JP2020140895A - Electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide an electrode for a lithium ion secondary battery, which is capable of increasing the capacity of a lithium ion secondary battery and also capable of imparting good charge/discharge cycle characteristics, and which includes a Si-based material, and a lithium ion secondary battery including the electrode for a lithium ion secondary battery.SOLUTION: An electrode for a lithium ion secondary battery of the present invention includes: a collector 10; a first electrode active material layer 20 disposed on a surface of the collector 10; and a second electrode active material layer 30 disposed between the collector 10 and the first electrode active material layer 20. The first electrode active material layer contains a Si-based material and a first electrode active material layer binder, and the second electrode active material layer contains graphite and a second electrode active material layer binder. A lithium ion secondary battery of the present invention includes the electrode for a lithium ion secondary battery of the present invention as a negative electrode.SELECTED DRAWING: Figure 1

Description

The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery.

Lithium-ion secondary batteries are used as large-scale stationary power sources for power storage, power sources for electric vehicles, etc. In recent years, lithium-ion secondary batteries with even higher energy densities have been used to further increase the capacity of batteries. It is desired. As a method of obtaining such a lithium ion secondary battery having a high energy density, for example, a method of using a Si-based material as a negative electrode material can be mentioned. The theoretical volume density of Si is 4200 mAh / g, which is more than 10 times higher than the theoretical volume density of carbon-based materials (for example, 372 mAh / g in the case of graphite). Therefore, by using a Si-based material as the negative electrode material, a high-capacity lithium ion secondary battery can be obtained.
However, the Si-based material has a problem that the volume increases up to about 4 times when Li ions are absorbed. Therefore, if the Si-based material is used as it is as the negative electrode material, the charge / discharge cycle characteristics of the lithium ion secondary battery deteriorate. Therefore, in order to reduce the influence of expansion and contraction of the Si-based material, a negative electrode for a lithium secondary battery using a mixture obtained by mixing a Si-based material and a carbon-based material as a negative electrode material is known as a prior art. (See, for example, Patent Document 1).

JP-A-2007-227239

However, in the negative electrode material using a mixture of a conventional Si-based material and a carbon-based material, the proportion of the carbon-based material must be increased in order to reduce the influence of expansion and contraction of the Si-based material. For example, the graphite content in the negative electrode for a lithium secondary battery described in Patent Document 1 was 70 to 100% by volume. Therefore, if the ratio of the Si-based material in the negative electrode material is increased for the purpose of increasing the capacity of the lithium secondary battery, there is a problem that the influence of expansion and contraction of the Si-based material becomes large and the charge / discharge cycle characteristics deteriorate. It was.
Therefore, the present invention includes an electrode for a lithium ion secondary battery containing a Si-based material and an electrode for the lithium ion secondary battery, which can increase the capacity of the lithium ion secondary battery and improve the charge / discharge cycle characteristics. An object of the present invention is to provide a lithium ion secondary battery.

As a result of diligent studies, the present inventors have increased the capacity of the lithium ion secondary battery by providing an electrode active material layer containing a carbon-based material between the electrode active material layer containing the Si-based material and the current collector. We have found that the charge / discharge cycle characteristics can be made large and the charge / discharge cycle characteristics can be improved, and the following invention has been completed. The gist of the present invention is the following [1] to [7].
[1] A second electrode provided between the current collector, a first electrode active material layer provided on the surface of the current collector, and the current collector and the first electrode active material layer. It is provided with an active material layer, the first electrode active material layer contains a Si-based material and a binder for the first electrode active material layer, and the second electrode active material layer is graphite and a second electrode active material layer. Electrodes for lithium ion secondary batteries containing a binder for.
[2] The lithium ion secondary battery according to the above [1], wherein the thickness of the first electrode active material layer is 10 to 70 μm, and the thickness of the second electrode active material layer is 5 to 60 μm. Electrode for.
[3] The ratio (D1 / D2) of the thickness (D1) of the first electrode active material layer to the thickness (D2) of the second electrode active material layer is 0.1 to 10 [1]. ] Or [2]. The electrode for a lithium ion secondary battery.
[4] The average particle size of the Si-based material in the first electrode active material layer is 1 to 30 μm, and the average particle size of the graphite in the second electrode active material layer is 1 to 30 μm. 1] The electrode for a lithium ion secondary battery according to any one of [3].
[5] The electrode for a lithium ion secondary battery according to any one of the above [1] to [4], wherein the content of the Si-based material in the first electrode active material layer is 95 to 99% by mass. ..
[6] The electrode for a lithium ion secondary battery according to any one of the above [1] to [5], wherein the graphite content in the second electrode active material layer is 95 to 99% by mass.
[7] A lithium ion secondary battery comprising the electrode for the lithium ion secondary battery according to any one of the above [1] to [6] as a negative electrode.

According to the present invention, an electrode for a lithium ion secondary battery containing a Si-based material and an electrode for the lithium ion secondary battery thereof, which can increase the capacity of the lithium ion secondary battery and improve the charge / discharge cycle characteristics, are provided. A lithium ion secondary battery can be provided.

It is schematic cross-sectional view which shows one Embodiment of the electrode for a lithium ion secondary battery of this invention.

<Electrodes for lithium-ion secondary batteries>
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 a current collector 10, a first electrode active material layer 20 provided on the surface of the current collector 10, and the current collector 10 and the first. A second electrode active material layer 30 provided between the electrode active material layer 20 and the electrode active material layer 20 is provided. The first electrode active material layer 20 and the second electrode active material layer 30 may be laminated on both surfaces of the current collector 10.

The electrode for a lithium ion secondary battery of the present invention is used as a negative electrode in a lithium ion secondary battery.

(First electrode active material layer)
The first electrode active material layer contains a Si-based material and a binder for the first electrode active material layer. As described above, since the Si-based material has a high theoretical capacity density, the capacity of the lithium ion secondary battery can be increased by using the Si-based material as the electrode active material of the first electrode active material layer.

The Si-based material is not particularly limited as long as it expands when it absorbs Li ions. Examples of the Si-based material include Si, a compound represented by the general formula SiOx (where x is a number of 0.5 to 1.5), and the like. Among the Si-based materials, the compound represented by the general formula SiOx (x is a number of 0.5 to 1.5 in the formula) is preferable because the expansion and contraction is relatively small. Here, when the above compound is viewed in units of "SiO", this SiO is an amorphous SiO or is around the Si of the nanocluster so that the molar ratio of Si: SiO ₂ is about 1: 1. It is a composite of Si and SiO _{2 in} which SiO ₂ is present. It is presumed that SiO ₂ has a buffering action against the expansion and contraction of Si during charging and discharging. Further, the Si-based material may be a material obtained by coating particles of a compound represented by the general formula SiOx (in the formula, x is a number of 0.5 to 1.5) with carbon such as nanocarbon.

The Si-based material is preferably in the form of particles. The average particle size of the Si-based material is preferably 1 to 30 μm. When the average particle size of the Si-based material is 1 μm or more, the binding force between the Si-based material particles is increased, and the charge / discharge cycle characteristics of the lithium ion secondary battery are improved. On the other hand, when the average particle size of the Si-based material is 30 μm or less, the expansion and contraction of the Si-based material is suppressed, and the charge / discharge cycle characteristics of the lithium ion secondary battery are improved. From the above viewpoint, the average particle size of the Si-based material is more preferably 2 to 20 μm, still more preferably 3 to 10 μm. Examples of the method for adjusting the average particle size of the Si-based material to a desired value include a method of pulverizing by a known method using a ball mill or the like. The average particle size means the particle size (D50) when the volume integration is 50% in the particle size distribution of the Si-based material obtained by the laser diffraction / scattering method.

The content of the Si-based material in the first electrode active material layer is preferably 95 to 99% by mass. When the content of the Si-based material in the first electrode active material layer is 95% by mass or more, the capacity of the lithium ion secondary battery can be increased. On the other hand, when the content of the Si-based material in the first electrode active material layer is 99% by mass or less, the amount of the binder can be increased to a certain amount or more, thereby increasing the binding force between the Si-based material particles and lithium ion secondary. Improve the charge / discharge cycle characteristics of the next battery. From the above viewpoint, the content of the Si-based material in the first electrode active material layer is more preferably 96 to 98% by mass.

A part or all of the Si-based material may be pre-doped with lithium or lithium ions. By the pre-doping treatment, silicon dioxide and lithium in the first electrode active material layer react irreversibly to form lithium silicate (Li ₄ SiO ₄ ). As a result, when lithium is occluded in the first electrode active material layer in the initial charging step, lithium silicate is not generated, so that a decrease in discharge capacity is suppressed.

The predoping method for the first electrode active material layer is not particularly limited, and the predoping method applied to a conventional lithium ion secondary battery can be applied. For example, a lithium layer may be formed on the surface of the first electrode active material layer by a sputtering method. Further, a lithium foil may be provided on the surface of the first electrode active material layer. The amount of lithium to be pre-doped is not particularly limited, and is preferably 1 to 4 times the molar amount of silicon oxide in the first electrode active material layer, for example.

The first electrode active material layer may contain a conductive auxiliary agent from the viewpoint of imparting conductivity and mitigating expansion and contraction of the Si-based material. As the conductive auxiliary agent, a material having higher conductivity than the Si-based material is used. Specifically, examples of the conductive auxiliary agent include carbon materials such as Ketjen black, acetylene black, carbon nanotubes, and rod-shaped carbon. These conductive auxiliaries may be used alone or in combination of two or more.
When the conductive auxiliary material is contained in the first electrode active material layer, the content of the conductive auxiliary agent is preferably 5% by mass or less, preferably 4% by mass or less, based on the total amount of the electrode active material layer. More preferably, it is more preferably 3% by mass or less, and particularly preferably 2% by mass or less.

The first electrode active material layer is formed by binding Si-based materials with a binder for the first electrode active material layer.
The binder for the first electrode active material layer is poly (meth) acrylic acid, lithium poly (meth) acrylate, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), poly. Fluorine-containing resin such as tetrafluoroethylene (PTFE), acrylic resin such as polymethylacrylate (PMA) and polymethylmethacrylate (PMMA), polyvinylidene acetate, polyimide (PI), polyamide (PA), polyvinylidene chloride (PVC) ), Polyethernitrile (PEN), polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyvinyl alcohol, etc. Can be mentioned. These binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose (CMC) or the like may be used in the form of a salt such as a sodium salt.
The content of the binder for the first electrode active material layer in the first electrode active material layer is preferably 1 to 5% by mass based on the total amount of the electrode active material layer. When the content of the binder for the first electrode active material layer is 1% by mass or more, the binding force between the Si-based material particles is increased, and the charge / discharge cycle characteristics of the lithium ion secondary battery are improved. On the other hand, when the content of the binder for the first electrode active material layer is 5% by mass or less, the amount of the binder, which is a highly resistant component in the first electrode active material layer, decreases, so that the lithium ion secondary battery Output characteristics are improved. From the above viewpoint, the content of the binder for the first electrode active material layer in the first electrode active material layer is more preferably 2 to 4% by mass based on the total amount of the electrode active material layer.

The thickness of the first electrode active material layer is preferably 10 to 70 μm per one side of the current collector. When the thickness of the first electrode active material layer is 10 μm or more per one side of the current collector, the amount of Si-based material which is a high-capacity component in the electrode increases, and the capacity of the lithium ion secondary battery is improved. On the other hand, when the thickness of the first electrode active material layer is 70 μm or less per one side of the current collector, the expansion amount of the electrode during charging / discharging of the lithium ion secondary battery decreases, and charging / discharging of the lithium ion secondary battery Cycle characteristics are improved. From the above viewpoint, the thickness of the first electrode active material layer is more preferably 20 to 40 μm and further preferably 20 to 38 μm per one side of the current collector.

The first electrode active material layer may contain an optional component other than the Si-based material, the conductive auxiliary agent, and the binder for the electrode as long as the effect of the present invention is not impaired. However, the total content of the Si-based material, the conductive auxiliary agent, and the binder for the electrode is preferably 96% by mass or more, and more preferably 98% by mass or more, based on the total mass of the electrode active material layer. ..

(Second electrode active material layer)
The second electrode active material layer contains graphite and a binder for the second electrode active material layer. Even if graphite absorbs Li ions, the graphite does not expand significantly, so that the second electrode active material layer functions as a buffer layer with respect to the first electrode active material layer that causes expansion and contraction. ..

Graphite is not particularly limited, and examples of graphite include natural graphite, artificial graphite, quiche graphite and the like. In addition, examples of natural graphite include scaly graphite, lump graphite, earth graphite and the like.

The average particle size of graphite is preferably 1 to 30 μm. When the average particle size of graphite is 1 μm or more, the binding force between the graphite particles is increased, and the charge / discharge cycle characteristics of the lithium ion secondary battery are improved. On the other hand, when the average particle size of graphite is 30 μm or less, the second electrode active material layer can be prevented from being too thick, and the capacity of the lithium ion secondary battery can be improved. From the above viewpoint, the average particle size of graphite is more preferably 2 to 20 μm, still more preferably 5 to 15 μm. Examples of the method of adjusting the average particle size of graphite to a desired value include a method of pulverizing by a known method using a ball mill or the like. The average particle size means the particle size (D50) when the volume integration is 50% in the particle size distribution of graphite obtained by the laser diffraction / scattering method.

The graphite content in the second electrode active material layer is preferably 95 to 99% by mass. When the graphite content in the second electrode active material layer is 95% by mass or more, the capacity of the lithium ion secondary battery can be increased. On the other hand, when the graphite content in the second electrode active material layer is 99% by mass or less, the amount of the binder can be made constant or more, thereby increasing the binding force between the graphite particles and charging / discharging the lithium ion secondary battery. Improve cycle characteristics. From the above viewpoint, the graphite content in the second electrode active material layer is more preferably 96 to 98% by mass.

The second electrode active material layer may contain a conductive auxiliary agent. Examples of the conductive auxiliary agent include carbon materials such as Ketjen black, acetylene black, carbon nanotubes, and rod-shaped carbon. These conductive auxiliaries may be used alone or in combination of two or more.
When the conductive auxiliary material is contained in the first electrode active material layer, the content of the conductive auxiliary agent is preferably 5% by mass or less, preferably 4% by mass or less, based on the total amount of the electrode active material layer. More preferably, it is more preferably 3% by mass or less, and particularly preferably 2% by mass or less.

The second electrode active material layer is formed by binding graphite with a binder for the second electrode active material layer.
As the binder for the second electrode active material layer, the same resin as that listed in the binder for the first electrode active material layer can be used. The binder for the second electrode active material layer may be the same as or different from the binder for the first electrode active material.
The content of the binder for the second electrode active material layer in the second electrode active material layer is preferably 1 to 5% by mass based on the total amount of the electrode active material layer. When the content of the binder for the second electrode active material layer is 1% by mass or more, the binding force between the graphite particles is increased, and the charge / discharge cycle characteristics of the lithium ion secondary battery are improved. On the other hand, when the content of the binder for the second electrode active material layer is 5% by mass or less, the amount of the binder, which is a highly resistant component in the second electrode active material layer, decreases, so that the lithium ion secondary battery Output characteristics are improved. From the above viewpoint, the content of the binder for the second electrode active material layer in the second electrode active material layer is more preferably 2 to 4% by mass based on the total amount of the electrode active material layer.

The thickness of the second electrode active material layer is not particularly limited, but is preferably 5 to 60 μm per one side of the current collector. When the thickness of the second electrode active material layer is 5 μm or more, the adhesion between the first electrode active material layer and the second electrode active material layer is enhanced, and the charge / discharge cycle of the lithium ion secondary battery is increased. The characteristics are improved. On the other hand, when the thickness of the second electrode active material layer is 60 μm or less, the ratio of the first electrode active material layer in the electrode increases, and the capacity of the lithium ion secondary battery is improved. From the above viewpoint, the thickness of the second electrode active material layer is more preferably 15 to 25 μm per one side of the current collector.

The ratio (D1 / D2) of the thickness (D1) of the first electrode active material layer per one side of the current collector to the thickness (D2) of the second electrode active material layer per one side of the current collector is preferably. It is 0.1 to 10. The ratio (D1 / D2) of the thickness (D1) of the first electrode active material layer per one side of the current collector to the thickness (D2) of the second electrode active material layer per one side of the current collector is 0.1. With the above, the ratio of the Si-based material in the electrode is high, so that the capacity of the lithium ion secondary battery is improved. On the other hand, the ratio (D1 / D2) of the thickness (D1) of the first electrode active material layer per one side of the current collector to the thickness (D2) of the second electrode active material layer per one side of the current collector is 10. When the following, the amount of expansion of the electrode during charging / discharging of the lithium ion secondary battery is reduced, and the charging / discharging cycle characteristics of the lithium ion secondary battery are improved. From the above viewpoint, the ratio (D1 / D2) of the thickness (D1) of the first electrode active material layer per one side of the current collector to the thickness (D2) of the second electrode active material layer per one side of the current collector. ) Is more preferably 0.1 to 5, and even more preferably 1.2 to 3.

The second electrode active material layer may contain arbitrary components other than graphite, a conductive auxiliary agent, and a binder for the second electrode active material layer as long as the effects of the present invention are not impaired. However, the total content of graphite, the conductive auxiliary agent, and the binder for the second electrode active material layer in the total mass of the electrode active material layer is preferably 96% by mass or more, preferably 98% by mass or more. Is more preferable.

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

(Insulation layer)
An insulating layer may be provided on the surface of the first electrode active material layer. The insulating layer includes insulating fine particles and a binder for the insulating layer. The insulating layer is a layer formed by binding insulating fine particles with a binder for an insulating layer, and has a porous structure. By providing the insulating layer on the surface of the first electrode active material layer, it is possible to prevent the first electrode active material layer from expanding in the plane direction due to expansion. When the first electrode active material layer expands in the plane direction due to expansion, the proportion of the first electrode active material layer that contributes to the charging / discharging of the lithium ion secondary battery decreases, and the charging / discharging of the lithium ion secondary battery The cycle characteristics deteriorate.

The thickness of the insulating layer is preferably 3 to 15 μm. When the thickness of the insulating layer is 3 μm or more, the insulating layer relaxes the expansion and contraction of the electrode, and the charge / discharge cycle characteristics of the lithium ion secondary battery are improved. On the other hand, when the thickness of the insulating layer is 15 μm or less, the distance between the positive electrode and the negative electrode becomes small, so that the output of the lithium ion secondary battery is improved. From the above viewpoint, the thickness of the insulating layer is more preferably 3 to 13 μm, further preferably 3 to 10 μm.

As described above, the insulating layer has a porous structure, and the porosity thereof is preferably 30 to 80%. When the porosity of the insulating layer is 30% or more, the conduction path of lithium ions in the insulating layer can be secured, and the output of the lithium ion secondary battery is improved. On the other hand, when the porosity of the insulating layer is 80% or less, the ratio of the insulating component in the insulating layer is increased, and the safety of the lithium ion secondary battery is improved. From the above viewpoint, the porosity of the insulating layer is more preferably 40 to 78%, further preferably 50 to 75%. The porosity of the insulating layer can be measured by the method described in Examples described later.

(Insulating fine particles)
The insulating fine particles are not particularly limited as long as they are insulating, and may be either organic particles or inorganic particles. Specific organic particles include, for example, crosslinked polymethyl methacrylate, crosslinked styrene-acrylic acid copolymer, crosslinked acrylonitrile resin, polyamide resin, polyimide resin, poly (lithium 2-acrylamide-2-methylpropanesulfonate), and the like. Examples thereof include particles composed of organic compounds such as polyacetal resin, epoxy resin, polyester resin, phenol resin, and melamine resin. 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 foot. Examples thereof include particles composed of inorganic compounds such as lithium pentoxide, clay, zeolite, and calcium carbonate. Further, the inorganic particles may be particles composed of known composite oxides such as niobium-tantalum composite oxide and magnesium-tantalum composite oxide.
The insulating fine particles may be particles in which one type of each of the above materials is used alone, or particles in which two or more types are used in combination. Further, the insulating fine particles may be fine particles containing both an inorganic compound and an organic compound. For example, it may be an inorganic-organic composite particle in which the surface of a particle made of an organic compound is coated with an inorganic oxide.
Among these, inorganic particles are preferable, and alumina particles and boehmite particles are particularly preferable.

The average particle size of the insulating fine particles is preferably 0.1 to 5.0 μm. When the average particle size of the insulating fine particles is 0.1 μm or more, the binding property between the insulating fine particles is improved, and the safety of the lithium ion secondary battery is improved. On the other hand, when the average particle size of the insulating fine particles is 5.0 μm or less, the reduction of the porosity of the insulating layer is suppressed, and the safety of the lithium ion secondary battery is improved. From the above viewpoint, the average particle size of the insulating fine particles is more preferably 0.2 to 3.0 μm, still more preferably 0.3 to 1.0 μm.
The average particle size means the particle size (D50) when the volume integration is 50% in the particle size distribution of the insulating fine particles obtained by the laser diffraction scattering method.
Further, as the insulating fine particles, one type having an average particle diameter within the above range may be used alone, or two types of insulating fine particles having different average particle diameters may be mixed and used.

The content of the insulating fine particles contained in the insulating layer is preferably 50 to 90% by volume with respect to 100% by volume of the total of the insulating fine particles and the binder for the insulating layer. When the content of the insulating fine particles is 50% by volume or more, the ratio of the insulating fine particles, which is a heat-resistant component, in the insulating layer is increased, and the safety of the lithium ion secondary battery is improved. On the other hand, when the content of the insulating fine particles is 90% by volume or less, the ratio of the binder for the insulating layer, which is a binding component, is increased, the strength of the insulating layer is increased, and the safety of the lithium ion secondary battery is improved. .. From the above viewpoint, the content of the insulating fine particles contained in the insulating layer is more preferably 70 to 85% by volume with respect to 100% by volume of the total of the insulating fine particles and the binder for the insulating layer.

(Binder for insulating layer)
Binders for the insulating layer include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), fluorine-containing resin such as polytetrafluoroethylene (PTFE), polymethylacrylate (PMA), and poly. Acrylic resin such as methyl methacrylate (PMMA), polyvinylidene acetate, polyimide (PI), polyamide (PA), polyvinylidene chloride (PVC), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP), poly Examples thereof include acrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene butadiene rubber, poly (meth) acrylic acid, carboxymethyl cellulose, hydroxyethyl cellulose, 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 content of the binder for the insulating layer contained in the insulating layer is preferably 10 to 50% by volume with respect to 100% by volume of the total of the insulating fine particles and the binder for the insulating layer. When the content of the binder for the insulating layer is 10% by volume or more, the ratio of the binder for the insulating layer, which is a binding component, is increased, the strength of the insulating layer is increased, and the safety of the lithium ion secondary battery is improved. On the other hand, when the content of the binder for the insulating layer is 50% by volume or less, the ratio of the insulating fine particles, which are heat-resistant components, in the insulating layer increases, and the safety of the lithium ion secondary battery is improved. From the above viewpoint, the content of the binder for the insulating layer contained in the insulating layer is more preferably 15 to 30% by volume with respect to 100% by volume of the total of the insulating fine particles and the binder for the insulating layer.
The insulating layer may contain 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.

<Manufacturing method of electrodes for lithium-ion secondary batteries>
Next, an embodiment of a method for manufacturing an electrode for a lithium ion secondary battery will be described in detail. In the method for producing an electrode for a lithium ion secondary battery of the present invention, first, a second electrode active material layer is formed, and the composition for the first electrode active material layer is applied on the surface of the electrode active material layer. To form the first electrode active material layer. Further, the insulating layer composition may be applied on the surface of the first electrode active material layer to form the insulating layer.

(Formation of the second electrode active material layer)
In the formation of the second electrode active material layer, first, a composition for the second electrode active material layer containing graphite, a binder for the second electrode active material layer, and a solvent is prepared. The composition for the second electrode active material layer may contain other components such as a conductive additive to be blended if necessary. Graphite, the binder for the second electrode active material layer, the conductive auxiliary agent and the like are as described above. The composition for the second electrode active material layer becomes a slurry.

Water is preferably used as the solvent in the composition for the second electrode active material layer. By using water, the binder for the second electrode active material layer described above can be easily dissolved in the composition for the second electrode active material layer.
The solid content concentration of the composition for the second electrode active material layer is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

The second electrode active material layer may be formed by a known method using the composition for the second electrode active material layer. For example, the composition for the second electrode active material layer may be formed on the current collector. It can be formed by applying to and drying.
Further, the second electrode active material layer may be formed by applying the composition for the second electrode active material layer on a base material other than the current collector and drying it. Examples of the base material other than the current collector include known release sheets. The second electrode active material layer formed on the base material may be transferred onto the current collector by peeling the second electrode active material layer from the base material.
The second electrode active material layer formed on the current collector or the base material is preferably pressure-pressed. By pressurizing, it becomes possible to increase the electrode density. The pressure press may be performed by a roll press or the like.

(Formation of the first electrode active material layer)
The composition for the first electrode active material layer used for forming the first electrode active material layer contains a Si-based material, a binder for the first electrode active material layer, and a solvent. The composition for the first electrode active material layer may contain other components such as a conductive additive to be blended if necessary. The Si-based material, the binder for the first electrode active material layer, the conductive additive, and the like are as described above. The composition for the first electrode active material layer is a slurry.

Water is preferably used as the solvent in the composition for the first electrode active material layer. By using water, the above-mentioned binder for the first electrode active material layer can be easily dissolved in the composition for the first electrode active material layer.
The solid content concentration of the composition for the first electrode active material layer is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

The first electrode active material layer can be formed by applying the composition for the first electrode active material layer on the second electrode active material layer and drying. The method of applying the composition for the first electrode active material layer to the surface of the second electrode active material layer is not particularly limited, and for example, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, and a bar coating method are used. Examples include the method, the gravure coating method, and the screen printing method. Among these, the bar coating method or the gravure coating method is preferable from the viewpoint of uniformly applying the composition for the first electrode active material layer.
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. The drying time is not particularly limited, but is, for example, 30 seconds to 10 minutes.

(Formation of insulating layer)
The composition for an insulating layer used for forming the insulating layer contains insulating fine particles, a binder for the insulating layer, and a solvent. The composition for the insulating layer may contain other optional components to be blended as needed. Details of the insulating fine particles, the binder for the insulating layer, and the like are as described above. The composition for the insulating layer is a slurry.

The solid content concentration of the composition for the insulating layer is preferably 5 to 75% by mass, more preferably 15 to 50% by mass. The viscosity of the composition for the insulating layer is preferably 1000 to 3000 mPa · s, more preferably 1700 to 2300 mPa · s. The viscosity is a viscosity measured with a B-type viscometer under the conditions of 60 rpm and 25 ° C.

The insulating layer can be formed by applying the composition for an insulating layer on the first electrode active material layer and drying it. The method of applying the composition for the insulating layer to the surface of the first electrode active material layer is not particularly limited, and for example, 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. , Screen printing method and the like. Among these, the bar coating method or the gravure coating method is preferable from the viewpoint of uniformly applying the composition for the insulating layer to thin the insulating layer.
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. 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 above-mentioned electrode for a lithium ion secondary battery as a negative electrode. 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 the negative electrode is the first electrode active material layer and the second electrode active material layer described above. It becomes an electrode for a lithium ion secondary battery having.

(Positive electrode)
The positive electrode of the lithium ion secondary battery of the present invention is not particularly limited. The positive electrode includes, for example, a positive electrode active material layer and a current collector, and the positive electrode active material layer contains a positive electrode active material and a binder for a positive electrode.
Examples of the positive electrode active material include lithium metallic acid compounds. Examples of the lithium metal acid compound include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like. Further, it may be olivine type lithium iron phosphate (LiFePO ₄ ) or the like. Further, a plurality of metals other than lithium may be used, and NCM (nickel cobalt manganese) oxides, NCA (nickel cobalt aluminum) oxides and the like, which are called ternary oxides, may be used.
As the binder for the positive electrode, the same binder as the binder for the first electrode active material layer or the binder for the second electrode active material described above can be used.
The material used as the current collector is the same as that of the compound used for the negative electrode current collector, but aluminum or copper is preferably used, and aluminum is more preferable.

(Separator)
The lithium ion secondary battery of the present invention preferably further includes 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 is more effectively prevented. Further, the separator may retain an electrolyte described later.
Examples of the separator include a porous polymer film, a non-woven fabric, and glass fiber, and among these, a porous polymer film is preferable. Examples of the porous polymer film include an olefin-based porous film.

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

(Electrolytes)
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. As the electrolyte, for example, an electrolytic solution is used.
Examples of the electrolytic solution include an organic solvent and an electrolytic solution containing an electrolyte salt. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, and tetrohydra. Polar solvents such as furan, 2-methyltetraethane, dioxolane, and methylacetamide, or mixtures of two or more of these solvents can be mentioned. Electrolyte salts include 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 , Examples thereof include lithium-containing salts such as LiN (COCF ₃ ) _2, LiN (COCF ₂ CF ₃ ) ₂ , and lithium bisoxalate boronate (LiB (C ₂ O ₄ ) ₂ ). Also, lithium organic acid salt-3 foot. Examples thereof include a boron carbonate complex, a complex such as a complex hydride such as LiBH _{4, and the} like. These salts or complexes may be used alone or as a mixture of two or more.
Further, the electrolyte may be a gel-like electrolyte in which the above-mentioned electrolyte solution further contains a polymer compound. Examples of the polymer compound include a fluorine-based polymer such as polyvinylidene fluoride and a polyacrylic polymer such as methyl poly (meth) acrylate. The gel electrolyte may be used as a separator.
The electrolyte may be arranged between the negative electrode and the positive electrode. For example, the electrolyte is filled in the above-mentioned negative electrode and positive electrode, or in a battery cell in which the negative electrode, the positive electrode, and the separator are housed. Further, the electrolyte may be applied on the negative electrode or the positive electrode and arranged between the negative electrode and the positive electrode, for example.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The obtained lithium ion secondary battery was evaluated by the following evaluation method.
(capacity)
The prepared lithium ion secondary battery was charged and discharged once at 20 ° C., and the discharge capacity was measured.
The measured discharge capacity was divided by the thickness of the negative electrode to calculate the capacity per thickness.
The measured discharge capacity was divided by the thickness of the negative electrode for the following reasons.
Generally, the capacity of a battery is determined by an electrode having a smaller capacity (usually a positive electrode) among a positive electrode and a negative electrode. Therefore, even if the total thickness of the negative electrode is kept constant and the thickness of the negative electrode is changed, the capacity of the battery is determined by the positive electrode, so that the capacity of the battery does not change. Therefore, the thickness of the negative electrode was redesigned to match the capacity of the positive electrode, and the measured discharge capacity was divided by the thickness of the negative electrode so that the capacity characteristics of the battery caused by the negative electrode could be evaluated.
Discharging and charging was performed under the following conditions.
Charging conditions: CCCV charging. The CC conditions were 4.2V and 1.0C. The CV conditions were 4.2 V and 0.05 C termination.
Discharge condition: CC discharge. The CC conditions were 2.5V and 1.0C.
The calculated capacity per thickness was evaluated as follows.
A: 3.5Ah / g / μm or more B: 3.1Ah / g / μm or more, less than 3.5Ah / g / μm C: 2.9Ah / g / μm or more, less than 3.1Ah / g / μm D: Less than 2.9 Ah / g / μm

(Cycle characteristics)
The prepared lithium ion secondary battery was subjected to a charge / discharge cycle under an environment of a temperature of 40 ° C. with a charge rate of 2C and a discharge rate of 1C.
The capacity retention rate was calculated by dividing the discharge capacity after 500 cycles by the discharge capacity after 10 cycles. The cycle characteristics were evaluated from the capacity retention rate as follows.
A: Capacity retention rate is 50% or more B: Capacity retention rate is 45% or more and less than 50% C: Capacity retention rate is 30% or more and less than 45% D: Capacity retention rate is less than 30%

The physical properties of the obtained electrode for the lithium ion secondary battery were measured by the following measuring method.
(Porosity)
The cross section of the electrode for the lithium ion secondary battery was exposed by the ion milling method. Next, the cross section of the exposed electrode for the lithium ion secondary battery is shown as a whole of the first electrode active material layer or the second electrode active material layer by using FE-SEM (field emission scanning electron microscope). Was observed at an observable magnification, and an image of the first electrode active material layer or the second electrode active material layer was obtained. The magnification was 5000 to 25000 times. Next, using the image analysis software "Image J", the real part of the first electrode active material layer or the second electrode active material layer is displayed in black, and the void part is displayed in white. The image was binarized. Then, the ratio of the area of the white portion was measured using the image analysis software "Image J". The ratio of the area of this white part is the porosity (%).

(thickness)
The cross section of the electrode for the lithium ion secondary battery was exposed by the same method as the above-mentioned method for evaluating the porosity. Then, the thickness of the electrode active material layer was measured using the above-mentioned SEM. The above-mentioned ImageJ was used for measuring the thickness.

[Example 1]
(Preparation of the second negative electrode active material layer)
97 parts by mass of graphite (average particle diameter 10 μm) as a negative electrode active material, 1.5 parts by mass of styrene-butadiene rubber (SBR) as a binder, 1.5 parts by mass of a sodium salt of carboxymethyl cellulose (CMC), and water as a solvent. Was mixed to adjust the solid content to 50% by mass to obtain a composition for a second negative electrode active material layer. This composition was applied to both sides of a copper foil having a thickness of 8 μm as a negative electrode current collector and vacuum dried at 100 ° C. Then, the negative electrode current collector coated with the composition for the second negative electrode active material layer on both sides is pressure-pressed at a linear pressure of 400 kN / m to obtain a negative electrode first layer electrode having the second negative electrode active material layer. did. The density of the second negative electrode active material layer was 1.21 g / cc. The thickness of the second negative electrode active material layer was 23 μm per side.

(Preparation of the first negative electrode active material layer)
97 parts by mass of silicon monoxide (SiO) (average particle size 5 μm) as the negative electrode active material, 1.5 parts by mass of styrene butadiene rubber (SBR) as the binder, and 1.5 parts by mass of the sodium salt of carboxymethyl cellulose (CMC). , Water was mixed as a solvent to adjust the solid content to 50% by mass to obtain a first composition for the negative electrode active material layer. This composition was applied to both surfaces of the negative electrode first layer electrode and vacuum dried at 100 ° C. Then, the negative electrode current collector coated with the composition for the first negative electrode active material layer on both surfaces was pressure-pressed at a linear pressure of 500 kN / m to obtain a negative electrode.
The thickness of the second negative electrode active material layer changed from 23 μm to 18 μm. The density of the second negative electrode active material layer changed from 1.21 g / cc to 1.55 g / cc. The thickness of the first negative electrode active material layer was 35 μm per side. The density of the first negative electrode active material layer was 1.21 g / cc.

(Formation of insulating layer)
Prepare a coating solution containing 80% by volume of alumina having an average particle diameter of 500 nm per solid content and 20% by volume of an acrylic resin, diluted with a solvent N-methyl-2-pyrrolidone, and having a solid content concentration of 40% by mass. did. The viscosity of the coating liquid at 25 ° C. was 1500 mPa · s. Then, the negative electrode of the coating liquid was coated using a bar coater type coating device. After applying the coating liquid to both surfaces of the negative electrode, it was dried at 60 ° C. for 1 hour to obtain an insulating layer-forming negative electrode. The thickness of the insulating layer after drying was 8 μm per side. The porosity of the insulating layer was 70%.

(Preparation of positive electrode)
100 parts by mass of Li (Ni-Co-Al) O ₂ (NCA-based oxide) having an average particle diameter of 10 μm as a positive electrode active material, 4 parts by mass of acetylene black as a conductive auxiliary agent, and polyvinylidene fluoride as an electrode binder. 4 parts by mass of (PVdF) and N-methylpyrrolidone (NMP) as a solvent were mixed to obtain a composition for a positive electrode active material layer adjusted to a solid content concentration of 60% by mass. This composition for a positive electrode active material layer was applied to both sides of an aluminum foil having a thickness of 15 μm as a positive electrode current collector, pre-dried, and then vacuum dried at 120 ° C. Then, a positive electrode current collector coated with the composition for a positive electrode active material layer on both sides was pressure-pressed at 400 kN / m to prepare a positive electrode. The thickness of the positive electrode active material layer was 50 μm per side.

(Preparation of electrolyte)
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 electrolytic solution was prepared. Prepared.

(Battery manufacturing)
A temporary laminate was obtained by laminating the two negative electrodes obtained above, two microporous membrane separators made of PE, and one positive electrode. Here, one positive electrode was placed between the two negative electrodes, and one microporous membrane separator was placed between the negative electrode and the positive electrode.
The exposed ends of the positive electrode current collectors of each positive electrode were joined together by ultrasonic fusion, and the terminal tabs protruding to the outside were joined. Similarly, the exposed ends of the negative electrode current collectors of each negative electrode were joined together by ultrasonic fusion, and the terminal tabs protruding to the outside were joined.
Next, the laminate was sandwiched between aluminum laminate films, the terminal tabs were projected to the outside, and the three sides were sealed by laminating. A laminated cell was manufactured by injecting the electrolytic solution obtained above from one side left unsealed and vacuum-sealing.
At this time, the area of the positive electrode was 50 mm × 50 mm, and the area of the negative electrode was 55 mm × 55 mm.

[Example 2]
The coating conditions for applying the composition for the first negative electrode active material layer were changed to change the thickness of the first negative electrode active material layer from 35 μm to 43 μm, and the composition for the second negative electrode active material layer was changed. The same procedure as in Example 1 was carried out except that the coating conditions at the time of coating were changed and the thickness of the second negative electrode active material layer was changed from 18 μm to 8 μm.

[Example 3]
The coating conditions for applying the composition for the first negative electrode active material layer were changed to change the thickness of the first negative electrode active material layer from 35 μm to 13 μm, and the composition for the second negative electrode active material layer was changed. The same procedure as in Example 1 was carried out except that the coating conditions at the time of coating were changed and the thickness of the second negative electrode active material layer was changed from 18 μm to 44 μm.

[Example 4]
The coating conditions for applying the composition for the first negative electrode active material layer were changed to change the thickness of the first negative electrode active material layer from 35 μm to 11 μm, and the composition for the second negative electrode active material layer was changed. The same procedure as in Example 1 was carried out except that the coating conditions at the time of coating were changed and the thickness of the second negative electrode active material layer was changed from 18 μm to 49 μm.

[Example 5]
The coating conditions for applying the composition for the first negative electrode active material layer were changed to change the thickness of the first negative electrode active material layer from 35 μm to 39 μm, and the composition for the second electrode active material layer was changed. The same procedure as in Example 1 was carried out except that the coating conditions at the time of coating were changed and the thickness of the second negative electrode active material layer was changed from 18 μm to 6 μm.

[Comparative Example 1]
The thickness of the second electrode active material layer was changed by changing the point that the composition for the first negative electrode active material layer was not applied and the coating conditions when applying the composition for the second negative electrode active material layer. Was changed from 18 μm to 70 μm, but the same procedure as in Example 1 was carried out.

[Comparative Example 2]
The coating conditions for applying the composition for the first negative electrode active material layer were changed to change the thickness of the first negative electrode active material layer from 35 μm to 53 μm, and for the second negative electrode active material layer. The procedure was carried out in the same manner as in Example 1 except that the composition was not applied.

[Comparative Example 3]
The same procedure as in Example 1 was carried out except that the negative electrode prepared as follows was used instead of the negative electrode provided with the first negative electrode active material layer and the second negative electrode active material layer.
A mixture was prepared by mixing graphite powder and SiO powder so that the ratio of graphite to silicon monoxide (SiO) (graphite / SiO) was 18/35% by volume. Mix 100 parts by mass of the mixture, 1.5 parts by mass of styrene-butadiene rubber (SBR) as a binder, 1.5 parts by mass of a sodium salt of carboxymethyl cellulose (CMC), and water as a solvent to obtain a solid content of 50% by mass. A prepared composition for the negative electrode active material layer was obtained. This composition was applied to both sides of a copper foil having a thickness of 8 μm as a negative electrode current collector and vacuum dried at 100 ° C. Then, the negative electrode current collector coated with the composition for the negative electrode active material layer on both sides was pressure-pressed at a linear pressure of 500 kN / m to obtain a negative electrode. The thickness of the negative electrode active material layer was 53 μm per side.

[Comparative Example 4]
The coating conditions for applying the composition for the first negative electrode active material layer were changed to change the thickness of the first electrode active material layer from 35 μm to 52 μm, and the second negative electrode active material layer Instead, it was carried out in the same manner as in Example 1 except that the conductive material undercoat prepared as follows was formed.
100 parts by mass of acetylene black having an average particle diameter of 50 nm, 5.0 parts by mass of styrene-butadiene rubber (SBR) as a binder, 5.0 parts by mass of a sodium salt of carboxymethyl cellulose (CMC), and water as a solvent are mixed. , A composition for undercoating a conductive material adjusted to a solid content of 50% by mass was obtained. This composition was applied to both sides of a copper foil having a thickness of 8 μm as a negative electrode current collector and vacuum dried at 100 ° C. to obtain a negative electrode. The thickness of the negative electrode active material layer was 1.0 μm.

[Comparative Example 5]
This was carried out in the same manner as in Comparative Example 2 except that the insulating layer was not provided.

Table 1 shows the evaluation results of the batteries manufactured in Examples 1 to 5, and Table 2 shows the evaluation results of the batteries manufactured in Comparative Examples 1 to 5, respectively.

As shown in Examples 1 to 5 above, lithium ions are provided by providing a second negative electrode active material layer containing graphite between the first negative electrode active material layer containing a Si-based material and the negative electrode current collector. It was found that the capacity of the secondary battery can be increased and the charge / discharge cycle characteristics can be improved.
From Comparative Example 1, it was found that the capacity of the lithium ion secondary battery could not be increased unless the first negative electrode active material layer containing the Si-based material was provided.
From Comparative Example 2, Comparative Example 4 and Comparative Example 5, if a second negative electrode active material layer containing graphite is not provided between the first negative electrode active material layer containing the Si-based material and the negative electrode current collector, lithium is used. It was found that the charge / discharge cycle characteristics of the ion secondary battery deteriorated.
By comparing Examples 1 to 5 with Comparative Example 3, the first method containing the Si-based material is compared with the conventional method of mixing the carbon-based material with the Si-based material to alleviate the expansion and contraction of the Si-based material. It was found that the method of providing the second negative electrode active material layer containing graphite between the negative electrode active material layer and the negative electrode current collector can improve the charge / discharge cycle characteristics of the lithium ion secondary battery.
By comparing Comparative Example 2 and Comparative Example 5, it was found that the presence or absence of the insulating layer did not significantly affect the capacity and charge / discharge cycle characteristics of the lithium ion secondary battery.

1 Electrode for lithium ion secondary battery 10 Current collector 20 First electrode active material layer 30 Second electrode active material layer

Claims (7)

A second electrode active material layer provided between the current collector, a first electrode active material layer provided on the surface of the current collector, and the current collector and the first electrode active material layer. With and
The first electrode active material layer contains a Si-based material and a binder for the first electrode active material layer.
An electrode for a lithium ion secondary battery in which the second electrode active material layer contains graphite and a binder for the second electrode active material layer. The electrode for a lithium ion secondary battery according to claim 1, wherein the thickness of the first electrode active material layer is 10 to 70 μm, and the thickness of the second electrode active material layer is 5 to 60 μm. Claim 1 or 2 in which the ratio (D1 / D2) of the thickness (D1) of the first electrode active material layer to the thickness (D2) of the second electrode active material layer is 0.1 to 10. The electrode for a lithium ion secondary battery described. Claims 1 to 3 in which the average particle size of the Si-based material in the first electrode active material layer is 1 to 30 μm, and the average particle size of the graphite in the second electrode active material layer is 1 to 30 μm. The electrode for a lithium ion secondary battery according to any one of the above items. The electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the content of the Si-based material in the first electrode active material layer is 95 to 99% by mass. The electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the graphite content in the second electrode active material layer is 95 to 99% by mass. A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of claims 1 to 6 as a negative electrode.