Coated diaphragm and preparation method thereof

Abstract

The present invention discloses a coated separator comprising a base membrane layer coated with an elastic material layer containing nanofibers. The coated separator exhibits high strength and toughness, preventing transverse tearing of the separator. This improves separator safety testing, such as high short-circuit rates, tearing during winding and cutting, and difficulty passing heavy object impact tests, which can occur in battery applications. This approach facilitates the large-scale application of thin separators in power and energy storage batteries.

Description

Technical Field

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

[0002] Lithium-ion batteries have gained market recognition and are experiencing rapid growth due to their high energy density, long cycle life, low self-discharge, excellent safety, and fast charge and discharge rates. Wet-process PE and dry-process PP separators are currently the most mature and cost-effective separators for lithium-ion batteries. Wet-process separators primarily use PE as a raw material, while dry-process separators primarily use PP. However, as market demands for lithium-ion battery performance, such as capacity, energy density, cycle life, and rapid charge and discharge, simply prepared PE or PP separators have certain performance shortcomings and are no longer able to meet application requirements.

[0003] At present, the preparation of dry-process diaphragms mainly improves the lateral strength and toughness of the diaphragm through fine-tuning of the process heat treatment and stretching technology, but the overall effect is not significant, which leads to problems such as high short-circuit rate and difficulty in passing safety tests such as cutting and tearing, and heavy object impact when using dry-process thin diaphragms in batteries. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a coated separator and a method for preparing the same. The coated separator has high strength and toughness, and can prevent the separator from transversely tearing. This can alleviate problems such as high short-circuit rates, tearing during winding and cutting, and difficulty passing safety tests such as heavy object impacts in battery applications. This will facilitate the large-scale application of thin separators in power and energy storage batteries.

[0005] The above technical objectives of the present invention are achieved through the following technical solutions:

[0006] A coating diaphragm comprises a base membrane layer, an elastic material layer is coated on the base membrane layer, and nanofibers are mixed in the elastic material layer.

[0007] Preferably, the elastic material layer has a network porous structure that is sparse inside and dense outside.

[0008] Preferably, the nanofibers include at least one of cellulose nanocrystals, polypropylene fibers, polyethylene fibers, polyacrylonitrile fibers, polyester fibers, carbon nanofibers, meta-aramid nanofibers, para-aramid nanofibers, polyimide fibers, boehmite fibers, ceramic fibers and glass fibers.

[0009] Preferably, the nanofiber has an aspect ratio L / D>10, a diameter D<100 nm, and a length L>100 nm.

[0010] Preferably, the base film layer is a PP diaphragm, the thickness of the base film layer is 9-25 μm, and the porosity of the base film layer is 40%-60%.

[0011] Preferably, the thickness of the elastic material layer is 0.5-3 μm.

[0012] A method for preparing a coated diaphragm as described above comprises the following steps: first coating a layer of elastomer slurry A1 having a solid content of TS1 on the base film layer, then coating a layer of elastomer slurry A2 having a solid content of TS2 on the surface of the elastomer slurry A1, and drying to obtain the diaphragm, wherein 1% < TS1 ≤ 10%, and 10% < TS2 ≤ 50%. The elastomer slurries A1 and A2 are obtained by dissolving an elastomer in a solvent, and nanofibers are dispersed in both the elastomer slurries A1 and A2. The solid content of the elastomer slurry A1 is limited to 1% < TS1 ≤ 10%. If the solid content of the elastomer slurry A1 is less than 1%, the slurry is too thin, the coating amount is too small, and a continuous network coating structure cannot be formed. If the slurry concentration is high, the material filling amount in the pores inside the diaphragm is high, thereby blocking the pores. The solid content of elastomer slurry A2 is limited to 10% < TS2 ≤ 50%. If the solid content of elastomer slurry A2 is less than 10%, the slurry is too thin, resulting in a low coating surface area and a poorly formed network coating structure. A high slurry concentration also results in a high filling rate in the pores of the diaphragm surface, leading to pore blockage. The same applies to the nanofiber addition ratio: too little will not provide sufficient strength support for the coating, while too much cellulose will cause pore blockage.

[0013] Preferably, the elastomer comprises at least one of thermoplastic polyurethane, ethylene-vinyl acetate copolymer, ethylene propylene diene monomer (EPDM), styrene-butadiene-styrene block copolymer, styrene-butadiene copolymer, styrene-isoprene-styrene block copolymer, ethylene-1-octene random copolymer, polypropylene thermoplastic elastomer, and nylon elastomer. The elastomer is composed of a flexible segment (soft segment) and a rigid segment (hard segment). The soft segment exhibits flexibility and toughness, imparting high elasticity to the material; the hard segment imparts hardness and rigidity to the material, and after crystallization, the hard segment serves as a physical crosslinking point. The combination of the soft segment and the hard segment imparts high strength and elasticity to the material. The soft segment can be selected from one or a combination of alcohols or polyols, lower esters (C atoms ≤ 8), non-conjugated dienes, butadiene, isoprene, polyether, or polyester; the hard segment can be selected from one or a combination of isocyanates, ethylene, propylene, octene, styrene, acrylonitrile, and polyamide.

[0014] Preferably, the elongation at break of the elastomer is greater than 100%.

[0015] Preferably, the difference between TS2 and TS1 is ≥5%.

[0016] Preferably, the solvent includes at least one of water, acetone, methanol, ethanol, butanol, petroleum ether, N-methylpyrrolidone, N,N-dimethylacetamide and ethyl acetate.

[0017] Preferably, the solvent further comprises water.

[0018] Preferably, the weight ratio of the elastomer to the nanofiber used in preparing the elastomer slurry A1 is 1:(0.01-0.2).

[0019] Preferably, the weight ratio of the elastomer to the nanofiber used in preparing the elastomer slurry A2 is 1:(0.1-1).

[0020] The beneficial effects of the present invention are:

[0021] (1) The air permeability of the coated diaphragm of the present invention is not higher than 250s / 100ml, the liquid absorption rate can reach more than 82%, and the transverse tensile strength can reach 192kgf / cm 2 The puncture strength can reach 250gf, the puncture strength test perforation is circular, the internal resistance of the battery prepared by the coated diaphragm of the present invention is not higher than 0.33mΩ, the short circuit rate is not higher than 0.5%, and the rate 3C / 0.5C can reach 86%;

[0022] (2) The coating membrane of the present invention is composed of a high-elastic polymer material and nanofibers. The nanofibers serve as a skeleton to improve the strength of the coating, and the high-elastic material provides flexibility and elasticity. Both are indispensable. The composite material composed of the high-elastic polymer material and the nanofibers is like a net filling the pores of the membrane and covering the outer surface of the membrane. In this way, the high-elastic polymer material and the nanofibers form an integral structure with the substrate, which can effectively improve the anisotropy problem of the membrane material, improve the overall strength and toughness of the membrane, and prevent the membrane from lateral tearing.

[0023] (3) The preparation method of the coating diaphragm of the present invention is to first coat a layer of elastomer slurry with a low solid content on the base film layer, then coat a layer of elastomer slurry with a high solid content, and then dry it so that an elastic material layer with a network porous structure that is sparse inside and dense outside can be formed on the base film layer, so that the obtained coating diaphragm has excellent strength and toughness, and after being prepared into a battery, it can effectively improve the electrochemical performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a cross-sectional SEM image of the coated diaphragm of Example 1 of the present invention;

[0025] Figure 2 This is an SEM image of the elastic material layer of the coated diaphragm of Example 2 of the present invention.

[0026] Reference numerals:

[0027] 100. Base film layer; 200. Elastic material layer. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to specific embodiments.

[0029] Example 1:

[0030] A coated diaphragm, such as Figure 1 As shown, the base film layer 100 is coated with an elastic material layer 200, and the elastic material layer 200 is mixed with nanofibers. Figure 1 and Figure 2 It can be seen that the elastic material layer 200 is a network porous structure with sparse inside and dense outside, wherein the base film layer 100 is a dry uniaxially stretched PP diaphragm with a thickness of 12 μm and a porosity of 45%, and the thickness of the elastic material layer 200 is 1 μm.

[0031] The method for preparing the coated diaphragm as described above comprises the following steps:

[0032] S1. Preparation of elastomer slurry:

[0033] ① Preparation of elastomer slurry A1: 10 parts of ethylene-vinyl acetate copolymer emulsion with a solid content of 50% was added to a mixed solvent consisting of 11 parts of ethanol and 89 parts of water, and the mixture was thoroughly dissolved and stirred at 500 rpm for 0.5 h. Then, 0.55 parts of cellulose nanocrystals with an aspect ratio (L / D) of 70, a diameter (D) of 6-8 nm, and a length (L) of 300-400 nm were added and uniformly dispersed using ultrasound at a power of 800 W for 1 h. The mixture was cooled and allowed to stand to obtain an elastomer slurry A1 with a solid content of approximately 4.55%.

[0034] ② Preparation of elastomer slurry A2: 10 parts of ethylene-vinyl acetate copolymer emulsion with a solid content of 50% was added to a mixed solvent consisting of 5 parts of methanol and 15 parts of water, and the mixture was thoroughly dissolved and stirred at 800 rpm for 1.0 h. Then, 3.33 parts of cellulose nanocrystals with an aspect ratio L / D ≈ 70, a diameter D of 6-8 nm, and a length L of 300-400 nm were added and uniformly dispersed using ultrasound at an ultrasonic power of 1200 W for 1.5 h. The mixture was cooled and allowed to stand to obtain an elastomer slurry A2 with a solid content of approximately 16.67%.

[0035] S2. Coating: The above-mentioned elastomer slurry is coated twice: ① The first coating: The above-mentioned elastomer slurry A1 is coated on the surface of the base film layer 100 by a dip coating machine at a coating speed of 60 m / min, and the excess liquid on the surface of the base film layer is scraped off by a double metering roller; ② The second coating: The above-mentioned elastomer slurry A2 is coated on the surface of the elastomer slurry A1 by micro-gravure coating at a coating speed of 60 m / min, and then sent to an oven for hot air drying to remove the solvent. The drying temperature is 70°C and the drying time is 20s, thereby obtaining a coated diaphragm containing an elastic material layer 200 with a sparse inner layer and a dense outer layer.

[0036] Example 2:

[0037] A coated diaphragm comprises a base membrane layer coated with an elastic material layer, the elastic material layer being mixed with nanofibers, the elastic material layer being a network porous structure with a sparse interior and a dense exterior, wherein the base membrane layer is a dry-process uniaxially stretched PP diaphragm with a thickness of 12 μm and a porosity of 45%, and the elastic material layer is 0.5 μm thick.

[0038] The method for preparing the coated diaphragm as described above comprises the following steps:

[0039] S1. Preparation of elastomer slurry:

[0040] ① Preparation of elastomer slurry A1: 21.8 parts of a styrene-butadiene copolymer emulsion having a solid content of 48% was added to a mixed solvent consisting of 150 parts of butanone and 850 parts of water, and the mixture was thoroughly dissolved and stirred at 400 rpm for 0.3 h. Then, 0.1 part of a polypropylene fiber having an aspect ratio (L / D) of 56, a diameter (D) of 10-15 nm, and a length (L) of 600-800 nm was added and ultrasonically dispersed at a power of 600 W for 0.8 h. The mixture was cooled and allowed to stand to obtain an elastomer slurry A1 having a solid content of approximately 1.02%.

[0041] ② Preparation of elastomer slurry A2: 23 parts of a styrene-butadiene-styrene block copolymer emulsion with a solid content of 50% was added to a mixed solvent consisting of 20 parts of acetone and 68 parts of water, and the mixture was thoroughly dissolved and stirred at 600 rpm for 0.7 h. One part of a polyethylene fiber with an aspect ratio (L / D) of ≈100, a diameter (D) of 10-12 nm, and a length (L) of 1000-1200 nm was then added and ultrasonically dispersed at a power of 1000 W for 1.0 h. The mixture was then cooled and allowed to stand to obtain an elastomer slurry A2 with a solid content of approximately 10.36%.

[0042] S2. Coating: The above-mentioned elastomer slurry is coated twice: ① The first coating: The above-mentioned elastomer slurry A1 is coated on the surface of the base film layer by a dip coating machine at a coating speed of 50 m / min, and the excess liquid on the surface of the base film layer is scraped off by a double metering roller; ② The second coating: The above-mentioned elastomer slurry A2 is coated on the surface of the elastomer slurry A1 by a wire rod coating method at a coating speed of 50 m / min, and then sent to an oven for hot air drying to remove the solvent. The drying temperature is 80°C and the drying time is 24 seconds, thereby obtaining a coated diaphragm containing an inner sparse and outer dense elastic material layer.

[0043] Example 3:

[0044] A coated diaphragm comprises a base membrane layer coated with an elastic material layer, the elastic material layer being mixed with nanofibers, the elastic material layer being a network porous structure with a sparse interior and a dense exterior, wherein the base membrane layer is a dry-process uniaxially stretched PP diaphragm with a thickness of 12 μm and a porosity of 45%, and the elastic material layer is 3 μm thick.

[0045] The method for preparing the coated diaphragm as described above comprises the following steps:

[0046] S1. Preparation of elastomer slurry:

[0047] ① Preparation of elastomer slurry A1: 10 parts of thermoplastic polyurethane were added to a mixed solvent consisting of 80 parts of N-methylpyrrolidone, 5 parts of water, and 14 parts of butanol, and the mixture was thoroughly dissolved and stirred at 1000 rpm, 60°C, and stirred for 2 hours. Then, 1 part of polyacrylonitrile fiber with an aspect ratio (L / D) of 40-60, a diameter (D) of 8-10 nm, and a length (L) of 400-600 nm was added and ultrasonically dispersed at 900 W for 1.0 hour. The mixture was cooled and allowed to stand to obtain an elastomer slurry A1 with a solids content of 9.17%.

[0048] ② Preparation of elastomer slurry A2: 20 parts of nylon elastomer were added to a mixed solvent consisting of 16 parts of N,N-dimethylacetamide, 2 parts of water, and 2 parts of ethyl acetate, and the mixture was thoroughly dissolved and stirred at a stirring speed of 1200 rpm, a stirring temperature of 80°C, and a stirring time of 1.5 hours. Then, 10 parts of boehmite fiber with an aspect ratio L / D ≈ 15, a diameter D of 40-60 nm, and a length L of 600-900 nm were added and ultrasonically dispersed uniformly at an ultrasonic power of 1500 W for 3.0 hours. The mixture was cooled and allowed to stand to obtain an elastomer slurry A2 with a solid content of 50%.

[0049] S2. Coating: The above-mentioned elastomer slurry is coated twice: ① The first coating: The above-mentioned elastomer slurry A1 is coated on the surface of the base film layer by a slit coater at a coating speed of 40 m / min, and the excess liquid on the surface of the base film layer is scraped off by a metering roller; ② The second coating: The above-mentioned elastomer slurry A2 is coated on the surface of the elastomer slurry A1 by a comma scraper coating method at a coating speed of 40 m / min, and then sent to an oven for hot air drying to remove the solvent. The drying temperature is 70°C and the drying time is 30s, thereby obtaining a coated diaphragm containing an inner sparse and outer dense elastic material layer.

[0050] Comparative Example 1:

[0051] A coated diaphragm is different from Example 1 in that the elastic material layer does not contain nanofibers, and the remaining components and preparation method are the same as those of Example 1.

[0052] Comparative Example 2:

[0053] A coating diaphragm differs from Example 1 in that: in step S1, the styrene-butadiene copolymer emulsion used to prepare elastomer slurry A1 and elastomer slurry A2 is replaced with polyacrylic acid emulsion (non-elastomeric material), and the remaining components and preparation method are the same as Example 1.

[0054] Comparative Example 3:

[0055] A coated diaphragm differs from Example 1 in that: step S1 omits the preparation of elastomer slurry A1, step S2 omits the first coating process, the thickness of the elastic material layer is 1.0 μm, and the remaining components and preparation methods are the same as those in Example 1.

[0056] Comparative Example 4:

[0057] A coated diaphragm differs from Example 1 in that: step S1 omits the preparation of elastomer slurry A2, step S2 omits the second coating process, the thickness of the elastic material layer is 0.2 μm, and the remaining components and preparation methods are the same as those in Example 1.

[0058] Comparative Example 5:

[0059] A diaphragm is the dry uniaxially stretched PP diaphragm in Example 1, without any coating process, the PP diaphragm has a thickness of 12μ and a porosity of 45%.

[0060] Test example:

[0061] The coating thickness, air permeability, liquid absorption, tensile strength, and puncture strength of the eight separators from Examples 1-3 and Comparative Examples 1-5 were tested. Prismatic aluminum-cased lithium-ion batteries were manufactured using the eight separators from Examples 1-3 and Comparative Examples 1-5. The parameters of the prismatic aluminum-cased lithium-ion batteries were as follows: rated capacity: 280 Ah; positive electrode: LiFePO4; negative electrode: graphite; electrolyte: EC / EMC = 3 / 7 (v / v), 1M LiPF6, 2% VC. The internal resistance, short-circuit rate, and rate performance of the resulting batteries were tested. The test results are shown in Table 1.

[0062] Table 1: Separator and battery performance test results

[0063]

[0064] As shown in Table 1, the air permeability of the coated membrane of the present invention is not higher than 250s / 100ml, the liquid absorption rate can reach more than 82%, and the transverse tensile strength can reach 192kgf / cm 2 The puncture strength can reach 250gf, the puncture strength test perforation is circular, the internal resistance of the battery prepared by the coated diaphragm of the present invention is not higher than 0.33mΩ, the short circuit rate is not higher than 0.5%, and the rate 3C / 0.5C can reach 86%.

[0065] Among them, the coating membrane obtained by the preparation method of Example 1 through a better formula combination and process has the best comprehensive physical and chemical properties: thin coating, low air permeability, high liquid absorption rate, high transverse tensile strength, high puncture strength, and circular puncture holes; the battery prepared by the coating membrane prepared in Example 1 has the best comprehensive battery performance: small battery internal resistance, low short circuit rate, and good 3C rate performance.

[0066] In Example 2, as the thickness of the elastic material layer is reduced to 0.5 μm, the liquid absorption rate of the coated separator decreases, and the transverse tensile strength and puncture strength also decrease slightly. The short circuit rate of the battery manufactured by the coated separator of Example 2 increases, and the rate ratio deteriorates, so it is not preferred.

[0067] In Example 3, as the thickness of the elastic material layer increases to 3 μm, the transverse tensile strength and puncture strength of the coated separator increase, and the liquid absorption rate increases. However, because the thickness of the elastic material layer increases, the pore blockage increases, and the air permeability of the coated separator increases. Although the short circuit rate of the battery manufactured by the coated separator of Example 3 decreases, the rate becomes worse. Therefore, it is not preferred;

[0068] Compared with Example 1, Comparative Example 1 lacks nanofibers in the elastic material layer. Nanofibers serve as the skeleton of the coating. The transverse tensile strength and puncture strength of the coated separator obtained after the nanofibers are missing are significantly reduced, the liquid absorption rate is also significantly reduced, and the puncture hole is torn; the short circuit rate of the battery manufactured by the coated separator of Comparative Example 1 is significantly increased, and the rate is deteriorated;

[0069] Comparative Example 2 Compared with Example 1, after the styrene-butadiene copolymer emulsion was replaced with a polyacrylic acid emulsion (non-elastomeric material), the coating had poor porosity, resulting in obvious pore blocking, and the air permeability of the prepared coated membrane was significantly increased, while the transverse tensile strength, puncture strength, and liquid absorption rate were all reduced to varying degrees, and the puncture holes tore; the short circuit rate of the battery manufactured by the coated membrane of Comparative Example 2 increased significantly, and the rate was poor;

[0070] Compared with Example 1, Comparative Example 3 omitted the preparation of the low-concentration elastomer slurry and the first coating process. Although the pore blockage was reduced, the elastic material layer did not form a sparse inner and dense outer porous network structure, resulting in a poor overall toughening and strengthening effect. The transverse tensile strength and puncture strength of the manufactured coated separator were reduced to a certain extent. The short-circuit rate of the battery manufactured with the separator of Comparative Example 3 increased significantly, and the rate capability was poor.

[0071] Compared with Example 1, Comparative Example 4 omitted the preparation of the high-concentration elastomer slurry and the second coating process. Although the pore blockage was reduced, the elastic material layer did not form a sparse inner and dense outer porous network structure, resulting in poor overall toughening and strengthening effects. The coating thickness was too thin, and the functional coating dry-process PP separator produced had a reduced liquid absorption rate, and the transverse tensile strength and puncture strength were reduced to a certain extent. The short-circuit rate of the battery manufactured with the separator of Comparative Example 4 increased significantly, and the rate capability was poor.

[0072] Comparative Example 5 directly uses a dry uniaxially stretched PP diaphragm with a thickness of 12 μm and a porosity of 45% without any processing. The product has the lowest transverse tensile strength and puncture strength, severe perforation and tearing, and the manufactured battery has a high short circuit rate, large internal resistance, and poor rate capability.

[0073] Compared to the single PP separator of Comparative Example 5, the coated separator of Example 1 showed a >5% increase in transverse tensile strength and a >5% increase in puncture strength. The perforations in the puncture strength test were circular and tear-free. After the separator was applied to the battery, the short-circuit rate decreased by over 20%. The separator's liquid absorption rate increased by >20%.

[0074] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A coated diaphragm, characterized in that: It includes a base film layer, on which an elastic material layer is coated, and nanofibers are mixed in the elastic material layer; the elastic material layer is a network porous structure with a sparse inside and a dense outside; the nanofibers have an aspect ratio L / D greater than 10, a diameter D less than 100 nm, and a length L greater than 100 nm; the coated diaphragm is prepared by the following preparation method: first, a layer of elastomer slurry A1 with a solid content of TS1 is coated on the base film layer, and then a layer of elastomer slurry A2 with a solid content of TS2 is coated on the surface of the elastomer slurry A1, and then dried to obtain the membrane, wherein 1% < TS1 ≤ 10%, 10% < TS2 ≤ 50%, the elastomer slurry A1 and the elastomer slurry A2 are obtained by dissolving an elastomer in a solvent, and nanofibers are dispersed in the elastomer slurry A1 and the elastomer slurry A2.

2. A coated diaphragm according to claim 1, characterized in that: The nanofibers include at least one of cellulose nanocrystals, polypropylene fibers, polyethylene fibers, polyacrylonitrile fibers, polyester fibers, carbon nanofibers, meta-aramid nanofibers, para-aramid nanofibers, polyimide fibers, boehmite fibers, ceramic fibers and glass fibers.

3. The coated diaphragm according to claim 1, characterized in that: The base film layer is a PP diaphragm, the thickness of the base film layer is 9-25 μm, and the porosity of the base film layer is 40%-60%.

4. The coated diaphragm according to claim 1, characterized in that: The thickness of the elastic material layer is 0.5-3 μm.

5. The coated diaphragm according to claim 1, characterized in that: The elastomer includes at least one of thermoplastic polyurethane, ethylene-vinyl acetate copolymer, ethylene propylene diene monomer rubber, styrene-butadiene-styrene block copolymer, styrene-butadiene copolymer, styrene-isoprene-styrene block copolymer, ethylene-1-octene random copolymer, polypropylene thermoplastic elastomer and nylon elastomer.

6. The coated diaphragm according to claim 1, characterized in that: The solvent includes at least one of water, acetone, methanol, ethanol, butanol, petroleum ether, N-methylpyrrolidone, N,N-dimethylacetamide and ethyl acetate.

7. The coated diaphragm according to claim 1, characterized in that: The weight ratio of the elastomer to the nanofiber used in preparing the elastomer slurry A1 is 1:(0.01-0.2).

8. The coated diaphragm according to claim 1, characterized in that: The weight ratio of the elastomer to the nanofiber used in preparing the elastomer slurry A2 is 1:(0.1-1).

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