JP2014235835A - Multilayer separator and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator minimizing curl at an end part during coating, in a wet type separator process in which a porous membrane is layered on another porous membrane.SOLUTION: A multilayer separator includes: a first porous membrane which comprises a laminate of a porous membrane and is a resin base material; and a second porous membrane which comprises a material having a higher heatproof temperature. An end part of the second porous membrane has a thinner thickness than a central part.

Description

  The present invention relates to a multilayer separator and a battery.

  In recent years, as mobile terminals and laptop PCs have become smaller and higher in performance, secondary batteries serving as power sources have been required to be smaller, have higher capacity, and have higher energy density. Lithium-ion secondary batteries (hereinafter sometimes referred to as “batteries”) have the lowest redox potential of metallic lithium and the lightest metal, so the battery size is reduced compared to conventional batteries. High capacity and high energy density are possible. For this reason, lithium ion secondary batteries are widely used, and demand for automotive power supplies is expected to increase in the future.

  However, a lithium ion secondary battery may generate heat due to an internal short circuit due to its high energy density. For this reason, various countermeasures have been taken, and separators play an important role as one of the countermeasures.

  Typical separators include polyolefin porous membranes such as polyethylene and polypropylene. For safety, the polyolefin porous membrane separator is provided with a shutdown function that closes the pores at high temperatures and stops the battery reaction.

  However, when the temperature exceeds the temperature at which the polyolefin porous membrane separator melts due to overcharge or short circuit, the separator cannot retain its shape and the shutdown function is lost. This can lead to a larger internal short circuit.

  Various measures have been taken for this problem. As one of them, there is a method of coating a porous film made of a heat-resistant resin on a conventional polyolefin porous film in order to maintain a shape at a high temperature as in Patent Document 1. As a result, it has been found that the heat-resistant resin layer can maintain its shape and suppress secondary short-circuiting even after the polyolefin layer melts during abnormal heat generation.

  As a general method for forming a porous membrane as in Patent Document 1, there are a melt extrusion method, a phase separation method, a solvent casting method, and the like. However, when a heat-resistant resin layer is coated on one side of the polyolefin layer by these methods, shrinkage stress is generated in the heat-resistant resin layer in the process of solidifying the heat-resistant resin layer in a poor solvent, and curling occurs from the end. there is a possibility. Then, due to this curl, the handling property of the separator is lowered, and there is a possibility that troubles occur when the battery is assembled.

JP 2009-70609 A

  An object of the present invention is to provide a multilayer separator that does not easily curl, and a battery using the same.

  In order to solve the above problems, a multilayer separator of the present invention is provided with a first porous membrane and a second porous membrane provided on the first porous membrane and having a heat resistant temperature higher than that of the first porous membrane. And the end portion of the second porous membrane is thinner than the other portions.

  Since the end portion of the second porous film is thinner than the other portions, the stress at the end portion is reduced, and curling that tends to occur from the end portion is less likely to occur.

  Moreover, it is preferable that the porosity in the edge part of a 2nd porous film is higher than the porosity of the other part of a 2nd porous film.

  Since the porosity of the end portion of the second porous membrane is higher than that of the other portions, the amount of the material of the second porous membrane is smaller than that of the other portions, so that the generated stress is small. And curling is less likely to occur.

  More preferably, the pores on the first porous membrane side of the second porous membrane are larger than the pores on the opposite side.

  Since the pores on the first porous membrane side of the second porous membrane are larger than the pores on the opposite side, the amount of material of the second porous membrane is reduced, resulting in less stress and curling. Can be further suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, the multilayer separator and battery which are hard to generate | occur | produce a curl can be provided.

It is a schematic cross section of multilayer separator 200 of a 1st embodiment. It is a schematic cross section of the multilayer separator 201 of 2nd Embodiment. It is a schematic cross section of the multilayer separator 202 of 3rd Embodiment. It is a schematic cross section of a lithium ion secondary battery.

(Multilayer separator)
Hereinafter, preferred embodiments of the multilayer separator of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(First embodiment)
FIG. 1 is a schematic cross-sectional view showing an embodiment of the multilayer separator of the present invention. The schematic cross-sectional view is a schematic view showing a cross section when cut in a direction perpendicular to the longitudinal direction, that is, a cross section in the width direction. As shown in this figure, the multilayer separator 200 of the present embodiment has a structure in which two layers of porous films of a first porous film 12 and a second porous film 14 are laminated. Both the first porous membrane 12 and the second porous membrane 14 each have a large number of pores (the pores of the first porous membrane 12 are not shown). Since the holes in FIG. 1 are schematically shown, they are sparsely present. However, in reality, there are a large number of holes close to each other, and a place where the holes are in contact with each other is a continuous hole. In the second porous membrane 14, the central portion 1 has a substantially uniform thickness, and the end portion 2 is thinner than the central portion 1. Further, the end portion 2 is inclined so as to gradually become thinner from the central portion 1 side to the end of the second porous membrane 14. Here, the end portion 2 has an inclination so that the cross section of the second porous film 14 is thinned linearly. Since the amount of the material of the second porous film 14 at the end 2 is reduced and the stress is reduced, curling is unlikely to occur from the end 2.

  In the present embodiment, the second porous film 14 is a porous film containing a heat resistant material, and the first porous film 12 is a porous film containing a polyolefin resin.

  Examples of the material of the second porous film 14 include various resin materials. It is preferable that it is stable against an electrolytic solution and a battery reaction and has sufficient heat resistance. Examples include polyamideimide, polyamide, polyimide, polycarbonate, polyacetal, polyphenylene ether, polyvinylidene fluoride, polytetrafluoroethylene, polyetherketone, polyethylene terephthalate, polysulfone, polyester, polyacrylonitrile, etc. It doesn't matter.

  As a method for producing the second porous film 14 using a resin material, for example, the resin material is dissolved in the good solvent, and then applied onto the substrate, that is, the first porous film 12 to remove the solvent. There is a method of forming a resin layer by doing so. The concentration of the resin material is preferably 1 wt% or more from the viewpoint of heat resistance, and preferably 30 wt% or less from the viewpoint of air permeability resistance. A more preferable concentration is 5 to 20 wt%. The type of the solvent is not particularly limited, but N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylformamide, dimethylacetamide and the like are used when the heat resistant resin is polyamideimide. Among these, N-methyl-2-pyrrolidone is particularly preferable in terms of solubility and viscosity.

  In addition to the resin material described above, an inorganic material can be used as the second porous film 14. As long as the inorganic material is an inorganic substance that can form a thin film having a higher mechanical strength than the polyolefin porous film used for the first porous membrane 12 and a higher softening point than the polyolefin porous film, the inorganic material can be formed. Although it does not specifically limit, the thing excellent in the chemical stability in a battery when using the porous film of this invention as a separator for batteries is preferable. Examples of such materials include silicon oxide, titanium oxide, aluminum oxide, magnesium oxide, tin oxide, and indium oxide. One or more of these can be used. In particular, when an inorganic layer is produced by a coating method, it is preferable to use a single substance of silicon oxide or a mixture of silicon oxide and other inorganic substances from the viewpoint of high stability of the coating solution.

  The production method of the second porous film 14 using an inorganic material is usually a conventional casting or coating method, for example, roll coater, air knife coater, blade coater, rod coater, bar coater, comma coater, gravure coater. , Silk screen coater, die coater, micro gravure coater method and the like.

  As a method for forming the end portion 2 of the second porous film 14 to be thinner than the center portion 1, for example, a method of reducing the thickness by controlling the coating amount at the time of coating can be mentioned. A shape with a thin end is formed by reducing the flow rate at the end during application. Moreover, after apply | coating normally, the method of shaving off the coating material of an edge part using a linear blade etc., and making it solidify after that is mentioned.

  The first porous membrane 12 in the present invention contains polyolefin. The first porous film 12 containing polyolefin may be a known one, and may be manufactured by any manufacturing method. Examples of polyolefins include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high molecular weight polyethylene, and two-stage polymerized polyethylene. As the high-density polyethylene, those produced by using a catalyst such as Ziegler-Natta, Cr, or metallocene can be used. Moreover, it can be used by mixing with other polyolefin resin in arbitrary ratios. Other polyolefin resins include polymers such as ethylene, propylene, 1-butene, 1-hexene, and 1-octene. The polyolefin resin used for the first porous membrane 12 preferably has a melting point of 150 ° C. or less from the viewpoint of obtaining a good function of closing the membrane pores during heat generation.

  Examples of the aforementioned polymer include polypropylene, polybutene, and ethylene propylene rubber. If necessary, the polyolefin resin may be phenolic, phosphorus or sulfur antioxidants, metal soaps such as calcium stearate or zinc stearate, UV absorbers, light stabilizers, antistatic agents, Known additives such as clouding agents and color pigments, and inorganic substances such as silica and alumina can be mixed and used.

  Moreover, it does not specifically limit about the manufacturing method of the porous film which has the polyolefin resin used as a main component for this invention. For example, a polyolefin resin and a solvent are melt-kneaded to obtain a uniform solution, and then extruded from a T-die and cooled and solidified to form a sheet-like porous film precursor. After stretching, the solvent is removed, or stretching is performed after removing the solvent. Can be obtained.

(Second embodiment)
Next, although 2nd embodiment is described, it demonstrates focusing on a different point from 1st embodiment of 2nd embodiment. FIG. 2 is a schematic cross-sectional view showing a multilayer separator 201 manufactured using a concavely curved blade in accordance with the method of Example 1. The point that the end 3 of the second porous membrane 141 is thinner than the other portions is the same as in the first embodiment, but the end 3 is formed in a convex curve shape from the central portion 1. Here, the convex curved surface shape means a curved surface so that the central portion 1 having substantially uniform thickness and flatness does not protrude upward from the uniform flat portion and has a convex upward. The state that you have. Since the amount of the material of the second porous film 141 at the end 3 is reduced and the stress is reduced, curling is less likely to occur from the end 3.

  The porosity at the end portion 3 of the second porous membrane 141 is preferably higher than that of the central portion 1. As a result, the amount of the second porous film coating material at the end portion 3 is reduced and the stress is reduced, so that curling of the end portion is suppressed.

  More preferably, the size of the pores at the end portion 3 of the second porous membrane 141 is larger than that of the central portion 1. As a result, the amount of the second porous film coating material at the end portion is reduced and the stress is reduced, so that curling of the end portion is suppressed.

(Third embodiment)
FIG. 3 is a schematic cross-sectional view showing an embodiment of the multilayer separator 202 of the present invention manufactured using a convex curved blade in accordance with the method of Example 1. The end portion 4 of the second porous membrane 142 is thinner than the other portions, and becomes a thin film with a concave curve from the central portion 1, which is a flat portion having a uniform thickness, to the end of the first porous membrane 12. An end 4 is formed. As a result, the amount of the second coating material for the porous lipid membrane at the end portion 4 is reduced and the stress is reduced, so that curling of the end portion 4 is suppressed.

  In the present embodiment, the end 4 that is a thin film is formed in a concave curve shape from the central portion 1, and the thickness of the end 4 of the second porous membrane 142 can be formed thinner, so that the stress is reduced, In order to suppress curling, this embodiment is more preferable.

  More preferably, the size of the pores at the end 4 of the second porous membrane 142 is larger than that of the central portion 1. As a result, the amount of the second porous film coating material at the end portion is reduced and the stress is reduced, so that curling of the multilayer separator 202 generated from the end portion 4 is suppressed.

  More preferably, the pores on the first porous membrane 12 side of the second porous membrane 142 are larger than the pores on the opposite side. Thereby, the amount of the second porous film coating material on the first porous 12 side is reduced and the stress is reduced, so that curling is suppressed.

(battery)
Hereinafter, each constituent member will be described in detail by taking a lithium ion secondary battery as an example. FIG. 4 is a schematic cross-sectional view showing the lithium ion secondary battery 100 according to the present embodiment. The lithium ion secondary battery 100 includes a positive electrode 20, a negative electrode 30 facing the positive electrode 20, a multilayer separator that is interposed between the positive electrode 20 and the negative electrode 30, and contacts the main surface of the positive electrode 20 and the main surface of the negative electrode 30, respectively. 200 is a lithium ion secondary battery. Here, the multilayer separator 200 has been described in the first to third embodiments.

  The lithium ion secondary battery 100 mainly includes a power generation element 40, a case 50 that houses the power generation element 40 in a sealed state, and a pair of leads 60 and 62 connected to the power generation element 40.

  In the power generation element 40, a pair of positive electrode 20 and negative electrode 30 are arranged to face each other with the multilayer separator 200 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-like (film-like) negative electrode current collector 32. The main surface of the positive electrode active material layer 24 and the main surface of the negative electrode active material layer 34 are in contact with the main surface of the multilayer separator 200. Leads 60 and 62 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 20 and the negative electrode 30 are collectively referred to as electrodes 20 and 30, and the positive electrode current collector 22 and the negative electrode current collector 32 are collectively referred to as current collectors 22 and 32. The positive electrode active material layer 24 and the negative electrode The active material layer 34 may be collectively referred to as the active material layers 24 and 34.

  First, the electrodes 20 and 30 will be specifically described.

  The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

The positive electrode active material layer 24 is mainly composed of a positive electrode active material, a binder, and a conductive auxiliary agent in an amount as necessary. The positive electrode active material includes insertion and extraction of lithium ions, desorption and insertion of lithium ions (intercalation), or doping and dedoping of lithium ions and a counter anion (for example, ClO ₄ ⁻ ) of the lithium ions. It is not particularly limited as long as it can be reversibly advanced, and a positive electrode active material used in a known lithium ion secondary battery can be used. For example, a lithium containing metal oxide is mentioned. Examples of the lithium-containing metal oxide include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a general formula: LiNi _x Co _y Mn _z O ₂ ( x + y + z = 1), a composite metal oxide, a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (wherein M represents Co, Ni, Mn or Fe), lithium titanate (Li ₄ Ti ₅ O _12), and the like.

  The binder binds the active materials to each other and binds the active material to the current collector 22. The binder is not particularly limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And the like.

  In addition to the above, as the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFPPTFE-based) Fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride Ride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber It may be used vinylidene fluoride-based fluorine rubbers such as rubber (VDF-CTFE-based fluorine rubber).

  In addition to the above, for example, polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, cellulose, styrene / butadiene rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber, and the like may be used as the binder. Also, thermoplastic elastomeric polymers such as styrene / butadiene / styrene block copolymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers, and hydrogenated products thereof. May be used. Further, syndiotactic 1,2-polybutadiene, ethylene / vinyl acetate copolymer, propylene / α-olefin (carbon number 2 to 12) copolymer may be used.

  Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant.

As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) A crosslinked polymer of a polyether compound, polyepichlorohydrin, polyphosphazene, polysiloxane, polyvinylpyrrolidone, polyvinylidene carbonate, polyacrylonitrile, etc.) monomers, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCl, LiBr, Examples thereof include a composite of a lithium salt such as Li (CF ₃ SO ₂ ) ₂ N, LiN (C ₂ F ₅ SO ₂ ) ₂ or an alkali metal salt mainly composed of lithium. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

  The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 24, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

  The content of the binder in the positive electrode active material layer 24 is not particularly limited, but is preferably 1% by mass to 15% by mass based on the sum of the masses of the active material, the conductive additive, and the binder, and is 3% by mass. More preferably, it is 10 mass%. By setting the content of the active material and the binder in the above range, the obtained electrode active material layer 24 can suppress the tendency that the amount of the binder is too small to form a strong active material layer. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

  The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited, but when added, it is usually preferably 0.5% by mass to 20% by mass with respect to the active material, and 1% by mass to It is more preferable to set it as 12 mass%.

  The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

The negative electrode active material layer 34 is mainly composed of a negative electrode active material, a binder, and a conductive auxiliary agent in an amount as necessary. The negative electrode active material reversibly advances the insertion and removal of lithium ions, the desorption and insertion of lithium ions, or the doping and dedoping of lithium ions and counter anions of the lithium ions (for example, ClO ₄ ⁻ ). If it can be made, it will not specifically limit, The negative electrode active material used for the well-known lithium ion secondary battery can be used. For example, it can be combined with natural graphite, artificial graphite, mesocarbon microbeads, mesocarbon fiber (MCF), coke, glassy carbon, carbon materials such as organic compound fired bodies, lithium such as Al, Si, Sn, etc. Examples thereof include amorphous compounds mainly composed of metal, oxides such as SiO ₂ and SnO ₂ , lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

  As the binder and the conductive additive, the same materials as those used for the positive electrode 20 described above can be used. Moreover, what is necessary is just to employ | adopt content similar to content in the positive electrode 20 mentioned above also about content of a binder and a conductive support agent.

  The electrodes 20 and 30 can be produced by a commonly used method. For example, it can manufacture by apply | coating the coating material containing an active material, a binder, a solvent, and a conductive support agent on a collector, and removing the solvent in the coating material apply | coated on the collector.

  As the solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylformamide and the like can be used.

  There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

  The method for removing the solvent in the paint applied to the current collectors 22 and 32 is not particularly limited, and the current collectors 22 and 32 to which the paint has been applied are dried, for example, in an atmosphere of 80 ° C. to 150 ° C. Just do it.

  Then, the electrodes on which the active material layers 24 and 34 are formed in this way may be pressed by a roll press device or the like, if necessary. The linear pressure of the roll press can be, for example, 10 to 50 kgf / cm.

  The electrodes 20 and 30 can be manufactured through the above steps.

  Next, other components of the lithium ion secondary battery 100 will be described.

The electrolyte is contained in the positive electrode active material layer 24, the negative electrode active material layer 34, and the battery separator 10. The electrolyte is not particularly limited, and, for example, in the present embodiment, an electrolyte solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, and the withstand voltage during charging is limited to a low level. As the electrolyte solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. Examples of the lithium salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN Salts such as (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Moreover, as an organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, etc. are mentioned preferably, for example. These may be used alone or in combination of two or more at any ratio.

  The case 50 seals the power generation element 40 and the electrolyte solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 5, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

  The leads 60 and 62 are made of a conductive material such as aluminum.

  Then, the leads 60 and 62 are respectively welded to the positive electrode current collector 22 and the negative electrode current collector 32 by a known method, and the battery is interposed between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. What is necessary is just to insert in the case 50 with electrolyte solution in the state which pinched | interposed the separator 10, and the entrance of the case 50 should be sealed.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
Polyamide imide resin solution (non-volatile content: 30% by mass, solvent: NMP, glass transition temperature: 283 ° C.) 50 parts by mass, polyethylene glycol (number average molecular weight Mn = 1000) 20 parts by mass and NMP 30 parts by mass were added. The mixture was mixed uniformly at room temperature to prepare a heat resistant resin solution.

  The obtained heat-resistant resin solution was 28 μm using a bar coater on one side of a polyethylene porous film (first porous film, porosity: 40%, shutdown temperature: 134 ° C.) having a width of 90 mm and a film thickness of 20 μm. The coating was shaved using a blade so that a thin film portion having a width of 5 mm was formed linearly from the center to the end. The polyethylene porous membrane was immersed in a coagulation liquid consisting of 50 parts by mass of water and 50 parts by mass of NMP for 3 minutes, then washed with ion-exchanged water and then dried with hot air at 60 ° C. for 30 minutes to make the polyethylene porous membrane porous. A heat-resistant porous film (second porous film) having a thickness of 2 μm at the center was formed on the surface. Thereby, a multilayer separator was obtained. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Example 2)
A multilayer separator was produced in the same manner as in Example 1 except that the thin film part was formed in a convex curve shape from the center part to the end part. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

Example 3
A multilayer separator was produced in the same manner as in Example 1 except that the thin film part was formed in a concave curve shape from the center part to the end part. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Comparative Example 1)
The heat-resistant resin solution used in Example 1 was applied to one side of the polyethylene porous film used in Example 1 using a bar coater, and then solidified to form a uniform heat-resistant porous film having a thickness of 4 μm as a polyethylene porous film. A multilayer separator formed on the surface was produced. The obtained multi-layer separator was significantly warped and difficult to handle.

(Comparative Example 2)
The heat-resistant resin solution used in Example 1 was applied to one side of the polyethylene porous film used in Example 1 using a bar coater and then solidified, and a uniform heat-resistant porous film having a thickness of 2 μm was formed of the polyethylene porous film. A multilayer separator formed on the surface was produced. The obtained multi-layer separator was significantly warped and difficult to handle.

<Measurement of warpage>
The length of the multi-layer separator obtained in the example and the comparative example was contracted inward from the position of the original end when viewed from above, one of the end in the width direction contracted by warping. Was the amount of warpage.

  Table 1 below collectively shows information and evaluation results regarding the multilayer separators produced in the examples and comparative examples. In Examples 1 to 3, in which the end is thinned in a shape as shown in FIGS. 1 to 3, the end is compared with Comparative Examples 1 and 2 in which a second porous film having a uniform thickness up to the end is formed. It can be seen that the warpage of the film is suppressed and shows good characteristics.

  According to the present invention, it is possible to provide a multilayer separator suitable for a lithium ion secondary battery excellent in reliability with suppressed warpage and a lithium ion secondary battery using the same. Therefore, the present invention greatly contributes to the industry.

  DESCRIPTION OF SYMBOLS 1 ... Center part, 2, 3, 4 ... End part, 12 ... 1st porous film, 14, 141, 142 ... 2nd porous film, 20 ... Positive electrode, 22 ... Positive electrode collector, 24 ... Positive electrode Active material layer, 30 ... negative electrode, 32 ... negative electrode current collector, 34 ... negative electrode active material layer, 40 ... power generation element, 50 ... case, 52 ... metal foil, 54 ... polymer film, 60, 62 ... lead, 100 ... Lithium ion secondary battery.

Claims (4)

A second porous film provided on the first porous film and having a heat resistant temperature higher than that of the first porous film, and the second porous film A multi-layer separator, characterized in that the end of each is thinner than the other parts.   2. The multilayer according to claim 1, wherein the porosity of the end portion of the second porous membrane is higher than the porosity of the other portion of the second porous membrane. Separator.   3. The multilayer separator according to claim 1, wherein a pore on the first porous membrane side of the second porous membrane is larger than a pore on the opposite side. 4.   A battery using the multilayer separator according to any one of claims 1 to 3.

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