Separator and non-aqueous electrolyte secondary battery

Abstract

To provide a nonaqueous electrolyte secondary battery separator enhanced in the effect of preventing short circuit by controlling the shape of dendrite.SOLUTION: A nonaqueous electrolyte secondary battery separator according to an embodiment of the present invention is 5 or larger in the kurtosis of a group of data of heights from a reference plane when unevenness data of at least one surface of the separator is analyzed; the standard deviation of the data group of heights from the reference plane is 12 or below.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a separator and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, have a high energy density and are therefore widely used as batteries in personal computers, mobile phones, personal digital assistants, and automobiles.

[0003] In recent years, there has been a growing demand for high-power lithium-ion secondary batteries. To meet this demand, development is underway on lithium-ion secondary batteries equipped with separators that have excellent ion permeability.

[0004] On the other hand, lithium-ion secondary batteries have a problem in that minute short circuits occur due to lithium deposits that grow in a fibrous manner, which easily leads to voltage drops. These minute short circuits are one of the factors that reduce the long-term reliability of the battery.

[0005] In this regard, Patent Document 1 discloses a porous film in which a skin layer having a specific porosity is formed on at least one surface of the porous film. Patent Document 2 also discloses a separator having a patterned region in which a pattern is formed and a non-patterned region in which no pattern is formed. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 10-055794 [Patent Document 2] Japanese Patent Publication No. 2020-068178 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, there was still room for further development in methods for suppressing dendrite growth in lithium-ion secondary batteries. For example, it is known that the shape of dendrites becomes closer to a fibrous shape as they grow, and the closer the shape is to a fiber, the easier it is for the dendrites to extend through the pores of the separator. Therefore, in order to prevent short circuits caused by dendrite growth, it is important to control the shape of the dendrites as well. However, the aforementioned prior art documents do not consider controlling the shape of dendrites.

[0008] One aspect of the present invention aims to provide a separator for a non-aqueous electrolyte secondary battery that controls the shape of dendrites and enhances the short-circuit prevention effect. [Means for solving the problem]

[0009] The present invention encompasses the following aspects. <1> A separator for non-aqueous electrolyte secondary batteries, When analyzing the surface irregularities data of at least one surface of the above separator, The kurtosis of the height data set from the reference plane is 5 or greater (however, a kurtosis of 0 or greater indicates that the peak is sharper than that of a normal distribution, and a larger kurtosis indicates a sharper distribution), The standard deviation of the height data set from the reference plane is 12 or less. Separator. <2> This is a laminated separator comprising a porous layer and a polyolefin porous film. <1> The separator described above. <3> The above porous layer contains a nitrogen-containing aromatic resin. <2> The separator described above. <4> The above nitrogen-containing aromatic resin is an aramid resin. <3> The separator described above. <5> The compressive modulus in the thickness direction is 50 MPa or more. <1> ~ <4> A separator as described in any of the following. <6> A member for a non-aqueous electrolyte secondary battery, in which a positive electrode, a separator according to any one of <1> to <5>, and a negative electrode are laminated in this order. <7> A non-aqueous electrolyte secondary battery including a separator according to any one of <1> to <5> or a member for a non-aqueous electrolyte secondary battery according to <6>.

Effects of the Invention

[0010] According to one aspect of the present invention, there is provided a separator for a non-aqueous electrolyte secondary battery that controls the shape of dendrites and enhances the short-circuit prevention effect.

Modes for Carrying Out the Invention

[0011] One embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to each configuration described below, and various modifications are possible within the scope shown in 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. Unless otherwise specified in this specification, "A to B" representing a numerical range means "A or more and B or less".

[0012] 〔1. Separator for Non-aqueous Electrolyte Secondary Battery〕 According to a newly discovered fact by the present inventors, by controlling the smoothness of the surface of the separator contained in a non-aqueous electrolyte secondary battery, the shape of dendrites growing on the surface of the negative electrode can be controlled. That is, a separator having a high surface smoothness on the side facing the negative electrode can control the shape of the growing dendrites into a granular or flat plate shape. Dendrites of such a shape are less likely to penetrate the separator even if they occur. Conversely, a separator with low smoothness on the side facing the negative electrode grows fibrous dendrites. Dendrites of such a shape are highly likely to penetrate the separator.

[0013] In this specification, the smoothness of the separator surface is expressed by parameters obtained by analyzing the unevenness data of the separator surface. These parameters are the kurtosis of the height data group from the reference plane and the standard deviation of the height data group from the reference plane. By definition, these values are the same for the same surface regardless of where the reference plane is set.

[0014] In one embodiment, the reference plane is set to a plane that includes a point deeper than the deepest point of the unevenness data of the separator surface. In this case, all the height data from the reference plane take positive values. In one embodiment, the reference plane represents a plane that includes the deepest point (point with zero height) in a standard sample in which the height of the unevenness is adjusted to a predetermined value. In this case, the height data group from the reference plane of the separator surface is obtained by comparing the standard sample and the separator surface. In one embodiment, the height of the unevenness of the standard sample is adjusted to 54.6 μm.

[0015] For the separator according to one embodiment of the present invention, for at least one surface, the kurtosis of the height data group from the reference plane is 5 or more, preferably 6 or more, and more preferably 8 or more. The upper limit of the kurtosis is, for example, 124 or less, 80 or less, 60 or less, 40 or less, or 30 or less. Kurtosis is an index representing the sharpness of the distribution curve. In one embodiment, the kurtosis of the normal distribution is set to 0. In other embodiments, the kurtosis of the normal distribution is set to 3.

[0016] For the separator according to one embodiment of the present invention, for at least one surface, the standard deviation of the height data group from the reference plane is 12 or less, preferably 10 or less, and more preferably 8 or less. The lower limit of the standard deviation can be, for example, 0 or more, or 0.1 or more.

[0017] Surface topography data for the separator can be obtained, for example, from an optical microscope image of the separator surface. The kurtosis and standard deviation of the height data set from the reference plane are calculated by analyzing the surface topography data for the separator. Here, the height data set from the reference plane, which forms the basis for calculating the kurtosis and standard deviation, preferably has a sampling rate of 100,000 pixels or more, and more preferably 150,000 pixels or more.

[0018] The following example illustrates how to determine the kurtosis and standard deviation of height data sets from a reference plane using ImageJ (provided by the National Institutes of Health (NIH)) as the image analysis software. For more detailed examples of measurement methods, please refer to the embodiments of this application. 1. Import the surface texture data of the separator (such as a 3D image of the separator surface) and use the polygon tool to select a rectangular area that includes all the textures. Be careful to ensure that areas unrelated to the separator surface (such as the image file name or black background) are not included in the rectangular area. 2. Select Edit-clear outside from the pull-down menu to trim the area outside the selection. 3. Select "Analyze-histogram" from the pull-down menu to automatically analyze the topography data. This will calculate the maximum, minimum, mean, and standard deviation values. This standard deviation value is the standard deviation of the height data set from the reference plane. 4. Check "Set measurement kurtosis" and select "Analyze measure". This operation calculates the kurtosis of the height data set from the reference plane.

[0019] The inventors have further discovered that the rigidity of the separator in the thickness direction is also a factor that controls the shape of the dendrites growing on the negative electrode surface. Separators with high rigidity in the thickness direction are less prone to deformation in that direction and are less likely to create a space between the negative electrode and the separator. Under such conditions, the shape of the growing dendrites tends to be granular or plate-like. Even if such dendrites are formed, they are unlikely to penetrate the separator. Conversely, separators with low rigidity in the thickness direction are more prone to deformation in that direction and are more likely to create a space between the negative electrode and the separator. Therefore, they tend to grow fibrous dendrites. Such dendrites are more likely to penetrate the separator.

[0020] In this specification, the rigidity of the separator in the thickness direction is expressed by the compressive modulus of the separator in the thickness direction. In one embodiment, the compressive modulus of the separator in the thickness direction is preferably 50 MPa or more, more preferably 55 MPa or more, even more preferably 60 MPa or more, and particularly preferably 75 MPa or more. The upper limit of the compressive modulus of the separator in the thickness direction is preferably 300 MPa or less, more preferably 250 MPa or less, even more preferably 175 MPa or less, and particularly preferably 125 MPa or less. Examples of combinations of lower and upper limits for the compressive modulus of the separator in the thickness direction include 50-300 MPa, 55-250 MPa, 60-175 MPa, and 75-125 MPa. The compressive modulus of the separator in the thickness direction is measured using a micro-compression testing machine. As a micro-compression testing machine, an MCT-510 (manufactured by Shimadzu Corporation) or the like is used.

[0021] In this specification, the shape of the dendrite is determined based on a 5000x SEM image. In this specification, the shape of the dendrite is classified by the aspect ratio (major axis / minor axis value) of the dendrite. Dendrites with an aspect ratio of less than 2 are referred to as granular. Dendrites in which multiple granular dendrites are linked together are referred to as plate-like. Dendrites with an aspect ratio greater than 13 are referred to as "fibrous". For a test method for generating dendrites, please refer to the examples of this application. In the dendrite generation test, the shape of the generated dendrite is most preferably granular, followed by plate-like. Fibrous dendrites are undesirable because they may penetrate the separator.

[0022] [1.1. Laminated Separator] In one embodiment, the separator is a laminated separator comprising a porous layer and a polyolefin porous film. The laminated separator has a porous layer on one or both sides of the polyolefin porous film.

[0023] When a porous layer is provided on one side of a porous polyolefin film, the surface that satisfies the above-mentioned smoothness condition may be the surface on the porous layer side or the surface on the porous polyolefin film side. The surface that satisfies the above-mentioned smoothness condition faces the negative electrode when assembled into a non-aqueous electrolyte secondary battery.

[0024] (1.1.1. Porous layer) The porous layer, as a component constituting a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention, can be disposed between a polyolefin porous film and at least one of the positive electrode and the negative electrode in a non-aqueous electrolyte secondary battery.

[0025] The porous layer may be formed on the active material layer of at least one of the positive electrode and the negative electrode in a non-aqueous electrolyte secondary battery. The porous layer may be arranged between the polyolefin porous film and at least one of the positive electrode and the negative electrode, in contact with them. Alternatively, although not in the form of a laminated separator, the porous layer itself, without the polyolefin porous film, may function as a separator for a non-aqueous electrolyte secondary battery. The porous layer may be one layer or two or more layers.

[0026] The porous layer contains a resin. Preferably, the porous layer is an insulating porous layer containing an insulating resin.

[0027] When the porous layer is laminated on one side of the polyolefin porous film, the porous layer is preferably laminated on the side of the polyolefin porous film facing the negative electrode in a non-aqueous electrolyte secondary battery. More preferably, the porous layer is laminated on the side in contact with the negative electrode in a non-aqueous electrolyte secondary battery.

[0028] (resin) Preferably, the resin is insoluble in the battery electrolyte and is electrochemically stable within the battery's operating range.

[0029] Examples of the aforementioned resins include polyolefins; (meth)acrylate resins; nitrogen-containing aromatic resins; fluorine-containing resins; polyamide resins; polyimide resins; polyester resins; rubbers; resins with a melting point or glass transition temperature of 180°C or higher; water-soluble polymers; polycarbonates, polyacetals, polyetheretherketones, etc.

[0030] Of the aforementioned resins, one or more resins selected from the group consisting of polyolefins, (meth)acrylate resins, fluororesins, nitrogen-containing aromatic resins, polyamide resins, polyester resins, and water-soluble polymers are preferred.

[0031] Furthermore, it is particularly preferable that the resin is a nitrogen-containing aromatic resin. Since the nitrogen-containing aromatic resin has nitrogen-mediated bonds such as amide bonds, it has excellent heat resistance. Therefore, by using a nitrogen-containing aromatic resin, the heat resistance of the porous layer can be suitably improved. As a result, the heat resistance of the separator for a non-aqueous electrolyte secondary battery containing the porous layer can be improved.

[0032] The polyolefins mentioned above are preferably polyethylene, polypropylene, polybutene, and ethylene-propylene copolymers.

[0033] Examples of fluororesins 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 fluororubber, which has a glass transition temperature of 23°C or lower among the fluororesins mentioned above.

[0034] The polyamide resin is preferably a polyamide resin that corresponds to a nitrogen-containing aromatic resin, and is particularly preferably an aramid resin such as an aromatic polyamide or a fully aromatic polyamide.

[0035] Examples of the aramid resin include poly(paraphenylene terephthalamide), poly(metaphenylene isophthalamide), poly(parabenzamide), poly(metabenzamide), poly(4,4'-benzanilide terephthalamide), poly(paraphenylene-4,4'-biphenylenedicarboxylic acid amide), poly(metaphenylene-4,4'-biphenylenedicarboxylic acid amide), poly(paraphenylene-2,6-naphthalenedicarboxylic acid amide), poly(metaphenylene-2,6-naphthalenedicarboxylic acid amide), poly(2-chloroparaphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer, metaphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer, etc. Of these, poly(paraphenylene terephthalamide) is more preferred.

[0036] As the polyester resin, aromatic polyesters such as polyarylate and liquid crystal polyesters are preferred.

[0037] Examples of the aforementioned rubbers include styrene-butadiene copolymers and their hydrogenates, methacrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers, styrene-acrylic acid ester copolymers, ethylene propylene rubber, and polyvinyl acetate.

[0038] 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, and polyetheramide.

[0039] Examples of the water-soluble polymers mentioned above include polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, and polymethacrylic acid.

[0040] The resin may be one type only, or a combination of two or more types of resins may be used. The resin content in the porous layer is preferably 25 to 80% by weight, and more preferably 30 to 70% by weight, when the total weight of the porous layer is 100% by weight.

[0041] (Filler) In one embodiment of the present invention, the porous layer preferably contains a filler. The filler may be an inorganic filler or an organic filler. More preferably, the filler is an inorganic filler consisting of one or more inorganic oxides selected from the group consisting of silica, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, zeolite, aluminum hydroxide, and boehmite.

[0042] Furthermore, in order to improve the water absorption of the inorganic filler, the surface of the inorganic filler may be treated to make it hydrophilic with a silane coupling agent or the like.

[0043] The lower limit of the filler content in the porous layer is preferably 20% by weight or more, and more preferably 30% by weight or more, when the total weight of the porous layer is 100% by weight. The upper limit of the filler content in the porous layer is preferably 80% by weight or less, and more preferably 70% by weight or less, when the total weight of the porous layer is 100% by weight. Examples of combinations of the lower and upper limits of the filler content include 20-80% by weight and 30-70% by weight. When the filler content is within the above range, it is easy to obtain a porous layer with sufficient ion permeability.

[0044] The porous layer is preferably placed between the polyolefin porous film and the negative electrode active material layer of the negative electrode. In the following description, the physical properties of the porous layer refer to at least the physical properties of the porous layer placed between the polyolefin porous film and the negative electrode active material layer of the negative electrode when a non-aqueous electrolyte secondary battery is constructed.

[0045] The basis weight of the porous layer, i.e., the weight per unit area, can be appropriately determined considering the strength, film thickness, weight, and handling characteristics of the porous layer. The basis weight of the porous layer is 0.5 to 3.5 g / m² per layer of the porous layer. 2 Preferably, it is 1.0 to 3.0 g / m 2 It is preferable that it be so.

[0046] By setting the basis weight of the porous layer within these numerical ranges, the gravimetric energy density and volumetric energy density of the non-aqueous electrolyte secondary battery equipped with the porous layer can be increased. If the basis weight of the porous layer exceeds the range, the non-aqueous electrolyte secondary battery equipped with the porous layer tends to become heavier.

[0047] The porosity of the porous layer is preferably 20 to 90 volume%, and more preferably 30 to 80 volume%, in order to obtain sufficient ion permeability.

[0048] Furthermore, the pore size of the pores in the porous layer is preferably 1.0 μm or less, and more preferably 0.5 μm or less. By setting the pore size to these sizes, the non-aqueous electrolyte secondary battery equipped with the porous layer can obtain sufficient ion permeability.

[0049] The air permeability of the porous layer is preferably 30 to 80 s / 100 mL in Gaarle values, and more preferably 40 to 75 s / 100 mL. If the air permeability of the porous layer is within the above range, it can be said that the porous layer has sufficient ion permeability.

[0050] The lower limit of the porous layer thickness is preferably 0.1 μm or more, more preferably 0.3 μm or more, and even more preferably 0.5 μm or more. The upper limit of the porous layer thickness is preferably 20 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. Examples of combinations of the lower and upper limits of the porous layer thickness include 0.1 to 20 μm, 0.3 to 10 μm, and 0.5 to 5 μm. If the porous layer thickness is within the above range, the function of the porous layer (such as providing heat resistance) can be fully exhibited, and the overall thickness of the separator can be reduced.

[0051] (Examples of preferred combinations of resin and filler) In one embodiment, the intrinsic viscosity of the resin contained in the porous layer is 1.4 to 4.0 dL / g, and the average particle size of the filler is 1 μm or less. By using a porous layer with such a composition, a laminated separator can be manufactured that achieves both thinness, heat resistance, and ion permeability.

[0052] The resin contained in the porous layer preferably has a lower limit of intrinsic viscosity of 1.4 dL / g or more, and more preferably 1.5 dL / g or more. Furthermore, the resin contained in the porous layer preferably has an upper limit of intrinsic viscosity of 4.0 dL / g or less, more preferably 3.0 dL / g or less, and even more preferably 2.0 dL / g or less. A porous layer containing a resin with an intrinsic viscosity of 1.4 dL / g or more can impart sufficient heat resistance to the laminated separator. A porous layer containing a resin with an intrinsic viscosity of 4.0 dL / g or less has sufficient ion permeability. For methods of measuring intrinsic viscosity, please refer to the examples of this application.

[0053] Resins with an intrinsic viscosity of 1.4 to 4.0 dL / g can be synthesized by adjusting the molecular weight distribution of the resin by appropriately setting the synthesis conditions (amount of monomer added, synthesis temperature, synthesis time, etc.). Alternatively, commercially available resins with an intrinsic viscosity of 1.4 to 4.0 dL / g may be used. In one embodiment, the resin with an intrinsic viscosity of 1.4 to 4.0 dL / g is an aramid resin.

[0054] The average particle size of the filler contained in the porous layer is preferably 1 μm or less, more preferably 800 nm or less, more preferably 500 nm or less, more preferably 100 nm or less, and more preferably 50 nm or less. Here, the average particle size of the filler is the average value of the spherical particle size of 50 fillers. The spherical particle size of the filler is a value measured by a transmission electron microscope. An example of a specific measurement method is as follows. 1. Using a transmission electron microscope (TEM; JEOL Ltd., Transmission Electron Microscope JEM-2100F), images were taken at an acceleration voltage of 200kV and a magnification of 10,000x using a Gatan Imaging Filter. 2. Using image analysis software (ImageJ), trace the contours of the particles in the obtained images and measure the spherical equivalent particle size of the filler particles (primary particles). 3. The above measurements are performed on 50 randomly selected filler particles. The arithmetic mean of the spherical equivalent particle sizes of the 50 filler particles is taken as the average particle size.

[0055] By making the average particle size of the filler 1 μm or less, the laminated separator can be made thinner. The lower limit of the average particle size of the filler is not particularly limited, but for example, it can be 5 nm or more.

[0056] (1.1.2. Polyolefin porous film) A laminated separator according to one embodiment of the present invention includes a polyolefin porous film. Alternatively, although not a laminated separator, a separator according to one embodiment of the present invention may consist only of a polyolefin porous film.

[0057] Here, "polyolefin porous film" refers to a porous film whose main component is a polyolefin resin. Furthermore, "main component is a polyolefin resin" means that the proportion of polyolefin resin in the porous film is 50% by volume or more, preferably 90% by volume or more, and more preferably 95% by volume or more, of the total material constituting the porous film.

[0058] The aforementioned polyolefin porous film is mainly composed of a polyolefin resin and has numerous interconnected pores inside, allowing gases and liquids to pass from one side to the other. Hereinafter, the aforementioned polyolefin porous film will also be simply referred to as "porous film."

[0059] The aforementioned polyolefin has a weight-average molecular weight of 5 × 10 5 ~15×10 6 It is more preferable that the polyolefin contains a high molecular weight component. In particular, it is more preferable that the polyolefin contains a high molecular weight component with a weight-average molecular weight of 1 million or more, because this improves the strength of the separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention.

[0060] Examples of the polyolefin include homopolymers or copolymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene.

[0061] Examples of the aforementioned homopolymers include polyethylene, polypropylene, and polybutene. Examples of the aforementioned copolymers include ethylene-propylene copolymers.

[0062] Of these, polyethylene is more preferred as the polyolefin because it can prevent excessive current from flowing at a lower temperature. This "prevention of excessive current flow" is also called shutdown.

[0063] Examples of the polyethylene include low-density polyethylene, high-density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), and ultra-high molecular weight polyethylene with a weight-average molecular weight of 1 million or more. Of these, ultra-high molecular weight polyethylene with a weight-average molecular weight of 1 million or more is even more preferred.

[0064] The basis weight of the porous film can be appropriately determined considering strength, film thickness, weight, and handling. However, in order to increase the gravimetric energy density and volumetric energy density of the non-aqueous electrolyte secondary battery, the basis weight should be 4 to 20 g / m². 2 Preferably, it is 4-12 g / m 2 It is more preferable that it be 5-10 g / m 2 It is even more preferable that this be the case.

[0065] The air permeability of the porous film is preferably 30 to 500 s / 100 mL in Gaarle values, and more preferably 50 to 300 s / 100 mL. By having an air permeability within the above range, the porous film can obtain sufficient ion permeability.

[0066] The porosity of the porous film is preferably 20 to 80 volume%, and more preferably 30 to 75 volume%, in order to increase the amount of electrolyte it can hold and to reliably prevent excessive current from flowing at lower temperatures.

[0067] Furthermore, the pore size of the pores in the porous film is preferably 0.3 μm or less, and more preferably 0.14 μm or less, in order to obtain sufficient ion permeability and to prevent particles from entering the positive and negative electrodes.

[0068] The laminated separator may further include other layers besides the polyolefin porous film, such as an adhesive layer, a heat-resistant layer, and a protective layer.

[0069] Furthermore, it is preferable that the film thickness of the separator for non-aqueous electrolyte secondary batteries is such that the distance the dendrites need to travel before the minute short circuit occurs is sufficiently long, while maintaining good ionic conductivity.

[0070] From this perspective, the lower limit of the film thickness of the polyolefin porous film is preferably 4 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. The upper limit of the film thickness of the polyolefin porous film is preferably 29 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less. Examples of combinations of the lower and upper limits of the film thickness of the polyolefin porous film include 4 to 29 μm, 5 to 20 μm, and 6 to 15 μm.

[0071] [2. Method for manufacturing separators for non-aqueous electrolyte secondary batteries] [2.1. Method for producing polyolefin porous film] As an example of a method for producing the porous film, the following method can be used. First, a polyolefin resin is kneaded with a pore-forming agent such as an inorganic filler or plasticizer, and optionally an antioxidant, to obtain a polyolefin resin composition. Then, the polyolefin resin composition is extruded to produce a sheet-like polyolefin resin composition. The pore-forming agent is then removed from the sheet-like polyolefin resin composition using a suitable solvent. Finally, the polyolefin resin composition from which the pore-forming agent has been removed is stretched to produce a polyolefin porous film.

[0072] The inorganic filler is not particularly limited and includes inorganic fillers, specifically calcium carbonate, etc. The plasticizer is not particularly limited and includes low molecular weight hydrocarbons such as liquid paraffin.

[0073] As a method for manufacturing the porous film, for example, a method including the following steps can be cited. (i) A step of kneading ultra-high molecular weight polyethylene with a weight-average molecular weight of 1 million or more, low molecular weight polyethylene with a weight-average molecular weight of 10,000 or less, a pore-forming agent such as calcium carbonate or a plasticizer, and an antioxidant to obtain a polyolefin resin composition. (ii) A step of cooling the obtained polyolefin resin composition in stages and forming a sheet. (iii) A step of removing the pore-forming agent from the obtained sheet with a suitable solvent. (iv) A step of stretching the sheet from which the pore-forming agent has been removed at an appropriate stretching ratio.

[0074] [2.2. Method for manufacturing porous layers] A porous layer can be formed using a coating solution obtained by dissolving or dispersing the resin described in the (Resin) section in a solvent. Alternatively, a porous layer containing the resin and the filler can be formed using a coating solution obtained by dissolving or dispersing the resin in a solvent and dispersing the filler in the same solution.

[0075] The solvent may be a solvent for dissolving the resin. The solvent may also be a dispersion medium for dispersing the resin or the filler. Examples of methods for forming the coating liquid include mechanical stirring, ultrasonic dispersion, high-pressure dispersion, and media dispersion.

[0076] Examples of methods for forming the porous layer include: applying the coating liquid directly to the surface of a substrate and then removing the solvent; applying the coating liquid to a suitable support, removing the solvent to form the porous layer, pressing the porous layer and the substrate together, and then peeling off the support; applying the coating liquid to a suitable support, pressing the substrate onto the coated surface, then peeling off the support and then removing the solvent; and performing dip coating by immersing the substrate in the coating liquid and then removing the solvent.

[0077] Preferably, the solvent does not adversely affect the substrate, dissolves the resin uniformly and stably, and disperses the filler uniformly and stably. Examples of the solvent include one or more solvents selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, acetone, and water.

[0078] The coating liquid may appropriately contain components other than the resin and the filler, such as dispersants, plasticizers, surfactants, and pH adjusters.

[0079] In addition to the aforementioned polyolefin porous film, other films, positive electrodes, and negative electrodes can be used as the substrate. If the substrate for forming the porous layer is a polyolefin porous film, a laminated separator according to one embodiment of the present invention can be manufactured.

[0080] Conventional known methods can be used as the method for applying the coating liquid to the substrate, and specifically, examples include the gravure coater method, the dip coater method, the bar coater method, and the die coater method.

[0081] If the coating liquid contains aramid resin, the aramid resin can be precipitated by applying humidity to the coated surface. This may form the porous layer.

[0082] As a method for removing the solvent from the coating liquid applied to the substrate, for example, a method of removing the solvent from the coating film, which is a film of the coating liquid, by air drying and heat drying.

[0083] Furthermore, by changing the amount of solvent in the coating solution, the porosity and average pore size of the resulting porous layer can be adjusted.

[0084] The preferred solid content concentration of the coating liquid may vary depending on the type of filler, but generally, it is preferable to have a concentration greater than 3% by weight and 40% by weight or less.

[0085] The coating shear rate when applying the coating liquid onto the substrate may vary depending on the type of filler, but is generally preferably 2 (1 / s) or higher, and more preferably 4 (1 / s) to 50 (1 / s).

[0086] (Method for preparing aramid resin) While not particularly limited, a condensation polymerization method of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide is one possible method for preparing aramid resins. In this case, the resulting aramid resin consists substantially of repeating units in which amide bonds are bonded at the para position or a similar orientation position of the aromatic ring. An orientation position similar to the para position refers to an orientation position that extends coaxially or parallel to the opposite direction, such as 4,4'-biphenylene, 1,5-naphthalene, and 2,6-naphthalene.

[0087] Specific methods for preparing a solution of poly(paraphenylene terephthalamide) include, for example, a method comprising the steps shown in (I) to (IV) below. (I) Place N-methyl-2-pyrrolidone in a dry flask, add calcium chloride that has been dried at 200°C for 2 hours, and raise the temperature to 100°C to completely dissolve the calcium chloride. (II) After the temperature of the solution obtained in step (I) is returned to room temperature, paraphenylenediamine is added and the paraphenylenediamine is completely dissolved. (III) While maintaining the temperature of the solution obtained in step (II) at 20±2℃, add the terephthalic acid dichloride in 10 portions at approximately 5-minute intervals. (IV) The solution obtained in step (III) is aged for 1 hour while maintaining the temperature at 20±2℃, and then stirred under reduced pressure for 30 minutes to remove air bubbles, thereby obtaining a solution of poly(paraphenylene terephthalamide).

[0088] [2.3. Method for controlling surface smoothness] One example of a factor that controls the smoothness of the surface of a porous layer is the dispersion state of the coating solution. If the coating solution is sufficiently dispersed, a separator that satisfies the smoothness requirements is more likely to be obtained.

[0089] For example, the preparation of a coating solution usually involves stirring, and if the time between the completion of stirring and the application of the coating solution is short, a well-dispersed coating solution can be applied to the substrate. In one embodiment, the time between the completion of the final stirring in the preparation of the coating solution and the application of the coating solution to the substrate is preferably less than 1 hour, more preferably within 30 minutes, and even more preferably within 10 minutes.

[0090] Furthermore, a coating solution treated with microbubbles can maintain a sufficiently dispersed state for a longer period of time. In the case of a coating solution treated with microbubbles, the time from the completion of the final stirring in the preparation of the coating solution to the application of the coating solution to the substrate is preferably within 20 days, more preferably within 48 hours, and even more preferably within 24 hours. The time for applying microbubble treatment to the coating solution is preferably 30 minutes or more, and more preferably 60 minutes or more. There is no particular upper limit to the time for applying microbubble treatment to the coating solution, but for example, it is 72 hours or less.

[0091] Other factors that control the smoothness of the porous layer surface include the particle size and content of the filler. If the particle size of the filler is too large or the filler content is too high, the surface of the porous layer tends to become uneven due to the filler. This is because filler particles have many irregularities and do not have a planar shape. On the other hand, since resin is more flexible than filler, the smoothness of the porous layer surface tends to increase as the resin content of the porous layer increases. The preferred average particle size (D50) and content of the filler are as described above.

[0092] Examples of factors that control the smoothness of the surface of a porous polyolefin film include coating speed and the flatness of the coating bar. Specifically, slowing down the coating speed makes it easier to obtain a separator that satisfies the smoothness requirements.

[0093] [2.4. Method for adjusting the compressive modulus in the thickness direction] The compressive modulus of the separator in the thickness direction can be adjusted, for example, by appropriately combining the materials of the porous layer and the polyolefin porous film.

[0094] [3. Components for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries] A component for a non-aqueous electrolyte secondary battery according to one aspect of the present invention comprises a positive electrode, the separator described above, and a negative electrode arranged in this order. A non-aqueous electrolyte secondary battery according to one aspect of the present invention includes the separator described above. In the component for a non-aqueous electrolyte secondary battery and the non-aqueous electrolyte secondary battery, the separator is arranged such that the surface facing the negative electrode satisfies the above-mentioned smoothness condition.

[0095] The shape of the non-aqueous electrolyte secondary battery is not particularly limited and may be a thin plate (paper) type, a disc type, a cylindrical type, a rectangular prism type, etc. The non-aqueous electrolyte secondary battery is, for example, a non-aqueous electrolyte secondary battery that obtains electromotive force by doping and dedoping lithium, and comprises a component for a non-aqueous electrolyte secondary battery in which a positive electrode, the above-mentioned separator, and a negative electrode are stacked in this order. Note that the components of the non-aqueous electrolyte secondary battery other than the above-mentioned separator are not limited to the components described below.

[0096] The aforementioned non-aqueous electrolyte secondary battery typically has a structure in which a battery element, impregnated with electrolyte in a structure where the negative electrode and positive electrode face each other via the aforementioned separator, is sealed within an outer casing. Note that "doping" refers to the phenomenon of lithium ions entering the active material of electrodes such as the positive electrode, through absorption, support, adsorption, or insertion.

[0097] Since the non-aqueous electrolyte secondary battery component is equipped with the separator described above, when incorporated into a non-aqueous electrolyte secondary battery, it can suppress the occurrence of minute short circuits in the non-aqueous electrolyte secondary battery and improve its safety. Furthermore, since the non-aqueous electrolyte secondary battery is equipped with the separator described above, the occurrence of minute short circuits is suppressed, resulting in superior safety.

[0098] A conventionally known manufacturing method can be used as the manufacturing method for the non-aqueous electrolyte secondary battery. For example, the components for the non-aqueous electrolyte secondary battery are formed by arranging the positive electrode, the separator described above, and the negative electrode in this order. Next, the components for the non-aqueous electrolyte secondary battery are placed in a container that will serve as the housing for the non-aqueous electrolyte secondary battery. Furthermore, the container is filled with a non-aqueous electrolyte and then sealed while the pressure is reduced. In this way, the non-aqueous electrolyte secondary battery can be manufactured.

[0099] [3.1. Positive electrode] The positive electrode in one embodiment of the present invention is not particularly limited, as long as it is generally used as the positive electrode of a non-aqueous electrolyte secondary battery. For example, as the positive electrode, a positive electrode sheet can be used, which has 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. The active material layer may further contain a conductive agent and / or a binder.

[0100] Examples of the positive electrode active material include materials that can be doped and dedoped with lithium ions. Specifically, examples of such materials include lithium composite oxides containing at least one transition metal such as V, Mn, Fe, Co, and Ni.

[0101] Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, coke, carbon black, pyrolytic carbons, carbon fibers, and calcined organic polymer compounds. The conductive agent may be used alone or in combination of two or more types.

[0102] Examples of the aforementioned binders include fluororesins such as polyvinylidene fluoride (PVDF), acrylic resins, and styrene-butadiene rubber. The binder also functions as a thickening agent.

[0103] Examples of the positive electrode current collector include conductors such as Al, Ni, and stainless steel. Among these, Al is more preferred because it is easy to process into a thin film and is inexpensive.

[0104] Examples of methods for manufacturing a positive electrode sheet include a method of pressurizing a positive electrode active material, a conductive agent, and a binder on a positive electrode current collector; a method of making a paste from the positive electrode active material, a conductive agent, and a binder using a suitable organic solvent, applying the paste to the positive electrode current collector, drying it, and then pressing it to fix it to the positive electrode current collector; and so on.

[0105] [3.2. Negative electrode] The negative electrode in one embodiment of the present invention is not particularly limited as long as it is generally used as the negative electrode of a non-aqueous electrolyte secondary battery. For example, as the negative electrode, a negative electrode sheet can be used which has 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. The active material layer may further contain a conductive agent and / or a binder.

[0106] Examples of the negative electrode active material include materials that can be doped and dedoped with lithium ions. Examples of such materials include carbonaceous materials. Examples of carbonaceous materials include natural graphite, artificial graphite, coke, carbon black, and pyrolytic carbons.

[0107] Examples of the negative electrode current collector include Cu, Ni, and stainless steel. Cu is more preferred because it is difficult to form alloys with lithium and is easy to process into thin films.

[0108] Examples of methods for manufacturing a negative electrode sheet include a method of pressurizing a negative electrode active material on a negative electrode current collector; a method of making a paste from the negative electrode active material using a suitable organic solvent, applying the paste to a negative electrode current collector, drying it, and then pressing it to fix it to the negative electrode current collector; and so on. The paste preferably contains the conductive agent and the binder.

[0109] [3.3.Nonaqueous electrolyte] The non-aqueous electrolyte in one embodiment of the present invention is not particularly limited as long as it is a non-aqueous electrolyte generally used in non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries. As the non-aqueous electrolyte, for example, a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent can be used. Examples of the lithium salt include LiClO4, LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, Li2B 10 Cl 10 , lithium lower aliphatic carboxylate salts, LiAlCl4, and the like. Only one kind of the lithium salt may be used, or two or more kinds may be used in combination.

[0110] Examples of the organic solvent constituting the non-aqueous electrolyte include carbonates, ethers, esters, nitriles, amides, carbamates, sulfur-containing compounds, and fluorine-containing organic solvents obtained by introducing a fluorine group into these organic solvents. Only one kind of the organic solvent may be used, or two or more kinds may be used in combination.

Examples

[0111] s Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[0112] [Measurement of Physical Properties] In Examples and Comparative Examples, the physical properties of the separator for non-aqueous electrolyte secondary batteries and the polyolefin porous film were measured by the following methods.

[0113] (1) Film Thickness The film thickness of the separator for non-aqueous electrolyte secondary batteries was measured using a high-precision digital length measuring instrument (manufactured by Mitutoyo Corporation). Specifically, each of the separators for non-aqueous electrolyte secondary batteries was cut into a square with a side length of 8 cm, and measurements were taken at five locations within the square, and the average value was taken as the film thickness.

[0114] (2) Surface Smoothness It was calculated by the following procedure. 1. The laminated separator was fixed to the glass plate with the porous layer facing upwards. 2. An optical microscope image of the porous layer surface was captured using an optical microscope (VHX-1000, manufactured by Keyence) (magnification: 1000x). 3. Using the built-in software of the optical microscope, the automatic 3D synthesis button was selected from the high-quality depth synthesis tab to create a 3D image of a standard sample with a surface elevation of 54.6 μm. This 3D image was obtained by imaging the standard sample at 0.5 μm intervals in the depth direction and superimposing the resulting images. In this 3D image of the standard sample, the highest point (height 54.6 μm) was colored red, and the lowest point (height 0 μm) was colored blue, displaying a gradient according to the surface elevation. A 3D image of the porous layer was created using the same procedure as in 4.3. Using the built-in software, the 3D comparison mode button was selected and compared with the 3D image of the standard sample created in 3. The 3D image of the porous layer was displayed as a gradient according to the height of the irregularities. 5. The obtained 3D image was loaded into the image analysis software ImageJ (provided by the National Institutes of Health, USA), and a rectangular area was selected to include all the bumps and depressions in the 3D image (using the polygon tool). Care was taken to ensure that areas unrelated to the separator surface (such as the image file name and black background) were not included in the rectangular area. 6. Select Edit-clear outside from the pull-down menu to trim the area outside the selection. 7. Select "Analyze-histogram" from the pull-down menu to automatically analyze the topography data. This operation calculates the maximum, minimum, mean, and standard deviation values. This standard deviation value corresponds to the standard deviation of the height data set from the reference plane. 8. Check "Set measurement kurtosis" and select "Analyze measure". This operation calculates the kurtosis of the height data set from the reference plane.

[0115] A positive kurtosis value for the height data set from the reference plane indicates a sharper distribution than a normal distribution. A negative kurtosis value for the height data set from the reference plane indicates a broader distribution than a normal distribution.

[0116] (3) Compression modulus The displacement rate was measured from the compression characteristics of a laminated separator using a micro-compression testing machine (MCT-510, Shimadzu Corporation). The measurement mode was set to the indentation depth setting load-unload test mode. Specifically, the separator film thickness was measured in advance, the separator was cut into 1 cm squares, attached to the measurement stage, and a compression test was performed using a flat indenter (50 μm diameter) at a loading speed of 0.45 mN / sec until the load reached 20 mN. Linear approximation was performed on the obtained displacement-load curve in the range of 5 mN to 15 mN to obtain the slope [N / μm]. The compressive modulus was calculated using the following formula. Compression modulus [P] = gradient [N / μm] * film thickness [μm] / indenter area [m²] 2 ]

[0117] (4) Shape of dendrites A CR2032 type coin cell with the following configuration was assembled in an argon glove box. The electrolyte used was a 1M solution of LiClO4 ethylene carbonate / diethyl carbonate (1 / 1=vol / vol, manufactured by Toyama Pharmaceutical Co., Ltd.). • Working electrode: Graphite mixture electrode and Cu foil • Laminated separator fabricated in the example or comparative example (porous layer positioned on the graphite mixture electrode side) Counter electrode: Li (thickness: 200 μm, manufactured by Honjo Metal).

[0118] The assembled coin cell was subjected to the following process. The material was left to stand for 1.4 hours to allow it to be impregnated with the electrolyte. 2. Current density at room temperature: 0.2 mA / cm² 2 The battery was charged and discharged for one cycle within a voltage range of 3V to 0.005V. 3.Current density: 0.2mA / cm 2 It was fully charged to 0.005V. 4.Current density: 6mA / cm2 The device was overcharged for 333 seconds. This caused lithium to be deposited on the graphite mixture electrode. 5. The electrolyte adhering to the graphite mixture electrode was washed with diethyl carbonate inside an argon glove box. 6. The removed samples were observed using a scanning electron microscope (SEM) while maintaining the argon-sealed state.

[0119] Dendrites with a granular and / or plate-like shape were determined to have a high short-circuit prevention effect because they do not grow in the thickness direction of the separator. Dendrites with a fibrous shape grew in the thickness direction of the separator and were deemed to have a low short-circuit prevention effect because they are likely to penetrate the separator.

[0120] (5) Intrinsic viscosity (i) A solution prepared by dissolving 0.5 g of aramid resin in 100 mL of concentrated sulfuric acid (96-98%), and (ii) concentrated sulfuric acid (96-98%) without the resin were prepared. The flow time was measured using an Ubbelohde capillary viscometer. The temperature during measurement was 30°C. The intrinsic viscosity was determined from the obtained flow time using the following formula. Intrinsic viscosity = ln(T / T0) / C (unit: dL / g) T: Flow time of concentrated sulfuric acid solution of aramid resin T0: Flow time of concentrated sulfuric acid C: Concentration of aramid resin in concentrated sulfuric acid solution (g / dL).

[0121] [Synthesis Example 1] Poly(paraphenylene terephthalamide) was synthesized using the following procedure. 1. A separable flask (capacity: 3L) equipped with a stirring blade, thermometer, nitrogen inlet tube, and powder addition port was prepared. 2. The separable flask was thoroughly dried and then charged with 2200 g of N-methyl-2-pyrrolidone (NMP). 3.151.07 g of calcium chloride powder (vacuum-dried at 200°C for 2 hours) was added, and the temperature was raised to 100°C to completely dissolve the calcium chloride powder. 4. The resulting calcium chloride NMP solution was allowed to return to room temperature. 5.68.23 g of paraphenylenediamine was added and completely dissolved. 6. While maintaining the obtained solution at 20°C ± 2°C, 124.61 g of terephthalic acid dichloride was divided into 10 portions and added to the solution at approximately 5-minute intervals. 7. The obtained solution was aged for 1 hour while stirring, maintaining a temperature of 20°C ± 2°C. The matured solution was filtered using an 8.1500 mesh stainless steel mesh. In this way, aramid polymerization liquid 1 was obtained.

[0122] The intrinsic viscosity of poly(paraphenylene terephthalamide) contained in aramid polymerization solution 1 was 1.7 g / dL.

[0123] [Synthesis Example 2] Aramid polymerization solution 2 was obtained using the same procedure as in Synthesis Example 1, except that the amount of terephthalic acid dichloride added was changed to 124.48 g. The intrinsic viscosity of poly(paraphenylene terephthalamide) contained in aramid polymerization solution 2 was 1.6 g / dL.

[0124] [Example 1] 100 g of aramid polymerization solution 1 was weighed into a flask, and 6.0 g of alumina C (manufactured by Nippon Aerosil Co., Ltd., average particle size 0.013 μm) and 6.0 g of AKP-3000 (manufactured by Sumitomo Chemical Co., Ltd., average particle size 0.7 μm) were added. At this time, the weight ratio of poly(paraphenylene terephthalamide) to the total amount of alumina was 33:67. Next, NMP was added so that the solid content was 6.0% by weight, and the mixture was stirred for 240 minutes. Here, "solid content" refers to the total weight of poly(paraphenylene terephthalamide) and alumina. Next, 0.73 g of calcium carbonate was added and the mixture was stirred for 240 minutes to neutralize the solution and prepare a slurry-like coating solution 1.

[0125] Coating solution 1 was allowed to stand for 8 minutes. Then, coating solution 1 was applied to a porous polyolefin film (thickness: 12 μm) made of polyethylene using the doctor blade method. The resulting coating 1 was allowed to stand in air at 50°C and 70% relative humidity for 1 minute to precipitate poly(paraphenylene terephthalamide). Next, coating 1 was immersed in deionized water to remove calcium chloride and solvent. Then, coating 1 was dried in an oven at 70°C to obtain a laminated separator 1. The physical properties of the laminated separator 1 are shown in Table 1.

[0126] [Example 2] A slurry-like coating solution 2 was obtained in the same manner as in Example 1, except that the neutralized solution was subjected to microbubble treatment for 30 minutes. The microbubble treatment was performed by stirring the coating solution with a stirring rod and stirring bar using a Three One Motor (manufactured by Shinto Kogyo Co., Ltd.) and supplying nitrogen from piping near the stirring bar. A laminated separator 2 was obtained in the same manner as in Example 1, except that the coating solution 2 that had been allowed to stand for 3 minutes was used. The physical properties of the laminated separator 2 are shown in Table 1.

[0127] [Example 3] 100 g of aramid polymerization solution 2 was weighed into a flask, and 6.0 g of alumina C (manufactured by Nippon Aerosil Co., Ltd., average particle size 0.013 μm) was added. At this time, the weight ratio of poly(paraphenylene terephthalamide) to the total amount of alumina was 1:1. Next, NMP was added so that the solid content was 4.5% by weight, and the mixture was stirred for 240 minutes. Here, "solid content" refers to the total weight of poly(paraphenylene terephthalamide) and alumina. Next, 0.73 g of calcium carbonate was added and the mixture was stirred for 240 minutes to neutralize the solution and prepare a slurry coating solution 3. A laminated separator 3 was obtained in the same manner as in Example 1, except that the coating solution 3 was used after being allowed to stand for 8 minutes. The physical properties of the laminated separator 3 are shown in Table 1.

[0128] [Example 4] A slurry-like coating solution 4 was obtained in the same manner as in Example 3, except that the neutralized solution was subjected to microbubble treatment for 30 minutes. The microbubble treatment was performed by stirring the coating solution with a stirring rod and stirring bar using a Three One Motor (manufactured by Shinto Kogyo Co., Ltd.) and supplying nitrogen from piping near the stirring bar. A laminated separator 4 was obtained in the same manner as in Example 3, except that the coating solution 4 that had been allowed to stand for 3 minutes was used. The physical properties of the laminated separator 4 are shown in Table 1.

[0129] [Comparative Example 1] Comparative coating solution 1 was prepared by mixing 9 g of polyvinylidene fluoride resin, 0.8 g of alumina with an average particle size of 500 nm, and 0.2 g of alumina with an average particle size of 50 nm. Comparative coating solution 1, which was allowed to stand for 8 minutes, was applied to one side of a polyethylene film (thickness: 12 μm) to obtain comparative laminated separator 1.

[0130] [Comparative Example 2] A comparative laminated separator 2 was obtained in the same manner as in Example 1, except that coating solution 1 was allowed to stand for 1 hour and 50 minutes before coating. The physical properties of comparative laminated separator 2 are shown in Table 1.

[0131] [Comparative Example 3] A comparative laminated separator 3 was obtained in the same manner as in Example 1, except that coating solution 1 was allowed to stand for 72 hours before coating. The physical properties of comparative laminated separator 3 are shown in Table 1.

[0132] [Table 1]

[0133] 〔result〕 Laminated separators 1-4 were smooth-surfaced separators with a kurtosis of 5 or more in the height data group from the reference plane and a standard deviation of 12 or less in the height data group from the reference plane. As a result, the dendrites generated by overcharging grew in a granular or plate-like manner. In other words, laminated separators 1-4 can be said to be separators with a high short-circuit prevention effect. This is thought to be because the porous layer was formed with fine bubbles remaining in the coating solution, either by applying the coating solution immediately after preparation or by treating the coating solution with microbubbles.

[0134] Comparative multilayer separators 1-3 were separators with non-smooth surfaces, where the kurtosis of the height data group from the reference plane was less than 5 and the standard deviation of the height data group from the reference plane was greater than 12. As a result, the dendrites generated by overcharging grew in a fibrous manner. In other words, comparative multilayer separators 1-3 can be said to be separators with a small short-circuit prevention effect.

[0135] Furthermore, all of the laminated separators 1 to 4 had a compressive modulus of 50 MPa or higher in the thickness direction. In other words, the laminated separators 1 to 4 have sufficient rigidity in the thickness direction, and voids are unlikely to form at the boundary between the negative electrode and the separator. In this respect as well, it can be said that the laminated separators 1 to 4 have a great short-circuit prevention effect. [Industrial applicability]

[0136] A separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention can be used in the manufacture of a non-aqueous electrolyte secondary battery that suppresses the occurrence of minute short circuits during charging and discharging and offers superior safety.

Claims

1. A separator for non-aqueous electrolyte secondary batteries, The above separator comprises a polyolefin porous film and a porous layer provided on one side of the polyolefin porous film. The porous layer described above faces the negative electrode when assembled into a non-aqueous electrolyte secondary battery. The above porous layer contains aramid resin. The above separator satisfies the following conditions when the surface irregularities data of the above porous layer are analyzed: The kurtosis of the height data set from the reference plane is 5 or greater (however, a kurtosis of 0 or greater indicates that the distribution curve is sharper than a normal distribution, and a larger kurtosis indicates a sharper distribution curve); The standard deviation of the height data set from the reference plane is 12 or less; The difference between the maximum and minimum values ​​of the height data set from the reference plane is 79 or less; The above separator has a compressive modulus of elasticity of 50 MPa or more in the thickness direction. Here, the kurtosis, standard deviation, maximum value, and minimum value of the data set for height from the above reference plane are obtained by the following procedure:

1. Fix the laminated separator to the glass plate so that the porous layer is facing upwards.

2. An optical microscope image of the porous layer surface is taken using an optical microscope (VHX-1000, manufactured by Keyence) (magnification: 1000x).

3. Using the built-in software of the optical microscope, select the automatic 3D synthesis button from the high-quality depth synthesis tab to create a 3D image of a standard sample with a surface elevation of 54.6 μm. In this 3D image of the standard sample, the highest point (height 54.6 μm) is colored red, and the lowest point (height 0 μm) is colored blue, displaying a gradient according to the surface elevation.

4. Create a 3D image of the porous layer using the same procedure as in step 3. Using the built-in software, select the 3D comparison mode button and compare it with the 3D image of the standard sample created in step 3 to display the 3D image of the porous layer as a gradient according to the height of the irregularities.

5. Load the obtained 3D image into the image analysis software Image J (provided by the National Institutes of Health, USA), and select a rectangular area that includes all the bumps and depressions in the 3D image (use the polygon tool).

6. Select Edit-clear outside from the pull-down menu to trim the area outside the selection.

7. Select "Analyze-histogram" from the pull-down menu to automatically analyze the topography data. This operation calculates the maximum, minimum, and standard deviation values. These values ​​correspond to the maximum, minimum, and standard deviation of the height from the reference plane.

8. Check "Set measurement kurtosis" and select "Analyze measure". This operation calculates the kurtosis of the height data set from the above reference plane.

2. A component for a non-aqueous electrolyte secondary battery, wherein a positive electrode, the separator described in claim 1, and a negative electrode are stacked in this order.

3. A non-aqueous electrolyte secondary battery comprising the separator described in claim 1 or the non-aqueous electrolyte secondary battery component described in claim 2.

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