Method for manufacturing a conductive paste

Abstract

A method for producing a conductive paste, wherein a mixture containing carbon nanotubes (CNT) and a solvent, said mixture being obtained by bringing the CNT and the solvent into contact with each other so that the wetting rate is from 25% to 125%, thereby impregnating the CNT with the solvent, is kneaded by means of a planetary stirrer, thereby obtaining a paste-like kneaded material, and subsequently, a diluted material obtained by mixing a solvent into the thus-obtained kneaded material is subjected to a dispersion treatment. This method is capable of producing a conductive paste which contains CNT at a high concentration even with a small amount of a dispersant, and which is easy to handle because of a low density. This conductive paste is capable of forming a positive electrode mixture by being mixed with a positive electrode active material of a lithium ion secondary battery.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a conductive paste.

Background Art

[0002] Conventionally, as substances excellent in various properties such as conductivity, heat conductivity, and mechanical properties, carbon nanotubes (hereinafter sometimes referred to as "CNT") and the like are known. In recent years, techniques for improving the properties of various products such as films and fibers by using CNT have been proposed. Specifically, a technique has been proposed in which a CNT dispersion liquid containing CNT and an organic solvent is used as a material, and products containing CNT are manufactured to improve the properties of the products. Among them, the positive electrode of a lithium-ion secondary battery mainly uses three main materials: an active material, a binder, and a conductive assistant. Among these, since the active material, which accounts for 90% or more of the positive electrode mixture, has poor conductivity, carbon black (acetylene black) has been used as a conductive assistant to solve this problem. In recent years, however, CNT, which has better conductivity than carbon black, has attracted attention.

[0003] For CNT to effectively act as a conductive assistant, it is necessary for CNT to be sufficiently dispersed in the active material. However, since CNT has a property of easily aggregating and being difficult to disperse, in order to improve its dispersibility, attempts have been made to prepare a CNT dispersion liquid containing CNT and a dispersant in advance and mix this with an active material or the like. For example, Patent Document 1 discloses a CNT dispersion liquid with a CNT concentration of 2 to 30% using 30 to 200 parts by weight of a nonionic dispersant with respect to 100 parts by weight of CNT. However, when a dispersant is used, although CNT can be dispersed in the active material, there is a concern that side reactions may occur during battery operation, adversely affecting battery characteristics and safety. In addition, there is an unfavorable problem that the content rate of other active ingredients decreases only by the content of the dispersant.

[0004] Therefore, the preparation of a CNT dispersion liquid that does not use a dispersant as much as possible has also been studied. For example, methods using ultrasonic treatment (Patent Documents 2 and 3), methods using a rolling mill (Patent Document 4), methods using a colloid mill (Patent Document 5), methods using a homogenizer or a wet jet mill (Patent Documents 6 and 7), etc. are known.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in order to disperse CNTs at a high concentration without adding unnecessary components such as dispersants as much as possible, although it is necessary to disperse CNTs while suppressing the amount of the solvent, when using a disperser such as a sand mill, a jet mill, or a disper, there are problems such as the dispersion becoming highly viscous, adversely affecting the operation of the disperser, and taking too much time. In addition, in the methods described in Patent Documents 2, 3, 5, 6, and 7, since a solvent is required to impart a certain degree of fluidity, it is considered that there is a limit to increasing the concentration. In particular, ultrasonic treatment, homogenizers, wet jet mills, etc. are indispensable for a liquid medium, and it is difficult to disperse in a paste state. On the one hand, although a high-concentration CNT dispersion is disclosed in Patent Document 4, when the present inventor conducted a follow-up test, the viscosity of the obtained CNT dispersion was almost the same as that of the undispersed CNTs. When the viscosity of the CNT dispersion is extremely high in this way, since the fluidity of the dispersion is poor, it becomes difficult to ensure the processability, homogeneity, and coatability of the pharmaceutical preparation, and it becomes difficult to obtain a uniform pharmaceutical preparation electrode. In addition, the possibility that CNTs aggregate in the positive electrode mixture can also be considered.

[0007] Therefore, through further investigation, it was found that by preparing a kneaded product containing CNTs at a high concentration using a planetary stirrer for CNTs and a solvent, and then subjecting the dilution obtained by mixing the solvent with the obtained kneaded product to a dispersion treatment, and further adjusting the wetting rate, which is represented by the ratio of the CNT concentration based on the maximum penetration weight of the solvent per 1 g of CNT at the time of preparing the kneaded product to the CNT concentration based on the weight of the mixture containing CNTs and the solvent, to a specific range, it becomes possible to produce a conductive paste containing CNTs at a higher concentration, and moreover, it has the property that aggregates of CNTs are less likely to occur in the positive electrode mixture mixed with this conductive paste.

[0008] Therefore, an object of the present invention is to provide a method for producing a conductive paste that can be used for a positive electrode mixture of a lithium-ion secondary battery, which can produce a conductive paste that contains CNTs at a high concentration with a small amount of dispersant and has a low viscosity and is easy to handle.

Means for Solving the Problems

[0009] The present invention relates to a method for producing a conductive paste, in which CNTs and a solvent are brought into contact so that the wetting rate represented by the following formula (1) is 25 to 125%, and the mixture containing CNTs and the solvent obtained by impregnating the CNTs with the solvent is kneaded with a planetary stirrer to obtain a paste-like kneaded product, and then the dilution obtained by mixing the solvent with the obtained kneaded product is subjected to a dispersion treatment. Wetting rate (%) = A / B × 100 (1) A: The CNT concentration when the solvent has penetrated to the maximum, calculated by the following formula A(%) = CNT1g / (CNT1g + maximum penetration weight of solvent per CNT1g [g]) × 100 B: CNT concentration in the mixture, calculated by the following formula B(%) = CNT weight [g] / mixture weight [g] × 100

[0010] In an embodiment of the present invention, the content of the dispersant in the mixture, the kneaded product, and the diluent may be less than 30 parts by weight with respect to 100 parts by weight of CNT.

[0011] In an embodiment of the present invention, the dispersion treatment may be performed by a stirrer that applies a shearing force to the diluent by rotating a stirring blade at high speed and / or a stirrer that disperses the diluent by refluxing or passing it while mechanically stirring the media in a container filled with natural or synthetic media.

[0012] In an embodiment of the present invention, a solvent may be further mixed with the dispersion-treated diluent to adjust the CNT concentration.

Advantages of the Invention

[0013] In the conductive paste obtained in the present invention, since the content of the dispersant is small, there is no concern about side reactions caused by the dispersant during battery operation, and the content ratio of the active ingredients (active material, binder, and conductive aids other than CNT) in the positive electrode mixture can be increased by the amount corresponding to the small content of the dispersant. Further, the conductive paste obtained in the present invention has an easy-to-handle viscosity while having a high concentration of CNT, so it is easy to mix with the positive electrode mixture, and since the concentration of CNT can be easily adjusted, an electrode member excellent in desired electron conductivity can be efficiently manufactured. For example, since the conductive paste obtained in the present invention has extremely excellent dispersibility of CNT in the active material, the discharge capacity can be significantly improved by adding and mixing it into the positive electrode mixture. Further, since the conductive paste obtained in the present invention does not use a dispersant as an essential component, the content of other components such as the active material, binder, and conductive aids can be increased, and the development of electrode members with various characteristics can be promoted.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of a method for manufacturing a conductive paste according to the present invention will be described.

[0015] A method for manufacturing a conductive paste according to an embodiment of the present invention (hereinafter also referred to as the manufacturing method of the present invention) is bringing a carbon nanotube (CNT) and a solvent into contact so that the wetting rate represented by the following formula (1) is 25 to 125%, impregnating the CNT with the solvent, and kneading the obtained mixture containing the CNT and the solvent with a planetary stirrer to obtain a paste-like kneaded product, and then dispersing a diluent obtained by mixing the solvent into the obtained kneaded product. Wetting rate (%) = A / B × 100 (1) A: The CNT concentration when the solvent penetrates to the maximum, calculated by the following formula A(%) = CNT 1g / (CNT 1g + maximum penetration weight of the solvent per CNT 1g [g]) × 100 B: The CNT concentration in the mixture, calculated by the following formula B(%) = CNT weight [g] / mixture weight [g] × 100

[0016] In the present invention, the conductive paste can be used as a conductive auxiliary for a positive electrode mixture of a lithium-ion secondary battery, and contains CNT and a solvent as main components.

[0017] The CNT used in the present invention is not particularly limited, and either multi-walled CNT (MWCNT) or single-walled CNT (SWCNT) can be used. The number of the multi-layers is not particularly limited, and those having 2 layers, 3 layers, 4 layers, 5 layers or more can all be used. Further, as the MWCNT having 5 layers or more, those having a diameter of 5 nm or more can be mentioned. For example, as the MWCNT, those manufactured by companies such as Showa Denko K.K., Cnano, Nanocyl, LG Chem, JEIO, Kumho Petrochemical, SUSN Sinotech New Materials, ARKEMA, etc. can be mentioned, but there is no particular limitation. As the SWCNT, for example, those manufactured by companies such as OCSiAl, Osaka Soda Co., Ltd., NanoIntegris, etc. can be mentioned, but there is no particular limitation. In the present invention, one type of CNT may be used alone, or a plurality of CNTs may be mixed and used.

[0018] Examples of the solvent used in the present invention include aprotic polar solvents such as N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide, and dimethylformamide; aliphatic hydrocarbon solvents such as pentane, normal hexane, octane, cyclopentane, and cyclohexane; aromatic hydrocarbon solvents such as benzene, toluene, xylene, and cymene; aldehyde solvents such as furfural; ketone solvents such as acetone, methyl ethyl ketone, cyclopentanone, and cyclohexanone; ester solvents such as butyl acetate, ethyl acetate, methyl acetate, butyl propionate, butyl butyrate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, 3-methoxybutyl acetate, and ethylene glycol diacetate; ether solvents such as tetrahydrofuran, dioxane, anisole, and ethylene glycol dimethyl ether; alcohol solvents such as methanol, ethanol, normal propyl alcohol, isopropyl alcohol, butyl alcohol, octyl alcohol, cyclohexanol, allyl alcohol, benzyl alcohol, cresol, and furfuryl alcohol; polyol solvents such as glycerol, ethylene glycol, and diethylene glycol; alcohol ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and diethylene glycol monobutyl ether; and water, etc., but there is no particular limitation. Further, two or more of these solvents can be used in combination.

[0019] The planetary stirrer used in the present invention is a machine that mixes by the material retention and shear stress due to the centrifugal force generated by the rotation and revolution (planetary motion), and is also called a rotation-revolution type mixer. As for the planetary stirrer, a commercially available manufacturing device may be used, and there is no particular limitation.

[0020] In the manufacturing method of the present invention, first, CNT and a solvent are brought into contact so that the wetting rate represented by the formula (1) is 25 to 125%, and the mixture containing CNT and the solvent obtained by impregnating CNT with the solvent is kneaded with a planetary stirrer to obtain a paste-like kneaded product.

[0021] The wetting rate represented by the formula (1) is the ratio of the CNT concentration (A) when the solvent has penetrated to the maximum to the CNT concentration (B) in the mixture. The lower this wetting rate, the lower the viscosity of the resulting mixture, and the higher the wetting rate, the higher the viscosity of the mixture. A wetting rate of 100% indicates the state when the solvent has penetrated to the maximum into the CNT. In the present invention, by adjusting the wetting rate to be 25 to 125%, while allowing the solvent to penetrate into the CNT, a paste-like mixture can be efficiently obtained. As a result, a kneaded product with a low viscosity can be obtained, and CNT can be dispersed at a high concentration in the subsequent dispersion treatment. If the wetting rate is less than 25% or exceeds 125%, it is difficult to obtain the effect of reducing the viscosity in the kneaded product. As described above, the wetting rate represented by the formula (1) can be said to be an excellent index in that the optimal dispersion formulation of the conductive paste that exhibits the desired effect can be predicted in advance according to the types of CNT and solvent used. In the formula (1), the value range of the CNT concentration (B) in the mixture is 0 < B < 100 according to the above definition. As the value of B increases, the wetting rate converges to A, which means that "even when the CNT concentration approaches 100%, the wetting rate does not approach 0%".

[0022] The maximum penetration weight of the solvent per 1 g of the CNT can be measured by the following procedure. 1. Prepare a powder measurement penetration rate measurement device "Penetrometer", a jig dedicated to the Penetrometer (Teflon (registered trademark) cylinder, stainless steel mesh, filter paper, etc.), and a tapping device (all manufactured by Hosokawa Micron). 2. Line a bottom of a Teflon (registered trademark) cylinder with a stainless-steel mesh and filter paper, and fill it with well-dispersed CNTs on top. Also, precisely weigh the weight of the CNTs (ensure at least three significant figures). 3. Use a tapping device to mechanically tap the CNTs. The tapping conditions shall comply with the pigment tap density measurement method (JIS K 5101-12-2). Specifically, fix the Teflon (registered trademark) cylinder containing the CNTs prepared in step 2 to the tapping device support base, and perform tapping at a nominal speed of 250 times per minute for 5 minutes from a height of 3 mm ± 0.2 mm. 4. Place the CNTs prepared in step 3 on the weighing section of a penetrometer, and set a beaker containing the solvent used for dispersion on the pedestal. 5. Start the measurement. When the CNTs come into contact with the solvent and begin to absorb it, the penetration weight of the solvent is plotted against time. Continue the measurement until the penetration weight of the solvent reaches saturation. Read the penetration weight (maximum penetration weight) in the saturation region, and divide it by the weight of the CNTs precisely weighed in step 2 to obtain the maximum penetration weight of the solvent per 1 g of CNTs. Note that the region where the solvent penetration rate per 1 g of CNTs (the slope of the plot) is 0.010 g / s or less is regarded as the saturation region.

[0023] In the present invention, from the perspective of enabling efficient kneading, the lower limit of the wetting rate is preferably 25% or more, more preferably 30% or more, and the upper limit of the wetting rate is preferably 125% or less, more preferably 100% or less.

[0024] Also, the amount of CNTs mixed with the solvent in the present invention cannot be specified unconditionally depending on the types of CNTs and the solvent. For example, even if the amount of CNTs is adjusted to 8.5% by weight or more, in the subsequent dispersion treatment step, a conductive paste can be efficiently produced without viscosity breakdown (a state where the entire paste lifts up and the stirring member of the disperser rotates idly).

[0025] Also, the concentration of the CNTs is preferably adjusted to a high viscosity (optimal viscosity) within a range where viscosity breakdown does not occur. Specifically, using the specifications of the disperser to be used (such as the shape of the stirring blades, the presence or absence of baffles, the diameter and amount of beads as the medium, etc.), the viscosity threshold at which viscosity breakdown occurs is examined in advance. Next, a kneaded product is actually prepared using CNTs, the viscosity is measured while changing its concentration, and the CNT concentration condition within the range of the optimum viscosity is adopted as this dispersion condition. Regarding the measurement of viscosity, there is no particular limitation, but it can be measured using an E-type viscometer, a rheometer, or the like.

[0026] In the present invention, as a method of bringing CNTs into contact with a solvent, there are mentioned a method of mixing a predetermined amount of the solvent with the CNTs once or in multiple portions, a method of mixing a predetermined amount of the CNTs with the solvent once or in multiple portions, etc., but there is no particular limitation.

[0027] As the conditions for kneading with the planetary stirrer, it may be adjusted to a temperature below the boiling point of the solvent to be used. Also, the kneading treatment time cannot be specified unconditionally because it depends on the specifications of the planetary stirrer, but the time when the torque converges may be taken as the end point of kneading. The conditions are not particularly limited, but a temperature of 0 to 80°C and a time of 1 to 24 hours are preferable. Regarding the speed during the kneading, it may be about 30 rpm (30 min -1 ), and there is no particular limitation. Also, the filling rate of the planetary stirrer may be appropriately adjusted according to the capacity of the tank and the charged amounts of the CNTs and the solvent to be used so that kneading can be performed efficiently. There is no particular limitation on the specific filling rate, but it may be adjusted to be 20 to 70%. Regarding the state where the torque has converged, when kneading is started, the torque increases, but as the dispersion progresses, the increase becomes gentle, and when the kneaded product is sufficiently dispersed, the torque converges to a constant value. Also, the torque may be confirmed by the monitor function installed in the planetary stirrer.

[0028] Incidentally, as confirmed by the inventors of the present invention, when kneading CNT and a solvent with a general disperser, even if a dispersant is used, if the CNT concentration is not 4% by weight or less, the kneaded product becomes highly viscous or the viscosity breaks down, so that it cannot be successfully made into a paste. Further, even when the obtained kneaded product is mixed with a positive electrode active material, the CNTs aggregate or the dispersion of the CNTs in the positive electrode active material does not proceed to the desired state. On the other hand, in the production method of the present invention, as in this step, first, by kneading with a planetary stirrer, a kneaded product can be obtained even when the CNT concentration is adjusted high without using a dispersant. Further, when a dispersant is used, a kneaded product with a higher CNT concentration can also be obtained. The CNT concentration is not limited because the adjustable concentration varies depending on the CNT. However, a kneaded product with a concentration of 6% by weight or more can be obtained without using a dispersant, and a kneaded product with a concentration of 10% by weight or more can be obtained when a dispersant is used.

[0029] In the production method of the present invention, a dilution obtained by mixing a solvent with the kneaded product is subjected to a dispersion treatment. By this dispersion treatment, the CNT concentration can be adjusted to produce a conductive paste capable of favorably dispersing the CNTs in the active material when mixed with the active material.

[0030] In this dispersion treatment step, a high-speed shearing device with high shearing force is used. Examples of high-speed shearing devices include stirrers that rotate stirring blades (such as turbine blades, paddle blades, propeller blades, anchor blades, etc.) at high speed to apply shearing force to the diluent; dispersers that allow the diluent to pass through the narrow gap between a high-speed rotating disk and a fixed disk to impart a strong shearing flow and strong velocity fluctuations before and after; dispersers that inject the diluent at high pressure and cause it to collide with a fixed plate or the diluents themselves; dispersers that place media such as balls in a rotating container to impart collision and frictional forces; stirrers (such as bead mills manufactured by Asizawa Fine Tech Co., Ltd.) that disperse the diluent by mechanically stirring the media while refluxing or passing the diluent through a container filled with natural or synthetic media (such as natural sand, glass beads, zirconia beads, etc.). Among them, from the perspective of being able to perform dispersion treatment efficiently and having less metal contamination, a stirrer that rotates stirring blades at high speed to apply shearing force to the diluent is preferred. Also, these high-speed shearing devices may be used in combination of two or more types.

[0031] As the conditions for dispersion treatment using the high-speed shearing device, it may be adjusted to a temperature below the flash point of the solvent used. Also, the treatment time cannot be specified unconditionally because it depends on the specifications of the disperser, but the time when the particle size distribution converges may be regarded as the end point of dispersion. Examples of the conditions include a range of 0 to 80°C and 1 to 10 hours. Regarding the speed during the dispersion treatment, it is only necessary that the fluidity of the content can be maintained, and there is no particular limitation.

[0032] The amount of the solvent mixed with the kneaded product is preferably adjusted so that the viscosity in the dispersion treatment is within a range where viscosity breakdown does not occur. For example, when performing dispersion treatment using a stirrer that rotates stirring blades at high speed to apply shearing force to the diluent, the shearing speed is 10 s -1When the shear viscosity at [a certain condition] exceeds 15,000 mPa·s, viscosity breakdown occurs. Therefore, the amount of the solvent is adjusted so that it is 15,000 mPa·s or less, more preferably around 10,000 mPa·s. Also, the CNT concentration at this time can be adjusted to be equal to or less than the CNT concentration of the kneaded product, for example, within the range of 4 to 10% by weight.

[0033] As the solvent used in this dispersion treatment step, the solvent used in the kneaded product may be used, but other solvents may also be used. Regarding the type of the solvent, any solvent that can be used in the kneaded product may be used, and there is no particular limitation.

[0034] In the production method of the present invention, the dispersant is not an essential component. However, from the viewpoints of adjusting the CNT concentration in the kneaded product to a high level and efficiently dispersing the conductive paste in the active material, a dispersant may be mixed in the conductive paste. Examples of the dispersant include polyvinylidene fluoride, 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, polyalkylene oxides, polyvinyl ether, polyvinyl pyrrolidone, chitins, chitosans, starch, polyimide, etc. Also, these dispersants can be used alone or in combination of two or more. The dispersant may be mixed when preparing the kneaded product or the diluted product, and there is no particular limitation on the mixing timing.

[0035] When using the dispersant, from the perspective of making side reactions less likely to occur during battery operation, the content of the dispersant in the mixture, the kneaded product, and the diluent is preferably adjusted to be low. For example, it can be adjusted to be at most less than 30 parts by weight with respect to 100 parts by weight of CNT1.

[0036] By further mixing a solvent with the dispersion-treated diluent to adjust the CNT concentration, it is possible to correspond to cathode active materials with various compositions. As a method for adjusting the CNT concentration, it may be carried out in the same manner as in the dilution step.

[0037] The conductive paste obtained as described above can be used as a conductive auxiliary agent and mixed with an active material and a binder to produce a cathode active material for a lithium-ion secondary battery.

Examples

[0038] (Example 1) Since the maximum penetration weight of NMP per 1 g of CNT1 (MWCNT, manufactured by LG Chem, LUCAN BT 1003M, the same below) was 12.6 g, when the CNT1 concentration was 7.4% by weight, 100% of CNT1 was in a wetted state (wetting rate 100%). To make the wetting rate of CNT1 74% (CNT1 concentration 10.0% by weight), 900 parts by weight of NMP was supplied to a planetary mixer (manufactured by Inoue Manufacturing Co., Ltd., product name "Trimix", the same below) with respect to 100 parts by weight of CNT1, and the planetary mixer was operated at about 30 °C and 35 rpm (35 min -1 ) for 6 hours to perform kneading, and a kneaded product with a CNT1 concentration of 10.0% by weight was obtained. The obtained paste-like kneaded product was placed in a high-speed shearing tank (tank inner diameter: 80 mm; the same applies hereinafter). To 100 parts by weight of CNT1, 667 parts by weight of NMP was added so that the CNT1 concentration became 6.0% by weight. An edge-driven turbine blade (on the radial outer periphery of a rotating plate with a diameter of 60 mm, there are six turbine blades rising at a height of 6 mm in the axial direction of the drive shaft, six on each of the upper and lower sides in the axial direction; the same applies hereinafter) and a three-phase induction motor (manufactured by Fuji Electric, MLH8065M) were used to perform dispersion treatment by rotating at high speed at 2000 rpm (2000 min -1 ) for 2 hours. As a result, a CNT1 conductive paste was obtained without viscosity breakdown (although it was on the verge of breakdown, the fluidity of the paste was consistently confirmed). Next, NMP was added to the diluted product after the dispersion treatment to obtain a conductive paste with a CNT1 concentration of 4.0% by weight.

[0039] (Example 2) After obtaining a paste-like kneaded product in the same manner as in Example 1, the concentration was adjusted so that the CNT1 concentration became 5.5% by weight, and dispersion treatment was performed with a stirring blade in a high-speed shearing tank for 2 hours. As a result, a CNT1 conductive paste was obtained without viscosity breakdown. Thereafter, a conductive paste with a CNT1 concentration of 4.0% by weight was obtained in the same manner as in Example 1.

[0040] (Example 3) After obtaining a paste-like kneaded product in the same manner as in Example 1, the concentration was adjusted so that the CNT1 concentration became 4.5% by weight, and dispersion treatment was performed with a stirring blade in a high-speed shearing tank for 2 hours. As a result, a CNT1 conductive paste was obtained without viscosity breakdown. Thereafter, a conductive paste with a CNT1 concentration of 4.0% by weight was obtained in the same manner as in Example 1.

[0041] (Example 4) Kneading was carried out in the same manner as in Example 1, except that 681 parts by weight of NMP was charged with respect to 100 parts by weight of CNT1 so that the wetting rate of CNT1 was 58% (CNT1 concentration: 12.8% by weight). The obtained paste-like kneaded product (CNT1 concentration: 12.8% by weight) was added with 757 parts by weight of NMP to 100 parts by weight of CNT1 so that the CNT1 concentration became 6.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1 ) in a high-speed shearing tank for 2 hours for high-speed rotation for dispersion treatment. As a result, a CNT1 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT1 concentration of 6.0% by weight.

[0042] (Example 5) Kneading was carried out in the same manner as in Example 1 except that 400 parts by weight of NMP was charged to 100 parts by weight of CNT1 so that the wetting rate of CNT1 was 37% (CNT1 concentration: 20.0% by weight). The obtained paste-like kneaded product (CNT1 concentration: 20.0% by weight) was added with 677 parts by weight of NMP to 100 parts by weight of CNT1 so that the CNT1 concentration became 8.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1 ) in a high-speed shearing tank for 2 hours for high-speed rotation for dispersion treatment. As a result, a CNT1 conductive paste was obtained without viscosity breakdown (although it was on the verge of breakdown, the fluidity of the paste was consistently confirmed). Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT1 concentration of 8.0% by weight.

[0043] (Example 6) Kneading was carried out in the same manner as in Example 1 except that 124 parts by weight of polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., PVP K-30) dissolved in NMP at an NV value of 20% and 775 parts by weight of NMP were charged to 100 parts by weight of CNT1 so that the wetting rate of CNT1 was 74% (CNT1 concentration: 10.0% by weight). The obtained paste-like kneaded product (CNT1 concentration: 10.0% by weight) was added with 1222 parts by weight of NMP to 100 parts by weight of CNT1 so that the CNT1 concentration became 4.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1) When it was rotated at high speed for 2 hours for dispersion treatment, a CNT1 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT1 concentration of 4.0% by weight.

[0044] (Example 7) Except that 124 parts by weight of polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., PVP K-30) dissolved in NMP at an NV value of 20% and 275 parts by weight of NMP were charged with respect to 100 parts by weight of CNT1 so that the wetting rate of CNT1 was 37% (CNT1 concentration 20.0% by weight), kneading was carried out in the same manner as in Example 1. The obtained paste-like kneaded product (CNT1 concentration 20.0% by weight) was added with 452 parts by weight of NMP with respect to 100 parts by weight of CNT1 so that the CNT1 concentration became 10.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1 ) When it was rotated at high speed for 2 hours for dispersion treatment, a CNT1 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT1 concentration of 9.8% by weight.

[0045] (Example 8) Since the maximum penetration weight of NMP per 1 g of CNT2 (MWCNT, manufactured by LG Chem, LUCAN BT 1001M, the same applies hereinafter) was 15.5 g, 100% of CNT2 was in a wet state (wetting rate 100%) when the CNT2 concentration was 6.1% by weight. Except that 502 parts by weight of NMP were charged with respect to 100 parts by weight of CNT2 so that the wetting rate of CNT2 was 37% (CNT2 concentration 16.6% by weight), kneading was carried out in the same manner as in Example 1. The obtained paste-like kneaded product (CNT2 concentration 16.6% by weight) was added with 936 parts by weight of NMP with respect to 100 parts by weight of CNT2 so that the CNT2 concentration became 6.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1) When it was rotated at high speed for 2 hours for dispersion treatment, a CNT2 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT2 concentration of 6.0% by weight.

[0046] (Example 9) Except that 124 parts by weight of polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., PVP K-30) dissolved in NMP at an NV value of 20% and 275 parts by weight of NMP were charged with respect to 100 parts by weight of CNT2 so that the wetting rate of CNT2 was 31% (CNT2 concentration: 20.0% by weight), kneading was carried out in the same manner as in Example 1. The obtained paste-like kneaded product (CNT2 concentration: 20.0% by weight) was added with 676 parts by weight of NMP with respect to 100 parts by weight of CNT2 so that the CNT2 concentration became 8.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1 ) When it was rotated at high speed for 2 hours for dispersion treatment, a CNT2 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT2 concentration of 8.0% by weight.

[0047] (Comparative Example 1) An attempt was made to disperse CNT1 and NMP with a stirring blade while changing the concentration without using a planetary stirrer. However, even when the CNT1 concentration was 4.0% by weight, the viscosity became extremely high and the paste had no fluidity (viscosity breakdown). When the CNT1 concentration was lowered to 2.0% by weight, fluidity could be ensured.

[0048] (Comparative Example 2) The amount of NMP was adjusted so that the wetting rate of CNT1 was 124% (CNT1 concentration: 5.9% by weight), and kneading (2 passes) was carried out using three rolls instead of a planetary stirrer. The viscosity was measured while changing the concentration of the kneaded product. However, the viscosity was as high as that of Comparative Example 1 and was far from the level at which dispersion treatment could be performed with a stirring blade.

[0049] (Comparative Example 3) The amount of NMP was adjusted so that the wetting rate of CNT1 was 74% (CNT1 concentration: 10.0% by weight), and kneading (2 passes) was carried out using three rolls instead of a planetary stirrer. The viscosity was measured by changing the concentration of the kneaded product. However, the viscosity was extremely high, equivalent to that of Comparative Example 1, and was far beyond the level that could be dispersed by a stirring blade.

[0050] (Comparative Example 4) The amount of NMP was adjusted so that the wetting rate of CNT1 was 37% (CNT1 concentration: 20.0% by weight), and kneading (1 pass, 2 passes, 5 passes) was carried out using three rolls instead of a planetary stirrer. The viscosity was measured by changing the concentration of the kneaded product. However, the viscosity was extremely high, equivalent to that of Comparative Example 1, and no effect of reducing the viscosity was observed even when the number of passes was increased. It was far beyond the level that could be dispersed by a stirring blade.

[0051] (Comparative Example 5) In Comparative Example 1, CNT2 was used instead of CNT1, and an attempt was made to disperse CNT2 and NMP with a stirring blade by changing the concentration. However, even when the CNT2 concentration was 4.0% by weight, the viscosity was extremely high and the paste had no fluidity (viscosity breakdown).

[0052] (Test Example 1) The electrode binder PVDF (manufactured by Kureha, KF Polymer L#1120, NV value: 12%) and the conductive assistant acetylene black (hereinafter referred to as AB) (manufactured by Denka, "Denka Black") were mixed based on a conventional method, and then the CNT1 conductive pastes obtained in Examples 2, 4, 5, 6, 7, 8, 9 and Comparative Example 1 were respectively mixed. Subsequently, the positive electrode active material LCO (manufactured by Nippon Chemical Industry Co., Ltd., lithium cobaltate) was mixed based on a conventional method to obtain a positive electrode mixture (LCO:PVDF:AB:CNT1 (mass ratio) = 94:3:3:0.3). The obtained positive electrode mixture was formed into a film on an aluminum current collector foil (manufactured by Nirako, AL-013225) using a tabletop mini coater (manufactured by Hozumi, MC-20), and dried with a hot plate and a vacuum dryer. When visually observing the surface of the obtained positive electrode mixture electrode, no aggregates of CNT1 were observed in the positive electrode mixture electrodes prepared from the CNT1 conductive pastes of Examples 2, 4, 5, 6, 7, 8, and 9. However, a large number of aggregates of CNT1 were confirmed in the positive electrode mixture electrode prepared from the CNT1 conductive paste of Comparative Example 1. The positive electrode mixture electrode prepared from the CNT1 conductive paste of Comparative Example 1 was not suitable for use in the battery evaluation of Test Example 2. From the above, it can be seen that the conductive paste obtained by the production method of the present invention maintains a dispersed state in the positive electrode mixture and in its coated electrode. Therefore, it can be understood that the production method of the present invention can produce a conductive paste that contains CNTs at a high concentration with a small amount of dispersant, has a low viscosity, and is easy to handle. Also, it can be understood that the conductive paste obtained in the present invention can be mixed with the positive electrode active material of a lithium secondary battery to produce a positive electrode mixture.

[0053] (Test Example 2) Using the positive electrode mixture electrode prepared in Test Example 1, a test battery (half cell) was prepared and battery evaluations (rate test and cycle test) were carried out. The test battery (product) was prepared as follows. The positive electrode mixture electrode prepared in Test Example 1 was pressed at about 32 kgN by a uniaxial press to adjust the porosity of the electrode to around 30%. Using an electrochemical measurement cell (manufactured by Nippon Tomcell, TJ-AC), the positive electrode was the positive electrode mixture electrode prepared in Test Example 1, the negative electrode was a metallic lithium foil (manufactured by Motoyama Chemical), the separator was a glass fiber filter paper (manufactured by Whatman, GF / B), and the electrolyte was 1 M LiPF6 EC:DEC (1:1 v / v%) (manufactured by Kishida Chemical). As a control, a test battery without mixing the CNT1 conductive paste (acetylene black (AB) alone as a conductive aid) was also prepared. In addition, after the assembly process using the electrolyte and lithium foil, it was carried out in a glove box (manufactured by UNICO, UN-650L) under an argon gas atmosphere. Also, the porosity of the electrode was calculated from the basis weight, film thickness, and true density of the material.

[0054] Also, the battery characteristics were measured as follows. The fabricated test battery was connected to a charge-discharge measurement device (HJ-1001SD8, manufactured by Hokuto Denko), and a rate test and a cycle test were conducted. In the rate test, charging was performed under constant current control at a fixed rate of 0.1C and a termination voltage of 4.2V. Discharging was performed under constant current control with rates varying from 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, 5C, 7C, to 10C, and a termination voltage of 3.0V. In the cycle test, basically, charging and discharging were repeated about 200 cycles under constant current control at a rate of 1C and a termination voltage from 4.3V to 3.0V. However, for the initial 5 cycles and the 51st, 101st, 151st, and 201st cycles, charging and discharging were performed at a rate of 0.1C. The obtained rate test results are shown in Table 1, and the cycle test results are shown in Table 2.

[0055]

Table 1

[0056]

Table 2

[0057] From the results shown in Table 1, in the implementation products (AB + Examples 2 to 9), compared with the control product (AB alone), by adding only slightly less than 0.3 wt% of CNT, the discharge capacity at 3C was improved by about 104 - 142%. Also, from the results shown in Table 2, when charging and discharging were repeated, in the implementation products (AB + Examples 2 to 9), compared with the control product (AB alone), by adding only slightly less than 0.3 wt% of CNT, the discharge capacity at the 100th cycle was about 107 - 182%. From the above results, it can be seen that by using the conductive paste obtained in the present invention, the discharge capacity of the positive electrode mixture can be efficiently improved, and an electrode member excellent in desired electronic conductivity can be efficiently manufactured.

[0058] (Example 10) Since the maximum penetration weight of NMP per 1 g of CNT3 (single-walled carbon nanotube, manufactured by OCSiAl, TUBALL, the same hereinafter) was 8.1 g, 100% of CNT3 was in a wetted state (wetting rate 100%) at a CNT3 concentration of 11.0% by weight. To achieve a wetting rate of 35% for CNT3 (CNT3 concentration of 31.4% by weight), 218 parts by weight of NMP was charged per 100 parts by weight of CNT3, and planetary stirring was carried out at about 30 °C and 35 rpm (35 min -1 ) for 4 hours to obtain a kneaded product with a CNT3 concentration of 31.4% by weight. Here, 62.5 parts by weight of polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., PVP K-30) previously dissolved in NMP at an NV value of 40% was added, and kneading was carried out at about 30 °C and 35 rpm (35 min -1 ) for 4 hours to obtain a kneaded product with a CNT3 concentration of 26.2% by weight. To the obtained paste-like kneaded product (CNT3 concentration 26.2% by weight), 2476 parts by weight of NMP was added per 100 parts by weight of CNT3 so that the CNT3 concentration became 3.5% by weight, and the stirring blade was rotated at high speed at 2000 rpm (2000 min -1 ) for 2 hours for dispersion treatment, and a CNT3 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the above dispersion treatment to obtain a conductive paste with a CNT3 concentration of 3.0% by weight.

[0059] (Example 11) Since the maximum penetration weight of NMP per 1 g of CNT4 (multi-walled carbon nanotube, manufactured by JEIO, JENOTUBE 6A, the same hereinafter) was 16.4 g, 100% of CNT4 was in a wetted state (wetting rate 100%) at a CNT4 concentration of 5.7% by weight. To achieve a wetting rate of 31% for CNT4 (CNT4 concentration of 18.4% by weight), 443 parts by weight of NMP was charged per 100 parts by weight of CNT4, and planetary stirring was carried out at about 30 °C and 35 rpm (35 min -1 ) for 21 hours to obtain a kneaded product with a CNT4 concentration of 18.4% by weight. The obtained paste-like kneaded product (CNT4 concentration: 18.4% by weight) was added with 789 parts by weight of NMP to 100 parts by weight of CNT4 so that the CNT4 concentration became 7.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1 ) in a high-speed shearing tank for 2 hours for high-speed rotation for dispersion treatment, and a CNT4 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT4 concentration of 7.0% by weight.

[0060] (Example 12) So that the wetting rate of CNT4 becomes 31% (CNT4 concentration: 18.4% by weight), 125 parts by weight of polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., PVP K-30) dissolved in NMP with an NV value of 20% in advance and 318 parts by weight of NMP were charged with respect to 100 parts by weight of CNT4, and kneading was carried out by operating a planetary stirrer at about 30 °C and 35 rpm (35 min -1 ) for 18 hours to obtain a kneaded product having a CNT4 concentration of 18.4% by weight. The obtained paste-like kneaded product (CNT4 concentration: 18.4% by weight) was added with 995 parts by weight of NMP to 100 parts by weight of CNT4 so that the CNT4 concentration became 6.5% by weight, and the stirring blade was rotated at 2000 rpm (2000 min -1 ) in a high-speed shearing tank for 2 hours for high-speed rotation for dispersion treatment, and a CNT4 conductive paste was obtained without viscosity breakdown. Next, NMP was added to the diluted product subjected to the dispersion treatment to obtain a conductive paste having a CNT4 concentration of 6.0% by weight.

[0061] Using the conductive pastes obtained in Examples 10, 11, and 12, a positive electrode mixture electrode was fabricated based on the method described in Test Example 1. As a result, no aggregates were observed in the positive electrode mixture, and the dispersed state of CNTs was maintained.

[0062] Therefore, it can be understood that in the manufacturing method of the present invention, regardless of whether MWCNT or SWCNT is used as the CNT, a conductive paste with a low viscosity and easy handling can be produced, which contains CNT at a high concentration even with a small amount of dispersant. Further, it can be understood that regardless of whether MWCNT or SWCNT is used as the CNT, the conductive paste obtained in the present invention can be mixed with the positive electrode active material of the lithium ion secondary battery to produce a positive electrode mixture.

Claims

1. A method for producing a conductive paste, comprising bringing a carbon nanotube (hereinafter referred to as "CNT") into contact with a solvent so that the wetting rate represented by the following formula (1) is 25 to 125%, impregnating the CNT with the solvent to obtain a mixture containing the CNT and the solvent, kneading the mixture with a planetary mixer to obtain a paste-like kneaded product, and then subjecting a diluent obtained by mixing the solvent with the obtained kneaded product to a dispersion treatment. Wetting rate (%) = A / B × 100 (1) Wetting rate (%) = A / B × 100 (1) A: The CNT concentration when the solvent has penetrated to the maximum, calculated by the following formula A (%) = CNT 1 g / (CNT 1 g + maximum penetration weight of the solvent per CNT 1 g [g]) × 100 B: The CNT concentration in the mixture, calculated by the following formula B (%) = CNT weight [g] / mixture weight [g] × 100

2. The method for producing a conductive paste according to Claim 1, wherein the content of the dispersant in the mixture, the kneaded product, and the diluent is less than 30 parts by weight with respect to 100 parts by weight of the CNT.

3. The method for producing a conductive paste according to Claim 1 or 2, wherein the dispersion treatment is performed by a stirrer that applies a shearing force to the diluent by rotating a stirring blade at a high speed and / or a stirrer that disperses the diluent by refluxing or passing it through while mechanically stirring the medium in a container filled with a natural or synthetic medium.

4. The method for producing a conductive paste according to any one of Claims 1 to 3, further comprising mixing a solvent with the dispersion-treated diluent to adjust the CNT concentration.

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