Separator for secondary battery, method of manufacturing the separator, and secondary battery including the separator

Abstract

The present disclosure relates to a separator for a secondary battery, including: a porous polymer substrate; a porous coating layer formed on at least one surface of the porous polymer substrate and including a plurality of inorganic particles and a first binder polymer for interconnecting and fixing the inorganic particles; and a binder layer formed on a top surface of the porous coating layer and including a second binder polymer, wherein the binder layer includes a first layer in contact with the top surface of the porous coating layer, and a second layer integrated with the first layer and facing an electrode, and an average pore diameter of the second layer is greater than an average pore diameter of the first layer. The separator for a secondary battery can improve problems related to electrical resistance while ensuring adhesion with an electrode.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2020-0089137, filed in Korea on July 17, 2020, the disclosure of which is incorporated herein by reference.

[0002] This disclosure relates to a separator for a secondary battery, a method for manufacturing the separator, and a secondary battery including the separator. Background Technology

[0003] Recently, energy storage technology has received increasing attention. As its applications have expanded to power mobile phones, cameras, and laptops, and even electric vehicles, there is a growing need for high-energy-density batteries to power these electronic devices. Because rechargeable batteries are the most suitable for meeting this need, active research has been conducted on them.

[0004] Typically, such a secondary battery includes: a positive electrode containing a positive active material; a negative electrode containing a negative active material; a non-aqueous electrolyte containing an electrolyte salt and an organic solvent; and a separator inserted between the positive and negative electrodes and configured to electrically insulate the two electrodes from each other.

[0005] Ensuring the safety of such secondary batteries is a crucial issue to be addressed in their manufacture and use. Separators typically utilize polyolefin-based porous substrates. However, due to their material properties and manufacturing processes, separators exhibit severe thermal shrinkage at high temperatures, posing safety-related problems such as internal short circuits. Recently, to address these issues, an organic-inorganic composite porous separator has been proposed, featuring a porous coating formed by coating a porous polymer substrate with a mixture of inorganic particles and a binder polymer.

[0006] However, such organic-inorganic composite porous separators have the following problems: due to their material properties, they exhibit insufficient interlayer adhesion when forming electrode assemblies by laminating with electrodes, which may lead to the separation of the electrodes from the separator.

[0007] To address this problem, a method has been developed that involves coating a binder polymer mixture onto a separator and allowing the binder polymer to migrate to the separator surface via a gas-phase induced phase separation process, thereby forming a binder layer with a high binder polymer content near the separator surface.

[0008] However, in this case, there is a problem that the adhesive polymer acts as a resistor, causing the output and cycle characteristics of the battery using this separator to deteriorate.

[0009] Therefore, there is still a need for a technology that can address the aforementioned resistance-related problems while ensuring adequate adhesion to the electrodes even when using porous coatings containing inorganic particles. Summary of the Invention

[0010] Technical issues

[0011] This disclosure aims to address the problems of the prior art, and therefore aims to provide a separator for a secondary battery and a secondary battery including the separator, which, despite using a porous coating including inorganic particles, is still able to ensure adhesion to the electrodes and improve resistance-related problems.

[0012] This disclosure also aims to provide a method for manufacturing a separator for secondary batteries that, despite using a porous coating including inorganic particles, still ensures adhesion to the electrodes and improves resistance-related issues.

[0013] Technical solution

[0014] In one aspect of this disclosure, a separator for a secondary battery is provided according to any of the following embodiments.

[0015] According to a first embodiment, a separator for a secondary battery is provided, comprising:

[0016] Porous polymer substrate;

[0017] A porous coating, said porous coating being formed on at least one surface of said porous polymer substrate, and comprising a plurality of inorganic particles and a first binder polymer for interconnecting and fixing the inorganic particles; and

[0018] An adhesive layer is formed on the top surface of the porous coating and includes a second adhesive polymer.

[0019] The adhesive layer comprises a first layer in contact with the top surface of the porous coating, and a second layer integral with the first layer and facing the electrode.

[0020] The average pore size of the second layer is greater than that of the first layer.

[0021] According to the second embodiment, a separator for a secondary battery as defined in the first embodiment is provided, wherein the ratio of the average pore size of the first layer to the average pore size of the second layer can be 0.75 or less.

[0022] According to a third embodiment, a separator for a secondary battery as defined in the first or second embodiment is provided, wherein the ratio of the average pore size of the first layer to the average pore size of the second layer can be 0.4 or less.

[0023] According to the fourth embodiment, a separator for a secondary battery as defined in any one of the first to third embodiments is provided, wherein the second layer may have an average pore size of 0.1-10 μm.

[0024] According to a fifth embodiment, a separator for a secondary battery as defined in any one of the first to fourth embodiments is provided, wherein the first layer may have an average pore size of 0.1-7 μm.

[0025] According to a sixth embodiment, a separator for a secondary battery as defined in any one of the first to fifth embodiments is provided, wherein the first adhesive polymer may include acrylic polymer, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullullan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullullan, carboxymethyl cellulose. methylcellulose), acrylonitrile-styrene-butadiene copolymer, polyimide, or a mixture of two or more thereof.

[0026] According to the seventh embodiment, a separator for a secondary battery as defined in any one of the first to sixth embodiments is provided, wherein the second adhesive polymer may include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polyvinylidene fluoride-co-tetrafluoroethylene, polyvinylidene fluoride-co-trifluoroethylene, or a mixture of two or more thereof.

[0027] According to the eighth embodiment, a separator for a secondary battery as defined in any one of the first to seventh embodiments is provided, the separator exhibiting an adhesion force to the electrode of 60-250 gf / 25 mm.

[0028] According to the ninth embodiment, a separator for a secondary battery as defined in any one of the first to eighth embodiments is provided, the separator having an air permeability of 10-500s / 100cc.

[0029] According to the tenth embodiment, a separator for a secondary battery as defined in any one of the first to ninth embodiments is provided, the separator exhibiting a resistance of 0.1-3Ω.

[0030] In another aspect of this disclosure, a secondary battery according to the following embodiments is provided.

[0031] According to the eleventh embodiment, a secondary battery is provided, including a positive electrode, a negative electrode, and a separator inserted between the positive electrode and the negative electrode, wherein the separator is the same as the separator defined in any one of the first to tenth embodiments.

[0032] Beneficial effects

[0033] A separator for a secondary battery according to embodiments of this disclosure includes: a porous coating containing inorganic particles; and an adhesive layer comprising a first layer in contact with the top surface of the porous coating and a second layer integral with the first layer and facing the electrode, wherein the average pore size of the second layer is larger than the average pore size of the first layer. In this way, thermal stability can be provided to the separator while addressing resistance-related issues and ensuring adhesion to the electrode.

[0034] The separator for secondary batteries according to embodiments of this disclosure ensures linear propagation of lithium ions by having a first layer with an average pore size smaller than that of the second layer, and can improve the resistance-related problems caused by the adhesive layer.

[0035] Furthermore, the separator for secondary batteries according to the embodiments of this disclosure ensures adhesion between the separator and the electrode by having an average pore size of the second layer that is larger than that of the first layer, and reduces the contact area between the separator and the electrode, thereby improving resistance-related issues.

[0036] A separator for a secondary battery according to an embodiment of the present disclosure includes an adhesive layer in which the average pore size of the second layer is larger than that of the average pore size of the first layer, and thus provides improved air permeability. Attached Figure Description

[0037] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, serve to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure is not limited to the accompanying drawings.

[0038] Figure 1 This is a cross-sectional view showing a separator for a secondary battery according to an embodiment of the present disclosure.

[0039] Figure 2 This is a scanning electron microscope (SEM) image showing the adhesive layer of the separator for a secondary battery obtained according to Example 1.

[0040] Figure 3 This is a SEM image showing the adhesive layer of the separator for a secondary battery obtained according to Example 2.

[0041] Figure 4 This is a SEM image showing the adhesive layer of the separator for a secondary battery obtained according to Example 3.

[0042] Figure 5 This is a SEM image showing the adhesive layer of the separator for a secondary battery obtained according to Example 4.

[0043] Figure 6 This is a SEM image showing the adhesive layer of the separator for a secondary battery obtained according to Comparative Example 1.

[0044] Figure 7 This is a SEM image showing the adhesive layer of the separator for a secondary battery obtained according to Comparative Example 2.

[0045] Figure 8 This is a SEM image showing the adhesive layer of the separator for a secondary battery obtained according to Comparative Example 3. Detailed Implementation

[0046] In the following, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terminology used in the specification and appended claims should not be construed as limited to its common or dictionary meaning, but should be interpreted based on its meaning and concept corresponding to the technical aspects of the present disclosure, on the basis of the principle that the inventors are allowed to appropriately define the terminology for the best interpretation.

[0047] Therefore, the description presented herein is merely a preferred embodiment for illustrative purposes and is not intended to limit the scope of this disclosure. It should be understood that other equivalents and modifications may be made without departing from the scope of this disclosure.

[0048] The specific terminology used in the following description is for ease of description and understanding, and the scope of this disclosure is not limited thereto. The terms "a surface" or "top surface" indicate a position or orientation in the referenced figures and are not intended for limitation. These terms include the words listed above, their derivatives, and their synonyms.

[0049] Throughout the specification, the statement "one layer is disposed on the top surface of another layer" covers not only embodiments where one layer is in contact with a surface of another layer, but also embodiments where there is another layer between the two layers.

[0050] As used herein, the terms “first,” “second,” or similar are used to distinguish one constituent element from another, and each constituent element is not limited to these terms.

[0051] In one aspect of this disclosure, a separator for a secondary battery is provided, comprising:

[0052] Porous polymer substrate;

[0053] A porous coating, said porous coating being formed on at least one surface of said porous polymer substrate, and comprising a plurality of inorganic particles and a first binder polymer for interconnecting and fixing the inorganic particles; and

[0054] An adhesive layer is formed on the top surface of the porous coating and includes a second adhesive polymer.

[0055] The adhesive layer comprises a first layer in contact with the top surface of the porous coating, and a second layer integral with the first layer and facing the electrode.

[0056] The average pore size of the second layer is greater than that of the first layer.

[0057] Figure 1This is a schematic diagram illustrating a separator for a secondary battery according to an embodiment of the present disclosure.

[0058] Reference Figure 1 The separator 1 for the secondary battery is provided with a porous polymer substrate 10.

[0059] According to embodiments of this disclosure, the porous polymer substrate 10 is not particularly limited, as long as it can be conventionally used as a separator material for secondary batteries. The porous polymer substrate can be a thin film comprising a polymer material. Non-limiting examples of polymer materials may include at least one selected from polymer resins such as polyolefin resins, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, and polyvinyl naphthalene. Furthermore, the porous polymer substrate can be a nonwoven web or porous polymer film comprising such polymer material, or a laminate comprising two or more of them. Specifically, the porous polymer substrate can be any of the following a) to e):

[0060] a) A porous membrane formed by melting and extruding a polymer resin;

[0061] b) A multilayer membrane formed by stacking two or more porous membranes from a);

[0062] c) Nonwoven webs formed by integrating filaments obtained from melt / spinned polymer resins;

[0063] d) Multilayer films formed by stacking two or more layers of nonwoven webs from c); and

[0064] e) A porous composite membrane having a multilayer structure including two or more of a) to d).

[0065] According to embodiments of this disclosure, the porous polymer substrate 10 may have a thickness of 5-50 μm. The thickness of the porous polymer substrate is not limited to the range defined above. However, when the thickness is within the range defined above, it is easier to prevent the separator from being easily damaged during battery use. Meanwhile, although there are no particular limitations on the average pore size and porosity of the pores present in the porous polymer substrate, the pore size and porosity can be 0.01-50 μm and 10-95%, respectively.

[0066] As used herein, the term "average aperture" refers to the average aperture value. Aperture refers to the average diameter of a hole, which is the average of the longest and shortest diameters of the hole.

[0067] According to this disclosure, the porosity and average pore size of the porous polymer substrate 10 can be determined using the 6-point BET method based on the nitrogen adsorption-flow method, scanning electron microscopy (SEM) images, a mercury porosimeter, a capillary flow porometer, or a porosimetry analyzer (Bell Japan Inc, Bellorp-II mini).

[0068] Reference Figure 1 The separator 1 for the secondary battery has a porous coating 20 on at least one surface of the porous polymer substrate 10. Specifically, the porous coating 20 may be formed on one or both surfaces of the porous polymer substrate 10.

[0069] The porous coating 20 comprises a plurality of inorganic particles (not shown) and a first adhesive polymer (not shown), the inorganic particles being attached by the first adhesive polymer (not shown) such that the inorganic particles can maintain their bonded state (in other words, the first adhesive polymer connects and fixes the inorganic particles to each other). Furthermore, the inorganic particles and the porous polymer substrate 10 can maintain their bonded state through the first adhesive polymer. The inorganic particles of the porous coating 20 prevent the porous polymer substrate 10 from exhibiting severe thermal shrinkage behavior, thereby providing improved security for the separator.

[0070] There are no particular restrictions on the inorganic particles, as long as they are electrochemically stable. In other words, there are no particular restrictions on the inorganic particles that can be used in this paper, as long as they are within the operating voltage range of the applicable battery (e.g., based on Li / Li). + It is sufficient that it does not cause oxidation and / or reduction reactions within 0-5V.

[0071] According to embodiments of this disclosure, inorganic particles may include high dielectric constant inorganic particles having a dielectric constant of 5 or greater, or 10 or greater. Non-limiting examples of inorganic particles having a dielectric constant of 5 or greater may include those selected from BaTiO3, BaSO4, Pb(Zr,Ti)O3(PZT), and Pb... 1-x La x Zr 1-y Ti y O3(PLZT, where 0 < x < 1 and 0 < y < 1), Pb(Mg) 1 / 3 Nb 2 / 3The invention relates to any one of the following: PbTiO3 (PMN-PT), hafnium dioxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, Y2O3, SiO2, Al2O3, γ-AlOOH, Al(OH)3, SiC, TiO2, or similar substances, or mixtures of two or more thereof. However, the scope of this disclosure is not limited thereto.

[0072] According to another embodiment of this disclosure, inorganic particles may include inorganic particles with lithium-ion transport capability, i.e., inorganic particles containing lithium but not storing lithium but transporting lithium ions. Non-limiting examples of inorganic particles with lithium-ion transport capability may include lithium phosphate (Li3PO4) and lithium titanium phosphate (Li... x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium titanium aluminum phosphate (Li x Al y Ti z (PO4)3, 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) x Oy-based glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Ge y P z S w 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), and P2S5-based glass (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7), or a mixture of two or more of them.

[0073] According to embodiments of this disclosure, while there is no particular limitation on the particle size of the inorganic particles in the porous coating, the average particle size of the inorganic particles can be about 0.01-10 μm or about 0.05-1.0 μm in order to form a coating with uniform thickness and suitable porosity. When the average particle size of the inorganic particles meets the above-defined range, the inorganic particles are more likely to maintain dispersion, thereby helping to control the physical properties of the separator used in lithium secondary batteries. Furthermore, it is easier to avoid an increase in the thickness of the porous coating, thereby improving mechanical properties. Additionally, it is less likely that an internal short circuit will occur during battery charging / discharging due to excessively large pore sizes.

[0074] The term "average particle size of inorganic particles" refers to D 50 Particle size, "D 50 "Particle size" refers to the particle size at the 50% point of the cumulative particle number distribution based on particle size. Particle size can be determined using a laser diffraction method. Specifically, the powder to be analyzed is dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The differences in the diffraction pattern depending on the particle size are then determined as the particles pass through the laser beam, and the particle size distribution is calculated. The particle size at the 50% point of the cumulative particle number distribution based on particle size is then calculated to determine D. 50 .

[0075] According to embodiments of this disclosure, the first adhesive polymer may include an acrylic polymer, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullullan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullullan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, or a mixture of two or more thereof.

[0076] Specifically, according to embodiments of this disclosure, the acrylic polymer may include acrylate polymers. For example, the acrylate polymer may include polybutyl acrylate, ethyl acrylate-N,N-dimethylacrylamide copolymer, ethyl acrylate-2-(dimethylamino)ethyl acrylate copolymer, ethyl acrylate-N,N-diethylacrylamide copolymer, ethyl acrylate-2-(d-ethylamino)ethyl acrylate copolymer, or mixtures of two or more thereof, but are not limited thereto.

[0077] According to embodiments of this disclosure, the weight ratio of inorganic particles to the first binder polymer can be determined by considering the thickness, average pore size, and porosity of the completed porous coating 20. The weight ratio of inorganic particles to the first binder polymer can be 20:80-99.9:0.1 or 50:50-99.5:0.5. When the weight ratio of inorganic particles to the first binder polymer meets the above ranges, it is easier to ensure sufficient voids formed between the inorganic particles, while also ensuring sufficient adhesion between the inorganic particles. Furthermore, the completed porous coating 20 can possess excellent mechanical properties.

[0078] According to embodiments of this disclosure, the thickness of the porous coating 20 can be 1-50 μm, 2-30 μm, or 2-20 μm.

[0079] According to embodiments of this disclosure, the average pore size of the porous coating 20 can be 0.001-10 μm or 0.001-1 μm. Furthermore, the porous coating 20 can have a porosity of 5-95%, 10-95%, 20-90%, or 30-80%. Porosity corresponds to a value obtained by subtracting the volume expressed as the weight and density of each component in the porous coating from the volume calculated from the thickness, width, and length of the porous coating.

[0080] According to this disclosure, the porosity and average pore size of the porous coating 20 can be determined using the 6-point BET method based on the nitrogen adsorption-flow method, scanning electron microscopy (SEM) images, a mercury porosimeter, a capillary flow porometer, or a porosimetry analyzer (Bell Japan Inc, Bellorp-II mini).

[0081] Reference Figure 1 The separator 1 for the secondary battery has an adhesive layer 30 comprising a second adhesive polymer on the top surface of the porous coating 20. The adhesive layer 30 imparts adhesion to the separator 1, allowing the surface of the separator 1 to adhere well to the electrode.

[0082] According to embodiments of this disclosure, the second adhesive polymer may include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polyvinylidene fluoride-co-tetrafluoroethylene, polyvinylidene fluoride-co-trifluoroethylene, or a mixture of two or more thereof.

[0083] Reference Figure 1The adhesive layer 30 includes a first layer 31 in contact with the top surface of the porous coating 20, and a second layer 32 integral with the first layer 31 and facing the electrode. The average pore size of the second layer 32 is larger than the average pore size of the first layer 31.

[0084] In the separator 1 for a secondary battery according to this disclosure, the first layer 31 and the second layer 32 are not formed separately from each other, but are integrally formed to form a single adhesive layer 30. As used herein, the terms "first layer" and "second layer" refer to layers present in the adhesive layer 30 that are different from each other in terms of average pore size.

[0085] According to embodiments of this disclosure, the ratio of the average pore size of the first layer to the average pore size of the second layer can be 0.75 or less, 0.7 or less, or 0.25-0.7, 0.4 or less, 0.25-0.4, or 0.25 or less. When the ratio of the average pore size of the first layer to the average pore size of the second layer meets the above-defined range, the first layer can have a smaller average pore size to provide reduced tortuosity, thus further improving resistance-related problems. Furthermore, the second layer can have a larger average pore size, thereby further ensuring adhesion between the separator and the electrode, and further minimizing the contact area between the separator and the electrode, thus further improving resistance-related problems.

[0086] Specifically, when the ratio of the average pore size of the first layer 31 to the average pore size of the second layer 32 is 0.4 or less, resistance-related issues can be further improved, and the adhesion between the separator and the electrode can be further enhanced.

[0087] In the separator 1 for a secondary battery according to this disclosure, a first layer 31 is formed on the top surface of the porous coating 20 in contact with the top surface of the porous coating 20. Since the average pore size of the first layer 31 is smaller than the average pore size of the second layer 32, linear propagation of lithium ions can be ensured. In other words, reverse transport of lithium ions can be minimized, thereby solving the problem related to the separator's resistance caused by the second adhesive polymer of the adhesive layer 30.

[0088] According to embodiments of this disclosure, the average pore size of the first layer 31 can be 0.1-7 μm, 0.1-5 μm, or 0.5-2 μm. When the average pore size of the first layer 31 meets the above-defined range, the tortuosity can be further reduced to facilitate the improvement of resistance-related problems.

[0089] In the separator 1 for a secondary battery according to this disclosure, a second layer 32 is formed at the portion where the separator adheres to the electrode. The average pore size of the second layer 32 is larger than the average pore size of the first layer 31. The second layer 32 reduces the contact area between the separator and the electrode, thereby improving resistance-related issues while ensuring adhesion between the separator and the electrode.

[0090] According to embodiments of this disclosure, the second layer 32 may have a well-developed Bernard cell pore structure. A Bernard cell pore structure refers to a pore structure formed in a mesh shape through partial openings in the portion including the adhesive polymer. When the second layer 32 is provided with a Bernard cell pore structure, significantly high adhesion between the separator and the electrodes can be achieved.

[0091] According to embodiments of this disclosure, the average pore size of the second layer 32 can be 0.1-10 μm or 1-4 μm. When the average pore size of the second layer meets the above-defined range, the adhesion between the separator and the electrode can be further ensured, and the contact area between the separator and the electrode can be further minimized, thus further improving resistance-related issues.

[0092] Specifically, when the average pore size of the second layer 32 is 1-4 μm, the contact area between the separator and the electrode can be minimized and resistance-related issues can be further improved, while providing higher adhesion between the separator and the electrode.

[0093] Furthermore, when the average pore size of the second layer 32 is 3-4 μm, the Bernard cell pore structure can be well formed to further minimize the contact area between the separator and the electrode and further improve the resistance-related issues, while providing higher adhesion between the separator and the electrode.

[0094] According to embodiments of the present disclosure, when the average pore size of the second layer 32 can be 0.1-10 μm, the average pore size of the first layer 31 can be smaller than the average pore size of the second layer 32 and range from 0.1 μm to 7 μm.

[0095] According to embodiments of this disclosure, the average pore size of the first layer 31 and the second layer 32 can be determined by scanning electron microscopy (SEM) images or by pore size distribution (PSD). For example, the average pore size can be determined using a mercury porosimeter.

[0096] A separator for a secondary battery according to an embodiment of this disclosure includes a first layer in contact with the top surface of a porous coating, and a second layer integral with the first layer and facing the electrode, wherein the average pore size of the second layer is larger than the average pore size of the first layer. In this way, excellent permeability can be provided, resistance-related problems can be improved, and excellent adhesion to the electrode can be achieved.

[0097] According to embodiments of this disclosure, the separator for a secondary battery can exhibit an adhesion force to the electrode of 60-200 gf / 25 mm or 100-200 gf / 25 mm.

[0098] According to embodiments of this disclosure, the adhesion force between the separator and the electrode for a secondary battery can be determined by: manufacturing the electrode, laminating the electrode and the separator at 60°C and 6.54 MPa using a press to obtain a sample, attaching and fixing the sample to a glass plate using double-sided tape, and measuring the strength when peeling the separator portion of the sample at 25°C, a rate of 25 mm / min, and an angle of 180°.

[0099] According to embodiments of this disclosure, the separator for a secondary battery may have a permeability of 10-500s / 100cc, 10-300s / 100cc, 100-300s / 100cc, 200-300s / 100cc, or 200-250s / 100cc.

[0100] According to embodiments of this disclosure, air permeability refers to the Gurley value, and specifically, the ability of 100 cc of air to pass through an area of ​​1 inch of surface area under a pressure of 12.2 inH2O. 2 The permeability of a separator is the time (in seconds) required for the separator cross-section to be permeable, i.e., the air permeability time. The air permeability of separators used in secondary batteries can be determined according to the method of ASTM D726-94.

[0101] According to embodiments of this disclosure, the separator for a secondary battery may display a resistance of 0.1-3Ω, 0.1-2Ω, or 0.5-1Ω.

[0102] According to embodiments of this disclosure, the resistance can be calculated by manufacturing a coin cell and measuring the impedance of the separator as a function of frequency using electrochemical impedance spectroscopy (EIS).

[0103] The separator for a secondary battery according to embodiments of this disclosure can be manufactured by, but is not limited to, the following methods.

[0104] A method for manufacturing a separator for a secondary battery according to embodiments of this disclosure includes the following steps:

[0105] Preparation of porous polymer substrates;

[0106] A slurry comprising inorganic particles, a first binder polymer, and a solvent for the first binder polymer is coated onto at least one surface of a porous polymer substrate and then dried to form a porous coating.

[0107] A coating solution comprising a second adhesive polymer, a solvent for the second adhesive polymer, and a non-solvent for the second adhesive polymer is applied to the top surface of the porous coating; and

[0108] While maintaining a relative humidity of 35-40%, dry the coating solution to form an adhesive layer.

[0109] The boiling point of the non-solvent is different from that of water.

[0110] In the following text, a method for manufacturing a separator for a secondary battery according to an embodiment of the present disclosure will be described in detail with reference to its key parts.

[0111] First, a porous polymer substrate is prepared. The porous polymer substrate can be one of those described above. The porous polymer substrate can be obtained from the aforementioned material by forming pores therein through methods known to those skilled in the art for ensuring excellent air permeability and porosity, such as wet processes using solvents, diluents, or pore-forming agents, or dry processes using orientation processes.

[0112] Next, a slurry comprising inorganic particles, a first binder polymer, and a solvent for the first binder polymer is coated onto at least one surface of a porous polymer substrate and then dried to form a porous coating.

[0113] According to embodiments of this disclosure, a slurry can be coated on one or both surfaces of a porous polymer substrate.

[0114] According to embodiments of this disclosure, reference will be made to the above description regarding the types of inorganic particles and the first binder polymer, as well as the weight ratio of the inorganic particles to the first binder polymer.

[0115] Here, the solvent used for the first adhesive polymer can be used as a solvent capable of dissolving the first adhesive polymer, or as a dispersion medium that does not dissolve the first adhesive polymer but disperses it, depending on the specific type of the first adhesive polymer. For example, the solvent used for the first adhesive polymer may include acetone, methyl ethyl ketone (MEK), tetrahydrofuran (THF), methylene chloride (MC), chloroform, dimethyl formamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), cyclohexane, water, methanol, ethanol, propanol, isopropanol, butanol, or mixtures of two or more of these.

[0116] According to embodiments of this disclosure, a slurry can be prepared by dissolving or dispersing a first adhesive polymer in a solvent for the first adhesive polymer, adding inorganic particles thereto, and dispersing them. The inorganic particles can be pre-crushed to a predetermined average particle size before being added. Alternatively, the inorganic particles can be added to a solution containing the first adhesive polymer dissolved therein, then crushed and dispersed, while controlling their predetermined average particle size using a ball milling process or a similar process.

[0117] According to embodiments of this disclosure, there are no particular limitations on the method of forming a porous coating by coating a slurry onto at least one surface of a porous polymer substrate. Specific examples of the method may include dip coating, die coating, roll coating, comma coating, blade coating, reverse roll coating, and direct roll coating or similar methods.

[0118] Phase separation processes can be performed to form pore structures with higher quality in porous coatings. Phase separation processes refer to the processes used to form pore structures in porous coatings through phase separation phenomena known in the art.

[0119] According to one embodiment, the drying step can be performed using a drying process typically used in the manufacture of separators. For example, the coated slurry can be dried in air for 3-45 seconds or 5-40 seconds. When the drying time is within the range defined above, it is easier to remove residual solvents used for the first adhesive polymer without reducing productivity.

[0120] Next, a coating solution comprising a second adhesive polymer, a solvent for the second adhesive polymer, and a non-solvent for the second adhesive polymer is applied to the top surface of the porous coating.

[0121] According to existing technology, a coating solution comprising an adhesive polymer is applied to at least one surface of a porous polymer substrate to form an adhesive layer. Consequently, the coating solution penetrates into the pores of the porous polymer substrate, leading to an increase in the resistance of the separator.

[0122] Conversely, in the method of manufacturing a separator for a secondary battery according to an embodiment of this disclosure, a coating solution comprising a second binder polymer is coated onto the top surface of a porous coating comprising inorganic particles. Therefore, the coating solution is less likely to penetrate into the pores of the porous polymer substrate. Consequently, resistance-related problems can be improved.

[0123] According to embodiments of this disclosure, reference will be made to the above description of the specific type of the second adhesive polymer. Based on 100% by weight of the solvent used for the second adhesive polymer, the second adhesive polymer can be used in amounts of 0.1-20% by weight or 1-10% by weight. When the second adhesive polymer is used in the coating solution within the range defined above, the separator can easily ensure sufficient adhesion to the electrode.

[0124] According to this disclosure, the solvent used for the second adhesive polymer refers to a liquid capable of dissolving the second adhesive polymer. According to embodiments of this disclosure, the solvent used for the second adhesive polymer may include acetone, methyl ethyl ketone (MEK), tetrahydrofuran (THF), methylene chloride (MC), chloroform, dimethyl formamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), cyclohexane, or mixtures of two or more of these.

[0125] According to this disclosure, the non-solvent used for the second adhesive polymer does not dissolve the second adhesive polymer and has a boiling point different from that of water. Because a non-solvent for the second adhesive polymer with a boiling point different from that of water is used, phase separation may occur between water and the non-solvent for the second adhesive polymer during the formation of the adhesive layer. In this way, the adhesive layer may include a first layer in contact with the top surface of the porous coating, and a second layer integral with the first layer, the second layer facing the electrode and having an average pore size larger than that of the first layer.

[0126] In existing methods for manufacturing separators for secondary batteries, the adhesive layer used to adhere the separator to the electrodes is formed by using gas-phase induced phase separation. Here, the term "gas-phase induced phase separation" refers to phase separation that occurs when a coating solution comprising an adhesive polymer and coated onto the separator surface is exposed to an environment rich in water, which serves as a non-solvent for the adhesive polymer. However, when this adhesive layer is formed solely by using gas-phase induced phase separation, i.e., by controlling humidity, numerous processing-related problems arise.

[0127] Conversely, in a method for manufacturing a separator for a secondary battery according to an embodiment of this disclosure, a vapor-induced phase separation is performed on a coating solution containing a non-solvent for a second binder polymer added thereto, such that phase separation can occur not only through water (moisture) but also through the non-solvent for the second binder polymer, thereby promoting the formation of an adhesive layer. The vapor-induced phase separation step will be described below. When a coating solution including a non-solvent for the second binder polymer is applied to the top surface of a porous coating and vapor-induced phase separation is performed, an adhesive layer can be formed comprising layers that differ in average pore size according to the difference in boiling points between water and the non-solvent for the second binder polymer. Therefore, resistance-related problems can be improved while ensuring adhesion between the separator and the electrodes.

[0128] According to embodiments of this disclosure, the boiling point of the non-solvent used in the second adhesive polymer may be lower than that of water.

[0129] Because water is applied to the outermost surface layer of the separator via gas-phase induced phase separation, the water content is higher than the non-solvent content of the second binder polymer in the electrode-facing portion, and the non-solvent content of the second binder polymer is higher than the water content in the portion of the binder layer in contact with the top surface of the porous coating. Since water has a higher boiling point and evaporates more slowly than the non-solvent used in the second binder polymer, a second layer with a relatively large average pore size can be formed in the electrode-facing portion. Simultaneously, since the non-solvent used in the second binder polymer has a lower boiling point and evaporates more rapidly than water, a first layer with a relatively small average pore size can be formed in the portion in contact with the top surface of the porous coating.

[0130] According to embodiments of this disclosure, the non-solvent used for the second adhesive polymer can be an alcohol. According to embodiments of this disclosure, the alcohol may include methanol, ethanol, propanol, isopropanol, butanol, or a mixture of two or more of these.

[0131] According to embodiments of this disclosure, based on the weight of the non-solvent when the second adhesive polymer gels in the coating solution, the non-solvent used for the second adhesive polymer can be present in the coating solution in amounts of 30-60% by weight, 35-55% by weight, or 40-60% by weight. When the non-solvent is used within the above-defined range, an appropriate ratio of the average pore size of the first layer to the average pore size of the second layer can be readily provided. For example, it can be helpful to set the ratio of the average pore size of the first layer to the average pore size of the second layer in the range of 0.4 or less. This can also facilitate the formation of a Bernard cell pore structure in the second layer.

[0132] Therefore, by making the average pore size of the first layer smaller than that of the second layer, resistance-related problems can be further improved by reducing tortuosity. By making the average pore size of the second layer larger than that of the first layer, resistance-related problems can also be easily improved by minimizing the contact area between the separator and the electrode while ensuring adhesion between them.

[0133] Specifically, when the amount of non-solvent used for the second adhesive polymer is 40-60% by weight, based on the weight of the non-solvent when the second adhesive polymer gels in the coating solution, the most suitable ratio of the average pore size of the first layer to the average pore size of the second layer can be provided.

[0134] According to this disclosure, the statement "weight of non-solvent when the second adhesive polymer gels in the coating solution" refers to the weight of non-solvent when precipitation occurs in the coating solution due to the gelation of the second adhesive polymer caused by the addition of non-solvent. Furthermore, when the coating solution is placed on a balance and non-solvent is continuously added to the coating solution, the weight can be determined by measuring the weight of non-solvent at the time point at which the clouding point occurs.

[0135] According to embodiments of this disclosure, based on 100% by weight of the solvent for the second adhesive polymer, the weight of the non-solvent can be 10-30% by weight, 10-20% by weight, or 13-17% by weight when the second adhesive polymer gels in the coating solution.

[0136] For example, when the solvent used for the second adhesive polymer is 5-10% by weight based on 100% by weight, the weight of the non-solvent can be 13-17% by weight when the second adhesive polymer gels in the coating solution.

[0137] According to embodiments of this disclosure, the volume ratio of the non-solvent used in the second adhesive polymer to the solvent used in the second adhesive polymer can be 20:80 to 60:40. When the volume ratio of the non-solvent used in the second adhesive polymer to the solvent used in the second adhesive polymer meets the above-defined range, the separator can further ensure sufficient adhesion to the electrode and have suitable pore size and porosity, thereby helping to improve resistance-related problems.

[0138] Next, while maintaining a relative humidity of 35-40%, dry the coating solution to form an adhesive layer.

[0139] In the method for manufacturing a separator for a secondary battery according to this disclosure, the step of drying the coating solution while maintaining a relative humidity of 35-40% is a process of forming pores in the adhesive layer using gas-phase induced phase separation. When phase separation of the coating solution is performed using gas-phase induced phase separation, phase separation can be performed in such a way that a concentration gradient of the second adhesive polymer can be formed in the thickness direction of the separator, and the second adhesive polymer can be present in a larger quantity on the separator surface to which the separator adheres to the electrode.

[0140] According to embodiments of this disclosure, water can be introduced in a gaseous state during the step of drying the coating solution while maintaining a relative humidity of 35-40%. When water is introduced and added in a gaseous state, phase separation can be achieved using a smaller amount of water, and the drying of the coating solution can be promoted.

[0141] Here, the temperature at which water is added in a gaseous or droplet state can be 15-70°C. When the temperature for adding water in a gaseous state meets the above-defined range, the water is more likely to remain in a gaseous state, ensuring the drying rate of the coating solution and thus improving productivity.

[0142] During the drying of the coating solution while maintaining a relative humidity of 35-40%, phase separation occurs in a water atmosphere where the water vapor pressure becomes 35-40% of the saturated vapor pressure. When the water vapor pressure meets the aforementioned defined range, the second binder polymer can be distributed in sufficient quantity on the surface of the separator, ensuring adhesion between the separator and the electrode. Furthermore, the Bernard cell pore structure can be well formed.

[0143] When the water vapor pressure meets the aforementioned limits, it is easier to prevent problems such as reduced adhesion between the porous coating (including inorganic particles) and the adhesive layer caused by excessive humidification levels and excessive migration of the second adhesive polymer toward the separator surface. Therefore, it is easier to ensure adequate adhesion between the separator and the electrodes.

[0144] Furthermore, it is easier to achieve a ratio of the average pore size of the first layer to the average pore size of the second layer of 0.25-0.4, and it is also easier to achieve an average pore size of 1-4 μm for the second layer.

[0145] Conversely, when the vapor pressure of water is less than 35%, this insufficient humidification level allows phase separation to occur solely through the non-solvent used in the second binder polymer. Therefore, it is difficult to form an binder layer comprising layers with varying average pore sizes.

[0146] The separator for a secondary battery, as described above, can be inserted between the positive and negative terminals to obtain a secondary battery.

[0147] Preferably, the secondary battery can be a lithium secondary battery. Lithium secondary batteries may include lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, lithium-ion polymer secondary batteries, or similar types.

[0148] There are no particular limitations on the electrodes used in combination with the separator according to this disclosure, and they can be obtained by bonding a layer of electrode active material, including electrode active material, conductive material and adhesive, to the electrode current collector via methods known in the art.

[0149] In electrode active materials, non-limiting examples of positive electrode active materials may include, but are not limited to: layered compounds, such as lithium cobalt composite oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or those compounds substituted with one or more transition metals; lithium manganese oxides, such as those composed of the chemical formula Li... 1+x Mn 2-xThose represented by O4 (where x is 0-0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxides (Li2CuO2); vanadium oxides, such as LiV3O8, LiV3O4, V2O5, or Cu2V2O7; and those represented by the chemical formula LiNi. 1-x M x Ni-site type lithium nickel oxide represented by O2 (where M is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x is 0.01-0.3); LiMn 2-x M x Lithium manganese composite oxides represented by O2 (where M is Co, Ni, Fe, Cr, Zn or Ta, and x is 0.01-0.1) or Li2Mn3MO5 (where M = Fe, Co, Ni, Cu or Zn); LiMn2O4, wherein Li is partially replaced by alkaline earth metal ions; disulfides; Fe2(MoO4)3; or similar.

[0150] Non-limiting examples of negative electrode active materials include conventional negative electrode active materials that can be used as negative electrodes in conventional electrochemical devices. Specific examples of negative electrode active materials may include lithium-intercalating materials such as lithium metal or lithium alloys, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials.

[0151] Non-limiting examples of a positive current collector may include a foil made of aluminum, nickel, or a combination thereof. Non-limiting examples of a negative current collector may include a foil made of copper, gold, nickel, copper alloys, or a combination thereof.

[0152] According to embodiments of this disclosure, the conductive materials used in the negative and positive electrodes are typically added in an amount of 1-30% by weight based on the total weight of each active material layer. There are no particular limitations on the conductive materials, as long as they do not cause chemical changes in the corresponding battery and are conductive. For example, conductive materials may include: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lampblack, or thermal black; conductive fibers, such as carbon fibers or metal fibers; fluorocarbons; metal powders, such as aluminum powder or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0153] According to embodiments of this disclosure, the binder used in the negative and positive electrodes is a component that facilitates the bonding between the electrode active material and the conductive material, as well as the bonding with the current collector. Typically, the binder is added in an amount of 1-30% by weight based on the total weight of each active material layer. Specific examples of binders may include polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers, or the like.

[0154] According to embodiments of this disclosure, the secondary battery includes an electrolyte, which may include an organic solvent and a lithium salt. Furthermore, the electrolyte may include an organic solid electrolyte or an inorganic solid electrolyte.

[0155] Specific examples of organic solvents may include aprotic organic solvents such as N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butenyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, ethyl propionate, or the like.

[0156] Lithium salts are materials that can be readily dissolved in non-aqueous electrolytes; specific examples include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB. 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborate, lower aliphatic carboxylic acids, lithium tetraphenylborate, imide or similar.

[0157] Furthermore, to improve charge / discharge characteristics, flame retardancy, etc., the electrolyte may further include pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-ethylene glycol dimethyl ether (glyme), triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride. Optionally, to impart non-flammability, the electrolyte may further include halogen-containing solvents, such as carbon tetrachloride or trifluoroethylene. To improve high-temperature storage characteristics, the electrolyte may further include carbon dioxide gas.

[0158] Specific examples of organic solid electrolytes may include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, agitation lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ionizable dissociable groups, or the like.

[0159] Specific examples of inorganic solid electrolytes may include nitrides, halides and sulfates of Li, such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH and Li3PO4-Li2S-SiS2.

[0160] Depending on the manufacturing process of the final product and the required performance of the final product, the electrolyte can be injected at an appropriate step in the process of manufacturing the battery. In other words, the electrolyte can be injected before battery assembly or as a final step in battery assembly.

[0161] According to embodiments of this disclosure, in addition to conventional winding processes, separators for secondary batteries can also be applied to batteries by lamination, stacking, and folding of the separators and electrodes.

[0162] According to embodiments of this disclosure, a separator for a secondary battery can be inserted between the positive and negative electrodes. When an electrode assembly is formed by assembling multiple batteries or electrodes, the separator can be inserted between adjacent batteries or electrodes. The electrode assembly can have various structures, such as simple stacked, jelly roll, stack-folded, laminated-stacked, or similar.

[0163] This is the method of disclosing the content.

[0164] Embodiments will be described more fully below to facilitate a clear understanding of this disclosure. However, the following embodiments may be embodied in many different forms and should not be construed as limited to the exemplary implementations set forth herein. Rather, these exemplary implementations are provided to make this disclosure thorough and complete, and to fully convey the scope of this disclosure to those skilled in the art.

[0165] Example 1

[0166] An adhesive polymer solution was prepared by adding 5 parts by weight of an acrylic copolymer (Toyo Chem, CSB-130) as the first adhesive polymer to water and stirring the resulting mixture at 25°C for approximately 2 hours. Next, 95 parts by weight of Al(OH)3 with an average particle size of 500 nm were added to the resulting adhesive polymer solution and subsequently dispersed to prepare a slurry. Here, the weight ratio of inorganic particles to the first adhesive polymer in the slurry was 95:5.

[0167] The prepared slurry was coated onto one surface of a 9 μm thick porous polyethylene substrate using a gravure coating process, and the coating thickness was controlled to be about 2 μm, thereby obtaining a separator substrate with a porous coating on one surface.

[0168] Next, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) as the second adhesive polymer is added to a mixture of acetone as the solvent for the second polymer and ethanol as the non-solvent for the second adhesive polymer, such that the content of the second adhesive polymer is 5% by weight based on 100% by weight of the solvent for the second adhesive polymer, and dissolved therein at 25°C for about 30 minutes to obtain a coating solution. Here, considering that when PVDF-HFP gels in the coating solution, the content of ethanol is 15% by weight based on 100% by weight of the solvent for the second adhesive polymer, the content of ethanol added to the coating solution is 7.5% by weight based on 100% by weight of the solvent for the second adhesive polymer.

[0169] The prepared coating solution is applied to the top surface of the porous coating of the separator substrate by a gravure coating process, and then dried at 40-60°C while maintaining a relative humidity of 35% to obtain a separator with an adhesive layer formed on the top surface of the porous coating of the separator substrate.

[0170] In the resulting separator, the thicknesses of the porous coating and the adhesive layer are 2 μm and 1 μm, respectively.

[0171] Example 2

[0172] The separator for the secondary battery was obtained in the same manner as in Example 1, except that: considering that when PVDF-HFP gels in the coating solution, the content of ethanol is 15% by weight based on 100% by weight of the solvent for the second binder polymer, the content of ethanol added to the coating solution is 1.5% by weight based on 100% by weight of the solvent for the second binder polymer.

[0173] Example 3

[0174] The separator for the secondary battery was obtained in the same manner as in Example 1, except that: considering that when PVDF-HFP gels in the coating solution, the content of ethanol is 15% by weight based on 100% by weight of the solvent for the second binder polymer, the content of ethanol added to the coating solution is 4.5% by weight based on 100% by weight of the solvent for the second binder polymer.

[0175] Example 4

[0176] The separator for the secondary battery was obtained in the same manner as in Example 1, except that: considering that when PVDF-HFP gels in the coating solution, the content of ethanol is 15% by weight based on 100% by weight of the solvent for the second binder polymer, the content of ethanol added to the coating solution is 10.5% by weight based on 100% by weight of the solvent for the second binder polymer.

[0177] Comparative Example 1

[0178] A separator for a secondary battery was obtained in the same manner as in Example 1, except that ethanol was not added and the coating solution was prepared by dissolving PVDF-HFP, which is a second binder polymer, in acetone, which is a solvent.

[0179] In the resulting separator, the thicknesses of the porous coating and the adhesive layer are 2 μm and 1 μm, respectively.

[0180] Comparative Example 2

[0181] A separator for a secondary battery was obtained in the same manner as in Example 1, except that the gas-induced phase separation step of drying while maintaining a specific relative humidity range was not performed.

[0182] In the resulting separator, the thicknesses of the porous coating and the adhesive layer are 2 μm and 1 μm, respectively.

[0183] Comparative Example 3

[0184] A separator for a secondary battery was obtained in the same manner as in Example 1, except that a gas-phase induced phase separation step was performed while maintaining a relative humidity of 25%.

[0185] In the resulting separator, the thicknesses of the porous coating and the adhesive layer are 2 μm and 1 μm, respectively.

[0186] Test Example 1: Pore Structure Analysis of the Adhesive Layer of the Partition

[0187] Figure 2-7 These are scanning electron microscope (SEM) images of the adhesive layer of the partitions according to Examples 1-4 and Comparative Examples 1-3.

[0188] from Figure 1 and Figure 2-5 It can be seen that each adhesive layer of the separator according to Examples 1-4 is provided with two layers with different average pore sizes, and the average pore size of the second layer facing the electrode is larger than the average pore size of the first layer in contact with the top surface of the porous coating.

[0189] Specifically, from Figure 2 and Figure 4 As can be seen, each adhesive layer of the separator according to Examples 1 and 3 exhibits a well-developed Bernard cell pore structure in the second layer facing the electrode.

[0190] On the contrary, from Figure 6 It can be seen that the adhesive layer of the partition according to Comparative Example 1 does not have layers with different average pore sizes.

[0191] from Figure 7 It can be seen that the adhesive layer of the partition according to Comparative Example 2 also does not have a layer with a different average pore size.

[0192] from Figure 8 It can be seen that the adhesive layer of the separator according to Comparative Example 3 also does not have layers with different average pore sizes. Furthermore, the adhesive layer of the separator according to Comparative Example 3 does not have a well-developed Bernard cell pore structure.

[0193] Test Example 2: Physical Performance Analysis of the Partition

[0194] In the adhesive layer of each separator according to Examples 1-4 and Comparative Examples 1-3, the average pore size of the second layer facing the electrode, the average pore size of the first layer in contact with the top surface of the porous coating, air permeability, electrical resistance, and adhesion to the electrode were measured. The results are shown in Table 1 below.

[0195] (1) Determination of the average pore size of the first and second layers

[0196] The average pore size of the first and second layers of the adhesive layer of each partition according to Examples 1-4 was determined by a mercury porosimeter.

[0197] The average pore size of the adhesive layer of each partition according to Comparative Examples 1-3 was determined by a mercury porosimeter.

[0198] (2) Measurement of air permeability

[0199] Air permeability (Gurley value) was determined using the method defined in ASTM D726-94. Here, the air permeability value was determined as 100 cc of air passing through a 1-inch section of the partition according to each of Examples 1-4 and Comparative Examples 1-3 at a pressure of 12.2 in H2O. 2 The time (in seconds) required for the cross-section to be ventilated is the air permeability time.

[0200] (3) Measurement of resistance

[0201] The resistance of the separators for secondary batteries according to Examples 1-4 and Comparative Examples 1-3 was determined as follows. A coin cell was obtained using each separator, and the impedance of each separator was then determined under predetermined multi-frequency conditions using frequency-dependent electrochemical impedance spectroscopy (EIS). The coin cell was manufactured as follows.

[0202] Manufacturing of negative electrode

[0203] First, artificial graphite as the negative electrode active material, denka black as the conductive material, and polyvinylidene fluoride (PVDF) as the binder are mixed in a weight ratio of 75:5:20, and N-methylpyrrolidone as the solvent is added to the resulting mixture to prepare a negative electrode slurry.

[0204] The negative electrode slurry was coated onto the copper current collector to a capacity of 3.8 mAh / cm². 2 The loading is then dried to obtain the negative electrode.

[0205] Manufacturing of the positive electrode

[0206] First, LiCoO2 (as the positive electrode active material), Denka black (as the conductive material), and polyvinylidene fluoride (PVDF) (as the binder) were mixed in a weight ratio of 85:5:10. The resulting mixture was then added to N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode active material slurry. This positive electrode active material slurry was coated onto a sheet aluminum current collector and subsequently dried to form a positive electrode active material layer, resulting in a final positive electrode loading of 3.3 mAh / cm³. 2 .

[0207] Manufacturing of coin batteries

[0208] A separator for a secondary battery according to each of Examples 1-4 and Comparative Examples 1-3 was inserted between the negative and positive electrodes obtained as described above, and a non-aqueous electrolyte (1M LiPF6, ethylene carbonate (EC) / propylene carbonate (PC) / diethyl carbonate (DEC), volume ratio = 3:3:4) was injected therein to obtain a coin battery.

[0209] (4) Determination of adhesion to the electrode

[0210] The adhesion between the separator and the electrode in each of Examples 1-4 and Comparative Examples 1-3 was measured as follows.

[0211] First, natural graphite, SBR, CMC, and conductive materials (weight ratio 90:2.5:2.5:5) are introduced into water to obtain a negative electrode slurry, and the negative electrode slurry is then mixed with water at a concentration of 5 mg / cm³. 2 The loading was coated onto copper foil (thickness: 20 μm) and then dried. The resulting structure was then pressed at 90 °C and 8.5 MPa and cut into 60 mm (length) × 25 mm (width) dimensions to obtain the negative electrode.

[0212] The separators obtained from Examples 1-4 and Comparative Examples 1-3 were cut into 70 mm (length) × 25 mm (width) dimensions, and each separator was laminated with a negative electrode using a press at 60°C and 6.54 MPa to obtain a sample. The prepared sample was attached and fixed to the glass plate with the negative electrode facing the glass plate using double-sided adhesive tape. Then, the separator portion of the sample was peeled off at 25°C at an angle of 180° and a rate of 25 mm / min, and the strength was measured.

[0213] [Table 1]

[0214]

[0215]

[0216] As can be seen from Table 1, the adhesive layer of the separator according to each of Examples 1-4 is provided with layers of different average pore sizes formed by phase separation caused by the non-solvent and water of the second adhesive polymer, and the average pore size of the second layer facing the electrode is larger than the average pore size of the first layer in contact with the top surface of the porous coating. Furthermore, the separator ensures sufficient adhesion to the electrode and exhibits excellent permeability and electrical resistance characteristics.

[0217] Furthermore, compared to Examples 2 and 4 (which included non-solvents in amounts of 10 wt% and 70 wt% respectively, based on the weight of non-solvents during gelation of PVDF-HFP in each coating solution), Examples 1 and 3 (which included non-solvents in amounts of 30 wt% and 50 wt% respectively, based on the weight of non-solvents during gelation of PVDF-HFP in each coating solution) showed a lower ratio of the average pore size of the first layer to the average pore size of the second layer, i.e., 0.4 or less. In addition, Examples 1 and 3 achieved greater improvement in resistance. Furthermore, it can be seen that Example 1 provided better results in terms of resistance and adhesion to the electrode compared to Example 3.

[0218] In contrast, the adhesive layer of the separator according to Comparative Example 1 comprises an adhesive layer formed solely by gas-phase induced phase separation, i.e., it does not contain water or any non-solvents in the coating solution. Therefore, the adhesive layer does not have layers with varying average pore sizes. Consequently, the separator exhibits significantly higher permeability time and electrical resistance.

[0219] The adhesive layer of the separator according to Comparative Example 2 comprises an adhesive layer formed solely by phase separation induced by a non-solvent-based polymer used for the second adhesive, rather than by gas-phase induced phase separation (i.e., water). Therefore, the adhesive layer does not have a layer with a different average pore size. Furthermore, it can be seen that the separator cannot ensure adequate adhesion to the electrode and exhibits high resistance and permeability time.

[0220] In the case of the adhesive layer of the separator according to Comparative Example 3, the humidification was too low, resulting in almost no phase separation, and phase separation occurred only through the non-solvent (ethanol) used for the second adhesive polymer. Therefore, the adhesive layer lacked layers with varying average pore sizes. Furthermore, it was observed that the separator exhibited a high permeability time and could not ensure adequate adhesion to the electrodes.

Claims

1. A separator for a secondary battery, comprising: a porous polymer substrate; a porous coating layer formed on at least one surface of the porous polymer substrate and including a plurality of inorganic particles and a first binder polymer for interconnecting and fixing the inorganic particles; and a binder layer formed on a top surface of the porous coating layer and including a second binder polymer, wherein the binder layer includes a first layer in contact with the top surface of the porous coating layer, and a second layer integrated with the first layer and facing an electrode, an average pore diameter of the second layer is greater than an average pore diameter of the first layer, and wherein a ratio of the average pore diameter of the first layer to the average pore diameter of the second layer is 0.4 or less. 2.The separator for a secondary battery according to claim 1, wherein the second layer has an average pore diameter of 0.1-10 µm. 3.The separator for a secondary battery according to claim 1, wherein the first layer has an average pore diameter of 0.1-7 µm. 4.The separator for a secondary battery according to claim 1, wherein the first binder polymer includes an acrylic polymer, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, or a mixture of two or more thereof. 5.The separator for a secondary battery according to claim 1, wherein the second binder polymer includes polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polyvinylidene fluoride-co-tetrafluoroethylene, polyvinylidene fluoride-co-trifluoroethylene, or a mixture of two or more thereof. 6.The separator for a secondary battery according to claim 1, which shows an adhesion force to an electrode of 60-250 gf / 25 mm. 7.The separator for a secondary battery according to claim 1, which has a gas permeability of 10-500 s / 100 cc. 8.The separator for a secondary battery according to claim 1, which shows an electrical resistance of 0.1-3 Ω. a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is identical to the separator as defined in any one of claims 1 to 8.

9. A secondary battery comprising: ​

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