Separator, component for non-aqueous electrolyte secondary battery, 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 0.20 or less in the coefficient of variation of ion resistances calculated from local ion resistances measured by a probe of 7 mm-length×250 μm-width at 10 points located at intervals of 800 μm in any region of 1.5 cm×1.5 cm in at least one surface of the separator.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a separator, a component for a non-aqueous electrolyte secondary battery, 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 fibrous shape, 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, A separator in which the coefficient of variation of ion resistance calculated from 10 local ion resistances measured at 800 μm intervals using a probe measuring 7 mm in length and 250 μm in width within any 1.5 cm × 1.5 cm area on at least one surface of the separator is 0.20 or less. <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> Positive electrode and, <1> ~ <5> A component for a non-aqueous electrolyte secondary battery, wherein a separator described in any of the above and a negative electrode are stacked in this order. <7> A non-aqueous electrolyte secondary battery comprising the separator according to any one of <1> to <5> or the 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.

Brief Description of the Drawings

[0011] [Figure 1] It is a schematic diagram showing an example of a working electrode used for measuring ion resistance distribution. [Figure 2] It is a schematic diagram showing an example of a method for measuring ion resistance distribution.

Mode for Carrying Out the Invention

[0012] An 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 can be made within the scope set forth in the claims. Embodiments obtained by appropriately combining technical means respectively 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 "not less than A and not more than B".

[0013] [1. Separator for Non-aqueous Electrolyte Secondary Battery] According to the inventors' new findings, the shape of dendrites growing on the negative electrode surface can be controlled by controlling the ion resistance distribution, i.e., the variation in ion resistance, on the surface of the separator contained in a non-aqueous electrolyte secondary battery. Specifically, a separator with small variation in ion resistance on the surface facing the negative electrode allows for a uniform ion flow within the separator, thus controlling the shape of the growing dendrites to be granular or plate-like. Dendrites of this shape are unlikely to penetrate the separator, even if they occur. Conversely, a separator with large variation in ion resistance on the side facing the negative electrode results in an uneven ion flow within the separator, causing fibrous dendrites to grow in areas with dense ion flow. Dendrites of this shape are highly likely to penetrate the separator.

[0014] In this specification, the variation in ion resistance on the separator surface is expressed by the coefficient of variation of ion resistance. In one embodiment of the present invention, the coefficient of variation of ion resistance calculated from 10 local ion resistances measured at 800 μm intervals using a probe with a length of 7 mm and a width of 250 μm within an arbitrary 1.5 cm × 1.5 cm area on at least one surface of the separator is 0.20 or less. The coefficient of variation of ion resistance is preferably 0.17 or less, more preferably 0.15 or less, and even more preferably 0.13 or less. The lower limit of the coefficient of variation of ion resistance is not particularly limited, and ideally it is 0 (zero).

[0015] The coefficient of variation of ion resistance is calculated by dividing the standard deviation of the local ion resistances at 10 points by the mean of the local ion resistances. In this specification, local ion resistance refers to the local ion resistance measured using a probe measuring 7 mm in length and 250 μm in width. The 10 local ion resistances can be measured, for example, using a working electrode with 10 probes (7 mm in length and 250 μm in width) spaced 800 μm apart. This allows for the measurement of 10 ion resistances at 800 μm intervals. The value of the ion resistance at each measurement point can be obtained by plotting the AC impedance measured at each measurement point using a Nyquist plot and taking the value at the intersection of the resulting curve and the X-axis. A more specific measurement method will be described later.

[0016] A known method for measuring the ion resistance distribution of a separator involves measuring the AC impedance of a coin cell assembled by sandwiching the separator between two metal plates, calculating the value at the intersection of the curve obtained from the Nyquist plot and the X-axis, and using this value as the ion resistance of the separator. However, this method is limited by the number of centimeters. 2 This means that the ion resistance is being measured based on the average pore structure over a relatively large area. Therefore, conventional measurement methods have not been able to adequately reflect the pore structure distribution, i.e., the variability of the pore structure. In one embodiment of the present invention, the ion resistance distribution due to the pore structure distribution of the separator can be measured by measuring the local ion resistance of the separator at multiple measurement points.

[0017] 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.

[0018] 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. An MCT-510 (manufactured by Shimadzu Corporation) or the like is used as a micro-compression testing machine.

[0019] 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.

[0020] [1.1. Multilayer 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.

[0021] When a porous layer is provided on one side of a porous polyolefin film, the surface that satisfies the above-mentioned conditions for the coefficient of variation of ion resistance may be the surface on the porous layer side or the surface on the porous polyolefin film side. In one embodiment, the surface that satisfies the above-mentioned conditions for the coefficient of variation of ion resistance is facing the negative electrode when assembled into a non-aqueous electrolyte secondary battery.

[0022] (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.

[0023] 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.

[0024] The porous layer contains a resin. Preferably, the porous layer is an insulating porous layer containing an insulating resin.

[0025] 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.

[0026] (resin) Preferably, the resin is insoluble in the battery electrolyte and is electrochemically stable within the battery's operating range.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] The polyolefins mentioned above are preferably polyethylene, polypropylene, polybutene, and ethylene-propylene copolymers.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] As the polyester resin, aromatic polyesters such as polyarylate and liquid crystal polyesters are preferred.

[0035] 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.

[0036] 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.

[0037] Examples of the water-soluble polymers mentioned above include polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, and polymethacrylic acid.

[0038] The resin may be one type only, or two or more types of resins may be used in combination. 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.

[0039] (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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] (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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] (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.

[0055] 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.

[0056] 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."

[0057] 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.

[0058] 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.

[0059] Examples of the aforementioned homopolymers include polyethylene, polypropylene, and polybutene. Examples of the aforementioned copolymers include ethylene-propylene copolymers.

[0060] 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.

[0061] 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.

[0062] 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 the amount be 5-10 g / m 2 It is even more preferable that this be the case.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] [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.

[0070] 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.

[0071] As a method for manufacturing the porous film, for example, a method including the following steps can be cited.

[0072] (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.

[0073] (ii) A step of cooling the obtained polyolefin resin composition in stages and forming a sheet.

[0074] (iii) A step of removing the pore-forming agent from the obtained sheet with a suitable solvent.

[0075] (iv) A step of stretching the sheet from which the pore-forming agent has been removed at an appropriate stretching ratio.

[0076] [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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] The coating liquid may appropriately contain components other than the resin and the filler, such as dispersants, plasticizers, surfactants, and pH adjusters.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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).

[0088] (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.

[0089] Specific methods for preparing a solution of poly(paraphenylene terephthalamide) include, for example, a method comprising the steps shown in (I) to (IV) below.

[0090] (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.

[0091] (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.

[0092] (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.

[0093] (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).

[0094] [2.3. Methods for controlling ion resistance distribution] One example of a factor that controls the ion resistance distribution on the surface of a porous layer is the dispersion state of the coating solution. If a coating solution that is sufficiently dispersed is applied to the substrate, a separator that satisfies the conditions for ion resistance distribution is more likely to be obtained.

[0095] 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.

[0096] 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.

[0097] Other examples of factors that control the ion resistance distribution on the surface of a porous layer 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 pore structure inside the porous layer tends to become non-uniform. This is because filler particles have many irregularities and tend to create large voids inside the porous layer. As a result, the ion flow inside the porous layer tends to become non-uniform. Consequently, the variation in ion resistance can become large. On the other hand, since resin is more flexible than filler, if the resin content of the porous layer is high, the pore structure inside the porous layer tends to become more uniform. As a result, the ion flow inside the porous layer becomes uniform, and the variation in ion resistance can become smaller. The preferred average particle size (D50) and content of the filler are as described above.

[0098] Examples of factors that control the ion resistance distribution on 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 conditions for ion resistance distribution.

[0099] [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.

[0100] [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 condition for the coefficient of variation of ion resistance.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] [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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] [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.

[0112] 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.

[0113] 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.

[0114] Examples of the method for producing a negative electrode sheet include: a method of pressure-molding a negative electrode active material on a negative electrode current collector; a method of forming a negative electrode active material into a paste using an appropriate organic solvent, applying the paste to a negative electrode current collector, drying the paste, then applying pressure to fix it to the negative electrode current collector; and the like. The paste preferably contains the conductive agent and the binder.

[0115] [3.3. Non-aqueous 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 for 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 , lower aliphatic carboxylic acid lithium salt, LiAlCl4, and the like. Only one type of the lithium salt may be used, or two or more types may be used in combination.

[0116] 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 fluorine groups into these organic solvents. Only one type of the organic solvent may be used, or two or more types may be used in combination.

[0117] [4. Method and Apparatus for Measuring Ionic Resistance Distribution] A method for measuring the ion resistance distribution of a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention includes the step of measuring 10 local ion resistances at 800 μm intervals using a probe with a length of 7 mm and a width of 250 μm within an arbitrary 1.5 cm × 1.5 cm area on at least one surface of the separator for a non-aqueous electrolyte secondary battery. This allows for the measurement of the ion resistance distribution, i.e., the variation in ion resistance, from the obtained 10 local ion resistances. The above measurement method may also include the step of calculating the coefficient of variation of ion resistance from the obtained 10 local ion resistances.

[0118] Local ion resistance can be measured using a working electrode having, for example, 10 probes, each 7 mm long and 250 μm wide, arranged at 800 μm intervals. Figure 1 is a schematic diagram showing an example of a working electrode used for measuring ion resistance distribution. The working electrode 10 comprises a substrate 1 and probes 2 placed on the substrate 1. As the substrate 1, glass epoxy substrates, glass substrates, epoxy substrates, bakelite substrates, paper-based phenolic resin laminated substrates, polycarbonate substrates, ABS resin substrates, etc., can be used. The probes 2 may be, for example, metal wiring. Examples of metal wiring materials include gold and copper. The substrate 1 is also provided with terminals 3. The probes 2 are connected to terminals 3. The terminals 3 can be connected to an AC impedance measuring device. The region including the wiring connecting the probes 2 and terminals 3 is insulated from the counter electrode 11, electrolyte, and electrolyte-impregnated separator 12 described later by covering it with an insulating material 4. Examples of insulating materials 4 include polytetrafluoroethylene, PEEK (polyetheretherketone), phenolic resin, silicone resin, and PP (polypropylene).

[0119] In this specification, the length of probe 2 refers to the length of the portion that contacts the surface of the separator being measured. That is, in Figure 1, the length of the portion of probe 2 exposed from the insulating material 4 is 7 mm.

[0120] Furthermore, more than 10 probes 2 may be arranged on the substrate 1. Also, more than 10 terminals 3 may be arranged. In Figure 1, 24 probes 2 and 12 terminals 3 are arranged. Every other probe 2 is connected to a terminal 3. At least 10 of these probes 2 may be used for measurement. In other words, it is not necessary to use all of the probes 2 on the substrate 1. For example, if irregular measurements are obtained from the probes 2 at both ends of a set of multiple probes, these probes 2 at both ends may not be used for measurement. Probes 2 not used for measurement may be useful for fixing the separator 12 described later.

[0121] Furthermore, in this specification, "10 points at 800 μm intervals" means that the distance (pitch) between the centerlines of the probes 2 used for measurement is 800 μm. For example, in Figure 1, when probes 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, and 2j are used for measurement, it means that the distance between the centerlines of these 10 probes 2 is 800 μm.

[0122] Figure 2 is a schematic diagram showing an example of a method for measuring ion resistance distribution. The separator 12 to be measured is sandwiched between the working electrode 10 and the metal counter electrode 11 described above. The working electrode 10 has the same configuration as in Figure 1, but is simplified in Figure 2 for convenience. Here, the surface of the separator 12 to be measured for ion resistance distribution is brought into contact with the surface of the working electrode 10 having the probe 2. In Figure 2, the working electrode 10, separator 12, and counter electrode 11 are arranged separately for convenience, but when actually measuring ion resistance distribution, it is preferable that the working electrode 10 and counter electrode 11 are in close contact with the separator 12. Furthermore, in order to maintain the working electrode 10 and counter electrode 11 in contact with the separator 12, pressure may be applied to the working electrode 10 and counter electrode 11. In this case, it is preferable to apply pressure so that the pressure is applied uniformly to the surface of the separator 12. Examples of materials for the counter electrode 11 include precious metals such as gold, silver, and platinum, transition metals such as Ni and Cu, and metals such as SUS. It is desirable that the material of the counter electrode and the material of the working electrode probe be of the same type. An ion resistance measurement cell 100 is fabricated by housing the working electrode 10, separator 12 and counter electrode 11 in a container 13 and pouring an electrolyte solution into the container 13. The working electrode 10 and counter electrode 11 are connected to an AC impedance measuring device 101. In Figure 2, for convenience, the working electrode 10 is shown connected to the AC impedance measuring device 101 with a single wire, but in reality, each of the terminals 3 used for measurement is connected to the AC impedance measuring device 101.

[0123] Since there are 10 probes 2 on the working electrode 10, the AC impedance can be measured at 10 measurement points. The value of the local ion resistance at each measurement point can be determined by plotting the AC impedance measured at each measurement point using a Nyquist plot and taking the value at the intersection of the resulting curve and the X-axis. The probes 2 and / or the working electrode 10 may be replaced each time a separator 12 is measured.

[0124] Furthermore, one embodiment of the present invention also includes a separator screening method. The separator screening method according to one embodiment of the present invention includes the steps of measuring the coefficient of variation of the ion resistance of the separator by the measurement method described above, and selecting separators in which the coefficient of variation of the ion resistance is 0.20 or less. This makes it possible to identify separators in which the shape of the dendrites is controlled to be granular or flat.

[0125] An apparatus for measuring the ion resistance distribution of a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention comprises 10 probes, each 7 mm long and 250 μm wide, arranged at 800 μm intervals. The apparatus comprises, for example, a working electrode having the 10 probes described above. The apparatus may also comprise the counter electrode described above. Furthermore, the apparatus may also comprise the AC impedance measuring device described above.

[0126] The present invention is not limited to the embodiments described above, 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. [Examples]

[0127] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0128] [Measurement of physical properties] In the examples and comparative examples, the physical properties of the separator for non-aqueous electrolyte secondary batteries and the porous polyolefin film were measured by the following methods.

[0129] (1) Film thickness The film thickness of separators for non-aqueous electrolyte secondary batteries was measured using a high-precision digital measuring instrument (manufactured by Mitutoyo Corporation). Specifically, each separator for non-aqueous electrolyte secondary batteries was cut into a square with sides of 8 cm, and measurements were taken at five locations within each square. The average value was used as the film thickness.

[0130] (2) Coefficient of variation of ionic resistance (Preparation for measurement) The working electrode 10 shown in Figure 1 above was prepared. A glass epoxy substrate was used as the substrate 1. Twenty-four gold wires with a width of 250 μm were placed on the substrate 1 as probes 2. Twelve terminals were also placed on the substrate 1. Every other probe 2 was connected to one of the twelve terminals. Of the 24 probes 2, two at each end (a total of four probes) and of the twelve terminals 3, one at each end (a total of two terminals) were not used for measurement because irregular measurements may be obtained due to the edge effect. The distance between the centerlines of the probes 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, and 2j used for measurement was set to 800 μm. Polytetrafluoroethylene was used as the insulating material 4. The length of the portion of the gold wire exposed from the insulating material 4 was set to 7 mm.

[0131] The porous layer of the multilayer separator was brought into close contact with the gold wiring of the working electrode. Also, the counter electrode (1cm 2 The gold plate electrode was placed in close contact with the polyethylene porous film of the laminated separator. This sandwiched the laminated separator between the working electrode and the counter electrode. The distance between the working electrode and the counter electrode was kept constant. Glass plates were placed on both the counter electrode side and the working electrode side to ensure that pressure was applied uniformly across the entire surface of the porous layer to be measured.

[0132] (Ionic resistance distribution) Using the working electrode described above, an ion resistance measurement cell was assembled in a glove box under an argon atmosphere. Specifically, a separator was placed between the working electrode and the counter electrode, and these were placed in a container, into which the electrolyte solution was poured. The electrolyte solution used was a 1M solution of ethylene carbonate / diethyl carbonate (1 / 1=vol / vol, manufactured by Toyama Pharmaceutical Co., Ltd.) of LiClO4. The container was sealed under an argon atmosphere. In this way, an ion resistance measurement cell was obtained. To allow the electrolyte solution to permeate the ion resistance measurement cell from pouring to measurement, the cell was left to stand for 4 hours after assembly. AC impedance was measured using an AC impedance meter manufactured by Bio-Logic, in the range of amplitude 10mV and frequency 100MHz to 0.1Hz. The AC impedance meter was connected to the working electrode and the counter electrode. The measurement temperature was 25°C. Ten measurement points were measured one by one. A measurement interval of 5 minutes or more was maintained between each point.

[0133] The AC impedance was plotted using the Nyquist method for each measurement point, and the intersection of the resulting curve with the X-axis was defined as the local ion resistance of the separator at that measurement point. In Figure 1, the 10 probes 2 used for measurement were designated probes 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, and 2j, starting from the end of the working electrode. Measurements were performed in the order of probes 2a, 2j, 2b, 2i, 2c, 2h, 2d, 2g, 2e, and 2f. This measurement method minimized the influence of ion flow from other parts. The standard deviation and mean of the local ion resistance were calculated from the obtained local ion resistances at the 10 points. The coefficient of variation was also calculated using the formula: coefficient of variation = standard deviation / mean. The working electrode was replaced after each measurement of a multilayer separator.

[0134] (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 ] (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).

[0135] 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 / cm 2 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.

[0136] 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.

[0137] (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).

[0138] [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.

[0139] The intrinsic viscosity of poly(paraphenylene terephthalamide) contained in aramid polymerization solution 1 was 1.7 g / dL.

[0140] [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.

[0141] [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.

[0142] 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.

[0143] [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.

[0144] [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.

[0145] [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.

[0146] [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.

[0147] [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.

[0148] [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.

[0149] [Table 1]

[0150] In Table 1, "max-min" represents the difference between the maximum and minimum values ​​of the ion resistance measured at 10 points.

[0151] 〔result〕 The multilayer separators 1-4 had a coefficient of variation of ion resistance of 0.20 or less. As a result, the dendrites generated by overcharging grew in a granular or plate-like form. In other words, multilayer 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.

[0152] Comparative multilayer separators 1-3 had a coefficient of variation of ion resistance greater than 0.20. 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 low short-circuit prevention effect.

[0153] Furthermore, all of the laminated separators 1 to 4 had a compressive modulus of 65 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]

[0154] 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. [Explanation of symbols]

[0155] 1 circuit board 2 probes 3 terminals 4. Insulating materials 10 Working electrode 11 Opposite 12 Separators 13 Container 100 Ion Resistance Measurement Cell 101 AC Impedance Measurement Device

Claims

1. A separator for non-aqueous electrolyte secondary batteries, The above separator is a laminated separator comprising a porous layer and a polyolefin porous film. The above porous layer is provided on one side of the polyolefin porous film. The above porous layer contains nitrogen-containing aromatic resin, The coefficient of variation of the ion resistance calculated from the local ion resistance measured at 800 μm intervals using a probe measuring 7 mm in length and 250 μm in width within an arbitrary 1.5 cm × 1.5 cm area on the surface of the porous layer of the above separator is 0.20 or less. A porous layer that satisfies the above conditions for the coefficient of variation of ion resistance is a separator that faces at least the negative electrode when assembled into a non-aqueous electrolyte secondary battery.

2. The separator according to claim 1, wherein the nitrogen-containing aromatic resin is an aramid resin.

3. The separator according to claim 1 or 2, wherein the compressive modulus in the thickness direction is 50 MPa or more.

4. A component for a non-aqueous electrolyte secondary battery, comprising a positive electrode, a separator according to any one of claims 1 to 3, and a negative electrode, stacked in this order.

5. A non-aqueous electrolyte secondary battery comprising a separator according to any one of claims 1 to 3 or a non-aqueous electrolyte secondary battery component according to claim 4.

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