An ultrathin high-safety high-orientation lithium battery separator base film and a preparation method thereof

Abstract

This invention discloses an ultrathin, high-safety, and highly oriented lithium-ion battery separator base film and its preparation method, belonging to the field of lithium-ion battery separator technology. The preparation method includes: mixing polyethylene, polypropylene, and oriented separator particles with white oil to obtain a mixture; melt-extruding the mixture and cooling and solidifying it to obtain a sheet; sequentially stretching the sheet longitudinally, transversely, and biaxially; extracting the stretched film to remove the white oil; and re-stretching and heat-setting the extracted film to obtain a separator base film with a thickness of 2.5 ± 0.5 μm. The oriented separator particles act as nucleation inducing agents, and combined with the multi-stage stretching process, stable preparation of the ultrathin separator is achieved. The ultrathin separator prepared by this invention exhibits excellent mechanical properties and thermal stability, and can be used in high-energy-density lithium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery separators, specifically relating to an ultra-thin, high-safety, and highly oriented lithium battery separator base film and its preparation method. Background Technology

[0002] Lithium-ion battery microporous membranes are crucial materials in lithium-ion batteries, possessing nanopores. Currently, the main methods for manufacturing lithium-ion battery microporous membranes include dry single-stretching, dry bi-stretching, and wet processes. The micropores in the lithium-ion battery microporous membrane prevent direct contact between the positive and negative electrodes, while allowing lithium ions to pass through and combine with electrons, thus undergoing redox reactions and effectively achieving the charging and discharging of the lithium-ion battery. Lithium-ion battery microporous membranes not only need to possess good micropore size, porosity, and liquid absorption rate, but also sufficient mechanical properties, such as puncture strength and tensile strength. However, current products, due to their thickness, cannot meet the higher energy density demands of the market.

[0003] However, during the fabrication of microporous membranes for lithium-ion batteries, process issues sometimes prevent the film thickness from meeting expectations, thus hindering energy density requirements. For example, most products on the market are 7µm, 9µm, or 12µm thick. After being installed in battery cells, the amount of electrolyte added is limited due to the thickness of the lithium-ion battery membrane, resulting in a limitation on the cell's energy density.

[0004] To improve the energy density of lithium-ion batteries, the industry is working to develop thinner separator products. Ultra-thin separators can significantly reduce the proportion of inactive materials inside the battery, providing more space for active materials and reducing the amount of electrolyte used, thereby effectively improving the battery's energy density. However, current technologies face many challenges in preparing ultra-thin separators. First, there is the issue of process stability. As the separator thickness decreases, problems such as membrane breakage, uneven thickness, and unstable pore size distribution easily occur during preparation, leading to a significant decrease in production yield. Second, there is the issue of degraded mechanical properties. Reducing the separator thickness directly leads to a decrease in its tensile strength, puncture strength, and other mechanical properties, affecting the battery's safety and reliability. Third, there is the issue of thermal stability. Ultra-thin separators are more prone to thermal shrinkage and deformation under high-temperature conditions, increasing the risk of battery thermal runaway.

[0005] Therefore, developing a technical solution that can stably prepare ultrathin, high-safety separators is of great significance for meeting the performance requirements of next-generation lithium batteries. Summary of the Invention

[0006] Based on the problems existing in the background technology, the present invention provides an ultra-thin, high-safety, and highly oriented lithium battery separator base film and its preparation method, which reduces the thickness of the lithium battery separator base film from 7 μm to 2.5 μm while ensuring the high safety performance of the lithium battery separator base film.

[0007] This invention is implemented through the following technical solutions: This invention discloses a method for preparing an ultrathin, high-safety, and highly oriented lithium battery separator base film, comprising the following steps: S1. A mixture is prepared by mixing polyethylene, polypropylene, and oriented membrane particles with white oil; S2. The mixture is melt-extruded and cooled to solidify to obtain a sheet; S3. The sheet is subjected to longitudinal stretching, transverse stretching, and biaxial stretching in sequence; S4. Extract the stretched film to remove white oil; S5. The extracted film is stretched and heat-set again to obtain an ultra-thin, high-safety, and highly oriented lithium battery separator base film.

[0008] Furthermore, the thickness of the ultra-thin, high-safety, and highly oriented lithium battery separator base film is 2.5 ± 0.5 μm.

[0009] Further, in step S1, the oriented membrane particles are specifically HC07 membrane particles with a particle size of 0.5~1.2μm. HC07 is a 7μm polyolefin lithium-ion battery base film, purchased from Taizhou Hengchuan New Energy Materials Technology Co., Ltd. The SEM image of the HC07 membrane is shown below. Figure 1 As shown, the DSC of the HC07 diaphragm is as follows: Figure 2 As shown, the HC07 membrane has a highly oriented structure. The HC07 membrane particles have a pre-formed oriented structure, which can play a nucleation-inducing role in the matrix material and promote the orientation and alignment of polymer molecular chains.

[0010] Furthermore, the method for preparing the oriented membrane particles specifically HC07 membrane particles includes the following steps: (1) Raw material preparation: Polyethylene or polypropylene is selected as raw material with a molecular weight of 100,000-600,000 and a melt index of 0.4-5 g / 10 min. No. 100 white oil is used. (2) Melt extrusion: Polyethylene and white oil are melt extruded at a temperature of 180-230℃ and then cooled and solidified; (3) Stretching and extraction: The cooled sheet is stretched biaxially with a longitudinal stretching ratio of 5-15 times and a transverse stretching ratio of 2-20 times. After biaxial stretching, it is extracted and then stretched transversely again with a stretching ratio of 1-3 times. (4) Heat setting: The stretched film is heat set at a temperature of 70-90℃; (5) Collection and freezing of membrane: The prepared membrane is collected and then placed in a liquid nitrogen environment for freezing treatment at a temperature of -196°C for 10 minutes to make the material completely brittle; the frozen material is then crushed by a mechanical crushing device to obtain granular material. (6) Screening and collection: The obtained granular material is screened and particles with a diameter of 0.5~1.2μm are collected to obtain HC07 membrane particles.

[0011] Furthermore, in step S1, the total solid content of polyethylene, polypropylene and oriented membrane particles in the mixture is 10-30%, and the white oil content is 70-90%. By controlling the solid content ratio, both the processability of extrusion molding is ensured, and a suitable basic structure is provided for subsequent stretching treatment.

[0012] Furthermore, the mass ratio of polyethylene, polypropylene, and oriented membrane particles is 85~97:1~5:2~10.

[0013] Furthermore, in step S2, a booster pump is used to feed the material into the extruder. The extruder is heated at a high temperature, with a processing temperature of 180℃~230℃, to make the mixture more uniform and improve the mechanical properties of the product. The basic melt mixture is extruded from the die and then enters the CAST cooling stage where low-temperature cooling is used, with a cooling temperature of 3~18℃.

[0014] Furthermore, in step S3, the longitudinal stretching ratio is 5 to 15 times, which causes the molecular chains of the separator material to be highly oriented in the longitudinal direction. High MD stretching ratio can improve the mechanical properties of the lithium battery separator base film. The transverse stretching ratio is 5 to 20 times. This is mainly because the molecular chain mobility of the material is enhanced under high temperature conditions. When a transverse stretching force is applied to the material, the molecular chains will be oriented along the stretching direction. This orientation makes the structure of the material more regular in the TD direction, thereby improving the mechanical properties of the material. In bidirectional stretching, the MD ratio is 2 to 5 times and the TD ratio is 1 to 3 times, which improves the degree of bidirectional orientation.

[0015] Furthermore, in step S4, the extraction process uses an ion extraction solution, and the oil content of the film after extraction is ≤1%.

[0016] Furthermore, in step S5, the stretching is TD transverse stretching, with a stretching ratio of 1 to 3 times and a stretching temperature of 100 to 150°C.

[0017] Furthermore, in step S5, the temperature of the heat setting treatment is 70~90℃, which can significantly reduce the internal stress of the film and reduce the collapse deformation of the diaphragm during winding.

[0018] The present invention also discloses an ultra-thin, high-safety, highly oriented lithium battery separator base film prepared by the above preparation method. The thickness of the separator base film is 2.5±0.5μm, and it is composed of polyethylene, polypropylene and separator particles with an oriented structure.

[0019] The beneficial effects of this invention are: 1. This invention effectively solves the process stability problem in the ultra-thinning process by using oriented membrane particles as nucleation inducing agents and combining them with an optimized multi-segment stretching process, significantly improving the production yield. The addition of HC07 membrane particles and polypropylene can serve as new nucleation sites, enabling the polymer chains to be highly oriented, further improving the orientation degree of the membrane. This significantly enhances the mechanical strength of the product while reducing its thickness, greatly improving the safety of the membrane during use.

[0020] 2. This invention significantly improves the mechanical properties of lithium battery separator base film through the synergistic effect of grain orientation induction and high-temperature high-rate stretching treatment. While reducing the product thickness, it can significantly improve the mechanical strength of the product, providing important technical support for the development of high-energy-density lithium batteries. Attached Figure Description

[0021] The accompanying drawings are provided to further explain the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 SEM image of the HC07 membrane; Figure 2 The DSC diagram of the HC07 membrane; Figure 3 SEM and DSC images of the membranes obtained by adding HC07 membrane particles in Example 1 and by not adding HC07 membrane particles in Comparative Example 2 are shown. Detailed Implementation

[0022] 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.

[0023] The polyethylene used in this invention has a molecular weight of 300,000-400,000 and a melt index of 1.2 g / 10 min (190℃, 2.16 kg); the polypropylene has a molecular weight of 400,000-500,000 and a melt index of 2.8 g / 10 min (230℃, 2.16 kg).

[0024] The HC07 separator particles used in this invention are 7-micron polyolefin lithium-ion battery base film particles with a particle size of 0.5~1.2μm. This product is available from Taizhou Hengchuan New Energy Materials Technology Co., Ltd. SEM images of the HC07 separator are shown below. Figure 1As shown, the DSC of the HC07 diaphragm is as follows: Figure 2 As shown, the HC07 membrane possesses excellent mechanical properties, with a thickness of 7.1 μm, a porosity of 34.6%, a needle punch strength of 520 gf, and both MD and TD tensile strengths exceeding 2500 kgf / cm². 2 .

[0025] HC07 membrane particles are prepared using the following process: (1) Raw material preparation: High-density polyethylene (HDPE) is selected as raw material with a molecular weight of 300,000-400,000 and a melt index of 1.2 g / 10 min (190℃, 2.16 kg). No. 100 white oil is used. (2) Melt extrusion: Polyethylene and white oil are melt extruded at 180°C and then cooled and solidified; (3) Stretching and extraction: The cooled sheet is stretched biaxially with a longitudinal stretching ratio of 10 times and a transverse stretching ratio of 10 times. After biaxial stretching, it is extracted and then stretched transversely again with a stretching ratio of 1.5 times. (4) Heat setting: The stretched film is heat set at a temperature of 80°C. (5) Collection and freezing of membrane: The prepared membrane is collected and then placed in a liquid nitrogen environment for freezing treatment at a temperature of -196°C for 10 minutes to make the material completely brittle; the frozen material is then crushed by a mechanical crushing device to obtain granular material. (6) Screening and collection: The obtained granular material is screened and particles with a diameter of 0.5~1.2μm are collected to obtain HC07 membrane particles.

[0026] The white oil used is No. 100 white oil. The production process of ultra-thin, high-safety lithium battery microporous separator base film of different thicknesses mainly depends on the stretching ratio. In addition, the stretching temperature during the lithium battery separator manufacturing process also has an important impact on the thickness of the product.

[0027] Example 1: This example describes the preparation of an ultrathin, high-safety, highly oriented lithium-ion battery separator base film with a thickness of 2.5 μm. The specific preparation steps are as follows: S1: Raw material mixing. Polyethylene, polypropylene, and HC07 membrane granules are mixed to a solid content of 25%, with polypropylene comprising 1% of the solid content, HC07 membrane granules comprising 5%, and the remainder being polyethylene. The above-mentioned mixed granules are then thoroughly mixed with white oil (No. 100), resulting in a white oil content of 75%.

[0028] S2: Melt extrusion involves mixing white oil, polyethylene, polypropylene, and HC07 diaphragm particles in a premixing reactor. After mixing, the mixture is fed into the extruder using a booster pump. The extruder is heated at high temperature, with the processing temperature set at 210℃ to ensure thorough melting and more uniform mixing, thereby improving the mechanical properties of the product. The molten mixture is extruded from the die.

[0029] S3: Cooling and curing. The extruded molten material enters the CAST cooling stage, using low-temperature cooling, with the cooling temperature controlled at 7°C, to rapidly cool and cure into a sheet.

[0030] S4: First stretching. The CAST cooling sheet continuously enters the first stretching stage, MDO longitudinal stretching, using high-ratio stretching (10 times) to ensure that the molecular chains of the separator material are highly oriented in the longitudinal direction. High-ratio stretching (MD) can improve the mechanical properties of the lithium battery separator base film.

[0031] S5: Second stretching. TD transverse stretching is performed using progressively increasing heating temperatures: preheating temperature 80℃ and stretching temperature 135℃. Under high-temperature conditions, the mobility of molecular chains is enhanced. When a transverse tensile force is applied to the material at this point, the molecular chains will align along the stretching direction. This alignment makes the material's structure more regular in the TD direction, thereby improving the material's mechanical properties.

[0032] S6: Third stretching. Perform bidirectional stretching (SBS synchronous stretching), MD ratio 4 times, TD ratio 2 times, to improve the degree of bidirectional orientation.

[0033] S7: Extraction Process. The film stretched for the third time is immersed in an extraction tank containing an ionic extractant to extract white oil, resulting in a film with an oil content of ≤1%. This ionic extractant is an environmentally friendly extractant, free of chlorine, suitable for overseas projects, and meets the safety and environmental requirements of the new energy industry in the United States, the European Union, and other regions.

[0034] S8: Fourth stretching. The extracted film is placed in an oven for the fourth TD transverse stretching, with a stretching ratio of 2 times and a stretching temperature of 130℃.

[0035] S9: Heat setting. The stretched film enters the heat setting stage, with a setting temperature of 80℃, which can significantly reduce the internal stress of the film and reduce the collapse deformation of the diaphragm during winding.

[0036] After the above process steps, an ultra-thin lithium battery separator base film with a thickness of 2.5μm can be obtained.

[0037] Example 2: This example provides an ultra-thin, high-safety lithium battery separator. The difference between this example and Example 1 is that the raw material HC07 separator particles account for 2% of the solid content.

[0038] Example 3: This example provides an ultra-thin, high-safety lithium battery separator. The difference between this example and Example 1 is that the MD ratio of the third synchronous stretching is 2 times.

[0039] Comparative Example 1: This comparative example provides an ultra-thin, high-safety lithium battery separator. The difference between this comparative example and Example 1 is that the raw materials used are a mixture of polyethylene and white oil.

[0040] Comparative Example 2: This comparative example provides an ultra-thin, high-safety lithium battery separator. The difference between this comparative example and Example 1 is that the raw materials used are a mixture of polyethylene, polypropylene and white oil, with polypropylene accounting for 1% of the solid content, and no HCO7 separator particles are added.

[0041] Comparative Example 3: This comparative example provides an ultra-thin, high-safety lithium battery separator. The difference between this comparative example and Example 1 is that the third stretching method is TD unidirectional stretching, while the total stretching ratio remains unchanged.

[0042] Comparative Example 4: This comparative example provides an ultra-thin, high-safety lithium battery separator. The difference between this comparative example and Example 1 is that the CAST cooling temperature is 20°C.

[0043] The membranes obtained in Example 1 with added HC07 membrane particles and in Comparative Example 2 without added HC07 membrane particles were characterized by SEM and DSC tests. The test results are as follows. Figure 3 As shown.

[0044] The membranes prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to performance testing, and the test results are shown in Table 1 below: Table 1. Performance comparison of membranes prepared in different embodiments and comparative examples As can be seen from Examples 1-3 and Comparative Examples 1-4 in the table above, the process method of the present invention effectively reduces the thickness of the product and improves its mechanical strength, significantly enhancing product quality while achieving thin-film production. The stretching process parameters of the diaphragm affect its mechanical properties; using CAST low-temperature cooling and adding a biaxial stretching process can increase the mechanical strength of the diaphragm. Adding HC07 diaphragm particles and polypropylene can significantly improve the needle punch strength and tensile strength of the diaphragm, increasing the needle punch strength by 66.7% and the TD tensile strength by 36.4% compared to the original PE raw material. Figure 3 It can be seen that the addition of HC07 membrane particles increased the crystallinity of the 2.5μm membrane by 16.5%, resulting in an excellent oriented structure. The added HC07 membrane particles can serve as new nucleation sites, aligning the polymer chains and further improving the orientation of the membrane, thereby increasing its mechanical strength and significantly enhancing its safety during use.

[0045] 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 preparation method of an ultra-thin high-safety high-orientation lithium battery separator base film, characterized in that, Includes the following steps: S1. A mixture is prepared by mixing polyethylene, polypropylene, and oriented membrane particles with white oil; the oriented membrane particles are specifically HC07 membrane particles with a particle size of 0.5~1.2μm. HC07 is a 7μm polyolefin lithium-ion battery base film, purchased from Taizhou Hengchuan New Energy Materials Technology Co., Ltd. S2. The mixture is melt-extruded and cooled to solidify to obtain a sheet, wherein the cooling and solidification temperature is 3~18℃; S3. The sheet is subjected to longitudinal stretching, transverse stretching, and biaxial stretching in sequence; S4. Extract the stretched film to remove white oil; S5. The extracted film is stretched and heat-set again to obtain an ultra-thin, high-safety, and highly oriented lithium battery separator base film.

2. The production method according to claim 1, characterized by, The thickness of the ultra-thin, high-safety, and highly oriented lithium battery separator base film is 2.5 ± 0.5 μm.

3. The preparation method according to claim 1, characterized in that, In step S1, the total solids content of polyethylene, polypropylene and oriented membrane particles in the mixture is 10-30%, and the white oil content is 70-90%. The mass ratio of polyethylene, polypropylene and oriented membrane particles is 85~97:1~5:2~10.

4. The production method according to claim 1, characterized by, In step S2, the temperature of melt extrusion is 180~230℃.

5. The method of claim 1, wherein, In step S3, the longitudinal stretching ratio is 5 to 15 times, the transverse stretching ratio is 5 to 20 times, and the biaxial stretching ratio is 2 to 5 times for MD and 1 to 3 times for TD.

6. The method of claim 1, wherein, In step S4, the extraction process uses an ion extraction solution, and the oil content of the film after extraction is ≤1%.

7. The preparation method according to claim 1, characterized in that, In step S5, the second stretching is a TD transverse stretching with a stretching ratio of 1 to 3 times and a stretching temperature of 100 to 150°C.

8. The preparation method according to claim 1, characterized in that, In step S5, the temperature for heat setting is 70~90℃.

9. The ultra-thin high safety high orientation lithium battery separator base film prepared by the preparation method according to any one of claims 1-8, characterized in that, The membrane base has a thickness of 2.5±0.5μm and is composed of polyethylene, polypropylene and membrane particles with an oriented structure.

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