Conductive material dispersion liquid, positive electrode for lithium ion secondary battery using the same, and manufacturing method of lithium ion secondary battery

Abstract

Disclosed is a conductive material dispersion liquid having excellent properties. The conductive material dispersion liquid contains a conductive material, a dispersion medium, a polyvinyl acetal resin, and a cellulose resin. The polyvinyl acetal resin is contained in an amount of 10 to 200 parts by weight per 100 parts by weight of the cellulose resin.

Description

TECHNICAL FIELD

[0001] The present application provides a conductive material dispersion liquid which is excellent in dispersibility and conductivity, and can maintain appropriate quality for a long period of time, is suitable as a material for an electrode paste for manufacturing an electrode, and a positive electrode for a lithium ion secondary battery and a manufacturing method of a lithium ion secondary battery using such a conductive material dispersion liquid. BACKGROUND

[0002] Lithium ion secondary batteries show the highest energy density among batteries that have been put into practical use, and are advancing toward being mounted in portable electronic devices such as smartphones and automobiles. Among others, in addition to miniaturization, weight reduction, and long life of lithium ion secondary batteries, further high performance such as improvement in safety and operation in a wide temperature range is required.

[0003] Generally, a lithium ion secondary battery is composed of an electrode, a separator, and an electrolyte solution containing an electrolyte. In addition, the electrode uses a positive electrode obtained by coating an electrode paste containing a positive electrode active material containing lithium ions and a conductive material, an organic binder, and the like on the surface of a current collector metal foil and adhering it, and a negative electrode obtained by adhering an electrode paste containing a negative electrode active material capable of deintercalating lithium ions and a conductive material, an organic binder, and the like on the surface of a current collector metal foil.

[0004] In particular, regarding the positive electrode, a transition metal such as LiCoO2, which is used as a positive electrode active material, has low electronic conductivity, and does not provide sufficient battery performance when used alone. Therefore, attempts have been made to reduce the internal resistance of the battery by using a carbon material such as carbon black or a carbon nanotube as a conductive material, to exert the original battery performance.

[0005] In order to reduce the internal resistance of the battery, it is necessary to form a good conductive path between the current collector and the active material, and between the active materials, and therefore, it is preferable to use a conductive material that shows high conductivity, such as carbon black having a small particle size and a high structure, or a carbon nanotube that is thin and long in outer diameter. However, such a conductive material is strong in cohesion, and therefore, easily becomes non-uniform, and if used directly, cannot exert sufficient performance, and therefore, a method of using a conductive material dispersion liquid in which the conductive material is dispersed in an optimal state in advance is known, and in order to disperse the conductive material more uniformly, and to suppress an increase in viscosity, the use of a dispersant is proposed (Patent Documents 1 and 2).

[0006] PRIOR ART DOCUMENTS

[0007] PATENT DOCUMENTS

[0008] Patent Document 1: Japanese Patent No. 5628503

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2011-184664 SUMMARY

[0010] PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] However, for the method described in Patent Literature 1, the effect of suppressing the viscosity increase is still insufficient. In particular, when the concentration of the conductive material is high, the viscosity of the conductive material dispersion liquid increases greatly, and thus the workability becomes poor. Furthermore, the storage stability of the conductive material dispersion liquid is also insufficient, and the viscosity increases with time. In the case where an electrode paste is produced by mixing an active material with a binder and an arbitrary additive using such a conductive material dispersion liquid, not only is the condition at the time of kneading deviated, and the productivity is decreased, but also a position where the balance of the amount of the conductive material covering the active material is disrupted is generated, and thus there is also a possibility that stable quality cannot be ensured.

[0012] In order to avoid this problem, it is also considered to use a conductive material dispersion liquid of low concentration, but in this case, the amount of the solvent used increases, and the load in the drying process increases. Furthermore, in order to make the electrode paste be in an appropriate viscosity range, there is a possibility that the degree of freedom of the electrode composition design is decreased, and becomes a fetter of development toward high performance.

[0013] METHOD FOR SOLVING THE PROBLEM

[0014] Therefore, the present inventors have made intensive studies in order to solve the above problem, and as a result, have found that by a specific combination of resins, the viscosity can be kept low even at a high concentration, and the storage stability is excellent, and thus have achieved the present invention.

[0015] That is, the present invention provides a conductive material dispersion liquid for a positive electrode of a lithium ion secondary battery, which has excellent storage stability, and in which a conductive material can be contained at a high concentration, and a method of manufacturing a lithium ion secondary battery and a positive electrode for a lithium ion secondary battery using such a conductive material dispersion liquid.

[0016] EFFECT OF THE INVENTION

[0017] According to the present invention, by using a conductive material dispersion liquid in which a conductive material is dispersed at a high concentration in a dispersion medium, the kneading time at the time of manufacturing an electrode paste can be shortened, and the in-plane deviation of a prepared positive electrode can be suppressed, and a lithium ion secondary battery of stable quality can be provided. Furthermore, since the solid content of the electrode paste can be increased by the present invention, not only can the time of the drying process be shortened, but also the absolute amount of the solvent used can be reduced, and thus the load of regenerating the solvent is also reduced. That is, the environmental load can be reduced. DETAILED DESCRIPTION

[0018] That is, the present invention is:

[0019] (1) A conductive material dispersion liquid containing at least a conductive material, a dispersion medium, a polyvinyl acetal-based resin, and a cellulose-based resin, wherein, with respect to 100 parts by weight of the cellulose-based resin, 10 to 200 parts by weight of the polyvinyl acetal-based resin is contained,

[0020] (2) The conductive material dispersion liquid according to the above (1), wherein the conductive material is carbon black having a primary particle diameter of 30 nm or less and a DBP oil absorption amount of 160 to 250 ml / 100 g,

[0021] (3) The conductive material dispersion liquid according to the above (1) or (2), wherein the polyvinyl acetal-based resin has an average degree of polymerization of 100 to 600,

[0022] (4) The conductive material dispersion liquid according to any one of the above (1) to (3), wherein the cellulose-based resin has a weight average molecular weight of 5,000 to 50,000,

[0023] (5) A method for producing a positive electrode for a lithium ion secondary battery, characterized by mixing the conductive material dispersion liquid according to the above (1), (2), (3), or (4), an electrode active material, and a binder, coating on an electrode substrate, and drying,

[0024] (6) A method for producing a lithium ion secondary battery, characterized by mixing the conductive material dispersion liquid according to the above (1), (2), (3), or (4), an electrode active material, and a binder, coating on an electrode substrate, drying, and incorporating as a positive electrode.

[0025] [Conductive material dispersion liquid]

[0026] The conductive material dispersion liquid of the present application contains at least a dispersion medium, a conductive material, a polyvinyl acetal-based resin, and a cellulose-based resin.

[0027] <About the dispersion medium>

[0028] The dispersion medium used in the present application is not particularly limited, but since it is used as a binder for a battery, polyvinylidene fluoride is generally used, and thus it is necessary to dissolve it. Therefore, N-methyl-2-pyrrolidone is generally suitable. As long as the binder can be uniformly dissolved, it is not problematic even if other components are mixed.

[0029] <About the polyvinyl acetal-based resin>

[0030] The conductive material dispersion liquid of the present application contains a polyvinyl acetal-based resin.

[0031] The polyvinyl acetal-based resin is a high molecular compound having at least three kinds of repeating units of a unit having an acetal group (a substance obtained by acetalizing an ethylene alcohol unit of PVA), a unit having a hydroxyl group (an ethylene alcohol unit derived from PVA), and a unit having an acetyl group (an unsaponifiable portion derived from the production of PVA). Therefore, it is represented by the following formula (Formula 1) on a representative basis.

[0032]

[0033] The polyvinyl acetal-based resin used in the present application is not particularly limited, and various commercially available products can be used alone or two or more kinds can be used together. Further, the acetal group is not particularly limited, and various polyvinyl acetal-based resins synthesized by publicly known methods can be used. For example, polyvinyl butyral resin in which R is a butyl group, polyvinyl acetoacetal resin in which R is an acetyl group, and the like can be mentioned.

[0034] In Formula 1, 1 is preferably 50 to 90 mol%, m is preferably 0 to 10 mol%, and n is preferably 10 to 50 mol%. Particularly preferably, 1 is 50 to 80 mol%, m is 0 to 5 mol%, and n is 15 to 40 mol%.

[0035] As commercially available products, Escored B BL-1, Escored B BL-10, Escored B BL-S, Escored B BX-L, Escored K KS-10 (trade names, manufactured by Nippon Shokubai Co., Ltd.), Mowital B14S, Mowital B16H, Mowital B20H, Mowital B30T, Mowital B30H, Mowital B30HH, Mowital B45M, Mowital B45H, Mowital B60T, Mowital B60H, Mowital B60HH, Mowital 75H (trade names, manufactured by Kuraray Co., Ltd.), and the like can be mentioned.

[0036] Among them, polyvinyl acetal resins having an average polymerization degree of 100 to 600 are good, preferably 150 to 600, and more preferably 200 to 500. The average polymerization degree can be measured in accordance with JIS K6726.

[0037] As long as a substance that is well dissolved according to the solvent is selected among them, in the case of using N-methyl-2-pyrrolidone, Escored BL-1, Escored BL-10, Escored BX-L, Mowital B14S, Mowital B16H, and Mowital B20H are suitable.

[0038] <About the cellulose-based resin>

[0039] The conductive material dispersion liquid of the present application contains a cellulose-based resin.

[0040] As the cellulose resin used in the present application, there is no particular limitation as long as it is a high molecule having a cellulose skeleton, and specifically, alcohol-solubilized butyrate of cellulose, cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, ethylhydroxyethyl cellulose, nitrocellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, ammonium carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, and the like can be mentioned.

[0041] Among them, particularly, a high molecule having a weight average molecular weight of 5,000 to 200,000, preferably a weight average molecular weight of 5,000 to 100,000, and further preferably a weight average molecular weight of 5,000 to 50,000 is a cellulose resin.

[0042] As long as a substance that is well dissolved according to the solvent is selected from among them, in the case of using N-methyl-2-pyrrolidone, cellulose acetate, cellulose acetate butyrate, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, ethylhydroxyethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and hydroxypropylmethyl cellulose are suitable. Furthermore, in terms of excellent resistance to electrolyte, cellulose acetate and methyl cellulose are good, and among them, methyl cellulose is most preferable.

[0043] <About the addition amount and mixing ratio of the polyvinyl acetal-based resin and the cellulose-based resin>

[0044] As the total amount of the polyvinyl acetal-based resin and the cellulose-based resin in the conductive material dispersion liquid, 3 to 30 parts by weight, further preferably 5 to 20 parts by weight, and more preferably 6 to 15 parts by weight, relative to 100 parts by weight of the conductive material is preferable.

[0045] If the content of the polyvinyl acetal-based resin and the cellulose-based resin is too small, the agglomerates of the conductive material cannot be sufficiently broken up, and the conductivity of the coating film made of the electrode paste obtained by mixing such a conductive material dispersion liquid with an active material and an organic binder in a non-uniform state has a tendency to decrease. If the content of the polyvinyl acetal-based resin and the cellulose-based resin in the conductive material dispersion liquid is too large, the resistance component in the coating film made of the electrode paste obtained using the conductive material dispersion liquid increases, and thus the conductivity decreases, and in the case of making a lithium ion secondary battery having a positive electrode including such a coating film, sometimes high capacity becomes difficult.

[0046] Further, in the present application, the above polyvinyl acetal-based resin and the cellulose-based resin are contained in a certain ratio. That is, it is characterized in that, with respect to 100 parts by weight of the cellulose-based resin, 10 to 200 parts by weight of the polyvinyl acetal-based resin is contained.

[0047] By being contained in this ratio, a conductive material dispersion liquid that is uniform and has good stability can be obtained, and the effects of low viscosity and optimization of stability of the conductive material dispersion liquid are excellent. More preferably, it is 10 to 150 parts by weight with respect to 100 parts by weight of the cellulose-based resin, and further preferably, it is 25 to 100 parts by weight.

[0048] <About the Conductive Material>

[0049] As the conductive material used in the present application, carbon black and carbon nanotubes, carbon nanofibers, graphite, graphene, hard carbon, and the like are suitable. As the carbon black, Ketjen black, furnace black, acetylene black, thermal cracking carbon black, and the like can be used. Further, one of these conductive materials can be used alone or two or more of them can be used together.

[0050] The average primary particle diameter of the carbon black is preferably 50 nm or less, particularly preferably 40 nm or less, and more preferably 30 nm or less. Further, the average primary particle diameter is preferably 10 nm or more, and particularly preferably 15 nm or more. If the average primary particle diameter of the carbon black is too large, there is a tendency for the conductivity of the coating film obtained from the electrode paste to decrease. Further, if it is too small, sometimes the viscosity of the conductive material dispersion liquid and the electrode paste becomes too high, the dispersion of the carbon black becomes difficult, and sufficient conductivity cannot be exerted.

[0051] The so-called average primary particle diameter indicates the arithmetic average particle diameter measured using a transmission electron microscope in accordance with ASTM: D3849-14. In addition, the average primary particle diameter is generally used for evaluating the physical properties of the conductive material.

[0052] The DBP oil absorption amount of the carbon black is preferably 160 to 250 ml / 100 g, more preferably 170 to 240 ml / 100 g, and most preferably 170 to 230 ml / 100 g. If the DBP oil absorption amount of the carbon black is too small, the connection of the carbon black particles to each other is short, and the conductivity is lacking. Further, if it is too large, sometimes the viscosity of the conductive material dispersion liquid and the electrode paste becomes too high, the dispersion of the carbon black becomes difficult, and sufficient conductivity cannot be exerted.

[0053] The DBP oil absorption amount can be measured in accordance with JIS6217-4. Further, the DBP oil absorption amount is used as an index of the conductivity because it reflects the degree of development of the aggregate, which is called structure, as a state in which the carbon black particles are fused to each other.

[0054] The dispersed particle diameter of the carbon black in the dispersion is preferably 40 μm or less as the maximum particle diameter, further preferably 30 μm or less, and more preferably 20 μm or less. In general, the particle state of the dispersion of the conductive material or the like is managed using the average particle diameter. However, in the case of using the average particle diameter, since the presence of coarse particles is not taken into account, even in the case where the value of the average particle diameter is small, there are sometimes coarse particles of 40 μm or more present as the maximum particle diameter in practice. In this case, the distribution of the active material and the conductive material in the electrode coating film of the lithium ion secondary battery is non-uniformized, and the possibility of impairing the battery performance arises.

[0055] The maximum particle diameter can be measured using a particle size meter in accordance with JIS K5600-2-5.

[0056] The purity of the carbon black is preferably 99.90 to 100% by mass, and more preferably 99.95 to 100% by mass. In addition, the purity of the carbon black can be calculated based on the amount of impurities, taking the ash content measured in accordance with JIS K1469 or JIS K6218 as the impurities.

[0057] As the carbon black having these characteristics, acetylene black can be given, and specifically, Denka Black Powdery, Denka Black Granular, Denka Black FX-35, Denka Black HS-100, Denka Black Li Li-100, Denka Black Li Li-250, Denka Black Li Li-400, Denka Black Li Li-435, and the like (all of which are trade names, manufactured by Denki Kagaku K.K.) can be given. Among these, Denka Black FX-35 and Denka Black Li Li-435 are particularly suitable.

[0058] The carbon nanotube is a carbon crystal that forms a substantially cylindrical shape. The average outer diameter of the carbon nanotube is preferably 90 nm or less, particularly preferably 30 nm or less, further preferably 20 nm or less, and most preferably 15 nm or less. In addition, the average outer diameter thereof is preferably 1 nm or more, or 5 nm or more. If the average outer diameter of the carbon nanotube is too large, the conductivity of the coating film obtained from the electrode paste has a tendency to decrease. In addition, if it is too small, the viscosity of the conductive material dispersion and the electrode paste sometimes becomes too high, and the dispersion of the carbon nanotube becomes difficult.

[0059] The average outer diameter of the carbon nanotube is an arithmetic mean of the outer diameters of a sufficient number of n, measured using an image of a magnification of 100,000 times or more of a transmission electron microscope.

[0060] As the carbon nanotube, specifically, there can be mentioned, for example, VGCF-X (average outer diameter 30 nm) manufactured by Showa Denko K.K., C100 (average outer diameter 10-15 nm), U100 (average outer diameter 10-15 nm high purity product) 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 Tokai Carbon Co., Ltd. In addition, the carbon nanotube can be used singly or two or more kinds can be used together.

[0061] In the case where the conductive material dispersion liquid of the present disclosure contains the carbon nanotube, it is more preferable that the carbon nanotube is dispersed independently as one by one without aggregation. This is because the conductive property of the coating film obtained from the electrode paste is excellent.

[0062] A plurality of conductive materials such as carbon black and carbon nanotube can also be used in combination.

[0063] As the amount of the conductive material contained in the conductive material dispersion liquid, 10 to 30% by weight is suitable, preferably 12 to 25% by weight, and more preferably 13 to 22% by weight.

[0064] If the content of the conductive material in the conductive material dispersion liquid is too small, the total solid content at the time of preparation of the electrode paste decreases, and the viscosity becomes low compared to the appropriate viscosity, and thus a non-uniform coating film is produced. The non-uniform coating film means a coating film in a state where the active material and the conductive material are biased, or a coating film in a state where the coating amount (coating amount on the current collector) is deviated depending on the position. In the case of the lithium ion secondary battery in which the positive electrode is constituted of the coating film in a state where the active material and the conductive material are biased, there is a possibility that the performance such as reduction of the conductivity, or deterioration of the high-speed charge and discharge due to the charge bias is impaired. In the case where the coating film in a state where the coating amount is deviated depending on the position is used to manufacture a plurality of lithium ion secondary batteries, since the capacity of each of the lithium ion secondary batteries is deviated, there is a possibility that the yield is deteriorated. If the content of the conductive material in the conductive material dispersion liquid is too large, sometimes the flowability of the conductive material dispersion liquid is reduced, and the handleability at the time of preparation of the electrode paste is deteriorated.

[0065] <Concerning Arbitrary Components>

[0066] The conductive material dispersion liquid of the present application can appropriately contain any component other than the conductive material, the cellulose-based resin, the polyvinyl acetal-based resin, and the dispersion medium within the scope of the object of the present application. As such any component, for example, a dispersant (a component other than the above-mentioned cellulose-based resin and polyvinyl acetal-based resin and having a function of dispersing the conductive material); a phosphorus compound; a sulfur compound; an organic acid; an amine compound, an ammonium compound, and the like nitrogen compound; an organic ester; and a coupling agent of various silane-based, titanium-based, and aluminum-based systems, and the like conventionally known additive can be cited. The any component can be used singly or two or more kinds can be used together.

[0067] As the dispersant, for example, poly 1,1-difluoroethylene, polytetrafluoroethylene, polyhexafluoropropylene, polyethylene, polypropylene, polymethyl methacrylate, polyvinyl chloride, poly 1,1-dichloroethylene, polyvinyl acetate, polyacrylic acid, polyvinyl butyral, polyacrylamide, polyurethane, polydimethylsiloxane, epoxy resin, acrylic resin, polyester resin, melamine resin, phenol resin, various rubber, lignin, pectin, gelatin, xanthan gum, welan gum, succinoglycan, polyvinyl alcohol, polyalkylene oxide, polyvinyl ether, polyvinyl pyrrolidone, chitin, chitosan, and starch, and the like nonionic dispersant can be cited.

[0068] The mixing amount of the dispersant is preferably 0.1 to 100 parts by mass, more preferably 0.1 to 50 parts by mass, relative to 100 parts by mass of the conductive material.

[0069] As the phosphorus compound, for example, tributylphosphine, triphenylphosphine, triethyl phosphite, and triphenyl phosphite can be cited.

[0070] As the sulfur compound, for example, butanethiol, n-hexanethiol, ethyl sulfide, and tetrahydrothiophene can be cited.

[0071] As the organic acid, for example, acetic acid, propionic acid, butyric acid, hexanoic acid, acrylic acid, crotonic acid, decanoic acid, stearic acid, oleic acid, oxalic acid, succinic acid, adipic acid, maleic acid, glutaric acid, benzoic acid, 2-methylbenzoic acid, 4-methylbenzoic acid, and a mixture of two or more kinds thereof can be cited.

[0072] As the amine compound, for example, methylamine, ethylamine, n-propylamine, n-butylamine, n-hexylamine, n-heptylamine, 2-ethylhexylamine, n-octylamine, nonylamine, decylamine, dodecylamine, dodecylamine, hexadecylamine, octadecylamine, isopropylamine, isobutylamine, isooctylamine, isopentylamine, 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, hydroxylamine, ethanolamine, diethanolamine, methyldiethanolamine, 2-hydroxypropylamine, N-ethyldiethanolamine, N-methyldiethanolamine, aminoethyl ethanolamine, dimethylethanolamine, triisopropanolamine, triethanolamine, ethylenediamine, propylenediamine, triethylenediamine, triethylenetetramine, 1,6-hexanediamine, 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, hexamethylenimine, polyallylamine, polyethylenimine, adipic acid dihydrazide, and the like can be given.

[0073] As the ammonium compound, for example, 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 isopropylcarbamate, isopropylammonium isopropylcarbonate, isopropylammonium isopropylcarbonate, ethylammonium ethylcarbamate, pyridinium ethylhexylcarbamate Ethylammonium ethylcarbonate, octadecylammonium octadecylcarbamate, octadecylammonium octadecylcarbonate, ammonium carbamate, di(octadecyl)ammonium di(octadecyl)carbamate, di(octadecyl)ammonium di(octadecyl)carbonate, dibutylammonium dibutylcarbamate, dibutylammonium dibutylcarbonate, triethoxysilylpropylammonium triethoxysilylpropylcarbamate, triethoxysilylpropylammonium triethoxysilylpropylcarbonate, hexamethyleneimine ammonium carbamate, hexamethyleneimine ammonium carbonate, benzylammonium benzylcarbamate, benzylammonium benzylcarbonate, methyldecylammonium methyldecylcarbamate, methyldecylammonium methyldecylcarbonate, morpholinium carbamate Morpholinium carbonate 2-Ethylhexylammonium bicarbonate, 2-cyanoethylammonium bicarbonate, 2-methoxyethylammonium bicarbonate, t-butylammonium bicarbonate, ammonium bicarbonate, isopropylammonium bicarbonate, di(octadecyl)ammonium bicarbonate, triethylenediamine bicarbonate, and pyridinium bicarbonate and the like, and derivatives or mixtures thereof, and the like.

[0074] As the organic ester, 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, and the like can be given.

[0075] As the silane coupling agent, vinyltrimethoxysilane, γ-methacryloyloxypropyl-tris(β-methoxyethoxy)silane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, vinyltriacetoxysilane, γ-mercaptopropyltrimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropylmethyimethoxysilane, N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane, N,N-bis(β-hydroxyethyl)-γ-aminopropyltriethoxysilane, γ-chloropropyltrimethoxysilane, vinyltris(2-methoxyethoxysilane), 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, and γ-chloropropyltrimethoxysilane, and the like can be given.

[0076] As the titanium coupling agent, for example, tetrabutyl titanate, tetraoctyl titanate, isopropyl triisostearyl titanate, isopropyl tridecylbenzenesulfonyl titanate, bis(dioctylpyrophosphato)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 a mixture thereof, and the like can be given.

[0077] As the aluminum coupling agent, for example, various aluminum chelates, aluminum alkylacetoacetate diisopropyl, aluminum • bisethylacetate • diisopropyl, acetoalkoxy aluminum diisopropyl, and a mixture thereof, and the like can be given.

[0078] <Viscosity of dispersion liquid>

[0079] The viscosity of the conductive material dispersion liquid of the present application is preferably 50 to 5000 mPa-s, more preferably 80 to 3000 mPa-s, and further preferably 80 to 2000 mPa-s. If the viscosity of the dispersion liquid is less than 50 mPa-s, it is difficult to suppress the settlement of the conductive material in the dispersion liquid, and it is difficult to maintain the quality for a long period of time. Further, the viscosity of the electrode paste prepared by mixing the active material, the binder, and the conductive material dispersion liquid is lower than the optimum range, and it is difficult to uniformly produce the coating film of the target film thickness. In addition, if the viscosity of the dispersion liquid exceeds 5000 mPa-s, the viscosity of the electrode paste is also increased, and kneading and coating become difficult.

[0080] The viscosity of the conductive material dispersion liquid can be measured by using a B-type viscometer according to JIS K7117-1.

[0081] <Method for producing conductive material dispersion liquid>

[0082] The method for producing the conductive material dispersion liquid of the present application is not limited as long as it can contain each component described above in a prescribed mixing ratio and produce the desired viscosity, but the following method is preferred.

[0083] First, the polyvinyl acetal resin and the cellulose resin are dissolved in a dispersion medium (preferably, N-methyl-2-pyrrolidone). In this solution, the arbitrary components and the conductive material are mixed as necessary, and then dispersed by a general dispersion device such as a bead mill while pulverizing the aggregated conductive material, and the dispersion is continued until the prescribed viscosity is obtained. This operation can obtain the conductive material-containing dispersion liquid having the prescribed dispersion particle diameter and viscosity at the prescribed concentration.

[0084] The dispersing device is preferably one that can disperse to a maximum particle diameter of 20 μm or less, but is not particularly limited to a bead mill, and examples include a ball mill, a jet mill, and the like.

[0085] [Use of the conductive material dispersion liquid to a lithium ion secondary battery]

[0086] As the method of using the conductive material dispersion liquid of the present application, the conductive material dispersion liquid of the present application, an active material for a positive electrode, and a binder or the like are mixed to make an electrode paste for coating on an electrode substrate, thereby obtaining a lithium ion secondary battery. As the method thereof, various methods known in the past can be adopted. As a representative example, the conductive material dispersion liquid of the present application is mixed with a positive electrode active material and a binder to be slurried, which is coated on an electrode substrate to be dried to form an electrode. This is used as a positive electrode of a lithium ion secondary battery, and between this and a negative electrode formed of a carbon material such as graphite, a separator of an insulating material as a porous substance is interposed, and is housed in a cylindrical shape or a flat shape according to the shape of the container, and an electrolyte is injected therein. The lithium ion secondary battery of the present application obtained by such an operation can maintain performance even if repeated charging and discharging for a long period of time.

[0087] Examples

[0088] Hereinafter, the present application will be explained in detail based on examples, but the present application is not limited only to these examples.

[0089] The carbon black, polyvinyl acetal-based resin, cellulose-based resin, active material, and binder used in the examples and comparative examples are shown below.

[0090] < Carbon Black >

[0091] • Denka Black Li Li-435 (trade name. Manufactured by Denki Kagaku K.K.): acetylene black, primary particle diameter 23 nm, DBP oil absorption 220 ml / 100 g, hereinafter abbreviated as Li-435.

[0092] < Polyvinyl Acetal-Based Resin >

[0093] • Esterex B BL-1 (trade name. Manufactured by Sekisui Chemical Co., Ltd.): polyvinyl butyral resin, average polymerization degree 300, hydroxyl group amount 36 mol%, butyralation degree 63 mol%, hereinafter abbreviated as BL-1.

[0094] • Esterex B BL-10 (trade name. Manufactured by Sekisui Chemical Co., Ltd.): polyvinyl butyral resin, average polymerization degree 250, hydroxyl group amount 28 mol%, butyralation degree 71 mol%, hereinafter abbreviated as BL-10.

[0095] • Escored B BX-L (trade name. Made by Sekisui Chemical Co., Ltd.): polyvinyl acetal resin, average degree of polymerization 250, hydroxyl group amount 32 mol%, acetalization degree 67 mol%, hereinafter abbreviated as BX-L.

[0096] • Mowiol B 16H (trade name. Made by Kuraray Co., Ltd.): polyvinyl butyral resin, average degree of polymerization 250 to 300, hydroxyl group amount 26 to 30 mol%, butyralization degree 70 to 73 mol%, hereinafter abbreviated as B 16H.

[0097] • Mowiol B 20H (trade name. Made by Kuraray Co., Ltd.): polyvinyl butyral resin, average degree of polymerization 250 to 500, hydroxyl group amount 26 to 30 mol%, butyralization degree 70 to 73 mol%, hereinafter abbreviated as B 20H.

[0098] < Cellulose Resin >

[0099] • Methyl cellulose: weight average molecular weight 35,000 to 45,000, methoxyl substitution degree 1.8, hereinafter abbreviated as MC. The weight average molecular weight can be measured using gel filtration chromatography. An example of the measurement conditions in the gel filtration chromatography is described below.

[0100] Apparatus: Prominence (made by Shimadzu Corporation)

[0101] Column: OHpak SB-802.5 HQ (made by Shodex Corporation),

[0102] OHpak SB-804 HQ (made by Shodex Corporation)

[0103] Detector: RI

[0104] Eluent: 0.5 M aqueous NaCl solution

[0105] Flow rate: 1.0 ml / min

[0106] Sample concentration: 0.2 wt / vol%

[0107] Column temperature: 40°C

[0108] < Active Material >

[0109] • Celtec C-5H (trade name. Made by Nippon Chemical Industrial Co., Ltd.): active material for positive electrode, lithium cobaltate (LiCoO2), D50 7.2 μm, specific surface area 0.46 m 2 / g, hereinafter abbreviated as LCO.

[0110] < Binder >

[0111] • KF Polymer W#1100 (trade name. Made by Kureha Corporation): Polyvinylidene fluoride, average molecular weight 280000, hereinafter abbreviated as PVDF.

[0112] The various evaluations in the examples and comparative examples were performed by the following methods.

[0113] <Viscosity of dispersion liquid, storage stability of dispersion liquid>

[0114] The viscosity of the conductive material dispersion liquid produced by the method described in Example 1 was measured using the following value: the value after 60 seconds when rotated at 60 rpm at 25°C using a B-type viscometer (TVB-10: trade name. Made by Tokimec, Inc.).

[0115] Further, the storage stability of the dispersion liquid was measured for the viscosity of each dispersion liquid after standing for 1 week at 25°C, and the degree of change in viscosity relative to the initial viscosity of each dispersion liquid was evaluated using the following criteria.

[0116] O: The absolute value of the amount of change in initial viscosity was 10% or less.

[0117] Δ: The absolute value of the amount of change in initial viscosity was more than 10% and 15% or less.

[0118] X: The absolute value of the amount of change in initial viscosity was more than 15%.

[0119] <Storage stability of electrode paste>

[0120] An electrode paste was obtained by adding 16.45 parts by mass of LCO, 3.22 parts by mass of a PVDF solution (PVDF: 0.48 parts by mass), 2.63 parts by mass of the conductive material dispersion liquid obtained in the examples and comparative examples, and 2.70 parts by mass of N-methyl-2-pyrrolidone to a plastic container, and mixing using an awatori Rikurito (trade name. Made by Sinkya Corporation), and the polyvinyl acetal resin and the cellulose resin contained in the conductive material dispersion liquid were considered as the binder component, and the amount was set to the total binder amount in combination with the added PVDF.

[0121] The viscosity of the obtained electrode paste was measured using a rheometer under the conditions shown below.

[0122] Apparatus: HAAKE MARS III (Made by Thermo Fisher Scientific, Inc.)

[0123] Sensor: Cone C35 / 1°

[0124] Measurement temperature: 25°C

[0125] Shear rate: 10 s-1

[0126] Measurement time: 60 seconds

[0127] Further, the degree of change in the viscosity of each electrode paste with respect to the initial viscosity was evaluated using the following criteria based on the viscosity of each electrode paste after storage at 25°C for 1 week. In addition, in measuring the viscosity of the electrode paste after 1 week, in order to eliminate sedimentation, the electrode paste was mixed in advance with an awatoritaro at 2000 rpm for 20 seconds.

[0128] : The absolute value of the amount of change in the initial viscosity is 10% or less.

[0129] : The absolute value of the amount of change in the initial viscosity is more than 10% and 15% or less.

[0130] : The absolute value of the amount of change in the initial viscosity is more than 15% and 20% or less.

[0131] : The absolute value of the amount of change in the initial viscosity is more than 20%.

[0132] (Example 1)

[0133] In a plastic bottle, 78.4 parts by mass of N-methyl-2-pyrrolidone, 1.12 parts by mass of MC, and 0.48 parts by mass of BL-1 were added as a dispersion medium, mixed, and dissolved. Then, 20 parts by mass of Li-435 was added, and zirconium oxide beads were used as a medium to disperse for 6 hours with a paint shaker to obtain a conductive material dispersion liquid. The "dispersion liquid viscosity", "storage stability of the dispersion liquid", and "storage stability of the electrode paste" of the obtained dispersion liquid were each determined by the above-described methods. The results are shown in Table 1.

[0134] (Examples 2 to 10, Comparative Examples 1 to 11)

[0135] The composition shown in Table 1 (polyvinyl acetal resin type, mixing ratio) was changed, and otherwise, the same operations as in Example 1 were performed to obtain a conductive material dispersion liquid. Various measurements of the obtained conductive material dispersion liquid were also performed in the same manner. The results are shown in Table 1.

[0136] [Table 1]

[0137]

[0138] According to the results shown in Table 1, the initial viscosity of the conductive material dispersion liquids of Examples 1 to 10 containing polyvinyl acetal resins and cellulose resins was 2000 mPa-s or less, and in particular, regarding the storage stability of the electrode paste of Examples 1 to 3, the absolute value of the amount of change in the initial viscosity was 10% or less, and the amount of change was suppressed to the lowest.

[0139] In Comparative Examples 1 to 6 containing only either one and Comparative Examples 7 to 11 in which the mixing ratio of the polyvinyl acetal resin and the cellulose resin is out of the range of the present application, the initial viscosity of the conductive material dispersion liquid exceeds 2000 mPa-s, significantly impairing the workability. It is known that the initial viscosity and the storage stability of the conductive material dispersion liquid of the present application are both excellent compared to the comparative examples, and the effect of improving the storage stability of the electrode paste is great.

[0140] Industrial applicability

[0141] As above, it is known that by the present application, a conductive material dispersion liquid of excellent performance, a positive electrode for lithium ion batteries using the conductive material dispersion liquid, and a manufacturing method of a lithium ion secondary battery can be provided.

Claims

1. An electrically conductive material dispersion liquid containing at least an electrically conductive material, a dispersion medium, a polyvinyl acetal-based resin, and a cellulose-based resin, wherein, 10 to 200 parts by weight of a polyvinyl acetal-based resin, the total amount of the polyvinyl acetal-based resin and the cellulose-based resin is 3 to 30 parts by weight relative to 100 parts by weight of the conductive material, the amount of the conductive material contained in the conductive material dispersion liquid is 10 to 30% by weight, the conductive material is carbon black and / or carbon nanotube, the viscosity of the conductive material dispersion liquid is 50 to 5000 mPa-s.

2. The conductive material dispersion liquid according to claim 1, wherein the conductive material is carbon black having a primary particle diameter of 30 nm or less and a DBP oil absorption amount of 160 to 250 ml / 100 g.

3. The conductive material dispersion liquid according to claim 1 or 2, wherein the polyvinyl acetal-based resin has an average degree of polymerization of 100 to 600.

4. The conductive material dispersion liquid according to claim 1 or 2, wherein the cellulose-based resin has a weight average molecular weight of 5000 to 50000.

5. A method for producing a positive electrode for a lithium-ion secondary battery, characterized by, The conductive material dispersion liquid according to claim 1, 2, 3 or 4, an electrode active material and a binder are mixed, coated on an electrode substrate and dried.

6. A method for manufacturing a lithium-ion secondary battery, characterized by, The conductive material dispersion liquid according to claim 1, 2, 3 or 4, an electrode active material and a binder are mixed, coated on an electrode substrate, dried and incorporated as a positive electrode.

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