Secondary battery separator and method for manufacturing secondary battery separator

Abstract

To provide a separator for a secondary battery that does not contain PFAS, has excellent heat press adhesion to electrodes and excellent potential resistance, and can be produced by a simple process.SOLUTION: A separator for a secondary battery includes a porous substrate and an adhesive layer provided on at least a portion of one or both surfaces of the porous substrate, and the adhesive layer contains a copolymer (A) containing, as essential constituent monomers, acrylic acid (a1) and an ethylenically unsaturated monomer (a2) having a homopolymer glass transition temperature of -15°C or lower, and the content of the acrylic acid (a1) based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A) is 25 to 70 wt.% and the content of the ethylenically unsaturated monomer (a2) is 30 to 75 wt.%, and the glass transition temperature of the copolymer (A) is 40 to 100°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator for a secondary battery and a method for manufacturing a separator for a secondary battery. [Background technology]

[0002] Separators, which are one of the components constituting secondary batteries, are required to have adhesive properties that prevent them from easily peeling off from electrodes even when they are subjected to external impacts or when the electrodes expand and contract during charging and discharging. A separator with enhanced adhesiveness to electrodes is known to have a porous substrate and a resin layer containing a resin that exhibits adhesiveness to electrodes (for example, Patent Document 1).

[0003] When manufacturing a battery, a laminate in which a separator is placed between a positive electrode and a negative electrode may be subjected to dry heat pressing (a heat pressing process performed without impregnating the separator with an electrolyte). If the separator and electrode are well bonded by dry heat pressing, the separator and electrode are less likely to become misaligned during the battery manufacturing process, which can improve the battery manufacturing yield. Separators are also required to have high potential resistance and not experience a decrease in battery performance (such as a decrease in capacity) even after repeated charge and discharge. Therefore, in recent years, there has been a demand for the development of a separator that can be well bonded to electrodes by dry heat pressing and has high potential resistance. For example, Patent Document 2 discloses a separator for secondary batteries that has an adhesive porous layer containing a polyvinylidene fluoride resin, an acrylic resin, and metal sulfate particles on a porous substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4127989 [Patent Document 2] International Publication No. 2020 / 189119 Summary of the Invention [Problem to be solved by the invention]

[0005] The secondary battery separator with an adhesive porous layer described in Patent Document 2 has good adhesion to electrodes when heat-pressed and good potential resistance (capacity retention rate after 500 charge / discharge cycles), but is undesirable in that it requires the use of polyvinylidene fluoride (PVDF) resin, a type of organic fluorine compound PFAS (an abbreviation for perfluoroalkyl compounds, polyfluoroalkyl compounds, and their salts), which is subject to stricter environmental regulations. Another issue is that the use of two types of resin in combination makes the manufacturing process complicated.

[0006] The present invention solves the above-mentioned problems and aims to provide a secondary battery separator comprising a porous substrate and an adhesive layer provided on at least a portion of one or both surfaces of the porous substrate, which separator has excellent heat press adhesion to electrodes and excellent potential resistance, does not contain PFAS, and can be produced by a simple process. [Means for solving the problem]

[0007] The present inventors have made extensive studies and arrived at the present invention. The present invention relates to a separator for a secondary battery, comprising a porous substrate and an adhesive layer provided on at least a portion of one or both surfaces of the porous substrate, wherein the adhesive layer contains a copolymer (A) containing, as essential constituent monomers, acrylic acid (a1) and an ethylenically unsaturated monomer (a2) having a homopolymer glass transition temperature of −15°C or lower, wherein the content of the acrylic acid (a1) based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A) is 25 to 70 wt % and the content of the ethylenically unsaturated monomer (a2) is 30 to 75 wt %, and the glass transition temperature of the copolymer (A) is 40 to 100°C; and a method for producing the separator for a secondary battery, comprising the step of adhering a resin solution of the copolymer (A) having a solids content of 2 to 40 wt % to at least a portion of one or both surfaces of the porous substrate. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a secondary battery separator that includes a porous substrate and an adhesive layer provided on one or both sides of the porous substrate, and that has excellent heat press adhesion to electrodes and excellent potential resistance, does not contain PFAS, and can be produced by a simple process. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described in detail below. The present invention relates to a separator for a secondary battery and a method for manufacturing the separator for a secondary battery. The separator for a secondary battery of the present invention may be a separator for a lithium ion battery or a separator for a sodium ion battery. In this specification, the terms lithium ion battery and sodium ion battery are used to refer to lithium ion secondary batteries and sodium ion secondary batteries, respectively.

[0010] <Separator for secondary batteries> The secondary battery separator of the present invention comprises a porous substrate and an adhesive layer provided on at least a portion of one or both surfaces of the porous substrate.

[0011] In the present invention, the term "porous substrate" refers to a substrate having pores or gaps therein. Examples of such substrates include microporous membranes and porous sheets made of fibrous materials (nonwoven fabrics, paper, etc.). In the present invention, microporous membranes are preferred from the viewpoint of thinning and strength of the separator. The term "microporous membrane" refers to a membrane having a large number of micropores therein, which are connected to each other, allowing gas or liquid to pass through from one surface to the other. The pore size of the porous substrate is not particularly limited and may include micropores, mesopores, or macropores. In some preferred embodiments, the pores are micropores, and may have an average pore size of 0.1 to 1.0 μm.

[0012] The material of the porous substrate is preferably an electrically insulating material, and preferably contains a thermoplastic resin. Examples of the thermoplastic resin include polyester (polyethylene terephthalate, etc.), polyolefin (polyethylene, polypropylene, etc.), and the like, and it is preferable that the resin contains polyolefin. In some preferred embodiments of the present invention, the porous substrate is a microporous polyolefin membrane.

[0013] The thickness of the porous substrate is not particularly limited, but is preferably 5 to 100 μm from the viewpoint of the strength of the secondary battery separator of the present invention and the performance of the resulting battery.

[0014] Examples of the polyolefin microporous membrane that can be suitably used for the secondary battery separator of the present invention include commercially available lithium ion battery separators (Hipore manufactured by Asahi Kasei Corporation, Celgard (registered trademark) manufactured by Asahi Kasei Corporation, and Upore (registered trademark) manufactured by Ube Industries, Ltd.).

[0015] The adhesive layer comprises a copolymer (A) containing, as essential constituent monomers, acrylic acid (a1) and an ethylenically unsaturated monomer (a2) having a homopolymer glass transition temperature of −15° C. or lower, and the content of the acrylic acid (a1) is 25 to 70% by weight and the content of the ethylenically unsaturated monomer (a2) is 30 to 75% by weight based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A).

[0016] The glass transition temperature (Tg) of the homopolymer of the ethylenically unsaturated monomer (a2) is −15° C. or lower. If it exceeds −15° C., it becomes difficult to adjust the Tg of the copolymer (A) described below to 40 to 100° C., and heat press adhesiveness deteriorates. From the viewpoint of achieving both heat press adhesiveness and resin strength, the Tg of (a2) is preferably −100° C. or higher and −15° C. or lower, more preferably −75° C. or higher and −15° C. or lower, and particularly preferably −75° C. or higher and −20° C. or lower.

[0017] In the present invention, the glass transition temperature (Tg) of a homopolymer of a monomer is determined based on a value listed in a publicly available document. Specifically, the values ​​are listed in "Polymer Handbook" (4th edition, John Wiley & Sons, Inc., 1999). For a monomer for which multiple values ​​are listed in the Polymer Handbook, the highest value is used.

[0018] For monomers for which the Tg of the homopolymer is not listed in the above-mentioned document "Polymer Handbook," the midpoint glass transition temperature measured by differential scanning calorimetry (DSC) in accordance with JIS K7121 is used as a sample of a cured product obtained by polymerizing the monomer to an extent that the Tg becomes constant, and the Tg of the homopolymer is taken as the midpoint glass transition temperature. As a DSC measuring device, DSC20, SSC / 580 manufactured by Seiko Instruments Inc., or the like can be used.

[0019] The ethylenically unsaturated monomer (a2) having a homopolymer glass transition temperature of −15° C. or lower includes a monofunctional ethylenically unsaturated monomer (a21) having one ethylenically unsaturated group and a polyfunctional ethylenically unsaturated monomer (a22) having two or more ethylenically unsaturated groups, and the monofunctional ethylenically unsaturated monomer (a21) having two ethylenically unsaturated groups is preferred. The ethylenically unsaturated group is preferably at least one group selected from the group consisting of an acrylate group, a methacrylate group, a vinyloxy group, a propenyloxy group, and an allyloxy group, more preferably an acrylate group or a methacrylate group, and even more preferably an acrylate group.

[0020] Examples of the monofunctional ethylenically unsaturated monomer (a21) having a glass transition temperature of a homopolymer of -15°C or lower include alkyl acrylates {ethyl acrylate (Tg: -20°C), n-butyl acrylate (Tg: -55°C), sec-butyl acrylate (Tg: -26°C), isobutyl acrylate (Tg: -24°C), isoamyl acrylate (Tg: -45°C), hexyl acrylate (Tg: -57°C), 2-ethylhexyl acrylate (Tg: -70°C), isooctyl acrylate (Tg: -58°C), isodecyl acrylate (Tg: -60°C), and isomistyryl acrylate (Tg: -56°C), etc.} Examples of the acrylate include alkyl methacrylates {octyl methacrylate (Tg: -20°C), isooctyl methacrylate (Tg: -45°C), lauryl methacrylate (Tg: -65°C), tetradecyl methacrylate (Tg: -72°C), and octadecyl methacrylate (Tg: -100°C)}, hydroxyalkyl acrylates {hydroxyethyl acrylate (Tg: -15°C), 4-hydroxybutyl acrylate (Tg: -40°C), and the like}, 2-methoxyethyl acrylate (Tg: -50°C), ethyl carbitol acrylate (Tg: -70°C), and phenoxyethyl acrylate (Tg: -22°C), etc.

[0021] (a2) may be used alone or in combination of two or more. Among these (a2), alkyl acrylates and hydroxyalkyl acrylates are preferred from the viewpoint of resin strength and heat press adhesiveness, and (a2) is particularly preferably an alkyl acrylate or a combination of an alkyl acrylate and a hydroxyalkyl acrylate. Particularly preferred examples of the alkyl acrylate include 2-ethylhexyl acrylate and n-butyl acrylate. Particularly preferred examples of the hydroxyalkyl acrylate include hydroxyethyl acrylate and hydroxybutyl acrylate.

[0022] The content of acrylic acid (a1) based on the total weight of acrylic acid (a1) and ethylenically unsaturated monomer (a2) contained in the copolymer (A) is 25 to 70% by weight, and the content of ethylenically unsaturated monomer (a2) is 30 to 75% by weight. If the contents of (a1) and (a2) are outside the above range, it becomes difficult to adjust the Tg of copolymer (A) to 40 to 100°C, and it becomes difficult to achieve both heat press adhesiveness and workability (handling) at an appropriate temperature. The appropriate temperature means a temperature at which the pores of the porous substrate do not collapse or deform, and is generally 120°C or lower. It is also preferred that the content of acrylic acid (a1) is 40 to 70% by weight and the content of ethylenically unsaturated monomer (a2) is 30 to 60% by weight, based on the total weight of acrylic acid (a1) and ethylenically unsaturated monomer (a2) contained in copolymer (A).

[0023] The copolymer (A) may contain, in addition to the acrylic acid (a1) and the ethylenically unsaturated monomer (a2), another copolymerizable monomer (a3) ​​as a constituent monomer. Examples of other copolymerizable monomers (a3) ​​include copolymerizable vinyl monomers containing no active hydrogen, etc. Examples of the copolymerizable vinyl monomers containing no active hydrogen include those of the following (a31) to (a35) whose homopolymers have a glass transition temperature of above -15°C.

[0024] (a31) Carbyl (meth)acrylate formed from a monool having 1 to 20 carbon atoms and (meth)acrylic acid Examples of the monools include (i) aliphatic monools [methanol, ethanol, n- or i-propyl alcohol, n-butyl alcohol, n-pentyl alcohol, n-octyl alcohol, nonyl alcohol, decyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, cetyl alcohol, and stearyl alcohol], (ii) alicyclic monools [cyclohexyl alcohol, and the like], and (iii) aromatic aliphatic monools [benzyl alcohol, and the like], and mixtures of two or more of these. In the present invention, (meth)acrylic acid means acrylic acid and / or methacrylic acid, and (meth)acrylate means acrylate and / or methacrylate.

[0025] (a32) Poly(n=2-30) oxyalkylene (carbon number 2-4) alkyl (carbon number 1-18) ether (meth)acrylate [(meth)acrylate) adduct of 10 moles of ethylene oxide (hereinafter abbreviated as EO) with methanol, (meth)acrylate) adduct of 10 moles of propylene oxide (hereinafter abbreviated as PO) with methanol, etc.]

[0026] (a33) Nitrogen-containing vinyl compounds (a33-1) Amide group-containing vinyl compound (i) (meth)acrylamide compounds having 3 to 30 carbon atoms, such as N,N-dialkyl (having 1 to 6 carbon atoms) or diaralkyl (having 7 to 15 carbon atoms) (meth)acrylamides [N,N-dimethylacrylamide, N,N-dibenzylacrylamide, etc.], and diacetone acrylamide (ii) Amide group-containing vinyl compounds having 4 to 20 carbon atoms, excluding the above (meth)acrylamide compounds, such as N-methyl-N-vinylacetamide and cyclic amides (pyrrolidone compounds (having 6 to 13 carbon atoms, for example, N-vinylpyrrolidone, etc.))

[0027] (a33-2) (Meth)acrylate compounds (i) Dialkyl (C1-4) aminoalkyl (C1-4) (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, etc.] (ii) Quaternary ammonium group-containing (meth)acrylates [quaternized products of tertiary amino group-containing (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, etc.] (those quaternized using the above-mentioned quaternizing agents), etc.]

[0028] (a33-3) Heterocycle-containing vinyl compounds Pyridine compounds (having 7 to 14 carbon atoms, for example, 2- or 4-vinylpyridine), imidazole compounds (having 5 to 12 carbon atoms, for example, N-vinylimidazole), pyrrole compounds (having 6 to 13 carbon atoms, for example, N-vinylpyrrole), pyrrolidone compounds (having 6 to 13 carbon atoms, for example, N-vinyl-2-pyrrolidone)

[0029] (a33-4) Nitrile group-containing vinyl compounds Nitrile group-containing vinyl compounds having 3 to 15 carbon atoms, such as (meth)acrylonitrile, cyanostyrene, and cyanoalkyl (carbon number 1 to 4) acrylates

[0030] (a33-5) Other vinyl compounds Nitro group-containing vinyl compounds (carbon numbers 8 to 16, e.g., nitrostyrene), etc.

[0031] (a34) Vinyl hydrocarbons (a34-1) Aliphatic vinyl hydrocarbons Olefins with 2 to 18 or more carbon atoms [ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.], dienes with 4 to 10 or more carbon atoms [butadiene, isoprene, 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene, etc.], etc.

[0032] (a34-2) Alicyclic vinyl hydrocarbons Cyclic unsaturated compounds having 4 to 18 carbon atoms or more, such as cycloalkenes (e.g., cyclohexene), (di)cycloalkadienes [e.g., (di)cyclopentadiene], and terpenes (e.g., pinene, limonene, and indene).

[0033] (a34-3) Aromatic vinyl hydrocarbons Aromatic unsaturated compounds having 8 to 20 carbon atoms or more and their derivatives, such as styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, and lithium styrenesulfonate

[0034] (a35) Vinyl esters, vinyl ethers, vinyl ketones, unsaturated dicarboxylic acid diesters (a35-1) Vinyl ester Aliphatic vinyl esters [having 4 to 15 carbon atoms, for example, alkenyl esters of aliphatic carboxylic acids (mono- or dicarboxylic acids) (for example, vinyl acetate, vinyl propionate, vinyl butyrate, diallyl adipate, isopropenyl acetate, vinyl methoxyacetate)] Aromatic vinyl esters [having 9 to 20 carbon atoms, for example, alkenyl esters of aromatic carboxylic acids (mono- or dicarboxylic acids) (e.g., vinyl benzoate, diallyl phthalate, methyl-4-vinylbenzoate), aromatic ring-containing esters of aliphatic carboxylic acids (e.g., acetoxystyrene)]

[0035] (a35-2) Vinyl ether Aliphatic vinyl ethers (C3-15, for example, vinyl alkyl (C1-10) ethers [vinyl methyl ether, vinyl butyl ether, vinyl 2-ethylhexyl ether, etc.], vinyl alkoxy (C1-6) alkyl (C1-4) ethers [vinyl-2-methoxyethyl ether, methoxybutadiene, 3,4-dihydro-1,2-pyran, 2-butoxy-2'-vinyloxydiethyl ether, vinyl-2-ethylmercaptoethyl ether, etc.], poly(2-4)(meth)allyloxyalkanes (C2-6) [diallyloxyethane, triallyloxyethane, tetraallyloxybutane, tetramethallyloxyethane, etc.]) Aromatic vinyl ethers (8 to 20 carbon atoms, e.g., vinyl phenyl ether, phenoxystyrene)

[0036] (a35-3) Vinyl ketone Aliphatic vinyl ketones (carbon numbers 4 to 25, e.g., vinyl methyl ketone, vinyl ethyl ketone) Aromatic vinyl ketones (carbon numbers 9 to 21, e.g., vinyl phenyl ketone)

[0037] (a35-4) Unsaturated dicarboxylic acid diester Unsaturated dicarboxylic acid diesters having 4 to 34 carbon atoms, such as dialkyl fumarates (wherein the two alkyl groups are linear, branched, or alicyclic groups having 1 to 22 carbon atoms), and dialkyl maleates (wherein the two alkyl groups are linear, branched, or alicyclic groups having 1 to 22 carbon atoms).

[0038] Among the examples of (a3) ​​above, (a31), (a32), (a33), and (a34) are preferred in terms of electrolyte absorption and potential resistance, with (a31 being more preferred. Furthermore, from the viewpoint of adjusting the Tg of the copolymer (A), among (a31), those having a homopolymer glass transition temperature of more than -15°C and not more than 20°C are preferred.

[0039] In the copolymer (A), the contents of the acrylic acid (a1), the ethylenically unsaturated monomer (a2) and the other copolymerizable monomer (a3) ​​are preferably such that, based on the weight of the copolymer (A), (a1) is 25 to 70% by weight, (a2) is 30 to 75% by weight, and (a3) ​​is 0 to 30% by weight. When the content of the monomer is within the above range, it is easy to achieve both the liquid absorption property for the electrolyte, the potential resistance property, and the heat press adhesive property to the electrode.

[0040] The glass transition temperature (Tg) of the copolymer (A) is 40 to 100°C. If the Tg of the copolymer (A) is less than 40°C, stickiness occurs even at room temperature, resulting in poor workability (handling), and the storage stability of the separator is also poor. If the Tg exceeds 100°C, adhesion to the electrode decreases, more specifically, heat press adhesion at the appropriate temperature decreases. The appropriate temperature refers to a temperature at which the pores of the porous substrate are not crushed or deformed, and is generally 120°C or lower. From the viewpoint of achieving both workability and heat press adhesiveness, the Tg of the copolymer (A) is preferably 45 to 96°C, more preferably 45 to 93°C, and even more preferably 45 to 90°C. The Tg of the copolymer (A) can be adjusted by the type (Tg) and ratio of the monomers constituting the copolymer (A).

[0041] The glass transition temperature (Tg) of the copolymer (A) in the present invention is a value (temperature at the inflection point) measured by the method specified in ASTM D3418-82 (DSC method) using DSC20 and SSC / 580 manufactured by Seiko Instruments Inc. [Measurement conditions] (1) Heat from 30°C to 150°C at 20°C / min (2) Hold at 150°C for 10 minutes (3) Cool to -35°C at 20°C / min (4) Keep at -35°C for 10 minutes (5) Heat up to 150°C at 20°C / min (6) The differential scanning calorimetry curve measured in the step (5) is analyzed, and the position of the inflection point is determined as the glass transition temperature.

[0042] The copolymer (A) preferably has a liquid absorption rate of 40% or more when immersed in an electrolytic solution at 50° C. for 3 days. The liquid absorption rate when immersed in the electrolytic solution is calculated by measuring the weight of the copolymer (A) before and after immersion in the electrolytic solution and using the following formula. Absorption rate (%) = [(weight of copolymer (A) after immersion in electrolyte solution - weight of copolymer (A) before immersion in electrolyte solution) / weight of copolymer (A) before immersion in electrolyte solution] × 100

[0043] As the electrolyte for determining the liquid absorption rate, when the secondary battery is a lithium ion battery, it is preferable to use an electrolyte solution (hereinafter sometimes referred to as electrolyte solution 1) prepared by dissolving LiPF as an electrolyte in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 to give a concentration of 1 mol / L. When the secondary battery is a sodium ion battery, it is preferable to use an electrolyte solution (hereinafter sometimes referred to as electrolyte solution 2) prepared by dissolving NaPF6 as an electrolyte in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7 to a concentration of 1 mol / L.

[0044] The sample for the liquid absorption test is prepared as follows. The method for preparing a sample for the liquid absorption test is the same whether the secondary battery is a lithium ion battery or a sodium ion battery. A 30% resin solution of copolymer (A) (using N'-dimethylformamide (DMF) as the solvent) is poured into a PP tray (14 cm x 10 cm) to a height of approximately 3 mm, and dried in a 70°C forward air dryer for 3 hours, and then in a reduced pressure (>-90 kPa) dryer for 3 hours to prepare a resin sheet. This resin sheet is cut into 1.5 cm squares, and the thickness of the sample is measured with a handy film thickness meter (manufactured by Mitutoyo Corporation) to confirm that it is about 1 mm, and then the sample is used for testing.

[0045] When determining the liquid absorption rate, the resin sheet is immersed in the electrolyte at 50°C for 3 days. The resin sheet is placed in a screw tube (No. 7, capacity 50 mL) and immersed in the electrolyte in such a manner that the resin sheet is completely immersed in the electrolyte. Immersion at 50°C for 3 days causes the copolymer (A) to reach a saturated liquid absorption state. The saturated liquid absorption state refers to a state in which the weight of the copolymer (A) does not increase even if it is further immersed in the electrolyte. The method of immersion in the electrolyte when determining the absorption rate is the same as above, except that electrolyte 1 is used when the secondary battery is a lithium ion battery, and electrolyte 2 is used when the secondary battery is a sodium ion battery.

[0046] When the liquid absorption rate is 40% or more, the electrolyte solution easily penetrates into the separator, the ion conductivity is good, and the secondary battery is more likely to exhibit sufficient performance. From the viewpoint of ion conductivity and preventing the adhesive layer from swelling too much due to absorption of the electrolyte solution, causing peeling between the adhesive layer and the porous substrate and between the adhesive layer and the electrode, the above liquid absorption rate is more preferably 40 to 70%. The liquid absorption rate of the copolymer (A) when immersed in an electrolyte solution at 50°C for 3 days can be adjusted by the carboxyl group content (which can be adjusted mainly by the content of acrylic acid (a1)), SP value (solubility parameter), weight average molecular weight, etc. of the copolymer (A). Methods for increasing the liquid absorption rate of the copolymer (A) include reducing the acrylic acid (a1) content of the copolymer (A), increasing the SP value (reducing the difference in SP value from the electrolyte), and reducing the weight average molecular weight. Methods for reducing the liquid absorption rate of the copolymer (A) include increasing the acrylic acid (a1) content of the copolymer (A), reducing the SP value (increasing the difference in SP value from the electrolyte), and increasing the weight average molecular weight. The above-mentioned preferable range of the liquid absorption rate and the method for adjusting the liquid absorption rate are the same whether the secondary battery is a lithium ion battery (using electrolytic solution 1 as the electrolyte) or a sodium ion battery (using electrolytic solution 2 as the electrolyte).

[0047] In the present invention, the SP value (solubility parameter) [unit: (cal / cm 3 ) 1 / 2 ] is a value calculated by the method described in formula (28) on page 153 of the Fedors method (Polymer Engineering and Science, February 1974, Vol. 14, No. 2, pp. 147-154) using the values ​​(heat of vaporization and molar volume at 25°C of atoms or functional groups) described on page 152 (Table 5) of the same. The SP value of the copolymer (A) is preferably 10.0 to 15.0.

[0048] The weight average molecular weight of the copolymer (A) is preferably 50,000 to 300,000, and more preferably 80,000 to 200,000. The weight-average molecular weight of the copolymer (A) can be determined by GPC (gel permeation chromatography) measurement under the following conditions: A sample polymer is dissolved in a solvent such as orthodichlorobenzene, N'-dimethylformamide (DMF), or tetrahydrofuran (THF) to prepare a 0.25 wt% solution, and the insoluble matter is filtered through a PTFE filter with a pore size of 1 μm to obtain a sample solution. Apparatus: Alliance GPC V2000 (Waters) Solvent: orthodichlorobenzene, DMF, THF Standard: Polystyrene sample Concentration: 3mg / ml Column stationary phase: PLgel 10um, MIXED-B, two columns in series (Polymer Laboratories) Column temperature: 135℃

[0049] The copolymer (A) has a lithium ion conductivity of 1×10 when immersed in the electrolyte solution at 50° C. for 3 days. -9 ~1×10 -5 Preferably, it is S / cm. The lithium ion conductivity is determined by measuring the conductivity of the copolymer (A) at 25° C. when immersed in the following electrolyte solution at 50° C. for 3 days by an AC impedance method.

[0050] The electrolyte used to determine the lithium ion conductivity is a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7, to which LiPF6 is dissolved to a concentration of 1 mol / L (electrolyte solution 1).

[0051] In the present application, the lithium ion conductivity of the copolymer (A) is determined by measuring it by an AC impedance method using a modular charge-discharge measurement system (Toyo Technica Co., Ltd.). The preparation of the sample for measuring lithium ion conductivity and the immersion in the electrolyte are carried out in the same manner as in the preparation of the sample for the liquid absorption test. AC impedance measurement was performed at 25°C and a frequency of 2.0 x 10 4 Hz~1.0×10 -1 The real impedance component R (Ω) is calculated using a frequency of 1000 Hz, a voltage swing of 1000 mA, and the ionic conductivity σ (S / cm) of the resin sheet is calculated using the impedance component R (Ω), the thickness d (cm) of the resin sheet, and the contact area A (cm) between the electrode and the resin sheet. 2 ) is calculated. Ionic conductivity σ (S / cm) = d / (R × A)

[0052] The copolymer (A) had a lithium ion conductivity of 1×10 when immersed in the electrolyte solution at 50° C. for 3 days. -9 ~1×10 -5 S / cm is preferable because it provides lithium ion conductivity that allows the secondary battery (lithium ion battery) to fully exhibit its performance, and can prevent the copolymer (A) from swelling too much in the electrolyte solution, causing the adhesive surface to peel off. The lithium ion conductivity of the copolymer (A) when immersed in an electrolyte solution at 50°C for 3 days can be adjusted by the carboxyl group content (which can be adjusted mainly by the content of acrylic acid (a1)), SP value (solubility parameter), weight average molecular weight, etc. of the copolymer (A). Methods for increasing the lithium ion conductivity of the copolymer (A) include reducing the acrylic acid (a1) content of the copolymer (A), increasing the SP value (reducing the difference in SP value from the electrolyte), and decreasing the weight average molecular weight. Methods for decreasing the lithium ion conductivity of the copolymer (A) include increasing the acrylic acid (a1) content of the copolymer (A), decreasing the SP value (increasing the difference in SP value from the electrolyte), and increasing the weight average molecular weight.

[0053] The copolymer (A) has a sodium ion conductivity of 1×10 when immersed in the electrolyte at 50° C. for 3 days. -10 ~1×10 -6 Preferably, it is S / cm. The sodium ion conductivity is determined by measuring the conductivity of the binder resin at 25° C. by an AC impedance method after immersing the binder resin in the following electrolyte at 50° C. for 3 days.

[0054] The electrolyte used to determine sodium ion conductivity is a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7, to which NaPF6 is dissolved to a concentration of 1 mol / L (electrolyte solution 2).

[0055] In the present application, the sodium ion conductivity of the copolymer (A) is determined by measuring it by an AC impedance method using a modular charge-discharge measurement system (Toyo Technica Co., Ltd.). The preparation of the sample for measuring sodium ion conductivity and the immersion in the electrolyte are carried out in the same manner as in the preparation of the sample for the liquid absorption test. AC impedance measurement was performed at 25°C and a frequency of 2.0 x 10 4 Hz~1.0×10 -1 The real impedance component R (Ω) is calculated using a frequency of 1000 Hz, a voltage swing of 1000 mA, and the ionic conductivity σ (S / cm) of the resin sheet is calculated using the impedance component R (Ω), the thickness d (cm) of the resin sheet, and the contact area A (cm) between the electrode and the resin sheet. 2 ) is calculated. Ionic conductivity σ (S / cm) = d / (R × A)

[0056] The sodium ion conductivity of copolymer (A) when immersed in the electrolyte at 50°C for 3 days is 1×10 -10 ~1×10 -6 S / cm is preferable because it provides sodium ion conductivity that allows the secondary battery (sodium ion battery) to fully exhibit its performance, and can prevent the copolymer (A) from swelling excessively in the electrolyte solution, causing the adhesive surface to peel off. The sodium ion conductivity of the copolymer (A) when immersed in an electrolyte solution at 50°C for 3 days can be adjusted by the carboxyl group content (which can be adjusted mainly by the content of acrylic acid (a1)), SP value (solubility parameter), weight average molecular weight, etc. of the copolymer (A). Methods for increasing the sodium ion conductivity of copolymer (A) include reducing the acrylic acid (a1) content of copolymer (A), increasing the SP value (reducing the difference in SP value from the electrolyte), and decreasing the weight average molecular weight. Methods for decreasing the sodium ion conductivity of the copolymer (A) include increasing the acrylic acid (a1) content of the copolymer (A), decreasing the SP value (increasing the difference in SP value from the electrolyte), and increasing the weight average molecular weight.

[0057] The adhesive layer in the secondary battery separator of the present invention may contain inorganic fine particles in addition to the copolymer (A) for the purpose of improving heat resistance and the like. Examples of inorganic fine particles include metal fine particles, metal oxide fine particles, and carbonaceous fine particles. Examples of metal fine particles include lithium fine particles, silver fine particles, gold fine particles, nickel fine particles, platinum fine particles, aluminum fine particles, tin fine particles, and fine particles of alloys made of the metals that make up these metal fine particles.

[0058] Examples of metal oxide fine particles include lithium transition metal oxides {lithium manganese composite oxides (chemical formula: LiMn2O4, LiMnO2, etc.), lithium nickel composite oxides (chemical formula: LiNiO2, etc.), lithium cobalt composite oxides (chemical formula: LiCoO4, etc.), lithium titanium composite oxides (chemical formula: Li4Ti5O 12 Lithium phosphate oxides with an olivine structure (chemical formula: LiFePO4, etc.), lithium nickel cobalt composite oxides (chemical formula: LiNi 1 / 3 Co 1 / 3 Mn 1 / 3Fine particles such as O2, etc. and lithium vanadium oxide (such as Li3V2O5, etc. when shown by chemical formula), etc., sodium ion-containing compounds {such as NaFeO2, NaNiO2, NaCoO2, NaCrO2, NaMnO2, NaVO2, Na(Ni X Mn 1-X )O2 (0 < X < 1), Na(Fe X Mn 1-X )O2 (0 < X < 1), NaVPO4F, Na2FePO4F, Na3V2(PO4)3, etc.}, silica fine particles, alumina fine particles, iron oxide fine particles, molybdenum oxide fine particles, ruthenium oxide fine particles, titanium oxide fine particles, barium titanate fine particles, zirconium oxide fine particles, etc. are included.

[0059] Examples of carbonaceous fine particles include diamond fine particles, carbon black fine particles (such as acetylene black fine particles, ketjen black fine particles, etc.), graphite fine particles, and carbon nanotube fine particles, etc.

[0060] In addition to these, nitride fine particles (such as aluminum nitride and silicon nitride, etc.), hydroxide fine particles (such as calcium hydroxide, magnesium hydroxide, and aluminum hydroxide, etc.), ion-crystalline fine particles (such as calcium fluoride, barium fluoride, barium sulfate, and calcium carbonate, etc.), clay fine particles (such as talc and montmorillonite, etc.), etc. can also be used as inorganic fine particles.

[0061] The inorganic fine particles can be appropriately selected according to the type of secondary battery used. For example, when the secondary battery is a lithium ion battery or a sodium ion battery, metal oxide fine particles and carbonaceous fine particles are preferable, and more preferably lithium transition metal fine particles, alumina fine particles, fine particles of sodium ion-containing compounds, carbon black fine particles, and graphite fine particles.

[0062] From the viewpoint of adhesiveness and the cycle characteristics of the resulting battery, the volume average particle diameter of the inorganic fine particles is preferably 0.05 to 10 μm and smaller than the thickness of each adhesive layer described below. In this specification, the volume average particle diameter refers to the 50% diameter value in the volume-based cumulative fraction determined by laser diffraction measurement.

[0063] The content of the copolymer (A) in the adhesive layer of the secondary battery separator of the present invention is preferably 50 to 100% by weight, more preferably 70 to 100% by weight, based on the weight of the adhesive layer, from the viewpoints of heat press adhesion to the electrode and the cycle characteristics of the resulting battery. The content of the inorganic fine particles is preferably less than 50% by weight, and more preferably less than 30% by weight, based on the weight of the adhesive layer.

[0064] In the secondary battery separator of the present invention, the adhesive layer is provided on at least a portion of one or both surfaces of the porous substrate. The adhesive layer may be provided on only one surface of the porous substrate, or on both surfaces thereof. When adhesive layers are provided on both sides of the porous substrate, the separator has good adhesion to both electrodes (positive and negative electrodes) of the secondary battery. Furthermore, the separator is less likely to curl, making it easier to handle during battery production. When adhesive layers are provided on only one side of the porous substrate, the overall thickness of the separator can be reduced, allowing for the production of secondary batteries with low internal resistance and high energy density. The adhesive layer may be provided on the entire surface of one or both surfaces of the porous substrate, or may be provided on only a portion of the surface (for example, only the outer edge portion). The method for providing the adhesive layer on at least a portion of one or both sides of the porous substrate is not particularly limited. For example, as described below, a resin solution containing the copolymer (A) is prepared and attached to at least a portion of one or both sides of the porous substrate, and then the solvent in the resin solution is volatilized off to provide the adhesive layer.

[0065] In the secondary battery separator of the present invention, the thickness of each adhesive layer relative to the thickness of the porous substrate is preferably 5 to 25%. When the thickness of each adhesive layer relative to the thickness of the porous substrate is in this range, the heat press adhesion to the electrode is improved, and the internal resistance of the obtained battery can be kept low. When adhesive layers are provided on both sides of the porous substrate, it is more preferable that the thickness of each adhesive layer is 5 to 20% of the thickness of the porous substrate, and the total thickness of the adhesive layers on both sides is 10 to 40% of the thickness of the porous substrate.

[0066] The thickness of each adhesive layer relative to the thickness of the porous substrate can be calculated by the following method. [Calculation method] (1) Using a film thickness meter [Mitutoyo Corporation, Thickness Gauge (High Precision Type)], measure the thickness (X) of the porous substrate. Measure five arbitrary points within a 10 cm × 10 cm area, and calculate the average value to obtain the measured value (X). (2) A porous substrate with an adhesive layer on one or both sides is used as a sample, and the thickness (Y) of each adhesive layer is measured. The thickness (Y) of each adhesive layer can be measured with a film thickness meter or observed and measured using a microscope (optical microscope, electron microscope) depending on the order of thickness. Observation and measurement using a microscope is performed by cutting a portion near the center of the sample with a razor in the direction of the thickness and observing the cross section. Measurements are taken at any five points, and the average value is calculated to obtain the measurement value (Y). If it is difficult to visually confirm the presence or absence of an adhesive layer, observation using a microscope is carried out. (3) Calculate the "thickness (%) of one adhesive layer relative to the thickness of the porous substrate" using the following formula. Thickness of one adhesive layer relative to the thickness of the porous substrate (%) = [(Y) / (X)] × 100

[0067] The secondary battery separator of the present invention has an adhesive layer containing the copolymer (A), which has excellent adhesion and electric potential resistance, on at least a portion of one or both sides of the porous substrate, and therefore has excellent heat-press adhesion to the electrode and electric potential resistance, while not containing the organic fluorine compound PFAS (an abbreviation for perfluoroalkyl compounds, polyfluoroalkyl compounds, and their salts), which has been subject to stricter environmental regulations in recent years. Because the secondary battery separator of the present invention has excellent electric potential resistance, a secondary battery using the secondary battery separator of the present invention is less likely to experience a decrease in battery performance (such as a decrease in capacity) even after repeated charge and discharge, and has good charge and discharge cycle characteristics.

[0068] <Method for manufacturing a separator for a secondary battery> The present invention provides a method for producing the secondary battery separator, comprising the step of adhering a resin solution containing the copolymer (A) at a solids content of 2 to 40% by weight to at least a portion of one or both surfaces of the porous substrate. This step may be referred to as the "adhering step" in this specification. The resin solution of the copolymer (A) having a solid content of 2 to 40% by weight is a solution in which the copolymer (A) is dissolved in a solvent so that the content of the copolymer (A) in the solution is 2 to 40% by weight. If the solids concentration of the resin solution in the adhesion step is outside the above range, it becomes difficult to adhere the copolymer (A) to the surface of the porous substrate at the desired basis weight (particularly to achieve a thin film thickness). When the adhesive layer of the secondary battery separator contains components other than the copolymer (A) (such as inorganic fine particles), it is preferable to add (dissolve or disperse) the components other than the copolymer (A) in the resin solution. When the adhesive layer contains components other than the copolymer (A) (hereinafter referred to as other components), the resin solution having a solids content of 2 to 40% by weight of the copolymer (A) is interpreted as "a solution prepared by dissolving or dispersing the copolymer (A) in a solvent so that the total weight of the copolymer (A) and the solids of the other components (the weight remaining after heating at 100°C for 8 hours) is 2 to 40% by weight."

[0069] The solvent used in the resin solution is preferably an organic solvent, and examples thereof include amides, alcohols, ketones, esters, ethers, and hydrocarbons.

[0070] Examples of the amide include cyclic amides having 4 to 14 carbon atoms (2-pyrrolidone, N-methylpyrrolidone, N-hexylpyrrolidone, N-decylpyrrolidone, etc.), and linear amides having 2 to 8 carbon atoms (N-methylformamide, N,N-dimethylformamide, N,N-dimethylacetamide, N-phenyl-N-methylformamide, etc.).

[0071] As the alcohol, alcohols having 1 to 10 carbon atoms can be used, and examples thereof include methanol, ethanol, normal propanol, isopropanol, normal butanol, isobutanol, secondary butanol, 3-methoxy-3-methyl-1-butanol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, propylene glycol monoethyl ether, dipropylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and propylene glycol monobutyl ether.

[0072] As the ketone, a ketone having 3 to 6 carbon atoms can be used, and examples thereof include acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone.

[0073] As the ester, an ester having 4 to 10 carbon atoms can be used, and examples thereof include ethyl acetate, butyl acetate, ethyl cellosolve acetate, butyl cellosolve acetate, butyl carbitol acetate, and propylene glycol monomethyl ether acetate.

[0074] As the ether, ethers having 4 to 10 carbon atoms can be used, and examples thereof include ethyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, propylene glycol dimethyl ether, dipropylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol diethyl ether, and 1,4-dioxane.

[0075] Hydrocarbons include aromatic hydrocarbons (such as benzene, toluene, xylene, ethylbenzene, and trimethylbenzene), alicyclic hydrocarbons (such as cyclopentane, cyclohexane, cycloheptane, cyclooctane, cyclononane, and cyclodecane), and aliphatic hydrocarbons (such as pentane, hexane, heptane, octane, nonane, and decane).

[0076] These solvents may be used alone or in combination of two or more. Among these solvents, from the viewpoint of the solubility and coating properties of the copolymer (A), amides and alcohols are preferred, and amides and isopropanol are particularly preferred. When the adhesive layer of the secondary battery separator contains inorganic fine particles in addition to the copolymer (A), it is preferable to select a solvent in which the inorganic fine particles do not dissolve but in which the inorganic fine particles can be dispersed.

[0077] Methods for adhering the resin solution to at least a portion of one or both sides of the porous substrate include (1) a method of applying the resin solution to the porous substrate and adhering it, and (2) a method of immersing the porous substrate in the resin solution and adhering it. When adhesive layers are provided on both sides of the porous substrate, the resin solution may be applied to both sides of the porous substrate simultaneously or to each side at a time.

[0078] As the coating method in "(1) the method of applying a resin solution to a porous substrate and adhering it to the substrate," known coating methods can be used, such as the doctor blade method, the reverse roll method, the direct Examples of the coating method include a roll method, a gravure method, an extrusion method, a bar coating method, and a brush coating method. When the resin solution is applied to the porous substrate to adhere it, the solid content of the resin solution is preferably 5 to 40% by weight.

[0079] As the immersion method in "(2) the method of immersing a porous substrate in a resin solution to adhere the resin," a dipping method can be mentioned. When the porous substrate is immersed in the resin solution to adhere the resin, the solid content of the resin solution is preferably 2 to 40% by weight.

[0080] After the adhesion step, the adhesion portion is dried to volatilize and remove the solvent, thereby providing an adhesive layer on at least a portion of one or both sides of the porous substrate. That is, a secondary battery separator including a porous substrate and an adhesive layer provided on at least a portion of one or both sides of the porous substrate can be obtained. The method for removing the solvent by evaporation is preferably drying using an oven or a vacuum oven. The drying temperature can be appropriately set depending on the type of solvent and solid concentration of the resin solution, but is, for example, 60 to 180°C from the viewpoint of preventing a decrease in separator performance and production efficiency.

[0081] According to the production method of the present invention, the copolymer (A) can be easily and uniformly adhered to the surface of the porous substrate with a desired thickness, dripping is less likely to occur, and the yield is good, so that the secondary battery separator of the present invention can be efficiently produced.

[0082] The secondary battery separator of the present invention may be produced by forming the adhesive layer as an independent sheet, overlaying this adhesive layer on a porous substrate, and combining them using heat pressing or an adhesive. The independent sheet can be produced by adhering and drying the copolymer (A) on a release sheet to form an adhesive layer.

[0083] The secondary battery separator of the present invention is disposed between a positive electrode and a negative electrode in a secondary battery. At this time, it is bonded (preferably by heat press bonding) to the positive electrode and / or the negative electrode via an adhesive layer provided on at least a portion of one or both sides of the porous substrate. When the adhesive layer is provided on only one side of the porous substrate, it is bonded to either the positive electrode or the negative electrode, and when the adhesive layer is provided on both sides of the porous substrate, it can be bonded to both the positive electrode and the negative electrode.

[0084] The secondary battery in which the secondary battery separator of the present invention is used is preferably a non-aqueous electrolyte secondary battery, and particularly preferred examples include lithium ion batteries and sodium ion batteries. Known materials can be used for the components (positive electrode, negative electrode, electrolyte, etc.) other than the separator that constitute the secondary battery in which the secondary battery separator of the present invention is used. The electrolyte may be the same as or different from the electrolyte used to determine the liquid absorption rate (%), but it is preferable to use ethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and mixtures thereof as the non-aqueous solvent. As the electrolyte contained in the electrolytic solution, electrolytes used in known electrolytic solutions can be used. For example, in the case of a lithium ion battery, a lithium salt can be used, and in the case of a sodium ion battery, a sodium salt can be used.

[0085] Examples of lithium salts include lithium salts of inorganic anions such as LiPF, LiBF, LiSbF, LiAsF, LiClO, and LiN(FSO), and lithium salts of organic anions such as LiN(CFSO), LiN(CFS0), and LiC(CFSO). Of these, LiPF (lithium hexafluorophosphate), LiFSI (lithium bis(fluorosulfonyl)imide), and the like are preferably used.

[0086] Examples of sodium salts include inorganic sodium salts such as NaPF6, NaBF4, NaClO4, and NaAsF6, and organic sodium salts such as NaCF3SO3, NaN(CF3SO2)2, NaN(C2F5SO2)2, NaN(FSO2)2, and NaC(CF3SO2)3. Of these, sodium hexafluorophosphate (NaPF6) is preferred from the viewpoint of battery output and charge / discharge cycle characteristics.

[0087] The present specification discloses the following:

[0088] The present disclosure (I) is a separator for a secondary battery comprising a porous substrate and an adhesive layer provided on at least a portion of one or both surfaces of the porous substrate, wherein the adhesive layer comprises a copolymer (A) containing, as essential constituent monomers, acrylic acid (a1) and an ethylenically unsaturated monomer (a2) having a homopolymer glass transition temperature of −15°C or lower, wherein the content of the acrylic acid (a1) based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A) is 25 to 70 wt % and the content of the ethylenically unsaturated monomer (a2) is 30 to 75 wt %, and the glass transition temperature of the copolymer (A) is 40 to 100°C.

[0089] The present disclosure (II) is the secondary battery separator according to the present disclosure (I), wherein the content of the acrylic acid (a1) is 40 to 70% by weight, and the content of the ethylenically unsaturated monomer (a2) is 30 to 60% by weight, based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A).

[0090] The present disclosure (III) provides that the copolymer (A) has a liquid absorption rate of 40% or more when immersed in an electrolytic solution at 50°C for 3 days, and the electrolytic solution is an electrolytic solution prepared by dissolving LiPF as an electrolyte in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7, to a concentration of 1 mol / L, and the liquid absorption rate is calculated based on the following formula: Absorption rate (%) = [(weight of copolymer (A) after immersion in electrolyte solution - weight of copolymer (A) before immersion in electrolyte solution) / weight of copolymer (A) before immersion in electrolyte solution] × 100 The secondary battery separator according to the present disclosure (I) or (II) is characterized in that it is obtained by:

[0091] The present disclosure (IV) provides that the copolymer (A) has a liquid absorption rate of 40% or more when immersed in an electrolytic solution at 50°C for 3 days, and the electrolytic solution is an electrolytic solution prepared by dissolving NaPF6 as an electrolyte in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7, to a concentration of 1 mol / L, and the liquid absorption rate is calculated based on the following formula: Absorption rate (%) = [(weight of copolymer (A) after immersion in electrolyte solution - weight of copolymer (A) before immersion in electrolyte solution) / weight of copolymer (A) before immersion in electrolyte solution] × 100 The secondary battery separator according to the present disclosure (I) or (II) is characterized in that it is obtained by:

[0092] The present disclosure (V) is the secondary battery separator according to any one of the present disclosures (I) to (IV), characterized in that the copolymer (A) has a glass transition temperature of 40 to 93°C.

[0093] The present disclosure (VI) is a separator for a secondary battery according to any one of the present disclosures (I) to (V), characterized in that the thickness of each of the adhesive layers is 5 to 25% of the thickness of the porous substrate.

[0094] The present disclosure (VII) is a method for producing a separator for a secondary battery according to any one of the present disclosures (I) to (VI), characterized by comprising a step of adhering a resin solution of the copolymer (A) with a solid content of 2 to 40% by weight to at least a portion of one or both surfaces of the porous substrate. [Example]

[0095] The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples as long as they do not depart from the gist of the present invention.

[0096] (Production Example 1: Production of Copolymer (A-1)) A four-neck flask equipped with a stirrer, thermometer, reflux condenser, dropping funnel, and nitrogen gas inlet tube was charged with 150 parts by weight of DMF and heated to 75°C. Next, a monomer composition containing 60 parts by weight of acrylic acid, 40 parts by weight of 2-ethylhexyl acrylate, and 50 parts by weight of DMF (dimethylformamide) was added dropwise over two hours to the four-neck flask with stirring while nitrogen was blown into the flask. A radical polymerization reaction was then carried out by continuously adding the monomer composition dropwise over two hours using the dropping funnel. After the addition, the reaction was continued for three hours at 75°C. The temperature was then raised to 80°C and the reaction was continued for three hours, yielding a resin solution with a resin concentration of 30% by weight. This resin solution was dried under reduced pressure at 160° C. and 0.01 MPa for 3 hours to remove the DMF, thereby obtaining a copolymer (A-1).

[0097] (Production Examples 2 to 10, Comparative Production Examples 1 to 5: Production of Copolymers (A-2) to (A-10) and Comparative Copolymers (AX-2) to (AX-6)) Copolymers (A-2) to (A-10) and comparative copolymers (AX-2) to (AX-6) were produced in the same manner as in Production Example 1, except that the types of constituent monomers and the weight parts charged were changed as shown in Table 1.

[0098] For each of the obtained copolymers (A) and (AX), the SP value, the weight-average molecular weight measured by the above-mentioned method, the glass transition temperature (Tg), the liquid absorption rate when immersed in the electrolyte solutions (electrolyte solution 1 and electrolyte solution 2) at 50°C for 3 days, and the ionic conductivity when immersed in the electrolyte solutions (electrolyte solution 1 and electrolyte solution 2) at 50°C for 3 days (lithium ionic conductivity for electrolyte solution 1 and sodium ionic conductivity for electrolyte solution 2) are shown in Table 1. For comparison, polyvinylidene fluoride (hereinafter referred to as PVDF) [Kishida Chemical Co., Ltd., #1100] was used as the copolymer (AX-1). The electrolyte solution 1 is prepared by dissolving LiPF6 as an electrolyte at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1, and the electrolyte solution 2 is prepared by dissolving NaPF6 as an electrolyte at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1.

[0099] [Table 1]

[0100] <Examples 1 to 14 and Comparative Examples 1 to 6: Production of Lithium-ion Battery Separator> (Preparation of porous substrate) A commercially available microporous polyolefin membrane [thickness: 13 μm] was prepared as the porous substrate.

[0101] (Preparation of copolymer solution) Each copolymer (A) or (AX) shown in Table 2 was mixed with DMF to obtain a resin solution of each copolymer with a solid content of 10% by weight.

[0102] (Attachment process) The resin solution of each copolymer obtained above, having a solid content of 10% by weight, was applied to the entire surface of one side of the porous substrate (approximately 10 cm×15 cm) using a gravure coater. At the time of application, the adhesive layer was applied so that the thickness after drying (solvent removal) would be 2.0 μm in Examples 1 to 10 and Comparative Examples 1 to 6. The adhesive layer was applied so that the thickness after drying (solvent removal) would be 0.5 μm in Example 11, 3.5 μm in Example 12, 1.0 μm in Example 13, and 3.0 μm in Example 14.

[0103] (drying process) After the adhesion step, the separator was dried at 60°C for 30 minutes using a wind dryer to obtain a lithium ion battery separator, which is a secondary battery separator of the present invention having an adhesive layer on one side of a porous substrate.

[0104] <Heat press adhesion to electrodes for lithium-ion batteries> (Preparation of lithium-ion battery positive electrodes for adhesive strength measurement) Cathode active material {NCA: LiNi 0.8 Co 0.15 Al 0.05 Ninety parts by weight of O2 powder (volume average particle diameter: 4 μm), 5.0 parts by weight of a conductive additive (hereinafter referred to as "Denka Black Li100" manufactured by Denka Co., Ltd., average primary particle diameter: 35 nm), and 5.0 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed with N-methyl-2-pyrrolidone (hereinafter referred to as NMP) to obtain a mixed solution (solids concentration: 10 wt%). The mixed solution was stirred at 2000 rpm for 1 minute using a planetary stirring mixer / kneader (Awatori Rentaro [manufactured by Thinky Corporation]) to prepare a slurry for the positive electrode layer. The resulting slurry was applied to one side of a current collector (carbon-coated aluminum foil [product name "Carbon-Coated Aluminum Foil"] manufactured by Toyo Aluminum Co., Ltd.) using a wire bar in the atmosphere and pre-dried overnight in a draft to form a positive electrode active material layer on the current collector. (Hereinafter, the current collector and positive electrode active material layer will be collectively referred to as the "electrode sheet.") The electrode sheet was then cut into three pieces measuring 25 mm wide x 50 mm long with a cutter, dried at 100°C for 2 hours, and pressed in a press to prepare positive electrodes for adhesive strength measurements.

[0105] (Preparation of lithium-ion battery negative electrodes for adhesive strength measurement) A mixture (solids concentration 10 wt%) was prepared by mixing 90.9 parts by weight of artificial graphite (FSN-1, manufactured by Shanshan Co., Ltd., China) as the negative electrode active material, 5.1 parts by weight of AB as the conductive additive, and 2.0 parts by weight of styrene butadiene rubber (hereinafter referred to as SBR) and 2.0 parts by weight of carboxymethyl cellulose (hereinafter referred to as CMC) as binders with NMP. The mixture was stirred at 2000 rpm for 1 minute using a planetary stirring mixer (Thinky Mixer, manufactured by Awatori Rentaro). The slurry for the negative electrode layer was prepared. The resulting slurry was applied to one side of a current collector (copper foil) using a wire bar in the atmosphere and pre-dried overnight in a draft to form a negative electrode active material layer on the current collector. (Hereinafter, the current collector and negative electrode active material layer will be collectively referred to as the electrode sheet.) The electrode sheet was then cut into three pieces measuring 25 mm wide x 50 mm long using a cutter, dried at 100°C for 2 hours, and pressed in a press to prepare negative electrodes for adhesive strength measurements.

[0106] (Preparation of test pieces for measuring adhesive strength) Each lithium-ion battery separator prepared as described above was placed with the adhesive layer facing up. Three lithium-ion battery electrodes (positive or negative electrodes) for adhesive strength measurement were then placed at intervals so that the electrode active material layer was in contact with the adhesive layer. The separator and electrode were then bonded by heat pressing from the current collector side of the electrode (temperature: 20°C higher than the Tg of the adhesive layer resin, pressure: 1.0 MPa, time: 30 seconds). The heat pressing was performed to bond only a 25 mm x 25 mm area of ​​the lithium-ion battery electrode for adhesive strength measurement, which was 25 mm wide x 50 mm long (both ends in the longitudinal direction were not bonded). The separator was then cut into a 25 mm strip to match the width of the electrode, creating a test specimen for adhesive strength measurement. In this test specimen, the separator and electrode were bonded over a 25 mm x 25 mm area; hereinafter, the unbonded area may be referred to as the blank area.

[0107] (Measurement of adhesive strength) For the test specimens used to measure adhesive strength, a Nichiban NISTACK (strong type: model number NW-K15) was attached to one end of the separator margin (adhesive layer side) for reinforcement. Similarly, a NISTACK was attached to one end of the electrode margin (electrode active material side) for reinforcement. The separator's NISTACK-attached portion was clamped in the chuck of the upper (movable part) of the autograph, and the electrode-free stack was clamped in the chuck of the lower (fixed part) of the autograph, and an interfacial fracture test was performed. A 20 N test jig was used, and the tensile speed was 150 mm / min. The force at which the adhesive layer and the electrode active material layer peeled was measured in accordance with JIS K 6854-2:1999 to determine the average peel force (N). The average peel force was divided by the width of the test specimen (0.025 m) to obtain the adhesive strength (N / m) between the separator and the electrode (positive or negative electrode). Table 2 shows this. For each example and comparative example, the adhesive strength (N / m) was calculated for three test pieces, and the arithmetic mean value was used as the analytical value.

[0108] (Determining heat press adhesion with electrodes for lithium-ion batteries) The heat-press adhesiveness between the lithium-ion battery separator and the lithium-ion battery electrodes (positive and negative electrodes) of each Example and Comparative Example was evaluated according to the following criteria, and the results are shown in Table 2. Note that "material failure" in Table 2 means that the heat-press adhesiveness was so high that the electrode broke before interfacial peeling occurred between the separator and the electrode (positive or negative electrode). In all of Example 2 (negative electrode), Example 10 (negative electrode), Example 12 (positive and negative electrodes), and Example 14 (negative electrode), the force (N) at which material failure occurred was greater than 2.0 N, and the adhesive strength was at least greater than 80 N / m. [Evaluation criteria] ◎:80N / m or more ○: 40N / m or more and less than 80N / m △: 10N / m or more and less than 40N / m ×: Less than 10N / m

[0109] <Battery cycle characteristics (capacity retention rate after charge / discharge)> (Preparation of lithium-ion battery positive electrodes for charge / discharge tests) A mixture (solid content: 10 wt%) was prepared by mixing 90 parts by weight of NCA as a positive electrode active material, 5.0 parts by weight of AB as a conductive additive, and 5.0 parts by weight of PVDF as a binder with NMP. The mixture was stirred at 2000 rpm for 1 minute using a planetary stirring mixer (Thinky Mixer, manufactured by THINKY CORPORATION) to prepare a slurry for the positive electrode layer. The resulting slurry was applied to one side of a current collector (carbon-coated aluminum foil [product name "Carbon-Coated Aluminum Foil"], manufactured by Toyo Aluminum Co., Ltd.) using a wire bar in the atmosphere and pre-dried overnight in a draft to form a positive electrode active material layer on the current collector. (Hereinafter, the current collector and positive electrode active material layer will be collectively referred to as the "electrode sheet.") Five 15 mm diameter pieces were then punched out near the center of the electrode sheet, dried at 100°C for 2 hours, and pressed in a press to prepare lithium-ion battery positive electrodes for charge / discharge tests.

[0110] (Preparation of lithium-ion battery negative electrode with separator for charge / discharge test) 90.9 parts by weight of artificial graphite (FSN-1, manufactured by Shanshan China Co., Ltd.) as the negative electrode active material, 5.1 parts by weight of AB as the conductive additive, and 2.0 parts by weight of SBR and 2.0 parts by weight of CMC as binders were mixed with NMP to obtain a mixed solution (solids concentration 10% by weight). The mixed solution was stirred at 2000 rpm for 1 minute using a planetary stirring mixer (Awatori Rentaro [manufactured by Thinky Corporation]) to prepare a slurry for the negative electrode layer. The resulting slurry was applied to one side of a current collector (copper foil) in the atmosphere using a wire bar and pre-dried overnight in a draft to form a negative electrode active material layer on the current collector. (Hereinafter, the current collector and negative electrode active material layer will be collectively referred to as the electrode sheet.) The electrode sheet was then dried at 100°C for 2 hours and pressed using a press. Next, the lithium-ion battery separators of each Example and Comparative Example were overlapped so that the adhesive layer of the separator contacted the negative electrode active material layer of the electrode sheet, and the separator and negative electrode were bonded together by heat pressing (temperature: 98°C, pressure: 1.0 MPa, time: 10 seconds) from the current collector side of the electrode sheet, obtaining a negative electrode integrated with the separator. Five 16 mm diameter pieces were punched out of this to prepare lithium-ion battery negative electrodes with separators for charge / discharge tests.

[0111] (Preparation of lithium-ion batteries for charge / discharge tests) From the positive electrode side, the lithium-ion battery positive electrode for charge / discharge tests and the lithium-ion battery negative electrode with a separator for charge / discharge tests were arranged so that the separator surface (porous substrate surface) was in contact with the positive electrode, and after injecting the electrolyte, they were vacuum laminated to prevent oxygen from entering, thereby producing a lithium-ion battery for charge / discharge tests. The electrolyte used was electrolyte solution 1 (a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1, in which LiPF6 was dissolved at a ratio of 1 mol / L as an electrolyte).

[0112] (Charge / discharge test) Each lithium-ion battery for charge / discharge test prepared above was subjected to 500 charge / discharge cycles in an environment at a temperature of 40°C. Charging was performed by constant current / constant voltage charging at 1C to 4.2V, and discharging was performed by constant current discharging at 1C with a cutoff voltage of 2.75V. The discharge capacity at the 500th cycle was divided by the initial discharge capacity, and the resulting value (%) was used as the capacity retention rate. For each example and comparative example, the capacity retention rate (%) was calculated for five batteries, and the arithmetic average value was used as the analytical value. Table 2 also shows the results of judging the cycle characteristics (potential resistance) of the battery according to the following criteria. [Judgment criteria] ◎: Capacity retention rate of 97% or more 〇: Capacity retention rate 95% or more but less than 97% △: Capacity retention rate: 90% to less than 95% ×: Capacity retention rate less than 90%

[0113] [Table 2]

[0114] From Table 2, it can be seen that the lithium ion battery separators obtained in each example, which are secondary battery separators of the present invention, do not contain PFAS such as PVDF, yet have excellent heat press adhesion to the electrodes (positive and negative electrodes) and good potential resistance, and therefore lithium ion batteries equipped with these secondary battery separators have excellent cycle characteristics.

[0115] <Examples 15 to 17 and Comparative Examples 7 to 9: Production of Separators for Sodium Ion Batteries> (Preparation of porous substrate) A commercially available microporous polyolefin membrane [thickness: 13 μm] was prepared as the porous substrate.

[0116] (Preparation of copolymer solution) Each copolymer (A) or (AX) shown in Table 3 was mixed with DMF to obtain a resin solution of each copolymer with a solid content of 10% by weight.

[0117] (Attachment process) The resin solution of each copolymer obtained above, having a solid content of 10% by weight, was applied to the entire surface of one side of the porous substrate (approximately 10 cm×15 cm) using a gravure coater. The coating was performed so that the thickness of the adhesive layer after drying (solvent removal) would be 2.0 μm.

[0118] (drying process) After the adhesion step, the separator was dried at 60°C for 30 minutes using a wind dryer to obtain a sodium ion battery separator, which is a secondary battery separator of the present invention having an adhesive layer on one side of a porous substrate.

[0119] <Heat press adhesion to sodium ion battery electrodes> (Preparation of sodium ion battery positive electrodes for adhesive strength measurement) A mixture (solids concentration 10 wt%) of 90 parts by weight of a positive electrode active material (NaCrO2 [sodium chromite, manufactured by Kojundo Chemical Laboratory]), 5.0 parts by weight of a conductive additive (hereinafter referred to as "Denka Black Li100," manufactured by Denka Co., Ltd., average primary particle size: 35 nm), and 5.0 parts by weight of polyvinylidene fluoride (PVDF) as a binder was mixed with N-methyl-2-pyrrolidone (NMP). The mixture was stirred at 2000 rpm for 1 minute using a planetary stirring mixer / kneader (Awatori Rentaro, manufactured by Thinky Corporation) to prepare a slurry for the positive electrode layer. The resulting slurry was applied to one side of a current collector (carbon-coated aluminum foil [product name "Carbon-Coated Aluminum Foil"], manufactured by Toyo Aluminum Co., Ltd.) using a wire bar in the atmosphere and pre-dried overnight in a draft to form a positive electrode active material layer on the current collector. (Hereinafter, the current collector and positive electrode active material layer will be collectively referred to as the "electrode sheet.") Three pieces measuring 25 mm wide and 50 mm long were then cut from the electrode sheet using a cutter, dried at 100°C for 2 hours, and pressed in a press to prepare sodium-ion battery positive electrodes for adhesive strength measurements.

[0120] (Preparation of sodium ion battery negative electrodes for adhesive strength measurement) A mixture (solids concentration 10 wt%) was prepared by mixing 90.9 parts by weight of non-graphitizable carbon (Carbotron® PS(F), volume average particle diameter 9 μm, manufactured by Kureha Battery Materials Japan Co., Ltd.) as the negative electrode active material, 5.1 parts by weight of AB as the conductive additive, and 2.0 parts by weight of styrene butadiene rubber (SBR) and 2.0 parts by weight of carboxymethyl cellulose (CMC) as binders with NMP. The mixture was stirred at 2000 rpm for 1 minute using a planetary stirring mixer (Thinky Mixer, Awatori Rentaro) to prepare a slurry for the negative electrode layer. The resulting slurry was applied to one side of a current collector (copper foil) using a wire bar in the atmosphere and pre-dried overnight in a draft to form a negative electrode active material layer on the current collector. (Hereinafter, the current collector and negative electrode active material layer will be collectively referred to as the electrode sheet.) The electrode sheet was then cut into three pieces measuring 25 mm wide x 50 mm long using a cutter, dried at 100°C for 2 hours, and pressed in a press to prepare sodium-ion battery negative electrodes for adhesive strength measurements.

[0121] (Preparation of test pieces for measuring adhesive strength) Each sodium-ion battery separator prepared as described above was placed with the adhesive layer facing up. Next, three sodium-ion battery electrodes (positive or negative electrodes) for adhesive strength measurement were placed at intervals so that the electrode active material layer side was in contact with the adhesive layer. The separator and electrode were then bonded by heat pressing from the current collector side of the electrode (temperature: 20°C higher than the Tg of the adhesive layer resin, pressure: 1.0 MPa, time: 30 seconds). The heat pressing was performed to bond only a 25 mm x 25 mm area of ​​the sodium-ion battery electrode for adhesive strength measurement, which was 25 mm wide x 50 mm long (both ends in the longitudinal direction were not bonded). The separator was then cut into a 25 mm strip to match the width of the electrode, creating a test specimen for adhesive strength measurement. In this test specimen, the separator and electrode were bonded over a 25 mm x 25 mm area; hereinafter, the unbonded area may be referred to as the blank area.

[0122] (Measurement of adhesive strength) For the test specimens used to measure adhesive strength, a Nichiban NISTACK (strong type: model number NW-K15) was attached to one end of the separator margin (adhesive layer side) for reinforcement. Similarly, a NISTACK was attached to one end of the electrode margin (electrode active material side) for reinforcement. The separator's NISTACK-attached portion was clamped in the chuck of the upper (movable part) of the autograph, and the electrode-free stack-attached portion was clamped in the chuck of the lower (fixed part) of the autograph, and an interfacial fracture test was performed. A 20 N test jig was used, and the tensile speed was 150 mm / min. The force at which the adhesive layer and the electrode active material layer peeled was measured in accordance with JIS K 6854-2:1999 to determine the average peel force (N). The average peel force was divided by the width of the test specimen (0.025 m) to obtain the adhesive strength (N / m) between the separator and the electrode (positive or negative electrode). Table 3 shows this. For each example and comparative example, the adhesive strength (N / m) was calculated for three test pieces, and the arithmetic mean value was used as the analytical value.

[0123] (Determining heat press adhesion with sodium ion battery electrodes) The heat press adhesion between the sodium ion battery separator and the sodium ion battery electrodes (positive and negative electrodes) of each Example and Comparative Example was evaluated according to the following criteria, and the results are shown in Table 3. Note that "material failure" in Table 3 means that the heat press adhesion was so high that the electrode broke before interfacial peeling occurred between the separator and the electrode (positive or negative electrode). The force (N) at which material failure occurred in Example 17 (negative electrode) was greater than 2.0 N, and the adhesive strength was at least greater than 80 N / m. [Evaluation criteria] ◎:80N / m or more ○: 40N / m or more and less than 80N / m △: 10N / m or more and less than 40N / m ×: Less than 10N / m

[0124] <Battery cycle characteristics (capacity retention rate after charge / discharge)> (Preparation of sodium ion battery positive electrode for charge / discharge test) 290 parts by weight of NaCrO as a positive electrode active material, 5.0 parts by weight of AB as a conductive additive, and 5.0 parts by weight of PVDF as a binder were mixed with NMP to obtain a mixed solution (solid content concentration 10 wt%). The mixed solution was stirred at 2000 rpm for 1 minute using a planetary stirring mixer (Awatori Rentaro [manufactured by Thinky Corporation]) to prepare a slurry for the positive electrode layer. The resulting slurry was applied to one side of a current collector (carbon-coated aluminum foil [product name "Carbon-Coated Aluminum Foil"], manufactured by Toyo Aluminum Co., Ltd.) using a wire bar in the atmosphere and pre-dried overnight in a draft to form a positive electrode active material layer on the current collector. (Hereinafter, the current collector and positive electrode active material layer will be collectively referred to as the "electrode sheet.") Five 15 mm diameter pieces were then punched out near the center of the electrode sheet, dried at 100°C for 2 hours, and pressed in a press to prepare sodium-ion battery positive electrodes for charge-discharge tests.

[0125] (Preparation of a sodium ion battery negative electrode with a separator for charge / discharge testing) A mixture (solids concentration 10 wt%) was prepared by mixing 90.9 parts by weight of non-graphitizable carbon as the negative electrode active material, 5.1 parts by weight of AB as the conductive additive, and 2.0 parts by weight of SBR and 2.0 parts by weight of CMC as the binder with NMP. The mixture was stirred at 2000 rpm for 1 minute using a planetary stirring mixer (Thinky Mixer, manufactured by THINKY CORPORATION) to prepare a slurry for the negative electrode layer. The resulting slurry was applied to one side of a current collector (copper foil) in the atmosphere using a wire bar and pre-dried overnight in a draft to form a negative electrode active material layer on the current collector. (Hereinafter, the current collector and negative electrode active material layer will be collectively referred to as the electrode sheet.) The electrode sheet was then dried at 100°C for 2 hours and pressed using a press. Next, the sodium-ion battery separators of each Example and Comparative Example were overlapped so that the adhesive layer of the separator contacted the negative electrode active material layer of the electrode sheet, and the separator and negative electrode were bonded together by heat pressing (temperature: 98°C, pressure: 1.0 MPa, time: 10 seconds) from the current collector side of the electrode sheet, obtaining a negative electrode integrated with the separator. Five 16 mm diameter pieces were punched out of this to prepare sodium-ion battery negative electrodes with separators for charge / discharge tests.

[0126] (Preparation of sodium ion battery for charge / discharge test) From the positive electrode side, a sodium ion battery positive electrode for charge / discharge tests and a sodium ion battery negative electrode with a separator for charge / discharge tests were placed so that the separator surface (porous substrate surface) was in contact with the positive electrode, and after injecting the electrolyte, they were vacuum laminated to prevent oxygen from entering, thereby producing a sodium ion battery for charge / discharge tests. The electrolyte used was electrolyte solution 2 (a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1, with NaPF6 dissolved at a ratio of 1 mol / L as the electrolyte).

[0127] (Preparation of sodium ion battery for charge / discharge test) From the positive electrode side, a sodium ion battery positive electrode for charge / discharge tests and a sodium ion battery negative electrode with a separator for charge / discharge tests were placed so that the separator surface (porous substrate surface) was in contact with the positive electrode, and after injecting the electrolyte, they were vacuum laminated to prevent oxygen from entering, thereby producing a sodium ion battery for charge / discharge tests. The electrolyte used was electrolyte solution 2 (a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1, with NaPF6 dissolved at a ratio of 1 mol / L as the electrolyte).

[0128] (Charge / discharge test) Each sodium-ion battery for charge / discharge test prepared above was subjected to 500 charge / discharge cycles in an environment at a temperature of 40°C. Charging was performed by constant current / constant voltage charging at 1C to 3.7V, and discharging was performed by constant current discharging at 1C with a cutoff voltage of 2.75V. The discharge capacity at the 500th cycle was divided by the initial discharge capacity, and the resulting value (%) was used as the capacity retention rate. For each example and comparative example, the capacity retention rate (%) was calculated for five batteries, and the arithmetic average value was used as the analytical value. Table 3 shows the results of judging the cycle characteristics (potential resistance) of the battery according to the following criteria. Furthermore, because sodium ions have a larger ionic radius than lithium ions and are less efficient at moving into and out of electrode materials than lithium ions, the capacity retention rate of sodium ion batteries is generally slightly lower than that of lithium ion batteries. For this reason, the evaluation criteria for lithium ion batteries and sodium ion batteries are different. [Judgment criteria] ◎: Capacity retention rate of 93% or more 〇: Capacity retention rate 91% or more but less than 93% △: Capacity retention rate 87% or more but less than 91% ×: Capacity retention rate less than 87%

[0129] [Table 3]

[0130] From Table 3, it can be seen that the sodium ion battery separators obtained in each example, which are secondary battery separators of the present invention, do not contain PFAS such as PVDF, like the lithium ion battery separators described above, but have excellent heat press adhesion to electrodes (positive and negative electrodes) and good potential resistance, and therefore sodium ion batteries equipped with these secondary battery separators have excellent cycle characteristics. [Industrial Applicability]

[0131] A secondary battery obtained from the secondary battery separator of the present invention is particularly useful as a secondary battery for use in mobile phones, personal computers, hybrid vehicles, and electric vehicles.

Claims

1. A separator for a secondary battery comprising a porous substrate and an adhesive layer provided on at least a portion of one or both surfaces of the porous substrate, the adhesive layer comprises a copolymer (A) containing, as essential constituent monomers, acrylic acid (a1) and an ethylenically unsaturated monomer (a2) whose homopolymer has a glass transition temperature of −15° C. or lower; the content of the acrylic acid (a1) is 25 to 70% by weight and the content of the ethylenically unsaturated monomer (a2) is 30 to 75% by weight based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A); The copolymer (A) has a glass transition temperature of 40 to 100°C.

2. 2. The separator for a secondary battery according to claim 1, wherein the content of the acrylic acid (a1) based on the total weight of the acrylic acid (a1) and the ethylenically unsaturated monomer (a2) contained in the copolymer (A) is 40 to 70% by weight, and the content of the ethylenically unsaturated monomer (a2) is 30 to 60% by weight.

3. the copolymer (A) has a liquid absorption rate of 40% or more when immersed in an electrolyte solution at 50°C for 3 days; The electrolyte solution was prepared by dissolving LiPF as an electrolyte in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:

7. ６ is an electrolyte solution in which the above is dissolved to a concentration of 1 mol / L, The liquid absorption rate is calculated by the following formula: Absorption rate (%)=[(weight of copolymer (A) after immersion in electrolyte solution−weight of copolymer (A) before immersion in electrolyte solution) / weight of copolymer (A) before immersion in electrolyte solution]×100 2. The separator for a secondary battery according to claim 1, wherein the separator is determined by the following formula:

4. the copolymer (A) has a liquid absorption rate of 40% or more when immersed in an electrolyte solution at 50°C for 3 days; The electrolytic solution was prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, and adding NaPF as an electrolyte. ６ is an electrolyte solution in which the above is dissolved to a concentration of 1 mol / L, The liquid absorption rate is calculated by the following formula: Absorption rate (%)=[(weight of copolymer (A) after immersion in electrolyte solution−weight of copolymer (A) before immersion in electrolyte solution) / weight of copolymer (A) before immersion in electrolyte solution]×100 2. The separator for a secondary battery according to claim 1, wherein the separator is determined by the following formula:

5. 2. The secondary battery separator according to claim 1, wherein the copolymer (A) has a glass transition temperature of 40 to 93°C.

6. 2. The secondary battery separator according to claim 1, wherein the thickness of each of the adhesive layers is 5 to 25% of the thickness of the porous substrate.

7. The method for producing a secondary battery separator according to any one of claims 1 to 6, further comprising a step of adhering a resin solution having a solid content of 2 to 40 wt% of the copolymer (A) to at least a part of one surface or both surfaces of the porous substrate.

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