Multilayer porous membrane

Abstract

An object of the present invention is to provide a multilayer porous membrane that, when incorporated into an electricity storage device, is superior in battery characteristics and safety to conventional multilayer porous membranes. The present invention provides a multilayer porous membrane having a porous membrane containing a polyolefin resin as a main component and a porous layer laminated on at least one surface of the porous membrane, the porous layer having a total thickness of 0.5 μm to 3.0 μm, and an area of ​​each pore in the porous layer of 0.001 μm. 2 The number of holes that are 10 μm or more 2 The number of particles per field of view is 65 to 180, and the area of ​​the porous layer is 0.001 μm 2 Among the pores that are 0.001 μm or more 2 ~0.05μm 2 The proportion of pores within this range is 90% or more, the proportion of inorganic particles in the porous layer is 90% by mass or more and 99% by mass or less, and the aspect ratio of the inorganic particles is 1.0 or more and 3.0 or less.

Description

[Technical Field]

[0001] The present invention relates to a multilayer porous membrane, and more particularly to a multilayer porous membrane that is suitably used as a separator disposed between a positive electrode and a negative electrode in a battery. [Background technology]

[0002] In conventional energy storage devices, a power generating element having a separator interposed between a positive electrode plate and a negative electrode plate is impregnated with an electrolyte. Generally, separators are required to have ion permeability and safety, such as a shutdown function, and therefore separators having a microporous membrane containing a polyolefin resin are used. Furthermore, from the viewpoints of electrical insulation during thermal runaway, heat resistance, strength, safety and cycle characteristics of the energy storage device, multilayer multilayer separators in which a polyolefin microporous membrane and a porous layer containing inorganic particles and a binder polymer are laminated are used. Porous membranes are also being considered as separators (Patent Documents 1 to 7).

[0003] In Patent Document 1, a multilayer porous membrane using kaolin-based particles as inorganic particles is studied in order to suppress the thermal shrinkage rate of the multilayer porous membrane.

[0004] In Patent Document 2, in order to enhance the heat resistance of the multilayer porous film, it is studied to set the content ratio of inorganic particles to binder polymer and the BET specific surface area of ​​the inorganic particles within a specific range, thereby improving the dispersibility of the coating liquid that forms the porous layer or the density of the porous layer.

[0005] In Patent Document 3, for a heat-resistant multilayer porous membrane comprising a substrate and heat-resistant layers formed on both sides of the substrate, the types or physical properties of the heat-resistant resin and heat-resistant particles contained in the heat-resistant layer are studied in order to suppress the thermal shrinkage rate without increasing the thickness of the heat-resistant layer.

[0006] Patent Document 4 proposes a battery separator having an inorganic porous layer on at least one side of a polyolefin microporous membrane, which has excellent mechanical stability and also has a heat shrinkage rate of less than 5.0% at 150°C and a tensile strength of 120 MPa or more, with the aim of providing a nonaqueous electrolyte battery using such a battery separator that has good processability, excellent charge / discharge characteristics, and high safety.

[0007] Patent Document 5 describes a multilayer porous membrane that includes a resin porous layer containing a thermoplastic resin as a main component and a heat-resistant porous layer containing heat-resistant fine particles as a main component, as a separator that can be used to construct a non-aqueous electrolyte battery that is excellent in load characteristics and safety, and also investigates the particle size, particle size distribution, average particle size, and aspect ratio of the heat-resistant fine particles.

[0008] Patent Document 6 proposes a unique pore structure for a polyolefin microporous membrane that can serve as a base material for a multilayer porous membrane from the viewpoint of ionic conductivity of the multilayer porous membrane.

[0009] In Patent Document 7, in consideration of the problem that curling occurs at the separator end during battery production and the curled portion is left folded when the battery is stacked, the relationship between the total thickness of the separator with heat-resistant insulation layers and the thickness of each of the multiple heat-resistant insulation layers formed on both sides of the resin porous substrate is studied. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2010 / 134585 [Patent Document 2] International Publication No. 2014 / 148577 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-181110 [Patent Document 4] Japanese Patent Application Laid-Open No. 2016-139490 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-15917 [Patent Document 6] International Publication No. 2013 / 147071 [Patent Document 7] Japanese Patent Application Laid-Open No. 2013-8481 Summary of the Invention [Problem to be solved by the invention]

[0011] In recent years, with the expansion of lithium-ion secondary batteries for in-vehicle applications, improving safety has become increasingly important. On the other hand, ensuring safety has become more difficult than ever before in light of the need for higher capacity, higher energy density, lighter weight, and thinner lithium-ion secondary batteries. While thinner inorganic porous layers are also required for separators, there is a problem in that if the porous layer becomes thinner than a certain value, the thermal shrinkage suppression effect on the separator is significantly impaired. The thermal shrinkage suppression ability of conventional multilayer porous films such as those described in Patent Documents 1 and 2 is insufficient to meet the demands for high safety and high capacity for in-vehicle applications, etc.

[0012] In addition, in the case of automotive batteries, in order to achieve higher capacity and thinner batteries, batteries are assembled using a stacking method in which separators are folded zigzag and positive and negative electrodes are alternately inserted between the separators. However, in the stacking method, the tension of the separator within the battery is lower than in the conventional wound method, which causes short circuits within the battery in nail penetration tests, and when the temperature of the nail rises, the zigzag-folded separator is likely to shrink, making it more likely to cause further short circuits. Furthermore, when the separator is folded zigzag, the top surfaces of the separators alternate along a predetermined direction. Therefore, the fact that the nail hole is less likely to widen when pierced from either side of the separator is thought to be effective in improving safety.

[0013] In view of the above circumstances, an object of the present invention is to provide a multilayer porous membrane that, when incorporated into an electricity storage device, is superior in battery characteristics and safety to conventional multilayer porous membranes. [Means for solving the problem]

[0014] As a result of intensive research into solving the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by specifying the pore structure of the porous layer in a multilayer porous film in which a porous layer containing inorganic particles and a binder polymer is laminated on a porous film, and / or by specifying the surface of the multilayer porous film that is pierced with a soldering iron and the hole area in a 400°C soldering test, and have completed the present invention. Examples of embodiments of the present invention are listed below. [1] A multilayer porous membrane comprising a porous membrane containing a polyolefin resin as a main component and a porous layer containing inorganic particles and a binder polymer laminated on at least one surface of the porous membrane, wherein the total thickness of the porous layer is 0.5 μm or more and 3.0 μm or less, and the area of ​​each pore in the porous layer is 0.001 μm. 2 The number of holes that are 10 μm or more 2 The number of particles per field of view is 65 to 180, and the area of ​​the porous layer is 0.001 μm 2 Among the holes with an area of ​​0.001 μm or more, 2 ~0.05μm 2 The proportion of pores within this range is 90% or more, the proportion of inorganic particles in the porous layer is 90% by mass or more and 99% by mass or less, and the aspect ratio of the inorganic particles is 1.0 or more and 3.0 or less. [2] The layer density of the porous layer is 1.10 g / (m 2 ·μm) or more 3.00g / (m 2 2. The multilayer porous membrane according to item 1, wherein the thickness is 1 / 2 μm or less. [3] The average particle size D of the inorganic particles in the porous layer 50 3. The multilayer porous membrane according to item 1 or 2, wherein the average particle size is 0.10 μm or more and 0.60 μm or less. [4] The particle size D of the inorganic particles in the porous layer 90 4. The multilayer porous membrane according to any one of items 1 to 3, wherein the average particle size is 0.30 μm or more and 1.20 μm or less. [5] 5. The multilayer porous membrane according to any one of items 1 to 4, wherein the ratio of the air permeability of the multilayer porous membrane to the air permeability of the porous membrane is 1.0 or more and 1.6 or less. [6] The puncture strength of the porous membrane converted into basis weight is 60 gf / (g / m 2 6. The multilayer porous membrane according to any one of items 1 to 5, wherein: [7] The air permeability of the multilayer porous membrane is 50 sec / 100 cm 3 More than 250sec / 100cm 3 7. The multilayer porous membrane according to any one of items 1 to 6, wherein: [8] 8. A separator for a non-aqueous electrolyte battery, comprising the multilayer porous membrane according to any one of items 1 to 7. [9] A non-aqueous electrolyte battery comprising the separator for a non-aqueous electrolyte battery according to item 8, a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[10] A porous membrane containing a polyolefin resin as a main component; a first porous layer disposed on one surface of the porous membrane, the first porous layer including inorganic particles and a binder polymer; a second porous layer disposed on the other surface of the porous membrane and including inorganic particles and a binder polymer; A multilayer porous membrane comprising: In a 400°C soldering test in which a soldering iron with a diameter of 1 mm and a temperature of 400°C is pierced into the multilayer porous film, the soldering iron is held in the pierced state for 3 seconds, and then removed, the area of ​​the hole formed in the multilayer porous film is 10.0 mm2 or less regardless of whether the soldering iron is inserted from the first porous layer side or the second porous layer side. 2 A multilayer porous membrane as follows:

[11] Item 11. The multilayer porous membrane according to item 10, wherein in a soldering test of the multilayer porous membrane at 400°C, an area ratio of holes formed in the multilayer porous membrane when the soldering iron is inserted from each of the first porous layer side and the second porous layer side is within a range of 0.8 to 1.2.

[12] Item 12. The multilayer porous membrane according to item 10 or 11, wherein the total thickness of the first porous layer and the second porous layer is 5 μm or less.

[13] 13. The multilayer porous membrane according to any one of items 10 to 12, wherein the thickness of either the first porous layer or the second porous layer is 1.5 μm or less.

[14] D of the inorganic particles constituting the first porous layer and the second porous layer 90 14. The multilayer porous membrane according to any one of items 10 to 13, wherein the average particle diameter is 1.5 μm or less.

[15] The puncture strength of the porous membrane converted into basis weight is 50 gf / (g / m 2 15. The multilayer porous membrane according to any one of items 10 to 14, wherein

[16] 16. The multilayer porous membrane according to any one of items 10 to 15, wherein the melt index (MI) of the porous membrane at 190° C. is 0.02 g / 10 min to 0.5 g / 10 min.

[17] 17. The multilayer porous membrane according to any one of items 10 to 16, wherein the multilayer porous membrane has a heat shrinkage rate at 150° C. of less than 10.0%.

[18] 18. The multilayer porous membrane according to any one of items 10 to 17, wherein the viscosity average molecular weight of the porous membrane is 400,000 or more and 1,300,000 or less.

[19] 19. The multilayer porous membrane according to any one of items 10 to 18, wherein the porous membrane contains polypropylene as the polyolefin resin.

[20] In a soldering test of the multilayer porous film at 400°C, the area of ​​the hole formed in the multilayer porous film is 1.0 mm2 or less regardless of whether the soldering iron is inserted from the first porous layer side or the second porous layer side. 2 20. The multilayer porous membrane according to any one of items 10 to 19, wherein the multilayer porous membrane exceeds [twenty one] The area of ​​each hole in the porous layer is 0.001 μm 2 The number of holes that are 10 μm or more 2 The number of particles per field of view is 65 to 180, and the area of ​​the porous layer is 0.001 μm 2 Among the holes with an area of ​​0.001 μm or more, 2 ~0.05μm 221. The multilayer porous membrane according to any one of items 10 to 20, wherein the proportion of pores in the range is 90% or more. [twenty two] A lithium ion secondary battery, in which a zigzag fold body of the multilayer porous membrane according to any one of items 10 to 21 is housed in an outer casing, and positive electrodes and negative electrodes are alternately inserted into the gaps between the zigzag fold body. [Effects of the Invention]

[0015] According to the present invention, a highly safe multilayer porous membrane can be provided, and by using the membrane, an electricity storage device can be provided which is excellent in safety, particularly safety in a nail penetration test, while maintaining battery properties. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is an example of a cross-sectional SEM image of the porous layer according to the first embodiment, obtained by BIB processing. [Figure 2] FIG. 2 shows an example of a selected area of ​​the porous layer for performing binarization processing on the porous layer in the first embodiment. [Figure 3] FIG. 3 is an example showing the field area U of the selected area in FIG. 2 for the porous layer in the first embodiment. [Figure 4] FIG. 4 is an example showing an image of the porous layer according to the first embodiment after Gusian Blur processing. [Figure 5] FIG. 5 shows an example of a luminance histogram of the image of FIG. 4 and a method of determining a threshold value when performing binarization processing for the porous layer in the first embodiment. [Figure 6] FIG. 6 shows an example of an image of the porous layer in the first embodiment after binarization processing. [Figure 7] FIG. 7 is a schematic diagram showing the shape of a soldering iron used in the 400° C. soldering test in this embodiment. [Figure 8] FIG. 8 is a photograph showing the appearance of a stage used in a 400° C. soldering test in this embodiment. [Figure 9] FIG. 9 is a schematic diagram showing the state before a soldering iron is pierced into the multilayer porous film in the 400° C. soldering test in this embodiment. [Figure 10] FIG. 10 is a schematic diagram showing a state in which a soldering iron is stuck into the multilayer porous film in a 400° C. soldering test in this embodiment. [Figure 11] FIG. 11 is a schematic diagram of the impact test. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, a detailed description will be given of embodiments (hereinafter abbreviated as "embodiments") for the purpose of illustrating the present invention, but the present invention is not limited to the following embodiments. In this specification, the upper and lower limits of each numerical range can be arbitrarily combined. Furthermore, when a certain component contains a specific component as a main component, it means that the content of the specific component is 50 mass% or more based on the mass of the component. Unless otherwise specified, the physical properties or numerical values ​​described in this specification are measured or calculated by the methods described in the examples.

[0018] [Embodiment 1] <Multilayer porous membrane> The multilayer porous membrane according to embodiment 1 is a multilayer porous membrane comprising a porous membrane containing a polyolefin resin as a main component (a PO microporous membrane) and a porous layer containing inorganic particles and a binder polymer laminated on at least one surface of the PO microporous membrane. The multilayer porous membrane according to embodiment 1 has a pore structure of the porous layer described below, and thereby exhibits excellent thermal shrinkage suppression ability even at a thinner total thickness while maintaining ion permeability.

[0019] <Porous layer> The porous layer according to embodiment 1 has a total thickness of 0.5 μm or more and 3.0 μm or less, and the area of ​​each pore in the porous layer is 0.001 μm 2 In holes that are greater than or equal to 10 μm 2 The number of holes per field of view S is 65 or more and 180 or less, and the area of ​​the porous layer is 0.001 μm 2The total number of holes is 0.001 μm or more. 2 ~0.05μm 2 The ratio T of the number of pores in the range is 90% or more, and the aspect ratio of the inorganic particles is 1.0 or more and 3.0 or less.

[0020] The number S and ratio T of the pores can be determined by observing the cross section of the porous layer using a scanning electron microscope (SEM) at a magnification of 30,000 times and binarizing the resulting cross-sectional SEM image.

[0021] Above 10 μm 2 The number of holes per field of view, S, and the total number of holes are 0.001 μm 2 More than ~0.05μm 2 Specifically, the ratio T of the number of pores that satisfy the condition can be calculated by the method described later in the Examples with reference to FIGS.

[0022] The porous layer of embodiment 1 has the above-described pore structure, and therefore maintains high ion permeability and exhibits excellent thermal shrinkage suppression even when thin, thereby enabling the realization of a nonaqueous electrolyte secondary battery with high safety performance.

[0023] The total thickness of the porous layers is preferably 0.5 μm to 3.0 μm, more preferably 0.6 μm to 2.5 μm, even more preferably 0.7 μm to 2.0 μm, and particularly preferably 1.0 μm to 1.5 μm. The total thickness of the porous layers refers to the thickness of the porous layers when the porous layers are laminated on one side of the PO microporous membrane, and refers to the sum of the thicknesses of the porous layers when the porous layers are laminated on both sides of the PO microporous membrane. A total thickness of the porous layers of 0.5 μm or more is preferred from the viewpoint of suppressing deformation at temperatures exceeding the melting point of the porous membrane, and a total thickness of the porous layers of 3.0 μm or less is preferred from the viewpoint of improving battery capacity and suppressing the amount of water adsorption by the multilayer porous membrane.

[0024] Above 10 μm 2The number S of holes in the field of view is preferably 65 to 180, more preferably 70 to 170, even more preferably 75 to 160, and particularly preferably 80 to 150.

[0025] In addition, the hole area for the total number of holes is 0.001 μm 2 More than 0.05μm 2 The proportion T of the number of pores that is equal to or less than 90% is preferably 90% or more, more preferably 91% or more, 92% or more, 93% or more, 94% or more, or 95% or more, and even more preferably 96% or more, 97% or more, 98% or more, or 99% or more, and theoretically may be 100%.

[0026] The number of holes S is 65 or more, and the hole area relative to the total number of holes X is 0.001 μm 2 More than 0.05μm 2 It is preferable that the pore number S is 180 or less and the pore ratio T is 90% or more from the viewpoint of suppressing deformation at temperatures exceeding the melting point of the microporous PO membrane. It is also preferable that the pore number S is 180 or less and the pore ratio T is 90% from the viewpoint of suppressing deterioration of battery capacity during repeated battery cycles.

[0027] The pore structure is not particularly limited, but may be determined, for example, by the shape of the inorganic particles used, the proportion of the inorganic particles in the porous layer, the average particle diameter D 50 , D 10 , D 90 These can be controlled by one or more of the above, the amount of dispersant added, the specific surface area of ​​the inorganic particles, the viscosity of the coating solution containing the inorganic particles and the binder polymer, the layer density of the porous layer, etc. For example, by reducing the particle size of the inorganic particles, the number of pores in the porous layer tends to increase. Also, by increasing the viscosity of the coating solution, the pore area of ​​0.001 μm relative to the total number of pores X can be reduced. 2 More than 0.05μm 2 The percentage T of the number of holes that is equal to or less than this tends to decrease.

[0028] The inorganic particles used in the porous layer are not particularly limited, but are preferably those that have high heat resistance and electrical insulation properties and are electrochemically stable within the range of use of lithium ion secondary batteries.

[0029] Examples of inorganic particle materials include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum oxide hydroxide, or boehmite, potassium titanate, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber. Among these, at least one selected from the group consisting of alumina, boehmite, and barium sulfate is preferred from the viewpoint of stability in lithium-ion secondary batteries. Furthermore, synthetic boehmite is preferred as it can reduce ionic impurities that adversely affect the performance of electrochemical devices. The inorganic particles may be used alone or in combination.

[0030] Examples of the shape of the inorganic particles include plate-like, scale-like, polyhedral, needle-like, columnar, granular, spherical, spindle-like, and block-like shapes, and a combination of multiple types of inorganic particles having the above shapes may be used. Among these, block-like shapes are preferred from the viewpoint of a balance between permeability and heat resistance.

[0031] The aspect ratio of the inorganic particles is preferably 1.0 to 3.0, more preferably 1.1 to 2.5. An aspect ratio of 3.0 or less is preferred from the viewpoints of suppressing the amount of water adsorption by the multilayer porous membrane, suppressing capacity degradation after repeated cycles, and suppressing deformation at temperatures exceeding the melting point of the microporous PO membrane.

[0032] The specific surface area of ​​inorganic particles is 5.5m 2 / g or more 17m 2 / g or less, and more preferably 6.0m2 / g or more 15m 2 / g or less, and more preferably 6.5m 2 / g or more 13m 2 / g or less. The specific surface area is 17m 2 / g or less is preferable from the viewpoint of suppressing the amount of water adsorption of the multilayer porous membrane and suppressing capacity deterioration during repeated cycles. 2 / g or more is preferable from the viewpoint of suppressing deformation of the microporous PO membrane at a temperature exceeding the melting point. The specific surface area of ​​the inorganic particles is measured by the BET adsorption method.

[0033] In the particle size distribution of a slurry containing inorganic particles, the average particle size D of the inorganic particles 50 is preferably 0.10 μm or more and 0.60 μm or less, more preferably 0.20 μm or more and 0.50 μm or less, and even more preferably 0.25 μm or more and 0.45 μm or less. 50 When the diameter is 0.10 μm or more, the amount of water adsorption of the multilayer porous film is suppressed, and it is preferable from the viewpoint of suppressing the capacity deterioration when the cycle is repeated. 50 It is preferable that the thickness is 0.60 μm or less from the viewpoint of suppressing deformation at a temperature exceeding the melting point of the microporous PO membrane.

[0034] In addition, in the particle size distribution of the slurry containing inorganic particles, the D 90 is preferably 0.30 μm or more and 1.20 μm or less, more preferably 0.40 μm or more and 1.10 μm or less, and even more preferably 0.50 μm or more and 1.00 μm or less. 90 When the diameter is 0.30 μm or more, the amount of water adsorption by the multilayer porous film is suppressed, and it is preferable from the viewpoint of suppressing the capacity deterioration when the cycle is repeated. 90 It is preferable that the thickness is 1.20 μm or less from the viewpoint of suppressing deformation at a temperature exceeding the melting point of the microporous PO membrane.

[0035] In addition, in the particle size distribution of the slurry containing inorganic particles, the D 10is preferably 0.08 μm or more and 0.50 μm or less, more preferably 0.09 μm or more and 0.45 μm or less, and even more preferably 0.10 μm or more and 0.35 μm or less. 10 When the diameter is 0.08 μm or more, the amount of water adsorption of the multilayer porous film is suppressed, and it is preferable from the viewpoint of suppressing the capacity deterioration when the cycle is repeated. 10 It is preferable that the thickness is 0.50 μm or less from the viewpoint of suppressing deformation at a temperature exceeding the melting point of the microporous PO membrane.

[0036] Methods for adjusting the particle size distribution of inorganic particles as described above include, for example, grinding inorganic particles using a ball mill, bead mill, jet mill, etc. to obtain the desired particle size distribution, and blending fillers with multiple particle size distributions.

[0037] The proportion of inorganic particles in the porous layer is preferably 90% by mass to 99% by mass, more preferably 91% by mass to 98% by mass, and even more preferably 92% by mass to 98% by mass. As the proportion of inorganic particles decreases, the number of pores S tends to increase due to the increased content of organic compounds such as binder polymers. However, a proportion of inorganic particles of 90% by mass or more is preferred from the viewpoints of ion permeability and suppression of deformation at temperatures exceeding the melting point of the PO microporous membrane. Furthermore, a proportion of inorganic particles of 99% by mass or less is preferred from the viewpoint of maintaining the binding strength between inorganic particles and the interfacial binding strength between the inorganic particles and the PO microporous membrane.

[0038] The binder polymer is a material that binds together multiple inorganic particles in the porous layer and binds the porous layer to the PO microporous membrane. When the multilayer porous membrane is used as a separator, it is preferable to use a binder polymer that is insoluble in the electrolyte of a lithium ion secondary battery and is electrochemically stable within the range of use of the lithium ion secondary battery.

[0039] Specific examples of the binder polymer include the following 1) to 7). 1) Polyolefins: for example, polyethylene, polypropylene, ethylene propylene rubber, and modified products thereof; 2) Conjugated diene polymers: for example, styrene-butadiene copolymers and hydrogenated products thereof, acrylonitrile-butadiene copolymers and hydrogenated products thereof, acrylonitrile-butadiene-styrene copolymers and hydrogenated products thereof; 3) Acrylic polymers: for example, methacrylate-acrylate copolymers, styrene-acrylate copolymers, acrylonitrile-acrylate copolymers; 4) Polyvinyl alcohol resins: for example, polyvinyl alcohol, polyvinyl acetate; 5) Fluorine-containing resins: for example, polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer; 6) Cellulose derivatives: for example, ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose; 7) Resins having a melting point and / or glass transition temperature of 180°C or higher, or polymers having no melting point but a decomposition temperature of 200°C or higher: for example, polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester.

[0040] From the viewpoint of further improving safety in the event of a short circuit, 3) acrylic polymers, 5) fluorine-containing resins, and 7) polyamides as polymers are preferred. As polyamides, from the viewpoint of durability, wholly aromatic polyamides, particularly polymetaphenylene isophthalamide, are preferred.

[0041] From the viewpoint of compatibility between the binder polymer and the electrode, the above 2) conjugated diene polymer is preferred, and from the viewpoint of voltage resistance, the above 3) acrylic polymer and 5) fluorine-containing resin are preferred.

[0042] The above-mentioned 2) conjugated diene polymer is a polymer containing a conjugated diene compound as a monomer unit.

[0043] Examples of the conjugated diene compound include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, substituted linear conjugated pentadiene, substituted and side-chain conjugated hexadienes, etc., which may be used alone or in combination of two or more. Among these, 1,3-butadiene is particularly preferred.

[0044] The acrylic polymer (3) is a polymer containing a (meth)acrylic compound as a monomer unit. The (meth)acrylic compound refers to at least one selected from the group consisting of (meth)acrylic acid and (meth)acrylic acid esters.

[0045] Examples of the (meth)acrylic acid used in the above 3) acrylic polymer include acrylic acid and methacrylic acid.

[0046] Examples of (meth)acrylic acid esters used in the acrylic polymer (3) include (meth)acrylic acid alkyl esters, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate; and epoxy group-containing (meth)acrylic acid esters, such as glycidyl acrylate and glycidyl methacrylate; these may be used alone or in combination of two or more. Among the above, 2-ethylhexyl acrylate (EHA) and butyl acrylate (BA) are particularly preferred.

[0047] From the viewpoint of safety in crash tests, the acrylic polymer is preferably a polymer containing EHA or BA as a main structural unit, where the main structural unit refers to a polymer portion corresponding to a monomer that accounts for 40 mol % or more of all raw materials used to form the polymer.

[0048] The above 2) conjugated diene polymer and 3) acrylic polymer may be obtained by copolymerizing other copolymerizable monomers with them. Examples of the copolymerizable other monomers include unsaturated carboxylic acid alkyl esters, aromatic vinyl monomers, vinyl cyanide monomers, unsaturated monomers containing hydroxyalkyl groups, unsaturated carboxylic acid amide monomers, crotonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, etc., which may be used alone or in combination of two or more. Among the above, unsaturated carboxylic acid alkyl ester monomers are particularly preferred. Examples of unsaturated carboxylic acid alkyl ester monomers include dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaconate, monomethyl fumarate, monoethyl fumarate, etc., which may be used alone or in combination of two or more.

[0049] The above-mentioned 2) conjugated diene polymer may be one obtained by copolymerizing the above-mentioned (meth)acrylic compound as another monomer.

[0050] The binder polymer is preferably in the form of a latex, and more preferably an acrylic polymer latex, from the viewpoint of having a strong binding force between multiple inorganic particles even at high temperatures exceeding room temperature and suppressing thermal shrinkage.

[0051] A dispersant such as a surfactant may be added to the coating solution to stabilize dispersion or improve coatability. The dispersant adsorbs to the surface of inorganic particles in the slurry and stabilizes the inorganic particles by electrostatic repulsion or the like, and examples of such dispersants include polycarboxylates, sulfonates, and polyoxyethers. The amount of dispersant added is preferably 0.2 to 5.0 parts by weight, more preferably 0.3 to 1.0 parts by weight, calculated as solid content.

[0052] The viscosity of the coating solution, as measured using a Brookfield viscometer (at 60 rpm), is preferably 10 mPa·sec to 200 mPa·sec, more preferably 40 mPa·sec to 150 mPa·sec, and even more preferably 50 mPa·sec to 130 mPa·sec. A viscosity of 10 mPa·sec or more is preferred from the viewpoint of suppressing sedimentation of inorganic particles in the coating solution, and a viscosity of 200 mPa·sec or less is preferred from the viewpoint of stabilizing the dispersion of the coating solution and suppressing surface patterns of the porous layer after applying the coating solution to a microporous PO membrane.

[0053] The layer density in the porous layer is 1.10 g / (m 2 ·μm) or more 3.00g / (m 2 ·μm) or less, and more preferably 1.20 g / (m 2 ·μm) or more 2.90g / (m 2 ·μm) or less, more preferably 1.40 g / (m 2 ·μm) or more 2.70g / (m 2 ·μm) or less, particularly preferably 1.50 g / (m 2 ·μm) or more 2.50g / (m 2 ·μm) or less. The layer density in the porous layer is 1.10 g / (m 2 ·μm) or more is preferable from the viewpoint of suppressing deformation at temperatures exceeding the melting point of the microporous PO membrane, and 2 A thickness of 0.1 μm or less is preferable from the viewpoint of maintaining the ion permeability of the porous layer and suppressing capacity degradation during repeated cycles.

[0054] <Relationship between the microporous PO membrane, porous layer, and multilayer porous membrane according to embodiment 1> In embodiment 1, the ratio of the air permeability of the multilayer porous membrane to the air permeability of the PO microporous membrane is preferably 1.0 or more and 1.6 or less, more preferably 1.5 or less, and even more preferably 1.4 or less. A ratio of the air permeability of the multilayer porous membrane to the air permeability of the PO microporous membrane of 1.6 or less is preferable from the viewpoint of maintaining ion permeability while suppressing an increase in clogging with repeated cycles, since the surface of the PO microporous membrane is appropriately covered with the porous layer.

[0055] In embodiment 1, the porous layer preferably has a porosity of more than 30%, more preferably 40% or more, and even more preferably 45% or more, from the viewpoint of the permeability of the multilayer porous membrane and the rate characteristics of the electricity storage device. The upper limit of the porosity of the porous layer is preferably less than 70%, more preferably 60% or less, and even more preferably 55% or less, from the viewpoint of heat resistance.

[0056] [Embodiment 2] <Multilayer porous membrane> The multilayer porous membrane according to embodiment 2 is A porous membrane (PO microporous membrane) containing polyolefin resin as a main component, a first porous layer disposed on one surface of the microporous PO membrane, the first porous layer including inorganic particles and a binder polymer; a second porous layer disposed on the other surface of the microporous PO membrane, the second porous layer including inorganic particles and a binder polymer; The multilayer porous membrane comprises inorganic porous layers disposed on both sides of a polyolefin microporous membrane, and can be used as a separator for an electricity storage device, for example, a nonaqueous electrolyte battery, a lithium ion secondary battery, etc.

[0057] <400℃ solder test> In a 400°C soldering test in which a soldering iron with a diameter of 1 mm and a temperature of 400°C is pierced into the multilayer porous film according to embodiment 2, the soldering iron is held in the pierced state for 3 seconds, and then removed, the area of ​​the hole formed in the multilayer porous film was 10.0 mm or less regardless of whether the soldering iron was inserted from the first porous layer side or the second porous layer side. 2 The following is the result.

[0058] One known safety test for internal short circuits is the nail penetration test. The nail penetration test is a simulation test for internal short circuits, in which a nail is pierced through a lithium-ion secondary battery to simulate an internal short circuit and verify that the battery does not experience thermal runaway. Previously, in this technical field, it was assumed that the smaller the separator hole area in a soldering test, the better the battery's safety in the nail penetration test. However, the hole area sometimes varied depending on the insertion side of the soldering iron. More specifically, in a given orientation of a battery including a zigzag-folded electrode and separator, both sides of the membrane constituting the separator can be the top surface, so the short circuit area must be small regardless of the insertion side. If the hole area is large on either side, heat generation due to Joule heating can progress rapidly, leading to thermal runaway. The present inventors have discovered that the hole area measured when a 400°C soldering test is performed from both sides of a multilayer porous membrane serving as a separator can be characterized as an indicator of the separator design range appropriate for improving safety (particularly the safety of batteries in the nail penetration test).

[0059] From the viewpoint explained above, in the 400°C soldering test, regardless of whether the soldering iron was inserted into the multilayer porous film from the first porous layer side or the second porous layer side, the area of ​​the hole formed in the multilayer porous film was 10.0 mm 2 Less than or equal to 8mm 2 Preferably less than 6 mm 2 In this case, the lower limit of the hole area is not particularly limited, and is preferably 0 mm 2 However, the soldering iron itself is forcibly destroyed, so it must be less than 1.0mm 2 can be exceeded.

[0060] The size of the hole area is not particularly limited, but may depend on, for example, the shape of the inorganic particles used, the proportion of the inorganic particles in the porous layer, the average particle diameter D 50 and / or D. 90 It can be controlled by the amount of dispersant added, the specific surface area of ​​the inorganic particles, the viscosity of the coating solution containing the inorganic particles and the binder polymer, the layer density of the porous layer, the basis weight converted strength of the porous film, and the maximum shrinkage stress. For example, the D of the inorganic particles90 The hole area tends to be smaller by reducing the area per unit area of ​​the porous membrane.

[0061] In a soldering test of the multilayer porous membrane at 400°C, when a soldering iron is inserted from each of the first porous layer side and the second porous layer side, the area ratio of holes formed in the multilayer porous membrane is preferably within the range of 0.8 to 1.2. In this case, the area ratio may be the ratio of the hole area on the second porous layer side to the hole area on the first porous layer side, or the ratio of the hole area on the first porous layer side to the hole area on the second porous layer side. Although the reason for this is unclear, if this area ratio is large (i.e., the difference in hole area when a soldering iron is inserted into both sides of the multilayer porous membrane is large), a temperature difference occurs within the battery during the nail penetration test of the battery, and this temperature difference increases the flux of the electrolyte, making side reactions more likely to proceed. From this perspective, when a soldering iron is inserted from each of the first porous layer side and the second porous layer side, if the area ratio of the holes formed in the multilayer porous membrane is within the range of 0.8 to 1.2, the progress of side reactions is suppressed, and the device characteristics and safety of the electricity storage device are excellent. From the perspective of further suppressing the progress of side reactions, the ratio of the hole area on the second porous layer side to the hole area on the first porous layer side is preferably 0.9 to 1.1, and more preferably 0.9 to 1.0.

[0062] In the 400°C soldering test, a discolored area may be formed on the surface where the soldering iron is inserted, and this discolored area is distinguishable from a hole formed on the surface. In the 400°C soldering test of embodiment 2, a schematic diagram showing the state before the soldering iron is pierced into the multilayer porous membrane is shown in FIG. 9, and a schematic diagram showing the state after the soldering iron is pierced into the multilayer porous membrane is shown in FIG. 10. In FIG. 10, the multilayer porous membrane (10) has a hole (11) formed by the 400°C soldering test, and a discolored area (12) around the hole. The hole is a through-hole formed by piercing the multilayer porous membrane with the soldering iron (20) and by the components around the soldering iron (20) melting down due to heating. The discolored area refers to a portion of the multilayer porous membrane (10) where no hole is formed, where the color has changed due to the deformation of the multilayer porous membrane structure due to heating. Whether or not discoloration has occurred can be determined using the image processing method described below. For example, in embodiment 2, the multilayer porous membrane before the 400°C soldering test is white because the porous portions diffusely reflect light, but when heated with a soldering iron, the components around the holes melt, closing the pores in the polyolefin microporous membrane, the first porous layer, or the second porous layer, changing to translucent or transparent. In embodiment 2, the discolored portion is a portion of the multilayer porous membrane where no holes are formed that changes from white to translucent or transparent due to deformation of the pores by heating.

[0063] <Multilayer structure> The multilayer porous membrane of embodiment 2 has a multilayer structure including, in order, a first porous layer containing inorganic particles and a binder polymer, a polyolefin microporous membrane (PO microporous membrane), and a second porous layer containing inorganic particles and a binder polymer. The multilayer structure is not limited to a three-layer structure of first porous layer-PO microporous membrane-second porous layer. For example, one or more additional layers may be formed between the first porous layer and the PO microporous membrane, between the second porous layer and the PO microporous membrane, or on the outside of the multilayer porous membrane. Examples of additional layers include an additional PO microporous membrane, an additional porous layer containing inorganic particles and a binder polymer, a resin layer containing 50% or more by mass of a resin other than polyolefin (PO), and an adhesive layer containing an adhesive polymer.

[0064] <Porous layer> In embodiment 2, the microporous PO membrane has at least two surfaces due to its membrane configuration, and the porous layer disposed on one surface of the microporous PO membrane is the first porous layer, and the porous layer disposed on the other surface of the microporous PO membrane is the second porous layer. The first and second porous layers may be the same or different, as long as they contain inorganic particles and a binder polymer.

[0065] The materials and shapes of the inorganic particles used in the porous layer according to the second embodiment may be the same as those described in the first embodiment.

[0066] In the particle size distribution of the slurry containing inorganic particles according to the second embodiment, the particle size D 50 is preferably in the range of 0.05 μm to 1.2 μm, more preferably 0.05 μm to 0.8 μm, and even more preferably 0.05 μm to 0.5 μm. 50 When the particle size is 0.05 μm or more, migration of inorganic particles from the porous layer into the pores of the microporous PO membrane is suppressed, and the permeability of the multilayer porous membrane may be improved. 50 When the thickness is 1.2 μm or less, the heat resistance of the porous layer is easily obtained.

[0067] In the particle size distribution of the slurry containing inorganic particles constituting the first porous layer and the second porous layer, the D 90 is preferably 1.5 μm or less, and more preferably 1.0 μm or less, so that destruction of the first and second porous layers does not start from the inorganic particles when the layers are thinned. 90 The lower limit of the thickness is preferably 0.05 μm or more from the viewpoint of suppressing migration of inorganic particles from the porous layer into the pores of the microporous PO membrane and improving the permeability of the multilayer porous membrane.

[0068] Examples of methods for adjusting the particle size distribution of the inorganic particles according to embodiment 2 as described above include a method of pulverizing the inorganic particles using a ball mill, bead mill, jet mill, etc. to obtain the desired particle size distribution, and a method of preparing fillers with multiple particle size distributions and then blending them.

[0069] The proportion of the inorganic particles according to embodiment 2 in the porous layer can be determined appropriately from the viewpoint of permeability, heat resistance, etc. This proportion is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, particularly preferably 90% by mass or more, and most preferably 95% by mass or more. In addition, this proportion is preferably less than 100% by mass, more preferably 99.9% by mass or less, even more preferably 99% by mass or less, and particularly preferably 98% by mass or less.

[0070] As the material and specific examples of the binder polymer used in the porous layer according to the second embodiment, the material and specific examples described in the first embodiment may be adopted.

[0071] As the coating liquid and its components for forming the porous layer according to the second embodiment, the coating liquid and its components described in the first embodiment may be used.

[0072] In embodiment 2, the thickness of the first porous layer or the second porous layer can be 1.5 μm or less, or 1 μm or less, from the viewpoint of balancing heat resistance with the capacity and cycle characteristics of the electricity storage device. From the same viewpoint, the thickness of each of the first porous layer and the second porous layer is preferably 0.1 μm to 5 μm, more preferably 0.3 μm to 3 μm, 0.3 μm to 1.5 μm, or 0.3 μm to 1 μm. When the multilayer porous membrane according to this embodiment is incorporated into an electricity storage device as a separator, from the viewpoint of achieving both the oxidation resistance of the separator and the cycle characteristics of the electricity storage device, the thickness of the porous layer facing the positive electrode is preferably smaller than the thickness of the porous layer facing the negative electrode.

[0073] In embodiment 2, the combined thickness of the first and second porous layers (total thickness of the porous layers) is preferably 5 μm or less, more preferably 4.5 μm or less, or 4 μm or less, from the viewpoint of achieving both the capacity and cycle characteristics of the power storage device. The lower limit of the combined thickness of the first and second porous layers is not limited, and may be, for example, 0.2 μm or more, 0.4 μm or more, 0.6 μm or more, 1 μm or more, or 2 μm or more. The thicknesses of the first and second porous layers can be controlled, for example, by adjusting the coating thickness when these layers are formed into a microporous polypropylene membrane by coating.

[0074] In embodiment 2, the porous layer containing inorganic particles and a binder polymer preferably has a porosity of more than 30%, more preferably 40% or more, and even more preferably 45% or more, from the viewpoint of the permeability of the multilayer porous membrane and the rate characteristics of the electricity storage device. The upper limit of the porosity of the porous layer is preferably less than 70%, more preferably 60% or less, and even more preferably 55% or less, from the viewpoint of heat resistance.

[0075] [Embodiment 3] <Multilayer porous membrane> In the third embodiment, a multilayer porous membrane is provided that combines the configurations according to the first and second embodiments.

[0076] The multilayer porous membrane according to embodiment 3 comprises a microporous PO membrane and first and second porous layers, each containing inorganic particles and a binder polymer, disposed on both sides of the microporous PO membrane, wherein the first or second porous layer has a total thickness of 0.5 μm to 3.0 μm, the proportion of inorganic particles in each porous layer is 90% by mass to 99% by mass, and the aspect ratio of the inorganic particles is 1.0 to 3.0, the number of holes S described in embodiment 1 is 65 to 180, and the proportion of the number of holes T is 90% or more, and in the 400°C soldering test described in embodiment 2, the area of ​​the holes formed in the multilayer porous membrane is 10.0 mm2 or less regardless of whether a soldering iron is inserted from the first porous layer side or the second porous layer side. 2 The following is the result.

[0077] In the first to third embodiments, common components, preferred components, or other components will be described below.

[0078] <Polyolefin microporous membrane> Porous membranes containing polyolefin as a primary component (microporous PO membranes) contain polyolefin and are preferably composed of polyolefin. The polyolefin may be in the form of a microporous polyolefin, such as a polyolefin membrane, a polyolefin fiber woven fabric, or a polyolefin fiber nonwoven fabric. Examples of polyolefins include homopolymers, copolymers, and multistage polymers obtained using monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. These polymers may be used alone or in combination of two or more. From the viewpoint of the melt viscosity, shutdown, and meltdown properties of microporous PO membranes usable as separators, the polyolefin is preferably at least one selected from the group consisting of polyethylene, polypropylene, and copolymers thereof, more preferably polypropylene, and even more preferably an ethylene-propylene copolymer or a mixture of polyethylene and polypropylene.

[0079] Specific examples of polyethylene include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), high-molecular-weight polyethylene (HMWPE), and ultra-high-molecular-weight polyethylene (UHMWPE).

[0080] In this specification, high molecular weight polyethylene (HMWPE) refers to polyethylene with a viscosity average molecular weight (Mv) of 100,000 or more. Generally, the Mv of ultra-high molecular weight polyethylene (UHMWPE) is 1,000,000 or more, and therefore, by definition, high molecular weight polyethylene (HMWPE) in this specification includes UHMWPE.

[0081] In this specification, high density polyethylene means polyethylene with a density of 0.942 to 0.970 g / cm 3 In the present invention, the density of polyethylene refers to a value measured in accordance with D) density gradient tube method described in JIS K7112 (1999).

[0082] Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene.

[0083] Specific examples of the copolymer of ethylene and propylene include an ethylene-propylene random copolymer and an ethylene-propylene rubber.

[0084] When the polyolefin (PO) contained in the PO microporous membrane contains polyethylene (PE), the PE content is from 50 to 100% by mass, based on the total mass of the resin components constituting the PO microporous membrane, and from the viewpoint of fuse properties or meltdown properties, is preferably from 85 to 100% by mass, more preferably from 90 to 95% by mass.

[0085] When the PO contained in the PO microporous membrane contains polypropylene (PP), the PP content is from 0 to less than 50% by mass, based on the total mass of the resin components constituting the PO microporous membrane, and from the viewpoints of melt viscosity and fuse properties, is preferably from 0 to 20% by mass, more preferably from 5 to 10% by mass.

[0086] In addition to the polyolefins listed above, the microporous PO membrane may further contain resins such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimideamide, polyaramid, polyvinylidene fluoride, nylon, and polytetrafluoroethylene.

[0087] The melt index (MI) of the PO microporous membrane at 190°C is preferably 0.02 g / 10 min to 0.5 g / 10 min, and more preferably 0.05 g / 10 min to 0.3 g / 10 min, from the viewpoint of suppressing high viscosity of the PO resin composition during membrane formation and suppressing the occurrence of defective products.

[0088] PO microporous membrane basis weight (g / m 2 ) (hereinafter referred to as the basis weight converted puncture strength) is 50gf / (g / m 2 ) or more, or 60gf / (g / m 2 ) or more. 2 ) or more than 60gf / (g / m 2 A PO microporous membrane having a puncture strength equivalent to 70 gf / (g / m) or more tends to be less likely to break in an impact test of an electricity storage device. From the viewpoint of improving the safety, for example, impact resistance, of an electricity storage device while maintaining the strength of the PO microporous membrane, the puncture strength equivalent to 70 gf / (g / m) is more preferably 2 ) or more, more preferably 80 gf / (g / m 2 ) or more. The puncture strength converted into basis weight is not limited, but for example, 200 gf / (g / m 2 ) or less, 150gf / (g / m 2 ) or less, or 140gf / (g / m 2 ) can be:

[0089] The lower limit of the pin puncture strength of the PO microporous membrane not converted into basis weight (hereinafter simply referred to as pin puncture strength) is preferably 100 gf or more, more preferably 200 gf or more, and even more preferably 300 gf or more. A pin puncture strength of 100 gf or more is preferred from the viewpoint of preventing the PO microporous membrane from breaking in an impact test. From the viewpoint of stability during membrane production, the upper limit of the pin puncture strength of the PO microporous membrane is preferably 1000 gf or less, more preferably 800 gf or less, and even more preferably 700 gf or less. Any lower limit can be used as long as it allows stable membrane production and battery manufacturing. The upper limit is set by balancing with other properties. The pin puncture strength can be increased by increasing the shear force applied to the molded product during extrusion or by increasing the molecular chain orientation due to stretching. However, as strength increases, thermal stability deteriorates due to increased residual stress, so the strength is controlled according to the purpose.

[0090] The thickness of the microporous PO membrane is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 5 μm or more to ensure voltage resistance, and is preferably 25 μm or less, more preferably 20 μm or less, even more preferably 16 μm or less, and particularly preferably 12 μm or less to ensure the capacity of the electricity storage device. The membrane thickness can be adjusted by controlling, for example, the die lip gap, the stretch ratio in the stretching step, etc.

[0091] The porosity of the microporous PO membrane is preferably 20% or more, more preferably 30% or more, and even more preferably 35% or more from the viewpoint of permeability, and is preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less from the viewpoint of membrane strength. The porosity of the microporous PO membrane can be adjusted, for example, by controlling the mixing ratio of the polyolefin resin composition and the plasticizer, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat setting, the relaxation rate during heat setting, etc., or by combining these.

[0092] The air permeability of the microporous PO membrane is preferably 10 sec / 100 cm from the viewpoint of preventing excessive current from flowing between multiple electrodes through the microporous PO membrane. 3More than 50sec / 100cm, preferably 3 More preferably, 80 sec / 100 cm 3 From the viewpoint of permeability, it is preferably 1000 sec / 100 cm 3 Less than 300sec / 100cm, preferably less than 300sec / 100cm 3 More preferably, 200 sec / 100 cm 3 Below 160 sec / 100 cm, particularly preferred 3 is.

[0093] The viscosity-average molecular weight (Mv) of the microporous PO membrane is preferably 400,000 or more and 1,300,000 or less, more preferably 450,000 or more and 1,200,000 or less, and even more preferably 500,000 or more and 1,150,000 or less. When the microporous PO membrane has an Mv of 400,000 or more, the membrane exhibits high melt tension during melt molding, improving moldability, and the entanglement of polymer molecules tends to result in high membrane strength. When the membrane has an Mv of 1,300,000 or less, the raw materials are easily melt-blended uniformly, which tends to result in excellent sheet moldability, particularly thickness stability. Furthermore, when the membrane is used as a separator for an electricity storage device, the pores tend to be easily blocked at elevated temperatures, resulting in good fuse function.

[0094] The average pore size of the microporous PO membrane is preferably 0.03 μm to 0.70 μm, more preferably 0.04 μm to 0.20 μm, even more preferably 0.05 μm to 0.10 μm, and still more preferably 0.055 μm to 0.09 μm. From the viewpoints of ionic conductivity and voltage resistance, the average pore size of the microporous PO membrane is preferably 0.03 μm to 0.70 μm. The average pore size can be adjusted by, for example, controlling the polyolefin composition ratio, the type of polyolefin or plasticizer, the cooling rate of the extruded sheet, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat setting, and the relaxation rate during heat setting, or by combining these factors.

[0095] The microporous PO membrane preferably has low electronic conductivity, ionic conductivity, high resistance to organic solvents, and fine pores. The microporous PO membrane can be used alone as a separator for a lithium ion secondary battery, and is particularly suitable for use as a laminate-type separator for a lithium ion secondary battery.

[0096] <Other properties of multilayer porous membrane> In order to ensure voltage resistance, the total thickness of the multilayer porous membrane in embodiments 1 to 3 is preferably more than 1.5 μm, more preferably 2.5 μm or more, and even more preferably 5.5 μm or more. Furthermore, if the total thickness of the multilayer porous membrane is less than 28 μm, the capacity of the electricity storage device in which the multilayer porous membrane is mounted is less likely to deteriorate, so it is preferable, and it is more preferably 23 μm or less, even more preferably 19 μm or less, and particularly preferably 15 μm or less.

[0097] The air permeability of the multilayer porous membrane according to embodiment 1 is preferably 50 sec / 100 cm from the viewpoint of ensuring the safety of the electricity storage device by preventing excessive current from flowing between the plurality of electrodes through the multilayer porous membrane. 3 More than 80sec / 100cm, preferably 3 The air permeability of the multilayer porous membrane according to embodiment 1 is preferably 250 sec / 100 cm from the viewpoint of ion permeability. 3 Less than 200sec / 100cm, preferably 3 The following is the result.

[0098] The air permeability of the multilayer porous membrane in embodiment 2 or 3 is preferably 10 sec / 100 cm from the viewpoint of ensuring the safety of the electricity storage device by preventing excessive current from flowing between the plurality of electrodes through the multilayer porous membrane. 3 More than 50sec / 100cm, preferably 3 More preferably, 80 sec / 100 cm 3 In addition, from the viewpoint of permeability, the air permeability of the multilayer porous membrane is preferably 1000 sec / 100 cm 3 Less than 300sec / 100cm, preferably less than 300sec / 100cm3 Less than 250 sec / 100 cm, more preferably 3 The following is the result.

[0099] The 130°C heat shrinkage rate of the multilayer porous membrane in embodiment 1 is not particularly limited, but is, for example, preferably 0.0% to 5.0% in both the MD and TD directions, more preferably 0.0% to 3.0%, and even more preferably 0.0% to 2.0%. A 130°C heat shrinkage rate of 5.0% or less in both the MD and TD directions is preferred from the viewpoint of suppressing rupture of the multilayer porous membrane when an abnormality occurs in the battery and suppressing a short circuit.

[0100] The heat shrinkage rate at 150°C of the multilayer porous membrane in embodiment 1 is not particularly limited, but is preferably 0.0% or more and 5.0% or less, more preferably 0.0% or more and 3.0% or less, in both the MD and TD directions. A heat shrinkage rate at 150°C of 5.0% or less in both the MD and TD directions is preferred from the viewpoint of suppressing rupture of the multilayer porous membrane when an abnormality occurs in the battery and suppressing a short circuit.

[0101] The heat shrinkage rate at 150°C of the multilayer porous membrane according to embodiment 2 or 3 is preferably less than 10.0%, more preferably 5.0% or less. Here, the larger of the MD heat shrinkage rate and the TD heat shrinkage rate is used as the heat shrinkage rate of the multilayer porous membrane according to embodiment 2 or 3. MD is the machine direction during continuous molding of the microporous membrane or multilayer porous membrane, and TD is the direction crossing the MD at an angle of 90°. If the heat shrinkage rate at 150°C is less than 10.0%, the area causing a short circuit in a nail penetration test of an electricity storage device including the multilayer porous membrane according to embodiment 2 or 3 as a separator tends to be reduced. The lower limit of the heat shrinkage rate at 150°C of the multilayer porous membrane according to embodiment 2 or 3 is not particularly limited, and can be, for example, -5.0% or more, -3.0% or more, 0.0% or more, or more than 0.0% in both MD and TD.

[0102] For the multilayer porous membrane of embodiment 2 or 3, the hole area, total thickness, air permeability, heat shrinkage rate, etc. in a 400°C soldering test can be adjusted by appropriately combining the production conditions of the PO microporous membrane and the production conditions of the porous layer.

[0103] <Method for manufacturing multilayer porous membrane> The multilayer porous membrane of embodiment 1 can be produced by a known method, for example, by forming a microporous PO membrane and then arranging a porous layer on at least one surface of the microporous PO membrane.

[0104] The multilayer porous membrane of Embodiment 2 or 3 can be produced by a known method. For example, the multilayer porous membrane of Embodiment 2 or 3 can be produced by forming a PO microporous membrane, and then disposing a first porous layer on one side of the PO microporous membrane and a second porous layer on the other side of the PO microporous membrane. Alternatively, the PO microporous membrane and the porous layer can be produced by coextrusion, or the first and second porous layers can be extruded onto both sides of the PO microporous membrane, respectively, or the PO microporous membrane and the porous layer can be bonded together after being separately produced.

[0105] <Method for producing polyolefin microporous membrane> The method for producing the polyolefin microporous membrane (PO microporous membrane) is not particularly limited, and known production methods can be used. For example, (1) A method in which a polyolefin resin composition and a pore-forming material are melt-kneaded to form a sheet, which is then stretched as necessary, and the pore-forming material is extracted to make the sheet porous; (2) A method in which a polyolefin resin composition is melt-kneaded and extruded at a high draw ratio, and then heat-treated and stretched to separate the polyolefin crystal interface, thereby making the resin porous; (3) A method in which a polyolefin resin composition and an inorganic filler are melt-kneaded and formed into a sheet, and then the interface between the polyolefin and the inorganic filler is peeled off by stretching to make the sheet porous; (4) A method in which a polyolefin resin composition is dissolved and then immersed in a poor solvent for the polyolefin to solidify the polyolefin and simultaneously remove the solvent, thereby making the composition porous; etc.

[0106] As an example of a method for producing a microporous PO membrane, a method in which a polyolefin resin composition and a pore-forming material are melt-kneaded and formed into a sheet, and then the pore-forming material is extracted will be described below.

[0107] First, the polyolefin resin composition and the pore-forming material are melt-kneaded. Examples of the melt-kneading method include a method in which the polyolefin resin and, if necessary, other additives are fed into a resin kneading device such as an extruder, a feeder, a lab plasto mill, a kneading roll, or a Banbury mixer, and the pore-forming material is introduced at an arbitrary ratio while the resin components are heated and melted, and then kneaded.

[0108] Examples of the pore-forming material include plasticizers, inorganic materials, and combinations thereof. The plasticizer is not particularly limited, but examples include non-volatile solvents capable of forming a homogeneous solution at or above the melting point of the polyolefin, such as hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. Among plasticizers, liquid paraffin is preferred because, when the polyolefin resin is polyethylene and / or polypropylene, it is highly compatible with these resins, and even when the molten mixture is stretched, interfacial peeling between the resin and the plasticizer is unlikely to occur, making it easier to perform uniform stretching. The inorganic material is not particularly limited and includes, for example, oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber. These may be used alone or in combination of two or more. Among these inorganic materials, silica, alumina, and titania are preferred from the viewpoint of electrochemical stability, and silica is particularly preferred from the viewpoint of ease of extraction.

[0109] Next, the melt-kneaded material is molded into a sheet. Examples of methods for producing a sheet-shaped product include extruding the melt-kneaded material into a sheet through a T-die or the like, contacting it with a thermal conductor, and solidifying it by cooling it to a temperature sufficiently lower than the crystallization temperature of the resin component. Examples of thermal conductors used for cooling and solidifying include metal, water, air, and plasticizers. Among these, metal rolls are preferred because of their high thermal conductivity. Furthermore, sandwiching the extruded material between metal rolls when contacting them is more preferred because it further increases the thermal conductivity efficiency, orients the sheet, increasing film strength, and tends to improve the surface smoothness of the sheet. When extruding the melt-kneaded material into a sheet through a T-die, the die lip spacing is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. When the die lip gap is 200 μm or more, the occurrence of scum and other deposits is reduced, and the impact on film quality such as streaks and defects is minimal, reducing the risk of film rupture in the subsequent stretching process.On the other hand, when the die lip gap is 3,000 μm or less, the cooling rate is fast, preventing uneven cooling and maintaining the thickness stability of the sheet.

[0110] The sheet-like molded body may also be rolled. Rolling can be performed, for example, by a pressing method using a double belt press or the like. Rolling can increase the orientation, particularly in the surface layer portion. The rolling area ratio is preferably more than 1 and not more than 3, and more preferably more than 1 and not more than 2. When the rolling ratio exceeds 1, the surface orientation increases, and the membrane strength of the finally obtained porous membrane tends to increase. On the other hand, when the rolling ratio is 3 or less, the difference in orientation between the surface layer portion and the central interior is small, and a uniform porous structure tends to be formed in the thickness direction of the membrane.

[0111] Next, the pore-forming material is removed from the sheet-like formed body to form a porous membrane. For example, a method for removing the pore-forming material includes immersing the sheet-like formed body in an extraction solvent to extract the pore-forming material, followed by thorough drying. The pore-forming material may be extracted by either a batch method or a continuous method. To prevent the porous membrane from shrinking, it is preferable to restrain the edges of the sheet-like formed body during the immersion and drying process. Furthermore, it is preferable that the amount of pore-forming material remaining in the porous membrane is less than 1% by mass relative to the total mass of the porous membrane.

[0112] The extraction solvent used to extract the pore-forming material is preferably a poor solvent for the polyolefin resin and a good solvent for the pore-forming material, with a boiling point lower than the melting point of the polyolefin resin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine-based halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation. In addition, when an inorganic material is used as the pore-forming material, an aqueous solution of sodium hydroxide, potassium hydroxide, or the like can be used as the extraction solvent.

[0113] It is also preferable to stretch the sheet-like formed body or porous membrane. Stretching may be performed before extracting the pore-forming material from the sheet-like formed body. Stretching may also be performed on the porous membrane from which the pore-forming material has been extracted from the sheet-like formed body. Furthermore, stretching may be performed both before and after extracting the pore-forming material from the sheet-like formed body.

[0114] As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is preferred from the viewpoint of improving the strength, etc., of the resulting microporous PO membrane. When the sheet-like molded article is stretched biaxially at a high ratio, the molecules are oriented in the planar direction, and the final microporous membrane becomes tear-resistant and has high pin puncture strength.

[0115] Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. From the viewpoints of improving puncture strength, uniformity of stretching, and shut-down property, simultaneous biaxial stretching is preferred. Furthermore, from the viewpoint of ease of control of plane orientation, sequential biaxial stretching is preferred.

[0116] Here, simultaneous biaxial stretching refers to a stretching method in which stretching in MD (the machine direction of continuous molding of a PO microporous membrane) and stretching in TD (the direction crossing the MD of the PO microporous membrane at an angle of 90°) are performed simultaneously, and the stretching ratios in each direction may be different. Sequential biaxial stretching refers to a stretching method in which stretching in MD and TD is performed independently, and while stretching is performed in MD or TD, the other direction is unconstrained or fixed at a fixed length.

[0117] The areal stretching ratio is preferably 20 to 100 times, more preferably 25 to 70 times. The stretching ratios in each axial direction are preferably 4 to 10 times in MD and 4 to 10 times in TD, more preferably 5 to 8 times in MD and 5 to 8 times in TD. A total areal stretching ratio of 20 times or more tends to impart sufficient strength to the resulting microporous PO membrane, while a total areal stretching ratio of 100 times or less tends to prevent membrane rupture during the stretching step and achieve high productivity.

[0118] To suppress shrinkage of the PO microporous membrane, a heat treatment for heat setting can be performed after the stretching step or after the formation of the PO microporous membrane. The PO microporous membrane may also be subjected to post-treatments such as hydrophilization treatment with a surfactant or crosslinking treatment with ionizing radiation.

[0119] The microporous polypropylene membrane is preferably heat-treated for heat setting to suppress shrinkage. Heat treatment methods include stretching at a predetermined temperature and at a predetermined stretch ratio to adjust physical properties, and / or relaxation at a predetermined temperature and at a predetermined relaxation ratio to reduce stretching stress. The relaxation may be performed after the stretching. These heat treatments can be performed using a tenter or roll stretching machine.

[0120] The stretching operation is preferably performed by stretching the membrane in MD and / or TD by 1.1 times or more, more preferably 1.2 times or more, from the viewpoint of obtaining a microporous PO membrane with even higher strength and higher porosity.

[0121] The relaxation operation is a shrinking operation of the membrane in MD and / or TD. The relaxation rate is the value obtained by dividing the membrane dimension after the relaxation operation by the membrane dimension before the relaxation operation. When both MD and TD are relaxed, the relaxation rate is the value obtained by multiplying the relaxation rate in MD by the relaxation rate in TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less. From the viewpoint of membrane quality, the relaxation rate is preferably 0.5 or more. The relaxation operation may be performed in both MD and TD, or may be performed in only one of MD or TD.

[0122] The stretching and relaxation operations after the plasticizer extraction are preferably performed in TD from the viewpoints of process control and hole area control in a 400°C soldering test. The temperature in the stretching and relaxation operations is preferably lower than the melting point (hereinafter also referred to as "Tm") of the polyolefin resin, more preferably in the range of 1°C to 25°C lower than Tm. A temperature in the above range in the stretching and relaxation operations is preferred from the viewpoint of a balance between reduced thermal shrinkage and porosity.

[0123] <Porous layer arrangement> The porous layer can be disposed on at least one surface of the microporous PO membrane by any known method, such as disposing, coating, laminating, or extruding. For example, a porous layer can be formed by applying the above-described coating liquid containing inorganic particles and a binder polymer to the microporous PO membrane.

[0124] The first porous layer can be disposed on one side of a polyolefin microporous membrane (PO microporous membrane) and the second porous layer can be disposed on the other side of the PO microporous membrane by any known disposing method, coating method, laminating method, extrusion method, etc. For example, a porous layer can be formed by applying a coating liquid containing the inorganic particles and the binder polymer described above to a PO microporous membrane.

[0125] The binder polymer in the coating solution may be in the form of an aqueous solution dissolved or dispersed in water or an organic medium solution dissolved or dispersed in a common organic medium. However, a resin latex is preferred, and an acrylic polymer latex is more preferred. "Resin latex" refers to a resin dispersed in a medium. When a resin latex is used as a binder, when a porous layer containing inorganic particles and a binder is laminated on at least one side of a microporous polypropylene membrane, ion permeability is less likely to decrease, making it easier to achieve high-output characteristics. In addition, even when temperature rises rapidly during abnormal heat generation, smooth shutdown characteristics are exhibited, making it easier to achieve high safety.

[0126] When forming the coating solution, the amount of binder polymer used relative to the amount of inorganic particles used is not limited, but as described below, it is preferable that the amount is sufficient to adjust the dynamic friction coefficient of the porous layer after separator formation to within the range of 0.1 to 0.6.

[0127] The average particle size of the resin latex binder is preferably 50 nm to 1,000 nm, more preferably 60 nm to 500 nm, even more preferably 65 nm to 250 nm, and particularly preferably 70 nm to 150 nm. When the average particle size is 50 nm or more, ion permeability is less likely to decrease, making it easier to achieve high-output characteristics. In addition, even when the temperature rises rapidly during abnormal heat generation, smooth shutdown characteristics are exhibited, making it easier to achieve high safety. When the average particle size is 1,000 nm or less, a porous layer containing inorganic particles and a binder polymer is laminated on at least one side of a microporous polypropylene membrane, exhibits good binding properties. When used as a separator, the binder tends to exhibit good thermal shrinkage and be highly safe. The average particle size can be controlled by adjusting the polymerization time, polymerization temperature, raw material composition ratio, raw material charging order, pH, stirring speed, etc. during the production of the binder polymer.

[0128] The medium for the coating liquid is preferably one that can uniformly and stably disperse or dissolve the inorganic particles and binder polymer, and examples thereof include N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, and hexane.

[0129] To the coating solution, various additives such as thickeners such as surfactants, wetting agents, antifoaming agents, and pH adjusters containing acids and alkalis may be added to stabilize the dispersion or improve the coatability, and further to adjust the contact angle on the surface of the porous layer. The total amount of these additives added is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, of the active ingredients (the mass of the dissolved additive components when the additives are dissolved in a solvent) per 100 parts by mass of inorganic particles.

[0130] Regarding additives, anionic surfactants include, for example, higher fatty acid salts, alkyl sulfonates, alpha olefin sulfonates, alkanesulfonates, alkyl benzene sulfonates, sulfosuccinate salts, alkyl sulfate salts, alkyl ether sulfate salts, alkyl phosphate salts, alkyl ether phosphate salts, alkyl ether carboxylate salts, alpha sulfo fatty acid methyl ester salts, and methyl taurate salts. Nonionic surfactants include, for example, glycerin fatty acid esters, polyglycerin fatty acid esters, sucrose fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene fatty acid esters, fatty acid alkanolamides, and alkyl glucosides. Amphoteric surfactants include, for example, alkyl betaines, fatty acid amidopropyl betaines, and alkyl amine oxides. Cationic surfactants include, for example, alkyl trimethyl ammonium salts, dialkyl dimethyl ammonium salts, alkyl dimethyl benzyl ammonium salts, and alkyl pyridinium salts. Other examples include fluorosurfactants and polymer surfactants such as cellulose derivatives, polycarboxylates, and polystyrene sulfonates.

[0131] The method for dispersing or dissolving the inorganic particles and the binder polymer in the medium of the coating liquid is not particularly limited as long as it can achieve the dispersion characteristics of the coating liquid required for the coating step, and examples thereof include a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, high-speed impeller dispersion, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade or the like.

[0132] The method for applying the coating liquid to the microporous PO membrane is not particularly limited as long as it can achieve the required layer thickness or coating area, and examples include gravure coater, small-diameter gravure coater, reverse roll coater, transfer roll coater, kiss coater, dip coater, knife coater, air doctor coater, blade coater, rod coater, squeeze coater, cast coater, die coater, screen printing, and spray coating.

[0133] Furthermore, it is preferable to subject the surface of the PO microporous membrane to a surface treatment prior to application of the coating solution, since this facilitates application of the coating solution and improves adhesion between the inorganic particle-containing porous layer and the PO microporous membrane surface after application. The surface treatment method is not particularly limited as long as it does not significantly impair the porous structure of the PO microporous membrane, and examples thereof include corona discharge treatment, plasma discharge treatment, mechanical graining, solvent treatment, acid treatment, and ultraviolet oxidation.

[0134] The method for removing the medium from the coated film after coating is not particularly limited as long as it does not adversely affect the PO microporous membrane, and examples include a method in which the PO microporous membrane is fixed while being dried at a temperature below its melting point, a method in which the membrane is dried under reduced pressure at a low temperature, and extraction drying. Some of the solvent may be allowed to remain as long as it does not significantly affect the properties of the electricity storage device. For a multilayer porous membrane comprising a laminate of a PO microporous membrane and a porous layer, it is preferable to appropriately adjust the drying temperature, winding tension, etc., from the viewpoint of controlling the shrinkage stress in the MD direction.

[0135] <Separators for electricity storage devices> The multilayer porous membranes according to Embodiments 1 to 3 can be used as separators for electricity storage devices. The electricity storage device comprises a positive electrode, a separator, a negative electrode, and, if desired, an electrolyte. Specific examples of electricity storage devices include lithium batteries, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, and zinc-air batteries. Among these, from the viewpoint of practicality, lithium batteries, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

[0136] The electricity storage device can be produced, for example, by stacking a positive electrode and a negative electrode via a separator made of the multilayer porous membrane according to any one of the first to third embodiments, and then winding or zigzag folding the stacked electrode body or the wound electrode body or the zigzag folding body, and then loading the stacked electrode body into an outer casing. The positive and negative electrodes are connected to the positive and negative electrode terminals of the outer casing via lead bodies or the like, and a non-aqueous electrolyte solution containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt is injected into the outer casing, and then the outer casing is sealed.

[0137] <Battery> The multilayer porous membrane according to any one of the first to third embodiments can be used as a separator and stacked on a plurality of electrodes with the separator interposed therebetween to obtain a laminate in which the separators and electrodes are stacked. The obtained laminate or a wound body or zigzag body obtained by winding or zigzag folding the laminate can be used to manufacture a nonaqueous electrolyte battery. The nonaqueous electrolyte battery using the multilayer porous membrane according to any one of the first to third embodiments as a separator may be excellent in nail penetration tests and impact tests.

[0138] The method for producing the laminate is not particularly limited, and may include, for example, a step of stacking a separator and an electrode, and heating and / or pressing as necessary. Heating and / or pressing can be performed when stacking the electrode and the separator. Heating and / or pressing may also be performed on a wound body obtained by stacking the electrode and the separator and then winding them into a circular or flat spiral shape. The heating and pressing steps for the laminate may be performed after the laminate is produced, and may be performed after the laminate is housed in an exterior case and an electrolyte solution is injected into the exterior case.

[0139] A non-aqueous electrolyte battery includes the above-described laminate, a wound body in which the laminate is wound, or a zigzag body in which the laminate is zigzag folded, inside an exterior body such as a cylindrical can, a pouch-type case, or a laminate case, together with a non-aqueous electrolyte.

[0140] When the nonaqueous electrolyte battery is a secondary battery, a positive electrode terminal is welded to an end of a positive electrode laminate consisting of a positive electrode current collector and a positive electrode active material layer, and a negative electrode terminal is welded to an end of a negative electrode laminate consisting of a negative electrode current collector and a negative electrode active material layer, thereby enabling charging and discharging of the secondary battery including the positive electrode laminate with terminals and the negative electrode laminate with terminals.

[0141] Furthermore, the positive electrode laminate with terminals and the negative electrode laminate with terminals are laminated via a separator, and wound or zigzag folded as desired. The resulting laminate, wound body, or zigzag folded body is housed in an outer casing, a nonaqueous electrolyte solution is injected into the outer casing, and the outer casing is sealed, thereby obtaining a secondary battery.

[0142] When a non-aqueous electrolyte secondary battery is produced using the multilayer porous membrane according to any one of the first to third embodiments as a separator, a known positive electrode, negative electrode and non-aqueous electrolyte may be used.

[0143] The positive electrode material is not particularly limited, but examples thereof include lithium-containing composite oxides such as LiCoO2, LiNiO2, spinel-type LiMnO4, and olivine-type LiFePO4.

[0144] The negative electrode material is not particularly limited, and examples thereof include carbon materials such as graphite, non-graphitizable carbon, graphitizable carbon, and composite carbon; silicon, tin, metallic lithium, and various alloy materials.

[0145] The non-aqueous electrolyte is not particularly limited, and an electrolyte in which an electrolyte is dissolved in an organic solvent can be used. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Examples of the electrolyte include lithium salts such as LiClO4, LiBF4, and LiPF6.

[0146] <Stacked Lithium Ion Secondary Battery> Another aspect of the present invention is a stacked lithium ion secondary battery in which a ninety-nine-fold body of the multilayer porous film according to Embodiment 2 or 3 is housed in an exterior body, and the positive electrode and the negative electrode are alternately inserted into the gaps of the ninety-nine-fold body. The stacked lithium ion secondary battery can further contain the electrolyte described above. A stacked lithium ion secondary battery using the multilayer porous film according to Embodiment 2 or 3 as a separator may be excellent in a nail penetration test and a collision test.

Examples

[0147] Hereinafter, embodiments of the present invention will be specifically described with reference to examples and comparative examples, but the present invention is not limited to these examples and comparative examples.

[0148] <Test Series I (related to Embodiment 1)>

[0149] Test and Evaluation Methods <BIB Cross-Section SEM Observation> The cross section of the multilayer porous membrane is processed using a broad ion beam (BIB). The cross section is processed using an IM4000 manufactured by Hitachi High-Tech Corporation under the following processing conditions: an acceleration voltage of 3 kV, and a beam current of 60 to 65 μA. During processing, the multilayer porous membrane is cooled down to just before processing as needed to suppress thermal damage. Specifically, the multilayer porous membrane is left overnight in a cooling device at -40°C. This results in a smooth cross section of the multilayer porous membrane.

[0150] Next, the cross section of the obtained multilayer porous film is subjected to a conductive treatment using C paste and Os coating, and then an electron image of the cross section SEM image is taken using a "HITACHI (trademark) S-4800" (manufactured by Hitachi High-Technologies) with a magnification of 30,000 times, an acceleration voltage of 1.0 kV, and a detector setting of secondary electrons (upper).

[0151] When photographing, an image is acquired with the porous layer at the center, as in the cross-sectional SEM image shown in Figure 1, and with the interface between the PO microporous membrane and the porous layer, as well as the PO microporous membrane within a thickness of 0.1 to 0.2 μm from the interface, at the bottom of the photographed image. In this procedure, images of three fields of view are acquired.

[0152] <Binarization processing method> The number of holes S and the total number of holes are 0.001 μm 2 More than ~0.05μm 2 The ratio T of the number of pores that are equal to or smaller than the number of pores is calculated using the image processing software "Fiji" (Fuji Is Just ImageJ) in the following manner. In order to show a specific example of the binarization processing method used in the calculation, reference is made to Figs. 2 to 6.

[0153] First, open "File" → "Open" and open the target cross-sectional SEM electron image. Next, measure a known distance in the image using the line selection tool "Straight." Open "Analyze" → "SET SCALE," enter the measurement unit and known distance, and set the scale.

[0154] Next, to select the evaluation area for binarization, the desired region (porous layer) is selected using "Rectangular selections." Regarding the desired region, in the height direction (vertical direction of the figure), if the thickness of the porous layer is 1.0 μm or more, the region from the interface between the PO microporous membrane and the porous layer to a thickness of 0.2 μm toward the porous layer and the region from the outermost surface of the porous layer to a thickness of 0.2 μm are excluded, as shown in Figure 2, and the remaining region is selected. If the outermost surface of the porous layer is not shown in the figure, the region from the top to 0.2 μm is excluded. On the other hand, if the thickness of the porous layer is less than 1.0 μm, 10% of the thickness from the interface and the outermost surface are excluded, and the remaining 80% is selected as the remainder. Then, in the horizontal direction of the figure, the entire image is selected.

[0155] Next, use "Image" → "Crop" to display only the selected area. At this time, calculate the visual field area U of the selected area as shown in Figure 3.

[0156] Next, we will perform a contrast equalization process on the cross-sectional SEM image. Specifically, open "Process" → "Enhance Contrast," set "Saturated pixels" to 0.3%, check "Equalize histogram," and click "OK." This process will enhance the contrast of the image, making bright areas (edges of inorganic filler particles) brighter and dark areas (pores) darker.

[0157] Next, open "Plugins" → "Process" → "Bilateral Filter", enter 16 for "Spatial radius" and 64 for "range radius", then click "OK". By performing this process, you can remove noise while preserving the edges of the filler particles.

[0158] Next, open "Process" → "Filters" → "Gausian Blur", enter 1.0 in "Sigma (radius)", and click "OK". As a result, the image in Figure 4 will be obtained.

[0159] Next, to perform the binarization process, set the threshold using Figure 4. Specifically, select "Analysis" → "Histgram" and use a graph showing the number of values ​​(vertical axis) for each brightness (horizontal axis) from 0 to 255, as shown in Figure 5, and "List." At this time, as shown in Figure 5, read the number E of the peaks of the largest peak in the center of the histogram from "List." From the peak of the peak, read the brightness F that is 20% or less of E in the direction toward brightness 0 (left side of the peak) and is the minimum value closest to the peak from List, and use this brightness F as the threshold.

[0160] Next, select "Image" → "Adjust" → "Threshold". Next, press "Set", enter 0 in "Lower threshold level" and brightness F (threshold) in "Upper threshold level", and click OK. Figure 6 will be obtained, with the holes in the image in Figure 4 filled in black.

[0161] Next, perform the binarization process. Specifically, select "Analyze" → "Analyze particles" and set "Size (μm 2 )" field, enter "0.001-Infinity", and then check "Display results", "Clear results", "Exclude on edges", "Include holes", and "Add to Manager". Click "OK" to set the field of view area to Uμm 2 0.001μm 2 The number of holes X and the area value of each hole are obtained. The obtained number of holes X and field area U μm 2 For 10 μm 2 The number of holes in the field of view is calculated. Furthermore, 0.001 μm 2 More than 0.05μm 2 Calculate the number of holes that are less than 0.001 μm relative to the total number of holes X. 2 More than 0.05μm 2 Calculate the percentage of holes that are: In the above method, from the three captured images, pores with a size of 10 μm 2 in the field of view, and the ratio of the number of pores with a size of 0.001 μm 2 or more and 0.05 μm 2 or less with respect to the total number of pores X are calculated respectively, and the average value thereof is taken as the ratio T of the number of pores with a size of 0.001 μm 2 or more and 0.05 μm 2 or less with respect to the number of pores S and the total number of pores X.

[0162] <Thickness (μm) of PO microporous membrane, multilayer porous membrane, and multilayer film> Using the micro-thickness gauge "KBM (trademark)" manufactured by Toyo Seiki Co., Ltd., the thicknesses of the polyolefin microporous membrane and the multilayer porous membrane were measured at room temperature (23 ± 2 °C), and the coating thickness of the porous layer was calculated from the respective thicknesses. Also, from the perspective of detection from the multilayer porous membrane, it is also possible to measure the thickness of each layer using a cross-sectional SEM image.

[0163] <Melt index (MI) (g / 10 min) of polyolefin microporous membrane> In accordance with JIS K7210:1999 (Plastics - Melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastic plastics), the melt index (MI) of the polyolefin microporous membrane (PO microporous membrane) was measured. A load of 21.6 kgf was applied to the membrane at 190 °C, and the amount of resin (g) that flowed out from an orifice with a diameter of 1 mm and a length of 10 mm in 10 minutes was measured, and the value obtained by rounding off the first decimal place was taken as MI. <A

[0164] <Aspect ratio of inorganic particles in the porous layer> The cross section of the multilayer film was photographed at a magnification of 10,000 times using a scanning electron microscope (SEM) "HITACHI (trademark) S-4800" (Hitachi High-Technologies), and the aspect ratio was determined by image processing of the inorganic particles in Layer B. Even when inorganic particles were bonded to each other, those for which the length and width of each inorganic particle were clearly recognizable were selected, and the aspect ratio was calculated based on these. Specifically, 10 particles for which the length and width were clearly recognizable were selected, and the aspect ratio was calculated by averaging the values ​​obtained by dividing the long axis of each inorganic particle by the length of its short axis. If there were fewer than 10 particles in one field of view with clearly recognizable length and width, 10 particles were selected from images of multiple fields of view.

[0165] <Average particle size and particle size distribution of inorganic particles> The particle size distribution and median diameter (μm) of the inorganic particle dispersion or slurry coating liquid were measured using a laser particle size distribution measuring device (Microtrac MT3300EX manufactured by Nikkiso Co., Ltd.). If necessary, the particle size distribution of the inorganic particle dispersion or slurry coating liquid was adjusted using the particle size distribution of water or the binder polymer as a baseline. The particle size at which the cumulative frequency is 50% was defined as D 50 , the particle size at which the cumulative frequency is 10% is D 10、 The particle size at which the cumulative frequency is 90% is D 90 It was decided.

[0166] <Air permeability (sec / 100cm 3 ), and the air permeability ratio of the multilayer porous membrane to the multilayer porous membrane and the polyolefin microporous membrane > The air permeabilities of the multilayer porous membrane and the PO microporous membrane, where the air resistance in accordance with JIS P-8117 was used as the air permeability, were measured in accordance with JIS P-8117 using a Gurley air permeability meter "G-B2 (trademark)" manufactured by Toyo Seiki Co., Ltd., in an atmosphere at a temperature of 23°C and a humidity of 40%. The air permeability of the multilayer porous membrane divided by the air permeability of the PO microporous membrane was defined as the air permeability ratio of the multilayer porous membrane to the multilayer porous membrane and polyolefin microporous membrane.

[0167] <Content of inorganic particles in porous layer (mass%)> It can be calculated from the blending ratio of the constituent materials when preparing the coating liquid. Furthermore, from the perspective of detecting from a multilayer porous membrane, it is also possible to measure the weight changes of organic and inorganic particles using TG-DTA. Specifically, the porous layer portion of the multilayer porous membrane is scraped off with a glass plate, and 8 mg to 10 mg of the sample is collected. The sampled porous layer is placed in the apparatus and heated from room temperature to 600°C at a rate of 10°C / min in an air atmosphere, and the weight change is measured and calculated.

[0168] <Porosity (%)> A 10cm x 10cm square sample was cut from the microporous membrane and its volume (cm 3 ) and mass (g), and compare them with the film density (g / cm 3 ) the porosity was calculated using the following formula: Porosity (%) = (volume - mass / density) / volume x 100

[0169] <Piercing strength (gf) and area weight conversion piercing strength (gf / (g / m 2 ))> Using a Kato Tech handy compression tester "KES-G5 (trademark)," the microporous membrane was fixed in a sample holder with an opening diameter of 11.3 mm. Next, a puncture test was performed on the center of the fixed microporous membrane at a temperature of 23°C and humidity of 40% with a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec. The raw puncture strength (gf) was obtained as the maximum puncture load. The obtained puncture strength (gf) was converted to basis weight (gf / (g / m 2 )) was also calculated.

[0170] <Heat shrinkage rate (%) at 130℃ and 150℃> The multilayer porous membrane was cut into a sample of 100 mm in the MD direction and 100 mm in the TD direction, and left to stand in an oven at 130°C or 150°C for 1 hour. At this time, the sample was sandwiched between 10 sheets of paper to prevent hot air from directly hitting the sample. After removing the sample from the oven and cooling, the length (mm) was measured and the thermal shrinkage was calculated using the following formula. Measurements were performed in the MD direction and the TD direction, and the larger value was taken as the thermal shrinkage. Heat shrinkage rate (%) = {(100 - length after heating) / 100} x 100

[0171] <Nail penetration test> (Preparation of positive electrode) The positive electrode active material is lithium nickel manganese cobalt composite oxide powder (LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 A cathode mixture paste was prepared by mechanically mixing 85 parts by weight of a mixed cathode active material (a 70:30 mass ratio of lithium manganese oxide powder (LiMnO2) and lithium manganese composite oxide powder (LiMnO4)), 6 parts by weight of acetylene black (a conductive additive), and 9 parts by weight of PVdF (a binder) in N-methyl-2-pyrrolidone (NMP) as a solvent. This cathode mixture paste was uniformly applied to both sides of a 20 μm-thick aluminum foil current collector, dried, and then compression-molded using a roll press to adjust the thickness of the cathode mixture layer to a total thickness of 130 μm. A cathode was fabricated from a rectangular sheet with 95 mm short sides and 120 mm long sides, with a 20 mm long, uncoated aluminum foil lead tab attached to the top of the short side.

[0172] (Preparation of negative electrode) A negative electrode mixture paste was prepared by uniformly mixing 91 parts by weight of graphite (negative electrode active material) and 9 parts by weight of PVdF (binder) using NMP as a solvent. This negative electrode mixture paste was uniformly applied to both sides of a 15 μm-thick copper foil current collector, dried, and then compression-molded using a roll press to adjust the thickness of the negative electrode mixture layer to a total thickness of 130 μm. A negative electrode was fabricated from a rectangular sheet with short sides of 95 mm and long sides of 120 mm, with a 20 mm-long lead tab of uncoated copper foil on the upper short side.

[0173] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 1:1:1 (volume ratio) to a concentration of 1.0 mol / L.

[0174] (Cell preparation) An electrode plate laminate was fabricated by alternately stacking 27 positive electrode sheets and 28 negative electrode sheets and isolating each sheet with a multilayer porous membrane as a separator. The separator was a 125 mm wide strip, which was folded zigzag to fabricate the electrode plate laminate. This electrode plate laminate was pressed into a flat plate, then housed in an aluminum laminate film, and three sides were heat-sealed. The positive electrode lead tab and the negative electrode lead tab each protruded from one side of the laminate film. After drying, the nonaqueous electrolyte solution was injected into the three-side sealed laminate film, and the remaining side was sealed. The laminated lithium-ion secondary battery fabricated in this manner was designed to have a capacity of 10 Ah.

[0175] (Nail penetration rating) A laminated lithium-ion secondary battery was placed on an iron plate in a temperature-controlled explosion-proof booth. The temperature in the explosion-proof booth was set to 40°C, and a 3.0 mm diameter iron nail was driven into the center of the laminated lithium-ion secondary battery at a speed of 2 mm / sec, and the nail was maintained in the penetrated state. After the nail penetrated, a thermocouple was installed inside the nail to measure the temperature inside the laminated battery, and the presence or absence of ignition and the maximum temperature reached were evaluated as follows. A: No fire, maximum temperature reached is less than 300°C B: No ignition, maximum temperature reached is 300°C or higher C: Ignition occurs 15 seconds after the start of the test D: Ignition occurs within 15 seconds of the start of the test

[0176] <Rate characteristics> (a. Preparation of Positive Electrode) The positive electrode active material was lithium nickel manganese cobalt composite oxide (Li[Ni 1 / 3 Mn 1 / 3 Co 1 / 3 91.2 parts by mass of ]O2), 2.3 parts by mass each of flake graphite and acetylene black as conductive materials, and 4.2 parts by mass of polyvinylidene fluoride (PVdF) as a resin binder were prepared, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to one side of a 20 μm thick aluminum foil that would serve as the positive electrode using a die coater, with the positive electrode active material coating amount being 120 g / m. 2 After drying at 130°C for 3 minutes, the positive electrode active material was pressed using a roll press so that the bulk density of the positive electrode active material was 2.90 g / cm 3 This positive electrode was compression molded to have an area of ​​2.00 cm. 2 It was punched into a circle.

[0177] (b. Preparation of negative electrode) 96.6 parts by mass of artificial graphite was prepared as the negative electrode active material, and 1.4 parts by mass of ammonium salt of carboxymethyl cellulose and 1.7 parts by mass of styrene-butadiene copolymer latex were prepared as the resin binder. These were dispersed in purified water to prepare a slurry. This slurry was applied to one side of a 16 μm-thick copper foil serving as a negative electrode current collector using a die coater so that the negative electrode active material was 53 g / m 2 After drying at 120°C for 3 minutes, the negative electrode active material was pressed using a roll press so that the bulk density of the negative electrode active material was 1.35 g / cm 3 This was compression molded to form a negative electrode with an area of ​​2.05 cm. 2 It was punched into a circle.

[0178] (c. Preparation of non-aqueous electrolyte) A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate:ethyl carbonate=1:2 (volume ratio) to a concentration of 1.0 ml / L.

[0179] (d. Battery assembly) The negative electrode, multilayer porous membrane, and positive electrode were stacked in this order from bottom to top, with the active material surfaces of the positive and negative electrodes facing each other. This stack was then placed in a lidded stainless steel metal container, the container body and lid of which were insulated, with the copper foil of the negative electrode and the aluminum foil of the positive electrode in contact with the container body and lid, respectively, to obtain a cell. This cell was then dried under reduced pressure at 70°C for 10 hours. A nonaqueous electrolyte was then poured into the container in an argon box, and the container was sealed to prepare a test battery.

[0180] (e. Evaluation of rate characteristics) The battery assembled in (d. Battery assembly) above was charged at a temperature of 25°C with a current value of 3 mA (approximately 0.5 C) up to a battery voltage of 4.2 V, and then the current value was reduced from 3 mA to maintain 4.2 V. This method was used for the first charge after battery fabrication for a total of approximately 6 hours, and the battery was then discharged at a current value of 3 mA down to a battery voltage of 3.0 V. Next, at 25°C, the battery was charged at a current of 6 mA (approximately 1.0 C) up to a battery voltage of 4.2 V, and then the current was reduced from 6 mA to maintain 4.2 V. This method of charging was continued for a total of approximately 3 hours, and the battery was then discharged at a current of 6 mA down to a battery voltage of 3.0 V, and the discharge capacity at this time was recorded as the 1 C discharge capacity (mAh). Next, at 25°C, the battery was charged at a current of 6 mA (approximately 1.0 C) up to a battery voltage of 4.2 V, and then the current was reduced from 6 mA to maintain 4.2 V, for a total of approximately 3 hours of charging. After that, the battery was discharged at a current of 60 mA (approximately 10 C) down to a battery voltage of 3.0 V, and the discharge capacity at that time was recorded as the 10 C discharge capacity (mAh). The ratio of the 10C discharge capacity to the 1C discharge capacity was calculated, and this value was taken as the rate characteristic. Rate characteristics at 10C (%) = (10C discharge capacity / 1C discharge capacity) x 100 The rate characteristics at 10C were evaluated according to the following criteria. A: Rate characteristics at 10C above 22% B: Rate characteristics at 10C of 20% or more and less than 22% C: Rate characteristics at 10C of 18% or more and less than 20% D: Rate characteristics at 10C below 18%

[0181] <Cycle test> The battery tested for the rate characteristics above was discharged at a discharge current of 1 C to a discharge cut-off voltage of 3 V at a temperature of 25°C, and then charged at a charge current of 1 C to a charge cut-off voltage of 4.2 V. This cycle was repeated, and the cycle characteristics were evaluated according to the following criteria, using the capacity retention rate after 300 cycles relative to the initial capacity (capacity at the first cycle). A: Capacity retention rate of 65% or more B: Capacity retention rate of 60% or more but less than 65% C: Capacity retention rate of less than 60%

[0182] [Example 1] Using a tumbler blender, a polymer mixture was prepared containing 46.5% by mass of homopolymer polyethylene (PE) with a viscosity average molecular weight (Mv) of 700,000, 46.5% by mass of homopolymer PE with a Mv of 250,000, and 7% by mass of homopolymer polypropylene (PP) with a Mv of 400,000, as shown in Table 1. One part by mass of pentaerythrityl tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as an antioxidant was added to 99 parts by mass of the polymer mixture, and the mixture was dry-blended again using the tumbler blender to obtain a polymer mixture. The resulting polymer mixture was then purged with nitrogen and fed to a twin-screw extruder using a feeder under a nitrogen atmosphere. Liquid paraffin (kinematic viscosity at 37.78°C: 7.59 x 10) was also used. -5 m 2 / s) was injected into the extruder cylinder by a plunger pump.

[0183] The mixture was melt-kneaded, and the feeder and pump were adjusted so that the ratio of liquid paraffin to the total mixture extruded was 68% by mass (resin composition concentration: 32% by mass). The melt-kneading conditions were a set temperature of 200°C, a screw rotation speed of 70 rpm, and a discharge rate of 145 kg / h.

[0184] Subsequently, the melt-kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled to 25°C, and cast to obtain a gel sheet having a thickness of 1350 µm.

[0185] The gel sheet was then introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched. The stretching conditions were an MD magnification of 7.0, a TD magnification of 6.38, and a set temperature of 122°C. The gel sheet was then introduced into a methylene chloride bath and thoroughly immersed in methylene chloride to extract and remove the liquid paraffin. The methylene chloride was then dried and removed to obtain a porous body.

[0186] The porous body was then introduced into a TD tenter and heat-set at a heat setting temperature of 132°C, a TD maximum magnification of 1.85, and a relaxation rate of 0.784 to obtain a 12 µm-thick microporous polyolefin membrane.

[0187] Next, as shown in Table 1, aluminum hydroxide oxide (boehmite, block-shaped, D 50 94.6 parts by mass of inorganic particles (D = 0.3 μm) were mixed with 100 parts by mass of water and 0.5 parts by mass of an aqueous solution of ammonium polycarboxylate (in terms of solid content), and the mixture was subjected to a bead mill treatment. The bead mill treatment was carried out under the conditions of a bead diameter of 0.1 mm and a mill rotation speed of 2000 rpm. To the mixture after the treatment, 0.2 parts by mass of xanthan gum (in terms of solid content) and 4.7 parts by mass of acrylic latex (solid content concentration 40%) were added as a thickener to prepare a coating solution. The particle diameter of the inorganic particles in the coating solution at this time was D 10 =0.13μm, D 50 = 0.25 μm, D 90 The viscosity of the coating solution was 130 mPa·sec.

[0188] After corona discharge treatment was performed on the surface of the polyolefin microporous membrane, a coating solution was applied to the treated surface using a gravure coater.The coating solution on the polyolefin microporous membrane was then dried at 60°C to remove water, and a porous layer with a coating thickness of 3.0 μm was formed on one side of the polyolefin microporous membrane to obtain a multilayer porous membrane.The membrane properties of the obtained multilayer porous membrane and the evaluation results of a battery equipped with the multilayer porous membrane as a separator are also shown in Table 1.

[0189] [Examples 2 to 13 and Comparative Examples 1 to 6] A multilayer porous membrane was formed in the same manner as in Example 1, except that the raw material composition and physical properties of the polyolefin microporous membrane, and the raw material type and coating conditions of the porous layer were set as shown in any one of Tables 1 to 3. The various properties of the obtained multilayer porous membrane and a battery including the same as a separator were evaluated by the above-mentioned methods. The evaluation results are shown in Tables 1 to 3.

[0190] [Example 14] The raw material composition of the polyolefin microporous membrane was 70% by mass of polyethylene (PE) homopolymer with an Mv of 1 million and 30% by mass of PE homopolymer with an Mv of 250,000. The porous membrane was produced in the same manner as in Example 4, except that the extrusion rate was 130 kg / h, the gel sheet thickness was 1200 μm, the simultaneous biaxial stretching temperature was 117°C, the heat setting temperature in the TD tenter was 135°C, and the maximum TD magnification was 1.90. The properties of the obtained multilayer porous membrane and the battery using it as a separator were evaluated by the above-mentioned methods. The evaluation results are shown in Table 2.

[0191] [Example 15] A multilayer porous membrane was formed in the same manner as in Example 4, except that the polyolefin microporous membrane was produced at a discharge rate of 125 kg / h, a gel sheet thickness of 1250 μm, a simultaneous biaxial stretching temperature of 120°C, a heat setting temperature in the TD tenter of 130°C, a TD maximum stretching ratio of 1.50, and a porous layer thickness of 1.0 μm. The various properties of the obtained multilayer porous membrane and a battery including the same as a separator were evaluated by the above-mentioned methods. The evaluation results are shown in Table 2.

[0192] [Table 1]

[0193] [Table 2]

[0194] [Table 3]

[0195] Test Series II (Related to Embodiment 2)

[0196] Testing and Evaluation Methods <400℃ solder test> The following measuring equipment and instruments were used: (1) High-power, compact, temperature-controlled soldering iron Hakko Corporation Model FM-202 (discontinued: current FM-203 can be used instead) Soldering iron part Model number FM-2027 Tip Model Number T7-C1 Cleaning sponge Model number A1519 7 is a schematic diagram showing the shape of a soldering iron. The tip of the soldering iron (20) is cylindrical with a diameter of 1 mm, and the tip is cut obliquely at an angle of 60° to the central axis of the cylinder. (2) Stage Automatic linear X-axis stage manufactured by Suruga Seiki Co., Ltd. X-axis linear ball guide Model number PG430-L05AG (C / N 120300125) Equipped with a unit motor manufactured by Oriental Motor Co., Ltd. Figure 8 shows the appearance of the stage. The stage (30) has a sample stage (31) whose height can be adjusted and fixed. The stage has a soldering iron holder (32) at the center top of the sample stage, which holds the soldering iron (20) facing vertically downward. (3) AC100V stepping motor controller Model number DS102 manufactured by Suruga Seiki Co., Ltd. The controller is equipped with a handheld terminal, model number DT100, which can operate the soldering iron holder on the stage up and down. The controller is programmed to lower the soldering iron holder (i.e., the soldering iron) 30 mm at a speed of 10 mm / s, hold it at the lowest point for 3 seconds, and then raise it back to its original position at a speed of 10 mm / s. (4) Sample holder Two metal frames of the following sizes were used as sample holders: Metal frame dimensions: 50mm x 60mm Inner dimensions: 30mm x 40mm The sample holder can be placed and fixed on the sample table of the stage.

[0197] The 400°C soldering test is performed according to the following procedure. The test was carried out under the conditions of a measurement environment of 25°C ± 5°C, a relative humidity of 40 ± 10%, and a location that was not affected by wind. Using tweezers, wipe the tip of the soldering iron with a Kimwipe or cotton swab soaked in ethanol. Connect the soldering iron to the heating device and heat it to 400°C. Once it reaches 400°C, leave it for at least 90 seconds until the temperature stabilizes. The multilayer porous membrane is cut out to fit the outer dimensions of the sample holder, and is sandwiched between two sample holders so as not to wrinkle the sample, and the four corners are fixed with clips (not shown). Turn on the controller and operate the soldering iron holder with the handheld terminal to raise the tip of the soldering iron to its highest position. The height of the sample stage is adjusted so that when the soldering iron is lowered to its lowest point, it penetrates the multilayer porous film to a position 5 mm from the tip of the soldering iron. The multilayer porous membrane held by the sample holder is placed and fixed in the center of the sample stage. Figure 9 is a schematic diagram showing the state before the soldering iron is inserted into the multilayer porous film. The figure is a schematic diagram and not to scale, but the distance from the multilayer porous film to the tip of the soldering iron is 25 mm. Operate the handheld terminal and execute the pre-programmed soldering iron operation: descend 30 mm at a speed of 10 mm / sec, hold at the lowest point for 3 seconds, then rise again to the original position at a speed of 10 mm / sec. FIG. 10 is a schematic diagram showing a state in which a soldering iron is pierced into a multilayer porous film. Although the diagram is a schematic diagram and not to scale, when the soldering iron is at its lowest point, it pierces the multilayer porous film to a position 5 mm from the tip of the soldering iron. A hole (11) is formed in the multilayer porous film by the soldering iron and its heat. In addition, discolored areas (12) may form around the hole due to deformation of the pores caused by the heat of the soldering iron. After performing the above soldering iron stroke once, the sample is removed from the sample stage and allowed to cool to room temperature.

[0198] (Image processing method) (1) Importing images The sample after the 400°C soldering test is scanned using the scanner function of the "RICOH MP C5503" (manufactured by Ricoh Co., Ltd.). When doing so, the sample is placed directly on the document glass, taking care not to fold or wrinkle the sample, and a metal ruler is placed next to the sample so that the scale can be seen. Black construction paper, "Recycled Construction Paper Fresh Color C-55" (manufactured by Daio Paper Co., Ltd.), is placed on top of the sample as a background, the document cover is closed, and the scan is performed under the following scanning conditions: "Full color: text and photos," "600 dpi," and the file format is "JPEG," obtaining an electronic image of the sample.

[0199] (2) Calculation of area The resulting electronic image of the sample is used to calculate the hole area (S) from the 400°C soldering test. The area S is calculated using the image processing software "ImageJ" (ver. 1.50i) in the following manner.

[0200] Click "Open" from "File," select the electronic image of the target sample, open the file, and use the line selection tool "Straight" to measure the distance between the markings of the metal ruler in the image as the known distance "Known Distance." Open "Set Scale" from "Analyze," and the distance (number of pixels) selected with the line selection tool will be displayed in "Distance in pixels," so enter "Known Distance" and "Unit of length" to set the scale. Next, use the rectangular selection tool "Rectangular" to draw a 4.5mm square, drag the square to a position inside the square where the area S fits, and click "Crop" under "Image" to extract the image of the selected area. To draw a 4.5mm square, for example, click "Plugins," "Macros," and "Record" to open the Recorder window, enter "makeRectangle(0, 0, X, X);," click "Create," and create a macro that draws a rectangle selection tool of the specified size. This makes the process easier. X in the above input formula is the number of pixels calculated from the scale setting mentioned above that corresponds to 4.5mm in the image. If the number of pixels is a decimal, enter the integer value obtained by rounding to the nearest whole number.

[0201] Next, the extracted image is binarized. Click "Image" and then "Type" to convert the image to 8-bit, then click "Image," "Adjust," and "Threshold" to set the threshold. When calculating the area S, leave the algorithm at default, set the lower threshold to 0, and determine the upper threshold using the method shown below. In the brightness histogram displayed by the above operation, when the gradation is increased by 1 from the peak top on the side closest to 0 (closest to black), the point at which the cumulative percentage change displayed at the bottom of the brightness histogram first becomes 0.3% or less is set as the upper limit of the threshold for the hole area (S). If it is difficult to identify the peaks in the above steps, click "Histogram" from "Analyze" to display a separate histogram, and click "Log" to change the display as necessary to help with peak detection. After determining the upper limit of the area (S) threshold in this way, click "Apply" to obtain each binarized image. In addition, when the gradation is decreased by one from the peak top of the peak closest to 255 (closest to white) in the brightness histogram, the point at which the cumulative percentage change displayed at the bottom of the brightness histogram first becomes 0.1% or less can be used as the upper limit value of the threshold for the total area of ​​the hole area and discolored areas. Depending on the state of the sample, two peaks may be consecutive and it may not be possible to select an appropriate threshold using the above method. In such cases, the lowest point of the valley of each peak is used as the threshold.

[0202] Finally, the hole area S is calculated from the obtained binarized image by the following operation. Select "Analyze" > "Set Measurements...", check the "Area", "Shape descriptors", and "Fit ellipse" boxes, press "OK", then select "Analyze" > "Analyze Particles...". Enter "1" in the "Size (mm^2)" column, select "Outlines" in the "Show" column, check the boxes for "Display results," "Clear results," "Exclude on edges," and "Include holes," and click "OK" to obtain calculation results for area S, aspect ratio, etc. The area will be displayed in the "Area" column of the analysis results, and the calculated aspect ratio will be displayed in the "AR" column of the analysis results.

[0203] <Viscosity average molecular weight (Mv)> The intrinsic viscosity [η] (dl / g) at 135°C in decalin solvent was determined based on ASTM-D4020. For the Mv of the polyethylene and polyolefin microporous membranes, it was calculated by the following formula. [η]=6.77×10 -4 Mv 0.67 For the Mv of the polypropylene, it was calculated by the following formula. [η]=1.10×10 -4 Mv 0.80

[0204] <Melt Index (MI) (g / 10 min) of the polyolefin microporous membrane> The MI of the sample was measured in the same manner as in Test Series I.

[0205] <Film thickness (μm)> Using the micro-thickness gauge "KBM (trademark)" manufactured by Toyo Seiki Co., Ltd., the thickness of the polyolefin microporous membrane or the multi-layer porous layer and the membrane coated with only the first porous layer was measured at room temperature (23 ± 2°C), and the coating thickness of the first porous layer and the second porous layer from those thicknesses was calculated. Also, from the perspective of detecting products of other companies, it is also possible to measure the thickness of each layer using a cross-sectional SEM image.

[0206] <Porosity (%)> A 10 cm × 10 cm square sample was cut from the microporous membrane, and its volume (cm 3 ) and mass (g) were determined. From these and the membrane density (g / cm 3 ), the porosity was calculated using the following formula. Porosity (%) = (Volume - Mass / Density) / Volume × 100

[0207] <Air permeability (seconds / 100 cm 3 )> The air permeability of the sample was measured in the same manner as in Test Series I.

[0208] <Puncture strength (gf) and puncture strength in terms of basis weight (gf / (g / m 2 ))> The puncture strength of the sample was measured in the same manner as in Test Series I, and the puncture strength in terms of basis weight was also calculated.

[0209] <Maximum shrinkage stress of TMA (gf)> The shrinkage stress of the sample was measured using a Shimadzu TMA50 (trademark). When measuring values ​​in the MD (TD) direction, a sample cut to a width of 3 mm in the TD (MD) direction was fixed to a chuck with a 10 mm inter-chuck distance and set on a dedicated probe. The initial load was 1.0 g, and the fixed length measurement mode was used. The sample was heated from 30°C to 200°C at a heating rate of 10°C / min, and the load (gf) generated at that time was measured. Measurements were performed in both the MD and TD, and the larger of the maximum load values ​​was taken as the maximum thermal shrinkage stress (gf).

[0210] <Average particle size and particle size distribution of inorganic particles> As in Test Series I, the particle size distribution of the samples, D 50 and D 90 was measured

[0211] <Heat shrinkage rate (%) at 150°C> The multilayer porous membrane was cut into a sample of 100 mm in the MD direction and 100 mm in the TD direction, and left to stand in an oven at 150°C for 1 hour. At this time, the sample was sandwiched between two pieces of paper to prevent the hot air from directly hitting the sample. After removing the sample from the oven and cooling, the length (mm) was measured and the thermal shrinkage was calculated using the following formula. Measurements were performed in the MD direction and the TD direction, and the larger value was taken as the thermal shrinkage. Heat shrinkage rate (%) = {(100 - length after heating) / 100} x 100

[0212] <Nail penetration test> The nail penetration test was carried out in the same manner as in Test Series I, and the test results were evaluated.

[0213] <Impact test> FIG. 11 is a schematic diagram of the impact test. In the impact test, a round bar (φ=15.8 mm) is placed on top of a sample placed on a test bench so that the sample and the round bar are roughly perpendicular, and an 18.2 kg weight is dropped onto the top of the round bar from a position 61 cm above the round bar to observe the effect of the impact on the sample. The procedure for the impact test is described below with reference to FIG. The laminated lithium-ion secondary batteries assembled and selected for evaluation in the above <Nail Penetration Test> were charged at a constant current and constant voltage (CCCV) for 3 hours at a current of 3000 mA (1.0 C) and a final battery voltage of 4.2 V. Next, in a 25°C environment, the battery was placed horizontally on a flat surface, and a 15.8 mm diameter stainless steel rod was placed across the center of the battery. The rod was positioned so that its long axis was parallel to the longitudinal direction of the separator. An 18.2 kg weight was dropped from a height of 61 cm from the rod positioned in the center of the battery, so that the impact was perpendicular to the longitudinal axis of the battery. After the impact, the surface temperature of the battery was measured. Tests were conducted on five cells at a time, and the results were evaluated according to the following criteria. For this evaluation item, A (good) and B (acceptable) were used as the pass criteria. The surface temperature of the battery was measured using a thermocouple (K-type seal type) at a position 1 cm from the bottom of the battery's exterior. A: For all cells, the surface temperature rise is less than 30°C. B: There are cells whose surface temperatures are above 30°C and below 100°C, but the surface temperatures of all cells are below 100°C. C: The surface temperature of one or more cells exceeds 100°C. D: One or more cells fire.

[0214] <Rate characteristics and cycle testing> The rate characteristic evaluation test and cycle test were carried out in the same manner as in Test Series I, except that the evaluation criteria for the cycle test were changed as follows. (Test Series II cycle performance evaluation criteria) A: 70% or more capacity retention rate B: Capacity retention rate of 65% or more but less than 70% C: Capacity retention rate of 60% or more but less than 65% D: Capacity retention rate of less than 60%

[0215] [Example II-1] Using a tumbler blender, a polymer mixture containing 93% polyethylene (PE) and 7% polypropylene (PP) was prepared, as shown in Table 4. One part by mass of pentaerythrityl tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant to 99 parts by mass of the polymer mixture, and the mixture was dry-blended again using the tumbler blender to obtain a polymer mixture. The resulting polymer mixture was purged with nitrogen and then fed into a twin-screw extruder using a feeder under a nitrogen atmosphere. Liquid paraffin (kinematic viscosity at 37.78°C: 7.59 x 10) was also used. -5 m 2 / s) was injected into the extruder cylinder by a plunger pump.

[0216] The feeder and pump were adjusted so that the ratio of liquid paraffin to the total mixture extruded after melt-kneading was 62% by mass (resin composition concentration: 38% by mass). The melt-kneading conditions were a set temperature of 200°C, a screw rotation speed of 100 rpm, and a discharge rate of 230 kg / h.

[0217] Subsequently, the melt-kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled to 25°C, and cast to obtain a gel sheet having a thickness of 1700 µm.

[0218] The gel sheet was then introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched. The stretching conditions were an MD magnification of 7.0, a TD magnification of 6.38, and a set temperature of 123°C. The gel sheet was then introduced into a methylene chloride bath and thoroughly immersed in methylene chloride to extract and remove the liquid paraffin. The methylene chloride was then dried and removed to obtain a porous body.

[0219] The porous body was then introduced into a TD tenter and heat-set at a heat setting temperature of 125°C, a TD maximum magnification of 1.47 times, and a relaxation rate of 0.864 to obtain a 12 µm-thick microporous polyolefin membrane.

[0220] The surface of the polyolefin microporous membrane was subjected to corona discharge treatment. As shown in Table 4, aluminum hydroxide oxide (boehmite, block-shaped, D 50 = 0.25 μm, D 90 A coating solution was prepared by uniformly dispersing 95.0 parts by mass of a polyolefin microporous membrane (0.49 μm), 4.0 parts by mass of an acrylic latex (solids concentration 40%, average particle size 145 nm) in terms of solids, and 1.0 part by mass of an aqueous solution of ammonium polycarboxylate (SN Dispersant 5468, manufactured by San Nopco) in terms of solids in 100 parts by mass of water. The coating solution was applied to the treated surface of the polyolefin microporous membrane using a gravure coater, and then dried at 60°C to remove water. A first porous layer with a coating thickness of 3 μm and a porosity of 50% was formed on one side of the polyolefin microporous membrane. The other side of the polyolefin microporous membrane was similarly corona-treated to form a second porous layer with a coating thickness of 1 μm and a porosity of 50%, to obtain a multilayer porous membrane. The resulting multilayer porous membrane had a total thickness of 16.0 μm and an air permeability of 170 sec / 100 cm. 3 The heat shrinkage rate at 150° C. was 1.5%. Table 4 also shows the evaluation results of a battery equipped with the multilayer porous membrane as a separator.

[0221] [Example II-2~ II-20, Reference Example II-21, Example II-22 II-27 and Comparative Examples II-1 to II-3] A multilayer porous membrane was formed in the same manner as in Example II-1, except that the raw material composition and physical properties of the polyolefin microporous membrane and the raw material types and coating conditions of the first or second porous layer were set as shown in Tables 4 to 9. In Comparative Examples II-1 and II-3, a porous layer was disposed on only one side of the PO microporous membrane. The various properties of the obtained multilayer porous membrane and batteries including the same as a separator were evaluated by the above-mentioned methods. The evaluation results are shown in Tables 4 to 9.

[0222] [Example II-28] An adhesive resin-containing coating solution (adhesive resin concentration 3% by mass) was prepared by mixing 80 parts by mass of an adhesive resin (acrylic polymer, glass transition temperature 90°C, average particle size 380 nm, electrolyte swelling index 2.8) with 20 parts by mass of an adhesive resin having a different glass transition temperature (acrylic polymer, glass transition temperature -6°C, average particle size 132 nm, electrolyte swelling index 2.5), and adding ion-exchanged water. This was applied in the form of dots using a gravure coater to the surface of the polyolefin microporous membrane having the porous layer described in Example II-1. The film was then dried at 60°C to remove water. The coating solution was then applied to the other side in the same manner, followed by drying, to obtain a coating solution with an adhesive resin weight of 0.2 g / m. 2 A separator was obtained having an adhesive layer with a surface coverage of 30%, an average major axis of the dots of 50 μm, and a thickness of 0.5 μm. The evaluation results are also shown in Table 9.

[0223] [Table 4]

[0224] [Table 5]

[0225] [Table 6]

[0226] [Table 7]

[0227] [Table 8]

[0228] [Table 9]

[0229] Test Series III (Related to Embodiment 3) [Examples III-1 to III-4] Multilayer porous membranes were formed in the same manner as in Test Series II, except that the raw material composition and physical properties of the polyolefin microporous membrane and the raw material types and coating conditions of the first or second porous layer were set as shown in Table 10. The formed polyolefin microporous membrane, porous layer, and multilayer porous membrane were measured and evaluated in the same manner as in Test Series I, a 400°C solder test of the multilayer porous membrane was performed and evaluated in the same manner as in Test Series II, and various properties of batteries comprising the multilayer porous membrane as a separator were measured and evaluated in the same manner as in Test Series II.

[0230] [Table 10] [Explanation of symbols]

[0231] 10 Multilayer porous membrane 11 holes 12 Discolored area 20 Soldering iron 30 stages 31 Sample stand 32 Soldering iron holder 40 sample holders

Claims

1. A porous membrane containing a polyolefin resin as a main component; a first porous layer disposed on one surface of the porous membrane, the first porous layer including inorganic particles and a binder polymer; a second porous layer disposed on the other surface of the porous membrane, the second porous layer including inorganic particles and a binder polymer; A multilayer porous membrane comprising: The puncture strength of the porous membrane converted into basis weight is 60 gf / (g / m ２ ) and above, the total thickness of the first porous layer and the second porous layer is 0.2 μm or more and 5 μm or less; The air permeability of the multilayer porous membrane is 10 sec / 100 cm ３ Over 1000sec / 100cm ３ is as follows: The heat shrinkage rate of the multilayer porous membrane at 150°C is less than 10.0%, and In a 400°C soldering test in which a soldering iron having a diameter of 1 mm and a temperature of 400°C is pierced into the multilayer porous film, the soldering iron is held in the pierced state for 3 seconds, and then removed, the area of ​​the hole formed in the multilayer porous film is 10.0 mm2 or less regardless of whether the soldering iron is inserted from the first porous layer side or the second porous layer side. ２ A multilayer porous membrane as follows:

2. The multilayer porous film according to claim 1, wherein in a soldering test of the multilayer porous film at 400°C, the area ratio of holes formed in the multilayer porous film when the soldering iron is inserted from each of the first porous layer side and the second porous layer side is within a range of 0.8 to 1.

2.

3. The multilayer porous membrane according to claim 1 or 2, wherein the thickness of either the first porous layer or the second porous layer is 1.5 μm or less.

4. D of the inorganic particles constituting the first porous layer and the second porous layer ９０ The multilayer porous membrane according to any one of claims 1 to 3, wherein the thickness is 1.5 µm or less.

5. The multilayer porous membrane according to any one of claims 1 to 4, wherein the melt index (MI) of the porous membrane at 190 ° C. is 0.02 g / 10 min to 0.5 g / 10 min.

6. The multilayer porous membrane according to any one of claims 1 to 5, wherein the viscosity average molecular weight of the porous membrane is 400,000 or more and 1,300,000 or less.

7. The multilayer porous membrane according to any one of claims 1 to 6, wherein the porous membrane contains polypropylene as the polyolefin resin.

8. In a soldering test of the multilayer porous film at 400°C, the area of ​​the hole formed in the multilayer porous film was 1.0 mm or less regardless of whether the soldering iron was inserted from the first porous layer side or the second porous layer side. ２ The multilayer porous membrane according to any one of claims 1 to 7, wherein the thickness of the multilayer porous membrane exceeds 100 nm.

9. The area of ​​each hole in the porous layer is 0.001 μm ２ The number of holes that are 10 μm or more ２ The number of particles per field of view is 65 to 180, and the area of ​​the porous layer is 0.001 μm ２ Among the holes with an area of ​​0.001 μm or more, ２ ~0.05μm ２ The multilayer porous membrane according to any one of claims 1 to 8, wherein the proportion of pores in the range is 90% or more.

10. A lithium ion secondary battery in which a zigzag folded body of the multilayer porous membrane according to any one of claims 1 to 9 is housed in an outer casing, and positive electrodes and negative electrodes are alternately inserted into the gaps of the zigzag folded body.

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