Conductive Material Dispersion Liquid and Electrode Paste for Positive Electrode of Lithium Ion Secondary Battery

Abstract

To provide a conductive material dispersion liquid for a lithium ion secondary battery positive electrode having excellent storage stability of viscosity.SOLUTION: A conductive material dispersion liquid for a lithium ion secondary battery positive electrode includes a conductive material, a methyloctyl cellulose, and a dispersion medium.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a conductive material dispersion liquid for a positive electrode of a lithium ion secondary battery and an electrode paste.

Background Art

[0002] In recent years, with the popularization of mobile phones, notebook personal computers, etc., lithium ion secondary batteries have attracted attention. A lithium ion secondary battery usually includes a negative electrode made of a carbon material, a positive electrode containing an active material that reversibly allows lithium ions to enter and exit, and a non-aqueous electrolyte that immerses them.

[0003] Among these, the positive electrode is manufactured by applying an electrode paste composed of a positive electrode active material, a conductive material, and a binder to a current collector plate. As the positive electrode active material, a lithium transition metal composite oxide or the like is used. Since such a positive electrode active material alone has poor electron conductivity, that is, poor conductivity, in order to impart conductivity, a highly structured conductive carbon black and a carbon material such as graphite having significant anisotropy in crystals are added as conductive materials, and together with a binder (binding material), they are dispersed in a non-aqueous solvent such as N-methyl-2-pyrrolidone (NMP) to prepare a slurry. This slurry is applied onto a metal foil and dried to form a positive electrode.

[0004] However, carbon black and graphite, which are carbon materials used as conductive materials, are fine powders with a small primary particle size. Due to their large structure and specific surface area, they have strong cohesive forces and are difficult to uniformly mix and disperse in the slurry for forming the electrode composite material of a lithium-ion secondary battery. When the dispersibility and particle size control of the carbon material as the conductive material are insufficient, a uniform conductive network cannot be formed, so the internal resistance of the electrode cannot be reduced. As a result, there is a problem that the performance of the lithium transition metal composite oxide as the positive electrode active material and graphite as the carbon material cannot be fully exhibited. In addition, if the dispersion of the conductive material (conductive aid) in the electrode composite material is insufficient, a resistance distribution occurs on the electrode plate due to partial aggregation. When used as a battery, current concentration occurs, and problems such as partial heat generation and accelerated deterioration may occur.

[0005] In order to uniformly disperse the conductive material in the electrode, it has been proposed to disperse it in a dispersion medium such as an organic solvent together with a dispersant to prepare a dispersed liquid (conductive material dispersion liquid) in a slurry state in advance, and knead this together with the active material and the binder to form an electrode (Patent Document 1). A dispersant for a battery has been proposed that achieves dispersion stabilization without inhibiting the conductivity of the conductive aid and improves the wettability of the conductive aid with respect to the electrolyte (Patent Document 2). In addition, a conductive material dispersion liquid capable of ensuring good dispersibility and conductivity has been proposed (Patent Document 3).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, conventional conductive material dispersions have insufficient storage stability and are prone to changes in viscosity such as thickening over time. When such a conductive material dispersion is kneaded together with an active material and a binder to form an electrode, its workability is poor.

[0008] An object of the present disclosure is to provide a conductive material dispersion for a positive electrode of a lithium-ion secondary battery, which has excellent storage stability of viscosity.

Means for Solving the Problems

[0009] The first aspect of the present disclosure relates to a conductive material dispersion for a positive electrode of a lithium-ion secondary battery, which contains a conductive material, methyl octyl cellulose, and a dispersion medium.

[0010] In the conductive material dispersion for a positive electrode of a lithium-ion secondary battery, the conductive material is at least one carbon black selected from the group consisting of acetylene black, furnace black, and ketjen black, the content of the carbon black in the dispersion is 5% by mass or more and 30% by mass or less, and the viscosity of the dispersion measured with a B-type viscometer may be 50 mPa·s or more and 2000 mPa·s or less.

[0011] In the conductive material dispersion for a positive electrode of a lithium-ion secondary battery, the conductive material is carbon nanotubes, the content of the carbon nanotubes in the dispersion is 0.1% by mass or more and 10% by mass or less, and the viscosity of the dispersion measured with a B-type viscometer may be 50 mPa·s or more and 2000 mPa·s or less.

[0012] In the conductive material dispersion for a positive electrode of a lithium-ion secondary battery, the methyl octyl cellulose has a degree of methyl substitution of 0.1 or more and less than 2.9, a degree of octyl substitution of 0.01 or more and less than 2.9, and the sum of the degree of methyl substitution and the degree of octyl substitution is less than 3.0, and the content of the methyl octyl cellulose may be 0.1 part by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the carbon black.

[0013] In the conductive material dispersion for the positive electrode of the lithium-ion secondary battery, the methyl octyl cellulose has a methyl group substitution degree of 0.1 or more and less than 2.9, an octyl group substitution degree of 0.01 or more and less than 2.9, and the sum of the methyl group substitution degree and the octyl group substitution degree is less than 3.0. The content of the methyl octyl cellulose may be 30 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the carbon nanotubes.

[0014] In the conductive material dispersion for the positive electrode of the lithium-ion secondary battery, the dispersion medium may be N-methyl-2-pyrrolidone.

[0015] The second aspect of the present disclosure relates to an electrode paste for a positive electrode of a lithium-ion secondary battery, which contains the conductive material dispersion, an active material, and a binder.

Effect of the Invention

[0016] The conductive material dispersion for the positive electrode of the lithium-ion secondary battery of the present disclosure is excellent in the storage stability of viscosity.

Mode for Carrying Out the Invention

[0017] [Conductive Material Dispersion] The conductive material dispersion of the present disclosure is a conductive material dispersion for the positive electrode of a lithium-ion secondary battery, which contains a conductive material, methyl octyl cellulose, and a dispersion medium.

[0018] (Dispersion) The conductive material dispersion refers to a liquid in which at least the conductive material among the contained components is dispersed in the dispersion medium. It is preferable that the conductive material and methyl octyl cellulose are in a dispersed state. Also, regardless of whether other optional components are contained or not, it is preferable that all the contained components are in a dispersed state in the dispersion medium. Here, the dispersed state includes both the suspension and solution states.

[0019] (Conductive Material) A conductive material is a substance that has conductivity and enhances the conductivity of an electrode. As the conductive material, conventionally known substances can be used, and examples thereof include carbon black and carbon nanotubes.

[0020] Carbon black is particulate carbon. Since carbon black is particles whose characteristics vary depending on the manufacturing method, the quality (particle diameter, structure, crystallinity, etc.) is controlled by the manufacturing method and is classified according to the manufacturing method. For example, acetylene black, furnace black, ketjen black, channel black, and thermal black can be mentioned. Further, carbon black can be used alone or in combination of two or more.

[0021] From the viewpoint of increasing the capacity and improving the cycle characteristics of the battery, at least one selected from the group consisting of acetylene black, furnace black, ketjen black, and carbon nanotubes is preferable as the conductive material. These can be used alone or in combination of two or more.

[0022] The average primary particle diameter of the carbon black may be 50 nm or less, preferably 40 nm or less, and more preferably 30 nm or less. Further, the average primary particle diameter may be 10 nm or more and may be 15 nm or more. If the average primary particle diameter of the carbon black is too large, the conductivity of the coating film obtained from the electrode paste tends to decrease. On the other hand, if it is too small, the viscosities of the conductive material dispersion liquid and the electrode paste become too high, making it difficult to disperse the carbon black, and sufficient conductivity may not be exhibited.

[0023] The average primary particle diameter indicates the arithmetic average particle diameter measured using a transmission electron microscope in accordance with ASTM:D3849-14. The average primary particle diameter is generally used to evaluate the physical properties of the conductive material.

[0024] The dispersed particle size of carbon black in the dispersion is preferably 40 μm or less, more preferably 30 μm or less, as the maximum particle size. Generally, the average particle size is used for managing the particle state of a dispersion such as a conductive material. However, when using the average particle size, since the existence of coarse particles is not considered, even if the value of the average particle size is small, there may actually be coarse particles with a maximum particle size exceeding 40 μm. In this case, the distribution of the active material and the conductive material in the electrode coating film of the lithium-ion secondary battery may become non-uniform, resulting in the possibility of deteriorating the battery performance.

[0025] The maximum particle size may be measured using a grind gauge in accordance with JIS K5600-2-5.

[0026] The purity of carbon black may be 99.90 to 100% by mass, preferably 99.95 to 100% by mass. The purity of carbon black can be calculated based on the amount of impurities, taking the ash measured in accordance with JIS K1469 or JIS K6218 as impurities.

[0027] Carbon nanotubes are carbon crystals having a substantially cylindrical shape. The average outer diameter of carbon nanotubes may be 90 nm or less, preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 15 nm or less. Also, the average outer diameter may be 1 nm or more, or 5 nm or more. If the average outer diameter of carbon nanotubes is too large, the conductivity of the coating film obtained from the electrode paste tends to decrease. Also, if it is too small, the viscosity of the conductive material dispersion and the electrode paste may become too high, making it difficult to disperse the carbon nanotubes.

[0028] The average outer diameter of carbon nanotubes is the arithmetic mean value of the outer diameters of a sufficient number of n measured using an image at a magnification of 100,000 times or more of a transmission electron microscope.

[0029] Examples of the carbon nanotubes include, specifically, VGCF-X (average outer diameter: 30 nm) manufactured by Showa Denko K.K., C100 (average outer diameter: 10-15 nm), U100 (high-purity product with an average outer diameter of 10-15 nm) manufactured by ARKEMA, NC7000 (average outer diameter: 10 nm), NC2150, NC3100 manufactured by Nanocyl, Baytubes C150 (average outer diameter: 13-16 nm), Baytubes C150P (average outer diameter: 13-16 nm) manufactured by BAYER, and MWNT (average outer diameter: 40-90 nm) manufactured by Hodogaya Chemical Co., Ltd. Further, the carbon nanotubes can be used alone or in combination of two or more kinds.

[0030] When the conductive material dispersion of the present disclosure contains carbon nanotubes, it is preferably dispersed one by one independently without aggregation. This is because the coating film obtained from the electrode paste has excellent conductivity.

[0031] The purity of the carbon nanotubes may be 90 to 100% by mass, and preferably 95 to 100% by mass. The purity of the carbon nanotubes can be calculated based on the amount of impurities, which is the ash content measured in accordance with JIS K1469 or JIS K6218 in the same manner as the purity of carbon black.

[0032] The content of the conductive material in the conductive material dispersion is not particularly limited. When the conductive material is carbon black, the content of carbon black in the conductive material dispersion is preferably 5% by mass or more, more preferably 8% by mass or more, and further preferably 12% by mass or more. Also, the content of carbon black in the conductive material dispersion is preferably 30% by mass or less, and more preferably 28% by mass or less.

[0033] When the conductive material is carbon nanotubes, the content of carbon nanotubes in the conductive material dispersion is preferably 0.1% by mass or more, more preferably 0.4% by mass or more, and further preferably 2% by mass or more. Also, the content of carbon nanotubes in the conductive material dispersion is preferably 10% by mass or less.

[0034] If the content of the conductive material in the conductive material dispersion liquid is too low, the total solid content during the preparation of the electrode paste decreases and becomes lower than the appropriate viscosity, resulting in unevenness and a non-uniform coating film. The non-uniform coating film refers to a coating film in which the active material and the conductive material are unevenly distributed, or a coating film in which the basis weight (coating amount on the current collector) varies depending on the position. When a lithium-ion secondary battery having a coating film in which the active material and the conductive material are unevenly distributed is used as the positive electrode, the conductivity may decrease or the charge may be uneven, impairing performance such as high-rate charge and discharge and durability. When a plurality of lithium-ion secondary batteries are manufactured using a coating film in which the basis weight varies depending on the position, the capacity of each lithium-ion secondary battery varies, which may result in a poor yield. If the content of the conductive material in the conductive material dispersion liquid is too high, the fluidity of the conductive material dispersion liquid decreases, and the handling during the preparation of the electrode paste may deteriorate.

[0035] (Methyloctyl cellulose) Methyloctyl cellulose is cellulose in which some or all of the hydrogens of the hydroxyl groups are substituted with methyl groups and octyl groups.

[0036] The degree of methyl substitution of methyloctyl cellulose is preferably 0.1 or more, more preferably 1.0 or more, and even more preferably 1.5 or more. Also, the degree of methyl substitution is preferably less than 2.9, more preferably less than 2.5, and even more preferably less than 2.0. If the degree of methyl substitution is too small, the solubility in the solvent deteriorates. If the degree of methyl substitution is too large, it becomes difficult to introduce an octyl group during the production of methyloctyl cellulose.

[0037] The degree of octyl substitution of methyloctyl cellulose is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.08 or more. Also, the degree of octyl substitution is preferably less than 2.9, more preferably less than 1.8, even more preferably less than 0.8, particularly preferably less than 0.7, and most preferably less than 0.5. This is because better temporal stability of the viscosity of the conductive material dispersion liquid can be obtained. If the degree of octyl substitution is too large, when the conductive material dispersion liquid is prepared, the viscosity of the conductive material dispersion liquid tends to increase.

[0038] The sum of the degree of methyl group substitution and the degree of octyl group substitution of methyl octyl cellulose is preferably less than 3.0, more preferably less than 2.5, and even more preferably less than 2.2. To increase the sum of the degrees of substitution, it is necessary to extend the reaction time, which may lead to a decrease in productivity and physical properties.

[0039] The sum of the degrees of substitution of each substituent of methyl octyl cellulose is referred to as the total degree of substitution. The total degree of substitution of methyl octyl cellulose is preferably 0.3 or more, more preferably 1.0 or more, and even more preferably 1.5 or more. Also, the total degree of substitution is preferably less than 3.0, more preferably less than 2.9, still more preferably less than 2.5, and even more preferably less than 2.2.

[0040] The degree of alkyl group substitution including the degree of methyl group substitution and the degree of octyl group substitution of methyl octyl cellulose can be measured by the following methods. A method according to ASTM: D - 817 - 91, or 13 C - NMR, 1 It can be measured by H - NMR.

[0041] The degree of methyl group substitution and the degree of octyl group substitution of methyl octyl cellulose 1 Examples of the conditions for quantification by H - NMR are described below.

[0042] Apparatus: JEOL JNM ECA - 500 Temperature: 80°C Solvent: DMSO Sample concentration: 0.8 wt% Calculation: Degree of methyl group substitution = 35β / (15α - 15β - 2γ) Degree of octyl group substitution = 7γ / (15α - 15β - 2γ) α: Integration value from 5.40 to 2.70 ppm β: Integration values from 3.51 to 3.41, 3.32 to 3.25 ppm γ: Integration value from 1.65 to 0.70 ppm

[0043] The weight-average molecular weight (Mw) of methyl octyl cellulose is not particularly limited, but is preferably 1.0×10 4 or more, more preferably 2.0×10 4 or more, and even more preferably 3.0×10 4 or more. Also, the weight-average molecular weight is preferably 1.0×10 6 or less, more preferably 5.0×10 5 or less, and even more preferably 2.0×10 5 or less. By being within this range, the dispersibility of methyl octyl cellulose in the conductive material dispersion liquid and the workability during the production of the dispersion liquid become good.

[0044] The weight-average molecular weight is the so-called weighted average value of the molecular weights obtained by multiplying the molecular weight by the weight of each molecule and can be measured by GPC.

[0045] The content of methyl octyl cellulose in the conductive material dispersion liquid is not particularly limited. However, if the content of methyl octyl cellulose in the conductive material dispersion liquid is too low, the dispersion of the conductive material becomes insufficient, and the conductivity of the coating film obtained from the electrode paste using this conductive material dispersion liquid tends to decrease. If the content of methyl octyl cellulose in the conductive material dispersion liquid is too high, the resistance component in the coating film obtained from the electrode paste using this conductive material dispersion liquid increases, resulting in a decrease in conductivity. When a lithium-ion secondary battery having a positive electrode made of such a coating film is configured, it may be difficult to increase the capacity.

[0046] For example, when the conductive material is carbon black, the content of methyl octyl cellulose in the conductive material dispersion liquid is preferably 0.1 part by mass or more, preferably 3 parts by mass or more, more preferably 5 parts by mass or more, and even more preferably 6 parts by mass or more with respect to 100 parts by mass of the conductive material (carbon black). Also, it is preferably 30 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less. When it is within the above range, the dispersibility of the conductive material dispersion liquid is excellent.

[0047] Also, for example, when the conductive material is carbon nanotubes, the content of methyl octyl cellulose in the conductive material dispersion is preferably 30 parts by mass or more, more preferably 50 parts by mass or more, based on 100 parts by mass of the conductive material (carbon nanotubes). Also, it is preferably 200 parts by mass or less, and more preferably 150 parts by mass or less. When it is within the above range, the dispersibility of the conductive material dispersion is excellent.

[0048] Methyl octyl cellulose can be produced, for example, as follows. A production method including a step of converting a cellulose raw material into alkali cellulose under basic conditions (activation step); and a step of reacting the alkali cellulose with an alkyl halide (etherification treatment) can be mentioned. More specifically, for example, a cellulose raw material is converted into alkali cellulose, and the alkali cellulose is reacted with methyl halide to produce methyl cellulose. Thereafter, a method of producing methyl octyl cellulose by reacting the methyl cellulose with octyl halide under basic conditions can be mentioned.

[0049] (Dispersion medium) As described above, the dispersion medium is a component that can prepare a dispersion liquid by dispersing at least a conductive material. Examples of the dispersion medium include aliphatic hydrocarbon-based dispersion media such as pentane, normal hexane, octane, cyclopentane, and cyclohexane; aromatic hydrocarbon-based dispersion media such as benzene, toluene, xylene, and cymene; aldehyde-based dispersion media such as furfural; ketone-based dispersion media such as acetone, methyl ethyl ketone, cyclopentanone, and cyclohexanone; ester-based dispersion media such as butyl acetate, ethyl acetate, methyl acetate, butyl propionate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, 3-methoxybutyl acetate, and ethylene glycol diacetate; ether-based dispersion media such as tetrahydrofuran, dioxane, and ethylene glycol dimethyl ether; alcohol-based dispersion media such as methanol, ethanol, normal propyl alcohol, isopropyl alcohol, butyl alcohol, octyl alcohol, cyclohexanol, allyl alcohol, benzyl alcohol, cresol, and furfuryl alcohol; polyol-based dispersion media such as glycerol, ethylene glycol, and diethylene glycol; alcohol ether-based dispersion media such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and diethylene glycol monobutyl ether; aprotic polar dispersion media such as N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide, and dimethylformamide; and water. These dispersion media can be used alone or in combination of two or more.

[0050] As the dispersion medium, it is preferable to use a dispersion medium with high solubility of methyl octyl cellulose. For example, when preparing an electrode paste for a positive electrode of a lithium-ion secondary battery containing the conductive material dispersion liquid of the present disclosure, applying it to a current collector (aluminum foil), and then evaporating the dispersion medium from the electrode paste to dry it to produce a positive electrode of a lithium-ion secondary battery, the conductive material can be uniformly dispersed even when the concentration of the dispersion medium decreases and other components become highly concentrated.

[0051] Among the above-mentioned various dispersion media, an aprotic polar dispersion medium is preferred, and it is more preferable to use N-methyl-2-pyrrolidone (NMP). This is because it is easy to prepare an electrode paste containing a conductive material dispersion liquid, and the coating property of the electrode paste on the current collector is also excellent.

[0052] The content of the dispersion medium in the conductive material dispersion liquid is not particularly limited. However, when the conductive material is carbon black, it is preferably contained so that the solid content concentration in the conductive material dispersion liquid is 5% by mass or more, more preferably 10% by mass or more, and even more preferably 13% by mass or more. When the conductive material is carbon nanotube, it is preferably contained so that the solid content concentration in the conductive material dispersion liquid is 0.2% by mass or more, more preferably 1% by mass or more, and even more preferably 3% by mass or more.

[0053] The solid content concentration in the conductive material dispersion liquid can be calculated based on the residue when about 1 g of the conductive material dispersion liquid sample is heated at 170 °C for 2 hours.

[0054] (Viscosity) The viscosity of the conductive material dispersion liquid is not particularly limited. However, at 25 °C under atmospheric pressure, it is preferably 50 mPa·s or more, more preferably 80 mPa·s or more, and even more preferably 100 mPa·s or more. Also, it is preferably 2000 mPa·s or less, more preferably 1800 mPa·s or less, and even more preferably 1500 mPa·s or less. This is because it is easy to prepare an electrode paste containing the conductive material dispersion liquid, and the coating property of the electrode paste on the current collector is also excellent. If the viscosity of the conductive material dispersion liquid is too high, the coating property of the electrode paste containing the conductive material dispersion liquid on the current collector may be poor.

[0055] The viscosity of the conductive material dispersion liquid may be measured using a B-type viscometer in accordance with JIS K7117-1.

[0056] (Optional component) Within the scope of the object of the present disclosure, the conductive material dispersion liquid of the present disclosure may appropriately contain optional components other than the conductive material, methyl octyl cellulose, and the dispersion medium. Examples of such optional components include a dispersant; a phosphorus compound; a sulfur compound; an organic acid; a nitrogen compound such as an amine compound or an ammonium compound; an organic ester; and conventionally known additives such as various silane-based, titanium-based, and aluminum-based coupling agents. The optional components can be used singly or in combination of two or more kinds.

[0057] Examples of the dispersant include nonionic dispersants such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyhexafluoropropylene, polyethylene, polypropylene, polymethyl methacrylate, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyacrylic acid, polyvinyl butyral, polyacrylamide, polyurethane, polydimethylsiloxane, epoxy resin, acrylic resin, polyester resin, melamine resin, phenol resin, various rubbers, lignin, pectin, gelatin, xanthan gum, welan gum, succinoglycan, polyvinyl alcohol, polyvinyl acetal, cellulose-based resins (excluding methyl octyl cellulose), polyalkylene oxide, polyvinyl ether, polyvinyl pyrrolidone, chitins, chitosans, and starch.

[0058] The blending amount of the dispersant is preferably 0.1 to 100 parts by mass, more preferably 0.1 to 50 parts by mass, based on 100 parts by mass of the conductive material.

[0059] Examples of the phosphorus compound include tributylphosphine, triphenylphosphine, triethyl phosphite, and triphenyl phosphite.

[0060] Examples of the sulfur compound include butanethiol, n-hexanethiol, diethyl sulfide, and tetrahydrothiophene.

[0061] Examples of the organic acid include acetic acid, propionic acid, butyric acid, caproic acid, acrylic acid, crotonic acid, capric acid, stearic acid, oleic acid, oxalic acid, succinic acid, adipic acid, maleic acid, glutaric acid, benzoic acid, 2-methylbenzoic acid, 4-methylbenzoic acid, and mixtures of two or more thereof.

[0062] Examples of the amine compound include methylamine, ethylamine, n-propylamine, n-butylamine, n-hexylamine, n-heptylamine, 2-ethylhexylamine, n-octylamine, nonylamine, decylamine, dodecylamine, docosylamine, hexadecylamine, octadecylamine, isopropylamine, isobutylamine, isooctylamine, isoamylamine, allylamine, cyanoethylamine, cyclopropylamine, cyclohexylamine, cyclopentylamine, aniline, N,N-dimethylaniline, benzylamine, anisidine, aminobenzonitrile, piperidine, pyrazine, pyridine, pyrrole, pyrrolidine, methoxyamine, methoxyethylamine, methoxyethoxyethylamine, methoxyethoxyethoxyethylamine, methoxypropylamine, ethoxyamine, n-butoxyamine, 2-hexyloxyamine, 2-amino-2-methyl-1-propanol, aminoacetaldehyde dimethyl acetal, hydroxyamine, ethanolamine, diethanolamine, methyldiethanolamine, 2-hydroxypropylamine, N-ethyldiethanolamine, N-methyldiethanolamine, aminoethylethanolamine, dimethylethanolamine, triisopropanolamine, triethanolamine, ethylenediamine, propylenediamine, tritriethylenediamine, triethylenetetramine, hexamethylenediamine, 2-ethyldiamine, 2,2-(ethylenedioxy)bisethylamine, tetramethylpropylenediamine, morpholine, N-methylmorpholine, N-ethylmorpholine, N-methylpiperidine, dimethylamine, diethylamine, dipropylamine, diethylenetriamine, tri-n-butylamine, ammonium hydroxide, imidazole, diazabicycloundecene, diazabicyclooctane, taurine, hydrazine, hexamethyleneimine, polyallylamine, polyethyleneimine, and adipic acid dihydrazide, etc.

[0063] Examples of ammonium compounds include 2-ethylhexylammonium 2-ethylhexylcarbamate, 2-ethylhexylammonium 2-ethylhexylcarbonate, 2-cyanoethylammonium 2-cyanoethylcarbamate, 2-cyanoethylammonium 2-cyanoethylcarbonate, 2-methoxyethylammonium 2-methoxyethylcarbamate, 2-methoxyethylammonium 2-methoxyethylcarbonate, n-butylammonium n-butylcarbamate, n-butylammonium n-butylcarbonate, t-butylammonium t-butylcarbamate, t-butylammonium t-butylcarbonate, isobutylammonium isobutylcarbamate, isobutylammonium isobutylcarbonate, isopropylammonium isopropylcarbamate, isopropylammonium triethylenediaminecarbamate, isopropylammonium isopropylcarbonate, isopropylammonium triethylenediaminecarbonate, ethylammonium ethylcarbamate, pyridinium 2-ethylhexylcarbamate, ethylammonium ethylcarbonate, octadecylammonium octadecylcarbamate, octadecylammonium octadecylcarbonate, ammonium carbamate, dioctadecylammonium dioctadecylcarbamate, dioctadecylammonium dioctadecylcarbonate, dibutylammonium dibutylcarbamate, dibutylammonium dibutylcarbonate, triethoxysilylpropylammonium triethoxysilylpropylcarbamate, triethoxysilylpropylammonium triethoxysilylpropylcarbonate, hexamethyleneiminium hexamethyleneiminiumcarbamate, hexamethyleneiminium hexamethyleneiminiumcarbonate ammonium, benzylammonium benzylcarbamate, benzylammonium benzylcarbonate, methyldecylammonium methyldecylcarbamate, methyldecylammonium methyldecylcarbonate, morpholinium morpholiniumcarbamate, morpholinium morpholiniumcarbonate, 2-ethylhexylammonium bicarbonate, 2-cyanoethylammonium bicarbonate, 2-methoxyethylammonium bicarbonate, t-butylammonium bicarbonate, ammonium bicarbonate, isopropylammonium bicarbonate, dioctadecylammonium bicarbonate, triethylenediamineammonium bicarbonate, and pyridinium bicarbonate, etc.And derivatives or mixtures thereof, etc. are included.

[0064] Examples of organic esters include ethyl acetate, isobutyl acetate, n-butyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl acrylate, dimethyl oxalate, dimethyl succinate, methyl crotonate, methyl benzoate, methyl 2-methylbenzoate, and mixtures thereof.

[0065] Examples of silane coupling agents include vinyltrimethoxysilane, γ-methacryloxypropyl-tris(β-methoxyethoxy)silane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, vinyltriacetoxysilane, γ-mercaptopropyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropylmethylmethoxysilane, N-β-(aminoethyl)-γ-aminopropyldimethylmethoxysilane, N,N-bis(β-hydroxyethyl)-γ-aminopropyltriethoxysilane, γ-chloropropyltrimethoxysilane, vinyltris(2-methoxyethoxysilane), 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, and γ-chloropropyltrimethoxysilane, etc.

[0066] Examples of the titanium coupling agent include tetrabutyl titanate, tetraoctyl titanate, isopropyltriisostearoyl titanate, isopropyltridecylbenzenesulfonyl titanate, bis(dioctylpyrophosphate)oxyacetate titanate, trimethoxy titanate, tetramethoxy titanate, triethoxy titanate, tetraethoxy titanate, tetrapropoxy titanate, chlorotrimethoxy titanate, chlorotriethoxy titanate, ethyltrimethoxy titanate, methyltriethoxy titanate, ethyltriethoxy titanate, diethyldiethoxy titanate, phenyltrimethoxy titanate, phenyltriethoxy titanate, and mixtures thereof.

[0067] Examples of the aluminum coupling agent include various aluminum chelates, alkyl acetoacetate aluminum diisopropylate, aluminum bisethyl acetate diisopropylate, acetoalkoxydialuminum diisoprobylate, and mixtures thereof.

[0068] (Production of Conductive Material Dispersion Liquid) The method for producing the conductive material dispersion liquid of the present disclosure is not particularly limited. For example, it can be produced by simultaneously or stepwise blending a conductive material, methyl octyl cellulose, and a dispersion medium and stirring them.

[0069] For example, after blending a conductive material, methyl octyl cellulose, and a dispersion medium, stirring may be performed using a known mixing device such as a bead mill or a ball mill. At this time, when the conductive material is carbon black, it is preferable to disperse until the viscosity of the conductive material dispersion liquid falls within the above viscosity range. Further, when the conductive material is a carbon nanotube, it is preferable to disperse until each one is independent.

[0070] [Electrode Paste] The electrode paste of the present disclosure is an electrode paste for a positive electrode of a lithium ion secondary battery, containing the conductive material dispersion liquid, active material, and binder of the present disclosure.

[0071] (Conductive material dispersion liquid) The content of the conductive material dispersion liquid in the electrode paste is preferably adjusted so that the content of the conductive material is in the following range. When the conductive material is carbon black, 0.5 to 15 parts by mass is preferable, and 1 to 9 parts by mass is more preferable with respect to 100 parts by mass of the active material. Further, when the conductive material is carbon nanotubes, 0.05 to 15 parts by mass is preferable, and 0.2 to 9 parts by mass is more preferable with respect to 100 parts by mass of the active material.

[0072] If the ratio of the conductive material to the active material is excessively small, the conductivity may be low and the battery characteristics may deteriorate. On the other hand, if the ratio of the conductive material to the active material is excessively large, the coating of the conductive material on the surface of the active material may be excessive, which may become a barrier to hinder the movement of lithium ions and the battery characteristics may deteriorate.

[0073] (Active material) The active material is an active material for a positive electrode of a lithium ion secondary battery. As the active material, conventionally known active materials can be used. For example, lithium transition metal oxides such as lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, and lithium iron oxide; lithium iron phosphate; nickel manganese cobalt oxide; and manganese oxide and the like can be mentioned. Among these, lithium transition metal oxides are preferable. Further, the active material can be used alone or in combination of two or more kinds.

[0074] The content of the active material in the electrode paste is preferably 50% by mass or more, and more preferably 54% by mass or more. Further, it is preferably 80% by mass or less, and more preferably 78% by mass or less. If it is less than 50% by mass, unevenness may occur during solvent drying and the coating film may become non-uniform. On the other hand, if it exceeds 80% by mass, the fluidity of the electrode slurry may be significantly reduced and coating may become difficult.

[0075] (Binder) As the binder, conventionally known active materials can be used. For example, polyvinylidene fluoride (PVDF); polytetrafluoroethylene; polyhexafluoropropylene; polyethylene; polypropylene; polymethyl methacrylate; polyvinyl chloride; polyvinylidene chloride; polyvinyl acetate; polyacrylic acid; polyvinyl butyral; polyacrylamide; polyurethane; polydimethylsiloxane; epoxy resin; acrylic resin; polyester resin; melamine resin; phenolic resin; various rubbers such as styrene-butadiene rubber; lignin; pectin; gelatin; xanthan gum; welan gum; succinoglycan; polyvinyl alcohol; polyvinyl acetal; cellulose-based resins; polyalkylene oxide; polyvinyl ether; polyvinyl pyrrolidone; chitins; chitosans; and starch, etc. can be mentioned. The binder can be used alone or in combination of two or more.

[0076] The content of the binder in the electrode paste is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, and preferably 25% by mass or less, more preferably 20% by mass or less. If it is less than 0.3% by mass, the coatability may be insufficient. On the other hand, if it exceeds 25% by mass, the battery characteristics may deteriorate.

[0077] Also, the form of the binder is not limited, and it may be, for example, a solid such as powder and granule; or a liquid such as a solution and a dispersion (dispersion, emulsion, etc.).

[0078] (Optional component) The electrode paste of the present disclosure may appropriately contain optional components other than the conductive material dispersion liquid, the active material, and the binder of the present disclosure as needed within the scope of the object of the present disclosure. Examples of such optional components include conventionally known additives such as flame retardant aids, thickeners, defoamers, leveling agents, and adhesion improvers. The optional components can be used alone or in combination of two or more.

[0079] (Manufacture of electrode paste) The method for manufacturing the electrode paste is not particularly limited. For example, the conductive material dispersion liquid, the active material, the binder, and, if necessary, the dispersion medium and various additives are blended simultaneously or step by step, and can be manufactured by mixing using various mixers such as a planetary mixer, a disperser, a ball mill, and a blender mill.

[0080] [Use] The conductive material dispersion liquid and the electrode paste of the present disclosure are preferably used for the positive electrode of a lithium-ion secondary battery.

Examples

[0081] Hereinafter, the present disclosure will be described in detail based on examples, but the technical scope is not limited by these examples.

[0082] Various measurements in the examples and comparative examples were performed by the following methods. [Degree of substitution] The degree of alkyl group substitution was quantified by 1 1H-NMR under the following conditions. Apparatus: JEOL JNM ECA-500 Temperature: 80 °C Solvent: DMSO Sample concentration: 0.8 wt% Calculation: · In the case of methyl octyl cellulose Degree of methyl group substitution = 35β / (15α - 15β - 2γ) Degree of octyl group substitution = 7γ / (15α - 15β - 2γ) α: Integration value from 5.40 to 2.70 ppm β: Integration values from 3.51 to 3.41 and from 3.32 to 3.25 ppm γ: Integration value from 1.65 to 0.70 ppm · In the case of methyl butyl cellulose Degree of methyl group substitution = 49β / 3(7α - 7β - 2γ) Degree of butyl group substitution = 7γ / (7α - 7β - 2γ) α: Integration value from 5.40 to 2.70 ppm β: Integration values from 3.51 to 3.41 and from 3.32 to 3.25 ppm Integrated value of γ: 1.65 to 0.70 ppm · In the case of methylhexyl cellulose Degree of methyl group substitution = 77β / 3(11α - 11β - 2γ) Degree of hexyl group substitution = 7γ / (11α - 11β - 2γ) Integrated value of α: 5.40 to 2.70 ppm Integrated values of β: 3.51 to 3.41, 3.32 to 3.25 ppm Integrated value of γ: 1.65 to 0.70 ppm

[0083] <N-methyl-2-pyrrolidone (NMP) solubility> The state after mixing 2.5 parts by mass of the sample and 47.5 parts by mass of NMP at room temperature (20 to 25 °C) and the state after mixing at 100 °C were visually observed and evaluated according to the following criteria. ◎: Easily and completely dissolved at room temperature 〇: Completely dissolved by adjusting the temperature to 100 °C. △: Partially undissolved gel remains even after adjusting the temperature to 100 °C. ×: Swells or is insoluble even after adjusting the temperature to 100 °C.

[0084] <Solvent resistance> 0.3 part by mass of the sample and 5.7 parts by mass of a mixed solvent of ethylene carbonate:diethyl carbonate = 1:1 as a solvent were added to a screw bottle with a volume of about 10 ml, and the state after holding at 85 °C for 6 hours was visually observed and evaluated according to the following criteria. 〇: Insoluble. △: Swells or gels. ×: Partially dissolved or completely dissolved.

[0085] <Dispersion viscosity and dispersion storage stability> A plastic bottle was charged with 1 part by mass of the sample, 13.5 parts by mass of Denka Black Li Li-435 as a conductive material, and 85.5 parts by mass of NMP, and zirconia beads were used as a medium to disperse with a paint shaker until the above viscosity (50 to 2000 mPa·s) was obtained to prepare a dispersion. The dispersion viscosity was measured using a B-type viscometer at 25 °C under atmospheric pressure in accordance with JIS K7117-1.

[0086] The viscosity of the dispersion immediately after preparation was calculated as the relative value when the viscosity value of the dispersion containing methyl cellulose in Comparative Example 1 below was set to 100. Note that the lower the numerical value of the dispersion viscosity, the better.

[0087] In addition, the storage stability of the dispersion was calculated as the relative value when the viscosity of each dispersion after standing at 25°C for one week was measured and the viscosity value of each dispersion immediately after preparation was set to 100. Note that the closer the numerical value of the dispersion storage stability is to 100, the better.

[0088] (Example 1) To a 5000 mL separable flask equipped with a three-one motor, a reflux condenser, a thermometer, and a dropping funnel, 100 g of methyl cellulose (manufactured by Fujifilm Wako Pure Chemical Corporation: degree of methyl group substitution 1.8) and 2000 mL of isopropyl alcohol were added and stirred at room temperature. Then, 250 g of a 48% by mass aqueous sodium hydroxide solution was added and stirred for another 1 hour. 120 mL of octyl iodide was added dropwise and stirred at room temperature for another 30 minutes. Then, it was stirred at 70°C for 5 hours and then returned to room temperature. After filtering off the white solid by suction filtration, it was washed twice with water. It was heated and dried at 80°C for 12 hours to obtain 95 g of methyl octyl cellulose.

[0089] The "degree of substitution", "NMP solubility", and "solvent resistance" of the obtained methyl octyl cellulose; and the "dispersion viscosity" and "dispersion storage stability" of the dispersion containing the methyl octyl cellulose were determined by the above methods, respectively. The results are shown in Table 1.

[0090] (Example 2) Except that the addition amount of octyl iodide was changed to 270 ml, in the same manner as in Example 1, 101 g of methyl octyl cellulose was obtained. Various measurements of the obtained methyl octyl cellulose and the dispersion containing the methyl octyl cellulose were also carried out in the same manner. The results are shown in Table 1.

[0091] (Example 3) Except that the addition amount of octyl iodide was changed to 510 ml, methyl octyl cellulose 111 g was obtained in the same manner as in Example 1. Various measurements of the obtained methyl octyl cellulose and the dispersion containing the methyl octyl cellulose were also carried out in the same manner. The results are shown in Table 1.

[0092] (Example 4) Except that the addition amount of octyl iodide was changed to 1160 ml, methyl octyl cellulose 150 g was obtained in the same manner as in Example 1. Various measurements of the obtained methyl octyl cellulose and the dispersion containing the methyl octyl cellulose were also carried out in the same manner. The results are shown in Table 1.

[0093] (Example 5) As the methyl cellulose, methyl cellulose (DS 1.0) obtained by the following Preparation Method 1 was used. Except that the addition amount of octyl iodide was changed to 440 ml, methyl octyl cellulose 92 g was obtained in the same manner as in Example 1. Various measurements of the obtained methyl octyl cellulose and the dispersion containing the methyl octyl cellulose were also carried out in the same manner. The results are shown in Table 1.

[0094] (Preparation Method 1) 100 g of crushed pulp and 390 ml of 48 mass% aqueous sodium hydroxide solution were added to a 3 L autoclave equipped with a stirrer, and stirred at 45 °C for 1 hour under a nitrogen atmosphere (first step). After cooling, it was cooled to -40 °C in a dry ice / methanol bath, and further stirred with 150 ml of toluene and 310 g of chloromethane at 60 °C for 1 hour and then at 100 °C for 3 hours (second step). After returning to room temperature, the residual gas in the system was exhausted and poured into 12 L of methanol with vigorous stirring to obtain a white solid (third step). The white solid was separated by suction filtration and washed 3 times with a large amount of isopropyl alcohol. The obtained white solid was vacuum dried at 80 °C for 15 hours to obtain methyl cellulose (DS 1.0) as a white powder.

[0095] (Example 6) As methyl cellulose, methyl octyl cellulose (101 g) was obtained in the same manner as in Example 1, except that methyl cellulose (DS 0.48) obtained by the following Preparation Method 2 was used and the addition amount of octyl iodide was changed to 760 ml. Various measurements were also carried out in the same manner for the obtained methyl octyl cellulose and the dispersion containing the methyl octyl cellulose. The results are shown in Table 1.

[0096] (Preparation Method 2) Methyl cellulose was obtained in the same manner as in Preparation Method 1, except that 200 ml of 48% aqueous sodium hydroxide solution and 170 g of chloromethane were used.

[0097] (Comparative Example 1) As methyl cellulose, methyl cellulose (manufactured by Fujifilm Wako Pure Chemical Corporation: degree of methyl group substitution 1.8) was used. Various measurements were also carried out in the same manner for this methyl cellulose and the dispersion containing the methyl cellulose. The results are shown in Table 1.

[0098] (Comparative Example 2) Methyl butyl cellulose (95 g) was obtained in the same manner as in Example 1, except that octyl iodide was changed to 190 ml of butyl iodide. Various measurements were also carried out in the same manner for the obtained methyl butyl cellulose and the dispersion containing the methyl butyl cellulose. The results are shown in Table 1.

[0099] (Comparative Example 3) Methyl hexyl cellulose (98 g) was obtained in the same manner as in Example 1, except that octyl iodide was changed to 238 ml of hexyl iodide. Various measurements were also carried out in the same manner for the obtained methyl hexyl cellulose and the dispersion containing the methyl hexyl cellulose. The results are shown in Table 1.

[0100]

Table 1

[0101] As shown in Table 1, the values of "dispersion storage stability" for Comparative Examples 1, 2, and 3 were 145, 247, and 267, respectively, all of which increased significantly from the values of the viscosities of the respective dispersions immediately after preparation (100). In contrast, the values of "dispersion storage stability" for the Examples were all 5 or more lower than 145 of Comparative Example 1, and were excellent in the storage stability of viscosity (a value 5 or more lower than 145 of Comparative Example 1 is a significant difference). Thus, the conductive material dispersion of the Examples was excellent in the storage stability of viscosity even after one week had passed since its preparation.

[0102] Among the Examples, Examples 1, 2, 3, and 5 in which the degree of octyl group substitution of methyl octyl cellulose was suitable had values of "dispersion storage stability" of 93, 85, 96, and 103, respectively, with almost no change from the values of the viscosities of the respective dispersions immediately after preparation (100), and were particularly excellent in the storage stability of viscosity.

Claims

1. Contains a conductive material, methyl octyl cellulose and a dispersion medium, The methyloctyl cellulose has a degree of methyl group substitution of 0.1 or more and less than 2.9, a degree of octyl group substitution of 0.01 or more and less than 2.9, and the sum of the degree of methyl group substitution and the degree of octyl group substitution is less than 3.

0.

2. An electrode paste for a positive electrode of a lithium ion secondary battery, comprising the conductive material dispersion according to claim 1, an active material, and a binder.

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