Ultrathin diaphragm and preparation method thereof

Abstract

The present invention discloses a method for preparing an ultrathin membrane, comprising the following steps: Step 1, ultrasonically stirring fumed alumina, deionized water, and a dispersant, adding sulfonated cellulose nanowhiskers (CNC-C), a binder, and a wetting agent, and ultrasonically mixing them, and evacuating the mixture to obtain a slurry; Step 2, coating the slurry described in Step 1 on the surface of a base film to form a coating on the base film surface, drying the base film, and winding the membrane to obtain the ultrathin membrane. The present invention improves the heat resistance of the ultrathin membrane by coating the surface of a PE base film with a slurry made of fumed alumina as the main material and a certain mass fraction of a CNC-C (sulfonated cellulose nanowhiskers) aqueous solution and other additives and solutions, thereby improving the safety of lithium batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and in particular relates to an ultra-thin diaphragm and a preparation method thereof. Background Art

[0002] As society develops, environmental and energy crises are global challenges. Therefore, energy conservation and emission reduction are essential for the survival and development of human society. To address this situation and adapt to social trends, all industries are undergoing transformation and innovation. In the automotive industry, this innovation is manifested in the development and use of electric vehicles. Battery technology has always been a bottleneck in the development of electric vehicles, and lithium batteries, as the most widely used power battery, have also become a major concern. The separator is a key internal component of a lithium battery. Its primary function is to separate the positive and negative electrodes of the battery, preventing contact and short circuits. It also allows the passage of electrolyte ions. Currently available lithium battery separators have poor high-temperature resistance and thermal stability. When exposed to high temperatures, separators in lithium batteries tend to expand, impacting the battery's performance. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method for preparing an ultra-thin membrane.

[0004] Another object of the present invention is to provide an ultra-thin diaphragm obtained by the above preparation method.

[0005] The purpose of the present invention is achieved through the following technical solutions.

[0006] A method for preparing an ultrathin diaphragm comprises the following steps:

[0007] Step 1, ultrasonically stirring fumed alumina, deionized water and a dispersant, adding sulfonated cellulose nanowhiskers (CNC-C), a binder and a wetting agent, ultrasonically mixing and uniformly mixing, and vacuuming to obtain a slurry, wherein, by weight, the fumed alumina is 25 to 39 parts, the dispersant is 0.5 to 1.0 parts, the deionized water is 36.2 to 59.2 parts, the CNC-C (sulfonated cellulose nanowhiskers) aqueous solution is 10 to 15 parts, the binder is 5 to 8 parts, and the wetting agent is 0.3 to 0.8 parts;

[0008] In the step 1, the ultrasonic stirring is performed uniformly by first rotating the material at a speed of 2000 to 2500 r / min and an orbital speed of 30 to 45 for 60 minutes, and then ultrasonicating the material at 5 to 8 kHz for 10 to 15 minutes.

[0009] In step 1, the vacuum degree of the vacuuming is greater than 0.6 MPa.

[0010] In step 1, the ultrasonic mixing is carried out at an autogenous rotation speed of 2000-2500 r / min, an orbital rotation speed of 35-45 r / min, an ultrasonic frequency of 5-8 kHz, and is maintained for 15-20 minutes.

[0011] In the step 1, the D50 of the slurry is 0.2-0.4 μm, and the D90 is 0.6-0.8 μm.

[0012] Step 2: coating the slurry described in step 1 on the surface of the base film to form a coating on the surface of the base film, drying, and rolling up to obtain an ultra-thin diaphragm.

[0013] In step 2, the drying temperature is 50-70° C., and the drying time is 2-4 minutes.

[0014] In the step 2, the coating speed is 40-50 m / min.

[0015] In step 2, the base film is a PE base film.

[0016] The ultra-thin membrane reduces thermal shrinkage while also increasing mechanical strength. When baked at 180°C for 1 hour, the transverse thermal shrinkage is 1.4-2.1%, and the longitudinal thermal shrinkage is 2.1-2.4%. The needle puncture strength of the ultra-thin membrane remains at 6.8-8.7N, and the transverse tensile strength remains at 1465-1486Kgf / cm 2 , longitudinal tensile strength remains at 1594~1643Kgf / cm 2 ;

[0017] The thickness of the base film is 9 μm, and the thickness of the coating is 1 to 2 μm.

[0018] The ultra-thin diaphragm obtained by the above preparation method.

[0019] The advantages and beneficial effects of the present invention are:

[0020] 1. The slurry of the present invention has good bonding with the PE base film and good support under the microscopic condition. The slurry is resistant to high temperatures and has good insulation properties. This is because fumed alumina is a nanomaterial, which is characterized by a small particle size and a large bulk density. When it is coated on the surface of the base film and dried to form a coating, the fumed alumina is densely packed. Compared with ordinary alumina, the gaps left are smaller, and by adding CNC-C (CNC-C is nanocellulose, and its diameter is generally tens of nanometers), it just fills the gaps between the fumed alumina, which is equivalent to weaving a mesh formed by the fibers of the fumed alumina together to form a support. Since the specific surface area of fumed alumina is large, compared with ordinary alumina, it has more contact area with the base film and the binder, so it has good bonding. Fumed alumina itself is resistant to high temperatures, and the dense coating formed can be heat-insulating. The reason is that the gaps in the formed coating are small, which can isolate heat, and CNC-C also fills the gaps, further reducing the porosity and isolating heat.

[0021] 2. The present invention improves the heat resistance of the ultra-thin diaphragm and thus improves the safety of the lithium battery by coating a layer of slurry made of fumed alumina as the main material and adding a certain mass fraction of CNC-C (sulfonated cellulose nanowhiskers) aqueous solution and other additives and solutions on the surface of the PE base film.

[0022] 3. The fumed alumina of the present invention is combined with CNC-C (sulfonated cellulose nanowhiskers) to form a dense network coating on the surface of the ultra-thin diaphragm. The fumed alumina is responsible for improving the heat resistance of the diaphragm, while the CNC-C (sulfonated cellulose nanowhiskers) is responsible for fixing the fumed alumina to the surface of the base membrane. This is like adding steel bars to a red brick wall, effectively improving the diaphragm's resistance to thermal shrinkage, thereby effectively improving the ultra-thin diaphragm's resistance to thermal shrinkage. Figuratively speaking, CNC-C is a nanofiber material with a small diameter and a large specific surface area. When mixed with a binder and in contact with the diaphragm, it acts like the feet of a creeper adhering to a wall, fixing and strengthening the fumed alumina.

[0023] 4. The present invention improves the thermal stability of the ultra-thin diaphragm and enhances the mechanical strength, thereby avoiding the short circuit problem caused by the large-area contact between the positive and negative electrodes of the lithium battery due to the shrinkage of the diaphragm.

[0024] 5. Since fumed alumina is a new type of nanomaterial, its particles are generally tens of nanometers in size. The ceramic slurry made from it has a particle size D90 of 0.6-0.8μm. CNC-C is also a nanofiber with a diameter of tens of nanometers and a length of 1-5μm. Compared with commonly used alumina (conventional alumina has a diameter of 1.3-2.6 microns and a slurry particle size D90 of 2.3-3.6 microns), the particles of these two materials are smaller, and the coating that can be formed is thinner. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the SEM of the coated separator prepared in Comparative Example 1.

[0026] Figure 2 This is the SEM image of the ultrathin membrane prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below with reference to specific embodiments.

[0028] Example 1

[0029] A method for preparing an ultrathin diaphragm comprises the following steps:

[0030] Step 1: In a dual planetary stirring device, fumed alumina, deionized water and a dispersant are ultrasonically stirred evenly, sulfonated cellulose nanowhiskers, a binder and a wetting agent are added and ultrasonically mixed evenly, and vacuum is evacuated to eliminate bubbles generated during the stirring and mixing process. The vacuum degree is greater than 0.6 MPa to obtain a slurry, wherein the ultrasonic stirring is first carried out at a rotation speed of 2000 r / min and a revolution speed of 35 r / min, maintained for 60 min, and then ultrasonicated at 5 kHz for 10 min. The rotation speed for ultrasonic mixing is 2000 r / min, the revolution speed is 35 r / min, and then ultrasonicated at 5 kHz for 15 min. The D50 of the slurry is 0.257 μm and the D90 is 0.685 μm. The slurry comprises 30 parts of fumed alumina, 0.5 parts of dispersant, 51.1 parts of deionized water, 12 parts of CNC-C (sulfonated cellulose nanowhiskers) aqueous solution, 6 parts of adhesive and 0.4 parts of wetting agent.

[0031] Step 2: Use a coating machine to coat the slurry of step 1 on the surface of a 9 μm thick PE base film to form a coating with a thickness of 1.1 μm on the surface of the base film. The film is then pulled into a drying device by a traction roller and dried at 50°C for 4 minutes, and rolled up to obtain an ultra-thin diaphragm (such as Figure 2 As shown), wherein the coating speed is 40m / min.

[0032] Example 2

[0033] A method for preparing an ultrathin diaphragm comprises the following steps:

[0034] Step 1: In a dual planetary stirring device, ultrasonically stir the fumed alumina, deionized water and dispersant, add sulfonated cellulose nanowhiskers, binders and wetting agents, mix them evenly, and evacuate the mixture. The vacuum degree is greater than 0.6 MPa to obtain a slurry, wherein the ultrasonic stirring is performed at a rotation speed of 2300 r / min and a revolution speed of 40 r / min for 60 min, and then ultrasonically stirred at 5 kHz for 10 min. The rotation speed for ultrasonic mixing is 2000 r / min, the revolution speed is 35 r / min, the ultrasonic frequency is 5 kHz, and the time is 15 min. The D50 of the slurry is 0.294 μm and the D90 is 0.716 μm. The slurry comprises 34 parts of fumed alumina, 0.7 parts of dispersant, 44.8 parts of deionized water, 13 parts of CNC-C (sulfonated cellulose nanowhiskers) aqueous solution, 7 parts of binder, and 0.5 parts of wetting agent.

[0035] Step 2: Use a coating machine to coat the slurry of step 1 on the surface of a PE base film with a thickness of 9 μm to form a coating with a thickness of 1.2 μm on the surface of the base film. The film is pulled into a drying device by a traction roller and dried at 60°C for 3 minutes. The film is rolled up to obtain an ultra-thin diaphragm, wherein the coating speed is 45 m / min.

[0036] Example 3

[0037] A method for preparing an ultrathin diaphragm comprises the following steps:

[0038] Step 1: In a dual planetary stirring device, ultrasonically stir the fumed alumina, deionized water and dispersant, add sulfonated cellulose nanowhiskers, binders and wetting agents, mix them evenly, and evacuate the mixture. The vacuum degree is greater than 0.6 MPa to obtain a slurry, wherein the ultrasonic stirring is performed at a rotation speed of 2500 r / min and a revolution speed of 45 r / min for 60 min, and then ultrasonically stirred at 5 kHz for 10 min. The rotation speed for ultrasonic mixing is 2000 r / min, the revolution speed is 35 r / min, the ultrasonic frequency is 5 kHz, and the time is 15 min. The D50 of the slurry is 0.255 μm and the D90 is 0.673 μm. The slurry comprises 38 parts of fumed alumina, 1.0 part of dispersant, 37.4 parts of deionized water, 15 parts of CNC-C (sulfonated cellulose nanowhiskers) aqueous solution, 8 parts of binder, and 0.6 part of wetting agent.

[0039] Step 2: Use a coating machine to coat the slurry of step 1 on the surface of a PE base film with a thickness of 9 μm to form a coating with a thickness of 1.1 μm on the surface of the base film. The film is pulled into a drying device by a traction roller and dried at 65°C for 2 minutes. The film is rolled up to obtain an ultra-thin diaphragm, wherein the coating speed is 50 m / min.

[0040] Comparative Example 1

[0041] A method for preparing a common alumina coating diaphragm comprises the following steps:

[0042] Step 1. In a double planetary mixing device, ordinary alumina, deionized water and dispersant are ultrasonically stirred evenly, and then a binder and a wetting agent are added and ultrasonically mixed evenly. Vacuum is evacuated to eliminate bubbles generated during the stirring and mixing process. The vacuum degree is greater than 0.6 MPa to obtain ordinary alumina slurry, wherein the ultrasonic stirring is first carried out at a rotation speed of 2000 r / min and a revolution speed of 35 r / min, maintained for 60 minutes, and then ultrasonically stirred at 5 kHz for 10 minutes. The rotation speed for ultrasonic mixing is 2000 r / min, the revolution speed is 35 r / min, and then ultrasonically stirred at 5 kHz for 15 minutes. The D50 of the ordinary alumina slurry is 1.225 μm and the D90 is 3.426 μm. In terms of mass, the components are 30 parts of alumina, 0.5 parts of dispersant, 51.1 parts of deionized water, 6 parts of binder, and 0.4 parts of wetting agent.

[0043] Step 2: Use a coating machine to coat the ordinary alumina slurry of step 1 on the surface of a 9 μm thick PE base film to form a coating with a thickness of 3 μm on the surface of the base film. The film is then pulled into a drying device by a traction roller and dried at 50°C for 4 minutes, and then rolled up to obtain a coated diaphragm (such as Figure 1 As shown), wherein the coating speed is 40m / min.

[0044] Comparative Example 2

[0045] A method for preparing a common alumina and CNC-C coating diaphragm comprises the following steps:

[0046] Step 1. In a double planetary mixing device, ordinary alumina, deionized water and dispersant are ultrasonically stirred evenly, and CNC-C, binder and wetting agent are added and ultrasonically mixed evenly. Vacuum is evacuated to eliminate bubbles generated during the stirring and mixing process. The vacuum degree is greater than 0.6 MPa to obtain ordinary alumina slurry, wherein the ultrasonic stirring is first carried out at a rotation speed of 2000 r / min and a revolution speed of 35 r / min, maintained for 60 minutes, and then ultrasonically stirred at 5 kHz for 10 minutes. The rotation speed for ultrasonic mixing is 2000 r / min, the revolution speed is 35 r / min, and then ultrasonically stirred at 5 kHz for 15 minutes. The D50 of the ordinary alumina slurry is 1.324 μm and the D90 is 3.126 μm. In terms of mass, the components are 30 parts of alumina, 0.5 parts of dispersant, 51.1 parts of deionized water, 15 parts of CNC-C, 6 parts of adhesive and 0.4 parts of wetting agent.

[0047] Step 2: Use a coating machine to coat the ordinary alumina slurry of step 1 on the surface of a PE base film with a thickness of 9 μm to form a coating with a thickness of 3 μm on the surface of the base film. The film is pulled into a drying device by a traction roller and dried at 60°C for 3 minutes, and then rolled up to obtain a coated diaphragm. The coating speed is 40 m / min.

[0048] The performance tests were conducted on the ultra-thin membranes prepared in Examples 1 to 3 and the coated membranes prepared in Comparative Examples 1 to 2. The test results are shown in Table 1.

[0049] Table 1

[0050]

[0051]

[0052] As can be seen from Table 1, the ultra-thin diaphragm obtained by coating a slurry of about 1 μm with a coating of gas-phase alumina mixed with CNC-C (sulfonated cellulose nanowhiskers) in the present invention has a thermal shrinkage rate of less than 1.5% when baked at 150°C for 1 hour, and a thermal shrinkage rate of less than 2.5% when baked at 180°C for 1 hour, while the thermal shrinkage rate of the conventional alumina 1 μm coated diaphragm at 150°C for 1 hour is about 50-60%. The present invention greatly improves the heat resistance of the ultra-thin diaphragm without affecting other properties, and the thickness is only about 1 μm. Ordinary alumina has a large particle size, generally with a D90 of 2.8-5 μm, and the coating formed by coating the slurry made from it is also thick, generally with a coating thickness of 3-6 μm.

[0053] The above is an exemplary description of the present invention. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by other skilled in the art without expending creative labor falls within the scope of protection of the present invention.

Claims

1. A method for preparing an ultrathin diaphragm, characterized in that: The following steps are involved: Step 1, ultrasonically stirring fumed alumina, deionized water and a dispersant, adding sulfonated cellulose nanowhiskers, a binder and a wetting agent, ultrasonically mixing and uniformly mixing, and evacuating to obtain a slurry, wherein, by weight, the fumed alumina comprises 25 to 39 parts, the dispersant 0.5 to 1.0 part, the deionized water 36.2 to 59.2 parts, the sulfonated cellulose nanowhiskers aqueous solution 10 to 15 parts, the binder 5 to 8 parts, and the wetting agent 0.3 to 0.8 parts; Step 2, coating the slurry described in step 1 on the surface of the base film to form a coating on the surface of the base film, drying, and rolling to obtain an ultra-thin diaphragm; The ultra-thin membrane has a transverse heat shrinkage of 1.4-2.1% and a longitudinal heat shrinkage of 2.1-2.4% when baked at 180°C for 1 hour. The ultra-thin membrane has a needle puncture strength of 6.8-8.7N and a transverse tensile strength of 1465-1486Kgf / cm 2 , longitudinal tensile strength is 1594~1643Kgf / cm 2 ; The thickness of the base film is 9 μm, and the thickness of the coating is 1 to 2 μm.

2. The preparation method according to claim 1, characterized in that In the step 1, the ultrasonic stirring is performed uniformly by first rotating the material at a speed of 2000 to 2500 r / min and an orbital speed of 30 to 45 for 60 minutes, and then ultrasonicating the material at 5 to 8 kHz for 10 to 15 minutes.

3. The preparation method according to claim 1, characterized in that In step 1, the vacuum degree of the vacuuming is greater than 0.6 MPa.

4. The preparation method according to claim 1, characterized in that In step 1, the ultrasonic mixing is carried out at an autogenous rotation speed of 2000-2500 r / min, an orbital rotation speed of 35-45 r / min, an ultrasonic frequency of 5-8 kHz, and is maintained for 15-20 minutes.

5. The preparation method according to claim 1, characterized in that In the step 1, the D50 of the slurry is 0.2-0.4 μm, and the D90 is 0.6-0.8 μm.

6. The preparation method according to claim 1, characterized in that In step 2, the drying temperature is 50-70° C., and the drying time is 2-4 minutes.

7. The preparation method according to claim 1, characterized in that In the step 2, the coating speed is 40-50 m / min.

8. The preparation method according to claim 1, characterized in that In step 2, the base film is a PE base film.

9. An ultrathin diaphragm obtained by the preparation method according to any one of claims 1 to 8.

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