Carbon black dispersion composition for battery, mixture paste for positive electrode, positive electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

To provide a carbon black dispersion composition for battery that is enhanced in dispersibility of carbon black, and also continues to maintain the dispersibility right after dispersion and also includes N-methyl-2-pyrrolidone with superior storage stability as a dispersant without taking much trouble and time.SOLUTION: There is provided a carbon black dispersion composition for battery that includes carbon black, methyl cellulose and N-methyl-2-pyrrolidone, wherein the polydispersity obtained through absolute molecular weight measurement using a combination of size exclusive chromatography (SEC) and multi-angle light scattering of the methyl cellulose is 1.9 or less, and the viscosity of a 2 mass% solution of the methyl cellulose at 20°C is 3 to 30 mPa s.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a carbon black dispersion composition for a battery, a composite paste for a positive electrode, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

Background Art

[0002] Lithium ion secondary batteries are characterized by high energy density, and the market has expanded due to the rapid spread of mobile terminals such as mobile phones and notebook computers. Along with this, the performance has also improved dramatically. In recent years, as part of the movement towards a sustainable society, the electrification of automobiles and the improvement of the efficiency of power storage systems have been promoted, and the market for lithium ion secondary batteries is expected to continue to expand in the future.

[0003] The positive electrode of a lithium ion secondary battery mainly includes a positive electrode active material, a conductive assistant, a binder, and a current collector. Since the capacity of a lithium ion secondary battery is mainly determined by the amount of the active material, it is preferable to reduce as much as possible the materials other than the active material such as the conductive assistant in order to increase the capacity of the battery. The conductive assistant is being developed to have a higher specific surface area and a higher structure so that high conductivity can be imparted with a smaller addition amount. However, due to the increase in the cohesiveness of the conductive assistant due to the increase in specific surface area and structure, it becomes difficult to achieve uniform dispersion, and as a result, there is a risk of an increase in internal resistance due to non-uniformity of the conductive path in the electrode and a shortening of the battery life due to uneven load distribution.

[0004] Therefore, in recent years, as a method for preventing the aggregation of the conductive assistant and creating a uniformly dispersed electrode, a method has generally been used in which a polymer dispersant is used to uniformly disperse the conductive assistant in a dispersion medium such as an organic solvent in advance to prepare a conductive assistant dispersion liquid, and the obtained conductive assistant dispersion liquid is mixed with a positive electrode active material and a binder to prepare a positive electrode composite slurry.

[0005] In the above conductive auxiliary agent dispersion liquid, it is required that the conductive auxiliary agent is in a uniform and good dispersion state, and in addition, the dispersion state and the viscosity of the dispersion liquid of the conductive auxiliary agent do not change and are stable during the storage period from after the production of the dispersion liquid to the preparation of the electrode composite material slurry.

[0006] This is because when the conductive auxiliary agent is poorly dispersed and aggregated, the viscosity of the dispersion liquid increases. Therefore, the coating property of the positive electrode composite material slurry produced using this becomes poor, and a smooth coating film of the composite material cannot be obtained. As a result, it leads to deterioration in performance in combination with the non-uniformity of the internal conductive path of the electrode described above.

[0007] Also, regarding the dispersion storage stability, the change over time in the dispersion state of the conductive auxiliary agent is reflected. If the change in the dispersion state due to storage is large, the variation in quality in the production of the positive electrode increases.

[0008] Generally, carbon black is used as the conductive auxiliary agent. As a method for improving the dispersibility and storage stability of a dispersion liquid using carbon black as the conductive auxiliary agent, for example, a dispersion liquid in which an acidic compound such as carboxylic acid is added as a further additive is known (Japanese Patent Application Laid-Open No. 2016-046188 (Patent Document 1)).

[0009] Also, as a method that does not use an additional additive in the conductive auxiliary agent dispersion liquid, a dispersion liquid in which the dispersibility and storage stability are improved by controlling the viscosity characteristics of the dispersion liquid within a specific range is known (Japanese Patent Application Laid-Open No. 2020-021632 (Patent Document 2)).

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0011] Patent Document 1 does not explain the mechanism by which acidic compounds act on the dispersibility of carbon black, but it is presumed that the dispersibility of carbon black is enhanced by controlling the pH environment on the surface of carbon black particles. However, since there is a concern that acidic compounds act as a factor of chemical modification on the composition of the electrode binder, the addition to the carbon black dispersion was not desirable. Also, in the method described in Patent Document 2, since it is required to continue the dispersion treatment until a dispersion having the target viscosity characteristics is obtained, a large amount of labor and time are often required, and it may be difficult to meet the recent demands of the battery market for cost reduction in production and increase in production volume.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a carbon black dispersion composition for a battery using N-methyl-2-pyrrolidone as a dispersion medium, which enhances the dispersibility of carbon black, maintains the dispersibility immediately after dispersion, and has excellent storage stability without consuming much labor and time, a binder paste for a positive electrode using the carbon black dispersion composition for a battery, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

Means for Solving the Problems

[0013] The present inventors diligently studied to achieve the above object, and various attempts were made to obtain a dispersant that acts on improving the dispersibility of carbon black and the storage stability of the dispersion in N-methyl-2-pyrrolidone from among numerous polymers. Among them, attention was focused on cellulose derivatives, and repeated trials and errors were made on the application of various cellulose derivatives.

[0014] As a result, surprisingly, among numerous cellulose derivatives, when methyl cellulose with a polydispersity of 1.9 or less as measured by absolute molecular weight measurement using a combination of size exclusion chromatography (SEC) and multi-angle light scattering (MALS) and a viscosity of 3 to 30 mPa·s for a 2% by mass aqueous solution at 20°C is used as a dispersant, the dispersibility and storage stability of the obtained carbon black are particularly good.

[0015] That is, the present invention provides a carbon black dispersion composition for a battery, a positive electrode composite paste, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery as described below. 1. A carbon black dispersion composition for a battery containing carbon black, methyl cellulose, and N-methyl-2-pyrrolidone, wherein the polydispersity of the methyl cellulose as measured by absolute molecular weight measurement using a combination of size exclusion chromatography (SEC) and multi-angle light scattering (MALS) is 1.9 or less, and the viscosity of a 2% by mass aqueous solution of the methyl cellulose at 20°C is 3 to 30 mPa·s. 2. The carbon black dispersion composition for a battery according to 1, wherein the content of the carbon black is 5 to 20% by mass. 3. The carbon black dispersion composition for a battery according to 1 or 2, wherein the BET specific surface area of the carbon black is 30 to 1500 m 2 / g. 4. The carbon black dispersion composition for a battery according to any one of 1 to 3, wherein the addition amount of the methyl cellulose is 1 to 20 parts by mass with respect to 100 parts by mass of the carbon black. 5. A positive electrode composite paste containing the carbon black dispersion composition for a battery according to any one of 1 to 4, a positive electrode active material, and a binder. 6. A positive electrode for a lithium ion secondary battery including a current collector and a positive electrode composite layer which is a coated and dried film of the positive electrode composite paste according to 5 formed on the current collector. 7. A lithium-ion secondary battery comprising the positive electrode, negative electrode, electrolyte, and separator for a lithium-ion secondary battery according to 6.

Advantages of the Invention

[0016] According to the present invention, it becomes possible to provide a carbon black dispersion composition for a battery having excellent dispersibility and storage stability of carbon black. Further, by using the carbon black dispersion composition for a battery, it is possible to expect a reduction in the defect rate and a reduction in production cost in the manufacture of a lithium-ion secondary battery.

Embodiments for Carrying Out the Invention

[0017] [Carbon Black Dispersion Composition for Battery] Hereinafter, the carbon black dispersion composition for a battery according to the present invention will be described. The carbon black dispersion composition for a battery according to the present invention is a carbon black dispersion composition for a battery containing carbon black, methylcellulose, and N-methyl-2-pyrrolidone, and has a polydispersity of 1.9 or less as measured by absolute molecular weight measurement by a combination of size exclusion chromatography (SEC) and multi-angle light scattering (MALS) of the methylcellulose, and a viscosity of 3 to 30 mPa·s of a 2 mass% aqueous solution of the methylcellulose at 20°C.

[0018] (Carbon Black) Carbon black is added as a conductive aid for enhancing the electrical conductivity of an electrode (specifically, a positive electrode in a lithium-ion secondary battery described later) formed of the carbon black dispersion composition for a battery, and examples thereof include acetylene black, furnace black, thermal black, ketjen black, and the like. Among them, acetylene black is preferable from the viewpoints of conductivity and dispersibility.

[0019] The BET specific surface area (specific surface area measured by the BET method) of the carbon black is 30 to 1500 m 2 / g, preferably 40 to 600 m 2 / g, more preferably 50 to 300 m 2 / g, particularly preferably.

[0020] From the viewpoints of the fluidity and operability of the dispersion composition to be prepared, the content of carbon black is preferably 5 to 20% by mass, more preferably 9 to 19% by mass (that is, preferably 5 to 20 parts by mass, more preferably 9 to 19 parts by mass, based on 100 parts by mass of the total amount of the carbon black dispersion composition for the battery (preferably the total of carbon black, methylcellulose and N-methyl-2-pyrrolidone)).

[0021] In addition, other carbon materials such as carbon nanotubes, graphene, graphite, etc. may be used in combination as a conductive aid together with the carbon black. When used in combination, the addition amount of other carbon materials is preferably 5 to 20 parts by mass, more preferably 9 to 19 parts by mass, based on 100 parts by mass of the total amount of the carbon black dispersion composition for the battery (preferably the total of carbon black, methylcellulose and N-methyl-2-pyrrolidone).

[0022] (Methylcellulose) The methylcellulose used in the present invention is used as a dispersant for improving the dispersibility of carbon black.

[0023] The degree of substitution (DS) of the methoxy group in methylcellulose is preferably 1.60 to 2.10, more preferably 1.70 to 2.00. The degree of substitution (DS) of the methoxy group in methylcellulose can be obtained by converting the value measured by the method for analyzing the degree of substitution of methylcellulose in the 18th revised Japanese Pharmacopoeia (the same applies in the following examples). The viscosity of a 2% by mass aqueous solution of methyl cellulose at 20°C is preferably 3.0 to 30.0 mPa·s, more preferably 3.5 to 20.0 mPa·s, from the viewpoints of the dispersibility and storage stability of carbon black. The viscosity of a 2% by mass aqueous solution of methyl cellulose at 20°C can be the measured viscosity value of a 2% by mass aqueous solution at 20°C using an Ubbelohde viscometer specified in JIS K2283-1993 (the same applies in the following examples).

[0024] The polydispersity by absolute molecular weight measurement in methyl cellulose is 1.9 or less, preferably 1.0 to 1.80, 1.40 to 1.80, from the viewpoints of the dispersibility and storage stability of carbon black. The polydispersity is defined as the ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn), and represents the spread of molecular weights (molecular weight distribution). The smaller the value of Mw / Mn, the narrower the molecular weight distribution, and the larger the value of Mw / Mn, the wider the molecular weight distribution. The weight average molecular weight (Mw) and the number average molecular weight (Mn) can be measured by an absolute molecular weight measurement method (Tablet s&Capsules, 14-20, July, 2007) using a combination of size exclusion chromatography (SEC) and multi-angle light scattering (MALS).

[0025] The addition amount of methyl cellulose is preferably 1 to 20 parts by mass, more preferably 3 to 15 parts by mass, per 100 parts by mass of carbon black in the carbon black dispersion composition for battery, from the viewpoints of the dispersibility of carbon black and the electrical properties of the electrode formed of the carbon black dispersion composition for battery. Also, the content of the dispersant is preferably 0.05 to 4% by mass, more preferably 0.15 to 3% by mass, from the viewpoints of the dispersibility of carbon black and the electrical properties of the electrode formed of the carbon black dispersion composition for battery (that is, preferably 0.05 to 4 parts by mass, more preferably 0.15 to 3 parts by mass, per 100 parts by mass of the total amount of the carbon black dispersion composition for battery (preferably the total of carbon black, methyl cellulose and N-methyl-2-pyrrolidone)).

[0026] As long as methyl cellulose with a polydispersity of 1.9 or less by the above absolute molecular weight measurement and a viscosity of 3 to 30 mPa·s in a 2 mass% aqueous solution at 20°C can be obtained, the production method thereof is not particularly limited. Such methyl cellulose can be produced, for example, by a method for producing methyl cellulose (hereinafter, also referred to as "Production Method 1 of Methyl Cellulose") including at least a step (S11) of obtaining alkali cellulose by bringing pulp into contact with an alkali metal hydroxide solution, a step (S12) of supplying oxygen to the alkali cellulose and subjecting it to a depolymerization reaction treatment, a step (S13) of obtaining a reaction product by an etherification reaction between the alkali cellulose subjected to the depolymerization reaction treatment and a methylating agent, a step (S14) of obtaining washed methyl cellulose by washing the reaction product, and a step (S15) of obtaining methyl cellulose by drying and pulverizing the washed methyl cellulose. Also, it can be produced by a method for producing methyl cellulose (hereinafter, also referred to as "Production Method 2 of Methyl Cellulose") including at least a step (S21) of obtaining alkali cellulose by bringing pulp into contact with an alkali metal hydroxide solution, a step (S22) of obtaining a reaction product by an etherification reaction between the alkali cellulose and a methylating agent (that is, adding a methylating agent and performing an etherification reaction without performing a depolymerization reaction treatment by supplying oxygen to the obtained alkali cellulose, which is performed in Production Method 1 of Methyl Cellulose), a step (S23) of obtaining washed methyl cellulose by washing the reaction product, a step (S24) of obtaining methyl cellulose before depolymerization by drying and pulverizing the washed methyl cellulose, a step (S25) of subjecting the methyl cellulose before depolymerization to a depolymerization reaction treatment with an acid to obtain depolymerized methyl cellulose, and a step (S26) of subjecting the depolymerized methyl cellulose to dialysis treatment to obtain methyl cellulose. Hereinafter, the details of the above-exemplified methods will be described.

[0027] [Production Method 1 of Methyl Cellulose] First, Production Method 1 of methyl cellulose will be described. (S11) By bringing pulp into contact with an alkali metal hydroxide solution, alkali cellulose is obtained.

[0028] Examples of the pulp include cellulose pulps such as wood pulp and linter pulp.

[0029] In addition to the main component cellulose, the solid components in the pulp contain very small amounts of organic substances such as low-degree-polymerization cellulose, hemicellulose, lignin, and resin components, and inorganic substances such as Si components and Fe components. In the case of commercially available pulp obtained by cooking and bleaching wood, since the content of low-degree-polymerization cellulose is very small, the cellulose content of the solid components in the pulp can be treated as being substantially the same as the alpha-cellulose content.

[0030] The solid components in the pulp can be calculated by the dry matter content determined by the test method for pulp - dry matter content in JIS P8203:1998. The dry matter content is the ratio of the mass when the sample is dried at 105 ± 2 °C until a constant weight is reached to the mass before drying, and is expressed in mass%.

[0031] From the viewpoint of reducing the polydispersity (Mw / Mn) by measuring the absolute molecular weight of methyl cellulose, the intrinsic viscosity, which is an index of the degree of polymerization of the pulp, is preferably less than 800 ml / g, more preferably less than 600 ml / g. The lower limit of the intrinsic viscosity is preferably 200 ml / g. The intrinsic viscosity can be measured by the viscosity measurement method described in JIS P8215.

[0032] Examples of the alkali metal hydroxide solution include aqueous solutions of alkali metal hydroxides such as aqueous sodium hydroxide solution and aqueous potassium hydroxide solution.

[0033] From the viewpoints of economy and ease of handling, the concentration of the alkali metal hydroxide in the alkali metal hydroxide solution is preferably 10 to 60% by mass.

[0034] The amount of the alkali metal hydroxide solution used can be appropriately set according to the DS of the methoxy group of the methyl cellulose to be produced and the concentration of the alkali metal hydroxide in the alkali metal hydroxide solution. However, it is preferably 0.01 to 2.0 parts by mass, more preferably 0.5 to 1.5 parts by mass, of the alkali metal hydroxide with respect to 1.00 part by mass of the unit mass of the pulp.

[0035] The contact between the pulp and the alkali metal hydroxide solution can be carried out using a reactor such as a reactor equipped with an internal stirrer.

[0036] (S12) Next, oxygen is supplied to the obtained alkali cellulose to perform a depolymerization reaction treatment to obtain depolymerized alkali cellulose.

[0037] Oxygen can be supplied, for example, by ventilating air through the reactor.

[0038] The supply of oxygen is carried out continuously or intermittently after the addition of the alkali metal hydroxide solution in the above step is completed. By supplying oxygen after the addition of the alkali metal hydroxide solution is completed in this way, the distribution of the alkali metal hydroxide solution on the pulp becomes uniform, and the depolymerization reaction by oxygen becomes uniform. Although the detailed mechanism has not been elucidated, it is presumed that the variation in molecular weight is suppressed and the polydispersity (Mw / Mn) by absolute molecular weight measurement becomes small due to the uniformity of the depolymerization reaction by oxygen.

[0039] From the viewpoint of reducing the polydispersity (Mw / Mn) by absolute molecular weight measurement of methyl cellulose, the temperature of the depolymerization reaction treatment by oxygen is preferably 30 to 85°C, more preferably 30 to 80°C.

[0040] The time for the depolymerization reaction treatment by oxygen can be appropriately set according to the viscosity of the 2% by mass aqueous solution of the target methyl cellulose at 20°C and the reaction temperature, but it is preferably 10 minutes to 2 hours.

[0041] (S13) Next, a reaction product is obtained by the etherification reaction of the depolymerized alkali cellulose and a methylating agent. Examples of the methylating agent include methyl chloride.

[0042] The amount of the methylating agent used can be appropriately determined according to the degree of substitution (DS) of methyl cellulose, but it is preferably 0.5 to 3.0 parts by mass per 1.0 part by mass of pulp.

[0043] The reaction temperature in the etherification reaction is preferably 40 to 100°C. Also, the reaction time in the etherification reaction is preferably 1 to 5 hours.

[0044] (S14) By washing the reaction product, washed methyl cellulose is obtained.

[0045] The washing can be performed using water. The temperature of the water used for washing is preferably 85°C to 100°C. The water content of the washed methyl cellulose is preferably 25 to 95% by mass from the viewpoints of impurity removal and ease of adjusting the water content in the next step. The water content of the washed methyl cellulose can be measured according to the "Loss on Drying Test Method" of the 18th Revised Japanese Pharmacopoeia.

[0046] (S15) Next, the washed methyl cellulose is dried and pulverized to obtain methyl cellulose. The devices that can be used in the drying process are not limited as long as the target water content can be adjusted. Examples include a hot air dryer. Also, the devices that can be used in the pulverization process are not limited as long as they can pulverize methyl cellulose. Examples include an impact pulverizer and a ball mill.

[0047] [Manufacturing Method 2 of Methyl Cellulose] Next, Manufacturing Method 2 of methyl cellulose will be described. In the second method for producing methyl cellulose, up to the step of obtaining methyl cellulose before depolymerization by drying and pulverizing the washed methyl cellulose in the first method for producing methyl cellulose, the process is the same as the first method for producing methyl cellulose, except that a step of obtaining a reaction product by subjecting the alkali cellulose to an etherification reaction without undergoing a depolymerization reaction treatment with oxygen after the step of obtaining the alkali cellulose is performed. That is, the step (S11) is performed as the step (S21), and then, a methylating agent is added to the obtained alkali cellulose to cause an etherification reaction (S22), and then, the steps (S23) and (S24) are performed in the same manner as the steps (S14) and (S15).

[0048] (S25) Next, the methyl cellulose before depolymerization is subjected to a depolymerization reaction treatment with an acid to obtain depolymerized methyl cellulose.

[0049] The depolymerization with an acid can be carried out according to a conventional method. For example, examples of the acid include hydrogen halides such as hydrogen chloride. The acid may be used as an aqueous solution or the like. For example, hydrogen chloride may be used as hydrochloric acid (aqueous hydrogen chloride solution). The concentration of hydrogen chloride in the aqueous hydrogen chloride solution is preferably 1 to 45% by mass.

[0050] The amount of the acid used is preferably 0.04 to 1 part by mass with respect to 100 parts by mass of the methyl cellulose before depolymerization. The reaction temperature in the depolymerization with an acid is preferably 40 to 85°C. The reaction time in the depolymerization with an acid is preferably 0.1 to 4 hours.

[0051] After the completion of the depolymerization with an acid, the acid may be removed by reducing the pressure in the reactor or the like. Further, if necessary, sodium bicarbonate or the like may be added to neutralize the acid used in the depolymerization.

[0052] (S26) Next, the depolymerized methyl cellulose is subjected to dialysis treatment to obtain methyl cellulose. The dialysis treatment can be carried out by dissolving the depolymerized methylcellulose in pure water to prepare an aqueous solution, filling the aqueous solution into a dialysis tube, immersing the filled dialysis tube in pure water, and drying the content of the tube after immersion to precipitate methylcellulose.

[0053] The fractionation molecular weight of the dialysis tube to be used is preferably 10,000 to 20,000, more preferably 12,000 to 14,000. The length and diameter of the tube are not particularly limited as long as the dialysis treatment can be carried out.

[0054] The immersion time is preferably 30 minutes to 5 days, more preferably 1 to 3 days. It is preferable to immerse in pure water that is 50 times or more the amount of the aqueous solution filled in the tube.

[0055] The drying method is not particularly limited. For example, there is a method of spreading the aqueous solution in the tube in a container and drying the container in a forced-air dryer. The precipitated methylcellulose obtained by drying may be further pulverized. Examples of the pulverizer include a force mill. As described above, methylcellulose having a polydispersity of 1.9 or less by absolute molecular weight measurement and a viscosity of 3 to 30 mPa·s in a 2 mass% aqueous solution at 20°C can be obtained.

[0056] (N-methyl-2-pyrrolidone) N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") is an organic solvent used in the production of the positive electrode of a lithium-ion battery, and is also added as a dispersion medium for carbon black in the carbon black dispersion composition for batteries of the present invention.

[0057] Although there is no particular limitation on the content of NMP, from the viewpoint of the operability of the carbon black dispersion composition for batteries, 75 to 95% by mass is preferable, and 81 to 91% by mass is more preferable (that is, based on 100 parts by mass of the total amount of the carbon black dispersion composition for batteries (preferably, the total of carbon black, methyl cellulose, and N-methyl-2-pyrrolidone), it is preferably 75 to 95 parts by mass, and more preferably 81 to 91 parts by mass).

[0058] In addition, from the viewpoint of improving the affinity of each component (components of the carbon black dispersion liquid for batteries and the paste for positive electrode composite materials), one or more other solvents may be used in combination. Examples of solvents other than N-methyl-2-pyrrolidone include water, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide, N,N-diethylacetamide, 4-acetylmorpholine, and the like. In the present invention, it is preferable to use NMP alone as the solvent.

[0059] (Other components) The carbon black dispersion composition for batteries of the present invention may contain other components within the scope of the object of the present invention or within the scope that does not inhibit its dispersibility and performance when polarized. Examples of other components include surfactants, antifoaming agents, pH adjusters, and viscosity adjusters.

[0060] The method for producing the carbon black dispersion composition of the present invention is not particularly limited as long as the above components are uniformly mixed and the carbon black can be dispersed. For example, it is prepared by a dispersion step of subjecting carbon black, methyl cellulose, NMP, etc. to a dispersion treatment using a dispersion device. At this time, carbon black and methyl cellulose, which are powder raw materials, may be premixed and then this mixture may be dispersed in NMP, or the dispersant may be dissolved in a solvent such as NMP in advance and then mixed with carbon black.

[0061] Examples of the dispersion device to be used include a homogenizer, a homodisper, a planetary mixer, a paint conditioner, a bead mill, and a thin-film swirling high-speed mixer.

[0062] In the above dispersion step, from the viewpoint of further improving the dispersibility of carbon black, the dispersion step may be carried out stepwise using a plurality of dispersion devices. As an example, after uniformly mixing the powder raw materials (carbon black, dispersant, hindered phenol compound) and the dispersion medium (NMP) with a homodisper or a planetary mixer, a method of dispersing carbon black as fine particles with a bead mill or a thin-film swirling high-speed mixer can be mentioned.

[0063] [Composite paste for positive electrode] The composite paste for positive electrode of the present invention contains the above-described carbon black dispersion composition for battery of the present invention, a positive electrode active material, and a binder, and can be used for manufacturing a positive electrode for a lithium-ion secondary battery.

[0064] The positive electrode active material is not particularly limited as long as it is a material used as a positive electrode of a lithium-ion secondary battery, but a transition metal oxide containing a lithium element, or a transition metal in which a part of the transition metal elements contained in the transition metal oxide is substituted with a different element can be used. Specifically, lithium cobaltate, lithium nickelate, lithium manganate, ternary (NCM) active materials, and NCA active materials correspond to this. Further, unlike the above-described oxide-based active materials, polyanion-based positive electrode active materials in which the ionic property of the transition metal is enhanced by introducing polyvalent anions such as phosphate ions (PO 4 3- ) and silicate ions (SiO 4 4- ) can also be used. Specifically, lithium iron phosphate, lithium iron silicate, etc. can be mentioned.

[0065] The content of the positive electrode active material is 35 to 80% by mass, preferably 40 to 70% by mass (35 to 80 parts by mass, preferably 40 to 70 parts by mass with respect to 100 parts by mass of the composite paste for positive electrode). If the content of the positive electrode active material is less than 35% by mass, the energy density when assembled into a battery becomes poor, and if it exceeds 80% by mass, the composite paste for positive electrode becomes hard, and there is a risk that the paste coating process on the current collector becomes difficult.

[0066] As the binder, a fluorine-based polymer material is preferred from the viewpoint of the durability of the formed positive electrode composite material layer, and examples thereof include polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene. Further, from the viewpoints of the binding property and coatability of the composite material paste for the positive electrode, the weight average molecular weight of the polymer material used as the binder is preferably 200,000 to 1,200,000, and more preferably 600,000 to 1,000,000. The weight average molecular weight of the polymer material used as the binder can be measured by a known method such as gel permeation chromatography.

[0067] The content of the binder is 0.3 to 10% by mass, preferably 0.5 to 6% by mass (that is, 0.3 to 10 parts by mass and preferably 0.5 to 6 parts by mass with respect to 100 parts by mass of the composite material paste for the positive electrode). If the content of the binder is less than 0.3% by mass, the strength of the positive electrode composite material layer is insufficient, and there is a risk of peeling or cracking of the composite material layer. Further, if it exceeds 10% by mass, the electrical resistance inside the electrode may increase.

[0068] The composite material paste for the positive electrode of the present invention can be prepared by mixing the carbon black dispersion composition for a battery, the positive electrode active material, and the binder of the present invention described above, and the dispersion device shown in the manufacturing method of the carbon black dispersion composition for a battery described above can be used as the mixing device.

[0069] When manufacturing the composite material paste for the positive electrode, a solvent may be added from the viewpoint of viscosity adjustment. As the solvent, the solvent (such as NMP) shown in the carbon black dispersion composition for a battery of the present invention described above can be used. The amount of the solvent (NMP) contained in the composite material paste for the positive electrode at this time (that is, the total of the amount of NMP contained in the carbon black dispersion composition for a battery and the amount of NMP added) is 10 to 70% by mass, preferably 20 to 60% by mass.

[0070] Also, the material charging procedure and the like during mixing are not particularly limited. All materials may be added and mixed simultaneously, or the mixing process may be carried out step by step, such as adding the battery carbon black dispersion composition after mixing the binder, the positive electrode active material, and the solvent and then further mixing. Regarding the binder, from the viewpoint of preventing foreign matters such as nodules due to undissolution from being mixed into the positive electrode composite paste, it is recommended to dissolve it in the solvent used in advance, prepare a solution, and then add it.

[0071] The positive electrode composite paste of the present invention is suitable for manufacturing a positive electrode for a lithium ion secondary battery. Specifically, the positive electrode composite paste is applied onto a current collector with an arbitrary thickness to form a coating film, the coating film is dried, and the obtained coated and dried film is used as a positive electrode composite layer to manufacture a positive electrode for a lithium ion secondary battery.

[0072] Here, as the current collector, metal or alloy film foils such as iron, stainless steel, copper, aluminum, and nickel are used, and aluminum is particularly preferable from the viewpoint of potential stability in the positive electrode. The current collector may be subjected to surface treatment such as carbon coating from the viewpoint of reducing the interfacial resistance.

[0073] The apparatus used for applying the positive electrode composite paste is not particularly limited, and examples include a knife coater, a comma coater, a die coater, and a gravure coater. Further, in order to improve the electrical resistance of the electrode and the binding property of the positive electrode composite layer, the positive electrode composite paste applied to the current collector or the coated and dried film of the positive electrode composite paste may be rolled by a roll press or the like.

[0074] The coating thickness of the positive electrode composite paste is preferably 50 to 1000 μm, and more preferably 100 to 500 μm.

[0075] The coating film of the positive electrode composite paste after coating is dried by a drying furnace. The drying temperature and time at this time are preferably 50 to 180 °C for 0.5 to 1200 minutes, and more preferably 60 to 140 °C for 1 to 600 minutes.

[0076] As a result, a positive electrode for a lithium-ion secondary battery is obtained, which includes a current collector and a positive electrode composite layer that is a coating and drying film of the positive electrode composite paste of the present invention. At this time, the thickness of the positive electrode composite layer is preferably 10 to 800 μm, and more preferably 30 to 400 μm.

[0077] [Lithium-ion secondary battery] The lithium-ion secondary battery of the present invention includes a positive electrode for a lithium-ion secondary battery including the positive electrode composite layer of the present invention described above, a negative electrode, an electrolyte, and a separator, and has a structure in which the electrolyte is impregnated in a sealed state with a separator sandwiched between the positive electrode for a lithium-ion secondary battery and the negative electrode.

[0078] Here, the negative electrode of the lithium-ion secondary battery has a negative electrode composite layer on one or both sides of the negative electrode current collector.

[0079] The negative electrode current collector is a metal or alloy film foil, similar to the current collector in the positive electrode for a lithium-ion secondary battery. From the perspective of potential stability in the negative electrode, copper foil and nickel foil are preferred.

[0080] The negative electrode composite layer contains one or more negative electrode active materials capable of occluding and releasing lithium ions, and may contain a negative electrode binder, a negative electrode conductive aid, and a negative electrode dispersant as required.

[0081] The negative electrode active material is not particularly limited as long as it is a material used as a negative electrode active material for a lithium-ion secondary battery. Specifically, natural graphite and artificial graphite, which are carbon-based negative electrode materials, lithium titanate, which is an oxide-based negative electrode material, nanosilicon, which is an Si-based negative electrode material, silicon alloy, silicon monoxide, etc. can be mentioned.

[0082] As the binder for the negative electrode, for example, any one or more of polymer materials, synthetic rubbers, etc. can be used. Examples of the polymer materials include polyvinylidene fluoride, polyimide, polyamideimide, aramid, polyacrylic acid, lithium polyacrylate, sodium carboxymethyl cellulose, etc. Examples of the synthetic rubbers include styrene-butadiene rubber, fluorine rubber, ethylene propylene diene, etc.

[0083] As the conductive assistant for the negative electrode, for example, any one or more of carbon materials such as acetylene black, ketjen black, graphite, carbon nanotubes, carbon nanofibers, etc. can be used.

[0084] As the dispersant for the negative electrode, for example, any one or more of methyl cellulose, hydroxypropyl methyl cellulose, ethyl cellulose, sodium carboxymethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone, polyurethane can be used.

[0085] The separator electronically insulates the positive electrode and the negative electrode for the lithium-ion secondary battery, prevents current short circuit due to contact between the two electrodes, and allows lithium ions to pass through by impregnating the electrolyte. This separator is formed of, for example, a porous membrane made of synthetic resin or ceramic, and may have a laminated structure in which two or more porous membranes are laminated. Examples of the synthetic resin include polytetrafluoroethylene, polypropylene, polyethylene, etc., and examples of the ceramic include alumina, etc.

[0086] The electrolyte is a composition that mediates ion conduction between the positive electrode and the negative electrode. The electrolyte is prepared, for example, by dissolving a lithium salt such as lithium hexafluorophosphate in a non-aqueous solvent in which ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate are mixed. Also, additives may be added to the electrolyte for the purpose of improving stability, improving safety, and reducing resistance.

[0087] As described above, the carbon black dispersion composition for a battery of the present invention has excellent dispersibility and storage stability of carbon black. By using the carbon black dispersion composition for a battery, it is possible to reduce the defect rate and production cost in the manufacture of a lithium ion secondary battery.

Examples

[0088] Hereinafter, the present invention will be described in detail by way of examples, but the present invention is not limited to the following examples. The BET specific surface area is a measured value by the BET method. The viscosity of a 2 mass% aqueous solution of methyl cellulose at 20°C was measured with an Ubbelohde viscometer specified in JIS K2283-1993.

[0089] The polydispersity (Mw / Mn) (molecular weight) was measured by the following method. <Measurement of polydispersity (Mw / Mn)> For the measurement of the molecular weight, a GPC-MALLS system composed of a pump (LC-20AD, manufactured by Shimadzu Corporation), a degasser (DGU-20A3R, manufactured by Shimadzu Corporation), an autosampler (SIL-20A, manufactured by Shimadzu Corporation), a column oven (CTO-20A, manufactured by Shimadzu Corporation), a guard column (SB-G, manufactured by Shoko Science Co., Ltd.), a size exclusion column (OHpakSB-806MHQ, manufactured by Shoko Science Co., Ltd.), an 18-angle light scattering detector (DAWN M3220, manufactured by Wyatt Technologies) and a differential refractive index measuring instrument (Optilab M1520, manufactured by Wyatt Technologies) was used. In the measurement, first, methyl cellulose (MC) prepared as described later was weighed into a 50 mL screw bottle, and then 0.1 M sodium nitrate salt (NaNO 3) 20 mL of buffer solution was added and dissolved by stirring at room temperature for 1 hour. Then, it was stirred for 1 hour under ice cooling to completely dissolve MC. Thereafter, it was left to stand at room temperature for 30 minutes to return the solution to room temperature. The concentration of the solution varied depending on the viscosity of a 2 mass% aqueous solution of MC at 20°C, and was adjusted to be 0.30 mass% when it was 3 mPa·s or more and less than 6 mPa·s, and 0.20 mass% when it was 6 mPa·s or more and 30 mPa·s or less. The prepared solution was filtered using a 0.45 μm membrane filter (DIMIC-13CP, manufactured by Advantec). As a standard product, 5 mg of pullulan (P50, manufactured by Shoko Science Co., Ltd.) was added to 0.1 M sodium nitrate salt (NaNO 3 ) 2 mL of buffer solution was added and dissolved, and then filtered using a 0.20 μm membrane filter (DIMIC-13CP, manufactured by Advantec). The molecular weights of the standard pullulan solution and the MC solution were measured under the measurement conditions of using 0.1 M sodium nitrate salt (NaNO 3 ) buffer solution as the mobile phase, a flow rate of 1.0 mL / min, a column temperature of 40°C, a differential refractive index detector temperature of 25°C, an injection volume of 200 μL, and a measurement time of 20 minutes. After the measurement, analysis was carried out by the following method using ASTRA (version 7.3.2) as analysis software. The dn / dc value of MC used was 0.139 mL / g. The peak range was selected from the obtained light scattering chromatogram, and Detector numbers 5 to 16 of the light scattering detector were used. The molecular weight was calculated by AUTOFitting using the extrapolation line method (primary), and the polydispersity (Mw / Mn) was measured. The polydispersity measurement result of MC was normalized using the measurement result of pullulan measured as a standard product.

[0090] <Preparation of Methylcellulose> (Synthesis Example 1: Preparation of MC-1) Wood pulp with an intrinsic viscosity of 370 ml / g was pulverized with a pulverizer to obtain powdered pulp. Among this powdered pulp, 6.0 kg of pulp corresponding to the cellulose content of the solid component in the pulp was charged into a pressure-resistant reactor with an internal stirrer equipped with a jacket, and vacuum nitrogen substitution was performed to sufficiently remove oxygen in the reactor. Next, while stirring, 13.54 kg of a 49% by mass aqueous sodium hydroxide solution was added over 20 minutes. Then, while maintaining the temperature inside the reactor at 80 °C, air was introduced into the reactor at a rate of 10 NL / min for about 38 minutes, and after the aeration was completed, mixing was carried out until 40 minutes had elapsed since the start of aeration. After the aeration was completed, vacuum nitrogen substitution was performed to sufficiently remove oxygen in the reactor. To this, 2.4 kg of dimethyl ether and 10.9 kg of methyl chloride were added, and the reaction was carried out at 60 - 90 °C for 110 minutes. After the reaction, washing was carried out with water to a water content of 85%, drying was carried out using a forced-air dryer to a water content of 2% by mass, and pulverization was carried out using a Victoreen mill impact pulverizer to obtain methyl cellulose - 1 (hereinafter referred to as "MC - 1").

[0091] The obtained MC - 1 had a viscosity of 15 mPa·s for a 2% by mass aqueous solution at 20 °C, a methoxy group substitution degree (DS) of 1.8, and a polydispersity (Mw / Mn) of 1.52 as measured by absolute molecular weight measurement.

[0092] (Synthesis Example 2: Preparation of MC - 2) Wood pulp with an intrinsic viscosity of 700 ml / g was pulverized with a pulverizer to obtain powdered pulp. Among this powdered pulp, 6.0 kg of pulp corresponding to the cellulose content of the solid component in the pulp was charged into a pressure-resistant reactor with an internal stirrer equipped with a jacket, and vacuum nitrogen substitution was performed to sufficiently remove oxygen in the reactor. Next, while stirring, 13.43 kg of a 49% by mass aqueous sodium hydroxide solution was added over 20 minutes. To this, 2.4 kg of dimethyl ether and 10.0 kg of methyl chloride were added, and the reaction was carried out at 60 - 90 °C for 110 minutes. After the reaction, washing was carried out with water to a water content of 85%, drying was carried out using a forced-air dryer to a water content of 2% by mass, and pulverization was carried out using a Victoreen mill impact pulverizer to obtain powdered methyl cellulose. Next, 10.5 mass% hydrochloric acid was sprayed onto the obtained powdered methyl cellulose so that the amount of hydrogen chloride was 0.15 mass part with respect to 100 mass parts of methyl cellulose. The methyl cellulose sprayed with hydrochloric acid was reacted for 70 minutes in a rotating glass reactor with a jacket temperature of 80°C to depolymerize it. By maintaining the jacket temperature at 80°C, bringing it to a reduced pressure state of 40 mmHg, and leaving it for 30 minutes, hydrogen chloride and water in the glass reactor were volatilized. Next, sodium bicarbonate corresponding to 1 / 2 mole of the added hydrogen chloride was added to the methyl cellulose in the glass reactor to neutralize the methyl cellulose, thereby obtaining methyl cellulose - 1 (hereinafter referred to as "MC - 2").

[0093] The obtained MC - 2 had a 2 mass% aqueous solution viscosity of 15 mPa·s at 20°C, a methoxy group substitution degree (DS) of 1.8, and a polydispersity (Mw / Mn) of 2.17 by absolute molecular weight measurement.

[0094] (Synthesis Example 3: Preparation of MC - 3) Methyl cellulose - 3 (hereinafter referred to as "MC - 3") was obtained in the same manner as MC - 2, except that the concentration of hydrochloric acid used for depolymerization was 14 mass%, and the jacket temperature during depolymerization was 95°C and the reaction time was 25 minutes.

[0095] The obtained MC - 3 had a 2 mass% aqueous solution viscosity of 15 mPa·s at 20°C, a methoxy group substitution degree (DS) of 1.8, and a polydispersity (Mw / Mn) of 6.74 by absolute molecular weight measurement.

[0096] (Synthesis Example 4: Preparation of MC - 4) Powdered methyl cellulose was obtained through the etherification reaction step, washing step, and drying and pulverization step using alkali cellulose and a methylating agent in the same manner as in Synthesis Example 2. To the obtained powdered methylcellulose, 14% hydrochloric acid was sprayed so that the amount of hydrogen chloride was 0.30 parts by mass with respect to 100 parts by mass of methylcellulose. The methylcellulose sprayed with hydrochloric acid was reacted for 70 minutes in a rotating glass reactor with a jacket temperature of 80°C to depolymerize it. By maintaining the jacket temperature at 80°C and leaving it under a reduced pressure of 40 mmHg for 30 minutes, hydrogen chloride and water in the glass reactor were volatilized. Next, sodium bicarbonate corresponding to 1 / 2 mole of the added hydrogen chloride was added to the methylcellulose in the glass reactor to neutralize the methylcellulose. By dissolving the neutralized methylcellulose above in pure water, a 2% by mass aqueous solution was prepared and filled into a dialysis tube (fractionation molecular weight: 12,000 - 14,000, diameter 28.6 mm). The tube filled with the solution was immersed in pure water and dialysis was carried out for 2 days (dialysis treatment). Then, a Teflon sheet (Teflon: registered trademark) was laid in a metal bath, and the solution in the dialysis cube was spread on it. By drying this in a dryer at 100°C for 2 hours, a film of methylcellulose was obtained. The obtained film was cut and pulverized with a force mill to obtain powdered methylcellulose-4 (hereinafter referred to as "MC-4").

[0097] The obtained MC-4 had a 2% by mass aqueous solution viscosity at 20°C of 4 mPa·s, a methoxy group substitution degree (DS) of 1.8, and a polydispersity (Mw / Mn) by absolute molecular weight measurement of 1.71.

[0098] (Synthesis Example 5: Preparation of MC-5) In Synthesis Example 4, after the depolymerization treatment with hydrochloric acid and neutralization, the methylcellulose that was not subjected to dialysis treatment was designated as methylcellulose-5 (hereinafter referred to as "MC-5").

[0099] The obtained MC-5 had a 2% by mass aqueous solution viscosity at 20°C of 4 mPa·s, a methoxy group substitution degree (DS) of 1.8, and a polydispersity (Mw / Mn) by absolute molecular weight measurement of 1.99.

[0100] [Test Materials] The materials used for the carbon black dispersion composition for batteries in the examples and comparative examples are shown below. [Carbon Black] · Acetylene black: DENKA BLACK Li-435 (hereinafter referred to as "Li-435"), manufactured by Denka Co., Ltd., BET specific surface area 136 m 2 / g · Acetylene black: DENKA BLACK Li-100 (hereinafter referred to as "Li-100"), manufactured by Denka Co., Ltd., BET specific surface area 68 m 2 / g · Furnace black: VULCAN XC72 (hereinafter referred to as "VXC72"), manufactured by Cabot Corporation, BET specific surface area 237 m 2 / g

[0101] [Dispersant] · Methyl cellulose - 1 (MC - 1): 2 mass% aqueous solution viscosity (20 °C) ··· 15 mPa·s, methoxy group substitution degree (DS) ··· 1.8, polydispersity (Mw / Mn) ··· 1.52 · Methyl cellulose - 2 (MC - 2): 2 mass% aqueous solution viscosity (20 °C) ··· 15 mPa·s, methoxy group substitution degree (DS) ··· 1.8, polydispersity (Mw / Mn) ··· 2.17 · Methyl cellulose - 3 (MC - 3): 2 mass% aqueous solution viscosity (20 °C) ··· 15 mPa·s, methoxy group substitution degree (DS) ··· 1.8, polydispersity (Mw / Mn) ··· 6.74 · Methyl cellulose - 4 (MC - 4): 2 mass% aqueous solution viscosity (20 °C) ··· 4 mPa·s, methoxy group substitution degree (DS) ··· 1.8, polydispersity (Mw / Mn) ··· 1.71 · Methyl cellulose - 5 (MC - 5): 2 mass% aqueous solution viscosity (20 °C) ··· 4 mPa·s, methoxy group substitution degree (DS) ··· 1.8, polydispersity (Mw / Mn) ··· 1.99

[0102] [Preparation of Carbon Dispersion Composition for Batteries] [Example 1] A carbon black dispersion composition for batteries was prepared according to the following procedure. As the carbon black, 2.6 g of acetylene black (“Li-435”) and 0.208 g of methyl cellulose-1 (MC-1) (8 parts by mass with respect to 100 parts by mass of the carbon black) as a dispersant were powder-mixed. To this, 12.0 g of N-methyl-2-pyrrolidone (NMP) was added, and pre-kneaded for 5 minutes under the condition of a rotation speed of 2000 rpm using a defoaming kneader (“ARV-310” manufactured by Shinky Co., Ltd.). Thereafter, 3.8 g of NMP was further added and mixed for another 5 minutes using a defoaming kneader. The obtained dispersion was further stirred for 1 minute under the condition of a rotation speed of 12500 rpm using a thin-film swirling disperser (“Filmix 30-L type” manufactured by Primix Corporation) to obtain a carbon black dispersion composition for batteries.

[0103] [Comparative Example 1] In Example 1, methyl cellulose-2 (MC-2) was used instead of MC-1, and otherwise in the same manner as in Example 1, a carbon black dispersion composition for batteries was obtained.

[0104] [Comparative Example 2] In Example 1, methyl cellulose-3 (MC-3) was used instead of MC-1, and otherwise in the same manner as in Example 1, a carbon black dispersion composition for batteries was obtained.

[0105] [Example 2] As the carbon black, 2.6 g of acetylene black (“Li-435”) and 0.286 g of methyl cellulose-4 (MC-4) (11 parts by mass with respect to 100 parts by mass of the carbon black) as a dispersant were powder-mixed. To this, 12.0 g of N-methyl-2-pyrrolidone (NMP) was added, and pre-kneaded for 5 minutes under the condition of a rotation speed of 2000 rpm using a defoaming kneader (“ARV-310” manufactured by Shinky Co., Ltd.). Thereafter, 3.7 g of NMP was further added and mixed for another 5 minutes using a defoaming kneader. The obtained dispersion was further stirred for 1 minute under the condition of a rotation speed of 12500 rpm using a thin-film swirling disperser (“Filmix 30-L type” manufactured by Primix Corporation) to obtain a carbon black dispersion composition for batteries.

[0106] [Comparative Example 3] In Example 2, methyl cellulose-5 (MC-5) was used instead of MC-4, and in other respects, the same procedure as in Example 2 was followed to obtain a carbon black dispersion composition for a battery.

[0107] [Example 3] 3.3 g of acetylene black (“Li-100”) as carbon black and 0.116 g of methyl cellulose-1 (MC-1) (3.5 parts by mass with respect to 100 parts by mass of carbon black) as a dispersant were powder-mixed. To this, 11.0 g of N-methyl-2-pyrrolidone (NMP) was added, and pre-kneading was carried out for 5 minutes under the condition of a rotation speed of 2000 rpm using a defoaming kneader (“ARV-310” manufactured by Shinky Co., Ltd.). Then, 3.9 g of NMP was further added, and mixing was carried out for another 5 minutes using a defoaming kneader. The obtained dispersion was further stirred for 1 minute under the condition of a rotation speed of 12500 rpm using a thin-film swirling disperser (“Filmix 30-L type” manufactured by Primix Corporation) to obtain a carbon black dispersion composition for a battery.

[0108] [Comparative Example 4] In Example 3, methyl cellulose-2 (MC-2) was used instead of MC-1, and in other respects, the same procedure as in Example 3 was followed to obtain a carbon black dispersion composition for a battery.

[0109] [Comparative Example 5] In Example 3, methyl cellulose-3 (MC-3) was used instead of MC-1, and in other respects, the same procedure as in Example 3 was followed to obtain a carbon black dispersion composition for a battery.

[0110] [Example 4] As carbon black, 2.5 g of furnace black ("VXC72") and 0.325 g of methyl cellulose - 1 (MC - 1) (13 parts by mass with respect to 100 parts by mass of carbon black) as a dispersant were powder - mixed. To this, 10.0 g of N - methyl - 2 - pyrrolidone (NMP) was added, and pre - kneaded for 5 minutes under the condition of a rotation speed of 2000 rpm using a defoaming kneader ("ARV - 310" manufactured by Shinki Co., Ltd.). Then, an additional 6.4 g of NMP was added and mixed for another 5 minutes using a defoaming kneader. The obtained dispersion was further stirred for 1 minute under the condition of a rotation speed of 12500 rpm using a thin - film swirling disperser ("Filmix 30 - L type" manufactured by Primix Co., Ltd.) to obtain a carbon black dispersion composition for batteries.

[0111] [Comparative Example 6] In Example 4, methyl cellulose - 3 (MC - 3) was used instead of MC - 1, and in other respects, it was the same as in Example 4 to obtain a carbon black dispersion composition for batteries.

[0112] (Evaluation Method) The dispersibility of the carbon black dispersion composition for batteries obtained as described above was evaluated using a rotational rheometer. Specifically, using a rotational rheometer ("MCR702" manufactured by Anton Paar), a cone - plate was used as a measurement jig, the set temperature was set to 20 °C, and the shear viscosity at a shear rate of 10 m / s was measured. A lower shear viscosity means better dispersibility of the carbon black. Here, if the initial viscosity is 600 mPa·s or less, it is determined that the dispersibility of the carbon black is good. When the carbon black dispersion composition for batteries was sandwiched between the cone - plates during the shear viscosity measurement, if it became overloaded due to the solidity of the slurry or the viscosity behavior could not be measured normally, it was regarded as unmeasurable (×). In addition, the storage stability evaluation was performed based on the change amount of viscosity from the shear viscosity value (viscosity after one-week storage) after allowing the carbon black dispersion composition for battery to stand at a temperature of 35°C for one week to the viscosity immediately after dispersion. A dispersion with less viscosity change means better storage stability. Here, if the change ratio of the viscosity after one-week storage to the viscosity immediately after dispersion is ±20% or less, it is determined that the storage stability of the carbon black dispersion composition for battery is good. The above results are shown in Table 1.

[0113]

Table 1

[0114] As shown in Example 1, the carbon black dispersion composition for battery using methylcellulose with a polydispersity of 1.9 or less by absolute molecular weight measurement as a dispersant had improved dispersibility and storage stability of carbon black compared to Comparative Examples 1 and 2 using methylcellulose with a polydispersity greater than 1.9. Also, from the results of Examples 2 to 4 and Comparative Examples 3 to 6, it was similarly confirmed that the dispersibility and storage stability of carbon black were improved in Examples 2 to 4 where the polydispersity was 1.9 or less and methylcellulose was used as a dispersant.

[0115] <Preparation of Composite Paste for Positive Electrode> According to the following procedure, a composite paste for positive electrode using the carbon dispersion composition for battery obtained in Examples 1 to 4 described above was prepared. 7.1 g of the carbon dispersion composition for battery of Example 1 and 14.3 g of an NMP solution (solution concentration: 7% by mass) of polyvinylidene fluoride (weight average molecular weight: 630,000) were added to a stirring container and kneaded for 2 minutes under the condition of a rotation speed of 2000 rpm using a defoaming kneader (“ARV-310” manufactured by Shinky Co., Ltd.). To this, 48.0 g of a ternary positive electrode active material (NCM622) and 7.6 g of N-methyl-2-pyrrolidone (NMP) were added, and kneaded for 3 minutes under the condition of a rotation speed of 2000 rpm using a defoaming kneader to obtain a composite paste for positive electrode (solid content concentration: 65% by mass). For the carbon dispersion compositions for batteries of Examples 2 to 4, positive electrode composite pastes were obtained by the same method as described above. All of the positive electrode composite pastes were uniformly dispersed and showed good coatability.

[0116] <Fabrication of Positive Electrode of Lithium-Ion Secondary Battery> The positive electrode of a lithium-ion secondary battery using the above positive electrode composite paste was fabricated by the following procedure. On a current collector made of an aluminum foil (thickness: 20 μm, width: 14 mm), coating and drying of the positive electrode composite paste were carried out using an electrode coating and drying machine ("LiB-W140" manufactured by Clean Technology Co., Ltd.). The coating and drying conditions were a coating thickness of 180 μm, a conveyance speed of 0.2 m / min, and a furnace internal temperature of 115 to 125°C in a drying furnace (furnace length: 50 cm). Thereafter, a lithium-ion secondary battery positive electrode was obtained by rolling the coating-dried film obtained by the above coating and drying treatment together with the current collector at a linear pressure of 200 kg / cm using a bench-top roll press machine ("SA-602" manufactured by Tester Sangyo Co., Ltd.). A lithium-ion secondary battery positive electrode could be normally fabricated using any of the above positive electrode composite pastes.

[0117] Although the present invention has been described with the above embodiments so far, the present invention is not limited to the embodiments, and can be changed within the range that those skilled in the art can conceive, such as other embodiments, additions, and changes. As long as the effects of the present invention are exhibited in any aspect, it is included in the scope of the present invention.

Claims

1. In a carbon black dispersion composition for a battery containing carbon black, methyl cellulose, and N-methyl-2-pyrrolidone, the polydispersity determined by absolute molecular weight measurement by a combination of size exclusion chromatography (SEC) and multi-angle light scattering (MALS) of the methyl cellulose is 1.9 or less, and the viscosity of a 2 mass% aqueous solution of the methyl cellulose at 20 °C is 3 to 30 mPa·s. A carbon black dispersion composition for a battery.

2. The carbon black dispersion composition for a battery according to Claim 1, wherein the content of the carbon black is 5 to 20 mass%.

3. The BET specific surface area of the carbon black is 30 to 1500 m 2 / g, and the carbon black dispersion composition for a battery according to claim 1.

4. The carbon black dispersion composition for a battery according to Claim 1, wherein the addition amount of the methyl cellulose is 1 to 20 parts by mass with respect to 100 parts by mass of the carbon black.

5. A positive electrode mixture paste for a positive electrode, comprising the carbon black dispersion composition for a battery according to any one of Claims 1 to 4, a positive electrode active material, and a binder.

6. A positive electrode for a lithium ion secondary battery, comprising a current collector and a positive electrode mixture layer which is a dried coating film of the positive electrode mixture paste according to Claim 5 formed on the current collector.

7. A lithium ion secondary battery, comprising the positive electrode for a lithium ion secondary battery according to Claim 6, a negative electrode, an electrolyte, and a separator.

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