WO2020050285A1

WO2020050285A1 - Lithium ion secondary battery, method for producing same, and electrode for lithium ion secondary batteries

Info

Publication number: WO2020050285A1
Application number: PCT/JP2019/034664
Authority: WO
Inventors: 寛大奥田; 利絵寺西
Original assignee: 積水化学工業株式会社
Priority date: 2018-09-05
Filing date: 2019-09-03
Publication date: 2020-03-12
Also published as: JPWO2020050285A1

Abstract

An electrode 10 for lithium ion secondary batteries according to the present invention is provided with a positive electrode 21 and a negative electrode 11. The negative electrode 11 is provided with: a negative electrode active material layer 12 that contains a negative electrode active material and a binder for negative electrodes; and an insulating layer 13 that is provided on the surface of the negative electrode active material layer 12, while containing insulating fine particles and a binder for insulating layers. The insulating layer 13 is arranged to be in contact with the positive electrode 21. The binder for negative electrodes contains a water-soluble polymer and a particulate binder. The binder for insulating layers contains an organic solvent-soluble polymer; and the content of the organic solvent-soluble polymer is 15-45% by volume based on the total amount of the insulating layer. The insulating layer 13 has a thickness of 10-20 μm.

Description

Lithium ion secondary battery, method of manufacturing the same, and electrode for lithium ion secondary battery

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

(4) Lithium-ion secondary batteries are used as large stationary power sources for power storage and power sources for electric vehicles, etc. In recent years, research into miniaturization and thinning of batteries has been advanced. Generally, a lithium ion secondary battery includes both electrodes having an electrode active material layer formed on a surface of a current collector made of a metal foil or the like, and a separator disposed between both electrodes. The separator plays a role in preventing a short circuit between the two electrodes and holding the electrolytic solution. As the separator, a polyolefin-based porous film such as polyethylene or polypropylene is generally used.

Conventionally, lithium-ion secondary batteries have been attempted to be separatorless without using the porous film for the purpose of reducing the number of parts. In order to eliminate the separator, it has been studied to form an insulating layer on the surface of the electrode active material layer and prevent a short circuit between both electrodes by the insulating layer. As disclosed in Patent Document 1, as the insulating layer, a layer having a three-dimensional network void structure including insulating particles and a binder for bonding the insulating particles to each other is known.

Patent No. 3253632

By the way, there is a demand for a lithium ion secondary battery to improve charge / discharge characteristics, output characteristics, and the like while ensuring safety such as not causing thermal runaway when heated. However, it cannot be said that the structure of the insulating layer and the combination with the electrode active material layer of the conventional insulating layer used without a separator have not been sufficiently studied, and the safety, charge / discharge characteristics, and output characteristics are not sufficient. It cannot be said that it has been raised.

Therefore, the present invention provides a lithium-ion secondary battery and a negative electrode for a lithium-ion secondary battery which can improve all of safety, charge / discharge characteristics, and output characteristics even without a separator. That is the task.

The present inventors have conducted intensive studies and as a result, used a predetermined binder as a negative electrode binder for forming a negative electrode active material layer and a predetermined binder for an insulating layer binder for forming an insulating layer. It has been found that the above problem can be solved by adjusting the thickness of the insulating layer within a predetermined range, and the present invention described below has been completed. The gist of the present invention is the following [1] to [9].
[1] A lithium ion secondary battery including a positive electrode and a negative electrode,
The negative electrode includes a negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on a surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder. ,
The insulating layer is disposed so as to contact the positive electrode,
The negative electrode binder contains a water-soluble polymer and a particulate binder,
The insulating layer binder contains an organic solvent-soluble polymer, and the content of the organic solvent-soluble polymer is 15 to 45% by volume based on the total amount of the insulating layer;
A lithium ion secondary battery, wherein the thickness of the insulating layer is 10 to 20 μm.
[2] The lithium ion secondary battery according to the above [1], wherein the organic solvent-soluble polymer is soluble in N-methylpyrrolidone.
[3] The lithium ion secondary battery according to the above [1] or [2], wherein the organic solvent-soluble polymer is at least one selected from the group consisting of polyvinylidene fluoride and an acrylic resin.
[4] The method according to any one of [1] to [3], wherein the water-soluble polymer is at least one selected from the group consisting of carboxymethylcellulose and salts thereof, hydroxyethylcellulose, polyvinyl alcohol, and polyvinylpyrrolidone. The lithium ion secondary battery according to the above.
[5] The lithium ion secondary according to any one of [1] to [4], wherein the particulate binder is at least one selected from the group consisting of styrene butadiene rubber and acrylic resin. battery.
[6] The lithium ion secondary according to any one of [1] to [5], wherein the positive electrode includes a positive electrode active material layer, and the insulating layer is arranged to be in contact with the positive electrode active material layer. battery.
[7] The lithium ion secondary battery according to any one of the above [1] to [6], which is a stacked type.
[8] The method for producing a lithium ion secondary battery according to any one of the above [1] to [7],
A step of applying an insulating layer composition to form an insulating layer on the surface of the negative electrode active material layer containing the negative electrode active material and the negative electrode binder, to obtain a negative electrode,
Pressure bonding the negative electrode to the positive electrode via the insulating layer,
A method for producing a lithium ion secondary battery, wherein the composition for an insulating layer contains the insulating fine particles, the binder for the insulating layer, and an organic solvent.
[9] There is no separator between the positive electrode and the negative electrode, a separator-less negative electrode for a lithium ion secondary battery,
A negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on the surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder,
The negative electrode binder contains a water-soluble polymer and a particulate binder,
The insulating layer binder contains an organic solvent-soluble polymer, and the content of the organic solvent-soluble polymer is 15 to 45% by volume based on the total amount of the insulating layer;
A negative electrode for a lithium ion secondary battery, wherein the thickness of the insulating layer is 10 to 20 μm.

According to the present invention, in a separator-less lithium ion secondary battery, safety, charge / discharge characteristics, and output characteristics can all be improved.

FIG. 1 is a schematic cross-sectional view showing one embodiment of a lithium ion secondary battery of the present invention.

<Lithium ion secondary battery>
Hereinafter, the lithium ion secondary battery of the present invention will be described in detail.
As shown in FIG. 1, the lithium ion secondary battery 10 includes a negative electrode 11 and a positive electrode 21. The negative electrode 11 includes a negative electrode active material layer 12 and an insulating layer 13 provided on a surface of the negative electrode active material layer 12. The lithium ion secondary battery 10 is a so-called separator-less type, and no separator is provided between the negative electrode 11 and the positive electrode 21 as a member separate from these electrodes. Therefore, the insulating layer 13 provided on the surface of the negative electrode active material layer 12 is arranged to be in contact with the positive electrode 21.
In addition, the negative electrode 11 is preferably bonded to the positive electrode 21 by pressing or the like via the insulating layer 13 so that the negative electrode 11 and the positive electrode 21 may form an integrated laminate. By adhering the negative electrode 11 to the positive electrode 21 by pressure bonding or the like, the charge / discharge characteristics, output characteristics, and the like can be easily improved.

In the lithium ion secondary battery 10, the negative electrode 11 usually includes a negative electrode current collector 14, and the negative electrode active material layer 12 is stacked on the negative electrode current collector 14. The positive electrode 21 includes a positive electrode active material layer 22. In the positive electrode 21, the positive electrode active material layer 22 is usually stacked on the electrode current collector 24. A surface layer (not shown) such as an insulating layer may be provided on the surface of the positive electrode active material layer 22 (the surface opposite to the surface on the positive electrode current collector 24 side). No layer is provided, and the insulating layer 13 of the negative electrode 11 directly contacts the positive electrode active material layer 22.

FIG. 1 shows a configuration in which the negative electrode active material layer 12 and the positive electrode active material layer 22 are provided only on one surface of each of the negative electrode current collector 14 and the positive electrode current collector 24. May be provided with a negative electrode active material layer 12. In that case, the insulating layer 13 is preferably provided on the surface of each negative electrode active material layer 12. Similarly, the positive electrode current collector 24 may be provided with the positive electrode active material layers 22 on both surfaces.
When the negative electrode 11 and the positive electrode 21 each having the negative electrode active material layer 12 and the positive electrode active material layer 22 on both surfaces are used, the negative electrode 11 and the positive electrode 21 are alternately arranged so that a plurality of layers are provided, respectively. It is preferable that the insulating layer 13 provided on the surface of the layer 12 be disposed so as to be in contact with the positive electrode 21 (the positive electrode active material layer 22).

[Negative electrode]
Hereinafter, each layer constituting the negative electrode 11 of the lithium ion secondary battery will be described in detail.
(Negative electrode active material layer)
The negative electrode active material layer 12 includes a negative electrode active material and a negative electrode binder.
Examples of the negative electrode active material used for the negative electrode active material layer include carbon materials such as graphite and hard carbon, a composite of a tin compound and silicon and carbon, and lithium. Among these, carbon materials are preferable, and graphite is preferable. More preferred. One kind of the negative electrode active material may be used alone, or two or more kinds may be used in combination.
The negative electrode active material is not particularly limited, but preferably has an average particle size of 0.5 to 50 μm, more preferably 1 to 30 μm.
The average particle diameter means the particle diameter (D50) at a volume integration of 50% in the particle size distribution of the negative electrode active material obtained by a laser diffraction / scattering method, and the same applies to the average particle diameter of other particles. .

負極 The content of the negative electrode active material in the negative electrode active material layer 11 is preferably 50 to 98.5% by mass, more preferably 60 to 98% by mass, and further preferably 70 to 98% by mass, based on the total amount of the negative electrode active material layer. By setting the lower limit or more, an appropriate amount of the negative electrode active material is blended in the negative electrode active material layer 12, and the charge / discharge characteristics and output characteristics of the lithium ion secondary battery can be easily improved. In addition, by setting the amount to be equal to or less than the upper limit, it becomes possible to make the anode active material layer 12 contain an appropriate amount of binder.

The negative electrode active material layer 12 may contain a conductive auxiliary. As the conductive additive, a material having higher conductivity than the above-described negative electrode active material is used, and specific examples thereof include carbon materials such as Ketjen black, acetylene black, carbon nanotube, and rod-like carbon. The conductive auxiliary may be used alone or in combination of two or more.
When the negative electrode active material layer 11 contains a conductive auxiliary, the content of the conductive auxiliary is preferably 1 to 30% by mass, and more preferably 2 to 25% by mass, based on the total amount of the negative electrode active material layer. Is more preferable. When the content is within these ranges, appropriate conductivity is imparted to the negative electrode active material, and various performances of the lithium ion secondary battery are easily improved. However, it is preferable that the negative electrode active material layer 12 does not contain a conductive auxiliary.

<Binder for negative electrode>
The negative electrode active material layer 12 is formed by binding a negative electrode active material or a negative electrode active material and a conductive auxiliary with a negative electrode binder. In the present invention, the binder for the negative electrode contains a water-soluble polymer and a particulate binder.

A water-soluble polymer is a polymer having high solubility in water. Since the water-soluble polymer has high solubility in water, the water-soluble polymer forms an insulating layer described later, and is hardly compatible with an insulating layer binder containing an organic solvent-soluble polymer. Thus, the negative electrode active material layer and the insulating layer are separated from each other as layers without the negative electrode active material and the insulating fine particles described later being mixed. Therefore, the negative electrode active material layer 12 and the insulating layer 13 can easily exhibit desired performance, respectively, and the charge / discharge characteristics and the output characteristics are likely to be good.

In the present specification, a solid content concentration decrease amount is used as an index indicating solubility. The solid content concentration decrease amount is an index indicating how much insoluble solids remain when a binder is dissolved in a solvent at a predetermined temperature and a predetermined concentration.The lower the value, the higher the solubility. Is shown. The specific method of measuring the amount of decrease in the solid content is as described in Examples.
Since the binder for the negative electrode has a water-soluble polymer, the amount of decrease in the solid content concentration with respect to water is reduced. The amount of decrease in the solid content of the binder for the negative electrode with respect to water is, for example, 5% by mass or less, more preferably 3% by mass or less, and further preferably 1.5% by mass or less. The lowering of the solid content concentration of the binder for the negative electrode with respect to water is preferably as low as possible, and may be 0% by mass or more, but practically 0.5% by mass or more.

On the other hand, water-soluble polymers have low solubility in organic solvents. Specifically, the water-soluble polymer has low solubility in N-methylpyrrolidone (NMP), and therefore, the binder for the negative electrode has a large decrease in the solid concentration with respect to NMP. Specifically, the amount of decrease in the solid concentration of the binder for the negative electrode with respect to NMP is, for example, 10% by mass or more, preferably 15% by mass or more, and more preferably 18% by mass or more. By setting the lower limit or more, the binder for the negative electrode becomes more difficult to be compatible with the binder for the insulating layer.
The lowering of the solid content concentration of the binder for the negative electrode with respect to NMP is preferably as high as 100% by mass or less, but practically 50% by mass or less. Here, the reason why NMP was used as an index of solubility in an organic solvent is that NMP has excellent solubility in various substances among organic solvents, and is generally used as a solvent for a composition for an insulating layer. It is.

Specific examples of the water-soluble polymer include a cellulose compound and a salt thereof, polyvinyl alcohol, and polyvinylpyrrolidone. These may be used alone or in combination of two or more.
Examples of the cellulose compound and its salt include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, and their sodium and ammonium salts.
The water-soluble polymer preferably contains at least one selected from the group consisting of carboxymethylcellulose and salts thereof, hydroxyethylcellulose and salts thereof, polyvinyl alcohol, and polyvinylpyrrolidone, and is selected from carboxymethylcellulose and salts thereof. At least one is particularly preferred. In addition, as a salt of carboxymethyl cellulose, a sodium salt is preferable.
The carboxymethyl cellulose salt has a viscosity of, for example, 10 to 5000 mPa · s, preferably 200 to 4000 mPa · s at 25 ° C. and 60 rpm measured by a B-type viscometer when dissolved in water at a concentration of 1% by mass. It is.

Rubber particles can be used as the particulate binder. Examples of the rubber include a conjugated diene-based polymer having a constituent unit derived from a conjugated diene-based monomer such as butadiene, and, for example, a rubber having a constituent unit derived from an aromatic vinyl monomer and a constituent unit derived from butadiene. Is mentioned. Specific preferred compounds include styrene butadiene rubber.
Another specific example of the particulate binder includes an acrylic resin. Examples of the acrylic resin include an acrylic polymer having a structural unit derived from a (meth) acrylate such as an alkyl (meth) acrylate. Specific examples include polymers of alkyl (meth) acrylates having about 1 to 8 carbon atoms, such as polymethyl methacrylate and polybutyl acrylate, and two or more copolymers selected from these. .
As the particulate binder, among the above, one or more selected from styrene-butadiene rubber and acrylic resin are preferable, and styrene-butadiene rubber is particularly preferable. Therefore, it is particularly preferable to use at least one selected from carboxymethylcellulose and its salts as the negative electrode binder, and to use styrene-butadiene rubber as the particulate binder.

平均 The average particle diameter of the particulate binder is not particularly limited, but is preferably 10 to 500 nm, more preferably 50 to 300 nm. When the average particle diameter of the particulate binder is within these ranges, the particulate binder can be appropriately dispersed in the negative electrode active material layer 12, and the performance of the negative electrode active material layer 12 is excellent. Becomes

In the present invention, the total amount of the water-soluble polymer and the particulate binder is preferably 1.5 to 40% by mass, more preferably 2.0 to 25% by mass, based on the total amount of the negative electrode active material layer. 2.0-20% by mass is more preferred. By setting the lower limit value or more, the negative electrode active material, or the negative electrode active material and the conductive assistant can be appropriately fixed to the negative electrode active material layer 12 by the negative electrode binder. In addition, when the content is not less than the upper limit, it is possible to prevent the amount of the binder from being excessively large and prevent the performance of the negative electrode from being lowered.

The mass ratio of the particulate binder to the water-soluble polymer (particulate binder / water-soluble polymer) is preferably 1/4 to 4, more preferably 1/2 to 2, and 2/3 to 3/2. More preferred. When the content is within these ranges, the negative electrode active material, or the negative electrode active material and the conductive assistant are appropriately held by the negative electrode binder. In addition, by being equal to or less than the upper limit, since the negative electrode active material layer contains a certain amount of water-soluble polymer, the compatibility with the insulating layer in which an organic solvent-soluble polymer described later is used as a binder is low. This makes it difficult for the negative electrode active material layer and the insulating layer to mix.

The thickness of each negative electrode active material layer 12 is not particularly limited, but is preferably 10 to 100 μm, and more preferably 20 to 80 μm.
The negative electrode active material layer 12 may contain other optional components other than the negative electrode active material, the conductive assistant, and the negative electrode binder as long as the effects of the present invention are not impaired. The negative electrode binder preferably comprises a water-soluble polymer and a particulate binder, but may contain other binders. Other binders may be used in amounts that do not impair the effects of the present invention. For example, the amount is less than 10% by mass, preferably less than 5% by mass, based on the total amount of the binder for the negative electrode.
However, in the total mass of the negative electrode active material layer 12, the total content of the negative electrode active material, the conductive auxiliary, the water-soluble polymer, and the particulate binder is preferably 90% by mass or more, and 95% by mass or more. More preferably, it is the above.

密度 The density (electrode density) of the negative electrode active material layer 12 is preferably 1.3 to 2.0 g / cc, and more preferably 1.4 to 1.8 g / cc. By setting the electrode density within these ranges, the performance of the negative electrode active material layer 12 is improved. Further, when the content is not less than these lower limits, the composition for an insulating layer described later (that is, the insulating layer 12) becomes more difficult to permeate into the negative electrode active material layer 12, and the negative electrode active material layer 12 and the insulating layer 13 are mixed. The charge / discharge characteristics, output characteristics, and the like are likely to be good.

The density of the negative electrode active material layer 12 can be calculated from the mass per unit area of the negative electrode active material layer and the thickness of the negative electrode active material layer.
For example, a plurality of measurement samples prepared by punching the negative electrode before forming the insulating layer into a predetermined size (for example, a diameter of 16 mm) are prepared. The mass of each measurement sample is weighed with a precision balance, and the mass is measured. By subtracting the previously measured mass of the negative electrode current collector from the measurement result, the mass of the negative electrode active material layer in the measurement sample can be calculated. In addition, the thickness of the negative electrode active material layer is measured by a known method such as observing a measurement sample subjected to cross-section processing with an SEM. The density of the negative electrode active material layer can be calculated from the average of the measured values based on the following equation (1).
Density of negative electrode active material layer (g / cc) = mass of negative electrode active material layer (g) / [(thickness of negative electrode active material (cm) × area of punched negative electrode (cm ² )] (1)

(Insulating layer)
The insulating layer 13 contains insulating fine particles and a binder for the insulating layer.
The insulating layer 13 is formed by binding insulating fine particles with an insulating layer binder. In the present invention, the binder for the insulating layer contains an organic solvent-soluble polymer.
The organic solvent-soluble polymer is a polymer that is soluble in an organic solvent and has high solubility in an organic solvent. In the present invention, since the organic solvent-soluble polymer is hardly compatible with the water-soluble polymer forming the negative electrode active material layer 12, the insulating layer 13 and the negative electrode active material layer 12 are separated from each other as layers, and each has a desired performance. Easy to demonstrate. Therefore, the lithium ion secondary battery has favorable charge / discharge characteristics and output characteristics.

<Content of each component>
In the present invention, the content of the organic solvent-soluble polymer in the insulating layer 13 is 15 to 45% by volume based on the total amount of the insulating layer. When the content is less than 15% by volume, the strength of the insulating layer becomes insufficient, so that it is difficult to maintain the insulating property and the safety is reduced. On the other hand, when the content exceeds 45% by volume, necessary voids are not formed in the insulating layer 13 and output characteristics are deteriorated.
The content of the organic solvent-soluble polymer is preferably 18% by volume or more, more preferably 20% by volume or more, and more preferably 23% by volume or more, from the viewpoint of enhancing safety and improving charge / discharge characteristics. It is more preferred that there be.
The content of the organic solvent-soluble polymer is preferably 40% by volume or less, more preferably 35% by volume or less, and further preferably 29% by volume or less, in order to make the void amount in the insulating layer 13 an appropriate amount and enhance the output characteristics. preferable.

On the other hand, the content of the insulating fine particles is, for example, 55 to 85% by volume based on the volume of the entire insulating layer. When the content is 55% by volume or more, necessary voids are formed in the insulating layer 13, and the output characteristics are improved. Further, when the content is 85% by volume or less, the strength of the insulating layer is increased, and the insulating property is easily ensured.
The content of the insulating fine particles is preferably 60% by volume or more, more preferably 65% by volume or more, and still more preferably 71% by volume or more, in order to make the void amount in the insulating layer 13 an appropriate amount and enhance output characteristics. .
The content of the insulating fine particles is preferably 82% by volume or less, more preferably 80% by volume or less, and further preferably 77% by volume or more, from the viewpoint of enhancing safety and improving charge / discharge characteristics. preferable.

<Thickness>
Further, in the present invention, the thickness of the insulating layer 13 is 10 to 20 μm. If the thickness is less than 10 μm, it becomes difficult to ensure insulation even if the content of the organic solvent-soluble polymer is within the above-mentioned predetermined range, and safety is reduced. On the other hand, when the thickness is larger than 20 μm, the insulating layer becomes thicker than necessary, and the output characteristics deteriorate.
The thickness of the insulating layer 13 is preferably 11 μm or more, and more preferably 13 μm or more, from the viewpoint of enhancing the charge / discharge characteristics and safety. In addition, from the viewpoint of improving output characteristics, the thickness of the insulating layer 13 is preferably equal to or less than 19 μm, and more preferably equal to or less than 17 μm.

<Insulating fine particles>
The insulating fine particles used in the present invention are not particularly limited as long as they are insulating, and may be either organic particles or inorganic particles. Specific organic particles include, for example, cross-linked polymethyl methacrylate, cross-linked styrene-acrylic acid copolymer, cross-linked acrylonitrile resin, polyamide resin, polyimide resin, poly (lithium 2-acrylamido-2-methylpropanesulfonate), Examples include particles composed of an organic compound such as a polyacetal resin, an epoxy resin, a polyester resin, a phenol resin, and a melamine resin. Examples of the inorganic particles include silicon dioxide, silicon nitride, alumina, boehmite, titania, zirconia, boron nitride, zinc oxide, tin dioxide, niobium oxide (Nb ₂ O ₅ ), tantalum oxide (Ta ₂ O ₅ ), potassium fluoride, and fluoride. Examples include particles composed of inorganic compounds such as lithium chloride, clay, zeolite, and calcium carbonate. Further, the inorganic particles may be particles composed of a known composite oxide such as a niobium-tantalum composite oxide or a magnesium-tantalum composite oxide.
The insulating fine particles may be particles in which each of the above-mentioned materials is used alone or in combination of two or more. Further, the insulating fine particles may be fine particles containing both an inorganic compound and an organic compound. For example, inorganic-organic composite particles in which an inorganic oxide is coated on the surface of particles made of an organic compound may be used.
Among these, inorganic particles are preferable, and among them, alumina particles and boehmite particles are preferable, and alumina particles are particularly preferable.

The average particle diameter of the insulating fine particles is smaller than the thickness of the insulating layer, and is, for example, 0.001 to 1 μm, preferably 0.05 to 0.8 μm, and more preferably 0.1 to 0.6 μm. . By setting the average particle diameter of the insulating layer within these ranges, the porosity can be easily adjusted within the above range.
As the insulating fine particles, one kind having an average particle diameter within the above range may be used alone, or two kinds of insulating fine particles having different average particle diameters may be mixed and used.

<Organic solvent soluble polymer>
The organic solvent-soluble polymer described above is soluble in an organic solvent, particularly an organic solvent for forming an insulating layer, but is preferably soluble in N-methylpyrrolidone (NMP). Since NMP is generally used as a diluting solvent for a composition for an insulating layer for forming an insulating layer, it is easy to form an insulating layer appropriately by being soluble in NMP.

Accordingly, the binder for the insulating layer contains the organic solvent-soluble polymer, so that the solid content concentration decrease with respect to NMP is reduced, for example, 1% by mass or less, more preferably 0.4% by mass or less, further preferably 0.1% by mass or less. % By mass or less. The lowering of the solid content concentration of the binder for the insulating layer with respect to the NMP is preferably low, and therefore it is sufficient that the amount is 0% by mass or more.

On the other hand, organic solvent-soluble polymers have low solubility in water. Therefore, the binder for the insulating layer containing the polymer soluble in an organic solvent has a large decrease in the solid concentration with respect to water. The amount of decrease in the solid content of the binder for the insulating layer with respect to water is, for example, 50% by mass or more, preferably 75% by mass or more, and more preferably 85% by mass or more. By setting the lower limit or more, the insulating layer becomes more difficult to be compatible with the negative electrode active material layer. The lowering of the solid content concentration of the binder for the insulating layer with respect to water is preferably as high as 100% by mass or less, but is practically 99% by mass or less.

In addition, the organic solvent-soluble polymer itself has a low decrease in solid content concentration with respect to NMP, and specifically, is 1% by mass or less, more preferably 0.4% by mass or less, and further preferably 0.1% by mass or less. % By mass or less. Further, the lowering of the solid content concentration of the organic solvent-soluble polymer with respect to NMP is better, and it is sufficient that the amount is 0% by mass or more.

Examples of the organic solvent-soluble polymer include fluorine-containing resins such as polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), acrylic resin, polyvinyl acetate, polyimide (PI), and polyamide (PA). ), Polyvinyl chloride (PVC), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), and the like. These may be used alone or in combination of two or more.
Among these, at least one selected from polyvinylidene fluoride and an acrylic resin is preferable, and an acrylic resin is more preferable. By using these resins, in the present invention, the binder for the insulating layer is hardly compatible with the binder for the negative electrode, and the insulating layer and the negative electrode active material layer are likely to be present as separate layers. It becomes easy to exhibit the function well. Therefore, the charge / discharge characteristics and the output characteristics of the lithium ion secondary battery are improved. When an acrylic resin is used, the adhesive strength of the insulating layer binder is increased, and the adhesiveness to the positive electrode active material layer 22 and the negative electrode active material layer 12 is increased.

The acrylic resin used as the organic solvent-soluble polymer includes an acrylic polymer having a structural unit derived from a (meth) acrylic ester. Specifically, it is preferable to have a structural unit derived from an alkyl (meth) acrylate, and the structural unit derived from an alkyl (meth) acrylate is, for example, 50% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more. contains.
The alkyl (meth) acrylate is preferably an alkyl acrylate having an alkyl group having 1 to 12, more preferably 2 to 8, carbon atoms. The acrylic polymer preferably contains 50% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass or more of a main unit of an alkyl acrylate-derived structural unit having 2 to 8 carbon atoms in the alkyl group. Contains on top.

Examples of the alkyl acrylate having 2 to 8 carbon atoms in the alkyl group include ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, hexyl acrylate, and octyl acrylate. The alkyl group in these may be a straight-chain alkyl group or a branched alkyl group which is a structural isomer thereof, such as 2-ethylhexyl acrylate.
Further, the acrylic polymer may be a copolymer of an alkyl (meth) acrylate and a vinyl monomer other than the alkyl (meth) acrylate. Examples of vinyl monomers other than alkyl (meth) acrylate include vinyl group-containing (meth) acrylates such as 2-hydroxyethyl (meth) acrylate, amino group-containing (meth) acrylates, nitrile group-containing vinyl monomers such as acrylonitrile, and (meth) acrylic. Examples include carboxyl group-containing vinyl monomers such as acid and itaconic acid, and aromatic ring-containing (meth) acrylates such as phenoxyethyl (meth) acrylate.

Specific examples of suitable acrylic polymers include polybutyl acrylate.
In addition, the acrylic polymer may be cross-linked, and specific examples thereof include cross-linked polybutyl acrylate.
In this specification, (meth) acrylate means one or both of acrylate and methacrylate, and the same applies to other similar terms.

The present invention, as described above, the binder for the negative electrode containing the water-soluble polymer, the binder for the insulating layer containing the organic solvent-soluble polymer becomes difficult to be compatible, the charge and discharge characteristics, and the more excellent output characteristics. Is what you do. From such a viewpoint, the water-soluble polymer is one or more selected from carboxymethylcellulose and salts thereof, hydroxyethylcellulose, polyvinyl alcohol, and polyvinylpyrrolidone, and the organic solvent-soluble polymer is polyvinylidene fluoride, And at least one selected from acrylic resins. It is particularly preferable that the water-soluble polymer is one or more selected from carboxymethylcellulose and salts thereof, and the organic solvent-soluble polymer is an acrylic resin.

The insulating layer may contain other optional components other than the insulating fine particles and the binder for the insulating layer as long as the effects of the present invention are not impaired. The binder for the insulating layer may be composed of an organic solvent-soluble polymer, but may contain other binders other than the organic solvent-soluble polymer as long as the effects of the present invention are not impaired. Other binders may be used in amounts that do not impair the effects of the present invention. For example, the amount is less than 10% by mass, preferably less than 5% by mass, based on the total amount of the binder for the insulating layer.
However, in the total volume of the insulating layer, the total volume of the insulating fine particles and the organic solvent-soluble polymer is preferably 85% by volume or more, more preferably 90% by volume or more, and more preferably 95% by volume or more. Is more preferred.

(Negative electrode current collector)
Examples of a material constituting the negative electrode current collector 14 include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum or copper is preferable, and copper is more preferable. The negative electrode current collector 14 is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

[Positive electrode]
Next, the positive electrode 21 of the lithium ion secondary battery 10 will be described in detail.
(Positive electrode active material layer)
The positive electrode 21 includes the positive electrode active material layer 22 as described above. The positive electrode active material layer 22 typically includes a positive electrode active material and a positive electrode binder.
Although it does not specifically limit as a positive electrode active material, a lithium metal oxide compound is mentioned. Examples of the lithium metal oxide compound include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like. Further, olivine-type lithium iron phosphate (LiFePO ₄ ) may be used. Further, a plurality of metals other than lithium may be used, and a ternary NCM (nickel-cobalt-manganese) -based oxide, an NCA (nickel-cobalt-aluminum-based) -based oxide, or the like may be used.

The binder for the positive electrode is not particularly limited, and a binder conventionally used for a positive electrode can be appropriately selected and used. For example, one or two or more selected from the above-mentioned organic solvent-soluble polymers and water-soluble polymers may be used. When a water-soluble polymer is used, one or more selected from the above-mentioned particulate binders may be further added.
Further, the positive electrode active material layer preferably further contains a conductive auxiliary. As the conductive aid, one or more of the conductive aids described above may be appropriately selected and used.

The content of the positive electrode active material in the positive electrode active material layer 22 is preferably 50 to 98.5% by mass, more preferably 60 to 98% by mass, based on the total amount of the positive electrode active material layer. Further, the content of the positive electrode binder in the positive electrode active material layer 22 is preferably 1.5 to 40% by mass, more preferably 2.0 to 25% by mass, based on the total amount of the positive electrode active material layer.
When the positive electrode active material layer 22 contains a conductive auxiliary, the content of the conductive auxiliary is preferably 1 to 30% by mass, and more preferably 2 to 25% by mass based on the total amount of the positive electrode active material layer. Is more preferable.
The thickness of each positive electrode active material layer 22 is not particularly limited, but is preferably 10 to 100 μm, and more preferably 20 to 80 μm.

(Positive electrode current collector)
The material constituting the positive electrode current collector 24 may be appropriately selected from the compounds used for the negative electrode current collector 14, and preferably used is aluminum or copper, and more preferably aluminum. The thickness of the positive electrode current collector 24 is not particularly limited, but is preferably 1 to 50 μm.

[casing]
The lithium ion secondary battery usually includes a casing, and the above-described positive electrode and negative electrode may be housed in the casing. The casing is not particularly limited, but may be an exterior can or an exterior film. The exterior film may be provided between two exterior films or one exterior film may be folded in two, for example, and the negative electrode and the positive electrode may be arranged between the exterior films.

[Structure of lithium ion secondary battery]
The lithium ion secondary battery includes a wound type and a stacked type, and the lithium ion secondary battery of the present invention is preferably a stacked type.
In the stacked lithium ion secondary battery, a negative electrode 11 in which a negative electrode active material layer 12 is provided on both surfaces of a negative electrode current collector 14 and a positive electrode 21 in which a positive electrode active material layer 22 is provided on both surfaces of a positive electrode current collector 24 , Respectively. Each of the negative electrode 11 and the positive electrode 21 has a planar shape, and these are stacked so as to be alternated along the thickness direction. The insulating layer 13 provided on the surface of each negative electrode active material layer 12 contacts the adjacent positive electrode 21 (for example, the positive electrode active material layer 22), and preferably contacts the positive electrode 21 (for example, the positive electrode active material layer 22). Glue.

The plurality of negative electrode current collectors 14 constituting each negative electrode 11 are put together and attached to a negative electrode tab or the like, and connected to the negative electrode terminal via the negative electrode tab or the like. The plurality of positive electrode current collectors 24 constituting each positive electrode 21 are put together and attached to a positive electrode tab or the like, and connected to a positive electrode terminal via the positive electrode tab or the like.

In the present invention, the type of the binder for the insulating layer, the amount of each component constituting the insulating layer, the thickness, and the like are adjusted as described above, so that the insulating layer 13 is cracked when bent or the like. Although there is a possibility that the negative electrode 11 is arranged in a plane in the stacked type, it is not bent. Therefore, in the present invention, a laminated lithium ion secondary battery having high mechanical strength and good durability can be provided.

[Electrolytes]
A lithium ion secondary battery usually includes an electrolyte. The electrolyte is not particularly limited, and a known electrolyte used in a lithium ion secondary battery may be used. For example, an electrolyte is used as the electrolyte.
Examples of the electrolyte include an electrolyte containing an organic solvent and an electrolyte salt. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane And polar solvents such as tetrohydrafuran, 2-methyltetrahydrofuran, dioxolane and methyl acetate, or a mixture of two or more of these solvents. As the electrolyte salt, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ CO ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) _And lithium-containing salts such as LiN (COCF ₂ CF ₃ ) ₂ and lithium bisoxalate borate (LiB (C ₂ O ₄ ) ₂ ). Further, a complex such as a lithium hydride of an organic acid-boron trifluoride complex or a complex hydride such as LiBH ₄ may be used. These salts or complexes may be used alone or as a mixture of two or more.
Further, the electrolyte may be a gel electrolyte further containing a polymer compound in the above-mentioned electrolytic solution. Examples of the polymer compound include a fluorine-based polymer such as polyvinylidene fluoride and a polyacryl-based polymer such as poly (methyl meth) acrylate. Note that the gel electrolyte may be used as a separator.
The electrolyte may be disposed between the negative electrode and the positive electrode. Therefore, for example, the electrolyte is filled in a casing in which the above-described negative electrode and positive electrode are housed. Further, the electrolyte may be, for example, applied on the negative electrode or the positive electrode and disposed between the negative electrode and the positive electrode.

<Production method of lithium ion secondary battery>
Next, an embodiment of a method for manufacturing an electrode for a lithium ion secondary battery will be described in detail. The method for producing an electrode for a lithium ion secondary battery according to one embodiment of the present invention includes a step of applying a composition for an insulating layer on the surface of a negative electrode active material layer to form an insulating layer, thereby producing a negative electrode. (A negative electrode manufacturing step) and a step of pressing the negative electrode to the positive electrode via the insulating layer (pressure bonding step). In addition, it usually includes a step of producing a positive electrode (positive electrode production step). Hereinafter, the present manufacturing method will be described in detail for each step.

[Negative electrode fabrication process]
Hereinafter, first, a method for manufacturing a negative electrode will be described.
(Formation of negative electrode active material layer)
In manufacturing a negative electrode, first, a negative electrode active material layer is formed. In forming the negative electrode active material layer, first, a negative electrode active material layer composition including a negative electrode active material, a negative electrode binder, and a solvent is prepared. The composition for a negative electrode active material layer may include other components such as a conductive auxiliary compounded as necessary. The negative electrode active material, the negative electrode binder, the conductive auxiliary, and the like are as described above. The composition for the negative electrode active material layer becomes a slurry.

水 Water is used as the solvent in the negative electrode active material layer composition. By using water, the water-soluble polymer used as the negative electrode binder can be easily dissolved in the negative electrode active material layer composition. Further, the particulate binder and other binders are preferably mixed with water in the form of an emulsion. The solid concentration of the composition for a negative electrode active material layer is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

The negative electrode active material layer may be formed by a known method using the negative electrode active material layer composition. For example, the negative electrode active material layer composition is applied on a negative electrode current collector and dried. Can be formed.
Further, the negative electrode active material layer may be formed by applying the composition for a negative electrode active material layer on a substrate other than the negative electrode current collector and drying the composition. As a substrate other than the negative electrode current collector, a known release sheet may be used. The negative electrode active material layer formed on the substrate is preferably formed by forming an insulating layer, and then peeling the negative electrode active material layer from the substrate and transferring the negative electrode active material layer onto the negative electrode current collector.
The negative electrode active material layer formed on the negative electrode current collector or the substrate is preferably pressed under pressure. By pressing under pressure, the density of the negative electrode can be increased. The pressure press may be performed by a roll press or the like.

(Formation of insulating layer)
In manufacturing a negative electrode, after forming a negative electrode active material layer, a composition for an insulating layer is applied over the surface of the negative electrode active material layer to form an insulating layer.
The composition for an insulating layer used for forming an insulating layer contains insulating fine particles, a binder for an insulating layer, and a solvent. The binder for the insulating layer contains the organic solvent-soluble binder as described above, but may also contain a binder other than the organic solvent-soluble binder. In addition, the composition for an insulating layer may include other optional components that are blended as necessary. Details of the insulating fine particles, the binder for the insulating layer, and the like are as described above. The composition for an insulating layer becomes a slurry (slurry for an insulating layer).

In the present production method, the solvent used for the composition for an insulating layer is an organic solvent. Specific examples of the organic solvent include one or more selected from N-methylpyrrolidone, N-ethylpyrrolidone, dimethylacetamide, and dimethylformamide. Among these, N-methylpyrrolidone is particularly preferred.
In the present production method, the composition for an insulating layer containing an organic solvent is applied on the surface of the negative electrode active material layer to form an insulating layer, but the negative electrode active material layer has low solubility in the organic solvent. Water-soluble polymers are used. Therefore, the insulating layer is formed with almost no compatibility of the composition for an insulating layer with the negative electrode active material layer. Therefore, the insulating fine particles are prevented from being mixed into the negative electrode active material layer, or the negative electrode active material is prevented from being mixed into the insulating layer, and the insulating layer and the negative electrode active material layer each easily exhibit desired functions, The charge and discharge characteristics, output characteristics, and the like are improved.

The solid content of the composition for an insulating layer is preferably 10 to 60% by mass, more preferably 20 to 50% by mass.
The viscosity of the composition for an insulating layer is preferably 100 to 4000 mPa · s, more preferably 1500 to 2500 mPa · s. The viscosity is a viscosity measured by a B-type viscometer at 60 rpm under a temperature condition at the time of coating.
By setting the viscosity and the solid content viscosity within the above ranges, an insulating layer having a predetermined thickness can be easily formed. By setting the viscosity within the above range, the composition for the insulating layer is prevented from seeping into the negative electrode active material layer, and the negative electrode active material layer and the insulating layer are easily separated as a layer.

The insulating layer can be formed by applying the composition for an insulating layer to the surface of the negative electrode active material layer and then drying the composition. The method for applying the composition for an insulating layer to the surface of the negative electrode active material layer is not particularly limited, and examples thereof include a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a bar coating method, a gravure coating method, and screen printing. And the like. Among these, the gravure coating method is preferable from the viewpoint of uniformly applying the insulating layer.
The drying temperature is not particularly limited as long as the solvent can be removed, but is, for example, 50 to 130 ° C., preferably 60 to 100 ° C. The drying time is not particularly limited, but is, for example, 30 seconds to 30 minutes, preferably 2 to 20 minutes.

[Positive electrode preparation process]
In manufacturing a positive electrode, a positive electrode active material layer is preferably formed over a positive electrode current collector. In forming the positive electrode active material layer, first, a positive electrode active material layer composition including a positive electrode active material, a positive electrode binder, and a solvent is prepared. The composition for a positive electrode active material layer may include other components such as a conductive auxiliary compounded as necessary. The positive electrode active material, the positive electrode binder, the conductive additive, and the like are as described above. The composition for the positive electrode active material layer becomes a slurry.

As the solvent in the composition for the positive electrode active material layer, it is preferable to use a solvent that dissolves the binder for the positive electrode, and may be appropriately selected depending on the type of the binder for the positive electrode, and may use water or an organic solvent. May be used. The organic solvent may be appropriately selected from the above-mentioned organic solvents. The solid concentration of the composition for a positive electrode active material layer is preferably 5 to 75% by mass, and more preferably 20 to 65% by mass.

The positive electrode active material layer may be formed by a known method using the positive electrode active material layer composition. For example, the positive electrode active material layer composition is applied on a positive electrode current collector and dried. Can be formed.
In addition, the positive electrode active material layer may be formed by applying the composition for a positive electrode active material layer on a substrate other than the positive electrode current collector and drying the composition. As a substrate other than the positive electrode current collector, a known release sheet may be used. The positive electrode active material layer formed on the substrate may be peeled off from the substrate and transferred onto the positive electrode current collector.
The positive electrode active material layer formed on the positive electrode current collector or the substrate is preferably pressed under pressure. By pressing under pressure, it is possible to increase the density of the positive electrode. The pressure press may be performed by a roll press or the like.

[Crimping process]
The negative electrode obtained as described above may be pressed against the positive electrode to form a laminate including the negative electrode and the positive electrode. Here, more specifically, in the negative electrode, the insulating layer may be disposed so as to be in contact with the positive electrode, typically the positive electrode active material layer, and the negative electrode may be press-bonded to the positive electrode via the insulating layer.
When the negative electrode and the positive electrode are laminated in a plurality of layers, respectively, the negative electrode and the positive electrode are laminated in a plurality of layers so as to be alternately arranged in the thickness direction. It is good to let.

The specific method of crimping the negative electrode and the positive electrode is to press the stacked negative electrode and the positive electrode (if there are multiple layers, alternately arranged and stacked) with a press machine or the like. It is good to do in. The pressing is preferably performed under such a condition that the negative electrode active material layer and the positive electrode active material layer are not compressed more than necessary and the insulating layer is bonded to the positive electrode. Specifically, the pressing temperature is 50 to 130 ° C., preferably 60 to 100 ° C., and the pressing pressure is, for example, 0.2 to 3 MPa, preferably 0.4 to 1.5 MPa. The pressing time is, for example, 15 seconds to 15 minutes, preferably 30 seconds to 10 minutes.

The laminated body of the negative electrode and the positive electrode obtained as described above is, for example, connected to the negative electrode current collector to the negative electrode terminal and the positive electrode current collector to the positive electrode terminal, and housed in a casing, so that the lithium ion The following battery can be obtained.
The above manufacturing method is one embodiment of the manufacturing method of the lithium ion secondary battery of the present invention, and is not limited to the above. For example, when the negative electrode does not adhere to the positive electrode, the negative electrode and the positive electrode may be simply overlapped without being pressed.

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

The method for evaluating the electrode for an ion secondary battery and the method for measuring various physical properties are as follows.
(Charge / discharge characteristics evaluation)
The lithium ion secondary batteries prepared in each of the examples and the comparative examples were charged at a constant current of 0.1 A, then the current was decreased as soon as the voltage reached 4.2 V, and the charging was completed when the current reached 0.02 A. Was done. Thereafter, a constant current discharge of 0.1 A was performed, and discharge was completed when the discharge was completed to 2.5 V. Thereafter, the battery was allowed to stand for 30 minutes, and the voltage was measured after 30 minutes. In each of Examples and Comparative Examples, a test was performed on a 15-cell lithium ion secondary battery, and an average value was calculated.
A: Average value of 2.5 V or more B: Average value of 2.0 V or more and less than 2.5 V C: Average value of 1.0 V or more and less than 2.0 V D: Average value of less than 1.0 V

(Output characteristic evaluation)
The lithium ion secondary batteries produced in each of the examples and comparative examples were evaluated by calculating the discharge capacity as described below.
The battery was charged at a constant current of 0.1 A, and then the current was reduced as soon as 4.2 V was reached. Thereafter, a constant current discharge of 1 A was performed, and when the discharge was completed to 2.5 V, a discharge was completed to complete the discharge, and the discharge capacity was calculated. The output characteristics were evaluated based on the following criteria.
A: The discharge capacity at 1 A is 30% or more as compared with the discharge capacity at a constant current of 0.1 A.
B: The discharge capacity at 1 A is 20% or more and less than 30% as compared with the discharge capacity at a constant current of 0.1 A.
C: The discharge capacity at 1 A is 10% or more and less than 20% as compared with the discharge capacity at a constant current of 0.1 A.
D: The discharge capacity at 1 A is less than 10% as compared with the discharge capacity at a constant current of 0.1 A.

(Safety evaluation)
The lithium ion secondary batteries prepared in each of the examples and the comparative examples were charged at a constant current of 0.1 A, then the current was decreased as soon as the voltage reached 4.2 V, and the charging was completed when the current reached 0.02 A. Was done. Thereafter, the battery was heated and stored at 110 ° C. The maximum temperature of the battery when it was held for 1 hour after reaching 110 ° C. was measured.
A: Maximum temperature of less than 115 ° C B: Maximum temperature of 115 ° C or more and less than 140 ° C C: Maximum temperature of 140 ° C or more and less than 200 ° C D: Maximum temperature of 200 ° C or more

(Decrease in solid content concentration)
The components used as the binder for the insulating layer, the binder for the negative electrode, or the organic solvent-soluble polymer are the same as the ratio of each component in the binder for the insulating layer, the binder for the negative electrode, or the organic solvent-soluble polymer, and the solid content concentration. Was adjusted to 5% by mass to prepare a binder solution diluted with water or NMP. 500 g of the prepared binder solution was filtered through a stainless steel filter having a mesh size of 72 μm. Preparation of the binder solution and filtration of the binder solution were performed at 25 ° C.
The filtered binder solution was dried on a hot plate at 150 ° C. for 30 minutes, the solid content after drying was measured, and the concentration of the filtered binder solution was determined by the following formula (2).
(Binder concentration) (%) = (solid content after heating) / (solution volume before heating) × 100 (2)
The binder liquid before filtration was also dried on a hot plate in the same manner, and the concentration of the binder liquid before filtration was determined by the above formula (2).
The amount of decrease in the solid content of the binder was measured by the following formula (3), and the solubility of the binder in water or NMP was determined. It should be noted that the lower the solid content concentration decrease amount, the smaller the insoluble content removed by filtration and the higher the solubility.
(Amount of decrease in solid content concentration) (%) = (concentration of binder solution before filtration) (%)-(concentration of binder solution after filtration) (%) (3)

The thickness of the insulating layer was measured by the following method.
A cross section of the negative electrode on which the negative electrode active material layer and the insulating layer were formed was exposed by an ion milling method. The exposed cross section was observed with a field emission scanning electron microscope (FE-SEM). The observation was made so that the surface of the insulating layer of the electrode could be seen from the surface to the bottom. The section magnification was 20000 times. For the obtained image, the length from the interface between the electrode active material and the insulating layer to the surface of the insulating layer was randomly measured using image analysis software (Image J) in a direction perpendicular to the electrode current collector. Ten points were measured for one image, and the average value was taken as the thickness of the insulating layer.
(Thickness of negative electrode active material layer and positive electrode active material layer)
The thicknesses of the negative electrode active material layer and the positive electrode active material layer were measured using “MF-101” manufactured by Nikon Corporation.

[Example 1]
[Preparation of positive electrode]
100 parts by mass of Li (Ni—Co—Al) O ₂ (NCA-based oxide) having an average particle diameter of 10 μm as a positive electrode active material, 4 parts by mass of acetylene black as a conductive additive, and a binder for an electrode 4 parts by mass of polyvinylidene fluoride (PVdF) and N-methylpyrrolidone (NMP) as a solvent were mixed to obtain a composition for a positive electrode active material layer adjusted to a solid concentration of 60% by mass. The composition for a positive electrode active material layer was applied on both sides of a 15 μm-thick aluminum foil as a positive electrode current collector, and was preliminarily dried and then vacuum dried at 120 ° C. Thereafter, the positive electrode current collector coated with the composition for a positive electrode active material layer on both sides is pressed under a pressure of 400 kN / m, and further punched into a 40 mm × 50 mm square of the electrode dimensions. A positive electrode having a certain positive electrode active material layer was obtained. Among these dimensions, the area where the positive electrode active material layer was formed was 40 mm × 45 mm.

[Preparation of negative electrode]
(Formation of negative electrode active material layer)
An aqueous dispersion of 100 parts by mass (97% by mass) of graphite (average particle size: 10 μm) as a negative electrode active material and styrene butadiene rubber (SBR, average particle size: 180 nm) as a particulate binder was used as a solid content. 0.5 mass parts (1.5 mass%) and sodium salt of carboxymethyl cellulose (CMC) as a water-soluble polymer (25 ° C., 60 rpm measured by a B-type viscometer when dissolved in water at a concentration of 1 mass%) Viscosity under the conditions: 3000 mPa · s) was mixed with 1.5 parts by mass (1.5% by mass) and water as a solvent to adjust the solid content concentration to 50% by mass. Obtained.
The composition for a negative electrode active material layer was applied to both surfaces of a copper foil having a thickness of 12 μm as a negative electrode current collector, and dried at 100 ° C. under vacuum. Thereafter, the negative electrode current collector having both sides coated with the negative electrode active material layer composition was pressed under a linear pressure of 500 kN / m to obtain a negative electrode active material layer having a thickness of 50 μm. The density of the negative electrode active material layer was 1.55 g / cc. In addition, the dimension of the negative electrode was 45 mm × 55 mm, and the area where the negative electrode active material layer was applied was 45 mm × 50 mm.

(Formation of insulating layer)
A polymer solution was prepared by dissolving crosslinked polybutyl acrylate as an organic solvent-soluble polymer in NMP at a concentration of 10% by mass. Alumina particles as insulating fine particles (manufactured by Nippon Light Metal Co., Ltd., product name: AHP200, average particle diameter 0.4 μm). The solution was mixed while applying moderate shear to prepare an insulating layer composition (insulating layer slurry). The solid content concentration in the insulating layer slurry was 40% by mass.
The obtained slurry for an insulating layer was applied to both surfaces of the negative electrode active material layer by gravure coating at a temperature of 25 ° C. while applying a shearing force. The viscosity of the slurry for the insulating layer at the time of coating was 2000 mPa · s. Thereafter, the coating film was dried at 90 ° C. for 10 minutes using a heating oven to form insulating layers on both surfaces of the negative electrode. The thickness of the insulating layer after drying was 15 μm per side.

(Preparation of electrolyte solution)
LiPF ₆ as an electrolyte salt was dissolved in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 (EC: DEC) so as to have a concentration of 1 mol / liter, and the electrolytic solution was dissolved. Prepared.

(Manufacture of lithium ion secondary batteries)
Twenty-five negative electrodes having the insulating layer obtained above and twenty-four positive electrodes were laminated to obtain a temporary laminate. Here, the negative electrode and the positive electrode were alternately arranged. The temporary laminate was pressed at 80 ° C. and 0.6 MPa for 1 minute using a flat plate hot press to obtain a laminate.
The ends of the exposed portions of the positive electrode current collector of each positive electrode were joined together by ultrasonic fusion, and a terminal tab projecting to the outside was joined. Similarly, the ends of the exposed portions of the negative electrode current collectors of the respective negative electrodes were joined together by ultrasonic fusion, and terminal tabs projecting to the outside were joined.
Next, the laminated body was sandwiched between aluminum laminated films, the terminal tabs were projected outside, and three sides were sealed by laminating. From one side left without sealing, the electrolyte solution obtained above was injected, and vacuum sealing was performed to produce a laminated lithium ion secondary battery (cell).

[Example 2]
Example 1 was carried out in the same manner as in Example 1 except that the solid content concentration of the slurry for the insulating layer was 45% by mass, and the thickness of the insulating layer formed on the negative electrode active material was changed to 19 μm.

[Example 3]
Example 1 was the same as Example 1 except that the solid content concentration of the slurry for the insulating layer was 35% by mass and the thickness of the insulating layer formed on the negative electrode active material was changed to 11 μm.

[Example 4]
The procedure was performed in the same manner as in Example 1 except that the composition for the insulating layer was prepared so that the organic solvent-soluble polymer was 40 parts by volume with respect to 60 parts by volume of the alumina particles.

[Example 5]
Example 1 was carried out in the same manner as in Example 1 except that the composition for an insulating layer was prepared so that the organic solvent-soluble polymer was 18 parts by volume with respect to 82 parts by volume of alumina particles.

[Comparative Example 1]
In addition to 25 negative electrodes and 24 positive electrodes having an insulating layer manufactured in the same manner as in Example 1, 50 separators were laminated to obtain a laminate. Here, the negative electrode and the positive electrode were alternately arranged, and the negative electrode or the positive electrode was arranged between the separators. As the separator, a polyethylene microporous membrane separator having a thickness of 16 μm and an air permeability of 110 sec / 100 cc air was used.
The ends of the exposed portions of the positive electrode current collector of each positive electrode were joined together by ultrasonic fusion, and a terminal tab projecting to the outside was joined. Similarly, the ends of the exposed portions of the negative electrode current collectors of the respective negative electrodes were joined together by ultrasonic fusion, and terminal tabs projecting to the outside were joined. Thereafter, a laminated lithium ion secondary battery (cell) was manufactured in the same manner as in Example 1.

[Comparative Example 2]
The same operation as in Example 1 was performed except that the solid content concentration of the slurry for the insulating layer was 15% by mass, and the thickness of the insulating layer formed on the negative electrode active material was changed to 4 μm.

[Comparative Example 3]
The procedure was performed in the same manner as in Example 1 except that the composition for an insulating layer was prepared so that the organic solvent-soluble polymer was 5 parts by volume with respect to 95 parts by volume of alumina particles.

[Comparative Example 4]
Example 1 was carried out in the same manner as in Example 1 except that the composition for an insulating layer was prepared such that the organic solvent-soluble polymer was 50 parts by volume with respect to 50 parts by volume of alumina particles.

[Comparative Example 5]
The procedure was the same as that of Example 1 except that the solid content concentration of the slurry for the insulating layer was 55% by mass, and the thickness of the insulating layer formed on the negative electrode active material was changed to 30 μm.

[Comparative Example 6]
Example 1 was carried out in the same manner as in Example 1 except that the insulating layer was formed using the insulating layer composition (insulating layer slurry) prepared as follows.
Alumina particles as insulating fine particles (product name: AHP200, manufactured by Nippon Light Metal Co., Ltd., average particle diameter 0.4 μm) and sodium salt of carboxymethylcellulose (CMC, B-type viscosity when dissolved in water at a concentration of 1% by mass) Viscosity at 25 ° C. and 60 rpm measured by a meter: 50 mPa · s), an aqueous dispersion of styrene-butadiene rubber (SBR, average particle diameter: 180 nm), and water as a solvent were subjected to moderate shearing. By mixing, a slurry for an insulating layer was prepared. The slurry for the insulating layer was adjusted so that CMC was 5 parts by volume, SBR was 20 parts by volume, and the solid content concentration was 50% by mass with respect to 100 parts by volume of alumina.

* A water-soluble polymer and a particulate binder were used as the binder for the insulating layer in Comparative Example 6.
* The value of the decrease in the solid concentration of the organic solvent-soluble polymer is the same as the value of the decrease in the solid concentration of the binder for the insulating layer.
* "-" In the evaluation column indicates that evaluation was not possible because charging / discharging was not possible.

As described above, in each example, the binder for the negative electrode contains the water-soluble polymer and the particulate binder, the binder for the insulating layer contains the polymer soluble in an organic solvent, and the volume% of the polymer soluble in the organic solvent, By adjusting the thickness of the layer within a predetermined range, all of the safety, charge / discharge characteristics, and output characteristics were improved.

Reference Signs List 10 lithium ion secondary battery 11 negative electrode 12 negative electrode active material layer 13 insulating layer 14 negative electrode current collector 21 positive electrode 22 positive electrode active material layer 24 positive electrode current collector

Claims

A lithium ion secondary battery including a positive electrode and a negative electrode,
The negative electrode includes a negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on a surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder. ,
The insulating layer is disposed so as to contact the positive electrode,
The negative electrode binder contains a water-soluble polymer and a particulate binder,
The insulating layer binder contains an organic solvent-soluble polymer, and the content of the organic solvent-soluble polymer is 15 to 45% by volume based on the total amount of the insulating layer;
A lithium ion secondary battery, wherein the thickness of the insulating layer is 10 to 20 μm.
(4) The lithium ion secondary battery according to (1), wherein the organic solvent-soluble polymer is soluble in N-methylpyrrolidone.
The lithium ion secondary battery according to claim 1 or 2, wherein the organic solvent-soluble polymer is at least one selected from the group consisting of polyvinylidene fluoride and an acrylic resin.
The lithium ion secondary according to any one of claims 1 to 3, wherein the water-soluble polymer is at least one selected from the group consisting of carboxymethylcellulose and salts thereof, hydroxyethylcellulose, polyvinyl alcohol, and polyvinylpyrrolidone. battery.
The lithium ion secondary battery according to any one of claims 1 to 4, wherein the particulate binder is at least one selected from the group consisting of styrene butadiene rubber and acrylic resin.
(6) The lithium ion secondary battery according to any one of (1) to (5), wherein the positive electrode includes a positive electrode active material layer, and the insulating layer is arranged to be in contact with the positive electrode active material layer.
The lithium ion secondary battery according to any one of claims 1 to 6, which is a stacked type.
The method for producing a lithium ion secondary battery according to any one of claims 1 to 7, wherein
A step of applying an insulating layer composition to form an insulating layer on the surface of the negative electrode active material layer containing the negative electrode active material and the negative electrode binder, and obtaining a negative electrode;
Pressure bonding the negative electrode to the positive electrode via the insulating layer,
A method for producing a lithium ion secondary battery, wherein the composition for an insulating layer includes the insulating fine particles, a binder for the insulating layer, and an organic solvent.
There is no separator between the positive electrode and the negative electrode, a separator-less negative electrode for a lithium ion secondary battery,
A negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on the surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder,
The negative electrode binder contains a water-soluble polymer and a particulate binder,
The insulating layer binder contains an organic solvent-soluble polymer, and the content of the organic solvent-soluble polymer is 15 to 45% by volume based on the total amount of the insulating layer;
A negative electrode for a lithium ion secondary battery, wherein the thickness of the insulating layer is 10 to 20 μm.