Diaphragm and preparation method and application thereof

Abstract

The invention provides a diaphragm as well as a preparation method and application thereof, and relates to the technical field of lithium ion battery diaphragm materials. The preparation method of the diaphragm comprises the following steps: S1, uniformly mixing a polyamide solution, lithium chloride and a solvent to obtain a spinning solution, and spinning and chopping the spinning solution to obtain polyamide short fibers; s2, dispersing polyamide short fibers, inorganic ceramic particles and auxiliaries in water to obtain water-based coating slurry; and S3, coating a base membrane with the water-based coating slurry, and drying to obtain the water-based composite membrane. The diaphragm provided by the invention has excellent heat resistance, and is helpful for improving the cycle performance and safety performance of the secondary battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery separator materials, and particularly to a separator and its preparation method and application. Background Art

[0002] Currently, the main commercialized separator is the polyolefin separator. The polyolefin separator has a low heat resistance temperature. For example, the heat distortion temperature of the polyethylene (PE) separator is 80 - 100 °C, the glass transition temperature is 135 - 140 °C, and the heat distortion temperature of the PP separator is about 100 °C, and the glass transition temperature is about 160 °C. Its safety boundary temperature is generally below 170 °C, making the traditional polyolefin separator unable to meet the usage requirements of current lithium-ion power batteries. There is an urgent need to develop a lithium battery separator with a higher service temperature.

[0003] Traditional high-heat-resistant lithium battery separators are mainly coated high-heat-resistant battery separators. The coated high-heat-resistant battery separator is based on a polyolefin separator base film and coated with an organic material (meta-aramid, para-aramid, polyvinylidene fluoride) or an inorganic ceramic (aluminum oxide, boehmite, etc.) material. The organic material coating mostly requires solvent phase conversion to form pores, uses a large amount of organic solvents, is not environmentally friendly and has a high cost. For example, the patents of Teijin Limited in Japan for meta-aramid coating are CN101558513B, CN101779311B, CN102160211B, and CN101180751B, and the patents of Sumitomo Chemical Company in Japan for para-aramid coating are CN101356665B, CN101657497B, CN106030857B, and CN106992279B. The principle is to coat the aramid polymer solution on the polyolefin base film, and then form pores through solvent exchange extraction, and then coat a layer of high-heat-resistant polymer on the surface of the base film to improve the heat resistance temperature of the separator. When only an inorganic material is coated, powder falling may occur during use, and it is easily broken at high temperatures, and the improvement of heat resistance cannot meet the current requirements. Currently, lithium batteries are continuously developing towards high-nickel, ultra-high-nickel, and lithium-rich manganese-based cathodes. There is an urgent need for a separator with a safety boundary temperature of 200 °C or even higher than 200 °C. In view of the problems existing in the above two solutions, it is particularly necessary to develop a new type of high-heat-resistant separator. Summary of the Invention

[0004] Based on the technical problems existing in the background art, the present invention proposes a separator and its preparation method and application.

[0005] A preparation method of a separator proposed by the present invention includes the following steps: S1. Mix a polyamide solution, lithium chloride, and a solvent evenly to obtain a spinning solution, and spin and cut the spinning solution to obtain polyamide short fibers; S2. Disperse the polyamide short fibers, inorganic ceramic particles, and an auxiliary agent in water to obtain an aqueous coating slurry; S3. Coat the aqueous coating slurry on a base film and dry it to obtain the product.

[0006] In the preparation process of the high heat-resistant lithium-ion battery separator, it is necessary to comprehensively consider process difficulty, production cost, the best performance that can be achieved, etc. The separator prepared by the present invention has an extremely low thermal shrinkage rate at high temperatures, improving the safety performance and energy density of the battery under extreme thermal conditions; it has good surface wettability, which can shorten the electrolyte infiltration time; the aqueous coating process is used, and the process route is environmentally friendly and has low cost. The present invention has great application advantages.

[0007] Preferably, in the S1, the preparation method of the polyamide solution includes adding a diamine monomer to dimethylacetamide at -5 to 5°C, then adding a diacyl chloride monomer, heating to 20 to 40°C, and stirring for 2 to 4 hours to obtain it.

[0008] The present invention uses a low-temperature synthesis technology to synthesize the polyamide solution, which can control the reaction rate and prevent explosive polymerization.

[0009] More preferably, the solid content of the polyamide solution is 5 to 20%.

[0010] More preferably, the thermal decomposition temperature of the polyamide solution is greater than 350°C.

[0011] More preferably, the diamine monomer is selected from one or more of m-phenylenediamine, p-phenylenediamine, p-diphenylether diamine, naphthalene-2,6-diamine, 3,3'-dihydroxy-[1,1'-biphenyl]-4,4'-diamine, 3,3'-dimethoxy-[1,1'-biphenyl]-4,4'-diamine, 1,4-cyclohexanediamine, 4,4'-diamino-α-cocoumaric acid.

[0012] More preferably, the diacyl chloride monomer is selected from one or more of isophthaloyl chloride, terephthaloyl chloride, 2,5-furandicarbonyl chloride, [1,1'-biphenyl]-4,4'-dicarbonyl chloride, naphthalene-2,3-dicarbonyl chloride, p-diphenylether dicarbonyl chloride, 1,4-cyclohexanedicarbonyl chloride, 1,3-cyclohexanedicarbonyl chloride, naphthalene-2,6-dicarbonyl chloride.

[0013] More preferably, the molar ratio of the diacyl chloride monomer to the diamine monomer is 1:1.

[0014] More preferably, the stirring speed is 500 to 800 rpm.

[0015] The present invention polycondenses a diacyl chloride monomer and a diamine monomer to form a polyamide. The diacyl chloride monomer and the diamine monomer used are mainly benzene ring-containing or cycloalkane monomers, making the molecular chain of the polyamide have a high rigidity.

[0016] In order to ensure good water dispersibility of the polyamide staple fibers, during the synthesis of polyamide, monomers containing hydrophilic groups need to be copolymerized to increase the hydrophilicity of the polyamide staple fibers. The hydrophilic groups include hydroxyl groups, carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, carboxylic acid esters, block polyethers, etc.

[0017] Preferably, in the above-mentioned S1, the solvent is selected from one or more of dimethylacetamide (DMAc) and dimethylformamide (DMF).

[0018] Preferably, in the above-mentioned S1, the mass of lithium chloride accounts for 1% - 3% of the mass of the polyamide solution.

[0019] As a co-solvent, controlling the mass of lithium chloride within a certain range is beneficial to improving the solubility of polyamide. After LiCl ionizes in the solvent, the Li + forms a complex structure with DMAc, and at the same time releases free Cl - , and Cl - has a very high electronegativity, which can break the hydrogen bond interaction between polyamide macromolecules and form a new complex structure, thereby improving the solubility of polyamide.

[0020] Preferably, in the above-mentioned S1, the viscosity of the spinning solution is below 10000 mPa·s, preferably 1000 - 5000 mPa·s.

[0021] Preferably, in the above-mentioned S1, the spinning parameter conditions include: spinning voltage: 20 - 30 kV, spinning temperature: 10 - 40 °C; spinning relative humidity: 20 - 95%, hot air drying temperature: 80 - 120 °C.

[0022] By controlling the spinning parameters, the prepared polyamide fibers have better dimensions, which is beneficial to their combination with inorganic ceramic particles.

[0023] Preferably, in the above-mentioned S1, the diameter of the polyamide staple fibers is 100 - 2000 nm and the length is 5 - 30 μm.

[0024] Controlling the dimensions of the polyamide staple fibers within a certain range can ensure good dispersion of the staple fibers in the coating slurry (it is difficult to disperse if the fibers are too long), and can also interpenetrate in the coating structure to form a "steel bar" network structure (the network structure cannot be formed if the fiber length is too short), so as to be both compatible and dispersed, and can effectively improve the heat resistance of the coating.

[0025] Preferably, in the above S2, the inorganic ceramic particles are selected from one or more of α-Al2O3, β-Al2O3, γ-Al2O3, boehmite (AlOOH), silica (SiO2), rutile titanium dioxide, anatase titanium dioxide, magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), zirconia, aluminum nitride (AlN), and molybdenum disulfide (MoS2).

[0026] Preferably, the particle size of the inorganic ceramic particles is 500 - 5000 nm.

[0027] Controlling the particle size of the inorganic ceramic particles within a certain range is beneficial to forming a "steel bar" network structure with the polyamide staple fibers, thereby enhancing the heat resistance.

[0028] Preferably, in the above S2, the additives include a dispersant, a thickener, and a wetting agent.

[0029] More preferably, the dispersant includes phosphate esters, cellulose derivatives, polyacrylamide, polyacrylate, styrene-maleic anhydride ester, modified styrene-maleic copolymer, alkyl ammonium salts of acidic copolymers, methyl amyl alcohol, guar gum, fatty acid polyethylene glycol esters, or cellulose ethers.

[0030] More preferably, the thickener includes polyvinyl alcohol, sodium carboxymethyl cellulose, polyurethane, acrylate, or sodium polyacrylate.

[0031] More preferably, the wetting agent includes monohydric alcohols, dihydric alcohols, trihydric alcohols, polyether-modified siloxanes, modified succinic acids, polyether-modified polydimethylsiloxanes, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE); the trihydric alcohols include ethanol, ethylene glycol, glycerol, isopropyl alcohol, and butanol.

[0032] Preferably, in the above S2, the mass ratio of the polyamide staple fibers, inorganic ceramic particles, additives, and water is (1 - 20):(20 - 35):(0 - 3):(42 - 80).

[0033] The mass ratio of the polyamide staple fibers, inorganic ceramic particles, additives, and water within a certain range helps to enhance the heat resistance of the separator.

[0034] Preferably, in the above S3, the base film is a wet-process polyvinyl film.

[0035] Preferably, in the above S3, the coating method is selected from one of knife coating, roll coating, and dip coating; more preferably, the coating is concave roll coating.

[0036] Preferably, in the step S3, the wet coating thickness is 4-8 μm, and the dry coating thickness is 1-3 μm.

[0037] In the present invention, the coating thickness is controlled within a certain range, which can not only control the cost but also improve the heat resistance of the separator. If the coating is too thin (<1 μm), firstly, it is difficult to achieve and the cost is relatively high; secondly, it is difficult to improve the heat resistance of the separator. If the coating is too thick (>3 μm), firstly, it increases the thickness of the separator, which is not conducive to improving the energy density of the battery; secondly, there is no obvious improvement in the heat resistance of the separator, but only an increase in cost.

[0038] Preferably, in the step S3, the drying temperature is 40-60 °C.

[0039] The present invention also provides a separator prepared by the above preparation method.

[0040] The application of a separator as described above or a separator prepared by the above preparation method in a secondary battery.

[0041] The beneficial effects of the present invention are as follows:

[0042] The present invention provides a high heat-resistant separator, which is mainly used as a separator in lithium-ion battery products. It can significantly reduce the thermal shrinkage of the lithium battery separator at high temperatures. The preparation method of this separator adopts a method of mixed coating of organic and inorganic materials. The organic material is in the form of fibers and can be directly coated with an aqueous solution without the need for additional pore formation. The process is simple and has the advantage of environmental protection. No additional solvent treatment is required during the coating process, solving the obvious problems existing in the separate coating of organic materials and inorganic materials.

[0043] The present invention selects a polyamide material with excellent heat resistance to prepare polymer fibers. Due to the presence of a large number of hydrogen bonds between molecular chains, its melting point and decomposition temperature are greater than 350 °C, with excellent heat resistance and good solubility. It can be completely dissolved in organic solvents, facilitating spinning. Its molecular chains are hydrophilically modified, making it easy to disperse and thus prepare an aqueous slurry.

[0044] The coating in the high heat-resistant separator prepared by the present invention contains polymer short fibers and inorganic ceramics, which together form a "reinforced concrete" composite coating structure. The coating does not shed powder, has excellent electrolyte wettability, and excellent heat resistance. Compared with a completely inorganic ceramic coating, the composite coating of the present invention contains some polymers. The density of the polymer is much lower than that of the inorganic substance. Under the same usage conditions, using the high heat-resistant composite coating separator of the present invention can reduce the mass of the battery cell and improve the mass energy density of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a schematic cross-sectional view of the coating of the separator proposed by the present invention; 1 represents the base film, and 2 represents the coating.

[0046] Figure 2 Schematic diagram of the coated surface of the separator proposed by the present invention; 21 represents alumina particles in the coating, and 22 represents polyamide short fibers in the coating. Detailed implementation manners

[0047] The technical solution of the present invention will be described in detail through specific examples.

[0048] Materials, reagents, etc. used in the following examples and comparative examples can be obtained from commercial channels without special instructions.

[0049] Example 1

[0050] A method for preparing a separator includes the following steps:

[0051] S1. First, add m-phenylenediamine monomer and 4,4'-diamino-α-cocamine diacid monomer to a reaction kettle containing dimethylacetamide. The temperature of the reaction kettle is controlled at 0 °C, and the molar ratio of m-phenylenediamine monomer to 4,4'-diamino-α-cocamine diacid monomer is 4:1. Subsequently, add isophthaloyl chloride dropwise to the reaction kettle, and the molar ratio of diacyl chloride monomer to diamine monomer is 1:1. Raise the temperature of the reaction kettle to 40 °C, and the stirring rate of the stirring paddle is 800 rpm. React for 3 h to obtain a polyamide solution with a solid content of 20%. Copolymerizing 4,4'-diamino-α-cocamine diacid monomer is to improve the hydrophilicity of polyamide.

[0052] Add 1.5% of the co-solvent lithium chloride LiCl to the polyamide solution, and further add DMAc solvent to adjust the viscosity of the slurry to 1500 mPa·s. Use an electrospinning machine to spin the polyamide slurry into fibers. Spinning voltage: 25 kV, spinning temperature: 40 °C; spinning relative humidity: 75%, hot air drying temperature: 100 °C. The fiber diameter obtained by spinning is 100 - 2000 nm; then use a closed crusher to cut the polyamide fibers into pieces to obtain polyamide short fibers with a length of 5 - 30 μm.

[0053] S2. Disperse polyamide short fibers, alumina particles and additives in water to prepare an aqueous coating slurry. In the aqueous coating slurry, the proportion of alumina particles is 25%, the proportion of polyamide short fibers is 2%, the proportion of the dispersant polyacrylamide is 0.15%, the proportion of the thickener sodium carboxymethylcellulose is 0.2%, the proportion of the wetting agent polyvinylidene fluoride - hexafluoropropylene (PVDF - HFP) is 0.2%, and the balance is water. After mixing the dispersant, polyamide short fibers, alumina particles, wetting agent, and thickener in sequence, stir at a stirring paddle speed of 1000 rpm for 2 h, and then stand for 2 h to naturally defoam for standby. The viscosity of the aqueous coating slurry is 203 mPa·s.

[0054] S3. Select a 9-μm wet-process polyethylene film as the coating substrate film, coat the aqueous coating slurry on the substrate film by double-sided concave roll coating, with a wet coating thickness of 6 μm, a drying air temperature of 60 °C, a dry coating thickness of 2 μm, and a double-sided dry coating thickness of 4 μm, and finally obtain a double-sided composite coating separator of 9+2+2.

[0055] Example 2

[0056] The difference from Example 1 is only that in S2, in the aqueous coating slurry, the proportion of alumina particles is 25%, the proportion of polyamide short fibers is 5%, the proportion of the dispersant polyacrylamide is 0.15%, the proportion of the thickener sodium carboxymethyl cellulose is 0.2%, and the proportion of the wetting agent polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) is 0.2%; the viscosity of the aqueous coating slurry is 256 mPa·s; the rest is the same as in Example 1.

[0057] Example 3

[0058] The difference from Example 1 is only that in S2, in the aqueous coating slurry, the proportion of alumina particles is 25%, the proportion of polyamide short fibers is 8%, the proportion of the dispersant polyacrylamide is 0.15%, the proportion of the thickener sodium carboxymethyl cellulose is 0.2%, and the proportion of the wetting agent polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) is 0.2%; the viscosity of the aqueous coating slurry is 355 mPa·s; the rest is the same as in Example 1.

[0059] Comparative Example 1

[0060] The difference from Example 1 is only that in S2, in the aqueous coating slurry, the proportion of alumina particles is 25%, the proportion of the dispersant polyacrylamide is 0.15%, the proportion of the thickener sodium carboxymethyl cellulose is 0.2%, and the proportion of the wetting agent polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) is 0.2%; the viscosity of the aqueous coating slurry is 178 mPa·s; the rest is the same as in Example 1.

[0061] Comparative Example 2

[0062] Wet-process biaxially oriented polyethylene separator (Yunnan Enjie HS9), with a separator thickness of 9 μm.

[0063] Comparative Example 3

[0064] Dry-process stretched polypropylene separator (Zhongxing New Materials ZM14), with a separator thickness of 14 μm.

[0065] Test the basic physical properties and high-temperature heat shrinkage performance of the above separators, and the test results are shown in Table 1.

[0066] Table 1

[0067]

[0068]

[0069] Application: Assemble the above diaphragm into a secondary battery. Specifically: Prepare a positive electrode sheet using ternary NCM712 material, prepare a negative electrode sheet using artificial graphite, use the above diaphragm as the separator, assemble a 3Ah soft-pack secondary battery using a stacking machine, and conduct 1C charge-discharge cycle tests and full-charge temperature rise thermal runaway tests on the soft-pack secondary battery. The test results are shown in Table 2.

[0070] Table 2

[0071]

[0072] As can be seen from Table 1 and Table 2, in the high heat-resistant diaphragm of the present invention, when the proportion of polyamide short fibers in the aqueous coating slurry reaches more than 5%, the safety performance of the soft-pack secondary battery is significantly improved. Examples 1 to 3 of the present invention can greatly reduce the thermal shrinkage of the diaphragm at high temperatures, and significantly improve the electrolyte wettability of the diaphragm, which is also beneficial to the improvement of the electrochemical cycling performance of the diaphragm. In terms of the first efficiency and capacity retention rate, the composite coating diaphragm of the present invention has obvious advantages.

[0073] In summary, the diaphragm provided by the present invention has excellent heat resistance, which helps to improve the cycling performance and safety performance of secondary batteries.

[0074] The above is only the preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A method for preparing a separator, characterized in that, It includes the following steps: S1. Mix the polyamide solution, lithium chloride, and solvent evenly to obtain a spinning solution, and then spin and cut the spinning solution to obtain polyamide short fibers; S2. Disperse the polyamide short fibers, inorganic ceramic particles, and additives in water to obtain an aqueous coating slurry; S3. Coat the aqueous coating slurry on a base film and dry it to obtain the product.

2. The preparation method according to claim 1, characterized in that, In the above S1, the mass of lithium chloride accounts for 1% - 3% of the mass of the polyamide solution.

3. The preparation method according to claim 1, characterized in that, In the above S1, the preparation method of the polyamide solution includes adding a diamine monomer to dimethylacetamide at -5 to 5 °C, then adding a diacyl chloride monomer, heating to 20 to 40 °C, and stirring for 2 to 4 hours to obtain it; the solid content of the polyamide solution is 5% - 20%.

4. The preparation method according to claim 3, characterized in that, The diamine monomer is selected from one or more of m-phenylenediamine, p-phenylenediamine, p-phenylenediamine ether, naphthalene-2,6-diamine, 3,3'-dihydroxy-[1,1'-biphenyl]-4,4'-diamine, 3,3'-dimethoxy-[1,1'-biphenyl]-4,4'-diamine, 1,4-cyclohexanediamine, 4,4'-diamino-α-cocamine diacid; the diacyl chloride monomer is selected from one or more of isophthaloyl chloride, terephthaloyl chloride, 2,5-furandicarbonyl chloride, [1,1'-biphenyl]-4,4'-dicarbonyl chloride, naphthalene-2,3-dicarbonyl chloride, p-phenylenediamine ether dicarbonyl chloride, 1,4-cyclohexanedicarbonyl chloride, 1,3-cyclohexanedicarbonyl chloride, naphthalene-2,6-dicarbonyl chloride; the molar ratio of the diacyl chloride monomer to the diamine monomer is 1:

1.

5. The preparation method according to claim 1, characterized in that, In the above S1, the parameter conditions for spinning include: spinning voltage: 20 - 30 kV, spinning temperature: 10 - 40 °C; relative humidity for spinning: 20 - 95%, hot air drying temperature: 80 - 120 °C; the diameter of the polyamide short fibers is 100 - 2000 nm, and the length is 5 - 30 μm.

6. The preparation method according to claim 1, characterized in that, In the above S2, the inorganic ceramic particles are selected from one or more of α-Al2O3, β-Al2O3, γ-Al2O3, boehmite, silica, rutile titanium dioxide, anatase titanium dioxide, magnesium oxide, magnesium hydroxide, zirconium oxide, aluminum nitride, molybdenum disulfide; the particle size of the inorganic ceramic particles is 500 - 5000 nm.

7. The preparation method according to claim 1, wherein In the above S2, the mass ratio of the polyamide short fibers, inorganic ceramic particles, additives, and water is (1 - 20):(20 - 35):(0 - 3):(42 - 80); the additives include a dispersant, a thickener, and a wetting agent.

8. The preparation method according to claim 1, wherein In the above S3, the coating method is selected from one of knife coating, roll coating, and dip coating; the wet coating thickness is 4 - 8 μm, and the dry coating thickness is 1 - 3 μm.

9. A separator, characterized in that, Prepared by the preparation method according to any one of claims 1 - 8.

10. Application of the separator according to claim 9 or the separator prepared by the preparation method according to any one of claims 1 - 8 in a secondary battery.

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