JP2014107191A - Dispersion slurry using carbon nanotube and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an electrode for a coating composition in which a suitable condition in a dispersion step of lithium ion battery electrode slurry can be determined by a numerical value and obtained performance of a battery can be improved.SOLUTION: Disclosed is a method of manufacturing electrode slurry for a positive electrode plate for a lithium ion secondary battery in which a carbon nanotube is used for at least a conductive material, and their carbon material and solvent are mixed and dispersed. Regarding carbon material dispersion slurry, an AC impedance value immediately after dispersion is controlled by a predetermined numerical value.

Description

  The present invention relates to a carbon material-dispersed slurry, and more particularly to a non-aqueous carbon material slurry for an electrode slurry for a lithium ion secondary battery positive electrode plate and a lithium ion secondary battery using the same.

  With the popularization of mobile phones, notebook computers, etc., lithium ion secondary batteries are attracting attention and demand is increasing. In the present lithium ion secondary battery, positive and negative electrodes are formed by applying a paint mixed with an electrode active material, a binder, and a conductive material on a strip-shaped metal foil in order to increase the efficiency of the battery reaction by increasing the electrode area. After being used and wound together with the separator, they are housed in a battery can (Patent Document 1, etc.).

  Among these, the positive electrode uses a lithium transition metal composite oxide or the like as an electrode active material. Since such an electrode active material alone has poor electronic conductivity, that is, conductivity, conductive carbon black having a highly developed structure for imparting conductivity, or carbon such as graphite in which crystals have significant anisotropy. The material is added as a conductive material and dispersed in a non-aqueous solvent such as N-methyl-2-pyrrolidone together with a binder (binder) to prepare a slurry (Patent Document 2), and this slurry is applied onto a metal foil. Dry to form a positive electrode.

However, current lithium ion secondary batteries are required to further improve electrode performance such as discharge capacity.
Particularly in recent years, there has been an increasing demand for lithium-ion batteries having a large battery capacity, such as in-vehicle batteries, and reducing the conductive network resistance by increasing the amount of conductive aid in the electrode. . However, it has been pointed out that when the blending amount of the conductive additive is increased, the amount of the active material in the electrode is decreased and the battery capacity is decreased (Patent Document 4).
Therefore, in order to increase the amount of the electrode active material, it is necessary to exhibit the conductive performance with a low addition amount of the conductive auxiliary agent.

  There has been proposed a method for improving conductivity by selecting fibrous carbon or carbon nanotubes as a conductive aid. For example, a method of combining a plurality of carbon particle types such as carbon black, non-fibrous graphite particles, and fibrous carbon (Patent Document 14), a method using scaly graphite (Patent Document 15), a plurality of fibers having different specific diameters There is a method (Patent Document 5) in which the conductivity, cycle characteristics, rate characteristics and battery capacity of a battery are improved by combining fibrous carbon or fibrous carbon having a specific diameter and non-fibrous carbon.

However, these conventional methods do not always provide the required high electrical characteristics. Carbon black, which is a carbon material used as a conductive material, fibrous graphite, carbon nanotubes, and the like are fine powders having a small primary particle size, and are strongly agglomerated and very difficult to uniformly disperse. In addition, the electrode active material is also a powder, and if the carbon materials are not agglomerated when they are mixed, there is a part in the positive electrode plate that is locally inferior in conductivity, and electrons are not sufficiently transferred. For this reason, it has been pointed out that the electrode active material is not effectively used, resulting in a low discharge capacity (Patent Document 1, etc.).

Therefore, a method of coating the surface of the electrode active material with a carbon material (Patent Document 1), or carbon black as a carbon material is previously dispersed in a dispersion medium such as an organic solvent together with a dispersant to form a slurry. A method of preparing a uniform electrode slurry by kneading together with a substance and a binder to form an electrode (Patent Document 6, Patent Document 7, Patent Document 8, Patent Document 9, Patent Document 10,
Patent Document 11, Patent Document 12, and Patent Document 13) have been proposed. In addition, in order to prevent fine fibrous carbon from aggregating during drying, it has also been proposed to disperse in a solvent in advance and then mix with the active material (Patent Document 5).

  In addition, the agglomerates of the electrode active material powder and the carbon material powder cannot be loosened, and surface defects such as streaks and protrusions due to the agglomerates occur on the positive electrode plate surface. However, it has also been pointed out that clogging occurs in a short period of time, and that kneading for a very long time is necessary to eliminate agglomerates, resulting in a cost increase (Patent Document 3). Therefore, a method has been proposed in which a solvent and a binder are mixed and dissolved or dispersed in advance, and then an electrode active material and a conductive material are additionally kneaded (Patent Document 3).

JP 2003-308845 A Japanese Patent Laid-Open No. 2003-157846 Japanese Patent Laid-Open No. 11-144714 JP 2007-73344 JP JP 2010-238575 A JP 2011-70908 JP JP 2011-113821 Japanese Patent No. 4235788 JP 2010-238575 A JP 2011-192020 gazette JP 2007-335175 A Japanese Patent Laid-Open No. 2004-281096 JP 2009-252683 A JP 2000-58066 A JP 2006-127823 A

However, even with these methods, the level and uniformity of battery performance are not sufficient. Even if the above-described method of dispersing the carbon material and the electrode active material in advance is adopted, it is presumed that the uniformity of the dispersed state at the micro level is not sufficient as the battery material.
The reason for this is that the causal relationship between the physical properties of the slurry and the obtained battery performance has not been fully elucidated, and the physical properties of the slurry, which is an index of performance when formed into an electrode, are not known. For this reason, battery performance cannot be controlled by observation of particle state and measurement of rheological properties, which are general means for evaluating a slurry.

The present invention has been made in view of such problems of the prior art, and the object of the present invention is to provide a carbon material-dispersed slurry that can exhibit excellent battery performance, and suitable conditions for the carbon material-dispersing step. An object of the present invention is to provide a method for producing a carbon material-dispersed slurry for a lithium ion secondary battery that can be judged numerically and can improve the performance of the obtained battery.

  As a result of intensive studies to achieve the above object, the present inventor used carbon nanotubes for the carbon material slurry, and paid attention to the AC impedance method and measured the AC impedance of the carbon material slurry. The inventors have found that when the value is set within a predetermined range, the performance of the obtained lithium ion secondary battery is improved, and the present invention has been completed.

That is, the present invention
(1) A slurry containing at least carbon nanotubes and a dispersion medium, wherein the content of carbon nanotubes in the slurry is 1% by mass or more and 20% by mass or less, and the viscosity measured with a B-type viscometer is 100 mPa · s or more and 5000 mPa · s. s or less, a carbon nanotube-dispersed slurry characterized in that the maximum particle size measured with a grind gauge is 20 μm or less, and the concentration dependency of admittance at an applied frequency of 2000 Hz obtained by AC impedance measurement is 30.0 μS / mass% or less,
(2) The carbon nanotube material slurry according to (1) above, which contains carbon nanotubes having a diameter of 200 nm or less,
(3) The carbon nanotube material slurry according to (2), wherein the carbon nanotubes having a diameter of 200 nm or less are multi-walled carbon nanotubes,
(4) The carbon nanotube-containing slurry according to any one of (1) to (3) above, which contains N-methyl-2-pyrrolidone as a dispersion medium,
(5) The carbon nanotube-containing slurry according to any one of (1) to (4), which contains a dispersibility-imparting agent,
(6) The carbon nanotube-containing slurry according to (5), wherein the dispersibility-imparting agent is a nonionic polymer resin,
(7) The carbon nanotube-containing slurry according to (6), wherein the nonionic polymer resin is a cellulose polymer or a butyral polymer,
(8) The carbon nanotube-containing slurry according to (6) or (7), wherein the nonionic polymer resin has a weight average molecular weight of 1,000 to 1,000,000.

(9) The carbon nanotube-containing slurry according to (8), wherein the nonionic polymer resin has a weight average molecular weight of 5,000 to 300,000,
(10) A lithium ion secondary battery, wherein the carbon nanotube-containing slurry according to any one of (1) to (9) is mixed with at least an electrode active material and a binder, applied to an electrode substrate, and dried. A method for producing a positive electrode,
(11) A lithium ion secondary battery comprising a positive electrode of a lithium ion secondary battery obtained by the production method according to (10) above,
(12) A method for producing a slurry containing at least carbon nanotubes and a dispersion medium and having a carbon nanotube content of 10% by mass to 30% by mass, wherein any of the following (i) to (iii): A method for producing a carbon nanotube-dispersed slurry, characterized by
(i) Viscosity measured with B-type viscometer
(ii) Maximum particle diameter of carbon nanotube material in slurry
(iii) Concentration dependence of admittance and retardation obtained by AC impedance measurement (13) A method for producing a slurry containing at least carbon nanotubes and a dispersion medium as described in (12) above, comprising a B-type viscometer At least carbon nanotubes and a dispersion medium, characterized in that the dispersion step is performed until the viscosity measured at 100 mPa · s to 5000 mPa · s and the maximum particle size measured with a grind gauge is 20 μm or less Slurry production method,
(14) The method for producing a slurry containing at least carbon nanotubes and a dispersion medium described in (12) above, wherein the concentration dependency of admittance at an applied frequency of 2000 Hz obtained by alternating current impedance measurement is 30.0 μS / mass% A method for producing a slurry containing at least carbon nanotubes and a dispersion medium, wherein the dispersion step is performed until:
(15) The slurry obtained by the method for producing a slurry according to any one of (12) to (14) above is mixed with at least an electrode active material and a binder, applied to an electrode substrate, and dried. Manufacturing method of positive electrode of lithium ion secondary battery,
(16) A lithium ion secondary battery comprising a positive electrode of a lithium ion secondary battery obtained by the production method according to (15) above,
It is in.

  According to the present invention, carbon nanotubes are used as a carbon material that is a conductive material. These carbon materials can be dispersed in a dispersion medium in advance, and at that time, the admittance value can be set within a predetermined range. In addition to greatly improved management, the performance of the resulting battery can also be improved.

FIG. 1 is a graph showing the relationship between the change in carbon concentration in the slurry and the equivalent admittance. FIG. 2 is a diagram showing dimensions of an aluminum foil flag type electrode. FIG. 3 is a diagram showing an admittance measurement cell.

  Hereinafter, the present invention will be specifically described.

[Conductive material]
In the present invention, carbon nanotubes are used as the conductive material. Carbon nanotubes are highly suitable for use as a conductive material for lithium ion batteries because of their highly developed crystallites and structures and excellent conductivity. Furthermore, the carbon nanotubes should have a predetermined physical property according to the present invention described below. Thus, the conductive performance can be exhibited with a low addition amount, and the amount of active material in the electrode slurry can be increased. Among the carbon nanotubes, those having a diameter of 200 nm or less and further multi-walled carbon nanotubes are suitable for obtaining the above-described particularly excellent conductive performance.

(Dispersibility imparting agent)
The slurry of the present invention can contain a dispersibility-imparting agent. Here, the dispersibility-imparting agent is a substance having a function of easily dispersing the carbon nanotubes in the dispersion medium, and a conventionally known substance can be used as a so-called dispersant. For example, as described in Patent Document 8, a resin system, a cationic surfactant, and a nonionic surfactant having a thickening action and / or a surface active action may be mentioned.
Among these dispersibility-imparting agents, in the present invention, preferably, a nonionic polymer resin is suitable so as not to inhibit the movement of lithium ions in the lithium ion secondary battery. The nonionic polymer resin has a hydrophilic portion where the hydrophilic portion is not ionized, and a cellulose polymer or a butyral polymer is representative. On the other hand, when the weight average molecular weight of the nonionic polymer resin exceeds 1,000,000, the viscosity of the carbon material-dispersed slurry becomes too high and the handling property is deteriorated. On the other hand, when the weight average molecular weight is less than 1,000, the dispersibility is poor, and it becomes difficult to produce a carbon material-dispersed slurry. More preferred is a weight average molecular weight of 5,000 to 300,000.

[Carbon nanotube dispersion slurry]
The slurry of the present invention is obtained using carbon nanotubes. Here, the slurry means a state in which carbon nanotubes are dispersed in a liquid dispersion medium. N-methyl-2-pyrrolidone is preferred as the dispersion medium.
When the content of the dispersion medium is less than 70% by mass of the slurry, the fluidity is poor and the handling property is lowered. At least 70% by mass or more, preferably 80% by mass or more is preferable.

〔concentration〕
The carbon nanotube content in the slurry is 1% by mass to 20% by mass, preferably 3% by mass to 15% by mass. If the carbon nanotube content is less than 1% by mass, the amount of solvent in the slurry increases, and thus the drying process in the coating process takes time. In addition, when the carbon nanotube content exceeds 20% by mass, it is difficult to disperse the carbon nanotubes.

[Relationship between physical properties of slurry]
As described above, the carbon nanotube dispersed slurry of the present invention contains carbon nanotubes in a specific concentration range. Furthermore, although the physical properties of viscosity and admittance concentration dependency are within a specific range, these reflect the dispersion state of the carbon nanotubes in the slurry, and the present inventors have a correlation with each other. It was found by. And it turned out that the carbon nanotube dispersion | distribution slurry which has the combination of the following physical properties exhibits the outstanding performance, when battery-ized.
First, the first form is a carbon nanotube-dispersed slurry having a concentration and a viscosity within a specific range. This is a carbon nanotube-dispersed slurry in which the concentration and the maximum particle size of the carbon material are within a specific range. A third form is a carbon nanotube-dispersed slurry in which the concentration dependency of concentration and admittance is within a specific range. Hereinafter, each physical property will be described.

〔viscosity〕
The slurry of the present invention is characterized in that the viscosity measured with a B-type viscometer is from 100 mPa · s to 5000 mPa · s, preferably from 100 mPa · s to 3000 mPa · s. It was found that the battery performance is excellent by adjusting the dispersion state so that the viscosity is in this concentration range and in this range. In addition, when the viscosity is lower than this range, there is a problem that the application work becomes difficult because the viscosity of the electrode paste applied to the electrode plate becomes too low.

[Dispersed particle size]
The dispersed particle diameter of the carbon nanotubes in the slurry is preferably a maximum particle diameter of 20 μm or less. In general, the average particle diameter is often used for the management of the particle state of a dispersion such as a carbon material. However, when the average particle size is used, the state of coarse particles is not shown, and even when the average particle size is small, if there are coarse particles of 20 μm or more, the thickness between the separators of the lithium ion battery is 20 μm. Therefore, there is a possibility of short-circuiting inside the lithium ion secondary battery through the separator. Therefore, a carbon material slurry having a maximum particle size of 20 μm or less is preferable. The maximum particle size is specified with a grind gauge. In order to maintain the maximum particle size at a particle size of 20 μm or less, it is extremely preferable to use the nonionic polymer resin described above as a dispersibility-imparting agent.

[Admittance concentration dependence]
The carbon nanotube-dispersed slurry of the present invention has an admittance concentration dependency of 30.0 μS / mass% or less, preferably 5.0 μS / mass% or less. Regarding the performance of the carbon material dispersed slurry, it is considered that the carbon material dispersed slurry should have a small admittance change with respect to the change in the carbon material concentration in order to exhibit uniform conductivity in the positive electrode plate of the lithium ion secondary battery. It is done. According to the study by the present inventors, it has been found that uniform conductivity can be exhibited when the concentration dependency is 30.0 μS / mass% or less, particularly preferably 5.0 μS / mass% or less.
There is a correlation between the concentration dependency of admittance and the dispersion state of the carbon material, and it has been found that the dispersion state must be controlled in order to obtain the concentration dependency of the admittance in a preferable range as described above. That is, if the dispersion is not sufficient, the battery performance is not sufficient. This is presumed to be due to the presence of coarse particles. On the other hand, it was surprisingly found that the battery performance is inhibited by excessive dispersion. The reason for this is not completely clear, but the present inventors speculate that this is due to a decrease in the connection between carbon nanotubes, which are conductive materials.

Similarly to the present invention, Patent Document 5 and Patent Document 7 that describe a slurry using carbon material such as carbon nanotubes, nonionic polymer resin, and N-methyl-2-pyrrolidone as a dispersion medium include compounding formulation and Although the dispersion method is described, the physical properties are not sufficiently controlled only by following the conditions described herein, the battery performance cannot be predicted, and the battery performance cannot be understood unless it is assembled into a lithium ion battery. On the other hand, by measuring various physical properties in the state of the dispersion defined in the present invention, it is possible to predict the battery performance and control the dispersion state.
That is, it is considered that the electric characteristics when the battery is formed are excellent by setting the viscosity within the above-mentioned concentration range and the above-described range and the admittance concentration dependency within the above-mentioned range.

[Slurry preparation method]
The carbon nanotube-dispersed slurry of the present invention is not limited in its production method as long as the carbon nanotube content, the viscosity measured with a B-type viscometer, the maximum particle diameter of the carbon material, and the concentration dependency of admittance are within the above-mentioned ranges. The following method is preferred.
First, carbon nanotubes are dispersed in a dispersion medium. At this time, the aforementioned dispersibility-imparting agent is added. Although other components that do not inhibit the function may be added, at least before adding the electrode active material and the binder, it is dispersed in a state having the predetermined physical properties defined in the present invention by the following method. .

That is, when the carbon nanotube is dispersed in the dispersion medium, the viscosity value is controlled. More preferably, first, a nonionic polymer resin as a dispersibility-imparting agent is dissolved in N-methyl-2-pyrrolidone as a dispersion medium. Carbon nanotubes are mixed into the solution, and then the aggregated carbon nanotubes are dispersed while being crushed by a dispersing device such as a bead mill, and the dispersion is continued until the maximum particle diameter reaches a predetermined range. Thus, a carbon nanotube-containing slurry having an admittance concentration dependency at an applied frequency of 2000 Hz obtained by measuring a predetermined dispersed particle size, viscosity, and AC impedance at a predetermined concentration can be obtained. The time to reach these physical properties depends on the amount charged and the equipment, so to manage these physical properties, mix and disperse the materials in the above equipment, take out a certain amount and measure each of the above physical properties. However, it is sufficient to determine the time to enter the predetermined range and continue dispersion until the next time, but there is a correlation as described above between each physical property, so it is not necessary to measure all physical property values It is good.
The dispersing device is not particularly limited to a bead mill, and examples include a ball mill, a sand mill, a roll mill, a homogenizer, an attritor, a paint shaker, and a jet mill.
During the dispersion process, the viscosity measured by the B-type viscometer, the maximum particle size, and the concentration dependence of the admittance obtained by AC impedance measurement are measured, and these physical properties are directly used as an index for obtaining a desired dispersion state. Also good.

[Lithium ion secondary battery]
A lithium ion secondary battery can be obtained by using the carbon nanotube-dispersed slurry of the present invention described above and mixing with an electrode active material, a binder and the like to form an electrode slurry for application to an electrode substrate. As the method at that time, various conventionally known methods can be adopted. Typically, the carbon nanotube dispersion slurry of the present invention is mixed with an electrode active material and a binder to form a slurry, which is applied to an electrode substrate and dried to form an electrode. This is used as the positive electrode of a lithium ion secondary battery, and a porous insulating material (separator) is sandwiched between the negative electrode made of a carbon material such as graphite and stored in a cylindrical or flat shape depending on the shape of the container. And an electrolyte is injected.
The lithium ion secondary battery of the present invention thus obtained can improve the discharge capacity maintenance rate during repeated charging and discharging.

[Production of carbon nanotube dispersion slurry 1]
In 95% by mass of N-methyl-2-pyrrolidone, 1% by mass of a methylcellulose polymer as a dispersibility imparting agent was dissolved. In the obtained solution, 5% by mass of “Baytubes C150P” (manufactured by Bayer) was mixed as carbon nanotubes, and the aggregated carbon nanotubes were dispersed while being crushed using a bead mill. As a result of taking out a sample, measuring the viscosity, and measuring the maximum particle size, it was confirmed that the obtained viscosity was 280 mPa · s and exceeded 100 mPa · s, and the maximum particle size was 12.5 μm. The process was finished. The obtained carbon nanotube dispersion slurry is referred to as “slurry 1”.
Slurry 1 has a maximum particle size of 12.5 μm, a viscosity of 280 mPa · s, a maximum particle size of 20 μm or less, a viscosity of 100 mPa · s or more, and an admittance concentration dependency of 30.0 μS / mass% at an applied frequency of 2000 Hz. It is in the following range.

[Production of carbon nanotube dispersion slurry 2]
Except that the dispersion was continued until the viscosity reached 470 mPa · s, the same operation as in Example 1 was performed, and the resulting carbon nanotube dispersion slurry was designated as “Slurry 2”. The maximum particle size of the slurry 2 was 12.5 μm, and the viscosity was 470 mPa · s.

[Production of carbon nanotube dispersion slurry 3]
Except that the dispersion was continued until the viscosity reached 520 mPa · s, the same operation as in Example 1 was performed, and the resulting carbon nanotube dispersion slurry was designated “Slurry 3”. The maximum particle size of the slurry 3 was 12.5 μm, and the viscosity was 520 mPa · s.

Comparative Example 1

[Production of carbon nanotube dispersion slurry 4]
In 95% by mass of N-methyl-2-pyrrolidone, 1% by mass of a methylcellulose polymer as a dispersibility imparting agent was dissolved. In the obtained solution, 5% by mass of “Baytubes C150P” (manufactured by Bayer) was mixed as carbon nanotubes, and the aggregated carbon nanotubes were dispersed while being crushed using a bead mill. As a result of taking out a sample, measuring the viscosity, and measuring the maximum particle size, it was confirmed that the obtained viscosity was 1000 mPa · s and exceeded 100 mPa · s, and the maximum particle size was 17.5 μm. The process was finished. The obtained carbon nanotube dispersion slurry is referred to as “slurry 4”.
The maximum particle size of the slurry 4 was 17.5 μm, and the viscosity was 1000 mPa · s.

Various physical properties of the slurries 1 to 4 are shown in Table 1. The evaluation methods of these physical properties are as follows.
(Measurement of viscosity)
The viscosity was measured using a B-type viscometer according to JIS K7117-1.
[Measurement of maximum particle size]
The maximum particle size was measured using a grind gauge according to JIS K5600-2-5: 1999.

A method for evaluating the performance of the slurry will be described.

[Measurement of admittance]
Slurries 1 to 4 were diluted with N-methyl-2-pyrrolidone twice to prepare a carbon material dispersion slurry and 4 times dilution carbon material dispersion slurry.
Using these 2-fold diluted slurry and 4-fold diluted slurry, the phase difference and admittance at an applied frequency of 2000 Hz were measured for these diluted slurries by the AC impedance method.

[Description of phase difference and admittance measurement cell]
Two aluminum foil flag-type electrodes were produced by cutting out an aluminum foil having a purity of 99.99% and a thickness of 0.1 mm so that the electrode portion was 7 × 7 mm (FIG. 2). Stainless steel lead wire 1 (SUS304, φ1.5mm, manufactured by Nilaco Co., Ltd.) 100mm tip with crimp terminal 3 (round terminal (R type), 1.25-3.7, manufactured by JST Co., Ltd.) 2 The aluminum foil was fixed to the crimp terminal portion with a screw (iron nabebis M3 × 5 mm) and a nut 4 (for iron nut M3) to form a measurement electrode 5. Further, a hole was made in a Teflon (registered trademark) cap 2 (# 10, upper diameter 32 mm, lower diameter 28 mm, height 41 mm, manufactured by SK Corporation), and the measurement electrode 5 was passed through and fixed.
The slurry was weighed in a tall beaker 6 (manufactured by Sansho, IWAKI GLASS CODE 7740), and an Al | slurry | Al bipolar cell was assembled (FIG. 3).

[AC impedance method]
For phase difference and admittance measurement, potentiostat (2020, manufactured by Toho Giken Co., Ltd.), function generator (WF1945B, Inc.)
NF circuit block), lock-in amplifier (LI575, manufactured by NF circuit block), recorder (GL900, manufactured by Graphtec), oscilloscope (2247A, manufactured by Tektronix).

[Calculation method of admittance]
Using the above AC impedance method, the phase difference, voltage amplitude, current range, frequency, effective value, maximum sensitivity of the lock-in amplifier, and sensitivity are read from each measuring instrument, and the cell constant and admittance are calculated according to the calculation formula shown in Table 2 below. calculate.

[Measurement of cell constant]
N-methyl-2-pyrrolidone is measured by the impedance method, and the cell constant is calculated by the above calculation method to obtain the cell constant.

[Measurement of admittance]
Using the cell whose cell constant has been measured, the slurry is measured by the impedance method, and the admittance is calculated by the above calculation method to obtain the admittance of the slurry.

As a condition for the AC impedance method, a voltage of 2000 Hz and an amplitude of 0.1 V _PP was applied.
Table 3 shows the results of admittance [μS] obtained by AC impedance measurement. A graph of these is shown in FIG.
The horizontal axis represents the solid content [%] of the carbon nanotubes in the entire slurry, and the vertical axis represents the admittance [μS]. For example, in the case of Example 1, the admittance at the carbon nanotube content of 5.0% was 22.90 μS, whereas the admittance decreased as the concentration decreased to 12.30 μS at 2.50% and 6.27 μS at 1.25%. There was a tendency to gradually decrease.

From Table 3, it was found that the carbon nanotube-dispersed slurries of Examples 1, 2, and 3 have small admittance dependency on the carbon material concentration.
From the above, in the method for producing a carbon material slurry that can be used in a lithium ion secondary battery, the carbon material concentration dependency of admittance at an applied frequency of 2000 Hz in the obtained carbon material slurry is 30.0 μS / mass% or less, and By defining the dispersion step such that the maximum particle size is 20 μm or less, the performance of the obtained battery can be improved. For example, when applied to a lithium ion secondary battery, it is possible to improve the discharge capacity maintenance rate during repeated discharge.

  Provided are a lithium ion secondary battery with improved battery performance, a carbon material-dispersed slurry suitable for the production thereof, a production method thereof, and a quality control method.

1 Stainless steel lead wire 2 Teflon (registered trademark) cap 3 Crimp terminal 4 Screw and nut 5 Measuring electrode 6 Tall beaker

Claims (16)

  A slurry containing at least carbon nanotubes and a dispersion medium, wherein the carbon nanotube content in the slurry is 1% by mass or more and 20% by mass or less, and the viscosity measured by a B-type viscometer is 100 mPa · s or more and 5000mPa · s or less, A carbon nanotube-dispersed slurry characterized in that the maximum particle size measured with a grind gauge is 20 μm or less, and the concentration dependency of admittance at an applied frequency of 2000 Hz obtained by AC impedance measurement is 30.0 μS / mass% or less. The carbon nanotube material slurry according to claim 1, comprising carbon nanotubes having a diameter of 200 nm or less. The carbon nanotube material slurry according to claim 2, wherein the carbon nanotubes having a diameter of 200 nm or less are multi-walled carbon nanotubes. The carbon nanotube-containing slurry according to any one of claims 1 to 3, comprising N-methyl-2-pyrrolidone as a dispersion medium. The carbon nanotube containing slurry in any one of Claims 1-4 containing a dispersibility imparting agent. The carbon nanotube-containing slurry according to claim 5, wherein the dispersibility-imparting agent is a nonionic polymer resin. The carbon nanotube-containing slurry according to claim 6, wherein the nonionic polymer resin is a cellulose polymer or a butyral polymer. The carbon nanotube-containing slurry according to claim 6 or 7, wherein the nonionic polymer resin has a weight average molecular weight of 1,000 to 1,000,000. The carbon nanotube-containing slurry according to claim 8, wherein the nonionic polymer resin has a weight average molecular weight of 5,000 to 300,000. A method for producing a positive electrode of a lithium ion secondary battery, comprising mixing the carbon nanotube-containing slurry according to any one of claims 1 to 9 with at least an electrode active material and a binder, coating the electrode substrate, and drying. A lithium ion secondary battery comprising a positive electrode of a lithium ion secondary battery obtained by the production method according to claim 10. A method for producing a slurry containing at least carbon nanotubes and a dispersion medium and having a carbon nanotube content of 10% by mass or more and 30% by mass or less, and managing any of the following (i) to (iii) A process for producing a carbon nanotube-dispersed slurry characterized by
(i) Viscosity measured with B-type viscometer
(ii) Maximum particle diameter of carbon nanotube material in slurry
(iii) Concentration dependence of admittance and phase difference obtained by AC impedance measurement A method for producing a slurry containing at least carbon nanotubes and a dispersion medium according to claim 12, wherein the viscosity measured with a B-type viscometer is 100 mPa · s to 5000 mPa · s and measured with a grind gauge. A method for producing a slurry containing at least carbon nanotubes and a dispersion medium, wherein the dispersion step is carried out until the maximum particle size becomes 20 μm or less. 13. A method for producing a slurry containing at least carbon nanotubes and a dispersion medium according to claim 12, wherein the concentration dependency of admittance at an applied frequency of 2000 Hz obtained by alternating current impedance measurement is 30.0 μS / mass% or less. A method for producing a slurry containing at least carbon nanotubes and a dispersion medium, wherein the dispersion step is performed.   A slurry obtained by the method for producing a slurry according to claim 12 is mixed with at least an electrode active material and a binder, applied to an electrode substrate, and dried. A method for producing a positive electrode. A lithium ion secondary battery comprising a positive electrode of a lithium ion secondary battery obtained by the production method according to claim 15.

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