Ultra-high molecular weight polyolefin separator and manufacturing method thereof

Abstract

The present application provides an ultra-high molecular weight polyolefin separator comprising ultra-high molecular weight polyethylene, the ultra-high molecular weight polyethylene having an average molecular weight of ≥ 1 million, wherein the ultra-high molecular weight polyolefin separator has a median pore size of 0.04 μm to 1 μm, a maximum pore size not exceeding 1.2 μm, and a puncture strength of ≥ 50 gf. The present application also provides a method for producing the ultra-high molecular weight polyolefin separator, which is safer than conventional nonwoven fabric separators and has higher ionic conductivity and a larger median pore size, thereby solving the problem that conventional nonwoven fabric separators in lithium batteries have high lithium ion permeability but are very dangerous and prone to short-circuiting the battery.

Description

[Technical Field]

[0001] The present application relates to the technical field of lithium ion battery materials, and in particular to an ultra-high molecular weight polyolefin separator and a method for producing the same. [Background technology]

[0002] As a porous, high-performance material, lithium-ion battery separators not only transport lithium ions but also separate the positive and negative electrodes to prevent short circuits caused by direct contact between the positive and negative electrode materials. As a channel for lithium ion transport within the battery, the separator's performance plays a crucial role in determining the speed of lithium ion transport and diffusion. The better the electrolyte penetration ability, the higher the ionic conductivity, which facilitates lithium ion transport within the separator, improving ionic conduction efficiency and contributing to rapid battery charging and discharging.

[0003] Currently, there are two main methods for improving the electrolyte permeation rate and ionic conductivity of the separator. The first is to improve the pore structure of the base film, increasing its porosity, reducing tortuosity, and increasing the number of straight through-holes. The higher the porosity, the more pores there are in the separator, allowing lithium ions to migrate more easily. The lower the tortuosity, the more straight through-holes there are, shortening the lithium ion transmission path and increasing the migration rate. However, the greater the number of pores and the greater the number of straight through-holes, the weaker the separator will be, increasing the risk of the separator being punctured by foreign objects during the manufacturing process, causing a short circuit. The second is to modify the surface of the base film with a coating. Typical coating layers include inorganic coating layers such as alumina or boehmite and organic coating layers such as PVDF. Traditional coating modifications can improve liquid absorption and retention capacity to a certain extent, but still have problems with low electrolyte permeation rate and ionic conductivity.

[0004] The median pore size of separators made by wet processes in conventional lithium batteries is generally 30 nm to 40 nm, which does not meet the lithium ion permeability requirements of the fast charging industry. Current fast charging separator processes generally use nonwoven fabrics, which can indeed meet the requirement for high lithium ion permeability, but they are also highly dangerous and prone to causing battery short circuits. Therefore, how to achieve both high ionic conductivity and high safety at the same time has become an important issue to be resolved. Summary of the Invention

[0005] Therefore, the main objective of the present application is to provide an ultra-high molecular weight polyolefin separator and a manufacturing method thereof so as to further optimize the above problems.

[0006] In order to solve the above technical problems, the present application aims to provide an ultra-high molecular weight polyolefin separator, which comprises ultra-high molecular weight polyethylene, and the ultra-high molecular weight polyethylene has an average molecular weight of 1 million or more, wherein the ultra-high molecular weight polyolefin separator has a median pore size of 0.04 μm to 1 μm, a maximum pore size not exceeding 1.2 μm, and a puncture strength of 50 gf or more.

[0007] In the ultra-high molecular weight polyolefin separator, the average molecular weight of the ultra-high molecular weight polyethylene is 1,000,000 to 1,500,000.

[0008] In the ultra-high molecular weight polyolefin separator as described above, the thickness of the ultra-high molecular weight polyolefin separator is 7 μm to 15 μm.

[0009] In the ultra-high molecular weight polyolefin separator, the ionic conductivity of the ultra-high molecular weight polyolefin separator is ≧1.8 mS / cm.

[0010] Another object of the present application is to provide a method for producing an ultra-high molecular weight polyolefin separator, which includes a step of producing micron or submicron sodium chloride granules, in which beads are added to sodium chloride and ball milled, and the milled product is sieved and dried, and then white oil is added and milled to obtain micron or submicron sodium chloride granules; and a raw material mixing step, in which ultra-high molecular weight polyethylene and the micron or submicron sodium chloride granules are selected and mixed with white oil, and thoroughly stirred with a mixer to form a premixed raw material, wherein the average molecular weight of the ultra-high molecular weight polyethylene is ≥ 1 million, and the content of the white oil is greater than the content of the ultra-high molecular weight polyethylene, and the content of the ultra-high molecular weight polyethylene is is greater than the content of the micron or submicron sodium chloride granules; an extrusion step, in which the premixed raw material is fed into an extruder and then extruded to form a sheet material; an extraction step, in which the extruded sheet material is first put into a mixture of acetone and water for extraction, and then ultrasonically treated, while simultaneously washing out the sodium chloride and some of the white oil in the pores, and then using pure water to wash out the acetone remaining on the surface, wiping it dry, and then putting it into pure dichloromethane for re-extraction, thereby forming an extracted raw material; and a stretching step, in which the extruded extracted raw material is put into a handkerchief machine for stretching, and the ultrahigh molecular weight polyolefin separator is obtained after stretching.

[0011] In the method for producing the ultrahigh molecular weight polyolefin separator, in the step of producing the micron or submicron sodium chloride granules, zirconium beads of at least three different sizes are added to general industrial sodium chloride, which is then crushed and sieved to dry. White oil is then added and the crushing continues. The addition of white oil prevents the sodium chloride from absorbing water, and micron or submicron sodium chloride granules are obtained after crushing. The general industrial sodium chloride is industrial salt containing sodium chloride with a purity of 99% or more and granules with a size of 0.5 mm to 2 mm, and the maximum diameter of the zirconium beads does not exceed 1 mm.

[0012] In the method for producing the ultra-high molecular weight polyolefin separator as described above, the diameter of the micron or submicron sodium chloride is 0.1 μm to 1 μm.

[0013] According to the method for manufacturing an ultra-high molecular weight polyolefin separator as described above, in the raw material mixing step, based on the total weight of the pre-mixed raw materials, the weight ratio of the content of the micron or submicron sodium chloride granules is 1 wt% to 20 wt%, the weight ratio of the content of the ultra-high molecular weight polyethylene is 1 wt% to 29 wt%, and the weight ratio of the content of the white oil is 60 wt% to 80 wt%.

[0014] In the method for producing the ultra-high molecular weight polyolefin separator, the average molecular weight of the ultra-high molecular weight polyethylene is 1,000,000 to 1,500,000.

[0015] In the method for producing the ultra-high molecular weight polyolefin separator as described above, in the step of producing the micron or submicron sodium chloride granules, the micron or submicron sodium chloride granules include spherical, pseudo-spherical or irregular spherical granules, and the particle size distribution thereof is approximately normal, with 80% of the sodium chloride granules being pseudo-spherical.

[0016] In the present invention, micron or submicron sodium chloride granules are introduced during the separator production process to limit the content and particle size distribution of sodium chloride granules, thereby controlling the separator to have optimal median and maximum pore sizes. A special process of extraction and stretching is also employed to ensure that the pore size is appropriate, i.e., neither too large nor too small. As a result, the resulting ultra-high molecular weight polyolefin separator has both high safety and high ionic conductivity. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a process block diagram illustrating a method for producing an ultra-high molecular weight polyolefin separator according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] In order to further explain the technical means and effects adopted by the present application to achieve the intended purpose of the invention, the ultra-high molecular weight polyolefin separator proposed by the present application and its manufacturing method, as well as its specific embodiments, structure, features and effects will now be described in detail in combination with the accompanying drawings and specific examples.

[0019] An ultra-high molecular weight polyolefin separator according to one embodiment of the present application comprises ultra-high molecular weight polyethylene, the ultra-high molecular weight polyethylene having an average molecular weight of 1,000,000 or more, the ultra-high molecular weight polyolefin separator having a median pore size of 0.04 μm to 1 μm, a maximum pore size not exceeding 1.2 μm, and a puncture strength of 50 gf or more.

[0020] In some embodiments, the ultra-high molecular weight polyethylene has an average molecular weight of 1,000,000 to 1,500,000. In some embodiments, the ultra-high molecular weight polyolefin separator may further comprise high-density polyethylene, high-molecular weight polyethylene, or an ultra-high molecular weight polyethylene graft material in addition to the ultra-high molecular weight polyethylene. In addition to the ultra-high molecular weight polyethylene, the ultra-high molecular weight polyethylene accounts for at least 80 wt% of the total weight of the ultra-high molecular weight polyolefin separator. In some embodiments, the ultra-high molecular weight polyolefin separator has a median pore size of 0.04 μm to 0.5 μm, preferably 0.04 μm, 0.05 μm, 0.1 μm, or 0.5 μm. In some embodiments, the ultra-high molecular weight polyolefin separator has a maximum pore size of 0.11 μm to 1.2 μm, preferably 0.11 μm, 0.15 μm, 0.25 μm, or 1.2 μm. In some embodiments, the puncture strength of the ultra-high molecular weight polyolefin separator is 50 gf to 150 gf, preferably 50 gf, 80 gf, 90 gf, or 150 gf.

[0021] In some embodiments, the thickness of the ultra-high molecular weight polyolefin separator is limited to 15 μm, the median pore size to 100 nm, and the maximum pore size to 0.25 μm, thereby ensuring battery safety (i.e., thickness requirements) while also ensuring higher ionic conductivity. Since ionic permeability is directly determined by pore size, a larger pore size, such as 1 μm, can achieve a higher ionic conductivity of ≥ 3 mS / cm. In other embodiments, the thickness of the ultra-high molecular weight polyolefin separator is 7 μm to 15 μm, preferably 7 μm, 9 μm, 12 μm, or 15 μm, and the ionic conductivity is ≥ 1.8 mS / cm, preferably 1.8 mS / cm, 2.1 mS / cm, 3 mS / cm, or 4.8 mS / cm.

[0022] As shown in FIG. 1, in another embodiment, the present application provides a method 1 for producing an ultra-high molecular weight polyolefin separator, which includes a step S11 for producing micron or submicron sodium chloride granules, in which beads are added to sodium chloride and ball milled, and the resulting mixture is sieved and dried, and then white oil is added and milled to obtain micron or submicron sodium chloride granules; and a raw material mixing step S12, in which ultra-high molecular weight polyethylene and the micron or submicron sodium chloride granules are selected and mixed with white oil, and the mixture is thoroughly stirred with a mixer to form a pre-mixed raw material, wherein the average molecular weight of the ultra-high molecular weight polyethylene is ≧1 million, and the content of the white oil is greater than that of the ultra-high molecular weight polyethylene, and the ultra-high molecular weight polyethylene is mixed with the white oil to obtain a pre-mixed raw material. the content of which is greater than the micron or submicron sodium chloride granules; an extrusion step S13, in which the premixed raw material is fed into an extruder and then extruded to form a sheet material; an extraction step S14, in which the extruded sheet material is first put into a mixture of acetone and water for extraction, and then ultrasonically treated, while simultaneously washing out the sodium chloride and some of the white oil in the pores, and then using pure water to wash out the acetone remaining on the surface, wiping it dry, and then putting it into pure dichloromethane for re-extraction, thereby forming an extracted raw material; and a stretching step S15, in which the extruded extracted raw material is put into a handkerchief machine for stretching, and the ultrahigh molecular weight polyolefin separator is obtained after stretching.

[0023] In some embodiments, in step S11 of producing the micron or submicron sodium chloride granules, zirconium beads of three sizes (large, medium, and small) are added to general industrial sodium chloride, and after pulverization for four hours, the mixture is sieved and dried, and then dried for one hour. White oil is then added and the mixture is continued to be pulverized. The addition of white oil prevents the sodium chloride from absorbing water. After 4 hours of pulverization, homogeneous micron or submicron sodium chloride granules are obtained. The general industrial sodium chloride is industrial salt containing sodium chloride with a purity of 99% or more and a granule size of 0.5 mm to 2 mm (millimeters). The diameter of the small size zirconium beads is 0.2 mm, the diameter of the medium size zirconium beads is 0.5 mm, and the diameter of the large size zirconium beads is 1 mm. Therefore, the diameter of the largest size of the zirconium beads in the embodiments does not exceed 1 mm.

[0024] However, without being limited thereto, in other embodiments, the zirconium beads can be replaced with other grinding tools capable of grinding conventional industrial sodium chloride so that the maximum particle size of the sodium chloride granules reaches 1 μm (micron) or less, and any of these can be applied to the present invention. In step S11 of producing the micron or submicron sodium chloride granules, the micron or submicron sodium chloride granules include spherical, pseudo-spherical, or irregular spherical shapes, and their particle size distribution is approximately normal, with 80% of the sodium chloride granules being pseudo-spherical. It can be understood that such shape and particle size distribution of the sodium chloride granules contributes to the pore size distribution and shape of the ultra-high molecular weight polyolefin separator.

[0025] In some embodiments, in the raw material mixing step S12, the ultra-high molecular weight polyethylene and the prepared micron or submicron sodium chloride granules are selected and mixed with white oil, and thoroughly stirred with a mixer to form a premixed raw material, wherein the average molecular weight of the ultra-high molecular weight polyethylene is 1 million to 1.5 million, and based on the total weight of the premixed raw material, the micron or submicron sodium chloride granules account for 10 wt%, the ultra-high molecular weight polyethylene accounts for 20 wt%, and the white oil accounts for 70 wt%, and the diameter of the micron or submicron sodium chloride granules is 0.1 μm to 1 μm. However, without being limited thereto, in another embodiment, based on the total weight of the premixed raw material, the micron or submicron sodium chloride granules preferably occupy 1 wt% to 20 wt%, more preferably occupy a range between any two values ​​of 1 wt%, 10 wt%, and 20 wt%, the ultra-high molecular weight polyethylene occupies 1 wt% to 29 wt%, more preferably occupy a range between any two values ​​of 1 wt%, 16 wt%, 18 wt%, 19.8 wt%, and 29 wt%, and the white oil occupies 60 wt% to 80 wt%, more preferably 60 wt%, 64 wt%, 72 wt%, 79.2 wt%, and 80 wt%, The weight percentage of the micron or submicron sodium chloride granules may range between any two values ​​of t%, preferably the micron or submicron sodium chloride granules account for 1 wt%, the ultra-high molecular weight polyethylene account for 19.8 wt%, and the white oil account for 79.2 wt%, preferably the micron or submicron sodium chloride granules account for 10 wt%, the ultra-high molecular weight polyethylene account for 18 wt%, and the white oil account for 72 wt%, preferably the micron or submicron sodium chloride granules account for 20 wt%, the ultra-high molecular weight polyethylene account for 16 wt%, and the white oil account for 64 wt%.

[0026] In this embodiment, in the extrusion step S13, 10% micron or submicron sodium chloride granules, 20% ultra-high molecular weight polyethylene, and 70% white oil are mixed and added to an extruder to form a homogeneous sheet material. In the extraction step S14, the extruded sheet material is first placed in a mixture of acetone and water for extraction, followed by ultrasonic treatment to simultaneously wash out the sodium chloride and some of the white oil from the pores. Next, the acetone remaining on the surface is washed with pure water, wiped dry, and then placed in pure dichloromethane for re-extraction.

[0027] In addition, this embodiment employs a special manufacturing process of extraction followed by stretching, where the first extraction step is to remove pore-forming agents such as white oil and sodium chloride, which causes the pores to retract and ensure that after reheating and stretching, the pores are expanded to an appropriate size, neither too large nor too small. Conversely, if a manufacturing process of stretching followed by extraction is employed, due to the presence of pore-forming agents, the pore size is relatively fixed after the initial sheet formation and only slightly expanded during the stretching process, so this manufacturing process cannot effectively enlarge the pores.

[0028] Some parameters are designed for experiment in this invention, and the experimental process and results are excerpted as follows:

[0029] Example 1

[0030] In this embodiment, the micron or submicron sodium chloride granules had a particle size of 100 nm. The rapid charging separator was manufactured as follows: Ultra-high molecular weight polyethylene powder and white oil were mixed in a mass ratio of 2:8, and then further mixed with sodium chloride granules to form a premixed raw material. The sodium chloride granules accounted for 1% of the premixed raw material, and the remainder was the ultra-high molecular weight polyethylene powder and white oil mixed in a mass ratio of 2:8. The premixed raw material was melt-mixed in a twin-screw extruder, cooled, extracted, and stretched to form the ultra-high molecular weight polyolefin separator. The stretching requirement was 10x10.

[0031] Example 2

[0032] In the rapid charging separator of this example, the weight ratio of the sodium chloride content was 5%, and all other factors were the same as in Example 1.

[0033] Example 3

[0034] In this embodiment, the micron or submicron sodium chloride granules in the fast charging separator had a particle size of 100 nm. The fast charging separator was manufactured as follows: Ultra-high molecular weight polyethylene powder and white oil were mixed in a 2:8 mass ratio, and then further mixed with sodium chloride granules to form a premixed raw material. The sodium chloride granules accounted for 10% of the premixed raw material, and the remainder was the ultra-high molecular weight polyethylene powder and white oil mixed in a 2:8 mass ratio. The premixed raw material was melt-mixed in a twin-screw extruder, cooled, extracted, and stretched to form the ultra-high molecular weight polyolefin separator. The stretching requirement was 6.5*6.5.

[0035] Example 4

[0036] In the rapid charging separator of this example, the weight ratio of the content of sodium chloride was 20%, and all other factors were the same as those of Example 3.

[0037] Comparative Example 1

[0038] Ultra-high molecular weight polyethylene powder and white oil are melt-mixed in a mass ratio of 3:7 through a twin-screw extruder, and then cooled, stretched and extracted to form a polyethylene base film.

[0039] Comparative Example 2

[0040] Ultra-high molecular weight polyethylene powder and white oil were mixed in a mass ratio of 2:8, and then further mixed with sodium chloride granules to form a pre-mixed raw material, of which the weight ratio of sodium chloride granules was 10%, and the remainder was ultra-high molecular weight polyethylene powder and white oil mixed in a mass ratio of 2:8. The pre-mixed raw material was melt-mixed in a twin-screw extruder, cooled, stretched and extracted, and then formed into a polyethylene base film.

[0041] Comparative Example 3

[0042] Ultra-high molecular weight polyethylene powder and white oil were melt-mixed in a mass ratio of 3:7 through a twin-screw extruder, and after cooling, extraction and stretching, a polyethylene base film was formed. Table 1: Median pore diameters and other values ​​in Examples and Comparative Examples JPEG2025530573000002.jpg221170

[0043] The above experimental results are summarized in Table 1. As can be seen from Table 1, Examples 1, 2, 3, and 4 show that the sodium chloride content mainly affects the median pore size and the uniformity of the pore size distribution. Example 4 shows that when the sodium chloride content is 20%, the maximum pore size is over 1000 nm, which cannot ensure safety. When the stretching ratio after increasing the solid content is 10x10, the thickness tends to decrease. Comparative Examples 1 and 2 use a conventional stretching process, i.e., stretching followed by extraction. However, this process results in small pore size and lower ionic conductivity than before, making it impossible to achieve fast charging of the battery. Comparative Example 3 uses stretching followed by extraction, which results in uneven pore size distribution and poor strength.

[0044] In summary, to improve the safety of current conventional wet-process separators for lithium batteries, the present invention proposes an ultra-high molecular weight polyolefin fast-charging separator. The separator manufacturing process differs from that of the sodium chloride raw material, which has a limited particle size range and is manufactured using a specific process. The number of extractions, the specific extractant, and the detailed steps are different, resulting in a fast-charging separator with a larger median pore size of 0.04 μm to 1 μm. This fast-charging separator has high ionic conductivity and provides high safety. Therefore, this solution actually solves the problem of prior art separators being unable to simultaneously achieve fast charging and high safety.

[0045] The above is only an embodiment of the present application and is not intended to limit the present application in any way. Although the present application has been disclosed above through specific embodiments, these embodiments are not used to limit the present application. Those skilled in the art can use the technical content disclosed above to make slight changes or modifications to equivalent embodiments as equivalent changes, as long as they do not deviate from the scope of the technical solutions of the present application. All simple changes, equivalent changes and modifications made to the above embodiments according to the technical substance of the present application, as long as they do not deviate from the technical solutions of the present application, still belong to the scope of the technical solutions of the present application.

Claims

1. An ultra-high molecular weight polyolefin separator comprising ultra-high molecular weight polyethylene, wherein the ultra-high molecular weight polyethylene has an average molecular weight of 1,000,000 or more, wherein the ultra-high molecular weight polyolefin separator has a median pore size of 0.04 μm to 1 μm, a maximum pore size not exceeding 1.2 μm, and a puncture strength of 50 gf or more.

2. 2. The ultra-high molecular weight polyolefin separator according to claim 1, wherein the average molecular weight of the ultra-high molecular weight polyethylene is 1,000,000 to 1,500,000.

3. 2. The ultra-high molecular weight polyolefin separator according to claim 1, wherein the thickness of the ultra-high molecular weight polyolefin separator is 7 μm to 15 μm.

4. 2. The ultra-high molecular weight polyolefin separator according to claim 1, wherein the ionic conductivity of the ultra-high molecular weight polyolefin separator is ≧1.8 mS / cm.

5. A method for producing an ultra-high molecular weight polyolefin separator, comprising: a step of preparing micron or submicron sodium chloride granules, which includes adding beads to sodium chloride and ball milling the mixture, sieving and drying the resulting mixture, and then adding white oil to continue milling the mixture to obtain micron or submicron sodium chloride granules; a raw material mixing step of selecting ultra-high molecular weight polyethylene and the micron or submicron sodium chloride granules and mixing them with white oil, and thoroughly stirring them with a mixer to form a premixed raw material, wherein the average molecular weight of the ultra-high molecular weight polyethylene is ≥ 1 million, and the content of the white oil is greater than the content of the ultra-high molecular weight polyethylene, and the content of the ultra-high molecular weight polyethylene is greater than the content of the micron or submicron sodium chloride granules; an extrusion step of adding the premixed ingredients to an extruder and then extruding to form a sheet material; an extraction step in which the extruded sheet material is firstly put into a mixture of acetone and water for extraction, then subjected to ultrasonic treatment, and simultaneously washed away the sodium chloride and some of the white oil in the pores; then washed away the acetone remaining on the surface with pure water, wiped dry, and then put into pure dichloromethane for re-extraction, thereby forming an extracted raw material; a stretching step in which the extracted raw material obtained by extrusion molding is placed in a handkerchief machine to stretch it, and after stretching, the ultra-high molecular weight polyolefin separator is obtained.

6. 6. The method for producing an ultrahigh molecular weight polyolefin separator according to claim 5, wherein in the step of producing the micron or submicron sodium chloride granules, zirconium beads of at least three different sizes are added to common industrial sodium chloride, which is then crushed and sieved to dry, and then white oil is added and the crushing continues to obtain micron or submicron sodium chloride granules after crushing, wherein the common industrial sodium chloride is an industrial salt containing sodium chloride with a purity of 99% or more and a granule size of 0.5 mm to 2 mm, and the maximum diameter of the zirconium beads does not exceed 1 mm.

7. 6. The method for producing an ultra-high molecular weight polyolefin separator according to claim 5, wherein the diameter of the micron or submicron sodium chloride granules is 0.1 μm to 1 μm.

8. 6. The method for producing an ultrahigh molecular weight polyolefin separator according to claim 5, wherein in the raw material mixing step, based on the total weight of the premixed raw materials, the weight ratio of the content of the micron or submicron sodium chloride granules is 1 wt% to 20 wt%, the weight ratio of the content of the ultrahigh molecular weight polyethylene is 1 wt% to 29 wt%, and the weight ratio of the content of the white oil is 60 wt% to 80 wt%.

9. 6. The method for producing an ultra-high molecular weight polyolefin separator according to claim 5, wherein the average molecular weight of the ultra-high molecular weight polyethylene is 1,000,000 to 1,500,000.

10. 6. The method for producing an ultrahigh molecular weight polyolefin separator according to claim 5, wherein in the step of producing micron or submicron sodium chloride granules, the micron or submicron sodium chloride granules include spherical, pseudo-spherical or irregular spherical granules, and the particle size distribution thereof is approximately normal, with 80% of the sodium chloride granules being pseudo-spherical.

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