Lithium ion secondary battery, separation membrane and method for producing these

Abstract

To provide a separation membrane which is used in a lithium ion secondary battery containing different solvents in a positive electrode mixture layer and a negative electrode mixture layer, has thin film thickness, and has a low resistance value, a lithium ion secondary battery having the separation membrane, and a method for producing them.SOLUTION: A lithium ion secondary battery includes a positive electrode mixture layer, a separation membrane, and a negative electrode mixture layer in this order. The positive electrode mixture layer contains a positive electrode active substance, a first lithium salt, and a first solvent. The negative electrode mixture layer contains a negative electrode active substance, a second lithium salt, and a second solvent different from the first solvent. The separation membrane contains a polymer having lithium ion conductivity, a third lithium salt and a third solvent. The polymer is a copolymer including a first monomer having two (meth)acryloyl groups and a second monomer having three or more (meth)acryloyl groups as monomer units.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery, a separator, and methods for producing the same. [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 without mixing between the positive and negative electrodes. The inventors have considered disposing a separator between the positive and negative electrodes to separate the solvents in the electrolyte in such a lithium-ion secondary battery. When using a separator, it is desirable that the separator be thin and have low resistance.

[0005] One aspect of the present invention aims to provide a separator 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 a thin membrane thickness and a low resistance value; a lithium ion secondary battery including the separator; and methods for manufacturing the separator and the lithium ion secondary battery. [Means for solving the problem]

[0006] One aspect of the present invention provides a lithium ion secondary battery comprising 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, the separator contains a polymer having lithium ion conductivity, a third lithium salt, and the third solvent, and the polymer is a copolymer containing, as monomer units, a first monomer having two (meth)acryloyl groups and a second monomer having three or more (meth)acryloyl groups.

[0007] 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 contains a polymer having lithium ion conductivity, a third lithium salt, and a third solvent, and the polymer is a copolymer containing, as monomer units, a first monomer having two (meth)acryloyl groups and a second monomer having three or more (meth)acryloyl groups.

[0008] Another aspect of the present invention provides a method for manufacturing a lithium ion secondary battery, the method comprising: obtaining a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a first lithium salt, and a first solvent; obtaining a negative electrode having a negative electrode mixture layer containing a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent; forming a slurry containing a first monomer having two (meth)acryloyl groups, a second monomer having three or more (meth)acryloyl groups, a third lithium salt, and the third solvent into a film shape, and then polymerizing the first monomer and the second monomer to obtain a separator; and providing the separator between the positive electrode and the negative electrode.

[0009] Another aspect of the present invention provides a method for producing a separator to be disposed between a positive electrode mixture layer and a 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 method comprising the steps of: forming a slurry containing a first monomer having two (meth)acryloyl groups, a second monomer having three or more (meth)acryloyl groups, a third lithium salt, and the third solvent into a membrane; and polymerizing the first monomer and the second monomer to obtain the separator.

[0010] In each aspect, the second monomer can be a monomer having four (meth)acryloyl groups. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a separator 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 a thin membrane thickness and a low resistance value; a lithium ion secondary battery including the separator; and methods for manufacturing the same. [Brief explanation of the drawings]

[0012] [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. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings as appropriate. However, the present invention is not limited to the following embodiments. In the following embodiments, it goes without saying that the components (including element steps, etc.) are not necessarily essential unless otherwise specified or considered to be clearly essential in principle. The same applies to numerical values ​​and ranges, and they should not be construed as unduly limiting the present invention.

[0014] In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended effect of the process is achieved. In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. When referring to the amount of each component in a composition in this specification, if there are multiple substances corresponding to each component in the composition, the total amount of those multiple substances present in the composition is meant unless otherwise specified.

[0015] 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 shape (such as a coin shape or a cylindrical shape).

[0016] 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.

[0017] 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 this 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.

[0018] 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.

[0019] 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.

[0020] In one embodiment, the positive electrode mixture layer 10 contains a positive electrode active material, a lithium salt (first lithium salt), and a solvent (first solvent).

[0021] 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, Lix 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 Co 0.2 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 Co0.15 Al 0.05 It may also be O2.

[0022] The positive electrode active material may be a lithium phosphate, such as lithium manganese phosphate (LiMnPO), lithium iron phosphate (LiFePO), lithium cobalt phosphate (LiCoPO), and lithium vanadium phosphate (LiV(PO)).

[0023] The content of the positive electrode active material may be 70 mass % or more, 80 mass % or more, or 85 mass % or more based on the total amount of the positive electrode mixture layer, and 95 mass % or less, 92 mass % or less, or 90 mass % or less based on the total amount of the positive electrode mixture layer.

[0024] The first lithium salt may be, for example, at least one selected from the group consisting of LiPF6, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, CF3SO2OLi, LiN(SO2F)2 (LiFSI, lithium bisfluorosulfonylimide), LiN(SO2CF3)2 (LiTFSI, lithium bistrifluoromethanesulfonylimide), and LiN(SO2CF2CF3)2.

[0025] 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.

[0026] 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.

[0027] A solvent preferably used as the first solvent is a solvent with excellent oxidation resistance, such as acetonitrile, ethylene carbonate, etc. This can enhance the oxidation resistance of the positive electrode mixture layer 10.

[0028] 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.

[0029] The positive electrode mixture layer 10 may further contain a binder and a conductive material as other components.

[0030] The binder may be a polymer containing at least one 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.

[0031] 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, based on the total amount of the positive electrode mixture layer, and may be 10% by mass or less, 8% by mass or less, 6% by mass or less, or 4% by mass or less.

[0032] 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.

[0033] 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.

[0034] 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, 70 μm or less, or 50 μm or less.

[0035] In one embodiment, the negative electrode mixture layer 12 contains a negative electrode active material, a lithium salt (second lithium salt), and a solvent (second solvent).

[0036] The negative electrode active material can be a material commonly used in the field of energy devices. Specific 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.

[0037] 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, specifically, for example, silicon oxides such as SiO, SiO, and LiSiO, silicon nitrides such as SiN and SiN0, and silicon carbides such as SiC.

[0038] 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, based on the total amount of the negative electrode mixture layer, and 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.

[0039] The type and content of the second lithium salt may be the same as those of the first lithium salt contained in the above-described positive electrode mixture layer 10. The second lithium salt may be the same type as or different from the first lithium salt.

[0040] 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.

[0041] The second solvent is preferably a solvent with excellent reduction resistance, such as γ-butyrolactone, tetrahydrofuran, etc. This can prevent the second solvent contained in the negative electrode mixture layer 12 from being reductively decomposed.

[0042] 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.

[0043] 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.

[0044] 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, 80 μm or less, 70 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less.

[0045] 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. This separation membrane serves to separate the first solvent and the second solvent contained in the positive electrode mixture layer 10 and the negative electrode mixture layer 12 from each other and prevent them from mixing with each other. Lithium ions can be exchanged through the separation membrane 7.

[0046] Separation membrane 7 contains a polymer having lithium ion conductivity, a lithium salt (third lithium salt), and a solvent (third solvent).

[0047] A polymer having lithium ion conductivity means a polymer that has the property of being able to conduct lithium ions derived from a lithium salt in the presence of the lithium salt. Whether a polymer can conduct lithium ions or not can be confirmed by measuring the ionic conductivity of the polymer. When 1 to 40 mass % of a lithium salt is added to the polymer, the peak of the ionic conductivity measured is 1×10 -6 If the value is S / cm or more, the polymer can be said to have lithium ion conductivity.

[0048] The polymer is a copolymer containing, as monomer units, a first monomer having two (meth)acryloyl groups and a second monomer having three or more (meth)acryloyl groups. It is believed that the polymer containing the first monomer and the second monomer in combination has an appropriate crosslinking density, which allows the formation of a thin separation membrane with low resistance, but the mechanism is not limited to this.

[0049] The first monomer is a compound having two (meth)acryloyl groups and a linking group connecting the two (meth)acryloyl groups. The linking group may contain a hydrocarbon group and / or a heteroatom-containing group. The linking group may contain an oxygen atom-containing group as the heteroatom-containing group, for example, an ether group (-O-). The linking group may be a divalent group consisting of a hydrocarbon group (e.g., an alkylene group) and a heteroatom-containing group (e.g., an ether group), for example, a polyoxyalkylene group or an oxyalkylene group.

[0050] The first monomer may be a monomer represented by the following formula (1-1). [ka]

[0051] In formula (1-1), R 11 and R 12 each independently represents a hydrogen atom or a methyl group (-CH3).

[0052] n represents an integer of 1 or greater. n may be, for example, 5 or greater, 10 or greater, 15 or greater, or 20 or greater, or may be 40 or less, 35 or less, 30 or less, or 25 or less.

[0053] Z 11 represents an alkylene group. 11 may be, for example, an alkylene group having 1 to 6 or 1 to 3 carbon atoms. 11 may be, for example, -CH2-CH2-, -CH(CH3)-CH2-.

[0054] The ionic conductivity of the first monomer at 25°C may be, for example, 0.01 mS / cm or more, 0.05 mS / cm or more, or 0.10 mS / cm or more, and may be 1.0 mS / cm or less, 0.50 mS / cm or less, or 0.30 mS / cm or less.

[0055] The ionic conductivity of the first monomer at 25° C. can be measured by the following method. <Preparation of separation membrane for measuring ionic conductivity> A first monomer, a lithium salt, a solvent, and a photopolymerization initiator are mixed to prepare a slurry. A silicone rubber frame (4 x 4 cm, 1 mm thick) is placed on a PET sheet (8 x 8 cm, 0.035 mm thick), and the prepared slurry is placed in the frame. The first monomer is then polymerized by irradiating it with ultraviolet light (365 nm wavelength) to obtain a separation membrane. The separation membrane is removed from the frame and subjected to the following tests. The lithium salt may be LiN(SOCF) (LiTFSI, lithium bistrifluoromethanesulfonylimide). The solvent may be 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI). The photopolymerization initiator may be 2-hydroxy-2-methyl-1-phenylpropanone. The irradiation time with ultraviolet light (365 nm wavelength) may be 15 minutes. <Measurement of ionic conductivity> The ionic conductivity of the separation membrane is evaluated by preparing a test cell using the separation membrane. The top cover (CR2032 cap, manufactured by Hosen Co., Ltd.), a 1.6 mm thick leaf spring, two 1.0 mm thick SUS spacers, the separation membrane, a gasket, and a bottom cover (CR2032 case, manufactured by Hosen Co., Ltd.) are stacked in this order, and the top and bottom covers are crimped together to prepare a test cell. The measurement equipment and conditions are as follows: Measurement device: VSP electrochemical measurement system (manufactured by BioLogic) Measurement temperature: 25℃ AC amplitude: 10mV Frequency range: 10mHz to 1MHz 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

[0056] Examples of the first monomer include polyethylene glycol #1000 diacrylate (for example, trade name: NK Ester A-1000, manufactured by Shin-Nakamura Chemical Co., Ltd.) and polyethylene glycol #800 diacrylate (for example, trade name: NK Ester A-800, manufactured by Shin-Nakamura Chemical Co., Ltd.).

[0057] The first monomer may be used alone or in combination of two or more.

[0058] The content of the first polymer contained as a monomer unit in the polymer may be 5% by mass or more, 10% by mass or more, or 15% by mass or more, based on the total mass of the separation membrane, and may be 70% by mass or less, 60% by mass or less, or 50% by mass or less.

[0059] The second monomer is a monomer having three or more (meth)acryloyl groups. The number of (meth)acryloyl groups in the second monomer may be, for example, 3 to 6, 3 to 4, or 4.

[0060] The second monomer may be a compound having three or more (meth)acryloyl groups and a linking group connecting these (meth)acryloyl groups. The linking group may contain a hydrocarbon group and / or a heteroatom-containing group. The linking group may contain an oxygen atom-containing group as the heteroatom-containing group, for example, an ether group (-O-). The linking group may be a divalent group consisting of a hydrocarbon group (e.g., an alkylene group) and a heteroatom-containing group (e.g., an ether group), for example, a polyoxyalkylene group or an oxyalkylene group.

[0061] The second monomer having three (meth)acryloyl groups may be a monomer represented by the following formula (1-2). [ka]

[0062] In formula (1-2), R 13 , R 14 and R 15each independently represents a hydrogen atom or a methyl group.

[0063] Z 2 , Z 3 and Z 4 each independently represents an alkylene group. 2 , Z 3 and Z 4 The alkylene group represented by the formula (I) may be an alkylene group having 1 to 6 or 1 to 3 carbon atoms, or may be a methylene group (-CH2-).

[0064] Z 5 represents an alkyl group. Z 5 may be, for example, an alkyl group having 1 to 10, 1 to 6, or 1 to 3 carbon atoms, or may be an ethyl group (-CH2-CH3). The monovalent hydrocarbon group represented by X may be, for example, an alkyl group.

[0065] An example of the second monomer having three (meth)acryloyl groups is trimethylolpropane triacrylate (for example, trade name: NK Ester A-TMPT, manufactured by Shin-Nakamura Chemical Co., Ltd.).

[0066] The second monomer having four (meth)acryloyl groups may be a monomer represented by the following formula (1-3). [ka]

[0067] In formula (1-3), R 16 , R 17 , R 18 and R 19 each independently represents a hydrogen atom or a methyl group.

[0068] Z 6 , Z 7 , Z 8 and Z 9 each independently represents an alkylene group. 2 , Z 3 and Z 4The alkylene group represented by the formula (I) may be an alkylene group having 1 to 6 or 1 to 3 carbon atoms, or may be an ethylene group (-CH2-CH2-).

[0069] a, b, c, and d each independently represent an integer of 1 or greater. a+b+c+d may be 4 or greater, 10 or greater, 20 or greater, or 30 or greater, and may be 50 or less, or 40 or less.

[0070] An example of the second monomer having three (meth)acryloyl groups is ethoxylated pentaerythritol tetraacrylate (for example, trade name: NK Ester A-TM35E, manufactured by Shin-Nakamura Chemical Co., Ltd.).

[0071] The ionic conductivity of the second monomer at 25°C may be, for example, 0.001 mS / cm or more, or 0.01 mS / cm or more, and 0.5 mS / cm or less, or 0.05 mS / cm or less. The ionic conductivity of the second monomer at 25°C can be measured using the second monomer by the same method as the above-mentioned method for measuring the ionic conductivity of the first monomer at 25°C. In measuring the ionic conductivity of the second monomer at 25°C, the irradiation time of ultraviolet light (wavelength 365 nm) may be 2 minutes.

[0072] The second monomer may be used alone or in combination of two or more.

[0073] The content of the second polymer contained as a monomer unit in the polymer may be 5% by mass or more, 10% by mass or more, or 15% by mass or more, based on the total mass of the separation membrane, and may be 70% by mass or less, 60% by mass or less, or 50% by mass or less.

[0074] The ratio (C2 / C1) of the mass (C2) of the second monomer to the mass (C1) of the first monomer contained as monomer units in the polymer may be 5 or less, 4 or less, 3 or less, 2 or less, 1 or less, or 1 / 2 or less, from the viewpoint of further reducing the resistance value of the separation membrane. The ratio (C2 / C1) of the mass (C2) of the second monomer to the mass (C1) of the first monomer contained as monomer units in the polymer may be 1 / 5 or more, 1 / 4 or more, 1 / 3 or more, 1 / 2 or more, 1 or more, or 2 or more, from the viewpoint of further improving the separation ability of the separation membrane.

[0075] The polymer content may be 60 mass % or more, 70 mass % or more, or 80 mass % or more, and may be 90 mass % or less, 80 mass % or less, or 70 mass % or less, based on the total mass of the separation membrane.

[0076] The type of the third lithium salt may be the same as the first lithium salt contained in the above-described positive electrode mixture layer 10. The third lithium salt may be the same type as the first lithium salt and / or the second lithium salt, or may be different from the first lithium salt and / or the second lithium salt.

[0077] The content of the third lithium salt is preferably 5% by mass or more, more preferably 13% by mass or more, and even more preferably 15% by mass or more, based on the total amount of the third lithium salt and the third solvent, from the viewpoint of excellent ionic conductivity of the separation membrane. The content of the third lithium salt is preferably 35% by mass or less, more preferably 23% by mass or less, and even more preferably 20% by mass or less, based on the total amount of the third lithium salt and the third solvent, from the viewpoint of viscosity of the solvent.

[0078] The content of the third lithium salt is preferably 2% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, based on the total amount of the separation membrane, from the viewpoint of further increasing the ionic conductivity of the separation membrane 7. The content of the third lithium salt is preferably 12% by mass or less, more preferably 9% by mass or less, and even more preferably 6% by mass or less, based on the total amount of the separation membrane, from the viewpoint of further increasing the ionic conductivity of the separation membrane 7.

[0079] The third solvent is a solvent for dissolving the third lithium salt. From the viewpoint of suppressing volatilization from the separation membrane, the third solvent is preferably an ionic liquid or a glyme represented by the following formula (2), and more preferably an ionic liquid. R 21 O-(CH2CH2O) k -R 22 (2) [In formula (2), R 21 and R 22 each independently represents an alkyl group having 1 to 4 carbon atoms, and k represents an integer of 3 to 6.

[0080] The ionic liquid contains the following anion component and cation component: In this specification, the ionic liquid is a substance that is liquid at temperatures of −20° C. or higher.

[0081] The anion component of the ionic liquid is not particularly limited, but may be Cl - , Br - , I - Anions of halogens such as BF4 - , N(SO2F)2 - ([FSI] - ), inorganic anions such as B(C6H5)4 - , CH3SO2O - , CF3SO2O - , N(SO2C4F9)2 - , N(SO2CF3)2 - ([TFSI] - ), N(SO2C2F5)2 - The anion component of the ionic liquid preferably contains at least one anion component represented by the following formula (3): N(SO2C m F 2m+1 )(SO2C n F 2n+1 ) - (3) [In formula (3), m and n each independently represent an integer of 0 to 5. m and n may be the same or different, and are preferably the same.]

[0082] The anion component represented by formula (3) is, for example, N(SO2C4F9)2 - , N(SO2F)2 - ([FSI] - ), N(SO2CF3)2 - ([TFSI] - ) and N(SO2C2F5)2 - From the viewpoint of improving the ionic conductivity in the lithium ion secondary battery 1, the anion component of the ionic liquid is more preferably N(SO2C4F9)2 - , CF3SO2O - , [FSI] - , [TFSI] - , and N(SO2C2F5)2 - and more preferably contains at least one selected from the group consisting of [FSI] - Contains:

[0083] The cationic component of the ionic liquid is not particularly limited, but is preferably at least one selected from the group consisting of a chain quaternary onium cation, a piperidinium cation, a pyrrolidinium cation, a pyridinium cation, and an imidazolium cation.

[0084] The chain quaternary onium cation is, for example, a compound represented by the following formula (4). [ka] [In formula (4), R 31 ~R 34 are each independently a chain alkyl group having 1 to 20 carbon atoms, or RO-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4), and X represents a nitrogen atom or a phosphorus atom. 31 ~R 34 The number of carbon atoms in the alkyl group represented by the formula (I) is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 5.]

[0085] The piperidinium cation is, for example, a nitrogen-containing six-membered ring compound represented by the following formula (5). [ka] [In formula (5), R 35 and R 36 are each independently an alkyl group having 1 to 20 carbon atoms or RO-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4). 35 and R 36 The number of carbon atoms in the alkyl group represented by the formula (I) is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 5.]

[0086] The pyrrolidinium cation is, for example, a five-membered ring compound represented by the following formula (6). [ka] [In formula (6), R 37 and R 38 are each independently an alkyl group having 1 to 20 carbon atoms or RO-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4). 37 and R 38 The number of carbon atoms in the alkyl group represented by the formula (I) is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 5.]

[0087] The pyridinium cation is, for example, a compound represented by the following formula (7). [ka] [In formula (7), R 39 ~R 43 are each independently an alkyl group having 1 to 20 carbon atoms, RO-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4), or a hydrogen atom. 39 ~R43 The number of carbon atoms in the alkyl group represented by the formula (I) is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 5.]

[0088] The imidazolium cation is, for example, a compound represented by the following formula (8). [ka] [In formula (8), R 44 ~R 48 are each independently an alkyl group having 1 to 20 carbon atoms, RO-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4), or a hydrogen atom. 44 ~R 48 The number of carbon atoms in the alkyl group represented by the formula (I) is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 5.]

[0089] More specifically, the ionic liquids include N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-bis(trifluoromethanesulfonyl)imide (DEME-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-bis(fluorosulfonyl)imide (DEME-FSI), 1-ethyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (EMI-TFSI), and 1-ethyl-3-methylimidazolium-bis(fluorosulfonyl)imide (EMI-FSI). , N-methyl-N-propylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (Py13-TFSI), N-methyl-N-propylpyrrolidinium-bis(fluorosulfonyl)imide (Py13-FSI), N-ethyl-N-methylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (Py12-TFSI), N-ethyl-N-methylpyrrolidinium-bis(fluorosulfonyl)imide (Py12-FSI), 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA), and the like.

[0090] In the glyme represented by the above formula (2), in formula (2), R 21 and R 22 each independently represents an alkyl group having 4 or less carbon atoms or a fluoroalkyl group having 4 or less carbon atoms, and k represents an integer of 1 to 6. 21 and R 22 are each independently preferably a methyl group or an ethyl group.

[0091] Specifically, the glyme may be monoglyme (k=1), diglyme (k=2), triglyme (k=3), tetraglyme (k=4), pentaglyme (k=5), or hexaglyme (k=6).

[0092] When separation membrane 7 contains glyme as a solvent, part or all of the glyme may form a complex with a lithium salt (third lithium salt).

[0093] The content of the third solvent may be 40% by mass or less, 38% by mass or less, 35% by mass or less, 33% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 18% by mass or less, 15% by mass or less, 13% by mass or less, or 10% by mass or less, based on the total amount of the separation membrane, from the viewpoint of obtaining a separation membrane 7 with better separation ability for the solvents (first solvent and second solvent). The content of the third solvent may be 5% by mass or more, 8% by mass or more, 18% by mass or more, or 27% by mass or more, based on the total amount of the separation membrane, from the viewpoint of further increasing the ionic conductivity of the separation membrane 7.

[0094] The content of the third solvent can be measured by the following method. First, the separation membrane is diluted approximately 10 times with methanol, and then subjected to ultrasonic extraction for 15 minutes to obtain an extract. 1.0 μL of this extract is injected into a gas chromatograph, and gas chromatography mass spectrometry is performed. The specific conditions for gas chromatography mass spectrometry are as follows: Device name: GC-4000 (GL Sciences) Carrier gas: Helium 5.0 mL / min Column: TC-WAX polyethylene glycol (0.53 mm ID x 30 m, 1.0 μL) Split ratio: 1 / 10 Injection temperature: 250℃ Detection temperature: 250℃ Oven temperature: 60°C (1 minute) then increase at 20°C / minute to 240°C Detector: Flame ionization detector (FID) Range: 10 2

[0095] Separation membrane 7 may further contain, for example, an inorganic filler as another component, or may not contain an inorganic filler from the viewpoint of further improving the ionic conductivity of the separation membrane.

[0096] The thickness of the separation membrane 7 may be 85 μm or more, or 90 μm or more, and may be 400 μm or less, 300 μm or less, 200 μm or less, or 100 μm or less.

[0097] The resistance of the separation membrane may be, for example, less than 180 Ω, 160 Ω or less, 140 Ω or less, 120 Ω or less, or 100 Ω or less. The resistance of the separation membrane may be, for example, 30 Ω or more or 50 Ω or more. The resistance of the separation membrane is measured by the following method. First, an upper cover (a cap for CR2032, manufactured by Hosen Co., Ltd.), a 1.6 mm thick leaf spring, two 1.0 mm thick SUS spacers, a separation membrane, a gasket, and a lower cover (a case for CR2032, manufactured by Hosen Co., Ltd.) are stacked in this order, and the upper and lower covers are crimped to prepare a test cell, and the resistance (bulk resistance) of the separation membrane is measured. The measurement device and measurement conditions are as follows. Measurement device: VSP electrochemical measurement system (manufactured by BioLogic) Measurement temperature: 25℃ AC amplitude: 10mV Frequency range: 10mHz to 1MHz

[0098] The ionic conductivity of the separation membrane may be, for example, 0.04 mS / cm or more, or 0.05 mS / cm or more, and may be, for example, 0.15 mS / cm or less.

[0099] The ionic conductivity of the separation membrane is 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

[0100] Next, a method for manufacturing the lithium-ion secondary battery 1 will be described. The method for manufacturing the lithium-ion secondary battery 1 according to one embodiment includes the steps of obtaining a positive electrode 6 including 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 including 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, forming a slurry containing a first monomer, a second monomer, a third lithium salt, and the third solvent into a film and then polymerizing the first monomer and the second monomer to obtain a separation membrane 7, and providing the separation membrane 7 between the positive electrode 6 and the negative electrode 8. The order of the steps is arbitrary.

[0101] In the above-described production 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, the third lithium salt, and the third solvent are as described above.

[0102] In the steps of obtaining a positive electrode and a negative electrode, 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, this 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. In this case, the dispersion medium may be water, N-methyl-2-pyrrolidone (NMP), or the like.

[0103] In one embodiment, the step of obtaining the separation membrane includes preparing a slurry containing a first monomer, a second monomer, a third lithium salt, and a third solvent. Specific aspects of the first monomer and the second monomer are as described above.

[0104] The content of the first monomer in the slurry may be 5 mass % or more, 10 mass % or more, or 15 mass % or more, and may be 70 mass % or less, 60 mass % or less, or 50 mass % or less, based on the total amount of the slurry.

[0105] The content of the second monomer in the slurry may be 5 mass % or more, 10 mass % or more, or 15 mass % or more, and may be 70 mass % or less, 60 mass % or less, or 50 mass % or less, based on the total amount of the slurry.

[0106] The mass of the second monomer relative to the mass of the first monomer in the slurry may be similar to the ratio of the mass of the second monomer to the mass of the first monomer contained as monomer units in the polymer.

[0107] The content of the third solvent in the slurry may be 40% by mass or less, 38% by mass or less, 35% by mass or less, 33% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 18% by mass or less, 15% by mass or less, 13% by mass or less, or 10% by mass or less, based on the total amount of the slurry. From the viewpoint of further increasing the ionic conductivity of separation membrane 7, the content of the third solvent may be 5% by mass or more, 8% by mass or more, 18% by mass or more, or 27% by mass or more, based on the total amount of the slurry.

[0108] A polymerization initiator may be added to the slurry. This allows the first monomer and the second monomer to be polymerized appropriately, and a separation membrane can be produced from the slurry. The polymerization initiator may be a thermal polymerization initiator or a photopolymerization initiator, and can be selected appropriately depending on the purpose.

[0109] Examples of the thermal polymerization initiator include azobisisobutyronitrile and azobis(2-methylbutyronitrile).

[0110] Examples of the photopolymerization initiator include 2-hydroxy-2-methyl-1-phenylpropanone and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0111] The content of the polymerization initiator may be 0.5% by mass or more, 1% by mass or more, 10% by mass or more, or 20% by mass or more, and may be 50% by mass or less, 40% by mass or less, 30% by mass or less, 10% by mass or less, 5% by mass or less, or 3% by mass or less, based on the total amount of the slurry.

[0112] The slurry may or may not further contain an inorganic filler as another component.

[0113] In the process of obtaining a separation membrane, the above-mentioned slurry is subsequently formed into a membrane, and then the first monomer and the second monomer are polymerized.

[0114] The method for forming the slurry into a film is, for example, to place a frame of a desired size on one surface of a substrate such as a PET sheet and pour the slurry into the frame, or to form the slurry into a film by applying the slurry onto one surface of the substrate by a doctor blade method, a dipping method, a spraying method, or the like.

[0115] When the slurry contains a thermal polymerization initiator, the first and second monomers are polymerized by applying heat under predetermined conditions. The heating temperature may be, for example, 80 to 90° C. The heating time may be adjusted appropriately depending on the heating temperature, but is, for example, 1 to 10 minutes.

[0116] When the slurry contains a photopolymerization initiator, the method for polymerizing the first and second monomers is a method of irradiating light under predetermined conditions. In one embodiment, the polymerizable compounds may be polymerized by irradiation with light (ultraviolet light) having a wavelength in the range of 200 to 400 nm.

[0117] In the step of providing the separator 7 between the positive electrode 6 and the negative electrode 8, the positive electrode 6, the separator 7, and the negative electrode 8 are stacked, for example, by lamination. This makes it possible to obtain an electrode group 2 including the positive electrode 6, the negative electrode 8, and the separator 7 provided between the positive electrode 6 and the negative electrode 8. Furthermore, by housing this electrode group 2 in a battery outer casing 3, a lithium ion secondary battery 1 can be obtained. [Example]

[0118] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0119] [Example 1] A slurry was prepared by mixing a monomer represented by the following formula (A) (n = 23, product name: NK Ester A-1000, manufactured by Shin-Nakamura Chemical Co., Ltd.), a monomer represented by the following formula (B) (a + b + c + d = 35, product name: NK Ester ATM-35E, manufactured by Shin-Nakamura Chemical Co., Ltd.), a lithium salt (LiTFSI), a solvent (EMI-TFSI), and a photopolymerization initiator (2-hydroxy-2-methyl-1-phenylpropanone). The composition of each material is shown in Table 1. A silicone rubber frame (4 × 4 cm, 1 mm thick) was placed on a PET sheet (8 × 8 cm, 0.035 mm thick), and the prepared slurry was placed in the frame. The monomer was then polymerized by irradiation with ultraviolet light (365 nm wavelength) for 5 minutes to obtain a separation membrane. The separation membrane was then removed from the frame and subjected to the following tests. [ka] [ka]

[0120] [Example 2] A separation membrane was produced in the same manner as in Example 1, except that the composition of the slurry was changed as shown in Table 1 and the irradiation time of ultraviolet light (wavelength 365 nm) was changed to 4 minutes.

[0121] [Example 3] A separation membrane was produced in the same manner as in Example 1, except that the composition of the slurry was changed as shown in Table 1 and the irradiation time of ultraviolet light (wavelength 365 nm) was changed to 3.5 minutes.

[0122] [Comparative Example 1] A separation membrane was produced in the same manner as in Example 1, except that the composition of the slurry was changed as shown in Table 1 and the irradiation time of ultraviolet light (wavelength 365 nm) was changed to 15 minutes.

[0123] Comparative Example 2 A separation membrane was produced in the same manner as in Example 1, except that the composition of the slurry was changed as shown in Table 1 and the irradiation time of ultraviolet light (wavelength 365 nm) was changed to 2 minutes.

[0124] <Film thickness measurement> The thickness of the separation membrane was measured by the following method. The membrane thickness was measured at three arbitrary locations using a micrometer (Mitutoyo Corporation, PMU150-25MX), and the average value was taken as the membrane thickness of the separation membrane.

[0125] <Resistance value (bulk resistance) measurement> The resistance of the separation membrane was evaluated by preparing test cells using the separation membranes according to the examples and comparative examples. First, an upper cover (a cap for CR2032, manufactured by Hosensha), a 1.6 mm thick leaf spring, two 1.0 mm thick SUS spacers, a separation membrane, a gasket, and a lower cover (a case for CR2032, manufactured by Hosensha) were stacked in this order, and the upper and lower covers were crimped to prepare a test cell, and the resistance (bulk resistance) of the separation membrane was measured. The measurement device and measurement conditions were as follows: Measurement device: VSP electrochemical measurement system (manufactured by BioLogic) Measurement temperature: 25℃ AC amplitude: 10mV Frequency range: 10mHz to 1MHz

[0126] [Table 1]

[0127] As shown in Table 1, the separation membranes according to the examples could be made thinner and had low resistance values.

[0128] With the slurry composition shown in Comparative Example 1, when the film thickness was thinned to about 90 μm, the separation membrane broke when peeled off from the PET sheet, and it was not possible to form a separation membrane.

[0129] <Evaluation of solvent separation ability> The solvent separation ability of the separation membranes according to Examples 1 to 3 was evaluated by the following method. The separation membrane according to Example 3 and a separator (UP3085, manufactured by Ube Industries, Ltd.) were stacked, sandwiched between two silicone rubber sheets (0.5 mm thick), and placed between H-shaped cells. Dimethyl carbonate (DMC) was placed in the cell on the separation membrane side, and the appearance of the separator after a predetermined number of days was visually observed. If the separation membrane has excellent solvent separation ability, DMC does not easily permeate the separation membrane, and therefore DMC does not easily permeate the separator. However, if the separation membrane has poor solvent separation ability, DMC permeates the separation membrane and permeates the separator. Therefore, the separation ability of the separation membrane for solvents (solvents corresponding to the first solvent and the second solvent) can be evaluated by observing the appearance of the separator and checking whether DMC has permeated the separator.

[0130] The separation membrane of Example 3 was superior in solvent separation ability to the separation membranes of Examples 1 and 2. With the separation membrane of Example 3, DMC did not permeate into the separator even 7 days after the start of the test. [Explanation of symbols]

[0131] 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

[Claim 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 separator contains a polymer having lithium ion conductivity, a third lithium salt, and a third solvent; A lithium ion secondary battery, wherein the polymer is a copolymer containing, as monomer units, a first monomer having two (meth)acryloyl groups and a second monomer having three or more (meth)acryloyl groups.

Images