JP7160573B2 - Electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a lithium ion secondary battery electrode having an insulating layer, and a lithium ion secondary battery.

Lithium-ion secondary batteries are used as large-scale stationary power sources for power storage and power sources for electric vehicles, etc. In recent years, research into miniaturization and thinning of batteries has progressed. Lithium-ion secondary batteries generally include both electrodes in which an electrode active material layer is formed on the surface of a metal foil, and a separator disposed between the two electrodes. The separator plays a role of preventing a short circuit between both electrodes and holding an electrolytic solution.
Conventionally, in lithium ion secondary batteries, it has been studied to provide a porous insulating layer on the surface of the electrode active material layer, for example, in order to have a good short-circuit suppressing function even when the separator shrinks. The insulating layer is formed, for example, by coating an insulating layer slurry containing insulating fine particles, a binder and a solvent on an electrode active material layer and drying the slurry, as disclosed in Patent Document 1.

WO2016/104782

However, in conventional lithium-ion secondary batteries, the insulating properties of the insulating layer may deteriorate during use, and it is necessary to further improve the insulating reliability of the insulating layer.
Accordingly, an object of the present invention is to provide an electrode for a lithium ion secondary battery in which the insulation reliability of the insulating layer is further improved.

As a result of intensive studies, the present inventors have found that in conventional lithium-ion secondary batteries, the insulation reliability of the insulating layer decreases due to migration of the electrode active material and conductive aid to the insulating layer during electrode formation. rice field. Then, they found that the use of carbon nanotubes as a part or all of the conductive aid can suppress migration of the electrode active material and the conductive aid to the insulating layer, and completed the present invention described below. The gist of the present invention is the following [1] to [10].
[1] An electrode active material layer and an insulating layer provided on the surface of the electrode active material layer, the insulating layer containing insulating fine particles and an insulating layer binder, and the electrode active material layer being an electrode active material and a carbon nanotube-containing electrode for a lithium ion secondary battery.
[2] The electrode for a lithium ion secondary battery according to [1] above, wherein the carbon nanotube comprises a single-walled carbon nanotube.
[3] The electrode for a lithium ion secondary battery according to [1] or [2] above, wherein the average length of the carbon nanotubes is 10 μm or less.
[4] The electrode for a lithium ion secondary battery according to any one of [1] to [3] above, wherein the carbon nanotube has an average ratio of length to diameter (length/diameter) of 500 or more. .
[5] The lithium ion diode according to any one of [1] to [4], wherein the content of the carbon nanotubes is 0.05 to 0.15% by mass based on the total amount of the electrode active material layer. Electrodes for secondary batteries.
[6] The electrode for a lithium ion secondary battery according to any one of [1] to [5] above, wherein the electrode active material layer further contains carbon black.
[7] The electrode for a lithium ion secondary battery according to [6] above, wherein the DBP oiling amount of the carbon black is 50 to 700 ml/100 g.
[8] The electrode for a lithium ion secondary battery according to [6] or [7] above, wherein the carbon black content is 0.5 to 5.0% by mass based on the total amount of the electrode active material layer.
[9] The electrode for a lithium ion secondary battery according to any one of [1] to [8] above, wherein in the electrode active material layer, the electrode active material has an average particle size of 0.2 to 15 μm.
[10] A lithium ion secondary battery comprising the electrode according to any one of [1] to [9] above as a positive electrode.

According to the present invention, it is possible to provide a lithium ion secondary battery electrode in which the insulation reliability of the insulating layer is further improved, and a lithium ion secondary battery having the electrode as a positive electrode.

FIG. 1 is a schematic cross-sectional view showing one embodiment of the lithium-ion secondary battery electrode of the present invention. FIG. 2 is a schematic cross-sectional view showing another embodiment of the lithium ion secondary battery electrode of the present invention.

<Electrodes for lithium ion secondary batteries>
Hereinafter, the electrode for lithium ion secondary batteries of the present invention will be described in detail.
A lithium ion secondary battery electrode of the present invention includes an electrode active material layer and an insulating layer provided on the surface of the electrode active material layer. For example, like the lithium ion secondary battery electrode 10 of one embodiment of the present invention shown in FIG. and an insulating layer 12 provided in. In the lithium-ion secondary battery electrode 10 , the electrode active material layer 11 is usually laminated on the electrode current collector 13 .
Further, the electrode active material layer 11 may be laminated on both surfaces of the electrode current collector 13 as in the lithium ion secondary battery electrode 10 according to another embodiment of the present invention shown in FIG. In that case, the insulating layer 12 is provided on the surface of each electrode active material layer 11 . Providing the insulating layers 12 on both sides of the lithium ion secondary battery electrode 10 effectively prevents short circuits between the positive electrodes and the negative electrodes even when a plurality of negative electrodes and positive electrodes are laminated to form a multilayer structure.

In the present invention, the electrode for a lithium ion secondary battery may be either a negative electrode or a positive electrode, but is preferably a positive electrode from the viewpoint that the insulation reliability of the insulating layer can be further improved.

[Insulating layer]
The insulating layer contains insulating fine particles and an insulating layer binder. The insulating layer has, for example, a porous structure in which insulating fine particles are bound by an insulating layer binder.

(insulating fine particles)
The insulating fine particles are not particularly limited as long as they are particles having insulating properties, and may be either organic particles or inorganic particles. Specific organic particles include, for example, crosslinked polymethyl methacrylate, crosslinked styrene-acrylic acid copolymer, crosslinked acrylonitrile resin, polyamide resin, polyimide resin, poly(lithium 2-acrylamide-2-methylpropanesulfonate), Particles composed of organic compounds such as polyacetal resins, epoxy resins, polyester resins, phenol resins, and melamine resins can be used. 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, Examples include particles composed of inorganic compounds such as lithium fluoride, clay, zeolite, and calcium carbonate. The inorganic particles may also be particles composed of composite oxides such as niobium-tantalum composite oxides and magnesium-tantalum composite oxides.
As the insulating fine particles, one of the above particles may be used alone, or two or more thereof may be used in combination. Also, the insulating fine particles may be fine particles containing both an inorganic compound and an organic compound. For example, the insulating fine particles may be inorganic-organic composite particles in which the surfaces of particles made of an organic compound are coated with an inorganic oxide.
Among these, inorganic particles are preferred, and alumina particles and boehmite particles are more preferred.

The average particle size of the insulating fine particles is not particularly limited as long as it is smaller than the thickness of the insulating layer. be. By setting the average particle size of the insulating layer within these ranges, it becomes easier to adjust the porosity within the above range.
The average particle size means the particle size (D50) at 50% volume integration in the particle size distribution of the insulating fine particles determined by the laser diffraction/scattering method.
Also, one type of insulating particles having an average particle size within the above range may be used alone, or two or more types of insulating fine particles having different average particle sizes may be mixed and used.

The content of the insulating fine particles in the insulating layer is preferably 15 to 95% by mass, more preferably 40 to 90% by mass, still more preferably 60 to 85% by mass, based on the total amount of the insulating layer. When the content of the insulating fine particles is within the above range, the insulating layer can have a uniform porous structure, and more appropriate insulating properties are imparted to the insulating layer.

(Binder for insulating layer)
The binder for the insulating layer is polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), fluorine-containing resin such as polytetrafluoroethylene (PTFE), polymethyl acrylate (PMA), poly Acrylic resins such as methyl methacrylate (PMMA), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polyethernitrile (PEN), polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene-butadiene rubber (SBR), poly(meth)acrylic acid, carboxymethylcellulose (CMC), hydroxyethylcellulose, and polyvinyl alcohol. As the insulating layer binder, these resins may be used singly or in combination of two or more. Carboxymethylcellulose or the like may also be used in the form of a salt such as a sodium salt as the insulating layer binder.

The content of the insulating layer binder in the insulating layer is preferably 5 to 50% by mass, more preferably 10 to 45% by mass, still more preferably 15 to 40% by mass based on the total amount of the insulating layer. Within the above range, a uniform porous structure can be formed in the insulating layer, and appropriate insulating properties can be imparted to the insulating layer.
The insulating layer may contain optional components other than the insulating fine particles and the insulating layer binder within a range that does not impair the effects of the present invention. However, the total content of the insulating fine particles and the binder for the insulating layer is preferably 85% by mass or more, more preferably 90% by mass or more, of the total mass of the insulating layer.

The thickness of the insulating layer is preferably 1 to 10 μm. When the thickness of the insulating layer is 10 μm or less, an increase in resistance due to the insulating layer is suppressed, and cycle characteristics are improved. In addition, when the thickness of the insulating layer is 1 μm or more, the coverage of the electrode active material layer with the insulating layer is increased, and the short circuit suppressing effect is improved. From the viewpoint of these cycle characteristics and short-circuit suppressing effect, the thickness of the insulating layer is more preferably 1.5 to 8.5 μm, still more preferably 3 to 7 μm.

The insulating layer has a porous structure as described above, and its porosity is preferably 50 to 90%. When the porosity is 90% or less, the coverage of the electrode active material layer with the insulating layer is increased, and the short circuit suppressing effect is improved. When the porosity is 50% or more, an increase in resistance due to the insulating layer is suppressed, and cycle characteristics are improved. From the viewpoint of these short-circuit suppressing effects and cycle characteristics, the porosity of the insulating layer is more preferably 60 to 85%, and even more preferably 70 to 80%.

[Electrode active material layer]
The electrode active material layer includes an electrode active material and carbon nanotubes. The electrode active material layer may further contain an electrode binder.
Further, when the electrode is a positive electrode, the electrode active material becomes the positive electrode active material, and the electrode active material layer becomes the positive electrode active material layer. On the other hand, when the electrode is the negative electrode, the electrode active material becomes the negative electrode active material, and the electrode active material layer becomes the negative electrode active material layer.

The thickness of the electrode active material layer is not particularly limited, but is preferably 10 to 100 μm, more preferably 20 to 80 μm per side of the electrode current collector.

(carbon nanotube)
A carbon nanotube is a tubular substance with a diameter on the order of nanometers. A carbon nanotube is composed of a carbon element and has a structure in which a sheet in which benzene rings are spread out on a plane is rolled into a cylindrical shape.
The electrode active material layer contains carbon nanotubes as part of or as a conductive aid. As a result, it is possible to suppress migration of the electrode active material and the conductive aid to the insulating layer. Further, deterioration of the insulating properties of the insulating layer is suppressed, and the insulating reliability of the insulating layer is improved.

Although the following reasons do not limit the present invention, the migration of the electrode active material and the conductive aid to the insulating layer can be suppressed by including the carbon nanotubes as part or all of the conductive aid. This is thought to be the reason. Carbon nanotubes are very elongated and flexible materials. Therefore, the carbon nanotubes are expected to be entangled with the electrode active material, with other carbon nanotubes, or with other conductive aids (for example, carbon black) other than the carbon nanotubes. It is believed that this can prevent the electrode active material and conductive aid in the electrode active material layer from migrating to the insulating layer during electrode formation.

Carbon nanotubes are roughly classified into single-walled carbon nanotubes and multi-walled carbon nanotubes. A single-walled carbon nanotube has a structure in which a single-walled sheet is rolled into a cylindrical shape, and a multi-walled carbon nanotube has a plurality of coaxially stacked carbon nanotubes.
Since single-walled carbon nanotubes are more flexible than multi-walled carbon nanotubes, the carbon nanotubes in the electrode active material layer preferably contain single-walled carbon nanotubes. As a result, the carbon nanotubes can be more reliably entangled with the electrode active material, other carbon nanotubes, conductive aids other than carbon nanotubes, and the like. As a result, migration of the electrode active material and the conductive aid in the electrode active material layer to the insulating layer during electrode formation can be more reliably suppressed.

The content of single-walled carbon nanotubes in the carbon nanotubes in the electrode active material layer is preferably 60-100% by mass, more preferably 70-100% by mass, based on the total amount of carbon nanotubes. When the content of single-walled carbon nanotubes in the carbon nanotubes is 60 to 100% by mass, the insulation reliability of the insulating layer can be further improved.

Examples of methods for producing carbon nanotubes include an arc discharge method, a laser vaporization method (laser ablation method), a chemical vapor deposition method (CVD method), and the like. A preferred method for producing carbon nanotubes is the CVD method, from the viewpoint of being excellent in mass productivity and from the viewpoint of being able to increase the content of single-walled carbon nanotubes in the carbon nanotubes.

From the viewpoint of facilitating dispersion of the carbon nanotubes in the electrode active material layer and further improving the insulation reliability of the insulating layer, the average length of the carbon nanotubes is, for example, 60 μm or less, preferably 10 μm or less. Yes, more preferably 0.5 to 10 μm, still more preferably 1 to 8 μm.
The average length of carbon nanotubes can be measured, for example, as follows. A small amount of the dispersion liquid is dropped on the mica cleavage surface and dried, and the length of the surface is measured using an AFM (atomic force microscope), and the average value of 30 or more carbon nanotubes is calculated. , the average length of the carbon nanotubes can be obtained.

From the viewpoint of making the carbon nanotubes more easily entangled and further improving the insulation reliability of the insulating layer, the average value of the ratio of the length to the diameter of the carbon nanotubes (length/diameter) is, for example, 100 or more, It is preferably 500 or more, more preferably 600 to 10,000, still more preferably 1,000 to 7,000. The average diameter of carbon nanotubes can be measured, for example, as follows. TEM (Transmission Electron Microscopy) is used to measure the diameter of 30 carbon nanotubes, and the average value of these values is calculated to obtain the average diameter of the carbon nanotubes. Then, the average length to diameter ratio (length/diameter) of the carbon nanotubes can be calculated by dividing the average length of the carbon nanotubes by the average diameter of the carbon nanotubes.

From the viewpoint of further improving the insulation reliability of the insulating layer, the content of carbon nanotubes in the electrode active material layer is, for example, 0.05 to 3% by mass, preferably 0.05, based on the total amount of the electrode active material layer. 0.15% by mass, more preferably 0.07 to 0.13% by mass.

(Conductive agents other than carbon nanotubes)
The electrode active material layer may contain a conductive aid other than carbon nanotubes. A material having higher conductivity than the electrode active material is usually used as the conductive aid other than carbon nanotubes. Carbon black is preferable as the conductive aid other than carbon nanotubes, from the viewpoints of being able to impart high conductivity to the electrode active material layer, and of being the same carbon material as carbon nanotubes and being easily captured by carbon nanotubes. Examples of carbon black include thermal black, acetylene black, channel black, lamp black, gas furnace black, oil furnace black and ketjen black. One of these carbon blacks may be used alone as the conductive aid, or two or more thereof may be used in combination. Among these carbon blacks, at least one of furnace black and acetylene black is more preferable.

When the electrode active material layer contains carbon black, from the viewpoint of being able to impart high conductivity to the electrode active material layer and from the viewpoint of the amount of carbon black that can be captured by carbon nanotubes, the content of carbon black is Based on the total amount, it is preferably 0.5 to 5.0% by mass, more preferably 1.0 to 3.0% by mass.
Further, from the viewpoint that the carbon black is easily entangled with the carbon nanotubes, it is preferable that the carbon black has a well-developed structure (aggregate).
The porosity between individual aggregates of carbon black has a positive correlation with the structure, and the structure is indirectly evaluated by DBP (a kind of plasticizer and an abbreviation of Di-Butyl Phthalate) absorption (ml / 100g). is possible.
Specifically, it can be measured according to JIS K 6217 (Test method for basic performance of carbon black for rubber).
The DBP feeding amount (ml/100 g) of carbon black is preferably 50-700, more preferably 100-550.

The conductive aid may contain conductive aids other than carbon nanotubes and carbon black within a range that does not impair the effects of the present invention. However, of the total mass of the conductive aid, the total content of carbon nanotubes and carbon black is preferably 85% by mass or more, more preferably 90% by mass or more, and 95% by mass or more. is more preferred.

(Binder for electrodes)
The electrode active material layer has, for example, a structure in which an electrode active material and a conductive aid are bound together by an electrode binder. Specific examples of the electrode binder include the resins described above as resins that can be used as the insulating layer binder. As the electrode binder, one of the above resins may be used alone, or two or more of them may be used in combination.

(Average particle size and content of electrode active material)
From the viewpoint of ease of entanglement with carbon nanotubes, the viewpoint of securing the diffusion path of lithium, and the viewpoint of improving the electronic conductivity of the electrode active material, the average particle diameter of the electrode active material is preferably 0.2 μm or more, for example. is 0.2 to 15 μm, more preferably 0.5 to 15 μm, still more preferably 1 to 15 μm.
When the electrode active material is a positive electrode active material, the average particle size of the electrode active material is, for example, 7 μm or more, preferably 8 to 15 μm.
On the other hand, when the electrode active material is a negative electrode active material, the average particle size of the electrode active material is preferably 1 to 25 μm, more preferably 5 to 20 μm.
Also in the case of the electrode active material, the average particle size means the particle size (D50) at 50% volume integration in the particle size distribution of the electrode active material obtained by the laser diffraction method.
The content of the electrode active material in the electrode active material layer is preferably 50 to 98.5% by mass, more preferably 60 to 98% by mass, based on the total amount of the electrode active material layer.

(Positive electrode active material)
As described above, when the electrode is the positive electrode, the electrode active material becomes the positive electrode active material, and the electrode active material layer becomes the positive electrode active material layer.
Examples of positive electrode active materials used in the positive electrode active material layer include lithium metal oxide compounds. Examples of lithium metal oxide compounds include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like. Moreover, olivine-type lithium iron phosphate (LiFePO ₄ ) or the like may be used as the positive electrode active material. Furthermore, as the positive electrode active material, a material using a plurality of metals other than lithium may be used, and NCM (nickel-cobalt-manganese)-based oxides called ternary system, NCA (nickel-cobalt-aluminum-based) oxides, and the like may be used. may be used. As the positive electrode active material, one of these substances may be used alone, or two or more of them may be used in combination.
In addition, since the positive electrode active material layer contains carbon nanotubes, the surface area of the positive electrode active material layer is increased, and the capacity of the lithium ion secondary battery is increased. Furthermore, flexibility is imparted to the positive electrode.

(Negative electrode active material)
As described above, when the electrode is the negative electrode, the electrode active material becomes the negative electrode active material, and the electrode active material layer becomes the negative electrode active material layer.
Examples of the negative electrode active material used in the negative electrode active material layer include carbon materials such as graphite and hard carbon, composites of tin compounds, silicon and carbon, and lithium. Among these, carbon materials are preferred, and graphite is preferred. more preferred. As the negative electrode active material, one of the above substances may be used alone, or two or more of them may be used in combination.
By including carbon nanotubes in the negative electrode active material layer, the strength of the negative electrode increases, the cycle characteristics of the lithium ion secondary battery improve, and the amount of power increases.

(Optional component)
The electrode active material layer may contain optional components other than the electrode active material, the conductive aid, and the electrode binder within the range that does not impair the effects of the present invention. However, in the total mass of the electrode active material layer, the total content of the electrode active material, conductive aid and electrode binder is preferably 90% by mass or more, more preferably 95% by mass or more.

(electrode current collector)
Materials constituting the electrode current collector include, for example, conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum, titanium, nickel and stainless steel are preferable, and aluminum is more preferable when the electrode current collector is a positive electrode current collector. When the electrode current collector is a negative electrode current collector, copper, titanium, nickel and stainless steel are preferred, and copper is more preferred. The electrode current collector is generally made of metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

<Method for producing electrode for lithium ion secondary battery>
Next, one embodiment of the manufacturing method for manufacturing the lithium ion secondary battery electrode of the present invention will be described in detail. In addition, the lithium ion secondary battery electrode of the present invention is not limited to one produced by the following production method.
In one embodiment of the method for producing a lithium ion secondary battery electrode of the present invention, for example, first, an electrode active material layer is formed, and the insulating layer composition is applied on the surface of the electrode active material layer. forming an insulating layer;

(Formation of electrode active material layer)
In forming the electrode active material layer, first, an electrode active material layer composition containing an electrode active material, carbon nanotubes, an electrode binder, and a solvent is prepared. The composition for an electrode active material layer may contain other components such as carbon black blended as necessary. The electrode active material, carbon nanotube, carbon black, electrode binder, etc. are as described above. The electrode active material layer composition is in the form of slurry.

Solvents used in the electrode active material layer composition include, for example, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, tetrahydrofuran, toluene, isopropyl alcohol, N-methylpyrrolidone (NMP), ethanol and water. Among them, N-methylpyrrolidone (NMP) and water are preferred. These solvents may be used alone or in combination of two or more.
The solid content concentration of the electrode active material layer composition is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

The electrode active material layer may be formed by a known method using the electrode active material layer composition. For example, the electrode active material layer composition is applied onto an electrode current collector and dried. can be formed by
Alternatively, the electrode active material layer may be formed by applying the electrode active material layer composition onto a substrate other than the electrode current collector and drying the composition. Examples of substrates other than the electrode current collector include known release sheets. The electrode active material layer formed on the base material may be peeled off from the base material and transferred onto the electrode current collector, preferably after forming an insulating layer on the electrode active material layer.
The electrode active material layer formed on the electrode current collector or substrate is preferably pressure-pressed. By pressurizing, it is possible to increase the electrode density. Pressure pressing can be performed by a roll press or the like.

(Formation of insulating layer)
The insulating layer composition used for forming the insulating layer contains insulating fine particles, an insulating layer binder, and a solvent. In addition, the composition for insulating layers may contain other arbitrary components mix|blended as needed. The details of the insulating fine particles, the insulating layer binder, and the like are as described above. The insulating layer composition is in the form of slurry.
Solvents used in the insulating layer composition include, for example, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, tetrahydrofuran, toluene, isopropyl alcohol, N-methylpyrrolidone (NMP), ethanol and water. Among them, N-methylpyrrolidone (NMP) and water are preferred. These solvents may be used alone or in combination of two or more.

The solid content concentration of the insulating layer composition is preferably 5 to 75% by mass, more preferably 15 to 50% by mass. The viscosity of the insulating layer composition is preferably 1000 to 3000 mPa·s, more preferably 1700 to 2300 mPa·s. By setting the viscosity and the solid content viscosity within the above ranges, it becomes easier to form an insulating layer having a predetermined thickness. The viscosity is the viscosity measured at 60 rpm and 25° C. with a Brookfield viscometer.

The insulating layer can be formed by applying an insulating layer composition onto an electrode current collector and drying the composition. The method of applying the insulating layer composition to the surface of the electrode active material layer is not particularly limited, and examples thereof include dip coating, spray coating, roll coating, doctor blade method, bar coating, gravure coating, and screen printing. law, etc. Among these, the bar coating method or the gravure coating method is preferable from the viewpoint that the insulating layer composition can be applied thinly and uniformly.
The drying temperature is not particularly limited as long as the solvent can be removed, but is, for example, 40 to 120°C, preferably 50 to 90°C. Also, the drying time is not particularly limited, but is, for example, 30 seconds to 10 minutes.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes the lithium ion secondary battery electrode having the insulating layer described above as a positive electrode. For example, the lithium-ion secondary battery of the present invention may have a positive electrode and a negative electrode arranged to face each other. Then, the positive electrode becomes the lithium-ion secondary battery electrode having the insulating layer described above. In this positive electrode, for example, an insulating layer is provided on the surface facing the negative electrode.

The lithium ion secondary battery of the present invention preferably further comprises a separator arranged between the positive electrode and the negative electrode. By providing the separator, a short circuit between the positive electrode and the negative electrode can be prevented more effectively. Also, the separator may hold an electrolyte, which will be described later. The insulating layer provided on the positive electrode or negative electrode may or may not be in contact with the separator, but is preferably in contact.
Examples of the separator include porous polymer membranes, non-woven fabrics, glass fibers, etc. Among these, porous polymer membranes are preferred. An olefin-based porous film is exemplified as the porous polymer film. The separator may be heated by heat generated when the lithium-ion secondary battery is driven, and may thermally shrink. However, even during such thermal contraction, the provision of the insulating layer suppresses the occurrence of a short circuit.
Moreover, the separator may be omitted in the lithium ion secondary battery of the present invention. Even if the separator is omitted, insulation between the negative electrode and the positive electrode can be ensured by the insulating layer provided on at least one of the negative electrode and the positive electrode.

The lithium-ion secondary battery of the present invention may have a multilayer structure in which a plurality of negative electrodes and positive electrodes are laminated. In this case, the negative electrodes and the positive electrodes may be alternately provided along the stacking direction. Also, if a separator is used, the separator may be placed between each negative electrode and each positive electrode.
In the lithium-ion secondary battery of the present invention, the negative electrode and positive electrode, or the negative electrode, positive electrode, and separator are housed in a battery cell, for example. The battery cell may be rectangular, cylindrical, laminated, or the like.

The lithium ion secondary battery of the present invention may have an electrolyte. The electrolyte is not particularly limited, and known electrolytes used in lithium ion secondary batteries can be used. As the electrolyte, for example, an electrolytic solution can be used.
As the electrolytic solution, an electrolytic solution containing an organic solvent and an electrolytic salt can be exemplified. Examples of organic solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethylsulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane. , tetrahydrafuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, or a mixture of two or more of these solvents. As electrolyte salts, LiClO4, _LiPF6 , _{LiBF4 , LiAsF6} , _LiCF6 , _LiCF3CO2 , _{LiPF6SO3 , LiN ( SO3CF3} ) ₂ , Li _{( SO2CF2CF3} ) _{2 ,} LiN(COCF ₃ ) ₂ , LiN(COCF ₂ CF ₃ ) ₂ , lithium bisoxalate borate (LiB(C ₂ O ₄ ) ₂ , and the like, and salts containing lithium. Complexes can also be used, such as salt-boron trifluoride complexes, complex hydrides such as LiBH _4. As electrolytes, these salts and complexes may be used singly or in combination of two or more. You may mix and use it.
Further, the electrolyte may be a gel electrolyte formed by adding a polymer compound to the electrolyte solution. Examples of the polymer compound include fluorine-based polymers such as polyvinylidene fluoride and polyacrylic polymers such as polymethyl(meth)acrylate. Since the gel electrolyte functions as a separator, the separator may not be provided when the gel electrolyte is used as the electrolyte.

The electrolyte may be placed between the negative electrode and the positive electrode. For example, the electrolyte is filled in a battery cell containing therein the negative electrode and positive electrode, or the negative electrode, the positive electrode, and the separator. Alternatively, the electrolyte may be applied, for example, on the negative electrode or the positive electrode and placed between the negative electrode and the positive electrode.

EXAMPLES The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

The obtained lithium ion secondary battery was evaluated by the following evaluation methods.
(Insulation reliability test of insulation layer)
After forming an insulating layer on the positive electrode, the volume resistance of the insulating layer was measured using a "super megohm meter SM7120" manufactured by Hioki Electric Co., Ltd. Measurement was performed at 10 points at a voltage of 0.1 V, and the average value was classified as insulation reliability as follows.
A: 10 ⁴ Ω/cm ³ or more B: 10 ³ Ω/cm ³ or more and less than 10 ⁴ Ω/cm ³ C: more than 10 Ω/cm ³ and less than 10 ³ Ω/cm ³ D: 10 Ω / cm ³ or less

[Methods for producing lithium-ion secondary batteries of Examples 1 to 5 and Comparative Example 1]
(Preparation of positive electrode)
The positive electrode active material, conductive aid, and electrode binder shown in Table 1 were mixed with N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode active material layer slurry adjusted to a solid content concentration of 60% by mass. This positive electrode active material layer slurry was applied to both surfaces of a 15 μm thick aluminum foil as a positive electrode current collector, pre-dried, and vacuum dried at 120°C. After that, it was press-pressed with a roller at a linear pressure of 400 kN/m, and punched into a 100 mm×200 mm square of the electrode size to prepare a positive electrode having positive electrode active material layers on both sides. Of the dimensions, the area coated with the positive electrode active material was 100 mm×180 mm. The thickness of the positive electrode active material layer formed on both sides was 50 μm per side.

(Preparation of negative electrode)
100 parts by mass of graphite (average particle diameter 10 μm) as a negative electrode active material, 1.5 parts by mass of sodium salt of carboxymethyl cellulose (CMC) as an electrode binder, and 1.5 parts by mass of styrene-butadiene rubber (SBR) as another binder. and water as a solvent to obtain a negative electrode active material layer slurry adjusted to a solid content of 50% by mass. This negative electrode active material layer slurry was applied to both sides of a copper foil having a thickness of 12 μm as a negative electrode current collector and vacuum dried at 100°C. After that, the negative electrode current collector coated with the slurry for the negative electrode active material layer on both sides was pressure-pressed with a roller at a linear pressure of 300 kN/m, and further punched into a 110 mm × 210 mm square of the electrode size, and the negative electrode active material was applied to both surfaces. A negative electrode having a layer was prepared. Of the dimensions, the area coated with the negative electrode active material was 110 mm×190 mm. The thickness of the negative electrode active material layer formed on both sides was 50 μm per side.

(Preparation of electrolytic 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 be 1 mol/liter, and the electrolyte solution was obtained. prepared.

(Formation of insulating layer)
Alumina particles (manufactured by Nippon Light Metal Co., Ltd., product name: AHP200, average particle size 0.4 μm) were prepared as insulating fine particles. This is added to a polyvinylidene fluoride solution (manufactured by Kureha Co., Ltd., product name: L#1710, 10% by mass solution, solvent: NMP) while applying a moderate shearing force, based on the solid content, 100 parts by mass of alumina particles , and polyvinylidene fluoride (PVdF) 22 parts by mass, and dispersed to obtain a slurry.
NMP was further added to this slurry so that the solid content concentration was 30% by mass, and the mixture was gently stirred with a stirrer for 30 minutes to prepare an insulating layer slurry. The viscosity of the insulating layer slurry was 2000 mPa·s. The viscosity was measured with a Brookfield viscometer at 60 rpm and 25°C.
The insulating layer slurry is applied to the surface of each positive electrode active material layer of the positive electrode with a bar coater, and the coating film is dried at 60 ° C. for 10 minutes to form an insulating layer on the surface of each positive electrode active material layer. Then, the opposite side was similarly coated to prepare a positive electrode having insulating layers on both sides. The thickness of the insulating layer formed on the surface of each positive electrode active material layer was 4 μm.

(Manufacture of lithium-ion secondary batteries)
A laminate was obtained by laminating 9 positive electrodes each having an insulating layer obtained above, 10 negative electrodes, and 18 separators. Here, the negative electrodes and the positive electrodes were alternately arranged, and a separator was arranged between each negative electrode and the positive electrode. A polyethylene porous film was used as the separator.
The ends of the exposed portions of the positive electrode current collectors of the positive electrodes were collectively joined by ultrasonic fusion, and terminal tabs protruding to the outside were joined. Similarly, the ends of the exposed portions of the negative electrode current collectors of the negative electrodes were collectively joined by ultrasonic fusion, and the terminal tabs protruding to the outside were joined.
Next, the laminate was sandwiched between aluminum laminate films, the terminal tab was protruded outside, and the three sides were sealed by lamination. A laminate type battery was manufactured by injecting the electrolytic solution obtained above from one side that was left unsealed and vacuum-sealing.

[Evaluation results of lithium ion secondary battery]
The evaluation results of the insulation reliability test of the insulating layer in the lithium ion secondary batteries of Examples 1 to 5 and Comparative Example 1 are shown in Table 1 below.

The positive electrode active material, conductive aid, and electrode binder used to prepare the positive electrode are as follows.
(Positive electrode active material)
・NCA: NCA-based (nickel-cobalt-aluminum-based) oxide (average particle size: 13.3 μm or 7.7 μm)
(Conductivity aid)
<Carbon nanotube>
・A: Single-walled carbon nanotube, diameter: 1.8 nm, length: 5 μm, single-walled carbon nanotube content: 74±1.5% by mass or more ・B: multi-walled carbon nanotube, diameter: 150 nm, length: 15 μm
・C: multi-walled carbon nanotube, diameter: 2.5 nm, length: 30 μm
・D: multi-walled carbon nanotube, diameter: 7.5 nm, length: 60 μm
・ E: multi-walled carbon nanotube, diameter: 15.0 nm, length: 9 μm
<Carbon Black>
・A: DBP oil supply amount: 150ml/100g
・B: DBP oil supply amount: 495ml/100g
(Binder for electrodes)
・PVdF: Polyvinylidene fluoride

As shown in Examples 1 to 5 above, the insulation reliability of the insulating layer in the lithium ion secondary batteries of Examples 1 to 5 was excellent because the electrode active material layer contained carbon nanotubes. Moreover, as shown in Example 1, the insulation reliability of the insulating layer in the lithium-ion secondary battery of Example 1 was further improved because the carbon nanotubes contained single-walled carbon nanotubes. On the other hand, in Comparative Example 1, since the electrode active material layer did not contain carbon nanotubes, the insulation reliability of the insulating layer was insufficient.

10 Lithium ion secondary battery electrode 11 Electrode active material layer 12 Insulating layer 13 Electrode current collector

Claims (9)

An electrode active material layer and an insulating layer provided on the surface of the electrode active material layer,
The insulating layer contains insulating fine particles and an insulating layer binder,
the electrode active material layer comprises an electrode active material and carbon nanotubes;
An electrode for a lithium ion secondary battery, wherein the carbon nanotube comprises a single-walled carbon nanotube . 2. The electrode for a lithium ion secondary battery according to claim 1, wherein the carbon nanotubes have an average length of 10 [mu]m or less. 3. The electrode for a lithium ion secondary battery according to claim 1, wherein the carbon nanotube has an average ratio of length to diameter (length/diameter) of 500 or more. The electrode for a lithium ion secondary battery according to any one of claims 1 to 3 , wherein the content of said carbon nanotubes is 0.05 to 0.15% by mass based on the total amount of said electrode active material layer. The electrode for a lithium ion secondary battery according to any one of claims 1 to 4 , wherein said electrode active material layer further contains carbon black. 6. The electrode for a lithium ion secondary battery according to claim 5 , wherein the DBP oiling amount of said carbon black is 50 to 700 ml/100 g. 7. The electrode for a lithium ion secondary battery according to claim 5 , wherein the content of said carbon black is 0.5 to 5.0% by mass based on the total amount of said electrode active material layer. The electrode for a lithium ion secondary battery according to any one of claims 1 to 7 , wherein the electrode active material in the electrode active material layer has an average particle size of 0.2 to 15 µm. An electrode active material layer and an insulating layer provided on the surface of the electrode active material layer,
The insulating layer contains insulating fine particles and an insulating layer binder,
A lithium ion secondary battery comprising, as a positive electrode, a lithium ion secondary battery electrode in which the electrode active material layer includes an electrode active material and carbon nanotubes .

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