JP2018200780A - Separator for lithium ion secondary battery - Google Patents

Abstract

To provide a separator for lithium ion secondary battery with excellent cycle characteristic and long-term storage stability, and a manufacturing method thereof.SOLUTION: A separator for lithium ion secondary battery includes: polyolefin resin (A); and resin (B) which is non-conductive particles or particles of a polymer having a core shell structure, in which ion resistance of the separator is 0.3 Ω or less, and satisfies expression (1) and/or expression (2) in a crushing test of a laminate cell including the separator. T1/T2≥1.0 (1) T3/T4≥1.0 (2) {in expressions, T1 and T2 denote time until voltage reaches 4.0 V from 4.1 V when using the separator and when using a microporous film made only of polyolefin resin (A) respectively, and T3 and T4 denote time until voltage reaches 3.0 V from 3.8 V when using the separator and when using the microporous film made only of polyolefin resin (A) respectively}.SELECTED DRAWING: None

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, microfiltration membranes, moisture permeable waterproof membranes, etc. 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 laptop computers, but also to electric vehicles such as electric vehicles and small electric motorcycles. A separator for a lithium ion secondary battery is required to have strength, air permeability, ion permeability, safety when incorporated in a battery, and the like.

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

  The dry method includes a method in which an incompatible sheet such as an inorganic particle and a polyolefin-containing unstretched sheet are subjected to stretching and extraction to separate different material interfaces to form pores, a lamellar pore opening method, a β crystal opening method There are laws.

  The lamella opening method obtains an unstretched sheet having a crystalline lamella structure by controlling the melt crystallization conditions during sheet formation by melt extrusion of the resin, and cleaves the lamella interface by stretching the obtained unstretched sheet This is a method of forming holes.

  The β crystal opening method produces an unstretched sheet having a β crystal having a relatively low crystal density at the time of melt extrusion of polypropylene (PP), and stretches the produced unstretched sheet to produce an α having a relatively high crystal density. In this method, a crystal is transformed to a crystal and a hole is formed by a difference in crystal density between the two.

  As a wet method, a pore forming material (extractable material) such as a plasticizer is added to polyolefin, dispersed, formed into a sheet and then extracted with a solvent to form pores. There is a method of performing stretching before and / or after extraction.

  In order to use a microporous film produced by a 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 discloses a biaxial sheet containing a first PP, a second PP or ethylene-octene copolymer that is incompatible with the first PP, and a β-crystal nucleating agent according to the β-crystal opening method. Stretching is described. It is described that the obtained microporous film has a vertical direction (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 PP having different melt flow rates (MFR) are subjected to melt extrusion and biaxial stretching in accordance with the β crystal opening method.

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

  Patent Document 4 discloses 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 resistance of 70 to 300 seconds / 100 ml. A puncture strength of 0.18 to 0.50 N / 1 μm, a width direction (TD) thermal shrinkage of 12% or less (at 135 ° C. for 60 minutes), and a 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 elastomer are coextruded according to the β crystal opening method, and biaxially stretched to be porous. The preparation of a conductive laminate film is described.

  Moreover, in order to use the microporous film manufactured by a wet method as a separator for secondary batteries, the intensity | strength of a microporous film is examined (patent document 6). Patent Document 6 describes a microporous polyolefin film whose degree of orientation changes in the film thickness direction and has a piercing elongation of 2.2 to 2.4 mm from the viewpoint of both breaking strength and piercing strength. Has been.

International Publication No. 2007/46226 JP 2012-7156 A JP 2012-131990 A International Publication No. 2013/54929 JP 2014-4771 A JP-A-7-188440

  In recent years, lithium ion secondary batteries have been incorporated into small electronic devices or electric vehicles, and have been used even in harsh environments. In this regard, separators including a microporous film have been required to further improve strength, cycle characteristics, safety, and the like when incorporated in a secondary battery.

  However, the microporous films described in Patent Documents 1 to 6 still have room for improvement in cycle characteristics and long-term storage stability when incorporated in a secondary battery.

  In view of the above circumstances, an object of the present invention is to provide a separator for a lithium ion secondary battery excellent in cycle characteristics and long-term storage stability when incorporated in a secondary battery.

The present inventors have developed the above-mentioned problem by supporting a specific resin on a microporous polyolefin film so that the ionic resistance of the separator and the crushing test characteristics when the separator is incorporated in a laminate cell satisfy a specific condition. As a result, the present invention has been completed.
That is, the present invention is as follows.
[1]
A separator for a lithium ion secondary battery comprising a polyolefin resin (A) and a resin (B) different from the polyolefin resin (A),
The resin (B) is at least selected from the group consisting of non-conductive particles or polymer particles having a core-shell structure present on the surface of a microporous film formed of the polyolefin resin (A). One,
The separator has an ionic resistance of 0.3Ω or less, and a laminate cell including the separator is charged to 4.2 V, and a step of 1 mm is provided between the sample stage and a sample stage placed in a thermostatic bath at 25 ° C. Set in a state, grip both ends of the laminate cell, press a 15.8 mm diameter SUS round bar against the laminate cell surface with a crushing speed of 0.2 mm / s and a force of 1.95 ton, When the cell is crushed and collapsed until the voltage reaches 4.2 V to 0.5 V, the following formula (1):
T1 / T2 ≧ 1.0 (1)
{Where,
T1 is the time until the voltage when evaluated with the separator reaches 4.1 V to 4.0 V, and T2 is when evaluated with a microporous film consisting only of the polyolefin resin (A). This is the time required for the voltage to reach 4.0V from 4.0V. }
And / or the following formula (2):
T3 / T4 ≧ 1.0 Formula (2)
{Where,
T3 is the time until the voltage when evaluated with the separator reaches 3.8 V to 3.0 V, and T4 is when evaluated with a microporous film consisting only of the polyolefin resin (A). This is the time it takes for the voltage to reach 3.0V from 3.8V. }
The separator for lithium ion secondary batteries which satisfy | fills the relationship represented by these.
[2]
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 machine direction (MD) to make it porous,
A dry microporous film obtained by stretching the precursor containing the polyolefin resin (A) in the transverse direction (TD) direction to make it porous, and the transverse direction (TD) stretching of the precursor containing the polyolefin resin (A) The separator for a lithium ion secondary battery according to [1], which is at least one selected from the group consisting of a dry microporous film that is made porous by calendaring.
[3]
The microporous film further includes a resin (C) different from the polyolefin resin (A) and the resin (B), and a precursor containing the polyolefin resin (A) at least in the machine direction (MD) or laterally. The separator for a lithium ion secondary battery according to [2], which is obtained by stretching in a direction (TD) and impregnating the stretched molded article with the resin (C).

  ADVANTAGE OF THE INVENTION According to this invention, the separator for lithium ion secondary batteries excellent in the cycling characteristics when integrated in a secondary battery and long-term storage stability can be provided.

  Hereinafter, modes for carrying out the present invention (hereinafter abbreviated as “embodiments”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

<Separator for lithium ion secondary battery>
One embodiment of the present invention is a separator for a lithium ion secondary battery. In this specification, a separator refers to a member that is disposed between a plurality of electrodes in a lithium ion secondary battery and that has lithium ion permeability and, if necessary, shutdown characteristics.

  The separator for a lithium ion secondary battery according to the first embodiment of the present invention includes a microporous film, and the microporous film has a specific resin (B) in the polyolefin resin (A) as a main component. The separator for a lithium ion secondary battery is supported. In the first embodiment, the resin (B) is different from the polyolefin resin (A).

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

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

  In 1st embodiment, the said resin (B) consists of the particle | grains of the polymer which exists in the surface of the microporous film formed with the said polyolefin resin (A) at least, and has a nonelectroconductive particle or a core-shell structure. It is at least one selected from the group.

In the first embodiment, the separator has an ionic resistance of 0.3Ω or less, and the laminate cell including the separator is charged to 4.2 V and 1 mm between the sample stage placed in a thermostat at 25 ° C. Set with a level difference of 2mm, grip both ends of the laminate cell, and press a SUS round bar with a diameter of 15.8mm against the laminate cell surface with a crushing speed of 0.2mm / s and a force of 1.95ton. When the laminate cell is crushed and collapsed until the voltage reaches 4.2 V to 0.5 V, the following formula (1):
T1 / T2 ≧ 1.0 (1)
{Where,
T1 is the time until the voltage when evaluated with the separator reaches 4.1 V to 4.0 V, and T2 is when evaluated with a microporous film consisting only of the polyolefin resin (A). This is the time required for the voltage to reach 4.0V from 4.0V. }
And / or the following formula (2):
T3 / T4 ≧ 1.0 Formula (2)
{Where,
T3 is the time until the voltage when evaluated with the separator reaches 3.8 V to 3.0 V, and T4 is when evaluated with a microporous film consisting only of the polyolefin resin (A). This is the time it takes for the voltage to reach 3.0V from 3.8V. }
The relationship represented by is satisfied.

Since the resin (B) is supported on the microporous film, the short circuit and / or the complete short circuit can be suppressed while maintaining the battery performance. In particular, it has been found for the first time that the present invention can prolong the time until a micro short circuit occurs.
Here, the micro short circuit is a phenomenon in which the terminal voltage gradually decreases in a no-discharge state due to a slight self-discharge compared to the short circuit. In the present invention, the time to reach a short circuit is specified by measuring the time until the voltage reaches 4.1 V to 4.0 V by a predetermined test defined in the present invention. When a minute short circuit occurs, the voltage drops. Specifically, for example, even when charging up to 4.2V, a voltage drop occurs immediately and a voltage drop of about 0.1V occurs, resulting in a decrease in battery capacity. That is, prolonging the time until the micro short circuit leads to improvement of cycle characteristics between 4.2V and 3.0V and improvement of long-term storage characteristics of the battery.
Here, the resin (B) is a polymer having non-conductive particles or a core-shell structure. When the resin (B) is a polymer having a core-shell structure, a rigid core portion and a soft shell portion coexist at the same point to prevent misalignment between both electrodes and to keep the distance between both electrodes constant. It is possible to suppress a short circuit of the separator.

  T1 / T2 represented by the formula (1) is preferably 1.1, 1.2, 1 from the viewpoint of further suppressing cycle characteristics and long-term storage stability when the separator is incorporated in a secondary battery. .3 or 1.4.

  T3 / T4 represented by the formula (2) is preferably 1.1, 1.2, 1 from the viewpoint of further suppressing cycle characteristics and long-term storage stability when the separator is incorporated in a secondary battery. .3 or 1.4.

  T1 / T2 represented by Formula (1) and T3 / T4 represented by Formula (2) will be described in detail in Examples. T1 / T2 and T3 / T4 optimize the type of polyolefin resin (A), the production conditions of the microporous film, the type of resin (B), the amount of resin (B) supported on the microporous film, etc. Can be adjusted.

  The film thickness of the lithium ion secondary battery separator is preferably 100 μm or less, more preferably in the range of 2 to 80 μm, and even more preferably 3 from the viewpoint of balance with puncture strength or puncture depth and downsizing of the secondary battery. Within the range of ~ 30 μm. The film thickness of the separator can be adjusted by optimizing the manufacturing conditions of the microporous film.

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

<Microporous film>
The microporous film according to the present 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 organic solvents, and a fine pore size. If desired, the microporous surface formed by the fibrils 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 proportion of the polyolefin resin (A) in the microporous film is 50% by mass or more based on 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 viewpoints of wettability, thickness, and shutdown characteristics of the film. Particularly preferably, it is 60% by mass or more and 98% by mass or less.

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

Examples of the polyolefin resin (A) include homopolymers, copolymers, and multistage polymerizations obtained using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and the like as monomers. Examples thereof include polymers. These polyolefin resins (A) may be used alone or in combination of two or more.
Among these, from the viewpoint of shutdown characteristics, polyethylene, polypropylene, and copolymers thereof, and mixtures thereof are preferable, and from the viewpoint of shutdown characteristics, octane insolubility and hexane insolubility, one or more polypropylenes are used. More preferred.

Specific examples of polyethylene include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, and ultrahigh molecular weight polyethylene. 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 in accordance with D) density gradient tube method described in JIS K7112 (1999).
Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene.
The ethylene-propylene copolymer may be in the form of a random or block structure, or ethylene propylene rubber.

  The microporous film containing the polyolefin resin (A) as a main component may be a single layer type or a laminate type. The single layer film is preferably formed of one layer containing polypropylene as the polyolefin resin (A) from the viewpoints 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 at least one selected from the group consisting of non-conductive particles or polymer particles having a core-shell structure. Non-conductive particles or polymer particles having a core-shell structure 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.

  When the resin (B) is present on the surface of the microporous film formed of the polyolefin resin (A), the resin (B) is supported on the polyolefin resin (A), and preferably the microporous film. And / or impregnated within the micropores of the microporous film.

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

(B1) Nitrile Resin A nitrile resin is a resin containing a polymer unit having a nitrile group as a main component. In this specification, including a polymerization unit as a main component means that it is 50 mol% or more based on the total mol of all monomers charged at the time of polymerization. The nitrile resin optionally has a linear alkylene polymer unit having 4 or more carbon atoms, an aromatic vinyl polymer unit, a polymer unit having a hydrophilic group, and a thermally crosslinkable group in addition to the polymer unit having a nitrile group. It may comprise at least one selected from the group consisting of polymerized units. Examples of the hydrophilic group include a carboxylic acid group, a hydroxyl group, and a sulfonic acid group. Examples of the thermally crosslinkable group include an epoxy group, an N-methylolamide group, and an oxazoline group.

  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 resin can be obtained by polymerization of a monomer having a nitrile group, or copolymerization of a monomer having a nitrile group and another monomer. Examples of the monomer having a nitrile group include (meth) acrylonitrile. (Meth) acrylonitrile means acrylonitrile or methacrylonitrile. Other monomers are ethylenically unsaturated compounds such as (meth) acrylic acid esters and unsaturated hydrocarbons having 4 or more carbon atoms. (Meth) acrylic acid ester means acrylic acid ester or methacrylic acid ester, and examples thereof include n-butyl acrylate, 2-ethylhexyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate, and the like. . The unsaturated hydrocarbon having 4 or more carbon atoms is, for example, 1,3-butadiene.

  Specifically, the nitrile resin may be polyacrylonitrile, acrylonitrile-butadiene copolymer, acrylonitrile-styrene copolymer, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene-acrylate copolymer, or hydrogenation thereof. Things.

(B2) Acrylic resin An acrylic resin is a resin obtained by using an acrylic compound as a main monomer. Using as a main monomer means that it is 50 mol% or more with respect to the total mol of all monomers charged at the time of superposition | polymerization. The acrylic compound is a monomer having a (meth) acryloyl group which is an acryloyl group or a methacryloyl group.

  Acrylic resin is optionally polymerized units derived from acrylic compounds, polymer units having nitrile groups, linear alkylene polymer units, aromatic vinyl polymer units, polymer units having hydrophilic groups, and thermal crosslinkability. It may comprise at least one selected from the group consisting of polymerized units having groups. Examples of the hydrophilic group include a carboxylic acid group, a hydroxyl group, and a sulfonic acid group. Examples of the thermally crosslinkable group include an epoxy group, an N-methylolamide group, and an oxazoline group.

  The acrylic resin can be obtained by polymerization of an acrylic compound or copolymerization of an acrylic compound and another monomer.

As the acrylic compound, the following monomers may be used:
(Meth) 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;
(Meth) acrylic acid ester;
Fluorine-containing acrylic ester;
Amide group-containing (meth) acrylic acid or amide group-containing (meth) acrylate;
(Meth) acryl functional silicon-containing monomers;
(Meth) 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 monomer units, for example, ethylenically unsaturated carboxylic acid monomers such as 3-butenoic acid, trans-butenedionic acid, cis-butenedionic acid, maleic acid and maleic acid derivatives, ethylenically unsaturated sulfonic acid A monomer, an ethylenically unsaturated phosphate monomer, and a vinyl monomer having an acid component;
Maleimide;
α, β-unsaturated nitrile monomer;
Vinyl monomers not containing an acid component, for example, aliphatic vinyl monomers such as vinyl acetate and aliphatic conjugated diene monomers, and aromatic vinyl monomers such as styrene;
Unsaturated polyalkylene glycol ether monomers;
Ethylene functional silicon-containing monomers;
Isothiazoline compounds;
Chelating compounds;
Fluorine-containing monomers, for example
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.

  Specific examples of the acrylic resin include an acrylic soft polymer, an acrylic hard polymer, an acrylic-styrene copolymer, a sulfonated acrylic polymer, or a seed polymer, a hydrogenated product, or a graft product thereof.

  The acrylic resin may be in the form of non-conductive organic particles. The acrylic resin may be water-soluble when formed from an acrylic compound and a 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 obtained using an aliphatic monomer having a conjugated diene as a main component. In this specification, using as a main component means that it is 50 mol% or more with respect to the total mol of all the monomers prepared at the time of superposition | polymerization.

  The aliphatic monomer having a conjugated diene is a substituted or unsubstituted chain diene, and may be linear or branched. Specific examples of the aliphatic monomer having a conjugated diene include 1,3-butadiene, 1,3-isoprene, 1,4-dimethyl-1,3-butadiene, 1,2-dimethyl-1,3- Examples thereof include butadiene, 1,3-dimethyl-1,3-butadiene, 1,2,3-trimethyl-1,3-butadiene, 1,3,5-hexatriene, and alloocimene.

  The aliphatic conjugated diene resin can be obtained by polymerization of an aliphatic monomer having a conjugated diene, or copolymerization of an aliphatic monomer having a conjugated diene and another monomer.

  Other monomers include ethylenically unsaturated carboxylic acids, sulfonic acid group-containing monomers, nitrile group-containing monomers, aromatic vinyl monomers, crosslinkable monomers, aromatic vinyl compounds, etc. You can do it.

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

(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, and cellulose resins. is there. The resin (b4) may be in the form of organic polymer particles, graft polymer, polymer latex or the like.

  Thermoplastic fluororesins include polytetrafluoroethylene, polyvinylidene fluoride, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-ethylene copolymer, perfluoroalkoxy fluororesin, tetrafluoroethylene-hexafluoropropylene copolymer And ethylene-chlorotrifluoroethylene copolymer.

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

  Examples of the cellulose resin include a cellulose semisynthetic polymer and a sodium salt or 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 part containing a polymer and a shell part containing a polymer. Moreover, it is preferable that resin which has a core shell structure has a segment which does not show the segment which shows the compatibility with respect to electrolyte solution. As the polymer of the core part or the shell part, the resins (b1) to (b4) described above can be used.

  For example, polymer particles having a core-shell structure are polymerized in stages by using a polymer monomer that forms the core part and a polymer monomer that forms the shell part, and changing the ratio of these monomers over time. By doing so, it can be manufactured. Specifically, first, polymer monomers forming the core part are polymerized to produce seed polymer particles. This seed polymer particle becomes the core of the particle. Thereafter, in a polymerization system including seed polymer particles, a polymer monomer that forms a shell portion is polymerized. Thereby, since a shell part is formed on the surface of the core part, polymer particles having a core-shell structure are obtained. At this time, if necessary, for example, a reaction medium, a polymerization initiator, a surfactant, or the like may be used.

(Core part)
The core part of the particles generally has a softening start point or decomposition point at 175 ° C. or higher, preferably 220 ° C. or higher, more preferably 225 ° C. or higher. A core portion having a softening start point or a decomposition point in a temperature range of 175 ° C. or higher is less likely to be deformed during the use environment of the secondary battery and heat press, and can prevent the pores of the microporous film from being blocked. Moreover, since it can suppress that the rigidity of a microporous film falls, shrinkage | contraction of a separator can also be suppressed. Therefore, it is possible to stably prevent a short circuit in a high temperature environment. Moreover, although there is no restriction | limiting in the upper limit of the softening start point or decomposition point of a core part, Usually, it is 450 degrees C or less.

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

A method for measuring the decomposition point will be described below.
Under a nitrogen atmosphere, the measurement sample is heated from 30 ° C. at a rate of temperature increase of 10 ° C./min with a differential thermogravimetric simultaneous measurement apparatus. At this time, the temperature at which the weight loss ratio reaches 10% by weight is defined as the decomposition point.
When both the softening start point and the decomposition point of the core part of the particle are observed, the lower temperature is regarded as the softening start point of the core part.

  As a polymer which forms a core part, the highly crosslinked polymer of resin (b1)-(b4) is mentioned, for example. Since the molecular motion of the polymer is suppressed even at a temperature equal to or higher than the glass transition point of the polymer due to the high degree of crosslinking, the core portion can maintain the shape.

  The polymer forming the core part is preferably obtained by polymerizing a crosslinkable vinyl monomer. Examples of the crosslinkable vinyl monomer include compounds having usually 2 or more, preferably 2 copolymerizable double bonds. Moreover, a crosslinking | crosslinked vinyl monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

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

Examples of non-conjugated divinyl compounds include divinylbenzene.
Examples of polyvalent acrylates include polyethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,6-hexane glycol diacrylate, neopentyl glycol diacrylate, polypropylene glycol diacrylate, 2,2′-bis (4 -Diacrylate compounds such as acryloxypropyloxyphenyl) propane and 2,2'-bis (4-acryloxydiethoxyphenyl) propane; trimethylolpropane triacrylate, trimethylolethane triacrylate, tetramethylolmethane triacrylate, etc. Triacrylate compounds; tetraacrylate compounds such as tetramethylolmethane tetraacrylate; ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, Liethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate, 1,6-hexane glycol dimethacrylate, neopentyl glycol dimethacrylate, dipropylene glycol dimethacrylate, polypropylene And dimethacrylate compounds such as glycol dimethacrylate and 2,2′-bis (4-methacryloxydiethoxyphenyl) propane; and tomimethacrylate compounds such as trimethylolpropane trimethacrylate and trimethylolethane trimethacrylate;

  The ratio 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 based on the total monomers of the polymer forming the core part. By setting the ratio of the crosslinkable vinyl monomer to 20% by weight or more, the hardness, heat resistance, and solvent resistance of the core part can be improved. Moreover, an upper limit is 100 weight% or less normally, Preferably it is 98 weight% or less, More preferably, it is 95 weight% or less. Here, the amount of the crosslinkable vinyl monomer is, for example, in terms of a pure product excluding diluents and impurities.

(Shell part)
The shell part of the particles has a softening start point in a temperature range of 85 ° C. or higher, preferably 87 ° C. or higher, more preferably 89 ° C. or higher, and 145 ° C. or lower, preferably 125 ° C. or lower, more preferably 115 ° C. or lower. Have. When the softening start point is 85 ° C. or higher, the blocking resistance of the microporous film can be improved. In addition, since the shell portion hardly melts at the use temperature of the secondary battery, it is possible to suppress the clogging of the separator holes, thereby improving the rate characteristics of the secondary battery. Moreover, since the softening start point is 145 ° C. or lower, the shell part can be easily melted during heat pressing, thereby improving the adhesion of the separator and thereby improving the cycle characteristics of the secondary battery. Can be made.

  As the polymer forming the shell part, it is preferable to use a polymer containing a (meth) acrylate unit. By forming the shell portion with a polymer containing a (meth) acrylate unit, the electrical stability of the porous film 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, and 2-ethylhexyl methacrylate.

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

(Size of non-conductive particles or polymer particles having a core-shell structure)
The number average particle diameter of the non-conductive particles 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 or less. By setting the number average particle diameter of the particles in such a range, a gap between the particles can be formed to such an extent that the particles have a contact portion and the movement of ions is not inhibited. Therefore, the strength of the microporous film is improved, the short circuit of the battery can be prevented, and the cycle characteristics of the secondary battery can be improved.

  The number average particle diameter of the particles can be measured as follows. 200 particles are arbitrarily selected from a photograph taken at a magnification of 25000 times with a field emission scanning electron microscope. When the longest side of the particle image is La and the shortest side is Lb, the particle size is (La + Lb) / 2. The average particle size of 200 particles is determined as the average particle size.

  The thickness of the shell part is 3% or more, preferably 5% or more, more preferably 7% or more, and 18% or less, preferably 16% or less, more preferably with respect to the number average particle diameter of the particles. 14% or less. If the thickness average particle diameter of the shell part is 3% or more, the adhesion of the separator can be improved 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 an extent that does not hinder the movement of ions, thereby improving the rate characteristics of the secondary battery. it can. Moreover, since the core portion can be relatively enlarged by making the shell portion thinner, the rigidity of the particles can be increased. For this reason, the rigidity of the microporous film can be increased and the shrinkage of the separator can be suppressed.

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

(Amount of non-conductive particles or polymer particles having a core-shell structure)
In the first embodiment, the content ratio of non-conductive particles or polymer particles having a core-shell structure in the porous film is usually 70% by weight or more, preferably 75% by weight or more, more preferably 80% by weight or more. Usually, it is 98 wt% or less, preferably 96 wt% or less, more preferably 94 wt% or less. When the content ratio of the particles is within the above range, a gap between the particles can be formed so that the movement of ions is not inhibited while the particles have a contact portion, thereby improving the separator strength and the battery. Short circuit can be stably prevented.

  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 the resin (A) alone. 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) are preferably the outer peripheral portion of the fibril constituting the microporous portion so as to cover at least part of the surface of the microporous portion of the microporous film. It may be arranged so as to cover.

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

  The melting point of the resin (C) is preferably 130 ° C. or less, more preferably 50 to 125 ° C. from the viewpoints of wettability to the polyolefin resin (A) and productivity of the microporous film. In the present specification, the melting point of a polymer refers to a temperature (° C.) corresponding to an endothermic peak accompanying melting of a crystalline polymer in a DSC curve obtained by differential scanning calorimetry (DSC) of the polymer. When two endothermic peaks are observed in the DSC curve, the temperature corresponding to the higher endothermic peak is taken as the melting point.

  The resin (C) is preferably a hydrophobic resin from the viewpoint of the air permeability, puncture strength, and puncture depth of the microporous film. In the present specification, the hydrophobic resin means a resin that does not dissolve in water at all 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 the air permeability and strength of the microporous film, and more preferably the resin (C) is dissolved in octane at 25 ° C. The 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 for the microporous film is approximately equal to the elastic modulus of the extract extracted from the formed microporous film with hexane or octane. .

  Resin (C) is a polyolefin resin represented by polyethylene, polypropylene, polybutene and copolymers thereof, polyethylene terephthalate, polycycloolefin, from the viewpoint of resin content, thickness, air permeability and strength of the microporous film. Preferred examples include polyethersulfone, polyamide, polyimide, polyimideamide, polyaramid, polyvinyl difluoride, nylon, polytetrafluoroethylene, polymethacrylate, polyacrylate, polystyrene, polyurethane, or a copolymer thereof. More preferably, a polyolefin resin represented by polyethylene, polypropylene, polybutene and a copolymer thereof, polymethacrylate, polyacrylate, polystyrene, polyurethane, or a copolymer thereof, and polyethylene terephthalate, polycycloolefin, polyethersulfone, Examples thereof include resins such as polyamide, polyimide, polyimide amide, polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene, and preferred are polyolefin resins typified by polyethylene, polypropylene, and polybutene, and copolymers thereof. The resin (C) is more preferably a polyolefin resin having an elastic modulus at 25 ° C. of 700 MPa or less, and more preferably a low-melting polyolefin resin having an elastic modulus at 25 ° C. of 110 to 520 MPa and a melting point of 130 ° C. or less. is there.

  Examples of the resin (C) include a 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 / subcomponent C4), polypropylene having low stereoregularity, and the like can be used.

  The resin (C) is preferably disposed on the microporous surface in the microporous film without being localized on the outermost surface (outer surface) of the microporous film, and the entire microporous surface or More preferably, a part is coated. Although not wishing to be bound by 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. It is conceivable that the resin (C) relaxes / avoids stress concentration with respect to the concentration site and improves the breaking depth and breaking strength of the microporous film.

  Regarding the ratio of the resin (C) to the polyolefin resin (A), the porosity of the microporous film formed of the resins (A) and (C) is the same as that of the microporous film formed of the resin (A) alone. 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). Examples of such components include inorganic fillers, woven or non-woven fabrics of resin fibers, paper, and aggregates of insulating substance particles.

  Especially, it is preferable that a microporous film contains an inorganic filler other than resin (A)-(C) from a viewpoint of improving safety | security, such as the heat resistance of a separator, suppression of an internal short circuit. The inorganic filler contained in the microporous film may be the same as the inorganic filler forming 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. 30% or more is preferable from the viewpoint of improving ion conductivity, and 95% or less is preferable from the viewpoint of strength. The porosity can be adjusted by controlling the resin composition, the mixing ratio of resin and plasticizer, stretching conditions, heat setting conditions, and the like.

  The puncture strength of the microporous film is preferably in the range of 0.25 kgf or more, more preferably in the range of 0.25 to 0.60 kgf, from the viewpoint of separator productivity and secondary battery safety. 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 a non-aqueous solvent and voltage resistance. Preferably, it is 2.6 mm or more and 4.5 mm or less, Especially preferably, it is 2.7 mm or more and 4.5 mm or less.

  The film thickness of the microporous film is preferably from 0.1 μm to 100 μm, more preferably from 1 μm to 50 μm, still more preferably from 3 μm to 25 μm, and particularly preferably from 5 μm to 20 μm. 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 diameter of the microporous film is preferably 0.03 μm or more and 0.80 μm or less, more preferably 0.04 μm or more and 0.70 μm or less. From the viewpoint of ion conductivity and voltage resistance, 0.03 μm or more and 0.80 μm or less is preferable. The average pore diameter can be adjusted by controlling the resin composition, extrusion conditions, stretching conditions, heat setting conditions, and the like.

  The viscosity average molecular weight of the microporous film is preferably from 30,000 to 12,000,000, more preferably from 50,000 to less than 4,000,000, still more preferably from 100,000 to 1,000. Less than 1,000. A viscosity average molecular weight of 30,000 or more is preferred because the melt tension during melt molding increases, moldability is improved, and the strength tends to increase due to entanglement between polymers. A viscosity average molecular weight of 12,000,000 or less is preferable because uniform melt kneading is facilitated and the sheet formability, particularly thickness stability, tends to be excellent.

<Filler porous layer>
The filler porous layer includes an inorganic filler and a resin binder.

(Inorganic filler)
The inorganic filler used in the filler porous layer is not particularly limited, but preferably has a melting point of 200 ° C. or higher, high electrical insulation, and electrochemically stable in the range of use of the lithium ion secondary battery. .
The inorganic filler is not particularly limited. For example, oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, 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 oxide, potassium titanate, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, Acerite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand and other ceramics; glass fiber and the like. These may be used alone or in combination.

Among these, from the viewpoint of improving the electrochemical stability and the heat resistance characteristics of the separator,
Aluminum oxide compounds such as alumina and aluminum hydroxide oxide; and aluminum silicate compounds having no ion exchange ability such as kaolinite, dickite, 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 mainly composed of kaolin mineral is more preferable because it is inexpensive and easily available. Known kaolins include wet kaolin and calcined kaolin obtained by calcining this. In the present invention, calcined kaolin is particularly preferred. The calcined kaolin is particularly preferable from the viewpoint of electrochemical stability because crystal water is released during the calcining process and impurities are also removed.

  The average particle size of the inorganic filler is preferably more than 0.01 μm and not more than 4.0 μm, more preferably more than 0.2 μm and not more than 3.5 μm, more than 0.4 μm and 3.0 μm. More preferably, it is as follows. Adjusting the average particle size of the inorganic filler to the above range is preferable from the viewpoint of suppressing thermal shrinkage at high temperature even when the filler porous layer is thin (for example, 7 μm or less). Examples of the method for adjusting the particle size and distribution of the inorganic filler include a method of reducing the particle size by pulverizing the inorganic filler using an appropriate pulverizer such as a ball mill, a bead mill, or a jet mill. .

  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 inorganic fillers having these shapes may be used in combination.

  The proportion of the inorganic filler in the filler porous layer can be appropriately determined from the viewpoints of the binding properties of the inorganic filler, the permeability of the separator, and the heat resistance. The proportion 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 filler porous layer is not particularly limited, but is insoluble in the electrolyte solution of the lithium ion secondary battery and is electrochemically stable in the usage range 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; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, ethylene -Fluorine-containing rubber such as tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, methacrylic acid Rubbers such as ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate Cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose; polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester, etc., melting point and / or glass transition temperature of 180 ° C. or higher And the like.

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

  A resin latex binder is preferably used as the resin binder. When a resin latex binder is used as the resin binder, the separator having the filler porous layer containing the binder and the inorganic filler binds the resin binder onto the porous film through a step of applying the resin binder solution onto the substrate. Compared to the separator, the ion permeability is less likely to decrease, and there is a tendency to provide an electricity storage device with high output characteristics. Furthermore, the electricity storage device having the separator has a smooth shutdown characteristic even when the temperature rise during abnormal heat generation is fast, and it tends to be easy to obtain high safety.

  The resin latex binder is obtained by copolymerizing an aliphatic conjugated diene monomer and an unsaturated carboxylic acid monomer and other monomers copolymerizable with these from the viewpoint of improving electrochemical stability and binding properties. Are preferred. The polymerization method in this case is not particularly limited, but emulsion polymerization is preferred. There is no restriction | limiting in particular as a method of emulsion polymerization, A known method can be used. The addition method of the monomer and other components is not particularly limited, and any of batch addition method, divided addition method, and continuous addition method can be adopted, and the polymerization method can be one-stage polymerization or two-stage polymerization. , Or multistage polymerization of three or more stages can be employed.

The average particle size of the resin binder is preferably 50 to 500 nm, more preferably 60 to 460 nm, and still more preferably 80 to 250 nm. When the average particle size of the resin binder is 50 nm or more, the separator including the filler porous layer containing the binder and the inorganic filler is less likely to have low ion permeability and easily provides an electricity storage device having high output characteristics. Furthermore, even when the temperature rises rapidly during abnormal heat generation, it is easy to obtain an electricity storage device that exhibits smooth shutdown characteristics and high safety. When the average particle size of the resin binder is 500 nm or less, good binding properties are exhibited, and when a multilayer porous film is formed, heat shrinkage is good and the safety tends to be excellent.
The average particle diameter of the resin binder can be controlled by adjusting the polymerization time, polymerization temperature, raw material composition ratio, raw material charging order, pH, and the like.

The layer thickness of the filler porous layer is preferably 0.5 μm or more from the viewpoint of improving heat resistance and insulation, and is preferably 50 μm or less from the viewpoint of increasing 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.

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

  The manufacturing method of the separator for lithium ion secondary batteries may further include the process of laminating | stacking the filler porous layer containing an inorganic filler on a microporous film depending on necessity.

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

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

  As a wet method, a polyolefin resin composition and a pore-forming material are melt-kneaded and formed into a sheet shape, stretched as necessary, and then the pore-forming material is extracted. And a method of removing the solvent at the same time as immersing in a poor solvent for coagulating polyolefin.

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

  The polyolefin resin composition can contain a resin other than the polyolefin resin (A), optional additives, and the like. Examples of the additive include inorganic fillers, antioxidants, metal soaps, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments.

  Melting and kneading of the polyolefin resin composition can be performed by, for example, an extruder, a kneader, a lab plast mill, a kneading roll, a Banbury mixer, and 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 inorganic fillers include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitriding such as silicon nitride, titanium nitride, and boron nitride Physical ceramics: Silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, zeolite , Ceramics such as calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; glass fiber.

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

  The hole forming step can be performed by a known dry method and / or wet method. The stretching step may be performed during the hole forming step or before or after the hole 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 and the like of the obtained microporous film. When the sheet-like molded body is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, the final product is difficult to tear, and has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoints of improvement of puncture strength, uniformity of stretching, and shutdown property. Further, sequential biaxial stretching is preferred from the viewpoint of easy control of plane orientation.

  Simultaneous biaxial stretching refers to a stretching method in which MD (machine direction of microporous membrane continuous molding) stretching and TD (direction crossing MD of microporous membrane at an angle of 90 °) are simultaneously performed. 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. When MD or TD is stretched, the other direction is fixed in an unconstrained state or a constant length. State.

  In order to suppress the shrinkage of the microporous film, heat treatment may be performed for the purpose of heat setting after stretching or after formation of pores. As heat treatment, for the purpose of adjusting physical properties, a stretching operation performed at a predetermined temperature atmosphere and a predetermined stretching rate and / or a relaxing operation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress are performed. 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 lamellar pore opening method will be described. The dry lamellar pore opening method uses a lamella interface by stretching a precursor in which a plurality of spherulites having a lamellar structure are bonded via an amorphous polymer without using a solvent such as water or an organic solvent. Is a method of forming a hole by cleaving.

  The dry lamellar pore opening method preferably includes (i) a step of extruding a precursor containing the 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 machine direction (MD) or the transverse direction (TD), for example, a molten resin, a resin composition, a resin molding. It can be a body. The precursor extruded in step (i) is, for example, an extrusion-molded body, an original sheet, an original film or the like. The microporous film obtained by the dry lamellar pore opening method including the steps (i) and (ii) is suitably used for supporting, coating, dipping, or impregnation with the resin (B).

  Step (i) can be performed by conventional extrusion methods. The extruder can include a T die having an elongated hole or an annular die.

  Uniaxial stretching of step (ii) can be performed as described above. Uniaxial stretching can be performed in the machine direction (MD) or the transverse direction (TD). Further, it is preferable to stretch the precursor in the transverse direction (TD) after uniaxial stretching and biaxial stretching. Biaxial stretching allows machine direction (MD) stretching and transverse direction (TD) stretching with simultaneous controlled MD direction relaxation. Here, MD stretching can include both cold stretching and hot stretching. The microporous film obtained by stretching in the step (ii) is suitably used for supporting, coating, dipping or impregnation with the resin (B).

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

  It is preferable to calender the molded product stretched in the MD and / or TD direction during or after step (ii). The calendered molded article is suitably subjected to loading with resin (B), coating, dipping or impregnation. The calendar treatment can be performed by passing the stretched molded article through at least a pair of calendar rolls. The pair of calendar rolls may comprise, for example, a set of steel rolls and elastic rolls, or a set of two steel rolls. During the calendar process, the pair of calendar rolls can be heated or cooled.

  From the viewpoint of the strength of the microporous film, in the dry lamellar pore opening method described above, a precursor containing a polyolefin resin (A) is sequentially or simultaneously stretched in the MD and TD directions and subjected to a calendar treatment ( Hereinafter, “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.

  A microporous film coated by coating at least part of the surface of the microporous portion of the microporous film obtained by the dry lamellar pore opening method with a coating resin (C) different from the polyolefin resin (A). Is preferably formed. 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 part 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) enters the microporous network 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. It is possible to improve the piercing depth. From the viewpoint of the voltage resistance 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) and 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, the coating of the microporous portion of the microporous film with the resin (C) is performed by applying the microporous film to a solution in which the resin (C) is dissolved or dispersed. It is preferably carried out by impregnation. The coating of the microporous portion of the microporous film with the resin (C) is carried out by using an electron beam from a monomer that is a constituent unit of the resin (C) with respect to the fibril composed of the polyolefin resin (A) from the viewpoint of the strength of the separator. More preferably, it does not include graft polymerization.

  The impregnation of the solution in which the resin (C) is dissolved or dispersed with the microporous film is carried out by dipping the microporous film in a solution tank in which the resin (C) is dispersed or dissolved, or the resin (C). The dispersion or dissolved solution can be applied to the outer surface of the microporous film to allow the resin (C) to penetrate into the micropores inside the microporous film.

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

  Coating of the solution in which the resin (C) is dispersed or dissolved on the outer surface of the microporous film is performed by a printing machine, a coater, manual application, resin (C) solution or resin (C) dispersion on the film. This can be done by dropwise addition or the like.

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

  From the viewpoint of fixing the resin (C) on the microporous surface, after coating the resin (C) on the microporous surface, the coated microparticles are coated at a temperature of 20 to 100 ° C. in an air atmosphere or an inert gas atmosphere. It is preferable to dry the porous film.

[Composite of resin (B) and microporous film]
The resin (B) and the microporous film are combined by dropping the resin (B) on the microporous film surface, coating the resin (B) on the microporous film, and resin (B) on the microporous film ( B) Printing of the containing paint, impregnation of the microporous film in the resin (B), dipping of the microporous film into the resin (B) can be performed. 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 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 filler porous layer, the arrangement pattern of the resin (B) is, for example, a dot shape, a diagonal shape, a stripe shape, a lattice shape, or a stripe shape. , Turtle shell, random, etc., and combinations thereof.

  The impregnation of the microporous film in the resin (B) and the dipping of the microporous film into the resin (B) are performed in the same manner as the coating of the microporous film with the resin (C) described above. Can do.

[Formation of filler porous layer]
Examples of the method for forming the porous filler layer include a method of applying a coating liquid containing an inorganic filler and a resin binder on at least one surface of a substrate. In this case, the coating liquid may contain a solvent, a dispersant, and the like in order to improve dispersion stability and coating property.
The method for applying the coating liquid to the substrate 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 may be laminated and extruded by a co-extrusion method, or may be bonded after the base material and the filler porous film are individually produced.

  In addition, the measured value of various physical properties mentioned above is a value measured according to the measuring method in the Example mentioned later unless there is particular notice.

  The present embodiment will be described more specifically 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. In addition, the raw material used and the evaluation method of various characteristics are as follows.

Melt flow rate (MFR) was measured in accordance with JIS K 7210. Polypropylene resin was measured at 210 ° C. and 2.16 kg, and polyethylene resin was measured at 190 ° C. and 2.16 kg. (Unit: g / 10 minutes) The density of the measured resin was shown as a value 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 23 ± 2 ° C. using a Digimatic Indicator IDC112 manufactured by Mitutoyo Corporation.

(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.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of resin composition (g / cm ³ )) / volume (cm ³ ) × 100

(3) Air permeability (sec / 100cc)
The air permeability of the porous film was measured with a Gurley type air permeability meter based on JIS P-8117.

(4) Production of positive electrode for micro short circuit test and complete short circuit test Lithium nickel, manganese and cobalt mixed oxide having a number average particle diameter of 11 μm as a positive electrode active material and graphite carbon powder having a number average particle diameter of 6.5 μm as a conductive additive And acetylene black powder having a number average particle diameter of 48 nm and polyvinylidene fluoride (PVDF) as a binder, mixed oxide: graphite carbon powder: acetylene black powder: PVDF = 100: 4.2: 1.8: 4.6 The mass ratio was mixed. 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 solution. This slurry-like solution was applied to one side of an aluminum foil having a thickness of 20 μm, and the solvent was removed by drying, followed by rolling with a roll press to obtain a positive electrode. The rolled product was punched into a 30 mm × 50 mm rectangular shape excluding the tab portion to obtain a positive electrode.

Production 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 of 1.83 as a binder) Mass%) and diene rubber (glass transition temperature: −5 ° C., number average particle size upon drying: 120 nm, dispersion medium: water, solid content concentration: 40 mass%), graphite carbon powder (III): graphite Carbon powder (IV): Carboxymethylcellulose solution: Diene rubber = 90: 10: 1.44: 1.76 The solid content is mixed so that the total solid content concentration is 45% by mass to form a slurry. A solution of was prepared. This slurry-like solution was applied to one side of a copper foil having a thickness of 18 μm, and the solvent was removed by drying, followed by rolling with 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 is mixed in a mixed solvent of ethylene carbonate (also expressed as “EC” in the table) and ethyl methyl carbonate (also expressed as “MEC” in the table) at a volume ratio of 1: 2. Electrolyte A was obtained by adding / L.

Production of Battery A positive electrode and a negative electrode produced as described above were laminated on both sides of a microporous film or a separator obtained in Examples or Comparative Examples, and both sides of an aluminum foil (thickness: 40 μm). After inserting the positive and negative terminals into a bag made of a laminate film coated with a resin layer, the electrolytic solution A is injected into a 0.5 mL bag, decompressed to −90 kPa, and then returned to −30 kPa. Was carried out twice and then 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 produce a sheet-like laminated lithium ion secondary battery. For decompression and temporary sealing, a decompression seal device (model: M-3295) manufactured by Techny Co., Ltd. was used.

Charging and Discharging of Lithium Ion Secondary Battery As a lithium ion secondary battery for measurement, a small battery with 1C = 15 mA was produced and used. The charge and discharge capacity of the lithium ion secondary battery was measured using a charge / discharge device ACD-01 (trade name) manufactured by Asuka Electronics Co., Ltd. and a thermostat PLM-63S (trade name) manufactured by Futaba Kagaku Co., Ltd.
The obtained lithium ion secondary battery is housed in a thermostatic chamber set at 25 ° C. (trade name “PLM-73S” manufactured by Futaba Kagaku Co., Ltd.), and a charge / discharge device (manufactured by Asuka Electronics Co., Ltd., product name “ACD”). -01 ") and allowed to stand for 20 hours. Subsequently, the battery was charged to 4.2 V with a constant current of 0.2 C, and maintained at 4.2 V, and the initial charge was performed until the charging current converged to 0.02 C. Then, after 10 minutes of rest, the battery was discharged to 3.0 V at a constant current of 0.2C. This charging / discharging was performed 3 times to complete the conditioning.

Confirmation of micro short circuit phenomenon and complete short circuit phenomenon After the conditioning was completed, the laminate cell charged to 4.2V was set again with a step of 1mm between the sample stage installed in the constant temperature bath at 25 ℃. Grasped both ends. The cell was crushed with a SUS round bar having a diameter of 15.8 mm with a crushing speed of 0.2 mm / s and a force of 1.95 ton, and the crushing test was performed until the voltage reached 0.5V. During the crushing test, the time until the voltage reaches 4.1V to 4.0V (the time when the micro short-circuit phenomenon occurs) and the time until the voltage reaches 3.8V to 3.0V (the complete short-circuit phenomenon) ) And the time during which each occurred.
T1: When the laminate cell including the separator was evaluated, the time until the voltage reached 4.0 V to 4.0 V was regarded as T1.
T2: When a laminate cell including a microporous film composed only of the polyolefin resin (A) was evaluated, the time required for the voltage to reach 4.0 V to 4.0 V was regarded as T2.
T3: When the laminate cell including the separator was evaluated, the time required for the voltage to reach 3.8 V to 3.0 V was regarded as T3.
T4: When evaluated with a laminate cell including a microporous film consisting only of polyolefin resin (A), the time until the voltage reached 3.8 V to 3.0 V was regarded as T4.

(5) Ion resistance (Ω)
A small piece was cut from the separator and placed between the positive electrode and the negative electrode prepared above, and the space between both electrodes was filled with the electrolytic solution A. By the AC impedance method, the ion resistance value of the separator was measured at a frequency at which the impedance of the separator becomes equal to the resistance value of the separator.

(6) Cycle characteristic test About the battery after the said formation, 1C charge / discharge cycle between 4.2V-3.0V was repeated 300 times at 50 degreeC, and the discharge capacity maintenance factor at the time of 300 cycle arrival was measured.

(7) Long-term storage stability test The battery after the above formation was stored at 60 ° C and 4.2 V for 30 days. The voltage of the battery after storage was measured.

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

  The obtained film was annealed for 1 hour in a hot air circulating oven heated to 145 ° C. Next, the annealed film was uniaxially stretched 1.2 times in the machine 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 machine direction at a temperature of 140 ° C. and heat-set at 150 ° C. to obtain a uniaxially stretched film. Furthermore, the uniaxially stretched film was uniaxially stretched 4.0 times in the transverse direction at a temperature of 145 ° C. and heat-set at 145 ° C. to obtain a microporous film (C1).

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

  The non-conductive particles A were coated and supported on the microporous film (C1) to obtain a separator.

[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 porosity method as in Example 1. A microporous film (C2) was coated with and supported by polymer particles B having a core-shell structure shown in Table 1 to obtain a separator.

[Comparative Example 1]
A microporous film (C1) was used as a separator.

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

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

Claims (3)

A separator for a lithium ion secondary battery comprising a polyolefin resin (A) and a resin (B) different from the polyolefin resin (A),
The resin (B) is at least selected from the group consisting of non-conductive particles or polymer particles having a core-shell structure present on the surface of a microporous film formed of the polyolefin resin (A). One,
The separator has an ionic resistance of 0.3Ω or less, and a laminate cell including the separator is charged to 4.2 V, and a step of 1 mm is provided between the sample stage and a sample stage placed in a thermostatic bath at 25 ° C. Set in a state, grip both ends of the laminate cell, press a 15.8 mm diameter SUS round bar against the laminate cell surface with a crushing speed of 0.2 mm / s and a force of 1.95 ton, When the cell is crushed and collapsed until the voltage reaches 4.2 V to 0.5 V, the following formula (1):
T1 / T2 ≧ 1.0 (1)
{Where,
T1 is the time until the voltage when evaluated with the separator reaches 4.1 V to 4.0 V, and T2 is when evaluated with a microporous film consisting only of the polyolefin resin (A). This is the time required for the voltage to reach 4.0V from 4.0V. }
And / or the following formula (2):
T3 / T4 ≧ 1.0 Formula (2)
{Where,
T3 is the time until the voltage when evaluated with the separator reaches 3.8 V to 3.0 V, and T4 is when evaluated with a microporous film consisting only of the polyolefin resin (A). This is the time it takes for the voltage to reach 3.0V from 3.8V. }
The separator for lithium ion secondary batteries which satisfy | fills the relationship represented by these. 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 machine direction (MD) to make it porous,
A dry microporous film obtained by stretching the precursor containing the polyolefin resin (A) in the transverse direction (TD) direction to make it porous, and the transverse direction (TD) stretching of the precursor containing the polyolefin resin (A) The separator for a lithium ion secondary battery according to claim 1, wherein the separator is at least one selected from the group consisting of a dry microporous film that is made porous by calendering.   The microporous film further includes a resin (C) different from the polyolefin resin (A) and the resin (B), and a precursor containing the polyolefin resin (A) at least in the machine direction (MD) or laterally. The separator for a lithium ion secondary battery according to claim 2, obtained by stretching in a direction (TD) and impregnating the stretched molded article with the resin (C).