A lightweight lithium ion battery composite diaphragm and a preparation method thereof

Abstract

The application discloses a lightweight lithium ion battery composite diaphragm and a preparation method, and belongs to the technical field of lithium ion batteries. The lightweight lithium ion battery composite diaphragm comprises a base film and a coating layer coated on the front and back surfaces of the base film. The coating layer is two layers, the first layer is an AlOOH nanowhisker layer coated on the surface of the base film, and the second layer is an aluminum oxide layer coated on the AlOOH nanowhisker layer. The lightweight lithium ion battery composite diaphragm provided by the application coats AlOOH nanowhiskers on the surface of the base film to form a three-dimensional network structure, the hydrophilic groups on the surface of the AlOOH whiskers can establish a "molecular bridge" between the aluminum oxide particles and the nanofibers, the adhesion of the coating layer is enhanced, the peeling strength of the coating layer to the base film is improved, and the coating layer is further lightened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lightweight lithium ion battery composite diaphragm and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries have been widely used in portable electronic products, electric vehicles and energy storage systems due to their high working voltage, long cycle life, low self-discharge rate and other advantages, and as the application range is widened, higher requirements are put forward for the performance of lithium ion batteries. As one of the four core materials of lithium ion batteries, the main function of the diaphragm is to separate the positive and negative electrodes of the battery and to ensure the transmission of lithium ions and hinder the transmission of electrons. The commonly used diaphragm coating is inorganic particles such as boehmite and aluminum oxide, and the size is generally greater than 500nm, and the coating area density is 4g / m 2 Above all, although the existence of inorganic coating improves the safety performance of the battery, the thick coating hinders the transmission of lithium ions and also affects the energy density of the battery to some extent. Lightweight design of the diaphragm has certain significance for improving the power density and energy density of the battery, such as using nano-powder particles, low-density and special force to reduce the unit volume powder mass of the coating.

[0003] Chinese patent CN108336279A discloses a kind of nanoparticle coated lithium ion battery diaphragm and its preparation method, wherein the nanoparticle has porous structure, and part of the pore channel penetrates the surface of nanoparticle, reduces the bulk density of powder particles while improving the lithium ion conductivity.

[0004] Although the construction of through-hole structure in powder particles can reduce the volume bulk density of the coating on one side, the difficulty of homogenate processing process and the peeling strength and thermal stability of the modified diaphragm have not been clearly concluded. The porous structure means greater capillary force, and part of the additives will enter the inner wall of the pore channel under the action of capillary force, so compared with the same order of magnitude of powder particles, the porous structure needs to be matched with more additives in the processing process, and the lack of part of the functional additives such as binder and the incomplete coating of powder surface additives will also cause the decrease of peeling strength and thermal stability of the coating. SUMMARY

[0005] The purpose of the present application is to provide a lightweight lithium ion battery composite diaphragm with high coating peeling strength and thermal stability, and the purpose of the present application is to provide a preparation method of lightweight lithium ion battery composite diaphragm with high coating peeling strength and thermal stability.

[0006] The application discloses a light lithium ion battery composite diaphragm, which comprises a base film and a coating layer coated on the front and back surfaces of the base film.

[0007] The application further discloses a preparation method of the light lithium ion battery composite diaphragm.

[0008] S1: placing the base film into AlOOH nanowhisker coating slurry, taking out the base film after the AlOOH nanowhisker coating slurry completely covers the surface of the base film, washing and drying;

[0009] S2: placing the base film with the AlOOH nanowhisker layer into aluminum oxide coating slurry, taking out the base film with the AlOOH nanowhisker layer after the aluminum oxide coating slurry completely covers the surface of the base film with the AlOOH nanowhisker layer, washing and drying, and obtaining the light lithium ion battery composite diaphragm as described above.

[0010] The AlOOH nanowhisker coating slurry comprises 45-85 parts of deionized water, 0-4 parts of a dispersing agent, 15-45 parts of AlOOH nanowhisker powder, 1-8 parts of a thickening agent, 0.1-7 parts of a binder and 0.05-0.3 parts of a wetting agent.

[0011] The aluminum oxide coating slurry comprises 45-85 parts of deionized water, 0-0.5 parts of a dispersing agent, 15-45 parts of aluminum oxide, 1.5-5 parts of nanofiber, 4-8 parts of a thickening agent, 0.5-7 parts of a binder and 0.05-0.15 parts of a wetting agent.

[0012] The base film is selected from one of a PE film, a PP film, an aramid diaphragm, a cellulose diaphragm and a non-woven fabric diaphragm.

[0013] The AlOOH nanowhisker is a one-dimensional material and has more short-range interaction forces than zero-dimensional particles such as aluminum oxide nanoparticles, and the AlOOH nanowhisker layer can improve the adhesion between the AlOOH nanowhisker layer and the base film.

[0014] The dispersing agent is a carboxylic acid dispersing agent, the thickening agent is a hydroxymethyl cellulose thickening agent, the binder is an acrylic ester binder and the wetting agent is an alkoxylated isomeric alcohol wetting agent.

[0015] The binder is a two-component binder, and the A and B components of the two-component binder are both acrylic emulsions, and the A and B components have a slight crosslinking synergistic effect after mixing, so that the adhesion of the coating layer to the base material is improved and the amount of the binder is reduced.

[0016] The present invention adopts the method of dipping to coat the slurry, and then uses deionized water to wash away the excess slurry. The preparation process is simple, the coating thickness is small and uniform, and compared with the scraping method, the dipping method allows less solution to penetrate into the pores of the base membrane, making it less likely to clog the pores and thus destroy the pore structure of the base membrane.

[0017] The D50 of the aluminum oxide is 0.2-0.5 μm.

[0018] The nanofibers include cellulose nanocrystals, cellulose nanofibers, microcrystalline cellulose, and bacterial nanocellulose, with an average length of 0.5-2 μm and an average diameter of 20-50 nm.

[0019] Furthermore, the AlOOH nanowhiskers have an average length of 0.5-2 μm and an average diameter of 20-50 nm.

[0020] Furthermore, the preparation method of the AlOOH nanowhiskers is as follows: AlCl3 is stirred and dissolved in deionized water, the pH value of the solution is adjusted to 2-9 with NaOH, the solution is placed in a sealed container and heated to 150-400°C for hydrothermal reaction for 6-48 hours, after naturally cooling to room temperature, filtering and washing the filter cake with deionized water, vacuum drying the filter cake at 50-100°C for 5-24 hours, and grinding to obtain AlOOH nanowhiskers powder.

[0021] Furthermore, the AlCl3 is AlCl3·6H2O, and the mass ratio of the added AlCl3·6H2O to the added deionized water is 0.5-2g:20-80ml.

[0022] Furthermore, the AlOOH nano whisker coating slurry is prepared by uniformly mixing deionized water, a dispersant, and a wetting agent, adding AlOOH nano whiskers and ball milling, and then adding a thickener and a binder and ball milling to obtain the AlOOH nano whisker coating slurry.

[0023] Furthermore, the alumina is saturated fatty acid modified nano alumina.

[0024] The saturated fatty acid modified nano-alumina is saturated fatty acid coated modified nano-alumina particles.

[0025] Furthermore, the preparation method of the saturated fatty acid modified nano-alumina comprises the following steps:

[0026] (1) Dissolve saturated fatty acids in deionized water at 60-90°C, add sodium lauryl sulfate and stir for 1.5-3 hours to form a water-in-oil emulsion;

[0027] (2) dissolving aluminum isopropoxide in deionized water at 60-90°C, adding an ammonia solution to adjust the solution pH to about 10 for peptization, and heating at 60-90°C to remove the alcohol after forming a stable sol;

[0028] (3) Add the sol prepared in step (2) to the water-in-oil emulsion prepared in step (1) at 75-85°C and stir for 3 hours.

[0029] Aluminum isopropoxide and surfactants self-assemble through electrostatic interaction, and aluminum isopropoxide undergoes a condensation reaction to form modified nano-aluminum oxide particles with aluminum oxide as a core layer and saturated fatty acids as a shell layer.

[0030] Furthermore, the saturated fatty acid is selected from one or more of caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and palmitic acid.

[0031] Furthermore, in step (1), the mass volume ratio of saturated fatty acid, sodium lauryl sulfate, and deionized water is 1-1.5 g:0.4-0.7 g:300-500 ml; in step (2), the mass volume ratio of aluminum isopropoxide and deionized water is 1.5-3 g:15-30 ml; and in step (3), the mass ratio of aluminum isopropoxide and saturated fatty acid is 1:1-2:1.

[0032] Furthermore, the preparation method of the alumina coating slurry is: deionized water and a dispersant are mixed evenly, saturated fatty acid modified nano-alumina, nanofibers and a thickener are added and stirred evenly, after ball milling, a binder and a wetting agent are added and stirred to obtain the alumina coating slurry.

[0033] The present invention provides a lightweight lithium-ion battery composite separator with AlOOH nanowhiskers coated on the surface of the base membrane to form a three-dimensional network structure. The hydrophilic groups on the surface of the AlOOH whiskers can establish "molecular bridges" between the aluminum oxide particles and the nanofibers, thereby enhancing the adhesion of the coating and improving the peel strength of the coating to the base membrane, and further lightweighting the coating. DETAILED DESCRIPTION

[0034] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to specific embodiments.

[0035] Example 1

[0036] Preparation of lightweight lithium-ion battery composite membrane:

[0037] Preparation of AlOOH nanowhiskers: Dissolve 0.724 g of AlCl₃·6H₂O in 30 ml of deionized water and stir at room temperature. Adjust the pH of the solution to 4 with a 1 mol / L aqueous solution of NaOH. Hydrothermally react the solution in an autoclave at 200°C for 24 hours. After cooling to room temperature, filter the mixture and wash the filter cake three times with deionized water. Dry the filter cake in a vacuum oven at 60°C for 12 hours and grind it to obtain AlOOH nanowhiskers.

[0038] Preparation of AlOOH nanowhisker coating slurry: 60 parts of ultrapure water, 3.5 parts of dispersant, 0.25 parts of wetting agent were mixed uniformly, then 30 parts of nanowhisker powder was added and ball milled for 4h at 600rpm / min. Then 1.5 parts of thickening agent and 2.5 parts of mixed two-component binder were mixed uniformly and added to the ball milled solution. The obtained slurry was ground by a ball mill for 2h to obtain the AlOOH nanowhisker coating slurry. The dispersant was polyacrylic acid, the thickening agent was carboxymethyl cellulose, the binder was two-component polyacrylate emulsion, the component ratio was 2:3, and the wetting agent was isomeric decanol alkoxylate.

[0039] Immersion coating of AlOOH nanowhisker coating slurry: the PE film was immersed in the AlOOH nanowhisker coating slurry for 10s, washed with deionized water for 1min, repeated for 3 times, then taken out and baked at 60℃ for 12h to obtain the AlOOH nanowhisker coated composite separator. The thickness of the PE film was 9μm.

[0040] Preparation of saturated fatty acid modified nano-alumina: 1g of saturated fatty acid was dissolved in 300ml of deionized water at 85℃, 0.4g of sodium dodecyl sulfate was added and stirred for 2h to form a water-in-oil emulsion; 1.5g of aluminum isopropoxide was dissolved in 15ml of deionized water at 85℃, and ammonia solution was added to adjust the pH of the solution to about 10 to form a sol; the aluminum isopropoxide / water sol was added to the water-in-oil emulsion at 80℃ and stirred for 3h, then precipitated and filtered, washed with ethanol and deionized water for 3 times, and the product was dried in a vacuum oven at 70℃ for 24h, then ground to obtain the saturated fatty acid modified nano-alumina particles. The saturated fatty acid was myristic acid.

[0041] Preparation of alumina coating slurry: 60 parts of ultrapure water and 0.5 parts of dispersant were mixed uniformly, then 27.5 parts of saturated fatty acid modified nano-alumina and 2.5 parts of nanofiber were added, the obtained mixture was stirred at 500rpm / min for 24h, then ground by a sand mill, 3 parts of thickening agent, 7 parts of two-component binder, and 0.1 parts of wetting agent were added to the obtained dispersion, and the mixture was stirred for 8h to obtain the alumina coating slurry. The modified alumina D50 was 0.4-0.5μm, the dispersant was polyacrylic acid, the thickening agent was carboxymethyl cellulose, the binder was two-component polyacrylate emulsion, the component ratio was 2:3, and the wetting agent was isomeric decanol alkoxylate.

[0042] Immersion coating of alumina coating slurry: the AlOOH nanowhisker coated composite separator was immersed in the alumina coating slurry for 10s, washed with deionized water for 1min, repeated for 3 times, then taken out and baked at 60℃ for 12h to obtain a lightweight lithium ion battery composite separator.

[0043] Example 2

[0044] The difference between this example and Example 1 is that "adjusting the pH value of the solution to 4 with a 1 mol / L NaOH aqueous solution" is replaced by "adjusting the pH value of the solution to 3 with a 1 mol / L NaOH aqueous solution".

[0045] Example 3

[0046] The difference between this embodiment and embodiment 1 is that in the step of preparing the AlOOH nano whisker coating slurry, "55 parts of ultrapure water, 2 parts of dispersant, and 0.15 parts of wetting agent are mixed evenly according to their mass parts, and then 35 parts of nano whisker powder are added and ball milled at 600 rpm / min for 4 hours. Thereafter, 1 part of thickener and 2.5 parts of mixed two-component adhesive are mixed evenly and added to the ball milling solution, and the resulting slurry is milled in a ball mill for 2 hours to obtain the AlOOH nano whisker coating slurry."

[0047] Example 4

[0048] The difference between this embodiment and embodiment 1 is that in the step of preparing saturated fatty acid modified nano-alumina, "dissolving 1.5 g of aluminum isopropoxide in 15 ml of deionized water at 85°C" is replaced with "dissolving 2 g of aluminum isopropoxide in 20 ml of deionized water at 85°C".

[0049] Example 5

[0050] The difference between this embodiment and embodiment 1 is that in the step of preparing the alumina coating slurry, "50 parts of ultrapure water and 0.15 parts of dispersant are mixed uniformly according to their mass proportions, and then 36.7 parts of modified alumina and 3.3 parts of nanofibers are added, the resulting mixed solution is stirred at 500 rpm for 24 hours, and after grinding with a sand mill, 3.5 parts of thickener, 5 parts of two-component adhesive, and 0.1 part of wetting agent are added to the resulting dispersion and stirring is continued for 8 hours to obtain the alumina coating slurry."

[0051] Example 6

[0052] The difference between this embodiment and embodiment 1 is that a single-component polyacrylate adhesive is used, and the other processes are normal.

[0053] Example 7

[0054] The difference between this comparative example and Example 1 is that conventional alumina with a D50 of 0.4-0.5 μm is used and the alumina is not modified with saturated fatty acid.

[0055] Comparative Example 1

[0056] The difference between this comparative example and Example 1 is that no AlOOH nano whisker layer is dip-coated.

[0057] Comparative Example 2

[0058] The difference between this comparative example and Example 1 is that no nanofibers are added in the preparation of the alumina coating slurry.

[0059] Comparative Example 3

[0060] The difference between this embodiment and embodiment 1 is that an AlOOH nano whisker layer and an aluminum oxide layer are superimposed on the surface of the base film by micro-gravure roller coating.

[0061] Performance testing and effect analysis

[0062] The performance of Examples 1 to 5 and Comparative Examples 1 to 5 was characterized, and the thickness, surface density, air permeability, heat shrinkage, puncture strength and peel strength were tested with reference to BG / T36363-2018; the peel strength was tested using a universal testing machine, and the diaphragm was cut into 100*15mm strips for testing. The test results are shown in Table 1.

[0063] Table 1 Performance test results of the diaphragms obtained in Examples 1-7 and Comparative Examples 1-3

[0064]

[0065]

[0066] Comparing the thermal shrinkage, puncture strength, and peel strength data of Example 1 and Comparative Example 1 reveals that the AlOOH nanowhisker layer forms a three-dimensional network structure on the surface of the base film. Furthermore, the presence of numerous hydrophilic groups on the whisker surface, capable of establishing "molecular bridges" with the alumina particles and nanofibers, significantly enhances the adhesion between the coatings and to the base film. Therefore, the peel strength and puncture strength of Example 1 are greater than those of Comparative Example 1, while the thermal shrinkage of Example 1 is also slightly less than that of Comparative Example 1. Comparing the areal density values ​​of the two, Example 1 has a lower areal density, indicating that the nanowhisker layer can significantly reduce the weight of the coating.

[0067] Comparing the surface density and air permeability data of Example 1, Example 7, and Comparative Example 3 reveals that coating and modifying the alumina surface with saturated fatty acids increases the interaction between the alumina and nanofibers. The addition of a small amount of nanofibers improves the uniformity of the nanoalumina dispersion in the slurry and coated separator, while also reducing the bulk density of the alumina. Comparing thermal shrinkage and puncture strength, the use of modified nanoalumina and nanofibers synergistically improves the crosslinking degree of the separator components, enhancing the separator's resistance to thermal shrinkage and mechanical strength.

[0068] By comparing the peel strength data of Example 1 and Example 6, it can be found that the two-component adhesive self-polymerizes and slightly cross-links after mixing, which improves the adhesion of the coating to the substrate while reducing the amount of adhesive added.

[0069] Comparing the thickness, coating area density and air permeability data of Example 1 and Comparative Example 3, it can be found that the area density and air permeability of the separator in Comparative Example 3 are much greater than those of Example 1, which shows that the use of dip coating can reduce the thickness of the coating layer and reduce the penetration of the solid phase component into the base film, thereby reducing the increase in air permeability.

[0070] The above-described examples only express several embodiments of the present application, which are described in a more specific and detailed manner, but should not be understood as limiting the scope of the patent of the present application. It should be noted that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the scope of protection of the present application. Therefore, the scope of protection of the patent of the present application should be subject to the appended claims.

Claims

1. A method for preparing a lightweight lithium-ion battery composite diaphragm, characterized in that: The following steps are involved: S1: placing the base film into the AlOOH nano whisker coating slurry, and after the AlOOH nano whisker coating slurry completely covers the surface of the base film, taking it out, washing it, and drying it; S2: placing the base film coated with the AlOOH nano whisker layer into an alumina coating slurry, and after the alumina coating slurry completely covers the surface of the base film coated with the AlOOH nano whisker layer, taking it out, washing it, and drying it to obtain a lightweight lithium-ion battery composite separator; The lightweight lithium-ion battery composite separator comprises a base film and a coating applied on both sides of the base film; the coating comprises two layers, the first layer being an AlOOH nano whisker layer applied on the surface of the base film; The second layer is an aluminum oxide layer coated on the AlOOH nanowhisker layer; The AlOOH nano whisker coating slurry comprises, by mass, 45-85 parts of deionized water, 0-4 parts of a dispersant, 15-45 parts of AlOOH nano whisker powder, 1-8 parts of a thickener, 0.1-7 parts of a binder, and 0.05-0.3 parts of a wetting agent. The aluminum oxide coating slurry comprises the following components, by mass: 45-85 parts of deionized water, 0-0.5 parts of a dispersant, 15-45 parts of aluminum oxide, 1.5-5 parts of nanofibers, 4-8 parts of a thickener, 0.5-7 parts of a binder, and 0.05-0.15 parts of a wetting agent.

2. The method for preparing a lightweight lithium-ion battery composite separator according to claim 1, characterized in that: The AlOOH nano whiskers have an average length of 0.5-2 μm and an average diameter of 20-50 nm.

3. The method for preparing a lightweight lithium-ion battery composite separator according to claim 2, characterized in that: The preparation method of the AlOOH nano whiskers comprises: stirring and dissolving AlCl3 in deionized water, adjusting the pH value of the solution to 2-9 with NaOH, placing the solution in a sealed container, heating to 150-400°C for hydrothermal reaction for 6-48 hours, naturally cooling to room temperature, filtering and washing the filter cake with deionized water, vacuum drying the filter cake at 50-100°C for 5-24 hours, and grinding to obtain AlOOH nano whisker powder.

4. The method for preparing a lightweight lithium-ion battery composite separator according to claim 1, characterized in that: The preparation method of the AlOOH nano whisker coating slurry comprises: uniformly mixing deionized water, a dispersant and a wetting agent, adding AlOOH nano whiskers and ball milling, and then adding a thickener and a binder and ball milling to obtain the AlOOH nano whisker coating slurry.

5. The method for preparing a lightweight lithium-ion battery composite separator according to claim 1, characterized in that: The aluminum oxide is saturated fatty acid modified nano aluminum oxide.

6. The method for preparing a lightweight lithium-ion battery composite separator according to claim 5, characterized in that: The preparation method of the saturated fatty acid modified nano-alumina comprises the following steps: (1) Dissolve saturated fatty acids in deionized water at 60-90°C, add sodium lauryl sulfate and stir for 1.5-3 hours to form a water-in-oil emulsion; (2) dissolving aluminum isopropoxide in deionized water at 60-90°C, adding an ammonia solution to adjust the solution pH to about 10 for peptization, and heating at 60-90°C to remove the alcohol after forming a stable sol; (3) Add the sol prepared in step (2) to the water-in-oil emulsion prepared in step (1) at 75-85°C and stir for 3 hours.

7. The method for preparing a lightweight lithium-ion battery composite separator according to claim 6, characterized in that: The saturated fatty acid is selected from one or more of caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid and palmitic acid.

8. The method for preparing a lightweight lithium-ion battery composite separator according to claim 6, characterized in that: In the step (1), the mass volume ratio of saturated fatty acid, sodium lauryl sulfate and deionized water is 1-1.5 g:0.4-0.7 g:300-500 ml; in the step (2), the mass volume ratio of aluminum isopropoxide and deionized water is 1.5-3 g:15-30 ml; in the step (3), the mass ratio of aluminum isopropoxide and saturated fatty acid is 1:1-2:

1.

9. The method for preparing a lightweight lithium-ion battery composite separator according to claim 1, characterized in that: The preparation method of the alumina coating slurry is as follows: deionized water and a dispersant are mixed evenly, saturated fatty acid modified nano alumina, nanofibers and a thickener are added and stirred evenly, after ball milling, a binder and a wetting agent are added and stirred to obtain the alumina coating slurry.

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