Composite separator and secondary battery using the same

Abstract

To provide a composite separator that has excellent electrode adhesion and is improved to prevent the occurrence of a blocking phenomenon when the separator is wound up.SOLUTION: The present invention provides a composite separator including a porous substrate, a ceramic coated separator including a ceramic layer formed on at least one surface of the porous substrate, and an electrode adhesive layer as the outermost layer on at least one surface of the ceramic coated separator, and the electrode adhesive layer is made of two or more types of organic particles having different average particle sizes (D50) and different glass transition temperatures.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a composite separator and a secondary battery including the same. [Background technology]

[0002] Conventional high-temperature separators made from porous substrates, porous substrates, and porous ceramic layers (or inorganic particle layers) on one or both sides of the porous substrate often have poor adhesion to electrodes, resulting in separation of the separator and electrodes during the cell assembly process, resulting in distortion and deformation of the electrode assembly. In particular, if a separator with a porous ceramic layer has poor adhesion to the electrodes, misalignment of the electrodes and separator within the jelly roll during cell stacking can occur.

[0003] When a stack cell battery with misalignment as described above is operated, local resistance due to the misalignment or physical damage due to continuous use can cause a short circuit between the electrodes, posing safety issues such as fire.

[0004] Furthermore, in recent years, secondary batteries have tended to have higher capacities and larger sizes for application in electric vehicles, etc. It is even more important to solve the above problems for a conventional separator (ceramic coated separator, CCS) in which a porous ceramic layer (inorganic particle layer) is formed on one or both sides of a porous substrate in order to be used in automotive batteries, which require high capacity and high heat resistance.

[0005] One method for improving the adhesion between a separator and an electrode involves applying a solution of an adhesive organic material to the surface of the separator and drying it to form an adhesive layer on the separator. However, the adhesive organic material reduces breathability and makes it difficult to form a thin film, resulting in poor electrode adhesion.

[0006] Therefore, when the separator is wound up, a blocking phenomenon occurs frequently, in which adhesive organic materials are transferred to the opposite surface and detached. Furthermore, there are still problems to be solved, such as a decrease in the ionic conductivity of the separator and / or a decrease in thickness during alignment of the electrode assembly, resulting in a decrease in battery performance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Registration No. 4414165 (Publication Date: 2005.03.03) Summary of the Invention [Problem to be solved by the invention]

[0008] According to one aspect of the present disclosure, there is provided a composite separator having an adhesive layer formed on one or both sides of a porous separator, the adhesive layer including a particulate organic binder having specific physical properties, thereby providing excellent electrode adhesion and reducing the occurrence of blocking during winding of the separator. The porous separator may be a porous separator made of a porous substrate, or a porous separator having a porous substrate and a porous ceramic layer formed on one or both sides of the porous substrate.

[0009] Another object of the present invention is to provide a composite separator that, when wound up and then unwound again to be used as a separator, can maintain its initial adhesive strength without a decrease in adhesive strength, does not cause blocking, and has excellent heat resistance.

[0010] Another object of the present invention is to provide a separator having uniform lithium ion conductivity over the entire area of ​​the separator.

[0011] Another object of the present invention is to provide a composite separator with excellent anti-blocking properties, in which blocking does not occur between adhesive layers, adhesive layers and ceramic layers, or adhesive layers and porous substrates in a wound roll, even when the wound separator is stored not only at room temperature of around 25°C but also at high temperatures of 50 to 70°C during transportation and storage.

[0012] Furthermore, it has been found that if no blocking occurs when the adhesive layers of a composite separator are brought into contact with each other and pressed at 50°C under a pressure of 1.7 MPa for 2 hours, and then peeled at a speed of 300 mm / min through a 180° angle, the above-mentioned conventional problems can be solved, and an object of the present invention is to provide a composite separator that satisfies this requirement.

[0013] Another object of the present invention is to provide a separator that provides excellent cell capacity retention in a lithium ion secondary battery using the separator according to one embodiment of the present disclosure, for example, a capacity retention rate of 85% or more or 90% or more of the initial capacity after 300 cycles.

[0014] The composite separator of the present disclosure and a secondary battery including the same can be widely applied in green technology fields such as electric vehicles, battery charging stations, and other battery-based solar and wind power generation. In addition, the composite separator of the present disclosure and a secondary battery including the same can be used in eco-friendly electric vehicles, hybrid vehicles, and the like that reduce air pollution and greenhouse gas emissions and prevent climate change. [Means for solving the problem]

[0015] One aspect of the present disclosure is a composite separator for a secondary battery including an adhesive layer as an outermost layer on at least one surface of a porous separator, the adhesive layer includes first organic particles and second organic particles having different average particle sizes (D50) and different glass transition temperatures, the first organic particles having a lower glass transition temperature and a smaller average particle size than the second organic particles; The present invention provides a composite separator for a secondary battery, wherein the first organic particles and the second organic particles satisfy the following relational expression 1:

[0016] [Equation 1] 3≦T2 / T1×R2 / R1≦8.5

[0017] In the above Relational Formula 1, T1 is the glass transition temperature (°C) of the first organic particles, T2 is the glass transition temperature (°C) of the second organic particles, R1 is the average particle size (μm) of the D50 of the first organic particles, and R2 is the average particle size (μm) of the D50 of the second organic particles.

[0018] In one embodiment, the first organic particles may have a glass transition temperature of 90° C. or lower, and the second organic particles may have a glass transition temperature of 95° C. or higher.

[0019] In one embodiment, the first organic particles may have an average particle size of 100 to 1000 nm, and the second organic particles may have an average particle size of 500 to 2000 nm.

[0020] In one embodiment, the first organic particles may have a glass transition temperature of 90°C or lower and an average particle size of 100 to 1000 nm, and the second organic particles may have a glass transition temperature of 95°C or higher and an average particle size of 500 to 2000 nm, but are not limited thereto.

[0021] In one embodiment, the difference in glass transition temperature between the first organic particles and the second organic particles may be 5° C. or more, but is not limited thereto.

[0022] In one embodiment, the average particle size of the second organic particles may be at least twice the average particle size of the first organic particles, but is not limited to this.

[0023] In one embodiment, the weight ratio of the first organic particles to the second organic particles may be 50 to 99:50 to 1, but is not limited thereto.

[0024] In one embodiment, the first organic particles and the second organic particles may be acrylic organic particles.

[0025] In one embodiment, the porous separator may be a porous substrate, or a porous substrate having a porous ceramic layer containing inorganic particles formed on one or both sides thereof.

[0026] In one embodiment, the inorganic particles of the porous ceramic layer may have an average particle size D50 of 50 nm to 2 μm, but is not limited thereto.

[0027] In one embodiment, the inorganic particles of the porous ceramic layer may include first inorganic particles having an average particle size D50 of 50 to 500 nm and second inorganic particles having an average particle size D50 of 500 to 2000 nm, but are not limited thereto.

[0028] In one embodiment, the porous ceramic layer may have inorganic particles bound together by a binder, with pores formed between the inorganic particles.

[0029] In one embodiment, the porous substrate may be a polyolefin-based porous film.

[0030] In one embodiment, the composite separator may have a heat shrinkage rate at 150°C of 3% or less in both the machine direction and the width direction.

[0031] Another aspect of the present disclosure provides a lithium secondary battery including the composite separator according to the above aspect. [Effects of the Invention]

[0032] A composite separator according to one embodiment of the present disclosure can provide a separator that has excellent adhesive strength to electrodes and is improved in terms of the blocking phenomenon that occurs during winding.

[0033] Furthermore, it is possible to provide a separator that has excellent heat resistance, little thermal shrinkage, and excellent battery safety.

[0034] Furthermore, when the separator is wound up and then unwound again for use, there is little change in adhesive strength, and it is possible to provide a separator that has excellent adhesive strength to electrodes.

[0035] Furthermore, a composite separator can be provided that does not experience a phenomenon in which the coating layer peels off due to blocking even when the wound separator is exposed to high temperatures during storage and transportation. Specifically, a composite separator with excellent anti-blocking properties can be provided in which blocking does not occur between adhesive layers, adhesive layers and ceramic layers, or adhesive layers and porous substrates of wound rolls even when the wound separator is stored at room temperatures of around 25°C or higher, or at high temperatures of 50 to 70°C during transportation and storage.

[0036] Furthermore, a composite separator can be provided in which no blocking occurs when adhesive layers of the separator of the present disclosure are brought into contact with each other, pressurized at 50°C and a pressure of 1.7 MPa for 2 hours, and then peeled off at a 180-degree angle at a rate of 300 mm / min.

[0037] In addition, during cell stacking, the electrodes and separators are well aligned within the jelly roll, preventing misalignment during the process and providing a uniform battery without thickness variations.

[0038] In addition, it is possible to provide a battery having uniform lithium ion conductivity across the entire area of ​​the separator, and maintaining a capacity retention rate of 85% or more, 90% or more, or 95% or more of the initial capacity after 300 cycles. [Brief explanation of the drawings]

[0039] [Figure 1]1 is an SEM photograph of a composite separator according to Example 1 of the present disclosure. [Figure 2] 1 is an SEM photograph of a composite separator according to Comparative Example 1 of the present disclosure. [Figure 3] FIG. 1 is a cross-sectional view of a composite separator according to one embodiment of the present disclosure. [Figure 4] FIG. 1 is a cross-sectional view of a composite separator according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0040] The present disclosure will now be described in detail, but this is for illustrative purposes only and the present disclosure is not limited to the specific embodiments described by way of example.

[0041] Furthermore, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms used in describing this disclosure are merely for the purpose of effectively describing specific embodiments and are not intended to limit this disclosure.

[0042] Also, as used in the specification and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0043] Furthermore, when a part is said to "comprise" a certain component, this means that it may further include other components, rather than excluding other components, unless otherwise specified.

[0044] Furthermore, unless otherwise defined, when a layer or member is said to be "on" another layer or member, this includes not only the case where a layer or member is in contact with the other layer or member, but also the case where another layer or member exists between the two layers or two members.

[0045] Furthermore, the terms "about," "substantially," and the like used in this specification are used to mean a numerical value or a value close to that numerical value when manufacturing and material tolerances inherent in the referred meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of the disclosure content in which precise or absolute numerical values ​​are mentioned for the purpose of understanding the present disclosure.

[0046] In this disclosure, "average particle size" refers to "D50," which means the particle size of inorganic particles corresponding to 50% of the total volume fraction. The average particle size can be derived from the particle size distribution results obtained by collecting samples of the inorganic and organic particles to be measured in accordance with ISO 13320-1 and analyzing them using a Microtrac S3500. Furthermore, "D90" means the particle size of particles corresponding to 90% of the total volume fraction, and "D10" means the particle size of inorganic and organic particles corresponding to 10% of the total volume fraction. D90 and D10 can be derived using the same method as D50.

[0047] In the present disclosure, "organic particles" refers to particulate organic binders. In addition, to distinguish between particles having different average particle sizes (D50) and different glass transition temperatures, they are referred to as first organic particles and second organic particles, respectively.

[0048] In this disclosure, the term "glass-transition temperature (T g "Glass transition temperature" refers to the temperature range in which glass transition occurs, and refers to the value measured using a dilatometer or a differential scanning calorimeter (DSC).

[0049] In the present disclosure, the term "composite separator" refers to a porous separator having an adhesive layer formed on one or both sides thereof, and the "porous separator" may be in the form of a porous substrate itself, or may be in the form of a porous ceramic layer (a "porous inorganic particle layer" may also be used in the same sense) formed on one or both sides of the porous substrate, in which inorganic particles formed on the porous substrate are fixed and connected with a binder, and pores are formed among the inorganic particles.

[0050] In the present disclosure, "blocking" refers to a measurement performed by placing two composite separators in contact with each other. Specifically, when an adhesive layer is formed on only one side of a composite separator, two composite separators are prepared, and the composite separators are laminated together with their adhesive layers facing each other. Then, pressure is applied at 50°C and a pressure of 1.7 MPa for two hours, followed by 180-degree peeling at a speed of 300 mm / min to evaluate blocking. When an adhesive layer is formed on both sides of a composite separator, two composite separators are prepared, and either one of the two separators is selected and laminated together with its adhesive layers facing each other. Then, pressure is applied at 50°C and a pressure of 1.7 MPa for two hours, followed by 180-degree peeling at a speed of 300 mm / min to evaluate blocking. In this case, the adhesive layers formed on both sides may have the same composition.

[0051] One aspect of the present disclosure will be described below.

[0052] One aspect of the present disclosure relates to a composite separator 100 including an adhesive layer 120 formed on one or both sides of a porous substrate or a porous separator 110 having a porous ceramic layer (which may also be referred to as a porous inorganic particle layer) in which inorganic particles formed on one or both sides of the porous substrate are connected and fixed by a binder, forming pores between the inorganic particles, as shown in FIGS. 3 and 4 , wherein the adhesive layer is made of two or more types of organic particles having different average particle sizes (D50) and different glass transition temperatures.

[0053] One aspect of the present disclosure provides a composite separator including an adhesive layer on at least one outermost layer of a porous separator, wherein the adhesive layer includes first organic particles and second organic particles having different average particle sizes (D50) and different glass transition temperatures, the first organic particles having a lower glass transition temperature and a smaller average particle size than the second organic particles, and the first organic particles and second organic particles satisfy the following relational expression 1:

[0054] [Equation 1] 3≦T2 / T1×R2 / R1≦8.5

[0055] In the above Relational Formula 1, T1 is the glass transition temperature (°C) of the first organic particles, T2 is the glass transition temperature (°C) of the second organic particles, R1 is the average particle size (μm) of the D50 of the first organic particles, and R2 is the average particle size (μm) of the D50 of the second organic particles.

[0056] By satisfying Relational Expression 1, it is possible to prevent blocking from occurring during storage and transportation of the composite separator after being wound into a roll, and also to achieve the effects of maintaining good alignment during assembly of a secondary battery, thereby improving the capacity retention rate of the battery.

[0057] In one embodiment, the adhesive layer may be an adhesive layer having only the first organic particles and the second organic particles.

[0058] In one embodiment, the adhesive layer may be laminated facing the negative electrode and the positive electrode of the lithium secondary battery. That is, the adhesive layer may be laminated on the negative electrode or the positive electrode during assembly of the lithium secondary battery, thereby exhibiting adhesiveness.

[0059] In one embodiment, the porous substrate, porous ceramic layer, and adhesive layer may have substantially the same or different areas. For example, the porous ceramic layer may be formed over the entire surface of the porous substrate with the same area, or may be formed with an area smaller than that of the porous substrate. Furthermore, the adhesive layer may be formed over the entire surface of the porous substrate or porous ceramic layer with the same area, or may be formed with an area smaller than that of the porous substrate or porous ceramic layer.

[0060] For example, the area on which the adhesive layer is formed may be 10 to 100% of the total area of ​​the porous ceramic layer or porous substrate, or may be 99% or less, 95% or less, 90% or less, 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, 30% or less, 20% or less, 10% or more, 20% or more, 50% or more, or an area between the above values. For example, the adhesive layer may be formed over an area of ​​10 to 90% or 20 to 80%, but is not particularly limited as long as the object of the present disclosure can be achieved.

[0061] In one embodiment, the porous ceramic layer may be formed to a thickness of 1 to 50%, 1 to 45%, 1 to 40%, or 1 to 35% of the total thickness of the composite separator. For example, the thickness may be 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, 1 μm or less, or 1 to 5 μm, or any range therebetween, but is not limited thereto.

[0062] In one embodiment, the adhesive layer has a coating amount of 0.05 to 1.0 g / m 2 , 0.1~0.8g / m 2 , 0.1~0.5g / m 2 , or 0.1 to 0.3 g / m 2 or any range therebetween, but is not limited thereto. In one embodiment, the adhesive layer may be formed by attaching the first organic particles and the second organic particles to at least one surface of the porous separator in the above-mentioned coating amount. In the coating amount, the weight is the total weight of the first organic particles and the second organic particles.

[0063] In one embodiment, the composite separator of the present disclosure includes first and second organic particles in the adhesive layer, the first organic particles having different average particle sizes (D50) and different glass transition temperatures. The first organic particles have a lower glass transition temperature and a smaller average particle size than the second organic particles. When the first and second organic particles satisfy the following relational expression 1, blocking does not occur between the adhesive layers. The present inventors discovered that if no blocking occurs when evaluating the presence or absence of blocking between the adhesive layers using the above method, blocking can be prevented even under harsh conditions encountered during storage and transportation of the composite separator after winding, leading to the completion of the presently disclosed invention. For example, it was confirmed that no blocking occurred even when the wound roll was stored at 50 to 70°C for 7 days. The presence or absence of blocking is evaluated using the measurement method described below. Furthermore, "no blocking occurs" refers to a range in which blocking is OK or PASS in the blocking evaluation method described below.

[0064] [Equation 1] 3≦T2 / T1×R2 / R1≦8.5

[0065] In the above Relational Formula 1, T1 is the glass transition temperature (°C) of the first organic particles, T2 is the glass transition temperature (°C) of the second organic particles, R1 is the average particle size (μm) of the D50 of the first organic particles, and R2 is the average particle size (μm) of the D50 of the second organic particles.

[0066] In one embodiment, the adhesive layer may have an adhesive strength to the positive electrode of 5 gf / cm or more, 6 gf / cm or more, 7 gf / cm or more, 8 gf / cm or more, 9 gf / cm or more, 10 gf / cm or more, or 20 gf / cm or less, or any range therebetween. While a higher adhesive strength is preferable, the adhesive strength to the positive electrode may be 5 to 15 gf / cm to prevent blocking after winding and facilitate alignment during battery assembly. When the adhesive strength satisfies the above range, the effect of preventing blocking from occurring not only at room temperature but also at 50 to 70°C can be more advantageously achieved.

[0067] The positive electrode may be formed from a positive electrode slurry prepared by adding, but not limited to, lithium cobalt composite oxide (LiCoO), carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent.

[0068] In one embodiment, the adhesive layer may have an adhesive strength to the negative electrode of 2 gf / cm or more, 3 gf / cm or more, 4 gf / cm or more, 5 gf / cm or more, 6 gf / cm or more, 7 gf / cm or more, or 15 gf / cm or less, or any range therebetween, but is not limited thereto.

[0069] The negative electrode may be formed from a negative electrode slurry prepared by adding, but not limited to, artificial graphite, acrylic latex as a binder, and carboxymethyl cellulose as a thickener to water as a solvent.

[0070] The composite separator according to one embodiment of the present disclosure may have a thermal shrinkage rate of 3% or less, 2% or less, 1.5% or less, 1% or less, or 0.5% or less after being left for 1 hour at 150° C. The low thermal shrinkage rate described above can prevent fires and explosions caused by abnormal phenomena such as a sudden temperature rise inside the lithium secondary battery.

[0071] In addition, a lithium secondary battery including a composite separator according to an embodiment of the present disclosure can be provided having a discharge capacity ratio of 90% or more, 95% or more, or 97% or more, calculated by the following formula during cycle evaluation, in which the discharge capacity is measured after charging and discharging the secondary battery 300 times at a discharge rate of 1C based on an initial cell capacity of 1800mAh.

[0072] Discharge capacity ratio = (measured capacity of battery after 300 cycles) / initial battery capacity

[0073] Furthermore, a composite separator according to one embodiment of the present disclosure is prepared by cutting an electrode into pieces 4 cm wide and 6 cm long, stacking four cut positive electrodes and four cut negative electrodes alternately on the surface of the composite separator, and then subjecting the electrodes to a pressure of 10 kgf / cm in an atmosphere at a temperature of 80°C. 2 It is possible to provide a composite separator in which substantially no electrodes fall off when the separator is lifted vertically after heating and pressurizing at 400°C for 30 seconds.

[0074] Furthermore, the present disclosure can provide a composite separator in which, when the composite separator having the ceramic layer and the adhesive layer is wound up to 1000 m or more, stored in ovens at 50°C and 70°C for 12 hours each, and then the wound composite separator is unwound, there is no inter-surface adhesion between the facing adhesive layers and no detachment of the ceramic layer.

[0075] Hereinafter, each configuration of the composite separator according to one embodiment of the present disclosure will be described with examples.

[0076] [Porous separator] In one embodiment of the present disclosure, the porous separator may be made of a porous substrate, or may be a porous substrate having a porous ceramic layer containing inorganic particles formed on one or both sides thereof.

[0077] The porous substrate may be a film, sheet, or the like made of a polyolefin resin, and may be any microporous membrane accepted in the art, such as a nonwoven fabric, paper, or a microporous membrane, without limitation, as long as it has pores and is applicable to batteries.

[0078] The polyolefin resin may be a polyolefin resin alone or in a mixture, specifically a mixture of one or more selected from polyethylene, polypropylene, and copolymers thereof. The porous substrate may be a polyolefin resin alone, or may be a substrate primarily composed of a polyolefin resin and further containing inorganic or organic particles. The porous substrate may be used in a laminated form, for example, a substrate composed of multiple layers of polyolefin resin. It is not excluded that any one or all of the substrate layers in the multilayer structure may contain inorganic and organic particles in the polyolefin resin.

[0079] The thickness of the porous substrate is not particularly limited, but may be 5 to 30 μm. The porous substrate may generally be a porous substrate prepared by stretching, but is not limited thereto.

[0080] The porous ceramic layer may be formed by binding inorganic particles with a binder, with pores formed between the inorganic particles.

[0081] The binder may be contained in an amount of 0.1 to 20 parts by weight, 0.1 to 10 parts by weight, or 1 to 5 parts by weight per 100 parts by weight of inorganic particles, and any binder commonly used in this field can be used without any particular limitation. As described above, by using a binder in a significantly smaller amount than the inorganic particles, the inorganic particles have a structure in which they are connected to each other, and pores are formed when the inorganic particles come into surface contact, thereby ensuring porosity.

[0082] Examples of the binder include various water-soluble and water-insoluble resins, such as polymethyl methacrylate and its copolymers, and acrylic resins such as polyacrylamide, ester resins, polyamides, polyimides, fluorine-based resins, polyacrylonitrile, polyethylene oxide, cellulose-based resins, polyvinyl alcohol-based resins, polyvinylpyrrolidone, ethylene-vinyl acetate copolymers, and cyanoethyl pullulan, and mixtures thereof. These binders can be used in the form of a solution in a solvent or in particulate form, and are not limited thereto.

[0083] The inorganic particles of the porous ceramic layer may be any inorganic particles commonly used in the relevant field, including, but not limited to, one or more inorganic particles selected from alumina, boehmite, aluminum hydroxide, titanium oxide, barium titanium oxide, magnesium oxide, magnesium hydroxide, silica, clay, and glass powder.

[0084] The inorganic particles may be included in an amount of 70 to 99.5 wt % based on the total weight of the porous ceramic layer (100 wt %). For example, but not limited to, 70 to 99 wt %, 70 to 98 wt %, 80 to 98 wt %, 85 to 98 wt %, or 90 to 98 wt %. When the porous ceramic layer contains the binder and inorganic particles in the above amounts, the pores of the porous ceramic layer can be secured, and adhesion between the porous substrate and the porous ceramic layer or between the inorganic particles can be secured.

[0085] The average particle size (D50) of the inorganic particles is not limited and may be, for example, 50 nm to 2 μm or 50 to 1000 nm.

[0086] In one embodiment, the inorganic particles may be a combination of two or more types of inorganic particles having different average particle sizes. For example, when the inorganic particles have two different average particle sizes, the first inorganic particles may have an average particle size of 50 to 500 nm or 100 to 400 nm, and the second inorganic particles may have an average particle size of 500 to 2000 nm or 600 to 1000 nm.

[0087] In one embodiment, the porous ceramic layer may have a thickness of 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, 1 μm or less, or 1 to 5 μm, or any range therebetween, but is not limited thereto.

[0088] [Adhesive layer] The adhesive layer of the present disclosure may be formed as the outermost layer on at least one surface of the porous separator.

[0089] The adhesive layer of the present disclosure includes first organic particles and second organic particles having different average particle sizes (D50) and different glass transition temperatures, and the first organic particles have a lower glass transition temperature and a smaller average particle size than the second organic particles, and the first organic particles and second organic particles satisfy the following relational formula 1, the object of the present disclosure can be more easily achieved.

[0090] [Equation 1] 3≦T2 / T1×R2 / R1≦8.5

[0091] In the above Relational Formula 1, T1 is the glass transition temperature (°C) of the first organic particles, T2 is the glass transition temperature (°C) of the second organic particles, R1 is the average particle size (μm) of the D50 of the first organic particles, and R2 is the average particle size (μm) of the D50 of the second organic particles.

[0092] A composite separator having an adhesive layer according to the present disclosure can achieve the effect of preventing blocking between adhesive layers even at high temperatures, preventing alignment defects during battery assembly, and imparting excellent battery capacity retention.

[0093] In one embodiment, the first organic particles and the second organic particles can be used without any limitation as long as they satisfy the relational expression 1.

[0094] In one embodiment, the first organic particles and the second organic particles may have a swelling ratio of 300 to 500% when immersed in an electrolyte solution, as determined by the following formula 1. A swelling ratio within the above range is more preferable because it is advantageous for achieving the object pursued by the present disclosure, but is not limited thereto.

[0095] [Formula 1] Swelling ratio = W2 / W1 x 100

[0096] The weight W2 is the weight measured after immersion in the electrolyte, and the weight W1 is the weight measured before immersion in the electrolyte.

[0097] The swelling ratio can be measured by the measurement method described in the Examples below. The electrolyte may be a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2. Particles are placed in the electrolyte and left at 50°C for 48 hours, after which the particles are removed and weighed to calculate the weight change rate.

[0098] In one embodiment for satisfying Relational Formula 1, the first organic particles may have a glass transition temperature of 90° C. or less, and the second organic particles may have a glass transition temperature of 95° C. or more. For example, the first organic particles may have a glass transition temperature of 40 to 90° C., 40 to 85° C., 45 to 85° C., 45 to 70° C., 50 to 70° C., or 50 to 65° C., but are not limited thereto. The second organic particles may have a glass transition temperature of 95 to 150° C., 95 to 130° C., 95 to 110° C., or 100 to 110° C., but are not limited thereto.

[0099] In one embodiment, the first organic particles may have a lower glass transition temperature than the second organic particles, and the difference in glass transition temperature between the first and second organic particles may be 5°C or more. That is, the glass transition temperature of the second organic particles may be 5°C or more, 10°C or more, 20°C or more, 30°C or more, 35°C or more, 40°C or more, 45°C or more, 50°C or more, 55°C or more, 60°C or more, 100°C or less, 90°C or less, 80°C or less, 70°C or less, 65°C or less, or any value between the above values. For example, the glass transition temperature may be higher by 10 to 100°C, 10 to 90°C, 10 to 70°C, 30 to 70°C, 30 to 65°C, 35 to 65°C, 30 to 60°C, 40 to 60°C, 50 to 60°C, or 45 to 55°C than the first organic particles, but is not limited thereto.

[0100] In one embodiment for satisfying the above-mentioned relational expression 1, the first organic particles may have an average particle size of 100 to 1000 nm, and the second organic particles may have an average particle size of 500 to 2000 nm. For example, the first organic particles may have an average particle size of 100 to 1000 nm, 200 to 800 nm, 200 to 600 nm, 300 to 700 nm, 400 to 600 nm, or 500 to 600 nm, but are not limited thereto. The second organic particles may have an average particle size of 500 to 2000 nm, 600 to 1800 nm, 800 to 1500 nm, 900 to 1500 nm, 1000 to 1500 nm, or 1300 to 1500 nm, but are not limited thereto. The average particle size refers to the average particle size defined by D50.

[0101] In one embodiment, the first organic particles may have a smaller average particle size than the second organic particles, and the average particle size of the second organic particles may be at least twice the average particle size of the first organic particles. For example, the difference in average particle size between the second organic particles and the first organic particles may be, but is not limited to, 100 to 2000 nm, 200 to 1500 nm, 300 to 1500 nm, 300 to 1100 nm, 600 to 1100 nm, or 400 to 900 nm.

[0102] In one embodiment, the adhesive layer has a total content of the first organic particles and the second organic particles of 0.05 to 1.0 g / m 2 , 0.1~0.8g / m 2 , 0.1~0.5g / m 2 , 0.2~0.4g / m 2 , or 0.1 to 0.3 g / m 2 The coating amount may be, but is not limited to,

[0103] In one embodiment, the adhesive layer may contain the first organic particles and the second organic particles in a weight ratio of, but not limited to, 50-99:50-1, 55-95:45-5, 60-90:40-10, or 70-90:30-10. Although not limited to the above ranges, by reducing the content of the second organic particles, which have a larger particle size and a relatively higher glass transition temperature, compared to the content of the first organic particles, it is possible to ensure bonding strength with the electrode even at low temperatures and to increase bonding with the porous ceramic layer.

[0104] In one embodiment, the first and second organic particles are polymer particles that can be produced by emulsion polymerization or suspension polymerization, and may be non-crosslinked or crosslinked. For example, the first and second organic particles may be organic particles made of an acrylic polymer, a fluorine-based polymer, or a copolymer thereof. The first and second organic particles may be particles made of the same polymer or different polymers.

[0105] Examples of the acrylic polymer include C1-C 10 It may be a polymer obtained by polymerizing one or more monomers selected from alkyl (meth)acrylates; (meth)acrylates; (meth)acrylonitriles such as acrylonitrile and methacrylonitrile; aromatic vinyl monomers such as styrene, α-methylstyrene, styrene sulfonic acid, butoxystyrene, and vinylnaphthalene; maleimide derivatives such as maleimide and phenylmaleimide; etc. However, it is not limited thereto and may be a polymer obtained by mixing various monomers.

[0106] For example, the first organic particles may be C1-C 10 The copolymer may be an acrylic copolymer obtained by copolymerizing alkyl (meth)acrylate and (meth)acrylonitrile. The (meth)acrylate includes acrylate and methacrylate. 10 The alkyl (meth)acrylate may be, for example, but is not limited to, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, and the like.

[0107] For example, the second organic particles may be an aromatic vinyl monomer such as styrene, C1-C 10 It may be an acrylic copolymer copolymerized with a monomer mixture containing alkyl (meth)acrylate, and the glass transition temperature can be increased by adjusting the content of the aromatic vinyl monomer.

[0108] [Manufacturing method for composite separators] A method for producing the composite separator of the present disclosure will now be described.

[0109] A method for manufacturing a composite separator for a secondary battery according to one embodiment of the present disclosure includes a step of forming an adhesive layer by applying an aqueous dispersion containing first organic particles and second organic particles to one or both surfaces of a porous separator and drying the aqueous dispersion.

[0110] Furthermore, a method for manufacturing a composite separator for a secondary battery according to one embodiment of the present disclosure includes: a) applying a slurry containing inorganic particles and a binder to one or both surfaces of a porous substrate and drying the slurry to form a porous ceramic layer; and b) applying an aqueous dispersion containing first organic particles and second organic particles to one or both surfaces of a ceramic coated separator on which the porous ceramic layer has been formed and drying the aqueous dispersion to form an adhesive layer.

[0111] The respective configurations are the same as those described above.

[0112] The slurry for forming the porous ceramic layer may be an aqueous slurry using water as a dispersion medium.

[0113] The dispersion for forming the adhesive layer may use water as a dispersion medium, and the first and second organic particles may be provided in the form of particles dispersed in water by emulsion or suspension polymerization.

[0114] The coating method may be any conventional method known in the art, without limitation. Non-limiting examples include roll coating, spin coating, dip coating, bar coating, die coating, slit coating, inkjet printing, and combinations thereof.

[0115] The drying step is not particularly limited, but the drying temperature may be 100° C. or lower, for example, 30 to 100° C., or 40 to 100° C. Drying at this temperature dries the coating layer uniformly without affecting the physical properties of the porous substrate, thereby preventing coating defects.

[0116] The method may also include a step of winding the dried film onto a roll for storage and transportation.

[0117] [Lithium secondary battery] One aspect of the present disclosure provides a lithium secondary battery including the composite separator for a secondary battery described above. The lithium secondary battery can be manufactured by including the composite separator for a secondary battery according to one aspect of the present disclosure, a positive electrode, a negative electrode, and a non-aqueous electrolyte solution.

[0118] In one embodiment, the lithium secondary battery is manufactured by a general manufacturing method in which a negative electrode, a composite separator, and a positive electrode are arranged and assembled, and an electrolyte solution is injected to complete the battery, and therefore, no further detailed description will be given here.

[0119] In this case, the positive electrode, negative electrode, and non-aqueous electrolyte may be any material that is commonly used in lithium secondary batteries, without any limitations.

[0120] In one embodiment, the positive electrode and negative electrode can be manufactured by mixing and stirring a positive electrode active material and a negative electrode active material with a solvent, and optionally a binder, a conductive material, a dispersant, etc., to prepare a composition, which is then applied to a metal current collector, dried, and then pressed. The positive electrode active material can be any active material typically used in positive electrodes of secondary batteries. For example, lithium metal oxide particles containing one or more metals selected from the group consisting of Ni, Co, Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, B, and combinations thereof may be used.

[0121] The negative electrode active material may be any active material commonly used in negative electrodes of secondary batteries. A material capable of lithium intercalation is preferred as the negative electrode active material for lithium secondary batteries. In a non-limiting embodiment, the negative electrode active material may be lithium (metallic lithium), soft carbon, hard carbon, graphite, silicon, Sn alloy, Si alloy, Sn oxide, Si oxide, Ti oxide, Ni oxide, Fe oxide (FeO), and lithium-titanium oxide (LiTiO, LiTiO). 12) may be one or more materials selected from the group of negative electrode active materials.

[0122] As the conductive material, a normal conductive carbon material may be used without any particular limitation.

[0123] The non-aqueous electrolyte solution contains a lithium salt as an electrolyte and an organic solvent. The lithium salt may be any one that is commonly used in electrolyte solutions for lithium secondary batteries, without any limitation. + X - It can be expressed as:

[0124] The anion of the lithium salt is not particularly limited, and may be F - , Cl - , Br - , I - , NO 3- , N(CN) 2- , B.F. 4- , ClO 4- , P.F. 6- , (CF3)2PF 4- , (CF3)3PF 3- , (CF3)4PF 2- , (CF3)5PF - , (CF3)6P - , CF3SO 3- , CF3CF2SO 3- , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO 3- , CF3CO 2- , CH3CO 2- , SCN - , and (CF3CF2SO2)2N -The organic solvent may be any one or a mixture of two or more selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone, tetrahydrofuran, and the like.

[0125] The non-aqueous electrolyte may be injected into an electrode structure including a positive electrode, a negative electrode, and a composite separator interposed between the positive electrode and the negative electrode.

[0126] The shape of the lithium secondary battery is not particularly limited, and may be selected from a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, and the like.

[0127] Although the embodiments of the present invention have been described in detail above, those skilled in the art may modify and implement the present invention in various ways without departing from the spirit and scope of the present invention as defined in the appended claims. Therefore, future changes to the embodiments of the present invention cannot depart from the technology of the present invention.

[0128] Hereinafter, the embodiments of the present disclosure will be further described with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are merely illustrative of the present invention and do not limit the scope of the appended claims. It is obvious to those skilled in the art that various changes and modifications of the embodiments are possible within the scope and technical spirit of the present disclosure, and it goes without saying that such changes and modifications also fall within the scope of the appended claims.

[0129] 1.Adhesion force to electrode The electrodes were cut into 4cm wide x 6cm long pieces, and four cut positive electrodes and four cut negative electrodes were alternately stacked on the surface of a composite separator. After that, they were subjected to a pressure of 10kgf / cm under an atmosphere of 80°C. 2 After adhering for 30 seconds, the electrode was spread out and the number of electrodes attached was counted. A: 8 sheets, B: 6-7 sheets, C: 4-5 sheets, D: Less than 4 sheets

[0130] The positive and negative electrodes used in the evaluation were manufactured as follows.

[0131] Fabrication of cathode: 94 wt% of lithium cobalt composite oxide (LiCoO2) as the cathode active material, 3.5 wt% of carbon black as the conductive material, and 2.5 wt% of polyvinylidene fluoride (PVDF) as the binder were added to N-methyl-2-pyrrolidone (NMP) as the solvent to fabricate cathode slurry. The resulting slurry was applied to a 30 μm-thick aluminum (Al) thin film, dried at 120°C, and then roll-pressed to fabricate a 150 μm-thick cathode.

[0132] Negative electrode fabrication: 95 wt%, 3 wt%, and 2 wt% of artificial graphite, binder (acrylic latex with a Tg of -52°C), and thickener (Carboxymethyl cellulose (CMC)) were added to water as a solvent to fabricate a negative electrode mixture slurry. The resulting slurry was applied to a 20 μm-thick copper (Cu) thin film, dried at 120°C, and pressed using a roll press to fabricate a 150 μm-thick negative electrode.

[0133] 2.Blocking Test Two samples were prepared, the adhesive layers of which were brought into contact with each other and pressed at 1.7 MPa at 50°C for two hours. After that, the samples were peeled at a speed of 300 mm / min at a 180° angle to evaluate whether or not the coating layer would separate between the adhesive layers. Visual observation and SEM confirmation were used to evaluate whether or not peeling of the coating layer would occur within a 50 x 50 μm area. PASS: When five 50 x 50 μm areas are randomly selected across the entire sample area and checked visually and by SEM, no peeling of the coating layer occurs. OK: No detachment of the coating layer is confirmed by visual observation, and peeling of less than 2% of the coating layer is confirmed when five 50 x 50 μm areas are randomly selected across the entire sample area using an SEM. Fail: Peeling of the coating layer is observed visually, or peeling of the coating layer of 2% or more is confirmed when five 50 x 50 μm areas are randomly selected across the entire sample area using an SEM. If peeling of 2% or more is confirmed in even one of the five random areas, the sample is deemed to have failed.

[0134] 3.Heat shrinkage rate A 10 cm x 10 cm separator was left at 150°C for 1 hour, after which the area reduction rate was measured and the thermal shrinkage rate was calculated using the following formula.

[0135] Heat shrinkage rate (%) = ((length before heating - length after heating) / length before heating) x 100

[0136] MD is the heat shrinkage in the machine direction, and TD is the heat shrinkage in the transverse direction.

[0137] 4. Increase in breathability The increase in air permeability after forming the adhesive layer on the ceramic coated separator was calculated by measuring the Gurley permeability and using the following formula.

[0138] Increase in air permeability = Air permeability of coated separator after adhesive layer binder coating - Air permeability of coated separator after ceramic coating

[0139] Gurley permeability is measured using a Toyo Seiki Densometer in accordance with ASTM D726 standard. It is the rate at which 100cc of air penetrates 1 inch of the separator. 2 Record the time it takes to pass through the area in seconds, in units of sec / 100cc.

[0140] 5. Ratio of battery life discharge capacity to initial capacity The pouch-type batteries were assembled by stacking in the same manner as in the electrode adhesion evaluation. Before injecting the electrolyte into each assembled battery, the batteries were heated at 80°C and 10 kgf / cm. 2 The resultant was subjected to thermocompression bonding for 30 seconds, and an electrolyte solution of 1M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) = 3:5:2 (volume ratio) was injected to fabricate a lithium secondary battery.

[0141] Each of the prepared batteries was charged and discharged 300 times at a discharge rate of 1C, and then the discharge capacity was measured to evaluate the degree of decrease in the initial capacity.

[0142] Discharge capacity ratio = (measured battery capacity after 300 cycles) / initial battery capacity (1.8Ah)

[0143] 6.Measuring method of average particle size The average particle size D50 was measured using a particle size analyzer, Microtrac S3500, in accordance with the ISO 13320-1 standard.

[0144] 7.Method for measuring glass transition temperature Analysis was performed using DSC (Differential Scanning Calorimetry, Mettler Toledo, DSC-822E). A 5 mg sample solidified by removing the solvent was heated from -50 to 200°C under nitrogen at a scanning rate of 10°C / min until the sample was completely melted. The sample was then cooled at a rate of 10°C / min until solidified, and the glass transition temperature was measured by heating again at a rate of 10°C / min from -50 to 200°C. The glass transition temperature was determined as the inflection point where the curve in the DSC graph changes significantly.

[0145] 8. Thickness (μm) The thickness of the separator was determined by the following method: After stacking 10 separators, the thickness was measured at five random locations in the width direction using a Mitutoyo thickness measuring device, and the sum of these measurements was divided by 5 to obtain the average thickness of the 10 separators. This value was then further divided by 10 to obtain the overall average thickness of a single separator.

[0146] [Example 1] 1) Manufacturing of ceramic coated separators (porous separators coated with a porous ceramic layer) The following slurry for the inorganic particle layer was coated on both sides of a 9 μm-thick polyethylene porous substrate (SK Innovation, ENPASS) with a Gurley permeability of 126 sec / 100 cc using a bar coater at a speed of 5 m / min to form a coating layer, which was then thoroughly dried at 40°C to form a porous ceramic layer. After drying, the coating thickness of the ceramic layer on both sides was 1.5 μm.

[0147] The slurry for the inorganic particle layer was prepared by mixing 29.1 wt % of boehmite particles with an average particle size of 300 nm, 67.9 wt % of boehmite particles with an average particle size of 700 nm, and 3 wt % of polyacrylamide resin, adding water as a solvent and stirring the mixture to produce a composition with a solid concentration of 25 wt %.

[0148] 2) Manufacturing of composite separators The adhesive layer was formed on both surfaces of the ceramic-coated separator by coating the adhesive layer coating liquid described below with a bar coater at a speed of 5 m / min, and the adhesive layer was thoroughly dried at 40°C and wound into a roll. The thickness of each adhesive layer on both sides was 0.5 μm, and the coating amount was 0.3 g / m. 2 It was.

[0149] The adhesive layer coating liquid was prepared by mixing 17.5 g of a first organic particle dispersion (acrylic polymer using butyl methacrylate, methyl methacrylate, and acrylonitrile as monomers, D50: 300 nm, Tg: 40°C, solid content: 40 wt%) and 15 g of a second organic particle dispersion (acrylic polymer using styrene and methyl methacrylate as monomers, D50: 900 nm, Tg: 100°C, solid content: 20 wt%). The solid content of the total solution was 70:30 by weight of the first organic particles:second organic particles.

[0150] The physical properties of the manufactured separator were evaluated and are shown in Table 1. The surface was also observed and is illustrated in Figure 1. As shown in Figure 1, it was confirmed that the first organic particles 10 and the second organic particles 20 were uniformly distributed.

[0151] [Examples 2 to 9] Composite separators were manufactured in the same manner as in Example 1, except that coating solutions for adhesive layers were used in which the types and contents of organic particles were changed as shown in Table 1 below.

[0152] The physical properties of the produced separator were evaluated and are shown in Table 1.

[0153] [Comparative Examples 1 to 4] A composite separator was manufactured in the same manner as in Example 1, except that the coating liquid for the adhesive layer was changed as shown in Table 2 below.

[0154] The physical properties of the produced separator were evaluated and are shown in Table 2.

[0155] [Table 1]

[0156] As shown in Table 1, satisfying Relational Formula 1 resulted in excellent electrode adhesion and anti-blocking properties. In addition, even after forming the adhesive layer, the increase in air permeability was less than that of the ceramic-coated separator, at 20 seconds or less, and the ratio of the battery life discharge capacity to the initial capacity was excellent, at 85% or more.

[0157] [Table 2]

[0158] As shown in Table 2, when Relational Formula 1 does not satisfy the range of the present invention, it is not possible to simultaneously satisfy adhesive strength and anti-blocking property, and it has been confirmed that the battery life is significantly reduced.

[0159] As shown in Comparative Example 1, when the second organic particles were not used, adhesive strength was exhibited, but there was a problem in that blocking occurred.

[0160] As shown in Comparative Example 2, it was confirmed that when the adhesive layer is made up of only second organic particles having a glass transition temperature higher than 80° C., which is the temperature in the bonding process, no adhesive strength is exhibited.

[0161] As shown in Comparative Example 3, it was confirmed that when Relational Expression 1 is not satisfied, adhesive strength is not exhibited and the effect of improving blocking is minimal.

[0162] As shown in Comparative Example 4, it was confirmed that when Relational Expression 1 was not satisfied, the adhesive strength was at Level B and blocking occurred.

[0163] The foregoing is merely illustrative of the application of the principles of the present disclosure, and other arrangements may be included without departing from the scope of the present invention.

[0164] As described above, the present disclosure has been described using specific details and limited examples, but these are merely provided for a more general understanding of the present disclosure, and the present disclosure is not limited to the above examples. Those skilled in the art will appreciate that various modifications and variations can be made from such descriptions.

[0165] Therefore, the idea of ​​the present disclosure should not be limited to the above-described embodiments, and it can be said that not only the scope of the attached claims, but also anything that is equivalent to or has an equivalent modification of the scope of the claims, falls within the scope of the idea of ​​the present disclosure. [Explanation of symbols]

[0166] 10 First organic particle 20 Second organic particle 100 Composite Separator 110 Porous separator 120 Adhesive layer

Claims

1. A composite separator including an adhesive layer on at least one outermost layer of a porous separator, the adhesive layer includes first organic particles and second organic particles having different average particle sizes (D50) and different glass transition temperatures, the first organic particles having a lower glass transition temperature and a smaller average particle size than the second organic particles; The composite separator for a secondary battery, wherein the first organic particles and the second organic particles satisfy the following relational expression 1: [Relationship 1] 3≦T ２ / T １ ×R ２ / R １ ≦8.5 (In the above-mentioned relational formula 1, T １ is the glass transition temperature (°C) of the first organic particles, and T ２ is the glass transition temperature (°C) of the second organic particles, and R １ is the average particle size (μm) of the D50 of the first organic particles, and R ２ is the average particle size (μm) of the D50 of the second organic particles.

2. 2. The composite separator according to claim 1, wherein the first organic particles have a glass transition temperature of 90°C or lower, and the second organic particles have a glass transition temperature of 95°C or higher.

3. 2. The composite separator according to claim 1, wherein the first organic particles have an average particle size of 100 to 1000 nm, and the second organic particles have an average particle size of 500 to 2000 nm.

4. the first organic particles have a glass transition temperature of 90° C. or less and an average particle size of 100 to 1000 nm; 2. The composite separator according to claim 1, wherein the second organic particles have a glass transition temperature of 95° C. or higher and an average particle size of 500 to 2000 nm.

5. 2. The composite separator according to claim 1, wherein the difference in glass transition temperature between the first organic particles and the second organic particles is 5°C or more.

6. 2. The composite separator according to claim 1, wherein the average particle size of the second organic particles is at least twice as large as the average particle size of the first organic particles.

7. 2. The composite separator according to claim 1, wherein the weight ratio of the first organic particles to the second organic particles is 50-99:50-1.

8. 2. The composite separator according to claim 1, wherein the first organic particles and the second organic particles are acrylic organic particles.

9. The composite separator according to claim 1 , wherein the porous separator is a porous substrate or a porous substrate having a porous ceramic layer containing inorganic particles formed on one or both sides thereof.

10. 10. The composite separator of claim 9, wherein the inorganic particles of the porous ceramic layer have an average particle size D50 of 50 nm to 2 μm.

11. 11. The composite separator of claim 10, wherein the inorganic particles of the porous ceramic layer include first inorganic particles having an average particle size D50 of 50 to 500 nm and second inorganic particles having an average particle size D50 of 500 to 2000 nm.

12. The composite separator according to claim 9 , wherein the porous ceramic layer comprises inorganic particles bound together by a binder, and pores are formed between the inorganic particles.

13. The composite separator according to claim 9 , wherein the porous substrate is a polyolefin-based porous film.

14. 2. The composite separator according to claim 1, wherein the composite separator has a heat shrinkage rate at 150°C of 3% or less in both the machine direction and the width direction.

15. A lithium secondary battery comprising the composite separator of any one of claims 1 to 14.

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