A chlorine-free porous wet-process membrane and its preparation method

Abstract

This invention discloses a chlorine-free porous wet-process separator and its preparation method, belonging to the field of lithium battery wet-process separator technology. The method includes: mixing and stirring a polyolefin material with an organic solvent in a specific ratio to obtain a mixture; then melting and extruding the mixture through an extruder, followed by casting, cooling, stretching, extraction, heat setting, and winding to obtain a porous polyolefin separator. The organic solvent is one or more of polyether-based, amino-based, carboxyl-based, amide-based modified silicone oils, glycerol ethers, and alkanes containing hydroxyl and ether groups. This invention uses modified silicone oil instead of traditional white oil as a pore-forming agent, utilizing its amphiphilic structure (both lipophilic and hydrophilic), eliminating the dichloromethane extraction process and its environmental and health hazards. It also simplifies the process, reduces production costs, and results in a separator with rich and uniform pores, excellent mechanical properties, and physicochemical properties that meet or exceed those of traditional wet-process separators.

Description

Technical Field

[0001] This invention relates to the field of wet-process separator technology for lithium batteries, and more specifically to a chlorine-free porous wet-process separator and its preparation method. Background Technology

[0002] As society transitions to a more sustainable energy model, lithium-ion batteries (LIBs) have become an indispensable part of the global energy landscape, providing a primary power source for portable electronics, electric vehicles (EVs), and grid-scale energy storage systems. The basic structure of a lithium-ion battery comprises four core components: the positive electrode, the negative electrode, the electrolyte, and the microporous separator. Each component performs a crucial function: the positive electrode determines the battery capacity, the negative electrode facilitates the insertion and extraction of lithium ions during charge-discharge cycles, and the electrolyte acts as an ion transport medium to maintain the overall conductivity of the battery. Among these components, the separator holds a uniquely vital position in terms of battery performance and safety. Located between the positive and negative electrodes, the separator not only plays a crucial role in promoting lithium ion migration in the electrolyte but also prevents accidental contact between the highly oxidizing positive electrode and the highly reducing negative electrode.

[0003] Currently, commercially available lithium-ion battery separators are mainly prepared using two routes: dry stretching and wet process (thermally induced phase separation, TIPS). The main principle of the wet process is to mix polymer materials such as polyethylene (PE) with high-boiling-point small-molecule pore-forming agents (such as paraffin oil) to form a homogeneous solution at high temperature. Subsequently, cooling induces phase separation, followed by biaxial stretching and extraction to remove the pore-forming agent, ultimately forming a separator with a uniform microporous structure. Separators prepared by the wet process outperform those prepared by the dry process in terms of thickness uniformity, mechanical properties, gas permeability, and physicochemical properties, making them particularly suitable for manufacturing high-capacity lithium-ion batteries. Therefore, the shipment growth rate of wet-process separators has consistently exceeded that of dry-process separators.

[0004] In existing wet-process separator production technologies, the extraction process generally relies on halogenated hydrocarbon organic solvents such as dichloromethane to remove residual pore-forming agents from the base membrane. For example, Chinese patent document CN103022401A discloses a method for preparing a polyolefin separator for lithium-ion batteries. This method involves melting and mixing polyolefin resin, inorganic particles, and plasticizers such as liquid paraffin, solid paraffin, or diisobutyl phthalate at 150–280°C to form a homogeneous solution. After extrusion through a die and cooling, the solution is cast into a thick sheet. The plasticizer in the thick sheet is then extracted using organic solvents such as n-hexane, heptane, dichloromethane, methanol, or ethanol. The sheet is then biaxially stretched and dried with hot air to obtain a polyolefin microporous membrane. In specific embodiments, liquid paraffin, solid paraffin, or diisobutyl phthalate are used as pore-forming agents, and dichloromethane is used as the extraction method to complete the extraction, thereby forming a microporous structure inside the base membrane. However, the industrial application of dichloromethane has the following serious drawbacks: Dichloromethane has an extremely low boiling point (approximately 40°C) and is a highly volatile solvent, easily escaping during use, leading to significant material losses; dichloromethane is strictly defined as a Group 2A carcinogen under relevant laws and regulations and is listed as a toxic and hazardous air pollutant and a toxic and hazardous water pollutant, posing serious harm to the environment and the health of workers. With increasingly stringent global regulations on chlorinated solvents, the use of dichloromethane in industrial production is facing increasingly strict restrictions, and there is a trend towards banning dichloromethane in the industry.

[0005] Therefore, developing a novel wet-process membrane preparation process that fundamentally eliminates the dichloromethane extraction step, and achieving environmentally friendly, low-cost, and high-performance polyethylene wet-process membrane production, is a key technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] Based on the problems existing in the background technology, the present invention provides a chlorine-free porous wet process separator and its preparation method. By replacing white oil with polyether modified silicone oil, and utilizing the dual characteristics of polyether modified silicone oil, which has both oleophilic (compatible with polyethylene matrix) and hydrophilic (can be washed away with water), the use of dichloromethane extractant is fundamentally eliminated, the production cost is reduced, and the environmental and health hazards of dichloromethane are eliminated. At the same time, a polyolefin wet process separator with rich and uniform pore structure and excellent mechanical properties is obtained.

[0007] This invention is implemented through the following technical solutions: This invention provides a method for preparing a chlorine-free porous wet-process separator, comprising the following steps: Step 1: Mix the polyolefin material and organic solvent in a certain proportion and stir evenly to obtain a mixture.

[0008] The polyolefin material is one or a mixture of polyethylene and polypropylene, with a molecular weight of 100,000 to 10 million, which can be selected according to the target thickness of the diaphragm and the mechanical strength requirements; the solid content of the polyolefin material in the mixture is 10% to 60%.

[0009] The organic solvent is selected from one or more of the following: polyether-modified silicone oil, amino-modified silicone oil, carboxyl-modified silicone oil, amide-modified silicone oil, glycerol ethers, and alkanes containing hydroxyl and ether groups, with a molecular weight of 200–20000 and a viscosity of 30–3000 mmHg. 2 / s. This type of modified silicone oil contains both a lipophilic siloxane backbone and hydrophilic side chain groups (such as polyether segments) in its molecular structure. Depending on the actual compatibility requirements, non-polar functional groups such as alkane groups and olefin groups can be further introduced into the branches or backbone for regulation and modification to enhance its compatibility with the polyolefin matrix. Glyceryl ethers and alkanes containing hydroxyl and ether groups have PE-loving alkane groups and hydrophilic ether groups. When melt-blended with polyolefins, they can form a compatible system with the polyolefin matrix. After cooling and casting, they can be uniformly distributed in the form of dispersed phase droplets inside the base film. After biaxial stretching, a microporous structure is formed in the film, and the channels can be distributed in a straight line. Because it also has a certain degree of hydrophilicity, it can be directly removed from the base film in subsequent processes by washing with water or environmentally friendly extractants, without relying on halogenated hydrocarbon organic solvents such as dichloromethane.

[0010] The mixing temperature of the mixture is 40-90℃, the mixing speed is 1000-2500 r / min, and the mixing time is 60-180 min, so as to ensure that the polyolefin material and the modified silicone oil pore-forming agent are fully dispersed and mixed evenly.

[0011] Step 2: Add the mixture obtained in Step 1 to an extruder for melt extrusion, and after casting, cooling, stretching, extraction, heat setting and winding, a porous polyolefin membrane is obtained.

[0012] The process involves several steps: The melt extrusion temperature is 160–250℃, ensuring the material is fully plasticized and forms a homogeneous melt within the extruder; the casting cooling temperature is 2–30℃, with a casting thickness of 0.5–30.0 mm. Controlling the cooling rate induces phase separation in the melt, forming a pore-forming agent dispersion phase and a polyolefin matrix phase; the stretching method is bidirectional asynchronous stretching, with a stretching temperature of 100–160℃ and a stretching ratio of 5–30 times. Stretching creates interconnected micropores in the aforementioned dispersed phase droplets, simultaneously enhancing the membrane's mechanical strength; the extraction process uses a low-boiling-point green solvent or aqueous solution as the extraction medium to remove modified silicone oil-based pore-forming agents from the base membrane, forming a porous structure; and the heat-setting temperature is 70–120℃ to eliminate internal stress generated during stretching and stabilize the membrane's pore structure and size.

[0013] The present invention also discloses a porous polyolefin membrane prepared according to the above method.

[0014] The beneficial effects of this invention are: This invention replaces traditional white oil as the pore-forming agent in polyolefin wet-process membranes with modified silicone oils such as polyether-modified silicone oils and amino-modified silicone oils. Utilizing the dual oleophilic and hydrophilic molecular structure of these substances, after melt blending with the polyolefin matrix, phase separation during casting, and biaxial stretching to form pores, the silicone oil can be directly removed by washing with low-boiling-point green solvents or aqueous solutions. This completely eliminates the dichloromethane extraction process, an indispensable step in traditional wet-process membrane production. On one hand, eliminating dichloromethane avoids raw material losses due to its high volatility during production, while also eliminating the need for dichloromethane raw material consumption and its associated recovery and distillation system, significantly simplifying the process and reducing equipment investment and production costs. On the other hand, it fundamentally eliminates the harm caused by dichloromethane as a Group 2A carcinogen and a toxic and harmful air and water pollutant to the production environment and the health of workers, truly making the wet-process membrane production process green and environmentally friendly, which is conducive to the large-scale implementation of related environmentally friendly production lines both domestically and internationally.

[0015] Furthermore, because modified silicone oil-based pore-forming agents exhibit good compatibility with the polyolefin matrix during the high-temperature melting stage, the resulting biaxially stretched micropores are more numerous and have a more uniform pore size distribution. The pores are mostly straight-through, reducing the tortuosity factor, which is beneficial for rapid electrolyte wetting and efficient lithium ion migration. Simultaneously, since these pore-forming agents are difficult to completely remove during water washing, the polar groups in the trace residual molecules can interact with lithium salts in the electrolyte, further enhancing the electrolyte wetting performance and ion transport performance of the membrane. Therefore, the porous polyolefin membrane prepared using the method described in this invention achieves or surpasses the level of traditional white oil-dichloromethane wet-process membranes in terms of pore structure uniformity, mechanical strength, wettability, and ionic conductivity, demonstrating promising prospects for industrial application. Detailed Implementation

[0016] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0017] Example 1:

[0018] This embodiment provides a chlorine-free pore-forming wet-process polyolefin separator and its preparation method. The polyolefin material is polyethylene with a molecular weight of 600,000, and the organic solvent is polyether-modified silicone oil with a molecular weight of 277 and a viscosity of 50 mmHg. 2 / s, the solid content of the mixture is 25%.

[0019] Step 1: In a mixing tank, add 250g of the above-mentioned 60W polyethylene to 750g of organic solvent polyether modified silicone oil, and stir for 120min at a stirring temperature of 70℃ and a stirring speed of 1600r / min to obtain a uniformly dispersed mixture.

[0020] Step 2: Add the above-mentioned well-stirred mixture to an extruder for melt extrusion at a temperature of 200°C. After extrusion, the mixture is cast and cooled to form a cast sheet at a temperature of 5°C with a thickness of 2.1 mm. Subsequently, the mixture undergoes bidirectional asynchronous stretching (stretching temperature 120°C, stretching ratio 14), extraction (extraction solvent is pure water, extraction temperature is 60°C, extraction time is 10 min, extraction method is continuous immersion extraction), heat setting (heat setting temperature 90°C), and winding to obtain a porous polyolefin membrane.

[0021] Example 2:

[0022] The preparation method provided in this embodiment is basically the same as that in Example 1, except that the solid content of the polyethylene raw material used in step 1 is 22%, and the remaining steps and process parameters are the same as those in Example 1.

[0023] Example 3:

[0024] The preparation method provided in this embodiment is basically the same as that in Example 1, except that the cooling temperature of the casting in step 2 is 10°C, and the other steps and process parameters are the same as those in Example 1.

[0025] Example 4:

[0026] The preparation method provided in this embodiment is basically the same as that in Example 1, except that the organic solvent used in step 1 is polyether epoxy modified silicone oil, with epoxy groups located on the side chain, a molecular weight of 5000, and a viscosity of 50 mm. 2 / s, the remaining steps and process parameters are the same as in Example 1.

[0027] Example 5:

[0028] The preparation method provided in this embodiment is basically the same as that in Example 1, except that the organic solvent used in step 1 is a double-terminated polyether modified silicone oil, with hydroxyl and vinyl groups at the double ends, a molecular weight of 6000, and a viscosity of 40 mm. 2 / s, the remaining steps and process parameters are the same as in Example 1.

[0029] Example 6:

[0030] The preparation method provided in this embodiment is basically the same as that in Example 1, except that the organic solvent used in step 1 is triethylene glycol monobutyl ether, with a molecular weight of 206 and a viscosity of 35 mm. 2 / s, the remaining steps and process parameters are the same as in Example 1.

[0031] Comparative Example 1: To illustrate the performance advantages of using polyether-modified silicone oil for pore formation, the difference between this comparative example and Example 1 is that the organic solvent used in step 1 is replaced with white oil, and the extraction solvent in step 2 is replaced with dichloromethane. All other steps and process parameters are the same as in Example 1.

[0032] Comparative Example 2: To illustrate the effect of casting cooling temperature on diaphragm performance, the difference between this comparative example and Example 1 is that the casting cooling temperature in step 2 is 35°C, while the remaining steps and process parameters are the same as in Example 1.

[0033] Experimental example: The porous membranes prepared in Examples 1-6 and Comparative Examples 1 and 2 were subjected to performance tests: 1. Ionic conductivity test; Test method: The ionic conductivity of the prepared porous polyethylene membrane was tested using an electrochemical workstation (Bio-Logic). The prepared polyethylene membrane was cut into 40mm*40mm pieces and then immersed in an electrolyte solution with a concentration of 1mol / L lithium hexafluorophosphate (LiPF6), ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1, and kept sealed for at least 60 minutes. The immersed membrane was then placed in a fixture, the test conditions were set, and the data was processed after the test. The goodness of fit was required to be greater than 0.99.

[0034] 2. Wettability Test; Test Method: Use tape to tautly attach a 60mm*40mm diaphragm to an H-shaped plate, ensuring the diaphragm surface is flat. After dropping 2μL of electrolyte onto the diaphragm surface, take a photograph immediately and use a machine to calibrate the diameter of the electrolyte in the MD and TD directions. After standing for 5 minutes, take another photograph of the same droplet and measure the diameter of the electrolyte in the MD and TD directions. Then calculate the wetting value.

[0035] 3. Air permeability test; Test method: Take a membrane the size of an A4 sheet of paper, measure five points with a Wang Yan-style air permeability meter and take the average value.

[0036] The test results are shown in Table 1.

[0037] Table 1. Performance test results of diaphragms in each example / comparative example

[0038] According to the results in Table 1: Comparing Example 1 with Comparative Example 1, it can be seen that replacing white oil with polyether-modified silicone oil as the pore-forming agent and replacing dichloromethane with aqueous solution as the extractant significantly improved the wettability and ionic conductivity of the membrane: wettability in the MD direction increased from 0.70 mm to 0.93 mm, an increase of approximately 32.8%; wettability in the TD direction increased from 0.53 mm to 0.72 mm, an increase of approximately 35.8%; and ionic conductivity increased from 1.2134 mS / cm to 1.5765 mS / cm, an increase of approximately 29.9%. Meanwhile, the air permeability of Example 1 (92 s / 100 ml) was significantly better than that of Comparative Example 1 (132 s / 100 ml), and its areal density was also lower (5.3 vs 5.5 g / m³). 2 This indicates that the membrane obtained by forming pores with polyether-modified silicone oil has richer and more uniform pores, and lower resistance to gas and ion transport.

[0039] In Examples 4 and 5, polyether-epoxy modified silicone oil and double-terminated polyether modified silicone oil were used to replace the polyether modified silicone oil in Example 1, respectively. The wettability (MD 0.88~0.94mm, TD 0.66~0.75mm) and ionic conductivity (1.53~1.58mS / cm) of the resulting membranes were also significantly better than those in Comparative Example 1. This indicates that the polyether modified silicone oil system has a certain degree of versatility in pore-forming agent structure and can achieve the technical effect of chlorine-free pore formation.

[0040] Comparative Example 2, with the casting cooling temperature increased to 35°C, showed wettability MD and TD of 0.75 mm and 0.53 mm, respectively, and an ionic conductivity of 1.4165 mS / cm. Its overall performance was lower than that of Examples 1 and 3. This indicates that the process parameters of the diaphragm affect its physicochemical properties. Using the CAST low-temperature cooling process can reduce diaphragm permeability and increase its electrolyte wettability.

[0041] Polyether-modified silicone oil molecules consist of a lipophilic siloxane backbone and a hydrophilic polyether side chain, while triethylene glycol monobutyl ether is composed of polyoxyethylene and terminal hydroxyl groups. Under the high-temperature conditions of melt extrusion, the siloxane segments and butyl polyoxyethylene segments are compatible with the polyethylene matrix to form a homogeneous system, while the polyether-modified silicone oil and triethylene glycol monobutyl ether tend to aggregate to form dispersed phase droplets. After biaxial stretching, the dispersed phase droplets are released to form micropores, resulting in a greater number of micropores with a more uniform pore size distribution. The channels are mostly straight cylindrical, reducing the tortuosity factor and facilitating rapid lithium ion migration. During subsequent water washing and extraction, polyether-modified silicone oil cannot be completely removed 100%, leaving trace amounts (ppm level) of molecules remaining in the porous polyethylene membrane. The residual polyether segments (especially ethylene oxide EO groups) can capture and dissociate lithium salts through Lewis acid-base interactions, thereby improving the membrane's affinity for the electrolyte and enhancing its wettability and ionic conductivity.

[0042] Compared to the traditional white oil-dichloromethane system, the polyether-modified silicone oil and aqueous solution extraction system can eliminate the need for dichloromethane raw material consumption and recovery systems, resulting in a significant reduction in costs.

[0043] Finally, it should be noted that the above embodiments are merely illustrative of several implementations of the present invention and are not intended to limit the scope of the invention. For those skilled in the art, any modifications, equivalent substitutions, or improvements made without departing from the concept of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a chlorine-free porous wet-process separator, characterized in that, Includes the following steps: Step 1: Mix the polyolefin material and the organic solvent, and stir until homogeneous to obtain a mixture; the organic solvent is selected from one or more of the following: polyether modified silicone oil, amino modified silicone oil, carboxyl modified silicone oil, amide modified silicone oil, glycerol ethers, and alkanes containing hydroxyl and ether groups. Step 2: Add the mixture obtained in Step 1 to an extruder for melt extrusion, and after casting, cooling, stretching, extraction, heat setting and winding, a porous polyolefin membrane is obtained.

2. The preparation method according to claim 1, characterized in that, The polyolefin material is one or a mixture of two of polyethylene and polypropylene, with a molecular weight of 100,000 to 10 million.

3. The preparation method according to claim 1, characterized in that, The organic solvent has a molecular weight of 200–20000 and a viscosity of 30–3000 mm. 2 / s.

4. The preparation method according to claim 1, characterized in that, In step 1, the solid content of the polyolefin material is 10% to 60%.

5. The preparation method according to claim 1, characterized in that, In step 1, the stirring temperature for uniform mixing is 40–90°C, the stirring speed is 1000–2500 r / min, and the stirring time is 60–180 min.

6. The preparation method according to claim 1, characterized in that, In step 2, the temperature of the melt extrusion is 160–250°C.

7. The preparation method according to claim 1, characterized in that, In step 2, the temperature at which the casting is cooled is 2 to 30°C, and the thickness of the casting is 0.5 to 30.0 mm.

8. The preparation method according to claim 1, characterized in that, In step 2, the stretching is bidirectional asynchronous stretching, the stretching temperature is 100-160℃, and the stretching ratio is 5-30; the heat setting temperature is 70-120℃.

9. The preparation method according to claim 1, characterized in that, In step 2, the extraction solvent used is one of a low-boiling-point green solvent and an aqueous solution.

10. A porous polyolefin membrane, prepared by any one of claims 1 to 9.

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