Multilayer porous film

Abstract

To provide a multilayer porous film which is a thin film, has high ion permeability, suppresses a meltdown phenomenon at a high temperature, and has sufficient mechanical strength.SOLUTION: The multilayer porous film is obtained by laminating an inorganic porous layer containing inorganic particles and a resin binder on at least one surface of a polyolefin porous substrate, wherein the polyolefin resin contains high molecular weight polyethylene having a viscosity average molecular weight of 750,000 or more and 5,000,000 or less. The inorganic particles have a mean particle size of 50nm or more and 500nm or less, the multilayer porous film has a thickness of 3 μm or more and 13 μm or less, the multilayer porous film has an air permeability of 30 seconds / 100ml or more and 150 seconds / 100ml or less, the multilayer porous film has a film resistance value at 200 °C. of 2000 Ω·cm2 or more and 100,000 Ω·cm2 or less, and the multilayer porous film has a pin puncture strength in terms of basis weight of 0. 64N / (g / m2) or more and 1. 23N / (g / ) or less. m2.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present invention relates to a membrane used for separating or purifying various substances, and to a multilayer porous membrane suitable for use as a separator disposed between a positive electrode and a negative electrode in a battery. [Background technology]

[0002] In recent years, lithium-ion secondary batteries, which have excellent energy density and input / output characteristics and a long life, have been used as power sources for vehicles such as electric vehicles. While lithium-ion secondary batteries have the above-mentioned excellent characteristics, they can sometimes experience thermal runaway and ignition when abnormal heat is generated in the battery due to a short circuit or other cause.

[0003] To prevent such a situation, various technologies for improving the safety of lithium-ion secondary batteries are used. Among these safety technologies, a separator-related technology is a method of using a multilayer porous membrane as a separator, in which a layer mainly composed of inorganic particles (hereinafter referred to as an inorganic porous layer) is provided on a polyolefin microporous membrane (hereinafter referred to as a polyolefin porous substrate).

[0004] In recent years, automotive lithium-ion secondary batteries have been required to achieve high levels of multiple characteristics, such as high energy density, high safety, and compatibility with fast charging. However, increasing the energy density of a battery generally involves trade-offs between characteristics, such as a loss of safety and difficulty in compatibility with fast charging, making it no easy task to achieve the multiple characteristics mentioned above at high levels.

[0005] Against this background, separators suitable for automotive lithium-ion secondary batteries have been extensively studied. To increase the energy density of batteries, one method is to thin the separator, thereby increasing the volume ratio of the positive and negative electrodes within the battery. As shown in Figure 12 of Non-Patent Document 1, the thickness of polyolefin porous substrates has been reduced to as little as 5 μm.

[0006] Furthermore, from the viewpoint of improving the input / output characteristics of a battery, there is a method of increasing the ion permeability of the separator by increasing the porosity of the separator. In the examples of Patent Document 1, the output characteristics of the battery are improved by adjusting the pore structure of the polyolefin porous substrate and increasing the porosity to 57% or more. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] "Development Trends of Separators for Lithium-ion Secondary Batteries," by Takeshi Ishihara, Toray Research Center, The TRC News, 201705-1 (May 2017) [Patent documents]

[0008] [Patent Document 1] International Publication No. 2013 / 147071 Summary of the Invention [Problem to be solved by the invention]

[0009] However, if the amount of resin contained in the separator is reduced by thinning the separator or increasing its porosity, it is expected that the shutdown mechanism, which causes the polyolefin porous substrate to melt when the battery abnormally heats up and block the micropores, thereby eliminating the ion permeability of the separator, will become less effective. It is also expected that the meltdown phenomenon, in which the insulation is destroyed after the separator shuts down, will become more likely to occur. In other words, thinning the separator or increasing its porosity acts in a way that impairs the separator's function of maintaining battery safety, so there is a demand for separators that can maintain battery safety even with thinner membranes and less resin.

[0010] In addition, if the amount of resin contained in the separator is reduced, it becomes difficult to ensure the mechanical strength of the separator. The polyolefin porous substrate described in Example 1 of Patent Document 1 has a film thickness of 17 μm, and therefore has a practically sufficient mechanical strength of 385 gf (3.78 N) in puncture strength. However, if the film thickness of the polyolefin porous substrate described in Example 1 of Patent Document 1 is halved to 8.5 μm, the puncture strength is expected to be approximately 190 gf (approximately 1.89 N), and therefore the mechanical strength is insufficient when the film is made thinner. The reason for this is that when the puncture strength per amount of polyolefin resin contained in the polyolefin porous substrate described in Example 1 of Patent Document 1 (hereinafter referred to as the puncture strength converted into basis weight in this specification) is calculated, it is 0.59 N / (g / m 2 ) and it is thought that the reason for this is that the strength converted into basis weight is insufficient.

[0011] In view of the above circumstances, an object of the present invention is to provide a multilayer porous membrane which is thin and has high ion permeability, while suppressing the meltdown phenomenon at high temperatures and having sufficient mechanical strength. [Means for solving the problem]

[0012] As a result of intensive research into achieving the above object, the present inventors have found that the above problems can be solved by a multilayer porous membrane having a structure in which an inorganic porous layer containing specific inorganic particles and a resin binder is laminated on at least one side of a polyolefin porous substrate containing a specific polyethylene, and which has specific membrane thickness, air permeability, membrane resistance at high temperatures, and pin puncture strength converted into basis weight. That is, the present invention is as follows. (1) A multilayer porous membrane in which an inorganic porous layer containing inorganic particles and a resin binder is laminated on at least one side of a polyolefin porous substrate containing a polyolefin resin as a main component, wherein the polyolefin resin contains high-molecular-weight polyethylene having a viscosity-average molecular weight of 750,000 to 5,000,000, the inorganic particles have an average particle size of 50 nm to 500 nm, the multilayer porous membrane has a thickness of 3 μm to 13 μm, the multilayer porous membrane has an air permeability of 30 sec / 100 ml to 150 sec / 100 ml, and the multilayer porous membrane has a membrane resistance at 200°C of 2,000 Ω cm 2 More than 100,000Ω cm 2 The multilayer porous membrane has a puncture strength converted into basis weight of 0.64 N / (g / m 2 ) or more 1.23N / (g / m 2 ) or less. (2) The multilayer porous membrane according to item 1, wherein the high-molecular-weight polyethylene is contained in the polyolefin porous substrate in an amount of 50% by mass or more and 100% by mass or less, when the entire polyolefin porous substrate is taken as 100% by mass. (3) The multilayer porous membrane according to item 1 or 2, wherein the polyolefin resin contains high-density polyethylene. (4) The multilayer porous membrane according to any one of items 1 to 3, wherein the polyolefin resin contains polypropylene. (5) The multilayer porous membrane according to any one of items 1 to 4, wherein the polyolefin porous substrate contains 1% by mass or more and 10% by mass or less of polypropylene when the entire polyolefin porous substrate is taken as 100% by mass. (6) The multilayer porous film according to any one of items 1 to 5, wherein the inorganic particles contain at least one selected from the group consisting of alumina, boehmite, magnesium oxide, and barium sulfate. (7) The multilayer porous membrane according to any one of items 1 to 6, wherein the resin binder contains a water-soluble polymer having a glass transition temperature (Tg) of 25°C or higher and a water-soluble polymer or polymer latex having a Tg of less than 25°C. (8) The multilayer porous membrane according to item 7, wherein the water-soluble polymer having a Tg of 25°C or higher contains one or more selected from polyacrylamide, poly-N-vinylacetamide, and polyvinylpyrrolidone. (9) The multilayer porous membrane according to any one of items 1 to 8, wherein the inorganic porous layer contains the resin binder in an amount of 1% by mass to 10% by mass, where the entire inorganic porous layer is taken as 100% by mass. [Effects of the Invention]

[0013] According to the present invention, when a multilayer porous membrane which is thin and has high ion permeability, which suppresses the meltdown phenomenon at high temperatures, and which has sufficient mechanical strength is used as a separator for a lithium ion secondary battery, it is possible to improve the energy density and input / output characteristics of the battery, as well as to improve the safety of the battery, and it is possible to provide a multilayer porous membrane which has sufficient mechanical strength as a separator and is easy to handle. [Brief explanation of the drawings]

[0014] [Figure 1A] FIG. 2 is a schematic diagram of a measuring device for film resistance values. [Figure 1B] FIG. 1B is a schematic diagram illustrating the overlapping of a plurality of members in the measurement device of FIG. 1A. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments for carrying out the present invention (hereinafter abbreviated as "present embodiments") will be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention. 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.

[0016] 《Multilayer porous membrane》 The multilayer porous membrane according to this embodiment is a multilayer porous membrane in which an inorganic porous layer containing inorganic particles and a resin binder is laminated on at least one surface of a polyolefin porous substrate containing a polyolefin resin as a main component, wherein the polyolefin resin contains high-molecular-weight polyethylene having a viscosity-average molecular weight of 750,000 or more and 5,000,000 or less, the inorganic particles have an average particle size of 50 nm or more and 500 nm or less, the multilayer porous membrane has a thickness of 3 μm or more and 13 μm or less, the air permeability of the multilayer porous membrane is 30 sec / 100 ml or more and 150 sec / 100 ml or less, and the multilayer porous membrane has a membrane resistance at 200°C of 2,000 Ω cm 2 More than 100,000Ω cm 2 The multilayer porous membrane has a puncture strength converted into basis weight of 0.64 N / (g / m 2 ) or more 1.23N / (g / m 2 ) is as follows.

[0017] The thickness of the multilayer porous membrane in this embodiment is adjusted to 3 to 13 μm. There are no particular limitations on the method as long as it can accurately measure the membrane thickness, and examples include a method using a micro-thickness gauge and a method of measuring the length by observing the cross section of the multilayer porous membrane with a scanning electron microscope. The membrane thickness of the multilayer porous membrane is 3 μm or more, preferably 4 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. When the membrane thickness is 3 μm or more, sufficient mechanical strength can be imparted to the multilayer porous membrane, making it easy to handle and improving the voltage resistance of the multilayer porous membrane.

[0018] The thickness of the multilayer porous membrane is 13 μm or less, preferably 12 μm or less, more preferably 11 μm or less, and even more preferably 10 μm or less. When the thickness of the multilayer porous membrane is 13 μm or less, it is possible to reduce the volume ratio of the separator in the internal volume of the battery, and the volume ratio of the electrodes is relatively increased, thereby increasing the energy density of the battery.

[0019] The air permeability of the multilayer porous membrane in this embodiment is measured in accordance with JIS P-8117:2009. The air permeability of the multilayer porous membrane is 30 seconds / 100 ml or more, preferably 40 seconds / 100 ml or more, more preferably 50 seconds / 100 ml or more, and even more preferably 60 seconds / 100 ml or more. When the multilayer porous membrane has an air permeability of 30 seconds / 100 ml or more, it becomes easier to impart a shutdown mechanism to the multilayer porous membrane, and meltdown of the multilayer porous membrane at high temperatures is suppressed, thereby improving the safety of the battery.

[0020] The air permeability of the multilayer porous membrane is 150 seconds / 100 ml or less, preferably 140 seconds / 100 ml or less, more preferably 130 seconds / 100 ml or less, and even more preferably 120 seconds / 100 ml or less. When the air permeability of the multilayer porous membrane is 150 seconds / 100 ml or less, the ion permeability of the multilayer porous membrane is improved, and sufficient input / output characteristics can be imparted to the battery.

[0021] The membrane resistance value (Ω cm) of the multilayer porous membrane in this embodiment at 200°C 2 ) is measured by the method described below. Figure 1A shows a schematic diagram of the measurement device. 1 is a multilayer porous membrane, 2A and 2B are a positive electrode and a negative electrode prepared by the method described in the Examples. 3 is a 2 cm 2 2A and 2B are connected to the current collectors 2A and 2B, and measure the resistance between 2A and 2B. 9 is a thermometer connected to the thermocouple 5 and measures the temperature. 10 is a data collector connected to the AC resistance measuring device 8 and the thermometer 9, and records the resistance and temperature.

[0022] More specifically, first, the separator 1 and the portions of the positive electrode 2A and negative electrode 2B coated with the electrode active materials are sufficiently impregnated with an electrolyte (lithium fluoroborate / propylene carbonate / ethylene carbonate / γ-butyl lactone / trioctyl phosphate = weight ratio 7.2 / 23.3 / 25.7 / 43.3 / 0.5). Then, the separator 1, positive electrode 2A, negative electrode 2B, aramid film 3, and silicone rubber 4 are stacked together as shown in FIG. 1B and placed on a ceramic plate 5.

[0023] Next, while applying a surface pressure of 4 MPa to members 1 to 4 using a hydraulic press 7, the heater 6 is heated and the temperature and resistance value are measured continuously. The temperature is raised from room temperature of 23°C to 200°C at a rate of 10°C / min, and the resistance value is measured at 1V, 1kHz AC. The obtained resistance value (Ω) is multiplied by the effective electrode area of ​​2cm. 2 Multiplying by this, the membrane resistance value (Ω cm 2 ) is calculated. 2 ) refers to the film resistance value when the temperature measured by a thermocouple embedded in the ceramic plate 5 is 200°C.

[0024] Multilayer porous membrane resistance at 200°C (Ω cm 2 ) is 2,000 Ω·cm 2 From the viewpoint of suppressing meltdown of the multilayer porous membrane at high temperatures and improving the safety of the battery, a resistance of 2,200 Ω cm 2 More than 2,400 Ω·cm is preferable. 2 More preferably, 2,600 Ω·cm 2 The above is more preferable.

[0025] Multilayer porous membrane resistance at 200°C (Ω cm 2 ) is 100,000 Ω·cm 2 From the viewpoint of reducing the thickness of the multilayer porous membrane and increasing the energy density of the battery, the resistance should be 50,000 Ω cm 2 Preferably less than 20,000 Ω·cm 2 Less than 10,000 Ω cm is preferable. 2 The following is even more preferred:

[0026] Multilayer porous membrane puncture strength converted into basis weight [N / (g / m 2 )] is the ratio of the puncture strength (N) of the multilayer porous membrane to the basis weight (g / m 2 ) and calculate it. The pin puncture strength converted into basis weight is an index showing the efficiency of manifestation of mechanical strength relative to the amount of polyolefin resin contained in the multilayer porous membrane. A high pin puncture strength converted into basis weight means that mechanical strength is manifested efficiently with a small amount of polyolefin resin.

[0027] The puncture strength (N) of the multilayer porous membrane is measured by the following method: Using a handy compression tester "KES-G5" (Kato Tech Co., Ltd.), the multilayer porous membrane is fixed with a sample holder having an opening diameter of 11.3 mm, and a puncture test is performed at room temperature of 23°C against the center of the fixed multilayer porous membrane under conditions of a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec, to measure the puncture strength (N) as the maximum puncture load.

[0028] From the viewpoint of imparting sufficient mechanical strength to the multilayer porous membrane, the pin puncture strength (N) of the multilayer porous membrane is preferably 1.96 N or more, more preferably 2.45 N or more, and even more preferably 2.94 N or more. Furthermore, from the viewpoint of mitigating the shrinkage stress of the multilayer porous membrane at high temperatures and suppressing meltdown, it is preferably 5.88 N or less, more preferably 5.39 N or less, and even more preferably 4.90 N or less.

[0029] The basis weight (g / m2) of the polyolefin resin contained in the multilayer porous membrane 2 ) is preferably 1.0 g / m from the viewpoint of suppressing meltdown of the multilayer porous membrane at high temperatures. 2 More preferably, 1.5 g / m 2 More preferably, 2.0 g / m 2 More than 2.5 g / m 2 In addition, from the viewpoint of increasing ion permeability while reducing the thickness of the multilayer porous membrane, it is preferable that the thickness is 7.0 g / m 2 Less than 6.5 g / m 2 More preferably 6.0 g / m or less2 Below 5.5 g / m, particularly preferably 2 The following is the result.

[0030] Multilayer porous membrane puncture strength converted into basis weight [N / (g / m 2 )] is 0.64N / (g / m 2 ) or more. From the viewpoint of ensuring sufficient mechanical strength while reducing the thickness of the multilayer porous membrane, the puncture strength converted into basis weight of the multilayer porous membrane is 0.69 N / (g / m 2 ) or more is preferable, and 0.74N / (g / m 2 ) or more is more preferable, and 0.78N / (g / m 2 ) or more is more preferable, and 0.83N / (g / m 2 ) or more are particularly preferred.

[0031] Multilayer porous membrane puncture strength converted into basis weight [N / (g / m 2 )] is 1.23N / (g / m 2 ) or less. The puncture strength converted into basis weight of the multilayer porous membrane is 1.18 N / (g / m) from the viewpoint of mitigating the shrinkage stress of the multilayer porous membrane at high temperatures and suppressing meltdown. 2 ) or less is preferable, and 1.13N / (g / m 2 ) or less is more preferable, and 1.08N / (g / m 2 ) The following is more preferred.

[0032] <Porous polyolefin substrate> The multilayer porous membrane according to this embodiment has a polyolefin porous substrate containing a polyolefin resin as a main component.

[0033] The thickness of the polyolefin porous substrate is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 5 μm or more. If the thickness of the polyolefin porous substrate is 3 μm or more, sufficient mechanical strength and voltage resistance can be imparted to the multilayer porous membrane. Furthermore, the thickness of the polyolefin porous substrate is preferably 12 μm or less, more preferably 11 μm or less, even more preferably 10 μm or less, and particularly preferably 9 μm or less. If the thickness of the polyolefin porous substrate is 12 μm or less, the thickness of the multilayer porous membrane can be reduced, and the energy density of the battery can be increased.

[0034] The porosity of the polyolefin porous substrate is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more from the viewpoint of increasing the ion permeability of the multilayer porous membrane, while it is preferably 65% ​​or less, more preferably 60% or less, even more preferably 55% or less, and particularly preferably 50% or less to prevent meltdown of the multilayer porous membrane at high temperatures.

[0035] From the viewpoint of achieving both ion conductivity and voltage resistance, the average pore size of the polyolefin porous substrate is preferably 10 nm to 100 nm, more preferably 20 nm to 90 nm, and even more preferably 30 nm to 80 nm. The average pore size can be adjusted by controlling the composition ratio, 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.

[0036] <<Method for producing a polyolefin porous substrate>> An example of a manufacturing method for obtaining a polyolefin porous substrate will be described below, but the method is not necessarily limited to this example. The manufacturing method of a polyolefin porous substrate may include, for example, the following steps: (1) a step of melt-kneading a polyolefin resin and a plasticizer to obtain a melt; (2) A step of transferring the molten material, forming it into a sheet, and then cooling and solidifying it. (3) stretching the sheet-shaped product at least uniaxially at an areal magnification of 20 times or more but less than 200 times; (4) A step of extracting the plasticizer from the sheet-shaped molded product after the stretching step; may also include:

[0037] In step (1), a polyolefin resin and a plasticizer are melt-kneaded. Examples of the melt-kneading method include a method in which a polyolefin resin, a plasticizer, and optionally other additives are fed into a resin kneading device such as an extruder, kneader, Labo Plastomill, kneading roll, or Banbury mixer, and the plasticizer is introduced at a desired ratio while the resin components are heated and melted, and then kneaded.

[0038] The plasticizer is not particularly limited, but it is preferable to use a non-volatile solvent that can form a homogeneous solution at or above the melting point of the polyolefin resin. Specific examples of such non-volatile solvents include 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. After extraction, these plasticizers may be recovered by distillation or other procedures and reused.

[0039] In addition, it is preferable to pre-mix the polyolefin resin and, if necessary, other additives in a predetermined ratio using a Henschel mixer or the like before adding them to the resin kneading machine. It is preferable not to add a plasticizer during pre-mixing. If a plasticizer is added, it is preferable to add only a portion of it, and the remaining plasticizer is preferably heated appropriately and side-fed into the resin kneading machine while being kneaded. Using such a kneading method increases the dispersibility of the plasticizer, which tends to enable the sheet-shaped molded product to be stretched at a high ratio without film rupture in the subsequent stretching process.

[0040] Among plasticizers, liquid paraffin is preferred because, when the polyolefin resin is polyethylene or polypropylene, it has high compatibility with these, and even when the molten kneaded product is stretched, interfacial peeling between the resin and the plasticizer is unlikely to occur, making it easier to perform uniform stretching.

[0041] The ratio of the polyolefin resin and the plasticizer is not particularly limited as long as they can be uniformly melt-kneaded and molded into a sheet. For example, the mass fraction of the plasticizer in a composition consisting of the polyolefin resin and the plasticizer is preferably 10% by mass or more and 90% by mass or less, more preferably 20% by mass or more and 80% by mass or less. When the mass fraction of the plasticizer is 90% by mass or less, the melt tension during melt molding tends to be sufficient to improve moldability. On the other hand, when the mass fraction of the plasticizer is 10% by mass or more, even when a mixture of the polyolefin resin composition and the plasticizer is stretched at a high ratio, scission of the polyolefin molecular chains does not occur, and a uniform and fine pore structure tends to be formed.

[0042] In step (2), the molten material is molded into a sheet. Examples of methods for producing a sheet-shaped molded product include extruding the molten 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 the plasticizer itself. However, metal rolls are preferred because of their high thermal conductivity. Furthermore, when the extruded kneaded material is brought into contact with the metal rolls, sandwiching it between the rolls is more preferred, as this further increases the thermal conductivity, orients the sheet, increasing film strength, and tends to improve the surface smoothness of the sheet.

[0043] When the melt-kneaded material is extruded into a sheet from a T-die, the die lip spacing is preferably 400 μm or more and 3000 μm or less, and more preferably 500 μm or more and 2500 μm or less. A die lip spacing of 400 μm or more reduces the occurrence of scum and other issues, minimizing the impact on film quality such as streaks and defects, and reducing the risk of film breakage in the subsequent stretching process. On the other hand, a die lip spacing of 3000 μm or less allows for a fast cooling rate, preventing uneven cooling, and maintaining the thickness stability of the sheet.

[0044] In the step (3), the sheet-form molded product is stretched. The stretching may be carried out after the plasticizer is extracted from the sheet-form molded product. Furthermore, the stretching may be carried out both before and after the plasticizer is extracted from the sheet-form molded product.

[0045] 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 of the resulting separator. When a sheet-shaped molded product is stretched biaxially at a high magnification, the molecules are oriented in the plane direction, and the finally obtained polyolefin porous substrate becomes less likely to tear and has high puncture strength. Examples of stretching methods include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferred from the viewpoint of improving puncture strength, uniformity of stretching, and fuse characteristics. Furthermore, sequential biaxial stretching is preferred from the viewpoint of ease of control of plane orientation.

[0046] Simultaneous biaxial stretching refers to a stretching method in which stretching in MD (the machine direction of separator continuous molding) and stretching in TD (the direction crossing the separator MD at an angle of 90°) are carried out 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 carried out independently, and while stretching is carried out in MD or TD, the other direction is in an unconstrained state or fixed at a fixed length.

[0047] The area stretching ratio is preferably in the range of 20 times or more and 200 times or less, more preferably in the range of 25 times or more and 150 times or less. The area stretching ratio in each axial direction is preferably in the range of 4 times or more and 15 times or less in MD and 4 times or more and 15 times or less in TD, more preferably in the range of 5 times or more and 12 times or less in MD and 5 times or more and 12 times or less in TD. If the total area stretching ratio is 25 times or more, the obtained polyolefin porous substrate tends to have sufficient strength, while if the total area stretching ratio is 200 times or less, breakage of the polyolefin porous substrate during the stretching step tends to be prevented and high productivity tends to be obtained.

[0048] In step (4), the plasticizer is removed from the sheet-shaped product to form a polyolefin porous substrate. For example, a method for removing the plasticizer includes immersing the sheet-shaped product in an extraction solvent to extract the plasticizer and then thoroughly drying the product. The method for extracting the plasticizer may be either a batch method or a continuous method. To prevent shrinkage of the polyolefin porous substrate, it is preferable to restrain the edges of the sheet-shaped product during the immersion and drying steps. Furthermore, it is preferable to adjust the amount of plasticizer remaining in the polyolefin porous substrate to less than 1% by mass relative to the total mass of the polyolefin porous substrate.

[0049] The extraction solvent used to extract the plasticizer is preferably a poor solvent for the polyolefin resin and a good solvent for the plasticizer, 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.

[0050] In order to suppress shrinkage of the polyolefin porous substrate, a heat treatment can be performed for the purpose of heat fixation after the stretching step or after the formation of the polyolefin porous substrate. In addition, the polyolefin porous substrate may be subjected to post-treatment such as hydrophilization treatment with a surfactant or crosslinking treatment with ionizing radiation.

[0051] The polyolefin porous substrate is preferably subjected to heat treatment for the purpose of heat setting in order to suppress shrinkage. The heat treatment method includes a stretching operation carried out in a predetermined temperature atmosphere and at a predetermined stretching rate for the purpose of adjusting physical properties, and / or a relaxation operation carried out in a predetermined temperature atmosphere and at a predetermined relaxation rate for the purpose of reducing stretching stress. The relaxation operation may be carried out after the stretching operation. These heat treatments can be carried out using a tenter or a roll stretching machine.

[0052] 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 polyolefin porous substrate with even higher strength and higher porosity. The relaxation operation is a reduction 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 and 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. The relaxation rate is preferably 0.5 or more from the viewpoint of improving the quality of the polyolefin porous substrate. The relaxation operation may be performed in both MD and TD, or may be performed in only one of MD and TD.

[0053] The stretching and relaxation operations after the plasticizer extraction are preferably carried out in TD. The temperature of the stretching and relaxation operations is preferably Tm-20°C or higher and Tm+20°C or lower, more preferably Tm-10°C or higher and Tm+10°C or lower, relative to the melting point (hereinafter also referred to as "Tm") of the polyolefin resin. The temperature of the stretching and relaxation operations within the above range is preferred from the viewpoint of the balance between reduced heat shrinkage and porosity.

[0054] <Polyolefin resin> The polyolefin resin used in this embodiment is a polyolefin resin used in ordinary extrusion, injection, inflation, and blow molding, and is a polymer containing an olefin hydrocarbon as a monomer component, such as a homopolymer, copolymer, or multistage polymer of ethylene, propylene, 1-butene, 2-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, etc. Polyolefins selected from the group consisting of these homopolymers, copolymers, and multistage polymers can also be used alone or in combination.

[0055] Specifically, the polyolefin resin is low-density polyethylene (density 0.910 g / cm 3 More than 0.930g / cm 3 less than 0.910 g / cm 3 More than 0.940g / cm 3 or less), medium density polyethylene (density 0.930 g / cm 3 More than 0.942g / cm 3 less than 0.942 g / cm 3 or more), high molecular weight polyethylene, ultra-high molecular weight polyethylene (density 0.910 g / cm 3 More than 0.970g / cm 3 hereinafter), isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene, polybutene, polymethylpentene, and ethylene propylene rubber.

[0056] The proportion of the polyolefin resin in the polyolefin porous substrate is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. When the proportion of the polyolefin resin is 70% by mass or more, the polyolefin porous substrate is easily formed, and the polyolefin porous substrate can be provided with physical strength and fuse properties. Furthermore, this proportion is preferably 100% by mass or less, more preferably 100% by mass. When the proportion of the polyolefin resin is 100% by mass or less, the mechanical strength of the polyolefin porous substrate is increased.

[0057] The viscosity average molecular weight of a polyolefin resin can be determined by measuring the intrinsic viscosity of a decalin solvent at 135°C in accordance with ASTM-D4020 and calculating from a formula appropriate for the polyolefin resin. For polyethylene, Mv can be calculated using the following formula: [η]=6.77×10 -4 Mv 0.67 For polypropylene, Mv can be calculated using the following formula: [η]=1.10×10 -4 Mv 0.80

[0058] The polyolefin resin contains high-molecular-weight polyethylene. The viscosity-average molecular weight (Mv) of the high-molecular-weight polyethylene is 750,000 or more, preferably 800,000 or more, and more preferably 850,000 or more. When the viscosity-average molecular weight (Mv) of the high-molecular-weight polyethylene is 750,000 or more, not only can the mechanical strength of the multilayer porous membrane be increased, but meltdown of the multilayer porous membrane at high temperatures can also be easily suppressed.

[0059] The viscosity average molecular weight (Mv) of the high-molecular-weight polyethylene is 5,000,000 or less, preferably 4,000,000 or less, and more preferably 3,000,000 or less. When the viscosity average molecular weight (Mv) of the high-molecular-weight polyethylene is 5,000,000 or less, the stability of the extrusion molding of the polyolefin resin is improved, and the quality of the multi-layer porous membrane can be improved.

[0060] The proportion of high-molecular-weight polyethylene in the polyolefin porous substrate is preferably 50% by mass or more, more preferably 65% ​​by mass or more, still more preferably 80% by mass or more, and may be 100% by mass. When the proportion of high-molecular-weight polyethylene is increased, not only can the mechanical strength of the multilayer porous membrane be increased, but also meltdown of the multilayer porous membrane at high temperatures can be easily suppressed.

[0061] The polyolefin resin preferably contains high-density polyethylene. The proportion of high-density polyethylene in the polyolefin porous substrate is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. When the proportion of high-density polyethylene in the polyolefin resin is 5% by mass or more, it becomes easy to adjust the fuse temperature of the multilayer porous membrane, and the safety of the battery is improved.

[0062] The polyolefin resin preferably contains polypropylene. The proportion of polypropylene in the polyolefin porous substrate is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more. The proportion of polypropylene in the polyolefin porous substrate is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, and even more preferably 10% by mass or less. Adjusting the proportion of polypropylene to 1% by mass or more is preferred from the viewpoint of suppressing meltdown of the multilayer porous membrane at high temperatures. On the other hand, from the viewpoint of improving the uniformity of the multilayer porous membrane, the proportion of polypropylene is preferably 30% by mass or less.

[0063] The polyolefin resin used in this embodiment may be mixed with additives such as phenol-based, phosphorus-based, and sulfur-based antioxidants; metal soaps such as calcium stearate and lithium stearate; ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments, as needed, within the scope of not impairing the advantages of the present invention.

[0064] 《Inorganic porous layer》 The multilayer porous membrane according to this embodiment has an inorganic porous layer containing inorganic particles and a resin binder. The inorganic porous layer is a single layer on at least one side of the polyolefin porous substrate, and may be a double layer on both sides.

[0065] The thickness of the inorganic porous layer is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. When the thickness of the inorganic porous layer is 1 μm or more, sufficient heat resistance can be imparted to the multilayer porous membrane. Furthermore, the thickness of the inorganic porous layer is preferably 6 μm or less, more preferably 5 μm or less, and even more preferably 4 μm or less. When the thickness of the inorganic porous layer is 6 μm or less, the thickness of the multilayer porous membrane can be reduced, and the energy density of the battery can be increased.

[0066] The porosity of the inorganic porous layer is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more from the viewpoint of increasing the ion permeability of the multilayer porous membrane, while it is preferably 65% ​​or less, more preferably 60% or less, even more preferably 55% or less, and particularly preferably 50% or less to prevent meltdown of the multilayer porous membrane at high temperatures.

[0067] <<Method for producing inorganic porous layer>> An example of a method for forming the inorganic porous layer is a method in which a coating liquid containing inorganic particles and a resin binder is applied to at least one surface of a porous film mainly composed of a polyolefin resin to form the inorganic porous layer.

[0068] The solvent for the coating liquid is preferably one that can disperse the inorganic particles and resin binder uniformly and stably, and examples thereof include N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, and hexane.

[0069] To stabilize dispersion and improve coating properties, various additives may be added to the coating solution, such as dispersants such as surfactants, thickeners, wetting agents, antifoaming agents, and pH adjusters containing acids and alkalis. These additives are preferably removable when the solvent is removed, but may remain in the inorganic porous layer as long as they are electrochemically stable within the range of use of the lithium ion secondary battery, do not inhibit the battery reaction, and are stable up to about 200°C.

[0070] The method for dispersing the inorganic particles and the resin binder in the solvent 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.

[0071] The method for applying the coating liquid to the porous membrane is not particularly limited as long as it is a method that can realize the required layer thickness and coating area, and examples thereof include gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, spray coating method, etc.

[0072] Furthermore, prior to application of the coating liquid, it is preferable to subject the surface of the porous membrane to a surface treatment, since this makes it easier to apply the coating liquid and improves the adhesion between the inorganic filler-containing porous layer and the porous membrane surface after application. The method of surface treatment is not particularly limited as long as it is a method that does not significantly impair the porous structure of the porous membrane, and examples thereof include a corona discharge treatment method, a mechanical roughening method, a solvent treatment method, an acid treatment method, and an ultraviolet oxidation method.

[0073] The method for removing the solvent from the coated film after coating is not particularly limited as long as it does not adversely affect the porous film, and examples thereof include a method of drying the porous film at a temperature below its melting point while fixing it, a method of drying under reduced pressure at a low temperature, etc. From the viewpoint of controlling the shrinkage stress in the MD direction of the porous film and the multilayer porous film, it is preferable to appropriately adjust the drying temperature, winding tension, etc.

[0074] 《Inorganic particles》 The inorganic particles preferably have a melting point of 200° C. or higher, high electrical insulation, are chemically stable inside the lithium ion secondary battery, and have excellent oxidation resistance and reduction resistance.

[0075] Specific examples of inorganic particles include inorganic oxide particles such as alumina, boehmite, silica, titanium oxide, zirconium oxide, and magnesium oxide; inorganic nitride particles such as silicon nitride, titanium nitride, and boron nitride; and sparingly soluble ionic crystal particles such as calcium fluoride and barium sulfate. These inorganic particles may be used alone or in combination of two or more. Considering chemical stability within a lithium ion secondary battery, one or more particles selected from the group consisting of alumina, boehmite, magnesium oxide, and barium sulfate are preferred.

[0076] The average particle size of inorganic particles is determined by the specific gravity of the inorganic particles (g / cm 3 ) and BET specific surface area (m 2 / g) are measured and calculated according to the following formula, assuming that the inorganic particles are spherical. The BET specific surface area is measured using nitrogen as the adsorbate in accordance with JIS Z 8830:2013. Average particle size (nm) = 6÷[specific gravity (g / cm 3 ) × BET specific surface area (m 2 / g)] × 1000

[0077] The inorganic particles have an average particle size of 50 nm or more, preferably 70 nm or more, more preferably 150 nm or more, and even more preferably 200 nm or more. When the inorganic particles have an average particle size of 50 nm or more, the pores present on the surface of the polyolefin porous substrate are prevented from being blocked by the inorganic particles, and the ion permeability of the multilayer porous membrane can be improved.

[0078] The inorganic particles have an average particle size of 500 nm or less, preferably 450 nm or less, more preferably 400 nm or less, and even more preferably 350 nm or less. When the inorganic particles have an average particle size of 500 nm or less, the density of the inorganic porous layer can be increased, making it easier to suppress meltdown of the multilayer porous film at high temperatures.

[0079] The shape of the inorganic particles may be plate-like, scale-like, needle-like, columnar, spherical, polyhedral, block-like, etc., and a combination of a plurality of inorganic particles having the above shapes may be used. The shape of the inorganic particles is not particularly limited, but from the viewpoint of suppressing meltdown of the multilayer porous membrane, polyhedral, columnar, spindle-like, and spherical shapes having multiple faces are preferred.

[0080] The proportion of inorganic particles in the inorganic porous layer can be determined appropriately from the viewpoint of the binding property of the inorganic particles, the ion permeability and heat resistance of the multilayer porous membrane, etc., but is preferably 50% by mass or more and less than 100% by mass, more preferably 70% by mass or more and 99.99% by mass or less, even more preferably 80% by mass or more and 99.9% by mass or less, and particularly preferably 90% by mass or more and 99% by mass or less.

[0081] Resin binder The resin binder is preferably one that interacts with the inorganic particles, binds the inorganic particles together, is chemically stable inside the lithium ion secondary battery, and has excellent oxidation resistance and reduction resistance.

[0082] Specific examples of the resin component of the resin binder include polyolefins such as polyethylene and polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; fluorine-containing rubbers such as vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer; rubbers such as styrene-butadiene copolymer and its hydrogenated product, acrylonitrile-butadiene copolymer and its hydrogenated product, acrylonitrile-butadiene-styrene copolymer and its hydrogenated product, methacrylate-acrylate copolymer, styrene-acrylate copolymer, acrylonitrile-acrylate copolymer, ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate; cellulose ethers such as carboxymethyl cellulose; polyphenylene ether, polyacrylamide, polyvinylpyrrolidone, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, and polyester.

[0083] The preferred form of the resin binder is a water-soluble polymer or a polymer latex. By using a water-soluble polymer or a polymer latex, a coating solution can be prepared using water as a solvent. In addition, when a water-soluble polymer is used, inorganic particles and the water-soluble polymer are linearly bonded at multiple points, which has the advantage of firmly fixing the inorganic particles in the inorganic porous layer. In addition, when a polymer latex is used, the polymer latex is less likely to block the pores of the polyolefin porous substrate, which has the advantage of easily increasing the ion permeability of the multilayer porous membrane.

[0084] Specific examples of the water-soluble polymer include cellulose ethers such as polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, poly-N-vinylacetamide, and carboxymethyl cellulose; and modified polymers such as modified polyamides.

[0085] From the viewpoint of improving electrochemical stability and binding properties, the polymer latex is preferably one obtained by emulsion polymerization of an aliphatic conjugated diene monomer, an unsaturated carboxylic acid monomer, and other monomers copolymerizable therewith. The emulsion polymerization method is not particularly limited, and a conventionally known method can be used. The method of adding the monomers and other components is not particularly limited, and any of a batch addition method, a divided addition method, and a continuous addition method can be used. Furthermore, any of single-stage polymerization, two-stage polymerization, and multi-stage polymerization can be used.

[0086] The aliphatic conjugated diene monomer is not particularly limited, and examples thereof include 1,3-butadiene, 2-Methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro- 1,3-butadiene, substituted straight-chain conjugated pentadienes, substituted and side-chain conjugated hexadienes, etc. These may be used alone or in combination of two or more. Of these, 1,3-butadiene is particularly preferred.

[0087] The unsaturated carboxylic acid monomer is not particularly limited, and examples thereof include acrylic acid and methacrylic acid. Mono- or dicarboxylic acids such as carboxylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, etc. These may be used alone or in combination of two or more. Among these, acrylic acid and methacrylic acid are particularly preferred.

[0088] The average particle size of the polymer latex is preferably 50 to 500 nm, more preferably 60 to 460 nm, and even more preferably 80 to 220 nm. When the average particle size is 50 nm or more, when an inorganic porous layer containing inorganic particles and a polymer latex is laminated on at least one side of a polyolefin porous substrate, the ion permeability of the multilayer porous membrane is less likely to decrease, and the input / output characteristics of the battery are easily improved. In addition, even when the temperature rises rapidly during abnormal heat generation of the battery, the multilayer porous membrane exhibits smooth shutdown characteristics, and the safety of the battery is easily improved. Furthermore, when the average particle size is 500 nm or less, the binding between the inorganic particles can be improved.

[0089] The average particle size of the polymer latex can be controlled by adjusting the polymerization time, polymerization temperature, raw material composition ratio, raw material charging order, pH, and the like.

[0090] The glass transition temperature (Tg) of a polymer is determined from a DSC curve obtained by differential scanning calorimetry (DSC). In this specification, the glass transition temperature is sometimes referred to as Tg. Specifically, it is determined by the intersection of a line extending the low-temperature baseline of the DSC curve toward the high-temperature side and a tangent to the inflection point of the step-like change in the glass transition.

[0091] Here, "glass transition" refers to the heat change that occurs on the endothermic side in DSC due to a change in the state of the polymer specimen. This heat change is observed as a step change or a combination of a step change and a peak in the DSC curve.

[0092] The glass transition temperature of a polymer can be adjusted appropriately by changing the monomer components and the ratio of each monomer used to produce the polymer. That is, the glass transition temperature can be estimated from the Tg of the homopolymer generally shown for each monomer used to produce the polymer (for example, as described in "Polymer Handbook" (A Wiley-Interscience Publication)) and the blending ratio of the monomers.

[0093] For example, copolymers with high Tg can be obtained by blending high proportions of monomers such as styrene, methyl methacrylate, and acrylonitrile, which give homopolymers with Tg of about 100°C, while copolymers with low Tg can be obtained by blending high proportions of monomers such as butadiene, which gives homopolymers with Tg of about -80°C, or n-butyl acrylate and 2-ethylhexyl acrylate, which give homopolymers with Tg of about -50°C.

[0094] The glass transition temperature of a polymer can be roughly calculated using the FOX formula (the following formula (1)). Note that the glass transition temperature of a polymer used in this specification is a value measured by the above-mentioned method using DSC. 1 / Tg=W1 / Tg1+W2 / Tg2++W i / Tg i +···W n / Tg n (1) In formula (1), Tg(K) is the Tg of the copolymer, Tg i (K) is the Tg of the homopolymer of each monomer i, W i indicates the mass fraction of each monomer.}

[0095] The resin binder preferably uses a water-soluble polymer having a glass transition temperature (Tg) of 25°C or higher in combination with a water-soluble polymer or polymer latex having a Tg of less than 25°C. When the inorganic porous layer contains a water-soluble polymer having a Tg of 25°C or higher, the elastic modulus of the water-soluble polymer is maintained even at high temperatures, even when the battery abnormally heats up, thereby maintaining the structure of the inorganic porous layer and suppressing meltdown. From the viewpoint of effectively suppressing meltdown of the multilayer porous membrane, the Tg of the water-soluble polymer is preferably 50°C or higher, more preferably 100°C or higher, and even more preferably 150°C or higher. Specific examples of water-soluble polymers having a Tg of 25°C or higher include polyacrylamide, poly-N-vinylacetamide, polyvinylpyrrolidone, etc., and one or more of these are preferred.

[0096] The proportion of the resin binder in the inorganic porous layer can be appropriately determined from the viewpoints of the binding property of the inorganic particles, the ion permeability and heat resistance of the multilayer porous membrane, etc., but is preferably 0.5 to 10% by mass, more preferably 1 to 8% by mass, even more preferably 1.5 to 6% by mass, and particularly preferably 2 to 4% by mass. From the same viewpoints as above, when the entire inorganic porous layer is taken as 100 parts by mass, the inorganic porous layer preferably contains 1 to 10 parts by mass of the resin binder, more preferably 1 to 8 parts by mass, and even more preferably 1.5 to 6 parts by mass.

[0097] <<Mechanism of action of multilayer porous membrane>> The reason why the multilayer porous membrane of the present embodiment is able to suppress the meltdown phenomenon at high temperatures and maintain a high membrane resistance while being thin and having high ion permeability is not necessarily clear. Therefore, although it is not limited to the action or principle described below, the assumed mechanism of action will be described below.

[0098] As a first factor, the multilayer porous membrane of the present embodiment contains high molecular weight polyethylene in the polyolefin porous substrate, and also controls the shrinkage stress of the high molecular weight polyethylene.At high temperatures, the polyolefin resin of the multilayer porous membrane melts, blocking the pores, and then the shutdown mechanism is activated, and the polyolefin resin penetrates into the positive and negative electrodes, resulting in meltdown.Since the multilayer porous membrane of the present embodiment contains high molecular weight polyethylene, the fluidity of the molten polyolefin resin is reduced, and it is thought that the occurrence of meltdown in the above mode is easily suppressed.

[0099] However, when using high molecular weight polyethylene, the polymer chains of polyolefin resin are more intertwined with each other, so it is easy to exhibit high strength, but it is expected that the shrinkage stress of polyolefin porous substrate will be increased.If the shrinkage stress of polyolefin porous substrate is high, when heated, polyolefin porous substrate is easy to heat shrink, so it is thought that the mode that multilayer porous membrane heat shrinks and meltdown occurs.In the multilayer porous membrane of the present embodiment, while using high molecular weight polyethylene, by devising composition and manufacturing conditions, it is thought that the shrinkage stress of high molecular weight polyethylene is controlled, and the occurrence of meltdown in the above-mentioned mode is suppressed.

[0100] The second factor is that the multilayer porous membrane of the present embodiment uses fine inorganic particles in the inorganic porous layer. When the inorganic porous layer is made of fine inorganic particles, the inorganic porous layer becomes dense, so that after the battery temperature rises and the shutdown mechanism of the multilayer porous membrane is activated, the molten polyolefin resin is less likely to penetrate into the inorganic porous layer, and meltdown is less likely to occur.

[0101] The third factor is that the multilayer porous membrane of the present embodiment may use a water-soluble polymer having a glass transition temperature (Tg) of 25° C. or higher in the inorganic porous layer. It is difficult for a water-soluble polymer having a Tg of 25° C. or higher to bond inorganic particles together by plastic deformation at room temperature, but it is thought that by maintaining its elastic modulus even when the battery is heated to a high temperature, the structure of the inorganic porous layer can be maintained and meltdown due to cracking or deformation of the inorganic porous layer can be suppressed.

[0102] Although the mechanism of action of the multilayer porous membrane may actually involve factors other than those mentioned above, it is believed that the effects of the present invention are achieved by the combined action of these multiple factors. [Example]

[0103] The present invention will be described in detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. The analytical methods and evaluation methods used in the examples and comparative examples are as follows.

[0104] (1) Film thickness (μm) The film thickness of the sample was measured at room temperature of 23° C. using a micro thickness gauge (Toyo Seiki Seisakusho Co., Ltd., Type KBM). The thickness of each layer of the sample was measured by observing the cross section of the sample with a scanning electron microscope.

[0105] (2) Weight (g / m 2 ) The mass (g) of a 10cm x 10cm square sample was measured using a precision electronic balance, and the obtained value was multiplied by 100 to obtain the basis weight (g / m 2 ) was calculated.

[0106] (3) Porosity (%) Sample 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 / film density) / volume × 100

[0107] (4) Air permeability (sec / 100ml) The air permeability of the samples was measured using an Oken air permeability meter "EG01-5-1MR" (Asahi Seiko Co., Ltd.) conforming to JIS P-8117:2009.

[0108] (5)Piercing strength (N) Using a handy compression tester "KES-G5" (Kato Tech Co., Ltd.), the sample was fixed in a sample holder with an opening diameter of 11.3 mm. A puncture test was performed on the center of the fixed sample at room temperature of 23°C under conditions of a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec, and the puncture strength (N) was measured as the maximum puncture load.

[0109] (6) Penetration strength converted to area weight [N / (g / m 2 )] The puncture strength (N) of the multilayer porous membrane was calculated based on the basis weight (g / m 2 ) to obtain the puncture strength converted into basis weight [N / (g / m 2 )] was calculated.

[0110] (7) Heat shrinkage rate at 150°C (%) A 100mm x 100mm square sample was cut from the multilayer porous membrane 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 in MD and TD was calculated using the following formula. MD heat shrinkage rate (%) = (100 - MD length after heating) / 100 x 100 TD heat shrinkage rate (%) = (100 - TD length after heating) / 100 x 100

[0111] (8) Film resistance (Ω·cm 2 ) A 40 mm × 30 mm square sample was cut from the multilayer porous membrane to prepare Sample 1. Also, a 55 mm × 20 mm square positive electrode and negative electrode were cut from the positive electrode and negative electrode fabricated by the method described below to prepare positive electrode 2A and negative electrode 2B. Of the 55 mm length of 2A and 2B, 10 mm was not coated with the electrode active material, leaving the current collector exposed.

[0112] Thereafter, the portions of sample 1, positive electrode 2A, and negative electrode 2B where the electrode active material was applied were thoroughly impregnated with an electrolyte (lithium fluoroborate / propylene carbonate / ethylene carbonate / γ-butyl lactone / trioctyl phosphate = weight ratio 7.2 / 23.3 / 25.7 / 43.3 / 0.5).

[0113] Then, a measuring device was prepared as shown in FIG. 1A, and a sample 1, a positive electrode 2A, a negative electrode 2B, and a 2 cm 2 An aramid film 3 (thickness 9 μm) with a circular hole drilled therein and a silicone rubber 4 (thickness 4 mm) were stacked together as shown in FIG. 1B and placed on a ceramic plate 5 with an embedded thermocouple.

[0114] Next, while applying a surface pressure of 4 MPa to members 1 to 4 using a hydraulic press 7, the heater 6 was heated, and the temperature and resistance were continuously measured using a thermometer 9 connected to the thermocouple 5 and an AC electrical resistance measuring device 8 "AG-4311" (Ando Electric Co., Ltd.) connected to the current collector portions of the positive electrode 2A and the negative electrode 2B. The temperature was raised from room temperature of 23°C to 200°C at a rate of 10°C / min, and the resistance was measured at 1 V, 1 kHz AC. The obtained resistance (Ω) was multiplied by the effective electrode area of ​​2 cm. 2 Multiplying by this, the membrane resistance value (Ω cm 2 The film resistance at a temperature of 200°C was calculated as the 200°C film resistance (Ω cm 2 ) are shown in Table 1.

[0115] (9) Test using lithium-ion secondary batteries <<Preparation of positive electrode sheet>> LiNi as the positive electrode active material 0.8 Mn 0.1 Co0.1 O2 (Dongsheng Technology), acetylene black powder and graphite powder as conductive additives, and polyvinylidene fluoride as a binder were mixed in a solids mass ratio of 100:3:3:3, and N-methyl-2-pyrrolidone was added as a dispersion solvent to a solids content of 40 mass%. The mixture was further mixed to prepare a slurry solution. This slurry solution was applied to both sides of a 20 μm-thick aluminum foil current collector, the solvent was dried and removed, and the coated aluminum foil was then compression-molded using a roll press to obtain a positive electrode sheet with a positive electrode mixture density adjusted to 3.1 g / cc.

[0116] <<Preparation of negative electrode sheet>> Artificial graphite (Rongtan Technology) was used as the negative electrode active material, and styrene butadiene rubber and carboxymethyl cellulose aqueous solution were used as binders, mixed in a solids mass ratio of 100:1.5:1.1, and water was added as a dispersion solvent to a solids content of 45% by mass. The mixture was further mixed to prepare a slurry solution. This slurry solution was applied to both sides of an 18 μm-thick copper foil current collector, the solvent was dried and removed, and the coated copper foil was then compression-molded using a roll press to obtain a negative electrode sheet with a negative electrode mixture density adjusted to 1.4 g / cc.

[0117] <<Preparation of non-aqueous electrolyte>> A solution containing 1 mol / L of LiPF6 was prepared in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:3 as a non-aqueous solvent. 2 parts by mass of vinylene carbonate was added to 100 parts by mass of this solution to prepare a non-aqueous electrolyte solution.

[0118] <<Fabrication of Lithium-ion Secondary Batteries>> The positive electrode sheet and negative electrode sheet prepared as described above were overlapped on both sides of the separator obtained in the Examples and Comparative Examples described below, and the resulting sheet was wound flat to obtain a wound body. The resulting wound body was inserted, with the positive and negative electrode terminals protruding, into a bag (exterior) made of an aluminum laminate film in which both sides of aluminum foil (40 μm thick) were coated with a resin layer. The nonaqueous electrolyte prepared as described above was then poured into the bag, and the bag was vacuum-sealed to produce a pouch-type lithium-ion secondary battery.

[0119] <<First charge / discharge of a lithium-ion secondary battery>> The pouch-type lithium-ion secondary battery was placed in a thermostatic chamber "PLM-73S" (Futaba Scientific Co., Ltd.) set at 25°C and connected to a charge / discharge device "ACD-01" (Asuka Electronics Co., Ltd.). The secondary battery was then charged at a constant current of 0.1 C until a voltage of 4.2 V was reached, then charged at a constant voltage of 4.2 V for one hour, and then discharged at a constant current of 0.1 C until a voltage of 3.0 V was reached. This charge / discharge cycle was repeated three times. The battery was then charged at a constant current of 0.1 C until a voltage of 4.2 V was reached, bringing the lithium-ion secondary battery into a fully charged state. Note that 1 C refers to the current value when the battery's full capacity is discharged in one hour; in this example, 1 C = 1090 mA.

[0120] <10C discharge test of lithium-ion secondary batteries> After the initial charge and discharge, the pouch-type lithium-ion secondary battery was charged at a constant current of 1 / 3C until a voltage of 4.2V was reached, then charged at a constant voltage of 4.2V for 1 hour, and discharged at a constant current of 1 / 3C to 3.0V. The battery was then charged at a constant current of 1 / 3C until a voltage of 4.2V was reached, then charged at a constant voltage of 4.2V for 1 hour, and discharged at a constant current of 10C to 3.0V. From the results of this series of charge and discharge, the 10C capacity retention rate (%) was calculated using the following formula. 10C capacity retention rate (%)=10C discharge capacity / (1 / 3C discharge capacity)×100

[0121] <Hot box testing of lithium-ion secondary batteries> After the 10C discharge test, the pouch-type lithium-ion secondary battery was fully charged, a thermocouple was attached to the surface of the battery, and the battery was fixed to a base installed inside an explosion-proof thermostatic chamber. The temperature of the explosion-proof thermostatic chamber was then raised from room temperature at a rate of 2°C / min, and the battery was observed until it went into thermal runaway. The temperature inside the explosion-proof thermostatic chamber when the temperature of the thermocouple attached to the battery first exceeded 200°C was defined as the "thermal runaway temperature," and the heat resistance of the battery was evaluated by measuring the thermal runaway temperature. The temperature data collection interval was 1 second.

[0122] [Example 1] A resin composition serving as the raw material for the polyolefin porous substrate was prepared by mixing 70 parts by mass of high-molecular-weight polyethylene (viscosity average molecular weight 900,000, melting point 137°C), 23 parts by mass of high-density polyethylene (viscosity average molecular weight 300,000, melting point 137°C), 7 parts by mass of polypropylene (viscosity average molecular weight 400,000, melting point 163°C), and 0.3 parts by mass of tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane as an antioxidant using a tumbler blender.

[0123] The resin composition was supplied to a twin-screw extruder, and liquid paraffin having a kinematic viscosity at 40°C of 70.6 cSt was side-fed into the cylinder of the twin-screw extruder so that the proportion of the resin composition in the melt-kneaded product was 26% by mass and the extrusion rate was 24 kg / h, and the mixture was melt-kneaded while being heated to 180°C in the twin-screw extruder. The melt-kneaded product was then discharged from a T-die and extruded onto a cooling roll whose surface temperature was controlled to 25°C, and the melt-kneaded product was cooled and solidified to obtain a sheet-like molded product.

[0124] Next, this sheet-like molded product was introduced into a simultaneous biaxial stretching machine and subjected to simultaneous biaxial stretching at a temperature of 124 ° C and a magnification of 7 × 6.38. Next, the sheet-like stretched product was introduced into an extraction tank and immersed in methylene chloride to extract and remove the liquid paraffin, and the methylene chloride was dried. Thereafter, the obtained porous film was introduced into a uniaxial stretching machine and stretched 1.7 times in the width direction at a temperature of 133 ° C, and then relaxed by about 10% in the width direction to obtain a polyolefin porous substrate with a film thickness of 9 μm. The physical property evaluation results of the obtained polyolefin porous substrate are shown in Table 1.

[0125] Next, 95 parts by mass of boehmite (average particle size 300 nm), 3.0 parts by mass of acrylic latex (solid content 40%, average particle size 145 nm, Tg -10°C) as solid content, 1.5 parts by mass of polyacrylamide (solid content 20%, Tg 150°C) as solid content, and 0.5 parts by mass of sodium polyacrylate (solid content 40%) as solid content were uniformly dispersed in 100 parts by mass of water to prepare an aqueous slurry solution, which was then applied to the surface of the polyolefin porous substrate using a microgravure coater. The resulting solution was dried at 60°C to remove water, yielding a 12-μm-thick multilayer porous membrane having a 3-μm-thick inorganic porous layer. The evaluation results of the multilayer porous membrane are shown in Table 1. The resulting multilayer porous membrane, despite its thinness, had high mechanical strength, and exhibited excellent output characteristics in lithium-ion secondary battery tests and good heat resistance in hot box tests.

[0126] [Example 2] Except for not adding polyacrylamide to the aqueous slurry solution, a multi-layer porous membrane was obtained in the same manner as in Example 1. Table 1 shows the results of various evaluations of the obtained multi-layer porous membrane.

[0127] [Example 3] A multilayer porous membrane was obtained in the same manner as in Example 2, except that 70 parts by mass of high molecular weight polyethylene (viscosity average molecular weight 900,000, melting point 137 ° C.), 30 parts by mass of high density polyethylene (viscosity average molecular weight 300,000, melting point 137 ° C.), and 0.3 parts by mass of tetrakis [methylene-3- (3',5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane as an antioxidant were mixed using a tumbler blender to prepare a resin composition that is the raw material for the polyolefin porous substrate. The evaluation results of the obtained multilayer porous membrane are shown in Table 1.

[0128] [Example 4] Except for providing inorganic porous layers with a thickness of 1.5 μm on both sides of the polyolefin porous substrate, a multilayer porous membrane was obtained in the same manner as in Example 2. Various evaluation results of the obtained multilayer porous membrane are shown in Table 1.

[0129] [Example 5] A multilayer porous membrane was obtained in the same manner as in Example 2, except that 93 parts by mass of high molecular weight polyethylene (viscosity average molecular weight 900,000, melting point 137 ° C), 7 parts by mass of polypropylene (viscosity average molecular weight 400,000, melting point 163 ° C), and 0.3 parts by mass of tetrakis [methylene-3- (3', 5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane as an antioxidant were mixed using a tumbler blender to prepare a resin composition that was the raw material for the polyolefin porous substrate. The evaluation results of the obtained multilayer porous membrane are shown in Table 1.

[0130] [Example 6] The same resin composition as in Example 1 was supplied to a twin-screw extruder, and liquid paraffin having a kinematic viscosity at 40°C of 70.6 cSt was side-fed into the cylinder of the twin-screw extruder so that the proportion of the resin composition in the melt-kneaded product was 26% by mass and the extrusion rate was 15 kg / h, and the mixture was melt-kneaded while being heated to 180°C in the twin-screw extruder. Thereafter, the melt-kneaded product was discharged from a T-die and extruded onto a cooling roll whose surface temperature was controlled to 25°C, and the melt-kneaded product was cooled and solidified to obtain a sheet-like molded product.

[0131] Next, this sheet-like molded product was introduced into a simultaneous biaxial stretching machine and subjected to simultaneous biaxial stretching at a temperature of 123 ° C and a magnification of 7 × 6.38. Next, the sheet-like stretched product was introduced into an extraction tank and immersed in methylene chloride to extract and remove the liquid paraffin, and the methylene chloride was dried. Thereafter, the obtained porous film was introduced into a uniaxial stretching machine and stretched 1.7 times in the width direction at a temperature of 134 ° C, and then relaxed by about 10% in the width direction to obtain a polyolefin porous substrate with a film thickness of 5 μm. The physical property evaluation results of the obtained polyolefin porous substrate are shown in Table 1.

[0132] Next, 95 parts by mass of boehmite (average particle size 300 nm), 3.0 parts by mass of acrylic latex (solid content 40%, average particle size 145 nm, Tg -10 ° C.) as solid content, 1.5 parts by mass of polyacrylamide (solid content 20%, Tg 150 ° C.) as solid content, and 0.5 parts by mass of sodium polyacrylate (solid content 40%) as solid content were uniformly dispersed in 100 parts by mass of water to prepare an aqueous slurry solution, which was then applied to the surface of the polyolefin porous substrate using a microgravure coater. The resulting solution was dried at 60 ° C. to remove water, and a 7 μm-thick multilayer porous membrane having a 2 μm-thick inorganic porous layer was obtained. The evaluation results of the multilayer porous membrane are shown in Table 1.

[0133] [Example 7] Except for not adding polyacrylamide to the aqueous slurry solution, a multi-layer porous membrane was obtained in the same manner as in Example 6. Table 1 shows the results of various evaluations of the obtained multi-layer porous membrane.

[0134] [Example 8] A multilayer porous membrane was obtained in the same manner as in Example 7, except that 70 parts by mass of high molecular weight polyethylene (viscosity average molecular weight 900,000, melting point 137 ° C.), 30 parts by mass of high density polyethylene (viscosity average molecular weight 300,000, melting point 137 ° C.), and 0.3 parts by mass of tetrakis [methylene-3- (3',5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane as an antioxidant were mixed using a tumbler blender to prepare a resin composition that is the raw material for a polyolefin porous substrate. The evaluation results of the obtained multilayer porous membrane are shown in Table 1.

[0135] [Example 9] A multilayer porous membrane was obtained in the same manner as in Example 7, except that an inorganic porous layer with a thickness of 1 μm was provided on both sides of the polyolefin porous substrate. Various evaluation results of the obtained multilayer porous membrane are shown in Table 1.

[0136] [Example 10] A multilayer porous membrane was obtained in the same manner as in Example 7, except that 93 parts by mass of high molecular weight polyethylene (viscosity average molecular weight 900,000, melting point 137 ° C), 7 parts by mass of polypropylene (viscosity average molecular weight 400,000, melting point 163 ° C), and 0.3 parts by mass of tetrakis [methylene-3- (3 ', 5 '-di-t-butyl-4 '-hydroxyphenyl) propionate] methane as an antioxidant were mixed using a tumbler blender to prepare a resin composition that is the raw material for a polyolefin porous substrate. The evaluation results of the obtained multilayer porous membrane are shown in Table 1.

[0137] [Example 11] Except for using alumina (average particle size 300 nm) instead of boehmite, a multilayer porous membrane was obtained in the same manner as in Example 7. Table 1 shows the results of various evaluations of the obtained multilayer porous membrane.

[0138] [Example 12] Except for using barium sulfate (average particle size 300 nm) instead of boehmite, a multilayer porous membrane was obtained in the same manner as in Example 7. Table 1 shows the results of various evaluations of the obtained multilayer porous membrane.

[0139] [Comparative Example 1] A resin composition serving as the raw material for the polyolefin porous substrate was prepared by mixing 47 parts by mass of high-density polyethylene (viscosity average molecular weight of 700,000, melting point of 137°C), 46 parts by mass of high-density polyethylene (viscosity average molecular weight of 300,000, melting point of 137°C), 7 parts by mass of polypropylene (viscosity average molecular weight of 400,000, melting point of 163°C), and 0.3 parts by mass of tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane as an antioxidant using a tumbler blender.

[0140] The resin composition was supplied to a twin-screw extruder, and liquid paraffin having a kinematic viscosity of 70.6 cSt at 40°C was side-fed into the cylinder of the twin-screw extruder so that the proportion of the resin composition in the melt-kneaded product was 33% by mass and the extrusion rate was 19 kg / h, and the mixture was melt-kneaded while being heated to 180°C in the twin-screw extruder. The melt-kneaded product was then discharged from a T-die and extruded onto a cooling roll whose surface temperature was controlled to 25°C, and the melt-kneaded product was cooled and solidified to obtain a sheet-like molded product.

[0141] Next, this sheet-like molded product was introduced into a simultaneous biaxial stretching machine and subjected to simultaneous biaxial stretching at a temperature of 124 ° C and a magnification of 7 × 6.38. Next, the sheet-like stretched product was introduced into an extraction tank and immersed in methylene chloride to extract and remove the liquid paraffin, and the methylene chloride was dried. Thereafter, the obtained porous film was introduced into a uniaxial stretching machine and stretched 1.7 times in the width direction at a temperature of 134 ° C, and then relaxed by about 10% in the width direction to obtain a polyolefin porous substrate with a film thickness of 9 μm. The physical property evaluation results of the obtained polyolefin porous substrate are shown in Table 1.

[0142] Next, 95 parts by mass of boehmite (average particle size 300 nm), 3.0 parts by mass of acrylic latex (solid content concentration 40%, average particle size 145 nm, Tg -10°C) as solid content, and 0.5 parts by mass of sodium polyacrylate (solid content concentration 40%) as solid content were uniformly dispersed in 100 parts by mass of water to prepare an aqueous slurry solution, which was then applied to the surface of the polyolefin porous substrate using a microgravure coater. The resulting solution was dried at 60°C to remove water, yielding a 12-μm-thick multilayer porous membrane having a 3-μm-thick inorganic porous layer. The results of various evaluations of the multilayer porous membrane are shown in Table 1. The resulting multilayer porous membrane had a pin puncture strength of 2.94 N, but as the polyolefin resin basis weight increased, its air permeability increased, and its 10C capacity retention rate was poor at 6%.

[0143] Comparative Example 2 A multilayer porous film was obtained in the same manner as in Comparative Example 1, except that the extrusion rate was changed to 11 kg / h. The obtained multilayer porous film had insufficient mechanical strength, so that the tension required for producing a lithium ion secondary battery could not be applied, and a wound body could not be produced with good quality. In addition, the membrane resistance value of the multilayer porous film at 200°C was 1256 Ω cm 2 This result raises concerns about the safety of the battery.

[0144] Comparative Example 3 A multilayer porous membrane was obtained in the same manner as in Example 7, except that boehmite (average particle size 600 nm) was used instead of the boehmite (average particle size 300 nm) of Example 7. The obtained multilayer porous membrane had a lower membrane resistance at 200°C and a lower thermal runaway temperature compared to the multilayer porous membrane of Example 7, resulting in inferior safety as a battery.

[0145] Comparative Example 4 A multilayer porous membrane was obtained in the same manner as in Example 7, except that 100 parts by mass of high molecular weight polyethylene (viscosity average molecular weight 900,000, melting point 137 ° C) and 0.3 parts by mass of tetrakis [methylene-3- (3 ', 5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane as an antioxidant were mixed using a tumbler blender to prepare a resin composition as a raw material for the polyolefin porous substrate. Compared with the multilayer porous membrane of Example 7, the obtained multilayer porous membrane has excellent pin puncture strength, but the heat shrinkage rate at 150 ° C. is increased and the membrane resistance at 200 ° C. is reduced. Therefore, it is estimated that the multilayer porous membrane of Comparative Example 4 has a polyolefin porous substrate with a too high shrinkage stress, which causes a short circuit due to thermal shrinkage during heating, resulting in a reduced thermal runaway temperature.

[0146] [Table 1-1]

[0147] [Table 1-2]

[0148] From Table 1, it can be seen from a comparison of Examples 1 and 2, 6 and 7 that when a water-soluble binder with a high Tg is used as the binder for the inorganic porous layer, the 200°C membrane resistance improves, the thermal runaway temperature of the battery also increases, and the safety of the battery tends to improve. Furthermore, from a comparison of Examples 2 and 3, 7 and 8, it can be seen that when polypropylene is used in the polyolefin porous substrate, the 200°C membrane resistance improves, the thermal runaway temperature of the battery also increases, and the safety of the battery tends to improve. [Explanation of symbols]

[0149] 1 Separator 2A, 2B positive electrode, negative electrode 3 Area 2cm 2 Aramid film with perfectly round holes 4 Silicone rubber sheet 5. Ceramic plate with embedded thermocouple 6 Heater 7. Hydraulic press 8. AC electrical resistance measuring device 9 Thermometer 10. Data Collector [Industrial Applicability]

[0150] The multilayer porous membrane in the present embodiment is thin and has high ion permeability, while suppressing the meltdown phenomenon at high temperatures and having sufficient mechanical strength, and therefore has industrial applicability as a separator for an in-vehicle lithium ion secondary battery.

Claims

1. A multilayer porous membrane comprising an inorganic porous layer containing inorganic particles and a resin binder laminated on at least one surface of a polyolefin porous substrate containing a polyolefin resin as a main component, wherein the polyolefin resin contains high-molecular-weight polyethylene having a viscosity-average molecular weight of 750,000 or more and 5,000,000 or less, the inorganic particles have an average particle size of 50 nm or more and 500 nm or less, the multilayer porous membrane has a thickness of 3 μm or more and 13 μm or less, the multilayer porous membrane has an air permeability of 30 sec / 100 ml or more and 150 sec / 100 ml or less, and the multilayer porous membrane has a membrane resistance at 200° C. of 2,000 Ω cm ２ More than 100,000Ω・cm ２ The multilayer porous membrane has a puncture strength converted into basis weight of 0.64 N / (g / m ２ ) or more 1.23N / (g / m ２ ) or less.

2. The multilayer porous film according to claim 1, wherein the polyolefin porous substrate contains 50% by mass or more and 100% by mass or less of the high molecular weight polyethylene when the entire polyolefin porous substrate is 100% by mass.

3. The multilayer porous membrane according to claim 1 or 2, wherein the polyolefin resin contains high-density polyethylene.

4. The multilayer porous membrane according to claim 1 or 2, wherein the polyolefin resin contains polypropylene.

5. The multilayer porous film according to claim 4, wherein the polyolefin porous substrate contains 1% by mass or more and 10% by mass or less of the polypropylene when the entire polyolefin porous substrate is 100% by mass.

6. The multilayer porous membrane according to claim 1 or 2, wherein the inorganic particles contain at least one selected from the group consisting of alumina, boehmite, magnesium oxide, and barium sulfate.

7. 3. The multilayer porous membrane according to claim 1, wherein the resin binder contains a water-soluble polymer having a glass transition temperature (Tg) of 25°C or higher and a water-soluble polymer or polymer latex having a Tg of less than 25°C.

8. 8. The multilayer porous membrane according to claim 7, wherein the water-soluble polymer having a Tg of 25° C. or higher contains one or more selected from the group consisting of polyacrylamide, poly-N-vinylacetamide, and polyvinylpyrrolidone.

9. The multilayer porous membrane according to claim 1 or 2, wherein the inorganic porous layer contains the resin binder in an amount of 1% by mass or more and 10% by mass or less, when the entire inorganic porous layer is taken as 100% by mass.

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