Separator for electrochemical element and method for manufacturing separator for electrochemical element

Abstract

To provide a separator for an electrochemical element with excellent adhesion between a polyolefin microporous membrane and a porous layer, stable shutdown characteristics even in a temperature range exceeding the shutdown temperature of the polyolefin microporous membrane, and excellent meltdown resistance and mechanical strength, and a manufacturing method thereof.SOLUTION: In a separator for an electrochemical device including a porous layer containing a resin binder and inorganic particles laminated on at least one side of a polyolefin microporous membrane, after the temperature of the separator for electrochemical elements is raised to 200°C, the temperature is cooled to the room temperature, and the ABSORBANCE of the peak appearing from 2800 cm-1 to 3000 cm-1 when measuring the porous layer side of the separator for electrochemical element by the total reflection method of infrared spectroscopy is 0.05 or more and less than 0.17.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator for an electrochemical element and a method for producing the same. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are widely used in a variety of applications, including portable digital devices such as smartphones, tablets, mobile phones, laptops, digital cameras, digital video cameras, and portable game consoles; portable device applications such as power tools, electric motorcycles, and electrically assisted bicycles; and automotive applications such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles.

[0003] A lithium ion secondary battery generally has a configuration in which a secondary battery separator and an electrolyte are interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector.

[0004] Polyolefin-based porous membrane substrates are used as separators for secondary batteries. The characteristics required for secondary battery separators include the ability to contain an electrolyte within the porous structure, allowing ion migration, and a shutdown characteristic, in which the porous structure closes due to heat melting when the lithium-ion secondary battery experiences abnormal heat generation, thereby exhibiting insulating properties. However, high-energy secondary batteries can generate a large amount of heat during abnormal heat generation, and the temperature may continue to rise even beyond the shutdown temperature. In such cases, separator rupture can impair the shutdown characteristic, causing a short circuit and potentially further heat generation. The ability of the separator to maintain its membrane structure even during abnormal heat generation in such secondary batteries is called meltdown resistance.

[0005] Under these circumstances, it has been proposed to form a porous layer containing highly heat-resistant inorganic particles on the surface of the porous substrate in order to prevent direct contact between the positive electrode and the negative electrode even if the porous substrate is broken.

[0006] Patent Document 1 proposes a separator for secondary batteries, characterized in that it comprises a porous insulating layer on the surface of a porous resin substrate, the resin substrate having a hydrophilic surface on which hydrophilic functional groups are present, the porous insulating layer contains an inorganic filler and a fibrillated polymer, the fibrillated polymer constitutes a mass aggregate of agglomerated cellulose-based fibers having a fiber length of 1 μm to 10 μm, fibril fibers protruding from the outer surface of the aggregate, the average fiber diameter of the fibril fibers protruding from the outer surface of the aggregate being 0.05 μm to 0.5 μm, and the content of the fibrillated polymer is 0.3 mass% to 5 mass% when the total mass of the porous insulating layer is 100 mass%.

[0007] In addition, Patent Document 2 discloses a microporous polyolefin resin film having a porous layer containing an inorganic filler and a resin binder on at least one surface thereof, the porous layer comprising the following (A) to (C): (A) the average particle size of the inorganic filler is 0.1 μm or more and 3.0 μm or less; (B) the ratio of the resin binder to the total amount of the inorganic filler and the resin binder is 0.5% or more and 8% or less in volume fraction; (C) the ratio of the thickness of the porous layer to the total thickness is 15% or more and 50% or less; The multilayer porous membrane is characterized in that it simultaneously satisfies the above requirements and the surface wettability index (measurement method: JIS K-6768) of the polyolefin resin porous membrane is 40 mN / m or more.

[0008] Furthermore, Patent Document 3 proposes a separator for an electricity storage device, characterized by having Layer A consisting of a porous substrate layer and Layer B containing a first water-soluble polymer selected from water-soluble (meth)acrylic resins (b1) and salts thereof having 2% by mass or more and less than 27% by mass of carboxyl group-containing monomer units. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-207059 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-97656 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-130425 Summary of the Invention [Problem to be solved by the invention]

[0010] Patent Document 1 proposes a secondary battery separator that includes a porous insulating layer containing an inorganic filler and a fibrillated polymer on the surface of a porous resin substrate with a hydrophilic surface. However, if the temperature rises above the shutdown temperature of the resin substrate and the resin substrate breaks, the polymer contained in the porous insulating layer may also melt, creating a region between the positive and negative electrodes where only the inorganic filler is present. Because ion migration continues in such a region, insulation sufficient to prevent abnormal heat generation cannot be obtained, and shutdown characteristics cannot be maintained.

[0011] Patent Document 2 proposes a multilayer porous membrane comprising a porous layer containing an inorganic filler and a resin binder on at least one side of a polyolefin resin microporous membrane, wherein the average particle size of the inorganic filler, the proportion of the resin binder in the total amount of the inorganic filler and the resin binder, the proportion of the porous layer in the total layer thickness, and the wettability of the polyolefin resin microporous membrane surface are all specified. However, the resin binder used, which has a melting point and / or midpoint glass transition temperature of 180°C or higher, has low polymer chain fluidity and low adhesion at room temperature, which reduces the peel strength between the polyolefin resin microporous membrane and the porous layer, potentially causing peeling of the porous layer. Furthermore, when the corona treatment applied to the surface of the polyolefin microporous membrane has a treatment intensity of 50 W·min / m 2 In the above cases, the network structure of the microporous polyolefin membrane may be damaged, resulting in a loss of mechanical strength.

[0012] Patent Document 3 proposes a separator for an electricity storage device, characterized by having an A layer made of a porous substrate layer and a B layer containing a water-soluble polymer. However, if the temperature rises beyond the shutdown temperature of the A layer and the A layer ruptures, the B layer may also rupture because the midpoint glass transition temperature of the water-soluble polymer contained in the B layer is 120°C or lower, and the shutdown properties may not be maintained.

[0013] In view of the above problems, an object of the present invention is to provide a separator for an electrochemical element that has mechanical strength, excellent adhesion between a polyolefin microporous membrane and a porous layer, and maintains stable shutdown properties even in a temperature range exceeding the shutdown temperature of the polyolefin microporous membrane, and has excellent meltdown resistance, and a method for producing the same. [Means for solving the problem]

[0014] Therefore, the present inventors have conducted extensive research to solve the above problems.

[0015] In order to solve the above problems, the separator for an electrochemical element of the present invention has the following configuration. (1) A separator for electrochemical elements, in which a porous layer containing a resin binder and inorganic particles is laminated on at least one side of a polyolefin microporous film, wherein the separator for electrochemical elements is heated to 200°C, then cooled to room temperature, and the porous layer side of the separator for electrochemical elements is measured by a total reflection method of infrared spectroscopy, and the measured value is 2800 cm -1 From 3000cm -1 The separator for an electrochemical element is characterized in that the absorbance of the peak appearing in the graph is 0.05 or more and less than 0.17. (2) The separator for an electrochemical element according to (1), wherein the surface oxygen concentration of the surface of the polyolefin microporous film on which the porous layer containing a resin binder and inorganic particles is laminated is 1% or more. (3) The separator for an electrochemical element according to (1) or (2), characterized in that the meltdown temperature measured by the impedance method is 180° C. or higher. (4) A separator for an electrochemical element according to any one of (1) to (3), characterized in that the resin binder contained in the porous layer is an acrylic resin having a midpoint glass transition temperature of −80° C. or higher and 0° C. or lower. (5) (A) A step of subjecting a polyolefin microporous membrane to corona treatment (B) applying a coating liquid containing a resin binder and inorganic particles to at least one surface of the polyolefin microporous membrane and drying the coating liquid; A method for producing a separator for an electrochemical element, comprising: 2 More than 50W min / m 2 The method for producing a separator for an electrochemical element according to any one of (1) to (4), wherein the total mass of the separator is less than 1000 sq.m. [Effects of the Invention]

[0016] According to the present invention, a separator for electrochemical devices can be obtained that has mechanical strength, excellent adhesion between a polyolefin microporous membrane and a porous layer, and stably maintains shutdown properties even in a temperature range above the shutdown temperature of the polyolefin microporous membrane, i.e., has excellent meltdown resistance. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below. The terms and words used in the specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed in accordance with the meaning and concept consistent with the technical idea of ​​the present invention.

[0018] (Polyolefin microporous membrane) Examples of polyolefin resins constituting the polyolefin microporous membrane of the present invention include homopolymers, two-stage polymers, copolymers, and mixtures thereof obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, etc. A single polyolefin resin or a mixture of two or more different polyolefin resins, such as a mixture of polyethylene and polypropylene, or a copolymer of different olefins, may be used. In particular, polyethylene is preferred from the viewpoint of shutdown properties, and the melting point (softening point) of polyethylene is more preferably 70 to 150°C.

[0019] The polyolefin resin constituting the polyolefin microporous membrane of the present invention preferably contains a polyethylene resin as a main component, and the content of the polyethylene resin is preferably 70% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass, based on 100% by mass of the total mass of the polyolefin resin.

[0020] Examples of polyethylene include ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene. The polymerization catalyst is not particularly limited, and Ziegler-Natta catalysts, Phillips catalysts, and metallocene catalysts can be used. These polyethylenes may be ethylene homopolymers or copolymers containing small amounts of other α-olefins. Suitable α-olefins other than ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, (meth)acrylic acid, esters of (meth)acrylic acid, and styrene. While a single polyethylene may be used, a polyethylene mixture containing two or more polyethylenes is preferred.

[0021] The polyethylene mixture may be a mixture of two or more ultra-high molecular weight polyethylenes, high-density polyethylenes, medium-density polyethylenes, or low-density polyethylenes, each having a different weight-average molecular weight (Mw). Alternatively, a mixture of two or more polyethylenes selected from the group consisting of ultra-high molecular weight polyethylenes, high-density polyethylenes, medium-density polyethylenes, and low-density polyethylenes may be used. In particular, a mixture of polyethylenes having an Mw of 5×10 5 Ultra-high molecular weight polyethylene with Mw of 1 x 10 4 ~5×10 5 A mixture containing polyethylene of less than 100 wt % is preferred. From the viewpoint of tensile strength, the content of ultra-high molecular weight polyethylene in the mixture is preferably 1 to 40 mass %. From the viewpoint of mechanical strength, the molecular weight distribution of the polyethylene (weight average molecular weight (Mw) / number average molecular weight (Mn)) is preferably within the range of 5 to 200.

[0022] The thickness of the polyolefin microporous membrane is preferably 3 μm to 50 μm, more preferably 5 μm to 30 μm. By making the thickness of the polyolefin microporous membrane 50 μm or less, an increase in the internal resistance of the polyolefin microporous membrane can be suppressed. Furthermore, by making the thickness of the polyolefin microporous membrane 3 μm or more, production becomes easier and sufficient mechanical properties can be obtained.

[0023] The pore size of the polyolefin microporous membrane is preferably 0.01 to 50 μm, and the porosity of the porous membrane substrate is preferably 10 to 95%. By having such thickness, pore size, and porosity, sufficient ion conductivity, mechanical strength, and insulating properties can be obtained.

[0024] The pore size here refers to the diameter of through-holes measured in accordance with the bubble point method (half-dry method) described in JIS K3832 or ASTM F316-86.

[0025] In addition, the porosity here is defined as the mass of the constituent materials consisting of a, b, and n, where Wa, Wb, and Wn (g cm 2), and their true densities are da, db, and dn (g / cm 3 ) and the film thickness of interest is t (cm), the value (ε (%)) is calculated by the following formula (1): ε={1-(Wa / da+Wb / db+···+Wn / dn) / t}×100···(1) The air resistance of the polyolefin microporous membrane is 50 seconds / 100 cm 3 More than 1,000 seconds / 100cm 3 Preferably, it is less than 50 seconds / 100 cm. 3 More than 500 seconds / 100cm 3 Air resistance is 1,000 seconds / 100 cm or less. 3 By setting the air permeation resistance to 50 seconds / 100 cm or less, sufficient ion mobility can be obtained, and good battery characteristics can be achieved. 3 By setting the above, it is possible to improve the mechanical properties.

[0026] A separator for electrochemical elements, in which a porous layer described later is laminated on a polyolefin microporous film, is heated to 200°C and then cooled to room temperature. The porous layer side of the separator for electrochemical elements is measured by the total reflection method of infrared spectroscopy. -1 From 3000cm -1The absorbance of the peak appearing in the graph must be 0.05 or greater but less than 0.17. An absorbance of 0.05 or greater means that the polyolefin contained in the polyolefin microporous membrane penetrates into the porous layer (which does not contain polyolefin at the time of manufacture), which is originally made of a resin binder and inorganic particles, filling the voids of the inorganic particles and partially reaching the surface. If the absorbance of the peak is 0.05 or greater, the shutdown function is stably maintained in a temperature range of 150°C or greater, which is above the melting point of the polyethylene contained in the polyolefin microporous membrane. This allows the electrochemical element separator, which has a porous layer (described below) laminated to the polyolefin microporous membrane, to exhibit a meltdown temperature of 180°C or greater as measured by the impedance method. Such an electrochemical element separator cannot be obtained by simply laminating a porous layer to a polyolefin microporous membrane. Therefore, the present inventors have conducted extensive research and found that such performance is exhibited when a polyolefin microporous membrane is subjected to corona treatment under the conditions described below and then laminated with a porous layer, thereby arriving at the present invention.Although the mechanism is not entirely clear, it is thought that the in-plane scission of polymer bonds in the polyolefin contained in the polyolefin microporous membrane or stress relaxation occurs when the polyolefin microporous membrane melts at a temperature range of 150°C or higher, suppressing in-plane shrinkage of the polyolefin microporous membrane and allowing the molten polyolefin to penetrate into the porous layer.The polyolefin contained in the polyolefin microporous membrane that has penetrated into the porous layer blocks the voids between the inorganic particles contained in the porous layer, so that the porous layer is responsible for the shutdown properties, and it is thought that the shutdown function can be stably maintained even in a temperature range exceeding the shutdown temperature of the polyolefin microporous membrane itself. On the other hand, if corona treatment is not performed or the corona treatment conditions are inappropriate, the polyolefin contained in the polyolefin microporous film melted into the porous layer does not penetrate into the porous layer, causing in-plane shrinkage of the polyolefin microporous film. As a result, when viewed as the entire electrochemical element separator, the voids in the porous layer remain unblocked by polyolefin and the shutdown mechanism cannot be stably maintained in a temperature range of 150°C or higher.

[0027] If the ABSORBANCE is 0.17 or higher, excessive cleavage of the polymer bonds of the polyolefin contained in the polyolefin microporous membrane in the in-plane direction occurs, leading to a decrease in mechanical strength and a decrease in safety as an electrochemical element. The ABSORBANCE is more preferably 0.1 or higher but less than 0.17. The meltdown temperature of the separator for electrochemical elements, as measured by the impedance method, is preferably 180°C or higher. Although there is no particular upper limit, it is substantially 230°C or lower, more preferably 250°C or lower. Having a meltdown temperature of 180°C or higher means that even when a battery using the separator for electrochemical elements is heated or when the temperature inside the battery rises due to an internal short circuit or the like, the separator for electrochemical elements maintains a high resistance, making it less likely to short-circuit, or even if a short circuit occurs, a large current does not flow, and the battery is less likely to experience thermal runaway.

[0028] The surface oxygen concentration of the polyolefin microporous membrane of the present invention on the surface on which the porous layer containing a resin binder and inorganic particles is laminated is preferably 1% or more. Although there is no particular upper limit, it is preferably 10% or less, more preferably 5% or less. A surface oxygen concentration of 1% or more means that the polymer bonds of the polyolefin are sufficiently severed to allow the polyolefin to penetrate into the porous layer. A surface oxygen concentration of less than 1% is considered to be insufficient in the severing of the polymer bonds of the polyolefin contained in the polyolefin microporous membrane. A surface oxygen concentration of more than 10% may damage the network structure of the polyolefin microporous membrane during corona treatment, resulting in a loss of mechanical strength. A polyolefin microporous membrane with a surface oxygen concentration of 1% or more is considered to have at least one oxygen-containing polar group selected from the following groups: (The above polar groups: -COOH, -OH, -CO-R, -COO-R (R is an alkyl group, and the carbon chain of the alkyl group may be linear or branched.)) Whether the polyolefin microporous membrane has the above polar groups on its surface can be determined from the surface oxygen concentration measured by X-ray photoelectron spectroscopy (XPS), which will be described later.

[0029] (porous layer) A porous layer is laminated on at least one surface of the polyolefin microporous membrane. In the present invention, the porous layer refers to a layer having pores therein. The porous layer contains a resin binder and inorganic particles. The resin binder is used to bond the inorganic particles together and to bond the polyolefin microporous membrane to the porous layer, and the inorganic particles are used to ensure the strength of the porous layer.

[0030] (resin binder) The resin binder contained in the porous layer preferably has a midpoint glass transition temperature of -80°C to 0°C, more preferably -40°C to 0°C. If the midpoint glass transition temperature of the resin binder is higher than 0°C, the polymer chains contained in the resin binder have low fluidity and low adhesiveness at room temperature, which reduces the peel strength between the polyolefin microporous film and the porous layer, potentially causing peeling of the porous layer. If the midpoint glass transition temperature of the resin binder is 0°C or lower, the polymer chains contained in the resin binder have high fluidity and high adhesiveness at room temperature, resulting in good adhesion between the polyolefin microporous film and the porous layer. If the midpoint glass transition temperature of the resin binder is lower than -80°C, the resin binder has high fluidity and high adhesiveness at room temperature, which may cause blocking and reduce productivity when the separator for electrochemical devices is wound alone. Furthermore, a glass transition temperature of -40°C or higher can suppress the fluidity of the resin binder at room temperature, thereby reducing the risk that, when a porous layer is laminated on a polyolefin microporous membrane, the resin binder contained in the porous layer will penetrate into the micropores in the polyolefin microporous membrane present at the lamination interface between the polyolefin microporous membrane and the porous layer, increasing the air resistance of the separator for electrochemical devices and deteriorating ion mobility. Here, the midpoint glass transition temperature refers to the intersection of a line drawn by extending the low-temperature baseline toward the high-temperature side during the second heating cycle after heating and cooling, and a tangent drawn at the point where the gradient of the stepwise change in the glass transition curve is maximized, in differential scanning calorimetry (DSC) in accordance with JIS K7121:2012, "Measurement Method for Transition Temperature of Plastics."

[0031] Examples of the resin binder used in the porous layer include polyvinyl alcohol, cellulose ether resin, acrylic resin, polyvinylidene fluoride resin, etc. The resin binder can be used as an aqueous solution or aqueous dispersion, and may be commercially available.

[0032] (Inorganic particles) Examples of inorganic particles include inorganic oxide particles such as aluminum oxide, boehmite, silica, titanium oxide, zirconium oxide, iron oxide, and magnesium oxide, inorganic nitride particles such as aluminum nitride and silicon nitride, and sparingly soluble ionic crystal particles such as calcium fluoride, barium fluoride, and barium sulfate. These particles may be used alone or in combination of two or more.

[0033] The shape of the inorganic particles used may be any of spherical, plate-like, needle-like, rod-like, elliptical, etc. Among these, spherical shapes are preferred from the viewpoints of surface modification, dispersibility, and coatability.

[0034] The lower limit of the content of the inorganic particles is preferably 50% by mass or more, more preferably 80% by mass or more but less than 97% by mass, based on the total mass of the resin binder and the inorganic particles. When the content of the inorganic particles is within this range, the strength of the porous layer is maintained at an appropriate level.

[0035] The primary average particle size of the inorganic particles is preferably 0.10 μm to 5.0 μm, more preferably 0.10 μm to 2.5 μm, from the viewpoint of the strength and porosity of the porous layer. Setting the lower limit of the primary average particle size to 0.10 μm or more makes the porous layer dense, preventing the pores of the polyolefin microporous membrane from being closed, thereby increasing the air resistance and deteriorating battery performance. Setting the upper limit of the primary average particle size to 5.0 μm or less prevents the first porous layer from having a non-uniform structure, resulting in an insufficient heat shrinkage rate, and also prevents the porous layer from becoming thicker, resulting in a deterioration of battery performance. The primary average particle size of the inorganic particles can be measured using a laser diffraction particle size analyzer (HORIBA, Ltd., LA-960V2) in accordance with JIS Z 8825 (2013). The value at a volumetric integration rate of 50% was defined as the primary average particle size of the inorganic particles.

[0036] (Method for producing a polyolefin microporous membrane) Next, a method for producing a polyolefin microporous membrane will be described.

[0037] Methods for producing a polyolefin microporous membrane include a dry method (a method of making the membrane porous by using a crystal nucleating agent or particles without using a molding solvent (also known as a stretching method)) and a wet method (phase separation method), of which the wet method is preferred from the viewpoint of uniformity and flatness of the micropores.

[0038] Examples of wet production methods include a method in which a polyolefin resin and a molding solvent are heated, melted, and kneaded, the resulting resin solution is extruded through a die and cooled to form an unstretched gel-like sheet, the unstretched gel-like sheet is stretched in at least one direction, the molding solvent is removed, and the sheet is dried to obtain a microporous membrane.

[0039] The polyolefin microporous membrane may be a single-layer membrane or a multilayer membrane consisting of two or more layers with different molecular weights or average pore sizes. In the case of a multilayer membrane, it is preferable that the polyethylene resin of at least one outermost layer satisfies the above-mentioned molecular weight and molecular weight distribution.

[0040] A multilayer polyolefin microporous membrane consisting of two or more layers can be produced by, for example, heat-melting and kneading the polyolefin resins constituting Layer A and Layer B with a molding solvent, feeding the resulting resin solutions from the respective extruders into a single die and combining them for co-extrusion, or by overlapping and heat-sealing the layers. The co-extrusion method is preferred because it is easier to obtain interlayer adhesive strength, it is easier to form interconnected pores between the layers, it is easier to maintain high permeability, and it is also superior in productivity.

[0041] The production method for obtaining the microporous polyolefin membrane of the present invention will be described in detail below.

[0042] In the present invention, an unstretched gel-like sheet is stretched at a predetermined magnification in two directions, namely, the machine direction (also referred to as "MD" or "longitudinal direction") and the width direction (also referred to as "TD" or "transverse direction"), by a roll method, a tenter method, or a combination of these methods. There are two types of stretching: a sequential biaxial stretching method in which stretching is performed in the longitudinal direction and the transverse direction sequentially, and a simultaneous biaxial stretching method in which stretching is performed in the longitudinal direction and the transverse direction simultaneously, but the present invention does not care about the stretching method.

[0043] The method for producing the microporous polyolefin membrane used in the present invention comprises the following steps (a) to (f). (a) A step of melt-kneading a polyolefin resin and a molding solvent to prepare a polyolefin resin solution. (b) extruding the polyolefin resin solution and cooling it to form an unstretched gel-like sheet. (c) a stretching step of stretching the gel-like sheet (d) removing the forming solvent from the biaxially stretched gel-like sheet and drying it; (e) heat-treating the dried sheet to obtain a polyolefin microporous membrane Each step will be described below.

[0044] (a) Preparation of polyolefin resin solution In the process of preparing a polyolefin resin solution, a molding solvent is added to a polyolefin resin, followed by melt-kneading to prepare a polyolefin resin solution. As a melt-kneading method, for example, a method using a twin-screw extruder as described in JP-B 06-104736 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well known, a description thereof will be omitted.

[0045] The molding solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin, and examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions having boiling points corresponding to these hydrocarbons, with nonvolatile solvents such as liquid paraffin being preferred.

[0046] The polyolefin resin concentration in the polyolefin resin solution is preferably 25 to 40 parts by weight, where the total of the polyolefin resin and the molding solvent is 100 parts by weight. When the polyolefin resin concentration is within this range, swelling and necking at the die outlet when the polyolefin resin solution is extruded can be prevented, and the moldability and self-supporting properties of the gel-like sheet can be maintained.

[0047] (b) forming an unstretched gel-like sheet In the process of forming an unstretched gel-like sheet, a polyolefin resin solution is fed from an extruder directly or via another extruder to a die, extruded into a sheet, and cooled to form an unstretched gel-like sheet. Multiple polyolefin solutions of the same or different compositions may be fed from extruders to a single die, where they are layered and extruded into a sheet.

[0048] The extrusion method may be either a flat die method or an inflation method. The extrusion temperature is preferably 140 to 250°C, and the extrusion speed is preferably 0.2 to 15 m / min. The film thickness can be adjusted by adjusting the extrusion rate of the polyolefin solution. Examples of extrusion methods that can be used include those disclosed in JP-B 06-104736 and JP-A 3347835.

[0049] The polyolefin resin solution extruded into a sheet is cooled to form a gel-like sheet. Cooling methods include contacting the solution with a refrigerant such as cold air or cooling water, or contacting the solution with a cooling roll. However, cooling by contacting the solution with a roll cooled with a refrigerant is preferred. For example, an unstretched gel-like sheet can be formed by contacting the polyolefin resin solution extruded into a sheet with a rotating cooling roll whose surface temperature has been set to 20°C to 40°C with a refrigerant. The extruded polyolefin resin solution is preferably cooled to 25°C or below.

[0050] (c) Stretching process Next, the resulting gel-like sheet is stretched at least uniaxially. Since the gel-like sheet contains the membrane-forming solvent, it can be stretched uniformly. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial or biaxial, with biaxial stretching being preferred. In the case of biaxial stretching, simultaneous biaxial stretching, sequential stretching, or multistage stretching (e.g., a combination of simultaneous biaxial stretching and sequential stretching) can all be used.

[0051] The stretching ratio (area stretching ratio) in this stretching step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The stretching ratios in the machine direction (MD) and the transverse direction (TD) may be the same or different. The stretching ratio in this stretching step refers to the area stretching ratio of the microporous substrate immediately before being subjected to the next step, based on the gel-like sheet immediately before the stretching step.

[0052] The stretching temperature in this stretching step is preferably within the range of the crystal dispersion temperature (Tcd) of the polyolefin to Tcd + 30°C, more preferably within the range of Tcd + 5°C to Tcd + 28°C, and particularly preferably within the range of Tcd + 10°C to Tcd + 26°C. For example, in the case of polyethylene, the stretching temperature is preferably 90 to 140°C, more preferably 100 to 130°C. The crystal dispersion temperature (Tcd) is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065.

[0053] When polyethylene is used as the polyolefin, stretching as described above causes cleavage between polyethylene lamellae, resulting in the polyethylene phase becoming finer and the formation of numerous fibrils. The fibrils form a network structure in which the fibrils are irregularly connected in three dimensions, and the gel-like sheet becomes a microporous substrate. Stretching improves mechanical strength and enlarges the pores, but by stretching under appropriate conditions, it is possible to control the through-pore diameter and achieve a high porosity even with a thin film thickness.

[0054] Depending on the desired physical properties, the film can be stretched by providing a temperature distribution in the film thickness direction, thereby obtaining a microporous substrate with excellent mechanical strength. Details of this method are described in Japanese Patent No. 3347854.

[0055] (d) A step of removing the forming solvent from the biaxially stretched gel-like sheet and drying it. The biaxially stretched gel-like sheet is washed to remove the molding solvent, followed by drying. Examples of suitable cleaning solvents include hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; fluorinated hydrocarbons such as trifluoroethane; and ethers such as diethyl ether and dioxane. These cleaning solvents are selected appropriately depending on the molding solvent used to dissolve the polyolefin, and are used alone or in combination. Cleaning methods include immersion in the cleaning solvent and extraction, showering the cleaning solvent, suctioning the cleaning solvent from the opposite side of the sheet, or a combination of these methods. The above-described washing is continued until the residual solvent in the sheet is less than 1% by mass. The sheet is then dried, which can be achieved by heat drying or air drying.

[0056] (e) A step of heat-treating the dried sheet to obtain a polyolefin microporous membrane. The dried sheet is heat-treated to obtain a polyolefin microporous membrane. From the viewpoint of heat shrinkage and air resistance, the heat treatment is preferably carried out at a temperature within a range of 90 to 150°C. The residence time in the heat treatment step is not particularly limited, but is usually 1 second to 10 minutes, preferably 3 seconds to 2 minutes. The heat treatment can be carried out by any of the tenter method, roll method, rolling method, and free method.

[0057] In the heat treatment step, it is preferable to shrink the film in at least one of the machine direction and the cross direction while fixing the film in both the machine direction and the cross direction. The heat treatment step can remove residual strain in the polyolefin microporous membrane. The shrinkage rate in the machine direction or the cross direction in the heat treatment step is preferably 0.01 to 50%, more preferably 3 to 20%, from the viewpoints of heat shrinkage rate and air resistance. Furthermore, the film may be reheated and re-stretched to improve mechanical strength. The re-stretching step may be performed using either a stretching roll system or a tenter system.

[0058] (Corona treatment process) It is preferable to subject the surface of the polyolefin microporous membrane obtained by the above method to be laminated with a porous layer to corona treatment. This may be performed continuously from step (e) above, or the film may be wound up after step (e) and then rewound and corona treated. Corona treatment has the effect of cleaving the polymer bonds of the polyolefin contained in the polyolefin microporous membrane, which reduces the shrinkage stress in the in-plane direction of the polyolefin microporous membrane when the polyolefin melts in a temperature range above the melting point of the polyolefin, allowing the polyolefin to penetrate into the porous layer in the inter-plane direction (thickness direction) of the polyolefin microporous membrane.

[0059] The treatment intensity of the corona treatment is 20W·min / m 2 More than 50W min / m 2 The treatment intensity of corona treatment is preferably less than 20W·min / m 2 By setting the treatment intensity to 50 W·min / m or more, sufficient cutting effect of the polymer bonds of the polyolefin can be obtained to allow the polyolefin contained in the microporous polyolefin film to penetrate into the porous layer when it melts. 2 When the corona treatment intensity is set to less than 50 W·min / m, the surface can be modified without damaging the network structure of the polyolefin microporous membrane and while maintaining its strength. To date, a known technique has been to subject a polyolefin microporous membrane to corona treatment to generate polar groups on the hydrophobic surface of the polyolefin microporous membrane, with the aim of improving the adhesion between the polyolefin microporous membrane and a porous layer or improving the uniform application of a porous layer to the polyolefin microporous membrane (Patent Documents 1 and 2). Generally, to improve the adhesion between the polyolefin microporous membrane and a porous layer or the uniform application of a porous layer to the polyolefin microporous membrane by corona treatment, the treatment intensity of the corona treatment should be 50 W·min / m 2The above is necessary. As a result, there is a concern that the network structure of the polyolefin microporous membrane will be damaged, resulting in problems such as a loss of mechanical strength of the polyolefin microporous membrane. In the present invention, focusing on the fact that the treatment intensity of the corona treatment required to cleave the polymer bonds of the polyolefin contained in the polyolefin microporous membrane, to allow the polyolefin contained in the polyolefin microporous membrane of the separator for electrochemical devices after heat treatment to penetrate into the porous layer and for some of the polyolefin that has penetrated into the porous layer to reach the surface of the porous layer, is lower than the treatment intensity of the corona treatment required to generate polar groups on the hydrophobic polyolefin microporous membrane surface and improve the adhesion between the polyolefin microporous membrane and the porous layer or the uniform coating of the porous layer on the polyolefin microporous membrane, the treatment intensity of the corona treatment is set to 20 W·min / m 2 More than 50W min / m 2 The porous layer is made of a resin binder having a midpoint glass transition temperature of -80 to 0° C. This makes it possible to obtain a separator for electrochemical elements that has excellent adhesion and meltdown resistance without compromising mechanical strength.

[0060] (Method of manufacturing porous layer) The composition and method of forming the porous layer are not particularly limited, but the porous layer can be obtained by carrying out a coating step and a film fixing step.

[0061] The order of preparing the coating liquid for forming the porous layer is not particularly limited, but from the viewpoint of uniformly dispersing the inorganic particles and making the primary average particle diameter of the inorganic particles in the coating liquid uniform, it is preferable to prepare the coating liquid by mixing a solution obtained by mixing and dissolving a resin binder and a polar solvent with a dispersion obtained by dispersing inorganic particles in a polar solvent, and further adding other organic resins, additives, etc. as necessary.

[0062] Here, as the solvent for dispersing the inorganic particles, polar solvents such as water, glycerol, ethylene glycol, propylene glycol, dimethyl sulfoxide, dimethylformamide, acetonitrile, ethylene carbonate, furfuryl alcohol, and methanol can be used.

[0063] In addition to the resin binder and inorganic particles, the coating liquid may contain organic resins, dispersants, thickeners, stabilizers, antifoaming agents, leveling agents, and the like, as needed.

[0064] The method for dispersing the coating liquid is not particularly limited, but from the viewpoint of flattening and uniforming the surface shape of the porous layer, it is important that the inorganic particles in the coating liquid are uniformly dispersed and that the inorganic particles have a uniform average primary particle size, and it is preferable to disperse the inorganic particles in a solvent using a ball mill, bead mill, sand mill, roll mill, etc., and then disperse the organic resin in the solvent. In particular, from the viewpoint of uniformity of the average primary particle size of the inorganic particles in the coating liquid, it is preferable to disperse using a bead mill, the diameter of the beads used in the bead mill is preferably 0.1 to 1 mm, and the material of the beads used is preferably aluminum oxide, zirconium oxide, zirconia-reinforced alumina, etc.

[0065] It is also preferable to add and disperse the fluorine-containing resin multiple times, and furthermore, from the viewpoint of uniformity of the average primary particle size of the inorganic particles in the coating liquid, it is preferable that the peripheral speed of the mixer used to mix the resin with the solvent is higher than the speed at which the inorganic particles are dispersed in the solvent and that the peripheral speed is increased stepwise.

[0066] The viscosity of the coating fluid is preferably 3 mPa·s or more and 200 mPa·s or less, more preferably 5 mPa·s or more and 150 mPa·s or less, and even more preferably 10 mPa·s or more and 100 mPa·s or less. A coating fluid viscosity of 200 mPa·s or less allows for high-speed coating and good productivity. A low viscosity coating fluid is preferable because it is suitable for high-speed coating, but from the perspective of stable coating film formation, for example, it is preferable that the viscosity does not fall below 3 mPa·s. The viscosity of the coating fluid can be controlled by the solids concentration of the coating fluid, the mixing ratio of the organic resin and inorganic particles, and the molecular weight of the organic resin.

[0067] Examples of coating methods used to coat at least one side of a polyolefin microporous membrane with the obtained coating liquid include dip coating, gravure coating, chamber doctor coating, slit die coating, knife coating, comma coating, kiss coating, roll coating, bar coating, spray coating, immersion coating, spin coating, screen printing, and pad printing. Among these, gravure coating, chamber doctor coating, and slit die coating are preferred from the viewpoint of mass productivity of the coating layer, and gravure coating and chamber doctor coating are particularly preferred when the coating liquid is coated on both sides of a polyolefin microporous membrane. As described above, a coating layer can be formed in the coating step.

[0068] When the coating process is carried out continuously, the conveying speed of the polyolefin microporous membrane can be set, for example, in the range of 5 m / min to 200 m / min, and can be appropriately set depending on the coating method from the viewpoints of productivity and uniformity of the coating thickness.

[0069] The porous layer can be obtained by carrying out a film fixing step in which the coating layer is fixed to form a porous layer. Film fixing refers to removing the solvent contained in the coating liquid by drying to obtain a porous layer. From the viewpoint of ion permeability, the porous layer preferably contains voids inside, and the film fixing step can cause the voids to be formed inside.

[0070] The film fixing step is not limited as long as it is a method that volatilizes the solvent contained in the coating liquid, and preferably includes at least one step selected from hot air drying, infrared drying, suction drying, vacuum drying, and microwave drying. Drying can be performed, for example, with hot air at 100°C or less.

[0071] The thickness of the porous layer (the total thickness when present on both sides of the microporous membrane) is preferably 1.0 to 5.0 μm, and more preferably 1.5 to 4.0 μm. If the thickness is less than 1.0 μm, it is not possible to prevent the polyolefin microporous membrane from melting and shrinking above the melting point of the polyolefin, which may cause membrane rupture and impair its insulating properties. If the thickness is 1.0 μm or more, it is possible to prevent the polyolefin microporous membrane from melting and shrinking above the melting point, ensuring membrane rupture strength and insulating properties. If the thickness is more than 5.0 μm, the polyolefin melted above the melting point of the polyolefin cannot reach the surface of the porous layer, making it difficult to maintain good meltdown resistance. If the thickness is 5.0 μm or less, it is possible to reduce the winding bulk and is suitable for increasing the capacity of electrochemical devices.

[0072] (electrochemical element) The electrochemical device referred to in the present invention comprises an electrode assembly and a battery case that houses the electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. A separator for an electrochemical device, in which a porous layer containing a resin binder and inorganic particles is laminated on at least one side of the polyolefin microporous membrane of the present invention, can be suitably used as the separator.

[0073] Examples of such electrochemical elements include primary batteries, secondary batteries, electric double layer capacitors, and aluminum electrolytic capacitors.

[0074] Examples of primary batteries include manganese dry batteries, alkaline manganese dry batteries, graphite fluoride-lithium batteries, manganese dioxide-lithium batteries, solid electrolyte batteries, water-injected batteries, and thermal batteries.

[0075] Examples of secondary batteries include lithium ion secondary batteries, lead acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-iron batteries, silver oxide-zinc batteries, manganese dioxide-lithium secondary batteries, lithium cobalt oxide-carbonate secondary batteries, and vanadium-lithium secondary batteries.

[0076] Among these, secondary batteries are preferred because they can be used for a long period of time, and lithium ion secondary batteries are more preferred because they achieve high energy density by using an organic solvent.

[0077] The battery case may be, for example, an aluminum case, an iron case with a nickel-plated inner surface, or a case made of an aluminum laminate film.

[0078] The shape of the battery case can be pouch-shaped, cylindrical, square, coin-shaped, etc. Among these, the pouch-shaped case is preferred because it can achieve a high energy density and can be freely designed in shape at low cost.

[0079] The positive electrode is formed by laminating a positive electrode material made of an active material, a binder resin, and a conductive additive on a current collector.

[0080] Examples of active materials include layered lithium-containing transition metal oxides such as LiCoO2, LiNiO2, and Li(NiCoMn)O2, spinel-type manganese oxides such as LiMn2O4, and iron-based compounds such as LiFePO4.

[0081] The binder resin may be a resin with high oxidation resistance, such as a fluorine-containing resin, an acrylic resin, or a styrene-butadiene resin.

[0082] Examples of the conductive additive include carbon materials such as carbon black and graphite.

[0083] As the current collector, a metal foil is suitable, and aluminum is particularly often used.

[0084] The negative electrode is formed by laminating a negative electrode material made of an active material and a binder resin on a current collector.

[0085] Active materials include carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon, lithium alloy materials such as tin and silicon, metal materials such as lithium, and lithium titanate (Li4Ti5O 12 ) etc.

[0086] Examples of the binder resin include fluorine-containing resin, acrylic resin, and styrene-butadiene resin.

[0087] As the current collector, a metal foil is suitable, and copper foil is particularly often used.

[0088] The electrochemical element of the present invention preferably contains an electrolytic solution, which serves as a site for ion migration between a positive electrode and a negative electrode in an electrochemical element such as a secondary battery, and is configured by dissolving an electrolyte in an organic solvent.

[0089] Examples of the electrolyte include LiPF6, LiBF4, and LiClO4, but LiPF6 is preferably used from the viewpoints of solubility in organic solvents and ionic conductivity.

[0090] Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and two or more of these organic solvents may be mixed and used.

[0091] Hereinafter, a method for producing a lithium ion secondary battery, which is preferably used among electrochemical devices, will be described.

[0092] The method for producing a lithium-ion secondary battery involves first dispersing an active material and a conductive additive in a binder resin solution to prepare a coating solution for electrodes, then applying the coating solution to a current collector and drying the solvent to obtain a positive electrode and a negative electrode. The thickness of the coating film after drying is preferably 50 μm to 500 μm.

[0093] A lithium ion secondary battery separator is placed between the obtained positive electrode and negative electrode so as to be in contact with the active material layer of each electrode, and the resultant is enclosed in an exterior material such as an aluminum laminate film. After injecting the electrolyte, a negative electrode lead and a safety valve are installed, and the exterior material is sealed.

[0094] The lithium ion secondary battery obtained in this manner has high adhesion to the electrode, has excellent battery characteristics, and can be produced at low cost.

[0095] (Other uses) The porous composite film of the present invention can be used as a material for separating substances, selectively permeating, and isolating substances, etc. Specific applications include, in addition to the electrochemical devices described above, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, moisture-permeable waterproof clothing, and medical materials. [Example]

[0096] The present invention will be described in more detail below with reference to examples, but the present invention is not limited thereto. It's not something like that. In the following, Example 10 will be read as Reference Example 10.

[0097] [Measurement method] (Thickness of separator and porous layer for electrochemical element) The thickness was measured using a contact-type film thickness meter (Mitutoyo Corporation's "Litematic" (registered trademark) series 318). Measurements were performed using a 9.5 mm diameter spherical carbide probe at 20 points under a load of 0.01 N, and the average of the measurements was taken as the thickness. The thickness t of the porous layer was calculated using the following formula. t = thickness of sample after porous layer formation (t1) - thickness of sample before porous layer formation (t2).

[0098] (Air resistance) One randomly selected spot was selected from each of three 100mm x 100mm size samples, and measurements were taken in accordance with JIS P 8117 (2009) using an Oken-type air resistance measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.). The average value was taken as the air resistance (seconds / 100cm 3 ) was decided.

[0099] (Piercing strength) The puncture strength was measured using an IMADA digital force gauge (Model DS2-20N) with a needle (Kato Tech P-3000S-102G-10) having a diameter of 1 mm (tip radius of 0.5 mm) to puncture a separator for electrochemical elements having a film thickness of T1 (μm) held at a speed of 2 mm / sec. The puncture strength P2 was calculated using the following formula, assuming a film thickness of 20 μm. Formula: P2=(P1×20) / T1 The 20 μm equivalent puncture strength was evaluated according to the following three levels. Evaluation criteria A (Good): 8.3N or more B (acceptable): 7.3N or more and less than 8.3N C (impossible): Less than 7.3N.

[0100] (Adhesion of porous layer) A duralumin plate (5 cm x 15 cm) was prepared and a 1.8 cm x 5 cm piece of 3M Scotch transparent double-sided tape (acrylic adhesive, part number 665-1-18) was attached to the center. Next, a sample (3 cm x 6 cm) was attached using a hand roller with the porous layer facing the double-sided tape and covering it, and then left at room temperature for 5 minutes. The duralumin plate was then fixed in place, and a peel test was performed using a Shimadzu Autograph AGS-J at a peel speed of 300 mm / min, with the edge of the sample gripped and a peel angle of 180°. The peel strength was measured using the average peel strength over a 3 cm length of tape. The peel strength was evaluated using the following four-point scale. Evaluation criteria A (excellent): 60gf / 25mm or more B (Good): 40gf / 25mm or more and less than 60gf / 25mm C (Acceptable): 20gf / 25mm or more, less than 40gf / 25mm D (unacceptable): Less than 20gf / 25mm.

[0101] (Shutdown temperature, meltdown temperature) A circular measurement sample with a diameter of 19 mm was cut out from the separator for electrochemical elements. In addition, components of a 2032-type coin cell manufactured by Hohsen Co., Ltd. (top lid, bottom lid, PFA gasket, circular spacer with a diameter of 15.5 mm and a thickness of 1.0 mm, and wave washer) were prepared.

[0102] First, in a dry room with a dew point temperature below -35°C, two cells were prepared, each with a measurement sample and a gasket placed on the bottom lid. Then, a solution of 1M Kishida Chemical Co., Ltd. electrolyte, consisting of LiBF4 in a 50 / 50 EC / PC mixed solvent (by weight), with 0.3 wt.% DIC surfactant F-444, was poured into the prepared cells. Next, a spacer was placed on top of the measurement sample in the hollow portion of the gasket, and the cell was left standing at approximately -50 kPa for 1 minute, repeating this cycle twice to impregnate the measurement sample with the electrolyte. Next, a wave washer and a top lid were placed on the spacer, starting from the spacer, and the cell was sealed with a coin cell crimper to obtain a sample cell.

[0103] The obtained sample cell was clamped by a coaxial contact probe placed in an oven, and the resistance of the cell was measured using a Hioki LCR meter at an amplitude of 50 mV and a frequency of 1 kHz. The temperature of the coin cell was measured by attaching a resistance thermometer to the top cover of the cell. After heating from room temperature to 50°C, the cell was left to stand for 10 minutes, and the resistance was measured while heating to 180°C at a rate of 5°C / min. 2 The temperature when it reaches the shutdown temperature is taken as the shutdown temperature. After reaching the shutdown temperature, the temperature continues to rise and the measured resistance again reaches 1 kΩcm. 2If the meltdown temperature is 180°C or higher, the separator for electrochemical elements maintains its shutdown properties even when the battery containing the separator for electrochemical elements is heated or when the temperature inside the battery rises due to an internal short circuit or the like, and is therefore deemed to have high meltdown resistance, being less likely to short circuit or not allowing a large current to flow even if a short circuit occurs.

[0104] (surface oxygen concentration) The surface oxygen concentration of a polyolefin microporous membrane can be determined by X-ray photoelectron spectroscopy (XPS) according to the following procedure. Measurements were performed using a KRATOS ULTRA2 instrument manufactured by Shimadzu Corporation. After cutting the polyolefin microporous membrane and spreading it on a stainless steel sample support, the photoelectron incident angle was set to 90 degrees, and an AlKα X-ray source was used. The sample chamber was heated to 1×10 -6 The vacuum was maintained at 100 [Pa]. To correct for peaks associated with charging during measurement, the binding energy of the main C1s peak was first adjusted to 284.6 eV. The O1s peak area was determined by drawing a linear baseline in the range of 528 to 536 eV, and the C1s peak area was determined by drawing a linear baseline in the range of 281 to 289 eV. When the surface of a polyolefin microporous membrane contains at least one polar group selected from -COOH, -OH, -CO-R, and -COO-R (where R is an alkyl group, and the carbon chain of the alkyl group may be linear or branched), a peak appears in the binding energy range of 528 to 536 eV. The surface oxygen concentration of the polyolefin microporous membrane was calculated as the ratio of the O1s peak area to the C1s peak area. The presence of oxygen-containing polar groups on the surface of the polyolefin microporous membrane was determined from the surface oxygen concentration ratio. The surface oxygen concentration was determined based on the following criteria. ◯ (Good): The surface oxygen concentration was 1% or more. × (bad): The surface oxygen concentration was less than 1%.

[0105] (Infrared spectroscopy total internal reflection method 2800cm -1 From 3000cm -1 The peak that appears in Absorption ) A rectangular measurement sample, 15 mm long (machine direction) x 7 mm wide, was cut from the electrochemical element separator. A 5 mm x 5 mm square hole was drilled in the center of a 13 mm wide piece of heat-resistant polyimide tape (API-114AFR, manufactured by Chukoh Flow), and the polyolefin microporous membrane side was attached and fixed in place while being gripped in all directions. The polyimide tape with the sample fixed to it was attached to a 30 mm long x 20 mm wide x 0.5 mm thick aluminum plate with a 5 mm diameter hole, and sandwiched between two other aluminum plates of the same shape. These were then fixed to a glass slide with polyimide tape to obtain a sand sample that was transparent to visible light for observation. The sand sample was mounted on a Mettler Toledo FP82HT hot stage and heated to 100°C under a microscope, then left to stand for 1 minute. The sample was then heated to 200°C at a rate of 2°C / min, and then cooled to 25°C at a rate of -30°C / min. The heat-treated electrochemical element separator was then removed from the slide glass.

[0106] Using a Horiba FT-720, the number of scans was 30, the instrument function was HG, the scanning speed was 5.0, the resolution was 4, the gain was AUTO, and the measurement wavenumber range was 600 cm. -1 ~4000cm -1 The porous layer side of the separator for electrochemical elements after the heat treatment was measured by the ATR method (ATR crystal: diamond) at the center position in both the length direction and width direction of the separator for electrochemical elements after the heat treatment. The measurement was carried out by placing the porous layer side of the separator for electrochemical elements after the heat treatment on the sample holder attached to the device so that it faced the ATR crystal side, and pressing it down with a pressure rod attached to the device. -1 and 3000cm -1 The line segment connecting these points was used as the baseline, and baseline correction was performed. -1 From 3000cm -1The absorption was recorded in a wavenumber range of 100 to 150. In infrared spectroscopy attenuated total reflection, the penetration depth of infrared light in the thickness direction of the separator is 2 μm or less. Therefore, in a separator for electrochemical elements having a porous layer thickness of more than 2 μm, if the absorption was less than 0.05, it was determined that the polyolefin contained in the polyolefin microporous membrane of the separator for electrochemical elements after the heat treatment had not permeated into the porous layer, and if the absorption was 0.05 or more, it was determined that the polyolefin contained in the polyolefin microporous membrane of the separator for electrochemical elements after the heat treatment had permeated into the porous layer, and that some of the polyolefin that had permeated into the porous layer had reached the surface of the porous layer.

[0107] [Example 1] (Preparation of microporous polyolefin membrane) Mass average molecular weight 2.7×10 6 40% by mass of ultra-high molecular weight polyethylene and a mass average molecular weight of 2.6 × 10 5 A polyethylene composition was prepared by dry-blending 100 parts by mass of a composition consisting of 60% by mass of high-density polyethylene (methicone-2-yl)methylpropionate with 0.375 parts by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane. 23 parts by mass of the resulting polyethylene composition was charged into a twin-screw extruder. 77 parts by mass of liquid paraffin was further fed from a side feeder of the twin-screw extruder and melt-kneaded to prepare a polyethylene resin solution in the extruder. The polyethylene resin solution was then extruded at 190°C through a die attached to the tip of the extruder and taken up by a cooling roll with internal cooling water maintained at 25°C to form an unstretched gel-like sheet.

[0108] The resulting unstretched gel-like sheet was introduced into a sequential biaxial stretching machine and stretched 5x5 times in the sheet conveying direction (MD) and the sheet width direction (TD). The preheating / stretching / heat setting temperatures were adjusted to 115 / 115 / 100°C. The resulting biaxially stretched gel-like sheet was cooled to 30°C, and the liquid paraffin was removed in a methylene chloride washing tank adjusted to 25°C. The sheet was then dried in a drying oven adjusted to 60°C to obtain a polyolefin microporous membrane.

[0109] (Corona treatment of polyolefin microporous membrane) Corona treatment was performed on one side of a polyolefin microporous membrane using a corona treatment device, model CTW-0112N, manufactured by Wedge Corporation, with two stainless steel electrode rods with an outer diameter of 5 mm and an electrode width of 200 mm. Corona discharge was generated under conditions of a frequency of 27.8 kHz, a voltage of 76 V, a current of 2 A, and a power of 0.16 kW. The polyolefin microporous membrane was transported at a speed of 20 m / min, resulting in a discharge rate of 40 W·min / m. 2 The corona treatment was carried out at a treatment intensity of 1000 ppm. The surface oxygen concentration of the corona-treated polyolefin microporous membrane thus obtained was determined. The results are shown in Table 1.

[0110] (Preparation of coating solution) Alumina (approximately spherical, particle size 0.5 μm), acrylic resin A (midpoint glass transition temperature -25°C), and carboxymethyl cellulose (radius of gyration 25 nm) were added to water in a weight ratio of 98:1:1 and stirred to obtain a uniformly dispersed slurry.

[0111] <Stacking of porous layers> The slurry was applied to the corona-treated surface of the polyolefin microporous membrane using gravure coating and dried for 20 seconds in a hot air oven (60°C) to obtain a separator for electrochemical devices with a porous layer thickness of 4 μm. This separator for electrochemical devices was evaluated for the thickness of the separator for electrochemical devices and the porous layer, air resistance, puncture strength, adhesion of the porous layer, shutdown temperature, meltdown temperature, and infrared spectroscopy (ATT). The results are shown in Table 1.

[0112] [Example 2] Except for changing the porous layer thickness to 3 μm, a polyolefin microporous membrane and a separator for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0113] [Example 3] Except for changing the porous layer thickness to 2 μm, a polyolefin microporous membrane and a separator for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0114] [Example 4] Polyolefin microporous membranes and separators for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 1, except that the discharge amount in the corona treatment was changed as shown in Table 1 by changing the conveying speed of the polyolefin microporous membrane during the corona discharge treatment. The results are shown in Table 1.

[0115] [Example 5] Except for changing the resin binder used to prepare the coating solution to acrylic resin B (intermediate glass transition temperature: -40°C), a separator for electrochemical elements was produced in the same manner as in Example 1, in which a porous layer was laminated on a polyolefin microporous membrane whose surface to be laminated with the porous layer had been subjected to corona treatment, and the separator was measured and evaluated. The results are shown in Table 1.

[0116] [Example 6] Separators for electrochemical devices each having a porous layer laminated on a polyolefin microporous membrane were produced, measured, and evaluated in the same manner as in Example 5, except that the discharge amount in the corona treatment was changed as shown in Table 1 by changing the conveying speed of the polyolefin microporous membrane during the corona discharge treatment. The results are shown in Table 1.

[0117] [Example 7] Except for changing the resin binder used to prepare the coating solution to acrylic resin C (intermediate glass transition temperature: −7° C.), a separator for electrochemical elements was produced in the same manner as in Example 1, in which a porous layer was laminated on a polyolefin microporous membrane whose surface to be laminated with the porous layer had been subjected to corona treatment, and the separator was measured and evaluated. The results are shown in Table 1.

[0118] [Example 8] Separators for electrochemical devices each having a porous layer laminated on a polyolefin microporous membrane were produced, measured, and evaluated in the same manner as in Example 7, except that the discharge amount in the corona treatment was changed as shown in Table 1 by changing the conveying speed of the polyolefin microporous membrane during the corona discharge treatment. The results are shown in Table 1.

[0119] [Example 9] Polyolefin microporous membranes and separators for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 1, except that the discharge amount in the corona treatment was changed as shown in Table 1 by changing the conveying speed of the polyolefin microporous membrane during the corona discharge treatment. The results are shown in Table 1.

[0120] [Example 10] Except for changing the resin binder used to prepare the coating solution to acrylic resin D (intermediate glass transition temperature 50°C), a separator for electrochemical elements was produced in the same manner as in Example 1, in which a porous layer was laminated on a polyolefin microporous membrane whose surface to be laminated with the porous layer had been subjected to corona treatment, and the separator was measured and evaluated. The results are shown in Table 1.

[0121] [Comparative Example 1] Except for not subjecting the polyolefin microporous membrane to corona treatment, a polyolefin microporous membrane and a separator for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0122] [Comparative Examples 2 and 3] Polyolefin microporous membranes and separators for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 1, except that the discharge amount in the corona treatment was changed as shown in Table 1 by changing the conveying speed of the polyolefin microporous membrane during the corona discharge treatment. The results are shown in Table 1.

[0123] Comparative Example 4 Except for not subjecting the polyolefin microporous membrane to corona treatment, a polyolefin microporous membrane and a separator for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 5. The results are shown in Table 1.

[0124] Comparative Example 5 Except for not subjecting the polyolefin microporous membrane to corona treatment, a polyolefin microporous membrane and a separator for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 7. The results are shown in Table 1.

[0125] Comparative Example 6 Except for not subjecting the polyolefin microporous membrane to corona treatment, a polyolefin microporous membrane and a separator for electrochemical devices were produced, measured, and evaluated in the same manner as in Example 10. The results are shown in Table 1.

[0126] [Table 1]

[0127] Table 1 and a comparison between the Examples and Comparative Examples shows that polyolefin microporous membranes in which a porous layer was formed after appropriate corona treatment had mechanical strength and maintained adhesion between the porous layer and the polyolefin microporous membrane, while also having a high meltdown temperature.

Claims

1. A separator for an electrochemical element, in which a porous layer containing a resin binder and inorganic particles is laminated on at least one side of a polyolefin microporous film, wherein the separator for an electrochemical element is heated to 200° C. and then cooled to room temperature, and the porous layer side of the separator for an electrochemical element is measured by a total reflection method of infrared spectroscopy, and the measured value is 2800 cm －１ From 3000 cm －１ The absorbance of the peak appearing at is 0.05 or more and less than 0.17, A separator for an electrochemical element, wherein the resin binder contained in the porous layer is an acrylic resin having a midpoint glass transition temperature of -80°C or higher and 0°C or lower.

2. 2. The separator for an electrochemical element according to claim 1, wherein the surface of the polyolefin microporous film on which the porous layer containing the resin binder and inorganic particles is laminated has a surface oxygen concentration of 1% or more.

3. 3. The separator for an electrochemical element according to claim 1, wherein the meltdown temperature measured by an impedance method is 180° C. or higher.

4. (A) Step of subjecting the polyolefin microporous membrane to corona treatment (B) A step of applying a coating liquid containing a resin binder and inorganic particles to at least one surface of the polyolefin microporous membrane and drying the coating liquid. A method for producing a separator for an electrochemical element, comprising: ２ More than 50W・min / m ２ The method for producing a separator for an electrochemical element according to claim 1 , wherein the total mass of the separator is less than 1000 mol / L.

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