
Mv of polypropylene can be calculated according to the following formula by measuring the intrinsic viscosity [η] (dl/g) at 135°C in decalin solvent based on ASTM-D4020.
[η]=1.10×10 ⁻⁴ Mv ^0.80

Polypropylene used as a raw material for the polyolefin microporous film is preferably a homopolymer from the viewpoint of enhancing heat resistance and melt viscosity at high temperatures. Among them, isotactic polypropylene is preferred. The amount of isotactic polypropylene is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 98% by mass or more, and even more preferably 100% by mass, based on the total mass of polypropylene in the entire polyolefin microporous membrane. % by mass (all). When the isotactic polypropylene is 90% by mass or more, it is possible to suppress further melting of the microporous membrane due to temperature rise at the time of short circuit. In addition, since isotactic polypropylene has high crystallinity, phase separation from the plasticizer tends to proceed, and a film with good porosity and high permeability tends to be obtained. Therefore, the output or cycle characteristics can be favorably affected. Furthermore, since the homopolymer has a small amorphous portion, it is possible to suppress an increase in thermal shrinkage when heat below the melting point is applied or when the amorphous portion shrinks due to residual stress. It becomes easy to suppress the problem that the short-circuit area increases due to shrinkage of the amorphous portion when the temperature reaches around 100°C.

The polyolefin resin that may be contained in the polyolefin raw material and its content are not limited to the above description. Therefore, the polyolefin raw material may contain polyolefin resins different from those described above as long as they do not significantly inhibit the exertion of the effects of the present invention. may be

Any additive can be contained in the polyolefin resin composition. Additives include, for example, polymers other than polyolefin resins; inorganic fillers; antioxidants such as phenol, phosphorus, and sulfur; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers. antistatic agents; antifogging agents; coloring pigments and the like. The total amount of these additives added is preferably 20% by mass or less with respect to 100% by mass of the polyolefin resin from the viewpoint of improving shutdown performance, more preferably 10% by mass or less, and still more preferably 5% by mass. % or less.

From the viewpoint of the physical properties of the polyolefin resin porous membrane or raw material properties, the polyolefin raw material preferably has a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (molecular weight distribution: Mw/Mn) of 1 or more and 25 or less. , is more preferably 3 or more and 25 or less, more preferably 5 or more and 25 or less, and particularly preferably 10 or more and 25 or less.

The Mv of the polyolefin resin porous membrane is preferably 400,000 to 1,000,000, more preferably 450,000 to 900,000, even more preferably 500,000 to 800,000. , 550,000 to 700,000, and/or the molecular weight distribution (Mw/Mn) of the polyolefin resin porous membrane is preferably 10 to 25, more preferably 15 to 20. . By adjusting the Mv and/or Mw/Mn of the polyolefin resin porous membrane within the above numerical range, the surface smoothness of the polyolefin resin porous membrane can be adjusted within a predetermined range.

The above weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) are measured by the method described in Examples.

<Other properties>
The thickness of the polyolefin microporous membrane is preferably 1.0 μm or more and 50 μm or less, more preferably 3.0 μm or more and 25.0 μm or less, still more preferably 4.0 μm or more and 15.0 μm or less, and particularly preferably 5.0 μm or more and 12.0 μm or more. 0 μm or less, most preferably 8.0 μm or more and less than 12.0 μm. The thickness of the polyolefin microporous film is preferably 0.1 μm or more from the viewpoint of mechanical strength and insulation retention during a short circuit, and preferably 100 μm or less from the viewpoint of increasing the capacity of LIB. The thickness of the entire polyolefin microporous membrane can be adjusted, for example, by controlling the die lip interval, the draw ratio in the drawing step, and the like.

The dynamic friction coefficient of one or both surfaces of the polyolefin microporous membrane is preferably 0.20 to 0.60, more preferably 0.25 or more and 0.55 or less, and 0.30 or more and 0.50. More preferably: When the dynamic friction coefficient of one or both surfaces of the polyolefin microporous membrane is 0.60 or less, the polyolefin microporous membrane can be prevented from wrinkling during transportation. The surface smoothness of the porous membrane can be controlled within an appropriate numerical range. The dynamic friction coefficient of the surface of the polyolefin microporous membrane can be adjusted within the above numerical range by controlling the molecular weight/molecular weight distribution of the raw material, the biaxial draw ratio, the HS temperature, etc. can. The above dynamic friction coefficient is measured by the method described in Examples.

The puncture strength (gf) of the polyolefin microporous membrane of the present embodiment is preferably 180 gf to 500 gf, more preferably 210 gf to 480 gf, still more preferably 225 gf to 460 gf. In addition, the microporous polyolefin membrane has a per unit weight equivalent puncture strength (gf/(g/m ² )) of preferably 38 gf/(g/m ² ) to 90 gf/(g/m ² ), more preferably 40 gf/ (g/m ² ) to 85 gf/(g/m ² ), more preferably 42 gf/(g/m ² ) to 82 gf/(g/m ² ). When the lower limits of the puncture strength and the per unit area-based puncture strength are as described above, it is possible to prevent minute thinning or film breakage when contacting irregularities on the electrode surface in the production of a battery using a polyolefin microporous membrane. , it is possible to suppress battery defects due to micro short circuits. When the upper limits of the puncture strength and the per unit area-converted puncture strength are as described above, the shrinkage stress of the battery including the polyolefin microporous membrane can be suppressed. The puncture strength and weight-converted puncture strength are measured by the method described in Examples.

The porosity of the polyolefin microporous membrane is preferably 40% or more and 60% or less, more preferably 42% or more and 55% or less, and still more preferably 45% or more and 50% or less. The porosity of the polyolefin microporous membrane is preferably 40% or more from the viewpoint of improving ion conductivity, and preferably 60% or less from the viewpoint of withstand voltage characteristics. The porosity of the polyolefin microporous membrane is adjusted by controlling the mixing ratio of the polyolefin resin composition and the plasticizer, the biaxial stretching temperature, the stretching ratio, the HS temperature, the stretching ratio during HS, the relaxation rate during HS, and the like. be able to. The above porosity is measured by the method described in Examples.

<<Method for producing polyolefin microporous membrane>>
The method for producing the polyolefin microporous membrane is not particularly limited, and known production methods can be employed. For example:
(1) A method in which a polyolefin resin composition and a pore-forming material are melt-kneaded, formed into a sheet, stretched as necessary, and made porous by extracting the pore-forming material;
(2) A method of melt-kneading a polyolefin resin composition, extruding it at a high draw ratio, and then exfoliating the polyolefin crystal interface by heat treatment and stretching to make it porous;
(3) A method in which a polyolefin resin composition and an inorganic filler are melt-kneaded to form a sheet, and then the interface between the polyolefin and the inorganic filler is exfoliated by stretching to make the sheet porous;
(4) After dissolving the polyolefin resin composition, it is immersed in a poor solvent for the polyolefin to solidify the polyolefin and at the same time remove the solvent to make it porous.

Hereinafter, as an example of a method for producing a polyolefin microporous membrane, (1) the method of melt-kneading the polyolefin resin composition and the pore-forming material to form a sheet, and then extracting the pore-forming material will be described.

First, the polyolefin resin composition and the pore-forming material are melt-kneaded. As a melt-kneading method, for example, a polyolefin resin and, if necessary, other additives are put into a resin-kneading device such as an extruder, a kneader, a Laboplastomill, a kneading roll, or a Banbury mixer to heat and melt the resin component. However, a method of introducing and kneading the pore-forming material in an arbitrary ratio can be mentioned. As the polyolefin resin, from the viewpoint of adjusting the surface smoothness and dynamic friction coefficient of the polyolefin microporous membrane to be obtained within the appropriate numerical ranges as described above, it is preferable to use the raw materials described in the above <Constituent Elements>. .

Pore formers can include plasticizers, inorganic materials, or combinations thereof.

Although the plasticizer is not particularly limited, it is preferable to use a non-volatile solvent capable of forming a homogeneous solution above the melting point of the polyolefin. Specific examples of such nonvolatile solvents include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. . After extraction, these plasticizers may be recovered and reused by an operation such as distillation. Furthermore, preferably, the polyolefin resin, other additives, and plasticizer are preliminarily kneaded in predetermined proportions using a Henschel mixer or the like before being charged into the resin kneading device. More preferably, in pre-kneading, a portion of the plasticizer to be used is added, and the remaining plasticizer is appropriately heated and kneaded while being side-fed to a resin kneading device. By using such a kneading method, the dispersibility of the plasticizer is enhanced, and when the sheet-like molded body of the melt-kneaded product of the resin composition and the plasticizer is stretched in a later step, the film is not broken and the stretching ratio is increased. tend to be stretchable.

Among plasticizers, liquid paraffin has high compatibility with polyethylene or polypropylene when the polyolefin resin is polyethylene or polypropylene. This is preferred because it tends to be easier to implement.

The mass fraction of the polyolefin raw material in the composition comprising the polyolefin resin composition and the plasticizer is preferably 18% by mass or more and less than 35% by mass, more preferably 20% by mass or more and less than 33% by mass, and still more preferably 22% by mass. It is more than mass % and less than 31 mass %. When the mass fraction of the polyolefin raw material is less than 35% by mass, the energy during kneading does not increase excessively, so deterioration of the molecular weight due to excessive entanglement between the polymers can be suppressed. No loss of properties. On the other hand, when the mass fraction of the polyolefin raw material is 18% by mass or more, sufficient energy can be given during melt-kneading, and the polymers are entangled with each other and kneaded uniformly. Even when the mixture is stretched at a high draw ratio, the entanglement of polyolefin molecular chains does not occur, a uniform and fine pore structure is easily formed, and the strength is easily increased.

When the pore-forming material and the polyolefin raw material are melt-kneaded by an extruder, the temperature in the melt-kneading section (kneading temperature) is the specific energy during melt-kneading, or the membrane strength and pore diameter of the polyolefin microporous membrane. From the viewpoint of uniformity, the temperature is preferably 140°C or higher and lower than 200°C, more preferably 150°C or higher and lower than 190°C.

Next, the melt-kneaded product is formed into a sheet. As a method for producing a sheet-shaped molding, for example, the melt-kneaded product is extruded into a sheet through a T-die or the like, brought into contact with a heat conductor, and cooled to a temperature sufficiently lower than the crystallization temperature of the resin component. and solidification. Heat conductors used for solidification by cooling include metals, water, air, plasticizers, and the like. Among these, it is preferable to use a metal roll because the efficiency of heat conduction is high. In addition, when the extruded kneaded product is brought into contact with metal rolls, sandwiching it between at least a pair of rolls further increases the efficiency of heat conduction, the orientation of the sheet increases the film strength, and the surface smoothness of the sheet is improved. It is more preferable because it tends to improve.

The die lip interval when the melt-kneaded product is extruded into a sheet from a T-die is preferably 200 μm or more and 3,000 μm or less, more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, die build-up and the like are reduced, streaks, defects and the like are less likely to affect the quality of the film, and the risk of film breakage and the like in the subsequent stretching step can be reduced. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high, and uneven cooling can be prevented, and the thickness stability of the sheet can be maintained.

From the viewpoint of adjusting the surface smoothness or the coefficient of dynamic friction of one surface and the other surface of the resulting polyolefin microporous membrane to an appropriate numerical range as described above, in the extrusion step, (i) the raw fabric surface It is preferable to control the cooling rate until the is crystallized, and (ii) the raw fabric surface temperature. (i) The cooling rate is preferably 7 to 20° C./sec, so that the compactness of the crystal structure can be controlled. (ii) It is preferable that the surface temperature of the original fabric is different on both sides of the original fabric. The surface temperature when one surface is molded into a sheet is preferably 80° C. or higher and 110° C. or lower, and the surface temperature when the other surface is molded into a sheet is 180° C. or higher and 220° C. The following are preferred. (i) Cooling rate and (ii) original fabric surface temperature can be achieved by, for example, die outlet temperature, cast roll temperature, air knife air volume/temperature, air gap, metal roll surface roughness, and the like. In this connection, the forming by the rolls can be sequential or simultaneous.

Moreover, you may roll a sheet-like molded object. Rolling can be carried out, for example, by a pressing method using a double belt press or the like. By rolling the sheet-like compact, the orientation of the surface layer portion can be particularly increased. The rolling surface ratio is preferably more than 1 times and 3 times or less, and more preferably more than 1 time and 2 times or less. When the rolling ratio exceeds 1, the plane orientation tends to increase and the film strength of the finally obtained porous film tends to increase. On the other hand, when the rolling ratio is 3 times or less, the orientation difference between the surface layer portion and the center portion is small, and there is a tendency that a uniform porous structure can be formed in the thickness direction of the film.

(stretching)
The stretching step of stretching the sheet-shaped compact or the porous membrane may be performed before the step of extracting the pore-forming material from the sheet-shaped compact (pore-forming step), or the step of extracting the pore-forming material from the sheet-shaped compact. You may perform to the porous membrane which carried out. Furthermore, the stretching step may be performed before and after extraction of the pore-forming material from the sheet-form compact.

As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is preferable from the viewpoint of improving the strength and the like of the obtained porous membrane. Moreover, from the viewpoint of heat shrinkability of the obtained porous membrane, it is preferable to perform the stretching step at least twice.
When the sheet-shaped molding is stretched in the biaxial direction at a high magnification, the molecules are oriented in the plane direction, and the finally obtained porous membrane becomes difficult to tear and has a high puncture strength. Examples of stretching methods include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoints of optimizing the surface smoothness of the obtained microporous membrane, uniformity of pore size, uniformity of stretching, and shutdown property.

Here, the simultaneous biaxial stretching is a stretching method in which stretching in the MD (machine direction for continuous molding of the microporous membrane) and stretching in the TD (the direction crossing the MD of the microporous membrane at an angle of 90°) are performed simultaneously. No, the draw ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD stretching are performed independently. be in a state where

The draw ratio is preferably in the range of 28 times or more and less than 100 times, more preferably in the range of 32 times or more and 70 times or less, and still more preferably 36 times or more and 50 times or less. The draw ratio in each axial direction is preferably in the range of 4 times or more and less than 10 times in MD, 4 times or more and less than 10 times in TD, 5 times or more and less than 9 times in MD, and 5 times or more and less than 9 times in TD. is more preferably in the range of 5.5 times or more and less than 8.5 times in MD, and more preferably in the range of 5.5 times or more and less than 8.5 times in TD. Adjusting the total area ratio or the stretching ratio in each axial direction as described above can optimize the air permeability of the obtained polyolefin microporous membrane.

The temperature during stretching of the molded sheet or polyolefin microporous membrane is preferably 120°C or higher, more preferably higher than 122°C. The temperature during stretching is preferably 130°C or lower, more preferably 129°C. When the temperature during stretching, particularly the temperature during biaxial stretching, is 120° C. or higher, an increase in heat shrinkage due to excessive residual stress can be suppressed. When the temperature during stretching, particularly the temperature during biaxial stretching, is 130° C. or less, it is possible to impart sufficient strength to the polyolefin microporous membrane, prevent disturbance of the pore size distribution due to melting of the membrane surface, and improve battery performance. A long life can be ensured when charging and discharging are repeated. Especially when the biaxial stretching temperature is in the range of 120° C. to 130° C., it tends to be easy to optimize the surface smoothness and dynamic friction coefficient of the resulting microporous membrane.

In order to suppress heat shrinkage of the polyolefin microporous membrane, heat treatment may be performed after the stretching step or after the formation of the polyolefin microporous membrane for heat setting.

From the viewpoint of suppressing heat shrinkage, it is preferable to subject the polyolefin microporous membrane to heat setting. As a method of heat setting, for the purpose of adjusting physical properties, a stretching operation is performed at a predetermined temperature atmosphere and a predetermined stretching rate, and/or for the purpose of reducing stretching stress, a predetermined temperature atmosphere and a predetermined relaxation rate. relaxation operations. The relaxation operation may be performed after the stretching operation. These heat setting can be performed using a tenter or a roll stretching machine.

Through the heat setting process, the HS temperature is preferably within the range of 125° C. to 133° C. from the viewpoint of obtaining a polyolefin microporous membrane having the air permeability, surface smoothness, or dynamic friction coefficient adjusted as described above.

From the viewpoint of obtaining a polyolefin microporous membrane with higher strength and higher porosity, the magnification of the stretching operation is preferably 1.1 times or more, more preferably 1.2 times or more, relative to the MD and/or TD of the membrane. It is more preferably more than 1.4 times, preferably less than 2.3 times, more preferably less than 2.0 times. Further, when stretching is performed in both MD and TD during heat setting, the product of the stretching ratios in MD and TD is preferably less than 3.5 times, more preferably less than 3.0 times. When the MD and/or TD draw ratio at the time of heat setting is 1.1 times or more, it is possible to obtain the effects of high porosity and low heat shrinkage, and when it is 2.3 times or less, excessive It is possible to prevent a large pore size or a decrease in tensile elongation. When the product of the draw ratios in MD and TD during heat treatment is less than 3.5 times, an increase in heat shrinkage can be suppressed. From the viewpoint of obtaining a polyolefin microporous membrane having the air permeability, surface smoothness, or dynamic friction coefficient adjusted as described above, the minimum heat setting magnification is preferably 1.4 to 1.9 times.

The stretching operation during heat setting after extraction of the plasticizer is preferably carried out in the TD. The temperature in the HS drawing operation is preferably 110° C. or higher and 140° C. or lower from the viewpoint of suppressing the TMA stress while maintaining the permeability and maintaining the pore size uniformity.

A relaxation operation during heat setting is a contraction operation to the MD and/or TD of the membrane. By performing the relaxation operation within a predetermined range of conditions, it is possible to moderate the decrease in stress accompanying the temperature rise after melting, and to obtain a polyolefin microporous membrane that does not easily break even at around 160°C. The relaxation rate is the value obtained by dividing the dimension of the membrane after the relaxation operation by the dimension of the membrane before the relaxation operation. When both MD and TD are relaxed, it is a value obtained by multiplying the relaxation rate of MD and the relaxation rate of TD. The relaxation rate is preferably less than 1.0, more preferably less than 0.97, more preferably less than 0.95, even more preferably less than 0.90 and most preferably less than 0.85. From the viewpoint of film quality, the relaxation rate is preferably 0.4 or more, more preferably 0.6 or more, and even more preferably 0.8 or more. The absolute value of the strain rate during relaxation is preferably 1.0%/sec or more and 9.0%/sec or less, more preferably 1.5%/sec or more and 8.5%/sec or less, and still more preferably 2.0%/sec or more and 8.0%/sec or less, more preferably 2.5%/sec or more and 7.5%/sec or less, most preferably 3.0%/sec or more and 7.0%/sec or less It is below. The relaxation operation may be performed in both MD and TD, or may be performed only in either MD or TD. By stretching and relaxing at the above ratio, the heat shrinkage in MD and/or TD can be controlled within an appropriate range.

The relaxation operation during heat setting after extraction of the plasticizer is preferably performed in TD. The temperature in the relaxation operation is preferably 125° C. or higher and 133° C. or lower from the viewpoint of suppressing TMA stress and maintaining pore size uniformity.

If desired, a slurry containing an inorganic filler, a thermoplastic resin, a binder, a dispersant, a solvent, etc. is applied to one or both sides of the polyolefin microporous membrane obtained by the above production method to form a coating layer. good. Even if a coating layer is formed on the surface of the polyolefin microporous membrane according to this embodiment, an increase in air permeability can be suppressed.

EXAMPLES The embodiments of the present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples and Comparative Examples.

<Viscosity average molecular weight (Mv)>
Based on ASTM-D4020, the intrinsic viscosity [η] (dl/g) at 135°C in decalin solvent was determined.
Polyethylene was calculated by the following formula.
[η]=6.77×10 ⁻⁴ Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[η]=1.10×10 ⁻⁴ Mv ^0.80

<Gel Permeation Chromatography (GPC) of Polyolefin Raw Material>
-Preparation of sample A polyolefin raw material was weighed, and an eluent 1,2,4-trichlorobenzene (TCB) was added so that the concentration was 1 mg/ml. Using a high-temperature dissolver, the sample was allowed to stand at 160°C for 30 minutes, then shaken at 160°C for 1 hour, and it was visually confirmed that the sample had completely dissolved. The mixture was filtered through a 0.5 μm filter while still at 160° C., and the filtrate was used as a sample for GPC measurement.
・GPC measurement As a GPC apparatus, PL-GPC220 (trademark) manufactured by Agilent was used, and two 30 cm columns of TSKgel GMHHR-H (20) HT (trademark) manufactured by Tosoh Corporation were used and adjusted as described above. 500 µl of the sample for GPC measurement was injected into the measuring machine, and GPC measurement was performed at 160°C.
A calibration curve was prepared using commercially available monodisperse polystyrene with a known molecular weight as a standard substance, and polystyrene-equivalent molecular weight distribution data for each sample was obtained. In the case of polyethylene, molecular weight distribution data in terms of polyethylene was obtained by multiplying the molecular weight distribution data in terms of polystyrene by 0.43 (Q factor of polyethylene/Q factor of polystyrene=17.7/41.3). In the case of polypropylene, molecular weight distribution data in terms of polypropylene was obtained by multiplying (Q factor of polypropylene/Q factor of polystyrene=26.4/41.3). Thereby, the weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of each sample were obtained.

<Film thickness (μm)>
Using a micropachymeter (type KBM manufactured by Toyo Seiki Co., Ltd.), the film thickness was measured under an atmosphere of room temperature of 23±2° C. and humidity of 40%. A terminal with a diameter of 5 mmφ was used, and a load of 44 gf was applied for measurement. In addition, a plurality of samples were stacked and measured so that the measured thickness was 14 μm or more.

<Porosity (%)>
A sample of 10 cm×10 cm square was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were obtained, and the porosity was calculated from these and density (g/cm ³ ) using the following formula.
Porosity (%) = (volume - mass / density) / volume x 100

<Air Permeability (sec/100 cm ³ )>
In accordance with JIS P-8117, using a Gurley air permeability meter, G-B2 (trademark) manufactured by Toyo Seiki Co., Ltd., the air permeability of the polyolefin microporous membrane was measured at a temperature of 23 ° C. and a humidity of 40%. The air permeability was obtained by measuring the resistance.

<Puncture Strength (gf), Weight-Converted Puncture Strength (gf/(g/m ² ))>
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed in a sample holder having an opening with a diameter of 11.3 mm. Next, the central portion of the fixed microporous membrane was subjected to a puncture test at a radius of curvature of the tip of the needle of 0.5 mm, a puncture speed of 2 mm/sec, a temperature of 23°C, and a humidity of 40%. Raw puncture strength (gf) was obtained as the load. The weight-converted puncture strength (gf/(g/m ² )) was calculated by dividing the pierce strength (gf) by the weight per unit area (g/m ² ).

<Smoothness (sec/10 cm ³ )>
In accordance with ISO 8791-5: 2020, using a stainless steel nozzle with an inner diameter of 0.15 mm and a length of 50 mm in an air permeability smoothness meter EYO-5 manufactured by Asahi Seiko Co., Ltd., a temperature of 23 ° C. and The smoothness of the polyolefin microporous membrane was measured in an atmosphere with a humidity of 40%. Surface smoothness measurements were performed on one surface and the other surface of the polyolefin microporous membrane, respectively, and the front-to-back smoothness ratio was also calculated as described above.

<Coefficient of dynamic friction>
For one surface and the other surface of the microporous membrane, the coefficient of dynamic friction was measured using a KES-SE friction tester manufactured by Kato Tech Co., Ltd., a load of 50 g, a contact area of 10 × 10 = 100 mm ² (hardness of 0.5 mmφ). 20 pieces of stainless steel SUS304 piano wire are wound without gaps and without overlapping), MD 50 mm under conditions of contact feed speed 1 mm / sec, tension 6 kPa, temperature 23 ° C., and humidity 50% A sample size of 200 mm×TD was measured three times each in MD and TD, and the average value was obtained for calculation. Tables 4 and 5 show the average values of three measurements in MD and TD for both surfaces of the microporous membrane sample.

<Increased air permeability>
Polymer PVdF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer, PVdF:HFP = 80:20 (mass ratio), weight average molecular weight 1.5 × 10) was added to acetone and heated at 50°C for about 12 hours. A polymer solution was prepared by dissolving the above. As inorganic particles, Al ₂ O ₃ powder was added to the polymer solution so that the mass ratio of the polymer:inorganic particles was 17:83, and the inorganic particles were milled using a ball mill method for 12 hours or more. was crushed and dispersed to prepare a slurry. The average particle size of the inorganic particles in the slurry thus produced was 600 nm.

After the microporous membrane is immersed in the prepared slurry and dip-coated, the thickness of the slurry coating layer coated on both sides is adjusted using a wire bar with a wire size of 0.5 mm, respectively, and the solvent is was dried. The thickness of the coating layers formed on both sides of the polyolefin microporous membrane substrate was 5.0 μm. The increase in air permeability of the polyolefin microporous membrane is expressed by the following formula:
Increase in air permeability (sec) = air permeability after coating - air permeability before coating. Also, the increase in air permeability was evaluated according to the following criteria.
(Evaluation Criteria for Increase in Air Permeability)
A: Increase in air permeability of 50 s or less B: Increase in air permeability of over 50 s to 70 s or less C: Increase in air permeability of over 70 s to 100 s or less D: Increase in air permeability of over 100 s

<Transportability>
A microporous film having a length of 1000 m was prepared and wound up with a winder, and the displacement of the end face after winding was measured and evaluated according to the following criteria. For this evaluation item, A (good) and B (acceptable) were used as criteria for acceptance.
A (Good): Deviation of the end face during winding is 1 mm or less.
B (Acceptable): The misalignment of the end face during winding is more than 1 mm and 5 mm or less.
C (impossible): Misalignment of the end face during winding is greater than 5 mm.

<Production of battery>
A positive electrode, a negative electrode, and a non-aqueous electrolyte were prepared by the following procedures a to c.

a. Fabrication of positive electrode 92.2% by mass (wt%) of lithium cobalt composite oxide LiCoO2 as an active material, _2.3 % by weight each of flake graphite and acetylene black as a conductive agent, and polyvinylidene fluoride (PVDF)3 as a binder. A slurry was prepared by dispersing .2 wt % in N-methylpyrrolidone (NMP). This slurry was applied with a die coater to both sides of a 20 μm-thick aluminum foil serving as a positive electrode current collector, dried at 130° C. for 3 minutes, and then compression-molded with a roll press. At this time, the positive electrode active material coating amount was 250 g/m ² and the active material bulk density was 3.00 g/cm ³ .

b. Preparation of Negative Electrode A slurry was prepared by dispersing 96.9 wt % of artificial graphite as an active material, 1.4 wt % of ammonium salt of carboxymethyl cellulose and 1.7 wt % of styrene-butadiene copolymer latex as binders in purified water. This slurry was applied by a die coater to both sides of a copper foil having a thickness of 12 μm as a negative electrode current collector, dried at 120° C. for 3 minutes, and then compression-molded by a roll press. At this time, the applied amount of the active material of the negative electrode was 106 g/m ² and the bulk density of the active material was 1.35 g/cm ³ .

c. Preparation of Non-Aqueous Electrolyte A solution was prepared by dissolving LiBF ₄ as an electrolyte at a concentration of 1.0 mol/L in a mixed solvent of ethylene carbonate:propylene carbonate:γ-butyrolactone=1:1:2 (volume ratio).

d. Evaluation of liquid absorption and liquid injection The positive electrode prepared in the previous section a was cut vertically by 96 mm and the horizontal direction was 40 mm, the negative electrode prepared in the previous section b was cut vertically by 98 mm and horizontally by 42 mm. It was cut to a size of 100 mm and 44 mm in TD. Next, the negative electrode, the separator, and the positive electrode were laminated in this order from the bottom side so that the central portions thereof were aligned to form a laminate. A load of 58.8 N (6.0 kg) was uniformly applied to the entire laminate, and the pressure was reduced to 5 torr. After being left in this state for 10 minutes, the pressure was returned to normal pressure, and after wiping off excess electrolyte, the laminate was dismantled. A when the area permeated by the electrolyte with respect to the area of the separator is 90% or more, B when 80% or more and less than 90%, C when 70% or more and less than 80%, less than 70% is D.

e. Cycle test Using the positive electrode, the negative electrode, and the non-aqueous electrolyte obtained in the above a to c, and using the microporous membranes obtained in the examples and comparative examples as separators, a current value of 1 A (0.3 C ), a laminated secondary battery having a size of 100 mm×60 mm and a capacity of 3 Ah was prepared by constant current constant voltage (CCCV) charging for 3 hours under the condition of a final battery voltage of 4.2 V.

The resulting battery was discharged at a discharge current of 1C to a final discharge voltage of 3V at a temperature of 25°C, and then charged to a final charge voltage of 4.2V at a charging current of 1C. This cycle was defined as one cycle, and charging and discharging were repeated. Then, using the capacity retention rate after 300 cycles with respect to the initial capacity (capacity in the first cycle), the cycle characteristics were evaluated according to the following criteria.
(Evaluation criteria for cycle characteristics)
A: Capacity retention rate of 70% or more B: Capacity retention rate of 65% or more and less than 70% C: Capacity retention rate of 60% or more and less than 65% D: Capacity retention rate of less than 60%

<<Example 1>>
<Production of polyolefin microporous membrane>
A polyolefin microporous membrane was produced by the following procedure. As shown in Tables 1 and 2, the resin raw material composition was PE1 as the first type of polyethylene, PE2 as the second type of polyethylene, and PP1 as the polypropylene. 0.3 parts by mass of tetrakis-(methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate)methane was mixed as an antioxidant with the above resin composition. Each mixture obtained was fed into a twin-screw extruder through a feeder. Furthermore, liquid paraffin (kinematic viscosity at 37.78° C. of 75.90 cSt, indicated as “LP” in Tables 2 and 3) was used as a pore-forming material. Injected into the extruder by side feed so as to be parts by mass, kneaded the resin raw material composition under the casting conditions shown in Table 2, extruded from the T die installed at the tip of the extruder, and then immediately cooled cast. It was cooled and solidified with a roll to form a sheet with a thickness of 1.3 mm. This sheet was stretched 7×7 times at 125° C. using a simultaneous biaxial stretching machine, and then immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, the sheet was dried and stretched 1.70 times in the TD at 131° C. using a tenter stretching machine. After that, the stretched sheet was subjected to a heat treatment at 131° C. to relax the TD to obtain a polyolefin microporous membrane. Table 4 shows the test results.

<<Examples 2 to 17 and Comparative Examples 1 to 12>>
Polyolefin microporous membranes of Examples 2 to 17 and Comparative Examples 1 to 12 were produced according to the production method of Example 1 under the conditions shown in Tables 1 to 5. In Table 1, the notations of PE1 to PE9 and PP1 to PP2 are for convenience, and the order of adding raw materials in the present invention is not limited to the order of these symbols.

Claims (9)

A polyolefin microporous membrane having one side and the other side,
The surface smoothness of one surface and the other surface is 50,000 sec/10 cm ³ or more and 130,000 sec/10 cm ³ or less, and the air permeability is 30 sec/100 cm ³ or more and 300 sec/100 cm ³ or less. A polyolefin microporous membrane. 2. The polyolefin microporous membrane according to claim 1, wherein the surface smoothness of at least one surface is 50,000 sec/10 cm ³ to 100,000 sec/10 cm ³ . 3. The polyolefin microporous membrane according to claim 1 or 2, wherein the smoothness ratio between one surface and the other surface of the polyolefin microporous membrane is 1.0 or more and 2.6 or less. One surface of the polyolefin microporous membrane has a smoothness of 50,000 sec/10 cm ³ or more and 100,000 sec/10 cm ³ or less, and the other surface has a smoothness of 60,000 sec/10 cm ³ or more and 130. ,000 sec/10 cm ³ or less, the polyolefin microporous membrane according to claim 3. 5. The polyolefin microporous membrane according to claim 3 or 4, wherein the smoothness ratio is more than 1.0 and less than or equal to 2.6. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein one or both surfaces of the polyolefin microporous membrane have a dynamic friction coefficient of 0.20 to 0.60. The polyolefin microporous membrane according to any one of claims 1 to 6, wherein one or both surfaces of the polyolefin microporous membrane have a dynamic friction coefficient of 0.25 or more and 0.55 or less. The polyolefin microporous membrane according to any one of claims 1 to 7, wherein the polyolefin microporous membrane has a porosity of 40% or more and 60% or less. The polyolefin microporous membrane has a viscosity average molecular weight (Mv) of 400,000 to 1,000,000, and is expressed as a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polyolefin microporous membrane. The polyolefin microporous membrane according to any one of claims 1 to 8, wherein the molecular weight distribution Mw/Mn is 10-25.