A method for reducing residual extractables in a wet-process separator membrane base membrane

Abstract

The application discloses a method for reducing residual extraction agent of a wet-process separator base film, which comprises the following steps: (1) raw material pretreatment; polyolefin particle drying and additive dispersion, (2) film sheet preparation, (3) extraction: first-stage extraction, second-stage extraction and supercritical fluid assisted extraction, (4) post-treatment: drying treatment and plasma treatment. The method can remove the extraction agent molecules possibly remaining on the surface of the base film, improve the microstructure of the surface of the base film, improve the compatibility of the base film with electrolyte, further improve the overall performance of the separator, and solve the problems of high residual extraction agent of the wet-process separator base film, high cost, low efficiency and poor effect of the traditional treatment process.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery separator technology, specifically relating to a method for reducing residual agents in wet-process separator base membrane extraction. Background Technology

[0002] Among the key components of lithium-ion batteries, the wet-process separator base film plays an extremely important role. It separates the positive and negative electrodes of the battery to prevent short circuits between the two electrodes, while ensuring that lithium ions can pass through smoothly. Its performance directly affects the battery's safety performance, charge and discharge efficiency, and cycle life.

[0003] Currently, wet processing is one of the mainstream methods for preparing high-performance separator base membranes. This process typically involves mixing high-molecular materials such as polyolefins with specific small-molecule additives (such as paraffin oil and hexane), extruding the mixture to form a membrane sheet, and then using an extractant to extract the small-molecule additives from the membrane sheet, thereby forming a wet-process separator base membrane with a specific pore structure. However, in actual production, the extraction process inevitably results in some extractant remaining inside the separator base membrane.

[0004] These residual extractants have numerous negative impacts on the performance of the wet-process separator base membrane. From a battery safety perspective, residual extractants may decompose or volatilize during battery charging and discharging, generating gas, increasing internal battery pressure, and in severe cases, even causing battery bulging, explosions, and other safety accidents. Regarding the battery's electrochemical performance, residual extractants interfere with lithium-ion transport within the separator, reducing ionic conductivity, which in turn leads to decreased charge / discharge efficiency and accelerated capacity decay. Furthermore, due to the residual extractants, the compatibility between the separator base membrane and the electrolyte is also affected, hindering the electrolyte's full wetting of the separator, significantly reducing the overall battery performance.

[0005] To address this issue, the industry currently employs conventional methods such as extending extraction time, increasing extraction temperature, and increasing the number of extractions. However, extending extraction time significantly reduces production efficiency and increases production costs; increasing extraction temperature not only requires more energy but may also damage the physical structure and performance of the separator base membrane; and increasing the number of extractions implies higher equipment investment and operating costs. Therefore, developing a new method that can effectively reduce residual agents in wet-process separator base membrane extraction without significantly increasing production costs and affecting production efficiency has become a key challenge that urgently needs to be overcome in the field of wet-process separator base membrane preparation. This is of vital practical significance for promoting the healthy development of the lithium-ion battery industry.

[0006] Defects and shortcomings of existing technology: Process operation Long extraction time and low efficiency: Although extending the extraction time can reduce residues to some extent, it will significantly extend the production cycle, severely reduce production efficiency, and fail to meet the needs of large-scale industrial production.

[0007] Temperature control is challenging: While increasing the extraction temperature can accelerate the extraction process, it requires extremely precise temperature control. Excessive temperature can damage the crystalline structure of the membrane base, leading to a decrease in the membrane's mechanical properties and potential changes in its pore structure, affecting key indicators such as ion conductivity.

[0008] Multiple extractions are costly: Increasing the number of extractions requires more extraction equipment and operating procedures, which not only increases the cost of equipment investment, but also significantly increases the cost of manpower, material resources and time. In addition, multiple operations may introduce new impurities.

[0009] Extractant selection Limitations of extractant performance: Existing extractants have shortcomings in solubility and selectivity, making it difficult to efficiently extract small molecule additives in a short time. They may also cause swelling or damage to the membrane substrate, affecting membrane performance. Significant environmental hazards: Some extractants, such as hexane, are highly volatile and toxic, volatilizing into the environment during extraction. This not only pollutes the air but may also harm the health of operators, and subsequent waste gas treatment costs are high.

[0010] Post-processing Incomplete drying: Conventional drying methods are insufficient to completely remove the extractant remaining in the micropores inside the membrane base, especially for extractants with high boiling points and tight bonding with the membrane material, leaving a significant residue after drying.

[0011] Lack of effective detection methods: Current methods for detecting extraction residues in diaphragm base membranes suffer from insufficient precision and slow detection speed, making it impossible to monitor and provide timely feedback on the extraction effect during the production process, which is not conducive to real-time adjustment and optimization of the process. Summary of the Invention

[0012] In view of this, the purpose of this invention is to provide a method for reducing the residual agent in wet membrane base extraction, so as to solve the problems of high residual agent in wet membrane base extraction, high cost, low efficiency and poor effect of traditional treatment process in the prior art.

[0013] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for reducing residual agents in wet-process membrane base extraction includes the following steps: (1) Raw material pretreatment: Drying polyolefin granules: Place the prepared polyolefin granules in a vacuum drying oven to dry. After drying, remove the polyolefin granules, seal and store them for later use. Additive dispersion: Weigh the small molecule additive, pour it into the container of the ultrasonic dispersion equipment, add an appropriate amount of dispersion medium, turn on the ultrasonic generator to disperse, and after dispersion, remove the dispersion medium by vacuum distillation to obtain uniformly dispersed small molecule additive for later use. (2) Preparation of film: The pretreated polyolefin particles and the well dispersed small molecule additives are fed into the twin-screw extruder in a mass ratio, the temperature distribution of the extruder is set, and the film is extruded through the die. (3) Extraction: First stage of extraction: Place the membrane in the extraction tank, add sufficient extractant, and control the stirring speed, extraction temperature and extraction time for extraction; Second stage extraction: The membrane after the first stage extraction is transferred to another extraction tank, new extractant is added, and the temperature is increased to carry out extraction; Supercritical fluid-assisted extraction: The membrane after multi-stage temperature-variable extraction is placed in a supercritical extraction device. Carbon dioxide is used as the supercritical fluid. Carbon dioxide and hexane are mixed and injected into the device. The pressure, temperature and extraction time are adjusted to further extract residual small molecule additives. (4) Post-processing: Drying process: The extracted membrane is placed in a vacuum freeze dryer, pre-cooled, and then the vacuum is adjusted for sublimation drying; the vacuum-freeze-dried membrane is then transferred to a hot air drying oven for further drying to remove residual extractant; Plasma treatment: The dried membrane is placed in a plasma treatment device and radio frequency plasma is used to treat the surface of the membrane to remove residual extractant, thus obtaining the membrane with reduced wet-process membrane base membrane extraction residue.

[0014] Preferably, in step (1), the dry air density of the polyolefin particles is adjusted to -0.1 MPa, the temperature is set to 80°C, and the drying time is 4 hours; the dispersion medium for the additive dispersion is anhydrous ethanol, the ultrasonic generator frequency is set to 40 kHz, the power is 500 W, and the dispersion time is 1 hour.

[0015] Preferably, in step (2), the mass ratio of polyolefin particles to well dispersed small molecule additives is 4:1, the temperature distribution of the extruder from the hopper to the die head is 150℃, 180℃, 200℃, 220℃, 230℃, and the screw speed is 120r / min; the small molecule additive is BASF antioxidant 1010.

[0016] Preferably, in step (3), the extractant for the first stage of extraction is hexane, the extraction temperature is 35°C, the extraction time is 2 hours, and the stirring speed is 100 r / min; the extraction temperature for the second stage of extraction is 55°C, the extraction time is 1 hour, and the stirring speed is 120 r / min.

[0017] Preferably, in step (3) supercritical fluid assisted extraction, the volume ratio of carbon dioxide to hexane is 1:3, the pressure is 10 MPa, the temperature is 40 °C, and the extraction time is 30 minutes.

[0018] Preferably, in step (4), the vacuum freeze-drying process is first pre-cooled to -40°C and maintained for 2 hours, and then the vacuum degree is adjusted to 10Pa for sublimation drying for 6 hours; the hot air drying temperature is 70°C and the drying time is 1 hour.

[0019] Preferably, the power in the plasma treatment in step (4) is 100W and the treatment time is 5 minutes.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention performs vacuum drying on polyolefin particles to remove moisture and volatile impurities that may be contained therein, ensuring the purity of the raw materials, making the subsequent mixing and extrusion process more stable, and reducing the problem of extraction residue caused by impurities; the small molecule additives are dispersed using ultrasonic dispersion equipment to improve their dispersion uniformity in polyolefin materials, ensuring that the additives can be extracted more uniformly during subsequent extraction, avoiding incomplete extraction and residue due to excessively high local concentrations; (2) This invention employs a multi-stage temperature-varying extraction process. In the first stage, at a relatively low temperature, most small-molecule additives begin to detach from the membrane substrate. The mild extraction conditions in this stage can avoid excessive impact on the substrate structure. In the second stage, the temperature is increased to further enhance the extraction effect, prompting the remaining small amount of additives to be extracted quickly. Through this temperature-varying method, both the high efficiency of extraction and the stability of the substrate structure are ensured. (3) This invention introduces supercritical fluid extraction technology to assist traditional solvent extraction. Carbon dioxide is used as supercritical fluid. In the supercritical state, carbon dioxide has good diffusion and solubility. After mixing it with traditional extractant in a certain proportion, it is used to extract the membrane base. Supercritical carbon dioxide can quickly penetrate into the micropores of the base membrane, dissolve and carry out small molecule additives, and at the same time promote the mass transfer efficiency of traditional extractant, significantly reducing extraction residue. (4) This invention employs a drying method combining vacuum freeze-drying and hot air drying. First, the extracted membrane substrate is placed in a vacuum freeze-drying environment to rapidly freeze the residual extractant. Then, it undergoes sublimation drying under vacuum conditions to remove most of the residual extractant. Afterward, hot air drying is performed to further remove the remaining trace extractant, ensuring thorough drying and reducing residue. (5) The present invention adds a plasma treatment process after drying. The high energy of plasma is used to treat the surface of the membrane base. On the one hand, it can remove the extractant molecules that may remain on the surface of the base. On the other hand, it can improve the microstructure of the base surface, improve the compatibility between the base and the electrolyte, and further improve the overall performance of the membrane, making up for the performance damage that may be caused to the base membrane by the extraction and drying process. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1 A method for reducing residual agents in wet-process membrane base extraction includes the following steps: (1) Raw material pretreatment: Drying of polyolefin granules: Place 5 kg of prepared polyolefin granules into a vacuum drying oven and dry them. Adjust the vacuum degree to -0.1 MPa, set the temperature to 80℃, and dry for 4 hours. After drying, take out the polyolefin granules, seal and store them for later use. Additive dispersion: Weigh out the small molecule additive BASF antioxidant 1010, pour it into the container of the ultrasonic dispersion device, add an appropriate amount of dispersion medium anhydrous ethanol, turn on the ultrasonic generator for dispersion, set the frequency to 40kHz, the power to 500W, and the dispersion time to 1 hour. After dispersion, remove the dispersion medium by vacuum distillation to obtain a uniformly dispersed small molecule additive for later use. (2) Preparation of film: The pretreated polyolefin particles and the well dispersed small molecule additives are fed into the twin-screw extruder at a mass ratio of 4:1. The temperature distribution of the extruder is set to 150℃, 180℃, 200℃, 220℃ and 230℃ from the hopper to the die head. The screw speed is 120r / min. A 0.1mm film is extruded through the die head. (3) Extraction: First stage of extraction: Place the membrane in the extraction tank, add sufficient hexane as the extractant, control the extraction temperature at 35℃, the extraction time is 2 hours, and control the stirring speed at 100r / min to ensure that the extractant is in full contact with the membrane for extraction. Second stage extraction: Transfer the membrane after the first stage extraction to another extraction tank, add new hexane as the extractant, raise the temperature to 55°C, extract for 1 hour, and maintain the stirring speed at 120 r / min for extraction. Supercritical fluid-assisted extraction: The membrane after multi-stage temperature-variable extraction is placed in a supercritical extraction device. Carbon dioxide is used as the supercritical fluid. Carbon dioxide and hexane are mixed at a volume ratio of 1:3 and injected into the device. The pressure is adjusted to 10MPa, the temperature to 40℃ and the extraction time to 30 minutes, so that the supercritical carbon dioxide and hexane work together to further extract the residual small molecule additives. (4) Post-processing: Drying process: The extracted membrane was placed in a vacuum freeze dryer, pre-cooled to -40°C and maintained for 2 hours. Then, the vacuum was adjusted to 10 Pa for sublimation drying for 6 hours. The freeze-dried membrane was then transferred to a hot air drying oven at 70°C for 1 hour to further remove residual extractant. Plasma treatment: The dried membrane is placed in a plasma treatment device and radio frequency plasma with a power of 100W is used to treat the surface of the membrane for 5 minutes to remove residual extractant, thus obtaining the membrane with reduced wet diaphragm base membrane extraction residue.

[0023] Example 2 A method for reducing residual agents in wet-process membrane base extraction includes the following steps: (1) Raw material pretreatment: Drying of polyolefin granules: Place 4 kg of prepared polyolefin granules into a vacuum drying oven and dry them. Adjust the vacuum degree to -0.1 MPa, set the temperature to 80℃, and dry for 4 hours. After drying, take out the polyolefin granules, seal and store them for later use. Additive dispersion: Weigh out the small molecule additive BASF antioxidant 1010, pour it into the container of the ultrasonic dispersion device, add an appropriate amount of dispersion medium anhydrous ethanol, turn on the ultrasonic generator for dispersion, set the frequency to 40kHz, the power to 500W, and the dispersion time to 1 hour. After dispersion, remove the dispersion medium by vacuum distillation to obtain a uniformly dispersed small molecule additive for later use. (2) Preparation of film: The pretreated polyolefin particles and the well dispersed small molecule additives are fed into the twin-screw extruder at a mass ratio of 3:1. The temperature distribution of the extruder is set to 150℃, 180℃, 200℃, 220℃ and 230℃ from the hopper to the die head. The screw speed is 120r / min. A 0.1mm film is extruded through the die head. (3) Extraction: First stage of extraction: Place the membrane in the extraction tank, add sufficient hexane as the extractant, control the extraction temperature at 35℃, the extraction time is 2 hours, and control the stirring speed at 100r / min to ensure that the extractant is in full contact with the membrane for extraction. Second stage extraction: Transfer the membrane after the first stage extraction to another extraction tank, add new hexane as the extractant, raise the temperature to 55°C, extract for 1 hour, and maintain the stirring speed at 120 r / min for extraction. Supercritical fluid-assisted extraction: The membrane after multi-stage temperature-variable extraction is placed in a supercritical extraction device. Carbon dioxide is used as the supercritical fluid. Carbon dioxide and hexane are mixed at a volume ratio of 1:3 and injected into the device. The pressure is adjusted to 10MPa, the temperature to 40℃ and the extraction time to 30 minutes, so that the supercritical carbon dioxide and hexane work together to further extract the residual small molecule additives. (4) Post-processing: Drying process: The extracted membrane was placed in a vacuum freeze dryer, pre-cooled to -40°C and maintained for 2 hours. Then, the vacuum was adjusted to 10 Pa for sublimation drying for 6 hours. The freeze-dried membrane was then transferred to a hot air drying oven at 70°C for 1 hour to further remove residual extractant. Plasma treatment: The dried membrane is placed in a plasma treatment device and radio frequency plasma with a power of 100W is used to treat the surface of the membrane for 5 minutes to remove residual extractant, thus obtaining the membrane with reduced wet diaphragm base membrane extraction residue.

[0024] Example 3 A method for reducing residual agents in wet-process membrane base extraction includes the following steps: (1) Raw material pretreatment: Drying of polyolefin granules: Place 6 kg of prepared polyolefin granules into a vacuum drying oven and dry them. Adjust the vacuum degree to -0.1 MPa, set the temperature to 80℃, and dry for 4 hours. After drying, take out the polyolefin granules, seal and store them for later use. Additive dispersion: Weigh out the small molecule additive BASF antioxidant 1010, pour it into the container of the ultrasonic dispersion device, add an appropriate amount of dispersion medium anhydrous ethanol, turn on the ultrasonic generator for dispersion, set the frequency to 40kHz, the power to 500W, and the dispersion time to 1 hour. After dispersion, remove the dispersion medium by vacuum distillation to obtain a uniformly dispersed small molecule additive for later use. (2) Preparation of film: The pretreated polyolefin particles and the well dispersed small molecule additives are fed into the twin-screw extruder at a mass ratio of 5:1. The temperature distribution of the extruder is set to 150℃, 180℃, 200℃, 220℃ and 230℃ from the hopper to the die head. The screw speed is 120r / min. A 0.1mm film is extruded through the die head. (3) Extraction: First stage of extraction: Place the membrane in the extraction tank, add sufficient hexane as the extractant, control the extraction temperature at 35℃, the extraction time is 2 hours, and control the stirring speed at 100r / min to ensure that the extractant is in full contact with the membrane for extraction. Second stage extraction: Transfer the membrane after the first stage extraction to another extraction tank, add new hexane as the extractant, raise the temperature to 55°C, extract for 1 hour, and maintain the stirring speed at 120 r / min for extraction. Supercritical fluid-assisted extraction: The membrane after multi-stage temperature-variable extraction is placed in a supercritical extraction device. Carbon dioxide is used as the supercritical fluid. Carbon dioxide and hexane are mixed at a volume ratio of 1:3 and injected into the device. The pressure is adjusted to 10MPa, the temperature to 40℃ and the extraction time to 30 minutes, so that the supercritical carbon dioxide and hexane work together to further extract the residual small molecule additives. (4) Post-processing: Drying process: The extracted membrane was placed in a vacuum freeze dryer, pre-cooled to -40°C and maintained for 2 hours. Then, the vacuum was adjusted to 10 Pa for sublimation drying for 6 hours. The freeze-dried membrane was then transferred to a hot air drying oven at 70°C for 1 hour to further remove residual extractant. Plasma treatment: The dried membrane is placed in a plasma treatment device and radio frequency plasma with a power of 100W is used to treat the surface of the membrane for 5 minutes to remove residual extractant, thus obtaining the membrane with reduced wet diaphragm base membrane extraction residue.

[0025] The relevant performance indicators of the wet-process separator base membranes of Examples 1-3 were measured, and the results are as follows:

[0026] As can be seen from the table above, the extractant residue and related properties of the base membrane in the embodiments of the present invention are both good.

[0027] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing residual agents in wet-process membrane base extraction, characterized in that, Includes the following steps: (1) Raw material pretreatment: Drying polyolefin granules: Place the prepared polyolefin granules in a vacuum drying oven to dry. After drying, remove the polyolefin granules, seal and store them for later use. Additive dispersion: Weigh the small molecule additive, pour it into the container of the ultrasonic dispersion equipment, add an appropriate amount of dispersion medium, turn on the ultrasonic generator to disperse, and after dispersion, remove the dispersion medium by vacuum distillation to obtain uniformly dispersed small molecule additive for later use. (2) Preparation of film: The pretreated polyolefin particles and the well dispersed small molecule additives are fed into the twin-screw extruder in a mass ratio, the temperature distribution of the extruder is set, and the film is extruded through the die. (3) Extraction: First stage extraction: The extractant is hexane. The membrane is placed in the extraction tank, and sufficient extractant is added. The stirring speed, extraction temperature and extraction time are controlled for extraction. Second stage extraction: The extractant is hexane. The membrane after the first stage extraction is transferred to another extraction tank, new extractant is added, and the temperature is increased to carry out extraction. Supercritical fluid-assisted extraction: The membrane after multi-stage temperature-variable extraction is placed in a supercritical extraction device. Carbon dioxide is used as the supercritical fluid. Carbon dioxide and hexane are mixed and injected into the device. The pressure, temperature and extraction time are adjusted to further extract residual small molecule additives. (4) Post-processing: Drying process: The extracted membrane is placed in a vacuum freeze dryer, pre-cooled, and then the vacuum is adjusted for sublimation drying; the vacuum-freeze-dried membrane is then transferred to a hot air drying oven for further drying to remove residual extractant; Plasma treatment: The dried membrane is placed in a plasma treatment device and radio frequency plasma is used to treat the surface of the membrane to remove residual extractant, thus obtaining the membrane with reduced wet-process membrane base membrane extraction residue.

2. The method as described in claim 1, characterized in that, In step (1), the vacuum degree of the polyolefin particles is adjusted to -0.1MPa, the temperature is set to 80℃, and the drying time is 4 hours. When the additive is dispersed, the dispersion medium is anhydrous ethanol, the ultrasonic generator frequency is set to 40kHz, the power is 500W, and the dispersion time is 1 hour.

3. The method according to any one of claims 1-2, characterized in that, In step (2), the mass ratio of polyolefin particles to well dispersed small molecule additives is 3-5:

1. The temperature distribution of the extruder from the hopper to the die head is 150℃, 180℃, 200℃, 220℃, and 230℃ respectively. The screw speed is 120r / min. The small molecule additive is BASF antioxidant 1010.

4. The method according to any one of claims 1-2, characterized in that, In step (3), the first stage of extraction is carried out at a temperature of 35°C for 2 hours and a stirring speed of 100 r / min; the second stage of extraction is carried out at a temperature of 55°C for 1 hour and a stirring speed of 120 r / min.

5. The method as described in claim 4, characterized in that, In step (3), the volume ratio of carbon dioxide to hexane in supercritical fluid-assisted extraction is 1:3, the pressure is 10 MPa, the temperature is 40 °C, and the extraction time is 30 minutes.

6. The method according to any one of claims 1-2, characterized in that, In step (4), the vacuum freeze dryer is first pre-cooled to -40°C and kept for 2 hours. Then the vacuum degree is adjusted to 10Pa and sublimation drying is carried out for 6 hours. The hot air drying temperature is 70°C and the drying time is 1 hour.

7. The method as described in claim 2, characterized in that, In step (4), the plasma treatment power is 100W and the treatment time is 5 minutes.

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