Method for producing aqueous slurry for secondary battery negative electrode

Abstract

To provide a manufacturing method of an aqueous slurry for secondary battery anode electrode capable of manufacturing a high-quality anode active material layer.SOLUTION: A method of manufacturing an aqueous slurry for secondary battery anode electrode includes: providing a first composition containing at least a non-conductive anode active material and an aqueous dispersant of carbon nanotubes; applying dispersion processing to the first composition; providing a second composition containing the first composition, to which the dispersion processing is applied, and polymers; and kneading the second composition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aqueous slurry for a secondary battery negative electrode. [Background technology]

[0002] Secondary batteries are widely used in various fields such as electric vehicles, power storage devices, and information devices. In the production of such secondary batteries, a dispersion for forming an electrode layer containing a conductive material, an active material, and the like is used.

[0003] Patent Document 1 discloses an electrode group including a positive electrode, a negative electrode, and a solid electrolyte layer located between the positive electrode and the negative electrode, the solid electrolyte layer including a first portion including a plurality of first solid electrolyte particles in contact with the positive electrode, a second portion including a plurality of second solid electrolyte particles in contact with the negative electrode, and a third portion located between the first portion and the second portion and including a plurality of third solid electrolyte particles, the average particle size of the third solid electrolyte particles being larger than the average particle size of the first solid electrolyte particles and larger than the average particle size of the second solid electrolyte particles. Patent Document 1 also discloses forming a negative electrode active material layer by coating a negative electrode current collector with a slurry obtained by dispersing materials constituting the negative electrode in water.

[0004] Patent Document 2 discloses a conductive material dispersion liquid containing a conductive material, a dispersant, and a dispersion medium, wherein the conductive material is carbon black having an average primary particle size of 40 nm or less and an average dispersed particle size of 400 nm or less, and the dispersant contains a nonionic dispersant.

[0005] Patent Document 3 discloses a binder for a non-aqueous storage element containing a given polymer, and an electrode for a non-aqueous storage element having an active material layer formed using the binder. Patent Document 3 also discloses the use of alcohol as a preservative. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-163870 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-70908 [Patent Document 3] International Publication No. 2015-16283 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a method for producing an aqueous slurry for a secondary battery negative electrode, which enables the production of a high-quality negative electrode active material layer. [Means for solving the problem]

[0008] The present inventors have conducted extensive research and found that the above problems can be solved by the following means, and have completed the present invention. That is, the present invention is as follows: Aspect 1: Providing a first composition containing at least a non-conductive negative electrode active material and an aqueous dispersion of carbon nanotubes; subjecting the first composition to a dispersion treatment; providing a second composition containing the first composition that has been subjected to a dispersion treatment and a polymer; and kneading the second composition Including, A method for producing aqueous slurry for secondary battery negative electrodes. Aspect 2: The method of aspect 1, wherein the first composition contains a polymer, and the content of the polymer contained in the first composition in the second composition is 70 mass% or less of the total polymer content contained in the second composition. Aspect 3 3. The method of claim 1 or 2, further comprising filtering the kneaded second composition through a filter having a pore size of 50 μm or less. Aspect 4: The method of any one of Aspects 1 to 3, wherein the kneaded second composition is applied to a colorless transparent film substrate using a coater having a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25°C, and then dried at 80°C for 5 minutes to obtain a coating film, and when the Y values ​​in the XYZ color system are measured at any five points within the surface of the obtained coating film, the standard deviation of the Y values ​​is 0.5 or less. Aspect 5: The kneaded second composition is applied to a colorless transparent film substrate using a coater with a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. The L * a * b * L in color space * value, a * value, and b * Find the values ​​of L * a * b * A method according to any one of aspects 1 to 4, wherein, when color difference ΔE in a color space is calculated from five points on the coating film, the maximum value of ΔE is 2.0 or less. Aspect 6: The method according to any one of aspects 1 to 5, wherein a color difference ΔE between the first composition before and after a dispersion treatment is calculated, the ΔE is 0.1 to 20.0, and the color difference is calculated by the following steps: (a) The first composition before dispersion treatment was applied to a colorless transparent film substrate using a coater having a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. * a * b * L in color space * value, a * value, and b * The values ​​were measured at three points on the coating, and * value, a * value, and b * calculating the average of each of the values; (b) After the dispersion treatment, the first composition is subjected to the same procedure as in (a) above to obtain L * value, a * value, and b* calculating the average of each of the values; and (c) L of the first composition obtained in (a) before dispersion treatment * value, a * value, and b * the average value of each of the values ​​and the L of the first composition after the dispersion treatment obtained in (a) * value, a * value, and b * and calculating the color difference ΔE between the first composition before dispersion treatment obtained in (a) and the first composition before dispersion treatment obtained in (a) from the average values ​​of the color difference ΔE and the average values ​​of the color difference ΔE. [Effects of the Invention]

[0009] According to the method of the present invention for producing an aqueous slurry for a secondary battery negative electrode, it is possible to provide an aqueous slurry for a secondary battery negative electrode that enables the production of a high-quality negative electrode active material layer. DETAILED DESCRIPTION OF THE INVENTION

[0010] <<Method for producing aqueous slurry for secondary battery negative electrode>> The method of the present invention for producing an aqueous slurry for a secondary battery negative electrode comprises: providing a first composition containing at least a non-conductive negative electrode active material and an aqueous dispersion of carbon nanotubes; subjecting the first composition to a dispersion treatment; providing a second composition containing the first composition that has been subjected to a dispersion treatment and a polymer; and kneading the second composition Includes:

[0011] Without being limited by theory, it is believed that the reason why the method of the present invention can provide an aqueous slurry for a secondary battery negative electrode that enables the production of a high-quality negative electrode active material layer is as follows: That is, by homogenizing a non-conductive negative electrode active material and carbon nanotubes in a mixed state, and by mixing and kneading the composition in this state with a polymer, the negative electrode active material and carbon nanotubes can be stabilized in a homogeneous state in the slurry, thereby providing an aqueous slurry for a secondary battery negative electrode that enables the production of a high-quality negative electrode active material layer.

[0012] The viscosity of the aqueous slurry for a secondary battery negative electrode of the present invention, measured at 25°C and a shear rate of 0.2 / s, can be less than 300 Pa·s. This viscosity can be 290 Pa·s or less, 280 Pa·s or less, 270 Pa·s or less, 260 Pa·s or less, 250 Pa·s or less, 240 Pa·s or less, 230 Pa·s or less, or 220 Pa·s or less, or 30 Pa·s or more, 50 Pa·s or more, 70 Pa·s or more, 100 Pa·s or more, 120 Pa·s or more, or 150 Pa·s or more. This viscosity can be measured, for example, using a rheometer (MCR302, Anton Paar, cone plate φ50 mm, 2°C, 25°C).

[0013] The aqueous slurry for a secondary battery negative electrode thus obtained is applied to a colorless transparent film substrate using a coater with a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. When the Y values ​​in the XYZ color system are measured at any five points on the surface of the obtained coating film, the standard deviation of the Y values ​​can be 0.5 or less. This standard deviation can be 0.4 or less, 0.3 or less, or 0.2 or less, or can be greater than 0.

[0014] Here, the Y value in the XYZ color system can be measured, for example, under the following conditions. Measurement equipment: Spectrophotometer (SC-T(P), Suga Test Instruments) Optical conditions: Diffused illumination 8°, d8 method (specular reflection excluded) Light source: 12V 50W halogen lamp Measurement conditions: D65 light, 2° field of view Measurement area: 5φ

[0015] The aqueous slurry for secondary battery negative electrodes thus obtained was applied to a colorless transparent film substrate using a coater with a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. The L * a * b * L in color space * value, a * value, and b * Measure the L value between the two points. * a * b * When the color difference ΔE in the color space is calculated between all two points that can be combined from the above-mentioned five arbitrary points, the maximum value of the color difference ΔE can be 2.0 or less. This maximum value of ΔE can be 2.0 or less, 1.6 or less, 1.3 or less, 1.0 or less, 0.8 or less, 0.6 or less, 0.5 or less, 0.4 or less, or 0.3 or less, or can be greater than 0.

[0016] where L * a * b * L in color space * value, a * value and b * The value can be measured, for example, under the following conditions. Measurement equipment: Spectrophotometer (SC-T(P), Suga Test Instruments) Optical conditions: Diffused illumination 8°, d8 method (specular reflection excluded) Light source: 12V 50W halogen lamp Measurement conditions: D65 light, 2° field of view Measurement area: 5φ

[0017] The color difference ΔE between two points is the L measured under the above conditions. * value, a * value and b* Using the values, it can be calculated from the following formula: {(ΔL * ) 2 +(Δa * ) 2 +(Δb * ) 2} (1 / 2) In this formula, ΔL * , Δa * and Δb * are two points of L * value, a * value and b * Represents the difference in values.

[0018] In this case, there are ten possible ΔE values ​​that can be calculated between any two points that can be combined from any five points, and the largest one among them can be set as the maximum value of ΔE described above.

[0019] Thus, the fact that the standard deviation of the Y value in the XYZ color system and the maximum value of the color difference ΔE are small means that a negative electrode active material layer in which each component is uniformly and well dispersed has been obtained, that is, a high-quality negative electrode active material layer has been obtained.

[0020] The method of the present invention preferably further comprises filtering the kneaded second composition through a filter having a pore size of 50 μm or less, in order to remove coarse particles remaining during dispersion and kneading, thereby obtaining a high-quality coating film. A pore size of 50 μm indicates a pore size that can remove particles of 50 μm or more with a collection efficiency of 90% or more.

[0021] Each component of the present invention will be described below.

[0022] <Provision of the first composition> The first composition contains at least a non-conductive negative electrode active material and an aqueous dispersion of carbon nanotubes, and may further contain other substances such as a pH adjuster, a preservative, and an organic solvent, which will be described below.

[0023] The first composition may be prepared by adding, for example, a non-conductive negative electrode active material to an aqueous dispersion of carbon nanotubes, but the order of addition of the materials constituting the first composition is not particularly limited. Because the non-conductive negative electrode active material has a large specific gravity and is prone to settling, it is preferable to add the negative electrode active material or the aqueous dispersion while stirring.

[0024] The stirring method is not particularly limited, and an appropriate method can be selected depending on the mode of providing the first composition. The stirring method may be a method that prevents clogging or unevenness in the subsequent dispersion treatment, and may be, for example, a method using a Disper (Homodisper, Primix Corporation) or a propeller mixer (Portable Mixer, Satake Multimix Corporation). The stirring method may also be a method mentioned in the dispersion treatment of the first composition or the kneading of the second composition, such as a method using a planetary mixer (Hibismix, Primix Corporation).

[0025] Here, the first composition does not contain a polymer, or the first composition contains a polymer, and the content of the polymer contained in the first composition in the second composition is 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less of the total polymer content contained in the second composition. Also, this proportion may be 10% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, or 50% by mass or more.

[0026] For the polymer contained in the first composition, reference can be made to the description of polymers below, and in particular, the polymer contained in the first composition may be a polymer referred to below as a thickener and a dispersant.

[0027] The aqueous dispersion of carbon nanotubes may contain water, carbon nanotubes, and an optional polymer. For the contents of these components, please refer to the descriptions of each component below.

[0028] <Dispersion treatment of first composition> The dispersion treatment of the first composition may be a treatment for uniformly dispersing each component contained in the first composition, such as the negative electrode active material and the carbon nanotubes, and can be performed using, for example, an ultrasonic disperser, a three-roll mill, a ball mill, a bead mill, a jet mill, a high-pressure homogenizer, a mixer, or the like.

[0029] When the first composition is dispersed, a change in color is observed as the particles disperse. Therefore, the color difference ΔE between before and after the dispersion treatment of the first composition can be used as an index of dispersion.

[0030] Specifically, from the viewpoint of sufficiently dispersing the negative electrode active material, it is preferable to perform the dispersion treatment of the first composition with a strength such that the color difference ΔE before and after the dispersion treatment of the first composition is 0.1 or more, 0.2 or more, 0.3 or more, or 0.4 or more. Furthermore, the dispersion treatment of the first composition can be performed with a strength such that the color difference ΔE is 20.0 or less, 15.0 or less, 12.0 or less, 10.0 or less, 8.0 or less, 5.0 or less, 3.0 or less, 1.0 or less, 0.8 or less, 0.7 or less, or 0.6 or less. In particular, it is preferable to perform the dispersion treatment of the first composition with a strength such that the color difference ΔE is 0.8 or less, 0.7 or less, or 0.6 or less, from the viewpoint of suppressing excessive pulverization of the negative electrode active material.

[0031] The color difference ΔE before and after the dispersion treatment of the first composition is measured by the following steps: (a) The first composition before dispersion treatment was applied to a colorless film substrate using a coater having a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. * a * b * L in color space * value, a * value, and b * The values ​​were measured at three points on the coating, and * value, a *value, and b * calculating the average of each of the values; (b) After the dispersion treatment, the first composition is subjected to the same procedure as in (a) above to obtain L * value, a * value, and b * calculating the average of each of the values; and (c) L of the first composition obtained in (a) before dispersion treatment * value, a * value, and b * the average value of each of the values ​​and the L of the first composition after the dispersion treatment obtained in (a) * value, a * value, and b * and calculating the color difference ΔE between the first composition before dispersion treatment obtained in (a) and the first composition before dispersion treatment obtained in (a) from the average values ​​of the color difference ΔE and the average values ​​of the color difference ΔE.

[0032] For example, when the dispersion treatment of the first composition is carried out using a bead mill, the peripheral speed of the bead mill, the diameter or material of the media used, or the packing rate can be set.

[0033] The peripheral speed of the bead mill is preferably set to 6 m / s or more, 7 m / s or more, or 8 m / s or more, and 17 m / s or less, 15 m / s or less, or 13 m / s or less, from the viewpoint of obtaining the above-mentioned color difference ΔE.

[0034] For media, sizes can be selected such as φ1.0mm, φ0.5mm, φ0.3mm, φ0.1mm, φ0.05mm, φ0.03mm, and φ0.01mm, and materials can also be selected such as glass, zirconia, and alumina.

[0035] Furthermore, the filling rate at which the media is filled into the bead mill can be selected arbitrarily within the allowable range of the bead mill.

[0036] When the dispersion treatment of the first composition is carried out using a high-pressure homogenizer, the pressure during the treatment can be set.

[0037] The pressure can be set arbitrarily up to the upper limit allowed by the device, and the lower limit can be set to 30 MPa or more, 50 MPa or more, 100 MPa or more, or 150 MPa or more.

[0038] The time for dispersing the first composition can be determined depending on the strength of dispersion. For example, in the case of using a bead mill, when the peripheral speed is 10 m / s, the residence time (dispersion time) in the bead mill is preferably 1 minute or more, 2 minutes or more, or 3 minutes or more, and 20 minutes or less, 15 minutes or less, 13 minutes or less, or 11 minutes or less, from the viewpoint of obtaining the above-mentioned color difference ΔE.

[0039] In the case of using a high-pressure homogenizer, when the pressure is 150 MPa, it is preferable from the viewpoint of obtaining the above-mentioned color difference ΔE that the number of times of passing through the device is 1 or more, 2 or more, or 3 or more, and 50 or less, 30 or less, or 20 or less.

[0040] <Provision of the second composition> The second composition contains the first composition that has been subjected to a dispersion treatment, and a polymer.

[0041] The second composition may be prepared by mixing a polymer into the first composition that has been subjected to a dispersion treatment, or by mixing the first composition that has been subjected to a dispersion treatment into a polymer. When other substances, such as a pH adjuster, a preservative, and an organic solvent, are further added in preparing the second composition, the order in which the first composition, the polymer, and these other substances are added is not particularly limited.

[0042] The mixing method is not particularly limited, and an appropriate method can be selected depending on the mode of providing the second composition. The mixing method may be, for example, a method using a Disper (Homodisper, Primix Corporation) or a propeller mixer (Portable Mixer, Satake Multimix Corporation). The stirring method may be a method mentioned in the dispersion treatment of the first composition or the kneading of the second composition, such as a method using a planetary mixer (Hibismix, Primix Corporation).

[0043] The polymer to be mixed here may be the same as or different from the polymer originally contained in the first composition. For the polymer to be mixed here, reference can be made to the description of polymers below, and in particular, the polymer to be mixed here may be a polymer referred to below as a polymer that can be used as a binder.

[0044] <Kneading of the second composition> Unlike simple stirring, the kneading of the second composition is generally a process in which a device having a plurality of media such as blades and balls is used, and these media are moved at various locations throughout the composition, thereby reliably applying shear to the entire second composition. The kneading of the second composition can be carried out using, for example, a bead mill, a ball mill, a planetary mixer, etc. In particular, the kneading of the second composition is preferably carried out using a planetary mixer capable of rotating and revolving a plurality of blades. In batch processing using a disperser or the like, so-called simple stirring, which involves flowing and homogenizing a liquid, does not reliably apply shear to the entire second composition.

[0045] For example, when the second composition is kneaded using a planetary mixer, it is preferable to set the revolution speed to 1 rpm or more, 2 rpm or more, 4 rpm or more, or 6 rpm or more, in order to further suppress uneven distribution of particles in the slurry and thereby improve the uniformity of the resulting coating film, particularly the color difference ΔE between surfaces. This revolution speed may be set to 40 rpm or less, 30 rpm or less, or 20 rpm or less.

[0046] <filtration> Filtration can be carried out using a filter with a pore size of 50 μm or less, which may be 50 μm or less, 40 μm or less, 35 μm or less, 30 μm or less, or 25 μm or less, or 1 μm or more, 3 μm or more, 5 μm or more, 7 μm or more, 9 μm or more, 10 μm or more, 12 μm or more, 15 μm or more, 17 μm or more, or 20 μm or more.

[0047] The items used in the method of the present invention will be described below.

[0048] <water> The water can be ion-exchanged water, distilled water, purified water, or the like.

[0049] <Negative electrode active material> The negative electrode active material can be any material without particular limitation as long as it does not have electrical conductivity, and for example, metal oxide-based active material particles, silicon-based active material particles, and particularly metal oxide-based negative electrode active material particles can be used. In the present invention, "not having electrical conductivity" or "non-conductive" means that the volume resistivity of the material is 10 Ω·cm or more, 10 2 Ω cm or more, 10 3 Ω cm or more, 10 4 Ω cm or more, 10 5 Ω cm or more, 10 6 Ω cm or more, 10 7 Ω cm or more, 10 8 Ω cm or more, 10 9 Ω·cm or more, or 10 10 This volume resistivity is measured in accordance with JIS C 2139:2008 at an ambient temperature of 25°C and a relative humidity of 40%.

[0050] The metal oxide-based negative electrode active material particles may be, for example, titanium oxide. The titanium oxide is not particularly limited as long as it can absorb and release lithium, but examples that can be used include spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxides, titanium dioxide (TiO(B)) having a monoclinic crystal structure, and anatase-type titanium dioxide.

[0051] Spinel-type lithium titanate includes Li 4+x Ti5O 12 (x varies in the range of -1≦x≦3 depending on the charge / discharge reaction). Ramsdellite-type lithium titanate includes Li 2+y Ti3O7 (where y varies in the range of -1≦y≦3 depending on the charge / discharge reaction). TiO2(B) and anatase titanium dioxide include Li 1+z Examples include TiO2 (z changes in the range of -1≦z≦0 depending on the charge / discharge reaction).

[0052] Examples of titanium-containing metal composite oxides include metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe. Examples of metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe include TiO2-P2O5, TiO2-V2O5, TiO2-P2O5-SnO2, and TiO2-P2O5-MeO (wherein Me is at least one element selected from the group consisting of Cu, Ni, and Fe).

[0053] Such a metal composite oxide preferably has a microstructure with low crystallinity, in which a crystalline phase and an amorphous phase coexist, or in which an amorphous phase exists alone, which can further improve cycle performance.

[0054] The content of metal oxide active material particles in the aqueous slurry for a secondary battery negative electrode may be 20% by mass or more, 25% by mass or more, 30% by mass or more, 35% by mass or more, or 40% by mass or more, based on the total mass of the aqueous slurry for a secondary battery negative electrode, and may be 70% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, or 50% by mass or less.

[0055] Carbon nanotubes As the carbon nanotube, known carbon nanotubes can be used.

[0056] The average particle diameter of carbon nanotubes can be 100 nm or more, 200 nm or more, 300 nm or more, 500 nm or more, 700 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and can be 20 μm or less, 15 μm or less, 10 μm or less, or 7 μm or less. The average particle diameter used herein is selected appropriately depending on the size of the carbon particles in question. For particles less than approximately 1 μm, it refers to the histogram mean particle diameter (D50) value based on the scattering intensity distribution measured by dynamic light scattering. For particles 1 μm or larger, it refers to the median diameter (D50) value calculated on a volume basis by laser diffraction. Measurements using dynamic light scattering can be performed using, for example, a DelsaMax CORE (Beckman Coulter). Measurements using laser diffraction can be performed using, for example, a particle size distribution analyzer MT3300II (Microtrac Bell Corporation).

[0057] The carbon nanotube content in the aqueous slurry for a secondary battery negative electrode may be 0.1 mass % or more, 0.3 mass % or more, 0.5 mass % or more, or 0.6 mass % or more, based on the total mass of the aqueous slurry for a secondary battery negative electrode, and may be 10.0 mass % or less, 8.0 mass % or less, 5.0 mass % or less, 3.0 mass % or less, 2.5 mass % or less, 2.0 mass % or less, 1.5 mass % or less, 1.0 mass % or less, or 0.8 mass % or less.

[0058] <Other conductive particles> As other conductive particles, for example, carbon-based conductive particles other than carbon nanotubes and metal-based conductive particles can be used.

[0059] The carbonaceous conductive particles may be carbon fibers and / or carbon particles.

[0060] Examples of carbon fibers include, but are not limited to, milled fibers and chopped fibers, which may be used alone or in combination.

[0061] The average length of the carbon fibers can be 100 nm or more, 300 nm or more, 500 nm or more, 700 nm or more, 1 μm or more, 3 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more, and can be 200 μm or less, 100 μm or less, 70 μm or less, 50 μm or less, or 30 μm or less. The average length of the carbon fibers can be measured by electron microscopy.

[0062] Examples of carbon particles include graphene, graphite, and carbon black such as acetylene black and ketjen black, which may be used alone or in combination.

[0063] The shape of the carbon particles is not particularly limited, and may be, for example, flat, array-like, spherical, or the like.

[0064] The average particle size of carbon particles can be 100 nm or more, 200 nm or more, 300 nm or more, 500 nm or more, 700 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and can be 20 μm or less, 15 μm or less, 10 μm or less, or 7 μm or less. The average particle size used herein is appropriately selected depending on the size of the carbon particles in question. For particles less than approximately 1 μm, it refers to the histogram mean particle size (D50) value based on the scattering intensity distribution measured by dynamic light scattering. For particles 1 μm or larger, it refers to the median diameter (D50) value calculated on a volume basis by laser diffraction. Measurements using dynamic light scattering can be performed using, for example, a DelsaMax CORE (Beckman Coulter). Measurements using laser diffraction can be performed using, for example, a particle size distribution analyzer MT3300II (Microtrack Bell Corporation).

[0065] The content of other conductive material particles in the aqueous slurry for a secondary battery negative electrode may be 0.1 mass % or more, 0.3 mass % or more, 0.5 mass % or more, or 0.6 mass % or more, based on the total mass of the aqueous slurry for a secondary battery negative electrode, and may be 10.0 mass % or less, 8.0 mass % or less, 5.0 mass % or less, 3.0 mass % or less, 2.5 mass % or less, 2.0 mass % or less, 1.5 mass % or less, 1.0 mass % or less, or 0.8 mass % or less.

[0066] The metallic conductive particles may be a metal element or a compound that has electrical conductivity.

[0067] <polymer> The polymer may be, for example, a polymer that can be used as a binder. Examples of such polymers include various emulsion-type polymers, such as fluorine-based emulsion-type polymers such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), elastomer-based emulsion-type polymers such as ethylene-propylene-diene copolymer (EPDM), nitrile-butadiene rubber (NBR), and styrene-butadiene rubber (SBR), and acrylic emulsion-type polymers.

[0068] As such a polymer, it is preferable to use an elastomer emulsion type polymer, particularly a styrene elastomer emulsion type polymer, particularly a styrene-butadiene rubber, from the viewpoint of electrical conductivity.

[0069] The polymer may be a natural polymer such as a polysaccharide or a synthetic polymer, and may be used as a thickener.

[0070] Examples of polysaccharides that can be used include gum arabic, tragacanth gum, guar gum, locust bean gum, alginic acid, carrageenan, gelatin, xanthan gum, welan gum, succinoglycan, diutan gum, dextran, methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, starch glycolic acid, and salts thereof. Of these, carboxymethylcellulose is preferred from the viewpoint of dispersion stability.

[0071] Examples of synthetic polymers that can be used include water-soluble resins such as polyvinylpyrrolidone, polyvinyl methyl ether, polyvinyl acetal, polyacrylic acid and its salts, polyethylene oxide, vinyl acetate-polyvinylpyrrolidone copolymer, styrene-acrylic acid copolymer and its salts, and isobutylene-maleic anhydride copolymer and its salts.

[0072] As the polymer, for example, nonionic dispersants such as polyalkylene oxides, polyvinyl acetals, polyvinyl ethers, chitins, chitosans, starch, etc. can be used. These polymers are sometimes used as dispersing aids.

[0073] The polymer may also be a dispersant. Specifically, the polymer may be a nonionic or anionic dispersant, or a polysaccharide. The nonionic dispersant may be the above-mentioned dispersing aid or polyvinylpyrrolidone, and the anionic dispersant may be an acrylic resin such as a styrene-acrylic resin, a urethane resin, a polyester resin, a polyvinyl chloride resin, or an epoxy resin.

[0074] The total polymer content, based on the total mass of the aqueous slurry for a secondary battery negative electrode, may be 0.5 mass% or more, 1.0 mass% or more, 1.5 mass% or more, 2.0 mass% or more, or 2.2 mass% or more, and may be 15.0 mass% or less, 12.0 mass% or less, 10.0 mass% or less, 8.0 mass% or less, 6.0 mass% or less, 5.0 mass% or less, 4.5 mass% or less, 4.0 mass% or less, 3.5 mass% or less, 3.0 mass% or less, or 2.5 mass% or less.

[0075] <pH adjuster> As the optional pH adjuster, at least one of ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, alkali metal salts of carbonate or phosphate such as sodium tripophosphate or sodium carbonate, and alkali metal hydroxides such as sodium hydroxide can be used.

[0076] <Preservatives> Optional preservatives that can be used include at least one of phenol, sodium omadine, sodium pentachlorophenol, 1,2-benzisothiazolin-3-one, 2,3,5,6-tetrachloro-4(methylphonyl)pyridine, alkali metal salts of benzoic acid, sorbic acid or dehydroacetic acid, benzimidazole compounds, alcohols such as phenoxyethanol, glycols such as 1,3-pentanediol, and the like.

[0077] <Organic solvents> The optional organic solvent may be, for example, an organic solvent containing a hydrophilic group, such as alcohols, polyhydric alcohols, glycol ethers, etc. These solvents may be used alone or in combination. [Example]

[0078] The present invention will be specifically explained with reference to examples and comparative examples, but the present invention is not limited to these.

[0079] <<Preparation of aqueous slurry for secondary battery negative electrode>> Example 1 A first composition was prepared by mixing 45 parts by mass of lithium titanate as a negative electrode active material, 40 parts by mass of a carbon nanotube (CNT) aqueous dispersion (carbon nanotube content: 2% by mass), 0.9 parts by mass of carbon black (CB), 0.8 parts by mass of carboxymethyl cellulose (CMC) as a polymer, and 10.3 parts by mass of water.

[0080] The prepared first composition was applied to a colorless transparent film substrate using a coater having a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. * a * b * L in color space * value, a * value, and b * The L values ​​were measured at three points on the coating film using a spectrophotometer (SC-T(P), manufactured by Suga Test Instruments Co., Ltd.).* value, a * value, and b * The average values ​​were calculated.

[0081] Next, this first composition was subjected to a dispersion treatment in a bead mill using zirconia beads of φ1.0 mm, with the peripheral speed set to 10 m / s. The residence time (dispersion time) in the bead mill was 10 minutes. The first composition after the dispersion treatment was subjected to L * value, a * value, and b * The average of each of the values ​​was calculated.

[0082] L of the first composition before dispersion treatment * value, a * value, and b * The average values ​​of the respective values ​​and the L of the first composition after the dispersion treatment * value, a * value, and b * The color difference ΔE between the first composition before the dispersion treatment and the first composition before the dispersion treatment was calculated from the average values ​​of the respective values.

[0083] Next, 93 parts by mass of the first composition was mixed with 1.5 parts by mass of styrene-butadiene rubber as a polymer and 5.5 parts by mass of water to prepare a second composition. Next, using a planetary mixer, the revolution speed was set to 10 rpm, and this second composition was kneaded for 120 minutes.

[0084] The defoamed second composition was filtered through a filter with a pore size of 50 μm to prepare an aqueous slurry for a secondary battery negative electrode of Example 1.

[0085] Examples 2 to 10 and Comparative Examples 1 to 3 Aqueous slurries for secondary battery negative electrodes of Examples 2 to 10 and Comparative Examples 1 to 3 were prepared in the same manner as in Example 1, except that the materials used and production conditions were changed as shown in Table 1.

[0086] "evaluation" <Evaluation as a slurry> (viscosity) The viscosity was measured using a rheometer (MCR302, Anton Paar, cone plate φ50 mm 2°), and the value at 25°C and a shear rate of 0.2 / s was taken as the viscosity value of the slurry.

[0087] (Coatability) The obtained slurry was applied to a colorless transparent film substrate using a coater with a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C. The appearance of the coating film was visually observed to evaluate the coatability of the slurry. The evaluation criteria were as follows: A: No streaks or unevenness B: There are streaks or unevenness in the coating

[0088] <Evaluation as a membrane> The obtained slurry was applied to a colorless transparent film substrate using a coater with a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a film. The obtained film was evaluated as follows.

[0089] (visual observation of the surface) The appearance of the film was evaluated according to the following criteria: A: No irregularities were visible on the surface of the film. B: Irregularities on the film surface were visible.

[0090] (standard deviation of Y values) The Y value of the obtained coating film in the XYZ color system was measured at five points on the coating film, and the standard deviation was calculated.

[0091] (color difference between surfaces) The maximum color difference ΔE of the obtained coating film was measured at five points on the coating film. * value, a * value, and b * The values ​​were measured and calculated.

[0092] The configurations and evaluation results of the examples and comparative examples are shown in Tables 1 and 2. In Table 2, a "-" next to the dispersion speed means that dispersion was not performed. In Table 2, a "-" next to the kneading speed means that a stirring treatment was performed for 120 minutes using a Disper (Homo Disper, Primix Corporation) instead of a planetary mixer.

[0093] [Table 1]

[0094] [Table 2]

[0095] It can be seen from Table 1 that the aqueous slurries for secondary battery negative electrodes of Examples 1 to 10, which were obtained by subjecting the first composition to a dispersion treatment and then kneading the second composition, provided negative electrode active material layers with uniform appearance, i.e., high-quality negative electrode active material layers.

Claims

1. providing a first composition containing at least a non-conductive negative electrode active material and an aqueous dispersion of carbon nanotubes; subjecting the first composition to a dispersion treatment; mixing the dispersed first composition with a polymer to provide a second composition; and kneading the second composition Including, a method for producing an aqueous slurry for a secondary battery negative electrode, wherein a color difference ΔE between the first composition before and after a dispersion treatment is calculated, and the ΔE is 0.1 to 20.0, and the calculation of the color difference is carried out by the following steps: (a) applying the first composition before dispersion treatment to a colorless film substrate using a coater having a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then drying the composition at 80° C. for 5 minutes to obtain a coating film; measuring the L* value, a* value, and b* value of the obtained coating film in the L*a*b* color space at three points on the coating film, and calculating the average values ​​of the L* value, a* value, and b* value; (b) calculating the average values ​​of the L* value, a* value, and b* value of the first composition after the dispersion treatment in the same manner as in (a) above; and (c) Calculating the color difference ΔE between the first composition before dispersion treatment obtained in (a) and the first composition before dispersion treatment obtained in (a) from the average values ​​of the L* value, a* value, and b* value of the first composition before dispersion treatment obtained in (a) and the average values ​​of the L* value, a* value, and b* value of the first composition after dispersion treatment obtained in (a).

2. 2. The method according to claim 1, wherein the first composition contains a polymer, and the content of the polymer contained in the first composition in the second composition is 70% by mass or less of the total polymer content contained in the second composition.

3. A method according to claim 1 or 2, further comprising filtering the kneaded second composition through a filter having a pore size of 1 μm or more and 50 μm or less.

4. The method according to any one of claims 1 to 3, wherein the kneaded second composition is applied to a colorless film substrate using a coater having a 50 µm slit at a coating speed of 25 mm / s and a temperature of 25°C, and then dried at 80°C for 5 minutes to obtain a coating film, and when the Y value in the XYZ color system is measured at any five points within the surface of the obtained coating film, the standard deviation of the Y values ​​is 0.5 or less.

5. The kneaded second composition was applied to a colorless film substrate using a coater having a 50 μm slit at a coating speed of 25 mm / s and a temperature of 25° C., and then dried at 80° C. for 5 minutes to obtain a coating film. The L ＊ a ＊ b ＊ L in color space ＊ Value, a ＊ value, and b ＊ Calculate the values ​​of L ＊ a ＊ b ＊ The method according to any one of claims 1 to 4, wherein when a color difference ΔE in a color space is calculated from five points on the coating film, the maximum value of the ΔE is 2.0 or less.

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