Lithium-ion secondary battery and separator

Abstract

According to one aspect of the present invention, provided is a lithium ion secondary battery comprising 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 different from the first solvent; the separation membrane includes a pore having an average pore diameter greater than or equal to 2Å and less than 20Å.

Description

Technical Field

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

Background Art

[0002] In recent years, with the spread of portable electronic devices, electric vehicles, etc., further performance improvement has been demanded for secondary batteries represented by lithium ion secondary batteries. For example, it has been studied to improve the performance of a lithium ion secondary battery by containing electrolytes of different types in a positive electrode and a negative electrode (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a lithium ion secondary battery in which electrolytes of different types are contained in a positive electrode and a negative electrode, it is important that the solvents contained in the electrolytes are sufficiently separated without mixing between the positive electrode and the negative electrode. The inventors of the present invention considered disposing a separator membrane between the positive electrode and the negative electrode in order to separate the solvents in such a lithium ion secondary battery. In this separator membrane, it is necessary to have a property that lithium ions pass through the separator membrane but solvents hardly pass through.

[0005] An object of the present invention is to provide a separator membrane having excellent separation ability for solvents used in a lithium ion secondary battery in which different solvents are contained in a positive electrode mixture layer and a negative electrode mixture layer, and a lithium ion secondary battery including the separator membrane.

Means for Solving the Problems

[0006] The inventors of the present invention have found that a separation membrane having pores with an average pore diameter adjusted to a predetermined range can effectively separate the solvents contained in the positive electrode mixture layer and the negative electrode mixture layer, and thus completed the present invention.

[0007] One aspect of the present invention is a lithium ion secondary battery including 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 different from the first solvent, and the separation membrane has pores with an average pore diameter of 2 Å or more and less than 20 Å, and provides a lithium ion secondary battery.

[0008] Another aspect of the present invention is a separation membrane for being disposed between 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 in a lithium ion secondary battery, the separation membrane having pores with an average pore diameter of 2 Å or more and less than 20 Å, and provides a separation membrane.

[0009] The separation membrane contains a polymer of a compound having two or more polymerizable groups and a lithium ion conductive compound having at least one group selected from the group consisting of a carbonyl group and a chain ether group, the weight average molecular weight of the polymer is 800 or more, and the molecular weight of the lithium ion conductive compound may be 150 or less.

[0010] The separation membrane may contain a crosslinked product of polymers having a group represented by the following formula (1).

Chemical formula

[0011] In the above lithium ion secondary battery, the molecular diameters of the first solvent and the second solvent are preferably each 20 Å or more.

Advantages of the Invention

[0012] According to the present invention, there can be provided a separator excellent in the separation ability of solvents, which is used in a lithium ion secondary battery containing solvents different from each other in the positive electrode mixture layer and the negative electrode mixture layer, and a lithium ion secondary battery including the separator.

Brief Description of Drawings

[0013]

Figure 1

Figure 2

Mode for Carrying Out the Invention

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

[0015] In this specification, (meth)acrylic acid means acrylic acid or its corresponding methacrylic acid. 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 an embodiment. As shown in FIG. 1, a lithium ion secondary battery 1 according to an 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 collecting tab 4 and a negative electrode current collecting tab 5. The positive electrode current collecting tab 4 and the negative electrode current collecting tab 5 project from the inside of the battery exterior 3 to the outside so that a positive electrode current collector and a negative electrode current collector (details will be described later) can be electrically connected to the outside of the lithium ion secondary battery 1, respectively. The lithium ion secondary battery 1 may have a shape other than the laminate type (coin type, cylindrical type, etc.) in another embodiment.

[0017] The battery exterior body 3 may be, for example, a container formed of a laminated film. The laminated film may be, for example, 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 an 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 collecting 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 collecting 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, fired carbon, a conductive polymer, conductive glass, or the like. 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, fired carbon, a conductive polymer, conductive glass, an aluminum-cadmium alloy, or the like. The thickness of the negative electrode current collector 11 may be, for example, 1 μm or more and 50 μm or less.

[0021] 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).

[0022] The positive electrode active material may be, for example, a lithium oxide. Examples of the 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 (however, M is an element different from the other elements in each formula). x = 0 to 1.2, y = 0 to 0.9, z = 2.0 to 2.3).) may be mentioned. Li x Ni 1-y M y O z The lithium oxide represented by is Li x Ni 1-(y1+y2) Co y1 Mn y2 O z (However, 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). It may be, 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 O2 may be. Li x Ni 1-y M y O z The lithium oxide represented by is Li x Ni 1-(y3+y4) Co y3 Al y4 O z (However, 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). It may be, for example, LiNi 0.8 Co0.15 Al 0.05 It may also be O2.

[0023] The positive electrode active material may be a phosphate of lithium. Examples of the phosphate of lithium include lithium manganese phosphate (LiMnPO4), lithium iron phosphate (LiFePO4), lithium cobalt phosphate (LiCoPO4), and lithium vanadium phosphate (Li3V2(PO4)3). The above-described positive electrode active materials are 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 based on the total amount of the positive electrode mixture layer. The content of the positive electrode active material 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 at least one selected from the group consisting of, for example, LiPF6, LiBF4, LiClO4, LiNO3, LiB(C6H5)4, LiCH3SO3, CF3SO2OLi, LiN(SO2F)2 (LiFSI, lithium bisfluorosulfonylimide), LiN(SO2CF3)2 (LiTFSI, lithium bistrifluoromethanesulfonylimide), and LiN(SO2CF2CF3)2.

[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 based on the total amount of the first solvent, and may be 1.5 mol / L or less, 1.3 mol / L or less, or 1.2 mol / L or less.

[0027] The first solvent is a solvent for dissolving the first lithium salt. The first solvent is preferably a compound having a molecular diameter of 20 Å or more. Thereby, in the lithium ion secondary battery 1, it is possible to easily separate the first solvent and a second solvent described later by the separation membrane 7.

[0028] From the viewpoint of facilitating the separation of the solvent by the separation membrane 7, the molecular diameter of the first solvent is preferably 20 Å or more, more preferably 25 Å or more, still more preferably 30 Å or more. From the viewpoint of the ease of stirring during the dissolution of the lithium salt, the molecular diameter of the first solvent is preferably 100 Å or less, more preferably 90 Å or less, still more preferably 80 Å or less.

[0029] The molecular diameter of the first solvent can be calculated based on the theoretical bond distance obtained from the most stable structure of the solvent determined by density functional theory (DFT), which is a type of quantum chemical calculation.

[0030] Examples of the first solvent having a molecular diameter of 20 Å or more include fluorinated phosphoric acid esters such as glyme represented by the following formula (2) and tris(1H,1H,5H-octafluoropentyl) phosphate. R 21 O-(CH2CH2O) k1 -R 22 (2) [In formula (2), R 21 and R 22 each independently represent an alkyl group having 1 to 4 carbon atoms, and k1 represents an integer of 4 to 6.]

[0031] More specifically, glyme may be tetraglyme (k1 = 4), pentaglyme (k1 = 5), or hexaglyme (k1 = 6).

[0032] The first solvent, in addition to the solvents described above, may also be cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, etc., chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc., cyclic esters such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, ε-caprolactone, γ-hexanolactone, etc., phosphate esters such as tetrahydrofuran, 1,3-dioxane, dimethoxyethane, diethoxyethane, methoxyethoxyethane, triphosphate ester, etc., nitriles such as acetonitrile, benzonitrile, adiponitrile, glutaronitrile, etc., chain sulfones such as dimethyl sulfone, diethyl sulfone, etc., cyclic sulfones such as sulfolane, etc., cyclic sulfonic acid esters such as propane sultone, etc. From the viewpoint of enhancing the oxidation resistance of the positive electrode mixture layer 10, the first solvent may be a solvent having oxidation resistance such as acetonitrile, ethylene carbonate, etc.

[0033] The first solvent is used alone or in combination of two or more of the solvents described above.

[0034] The content of the first solvent contained in the positive electrode mixture layer 10 can be appropriately set within a range where the first lithium salt can be dissolved. For example, based on the total amount of the positive electrode mixture layer, it may be 10% by mass or more and 80% by mass or less.

[0035] The positive electrode mixture layer 10 may further contain a binder and a conductive material as other components.

[0036] The binder may be a polymer containing at least one selected from the group consisting of ethylene tetrafluoride, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, methyl methacrylate, and acrylonitrile as monomer units, 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.

[0037] 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.

[0038] The conductive material may be a carbon material such as carbon black, acetylene black, graphite, carbon fiber, or carbon nanotube. These conductive materials are used alone or in combination of two or more.

[0039] 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 a decrease in the energy density of the lithium-ion secondary battery 1 associated therewith, the content of the conductive material is preferably 15% by mass or less, more preferably 10% by mass or less, and still more preferably 8% by mass or less, based on the total amount of the positive electrode mixture layer.

[0040] 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.

[0041] 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).

[0042] As the negative electrode active material, those commonly used in the field of energy devices can be used. Specifically, examples of the negative electrode active material include, for example, metallic lithium, lithium titanate (Li4Ti5O 12 ), lithium alloys or other metal compounds, carbon materials, metal complexes, organic polymer compounds, and the like. These negative electrode active materials can be used alone or in combination of two or more. Examples of the carbon material include graphite such as natural graphite (scaly graphite, etc.) 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 (for example, 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, or the like. Among these, the negative electrode active material may be a negative electrode active material containing silicon as a constituent element.

[0043] The negative electrode active material containing silicon as a constituent element may be an alloy containing silicon as a constituent element. For example, it may be an alloy containing silicon and at least one 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 also be an oxide, nitride, or carbide. Specifically, for example, it may be a silicon oxide such as SiO, SiO2, or LiSiO, a silicon nitride such as Si3N4 or Si2N2O, or a silicon carbide such as SiC.

[0044] 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 binder layer. The content of the negative electrode active material 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 binder layer.

[0045] The type and content of the second lithium salt may be the same as those of the first lithium salt contained in the positive electrode binder layer 10 described above. The second lithium salt may be of the same type as or different from the first lithium salt.

[0046] The second solvent is a solvent for dissolving the second lithium salt. As the second solvent, those similar to those used as the first solvent described above can be used, but a solvent different from the first solvent is used. Thereby, since solvents suitable for the positive electrode 6 and the negative electrode 8 can be used respectively, it becomes possible to improve various performances of the lithium ion secondary battery 1 such as energy density and life improvement.

[0047] The second solvent is preferably a compound having a molecular diameter of 20 Å or more. Specific examples of the solvent having a molecular diameter of 20 Å or more are as described above. From the viewpoint of suppressing the reductive decomposition of the second solvent contained in the negative electrode binder layer 12, the second solvent may be a solvent having reductive resistance such as γ-butyrolactone or tetrahydrofuran. The second solvent is used alone or in combination of two or more of the above-described solvents.

[0048] The content of the second solvent contained in the negative electrode binder layer 12 can be appropriately set within a range in which the second lithium salt can be dissolved. For example, based on the total amount of the negative electrode binder layer, it may be 10% by mass or more and may be 80% by mass or less.

[0049] The negative electrode binder layer 12 may further contain a binder and a conductive material as other components. The types and contents of the binder and the conductive material may be the same as those of the binder and the conductive material in the positive electrode binder layer 10 described above.

[0050] The thickness of the negative electrode binder 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.

[0051] The separator membrane 7 is a separator membrane for being disposed between the positive electrode mixture layer 10 and the negative electrode mixture layer 12 in the lithium-ion secondary battery 1. This separator membrane has the role of separating 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 so that they do not mix with each other. It is possible to transfer lithium ions through the separator membrane 7.

[0052] The separator membrane 7 is a porous body having a porous structure and has pores with an average pore diameter of 2 Å or more and less than 20 Å.

[0053] From the viewpoint of making it difficult to inhibit the movement of lithium ions, the average pore diameter of the separator membrane 7 is 2 Å or more, preferably 5 Å or more, more preferably 7 Å or more, and still more preferably 10 Å or more. From the viewpoint of further enhancing the separation ability of the solvent, the average pore diameter of the separator membrane 7 is less than 20 Å, preferably 18 Å or less, more preferably 15 Å or less, and still more preferably 13 Å or less. The average pore diameter of the separator membrane 7 can be measured by a gas adsorption method using argon gas. More specifically, the average pore diameter can be calculated from the adsorption amount of argon gas adsorbed in the pores of the separator membrane 7.

[0054] In one embodiment (the first embodiment), the separator membrane 7 having the above-described average pore diameter contains a polymer of a compound having two or more polymerizable groups and a lithium ion conductive compound having at least one group selected from the group consisting of a carbonyl group and a chain ether group, and the weight average molecular weight of the polymer is 800 or more, and the molecular weight of the lithium ion conductive compound is 150 or less. In this separator membrane, by adjusting the weight average molecular weight of the polymer and the molecular weight of the lithium ion conductive compound within the above ranges, the average pore diameter of the separator membrane 7 can be adjusted to 2 Å or more and less than 20 Å.

[0055] In a compound having two or more polymerizable groups (hereinafter also referred to as "polymerizable compound"), the polymerizable group is, for example, a group containing an ethylenically unsaturated bond. The polymerizable group may be a radically polymerizable group, and may be a vinyl group, an allyl group, a styryl group, an alkenyl group, an alkenylene group, a (meth)acryloyl group, a maleimide group, or the like.

[0056] In one embodiment, the polymerizable compound may be a polyfunctional (meth)acrylate (poly(meth)acrylate). Examples of the polyfunctional (meth)acrylate include polyalkylene glycol di(meth)acrylate such as polyethylene glycol di(meth)acrylate.

[0057] In the separation membrane of the first embodiment, the weight average molecular weight of the polymer of the polymerizable compound is 800 or more. From the viewpoint of increasing the weight average molecular weight of the polymer in the range where the average pore diameter of the separation membrane 7 is 2 Å or more and less than 20 Å, preferably, it is 900 or more, 1000 or more, 3000 or more, 5000 or more, 7000 or more, or 9000 or more. From the viewpoint of decreasing the weight average molecular weight of the polymer in the range where the average pore diameter of the separation membrane 7 is 2 Å or more and less than 20 Å, preferably it is 200000 or less, more preferably 100000 or less, and still more preferably 50000 or less. The weight average molecular weight is a value measured by gel permeation chromatography (GPC) method and converted from a standard polystyrene calibration curve (the same shall apply hereinafter).

[0058] From the viewpoint of the strength of the membrane, the content of the polymer of the polymerizable compound is preferably 5% by mass or more, more preferably 15% by mass or more, and still more preferably 30% by mass or more based on the total amount of the separation membrane. From the viewpoint of making it difficult to inhibit the movement of lithium ions, the content of the polymer of the polymerizable compound is preferably 95% by mass or less, more preferably 85% by mass or less, and still more preferably 80% by mass or less based on the total amount of the separation membrane.

[0059] The lithium ion conductive compound means a compound having lithium ion conductivity, and means a compound having the property of being able to conduct lithium ions derived from the lithium salt in the presence of the lithium salt. Whether the compound can conduct lithium ions can be confirmed by measuring the ionic conductivity of the compound, and when 1 to 40% by mass of the lithium salt is added to the compound, the peak of the ionic conductivity measured -6 is 1×10

[0060] The lithium ion conductive compound has at least one kind of group selected from the group consisting of a carbonyl group and a chain ether group.

[0061] In one embodiment, the lithium ion conductive compound having at least one kind of group selected from the group consisting of a carbonyl group and a chain ether group may be a carbonate such as a chain carbonate such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate.

[0062] In another embodiment, the lithium ion conductive compound having at least one kind of group selected from the group consisting of a carbonyl group and a chain ether group may be glyme represented by the following formula (3). R 23 O-(CH2CH2O) k2 -R 24 (3) [In formula (3), R 23 and R 24 each independently represent an alkyl group having 1 to 4 carbon atoms, and k2 represents an integer of 1 to 6.]

[0063] More specifically, glyme may be monoglyme (k2 = 1), diglyme (k2 = 2), triglyme (k2 = 3), tetraglyme (k2 = 4), pentaglyme (k2 = 5), or hexaglyme (k2 = 6).

[0064] In the separation membrane of the first embodiment, the molecular weight of the lithium ion conductive compound is 150 or less. From the viewpoint of increasing the average pore diameter of the separation membrane 7 in the range of 2 Å or more and less than 20 Å, the molecular weight of the lithium ion conductive compound is preferably 10 or more, more preferably 20 or more, and still more preferably 30 or more. From the viewpoint of decreasing the average pore diameter of the separation membrane 7 in the range of 2 Å or more and less than 20 Å, the molecular weight of the lithium ion conductive compound is preferably 145 or less, more preferably 140 or less, and still more preferably 135 or less.

[0065] From the viewpoint of further increasing the ionic conductivity of the separation membrane 7, the content of the lithium ion conductive compound is preferably 5% by mass or more, more preferably 10% by mass or more, and still more preferably 15% by mass or more based on the total amount of the separation membrane. From the viewpoint of film-forming properties, the content of the lithium ion conductive compound is preferably 90% by mass or less, more preferably 80% by mass or less, and still more preferably 70% by mass or less based on the total amount of the separation membrane.

[0066] The separation membrane according to the first embodiment may further contain, as other components, a lithium salt (third lithium salt), a polymerization initiator, and the like. As the third lithium salt, the same ones as the above-described first lithium salt can be used. The third lithium salt may be of the same type as or different from the above-described first lithium salt and second lithium salt.

[0067] The content of the third lithium salt may be 1% by mass or more, 1.5% by mass or more, or 2% by mass or more, and may be 30% by mass or less, 25% by mass or less, or 20% by mass or less based on the total amount of the separation membrane. The content of the polymerization initiator may be 0.1% by mass or more and may be 10% by mass or less based on the total amount of the separation membrane.

[0068] The separation membrane 7 having the above average pore diameter is, in another embodiment (the second embodiment), a separation membrane containing a crosslinked product of polymers having a group represented by the following formula (1). In this separation membrane, the polymer having the group represented by the following formula (1) crosslinks using lithium ions present in the lithium-ion secondary battery 1 or in the separation membrane 7 as an initiator, whereby the groups represented by formula (1) are bonded to each other, and pores having the above-described average pore diameter are formed. The average pore diameter of the separation membrane 7 is adjusted by adjusting the number of carbon atoms represented by R in the following formula (1). 1 It can be adjusted by adjusting the number of carbon atoms represented by 1 . [Chemical formula] In formula (1), R 1 represents an alkylene group having 1 to 6 carbon atoms, and * represents a bond.

[0069] The polymer having the group represented by formula (1) may be, for example, a polymer represented by the following formula (4). [Chemical formula] In formula (4), R 1 has the same meaning as R in formula (1), R 1 , R 11 represents a linear or branched alkylene group or a single bond, and r represents an integer of 2 or more.

[0070] In formula (1) or formula (4), the number of carbon atoms of the alkylene group represented by R 1 is preferably 2 or more or 3 or more from the viewpoint of increasing the average pore diameter of the separation membrane 7 in the range of 2 Å or more and less than 20 Å, and is preferably 5 or less or 4 or less from the viewpoint of decreasing the average pore diameter of the separation membrane 7 in the range of 2 Å or more and less than 20 Å.

[0071] In formula (4), the number of carbon atoms of the alkylene group represented by R 11 may be, for example, 2 or more, and may be 5 or less. r may be, for example, 5 or more, and may be 20 or less.

[0072] Examples of the polymer having the group represented by the formula (1) include polyglycidyl (meth) acrylate, poly(3-ethyloxetan-3-yl)methyl (meth) acrylate, and the like.

[0073] The weight average molecular weight of the polymer having the group represented by the formula (1) may be 900 or more, 1000 or more, or 3000 or more, and may be 200000 or less, 100000 or less, or 50000 or less.

[0074] From the viewpoint of the strength of the membrane, the content of the cross-linked product of the polymers having the group represented by the formula (1) is preferably 5% by mass or more, more preferably 15% by mass or more, and still more preferably 30% by mass or more, based on the total amount of the separation membrane. From the viewpoint of making it difficult to inhibit the movement of lithium ions, the content of the cross-linked product is preferably 95% by mass or less, more preferably 85% by mass or less, and still more preferably 80% by mass or less, based on the total amount of the separation membrane.

[0075] The separation membrane according to the second embodiment may further contain, as other components, a lithium salt (the third lithium salt), a polymerization initiator, and the like. The types and the contents in the separation membrane 7 of the third lithium salt and the polymerization initiator may be the same as those in the case of the separation membrane according to the first embodiment described above.

[0076] From the viewpoint of further enhancing the separation ability of the separation membrane 7, the thickness of the separation membrane 7 according to the embodiment described above is preferably 100 μm or more, 200 μm or more, or 500 μm or more. From the viewpoint of increasing the energy density of the separation membrane 7, the thickness is preferably 800 μm or less, 600 μm or less, or 400 μm or less.

[0077] 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 a step 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, a step of 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, and a step of providing a separator 7 between the positive electrode 6 and the negative electrode 8. The order of each step is arbitrary.

[0078] In the above manufacturing method, the specific embodiments 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 separator 7 are as described above.

[0079] In the step of obtaining the positive electrode and the step of obtaining the negative electrode, the positive electrode 6 and the negative electrode 8 can be obtained by using known methods. For example, the materials used for the positive electrode mixture layer 10 or the negative electrode mixture layer 12 are dispersed in an appropriate amount of a dispersion medium using a kneader, a disperser, etc. to obtain a slurry-like positive electrode mixture or negative electrode mixture. Then, this positive electrode mixture or negative electrode mixture is applied onto the positive electrode current collector 9 or the negative electrode current collector 11 by a doctor blade method, a dipping method, a spraying method, etc., and the positive electrode 6 and the negative electrode 8 are obtained by volatilizing the dispersion medium. At this time, the dispersion medium may be water, N-methyl-2-pyrrolidone (NMP), etc.

[0080] The step of providing the separator 7 between the positive electrode 6 and the negative electrode 8 may include a step of manufacturing the separator 7. In one embodiment, the step of manufacturing the separator 7 includes a step of preparing a slurry containing the material of the separator 7 and forming the slurry into a film shape.

[0081] When the separator 7 is the separator according to the above-described first embodiment, the slurry contains a compound having two or more polymerizable groups (polymerizable compound) and a lithium ion conductive compound having at least one group selected from the group consisting of a carbonyl group and a chain ether group. The embodiments of the polymerizable compound and the lithium ion conductive compound are as described above.

[0082] The slurry may further contain the third lithium salt described above. The content of the third lithium salt may be 1% by mass or more, 3% by mass or more, or 5% by mass or more, and may be 30% by mass or less, 25% by mass or less, or 20% by mass or less based on the total amount of the slurry.

[0083] The slurry may further contain a polymerization initiator. Thereby, the polymerizable compound can be preferably polymerized, and the separation membrane can be preferably produced from the slurry. The polymerization initiator may be a thermal polymerization initiator or a photoinitiator, and can be appropriately selected according to the purpose.

[0084] Examples of the thermal polymerization initiator include azobisisobutyronitrile and azobis(2-methylbutyronitrile).

[0085] Examples of the photoinitiator include 2-hydroxy-2-methyl-1-phenylpropanone and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0086] 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.

[0087] A method for forming the slurry into a film shape is, for example, a method in which a frame of an arbitrary size is installed on one surface of a substrate such as a PET sheet, and the slurry is poured therein. Alternatively, the slurry may be formed into a film shape by applying the slurry on one surface of the substrate by a doctor blade method, a dipping method, a spraying method, or the like.

[0088] The step of manufacturing the separation membrane 7 may further include a step of polymerizing a compound having two or more polymerizable groups contained in the slurry formed into a film shape. Thereby, the separation membrane according to the first embodiment is manufactured.

[0089] When the slurry contains a thermal polymerization initiator, the method for polymerizing the polymerizable compound is to apply heat under predetermined conditions. The heating temperature may be, for example, 50 to 90 °C. The heating time may be appropriately adjusted according to the heating temperature, but is, for example, 1 minute to 2 hours.

[0090] When the slurry contains a photoinitiator, the method for polymerizing the polymerizable compound is to irradiate light under predetermined conditions. In one embodiment, the polymerizable compound may be polymerized by irradiation with light (ultraviolet light) having a wavelength within the range of 200 to 400 nm.

[0091] When the separation membrane 7 is the separation membrane according to the second embodiment described above, the slurry used in the process of manufacturing the separation membrane 7 may contain a polymer having a group represented by the above formula (1).

[0092] In this case, the slurry may further contain a third lithium salt and / or a polymerization initiator. The types of the third lithium salt and the polymerization initiator and their contents in the slurry may be the same as those in the case of the separation membrane according to the first embodiment described above.

[0093] The method for forming the slurry into a film may be the same as that in the case of the separation membrane according to the first embodiment described above.

[0094] When the separation membrane 7 is the separation membrane according to the second embodiment described above, the process of manufacturing the separation membrane 7 may further include a step of crosslinking the polymer having a group represented by the formula (1) contained in the slurry formed into a film. Thereby, the separation membrane according to the second embodiment is manufactured.

[0095] The method for crosslinking the polymer may be carried out in the same manner as the step of polymerizing the compound having two or more polymerizable groups in the process of manufacturing the separation membrane 7 according to the first embodiment described above. In this case, the lithium salt contained in the slurry acts as a crosslinking agent, and the polymers having a group represented by the formula (1) are crosslinked to form a crosslinked product.

[0096] In the step of providing the separator 7 between the positive electrode 6 and the negative electrode 8, subsequently, the positive electrode 6, the separator 7, and the negative electrode 8 are laminated, for example, by lamination. Thereby, the 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 can be obtained. Further, by housing this electrode group 2 in the battery case 3, the lithium ion secondary battery 1 can be obtained.

Example

[0097] Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

[0098] [Example 1] 6.0 g of polyethylene glycol diacrylate represented by the following formula (2) (where n = 14, (trade name: NK Ester A-600, manufactured by Shin-Nakamura Chemical Co., Ltd.)), 4.0 g of diethyl carbonate (molecular weight 118, manufactured by Fujifilm Wako Pure Chemical Corporation), 1.4 g of lithium nitrate (manufactured by Fujifilm Wako Pure Chemical Corporation), and 1.0 g of azobisisobutyronitrile (manufactured by Fujifilm Wako Pure Chemical Corporation) were mixed to prepare a slurry. A silicon rubber frame (4 × 4 cm, thickness 1 mm) was placed on a PET sheet (8 × 8 cm, thickness 0.035 mm), and the prepared slurry was put into the frame. Then, using a hot plate, it was heated at 60° C. for 1 hour to polymerize polyethylene glycol diacrylate, thereby obtaining a separator. The weight average molecular weight (Mw) of the polymer of polyethylene glycol diacrylate was 1000. The separator was removed from the frame and subjected to the tests shown below.

Chemical formula

[0099] [Example 2] A separator was produced in the same manner as in Example 1, except that diethyl carbonate was changed to diglyme (molecular weight 134, manufactured by Fujifilm Wako Pure Chemical Corporation) in Example 1.

[0100] [Example 3] In Example 2, a separation membrane was prepared in the same manner as in Example 2, except that the amount of azobisisobutyronitrile (manufactured by Fuji Film Wako Pure Chemical Corporation) added was changed to 0.1 g. The weight average molecular weight of the polymer of polyethylene glycol diacrylate was 10,000.

[0101] [Example 4] In Example 1, a separation membrane was prepared in the same manner as in Example 1, except that diethyl carbonate was changed to monoglyme (molecular weight 32, manufactured by Fuji Film Wako Pure Chemical Corporation).

[0102] [Example 5] 6.0 g of a polymer represented by the following formula (4-1) (where r ≒ 366 and the weight average molecular weight is 10,000), 4.0 g of diglyme (manufactured by Fuji Film Wako Pure Chemical Corporation), 1.4 g of lithium nitrate (manufactured by Fuji Film Wako Pure Chemical Corporation), and 0.1 g of azobisisobutyronitrile (manufactured by Fuji Film Wako Pure Chemical Corporation) were mixed to prepare a slurry. A silicon rubber frame (4×4 cm, thickness 1 mm) was placed on a PET sheet (8×8 cm, thickness 0.035 mm), and the prepared slurry was put into the frame. Then, using a hot plate, it was heated at 60°C for 1 hour to crosslink the polymer represented by formula (4-1) to obtain a separation membrane. The separation membrane was removed from the frame and subjected to the tests shown below. [Chemical formula]

[0103] [Comparative Example 1] In Example 2, a separation membrane was prepared in the same manner as in Example 1, except that polyethylene glycol diacrylate was changed to one with a weight average molecular weight of 250 (manufactured by Sigma-Aldrich), and further the amount of azobisisobutyronitrile (manufactured by Fuji Film Wako Pure Chemical Corporation) added was changed to 2.0 g. The weight average molecular weight of the polymer of polyethylene glycol diacrylate was 500. As a result, film formation could not be achieved and a separation membrane could not be obtained.

[0104] [Comparative Example 2] In Example 1, a separation membrane was prepared in the same manner as in Example 1, except that diethyl carbonate was changed to pentaethylene glycol monomethyl ether (molecular weight 252, manufactured by Fujifilm Wako Pure Chemical Corporation).

[0105] [Comparative Example 3] In Example 5, a separation membrane was prepared in the same manner as in Example 5, except that the polymer represented by the formula (4-1) was changed to the polymer represented by the following formula (10) (r≈364 in the formula, weight average molecular weight 10,000). [Chemical formula]

[0106] <Average pore diameter of the separation membrane> The average pore diameter of the separation membranes according to the Examples and Comparative Examples was measured by a gas adsorption method using argon gas. Specifically, using a nitrogen adsorption measurement device (AUTOSORB-1, manufactured by QUANTACHROME), the evaluation temperature was set to 87 K, and the average pore diameter was measured under the condition that the evaluation pressure range was less than 1 in terms of relative pressure (equilibrium pressure with respect to the saturated vapor pressure). As shown in Tables 1 to 2, the average pore diameters of the separation membranes according to the Examples were all in the range of 2 Å or more and less than 20 Å, while the average pore diameters of the separation membranes according to the Comparative Examples were outside the range of 2 Å or more and less than 20 Å.

[0107] <Evaluation of solvent separation ability> The separation membrane according to the example or comparative example and a separator (UP3085, manufactured by Ube Industries, Ltd.) were overlapped, and these were sandwiched between two sheets of silicone rubber (thickness: 0.5 mm) and placed between the H-type cells. Tetraglyme (molecular diameter: 20 Å) was put into the cell on the separation membrane side, and the appearance of the separator after a lapse of a predetermined number of days was visually observed. If the separation membrane is excellent in solvent separation ability, since tetraglyme hardly permeates through the separation membrane, tetraglyme hardly penetrates into the separator. However, if the separation membrane is inferior in solvent separation ability, tetraglyme permeates through the separation membrane and penetrates into the separator. Therefore, by observing the appearance of the separator and confirming the presence or absence of penetration of tetraglyme into the separator, the separation ability of the solvent (solvent corresponding to the first solvent and the second solvent) of the separation membrane can be evaluated. When there is no penetration of tetraglyme into the separator even after one day has passed since the start of the test, it is indicated as "≧1 day" in Tables 1 to 2. In this case, it can be said that the solvent separation ability of the separation membrane is excellent. On the other hand, in Tables 1 to 2, when penetration of tetraglyme is observed after one day has passed, it is indicated as "<1 day". As shown in Tables 1 to 2, in the separation membrane according to the example, there was no penetration of tetraglyme into the separator even after one or more days had passed, but in the separation membrane according to the comparative example, tetraglyme had penetrated into the separator after one day had passed.

[0108]

Table 1

[0109]

Table 2

Explanation of Reference Numerals

[0110] 1... Lithium ion secondary battery, 2... Electrode group, 3... Battery exterior body, 4... Positive electrode current collector tab, 5... Negative electrode current collector tab, 6... Positive electrode, 7... Separation membrane, 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 comprising a positive electrode active material layer, a separator, and a negative electrode active material layer in this order, wherein the positive electrode active material layer contains a positive electrode active material, a first lithium salt, and a first solvent, the negative electrode active material layer contains a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, the separator has pores with an average pore diameter of 2 Å or more and less than 20 Å, the molecular diameters of the first solvent and the second solvent are each 20 Å or more, the first solvent is at least one selected from the group consisting of glyme represented by the following formula (2) and tris(1H,1H,5H-octafluoropentyl) phosphate, the second solvent is at least one selected from the group consisting of glyme represented by the following formula (2), tris(1H,1H,5H-octafluoropentyl) phosphate, γ-butyrolactone, and tetrahydrofuran, a lithium-ion secondary battery in which the molecular diameters of the first solvent and the second solvent are calculated based on the theoretical bond distance from the most stable structure of the solvent determined by density functional theory (DFT), which is one type of quantum chemical calculation. R21O-(CH2CH2O)k1-R22 (2) [In formula (2), R21 and R22 each independently represent an alkyl group having 1 to 4 carbon atoms, and k1 represents an integer of 4 to 6.]

2. The lithium-ion secondary battery according to claim 1, wherein the separator contains a polymer of a compound having two or more polymerizable groups and a lithium-ion conductive compound having at least one group selected from the group consisting of a carbonyl group and a chain ether group, the weight average molecular weight of the polymer is 800 or more, and the molecular weight of the lithium-ion conductive compound is 150 or less.

3. The lithium-ion secondary battery according to claim 1, wherein the separator contains a cross-linked product of polymers having a group represented by the following formula (1). 【Chemical 1】 [In formula (1), R １ represents an alkylene group having 1 to 6 carbon atoms, and * represents a bond.]

4. In a lithium-ion secondary battery comprising a positive electrode active material layer containing a positive electrode active material, a first lithium salt, and a first solvent, and a negative electrode active material layer containing a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent, a separator for being disposed between the positive electrode active material layer and the negative electrode active material layer, having pores with an average pore diameter of 2 Å or more and less than 20 Å, The molecular diameters of the first solvent and the second solvent are each 20 Å or more, The first solvent is at least one selected from the group consisting of glyme represented by the following formula (2) and tris(1H,1H,5H-octafluoropentyl) phosphate, The second solvent is at least one selected from the group consisting of glyme represented by the following formula (2), tris(1H,1H,5H-octafluoropentyl) phosphate, γ-butyrolactone, and tetrahydrofuran, The molecular diameters of the first solvent and the second solvent are calculated based on the theoretical bond distance from the most stable structure of the solvent obtained by density functional theory (DFT), which is a type of quantum chemical calculation, for the separation membrane. R 21 O-(CH 2 CH 2 O) k1 -R 22 (2) [In formula (2), R 21 and R 22 each independently represent an alkyl group having 1 to 4 carbon atoms, and k1 represents an integer of 4 to 6.]

5. A polymer of a compound having two or more polymerizable groups and a lithium ion conductive compound having at least one group selected from the group consisting of a carbonyl group and a chain ether group, The weight average molecular weight of the polymer is 800 or more, The separation membrane according to claim 4, wherein the molecular weight of the lithium ion conductive compound is 150 or less.

6. The separation membrane according to claim 4, containing a crosslinked product of polymers having a group represented by the following formula (1). 【Chemical 2】 [In formula (1), R １ represents an alkylene group having 1 to 6 carbon atoms, and * represents a bond.]

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