JP6960243B2 - Separator for lithium ion secondary battery - Google Patents

Description

The present invention relates to a separator for a lithium ion secondary battery.

Since the microporous resin film exhibits electrical insulation or ion permeability, it is used for battery separators, capacitor separators, fuel cell materials, precision filtration films, moisture permeable waterproof films, etc., and in particular, lithium ions. It is used as a separator for secondary batteries.

In recent years, lithium-ion secondary batteries have been applied not only to small electronic devices such as mobile phones and notebook personal computers, but also to electric vehicles such as electric vehicles and small electric motorcycles. The separator for a lithium ion secondary battery is required to have strength, air permeability, ion permeability, safety when incorporated into a battery, and the like.

Generally, a resin film is formed by melt extrusion of a resin, subsequent stretching, or the like. The porosity of the resin film is roughly classified into a dry method and a wet method.

As the dry method, an unstretched sheet containing incompatible particles such as inorganic particles and polyolefin is subjected to stretching and extraction to exfoliate the interface between different materials to form pores, a lamella pore-opening method, and a β-crystal pore-opening method. There are laws, etc.

In the lamella perforation method, an unstretched sheet having a crystalline lamellar structure is obtained by controlling the melt crystallization conditions at the time of sheet formation by melt extrusion of a resin, and the obtained unstretched sheet is stretched to cleave the lamella interface. It is a method of forming a hole.

In the β-crystal opening method, an unstretched sheet having β crystals having a relatively low crystal density is produced during melt extrusion of polypropylene (PP), and the produced unstretched sheet is stretched to obtain α having a relatively high crystal density. This is a method in which a crystal is transferred to a crystal and pores are formed by the difference in crystal density between the two.

As a wet method, a pore-forming material (extract) such as a plasticizer is added to polyolefin, dispersed, and sheeted, and then the extract is extracted with a solvent to form pores, if necessary. There is a method of performing stretching processing before and / or after extraction.

In order to use the microporous film produced by the dry method as a separator for a secondary battery, the strength of the microporous film has been studied (Patent Documents 1 to 5).

Patent Document 1 describes a sheet containing a first PP, a second PP or ethylene-octene copolymer incompatible with the first PP, and a β crystal nucleating agent according to the β crystal pore method. It is described to be stretched. It is described that the obtained microporous film has a vertical (MD) strength of 40 MPa or more, a porosity of 70% or more, and an average pore diameter of 40 to 400 nm.

Patent Document 2 describes that two types of PPs having different melt flow rates (MFRs) are subjected to melt extrusion and biaxial stretching according to the β crystal opening method.

Patent Document 3 describes that a mixture of PP and styrene-butadiene elastomer is sequentially subjected to biaxial stretching according to the β-crystal opening method.

Patent Document 4 describes a microporous PP film obtained by biaxial stretching according to the β-crystal opening method, having a thickness of 10 to 30 μm, a porosity of 55 to 85%, and an air permeation resistance of 70 to 300 seconds / 100 ml. , 0.18 to 0.50 N / 1 μm thick puncture strength, width direction (TD) heat shrinkage of 12% or less (at 135 ° C. for 60 minutes), and tensile strength of 60 to 200 MPa are described.

In Patent Document 5, in order to control the strength and air permeability, PP pellets and pellets made of polyethylene (PE) and a styrene-based elastomer are co-extruded according to the β-crystal perforation method, and biaxially stretched to be porous. It is described to prepare a sex-laminated film.

In order to use the microporous film produced by the wet method as a separator for a secondary battery, the strength of the microporous film has been studied (Patent Document 6). Patent Document 6 describes a microporous polyolefin film having a degree of orientation changing in the film thickness direction and having a piercing elongation of 2.2 to 2.4 mm from the viewpoint of both breaking strength and piercing strength. Has been done.

International Publication No. 2007/46226 Japanese Unexamined Patent Publication No. 2012-7156 Japanese Unexamined Patent Publication No. 2012-131990 International Publication No. 2013/5949 Japanese Unexamined Patent Publication No. 2014-4771 Japanese Unexamined Patent Publication No. 7-188440

In recent years, lithium-ion secondary batteries have been incorporated into small electronic devices or electric vehicles and used even in harsh environments. In connection with this, a separator containing a microporous film produced by a dry method has also been required to be further improved in strength, short-circuit characteristics, safety and the like when incorporated into a secondary battery.

However, with respect to the microporous film obtained by the dry method described in Patent Documents 1 to 5, there is still room for suppressing the complete short-circuit phenomenon in the crushing test of the secondary battery when incorporated into the secondary battery. be.

In view of the above circumstances, it is an object of the present invention to provide a separator for a lithium ion secondary battery capable of suppressing a complete short circuit phenomenon in a crushing test of the secondary battery when incorporated in the secondary battery. And.

The present inventors have solved the above problems by supporting specific non-conductive particles on a microporous polyolefin film and specifying the relationship between TD direction tearing and MD direction tearing of the separator surface during a separator puncture test. The present invention was completed by finding out what can be done.
That is, the present invention is as follows.
[1]
A separator for a lithium ion secondary battery containing a polyolefin resin (A) and a resin (B) different from the polyolefin resin (A).
The resin (B) is present on the surface of at least the microporous film formed of the polyolefin resin (A), and is non-conductive particles.
The microporous film formed of the polyolefin resin (A) is a dry microporous film formed by stretching a precursor containing the polyolefin resin (A) in the longitudinal direction (MD) to make it porous, and the polyolefin resin. A dry microporous film obtained by stretching the precursor containing (A) in the lateral direction (TD) to make it porous, and a calendar treatment after stretching the precursor containing the polyolefin resin (A) in the lateral direction (TD). The separator is a hemispherical needle having a needle tip having a radius of curvature of 0.5 mm, which is at least one selected from the group consisting of a dry microporous film obtained by making the separator porous in the Z-axis direction. When pressed at a speed of 2 mm / sec, the following equation (1):
0.3 ≤ tear length in the TD direction (mm) / tear length in the MD direction (mm) ≤ 1
... Equation (1)
A separator for a lithium ion secondary battery that satisfies the relationship represented by.
[2]
The separator for a lithium ion secondary battery according to [1], wherein the non-conductive particles have a softening start temperature of 20 ° C. or higher and 50 ° C. or lower.
[3]
The microporous film further contains the polyolefin resin (A) and a resin (C) different from the resin (B), and contains a precursor containing the polyolefin resin (A) at least in the longitudinal direction (MD) or laterally. The separator for a lithium ion secondary battery according to [1] or [2], which is obtained by stretching in the direction (TD) and impregnating the stretched molded product with the resin (C).

According to the present invention, there is provided a separator for a lithium ion secondary battery capable of suppressing a complete short circuit phenomenon in a crush test of the secondary battery when incorporated in the secondary battery.

FIG. 1 is a schematic view for explaining a test for measuring the tear length of the separator, and FIG. 1A is a side view showing the relationship between the needle, the evaluation sample, and the holding plate at the start of the puncture test, and the upper surface of the plate. FIG. 3B is a side view showing the positional relationship between the needle and the evaluation sample at the time of contact. 2A and 2B are photographs of the surface of the separator after the puncture test shown in FIG. 1, FIG. 2A is a photograph of the surface of the separator porous by the wet method of Reference Example 1, and FIG. 2B is a comparative example. It is a separator surface photograph obtained in 1, (c) is a separator surface photograph obtained in Comparative Example 3, and (d) is a separator surface photograph obtained in Example 1.

Hereinafter, embodiments for carrying out the present invention (hereinafter, abbreviated as “embodiments”) will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist thereof.

<Separator for lithium ion secondary battery>
One aspect of the present invention is a separator for a lithium ion secondary battery. As used herein, a separator refers to a member arranged between a plurality of electrodes in a lithium ion secondary battery and having lithium ion permeability and, if necessary, shutoff characteristics.

The separator for a lithium ion secondary battery according to the first embodiment of the present invention includes a microporous film formed of a polyolefin resin (A), and has a specific resin (B) on the surface of the microporous film. Is a separator for a lithium ion secondary battery, and in the first embodiment, the resin (B) is different from the polyolefin resin (A).

The separator for a power storage device according to the first embodiment may be composed of only the microporous film formed of the polyolefin resin (A) and the resin (B), and further has a filler porous layer in addition to these. You may. When the separator for a power storage device of the present invention has a porous filler layer, the porous filler layer may be one side of a microporous film or as long as at least a part of the surface composed of the polyolefin resin (A) and the resin (B) is exposed. It can be arranged on both sides or as an intermediate layer of laminated porous substrate layers.

The microporous portion of the microporous film is based on a network structure (hereinafter, also referred to as “fibril structure”) composed of the polyolefin resin (A). At least one of the microporous portions is defined by the smallest unit (hereinafter, referred to as "fibril") of the network structure composed of the polyolefin resin (A).

In the first embodiment, the resin (B) is present on the surface of at least the microporous film formed of the polyolefin resin (A) and is non-conductive particles.

In the first embodiment, when the separator is pressed at a speed of 2 mm / sec in the Z-axis direction with a hemispherical needle having a needle tip having a radius of curvature of 0.5 mm, the following equation (1):
0.3 ≤ tear length in the TD direction (mm) / tear length in the MD direction (mm) ≤ 1
... Equation (1)
Satisfy the relationship represented by.

By supporting the resin (B) on the microporous film so as to satisfy the relationship represented by the above formula (1), stress concentration on the microporous film can be prevented and the complete short-circuit time of the battery can be reduced. It can be prolonged.
Here, the resin (B) is non-conductive particles, and the softening start temperature is preferably 20 ° C. or higher and 50 ° C. or lower. When the softening start temperature is 20 ° C. or higher and 50 ° C. or lower, the semi-molten resin (B) covers the outer periphery of the fibril of the anisotropic polyolefin resin (A), so that the entire microporous film is anisotropic. Is isotropic, and as a result, the stress concentration caused in the Z-axis direction can be eliminated and the complete short circuit phenomenon can be suppressed.

Regarding the formula (1), the tear length (mm) in the TD direction / the tear length (mm) in the MD direction is preferably more than 0.3 and less than 1 from the viewpoint of suppressing the complete short-circuit phenomenon of the separator-containing cell. , Or 0.31 or more and 0.99 or less.

A method for measuring the tear length (mm) in the TD direction / the tear length (mm) in the MD direction of the separator will be described in detail in Examples. The tear length (mm) in the TD direction / tear length (mm) in the MD direction of the separator is the type of polyolefin resin (A), the conditions for producing a microporous film, the type of resin (B), and the resin (B). By optimizing the softening start temperature of the resin (B), the amount of the resin (B) supported on the microporous film, and the like, it can be adjusted within the range of 0.3 or more and 1 or less.

The film thickness of the separator for a lithium ion secondary battery is preferably 100 μm or less, more preferably within the range of 2 to 80 μm, and further preferably 3 from the viewpoint of the balance with the puncture strength or the puncture depth and the miniaturization of the secondary battery. It is in the range of ~ 30 μm. The film thickness of the separator can be adjusted by optimizing the production conditions of the microporous film.

Each component in the embodiment described above will be described below.

<Microporous film>
The microporous film according to the first embodiment is produced by a dry porous method. Specifically, the microporous film is a dry microporous film obtained by stretching a precursor containing a polyolefin resin (A) in the longitudinal direction (MD) to make it porous, and a precursor containing a polyolefin resin (A). A dry microporous film formed by stretching the film in the lateral direction (TD) to make it porous, and a dry microporous film formed by stretching the precursor containing the polyolefin resin (A) in the lateral direction (TD) and then performing a calendar treatment to make the film porous. At least one selected from the group consisting of porous films.

The microporous film according to the first embodiment contains a polyolefin resin (A) as a main component and carries a resin (B) different from the polyolefin resin (A). The microporous film preferably has low electron conductivity, ionic conductivity, high resistance to an organic solvent, and a fine pore size. If desired, the microporous surface formed by the fibril of the polyolefin resin (A) is further coated with the resin (B) or with a resin (C) different from the polyolefin resin (A) and the resin (B). It's okay.

[Polyolefin resin (A)]
The fact that the microporous film contains the polyolefin resin (A) as a main component means that the ratio of the polyolefin resin (A) in the microporous film is 50% by mass or more with respect to the mass of the microporous film. means. The proportion of the polyolefin resin (A) in the microporous film is preferably 50% by mass or more and 100% by mass or less, more preferably 55% by mass or more and 99% by mass or less, from the viewpoint of film wettability, thickness and shutdown characteristics. Particularly preferably, it is 60% by mass or more and 98% by mass or less.

The polyolefin resin (A) forms a fibril structure so as to have microporous as the main component of the microporous film, and a resin (B) different from the polyolefin resin (A) is supported. The polyolefin resin (A) is preferably insoluble in octane or hexane at room temperature from the viewpoint of maintaining the fibril structure of the microporous film. That is, it is preferable that the extract extracted from the film with octane or hexane does not contain the component derived from the polyolefin resin (A).

As the polyolefin resin (A), for example, a homopolymer, a copolymer, or a multi-stage polymerization obtained by using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene or the like as a monomer. Examples include polymers. These polyolefin resins (A) may be used alone or in combination of two or more.
Among these, polyethylene, polypropylene, and copolymers thereof, and mixtures thereof are preferable from the viewpoint of shutdown characteristics, and one or more polypropylenes are preferable from the viewpoint of shutdown characteristics, octane insolubility, and hexane insolubility. More preferred.

Specific examples of polyethylene include low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, ultra-high molecular weight polyethylene, and the like. In the present specification, high-density polyethylene refers to polyethylene having a ^{density of 0.942 to 0.970 g / cm 3.} The density of polyethylene means a value measured according to the D) density gradient tube method described in JIS K7112 (1999).
Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene and the like.
The ethylene-propylene copolymer may be of random or block structure, or in the form of ethylene propylene rubber.

The microporous film containing the polyolefin resin (A) as a main component may be a single-layer type or a laminated type. The single-layer film is preferably formed of one layer containing polypropylene as the polyolefin resin (A) from the viewpoint of air permeability, strength and octane insolubility. From the viewpoint of air permeability and strength, the laminated film preferably has an outer layer containing polypropylene as the polyolefin resin (A) and an inner layer containing polyethylene as the polyolefin resin (A).

[Resin (B)]
The resin (B) is a resin different from the polyolefin resin (A) described above, and is non-conductive particles. The non-conductive particles can be used alone or as a dispersoid in an organic solvent, a dispersoid in water, or a dispersoid in a mixture of an organic solvent and water. The non-conductive particles can be polymer particles having a core-shell structure.

The resin (B) present on the surface of the microporous film formed of the polyolefin resin (A) is supported on the polyolefin resin (A), and is preferably coated on the microporous film and /. Alternatively, it is impregnated in the microporous film.

The softening start temperature of the resin (B) is preferably 20 ° C. or higher and 50 ° C. or lower, more preferably, from the viewpoint of eliminating the stress concentration caused by the Z-axis direction of the separator surface and suppressing the complete short-circuit phenomenon of the separator-containing cell. Is 22 ° C. or higher and 48 ° C. or lower.

The method for measuring the softening start temperature will be described below.
First, 10 mg of the measurement sample is weighed in an aluminum pan, an empty aluminum pan is used as a reference in a differential thermal analysis measuring device, and the temperature rise rate is 10 ° C./min in the measurement temperature range of -100 ° C to 500 ° C. , Measure the DSC curve under normal temperature and humidity. In this heating process, the baseline immediately before the heat absorption peak of the DSC curve whose differential signal (DDSC) becomes 0.05 mW / min / mg or more and the tangent line of the DSC curve at the inflection point that first appears after the heat absorption peak. The intersection with is defined as the glass transition point (Tg). Further, a temperature 25 ° C. higher than the glass transition point is set as the softening start point.
When the decomposition point is lower than the softening start temperature of the non-conductive particles, the softening start temperature may not be observed due to decomposition.

The method for measuring the decomposition point will be described below.
In a nitrogen atmosphere, the measurement sample is heated from 30 ° C. to a heating rate of 10 ° C./min by a differential thermogravimetric simultaneous measuring device. At this time, the temperature at which the weight loss rate reaches 10% by weight is defined as the decomposition point.
When both the softening start temperature and the decomposition point of the particles are observed, the lower temperature is regarded as the softening start temperature.

Non-conductive particles or polymer particles having a core-shell structure include resins roughly classified into the following (b1) to (b4):
(B1) Nitrile-based resin (b2) Acrylic resin (b3) Aliphatic conjugated diene-based resin (b4) Resin different from the above (b1) to (b3)

(B1) Nitrile-based resin The nitrile-based resin is a resin containing a polymerization unit having a nitrile group as a main component. In the present specification, the inclusion of a certain polymerization unit as a main component means that the polymerization unit is 50 mol% or more with respect to the total mole of all the monomers charged at the time of polymerization. If desired, the nitrile-based resin has, in addition to the polymerization unit having a nitrile group, a linear alkylene polymerization unit having 4 or more carbon atoms, an aromatic vinyl polymerization unit, a polymerization unit having a hydrophilic group, and a thermally crosslinkable group. It may contain at least one selected from the group consisting of polymerization units. Examples of the hydrophilic group include a carboxylic acid group, a hydroxyl group, a sulfonic acid group and the like. Examples of the thermally crosslinkable group include an epoxy group, an N-methylolamide group, an oxazoline group and the like.

The iodine value of the nitrile resin is preferably 3 to 10 mg / 100 mg, 4 to 9 mg / 100 mg, or 5 to 8 mg / 100 mg.

The nitrile-based resin can be obtained by polymerizing a monomer having a nitrile group or copolymerizing a monomer having a nitrile group with another monomer. The monomer having a nitrile group is, for example, (meth) acrylonitrile. (Meta) acrylonitrile means acrylonitrile or methacrylonitrile. Other monomers include ethylenically unsaturated compounds, such as (meth) acrylic acid esters, unsaturated hydrocarbons having 4 or more carbon atoms, and the like. The (meth) acrylic acid ester means an acrylic acid ester or a methacrylic acid ester, and is, for example, n-butyl acrylate, 2-ethylhexyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate and the like. .. Unsaturated hydrocarbons having 4 or more carbon atoms are, for example, 1,3-butadiene.

Specifically, the nitrile resin is polyacrylonitrile, acrylonitrile-butadiene copolymer, acrylonitrile-styrene copolymer, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene-acrylate copolymer, or hydrogenation thereof. Things and so on.

(B2) Acrylic resin The acrylic resin is a resin containing a polymerization unit derived from an acrylic compound as a main component. The inclusion of a polymerization unit derived from an acrylic compound as a main component means that the amount of the acrylic compound is 50 mol% or more based on the total moles of all the monomers charged at the time of polymerization. The acrylic compound is a monomer having a (meth) acryloyl group which is an acryloyl group or a methacryloyl group.

If desired, the acrylic resin has a polymerization unit having a nitrile group, a linear alkylene polymerization unit, an aromatic vinyl polymerization unit, a polymerization unit having a hydrophilic group, and a heat crosslinkability, in addition to the polymerization unit derived from the acrylic compound. It may contain at least one selected from the group consisting of polymerization units having groups. Examples of the hydrophilic group include a carboxylic acid group, a hydroxyl group, a sulfonic acid group and the like. Examples of the thermally crosslinkable group include an epoxy group, an N-methylolamide group, an oxazoline group and the like.

The acrylic resin can be obtained by polymerizing an acrylic compound or copolymerizing an acrylic compound with another monomer.

The following monomers may be used as the acrylic compound:
(Meta) acrylic acid, such as acrylic acid, 2-methacrylic acid, 2-pentenoic acid, 2,3-dimethylacrylic acid, 3,3-dimethylacrylic acid, itaconic acid, and alkali metal salts thereof;
(Meta) acrylic acid ester;
Fluorine-containing acrylic acid ester;
Amide group-containing (meth) acrylic acid or amide group-containing (meth) acrylate;
(Meta) Acrylic functional silicon-containing monomer;
(Meta) Acrylic polyfunctional monomers such as diacrylate compounds, triacrylate compounds, tetraacrylate compounds, dimethacrylate compounds, and trimethacrylate compounds.

As other monomers, the following monomers may be used:
Acid group-containing monomeric units, eg, ethylenically unsaturated carboxylic acid monomers such as 3-butenoic acid, trans-butendionic acid, cis-butendionic acid, maleic acid and maleic acid derivatives, ethylenically unsaturated sulfonic acid mono A metric, an ethylenically unsaturated sulfonic acid monomer, and a vinyl monomer having an acid component;
Maleimide;
α, β-unsaturated nitrile monomer;
Vinyl monomers that do not contain acid components, such as vinyl acetate, aliphatic vinyl monomers such as aliphatic conjugated diene monomers, and aromatic vinyl monomers such as styrene;
Unsaturated polyalkylene glycol ether-based monomer;
Ethylene functional silicon-containing monomer;
Isothiazolinone compounds;
Chelate compound;
Fluorine-containing monomers such as
Reactive surfactants, such as sulfonic acid group-containing monomers;
Monomers that impart water solubility to the polymer, such as trifluoromethyl methacrylate, trifluoromethyl acrylate, perfluorooctyl methacrylate, monomethyl-2-methacryloyloxyethyl phosphate, dioctyl-2-methacryloyloxyethyl phosphate, diphenyl -2-Methacryloyloxyethyl phosphate, etc.

Specifically, the acrylic resin is an acrylic soft polymer, an acrylic hard polymer, an acrylic-styrene copolymer, a sulfonated acrylic polymer, a seed polymer thereof, a hydrogenated product, a graft, or the like.

The acrylic resin may be in the form of non-conductive organic particles. The acrylic resin may be water-soluble when formed from the acrylic compound and the silicon-containing monomer. The acrylic resin may contain carboxymethyl cellulose as a thickener.

(B3) Aliphatic Conjugated Diene Resin The aliphatic conjugated diene resin is a resin containing an aliphatic monomer having a conjugated diene as a main component. In the present specification, the inclusion of a certain polymerization unit as a main component means that the polymerization unit is 50 mol% or more with respect to the total mole of all the monomers charged at the time of polymerization.

The aliphatic monomer having a conjugated diene is a substituted or unsubstituted chain diene, and may be a straight chain or a branched chain. Specifically, the aliphatic monomer having a conjugated diene is 1,3-butadiene, 1,3-isoprene, 1,4-dimethyl-1,3-butadiene, 1,2-dimethyl-1,3-. Butadiene, 1,3-dimethyl-1,3-butadiene, 1,2,3-trimethyl-1,3-butadiene, 1,3,5-hexatriene, aloosimenes and the like.

The aliphatic conjugated diene-based resin can be obtained by polymerizing an aliphatic monomer having a conjugated diene or copolymerizing an aliphatic monomer having a conjugated diene with another monomer.

As other monomers, ethylene-based unsaturated carboxylic acid, sulfonic acid group-containing monomer, nitrile group-containing monomer, aromatic vinyl monomer, crosslinkable monomer, aromatic vinyl compound and the like are used. You can do it.

Specifically, the aliphatic conjugated diene-based resin may be a 1,3-butadiene polymer, a diene-based rubber, a thermoplastic elastomer, or a random copolymer, block copolymer, hydride or acid-modified product thereof. .. The aliphatic conjugated diene resin may optionally contain an antiaging agent such as a combination of a phenolic compound and a thioether compound, or a combination of a phenolic compound and a phosphite ester compound.

(B4) Resins different from resins (b1) to (b3) Resins (b4) different from resins (b1) to (b3) are, for example, thermoplastic fluororesins, sulfonic acid group-containing resins, cellulose-based resins, and the like. be. The resin (b4) may be in the form of organic polymer particles, graft polymer, polymer latex or the like.

Examples of the thermoplastic fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-ethylene copolymer, perfluoroalkoxyfluororesin, and tetrafluoroethylene-hexafluoropropylene copolymer. , Teflon-chlorotrifluoroethylene copolymer and the like.

Examples of the sulfonic acid group-containing resin include sulfonated polymers such as sulfonated polyether sulfone and sulfonated polysulfone.

Examples of the cellulosic resin include a cellulosic semi-synthetic polymer and a sodium salt or an ammonium salt thereof. The cellulosic resin may have a sulfur atom, a cationic group, an acid group, a propargyl group and the like.

[Polymer particles having a core-shell structure]
The polymer particles having a core-shell structure have a core portion containing the polymer and a shell portion containing the polymer. Further, the resin having a core-shell structure preferably has a segment showing compatibility with the electrolytic solution and a segment not showing compatibility with the electrolytic solution. As the polymer of the core portion or the shell portion, the resins (b1) to (b4) described above can be used.

As the polymer particles having a core-shell structure, for example, the monomer of the polymer forming the core portion and the monomer of the polymer forming the shell portion are used, and the ratio of these monomers is changed over time to carry out the polymerization stepwise. By doing so, it can be manufactured. Specifically, first, seed polymer particles are produced by polymerizing the monomer of the polymer forming the core portion. The seed polymer particles form the core of the particles. Then, in the polymerization system containing the seed polymer particles, the monomer of the polymer forming the shell portion is polymerized. As a result, the shell portion is formed on the surface of the core portion, so that polymer particles having a core-shell structure can be obtained. At this time, for example, a reaction medium, a polymerization initiator, a surfactant, or the like may be used, if necessary.

(Core part)
Examples of the polymer forming the core portion include highly crosslinked polymers of resins (b1) to (b4). Due to the high degree of cross-linking, the molecular motion of the polymer is suppressed even at a temperature above the glass transition point of the polymer, so that the core portion can maintain its shape.

The polymer forming the core portion is preferably obtained by polymerizing a crosslinkable vinyl monomer. Examples of the crosslinkable vinyl monomer include compounds having two or more, preferably two copolymerizable double bonds. Further, one type of crosslinkable vinyl monomer may be used alone, or two or more types may be used in combination at an arbitrary ratio.

Suitable crosslinkable vinyl monomers include, for example, non-conjugated divinyl compounds, polyvalent acrylate compounds and the like.

Examples of non-conjugated divinyl compounds include divinylbenzene and the like.
Examples of polyvalent acrylates include polyethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,6-hexane glycol diacrylate, neopentyl glycol diacrylate, polypropylene glycol diacrylate, and 2,2'-bis (4). -Diacrylate compounds such as acryloxipropyroxyphenyl) propane, 2,2'-bis (4-acryloxydiethoxyphenyl) propane; trimethylolpropane triacrylate, trimethylolethanetriacrylate, tetramethylolmethanetriacrylate, etc. Triacrylate compound; Tetraacrylate compound such as tetramethylolmethane tetraacrylate; Ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol Dimethacrylate compounds such as dimethacrylate, 1,6-hexaneglycol dimethacrylate, neopentyl glycol dimethacrylate, dipropylene glycol dimethacrylate, polypropylene glycol dimethacrylate, and 2,2'-bis (4-methacryloxydiethoxyphenyl) propane. Tomimethacrylate compounds such as trimethylolpropane trimethacrylate and trimethylolethanetrimethacrylate; and the like.

The proportion of the crosslinkable vinyl monomer is 20% by weight or more, preferably 25% by weight or more, and more preferably 30% by weight or more with respect to all the monomers of the polymer forming the core portion. By setting the proportion of the crosslinkable vinyl monomer to 20% by weight or more, the hardness, heat resistance and solvent resistance of the core portion can be improved. The upper limit is usually 100% by weight or less, preferably 98% by weight or less, and more preferably 95% by weight or less. Here, the amount of the crosslinkable vinyl monomer is based on a pure product equivalent excluding, for example, a diluent and impurities.

(Shell part)
As the polymer forming the shell portion, it is preferable to use a polymer containing a (meth) acrylate unit. By forming the shell portion with a polymer containing (meth) acrylate units, the electrical stability of the porous membrane can be improved. Examples of the acrylate include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl ethyl acrylate and the like. Examples of the methacrylate include methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate and the like.

From the viewpoint of electrical stability, the ratio of the (meth) acrylate unit in the polymer forming the shell portion is preferably 50% by weight or more, more preferably 60% by weight or more, still more preferably 70% by weight or more. It is 100% by weight or less.

(Size of non-conductive particles or particles of a polymer having a core-shell structure)
The number average particle size of the non-conductive particles or the polymer particles having a core-shell structure is 100 nm or more, preferably 200 nm or more, more preferably 300 nm or more, and 1500 nm or less, preferably 1200 nm or less, more preferably 1000 nm. It is as follows. By setting the average particle size of the number of particles in such a range, it is possible to form gaps between the particles to the extent that the movement of ions is not hindered while the particles have contact portions. Therefore, the strength of the microporous film can be improved, the short circuit of the battery can be prevented, and the cycle characteristics of the secondary battery can be improved.

The average particle size of the number of particles can be measured as follows. 200 particles are arbitrarily selected from photographs taken with a field emission scanning electron microscope at a magnification of 25,000 times. When the longest side of the particle image is La and the shortest side is Lb, (La + Lb) / 2 is the particle size. The average particle size of 200 particles is calculated as the average particle size.

The thickness of the shell portion of the polymer particles having a core-shell structure is 3% or more, preferably 5% or more, more preferably 7% or more, and 18% or less, based on the average particle size of the particles. It is preferably 16% or less, more preferably 14% or less. When the thickness of the shell portion is 3% or more with respect to the average particle size, the adhesiveness of the separator can be enhanced and the cycle characteristics of the secondary battery can be improved. Further, when the thickness of the shell portion is 18% or less with respect to the number average particle diameter, the pore diameter of the separator can be increased to such an extent that the movement of ions is not hindered, thereby improving the rate characteristics of the secondary battery. can. Further, since the core portion can be made relatively large by making the shell portion thin, the rigidity of the particles can be increased. Therefore, the rigidity of the microporous film can be increased and the shrinkage of the separator can be suppressed.

The thickness (S) of the shell portion is, for example, from the number average particle diameter (D1) of the seed polymer particles before forming the shell portion and the number average particle diameter (D2) of the non-conductive particles after forming the shell portion. , Can be calculated by the following formula.
(D2-D1) / 2 = S

(Amount of non-conductive particles or polymer particles having a core-shell structure)
The content ratio of the non-conductive particles or the particles of the polymer having a core-shell structure in the porous film may be 70% by weight or more, preferably 75% by weight or more, more preferably 80% by weight or more, and usually 98% by weight. % Or less, preferably 96% by weight or less, more preferably 94% by weight or less. When the content ratio of the particles is within the above range, gaps can be formed between the particles to the extent that the particles have contact portions and the movement of ions is not hindered, thereby improving the separator strength and the battery. Short circuit can be prevented stably.

Regarding the ratio of the resin (B) to the polyolefin resin (A), the porosity of the microporous film formed of the resins (A) and (B) is that of the microporous film formed of only the resin (A). It is preferable to apply the resin (B) to the resin (A) so that the porosity is 30% to 85%.

[Resin (C)]
The polyolefin resin (A) and the resin (C) different from the resin (B) preferably cover at least a part of the surface of the microporous portion of the microporous film, and preferably the outer peripheral portion of the fibril constituting the microporous portion. It may be arranged so as to cover the.

Elasticity of the resin (C) at 25 ° C. in order to deliver a relatively soft resin (C) to a large number of microporous portions, prevent stress concentration in the microporous portions in the microcracks, and improve the puncture strength of the separator. The ratio is preferably 50 to 700 MPa, more preferably 80 to 700 MPa, still more preferably 100 to 700 MPa, still more preferably 110 to 650 MPa.

The melting point of the resin (C) is preferably 130 ° C. or lower, more preferably 50 to 125 ° C., from the viewpoint of wettability to the polyolefin resin (A) and the productivity of the microporous film. As used herein, the melting point of a polymer refers to the temperature (° C.) corresponding to the heat absorption peak associated with the melting of a crystalline polymer in the DSC curve obtained by differential scanning calorimetry (DSC) of the polymer. When two endothermic peaks are observed on the DSC curve, the temperature corresponding to the endothermic peak on the higher temperature side is set as the melting point.

The resin (C) is preferably a hydrophobic resin from the viewpoint of air permeability, piercing strength and piercing depth of the microporous film. As used herein, the hydrophobic resin means a resin that is completely insoluble in water or has a solubility in water at 25 ° C. of less than 1 g / kg.

The resin (C) is preferably soluble in hexane or octane from the viewpoint of air permeability and strength of the microporous film, and more preferably the resin (C) is converted to octane at 25 ° C. Solubility is 20 g / kg or more. From the same viewpoint, it is preferable that the elastic modulus of the resin (C) used as a raw material of the microporous film is substantially equal to the elastic modulus of the extract extracted with hexane or octane from the formed microporous film. ..

The resin (C) is a polyolefin resin typified by polyethylene, polypropylene, polybutene and a copolymer thereof, polyethylene terephthalate, polycycloolefin, from the viewpoint of the resin content, thickness, air permeability and strength of the microporous film. Polyether sulfone, polyamide, polyimide, polyimideamide, polyaramid, polyvinyldifluoride, nylon, polytetrafluoroethylene, polymethacrylate, polyacrylate, polystyrene, polyurethane, or a copolymer thereof are preferable. More preferably, polyolefin resins typified by polyethylene, polypropylene, polybutene and their copolymers, polymethacrylates, polyacrylates, polystyrenes, polyurethanes, or polymers thereof, and polyethylene terephthalates, polycycloolefins, polyether sulfones, and the like. Examples thereof include resins such as polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene, and polyolefin resins typified by polyethylene, polypropylene, and polybutene, and copolymers thereof are preferable. The resin (C) is more preferably a polyolefin resin having an elastic modulus at 25 ° C. of 700 MPa or less, and further preferably a low melting point polyolefin resin having an elastic modulus of 110 to 520 MPa at 25 ° C. and a melting point of 130 ° C. or less. be.

Examples of the resin (C) include low molecular weight polypropylene having a weight average molecular weight of 140,000 or less, a copolymer of a monomer having 3 carbon atoms and a monomer having 4 carbon atoms (for example, an α-olefin copolymer of main component C4 / subcomponent C3). , Α-olefin copolymer of main component C3 / sub-component C4, etc.), polypropylene with low stereoregularity, etc. can be used.

The resin (C) is preferably arranged on the microporous surface in the microporous film without being localized on the outermost surface (outer surface) of the microporous film, and is preferably all or all of the microporous surface. It is more preferable that a part is coated. Although it is not desired to be bound by the theory, if the resin (C) is present on the microporous surface to such an extent that the air permeability of the microporous film can be maintained at a practical level, stress such as defects in the microporous film will occur. It is considered that the resin (C) relaxes / avoids stress concentration on the concentrated portion, and the breaking depth and breaking strength of the microporous film are improved.

Regarding the ratio of the resin (C) to the polyolefin resin (A), the porosity of the microporous film formed of the resin (A) and (C) is that of the microporous film formed of only the resin (A). It is preferable to apply the resin (C) to the resin (A) so that the porosity is 30% to 85%.

[Other components]
The microporous film may contain components other than the resins (A) to (C). Such components include, for example, inorganic fillers, woven or non-woven fabrics of resin fibers, paper, aggregates of insulating material particles, and the like.

Above all, the microporous film preferably contains an inorganic filler in addition to the resins (A) to (C) from the viewpoint of improving safety such as heat resistance of the separator and suppression of internal short circuits. The inorganic filler contained in the microporous film may be the same as the inorganic filler that forms the filler porous layer described later.

[Details of microporous film]
The porosity of the microporous film is preferably 30% or more, more preferably more than 30% and 95% or less, still more preferably 35% or more and 75% or less, and particularly preferably 35% or more and 55% or less. From the viewpoint of improving ionic conductivity, 30% or more is preferable, and from the viewpoint of strength, 95% or less is preferable. The porosity can be adjusted by controlling the resin composition, the mixing ratio of the resin and the plasticizer, the stretching conditions, the heat fixing conditions, and the like.

The puncture strength of the microporous film is preferably in the range of 0.25 kgf or more, more preferably 0.25 to 0.60 kgf, from the viewpoint of the productivity of the separator and the safety of the secondary battery. The piercing depth of the microporous film is preferably 2.5 mm or more, more preferably more than 2.5 mm and 4.5 mm or less, from the viewpoint of piercing strength, wettability to non-aqueous solvents, and withstand voltage resistance. It is preferably 2.6 mm or more and 4.5 mm or less, and particularly preferably 2.7 mm or more and 4.5 mm or less.

The film thickness of the microporous film is preferably 0.1 μm or more and 100 μm or less, more preferably 1 μm or more and 50 μm or less, still more preferably 3 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. From the viewpoint of mechanical strength, 0.1 μm or more is preferable, and from the viewpoint of increasing the capacity of the battery, 100 μm or less is preferable. The film thickness can be adjusted by controlling the die lip interval, stretching conditions, and the like.

The average pore size of the microporous film is preferably 0.03 μm or more and 0.80 μm or less, and more preferably 0.04 μm or more and 0.70 μm or less. From the viewpoint of ionic conductivity and withstand voltage, 0.03 μm or more and 0.80 μm or less are preferable. The average pore size can be adjusted by controlling the resin composition, extrusion conditions, stretching conditions, heat fixing conditions, and the like.

The viscosity average molecular weight of the microporous film is preferably 30,000 or more and 120,000 or less, more preferably 50,000 or more and less than 4,000,000, and further preferably 100,000 or more and 1,000 or more. Less than 000. When the viscosity average molecular weight is 30,000 or more, the melt tension during melt molding is increased, the moldability is improved, and the polymer tends to be entangled with each other to increase the strength, which is preferable. When the viscosity average molecular weight is 12,000,000 or less, it becomes easy to uniformly melt-knead, and the formability of the sheet, particularly the thickness stability tends to be excellent, which is preferable.

<Pollous filler layer>
The filler porous layer contains an inorganic filler and a resin binder.

(Inorganic filler)
The inorganic filler used for the porous filler layer is not particularly limited, but is preferably one having a melting point of 200 ° C. or higher, high electrical insulation, and electrochemically stable within the range of use of the lithium ion secondary battery. ..
The inorganic filler is not particularly limited, and is, for example, oxide-based ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, itria, zinc oxide, and iron oxide; nitrides such as silicon nitride, titanium nitride, and boron nitride. Ceramics: Silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, aluminum hydroxide, aluminum hydroxide, aluminum hydroxide, potassium titanate, talc, kaolinite, decite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, Ceramics such as amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, kaolin, and silica sand; glass fibers and the like can be mentioned. These may be used alone or in combination of two or more.

Among these, from the viewpoint of improving electrochemical stability and heat resistance of the separator,
Aluminum oxide compounds such as alumina and aluminum hydroxide; and aluminum silicate compounds having no ion exchange ability such as kaolinite, dekite, nacrite, halloysite, and pyrophyllite are preferable.

As the aluminum oxide compound, aluminum hydroxide oxide (AlO (OH)) is particularly preferable. As the aluminum silicate compound having no ion exchange ability, kaolin, which is mainly composed of kaolin minerals, is more preferable because it is inexpensive and easily available. As kaolin, wet kaolin and calcined kaolin obtained by calcining the wet kaolin are known. In the present invention, calcined kaolin is particularly preferable. Calcined kaolin is particularly preferable in terms of electrochemical stability because water of crystallization is released during the calcining treatment and impurities are also removed.

The average particle size of the inorganic filler is preferably more than 0.01 μm and 4.0 μm or less, more preferably more than 0.2 μm and 3.5 μm or less, and more than 0.4 μm and 3.0 μm. The following is more preferable. Adjusting the average particle size of the inorganic filler to the above range is preferable from the viewpoint of suppressing heat shrinkage at high temperatures even when the thickness of the porous filler layer is thin (for example, 7 μm or less). Examples of the method for adjusting the particle size of the inorganic filler and its distribution include a method of pulverizing the inorganic filler using an appropriate pulverizer such as a ball mill, a bead mill, or a jet mill to reduce the particle size. ..

Examples of the shape of the inorganic filler include a plate shape, a scale shape, a needle shape, a columnar shape, a spherical shape, a polyhedral shape, and a lump shape. A plurality of types of inorganic fillers having these shapes may be used in combination.

The ratio of the inorganic filler in the porous filler layer can be appropriately determined from the viewpoints of the binding property of the inorganic filler, the permeability of the separator, the heat resistance, and the like. The ratio of the inorganic filler in the filler porous layer is preferably 20% by mass or more and less than 100% by mass, more preferably 50% by mass or more and 99.99% by mass or less, and further preferably 80% by mass or more and 99.9% by mass. % Or less, particularly preferably 90% by mass or more and 99% by mass or less.

(Resin binder)
The type of resin binder contained in the porous filler layer is not particularly limited, but is insoluble in the electrolyte of the lithium ion secondary battery and is electrochemically stable in the range of use of the lithium ion secondary battery. It is preferable to use a resin binder.
Specific examples of such a resin binder include, for example, polyolefins such as polyethylene and polypropylene; fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and ethylene. Fluorine-containing rubber such as a tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, methacrylic acid Ester-acrylic acid ester copolymers, styrene-acrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers, rubbers such as ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate; ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxy Cellulous derivatives such as methyl cellulose; resins having a melting point and / or glass transition temperature of 180 ° C. or higher, such as polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester, etc., can be mentioned.

In order to reduce the ionic resistance of the porous filler layer, it is also preferable to include a polyalkylene glycol group-containing thermoplastic polymer described later in the porous filler layer as a resin binder.

As the resin binder, it is preferable to use a resin latex binder. When a resin latex binder is used as the resin binder, the separator provided with the filler porous layer containing the binder and the inorganic filler binds the resin binder onto the porous film through the step of applying the resin binder solution on the substrate. Compared with the separated separator, the ion permeability is less likely to decrease, and there is a tendency to provide a power storage device having high output characteristics. Further, the power storage device having the separator exhibits smooth shutdown characteristics even when the temperature rises rapidly at the time of abnormal heat generation, and tends to easily obtain high safety.

The resin latex binder is obtained by copolymerizing an aliphatic conjugated diene-based monomer, an unsaturated carboxylic acid monomer, and another monomer copolymerizable with the aliphatic conjugated diene-based monomer and the unsaturated carboxylic acid monomer from the viewpoint of improving the electrochemical stability and the binding property. Is preferred. The polymerization method in this case is not particularly limited, but emulsion polymerization is preferable. The method of emulsion polymerization is not particularly limited, and a known method can be used. The method for adding the monomer and other components is not particularly limited, and any of a batch addition method, a split addition method, and a continuous addition method can be adopted, and the polymerization method is one-stage polymerization or two-stage polymerization. , Or any of three or more stages of multi-step polymerization can be adopted.

The average particle size of the resin binder is preferably 50 to 500 nm, more preferably 60 to 460 nm, and even more preferably 80 to 250 nm. When the average particle size of the resin binder is 50 nm or more, the separator provided with the filler porous layer containing the binder and the inorganic filler is unlikely to have reduced ion permeability, and tends to provide a power storage device having high output characteristics. Further, even when the temperature rises rapidly at the time of abnormal heat generation, it is easy to obtain a power storage device that exhibits smooth shutdown characteristics and has high safety. When the average particle size of the resin binder is 500 nm or less, good binding property is exhibited, and when a multilayer porous film is formed, heat shrinkage is good, and the safety tends to be excellent.
The average particle size of the resin binder can be controlled by adjusting the polymerization time, the polymerization temperature, the raw material composition ratio, the raw material charging order, the pH, and the like.

The thickness of the porous filler layer is preferably 0.5 μm or more from the viewpoint of improving heat resistance and insulating properties, and preferably 50 μm or less from the viewpoint of improving the capacity and permeability of the battery.
Layer density of the filler porous layer is preferably from 0.5 to 3.0 g / cm ^3, more preferably 0.7~2.0cm ^3. When the layer density of the filler porous layer is 0.5 g / cm ³ or more, the heat shrinkage rate at high temperature tends to be good, and when it is 3.0 g / cm ³ or less, the air permeability tends to decrease. It is in.

<Manufacturing method of separator for lithium ion secondary battery>
Another aspect of the present invention is a method for producing a separator for a lithium ion secondary battery using the microporous film described above.

An example of a method for manufacturing a separator for a lithium ion secondary battery according to a second embodiment of the present invention is as follows:
A step of producing a microporous film by making a melt-kneaded product or a molded sheet of a polyolefin resin composition porous by a dry method or a wet method; and composed of a nitrile resin, an acrylic resin, and an aliphatic conjugated diene resin. A step of compounding at least one resin (B) selected from the group with a microporous film;
including.

The method for producing a separator for a lithium ion secondary battery may further include a step of laminating a filler porous layer containing an inorganic filler on a microporous film, if desired.

[Manufacturing of microporous film]
The microporous film can be produced by making a melt-kneaded product or a molded sheet of a polyolefin resin composition porous by a dry method or a wet method.

As a dry method, the polyolefin resin composition is melt-kneaded and extruded, and then the polyolefin crystal interface is peeled off by heat treatment and stretching. The polyolefin resin composition and the inorganic filler are melt-kneaded and molded on a sheet. , A method of peeling the interface between the polyolefin and the inorganic filler by stretching and the like.

As a wet method, a polyolefin resin composition and a pore-forming material are melt-kneaded to form a sheet, stretched as necessary, and then the pore-forming material is extracted. After the polyolefin resin composition is dissolved, the polyolefin is used. Examples thereof include a method of immersing the polyolefin in a poor solvent to coagulate the polyolefin and at the same time removing the solvent.

The polyolefin resin composition contains the polyolefin resin (A) in an amount of preferably 50% by mass or more, more preferably 60% by mass or more and 100% by mass or less.

The polyolefin resin composition may contain a resin other than the polyolefin resin (A), an arbitrary additive, or the like. Examples of the additive include an inorganic filler, an antioxidant, a metal soap, an ultraviolet absorber, a light stabilizer, an antistatic agent, an anti-fog agent, a coloring pigment and the like.

The melt-kneading of the polyolefin resin composition can be carried out by, for example, an extruder, a kneader, a lab plast mill, a kneading roll, a Banbury mixer or the like.

Examples of the pore-forming material include a plasticizer, an inorganic filler, or a combination thereof.

Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol.

Examples of the inorganic filler include oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, ittoria, zinc oxide, and iron oxide; nitrided silicon nitride, titanium nitride, boron nitride, etc. Physical ceramics; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, cericite, mica, amesite, bentonite, asbestos, zeolite , Calcium silicate, magnesium silicate, kaolin, kaolin, and other ceramics; glass fibers can be mentioned.

Sheet molding can be performed using, for example, a T-die, a metal roll, or the like. The molded sheet may be rolled with a double belt press or the like.

The pore forming step can be performed by a known dry method and / or a wet method. A stretching step may also be performed during the pore forming step or before or after the pore forming step. As the stretching treatment, either uniaxial stretching or biaxial stretching can be used, but biaxial stretching is preferable from the viewpoint of improving the strength of the obtained microporous film. When the sheet-shaped molded product is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, the final product is less likely to tear, and the sheet-like molded product has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoint of improving puncture strength, stretching uniformity, and shutdown property. Further, from the viewpoint of ease of controlling the plane orientation, successive biaxial stretching is preferable.

Simultaneous biaxial stretching refers to a stretching method in which MD (mechanical direction of continuous molding of microporous membrane) and TD (direction of crossing MD of microporous membrane at an angle of 90 °) are simultaneously stretched. The draw ratio in the direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD are stretched independently, and when MD or TD is stretched, the other direction is fixed in an unconstrained state or a fixed length. Make it a state.

In order to suppress the shrinkage of the microporous film, heat treatment may be performed for the purpose of heat fixation after stretching or pore formation. The heat treatment includes a stretching operation performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting physical properties, and / or a relaxing operation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress. Can be mentioned. The relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

As an example, a method for producing a microporous film by a dry lamella perforation method will be described. The dry lamellar perforation method is a lamellar interface by stretching a precursor in which a plurality of spherulites having a lamellar structure are bonded to each other via an amorphous polymer without using a solvent such as water or an organic solvent. It is a method of forming a hole by cleaving the solvent.

The dry lamellar pore opening method preferably includes (i) a step of extruding a precursor containing a polyolefin resin (A), and (ii) a step of uniaxially stretching the extruded precursor. Here, the precursor containing the polyolefin resin (A) is subjected to at least one stretching in the mechanical direction (MD) or the lateral direction (TD), and is, for example, a molten resin, a resin composition, or resin molding. It may be the body. The precursor extruded in the step (i) is, for example, an extruded product, a raw fabric sheet, a raw fabric film, or the like. The microporous film obtained by the dry lamellar perforation method including steps (i) and (ii) is suitably used for supporting, coating, dipping, or impregnating with the resin (B).

Step (i) can be performed by a conventional extrusion method. The extruder can include a T-die or an annular die with elongated holes.

The uniaxial stretching of the step (ii) can be performed as described above. Uniaxial stretching can be performed in the mechanical direction (MD) or the lateral direction (TD). Further, it is preferable that the precursor is stretched laterally (TD) after uniaxial stretching to be biaxially stretched. In biaxial stretching, mechanical (MD) stretching and lateral (TD) stretching with simultaneous controlled MD directional relaxation can be performed. Here, MD stretching can include both cold stretching and hot stretching. The microporous film obtained by stretching in step (ii) is suitably used for supporting, coating, dipping or impregnating with the resin (B).

From the viewpoint of suppressing the internal strain of the precursor, the precursor can be annealed during step (i), after step (ii), or before stretching in step (ii). Annealing is between a temperature 50 ° C. lower than the melting point of the polyolefin resin (A) and a temperature 10 ° C. lower than the melting point of the polyolefin resin (A), or 50 ° C. lower than the melting point of the polyolefin resin (A). It can be carried out within the range between the temperature and the temperature 15 ° C. lower than the melting point of the polyolefin resin (A).

It is preferable to perform calendar processing on the molded product stretched in the MD and / or TD directions during or after the step (ii). The calendered molded product is suitably subjected to supporting, coating, dipping or impregnating with the resin (B). The calendar processing can be performed by passing the stretched molded product through at least a pair of calendar rolls. The pair of calendar rolls may include, for example, a set of steel rolls and elastic rolls, or a set of two steel rolls. During calendar processing, the pair of calendar rolls can be heated or cooled.

From the viewpoint of the strength of the microporous film, in the dry lamella opening method described above, the precursor containing the polyolefin resin (A) is sequentially or simultaneously stretched in the MD and TD directions and subjected to calendar treatment (hereinafter referred to as “Calendar treatment”). , "MD / TD / calendar process") is also preferable. From the viewpoint of improving the strength, in the MD / TD / calendar process, sequential stretching in which TD stretching is performed after MD stretching is more preferable.

At least a part of the surface of the microporous portion of the microporous film obtained by the dry lamella perforation method is coated with a coating resin (C) different from the polyolefin resin (A), and the microporous film is coated. It is preferable to form. The polyolefin resin (A) and the coating resin (C) different from the polyolefin resin (A) are as described above.

By covering a part or all of the surface of the microporous portion of the microporous film with the resin (C), all the resin (C) is not fixed on the outermost surface of the microporous film (that is, the surface of the film). , The resin (C) penetrates into the microporous mesh of the microporous film formed of the polyolefin resin (A) and is delivered to the microporous surface, thereby maintaining the air permeability of the microporous film. However, the piercing depth can be improved. From the viewpoint of withstand voltage of the separator, it is preferable to cover the fibrils constituting the microporous portion with the resin (C).

From the viewpoint of covering more fibrils with the resin (C), it is preferable to coat the microporous portion of the microporous film with a solution in which the resin (C) is dissolved or dispersed. From the same viewpoint, the solution in which the resin (C) is dissolved is more preferably a solution in which the resin (C) is dissolved in an organic solvent such as hexane, octane, or methylene chloride. The solution in which the resin (C) is dispersed is more preferably an aqueous latex containing the resin (C), isopropyl alcohol (IPA) and / or a surfactant and water.

From the viewpoint of the strength of the separator and the wettability to the non-aqueous electrolyte solution, the coating of the microporous part of the microporous film with the resin (C) is performed by adding the microporous film to a solution in which the resin (C) is dissolved or dispersed. It is preferably carried out by impregnation. In the coating of the microporous portion of the microporous film with the resin (C), from the viewpoint of the strength of the separator, the monomer, which is the constituent unit of the resin (C), is electron-electron with respect to the fibril composed of the polyolefin resin (A). It is more preferable not to include graft polymerization by wire.

To impregnate the solution in which the resin (C) is dissolved or dispersed with the microporous film, the microporous film is dipped in a tank of the solution in which the resin (C) is dispersed or dissolved, or the resin (C) is mixed. This can be done by applying the dispersed or dissolved solution to the outer surface of the microporous film and allowing the resin (C) to permeate the microporous inside the microporous film.

The dipping of the microporous film into the tank of the solution in which the resin (C) is dispersed or dissolved is preferably performed at 20 to 60 ° C. for 0.5 to 15 minutes.

The coating of the solution in which the resin (C) is dispersed or dissolved on the outer surface of the microporous film can be applied to the film by a printing machine, a coater, manual application, the resin (C) solution or the resin (C) dispersion liquid. It can be done by dropping or the like.

A microporous film coated with a microporous portion with the resin (C) may be passed through at least a pair of calendar rolls. The pair of calendar rolls is as used in the dry lamella perforation method described above.

From the viewpoint of fixing the resin (C) on the microporous surface, the microporous surface is coated with the resin (C) and then coated at a temperature of 20 to 100 ° C. in an atmospheric atmosphere or an inert gas atmosphere. It is preferable to dry the porous film.

[Composite of resin (B) and microporous film]
The composite of the resin (B) and the microporous film includes dropping the resin (B) on the surface of the microporous film, coating the resin (B) on the microporous film, and resin (B) on the microporous film. B) It can be carried out by a method such as printing of the contained paint, impregnation of the microporous film in the resin (B), or dipping of the microporous film in the resin (B). The resin (B) is as described above.

In the second embodiment, the coating amount or coating area of the resin (B) on the microporous film is, for example, the concentration of the resin (B), the coating amount of the coating liquid containing the resin (B), the coating method, and the coating method. It can be adjusted by changing the coating conditions.

When the resin (B) is arranged only on a part of the surface of the microporous film or the filler porous layer, the arrangement pattern of the resin (B) includes, for example, a dot shape, a diagonal line shape, a striped shape, a lattice shape, and a striped shape. , Tortoise shell shape, random shape, etc., and combinations thereof.

The impregnation of the microporous film in the resin (B) and the dipping of the microporous film in the resin (B) can be performed in the same manner as the coating of the microporous film by the resin (C) described above. ..

[Formation of porous filler layer]
As a method for forming the porous filler layer, for example, a method of applying a coating liquid containing an inorganic filler and a resin binder to at least one surface of the base material can be mentioned. The coating liquid in this case may contain a solvent, a dispersant and the like in order to improve the dispersion stability and the coatability.
The method of applying the coating liquid to the base material is not particularly limited as long as the required layer thickness and coating area can be realized. The filler raw material containing the resin binder and the polymer base material raw material may be laminated and extruded by a coextrusion method, or the base material and the filler porous membrane may be individually prepared and then bonded together.

Unless otherwise specified, the above-mentioned measured values of various physical properties are values measured according to the measuring method in the examples described later.

The present embodiment will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples as long as the gist of the present invention is not exceeded. The raw materials used and the evaluation methods for various properties are as follows.

The melt flow rate (MFR) was measured according to JIS K 7210, and the polypropylene resin was measured under the conditions of 210 ° C. and 2.16 kg, and the polyethylene resin was measured under the conditions of 190 ° C. and 2.16 kg, respectively. The unit is g / 10 minutes). The measured resin densities are all measured in accordance with JIS K 7112 (unit: kg / m ³ ).

The characteristics of various films were measured as follows.

(1) Thickness (μm)
The thickness of the porous film was measured at room temperature of 23 ± 2 ° C. using the Digimatic Indicator IDC112 manufactured by Mitutoyo.

(2) Porosity (%)
A 5 cm × 5 cm square sample was cut out from the porous film, and the porosity was calculated from the volume and mass of the sample using the following formula.
Pore ratio (%) = (volume (cm ³ ) -mass (g) / density of resin composition (g / cm ³ )) / volume (cm ³ ) x 100

(3) Air permeability (seconds / 100cc)
The air permeability of the porous film was measured with a Garley type air permeability meter conforming to JIS P-8117.

(4) Glass transition point, softening start point and decomposition point Weigh 10 mg of the measurement sample into an aluminum pan, and use an empty aluminum pan as a reference with a differential thermal analysis measuring device (“EXSTAR DSC6220” manufactured by SII Nanotechnology Co., Ltd.). The DSC curve was measured in the measurement temperature range of −100 ° C. to 500 ° C. at a heating rate of 10 ° C./min and under normal temperature and humidity. In this heating process, the baseline immediately before the heat absorption peak of the DSC curve whose differential signal (DDSC) becomes 0.05 mW / min / mg or more and the tangent line of the DSC curve at the inflection point that first appears after the heat absorption peak. The intersection with was determined as the glass transition point (Tg). Further, a temperature 25 ° C. higher than the glass transition point was set as the softening start point.

In a nitrogen atmosphere, the differential thermogravimetric analysis device (“EXSTAR TG / DTA6000” manufactured by SII Nanotechnology Co., Ltd.) heats from 30 ° C. at a heating rate of 10 ° C./min, and the weight loss rate reaches 10% by weight. The temperature was taken as the decomposition point.

(5) Puncture test A schematic diagram for explaining a test for measuring the tear length of the separator is shown in FIG. When performing this test, a needle (1) having a hemispherical tip with a radius of curvature of 0.5 mm is prepared, and a separator (3,3) is provided between two plates (3, 3) having an opening with a diameter (dia.) Of 11 mm. 2) was sandwiched, and the needle (1), the separator (2) and the plate (3, 3) were set according to the positional relationship shown in FIG. 5 (a). Using "MX2-50N" manufactured by Imada Co., Ltd., the needle (1) and separator (2) are used under the conditions of a radius of curvature of the needle tip of 0.5 mm, an opening diameter of the separator holding plate of 11 mm, and a piercing speed of 2 mm / sec. (Fig. 5 (b)) and pierced to tear the separator. The maximum length (mm) of the tear was measured in the MD direction and the TD direction of the separator after the measurement.

(6) Complete short circuit test

Preparation of positive electrode As a positive electrode active material, lithium nickel, manganese and cobalt mixed oxide with a number average particle diameter of 11 μm, graphite carbon powder with a number average particle diameter of 6.5 μm and acetylene black powder with a number average particle diameter of 48 nm as conductive aids. And vinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of mixed oxide: graphite carbon powder: acetylene black powder: PVDF = 100: 4.2: 1.8: 4.6. N-Methyl-2-pyrrolidone was added to the obtained mixture so as to have a solid content of 68% by mass and further mixed to prepare a slurry-like solution. This slurry-like solution was applied to one side of an aluminum foil having a thickness of 20 μm, the solvent was dried and removed, and then rolled with a roll press to obtain a positive electrode. The rolled product was punched into a rectangular shape of 30 mm × 50 mm except for the tab portion to obtain a positive electrode.

Preparation of negative electrode Graphite carbon powder (III) having a number average particle diameter of 12.7 μm and graphite carbon powder (IV) having a number average particle diameter of 6.5 μm as a negative electrode active material, and a carboxymethyl cellulose solution (solid content concentration 1.83) as a binder. Mass%) and diene rubber (glass transition temperature: -5 ° C, number average particle size when dried: 120 nm, dispersion medium: water, solid content concentration 40% by mass), graphite carbon powder (III): graphite Carbon powder (IV): Carboxymethyl cellulose solution: Diene-based rubber = 90:10: 1.44: 1.76, mixed so that the total solid content concentration is 45% by mass in a slurry form. Solution was prepared. This slurry-like solution was applied to one side of a copper foil having a thickness of 18 μm, the solvent was dried and removed, and then rolled by a roll press. The rolled product was punched into a rectangular shape of 32 mm × 52 mm excluding the tab portion to obtain a negative electrode.

Preparation of electrolytic solution 1 mol _{of LiPF 6} salt was added to a mixed solvent in which ethylene carbonate (also referred to as "EC" in the table) and ethyl methyl carbonate (also referred to as "MEC" in the table) were mixed at a volume ratio of 1: 2. The electrolyte A was obtained by adding / L.

Preparation of Battery A laminate obtained by superimposing the positive electrode and the negative electrode prepared as described above on both sides of the separator obtained in Example or Comparative Example is coated on both sides of an aluminum foil (thickness 40 μm) with a resin layer. After inserting the positive and negative electrode terminals into the bag made of the laminated film while projecting them, the electrolytic solution A was injected into a 0.5 mL bag, the pressure was reduced to -90 kPa, and then the operation was returned to -30 kPa twice. After that, it was held at −95 kPa for 5 minutes. After returning to normal pressure, the pressure was reduced to −85 kPa, and then temporary sealing was performed to prepare a sheet-shaped laminated lithium ion secondary battery. For depressurization and temporary sealing, a decompression sealing device (model: M-3295) manufactured by Techney Co., Ltd. was used.

As a lithium ion secondary battery for charging and discharging measurement of the lithium ion secondary battery, a small battery having 1C = 15 mA was produced and used. The charge and discharge capacity of the lithium ion secondary battery was measured using a charging / discharging device ACD-01 (trade name) manufactured by Asuka Electronics Co., Ltd. and a constant temperature bath PLM-63S (trade name) manufactured by Futaba Kagaku Co., Ltd.
The obtained lithium ion secondary battery is housed in a constant temperature bath (manufactured by Futaba Kagaku Co., Ltd., trade name "PLM-73S") set at 25 ° C. It was connected to -01 ") and allowed to stand for 20 hours. Next, the battery was charged to 4.2 V with a constant current of 0.2 C, and the battery was charged for the first time until the charging current converged to 0.02 C so as to hold 4.2 V. Then, after a 10-minute rest, the battery was discharged to 3.0 V with a constant current of 0.2 C. This charging / discharging was performed three times to complete the conditioning.

Confirmation of complete short-circuit phenomenon After finishing the conditioning, set the laminated cell charged to 4.2 V again with a step of 1 mm between it and the sample table installed in the constant temperature bath at 25 ° C, and grasp both ends of the cell. bottom. The cell was crushed with a SUS round bar having a diameter of 15.8 mm at a crushing speed of 0.2 mm / s and 1.95 tons, and a crushing test was conducted until the voltage reached 0.5 V. At the time of the crushing test, the time until the voltage reached 3.0V from 3.8V (the time during which the complete short circuit phenomenon occurred) was measured.

[Example 1]
Polypropylene resin (MFR2.0, density 0.91) as the polyolefin resin (A), pore diameter 30 mm, L / D (L: distance from raw material supply port to discharge port of extruder (m), D: of extruder Inner diameter (m). The same shall apply hereinafter.) = 30 and a T-die (200 ° C.) with a lip thickness of 2.5 mm installed at the tip of the extruder by feeding it into a single-screw extruder set at a temperature of 200 ° C. via a feeder. Extruded from. Immediately after that, the molten resin was blown with cold air at 25 ° C. using an air knife and wound up with a cast roll set at 95 ° C. under the conditions of a draw ratio of 200 and a winding speed of 20 m / min to form a film.

The resulting film was annealed for 1 hour in a hot air circulation oven heated to 145 ° C. Next, the annealed film was uniaxially stretched 1.2 times in the longitudinal direction at a temperature of 25 ° C. to obtain a cold stretched film. Next, the cold stretched film was uniaxially stretched 2.5 times in the longitudinal direction at a temperature of 140 ° C. and heat-fixed at 150 ° C. to obtain a uniaxially stretched film. Further, the uniaxially stretched film was uniaxially stretched 4.0 times in the lateral direction at a temperature of 145 ° C. and heat-fixed at 145 ° C. to obtain a microporous film (C1).

Methyl methacrylate and butyl acrylate were copolymerized at a ratio of 87:13 (mass ratio) to obtain non-conductive particles A as the resin (B).

Non-conductive particles A were coated on a microporous film (C1) and supported to obtain a separator. A photograph of the surface of the separator after the puncture test is shown in FIG. 2 (d).

[Example 2]
A microporous film (C2) having a three-layer structure of PP / PE / PP shown in Table 1 was obtained by the same dry porosification method as in Example 1. As the core, butyl acrylate, methacrylic acid, acrylonitrile, allyl glycidyl ether, N-methylol acrylamide, ethylene glycol dimethacrylate (mass ratio 93.8: 2: 2: 1: 1.2: 500); and as the shell, Particles B of a polymer having a core-shell structure containing methyl methacrylate and butyl acrylate (mass ratio 87:13) were prepared. Particles B of a polymer having a core-shell structure were coated and supported on a microporous film (C2) to obtain a separator.

[Comparative Example 1]
A microporous film (C1) was used as the separator. A photograph of the surface of the separator after the puncture test is shown in FIG. 2 (b).

[Comparative Example 2]
A microporous film (C2) was used as the separator.

[Comparative Example 3]
Using the same polyolefin resin (A), extruder and stretching machine as in Example 1, a microporous film (C3) made porous by uniaxial stretching in the TD direction was produced. A surface photograph of the microporous film (C3) after the puncture test is shown in FIG. 2 (c).

[Reference example 1]
Using the same polyolefin resin (A) as in Example 1, a microporous film (C4) made porous by a wet method was prepared. A surface photograph of the microporous film (C4) after the puncture test is shown in FIG. 2 (a).

Table 1 shows the resins used for Examples 1 and 2 and Comparative Examples 1 and 2, and the evaluation results of the microporous film and the separator.

1 Needle whose tip is hemispherical with a radius of curvature of 0.5 mm 2 Separator 3 Plate for holding the separator dia. Diameter of plate opening (11 mm)

Claims (2)

A separator for a lithium ion secondary battery containing a polyolefin resin (A) and a resin (B) different from the polyolefin resin (A).
The resin (B) is present on the surface of at least the microporous film formed of the polyolefin resin (A), and is non-conductive particles.
The microporous film formed of the polyolefin resin (A) is a dry microporous film formed by stretching a precursor containing the polyolefin resin (A) in the longitudinal direction (MD) to make it porous, the polyolefin resin. A dry microporous film obtained by stretching the precursor containing (A) in the lateral direction (TD) to make it porous, and a calendar treatment after stretching the precursor containing the polyolefin resin (A) in the lateral direction (TD). The separator is a hemispherical needle having a needle tip having a radius of curvature of 0.5 mm, which is at least one selected from the group consisting of a dry microporous film obtained by making the separator porous in the Z-axis direction. When pressed at a speed of 2 mm / sec, the following equation (1):
0.3 ≤ tear length in the TD direction (mm) / tear length in the MD direction (mm) ≤ 1
... Equation (1)
In meet the relationship represented,
The non-conductive particles are separators for lithium ion secondary batteries having a softening start temperature of 20 ° C. or higher and 50 ° C. or lower. The microporous film further contains the polyolefin resin (A) and a resin (C) different from the resin (B), and contains a precursor containing the polyolefin resin (A) at least in the longitudinal direction (MD) or laterally. The separator for a lithium ion secondary battery according to claim 1, which is obtained by stretching in the direction (TD) and impregnating the stretched molded product with the resin (C).

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