Aramid fiber coated polyolefin battery diaphragm as well as preparation method and application thereof

Abstract

The invention discloses an aramid fiber coated polyolefin battery diaphragm and a preparation method and application thereof, the aramid fiber coated polyolefin battery diaphragm can be used for manufacturing a lithium ion battery, and the aramid fiber coated polyolefin battery diaphragm is prepared by the following method: (1) treating a polyolefin base membrane by using a strong oxidant to obtain a modified base membrane; (2) dissolving aramid resin in a solvent to obtain an aramid coating solution; and (3) coating the surface of the modified base membrane with the aramid fiber coating liquid, and then curing. According to the invention, the interface bonding strength of the base membrane and the aramid fiber coating is obviously enhanced, so that the aramid fiber coating can give full play to high heat resistance, on the basis of ensuring the tensile strength of the diaphragm, the overall thermal shrinkage rate of the diaphragm is reduced, the risk of high-temperature short circuit is effectively inhibited, and the composite diaphragm with high porosity, strong mechanical property, high uniformity and no surface defect is formed; the method is suitable for high-energy-density lithium ion batteries, and the safety and high-temperature working condition stability of the batteries are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separator beneficiation, and particularly relates to an aramid-coated polyolefin battery separator with good heat resistance, a preparation method thereof, and an application thereof. Background Art

[0002] Traditional lithium battery separators are mainly polyolefin separators, such as polyethylene, polypropylene, and polyethylene / polypropylene composite separators. Although such separators have strong mechanical properties, their electrolyte wettability and thermal stability are poor, and they will undergo thermal shrinkage in a high-temperature environment, resulting in short circuits in the battery and safety accidents. Aramid, that is, aromatic polyamide, has a high heat resistance due to the aromatic groups in its molecular structure, and the amide groups it contains are polar and have good compatibility with electrolytes. Therefore, the separator modified by aramid coating has excellent mechanical properties, heat resistance, and electrolyte wetting properties.

[0003] The aramid coating improves the heat resistance of the separator by supporting the polyolefin base film, preventing the positive and negative electrodes of the battery from contacting and short-circuiting due to the shrinkage of the separator at high temperatures. However, the adhesion between the polyolefin base film and the aramid coating is poor, and the advantages of the high heat resistance of aramid cannot be fully utilized. In addition, due to the large polarity difference between the coating solution and the base film, it is difficult for the coating solution to fully wet and completely spread on the surface of the base film, resulting in uneven coating. Aramid will shrink during the curing process, and uneven coating will lead to local undercoating / missing coating, and surface defects will be formed after curing. Summary of the Invention

[0004] The present invention provides an aramid-coated polyolefin battery separator, a preparation method thereof, and an application thereof, so as to solve the technical problems mentioned in the background art that the heat resistance of the existing separator with an aramid coating does not reach the ideal effect and surface defects are easily generated during preparation.

[0005] To solve the above technical problems, the technical solution proposed by the present invention is as follows: A preparation method of an aramid-coated polyolefin battery separator, comprising the following steps: (1) Treating a polyolefin base film with a strong oxidant to obtain a hydroxylated modified polyolefin base film; (2) Dissolving aramid resin in a solvent to obtain an aramid coating solution; (3) Coating the aramid coating solution on the surface of the hydroxylated modified polyolefin base film to obtain an aramid-coated separator; curing the aramid-coated separator to obtain the aramid-coated polyolefin battery separator.

[0006] The design concept of the above technical solution is that the inventor found that the commonly used polyolefin separator in existing lithium-ion batteries has poor heat resistance and is prone to diaphragm shrinkage at high temperature working conditions, resulting in battery short circuit. Even if the polyolefin separator is enhanced with an aramid coating, the ideal heat resistance effect cannot be achieved. The inventor further studied and found that this is because there are no polar groups on the surface of the polyolefin base film, resulting in poor bonding effect between the polyolefin base film and the aramid coating, and the high heat resistance advantage of aramid cannot be fully exerted. For the above discovery, in the present invention, a strong oxidant is first used to treat the polyolefin base film to hydroxylate its surface, and then an aramid coating is made on its surface. The hydroxyl groups on the surface of the base film are used to strengthen the bonding strength between the base film and the aramid coating, so that the aramid coating can better enhance the support effect for the base film, reduce the overall thermal shrinkage rate of the diaphragm, and thus improve its heat resistance performance.

[0007] As a further preference of the above technical solution, in step (1), the strong oxidant includes at least one of potassium persulfate, sodium persulfate, chromic acid, and potassium permanganate.

[0008] As a further preference of the above technical solution, in step (1), the polyolefin base film is immersed in a solution of a strong oxidant with a concentration of 0.1~1 mol / L for treatment, the immersion temperature is 40~100 °C, and the immersion time is 2~16 h. The above parameters can ensure the hydroxylation modification of the base film while the mechanical strength of the base film will not decrease significantly.

[0009] As a further preference of the above technical solution, in step (2), the aramid coating solution further includes a pore-forming agent and a surfactant, and the mass ratio of the aramid resin, the pore-forming agent, and the surfactant is (5~10):(1~5):(0.1~1). The pore-forming agent can form pores in the aramid coating to increase the porosity of the aramid coating. The surfactant can assist the wetting and complete spreading of the aramid coating solution on the surface of the base film, improve the thickness and mass uniformity after the formation of the aramid coating, and thus reduce the problems of the decrease in the bonding effect between the base film coating and the surface defects after the coating is cured due to uneven coating. The specific ratio can control the viscosity of the coating solution within a reasonable range to ensure the coating uniformity.

[0010] As a further preference of the above technical solution, the aramid resin includes at least one of meta-aramid, para-aramid, and heterocyclic aramid; the pore-forming agent includes at least one of polyvinylpyrrolidone and polyethylene glycol; the surfactant is at least one of fluorocarbon surfactants, silicone surfactants, and polyoxyethylene ether surfactants; the solvent includes at least one of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0011] As a further preference of the above technical solution, the apparent viscosity of the aramid coating solution is 100~3000 cp.

[0012] As a further preference of the above technical solution, in step (3), the aramid-coated diaphragm is placed in a coagulation bath for curing, and the coagulation bath is composed of a good solvent and a poor solvent of the aramid resin.

[0013] As a further preference of the above technical solution, the good solvent includes at least one of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; the poor solvent includes at least one of water, methanol, ethanol, ethylene glycol, and n-butanol.

[0014] As a further preference of the above technical solution, the coagulation bath includes a first coagulation bath and a second coagulation bath. The aramid-coated diaphragm is sequentially placed in the first coagulation bath and the second coagulation bath for curing. The volume fraction of the good solvent in the first coagulation bath is 40-70%, and the volume fraction of the good solvent in the second coagulation bath is 0-40%; the curing time of the aramid-coated diaphragm in the first coagulation bath and the second coagulation bath is both 1-10 min. The first coagulation bath is slow coagulation, and the second coagulation bath is fast coagulation. If fast coagulation is carried out first, the aramid coating is likely to separate from the base film. Therefore, one-stage coagulation (slow coagulation) is carried out first to enable the aramid coating to have a certain bonding force with the base film, and then two-stage coagulation is carried out to increase the coagulation speed, which helps to save the overall coagulation bath time.

[0015] As a further preference of the above technical solution, in step (3), the cured aramid-coated diaphragm is washed with deionized water and then dried at 40-80 °C to obtain the aramid-coated polyolefin battery diaphragm.

[0016] Based on the same inventive concept, the present invention also provides an aramid-coated polyolefin battery diaphragm prepared by using the preparation method described in the above technical solution.

[0017] Based on the same inventive concept, the present invention also provides an application of the above aramid-coated polyolefin battery diaphragm, and the aramid-coated polyolefin battery diaphragm can be used to manufacture lithium-ion batteries.

[0018] The present invention has the following beneficial effects: The present invention pre-treats a polyolefin separator by a simple chemical oxidation method to introduce polar groups (-OH) on its surface, enhancing the polarity and wettability of the polyolefin-based film, significantly enhancing the interfacial bonding strength between the base film and the aramid coating, enabling the aramid coating to fully exert its high heat resistance, reducing the overall thermal shrinkage rate of the separator to ≤6% (130 °C / 1 h) while ensuring the tensile strength of the separator, effectively suppressing the risk of high-temperature short circuit; at the same time, the present invention enhances the wetting of the aramid solution to the base film by adding a surfactant to the aramid solution, making the aramid solution more easily spread on the surface of the base film, reducing holes caused by uneven coating, and then combining a pore-forming agent to construct a uniform microporous structure and a two-stage coagulation bath gradient curing process to form a composite separator with high porosity, strong mechanical properties, high uniformity, and no surface defects, which is particularly suitable for high-energy-density lithium-ion batteries, significantly improving the battery safety and high-temperature operating stability.

[0019] The following will further elaborate on the present invention with reference to specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a physical picture of the aramid-coated separators prepared in Example 1, 2 and Comparative Example 1, 3; Figure 2 It is a scanning electron microscope image of the aramid-coated separator prepared in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The following will describe the embodiments of the present invention in detail with reference to the accompanying drawings, but the present invention can be implemented in many different ways defined and covered by the claims.

[0022] Example 1: The aramid-coated polyolefin battery separator of this example, its SEM image is as Figure 2 shown, and it can be used to make lithium-ion batteries, and is prepared by the following method: (1) Immerse a polyethylene base film with a thickness of 16 μm in a 0.1 mol / L potassium persulfate solution for 8 h, and the solution temperature is 80 °C. After immersion, take it out, wash it with deionized water and dry it at 60 °C to obtain a modified polyethylene base film.

[0023] (2) Dissolve 60 g of meta-aramid, 24 g of polyvinylpyrrolidone, and 5 g of fluorocarbon surfactant in 911 g of dimethylacetamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0024] (3) Coating the aramid coating solution obtained in step (2) on the modified polyethylene base film obtained in step (1) by wire bar coating.

[0025] (4) Immerse the aramid-coated separator obtained in step (3) successively in two coagulation baths. The first coagulation bath consists of 60% dimethylacetamide and 40% deionized water, and the immersion time is 5 min; the second coagulation bath consists of 100% deionized water, and the immersion time is 5 min.

[0026] (5) Wash the solidified aramid-coated separator in step (4) with deionized water, and then dry it at 60 °C to obtain the aramid-coated polyolefin battery separator of this example.

[0027] Example 2: The aramid-coated polyolefin battery separator of this example can be used to fabricate lithium-ion batteries and is prepared by the following method: (1) Immerse a polyethylene film with a thickness of 16 μm in a 1 mol / L potassium persulfate solution at 80 °C for 16 h. After immersion, take it out, wash it with deionized water and dry it at 60 °C to obtain a modified polyethylene film.

[0028] (2) Dissolve 60 g of meta-aramid, 24 g of polyvinylpyrrolidone, and 5 g of fluorocarbon surfactant in 911 g of dimethylacetamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0029] (3) Coating the aramid coating solution obtained in step (2) onto the modified polyethylene film obtained in step (1) by bar coating.

[0030] (4) Immerse the aramid-coated separator obtained in step (3) in a coagulation bath. The first coagulation bath consists of 60% dimethylacetamide and 40% deionized water, and the immersion time is 5 min; the second coagulation bath consists of 100% deionized water, and the immersion time is 5 min.

[0031] (5) Wash the solidified aramid-coated separator in step (4) with deionized water, and then dry it at 60 °C to obtain the aramid-coated polyolefin battery separator of this example.

[0032] Example 3: The aramid-coated polyolefin battery separator of this example can be used to fabricate lithium-ion batteries and is prepared by the following method: (1) Immerse a polyethylene film with a thickness of 16 μm in a 1 mol / L sodium persulfate solution at 80 °C for 10 h. After immersion, take it out, wash it with deionized water and dry it at 60 °C to obtain a modified polyethylene film.

[0033] (2) Dissolve 70 g of para-aramid, 35 g of polyethylene glycol, and 5 g of fluorocarbon surfactant in 890 g of dimethylacetamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0034] (3) Coat the aramid coating solution obtained in step (2) onto the modified polyethylene film obtained in step (1) by coating.

[0035] (4) Immerse the aramid-coated separator obtained in step (3) in a coagulation bath. The first coagulation bath consists of 60% dimethylacetamide and 40% ethanol, and the immersion time is 5 min; the second coagulation bath consists of 30% dimethylacetamide and 70% ethanol, and the immersion time is 5 min.

[0036] (5) Wash the cured aramid-coated separator in step (4) with deionized water, and then dry it at 60 °C to obtain the aramid-coated polyolefin battery separator of this example.

[0037] Example 4: The aramid-coated polyolefin battery separator of this example can be used to make lithium-ion batteries and is prepared by the following method: (1) Immerse a polyethylene film with a thickness of 16 μm in a 1 mol / L potassium persulfate solution for 6 h at a solution temperature of 80 °C. After immersion, take it out, wash it with deionized water, and dry it at 60 °C to obtain a modified polyethylene film.

[0038] (2) Dissolve 70 g of meta-aramid, 28 g of polyvinylpyrrolidone, and 5 g of fluorocarbon surfactant in 897 g of dimethylformamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0039] (3) Coat the aramid coating solution obtained in step (2) onto the modified polyethylene film obtained in step (1) by wire bar coating.

[0040] (4) Immerse the aramid-coated separator obtained in step (3) in a coagulation bath. The first coagulation bath consists of 70% dimethylformamide and 30% deionized water, and the immersion time is 5 min; the second coagulation bath consists of 20% dimethylformamide and 80% deionized water, and the immersion time is 5 min.

[0041] (5) Wash the cured aramid-coated separator in step (4) with deionized water, and then dry it at 60 °C to obtain the aramid-coated polyolefin battery separator of this example.

[0042] Comparative Example 1: The battery separator of this comparative example is prepared by the following method: (1) Dissolve 60 g of meta-aramid and 24 g of polyvinylpyrrolidone in 916 g of dimethylacetamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0043] (2) Coating the aramid coating solution obtained in step (1) onto a polyethylene film by bar coating.

[0044] (3) Immerse the aramid-coated separator obtained in step (2) in a coagulation bath. The first-stage coagulation bath consists of 60% dimethylacetamide and 40% deionized water, and the immersion time is 5 min; the second-stage coagulation bath consists of 100% deionized water, and the immersion time is 5 min.

[0045] (4) Wash the solidified aramid-coated separator in step (3) with deionized water, and then dry it at 60 °C to obtain the battery separator of this comparative example.

[0046] Comparative Example 2: The battery separator of this comparative example was prepared by the following method: (1) Dissolve 60 g of meta-aramid, 24 g of polyvinylpyrrolidone, and 5 g of fluorocarbon surfactant in 911 g of dimethylacetamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0047] (2) Coating the aramid coating solution obtained in step (1) onto a polyethylene film by bar coating.

[0048] (3) Immerse the aramid-coated separator obtained in step (2) in a coagulation bath. The first-stage coagulation bath consists of 60% dimethylacetamide and 40% deionized water, and the immersion time is 5 min; the second-stage coagulation bath consists of 100% deionized water, and the immersion time is 5 min.

[0049] (4) Wash the solidified aramid-coated separator in step (3) with deionized water, and then dry it at 60 °C to obtain the battery separator of this comparative example.

[0050] Performance tests were carried out on the battery separators of Examples 1 and 2 and Comparative Examples 1 and 2, and the results are shown in Table 1.

[0051] Table 1 Performance test results of the battery separators of Examples 1 and 2 and Comparative Examples 1 and 2

[0052] As can be seen from Table 1, compared with Comparative Examples 1 and 2, the tensile strength of Examples 1 and 2 did not decrease, but even slightly increased, and at the same time, the thermal shrinkage rate decreased significantly, and the heat resistance of the separator was better. This is because after the polyolefin base film was hydroxylated and modified, the bonding between the base film and the coating was enhanced.

[0053] Comparative Example 3: The battery separator of this comparative example was prepared by the following method: (1) Immerse a polyethylene film with a thickness of 16 μm in a 0.1 mol / L potassium persulfate solution for 8 h at a solution temperature of 80 °C. After immersion, take it out, wash it with deionized water and dry it at 60 °C to obtain a modified polyethylene film.

[0054] (2) Dissolve 60 g of meta-aramid and 24 g of polyvinylpyrrolidone in 916 g of dimethylacetamide, and mechanically stir at room temperature until completely dissolved to obtain an aramid coating solution.

[0055] (3) Coating the aramid coating solution obtained in step (2) onto the modified polyethylene film obtained in step (1) by bar coating.

[0056] (4) Immerse the aramid-coated separator obtained in step (3) in a coagulation bath. The first-stage coagulation bath consists of 60% dimethylacetamide and 40% deionized water, and the immersion time is 5 min; the second-stage coagulation bath consists of 100% deionized water, and the immersion time is 5 min.

[0057] (5) Wash the solidified aramid-coated separator in step (4) with deionized water, and then dry it at 60 °C to obtain the battery separator of this comparative example.

[0058] Perform performance tests on the battery separators of Examples 1 and 2 and Comparative Examples 1 and 3. The results are shown in Table 2.

[0059] Table 2 Performance test results of the separators prepared by the methods of Examples 1 and 2 and Comparative Examples 1 and 3

[0060] Compared with Comparative Examples 1 and 3, the performance of Examples 1 and 2 did not significantly weaken. However, the aramid-coated separators prepared in Examples 1 and 2 had uniform coating and no obvious appearance defects, as Figure 1 shown, while in Comparative Examples 1 and 3, due to the poor wetting of the base film surface, the coating was uneven and hole defects appeared.

[0061] The above is only the preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. For those skilled in the art of this technology, the improvements and transformations obtained without departing from the technical concept of the present invention should also be regarded as the protection scope of the present invention.

[0062] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above-described embodiments within the scope of the present invention.

Claims

1. A preparation method of an aramid-coated polyolefin battery separator, characterized in that, It includes the following steps: (1) Treat the polyolefin-based film with a strong oxidant to obtain a hydroxylated modified polyolefin-based film; (2) Dissolve the aramid resin in a solvent to obtain an aramid coating solution; (3) Coat the aramid coating solution on the surface of the hydroxylated modified polyolefin-based film to obtain an aramid-coated separator; cure the aramid-coated separator to obtain the aramid-coated polyolefin battery separator.

2. The preparation method of the aramid-coated polyolefin battery separator according to claim 1, wherein In step (1), the strong oxidant includes at least one of potassium persulfate, sodium persulfate, chromic acid, and potassium permanganate.

3. The preparation method of the aramid-coated polyolefin battery separator according to claim 2, characterized in that, In step (1), soak the polyolefin-based film in a solution of the strong oxidant with a concentration of 0.1 - 1 mol / L for treatment, with a soaking temperature of 40 - 100 °C and a soaking time of 2 - 16 h.

4. The preparation method of the aramid-coated polyolefin battery separator according to claim 1, characterized in that, In step (2), the aramid coating solution further includes a pore-forming agent and a surfactant, and the mass ratio of the aramid resin, pore-forming agent, and surfactant is (5 - 10):(1 - 5):(0.1 - 1).

5. The preparation method of the aramid-coated polyolefin battery separator according to claim 4, wherein, The aramid resin includes at least one of meta-aramid, para-aramid, and heterocyclic aramid; the pore-forming agent includes at least one of polyvinylpyrrolidone and polyethylene glycol; the surfactant is at least one of fluorocarbon surfactants, silicone surfactants, and polyoxyethylene ether surfactants; the solvent includes at least one of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

6. The preparation method of the aramid-coated polyolefin battery separator according to any one of claims 1-5, characterized in that, In step (3), place the aramid-coated separator in a coagulation bath for curing, and the coagulation bath is composed of a good solvent and a poor solvent for the aramid resin.

7. The preparation method of the aramid-coated polyolefin battery separator according to claim 6, characterized in that The good solvent includes at least one of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; the poor solvent includes at least one of water, methanol, ethanol, ethylene glycol, and n-butanol.

8. The preparation method of the aramid-coated polyolefin battery separator according to claim 6, wherein The coagulation bath includes a first coagulation bath and a second coagulation bath. Place the aramid-coated separator in the first coagulation bath and the second coagulation bath in sequence for curing. The volume fraction of the good solvent in the first coagulation bath is 40 - 70%, and the volume fraction of the good solvent in the second coagulation bath is 0 - 40%; the curing time of the aramid-coated separator in the first coagulation bath and the second coagulation bath is 1 - 10 min.

9. An aramid-coated polyolefin battery separator, characterized in that, It is prepared by the preparation method according to any one of claims 1 - 8.

10. Use of the aramid-coated polyolefin battery separator according to claim 9, characterized in that, The aramid-coated polyolefin battery separator is used to make a lithium-ion battery.

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