Carbon nanotube slurry, carbon nanotube slurry for electrodes, and electrode film

Abstract

The purpose of the present invention is to provide a carbon nanotube slurry that has low viscosity and excellent handleability, and also has good conductivity when used to form an electrode film. The purpose of the present invention is also to provide a carbon nanotube slurry for electrodes that uses said carbon nanotube slurry, and an electrode film that uses the carbon nanotube slurry for electrodes. One form of the present invention pertains to a carbon nanotube slurry containing: carbon nanotubes that have a fiber length of at least 50 μm, an average diameter of at least 3 nm and at most 20 nm, and a Raman spectroscopy peak intensity ratio G / D of at least 1.0 and at most 2.6; polyvinylpyrrolidone that has a weight average molecular weight of at most 70,000; and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water (in a Raman spectrum obtained using Raman spectroscopy, where the maximum intensity of a G-band scattered light peak in the range of 1570 cm-1 to 1620 cm-1 is G and the maximum intensity of a D-band scattered light peak in the range of 1320 cm-1 to 1370 cm-1 is D, the intensity ratio G / D represents the ratio thereof). Additionally, one form of the present invention pertains to a carbon nanotube slurry containing: carbon nanotubes that have a fiber length of at least 50 μm, an average diameter of at least 3 nm and at most 20 nm, and a BET specific surface area of at least 70 m2 / g and at most 180 m2 / g; polyvinylpyrrolidone that has a weight average molecular weight of at most 70,000; and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water.

Description

Carbon nanotube slurry, carbon nanotube slurry for electrodes, and electrode film

[0001] The present invention relates to a carbon nanotube slurry, a carbon nanotube slurry for electrodes, and an electrode film.

[0002] In recent years, with the spread of electronic devices and mobility that takes environmental aspects into consideration, lithium ion secondary batteries have attracted attention. Conductive aids are used in lithium ion secondary batteries to reduce the resistance of the electrodes.

[0003] Carbon nanotubes (CNTs), which can reduce resistance with a small amount of use compared to carbon materials used as conventional conductive aids, have attracted attention, and the development of their dispersions has been underway (see, for example, Patent Documents 1 and 2).

[0004] By the way, when using a carbon nanotube slurry (dispersion) for forming an electrode of a lithium ion secondary battery or the like, a carbon nanotube slurry with good handling properties and high conductivity is required. However, according to the studies of the present inventors, depending on the balance between the fiber length and fiber diameter of the carbon nanotubes and the molecular weight of the dispersant, the viscosity of the slurry may become high, resulting in poor handling properties, or sufficient conductivity may not be obtained when forming an electrode film.

[0005] Specifically, generally, as the fiber length of carbon nanotubes increases, the conductivity tends to improve. On the other hand, as the fiber length increases, the viscosity increases, making it difficult to obtain a slurry with good handling properties.

[0006] Furthermore, when dispersing carbon nanotubes, the general procedure involves pre-mixing (hereinafter referred to as "premixing") the powdered carbon nanotubes into a solvent to make them homogeneous, and then dispersing the pre-mixed mixture using a disperser. In this case, depending on the properties of the carbon nanotubes used, the carbon nanotube slurry may become extremely viscous, making pre-mixing difficult. Moreover, the load on the disperser may become too great in the initial stages of dispersion, such as making it difficult to supply the liquid to the disperser, which can make it difficult to operate the disperser.

[0007] Furthermore, while highly crystalline carbon nanotubes could be used to improve conductivity, highly crystalline carbon nanotubes have fewer defects, which reduces the number of adsorption sites for dispersants. As a result, they tend to have poorer dispersibility.

[0008] In other words, there tends to be a trade-off between the handling properties of the slurry and the conductivity of the electrode film obtained using the slurry, but it is necessary to achieve a balance between the two.

[0009] Japanese Patent Publication No. 2006-309958 Japanese Patent Publication No. 2020-11873

[0010] The object of the present invention is to provide a carbon nanotube slurry that has low viscosity, excellent handling properties, and good conductivity when used as an electrode film, a carbon nanotube slurry for electrodes using this carbon nanotube slurry, and an electrode film using this carbon nanotube slurry for electrodes.

[0011] As a result of diligent research, the inventors discovered that the above problems can be solved by using carbon nanotubes having predetermined physical properties and a dispersant having a predetermined molecular weight, and thus completed the present invention.

[0012] In other words, according to the present invention, (1) a carbon nanotube slurry comprising: carbon nanotubes having a fiber length of 50 μm or more, an average diameter of 3 nm or more and 20 nm or less, and a peak intensity ratio G / D of 1.0 or more and 2.6 or less in Raman spectroscopy; polyvinylpyrrolidone having a weight-average molecular weight of 70,000 or less; and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water (wherein the intensity ratio G / D is 1570 cm⁻¹ in the Raman spectrum obtained by the Raman spectroscopy). －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１ (1) When D is the maximum intensity of the D-band scattered light peak in the range, this represents the ratio. (2) The fiber length is 50 μm or more, the average diameter is 3 nm or more and 20 nm or less, and the BET specific surface area is 70 m ２ / g or more 180m ２ (3) A carbon nanotube slurry for electrodes is provided, characterized by containing the carbon nanotube slurry described in (1) or (2) and an active material, comprising: carbon nanotubes having a weight-average molecular weight of 70,000 or less per gram, polyvinylpyrrolidone having a weight-average molecular weight of 70,000 or less; (4) An electrode film formed using the carbon nanotube slurry for electrodes described in (3).

[0013] According to the present invention, a carbon nanotube slurry having low viscosity and excellent handling properties, as well as good conductivity when used as an electrode film, a carbon nanotube slurry for electrodes using this carbon nanotube slurry, and an electrode film using this carbon nanotube slurry for electrodes are provided.

[0014] Hereinafter, the carbon nanotube slurry of the present invention will be described. The carbon nanotube slurry of the present invention contains carbon nanotubes having a fiber length of 50 μm or more, an average diameter of 3 nm or more and 20 nm or less, and a peak intensity ratio G / D in Raman spectroscopy of 1.0 or more and 2.6 or less, polyvinylpyrrolidone having a weight average molecular weight of 70,000 or less, and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water. Also, the carbon nanotube slurry of the present invention contains carbon nanotubes having a fiber length of 50 μm or more, an average diameter of 3 nm or more and 20 nm or less, and a BET specific surface area of 70 m ２ / g or more and 180 m ２ / g or less, polyvinylpyrrolidone having a weight average molecular weight of 70,000 or less, and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water.

[0015] Here, the intensity ratio G / D represents the ratio when the maximum intensity of the G-band scattered light peak in the Raman spectrum obtained by the Raman spectroscopy in the range of 1570 cm －１ to 1620 cm －１ is G, and the maximum intensity of the D-band scattered light peak in the range of 1320 cm －１ to 1370 cm －１ is D.

[0016] (Carbon Nanotubes) The carbon nanotubes used in the present invention have a fiber length of 50 μm or more, an average diameter of 3 nm or more and 20 nm or less, and a peak intensity ratio G / D in Raman spectroscopy of 1.0 or more and 2.6 or less. Also, the carbon nanotubes used in the present invention have a fiber length of 50 μm or more, an average diameter of 3 nm or more and 20 nm or less, and a BET specific surface area of 70 m ２ / g or more and 180 m ２ / g or less.

[0017] More specifically, the fiber length of the carbon nanotubes used in this invention is 50 μm or more, preferably 150 μm or more, more preferably 250 μm or more, preferably 800 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. If the fiber length is less than 50 μm, it is difficult to form conductive paths and the conductivity decreases, and if it is greater than 800 μm, the fibers tend to aggregate with each other when preparing the slurry, reducing fluidity and making handling difficult. Furthermore, if the fiber length is within the above range, fluidity can be maintained and good conductive paths can be formed.

[0018] Furthermore, the average diameter of the carbon nanotubes used in this invention is 3 nm or more, preferably 6 nm or more, and 20 nm or less, preferably 10 nm or less.

[0019] The fiber length and average diameter of carbon nanotubes were measured using electron microscope images, and the arithmetic mean values ​​of the fiber length and average diameter were taken from a sufficient number of samples (e.g., 10 to 20 nanotubes). Specifically, the fiber length was measured using a scanning electron microscope (S-3400N; SEM) on carbon nanotubes collected and observed at 1000x magnification, and the arithmetic mean value of 10 carbon nanotubes was obtained. The average diameter was measured using a transmission electron microscope (H-7650; TEM) on carbon nanotubes and the arithmetic mean value of 10 carbon nanotubes measured at 50,000x magnification.

[0020] Furthermore, the peak intensity ratio G / D of the carbon nanotubes used in the present invention in Raman spectroscopy is 1.0 to 2.6, preferably 1.3 or higher, more preferably 1.5 or higher, even more preferably 1.8 or higher, and 2.6 or lower. Here, the intensity ratio G / D is the peak intensity at 1570 cm⁻¹ in the Raman spectrum obtained by the Raman spectroscopy. －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１This expression represents the ratio of the maximum intensity of the D-band scattered light peak in the specified range, where D is the maximum intensity of the D-band scattered light peak. Here, the Raman spectrum is obtained, for example, by placing a carbon nanotube in a Raman microscope (ThermoScientific DXR2xi) and performing measurements using a laser wavelength of 532 nm. The measurement conditions are: objective lens magnification 20x, aperture 50 μm confocal pinhole, exposure time 0.1 s, laser output 2 mW, number of scans 10, and measurement wavelength 100–3400 cm. -1 That's what I decided.

[0021] Here, if the fiber length of the carbon nanotube is large and the strength ratio G / D is low, the number of defects present in the carbon nanotube increases, resulting in more adsorption sites for the dispersant. Therefore, the resulting carbon nanotube slurry tends to have excellent dispersibility, but the resulting electrode film tends to have poor conductivity.

[0022] On the other hand, when the fiber length of carbon nanotubes is large and the strength ratio G / D is high, the crystallinity is high and there are fewer defects in the carbon nanotubes, resulting in fewer adsorption sites for the dispersant. Therefore, the resulting carbon nanotube slurry tends to have poor dispersibility, while the resulting electrode film tends to have excellent conductivity.

[0023] The carbon nanotubes used in this invention have fiber length and strength ratio G / D within the above range, which allows for the production of a carbon nanotube slurry with excellent dispersibility, and furthermore, the resulting electrode film can have excellent conductivity.

[0024] Furthermore, the present invention can overcome the drawbacks of the carbon nanotube slurry described above and is suitable for the manufacture of electrodes for highly efficient lithium-ion secondary batteries and the like.

[0025] Furthermore, the BET specific surface area of ​​the carbon nanotubes used in this invention is 70 m². ２／ g or more 180m ２ It is less than / g, preferably 100m ２ / g exceeds 170m ２ Less than / g, more preferably 110m ２ / g exceeds 160m ２It is less than / g. The BET specific surface area of ​​carbon nanotubes can be measured using a specific surface area measuring device. More specifically, carbon nanotubes can be collected, accurately weighed using an electronic balance, dried at 110°C for 30 minutes while degassing, and then the BET specific surface area can be measured using the BET single-point method with a fully automatic specific surface area measuring device (Macsorb model HM-1208, manufactured by Mountec Co., Ltd.).

[0026] Generally, as carbon nanotubes become longer and thinner, that is, as the fiber length increases and the average diameter decreases, the BET specific surface area increases. As a result, wettability to water decreases and dispersibility tends to decline. The carbon nanotubes used in this invention have a relatively small BET specific surface area relative to their fiber length and small average diameter. Therefore, the resulting carbon nanotube slurry tends to have excellent dispersibility. Furthermore, because the fiber length of the carbon nanotubes used in this invention is relatively large, the electrode film obtained using the carbon nanotube slurry of this invention also exhibits good conductivity. In other words, the carbon nanotube slurry of this invention can be suitably used in the manufacture of electrodes for high-efficiency lithium-ion secondary batteries and the like.

[0027] The carbon nanotubes used in the present invention are not particularly limited as long as they have a shape substantially formed by winding a single sheet of graphite into a tube. Single-walled carbon nanotubes, in which a single sheet of graphite is wound in one layer, and multi-walled carbon nanotubes, in which two or three or more layers of graphite are wound, can both be used.

[0028] Furthermore, examples of carbon nanotube forms include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, but are not limited to these. These may be used individually or in combination of two or more types.

[0029] Furthermore, the purity of the carbon nanotubes used in this invention is preferably 90 to 100% by mass, and particularly preferably 95 to 100% by mass. The purity of the carbon nanotubes is calculated based on the amount of impurities, with the ash content measured in accordance with JIS K 1469 or JIS K 6218 being treated as an impurity.

[0030] The carbon nanotube content in the carbon nanotube slurry of the present invention is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, even more preferably 0.5 parts by mass or more, preferably 3.0 parts by mass or less, more preferably 2.5 parts by mass or less, and even more preferably 2.0 parts by mass or less, per 100 parts by mass of carbon nanotubes. When the carbon nanotube content is within this range, the carbon nanotubes are uniformly mixed during electrode slurry formation, and as a result, they can be uniformly coated onto the current collector, and the performance of the secondary battery electrode made from the slurry can be ensured.

[0031] (Polyvinylpyrrolidone) The polyvinylpyrrolidone (hereinafter sometimes referred to as "PVP") used in this invention has a weight-average molecular weight of 70,000 or less. Polyvinylpyrrolidone functions as a dispersant in carbon nanotube slurry. That is, it functions as a polymer that disperses carbon nanotubes well in the solvent and provides a stable slurry.

[0032] The weight-average molecular weight of polyvinylpyrrolidone is 70,000 or less, preferably less than 70,000, more preferably 60,000 or less, even more preferably 5 or less, preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 20,000 or more. The weight-average molecular weight of the dispersant of PVP can be measured, for example, by gel permeation chromatography (GPC); for example, under the following measurement conditions: apparatus: HLC-8320GPC (manufactured by Tosoh), columns: two TSKgel GMPW XL, detector: HLC-8320GPC built-in RI detector, sample concentration: 0.2-0.6 wt%, flow rate: 0.6 mL / min, injection volume: 10 μL, column temperature: 40°C, eluent: 100 nM NaCl aqueous solution / acetonitrile = 70 / 30. In other words, the above weight-average molecular weight is different from the calculated molecular weight.

[0033] The PVP content in the carbon nanotube slurry of the present invention is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, even more preferably 8 parts by mass or more, particularly preferably 10 parts by mass or more, preferably 300 parts by mass or less, more preferably 280 parts by mass or less, even more preferably 260 parts by mass or less, and particularly preferably 250 parts by mass or less, based on 100 parts by mass of carbon nanotubes in the carbon nanotube slurry. When the PVP content is within this range, it is possible to ensure the performance of the secondary battery electrodes made from the slurry while exhibiting the dispersibility of carbon nanotubes in the slurry.

[0034] (Solvent) The carbon nanotube slurry of the present invention contains at least one solvent selected from the group consisting of N-methyl-2-pyrrolidone (hereinafter sometimes referred to as "NMP"), ethanol, and water. In addition, as water, for example, deionized water, distilled water, tap water, etc., can be used, and a mixed solvent of water and a water-soluble solvent can also be used within a range that can exhibit the effects of the present invention.

[0035] Examples of water-soluble solvents include ethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2,5-hexanediol, 3-methyl-1,3-butanediol, 2-methylpentane-2,4-diol, 3-methylpentane-1,3,5-triol, 1,2 Examples include alkylene glycols such as 3-hexanetriol, polyalkylene glycols such as polyethylene glycol and polypropylene glycol, glycerols such as glycerol, diglycerol and triglycerol, lower alkyl ethers of glycols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether and diethylene glycol mono-n-butyl ether, and at least one of N-methyl-2-pyrrolidone and 1,3-dimethyl-2-imidalidinone.

[0036] When a mixed solvent is used, the content of the water-soluble solvent in the mixed solvent is preferably 0.1 to 30% by mass, and more preferably 1 to 20% by mass, relative to the total amount of the mixed solvent, from the viewpoint of achieving the desired fluidity of the carbon nanotube slurry of the present invention.

[0037] (Conductive Materials) In addition to the carbon nanotubes, dispersant, and solvent mentioned above, the carbon nanotube slurry of the present invention may further contain conductive materials other than carbon nanotubes. By incorporating conductive materials, the conductivity of the electrodes for secondary batteries made from the slurry can be improved.

[0038] The content of the conductive material to be incorporated is preferably 0.5 to 10% by mass, more preferably 0.5 to 7% by mass, and particularly preferably 0.5 to 5% by mass, based on the total amount of carbon nanotube slurry. Examples of conductive materials include carbon black particles such as acetylene black and Ketjenblack, and carbon nanofibers.

[0039] (Other Components) In addition to the components described above, other components may be added to the carbon nanotube slurry of the present invention depending on its application. Examples of other components include pH adjusters, anti-settling agents, wetting agents, emulsifiers, anti-sagging agents, defoaming agents, leveling agents, and plasticizers.

[0040] For example, pH adjusters can be used from the viewpoint of preventing corrosion of current collectors and ensuring compositional stability. At least one of the following can be used: ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, sodium tripophosphate, alkali metal salts of carbonic acid or phosphoric acid such as sodium carbonate, or alkali metal hydroxides such as sodium hydroxide.

[0041] (Method for producing carbon nanotube slurry) The carbon nanotube slurry of the present invention can be obtained, for example, by adding a carbon material containing carbon nanotubes, PVP as a dispersant, a solvent, and other components used as needed, stirring and mixing, and then dispersing.

[0042] The dispersion of the above slurry can be carried out by using, for example, ultrasonic dispersers, mixers such as dispersers, homomixers, rotational mixers, Henschel mixers, and planetary mixers, media-type dispersers such as paint conditioners, colloid mills, bead mills, ball mills, sand mills, attritors, pearl mills, and coball mills, media-less dispersers such as (high-pressure) homogenizers, wet jet mills, wet cavitation mills, thin-film swirling high-speed mixers, and cone mills, as well as other dispersion devices such as roll mills.

[0043] From the standpoint of the stability and efficiency of the dispersion process, preferred dispersion devices are (high-pressure) homogenizers, wet cavitation mills, and bead mills.

[0044] Distributed processing may be performed using the same distribution device or multiple times using multiple distribution devices. For example, premixing may be performed using mixers, and then distributed processing may be performed using a media-type distributer.

[0045] In this case, the viscosity after premixing is preferably 20 mPa·s or more, more preferably 30 mPa·s or more, even more preferably 50 mPa·s or more, preferably 2000 mPa·s or less, more preferably 1500 mPa·s or less, and even more preferably 1000 mPa·s or less. Furthermore, the viscosity after dispersion treatment (for example, product viscosity) is preferably 80 mPa·s or more, more preferably 90 mPa·s or more, even more preferably 100 mPa·s or more, preferably 1500 mPa·s or less, more preferably 1200 mPa·s or less, and even more preferably 900 mPa·s or less. Here, the above viscosity is measured using an E-type rotational viscometer (TV-22 model, manufactured by Toki Sangyo Co., Ltd.) at a shear rate of 38.3 s. －１ These values ​​were measured under conditions of 25°C.

[0046] If the viscosity after premixing is within the above range, the load on the disperser during the dispersion process can be reduced, and it is also possible to prevent the dispersion process from becoming difficult due to an excessively high load on the disperser.

[0047] Furthermore, if the viscosity after dispersion treatment (for example, the product viscosity) is within the above range, the fluidity of the electrode carbon nanotube slurry, as described later, will increase, making it possible to uniformly coat the electrode carbon nanotube slurry onto the current collector at a high concentration.

[0048] (Applications of Carbon Nanotube Slurry) The carbon nanotube slurry of the present invention can be mixed with electrode active material, binders as needed, and other components to form an electrode carbon nanotube slurry (positive electrode slurry or negative electrode slurry). An electrode film can be formed using the electrode carbon nanotube slurry and used as a positive or negative electrode.

[0049] (Positive electrode slurry) The positive electrode slurry of the present invention, as a carbon nanotube slurry for electrodes, comprises the carbon nanotube slurry and positive electrode active material having the above configuration. The positive electrode active material can be a material that helps lithium ions to reversibly enter and exit the positive electrode of a lithium-ion secondary battery.

[0050] Examples of positive electrode active materials include lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, lithium-nickel-cobalt-manganese-aluminum composite oxide, and other composite oxides of lithium and transition metals, as well as TiS ２ FeS, MoS ２ Transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２ Examples include transition metal oxides and olivine-type lithium phosphate oxides.

[0051] Olivine-type lithium phosphate oxide is a compound containing at least one element from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, lithium, phosphorus, and oxygen. Olivine-type lithium phosphate oxide may also be a compound in which some of the aforementioned elements are substituted with other elements to improve its properties.

[0052] A preferred cathode active material is a lithium-nickel composite oxide, and more preferably, a material of the formula: LiNi Ｘ M1 Ｙ M2 Ｚ O ２ The active materials for the positive electrode are lithium-nickel composite oxides or lithium phosphate, represented as follows: (M1 and M2 are at least one metallic element from among Al, B, alkali metals, alkaline earth metals, and transition metals; 0.8 ≤ X ≤ 1.0, 0 ≤ Y ≤ 0.2, 0 ≤ Z ≤ 0.2). These positive electrode active materials may be used individually or in combination of two or more.

[0053] In the cathode slurry of the present invention, the content of the positive electrode active material is preferably 50 to 70% by mass, and more preferably 50 to 63% by mass, relative to the total amount of the positive electrode slurry. When the content of the positive electrode active material in the positive electrode slurry is within this range, the fluidity of the slurry can be maintained while ensuring the performance of the electrode produced.

[0054] Furthermore, the carbon nanotube content in the cathode slurry is preferably 0.05 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, and even more preferably 0.05 to 1 part by mass, per 100 parts by mass of the cathode active material.

[0055] (Negative electrode slurry) The negative electrode slurry of the present invention, as an electrode carbon nanotube slurry, comprises the carbon nanotube slurry and negative electrode active material having the above configuration. The negative electrode active material can be metal oxide-based active material particles, silicon-based active material particles, or spheroidal graphite, and metal oxide-based negative electrode active material particles can be used in particular.

[0056] As metal oxide-based negative electrode active material particles, for example, titanium oxide can be used. The titanium oxide is not particularly limited as long as it is capable of intercalating and deintercalating lithium, but examples include spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxides, and titanium dioxide (TiO2) having a monoclinic crystal structure. ２ (B)), as well as anatase-type titanium dioxide, etc., can be used.

[0057] As for spinel-type lithium titanate, Li ４＋ｘ Ti ５ O １２ Examples include (where x changes in the range of -1 ≤ x ≤ 3 due to the charge-discharge reaction). As for ramsdelite-type lithium titanate, Li ２＋ｙ Ti ３ O ７ (Y changes in the range of -1 ≤ y ≤ 3 due to the charge-discharge reaction), etc. ２ (B) and as anatase-type titanium dioxide, Li １＋ｚ TiO ２ (z changes in the range of -1 ≤ z ≤ 0 due to the charge-discharge reaction.)

[0058] Examples of titanium-containing metal composite oxides include metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe. For example, a metal composite oxide containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe is TiO ２ -P ２ O ５ , TiO ２ -V ２ O ５ , TiO ２ -P ２ O ５ -SnO ２ , TiO ２ -P ２ O ５ Examples include -MeO (where Me is at least one element selected from the group consisting of Cu, Ni, and Fe).

[0059] Such metal composite oxides preferably have low crystallinity and a microstructure in which crystalline and amorphous phases coexist, or in which the amorphous phase exists alone. This microstructure can further improve the cycling performance.

[0060] In the anode slurry of the present invention, the content of the anode active material is preferably 30 to 60% by mass, and more preferably 35 to 55% by mass, relative to the total amount of the anode slurry. When the content of the anode active material in the anode slurry is within this range, the fluidity of the slurry can be maintained while ensuring the performance of the electrode produced.

[0061] Furthermore, the carbon nanotube content in the negative electrode slurry is preferably 0.05 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, and even more preferably 0.05 to 1 part by mass, per 100 parts by mass of the negative electrode active material. When the content of the negative electrode active material in the negative electrode slurry is within this range, the fluidity of the slurry can be maintained while ensuring the performance of the manufactured electrode.

[0062] (Binder) The above-mentioned positive electrode slurry and negative electrode slurry preferably each further contain a binder. Examples of binders include hydrogenated nitrile rubber (H-NBR), polyimide resins, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, fluororesins such as tetrafluoroethylene / hexafluoropropylene / vinylidene fluoride copolymers, hexafluoropropylene / vinylidene fluoride copolymers, and tetrafluoroethylene / perfluorovinyl ether copolymers, polyolefin resins such as polyethylene and polypropylene, polyvinylpyrrolidone, polyvinyl alcohol, styrene-butadiene rubber (SBR), acrylic resins, carboxymethylcellulose or its metal salts. These binders may be used individually or in combination of two or more types.

[0063] The amount of binder added is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 0.5 parts by mass or more, preferably 5 parts by mass or less, and more preferably 4.5 parts by mass or less, per 100 parts by mass of active material in the positive electrode slurry or negative electrode slurry. When the amount of binder added is within this range, electrodes with high adhesion to the current collector can be obtained without adversely affecting the battery capacity or charge / discharge characteristics.

[0064] The amount of solvent contained in the electrode carbon nanotube slurry is preferably 0.5% by mass or more, more preferably 1% by mass or more, preferably 80% by mass or less, and more preferably 70% by mass or less, relative to the total amount of the electrode carbon nanotube slurry, because an appropriate viscosity is required when coating the electrode carbon nanotube slurry onto the current collector. Note that additional solvent may be added to adjust the amount of solvent contained in the electrode carbon nanotube slurry. Any solvent suitable for use in the above-mentioned carbon nanotube slurry can be used as the solvent.

[0065] In addition to the above components, the electrode carbon nanotube slurry may optionally contain leveling agents, solid electrolytes (sulfide solid electrolytes, oxide solid electrolytes, dry polymer electrolytes, gel polymer electrolytes, pseudo-solid electrolytes, etc.), preservatives, and other additives as needed.

[0066] The electrode carbon nanotube slurry can be prepared by mixing the carbon nanotube slurry of the present invention with a positive electrode active material or a negative electrode active material, and optionally adding a binder, solvent, and other components as appropriate. For the mixing operation, for example, a twin-screw kneader can be used.

[0067] An electrode film can be formed by applying the electrode carbon nanotube slurry of the present invention onto a current collector and drying it, thereby producing an electrode (positive or negative electrode). Since the carbon nanotube slurry of the present invention uniformly disperses carbon nanotubes and the like, and is a low-viscosity slurry, using this electrode carbon nanotube slurry of the present invention allows for a high-concentration and uniform coating of carbon nanotubes and the like onto a current collector.

[0068] Electrodes can be prepared from the carbon nanotube slurry for electrodes of the present invention (slurry for positive electrode or slurry for negative electrode) in the following manner.

[0069] First, a carbon nanotube slurry for electrodes is coated onto the current collector. The current collector is a conductive material that serves as the electrode substrate for a secondary battery such as a lithium-ion secondary battery. The material and shape of the current collector used as the electrode substrate are not particularly limited, and one can be appropriately selected to suit the secondary battery to which it is applied. Examples of current collector materials include metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel. In addition, while a flat metal foil is generally used as the shape of the current collector, foils with roughened surfaces, perforated foils, and mesh-shaped foils can also be used.

[0070] Methods for coating a current collector with an electrode carbon nanotube slurry include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, or electrostatic coating. Surface smoothing treatment using a flatbed press or calender roll may also be performed after coating. Next, the current collector coated with the electrode carbon nanotube slurry is dried. This produces an electrode with an electrode film formed on the current collector.

[0071] Methods for drying the coated carbon nanotube slurry for electrodes include natural drying, forced-air drying, hot-air drying, infrared heating, and far-infrared heating. The thickness of the manufactured electrodes, including the thickness of the current collector, is generally between 1 μm and 500 μm, and preferably between 10 μm and 300 μm.

[0072] Electrodes made from the carbon nanotube slurry for electrodes (positive electrode slurry or negative electrode slurry) using the carbon nanotube slurry of the present invention have high conductivity and can therefore be suitably used as electrodes for lithium-ion secondary batteries and the like.

[0073] (Lithium-ion secondary battery) A lithium-ion secondary battery typically consists of a positive electrode, a negative electrode, an electrolyte, a non-aqueous solvent, and a separator provided as needed. It can be in various shapes depending on the intended use, such as cylindrical, prismatic, gum-shaped, coin-shaped, button-shaped, pin-shaped, or paper-shaped. It is preferable to use an electrode made by coating at least one of the positive or negative electrode of a lithium-ion secondary battery with the electrode carbon nanotube slurry for the present invention (slurry for positive electrode or slurry for negative electrode).

[0074] The following describes a lithium-ion secondary battery constructed using electrodes made from the positive electrode slurry or negative electrode slurry of the present invention.

[0075] For the positive electrode, an electrode prepared by coating and drying the above-mentioned positive electrode slurry containing the positive electrode active material onto a current collector can be used.

[0076] For the negative electrode, an electrode prepared by coating and drying the aforementioned negative electrode slurry containing the negative electrode active material onto a current collector can be used.

[0077] For the electrolyte, a lithium salt that allows ion movement can be used. For example, LiBF ４ LiClO ４ LiPF ６ LiAsF ６ LiSbF ６ LiCF ３ SO ３ , Li(CF ３ SO ２ ) ２ N, LiC ４ F ９ SO ３ , Li(CF ３ SO ２ ) ３ C, LiI, LiBr, LiCl, LiAlCl, LiHF ２ Li SCN and LiBPh ４ (However, Ph represents a phenyl group.)

[0078] Examples of non-aqueous solvents include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and aprotic polar solvents such as nitriles such as acetonitrile. These solvents may be used individually or in mixtures of two or more.

[0079] Examples of separators include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those made by applying a hydrophilic treatment to these materials.

[0080] The carbon nanotube slurry of the present invention has low viscosity and excellent handling properties, and when used as an electrode film, it exhibits good conductivity. In other words, it is suitable for the manufacture of electrodes for high-efficiency lithium-ion secondary batteries and the like.

[0081] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following, "parts" refers to mass unless otherwise specified.

[0082] (Example 1) <Preparation of carbon nanotube slurry> Carbon nanotube (CNT) A (fiber length: 250 μm, average diameter: 8 nm, peak intensity ratio G / D by Raman spectroscopy: 1.5, BET specific surface area 154 m²) ２ 0.8 parts of ( / g), 0.8 parts of PVP (Disp. A; PVP-K30, weight-average molecular weight (Mw) 40,000) as a dispersant, and the remainder (198.4 parts) were mixed with NMP, and the mixture was stirred (premixed) for 1 hour using a disperser at a stirring speed that did not entrain foam to obtain a premixed liquid. A shear rate of 38.3 s was measured using an E-type rotational viscometer (Toki Sangyo Co., Ltd., TV-22 model) on this premixed liquid. －１ The viscosity value was measured at 25°C.

[0083] The above premixing liquid was introduced into a horizontal bead mill disperser, followed by the addition of 970 parts by mass of zirconia beads (bead diameter Φ0.5 mm), and then the mixture was dispersed at a peripheral speed of 14 m / s. Afterward, the beads were separated to obtain a carbon nanotube slurry in which carbon nanotubes were uniformly dispersed. The obtained carbon nanotube slurry was then subjected to a shear rate of 38.3 s using an E-type rotational viscometer (TV-22, manufactured by Toki Sangyo Co., Ltd.). －１ The viscosity value was measured at 25°C.

[0084] (Examples 2-5 and Comparative Examples 1-7) Carbon nanotube slurries were prepared in the same manner as in Example 1, except that the types and amounts (in parts) of carbon nanotubes (CNTs), dispersants (PVP), and solvents were changed to those listed in Table 1.

[0085] In the examples and comparative examples, the fiber length and average diameter of carbon nanotubes were measured using electron microscope images, and the arithmetic mean values ​​of the fiber length and average diameter of a sufficient number of samples (e.g., 10 to 20 nanotubes) were taken. Specifically, carbon nanotubes were collected and observed using a scanning electron microscope (Hitachi High-Tech Corporation, S-3400N; SEM), and the arithmetic mean of 10 carbon nanotubes measured using images at 1000x magnification was calculated and used as the fiber length. In addition, carbon nanotubes were observed using a transmission electron microscope (Hitachi High-Tech Corporation, H-7650; TEM), and the arithmetic mean of the diameters of 10 carbon nanotubes measured using images at 50,000x magnification was calculated and used as the average diameter.

[0086] Furthermore, the peak intensity ratio G / D of carbon nanotubes in Raman spectroscopy is 1570 cm⁻¹ in the Raman spectrum obtained by Raman spectroscopy. －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１ This represents the ratio of the maximum intensity of the D-band scattered light peak within the specified range, where D is denoted as the maximum intensity. Here, the Raman spectrum was obtained by placing a carbon nanotube in a Raman microscope (ThermoScientific DXR2xi) and measuring it using a laser wavelength of 532 nm. The measurement conditions were: objective lens magnification 20x, aperture 50 μm confocal pinhole, exposure time 0.1 s, laser output 2 mW, number of scans 10, and measurement wavelength 100–3400 cm. -1 That's what I decided.

[0087] Furthermore, the BET specific surface area of ​​carbon nanotubes was measured using the BET single-point method after sampling the carbon nanotubes, accurately weighing them using an electronic balance, drying them at 110°C for 30 minutes while degassing, and then using a fully automatic specific surface area measuring device (Macsorb model HM-1208, manufactured by Mountec Co., Ltd.).

[0088] <Evaluation Method> The evaluation (premixing viscosity, product viscosity, and conductivity) in the examples and comparative examples was performed as follows.

[0089] (Premixing Viscosity) The premixing liquids obtained in the "Preparation of Carbon Nanotube Slurry" in the Examples and Comparative Examples were evaluated according to the following criteria. The results are shown in Table 1. A: Fluid, beads move well. B: Poor fluidity, beads do not move.

[0090] (Product Viscosity) Table 1 shows the viscosity values ​​of the carbon nanotube slurries measured in the examples and comparative examples.

[0091] (Surface resistance (conductivity)) To 100 parts of titanium oxide as the negative electrode active material, the carbon nanotube slurry obtained in the examples and comparative examples was added so that the amount of carbon nanotubes was 0.3 parts and the amount of binder was 2.5 parts, and the negative electrode active material content was adjusted to 50% by mass of the total amount of the negative electrode slurry. This mixture was then mixed using a twin-screw kneader to obtain an electrode carbon nanotube slurry.

[0092] A carbon nanotube slurry for electrodes was applied to a current collector by hand using a bar coater (manufactured by Yasuda Seiki Seisakusho Co., Ltd.), and dried in an oven at 100°C for 10 minutes to form an electrode film of 0.5 to 3 μm thickness, thereby obtaining electrodes. The surface resistivity (Ω / □) of the fabricated electrodes was measured using a resistivity meter (Mitsubishi Chemical Analytec Co., Ltd., Rolester GP, MCP-T610, four-probe, ASP pin spacing 5 mm). A lower surface resistivity indicates better conductivity. The measurement results for surface resistivity are shown in Table 1.

[0093]

[0094] As shown in Table 1, a carbon nanotube slurry containing carbon nanotubes with a fiber length of 50 μm or more, an average diameter of 3 nm to 20 nm, and a peak intensity ratio G / D of 1.0 to 2.6 in Raman spectroscopy, polyvinylpyrrolidone with a weight-average molecular weight of 70,000 or less, and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water, exhibits excellent handling properties due to its low pre-mixing viscosity and product viscosity, and electrodes obtained using this carbon nanotube slurry exhibit high conductivity. Furthermore, as shown in Table 1, a carbon nanotube slurry with a fiber length of 50 μm or more, an average diameter of 3 nm to 20 nm, and a BET specific surface area of ​​70 m² is also found. ２ / g or more 180m ２ A carbon nanotube slurry containing carbon nanotubes with a molecular weight of 70,000 or less, polyvinylpyrrolidone with a weight-average molecular weight of 70,000 or less, and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water, exhibits excellent handling properties due to its low pre-mixing viscosity and product viscosity, and electrodes obtained using this carbon nanotube slurry exhibit high conductivity.

[0095] The carbon nanotube slurry of the present invention can be mixed with an electrode active material and suitably used as an electrode carbon nanotube slurry for the production of electrodes such as electrodes for lithium secondary batteries.

Claims

1. A carbon nanotube slurry comprising: carbon nanotubes having a fiber length of 50 μm or more, an average diameter of 3 nm or more and 20 nm or less, and a peak intensity ratio G / D of 1.0 or more and 2.6 or less in Raman spectroscopy; polyvinylpyrrolidone having a weight-average molecular weight of 70,000 or less; and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water. (However, the intensity ratio G / D is defined as the peak intensity at 1570 cm⁻¹ in the Raman spectrum obtained by the Raman spectroscopy.) －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１ (When D is the maximum intensity of the D-band scattered light peak in the specified range, this represents the ratio.) 2. The fiber length is 50 μm or more, the average diameter is 3 nm to 20 nm, and the BET specific surface area is 70 m². ２ / g or more 180m ２ A carbon nanotube slurry comprising carbon nanotubes having a weight-average molecular weight of 70,000 or less, polyvinylpyrrolidone having a weight-average molecular weight of 70,000 or less, and at least one selected from the group consisting of N-methyl-2-pyrrolidone, ethanol, and water.

3. A carbon nanotube slurry for electrodes, characterized by containing the carbon nanotube slurry described in claim 1 or 2 and an active material.

4. An electrode film formed using the electrode carbon nanotube slurry described in claim 3.

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