A battery separator and method of making the same

Abstract

The application discloses a battery diaphragm and a preparation method thereof, and belongs to the technical field of battery materials. The battery diaphragm comprises an acrylic resin coating layer, and the coating layer contains acrylic resin particles of different specifications. Among them, resin particle A has small particle size and low Tg, and has a high addition ratio, so as to ensure the appropriate bonding strength between the coating layer and the base material at room temperature. Resin particle B has large particle size and high Tg, and has a low addition ratio, and does not bond at room temperature. Resin particle A is distributed in the gap between resin particle B in a mutually adjacent manner, resin particle B plays a space supporting role, the contact area between the coating layers is reduced, and mutual adhesion of the diaphragm coating layer is prevented. Under the action of heat pressing, resin particle B is deformed and softened, and cooperates with resin particle A to improve the bonding strength of the diaphragm and the positive plate. In the application, the difference between the bonding strength of the battery diaphragm and the positive plate at room temperature and at medium temperature is controlled to be 13-21 N / m, mutual adhesion at room temperature and high adhesion at medium temperature are realized, the change of the air permeability of the battery diaphragm after drying and wetting is maintained to be 15%-27%, the diaphragm has a stable pore structure, and stable transmission of lithium ions is achieved.

Description

Technical Field

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

[0002] As one of the most important electrochemical energy storage devices of our time, the performance, safety, and cost of lithium-ion batteries largely depend on their internal key component—the battery separator. The battery separator is a porous insulating film placed between the positive and negative electrodes. Its core function is to prevent direct contact between the electrodes, which could lead to internal short circuits, while allowing lithium ions to freely pass through the electrolyte, forming an ionic conductivity pathway. In the commercialization process, polyolefin separators have long dominated due to their stable chemical properties, good mechanical strength, and mature manufacturing processes. However, with electric vehicles, large-scale energy storage systems, and high-end consumer electronics placing higher demands on battery energy density, power density, and safety, the limitations of traditional polyolefin separators are becoming increasingly apparent.

[0003] To overcome the aforementioned bottlenecks, researchers opted to coat the battery separator surface with protective materials to enhance its performance. For example, a PVDF (polyvinylidene fluoride) adhesive layer is applied after coating the surface with ceramic particles. The ceramic coating provides a protective layer for the PE (polyethylene) base membrane, improving its resistance to heat shrinkage and mechanical strength, while the PVDF adhesive layer enhances the adhesion between the separator and the electrode. However, PVDF is a fluorine-containing material, causing environmental pollution during production, high separator coating swelling rate, and low battery conductivity. Following the EU's 2025 fluorine-free policy, acrylic resin coatings as adhesives emerged. Acrylic resin is an amorphous organic adhesive, consisting of individual particles with a particle size ranging from 0.4 to 10 μm. The introduction of acrylic resin coatings not only improves the adhesion strength between the battery separator and the electrode and the hydrophilicity of the electrolyte, but also reduces interfacial side reactions and lithium dendrite growth. However, acrylic resin has a low glass transition temperature (Tg). If the storage temperature of the separator is too high after coating, or if the storage time is prolonged, the coatings may stick together, leading to the exposure of the base film. Increasing the Tg of acrylic resin requires increasing the bonding temperature between the separator and the electrode to ensure good process adhesion. However, prolonged operation of the battery in this environment can lead to electrolyte decomposition and consumption, electrode material degradation, and reduced battery life.

[0004] Based on the above, Chinese invention patent application CN108149785A discloses a composition for a non-aqueous secondary battery adhesive layer. The composition comprises a particulate polymer A with a glass transition temperature of 20°C or lower and a volume average particle size of 100 nm or more and less than 450 nm, and a particulate polymer B with a glass transition temperature of 30°C or higher and less than 60°C and a volume average particle size larger than that of the particulate polymer A. The battery structure formed by using the adhesive layer of this non-aqueous secondary battery adhesive layer composition has both high process adhesion and high anti-adhesion properties. However, the applicant found that in the composition described in the patent, the content (by mass) of the particulate polymer B (large particle size) is greater than that of the particulate polymer A (small particle size), with a specific content ratio of (1.4~5):1, which may cause the following problems: (1) The amount of large particle size polymer B in the adhesive layer is large, which will block the micropore channels when covering the surface of the separator, resulting in a sharp increase in air permeability. Moreover, it is more prone to swelling in the wet state (the swelling degree of polymer B in the electrolyte in Table 1 is as high as 20 times), which leads to excessive changes in the porosity of the separator material in the dry and wet states, that is, poor pore structure stability, which may affect the ion transport efficiency and thus reduce the overall performance of the battery; (2) After hot pressing, the large particle size polymer B covers a large area on the surface of the separator. Finally, the good process adhesion of the adhesive layer may mainly depend on the large particle size polymer B playing a leading role, and fails to work well with the adhesive effect of polymer A.

[0005] In summary, developing a battery separator material that is non-adhesive at room temperature, highly adhesive at medium temperature (50~70℃), and has a relatively stable pore structure is of great significance for meeting customers' demands for high-performance and high-safety lithium-ion battery products. Summary of the Invention

[0006] 1. The problem to be solved Traditional battery separator adhesive layers are prone to mutual adhesion during room temperature storage. Existing adhesive layers, while addressing this issue, do not consider the impact on the stability of the battery separator's pore structure. This can lead to significant variations in the separator's permeability under dry and wet conditions, affecting ion transport efficiency and ultimately reducing battery performance. This invention provides a battery separator comprising an acrylic resin coating. This coating contains two types of acrylic resin particles with different specifications (particle size, Tg). Resin particle A has a smaller particle size (0.40~0.70 μm), a lower Tg (25~30℃), and is added in a higher proportion to provide suitable adhesive strength at room temperature, preventing the coating from detaching from the substrate. Resin particles B have a larger particle size (0.95~1.45 μm) and a higher Tg (50~56℃). They are added in a lower proportion and do not adhere at room temperature. Resin particles A are distributed in the gaps between resin particles B in an adjacent manner. Resin particles B act as spatial supports, reducing the contact area between coatings and thus preventing them from sticking together. Under hot pressing, resin particles B deform and soften, working synergistically with resin particles A to increase the adhesion strength between the separator and the positive electrode. This invention, through precise control of the reasonable ratio of resin particles in the coating (solid content ratio of resin particles A to resin particles B (3~4):1) and the coating coverage (60~70%), controls the difference in adhesion strength between the battery separator and the positive electrode at room temperature and 60℃ to 13~21 N / m, achieving no adhesion at room temperature and high adhesion at medium temperature. The air permeability of the battery separator after drying and electrolyte wetting remains at 15%~27%, exhibiting a relatively stable pore structure to ensure stable lithium-ion transport. This means providing a high-performance medium-temperature bonding diaphragm.

[0007] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: This invention provides a battery separator comprising an acrylic resin coating; the acrylic resin coating comprises acrylic resin particles A and acrylic resin particles B, wherein the resin particles A are distributed in the gaps between the resin particles B in an adjacent manner; the D50 particle size of the resin particles A is smaller than the D50 particle size of the resin particles B, and the particle size ratio is 1:(1.5~3.5); the bonding strength between the battery separator and the positive electrode at room temperature is less than 1.5 N / m; the difference between the bonding strength between the battery separator and the positive electrode at room temperature and the bonding strength under hot pressing at 60°C is greater than 13 N / m.

[0008] Furthermore, the difference between the bonding strength of the battery separator and the positive electrode at room temperature and the bonding strength under hot pressing at 60°C is 13~21 N / m.

[0009] Furthermore, the difference between the bonding strength of the battery separator and the positive electrode at room temperature and the bonding strength under hot pressing at 60°C is 14~21 N / m.

[0010] More preferably, the difference between the bonding strength of the battery separator and the positive electrode at room temperature and the bonding strength under hot pressing at 60°C is 16~21 N / m.

[0011] Furthermore, the bonding strength between the battery separator and the positive electrode at room temperature is 0.7~1.5 N / m.

[0012] Furthermore, the solid content ratio of the above-mentioned resin particles A to resin particles B in the coating is (3~4):1.

[0013] Furthermore, the change in air permeability of the battery separator after drying and electrolyte wetting is within the range of 15% to 27%.

[0014] Furthermore, the glass transition temperature (Tg) of the aforementioned resin particles A is 25-30°C, more preferably 25-29°C, for example 25°C, 26°C, 27°C, 28°C, or 29°C; the glass transition temperature (Tg) of the resin particles B is 50-56°C, more preferably 52-56°C, for example 52°C, 53°C, 54°C, 55°C, or 56°C. It should be noted that this invention utilizes two types of acrylic resin particles with different particle sizes and different Tg as the coating of the battery separator. At room temperature, when the membrane is in contact with the membrane, even if the resin… Particle A softens upon reaching the Tg temperature, while resin particles B do not adhere at room temperature. Resin particles A are distributed in the gaps between resin particles B in an adjacent manner, and resin particles B provide spatial support, reducing the contact area between coatings. At the same time, resin particles A are bonded and fixed to the underlying substrate, so there is no mutual adhesion between the coatings of the separator. When the separator is in contact with the positive electrode for hot pressing, resin particles B soften and flatten upon reaching the Tg temperature, and their height matches that of resin particles A, thus working together with resin particles A to provide medium-temperature adhesion.

[0015] Further, the D50 particle size of the above-mentioned resin particles A is 0.40~0.70 μm, for example 0.40 μm, 0.50 μm, 0.60 μm, 0.70 μm; the D50 particle size of the resin particles B is 0.95~1.45 μm, for example 0.95 μm, 1 μm, 1.10 μm, 1.20 μm, 1.30 μm, 1.40 μm, 1.45 μm, and more preferably 0.95~1.20 μm; it should be noted that the acrylic resin coating formed by the present invention has a microscopic surface uneven structure. When the membrane is wound at room temperature, causing membrane-to-membrane contact, some resin particles B will be embedded between the resin particles B on the opposite side, thereby increasing the friction of the membrane surface and making it less likely to slip off when subjected to external force.

[0016] Furthermore, the D50 particle size ratio of the aforementioned resin particles A to resin particles B is 1:(1.6~2.8), for example, 1:1.8, 1:2.0, 1:2.2, 1:2.4, 1:2.6, 1:2.8. It should be noted that when the particle size ratio is less than this range, the resin particles A are more likely to come into contact with each other and thus self-adhede when the bottom membrane of the battery separator roll is under pressure. If it exceeds this range, the contact stress between the separators is more likely to concentrate in the resin particles B, leading to many adverse factors such as the resin particles B being flattened and falling off.

[0017] Furthermore, the solid content ratio of the above-mentioned resin particles A to resin particles B in the coating is (3~4):1.

[0018] Furthermore, the adhesion strength between the battery separator and the positive electrode at 60°C is 16~22 N / m.

[0019] Furthermore, the wet bonding strength between the battery separator and the positive electrode at 60°C is 6~13 N / m, and more preferably 8~13 N / m.

[0020] Furthermore, the air permeability of the battery separator during drying is 170~190 s / 100cc, for example, it can be 170s / 100cc, 172 s / 100cc, 174 s / 100cc, 176 s / 100cc, 178 s / 100cc, 180 s / 100cc, 182 s / 100cc, 184 s / 100cc, 186 s / 100cc, 188 s / 100cc, or 190 s / 100cc.

[0021] Furthermore, the air permeability of the battery separator after wetting is 190~230 s / 100cc, for example, it can be 195 s / 100cc, 200 s / 100cc, 205 s / 100cc, 210 s / 100cc, 215 s / 100cc, 220 s / 100cc, 225 s / 100cc, or 230 s / 100cc.

[0022] Furthermore, the thickness of the above-mentioned acrylic resin coating is 0.1~2 μm, preferably 0.1~0.6 μm, and more preferably 0.4~0.6 μm. It should be noted that the coating thickness in this invention is obtained by measuring the thickness 5 times after stacking 8 layers of battery separator, taking the average value and dividing it by 8. When the amount of large-diameter resin particles B increases, the coating thickness will increase slightly, indirectly affecting the air permeability of the separator.

[0023] Furthermore, the acrylic resin coating has a coverage of 60-70% on the surface of the battery separator.

[0024] Furthermore, the coverage of the acrylic resin coating on the surface of the battery separator after hot pressing at 60°C is 70-90%, more preferably 70-88%, for example 72%, 75%, 77%, 80%, 82%, 84%, 86%, 88%.

[0025] Furthermore, the aforementioned acrylic resin coating also includes a thickener, which can be any one or more of nanocellulose, carboxymethylcellulose, and hydroxyethylcellulose.

[0026] Furthermore, in the above-mentioned acrylic resin coating, the solid content ratio of acrylic resin A, acrylic resin B and thickener is (70~80):(15~25):(0.05~1), and more preferably (73~80):(18~25):(0.08~0.8).

[0027] Furthermore, the battery separator also includes a base film and a ceramic coating, wherein the ceramic coating is located between the base film and the acrylic resin coating.

[0028] Furthermore, the base film is a polyethylene film with a thickness of 5 to 15 μm, and more preferably 6.5 to 7.5 μm.

[0029] Furthermore, the thickness of the aforementioned ceramic coating is 0.5~3 μm.

[0030] Furthermore, the above-mentioned ceramic coating is composed of ceramic particles, dispersant, binder and wetting agent in a solid content ratio of (95~98):(0.5~3):(0.1~2):(0.0001~0.5).

[0031] Furthermore, the solid content ratio of the above-mentioned ceramic particles, dispersant, binder and wetting agent is (95~97):(2~3):(1~2):(0.0001~0.06).

[0032] Furthermore, the aforementioned ceramic particles include any one or more of alumina, boehmite, calcium carbonate, barium titanate, barium sulfate, montmorillonite, titanium dioxide, and calcium oxide.

[0033] Furthermore, the dispersants mentioned above include sodium polyacrylate and / or polyacrylate amine.

[0034] Furthermore, the adhesives mentioned above include any one or more of polyacrylic acid, polyvinyl alcohol, and polyacrylate.

[0035] The present invention also provides a method for preparing the above-mentioned battery separator, the steps of which are as follows: S1. Prepare ceramic coating slurry, coat the ceramic coating slurry onto the surface of the base film, and dry to obtain battery separator intermediate; S2. Prepare a dispersion of resin particles A and a dispersion of resin particles B, and mix them with a thickener and water to obtain an acrylic resin coating slurry. S3. The acrylic resin coating slurry in S2 is coated onto the surface of the battery separator intermediate in S1, and then dried to obtain the battery separator.

[0036] Furthermore, the preparation of the dispersion of the above-mentioned resin particles A includes: dispersing the resin particles A in deionized water at a solid content of 24-26%.

[0037] Furthermore, the preparation of the dispersion of the above-mentioned resin particles B includes: dispersing the resin particles B in deionized water at a solid content of 14-16%.

[0038] Furthermore, the thickener mentioned above is a 1-2 wt% nanocellulose solution.

[0039] Furthermore, the dispersions of resin particles A and B, along with the thickener and water, are mixed in a mass ratio of (20.0~30.0):(8.0~20.5):(6.0~10.0):(50.0~60.0) based on a total mass of 100%.

[0040] Preferably, the dispersions of resin particles A and resin particles B are mixed in a mass ratio of (21.0~27.0):(8.0~18):(7.0~9.0):(50.0~60.0).

[0041] Furthermore, the aforementioned acrylic resin particles A and B are each independently polymerized from the first component (a) and the second component (b), wherein the first component (a) is an acrylate monomer, and the second component (b) is selected from one or more of the following: (meth)acrylate derivatives (such as monomers containing hydroxyl or epoxy groups), acrylamide monomers, styrene monomers, acrylics, acrylonitrile, vinyl acetate, and N-vinylpyrrolidone.

[0042] Furthermore, the particle size of the resin particles A is 0.43~0.60 μm, and the glass transition temperature is 26℃.

[0043] Furthermore, the particle size of the resin particles A is 0.43 μm, and the glass transition temperature is 26°C.

[0044] Furthermore, the particle size of the resin particles A is 0.60 μm, and the glass transition temperature is 26°C.

[0045] Furthermore, the particle size of the resin particles B is 0.98~1.17 μm, and the glass transition temperature is 54℃.

[0046] Furthermore, the particle size of the resin particles B is 0.98 μm, and the glass transition temperature is 54°C.

[0047] Furthermore, the particle size of the resin particles B is 1.17 μm, and the glass transition temperature is 54°C.

[0048] Furthermore, in the above S1, the coating adopts a double-sided roller coating method, with a roller line count of 110 L / 65 U to 130 L / 65 U, a coating speed of 120 to 135 m / min, and a roller coating unit speed ratio of 0.8 to 1.

[0049] Furthermore, in the above S2, the coating adopts a double-sided roller coating method, with a roller line count of 130 L / 50 U to 150 L / 50 U, a coating speed of 140 to 160 m / min, and a roller coating unit speed ratio of 1 to 1.1.

[0050] Furthermore, in the above-mentioned S1, the drying process is carried out by an oven unit, which consists of 7 ovens connected in series. The temperature of the first preheating oven is 60~64℃, and the temperature of the subsequent ovens is maintained at 62~70℃.

[0051] Furthermore, in the above S2, the drying process is carried out by an oven unit, which consists of 7 ovens connected in series. The temperature of the first preheating oven is 60~65℃, and the temperature of the subsequent ovens is maintained at 65~70℃.

[0052] The present invention also provides a lithium-ion secondary battery, which includes the battery separator described above.

[0053] 3. Beneficial effects Compared with the prior art, the advantages of this invention are as follows: (1) The present invention provides a battery separator and its preparation method. The acrylic resin coating of the battery separator contains two types of acrylic resin particles with different specifications (particle size, Tg). Among them, resin particles A have small particle size (0.40~0.7 μm) and low Tg (25~30℃), and are added in a higher proportion. At room temperature, their suitable adhesive strength can prevent the acrylic resin coating from falling off the substrate. Resin particles B have large particle size (0.95~1.45 μm) and high Tg (50~56℃), and are added in a lower proportion. They do not adhere at room temperature. Resin particles A are distributed in the gaps between resin particles B in an adjacent manner. The spatial support provided by resin particles B reduces the contact area between the separator coatings and prevents the coatings from sticking together. After hot pressing, resin particles B will deform and soften, which can work with resin particles A to improve the adhesion strength between the separator and the electrode. The present invention ultimately achieves a bonding strength difference of 13~21 N / m between the battery separator and the positive electrode at room temperature and medium temperature (50~70℃), which not only ensures good adhesion between the coating and the substrate at room temperature and prevents the coating from sticking to each other; at the same time, it also ensures high adhesion between the positive electrode and the battery separator (≥16.9 N / m) under medium temperature (50~70℃) hot pressing.

[0054] (2) The battery separator and its preparation method provided by the present invention, through the reasonable ratio of acrylic resin particles in the coating (solid content ratio of resin particles A to resin particles B (3~4):1) and the coating coverage (60~70%), and the addition of a ceramic coating to increase mechanical strength, the thickness of the battery separator is controlled at about 8~28 μm. The air permeability of the battery separator after drying and electrolyte wetting is 15%~27%, indicating that it has a relatively stable pore structure, which provides a guarantee for the stable transport of lithium ions. Attached Figure Description

[0055] Figure 1 This is a schematic diagram of the battery separator winding and hot pressing process between the battery separator and the positive electrode sheet in this invention.

[0056] Figure 2 These are SEM images of the surface of the battery separator A1 prepared in Example 1 of the present invention. The left side is the SEM image of the surface of the battery separator A1 before hot pressing, and the right side is the SEM image of the surface of the battery separator A1 after hot pressing at 60°C. Detailed Implementation

[0057] The present invention will be further described below with reference to specific embodiments.

[0058] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0059] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0060] As used herein, the term “about” is used to provide for the flexibility and imprecision associated with a given term, measure, or value. Those skilled in the art can readily determine the degree of flexibility for a particular variable. As used herein, the term “at least one of…” is intended to be synonymous with “one or more of…”. For example, “at least one of A, B, and C” explicitly includes only A, only B, only C, and combinations thereof. Concentration, amount, and other numerical data may be presented in range format herein. It should be understood that such range format is used solely for convenience and brevity and should be flexibly interpreted to include not only the values ​​explicitly stated as the limits of the range, but also all individual values ​​or subranges encompassed within the range, as if each value and subrange were explicitly stated. For example, a range of values ​​from about 1 to about 4.5 should be interpreted to include not only the explicitly stated limits of 1 to 4.5, but also individual numbers (such as 2, 3, 4) and subranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that describe only a single value, such as "less than about 4.5," which should be interpreted to include all the values ​​and ranges described above. Furthermore, this interpretation should apply regardless of the breadth of the range or characteristic described.

[0061] Materials used in the embodiments of this invention: Resin particles A1, Shenzhen Haodian Technology Co., Ltd., grade HD-2205: particle size 0.43 μm, glass transition temperature 26℃; Resin particles A2, Shenzhen Haodian Technology Co., Ltd., grade HD-2205-B: particle size 0.60 μm, glass transition temperature 26℃; Resin particles A3, Shenzhen Haodian Technology Co., Ltd., grade HD-2248: particle size 0.75 μm, glass transition temperature 26℃; Resin particles B1, Guangzhou Yike Juneng New Materials Co., Ltd., brand name YK-331A: particle size 0.98 μm, glass transition temperature 54℃; Resin particles B2, Guangzhou Yike Juneng New Materials Co., Ltd., grade YK-331B: particle size 1.17 μm, glass transition temperature 54℃; Thickener: 1 wt% nanocellulose solution.

[0062] Example 1 This embodiment provides a method for preparing a battery separator and the prepared battery separator A1.

[0063] The steps for preparing the battery separator are as follows: S1. Prepare ceramic coating slurry, coat the ceramic coating slurry onto the surface of the base film, and dry to obtain battery separator intermediate; S2. Prepare dispersions of resin particles A1 and B1, and mix them with thickener and water to obtain acrylic resin coating slurry. S3. The acrylic resin coating slurry in S2 is coated onto the surface of the battery separator intermediate in S1, and then dried to obtain the battery separator.

[0064] S1, the preparation of the ceramic coating slurry and the specific coating process are as follows: Boehmite (D50 of 0.67 μm), binder (polyacrylate, solid content of 45%), dispersant (sodium polyacrylate, solid content of 40%), wetting agent (polyether siloxane, solid content of 100%), and water were mixed in a mass ratio of 32:1.2:1.79:0.01:65 and stirred at 300 r / min for 1 h to obtain a heat-resistant ceramic coating slurry with a solid content of 33%.

[0065] The ceramic coating slurry prepared above was introduced into the feeding system and uniformly coated onto both surfaces of a polyethylene-based film (7 μm thick, porosity approximately 42±1%) using a double-sided roller coating method. After drying, a battery separator intermediate with a thickness of 7 μm + 1.5 μm + 1.5 μm was obtained. The roller count was 120 L / 65 U, the coating speed was 130 m / min, and the roller coating unit speed ratio was 0.95. Drying was performed using an oven unit consisting of six ovens connected in series. The drying temperatures were set to 62℃, 64℃, 70℃, 70℃, 64℃, and 62℃, respectively, with each oven's throughput lasting 3.5 seconds.

[0066] S2, The specific process for preparing acrylic resin slurry and coating is as follows: Based on the dry weight percentage of solid components in the final acrylic resin coating, the composition is as follows: resin particles A1 account for 4 parts, resin particles B1 account for 1 part, and thickener accounts for 0.05 parts. Specifically: Preparation of a dispersion of resin particles A1: Resin particles A1 with a particle size of 0.43 μm and a glass transition temperature of 26℃ were dispersed in deionized water at a solid content of 25% to obtain a dispersion of resin particles A1. Preparation of a dispersion of resin particles B1: Resin particles B1 with a particle size of 0.98 μm and a glass transition temperature of 54℃ were dispersed in deionized water at a solid content of 15% to obtain a dispersion of resin particles B1. The thickener is a 1 wt% nanocellulose solution; The dispersions of acrylic resin particles A1 and B1, along with thickener and water, were mixed at a mass ratio of 25.6:10.6:8:55.7 and stirred for 10 minutes to obtain an 8wt% acrylic resin slurry.

[0067] S3, the acrylic resin slurry prepared above is introduced into the feeding system, and the slurry is uniformly coated on both surfaces of the battery separator intermediate prepared above using a double-sided roller coating method. After drying, a finished battery separator A1 with a thickness of 7 μm + 1.5 μm + 1.5 μm + 0.5 μm + 0.5 μm is obtained. Its surface SEM image is shown below. Figure 2 As shown in the left-hand image, small resin particles A and large resin particles B are distributed on the diaphragm surface. The roller count is 140 L / 50 U, the coating speed is 150 m / min, and the roller coating unit speed ratio is 1.05. Drying is performed using an oven unit consisting of six ovens connected in series. The drying temperatures are set to 62℃, 65℃, 68℃, 70℃, 70℃, and 65℃, respectively, with each oven's pass-through time being 4 seconds.

[0068] Finally, the battery separator A1 prepared above is cut into small rolls and tightened by the winding unit to obtain its master roll ( Figure 1 (As shown).

[0069] Example 2 This embodiment provides a method for preparing a battery separator and the prepared battery separator A2.

[0070] The battery separator preparation method differs from Example 1 only in the mass ratio of the two acrylic resin particle dispersions in S2. Based on the dry weight of the solid components in the final coating, resin particles A1 account for 3 parts, resin particles B1 account for 1 part, and the thickener remains unchanged. Specifically, the dispersions of acrylic resin particles A1 and B1, along with the thickener and water, are adjusted to a mass ratio of 24:13.3:8:54.7 and mixed evenly to obtain an acrylic resin slurry diluted to 8 wt%. Other conditions and operations remain unchanged, and the resulting battery separator A2 is obtained after drying.

[0071] Example 3 This embodiment provides a method for preparing a battery separator and the prepared battery separator A3.

[0072] The battery separator preparation method differs from Example 1 only in that the D50 particle size ratio of the resin particles used in S2 decreases, that is, the particle size of resin particles B becomes larger. Specifically, resin particles B1 are adjusted to resin particles B2 (particle size 1.17 μm), while other conditions and operations remain unchanged. After drying, battery separator A3 is obtained.

[0073] Example 4 This embodiment provides a method for preparing a battery separator and the prepared battery separator A4.

[0074] The battery separator preparation method differs from Example 1 only in that the D50 particle size ratio of the resin particles used in S2 is increased, that is, the particle size of resin particle A becomes larger. Specifically, resin particle A1 is adjusted to resin particle A2 (particle size 0.60 μm), and other conditions and operations remain unchanged. After drying, battery separator A4 is obtained.

[0075] Comparative Example 1 This embodiment provides a method for preparing a battery separator and the prepared battery separator B1.

[0076] The battery separator preparation method differs from Example 1 only in the mass ratio of the two acrylic resin particle dispersions in S2. Based on the dry weight of the solid components in the final coating, resin particles A1 account for 5 parts, resin particles B1 account for 1 part, and the thickener remains unchanged. Specifically, the dispersions of acrylic resin particles A1 and B1, along with the thickener and water, are adjusted to a mass ratio of 26.7:8.8:8:56.5 and mixed thoroughly. After stirring for 10 minutes, an 8wt% acrylic resin slurry is obtained. Other conditions and operations remain unchanged. The resulting battery separator B1 is then dried.

[0077] Comparative Example 2 This comparative example provides a method for preparing a battery separator and the prepared battery separator B2.

[0078] The battery separator preparation method differs from Example 1 only in the mass ratio of the two acrylic resin particle dispersions in S2. Based on the dry weight of the solid components in the final coating, resin particles A1 account for 2 parts, resin particles B1 account for 1 part, and the thickener remains unchanged. Specifically, the dispersions of acrylic resin particles A1 and B1, along with the thickener and water, are adjusted to a mass ratio of 21.3:17.8:8:52.9 and mixed evenly to obtain an acrylic resin slurry diluted to 8 wt%. Other conditions and operations remain unchanged, and the battery separator B2 is obtained after drying.

[0079] Comparative Example 3 This comparative example provides a method for preparing a battery separator and the prepared battery separator B3.

[0080] The battery separator preparation method differs from Example 1 only in that the solid content of resin particles B1 in the dispersion of resin particles B1 in S2 is adjusted from 15% to 25%. Then, the dispersion of acrylic resin particles A1, the dispersion of acrylic resin particles B1 (solid content 25%), the thickener and water are adjusted to a mass ratio of 25.6:10.6:8:55.7 and mixed to obtain an acrylic resin coating slurry with a content of 9.2 wt%. Other conditions and operations remain unchanged. After drying, battery separator B3 is obtained.

[0081] Comparative Example 4 This comparative example provides a method for preparing a battery separator and the prepared battery separator B4.

[0082] The battery separator preparation method differs from Example 1 only in that acrylic resin particles B1 are not used in S2. Specifically, the dispersion of acrylic resin particles A1 is mixed with thickener solution and water in a mass ratio of 31.7:8:60.3 to obtain an 8wt% acrylic resin coating slurry. Other conditions and operations remain unchanged. After drying, battery separator B4 is obtained.

[0083] Comparative Example 5 This comparative example provides a method for preparing a battery separator and the prepared battery separator B5.

[0084] The battery separator preparation method differs from Example 1 only in that acrylic resin particles A1 are not used in S2. Specifically, the dispersion of acrylic resin particles B1 is mixed with thickener and water in a mass ratio of 53.3:5.3:41.4 to obtain an 8wt% acrylic resin coating slurry. Other conditions and operations remain unchanged. After drying, battery separator B5 is obtained.

[0085] Comparative Example 6 This embodiment provides a method for preparing a battery separator and the prepared battery separator B6.

[0086] The battery separator preparation method differs from Example 1 only in that the particle size of the resin particles A1 used in S2 is increased. Specifically, the numerical particle A1 (particle size 0.43 μm) is adjusted to resin particles A2 (0.75 μm), while other conditions and operations remain unchanged. After drying, battery separator B6 is obtained.

[0087] Example 5 This embodiment provides the determination of the relevant performance of battery separators A1-A4 and B1-B6.

[0088] (1) Measurement scheme a. Bond strength at room temperature M1. Preparing Materials The separator of the battery to be tested was cut into a sample separator with a size of 2.5 cm × 13 cm; two rectangular pieces of paper with a size of 21 cm × 5 cm were cut from 80 g of printing paper and folded in half along the short side; a positive electrode plate with a size of 2.5 cm × 6 cm was then cut out. The positive electrode plate consists of a positive current collector and a positive electrode material layer, wherein the positive electrode material layer is: lithium iron phosphate + carbon black + PVDF binder, and the mass ratio of the three is 8:1:1; two polyethylene terephthalate films (PET films) were also prepared, with a single-sided release 15 g, 36 μm × 1040 mm × 100 m.

[0089] M2. Fabrication of stacked samples: The above-mentioned folded rectangular paper piece ×1, PET film ×1, positive electrode ×1, sample separator ×1, positive electrode ×1, PET film ×1 and folded rectangular paper piece ×1 are stacked from top to bottom to form a stacked sample, ensuring that both sides of the battery separator are in full contact with the surface of the positive electrode.

[0090] M3, cold pressing The above-mentioned laminated samples were made into cold-pressed samples using a hot press (model QMESYS QM940AS). The hot press parameters were set as follows: temperature 25℃, pressure 0.8 T. After reaching the set temperature and waiting for the temperature to stabilize for 10 min, the above-mentioned laminated samples were placed in the hot press for cold pressing for 10 s.

[0091] M4, Peel Test Peel the coating of the hot-pressed sample from the positive electrode sheet by hand to a portion 5 mm from the end, and then test the tensile strength using a tensile testing machine (model AI-3000-SU high-speed rail tensile testing machine). Fix the peeled positive electrode sheet of the hot-pressed sample onto the lower clamp of the tensile testing machine, and fix the other end that is not bonded to the positive electrode sheet onto the upper clamp of the tensile testing machine. Clamp the hot-pressed sample in the order of clamping the lower clamp (with steel plate assistance) first, and then clamping the upper clamp, keeping the sample flat, wrinkle-free, vertical and not skewed. The distance between the clamps is 65 mm, the test speed is 300 mm / min, and the adhesion strength between the battery separator coating and the positive electrode sheet is tested. 5 sets of tests are performed, and the average value is taken.

[0092] b. Hot-pressed (medium temperature) bond strength The only difference between this test and the cold-pressed bond strength test is M3; all other steps are the same. Specifically, M3 is adjusted as follows: a hot press (model QMESYS QM940AS) is used to prepare the above-mentioned laminated sample into a hot-pressed sample; the hot-pressing parameters are set as follows: temperature 70-60℃, pressure 0.8 T; after reaching the set temperature and waiting for the temperature to stabilize for 10 minutes, the above-mentioned laminated sample is placed in the hot press for hot pressing for 10 seconds. The hot-pressing effect is as follows. Figure 1As shown; the surface of the separator after hot pressing was observed using an electron high-resolution microscope (ZEISS SIGMA300) (taking battery separator A1 in Example 1 as an example), as shown. Figure 2 As shown in the right-hand diagram, the large-particle acrylic resin B, after being flattened, is at the same height as the small-particle acrylic resin A, and the two can work together to perform medium-temperature bonding.

[0093] c. Hot-pressed wet bond strength The only difference between this test and the hot-press bonding strength test is M3; all other steps are the same. Specifically, M3 is adjusted as follows: the above-mentioned laminated samples are made into hot-pressed samples using a hot press (the hot-pressing parameters and operations are the same); after hot pressing, the laminated samples are placed in a 40 mm × 120 mm plastic-sealed aluminum bag. After ensuring the sample is flat, 2~3 μL of electrolyte (LiPF6 1M, EC:EMC=3 / 7, VC=2%) is injected inside. The plastic bag is then sealed tightly, allowing the laminated samples to be fully immersed in the electrolyte for 24 h to obtain a hot-pressed wet-bonded sample. Then, the bonding strength test is performed in the same manner as in M4.

[0094] d. Anti-coating transfer Take a small roll of the battery separator to be tested, cut off the outermost layer of the separator (to eliminate possible surface contamination or deformation during storage), and cut the separator at the bottom (core end) into a rectangular sample of 10 cm × 1 m. Observe the separator surface using an electron high-resolution microscope (ZEISSSIGMA 300). A smooth and flat surface indicates a normal (OK) sample, while protrusions or depressions indicate an NG sample. The storage conditions for the small rolls of battery separator were: 25℃ for 3 days; 25℃ for 15 days; and 50℃ for 3 days.

[0095] e. Coating coverage N1. Cut the diaphragm into small cubes of 5mm*5mm and place them under a scanning electron microscope (ZEISS SIGMA 300 field emission scanning electron microscope) with a 5kx lens to photograph the surface of the diaphragm. After adjusting the sharpness, find the area where the small ball and the plane form a strong contrast and take a picture.

[0096] N2. Import the captured images into digital analysis software for analysis to obtain the coverage of the acrylic resin coating on the battery separator.

[0097] f. Coating coverage after hot pressing The only difference from the above coating coverage is N1; all other steps are the same. Specifically, N1 is adjusted as follows: the battery separator to be tested is cut into sample separators of 5 cm × 5 cm; the above-mentioned stacked samples are made into hot-pressed samples using a hot press (temperature 60℃, pressure 0.8 T); after hot pressing, the separator is cut into small cubes of 5 mm × 5 mm and placed under a scanning electron microscope (Zeiss field emission scanning electron microscope (ZEISS SIGMA 300)) with a 5 kx lens to photograph the separator surface. After adjusting the sharpness, the area where the black shadow area forms a strong contrast with the plane is found and photographed.

[0098] g. Air permeability test Referring to the operating requirements of GB / T 36363-2018 "Determination of air permeability of polyolefin separators for lithium-ion batteries", a 600 mm × 100 mm battery separator sample was cut. A Wang Yan-type air permeability meter (ASAHI Corporation, EG01-55-1MR) was used. The test time was 3 s. Along the 600 mm TD direction, the air permeability of the battery separator was measured at any position at 100 mm intervals. The average value of the above 5 test points was recorded as the air permeability of the battery separator.

[0099] h. Air permeability test after wetting Five 600 mm × 100 mm separator samples were cut and immersed in electrolyte (LiPF6 1M, EC:EMC=3 / 7, VC=2%) at 25°C for one day. Using a Wang Yan-type air permeability meter (ASAHI Corporation, EG01-55-1MR), the air permeability of the battery separator was measured at arbitrary positions every 100 mm along the 600 mm TD direction for 3 seconds. The average value of these five measurements was recorded as the air permeability of the battery separator after wetting.

[0100] I. Thickness The test was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0101] Five square samples of the base film were cut along the TD direction using a 10cm×10cm mold for testing. If the TD direction was less than 10cm, a 10cm sample was cut along the MD direction; in this case, the sample was not square. The four corners and the center point of the sample were measured using a Mahr thickness gauge (Mahr, C1202). The average value of these five points was taken as the thickness of a single sample. The average value of the five samples was taken as the thickness of the base film.

[0102] Eight square battery separator samples with coating were cut along the TD direction using a 10cm×10cm mold and stacked on top of each other for testing. If the TD direction was less than 10cm, a 10cm sample was cut along the MD direction; in this case, the sample was not square. The four corners and the center point of the sample were measured using a Mahr thickness gauge (C1202). The average value of these five points divided by eight was taken as the thickness of a single sample. The average value of the five samples tested in the above manner was taken as the thickness of the battery separator. The coating thickness = battery separator thickness - base film thickness.

[0103] (2) Measurement results Table 1. Relevant performance of battery separators A1-A4 and B1-B6

[0104] Inter-adhesion refers to the diaphragms being wound into rolls and sticking together at room temperature. When they are torn apart, they may carry away the coating from the other side or have their own coating carried away by the other side, resulting in coating transfer. Non-adhesion refers to the diaphragms being wound into rolls and not sticking together at room temperature.

[0105] As shown in Table 1, the battery separator provided by this invention exhibits good resistance to coating transfer after 15 days of storage at room temperature, effectively preventing inter-coating adhesion. Under medium temperature conditions (60℃), the synergistic effect of resin particles A and B ensures good adhesion between the electrode and the battery separator (adhesion strength ≥16.9 N / m). This invention, through a reasonable ratio of resin particles A and B, combined with control of coating coverage (60-70%), and supplemented by a ceramic coating to increase mechanical strength, controls the battery separator thickness to approximately 11 μm. The final air permeability of the separator material in the dry state is maintained at 170-190 s / 100cc, and the change in air permeability in the humid state is controlled within 15%-27%, indicating that the prepared battery separator has a relatively stable pore structure, providing a good material basis for the stable transport of lithium ions. When the D50 particle size, glass transition temperature, and dosage ratio of resin particles A and B selected for the battery separator coating are not within the scope of this invention, the requirements for room temperature anti-adhesion, medium-temperature high adhesion, and suitable air permeability cannot be guaranteed. For example, in Comparative Example 1 and Comparative Example 2, after adjusting the solid content of resin particles A and resin particles B in the battery separator coating to 5:1 and 2:1 respectively, the former, due to the increased proportion of resin particles A, showed increased coating adhesion strength at room temperature, and mutual adhesion appeared after 15 days in the room temperature anti-coating transfer test; the latter, due to the reduced proportion of resin particles A, showed no adhesion at room temperature, and the adhesion to the substrate could not be guaranteed. Good adhesion to the substrate; similarly, in Comparative Example 5, the coating using only resin particles B also failed to adhere to the substrate; in Comparative Example 3, the solid content of the resin particle B dispersion was adjusted from 15% to 25% (i.e., the amount of resin particles B in the coating was increased as in Comparative Example 2), and it can be seen that the coverage of the coating increased by about 10% after hot pressing, and the air permeability of the separator material changed significantly in both dry and wet states, further confirming that the amount of resin particles B has a significant impact on the stability of the pore structure of the separator material; in Comparative Example 6, after selecting large-diameter (0.75μm) resin particles A, the prepared separator material developed adhesion problems after 15 days of storage at room temperature; while the battery separator provided by this invention not only ensures good adhesion at medium temperature, but also successfully overcomes the adhesion problem at room temperature.

Claims

1. A battery separator, characterized in that, The battery separator includes an acrylic resin coating; The acrylic resin coating comprises acrylic resin particles A and acrylic resin particles B, wherein the resin particles A are distributed in the gaps between the resin particles B in an adjacent manner; the D50 particle size of the resin particles A is smaller than the D50 particle size of the resin particles B, and the particle size ratio is 1:(1.5~3.5). The bonding strength between the battery separator and the positive electrode at room temperature is less than 1.5 N / m; The difference in bonding strength between the battery separator and the positive electrode sheet under room temperature and 60°C hot pressing is greater than 13 N / m.

2. The battery separator according to claim 1, characterized in that, The air permeability of the battery separator changes by 15% to 27% after drying and electrolyte wetting. The difference between the bonding strength of the battery separator and the positive electrode at room temperature and the bonding strength under hot pressing at 60°C is 13~21N / m; The bonding strength between the battery separator and the positive electrode at room temperature is 0.7~1.5 N / m.

3. The battery separator according to claim 1, characterized in that, The solid content ratio of resin particles A to resin particles B in the coating is (3~4):1; and / or The glass transition temperature (Tg) of resin particle A is 25~30℃, and the glass transition temperature (Tg) of resin particle B is 50~56℃; and / or The D50 particle size of resin particle A is 0.40~0.70 μm, and the D50 particle size of resin particle B is 0.95~1.45 μm.

4. The battery separator according to claim 3, characterized in that, The acrylic resin coating has a coverage rate of 60-70% on the surface of the battery separator; and / or The acrylic resin coating, after being hot-pressed at 60°C, has a coverage rate of 70-90% on the surface of the battery separator.

5. The battery separator according to claim 4, characterized in that, The battery separator also includes a base film and a ceramic coating, wherein the ceramic coating is located between the base film and the acrylic resin coating.

6. The battery separator according to claim 5, characterized in that, The ceramic coating is composed of ceramic particles, dispersant, binder and wetting agent in a solid content ratio of (95~98):(0.5~3):(0.1~2):(0.0001~0.5).

7. The battery separator according to claim 6, characterized in that, The acrylic resin coating further includes a thickener, and the solid content ratio of the acrylic resin A, the acrylic resin B and the thickener is (70~80):(15~25):(0.05~1).

8. The method for preparing the battery separator according to any one of claims 1-7, characterized in that, The method steps are as follows: S1. Prepare ceramic coating slurry, coat the ceramic coating slurry onto the surface of the base film, and dry to obtain battery separator intermediate; S2. Prepare a dispersion of resin particles A and a dispersion of resin particles B, and mix them with a thickener and water to obtain an acrylic resin coating slurry. S3. The acrylic resin coating slurry in S2 is coated onto the surface of the battery separator intermediate in S1, and then dried to obtain the battery separator.

9. The method according to claim 8, characterized in that, The preparation of the dispersion of resin particles A includes: dispersing resin particles A in deionized water at a solid content of 24-26%; and / or The preparation of the dispersion of resin particles B includes: dispersing resin particles B in deionized water at a solid content of 14-16%; and / or The thickener is a 1-2 wt% nanocellulose solution.

10. A lithium-ion secondary battery, characterized in that, Includes the battery separator as described in any one of claims 1-7.

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