Battery diaphragm as well as preparation method and application thereof

Abstract

The invention provides a battery diaphragm as well as a preparation method and application thereof. The battery diaphragm comprises a base material and a polymer dotted coating arranged on the surface of the base material, the surface characteristic parameter M of the battery diaphragm is [(Sa * eta) / R] * 1000, and M is more than or equal to 0.012 and less than or equal to 7.8; wherein Sa is the surface roughness of the battery diaphragm, eta is the coverage rate of the polymer point-like coatings on the base material, and R is the average diameter of the single polymer point-like coating. The battery diaphragm provided by the invention is low in preparation cost and good in controllability, and the prepared battery diaphragm has good adhesive property and air permeability.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separators, and particularly relates to a battery separator, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, the adhesive coating of lithium battery separators is usually prepared by uniformly mixing an adhesive polymer with additives such as adhesives and wetting agents in an aqueous system, then coating it on the surface of the separator, and drying it. When assembling a lithium battery, the adhesive coating can bond the separator and the electrode sheet together after hot pressing.

[0003] Currently, the main coating method for the adhesive coating is microgravure coating. By contacting and rolling the microgravure roll with the separator, the slurry is transferred from the microcavities to the surface of the separator. However, the coating amount of microgravure coating is high, and the corresponding cost is also higher. Moreover, due to the high coverage rate, the pores of the separator are reduced, the air permeability becomes poor, which is not conducive to the passage of lithium ions, so the internal resistance of the battery will be greater.

[0004] Therefore, it is necessary to develop a battery separator with low preparation cost, excellent adhesive performance and air permeability. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a battery separator, a preparation method thereof, and an application thereof. The battery separator has excellent adhesive performance and air permeability, and the preparation cost is low.

[0006] To achieve this purpose, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a battery separator, which includes a substrate and a polymer dot coating provided on the surface of the substrate. The surface characteristic parameter M of the battery separator is M = [(Sa × η) / R] × 1000, where 0.012 ≤ M ≤ 7.8;

[0008] Wherein, Sa is the surface roughness of the battery separator, the unit is μm, η is the coverage rate of the polymer dot coating on the substrate, the unit is %, and R is the average diameter of a single polymer dot coating, the unit is μm.

[0009] In the present invention, by adjusting the surface roughness of the battery separator, the coverage rate of the polymer dot coating on the substrate, and the average diameter of a single polymer dot coating, so that the surface characteristic parameter M of the battery separator is in the range of 0.012 - 7.8, more effective contact points between the adhesive components can be obtained when the battery separator contacts the electrode sheet, thereby increasing the adhesive force. At the same time, by reducing the blockage of the pores of the substrate by the polymer dot coating, a higher air permeability can be maintained. Therefore, a battery separator with excellent adhesive performance and air permeability can be prepared by controlling the parameter M.

[0010] In the present invention, if the surface characteristic parameter M is too large or too small, then in the case of similar coating amounts, if at least one of the average diameter of a single polymer dot-shaped coating, the coverage rate of the polymer dot-shaped coating on the substrate, and the surface roughness of the battery separator is not controlled within a specific range, the effective contact points between the battery separator and the electrode and the air permeability change amount of the polymer coating cannot be controlled within a suitable range, resulting in a relatively low adhesion force or a relatively high air permeability increase. Specifically, the inventors found that when the surface characteristic parameter M is too small, the effective contact points between the battery separator per unit area and the battery electrode are fewer, and the improvement in the adhesion force of the battery separator compared to the prior art is not obvious. The overall air permeability increase of the battery separator shows an insignificant increase but a large deviation in the air permeability test at different positions (for example, there is a situation where the test position is completely not covered by the coating); when the surface characteristic parameter M is too large, the effective contact points between the battery separator per unit area and the electrode are more, resulting in an improvement in the adhesion force, and the air permeability increase is significantly improved. In addition, if the surface characteristic parameter M cannot be controlled within a certain range, it is also easy to occur film surface coating adhesion due to a relatively large diameter or a relatively large coverage rate of a single dot-shaped coating, which is not conducive to the control of the film surface coating morphology and the balance between the adhesion performance and the air permeability performance, and the production cost will also increase accordingly.

[0011] In the present invention, the surface roughness of the battery separator is the surface roughness of the side of the substrate where the polymer dot-shaped coating is provided.

[0012] In the present invention, the surface characteristic parameter M can be, for example, any one value among 0.012, 0.015, 0.02, 0.05, 0.06, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.4, 0.45, 0.5, 1, 2, 3, 4, 5, 6, 7, 7.2, 7.4, 7.6 or 7.8, etc., or a range value between any two values.

[0013] Preferably, the surface roughness of the battery separator is 0.1 to 1 μm, such as 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm or 0.9 μm, etc.

[0014] Preferably, the coverage rate of the polymer dot-shaped coating on the substrate in the battery separator is 10% to 75% (for example, any one value among 20%, 30%, 40%, 50%, 60% or 70%, etc., or a range value between any two values), and more preferably 15% to 50%.

[0015] Preferably, the average diameter of the single polymer dot-like coating is 100 - 800 μm (such as any value among 200 μm, 220 μm, 240 μm, 260 μm, 280 μm, 300 μm, 320 μm, 340 μm, 360 μm, 380 μm, 400 μm, 420 μm, 440 μm, 460 μm, 480 μm, 500 μm, 520 μm, 540 μm, 560 μm, 580 μm, 600 μm, 620 μm, 640 μm, 660 μm, 680 μm, 700 μm, etc. or the range value between any two numerical values), and more preferably 200 - 500 μm (such as 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, etc.).

[0016] Preferably, the range of the surface characteristic parameter M of the battery separator is 0.3 ≤ M ≤ 3, such as 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, etc.

[0017] Preferably, the substrate includes any one of a polypropylene-based film, a polyethylene-based film, a polyolefin composite film, or a coated film.

[0018] In the present invention, the polyolefin composite separator can be a double-layer composite film or a triple-layer composite film; it can be a composite film of polypropylene and polypropylene, a composite film of polypropylene and polyethylene, or a composite film of polyethylene and polyethylene. The composite microporous film described in the present invention can be prepared by the melt co-extrusion method or by laminating multiple microporous films. The coated film can be a composite separator obtained by coating a layer of a coating containing inorganic particles on a base film, or a composite separator obtained by coating a layer of a coating containing an adhesive polymer on a base film. Of course, it can also be a composite separator with inorganic particle coatings and / or polymer coatings coated on both sides.

[0019] Preferably, the thickness of the substrate is 5 - 15 μm, such as 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, etc.

[0020] Preferably, the polymer dot-like coating includes a polymer.

[0021] Preferably, the polymer includes any one or a combination of at least two of a polyvinylidene fluoride copolymer, a polyvinylidene fluoride homopolymer, an acrylate polymer, a methacrylate polymer, or a polyethylene wax.

[0022] In the second aspect, the present invention provides a method for preparing a battery separator as described in the first aspect, characterized in that the preparation method includes the following steps:

[0023] S1. Prepare the polymer into a slurry, where the solid content of the slurry is 1% - 40%, such as 5%, 10%, 15%, 20%, 25%, 30% or 35%, etc.

[0024] S2. Coat the slurry obtained in step S1 onto the surface of the substrate and dry it to obtain the battery separator.

[0025] Preferably, step S2 includes transferring the slurry obtained in step S1 at least twice and embossing and coating it onto the surface of the substrate, and then drying it to obtain the battery separator.

[0026] Preferably, the solid content of the slurry in step S1 is 1 - 30%, such as 4%, 7%, 10%, 13%, 16%, 19%, 22%, 25% or 28%, etc.

[0027] Preferably, the slurry in step S1 includes the following components by weight: 1 - 30 parts of polymer (such as 3 parts, 6 parts, 9 parts, 12 parts, 15 parts, 18 parts, 21 parts, 24 parts or 27 parts, etc.), 0 - 10 parts of adhesive (such as 1 part, 2 parts, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts or 9 parts, etc.), 0 - 10 parts of additive (such as 1 part, 2 parts, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts or 9 parts, etc.) and 50 - 99 parts of solvent (such as 55 parts, 60 parts, 65 parts, 70 parts, 75 parts, 80 parts, 85 parts, 90 parts or 95 parts, etc.).

[0028] Preferably, the adhesive includes polyacrylate.

[0029] Preferably, the additive includes any one or a combination of at least two of a wetting agent, a dispersant, a thickening agent or a suspending agent.

[0030] Preferably, the wetting agent includes any one or a combination of at least two of a silicone type, a polyether type, a polyether modified silicone or a fluorine-containing wetting agent.

[0031] Preferably, the solvent includes water.

[0032] Preferably, the transfer in step S2 includes transferring the slurry obtained in step S1 through a transfer roller, and the rotational speed of the transfer roller is 10 - 250 m / min, such as 40 m / min, 70 m / min, 100 m / min, 130 m / min, 160 m / min, 190 m / min or 220 m / min, etc.

[0033] Preferably, the transfer roller includes a first transfer roller and a second transfer roller.

[0034] In the present invention, the at least two transfers mean that the slurry in the cartridge is first transferred to the first transfer roller, where there can be one or more first transfer rollers. Then, when the first transfer roller contacts the second transfer roller, the slurry is transferred to the second transfer roller. Finally, when the second transfer roller contacts the substrate, the slurry is imprinted on the surface of the substrate.

[0035] Preferably, the surface of the first transfer roller is provided with reticulations, and the number of reticulations per inch in the direction perpendicular to the inclined direction of the reticulations or in the direction perpendicular to the tangent of the reticulations is 200 - 500 LPI (such as 230 LPI, 260 LPI, 290 LPI, 320 LPI, 350 LPI, 380 LPI, 410 LPI, 440 LPI or 470 LPI, etc.), and more preferably 200 - 300 LPI.

[0036] Preferably, the angle between the reticulations on the surface of the first transfer roller and the roller shaft is 30° - 80° (such as 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70° or 75°, etc.), and more preferably 45° - 60°.

[0037] In the present invention, controlling the number of reticulations and the angle between the reticulations and the roller shaft of the first transfer roller within a certain range can control the slurry carrying capacity on the first transfer roller, thereby controlling the surface roughness of the finally prepared battery separator. The number of reticulations of the first transfer roller is preferably 200 - 300 LPI and the angle between the reticulations and the roller shaft is preferably 45° - 60°, which can ensure that the first transfer roller has appropriate strength and slurry carrying capacity, and does not need to be replaced frequently, reducing the coating cost.

[0038] Preferably, the second transfer roller is a transfer roller that directly contacts the substrate, and the roller surface of the second transfer roller is provided with regularly arranged dot-shaped printing structures.

[0039] Preferably, the difference between the diameter of the surface of a single dot-shaped printing structure of the second transfer roller for contacting the substrate and the average diameter of a single polymer dot-shaped coating obtained is 0 - 50 μm (such as 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm or 45 μm, etc.), and more preferably 0 - 20 μm.

[0040] In the present invention, the absolute value of the difference between the diameter of the surface of a single dot-shaped printing structure of the second transfer roller for contacting the substrate and the average diameter of a target single polymer dot-shaped coating is preferably controlled to be 0 - 20 μm, so as to more precisely control the morphology of the coating layer on the substrate surface.

[0041] Preferably, the center distance between two adjacent dot printing structures on the second transfer roller is 100-800 μm, such as 200 μm, 220 μm, 240 μm, 260 μm, 280 μm, 300 μm, 320 μm, 340 μm, 360 μm, 380 μm, 400 μm, 420 μm, 440 μm, 460 μm, 480 μm, 500 μm, 520 μm, 540 μm, 560 μm, 580 μm, 600 μm or 700 μm, etc.

[0042] In the present invention, by controlling the diameter and center distance of the dot printing structures on the second transfer roller, it is beneficial to obtain a coated separator with a uniform surface topography and an appropriate coverage rate of the coating.

[0043] Preferably, the temperature of the drying in step S2 is 40-100 °C (such as 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C or 95 °C, etc.), and the drying time is 1-30 s (such as 4 s, 7 s, 10 s, 13 s, 16 s, 19 s, 22 s, 25 s or 28 s, etc.).

[0044] Preferably, after the drying in step S2, a winding step is further included.

[0045] Preferably, the winding tension of the winding is 3-30 N, such as 6 N, 9 N, 12 N, 15 N, 18 N, 21 N, 24 N or 27 N, etc.

[0046] In the present invention, by controlling the winding tension to be preferably in the range of 3-30 N, a battery separator with a better appearance can be obtained, and the adhesion and shedding of the coating caused by unwinding during the subsequent battery manufacturing process can be prevented.

[0047] Compared with the battery separator prepared by microgravure coating, the polymer dot coating on the battery separator prepared by the preparation method of the present invention has a lower coverage rate of the substrate, better air permeability and lower internal resistance of the whole battery separator; the shape of the polymer dot coating can be controlled, there is no waste and pollution problem caused by dust splashing, the cost is lower, and it is more environmentally friendly. At the same time, the uniformity is the same as that of roll coating, and the adhesion performance and air permeability performance are better when the coating amount is similar. It can be understood that the shape of the polymer dot coating described in the present application can be a solid dot, a hollow circular ring or a dot with a hollow inside (such as a dot with a cross hollow inside). When the dot coating is not a solid dot, the diameter R refers to the diameter of the circle at the outermost edge of a single polymer dot coating.

[0048] In the second aspect, the present invention provides an electrochemical device, and the electrochemical device includes the battery separator as described in the first aspect.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] The battery separator described in the present invention comprises a substrate and polymer dot coatings provided on the surface of the substrate. The surface characteristic parameter M of the battery separator is M = [(Sa × η) / R] × 1000, where 0.012 ≤ M ≤ 7.8. The battery separator has excellent adhesion performance and air permeability. The change in air permeability of the battery separator compared to the substrate is ≤ 11.9 s, and the pole piece adhesion force is 1.8 - 6.6 N / m. Preferably, the change in air permeability of the battery separator compared to the substrate is ≤ 10.2 s, and the pole piece adhesion force is 2.8 - 6.6 N / m. Description of the Drawings

[0051] Figure 1 SEM image of the battery separator provided in Example 1 with a magnification of 50 times on the surface;

[0052] Figure 2 SEM image of the battery separator provided in Example 14 with a magnification of 50 times on the surface. Detailed Description of the Invention

[0053] The technical solution of the present invention will be further described below in conjunction with the drawings and specific embodiments. However, the following examples are merely simple examples of the present invention and do not represent or limit the scope of the claimed protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0054] Some of the raw material components in the following examples and comparative examples are as follows:

[0055] Polyvinylidene fluoride copolymer: grade LBG, manufacturer is Arkema France;

[0056] PMMA polymer: grade HD6117, manufacturer is Shenzhen Haodian Technology Co., Ltd.;

[0057] Polyacrylate: grade GR - 406, manufacturer is Hunan Gaorui Power Materials Co., Ltd.;

[0058] Polyether silicone: grade QS - 446, manufacturer is Jiangxi Tiansheng New Materials Co., Ltd.;

[0059] Substrate: grade SH511J22A, manufacturer is Shenzhen Xingyuan Materials Technology Co., Ltd., which consists of a polyethylene microporous membrane and inorganic coatings coated on the upper and lower surfaces of the polyethylene microporous membrane. The thickness of the polyethylene microporous membrane is 7 μm, and the thickness of the inorganic coatings on the upper and lower surfaces is 2 μm each.

[0060] Example 1

[0061] This example provides a battery separator and a preparation method thereof. The battery separator comprises a substrate and polymer dot coatings provided on the surface of the substrate, as Figure 1As shown, the surface roughness of the battery separator is 0.721 μm. Each single polymer dot coating is dot-shaped, with an average diameter of 281 μm. The polymer dot coatings are regularly arranged on the substrate, with a coverage rate of 25.1%, and the surface characteristic parameter M = 0.644.

[0062] The battery separator is prepared by the following method:

[0063] S1. Slurry preparation: Mix 13.5 parts by weight of polymer (polyvinylidene fluoride copolymer), 0.9 parts by weight of adhesive (polyacrylate), 0.6 parts by weight of wetting agent (polyether siloxane), and 85 parts by weight of deionized water to obtain a slurry.

[0064] S2. Add the slurry to the cartridge. First, transfer the slurry in the cartridge to the first transfer roller by the rolling of the first transfer roller. Then, when the first transfer roller contacts the second transfer roller, transfer the slurry to the second transfer roller. Finally, when the second transfer roller contacts the substrate, imprint the slurry on the surface of the substrate. The roller rotation speeds of the first transfer roller and the second transfer roller are both controlled at 100 m / min. Then, dry it in an oven at a drying temperature of 80 °C for 15 s, and control the winding tension at 15 N to obtain the battery separator.

[0065] The surface of the above-mentioned first transfer roller is provided with diagonal-shaped reticulations, where the number of reticulations per inch in the direction perpendicular to the reticulations is 200 LPI, and the reticulations form a 60° angle with the axis of the first transfer roller.

[0066] The roller surface of the above-mentioned second transfer roller is provided with dot-shaped printing structures. The diameter of the surface of each single dot-shaped printing structure for contacting the substrate is 280 μm, and the center distance between the dot-shaped printing structures is controlled at 400 μm.

[0067] Example 2

[0068] This example provides a battery separator and its preparation method. The difference from Example 1 is only that the battery separator includes a substrate and polymer dot coatings provided on the surface of the substrate. The surface roughness of the battery separator is 0.738 μm. Each single polymer dot coating is dot-shaped, with an average diameter of 282 μm. The coverage rate of the polymer dot coatings on the substrate is 43.3%, and the surface characteristic parameter M = 1.133. In the preparation method of the battery separator, the second transfer roller is replaced. The diameter of the surface of each single dot-shaped printing structure on the second transfer roller for contacting the substrate is 280 μm, and the center distance between two adjacent dot-shaped printing structures is 300 μm. Other conditions are the same as those in Example 1.

[0069] Example 3

[0070] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.706 μm, a single polymer dot coating is in a dot shape, the average diameter is 285 μm, the coverage rate of the polymer dot coating on the substrate is 19.7%, and the surface characteristic parameter M = 0.488. In the preparation method of the battery separator, the second transfer roller is replaced. The diameter of the surface of a single dot printing structure on the second transfer roller for contacting the substrate is 280 μm, and the center distance between the centers of two adjacent dot printing structures is 450 μm. Other conditions are the same as those in Embodiment 1.

[0071] Embodiment 4

[0072] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.683 μm, a single polymer dot coating is in a dot shape, the average diameter is 122 μm, the coverage rate of the polymer dot coating on the substrate is 9.0%, and the surface characteristic parameter M = 0.503. In the preparation method of the battery separator, the second transfer roller is replaced. The diameter of the surface of a single dot printing structure on the second transfer roller for contacting the substrate is 120 μm, and the center distance between the centers of two adjacent dot printing structures is 360 μm. Other conditions are the same as those in Embodiment 1.

[0073] Embodiment 5

[0074] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.815 μm, a single polymer dot coating is in a dot shape, the average diameter is 143 μm, the coverage rate of the polymer dot coating on the substrate is 50.2%, and the surface characteristic parameter M = 2.861. In the preparation method of the battery separator, the second transfer roller is replaced. The diameter of the surface of a single dot printing structure on the second transfer roller for contacting the substrate is 140 μm, and the center distance between the centers of two adjacent dot printing structures is 200 μm. Other conditions are the same as those in Embodiment 1.

[0075] Embodiment 6

[0076] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot-shaped coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.649 μm, a single polymer dot-shaped coating is dot-shaped, the average diameter is 556 μm, the coverage rate of the polymer dot-shaped coating on the substrate is 12.1%, and the surface characteristic parameter M = 0.141. In the preparation method of the battery separator, the second transfer roller is replaced. The diameter of the surface of a single dot-shaped printing structure on the second transfer roller for contacting the substrate is 550 μm, and the center distance between adjacent two dot-shaped printing structures is 750 μm. Other conditions are the same as those in Embodiment 1.

[0077] Embodiment 7

[0078] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot-shaped coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.792 μm, a single polymer dot-shaped coating is dot-shaped, the average diameter is 81 μm, the coverage rate of the polymer dot-shaped coating on the substrate is 75.0%, and the surface characteristic parameter M = 7.333. In the preparation method of the battery separator, the second transfer roller is replaced. The diameter of the surface of a single dot-shaped printing structure on the second transfer roller for contacting the substrate is 80 μm, and the center distance between adjacent two dot-shaped printing structures is 120 μm. Other conditions are the same as those in Embodiment 1.

[0079] Embodiment 8

[0080] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot-shaped coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.526 μm, a single polymer dot-shaped coating is dot-shaped, the average diameter is 277 μm, the coverage rate of the polymer dot-shaped coating on the substrate is 24.8%, and the surface characteristic parameter M = 0.471. In the preparation method of the battery separator, the first transfer roller is replaced. The surface of the first transfer roller is provided with diagonal-shaped reticulations, where the number of reticulations per inch in the direction perpendicular to the reticulations is 300 LPI, and the reticulations form an angle of 60° with the axis of the first transfer roller. Other conditions are the same as those in Embodiment 1.

[0081] Embodiment 9

[0082] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot-shaped coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.794 μm, a single polymer dot-shaped coating is dot-shaped, the average diameter is 810 μm, the coverage rate of the polymer dot-shaped coating on the substrate is 25.2%, and the surface characteristic parameter M = 0.247. In the preparation method of the battery separator, the second transfer roller is replaced, so that the diameter of the surface of a single dot-shaped printing structure on the second transfer roller for contacting the substrate is 800 μm, and the center distance between the centers of two adjacent dot-shaped printing structures is 1400 μm. Other conditions are the same as those in Embodiment 1.

[0083] Embodiment 10

[0084] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot-shaped coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.461 μm, a single polymer dot-shaped coating is dot-shaped, the average diameter is 330 μm, the coverage rate of the polymer dot-shaped coating on the substrate is 26.8%, and the surface characteristic parameter M = 0.374. In the preparation method of the battery separator, step (1) is adjusted to slurry preparation: 13.5 parts by weight of a polymer (PMMA polymer) and 86.5 parts by weight of deionized water are mixed to obtain a slurry; the second transfer roller is replaced, and the diameter of the surface of a single dot-shaped printing structure on the second transfer roller for contacting the substrate is 330 μm, and the center distance between the centers of two adjacent dot-shaped printing structures is 400 μm. Other conditions are the same as those in Embodiment 1.

[0085] Embodiment 11

[0086] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot-shaped coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.524 μm, a single polymer dot-shaped coating is dot-shaped, the average diameter is 282 μm, the coverage rate of the polymer dot-shaped coating on the substrate is 25.6%, and the surface characteristic parameter M = 0.475. In the preparation method of the battery separator, step (1) is adjusted to: 7.5 parts by weight of a polymer (polyvinylidene fluoride copolymer), 0.9 part by weight of an adhesive (polyacrylate), 0.6 part by weight of a wetting agent (polyether siloxane) and 91 parts by weight of deionized water are mixed to obtain a slurry. Other conditions are the same as those in Embodiment 1, and the above-mentioned battery separator is prepared.

[0087] Embodiment 12

[0088] This embodiment provides a battery separator and a method for preparing the same. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot coating provided on the surface of the substrate. The surface roughness of the battery separator is 0.936 μm, a single polymer dot coating is dot-shaped, the average diameter is 278 μm, the coverage rate of the polymer dot coating on the substrate is 25.1%, and the surface characteristic parameter M = 0.845. In the preparation method of the battery separator, step (1) is adjusted as follows: 23.5 parts by weight of a polymer (polyvinylidene fluoride copolymer), 0.9 part by weight of an adhesive (polyacrylate), 0.6 part by weight of a wetting agent (polyether siloxane), and 75 parts by weight of deionized water are mixed to obtain a slurry. Other conditions are the same as those in Embodiment 1, and the above-mentioned battery separator is prepared.

[0089] Embodiment 13

[0090] This embodiment provides a battery separator and a method for preparing the same. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot coating provided on the surface of the substrate. The surface roughness of the battery separator is 1.034 μm, a single polymer dot coating is dot-shaped, the average diameter is 281 μm, the coverage rate of the polymer dot coating on the substrate is 25.7%, and the surface characteristic parameter M = 0.924. In the preparation method of the battery separator, step (1) is adjusted as follows: 32 parts by weight of a polymer (polyvinylidene fluoride copolymer), 0.9 part by weight of an adhesive (polyacrylate), 0.6 part by weight of a wetting agent (polyether siloxane), and 66.5 parts by weight of deionized water are mixed to obtain a slurry. Other conditions are the same as those in Embodiment 1, and the above-mentioned battery separator is prepared.

[0091] Embodiment 14

[0092] This embodiment provides a battery separator and a method for preparing the same. The difference from Embodiment 1 is only that the battery separator includes a substrate and a polymer dot coating provided on the surface of the substrate, as Figure 2 shown. The surface roughness of the battery separator is 0.714 μm, a single polymer dot coating is approximately dot-shaped, the average value of the diameters of multiple polymer dot coatings is 522 μm, the coverage rate of the polymer dot coating on the substrate is 19.7%, and the surface characteristic parameter M = 0.270. In the preparation method of the battery separator, it is adjusted to coat the slurry on the surface of the substrate by a rotary spraying method. The rotor speed used is 6100 rpm, and the flow rate of the slurry is 1500 mL / min to form an irregular polymer dot coating. Other conditions are the same as those in Embodiment 1.

[0093] Embodiment 15

[0094] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 10 is only that the battery separator includes a substrate and polymer dot-shaped coatings provided on the surface of the substrate. The surface roughness of the battery separator is 0.420 μm. Each single polymer dot-shaped coating is in a hollow circular ring shape, that is, there is an area where the polymer coating is not covered inside the dot. The average diameter of the circles formed by the outermost edges of multiple polymer dot-shaped coatings is 330 μm. The coverage rate of the polymer dot-shaped coatings on the substrate is 26.5%, and the surface characteristic parameter M = 0.337. In the preparation method of the battery separator, the second transfer roller is replaced. The surface of the second transfer roller where each single dot-shaped printing structure contacts the substrate has circular grooves with a diameter of 100 μm, corresponding to the inner circle diameter of the circular ring-shaped polymer dot-shaped coatings. The diameter of the circle formed by the outermost edge of the surface of the dot-shaped printing structure contacting the substrate is 330 μm, and the center distance between the centers of adjacent two dot-shaped printing structures is 500 μm. Other conditions are the same as those in Embodiment 10.

[0095] Comparative Example 1

[0096] This comparative example provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and polymer coatings provided on the surface of the substrate. The surface roughness of the battery separator is 0.533 μm. The coverage rate of the polymer coatings on the substrate is 82.1%. In the preparation method of the battery separator, it is adjusted to directly transfer and coat the slurry onto the surface of the substrate through the first transfer roller once, without using the second transfer roller for secondary transfer of the slurry. Other conditions are the same as those in Embodiment 1.

[0097] Comparative Example 2

[0098] This embodiment provides a battery separator and a preparation method thereof. The difference from Embodiment 1 is only that the battery separator includes a substrate and polymer dot-shaped coatings provided on the surface of the substrate. The surface roughness of the battery separator is 0.147 μm. Each single polymer dot-shaped coating is in a dot shape with an average diameter of 1386 μm. The coverage rate of the polymer dot-shaped coatings on the substrate is 4.9%, and the surface characteristic parameter M = 0.005. In the preparation method of the battery separator, the first transfer roller and the second transfer roller are replaced. The surface of the first transfer roller is provided with diagonal-shaped reticulations, where the number of reticulations per inch along the direction perpendicular to the reticulations is 300 LPI, and the reticulations form an angle of 60° with the axis of the first transfer roller. The diameter of the surface of each single dot-shaped printing structure on the second transfer roller that contacts the substrate is 1400 μm, and the center distance between the centers of adjacent two dot-shaped printing structures is 5400 μm. Other conditions are the same as those in Embodiment 1.

[0099] Comparative Example 3

[0100] This embodiment provides a battery separator and its preparation method. The difference from Embodiment 1 is only that the battery separator includes a substrate and polymer dot coatings provided on the surface of the substrate. The surface roughness of the battery separator is 0.975 μm. Each single polymer dot coating is in a round dot shape, with an average diameter of 84 μm. The coverage rate of the polymer dot coatings on the substrate is 78.2%, and the surface characteristic parameter M = 9.077. In the preparation method of the battery separator, the first transfer roller and the second transfer roller are replaced, such that the surface of the first transfer roller is provided with diagonal screen patterns, where the number of screen patterns per inch in the direction perpendicular to the screen patterns is 300 LPI, and the screen patterns form an angle of 60° with the axis of the first transfer roller. The diameter of the surface of each single dot printing structure on the second transfer roller for contacting the substrate is 85 μm, and the center distance between adjacent two dot printing structures is 80 μm. Other conditions are the same as those in Embodiment 1.

[0101] The battery separators provided in Embodiments 1 to 15 and Comparative Examples 1 to 3 are tested as follows.

[0102] (1) Surface roughness Sa: Using a confocal microscope (model OLYMPUS OLS5100), select 3 equally spaced positions horizontally and 3 equally spaced positions vertically at a magnification of 10 times, with a reference length interval of 3 cm, for a total of 9 positions. The surface roughness is measured separately by the Analysis application software, and the average value is taken.

[0103] (2) Coverage rate η: Using an optical microscope (model OLYMPUS MX51), scan at a magnification of 2.5 times, select 3 equally spaced positions horizontally and 3 equally spaced positions vertically, with a reference length interval of 3 cm, for a total of 9 positions. The coverage rate of the polymer dot coatings is measured separately by the OLYMPUS software, and the average value is taken.

[0104] (3) Dimension characteristic parameter R:

[0105] According to the different sizes of the diameters of the polymer dot coatings, use a confocal microscope (model OLYMPUS OLS5100) to scan 5 independent regions at magnifications of 10 to 25 times. The diameter of each single polymer dot coating is measured separately by the Analysis application software, and the average value of the diameters of multiple polymer dot coatings in the region is used as the dimension characteristic parameter.

[0106] (4) Areal density increment: Take 3 positions along the TD direction for both the battery separator and the substrate, cut specimens with an area size of A4 (210 mm × 297 mm), weigh them using an analytical balance, and calculate the areal density by dividing the average value of the specimen weights by the specimen area. Obtain the areal density of the battery separator and the areal density of the substrate, take the difference between the two, and then convert the unit to g / m 2 This is the areal density increment of the battery separator.

[0107] (5) Air permeability change: Along the TD direction, place the battery separator and the substrate 10 cm apart, and take 5 positions respectively. Test the air permeability through an air permeability analyzer, and take the average values as the air permeability of the battery separator and the substrate respectively. The difference between the two is the air permeability change.

[0108] (6) Electrode sheet adhesion: Along the TD direction of the battery separator, take 3 positions and cut them into 3 strips with a size of 210 mm × 25 mm. The long side of the strip is along the MD direction, and the short side is along the TD direction; cut the negative electrode sheet (graphite negative electrode) into 3 negative electrode strips with a size of 60 mm × 25 mm; place the side of the strip with the polymer dot coating opposite to the negative electrode strip, and hot-press and bond them under the conditions of 60 °C, 1000 kgf, and 120 s with a hot press; use a tensile machine (EM6.202 of Shenzhen Tesmate Instrument Equipment Co., Ltd.) to test the peeling strength between the battery separator and the negative electrode strip under the conditions of a width of 25 mm, a displacement of 50 mm, and a tensile speed of 200 m / min, and take the average value of the peeling strength as the electrode sheet adhesion.

[0109] The test results are shown in Table 1 below.

[0110] Table 1

[0111]

[0112]

[0113] In Table 1, " / " represents that the result of this test cannot be obtained (that is, there is no index corresponding to this test), and "0 - 3" means that the average value of multiple rounds of air permeability change tests is in the range of 0 - 3 s.

[0114] It can be seen from the test results in Table 1 that the air permeability change of the battery separators provided in Examples 1 - 15 compared with the substrate is ≤ 11.9 s, and the electrode sheet adhesion is 1.8 - 6.6 N / m.

[0115] Compared with Example 1, when the coating amount is similar, if the surface characteristic parameter M is relatively small (Example 6), the number of effective contact points between the battery separator per unit area and the electrode sheet is small. As a result, compared with the prior art, the adhesion of the battery separator is not significantly improved. Although the increase in air permeability is not significantly improved, the deviation of the air permeability increase data at different positions is relatively large (the 5 test data of the air permeability increase in Example 1 are 4s, 4.5s, 4.2s, 4.3s, and 3.9s respectively; the 5 test data of the air permeability increase in Example 6 are 3.2s, 4.8s, 6.2s, 3.5s, and 5s respectively). If the surface characteristic parameter M is relatively large (Example 7), since the size of a single polymer dot coating is too small and the spacing is too narrow, the polymer dot coatings will be connected to each other and become irregular. The coverage rate of the polymer dot coatings on the substrate is large, resulting in a relatively large change in air permeability of the prepared battery separator. Therefore, it can be seen that the range of M is preferably 0.3 ≤ M ≤ 3, and the performance of the prepared battery separator is better.

[0116] Compared with Example 1, when the first transfer roller is replaced and the number of screen lines is increased (Example 8), the content of the slurry carried on the first transfer roller decreases. Although the change in air permeability decreases, the adhesion of the electrode sheet decreases.

[0117] Compared with Example 1, if the solid content of the slurry is relatively low (Example 11), the amount of solids in the slurry carried on the first transfer roller decreases, and the change in air permeability of the prepared battery separator decreases, with good air permeability performance, but the adhesion of the electrode sheet decreases. If the solid content of the slurry is relatively high (Example 13), the amount of solids in the slurry carried on the first transfer roller increases, and the adhesion of the electrode sheet of the prepared battery separator increases, but the change in air permeability also increases, resulting in a decrease in air permeability performance.

[0118] Compared with Example 3, under the condition of similar coating amounts, the battery separator prepared by rotary spraying (Example 14) has an irregular distribution of polymer dot coatings, and the diameters of single polymer dot coatings vary greatly, resulting in a relatively low adhesion of the electrode sheet. At the same time, the deviation of the air permeability increase at different positions is relatively large, and the air permeability performance is poor.

[0119] Compared with Example 1, under the condition of similar coating amounts, the slurry is transferred once and no polymer dot coating is formed (Comparative Example 1). Then, in the prepared battery separator, the coverage rate of the polymer dot coating on the substrate is high, the change in air permeability is large, the air permeability effect is greatly reduced, the surface roughness of the battery separator is low, the adhesion of the electrode sheet decreases slightly, and the coating cost increases.

[0120] Compared with Example 1, when the coating amount is similar, if the surface characteristic parameter M is too small (Comparative Example 2), the coverage rate of the polymer dot coating on the substrate is low, and the distance between individual polymer dot coatings is far, resulting in a large fluctuation value in the air permeability change measurement, unstable performance, few effective contact points between the bonding components when the battery separator contacts the electrode sheet, and low electrode sheet adhesion; if the surface characteristic parameter M is too large (Comparative Example 3), since the size of individual polymer dot coatings is too small and the spacing is too narrow, the polymer dot coatings will be connected to each other and irregular, and the coverage rate of the polymer dot coating on the substrate is large, then the air permeability change of the prepared battery separator is too large. Therefore, it can be seen that by controlling M within a specific range, the performance of the prepared battery separator is better.

[0121] The applicant declares that the present invention uses the above embodiments to illustrate the process method of the present invention, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A battery separator, characterized in that, The battery separator includes a substrate and polymer dot coatings provided on the surface of the substrate. The surface characteristic parameter M of the battery separator is M = [(Sa × η) / R] × 1000, where 0.012 ≤ M ≤ 7.8; wherein, Sa is the surface roughness of the battery separator, with the unit of μm, η is the coverage rate of the polymer dot coatings on the substrate, with the unit of %, and R is the average diameter of a single polymer dot coating, with the unit of μm.

2. The battery separator according to claim 1, characterized in that The surface roughness of the battery separator is 0.1 - 1 μm; Preferably, the coverage rate of the polymer dot coatings on the substrate in the battery separator is 10% - 75%, and more preferably 15% - 50%.

3. The battery separator according to claim 1 or 2, characterized in that, The average diameter of a single polymer dot coating is 100 - 800 μm, and more preferably 200 - 500 μm.

4. The battery separator according to any one of claims 1 to 3, characterized in that, The range of the surface characteristic parameter M of the battery separator is 0.3 ≤ M ≤ 3.

5. The battery separator according to any one of claims 1 to 4, characterized in that The substrate includes any one of a polypropylene-based film, a polyethylene-based film, a polyolefin composite film, or a coated film.

6. The battery separator according to any one of claims 1 to 5, characterized in that, The thickness of the substrate is 5 - 15 μm.

7. The battery separator according to any one of claims 1 to 6, characterized in that, The polymer dot coatings include a polymer; Preferably, the polymer includes any one or a combination of at least two of a polyvinylidene fluoride copolymer, a polyvinylidene fluoride homopolymer, an acrylate polymer, a methacrylate polymer, or a polyethylene wax.

8. A method for preparing a battery separator according to any one of claims 1 to 7, characterized in that, The preparation method includes the following steps: S1. Prepare a slurry from the polymer, and the solid content of the slurry is 1% - 40%; S2. Coat the slurry obtained in step S1 on the surface of the substrate and dry it to obtain the battery separator.

9. The method for preparing a battery separator according to claim 8, characterized in that, Step S2 includes transferring the slurry obtained in step S1 at least twice and imprinting and coating it on the surface of the substrate, and then drying it to obtain the battery separator; Preferably, the solid content of the slurry in step S1 is 1 - 30%; Preferably, the slurry in step S1 includes the following components by weight: 1 - 30 parts of a polymer, 0 - 10 parts of an adhesive, 0 - 10 parts of an auxiliary agent, and 50 - 99 parts of a solvent; Preferably, the transfer in step S2 includes transferring the slurry obtained in step S1 through a transfer roller, and the rotation speed of the transfer roller is 10 - 250 m / min; Preferably, the drying temperature in step S2 is 40 - 100 °C, and the drying time is 1 - 30 s; Preferably, after drying in step S2, there is also a winding step; Preferably, the winding tension during winding is 3 - 30 N.

10. An electrochemical device, characterized in that, The electrochemical device includes the battery separator according to any one of claims 1 - 7 or the battery separator prepared by the preparation method according to claim 8 or 9.

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