Secondary battery separator and secondary battery

Abstract

A secondary battery separator and a secondary battery. The secondary battery separator comprises a base material and a covering layer. The base material comprises a polyolefin microporous membrane with a polyolefin as the main component. The covering layer is configured on at least one surface of the base material. The covering layer comprises inorganic particles and polymer particles, wherein the inorganic particles are consecutively distributed in the covering layer, and the polymer particles are discretely distributed among the inorganic particles and protrude out of the surface of the covering layer. The inorganic particles are accumulated as few as possible in the direction of the polymer particles to form a slope, such that the problem of the film surface being uneven after hot pressing can be mitigated.

Description

A secondary battery separator, a secondary battery Technical Field

[0001] This invention belongs to the field of secondary battery separator technology, specifically relating to a secondary battery separator and a secondary battery. Background Technology

[0002] With the rapid development of energy technologies, energy storage devices are playing an increasingly important role in various fields, especially in electric vehicles and renewable energy. As a key component in energy storage devices, the separator has a significant impact on battery performance and safety. In energy storage devices, the main function of the separator is to separate the positive and negative electrodes to prevent short circuits, while allowing ions to pass through during charging and discharging.

[0003] Traditional membrane designs typically use a single polyolefin microporous membrane as the substrate. While this offers some ion permeability and mechanical strength, improvements are still needed in areas such as enhancing adhesion and reducing thermal shrinkage. To address these issues, researchers have explored introducing a capping layer onto the membrane substrate to enhance its performance.

[0004] Currently, in existing technologies, the capping layer of a diaphragm is typically composed of inorganic fillers and particulate polymers. These inorganic fillers and particulate polymers not only improve the mechanical strength of the diaphragm but also help improve the ion transport performance within the diaphragm. Furthermore, patent application CN116569294A discloses a capping layer comprising inorganic fillers and particulate polymers protruding from the capping layer, wherein the thickness of the particulate polymer is at least 0.1 times that of the inorganic filler portion. The capping layer is formed in a manner that slopes towards the protruding particulate polymer (climbing effect). This arrangement improves the adhesion of the particulate polymer and reduces thermal shrinkage.

[0005] Although the polymer particles in the coating layer are quite prominent, they partially melt and deform during hot-pressing, thus not affecting the smoothness of the coating. However, this type of mixed-coating separator with a climbing effect struggles to maintain ideal smoothness after hot-pressing due to the distribution characteristics of the particulate polymer and inorganic fillers, as well as their interaction with the substrate. Uneven separators not only affect the uniform distribution and efficient transport of ions within the battery but may also pose a risk of internal short circuits, thereby threatening the battery's safety and performance stability. Summary of the Invention

[0006] The purpose of this invention is to design a granular polymer-inorganic particle mixed coating separator with good flatness after hot pressing, and a secondary battery containing the separator.

[0007] In a first aspect, the present invention provides a secondary battery separator comprising a substrate and a cover layer, wherein the substrate comprises a polyolefin microporous membrane with polyolefin as the main component, and the cover layer is disposed on at least one surface of the substrate, characterized in that the cover layer comprises inorganic particles and polymer particles, wherein the inorganic particles are continuously distributed in the cover layer, and the polymer particles are discretely distributed in the inorganic particles and protrude from the surface of the cover layer.

[0008] The maximum distance from the boundary line of the substrate-covering layer to the outline of the protruding polymer particles is defined as L1, the maximum distance from the boundary line of the substrate-covering layer to the outer surface of the inorganic particles of the covering layer is defined as L2, and the thickness of the inorganic particle portion in the covering layer is defined as L0.

[0009] The height of the climbing ball in the covering layer is H < (L1-L2) / 2R, where H = L2-L0, and 2R is the D50 of the polymer particles;

[0010] The degree of sphere climbing of the cover layer is M = H / (L1-L0), where M < 15%.

[0011] Preferably, the secondary battery separator satisfies More preferably, (L1-L2) / R is 0.5-1.5.

[0012] Preferably, the areal density per unit thickness of the covering layer is 0.8-2.0 g / (cm³). 2 ·μm).

[0013] Preferably, the covering layer contains 50-99 parts of inorganic particles and 1-50 parts of polymer particles, based on a total mass of 100 parts of inorganic and polymer particles; more preferably, the mass ratio of inorganic particles to polymer particles is 1-20:1.

[0014] Preferably, the polymer particles have a D50 of 0.5-10 μm, an L0 of 0.1-5 μm, and an L1 of 0.5-10 μm.

[0015] Preferably, the polymer particles include one or more of the following: polyimide water-soluble microspheres, styrene-butadiene rubber microspheres, polyacrylate modified microspheres, polyetherimide, aramid, aramid sulfone, polyvinylidene fluoride, poly(meth)acrylate polymers, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polyvinylidene fluoride-hexafluoropropylene.

[0016] Preferably, the inorganic particles include a first inorganic particle and a second inorganic particle, wherein the mass ratio of the first inorganic particle to the second inorganic particle is (15-35):(1-30); the first inorganic particle has a D50 diameter of D, the second inorganic particle has a D50 diameter of d, D / d≥2, D is 0.3-0.9μm, and d is 0.05-0.5μm.

[0017] Preferably, the inorganic particles include a third inorganic particle, wherein the D50 / D10 of the third inorganic particle is ≥2.

[0018] Preferably, the inorganic particles include one or more of silicon dioxide, aluminum oxide, magnesium oxide, zirconium oxide, titanium oxide, calcium oxide, boehmite, aluminum nitride, boron nitride, barium sulfate, barium titanate, calcium fluoride, and barium fluoride.

[0019] Preferably, the coating layer is formed by coating a slurry onto a substrate, the slurry comprising inorganic particles and polymer particles, the slurry viscosity being 100-260 mPa·s. More preferably, the polymer particles in the slurry have a dry weight of 1%-50% and a wet weight of 0.3%-30%.

[0020] As a preferred embodiment of the slurry, the slurry further includes one or more of carboxymethyl cellulose, binder, regulator, wetting agent, and dispersant. Based on 100 parts by weight of the total slurry, the amount of carboxymethyl cellulose is 0-15 parts by weight, the amount of binder is 0-6 parts by weight, the amount of regulator is 0-5 parts by weight, the amount of wetting agent is 0-1 parts by weight, and the amount of dispersant is 0-2 parts by weight. The amounts of carboxymethyl cellulose, binder, regulator, wetting agent, and dispersant are not all 0. The binder is one or more of styrene-butadiene rubber, acrylic resin, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride, polyethylene oxide, polytetrafluoroethylene, acrylic acid and its derivative copolymers, and acrylamide and its derivative copolymers. The regulator is one or more of polyacrylic acid, polyvinyl alcohol, polyimide, and cellulose. The wetting agent is one or more of anionic surfactant, polyethylene surfactant, and polyol surfactant. The dispersant is one or more of polyvinylpyrrolidone, polyacrylamide, and polyvinyl alcohol.

[0021] Secondly, the present invention provides a secondary battery comprising the secondary battery separator described above.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] The secondary battery separator of this invention features a small height and low degree of inorganic particle "climbing" on the polymer particle surface. The inorganic particles accumulate as little as possible towards the polymer particles to form a slope, thus reducing the problem of uneven film surface after hot pressing. Attached Figure Description

[0024] Figure 1 is a schematic diagram of the cross-section of the battery separator;

[0025] Figure 2 is a SEM image (5k magnification) of the diaphragm prepared using the finished slurry obtained in Example 1;

[0026] Figures 3 and 4 are SEM images of the diaphragm prepared by vertical cutting using the finished slurry obtained in Example 1. Figure 3 is at 10k magnification and Figure 4 is at 5k magnification.

[0027] Figure 5 is a magnified SEM image of the diaphragm prepared from the finished slurry obtained in Comparative Example 1 after vertical cutting (photographed at 5K magnification, partial magnification). Detailed Implementation

[0028] The present invention will now be described in detail through embodiments. It should be noted that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0029] The present invention provides a secondary battery separator comprising a substrate and a cover layer. The substrate comprises a polyolefin microporous membrane with polyolefin as the main component, and the cover layer is disposed on at least one surface of the substrate. The cover layer comprises inorganic particles and polymer particles, wherein the inorganic particles are continuously distributed in the cover layer, and the polymer particles are discretely distributed in the inorganic particles and protrude from the surface of the cover layer.

[0030] The maximum distance from the boundary line of the substrate-covering layer to the outline of the protruding polymer particles is defined as L1, the maximum distance from the boundary line of the substrate-covering layer to the outer surface of the inorganic particles of the covering layer is defined as L2, and the thickness of the inorganic particle portion in the covering layer is defined as L0.

[0031] The height of the climbing ball in the covering layer is H < (L1-L2) / 2R, where H = L2-L0, and 2R is the D50 of the polymer particles;

[0032] The degree of sphere climbing of the cover layer is M = H / (L1-L0), where M < 15%. Specifically, M can be 0%, 3%, 5%, 7%, 9%, 11%, and 13%.

[0033] The battery separator of this invention has limited ball-climbing height and degree, which can achieve an uneven effect during hot pressing. Specifically, the hot-pressed thickness difference is <0.3μm.

[0034] In this invention, Specifically, (L1-L2) / R can be 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, and 1.7. Within this range, the coating achieves a balance between adhesion and dust-removal performance, specifically exhibiting adhesion > 0.8 N / m and dust removal rate < 5%. Preferably, (L1-L2) / R is 0.5-1.5. A value greater than 1.5 indicates that the polymer particles are embedded too shallowly in the coating, increasing the risk of dust removal, while a value less than 0.5 indicates that the polymer particles are embedded too deeply, resulting in insufficient adhesion.

[0035] In a preferred embodiment, L0 is between 0.1 and 5 μm, specifically 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm and 5 μm, and L1 is between 0.5 and 10 μm, specifically 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm and 10 μm.

[0036] The particle size of various organic and inorganic substances described in this invention refers to the volume particle size, for example, D50 refers to Dv50.

[0037] In this invention, the D50 of the polymer particles is 0.5-10 μm, specifically 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, and 10 μm. The D50 of the polymer particles depends on the selection of the polymer raw materials. In particular, the D50 value can be determined by the raw materials, or it can be obtained by taking any five polymer particle samples, measuring their diameters, and taking the average value as the D50 value using electron microscopy.

[0038] It should be noted that units should not be substituted into the formula when calculating. The formula and range proposed in this invention are designed based on the numerical relationship of these parameters and are not affected by whether the dimensions are equivalent.

[0039] In this invention, the areal density per unit thickness of the covering layer is 0.8-2.0 g / (cm³). 2 ·μm), specifically 0.8g / (cm³). 2 ·μm), 1.1g / (cm 2 ·μm), 1.2g / (cm 2 ·μm), 1.4g / (cm 2 ·μm), 1.6g / (cm 2 ·μm), 1.8g / (cm) 2 ·μm) and 2.0g / (cm 2 • μm). The areal density per unit thickness of the capping layer is controlled between 0.8 and 2.0 g / (cm³).2 Within a range of μm, the coating layer exhibits good liquid absorption and retention properties, as well as ion conductivity, while maintaining good thermal shrinkage resistance. Specifically, this is reflected in liquid absorption rate > 80%, thermal shrinkage rate < 3%, and air permeability enhancement rate < 20%. In this invention, the inorganic particles can be a mixture of first inorganic particles and second inorganic particles, with the mass ratio of the first inorganic particles to the second inorganic particles being (15-35):(1-30). The first inorganic particle has a D50 diameter of D, and the second inorganic particle has a D50 diameter of d, with D / d ≥ 2, preferably D / d being 2-5, specifically 2, 2.1, 2.3, 2.5, 3, 3.2, 3.5, 3.8, 4, 4.5, and 5. The first inorganic particle has a D50 diameter of 0.3-0.9 μm, specifically 0.3 μm, 0.5 μm, 0.7 μm, and 0.9 μm; the second inorganic particle has a D50 diameter of 0.05-0.5 μm, specifically 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.3 μm, 0.4 μm, and 0.5 μm. In this invention, the inorganic particles can also be third inorganic particles with a certain particle size distribution. In the particle size distribution, D50 / D10≥2, preferably 2-10, specifically 2, 3, 4, 5, 6, 7, 8, 9 and 10.

[0040] When using two or more inorganic particles with significant size differences, or inorganic particles with a certain size distribution, the relatively smaller inorganic particles have a higher specific surface area. These smaller particles adhere to the surface of the larger inorganic particles, forming a relatively dense, bud-like aggregate. Because the size of the buds randomly generates within a certain range, a loose and porous packing morphology is formed. The van der Waals forces between these inorganic particle aggregates are much greater than the surface tension generated on the polymer particle surface. Therefore, as the slurry deposits and dries, the inorganic particle aggregates tend to settle uniformly near the polymer particles, reducing the climbing effect during slurry deposition and thus minimizing unevenness after diaphragm hot pressing.

[0041] This invention does not impose any particular limitation on the substrate. A polyolefin microporous membrane containing polyolefin as the main component can be selected based on existing lithium-ion battery separators. Typically, the substrate can be a polyolefin-based membrane, such as a polypropylene-based membrane or a polyethylene-based membrane. As a preferred embodiment, the base membrane is a polyethylene-based membrane. The thickness of the base membrane can be 5-12 μm.

[0042] In this invention, the coating layer is formed by coating a slurry onto a substrate. The slurry comprises inorganic particles and polymer particles, and may also contain one or more of CMC, binder, regulator, wetting agent, and dispersant. The viscosity of the slurry is controlled between 100-260 mPa·s. This invention further improves the diaphragm's climbing effect by adjusting the slurry viscosity within a specific range. When the slurry viscosity is appropriate, clusters of inorganic particles resembling flower buds are more easily formed. Adjusting the slurry viscosity can be achieved by adjusting the mass fractions of the regulator, binder, and CMC. Those skilled in the art can obtain the desired slurry viscosity through a limited number of experiments.

[0043] In this invention, the inorganic particles, polymer particles, CMC (carboxymethyl cellulose), binders, regulators, wetting agents, and dispersants are commonly used substances in the art, and this invention does not impose specific limitations. For example, the inorganic particles can be one or more of the following: oxide particles such as silicon dioxide (SiO2), aluminum oxide (Al2O3), magnesium oxide (MgO), zirconium oxide (ZrO2), titanium oxide (TiO2), and calcium oxide (CaO); nitride particles such as boehmite (ALOOH), aluminum nitride (AlN), and boron nitride (BN); and sparingly soluble ionic crystalline particles such as barium sulfate (BaSO4), barium titanate (BaTiO3), calcium fluoride (CaF2), and barium fluoride (BaF2). When the inorganic particles are a mixture of multiple substances, the amounts of different substances can be in any proportion, as long as the total amount meets the limits for the amount of inorganic particles. Preferably, the inorganic particles are boehmite.

[0044] The polymer particles can be one or more of the following: PI (polyimide) water-soluble microspheres, SBR (styrene-butadiene rubber) microspheres, PAA (polyacrylate) modified microspheres, polyetherimide, aramid, aramid sulfone, polyvinylidene fluoride, poly(meth)acrylate polymers, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polyvinylidene fluoride-hexafluoropropylene; preferably, at least one of poly(meth)acrylate polymers, SBR (styrene-butadiene rubber) microspheres, and PI (polyimide) water-soluble microspheres and PAA (polyacrylate) modified microspheres. One-piece molded poly(meth)acrylate polymers are particularly preferred.

[0045] The binder can be one or more of styrene-butadiene rubber, acrylic resin, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride, polyethylene oxide, polytetrafluoroethylene, acrylic acid and its derivative copolymers, and acrylamide and its derivative copolymers; the modifier can be one or more of polyacrylic acid, PVA, polyimide, and cellulose; the wetting agent can be one or more of anionic surfactant, polyethylene surfactant, and polyol surfactant; the dispersant can be one or more of polyvinylpyrrolidone (PVP), polyacrylamide (PAM), and polyvinyl alcohol (PVA). The amount of each raw material can be adjusted conventionally according to actual needs. This invention does not impose specific limitations. For example, the amount of CMC can be 0-15 parts by weight, the amount of binder can be 0-6 parts by weight, the amount of modifier can be 0-5 parts by weight, the amount of wetting agent can be 0-1 parts by weight, and the amount of dispersant can be 0-2 parts by weight.

[0046] The raw materials used above can be obtained through commercial purchase or self-production, and no special designation is made.

[0047] In this invention, the dry weight of polymer particles in the slurry is 1%-50%, specifically 1%, 5%, 10%, 15%, 20%, 30%, 40%, and 50%, and the wet weight is 0.3%-30%, specifically 0.3%, 1%, 5%, 10%, 15%, 20%, and 30%.

[0048] In this invention, taking the total mass of inorganic particles and polymer particles in the covering layer as 100 parts, the covering layer contains 50-99 parts of inorganic particles, specifically 50, 60, 70, 80, 90, and 99 parts, and 1-50 parts of polymer particles, specifically 1, 10, 20, 30, 40, and 50 parts. The mass ratio of inorganic particles to polymer particles is 1-20:1, specifically 1:1, 5:1, 10:1, 15:1, and 20:1.

[0049] The present application will now be described in detail through specific embodiments. In the following embodiments and comparative examples, unless otherwise stated, all parts refer to parts by weight.

[0050] Example 1:

[0051] 7.66 parts of boehmite B (small particle size), 0.3 parts of dispersant polyvinylpyrrolidone, and 54.61 parts of ultrapure water were dispersed at 1300 rpm for 40 min to form a homogeneous slurry. 17.88 parts of boehmite A (large particle size) were added and dispersed at 1300 rpm for 20 min. Then, 6 parts of CMC solution were added and dispersed at 1000 rpm for 20 min. The mixture was then ground at 1000 rpm for 20 min (once) to form a boehmite slurry. 9.6 parts of polymer microspheres (PMMA emulsion) were added to the slurry and dispersed at 40 rpm for 20 min. 3.33 parts of binder styrene-butadiene rubber were added to the slurry and dispersed at 40 rpm for 20 min. 0.57 parts of modifier polyacrylic acid were added to the slurry and dispersed for 20 min. 0.05 parts of wetting agent alkylphenol polyoxyethylene ether were added to the slurry and dispersed for 10 min to form the finished slurry.

[0052] Examples 2 and 3:

[0053] 25.54 parts boehmite C, 0.3 parts dispersant polyvinylpyrrolidone, and 54.61 parts ultrapure water were dispersed at 1300 rpm for 40 min to form a homogeneous slurry. Then, 6 parts CMC solution were added and dispersed at 1000 rpm for 20 min. The mixture was then ground at 1000 rpm for 20 min (once) to form a boehmite slurry. 9.6 parts polymer microspheres (PMMA emulsion) were added to the slurry and dispersed at 40 rpm for 20 min. 3.33 parts styrene-butadiene rubber binder were added to the slurry and dispersed at 40 rpm for 20 min. 0.57 parts polyacrylic acid modifier were added to the slurry and dispersed for 20 min. 0.05 parts alkylphenol polyoxyethylene ether wetting agent were added to the slurry and dispersed for 10 min to form the finished slurry.

[0054] The difference between Example 2 and Example 3 lies in the different boehmite particle sizes used, as detailed in Table 1.

[0055] Example 4:

[0056] The preparation method of Example 4 is the same as that of Example 1, except that the boehmite particle size used is different, as detailed in Table 1.

[0057] Example 5 and Comparative Examples 1-3:

[0058] The preparation methods of Examples 5 and Comparative Examples 1-3 are the same as those of Examples 2-3, except that the boehmite particle size used is different, as detailed in Table 1. The slurry viscosity is lower in Comparative Examples 2 and 3.

[0059] Example 6:

[0060] The preparation method of Example 6 is the same as that of Example 1, except that the type of polymer used is different and the boehmite particle size is different, as detailed in Table 1.

[0061] Example 7:

[0062] The preparation method of Example 7 is the same as that of Example 1, except that the type of polymer used is different, as detailed in Table 1.

[0063] Example 8:

[0064] The preparation method of Example 8 is the same as that of Example 2, except that the boehmite particle size, polymer D50 and coating thickness are different. See Table 1 for details.

[0065] Example 9:

[0066] The preparation method of Example 9 is the same as that of Example 2, except that the boehmite particle size and polymer D50 are different, as detailed in Table 1.

[0067] Comparative Example 4:

[0068] The preparation method of Comparative Example 4 is the same as that of Example 1, except that the D50 of small-particle boehmite and the D50 of polymer are different, as detailed in Table 1.

[0069] Coating process: The finished slurry of the examples and comparative examples is coated on both sides of a 7μm thick base film using a gravure roller or wire rod. After the finished slurry is coated onto the surface of the base film, it is dried in an oven (drying temperature is 75℃, time is 10s) to obtain a battery separator with a coated film.

[0070] Figure 2 is a planar SEM image of the diaphragm prepared from the finished slurry obtained in Example 1; Figures 3 and 4 are SEM images of the diaphragm prepared from the finished slurry obtained in Example 1 after vertical cutting at different magnifications. It can be seen that there is no obvious climbing effect. Figure 5 is an SEM image of the diaphragm prepared from the finished slurry of Comparative Example 1 after vertical cutting. It can be seen that the inorganic particles tend to tilt around the polymer particles.

[0071] [Testing Method]

[0072] Test methods for L0, L1, and L2:

[0073] Figure 1 is a schematic diagram of the diaphragm cross-section, which provides a visual understanding of L0, L1, and L2 as defined in this invention. Reference numeral ① represents polymer particles, and reference numeral ② represents inorganic particles.

[0074] Cross-sectional samples of the battery separator were prepared under conditions of 21±4℃ and ≤65%RH. Electron micrographs of the cross-section were obtained using a Zeiss SIGMA 300 electron microscope. Five arbitrary measurements were taken at five locations on the 5K electron microscope images, and the average value was recorded.

[0075] L0: The distance from the substrate-cover layer boundary line to the outer surface of the cover layer in the inorganic particle portion of the cover layer. It should be noted that the "inorganic particle portion" does not include the portion within 1.5 times the polymer particle D50 (or 3 times R) from the center line of the protruding polymer particles.

[0076] L2: The maximum distance from the boundary line of the substrate-capping layer to the outer surface of the inorganic particles of the capping layer.

[0077] L1: The maximum distance from the boundary line of the substrate-cover layer to the outline of the protruding polymer particles.

[0078] The D50 of polymer particles can also be measured using the method described above.

[0079] The testing methods for various performance and physicochemical parameters involved in this invention include:

[0080] 1. Weight measurement method:

[0081] (1) Take the base film, cut it using a 10cm×10cm mold, and weigh it on an electronic balance as m0;

[0082] (2) Take the coated finished film, cut it with a 10cm×10cm mold, place it on an electronic balance and weigh it as m1;

[0083] (3) Coating weight = m1 - m0.

[0084] 2. Method for measuring unit surface density:

[0085] Single-layer density = coating weight / coating thickness

[0086] 3. Adhesion: The adhesion referred to in this invention is the tightness and uniformity of the bond between the battery separator and the battery electrodes during cell assembly. Specifically, adhesion is evaluated according to the following method: testing is conducted in accordance with the requirements of GB / T 2792.

[0087] (1) Stack the A4 paper and the separator together in the order of A4 paper / separator / separator / A4 paper, with the separator coating facing each other;

[0088] (2) The stacked A4 paper and diaphragm are subjected to thermoplastic treatment at a temperature of 100℃;

[0089] (3) Cut the thermoplastic diaphragm into strips 200 mm long and 25 mm wide, with a distance of (100±5) mm between the clamps and a test speed of (50±10) mm / min.

[0090] 4. Heat resistance:

[0091] The heat shrinkage rate is measured according to the method specified in GB / T12027-2004. The average value of the heat shrinkage rate in the TD direction and the heat shrinkage rate in the MD direction is taken.

[0092] 5. Breathability Enhancement Rate:

[0093] The sample is placed in a Wang Yan-style air permeability tester. The time required for 100 mL of air to pass through a 1 square inch diaphragm under a pressure of 1.22 kPa is the air permeability value of the diaphragm. The air permeability measurement value of the base film is A1, and the air permeability measurement value of the coated film is A2. Then the value-added rate = (A2-A1) / A1.

[0094] 6. Methods for measuring slurry viscosity

[0095] The test was conducted using a Brookfield DV2T viscometer with a #2 rotor at 60 rpm.

[0096] 7. Hot-press thickness measurement method

[0097] (1) Sample preparation: PET & diaphragm 25×150mm, A4+PET+diaphragm+PET+A4

[0098] (2) Test method: Use a pressure of 1000 kgf, a temperature of 60°C, and a time of 1 second, and use PET clamps for hot pressing.

[0099] (3) After hot pressing, the thickness is measured at 10 points using a micrometer. The maximum value minus the minimum value of these 10 data points is used as the hot pressing thickness range = MAX - MIN as the basis for judgment.

[0100] 8. Liquid absorption rate

[0101] Take a 25mm×50mm coated film sample, weigh it and record the weight as W1. Soak it in the electrolyte for 2 hours, then take it out and wipe the electrolyte off the sample surface. Weigh it and record the weight as W2. Then the liquid absorption rate = (W2-W1) / W1×100%.

[0102] 9. Fan drop rate

[0103] After passing the finished battery separator through a 90-degree corner roller 20 times, random samples are taken and 1Kx electron microscope images are taken (500-5Kx is acceptable). The probability of polymer particles falling off is calculated (number of falling polymer particles / total number of polymer particles). This process is repeated 5 times and the average value is calculated as the powder removal rate.

[0104] The relevant parameters and indicators for the above embodiments and comparative examples are listed below:

[0105] Table 1

[0106] Table 1 (continued)

[0107] As can be seen from the table above, Examples 1 to 9 meet the climbing height and climbing degree specified in this invention, and the difference in hot-pressed thickness after hot pressing is small (all <0.3μm), resulting in a battery separator with a smooth film surface; while the climbing height or climbing degree in Comparative Examples 1 to 4 do not meet the requirements of this application, and the difference in hot-pressed thickness after hot pressing is significantly increased.

[0108] The battery separator provided by this invention is not limited to the preparation method provided by this invention. Battery separators that conform to this invention and are obtained by other methods should be protected within the scope of the claims of this invention.

[0109] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A secondary battery separator, comprising a substrate and a capping layer, wherein the substrate comprises a polyolefin microporous membrane with polyolefin as the main component, and the capping layer is disposed on at least one surface of the substrate, characterized in that, The capping layer comprises inorganic particles and polymer particles, wherein the inorganic particles are continuously distributed in the capping layer, and the polymer particles are discretely distributed among the inorganic particles and protrude from the surface of the capping layer; the maximum distance measured in the thickness direction of the membrane from the boundary line of the substrate-capping layer to the outline of the protruding polymer particles is defined as L1, the maximum distance from the boundary line of the substrate-capping layer to the outer surface of the inorganic particles of the capping layer is defined as L2, and the thickness of the inorganic particle portion in the capping layer is defined as L0, the inorganic particle portion excluding the portion within a range of 1.5 times the polymer particle D50 or 3 times R from the center line of the protruding polymer particles; The height of the climbing ball in the covering layer is H < (L1-L2) / 2R, where H = L2-L0, and 2R is the D50 of the polymer particles; The degree of sphere climbing of the cover layer is M = H / (L1-L0), M < 15%.

2. The secondary battery separator according to claim 1, characterized in that, The following relationship must be satisfied:

3. The secondary battery separator according to claim 1, characterized in that, The areal density per unit thickness of the covering layer is 0.8-2.0 g / (cm³). 2 ·μm).

4. The secondary battery separator according to claim 1, characterized in that, The polymer particles have a D50 of 0.5-10 μm, an L0 of 0.1-5 μm, and an L1 of 0.5-10 μm.

5. The secondary battery separator according to claim 2, characterized in that, (L1-L2) / R is 0.5-1.

5.

6. The secondary battery separator according to claim 1, characterized in that, The inorganic particles include a first inorganic particle and a second inorganic particle, with the mass ratio of the first inorganic particle to the second inorganic particle being (15-35):(1-30); the first inorganic particle has a D50 diameter of D, the second inorganic particle has a D50 diameter of d, D / d≥2, D is 0.3-0.9μm, and d is 0.05-0.5μm.

7. The secondary battery separator according to claim 1, characterized in that, The inorganic particles include a third inorganic particle, wherein the D50 / D10 of the third inorganic particle is ≥2.

8. The secondary battery separator according to claim 1, characterized in that, The coating layer is formed by coating a slurry onto a substrate, the slurry comprising inorganic particles and polymer particles, the slurry having a viscosity of 100-260 mPa·s.

9. The secondary battery separator according to claim 8, characterized in that, The polymer particles in the slurry have a dry weight of 1%-50% and a wet weight of 0.3%-30%.

10. The secondary battery separator according to claim 1, characterized in that, With a total mass of 100 parts of inorganic and polymer particles in the coating layer, the coating layer contains 50-99 parts of inorganic particles and 1-50 parts of polymer particles.

11. The secondary battery separator according to claim 10, characterized in that, The mass ratio of the inorganic particles to the polymer particles is 1-20:

1.

12. The secondary battery separator according to claim 1, characterized in that, The polymer particles include one or more of the following: polyimide water-soluble microspheres, styrene-butadiene rubber microspheres, polyacrylate modified microspheres, polyetherimide, aramid, aramid sulfone, polyvinylidene fluoride, poly(meth)acrylate polymers, polyethylene oxide, polyacrylonitrile, and polyvinylidene fluoride-hexafluoropropylene. The inorganic particles include one or more of the following: silicon dioxide, aluminum oxide, magnesium oxide, zirconium oxide, titanium oxide, calcium oxide, boehmite, aluminum nitride, boron nitride, barium sulfate, barium titanate, calcium fluoride, and barium fluoride.

13. The secondary battery separator according to claim 8, characterized in that, The slurry also includes one or more of carboxymethyl cellulose, binder, regulator, wetting agent and dispersant. Based on 100 parts by weight of the total slurry, the amount of carboxymethyl cellulose is 0-15 parts by weight, the amount of binder is 0-6 parts by weight, the amount of regulator is 0-5 parts by weight, the amount of wetting agent is 0-1 parts by weight, and the amount of dispersant is 0-2 parts by weight. The amounts of carboxymethyl cellulose, binder, regulator, wetting agent and dispersant are not all 0 at the same time. The adhesive is one or more of the following: styrene-butadiene rubber, acrylic resin, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride, polyethylene oxide, polytetrafluoroethylene, acrylic acid and its derivative copolymers, and acrylamide and its derivative copolymers. The regulator is one or more of polyacrylic acid, polyvinyl alcohol, polyimide, and cellulose; The wetting agent is one or more of anionic surfactants, polyethylene surfactants, and polyol surfactants; The dispersant is one or more of polyvinylpyrrolidone, polyacrylamide, and polyvinyl alcohol.

14. A secondary battery, characterized in that, The secondary battery separator comprises any one of claims 1-13.

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