JP2022145553A - Separator, member for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

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 The present invention relates to a separator, a non-aqueous electrolyte secondary battery member, and a non-aqueous electrolyte secondary battery.

Non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, have high energy density and are widely used as batteries for personal computers, mobile phones, personal digital assistants, vehicles, and the like.

In recent years, the demand for lithium-ion secondary batteries with high output has increased. In order to satisfy the demand, development of lithium ion secondary batteries equipped with separators having excellent ion permeability is underway.

On the other hand, the lithium ion secondary battery has a problem in that the fibrous lithium deposits tend to cause micro short circuits and voltage drop. The micro short circuit is one of the factors that reduce the long-term reliability of the battery.

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. Further, Patent Literature 2 discloses a separator having a patterned area where a pattern is formed and a non-patterned area where no pattern is formed.

JP-A-10-055794 Japanese Patent Application Laid-Open No. 2020-068178

However, there is still room for further development of methods for suppressing the growth of dendrites in lithium-ion secondary batteries. For example, it is known that the shape of a dendrite becomes closer to a fibrous shape as the dendrite grows. Therefore, it is important to also control the shape of dendrites in order to prevent short circuits due to dendrite growth. However, the aforementioned prior art documents do not consider controlling the shape of dendrites.

An object of one aspect of the present invention is to provide a separator for a non-aqueous electrolyte secondary battery in which the shape of dendrites is controlled and the short-circuit prevention effect is enhanced.

The present invention includes the following aspects.
<1>
A separator for a non-aqueous electrolyte secondary battery,
Ions calculated from the local ionic resistance of 10 points measured at intervals of 800 μm using a probe of length 7 mm × width 250 μm in an arbitrary 1.5 cm × 1.5 cm region on at least one surface of the separator A separator having a resistance variation coefficient of 0.20 or less.
<2>
The separator according to <1>, which is a laminated separator comprising a porous layer and a polyolefin porous film.
<3>
The separator according to <2>, wherein the porous layer contains a nitrogen-containing aromatic resin.
<4>
The separator according to <3>, wherein the nitrogen-containing aromatic resin is an aramid resin.
<5>
The separator according to any one of <1> to <4>, which has a compressive elastic modulus of 50 MPa or more in the thickness direction.
<6>
A member for a non-aqueous electrolyte secondary battery, comprising a positive electrode, a separator according to any one of <1> to <5>, and a negative electrode, which are laminated 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>.

According to one aspect of the present invention, there is provided a separator for a non-aqueous electrolyte secondary battery in which the shape of dendrites is controlled to enhance the short-circuit prevention effect.

FIG. 2 is a schematic diagram showing an example of a working electrode used for measuring ionic resistance distribution; FIG. 2 is a schematic diagram showing an example of a method for measuring ionic resistance distribution;

An embodiment of the invention will be described below, but the invention is not limited thereto. The present invention is not limited to the configurations described below, and can be modified in various ways within the scope of the claims, by appropriately combining technical means disclosed in different embodiments. The resulting embodiment is also included in the technical scope of the present invention. In this specification, unless otherwise specified, "A to B" representing a numerical range means "A or more and B or less".

[1. Separator for non-aqueous electrolyte secondary battery]
The present inventors have newly discovered that by controlling the ionic resistance distribution on the surface of the separator contained in the non-aqueous electrolyte secondary battery, that is, the variation in ionic resistance, dendrite growth on the surface of the negative electrode can be controlled. You can control the shape. That is, a separator having a small variation in ionic resistance on the side facing the negative electrode has a uniform flow of ions flowing through the separator, so that the shape of growing dendrites can be controlled to be granular or flat. Such shaped dendrites are less likely to penetrate the separator, if at all. Conversely, in a separator with a large variation in ion resistance on the side facing the negative electrode, the flow of ions flowing inside the separator becomes non-uniform, causing fibrous dendrites to grow in areas where the flow of ions is dense. A dendrite with such a shape is likely to penetrate the separator.

In this specification, the variation in ionic resistance on the surface of the separator is expressed by the coefficient of variation of ionic resistance. The separator according to one embodiment of the present invention was measured at intervals of 800 μm using a probe of length 7 mm×width 250 μm in an arbitrary 1.5 cm×1.5 cm region on at least one surface of the separator. The variation coefficient of ionic resistance calculated from 10 points of local ionic resistance is 0.20 or less. The variation coefficient of ionic resistance is preferably 0.17 or less, more preferably 0.15 or less, and still more preferably 0.13 or less. The lower limit of the coefficient of variation of ionic resistance is not particularly limited, and is ideally 0 (zero).

The coefficient of variation of the ionic resistance is calculated by dividing the standard deviation of the 10-point local ionic resistance by the mean of the local ionic resistance. As used herein, local ionic resistance means local ionic resistance measured using a 7 mm long by 250 μm wide probe. The local ionic resistance at 10 points can be measured, for example, using a working electrode having 10 probes (length 7 mm×width 250 μm) at intervals of 800 μm. As a result, ionic resistance can be measured at 10 points at intervals of 800 μm. The value of the ionic resistance at each measurement point can be obtained as the value at the intersection of the curve obtained by Nyquist-plotting the AC impedance measured at each measurement point and the X-axis. A more specific measuring method will be described later.

As a method for measuring the ionic resistance distribution of the separator, the AC impedance of the coin cell assembled by sandwiching the separator between two metal plates was measured, and the value at the intersection of the curve obtained from the Nyquist plot and the X axis was calculated. , and a method of using this value as the ionic resistance of the separator is known. However, in this method, the ionic resistance due to the average pore structure in a relatively large area of about 2 cm ² is measured. Therefore, the conventional measurement method cannot sufficiently reflect the pore structure distribution, that is, the variation in the pore structure. In one embodiment of the present invention, the ionic resistance distribution resulting from the pore structure distribution of the separator can be measured by measuring the local ionic resistance of the separator at a plurality of measurement points.

The present inventors have further discovered that the rigidity of the separator in the thickness direction is also a factor that controls the shape of dendrites growing on the surface of the negative electrode. A separator having high rigidity in the thickness direction is less likely to be deformed in the thickness direction and less likely to create a space between the negative electrode and the separator. Under such conditions, the shape of growing dendrites tends to be granular or tabular. Such shaped dendrites are less likely to penetrate the separator, if at all. On the contrary, a separator having low rigidity in the thickness direction is likely to be deformed in the thickness direction, and thus tends to create a space between the negative electrode and the separator. Therefore, it tends to grow fibrous dendrites. A dendrite with such a shape is likely to penetrate the separator.

In this specification, the rigidity in the thickness direction of the separator is expressed by the compressive elastic modulus in the thickness direction of the separator. In one embodiment, the compressive elastic 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 elastic modulus in the thickness direction of the separator is preferably 300 MPa or less, more preferably 250 MPa or less, still more preferably 175 MPa or less, and particularly preferably 125 MPa or less. Examples of combinations of the lower limit and upper limit of the compressive modulus in the thickness direction of the separator include 50 to 300 MPa, 55 to 250 MPa, 60 to 175 MPa, and 75 to 125 MPa. The compression elastic modulus in the thickness direction of the separator is measured using a microcompression tester. MCT-510 (manufactured by Shimadzu Corporation) or the like is used as a microcompression tester.

In this specification, the determination of the shape of the dendrite is based on the 5000x SEM image. In this specification, the shape of the dendrite is classified according to the aspect ratio (major axis/minor axis) of the dendrite. Dendrites with an aspect ratio of less than 2 are referred to as granular. A dendrite in which a plurality of granular dendrites are connected is called a tabular dendrite. Dendrites with aspect ratios greater than 13 are referred to as "fibrous". See the examples of the present application for the test method for generating dendrites. The shape of dendrites generated in the dendrite generation test is most preferably granular, followed by tabular. Fibrous dendrites are not preferred because they may penetrate the separator.

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

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

(1.1.1. Porous layer)
The porous layer is a member constituting a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. can be placed between either

The porous layer can be formed on the active material layer of at least one of the positive electrode and the negative electrode in the non-aqueous electrolyte secondary battery. The porous layer may be arranged between and in contact with the polyolefin porous film and at least one of the positive electrode and the negative electrode. Alternatively, although not in the form of a laminated separator, the porous layer itself that does not contain 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.

The porous layer contains a resin. The porous layer is preferably an insulating porous layer containing an insulating resin.

When the porous layer is laminated on one side of the polyolefin porous film, the porous layer preferably faces the negative electrode of the surface of the polyolefin porous film in the non-aqueous electrolyte secondary battery. Laminated on the surface to be covered. More preferably, the porous layer is laminated on a surface in contact with the negative electrode in the non-aqueous electrolyte secondary battery.

(resin)
The resin is preferably insoluble in the electrolyte of the battery and electrochemically stable within the range of use of the battery.

Examples of the resin include polyolefins; (meth)acrylate resins; nitrogen-containing aromatic resins; fluorine-containing resins; polyamide resins; polyimide resins; polyester resins; resin; water-soluble polymer; polycarbonate, polyacetal, polyetheretherketone and the like.

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

Moreover, it is particularly preferable that the resin is a nitrogen-containing aromatic resin. The nitrogen-containing aromatic resin has a bond such as an amide bond via nitrogen, and thus has excellent heat resistance. Therefore, by using the nitrogen-containing aromatic resin as the resin, the heat resistance of the porous layer can be preferably improved. As a result, the heat resistance of the non-aqueous electrolyte secondary battery separator including the porous layer can be improved.

As the polyolefin, polyethylene, polypropylene, polybutene, ethylene-propylene copolymer and the like are preferable.

Examples of the fluorine-containing resin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer. Polymer, Vinylidene Fluoride-Tetrafluoroethylene Copolymer, Vinylidene Fluoride-Trifluoroethylene Copolymer, Vinylidene Fluoride-Trichlorethylene Copolymer, Vinylidene Fluoride-Vinyl Fluoride Copolymer, Vinylidene Fluoride-Hexa Fluoropropylene-tetrafluoroethylene copolymers, ethylene-tetrafluoroethylene copolymers, etc., and among the fluorine-containing resins, fluorine-containing rubbers having a glass transition temperature of 23° C. or less can be mentioned.

The polyamide-based resin is preferably a polyamide-based resin corresponding to a nitrogen-containing aromatic resin, and particularly preferably an aramid resin such as an aromatic polyamide or a wholly aromatic polyamide.

Examples of the aramid resin include poly(paraphenylene terephthalamide), poly(metaphenylene isophthalamide), poly(parabenzamide), poly(methabenzamide), 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-chloro paraphenylene terephthalamide), paraphenylene terephthalamide/2,6-dichloro paraphenylene terephthalamide copolymer, metaphenylene terephthalamide/2,6-dichloro Examples include paraphenylene terephthalamide copolymers. Among these, poly(paraphenylene terephthalamide) is more preferable.

As the polyester-based resin, aromatic polyester such as polyarylate and liquid crystal polyester are preferable.

Examples of the rubbers include styrene-butadiene copolymer and its hydride, methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl acetate, and the like. can be mentioned.

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.

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

As the resin, only one kind of resin may be used, or two or more kinds of resin may be used in combination. The content of the resin in the porous layer is preferably 25 to 80% by weight, more preferably 30 to 70% by weight, when the total weight of the porous layer is 100% by weight.

(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. The filler is more preferably an inorganic filler made of one or more inorganic oxides selected from the group consisting of silica, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, zeolite, aluminum hydroxide, boehmite, and the like.

In addition, in order to improve the water absorbability of the inorganic filler, the surface of the inorganic filler may be hydrophilized with a silane coupling agent or the like.

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

The porous layer is preferably arranged between the polyolefin porous film and a negative electrode active material layer of the negative electrode. The physical properties of the porous layer in the following description refer to the physical properties of the porous layer disposed between the polyolefin porous film and the negative electrode active material layer of the negative electrode when the non-aqueous electrolyte secondary battery is constructed. Point at least.

The basis weight of the porous layer, that is, the weight per unit area can be appropriately determined in consideration of the strength, film thickness, weight and handleability of the porous layer. The basis weight of the porous layer is preferably 0.5 to 3.5 g/m ² and more preferably 1.0 to 3.0 g/m ² per one porous layer.

By setting the basis weight of the porous layer within these numerical ranges, the weight energy density and the volume energy density of the non-aqueous electrolyte secondary battery including the porous layer can be further increased. If the basis weight of the porous layer exceeds the above range, the non-aqueous electrolyte secondary battery including the porous layer tends to be heavy.

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

Moreover, the pore size of the pores of the porous layer is preferably 1.0 μm or less, more preferably 0.5 μm or less. By setting the pore diameters of the pores to these sizes, the non-aqueous electrolyte secondary battery having the porous layer can obtain sufficient ion permeability.

The air permeability of the porous layer is preferably 30 to 80 s/100 mL, more preferably 40 to 75 s/100 mL in Gurley value. 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.

The lower limit of the film thickness of the porous layer 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 film thickness of the porous layer 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 limit and upper limit of the thickness of the porous layer include 0.1 to 20 μm, 0.3 to 10 μm, and 0.5 to 5 μm. If the film thickness of the porous layer is within the above range, the function of the porous layer (imparting heat resistance, etc.) can be sufficiently exhibited, and the thickness of the entire separator can be reduced.

(Preferred combination example of resin and filler)
In one embodiment, the resin contained in the porous layer has an intrinsic viscosity of 1.4 to 4.0 dL/g, and the filler has an average particle size of 1 μm or less. By using a porous layer having such a composition, it is possible to produce a laminated separator that achieves both thinness, heat resistance, and ion permeability.

The lower limit of intrinsic viscosity of the resin contained in the porous layer is preferably 1.4 dL/g or more, more preferably 1.5 dL/g or more. 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 2.0 dL/g or less. is more preferable. A porous layer containing a resin having 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 having an intrinsic viscosity of 4.0 dL/g or less has sufficient ion permeability. For the method of measuring the intrinsic viscosity, refer to Examples of the present application.

A resin having 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 charged, synthesis temperature, synthesis time, etc.). Alternatively, commercially available resins with intrinsic viscosities between 1.4 and 4.0 dL/g may be used. In one embodiment, the resin with an intrinsic viscosity of 1.4-4.0 dL/g is an aramid resin.

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 diameter of the filler is the average value of the sphere-equivalent particle diameters of 50 fillers. Also, the spherical conversion particle size of the filler is a value actually measured by a transmission electron microscope. A specific measuring method is exemplified below.
1. Using a transmission electron microscope (TEM; JEOL Ltd., transmission electron microscope JEM-2100F), the acceleration voltage is 200 kV, and the magnification is 10000 using a Gatan Imaging Filter.
2. Using image analysis software (ImageJ), the contours of the particles are traced on the resulting image, and the sphere-converted particle diameter of the filler particles (primary particles) is measured.
3. The above measurements are performed on 50 randomly selected filler particles. Let the arithmetic mean of the spherical conversion particle diameter of 50 filler particles be an average particle diameter of particle|grains.

By setting the average particle diameter of the filler to 1 μm or less, the thickness of the laminated separator can be reduced. Although the lower limit of the average particle size of the filler is not particularly limited, it can be, for example, 5 nm or more.

(1.1.2. Polyolefin porous film)
A laminated separator according to one embodiment of the present invention includes a polyolefin porous film. Alternatively, only the polyolefin porous film may be used as the separator according to one embodiment of the present invention, although this is not the mode of the laminated separator.

Here, the "polyolefin porous film" is a porous film containing a polyolefin resin as a main component. In addition, "mainly composed of a polyolefin resin" means that the ratio of the polyolefin resin in the porous film is 50% by volume or more, preferably 90% by volume or more, of the entire material constituting the porous film, It means that it is more preferably 95% by volume or more.

The polyolefin porous film is mainly composed of a polyolefin resin and has a large number of pores connected to its interior, allowing gas and liquid to pass from one surface to the other surface. . Hereinafter, the polyolefin porous film is also simply referred to as "porous film".

More preferably, the polyolefin contains a high molecular weight component having a weight average molecular weight of 5×10 ⁵ to 15×10 ⁶ . In particular, when the polyolefin contains a high molecular weight component having a weight average molecular weight of 1,000,000 or more, the strength of the non-aqueous electrolyte secondary battery separator according to the embodiment of the present invention is improved, which is more preferable.

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

Examples of the homopolymer include polyethylene, polypropylene, and polybutene. Further, examples of the copolymer include an ethylene-propylene copolymer.

Among these, polyethylene is more preferable as the polyolefin because it can prevent the flow of excessive current at a lower temperature. This "preventing the flow of excessive current" is also called shutdown.

Examples of the polyethylene include low-density polyethylene, high-density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), and ultra-high molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more. Among these, ultra-high molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more is more preferable as the polyethylene.

The basis weight of the porous film can be appropriately determined in consideration of strength, film thickness, weight and handleability. However, the basis weight is preferably 4 to 20 g/m ² and more preferably 4 to 12 g/m ² so that the weight energy density and volume energy density of the non-aqueous electrolyte secondary battery can be increased. more preferably 5 to 10 g/m ² .

The air permeability of the porous film is preferably 30 to 500 s/100 mL, more preferably 50 to 300 s/100 mL in Gurley value. When the porous film has an air permeability within the above range, the porous film can obtain sufficient ion permeability.

The porosity of the porous film is preferably 20 to 80% by volume so as to increase the holding amount of the electrolytic solution and to obtain the function of reliably preventing the flow of excessive current at a lower temperature. , more preferably 30 to 75% by volume.

In addition, the pore diameter of the pores of the porous film is 0.3 μm or less so that sufficient ion permeability can be obtained and particles can be prevented from entering the positive electrode and the negative electrode. is preferable, and 0.14 μm or less is more preferable.

The laminated separator may further include other layers such as an adhesive layer, a heat-resistant layer, and a protective layer in addition to the polyolefin porous film.

Furthermore, the film thickness of the separator for a non-aqueous electrolyte secondary battery is such that the dendrite migration distance necessary for the occurrence of the micro short circuit is sufficiently long and good ionic conductivity can be maintained. Preferably.

From this point of view, 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 limit and upper limit of the film thickness of the polyolefin porous film include 4 to 29 μm, 5 to 20 μm and 6 to 15 μm.

[2. Manufacturing method of separator for non-aqueous electrolyte secondary battery]
[2.1. Method for producing polyolefin porous film]
Examples of the method for producing the porous film include the following methods. That is, first, a polyolefin resin, a pore-forming agent such as an inorganic filler or a plasticizer, and optionally an antioxidant or the like are kneaded to obtain a polyolefin resin composition. Thereafter, the polyolefin resin composition is extruded to prepare a sheet-like polyolefin resin composition. Then, the pore-forming agent is removed from the sheet-like polyolefin resin composition with an appropriate solvent. Thereafter, by stretching the polyolefin-based resin composition from which the pore-forming agent has been removed, a polyolefin porous film can be produced.

The inorganic filler is not particularly limited, and includes inorganic fillers, specifically calcium carbonate and the like. The plasticizer is not particularly limited, and examples thereof include low-molecular-weight hydrocarbons such as liquid paraffin.

Examples of the method for producing the porous film include a method including the steps shown below.

(i) Ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more, low molecular weight polyethylene having a weight average molecular weight of 10,000 or less, a pore forming agent such as calcium carbonate or a plasticizer, and an antioxidant are kneaded. A step of obtaining a polyolefin resin composition.

(ii) step of cooling the obtained polyolefin resin composition in stages to form a sheet;

(iii) removing the pore-forming agent from the obtained sheet with a suitable solvent;

(iv) stretching the sheet from which the pore-forming agent has been removed at an appropriate stretching ratio;

[2.2. Method for manufacturing porous layer]
A porous layer can be formed using a coating liquid in which the resin described in the section (Resin) is dissolved or dispersed in a solvent. A porous layer containing the resin and the filler can be formed using a coating liquid obtained by dissolving or dispersing the resin in a solvent and dispersing the filler.

The solvent may be a solvent that dissolves the resin. Also, the solvent may be a dispersion medium for dispersing the resin or the filler. Examples of the method for forming the coating liquid include a mechanical stirring method, an ultrasonic dispersion method, a high pressure dispersion method, a media dispersion method, and the like.

Examples of the method for forming the porous layer include a method of directly applying the coating liquid to the surface of a substrate and then removing the solvent; A method of removing to form the porous layer, pressing the porous layer and the base material together, and then peeling off the support; A method of pressing a substrate and then removing the solvent after peeling off the support; and a method of performing dip coating by immersing the substrate in the coating solution and then removing the solvent. mentioned.

The solvent is preferably a solvent that does not adversely affect the substrate, uniformly and stably dissolves the resin, and uniformly and stably disperses the filler. 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.

The coating liquid may appropriately contain a dispersant, a plasticizer, a surfactant, a pH adjuster, and the like as components other than the resin and the filler.

In addition to the polyolefin porous film described above, other films, positive and negative electrodes, and the like can be used as the substrate. A laminated separator according to an embodiment of the present invention can be produced by using a polyolefin porous film as a substrate for forming a porous layer.

As a method for applying the coating liquid to the base material, a conventionally known method can be employed, and specific examples include a gravure coater method, a dip coater method, a bar coater method and a die coater method. be done.

When the coating liquid contains an aramid resin, the aramid resin can be precipitated by moistening the coated surface. Thereby, the porous layer may be formed.

As a method of 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 the film of the coating liquid, by ventilation drying, heat drying, or the like. can be mentioned.

Moreover, the porosity and average pore size of the obtained porous layer can be adjusted by changing the amount of the solvent in the coating liquid.

A suitable solid content concentration of the coating liquid may vary depending on the type of filler and the like, but in general, it is preferably more than 3% by weight and 40% by weight or less.

The coating shear rate at which the coating liquid is applied onto the substrate may vary depending on the type of filler, but is generally preferably 2 (1/s) or more, and 4 (1/s). s) to 50 (1/s) is more preferable.

(Method for preparing aramid resin)
The method for preparing the aramid resin is not particularly limited, but includes a condensation polymerization method of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide. In that case, the resulting aramid resin consists essentially of repeating units in which the amide linkages are attached at the para-position of the aromatic ring or a corresponding orientation. Orientation according to the para-position is orientation that extends coaxially or parallel in opposite directions, such as 4,4′-biphenylene, 1,5-naphthalene, 2,6-naphthalene, and the like.

Specific methods for preparing a solution of poly(paraphenylene terephthalamide) include, for example, methods including the following steps (I) to (IV).

(I) A dry flask is charged with N-methyl-2-pyrrolidone, calcium chloride dried at 200° C. for 2 hours is added, and the temperature is raised to 100° C. to completely dissolve the calcium chloride.

(II) After returning the temperature of the solution obtained in step (I) to room temperature, paraphenylenediamine is added and completely dissolved.

(III) While maintaining the temperature of the solution obtained in step (II) at 20.+-.2.degree.

(IV) The temperature of the solution obtained in step (III) was kept at 20±2° C. and aged for 1 hour. amide) is obtained.

[2.3. Control method of ionic resistance distribution]
An example of a factor that controls the ionic resistance distribution on the surface of the porous layer is the state of dispersion of the coating liquid. By coating the base material with the coating liquid in a sufficiently dispersed state, it is easy to obtain a separator that satisfies the conditions for the ionic resistance distribution.

For example, the preparation of the coating liquid is usually accompanied by stirring, and if the time from the completion of stirring to the coating of the coating liquid is short, the coating liquid in a sufficiently dispersed state can be used as the base material. can be coated on In one embodiment, 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 less than 1 hour, more preferably within 30 minutes, and 10 minutes. Within is more preferable.

Also, the microbubble-treated coating fluid can remain well dispersed for a longer period of time. In the case of a microbubble-treated coating liquid, the time from the completion of the final stirring in preparation of the coating liquid to the application of the coating liquid to the substrate is preferably within 20 days, and within 48 hours. More preferably, within 24 hours is even more preferable. The time for applying the microbubble treatment to the coating liquid is preferably 30 minutes or longer, more preferably 60 minutes or longer. The upper limit of the time for applying the microbubble treatment to the coating liquid is not particularly limited, but is, for example, 72 hours or less.

Another example of factors controlling the ionic resistance distribution on the surface of the porous layer is the particle size and content of the filler. If the particle size of the filler is too large or the content of the filler is too high, the pore structure inside the porous layer tends to become uneven. This is because the filler particles have many irregularities and tend to generate large voids inside the porous layer. Therefore, the flow of ions flowing inside the porous layer tends to be uneven. As a result, variations in ionic resistance can increase. On the other hand, since the resin has higher flexibility than the filler, the pore structure inside the porous layer tends to become uniform when the content of the resin in the porous layer increases. As a result, the ion flow inside the porous layer becomes uniform, and variations in ion resistance can be reduced. The preferred average particle size (D50) and content of the filler are as described above.

Examples of factors that control the ionic resistance distribution on the polyolefin porous film surface include the coating speed and the flatness of the coating bar. Specifically, if the coating speed is slowed down, it is easier to obtain a separator that satisfies the conditions for the ionic resistance distribution.

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

[3. Non-aqueous electrolyte secondary battery member and non-aqueous electrolyte secondary battery]
A non-aqueous electrolyte secondary battery member according to an aspect of the present invention includes a positive electrode, the separator described above, and a negative electrode arranged in this order. A non-aqueous electrolyte secondary battery according to an aspect of the present invention includes the separator described above. In the non-aqueous electrolyte secondary battery member and the non-aqueous electrolyte secondary battery, the separator is arranged such that the surface facing the negative electrode satisfies the above-described condition of the variation coefficient of ionic resistance.

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 prism type such as a rectangular parallelepiped, or the like. The non-aqueous electrolyte secondary battery is, for example, a non-aqueous electrolyte secondary battery that obtains an electromotive force by doping and dedoping lithium, in which a positive electrode, the above separator, and a negative electrode are laminated in this order. A member for a non-aqueous electrolyte secondary battery is provided. Components of the non-aqueous electrolyte secondary battery other than the separator described above are not limited to those described below.

The non-aqueous electrolyte secondary battery generally has a structure in which a battery element, in which a negative electrode and a positive electrode are opposed to each other with the separator interposed therebetween and which is impregnated with an electrolyte, is enclosed in an exterior material. Doping means occluded, carried, adsorbed, or inserted, and means a phenomenon in which lithium ions enter an active material of an electrode such as a positive electrode.

Since the non-aqueous electrolyte secondary battery member includes the separator described above, when it is incorporated in the non-aqueous electrolyte secondary battery, it suppresses the occurrence of micro short circuits in the non-aqueous electrolyte secondary battery. and improve its safety. In addition, since the non-aqueous electrolyte secondary battery includes the separator described above, the occurrence of micro short circuits is suppressed, and safety is excellent.

As a method for manufacturing the non-aqueous electrolyte secondary battery, a conventionally known manufacturing method can be adopted. For example, the non-aqueous electrolyte secondary battery member is formed by arranging the positive electrode, the separator, and the negative electrode in this order. Next, the non-aqueous electrolyte secondary battery member is placed in a container that serves as a housing for the non-aqueous electrolyte secondary battery. Further, after filling the inside of the container with the non-aqueous electrolyte, the container is sealed while reducing the pressure. Thereby, the non-aqueous electrolyte secondary battery can be manufactured.

[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 a positive electrode for non-aqueous electrolyte secondary batteries. For example, a positive electrode sheet having a structure in which an active material layer containing a positive electrode active material and a binder is formed on a positive electrode current collector can be used as the positive electrode. The active material layer may further contain a conductive agent and/or a binder.

Examples of the positive electrode active material include materials capable of doping and dedoping lithium ions. Specific examples of such materials include lithium composite oxides containing at least one type of transition metal such as V, Mn, Fe, Co and Ni.

Examples of the conductive agent include natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and carbonaceous materials such as sintered organic polymer compounds. Only one type of the conductive agent may be used, or two or more types may be used in combination.

Examples of the binder include fluorine-based resins such as polyvinylidene fluoride (PVDF), acrylic resins, and styrene-butadiene rubber. In addition, the binder also has a function as a thickener.

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

Methods for producing the positive electrode sheet include, for example, a method of pressure-molding a positive electrode active material, a conductive agent and a binder on a positive electrode current collector; is made into a paste, the paste is applied to the positive electrode current collector, and after drying, pressure is applied to fix it to the positive electrode current collector;

[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 a negative electrode for non-aqueous electrolyte secondary batteries. For example, a negative electrode sheet having a structure in which an active material layer containing a negative electrode active material and a binder is formed on a negative electrode current collector can be used as the negative electrode. The active material layer may further contain a conductive agent and/or a binder.

Examples of the negative electrode active material include materials capable of doping and dedoping lithium ions. Examples of such materials include carbonaceous materials. Examples of carbonaceous materials include natural graphite, artificial graphite, cokes, carbon black and pyrolytic carbons.

Examples of the negative electrode current collector include Cu, Ni, and stainless steel. Cu is more preferable because it is difficult to form an alloy with lithium and is easy to process into a thin film.

Examples of the method for producing the negative electrode sheet include a method of pressure-molding a negative electrode active material on a negative electrode current collector; a method in which the coating is applied to the surface, dried, and then pressed to adhere to the negative electrode current collector; and the like. The paste preferably contains the conductive agent and the binder.

[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 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 lithium salts include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiC(CF ₃ SO ₂ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , lower aliphatic carboxylic acid lithium salts and _LiAlCl4 . Only one type of the lithium salt may be used, or two or more types may be used in combination.

Organic solvents constituting the non-aqueous electrolyte include, for example, carbonates, ethers, esters, nitriles, amides, carbamates, sulfur-containing compounds, and fluorine groups introduced into these organic solvents. Fluorinated organic solvents and the like are included. Only one type of the organic solvent may be used, or two or more types may be used in combination.

[4. Method and apparatus for measuring ionic resistance distribution]
A method for measuring the ionic resistance distribution of a separator for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention is an arbitrary 1.5 cm × 1.5 cm of at least one surface of the separator for a non-aqueous electrolyte secondary battery. A step of measuring the local ionic resistance at 10 points at 800 μm intervals using a probe of length 7 mm×width 250 μm in the area of . As a result, the ionic resistance distribution, that is, the variation in ionic resistance, can be measured from the local ionic resistance obtained at 10 points. The measuring method may include a step of calculating a coefficient of variation of ionic resistance from the obtained 10 points of local ionic resistance.

Local ionic resistance can be measured, for example, using a working electrode with ten probes of 7 mm length×250 μm width arranged at 800 μm intervals. FIG. 1 is a schematic diagram showing an example of a working electrode used for measuring ionic resistance distribution. A working electrode 10 comprises a substrate 1 and a probe 2 arranged on the substrate 1 . As the substrate 1, a glass epoxy substrate, a glass substrate, an epoxy substrate, a baked substrate, a paper-based phenol resin laminated substrate, a polycarbonate substrate, an ABS resin substrate, or the like is used. The probes 2 can be metal wires, for example. Materials for the metal wiring include gold and copper. Terminals 3 are also provided on the substrate 1 . A probe 2 is connected to a terminal 3 . The terminal 3 can be connected with an AC impedance measuring device. A region including wiring connecting the probe 2 and the terminal 3 is covered with an insulating material 4 to be insulated from a later-described counter electrode 11, an electrolytic solution, and a separator 12 impregnated with the electrolytic solution. Examples of the insulating material 4 include polytetrafluoroethylene, PEEK (polyetheretherketone), phenol resin, silicone resin, and PP (polypropylene).

In this specification, the length of the probe 2 means the length of the portion that contacts the surface of the separator to be measured. That is, in FIG. 1, the length of the portion of the probe 2 exposed from the insulating material 4 is 7 mm.

Also, ten or more probes 2 may be arranged on the substrate 1 . Also, ten or more terminals 3 may be arranged. In FIG. 1, 24 probes 2 and 12 terminals 3 are arranged. Every other probe 2 is connected to a terminal 3 . At least ten of these probes 2 may be used for measurement. In other words, not all probes 2 on substrate 1 need to be used. For example, if irregular measurement values are obtained with the probes 2 at both ends of the plurality of arranged probes 2, the probes 2 at both ends may not be used for the measurement. A probe 2 not used for measurement may be useful for fixing a separator 12, which will be described later.

Further, in this specification, "10 points at intervals of 800 µm" means that the distance (pitch) between the center lines of the probes 2 used for measurement is 800 µm. For example, when probes 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, and 2j are used for measurement in FIG. ing.

FIG. 2 is a schematic diagram showing an example of a method for measuring the ionic resistance distribution. A separator 12 to be measured is sandwiched between the working electrode 10 and the counter electrode 11 made of metal. Although the working electrode 10 has the same configuration as in FIG. 1, it is simplified and illustrated in FIG. 2 for convenience. Here, the surface of the separator 12 to be measured for the ionic resistance distribution is brought into contact with the surface of the working electrode 10 having the probe 2 . In FIG. 2, the working electrode 10, the separator 12 and the counter electrode 11 are arranged separately for the sake of convenience. preferable. In order to keep the working electrode 10 and the counter electrode 11 in contact with the separator 12, the working electrode 10 and the counter electrode 11 may be pressurized. In this case, it is preferable to pressurize the surface of the separator 12 so that the pressure is applied uniformly. Materials for the counter electrode 11 include noble metals such as gold, silver and platinum, transition metals such as Ni and Cu, and metals such as SUS. The material of the counter electrode and the material of the working electrode probe are preferably of the same type. The working electrode 10 , the separator 12 and the counter electrode 11 are placed in a container 13 , and an electrolytic solution is injected into the container 13 to fabricate an ionic resistance measurement cell 100 . The working electrode 10 and counter electrode 11 are connected to an AC impedance measuring device 101 . In FIG. 2, the working electrode 10 is connected to the AC impedance measuring device 101 by one line for convenience, but actually each of the terminals 3 used for measurement is connected to the AC impedance measuring device 101. .

Since there are 10 probes 2 on the working electrode 10, AC impedance can be measured at 10 measurement points. The value of the local ionic resistance at each measurement point can be obtained as the value at the intersection of the curve obtained by Nyquist-plotting the AC impedance measured at each measurement point and the X-axis. The probe 2 and/or the working electrode 10 may be replaced each time one separator 12 is measured.

An embodiment of the invention also includes a method of screening a separator. A separator screening method according to an embodiment of the present invention includes the steps of measuring the coefficient of variation of ionic resistance of a separator by the above-described measurement method, and selecting a separator having a coefficient of variation of ionic resistance of 0.20 or less. include. As a result, separators in which the shape of dendrites is controlled to be granular or tabular can be identified.

An apparatus for measuring the ionic resistance distribution of a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention comprises ten probes of length 7 mm×width 250 μm arranged at intervals of 800 μm. The measuring device includes a working electrode having, for example, ten probes as described above. The measuring device may also comprise the counter electrode described above. Furthermore, the measuring device may comprise the AC impedance measuring device described above.

The present invention is not limited to the above-described embodiments, but can be modified in various ways within the scope of the claims, and can be obtained by appropriately combining technical means disclosed in different embodiments. is also included in the technical scope of the present invention.

EXAMPLES 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.

[Measurement of physical properties]
In Examples and Comparative Examples, the physical properties and the like of the non-aqueous electrolyte secondary battery separator and the polyolefin porous film were measured by the following methods.

(1) Film thickness The film thickness of the separator for non-aqueous electrolyte secondary batteries was measured using a high-precision digital length measuring machine (manufactured by Mitutoyo Corporation). Specifically, each of the separators for non-aqueous electrolyte secondary batteries was cut into squares each having a side length of 8 cm, measurements were performed at five points in each square, and the average value was taken as the film thickness.

(2) Coefficient of variation of ionic resistance (measurement preparation)
A working electrode 10 shown in FIG. 1 was prepared. A glass epoxy substrate was used as the substrate 1 . As probes 2 , 24 gold wirings with a width of 250 μm were arranged on substrate 1 . Also, 12 terminals are arranged on the substrate 1 . Every other probe 2 was connected to 12 terminals. There is a possibility that irregular measurement values may be obtained due to the edge effect of 2 terminals at each end (4 in total) out of 24 probes 2 and 1 terminal at each end (2 terminals in total) out of 12 terminals 3. Not used for measurements. The distance between the center lines of the probes 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, and 2j used for measurement was 800 µm. Polytetrafluoroethylene was used as the insulating material 4 . The length of the portion of the gold wiring exposed from the insulating material 4 was set to 7 mm.

The porous layer of the laminated separator was adhered to the gold wire of the working electrode. A counter electrode (metal plate electrode of 1 cm ² ) was brought into close contact with the polyethylene porous film of the laminated separator. As a result, the laminated separator was sandwiched between the working electrode and the counter electrode. The distance between the working electrode and the counter electrode was kept constant. The porous layer to be measured was sandwiched between glass plates from both the counter electrode side and the working electrode side so that pressure was uniformly applied to the entire surface of the porous layer.

(ion resistance distribution)
Using the working electrode described above, an ionic resistance measurement cell was assembled in an argon atmosphere glove box. Specifically, the working electrode and the counter electrode with the separator interposed therebetween were placed in a container, and the electrolytic solution was injected into the container. A solution (1 M) of LiClO ₄ in ethylene carbonate/diethyl carbonate (1/1=vol/vol, manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte. The vessel was sealed under an argon atmosphere. Thus, an ionic resistance measuring cell was obtained. The ionic resistance measurement cell was allowed to stand still for 4 hours after the cell was assembled for the purpose of impregnating the electrolyte with the electrolyte before measurement. Using an AC impedance measuring device manufactured by Bio-Logic, AC impedance was measured at an amplitude of 10 mV and a frequency range of 100 MHz to 0.1 Hz. An AC impedance measuring device was connected to the working electrode and the counter electrode. The measurement temperature was 25°C. Ten measurement points were measured one by one. An interval of 5 minutes or more was provided between each measurement.

The AC impedance was Nyquist-plotted for each measurement point, and the intersection of the obtained curve and the X-axis was defined as the ionic resistance at the local measurement point of the separator. The ten probes 2 used for the measurement in FIG. 2b, 2i, 2c, 2h, 2d, 2g, 2e, 2f were performed in this order. This measurement method minimizes the influence of ion currents in other parts. The standard deviation and average of the local ionic resistance were calculated from the obtained local ionic resistance at 10 points. Also, the coefficient of variation was calculated from the formula: coefficient of variation=standard deviation/average. The working electrode was replaced each time one laminated separator was measured.

(3) Compressive Modulus Using a microcompression tester (MCT-510, manufactured by Shimadzu Corporation), the displacement rate was measured from the compression characteristics of the laminated separator. The measurement mode was set indentation depth load-unload test mode. Specifically, the thickness of the separator was measured in advance, the separator was cut into 1 cm squares, attached on the measurement stage, and a compression test was performed using a flat indenter (50 μm diameter) at a load rate of 0.45 mN/sec until the load reached 20 mN. did The obtained displacement-load curve was linearly approximated in the range of 5 mN to 15 mN to obtain the slope [N/μm]. The compression elastic modulus was calculated by the following formula.
Compressive elastic modulus [P]=slope [N/μm]×film thickness [μm]/indenter area [m ² ]
(4) Shape of Dendrite A CR2032 type coin cell having the following configuration was assembled in an argon glove box. A solution (1 M) of LiClO ₄ in ethylene carbonate/diethyl carbonate (1/1=vol/vol, manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte.
・Working electrode: graphite mixture electrode and Cu foil ・Laminated separator prepared in Example or Comparative Example (Porous layer placed on the graphite mixture electrode side)
- Counter electrode: Li (thickness: 200 μm, manufactured by Honjo Metal).

The assembled coin cell was subjected to the following steps.
It was left standing for 1.4 hours to impregnate with the electrolytic solution.
2. At room temperature, one cycle of charging and discharging was performed at a current density of 0.2 mA/cm ² and a voltage range of 3V to 0.005V.
3. Current density: Fully charged to 0.005 V at 0.2 mA/cm ² .
4. Overcharged for 333 seconds at a current density of 6 mA/cm ² . As a result, Li was deposited on the graphite mixture electrode.
5. In an argon glove box, the electrolyte adhering to the graphite mixture electrode was washed with diethyl carbonate.
6. While maintaining the argon-sealed state, the sample taken out was observed with a scanning electron microscope (SEM).

Granular and/or tabular dendrites did not grow in the thickness direction of the separator, so it was determined that the short-circuit prevention effect was great. Dendrites having a fibrous shape grow in the thickness direction of the separator and are highly likely to penetrate the separator.

(5) Intrinsic viscosity (i) A solution of 0.5 g of aramid resin dissolved in 100 mL of concentrated sulfuric acid (96-98%), and (ii) Concentrated sulfuric acid (96-98%) without dissolving the resin ), the flow time was measured with an Ubbelohde capillary viscometer. The temperature at the time of measurement was 30°C. From the flow time obtained, the intrinsic viscosity was obtained by the following formula.
Intrinsic viscosity = ln (T/T ₀ )/C (unit: dL/g)
T: Flow time of concentrated sulfuric acid solution of aramid resin T ₀ : Flow time of concentrated sulfuric acid C: Concentration of aramid resin in concentrated sulfuric acid solution of aramid resin (g/dL).

[Synthesis Example 1]
Poly(paraphenylene terephthalamide) was synthesized by the following procedure.
1. A separable flask (capacity: 3 L) having a stirring blade, a thermometer, a nitrogen inlet tube and a powder addition port was prepared.
2. The separable flask was thoroughly dried and charged with 2200 g of N-methyl-2-pyrrolidone (NMP).
3. Add 151.07 g of calcium chloride powder (vacuum dried at 200° C. for 2 hours) and raise the temperature to 100° C. to completely dissolve the calcium chloride powder.
4. The resulting NMP solution of calcium chloride was allowed to cool to room temperature.
5.68.23 g of paraphenylenediamine was added and completely dissolved.
6. While the resulting solution was maintained at 20°C ± 2°C, 124.61 g of terephthaloyl dichloride was added in 10 portions to the solution about every 5 minutes.
7. The resulting solution was aged for 1 hour while being stirred while being kept at 20°C ± 2°C.
8. The aged solution was filtered using a 1500 mesh stainless wire mesh. Thus, an aramid polymerization liquid 1 was obtained.

The intrinsic viscosity of poly(paraphenylene terephthalamide) contained in the aramid polymerization liquid 1 was 1.7 g/dL.

[Synthesis Example 2]
Aramid polymer solution 2 was obtained in the same manner 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 the aramid polymerization liquid 2 was 1.6 g/dL.

[Example 1]
100 g of aramid polymerization liquid 1 was weighed into a flask, 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) was 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. As used herein, "solids" refers to the total weight of poly(paraphenylene terephthalamide) and alumina. Next, 0.73 g of calcium carbonate was added and stirred for 240 minutes to neutralize the solution and prepare a slurry-like coating liquid 1 .

The coating liquid 1 was allowed to stand for 8 minutes. After that, the coating liquid 1 was applied onto a polyolefin porous film (thickness: 12 μm) made of polyethylene by a doctor blade method. The resulting coated material 1 was allowed to stand in air at 50° C. and a relative humidity of 70% for 1 minute to precipitate poly(paraphenylene terephthalamide). Next, the coated article 1 was immersed in ion-exchanged water to remove calcium chloride and the solvent. Next, the coated material 1 was dried in an oven at 70° C. to obtain a laminated separator 1 . Table 1 shows the physical properties of Laminated Separator 1.

[Example 2]
A slurry-like coating liquid 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 liquid with a stirring rod and a stirrer using a three-one motor (manufactured by Sintokogyo Co., Ltd.) and supplying nitrogen from a pipe near the stirrer. A laminated separator 2 was obtained in the same manner as in Example 1, except that the coating liquid 2 that had been allowed to stand for 3 minutes was used. Table 1 shows the physical properties of the laminated separator 2 .

[Example 3]
100 g of aramid polymerization liquid 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. As used herein, "solids" 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 coating liquid 3 in the form of a slurry. A laminated separator 3 was obtained in the same manner as in Example 1, except that the coating liquid 3 that had been allowed to stand for 8 minutes was used. Table 1 shows the physical properties of the laminated separator 3 .

[Example 4]
A slurry coating liquid 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 liquid with a stirring rod and a stirrer using a three-one motor (manufactured by Sintokogyo Co., Ltd.) and supplying nitrogen from a pipe near the stirrer. A laminated separator 4 was obtained in the same manner as in Example 3, except that the coating liquid 4 that had been allowed to stand for 3 minutes was used. Table 1 shows the physical properties of the laminated separator 4 .

[Comparative Example 1]
Comparative coating liquid 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. The comparative coating solution 1 was allowed to stand for 8 minutes and then applied to one side of a polyethylene film (thickness: 12 μm) to obtain a comparative laminated separator 1 .

[Comparative Example 2]
A comparative laminated separator 2 was obtained in the same manner as in Example 1, except that the coating liquid 1 was allowed to stand for 1 hour and 50 minutes before being applied. Table 1 shows the physical properties of Comparative Laminated Separator 2.

[Comparative Example 3]
A comparative laminated separator 3 was obtained in the same manner as in Example 1, except that the coating liquid 1 was allowed to stand for 72 hours before being applied. Table 1 shows the physical properties of Comparative Laminated Separator 3.

In Table 1, "max-min" represents the difference between the maximum and minimum values of ionic resistance measured at 10 points.

〔result〕
Laminated separators 1 to 4 had coefficients of variation of ionic resistance of 0.20 or less. As a result, dendrites generated by overcharging grew in granular or tabular form. In other words, it can be said that the laminated separators 1 to 4 are separators having a large short-circuit prevention effect. This is because the porous layer was formed with fine air bubbles remaining in the coating liquid by applying the coating liquid immediately after preparation or by subjecting the coating liquid to microbubble treatment. it is conceivable that.

Comparative laminated separators 1 to 3 had coefficients of variation of ionic resistance exceeding 0.20. As a result, dendrites generated by overcharging grew in fibrous form. In other words, it can be said that the comparative laminated separators 1 to 3 are separators having a small short-circuit prevention effect.

Moreover, all of the laminated separators 1 to 4 had a compressive elastic modulus of 65 MPa or more in the thickness direction. That is, the laminated separators 1 to 4 have sufficient rigidity in the thickness direction, and voids are less likely to occur 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 large short circuit prevention effect.

A non-aqueous electrolyte secondary battery separator according to one embodiment of the present invention is used for manufacturing a non-aqueous electrolyte secondary battery that suppresses the occurrence of micro short circuits during charging and discharging and has excellent safety. be able to.

REFERENCE SIGNS LIST 1 substrate 2 probe 3 terminal 4 insulating material 10 working electrode 11 counter electrode 12 separator 13 container 100 ionic resistance measuring cell 101 AC impedance measuring device

Claims (7)

A separator for a non-aqueous electrolyte secondary battery,
Ions calculated from the local ionic resistance of 10 points measured at intervals of 800 μm using a probe of length 7 mm × width 250 μm in an arbitrary 1.5 cm × 1.5 cm region on at least one surface of the separator A separator having a resistance variation coefficient of 0.20 or less. 2. The separator of claim 1, which is a laminated separator comprising a porous layer and a polyolefin porous film. 3. The separator according to claim 2, wherein said porous layer contains a nitrogen-containing aromatic resin. 4. The separator according to claim 3, wherein said nitrogen-containing aromatic resin is an aramid resin. The separator according to any one of claims 1 to 4, which has a compressive elastic modulus of 50 MPa or more in the thickness direction. A member for a non-aqueous electrolyte secondary battery, comprising a positive electrode, a separator according to any one of claims 1 to 5, and a negative electrode, which are laminated in this order. A non-aqueous electrolyte secondary battery comprising the separator according to any one of claims 1 to 5 or the non-aqueous electrolyte secondary battery member according to claim 6.

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