Method for producing coating liquid

Abstract

[Problem] To provide a method for producing a coating liquid with which thermal shrinkage resistance is high even if the film thickness of a coating layer is thin. [Solution] Provided is a method for producing a coating liquid including an inorganic filler and a poly-N-vinyl carboxylic acid amide, said method being characterized by including a step in which high shearing treatment is performed using a stirring means.

Description

Method for producing coating liquid

[0001] The present invention relates to a method for producing a coating film that can significantly improve the heat shrinkage resistance of the coating film after coating.

[0002] In recent years, the performance of lithium-ion batteries (hereinafter LIBs) has been improving, and their applications to electric vehicles, next-generation mobility, mobile devices, etc. have been expanding more and more. In addition, the load on the internal structure of the battery, such as heat generation and external environmental temperature associated with performance improvement, tends to increase. Also, according to standards such as JIS C 62133-2, stricter safety is required for LIBs. Technical improvements to ensure such safety also need to keep up with the improvement of battery performance.

[0003] One aspect of performance improvement is increasing the capacity. To increase the capacity, an increase in the amount around the electrode such as an active material is required. However, it is difficult to significantly change the size of the battery itself, and there is a limit to the volume inside the LIB. Therefore, if the amount around the electrode increases, the separator needs to be thinner.

[0004] As a result, thinning, high strength, and high heat resistance of the separator have become essential. However, the shutdown function, which is the original role of the separator, needs to operate at a predetermined temperature. Usually, a polyolefin material is used for the separator. Due to the characteristics of the polyolefin material, achieving high performance including high heat resistance of the separator has reached a stage where it is difficult to achieve from the separator base film itself.

[0005] Therefore, in order to enhance the heat resistance of the separator surface, a coated separator provided with a coating layer containing an inorganic substance has been proposed, and improvement is desired in terms of performance, cost, etc.

[0006] The coating layer of the coated separator contains a resin binder together with an inorganic filler. On the other hand, poly-N-vinylcarboxamide is a water-soluble polymer and a non-ionic polymer, so it is not affected by salts or pH, and has the characteristic of high stability against heat. Utilizing these characteristics, it is applied as a coating layer binder constituting the coated separator.

[0007] As a coated separator using poly-N-vinyl carboxylic acid amide, for example, Patent Document 1 discloses a battery separator comprising a multilayer porous membrane having a resin porous membrane mainly composed of a thermoplastic resin and a heat-resistant porous layer mainly composed of heat-resistant fine particles, wherein the thickness of the heat-resistant porous layer is 1 to 15 μm, and the peel strength at 180° between the resin porous membrane and the heat-resistant porous layer is 0.6 N / cm or more. Patent Document 1 discloses that this heat-resistant porous membrane contains a polymer of heat-resistant fine particles and the binder N-vinylacetamide, and a crosslinked acrylic resin. Furthermore, Patent Document 1 states that such a battery separator has excellent dimensional stability at high temperatures and suppresses thermal shrinkage of the separator.

[0008] Furthermore, the technical idea of ​​improving heat resistance by providing a coating layer containing inorganic materials is known as a method for suppressing thermal deformation of resin films used in packaging films, various displays such as liquid crystals and organic ELs, and flexible substrates. For example, Patent Document 2 discloses a method for suppressing thermal deformation in which an inorganic fine particle layer having a film thickness of 0.05 to 1 μm and containing silica and an organic polymer is laminated on at least one side of a resin film.

[0009] International Publication No. 2010 / 104127, Patent No. 4867211

[0010] However, in order to increase capacity, the thickness of the coating layer of the coated separator also needs to be reduced. When the thickness of the coating layer is reduced, the performance in preventing thermal shrinkage tends to decrease significantly, and countermeasures to improve this are necessary.

[0011] On the other hand, in coated separators, the slurry composition (referred to as the coating liquid) that constitutes the coating layer, mainly composed of inorganic fillers, plays an important role in improving the heat resistance of the separator. Conventionally, much of the research on coating liquids has focused on their constituent components and application methods.

[0012] However, simply dispersing inorganic fillers does not always guarantee high heat shrinkage resistance. Our research has revealed that the mechanical energy the coating solution receives during preparation significantly affects its heat shrinkage resistance.

[0013] The inventors have discovered that mechanical energy is influenced by the degree of expansion and entanglement of the molecular chains of poly-N-vinylcarboxylic acid amide, and that increased entanglement of the molecular chains affects the heat shrinkage resistance.

[0014] For example, Patent Document 1 does not describe anything other than stirring with a three-one motor for one hour when poly-N-vinylacetamide, which is a thickening agent in the document, is added. Patent Document 1 is a technology for preventing thermal shrinkage and film rupture of the resin porous membrane that serves as the base material by coating it with an inorganic particle layer. Patent Document 2 describes a method for stirring and mixing the coating liquid, but the various mixing methods are merely descriptions of the dispersibility of inorganic particles. In the first place, Patent Document 2 does not create a coated separator, and it is highly likely that a coated separator with high heat shrinkage resistance cannot be obtained by simply relying on the description in Patent Document 2.

[0015] Under these circumstances, the inventors conducted further investigations and found that by stirring and mixing the coating solution containing inorganic fillers and vinyl carboxylic acid amide under predetermined shear conditions, the heat shrinkage resistance of the coated film after application can be significantly improved, leading to the present invention.

[0016] The present invention has the following characteristics: [1] A method for producing a coating liquid containing an inorganic filler and a poly-N-vinylcarboxylic acid amide, characterized by including a step of performing a high shear treatment using a stirring means. [2] A method for producing the coating liquid according to [1], wherein the step of performing the high shear treatment is performed by stirring and mixing under the condition that the high shear coefficient expressed by the following formula is 1.0 or more: High shear coefficient = Stirring blade tip speed (m / s) × Stirring time (hours) × Number of stirring blades (blades) × Volume ratio of stirring blades to the coating liquid (%) ÷ 100 [3] A method for producing the coating liquid according to [1] or [2], wherein the TI value of the coating liquid after high shear treatment / initial TI value is 1.05 or more and 1.25 or less. [4] A method for producing the coating liquid according to any of [1] to [3], wherein the inorganic filler is alumina. [5] A method for producing the coating liquid according to any of [1] to [4], wherein the poly-N-vinylcarboxylic acid amide is poly-N-vinylacetamide. [6] A method for producing a coating solution according to any one of [1] to [5], wherein the initial mixing step is included prior to the high shear treatment step, and the initial mixing step involves stirring a mixture containing an inorganic filler, a solvent, and poly-N-vinylcarboxylic acid amide. [7] A method for producing a coating solution according to any one of [1] to [6], wherein an inorganic filler, a dispersion medium, poly-N-vinylcarboxylic acid amide, a binder, a dispersant, and a wetting agent are charged together into a mixer and stirred. [8] A method for producing a coating solution according to any one of [1] to [7], wherein the softening point temperature of the poly-N-vinylcarboxylic acid amide is 100°C or higher. [9] A method for producing a coating solution according to any one of [1] to [8], wherein the glass transition temperature of the poly-N-vinylcarboxylic acid amide is 120°C or higher.

[10] A method for producing a coating solution according to any one of [1] to [9], wherein the coating solution contains polyvinyl alcohol as a wetting agent, and its content is 0.05 parts by mass to 1.0 parts by mass per 100 parts by mass of inorganic filler.

[11] A method for producing the coating liquid of

[10] , wherein the viscosity of the polyvinyl alcohol at 20°C and a 4% by mass concentration is 20 to 80 mPa·s.

[12] A coating liquid produced by the production methods of [1] to

[11] .

[13] A coated substrate including a coating film and a substrate formed from the coating liquid produced by the production methods of [1] to

[11] .

[14] A separator formed from the coated substrate of

[13] .

[15] A secondary battery using the separator of

[14] .

[0017] According to the present invention, the heat shrinkage resistance of the coating film can be improved by including a step of performing a predetermined high shear treatment. Here, improvement in heat shrinkage resistance means that the thermal shrinkage rate of the coating film can be reduced. Furthermore, by adding a poly-N-vinylcarboxylic acid amide with a high softening point temperature and a high glass transition temperature to the coating liquid, a coating liquid with excellent heat shrinkage resistance can be obtained even when thinned. The obtained coating liquid can be suitably used in coated separators, and by optimizing the type and amount of each component in the coating liquid, it is possible to suppress the increase in the air permeability of the separator, thereby providing a coated separator that satisfies both high heat resistance and high air permeability. Such a separator can be used in both wet and dry applications.

[0018] The embodiment will now be described in detail. The method for producing the coating solution of this embodiment is a method for producing a coating solution containing an inorganic filler and a poly-N-vinylcarboxylic acid amide, and is characterized by including a step of performing a high-shear treatment using a stirring means. Details of the constituent materials of the coating solution will be described later.

[0019] <Step of applying high shear treatment> The method for producing the coating liquid of the present invention includes a step of applying high shear treatment. The high shear treatment is not particularly limited as long as it applies a high shear force to the coating liquid, and usually a stirring and mixing method using a stirring means is employed.

[0020] For the stirring and mixing method used in the present invention, known methods can be used. For example, methods using general impellers, mill mixers, ball mills, and tumblers can be used, but methods using shear force with impellers or screws are preferred. A mixing method using impellers is even more preferable because it provides high stirring and shearing efficiency even when the viscosity of the coating liquid is low.

[0021] The shape of the impeller blades used can also be any known shape. Specifically, propeller blades, flat paddle blades, inclined paddle blades, turbine blades, Faudler blades, anchor blades, helical ribbon blades, etc., can be used individually or in combination of two or more types. In addition, any other shape that can perform stirring and mixing can be used.

[0022] Among these, four or more paddle blades with high shear force are preferred, but more preferably, composite blades combining four or more paddle blades and turbine blades are preferred as they are good for shearing the compound liquid and dispersing inorganic particles at the bottom. Typically, the coating liquid used to form the coating layer of LIB-coated separators has a relatively low viscosity, so such stirring means are preferred because they have high stirring efficiency and shear efficiency.

[0023] Furthermore, regarding the stirring speed, it is preferable to stir and mix under conditions where the high shear coefficient is 1.0 or higher, as shown by the following formula: High shear coefficient = Stirring blade tip speed (m / s) × Stirring time (hours) × Number of stirring blades (blades) × Volume ratio of stirring blades to coating liquid (%) ÷ 100

[0024] The high-shear treatment of the coating solution can be optimized by adjusting the stirring and mixing time to achieve a predetermined high shear coefficient. When multiple stirring blades are used, the product of the stirring blade tip velocity and the number of stirring blades is the sum of the products of each stirring blade tip velocity, stirring time, and the number of stirring blades (this is also called the "shear coefficient").

[0025] The shear coefficient is preferably 2.0 to 8.0, more preferably 3.0 to 6.5. Within this range, it is possible to promote the entanglement of poly-N-vinylcarboxylic acid amide molecular chains in the coating solution and to disperse the inorganic filler well. This also reduces the time required for stirring and mixing, and prevents the severance of polymer chains due to excessive shear force. As a result, a coating film with excellent heat shrinkage resistance can be formed.

[0026] Furthermore, it is preferable that the TI value of the coating solution after high shear treatment / initial TI value be between 1.05 and 2.50, more preferably between 1.05 and 1.25, even more preferably between 1.08 and 1.25, and even more preferably between 1.20 and 1.25. Each TI value can be determined by the viscosity at 10 rpm / viscosity at 100 rpm. Within this range, it is considered that sufficient entanglement of polymer chains has occurred, and the effect of reinforcing heat shrinkage resistance can be obtained.

[0027] Furthermore, a higher TI value suggests that the polymer chains are more entangled. Therefore, it is presumed that as the TI value improves, the entanglement rate of the polymer chains increases, and the reinforcing effect of the coating film becomes stronger.

[0028] Furthermore, since it is extremely difficult to uniformly compare the shear force of mixers equipped with various types of stirring means, judging the effect of stirring and mixing by the rate of increase in thixotropy of the coating solution subjected to a high-shear treatment process is industrially simple and preferable. In the high-shear treatment process, the coating solution only needs to contain an inorganic filler and poly-N-vinylcarboxylic acid amide, but a dispersion medium, binder, dispersant, and wetting agent may also be used in combination.

[0029] Furthermore, after the high-shear treatment, a mixing treatment may be performed as needed. In such a mixing treatment, any materials to be optionally added to the coating liquid may be added, and immediately afterward, mixing may be performed using known mixing techniques, such as a rotation-and-revolution type mixer. The manufactured coating liquid can be adjusted in terms of solid content concentration, and necessary additives may be added depending on the application.

[0030] <Initial Mixing Step> In this embodiment, the method for producing the coating liquid preferably includes an initial mixing step prior to the high shear treatment step.

[0031] In the initial mixing step, it is preferable to use a known kneading technique, such as a rotation-and-revolving kneader, immediately after the material is added. At the start of the initial mixing step, it is preferable that the dispersion medium, poly-N-vinylcarboxylic acid amide, and inorganic filler are included. Furthermore, the inorganic filler may be a slurry in which inorganic particles are pre-dispersed in the dispersion medium in the presence of a dispersant with low affinity for the inorganic filler.

[0032] The order in which the materials are added is preferably such that it does not hinder contact between the inorganic filler and the poly-N-vinylcarboxylic acid amide. Preferably, the dispersion medium, inorganic filler, and poly-N-vinylcarboxylic acid amide are mixed together. In addition, in the present invention, a dispersant, wetting agent, binder, etc. may be included in the coating liquid as needed. When these are included, preferably, the dispersion medium, inorganic filler, and poly-N-vinylcarboxylic acid amide are stirred and mixed, then the dispersant, wetting agent, and binder are added and stirred and mixed, and more preferably, the dispersion medium, dispersant, wetting agent, and binder are put into a container, then the poly-N-vinylcarboxylic acid amide is added, and then the inorganic filler is added. In this case, the probability of the inorganic filler coming into contact with materials other than the poly-N-vinylcarboxylic acid amide is high, but if there is more than a certain amount of poly-N-vinylcarboxylic acid amide added, the poly-N-vinylcarboxylic acid amide, which has a high affinity for the inorganic filler, will preferentially come into contact with the inorganic filler, thereby further improving the affinity between the coating film formed from the coating liquid and the separator.

[0033] Furthermore, when performing a high-shear treatment without an initial mixing step, it is preferable to add the materials in the order described above. Also, as in Patent Document 1, if water, an inorganic filler such as alumina, and a dispersant are added first and an alumina dispersion is created using a ball mill, the dispersant will cover the alumina surface before the poly-N-vinylcarboxylic acid amide, which may reduce the binding force between the alumina particles.

[0034] This order of input makes it easier to achieve a dispersion effect of the inorganic filler, ensures good mixing with the binder, prevents performance degradation of the binder during the high-shear treatment process, and improves the bonding between the separator and the inorganic filler layer, as well as the bonding strength between the inorganic fillers.

[0035] Furthermore, in the manufacturing method of this embodiment, a defoaming step can be added as appropriate. By removing the mixed gas (foam) in the defoaming step, the stirring efficiency and shearing efficiency can be improved. For defoaming, a centrifugal defoamer that removes the mixed foam by centrifugal force, an ultrasonic defoamer that uses ultrasound, or a vacuum defoamer that reduces the pressure in the tank can be used.

[0036] The constituent materials and formulation of the coating liquid used in the manufacturing method of this embodiment are shown below. <Constituent Materials and Formulation> The constituent materials and formulation amounts of the coating liquid are described below.

[0037] [Poly-N-vinyl carboxylic acid amide] Poly-N-vinyl carboxylic acid amide may be a homopolymer obtained by polymerizing N-vinyl carboxylic acid amide as a monomer, or a copolymer with other monomers. Specific examples of monomers constituting N-vinyl carboxylic acid amide include N-vinylformamide, N-vinylacetamide, N-vinylpropionamide, N-vinylbenzamide, N-vinyl-N-methylformamide, N-vinyl-N-ethylformamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, and N-vinylpyrrolidone. Of these, N-vinylacetamide is particularly preferred due to its coating properties and affinity to water-containing solvents.

[0038] When poly-N-vinylcarboxylic acid amide is a copolymer, other monomers besides N-vinylcarboxylic acid amide include, but are not limited to, acrylonitrile, vinyl acetate, (meth)acrylic acid, itaconic acid, maleic acid, crotonic acid, and their salts.

[0039] The preferred weight-average molecular weight of poly-N-vinylcarboxylic acid amide is 300,000 to 2,000,000. More preferably, it is 500,000 to 1,000,000, and even more preferably, 700,000 to 1,000,000. Within this range, poly-N-vinylcarboxylic acid amide can be blended in an amount that contributes to the heat shrinkage resistance of the separator, and the viscosity of the final coating solution can be set within an appropriate range. From these, a coating solution with excellent coating properties for separators can be produced.

[0040] The softening point temperature of poly-N-vinylcarboxamide is preferably 100 °C or higher, more preferably 130 °C or higher, and still more preferably 150 °C or higher. The softening point temperature herein is the softening point measured by the TMA penetration method.

[0041] The glass transition temperature of poly-N-vinylcarboxamide is preferably 120 °C or higher, more preferably 150 °C or higher, and still more preferably 170 °C or higher. Further, the upper limit of the glass transition temperature is preferably 400 °C or lower, more preferably 380 °C or lower, still more preferably 350 °C or lower, and particularly preferably 190 °C or lower. The glass transition temperature is measured by the DSC method.

[0042] With the softening point temperature and glass transition temperature within this range, a high shape retention ability at high temperatures can be obtained in the separator after coating. The amount of poly-N-vinylcarboxamide in the coating liquid is preferably 0.1 part by mass or more and 10.0 parts by mass or less with respect to 100 parts by mass of the inorganic filler. More preferably, it is 0.5 part by mass or more and 4.0 parts by mass or less, and still more preferably, it is 2.0 parts by mass or more and 2.5 parts by mass or less.

[0043] Within this range, the shape retention performance of the separator by poly-N-vinylcarboxamide can be sufficiently exhibited. It is presumed that poly-N-vinylcarboxamide having a high softening point and a high glass transition temperature binds the inorganic particles, and can highly contribute to enhancing the binding property between the inorganic particles and preventing the thermal shrinkage of the separator at high temperatures.

[0044] [Inorganic filler] The inorganic filler is not particularly limited, but alumina, boehmite, talc, kaolin, calcium carbonate, calcium phosphate, magnesium oxide, amorphous silica, crystalline glass filler, titanium dioxide, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, mica, etc. are preferable. Among these, alumina and boehmite are more preferable, and alumina is particularly preferable.

[0045] The shape of the filler is not particularly limited and may be spherical, polyhedral, plate-like, scaly, columnar, tubular, fibrous or the like. The average particle diameter of the inorganic filler is preferably 0.1 to 5.0 μm, more preferably 0.3 to 1.0 μm.

[0046] Regarding the amount of the inorganic filler, 30 to 200 parts by mass is preferable with respect to 100 parts by mass of the dispersion medium. More preferably, it is 50 to 130 parts by mass, and still more preferably, it is 80 to 110 parts by mass. Within this range, the binding property between the particles of the inorganic filler layer can be ensured, and at the same time, a coating liquid with good coating property and viscosity can be obtained.

[0047] [Dispersion medium] The solvent is not particularly limited as long as it disperses the above inorganic filler without dissolving it, dissolves poly-N-vinylcarboxamide, and dissolves or disperses other components as necessary and does not react with these components. In view of drying property, a volatile dispersion medium is used. Specifically, water or a polar solvent compatible with water is used, but those containing 50% by mass or more of water are preferable, more preferably 95% by mass or more, and particularly preferably water alone. Ion-exchanged water (pure water subjected to ion exchange) is particularly preferable for water. Examples of solvents other than water include alcohols such as methanol, ethanol, and isopropanol.

[0048] [Dispersant] The dispersant is not particularly limited as long as it disperses the inorganic filler, and known ones can be used. Note that the dispersant does not include poly-N-vinylcarboxamide. In the present embodiment, as the dispersant, those of non-metal salts of acrylic acid are preferable. Preferably, anionic ones are preferable. A suitable example is an ammonium salt, which is also preferable from the meaning of preventing deterioration because of the interaction with metal oxides such as alumina and the deterioration of many poly-N-vinylcarboxamides due to acidity.

[0049] The dispersant is preferably 0.05 parts by mass or more and 3.0 parts by mass or less per 100 parts by mass of inorganic filler. More preferably 0.1 parts by mass or more and 1.0 part by mass or less, and even more preferably 0.20 parts by mass or more and 0.5 parts by mass or less. Within this range, it is possible to disperse the inorganic particles without adversely affecting the heat shrinkage properties.

[0050] [Wetting agent] Poly-N-vinyl carboxylic acid amide functions as a wetting agent, but other wetting agents may also be included. The wetting agent does not include the dispersant mentioned above. The wetting agent enhances affinity with the separator surface, and known surfactants, amphiphilic resins, and alcohols that enhance wettability with the resin can be used, but linear alcohol modified products are preferred. More preferably, the linear portion has a linkage of C10 to C40 carbon atoms, and even more preferably, a linkage of C15 to C25 carbon atoms. The end groups are preferably hydrophilic end groups, preferably sulfonate end groups, sulfate end groups, carboxylate end groups, phosphate end groups, or ammonium end groups. These have sufficient surfactant effect and have little adverse effect on the thermal shrinkage of the coating film.

[0051] Furthermore, nonionic surfactants having functional groups can also be used as wetting agents, specifically including polyalcohols, polyethers, and polyesters. Polyvinyl alcohol is also suitably used as one type of polyalcohol.

[0052] When using polyvinyl alcohol, it can be partially or fully saponified. The viscosity of the polyvinyl alcohol, which represents the molecular weight, is preferably 20 to 80 mPa·s at 20°C and a 4% by mass concentration, more preferably 30 to 70 mPa·s, and even more preferably 40 to 70 mPa·s. Within this range, the affinity to the separator surface can be increased, regardless of whether the application is wet or dry, which prevents liquid repulsion on the separator surface during coating and maintains high thermal shrinkage performance.

[0053] The wetting agent is preferably 0.005 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the inorganic filler described later, more preferably 0.01 parts by mass or more and 0.5 parts by mass or less, and even more preferably 0.02 parts by mass or more and 0.2 parts by mass or less. Within this range, affinity of the coating liquid to the separator can be obtained, preventing liquid repulsion during coating and suppressing foaming. Furthermore, it does not affect air permeability and heat shrinkage.

[0054] [Binder] The binder is not particularly limited as long as it shapes the coated material when dry and functions as an adhesive or fixative such as an inorganic filler. However, the binder used may be something other than poly-N-vinylcarboxylic acid amide, the aforementioned dispersant, or wetting agent.

[0055] Although binders containing resin components and those with high affinity to the dispersion medium are used, even if the resin itself has low affinity to the dispersion medium, it is possible to use, for example, known water-dispersion type emulsion resins or rubber latex.

[0056] Suitable materials include emulsions of styrene-butadiene resin or acrylic resin dispersed in water, and various rubber-based latexes, with acrylic emulsion being preferred. The acrylic binder may be either a homopolymer or a copolymer. It may also be used as an emulsion or after solidification.

[0057] Non-crosslinked or partially crosslinked acrylic binders are more preferable. Acrylic binders with a glass transition temperature of 25°C or lower are even more preferable. Furthermore, crosslinked acrylic resins are capable of forming a crosslinked structure within the coating layer. Additionally, binders with a low glass transition temperature (Tg) are highly flexible, and using them allows for the formation of a coated substrate with excellent flexibility.

[0058] According to the present invention, since a process of performing a predetermined high shear treatment is employed, the mixing state of the binder and N-vinylcarboxylic acid amide is improved, and the crosslinked material constituting the binder is not damaged by the shear force. As a result, the binder does not deteriorate in performance during the high shear treatment process, and it is possible to improve the bonding between the substrate and the inorganic filler layer, as well as the bonding strength between the inorganic fillers.

[0059] The amount of binder is preferably 0.5 to 5.0 parts by mass per 100 parts by mass of filler. More preferably 1.0 to 3.0 parts by mass, and even more preferably 1.5 to 2.5 parts by mass. Within this range, it is possible to obtain a sufficient effect in binding the inorganic filler to the substrate and to impart flexibility to the substrate after coating.

[0060] <Coating liquid, coated substrate and its uses> The present invention provides a coating liquid that can be used for coated separators and the like by the above manufacturing method.

[0061] The coated substrate of the present invention comprises a substrate and a coating film formed on the surface of the substrate from the coating liquid. The coating film can be formed by applying the coating liquid to the surface of the substrate and then drying it. The coating method is not particularly limited and conventionally known methods can be used, specifically, coating methods using gravure coaters, knife coaters, roll coaters, die coaters, etc.

[0062] By using a porous resin membrane as the substrate, it is also possible to construct a coated separator. Such a separator is composed of a multilayer porous membrane having a coating layer mainly composed of inorganic filler and poly-N-vinylcarboxylic acid amide, and a porous resin membrane, and is suitable for use as a separator in lithium-ion secondary batteries. The porous resin membrane is a layer that has the original function of a separator, preventing short circuits between the positive and negative electrodes while allowing ions to pass through, and the coating layer is a layer that provides heat resistance to the separator. In addition, the inorganic filler and poly-N-vinylcarboxylic acid amide contained in the coating layer play a role in preventing thermal shrinkage and rupture of the porous resin membrane that serves as the substrate. Furthermore, even if the inside of the battery overheats abnormally and the porous resin membrane melts, the coating layer mainly composed of inorganic filler and poly-N-vinylcarboxylic acid amide separates the positive and negative electrodes, ensuring the safety and reliability of the battery.

[0063] In the separator of the present invention, the thickness of the coating layer can be made thin, between 1 μm and 15 μm, and the adhesion between the porous resin film and the coating layer is also high. Therefore, even if the above-mentioned abnormalities occur during battery use, safety and reliability can be well ensured.

[0064] The porous resin membrane of the separator according to the present invention preferably mainly comprises a thermoplastic resin that softens at 80 to 180°C, closing its pores, and does not dissolve in the non-aqueous electrolyte of the battery. Examples of thermoplastic resins that soften at 80 to 180°C include thermoplastic resins whose melting temperature, as measured using a differential scanning calorimeter (DSC) in accordance with the provisions of Japanese Industrial Standard (JIS) K 7121, is 80 to 180°C. Specific examples of thermoplastic resins include polyolefins and thermoplastic polyurethanes. Examples of polyolefins include polyethylene such as low-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene; polypropylene; and others. The thermoplastic resin may be used alone or in combination of two or more of the examples above.

[0065] The thickness of the porous resin membrane is preferably 8 μm or more, and more preferably 10 μm or more, from the viewpoint of ensuring good battery shutdown characteristics. Furthermore, from the viewpoint of reducing the total thickness of the separator and further improving the battery capacity and load characteristics, the thickness of the porous resin membrane is preferably 40 μm or less, and more preferably 30 μm or less.

[0066] Furthermore, it is preferable that the porous resin membrane has a pore diameter of 3 μm or less. If the porous resin membrane has such a small pore diameter as described above, even if small pieces detach from the positive or negative electrode in a battery using the separator, the occurrence of short circuits caused by this can be effectively suppressed. The multilayer porous membrane constituting the separator may have a two-layer structure with one layer each of the porous resin membrane and the coating layer, but it can also have a three-layer structure, for example, with coating layers on both sides of the porous resin membrane.

[0067] The total thickness of the separator is preferably 5 μm or more, and more preferably 10 μm or more, particularly from the viewpoint of ensuring a good battery shutdown function. Furthermore, the total thickness of the separator is preferably 30 μm or less, and more preferably 20 μm or less, from the viewpoint of further improving the battery capacity and load characteristics.

[0068] In the case of a separator consisting of a multilayer porous membrane having a resin porous membrane mainly composed of a thermoplastic resin and a coating layer mainly composed of an inorganic filler, as in the present invention, the air permeability of the multilayer porous membrane is often greater than that of the resin porous membrane alone due to the formation of the coating layer. In other words, the coating layer tends to be a factor that hinders the ion permeability of the separator as a whole, but in the present invention, the function can be maintained even if the coating layer is thin, so a separator with suitable ion permeability can be constructed.

[0069] The separator of the present invention preferably has a thermal shrinkage rate of 10% or less when left standing for 1 hour in an atmosphere of 150°C. In a battery using a separator with such a thermal shrinkage rate, short circuits due to the shrinkage of the separator at high temperatures can be effectively suppressed. By adopting the configuration described above, a separator having the above thermal shrinkage rate can be obtained. The application of the coating liquid to the surface of the porous resin film can be carried out, for example, by applying the coating liquid to the surface of the porous resin film using a known coating apparatus, or by impregnating the porous resin film in the coating liquid.

[0070] The secondary battery of the present invention is not particularly limited as long as it comprises a positive electrode, a negative electrode, the separator of the present invention, and a non-aqueous electrolyte, and the configurations and structures used in conventionally known non-aqueous electrolyte batteries can be applied.

[0071] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0072] The samples and measuring instruments used in the examples and comparative examples are as follows: Inorganic filler: Alumina (Resonac Co., Ltd.) AL-160SG-4; Dispersant: Ion-exchanged water; Dispersant: Ammonium polyacrylate (Toagosei Co., Ltd.) Aron A-30SL

[0073] Water-soluble polymers (1) Poly-N-vinylcarboxylic acid amide (PNVA®) Manufactured by Resonaq Corporation GE-191-103, solids content 10% by mass, viscosity 14,000 to 20,000 mPa·s Mw 80 to 1,100,000 Softening point temperature 166°C Glass transition temperature 179°C Manufactured by Resonaq Corporation GE-191-053, solids content 5% by mass, viscosity 8,000 to 15,000 mPa·s Mw 1,500 to 2,500,000 Softening point temperature 165°C Glass transition temperature 178°C (2) Carboxymethylcellulose (CMC) Daicel Corporation CMC Daicel 2200 Softening point temperature 48°C No glass transition temperature Note that CMC Daicel is sodium carboxymethylcellulose, and 2200 is the product number.

[0074] Binder Acrylic Emulsion BYK Corporation C25002 Wetting Agent (1) BYK Corporation Dynwet800 Alcohol Alkoxylate (2) Polyvinyl Alcohol PVA Kurarepoval Co., Ltd. PVA124, PVA224, PVA105

[0075] Separators (1) Dry separator Ube Industries, Ltd. UP3093 (2) Wet separator Kahoku Kinriki Co., Ltd. GELLEC SU09 Note that the dimensions of the separators used were smaller than the A4 size of the backing sheet described later.

[0076] Other (evaluation sheet): Polypropylene sheet clear folder A4 (Nakabayashi Co., Ltd.). Note: The clear folder will be divided into two sections for use. The A4 size is 210 x 297 mm, 0.2 mm thick. Protective tape: Monotaro CURING TAPE Polyethylene cloth.

[0077] Dryer: AS ONE Corporation ON-600S Test Oven: AS ONE Corporation ONW-300 Constant Speed ​​Coating Machine: Tester Sangyo Co., Ltd. Thickness Gauge: Nikon DIGIMICRO STAND MS-11C Digital Caliper: Mitutoyo DIGIMATIC

[0078] The proportions of each ingredient are listed in Table 1.

[0079] [Example 1] (Preparation of coating solution) 32 g of deionized water was placed in a 150 ml cylindrical polypropylene container with a lid. Then, the following ingredients were added in the order shown in Table 1: dispersant (ammonium polyacrylate), wetting agent (Dynwet 800), binder (acrylic emulsion), poly-N-vinyl carboxylic acid amide (poly-N-vinylacetamide (PNVA GE191-053)), and alumina. The container was covered with a lid and mixed in a rotary-orbiting kneader (Sinky ARE-250) under the conditions of mixing for 1 minute and defoaming for 1 minute. The viscosity measured at this time was taken as the initial viscosity.

[0080] Subsequently, a stirring blade, consisting of four flat paddle blades on the upper part of the shaft and disc turbine blades on the lower part, was fixed 2 mm below the bottom of the container. The stirring motor was started at 200 rpm, and the rotation speed was gradually increased to 730 rpm over 60 seconds, and stirring was continued for 10 hours (a high shear treatment process). Afterwards, the mixture was mixed in a rotation-orbit type kneader (Sinky ARE-250) under the conditions of mixing for 1 minute and defoaming for 1 minute. The viscosity measured at this time was defined as the final viscosity.

[0081] (Separator Fabrication) Separators were fabricated by applying the prepared coating solution to a dry separator using a constant-speed coating machine as follows. The dry separator was placed on a polypropylene sheet, which served as a base, and then placed on the glass plate of the constant-speed coating machine. One side of the dry separator was secured to the polypropylene sheet and the glass plate on the surface of the constant-speed coating machine with masking tape. The coating solution was then applied to one side of the separator at a speed of 10 mm / second using a 6 μm thick bar coater. After that, the separator and the polypropylene sheet were removed from the coating machine and dried in a 40°C oven for 30 minutes to obtain a coated separator. The thickness of the coating layer after drying was 3.5 μm.

[0082] [Example 2] A coating solution was prepared and a coating-type separator was manufactured in the same manner as in Example 1, except that the stirring time in the high-shear treatment process was changed to 5 hours.

[0083] [Example 3] A coating solution was prepared in the same manner as in Example 2, except that Dynwet 800 and PVA 124 were added in that order as wetting agents, and the amount of deionized water and the poly-N-vinyl carboxylic acid amide were changed to poly-N-vinylacetamide (PNVA GE191-103).

[0084] (Separator Fabrication) Separators were fabricated by applying the prepared coating solution to a wet separator using a constant-speed coating machine as follows: The wet separator was placed on a polypropylene sheet, which served as a base, and then placed on the glass plate of the constant-speed coating machine. One side of the wet separator was secured to the polypropylene sheet and the glass plate on the surface of the constant-speed coating machine with masking tape. The obtained coating solution was then applied to one side of the wet separator at a speed of 10 mm / second using a bar coater with a thickness of 5 μm. After that, the polypropylene sheet and the wet separator were removed from the coating machine and dried in a 40°C oven for 30 minutes. A separator coated on one side was removed from a polypropylene sheet, the wet separator was turned over, and the side without the coating liquid was placed on a polypropylene sheet. The separator was then fixed to the polypropylene sheet and glass plate with masking tape as described above, and the coating liquid was applied and dried to obtain a coated separator with a coating layer on both sides. The thickness of the coating layer after drying was 2.0 μm in total for both sides.

[0085] [Example 4] A coating solution was prepared in the same manner as in Example 3, except that PVA224 was used instead of PVA124, and a coated separator was manufactured.

[0086] [Example 5] A coating solution was prepared in the same manner as in Example 3, except that PVA105 was used instead of PVA124, and a coated separator was manufactured.

[0087] [Example 6] (Preparation of coating solution) Ion-exchanged water was placed in a 150 ml cylindrical polypropylene container with a lid, then a dispersant and alumina were added, the container was covered with a lid, and the mixture was mixed for 1 minute and defoamed for 1 minute in a rotary-type kneader. A wetting agent, acrylic emulsion, and poly-N-vinylacetamide were added, and the mixture was mixed again for 1 minute and defoamed for 1 minute in the rotary-type kneader.

[0088] A stirring blade, consisting of four flat paddle blades on the upper part of the shaft and disc turbine blades on the lower part, was fixed at a point 2 mm below the bottom of the container. The stirring motor was started at a rotation speed of 200 rpm and gradually increased to 730 rpm over 60 seconds. Stirring was continued for 10 hours, and then a mixing and defoaming process was carried out in a rotary-orbiting kneader for 1 minute to prepare the coating liquid. Using the obtained coating liquid, a coating-type separator was prepared in the same manner as in Example 1.

[0089] [Comparative Example 1] (Preparation of coating solution) Ion-exchanged water was placed in a 150 ml cylindrical polypropylene container with a lid, and then a dispersant, wetting agent, acrylic emulsion, PNVA GE191-053, and alumina were added in that order. The container was covered with a lid, and the mixture was mixed for 1 minute and degassed for 1 minute in a rotary-orbiting kneader, and the viscosity was measured. A stirring blade, consisting of four flat paddle blades on the upper part of the shaft and a disk turbine blade on the lower part, was fixed at a point 2 mm from the bottom of the container. The stirring motor was started at a rotation speed of 200 rpm and the rotation speed was gradually increased to 730 rpm over 60 seconds. After continuing stirring for 1 hour, the mixture was mixed for 1 minute and degassed for 1 minute in a rotary-orbiting kneader to prepare the coating solution. Using the obtained coating solution, a coated separator was manufactured in the same manner as in Example 1.

[0090] [Comparative Example 2] (Preparation of coating solution) Ion-exchanged water was placed in a 150 ml cylindrical polypropylene container with a lid, and then a dispersant, wetting agent, acrylic emulsion, PNVA GE191-053, and alumina were added in that order.

[0091] The container was covered, and the mixture was mixed for 1 minute and degassed for 1 minute in a rotary-type kneader, after which the viscosity was measured. Two flat paddle blades were attached to the shaft and fixed 2 mm from the bottom of the container. The stirring motor was started at 200 rpm and gradually increased to 730 rpm over 60 seconds. After continuing stirring for 10 hours, the mixture was mixed for 1 minute and degassed for 1 minute in a rotary-type kneader. Using the obtained coating liquid, a coated separator was prepared in the same manner as in Example 1.

[0092] [Comparative Example 3] (Preparation of coating solution) Ion-exchanged water was placed in a 150 ml cylindrical polypropylene container with a lid, and then a dispersant, wetting agent, non-crosslinked emulsion, PNVA GE191-053, and alumina were added in that order.

[0093] The container was covered, and the mixture was mixed for 1 minute and degassed for 1 minute in a rotary-type kneader, after which the viscosity was measured. This procedure was then repeated six times. Using the obtained coating liquid, a coated separator was prepared in the same manner as in Example 1.

[0094] [Comparative Example 4] (Preparation of coating solution) Ion-exchanged water was placed in a 150 ml cylindrical polypropylene container with a lid, and then a dispersant, wetting agent, acrylic emulsion, PNVA GE191-053, and alumina were added in that order. The container was covered with a lid and mixed for 1 minute and defoamed for 1 minute in a rotary-orbiting kneader, and the viscosity was measured.

[0095] A stirring blade, consisting of four flat paddle blades at the top of the shaft and disc turbine blades at the bottom, was fixed 2 mm from the bottom of the container. The rotation speed of the stirring motor was increased to 50 rpm. After stirring continued for 10 hours, a mixing process of 1 minute and defoaming process of 1 minute was carried out in a rotation-orbit type kneader. Using the obtained coating liquid, a coated separator was manufactured in the same manner as in Example 1.

[0096] [Comparative Example 5] (Preparation of coating solution) Ion-exchanged water was placed in a 150 ml cylindrical polypropylene container with a lid, and then a dispersant, wetting agent, acrylic emulsion, sodium carboxymethylcellulose, and alumina were added in that order.

[0097] The container was covered with a lid, and the mixture was mixed for 1 minute and degassed for 1 minute in a rotary-type kneader. A stirring blade, consisting of four flat paddle blades on the upper part of the shaft and a disc turbine blade on the lower part, was fixed 2 mm from the bottom of the container. The stirring motor was started at a rotation speed of 200 rpm and gradually increased to 730 rpm over 60 seconds. After continuing stirring for 10 hours, the mixture was mixed for 1 minute and degassed for 1 minute in the rotary-type kneader. Using the obtained coating liquid, a coated separator was manufactured in the same manner as in Example 3.

[0098]

[0099] Coat-type separators were prepared using the coating solutions obtained in the examples and comparative examples under the following coating and drying conditions. The poly-N-vinylcarboxylic acid amide, coating solution, and coating film in these examples and comparative examples were evaluated using the following test methods.

[0100] <Softening Point Measurement> TMA Penetration Method Test conditions in accordance with JIS K 7196-2012: Temperature increase 5°C / min, Load 0.5N, Equipment name: TMA8310 (manufactured by Rigaku Corporation), Specimen preparation method: 70 g of an aqueous solution of poly-N-vinylacetamide diluted with ion-exchanged water to a concentration of 2% by mass was placed in a polyethylene container measuring 10 cm × 15 cm × 1.5 cm, and left to stand in a 40°C oven for 48 hours. After that, it was dried in a vacuum dryer with the pressure reduced to 10 kPa (absolute pressure) at a temperature of 50°C for 24 hours. The resulting plate was used as the test specimen.

[0101] <Glass Transition Temperature Measurement> Measurement was performed using differential heat flux operation calorimetry as specified in JIS K 7121-1987. Test conditions: Cooling / increasing temperature 10°C / min. Equipment name: DSC7000. Hitachi High-Tech. Specimen preparation method: Specimens were prepared using the same method as for softening point measurement.

[0102] <Weight-Average Molecular Weight> The weight-average molecular weight of poly-N-vinyl carboxylic acid amide was measured by adjusting the poly-N-vinylacetamide concentration to 0.05% by mass in a 0.1 mol / L phosphate buffer solution (0.1 mol / L sodium hydrogen phosphate + 0.1 mol / L disodium hydrogen phosphate) and allowing it to stand for 20 hours. This solution was filtered through a 0.45 μm pore size membrane filter, and the weight-average molecular weight was measured using GPC-MALS with the filtrate. Analytical equipment and measurement conditions GPC: SHODEX® SYSTEM-21, manufactured by Resonaq Corporation Column: SHODEX® SB807, SB807, and SB807 connected in series, manufactured by Resonaq Corporation Column temperature: 40°C Eluent: 0.1 mol / L NaH2PO4 + 0.1 mol / L Na2HPO4 (pH = 6.8) Flow rate: 0.7 mL / min Sample injection volume: 100 μL MALS (Multi-angle Light Scattering) detector: Wyatt Technology, DAWN DSP Laser wavelength: 633 nm Multi-angle fitting method: Berry method Analysis program: ASTRA 7.2.2

[0103] <Viscosity> Place the coating solution in a 100 ml beaker and leave it in a 20°C constant temperature bath for 12 hours or more to ensure that there are no air bubbles inside. Then, place the beaker in a constant temperature water bath heated to 20°C and confirm with a thermometer that the test sample temperature is 20 ± 0.5°C. Measure the viscosity using a Type B viscometer as shown in JIS K-7117-1-1999 under the following conditions. Place the sample in the viscometer at the following rotation speeds and record the viscosity after 10 minutes. Rotation speed: 10 rpm, 100 rpm Temperature: 20°C Viscometer: Brookfield DVE-LV type spindle: No. 63 spindle The TI value (thixotropy index) is calculated using the following formula: TI value = viscosity at rotation speed 10 rpm (mPa·s) ÷ viscosity at rotation speed 100 rpm (mPa·s) The TI value before high shear treatment, i.e., the initial TI value ｉｎｉ TI, TI value after high shear treatment ｅｎｄ Let TI be ΔTI = ｅｎｄ TI / ｉｎｉ It was requested as TI.

[0104] <Dispersion Measurement> Place the coating solution in a 100 ml beaker and leave it in a 20°C constant temperature bath for 12 hours or more until all air bubbles are completely removed. Remove the coating solution from the constant temperature bath, place 5 g of deionized water and a small stirrer tip into a quartz cell (0.5 cm deep) for particle size distribution measurement, and add 0.5 ml of the coating solution using a Pasteur pipette. Set the quartz cell in the particle size distribution analyzer and measure the median diameter (the particle size at the point where the cumulative volume reaches 50%). Laser Particle Size Distribution Analyzer HORIBA LA-350 Manufactured by HORIBA, Ltd. Detector: Silicon photodetector batch cell unit Liquid temperature 25°C Sample volume Coating solution 0.5 ml Dispersion medium (deionized water) 5 ml

[0105] <Thickness of the separator> The thickness of the cut separator test piece was measured using a thickness gauge. Measurements were taken at two points each on the top, middle, and bottom, for a total of six points, and the average of these measurements was taken as the thickness of the test piece.

[0106] <Heat Shrinkage Test> Using a mold measuring 2 cm wide by 3 cm long, cut the coated separator so that the 3 cm side aligns with the MD (direction of coating) and the 2 cm side aligns with the TD (direction perpendicular to the direction of coating). Place a piece of copy paper of the same size on a glass plate measuring 10 cm long x 20 cm wide x 0.18 cm thick, and place three cut coated separators on top of the copy paper at equal intervals.

[0107] Next, a cover glass measuring 76 mm (length) x 52 mm (width) x 1.4 mm (thickness) is placed on top of the coated separator. The length of each part of the test piece is measured and recorded using a digital caliper on top of the cover glass, and the sample is placed in a stable oven that has reached 150°C, with the bottom glass plate facing down. After 60 minutes, it is removed and placed on a wooden workbench to cool.

[0108] When the coated separator reached 25°C, the length of each side of the test specimen was measured using the same procedure on the cover glass. The difference between the length of each side of the test specimen before heating and the length of each side of the test specimen after heating was determined, and the rate of change of each side relative to the length before heating was calculated using the following formula. The average of the MD2 points and TD2 points was taken as the MD and TD shrinkage rate of the test specimen. The average of the MD and TD was taken as the thermal shrinkage rate.

[0109] The results are shown in Tables 2 and 3.

[0110]

[0111]

[0112] Examples 1 and 2 had appropriate stirring conditions, resulting in increased ΔTI and an average thermal shrinkage rate of 10% or less. In contrast, Comparative Examples 1 to 4 did not have sufficient high-shear treatment, resulting in a low increase in TI and an average thermal shrinkage rate exceeding 35%.

[0113] Furthermore, in Examples 3 and 4, where wet separators were coated on both sides, sufficient high shear treatment was performed, resulting in an average thermal shrinkage rate of 4% or less.

[0114] Furthermore, in Examples 5 and 6, the thermal shrinkage rate is lower compared to the comparative example.

[0115] In Comparative Example 5, since carboxymethylcellulose with a low softening point is used, the thermal shrinkage rate remains high even after undergoing a high-shear treatment process.

[0116] In both the example and comparative example, the degree of alumina dispersion was almost the same in terms of median diameter, indicating that it was dispersed without any problems.

[0117] As described above, the present invention makes it possible to obtain a coated separator with low thermal shrinkage and high heat resistance. By utilizing the present invention, it becomes possible to manufacture secondary batteries more easily and safely, which is considered to have significant industrial advantages.

Claims

1. A method for producing a coating solution containing an inorganic filler and a poly-N-vinylcarboxylic acid amide, characterized by comprising a step of performing a high-shear treatment using a stirring means.

2. A method for producing a coating liquid according to claim 1, wherein the process of performing high shear treatment is carried out by stirring and mixing under the condition that the high shear coefficient expressed by the following formula is 1.0 or greater. High shear coefficient = Stirring blade tip speed (m / s) × Stirring time (hours) × Number of stirring blades (blades) × Volume ratio of stirring blades to the coating liquid (%) ÷ 100 3. A method for producing a coating liquid according to claim 1 or claim 2, wherein the TI value of the coating liquid after high shear treatment / initial TI value is 1.05 or more and 1.25 or less.

4. The method for producing a coating liquid according to claim 1 or claim 2, wherein the inorganic filler is alumina.

5. The method for producing a coating solution according to claim 1 or claim 2, wherein the poly-N-vinyl carboxylic acid amide is poly-N-vinylacetamide.

6. A method for producing a coating liquid according to claim 1 or 2, comprising an initial mixing step preceding the high shear treatment step, wherein the initial mixing step involves stirring a mixture containing an inorganic filler, a solvent, and a poly-N-vinylcarboxylic acid amide.

7. A method for producing a coating solution according to claim 1 or claim 2, comprising placing an inorganic filler, a dispersion medium, a poly-N-vinylcarboxylic acid amide, a binder, a dispersant, and a wetting agent into a mixer and stirring them together.

8. The method for producing a coating solution according to claim 1 or claim 2, wherein the softening point temperature of the poly-N-vinylcarboxylic acid amide is 100°C or higher.

9. The method for producing a coating solution according to claim 1 or claim 2, wherein the glass transition temperature of the poly-N-vinylcarboxylic acid amide is 120°C or higher.

10. A method for producing a coating liquid according to claim 1 or claim 2, wherein the coating liquid contains polyvinyl alcohol as a wetting agent, and its content is 0.05 parts by mass to 1.0 part by mass per 100 parts by mass of inorganic filler.

11. The method for producing a coating solution according to claim 10, wherein the viscosity of the polyvinyl alcohol at 20°C and a 4% by mass concentration is 20 to 80 mPa·s.

12. A coating liquid manufactured by the manufacturing method described in claim 1 or claim 2.

13. A coated substrate comprising a coating film formed from a coating liquid manufactured by the manufacturing method described in claim 1 or claim 2, and a substrate.

14. A separator formed from a coated substrate according to claim 13.

15. A secondary battery using the separator described in claim 14.

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