JP2012054230A - Separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for nonaqueous electrolyte secondary batteries which, in addition to its shutdown performance, hardly causes shape changes such as membrane breakage and curl in overheating and which is excellent in shape maintaining properties.SOLUTION: In the separator, a porous membrane containing water-soluble polymer, a porous membrane containing polyolefin, and a porous membrane containing nitrogen-containing aromatic polymer are laminated in this order.

Description

  The present invention relates to a separator, and more particularly to a separator for a non-aqueous electrolyte secondary battery.

  Non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are widely used as batteries for personal computers, mobile phones, portable information terminals and the like because of their high energy density.

  Non-aqueous electrolyte secondary batteries represented by these lithium ion secondary batteries have high energy density. If an internal short circuit or external short circuit occurs due to damage to the battery or equipment using the battery, a large current may flow and abnormal heat generation may occur. Therefore, non-aqueous electrolyte secondary batteries are required to prevent heat generation beyond a certain level and ensure high safety. In the case of abnormal heat generation, the separator generally has a shutdown function that blocks passage of ions between the positive and negative electrodes to prevent further heat generation. Examples of the separator having a shutdown function include a separator having a porous film made of a material that melts when abnormal heat is generated. That is, in the battery using the separator, when the porous film melts and becomes non-porous during abnormal heat generation, the passage of ions can be blocked and further heat generation can be suppressed.

  As a separator having such a shutdown function, for example, a polyolefin porous film is used. A separator made of a polyolefin porous membrane can block the passage of ions (shut down) by melting and non-porous at about 80 to 180 ° C. during abnormal heat generation of the battery, and can suppress further heat generation. . However, when the temperature is further increased, a separator made of a polyolefin porous membrane may cause a short circuit due to direct contact between the positive electrode and the negative electrode due to shrinkage or membrane breakage. As described above, a separator made of a polyolefin porous film has insufficient shape stability when the temperature is further increased, and abnormal heat generation due to a short circuit may not be suppressed.

  On the other hand, a method of imparting shape stability at high temperature to a separator by laminating a porous film made of a heat-resistant material on a polyolefin porous film has been studied. As such a highly heat-resistant separator, for example, a separator in which a porous film obtained by immersing a regenerated cellulose film in an organic solvent and a polyolefin porous film are laminated is proposed (for example, Patent Document 1). reference.). Such a separator can provide a non-aqueous electrolyte secondary battery having excellent shape stability at high temperatures and higher safety, but the load of the non-aqueous electrolyte secondary battery obtained by using the separator There was a problem that the characteristics were insufficient.

Patent Document 2 discloses a separator in which a porous film containing fine particles and a water-soluble polymer and a polyolefin porous film are laminated as a separator excellent in shape stability at high temperature and shutdown property. By using the separator for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery having excellent load characteristics and cycle characteristics is obtained.
The separator is formed by applying a slurry containing a water-soluble polymer, fine particles, and a medium to one surface of the polyolefin porous membrane, and removing the medium from the obtained coated film, thereby forming the water-soluble polymer and the fine particles. Can be obtained by a step of laminating a porous membrane containing However, the water-soluble polymer may not be so high in heat resistance. In this case, the separator may be broken or curled due to abnormal heat generation in the battery, and the insulation between the positive and negative electrodes may not be maintained. .

Japanese Patent Laid-Open No. 10-3898 JP 2004-227972 A

  An object of the present invention is to provide a separator for a non-aqueous electrolyte secondary battery in which shape change such as a film breakage or curl is unlikely to occur in addition to shutdown performance during abnormal heat generation in the battery.

The present invention provides the following.
<1> A separator in which a porous film containing a water-soluble polymer, a porous film containing polyolefin, and a porous film of a nitrogen-containing aromatic polymer are laminated in this order.
<2> The separator according to <1>, wherein the porous film containing the water-soluble polymer further contains fine particles.
<3> The separator according to <2>, wherein an average particle size of the fine particles is 3 μm or less.
<4> The separator according to any one of <1> to <3>, wherein the water-soluble polymer is at least one polymer selected from the group consisting of cellulose ether, polyvinyl alcohol, and sodium alginate.
<5> The separator according to <4>, wherein the cellulose ether is carboxymethylcellulose.
<6> The separator according to any one of <1> to <5>, wherein the nitrogen-containing aromatic polymer is one or more polymers selected from the group consisting of aromatic polyamide, aromatic polyimide, and aromatic polyamideimide.
<7> The separator according to any one of <1> to <6>, wherein the polyolefin is polyethylene.
<8> A nonaqueous electrolyte secondary battery having the separator according to any one of <1> to <7>.

  The separator of the present invention is excellent in shape maintainability in addition to shutdown performance in a nonaqueous electrolyte secondary battery, hardly causing film breakage or curling due to abnormal heat generation.

The present invention will be described in detail below.
The separator of the present invention includes a porous film containing a water-soluble polymer (hereinafter sometimes referred to as “A film”), a porous film containing polyolefin (hereinafter sometimes referred to as “B film”), and nitrogen-containing. A porous film (hereinafter sometimes referred to as “C film”) containing an aromatic polymer is a separator that is laminated in this order. That is, it has a structure in which the A film is laminated on one side of the B film and the C film is laminated on the other side. The A film and the C film have heat resistance at a high temperature at which shutdown occurs. A film | membrane provides shape stability, without impairing the shutdown property of B film | membrane excessively. On the other hand, the C film has higher heat resistance than the A film, and can impart shape stability at a higher temperature than the A film. The B film melts and becomes nonporous due to abnormal heat generation of the battery, thereby giving a shutdown function to the separator.

  First, the A film will be described. The A film is a porous film containing a water-soluble polymer. Examples of water-soluble polymers include polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, polymethacrylic acid, etc., cellulose ether, polyvinyl alcohol, sodium alginate are preferred, and cellulose ether is further included. preferable. Specific examples of the cellulose ether include carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), carboxyethyl cellulose, methyl cellulose, ethyl cellulose, cyanethyl cellulose, oxyethyl cellulose, and the like, and CMC having excellent chemical stability. Particularly preferred.

  The A film preferably further contains fine particles. When the A film contains fine particles, it may be possible to provide a non-aqueous electrolyte secondary battery with better load characteristics. As the fine particles, fine particles made of an inorganic material or an organic material can be used. Specific examples of organic materials include homopolymers such as styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate, and methyl acrylate, or two or more types of copolymers; Fluororesin such as fluoroethylene, tetrafluoroethylene-6-propylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, melamine resin, urea resin, polyethylene, polypropylene, polymethacrylate, etc. Can be mentioned. Examples of inorganic materials include calcium carbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate, barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide, magnesium hydroxide, oxidation Examples include calcium, magnesium oxide, titanium oxide, alumina, mica, zeolite, and glass. The fine particle material is preferably an inorganic material and more preferably alumina from the viewpoints of heat resistance and chemical stability. These fine particle materials can be used alone. Two or more materials can be mixed and used.

  The average particle diameter of the fine particles is preferably 3 μm or less, and more preferably 1 μm or less. Regarding the lower limit of the average particle diameter, the average particle diameter of the fine particles is usually 0.01 μm or more, and preferably 0.2 μm or more. Examples of the shape of the fine particles include a spherical shape and a bowl shape. The average particle size of the fine particles is determined by arbitrarily extracting 25 particles with a scanning electron microscope (SEM) and measuring the particle size (diameter) of each particle. Can be calculated as When the shape of the fine particles is other than a spherical shape, the length in the direction indicating the maximum length of the particles is defined as the particle size. Two or more kinds of fine particles having different particle diameters and / or specific surface areas may be simultaneously contained.

  The thickness of the A film is usually 0.1 μm or more and 10 μm or less, preferably 2 μm or more and 6 μm or less. If the non-aqueous electrolyte secondary battery is manufactured if the thickness of the A film is too thick, the load characteristics of the battery may be reduced. If the thickness of the A film is too thin, the polyolefin porous film may be used when abnormal heat generation of the battery occurs. There is a possibility that the separator shrinks without being able to resist the heat shrinkage.

  The A film is a porous film, and the pore diameter is preferably 3 μm or less, more preferably 1 μm or less, as the diameter of the sphere when the hole is approximated to a sphere. If the average pore size or the pore size exceeds 3 μm, a short circuit may occur when the carbon powder, which is the main component of the positive electrode or the negative electrode, or a small piece thereof falls off. The porosity of the A film is preferably 30 to 95% by volume, more preferably 40 to 90% by volume.

  A film | membrane can contain a dispersing agent, a plasticizer, etc. in the range which does not impair the objective of this invention remarkably as compositions other than a water-soluble polymer and microparticles | fine-particles.

Next, the B film will be described.
The B film is a porous film containing polyolefin, and preferably contains a high molecular weight component having a molecular weight of 5 × 10 ⁵ to 15 × 10 ⁶ . Examples of the polyolefin include a homopolymer or a copolymer obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. The B film preferably contains polyethylene or polypropylene, more preferably contains high molecular weight polyethylene having a weight average molecular weight of 1 × 10 ⁵ or more, and contains ultra high molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more. Even more preferred.

  The porosity of the B film is preferably 20 to 80% by volume, more preferably 30 to 70% by volume. If the porosity is less than 20% by volume, the amount of electrolyte retained may be small, and if it exceeds 80% by volume, non-porous formation at a high temperature that causes shutdown will be insufficient, and current will be generated when abnormal heat generation of the battery occurs. May not be able to be blocked.

  The thickness of B film is 4-50 micrometers normally, Preferably it is 5-30 micrometers. If the thickness is less than 4 μm, there is a risk that the non-porous formation at a high temperature at which shutdown occurs will be insufficient, and if it exceeds 50 μm, the thickness of the entire separator becomes thick and the electric capacity of the battery may be reduced. The pore size of the B film is preferably 3 μm or less, and more preferably 1 μm or less.

  The B membrane has a structure having pores connected to the inside thereof, and allows gas and liquid to pass from one surface to the other surface. The permeability of the B film is usually 50 to 400 seconds / 100 cc in terms of Gurley value, and preferably 50 to 300 seconds / 100 cc.

The method for producing the B film is not particularly limited. For example, as described in JP-A-7-29563, a plasticizer is added to polyolefin to form a film, and then the plasticizer is removed with an appropriate solvent. As described in JP-A-7-304110, a film made of polyolefin produced by a known method is used, and a structurally weak amorphous portion of the film is selectively stretched to form fine pores. The method of forming is mentioned. In addition, for example, when the B film is formed from an ultrahigh molecular weight polyethylene and a polyolefin containing a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, it is produced by the following method from the viewpoint of production cost. Is preferred. That is,
(1) A polyolefin resin composition obtained by kneading 100 parts by weight of ultrahigh molecular weight polyethylene, 5 to 200 parts by weight of a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, and 100 to 400 parts by weight of an inorganic filler such as calcium carbonate. (2) The step of molding a sheet using the polyolefin resin composition (3) The step of removing the inorganic filler from the sheet obtained in the step (2) (4) The step (3) A method comprising a step of drawing a sheet to obtain a B film,
Or (1) a step of kneading 100 parts by weight of ultrahigh molecular weight polyethylene, 5 to 200 parts by weight of a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, and 100 to 400 parts by weight of an inorganic filler to obtain a polyolefin resin composition (2) Step of forming a sheet using the polyolefin resin composition (3) Step of stretching the sheet obtained in step (2) to obtain a stretched sheet (4) Stretched sheet obtained in step (3) It is a method including the process of removing an inorganic filler from and obtaining B film | membrane.

  In addition, about B film, the commercial item which has the characteristic of the said description can be used.

Next, the C film will be described.
Examples of the nitrogen-containing aromatic polymer constituting the C film include aromatic polyamide (para-oriented aromatic polyamide, meta-oriented aromatic polyamide; hereinafter referred to as “aramid”), aromatic polyimide, aromatic polyamide. Examples thereof include imides, preferably aromatic polyamides, and particularly preferably para-oriented aromatic polyamides (hereinafter sometimes referred to as “para-aramids”) from the viewpoint of easy use. When the C film has these nitrogen-containing aromatic polymers, the heat resistance of the separator, that is, the thermal film breaking temperature is further increased. In addition, the C film generally has a high elastic modulus and may improve the shape maintainability during overheating, and thus it is possible to provide a nonaqueous electrolyte secondary battery that is superior in safety.

  The para-aramid is obtained by polycondensation of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide, and the amide bond is in the para position of the aromatic ring or an oriented position equivalent thereto (for example, 4,4′-position in biphenylene, 1 to 5 positions in naphthalene and 2,6 positions in naphthalene). Specific examples of para-aramid include poly (paraphenylene terephthalamide), poly (parabenzamide), poly (4,4′-benzanilide terephthalamide), poly (paraphenylene-4,4′-biphenylenedicarboxylic acid). Amide), poly (paraphenylene-2,6-naphthalenedicarboxylic acid amide), poly (2-chloro-paraphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer, and the like. These are para-aramids having a para-oriented or para-oriented structure.

  The aromatic polyimide is preferably a wholly aromatic polyimide produced by condensation polymerization of an aromatic dianhydride and a diamine. Specific examples of the dianhydride include pyromellitic dianhydride, 3,3 ′, 4,4′-diphenylsulfone tetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenone tetracarboxylic Examples thereof include acid dianhydride, 2,2′-bis (3,4-dicarboxyphenyl) hexafluoropropane, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, and the like. Specific examples of the diamine include oxydianiline, paraphenylenediamine, benzophenonediamine, 3,3′-methylenedianiline, 3,3′-diaminobenzophenone, 3,3′-diaminodiphenylsulfone, 1,5 '-Naphthalenediamine etc. are mentioned. Moreover, a polyimide soluble in a solvent can be preferably used. An example of such a polyimide is a polycondensate polyimide of 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic dianhydride and an aromatic diamine.

  Examples of the aromatic polyamideimide include those obtained by condensation polymerization of aromatic dicarboxylic acid and aromatic diisocyanate, and those obtained by condensation polymerization of aromatic diacid anhydride and aromatic diisocyanate. Specific examples of the aromatic dicarboxylic acid include isophthalic acid and terephthalic acid. Specific examples of the aromatic dianhydride include trimellitic anhydride. Specific examples of the aromatic diisocyanate include 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, orthotolylane diisocyanate, m-xylene diisocyanate, and the like.

  The thickness of the C film in the present invention is preferably 1 μm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less, and particularly preferably 1 μm or more and 4 μm or less from the viewpoint of further improving ion permeability. The C film has fine holes, and the size (diameter) of the holes is usually 3 μm or less, preferably 1 μm or less.

  The C film can further contain fine particles. The material of the fine particles in the C film can be selected from any of organic materials, inorganic powders, and mixtures thereof, and the same material as that of the A film can be used. The fine particles preferably have an average particle size of 0.01 μm or more and 1 μm or less.

  When the C film contains fine particles, the content of the fine particles depends on the specific gravity of the material of the fine particles. The weight ratio of the fine particles is, for example, usually 5 or more and 95 or less, preferably 20 or more and 95 or less, and more preferably 30 or more and 90 or less with respect to the total weight 100 of the C film. These ranges are particularly suitable when all particulate materials are alumina particles.

  The C film is a porous film similar to the A film, but the pore diameter is preferably 3 μm or less, more preferably 1 μm or less, as the diameter of the sphere when the hole is approximated to a sphere. If the average pore size or the pore size exceeds 3 μm, a short circuit may occur when the carbon powder, which is the main component of the positive electrode or the negative electrode, or a small piece thereof falls off. Further, the porosity of the C film is preferably 30 to 90% by volume, more preferably 40 to 80% by volume.

  As described above, the separator of the present invention is a separator in which the A film, the B film, and the C film are laminated in this order. The separator may include a porous film other than the A film, the B film, and the C film, for example, a porous film such as an adhesive film and a protective film as long as the object of the present invention is not significantly impaired.

  When a non-aqueous electrolyte secondary battery is manufactured using the separator of the present invention, high load characteristics can be obtained, but the air permeability of the separator is preferably 50 to 2000 seconds / 100 cc, more preferably 50 to 1000 seconds / 100 cc. preferable. If the air permeability is less than 50 seconds / 100 cc, the separator becomes fragile, and when used in a non-aqueous electrolyte secondary battery, the insulation between the positive and negative electrodes may not be maintained. If the air permeability exceeds 2000 seconds / 100 cc, when used in a non-aqueous electrolyte secondary battery, the ion permeability may be reduced, and load characteristics and the like may be lowered.

  Next, the manufacturing method of a separator is demonstrated. The separator is a method of laminating the A film on one side of the B film and then laminating the C film on the other side, or a method of laminating the C film on one side of the B film and then laminating the A film on the other side Any of these can be manufactured. An appropriate method can be adopted according to the situation of the manufacturing apparatus, manufacturing process, and the like. Hereinafter, a method for laminating the B film and the A film and a method for laminating the B film and the C film will be described.

  As a method of laminating the B film and the A film, a coating liquid containing a water-soluble polymer and a medium or a coating liquid containing a water-soluble polymer, fine particles and a medium is applied on an appropriate support. A method of forming a film A by removing the medium from the coating film by drying or the like, removing the film A from the support, and pressing the film A and the film B; A film is formed by extrusion to form a film A, and the film A and the film B are pressure-bonded; the coating liquid is applied onto a film B fixed on a suitable support and coated. Examples of the method include obtaining a film, removing the medium from the coating film by drying or the like, forming an A film on the B film, and peeling the obtained laminated film from the support. Note that a B film on which a C film is stacked can also be used. In that case, the A film may be stacked on the surface of the B film on which the C film is not stacked, and a method similar to the above can be performed.

  A support in which a coating solution containing a water-soluble polymer is fixed on a suitable support using a coating method commonly used in the industry such as coating by a coater (also referred to as a doctor blade) or brush coating. Can be applied to body or B film. When the coating liquid contains a water-soluble polymer, fine particles, and a medium, a slurry in which the fine particles are dispersed in a solution in which the water-soluble polymer is dissolved in the medium can be used as the coating liquid. As the dispersing device, a pressure disperser (gorin homogenizer, nanomizer) or the like may be used. The thickness of the A film can be controlled by adjusting the thickness of the coating film, the concentration of the water-soluble polymer in the coating solution, and the ratio of the fine particles to the water-soluble polymer. As the support, a resin film, a metal belt, a drum, or the like can be used.

  The removal of the medium from the coating film is generally performed by drying. As an example of a removal method, a solvent that can dissolve in the medium but does not dissolve the water-soluble polymer is prepared, and the coating film is immersed in the solvent to replace the medium with the solvent. The method of depositing a conductive polymer, removing the medium by evaporating under dry heating, and removing the solvent by drying may be mentioned. When the coating liquid is applied onto the B film, the drying temperature of the medium or solvent is preferably a temperature that does not decrease the air permeability of the B film.

As a method of laminating the B film and the C film, a method of separately producing the C film and the B film and laminating each of them, containing a nitrogen-containing aromatic polymer on one side of the B film, and optionally fine particles In the present invention, when the C film is relatively thin, the latter method is preferable from the viewpoint of productivity. Examples of a method for forming a C film by applying a coating solution containing a nitrogen-containing aromatic polymer and fine particles on one surface of the B film include a method including the following steps.
(A) A slurry in which fine particles are dispersed in a polar organic solvent solution in which a nitrogen-containing aromatic polymer is dissolved in a polar organic solvent is prepared. Here, the weight ratio of the fine particles is set to 5 or more and 95 or less with respect to the total weight 100 of the nitrogen-containing aromatic polymer and fine particles.
(B) The coating solution is applied to one side of the B film to form a coating film.
(C) The nitrogen-containing aromatic polymer is precipitated from the coating film by means such as humidification, solvent removal, or immersion in a solvent that does not dissolve the nitrogen-containing aromatic polymer, and is dried as necessary.

  The coating solution is preferably applied continuously by, for example, a coating apparatus described in JP-A-2001-316006 and a method described in JP-A-2001-23602.

  In the polar organic solvent solution, when the nitrogen-containing aromatic polymer is an aromatic polyamide (aramid), a polar amide solvent or a polar urea solvent can be used as the polar organic solvent. Specific examples include, but are not limited to, N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone (NMP), tetramethylurea and the like.

  When para-aramid is used as the nitrogen-containing aromatic polymer, an alkali metal or alkaline earth metal chloride is preferably added during the para-aramid polymerization for the purpose of improving the solubility of para-aramid in a solvent. Specific examples include, but are not limited to lithium chloride or calcium chloride. The amount of the chloride added to the polymerization system is preferably in the range of 0.5 to 6.0 mol, more preferably in the range of 1.0 to 4.0 mol, per 1.0 mol of the amide group generated by condensation polymerization. . If the chloride is less than 0.5 mol, the solubility of the resulting para-aramid may be insufficient, and if it exceeds 6.0 mol, the solubility of the chloride in the solvent may be substantially exceeded, which may be undesirable. . In general, if the alkali metal or alkaline earth metal chloride is less than 2% by weight, the solubility of para-aramid may be insufficient. If it exceeds 10% by weight, the alkali metal or alkaline earth metal chloride may be insufficient. May not dissolve in polar organic solvents such as polar amide solvents or polar urea solvents.

  When the nitrogen-containing aromatic polymer is an aromatic polyimide, as the polar organic solvent for dissolving the aromatic polyimide, in addition to those exemplified as the solvent for dissolving aramid, dimethyl sulfoxide, cresol, and o-chlorophenol Etc. can be used suitably.

  As a device for dispersing the fine particles, a pressure type disperser (gorin homogenizer, nanomizer) or the like can be used.

  Examples of the method of coating the slurry include, for example, a coating method such as a knife, blade, bar, gravure, die, etc., and a coating method such as a bar or knife is simple, but industrially, A bar coater or a gravure coater excellent in film thickness control is preferred. Moreover, coating can also be performed twice or more. In this case, it is usual to perform the next coating after the nitrogen-containing aromatic polymer is precipitated in the step (c).

  When the C film and the B film are separately manufactured and laminated, the films can be laminated to each other by a method such as adhesion by an adhesive or heat fusion.

  Next, the nonaqueous electrolyte secondary battery of the present invention will be described. The battery of the present invention includes the separator of the present invention. Below, an example of a lithium ion secondary battery is demonstrated as a nonaqueous electrolyte secondary battery. Although components other than a separator are demonstrated especially, it is not limited to these.

As the nonaqueous electrolytic solution, for example, a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent can be used. Examples of the lithium salt include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , One or a mixture of two or more of lower aliphatic carboxylic acid lithium salts, LiAlCl _{4 and the} like can be mentioned. Among these, at least one selected from the group consisting of fluorine-containing LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiC (CF ₃ SO ₂ ) _3. Certain fluorine-containing lithium salts are preferred.

  Examples of the non-aqueous electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di (methoxycarbonyloxy) Carbonates such as ethane; ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran Esters such as methyl formate, methyl acetate and Y-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N, N-dimethylformamide and N, N-dimethylacetamide Carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone, or those obtained by introducing a fluorine group into the above-mentioned substances can be used. Two or more of these are mixed and used.

  Among these, those containing carbonates are preferred, and cyclic carbonates and acyclic carbonates, or mixtures of cyclic carbonates and ethers are more preferred. As a mixture of cyclic carbonate and acyclic carbonate, ethylene carbonate and dimethyl have a wide operating temperature range and are hardly decomposable even when a graphite material such as natural graphite or artificial graphite is used as the negative electrode active material. A mixture comprising carbonate and ethyl methyl carbonate is preferred.

As the positive electrode sheet, a sheet in which a positive electrode mixture containing a positive electrode active material, a conductive material and a binder is supported on a positive electrode current collector is usually used. As a method of supporting the positive electrode mixture on the positive electrode current collector, a method of pressure molding; a positive electrode mixture paste is obtained by further using an organic solvent, the paste is applied to the positive electrode current collector, and dried. And a method of pressing the obtained sheet and fixing the positive electrode mixture to the positive electrode current collector. Specifically, as the positive electrode active material, a material containing a material that can be doped / undoped with lithium ions, a carbonaceous material as a conductive material, and a thermoplastic resin as a binder can be used. As the positive electrode current collector, a conductor such as Al, Ni, and stainless steel can be used, but Al is preferable in that it is easily processed into a thin film and is inexpensive. Examples of the material that can be doped / undoped with lithium ions include lithium composite oxides containing at least one transition metal such as V, Mn, Fe, Co, and Ni. Among these, lithium composite oxides having an α-NaFeO ₂ type structure such as lithium nickelate and lithium cobaltate, and lithium composite oxides having a spinel type structure such as lithium manganese spinel are preferable in that the average discharge potential is high. Can be mentioned.

  The lithium composite oxide may contain various metal elements, particularly at least selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al, Ga, In, and Sn. The metal element is included so that the at least one metal element is 0.1 to 20 mol% with respect to the sum of the number of moles of one metal element and the number of moles of Ni in lithium nickelate. It is preferable to use composite lithium nickelate because the cycle performance in use at a high capacity is improved.

  As the binder, polyvinylidene fluoride, vinylidene fluoride copolymer, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, Examples include ethylene-tetrafluoroethylene copolymers, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, thermoplastic resins such as thermoplastic polyimide, polyethylene, and polypropylene.

  Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, cokes, and carbon black. As the conductive material, each may be used alone, for example, artificial graphite and carbon black may be mixed and used.

As the negative electrode sheet, for example, a material capable of doping and dedoping lithium ions, lithium metal, or a lithium alloy can be used. Materials that can be doped / undoped with lithium ions include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds, and lower potential than the positive electrode. And chalcogen compounds such as oxides and sulfides for doping and dedoping lithium ions.
As a carbonaceous material, a carbonaceous material mainly composed of graphite materials such as natural graphite and artificial graphite, because it has a high potential flatness and a low average discharge potential, so that a large energy density can be obtained when combined with a positive electrode. Is preferred.

  As the negative electrode current collector, Cu, Ni, stainless steel, or the like can be used. In particular, in a lithium ion secondary battery, Cu is preferable because it is difficult to form an alloy with lithium and it can be easily processed into a thin film. As a method of supporting the negative electrode mixture containing the negative electrode active material on the negative electrode current collector, a pressure molding method; a negative electrode mixture paste is obtained by further using a solvent or the like, and the paste is applied to the negative electrode current collector. Examples thereof include a method of pressing and drying the obtained sheet and fixing the negative electrode mixture to the negative electrode current collector.

  The shape of the battery is not particularly limited, and may be any of a paper type, a coin type, a cylindrical type, a rectangular shape, and the like.

  When producing a non-aqueous electrolyte secondary battery using the non-aqueous electrolyte secondary battery separator of the present invention, the separator has a high load characteristic, and even when abnormal heat generation occurs, the separator exhibits a shutdown function, and further A non-aqueous electrolyte secondary battery that can suppress heat generation and avoid contact between the positive electrode and the negative electrode due to shrinkage of the separator even when the temperature is further increased can be obtained.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples. In Examples and Comparative Examples, the physical properties of porous films and separators were measured by the following methods.
(1) Thickness measurement (unit: μm)
The thickness of the separator was measured according to the JIS standard (K7130-1992).
(2) Measurement of basis weight (unit: g / m ² )
A sample of the obtained separator was cut into a 10 cm long square and the weight W (g) was measured. The weight per unit area (g / m ² ) = W / (0.1 × 0.1) was calculated. The basis weight of the A film was calculated from the basis weight difference before and after the A film lamination, while the basis weight of the C film was calculated from the basis weight difference before and after the C film lamination.
(3) Porosity (unit: volume%)
A sample of the obtained separator was cut into a 10 cm long side, and weight W (g) and thickness D (cm) were measured. The weight (Wi (g)) of each layer in the sample is obtained, and the volume of each layer is obtained from Wi and the true specific gravity (true specific gravity i (g / cm ³ )) of each material. From this, the porosity (volume%) was determined.

Porosity (volume%) = 100 − [{(W1 / true specific gravity 1) + (W2 / true specific gravity 2) + ·· + (Wn / true specific gravity n)} / (10 × 10 × D)] × 100

(4) Air permeability (Gurley method) (Unit: sec / 100cc)
The air permeability of the separator was measured with a digital timer type Gurley type densometer manufactured by Toyo Seiki Seisakusho Co., Ltd. based on JIS P8117.

(5) Film shape at the time of heating The film is cut into a square of 15 cm square, a square ruled line is written at the center of 10 cm square, and sandwiched between two 0.5 mm thick aluminum plates coated with a release agent. In an oven heated to 60 ° C. The temperature of the oven was raised to 150 ° C. at a rate of 1 ° C./min and held for 10 minutes, then taken out and visually checked for the shape of the film.
(6) Thermal film breaking temperature (unit: ° C)
An aluminum frame having an outer dimension of 13 cm × 13 cm square and an inner dimension of 8 cm × 8 cm square was prepared, an 11 cm × 11 cm sample was placed on top, and four pieces were fixed on the whole surface with aluminum tape. The sample fixed to the aluminum frame was put in a hot air oven set at 120 to 160 ° C. (in increments of 5 ° C.) and 160 to 200 ° C. (in increments of 10 ° C.), and the sample was taken out after standing for 5 minutes to confirm the state of the sample. The temperature at which the sample began to break was defined as the thermal breakage temperature of the sample.
(7) Shutdown (SD) property Shutdown resistance was measured in a cell for measuring shutdown (hereinafter referred to as cell).
A 2 × 3 cm square rectangular membrane was impregnated with an electrolytic solution, and then sandwiched between two SUS electrodes and fixed with clips to prepare a cell. As the electrolytic solution, one obtained by dissolving 1 mol / L LiBF ₄ in a mixed solvent of ethylene carbonate 50 vol%: diethyl carbonate 50 vol% was used. A terminal of an impedance analyzer was connected to both electrodes of the assembled cell, and a resistance value at 1 kHz was measured. The resistance at 145 ° C. was measured while the temperature was raised in the oven at a rate of 15 ° C./min. The shutdown property was evaluated according to the following criteria.
Evaluation of shutdown performance:
○: Resistance value at 145 ° C is 1000Ω or more × Resistance value at 145 ° C is less than 1000Ω

Example 1
(1) Preparation of slurry 1 (CMC coating solution) 100 g of CMC (Dell Daigaku Seiyaku Co., Ltd., Serogen 4H) is weighed and added to 4.9 kg of water with stirring, and mixed and dissolved for 1 hour. I left still overnight. Thereafter, 5 kg of water and 2.5 kg of ethanol were added to the above CMC aqueous solution, stirred for 30 minutes, and as fine particles (a), fine alumina powder (AKP-G008 manufactured by Sumitomo Chemical Co., Ltd., average particle size: 0.1 μm or less) , Specific surface area: average 70 m ² / g) and 500 g of alumina powder (Sumitomo Chemical AA03, average particle size: 0.4 μm, specific surface area: 5 m ² / g) as fine particles (b) After adding and stirring with a homogenizer at 3000 rpm for 30 minutes, alumina was dispersed in the solution at a pressure of 60 MPa with a gorin homogenizer. Dispersion by a gorin homogenizer was further repeated twice to obtain slurry 1 (CMC coating solution) for A film. The composition of slurry 1 (CMC coating solution) is shown in Table 1.

(2) Preparation of slurry 2 (aramid coating solution) (2-1) Preparation of nitrogen-containing aromatic polymer N-methyl-2-pyrrolidone (hereinafter referred to as “NMP”) / calcium chloride solution (calcium chloride concentration) = 7.1 wt%) To 5000 g, 150.00 g of paraphenylenediamine (hereinafter referred to as “PPD”) was added and stirred and dissolved in a nitrogen atmosphere. Subsequently, 273.94 g of terephthalic acid dichloride (hereinafter referred to as “TPC”) was added at 15 ° C., and the mixture was stirred and reacted for 1 hour to obtain a polyparaphenylene terephthalamide polymerization solution.
(2-2) Preparation of coating liquid 1000 g of the polymerization liquid was collected, 3000 g of NMP, 23.4 g of calcium carbonate (manufactured by Ube Material Co., Ltd.), and fine particles (a) as fine alumina powder (alumina C (ALC manufactured by Aerosil) ), 60 g of average particle size: 0.013 μm), and 60 g of alumina powder (Sumitomo Random AA03 manufactured by Sumitomo Chemical Co., Ltd., average particle size: 0.3 μm) as fine particles (b) were added, stirred and mixed. A homogenizer (manufactured by APV) was subjected to dispersion treatment once at a pressure of 50 MPa to obtain slurry 2 (aramid coating solution) for C film. The composition of slurry 2 (aramid coating solution) is shown in Table 2.

(3) Manufacture of separator (3-1) Lamination of B membrane and A membrane A polyethylene porous membrane was used as the B membrane. The polyethylene porous membrane was fixed to a drum, and a weight of 0.6 kg was suspended on the other side so that a load was evenly applied to the substrate. A stainless steel coating bar having a diameter of 20 mm was arranged in parallel at the top of the drum so that the clearance from the drum was 30 μm. The drum was rotated and stopped so that the end on the side fixed with the polyethylene porous film tape came between the drum and the coating bar. While supplying slurry 1 (CMC coating solution) onto the polyethylene porous membrane in front of the coating bar, the drum was rotated at 0.5 rpm to coat the polyethylene porous membrane. After coating, the rotation of the drum was stopped and the drum was left in an atmosphere of 70 ° C. for 30 minutes, and the film was dried to obtain a laminated film (AB) in which the A film was laminated on one side of the B film.

(3-2) Lamination of C film on laminated film (AB) Slurry 2 (aramid coating solution) is applied to one side opposite to A film of laminated film (AB), and clearance between coating bar and drum is set to 70 μm Coating was carried out in the same manner as (3-1) except that the change was made.

  After coating, the drum was stopped and placed in an atmosphere of 70 ° C. for 30 minutes as it was, the film was dried and removed from the drum to obtain the separator of Example 1.

(4) Physical properties of the separator The above-mentioned separator has an overall thickness of 21.7 μm and a basis weight of 12.7 g / m ² (polyethylene 6.8 g / m ² , aramid 3.2 g / m ² , CMC 2.7 g / m ² ). The air permeability was 377 sec / 100 cc. Table 3 shows the physical property values of the separator. In Table 3, the heat resistant film laminated on one surface of the B film is denoted as a first heat resistant film, and the heat resistant film laminated on the other surface is denoted as a second heat resistant film (hereinafter the same). Table 4 shows the evaluation results of the film shape, the thermal film breaking temperature, and the shutdown (SD) property during heating.

Example 2
Example 1 except that the coating amount and thickness of the A film (second heat-resistant film) were changed to the values shown in Table 3 by arranging the coating bar in parallel so that the clearance with the drum was 70 μm. Similarly, the separator of Example 2 was produced. Table 1 shows the composition of the CMC coating solution, Table 2 shows the composition of the aramid coating solution, Table 3 shows the physical properties of the separators obtained, Table 4 shows the film shape, thermal film breaking temperature, and shutdown (SD) during heating. The evaluation result of sex is shown.

Comparative Examples 1-5
Using separators having the compositions shown in Tables 1 and 2, separators of Comparative Examples 1 to 5 were produced in the same manner as in Example 1. In the separators of Comparative Examples 1 to 5, Comparative Example 1 is a separator in which an aramid porous film (C film) is laminated on both sides of a polyethylene porous film (B film), and Comparative Example 2 is a polyethylene porous film (B The separator is formed by laminating an aramid porous film (C film) on one side of the film. Comparative Examples 3 and 4 are examples of a separator in which a CMC porous film (A film) is laminated on one side of a polyethylene porous film (B film), and Comparative Example 5 is laminated on both sides of a polyethylene porous film (B film).
Table 1 shows the composition of the CMC coating solution, Table 2 shows the composition of the aramid coating solution, Table 3 shows the physical properties of the separators obtained, Table 4 shows the film shape, thermal film breaking temperature, and shutdown (SD) during heating. The evaluation result of sex is shown. Note that the shutdown property was evaluated only for the separator that was judged to be free of curl in the film shape during heating.
In Comparative Example 1, the ultimate resistance was slow at the time of shutdown (SD), and in Comparative Example 2, curl occurred in the obtained laminated porous film. Further, in Comparative Examples 3 and 4, it was found that curl occurred in the obtained separator and that the thermal film breaking temperature was low. Moreover, in the comparative example 5, the thermal film breaking temperature was low.

  ADVANTAGE OF THE INVENTION According to this invention, in addition to shutdown property, the film separator, curl, etc. are hard to produce and the separator for nonaqueous electrolyte secondary batteries excellent in shape maintainability is provided. By using the separator of the present invention, a non-aqueous electrolyte secondary battery that can maintain the insulation between the positive electrode and the negative electrode is obtained without changing the shape of the separator even when abnormal heat generation occurs.

Claims (8)

  A separator in which a porous film containing a water-soluble polymer, a porous film containing polyolefin, and a porous film containing a nitrogen-containing aromatic polymer are laminated in this order.   The separator according to claim 1, wherein the porous film containing the water-soluble polymer further contains fine particles.   The separator according to claim 2, wherein the fine particles have an average particle size of 3 μm or less.   The separator according to any one of claims 1 to 3, wherein the water-soluble polymer is at least one polymer selected from the group consisting of cellulose ether, polyvinyl alcohol, and sodium alginate.   The separator according to claim 4, wherein the cellulose ether is carboxymethyl cellulose.   The separator according to any one of claims 1 to 5, wherein the nitrogen-containing aromatic polymer is at least one polymer selected from the group consisting of aromatic polyamide, aromatic polyimide, and aromatic polyamideimide.   The separator according to any one of claims 1 to 6, wherein the polyolefin is polyethylene.   A non-aqueous electrolyte secondary battery comprising the separator according to claim 1.