WO2013191239A1

WO2013191239A1 - Slurry for lithium ion secondary battery negative electrodes, electrode for lithium ion secondary batteries, method for producing electrode for lithium ion secondary batteries, and lithium ion secondary battery

Info

Publication number: WO2013191239A1
Application number: PCT/JP2013/066937
Authority: WO
Inventors: 高橋　直樹; 智一佐々木
Original assignee: 日本ゼオン株式会社
Priority date: 2012-06-20
Filing date: 2013-06-20
Publication date: 2013-12-27
Also published as: KR20150032943A; CN104471762B; JPWO2013191239A1; JP6237622B2; CN104471762A; KR102129829B1

Abstract

A slurry for lithium ion secondary battery negative electrodes, which contains a binder, a negative electrode active material, and a water-soluble polymer. The binder is a particulate polymer that contains 50-80% by weight of an aromatic vinyl monomer unit and 0.5-10% by weight of an ethylenically unsaturated carboxylic acid monomer unit. The particulate polymer has a surface acid amount of 0.20 meq/g or more, and the contact angle of the particulate polymer with a mixed solvent of ethylene carbonate and diethyl carbonate (ethylene carbonate/diethyl carbonate volume ratio = 1/2) is 50° or less.

Description

Slurry for lithium ion secondary battery negative electrode, electrode for lithium ion secondary battery and method for producing the same, and lithium ion secondary battery

The present invention relates to a negative electrode slurry for a lithium ion secondary battery, an electrode for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery.

In recent years, portable terminals such as notebook personal computers, mobile phones, and PDAs (Personal Digital Assistants) have become widespread. Lithium ion secondary batteries are frequently used as secondary batteries used as power sources for these portable terminals. Mobile terminals are required to have more comfortable portability, and are rapidly becoming smaller, thinner, lighter, and higher performance. As a result, mobile terminals are used in various places. In addition, secondary batteries are also required to be smaller, thinner, lighter, and have higher performance as with mobile terminals.

In order to improve the performance of secondary batteries, improvements to electrodes, electrolytes and other battery members are being studied. Of these, the electrode is usually prepared by mixing an electrode active material and, if necessary, a conductive material such as conductive carbon, in a liquid composition in which a polymer serving as a binder (binder) is dispersed or dissolved in a solvent. Thus, a slurry composition is obtained, and this slurry composition is applied to a current collector and dried.

Conventionally, an organic solvent has often been used as the solvent. However, the use of an organic solvent has a problem that it requires a cost for recycling the organic solvent or requires safety by using the organic solvent. Therefore, in recent years, it has been studied to produce an electrode using water as a solvent (see Patent Documents 1 and 2).

JP 2010-140841 A JP 2010-140684 A

However, the conventional negative electrode manufactured using water as a solvent has a problem in the adhesion of the negative electrode active material layer to the current collector. If the adhesiveness is low, the negative electrode active material layer cannot be held on the current collector, which may be a factor of reducing battery performance. In particular, the cycle characteristics in a high temperature environment may be deteriorated. For this reason, the technique which improves the adhesiveness of the negative electrode active material layer with respect to a collector is desired.

The present invention was devised in view of the above problems, and is a lithium ion secondary battery excellent in adhesion of a negative electrode active material layer to a current collector and excellent in cycle characteristics in a high temperature environment; the lithium ion secondary battery The manufacturing method of the negative electrode for lithium ion secondary batteries which can implement | achieve; and the slurry for lithium ion secondary battery negative electrodes which can manufacture the negative electrode for lithium ion secondary batteries is provided.

As a result of intensive studies to solve the above problems, the inventor of the present invention is a slurry for a secondary battery negative electrode containing a binder, a negative electrode active material, and a water-soluble polymer. Using a particulate polymer containing unsaturated carboxylic acid monomer units in a specific ratio as a binder, and controlling the surface acid amount of the binder and the contact angle with a predetermined mixed solvent within a predetermined range. Thus, it was found that the adhesion of the negative electrode active material layer to the current collector can be improved and a lithium ion secondary battery excellent in cycle characteristics in a high temperature environment can be realized, and the present invention has been completed.
That is, the present invention is as follows.

[1] including a binder, a negative electrode active material, and a water-soluble polymer,
The binder is a particulate polymer containing 50 wt% to 80 wt% of aromatic vinyl monomer units and 0.5 wt% to 10 wt% of ethylenically unsaturated carboxylic acid monomer units;
The surface acid amount of the particulate polymer is 0.20 meq / g or more,
A slurry for a negative electrode of a lithium ion secondary battery, wherein a contact angle of the particulate polymer with a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio: ethylene carbonate / diethyl carbonate = 1/2) is 50 ° or less.
[2] The lithium ion secondary battery negative electrode slurry according to [1], wherein the negative electrode active material includes at least one selected from the group consisting of tin, silicon, germanium, and lead.
[3] The lithium ion secondary battery negative electrode according to [1] or [2], wherein the water-soluble polymer includes a polymer containing 20% by weight or more of an ethylenically unsaturated monomer unit having an acidic functional group. Slurry.
[4] The structure according to any one of [1] to [3], wherein the ethylenically unsaturated carboxylic acid monomer unit is a structural unit formed by polymerizing an ethylenically unsaturated dicarboxylic acid monomer. The slurry for lithium ion secondary battery negative electrode of description.
[5] The slurry for a lithium ion secondary battery negative electrode according to [4], wherein the ethylenically unsaturated dicarboxylic acid monomer is itaconic acid.
[6] The slurry for a negative electrode of a lithium ion secondary battery according to any one of [1] to [5], wherein the particulate polymer further contains a hydroxyl group-containing monomer unit.
[7] The slurry for a negative electrode of a lithium ion secondary battery according to [6], wherein the hydroxyl group-containing monomer is 2-hydroxyethyl acrylate.
[8] The THF-insoluble content of the particulate polymer is 70% by weight or more,
The slurry for a negative electrode of a lithium ion secondary battery according to any one of [1] to [7], wherein the particulate polymer has a THF swelling degree of 25 times or less.
[9] A negative electrode for a lithium ion secondary battery, comprising applying the slurry for a lithium ion secondary battery negative electrode according to any one of [1] to [8] onto a current collector and drying the current collector. Production method.
[10] A positive electrode, a negative electrode, an electrolytic solution, and a separator are provided.
A lithium ion secondary battery, wherein the negative electrode is a negative electrode for a lithium ion secondary battery produced by the production method according to [9].

According to the slurry for the negative electrode of the lithium ion secondary battery of the present invention, it is possible to realize a lithium ion secondary battery having excellent adhesion of the negative electrode active material layer to the current collector and excellent cycle characteristics in a high temperature environment.
According to the method for producing a negative electrode for a lithium ion secondary battery of the present invention, lithium capable of realizing a lithium ion secondary battery having excellent adhesion of the negative electrode active material layer to the current collector and excellent cycle characteristics in a high temperature environment. An anode for an ion secondary battery can be manufactured.
The lithium ion secondary battery of this invention is excellent in the adhesiveness of the negative electrode active material layer with respect to a collector, and is excellent in the cycling characteristics in a high temperature environment.

Hereinafter, the present invention will be described in detail with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples described below, and may be arbitrarily modified within the scope of the claims of the present invention and its equivalents.

In the following description, (meth) acrylic acid means acrylic acid and methacrylic acid. Moreover, (meth) acrylate means an acrylate and a methacrylate. Furthermore, (meth) acrylonitrile means acrylonitrile and methacrylonitrile.

Furthermore, that a certain substance is water-soluble means that an insoluble content is less than 0.5% by weight when 0.5 g of the substance is dissolved in 100 g of water at 25 ° C. Further, that a certain substance is water-insoluble means that an insoluble content is 90% by weight or more when 0.5 g of the substance is dissolved in 100 g of water at 25 ° C.

In addition, “meq” contained in the unit of surface acid amount means milliequivalents.

[1. Slurry for negative electrode of lithium ion secondary battery]
The slurry for a negative electrode of a lithium ion secondary battery of the present invention (hereinafter sometimes referred to as “slurry for negative electrode” as appropriate) is a fluid composition containing a binder, a negative electrode active material, and a water-soluble polymer. Further, the negative electrode slurry of the present invention usually contains a solvent.

[1.1. (Binder)
A particulate polymer is used as the binder. This particulate polymer can bind negative electrode active materials to each other in the negative electrode active material layer, or bind the negative electrode active material and the current collector. In the negative electrode for a lithium ion secondary battery of the present invention (hereinafter sometimes referred to as “negative electrode” as appropriate), since the particulate polymer can hold the negative electrode active material firmly, the negative electrode active material layer for the current collector It is possible to improve the adhesion. Further, the particulate polymer can also bind particles other than the negative electrode active material usually contained in the negative electrode active material layer, and can also serve to maintain the strength of the negative electrode active material layer. In particular, since the particulate polymer has a particle shape, the binding property is particularly high, and the deterioration of the lithium ion secondary battery due to capacity reduction and repeated charge / discharge can be remarkably suppressed.

[1.1.1. (Aromatic vinyl monomer unit)
The particulate polymer according to the present invention contains an aromatic vinyl monomer unit. An aromatic vinyl monomer unit is a structural unit formed by polymerizing an aromatic vinyl monomer. Since the aromatic vinyl monomer unit is a structural unit having high rigidity, the rigidity of the particulate polymer can be increased by including the aromatic vinyl monomer unit. For this reason, the breaking strength of the particulate polymer can be improved. Further, since the rigidity of the particulate polymer is high, for example, even when a negative electrode active material such as a silicon compound repeatedly expands and contracts due to charge and discharge, the particulate polymer does not impair contact with the negative electrode active material. Can contact the negative electrode active material. Therefore, the adhesion of the negative electrode active material layer to the current collector can be improved. In particular, when the charge and discharge are repeated, the effect of improving the adhesion is remarkable. Moreover, since the rigidity of a particulate polymer will become high when there are many aromatic vinyl monomer units, the negative electrode active material which moved by the stress produced by expansion | swelling and shrinkage | contraction can be returned to an original position with a strong force. Therefore, the negative electrode active material layer can be hardly expanded even if the negative electrode active material repeatedly expands and contracts.

Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyl toluene, and divinylbenzene. Of these, styrene is preferred. Moreover, an aromatic vinyl monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The proportion of the aromatic vinyl monomer unit in the particulate polymer is usually 50% by weight or more, preferably 55% by weight or more, particularly preferably 60% by weight or more, and usually 80% by weight or less, preferably 75%. % By weight or less. When the ratio of the aromatic vinyl monomer unit is not less than the lower limit of the above range, as described above, the adhesion of the negative electrode active material layer to the current collector can be improved, and the negative electrode active material Even when the expansion and contraction are repeated, the negative electrode active material layer can be made difficult to expand. On the other hand, when the ratio of the aromatic vinyl monomer unit is not more than the upper limit of the above range, the ratio of the ethylenically unsaturated carboxylic acid monomer unit contained in the particulate polymer can be relatively increased. . For this reason, since the carboxyl group (—COOH group) contained in the particulate polymer can be increased, the adhesion of the negative electrode active material layer to the current collector can also be increased. Therefore, by keeping the ratio of the aromatic vinyl monomer unit in the particulate polymer within the above range, the adhesion of the negative electrode active material layer to the current collector can be effectively enhanced within the range where there is no problem in productivity. Can do.
Here, the ratio of the aromatic vinyl monomer unit in the particulate polymer usually corresponds to the ratio (preparation ratio) of the aromatic vinyl monomer in all the monomers of the particulate polymer.

[1.1.2. (Ethylenically unsaturated carboxylic acid monomer unit)
The particulate polymer according to the present invention contains an ethylenically unsaturated carboxylic acid monomer unit. An ethylenically unsaturated carboxylic acid monomer unit is a structural unit formed by polymerizing an ethylenically unsaturated carboxylic acid monomer. The carboxyl group (—COOH group) of the ethylenically unsaturated carboxylic acid monomer unit has a high polarity and has an effect of enhancing the binding property of the particulate polymer to the negative electrode active material and the current collector. The ethylenically unsaturated carboxylic acid monomer unit is a structural unit having high strength. For this reason, by including an ethylenically unsaturated carboxylic acid monomer unit, it is possible to increase the strength of the particulate polymer and increase the surface acid amount, thereby improving the adhesion of the negative electrode active material layer to the current collector. it can. Moreover, the affinity with respect to the water of a particulate polymer can be improved with the polarity which a carboxyl group has. Therefore, if an ethylenically unsaturated carboxylic acid monomer unit is used, the particulate polymer can be stably dispersed in water, and the stability of the negative electrode slurry can be improved. Furthermore, the affinity of the particulate polymer for the polar solvent is improved by the polarity of the carboxyl group, so that the wettability of the particulate polymer to the electrolytic solution can be improved.

Examples of the ethylenically unsaturated carboxylic acid monomer include ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid; ethylenically unsaturated dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid; and Anhydrides; and the like. Among these, an ethylenically unsaturated dicarboxylic acid monomer is preferable, and itaconic acid is particularly preferable. Generally, an ethylenically unsaturated carboxylic acid monomer is hydrophilic because it has a carboxyl group. Therefore, when a particulate polymer is produced by emulsion polymerization using water as a reaction medium, a large amount of ethylenically unsaturated carboxylic acid monomer units are collected on the surface portion of the particulate polymer. Of the ethylenically unsaturated carboxylic acid monomers, itaconic acid has a slow reaction rate in the synthesis reaction of the particulate polymer. Therefore, when itaconic acid is used, a large amount of structural units formed by polymerizing itaconic acid are particularly concentrated on the surface of the particulate polymer. By these, the surface acid amount of the particulate polymer can be increased. Moreover, an ethylenically unsaturated carboxylic acid monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The proportion of the ethylenically unsaturated carboxylic acid monomer unit in the particulate polymer is usually 0.5% by weight or more, preferably 2% by weight or more, particularly preferably 3% by weight or more, and usually 10% by weight or less. Preferably it is 7.5 weight% or less, More preferably, it is 5.0 weight% or less. When the ratio of the ethylenically unsaturated carboxylic acid monomer unit is not less than the lower limit of the above range, the adhesion of the negative electrode active material layer to the current collector can be enhanced. Further, the stability of the negative electrode slurry can be improved. For example, even when the negative electrode slurry is stored for a long period of time, it is difficult to increase the viscosity. On the other hand, when the ratio of the ethylenically unsaturated carboxylic acid monomer unit is not more than the upper limit of the above range, the particulate polymer according to the present invention can be easily produced.
Here, the ratio of the ethylenically unsaturated carboxylic acid monomer unit in the particulate polymer is usually equal to the ratio of the ethylenically unsaturated carboxylic acid monomer in all the monomers of the particulate polymer (feeding ratio). Match.

[1.1.3. Hydroxyl-containing monomer unit]
The particulate polymer according to the present invention preferably contains a hydroxyl group-containing monomer unit. A hydroxyl group-containing monomer unit is a structural unit formed by polymerizing a hydroxyl group-containing monomer. The hydroxyl group (—OH group) of the hydroxyl group-containing monomer unit has a high polarity and has an effect of enhancing the binding property of the particulate polymer to the negative electrode active material and the current collector. For this reason, the adhesiveness of the negative electrode active material layer with respect to a collector can further be improved by including a hydroxyl group-containing monomer unit. Moreover, the affinity with respect to the water of a particulate polymer can be improved with the polarity which a hydroxyl group has. Therefore, when the hydroxyl group-containing monomer unit is used, the particulate polymer can be more stably dispersed in water, and the stability of the negative electrode slurry can be improved. Further, the affinity of the particulate polymer for the polar solvent is improved by the polarity of the hydroxyl group, so that the wettability of the particulate polymer to the electrolytic solution can be further improved.

Examples of the hydroxyl group-containing monomer include 2-hydroxyethyl acrylate, 2-hydroxy methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate. Hydroxyalkyl acrylates such as di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, and 2-hydroxyethylmethyl fumarate; Examples include alcohols and monoallyl ethers of polyhydric alcohols. Of these, hydroxyalkyl acrylate is preferable, and 2-hydroxyethyl acrylate is particularly preferable. Moreover, a hydroxyl-containing monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The proportion of the hydroxyl group-containing monomer unit in the particulate polymer is usually 0.1% by weight or more, preferably 0.5% by weight or more, and usually 5% by weight or less, preferably 1.5% by weight or less. . The wettability with respect to the electrolyte solution of a particulate polymer can be improved because the ratio of a hydroxyl-containing monomer unit is more than the lower limit of the said range. Moreover, by being below an upper limit, the stability at the time of manufacture of a particulate polymer and the wettability with respect to electrolyte solution can be made compatible.
Here, the ratio of the hydroxyl group-containing monomer unit in the particulate polymer usually corresponds to the ratio (preparation ratio) of the hydroxyl group-containing monomer in all monomers of the particulate polymer.

[1.1.4. Arbitrary structural unit)
The particulate monomer according to the present invention includes an arbitrary structural unit in addition to the aromatic vinyl monomer unit, the ethylenically unsaturated carboxylic acid monomer unit, and the hydroxyl group-containing monomer unit as necessary. You may go out. Examples of monomers corresponding to these arbitrary structural units include aliphatic conjugated diene monomers, vinyl cyanide monomers, unsaturated carboxylic acid alkyl ester monomers, and unsaturated carboxylic acid amide monomers. Etc.
Examples of the aliphatic conjugated diene monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene and the like. Can be mentioned.
Examples of the vinyl cyanide monomer include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and α-ethylacrylonitrile.
Examples of unsaturated carboxylic acid alkyl ester monomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, and dimethyl itaco. Nates, monomethyl fumarate, monoethyl fumarate, and 2-ethylhexyl acrylate.
Examples of the unsaturated carboxylic acid amide monomer include acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, and N, N-dimethylacrylamide.
Moreover, these monomers may be used individually by 1 type, and may be used even if they combine 2 or more types by arbitrary ratios.

[1.1.5. Physical properties and amount of particulate polymer]
(Surface acid amount)
The surface acid amount of the particulate polymer according to the present invention is usually 0.20 meq / g or more, preferably 0.23 meq / g or more, usually 0.8 meq / g or less, preferably 0.60 meq / g or less. is there. By increasing the surface acid amount, the wettability of the particulate polymer to water can be improved. Thereby, since the dispersion stability of the particulate polymer in water can be improved, the viscosity increase of the slurry for negative electrodes can be suppressed. Therefore, since the applicability | paintability of the slurry for negative electrodes can be improved, a negative electrode active material with few defects can be manufactured now, and the low temperature output characteristic of a lithium ion secondary battery can be improved. Further, when the surface acid amount of the particulate polymer is large, the surface tension of the aqueous dispersion containing the particulate polymer is lowered, and the aqueous dispersion containing the particulate polymer wets the negative electrode active material and the current collector. Can improve sex. For this reason, since migration can be prevented when the negative electrode slurry is applied to the current collector, the adhesion of the negative electrode active material layer to the current collector can be increased. Therefore, even if charging / discharging is repeated, the negative electrode active material layer is hardly peeled off from the current collector, and the cycle characteristics (particularly, the cycle characteristics in a high temperature environment) of the lithium ion secondary battery can be improved.

The surface acid amount of the particulate polymer can be controlled by, for example, the type and ratio of the structural units of the particulate polymer. Specifically, the amount of surface acid can be efficiently controlled by adjusting the type and ratio of the ethylenically unsaturated carboxylic acid monomer unit among the structural units. Usually, using a highly hydrophilic ethylenically unsaturated carboxylic acid monomer makes it easier for the ethylenically unsaturated carboxylic acid monomer to copolymerize on the surface of the particulate polymer. There is a tendency to easily control the amount. Further, by using a combination of hydroxyl group-containing monomers, it is possible to enhance the copolymerizability of the ethylenically unsaturated carboxylic acid monomer and to control the surface acid amount more easily.

Here, the measuring method of the surface acid amount of the particulate polymer is as follows.
An aqueous dispersion containing a particulate polymer (solid content concentration 2%) is prepared. In a glass container having a capacity of 150 ml washed with distilled water, 50 g of the aqueous dispersion containing the particulate polymer is added by the weight of the particulate polymer, and set in a solution conductivity meter and stirred. Thereafter, stirring is continued until the addition of hydrochloric acid is completed.
0.1 N sodium hydroxide is added to the aqueous dispersion containing the particulate polymer so that the electrical conductivity of the aqueous dispersion containing the particulate polymer is 2.5 to 3.0 mS. Thereafter, after 6 minutes, the electrical conductivity is measured. This value is the electrical conductivity at the start of measurement.

Further, 0.5 ml of 0.1N hydrochloric acid is added to the aqueous dispersion containing the particulate polymer, and the electrical conductivity is measured after 30 seconds. Thereafter, 0.5 ml of 0.1 N hydrochloric acid is added again, and the electrical conductivity is measured after 30 seconds. This operation is repeated at intervals of 30 seconds until the electrical conductivity of the aqueous dispersion containing the particulate polymer becomes equal to or higher than the electrical conductivity at the start of measurement.

The obtained electrical conductivity data is plotted on a graph with the electrical conductivity (unit “mS”) as the vertical axis (Y coordinate axis) and the cumulative amount of added hydrochloric acid (unit “mmol”) as the horizontal axis (X coordinate axis). Plot. As a result, a hydrochloric acid amount-electric conductivity curve having three inflection points is obtained. The X coordinate of the three inflection points and the X coordinate at the end of the addition of hydrochloric acid are P1, P2, P3, and P4 in order from the smallest value. X-coordinates are approximate straight lines by the least squares method for the data in the four sections, from zero to coordinate P1, from coordinate P1 to coordinate P2, from coordinate P2 to coordinate P3, and from coordinate P3 to coordinate P4. L1, L2, L3 and L4 are obtained. The X coordinate of the intersection of the approximate line L1 and the approximate line L2 is A1 (mmole), the X coordinate of the intersection of the approximate line L2 and the approximate line L3 is A2 (mmol), and the X point of the intersection of the approximate line L3 and the approximate line L4 The coordinates are A3 (mmol).

The surface acid amount per gram of the particulate polymer and the acid amount in the aqueous phase per gram of the particulate polymer are given as milliequivalents converted to hydrochloric acid from the following formulas (a) and (b), respectively. . Further, the total acid amount per 1 g of the particulate polymer dispersed in water is the sum of the formula (a) and the formula (b) as represented by the following formula (c).
(A) Surface acid amount per 1 g of the particulate polymer = A2-A1
(B) Acid amount in aqueous phase per gram of particulate polymer = A3-A2
(C) Total acid group amount per gram of particulate polymer dispersed in water = A3-A1

(Contact angle)
The contact angle of the particulate polymer according to the present invention with respect to a mixed solvent of ethylene carbonate and diethyl carbonate is usually 50 ° or less, preferably 45 ° or less. The lower limit is ideally 0 °, but is usually 30 ° or more. Here, the volume ratio of ethylene carbonate to diethyl carbonate in the mixed solvent is ethylene carbonate / diethyl carbonate = 1/2. Such a small contact angle with respect to the mixed solvent means that the wettability of the particulate polymer with respect to the electrolytic solution is excellent. Since the particulate polymer has excellent wettability with respect to the mixed solvent, the electrolytic solution can easily enter the negative electrode active material layer even at a low temperature. Therefore, since the field of ion exchange between the negative electrode active material and the electrolytic solution can be widened, the resistance can be lowered, and the low temperature output characteristics of the lithium ion secondary battery can be improved.

Generally, the contact angle of the particulate polymer can be controlled by adjusting the polarity of the surface of the particulate polymer. When the contact angle of the particulate polymer is adjusted by adjusting the polarity of the surface in this way, the contact angle of the particulate polymer is controlled by, for example, the type of the structural unit of the particulate polymer and its ratio. Yes. As a specific example, the contact angle can be efficiently controlled by adjusting the type and ratio of the ethylenically unsaturated carboxylic acid monomer unit among the structural units. Normally, using a highly hydrophilic ethylenically unsaturated carboxylic acid monomer makes it easier for the ethylenically unsaturated carboxylic acid monomer to copolymerize on the surface of the particulate polymer. There is a tendency that the contact angle is easily controlled by adjusting the polarity of the surface of the polymer. Furthermore, by using a hydroxyl group-containing monomer in combination, it is possible to enhance the copolymerizability of the ethylenically unsaturated carboxylic acid monomer and to control the contact angle more easily.

Here, the method for measuring the contact angle of the particulate polymer is as follows.
An aqueous dispersion containing a particulate polymer is prepared, and the aqueous dispersion is dried at room temperature to form a film having a thickness of 0.2 mm to 0.5 mm. In a 25 ° C. dry room (in an environment with a dew point temperature of −40 ° C. or lower), the mixed solvent is dropped onto the film, and a measuring device (for example, “DMs-400” manufactured by Kyowa Interface Science Co., Ltd.) is used from the horizontal direction. ) To observe. The contact angle is obtained from the observed image by the tangent method.

(THF insoluble matter and THF swelling degree)
The THF-insoluble content of the particulate polymer of the present invention is preferably 70% by weight or more, more preferably 75% by weight or more, particularly preferably 80% by weight or more, and ideally 100% by weight. Here, the THF-insoluble matter refers to a component that does not dissolve in THF (ie, tetrahydrofuran). When the THF-insoluble matter in the particulate polymer is large, the particulate polymer is difficult to dissolve in the electrolytic solution, and a decrease in the adhesion between the negative electrode active material layer and the current collector due to the electrolytic solution can be suppressed. For this reason, the cycling characteristics (especially cycling characteristics in a high temperature environment) of a lithium ion secondary battery can be improved. Moreover, since the rigidity of the particulate polymer can be increased by increasing the proportion of the THF insoluble matter, the breaking strength of the particulate polymer is improved, and the adhesion between the current collector and the negative electrode active material layer is increased. It can also improve sex. Further, the negative electrode active material layer can be made difficult to expand even when the negative electrode active material repeatedly expands and contracts. The proportion of the THF-insoluble matter in the particulate polymer can be controlled by, for example, the molecular weight of the particulate polymer.

Further, the THF swelling degree of the particulate polymer of the present invention is preferably 25 times or less, more preferably 15 times or less. Further, the lower limit of the degree of swelling of the particulate polymer is usually 1 time or more, and practically 1.1 times or more. Here, the degree of THF swelling refers to the degree of swelling when immersed in THF. When the THF swelling degree of the particulate polymer is small, it becomes difficult for the particulate polymer to swell with the electrolytic solution, and a decrease in the adhesion between the negative electrode active material layer and the current collector due to the electrolytic solution can be suppressed. For this reason, the cycling characteristics (especially cycling characteristics in a high temperature environment) of a lithium ion secondary battery can be improved. The THF swelling degree of the particulate polymer can be controlled by, for example, the type and ratio of the structural unit of the particulate polymer.

Here, the measurement method of the ratio of THF insoluble matter and the THF swelling degree of the particulate polymer is as follows.
An aqueous dispersion containing a particulate polymer is prepared, and the aqueous dispersion is dried at room temperature to form a film having a thickness of 0.2 mm to 0.5 mm. This film is cut into 1 mm square, and about 1 g is precisely weighed. The weight of the film piece obtained by cutting is defined as W0.
This film piece is immersed in 100 g of tetrahydrofuran (THF) for 24 hours. Thereafter, the weight W1 of the film piece lifted from THF is measured. The change in weight is calculated according to the following formula, and this is taken as the degree of THF swelling.
THF swelling purity (%) = W1 / W0 × 100

Furthermore, the film piece pulled up from THF is vacuum-dried at 105 ° C. for 3 hours, and the weight W2 of THF-insoluble matter is measured. And the ratio (%) of THF insoluble matter is calculated according to the following formula.
Ratio of THF insoluble matter (%) = W2 / W0 × 100

(Other physical properties)
The weight average molecular weight of the particulate polymer is preferably 2,000,000 or less. When the weight average molecular weight of the particulate polymer is in the above range, the strength of the negative electrode of the present invention and the dispersibility of the negative electrode active material are easily improved. The weight average molecular weight of the particulate polymer can be determined as a value in terms of polystyrene using tetrahydrofuran as a developing solvent by gel permeation chromatography (GPC).

The glass transition temperature of the particulate polymer is preferably −75 ° C. or higher, more preferably −55 ° C. or higher, particularly preferably −35 ° C. or higher, preferably 20 ° C. or lower, more preferably 15 ° C. or lower. When the glass transition temperature of the particulate polymer is within the above range, the binding property between the negative electrode active material and the particulate polymer, the flexibility and winding property of the negative electrode, and the adhesion between the negative electrode active material layer and the current collector Properties such as sex are highly balanced and suitable.

The particulate polymer is particulate in the negative electrode slurry, and is usually contained in the negative electrode while maintaining the particle shape.
The number average particle diameter of the particulate polymer is preferably 50 nm or more, more preferably 70 nm or more, preferably 500 nm or less, more preferably 400 nm or less. When the number average particle diameter of the particulate polymer is in the above range, the strength and flexibility of the obtained negative electrode can be improved.
Here, the number average particle diameter is a number average particle diameter calculated as an arithmetic average value obtained by measuring the diameters of 100 particulate polymers randomly selected in a transmission electron micrograph. The shape of the particles may be either spherical or irregular.

(Amount of particulate polymer)
The amount of the particulate polymer is usually 0.1 parts by weight or more, preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and usually 50 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. , Preferably 20 parts by weight or less, more preferably 10 parts by weight or less. By making the amount of the particulate polymer within this range, sufficient adhesion between the negative electrode active material layer and the current collector can be secured, the capacity of the lithium ion secondary battery can be increased, and the lithium ion secondary battery The internal resistance of the working electrode can be lowered.

In addition, one kind of particulate polymer may be used alone, or two or more kinds of particulate polymers may be used in combination at any ratio.

[1.1.6. Method for producing particulate polymer]
The particulate polymer is, for example, a single monomer containing the above-described aromatic vinyl monomer and ethylenically unsaturated carboxylic acid monomer, and a hydroxyl group-containing monomer and any monomer used as necessary. The body composition may be polymerized in an aqueous solvent to obtain polymer particles.
The ratio of each monomer in the monomer composition is usually the structural unit in the particulate polymer (for example, an aromatic vinyl monomer unit, an ethylenically unsaturated carboxylic acid monomer unit, and a hydroxyl group-containing monomer). Same as the ratio of body unit.

The aqueous solvent is not particularly limited as long as the particulate polymer can be dispersed. Usually, the boiling point at normal pressure is usually 80 ° C. or higher, preferably 100 ° C. or higher, and usually 350 ° C. or lower. Preferably, it is selected from aqueous solvents at 300 ° C. or lower. Examples of the aqueous solvent will be given below. In the following examples, the number in parentheses after the solvent name is the boiling point (unit: ° C) at normal pressure, and the value after the decimal point is a value rounded off or rounded down.

Examples of aqueous solvents include water (100); ketones such as diacetone alcohol (169) and γ-butyrolactone (204); ethyl alcohol (78), isopropyl alcohol (82), and normal propyl alcohol (97). Alcohols: propylene glycol monomethyl ether (120), methyl cellosolve (124), ethyl cellosolve (136), ethylene glycol tertiary butyl ether (152), butyl cellosolve (171), 3-methoxy-3-methyl-1-butanol (174), Ethylene glycol monopropyl ether (150), diethylene glycol monobutyl pyrether (230), triethylene glycol monobutyl ether (271), dipropylene glycol monomethyl ether (18 Glycol ethers, etc.); and 1,3-dioxolane (75), 1,4-dioxolane (101), ethers such as tetrahydrofuran (66) and the like. Among these, water is particularly preferable from the viewpoint that it is not flammable and a dispersion of a particulate polymer is easily obtained. Further, water may be used as a main solvent, and an aqueous solvent other than the above-mentioned water may be mixed and used within a range in which the dispersed state of the particulate polymer can be ensured.

The polymerization method is not particularly limited, and any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method can be used. As the polymerization method, any method such as ion polymerization, radical polymerization, and living radical polymerization can be used. From the viewpoint of production efficiency, it is easy to obtain a high molecular weight substance, and since the polymer is obtained as it is dispersed in water, redispersion treatment is unnecessary and it can be used for production of a slurry for a negative electrode as it is. Of these, the emulsion polymerization method is particularly preferable.

The emulsion polymerization method is usually performed by a conventional method. For example, the method is described in “Experimental Chemistry Course” Vol. 28, (Publisher: Maruzen Co., Ltd., edited by The Chemical Society of Japan). That is, water, an additive such as a dispersant, an emulsifier, a crosslinking agent, a polymerization initiator, and a monomer are added to a sealed container equipped with a stirrer and a heating device so as to have a predetermined composition, and the composition in the container In this method, the monomer is emulsified in water by stirring the product, and the temperature is increased while stirring to initiate polymerization. Or after emulsifying the said composition, it is the method of putting into a sealed container and starting reaction similarly.

Examples of polymerization initiators include organic compounds such as lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexanoyl peroxide, and the like. Peroxides; azo compounds such as α, α′-azobisisobutyronitrile; ammonium persulfate; and potassium persulfate. A polymerization initiator may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

For example, emulsifiers, dispersants, polymerization initiators, and the like are generally used in these polymerization methods, and the amount used is generally the amount generally used. The polymerization usually proceeds in one stage, but it may be carried out in two or more stages, such as seed polymerization employing seed particles.

The polymerization temperature and polymerization time can be arbitrarily selected depending on, for example, the polymerization method and the type of polymerization initiator. Usually, the polymerization temperature is about 30 ° C. or more, and the polymerization time is about 0.5 to 30 hours.
Further, additives such as amines may be used as a polymerization aid.

Further, an aqueous dispersion of particulate polymer particles obtained by these methods is mixed with, for example, a basic aqueous solution, and the pH is adjusted to be usually in the range of 5 to 10, preferably 5 to 9. Also good. In this case, examples of the basic aqueous solution include hydroxides of alkali metals (for example, Li, Na, K, Rb, Cs), ammonia, inorganic ammonium compounds (for example, NH ₄ Cl), and organic amine compounds (for example, ethanol). An aqueous solution containing amine, diethylamine, etc.). Among these, pH adjustment with an alkali metal hydroxide is preferable because it improves the adhesion (peel strength) between the current collector and the negative electrode active material layer.

[1.2. Negative electrode active material)
The negative electrode active material is an electrode active material for a negative electrode, and is a material that transfers electrons in the negative electrode of a lithium ion secondary battery. As the negative electrode active material, a material that can occlude and release lithium is usually used.
An example of a suitable negative electrode active material is carbon. Examples of carbon include natural graphite, artificial graphite, and carbon black. Among these, natural graphite is preferably used.

In the negative electrode slurry according to the present invention, it is preferable to use a negative electrode active material containing at least one selected from the group consisting of tin, silicon, germanium and lead. This is because a negative electrode active material containing these elements has a small irreversible capacity.
Among these, a negative electrode active material containing silicon is preferable. By using a negative electrode active material containing silicon, the electric capacity of the lithium ion secondary battery can be increased. In general, a negative electrode active material containing silicon expands and contracts greatly (for example, about 5 times) with charge and discharge. However, in a negative electrode using the slurry for negative electrode of the present invention, a negative electrode active material containing silicon The deterioration of the battery performance due to the expansion and contraction of the battery can be suppressed.

Also, one type of negative electrode active material may be used alone, or two or more types may be used in combination at any ratio. Therefore, two or more kinds of the negative electrode active materials may be used in combination. Among these, it is preferable to use a negative electrode active material containing a combination of carbon and one or both of metallic silicon and a silicon-based active material. In a negative electrode active material containing a combination of carbon and one or both of metallic silicon and a silicon-based active material, Li insertion and desorption from one or both of metallic silicon and a silicon-based active material occurs at a high potential, It is presumed that Li insertion and desorption from carbon occur at low potential. For this reason, since expansion and contraction are suppressed, the cycle characteristics of the lithium ion secondary battery can be improved.

Examples of the silicon-based active material include SiO, SiO ₂ , SiO _x (0.01 ≦ x <2), SiC, SiOC, and the like, and SiO _x , SiC, and SiC are preferable. Among these, it is particularly preferable to use SiO _x as the silicon-based active material from the viewpoint of suppressing the swelling of the negative electrode active material itself. SiO _x is a compound formed from one or both of SiO and SiO ₂ and metallic silicon. This SiO _x can be produced, for example, by cooling and precipitating silicon monoxide gas generated by heating a mixture of SiO ₂ and metal silicon.

When carbon is used in combination with one or both of metallic silicon and a silicon-based active material, it is preferable that one or both of metallic silicon and the silicon-based active material is combined with conductive carbon. By combining with conductive carbon, swelling of the negative electrode active material itself can be suppressed.
Examples of the compounding method include a method of compounding one or both of metallic silicon and silicon-based active material with carbon; conductive carbon and one or both of metallic silicon and silicon-based active material The method of compounding by granulating a mixture; etc. are mentioned.

Examples of the method for coating one or both of metallic silicon and silicon-based active material with carbon include, for example, a method in which one or both of metallic silicon and silicon-based active material are subjected to heat treatment, and disproportionation; A method of performing chemical vapor deposition by subjecting one or both of the materials to a heat treatment; and the like.

Specific examples of these methods include a method of subjecting SiO _x to heat treatment in an atmosphere containing at least one or both of an organic gas and an organic vapor. This heat treatment is usually performed at a temperature of 900 ° C. or higher, preferably 1000 ° C. or higher, more preferably 1050 ° C. or higher, more preferably 1100 ° C. or higher, and usually 1400 ° C. or lower, preferably 1300 ° C. or lower, more preferably 1200 ° C. or lower. In the area. According to this method, SiO _x can be disproportionated into a composite of silicon and silicon dioxide, and carbon can be chemically deposited on the surface.

Another specific example is the following method. That is, one or both of metallic silicon and silicon-based active material is heat-treated in an inert gas atmosphere to disproportionate to obtain a silicon composite. The heat treatment at this time is usually performed at 900 ° C. or higher, preferably 1000 ° C. or higher, more preferably 1100 ° C. or higher, and usually 1400 ° C. or lower, preferably 1300 ° C. or lower. The silicon composite thus obtained is preferably pulverized to a particle size of 0.1 μm to 50 μm. The pulverized silicon composite is heated at 800 ° C. to 1400 ° C. under an inert gas stream. The heated silicon composite is subjected to a heat treatment in an atmosphere containing at least one or both of an organic gas and an organic vapor to chemically vapor-deposit carbon on the surface. The heat treatment at this time is usually performed at 800 ° C. or higher, preferably 900 ° C. or higher, more preferably 1000 ° C. or higher, and usually 1400 ° C. or lower, preferably 1300 ° C. or lower, more preferably 1200 ° C. or lower.

As another specific example, the following method may be mentioned. That is, one or both of the metal silicon and the silicon-based active material is usually one of an organic gas and an organic vapor in a temperature range of 500 ° C. to 1200 ° C., preferably 500 ° C. to 1000 ° C., more preferably 500 ° C. to 900 ° C. Alternatively, chemical vapor deposition is performed on both. This is heat-treated in an inert gas atmosphere to disproportionate. The heat treatment at this time is usually performed at 900 ° C. or higher, preferably 1000 ° C. or higher, more preferably 1100 ° C. or higher, and usually 1400 ° C. or lower, preferably 1300 ° C. or lower.

In the case of using a negative electrode active material containing a combination of carbon and one or both of metallic silicon and a silicon-based active material, the amount of silicon atoms in the negative electrode active material is 0.1 parts by weight with respect to 100 parts by weight of the total carbon atoms. It is preferable that the amount be ˜50 parts by weight. Thereby, a conductive path is formed satisfactorily and the conductivity of the negative electrode can be improved.

When a negative electrode active material including a combination of carbon and one or both of metallic silicon and a silicon-based active material is used, a weight ratio of carbon to one or both of metallic silicon and a silicon-based active material (“carbon weight” / It is preferable that “weight of metal silicon and silicon-based active material”) be within a predetermined range. Specifically, the weight ratio is preferably 50/50 or more, more preferably 70/30 or more, preferably 97/3 or less, more preferably 90/10 or less. Thereby, the cycling characteristics of a lithium ion secondary battery can be improved.

The negative electrode active material is preferably sized in the form of particles. When the shape of the particles is spherical, a higher density electrode can be formed during electrode molding.
When the negative electrode active material is particles, the volume average particle diameter is appropriately selected in view of other constituent requirements of the secondary battery. The volume average particle diameter of the particles of the specific negative electrode active material is usually 0.1 μm or more, preferably 1 μm or more, more preferably 5 μm or more, and usually 100 μm or less, preferably 50 μm or less, more preferably 20 μm or less. . Here, the volume average particle diameter employs a particle diameter at which the cumulative volume calculated from the small diameter side is 50% in the particle size distribution measured by the laser diffraction method.

The specific surface area of the negative electrode active material is usually 2 m ² / g or more, preferably 3 m ² / g or more, more preferably 5 m ² / g or more, and usually 20 m ² / g or less, preferably from the viewpoint of improving the output density. It is 15 m ² / g or less, more preferably 10 m ² / g or less. The specific surface area of the negative electrode active material can be measured by, for example, the BET method.

[1.3. Water-soluble polymer)
The water-soluble polymer usually has an effect of uniformly dispersing the negative electrode active material and the particulate polymer in the negative electrode slurry, and an effect of adjusting the viscosity of the negative electrode slurry. In addition, the water-soluble polymer reduces the surface tension of the negative electrode slurry, improves the wettability of the negative electrode slurry to the current collector, and improves the adhesion of the negative electrode active material layer to the current collector. sell. Furthermore, in the negative electrode, the water-soluble polymer is usually interposed between the negative electrode active materials and between the negative electrode active material and the current collector, and can act to bind the negative electrode active material and the current collector.

As the water-soluble polymer, a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is preferably used. Here, the ethylenically unsaturated monomer unit having an acidic functional group is a structural unit formed by polymerizing an ethylenically unsaturated monomer having an acidic functional group. A polymer containing an ethylenically unsaturated monomer unit having an acidic functional group can exhibit water solubility by the action of the acidic functional group.

Examples of the ethylenically unsaturated monomer having an acidic functional group include an ethylenically unsaturated carboxylic acid monomer, an ethylenically unsaturated sulfonic acid monomer, and an ethylenically unsaturated phosphoric acid monomer. .

Examples of the ethylenically unsaturated carboxylic acid monomer include an ethylenically unsaturated monocarboxylic acid monomer and derivatives thereof, an ethylenically unsaturated dicarboxylic acid monomer and acid anhydrides thereof, and derivatives thereof. Examples of ethylenically unsaturated monocarboxylic acid monomers include acrylic acid, methacrylic acid, and crotonic acid. Examples of derivatives of ethylenically unsaturated monocarboxylic acid monomers include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxy Examples include acrylic acid and β-diaminoacrylic acid. Examples of ethylenically unsaturated dicarboxylic acid monomers include maleic acid, fumaric acid, and itaconic acid. Examples of acid anhydrides of ethylenically unsaturated dicarboxylic acid monomers include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. Examples of ethylenically unsaturated dicarboxylic acid monomer derivatives include maleic acid substituted with substituents such as methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, etc. And maleic esters such as methylallyl maleate, diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Among these, ethylenically unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid are preferable. It is because the dispersibility with respect to the water of the obtained water-soluble polymer can be improved more.

Examples of ethylenically unsaturated sulfonic acid monomers include monomers sulfonated one of conjugated double bonds of diene compounds such as isoprene and butadiene, vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfone. Examples thereof include ethyl methacrylate, sulfopropyl methacrylate, sulfobutyl methacrylate, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), 3-allyloxy-2-hydroxypropanesulfonic acid (HAPS), and salts thereof. Examples of the salt include lithium salt, sodium salt, potassium salt and the like.

Examples of the ethylenically unsaturated phosphoric acid monomer include monomers having an ethylenically unsaturated group and a group —O—P (═O) (— OR ⁴ ) —OR ⁵ group (R ⁴ and R ⁵ is independently a hydrogen atom or any organic group.), Or a salt thereof. Specific examples of the organic group as R ⁴ and R ⁵ include an aliphatic group such as an octyl group, an aromatic group such as a phenyl group, and the like. Specific examples of the ethylenically unsaturated phosphoric acid monomer include a compound containing a phosphoric acid group and an allyloxy group, and a phosphoric acid group-containing (meth) acrylic acid ester. Examples of the compound containing a phosphoric acid group and an allyloxy group include 3-allyloxy-2-hydroxypropane phosphoric acid. Examples of phosphate group-containing (meth) acrylic acid esters include dioctyl-2-methacryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethyl phosphate, monomethyl-2-methacryloyloxyethyl phosphate, dimethyl-2-methacrylate. Leuoxyethyl phosphate, monoethyl-2-methacryloyloxyethyl phosphate, diethyl-2-methacryloyloxyethyl phosphate, monoisopropyl-2-methacryloyloxyethyl phosphate, diisopropyl-2-methacryloyloxyethyl phosphate, mono n-butyl -2-Methacryloyloxyethyl phosphate, di-n-butyl-2-methacryloyloxyethyl phosphate, monobutoxyethyl-2-methacryloyloxyethyl phosphate , Dibutoxyethyl-2-methacryloyloxyethyl phosphate, mono (2-ethylhexyl) -2-methacryloyloxyethyl phosphate, and di (2-ethylhexyl) -2-methacryloyloxyethyl phosphate.

Among the above-mentioned examples, preferred are ethylenically unsaturated carboxylic acid monomers and ethylenically unsaturated sulfonic acid monomers, and more preferred are acrylic acid, methacrylic acid, itaconic acid and 2- Examples include acrylamido-2-methylpropanesulfonic acid, acrylic acid and methacrylic acid are more preferable, and methacrylic acid is particularly preferable.
One type of ethylenically unsaturated monomer having an acidic functional group may be used alone, or two or more types may be used in combination at any ratio.

In the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group, the proportion of the ethylenically unsaturated monomer unit having an acidic functional group is preferably 20% by weight or more, more preferably 25% by weight or more. It is preferably 50% by weight or less, more preferably 40% by weight or less. By making the ratio of the ethylenically unsaturated monomer unit having an acidic functional group more than the lower limit of the above range, the polymer containing the ethylenically unsaturated monomer unit having an acidic functional group has good water solubility. Can be expressed. Moreover, by making it into an upper limit or less, excessive contact with an acidic functional group and electrolyte solution can be avoided, and durability can be improved. Here, the ratio of the ethylenically unsaturated monomer unit having an acidic functional group in the polymer containing the ethylenically unsaturated monomer unit having an acidic functional group is usually the ethylenically unsaturated monomer unit having an acidic functional group. This corresponds to the ratio (preparation ratio) of ethylenically unsaturated monomers having acidic functional groups in all monomers of the polymer containing the monomer unit.

Further, the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may contain any constituent unit other than the ethylenically unsaturated monomer unit having an acidic functional group. For example, a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may contain a fluorine-containing (meth) acrylate monomer unit as an optional component. Here, the fluorine-containing (meth) acrylic acid ester monomer unit is a structural unit formed by polymerizing a fluorine-containing (meth) acrylic acid ester monomer.

Examples of the fluorine-containing (meth) acrylic acid ester monomer include monomers represented by the following formula (I).

In the above formula (I), R ¹ represents a hydrogen atom or a methyl group.
In the above formula (I), R ² represents a hydrocarbon group containing a fluorine atom. The carbon number of the hydrocarbon group is usually 1 or more and usually 18 or less. Moreover, the number of fluorine atoms contained in R ² may be one or two or more.

Examples of fluorine-containing (meth) acrylic acid ester monomers represented by formula (I) include (meth) acrylic acid alkyl fluoride, (meth) acrylic acid fluoride aryl, and (meth) acrylic acid fluoride. Aralkyl is mentioned. Of these, alkyl fluoride (meth) acrylate is preferable. Specific examples of such monomers include 2,2,2-trifluoroethyl (meth) acrylate, β- (perfluorooctyl) ethyl (meth) acrylate, 2,2, (meth) acrylic acid. 3,3-tetrafluoropropyl, (meth) acrylic acid 2,2,3,4,4,4-hexafluorobutyl, (meth) acrylic acid 1H, 1H, 9H-perfluoro-1-nonyl, (meth) 1H, 1H, 11H-perfluoroundecyl acrylate, perfluorooctyl (meth) acrylate, trifluoromethyl (meth) acrylate, 3 [4 [1-trifluoromethyl-2, 2-methacrylic acid] Perfluoroalkyl esters of (meth) acrylic acid such as bis [bis (trifluoromethyl) fluoromethyl] ethynyloxy] benzooxy] 2-hydroxypropyl Le, and the like. Moreover, a fluorine-containing (meth) acrylic acid ester monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The ratio of the fluorine-containing (meth) acrylate monomer unit in the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is preferably 1% by weight or more, more preferably 2% by weight or more, and particularly Preferably it is 5 weight% or more, Preferably it is 20 weight% or less, More preferably, it is 15 weight% or less. By setting the ratio of the fluorine-containing (meth) acrylic acid ester monomer unit to the lower limit of the above range or more, the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group has a repulsive force against the electrolytic solution. And the swellability can be within an appropriate range. On the other hand, by setting it to the upper limit value or less of the above range, the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group can be given wettability to an electrolytic solution, and the resulting lithium ion secondary battery It is possible to improve the low temperature output characteristics. Here, the ratio of the fluorine-containing (meth) acrylic acid ester monomer unit in the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is usually an ethylenically unsaturated monomer having an acidic functional group. This corresponds to the ratio (preparation ratio) of fluorine-containing (meth) acrylic acid ester monomers in all monomers of the polymer including the body unit.

Examples of arbitrary structural units that the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may have are not limited to the fluorine-containing (meth) acrylate monomer units described above, Other structural units may be included. For example, (meth) acrylic acid ester monomer units other than fluorine-containing (meth) acrylic acid ester monomer units can be mentioned. The (meth) acrylic acid ester monomer unit is a structural unit formed by polymerizing a (meth) acrylic acid ester monomer. However, among the (meth) acrylate monomers, those containing fluorine are distinguished from (meth) acrylate monomers as fluorine-containing (meth) acrylate monomers.

Examples of (meth) acrylic acid ester monomers include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, Acrylic acid alkyl esters such as 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate; and methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t -Butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl Methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n- tetradecyl methacrylate, methacrylic acid alkyl esters such as stearyl methacrylate. Moreover, a (meth) acrylic acid ester monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

In the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group, the proportion of the (meth) acrylic acid ester monomer unit is usually 30% by weight or more, preferably 35% by weight or more, more preferably 40%. It is not less than wt% and usually not more than 80 wt%. By making the amount of the (meth) acrylic acid ester monomer unit equal to or higher than the lower limit value of the above range, the adhesion of the negative electrode active material layer to the current collector can be increased, and the upper limit value of the above range is set. Thus, the flexibility of the negative electrode can be increased. Here, the ratio of the (meth) acrylic acid ester monomer unit in the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is usually an ethylenically unsaturated monomer unit having an acidic functional group. This corresponds to the ratio (preparation ratio) of the (meth) acrylic acid ester monomer in all monomers of the polymer containing.

As a further example of an arbitrary structural unit that the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may have, a structural unit obtained by polymerizing the following monomers may be mentioned. That is, aromatic vinyl monomers such as styrene, chlorostyrene, vinyltoluene, t-butylstyrene, vinylbenzoic acid, methyl vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene, α-methylstyrene, divinylbenzene, etc. Amide monomers such as acrylamide; α, β-unsaturated nitrile compound monomers such as acrylonitrile and methacrylonitrile; olefin monomers such as ethylene and propylene; halogen atoms such as vinyl chloride and vinylidene chloride Monomers; vinyl ester monomers such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl benzoate; vinyl ether monomers such as methyl vinyl ether, ethyl vinyl ether, butyl vinyl ether; methyl vinyl ketone, ethyl vinyl ketone, Butyl vinyl Units obtained by polymerizing one or more of vinyl ketone monomers such as ketone, hexyl vinyl ketone and isopropenyl vinyl ketone; and heterocyclic-containing vinyl compound monomers such as N-vinyl pyrrolidone, vinyl pyridine and vinyl imidazole Is mentioned.

The weight average molecular weight of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is usually smaller than that of the particulate polymer, preferably 100 or more, more preferably 500 or more, particularly preferably 1000 or more. Yes, preferably 500,000 or less, more preferably 250,000 or less, particularly preferably 100,000 or less. By making the weight average molecular weight of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group more than the lower limit of the above range, the strength of the water-soluble polymer is increased and the negative electrode active material is stably covered. A protective layer can be formed. For this reason, for example, the dispersibility of the negative electrode active material and the high-temperature storage characteristics of the lithium ion secondary battery can be improved. On the other hand, the flexibility of the water-soluble polymer can be increased by setting it to the upper limit of the above range. For this reason, for example, suppression of the swelling of the negative electrode and improvement of the adhesion of the negative electrode active material layer to the current collector can be achieved.
Here, the weight average molecular weight of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group was 0.85 g / ml in a 10% by volume aqueous solution of dimethylformamide by gel permeation chromatography (GPC). It can be determined as a value in terms of polystyrene using a solution in which sodium nitrate is dissolved as a developing solvent.

The glass transition temperature of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is usually 0 ° C. or higher, preferably 5 ° C. or higher, and usually 100 ° C. or lower, preferably 50 ° C. or lower. When the glass transition temperature of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is in the above range, both the adhesion and flexibility of the negative electrode active material layer can be achieved. The glass transition temperature can be adjusted by combining appropriate monomers.

The polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is, for example, a monomer composition containing an ethylenically unsaturated monomer having an acidic functional group and, if necessary, any monomer. Can be produced by polymerizing in an aqueous solvent. In this case, the ratio of each monomer in the monomer composition is usually the structural unit in the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group (for example, an ethylenic group having an acidic functional group). The ratio of unsaturated monomer units, fluorine-containing (meth) acrylate monomer units, and (meth) acrylate monomer units) is the same.

The kind of the aqueous solvent used for the polymerization reaction can be the same as in the production of the particulate polymer, for example.
The procedure for the polymerization reaction can be the same as the procedure for producing the particulate polymer. Thereby, an aqueous solution in which a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is usually dissolved in an aqueous solvent is obtained. The polymer may be taken out from the aqueous solution thus obtained, but usually, a negative electrode slurry can be produced using a polymer dissolved in an aqueous solvent, and the negative electrode slurry can be produced using the negative electrode slurry.

The aqueous solution containing a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group in an aqueous solvent is usually acidic. Therefore, it may be alkalized to pH 7 to pH 13 as necessary. Thereby, the handleability of aqueous solution can be improved and the coating property of the slurry for negative electrodes can be improved. Examples of the method for alkalinizing to pH 7 to pH 13 include alkali metal aqueous solutions such as lithium hydroxide aqueous solution, sodium hydroxide aqueous solution and potassium hydroxide aqueous solution; alkaline earth metal aqueous solutions such as calcium hydroxide aqueous solution and magnesium hydroxide aqueous solution; The method of mixing aqueous alkali solution, such as aqueous ammonia solution, is mentioned. One kind of the alkaline aqueous solution may be used alone, or two or more kinds may be used in combination at any ratio.

In the present invention, a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may be used alone as a water-soluble polymer, or may be used in combination with any other water-soluble polymer. .
When a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is used in combination with any water-soluble polymer, the ethylenically unsaturated monomer having an acidic functional group in the total amount of the water-soluble polymer. The amount of the polymer including the monomer unit is preferably within a predetermined range. The specific amount of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is usually 0.1% by weight or more, preferably 0.5% by weight or more, more preferably 1% by weight or more. The amount is usually 15% by weight or less, preferably 10% by weight or less, more preferably 7% by weight or less. Adhesiveness between the negative electrode active material layer and the current collector can be sufficiently ensured by setting the amount of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group to be not less than the lower limit of the above range. Moreover, the viscosity stability of the slurry for negative electrodes is securable by setting it as below an upper limit.

One type of water-soluble polymer may be used alone, or two or more types may be used in any combination in any ratio. Therefore, for example, a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may be used in combination of two or more. Further, for example, a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group may be used in combination with another water-soluble polymer.

A preferred example of a water-soluble polymer that can be used in combination with a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group is carboxymethyl cellulose (CMC). By using a combination of a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group and carboxymethyl cellulose, the negative electrode active material while maintaining uniform dispersibility of particles such as the electrode active material in the negative electrode slurry. Adhesion between the layer and the current collector can be ensured.

When carboxymethyl cellulose is an aqueous solution having a concentration of 1% by weight, the viscosity of the aqueous solution (1% aqueous solution) is preferably 1000 mPa · s or more, preferably 2500 mPa · s or more. Thereby, the swelling of the negative electrode accompanying charging / discharging can be suppressed and the cycling characteristics of a lithium ion secondary battery can be improved. Moreover, the upper limit of the viscosity is usually 10,000 mPa · s or less.

When combining a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group with carboxymethyl cellulose, the weight ratio of the polymer containing an ethylenically unsaturated monomer unit having an acidic functional group to carboxymethyl cellulose ( It is preferable that “weight of carboxymethyl cellulose” / “weight of polymer containing an ethylenically unsaturated monomer unit having an acidic functional group”) falls within a predetermined range. Specifically, the weight ratio is preferably 70/30 or more, more preferably 85/15 or more, preferably 99.9 / 0.1 or less, more preferably 98/2 or less. Thereby, the adhesiveness of a negative electrode active material layer and a collector can be improved effectively.

The amount of the water-soluble polymer is usually 0.3 parts by weight or more, preferably 0.5 parts by weight or more, usually 5 parts by weight or less, preferably 3 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. is there. By keeping the amount of the water-soluble polymer within the above range, the dispersibility of the negative electrode active material in the negative electrode slurry can be improved, and the cycle characteristics of the lithium ion secondary battery can be improved.

[1.4. solvent〕
In the negative electrode slurry of the present invention, water is usually used as a solvent. In the slurry for the negative electrode, the solvent can disperse the negative electrode active material, disperse the particulate polymer, or dissolve the water-soluble polymer. At this time, in the slurry for the negative electrode, a part of the water-soluble polymer is dissolved in water, but another part of the water-soluble polymer is adsorbed on the surface of the negative electrode active material. Since the water-soluble polymer adsorbed on the negative electrode active material covers the surface of the negative electrode active material with a stable layer, the dispersibility of the negative electrode active material in the solvent is improved. Furthermore, the particulate polymer according to the present invention also has high dispersibility in a solvent as described above. For this reason, the slurry for negative electrodes of this invention has the favorable coating property at the time of apply | coating to a collector.

Further, as the solvent, a solvent other than water may be used in combination with water. For example, when a liquid that can dissolve a particulate polymer and a water-soluble polymer is combined with water, the dispersion of the negative electrode active material is stable because the particulate polymer and the water-soluble polymer are adsorbed on the surface of the negative electrode active material. Therefore, it is preferable.

The type of liquid to be combined with water is preferably selected from the viewpoint of drying speed and environment. Preferred examples include cycloaliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene and xylene; ketones such as ethyl methyl ketone and cyclohexanone; ethyl acetate, butyl acetate, γ-butyrolactone, Esters such as ε-caprolactone; Acylonitriles such as acetonitrile and propionitrile; Ethers such as tetrahydrofuran and ethylene glycol diethyl ether: Alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; N— Examples include amides such as methylpyrrolidone and N, N-dimethylformamide, among which N-methylpyrrolidone (NMP) is preferable. One of these may be used alone, or two or more of these may be used in combination at any ratio.

The amount of the solvent is preferably adjusted so that the viscosity of the negative electrode slurry is suitable for coating. Specifically, the solid content concentration of the negative electrode slurry is preferably 30% by weight or more, more preferably 40% by weight or more, preferably 90% by weight or less, more preferably 80% by weight or less. Used by adjusting.

[1.5. (Optional ingredients)
The negative electrode slurry may contain an optional component in addition to the particulate polymer, negative electrode active material, water-soluble polymer and solvent described above. Examples thereof include a conductive material, a reinforcing material, a leveling agent, nanoparticles, an electrolytic solution additive, and the like. Moreover, these components may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The conductive material is a component that can improve electrical contact between the negative electrode active materials. By including a conductive material, the discharge rate characteristics of the lithium ion secondary battery can be improved.
Examples of the conductive material include furnace black, acetylene black, ketjen black, carbon black, graphite, vapor grown carbon fiber, and conductive carbon such as carbon nanotube. A conductive material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
The amount of the conductive material is preferably 1 to 20 parts by weight, more preferably 1 to 10 parts by weight with respect to 100 parts by weight of the negative electrode active material.

As the reinforcing material, for example, various inorganic and organic spherical, plate-like, rod-like or fibrous fillers can be used. A reinforcing material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. By using the reinforcing material, a tough and flexible negative electrode can be obtained, and a lithium ion secondary battery exhibiting excellent long-term cycle characteristics can be realized.
The amount of the reinforcing material is usually 0.01 parts by weight or more, preferably 1 part by weight or more, and usually 20 parts by weight or less, preferably 10 parts by weight or less, with respect to 100 parts by weight of the negative electrode active material. By setting the amount of the reinforcing material in the above range, the lithium ion secondary battery can exhibit high capacity and high load characteristics.

Examples of the leveling agent include surfactants such as alkyl surfactants, silicone surfactants, fluorine surfactants, and metal surfactants. A leveling agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. By using the leveling agent, it is possible to prevent the repelling that occurs during the application of the negative electrode slurry, and to improve the smoothness of the negative electrode.
The amount of the leveling agent is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the negative electrode active material. When the leveling agent is in the above range, the productivity, smoothness, and battery characteristics during the production of the negative electrode are excellent. Moreover, by containing a surfactant, the dispersibility of the negative electrode active material and the like in the negative electrode slurry can be improved, and the smoothness of the negative electrode obtained thereby can be improved.

Examples of the nanoparticles include particles such as fumed silica and fumed alumina. One kind of nanoparticles may be used alone, or two or more kinds of nanoparticles may be used in combination at any ratio. In the case of containing nanoparticles, the thixotropy of the negative electrode slurry can be adjusted, so that the leveling property of the negative electrode obtained thereby can be improved.
The amount of the nanoparticles is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the negative electrode active material. When the nanoparticles are in the above range, the stability and productivity of the negative electrode slurry can be improved, and high battery characteristics can be realized.

Examples of the electrolytic solution additive include vinylene carbonate. One electrolyte solution additive may be used alone, or two or more electrolyte solution additives may be used in combination at any ratio. By using the electrolytic solution additive, for example, decomposition of the electrolytic solution can be suppressed.
The amount of the electrolytic solution additive is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the negative electrode active material. By setting the amount of the electrolytic solution additive in the above range, a secondary battery excellent in cycle characteristics and high temperature characteristics can be realized.

[1.6. Method for producing negative electrode slurry]
The slurry for negative electrode can be manufactured by mixing, for example, a negative electrode active material, a particulate polymer, a water-soluble polymer and a solvent, and optional components used as necessary. The specific procedure at this time is arbitrary. For example, in the case of producing a negative electrode slurry containing a negative electrode active material, a particulate polymer, a water-soluble polymer and a conductive material, the negative electrode active material, the particulate polymer, the water-soluble polymer and the conductive material are simultaneously added to the solvent. Method of mixing; Method of mixing water-soluble polymer in solvent, then mixing particulate polymer dispersed in solvent, and then mixing negative electrode active material and conductive material; Particle polymer dispersed in solvent And a method in which a negative electrode active material and a conductive material are mixed together, and a water-soluble polymer dissolved in a solvent is mixed into the mixture.

Examples of the mixing means include, for example, mixing equipment such as a ball mill, a sand mill, a bead mill, a roll mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, a homomixer, and a planetary mixer.

[2. Negative electrode for lithium ion secondary battery]
A negative electrode can be produced by using the above-described negative electrode slurry of the present invention. The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. Since the negative electrode active material layer includes the particulate polymer, the negative electrode active material, and the water-soluble polymer included in the negative electrode slurry of the present invention, the adhesion between the current collector and the negative electrode active material layer is increased. Yes.

Examples of a method for producing a negative electrode using the negative electrode slurry of the present invention include a production method including applying a negative electrode slurry on a current collector and drying. Hereinafter, this manufacturing method will be described.

The current collector is not particularly limited as long as it has electrical conductivity and is electrochemically durable, but a metal material is preferable because it has heat resistance. Examples of the material for the current collector for the negative electrode include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. Among these, copper is particularly preferable as the current collector used for the secondary battery negative electrode. One kind of the above materials may be used alone, or two or more kinds thereof may be used in combination at any ratio.

The shape of the current collector is not particularly limited, but a sheet shape having a thickness of about 0.001 mm to 0.5 mm is preferable.
In order to increase the adhesion strength with the negative electrode active material layer, the current collector is preferably used after the surface is roughened. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, for example, an abrasive cloth paper to which abrasive particles are fixed, a grindstone, an emery buff, a wire brush provided with a steel wire, or the like is used. In order to increase the adhesion strength and conductivity of the negative electrode active material layer, an intermediate layer may be formed on the surface of the current collector.

After preparing the current collector, the negative electrode slurry is applied on the current collector. The negative electrode slurry of the present invention is excellent in dispersion stability. Therefore, the slurry for negative electrode of the present invention can be easily applied uniformly without causing migration. At this time, the slurry for negative electrode may be applied to one side of the current collector or may be applied to both sides.

Application method is not limited, and examples thereof include a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. By applying the negative electrode slurry, a negative electrode slurry film is formed on the surface of the current collector. At this time, the thickness of the negative electrode slurry can be appropriately set according to the target thickness of the negative electrode active material layer.

Thereafter, the liquid such as water is removed from the negative electrode slurry by drying. Thereby, the negative electrode active material layer containing a negative electrode active material, a particulate polymer, and a water-soluble polymer is formed on the surface of a collector, and a negative electrode is obtained.

Examples of the drying method include drying with warm air, hot air, low-humidity air, or the like; vacuum drying; drying with irradiation of energy rays such as infrared rays, far infrared rays, or electron beams. Among these, a drying method by irradiation with far infrared rays is preferable.
The drying temperature and drying time are preferably a temperature and a time at which the solvent contained in the negative electrode slurry applied to the current collector can be removed. Specifically, the drying time is usually from 1 minute to 30 minutes, and the drying temperature is usually from 40 ° C. to 180 ° C.

After applying and drying the negative electrode slurry on the surface of the current collector, the negative electrode active material layer is preferably subjected to pressure treatment using, for example, a die press or a roll press as necessary. By the pressure treatment, the porosity of the negative electrode active material layer can be lowered. The porosity is preferably 5% or more, more preferably 7% or more, preferably 30% or less, more preferably 20% or less. By setting the porosity to the lower limit value or more of the above range, a high volume capacity can be easily obtained, the negative electrode active material layer can be made difficult to peel from the current collector, and by setting the porosity to the upper limit value or less, high charging efficiency And discharge efficiency is obtained.

Furthermore, when the negative electrode active material layer contains a curable polymer, the polymer may be cured after the formation of the negative electrode active material layer.

The thickness of the negative electrode active material layer is usually 5 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and usually 1000 μm or less, preferably 500 μm or less, more preferably 300 μm or less, and particularly preferably 250 μm or less. When the thickness of the negative electrode active material layer is in the above range, load characteristics and cycle characteristics can be improved.

The content ratio of the negative electrode active material in the negative electrode active material layer is preferably 85% by weight or more, more preferably 88% by weight or more, preferably 99% by weight or less, more preferably 97% by weight or less. By making the content rate of a negative electrode active material into the said range, the negative electrode which shows a softness | flexibility and adhesiveness is demonstrated, showing a high capacity | capacitance.

The water content in the negative electrode active material layer is preferably 1000 ppm or less, and more preferably 500 ppm or less. By setting the water content of the negative electrode active material layer within the above range, a negative electrode having excellent durability can be realized. The amount of water can be measured by a known method such as the Karl Fischer method. Such a low water content can be achieved by appropriately adjusting the composition of the structural unit in the water-soluble polymer. In particular, by making the fluorine-containing (meth) acrylic acid ester monomer unit in the range of usually 0.5% by weight or more, preferably 1% by weight or more, and usually 20% by weight or less, preferably 10% by weight or less. , Moisture content can be reduced.

[3. Lithium ion secondary battery]
The lithium ion secondary battery of this invention is equipped with the negative electrode mentioned above. Specifically, the lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolytic solution, and a separator, and the negative electrode is a negative electrode manufactured using the negative electrode slurry of the present invention by the manufacturing method described above. ing.
Since the above-described negative electrode is provided, the lithium ion secondary battery of the present invention is excellent in cycle characteristics, and particularly excellent in cycle characteristics in a high temperature environment. Moreover, normally, the swelling of the negative electrode accompanying charging / discharging can be suppressed, or low temperature output characteristics can be improved.

[3.1. (Positive electrode)
The positive electrode usually includes a current collector and a positive electrode active material layer including a positive electrode active material and a positive electrode binder formed on the surface of the current collector.

The current collector of the positive electrode is not particularly limited as long as it is a material having electrical conductivity and electrochemical durability. As the current collector for the positive electrode, for example, the current collector used for the negative electrode of the present invention may be used. Among these, aluminum is particularly preferable.

As the positive electrode active material, a material capable of inserting and desorbing lithium ions is usually used. Such positive electrode active materials are roughly classified into those made of inorganic compounds and those made of organic compounds.

Examples of the positive electrode active material made of an inorganic compound include transition metal oxides, transition metal sulfides, lithium-containing composite metal oxides of lithium and transition metals, and the like. Examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

Examples of the transition metal oxide include MnO, MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO ₃ , V ₂ O ₅ , V ₆ O _{13 and the} like can be mentioned. Among them, MnO, V ₂ O ₅ , V ₆ O ₁₃ and TiO ₂ are preferable from the viewpoint of cycle stability and capacity.

Examples of the transition metal sulfide include TiS ₂ , TiS ₃ , amorphous MoS ₂ , FeS, and the like.

Examples of the lithium-containing composite metal oxide include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

Examples of the lithium-containing composite metal oxide having a layered structure include lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), lithium composite oxide of Co—Ni—Mn, Ni—Mn— Examples thereof include lithium composite oxides of Al and lithium composite oxides of Ni—Co—Al.

Examples of the lithium-containing composite metal oxide having a spinel structure include lithium manganate (LiMn ₂ O ₄ ) or Li [Mn _3/2 M] in which a part of Mn of lithium manganate is substituted with another transition metal. _1/2 ] O ₄ (where M is Cr, Fe, Co, Ni, Cu, etc.).

Examples of the lithium-containing composite metal oxide having an olivine type structure include Li _X MPO ₄ (wherein M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti). Olivine type lithium phosphate compound represented by the formula: at least one selected from the group consisting of Al, Si, B and Mo, wherein X represents a number satisfying 0 ≦ X ≦ 2.

Examples of the positive electrode active material made of an organic compound include conductive polymer compounds such as polyacetylene and poly-p-phenylene.

Alternatively, a positive electrode active material made of a composite material in which an inorganic compound and an organic compound are combined may be used. For example, a composite material covered with a carbon material may be produced by reducing and firing an iron-based oxide in the presence of a carbon source material, and this composite material may be used as a positive electrode active material. Iron-based oxides tend to have poor electrical conductivity, but can be used as a high-performance positive electrode active material by using a composite material as described above.

Furthermore, you may use as a positive electrode active material what carried out the element substitution of the said compound partially.
Moreover, you may use the mixture of said inorganic compound and organic compound as a positive electrode active material.
As the positive electrode active material, one type may be used alone, or two or more types may be used in combination at any ratio.

The volume average particle diameter of the positive electrode active material particles is usually 1 μm or more, preferably 2 μm or more, and usually 50 μm or less, preferably 30 μm or less. By setting the average particle diameter of the positive electrode active material particles in the above range, the amount of the binder in preparing the positive electrode active material layer can be reduced, and the decrease in the capacity of the lithium ion secondary battery can be suppressed. . In order to form the positive electrode active material layer, a positive electrode slurry containing a positive electrode active material and a binder is usually prepared. The viscosity of the positive electrode slurry can be adjusted to an appropriate viscosity that is easy to apply. It becomes easy and a uniform positive electrode can be obtained.

The content ratio of the positive electrode active material in the positive electrode active material layer is preferably 90% by weight or more, more preferably 95% by weight or more, preferably 99.9% by weight or less, more preferably 99% by weight or less. By setting the content of the positive electrode active material in the above range, the capacity of the lithium ion secondary battery can be increased, and the flexibility of the positive electrode and the adhesion between the current collector and the positive electrode active material layer can be improved. .

Examples of the binder for the positive electrode include polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyacrylic acid derivatives, and polyacrylonitrile derivatives. Resins such as acrylic soft polymers, diene soft polymers, olefin soft polymers, vinyl soft polymers, and the like can be used. The binder for positive electrodes may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

In addition, the positive electrode active material layer may contain components other than the positive electrode active material and the binder as necessary. Examples thereof include a viscosity modifier, a conductive agent, a reinforcing material, a leveling agent, an electrolytic solution additive, and the like. Moreover, these components may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The thickness of the positive electrode active material layer is usually 5 μm or more, preferably 10 μm or more, and usually 300 μm or less, preferably 250 μm or less. When the thickness of the positive electrode active material layer is in the above range, high characteristics can be realized in both load characteristics and energy density.

The positive electrode can be manufactured, for example, in the same manner as the above-described negative electrode.

[3.2. Electrolyte)
As the electrolytic solution, for example, a solution obtained by dissolving a lithium salt as a supporting electrolyte in a non-aqueous solvent may be used. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and other lithium salts. In particular, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li that are easily soluble in a solvent and exhibit a high degree of dissociation are preferably used. One of these may be used alone, or two or more of these may be used in combination at any ratio.

The amount of the supporting electrolyte is usually 1% by weight or more, preferably 5% by weight or more, and usually 30% by weight or less, preferably 20% by weight or less with respect to the electrolytic solution. If the amount of the supporting electrolyte is too small or too large, the ionic conductivity is lowered, and the charging characteristics and discharging characteristics of the secondary battery may be lowered.

The solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of the solvent include alkyl carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (MEC); Esters such as butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; In particular, dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate are preferred because high ion conductivity is easily obtained and the use temperature range is wide. A solvent may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

Further, an additive may be included in the electrolytic solution as necessary. As the additive, for example, carbonate compounds such as vinylene carbonate (VC) are preferable. An additive may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

Examples of the electrolytic solution other than the above include a gel polymer electrolyte obtained by impregnating a polymer electrolyte such as polyethylene oxide and polyacrylonitrile with an electrolytic solution; an inorganic solid electrolyte such as lithium sulfide, LiI, and Li ₃ N; Can do.

[3.3. separator〕
As the separator, a porous substrate having a pore portion is usually used. Examples of separators include (a) a porous separator having pores, (b) a porous separator having a polymer coating layer formed on one or both sides, and (c) a porous resin coat containing inorganic ceramic powder. And a porous separator having a layer formed thereon. Examples of these are for solid polymer electrolytes such as polypropylene-based, polyethylene-based, polyolefin-based or aramid-based porous separators, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile or polyvinylidene fluoride hexafluoropropylene copolymer. Or a polymer film for a gel polymer electrolyte; a separator coated with a gelled polymer coat layer; a separator coated with a porous film layer composed of an inorganic filler and an inorganic filler dispersant; and the like.

[3.4. Method for producing lithium ion secondary battery]
The manufacturing method of the lithium ion secondary battery of the present invention is not particularly limited. For example, the above-described negative electrode and positive electrode may be overlapped via a separator, and this may be wound or folded in accordance with the shape of the battery and placed in the battery container, and the electrolyte may be injected into the battery container and sealed. Furthermore, if necessary, an expanded metal; an overcurrent prevention element such as a fuse or a PTC element; a lead plate or the like may be inserted to prevent an increase in pressure inside the battery or overcharge / discharge. The shape of the battery may be any of, for example, a laminate cell type, a coin type, a button type, a sheet type, a cylindrical type, a square type, and a flat type.

Hereinafter, the present invention will be specifically described with reference to examples. However, this invention is not limited to the Example shown below, You may implement arbitrarily changing in the range which does not deviate from the claim of this invention, and its equivalent range.
In the following description, “%” and “part” representing amounts are based on weight unless otherwise specified. In addition, the operations described below were performed under normal temperature and normal pressure conditions unless otherwise specified.

[Evaluation methods]
[1. Method for measuring surface acid amount of particulate polymer]
The solid content concentration of the aqueous dispersion containing the particulate polymer is adjusted to 2%. In a glass container having a capacity of 150 ml washed with distilled water, 50 g of the aqueous dispersion containing the particulate polymer is placed by weight of the particulate polymer, and a solution conductivity meter (“CM-117” manufactured by Kyoto Electronics Industry Co., Ltd., Use cell type: K-121) and stir. Thereafter, stirring is continued until the addition of hydrochloric acid is completed.

0.1 N sodium hydroxide (manufactured by Wako Pure Chemical Industries, Ltd .: reagent grade) was added to the particulate polymer so that the electrical conductivity of the aqueous dispersion containing the particulate polymer was 2.5 mS to 3.0 mS. Add to aqueous dispersion containing. Thereafter, after 6 minutes, the electrical conductivity is measured. This value is the electrical conductivity at the start of measurement.

Furthermore, 0.5 ml of 0.1 N hydrochloric acid (manufactured by Wako Pure Chemical Industries, Ltd .: reagent grade) is added to the aqueous dispersion containing the particulate polymer, and the electrical conductivity is measured after 30 seconds. Thereafter, 0.5 ml of 0.1 N hydrochloric acid is added again, and the electrical conductivity is measured after 30 seconds. This operation is repeated at intervals of 30 seconds until the electrical conductivity of the aqueous dispersion containing the particulate polymer becomes equal to or higher than the electrical conductivity at the start of measurement.

[2. (Measurement of THF swelling degree and THF insoluble matter)
An aqueous dispersion containing a particulate polymer was prepared, and this aqueous dispersion was dried at room temperature to form a film having a thickness of 0.2 mm to 0.5 mm. The film formed was cut into 1 mm square, and about 1 g was precisely weighed. The weight of the film piece obtained by this cutting is defined as W0.
This film piece was immersed in 100 g of tetrahydrofuran (THF) for 24 hours. Thereafter, the weight W1 of the film piece lifted from THF was measured. And weight change was calculated according to the following formula, and this was made into THF swelling degree.
THF swelling purity (%) = W1 / W0 × 100

Furthermore, the film piece pulled up from THF was vacuum-dried at 105 degreeC for 3 hours, and the weight W2 of THF insoluble matter was measured. And the ratio (%) of THF insoluble matter was computed according to the following formula.
Ratio of THF insoluble matter (%) = W2 / W0 × 100

[3. Contact angle measurement)
[2. Using the film obtained by forming a film of an aqueous dispersion containing the particulate polymer in [THF swelling degree and insoluble matter measurement], the contact angle was measured in the following manner.
A contact angle meter (“DMs-400” manufactured by Kyowa Interface Science Co., Ltd.) was prepared as a measuring device. Moreover, the mixed solvent which contains ethylene carbonate and diethyl carbonate by ethylene carbonate / diethyl carbonate = 1/2 (volume ratio) was prepared as a sample for measuring a contact angle. This mixed solvent was dropped on the film in a dry room at 25 ° C. (in an environment with a dew point temperature of −40 ° C. or lower), and image analysis was performed on the observed image using the measuring device from the horizontal direction. The contact angle was determined by the tangent method.

[4. Adhesion with copper foil)
The negative electrodes produced in the examples and comparative examples were cut into rectangles having a length of 100 mm and a width of 10 mm to obtain test pieces. A cellophane tape was affixed on the surface of the negative electrode active material layer of the test piece with the surface of the negative electrode active material layer facing down. At this time, a cellophane tape defined in JIS Z1522 was used. The cellophane tape was fixed on a horizontal test bench. Then, the stress when one end of the current collector was pulled vertically upward at a pulling speed of 50 mm / min and peeled was measured. This measurement was performed 3 times, the average value was calculated | required, and the said average value was made into peel strength (N / m). The higher the peel strength, the greater the binding force of the negative electrode active material layer to the copper foil, that is, the higher the adhesion strength.

[5. Battery low-temperature output characteristics)
The lithium ion secondary battery of the laminate type cell manufactured by the Example and the comparative example was left still for 24 hours. Thereafter, a charging operation of 0.1 C for 5 hours was performed under an environment of 25 ° C., and the voltage V0 at this time was measured. Thereafter, a discharge operation of 0.1 C was performed in an environment of −25 ° C., and the voltage V10 10 seconds after the start of discharge was measured. The low temperature output characteristics were evaluated by a voltage change ΔV (mV) indicated by ΔV = V0−V10. It shows that it is excellent in low temperature output characteristics, so that the value of this voltage change (DELTA) V is small.

[6. (High temperature cycle characteristics)
The lithium ion secondary battery of the laminate type cell manufactured by the Example and the comparative example was left still for 24 hours in 25 degreeC environment. Thereafter, the charge / discharge operation of charging to 4.2 V and discharging to 3.0 V was performed by a constant current method of 0.1 C, and the electric capacity at that time (initial capacity C0) was measured. Furthermore, in an environment of 60 ° C., the charge / discharge operation of charging to 4.2 V and discharging to 3.0 V by a constant current method of 0.1 C was repeated 100 times (100 cycles), and the electric capacity C2 after 100 cycles Was measured. The high-temperature cycle characteristics were evaluated by a capacity change rate ΔCC (%) represented by ΔCC = C2 / C0 × 100 (%). It shows that it is excellent in high temperature cycling characteristics, so that the value of this capacity change rate (DELTA) CC is high.

[Example 1]
(1-1. Production of particulate polymer)
In a 5 MPa pressure vessel with a stirrer, 30 parts of 1,3-butadiene, 4 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, 65 parts of styrene as an aromatic vinyl monomer, 2-hydroxy as a hydroxyl group-containing monomer Put 1 part of ethyl acrylate, 4 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water, and 0.5 part of potassium persulfate as a polymerization initiator, and after stirring sufficiently, warm to 50 ° C. Polymerization was started.

When the polymerization conversion rate reached 96%, the reaction was stopped by cooling to obtain a mixture containing particulate polymer (SBR) as a binder. A 5% aqueous sodium hydroxide solution was added to the mixture containing the particulate polymer to adjust the pH to 8. Then, the unreacted monomer was removed by heating under reduced pressure. Furthermore, it cooled to 30 degrees C or less after that, and obtained the aqueous dispersion containing a desired particulate polymer. Using the aqueous dispersion containing this particulate polymer, the surface acid amount, the degree of swelling of THF, the proportion of THF insoluble matter, and the contact angle of the particulate polymer were measured as described above.

(1-2. Production of water-soluble polymer 1)
In a 5 MPa pressure vessel with a stirrer, 50 parts of butyl acrylate, 20 parts of ethyl acrylate, 30 parts of methacrylic acid as an ethylenically unsaturated carboxylic acid monomer, 4 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water as a solvent, Then, 0.5 part of potassium persulfate was added as a polymerization initiator, and after sufficiently stirring, the polymerization was started by heating to 60 ° C.

When the polymerization conversion rate reached 96%, the reaction was stopped by cooling to obtain a mixture containing the water-soluble polymer 1. A 5% aqueous sodium hydroxide solution was added to the mixture containing the water-soluble polymer 1 to adjust the pH to 8, and an aqueous solution containing the desired water-soluble polymer 1 was obtained.

(1-3. Production of slurry for negative electrode)
In a planetary mixer with a disper, 75 parts of artificial graphite (volume average particle diameter: 24.5 μm) having a specific surface area of 4 m ² / g and 25 parts of SiOx (manufactured by Shin-Etsu Chemical; average particle diameter of 5 μm) as a negative electrode active material, As a water-soluble polymer capable of functioning as a dispersant, 1% aqueous solution of carboxymethyl cellulose (“BSH-6” manufactured by Daiichi Kogyo Seiyaku Co., Ltd., 1400% aqueous solution viscosity 3400 mPa · s) In addition, the solid concentration was adjusted to 55% with ion-exchanged water. Then, it mixed for 60 minutes at 25 degreeC. Next, the solid content concentration was adjusted to 52% with ion-exchanged water. Then, it further mixed for 15 minutes at 25 degreeC, and obtained the liquid mixture.

An aqueous dispersion containing the particulate polymer obtained in (1-1. Production of particulate polymer) is added to the mixed solution in an amount of the particulate polymer with respect to 100 parts of the total amount of the negative electrode active material. 2 parts of the aqueous solution containing the water-soluble polymer 1 obtained in (1-2. Production of water-soluble polymer 1) was added in an amount of 0.10 parts of the water-soluble polymer 1. Further, ion exchange water was added to adjust the final solid content concentration to 50% and mixed for 10 minutes. This was defoamed under reduced pressure to obtain a negative electrode slurry having good fluidity.

(1-4. Production of negative electrode)
Using a comma coater, the slurry for negative electrode obtained in the above (1-3. Production of slurry for negative electrode) is deposited on a copper foil having a thickness of 20 μm, which is a current collector, so that the film thickness after drying becomes about 150 μm. And then dried. This drying was performed by conveying the copper foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Then, the negative electrode original fabric was obtained by heat-processing at 120 degreeC for 2 minute (s). This negative electrode original fabric was rolled with a roll press to obtain a negative electrode having a negative electrode active material layer thickness of 80 μm.
About the obtained negative electrode, the adhesive strength to the copper foil of a negative electrode active material layer was measured in the way mentioned above.

(1-5. Production of positive electrode)
As a positive electrode binder, a 40% aqueous dispersion of an acrylate polymer having a glass transition temperature Tg of −40 ° C. and a number average particle size of 0.20 μm was prepared. This acrylate polymer is a copolymer obtained by emulsion polymerization of a monomer mixture containing 78% by weight of 2-ethylhexyl acrylate, 20% by weight of acrylonitrile, and 2% by weight of methacrylic acid.

100 parts of lithium cobaltate having a volume average particle diameter of 10 μm as the positive electrode active material, and 1 part of a 1% aqueous solution of carboxymethyl cellulose (“BSH-12” manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as the dispersant, As a binder, 5 parts of a 40% aqueous dispersion of the above acrylate polymer corresponding to the solid content and ion-exchanged water were mixed. The amount of ion-exchanged water was such that the total solid concentration was 40%. These were mixed by a planetary mixer to prepare a positive electrode slurry.

The above slurry for positive electrode was applied on a current collector 20 μm thick aluminum foil with a comma coater so that the film thickness after drying was about 200 μm and dried. This drying was performed by conveying the aluminum foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Then, it heat-processed for 2 minutes at 120 degreeC, and obtained the positive electrode.

(1-6. Preparation of separator)
A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 μm; produced by a dry method; porosity 55%) was prepared. This separator was cut into a 5 × 5 cm ² square.

(1-7. Lithium ion secondary battery)
An aluminum packaging exterior was prepared as the battery exterior. The positive electrode obtained in the above (1-5. Production of positive electrode) was cut into a square of 4 × 4 cm ² and arranged so that the surface on the current collector side was in contact with the aluminum packaging exterior. On the surface of the positive electrode active material layer of the positive electrode, the square separator obtained in the above (1-6. Preparation of separator) was disposed. Further, the negative electrode obtained in (1-4. Production of negative electrode) was cut into a square of 4.2 × 4.2 cm ² , and this was cut on the separator so that the surface on the negative electrode active material layer side faced the separator. Arranged. This was filled with a LiPF ₆ solution having a concentration of 1.0 M (solvent is a mixed solvent of EC / DEC = 1/2 (volume ratio)) as an electrolytic solution. Furthermore, in order to seal the opening of the aluminum packaging material exterior, heat sealing at 150 ° C. was performed to close the aluminum packaging material exterior, and a lithium ion secondary battery was manufactured.
About the obtained lithium ion secondary battery, the high temperature cycle characteristic and the low temperature output characteristic were evaluated.

[Example 2]
In the above (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 20 parts, and the amount of styrene was changed to 75 parts.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 3]
In the above (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 33 parts, and the amount of itaconic acid was changed to 1 part.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 4]
In (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 26 parts, and the amount of itaconic acid was changed to 8 parts.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 5]
In (1-1. Production of particulate polymer), maleic acid was used instead of itaconic acid.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 6]
In the above (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 15 parts, and the amount of styrene was changed to 80 parts.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 7]
In the above (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 31 parts, and 2-hydroxyethyl acrylate was not used.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 8]
In the above (1-3. Production of slurry for negative electrode), the amount of artificial graphite was changed to 65 parts, the amount of SiOx was changed to 45 parts, and the amount of 1% aqueous solution of carboxymethylcellulose was reduced to 0. The amount was changed to 97 parts, and the amount of the aqueous solution containing the water-soluble polymer 1 was changed to 0.03 parts by the amount of the water-soluble polymer 1.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 9]
In the above (1-3. Production of slurry for negative electrode), the amount of artificial graphite was changed to 90 parts, the amount of SiOx was changed to 10 parts, and the amount of 1% aqueous solution of carboxymethylcellulose was reduced to 0. The amount was changed to 97 parts, and the amount of the aqueous solution containing the water-soluble polymer 1 was changed to 0.03 parts by the amount of the water-soluble polymer 1.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 10]
In the above (1-3. Production of slurry for negative electrode), the amount of the 1% aqueous solution of carboxymethyl cellulose was changed to 1.00 parts corresponding to the solid content, and the aqueous solution containing the water-soluble polymer 1 was not used.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 11]
In a 5 MPa pressure vessel equipped with a stirrer, 40 parts of butyl acrylate, 20 parts of ethyl acrylate, 10 parts of 2,2,2-trifluoroethyl methacrylate, 30 parts of methacrylic acid as an ethylenically unsaturated carboxylic acid monomer, dodecylbenzenesulfone as an emulsifier 4 parts of sodium acid, 150 parts of ion-exchanged water as a solvent, and 0.5 part of potassium persulfate as a polymerization initiator were added and sufficiently stirred, and then heated to 60 ° C. to initiate polymerization.

When the polymerization conversion reached 96%, the reaction was stopped by cooling to obtain a mixture containing the water-soluble polymer 2. A 5% aqueous sodium hydroxide solution was added to the mixture containing the water-soluble polymer 2 to adjust the pH to 8, and an aqueous solution containing the desired water-soluble polymer 2 was obtained.

In the above (1-3. Production of slurry for negative electrode), instead of the aqueous solution of the water-soluble polymer 1, an aqueous solution containing the water-soluble polymer 2 produced in Example 11 was changed to 0 in the amount of the water-soluble polymer 2. 0.03 part was used. Further, the amount of 1% aqueous solution of carboxymethylcellulose was changed to 0.97 parts corresponding to the solid content.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 12]
In a 5 MPa pressure vessel equipped with a stirrer, 40 parts of butyl acrylate, 20 parts of ethyl acrylate, 10 parts of 2,2,2-trifluoroethyl methacrylate, 20 parts of methacrylic acid as an ethylenically unsaturated carboxylic acid monomer, ethylenically unsaturated sulfone 10 parts 2-acrylamido-2-methylpropanesulfonic acid as acid monomer, 4 parts sodium dodecylbenzenesulfonate as emulsifier, 150 parts ion-exchanged water as solvent, and 0.5 part potassium persulfate as polymerization initiator After sufficiently stirring, the polymerization was started by heating to 60 ° C.

When the polymerization conversion rate reached 96%, the reaction was stopped by cooling to obtain a mixture containing the water-soluble polymer 3. A 5% aqueous sodium hydroxide solution was added to the mixture containing the water-soluble polymer 3 to adjust the pH to 8, and an aqueous solution containing the desired water-soluble polymer 3 was obtained.

In (1-3. Production of slurry for negative electrode), instead of the aqueous solution of the water-soluble polymer 1, an aqueous solution containing the water-soluble polymer 3 produced in Example 12 was added in an amount of 0 to the amount of the water-soluble polymer 3. 0.03 part was used. Further, the amount of 1% aqueous solution of carboxymethylcellulose was changed to 0.97 parts corresponding to the solid content.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Example 13]
In the above (1-3. Production of slurry for negative electrode), the amount of artificial graphite was changed to 100 parts, and SiOx was not used.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Comparative Example 1]
In (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 47 parts, and the amount of styrene was changed to 48 parts.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Comparative Example 2]
In (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 33.8 parts, and the amount of itaconic acid was changed to 0.2 parts.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Comparative Example 3]
In (1-1. Production of particulate polymer), the amount of 1,3-butadiene was changed to 5 parts, and the amount of styrene was changed to 90 parts.
In the above (1-3. Production of slurry for negative electrode), the amount of 1% aqueous solution of carboxymethyl cellulose was changed to 0.97 parts corresponding to the solid content, and the amount of aqueous solution containing water-soluble polymer 1 was changed to water-soluble polymer. The amount of 1 was changed to 0.03 parts.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[Comparative Example 4]
Example 1-2 (1-2. Water-soluble polymer) of Example 1 except that the amount of butyl acrylate was changed to 60 parts, the amount of ethyl acrylate was changed to 30 parts, and the amount of methacrylic acid was changed to 10 parts. 1), an aqueous dispersion containing the water-insoluble polymer 4 was produced.

In the above (1-3. Production of slurry for negative electrode), an aqueous dispersion containing the water-insoluble polymer 4 produced in Comparative Example 4 was used instead of the aqueous solution of the water-soluble polymer 1. In an amount of 0.03 parts. Further, the amount of 1% aqueous solution of carboxymethylcellulose was changed to 0.97 parts corresponding to the solid content.
Except for the above, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1.

[result]
The results are shown in Tables 1 to 4. Here, the meanings of the abbreviations in the table are as follows.
Surface acid amount: The surface acid amount of the particulate polymer.
THF-insoluble matter: The proportion of the particulate polymer insoluble in THF.
THF swelling degree: THF swelling degree of a particulate polymer.
Contact angle: Contact angle of the particulate polymer with a mixed solvent of ethylene carbonate and diethyl carbonate.
Monomer combination: A combination of monomers of water-soluble polymers 1 to 3 or water-insoluble polymer 4 produced in Examples and Comparative Examples.
Monomer ratio: Amount ratio of monomers described in the “Combination of monomers” column.
1% aqueous solution viscosity: 1% aqueous solution viscosity of carboxymethyl cellulose used in each example or comparative example.
Amount of water-soluble polymer (parts): Total amount of water-soluble polymer with respect to 100 parts by weight of the negative electrode active material. Here, the total amount of the water-soluble polymer includes the amount of carboxymethyl cellulose.
Polymer ratio: A quantitative ratio of carboxymethyl cellulose to water-soluble polymers 1 to 3 or water-insoluble polymer 4 used in each example or comparative example.
Adhesiveness: Adhesiveness between the copper foil and the negative electrode active material layer. Represents peel strength.
Low temperature output characteristics: Low temperature output characteristics of lithium ion secondary batteries. This represents the voltage change ΔV.
High temperature cycle characteristics: High temperature cycle characteristics of lithium ion secondary batteries. It represents the capacity change rate ΔCC.
Monomer I: an ethylenically unsaturated carboxylic acid monomer.
IA: Itaconic acid.
Monomer II: a hydroxyl group-containing monomer.
2-HEA: 2-hydroxyethyl acrylate.
BA: Butyl acrylate.
EA: ethyl acrylate.
MAA: methacrylic acid.
V3FM: 2,2,2-trifluoroethyl methacrylate.
AMPS: 2-acrylamido-2-methylpropanesulfonic acid.

[Consideration]
As can be seen from Tables 1 to 4, according to the examples, better results than the comparative examples are obtained in the adhesion and high-temperature cycle characteristics. Therefore, according to the present invention, it was confirmed that the adhesion of the negative electrode active material layer to the current collector can be improved and that a lithium ion secondary battery excellent in cycle characteristics in a high temperature environment can be realized.

Focusing on Example 8, in Example 8, the amount of SiOx as the negative electrode active material is particularly increased to 45 parts. Thus, when there is much SiOx, a negative electrode active material will expand | swell greatly and shrink | contract by charging / discharging. For this reason, in the case of the conventional technique, when there is a large amount of SiOx, the conductive path is cut by charging / discharging in the negative electrode active material layer, so that the cycle characteristics are low. However, in Example 8, while having a large amount of SiOx, a cycle characteristic superior to that of the comparative example having a small amount of SiOx is obtained. For this reason, it turns out that this invention is especially effective when it uses for the negative electrode active material with a large degree of expansion | swelling and shrinkage | contraction accompanying charging / discharging.

Further, pay attention to Example 1 and Example 10. In Example 1 and Example 10, a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group as a water-soluble polymer is used in Example 1 and not used in Example 10. Different. Moreover, in Example 1, compared with Example 10, excellent results were obtained in two points of adhesion of the negative electrode active material layer to the current collector and cycle characteristics of the lithium ion secondary battery. Therefore, it can be seen that the effect of the slurry for negative electrode according to the present invention is greatly improved by using a polymer containing an ethylenically unsaturated monomer unit having an acidic functional group as the water-soluble polymer.

Claims

Including a binder, a negative electrode active material, and a water-soluble polymer,
The binder is a particulate polymer containing 50 wt% to 80 wt% of aromatic vinyl monomer units and 0.5 wt% to 10 wt% of ethylenically unsaturated carboxylic acid monomer units;
The surface acid amount of the particulate polymer is 0.20 meq / g or more,
A slurry for a negative electrode of a lithium ion secondary battery, wherein a contact angle of the particulate polymer with a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio: ethylene carbonate / diethyl carbonate = 1/2) is 50 ° or less.
The slurry for a lithium ion secondary battery negative electrode according to claim 1, wherein the negative electrode active material contains at least one selected from the group consisting of tin, silicon, germanium and lead.
The slurry for a lithium ion secondary battery negative electrode according to claim 1 or 2, wherein the water-soluble polymer contains a polymer containing 20% by weight or more of an ethylenically unsaturated monomer unit having an acidic functional group.
The lithium ion secondary unit according to any one of claims 1 to 3, wherein the ethylenically unsaturated carboxylic acid monomer unit is a structural unit formed by polymerizing an ethylenically unsaturated dicarboxylic acid monomer. Slurry for secondary battery negative electrode.
The slurry for a negative electrode of a lithium ion secondary battery according to claim 4, wherein the ethylenically unsaturated dicarboxylic acid monomer is itaconic acid.
The slurry for a negative electrode of a lithium ion secondary battery according to any one of claims 1 to 5, wherein the particulate polymer further contains a hydroxyl group-containing monomer unit.
The lithium ion secondary battery negative electrode slurry according to claim 6, wherein the hydroxyl group-containing monomer is 2-hydroxyethyl acrylate.
THF-insoluble matter of the particulate polymer is 70% by weight or more,
The slurry for a negative electrode of a lithium ion secondary battery according to any one of claims 1 to 7, wherein the particulate polymer has a THF swelling degree of 25 times or less.
A method for producing a negative electrode for a lithium ion secondary battery, comprising applying the slurry for a lithium ion secondary battery negative electrode according to any one of claims 1 to 8 onto a current collector and drying the slurry.
A positive electrode, a negative electrode, an electrolyte and a separator are provided.
The lithium ion secondary battery whose said negative electrode is a negative electrode for lithium ion secondary batteries manufactured by the manufacturing method of Claim 9.