Polyacrylamide lithium ion battery base film and preparation method thereof

Abstract

The present invention discloses a polyacrylamide lithium-ion battery base membrane and a preparation method thereof, comprising the following steps: heating and mixing polyacrylamide, a primary plasticizer, and a secondary plasticizer to form a suspension; adding the suspension to a twin-screw extruder for melt extrusion; cooling and shaping the suspension with shaping rollers to form a cast sheet; winding the cast sheet; cutting the cast sheet; stretching the cut sheet; and then washing and drying the cut sheet under a clamp to obtain the polyacrylamide lithium-ion battery base membrane. The present invention uses glycerol, a primary plasticizer that is soluble in water, to replace the traditional white oil plasticizer. Low-cost, pollution-free water is used instead of the high-cost, volatile, and toxic dichloromethane extractant, thereby reducing production costs and safety risks and having good application prospects and economic benefits.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and in particular relates to a polyacrylamide lithium ion battery base membrane and a preparation method thereof. Background Art

[0002] Lithium battery separators are crucial components used to isolate the positive and negative electrodes from contact and prevent short circuits. Their nanoporous structure also provides a pathway for lithium ions to migrate between the electrodes. The primary substrate for lithium battery separators is a high-molecular-weight linear polyolefin resin, a thermoplastic engineering plastic characterized by wear resistance, self-lubrication, and excellent low-temperature performance. Due to its stable physicochemical properties and superior mechanical properties, these separators are widely used in a variety of fields, including separator materials, ropes, lightweight composite materials, medical treatments, and aerospace.

[0003] However, the following problems still exist in the application of lithium-ion battery separators: First, due to the low surface energy of polyolefin-based membranes, they have poor affinity with aqueous coating slurries and carbonate electrolytes (the contact angle of polyethylene-based membranes with water is about 120°), resulting in low coating peeling force, low liquid absorption and retention rate, and low ionic conductivity of the base membrane; second, as a flexible chain fiber, linear polyethylene has a simple molecular structure, and the van der Waals force is only a dispersion force, the intermolecular force is small, and the viscous deformation caused by intermolecular slip makes its creep resistance and thermal stability poor (the thermal shrinkage of polyethylene-based membranes under the test conditions of 130℃*1h is more than 20% (longitudinal) and 5% (transverse) respectively). There is a safety risk of dimensional shrinkage and internal short circuit in exothermic or high-temperature application environments. In this regard, improving the surface interfacial energy of the base film material and enhancing its hydrophilicity; increasing the intermolecular interaction force and improving the creep resistance and thermal stability of the base film will significantly improve the application effect of the base film in the field of lithium-ion batteries; thirdly, the current wet-process lithium-ion battery separator uses polyethylene as the base material, the plasticizing pore-forming agent is white oil, and the extractant is dichloromethane. Dichloromethane is a high-cost, volatile, and toxic liquid. Therefore, the use of dichloromethane extractant in the manufacture of wet-process lithium battery separators has problems such as environmental pollution, harm to employee health, and high cost of use; fourthly, although the PAM base film can provide the base film with excellent hydrophilicity and creep resistance, the strong hydrogen bonds between high-polymerization PAM molecules also make its melting point higher, about 210°C, which is close to the oxidative decomposition temperature of PAM. Severe oxidation and decomposition occur during plasticizing and melting processing, causing changes and loss of its functional structure. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method for preparing a polyacrylamide lithium ion battery base membrane.

[0005] Another object of the present invention is to provide a polyacrylamide lithium ion battery base film obtained by the above preparation method.

[0006] The purpose of the present invention is achieved through the following technical solutions.

[0007] A method for preparing a polyacrylamide lithium ion battery base membrane comprises the following steps: heating and mixing polyacrylamide, a primary plasticizer, and an auxiliary plasticizer to form a suspension; adding the suspension into a twin-screw extruder for melt extrusion; cooling and shaping the suspension with shaping rollers to form a cast sheet; winding the cast sheet; cutting the cast sheet; stretching the cut sheet; and then washing and drying the sheet under clamping to obtain the polyacrylamide lithium ion battery base membrane. The polyacrylamide, the primary plasticizer, and the auxiliary plasticizer have a ratio of (60-68): (22-32): (8-10) by mass.

[0008] In the above technical solution, the main plasticizer is glycerol.

[0009] In the above technical solution, the auxiliary plasticizer is dimethyl sulfoxide.

[0010] In the above technical solution, the temperature of the heating and mixing is 80-90°C.

[0011] In the above technical solution, the temperature of melt extrusion in the twin-screw extruder is 170-190° C., and the screw speed is 80-100 rpm.

[0012] In the above technical solution, the screw diameter of the twin-screw extruder is 50-100 mm, and the aspect ratio is 60-68.

[0013] In the above technical solution, the cooling and shaping temperature of the shaping roller is 15-25°C.

[0014] In the above technical solution, the stretching ratio is 7 to 9 times, and the stretching temperature is 110 to 120°C.

[0015] In the above technical solution, the extraction agent for the extraction and washing is water, and the extraction and washing time is 3 to 5 minutes.

[0016] In the above technical solution, the drying temperature is 80-100° C., and the drying time is 10-20 minutes.

[0017] The polyacrylamide lithium ion battery base membrane is obtained by the above preparation method.

[0018] The advantages and beneficial effects of the present invention are:

[0019] 1. The polyacrylamide lithium ion battery base membrane of the present invention uses polyacrylamide instead of traditional polyethylene as the diaphragm substrate material (base membrane). Firstly, the alcoholic hydroxyl functional groups in the polyacrylamide molecules can form hydrogen bonds with water, thereby enhancing the affinity of the base membrane for aqueous coating slurries and carbonate electrolytes. The contact angle of the polyacrylamide lithium ion battery base membrane with water is reduced by 75.3% compared with that of the traditional polyethylene base membrane, thereby solving the problem of poor affinity of the traditional polyethylene diaphragm base membrane with aqueous coating slurries and polar electrolytes. Secondly, the strong hydrogen bonds formed between the alcoholic hydroxyl groups in the polyacrylamide molecules reduce the slip between the molecular chains, thereby solving the problems of large viscosity deformation and poor thermal stability of the polyethylene diaphragm, and can enhance the creep resistance and thermal stability of the polyacrylamide lithium ion battery base membrane.

[0020] 2. In order to solve the technical problem that polyacrylamide has a high melting point and is difficult to be melt-extruded, the preparation method of the present invention adopts a plasticizing processing system in which main and auxiliary plasticizers (i.e., propylene glycol and dimethyl sulfoxide) are mixed, and the optimal types and mixing ratios of the main and auxiliary plasticizers are determined through experiments to broaden the plasticizing extrusion processing window of polyacrylamide.

[0021] 3. The main plasticizer glycerol used in the present invention is soluble in water and replaces the traditional white oil plasticizer. Low-cost, pollution-free water is used instead of the high-cost, volatile, and toxic dichloromethane extractant, which reduces production costs and safety risks and has good application prospects and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the thermal shrinkage change curve of the polyethylene film.

[0023] Figure 2 This is the thermal shrinkage change curve of the polyacrylamide lithium ion battery base film of Example 1 of the present invention.

[0024] Figure 3 This is the contact angle test diagram of polyethylene film and water.

[0025] Figure 4 This is a test diagram of the contact angle between the polyacrylamide lithium-ion battery base membrane and water in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below with reference to specific embodiments.

[0027] The relevant drugs used in the specific embodiments of the present invention are as follows:

[0028] Polyacrylamide (PAM): Anhui Wanwei High-tech Materials Co., Ltd., industrial grade;

[0029] Glycerol: Tianjin Jinweier Chemical Co., Ltd., industrial grade;

[0030] Dimethyl sulfoxide (DMSO): Tianjin Jinweier Chemical Co., Ltd., industrial grade.

[0031] The equipment used in the specific implementation of the present invention is as follows:

[0032] Co-rotating twin-screw extruder: KET-65B, Changzhou JWELL Extrusion Machinery Manufacturing Co., Ltd.

[0033] Secondary filter: HZ013 type, filtration accuracy is 150um and 20um respectively, Weihai Haichao Machinery Factory;

[0034] Gear pump: GPE-70 / 70-03, Zhengzhou Haike Melt Pump Co., Ltd.

[0035] T-die: JC-013, EDI Extrusion Die Company, USA, die width 480mm, die lip opening 0.2-1.2mm;

[0036] Film high-temperature biaxial stretching machine: FICO-10, Guangzhou General Experimental Analytical Instrument Co., Ltd., maximum stretching ratio 10×10;

[0037] TMA (thermomechanical analyzer): Q-400, TA Instruments, USA;

[0038] Biolin-Optical Contact Angle Tester: Thete Flex model, Beijing Orange Times Import and Export Co., Ltd.

[0039] Example 1

[0040] A preparation method of a polyacrylamide lithium ion battery base film comprises the following steps: in a premixing tank, polyacrylamide powder, a main plasticizer and an auxiliary plasticizer are heated and mixed at 85°C to form a suspension; the suspension is added by a plunger pump to a co-rotating twin-screw extruder with a screw diameter of 80 mm and an aspect ratio of 64 for melting; the barrel is heated at a temperature of 180°C and the screw speed is 90 rpm; the suspension is quantitatively conveyed by a gear pump through a secondary filter and extruded along a slit die of a T-die head along a melt pipe; the suspension is cooled and shaped at 20°C by a shaping roller; the polyacrylamide and the main plasticizer and the auxiliary plasticizer are rapidly phase-separated during the quenching process to form a cast sheet with floating island crystals; the cast sheet is wound by a shaping roller and a winding roller; the cast sheet is cut into a square cast sheet; the cut cast sheet is stretched at a ratio of 8 in a high-temperature biaxial stretching machine for a thin film; and the film is stretched at a ratio of 8 in a high-temperature biaxial stretching machine. During the stretching process, the polyacrylamide microfibers are further oriented and crystallized under the action of stress, and the solid-liquid phases are further separated to expand the pores. Then, under the clamping of the extraction fixture (the clamp is used mainly to prevent the pores from shrinking and closing due to capillary force during the extraction process), water is used as the extractant for extraction and washing for 4 minutes at room temperature (the main plasticizer and auxiliary plasticizer that are miscible with water in the pores are extracted), and dried in an oven at 90°C for 15 minutes (the oven heating process is one to dry the water on the base film, and the second is to further improve the oriented crystallization of the base film and enhance its dimensional stability), to obtain a polyacrylamide lithium ion battery base film, wherein, by mass, the ratio of the polyacrylamide, the main plasticizer and the auxiliary plasticizer is 64:26:10, the main plasticizer is glycerol, the auxiliary plasticizer is dimethyl sulfoxide, and the stretching temperature is 115°C.

[0041] Comparative Example 1

[0042] A method for preparing a lithium-ion battery base film comprises the following steps: in a premixing tank, polyacrylamide powder, a main plasticizer and an auxiliary plasticizer are heated and mixed at 85°C to form a suspension; the suspension is added to a co-rotating twin-screw extruder with a screw diameter of 80 mm and an aspect ratio of 64 by a plunger pump for melting; the barrel is heated at a temperature of 180°C and a screw speed of 90 rpm; the suspension is quantitatively conveyed by a gear pump through a secondary filter and extruded along a slit die of a T-die head along a melt pipe; the suspension is cooled and shaped at 20°C by a shaping roller to form a cast sheet; the cast sheet is formed by a shaping roller. The casting sheet is pulled and wound by a forming roller and a winding roller, and is cut into a square casting sheet. The cut casting sheet is stretched at a ratio of 8 in a film high-temperature biaxial stretching machine, and then clamped in an extraction fixture, extracted with water as an extractant at room temperature for 4 minutes, and dried in an oven at 90°C for 15 minutes to obtain a lithium-ion battery base film, wherein the ratio of the polyacrylamide, the main plasticizer and the auxiliary plasticizer is 80:15:5 in parts by mass, the main plasticizer is glycerol, the auxiliary plasticizer is dimethyl sulfoxide, and the stretching temperature is 115°C.

[0043] Comparative Example 2

[0044] A method for preparing a lithium-ion battery base film comprises the following steps: in a premixing tank, polyacrylamide powder, a main plasticizer and an auxiliary plasticizer are heated and mixed at 85°C to form a suspension; the suspension is added to a co-rotating twin-screw extruder with a screw diameter of 80 mm and an aspect ratio of 64 by a plunger pump for melting; the barrel is heated at a temperature of 180°C and a screw speed of 90 rpm; the suspension is quantitatively conveyed by a gear pump through a secondary filter and extruded along a slit die of a T-die head along a melt pipe; the suspension is cooled and shaped at 20°C by a shaping roller to form a cast sheet; the cast sheet is formed by a shaping roller. The casting sheet is pulled and wound by a forming roller and a winding roller, and is cut into a square casting sheet. The cut casting sheet is stretched at a ratio of 8 in a film high-temperature biaxial stretching machine, and then clamped in an extraction fixture, extracted with water as an extractant at room temperature for 4 minutes, and dried in an oven at 90°C for 15 minutes to obtain a lithium-ion battery base film, wherein the ratio of the polyacrylamide, the main plasticizer and the auxiliary plasticizer is 70:20:10 in parts by mass, the main plasticizer is glycerol, the auxiliary plasticizer is dimethyl sulfoxide, and the stretching temperature is 115°C.

[0045] Comparative Example 3

[0046] A preparation method of a lithium-ion battery base film comprises the following steps: in a premixing tank, polyvinyl alcohol powder, a main plasticizer and an auxiliary plasticizer are heated and mixed at 85°C to form a suspension; the suspension is added to a co-rotating twin-screw extruder with a screw diameter of 80 mm and an aspect ratio of 64 by a plunger pump to melt; the barrel is heated at a temperature of 200°C and the screw speed is 110 rpm; the suspension is quantitatively transported by a gear pump through a secondary filter and extruded along a slit die of a T-die head along a melt pipe; and the suspension is shaped by a molten metal mold. The roller is cooled and shaped at 20°C to form a casting sheet, which is then drawn and wound by a shaping roller and a winding roller. The casting sheet is cut into a square casting sheet, and the cut casting sheet is stretched at a ratio of 8 times in a high-temperature biaxial stretching machine for a thin film. When the casting sheet is stretched to 2 to 3 times, it breaks and cannot be formed into a film. The ratio of the polyvinyl alcohol, the main plasticizer and the auxiliary plasticizer is 64:26:10 by mass, the main plasticizer is glycerol, the auxiliary plasticizer is dimethyl sulfoxide, and the stretching temperature is 125°C.

[0047] After testing and analysis, the reason for the film breakage was that the homogeneous melt formed by the polyvinyl alcohol and plasticizer system had excellent stability. During the rapid cooling process, phase separation occurred and the proportion of polyvinyl alcohol separation crystallization was low. The crystallinity of the polyvinyl alcohol flakes was about 50% lower than that of the polyacrylamide flakes in Example 1, resulting in insufficient strength of the flakes and poor ductility. The flakes broke during the high-temperature stretching process and could not form a film.

[0048] The performance tests were conducted on the polyacrylamide lithium ion battery base membrane prepared in Example 1 and the lithium ion battery base membranes prepared in Comparative Examples 1 and 2. The test results and test methods are as follows.

[0049] The thermal shrinkage test method is as follows:

[0050] ①Test preparation: Start the TMA test equipment, open the auxiliary test software, zero and calibrate the probe, set the test temperature to 135℃, and the test time to 1 hour;

[0051] ② Sample preparation and testing: Samples were taken in the MD and TD directions of the base films prepared in Example 1 and Comparative Examples 1-2, and the sample size was 80 mm * 4 mm. The sample was placed on the measuring instrument fixture with tweezers and tightened, and then placed into the sample chamber of the testing instrument. When placing it, hold the probe to the zero position with your left hand, hold the fixture with your right hand and place it under the probe. After releasing your hand, adjust the position of the standard block to keep it in the center clamping state, return the chamber door to its original position, close the furnace body, and click the software to start the test.

[0052] ③Data and graphics export: Export the test data and graphics results. The thermal shrinkage calculation formula is as follows:

[0053] Thermal shrinkage (%) = L1-L2 / L1

[0054] Wherein, L1 is the width of the base film before detection (MD / TD), and L2 is the width of the base film after detection (MD / TD).

[0055] The contact angle test method is as follows:

[0056] ① Sample preparation: Cut the base film prepared in Example 1 and Comparative Examples 1-2 into 50mm*50mm squares, lay them flat on a glass slide, and place the glass slide on a workbench. The volume of the test solution should be greater than 1ml;

[0057] ②Test: A. Turn on the computer, select AngleM in the stand-alone contact angle test software, turn on the light source knob, turn clockwise to see the light source brightness increase, and adjust the light source brightness according to the computer display image;

[0058] B. Adjust the hand wheel until the image is clear, rotate the micrometer head to let the liquid flow out, and move the workbench position according to the knob so that the sample to be measured appears in the center of the light source;

[0059] C. Rotate the micrometer head to release a drop of liquid onto the surface of the basement membrane. Wait for 1 second and click to capture the currently displayed image.

[0060] D. The contact angle of the liquid on the base film surface can be output by manual circle making and tangent line method.

[0061] like Figure 2 and 4As shown, the melt casting of the melt extrusion in Example 1 is continuous, uniform, smooth and consistent. The polyacrylamide lithium ion battery base membrane finally prepared has a thermal shrinkage of 135°C*1h: MD thermal shrinkage is 10.04%, TD thermal shrinkage is -2.11%; and its contact angle with deionized water is 31.1°.

[0062] Comparative Example 1 melt extrusion melt casting uniformity is poor, there is a small amount of raw materials that are not completely plasticized, the final prepared lithium ion battery base film, its 135 ℃ * 1h thermal shrinkage: MD thermal shrinkage is 11.89%, TD thermal shrinkage is -1.95%, but the same specification polyethylene base film MD thermal shrinkage is 21.47%, TD thermal shrinkage is 7.64% (such as Figure 1 As shown), therefore, the MD thermal shrinkage and TD thermal shrinkage of the lithium ion battery base film prepared in Comparative Example 1 are reduced by 44.6% and 125.5% respectively compared with the traditional polyethylene base film; the contact angle of the lithium ion battery base film of Comparative Example 1 with water is 47.1°, but the contact angle of the polyethylene base film of the same specification with water is 125.8° (as shown). Figure 3 As shown), the contact angle of water with the traditional polyethylene-based film is reduced by about 62.6%; however, in Comparative Example 1, the proportion of propylene glycol and dimethyl sulfoxide is small, resulting in poor plasticizing and mixing effect of acrylamide.

[0063] Comparative Example 2 melt extrusion and melt casting are continuous and uniform, and the lithium ion battery base film finally prepared has a thermal shrinkage of 135°C*1h: MD thermal shrinkage is 11.34%, TD thermal shrinkage is -2.03%; its contact angle with deionized water is 40.3°.

[0064] It can be seen that in Example 1, due to the large proportion of propylene glycol, the plasticizing and mixing effect of polyacrylamide is significantly improved. At the same time, the increased proportion of dimethyl sulfoxide also makes the hydrophilicity of the polyacrylamide lithium ion battery base membrane better and the contact angle smaller.

[0065] 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 a polyacrylamide lithium ion battery base film, characterized in that: The method comprises the following steps: heating and mixing polyacrylamide, a main plasticizer and an auxiliary plasticizer to form a suspension; adding the suspension into a twin-screw extruder for melt extrusion; cooling and shaping the suspension with a shaping roller to form a cast sheet; winding the cast sheet; cutting the cast sheet; stretching the cast sheet; and then washing and drying the cast sheet under a clamp to obtain a polyacrylamide lithium ion battery base film. The polyacrylamide, the main plasticizer and the auxiliary plasticizer are in a ratio of (60-68): (22-32): (8-10) by weight. The main plasticizer is glycerol, and the auxiliary plasticizer is dimethyl sulfoxide; The temperature of the heating and mixing is 80-90°C.

2. The preparation method according to claim 1, characterized in that The temperature of melt extrusion in the twin-screw extruder is 170-190° C., and the screw speed is 80-100 rpm.

3. The preparation method according to claim 1, characterized in that The screw diameter of the twin-screw extruder is 50-100 mm, and the aspect ratio is 60-68.

4. The preparation method according to claim 1, characterized in that The cooling and shaping temperature of the shaping roller is 15-25°C.

5. The preparation method according to claim 1, characterized in that The stretching ratio is 7 to 9 times, and the stretching temperature is 110 to 120°C.

6. The preparation method according to claim 1, characterized in that The extraction agent for the extraction and washing is water, and the extraction and washing time is 3 to 5 minutes.

7. The preparation method according to claim 1, characterized in that The drying temperature is 80-100° C., and the drying time is 10-20 minutes.

8. The polyacrylamide lithium ion battery base membrane obtained by the preparation method according to any one of claims 1 to 7.

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