Conductive pigment dispersion, cathode slurry composition and battery electrode layer

Abstract

To provide a low-viscosity conductive pigment dispersion containing high concentration carbon nanotubes.SOLUTION: There is provided a conductive pigment dispersion comprising at least (A) a conductive pigment, (B) a dispersant and (C) a solvent, wherein the conductive pigment (A) contains at least carbon nanotubes (A-1) and 2.5 to 10.0 pts.mass of carbon nanotubes (A-1) based on 100 pts.mass of the conductive pigment dispersion and the conductive pigment dispersion has a viscosity (23°C) at a shear rate of 1.0 s-1 of less than 20000 mPa s when containing bundle-type carbon nanotubes (A-1-1) and less than 150000 mPa s when containing entangled-type carbon nanotubes (A-1-2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a conductive pigment dispersion, more specifically to a conductive pigment dispersion containing carbon nanotubes, a positive electrode slurry containing the conductive pigment dispersion, an electrode active material, and a binder, and a battery electrode layer obtained by coating the same on a current collector. [Background technology]

[0002] With the spread of electric vehicles and the trend toward smaller, lighter, and more powerful portable devices, secondary batteries with high energy density and even higher capacity are required. Against this background, non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, which use a non-aqueous electrolyte solution, are being used in many devices due to their high energy density and high voltage characteristics.

[0003] A lithium-ion secondary battery typically comprises a negative electrode made of a carbonaceous material, a positive electrode containing an active material that allows lithium ions to reversibly enter and exit the battery, and a non-aqueous electrolyte in which these are immersed. The positive electrode is produced by applying an electrode paste made of the active material, a conductive material, and a binder to a current collector.

[0004] In the positive electrode, the addition of a conductive material reduces the resistance in the positive electrode layer and increases the conductivity. Therefore, a slurry for forming an electrode for a lithium-ion secondary battery has been proposed, which contains an electrode active material, a conductive material, a binder, and a polar solvent, and in which the conductive material is dispersed (see, for example, Patent Document 1). However, if the conductive material is not sufficiently dispersed in the positive electrode layer, the conductivity may decrease.

[0005] Examples of conductive materials that have been used include carbon black, acetylene black, furnace black, ketjen black, fullerene, graphene, and fine carbon materials. Carbon nanotubes, a type of fine carbon fiber, are tubular carbons with a diameter of 1 μm or less, and are being considered for use as conductive materials in lithium-ion secondary batteries due to their high conductivity based on their unique structure (see, for example, Patent Documents 2 and 3). Among these, multi-walled carbon nanotubes are relatively inexpensive and are expected to be put to practical use.

[0006] When carbon nanotubes with a small average outer diameter and long fiber length are used as a conductive material in a positive electrode, it is expected that a small amount will efficiently form a conductive network. However, carbon nanotubes with a small average outer diameter tend to aggregate due to strong interactions between the fibers, making it difficult to uniformly disperse them at high concentrations, making it difficult to obtain a carbon nanotube dispersion with sufficient dispersibility.

[0007] Therefore, various attempts have been made to improve the dispersibility of carbon nanotubes in dispersion media. For example, a method has been proposed in which carbon nanotubes are dispersed in acetone while being irradiated with ultrasound (see Patent Document 4). However, even if the carbon nanotubes can be dispersed while being irradiated with ultrasound, they begin to aggregate when the irradiation ends, and there is a problem that they aggregate when the concentration of carbon nanotubes becomes high.

[0008] Furthermore, methods for stabilizing the dispersion of carbon nanotubes using various dispersants have been proposed. For example, dispersing carbon nanotubes in water and N-methyl-2-pyrrolidone (NMP) using a polymer dispersant such as water-soluble polymer polyvinylpyrrolidone has been proposed (Patent Document 2). However, while Patent Document 2 evaluated electrodes fabricated using carbon nanotubes with outer diameters of 10 to 150 nm, it encountered the problem of high electrode resistance. Furthermore, Patent Document 3 proposed a dispersion using carbon nanotubes with low DBP oil absorption. However, although the dispersion improved, it was difficult to achieve high conductivity. Therefore, Patent Document 5 investigated a carbon nanotube dispersion using N-methyl-2-pyrrolidone as a solvent, which contains polyvinylpyrrolidone and an amine compound as a dispersing aid, thereby achieving low viscosity even at high carbon nanotube concentrations, and excellent dispersibility and storage stability of the dispersion. While this method has some effectiveness, it poses challenges when dispersing carbon nanotubes with small outer diameters and long fiber lengths or entangled carbon nanotubes at high concentrations.

[0009] Thus, obtaining a carbon nanotube dispersion in which small outer diameter, long fiber length carbon nanotubes or entangled carbon nanotubes are uniformly dispersed in a dispersion medium at high concentrations is challenging, but has been an important challenge for expanding applications. In particular, carbon nanotubes with small outer diameters and long fiber length tend to have high dispersion viscosity due to the interaction between the fibers. However, the viscosity of the carbon nanotube dispersion significantly affects the dispersibility during the preparation of a positive electrode slurry. Obtaining a carbon nanotube dispersion with a higher concentration could increase the solids content of the positive electrode slurry and potentially shorten the drying time during the preparation of a positive electrode. Furthermore, when a strong shear force is applied during the production of a dispersion containing carbon nanotubes, the carbon nanotubes shorten, reducing their conductive properties. Therefore, a method for obtaining a high-concentration, low-viscosity carbon nanotube dispersion while suppressing the shortening of the carbon nanotubes during production has been sought. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-309958 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-70908 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-19619 [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-86219 [Patent Document 5] Japanese Patent Application Publication No. 2019-192537 Summary of the Invention [Problem to be solved by the invention]

[0011] The problem to be solved by the present invention is to solve the above-mentioned conventional problems, and to provide a low-viscosity conductive pigment dispersion containing a high concentration of carbon nanotubes.

[0012] Another problem to be solved by the present invention is to provide a positive electrode slurry containing an electrode active material and a binder, which has excellent drying properties when applied by using a high-concentration, low-viscosity conductive pigment dispersion.Furthermore, to provide a battery electrode layer that has low resistivity and excellent conductivity by applying the positive electrode slurry, which has excellent drying properties when applied, to a current collector. [Means for solving the problem]

[0013] The inventors of the present invention conducted extensive research to solve the above-mentioned problems, and discovered that the problems can be solved by adjusting the amount of carbon nanotubes and viscosity of a conductive pigment dispersion containing a conductive pigment containing carbon nanotubes, a dispersant, and a solvent within certain ranges, which led to the completion of the present invention.

[0014] That is, the present invention provides a conductive pigment dispersion containing at least (A) a conductive pigment, (B) a dispersant, and (C) a solvent, wherein the conductive pigment (A) contains at least (A-1) carbon nanotubes, and the conductive pigment dispersion contains 2.5 to 10.0 parts by mass of the (A-1) carbon nanotubes per 100 parts by mass of the conductive pigment dispersion, and the conductive pigment dispersion is dispersed at a shear rate of 1.0 s at 23°C. -1 The conductive pigment dispersion is characterized in that the viscosity at the time of the dispersion is less than 20,000 mPa·s.

[0015] In a preferred embodiment of the present invention, the (A-1) carbon nanotube has an outer diameter of 1 to 20 nm.

[0016] Another aspect of the present invention is a conductive pigment dispersion liquid containing at least (A) a conductive pigment, (B) a dispersant, and (C) a solvent, wherein the (A) conductive pigment contains at least (A-1) carbon nanotubes, and the (A-1) carbon nanotubes are (A-1-2) entangled carbon nanotubes, and the conductive pigment dispersion liquid contains 2.5 to 10.0 parts by mass of the (A-1-2) entangled carbon nanotubes per 100 parts by mass of the conductive pigment dispersion liquid, and the conductive pigment dispersion liquid is dispersed at a shear rate of 1.0 s at 23°C. -1 The conductive pigment dispersion is characterized in that the viscosity at the time of

[0017] In a preferred embodiment of the present invention, the outer diameter of the (A-1-2) entangled carbon nanotube is 1 to 15 nm.

[0018] In another preferred embodiment of the present invention, the solid content of the (B) dispersant is 10 to 90 parts by mass relative to 100 parts by mass of the (A) conductive pigment.

[0019] In another preferred embodiment of the present invention, the (A-1) carbon nanotubes contain at least two or more types of carbon nanotubes having different fiber lengths.

[0020] Another aspect of the present invention is a positive electrode slurry composition containing at least a conductive pigment dispersion, (D) an electrode active material, and (E) a binder.

[0021] Another aspect of the present invention is a battery electrode layer obtained by coating a current collector with the positive electrode slurry composition. [Effects of the Invention]

[0022] According to the present invention, a conductive pigment dispersion having a high concentration and a low viscosity can be obtained. In particular, when used for electrodes of lithium ion batteries, the dispersion can be used to obtain a battery electrode layer that is homogeneous and has low electrode plate resistance, thereby contributing to improving the characteristics of the lithium ion battery. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention relates to a conductive pigment dispersion, a positive electrode slurry, and an electrode layer for a battery.

[0024] The conductive pigment dispersion of the present invention is characterized by containing at least (A) a conductive pigment containing at least carbon nanotubes, (B) a dispersant, and (C) a solvent. Details are described below. In this specification, "conductive pigment dispersion" and "carbon nanotube dispersion" may be abbreviated as "dispersion," "carbon nanotubes" as "CNT," and "N-methyl-2-pyrrolidone" as "NMP."

[0025] <(A) Conductive pigment> The (A) conductive pigment contains at least (A-1) carbon nanotubes. The (A-1) carbon nanotubes have a cylindrical shape formed by rolling up planar graphite. The carbon nanotubes may be multi-walled carbon nanotubes, single-walled carbon nanotubes, or a mixture of these. Single-walled carbon nanotubes have a structure in which one layer of graphite is rolled up. Multi-walled carbon nanotubes have a structure in which two or more layers of graphite are rolled up. Furthermore, the sidewalls of the (A-1) carbon nanotubes do not have to have a graphite structure. For example, carbon nanotubes with sidewalls having an amorphous structure can also be used as the (A-1) carbon nanotubes.

[0026] The shape of the (A-1) carbon nanotube of this embodiment is not limited. Examples of such shapes include needle-like, cylindrical, fishbone-like (fishbone or cup-stacked), trump-like (platelet), and coil-like. The (A-1) carbon nanotube may have a single shape or a combination of two or more shapes.

[0027] The carbon nanotubes may be of a bundle type or an entangled type. The entangled type refers to a form in which carbon nanotubes entangled with each other form a single particle, and is also called a non-bundle type. The particle type refers to particles that do not have a specific shape.

[0028] In the case of the (A-1-1) bundle type carbon nanotube of this embodiment, the outer diameter of the (A-1) carbon nanotube is preferably 1 to 20 nm, more preferably 2 to 20 nm, and even more preferably 5 to 15 nm.

[0029] In addition, the outer diameter of the (A-1) carbon nanotube of this embodiment is preferably 1 to 15 nm, more preferably 2 to 13 nm, and even more preferably 5 to 12 nm, in the case of the (A-1-2) entangled type.

[0030] Examples of such (A-1) carbon nanotubes include, but are not limited to, single-walled carbon nanotubes such as ZEONANO SG101 (outer diameter: 3 to 5 nm) manufactured by Zeon Corporation and TUBALL (outer diameter: approximately 2 nm) manufactured by OCSiAl. Examples of multi-walled carbon nanotubes include Graphistrength (outer diameter: 10-15 nm) manufactured by ARKEMA, HCNTs10 (outer diameter: 10-20 nm) and HCNTs40 (outer diameter: 30-50 nm) manufactured by SUSUN Sinotech New Materials, NC7000 (outer diameter: 10 nm, entangled type), NX7100 (outer diameter: 10 nm, entangled type), NC8000 (outer diameter: 10 nm, entangled type), and NX5000 (outer diameter: 7-9 nm, entangled type) manufactured by Nanocyl, JENOTUBE8A (outer diameter: 6-9 nm), JENOTUBE10A (outer diameter: 7-20 nm, bundled type), and JENOTUBE10B (outer diameter: 7-10 nm, bundled type) manufactured by JEIO, and CNano. Examples of suitable nanotubes include, but are not limited to, FloTube9100 (outer diameter: 10 to 15 nm), FloTube9110 (outer diameter: 10 to 15 nm), FloTube7010 (outer diameter: 7 to 11 nm), and Kumho Petrochemical's K-Nanos100P (outer diameter: 10 to 15 nm), K-Nanos100T (outer diameter: 10 to 15 nm, bundle type), K-Nanos200P (outer diameter: 5 to 15 nm, bundle type), and K-Nanos300T (outer diameter: 10 to 15 nm, bundle type).

[0031] In addition to (A-1) carbon nanotubes, (A) conductive pigments can also include carbon black, acetylene black, furnace black, ketjen black, fullerene, graphene, and other fine carbon materials in shapes other than carbon nanotubes. In particular, combining carbon nanofibers or acetylene black with (A-1) carbon nanotubes makes it easier to obtain a low-viscosity, high-concentration conductive pigment dispersion. Carbon nanofibers are fine carbon materials with a fibrous shape and a fiber diameter of approximately 100 nm to 1 μm.

[0032] In one preferred embodiment, two or more types of (A-1) carbon nanotubes and / or carbon nanofibers with different average fiber lengths are used in combination. The fiber length of the (A-1) carbon nanotubes is, for example, 1 μm or more, and preferably 10 μm or more. The upper limit is not particularly limited, but is, for example, 300 μm or less. By combining two or more types of CNTs with different average fiber lengths, such as (A-1-2) entangled CNTs with an average fiber length of 15 μm or less and (A-1-1) bundled CNTs with an average fiber length of 50 μm or more, the conductive pigment dispersion liquid of the present invention, which has high concentration, low viscosity, and excellent stability, can be easily obtained. The difference in average fiber length between the two types of CNTs with different average fiber lengths is, for example, 10 μm to 200 μm, and more preferably about 20 to 100 μm. In this case, the average fiber length of the (A-1) carbon nanotubes and / or carbon nanofibers is a characteristic of each raw material before dispersion.

[0033] <(B) Dispersant> The (B) dispersant may be a nonionic or anionic dispersant or a polysaccharide. Examples of nonionic dispersants include polyvinylpyrrolidone, and examples of anionic dispersants include acrylic resins such as styrene-acrylic resins, urethane resins, polyester resins, polyvinyl chloride resins, and epoxy resins. Among these, polymers containing N-vinyl lactam polymerization units, such as N-vinylpyrrolidone, and comb copolymer dispersants are preferred, and their combined use is also preferred. The preferred molecular weight range for polymers containing N-vinyl lactam polymerization units is 2,000 to 500,000 g / mol in weight-average molecular weight. The preferred comb copolymer dispersant has a polymer main chain based on a vinyl aromatic compound and an ethylenically unsaturated polymerizable carboxylic acid anhydride, and at least two polyalkylene oxide side chains. The preferred molecular weight of such comb dispersants is 4,000 to 100,000 g / mol in weight-average molecular weight. The content of the (B) dispersant in the conductive pigment dispersion is preferably 1.0 to 10.0 mass%, more preferably 1.5 to 7.0 mass%. The blending ratio of the (A) conductive pigment to the (B) dispersant is preferably (A) conductive pigment / (B) dispersant = 0.5 to 5.0 mass%, more preferably 1.0 to 4.0 mass. One type of (B) dispersant may be used, or two or more types may be used in appropriate combination. The solid content of the (B) dispersant is preferably 10 to 90 mass parts per 100 mass parts of the (A) conductive pigment.

[0034] <(C) Solvent> The solvent (C) preferably contains an aprotic dipolar solvent, and N-methyl-2-pyrrolidone (NMP) is preferably used as the dispersion medium. As the dispersion medium, one or more other solvents may be used in combination as long as the performance of the conductive pigment dispersion and the battery performance are not impaired. However, in view of the industrial applicability envisioned by the present invention, it is preferable to use NMP alone. The content of the solvent (C) in the conductive pigment dispersion is preferably 80 to 96% by mass, more preferably 85 to 95% by mass.

[0035] The conductive pigment dispersion of the present invention contains the above-mentioned (A) conductive pigment, (B) dispersant, and (C) solvent. Because the (A) conductive pigment contains (A-1) carbon nanotubes, it is important to obtain a dispersion in which the carbon nanotubes are defibrated while suppressing shortening of the carbon nanotubes. The inventors' studies have revealed that the following dispersion method is effective for this purpose.

[0036] (A-1) Preparation of conductive pigment dispersion containing carbon nanotubes <(A-1-1) Preparation of Conductive Pigment Dispersion Containing Bundled Carbon Nanotubes> (A-1-1) Conductive pigment dispersions containing bundled carbon nanotubes are preferably prepared through two or more dispersion steps: pre-dispersion and main dispersion. The pre-dispersion step involves blending (A) a conductive material containing carbon nanotubes with (B) a dispersant and (C) a solvent, and roughly defibrating the mixture. Examples of methods for this include using a high-speed shearing device with high shearing force. Examples of high-speed shearing devices include a mixer that applies shearing force to diluted materials by rotating stirring blades (turbine blades, paddle blades, propeller blades, anchor blades, etc.) at high speed. For example, a pre-dispersion of conductive pigment dispersion can be obtained by stirring for 5 to 30 minutes using a dispersing / disintegrating device such as Disper Dispersion, Filmix (manufactured by Primix Corporation), or Damatri System (manufactured by Yoshida Kikai Kogyo Co., Ltd.).

[0037] The resulting pre-dispersion can be further dispersed using a high-pressure homogenizer to produce a conductive pigment dispersion with low viscosity and excellent dispersion stability. High-pressure homogenizers include those that collide raw materials with each other under high pressure, those that collide high-pressure raw materials with ceramic balls or pass them through slits and process them using the resulting shear force, and those that utilize cavitation caused by a jet of high-pressure raw materials. While not limited to these, it is preferable to use a disperser that passes a paste through a narrow gap under pressure using a pump, thereby dispersing, micronizing, and homogenizing it using the shear force caused by inter-particle collisions and pressure differences. Commercially available high-pressure homogenizers include wet jet mills such as the "Genus PY" manufactured by Genus Corporation, the "NanoVeita" manufactured by Yoshida Kikai Kogyo Co., Ltd., the "Starburst" manufactured by Sugino Machine Co., Ltd., the "Nanomizer" manufactured by Nanomizer Inc., and the "NAGS" manufactured by Jokosha Co., Ltd., but are not limited to these. High-pressure homogenizers comprise a pump and one or more nozzles, and various nozzle shapes are available for the dispersion process.

[0038] The nozzle diameter can be changed depending on the stage of the dispersion treatment. In the early stage when the dispersion treatment has not yet progressed, it is preferable to use a nozzle with a diameter larger than the maximum aggregated carbon nanotubes to prevent clogging. On the other hand, if the nozzle diameter is large, the energy input decreases, which may result in a decrease in productivity.

[0039] The pressure used in a high-pressure homogenizer is generally 70 to 250 MPa, but in the present invention, it is preferable to carry out dispersion at a low pressure of about 70 to 150 MPa in order to suppress shortening of the carbon nanotubes and to improve dispersion stability.

[0040] Dispersion using a high-pressure homogenizer is preferably performed repeatedly under low pressure. In the case of a batch-type homogenizer in which dispersion is performed with each pass, 2 to 15 passes, preferably 4 to 10 passes, can be performed to obtain a conductive pigment dispersion with low viscosity and high dispersion stability. The number of passes is adjusted while checking the degree of dispersion and viscosity. Furthermore, when producing a dispersion using a circulation system in which the dispersed dispersion is sent directly to a raw material tank, the time equivalent to the number of passes in the batch-type system is calculated from the processing volume and processing flow rate of the dispersion, and adjusted accordingly. For example, when producing a conductive pigment dispersion using a circulation system that requires six passes in a batch-type system, if the processing volume is 10 L and the processing rate is 20 L / hour, a processing time of 3 hours can be used to obtain a conductive pigment dispersion equivalent to six passes in a batch-type system.

[0041] The conductive pigment dispersion containing (A-1-1) bundled carbon nanotubes contains 2.5 to 10.0 parts by mass of (A-1-1) bundled carbon nanotubes in the conductive pigment dispersion. A more preferred content is 2.5 to 5.0% by mass. If the amount of carbon nanotubes is less than 2.5% by mass, the solid content when forming a positive electrode slurry is low, resulting in poor drying when coating the positive electrode. If the amount of carbon nanotubes is more than 10.0% by mass, the viscosity becomes too high, resulting in poor dispersibility when forming a positive electrode slurry, which may impair coating workability and performance as a battery electrode layer. The viscosity of the conductive pigment dispersion containing (A-1-1) bundled carbon nanotubes is measured at a temperature of 23°C and a shear rate of 1.0 s -1 When the viscosity is less than 20,000 mPa·s,

[0042] <(A-1-2) Preparation of Conductive Pigment Dispersion Containing Entangled Carbon Nanotubes> The conductive pigment dispersion containing (A-1-2) entangled carbon nanotubes is preferably prepared through two or more dispersion steps: pre-dispersion and main dispersion. The pre-dispersion step involves blending (A-1-2) the conductive material containing entangled carbon nanotubes with (B) a dispersant and (C) a solvent, resulting in coarse defibration. Examples of methods for this include manual mixing or using a pre-dispersion device for light defibration and light defibration, such as an orifice-type shear-generating disperser. An orifice-type shear-generating disperser is a device that has a piping system with branching and merging flow paths and multiple orifices (small holes through which the process fluid passes) along the flow paths. The device uses the energy from collisions and shearing as the process fluid passes through to pulverize solid components in the process fluid and disperse them in the liquid. For example, the Damatori System manufactured by Yoshida Kikai Kogyo Co., Ltd. is an example. Even when a small amount of entangled carbon nanotubes is added, the viscosity tends to increase as the defibrillation progresses, making it difficult to disperse them uniformly without destroying their shape. However, by performing pre-dispersion followed by main dispersion, it becomes easier to obtain a low-viscosity, high-concentration dispersion.

[0043] The obtained pre-dispersion can be made into a conductive pigment dispersion liquid with low viscosity and excellent dispersion stability by main dispersion using a high-pressure homogenizer, which is the same as that used in the preparation of the conductive pigment dispersion liquid containing bundled carbon nanotubes (A-1-1) described above.

[0044] The conductive pigment dispersion containing (A-1-2) entangled carbon nanotubes contains 2.5 to 10.0 parts by mass of (A-1-2) entangled carbon nanotubes in the conductive pigment dispersion. A more preferred content is 2.5 to 5.0% by mass. If the amount of carbon nanotubes is less than 2.5% by mass, the solid content when forming a positive electrode slurry is low, resulting in poor drying when coating the positive electrode. If the amount of carbon nanotubes is more than 10.0% by mass, the viscosity becomes too high, resulting in poor dispersibility when forming a positive electrode slurry, which may impair coating workability and performance as a battery electrode layer. The viscosity of the conductive pigment dispersion containing (A-1-2) entangled carbon nanotubes is measured at a temperature of 23°C and a shear rate of 1.0 s-1 When the viscosity is less than 150,000 mPa·s,

[0045] The viscosity of the conductive pigment dispersion is preferably a so-called pseudoplastic paste, which has a high viscosity during storage and a low viscosity during production (blending, stirring, pumping, etc.) or application, where shear stress is applied. This is preferable from the viewpoints of storage stability (settling of the pigment), manufacturability, coatability, and finish. However, excessive dispersion can cause the dispersion to become unstable at high shear rates (for example, at a shear rate of 100 s -1 If the viscosity at a shear rate of 1.0 s becomes too low, the fiber length of the carbon nanotubes in the dispersion may become shorter, resulting in a decrease in conductivity. -1 Viscosity V1 at a shear rate of 100 s -1 The viscosity ratio (V1 / V2) of the viscosity V1 to the viscosity V2 in the dispersion liquid is preferably in the range of 5.0 to 30.0, and more preferably in the range of 5.0 to 20.0. The viscosity can be adjusted based on the type and amount of solvent, the NV value (solid content), the type and amount of dispersant, and the dispersion conditions (time, dispersing capacity, etc.).

[0046] The volume resistivity of the carbon nanotube of this embodiment is 1.0×10 -3 ~1.0×10 -1 Ω·cm is preferred, and 1.0×10 -3 ~1.0×10 -2 It is more preferable that the volume resistivity is Ω·cm. The volume resistivity of the conductive material can be measured using a powder resistivity measuring device (Loresta GP Powder Resistivity Measuring System MCP-PD-51, manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0047] (Measurement of volume resistivity of positive electrode layer) The positive electrode slurry was applied to an OHP sheet using an applicator, and then heated in an electric oven at 120°C ± 5°C for 25 minutes to dry the coating. The dried coating was then pressed in a press at 50,000 kg / cm. 2After applying a pressure of 100 Ω / sq. to form the positive electrode layer, the surface resistance (Ω / sq.) of the dried coating film was measured using a Loresta GP MCP-T600 manufactured by Mitsubishi Chemical Analytech Co., Ltd. After measurement, the surface resistance was multiplied by the thickness of the electrode mixture layer formed on the OHP sheet to obtain the volume resistivity (Ω·cm) of the electrode film for the positive electrode. The thickness of the electrode mixture layer was calculated by subtracting the film thickness of the OHP sheet from the average value measured at three points in the electrode film using a film thickness meter (DIGIMICRO MH-15M manufactured by NIKON Corporation). The volume resistivity of the positive electrode layer is preferably 100 Ω·cm or less, and more preferably 30 Ω·cm or less.

[0048] [Positive electrode slurry (for lithium-ion batteries)] The present invention provides a positive electrode slurry obtained by further blending the conductive pigment dispersion with (D) an electrode active material and (E) a binder. The positive electrode slurry is suitable for use as a positive or negative electrode for a lithium ion battery, and is preferably used as a positive electrode.

[0049] (D) Electrode active material (D) Examples of electrode active materials include lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium cobalt oxide (LiCoO2), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 O2, LiNi 6 / 10 Co 2 / 10 Mn 2 / 10 Examples of the electrode active material (D) include lithium composite oxides such as O2; lithium iron phosphate (LiFePO4); sodium composite oxides; and potassium composite oxides. These electrode active materials (D) can be used singly or in combination of two or more. Electrode active materials containing the lithium iron phosphate are suitable for use because they are inexpensive and have relatively good cycle characteristics and energy density. The particle size of the electrode active material is usually 0.5 μm or more, preferably 1.5 μm or more, and usually 30 μm or less, preferably 20 μm or less.

[0050] The solid content of the (D) electrode active material in the solid content of the positive electrode slurry for a lithium ion battery electrode of the present invention is usually 80 mass % or more, preferably 95 mass % or more, and more preferably 97.5 mass % or more, and is preferably less than 100 mass % from the viewpoints of battery capacity, battery resistance, etc.

[0051] (E) Binder Examples of the (E) binder include binders typically used for positive electrodes, such as polyvinylidene fluoride (hereinafter also referred to as "PVDF"), vinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, and polyacrylonitrile, which can be used alone or in combination. One type of binder may be used, or two or more types may be combined. When the binder is PVDF (polyvinylidene fluoride resin), the (C) solvent is preferably N-methylpyrrolidone (NMP), which exhibits high solubility for the binder.

[0052] The content of the binder in the positive electrode slurry of the present invention is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.5% by mass or more, from the viewpoint of the coating properties of the composite layer and the binding property with the current collector, and is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1.5% by mass or less, from the viewpoint of the energy density of the battery. When two or more binders are used in combination, the content of the binder is the total content thereof.

[0053] The content of the binder in the total solid content of the positive electrode slurry of the present invention is preferably 0.07% by mass or more, more preferably 0.15% by mass or more, and even more preferably 0.7% by mass or more, from the viewpoint of the coating properties of the mixture layer and the binding property with the current collector, and is preferably 6% by mass or less, more preferably 4% by mass or less, and even more preferably 2% by mass or less, from the viewpoint of the energy density of the battery.

[0054] [Positive electrode slurry manufacturing method] The positive electrode slurry of the present invention can be obtained by first preparing the conductive pigment dispersion described above, and then blending the dispersion with at least one electrode active material (D) and a binder (E). Alternatively, the positive electrode slurry of the present invention may be prepared by mixing the components (A), (B), and (C) described above with the electrode active material (D) and the binder (E).

[0055] The solid content of (A-1) carbon nanotubes in the solid content of the positive electrode slurry of the present invention is usually 0.01 mass% or more, preferably 0.05 mass% or more, more preferably 0.1 mass% or more, and is usually 10 mass% or less, preferably 3.0 mass% or less, more preferably 1.0 mass% or less, which is suitable from the viewpoint of battery performance. Also, the content of (C) solvent in the positive electrode slurry of the present invention is usually 1 mass% or more, preferably 5 mass% or more, more preferably 10 mass% or more, and is usually 70 mass% or less, preferably 60 mass% or less, more preferably 50 mass% or less, which is suitable from the viewpoint of electrode drying efficiency and slurry viscosity.

[0056] The solid content of the dispersant (B) in the solid content of the positive electrode slurry of the present invention is generally 0.01% by mass or more, preferably 0.02% by mass or more, and generally 9.0% by mass or less, preferably 2.7% by mass or less, and more preferably 0.9% by mass or less, which is suitable from the viewpoints of battery performance, paste viscosity, etc.

[0057] [Manufacturing method for lithium-ion battery electrodes] As described above, the electrode composite layer (also referred to as the electrode layer or composite layer) of a lithium-ion secondary battery can be produced by applying a lithium-ion battery positive electrode slurry to the surface of a positive electrode core material and drying the coating. The conductive pigment dispersion of the present invention can be used not only as a paste for the positive electrode layer, but also as a primer layer between the electrode core material and the composite layer. The lithium-ion battery positive electrode slurry can be applied by a known method using a die coater or the like. The amount of application of the lithium-ion battery positive electrode slurry is not particularly limited, but can be set so that the thickness of the dried positive electrode layer is, for example, 0.04 mm or more, preferably 0.06 mm or more, and, for example, 0.30 mm or less, preferably 0.24 mm or less. The temperature of the drying step can be appropriately set within a range of, for example, 80°C or more, preferably 100°C or more, and, for example, 200°C or less, preferably 180°C or less. The drying step time can be appropriately set within a range of, for example, 5 seconds or more, for example, 120 seconds or less, preferably 60 seconds or less.

[0058] In the drying step, all or part of the (B) dispersant and (C) solvent volatilize. As described above, it is preferable to recover and reuse the volatilized components (B) and (C) in order to reduce waste, be environmentally friendly, and / or reduce costs. [Example]

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0060] <Examples 1 to 14 and Comparative Examples 1 to 11> The preparation of conductive pigment dispersions in Examples and Comparative Examples will be described. The raw materials used in the conductive pigment dispersions and their blending amounts are shown in Tables 1 to 4. The values ​​in the tables are in mass %. Here, the solid content refers to the components remaining in the thin film when it is formed.

[0061] Details of the raw materials listed in Table 1 are as follows: (A) Conductive pigment (A-1) Carbon nanotubes (A-1-1) Bundle-type carbon nanotubes CNT-1: Average outer diameter: 10-15 nm, fiber length: 60-80 μm, BET specific surface area: 155-195 m 2 / g CNT-2: Average outer diameter: 7-12 nm, fiber length: 100-200 μm, BET specific surface area: 200-250 m 2 / g CNT-3: Average outer diameter: 6-9 nm, fiber length: 100-200 μm, BET specific surface area: 400-600 m 2 / g CNT-4: Average outer diameter: 150 nm, fiber length: approximately 15 μm, BET specific surface area: 5 to 20 m 2 / g (A-1-2) Entangled carbon nanotubes CNT-5: Average outer diameter: 9-10 nm, fiber length: approx. 15 μm, BET specific surface area: 230-280 m 2 / g (A-2) Carbon nanofiber CNF-1: Average outer diameter: 300 nm, fiber length: approximately 15 μm, BET specific surface area: 9 to 10 m 2 / g (A-3) Other conductive pigments ·CB-1: Average particle diameter: 35nm, BET specific surface area: 100~200m 2 / g ·CB-2: Average particle diameter: 50nm, BET specific surface area: 50m 2 / g (B) Dispersant Dispersant-1: A composition containing a comb copolymer described in JP-A-2022-524181 (solid content 16% by mass) Dispersant 2: KF Polymer #1100 (Kureha Corporation, polyvinylidene fluoride, low molecular weight homopolymer) Dispersant-3: PVP K-30 (Fujifilm Wako Pure Chemical Industries, Ltd., polyvinylpyrrolidone) Dispersant-4: PVP K-90 (Fujifilm Wako Pure Chemical Industries, Ltd., polyvinylpyrrolidone) (C) Solvent NMP: N-methyl-2-pyrrolidone (Fujifilm Wako Pure Chemical Industries, Ltd., special grade)

[0062] Example 1 95 parts by mass of NMP and 2 parts by mass of Dispersant-1 were placed in a stainless steel container and stirred with a Disper to make the mixture uniform, after which 3 parts by mass of CNT-1 were added and stirred with a Disper for 20 minutes (blade diameter: 1 / 2 the container diameter, rotation speed: 1,000 rpm) to obtain Pre-Dispersion 1. The obtained Pre-Dispersion 1 was further dispersed using a high-pressure homogenizer (Starburst LABO, manufactured by Sugino Machine Ltd.) under the following conditions to obtain Conductive Pigment Dispersion 1 of Example 1. <High-pressure homogenizer conditions> Chamber: Single nozzle chamber Nozzle diameter: 0.20 mm Discharge pressure: 70MPa Number of passes: 5 passes

[0063] (Examples 2 to 6, Comparative Examples 1 to 6) Conductive pigment dispersions 2 to 6 and 15 to 20 of Examples 2 to 6 and Comparative Examples 1 to 6 were prepared in the same manner as in Example 1, according to the formulations and dispersion conditions shown in Tables 1 and 3. Note that, for Example 6 only, the main dispersion was carried out with the discharge pressure of the high-pressure homogenizer set to 150 MPa.

[0064] Example 7 A stainless steel container was charged with 92 parts by weight of NMP and 5 parts by weight of Dispersant-1, and after stirring with a Disperser to achieve uniformity, 2.4 parts by weight of CNT-2 and 0.6 parts by weight of CNT-5 were added and stirred with a Disperser for 5 minutes (blade diameter: 1 / 2 the container diameter, rotation speed: 1,000 rpm). A Multi-Damatri 1 / 2S (Yoshida Kikai Kogyo Co., Ltd.) equipped with a single 1.2 mm nozzle was then used for 30 minutes of circulation treatment at a throughput of 5,000 mL / min to obtain Pre-Dispersion 7. The resulting Pre-Dispersion 7 was further dispersed using a high-pressure homogenizer (Starburst LABO, Sugino Machine Co., Ltd.) under the following conditions to obtain Conductive Pigment Dispersion 7 of Example 7. <High-pressure homogenizer conditions> Chamber: Single nozzle chamber Nozzle diameter: 0.25 mm Discharge pressure: 70MPa Number of passes: 10 passes

[0065] (Examples 8 to 13) Conductive pigment dispersions 8 to 13 of Examples 8 to 13 were prepared in the same manner as in Example 7 according to the formulation and dispersion conditions shown in Table 2.

[0066] Example 14 89.33 parts by mass of NMP and 6.67 parts by mass of Dispersant-1 were added to a stainless steel container and stirred with a disperser until uniform, after which 4 parts by mass of CNT-5 were added and stirred for 5 minutes with a disperser (blade diameter: 1 / 2 the container diameter, rotation speed: 1,000 rpm) to obtain pre-dispersion 14. The obtained pre-dispersion 14 was further dispersed using a bead mill to obtain conductive pigment dispersion 14 of Example 14. <Bead mill conditions> Beads type: Zirconia beads Bead diameter: 2.00mm Dispersion time: 8.5 hours

[0067] (Comparative Examples 7 and 8) Conductive pigment dispersions 21 and 22 of Comparative Examples 7 and 8 were prepared in the same manner as in Example 14 according to the formulation and dispersion conditions shown in Table 4.

[0068] (Comparative Example 9) 92 parts by mass of NMP and 5 parts by mass of Dispersant-1 were added to a stainless steel container and stirred with a Disper to make the mixture uniform, after which 3 parts by mass of CNT-5 were added and stirred with a Disper for 60 minutes (blade diameter: 1 / 2 the diameter of the container, rotation speed: 1,000 rpm) to obtain Pre-Dispersion 23. The obtained Pre-Dispersion 23 was further dispersed using a high-pressure homogenizer (Starburst LABO, manufactured by Sugino Machine Co., Ltd.) under the conditions below, but the nozzle clogged on the first pass and the desired dispersion was not obtained, so further evaluation was not carried out. <High-pressure homogenizer conditions> Chamber: Single nozzle chamber Nozzle diameter: 0.25 mm Discharge pressure: 70MPa Number of passes: 1 pass

[0069] (Comparative Example 10) 92 parts by mass of NMP, 5 parts by mass of Dispersant-1, 3 parts by mass of CNT-2, and 300 parts by mass of zirconia beads with a diameter of 2.0 mm were placed in a glass bottle, and a dispersion treatment was carried out for 2.5 hours using a paint shaker, thereby obtaining conductive pigment dispersion 24 of Comparative Example 10.

[0070] (Comparative Example 11) Conductive pigment dispersion 25 of Comparative Example 11 was prepared in the same manner as in Comparative Example 11 according to the formulation and dispersion conditions shown in Table 4.

[0071] <Evaluation of the properties of conductive pigment dispersions and positive electrode layers using conductive pigment dispersions> The conductive pigment dispersions obtained in the above Examples and Comparative Examples were evaluated as follows. The evaluation results are shown in Tables 1 to 4. The viscosities V1 and V2 at each shear rate were measured using a rheometer (such as the MCR rheometer manufactured by Anton Parr) after adjusting the liquid temperature to 23°C.

[0072] (Viscosity of conductive pigment dispersion) Shear rate of each conductive pigment dispersion: 1.0 s -1 The viscosity V1 in each sample was evaluated according to the following criteria: The conductive pigment dispersions (Examples 9 to 14, Comparative Example 9) containing 2.5 parts by mass or more of (A-1-2) entangled carbon nanotubes per 100 parts by mass of the conductive pigment dispersion were separately evaluated according to the following criteria. <Evaluation criteria> ◎: Less than 10,000 mPa·s ○: 10,000 mPa·s or more, less than 20,000 mPa·s ×:20,000mPa·s or more <Evaluation criteria: When containing 2.5 parts by mass or more of entangled carbon nanotubes> ◎: Less than 50,000 mPa·s ○: 50,000 mPa·s or more, less than 150,000 mPa·s ×: 150,000mPa·s or more

[0073] (Preparation of Positive Electrode Slurry) Capacity 150cm 3 To this plastic container, 98.5 parts by mass of NMC811-S800 (a high-nickel active material manufactured by Yonghye Technology Co., Ltd.) as (D) electrode active material, 20 parts by mass of the conductive pigment dispersion 1 of Example 1, and 11.25 parts by mass of an 8% NMP solution of PVDF (Solef 5130, manufactured by Solvay, polyvinylidene fluoride) as (E) binder were added, and then mixed using a spatula until the powder was uniform. The mixture was then stirred at 2,000 rpm for 60 seconds using a planetary centrifugal mixer (Thinky Corporation, Awatori Rentaro, ARE-310). An appropriate amount of N-methyl-2-pyrrolidone was then added to achieve a viscosity suitable for coating. The mixture in the plastic container was mixed using a spatula until uniform, and then stirred again using a planetary centrifugal mixer at 2,000 rpm for 60 seconds to obtain positive electrode slurry 1. For conductive pigment dispersions 2 to 25 in each of the Examples and Comparative Examples, positive electrode slurries 2 to 25 were prepared in the same manner as for conductive pigment dispersion 1 in Example 1, so that the positive electrode slurry solid content contained 98.5 parts by mass of the (D) electrode active material, 0.6 parts by mass of the (A) conductive pigment, and 0.9 parts by mass of the (E) binder.

[0074] (Drying property when applying positive electrode slurry) Each of the prepared positive electrode slurries was evaluated according to the following criteria. <Evaluation criteria> ◯: The positive electrode slurry has a high solid content and is excellent in drying properties when applied. ×: The solid content of the positive electrode slurry is low, and the drying property when the positive electrode slurry is applied is insufficient.

[0075] (Conductivity in the positive electrode layer) Each of the prepared positive electrode slurries was applied to an OHP sheet using a 10-mil applicator, and the coating was then dried in an electric oven at 120°C ± 5°C for 25 minutes. The dried coating was then pressed in a press at 50,000 kg / cm. 2After applying a pressure of 1000 kJ / cm to form the positive electrode layer, the surface resistance (Ω / □) of the positive electrode layer was measured using a Loresta GP MCP-T600 manufactured by Mitsubishi Chemical Analytech Co., Ltd. After the measurement, the volume resistivity (Ω·cm) of the positive electrode layer was determined by multiplying it by the thickness of the positive electrode layer formed on the OHP sheet. The thickness of the positive electrode layer was calculated by subtracting the thickness of the OHP sheet from the average value measured at three points in the electrode film using a film thickness meter (NIKON DIGIMICRO MH-15M). The conductivity of each positive electrode layer was evaluated according to the following criteria. <Evaluation criteria> ◎: Less than 30Ω·cm ○: 30 Ω·cm or more, less than 100 Ω·cm ×: 100Ω cm or more

[0076] Therefore, it is clear that the manufacturing method of the present invention can produce a conductive pigment dispersion that contains CNTs at a high concentration, has low viscosity, and is easy to handle, regardless of whether bundled or entangled carbon nanotubes are used.Furthermore, it is clear that the conductive pigment dispersion obtained by the present invention can be mixed with a positive electrode active material of a lithium ion secondary battery to produce a positive electrode mixture.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] [Table 4]

Claims

1. A conductive pigment dispersion liquid containing at least (A) a conductive pigment, (B) a dispersant, and (C) a solvent, wherein the conductive pigment (A) contains at least (A-1) carbon nanotubes, and the conductive pigment dispersion liquid contains 2.5 to 10.0 parts by mass of the (A-1) carbon nanotubes per 100 parts by mass of the conductive pigment dispersion liquid, and the conductive pigment dispersion liquid is dispersed at a shear rate of 1.0 s －１ 1. A conductive pigment dispersion, characterized in that the viscosity (23°C) at

2. 2. The conductive pigment dispersion according to claim 1, wherein the carbon nanotubes (A-1) have an outer diameter of 1 to 20 nm.

3. A conductive pigment dispersion liquid containing at least (A) a conductive pigment, (B) a dispersant, and (C) a solvent, wherein the conductive pigment (A) contains at least (A-1) carbon nanotubes, and the (A-1) carbon nanotubes are (A-1-2) entangled carbon nanotubes, and the conductive pigment dispersion liquid contains 2.5 to 10.0 parts by mass of the (A-1-2) entangled carbon nanotubes per 100 parts by mass of the conductive pigment dispersion liquid, and the conductive pigment dispersion liquid is dispersed at a shear rate of 1.0 s －１ 1. A conductive pigment dispersion, characterized in that the viscosity (23°C) at

4. 4. The conductive pigment dispersion according to claim 3, wherein the entangled carbon nanotubes (A-1-2) have an outer diameter of 1 to 15 nm.

5. 4. The conductive pigment dispersion according to claim 1, wherein the solid content of the dispersant (B) is 10 to 90 parts by mass relative to 100 parts by mass of the conductive pigment (A).

6. 4. The conductive pigment dispersion according to claim 1, wherein the carbon nanotubes (A-1) contain at least two or more types of carbon nanotubes having different fiber lengths.

7. 4. The conductive pigment dispersion according to claim 1, wherein the conductive pigment (A) further contains (A-2) carbon nanofibers.

8. A positive electrode slurry composition comprising at least the conductive pigment dispersion according to claim 1 or 3, (D) an electrode active material, and (E) a binder.

9. An electrode layer for a battery obtained by coating a current collector with the positive electrode slurry composition according to claim 8 .

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