WO2020054849A1

WO2020054849A1 - Lithium-ion secondary battery electrode, lithium-ion secondary battery, and production method for lithium-ion secondary battery electrode

Info

Publication number: WO2020054849A1
Application number: PCT/JP2019/036115
Authority: WO
Inventors: 寛大奥田; 和徳小関
Original assignee: 積水化学工業株式会社
Priority date: 2018-09-14
Filing date: 2019-09-13
Publication date: 2020-03-19
Also published as: JPWO2020054849A1; JP6876879B2

Abstract

This lithium-ion secondary battery electrode (1) is provided with an electrode active material layer (10) and an insulating layer (20) disposed on a surface of the electrode active material layer (10). The insulating layer (20) has: two or more laminar dense regions (21), (23), (25); and laminar sparse regions (22), (24) each sandwiched between two adjoining dense regions (21), (23), (25) and having a higher porosity than those of the two adjoining dense regions (21), (23), (25). According to the present invention, a lithium-ion secondary battery electrode which features improved safety against short-circuiting by means of an insulating layer and exhibits good lithium-ion secondary battery output characteristics can be provided.

Description

Electrode for lithium ion secondary battery, lithium ion secondary battery, and method of manufacturing electrode for lithium ion secondary battery

The present invention relates to an electrode for a lithium ion secondary battery including an insulating layer, a lithium ion secondary battery including the electrode for a lithium ion secondary battery, and a method for manufacturing an electrode for a lithium ion secondary battery.

Lithium ion secondary batteries are used as power sources for large stationary power storage devices, power sources for electric vehicles, and the like. In recent years, research into miniaturization and thinning of batteries has been progressing. A lithium ion secondary battery generally includes two electrodes each having an electrode active material layer formed on a surface of a metal foil, and a separator disposed between the two electrodes. The separator plays a role in preventing a short circuit between the two electrodes and holding the electrolytic solution.
Conventionally, lithium ion secondary batteries, for example, even when the separator shrinks, for example, in order to have a good short-circuit suppression function, it has been studied that a porous insulating layer is provided on the surface of the electrode active material layer. I have. For example, as disclosed in Patent Document 1, the insulating layer is known to be formed by applying an insulating layer slurry containing insulating fine particles, a binder and a solvent on the electrode active material layer and drying the slurry. Have been.

WO 2016/104782

However, the conventional insulating layer may have insufficient safety against short-circuits, and needs to further improve safety against short-circuits. Further, the output characteristics of a conventional lithium ion secondary battery having an insulating layer may be insufficient, and it is necessary to further improve the output characteristics.
Therefore, an object of the present invention is to provide an electrode for a lithium ion secondary battery which can improve the safety against short circuit by the insulating layer and improve the output characteristics of the lithium ion secondary battery.

The present inventors have made intensive studies and found that by forming a sparse region and a dense region in the insulating layer, the safety against short circuit of the insulating layer is improved and the output characteristics of the lithium ion secondary battery are improved. The present invention described below has been completed. The gist of the present invention is the following [1] to [13].
[1] An electrode active material layer and an insulating layer provided on a surface of the electrode active material layer, wherein the insulating layer is sandwiched between at least two or more layered dense regions and two adjacent dense regions. An electrode for a lithium ion secondary battery having a layered sparse region having a higher porosity than the two adjacent dense regions.
[2] The lithium ion secondary battery according to [1], wherein the insulating layer has two or more sparse regions and two or more dense regions, and the sparse regions and the dense regions are alternately arranged. Electrodes.
[3] The electrode for a lithium ion secondary battery according to [2], wherein the insulating layer has two or three sparse regions alternately arranged with the dense regions.
[4] The electrode for a lithium ion secondary battery according to [3], wherein the insulating layer has two of the sparse regions alternately arranged with the dense regions.
[5] The electrode for a lithium ion secondary battery according to any one of [1] to [4], wherein the porosity of the sparse region is 30% by volume or more.
[6] The electrode for a lithium ion secondary battery according to any one of [1] to [5], wherein the thickness of the insulating layer is 7 to 30 μm.
[7] The electrode for a lithium ion secondary battery according to any one of the above [1] to [6], wherein the insulating layer contains insulating fine particles and a binder for the insulating layer.
[8] The electrode for a lithium ion secondary battery according to any one of the above [1] to [7], wherein the electrode active material layer is a negative electrode active material layer.
[9] A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of the above [1] to [8].
[10] The lithium ion secondary battery according to the above [9], comprising a negative electrode and a positive electrode, wherein the negative electrode is the electrode for the lithium ion secondary battery.
[11] a step of applying a composition for a first insulating layer on the surface of the electrode active material layer to form a first composition layer, and forming a second composition on the surface of the first composition layer. Forming a second composition layer by applying the composition for an insulating layer of (a), wherein the step of forming the second composition layer has a high porosity in the second composition layer. Further, a region having a higher porosity than a region of the first composition layer near the second composition layer is formed in a region of the second composition layer near the first composition layer. As described above, a method for producing an electrode for a lithium ion secondary battery in which the second composition for an insulating layer is applied.
[12] The electrode for a lithium ion secondary battery according to the above [11], wherein the second insulating layer composition has a shear rate of 10,000 to 5,000,000 (1 / s) when applied. Production method.
[13] The method for producing an electrode for a lithium ion secondary battery according to the above [11] or [12], wherein the viscosity at the time of application of the second composition for an insulating layer is 2000 to 4000 mPa · s.

According to the present invention, it is possible to provide an electrode for a lithium ion secondary battery capable of improving safety against a short circuit by an insulating layer and improving output characteristics of the lithium ion secondary battery.

FIG. 1 is a schematic sectional view showing a preferred embodiment of the electrode for a lithium ion secondary battery of the present invention. FIG. 2 is a schematic cross-sectional view for explaining a step of forming a first composition layer in the method for producing an electrode for a lithium ion secondary battery of the present invention. FIG. 3 is a schematic cross-sectional view for explaining a step of forming a second composition layer in the method for producing an electrode for a lithium ion secondary battery of the present invention. FIG. 4 is a schematic cross-sectional view for explaining an electrode for a lithium ion secondary battery in which a third composition layer is formed on a second composition layer. FIG. 5 is a diagram for explaining a method of measuring the porosity of the sparse region and the dense region. FIG. 6 is a diagram for explaining a method of determining a sparse region and a dense region from the space ratio obtained by measurement.

<Electrode for lithium ion secondary battery>
Hereinafter, the electrode for a lithium ion secondary battery of the present invention will be described in detail.
As shown in FIG. 1, the electrode 1 for a lithium ion secondary battery includes an electrode active material layer 10 and an insulating layer 20 provided on a surface of the electrode active material layer 10. In the electrode 1 for a lithium ion secondary battery, the electrode active material layer 10 is usually laminated on the electrode current collector 30.
The electrode active material layers 10 may be laminated on both surfaces of the electrode current collector 30, and in that case, the insulating layer 20 may be provided on the surface of each electrode active material layer 10. As described above, when the insulating layer 20 is provided on both surfaces of the electrode 1 for a lithium ion secondary battery, a short circuit between each positive electrode and each negative electrode can be effectively prevented even when a plurality of negative electrodes and positive electrodes are laminated to form a multilayer structure. it can.

In the present invention, the electrode 1 for a lithium ion secondary battery may be either a negative electrode or a positive electrode, but is preferably a negative electrode. Hereinafter, the configuration in the case where the electrode 1 for a lithium ion secondary battery is a negative electrode will be described in detail.

[Insulating layer]
FIG. 1 shows a preferred embodiment of the insulating layer 20. The insulating layer 20 according to the embodiment shown in FIG. 1 has three layered

dense regions

21, 23, 25. Thereby, safety against short circuit of the insulating layer 20 can be improved. The insulating layer 20 has two layered sparse regions 22, 24 sandwiched between two adjacent

dense regions

21, 23, 25 and having a higher porosity than the adjacent two

dense regions

21, 23, 25. . As a result, the electrolyte can sufficiently penetrate into the electrode active material layer 10 through the sparse regions 22 and 24, so that the output characteristics of the lithium ion secondary battery can be improved. Therefore, since the insulating layer 20 has the

dense regions

21, 23, 25 and the sparse regions 22, 24, the safety against short circuit can be improved and the output characteristics of the lithium ion secondary battery can be improved.

However, the electrode 1 for a lithium ion secondary battery shown in FIG. 1 is an example, and from the viewpoint that the safety against a short circuit can be improved and the output characteristics of the lithium ion secondary battery can be improved, It is only necessary to have at least one sparse region sandwiched between two adjacent dense regions. From the same viewpoint, the insulating layer preferably has two or more sparse regions, more preferably two or three sparse regions. As in the lithium ion secondary battery electrode 1 shown in FIG. More preferably, it has two sparse regions. In these cases, the sparse regions and the dense regions may be alternately arranged. In addition, from the viewpoint of safety against a short circuit, the number of sparse regions included in the insulating layer is preferably 4 or less, and more preferably 3 or less.

From the viewpoint that the output characteristics of the lithium ion secondary battery can be improved, the porosity of each sparse region is preferably 30% by volume or more. Further, from the viewpoint that the safety against short circuit can be improved, the porosity of each sparse region is more preferably 30 to 60% by volume, still more preferably 30 to 42% by volume, and more preferably 30 to 37% by volume. % Is particularly preferred.
In addition, from the viewpoint that safety against short circuits can be improved, the porosity of each dense region is preferably less than 30 vol%, more preferably 29 vol% or less, and 28 vol% or less. Is more preferred.
Note that the porosity of the sparse region and the dense region in the insulating layer can be measured by, for example, a method described in Examples described later.

Any difference between the porosity of each sparse region and each of two dense regions adjacent to the sparse region is preferably 2% by volume or more, more preferably 3% by volume or more, and further preferably 5% by volume or more. When the difference in the porosity between each sparse region and the adjacent dense region is 2% by volume or more, the safety against short circuit can be further improved and the output characteristics of the lithium ion secondary battery can be further improved.

From the viewpoint that the safety against short circuit can be improved, the region closest to the electrode active material layer in the insulating layer is preferably a dense region (that is, the dense region 21 in FIG. 1). Further, from a similar viewpoint, it is preferable that the region farthest from the electrode active material layer in the insulating layer is also a dense region (that is, the dense region 25 in FIG. 1).

中で It is preferable that the dense area closest to the electrode active material layer has the highest porosity among the plurality of dense areas included in the insulating layer. It is considered that such a configuration accelerates the diffusion of the electrolytic solution into the active material, and improves the output characteristics. Further, it is preferable that the porosity of the plurality of dense regions included in the insulating layer decreases as the position of the dense region is away from the electrode active material layer. It is considered that such a configuration improves output characteristics. Further, among a plurality of dense regions included in the insulating layer, the porosity of the dense region farthest from the electrode active material layer is preferably the lowest. It is considered that such a configuration improves output characteristics.

厚み The thickness of the insulating layer is preferably 7 to 30 μm, more preferably 7 to 20 μm, and further preferably 10 to 20 μm. By setting the thickness of the insulating layer to 30 μm or less, an increase in resistance due to the insulating layer is suppressed, and the output characteristics of the lithium ion secondary battery are improved. Further, when the thickness is 7 μm or more, the coverage of the electrode active material layer by the insulating layer is increased, and the safety against short circuit is improved.

The insulating layer preferably contains insulating fine particles and a binder for the insulating layer. Thereby, an insulating layer having a porous structure can be easily formed.

(Insulating fine particles)
The insulating fine particles are not particularly limited as long as they are insulating, and may be 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 alumina particles and boehmite particles are particularly preferable.

The average particle diameter of the insulating fine particles is usually smaller than the average particle diameter of the electrode active material, for example, 0.001 to 2 μm, preferably 0.05 to 1.5 μm, more preferably 0.1 to 1. 0 μm. By setting the average particle diameter of the insulating layer within these ranges, the porosity can be easily adjusted within the above range. Further, when the content is not more than these upper limits, the surface of each composition layer to be described later becomes gently uneven, and a sparse region is easily formed.
The average particle size means the particle size (D50) at a volume integration of 50% in the particle size distribution of the insulating fine particles obtained by the laser diffraction / scattering method.
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 used as a mixture.

含有 The content of the insulating fine particles contained in the insulating layer is preferably 15 to 95% by mass, more preferably 40 to 90% by mass, and 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, appropriate insulating properties are imparted to the insulating layer.

(Binder for insulating layer)
Insulating layer binders include fluorine-containing resins such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene (PTFE), polymethyl acrylate (PMA), and polymethyl acrylate (PMA). Acrylic resin such as methyl methacrylate (PMMA), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene-butadiene rubber, poly (meth) acrylic acid, carboxymethylcellulose, hydroxyethylcellulose, polyvinyl alcohol and the like. These binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose and the like may be used in the form of a salt such as a sodium salt.
The binder for the insulating layer is preferably polyvinylidene fluoride.

The content of the binder for the insulating layer contained in the insulating layer is preferably 5 to 50% by mass, more preferably 10 to 45% by mass, and still more preferably 15 to 40% by mass, based on the total amount of the insulating layer. When the content is within the above range, appropriate insulating properties can be imparted to the insulating layer.
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. However, of the total mass of the insulating layer, the total content of the insulating fine particles and the binder for the insulating layer is preferably 85% by mass or more, and more preferably 90% by mass or more.

The weight average molecular weight (Mw) of the binder for the insulating layer contained in the insulating layer is preferably from 10,000 to 10,000,000, and more preferably from 100,000 to 1,000,000. When the Mw of the insulating layer binder is within the above range, a sparse region is easily formed in the insulating layer.
In addition, Mw of the binder for the insulating layer is a value measured by a known gel filtration chromatography method (GPC method) using a polystyrene standard substance.

(Electrode active material layer)
The electrode active material layer typically contains an electrode active material and a binder for an electrode. When the electrode is a negative electrode, the electrode active material becomes a negative electrode active material, and the electrode active material layer becomes a negative electrode active material layer.
Examples of the negative electrode active material used in the negative electrode active material layer include graphite, a carbon material such as hard carbon, a composite of a tin compound and silicon and carbon, and lithium. Among these, a carbon material is 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 average particle diameter of the electrode active material is not particularly limited, but is usually larger than the insulating fine particles, preferably 0.5 to 50 μm, more preferably 1 to 30 μm, and more preferably 6 to 25 μm. Is more preferable.
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.

The electrode active material layer may contain a conductive assistant. As the conductive additive, a material having higher conductivity than the above-mentioned 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 a conductive additive is contained in the electrode active material layer, the content of the conductive additive is preferably 1 to 30% by mass, and more preferably 2 to 25% by mass, based on the total amount of the electrode active material layer. Is more preferred.

The electrode active material layer is formed by binding an electrode active material or an electrode active material and a conductive auxiliary with an electrode binder. Specific examples of the electrode binder include the compounds exemplified as the compounds usable in the insulating layer binder. The electrode binder is preferably carboxymethyl cellulose, styrene butadiene rubber, acrylic rubber and nitrile butadiene rubber, more preferably carboxymethyl cellulose and styrene butadiene rubber.
The binder used for the electrode active material layer may be used alone or in combination of two or more. The content of the electrode binder in the electrode active material layer is preferably from 1.5 to 40% by mass, more preferably from 2.0 to 25% by mass, based on the total amount of the electrode active material layer.

The thickness of the electrode active material layer is not particularly limited, but is preferably from 10 to 100 μm, more preferably from 20 to 80 μm, per one surface of the electrode current collector.
The electrode active material layer may contain other optional components other than the electrode active material, the conductive additive, and the electrode binder as long as the effects of the present invention are not impaired. However, of the total mass of the electrode active material layer, the total content of the electrode active material, the conductive additive, and the electrode binder is preferably 90% by mass or more, and more preferably 95% by mass or more. .

(Electrode current collector)
When the electrode is a negative electrode, the electrode current collector becomes a negative electrode current collector. Examples of a material constituting the negative electrode current collector (electrode current collector) include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum or copper is preferable, Copper is more preferred. The electrode current collector is generally made of a 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, a method for producing an electrode for a lithium ion secondary battery of the present invention will be described in detail. In the method for producing an electrode for a lithium ion secondary battery of the present invention, a step of applying a composition for a first insulating layer on a surface of an electrode active material layer to form a first composition layer; Forming a second composition layer by applying a composition for a second insulating layer on the surface of the composition layer. In the step of forming the second composition layer, the porosity is high in the second composition layer, and the porosity is higher than the area of the first composition layer in the vicinity of the second composition layer. The second insulating layer composition is applied such that a region having a high N is formed in a region of the second composition layer near the first composition layer. Thereby, the electrode for a lithium ion secondary battery of the present invention can be manufactured.

(Step of Forming First Composition Layer)
In the step of forming the first composition layer, for example, as shown in FIG. 2, a first insulating layer composition is applied on the surface of the electrode active material layer 10 to form a first composition layer 41. To form

The first insulating layer composition used for forming the first composition layer 41 preferably contains insulating fine particles, an insulating layer binder, and a solvent. Further, the first insulating layer composition may contain 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 first insulating layer composition becomes a slurry.

固形 The solid content of the first insulating layer composition is preferably 5 to 75% by mass, more preferably 15 to 50% by mass. By setting the solid content viscosity within the above range, it becomes easy to form the first composition layer 41 having high safety against short circuit. The viscosity at the time of application of the first insulating layer composition is preferably from 1,000 to 5,000 mPa · s, and more preferably from 2,000 to 4,000 mPa · s. By setting the viscosity within the above range, it becomes easy to form the first composition layer 41 having high safety against short circuit. The viscosity at the time of application is a viscosity measured by a B-type viscometer at 60 rpm and 25 ° C. The shear rate at the time of applying the first insulating layer composition is preferably 5,000 to 7,000,000 (1 / s), and more preferably 10,000 to 5,000,000 ( 1 / s). By setting the shear rate within the above range, the first composition layer 41 having high safety against short circuit can be easily formed.

The method for applying the first insulating layer composition to the surface of the electrode active material layer 10 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, and a gravure coating method. Method, screen printing method and the like. Among these, a bar coating method or a gravure coating method is preferable from the viewpoint of applying the first insulating layer composition uniformly to make the first composition layer 41 thin.

前 Before applying the second insulating layer composition on the surface of the first composition layer 41, it is preferable to dry the first composition layer 41. The drying temperature is not particularly limited as long as the solvent can be removed, and is, for example, 40 to 120 ° C, preferably 50 to 90 ° C. The drying time is not particularly limited, but is, for example, 30 seconds to 10 minutes.

(Step of forming second composition layer)
In the step of forming the second composition layer, for example, as shown in FIG. 3, the second composition for an insulating layer is applied on the surface of the first composition layer 41 to form the second composition layer. A layer 42 is formed. Then, in the step of forming the second composition layer, the porosity is high in the second composition layer 42, and the step of forming the second composition layer is more difficult than the region 411 of the first composition layer 41 near the second composition layer. The second insulating layer composition is applied so that the region 421 having a high porosity is formed in a region of the second composition layer 42 near the first composition layer. Thereby, the above-described sparse region and dense region can be formed in the second composition layer 42.

The second insulating layer composition used for forming the second composition layer 42 preferably contains insulating fine particles, an insulating layer binder, and a solvent. The composition for the second insulating layer may include other optional components blended as necessary. Details of the insulating fine particles, the binder for the insulating layer, and the like are as described above. The second insulating layer composition becomes a slurry.

固形 The solid content of the second insulating layer composition is preferably 20 to 80% by mass, more preferably 25 to 60% by mass. By setting the solid content viscosity within the above range, the viscosity at the time of application of the second insulating layer composition can be easily set within the range described below.

粘度 The viscosity of the second insulating layer composition at the time of application is preferably 2000 to 4000 mPa · s, more preferably 2500 to 3500 mPa · s. By setting the viscosity at the time of application of the second insulating layer composition within the above range, it becomes easy to form a sparse region in the second composition layer 42. The viscosity at the time of application is a viscosity measured by a B-type viscometer at 60 rpm and 25 ° C.

The shear rate at the time of application of the second insulating layer composition is preferably 10,000 to 5,000,000 (1 / s), and more preferably 15,000 to 100,000 (1 / s). It is. By setting the shear rate within the above range, a sparse region is easily formed in the second composition layer 42. The shear rate can be adjusted by the transport speed of the substrate and the liquid level distance between the substrate and the liquid contact portion. The shear rate at the time of application can be calculated, for example, by the following equation.
(Shear speed (1 / s)) = (transport speed m / sec) ÷ (liquid level distance m)

The method for applying the composition for the second insulating layer to the surface of the first composition layer 41 is not particularly limited. For example, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a bar coating method, A gravure coating method, a screen printing method and the like can be mentioned. Among these, a bar coating method or a gravure coating method is preferred from the viewpoint of applying the second insulating layer composition to a uniform thickness to make the second composition layer 42 thin.

後に After applying the composition for the second insulating layer to the surface of the first composition layer 41, it is preferable to dry the second composition layer. The drying temperature is not particularly limited as long as the solvent can be removed, and is, for example, 40 to 120 ° C, preferably 50 to 90 ° C. The drying time is not particularly limited, but is, for example, 30 seconds to 10 minutes.

The following principle does not limit the present invention, but the reason why the sparse region of the second composition layer 42 is formed is considered as follows. The surface of the electrode active material layer 10 usually has irregularities due to the electrode active material, although not shown in the drawing. When the first composition layer 41 is formed on the surface of the electrode active material layer 10, the average particle diameter of the insulating fine particles in the first insulating layer composition is smaller than the unevenness of the surface of the electrode active material layer 10. Therefore, the surface of the first composition layer 41 becomes smoother than the surface of the electrode active material layer 10. However, the influence of the irregularities on the surface of the electrode active material layer 10 remains, so that gentle irregularities remain on the surface of the first composition layer 41. Since the gentle irregularities on the surface of the first composition layer 41 hinder the packing of the composition for the second insulating layer, it is considered that a sparse region is formed in the second composition layer 42. In particular, when the viscosity and the shear rate at the time of application of the second insulating layer composition and both of them are within the above ranges, the packing of the second insulating layer composition tends to be hindered, and the second composition It is considered that a sparse region is likely to be formed in the material layer 42. The same occurs when another composition layer is further formed on the second composition layer 42, and it is considered that a sparse region is also formed in the composition layer. When the viscosity at the time of application of the second insulating layer composition is too low or the shear rate is too low, the second insulating layer composition may have the above-mentioned gentle unevenness. Is not affected, and it is considered that a sparse region is hardly formed in the second composition layer 42.

As shown in FIG. 4, a second composition for an insulating layer may be applied on the surface of the second composition layer 42 to further form a third composition layer 43. Also in this case, the third composition layer has a high porosity, and a region 431 having a higher porosity than the region 422 of the second composition layer near the third composition layer is the third composition. It is preferable to apply the second insulating layer composition so as to be formed in a region of the layer 43 near the second composition layer. In the third composition layer 43, similarly to the second composition layer, by adjusting the viscosity and the shear rate at the time of application so as to be within the above ranges, the area 431 in which the porosity is adjusted to a predetermined range. Is formed in the vicinity of the second composition layer. By forming the third composition layer in this way, the electrode 1 for a lithium ion secondary battery shown in FIG. 1 can be manufactured. Further, one or two or more composition layers may be formed on the third composition layer 43 in a similar manner.
Note that as the distance from the electrode active material layer 10 increases, the influence of the unevenness of the surface of the electrode active material layer on the surface of the composition layer decreases. For this reason, it is considered that the porosity of the region corresponding to the dense region decreases as the distance from the electrode active material layer increases.

(Formation of electrode active material layer)
In the method for manufacturing an electrode for a lithium ion secondary battery of the present invention, first, the electrode active material layer 10 is formed on the surface of the electrode current collector 30, and the electrode active material layer 10 is formed on the surface of the electrode active material layer 10 as described above. It is preferable to form an insulating layer.
In forming the electrode active material layer 10, first, an electrode active material layer composition including an electrode active material (negative electrode active material), an electrode binder, and a solvent is prepared. Further, the composition for an electrode active material layer may include other components such as a conductive auxiliary compounded as necessary. The negative electrode active material, the binder for the electrode, the conductive assistant, and the like are as described above. The composition for an electrode active material layer becomes a slurry.

As the solvent in the composition for an electrode active material layer, water is preferably used. By using water, the above-mentioned second binder can be easily dissolved in the composition for an electrode active material layer. Further, other binders may be mixed with water in the form of an emulsion.
The solid concentration of the composition for an electrode active material layer is preferably 5 to 75% by mass, and more preferably 20 to 65% by mass.

The electrode active material layer may be formed by a known method using the composition for an electrode active material layer. For example, the composition for an electrode active material layer is applied on an electrode current collector and dried. Can be formed.
In addition, the electrode active material layer may be formed by applying the composition for an electrode active material layer on a substrate other than the electrode current collector and drying the composition. As a substrate other than the electrode current collector, a known release sheet may be used. The electrode active material layer formed on the base material is preferably formed by forming an insulating layer, and then peeling the electrode active material layer from the base material and transferring it to the electrode current collector.
The electrode active material layer formed on the electrode current collector or the substrate is preferably pressed under pressure. Pressing can increase the electrode density. The pressure press may be performed by a roll press or the like.

In the above description, the case where the electrode for a lithium ion secondary battery of the present invention is a negative electrode has been described, but the electrode for a lithium ion secondary battery may be a positive electrode. In the case of a positive electrode, the electrode active material layer becomes a positive electrode active material layer, and the electrode active material becomes a positive electrode active material.
Examples of the positive electrode active material include a lithium metal oxide compound. 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) oxide, or the like may be used.
The electrode current collector becomes a positive electrode current collector. The material for the positive electrode current collector is the same as the compound used for the negative electrode current collector, but preferably aluminum or copper, and more preferably aluminum.
Other configurations when the electrode for the lithium ion secondary battery is the positive electrode are the same as those in the case where the electrode is the negative electrode, and the description of the other configurations is omitted.

<Lithium ion secondary battery>
A lithium ion secondary battery of the present invention includes a lithium ion secondary battery electrode having the above-described insulating layer. Specifically, the lithium ion secondary battery of the present invention includes a positive electrode and a negative electrode arranged so as to face each other, and at least one of the negative electrode and the positive electrode has the above-described lithium ion secondary battery having the insulating layer. It becomes a battery electrode. In this lithium ion secondary battery electrode (negative electrode or positive electrode), an insulating layer may be provided on a surface facing the other electrode (positive electrode or negative electrode). It is preferable that the lithium ion secondary battery of the present invention includes a lithium ion secondary battery electrode having the above-described insulating layer as a negative electrode.

The lithium ion secondary battery of the present invention may further include a separator disposed between the positive electrode and the negative electrode. By providing the separator, a short circuit between the positive electrode and the negative electrode is more effectively prevented. Further, the separator may hold an electrolyte described later. The insulating layer provided on the positive electrode or the negative electrode may or may not be in contact with the separator, but is preferably in contact with the separator.
Examples of the separator include a porous polymer film, a nonwoven fabric, and a glass fiber. Among these, a porous polymer film is preferable. As the porous polymer film, an olefin-based porous film is exemplified. The separator may be heated by heat generated when the lithium ion secondary battery is driven, and may be thermally contracted. Even during such thermal contraction, the short circuit can be easily suppressed by providing the insulating layer.
Further, in the lithium ion secondary battery of the present invention, the separator may be omitted. Even if the separator is omitted, insulation between the negative electrode and the positive electrode is ensured by the insulating layer provided on at least one of the negative electrode and the positive electrode.

The lithium ion secondary battery may have a multilayer structure in which a plurality of negative electrodes and a plurality of positive electrodes are stacked. In this case, the negative electrode and the positive electrode may be provided alternately along the laminating direction. When a separator is used, the separator may be disposed between each negative electrode and each positive electrode.
In the lithium ion secondary battery, the negative electrode and the positive electrode, or the negative electrode, the positive electrode, and the separator are housed in a battery cell. The battery cell may be any of a square type, a cylindrical type, a laminated type, and the like.

Lithium ion secondary batteries include 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, ethyl methyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, and 1,2. Polar solvents such as -diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 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 , LiPF ₆ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ and salts containing lithium such as LiN (COCF ₂ CF ₃ ) ₂ and lithium bisoxalate borate (LiB (C ₂ O ₄ ) ₂ . Examples include complexes such as boron hydride complexes and complex hydrides such as LiBH _4. These salts or complexes may be used alone or in 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. For example, the electrolyte is filled in the battery cell in which the negative electrode and the positive electrode, or the negative electrode, the positive electrode, and the separator 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.

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 obtained electrode for a lithium ion secondary battery was evaluated by the following evaluation method.
(Output characteristics)
The cells produced in the examples and comparative examples were charged at a constant current of 1 C, and when the voltage reached 4.2 V, the current was reduced to 0.05 C, and constant voltage charging was performed until the charging was completed. Then, a constant current discharge of 1 C was performed, and the discharge was completed at the time when the discharge was performed to 2.5 V, and the discharge capacity of the constant current discharge of 1 C was calculated. Next, after performing the same charge, the battery was discharged at a constant current of 10 C. When the battery was discharged to 2.5 V, the discharge was completed, and the discharge capacity of the constant current discharge at 10 C was calculated. Then, the ratio of the discharge capacity of 10 C to 1 C was calculated from the following equation (1), and evaluated as follows. Note that the larger the ratio of the discharge capacity of 10C to 1C, the better the output characteristics.
(Ratio of discharge capacity of 10C to 1C) = (Discharge capacity of constant current discharge of 10C / discharge capacity of constant current discharge of 1C) × 100 (1)
A: Ratio of discharge capacity of 10C to 1C ≧ 20%
B: ratio of 10C discharge capacity to 20%> 1C ≧ 10%
C: ratio of 10C discharge capacity to 10%> 1C ≧ 5%
D: ratio of 10C discharge capacity to 5%> 1C

(Short circuit safety)
The cells prepared in the Examples and Comparative Examples were charged at a constant current of 1.0 C, and the current was decreased as soon as the voltage reached 4.2 V. When the voltage reached 0.05 C, the constant voltage charging was performed until the charging was completed. went. After that, it was placed in a thermostatic bath capable of controlling the temperature and set at 130 ° C. After reaching 130 ° C., it was left for 1 hour. During this period, the maximum temperature of the surface temperature of the cell was measured and classified as follows. The lower the maximum surface temperature, the higher the short-circuit safety.
A: The maximum surface temperature was less than 135 ° C.
B: The maximum surface temperature was 135 ° C or higher, but was lower than 140 ° C.
C: The maximum surface temperature was 140 ° C. or higher, but lower than 200 ° C.
D: The maximum surface temperature was 200 ° C. or higher.

The physical properties of the obtained electrode for a lithium ion secondary battery were measured by the following measurement methods.
(Space ratio)
A section of the electrode for a lithium ion secondary battery on which an insulating layer was formed was exposed by an ion milling method. Next, the exposed cross section of the electrode for a lithium ion secondary battery is observed using a FE-SEM (field emission scanning electron microscope) at a magnification that allows the entire insulating layer to be observed, and an image of the insulating layer is obtained. Was. The magnification was 5000 to 25000 times. Next, using an image analysis software “Image J”, the obtained image was binarized so that the real part of the insulating layer was displayed in black and the void part of the insulating layer was displayed in white (FIG. 5). Then, the insulating layer is divided into ten in the thickness direction, and the ratio of the area of the white part in each of the ten divided regions (for example, reference numeral 50 in FIG. 5) is measured using image analysis software “Image J”. did. The ratio of the area of the white portion is the porosity (volume%) of the divided region.
Then, as shown in FIG. 6, a divided area having a porosity higher by 2% by volume or more than the porosity of two adjacent divided areas is defined as a sparse divided area. In addition, in consideration of the measurement error of the porosity, a region where the porosity is higher than 2% by volume is defined as a sparsely divided region. As for the two divided regions at both ends, there is only one adjacent divided region. Therefore, when the porosity is higher than the porosity of one adjacent divided region by 2% by volume or more, the divided region is regarded as a sparse divided region. did.
Further, when two or more divided regions having a high space ratio are continuous, the continuous divided region is regarded as one continuous region, and the space ratio of the continuous region (that is, the average value of the space ratio of a plurality of divided regions) However, if the volume was higher than the two regions adjacent to the continuous region by 2% by volume or more, each of the divided regions constituting the continuous region was regarded as a sparsely divided region. The “continuous region in which two or more divided regions having a high space ratio are continuous” means that the space ratio of each of the divided regions constituting the continuous region is equal to the space ratio of two divided regions adjacent to the continuous region. It means the area higher than either.
As described above, the sparsely divided area is specified, and the divided area not specified as the sparsely divided area is defined as the densely divided area.
When the densely divided area and the sparsely divided area are respectively continuous, the dense and sparsely divided areas are combined into one dense area and one sparsely divided area. Further, when the densely divided region and the sparsely divided region are not continuous, one densely divided region and one sparsely divided region are regarded as one densely divided region and one sparsely divided region, respectively. When the continuous densely divided region and the sparsely divided region are combined into one dense region and a sparse region, respectively, the average spatial ratio of the continuous densely divided region and the sparsely divided region is defined as the spatial ratio of the dense region and the sparsely divided region. did.

(Thickness of insulating layer)
The thickness of the insulating layer was measured from the above-mentioned SEM image.

The physical properties of the obtained composition (slurry) for an insulating layer were measured by the following measurement methods.
(viscosity)
The viscosity of the insulating layer composition (slurry) was measured with a B-type viscometer at 60 rpm and 25 ° C.

[Example 1]
(Preparation of positive electrode)
100 parts by mass of an NCA-based oxide (average particle diameter: 10 μm) as a positive electrode active material, 4 parts by mass of acetylene black as a conductive additive, 4 parts by mass of polyvinylidene fluoride as an electrode binder, and N as a solvent -Methylpyrrolidone (NMP) was mixed to obtain a slurry for a positive electrode active material layer adjusted to a solid concentration of 60% by mass. This slurry for a positive electrode active material layer was applied to both sides of a 15-μm-thick aluminum foil as a positive electrode current collector, preliminarily dried, and then vacuum dried at 120 ° C. Then, the positive electrode current collector coated with the slurry for the positive electrode active material layer on both sides is pressed with a roller at a linear pressure of 400 kN / m using a roller, and further punched into a 100 mm × 200 mm square of the electrode dimensions, thereby forming a positive electrode active material on both sides. A positive electrode having a layer was obtained. Of the dimensions, the area where the positive electrode active material was applied was 100 mm × 180 mm. In addition, the thickness of the positive electrode active material layers formed on both surfaces was 50 μm per one surface.

(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 a binder for an electrode, 1.5 parts by mass of styrene butadiene rubber (SBR), and a solvent Was mixed with water to obtain a slurry for a negative electrode active material layer adjusted to a solid content of 50% by mass. This slurry 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 coated with the slurry for the negative electrode active material layer on both sides is pressed with a roller at a linear pressure of 500 kN / m using a roller, and further punched into a 110 mm × 210 mm square of the electrode dimensions to form a negative electrode active material on both sides. A negative electrode having a layer was obtained. Of the dimensions, the area where the negative electrode active material was applied was 110 mm × 190 mm. The thickness of the negative electrode active material layers formed on both sides was 50 μm per one side. The density of the negative electrode active material layer was 1.55 g / cc.

(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 an electrolytic solution was prepared. Was prepared.

(Formation of insulating layer)
Alumina particles (manufactured by Nippon Light Metal Co., Ltd., product name: AHP200, average particle size) in a polyvinylidene fluoride solution (manufactured by Kureha Corporation, product name: L # 1710, 10% by mass solution, solvent: NMP) as insulating fine particles 0.4 μm in diameter) was mixed and dispersed while applying moderate shearing force to obtain a slurry. The blending amount of the alumina particles and the polyvinylidene fluoride solution is such that the blending amount of the alumina particles in the slurry is 100 parts by mass on a solid basis, and the content of the polyvinylidene fluoride solution is 15 parts by mass on a solids basis. It was a proper blending amount.
NMP was further added to this slurry so that the solid content concentration became 35% by mass, and the mixture was gently stirred with a stirrer for 30 minutes to obtain a slurry for an insulating layer. The viscosity of the slurry for the insulating layer was 2800 mPa · s.
The slurry for the insulating layer is applied to the surface of each negative electrode active material layer of the negative electrode by a gravure coater, and the coating film is dried at 90 ° C. for 1 minute to form the first composition on the surface of each negative electrode active material layer. A layer was formed. Next, the opposite side was similarly coated to form a first composition layer on both surfaces of the negative electrode active material layer.

Next, the slurry for an insulating layer is applied to the surface of the first composition layer of the negative electrode by a gravure coater, and the coating film is dried at 90 ° C. for 1 minute to form a surface of each first composition layer. Then, a second composition layer was formed. Next, the opposite side was similarly coated to form a second composition layer on both sides of the negative electrode active material layer.

Next, the slurry for an insulating layer is applied to the surface of the second composition layer of the negative electrode by a gravure coater, and the coating film is dried at 90 ° C. for 1 minute, so that the surface of each second composition layer is formed. Then, a third composition layer was formed. Next, the opposite side was similarly coated to form a third composition layer on both sides of the negative electrode active material layer.
In addition, in Example 1, the shear rate at the time of applying each insulating layer slurry with a gravure coater was set to 20,000 (1 / s).

(Manufacture of batteries)
A laminate was obtained by laminating 10 negative electrodes, 9 positive electrodes, and 18 separators each having the insulating layer obtained above. Here, the negative electrode and the positive electrode were alternately arranged, and a separator was arranged between each negative electrode and the positive electrode. In addition, a polyethylene porous film was used as a separator.
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 laminate type cell.

[Example 2]
The slurry for an insulating layer is applied to the surface of the third composition layer of the negative electrode by a gravure coater, and the coating film is dried at 60 ° C. for 10 minutes to form the fourth composition layer on the surface of the third composition layer. A composition layer was formed. Next, the opposite side was similarly coated to form a fourth composition layer on both sides of the negative electrode active material layer. Other than that, it carried out similarly to Example 1.

[Example 3]
The operation was performed in the same manner as in Example 1 except that the third composition layer was not formed on both sides of the negative electrode active material layer.

[Example 4]
The slurry for an insulating layer is applied to the surface of the fourth composition layer of the negative electrode by a gravure coater, and the coating film is dried at 60 ° C. for 10 minutes, so that the fifth composition layer is applied to the surface of the fourth composition layer. A composition layer was formed. Next, the opposite side was similarly coated to form a fifth composition layer on both sides of the negative electrode active material layer. Other than that, it carried out similarly to Example 2.

[Example 5]
The same procedure as in Example 1 was carried out except that the slurry for the insulating layer was prepared so that the solid content concentration was changed from 35% by mass to 40% by mass so that the viscosity of the slurry for the insulating layer was 3300 mPa · s.

[Example 6]
Same as Example 1 except that the first to third composition layers were formed on both sides of the positive electrode active material layer instead of forming the first to third composition layers on both sides of the negative electrode active material layer It was carried out.

[Comparative Example 1]
Example 2 Except that the second and third composition layers were not formed on both sides of the negative electrode active material layer, and the thickness of the first composition layer was changed to 15 μm using a comma coater instead of a gravure coater. Performed in a similar manner to 1.

[Comparative Example 2]
The slurry for the insulating layer was prepared so that the viscosity of the slurry for the insulating layer was 90 mPa · s, and the slurry for the insulating layer was applied using a gravure coater. Further, the second and third composition layers were not formed on both sides of the negative electrode active material layer. Other than that, it carried out similarly to Example 1.

As shown in the above Examples 1 to 6, the insulating layer is sandwiched between at least two or more layered dense regions and between two adjacent dense regions and has a higher space ratio than the two adjacent dense regions. , The safety against short-circuit and the output characteristics were excellent. On the other hand, in Comparative Examples 1 and 2, the output characteristics were poor because the insulating layer had no sparse region.

Reference Signs List 1 electrode for lithium ion secondary battery 10 electrode active material layer 20 insulating

layer

21, 23, 25 dense region 22, 24 sparse region 30 electrode current collector 41 first composition layer 42 second composition layer 43 third Composition layer

Claims

An electrode active material layer, comprising an insulating layer provided on the surface of the electrode active material layer,
For a lithium ion secondary battery, the insulating layer has at least two or more layered dense regions and a layered sparse region sandwiched between two adjacent dense regions and having a higher porosity than the two adjacent dense regions. electrode.
The insulating layer has two or more of the sparse regions and two or more of the dense regions,
The electrode for a lithium ion secondary battery according to claim 1, wherein the sparse regions and the dense regions are alternately arranged.
3. The electrode for a lithium ion secondary battery according to claim 2, wherein the insulating layer has two or three of the sparse regions alternately arranged with the dense regions.
4. The electrode for a lithium ion secondary battery according to claim 3, wherein the insulating layer has two of the sparse regions alternately arranged with the dense regions.
(5) The electrode for a lithium ion secondary battery according to any one of (1) to (4), wherein the porosity of the sparse region is 30% by volume or more.
(6) The electrode for a lithium ion secondary battery according to any one of (1) to (5), wherein the thickness of the insulating layer is 7 to 30 μm.
(7) The electrode for a lithium ion secondary battery according to any one of (1) to (6), wherein the insulating layer contains insulating fine particles and a binder for the insulating layer.
(8) The electrode for a lithium ion secondary battery according to any one of (1) to (7), wherein the electrode active material layer is a negative electrode active material layer.
A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of claims 1 to 8.
Comprising a negative electrode and a positive electrode,
The lithium ion secondary battery according to claim 9, wherein the negative electrode is the electrode for the lithium ion secondary battery.
A step of applying a first insulating layer composition on the surface of the electrode active material layer to form a first composition layer; and a second insulating layer on the surface of the first composition layer. Applying a composition for forming a second composition layer,
In the step of forming the second composition layer, the porosity is high in the second composition layer, and the first composition layer has a higher porosity than a region near the second composition layer of the first composition layer. An electrode for a lithium ion secondary battery, in which the second insulating layer composition is applied so that a region having a high porosity is formed in a region of the second composition layer near the first composition layer. Manufacturing method.
The method for producing an electrode for a lithium ion secondary battery according to claim 11, wherein the shear rate at the time of applying the second composition for an insulating layer is 10,000 to 5,000,000 (1 / s).
13. The method for producing an electrode for a lithium ion secondary battery according to claim 11, wherein the viscosity of the second insulating layer composition at the time of application is 2,000 to 4,000 mPa · s.