Lithium-ion secondary battery and separator

Abstract

One aspect of the present invention provides a lithium ion secondary battery which sequentially comprises a positive electrode mixture layer, a separation membrane, and a negative electrode mixture layer in this order, wherein: the positive electrode mixture layer contains a positive electrode active material, a first lithium salt, and a first solvent; the negative electrode mixture layer contains a negative electrode active material, a second lithium salt, and a second solvent that is different from the first solvent; and the separation membrane contains a third lithium salt and a polymer that comprises, as a monomer unit, a monomer which is represented by formula (1). (In formula (1), R1 represents a hydrogen atom or a methyl group; R2 represents a divalent organic group; each of R3, R4 and R5 independently represents a hydrogen atom or a monovalent organic group; and X- represents a counter anion.)

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery and a separator. [Background technology]

[0002] In recent years, with the spread of portable electronic devices, electric vehicles, etc., further improvements in the performance of secondary batteries, such as lithium-ion secondary batteries, are being demanded. For example, studies have been conducted to improve the performance of lithium-ion secondary batteries by incorporating different types of electrolytes into the positive electrode and the negative electrode (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-110447 Summary of the Invention [Problem to be solved by the invention]

[0004] In a lithium-ion secondary battery in which different types of electrolytes are contained in the positive and negative electrodes, it is important that the solvents contained in the electrolytes are sufficiently separated between the positive and negative electrodes without mixing. The inventors have considered disposing a separator between the positive and negative electrodes to separate the solvent in the electrolyte in such a lithium-ion secondary battery. This separator must have the properties of allowing lithium ions to pass through the separator but preventing the solvent in the electrolyte from passing through the separator, i.e., lithium ion conductivity and solvent separation ability.

[0005] An object of the present invention is to provide a separator membrane for use in a lithium ion secondary battery containing different solvents in a positive electrode mixture layer and a negative electrode mixture layer, which has excellent ability to separate these solvents and has lithium ion conductivity, and a lithium ion secondary battery including the separator membrane. [Means for solving the problem]

[0006] One aspect of the present invention provides a lithium ion secondary battery including a positive electrode mixture layer, a separator, and a negative electrode mixture layer in this order, in which the positive electrode mixture layer contains a positive electrode active material, a first lithium salt, and a first solvent, the negative electrode mixture layer contains a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, and the separator contains a polymer including, as a monomer unit, a monomer represented by the following formula (1), and a third lithium salt:

[0007] [ka]

[0008] In formula (1), R 1 represents a hydrogen atom or a methyl group, and R 2 represents a divalent organic group, and R 3 , R 4 and R 5 each independently represents a hydrogen atom or a monovalent organic group, and X - indicates a counter anion.

[0009] Another aspect of the present invention provides a separator to be disposed between the positive electrode mixture layer and the negative electrode mixture layer in a lithium ion secondary battery including a positive electrode mixture layer containing a positive electrode active material, a first lithium salt, and a first solvent, and a negative electrode mixture layer containing a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, the separator containing a polymer including a monomer represented by the above formula (1) as a monomer unit, and a third lithium salt.

[0010] The third lithium salt may include LiN(SO2CF3)2. The content of the third lithium salt may be 10 mass % or less based on the total mass of the separation membrane.

[0011] In the above formula (1), R 2 may be an alkylene group having 1 to 3 carbon atoms, and R 3 , R 4 and R 5may each independently be an alkyl group having 1 to 3 carbon atoms. - is N(SO2CF3)2 - It may be. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a separation membrane that is used in a lithium ion secondary battery containing different solvents in a positive electrode mixture layer and a negative electrode mixture layer, and that has excellent separation ability for these solvents and lithium ion conductivity, and a lithium ion secondary battery including the separation membrane. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a perspective view showing a lithium ion secondary battery according to one embodiment; [Figure 2] 2 is an exploded perspective view showing one embodiment of an electrode group in the lithium ion secondary battery shown in FIG. 1. FIG. [Figure 3] FIG. 1 is a schematic cross-sectional view showing one embodiment of a separation membrane. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, although the present invention is not limited to the following embodiments.

[0015] In this specification, the term "(meth)acryloyl group" refers to an acryloyl group or the corresponding methacryloyl group. The same applies to other similar expressions such as (meth)acrylate.

[0016] FIG. 1 is a perspective view showing a lithium-ion secondary battery according to one embodiment. As shown in FIG. 1, the lithium-ion secondary battery 1 according to one embodiment is a so-called laminate-type secondary battery including an electrode group 2 and a bag-shaped battery exterior 3 that houses the electrode group 2. The electrode group 2 is provided with a positive electrode current collector tab 4 and a negative electrode current collector tab 5. The positive electrode current collector tab 4 and the negative electrode current collector tab 5 protrude from the inside to the outside of the battery exterior 3 so that the positive electrode current collector and the negative electrode current collector (described in detail below) can be electrically connected to the outside of the lithium-ion secondary battery 1, respectively. In another embodiment, the lithium-ion secondary battery 1 may have a shape other than a laminate type, such as a coin type or a cylindrical type.

[0017] The battery outer casing 3 may be a container formed of, for example, a laminated film. The laminated film may be a laminated film in which a polymer film such as a polyethylene terephthalate (PET) film, a metal foil such as aluminum, copper, or stainless steel, and a sealant layer such as polypropylene are laminated in this order.

[0018] FIG. 2 is an exploded perspective view showing one embodiment of the electrode group 2 in the lithium-ion secondary battery 1 shown in FIG. 1. As shown in FIG. 2, the electrode group 2 according to one embodiment includes a positive electrode 6, a separator 7, and a negative electrode 8, in this order. The positive electrode 6 includes a positive electrode current collector 9 and a positive electrode mixture layer 10 provided on the positive electrode current collector 9. A positive electrode current collector tab 4 is provided on the positive electrode current collector 9. The negative electrode 8 includes a negative electrode current collector 11 and a negative electrode mixture layer 12 provided on the negative electrode current collector 11. A negative electrode current collector tab 5 is provided on the negative electrode current collector 11.

[0019] The positive electrode current collector 9 is formed of, for example, aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymer, conductive glass, etc. The thickness of the positive electrode current collector 9 may be, for example, 1 μm or more and 50 μm or less.

[0020] The negative electrode current collector 11 is formed of, for example, copper, stainless steel, nickel, aluminum, titanium, baked carbon, conductive polymer, conductive glass, aluminum-cadmium alloy, etc. The thickness of the negative electrode current collector 11 may be, for example, 1 μm or more and 50 μm or less.

[0021] The positive electrode mixture layer 10 contains a positive electrode active material, a first lithium salt, and a first solvent.

[0022] The positive electrode active material may be, for example, lithium oxide. Examples of lithium oxide include Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4 and Li x Mn 2-y M y O4 (in each formula, M represents at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Cu, Zn, Al, Cr, Pb, Sb, V, and B (provided that M is an element different from the other elements in each formula), and x = 0 to 1.2, y = 0 to 0.9, and z = 2.0 to 2.3). x Ni 1-y M y O z Lithium oxide represented by Li x Ni 1-(y1+y2) Co y1 Mn y2 O z (where x and z are the same as those described above, y1=0 to 0.9, y2=0 to 0.9, and y1+y2=0 to 0.9), for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O 2、 LiNi 0.8 Co 0.1 Mn 0.1 It may be O2. x Ni 1-y M y O z Lithium oxide represented by Li x Ni 1-(y3+y4) Co y3 Al y4 O z (where x and z are the same as those described above, y3 = 0 to 0.9, y4 = 0 to 0.9, and y3 + y4 = 0 to 0.9), for example, LiNi 0.8 Co 0.15 Al 0.05 It can be O2.

[0023] The positive electrode active material may be a lithium phosphate. Examples of lithium phosphate include lithium manganese phosphate (LiMnPO), lithium iron phosphate (LiFePO), lithium cobalt phosphate (LiCoPO), and lithium vanadium phosphate (LiV(PO)). The above-mentioned positive electrode active materials may be used alone or in combination of two or more.

[0024] The content of the positive electrode active material may be 70% by mass or more, 80% by mass or more, or 85% by mass or more, and may be 95% by mass or less, 92% by mass or less, or 90% by mass or less, based on the total amount of the positive electrode mixture layer.

[0025] The first lithium salt may be, for example, at least one selected from the group consisting of LiPF, LiBF, LiClO, LiNO, LiB(C H ), LiCH SO, CF SO OLi, LiN(SO F) (LiFSI, lithium bis(fluorosulfonyl)imide), LiN(SO CF) (LiTFSI, lithium bis(trifluoromethanesulfonyl)imide), and LiN(SO CF CF) .

[0026] The content of the first lithium salt may be 0.5 mol / L or more, 0.7 mol / L or more, or 0.8 mol / L or more, and may be 1.5 mol / L or less, 1.3 mol / L or less, or 1.2 mol / L or less, based on the total amount of the first solvent.

[0027] The first solvent is a solvent for dissolving the first lithium salt. Examples of the first solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate, and difluoroethylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic esters such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, ε-caprolactone, and γ-hexanolactone; ethers such as tetrahydrofuran, 1,3-dioxane, dimethoxyethane, diethoxyethane, methoxyethoxyethane, glyme, diglyme, triglyme, and tetraglyme; phosphate esters such as phosphate triesters; nitriles such as acetonitrile, benzonitrile, adiponitrile, and glutaronitrile; chain sulfones such as dimethyl sulfone and diethyl sulfone; cyclic sulfones such as sulfolane; and cyclic sulfonate esters such as propane sultone. The first solvent may be used alone or in combination of two or more.

[0028] The first solvent is preferably a solvent having oxidation resistance. Examples of the solvent having oxidation resistance include acetonitrile and ethylene carbonate. By using a solvent having oxidation resistance, the oxidation resistance of the positive electrode mixture layer 10 can be improved.

[0029] The content of the first solvent contained in the positive electrode mixture layer 10 can be set appropriately within a range in which the first lithium salt can be dissolved, and may be, for example, 10 mass % or more and 80 mass % or less based on the total amount of the positive electrode mixture layer.

[0030] The positive electrode mixture layer 10 may further contain a binder and a conductive material as other components.

[0031] The binder may be a polymer containing at least one monomer selected from the group consisting of tetrafluoroethylene, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, methyl methacrylate, and acrylonitrile as a monomer unit, or a rubber such as styrene-butadiene rubber, isoprene rubber, or acrylic rubber. The binder is preferably polyvinylidene fluoride or a copolymer containing hexafluoropropylene and vinylidene fluoride as monomer units.

[0032] The content of the binder may be 0.3% by mass or more, 0.5% by mass or more, 1% by mass or more, or 1.5% by mass or more, and may be 10% by mass or less, 8% by mass or less, 6% by mass or less, or 4% by mass or less, based on the total amount of the positive electrode mixture layer.

[0033] The conductive material may be a carbon material such as carbon black, acetylene black, graphite, carbon fiber, carbon nanotube, etc. These conductive materials may be used alone or in combination of two or more.

[0034] The content of the conductive material may be 0.1% by mass or more, 1% by mass or more, or 3% by mass or more, based on the total amount of the positive electrode mixture layer. From the viewpoint of suppressing an increase in the volume of the positive electrode 6 and an accompanying decrease in the energy density of the lithium-ion secondary battery 1, the content of the conductive material is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less, based on the total amount of the positive electrode mixture layer.

[0035] The thickness of the positive electrode mixture layer 10 may be 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more, and may be 100 μm or less, 80 μm or less, or 70 μm or less.

[0036] The negative electrode mixture layer 12 contains a negative electrode active material, a second lithium salt, and a second solvent.

[0037] The negative electrode active material can be a material commonly used in the field of energy devices. Examples of the negative electrode active material include metallic lithium and lithium titanate (Li4Ti5O 12 ), lithium alloys or other metal compounds, carbon materials, metal complexes, organic polymer compounds, etc. These negative electrode active materials may be used singly or in combination of two or more. Examples of carbon materials include graphite such as natural graphite (e.g., flake graphite) and artificial graphite, amorphous carbon, carbon fiber, and carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. From the viewpoint of obtaining a larger theoretical capacity (e.g., 500 to 1500 Ah / kg), the negative electrode active material may be a negative electrode active material containing silicon as a constituent element, a negative electrode active material containing tin as a constituent element, etc. Among these, the negative electrode active material may be a negative electrode active material containing silicon as a constituent element.

[0038] The negative electrode active material containing silicon as a constituent element may be an alloy containing silicon as a constituent element, for example, an alloy containing silicon and at least one element selected from the group consisting of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as constituent elements. The negative electrode active material containing silicon as a constituent element may be an oxide, nitride, or carbide, for example, silicon oxides such as SiO, SiO, and LiSiO, silicon nitrides such as SiN and SiN0, and silicon carbides such as SiC.

[0039] The content of the negative electrode active material may be 60% by mass or more, 65% by mass or more, or 70% by mass or more, and may be 99% by mass or less, 95% by mass or less, or 90% by mass or less, based on the total amount of the negative electrode mixture layer.

[0040] The type and content of the second lithium salt may be the same as those of the first lithium salt described above. The second lithium salt may be the same type as the first lithium salt or may be different from the first lithium salt.

[0041] The second solvent is a solvent for dissolving the second lithium salt. The second solvent may be the same as the first solvent described above, but may be different from the first solvent. This allows suitable solvents to be used for the positive electrode 6 and the negative electrode 8, respectively, thereby improving various performances of the lithium-ion secondary battery 1, such as energy density and lifespan.

[0042] The second solvent is preferably a solvent that is resistant to reduction. Examples of the solvent that is resistant to reduction include γ-butyrolactone and tetrahydrofuran. By using a solvent that is resistant to reduction, it is possible to suppress reductive decomposition of the second solvent contained in the negative electrode mixture layer 12.

[0043] The content of the second solvent contained in the negative electrode mixture layer 12 can be set appropriately within a range in which the second lithium salt can be dissolved, and may be, for example, 10 mass % or more and 80 mass % or less based on the total amount of the negative electrode mixture layer.

[0044] The negative electrode mixture layer 12 may further contain a binder and a conductive material as other components. The types and contents of the binder and conductive material may be the same as those of the binder and conductive material in the positive electrode mixture layer 10 described above.

[0045] The thickness of the negative electrode mixture layer 12 may be 10 μm or more, 15 μm or more, or 20 μm or more, and may be 100 μm or less, or 80 μm or less.

[0046] The separation membrane 7 is a separation membrane to be disposed between the positive electrode mixture layer 10 and the negative electrode mixture layer 12 in the lithium-ion secondary battery 1. The separation membrane 7 serves to separate the first solvent contained in the positive electrode mixture layer 10 and the second solvent contained in the negative electrode mixture layer 12 from each other and to prevent the first solvent and the second solvent from mixing with each other. Lithium ions can be exchanged through the separation membrane 7.

[0047] Separation membrane 7 contains a polymer (hereinafter also referred to as "(meth)acrylate polymer") containing a monomer represented by the following formula (1) (hereinafter also referred to as "(meth)acrylate monomer") as a monomer unit, and a third lithium salt. Separation membrane 7 contains the above-mentioned (meth)acrylate polymer and third lithium salt, and thus has excellent solvent separation ability and lithium ion conductivity. Lithium ion conductivity refers to the property of being able to conduct lithium ions derived from a lithium salt in the presence of the lithium salt. Whether or not lithium ions can be conducted can be confirmed by measuring the ionic conductivity of separation membrane 7. For example, when 1 to 40 mass % of a lithium salt is added to separation membrane 7, the peak of ionic conductivity measured is 1×10 -6 3 is a schematic cross-sectional view showing one embodiment of the separation membrane 7. The separation membrane 7 has a lithium ion conductivity of 100 S / cm or more.

[0048] [ka]

[0049] In formula (1), R 1 represents a hydrogen atom or a methyl group, and R 2 represents a divalent organic group, and R 3 , R 4 and R 5 each independently represents a hydrogen atom or a monovalent organic group, and X - indicates a counter anion. 3 , R 4 and R 5 may be the same or different. From the viewpoint of superior solvent separation ability, R 3 , R 4 and R 5 are preferably the same as each other.

[0050] From the viewpoint of superior solvent separation ability, R 1 is preferably a methyl group. 2Examples of the divalent organic group represented by the formula (I) include a divalent hydrocarbon group. Examples of the divalent hydrocarbon group include an alkylene group. The number of carbon atoms in the divalent organic group may be 1 to 3 or 1 to 2. From the viewpoint of achieving a more excellent solvent separation ability, R 2 is preferably an alkylene group having 1 to 3 carbon atoms, more preferably an alkylene group having 1 to 2 carbon atoms, and even more preferably an ethylene group.

[0051] R 3 , R 4 and R 5 Examples of the monovalent organic group represented by the formula (I) include a monovalent hydrocarbon group. Examples of the monovalent hydrocarbon group include an alkyl group. The number of carbon atoms in the monovalent organic group may be 1 to 3 or 1 to 2. From the viewpoint of achieving a more excellent solvent separation ability, R 3 , R 4 and R 5 are preferably each independently an alkyl group having 1 to 3 carbon atoms, more preferably each independently an alkyl group having 1 to 2 carbon atoms, and even more preferably a methyl group.

[0052] X - For example, PF6 - , BF4 - , ClO4 - , NO3 - , B(C6H5)4 - , CH3SO3 - , CF3SO2O - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2CF2CF3)2 - X - is preferably the same as the anion of the third lithium salt, more preferably N(SO2CF3)2 - ([TFSI] - )

[0053] The (meth)acrylate polymer may contain only (meth)acrylate monomers as monomer units, or may contain (meth)acrylate monomers and other monomers copolymerizable with the (meth)acrylate monomers as monomer units. The (meth)acrylate polymer may be a homopolymer containing only one type of (meth)acrylate monomer as a monomer unit, a copolymer containing one or more types of (meth)acrylate monomers and one or more types of other monomers as monomer units, or a copolymer containing two or more types of (meth)acrylate monomers as monomer units. The content of (meth)acrylate monomer units may be 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more based on the total amount of monomer units contained in the (meth)acrylate polymer.

[0054] The weight average molecular weight of the (meth)acrylate polymer may be 1,000 or more, 5,000 or more, or 10,000 or more, and may be 200,000 or less, 100,000 or less, or 50,000 or less. In this specification, the weight average molecular weight refers to a value measured using gel permeation chromatography (GPC) under the following conditions and determined using polystyrene as a standard substance. Measuring equipment: HLC-8320GPC (product name, manufactured by Tosoh Corporation) Analytical column: TSKgel SuperMultipore HZ-H (3 columns connected) (product name, manufactured by Tosoh Corporation) Guard column: TSKguardcolumn SuperMP(HZ)-H (product name, manufactured by Tosoh Corporation) ·Eluent:THF ·Measurement temperature: 25℃

[0055] The content of the (meth)acrylate polymer may be 80% by mass or more, 82% by mass or more, 84% by mass or more, 86% by mass or more, 87% by mass or more, or 88% by mass or more, based on the total mass of the separation membrane, and may be 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 94% by mass or less, or 93% by mass or less. From the viewpoint of achieving better solvent separation ability, the content of the (meth)acrylate polymer is preferably 84% by mass or more, 86% by mass or more, 87% by mass or more, or 88% by mass or more, based on the total mass of the separation membrane.

[0056] The third lithium salt can be the same as the first lithium salt described above. The third lithium salt may be the same as or different from the first and second lithium salts described above. From the viewpoint of achieving superior lithium ion conductivity, the third lithium salt preferably includes at least one selected from the group consisting of LiPF6, LiBF4, LiClO4, LiNO3, LiB(C6H5)4, LiCH3SO3, CF3SO2OLi, LiN(SO2F)2, LiN(SO2CF3)2, and LiN(SO2CF2CF3)2, and more preferably includes LiN(SO2CF3)2.

[0057] The content of the third lithium salt may be 2% by mass or more, 2.5% by mass or more, 3% by mass or more, 3.5% by mass or more, 4% by mass or more, 4.2% by mass or more, 4.4% by mass or more, 4.6% by mass or more, or 4.8% by mass or more, based on the total weight of the separation membrane, and may be 20% by mass or less, 15% by mass or less, 14% by mass or less, 13% by mass or less, 12% by mass or less, 11.5% by mass or less, 11% by mass or less, 10.5% by mass or less, or 10% by mass or less. From the viewpoint of achieving better lithium ion conductivity, the content of the third lithium salt is preferably 3% by mass or more, 3.5% by mass or more, 4% by mass or more, 4.2% by mass or more, 4.4% by mass or more, 4.6% by mass or more, or 4.8% by mass or more, based on the total weight of the separation membrane. From the viewpoint of achieving better solvent separation ability, the content of the third lithium salt is preferably 15 mass% or less, 14 mass% or less, 13 mass% or less, 12 mass% or less, 11.5 mass% or less, 11 mass% or less, 10.5 mass% or less, or 10 mass% or less, based on the total amount of the separation membrane.

[0058] The thickness of the separation membrane 7 is preferably 20 μm or more, 50 μm or more, or 100 μm or more from the viewpoint of further enhancing the solvent separation ability of the separation membrane 7. The thickness of the separation membrane 7 is preferably 2000 μm or less, 1500 μm or less, or 1200 μm or less from the viewpoint of enhancing the energy density of the lithium ion secondary battery 1.

[0059] The separation membrane 7 can be produced, for example, by the following method. That is, the production method of the separation membrane 7 can include the steps of mixing a monomer containing the monomer represented by the above formula (1), a third lithium salt, and a third solvent to obtain a slurry, applying the obtained slurry to a substrate, drying the applied slurry to remove the third solvent, and irradiating with light to polymerize the monomer. By including the step of drying the applied slurry to remove the third solvent, such a production method can obtain a separation membrane 7 that has excellent solvent separation ability and lithium ion conductivity. That is, the separation membrane 7 preferably does not contain a solvent.

[0060] The third solvent is not particularly limited as long as it is an organic solvent capable of dissolving the third lithium salt, and examples of the third solvent include methyl ethyl ketone (MEK).

[0061] Examples of methods for applying the slurry to the substrate include a method in which a frame of a given size is placed on one surface of the substrate and the slurry is poured into the frame, and a method in which the slurry is applied to one surface of the substrate by a coating method such as a doctor blade method, a dipping method, a spray method, etc. Examples of the substrate include a PET sheet.

[0062] Examples of methods for drying the slurry to remove the solvent include a method of heating the slurry.

[0063] To polymerize the monomers, a photopolymerization initiator may be added to the slurry, such as 2-hydroxy-2-methyl-1-phenylpropanone or diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0064] When a photopolymerization initiator is added, the content of the photopolymerization initiator may be 0.01 parts by mass or more, 0.05 parts by mass or more, 0.1 parts by mass or more, or 1 part by mass or more, and may be 15 parts by mass or less, 10 parts by mass or less, or 5 parts by mass or less, relative to 100 parts by mass of the monomer.

[0065] Examples of the method for polymerizing the monomer by irradiating with light include a method of irradiating with light under predetermined conditions, etc. In one embodiment, the monomer may be polymerized by irradiating with light having a wavelength in the range of 200 to 400 nm (ultraviolet light).

[0066] Next, a description will be given of a method for manufacturing the lithium ion secondary battery 1. The method for manufacturing the lithium ion secondary battery 1 according to one embodiment includes the steps of obtaining a positive electrode 6 having a positive electrode mixture layer 10 containing a positive electrode active material, a first lithium salt, and a first solvent, obtaining a negative electrode 8 having a negative electrode mixture layer 12 containing a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, and providing a separation membrane 7 between the positive electrode 6 and the negative electrode 8. The order of the steps is arbitrary.

[0067] In the above manufacturing method, specific aspects of the positive electrode active material, the first lithium salt, the first solvent, the negative electrode active material, the second lithium salt, the second solvent, and the separation membrane 7 are as described above.

[0068] In the steps of obtaining the positive electrode 6 and the negative electrode 8, the positive electrode 6 and the negative electrode 8 can be obtained using known methods. For example, the material used for the positive electrode mixture layer 10 or the negative electrode mixture layer 12 is dispersed in an appropriate amount of a dispersion medium using a kneader, disperser, or the like to obtain a slurry of the positive electrode mixture or the negative electrode mixture. Thereafter, the positive electrode mixture or the negative electrode mixture is applied to the positive electrode current collector 9 or the negative electrode current collector 11 by a doctor blade method, a dipping method, a spray method, or the like, and the dispersion medium is volatilized to obtain the positive electrode 6 and the negative electrode 8. The dispersion medium may be water, N-methyl-2-pyrrolidone (NMP), or the like.

[0069] The step of providing the separator 7 between the positive electrode 6 and the negative electrode 8 may include the step of manufacturing the separator 7 described above. In this case, after the separator 7 is obtained, the positive electrode 6, the separator 7, and the negative electrode 8 are stacked, for example, by lamination, to obtain an electrode group 2 including the positive electrode 6, the separator 7, and the negative electrode 8 in this order. The electrode group 2 is housed in a battery exterior casing 3, and a lithium-ion secondary battery 1 can be obtained. [Example]

[0070] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0071] [Example 1] Monomer (in formula (1), R 1 , R 3 , R 4 and R 5 is a methyl group, R 2 is an ethylene group, X - [TFSI] - 3.6 g of a monomer (a compound of the formula (1)), 0.19 g of a lithium salt (Li[TFSI]), 0.1 g of a photopolymerization initiator (2-hydroxy-2-methylpropiophenone, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 2.4 g of a solvent (methyl ethyl ketone) were added to a brown screw tube and mixed with a stirrer for 1 hour to obtain a slurry. A silicone rubber frame (4 cm x 4 cm, 1 mm thick) was placed on a PET sheet (8 cm x 8 cm, 0.035 mm thick). The slurry was then added to the frame and dried at 100 °C for 30 minutes to remove the solvent. The mixture was then irradiated with UV light at a distance of 30 cm and an irradiance of approximately 335 mW / cm. 2 The monomer was polymerized by irradiating it with UV light at 400 K for 3 minutes to produce a separation membrane. The separation membrane was removed from the frame and subjected to the following tests.

[0072] [Example 2] A separation membrane was produced in the same manner as in Example 1, except that the amount of the monomer was changed to 4.6 g and the amount of the lithium salt was changed to 0.52 g.

[0073] <Evaluation of solvent separation ability> The separation membrane obtained in the examples was stacked on a separator (UP3085, manufactured by Ube Industries, Ltd.), and the resulting laminate was sandwiched between two silicone rubber sheets (0.5 mm thick). The resulting laminate was placed in an H-shaped cell. Dimethyl carbonate (DMC) was placed in the cell on the separation membrane side, and the appearance of the separator was visually observed after a predetermined number of days. If the separation membrane has excellent solvent separation ability, DMC does not easily permeate the separation membrane, and therefore does not easily penetrate the separator. On the other hand, if the separation membrane has poor solvent separation ability, DMC permeates the separation membrane and penetrates into the separator. Therefore, by observing the appearance of the separator and confirming whether or not DMC has penetrated into the separator, the separation ability of the separation membrane for solvents (solvents corresponding to the first and second solvents) can be evaluated. If DMC has not permeated the separator even after one day has passed since the start of the test, this is indicated as "≧1 day" in Table 1, and in this case, the separation membrane can be said to have excellent solvent separation ability. As shown in Table 1, the separation membranes obtained in the examples did not allow DMC to permeate into the separator even after one day or more had passed.

[0074] <Evaluation of ionic conductivity> The ionic conductivity of the separation membrane was evaluated by preparing a test cell using the separation membrane obtained in the examples. First, a top cover (a cap for CR2032, manufactured by Hohsen Co., Ltd.), a 1.6 mm thick leaf spring, two 1.0 mm thick SUS spacers, a separation membrane, a gasket, and a bottom cover (a case for CR2032, manufactured by Hohsen Co., Ltd.) were stacked in this order, and the top and bottom covers were crimped to prepare a test cell.

[0075] Next, the bulk resistance of the separation membrane was measured using the following measuring device and under the following measuring conditions. Measurement device: VSP electrochemical measurement system (manufactured by BioLogic) Measurement temperature: 80℃ AC amplitude: 10mV Frequency range: 10mHz to 1MHz

[0076] After the measurement, the ionic conductivity of the separation membrane was calculated according to the following formula (α). σ=L / RA (α) σ (S / cm): ionic conductivity L (cm): thickness of the separation membrane R(Ω): bulk resistance A(cm 2 ): Cross-sectional area of ​​SUS spacer

[0077] [Table 1] [Explanation of symbols]

[0078] 1... lithium ion secondary battery, 2... electrode group, 3... battery exterior, 4... positive electrode current collecting tab, 5... negative electrode current collecting tab, 6... positive electrode, 7... separator, 8... negative electrode, 9... positive electrode current collector, 10... positive electrode mixture layer, 11... negative electrode current collector, 12... negative electrode mixture layer.

Claims

1. A lithium ion secondary battery including a positive electrode mixture layer, a separator, and a negative electrode mixture layer in this order, the positive electrode mixture layer contains a positive electrode active material, a first lithium salt, and a first solvent, the negative electrode mixture layer contains a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, The separation membrane contains a polymer including a monomer represented by the following formula (1) as a monomer unit, and a third lithium salt: The content of the polymer containing a monomer represented by the following formula (1) as a monomer unit is 88 mass% or more based on the total amount of the separation membrane, A lithium ion secondary battery, wherein the content of the third lithium salt is 4.8 mass % or more based on the total amount of the separator. 【Chemistry 1】 [In formula (1), R １ represents a hydrogen atom or a methyl group, and R ２ represents a divalent organic group, and R ３ , R ４ and R ５ each independently represents a hydrogen atom or a monovalent organic group; X － indicates a counter anion.]

2. The third lithium salt is LiN(SO ２ CF ３ ) ２ The lithium ion secondary battery according to claim 1 , comprising:

3. 3. The lithium ion secondary battery according to claim 1, wherein the content of the third lithium salt is 10 mass % or less based on the total mass of the separator.

4. In the formula (1), R ２ is an alkylene group having 1 to 3 carbon atoms, and R ３ , R ４ and R ５ The lithium ion secondary battery according to any one of claims 1 to 3, wherein each independently represents an alkyl group having 1 to 3 carbon atoms.

5. In the formula (1), X － is N(SO ２ CF ３ ) ２ － The lithium ion secondary battery according to any one of claims 1 to 4.

6. In the formula (1), R ２ is an alkylene group having 1 to 3 carbon atoms, and R ３ , R ４ and R ５ are each independently an alkyl group having 1 to 3 carbon atoms, and X － is N(SO ２ CF ３ ) ２ － The lithium ion secondary battery according to any one of claims 1 to 5.

7. A lithium ion secondary battery includes a positive electrode mixture layer containing a positive electrode active material, a first lithium salt, and a first solvent, and a negative electrode mixture layer containing a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, the lithium ion secondary battery comprising a separator disposed between the positive electrode mixture layer and the negative electrode mixture layer, the separator comprising: The polymer contains a monomer represented by the following formula (1) as a monomer unit, and a third lithium salt: The content of the polymer containing a monomer represented by the following formula (1) as a monomer unit is 88 mass% or more based on the total amount of the separation membrane, A separation membrane, wherein the content of the third lithium salt is 4.8 mass % or more based on the total mass of the separation membrane. 【Chemistry 2】 [In formula (1), R １ represents a hydrogen atom or a methyl group, and R ２ represents a divalent organic group, and R ３ , R ４ and R ５ each independently represents a hydrogen atom or a monovalent organic group; X － indicates a counter anion.]

8. The third lithium salt is LiN(SO ２ CF ３ ) ２ The separation membrane of claim 7 , comprising:

9. The separation membrane according to claim 7 or 8, wherein the content of the third lithium salt is 10 mass % or less based on the total mass of the separation membrane.

10. In the formula (1), R ２ is an alkylene group having 1 to 3 carbon atoms, and R ３ , R ４ and R ５ The separation membrane according to any one of claims 7 to 9, wherein each independently represents an alkyl group having 1 to 3 carbon atoms.

11. In the formula (1), X － is N(SO ２ CF ３ ) ２ － The separation membrane according to any one of claims 7 to 10,

12. In the formula (1), R ２ is an alkylene group having 1 to 3 carbon atoms, and R ３ , R ４ and R ５ are each independently an alkyl group having 1 to 3 carbon atoms, and X － is N(SO ２ CF ３ ) ２ － The separation membrane according to any one of claims 7 to 11,

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