WO2020158306A1

WO2020158306A1 - Electrode for lithium ion secondary battery and lithium ion secondary battery

Info

Publication number: WO2020158306A1
Application number: PCT/JP2020/000218
Authority: WO
Inventors: 章弘鈴木
Original assignee: 積水化学工業株式会社
Priority date: 2019-01-28
Filing date: 2020-01-08
Publication date: 2020-08-06
Also published as: JPWO2020158306A1; CN112840479A

Abstract

This electrode 1 for lithium ion secondary batteries includes an electrode active material layer 10 and an insulating layer 20 that is provided on a surface of the electrode active material layer 10, and the insulating layer 20 includes a polymer solid electrolyte and has a porosity of 10% or less. This lithium ion secondary battery includes the electrode for lithium ion secondary batteries and an electrolyte. The present invention makes it possible to provide: an electrode for lithium ion secondary batteries which has an insulating layer that is capable of having good electronically insulating properties and improving the volumetric energy density of the lithium ion secondary batteries even when the insulating layer is thinned; and a lithium ion secondary battery that includes the electrode for lithium ion secondary batteries.

Description

Electrode for lithium-ion secondary battery and lithium-ion secondary battery

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

BACKGROUND OF THE INVENTION Lithium ion secondary batteries are used as large stationary power sources for power storage, power sources for electric vehicles, etc., and research into miniaturization and thinning of batteries has been progressing in recent years. A lithium ion secondary battery is generally provided with both electrodes in which an electrode active material layer is formed on the surface of a metal foil, and a separator arranged between both electrodes. The separator plays a role of preventing a short circuit between both electrodes and holding an electrolytic solution.
Conventionally, lithium ion secondary batteries, for example, in order to have a good short-circuit suppressing function, even when the separator is contracted, it has been considered that a porous insulating layer is provided on the surface of the electrode active material layer. There is. It is known that the insulating layer is formed by, for example, as disclosed in Patent Document 1, applying an insulating layer slurry containing insulating fine particles, a binder and a solvent onto the electrode active material layer and drying the slurry. Has been.

International publication 2016/104782

However, if the conventional insulating layer is thinned to improve the volume energy density of the lithium ion secondary battery, the electronic insulating property of the insulating layer may be insufficient.
Therefore, the present invention provides a lithium ion secondary battery electrode that includes an insulating layer that can improve the electronic insulating property even when the layer is thinned and can improve the volume energy density of the lithium ion secondary battery, and It is an object to provide a lithium ion secondary battery including the electrode for the lithium ion secondary battery.

As a result of intensive studies, the inventors of the present invention used a polymer solid electrolyte as a binder for the insulating layer, and set the porosity of the insulating layer to 10% or less, so that even if the insulating layer was thinned, the electrons in the insulating layer were reduced. It was found that the insulating property can be improved, and the following invention was completed. The gist of the present invention is the following [1] to [7].
[1] Lithium including an electrode active material layer and an insulating layer provided on the surface of the electrode active material layer, the insulating layer containing a polymer solid electrolyte, and the porosity of the insulating layer being 10% or less. Electrode for ion secondary battery.
[2] The insulating layer optionally contains insulating fine particles, and the content of the insulating fine particles in the insulating layer is 20% by volume or less based on 100% by volume of the total of the polymer solid electrolyte and the insulating fine particles. The electrode for a lithium ion secondary battery according to the above [1], which is
[3] The electrode for a lithium ion secondary battery according to the above [1] or [2], wherein the polymer solid electrolyte is a polyether electrolyte.
[4] The electrode for a lithium ion secondary battery according to the above [3], wherein the polymer serving as the matrix of the polyether-based electrolyte is a polymer having at least an ethylene oxide structure.
[5] The electrode for a lithium ion secondary battery according to any one of the above [1] to [4], wherein the polymer solid electrolyte contains a lithium salt.
[6] The electrode for a lithium ion secondary battery according to any one of the above [1] to [5], wherein the insulating layer has a thickness of less than 30 μm.
[7] A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of the above [1] to [6] and an electrolytic solution.

According to the present invention, an electrode for a lithium ion secondary battery, which is provided with an insulating layer that can improve the electronic insulating property even when the layer is thinned and can improve the volume energy density of the lithium ion secondary battery, and A lithium ion secondary battery including the electrode for the lithium ion secondary battery can be provided.

FIG. 1 is a schematic sectional view showing a preferred embodiment of an electrode for a lithium ion secondary battery of the present invention. FIG. 2 is a diagram for explaining a battery for characteristic evaluation.

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

In the present invention, the lithium ion secondary battery electrode may be either a positive electrode or a negative electrode, but is preferably a positive electrode.

[Insulation layer]
The insulating layer contains a solid polymer electrolyte, and the porosity of the insulating layer is 10% or less.

(Polymer solid electrolyte)
A polymer solid electrolyte is a material mainly composed of a polymer and exhibiting ion conductivity. Examples of the polymer solid electrolyte include dry type polymer electrolytes and gel type polymer electrolytes. In the dry type polyelectrolyte, it is considered that ion conduction is essentially caused by the movement of the polymer skeleton. On the other hand, in the gel type polymer electrolyte, the conduction of ions occurs through the electrolytic solution containing a large amount. From the viewpoint of high mechanical strength, a preferable polymer solid electrolyte is a dry type polymer electrolyte.

A preferred dry-type solid polymer electrolyte is a polyether-based electrolyte, from the viewpoints of high ionic conductivity and high mechanical strength, and from the viewpoint of enormous studies on molecular design. The polymer serving as the matrix of the polyether-based electrolyte preferably has an ethylene oxide structure, a propylene oxide structure, or both structures. Examples of the polymer serving as the matrix of the polyether electrolyte include polyethylene oxide, polypropylene oxide, ethylene oxide-propylene copolymer, dimethylsiloxane-ethylene oxide copolymer and the like. Further, a comb polymer having a polyether side chain containing an ethylene oxide structure, a copolymer of a monomer other than ethylene oxide and ethylene oxide, a product obtained by crosslinking polyethylene oxide or a polyether oligomer with a crosslinking agent, and having a branch. Other examples include branched polyether-based polymers, and those obtained by thermally or photopolymerizing macromonomers having a molecular weight of several hundreds to several thousands. These polymers may be used alone or in combination of two or more. From the viewpoint of high ionic conductivity and high mechanical strength, the polymer serving as the matrix of the polyether electrolyte is more preferably a polymer having at least an ethylene oxide structure, and further preferably polyethylene oxide. The ethylene oxide structure is composed of a basic unit composed of ethylene and oxygen.

From the viewpoint of increasing the ionic conductivity of the insulating layer, the content of the polymer solid electrolyte in the insulating layer is preferably 80% by volume or more, more preferably 90% by volume or more, and further preferably 95% by volume. It is above, and particularly preferably 98% by volume or more. The upper limit of the content of the polymer solid electrolyte is 100% by volume.

(Lithium salt)
From the viewpoint of increasing the ionic conductivity of the polymer solid electrolyte, the polymer solid electrolyte preferably contains a lithium salt. For example, when the solid polymer electrolyte is a polyether-based electrolyte, cations (lithium ions) of the lithium salt and ion-dipole interaction between the lone electron pair of ether oxygen in the polymer serving as the matrix of the polyether-based electrolyte cause It is believed that the lithium salt is complexed and the lithium salt is dissolved in the matrix polymer. Then, it is considered that a part of the dissolved lithium salt is in a dissociated state, and the ionic conductivity of the polyether electrolyte is higher.

Examples of the lithium salt used for the polymer solid electrolyte include LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , and lower. Examples thereof include lithium aliphatic carboxylate, lithium chloroborane, LiBPh ₄ (lithium tetraphenylborate), LiTFSA (lithium bistrifluoromethylsulfonylamide), and LiTFSI (lithium bistrifluoromethylsulfonylimide). These lithium salts may be used alone or in combination of two or more. Among these, LiTFSA and LiTFSI are preferable, and LiTFSI is more preferable, from the viewpoint that the dissociation property of the lithium salt in the polymer solid electrolyte can be increased.

From the viewpoint of the ionic conductivity of the solid polymer electrolyte and the mechanical strength of the solid polymer electrolyte, the amount of the lithium salt blended is preferably 1 to 100 parts by mass with respect to 100 parts by mass of the polymer serving as the matrix of the solid polymer electrolyte. , More preferably 5 to 80 parts by mass, further preferably 10 to 50 parts by mass.

(Porosity of insulating layer)
The porosity of the insulating layer is 10% or less. When the porosity of the insulating layer is larger than 10%, a minute short circuit may occur in the void portion of the insulating layer, and the electronic insulating property of the insulating layer may deteriorate. In particular, when the thickness of the insulating layer is less than 30 μm, the electronic insulating property of the insulating layer is likely to deteriorate when the porosity of the insulating layer is larger than 10%. From the viewpoint of further enhancing the electronic insulating property of the insulating layer, the porosity of the insulating layer is preferably 7% or less, more preferably 5% or less, further preferably 3% or less, and particularly preferably It is 1% or less. The lower limit of the range of the porosity of the insulating layer is 0%. Also, the thinner the insulating layer, the higher the volume energy density of the lithium-ion secondary battery can be. From this point of view, the insulating layer used in the electrode for a lithium ion secondary battery of the present invention can secure electronic insulation even if it is thin, so that the volume energy density of the lithium ion secondary battery can be improved. The porosity of the insulating layer can be measured by the method described in the item of Examples below.

(Thickness of insulating layer)
From the viewpoint of improving the volume energy density of the lithium ion secondary battery, the thickness of the insulating layer is preferably less than 30 μm, more preferably 25 μm or less, further preferably 20 μm or less, and even more preferably It is 15 μm or less, and particularly preferably 13 μm or less. In addition, the thickness of the insulating layer is preferably 1 μm or more, more preferably 3 μm or more, and further preferably 5 μm or more, from the viewpoint of more reliably ensuring the electronic insulating property of the insulating layer. Further, from the viewpoint of workability when forming the insulating layer, the thickness of the insulating layer is more preferably 7 μm or more, and particularly preferably 9 μm or more. The upper limit and the lower limit of the thickness range of the insulating layer can be arbitrarily combined.

(Insulating fine particles)
The insulating layer may optionally contain insulating fine particles. Thereby, the mechanical strength of the insulating layer can be increased. From the viewpoint of reducing the porosity of the insulating layer, the content of the insulating fine particles in the insulating layer is preferably 20% by volume or less based on 100% by volume of the total of the solid polymer electrolyte and the insulating fine particles. It is preferably 10% by volume or less, more preferably 7% by volume or less, particularly preferably 5% by volume or less, and even more preferably 1% by volume or less. From the viewpoint of surely reducing the porosity of the insulating layer, it is particularly preferable that the insulating layer does not contain 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, crosslinked polymethylmethacrylate, crosslinked styrene-acrylic acid copolymer, crosslinked acrylonitrile resin, polyamide resin, polyimide resin, poly(lithium 2-acrylamido-2-methylpropanesulfonate), Examples thereof include particles composed of organic compounds such as polyacetal resin, epoxy resin, polyester resin, phenol resin, and melamine resin. 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, fluorine. Examples thereof include particles composed of an inorganic compound such as lithium chloride, 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 each of the above-mentioned materials is used alone, or particles in which two or more kinds are used in combination. The insulating fine particles may be fine particles containing both an inorganic compound and an organic compound. For example, it may be inorganic-organic composite particles in which the surface of particles 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 diameter of the insulating fine particles is usually smaller than the average particle diameter of the electrode active material, and is, for example, 0.001 to 2 μm, preferably 0.05 to 1.5 μm, and more preferably 0.1 to 1. It is 0 μm, and particularly preferably 0.1 to 0.5 μm. By setting the average particle diameter of the insulating layer within these ranges, the mechanical strength of the insulating layer can be further increased.
The average particle size means the particle size (D50) at a volume cumulative of 50% in the particle size distribution of insulating fine particles determined 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 mixed and used.

(Method of forming insulating layer)
The insulating layer can be formed, for example, by disposing a polymer solid electrolyte film on the surface of the electrode active material layer. The polymer solid electrolyte film may or may not be adhered to the electrode active material layer by heating, pressure bonding, or the like. The polymer solid electrolyte film can be prepared, for example, by dissolving a polymer serving as a matrix of the polymer solid electrolyte in a suitable solvent together with a lithium salt, and then evaporating the solvent. Further, by polymerizing a raw material monomer (for example, a macromonomer having a molecular weight of several hundreds to several thousands) to which a lithium salt is added and which serves as a matrix of the polymer solid electrolyte, by polymerizing by thermal polymerization or photopolymerization, A solid electrolyte film may be prepared.
You may form an insulating layer by apply|coating the coating material of a polymer solid electrolyte on the surface of an electrode active material layer, and hardening the applied coating material.
When the insulating layer contains insulating fine particles, for example, a polymer serving as a matrix of the polymer solid electrolyte is dispersed or dissolved in a suitable solvent together with the insulating fine particles and the lithium salt, and then the solvent is evaporated. , A film of an insulating layer can be produced. In addition, a coating material obtained by mixing a polymer serving as a matrix of a polymer solid electrolyte, insulating fine particles, a lithium salt and a solvent is applied to the surface of an electrode active material layer, and the applied coating material is cured to form an insulating layer. May be formed.

[Electrode active material layer]
The electrode active material layer typically includes an electrode active material and an electrode binder. When the electrode is a positive electrode, the electrode active material becomes the positive electrode active material and the electrode active material layer becomes the positive electrode active material layer. On the other hand, when the electrode is the negative electrode, the electrode active material becomes the negative electrode active material and the electrode active material layer becomes the negative electrode active material layer.

(Cathode active material)
Examples of the positive electrode active material used for the positive electrode active material layer include lithium metal oxide compounds. Examples of the lithium metal oxide compound include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganate (LiMn ₂ O ₄ ). Further, olivine-type lithium iron phosphate (LiFePO ₄ ) or the like may be used as the positive electrode active material. Further, as the positive electrode active material, a material using a plurality of metals other than lithium may be used, and NCM (nickel cobalt manganese)-based oxide, NCA (nickel cobalt aluminum-based) oxide, etc. called ternary system may be used. May be used. As the positive electrode active material, these materials may be used alone or in combination of two or more.

(Negative electrode active material)
Examples of the negative electrode active material used in the negative electrode active material layer include graphite, carbon materials such as hard carbon, composites of tin compounds and silicon and carbon, lithium, and the like. Among these, carbon materials are preferable, and graphite is preferable. More preferable. As the negative electrode active material, the above substances may be used alone or in combination of two or more.

(Average particle size of electrode active material)
The average particle size of the electrode active material is not particularly limited, but is preferably 0.5 to 50 μm, more preferably 1 to 30 μm, and further preferably 5 to 25 μm. The average particle size means the particle size (D50) at a volume cumulative of 50% in the particle size distribution of the electrode active material obtained by the laser diffraction/scattering method.

(Content of electrode active material)
The content of the electrode active material in the electrode active material layer is, based on the total amount of the electrode active material layer, preferably 50 to 99% by mass, more preferably 60 to 99% by mass, further preferably 80 to 99% by mass, and 90 to 98% by mass. Mass% is particularly preferred.

(binder)
Specific examples of the binder include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), fluorine-containing resin such as polytetrafluoroethylene (PTFE), polymethyl acrylate (PMA), Acrylic resin such as polymethylmethacrylate (PMMA), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP), poly Examples thereof include acrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene-butadiene rubber (SBR), poly(meth)acrylic acid, carboxymethyl cellulose (CMC), hydroxyethyl cellulose, and polyvinyl alcohol. These binders may be used alone or in combination of two or more. In addition, carboxymethyl cellulose and the like may be used in the form of salts such as sodium salt.
The content of the binder in the electrode active material layer is, based on the total amount of the electrode active material layer, preferably 0.5% by mass or more, more preferably 0.5 to 20% by mass, and 1.0 to 10% by mass. Mass% is more preferable.

(Conductive agent)
The electrode active material layer may further contain a conductive auxiliary agent, and the positive electrode active material layer preferably contains a conductive auxiliary agent. A material having higher conductivity than the electrode active material is used as the conduction aid, and specific examples thereof include carbon materials such as Ketjen black, acetylene black (AB), carbon nanotubes and rod-shaped carbon. The conductive additive 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 0.5 to 15% by mass, and preferably 1 to 10% by mass, based on the total amount of the electrode active material layer. More preferably.

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

[Electrode current collector]
Examples of the material forming the electrode current collector include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, when the electrode current collector is the positive electrode current collector, aluminum, titanium, nickel and stainless steel are preferable, and aluminum is more preferable. When the electrode current collector is the negative electrode current collector, copper, titanium, nickel and stainless steel are preferable, and copper is more preferable. The electrode current collector is generally composed of a metal foil, and the thickness thereof is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm. When the thickness of the electrode current collector is 1 to 50 μm, handling of the electrode current collector is facilitated and reduction in energy density can be suppressed.

<Lithium-ion secondary battery>
The lithium-ion secondary battery of the present invention includes an electrode for a lithium-ion secondary battery having the above-mentioned insulating layer and an electrolytic solution. Specifically, the lithium ion secondary battery of the present invention includes a positive electrode and a negative electrode that are arranged so as to face each other, and at least one of the negative electrode and the positive electrode has a lithium ion secondary battery having the above-mentioned insulating layer. It becomes an electrode for batteries. In this lithium ion secondary battery electrode (negative electrode or positive electrode), an insulating layer may be provided on the surface facing the other electrode (positive electrode or negative electrode). The lithium-ion secondary battery of the present invention preferably includes, as a positive electrode, the lithium-ion secondary battery electrode having the above-described insulating layer. Then, the electrolytic solution exists in the voids of the electrode active material layer, the voids of the insulating layer, and the like.

[Separator]
The lithium ion secondary battery of the present invention may further include a separator arranged between the positive electrode and the negative electrode. By providing the separator, a short circuit between the positive electrode and the negative electrode can be prevented more effectively. Moreover, the separator may hold an electrolyte. 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 porous polymer films, non-woven fabrics, glass fibers and the like, and among these, porous polymer films are preferable. An example of the porous polymer film is an olefin-based porous film. The separator may be heated by heat generated when the lithium-ion secondary battery is driven to cause thermal contraction. However, even when such a thermal contraction occurs, the insulating layer is provided so that a short circuit is easily suppressed.

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 electrodes and the positive electrodes 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 above-described negative electrode and positive electrode, or the negative electrode, positive electrode, and separator are housed in a battery cell. The battery cell may be a square type, a cylindrical type, a laminated type or the like.

[Electrolyte]
The electrolyte of the lithium ion secondary battery is not particularly limited, and a known electrolyte used in the lithium ion secondary battery may be used. For example, an electrolytic solution is used as the electrolyte.
Examples of the electrolytic solution include an electrolytic solution containing an organic solvent and an electrolyte salt. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2. Examples include polar solvents such as diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane and methyl acetate, or a mixture of two or more kinds 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 , Examples thereof include lithium-containing salts such as LiN(COCF ₃ ) ₂ and LiN(COCF ₂ CF ₃ ) ₂ and lithium bisoxalate borate (LiB(C ₂ O ₄ ) ₂ ) Organic acid lithium salt-trifluoride Examples thereof include boron hydride complexes and complexes such as complex hydrides such as LiBH _4. These salts or complexes may be used alone or in a mixture of two or more.
The electrolyte may be arranged 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 described above, or the negative electrode, the positive electrode, and the separator are housed inside. Further, the electrolyte may be applied, for example, on the negative electrode or the positive electrode and arranged between the negative electrode and the positive electrode.

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

The obtained lithium ion secondary battery electrode was evaluated by the following evaluation methods.
(Volume energy density)
Under the environment of a temperature of 45° C., 1 C of constant current charging was performed on the cells prepared in Examples and Comparative Examples. Then, the constant current discharge of 1C was performed, and the discharge was completed at the time of discharging to 2.5V, and the discharge capacity of the constant current discharge of 1C was calculated.
Then, the volume energy density was calculated from the following formula. The nominal voltage was 1V.
(Volume energy density)=(nominal voltage) [V]×(discharge capacity of constant current discharge of 1 C) [mAh]÷(volume of battery) [mm ³ ]
Since the electrodes are circular with a diameter of 14 mm as described later, the battery volume can be calculated by the following formula.
(Battery volume) [mm ³ ]=7 [mm]×7 [mm]×π× (total thickness of insulating layer, positive electrode active material layer, positive electrode current collector, negative electrode active material layer, and negative electrode current collector) [Mm]

(Electronic insulation)
In an environment of a temperature of 45° C., the presence or absence of a voltage drop 60 minutes after full charge with respect to the voltage at full charge at the first charge was examined. Further, the charging/discharging efficiency of the second charging/discharging was examined under the environment of the temperature of 45°C. The charge/discharge efficiency was calculated by dividing the discharge capacity by the charge capacity. And the following criteria evaluated.
◯: Voltage drop is 0.1 V or less and charge/discharge efficiency is 95% or more X: Voltage drop is greater than 0.1 V or charge/discharge efficiency is less than 95%

(Porosity)
The cross section of the lithium ion secondary battery electrode on which the insulating layer was formed was exposed by the ion milling method. Next, the exposed cross section of the lithium-ion secondary battery electrode is observed with a FE-SEM (field emission scanning electron microscope) at a magnification that allows observation of the entire insulating layer to obtain an image of the insulating layer. It was The magnification was 5000 to 25000 times. Next, the obtained image was binarized using image analysis software "Image J" 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. Then, the area ratio of the white portion was measured. The ratio of the area of this white part becomes the porosity (%) of the insulating layer.

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

[Example 1]
(Preparation of positive electrode)
100 parts by mass of NCA-based oxide (average particle size 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 a binder for electrodes, and N as a solvent. -Methylpyrrolidone (NMP) was mixed to obtain a slurry for a positive electrode active material layer adjusted to a solid content concentration of 60% by mass. This positive electrode active material layer slurry was applied to an aluminum foil having a thickness of 15 μm as a positive electrode current collector, preliminarily dried, and then vacuum dried at 120° C. Then, the positive electrode current collector coated with the positive electrode active material layer slurry is pressed by a roller at a linear pressure of 400 kN/m and further punched into a circular shape having an electrode size of 14 mm to obtain a positive electrode having a positive electrode active material layer. And The thickness of the positive electrode active material layer was 45.5 μm.

(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 electrodes, 1.5 parts by mass of styrene-butadiene rubber (SBR), and solvent Was mixed with water to obtain a slurry for negative electrode active material layer adjusted to a solid content of 50% by mass. This negative electrode active material layer slurry was applied to a copper foil having a thickness of 12 μm as a negative electrode current collector and vacuum dried at 100° C. After that, the negative electrode current collector coated with the slurry for the negative electrode active material layer was pressed by a roller at a linear pressure of 500 kN/m and punched into a circular shape having an electrode size of 14 mm to obtain a negative electrode having a negative electrode active material layer. And The thickness of the negative electrode active material layer was 52.5 μm.

(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) to a concentration of 1 mol/liter to prepare an electrolyte solution. Was prepared.

(Formation of insulating layer)
To a solution obtained by dissolving 2.5 parts by mass of LiTFSI (lithium bistrifluoromethylsulfonylimide) in 16 parts by mass of acetonitrile, 0.1 part by mass of a UV curing agent (trade name: Esacure KTO 46, manufactured by Sartomer Co., Ltd. Manufactured) was added to prepare a lithium salt-containing solution. Next, 10 parts by mass of ethylene oxide was added to the lithium salt-containing solution and stirred to prepare a polymer electrolyte solution. The obtained polymer electrolyte solution was applied to a Teflon (registered trademark) sheet and dried under reduced pressure at a drying temperature of 60° C. for 30 minutes. Another Teflon (registered trademark) sheet was placed on the dried polymer electrolyte, and the polymer electrolyte was sandwiched between two Teflon (registered trademark) sheets. Then, both sides of the polymer electrolyte sandwiched between two Teflon (registered trademark) sheets are irradiated with ultraviolet rays through the Teflon (registered trademark) sheet to cure the polymer electrolyte and produce a polymer solid electrolyte film. did. The polymer used as the matrix of this polymer solid electrolyte film is polyethylene oxide, and the lithium salt is LiTFSI. Then, the polymer solid electrolyte film was punched out into a circular shape having an electrode size of 14 mm in diameter to prepare an insulating layer. The insulating layer had a thickness of 10 μm. The porosity of the insulating layer was 0%.

(Manufacture of batteries)
A battery for characteristic evaluation was prepared by placing the positive electrode, the insulating layer, and the negative electrode in the battery characteristic evaluation jig 100 shown in FIG. 2 and injecting the electrolytic solution. Specifically, between the negative electrode body 106 and the positive electrode body 107, in order from the negative electrode body 106 side, a negative electrode 108, an insulating layer 109, an electrode guide 110, a positive electrode 111, an electrode retainer 112, and a spring 113 are attached to a battery characteristic evaluation jig. Placed at 100. Then, the above electrolyte solution was injected into the battery characteristic evaluation jig 100 to manufacture a battery.

[Examples 2 and 3]
Into a solution obtained by dissolving 2.5 parts by mass of LiTFSI (lithium bistrifluoromethylsulfonylimide) in 20 parts by mass of acetonitrile, alumina particles as insulating fine particles (manufactured by Nippon Light Metal Co., Ltd., product name: low soda) Alumina and an average particle diameter of 500 nm) were mixed and dispersed while applying a medium shearing force to obtain a slurry. To this slurry, 0.1 part by mass of a UV curing agent (trade name: Esacure KTO 46, manufactured by Sartomer) was added to prepare a lithium salt-containing solution. Next, ethylene oxide was added to the lithium salt-containing solution and stirred to prepare a polymer electrolyte slurry. The proportions of alumina particles and ethylene oxide are shown in the table. Thereafter, in the same manner as in Example 1, insulating layers used in the batteries for characteristic evaluation of Examples 2 and 3 were produced. The insulating layers both had a thickness of 10 μm. The porosity of the insulating layer was 3% and 8%, respectively. Except for this, batteries for characteristic evaluation of Examples 2 and 3 were produced in the same manner as in Example 1.

[Comparative Example 1]
After adding NMP to a polyvinylidene fluoride solution (manufactured by Kureha Co., Ltd., product name: L#1710, 10 mass% solution, solvent: NMP), it was applied to a Teflon (registered trademark) sheet and dried at 90°C. And dried for 1 minute to prepare a polyvinylidene fluoride film. The obtained polyvinylidene fluoride film was punched into a circular shape having an electrode size of 14 mm in diameter to form an insulating layer. The thickness of the insulating layer was 30 μm. The porosity of the insulating layer was 0%. A battery for characteristic evaluation of Comparative Example 1 was produced in the same manner as in Example 1 except for the above.

[Comparative example 2]
The thickness of the insulating layer was changed from 30 μm to 10 μm by adjusting the amount of NMP added to the polyvinylidene fluoride solution. A battery for characteristic evaluation of Comparative Example 2 was manufactured in the same manner as Comparative Example 1 except for the above. The porosity of the insulating layer was 0%.

[Comparative Example 3]
Polyvinylidene fluoride solution (manufactured by Kureha Co., Ltd., product name: L#1710, 10 mass% solution, solvent: NMP), and alumina particles (manufactured by Nippon Light Metal Co., Ltd., product name: low soda alumina, average) as insulating fine particles. (Particle size: 500 nm) was mixed and dispersed while applying a medium shearing force to obtain a slurry. In addition, the compounding amount of the alumina particles and the polyvinylidene fluoride solution is such that the solid content ratio of the polyvinylidene fluoride is 20% by volume with respect to the total solid content of the polyvinylidene fluoride and the alumina particles of 100% by volume. The blending amount was such that the ratio was 80% by volume.
A predetermined amount of NMP was further added to this slurry and gently stirred with a stirrer for 30 minutes to obtain a slurry for an insulating layer.
The surface of the positive electrode active material layer is formed by applying the insulating layer slurry to the surface of the positive electrode active material layer of the positive electrode after pressure pressing and before punching with a gravure coater and drying the coating film at 90° C. for 1 minute. A positive electrode plate having an insulating layer was prepared. A positive electrode plate having an insulating layer was punched into a circular shape having a diameter of 14 mm, which is the electrode size, to produce a positive electrode having an insulating layer. The insulating layer had a thickness of 30 μm. The porosity of the insulating layer was 70%. A battery for characteristic evaluation of Comparative Example 3 was produced in the same manner as in Example 1 except for the above.

[Comparative Example 4]
The thickness of the insulating layer was changed from 30 μm to 10 μm by adjusting the amount of NMP added to the slurry. A battery for characteristic evaluation of Comparative Example 4 was produced in the same manner as Comparative Example 3 except for the above. The porosity of the insulating layer was 70%.

[Comparative Example 5]
A battery for characteristic evaluation of Comparative Example 5 was produced in the same manner as in Example 2 except that the ratio of alumina particles to ethylene oxide was changed to the ratio shown in the table. The insulating layer had a thickness of 10 μm. The porosity of the insulating layer was 14%.

The evaluation results of the batteries of Examples 1 to 3 and Comparative Examples 1 to 5 are shown in Table 1 below.

From the results of Examples 1 to 3, when the insulating layer contains the solid polymer electrolyte and the porosity of the insulating layer is 10% or less, the volume energy of the lithium ion secondary battery is improved while improving the electronic insulating property of the insulating layer. It was found that the density can be increased. On the other hand, from the results of Comparative Examples 1 and 2, by setting the porosity of the insulating layer to 10% or less, the electronic insulating property can be improved even if the insulating layer is made thin, but the insulating layer contains the polymer solid electrolyte. It turns out that the battery cannot be charged without it. Further, the results of Comparative Example 3 show that even if the porosity of the insulating layer is large, by increasing the thickness of the insulating layer, the electronic insulating property of the insulating layer can be improved, but the volume energy density of the battery becomes low. It was Furthermore, from the results of Comparative Examples 4 and 5, it was found that when the porosity of the insulating layer is high, the electronic insulating property of the insulating layer cannot be secured when the insulating layer is thinned in order to increase the volume energy density of the battery. It was In Comparative Examples 4 and 5, the electronic insulating property could not be ensured, so the 1C discharge capacity could not be measured, and therefore the volume energy density could not be calculated.

DESCRIPTION OF SYMBOLS 1 Electrode for lithium-ion secondary battery 10 Electrode active material layer 20,109 Insulating layer 30 Electrode current collector 106 Negative electrode body 107 Positive electrode body 108 Negative electrode 110 Electrode guide 111 Positive electrode 112 Electrode pressing 113 Spring

Claims

An electrode active material layer, and an insulating layer provided on the surface of the electrode active material layer,
The insulating layer includes a solid polymer electrolyte,
An electrode for a lithium ion secondary battery, wherein the porosity of the insulating layer is 10% or less.
The insulating layer optionally contains insulating fine particles,
The lithium ion secondary battery electrode according to claim 1, wherein the content of the insulating fine particles in the insulating layer is 20% by volume or less based on 100% by volume of the total amount of the polymer solid electrolyte and the insulating fine particles. ..
The electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the polymer solid electrolyte is a polyether electrolyte.
The electrode for a lithium ion secondary battery according to claim 3, wherein the polymer serving as the matrix of the polyether-based electrolyte is a polymer having at least an ethylene oxide structure.
The lithium ion secondary battery electrode according to any one of claims 1 to 4, wherein the polymer solid electrolyte contains a lithium salt.
The lithium-ion secondary battery electrode according to any one of claims 1 to 5, wherein the insulating layer has a thickness of less than 30 μm.
A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of claims 1 to 6 and an electrolytic solution.