Carbon black, slurry and lithium ion secondary battery

Abstract

The present invention provides carbon black wherein if the carbon black is divided into first primary aggregates having an X value of more than 1.7, second primary aggregates having an X value of 1.7 or less and a Y value of 1.2 or less, third primary aggregates having an X value of 1.7 or less, a Y value of more than 1.2 and a Z value of 2.0 or less, and fourth primary aggregates having an X value of 1.7 or less, a Y value of more than 1.2 and a Z value of more than 2.0, the proportion of the total number of the second primary aggregates and the third primary aggregates is 23% or more.　(X): X = L / W (Y): Y = P2 / 4πA (Z): Z = (L×W) / A (In the formulae, W (µm) is the Feret's diameter of a primary aggregate in the minor axis direction, L (µm) is the Feret's diameter of the primary aggregate in the major axis direction, P (µm) is the length of the periphery of the primary aggregate, and A (µm2) is the projected area of the primary aggregate in a two-dimensional projection image of the primary aggregate obtained by a transmission electronic microscope.)

Description

[Technical Field]

[0001] The present invention relates to a carbon black slurry and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are widely used as power sources for small electronic devices such as smartphones and tablet computers. A lithium-ion secondary battery generally comprises electrodes, a separator, and an electrolyte. Electrodes are manufactured by coating and drying a composite slurry, in which active materials, conductive agents, binders, etc. are dispersed in a dispersion medium, onto a metal collector plate to form a composite layer.

[0003] For example, carbon black is used as the conductive agent (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-193986 Summary of the Invention [Problem to be solved by the invention]

[0005] The role of the conductive agent is to form a conductive path within the electrode, so if it aggregates excessively within the electrode, localized areas of poor conductivity will appear, resulting in ineffective use of the active material, a decrease in discharge capacity, and a deterioration in battery performance.

[0006] In recent years, there has been a demand for higher capacity lithium-ion secondary batteries, leading to a trend toward increasing the active material content in the composite layer and decreasing the conductive agent and binder content. A reduced conductive agent content makes it difficult to form conductive paths within the electrode, resulting in reduced battery performance. Therefore, efforts have been made to improve conductivity by using smaller particle diameter conductive agents to increase the number of particles per unit mass, shorten the proximity distance between conductive agents within the electrode, and increase the number of contact points with the active material and current collector. However, as the specific surface area increases with the smaller particle diameter of the conductive agent, the viscosity of the composite slurry increases significantly, making uniform dispersion difficult.

[0007] To achieve high dispersion of the conductive agent and low viscosity of the composite slurry, methods such as dispersing by applying strong collision energy using equipment such as a high-pressure jet mill and extending the dispersion processing time have been investigated, but these have presented problems such as impurities being mixed in due to wear of the equipment.Addition of dispersants has also been investigated, but there has been a problem that increasing the amount of dispersant added due to the reduction in particle size of the conductive agent leads to a decrease in battery performance.

[0008] Therefore, an object of the present invention is to provide carbon black that has excellent dispersibility and can be used to form a low-viscosity slurry, enabling the realization of a lithium-ion secondary battery with excellent discharge rate characteristics and cycle characteristics. Another object of the present invention is to provide a slurry containing the carbon black, and a lithium-ion secondary battery containing the carbon black. [Means for solving the problem]

[0009] One aspect of the present invention relates to, for example, the following [1] to [6]. [1] First primary aggregates having an X value calculated by the following formula (X) of greater than 1.7; second primary aggregates in which the X value is 1.7 or less and the Y value calculated by the following formula (Y) is 1.2 or less; a third primary aggregate in which the X value is 1.7 or less, the Y value is greater than 1.2, and the Z value calculated by the following formula (Z) is 2.0 or less; fourth primary aggregates having an X value of 1.7 or less, an Y value of more than 1.2, and an Z value of more than 2.0; When divided into Carbon black, wherein the ratio of the total number of the second primary agglomerates and the third primary agglomerates to the total number of the first primary agglomerates, the second primary agglomerates, the third primary agglomerates, and the fourth primary agglomerates is 23% or more. X=L / W(X) Y=P 2 / 4πA (Y) Z=(L×W) / A(Z) [wherein, in a two-dimensional projection image of the primary aggregates by a transmission electron microscope, the Feret diameter in the minor axis direction of the primary aggregates is W (μm), the Feret diameter in the major axis direction of the primary aggregates is L (μm), the perimeter of the primary aggregates is P (μm), and the projected area of ​​the primary aggregates is A (μm 2 ). [2] Specific surface area is 150m 2 / g or more 400m 2 / g or less. [3] [1] or [2], which has a DBP absorption of 165 mL / 100 g or more and 285 mL / 100 g or less. [4] The carbon black according to any one of [1] to [3], having an iron content of less than 2000 ppb by mass. [5] A slurry comprising the carbon black according to any one of [1] to [4] and a dispersion medium. [6] A positive electrode, a negative electrode, and a separator are provided, A lithium ion secondary battery, wherein at least one of the positive electrode and the negative electrode contains the carbon black according to any one of [1] to [4]. [Effects of the Invention]

[0010] The present invention provides carbon black that has excellent dispersibility and can be used to form a low-viscosity slurry, enabling the realization of a lithium-ion secondary battery that is excellent in discharge rate characteristics and cycle characteristics. The present invention also provides a slurry containing the carbon black and a lithium-ion secondary battery containing the carbon black. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram for explaining a two-dimensional projection image of a primary aggregate. [Figure 2] FIG. 1 is a diagram showing a two-dimensional projection image of primary aggregates of carbon black (A1) of Example A1, taken by a transmission electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the present invention will be described in detail below. However, the present invention is not limited to the embodiments described below. In this specification, unless otherwise specified, a numerical range indicated with "to" means a range "greater than or equal to" the number on the left and "less than or equal to" the number on the right. For example, "A to B" means greater than or equal to A and less than or equal to B.

[0013] <Carbon black> When the carbon black of this embodiment is divided into first primary aggregates having an X value of more than 1.7 as calculated by formula (X), second primary aggregates having an X value of 1.7 or less and a Y value of 1.2 or less as calculated by formula (Y), third primary aggregates having an X value of 1.7 or less, a Y value of more than 1.2 and a Z value of 2.0 or less as calculated by formula (Z), and fourth primary aggregates having an X value of 1.7 or less, a Y value of more than 1.2 and a Z value of more than 2.0, the proportion of the total number of second primary aggregates and third primary aggregates (N2+N3) to the total number of first primary aggregates, second primary aggregates, third primary aggregates and fourth primary aggregates (N1+N2+N3+N4) is 23% or more. X=L / W(X) Y=P2 / 4πA (Y) Z=(L×W) / A(Z)

[0014] In the formulas (X), (Y), and (Z), in a two-dimensional projection image of the primary aggregates by a transmission electron microscope, the Feret diameter (minor diameter) in the minor axis direction of the primary aggregates is W (μm), the Feret diameter (major diameter) in the major axis direction of the primary aggregates is L (μm), the perimeter of the primary aggregates is P (μm), and the projected area of ​​the primary aggregates is A (μm 2 )

[0015] FIG. 1 is a schematic diagram for explaining a two-dimensional projected image of a primary aggregate. The width of the rectangle circumscribing the primary aggregate is W (μm), the length of the rectangle circumscribing the primary aggregate is L (μm), the perimeter of the primary aggregate is P (μm), and the projected area of ​​the primary aggregate is A (μm 2 )

[0016] The first primary aggregates are primary aggregates having an X value of greater than 1.7 in formula (X). Here, the X value in formula (X) indicates the aspect ratio of the primary aggregate, and the greater the difference between the major axis and the minor axis, the greater the X value. Since the first primary aggregates have an X value of greater than 1.7, they can be said to be primary aggregates having a shape close to a linear shape.

[0017] The second primary aggregates are primary aggregates in which the X value in formula (X) is 1.7 or less and the Y value in formula (Y) is 1.2 or less. Here, the Y value in formula (Y) is an index of the complexity of the primary aggregates, and it can be said that the closer the Y value is to 1, the closer the shape is to a perfect circle. Since the second primary aggregates have an X value of 1.7 or less and a Y value of 1.2 or less, they can be said to be primary aggregates having a shape close to a spheroid.

[0018] The third primary aggregates are primary aggregates in which the X value of formula (X) is 1.7 or less, the Y value of formula (Y) is greater than 1.2, and the Z value of formula (Z) is 2.0 or less. Here, the Z value of formula (Z) is the ratio of the area (L × W) of a rectangle circumscribing a projection of the primary aggregate to the projected area (A) of the primary aggregate. The larger the Z value, the more branched the primary aggregates are. Since the third primary aggregates have an X value of 1.7 or less, a Y value of greater than 1.2, and a Z value of 2.0 or less, they can be said to be primary aggregates having a shape close to an ellipsoid.

[0019] The fourth primary aggregates are primary aggregates in which the X value of formula (X) is 1.7 or less, the Y value of formula (Y) is greater than 1.2, and the Z value of formula (Z) is greater than 2.0. Because the fourth primary aggregates have an X value of 1.7 or less, a Y value greater than 1.2, and a Z value greater than 2.0, they can be said to be branched primary aggregates with many branches.

[0020] According to the findings of the present inventors, the second and third primary agglomerates, which have spheroidal or nearly spheroidal shapes, are less likely to cause an increase in the viscosity of the slurry due to entanglement between the primary agglomerates than the first and fourth primary agglomerates. The carbon black of this embodiment has excellent dispersibility and can form a low-viscosity slurry because the proportion of the total number of the second and third primary agglomerates is 23% or more.

[0021] The first and fourth primary aggregates are advantageous for forming conductive paths, and from the viewpoint of excellent performance as a conductive agent, it is desirable that the first and fourth primary aggregates are present in a predetermined amount or more. Therefore, the ratio of the total number of the second and third primary aggregates (N2+N3) to the total number of the first, second, third, and fourth primary aggregates (N1+N2+N3+N4) may be, for example, 50% or less, preferably 45% or less, more preferably 40% or less, and even more preferably 35% or less. That is, the ratio of the total number of second primary aggregates and third primary aggregates (N2+N3) to the total number of first primary aggregates, second primary aggregates, third primary aggregates and fourth primary aggregates (N1+N2+N3+N4) may be, for example, 23 to 50%, 23 to 45%, 23 to 40%, or 23 to 35%.

[0022] The number proportion of first primary aggregates (N1) is not particularly limited, and may be, for example, 25% or more, 30% or more, or 35% or more of the total number of first, second, third, and fourth primary aggregates (N1+N2+N3+N4). Furthermore, the number proportion of first primary aggregates (N1) may be, for example, 70% or less, 65% or less, or 60% or less of the total number of first, second, third, and fourth primary aggregates (N1+N2+N3+N4). That is, the number ratio of first primary aggregates (N1) to the total number of first primary aggregates, second primary aggregates, third primary aggregates and fourth primary aggregates (N1+N2+N3+N4) may be, for example, 25 to 70%, 25 to 65%, 25 to 60%, 30 to 70%, 30 to 65%, 30 to 60%, 35 to 70%, 35 to 65% or 35 to 60%.

[0023] The number proportion of second primary aggregates (N2) is not particularly limited, and may be, for example, 1% or more, 2% or more, or 3% or more relative to the total number (N1+N2+N3+N4) of first, second, third, and fourth primary aggregates. Furthermore, the number proportion of second primary aggregates (N2) may be, for example, 20% or less, 10% or less, or 8% or less relative to the total number (N1+N2+N3+N4) of first, second, third, and fourth primary aggregates. That is, the number proportion of second primary aggregates (N2) may be, for example, 1 to 20%, 1 to 10%, 1 to 8%, 2 to 20%, 2 to 10%, 2 to 8%, 3 to 20%, 3 to 10%, or 3 to 8% of the total number of first primary aggregates, second primary aggregates, third primary aggregates, and fourth primary aggregates (N1+N2+N3+N4).

[0024] The number proportion of the third primary aggregates (N3) is not particularly limited, and may be, for example, 5% or more, 10% or more, or 15% or more of the total number of the first, second, third, and fourth primary aggregates (N1+N2+N3+N4). Furthermore, the number proportion of the third primary aggregates (N3) may be, for example, 55% or less, 50% or less, or 45% or less of the total number of the first, second, third, and fourth primary aggregates (N1+N2+N3+N4). That is, the number proportion of the third primary aggregates (N3) relative to the total number of the first primary aggregates, second primary aggregates, third primary aggregates and fourth primary aggregates (N1+N2+N3+N4) may be, for example, 5 to 55%, 5 to 50%, 5 to 45%, 10 to 55%, 10 to 50%, 10 to 45%, 15 to 55%, 15 to 50% or 15 to 45%.

[0025] The number proportion of the fourth primary aggregates (N4) is not particularly limited, and may be, for example, 5% or more, 10% or more, or 15% or more of the total number of the first, second, third, and fourth primary aggregates (N1+N2+N3+N4). Furthermore, the number proportion of the fourth primary aggregates (N4) may be, for example, 45% or less, 40% or less, or 35% or less of the total number of the first, second, third, and fourth primary aggregates (N1+N2+N3+N4). That is, the number proportion of the fourth primary aggregates (N4) relative to the total number of the first primary aggregates, second primary aggregates, third primary aggregates and fourth primary aggregates (N1+N2+N3+N4) may be, for example, 5 to 45%, 5 to 40%, 5 to 35%, 10 to 45%, 10 to 40%, 10 to 35%, 15 to 45%, 15 to 40% or 15 to 35%.

[0026] In this specification, the two-dimensional projection image of the primary aggregates taken by a transmission electron microscope and the image analysis can be performed by the following method. First, carbon black is dispersed in chloroform for 10 minutes using ultrasonic waves at an output of 90 W to break up secondary agglomerates into primary agglomerates. This is then scooped onto a collodion membrane mesh and photographed under a transmission electron microscope at a magnification of 2000x. Next, the Feret diameter W (μm) in the minor axis direction, Feret diameter L (μm) in the major axis direction, perimeter P (μm), and projected area A (μm) of 100 or more carbon black primary aggregates randomly selected from the captured two-dimensional projection image were measured. 2 ) is measured using the image analysis software "Image-Pro Plus 6.2J (Media Cybernetics)." Specifically, the two-dimensional projection image is filtered (median filter, option 7x7, number of times: 3), and then the brightness range is manually extracted to match the primary aggregates. Measurement items are selected and measured: "Size (width)," "Size (length)," "Perimeter," and "Area." Note that primary aggregates and scale bars that extend to the edges of the two-dimensional projection image, as well as background noise, are excluded from the measurements.

[0027] Carbon black has significantly different shapes of primary aggregates due to differences in thermal history during synthesis (for example, thermal history resulting from the thermal decomposition and combustion reaction of fuel oil, the thermal decomposition and combustion reaction of raw materials, rapid cooling by a cooling medium and reaction termination, etc.), differences in the frequency of collisions between primary particles, etc.

[0028] The specific surface area of ​​the carbon black of this embodiment is, for example, 130 m 2 The specific surface area of ​​the carbon black is preferably 140 m / g or more from the viewpoint of further improving the conductivity-imparting ability.2 / g or more, more preferably 150m 2 / g or more, more preferably 160m 2 The specific surface area of ​​carbon black can be increased by reducing the size of the primary particles, making them hollow, or making the particle surface porous. The specific surface area of ​​the carbon black of this embodiment is, for example, 500 m 2 The specific surface area of ​​the carbon black is preferably 450 m / g or less from the viewpoint of further improving dispersibility. 2 / g or less, more preferably 400m 2 / g or less. That is, the specific surface area of ​​carbon black is, for example, 130 to 500 m 2 / g, 130-450m 2 / g, 130-400m 2 / g, 140-500m 2 / g, 140-450m 2 / g, 140-400m 2 / g, 150-500m 2 / g, 150-450m 2 / g, 150-400m 2 / g, 160-500m 2 / g, 160-450m 2 / g or 160-400m 2 / g.

[0029] In this specification, the specific surface area is measured in accordance with JIS K6217-2:2017, Method A flow method (thermal conductivity measurement method).

[0030] The DBP absorption of the carbon black of this embodiment may be, for example, 150 mL / 100 g or more, preferably 160 mL / 100 g or more, more preferably 165 mL / 100 g or more, and even more preferably 190 mL / 100 g or more. The DBP absorption of the carbon black of this embodiment is, for example, 300 mL / 100 g or less, more preferably 285 mL / 100 g or less. That is, the DBP absorption amount of carbon black may be, for example, 150 to 300 mL / 100 g, 150 to 285 mL / 100 g, 160 to 300 mL / 100 g, 160 to 285 mL / 100 g, 165 to 300 mL / 100 g, 165 to 285 mL / 100 g, 190 to 300 mL / 100 g, or 190 to 285 mL / 100 g.

[0031] DBP absorption is an index used to evaluate the ability of carbon black to absorb dibutyl phthalate (DBP) on the particle surface and in the voids formed by primary aggregates. Carbon black with developed primary aggregates has a high DBP absorption capacity due to the increased number of necks formed by fusion of primary particles and the increased voids formed between particles. High DBP absorption tends to enhance the ability to impart conductivity within the electrode due to the development of primary aggregates. It also tends to better adapt to volume changes in the active material associated with charging and discharging in lithium-ion secondary batteries, improving battery performance, including cycle characteristics. Furthermore, low DBP absorption tends to prevent the binder in the composite layer from being trapped by the carbon black primary aggregates, thereby helping to maintain good adhesion with the active material and current collector.

[0032] In this specification, the DBP absorption amount refers to a value measured according to the method described in JIS K6221, Method B, converted into a value corresponding to JIS K6217-4:2008 using the following formula (a). DBP absorption amount = (A-10.974) / 0.7833 …(a) [In the formula, A represents the value of DBP absorption measured by the method described in JIS K6221, Method B.]

[0033] The average primary particle size of the carbon black of this embodiment may be, for example, less than 35 nm, preferably less than 30 nm, and more preferably less than 25 nm. The average primary particle size of the carbon black of this embodiment may be, for example, 1.0 nm or more.

[0034] Conventionally, carbon black used as a conductive agent in lithium-ion secondary batteries has been difficult to form into a slurry when its average primary particle diameter is small (for example, less than 30 nm), but the carbon black of this embodiment can easily form a low-viscosity slurry even when its average primary particle diameter is small (for example, less than 30 nm).By making it possible to use carbon black with such a small particle diameter, high conductivity can be achieved even when the blending ratio in the composite layer is low.

[0035] The average primary particle size of carbon black can be determined by measuring the primary particle sizes of 100 or more carbon black particles randomly selected from a 50,000x magnification image taken with a transmission electron microscope (TEM) and calculating the average value. Primary particles of carbon black have a small aspect ratio and are close to being spherical, but are not perfectly spherical. Therefore, in this embodiment, the primary particle size of carbon black is determined as the largest line segment connecting two points on the periphery of a primary particle in a TEM image.

[0036] The ash content of the carbon black of this embodiment may be, for example, 0.05% by mass or less, preferably 0.03% by mass or less, and more preferably 0.02% by mass or less. The ash content can be measured in accordance with JIS K1469:2003, and can be reduced, for example, by classifying the carbon black using a device such as a dry cyclone.

[0037] The iron content of the carbon black of this embodiment may be, for example, less than 2500 ppb by mass, preferably less than 2300 ppb by mass, and more preferably less than 2000 ppb by mass. The iron content can be reduced, for example, by bringing the carbon black into contact with a magnet.

[0038] The iron content of carbon black can be measured by inductively coupled plasma mass spectrometry after pretreatment using the acid decomposition method in accordance with JIS K0116:2014. Specifically, it can be measured as follows. First, 1 g of carbon black is precisely weighed into a quartz beaker and heated in an electric furnace in an air atmosphere at 800°C for 3 hours. Next, 10 mL of mixed acid (70% hydrochloric acid by mass, 30% nitric acid by mass) and at least 10 mL of ultrapure water are added to the residue, and the mixture is heated and dissolved on a hot plate at 200°C for 1 hour. After cooling, the solution is diluted to 25 mL with ultrapure water and measured using an inductively coupled plasma mass spectrometer (Agilent 8800, manufactured by Agilent).

[0039] When the ash and iron contents of carbon black are low, contamination with metals, ceramics, and other foreign matter due to damage to equipment during the kneading process can be more significantly suppressed. Furthermore, a decrease in conductivity within the electrode due to contamination with ash, insulating foreign matter, and the like can also be suppressed. Therefore, carbon black with low ash and iron contents can be suitably used in lithium-ion secondary batteries, which require high safety.

[0040] The method for producing carbon black according to the present embodiment is not particularly limited. For example, a raw material such as a hydrocarbon may be supplied through a nozzle installed upstream of a reactor, carbon black may be produced through a pyrolysis reaction and / or a combustion reaction, and carbon black may be collected through a bag filter directly connected downstream of the reactor.

[0041] The raw materials used are not particularly limited, and include gaseous hydrocarbons such as acetylene, methane, ethane, propane, ethylene, propylene, and butadiene, as well as oily hydrocarbons such as toluene, benzene, xylene, gasoline, kerosene, light oil, and heavy oil. Among these, acetylene, which contains few impurities, is preferred. Acetylene generates more heat of decomposition than other raw materials, allowing the temperature inside the reactor to be raised. This allows carbon black nucleation to prevail over particle growth due to addition reactions, thereby reducing the primary particle size of the carbon black.

[0042] In addition, it is preferable to supply oxygen, carbon dioxide, hydrogen, nitrogen, steam, or the like to the reactor in addition to the raw material carbon source. These non-raw material gases promote gas agitation in the reactor and increase the frequency of collisions and fusion between primary particles of carbon black produced from the raw materials. Therefore, the use of non-raw material gases tends to promote the development of primary aggregates of carbon black and increase DBP absorption. Oxygen is preferably used as the non-raw material gas. The use of oxygen burns part of the raw material, raising the temperature in the reactor, making it easier to obtain carbon black with a small particle size and a high specific surface area. Multiple gases can also be used as non-raw material gases. The non-raw material gases are preferably supplied upstream of the reactor, preferably from a nozzle separate from the raw material. This effectively agitates the raw material, which is also supplied upstream, and facilitates the development of primary aggregates.

[0043] Furthermore, the inventors conducted extensive research to control the shape of the primary aggregates and found that supplying a gas other than the raw material gas into the reactor in a direction perpendicular to the flow of the raw material gas is effective in producing the carbon black of this embodiment. This method is thought to exert a rotational effect on the produced carbon black, compared to the conventional method of supplying the raw material gas and a gas other than the raw material gas in parallel, making it easier to form primary aggregates with a spheroidal or ellipsoidal shape rather than linear or branched primary aggregates. The inventors also found that the injection speed of the gas other than the raw material gas into the reactor affects the shape of the primary aggregates.

[0044] In conventional carbon black production, a cooling medium such as water may be introduced from the downstream part of the reactor to stop the thermal decomposition and combustion reaction of the raw material. However, this has no effect on the development of primary aggregates, and there is a risk of large variations in properties due to sudden temperature changes. Therefore, in this embodiment, it is preferable not to introduce a cooling medium from the downstream part of the reactor.

[0045] The carbon black of the present embodiment is not limited to carbon black obtained directly by production using a reactor as described above, but can also be obtained, for example, by pulverizing the obtained carbon black or by mixing carbon blacks produced under different conditions.

[0046] <Slurry> The slurry of the present embodiment contains the carbon black of the present embodiment and a dispersion medium.

[0047] If the viscosity of the slurry is too high, strong shear is applied during kneading with the active material, which can destroy the primary aggregates of the carbon black, resulting in reduced conductivity, or can lead to the inclusion of foreign matter due to wear of the equipment. On the other hand, if the viscosity of the slurry is too low, the carbon black is more likely to settle in the slurry, making it difficult to maintain uniformity. In this embodiment, the use of the above-mentioned carbon black allows the slurry viscosity to be reduced, significantly suppressing the destruction of the primary aggregates of the carbon black, thereby maintaining excellent conductivity-imparting capabilities and significantly suppressing the inclusion of foreign matter due to wear of the equipment. In other words, in this embodiment, the active material content in the composite layer can be increased without compromising the viscosity characteristics and conductivity of the slurry, thereby achieving a high capacity lithium-ion secondary battery.

[0048] In order to obtain the above effect more significantly, the viscosity of the slurry (25°C, shear rate 10 s -1 ) is preferably 100 mPa·s or more, more preferably 200 mPa·s or more. This suppresses the sedimentation of carbon black and improves the uniformity of the slurry. In addition, from the viewpoint of obtaining the above-mentioned effect more significantly, the slurry viscosity (at 25°C and a shear rate of 10 s -1 ) is preferably 1500 mPa·s or less, more preferably 1200 mPa·s or less. That is, the slurry viscosity (25°C, shear rate 10 s -1 ) may be, for example, 100 to 1500 mPa·s, 100 to 1200 mPa·s, 200 to 1500 mPa·s, or 200 to 1200 mPa·s.

[0049] The dispersion medium is not particularly limited, and for example, N-methyl-2-pyrrolidone, ethanol, ethyl acetate, etc. can be used.

[0050] The slurry of the present embodiment may further contain other carbon blacks, graphite, carbon nanotubes, carbon nanofibers, etc., as long as the conductivity-imparting ability and dispersibility of the carbon black of the present embodiment are not impaired.

[0051] The slurry of this embodiment may further contain an active material and additives such as a dispersant.

[0052] In the slurry of this embodiment, the content of the carbon black of this embodiment may be, for example, 0.5% by mass or more, and preferably 1% by mass or more. In addition, in the slurry of this embodiment, the content of the carbon black of this embodiment may be, for example, 50% by mass or less, and preferably 20% by mass or less. That is, in the slurry of this embodiment, the content of the carbon black of this embodiment may be, for example, 0.5 to 50 mass %, 0.5 to 20 mass %, 1 to 50 mass %, or 1 to 20 mass %.

[0053] The method for producing the slurry of this embodiment is not particularly limited, and the slurry can be produced by kneading the components using a general device such as a mixer, a kneader, a disperser, a mill, or an automatic orbital rotating device.

[0054] The slurry of this embodiment can be suitably used as an electrode-forming slurry for forming an electrode of a lithium-ion secondary battery. The electrode-forming slurry may be a positive electrode-forming slurry or a negative electrode-forming slurry.

[0055] When the slurry of the present embodiment is a slurry for forming an electrode, the slurry of the present embodiment may contain an active material, a conductive agent, and a dispersion medium, and in this case, the slurry contains the carbon black of the present embodiment as the conductive agent.

[0056] The content of the conductive agent in the electrode-forming slurry may be, for example, 0.01% by mass or more, preferably 0.05% by mass or more, and more preferably 0.08% by mass or more. The content of the conductive agent in the electrode-forming slurry may be, for example, 20% by mass or less, preferably 15% by mass or less, and more preferably 10% by mass or less. That is, the content of the conductive agent in the electrode-forming slurry may be, for example, 0.01 to 20 mass%, 0.01 to 15 mass%, 0.01 to 10 mass%, 0.05 to 20 mass%, 0.05 to 15 mass%, 0.05 to 10 mass%, 0.08 to 20 mass%, 0.08 to 15 mass%, or 0.08 to 10 mass%.

[0057] The electrode-forming slurry may further contain a conductive agent other than carbon black, such as graphite, carbon nanotubes, and carbon nanofibers.

[0058] In the electrode-forming slurry, the proportion of carbon black in the conductive agent may be, for example, 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass or more, and may be 100% by mass.

[0059] The active material is not particularly limited, and known active materials used in lithium-ion secondary batteries can be used without particular limitation. Examples of positive electrode active materials include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel-manganese-cobalt oxide, and lithium iron phosphate. Examples of negative electrode active materials include carbonaceous materials such as natural graphite, artificial graphite, graphite, activated carbon, coke, needle coke, fluid coke, mesophase microbeads, carbon fiber, and pyrolytic carbon.

[0060] The electrode-forming slurry may further contain a binder. The binder is not particularly limited, and any known binder used in lithium-ion secondary batteries can be used without particular limitation. Examples of binders include polyethylene, nitrile rubber, polybutadiene, butyl rubber, polystyrene, styrene-butadiene rubber, polysulfide rubber, nitrocellulose, carboxymethyl cellulose, polyvinyl alcohol, tetrafluoroethylene resin, polyvinylidene fluoride, and polychloroprene fluoride.

[0061] The method for forming an electrode using the electrode-forming slurry is not particularly limited. For example, the electrode-forming slurry can be applied to a current collector and dried to form an electrode including a current collector and a composite layer.

[0062] The current collector is not particularly limited, and examples thereof include metal foils of gold, silver, copper, platinum, aluminum, iron, nickel, chromium, manganese, lead, tungsten, titanium, and alloys containing these as main components. For example, aluminum foil is preferably used as the positive electrode current collector, and copper foil is preferably used as the negative electrode current collector.

[0063] <Lithium-ion secondary battery> The lithium ion secondary battery of this embodiment includes a positive electrode, a negative electrode, and a separator. At least one of the positive electrode and the negative electrode of this embodiment contains the carbon black of this embodiment. In this lithium ion secondary battery of this embodiment, at least one of the positive electrode and the negative electrode may be formed from the electrode-forming slurry, or at least one of the positive electrode and the negative electrode may include a composite layer formed on a current collector using the electrode-forming slurry.

[0064] The lithium ion secondary battery of this embodiment has a high capacity because it uses the carbon black of this embodiment, and can be produced with good productivity by using the above-described electrode-forming slurry.

[0065] In the lithium ion secondary battery of this embodiment, the positive electrode preferably contains the carbon black of this embodiment. Also, in the lithium ion secondary battery of this embodiment, the positive electrode is preferably formed from the electrode-forming slurry, and more preferably includes a composite layer formed on a current collector using the electrode-forming slurry.

[0066] The lithium ion secondary battery of this embodiment may have the same configuration as known lithium ion secondary batteries, except for the electrode containing the carbon black of this embodiment.

[0067] The separator is not particularly limited, and any separator known for lithium ion secondary batteries can be used without particular limitation. Examples of separators include synthetic resins such as polyethylene and polypropylene. The separator is preferably a porous film because it has good electrolyte retention.

[0068] The lithium ion secondary battery of this embodiment may include an electrode group in which a positive electrode and a negative electrode are stacked or wound with a separator interposed therebetween.

[0069] In the lithium ion secondary battery of this embodiment, the positive electrode, the negative electrode, and the separator may be immersed in the electrolyte solution.

[0070] The electrolyte is not particularly limited and may be, for example, a non-aqueous electrolyte containing a lithium salt. Examples of non-aqueous solvents in non-aqueous electrolytes containing a lithium salt include ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. Examples of lithium salts that can be dissolved in non-aqueous solvents include lithium hexafluorophosphate, lithium borotetrafluoride, and lithium trifluoromethanesulfonate.

[0071] The lithium ion secondary battery of this embodiment may use an ion-conductive polymer or the like as an electrolyte.

[0072] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments.

[0073] For example, one aspect of the present invention may be a method for evaluating carbon black, comprising: an image acquisition step of acquiring two-dimensional projection images of primary aggregates of carbon black using a transmission electron microscope; and an evaluation step of determining the numbers of first primary aggregates, second primary aggregates, third primary aggregates, and fourth primary aggregates from the two-dimensional projection images, and determining the number ratio of the total number of second primary aggregates and third primary aggregates to the total number of the first primary aggregates, second primary aggregates, third primary aggregates, and fourth primary aggregates.

[0074] Another aspect of the present invention may be a method for sorting carbon black, comprising: an image acquisition step of acquiring two-dimensional projection images of primary aggregates of carbon black using a transmission electron microscope; and a sorting step of determining the numbers of first primary aggregates, second primary aggregates, third primary aggregates, and fourth primary aggregates from the two-dimensional projection images, determining the number ratio of the total number of second primary aggregates and third primary aggregates to the total number of first primary aggregates, second primary aggregates, third primary aggregates, and fourth primary aggregates, and sorting carbon black having this number ratio of 23% or more. [Example]

[0075] (Example A1: Production of carbon black (A1)) Acetylene (110°C) was injected as raw material gas at 20 Nm from a nozzle installed upstream of a cylindrical reactor (diameter 1 m, length 7 m). 3 / h, oxygen (25°C) was injected as a gas other than the raw material from a nozzle installed perpendicular to the raw material gas at 8 Nm 3 / h to produce carbon black, which was then collected in a bag filter installed downstream of the reactor. It then passed through a dry cyclone device and an iron-removing magnet before being collected in a tank. The nozzle diameter was adjusted so that the ejection velocity of the raw material gas into the reactor was 5 m / s, and the ejection velocity of gases other than the raw material was 15 m / s.

[0076] (Example A2: Production of Carbon Black (A2)) Carbon dioxide (25°C) was used as a gas other than the raw material, and the ejection velocity was 15 m / s and 14 Nm 3 Carbon black was produced in the same manner as in Example 1, except that the feed rate was changed to 1 / h.

[0077] (Example A3: Production of Carbon Black (A3)) Ethylene (110°C) was used as the raw material gas, and the ejection velocity was 5 m / s and 20 Nm 3 Carbon black was produced in the same manner as in Example 1, except that the feed rate was changed to 1 / h.

[0078] (Examples A4 to A12: Production of Carbon Blacks (A4) to (A12)) Carbon black was produced in the same manner as in Example 1, except that oxygen (25° C.) was supplied as a gas other than the raw materials at the supply amount and jetting speed shown in Table 1.

[0079] (Comparative Examples X1 to X3, X5: Production of Carbon Blacks (X1) to (X3), (X5)) Carbon black was produced in the same manner as in Example 1, except that oxygen (25° C.) was supplied as a gas other than the raw materials at the supply amount and jetting speed shown in Table 1.

[0080] (Comparative Example X4: Production of Carbon Black (X4)) Carbon black was produced in the same manner as in Example 10, except that oxygen (25°C) was supplied as a gas other than the raw material in parallel with the raw material gas.

[0081] [Table 1]

[0082] The carbon black obtained in the examples and comparative examples was subjected to the following measurements. The average primary particle diameter of the carbon black obtained in each example was 1.0 nm or more and less than 25 nm. (1) Specific surface area Measurement was performed according to JIS K6217-2:2017, Method A Flow Method (thermal conductivity measurement method). (2) DBP absorption DBP absorption: The value measured according to the method described in JIS K6221, Method B, was converted into a value corresponding to JIS K6217-4:2008 using the above formula (a). (3) Iron content The sample was pretreated by acid decomposition according to JIS K0116:2014, and then measured by high-frequency inductively coupled plasma mass spectrometry.

[0083] Next, the carbon black obtained in each of the examples and comparative examples was dispersed in chloroform for 10 minutes with an ultrasonic output of 90 W to break up the secondary agglomerates into primary agglomerates, which were then scooped onto a collodion membrane mesh and photographed with a transmission electron microscope at a magnification of 2000x.

[0084] The obtained two-dimensional projection images were filtered (median filter, option 7x7, number of times: 3) using the image analysis software "Image-Pro Plus 6.2J (Media Cybernetics)" and then the brightness range was manually extracted to match the primary aggregates. "Size (width)," "Size (length)," "Perimeter," and "Area" were selected from the measurement items, and the Feret diameter W (μm) in the minor axis direction, Feret diameter L (μm) in the major axis direction, perimeter P (μm), and projected area A (μm) of 100 or more randomly selected primary aggregates were measured. 2 ) was measured. Primary aggregates that overlapped the edges of the image, the scale bar, and background noise were excluded. Based on formulas (X), (Y), and (Z), the primary aggregates were divided into first, second, third, and fourth primary aggregates, and the proportion of each was calculated. The results are shown in Table 2.

[0085] FIG. 2 is a two-dimensional projection image of primary aggregates of the carbon black (A1) of Example A1, taken by a transmission electron microscope.

[0086] [Table 2]

[0087] (Example B1: Preparation of Slurry (B1)) 3 parts by mass of carbon black (A1) and 97 parts by mass of N-methyl-2-pyrrolidone (Kanto Chemical Co., Ltd.) as a dispersion medium were kneaded for 40 minutes at 1800 rpm in a planetary centrifugal mixer (Thinky Corporation's "Awatori Rentaro ARV-310") to prepare a carbon black slurry. The viscosity of this slurry at 25°C was evaluated using a viscoelasticity measuring device (Anton Paar's "MCR102," using a 30 mm diameter, 3° angle cone plate, and a 1 mm gap). The shear rate was 0.01 s -1 From 100s -1 The shear rate was changed to 10 s -1 The viscosity was measured at 100°C, and the results are shown in Table 3.

[0088] (Examples B2 to B12: Preparation of Slurries (B2) to (B12)) Slurries were prepared in the same manner as in Example B1, except that carbon black (A2) to (A12) were used instead of carbon black (A1), and the viscosities thereof were determined. The results are shown in Table 3.

[0089] (Comparative Examples Y1 to Y5: Preparation of Slurries (Y1) to (Y5)) Slurries were prepared in the same manner as in Example B1, except that carbon black (X1) to (X5) were used instead of carbon black (A1), and the viscosities thereof were determined. The results are shown in Table 3.

[0090] [Table 3]

[0091] As shown in Table 3, the slurries of the Examples had lower viscosities than the slurries of the Comparative Examples, which used carbon blacks with similar specific surface areas. These results confirmed that the carbon black of the present invention has excellent dispersibility and can form low-viscosity slurries.

[0092] (Example C1: Preparation of Battery (C1)) Slurry (B1) 40 parts by mass (carbon black 1.2 parts by mass, N-methyl-2-pyrrolidone 38.8 parts by mass), LiNi as a positive electrode active material 0.5 Mn 0.3 Co 0.2 96.8 parts by weight of O2 (Umicore "TX10"), 2 parts by weight of polyvinylidene fluoride (Arkema "HSV900") as a binder, 0.1 parts by weight of polyvinyl alcohol (Denka "B05") as a dispersant, and 10 parts by weight of N-methyl-2-pyrrolidone (Kanto Chemical Co., Ltd.) as a dispersion medium were mixed in a centrifugal mixer (Thinky Corporation "Awatori Rentaro ARV-310") at a rotation speed of 2000 rpm for 10 minutes to prepare a positive electrode-forming composite slurry. The resulting positive electrode-forming composite slurry was applied to a 15 μm thick aluminum foil (UACJ Corporation) using an applicator and pre-dried at 105 °C for 1 hour. Next, the mixture was pressed in a roll press at a linear pressure of 200 kg / cm, and the sum of the thicknesses of the aluminum foil and the coating film was adjusted to 80 μm. To remove volatile components, the cathode was prepared by vacuum drying at 170°C for 3 hours.

[0093] 97 parts by weight of artificial graphite (Hitachi Chemical Co., Ltd. "MAG-D") as the negative electrode active material, 2 parts by weight of styrene-butadiene rubber (Zeon Corporation "BM-400B") as the binder, and 1 part by weight of carboxymethyl cellulose (Daicel Corporation "D2200") as the dispersant were weighed, and pure water was added and mixed using a centrifugal mixer (Thinky Corporation, Awatori Rentaro ARV-310) to prepare a negative electrode composite slurry. The resulting negative electrode composite slurry was applied to a 10 μm-thick copper foil (UACJ Corporation) using an applicator and pre-dried at 60 °C for 1 hour. Next, the mixture was pressed using a roll press at a linear pressure of 100 kg / cm, and the combined thickness of the copper foil and the coating film was adjusted to 40 μm. To completely remove moisture, the mixture was vacuum-dried at 120 °C for 3 hours to prepare a negative electrode.

[0094] The positive electrode and the negative electrode were processed to 40 × 40 mm and 44 × 44 mm, respectively, and a polyolefin microporous membrane was placed between the electrodes as a separator to fabricate a battery. The electrolyte used was a solution of ethylene carbonate (manufactured by Aldrich) and dimethyl carbonate (manufactured by Aldrich) mixed at a volume ratio of 1:1, with 1 mol / L of lithium hexafluorophosphate (manufactured by Stella Chemifa Corporation) dissolved therein.

[0095] For the battery discharge test, the fabricated battery was charged at 25°C to 4.35 V at a constant current and voltage with a limit of 0.2 C, and then discharged to 3.0 V at a constant current of 0.2 C. The discharge current was then varied to 0.2 C, 0.5 C, 1 C, 2 C, and 3 C, and the discharge capacity was measured for each discharge current. The capacity retention ratio at 3 C discharge relative to 0.2 C discharge was calculated and evaluated as discharge rate characteristics. The fabricated battery was also charged at 25°C to 4.35 V at a constant current and voltage with a limit of 1 C, and then discharged to 3.0 V at a constant current of 1 C. The above charge / discharge cycle was then repeated 500 times, and the discharge capacity was measured. The capacity retention ratio at 500 discharge cycles relative to 1 discharge cycle was calculated and evaluated as cycle characteristics. The results are shown in Table 4.

[0096] (Examples C2 to C12: Preparation of Batteries (C2) to (C12)) Batteries were fabricated and evaluated in the same manner as in Example C1, except that slurries (B2) to (B12) were used instead of slurry (B1). The results are shown in Table 4.

[0097] (Examples Z1 to Z5: Preparation of batteries (Z1) to (Z5)) Batteries were fabricated and evaluated in the same manner as in Example C1, except that slurries (Y1) to (Y5) were used instead of slurry (B1). The results are shown in Table 4.

[0098] [Table 4]

Claims

1. First primary aggregates having an X value calculated by the following formula (X) of more than 1.7; second primary aggregates in which the X value is 1.7 or less and the Y value calculated by the following formula (Y) is 1.2 or less; third primary aggregates in which the X value is 1.7 or less, the Y value is greater than 1.2, and the Z value calculated by the following formula (Z) is 2.0 or less; fourth primary aggregates having an X value of 1.7 or less, an Y value of more than 1.2, and an Z value of more than 2.0; When divided into a carbon black in which the ratio of the total number of the second primary agglomerates and the third primary agglomerates to the total number of the first primary agglomerates, the second primary agglomerates, the third primary agglomerates, and the fourth primary agglomerates is 23% or more, the ratio of the number of the first primary agglomerates is 25% or more, and the ratio of the number of the fourth primary agglomerates is 5% or more. X = L / W (X) Y=P ２ / 4πA (Y) Z=(L×W) / A (Z) [wherein, in a two-dimensional projection image of the primary aggregates taken by a transmission electron microscope, the Feret diameter in the minor axis direction of the primary aggregates is W (μm), the Feret diameter in the major axis direction of the primary aggregates is L (μm), the perimeter of the primary aggregates is P (μm), and the projected area of ​​the primary aggregates is A (μm ２ )

2. Specific surface area is 150m ２ / g or more 400m ２ 2. The carbon black according to claim 1, wherein the molecular weight of the carbon black is 1 / g or less.

3. 2. The carbon black according to claim 1, which has a DBP absorption of 165 mL / 100 g or more and 285 mL / 100 g or less.

4. 2. The carbon black according to claim 1, having an iron content of less than 2,000 ppb by mass.

5. A slurry comprising the carbon black according to any one of claims 1 to 4 and a dispersion medium.

6. A positive electrode, a negative electrode, and a separator are provided, A lithium ion secondary battery, wherein at least one of the positive electrode and the negative electrode contains the carbon black according to any one of claims 1 to 4.

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