Porous membrane, separator for electrochemical element, member for electrochemical element, and electrochemical element

Abstract

To provide a porous film that can be used to obtain an electrochemical element with excellent cycle characteristics.SOLUTION: A porous membrane according to the present disclosure has a surface in which the ratio R5 / R0.3 of the pore area ratio R5 [%] measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 5 kV to the pore area ratio R0.3 [%] measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 0.3 kV is 5 or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a porous membrane, a separator for an electrochemical device, a member for an electrochemical device, and an electrochemical device. [Background technology]

[0002] Electrochemical devices such as non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, have high energy densities and are therefore widely used as batteries for personal computers, mobile phones, personal digital assistants, and vehicle-mounted devices.

[0003] As a component of such non-aqueous electrolyte secondary batteries, separators with excellent heat resistance have been developed. For example, as disclosed in Patent Document 1, a separator is known in which a heat-resistant layer containing an aramid resin and inorganic particles is laminated on a porous substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 176421 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when separators such as those of the above-mentioned conventional techniques are used in electrochemical elements, there is room for improvement in terms of cycle characteristics. One aspect of the present invention aims to realize a porous membrane that can be used to obtain an electrochemical element with excellent cycle characteristics. [Means for solving the problem]

[0006] In order to solve the above problems, a porous membrane according to one embodiment of the present invention has a pore area ratio R5 [%] measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 5 kV and a pore area ratio R1 [%] measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 0.3 kV.0.3 [%] and the ratio R5 / R 0.3 has a face where is 5 or more.

[0007] A porous membrane according to another embodiment of the present invention has a first surface and a second surface, and has a pore area ratio R measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A [%] and the pore area ratio R measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. B [%] Ratio A / R B is 2 or more, and R B is less than 5%.

[0008] A porous membrane according to yet another embodiment of the present invention has a first surface and a second surface, and has an average pore diameter d measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A [nm] and the average pore diameter d measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. B [nm] A / d B is less than 1.2, and the pore area ratio measured from the scanning electron microscope image of the first surface or the second surface is 5% or less. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide a porous film that can be used to obtain an electrochemical device with excellent cycle characteristics. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 shows scanning electron microscope images of the front and back surfaces of the porous membranes of Examples 1 and 2, respectively, obtained at an accelerating voltage of 0.3 kV. [Figure 2] FIG. 2 shows scanning electron microscope images of the front and back surfaces of the porous membranes of Comparative Examples 1 and 2, obtained at an accelerating voltage of 0.3 kV. [Figure 3] FIG. 2 shows scanning electron microscope images of the backside surfaces of the porous membranes of Examples 1 and 2, obtained at accelerating voltages of 0.3 kV and 5 kV. [Figure 4] FIG. 1 shows scanning electron microscope images of the back surfaces of the porous membranes of Comparative Examples 1 and 2, obtained at acceleration voltages of 0.3 kV and 5 kV. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present invention will be described below, but the present invention is not limited thereto. In this specification, unless otherwise specified, "A to B" representing a numerical range means "A or more and B or less."

[0012] [1. Porous membrane] [First embodiment] The porous membrane according to the first embodiment has a pore area ratio R5 [%] measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 5 kV and a pore area ratio R 0.3 [%] and the ratio R5 / R 0.3 has a face where is 5 or more.

[0013] In this specification, a scanning electron microscope image refers to an image obtained by a scanning electron microscope (SEM). Hereinafter, a scanning electron microscope image is also referred to as an SEM image. The SEM image is an image obtained by observing the surface of a porous membrane from a direction perpendicular to the in-plane direction, and is not an image obtained by observing a cross section. The SEM image may be, for example, an image obtained under the conditions of a working distance (WD) of 3 mm, a backscattered electron image, and an image resolution of 49.6 nm / pix.

[0014] In this specification, the pore area ratio [%] is the area [nm 2 ] to the total pore area [nm 2The total pore area refers to the sum of the areas of each pore included in the obtained SEM image.

[0015] The higher the accelerating voltage in an SEM, the deeper the electron beam penetrates from the surface of the sample, allowing images of the structure of deeper regions to be obtained. SEM images obtained at an accelerating voltage of 0.3 kV are images of a very thin layer near the outermost surface of the porous film. SEM images obtained at an accelerating voltage of 5 kV are images of a deeper region, i.e., a region further inside, than SEM images obtained at an accelerating voltage of 0.3 kV. For example, an SEM image obtained at an accelerating voltage of 0.3 kV may be an image of a region less than 1 μm deep from the outermost surface of the porous film, while an SEM image obtained at an accelerating voltage of 5 kV may be an image of a region 1 μm or deeper from the outermost surface of the porous film.

[0016] R5 / R 0.3 R5 / R is 5 or more, which means that the pore area ratio in the region deeper inside is larger than that in the very thin layer near the outermost surface of the porous membrane, and the difference in the pore area ratio is large. In other words, in the very thin layer near the outermost surface of the porous membrane, the number of pores is small and they are blocked. This layer is also referred to as a blocked layer in this specification. In this specification, blocked does not mean that there are no pores at all, but that the number of pores is relatively small. 0.3 is 5 or more, it is suggested that the blockage layer exists only in a very thin region near the outermost surface of the porous membrane.

[0017] For example, repeated charge and discharge in an electrochemical element can cause dendrites derived from metals contained in the electrolyte. If a porous film having the above-described blocking layer is used as a separator, dendrites are less likely to penetrate into the separator, and the growth of dendrites is suppressed, resulting in an electrochemical element with excellent cycle characteristics.

[0018] In the porous membrane, R5 / R 0.3The surface on which the value of the blocking layer is present in a very thin region near the outermost surface may be referred to as a blocked surface.

[0019] R5 / R 0.3 is preferably 10 or more, and more preferably 15 or more. 0.3 The upper limit of R is not particularly limited, but may be, for example, 30 or less. 0.3 is preferably 5% or less, more preferably 3% or less. 0.3 R5 may be, for example, 0.5% or more, or 1% or more. R5 is preferably 25% or more, and more preferably 30% or more. R5 is preferably 50% or less, and more preferably 40% or less.

[0020] As mentioned above, the porous membrane may have a closed surface on both sides or on one side. In other words, the porous membrane has a closed surface on one side and an R5 / R 0.3 For example, on one side of the porous membrane, R5 / R 0.3 may be 3 or less, or may be 1.5 or less. 0.3 On one side of the porous membrane, R 0.3 may be 10% or less, or may be 9% or less. 0.3 For example, R5 may be 7% or more, or 8% or more. On one side of the porous membrane, R5 may be 6% or more, or 7% or more. Furthermore, R5 may be 10% or less, or 9% or less.

[0021] The average pore area S measured from SEM images obtained at an accelerating voltage of 0.3 kV 0.3 is 100000nm 2 It is preferable that the wavelength is 150,000 nm or more. 2It is more preferable that the value is equal to or greater than S. 0.3 is 400000nm 2 Preferably, it is 350,000 nm or less. 2 The average pore area S5 measured from an SEM image obtained at an accelerating voltage of 5 kV is more preferably 200,000 nm or less. 2 It is preferable that the wavelength is 250,000 nm or more. 2 More preferably, S5 is 350,000 nm or more. 2 Preferably, it is 300,000 nm or less. 2 It is more preferable that it is S5 / S 0.3 is preferably 0.7 or more, and more preferably 0.9 or more. 0.3 is preferably 1.5 or less, more preferably 1.3 or less. That is, in the porous membrane, the ratio of the average pore area to the ratio of the pore area ratios of the outermost surface and the interior may be small.

[0022] On one side of the porous membrane, S 0.3 is 200000nm 2 May be greater than 250,000 nm 2 In addition, on one side of the porous membrane, S 0.3 is 400000nm 2 May be less than 350,000 nm 2 On one side of the porous membrane, S5 may be 100,000 nm or less. 2 It may be more than 150,000 nm 2 On one side of the porous film, S5 may be 250,000 nm or more. 2 May be less than 200,000 nm 2 On one side of the porous membrane, S5 / S 0.3 may be 0.4 or more, or may be 0.5 or more. 0.3 The R5 / R may be 0.7 or less, or may be 0.6 or less. 0.3 It may be a surface where is less than 5.

[0023] In this specification, the average pore area means the value obtained by dividing the sum of the areas of the individual pores (pore areas) contained in the obtained SEM image by the number of pores.

[0024] The porous membrane has an average pore area S measured from an SEM image obtained by imaging at an accelerating voltage of 0.3 kV. 0.3 500000nm 2 It is preferable that the porous membrane has a surface with an average pore area of ​​less than 500,000 nm on both sides. This can further suppress the growth of dendrites. 2 The average pore area on only one side may be less than 500,000 nm 2 It may be less than.

[0025] The average pore diameter d measured from SEM images obtained at an accelerating voltage of 0.3 kV 0.3 is preferably 250 nm or more, more preferably 300 nm or more. 0.3 is preferably 500 nm or less, more preferably 450 nm or less. The average pore diameter d5 measured from an SEM image obtained at an accelerating voltage of 5 kV is preferably 250 nm or more, more preferably 300 nm or more. Furthermore, d5 is preferably 450 nm or less, more preferably 400 nm or less. d5 / d 0.3 is preferably 0.5 or more, and more preferably 0.7 or more. 0.3 is preferably 1.5 or less, more preferably 1.3 or less, and may be 1.1 or less. That is, in the porous membrane, the ratio of the average pore diameter to the ratio of the pore area ratio of the outermost surface to the interior may be small.

[0026] On one side of the porous membrane, d 0.3 may be 300 nm or more, or 350 nm or more. 0.3On one side of the porous membrane, d5 may be 450 nm or less, or 400 nm or less. On one side of the porous membrane, d5 may be 200 nm or more, or 250 nm or more. On one side of the porous membrane, d5 may be 400 nm or less, or 350 nm or less. On one side of the porous membrane, d5 / d 0.3 may be 0.6 or more, or may be 0.7 or more. 0.3 The R5 / R may be 0.9 or less, or may be 0.8 or less. 0.3 It may be a surface where is less than 5.

[0027] In this specification, the average pore diameter means the value obtained by dividing the sum of the circle-equivalent diameters of each pore contained in the obtained SEM image by the number of pores. The circle-equivalent diameter is a value that satisfies the following formula (1). S = π × (d / 2) 2 (1) In equation (1), S is the pore area [nm 2 ] and d represents the equivalent circular diameter of the pore [nm].

[0028] The porous membrane may have a first surface and a second surface having a smaller pore area than the first surface, and the second surface may be a blocked surface.

[0029] The pore area ratio R was measured from an SEM image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A is preferably more than 5%, more preferably 7% or more. A The pore area ratio R is preferably 15% or less, and more preferably 10% or less. B is preferably more than 0.5%, and more preferably 1% or more. B is preferably 5% or less, and more preferably 3% or less. A >R B and R A / R Bis preferably 2 or more, more preferably 3 or more. A / R B is preferably 7 or less, more preferably 5 or less.

[0030] The average pore area S measured from the SEM image obtained by imaging the first surface at an accelerating voltage of 0.3 kV A is 200000nm 2 It is preferable that the wavelength is 250,000 nm or more. 2 It is more preferable that the value is equal to or greater than S. A is 400000nm 2 Preferably, it is 350,000 nm or less. 2 The average pore area S measured from an SEM image obtained by imaging the second surface at an accelerating voltage of 0.3 kV is more preferably equal to or less than 0.3 kV. B is 100000nm 2 It is preferable that the wavelength is 150,000 nm or more. 2 More preferably, it is S or more. B is 400000nm 2 Preferably, it is 350,000 nm or less. 2 It is more preferable that the value is less than S. A / S B is preferably 0.9 or more, and more preferably 1.0 or more. A / S B is preferably less than 1.5. That is, in the porous membrane, the ratio of the average pore area to the ratio of the pore area ratios of the first surface and the second surface may be small.

[0031] The average pore diameter d measured from an SEM image obtained by imaging the first surface at an accelerating voltage of 0.3 kV A is preferably 300 nm or more, more preferably 350 nm or more. A The average pore diameter d measured from an SEM image obtained by photographing the second surface at an accelerating voltage of 0.3 kV is preferably 500 nm or less, and more preferably 450 nm or less. Bis preferably 250 nm or more, and more preferably 300 nm or more. B is preferably 500 nm or less, more preferably 450 nm or less. A / d B is preferably 0.9 or more, and more preferably 1.0 or more. A / d B is preferably less than 1.5, and more preferably less than 1.2. That is, in the porous membrane, the ratio of the average pore diameter to the ratio of the pore area ratios of the first surface and the second surface may be small.

[0032] The above description assumes that the porous membrane has a first surface and a second surface having a smaller pore area ratio than the first surface, but is not limited to this. The porous membrane may have a first surface and a second surface having a smaller average pore area than the first surface, or may have a first surface and a second surface having a smaller average pore diameter than the first surface.

[0033] It can also be said that the porous membrane itself may be a self-supporting membrane. The porous membrane may be a single layer or a multilayer, but is preferably a single layer. Such a porous membrane may be a porous membrane for an electrochemical device, but the use of the porous membrane is not limited thereto and may be used, for example, as a separation membrane, a substrate, a protective membrane, etc.

[0034] In one embodiment, the porous membrane is a single layer. In one embodiment, the porous membrane does not include two or more layers with different compositions, or is not in contact with a layer with a different composition in the thickness direction. In one embodiment, the porous membrane does not include another layer containing a resin as a main component (e.g., a layer in which resin accounts for 30 wt % or more, 50 wt % or more, 70 wt % or more, or 90 wt % or more of the total weight of the layer), or is not in contact with such another layer in the thickness direction. This configuration eliminates the need to stack multiple layers, thereby simplifying the manufacturing process. Note that the porous membrane can be used as a separator for an electrochemical device, and in this regard, it can be stacked with other components (such as the positive and negative electrodes of a secondary battery). Therefore, it is not necessarily understood that the porous membrane included in the electrochemical device component described below is not a single layer.

[0035] The porous membrane can be disposed between a positive electrode and a negative electrode as a component of an electrochemical device. The porous membrane may be disposed on the active material layer of at least one of the positive electrode and the negative electrode. The porous membrane may be disposed between the positive electrode and the negative electrode so as to be in contact with them. The porous membrane is preferably an insulating membrane.

[0036] Porous membranes usually contain a resin. Examples of resins include nitrogen-containing resins. In this specification, nitrogen-containing resins refer to resins containing nitrogen atoms. Examples of nitrogen-containing resins include polyamide, polyimide, polyamideimide, polybenzimidazole, polyurethane, and melamine resin. In particular, from the viewpoint of heat resistance, the porous membrane preferably contains one or more resins selected from the group consisting of polyamide, polyimide, and polyamideimide.

[0037] The resin may be a nitrogen-containing aromatic resin. An aromatic resin refers to a resin containing at least a structural unit having an aromatic group. Examples of nitrogen-containing aromatic resins include aromatic polyamides such as fully aromatic polyamides (aramid resins) and semi-aromatic polyamides, aromatic polyimides, aromatic polyamideimides, polybenzimidazole, aromatic polyurethanes, and melamine resins. Among these, from the viewpoint of heat resistance, it is preferable that the resin contained in the porous membrane is an aramid resin.

[0038] Examples of aramid resins include para-aramid and meta-aramid, with para-aramid being preferred. Examples of para-aramid include para-oriented or para-oriented para-aramids having a structure similar to the para-oriented type, such as poly(paraphenylene terephthalamide), poly(parabenzamide), poly(4,4'-benzanilide terephthalamide), poly(paraphenylene-4,4'-biphenylenedicarboxylic acid amide), poly(paraphenylene-2,6-naphthalenedicarboxylic acid amide), poly(2-chloro-paraphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer, poly(4,4'-diphenylsulfonyl terephthalamide), paraphenylene terephthalamide / 4,4'-diphenylsulfonyl terephthalamide copolymer, and paraphenylene terephthalamide / 3,4'-oxydiphenylene terephthalamide copolymer. Examples of meta-aramids include poly(metaphenylene terephthalamide), poly(metaphenylene isophthalamide), poly(metabenzamide), poly(metaphenylene-4,4'-biphenylenedicarboxylic acid amide), and poly(metaphenylene-2,6-naphthalenedicarboxylic acid amide).

[0039] From the viewpoint of facilitating pore formation, the porous membrane preferably contains two or more nitrogen-containing aromatic resins, and the two or more nitrogen-containing aromatic resins preferably contain resins with different precipitability. It is speculated that pores are likely to be formed due to the following mechanism: When there is a difference in the solubility (precipitability) of two or more nitrogen-containing aromatic resins, the less soluble resin (first resin) precipitates first during the precipitation process, while the more soluble resin (second resin) precipitates later. Due to the compatibility between the first resin and the solvent, the second resin precipitates near the first resin that precipitated first. It is believed that this uneven precipitation of the resins facilitates pore formation.

[0040] For example, it is preferable to combine resins with different structures, such as a resin with a rigid structure and a resin with flexibility. For example, poly(paraphenylene terephthalamide), poly(2-chloro-paraphenylene terephthalamide), poly(parabenzamide), and poly(4,4'-benzanilide terephthalamide) have rigid structures. On the other hand, poly(4,4'-diphenylsulfonyl terephthalamide), paraphenylene terephthalamide / 4,4'-diphenylsulfonyl terephthalamide copolymer, and meta-aramid have flexibility. However, the combination of two or more nitrogen-containing aromatic resins is not limited to these combinations.

[0041] Of the 100% by weight of resin contained in the porous membrane, the nitrogen-containing aromatic resin is preferably more than 50% by weight, more preferably 70% by weight or more, and even more preferably 90% by weight or more. Of the 100% by weight of resin contained in the porous membrane, the nitrogen-containing aromatic resin may be 100% by weight or less, or may be less than 100% by weight. It is particularly preferable that the resin contained in the porous membrane consists solely of the nitrogen-containing aromatic resin.

[0042] The porous membrane may contain a nitrogen-containing aromatic resin and a resin other than the nitrogen-containing aromatic resin, but the resin other than the nitrogen-containing aromatic resin is preferably less than 50 wt%, more preferably 30 wt% or less, and even more preferably 10 wt% or less of 100 wt% of the resin contained in the porous membrane. The resin other than the nitrogen-containing aromatic resin may be 0 wt% or more, or may exceed 0 wt% of 100 wt% of the resin contained in the porous membrane.

[0043] Examples of resins other than nitrogen-containing aromatic resins include polyolefin resins, (meth)acrylate resins, fluorine-containing resins, polyester resins, rubbers, resins with a melting point or glass transition temperature of 180°C or higher, water-soluble polymers, polycarbonate, polyacetal, etc. In one embodiment, the resin contained in the porous membrane may be a resin other than polyester resins.

[0044] Examples of polyester resins include aromatic polyesters such as polyarylates and liquid crystal polyesters.

[0045] Examples of rubbers include styrene-butadiene copolymers and hydrogenated products thereof, methacrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers, styrene-acrylic acid ester copolymers, ethylene propylene rubber, and polyvinyl acetate.

[0046] Examples of fluorine-containing resins include polyvinylidene fluoride (PVdF), polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-trichloroethylene copolymer, vinylidene fluoride-vinyl fluoride copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene copolymer, as well as fluorine-containing rubbers having a glass transition temperature of 23°C or lower among the above-mentioned fluorine-containing resins.

[0047] Examples of resins having a melting point or glass transition temperature of 180° C. or higher include polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyetheramide, and polyetheretherketone.

[0048] Examples of the water-soluble polymer include polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, and polymethacrylic acid.

[0049] The porous membrane may or may not contain a filler. The filler may be an inorganic filler or an organic filler. As the filler, a filler made of an inorganic oxide such as silica, calcium oxide, magnesium oxide, magnesium hydroxide, titanium oxide, alumina, mica, zeolite, aluminum hydroxide, or boehmite is preferred, a filler made of calcium oxide, magnesium oxide, or alumina is more preferred, and a filler made of alumina is even more preferred.

[0050] The content of the filler in 100% by weight of the porous film is preferably 0 to 50% by weight, more preferably 0 to 30% by weight, and may be 0% by weight or more but less than 20% by weight, may be 0 to 15% by weight, may be 0 to 10% by weight, or may be 0 to 5% by weight. A filler content of 0% by weight means that the porous film does not contain a filler. The content of the filler in 100% by weight of the porous film may be more than 0% by weight or may be 1% by weight or more. In particular, from the viewpoint of strength, the content of the inorganic filler in 100% by weight of the porous film is preferably 0 to 50% by weight.

[0051] The average particle size of the filler is preferably 1 μm or less, more preferably 800 nm or less, even more preferably 500 nm or less, particularly preferably 100 nm or less, and most preferably 50 nm or less. The lower limit of the average particle size of the filler is not particularly limited, but can be, for example, 5 nm or more. Here, the average particle size of the filler is the average value of the equivalent sphere diameters of 50 filler particles. The equivalent sphere diameter of the filler is a value measured using a transmission electron microscope. Specific examples of measurement methods are as follows. 1. Using a transmission electron microscope (TEM; JEOL Ltd., JEM-2100F), images were taken at an accelerating voltage of 200 kV and a magnification of 10,000x using a Gatan Imaging Filter. 2. Using image analysis software (ImageJ), trace the particle contours of the obtained image and measure the spherical equivalent particle size of the filler particles (primary particles). 3. The above measurement is carried out on 50 randomly selected filler particles. The arithmetic mean of the spherical equivalent particle diameters of the 50 filler particles is taken as the average particle diameter of the particles.

[0052] The thickness of the porous membrane is preferably 0.3 to 35 μm, more preferably 5.5 to 35 μm, from the viewpoint of ensuring adhesion to the electrodes and high energy density. When the thickness of the porous membrane is 0.3 μm or more, internal short circuits due to breakage of the electrochemical device can be sufficiently suppressed, and the porous membrane can retain a sufficient amount of electrolyte. Furthermore, when the thickness of the porous membrane is 35 μm or less, the permeation resistance of metal ions in the electrochemical device can be suppressed, thereby suppressing deterioration in rate characteristics and cycle characteristics. Furthermore, an increase in the distance between the positive electrode and the negative electrode can be suppressed, thereby suppressing a decrease in the internal volume efficiency of the electrochemical device.

[0053] The basis weight of the porous membrane, i.e., the weight per unit area, can be appropriately determined in consideration of the strength, thickness, weight and handling properties of the porous membrane. The basis weight of the porous membrane is 0.3 to 30 g / m 2 It is preferable that the density is 0.5 to 10 g / m 2 By setting the basis weight of the porous film within these numerical ranges, it is possible to increase the weight energy density and volume energy density of the electrochemical device.

[0054] The porosity of the porous membrane is preferably 20 to 90% by volume, more preferably 30 to 80% by volume, so as to obtain sufficient ion permeability.

[0055] The air permeability of the porous membrane is preferably 150 sec / 100 cc or less, more preferably 120 sec / 100 cc or less, from the viewpoint of obtaining sufficient ion permeability. The air permeability of the porous membrane is a value exceeding 0 sec / 100 cc or more, and may usually be 10 sec / 100 cc or more, or even 20 sec / 100 cc or more. The air permeability is a value measured using an Oken air permeability tester in accordance with JIS P8117.

[0056] The porous membrane may contain additives as long as they do not impair the effects of the present invention. Examples of additives include paper strength agents. The addition of paper strength agents can improve the mechanical strength of the porous membrane. Examples of paper strength agents include adipic acid-diethylenetriamine-epichlorohydrin resin and diallylamine hydrochloride-acrylamide copolymer.

[0057] In one embodiment, the porous membrane does not contain or consist of fabric. As used herein, fabric refers to an article made of fibers, including woven fabrics, knitted fabrics, and nonwoven fabrics. In one embodiment, the porous membrane does not contain or consist of nonwoven fabric. Nonwoven fabric refers to an article formed by collecting unidirectionally or randomly oriented fibers, chemically or physically bonding the fibers, and processing them into a sheet. This manufacturing method is significantly different from, for example, the manufacturing method disclosed in the examples of the present application, in which a resin is precipitated from a solvent.

[0058] The porous membrane may be one other than a nonwoven fabric. The porous membrane may have a surface that is smoother than a nonwoven fabric. For example, the porous membrane may have a surface with a surface roughness of 0.1 μm or less. In this specification, surface roughness refers to the arithmetic mean height (Sa) defined in ISO 25178. A surface roughness of 0.1 μm or less is preferable from the viewpoint of improving adhesion to the electrode. The surface roughness is more preferably 0.05 μm or less, and even more preferably 0.02 μm or less. The lower limit of the surface roughness is not particularly limited, and may be 0.005 μm or more.

[0059] [Second embodiment] The porous membrane according to the second embodiment has a first surface and a second surface, and has a pore area ratio R measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A [%] and the pore area ratio R measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. B [%] Ratio A / RB is 2 or more, and R B The SEM image and the pore area ratio are not more than 5%. The same description as in the first embodiment can be applied to the SEM image and the pore area ratio, so a description thereof will be omitted.

[0060] R A / R B is 2 or more, and R B is 5% or less, this indicates that the pore area ratio is smaller on one side of the porous membrane than on the other side, and that the difference in pore area ratio is large. In other words, the second side has fewer pores than the first side. That is, the porous membrane according to the second embodiment has a blocking layer on the second side. Therefore, the porous membrane according to the second embodiment can also achieve the same effects as the first embodiment.

[0061] Other configurations and properties of the porous membrane can be similar to those described in the first embodiment, and therefore will not be described here.

[0062] [Third embodiment] The porous membrane according to the third embodiment has a first surface and a second surface, and has an average pore diameter d measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A [nm] and the average pore diameter d measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. B [nm] A / d B is less than 1.2, and the pore area ratio measured from the scanning electron microscope image of the first surface or the second surface is 5% or less. In the third embodiment, the second surface does not have to have a smaller pore area ratio than the first surface, and the second surface may have a smaller average pore diameter than the first surface. The details of the SEM image, average pore diameter, and pore area ratio described in [First Embodiment] can be used, so further explanation will be omitted.

[0063] The pore area ratio of the first or second surface being 5% or less means that the porous membrane has a surface with a relatively small pore area ratio, i.e., a surface with a relatively small number of pores. That is, the porous membrane according to the third embodiment has a blocking layer on the first or second surface. Therefore, the porous membrane according to the third embodiment can also achieve the same effects as the first embodiment. In addition, d A / d B is less than 1.2, this indicates that the difference in diameter of each pore between one surface and the other surface of the porous membrane is small. In other words, the porous membrane according to the third embodiment has pores of equal size on both surfaces, and therefore is less likely to inhibit the permeation of Li ions when used as a separator for an electrochemical element.

[0064] Other configurations and properties of the porous membrane can be similar to those described in the first embodiment, and therefore will not be described here.

[0065] 2. Method for producing porous membrane A porous film can be formed using a coating liquid obtained by dissolving or dispersing a resin in a solvent. The solvent can also be considered as a dispersion medium for dispersing the resin. Examples of methods for forming the coating liquid include mechanical stirring, ultrasonic dispersion, high-pressure dispersion, and media dispersion.

[0066] As the method for forming porous film, for example, coating liquid is applied to suitable support, then resin is precipitated, then solvent is removed to form porous film, and this porous film is peeled off from support.Here, by using non-porous support as support, it can form the porous film with the above-mentioned blocking layer.In addition, for example, after coating liquid on support, by contacting another support with the surface opposite to the surface of the coating film that contacts with this support, it can also form the porous film with blocking layer on both sides.

[0067] It is also preferable to set the resin deposition time to a relatively long time. For example, the treatment time in the deposition tank may be 3 minutes or more, 5 minutes or more, or 10 minutes or more. This results in the formation of a blocked layer on the surface in contact with the non-porous support, and the formation of a sufficient number of pores on the surface of the porous membrane not in contact with the support and inside the membrane.

[0068] The solvent is preferably one that does not adversely affect the support, dissolves the resin uniformly and stably, and, if necessary, disperses the filler uniformly and stably. Examples of the solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide, N,N-dimethylformamide, acetone, and water.

[0069] The coating liquid may contain a filler. The coating liquid may also contain, as appropriate, components other than the resin and the filler, such as a dispersant, a plasticizer, a surfactant, and a pH adjuster.

[0070] The coating liquid can be applied to the support by a conventionally known method, and specific examples thereof include a gravure coater method, a dip coater method, a bar coater method, and a die coater method.

[0071] When the coating liquid contains an aramid resin, the aramid resin can be precipitated by adding moisture to the coating surface. This may form a porous film. Specific methods for adding moisture to the coating surface include, but are not limited to, exposing the surface to a humid atmosphere, spraying water onto the surface with a spray or the like, and spraying water vapor onto the surface with a nozzle or the like.

[0072] In addition to the above-mentioned porous film forming methods, a method of forming a porous film by, for example, discharging a coating liquid from a slit die and then immersing the coating liquid in an immersion tank filled with a solvent can also be mentioned. From the viewpoint of uniformly precipitating a porous film, it is preferable to discharge the coating liquid while the slit die is immersed in the solvent in the immersion tank. Furthermore, from the viewpoint of obtaining a homogeneous porous film, it is preferable to apply tension to the porous film precipitated in the immersion tank. The solvent used in the immersion tank is not particularly limited as long as it is a poor solvent for the solvent used in the coating liquid. According to this method, porous films having similar structures can be formed on both sides of the porous film.

[0073] [3. Separators for electrochemical elements, electrochemical element components, electrochemical elements] A separator for an electrochemical element according to one embodiment of the present invention includes the porous membrane described above. Hereinafter, the separator for an electrochemical element will also be simply referred to as a "separator." The separator may consist solely of the porous membrane described above.

[0074] An electrochemical device member according to one embodiment of the present invention comprises a positive electrode, the above-described separator for an electrochemical device, and a negative electrode arranged in this order. An electrochemical device according to one embodiment of the present invention includes the above-described separator for an electrochemical device.

[0075] Examples of electrochemical elements include secondary batteries and capacitors. Examples of secondary batteries include non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries. Examples of capacitors include electric double layer capacitors. The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and may be a thin plate (paper) type, a disk type, a cylinder type, a prismatic type such as a rectangular parallelepiped, or the like.

[0076] For example, a member for an electrochemical device can be formed by arranging a positive electrode, the separator described above, and a negative electrode in this order. The member for an electrochemical device is then placed in a container that will serve as the housing for the electrochemical device. This completes the manufacture of an electrochemical device. In the case of a nonaqueous electrolyte secondary battery, the container is filled with the nonaqueous electrolyte and then sealed under reduced pressure.

[0077] <Positive electrode> The positive electrode is not particularly limited as long as it is generally used as a positive electrode for an electrochemical element. For example, a positive electrode sheet having a structure in which an active material layer containing a positive electrode active material and a binder is formed on a positive electrode current collector can be used as the positive electrode. The active material layer may further contain a conductive agent.

[0078] The positive electrode active material may be, for example, a material capable of doping and dedoping metal ions such as lithium ions or sodium ions. Specific examples of such materials include lithium-containing composite metal oxides containing lithium (Li) and at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, and Al. Examples of such lithium-containing composite metal oxides include LiCoO2, LiNiO2, LiMn2O4, Li2MnO3, and LiNi x Mn y Co 1-x-y O2[0 <x+y<1]、LiNi x Co y Al 1-x-y O2[0 <x+y<1]、LiCr 0.5 Mn 0.5 Examples include O2, LiFePO4, Li2FeP2O7, LiMnPO4, LiFeBO3, Li3V2(PO4)3, Li2CuO2, Li2FeSiO4, and Li2MnSiO4.

[0079] Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black (e.g., acetylene black), pyrolytic carbons, fibrous carbon materials, and baked organic polymer compounds. The conductive agent may be used alone or in combination of two or more. The proportion of the conductive agent in the positive electrode mixture is preferably 5 to 20 parts by mass per 100 parts by mass of the positive electrode active material. When a fibrous carbon material such as graphitized carbon fiber or carbon nanotubes is used as the conductive agent, this proportion can be reduced.

[0080] Thermoplastic resins can be used as the binder. Examples include fluororesins such as PVdF, polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; acrylic resins; styrene-butadiene rubber; polyimide resins; and polyolefin resins. The binder also functions as a thickener. Two or more of these thermoplastic resins may be mixed. By using a fluororesin and a polyolefin resin as binders and setting the ratio of the fluororesin to the total positive electrode mixture to 1% by mass or more and 10% by mass or less and the ratio of the polyolefin resin to 0.1% by mass or more and 2% by mass or less, a positive electrode mixture with high adhesion to the positive electrode current collector and high internal bonding strength can be obtained.

[0081] Examples of the positive electrode current collector include conductors such as Al, Ni, stainless steel, etc. Among these, Al is more preferred because it can be easily processed into a thin film and is inexpensive.

[0082] Examples of methods for producing a positive electrode sheet include a method of pressurizing a positive electrode active material, a conductive agent, and a binder (positive electrode mixture) onto a positive electrode current collector; a method of forming the positive electrode mixture into a paste using an appropriate organic solvent, applying the paste to a positive electrode current collector, drying it, and then pressurizing it to adhere it to the positive electrode current collector.

[0083] Examples of organic solvents that can be used in the above method include amine solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; and amide solvents such as dimethylacetamide and NMP.

[0084] Examples of methods for applying the positive electrode mixture paste to the positive electrode current collector include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.

[0085] <Negative electrode> The negative electrode is not particularly limited as long as it is generally used as a negative electrode for an electrochemical element. For example, a negative electrode sheet having a structure in which an active material layer containing a negative electrode active material and a binder is formed on a negative electrode current collector can be used as the negative electrode. The active material layer may further contain a conductive agent.

[0086] Examples of the negative electrode active material include materials that can be doped and dedoped with metal ions such as lithium ions or sodium ions. Examples of such materials include carbonaceous materials, chalcogen compounds (oxides, sulfides, etc.), nitrides, metals, and alloys that can be doped and dedoped with lithium ions at a lower potential than the positive electrode. Examples of carbonaceous materials include natural graphite, artificial graphite, cokes, carbon black, and pyrolytic carbons.

[0087] Oxides that can be used as negative electrode active materials include SiO2, SiO, and the like, which are compounds of the formula SiO x (where x is a positive real number); oxides of silicon such as TiO2 and TiO with the formula TiO x (where x is a positive real number); oxides of titanium, such as V2O5 and VO2, with the formula VO x (where x is a positive real number) oxides of vanadium; Fe3O4, Fe2O3, FeO, etc., with the formula FeO x (where x is a positive real number) Iron oxides such as SnO2 and SnO with the formula SnO x (where x is a positive real number); tin oxides such as WO3 and WO2 with the formula WO x (where x is a positive real number) is the oxide of tungsten; Li4Ti5O 12 and composite metal oxides containing lithium, such as LiVO2, and titanium or vanadium.

[0088] Sulfides that can be used as negative electrode active materials include Ti2S3, TiS2, TiS, and other sulfides with the formula TiS xTitanium sulfides represented by (where x is a positive real number); sulfides of vanadium represented by V3S4, VS2, VS, etc. with the formula VS x Sulfides of vanadium represented by (where x is a positive real number); sulfides of iron such as Fe3S4, FeS2, FeS, etc. with the formula FeS x Sulfides of iron represented by (where x is a positive real number); sulfides of molybdenum such as Mo2S3, MoS2, etc. with the formula MoS x Sulfides of molybdenum represented by (where x is a positive real number); sulfides of tin such as SnS2, SnS, etc. with the formula SnS x Sulfides of tin represented by (where x is a positive real number); sulfides of tungsten such as WS2, etc. with the formula WS x Sulfides of tungsten represented by (where x is a positive real number); sulfides of antimony such as Sb2S3, etc. with the formula SbS x Sulfides of antimony represented by (where x is a positive real number); sulfides of selenium such as Se5S3, SeS2, SeS, etc. with the formula SeS x Examples include sulfides of selenium represented by (where x is a positive real number), etc.

[0089] As nitrides that can be used as negative electrode active materials, there are Li3N, Li 3-x A x Lithium-containing nitrides such as N(where A is either one or both of Ni and Co, and 0 < x < 3) can be mentioned.

[0090] These carbonaceous materials, oxides, sulfides, and nitrides may be used alone or in combination of two or more. Also, these carbonaceous materials, oxides, sulfides, and nitrides may be either crystalline or amorphous.

[0091] Also, as metals that can be used as negative electrode active materials, there are lithium metal, silicon metal, tin metal, etc.

[0092] As alloys that can be used as negative electrode active materials, there are lithium alloys such as Li-Al, Li-Ni, Li-Si, Li-Sn, Li-Sn-Ni; silicon alloys such as Si-Zn; tin alloys such as Sn-Mn, Sn-Co, Sn-Ni, Sn-Cu, Sn-La; alloys such as Cu2Sb, La3Ni2Sn7, etc.

[0093] These metals and alloys are typically used alone as electrodes after being processed into foils, for example. Among the above-mentioned negative electrode active materials, carbonaceous materials primarily composed of graphite, such as natural graphite and artificial graphite, are preferred. This is because the negative electrode potential hardly changes from an uncharged state to a fully charged state during charging (good potential flatness), the average discharge potential is low, and the capacity retention rate after repeated charging and discharging is high (good cycle characteristics). The carbonaceous material may be in the form of, for example, flakes like natural graphite, spheres like mesocarbon microbeads, fibers like graphitized carbon fibers, or aggregates of fine powder.

[0094] The negative electrode current collector may be made of, for example, Cu, Ni, stainless steel, etc. Cu is more preferred because it is less likely to form an alloy with lithium and is easy to process into a thin film.

[0095] Examples of methods for producing a negative electrode sheet include a method of press-molding a negative electrode active material onto a negative electrode current collector, a method of forming a paste of a negative electrode active material using an appropriate organic solvent, applying the paste to a negative electrode current collector, drying, and then pressing to adhere it to the negative electrode current collector, etc. The paste preferably contains the conductive agent and the binder described above.

[0096] The negative electrode sheet may contain a binder as needed. Examples of the binder include thermoplastic resins, such as PVdF, thermoplastic polyimide, carboxymethyl cellulose, and polyolefin resins.

[0097] <Nonaqueous electrolyte> The non-aqueous electrolyte is not particularly limited as long as it is a non-aqueous electrolyte generally used in electrochemical elements, such as non-aqueous electrolyte secondary batteries. For example, a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent can be used as the non-aqueous electrolyte. Examples of lithium salts include LiClO4, LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiN(SO2C2F5)2, LiN(SO2CF3)(COCF3), Li(C4F9SO3), Li2B 10 Cl 10 Examples of the lithium salt include LiBOB (here, BOB stands for bis(oxalato)borate), LiFSI (here, FSI stands for bis(fluorosulfonyl)imide), lithium salts of lower aliphatic carboxylic acids, and LiAlCl4. The lithium salts may be used alone or in combination of two or more. Among these, it is preferable to use an electrolyte containing at least one fluorine-containing material selected from the group consisting of LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(SO2CF3)2, and LiC(SO2CF3)3.

[0098] Examples of organic solvents include carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of suitable organic solvents include ethers such as methyl formate, methyl acetate, and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propanesultone; and solvents obtained by further introducing fluoro groups into these organic solvents (solvents in which one or more hydrogen atoms in the organic solvent are substituted with fluorine atoms). These organic solvents may be used alone or in combination. Among these, mixed solvents containing carbonates are preferred, with mixed solvents of cyclic carbonates and acyclic carbonates and mixed solvents of cyclic carbonates and ethers being more preferred. Mixed solvents of cyclic carbonates and acyclic carbonates are preferably mixed solvents containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. An electrolyte solution using such a mixed solvent has many advantages, including a wide operating temperature range, resistance to deterioration even when charged and discharged at a high current rate, resistance to deterioration even when used for a long period of time, and resistance to decomposition even when a graphite material such as natural graphite or artificial graphite is used as the active material of the negative electrode.

[0099] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0100] An embodiment of the present invention may include the following features. <1> Pore ​​area ratio R5 [%] measured from scanning electron microscope images taken at an accelerating voltage of 5 kV and pore area ratio R measured from scanning electron microscope images taken at an accelerating voltage of 0.3 kV 0.3 [%] and the ratio R5 / R 0.3 A porous membrane having a surface where is 5 or more. <2> A porous membrane having a first surface and a second surface, wherein the porous membrane has a pore area ratio R measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A [%] and the pore area ratio R measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. B [%] Ratio A / R B is 2 or more, and R B A porous membrane having a porosity of 5% or less. <3> The porous membrane has a first surface and a second surface, and has an average pore diameter d measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV. A [nm] and the average pore diameter d measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. B [nm] A / d B is less than 1.2, and the pore area ratio measured from the scanning electron microscope image of the first surface or the second surface is 5% or less. <4> The average pore area measured from scanning electron microscope images taken at an accelerating voltage of 0.3 kV was 500,000 nm 2 having a surface that is less than <1> ~ <3> 10. The porous membrane according to any one of the preceding items. <5> It is a single layer, <1> ~ <4> 10. The porous membrane according to any one of the preceding items. <6> Contains one or more resins selected from the group consisting of polyamide, polyimide, and polyamideimide, <1> ~ <5> 10. The porous membrane according to any one of the preceding items. <7> The resin is an aramid resin. <6> The porous membrane according to claim 1. <8> <1> ~ <7> A separator for an electrochemical element, comprising the porous film according to any one of the above items. <9> A positive electrode and <8> 10. A member for an electrochemical element comprising the separator for an electrochemical element according to claim 1, and a negative electrode arranged in this order. <10> <8> An electrochemical element comprising the separator for an electrochemical element according to claim 1. <11> a secondary battery or a capacitor; <10> The electrochemical element according to claim 1. [Example]

[0101] An embodiment of the present invention will now be described.

[0102] [Evaluation method] (pore area ratio, pore diameter, pore area of ​​porous membrane) The porous membranes produced in the examples and comparative examples were used as measurement samples. Os vapor deposition was performed on the surface of the porous membrane of the examples, and a scanning electron microscope (SEM, S-4800 (manufactured by Hitachi High-Tech Corporation)) was used to observe the surface of the porous membrane at an acceleration voltage of 0.3 kV, a working distance (WD) of 3 mm, a backscattered electron image, and an image resolution of 49.6 nm / pix to obtain a surface image of the porous membrane. In this way, 15 images were obtained for one side. For the comparative examples, the same operation as in the examples was performed, except that the image resolution was changed to 4.96 nm / pix, to obtain a surface image of the porous membrane.

[0103] Using the surface image as a target, two-dimensional quantitative analysis of the pores was performed using software from Ratoc Systems Engineering (TRI / 3D-BON-FCS: 2D particle analysis option), and the pore area and pore diameter were calculated.

[0104] The two-dimensional quantitative analysis was carried out using the software by a method consisting of the following steps (a) to (i). (a) The surface image was subjected to two-tone gradation of the pores and the substance of the porous membrane (the part other than the pores), and image Ia in which the pores and the substance of the porous membrane were two-toned was obtained. (b) Using image Ia obtained in step (a), two-dimensionally isolated pores were identified and labeled. The number of labeled pores was over 500 per image. (c) The area of ​​each of the pores labeled in step (b), i.e., the pore area S [nm 2 ] was calculated. (d) The pore area S [nm of each pore in the surface image that is the measurement target calculated in step (c) 2 ], the circle equivalent diameter d [nm] of each pore was calculated, satisfying the following formula (1). S = π × (d / 2) 2 (1) (e) The pore areas of all pores calculated in step (c) are added together to obtain the total area S of the pores in the surface image to be measured. tot [nm 2 ] was calculated. (f) The total area of ​​the pores calculated in step (e) S tot [nm 2 ] and the number n of pores labeled in step (b), the average pore area S in the surface image to be measured is calculated based on the following formula (2): av was calculated. S av =S tot / n (2) (g) The circular equivalent diameters of all pores calculated in step (d) are added together to obtain the total circular equivalent diameters d of the pores in the surface image to be measured. tot [nm] was calculated. (h) The sum of the circle equivalent diameters of each pore calculated in step (g), d tot Using the average pore diameter d in the surface image to be measured, the average pore diameter d in the surface image to be measured is calculated based on the following formula (3) using the average pore diameter d in the surface image to be measured and the number n of pores labeled in step (b). av was calculated. d av =d tot / n (3) (i) The total area S of the pores in the surface image to be measured calculated in step (e) tot [nm 2] and the area of ​​the surface image to be measured [nm 2 ] was used to calculate the pore area ratio in the surface image to be measured based on the following formula (4). Pore ​​area ratio [%] = 100 × (total pore area) / (surface image area) (4) Next, the average pore area, average pore diameter, and pore area ratio were calculated for the other surface of the measurement sample, which was different from the one surface that had been measured, using the same method as for the one surface that had been measured.

[0105] Next, the average pore area on the two surfaces is compared, and the average pore area S of the surface with the larger average pore area is determined. A and the average pore area S of the face with the smaller average pore area B Relative to S A / S B The average pore diameters on the two surfaces were compared, and the average pore diameter d A and the average pore diameter d of the surface with the smaller average pore diameter B Relative to d A / d B Furthermore, the pore area ratios of the two surfaces were compared, and the pore area ratio R of the surface with the larger pore area ratio was calculated. A and the pore area ratio R of the surface with the smaller pore area ratio B Ratio to A / R B was calculated. Image condition details Acceleration voltage: 0.3 kV Working distance (WD): 3mm Probe current: norm Condenser lens 1:16 Condenser lens 2:1 Emission current: 20μA ·Backscattered electron image In the following examples and comparative examples, the surface with a large pore area ratio is also referred to as the front side, and the surface with a small pore area ratio is also referred to as the back side. In the porous membranes of Examples 1 and 2, the surface that was in contact with the PET film during production corresponded to the back side, and the opposite surface corresponded to the front side.

[0106] (Pore area ratio, pore diameter, and pore area of ​​porous membrane measured at different accelerating voltages) The porous membranes produced in Examples and Comparative Examples were used as measurement samples. Os vapor deposition was performed on the back surface of the porous membrane of each Example, and a scanning electron microscope (SEM, S-4800 (manufactured by Hitachi High-Technologies Corporation)) was used to observe the surface of the porous membrane at an acceleration voltage of 0.3 kV, a working distance (WD) of 3 mm, a backscattered electron image, and an image resolution of 49.6 nm / pix, thereby obtaining a surface image of the porous membrane at an acceleration voltage of 0.3 kV.

[0107] Subsequently, the acceleration voltage was changed to 5 kV, and the same operation was performed on the same observation point as in the case of an acceleration voltage of 0.3 kV, thereby obtaining a surface image of the porous membrane at an acceleration voltage of 5 kV. In this way, 15 images were obtained at each acceleration voltage. For the comparative example, the same operation as in the example was performed except that the image resolution was changed to 4.96 nm / pix, and surface images of the porous membrane at each acceleration voltage were obtained. Thereafter, the analysis method described in (pore area ratio, pore diameter, pore area of ​​the porous membrane) was performed on the surface images obtained from each acceleration voltage.

[0108] Next, the pore area ratio R5, average pore diameter d5, and average pore area S5 obtained from the analysis of the surface image at an accelerating voltage of 5 kV were compared with the pore area ratio R 0.3 , average pore diameter d 0.3 , average pore area S 0.3 As a result, the ratio of the pore area ratios on the back side of the porous membrane, R5 / R 0.3 , average pore diameter ratio d5 / d 0.3 and the ratio of average pore area S5 / S 0.3 The same measurement was carried out on the front surface.

[0109] (Cycle characteristics) (Fabrication of non-aqueous electrolyte secondary battery) A test non-aqueous electrolyte secondary battery incorporating the porous membrane as a separator was fabricated according to the following procedure. 1. (Positive electrode) Thickness: 49.9 μm, Density: 2.97 g / cm 3 The positive electrode active material layer was composed of, by weight, LiNi 0.78 Co 0.19 Al 0.03 The ratio of O2:conductive agent:polyvinylidene fluoride was 92:4:4. 2. (Negative electrode) Thickness: 71.2 μm, Density: 1.45 g / cm 3 A negative electrode material having a void volume of 41.3 μL was prepared. The composition of the negative electrode active material layer was artificial graphite:styrene butadiene rubber:carboxymethyl cellulose=96.5:2.0:1.5 in weight ratio. 3. A negative electrode, a separator, and a positive electrode were stacked in this order to prepare a non-aqueous electrolyte secondary battery member. Here, the separator was placed so that the back side of the porous membrane was in contact with the negative electrode. 4. The nonaqueous electrolyte secondary battery components were placed in a bag formed by laminating an aluminum layer and a heat-seal layer, and a nonaqueous electrolyte was poured into the bag. The amount of nonaqueous electrolyte poured was 2.8 times the total void volume of the electrodes and separator. The nonaqueous electrolyte was prepared by dissolving vinylene carbonate to a concentration of 1 wt % and LiPF6 to a concentration of 1 mol / L in a mixed solvent of ethylene carbonate: ethyl methyl carbonate: diethyl carbonate = 3:5:2 (volume ratio). 5. The bag was heat-sealed while the pressure inside the bag was reduced, thereby completing a non-aqueous electrolyte secondary battery for testing.

[0110] (Capacity retention rate measurement) The discharge capacity of the prepared battery was measured according to the following procedure. 1. An initial charge / discharge cycle was performed under the following conditions: temperature: 25°C, voltage range: 2.7 to 4.2 V, current value: 0.1 C (charging) or 0.2 C (discharging). Here, 1 C is the current value at which the rated capacity is discharged in 1 hour based on the hourly rate of discharge capacity. 2. The non-aqueous electrolyte secondary battery was aged by performing 10 cycles of charge and discharge under the conditions of a temperature of 25°C, a voltage range of 2.7 to 4.2 V, and a current value of 1 C (when charging) or 5 C (when discharging). 3. One charge / discharge cycle was performed under the following conditions: temperature: 25°C, voltage range: 2.7 to 4.2 V, current value: 0.2 C (charging) or 0.2 C (discharging). The discharge capacity obtained here was defined as the initial capacity [mAh]. 4. 200 charge / discharge cycles were performed under the following conditions: temperature: 45°C, voltage range: 2.7 to 4.2 V, current value: 1 C (charging) or 5 C (discharging). The discharge capacity [mAh] after 200 cycles was divided by the initial capacity [mAh] and expressed as a percentage, which was used as the capacity retention rate after 200 cycles.

[0111] (surface roughness) The non-contact surface roughness measuring device used was the "LEXT 3D MEASURING LASER MICROSCOPE OLS4100" manufactured by OLYMPUS Corp. The measurement conditions were as follows, and when the surface roughness differed between the front and back sides, the smaller value was used as the surface roughness of the sample. Objective lens: 100x Wavelength filter: 405nm Photography: After adjusting the focus using color photography, the upper and lower limits of brightness in the film thickness direction were manually adjusted using laser observation before photography. Measurement: After correcting the tilt using image correction, the surface roughness was calculated. Cutoff: None.

[0112] The specific method for calculating the surface roughness was as follows. First, the one-dimensional surface roughness Sa of a 130 μm length was obtained from two-dimensional data obtained at one point on the porous membrane. Next, this operation was repeated at 10 randomly selected points on the porous membrane. The average surface roughness was calculated using the measured values ​​obtained at the 10 points.

[0113] [Synthesis Example 1: Synthesis of Resin A] Resin A (poly(4,4'-diphenylsulfonyl terephthalamide)) was synthesized according to the following procedure. 1. A 0.5 L separable flask equipped with a stirring blade, a thermometer, a nitrogen inlet, and a powder addition port was thoroughly dried. 2. 408.6 g of NMP was charged into a flask. 31.4 g of calcium chloride was added, and the temperature was raised to 100°C. The calcium chloride was dried at 200°C for 2 hours before being added. 3. After calcium chloride was completely dissolved, 31.97 g of 4,4'-diaminodiphenyl sulfone was added at 100°C and completely dissolved. 4. The resulting solution was cooled to room temperature. While maintaining the temperature of the solution at 25±2°C, 26.14 g of terephthalic acid dichloride in total was added in three portions. 5. The temperature of the obtained solution was maintained at 25±2°C, and the solution was aged for 1 hour to obtain a solution containing Resin A.

[0114] [Synthesis Example 2: Synthesis of Resin B] Resin B (poly(paraphenylene terephthalamide)) was synthesized according to the following procedure. 1. A 0.5 L separable flask equipped with a stirring blade, a thermometer, a nitrogen inlet, and a powder addition port was thoroughly dried. 2. 408.6 g of NMP was charged into a flask. 31.4 g of calcium chloride was added, and the temperature was raised to 100°C. The calcium chloride was dried at 200°C for 2 hours before being added. 3. After calcium chloride was completely dissolved, the solution was allowed to return to room temperature, and then 13.20 g of paraphenylenediamine was added and completely dissolved. 4. While maintaining the temperature of the solution at 25±2°C, a total of 24.24 g of terephthalic acid dichloride was added in three portions. 5. The temperature of the obtained solution was maintained at 25±2°C, and the solution was aged for 1 hour to obtain a solution containing Resin B.

[0115] [Example 1] The solutions obtained in Synthesis Examples 1 and 2 were mixed so that the weight ratio of resin A to resin B was 90:10 to obtain a mixture (1). 3.55 g of calcium carbonate was added to 100 g of the obtained mixture (1) and the mixture was stirred for 10 minutes to neutralize the solution, obtaining a neutralized liquid (1). The neutralized liquid (1) was then diluted with NMP and degassed under reduced pressure to prepare a slurry coating liquid (1). The solids concentration of the coating liquid (1) was 6.0 wt%.

[0116] The coating liquid (1) was applied to a release-treated PET film (75 μm thick) and treated in a deposition tank at 50°C and 70% relative humidity for 5 minutes to deposit a porous layer (1). The film was then washed with water in a water washing tank and dried in a drying oven at 80°C. After drying, the PET film was peeled off from the porous layer (1) to obtain a porous membrane (1).

[0117] [Example 2] The same procedure as in Example 1 was carried out except that the treatment was carried out in a precipitation bath at 50°C and a relative humidity of 70% for 15 minutes, to obtain a porous membrane (2).

[0118] [Comparative Example 1] The solution containing Resin B prepared in Synthesis Example 2 was mixed with alumina (average particle size: 13 nm) so that the weight ratio of Resin B:alumina was 50:50. 2.30 g of calcium carbonate was added to 100 g of the resulting mixture (3) and stirred for 10 minutes to neutralize the solution, obtaining a neutralized liquid (3). The neutralized liquid (3) was then diluted with NMP and degassed under reduced pressure to prepare a slurry coating liquid (3). The solids concentration of the coating liquid (3) was 4.5 wt %.

[0119] The coating liquid (3) was applied to a polyethylene porous film (thickness: 10.3 μm, air permeability: 180 s / 100 mL) and treated in an oven at 50°C and a relative humidity of 70% for 1 minute to precipitate a porous layer (3). After that, the film was washed with water and dried to obtain a porous membrane (3) having a porous layer (3) on a polyethylene porous film.

[0120] Comparative Example 2 The polyethylene porous film used in Comparative Example 1 was used as the porous membrane (4).

[0121] [Evaluation results] FIG. 1 shows SEM images of the front and back surfaces of the porous membranes of Examples 1 and 2, respectively, obtained at an accelerating voltage of 0.3 kV. In FIGS. 1 to 4, "×2k" and "×20k" indicate the magnification. It can be seen that the back surfaces of Examples 1 and 2 have fewer pores than the front surfaces, and are blocked. FIG. 2 shows SEM images of the front and back surfaces of the porous membranes of Comparative Examples 1 and 2, respectively, obtained at an accelerating voltage of 0.3 kV. It can be seen that the difference between the front and back surfaces is small in Comparative Examples 1 and 2.

[0122] FIG. 3 shows SEM images of the backside of the porous membranes of Examples 1 and 2, taken at accelerating voltages of 0.3 kV and 5 kV. In Examples 1 and 2, the SEM images taken at an accelerating voltage of 0.3 kV show fewer pores and are blocked than the SEM images taken at an accelerating voltage of 5 kV. This suggests that a thin blocked layer (a layer with a small number of pores) is present on the backside of the porous membranes of Examples 1 and 2. FIG. 4 shows SEM images of the backside of the porous membranes of Comparative Examples 1 and 2, taken at accelerating voltages of 0.3 kV and 5 kV. In Comparative Examples 1 and 2, the difference between the SEM images taken at an accelerating voltage of 0.3 kV and the SEM images taken at an accelerating voltage of 5 kV is small. This suggests that a blocked layer like that in Examples 1 and 2 is not present in Comparative Examples 1 and 2.

[0123] The evaluation results of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Tables 1 to 4. Table 2 shows the results measured on the back side at different accelerating voltages, and Table 3 shows the results measured on the front side at different accelerating voltages.

[0124] [Table 1]

[0125] [Table 2]

[0126] [Table 3]

[0127] [Table 4]

[0128] From Tables 2, 3 and 4, R5 / R 0.3 When the porous membranes of Examples 1 and 2 having a surface with a ratio of R5 / R of 5 or more were used, batteries with a high capacity retention rate and excellent cycle characteristics were obtained. 0.3 On the other hand, the R5 / R 0.3 The batteries using the porous membranes of Comparative Examples 1 and 2, in which the value was less than 5, were inferior in cycle characteristics to the batteries using the porous membranes of Examples 1 and 2.

[0129] In addition, from Table 1, in the porous membranes of Examples 1 and 2, the back surface with a small pore area ratio is a blocked surface, and R B is 5% or less, and R A / R B On the other hand, the porous membranes of Comparative Examples 1 and 2 had a pore area ratio R B exceeds 5%, and R A / R B was less than 2. The porous membranes of Examples 1 and 2 and the porous membranes of Comparative Examples 1 and 2 were different from each other in this respect as well.

[0130] Furthermore, from Table 1, the porous membranes of Examples 1 and 2 have a d A / d B On the other hand, the porous membrane of Comparative Example 1 had a surface with a d A / d BThe porous membrane of Comparative Example 2 had a d A / d B Although the pore area ratio was less than 1.2, the pore area ratio exceeded 5% on both the front and back sides. The porous membranes of Examples 1 and 2 differed from the porous membranes of Comparative Examples 1 and 2 in this respect as well. [Industrial Applicability]

[0131] One aspect of the present invention can be used in an electrochemical device.

Claims

1. The pore area ratio R measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 5 kV ５ [%] and the pore area ratio R measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 0.3 kV. ０．３ Ratio R ５ / R ０．３ A porous membrane having a surface where is 5 or more.

2. having a first surface and a second surface; The pore area ratio R measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV Ａ [%] and the pore area ratio R measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV. Ｂ Ratio R Ａ / R Ｂ is 2 or more, and R Ｂ A porous membrane having a porosity of 5% or less.

3. having a first surface and a second surface; The average pore diameter d measured from a scanning electron microscope image obtained by imaging the first surface at an accelerating voltage of 0.3 kV Ａ [nm] and the average pore diameter d measured from a scanning electron microscope image obtained by imaging the second surface at an accelerating voltage of 0.3 kV Ｂ Ratio d to [nm] Ａ / d Ｂ is less than 1.2, and the pore area ratio measured from the scanning electron microscope image of the first surface or the second surface is 5% or less.

4. The average pore area measured from a scanning electron microscope image obtained by imaging at an accelerating voltage of 0.3 kV was 500,000 nm ２ The porous membrane of any one of claims 1 to 3, having a surface that is less than 1 / 2 mm.

5. The porous membrane according to any one of claims 1 to 3, which is a single layer.

6. The porous membrane according to any one of claims 1 to 3, comprising one or more resins selected from the group consisting of polyamide, polyimide, and polyamideimide.

7. 7. The porous membrane of claim 6, wherein the resin is an aramid resin.

8. A separator for an electrochemical element, comprising the porous film according to any one of claims 1 to 3.

9. A member for an electrochemical element, comprising a positive electrode, the separator for an electrochemical element according to claim 8, and a negative electrode arranged in this order.

10. An electrochemical element comprising the separator for an electrochemical element according to claim 8 .

11. The electrochemical device according to claim 10, which is a secondary battery or a capacitor.

Images