Method for manufacturing conductive paste for secondary battery

Abstract

The present disclosure relates to a method for manufacturing a conductive paste for a secondary battery, the paste including carbon nanotubes, a dispersant, and a dispersion medium, the dispersant including a rubber-based compound. The method comprises: a first dispersion step for obtaining a first dispersion liquid by dispersing the carbon nanotubes in the dispersion medium by utilizing the collision energy between media particles using a media-type disperser; and a second dispersion step for obtaining a second dispersion liquid by further dispersing the carbon nanotubes in the dispersion medium by applying shear force to the carbon nanotubes in the first dispersion liquid. In the second dispersion step, the second dispersion liquid is obtained by passing the first dispersion liquid through a narrow path through application of a pressure exceeding atmospheric pressure so that, in a particle distribution curve of the carbon nanotubes indicating a frequency distribution obtained by particle size distribution measurement on an area basis, the frequency of the peak top of the highest peak appearing in a particle diameter range of 0.1 μm to 0.5 μm inclusive is 30% or less.

Description

Method for manufacturing conductive paste for secondary batteries

[0001] The present invention relates to a method for producing a conductive paste for a secondary battery, and more particularly to a method for producing a conductive paste for a secondary battery containing carbon nanotubes and a rubber-based compound.

[0002] Conductive pastes for secondary batteries containing a conductive agent, a dispersant, and a dispersion medium have been known. Such conductive pastes for secondary batteries are used to prepare a positive electrode mixture paste by adding a positive electrode active material, or to prepare a negative electrode mixture paste by adding a negative electrode active material. The positive electrode mixture paste is applied to a positive electrode current collector to form a positive electrode mixture layer, and the negative electrode mixture paste is applied to a negative electrode current collector to form a negative electrode mixture layer.

[0003] Patent Document 1 below describes a conductive agent dispersion containing a polymer A (e.g., hydrogenated nitrile rubber) containing a nitrile group-containing monomer, a substituted phenol (an alkylated phenol compound having at least one phenolic hydroxyl group and at least one alkyl group), a conductive agent (carbon nanotubes, etc.), and a solvent. Patent Document 1 below also describes that the conductive agent dispersion is obtained by mixing the components using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, or Filmix. Patent Document 1 below also describes that the dispersion stability of the conductive agent is improved in the conductive agent dispersion.

[0004] The following Patent Document 2 describes a conductive material containing a conductive agent, a dispersant, and water, in which the conductive agent has an acidic group content of 0.1 to 1.0 μmol / m ２and 60 to 300 μmol / g of carbon nanotubes, and the dispersant is a copolymer (e.g., hydrogenated nitrile rubber) containing units derived from (meth)acrylonitrile and carboxyl group-containing monomer units, the copolymer containing 40 to 99 mass% of units derived from (meth)acrylonitrile among all monomer units constituting the copolymer, and having an acid value of 30 to 400 mg KOH. Furthermore, Patent Document 2 listed below describes a method for producing a conductive material dispersion, which includes Step 1 of applying shear stress to a conductive material composition to disperse the conductive material until the dispersed particle size becomes 250 μm or less, and Step 2 of dispersing the conductive material composition using a high-pressure homogenizer at a pressure of 60 to 150 MPa until the median diameter becomes 100 μm or less. Furthermore, Patent Document 2 listed below describes that the conductive material can be well dispersed in water in the conductive material dispersion.

[0005] Patent No. 6911987 JP 2021-190331 A

[0006] In order to prevent cracking of the positive electrode mixture layer or the negative electrode mixture layer, conductive pastes for secondary batteries often contain a rubber-based compound (e.g., hydrogenated nitrile rubber) as a dispersant, as described above. When the conductive paste for secondary batteries contains carbon nanotubes as a conductive agent, the elasticity of the rubber-based compound can hinder the dispersion of the carbon nanotubes in the dispersion medium. Therefore, it is difficult to say that the method for producing a conductive agent dispersion liquid described in Patent Document 1 and the method for producing a conductive material dispersion described in Patent Document 2 necessarily achieve a sufficient dispersion state of the carbon nanotubes in the dispersion medium.

[0007] However, in a conductive paste for a secondary battery containing carbon nanotubes, a dispersant containing a rubber-based compound, and a dispersion medium, it cannot be said that sufficient research has yet been conducted into how to more fully disperse carbon nanotubes in the dispersion medium.

[0008] Therefore, an object of the present disclosure is to provide a method for producing a conductive paste for a secondary battery that can more thoroughly disperse carbon nanotubes in a dispersion medium.

[0009] One aspect of the present invention relates to a method for producing a conductive paste for secondary batteries, which contains carbon nanotubes, a dispersant, and a dispersion medium, wherein the dispersant contains a rubber-based compound, and the method comprises: a first dispersion step in which a media-type disperser is used to disperse the carbon nanotubes in the dispersion medium by utilizing collision energy between media particles to obtain a first dispersion; and a second dispersion step in which shear force is applied to the carbon nanotubes in the first dispersion to further disperse the carbon nanotubes in the dispersion medium to obtain a second dispersion, wherein in the second dispersion step, a pressure exceeding atmospheric pressure is applied to pass the first dispersion through a narrow passage to obtain the second dispersion, so that the frequency of the highest peak appearing in the particle size range of 0.1 μm or more and 0.5 μm or less in a particle distribution curve of the carbon nanotubes, which shows a frequency distribution obtained by area-based particle size distribution measurement, is 30% or less.

[0010] According to the present disclosure, it is possible to provide a method for producing a conductive paste for a secondary battery that can more thoroughly disperse carbon nanotubes in a dispersant.

[0011] 1 shows particle distribution curves for Example 3, Comparative Example 1, and Comparative Example 2. 2 shows particle distribution curves for Example 2, Example 6, and Comparative Example 1. 3 shows particle distribution curves for Example 1, Example 4, Example 5, Comparative Example 1, and Comparative Example 3.

[0012] Below, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that known components may be applied to components characteristic of the present disclosure. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0013] In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties, conditions, etc. are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of them can be selected and used alone, or two or more can be used in combination, unless otherwise specified.

[0014] The present disclosure encompasses any combination of two or more features arbitrarily selected from the appended claims, i.e., any combination of two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0015] [Method for Manufacturing Conductive Paste for Secondary Batteries] A method for manufacturing a conductive paste for secondary batteries according to an embodiment of the present disclosure is a method for manufacturing a conductive paste for secondary batteries including carbon nanotubes, a dispersant, and a dispersion medium. In the method for manufacturing a conductive paste for secondary batteries according to an embodiment of the present disclosure, the dispersant includes a rubber-based compound. The method for manufacturing a conductive paste for secondary batteries according to an embodiment of the present disclosure includes a first dispersion step in which carbon nanotubes are dispersed in a dispersion medium using a media-type disperser by utilizing collision energy between media particles to obtain a first dispersion, and a second dispersion step in which shear force is applied to the carbon nanotubes in the first dispersion to further disperse the carbon nanotubes in the dispersion medium to obtain a second dispersion. In the method for manufacturing a conductive paste for secondary batteries according to an embodiment of the present disclosure, the second dispersion step involves passing the first dispersion through a narrow passage under pressure exceeding atmospheric pressure so that the frequency of the highest peak appearing in the particle size range of 0.1 μm to 0.5 μm in a carbon nanotube particle distribution curve, which indicates a frequency distribution obtained by area-based particle size distribution measurement, is 30% or less.

[0016] (Carbon nanotubes) Carbon nanotubes function as a conductive material in the conductive paste for secondary batteries. In the conductive paste for secondary batteries, the carbon nanotubes may be contained in an amount of 0.1 parts by mass or more, 0.4 parts by mass or more, or 1 part by mass or more relative to 100 parts by mass of the dispersion medium. In the conductive paste for secondary batteries, the carbon nanotubes may be contained in an amount of 6 parts by mass or less, 4 parts by mass or less, or 2 parts by mass or less relative to 100 parts by mass of the dispersion medium.

[0017] Examples of carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Among these, single-walled carbon nanotubes are preferred because they can further suppress deterioration in the charge-discharge cycle characteristics of secondary batteries. A single-walled carbon nanotube is a cylindrical carbon nanostructure composed of a single graphene sheet. A double-walled carbon nanotube is a cylindrical carbon nanostructure composed of two concentrically stacked graphene sheets. A multi-walled carbon nanotube is a cylindrical carbon nanostructure composed of three or more concentrically stacked graphene sheets. A graphene sheet refers to a layer in which carbon atoms in sp2 hybrid orbitals constituting a graphite crystal are located at the vertices of a regular hexagon. Carbon nanotubes typically have a cylindrical tube shape. Carbon nanotubes may also have a spirally wound coil shape.

[0018] The average length of the carbon nanotubes may be 1 μm or more. In this case, the aspect ratio of the carbon nanotubes (ratio of length to diameter of the fiber) becomes extremely large. Carbon nanotubes with a large aspect ratio tend to make linear contact with the positive electrode active material and the positive electrode current collector at the positive electrode, and tend to make linear contact with the negative electrode active material and the negative electrode current collector at the negative electrode. Furthermore, carbon nanotubes have excellent electrical conductivity. Therefore, the use of carbon nanotubes can significantly reduce the direct current resistance (DCR) of the secondary battery. Carbon nanotubes present in the positive electrode may be present in the positive electrode mixture layer in the form of a bundle of multiple carbon nanotubes. Similarly to the positive electrode, multiple carbon nanotubes may also be present in the negative electrode mixture layer in the form of a bundle of multiple carbon nanotubes. In such cases, the length and diameter of the carbon nanotube refer to the length and diameter of a single carbon nanotube present within the bundle of carbon nanotubes.

[0019] The average length of the carbon nanotubes is preferably 1 μm or more from the viewpoint of increasing electrical conductivity. On the other hand, although there is no particular upper limit on the length of the carbon nanotubes, it is preferable that the length is not significantly greater than the particle size of the positive electrode active material or the negative electrode active material. The average length of the carbon nanotubes may be 1 μm or more or 5 μm or more, or may be 20 μm or less or 10 μm or less.

[0020] The average length of carbon nanotubes is determined by image analysis using a scanning electron microscope (SEM). The average length of carbon nanotubes is determined by measuring the lengths of 100 randomly selected carbon nanotubes and calculating the arithmetic mean. The length refers to the length of the carbon nanotubes when stretched linearly.

[0021] The average diameter of the carbon nanotubes may be 20 nm or less, 15 nm or less, or 1 nm or more. By setting the average diameter to 20 nm or less, a high effect can be obtained with a small amount. The average diameter of the single-walled carbon nanotubes, the average diameter of the double-walled carbon nanotubes, and the average diameter of the multi-walled carbon nanotubes may each be, for example, 5 nm or less.

[0022] The average diameter of carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM). The average diameter of carbon nanotubes can be measured by the following method. First, 100 carbon nanotubes are randomly selected, and the diameter (outer diameter) of each is measured at one arbitrary point. The average diameter is then determined by arithmetically averaging the measured diameters.

[0023] The conductive paste for secondary batteries may contain a conductive material other than carbon nanotubes. Examples of conductive materials other than carbon nanotubes include carbon materials such as carbon black (CB), acetylene black (AB), and ketjen black. The conductive material may contain 50% by mass or more of carbon nanotubes, 70% by mass or more, or 90% by mass or more of carbon nanotubes. Furthermore, the conductive material may contain 100% by mass of carbon nanotubes. In other words, the entire conductive material may be carbon nanotubes.

[0024] (Dispersant) In the conductive paste for secondary batteries, the dispersant may be contained in an amount of 0.1 parts by mass or more, 0.5 parts by mass or more, or 1 part by mass or more relative to 100 parts by mass of the dispersion medium. In the conductive paste for secondary batteries, the dispersant may be contained in an amount of 6 parts by mass or less, 4 parts by mass or less, or 2 parts by mass or less relative to 100 parts by mass of the dispersion medium.

[0025] As described above, the dispersant contains a rubber-based compound. The rubber-based compound can also function as a binder in at least one of the positive electrode mixture layer and the negative electrode mixture layer. By containing a rubber-based compound in at least one of the positive electrode mixture layer and the negative electrode mixture layer, these mixture layers become elastic. This can prevent cracks from occurring in the positive electrode mixture layer or the negative electrode mixture layer due to expansion and contraction of the positive electrode active material or the negative electrode active material during charge and discharge.

[0026] The rubber-based compound may contain a nitrile group-containing rubber. The nitrile group-containing rubber contains a nitrile group. Examples of the nitrile group-containing rubber include copolymers of a monomer containing acrylonitrile and a diene (e.g., butadiene). Specific examples of the nitrile group-containing rubber include nitrile rubber (NBR), hydrogenated nitrile rubber (H-NBR), and modified products thereof. The weight average molecular weight of the nitrile group-containing rubber may be in the range of 40,000 to 5,000,000.

[0027] The dispersant may contain compounds other than rubber compounds and is selected from compounds soluble in the dispersion medium. Examples of such compounds include various known thickeners such as polyvinylpyrrolidone, carboxymethylcellulose or its salts (hereinafter sometimes referred to as CMC or CMC salts), methylcellulose, ethylcellulose, polyethylene glycol, and polyethylene oxide, as well as anionic, cationic, nonionic, or amphoteric surfactants. Among these, CMC or CMC salts are preferred because they function as binders. Examples of CMC salts include ammonium salts, sodium salts, potassium salts, and lithium salts.

[0028] The dispersant preferably contains 25% by mass or more and 60% by mass or less of the rubber compound. The dispersant may contain 30% by mass or more, or 35% by mass or more of the rubber compound. The dispersant may contain 55% by mass or less, or 50% by mass or less of the rubber compound. When the dispersant contains the rubber compound within the above numerical range, excellent elasticity can be exhibited in at least one of the positive electrode mixture layer and the negative electrode mixture layer.

[0029] (Dispersion Medium) Water may be used as the dispersion medium because it is easy to recover during drying and is highly environmentally friendly. The type of water is not particularly limited, and ultrapure water, pure water, industrial water, etc. can be used, but in consideration of the grade that is not restricted by treatment costs and usage amount, it is common to use water of Japanese Industrial Standards A1 class. Various known organic solvents can also be used as the dispersion medium. Examples of organic solvents include N-methylpyrrolidone (NMP).

[0030] Hereinafter, each step of the method for producing a conductive paste for a secondary battery according to an embodiment of the present disclosure will be described.

[0031] As described above, in the first dispersion step, a media-type disperser is used to disperse carbon nanotubes in a dispersion medium by utilizing collision energy between media particles to obtain a first dispersion. A mixed solution containing carbon nanotubes, a dispersant, and a dispersion medium and prior to being subjected to the first dispersion step is referred to as a "liquid mixture."

[0032] The media-type disperser, for example, comprises a cylindrical vessel having a supply port for the liquid mixture and an outlet for the first dispersion, a drive shaft housed in the cylindrical vessel so as to extend along the cylindrical axial direction, a plurality of stirring blades arranged on the drive shaft in a direction intersecting the axial direction, and a plurality of media particles housed in the cylindrical vessel. The plurality of stirring blades rotate around the drive shaft as the drive shaft rotates. The combination of the drive shaft and the plurality of stirring blades is also called an agitator. The above media-type disperser is a media-type wet disperser.

[0033] In the media-type disperser described above, a liquid mixture is supplied from a supply port into a cylindrical vessel. As the drive shaft rotates, multiple agitator blades rotate around the drive shaft, accelerating and colliding multiple media particles contained in the cylindrical vessel, generating collision energy. In the liquid mixture, carbon nanotubes are typically molded into pellets, briquettes, granules, or other shapes. These molded carbon nanotubes (hereinafter referred to as molded carbon nanotubes) have sizes ranging from several millimeters to 1 cm. For example, the width of the molded carbon nanotubes is 1 mm to 1 cm. However, by applying the above-described collision energy to the molded carbon nanotubes contained in the liquid mixture, the molded carbon nanotubes can be crushed to obtain, for example, multiple small bundles (hereinafter referred to as second bundles) having a D50 of 1.5 μm to 2.5 μm and a D90 of 3.5 μm to 5 μm. In other words, a first dispersion in which carbon nanotubes are dispersed in a dispersion medium is obtained. Thereafter, the first dispersion is discharged to the outside from the discharge port of the cylindrical vessel.

[0034] D50 and D90 can be measured using a laser diffraction particle size analyzer (for example, a particle size distribution analyzer MT3300EX II manufactured by Microtrac). Specifically, first, a mixture is obtained by diluting the carbon nanotubes in the first dispersion 20 times with the dispersion medium used. Next, this mixture is subjected to ultrasonic treatment for 1 minute to obtain a dispersion. This dispersion is then placed in an apparatus in which the dispersion medium used is circulated so that the light transmittance becomes 85%, and measurement is performed. Note that when the aspect ratio of the second bundle is large, the particle size of the second bundle may be determined from an area-based particle distribution curve obtained by the start line method described in JIS Z 8823-2, which will be described later.

[0035] In the media-type disperser described above, the media particles preferably have a spherical shape. Furthermore, when the media particles have a spherical shape, their diameter, d1, is preferably 0.3 mm or more and 1 mm or less. That is, in the first dispersion step, it is preferable to use a bead mill as the media-type disperser. By using such small-diameter media particles, collision energy can be efficiently applied to the molded carbon nanotubes between the colliding media particles. This allows the carbon nanotubes to be sufficiently dispersed in the dispersion medium in the first dispersion step.

[0036] Furthermore, in the media-type disperser described above, the peripheral speed of the agitator is preferably 6 m / s or more, more preferably 8 m / s or more, and even more preferably 10 m / s or more. By setting the peripheral speed of the agitator within the above range, it is possible to sufficiently accelerate the plurality of media particles within the cylindrical vessel. This allows for a sufficient amount of collision energy to be obtained. Furthermore, the upper limit of the peripheral speed of the agitator is usually 20 m / s. This makes it possible to prevent the plurality of media particles from being excessively accelerated within the cylindrical vessel. As a result, it is possible to prevent the carbon nanotubes from being excessively crushed due to the generation of excessive collision energy.

[0037] The media-type disperser as described above preferably has a separation mechanism in the cylindrical vessel for separating the plurality of media particles from the first dispersion liquid, which allows only the first dispersion liquid to be discharged to the outside from the outlet of the cylindrical vessel.

[0038] Examples of the separation mechanism include a gap separator, a screen, a slit, a mesh, etc. Among these, the gap separator method is preferable. Here, the gap separator method is a method that has a rotor that can be rotated by rotation of a drive shaft and a stator that is a fixed ring, and separates the media particles and the dispersion liquid by allowing only the dispersion liquid to pass through the gap between the rotor and the stator.

[0039] <Second Dispersion Step> As described above, in the second dispersion step, a shear force is applied to the carbon nanotubes in the first dispersion to further disperse the carbon nanotubes in the dispersion medium to obtain a second dispersion. Specifically, as described above, in the second dispersion step, a pressure exceeding atmospheric pressure is applied to pass the first dispersion through a narrow passage so that the frequency of the highest peak appearing in the particle size range of 0.1 μm to 0.5 μm in a particle distribution curve of carbon nanotubes, which indicates a frequency distribution obtained by area-based particle size distribution measurement, becomes 30% or less.

[0040] The second dispersion step is preferably carried out using a pressure homogenizer. Examples of pressure homogenizers include nozzle-type high-pressure homogenizers. A nozzle-type high-pressure homogenizer includes, for example, a first flow path having a supply port at one end and extending downstream; a second flow path having a larger inner diameter than the first flow path, one end of which is connected to the other end of the first flow path and extending downstream; and a third flow path having a larger inner diameter than the second flow path, one end of which is connected to the other end of the second flow path and extending downstream, with an outlet at the other end. In a nozzle-type high-pressure homogenizer, the first flow path is a narrow path. In a nozzle-type high-pressure homogenizer, the first flow path may be a nozzle. In a nozzle-type high-pressure homogenizer, a first covering portion is arranged to cover a first connection portion between the other end of the first flow path and one end of the second flow path. The portion of the opening on one end of the second flow path that does not overlap with the cross section of the narrow passage is closed by a first covering part. Furthermore, in the nozzle-type high-pressure homogenizer, a second covering part is arranged to cover a second connecting part between the other end of the second flow path and one end of the third flow path. The portion of the opening on one end of the third flow path that does not overlap with the cross section of the second flow path is closed by the second covering part. In other words, the nozzle-type high-pressure homogenizer is configured to prevent liquid leakage due to differences in inner diameter between the first connecting part and the second connecting part.

[0041] In the pressure homogenizer described above, a pressure exceeding atmospheric pressure is applied to cause the first dispersion to flow from the supply port into the narrow passage (first flow path), thereby applying shear force to the carbon nanotubes in the first dispersion within the narrow passage. Specifically, shear force can be applied to the carbon nanotubes forming the second bundles. This shear force can then detach at least some of the carbon nanotubes forming the second bundles from the second bundles. This allows the carbon nanotubes to be further dispersed in the dispersion medium, thereby obtaining a second dispersion. Specifically, as described above, a second dispersion can be obtained in which carbon nanotubes are dispersed in a dispersion medium such that the frequency of the highest peak appearing in the particle size range of 0.1 μm to 0.5 μm in a carbon nanotube particle distribution curve, which indicates a frequency distribution obtained by area-based particle size distribution measurement, is 30% or less.

[0042] The particle distribution curve of the carbon nanotubes can be obtained in accordance with the line start method described in JIS Z 8823-2, "Method for measuring particle size distribution by liquid phase centrifugal sedimentation - Part 2: Light transmission centrifugal sedimentation method." The particle distribution curve can also be obtained by acquiring area-based absorbance data. Furthermore, the area-based absorbance data can be measured using, for example, a "Partica CENTRIFUGE" manufactured by Horiba, Ltd.

[0043] The carbon nanotubes in the second dispersion have, for example, a D50 of 1.3 μm or more and 2 μm or less, and a D90 of 2.5 μm or more and 4 μm or less. The D50 and D90 of the carbon nanotubes in the second dispersion can be measured in the same manner as described above.

[0044] The pressure homogenizer may be a valve-type high-pressure homogenizer. The valve-type high-pressure homogenizer includes a cylindrical homogenizing valve and a homogenizing valve seat having a hollow cylindrical portion. In the homogenizing valve seat, the hollow cylindrical portion is formed by covering the hollow portion with an outer wall portion. In the valve-type high-pressure homogenizer, one end face of the cylindrical homogenizing valve and one opening of the hollow cylindrical portion of the homogenizing valve seat are arranged opposite to each other with a predetermined distance between them. A narrow passage is formed between the one end face of the cylindrical homogenizing valve and the outer wall portion of the hollow cylindrical portion. That is, in the valve-type high-pressure homogenizer, the narrow passage is formed so as to circumnavigate one opening of the hollow cylindrical portion. In the valve-type high-pressure homogenizer, the first dispersion that has passed through the hollow portion of the homogenizing valve seat passes through the narrow passage formed as described above, thereby applying shear force to the carbon nanotubes in the first dispersion within the narrow passage.

[0045] The frequency of the peak tops described above can be obtained, for example, by adjusting the magnitude of the pressure above atmospheric pressure. The higher the pressure above atmospheric pressure, the greater the shear force that can be applied to the carbon nanotubes in the first dispersion (e.g., the carbon nanotubes forming the second bundles) in the narrow channel, thereby enabling more thorough disintegration of the carbon nanotubes in the dispersion medium. In other words, the frequency of the peak tops can be reduced. Furthermore, when the pressure-type homogenizer is a nozzle-type high-pressure homogenizer, reducing the inner diameter of the narrow channel (first flow path) can apply greater shear force to the carbon nanotubes in the first dispersion (e.g., the carbon nanotubes forming the second bundles) in the narrow channel, thereby also reducing the frequency of the peak tops. Furthermore, when the pressure-type homogenizer is a valve-type high-pressure homogenizer, reducing the distance between one end face of the cylindrical homogenizing valve and one opening of the hollow cylindrical portion of the homogenizing valve seat can apply greater shear force to the carbon nanotubes in the first dispersion in the narrow channel, thereby also reducing the frequency of the peak tops.

[0046] As described above, in the method for producing a conductive paste for a secondary battery according to an embodiment of the present disclosure, a first dispersion liquid containing second bundles is obtained by crushing molded carbon nanotubes in a media-type disperser using collision energy between a plurality of media particles, and then a pressure exceeding atmospheric pressure is applied to apply a shear force to the second bundles contained in the first dispersion liquid, thereby separating at least some of the carbon nanotubes from the second bundles. This makes it possible to obtain a second dispersion liquid in which the carbon nanotubes are more thoroughly dispersed in the dispersion medium, i.e., a conductive paste for a secondary battery.

[0047] When the pressure homogenizer is a nozzle-type high-pressure homogenizer, the inner diameter of the narrow passage (first flow path) may be 0.05 mm or more, 0.10 mm or more, or 0.15 mm or more. The inner diameter of the narrow passage may be 0.50 mm or less, 0.40 mm or less, or 0.30 mm or less. The cross-sectional shape of the narrow passage is preferably circular.

[0048] When the pressure homogenizer is a valve-type high-pressure homogenizer, the width of the narrow passage formed between one end face of the cylindrical homogenizer valve and the outer wall of the hollow cylindrical portion may be 0.01 mm or more and 1 mm or less.

[0049] As described above, in a nozzle-type high-pressure homogenizer, the inner diameter of the second flow path is larger than the inner diameter of the narrow passage (first flow path). That is, the cross-sectional area of ​​the second flow path is larger than the cross-sectional area of ​​the first flow path. This allows back pressure to be applied to the narrow passage, preventing an instantaneous return to atmospheric pressure. As a result, bubbling in the narrow passage can be suppressed. If the inner diameter of the narrow passage is ID1 and the inner diameter of the second flow path is ID2, it is preferable that the relationship ID2 ≧ 2ID1 is satisfied. ID1 and ID2 may also satisfy the relationship ID2 ≧ 3ID1. Furthermore, it is preferable that ID1 and ID2 satisfy the relationship ID2 ≦ 5ID1. Furthermore, it is preferable that the second flow path extends downstream in a spiral. This ensures that the second flow path has a sufficient length, allowing even more sufficient back pressure to be applied to the narrow passage.

[0050] As explained above, in a nozzle-type high-pressure homogenizer, the inner diameter of the third flow path is larger than the inner diameter of the second flow path. This allows the pressure generated in the nozzle-type high-pressure homogenizer to gradually return to near atmospheric pressure from the supply port to the discharge port. If the inner diameter of the third flow path is ID3, it is preferable that ID2 and ID3 satisfy the relationship ID3 ≧ 2ID2. It is also preferable that ID2 and ID3 satisfy the relationship ID3 ≦ 5ID2.

[0051] When the pressure homogenizer is a valve-type high-pressure homogenizer, it is preferable that the valve-type high-pressure homogenizer has another pair of homogenizing valves and homogenizing valve seats downstream of the above-described pair of homogenizing valves and homogenizing valve seats. In the other pair of homogenizing valves and homogenizing valve seats, it is preferable that the homogenizing valves are cylindrical and the homogenizing valve seats have a hollow cylindrical portion. Furthermore, in the other pair of homogenizing valves and homogenizing valve seats, it is preferable that one end face of the cylindrical homogenizing valve and one opening of the hollow cylindrical portion of the homogenizing valve seat are arranged opposite each other with a predetermined distance between them, and that a narrow passage is formed between the one end face of the cylindrical homogenizing valve and the outer wall of the hollow cylindrical portion. On the other hand, it is preferable that the width of the narrow passage (downstream narrow passage) formed between the other pair of homogenizing valves and homogenizing valve seats is wider than the width of the narrow passage (upstream narrow passage) formed between the pair of homogenizing valves and homogenizing valve seats. This allows back pressure to be applied to the upstream narrow passage even in a valve-type high-pressure homogenizer, preventing an instantaneous return to atmospheric pressure. As a result, bubbling in the upstream narrow passage can be suppressed.

[0052] In the second dispersion step, the shear rate RS (1 / s) calculated from the flow rate and clearance when the first dispersion is passed through the narrow passage is preferably 1,000,000 (1,000,000) or more. Here, clearance refers to the narrowest passage in a pressure homogenizer. The largest differential pressure occurs at the clearance. Furthermore, the largest shear force is applied to the carbon nanotubes at the clearance. When the pressure homogenizer is a nozzle-type high-pressure homogenizer, the clearance refers to the inner diameter of the first passage (narrow passage). When the pressure homogenizer is a valve-type high-pressure homogenizer, the clearance refers to the width of the narrow passage formed between one end face of the cylindrical homovalve and the outer wall of the hollow cylindrical portion. Note that when a valve-type high-pressure homogenizer has a pair of homovalves and homovalve seats on the upstream side and another pair of homovalves and homovalve seats on the downstream side, the width of the narrow passage formed between the upstream homovalve and homovalve seats is the clearance.

[0053] By setting the shear rate RS within the above range, the carbon nanotubes can be more thoroughly separated from the second bundles. RS (1 / s) may be 1,500,000 (1.5 million) or more, 2,000,000 (2 million) or more, or 3,000,000 (3 million) or more. In the second dispersion step, the higher the shear rate RS, the more thoroughly the carbon nanotubes can be dispersed. The upper limit of the shear rate RS is 100,000,000 (100 million).

[0054] The pressure P1 exceeding atmospheric pressure is preferably 100 MPa or less. P1 may be 80 MPa or less, 60 MPa or less, or 50 MPa or less. P1 may be greater than 0.1 MPa, and may be 1 MPa or more, or 5 MPa or more. When the pressure P1 is in the above-mentioned numerical range, a more sufficient shear force can be applied to the second bundles when the first dispersion passes through the narrow passage. This allows the carbon nanotubes to be more sufficiently separated from the second bundles.

[0055] It is preferable to perform the second dispersion step once or twice. By performing the second dispersion step fewer times, the method for producing a conductive paste for a secondary battery according to an embodiment of the present disclosure can be performed efficiently.

[0056] <Coarse Crushing Step> The method for producing a conductive paste for a secondary battery according to an embodiment of the present disclosure preferably further includes a coarse crushing step of coarsely crushing carbon nanotubes in a dispersion medium prior to the first dispersion step. The coarse crushing step roughly crushes molded carbon nanotubes for purposes such as preventing scattering. As described above, such carbon nanotubes are molded into, for example, pellets, briquettes, or granules, each having a size (length) of approximately several millimeters to 1 cm. In the coarse crushing step, the molded carbon nanotubes are crushed to obtain bundles of carbon nanotubes (first bundles) having, for example, a D50 of 20 μm or more and a D90 of 40 μm or more and 100 μm or less. Furthermore, the coarse crushing step can reduce the load on the equipment in the first dispersion step. The D50 and D90 of the carbon nanotubes after the coarse crushing step can be measured in the same manner as described above.

[0057] The coarse grinding step can be carried out using, for example, a media-type disperser. As described above, the media-type disperser includes, for example, a cylindrical vessel having a supply port for the liquid mixture and a discharge port for the first dispersion, a drive shaft housed in the cylindrical vessel so as to extend along the cylindrical axial direction, a plurality of stirring blades arranged on the drive shaft in a direction intersecting the drive shaft direction, and a plurality of media particles housed in the cylindrical vessel.

[0058] Even when a media-type disperser is used in the coarse pulverization step, the media particles preferably have a spherical shape. On the other hand, the coarse pulverization step is a step of roughly crushing the molded carbon nanotubes to obtain the first bundles, as described above. Therefore, the diameter of the media particles used in the coarse pulverization step is preferably larger than the diameter (diameter d1) of the media particles used in the first dispersion step. In other words, when the diameter of the media particles used in the coarse pulverization step is d2, it is preferable that the relationship d2 > d1 is satisfied.

[0059] d2 is preferably 0.9 mm or more and 50 mm or less. d2 may be 10 mm or more, 15 mm or more, or 20 mm or more. d2 may be 40 mm or less, or 30 mm or less. That is, in the coarse grinding step, a ball mill or an attritor (for example, manufactured by Union Process Co., Ltd.) is preferably used as the media-type disperser. By using such large-diameter media particles, the molded carbon nanotubes are roughly crushed between the colliding media particles, and first bundles smaller in size can be obtained. This allows collision energy to be applied more efficiently to the bundles (first bundles) formed by the carbon nanotubes in the first dispersion step.

[0060] The carbon nanotubes after the coarse pulverization step preferably have a D50 of 1.3 μm or more and 2.0 μm or less in the cumulative particle size distribution on a volume basis, and a D90 of 3 μm or more and 10 μm or less in the cumulative particle size distribution on a volume basis. Since the carbon nanotubes after the coarse pulverization step have the above-mentioned D50 and D90, collision energy can be applied more efficiently to the first bundles formed by the carbon nanotubes in the first dispersion step.

[0061] The conductive paste for a secondary battery obtained as described above is used to prepare at least one of a positive electrode mixture paste and a negative electrode mixture paste. A method for producing the positive electrode mixture paste will be described below.

[0062] [Method for Manufacturing Positive Electrode Mixture Paste] The method for manufacturing a positive electrode mixture paste includes a mixing step of mixing a conductive paste for a secondary battery manufactured by the method for manufacturing a conductive paste for a secondary battery according to an embodiment of the present disclosure with a positive electrode active material.

[0063] In the mixing step, the conductive paste for a secondary battery and the positive electrode active material are mixed to disperse the positive electrode active material in the dispersion medium. As described above, in the conductive paste for a secondary battery manufactured using the conductive paste for a secondary battery according to the embodiment of the present disclosure, the carbon nanotubes are sufficiently dispersed in the dispersion medium. Therefore, the carbon nanotubes are also sufficiently dispersed in the positive electrode mixture paste obtained by the mixing step. Therefore, the carbon nanotubes are also sufficiently dispersed in the positive electrode mixture layer produced using such a positive electrode mixture paste, and the positive electrode mixture layer can exhibit high conductivity.

[0064] The mixing step can be carried out using, for example, various known mixing devices, such as a planetary mixer, a media-type disperser, a kneader, a pressure homogenizer, and an ultrasonic homogenizer.

[0065] As the positive electrode active material, for example, a lithium-containing composite oxide containing a transition metal element and having nickel (Ni) as a main component is used. As the lithium-containing metal composite oxide, for example, Li ｘ Ni ｙ M （１－ｙ） O ２ (wherein M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, and 0.95≦x≦1.2, and 0.30≦y≦0.95). ｘ Ni ｙ M （１－ｙ） O ２ In the above formula, the Ni content y is preferably 0.50≦y≦0.95, and more preferably 0.80≦y≦0.95.

[0066] The positive electrode mixture paste may contain a binder in addition to the positive electrode active material layer. Examples of the binder include resin materials. Examples of the resin material include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; and polyethersulfone.

[0067] In the above embodiment, an example of producing a positive electrode mixture paste using a conductive paste for a secondary battery has been described, but the conductive paste for a secondary battery may also be used to produce a negative electrode mixture paste. In the example of producing a negative electrode mixture paste, a negative electrode active material may be mixed with the conductive paste for a secondary battery in place of a positive electrode active material.

[0068] (Additional Notes) The above description discloses the following technology: (Technology 1) A method for producing a conductive paste for a secondary battery containing carbon nanotubes, a dispersant, and a dispersion medium, wherein the dispersant contains a rubber-based compound, the method comprising: a first dispersion step of using a media-type disperser to disperse the carbon nanotubes in the dispersion medium by utilizing collision energy between media particles to obtain a first dispersion; and a second dispersion step of applying a shear force to the carbon nanotubes in the first dispersion to further disperse the carbon nanotubes in the dispersion medium to obtain a second dispersion, wherein in the second dispersion step, a pressure exceeding atmospheric pressure is applied to pass the dispersion through a narrow passage to obtain the second dispersion, so that the frequency of the highest peak appearing in a particle size range of 0.1 μm to 0.5 μm inclusive is 30% or less in a particle distribution curve of the carbon nanotubes, which shows a frequency distribution obtained by area-based particle size distribution measurement. (Technology 2) The method for producing a conductive paste for a secondary battery according to Technology 1, wherein in the second dispersion step, the shear force is applied so that a shear rate (1 / s) calculated from the flow rate and clearance when the first dispersion is passed through the narrow passage is 1,000,000 or more. (Technology 3) The method for producing a conductive paste for a secondary battery according to Technology 1 or 2, wherein a flow path having a larger cross-sectional area than the narrow passage is arranged downstream of the narrow passage, and the second dispersion step is carried out by passing the first dispersion through the narrow passage and the flow path. (Technology 4) The method for producing a conductive paste for a secondary battery according to any one of Technology 1 to 3, wherein the pressure exceeding atmospheric pressure is 100 MPa or less. (Technology 5) The method for producing a conductive paste for a secondary battery according to Technology 4, wherein the second dispersion step is carried out once or twice. (Technology 6) The method for producing a conductive paste for a secondary battery according to any one of Technology 1 to 5, wherein the second dispersion step is carried out using a pressure homogenizer. (Technology 7) The method for producing a conductive paste for a secondary battery according to any one of Technologies 1 to 6, further comprising a coarse pulverization step of coarsely pulverizing the carbon nanotubes in the dispersion medium before the first dispersion step.(Technology 8) The carbon nanotubes after the coarse pulverization step have a D50 in a volume-based cumulative particle size distribution of 20 μm or more and 50 μm or less and a D90 in a volume-based cumulative particle size distribution of 40 μm or more and 100 μm or less, and the carbon nanotubes after the second dispersion step have a D50 in a volume-based cumulative particle size distribution of 1.3 μm or more and 2.0 μm or less and a D90 in a volume-based cumulative particle size distribution of 2.5 μm or more and 4.0 μm or less, the method for manufacturing a conductive paste for a secondary battery according to Technology 7. (Technology 9) The dispersant contains 25 mass % or more and 60 mass % or less of the rubber-based compound.

[0069] While the present invention has been described in terms of presently preferred embodiments, such disclosure should not be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0070] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0071] Example 1 Coarse Grinding Process A liquid mixture containing carbon nanotubes as a conductive material, hydrogenated nitrile rubber (H-NBR) as a dispersant, polyvinylpyrrolidone as a dispersant, ethyl cellulose as a dispersant, and N-methylpyrrolidone (NMP) as a dispersion medium was obtained in a mass ratio of hydrogenated nitrile rubber:polyvinylpyrrolidone:ethyl cellulose:N-methylpyrrolidone = 4.0:0.9:0.3:0.7:94.1. A ball mill was used as a media-type disperser to crush the carbon nanotubes contained in the liquid mixture to obtain a coarsely crushed liquid. The ball diameter was 1.0 cm. The rotation speed of the drive shaft of the ball mill was 120 rpm, and the peripheral speed of the agitator was 2.3 m / s. As described above, the agitator is a combination of a drive shaft and multiple stirring blades arranged on the drive shaft.

[0072] <First Dispersion Step> The carbon nanotubes contained in the coarsely pulverized liquid were further crushed using a bead mill as a media-type disperser to obtain a first dispersion liquid. The diameter of the ball was 0.5 mm. In the bead mill, the peripheral speed of the agitator was set to 13.3 m / s.

[0073] <Second Dispersion Step> A pressure homogenizer was used to further disintegrate the carbon nanotubes contained in the first dispersion to obtain a second dispersion. That is, a conductive paste for a secondary battery according to Example 1 was obtained. The pressure homogenizer used was a high-pressure homogenizer (product name "BERYU MINI") manufactured by Biryu Co., Ltd. The operating conditions of the pressure homogenizer were as shown in Table 1 below. In Table 1, the term "nozzle" is a concept that includes a narrow passage.

[0074] Example 2 A conductive paste for a secondary battery according to Example 2 was obtained in the same manner as in Example 1, except that in the second dispersion step, the operating conditions of the pressure homogenizer were set as shown in Table 1 below.

[0075] Example 3 A conductive paste for a secondary battery according to Example 3 was obtained in the same manner as in Example 1, except that in the second dispersion step, the operating conditions of the pressure homogenizer were set as shown in Table 1 below.

[0076] Example 4 A conductive paste for a secondary battery according to Example 4 was obtained in the same manner as in Example 1, except that in the second dispersion step, the operating conditions of the pressure homogenizer were set as shown in Table 1 below.

[0077] [Example 5] In the second dispersion step, a pressure homogenizer, OMEGA500 manufactured by Netzsch, was used, and the operating conditions were as shown in Table 1 below. Except for this, the conductive paste for a secondary battery according to Example 5 was obtained in the same manner as in Example 1.

[0078] [Example 6] A conductive paste for a secondary battery according to Example 6 was obtained in the same manner as in Example 1, except that an in-line mixer was used in the second dispersion step under the operating conditions shown in Table 2. As the in-line mixer, a tabletop experimental machine, magic LAB, manufactured by IKA Corporation was used.

[0079] Comparative Example 1 A conductive paste for a secondary battery according to Comparative Example 1 was obtained in the same manner as in Example 1, except that the second dispersion step was not carried out.

[0080] Comparative Example 2 A conductive paste for a secondary battery according to Comparative Example 2 was obtained in the same manner as in Example 1, except that the coarse pulverization step and the first dispersion step were not performed, and only the second dispersion step was performed.

[0081] Comparative Example 3 A conductive paste for a secondary battery according to Comparative Example 3 was obtained in the same manner as in Example 1, except that in the second dispersion step, the mixture was passed through a thin tube having an inner diameter of 0.5 mm without using a nozzle of the pressure homogenizer, and the operating conditions were as shown in Table 1 below.

[0082]

[0083]

[0084] Evaluation: For the conductive pastes for secondary batteries according to Examples 1 to 6 and Comparative Examples 1 to 3, particle distribution curves of carbon nanotubes were obtained, which show the frequency distribution obtained by measuring particle size distribution on an area basis. The results are shown in Figures 1 to 3. These particle distribution curves were obtained according to the method described in the above embodiment section.

[0085] 1 to 3 show that in the conductive pastes for secondary batteries according to Examples 1 to 6, the peak top frequency of the highest peak appearing in the range of 0.1 μm or more and 0.5 μm or less is 30% or less. This suggests that the amount of carbon nanotubes with large particle diameters is small in the conductive pastes for secondary batteries according to these examples, and that the carbon nanotubes are sufficiently dispersed in the dispersion medium. On the other hand, in the conductive pastes for secondary batteries according to Comparative Examples 1 to 3, the peak top frequency of the highest peak appearing in the range of 0.1 μm or more and 0.5 μm or less is greater than 30%. This suggests that the amount of carbon nanotubes with large particle diameters is large in the conductive pastes for secondary batteries according to these examples, and that the carbon nanotubes are not sufficiently dispersed in the dispersion medium.

[0086] The method for producing a conductive paste for a secondary battery according to the present disclosure can be used in applications where sufficient dispersion of carbon nanotubes is required.

Claims

1. A method for producing a conductive paste for secondary batteries containing carbon nanotubes, a dispersant, and a dispersion medium, wherein the dispersant contains a rubber-based compound, the method comprising: a first dispersion step of using a media-type disperser to disperse the carbon nanotubes in the dispersion medium by utilizing collision energy between media particles to obtain a first dispersion; and a second dispersion step of applying shear force to the carbon nanotubes in the first dispersion to further disperse the carbon nanotubes in the dispersion medium to obtain a second dispersion, wherein in the second dispersion step, the first dispersion is passed through a narrow passage under application of a pressure exceeding atmospheric pressure so that the frequency of the highest peak appearing in the particle size range of 0.1 μm to 0.5 μm inclusive in a particle distribution curve of the carbon nanotubes, which shows a frequency distribution obtained by area-based particle size distribution measurement, is 30% or less.

2. The method for producing a conductive paste for a secondary battery according to claim 1, wherein in the second dispersion step, the shear force is applied so that the shear rate (1 / s) calculated from the flow rate and clearance when the first dispersion is passed through the narrow passage is 1,000,000 or more.

3. The method for producing a conductive paste for a secondary battery according to claim 1 or 2, wherein a flow path having a cross-sectional area larger than that of the narrow path is disposed downstream of the narrow path, and the second dispersion step is carried out by passing the paste through the narrow path and the flow path.

4. The method for producing a conductive paste for a secondary battery according to claim 1 or 2, wherein the pressure exceeding atmospheric pressure is 100 MPa or less.

5. The method for producing a conductive paste for a secondary battery according to claim 4, wherein the second dispersion step is carried out once or twice.

6. The method for producing a conductive paste for a secondary battery according to claim 1 or 2, wherein the second dispersion step is carried out using a pressure homogenizer.

7. The method for producing a conductive paste for a secondary battery according to claim 1 or 2, further comprising a coarse pulverization step of coarsely pulverizing the carbon nanotubes in the dispersion medium before the first dispersion step.

8. The method for producing a conductive paste for a secondary battery according to claim 7, wherein the carbon nanotubes after the coarse pulverization step have a D50 in a volume-based cumulative particle size distribution of 20 μm or more and 50 μm or less and a D90 in a volume-based cumulative particle size distribution of 40 μm or more and 100 μm or less, and the carbon nanotubes after the second dispersion step have a D50 in a volume-based cumulative particle size distribution of 1.3 μm or more and 2.0 μm or less and a D90 in a volume-based cumulative particle size distribution of 2.5 μm or more and 4.0 μm or less.

9. The method for producing a conductive paste for a secondary battery according to claim 1 or 2, wherein the dispersant contains 25% by mass or more and 60% by mass or less of the rubber-based compound.

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