Separator for secondary battery, method for manufacturing the same, method for manufacturing secondary battery including the separator, and secondary battery manufactured by the method

Abstract

The present disclosure relates to a separator for a secondary battery, including: a porous polymer substrate; a first layer formed on at least one surface of the porous polymer substrate and including inorganic particles and a non-particulate acrylic polymer having a glass transition temperature of 15°C or less, wherein the non-particulate acrylic polymer connects and fixes the inorganic particles; and a second layer formed on an upper surface of the first layer and including a particulate acrylic polymer having a glass transition temperature of 20°C to 50°C. The separator for a secondary battery according to the present disclosure has good adhesion to an electrode and can solve a problem of electrical resistance in the presence of inorganic particles.

Description

TECHNICAL FIELD

[0001] This application claims priority to Korean Patent Application No. 10-2020-0089709, filed on July 20, 2020, in the Republic of Korea, the disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to a separator for a secondary battery, a method of manufacturing the same, a method of manufacturing a secondary battery including the same, and a secondary battery manufactured by the method. BACKGROUND

[0003] Recently, energy storage technology has been increasingly attracting attention. As the application field of the energy storage technology has been expanded to mobile phones, camcorders, notebook computers, and even electric vehicles, the demand for high energy density batteries used as power sources for electronic devices is growing. Secondary batteries are the best batteries to meet the demand, and much research is being conducted on secondary batteries.

[0004] In general, a secondary battery includes a cathode containing a cathode active material, an anode containing an anode active material, a non-aqueous electrolyte containing an electrolyte salt and an organic solvent, and a separator interposed between the cathode and the anode to electrically separate them.

[0005] In the manufacturing and use of secondary batteries, it is challenging to ensure the safety of the secondary batteries. The separator generally uses a polyolefin-based porous substrate, and due to its material properties and process properties, the separator exhibits a severe thermal shrinkage behavior under high temperature conditions, leading to safety problems such as internal short circuit. Recently, to solve this problem, an organic-inorganic composite porous separator including a porous polymer substrate coated with a mixture of inorganic particles and a binder polymer has been proposed.

[0006] However, the organic-inorganic composite porous separator has insufficient interlayer adhesion due to its material properties in the step of stacking electrodes to form an electronic assembly, and there is a risk that the separator and the electrodes can be separated from each other.

[0007] To solve this problem, a technology of coating a mixture of a binder polymer on a separator and moving the binder polymer to the surface of the separator by a vapor-induced phase separation method to form a binder layer containing a high content of the binder polymer near the surface of the separator has been developed.

[0008] However, the vapor-induced phase separation method is difficult to adjust humidity and thus to ensure processability, and the use of an organic solvent increases environmental problems. In addition, the binder polymer dissolved in the organic solvent penetrates into the pores of the porous polymer substrate.

[0009] Therefore, there is still a high need for a separator for a secondary battery that can solve the above problems in the presence of inorganic particles and has good adhesion to electrodes. SUMMARY

[0010] TECHNICAL PROBLEM

[0011] The present disclosure relates to providing a separator for a secondary battery having good adhesion to an electrode in the presence of inorganic particles and a method of manufacturing the same.

[0012] The present disclosure further relates to providing a method of manufacturing a secondary battery including the separator and a secondary battery manufactured by the method.

[0013] TECHNICAL SOLUTION

[0014] To solve the above problems, according to one aspect of the present disclosure, there is provided a separator for a secondary battery of the following embodiments.

[0015] A first embodiment relates to a separator for a secondary battery, including: a porous polymer substrate; a first layer formed on at least one surface of the porous polymer substrate and including inorganic particles and a non-particulate acrylic polymer having a glass transition temperature of 15°C or less, wherein the non-particulate acrylic polymer connects and fixes the inorganic particles; and a second layer formed on an upper surface of the first layer and including a particulate acrylic polymer having a glass transition temperature of 20°C to 50°C.

[0016] In the first embodiment, according to a second embodiment,

[0017] The non-particulate acrylic polymer can have a glass transition temperature of 0°C or less.

[0018] In the first or second embodiment, according to a third embodiment,

[0019] The non-particulate acrylic polymer can include a repeating unit derived from a first monomer and a repeating unit derived from a second monomer, and the first monomer can have a higher glass transition temperature than the second monomer.

[0020] In the third embodiment, according to a fourth embodiment,

[0021] The first monomer can include at least one of methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, or t-butyl methacrylate.

[0022] In the third or fourth embodiment, according to a fifth embodiment,

[0023] The second monomer can include at least one of 2-ethylhexyl methacrylate, dodecyl methacrylate, or octadecyl methacrylate.

[0024] In any one of the third to fifth embodiments, according to a sixth embodiment,

[0025] The repeating units derived from the second monomer can be included in an amount of 60% by weight or more, based on 100% by weight of the non-particulate acrylic polymer.

[0026] In any one of the first to sixth embodiments, according to a seventh embodiment,

[0027] The particulate acrylic polymer can include repeating units derived from a third monomer and repeating units derived from a fourth monomer, and the glass transition temperature of the third monomer can be higher than the glass transition temperature of the fourth monomer.

[0028] In the seventh embodiment, according to an eighth embodiment,

[0029] The third monomer can include at least one of styrene, vinyl acetate, or acrylonitrile.

[0030] In the seventh or eighth embodiment, according to a ninth embodiment,

[0031] The fourth monomer can include at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, or ethylene.

[0032] In any one of the seventh to ninth embodiments, according to a tenth embodiment,

[0033] The weight ratio of the repeating units derived from the third monomer to the repeating units derived from the fourth monomer can be 1.5:8.5 to 4:6.

[0034] In any one of the first to tenth embodiments, according to an eleventh embodiment,

[0035] The average particle size of the particulate acrylic polymer can be 200 nm to 800 nm.

[0036] In any one of the first to eleventh embodiments, according to a twelfth embodiment,

[0037] The ratio of the density of the particulate acrylic polymer to the density of the inorganic particles can be 0.5 or less.

[0038] In any one of the first to twelfth embodiments, according to a thirteenth embodiment,

[0039] The density of the particulate acrylic polymer can be 1.5 g / m 3 or less.

[0040] In any one of the first to thirteenth embodiments, according to a fourteenth embodiment,

[0041] The inorganic particles can have a density of 2.0 g / m 3 or more.

[0042] In any one of the first to fourteenth embodiments, according to a fifteenth embodiment,

[0043] The separator for a secondary battery can have an adhesion strength to an electrode of 30 gf / 25 mm to 200 gf / 25 mm.

[0044] In any one of the first to fifteenth embodiments, according to a sixteenth embodiment,

[0045] The separator for a secondary battery can have an air permeability of 10 sec / 100 cc to 300 sec / 100 cc.

[0046] In any one of the first to sixteenth embodiments, according to a seventeenth embodiment,

[0047] The adhesion strength between the porous polymer substrate and the first layer can be 10 gf / 15 mm to 300 gf / 15 mm.

[0048] To solve the above problems, according to one aspect of the present disclosure, there is provided a secondary battery of the following embodiments.

[0049] An eighteenth embodiment relates to a secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the separator for a secondary battery according to any one of the first to seventeenth embodiments.

[0050] To solve the above problems, according to one aspect of the present disclosure, there is provided a method of manufacturing a secondary battery of the following embodiments and a secondary battery manufactured by the method.

[0051] A nineteenth embodiment relates to a method of manufacturing a secondary battery, including interposing the separator for a secondary battery according to any one of the first to seventeenth embodiments between a positive electrode and a negative electrode and laminating them by heat and pressure.

[0052] A twentieth embodiment relates to a secondary battery manufactured by the manufacturing method according to the nineteenth embodiment.

[0053] Advantageous Effects

[0054] The separator for a secondary battery according to the embodiments of the present disclosure includes a second layer containing a particulate acrylic polymer having a glass transition temperature of 20 to 50°C on the upper surface of a first layer including inorganic particles to have good adhesion to an electrode in the presence of the inorganic particles.

[0055] Further, since the separator for a secondary battery according to the embodiments of the present disclosure uses a particulate acrylic polymer having a glass transition temperature of 20 to 50°C, the second layer can be formed without clogging the pores of the first layer, thereby solving the problem of electrical resistance. BRIEF DESCRIPTION OF DRAWINGS

[0056] The accompanying drawings illustrate preferred embodiments of the present disclosure and are used in conjunction with the following disclosure to provide a further understanding of the technical spirit of the present disclosure. However, the present disclosure should not be construed as being limited to the accompanying drawings.

[0057] Figure 1 is a cross-sectional view of a separator for a secondary battery according to the embodiments of the present disclosure.

[0058] Figure 2 is a scanning electron microscope (SEM) image of a particulate acrylic polymer used in the second layer in Example 1.

[0059] Figure 3 is a SEM image of a particulate acrylic polymer used in the second layer in Example 2.

[0060] Figure 4 is a SEM image of a particulate acrylic polymer used in the second layer in Example 3.

[0061] Figure 5 is a SEM image of an acrylic polymer used in the second layer in Comparative Example 2.

[0062] Figure 6 is a SEM image of an acrylic polymer used in the second layer in Comparative Example 3.

[0063] Figure 7 is a SEM image of a particulate acrylic polymer used in the second layer in Comparative Example 4. DETAILED DESCRIPTION

[0064] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it is to be understood that the term used in the specification and the appended claims should not be construed as being limited to general and dictionary meanings and should be construed as having a concept matching the technical aspect of the present disclosure based on the principle that the inventor is allowed to define terms appropriately for the best explanation of the invention.

[0065] Accordingly, the embodiments described herein and illustrated in the drawings are set forth only for a best understanding of the present disclosure and are not intended to limit the technical aspects of the present disclosure, and it should be understood that various other equivalents or modifications can be made thereto at the time of filing this application.

[0066] The specific terms used in the following detailed description of the present disclosure are provided for convenience and are not intended to limit the present disclosure. In the present specification, "a surface" or "an upper surface" indicates a position and orientation in the corresponding drawing and is not limited thereto. These terms include the above-mentioned words, their derivatives, and words having similar meanings.

[0067] Throughout the specification, it is to be understood that when a layer is referred to as being on the "upper surface" of another layer, the layer can be placed in contact with the surface of the other layer, and there can be an intermediate layer.

[0068] An electrode for a secondary battery according to an embodiment of the present disclosure includes:

[0069] A porous polymer substrate;

[0070] a first layer formed on at least one surface of the porous polymer substrate and including inorganic particles and a non-particulate acrylic polymer having a glass transition temperature of 15°C or less, wherein the non-particulate acrylic polymer connects and fixes the inorganic particles; and

[0071] a second layer formed on an upper surface of the first layer and including a particulate acrylic polymer having a glass transition temperature of 20°C to 50°C.

[0072] Figure 1 is a schematic view of a separator for a secondary battery according to an embodiment of the present disclosure.

[0073] Referring to Figure 1 , a separator 1 for a secondary battery includes a porous polymer substrate 10.

[0074] In an embodiment of the disclosure, the porous polymer substrate 10 can include any material for a separator of a secondary battery commonly found in the corresponding technical field without limitation. The porous polymer substrate can be a thin film including a polymer material, and non-limiting examples of the polymer material can include at least one polymer resin of polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, or polyethylene naphthalate. In addition, the porous polymer substrate can include a non-woven fabric or a porous polymer film made of the above-described polymer material or a stack of two or more thereof. Specifically, the porous polymer substrate can be any one of the following a) to e).

[0075] a) a porous film formed by melting and extruding a polymer resin,

[0076] b) a multi-layer film formed by stacking the porous film of a) in two or more layers,

[0077] c) a non-woven web made of a filament obtained by melting / spinning a polymer resin,

[0078] d) a multi-layer film formed by stacking the non-woven web of b) in two or more layers,

[0079] e) a porous composite film including a multi-layer structure of at least two of a) to d).

[0080] In an embodiment of the disclosure, the thickness of the porous polymer substrate 10 can be 5 μm to 50 μm, but is not limited thereto. When the thickness of the porous polymer substrate is in the above-described range, it is easier to prevent the problem that the separator is easily damaged while the battery is used. Meanwhile, there is no limitation on the average pore size and the porosity in the porous polymer substrate, but the average pore size can be 0.01 μm to 50 μm, and the porosity can be 10% to 95%.

[0081] In the disclosure, the porosity and the average pore size of the porous polymer substrate 10 can be measured by a 6-point BET method according to nitrogen adsorption flow method, a scanning electron microscope (SEM) image, a mercury porosimeter, a capillary flow porometer, or a porosimetry analyzer (Porosimetry analyzer; Bell Japan Inc, Belsorp-II mini).

[0082] Referring to Figure 1A separator 1 for a secondary battery includes a first layer 20 on at least one surface of a porous polymer substrate 10. Specifically, the first layer 20 can be formed on one or both surfaces of the porous polymer substrate 10.

[0083] The first layer 20 includes inorganic particles 21, and a non-particulate acrylic polymer 22 having a glass transition temperature of 15°C or less and connecting and fixing the inorganic particles 21. The inorganic particles 21 of the first layer 20 can prevent the porous polymer substrate 10 from exhibiting a severe thermal shrinkage behavior at a high temperature, thereby improving the safety of the separator.

[0084] The inorganic particles 21 can include, without limitation, any type of inorganic particles that are electrochemically stable. That is, the inorganic particles 21 usable in the present disclosure can include, without limitation, any type of inorganic particles that do not cause an oxidation or reduction reaction within the operating voltage range of the applied battery (e.g., 0 ~ 5V vs. Li / Li + +). In particular, the use of high dielectric constant inorganic particles as the inorganic particles 21 helps to increase the degree of dissociation of an electrolyte salt, e.g., a lithium salt, in a liquid electrolyte, thereby improving the ionic conductivity of the electrolyte.

[0085] In an embodiment of the present disclosure, the inorganic particles 21 can include high dielectric constant inorganic particles having a dielectric constant of 5 or more, or preferably 10 or more. Non-limiting examples of inorganic particles having a dielectric constant of 5 or more can include at least one of BaTiO3, Pb(Zr,Ti)O3(PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT, 0 < x < 1, 0 < y < 1), Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3(PMN-PT), hafnium oxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, Al(OH)3, SiC, or TiO2.

[0086] Further, in another embodiment of the present disclosure, the inorganic particles 21 can include inorganic particles capable of transporting lithium ions, i.e., inorganic particles that include lithium but do not store lithium and have a function of moving lithium ions. Non-limiting examples of inorganic particles capable of transporting lithium ions can include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Tiz (LiAlTiP) x O y (Li x La y (Li x Ge y P z S w (Li x N y (Li x Si y S z (Li x P y S z (Li

[0087] In embodiments of the present disclosure, there is no limitation on the average particle size of the inorganic particles 21, but the inorganic particles 21 can have an average particle size of 0.01 to 10 µm, or 0.05 to 1.0 µm, or 0.2 to 1.0 µm, or 0.5 to 1.0 µm to form the first layer having a uniform thickness and to ensure the best porosity. When the average particle size of the inorganic particles 21 satisfies the above range, it is easier to maintain dispersion, which makes it easy to control the properties of the separator, and it is easier to prevent the thickness of the first layer 20 from increasing, thereby improving the mechanical properties. In addition, it is easier to reduce the possibility of internal short-circuiting due to an excessively large pore size during charging / discharging of the battery.

[0088] In this case, the average particle size of the inorganic particles 21 refers to D 50 , and the "particle size D 50 " refers to the particle size at 50% in the cumulative particle size distribution. The particle size can be measured using a laser diffraction method. Specifically, after the powder to be measured is dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500), the particle size distribution is calculated by measuring the difference in diffraction pattern according to the particle size as the particle passes through the laser beam. The particle size D 50 can be measured by calculating the particle diameter at 50% in the cumulative particle size distribution in the measuring device.

[0089] In the present disclosure, the term "non-particulate acrylic polymer" refers to an acrylic polymer used in the first layer that does not have a particulate shape, and is used to distinguish from the particulate acrylic polymer included in the second layer. The non-particulate acrylic polymer 22 adheres the inorganic particles 21 to each other to hold the inorganic particles 21 together (i.e., the non-particulate acrylic polymer connects and fixes the inorganic particles), and binds the inorganic particles 21 and the porous polymer substrate 10 together.

[0090] The non-particulate acrylic polymer has a glass transition temperature of 15°C or less. The non-particulate acrylic polymer 22 having a glass transition temperature of 15°C or less does not have a particulate shape, and is entangled with the inorganic particles 21 to form the first layer 20 on at least one surface of the porous polymer substrate 10. Accordingly, the adhesion between the porous polymer substrate 10 and the first layer 20 can be ensured by the non-particulate acrylic polymer 22. Further, since the non-particulate acrylic polymer 22 has a relatively low glass transition temperature, it can be advantageous in ensuring the adhesion between the porous polymer substrate 10 and the first layer 20.

[0091] In an embodiment of the present disclosure, the non-particulate acrylic polymer 22 can have a glass transition temperature of 12°C or less, or 8°C or less, or 0°C or less, or -40°C or less, or -80°C to 12°C, or -80°C to 8°C, or -40°C to 8°C, or -80°C to 0°C, or -40°C to 0°C. Since the non-particulate acrylic polymer 22 has a lower glass transition temperature, it can be more advantageous in ensuring the adhesion between the porous polymer substrate 10 and the first layer 20.

[0092] In the present specification, the glass transition temperature can be measured using differential scanning calorimetry (DSC). Specifically, the glass transition temperature can be measured using DSC at a temperature increase rate of 10°C / min (-50°C to 250°C). For example, the glass transition temperature can be measured using DSC 250 (TA).

[0093] The glass transition temperature of the non-particulate acrylic polymer 22 can vary depending on the type of monomer used in making the non-particulate acrylic polymer 22.

[0094] In an embodiment of the present disclosure, the non-particulate acrylic polymer 22 can include a repeating unit derived from a first monomer and a repeating unit derived from a second monomer, and the glass transition temperature of the first monomer can be higher than the glass transition temperature of the second monomer.

[0095] In an embodiment of the disclosure, the first monomer can include at least one of methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, or t-butyl methacrylate.

[0096] The higher glass transition temperature of the first monomer than the second monomer can make the first layer 20 less likely to be pressed in a lamination process of the electrode and the separator, and the first layer 20 can maintain porosity after lamination. For example, the glass transition temperature of the first monomer can be 20℃ or more, or 20℃ to 107℃, or 20℃ to 47℃, or 47℃ to 107℃.

[0097] In an embodiment of the disclosure, the second monomer can include at least one of 2-ethylhexyl methacrylate, dodecyl methacrylate, or octadecyl methacrylate.

[0098] The second monomer having a lower glass transition temperature than the first monomer can secure adhesion of the first layer 20 to the porous polymer substrate 10. For example, the glass transition temperature of the second monomer can be 10℃ or less, or -124℃ to 10℃, or -100℃ to -10℃, or -100℃ to -65℃, or -65℃ to -10℃.

[0099] In an embodiment of the disclosure, the non-particulate acrylic polymer can include a repeating unit derived from methyl methacrylate and a repeating unit derived from 2-ethylhexyl methacrylate.

[0100] Even if the same type of monomer is used to make the non-particulate acrylic polymer 22, the glass transition temperature of the non-particulate acrylic polymer 22 can differ depending on the content of each monomer.

[0101] In an embodiment of the disclosure, the repeating unit derived from the second monomer can be included in an amount of 60% by weight or more, or 90% by weight or more, or 95% by weight or more, based on 100% by weight of the non-particulate acrylic polymer. When the amount of the repeating unit derived from the second monomer satisfies the above range, it can be more advantageous in securing adhesion of the first layer 20 to the porous polymer substrate 10 since the amount of the second monomer having a lower glass transition temperature than the first monomer is higher than that of the first monomer.

[0102] In an embodiment of the disclosure, the weight ratio of the inorganic particles 21 and the non-particulate acrylic polymer 22 included in the first layer 20 can be decided in consideration of the thickness, the average pore size, and the porosity of the first layer 20 finally formed.

[0103] In the detailed description of the present disclosure, the weight ratio of the inorganic particles 21 and the non-particulate acrylic polymer 22 can be 20:80 to 99.9:0.1, or 50:50 to 99.5:0.5. When the weight ratio of the inorganic particles 21 and the non-particulate acrylic polymer 22 is within the above range, it is easier to ensure sufficient adhesion between the inorganic particles 21 and sufficient empty spaces formed between the inorganic particles 21. In addition, the finally formed first layer 20 can have good mechanical properties.

[0104] In an embodiment of the present disclosure, the first layer 20 can have a thickness in the range of 1 μm to 50 μm, or 2 μm to 30 μm, or 2 μm to 20 μm.

[0105] In an embodiment of the present disclosure, the average pore size of the first layer 20 can vary from 0.001 μm to 10 μm, or from 0.001 μm to 1 μm. In addition, the porosity of the first layer 20 can vary between 5% and 95%, between 10% and 95%, between 20% and 90%, or between 30% and 80%. The porosity corresponds to a value obtained by subtraction of a volume converted from the weight and the density of each component of the first layer from a volume calculated using the thickness, the width, and the height of the first layer.

[0106] Meanwhile, in the present disclosure, the porosity and the average pore size of the first layer 20 can be measured by a 6-point BET method according to nitrogen adsorption flow method, a SEM image, a mercury porosimeter, a capillary flow porometer, or a porosimetry analyzer (Porosimetry analyzer; Bell Japan Inc, Belsorp-II mini).

[0107] In an embodiment of the present disclosure, the first layer 20 can further include a dispersant.

[0108] The dispersant can be used to improve the dispersion of the inorganic particles 21. Specifically, in an embodiment of the present disclosure, the dispersant can include at least one of carboxymethyl cellulose (CMC), poly acrylic acid (PAA), or polymethyl acrylic acid (PMAA), but is not limited thereto.

[0109] Referring to Figure 1 The separator 1 for a secondary battery includes a second layer 30 on the upper surface of the first layer 20. The second layer 30 imparts adhesion properties to the separator 1 to allow good adhesion of the surface of the separator 1 to an electrode.

[0110] The second layer 30 includes a particulate acrylic polymer 31 having a glass transition temperature of 20°C to 50°C. In the adhesion of the electrode and the separator, the shape of the particulate acrylic polymer 31 is changed by lamination, thereby obtaining the adhesion between the electrode and the separator. In the present disclosure, the term "particulate acrylic polymer" refers to an acrylic polymer included in the second layer that exhibits a particulate shape, and is used to be distinguished from the non-particulate acrylic polymer 22 included in the first layer.

[0111] In the present disclosure, the particulate acrylic polymer 31 has a glass transition temperature of 20°C or more and independently exhibits a particulate shape.

[0112] In the embodiments of the present disclosure, due to its lower density than the inorganic particles 21, the particulate acrylic polymer 31 can move to the upper portion of the inorganic particles in the coating process to form the second layer 30 on the upper surface of the first layer 20 including the inorganic particles 21.

[0113] In contrast, in the present disclosure, when the glass transition temperature of the particulate acrylic polymer 31 exceeds 50°C, the particulate acrylic polymer 31 has a particulate shape, but in the adhesion of the electrode and the separator 1, the particulate shape is not changed by lamination, which makes it difficult to obtain the adhesion between the separator and the electrode.

[0114] In the embodiments of the present disclosure, the glass transition temperature of the particulate acrylic polymer 31 can be 30°C to 45°C. When the glass transition temperature of the particulate acrylic polymer 31 is 30°C to 45°C, it is easier to improve the air permeability of the separator 1 and the adhesion to the electrode by adjusting the degree of shape change of the particulate acrylic polymer by lamination of the electrode and the separator 1.

[0115] In the embodiments of the present disclosure, the ratio of the density of the particulate acrylic polymer 31 to the density of the inorganic particles 21 can be 0.5 or less, or 0.45 or less, or 0.42 or less. For example, the density of the particulate acrylic polymer : the density of the inorganic particles can be 0.5: 1 or less, or 0.45: 1 or less, or 0.42: or less. When the ratio of the density of the particulate acrylic polymer 31 to the density of the inorganic particles 21 is within the range defined above, the particulate acrylic polymer 31 is more likely to move to the upper portion of the inorganic particles in the coating process due to the density difference between the particulate acrylic polymer 31 and the inorganic particles 21, which makes it easier to form the second layer 30 on the upper surface of the first layer 20 including the inorganic particles 21.

[0116] In the present disclosure, the density of the inorganic particles 21 refers to a true density. The true density refers to a density of a volume of the particles other than a gap between the particles. In an embodiment of the present disclosure, the density of the inorganic particles 21 can be measured by a commonly used true density measurement method, and for example, can be measured using a Micromeritics AccuPyc II-1340.

[0117] In an embodiment of the present disclosure, the density value of the inorganic particles 21 can be 2.0 g / m 3 or more, or 2 g / cm 3 to 6 g / cm 3 , or 2 g / cm 3 to 4 g / cm 3 .

[0118] In the present disclosure, the density of the particulate acrylic polymer 31 refers to a true density. In an embodiment of the present disclosure, the density of the particulate acrylic polymer 31 can be measured by a commonly used true density measurement method, and for example, can be measured using a Micromeritics AccuPyc II-1340.

[0119] In an embodiment of the present disclosure, the density value of the particulate acrylic polymer 31 can be 1.5 g / m 3 or less, or 0.5 g / cm 3 to 1.5 g / cm 3 , or 1 g / cm 3 to 2 g / cm 3 .

[0120] In an embodiment of the present disclosure, the density value of the particulate acrylic polymer 31 can be 1.5 g / m 3 or less, and the density value of the inorganic particles can be 2.0 g / m 3 or more. When the density values of the particulate acrylic polymer 31 and the inorganic particles 21 satisfy the ranges defined above, the particulate acrylic polymer 31 is more easily moved to the upper portion of the inorganic particles in the coating process due to the density difference between the particulate acrylic polymer 31 and the inorganic particles 21, which makes it easier to form the second layer 30 on the upper surface of the first layer 20 including the inorganic particles 21.

[0121] The glass transition temperature of the particulate acrylic polymer 31 can vary depending on the type of monomer used to make the particulate acrylic polymer 31.

[0122] In embodiments of the present disclosure, the particulate acrylic polymer can include repeating units derived from a third monomer and repeating units derived from a fourth monomer, and the glass transition temperature of the third monomer can be higher than the glass transition temperature of the fourth monomer.

[0123] In embodiments of the present disclosure, the third monomer can include at least one of styrene, vinyl acetate, or acrylonitrile.

[0124] The relatively high glass transition temperature of the third monomer can make it less likely for the second layer 30 to be pressed during a lamination process of the electrode and the separator, and the second layer 30 can maintain porosity after lamination. For example, the glass transition temperature of the third monomer can be 20℃ or more, or 20℃ to 107℃, or 28℃ to 105℃, or 28℃ to 100℃, or 100℃ to 105℃.

[0125] In embodiments of the present disclosure, the fourth monomer can include at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, or ethylene.

[0126] The fourth monomer can be advantageous in securing adhesion of the second layer 30 to the electrode due to its relatively low glass transition temperature. For example, the glass transition temperature of the fourth monomer can be 10℃ or less, or -124℃ to 10℃, or -124℃ to -55℃, or -55℃ to -22℃, or -22℃ to 10℃.

[0127] In embodiments of the present disclosure, the particulate acrylic polymer can include repeating units derived from styrene and repeating units derived from butyl acrylate.

[0128] Even if the same type of monomer is used to make the particulate acrylic polymer 31, the glass transition temperature of the particulate acrylic polymer 31 can differ depending on the content of each monomer.

[0129] In embodiments of the present disclosure, the weight ratio of the repeating units derived from the third monomer to the repeating units derived from the fourth monomer can be 1.5:8.5 to 4:6, or 2:8 to 4:6, or 2.5:7.5 to 3:7. When the weight ratio of the repeating units derived from the third monomer to the repeating units derived from the fourth monomer satisfies the above-defined range, it can be more powerful in securing adhesion of the second layer 30 to the electrode.

[0130] In embodiments of the present disclosure, the average particle size of the particulate acrylic polymer 31 can be 200 nm to 800 nm, 250 nm to 500 nm, or 300 nm to 450 nm. When the average particle size of the particulate acrylic polymer 31 is within the range defined above, it is easier to produce the particulate acrylic polymer 31, and the adhesion area with the electrode increases, thereby obtaining better adhesion between the second layer 30 including the particulate acrylic polymer 31 and the electrode.

[0131] In the present disclosure, the average particle size of the particulate acrylic polymer 31 refers to D50 50 , and "particle size D 50 " refers to the particle size at 50% in the cumulative particle size distribution. The particle size can be measured using a laser diffraction method. Specifically, after the powder to be measured is dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500), the particle size distribution is calculated by measuring the difference in the diffraction pattern as the particle passes through the laser beam. The particle size D 50 can be measured by calculating the particle diameter at 50% in the cumulative particle size distribution in the measuring device.

[0132] The separator 1 for a secondary battery according to embodiments of the present disclosure includes two types of acrylic polymers: a particulate acrylic polymer 31 having a glass transition temperature of 20°C to 50°C in the second layer 30, and a non-particulate acrylic polymer 22 having a glass transition temperature of 15°C or lower in the first layer 20. Accordingly, the separator 1 has good adhesion with the electrode and ensures adhesion between the first layer 20 and the porous polymer substrate 10.

[0133] Further, the separator 1 for a secondary battery according to the present disclosure uses a particulate acrylic polymer 31 in the second layer 30, thereby preventing the particulate acrylic polymer 31 from clogging the pores of the first layer 20, resulting in good air permeability and solving the problem of electrical resistance.

[0134] In embodiments of the present disclosure, the adhesion strength of the separator 1 for a secondary battery with the electrode can be 30 gf / 25 mm to 200 gf / 25 mm, or 30 gf / 25 mm to 100 gf / 25 mm, or 35 gf / 25 mm to 60 gf / 25 mm, or 40 gf / 25 mm to 60 gf / 25 mm.

[0135] In an embodiment of the present disclosure, the adhesion strength of the separator 1 for a secondary battery to an electrode can be measured from the strength when peeling the separator portion of the sample after the sample is manufactured by laminating the electrode and the separator under the condition of 60℃, 6.5Mpa by using a press, and the sample is attached and fixed to a glass plate using a double-sided tape, at a rate of 25mm / min at an angle of 180° at 25℃.

[0136] In an embodiment of the present disclosure, the air permeability of the separator 1 for a secondary battery can be 10sec / 100cc to 300sec / 100cc, or 100sec / 100cc to 300sec / 100cc, or 110sec / 100cc to 200sec / 100cc, or 100sec / 100cc to 160sec / 100cc.

[0137] In an embodiment of the present disclosure, the air permeability of the separator 1 for a secondary battery is a Gurley value, and can refer to the time (sec) required for 100cc of air to pass through the cross-section of the separator 1 under the pressure of 12.2in of water column, i.e., air permeation time. The air permeability of the separator 1 for a secondary battery can be measured by ASTM D726-94 method. 2

[0138] In an embodiment of the present disclosure, the adhesion strength between the porous polymer substrate 10 and the first layer 20 can be 10gf / 15mm to 300gf / 15mm, or 40gf / 15mm to 100gf / 15mm.

[0139] In an embodiment of the present disclosure, the adhesion strength between the porous polymer substrate 10 and the first layer 20 can be measured by fixing the separator 1 to a glass plate using a double-sided tape, firmly attaching the tape (3M transparent tape) to the exposed first layer 20, and measuring the force (gf / 15mm) required to peel the tape using a tensile strength measuring device. For example, the tensile strength measuring device can be an 11oyd LS-1.

[0140] The separator for a secondary battery according to an embodiment of the present disclosure can be manufactured by the following manufacturing method, but is not limited thereto.

[0141] The method of manufacturing a separator for a secondary battery according to an embodiment of the present disclosure includes:

[0142] (S1) preparing a porous polymer substrate;

[0143] (S2) preparing an aqueous slurry including inorganic particles, a non-particulate acrylic polymer having a glass transition temperature of 15℃ or less, and a particulate acrylic polymer having a glass transition temperature of 20℃ to 50℃;​

[0144] (S3) coating the aqueous slurry on at least one surface of the porous polymer substrate; and

[0145] (S4) Drying the result of step (S3).

[0146] Hereinafter, a method of manufacturing a separator for a secondary battery according to an embodiment of the present disclosure will be described based on main parts.

[0147] First, a porous polymer substrate is prepared. The porous polymer substrate can be used as described above, and the porous polymer substrate can be made from the above materials by forming pores using a method commonly used in the corresponding technical field, such as a wet method using a solvent and a pore-forming agent or a dry method using a stretching process, to ensure good air permeability and porosity.

[0148] Subsequently, an aqueous slurry including inorganic particles, a non-granular acrylic polymer having a glass transition temperature of 15° C. or less, and a granular acrylic polymer having a glass transition temperature of 20° C. to 50° C. is prepared.

[0149] In the aqueous slurry, water serves as a dispersion medium for the non-particulate acrylic polymer and the particulate acrylic polymer. The method of manufacturing a separator for a secondary battery according to an embodiment of the present disclosure is environmentally friendly due to the use of water that can disperse the non-particulate acrylic polymer and the particulate acrylic polymer.

[0150] In the embodiment of the present disclosure, for details of the inorganic particles, the non-granular acrylic polymer having a glass transition temperature of 15°C or less, and the granular acrylic polymer having a glass transition temperature of 20°C to 50°C, refer to the above description.

[0151] In an embodiment of the present disclosure, an aqueous slurry can be prepared by dispersing a non-granular acrylic polymer and a granular acrylic polymer in water, and adding and dispersing inorganic particles. The inorganic particles may be added in a pre-crushed state to have a predetermined average particle size, or the inorganic particles may be added to a solution in which the non-granular acrylic polymer and the granular acrylic polymer are dispersed, and crushed and dispersed while being controlled to have a predetermined average particle size using a ball milling method.

[0152] In an embodiment of the present disclosure, the weight ratio of the inorganic particles, the non-particulate acrylic polymer having a glass transition temperature of 15°C or less, and the particulate acrylic polymer having a glass transition temperature of 20°C to 50°C in the aqueous slurry can be 70:1:29 to 80:5:15. When the weight ratio of the inorganic particles, the non-particulate acrylic polymer having a glass transition temperature of 15°C or less, and the particulate acrylic polymer having a glass transition temperature of 20°C to 50°C is within the above defined range, it is easier to minimize the increase in electrical resistance caused by the clogging of the pores of the porous polymer substrate by the non-particulate acrylic polymer and to ensure the adhesion between the inorganic particles and the porous polymer substrate.

[0153] In an embodiment of the present disclosure, the aqueous slurry can further include a dispersant. For details of the dispersant, refer to the above description.

[0154] Subsequently, the aqueous slurry is coated on at least one surface of the porous polymer substrate.

[0155] In an embodiment of the present disclosure, the method of coating the aqueous slurry on at least one surface of the porous polymer substrate to form the first layer is not limited to a specific method, and can include a method suitable for use in general in the technical field of the present disclosure, non-limiting examples of which can include a dip coating method, a die coating method, a roll coating method, a comma coating method, a doctor blade coating method, a reverse roll coating method, and a direct roll coating method.

[0156] In the method of manufacturing a separator for a secondary battery according to an embodiment of the present disclosure, the particulate acrylic polymer having a glass transition temperature of 20°C to 50°C can have a lower density than the inorganic particles. Accordingly, when the aqueous slurry is coated on the porous polymer substrate, the particles can move to the upper portion of the inorganic particles and be disposed on the surface of the separator that contacts the electrode.

[0157] In contrast, the non-particulate acrylic polymer does not move to the upper portion of the inorganic particles, and can be entangled with the inorganic particles and sink toward the porous polymer substrate. Accordingly, using the method of manufacturing a separator for a secondary battery according to an embodiment of the present disclosure, the first layer and the second layer can be formed together without performing a process of forming the second layer.

[0158] Further, since the particulate acrylic polymer can have a lower density than the inorganic particles, the particulate acrylic polymer can move to the upper portion of the inorganic particles, and thus the particulate acrylic polymer does not clog the pores of the first layer, thereby improving air permeability and solving the problem of electrical resistance.

[0159] Subsequently, the coated aqueous slurry is dried.

[0160] In embodiments of the present disclosure, drying can be performed using a drying method commonly used in separator manufacturing. For example, when drying the applied aqueous slurry, temperatures at least 10°C higher than the glass transition temperature of the particulate acrylic polymer should not be transferred to the separator. When the drying conditions are the same as described above, it is easier to remove the residual dispersion medium without destroying the particle shape of the particulate acrylic polymer.

[0161] As described above, a separator for a secondary battery that ensures good adhesion to an electrode and can solve a resistance problem may be manufactured by a method of manufacturing a separator for a secondary battery according to an embodiment of the present disclosure.

[0162] The separator for a secondary battery includes two types of acrylic polymers: a granular acrylic polymer having a glass transition temperature of 20°C to 50°C and a non-granular acrylic polymer having a glass transition temperature of 15°C or less, thereby obtaining good adhesion between the separator and the electrode and ensuring adhesion between the first layer and the porous polymer substrate.

[0163] In an embodiment of the present disclosure, a secondary battery may be manufactured using the separator for a secondary battery according to the present disclosure.

[0164] A method for manufacturing a secondary battery according to an embodiment of the present disclosure includes interposing a separator for a secondary battery between a positive electrode and a negative electrode and laminating them using heat and pressure. Lamination can improve adhesion between the electrodes and the separator by changing the shape of a particulate acrylic polymer having a glass transition temperature of 20°C to 50°C included in the second layer of the separator for a secondary battery.

[0165] In an embodiment of the present disclosure, lamination may be performed at 30° C. to 150° C., or 60° C. to 100° C., or 60° C. to 80° C. Since lamination is performed within the above temperature range, the particle shape of the particulate acrylic polymer is changed, which makes it easier to obtain adhesion between the electrode and the separator.

[0166] Furthermore, in an embodiment of the present disclosure, lamination may be performed at a pressure of 3.5 MPa to 7.8 MPa.

[0167] In an embodiment of the present disclosure, lamination may be performed under a temperature condition of 30° C. to 150° C. and a pressure condition of 3.5 MPa to 7.8 MPa.

[0168] In the embodiment of the present disclosure, for details of the positive electrode and the negative electrode, refer to the following description.

[0169] In an embodiment of the present disclosure, a separator for a secondary battery may be interposed between a positive electrode and a negative electrode of the secondary battery, and may be interposed between adjacent battery cells or electrodes when an electrode assembly is constructed by assembling a plurality of battery cells or electrodes. The electrode assembly may have various structures, for example, a simple stack type, a jelly roll type, a stack-folding type, and a laminate-stack type.

[0170] The secondary battery can be manufactured by the method of manufacturing a secondary battery as described above.

[0171] The secondary battery of the present disclosure may preferably be a lithium secondary battery. The lithium secondary battery may include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0172] The electrode to be used with the separator for a secondary battery of the present disclosure is not limited to a specific type and can be manufactured by bonding an electrode active material to an electrode current collector by a conventional method known in the technical field appropriate to the present disclosure.

[0173] Among the electrode active materials, non-limiting examples of the positive electrode active material may include layered compounds such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or compounds having one or more transition metals; 1+x Mn 2-x Lithium manganese oxides such as LiMnO4 (x = 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxides (Li2CuO2); vanadium oxides such as LiV3O5, LiV3O4, V2O5, Cu2V2O7; 1-x M x Ni-site lithium nickel oxide represented by O2 (M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, x = 0.01 to 0.3); 2-x M x Lithium manganese composite oxides represented by Li2Mn3MO5 (M=Fe, Co, Ni, Cu, or Zn); LiMn2O4 having alkaline earth metal ions partially substituted for Li; disulfide compounds; and Fe2(MoO4)3, but not limited thereto.

[0174] Non-limiting examples of the negative electrode active material may include any negative electrode active material commonly used in the negative electrode of an electrochemical device, and in particular, may include a lithium adsorption material such as lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbon.

[0175] Non-limiting examples of the positive current collector can include a foil made of aluminum, nickel, or a combination thereof, and non-limiting examples of the negative current collector can include a foil made of copper, gold, nickel, or a copper alloy, or a combination thereof.

[0176] In embodiments of the present disclosure, the conductive material used in the negative and positive electrodes can generally be added in an amount of 1 to 30% by weight, respectively, based on the total weight of the active material layer. The conductive material is not limited to any particular type while the material has electrical conductivity without causing chemical changes to the corresponding battery, and can include conductive materials, for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers or metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyaniline derivatives.

[0177] In embodiments of the present disclosure, the binder assists in binding the active material and the conductive material and binding the active material and the current collector in the negative and positive electrodes, and can generally be added in an amount of 1 to 30% by weight, respectively, based on the total weight of the active material layer. Examples of the binder can 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, fluoro rubber, and various types of copolymers.

[0178] In embodiments of the present disclosure, the electrochemical device includes an electrolyte solution, and the electrolyte solution can include an organic solvent and a lithium salt. In addition, the electrolyte solution can include an organic solid electrolyte or an inorganic solid electrolyte.

[0179] For example, the organic solvent can include an aprotic organic solvent such as N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfranc, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0180] Lithium salts are materials that are easily dissolved in organic solvents, and can include, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, and imide.

[0181] In addition, in order to improve charge / discharge characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, a cyclic ether, ethylenediamine, n- glyme, triamide hexaphosphoric acid, a nitrobenzene derivative, sulfur, a quinone imine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidinone, a glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, and aluminum trichloride can be added to the non-aqueous electrolyte. In some cases, a halogen-containing solvent such as carbon tetrachloride and trifluoroethylene can be added to give non-flammability, and carbon dioxide gas can be added to improve high-temperature storage characteristics.

[0182] For example, the organic solid electrolyte can include a polyvinyl derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, a polyagitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, and a polymer including an ionic dissociation group.

[0183] For example, the inorganic solid electrolyte can include a nitride, a halide, and a sulfate of lithium (Li), for example, Li3N, Lil, Li5NI2, Li3N-Lil-LiOH, LiSiO4, LiSiO4-Lil-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-Lil-LiOH, Li3PO4-Li2S-SiS2.

[0184] The injection of the electrolyte solution can be performed in any appropriate step of the battery manufacturing process according to the manufacturing process of the final product and the desired properties. That is, the injection of the electrolyte solution can be applied before the battery assembly or in the final step of the battery assembly.

[0185] In an embodiment of the disclosure, the process of applying a separator for an electrochemical device to a battery can include a winding process commonly used and a lamination or stack and folding process of the separator and the electrode.

[0186] In embodiments of the present disclosure, a separator for an electrochemical device can be interposed between a positive electrode and a negative electrode of the electrochemical device, and can be interposed between adjacent electrode assemblies when the electrode assemblies are constructed by assembling a plurality of electrode assemblies or electrodes. The electrode assemblies can have various structures, for example, a simple stack type, a jelly-roll type, a stack-fold type, and a laminate-stack type.

[0187] Hereinafter, the present disclosure will be described in detail by way of Examples and Comparative Examples to assist in the understanding of the present disclosure. However, the embodiments according to the present disclosure can be embodied in many different forms, and the scope of the present disclosure should not be construed as being limited to the following Examples. The Examples of the present disclosure are provided to fully explain the present disclosure to those having ordinary knowledge in the technical field to which the present disclosure pertains.

[0188] Example 1

[0189] A particulate acrylic polymer (weight ratio of butyl acrylate:styrene = 7:3, glass transition temperature: 43°C, average particle size: 200 nm, true density: 1.02 g / cm 3 ), a non-particulate acrylic polymer (weight ratio of 2-ethylhexyl acrylate:methyl methacrylate = 6:4, glass transition temperature: 8°C, true density: 1.05 g / cm 3 ), and aluminum hydroxide (Al(OH)3) (Huber, average particle size: 800 nm, true density: 2.4 g / cm 3 ) were added to water as a dispersion medium in a weight ratio of 28.5:1.5:70, followed by dispersion using a bead mill at 25°C for 120 minutes to prepare an aqueous slurry. Here, water was added so that the total solid content was 40% by weight. The aqueous slurry was coated on one surface of a 9 μm thick polyethylene porous substrate (Toray) to a thickness of 3 μm by a bar coating method, and dried at 60 to 80°C for 30 seconds to manufacture a separator for a secondary battery.

[0190] Example 2

[0191] A separator for a secondary battery was manufactured in the same manner as in Example 1, except that a particulate acrylic polymer (weight ratio of butyl acrylate:styrene = 7.5:2.5, glass transition temperature: 40°C, average particle size: 380 nm, true density: 1.09 g / cm 3 ) was used instead of the particulate acrylic polymer (glass transition temperature: 43°C) of Example 1.

[0192] Example 3

[0193] A separator for a secondary battery was produced in the same manner as in Example 1, except that a non-particulate acrylic polymer (weight ratio of butyl acrylate : styrene = 8.5 : 1.5, glass transition temperature : 30°C, average particle size : 180 nm, true density : 1.09 g / cm 3 ) was used instead of the particulate acrylic polymer (glass transition temperature : 43°C) of Example 1.

[0194] Example 4

[0195] A separator for a secondary battery was produced in the same manner as in Example 1, except that a non-particulate acrylic polymer (weight ratio of 2-ethylhexyl acrylate : methyl methacrylate = 9.5 : 0.5, glass transition temperature : -40°C, true density : 1.02 g / cm 3 ) was used instead of the non-particulate acrylic polymer (glass transition temperature : 8°C) of Example 1.

[0196] Comparative Example 1

[0197] A separator for a secondary battery was produced in the same manner as in Example 1, except that no particulate acrylic polymer was used.

[0198] Comparative Example 2

[0199] A separator for a secondary battery was produced in the same manner as in Example 1, except that an acrylic polymer (weight ratio of butyl acrylate : styrene = 9.5 : 0.5, glass transition temperature : 0°C, true density : 1.02 g / cm 3 ) was used instead of the particulate acrylic polymer (glass transition temperature : 43°C) of Example 1.

[0200] This acrylic polymer did not have a particulate shape, which made it difficult to measure the average particle size.

[0201] Comparative Example 3

[0202] A separator for a secondary battery was produced in the same manner as in Example 1, except that an acrylic polymer (weight ratio of butyl acrylate : styrene = 9 : 1, glass transition temperature : 12°C, true density : 1.10 g / cm 3 ) was used instead of the particulate acrylic polymer (glass transition temperature : 43°C) of Example 1.

[0203] This acrylic polymer did not have a particulate shape, which made it difficult to measure the average particle size.

[0204] Comparative Example 4

[0205] A separator for a secondary battery was produced in the same manner as in Example 1, except that a particulate acrylic polymer (weight ratio of butyl acrylate : styrene = 5 : 5, glass transition temperature : 60°C, average particle size : 360 nm, true density : 1.08 g / cm 3 ) was used instead of the particulate acrylic polymer (glass transition temperature : 43°C) of Example 1.

[0206] Comparative Example 5

[0207] A separator for a secondary battery was produced in the same manner as in Example 1, except that no non-particulate acrylic polymer was used.

[0208] Evaluation Example 1 : Observation of the shape of the acrylic polymers by glass transition temperature

[0209] Figures 2 to 7 SEM images of the particulate acrylic polymers used in Examples 1 to 3, the acrylic polymer having a glass transition temperature of 0°C used in Comparative Example 2, the acrylic polymer having a glass transition temperature of 12°C used in Comparative Example 3, and the particulate acrylic polymer used in Comparative Example 4 are shown, respectively.

[0210] From Figures 2 to 7 it can be seen that only the particulate acrylic polymers having a glass transition temperature of 20°C or higher have a particle shape, while the acrylic polymers of Comparative Examples 2 and 3 having glass transition temperatures of 0°C and 12°C, respectively, have a glass transition temperature lower than 20°C do not have a particle shape.

[0211] Evaluation Example 2 : Evaluation of the properties of the second layer depending on the glass transition temperature of the acrylic polymer

[0212] The thickness, coating amount, air permeability, and adhesion to the electrode of the separators produced in Examples 1 to 3 and Comparative Examples 1 to 4 were evaluated, and the results are shown in Table 1.

[0213] Air permeability measurement method

[0214] The air permeability (Gurley) was measured by the ASTM D726-94 method. In this case, the air permeability value was expressed as the time (seconds) required for 100 cc of air to pass through a 1 in 2 cross section of the separators produced in Examples 1 to 3 and Comparative Examples 1 to 4 under a pressure of 12.2 in water column, i.e., the air permeation time.

[0215] Method for evaluating the adhesion strength to an electrode

[0216] The adhesion strength between each separator and electrode manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 was measured by the following method.

[0217] First, natural graphite, SRB, CMC, and a conductive material (weight ratio of 90:2.5:2.5:5) were added to water to prepare a negative electrode slurry, which was coated on a copper film (20 μm thick) at a loading of 5 mg / cm 2 and dried. Subsequently, roll-pressing was performed at 90°C, 8.5 Mpa, and cutting was performed to a size of 60 mm (length) x 25 mm (width) to manufacture a negative electrode.

[0218] The separators manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were cut to a size of 70 mm (length) x 25 mm (width), and a sample was manufactured by laminating the separators using a press at 60°C, 6.5 Mpa, using the prepared negative electrode. Each of the prepared samples was attached and fixed to a glass plate using double-sided tape, and in this case, the negative electrode was placed in contact with the glass plate. The separator portion of the sample was peeled at 25°C at a rate of 180° and 25 mm / min, and the strength at that time was measured.

[0219] [Table 1]

[0220]

[0221] As can be seen from Table 1 above, Examples 1 to 3 use a particulate acrylic polymer in a particle shape having a glass transition temperature of 20°C to 50°C, and a second layer including the particulate acrylic polymer can be formed on the upper surface of a first layer including inorganic particles, and thus it can be seen that the adhesion strength with the electrode is good. In addition, the particulate acrylic polymer does not clog the pores of the first layer, and thus it can be seen that the air permeability is good.

[0222] In addition, in the case of Examples 1 and 2 using a particulate acrylic polymer having a particle size of 200 nm or more, it can be seen that the adhesion strength with the electrode is much better than Example 3 using a particulate acrylic polymer having a particle size of less than 200 nm. It is presumed that this is because, when the separator and the electrode are stacked by lamination, the adhesion area with the electrode increases by the shape of the particulate acrylic polymer as the particle size of the particulate acrylic polymer increases.

[0223] In contrast, Comparative Example 1 used only a non-particulate acrylic polymer having a glass transition temperature of 15°C or less, and did not use a particulate acrylic polymer having a glass transition temperature of 20 to 50°C, and thus the non-particulate acrylic polymer was entangled with the inorganic particles and sank toward the porous polymer substrate, failed to form a second layer on the upper surface of the first layer, and it was difficult to ensure sufficient adhesion strength to the electrode.

[0224] In the case of Comparative Example 2 and Comparative Example 3, two different types of acrylic polymers were used, but both of the acrylic polymers had a glass transition temperature of 15°C or less and did not have a particulate shape, and thus they were entangled with the inorganic particles and sank toward the porous polymer substrate, failed to form a second layer on the upper surface of the first layer. In addition, both of the types of acrylic polymers did not exhibit a particulate shape, resulting in the first layer being clogged pores, and thus it can be seen that the air permeation time was long.

[0225] Since Comparative Example 4 used a particulate acrylic polymer having a glass transition temperature of 20°C or more and a particulate shape, and a second layer could be formed on the upper surface of the first layer, but since the glass transition temperature of the particulate acrylic polymer was higher than 50°C, the particulate shape did not change in the lamination step of the electrode and the separator, which made it difficult to ensure sufficient adhesion strength to the electrode.

[0226] Evaluation Example 3: Measurement of Adhesion Strength between the First Layer and the Porous Polymer Substrate

[0227] The adhesion strength between the first layer and the porous polymer substrate in the separators manufactured in Example 1 and Example 4 and Comparative Example 5 was measured, and the results are shown in Table 2.

[0228] Each of the separators manufactured in Example 1 and Example 4 and Comparative Example 5 was fixed to a glass plate using a double-sided tape, the tape (3M transparent tape) was firmly attached to the exposed first layer, and the adhesion strength between the first layer and the porous polymer substrate of the separator was measured as the force (gf / 15mm) required to peel the tape using an 110yd LS-1.

[0229] [Table 2]

[0230]

[0231] As can be seen from the above Table 2, it can be seen that Example 1 and Example 4 have sufficient adhesion strength between the first layer and the porous polymer substrate. It is presumed that this is because the non-particulate acrylic polymer having a glass transition temperature of 15°C or less is entangled with the inorganic particles and sinks toward the porous polymer substrate, inducing the first layer and the porous polymer substrate to be in a bonded state.

[0232] In contrast, in the case of Comparative Example 5, it can be seen that it is difficult to ensure sufficient adhesion strength between the first layer and the porous polymer substrate. It is presumed that this is because the particulate acrylic polymer having a glass transition temperature of 20 to 50°C is not entangled with the inorganic particles and sinks toward the porous polymer substrate, and thus there is no material having the ability to obtain adhesion strength between the porous polymer substrate and the first layer.

Claims

1. A separator for a secondary battery, comprising: a porous polymer substrate; a first layer formed on at least one surface of the porous polymer substrate and including inorganic particles and a non-particulate acrylic polymer having a glass transition temperature of 15℃ or less, wherein the non-particulate acrylic polymer connects and fixes the inorganic particles; and a second layer formed on a surface of the first layer and including a particulate acrylic polymer having a glass transition temperature of 20 to 50℃, wherein in the first layer and the second layer, a slurry including the inorganic particles, the non-particulate acrylic polymer, and the particulate acrylic polymer is coated on at least one surface of the porous polymer substrate, and the particulate acrylic polymer has a lower density than the inorganic particles to move to an upper portion of the inorganic particles so that the second layer is formed on an upper surface of the first layer. 2.The separator for a secondary battery according to claim 1, wherein the non-particulate acrylic polymer has a glass transition temperature of 0℃ or less. 3.The separator for a secondary battery according to claim 1, wherein the non-particulate acrylic polymer includes a repeating unit derived from a first monomer and a repeating unit derived from a second monomer, and the first monomer has a higher glass transition temperature than the second monomer. 4.The separator for a secondary battery according to claim 3, wherein the first monomer includes at least one of methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, and t-butyl methacrylate. 5.The separator for a secondary battery according to claim 3, wherein the second monomer includes at least one of 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate. 6.The separator for a secondary battery according to claim 3, wherein the repeating unit derived from the second monomer is included in an amount of 60% by weight or more based on 100% by weight of the non-particulate acrylic polymer. 7.The separator for a secondary battery according to claim 1, wherein the particulate acrylic polymer includes a repeating unit derived from a third monomer and a repeating unit derived from a fourth monomer, and the third monomer has a higher glass transition temperature than the fourth monomer. 8.The separator for a secondary battery according to claim 7, wherein the third monomer includes at least one of styrene, vinyl acetate, and acrylonitrile. 9.The separator for a secondary battery according to claim 7, wherein the fourth monomer includes at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, and ethylene. 10.The separator for a secondary battery according to claim 7, wherein a weight ratio of the repeating unit derived from the third monomer to the repeating unit derived from the fourth monomer is 1.5:8.5 to 4:

6.

11. The separator for a secondary battery according to claim 1, wherein the particulate acrylic polymer has an average particle size of 200 nm to 800 nm.

12. The separator for a secondary battery according to claim 1, wherein a ratio of a density of the particulate acrylic polymer to a density of the inorganic particles is 0.5 or less.

13. The separator for a secondary battery according to claim 1, wherein the density of the particulate acrylic acid polymer is 1.5 g / m2 or less. 3 or less.

14. The separator for a secondary battery according to claim 1, wherein the density of the inorganic particles is 2.0 g / m2 or more. 3 or more.

15. The separator for a secondary battery according to claim 1, wherein the separator for a secondary battery has an adhesion strength to an electrode of 30 gf / 25 mm to 200 gf / 25 mm.

16. The separator for a secondary battery according to claim 1, wherein the separator for a secondary battery has an air permeability of 10 sec / 100 cc to 300 sec / 100 cc.

17. The separator for a secondary battery according to claim 1, wherein an adhesion strength between the porous polymer substrate and the first layer is 10 gf / 15 mm to 300 gf / 15 mm.

18. A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the separator for a secondary battery according to any one of claims 1 to 17.

19. A method of manufacturing a lithium secondary battery, comprising: (S1) preparing a porous polymer substrate; (S2) preparing an aqueous slurry including inorganic particles, a non-particulate acrylic polymer having a glass transition temperature of 15°C or less, and a particulate acrylic polymer having a glass transition temperature of 20°C to 50°C; (S3) coating the aqueous slurry on at least one surface of the porous polymer substrate; and (S4) drying a result of the (S3), wherein the non-particulate acrylic polymer adheres the inorganic particles to each other to hold the inorganic particles together, and wherein the particulate acrylic polymer has a lower density than the inorganic particles to move to an upper portion of the inorganic particles to form a second layer on an upper surface of a first layer including the inorganic particles.

20. The method of manufacturing a lithium secondary battery according to claim 19, wherein a ratio of a density of the particulate acrylic polymer to a density of the inorganic particles is 0.5 or less.

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