PTC functional composition, PTC functional composition layer, coated current collector for non-aqueous electrolyte secondary battery, electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

To provide a PTC functional composition with excellent coating properties and slurry stability.SOLUTION: A PTC functional composition includes a conductive material, a water-soluble resin, water-dispersed resin particles, and an aqueous medium, and the water-soluble resin is a nonionic polymer containing a (meth)acrylamide monomer unit or a nonionic polymer containing an N-vinylamide monomer unit.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a PTC functional composition, a PTC functional composition layer using the PTC functional composition, a coated current collector for a non-aqueous electrolyte secondary battery, an electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte rechargeable batteries, including lithium-ion batteries, are widely used as power sources for smartphones and laptop computers, and recently they have also been used in large batteries for automotive applications. On the other hand, while lithium-ion batteries have the advantage of high energy density, they use non-aqueous electrolytes, requiring sufficient safety measures. In recent years, as batteries have become larger, ensuring safety has become even more important.

[0003] Therefore, lithium-ion secondary batteries are required to have a so-called shutdown function that automatically and safely stops charging and discharging in the event of an accident such as a malfunction, and this function is provided to the separator inside the battery. However, there are cases where the shutdown by the separator is incomplete and the temperature rises above the melting point of the separator, or where the separator melts due to rising external temperatures, causing an internal short circuit, so further measures to improve safety are required.

[0004] As a countermeasure, a technique has been proposed to form a positive temperature coefficient (PTC) functional composition within the current collector or active material layer that constitutes the electrodes of a lithium secondary battery (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. WO2019 / 003721 [Patent Document 2] International Publication No. WO2016 / 158480 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, in conventionally known PTC functional compositions, the interaction of functional groups of multiple types of resins used in their formation leads to the formation of aggregates in the slurry, causing problems such as deterioration of coating properties and slurry stability. Therefore, when a non-aqueous electrolyte secondary battery is created using a current collector with such a PTC functional composition as the base layer, the thickness of the PTC functional composition becomes large, which hinders the improvement of energy density.

[0007] Therefore, the primary objective of the present invention is to provide a PTC functional composition that exhibits excellent coating properties and slurry stability. [Means for solving the problem]

[0008] In other words, Embodiment 1 of the present invention is a PTC functional composition containing a conductive material, a water-soluble resin, water-dispersible resin fine particles, and an aqueous medium, wherein the water-soluble resin is a nonionic polymer containing (meth)acrylamide monomer units, or a nonionic polymer containing N-vinylamide monomer units.

[0009] Furthermore, aspect 2 of the present invention is a PTC functional composition according to aspect 1, wherein the water-dispersible resin particles are made of a polyolefin-based material.

[0010] A third aspect of the present invention is the PTC functional composition of the second aspect, wherein the water-dispersible resin particles have some or all of their repeating units modified by maleic acid.

[0011] Aspect 4 of the present invention is a PTC functional composition according to any of aspects 1 to 3, wherein the conductive material consists of one or more types of carbon materials.

[0012] Aspect 5 of the present invention is a PTC functional composition according to any of aspects 1 to 4, wherein the aqueous medium is water or a water-compatible liquid medium.

[0013] Aspect 6 of the present invention is the PTC functional composition according to any one of Aspects 1 to 5 above, wherein, based on 100% by mass of the total solid content of the PTC functional composition, the content of the conductive material is 10 to 70% by mass, the content of the water-soluble resin is 1 to 50% by mass, and the content of the water-dispersed resin particles is 5 to 70% by mass.

[0014] Another aspect of the present invention includes a PTC functional composition layer containing the PTC functional composition of each aspect of the present invention as described above, a coated current collector for a non-aqueous electrolyte secondary battery in which the PTC functional composition layer is formed on a current collector, an electrode for a non-aqueous electrolyte secondary battery provided with the coated current collector for a non-aqueous electrolyte secondary battery, or a non-aqueous electrolyte secondary battery provided with the electrode for a non-aqueous electrolyte secondary battery.

Advantages of the Invention

[0015] According to the present invention as described above, it is possible to provide a PTC functional composition excellent in coating properties and slurry stability.

Modes for Carrying Out the Invention

[0016] Regarding the reason why excellent coating properties and slurry stability are obtained by the PTC functional composition according to the embodiment of the present invention, there are still some unclear points. The mechanism considered by the present inventors based on the knowledge obtained so far will be described below.

[0017] That is, the PTC functional composition of the present embodiment contains a conductive material, a water-soluble resin, water-dispersed resin fine particles, and an aqueous medium. Since the water-soluble resin is a nonionic polymer containing (meth)acrylamide-based monomer units or a nonionic polymer containing N-vinylamide-based monomer units, the interaction with ions present on the surface of the water-dispersed resin particles that contributes to the stabilization of the dispersion state of the water-dispersed resin particles is suppressed. Therefore, the formation of aggregates in the slurry can be suppressed, and thus it is considered that excellent coating properties and slurry stability can be exhibited. It should be noted that the explanation of this mechanism is not intended to limit the technical scope of the present invention.

[0018] The specific configuration of the secondary battery according to an embodiment of the present invention will be described below.

[0019] <1. Basic Configuration of Non-Aqueous Electrolyte Secondary Battery> The lithium-ion secondary battery according to this embodiment includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The form of the lithium-ion secondary battery is not particularly limited, and for example, it may be any of a cylindrical shape, a rectangular shape, a laminated shape, or a button shape.

[0020] (1-1. Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector.

[0021] The positive electrode current collector may be any conductor, and for example, it is preferably in the form of a plate or foil and is composed of aluminum, stainless steel, nickel-plated steel, or the like.

[0022] The positive electrode mixture layer contains at least a positive electrode active material and may further contain a conductive agent and a positive electrode binder.

[0023] The positive electrode active material is, for example, a transition metal oxide or a solid solution oxide containing lithium, and is not particularly limited as long as it can electrochemically occlude and release lithium ions. Examples of the transition metal oxide containing lithium include Li 1.0 Ni 0.88 Co 0.1 Al [[ID=3s4]] 0.01 Mg 0.01 O2 and the like. In addition to these, Li·Co-based composite oxides such as LiCoO2, LiNi x Co y Mn z O2 and other Li·Ni·Co·Mn-based composite oxides, Li·Ni-based composite oxides such as LiNiO2, or Li·Mn-based composite oxides such as LiMn2O4 can be exemplified. Examples of the solid solution oxide include Li a Mn x Coy Ni z O2 (1.150≦a≦1.430, 0.45≦x≦0.6, 0.10≦y≦0.15, 0.20≦z≦0.28), LiMn 1.5 Ni 0.5 Examples include O4. The content (ratio) of the positive electrode active material is not particularly limited and should be any content applicable to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery. Furthermore, these compounds may be used individually or in combination of multiple types.

[0024] The conductive agent is not particularly limited as long as it enhances the conductivity of the positive electrode. Specific examples of conductive agents include those containing one or more selected from carbon black, natural graphite, artificial graphite, and fibrous carbon. Examples of carbon black include furnace black, channel black, thermal black, Ketjen black, and acetylene black. Examples of fibrous carbon include carbon nanotubes, graphene, and carbon nanofibers. The content of the conductive agent is not particularly limited and should be any amount applicable to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery.

[0025] Examples of positive electrode binders include fluorine-containing resins such as polyvinylidene fluoride, ethylene-containing resins such as styrene-butadiene rubber, ethylene-propylene-diene terpolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, polyvinyl alcohol, carboxymethylcellulose or carboxymethylcellulose derivatives (such as salts of carboxymethylcellulose), or nitrocellulose. The positive electrode binder is not particularly limited as long as it can bind the positive electrode active material and conductive agent onto the positive electrode current collector.

[0026] (1-2. Negative electrode) The negative electrode comprises a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector.

[0027] The negative electrode current collector can be any conductive material, for example, a plate or foil, and is preferably made of copper, stainless steel, or nickel-plated steel.

[0028] The negative electrode mixture layer may further contain at least a negative electrode active material, a conductive agent, and a negative electrode binder.

[0029] The negative electrode active material is not particularly limited as long as it can electrochemically intercept and release lithium ions, but examples include graphite active material (artificial graphite, natural graphite, mixture of artificial and natural graphite, natural graphite coated with artificial graphite, etc.), Si-based active material or Sn-based active material (for example, a mixture of fine particles of silicon (Si) or tin (Sn) or their oxides and graphite active material, fine particles of silicon or tin, alloys based on silicon or tin), metallic lithium and Li4Ti5O 12 Examples include titanium oxide compounds and lithium nitrides. As the negative electrode active material, one of the above may be used, or two or more may be used in combination. Note that silicon oxides are represented by SiOx (0 ≤ x ≤ 2).

[0030] The conductive agent is not particularly limited as long as it is used to enhance the conductivity of the negative electrode; for example, the same type as described in the section on the positive electrode can be used.

[0031] The negative electrode binder can be any binder capable of binding the negative electrode active material and conductive agent onto the negative electrode current collector, and is not particularly limited. Examples of negative electrode binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene-butadiene copolymer (SBR), and metal salts of carboxymethylcellulose (CMC). One type of binder may be used alone, or two or more types may be included.

[0032] (1-3. Separator) The separator is not particularly limited and can be any type that can be used as a separator in a lithium-ion secondary battery. Preferably, the separator is a porous membrane or nonwoven fabric that exhibits excellent high-rate discharge performance, used alone or in combination. Examples of resins that make up the separator include polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, and vinylidene fluoride-ethylene-tetrafluoroethylene copolymer. Furthermore, the porosity of the separator is not particularly limited, and any porosity of a conventional lithium-ion secondary battery separator can be arbitrarily applied.

[0033] The separator surface may have a heat-resistant layer containing inorganic particles to improve heat resistance, or a layer containing an adhesive to bond to the electrodes and fix the battery element. Examples of the aforementioned inorganic particles include Al2O3, AlOOH, Mg(OH)2, and SiO2. Examples of adhesives include vinylidene fluoride-hexafluoropropylene copolymer, acid-modified vinylidene fluoride polymer, and styrene-(meth)acrylic acid ester copolymer.

[0034] (1-4.Non-aqueous electrolyte) The non-aqueous electrolyte can be the same as the non-aqueous electrolyte conventionally used in lithium-ion secondary batteries. The non-aqueous electrolyte has a composition in which an electrolyte salt is contained in a non-aqueous solvent, which is the solvent for the electrolyte.

[0035] As non-aqueous solvents, for example, cyclic carbonate esters such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; linear carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; linear esters such as methyl formate, methyl acetate, methyl butyrate, ethyl propionate, and propyl propionate; tetrahydrofuran or its derivatives; ethers such as 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, or methyl diglyme, ethylene glycol monopropyl ether, and propylene glycol monopropyl ether; nitriles such as acetonitrile and benzonitrile; dioxolane or its derivatives; ethylene sulfide, sulfolane, sultone or its derivatives, etc., can be used individually or in mixtures of two or more of these. Furthermore, when using a mixture of two or more non-aqueous solvents, the mixing ratio of each non-aqueous solvent can be the same as that used in conventional lithium-ion secondary batteries.

[0036] Examples of electrolyte salts include LiClO4, LiBF4, LiAsF6, LiPF6, and LIPF6-x(C). n F 2n+1 )x[However, 1 <x<6、n=1or2]、LiSCN、LiBr、LiI、Li2SO4、Li2B 10 Cl 10, inorganic ionic salts containing lithium (Li), sodium (Na), or potassium (K), such as NaClO4, NaI, NaSCN, NaBr, KClO4, KSCN, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, (CH3)4NBF4, (CH3)4NBr, (C2H5)4NClO4, (C2H 5) Examples of organic ionic salts include 4NI, (C3H7)4NBr, (n-C4H9)4NClO4, (n-C4H9)4NI, (C2H5)4N-maleate, (C2H5)4N-benzoate, (C2H5)4N-phtalate, lithium stearylsulfonate, lithium octylsulfonate, lithium dodecylbenzenesulfonate, etc. These ionic compounds can be used individually or in combination of two or more types. The concentration of the electrolyte salt may be the same as that of the non-aqueous electrolyte used in conventional lithium-ion secondary batteries, and there are no particular restrictions. In this embodiment, it is preferable to use a non-aqueous electrolyte containing the above-mentioned lithium compounds (electrolyte salts) at a concentration of about 0.8 mol / l to 1.5 mol / l.

[0037] Furthermore, various additives may be added to the non-aqueous electrolyte. Examples of such additives include negative electrode additives, positive electrode additives, ester-based additives, carbonate ester-based additives, sulfate ester-based additives, phosphate ester-based additives, borate ester-based additives, acid anhydride-based additives, and electrolyte-based additives. Any one of these may be added to the non-aqueous electrolyte, or multiple types of additives may be added to the non-aqueous electrolyte.

[0038] <2. Method for manufacturing a non-aqueous electrolyte secondary battery according to this embodiment> Next, we will explain the manufacturing method of lithium-ion secondary batteries. The positive electrode is manufactured as follows. First, a mixture of positive electrode active material, conductive agent, and positive electrode binder in desired proportions is dispersed in a solvent for positive electrode slurry to form a positive electrode slurry. Next, this positive electrode slurry is applied to the positive electrode current collector and dried to form a positive electrode mixture layer. The application method is not particularly limited, and examples include knife coater, gravure coater, reverse roll coater, and slit die coater. Each of the following application steps is performed in the same manner. Next, the positive electrode mixture layer is pressed with a press machine to the desired density. This completes the production of the positive electrode.

[0039] The negative electrode is manufactured in the same way as the positive electrode. First, the materials constituting the negative electrode mixture layer are mixed and dispersed in a solvent for the negative electrode slurry to create the negative electrode slurry. Next, the negative electrode slurry is applied onto the negative electrode current collector and dried to form the negative electrode mixture layer. Then, the negative electrode mixture layer is pressed using a press machine to achieve the desired density. This completes the production of the negative electrode.

[0040] Next, an electrode structure is fabricated by sandwiching a separator between the positive and negative electrodes. Then, the electrode structure is processed into a desired shape (e.g., cylindrical, rectangular, laminated, button-shaped, etc.) and inserted into a container of that shape. Next, a non-aqueous electrolyte is injected into the container, impregnating the pores in the separator and the gaps between the positive and negative electrodes with the electrolyte. This completes the fabrication of a lithium-ion secondary battery.

[0041] <3. Characteristic configuration of the non-aqueous electrolyte secondary battery according to this embodiment> The characteristic configuration of the non-aqueous electrolyte secondary battery according to this embodiment will be described below.

[0042] (3-1.PTC functional composition layer) The aforementioned positive electrode and negative electrode further comprise a PTC functional composition layer containing a positive temperature coefficient (PTC) functional composition as an underlayer.

[0043] The PTC functional composition layer contains at least a conductive material, a water-soluble resin, water-dispersible resin fine particles, and an aqueous medium. The PTC functional composition layer only needs to be formed between the positive electrode current collector and the negative electrode current collector, and is preferably provided, for example, between the positive electrode current collector and the positive electrode mixture layer, and / or between the negative electrode current collector and the negative electrode mixture layer. In this embodiment, the PTC functional composition layer is provided both between the positive electrode current collector and the positive electrode mixture layer, and between the negative electrode current collector and the negative electrode mixture layer.

[0044] <Conductive material> The conductive material is a particulate material that has electrical conductivity and consists of one or more types of carbon materials such as graphite, carbon black, furnace black, graphitized furnace black, conductive carbon fibers (carbon nanotubes, carbon nanofibers, carbon fibers), and fullerenes.

[0045] If the conductive material content is low, the electrical resistance of the PTC functional composition layer at the battery operating temperature will increase, leading to an increased voltage drop and potentially degrading battery characteristics such as cell capacity, cell voltage, load characteristics, and cycle characteristics. On the other hand, if the conductive material content is excessive, the content of the water-dispersible resin will relatively decrease, potentially resulting in insufficient PTC functionality. Therefore, the conductive material content is preferably 10% to 70% by mass, more preferably 17.5% to 35% by mass, and even more preferably 22.5% to 35% by mass, based on 100% by mass of the total solid content of the PTC functional composition excluding liquid components such as solvents.

[0046] Furthermore, the total solid content of the PTC functional composition refers to the total mass of solids remaining after all the liquid portion has evaporated by drying, for example, and is equal to the total mass of each component used in powder form when preparing the PTC functional composition. The same applies hereafter.

[0047] <Water-soluble resin> The water-soluble resin possesses water solubility and binds conductive materials to water-dispersible resin fine particles. Here, "water solubility" means, for example, that when 1 g of water-soluble resin (E) is added to 99 g of water at 25°C, stirred, and left at 25°C for 24 hours, the resin remains soluble in water without separation or precipitation. Specifically, the water-soluble resin of the present invention is characterized by being a nonionic polymer containing (meth)acrylamide monomer units, or a nonionic polymer having N-vinylamide monomer units.

[0048] Examples of (meth)acrylamide monomer units include acrylamide, methacrylamide, N-isopropylacrylamide, N-methylolacrylamide, N-(2-hydroxyethyl)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-butylacrylamide, N-tertbutylacrylamide, and acroylmorpholine.

[0049] Among (meth)acrylamide monomer units, acrylamide is preferred from the viewpoint of water solubility, adhesion, and low electrolyte swelling. Examples of N-vinylamide monomer units include N-vinylformamide, N-vinylacetamide, and N-vinylisobutylamide. These (meth)acrylamide monomer units and N-vinylamide monomer units may be contained individually or in any ratio of two or more types.

[0050] Furthermore, the above-mentioned water-soluble resin may contain monomer units other than (meth)acrylamide monomer units and N-vinylamide monomer units. The other monomer units are not particularly limited as long as they are nonionic monomer units that can copolymerize with (meth)acrylamide monomers and N-vinylamide monomers. Examples of other nonionic monomer units include (meth)acrylic acid ester monomer units, vinyl cyanide monomer units, vinyl alcohol, vinylpyrrolidone, and the like.

[0051] If the water-soluble resin content is low, the binding properties of the water-soluble resin will be impaired, and components of the PTC functional composition layer, such as conductive agents, may detach from the current collector during manufacturing, potentially resulting in insufficient PTC functionality. On the other hand, if the water-soluble resin content is excessive, there is a risk of reduced battery characteristics due to insufficient conductive agents or reduced PTC functionality due to insufficient water-dispersible resin. Therefore, the water-soluble resin content is preferably 1% to 50% by mass, more preferably 10% to 40% by mass, and even more preferably 22.5% to 32.5% by mass, based on 100% by mass of the total solid content of the PTC functional composition excluding liquid components such as solvents.

[0052] <Water-dispersed resin particles> Water-dispersible resin microparticles, also commonly known as aqueous emulsions, are resin particles that are dispersed in water in the form of microparticles without dissolving.

[0053] Specifically, examples of these water-dispersible resin fine particles include polyolefin materials whose olefin components include ethylene, propylene, isobutylene, isobutene, 1-butene, 2-butene, 1-pentene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-hexene, 1-octene, norvonene, etc. The polyolefin material constituting the water-dispersible resin fine particles may be a polymer of a single olefin component or a copolymer of two or more components.

[0054] Furthermore, it is preferable that the polyolefin material constituting the water-dispersible resin fine particles has some or all of its repeating units modified with a compound having maleic acid or ethyl acrylate, and it is particularly preferable that it is modified with maleic acid.

[0055] If the content of water-dispersible resin fine particles is low, the so-called shutdown function, which spontaneously and safely stops charging and discharging when the battery temperature rises due to an unforeseen accident such as a malfunction, may be insufficient. On the other hand, if the content of water-soluble resin fine particles is excessive, the shutdown function may be activated at a temperature rise that does not require the activation of the so-called shutdown function, potentially degrading battery performance. Therefore, the content of water-dispersible resin fine particles is preferably 5% to 70% by mass, more preferably 20% to 60% by mass, and even more preferably 35% to 50% by mass, based on 100% by mass of the total solid content of the PTC functional composition excluding liquid components such as solvents.

[0056] <Aqueous medium> As the aqueous medium, water is preferred, but if necessary, a liquid medium that is compatible with water may be used, for example, to improve the coating properties on the current collector.

[0057] Examples of liquid media that are compatible with water include alcohols, glycols, cellosolves, amino alcohols, amines, ketones, carboxylic acid amides, phosphate amides, sulfoxides, carboxylic acid esters, phosphate esters, ethers, nitriles, etc., and may be used within the range of compatibility with water.

[0058] <Other additives> The PTC functional composition may further contain other additives as needed, such as surfactants, film-forming aids, defoamers, leveling agents, preservatives, pH adjusters, and viscosity modifiers. The other additives, such as surfactants, film-forming aids, defoamers, leveling agents, preservatives, pH adjusters, and viscosity modifiers, are preferably nonionic. Examples of nonionic additives include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, and polyoxyethylene cetyl ether; polyoxyethylene octylphenyl ether, polyoxyethylene alkyl allyl ether; polyoxyethylene derivatives; oxyethylene-oxypropylene block copolymers; sorbitan fatty acid esters such as sorbitan monolaurate, sorbitan monostearate, and sorbitan trioleate; glycerin fatty acid esters; polyoxyethylene fatty acid esters; sucrose fatty acid esters; polyethylene glycol; and polypropylene glycol. By using such additives as surfactants, film-forming aids, defoamers, leveling agents, preservatives, pH adjusters, viscosity adjusters, etc., it is possible to suppress the formation of aggregates in the PTC functional composition that may occur when the aforementioned additives are added.

[0059] (3-2. Method for preparing PTC functional composition layer) The aforementioned PTC functional composition can be suspended in a solvent such as water or NMP to form a slurry, which is then applied to a positive or negative electrode current collector so that the thickness after drying is 0.1 μm to 5 μm, more preferably 0.3 μm to 2 μm, and then dried to form the PTC functional composition layer. In this specification, a current collector with a PTC functional composition layer formed on its surface will be referred to as a coated current collector. A thickness of 0.1 μm or more for the PTC functional composition layer of the coated current collector is preferable because it allows the PTC function to be fully exhibited during abnormal heat generation. Furthermore, a thickness of 5 μm or less for the PTC functional composition layer of the coated current collector is preferable because it ensures the ratio of active material in the electrode and suppresses a decrease in battery capacity. By forming a composite layer on the PTC functional composition layer of this coated current collector, an electrode for a secondary battery, which will be the positive or negative electrode, can be manufactured.

[0060] More specifically, the positive electrode can be formed by forming a positive electrode mixture layer on the PTC functional composition layer of a positive electrode coated current collector comprising a PTC functional composition layer and a positive electrode current collector. Similarly, the negative electrode can be formed by forming a negative electrode mixture layer on the PTC functional composition layer of a negative electrode coated current collector comprising a PTC functional composition layer and a negative electrode current collector. [Examples]

[0061] The present invention will be described in more detail below based on specific examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to these examples.

[0062] <Synthesis of water-soluble resins> (Synthesis of water-soluble resin (B-1)) In a 2000 mL separable flask equipped with a mechanical stirrer, stirring rod, and thermometer, 60.0 g of acrylamide and 1125.0 g of deionized water were charged and stirred at 400 rpm. The system was then purged with nitrogen, and the temperature was raised to a jacket temperature of 85°C. When the system temperature reached 60°C, an initiator aqueous solution of 1217 mg of 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) dissolved in 15.0 g of deionized water was added. With the jacket temperature set to 85°C, stirring was continued for 12 hours from the addition of the initiator to obtain a colorless, transparent polymer aqueous solution. The non-volatile content of the aqueous solution after the reaction was measured to be 5.1% by mass. This was collected to obtain an aqueous solution with a pH of 7.2 containing 5.1% by mass of water-soluble resin (B-1).

[0063] (Synthesis of water-soluble resin (B-2)) In a 2000 mL separable flask equipped with a mechanical stirrer, stirring rod, and thermometer, 45.0 g of acrylamide, 45.0 g of methacrylamide, and 790.0 g of deionized water were charged and stirred at 400 rpm. The system was then purged with nitrogen, and the temperature was raised to a jacket temperature of 85°C. When the system temperature reached 60°C, an initiator aqueous solution of 1675 mg of 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) dissolved in 20.0 g of deionized water was added. Stirring was continued for 12 hours from the addition of the initiator at a jacket temperature of 85°C to obtain a colorless, transparent polymer aqueous solution. The non-volatile content of the aqueous solution after the reaction was measured to be 10.0% by mass. This was collected to obtain an aqueous solution with a pH of 7.8 containing 10.0% by mass of water-soluble resin (B-2).

[0064] (Synthesis of water-soluble resin (B-3)) Into a 2000 mL separable flask equipped with a mechanical stirrer, a stirring rod, and a thermometer, 100.0 g of N-vinylformamide and 880.0 g of ion-exchanged water were charged and stirred at 400 rpm. Then, the inside of the system was purged with nitrogen, and the jacket temperature was set to 85 °C and the temperature was raised. When the internal temperature reached 60 °C, an initiator aqueous solution prepared by dissolving 2028 mg of 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) in 20.0 g of ion-exchanged water was added. With the jacket temperature set at 85 °C, stirring was continued for 6 hours from the addition of the aforementioned initiator. Then, an initiator aqueous solution prepared by dissolving 2028 mg of 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) in 20.0 g of ion-exchanged water was added, and stirring was continued for another 6 hours to obtain a colorless and transparent polymer aqueous solution. When the non-volatile content of the aqueous solution after the reaction was measured, it was 10.0 mass%. This was recovered to obtain an aqueous solution of pH 7.5 containing 10.0 mass% of the water-soluble resin (B-3).

[0065] <Selection of water-soluble resin> As the water-soluble resin (B-4), a 1.36 mass% aqueous solution of sodium carboxymethyl cellulose was used. Also, as the water-soluble resin (B-5), a 25.0 mass% solution of sodium polyacrylate was used.

[0066] <Preparation of PTC functional composition> (Example 1) As the conductive carbon material, 70.0 g of conductive material (A-1), 588 g of a 5.1% aqueous solution of water-soluble resin (B-1) as a binder (30 g as solid content), and 100.0 g of water were put in and mixed at 3000 rpm for 20 minutes using a disper. The above mixture was subjected to high-pressure dispersion treatment at a pressure of 80 MPa using NanoVator manufactured by Yoshida Kogyo Kikai Co., Ltd. The high-pressure dispersion treatment was repeated 3 times to obtain a dispersion of conductive material (A-1). Then, 120 g of water-dispersed resin particles (C-1) (30 g as solid content) were mixed with the above dispersion at 500 rpm for 10 minutes using a mixer to obtain a PTC functional composition.

[0067] ]>(Examples 2 to 20) PTC functional compositions, secondary battery cells, and positive electrode symmetric cells were fabricated using the same procedure as shown in Table 4, except that the types and amounts of conductive material, water-soluble resin, and water-dispersible resin particles were changed.

[0068] (Comparative Examples 1-11) PTC functional compositions, secondary battery cells, and positive electrode symmetric cells were fabricated using the same procedure as shown in Table 4, except that the types and amounts of conductive material, water-soluble resin, and water-dispersible resin particles were changed.

[0069] <Manufacturing of lithium-ion secondary batteries> (Insulated current collector) A PTC functional composition was applied to a 10 μm thick aluminum current collector foil using a gravure coater to a thickness of 1.5 μm or 3 μm and dried to produce a coated current collector.

[0070] (Positive electrode fabrication) A cathode mixture slurry was prepared by dispersing LiCoO2, acetylene black, and polyvinylidene fluoride in an N-methyl-2-pyrrolidone solvent in a solid content mass ratio of 97.7:1.0:1.3 and mixing them. Subsequently, the amount of mixture applied (surface density) after drying was 18.9 mg / cm² on one side. 2 The slurry was coated onto one side of a coated current collector coated with a PTC functional composition and then dried. After drying, the mixture was pressed using a roll press machine to achieve a composite layer density of 4.15 g / cc, thereby producing a positive electrode.

[0071] (Negative electrode fabrication) A negative electrode slurry was prepared by dispersing and mixing 100.0 g of artificial graphite active material, 75.42 g of a 1.36% by mass aqueous solution of carboxymethylcellulose sodium salt (CMC), and 25.86 g of deionized water. Then, 3.85 g of a 40% by mass aqueous dispersion of particulate dispersion a, in which modified styrene-butadiene copolymer was dispersed in an aqueous solvent, was added to the mixture. Subsequently, the amount of mixture coated (surface density) after drying was 11.15 mg / cm² on one side. 2 After coating one side of the copper foil with the slurry and allowing it to dry, the mixture was pressed using a roll press machine to create a negative electrode with a composite layer density of 1.42 g / cc.

[0072] (Fabrication of a secondary battery cell) After welding nickel and aluminum lead wires to the above-mentioned single-sided negative electrode and single-sided positive electrode respectively, an electrode laminate was fabricated by laminating one single-sided negative electrode and one single-sided positive electrode with a porous polyethylene separator interposed therebetween. Next, the above electrode laminate was housed in an aluminum laminate film with the lead wires drawn out to the outside, and an electrolyte was injected and sealed under reduced pressure to fabricate a secondary battery cell before initial charging. As the electrolyte, a solvent in which ethylene carbonate / dimethyl carbonate / ethyl propionate / propyl propionate were mixed at 15 / 15 / 30 / 40 (volume ratio) was used, and 1.3 M of LiPF6, 6.0 mass% of fluoroethylene carbonate, and 0.5 mass% of vinylene carbonate were dissolved therein.

[0073] (Fabrication of a positive electrode symmetric cell) In the fabrication of the above secondary battery cell, a positive electrode symmetric cell was fabricated in the same procedure except that the single-sided negative electrode was changed to a single-sided positive electrode.

[0074] <Evaluation of PTC functional composition, coated current collector, electrode, and non-aqueous secondary battery> (Presence or absence of aggregates) In Examples 1 to 20 and Comparative Examples 1 to 11, a grind meter (0 - 50 μm) manufactured by BYK was used to measure the particle diameter of the fine particles dispersed in the PTC functional composition in accordance with JIS-K5600-2-5. The point at which distinct spots began to appear was read, and when it was less than 15 μm, it was regarded as "no aggregation", and when it was 15 μm or more, it was regarded as "aggregation present".

[0075] (Resistance increase)<0000This embodiment describes the method for evaluating the PTC functionality of the PTC functional composition. A positive electrode symmetric cell was placed in an electric furnace and connected to a battery tester. Next, a K-type thermocouple was attached to the surface of the positive electrode symmetric cell with heat-resistant tape. Then, the temperature inside the electric furnace was raised from room temperature to 100°C, and the 1kHz impedance and cell surface temperature were measured using the battery tester and the K-type thermocouple. The resistance value of Comparative Example 1 at the time of heating was set to 100%, and the relative percentage of the resistance values ​​of each sample at the time of heating was calculated as "resistance increase (%)" and is shown in Table 4.

[0076] (High-temperature cycle life) The secondary battery cells prepared in Examples 1-20 and Comparative Examples 1-11 were subjected to one cycle of constant current charging at 0.1 CA, constant voltage charging at 0.05 CA, and constant current discharge at 0.1 CA in a constant temperature bath at 25°C, under conditions of a charging termination voltage of 4.45 V and a discharge termination voltage of 3.0 V. Subsequently, two cycles of constant current charging at 0.2 CA, constant voltage charging at 0.05 CA, and constant current discharge at 0.2 CA were performed under conditions of a charging termination voltage of 4.45 V and a discharge termination voltage of 3.0 V. The secondary battery was placed in a constant temperature bath at 45°C, and under the conditions of a charge termination voltage of 4.45V and a discharge termination voltage of 3.0V, it underwent one cycle of constant current charging at 0.2CA, constant voltage charging at 0.05CA, and constant current discharge at 0.2CA. Then, under the conditions of a charge termination voltage of 4.45V and a discharge termination voltage of 3.0V, it underwent 49 cycles of constant current charging at 0.2CA, constant voltage charging at 0.05CA, and constant current discharge at 0.2CA. From the 51st cycle onward, the above charge-discharge pattern was repeated up to 200 cycles. The capacity retention rate (%) at the 200th cycle was calculated as (discharge capacity at the 200th cycle) ÷ (initial discharge capacity) × 100.

[0077] (High-temperature storage) The secondary battery cells prepared in Examples 1-20 and Comparative Examples 1-11 were subjected to one cycle of constant current charging at 0.1 CA, constant voltage charging at 0.05 CA, and constant current discharge at 0.1 CA under the conditions of a charge termination voltage of 4.45 V and a discharge termination voltage of 3.0 V in a constant temperature bath at 25°C. Then, two cycles of constant current charging at 0.2 CA, constant voltage charging at 0.05 CA, and constant current discharge at 0.2 CA were performed under the conditions of a charge termination voltage of 4.45 V and a discharge termination voltage of 3.0 V. Finally, constant current charging at 0.2 CA and constant voltage charging at 0.05 CA were performed under the condition of a charge termination voltage of 4.45 V, until the secondary batteries were fully charged. The cell voltage and 1 kHz impedance of the fully charged secondary batteries were measured using a battery tester and stored in a constant temperature bath at 60°C. Fully charged secondary batteries were returned to room temperature on days 1, 3, and 7, and their OCV and 1kHz impedance were measured using a battery tester. They were then stored in a 60°C constant temperature chamber. On day 14, the fully charged secondary batteries were returned to room temperature, and their OCV and 1kHz impedance were measured using a battery tester. They were then discharged at a constant current of 0.2CA in a 25°C constant temperature chamber with a discharge termination voltage of 3.0V. Subsequently, they underwent two cycles of constant current charging at 0.2CA with a charge termination voltage of 4.45V and a discharge termination voltage of 3.0V, followed by constant voltage charging at 0.05CA and constant current discharge at 0.2CA. The remaining capacity and recovered capacity were then measured.

[0078] (Evaluation of resistance increase during high-temperature storage) The resistance increase (%) during high-temperature storage was calculated as (1kHz impedance of the secondary battery measured on the 14th day of high-temperature storage) ÷ (1kHz impedance of the secondary battery measured immediately before high-temperature storage) × 100%.

[0079] (Evaluation of voltage drop during high-temperature storage) The voltage drop (V) during high-temperature storage was calculated as (OCV of the secondary battery measured immediately before high-temperature storage) - (OCV of the secondary battery measured on the 14th day of high-temperature storage).

[0080] (Remaining capacity after high-temperature storage) After measuring the 1kHz impedance and OCV on the 14th day of high-temperature storage, constant current discharge was performed at 0.2CA in a 25°C constant temperature bath with a discharge termination voltage of 3.0V. The discharge capacity obtained from this measurement was divided by the full charge capacity before high-temperature storage and multiplied by 100% to calculate the remaining capacity during high-temperature storage.

[0081] (Recovery capacity after high-temperature storage) After measuring the remaining capacity during high-temperature storage, two cycles of constant-current charging at 0.2CA and constant-voltage charging at 0.05CA and constant-current discharge at 0.2CA were performed in a 25°C constant-temperature bath under the conditions of a charging termination voltage of 4.45V and a discharge termination voltage of 3.0V. The discharge capacity of the second cycle was divided by the full charge capacity before high-temperature storage and multiplied by 100% to calculate the recovery capacity during high-temperature storage.

[0082] (Evaluation results) Table 1 shows the physical properties of the conductive materials (A-1) to (A-5) used in the examples and comparative examples described above. Table 2 shows the structural formulas of the water-soluble resins (B-1) to (B-3), and Table 3 shows the physical properties and solid content concentrations of the water-dispersible resin particles (C-1) to (C-5). Table 4 summarizes the evaluation results of Examples 1 to 20 and Comparative Examples 1 to 11.

[0083] [Table 1]

[0084] [Table 2]

[0085] [Table 3]

[0086] [Table 4]

[0087] First, in Comparative Example 1, which lacked an undercoat, the increase in resistance due to temperature rise was insufficient. Furthermore, in Comparative Examples 2 to 11, in which PTC functional compositions were prepared using anionic water-soluble resins, aggregation occurred, and a coated current collector suitable for practical use could not be obtained. In the embodiments of the present invention, the polyolefin aqueous dispersion used as water-dispersible resin particles is thought to maintain an emulsion state through the introduction of modified functional groups, pH adjustment, and addition of surfactants. However, the water-soluble resins (B-4) and (B-5) used in Comparative Examples 2 to 11 are ionic resins, and it is thought that their addition destabilizes the dispersion state of the polyolefin aqueous dispersion, leading to aggregation.

[0088] On the other hand, in Examples 1 to 20, no aggregation occurred when preparing the PTC functional compositions, and it was confirmed that they exhibited excellent coating properties and slurry stability. This is thought to be because the water-soluble resins (B-1), (B-2), and (B-3) used in Examples 1 to 20 are nonionic and did not destabilize the dispersion state of the polyolefin particles, which is maintained by the introduction of modified functional groups, pH adjustment, and addition of surfactants. Furthermore, in Examples 1 to 20, it was confirmed that the electrical resistance increased sufficiently when the temperature rose, exhibiting PTC (positive temperature coefficient) functionality.

[0089] Comparing Examples 1-12 with Examples 13-16, it was confirmed that Examples 1-12 exhibited superior high-temperature cycle life and high-temperature storage characteristics. The water-dispersible resin particles (C-1), (C-4), and (C-5) used in Examples 1-12 were modified with a compound containing maleic acid, while the water-dispersible resin particles (C-2) and (C-3) used in Examples 13-16 were modified with a compound containing ethyl acrylate. From the viewpoint of improving high-temperature cycle characteristics and high-temperature storage characteristics, it is considered preferable to modify the water-dispersible resin particles using a compound containing maleic acid.

[0090] Comparing Examples 1 to 10, it can be seen that the increase in resistance tends to be greater as the content of water-dispersible resin particles in the PTC functional composition increases. From the viewpoint of improving PTC functionality, a higher content of water-dispersible resin particles in the PTC functional composition is considered preferable. On the other hand, comparing the high-temperature cycling characteristics and the increase in resistance during high-temperature storage of Examples 1 to 10, it can be seen that the lower the content of water-dispersible resin particles in the PTC functional composition, the better the high-temperature cycling characteristics and cell characteristics during high-temperature storage. For example, from the viewpoint of improving cell characteristics at high temperatures such as 45°C and 60°C, a lower content of water-dispersible resin particles in the PTC functional composition is considered preferable. As described above, when the content of water-dispersible resin particles in a PTC functional composition is changed, a trade-off relationship exists between PTC function and cell characteristics. However, if the total solid content of the PTC functional composition, excluding liquid components such as solvents, is taken as 100% by mass, then PTC function and cell characteristics can be achieved simultaneously if the content of water-dispersible resin particles is 5% by mass or more and 70% by mass or less; if it is 20% by mass or more and 60% by mass or less, PTC function and cell characteristics can be achieved to a higher degree; and if it is 35% by mass or more and 50% by mass or less, PTC function and cell characteristics can be achieved to an even higher degree.

[0091] Comparing Examples 1 to 10, it can be seen that the lower the carbon material content in the PTC functional composition, the greater the increase in resistance. From the viewpoint of improving PTC functionality, a lower carbon material content in the PTC functional composition is considered preferable. On the other hand, comparing the high-temperature cycle characteristics and the increase in resistance during high-temperature storage of Examples 1 to 10, it can be seen that the higher the carbon material content in the PTC functional composition, the better the high-temperature cycle characteristics and the cell characteristics during high-temperature storage. For example, from the viewpoint of improving cell characteristics at high temperatures such as 45°C and 60°C, a higher carbon material content in the PTC functional composition is considered preferable. As described above, when the carbon material content in a PTC functional composition is changed, a trade-off relationship exists between PTC function and cell properties. However, if the total solid content of the PTC functional composition, excluding liquid components such as solvents, is taken as 100% by mass, then both PTC function and cell properties can be achieved if the carbon material content is between 10% by mass and 70% by mass, then a higher degree of compatibility can be achieved if the carbon material content is between 17.5% by mass and 35% by mass, and an even higher degree of compatibility can be achieved if the carbon material content is between 22.5% by mass and 35% by mass.

[0092] Comparing the resistance increase in Example 1 with that of Example 17, it can be confirmed that the resistance increase in Example 17 is greater. In Example 1 and Example 17, the composition of the PTC functional composition is the same, but the thickness of the PTC functional composition is different. When the thickness of the PTC functional composition is large, the electrical resistance of the PTC functional composition layer after the PTC function has manifested increases, which is thought to result in a greater PTC function. On the other hand, the PTC functional composition layer provided in a secondary battery cell as described in the embodiments of the present invention does not contain the active material for general non-aqueous electrolyte secondary battery electrodes. From the viewpoint of improving the energy density of the secondary battery cell, it is considered preferable for the thickness of the PTC functional composition layer to be smaller. When the thickness of the PTC functional composition layer is changed in this way, there is a trade-off relationship between the PTC function and the cell characteristics, but if the thickness of the PTC functional composition after coating and drying is 0.1 μm or more and 5 μm or less, the PTC function and cell characteristics can be balanced, and if it is 0.3 μm or more and 2 μm or less, a higher degree of balance can be achieved.

Claims

1. It contains a conductive material, a water-soluble resin, water-dispersible resin fine particles, and an aqueous medium. The aforementioned water-dispersible resin fine particles are a polyolefin-based material in which some or all of its repeating units are modified by a compound containing maleic acid. The PTC functional composition wherein the water-soluble resin is a nonionic polymer containing (meth)acrylamide monomer units, or a nonionic polymer containing N-vinylamide monomer units, The total solid content of the PTC functional composition is set to 100% by mass. The content of the conductive material is 10 to 70% by mass, The content of the water-soluble resin is 1 to 50% by mass. A PTC functional composition having a content of 5 to 70% by mass of the aforementioned water-dispersible resin fine particles.

2. The PTC functional composition according to claim 1, wherein the conductive material consists of one or more types of carbon materials.

3. The PTC functional composition according to claim 1 or 2, wherein the aqueous medium is water or a water-compatible liquid medium.

4. A PTC functional composition layer containing the PTC functional composition according to any one of claims 1 to 3.

5. A coated current collector for a non-aqueous electrolyte secondary battery comprising the PTC functional composition layer according to claim 4.

6. An electrode for a non-aqueous electrolyte secondary battery, comprising a coated current collector for a non-aqueous electrolyte secondary battery as described in claim 5.

7. A non-aqueous electrolyte secondary battery comprising electrodes for a non-aqueous electrolyte secondary battery as described in claim 6.

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