JP6219981B2 - Carbon black having excellent slurry viscosity characteristics, slurry composition for lithium ion secondary battery electrode, and lithium ion secondary battery - Google Patents

Description

The present invention relates to a carbon black having excellent slurry viscosity characteristics, a slurry composition for a lithium ion secondary battery electrode, and a lithium ion secondary battery.

Lithium ion secondary batteries are widely used as power sources for small electronic devices such as smartphones and tablet computers. Generally, a lithium ion secondary battery includes an electrolyte solution including an electrode, a separator, and an electrolyte. The electrode is manufactured by coating and drying a mixture slurry in which an active material, a conductive agent, a binder, and the like are dispersed in a solvent, on a metal plate for a current collector to form a mixture layer.

The role of the conductive agent is to impart conductivity to an active material having low conductivity, and to prevent the conductivity from being impaired due to repeated expansion and contraction of the active material during charging and discharging. Therefore, if the dispersion state of the active material and the conductive agent is poor in the electrode, a portion with poor conductivity appears locally, the active material is not effectively used, the discharge capacity is reduced, and the battery characteristics are reduced. Become. That is, the battery characteristics are not only the material characteristics such as the chemical composition of the active material and the conductivity of the conductive agent, but also the structural characteristics such as the compounding ratio and mixing / dispersing state of the material in the composite material layer, the density of the composite material layer, and the porosity. It is necessary to optimize them. In addition, in order to continue producing electrodes stably while maintaining the structure of the composite material layer in an optimum state, the unevenness of coating slurry application and the thickness of the composite material layer after drying are reduced to the utmost limit. There is a need.

The specific surface area of carbon black conventionally used for the conductive agent is about 40 to 70 m ² / g, which is very high compared to the specific surface area of the positive electrode active material (0.2 to 1.0 m ² / g). For example, when 92 parts by mass of the positive electrode active material having a specific surface area of 0.7 m ² / g and 8 parts by mass of the conductive agent having a specific surface area of 65 m ² / g are mixed, the ratio of the surface area occupied by the conductive agent reaches 89%. Thus, it can be seen that the characteristics of the conductive agent are dominant in the viscosity characteristics of the composite slurry.

In recent years, further increase in capacity of lithium ion secondary batteries has been demanded, and the blending ratio of the active material in the composite layer is increasing, and conversely, the blending ratio of the conductive agent and the binder tends to decrease. When the blending ratio of the conductive agent decreases, it becomes difficult to form a conductive path in the electrode, and the battery characteristics deteriorate. Thus, studies have been made to maintain the conductive path by using a conductive agent having a small particle diameter. However, the specific surface area increases as the particle size is reduced, and the viscosity of the mixture slurry increases remarkably, making uniform dispersion difficult. Moreover, since the liquid absorbency also becomes high, there is a problem that the binder in the composite layer is trapped and the adhesiveness (electrode adhesiveness) between the composite layer and the current collector is lowered.

Therefore, for the purpose of uniform dispersion of the mixture slurry, Patent Document 1 discusses an attempt to disperse the conductive agent using a high-pressure jet mill. However, according to this method, processing by a special apparatus is necessary, and since the structure is miniaturized to submicron order, there is a problem that the fine structure of the conductive agent is changed and the conductivity is lowered. In Patent Document 2, studies have been made to control the aggregation state of a solid material composed of an active material and a binder. However, the aggregation state of the conductive agent is not controlled, and the target is used when a conductive agent having a small particle size is used. It was difficult to achieve the composite material slurry characteristics. Further, in Patent Document 3, it is considered to prepare a conductive agent slurry having a wide particle size distribution in advance, but since the fine structure of the conductive agent is processed under the condition that the fine structure is not crushed, The particle size distribution of the conductive agent having a diameter could not be controlled.

JP 2004-289696 A Japanese Patent No. 5500395 Japanese Patent No. 5561559

An object of the present invention is to provide a carbon black, a slurry composition for a lithium ion secondary battery electrode, and a lithium ion secondary battery excellent in slurry viscosity characteristics.

The present invention employs the following means in order to solve the above problems.
(1) Presence of a structure having a specific surface area of 100 m ² / g or more , a DBP absorption amount of 150 ml / 100 g or more and less than 250 ml / 100 g, a primary average particle size of less than 30 nm , and an aspect ratio of a circumscribed rectangle of less than 1.50 Carbon black whose number ratio is 70% or more.
( 2 ) A slurry composition for a lithium ion secondary battery electrode, comprising an active material, a conductive agent, a binder, and a solvent, wherein the conductive agent is carbon black according to (1).
( 3 ) The solid content is 80 to 99.8 mass% active material, 0.1 to 10 mass% conductive agent, 0.1 to 10 mass% binder, and the solid content concentration is 50 to 85 mass%, The slurry composition as described in said ( 2 ) whose viscosity in shear rate 1s- ¹ is 10 Pa.s or less.
( 4 ) A lithium ion secondary battery using the slurry composition according to ( 2 ) or ( 3 ).

Since the carbon black of the present invention is excellent in conductivity, slurry viscosity characteristics, and electrode adhesion, a high-performance lithium ion secondary battery can be provided with high productivity.

An example of a structure whose aspect ratio of circumscribed rectangle is less than 1.50 An example of a structure in which the aspect ratio of the circumscribed rectangle is 1.50 or more

1 Carbon black 2 circumscribed rectangle 3 circumscribed rectangle long side length 4 circumscribed rectangular short side length

The specific surface area of the carbon black of the present invention is 100 m ² / g or more, which is characterized by being higher than the specific surface area of carbon black conventionally used as a conductive agent for lithium ion secondary batteries. The specific surface area can be measured according to JIS K6217-2: 2001, and can be increased by reducing the particle size of the particles, making them hollow, making the particle surface porous, or the like. As described above, carbon black having a high specific surface area is effective as a conductive agent because the conductivity imparting ability is enhanced by the percolation effect in the matrix. When the specific surface area is less than 100 m ² / g, the number of contact points with the active material is reduced in the composite layer, and sufficient conductivity cannot be exhibited. On the other hand, the specific surface area is preferably 250 m ² / g or less. When the specific surface area exceeds 250 m ² / g, the dispersibility in the mixture slurry is lowered, so that a portion having poor conductivity is locally generated in the electrode, and the battery characteristics are lowered.

The carbon black of the present invention is characterized in that the ratio of the number of structures having an aspect ratio of a circumscribed rectangle of less than 1.50 is 70% or more. Here, the structure of carbon black is a structure in which primary particles are connected, and it is known that conventional carbon black develops in a complicatedly entangled shape as the particle size is reduced. The circumscribed rectangle is defined as a rectangle that completely surrounds the carbon black structure and has the smallest area. The aspect ratio of the circumscribed rectangle of the structure was obtained by photographing carbon black dispersed with chloroform and scooped with a collodion membrane mesh with a transmission electron microscope (observation magnification 2000 times), and using the image analysis software (for example, Media Cybernetic)
It is calculated | required by remove | dividing the long side length of the circumscribed rectangle measured using "Image-Pro Plus 6.2J" by s company by the short side length. Although the structure has a three-dimensional structure, the length in the longitudinal direction, which is a feature of the structure, can be grasped by observing as described above. In addition, by measuring the aspect ratio of the circumscribed rectangle for 100 or more randomly selected carbon black structures, it is possible to calculate the ratio of the number of structuring in which the aspect ratio of the circumscribed rectangle is less than 1.50. .
As a result of intensive studies to improve the slurry viscosity characteristics of carbon black, the present inventor has found that the structure shape of carbon black greatly affects this characteristic. That is, a structure having an aspect ratio of a circumscribed rectangle of less than 1.50 has a small aspect ratio and a nearly spherical shape. Carbon black in which the number of structures having such a shape is 70% or more can reduce the flow resistance between adjacent carbon black and other materials during kneading of the composite slurry. Further, since the primary particles are coalesced in a dense state and there are few voids between the particles, the bubbles in the voids can be easily replaced with the solvent, and the wettability is improved. Due to the above effects, even in the case of carbon black having a small particle size and a high specific surface area, uneven coating due to an increase in viscosity of the mixture slurry and variations in the thickness of the mixture layer after drying can be reduced to the utmost. Furthermore, since the dispersed state and the contact state of the active material and the conductive agent in the electrode are improved, a local decrease in conductivity and a decrease in the discharge capacity of the battery can be suppressed. When the proportion of the number of structures having an aspect ratio of the circumscribed rectangle of less than 1.50 is less than 70%, the effect of suppressing the increase in the viscosity of the composite slurry is reduced. When the aspect ratio of the circumscribed rectangle is 1.50 or more, the branches have a structure shape extending in multiple directions, so that they are entangled with each other at the time of kneading the mixture slurry, and a sudden increase in viscosity occurs.

The DBP absorption of the carbon black of the present invention is less than 250 ml / 100 g. Here, the DBP absorption amount is an index for evaluating the ability to absorb dibutyl phthalate in the voids formed by the carbon black particle surface and structure or aggregated particles, and can be measured according to JIS K6217-4: 2008. In carbon black having a developed structure, the neck portion formed by fusing primary particles and the voids formed by agglomeration of particles increase, so the DBP absorption amount increases. When the DBP absorption amount is 250 ml / 100 g or more, the binder in the composite layer is trapped in the carbon black structure, and the adhesion to other materials such as an active material and a current collector is lowered. On the other hand, the DBP absorption is preferably 150 ml / 100 g or more. When the DBP absorption is less than 150 ml / 100 g, the structure is not sufficiently developed, so that a conductive path is not formed, and the conductivity imparting ability is lowered. Moreover, when used for a secondary battery, the volume change of the active material accompanying charge / discharge cannot be buffered, and battery characteristics such as cycle characteristics are deteriorated.

The average primary particle size of the carbon black of the present invention is preferably less than 50 nm, and more preferably less than 30 nm. Conventionally, carbon black used as a conductive agent for lithium ion secondary batteries has a ratio of the number of structures having an aspect ratio of a circumscribed rectangle of less than 1.50 to less than 70%, so the average primary particle diameter is less than 30 nm. Then, slurrying was difficult. On the other hand, in the carbon black of the present invention, the ratio of the number of structures having an aspect ratio of the circumscribed rectangle of less than 1.50 is 70% or more, and the dispersibility is excellent, so the average primary particle diameter is less than 30 nm. Can also be slurried. Thus, by using carbon black having a small particle diameter, high conductivity can be exhibited even if the compounding ratio in the composite layer is low. The average primary particle diameter of carbon black can be determined by measuring the primary particle diameter of 100 or more carbon blacks randomly selected from 50,000 times images of a transmission electron microscope and calculating the average value. The primary particles of carbon black have a small aspect ratio and a shape close to a true sphere, but are not perfect spheres. Therefore, the largest of the line segments connecting the two outer peripheral points of the primary particles in the TEM image is the primary particle diameter of carbon black.

The method for producing carbon black according to the present invention is not particularly limited. For example, a raw material gas such as a hydrocarbon is supplied from a nozzle installed at the top of the reactor and is subjected to a pyrolysis reaction and / or a partial combustion reaction. Carbon black can be produced and collected from a bag filter directly connected to the bottom of the reactor. The raw material gas to be used is not particularly limited, and gaseous hydrocarbons such as acetylene, methane, ethane, propane, ethylene, propylene, and butadiene, and oils such as toluene, benzene, xylene, gasoline, kerosene, light oil, and heavy oil A hydrocarbon can be used. A plurality of these can be mixed and used. Among them, it is preferable to use acetylene gas with few impurities. Since the temperature in the reactor is increased by the heat of decomposition of the acetylene gas, uniform nucleation of carbon black occurs, and carbon black having a primary particle size of less than 30 nm can be obtained.

Apart from the source gas, oxygen gas, hydrogen gas, nitrogen gas and the like can be used. Among these, it is preferable to use oxygen gas. When oxygen gas is used, carbon black is made porous by the activation action, and the specific surface area can be increased.

The structure shape of carbon black can be adjusted by the shape of the reaction furnace, the supply amount of raw material gas, and the like. For example, when the supply amount of the source gas is increased, the frequency of primary particles of generated carbon black colliding with each other in the reaction furnace increases, so that the structure develops and the DBP absorption amount increases. In order to obtain carbon black having the structure shape of the present invention, it is preferable that the jetting speed of the raw material gas into the reaction furnace is 6 m / s or more. Here, carbon black structure formation is a reaction during which the raw material gas supplied into the reactor is instantly pyrolyzed and / or partially burned to cause nucleation and grain growth of the carbon black and move to a collection facility. This is explained by the fact that the carbon black particles are fused together in a low temperature region in the furnace and connected in a chain form. When the injection speed of the raw material gas into the reaction furnace is 6 m / s or more, the gas stirring in the reaction furnace is promoted, and a large amount of carbon black particles collide in the structure formation region in a short time. It is inferred that it is difficult to form a complex structure. Moreover, since the residence time in the reactor is shortened, an increase in the amount of DBP absorption due to excessive structure growth can be prevented.

The slurry composition for a lithium ion secondary battery electrode of the present invention includes an active material, a conductive agent, a binder, and a solvent. Depending on the purpose, additives such as a dispersant and a flame retardant can also be contained. The solid content of the slurry composition is preferably 80 to 99.8% by mass of the active material, 0.1 to 10% by mass of the conductive agent, and 0.1 to 10% by mass of the binder. By using the carbon black of the present invention as a conductive agent and blending the above solid content, the mixing ratio of the active material in the composite layer can be increased without impairing the viscosity characteristics and conductivity of the composite slurry, and lithium The capacity of the ion secondary battery can be increased. Further, by using the carbon black of the present invention, the conductivity can be improved even with the same solid content blending, and high output of the lithium ion secondary battery can be achieved. In addition, other carbon black, graphite, a carbon nanotube, carbon nanofiber, etc. can be added to a electrically conductive agent in the range which does not inhibit the electroconductivity provision ability and dispersibility of the carbon black of this invention. The solid content concentration of the slurry composition for a lithium ion secondary battery electrode of the present invention is preferably 50 to 85% by mass. When solid content concentration exceeds 85 mass%, since strong shearing will be added at the time of kneading | mixing of mixture slurry, the primary aggregation of a electrically conductive agent will be destroyed and electroconductivity will fall. In addition, cracks are likely to occur in the drying process. On the other hand, if the solid content concentration is less than 50% by mass, the drying process after coating takes time, and the productivity is lowered.

The slurry composition for lithium ion secondary battery electrodes of the present invention preferably has a viscosity at a shear rate of 1 s ⁻¹ of 10 Pa · s or less. There are a doctor blade method, a roll coater method, a die coater method and the like as a method for applying a mixture slurry to a current collector, but the coating speed is generally about 10 to 50 m / min. If the viscosity at a shear rate of 1 s ⁻¹ corresponding to this region is 10 Pa · s or less, uniform coating is possible. The viscosity at a shear rate of 1 s ⁻¹ is preferably 1 Pa · s or higher. If it is less than 1 Pa · s, the active material tends to settle after coating, and the structure of the composite layer may be biased. The viscosity of the composite slurry can be measured by a rheometer. The viscosity in the present invention is the viscosity at a measurement temperature of 25 ° C. The viscosity of the composite slurry can be adjusted by changing the blending ratio of solids such as active material, conductive agent, binder, solid content concentration, dispersion conditions, addition of dispersant, and structure of carbon black, which is a conductive agent. .

The active material used in the slurry composition for a lithium ion secondary battery electrode of the present invention is not particularly limited, but if it is a positive electrode active material, lithium cobaltate, nickel / manganese / A lithium-containing transition metal oxide containing cobalt or manganese such as lithium cobaltate is preferable. Moreover, it is preferable that the average particle diameter of an active material is 1-20 micrometers. If it is less than 1 μm, the mixture slurry will thicken, and if it exceeds 20 μm, it will be difficult to produce a smooth electrode. As the negative electrode active material, various carbonaceous materials such as natural graphite, artificial graphite, graphite, activated carbon, coke, needle coke, fluid coke, mesophase microbead, carbon fiber, pyrolytic carbon, and the like can be used.

The binder used in the lithium ion secondary battery electrode slurry composition of the present invention is not particularly limited, for example, polyethylene, nitrile rubber, polybutadiene, butyl rubber, polystyrene, styrene-butadiene rubber, polysulfide rubber, nitrocellulose, Carboxymethylcellulose, polyvinyl alcohol, tetrafluoroethylene resin, polyvinylidene fluoride, polychlorochloroprene, and the like can be used. Moreover, what melt | dissolved these in the solvent previously can also be used.

The solvent used for the slurry composition for lithium ion secondary battery electrodes of the present invention is not particularly limited, and for example, N-methylpyrrolidone, ethanol, ethyl acetate, or the like can be used.

The kneading method at the time of preparing the slurry composition for the lithium ion secondary battery electrode of the present invention is not particularly limited, and for example, a general apparatus such as a mixer, a kneader, a disperser, a mill, a rotation and revolution type rotating apparatus is used. can do.

The lithium ion secondary battery of the present invention can be produced, for example, by applying the mixture slurry of the present invention to a current collector made of a metal foil, and then drying and depositing the mixture slurry. A secondary battery can be manufactured by immersing the electrolytic solution in an electrode group formed by laminating or winding a positive electrode and a negative electrode through a separator. Since the composite slurry of the present invention has excellent coating properties while containing a conductive agent having a small particle size and a high specific surface area, a high-capacity lithium ion secondary battery can be provided with high productivity.

The current collector is not particularly limited, and a metal foil of gold, silver, copper, platinum, aluminum, iron, nickel, chromium, manganese, lead, tungsten, titanium, or an alloy containing these as a main component is used. Is done. It is preferable to use aluminum for the positive electrode and copper for the negative electrode.

The electrolytic solution is not particularly limited, and a nonaqueous electrolytic solution containing lithium salt or an ion conductive polymer can be used. Examples of the nonaqueous solvent for the nonaqueous electrolyte in the nonaqueous electrolytic solution containing a lithium salt include ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. Examples of the lithium salt that can be dissolved in the non-aqueous solvent include lithium hexafluorophosphate, lithium borotetrafluoride, and lithium trifluoromethanesulfonate.

The separator is not particularly limited, and synthetic resins such as polyethylene and polypropylene can be used. It is preferable to use a porous film because the electrolyte retainability is good.

Hereinafter, the present invention will be described in detail by way of examples. However, the scope of the present invention is not limited to the following examples.

Example 1
From the nozzle installed at the top of the carbon black reactor (furnace length 5m, furnace diameter 0.5m), 15m ³ / h of acetylene gas as raw material gas and 9m ³ / h of oxygen gas were jetted, and heat of acetylene gas Carbon black was produced using decomposition and combustion reactions. In addition, the jet speed of the raw material gas was set to 6.5 m / s by adjusting the nozzle diameter. The obtained carbon black was measured for the following physical properties. The evaluation results are shown in Table 1.
(1) Specific surface area: Measured according to JIS K6217-2: 2001.
(2) The ratio of the number of structures in which the aspect ratio of the circumscribed rectangle is less than 1.50: From a 2000 times magnification image of a transmission electron microscope, image analysis software (“Image-Pro Plus 6.2J” manufactured by Media Cybernetics) The aspect ratio of the circumscribed rectangle was measured for 100 or more randomly selected carbon black structures, and the ratio of the number of structures having an aspect ratio of the circumscribed rectangle of less than 1.50 was calculated.
(3) DBP absorption: Measured according to JIS K6217-4: 2008.
(4) Average primary particle size: 100 or more randomly selected primary particle sizes of carbon black were measured from a 50,000-magnification image of a transmission electron microscope, and an average value was calculated.

3 parts by mass of the obtained carbon black and LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (“CELLSEED NMC Ni: Mn: Co = 1: 1: 1” manufactured by Nippon Chemical Industry Co., Ltd.) 94 3 parts by mass of polyvinylidene fluoride (“Kureha KF Polymer W # 1100” manufactured by Kureha Co., Ltd.) as a binder and 60 parts by mass of N-methylpyrrolidone (manufactured by Aldrich) as a solvent “NBK-1”) Kneaded for 15 minutes under the condition of a rotational speed of 1000 rpm to produce a mixture slurry having a solid content concentration of 62.5 mass%. The viscosity of this mixture slurry at 25 ° C. was evaluated with a viscoelasticity measuring device (“MCR102” manufactured by Anton Paar, φ30 mm, using a cone plate with an angle of 3 °, a gap of 1 mm). The shear rate was measured while changing from 0.01 to 100 s ^−1, and the viscosity at the shear rate of 1 s ⁻¹ was determined. The measurement results are shown in Table 1.

The obtained mixture slurry was coated on a 20 μm-thick aluminum foil (current collector) using a coating machine (“MINI-CATOR MODEL: MC10” manufactured by Hosen Co., Ltd.), and the dried one was pressed. The positive electrode was produced by cutting. This positive electrode was cut to a width of 25 mm and a length of 100 mm, and an adhesive tape was attached to the surface of the composite material layer, and then one end of the aluminum foil was pulled using a tensile tester (“Tensilon Universal Tester” manufactured by A & D). The peel strength when peeled at a peeling direction of 180 ° and a peeling speed of 100 mm / min was measured.
Moreover, the electrode plate resistance of the positive electrode was evaluated by an AC impedance method. Specifically, the positive electrode is set in a SUS tripolar cell, and an impedance analyzer (“electrochemical measurement system 12608W type” manufactured by Toyo Technica Co., Ltd., electrode area 1.54 cm ² , sweep frequency 1 MHz → 1 Hz, applied voltage 10 mV) AC resistance was measured at a measurement temperature of 20 degrees). The evaluation results are shown in Table 1.

Metallic lithium (manufactured by Honjo Metal Co., Ltd.) was used as the counter electrode with respect to the positive electrode, and a polyolefin cell non-woven fabric was used as a separator to electrically isolate them to produce a coin cell (CR-2032 type). In the electrolyte solution, 1 mol / L of lithium hexafluorophosphate (manufactured by Stella Chemifa) is dissolved in a solution in which ethylene carbonate (manufactured by Aldrich) / dimethyl carbonate (manufactured by Aldrich) is mixed at a volume ratio of 1/1. Was used. As a discharge test of the battery, after confirming that the charge / discharge efficiency is close to 100% after the initial charge of the battery, discharge when performing constant current discharge to 3.0 V at a current density of 0.7 mA / cm ² The capacity was measured. The measurement results are shown in Table 1.

Examples 2-7 , Reference Example 8, Comparative Examples 1-2
Carbon black was obtained in the same manner as in Example 1 except that the supply amount of the raw material gas, the supply amount of the oxygen gas, and the injection speed of the raw material gas were changed as shown in Table 1. The measurement results are shown in Table 1.

Example 9
A mixture slurry was obtained in the same manner as in Example 1 except that the mixing ratio in the mixture layer was changed to 96 parts by mass of the active material, 2 parts by mass of carbon black, and 2 parts by mass of the binder. The measurement results are shown in Table 1.

From the measurement results shown in Table 1, the carbon black of the present invention is excellent in conductivity, slurry viscosity characteristics, and electrode adhesion, and can provide a high-performance lithium ion secondary battery with high productivity.

The carbon black of the present invention can be suitably used for a slurry composition for a lithium ion secondary battery electrode and a lithium ion secondary battery.

Claims (4)

The ratio of the number of existing structures having a specific surface area of 100 m ² / g or more , a DBP absorption of 150 ml / 100 g or more and less than 250 ml / 100 g, an average primary particle diameter of less than 30 nm , and a circumscribed rectangle having an aspect ratio of less than 1.50. Carbon black that is 70% or more. A slurry composition for a lithium ion secondary battery electrode, comprising an active material, a conductive agent, a binder, and a solvent, wherein the conductive agent is carbon black according to claim 1. The solid content is 80 to 99.8% by mass of the active material, the conductive agent is 0.1 to 10% by mass, the binder is 0.1 to 10% by mass, the solid content is 50 to 85% by mass, and at 25 ° C. The slurry composition according to claim 2 , wherein the viscosity at a shear rate of 1 s ⁻¹ is 10 Pa · s or less. The lithium ion secondary battery using the slurry composition of Claim 2 or Claim 3 .