Ultra-high molecular weight polyolefin diaphragm and preparation method thereof

Abstract

The present application provides an ultra-high molecular weight polyolefin separator, which includes ultra-high molecular weight polyethylene, wherein the average molecular weight of the ultra-high molecular weight polyethylene is ≥ 1 million, wherein the median pore size of the ultra-high molecular weight polyolefin separator is 0.0.4μm to 1μm, the maximum pore size does not exceed 1.2μm, and the puncture strength is ≥ 50gf. At the same time, the present application also provides a method for preparing the ultra-high molecular weight polyolefin separator. Since the polyolefin separator is safer than ordinary non-woven separators and has higher ionic conductivity and larger median pore size, it solves the problem that ordinary lithium battery non-woven separators have high lithium ion transmission rate but are very dangerous and easily cause battery short circuits.

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 preparation method thereof. Background Art

[0002] As a porous, highly functional material, lithium-ion battery separators not only transport lithium ions but also isolate the positive and negative electrodes, preventing direct contact and short circuits. As a channel for lithium ion transmission within the battery, lithium-ion battery separators are crucial to the speed of lithium ion transmission and diffusion. Their performance plays a key role in battery characteristics. A separator with better electrolyte wettability exhibits higher ionic conductivity, facilitating lithium ion transmission within the separator, increasing ion conduction efficiency, and facilitating rapid battery charging and discharging.

[0003] At present, there are two main means to improve the electrolyte infiltration speed and ion conductivity of the diaphragm. One is to improve the pore structure of the base membrane, increase the porosity of the base membrane, reduce the tortuosity and increase the through holes. The greater the porosity, the more pores in the diaphragm, and the easier it is for lithium ions to migrate. The smaller the tortuosity and the more through holes, the shorter the lithium ion transmission path and the faster the migration speed. However, the large number of holes and the increase in through holes will lead to a decrease in the strength of the diaphragm. During the preparation process, the risk of the diaphragm being pierced by foreign objects and causing a short circuit will increase; the second is to perform coating modification on the surface of the base membrane. Common coatings include inorganic coatings such as alumina or boehmite, and organic coatings such as PVDF. Although traditional coating modifications can improve the liquid absorption and liquid retention capabilities to a certain extent, there are still problems with slow electrolyte infiltration and low ionic conductivity.

[0004] The median pore size of ordinary lithium battery wet-process diaphragms is generally between 30nm and 40nm, which cannot meet the fast charging industry's requirements for lithium ion pass rate. The current fast charging diaphragm process is generally a non-woven fabric process. This process can indeed meet a high lithium ion pass rate, but it is very dangerous and can easily cause battery short circuits. Therefore, how to simultaneously have high ionic conductivity and high safety performance is a key issue that needs to be solved. Summary of the Invention

[0005] Therefore, the main purpose of this application is to provide an ultra-high molecular weight polyolefin membrane and a preparation method thereof to further optimize the above-mentioned problems.

[0006] In order to solve the above technical problems, the purpose of this application is to provide an ultra-high molecular weight polyolefin membrane, characterized in that it includes ultra-high molecular weight polyethylene, the average molecular weight of the ultra-high molecular weight polyethylene is ≥1 million, wherein the median pore size of the ultra-high molecular weight polyolefin membrane is 0.04μm~1μm, the maximum pore size does not exceed 1.2μm, and the puncture strength is ≥50gf.

[0007] The ultra-high molecular weight polyolefin separator as described above, wherein the average molecular weight of the ultra-high molecular weight polyethylene is 1 million to 1.5 million.

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

[0009] The ultra-high molecular weight polyolefin membrane as described above, wherein the ionic conductivity of the ultra-high molecular weight polyolefin membrane is ≥1.8 mS / cm.

[0010] Another object of the present application is to provide a method for preparing an ultra-high molecular weight polyolefin diaphragm, characterized in that it includes the following steps: a step of preparing micron-sized or submicron-sized sodium chloride particles: adding balls to sodium chloride for ball milling, sieving and drying after grinding, then adding white oil and continuing grinding to obtain micron-sized or submicron-sized sodium chloride particles; a step of mixing raw materials: selecting ultra-high molecular weight polyethylene and the micron-sized or submicron-sized sodium chloride particles and mixing them in white oil, and fully stirring them with a stirrer 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 that of the ultra-high molecular weight polyethylene, and the The content of ultra-high molecular weight polyethylene is greater than the micron-sized or submicron-sized sodium chloride particles; an extrusion molding step: adding the premixed raw material into an extruder, followed by extrusion molding to form a cast sheet; an extraction step: first extracting the cast sheet obtained by extrusion molding in a mixture of acetone and water, then performing ultrasound to simultaneously clean out the sodium chloride and part of the white oil in the pores, then cleaning the residual acetone on the surface with pure water, wiping it dry, and then placing it in pure dichloromethane for re-extraction to form an extracted raw material; and a stretching step: subsequently placing the extracted raw material obtained by extrusion molding in a handkerchief machine for stretching, and obtaining the ultra-high molecular weight polyolefin diaphragm after stretching.

[0011] As described above, in the preparation method of the ultra-high molecular weight polyolefin diaphragm, in the step of preparing the micron- or submicron-sized sodium chloride particles, zirconium beads mixed from at least three different sizes are added to ordinary industrial-grade sodium chloride, which is ground, sieved, and dried, and then white oil is added and continued to be ground. The addition of white oil can ensure that the sodium chloride does not absorb water. After grinding, micron- or submicron-sized sodium chloride particles are obtained, wherein the ordinary industrial-grade sodium chloride is industrial salt with a sodium chloride purity of more than 99% and a particle 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 preparing the ultra-high molecular weight polyolefin separator as described above, the diameter of the micron-sized or submicron-sized sodium chloride particles is between 0.1 μm and 1 μm.

[0013] The method for preparing an ultra-high molecular weight polyolefin diaphragm as described above, wherein, in the raw material mixing step, based on the total weight of the premixed raw materials, the weight proportion of the micron-sized or submicron-sized sodium chloride particles is 1wt% to 20wt%, the weight proportion of the ultra-high molecular weight polyethylene content is 1wt% to 29wt%, and the weight proportion of the white oil is 60wt% to 80wt%.

[0014] In the method for preparing the ultra-high molecular weight polyolefin separator as described above, the average molecular weight of the ultra-high molecular weight polyethylene is 1 million to 1.5 million.

[0015] The method for preparing the ultra-high molecular weight polyolefin diaphragm as described above, wherein, in the step of preparing the micron-sized or submicron-sized sodium chloride particles, the micron-sized or submicron-sized sodium chloride particles include spherical, quasi-spherical or irregular spherical shapes, and their particle size distribution is approximately normal distribution, wherein 80% of the sodium chloride particles are quasi-spherical.

[0016] The present application introduces micron-sized or submicron-sized sodium chloride particles during the membrane production process, controls the membrane to achieve an optimal median pore size and maximum pore size by limiting the sodium chloride particle content and particle size distribution, and adopts a special process of first extracting and then stretching to ensure the pore size is appropriate, neither too large nor too small. As a result, the resulting ultra-high molecular weight polyolefin membrane has both high safety performance and ionic conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic block diagram of a process for preparing an ultra-high molecular weight polyolefin membrane according to an embodiment of the present application. DETAILED DESCRIPTION

[0018] In order to further illustrate the technical means and effects adopted by this application to achieve the predetermined purpose of the invention, the following, in combination with the drawings and specific embodiments, describes in detail an ultra-high molecular weight polyolefin diaphragm and its preparation method proposed in this application, its specific implementation method, structure, characteristics and effects.

[0019] An ultra-high molecular weight polyolefin membrane according to an embodiment of the present application is characterized in that it comprises ultra-high molecular weight polyethylene, the average molecular weight of the ultra-high molecular weight polyethylene being ≥ 1 million, wherein the median pore size of the ultra-high molecular weight polyolefin membrane is 0.04 μm to 1 μm, the maximum pore size thereof does not exceed 1.2 μm, and the puncture strength thereof is ≥ 50 gf.

[0020] In some embodiments, the average molecular weight of the ultra-high molecular weight polyethylene is 1 million to 1.5 million. In some embodiments, in addition to ultra-high molecular weight polyethylene, the ultra-high molecular weight polyolefin membrane may also use high-density polyethylene, high molecular weight polyethylene, or ultra-high molecular weight polyethylene grafted material. Under conditions other than ultra-high molecular weight polyethylene, the ultra-high molecular weight polyethylene accounts for at least 80 wt% based on the total weight of the ultra-high molecular weight polyolefin membrane. In some embodiments, the median pore size of the ultra-high molecular weight polyolefin membrane is 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 maximum pore size of the ultra-high molecular weight polyolefin membrane is 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 ultra-high molecular weight polyolefin (UHMWPE) separator is limited to a thickness of 15 μm, a median pore size of 100 nm, and a maximum pore size of 0.25 μm. This ensures both battery safety (i.e., thickness) and high ionic conductivity. This is because ion flow is directly determined by pore size, and a larger pore size of 1 μm can achieve a higher ionic conductivity of ≥3 mS / cm. In other embodiments, the UHMWPE separator has a thickness of 7 μm to 15 μm, preferably 7 μm, 9 μm, 12 μm, or 15 μm, and an ionic conductivity of ≥1.8 mS / cm, preferably 1.8 mS / cm, 2.1 mS / cm, 3 mS / cm, or 4.8 mS / cm.

[0022] See also Figure 1In another embodiment, the present application provides a method 1 for preparing an ultra-high molecular weight polyolefin membrane, characterized in that it comprises the following steps: a micron- or submicron-sized sodium chloride particle preparation step S11: adding balls to sodium chloride for ball milling, sieving and drying after grinding, then adding white oil and continuing grinding to obtain micron- or submicron-sized sodium chloride particles; a raw material mixing step S12: selecting ultra-high molecular weight polyethylene and the micron- or submicron-sized sodium chloride particles and mixing them in white oil, and fully stirring them with a stirrer 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 that of the ultra-high molecular weight polyethylene, and the ultra-high molecular weight polyethylene is ≥1 million. The content of high molecular weight polyethylene is greater than the micron-sized or submicron-sized sodium chloride particles; an extrusion molding step S13: adding the premixed raw material into an extruder, followed by extrusion molding to form a casting; an extraction step S14: firstly extracting the casting obtained by extrusion molding in a mixture of acetone and water, then performing ultrasound to simultaneously clean out the sodium chloride and part of the white oil in the pores, then cleaning the residual acetone on the surface with pure water, wiping it dry and placing it in pure dichloromethane for re-extraction to form an extracted raw material; and a stretching step S15: then placing the extracted raw material obtained by extrusion molding into a handkerchief machine for stretching, and obtaining the ultra-high molecular weight polyolefin diaphragm after stretching.

[0023] In some embodiments, in the micron- or submicron-sized sodium chloride particle preparation step S11, zirconium beads of a mixture of large, medium, and small sizes are added to ordinary industrial-grade sodium chloride. After grinding for 4 hours, the beads are opened and sieved, and then dried for 1 hour after sieving. Then, white oil is added and grinding is continued. The addition of white oil can ensure that the sodium chloride does not absorb water. After grinding for 4 hours, uniform micron- or submicron-sized sodium chloride particles are obtained. The ordinary industrial-grade sodium chloride is industrial salt with a sodium chloride purity of more than 99% and a particle size of 0.5 mm to 2 mm (millimeter). The diameter of the small-sized zirconium beads is 0.2 mm, the diameter of the medium-sized zirconium beads is 0.5 mm, and the diameter of the large-sized zirconium beads is 1 mm. Therefore, the maximum diameter of the zirconium beads described in the embodiment does not exceed 1 mm.

[0024] However, it is not limited to this. In other embodiments, the zirconium beads can be replaced with other grinding equipment, and ordinary industrial-grade sodium chloride can be ground so that the maximum particle size of the sodium chloride particles reaches 1 μm (micrometer), which can be applied to the present invention. In the micron- or submicron-grade sodium chloride particle preparation step S11, the micron- or submicron-grade sodium chloride particles include spherical, quasi-spherical or irregular spherical shapes, and their particle size distribution is approximately normal, wherein 80% of the sodium chloride particles are quasi-spherical. It is understood that the shape and particle size distribution of such sodium chloride particles will be conducive to forming the size distribution and shape of the pore size of the ultra-high molecular weight polyolefin diaphragm.

[0025] In some embodiments, in the raw material mixing step S12, ultra-high molecular weight polyethylene is selected and mixed with the previously prepared micron or submicron sodium chloride particles in white oil, and the mixture is fully stirred with a blender 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 particles account for 10wt%, the ultra-high molecular weight polyethylene accounts for 20wt%, and the white oil accounts for 70wt%, and the diameter of the micron or submicron sodium chloride particles is between 0.1μm and 1μm. However, not limited thereto, in other embodiments, based on the total weight of the premixed raw materials, the micron- or submicron-sized sodium chloride particles preferably account for 1 wt% to 20 wt%, more preferably in the range between any two values ​​of 1 wt%, 10 wt%, and 20 wt%, the ultra-high molecular weight polyethylene accounts for 1 wt% to 29 wt%, more preferably in the range between any two values ​​of 1 wt%, 16 wt%, 18 wt%, 19.8 wt%, and 29 wt%, the white oil accounts for 60 wt% to 80 wt%, more preferably 60 wt%, 64 wt%, 72 wt%. In the range between any two values ​​of %, 79.2wt%, and 80wt%, preferably, the micron or submicron sodium chloride particles account for 1wt%, the ultra-high molecular weight polyethylene accounts for 19.8wt%, and the white oil accounts for 79.2wt%. Preferably, the micron or submicron sodium chloride particles account for 10wt%, the ultra-high molecular weight polyethylene accounts for 18wt%, and the white oil accounts for 72wt%. Preferably, the micron or submicron sodium chloride particles account for 20wt%, the ultra-high molecular weight polyethylene accounts for 16wt%, and the white oil accounts for 64wt%.

[0026] In this embodiment, in the extrusion molding step S13, a mixture containing 10% micron- or submicron-sized sodium chloride particles, 20% ultra-high molecular weight polyethylene, and 70% white oil is added to an extruder and extruded to form a uniform cast sheet. In the extraction step S14, the cast sheet obtained by extrusion is first extracted in a mixture of acetone and water, then ultrasonically cleaned to remove the sodium chloride and some white oil from the pores. The remaining acetone on the surface is then cleaned with pure water, dried, and then re-extracted in pure dichloromethane.

[0027] It's worth noting that this embodiment utilizes a unique process involving extraction followed by stretching. The initial extraction step removes pore-forming agents such as white oil and sodium chloride, causing the pores to shrink. The subsequent warming and stretching ensures that the pores expand to an appropriate size, neither too large nor too small. In contrast, a process that involves stretching followed by extraction would not effectively expand the pores due to the presence of pore-forming agents, resulting in a relatively fixed pore size after the initial casting. The pores would then only expand slightly during the stretching process, making this process ineffective in achieving effective pore expansion.

[0028] In the invention, some parameters are designed to conduct experiments, and the experimental process and results are summarized as follows.

[0029] Example 1

[0030] The fast-charging diaphragm of this embodiment has a micron- or submicron-sized sodium chloride particle size of 100 nm. The fast-charging diaphragm is prepared as follows: ultra-high molecular weight polyethylene powder and white oil are mixed at a mass ratio of 2:8, and then further mixed with sodium chloride particles to form a premix. The sodium chloride particles in the premix account for 1% by weight, and the remainder is ultra-high molecular weight polyethylene powder and white oil mixed at a mass ratio of 2:8. The mixture is melt-mixed in a twin-screw extruder, cooled, extracted, and stretched to form the ultra-high molecular weight polyolefin diaphragm. The stretching condition is 10*10.

[0031] Example 2

[0032] The fast-charging diaphragm of this embodiment is the same as that of Example 1 except that the weight percentage of sodium chloride is 5%.

[0033] Example 3

[0034] In the fast-charging diaphragm of this embodiment, the micron- or submicron-sized sodium chloride particles have a particle size of 100 nm. The fast-charging diaphragm is prepared as follows: ultra-high molecular weight polyethylene powder and white oil are mixed at a mass ratio of 2:8, and then further mixed with sodium chloride particles to form a premixed raw material. The sodium chloride particles in the premixed raw material account for 10% by weight, and the rest is ultra-high molecular weight polyethylene powder and white oil mixed at a mass ratio of 2:8. The mixture is melt-mixed in a twin-screw extruder, cooled, extracted, and stretched to form the ultra-high molecular weight polyolefin diaphragm. The stretching condition is 6.5*6.5.

[0035] Example 4

[0036] The fast-charging diaphragm of this embodiment is the same as that of Example 3 except that the weight percentage of sodium chloride is 20%.

[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-based film.

[0039] Comparative Example 2

[0040] Ultra-high molecular weight polyethylene powder and white oil are mixed in a mass ratio of 2:8, and then further mixed with sodium chloride particles to form a premixed raw material. The weight proportion of sodium chloride particles in the premixed raw material is 10%, and the rest is ultra-high molecular weight polyethylene powder and white oil mixed in a mass ratio of 2:8. The mixture is melt-mixed through a twin-screw extruder, and then cooled, stretched, and extracted to form a polyethylene-based film.

[0041] Comparative Example 3

[0042] 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, extracted, and stretched to form a polyethylene-based film. Table 1: Median pore diameter and other values ​​in the examples and comparative examples

[0043]

[0044]

[0045] The above experimental results are summarized as shown in Table 1. From the results in Table 1, it can be seen that in Examples 1, 2, 3, and 4, the content of sodium chloride mainly affects the size of the median pore size and the uniformity of the pore size distribution. It can be seen from Example 4 that when the sodium chloride content is 20%, the maximum pore size is above 1000 nm, and the safety performance cannot be guaranteed. When the solid content is increased and the stretching ratio is 10*10, the thickness tends to decrease; Comparative Examples 1 and 2 adopt a general stretching process, that is, stretching first and then extracting, but this process has a smaller pore size and a smaller ionic conductivity than before, and cannot achieve fast charging of the battery. Comparative Example 3 adopts extraction first and then stretching, and the pore size distribution is uneven and the strength is not very high.

[0046] In summary, to improve the safety performance of today's common wet-process lithium battery diaphragms, the present invention proposes an ultra-high molecular weight polyolefin fast-charging diaphragm. This fast-charging diaphragm is prepared by a specific process using a sodium chloride raw material with a limited particle size range. The diaphragm preparation process is different, and the number of extractions, specific extractants, and detailed steps are different to obtain a fast-charging diaphragm with a larger median pore size of 0.04 μm to 1 μm. This fast-charging diaphragm has higher ionic conductivity and can provide higher safety performance. Therefore, this case can actually solve the problem that diaphragms in the prior art cannot simultaneously achieve both fast charging and high safety.

[0047] The above description is merely an embodiment of the present application and does not constitute any form of limitation to the present application. Although the present application has been disclosed as above with specific embodiments, it is not intended to limit the present application. Any technician familiar with this profession can make slight changes or modifications to equivalent embodiments of the technical contents disclosed above without departing from the scope of the technical solution of the present application. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present application without departing from the content of the technical solution of the present application are still within the scope of the technical solution of the present application.

Claims

1. An ultra-high molecular weight polyolefin diaphragm, characterized in that: It includes ultra-high molecular weight polyethylene, the average molecular weight of the ultra-high molecular weight polyethylene is ≥1 million, the thickness of the ultra-high molecular weight polyolefin membrane is 7μm~15μm, the median pore size is 0.04μm~0.5μm, the maximum pore size is 0.11μm~1.2μm, the puncture strength is ≥50gf, and the ionic conductivity is ≥1.8mS / cm.

2. The ultra-high molecular weight polyolefin membrane according to claim 1, wherein: The average molecular weight of the ultra-high molecular weight polyethylene is 1 million to 1.5 million.

3. A method for preparing an ultra-high molecular weight polyolefin membrane according to claim 1 or 2, characterized in that: The following steps are involved: The steps for preparing micron-sized or submicron-sized sodium chloride particles are as follows: adding beads to sodium chloride for ball milling, sieving and drying after grinding, then adding white oil and continuing grinding to obtain micron-sized or submicron-sized sodium chloride particles; a raw material mixing step: selecting ultra-high molecular weight polyethylene and the micron-sized or submicron-sized sodium chloride particles, mixing them in white oil, and fully stirring them with a blender to form a premixed raw material, wherein the average molecular weight of the ultra-high molecular weight polyethylene is ≥ 1,000,000, the content of the white oil is greater than that of the ultra-high molecular weight polyethylene, and the content of the ultra-high molecular weight polyethylene is greater than that of the micron-sized or submicron-sized sodium chloride particles; An extrusion molding step: adding the premixed raw material into an extruder, and then extruding and molding to form a cast sheet; An extraction step: extracting the extruded sheet in a mixture of acetone and water, then ultrasonically cleaning the pores of the sheet to remove sodium chloride and some white oil, then washing the remaining acetone on the surface with pure water, drying the sheet, and re-extracting the sheet in pure dichloromethane to form an extracted raw material. as well as A stretching step: subsequently placing the extracted raw material obtained by extrusion molding into a handkerchief machine for stretching, to obtain the ultra-high molecular weight polyolefin diaphragm; The extraction step is not preceded by stretching.

4. The method for preparing an ultra-high molecular weight polyolefin membrane according to claim 3, wherein: In the step of preparing the micron- or submicron-sized sodium chloride particles, zirconium beads mixed from at least three different sizes are added to ordinary industrial-grade sodium chloride, which is ground, sieved, and dried, and then white oil is added and further ground. After grinding, micron- or submicron-sized sodium chloride particles are obtained, wherein the ordinary industrial-grade sodium chloride is industrial salt with a sodium chloride purity of more than 99% and a particle size of 0.5 mm to 2 mm, and the maximum diameter of the zirconium beads does not exceed 1 mm.

5. The method for preparing an ultra-high molecular weight polyolefin membrane according to claim 3, wherein: The diameter of the micron-sized or submicron-sized sodium chloride particles is between 0.1 μm and 1 μm.

6. The method for preparing an ultra-high molecular weight polyolefin membrane according to claim 3, wherein: In the raw material mixing step, based on the total weight of the premixed raw materials, the weight proportion of the micron-sized or submicron-sized sodium chloride particles is 1 wt% to 20 wt%, the weight proportion of the ultra-high molecular weight polyethylene is 1 wt% to 29 wt%, and the weight proportion of the white oil is 60 wt% to 80 wt%.

7. The method for preparing an ultra-high molecular weight polyolefin membrane according to claim 3, wherein: The average molecular weight of the ultra-high molecular weight polyethylene is 1 million to 1.5 million.

8. The method for preparing an ultra-high molecular weight polyolefin membrane according to claim 3, wherein: In the step of preparing the micron-sized or submicron-sized sodium chloride particles, the micron-sized or submicron-sized sodium chloride particles include spherical, quasi-spherical or irregular spherical shapes, and their particle size distribution is approximately normal distribution, wherein 80% of the sodium chloride particles are quasi-spherical.

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