A high-strength lithium-ion battery separator and a method for preparing the same

Abstract

This invention provides a high-strength lithium-ion battery separator and its preparation method, belonging to the field of lithium battery separator preparation technology. The preparation method involves mixing a high-density polyethylene or polyethylene-polypropylene blend system, additives, and white oil, followed by twin-screw plasticizing melt extrusion to obtain a cast sheet. This sheet is then biaxially stretched to form a film, followed by extraction degreasing, secondary stretching, and heat setting before winding to obtain the final product. This method utilizes a high-solids-content, low-molecular-weight high-density polyethylene or polyethylene-polypropylene blend system, combined with low-grade white oil and shear additives to reduce the viscosity of the extruded melt. Under ordinary stretching conditions, this effectively improves the separator strength and enhances the pore size uniformity. This method is simple, with controllable conditions, requires no additional equipment, and produces a separator with strengths, including puncture and tensile strength, far exceeding that of ultra-high molecular weight polyethylene separators produced using conventional processes. It also exhibits good pore size uniformity and has broad application prospects.

Description

Technical Field

[0001] This invention relates to a high-strength lithium-ion battery separator and its preparation method, belonging to the field of lithium battery separator preparation technology. Background Technology

[0002] Currently, lithium-ion battery separators are mainly polyolefin membranes, including polyethylene and polypropylene separators. Processing methods include dry polypropylene molding technology and wet polyethylene (thermally induced phase separation) molding technology. Wet-molded polyethylene separators have advantages such as high porosity, uniform pore size distribution, and high strength, and are widely used. Separator strength is closely related to battery safety. As battery energy density increases, battery separators are becoming thinner, and the requirements for separator strength are also increasing. Improving separator strength will significantly reduce the risk of short circuits caused by separator breakage during abnormal extrusion deformation, greatly enhancing battery safety and representing the main development direction for future separator products.

[0003] Currently, there are generally three main methods to improve the strength of polyethylene lithium-ion battery separators:

[0004] First, it uses ultra-high molecular weight polyethylene (UHMWPE). UHMWPE has a molecular weight close to or exceeding 1 million, which makes twin-screw plasticizing difficult. It requires high temperature and strong shear melting, resulting in severe molecular weight degradation. At the same time, it is easy to form carbon compounds at high temperatures, forming appearance defects such as crystal points and black spots, which affect the quality of the diaphragm. In addition, a suitable die head needs to be matched to adapt to the high melt viscosity of UHMWPE.

[0005] Secondly, high-ratio stretching is employed. Patent CN 103687901A describes the extrusion of sheets by mixing polyethylene and solvent, followed by stretching and extraction to obtain a separator. A high-strength separator is obtained by using a stretching ratio of 1.1-2.0 times in the length direction and a high stretching ratio of 4-50 times in both the length and width directions. The polyethylene used is a blend of 2 million and 300,000 molecular weights with a solid content of 30%, and the longitudinal tensile strength is approximately 150 MPa. Patent CN 105552280B describes the cooling and molding of a liquid-phase stable system containing polyethylene, followed by biaxial stretching and pore expansion. A lithium-ion battery separator is obtained through solvent extraction and heat setting. A high-strength separator is obtained by pre-stretching before the simultaneous biaxial stretching step, with a pre-stretching ratio of 1.01-6 times to increase the overall stretching ratio. The polyethylene used has a molecular weight of 800,000-1,600,000, a solid content of 30%, a pre-stretching ratio of 2 times, and a biaxial tensile strength of approximately 180 MPa. However, the greater the stretching ratio, the greater the internal stress of the diaphragm. If the stress cannot be fully released in subsequent processes, it can easily lead to large deformation of the diaphragm after placement. In addition, increasing the stretching ratio requires large-scale equipment modification, which is costly.

[0006] Thirdly, by adding inorganic whiskers, elastomer materials, and crosslinking agents, patent CN 112490582A describes a method that uses high-molecular-weight ultra-high molecular weight polyethylene (UHMWPE) to add elastomer materials to a polyethylene mixture to prepare a lithium battery separator with high mechanical strength and high toughness. The UHMWPE used has a weight-average molecular weight of 1.5-2.5 million, a polyethylene solid content of 23-30%, and a biaxial tensile strength of approximately 300 MPa. However, this method suffers from poor melting effect, and the crystallization precipitation can easily lead to impurity crystal points in the separator, affecting its quality. Furthermore, the added elastomers and additives, after precipitation, can easily cause tensile cracking during stretching, affecting the safety of separator production. Therefore, there is a need to develop a polyethylene lithium battery separator with a simple process, high equipment acceptance, and high strength, which can be widely used in 3C consumer products, power batteries, and other fields. Summary of the Invention

[0007] The purpose of this invention is to provide a high-strength lithium-ion battery separator and its preparation method. It employs a high-solids-content, low-molecular-weight high-density polyethylene or polyethylene-polypropylene blend system, combined with low-grade white oil and shear additives to reduce the viscosity of the extruded melt. Under conventional stretching processes, this effectively improves the separator strength and enhances the pore size uniformity. This method is simple, with controllable conditions, requires no additional equipment, and produces a separator with significantly higher strength (including puncture and tensile strength) than ultra-high molecular weight polyethylene separators produced using conventional processes. It also exhibits excellent pore size uniformity and has broad application prospects.

[0008] To achieve the above objectives, the present invention adopts the following specific technical solution:

[0009] A method for preparing a high-strength lithium-ion battery separator includes the following steps:

[0010] S1: High-density polyethylene or a polyethylene-polypropylene blend, additives, and white oil are mixed and then plasticized and melt-extruded using a twin-screw extruder. Preferably, the extrusion temperature of the twin-screw extruder is 130-230℃, and the screw speed is 30-150 rpm.

[0011] The solid content of the high-density polyethylene or polyethylene-polypropylene blend system can be 30-50%; preferably 35-45%. If the solid content is too low, the diaphragm strength will be low; if the solid content is too high, the twin-screw plasticizing and melting will be difficult, and the processing will be difficult.

[0012] The molecular weight of the polyethylene can be 100,000 to 1,000,000, preferably 300,000 to 800,000; the molecular weight of the polypropylene can be 500,000 to 2,000,000, preferably 700,000 to 1,500,000. If the molecular weight is too low, the membrane strength will be low; if the molecular weight is too high, the twin-screw plasticizing and melting will be difficult, and the processing will be challenging.

[0013] The grade of the white oil can be 10#-70#, preferably 15#-50#. Because increasing the solid content will lead to an increase in the viscosity of the extruded melt, it is necessary to use the aforementioned low-grade white oil and add shearing aids to reduce the melt viscosity, thereby eliminating the need to change the existing die structure and reducing the complexity of the process.

[0014] The additive can be a fluorinated polymer with a particle size range of 50-200 micrometers and a fluorine content of 40-80%.

[0015] Preferably, the additive is a polyperfluoroalkoxy resin and its copolymer, a polyvinylidene fluoride resin and its copolymer, a polychlorotrifluoroethylene resin and its copolymer, or a polyvinylidene fluoride resin and its copolymer;

[0016] More preferably, the additive is polyvinylidene fluoride resin and its copolymers.

[0017] S2: The melt after S1 is extruded is cooled by a chiller roller to obtain a casting.

[0018] Preferably, the cooling temperature is 10-80℃ and the thickness of the casting sheet is 0.4-5mm.

[0019] S3: The cast sheet obtained in S2 is biaxially stretched into a thin film by a synchronous or asynchronous biaxial stretching machine.

[0020] The stretching temperature is 105-130℃, and the biaxial stretching ratio is 5*5-13*13 times.

[0021] S4: Extract and remove oil from the film obtained in S3.

[0022] The extraction is carried out at room temperature, typically 20-25°C; the extractant is dichloromethane.

[0023] S5: After drying the degreased film obtained in S4, it is then stretched a second time using a cross-stretching machine.

[0024] The horizontal stretching ratio is 1.2-1.8 times, and the horizontal stretching temperature is 120-150℃.

[0025] S6: The film obtained in S5 is heat-set and then wound up. The heat-setting temperature is 25-100℃, thus preparing the high-strength lithium-ion battery separator.

[0026] The present invention also provides a high-strength lithium-ion battery separator, which is prepared by the above-described preparation method.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) High-density polyethylene or polyethylene-polypropylene blend system is used. High solid content is used. Shear plasticization can be carried out at low temperature. The melt plasticization effect is good. The preparation process is simple and does not require additional modification or addition of large equipment.

[0029] (2) Use low-grade white oil and add shearing aids to reduce the viscosity of the extruded melt and improve the melt extrusion effect.

[0030] (3) Through normal multiple stretching, the high solid content increases the number of internal fiber connections in the diaphragm during stretching, thereby improving the strength performance of the diaphragm. The diaphragm has high tensile and puncture strength. At the same time, due to the increase in the number of internal fiber connections during stretching, the diaphragm pore size is reduced, the pore size uniformity is improved, which is beneficial to the long-term cycle performance of the battery and the service life is long.

[0031] (4) Due to the use of normal stretching, the internal stress of the diaphragm is small, the deformation after placement is small, and the diaphragm is easy to preserve for a long time. Attached Figure Description

[0032] To more clearly illustrate the technical solutions of the background technology and embodiments of the present invention, the accompanying drawings used in the background technology and embodiments will be briefly introduced below. It should be understood that the following drawings may only show some embodiments of the present invention, and therefore should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0033] Figure 1 This is a schematic diagram of the process and equipment for preparing the high-strength lithium-ion battery separator in Embodiment 1 of the present invention;

[0034] Figure 2 This is an electron microscope image of the high-strength lithium-ion battery separator in Embodiment 1 of the present invention. Detailed Implementation

[0035] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0036] Example 1

[0037] A method for preparing a high-strength lithium-ion battery separator, wherein the raw material is high-density polyethylene with a solid content of 40% and a molecular weight of 600,000; and the additives are polyvinylidene fluoride resin and its copolymers with a particle size range of about 100 micrometers and a fluorine content of 60%.

[0038] like Figure 1 The process flow shown is as follows:

[0039] Raw materials, additives, and 25# white oil are mixed and then plasticized and melted using a twin-screw extruder. The extrusion temperature is 130℃, and the screw speed is 80 rpm. The melt, after being cast and extruded from the die under the control of a metering pump, is cooled by a 30℃ chiller roller to obtain a 1.2mm polyethylene casting. The casting is then biaxially stretched into a film using a synchronous biaxial stretching machine at 120℃, with a biaxial stretching ratio of 6*6. The film is then extracted and degreased using dichloromethane at 25℃, dried, and then subjected to a second stretching using a cross-stretching machine at 1.5 times the stretching ratio and 120℃. Finally, it is heat-set at 50℃ and then wound up to obtain the high-strength lithium-ion battery separator. Figure 2 As shown, the fibers are densely connected and have uniform pore size.

[0040] Example 2

[0041] A method for preparing a high-strength lithium-ion battery separator, wherein the raw material is a blend of high-density polyethylene with a molecular weight of 600,000 and polypropylene with a molecular weight of 800,000, with a solid content of 45%; the additive is polyvinylidene fluoride resin and its copolymers with a particle size range of about 50 micrometers and a fluorine content of 40%.

[0042] Raw materials, additives, and 50# white oil are mixed and then plasticized and melted using a twin-screw extruder. The extrusion temperature is 230℃, and the screw speed is 100 rpm. The melt, after being cast and extruded from the die under the control of a metering pump, is cooled by an 80℃ chiller roller to obtain a 4mm thick cast sheet. The cast sheet is then biaxially stretched into a film using a synchronous biaxial stretching machine at 130℃, with a biaxial stretching ratio of 13*13. The film is then extracted and degreased using dichloromethane at 25℃, dried, and then subjected to a second stretching using a cross-stretching machine at 150℃, with a cross-stretching ratio of 1.8. Finally, it is heat-set at 80℃ and then wound up to obtain the final product.

[0043] Example 3

[0044] A method for preparing a high-strength lithium-ion battery separator, wherein the raw material is a blend of high-density polyethylene with a molecular weight of 100,000 and polypropylene with a molecular weight of 1.5 million, with a solid content of 32%; the additive is a polyperfluoroalkoxy resin and its copolymer with a particle size range of about 200 micrometers and a fluorine content of 80%.

[0045] Raw materials, additives, and No. 15 white oil are mixed and then plasticized and melted using a twin-screw extruder. The extrusion temperature is 200℃, and the screw speed is 150 rpm. The melt, after being cast and extruded from the die under the control of a metering pump, is cooled by a 60℃ chiller roller to obtain a 1mm thick cast sheet. The cast sheet is then biaxially stretched into a film using a synchronous biaxial stretching machine at 105℃, with a biaxial stretching ratio of 13*13. The film is then extracted and degreased using dichloromethane at 20℃, dried, and then subjected to a second stretching using a cross-stretching machine at 130℃, with a cross-stretching ratio of 1.5. Finally, it is heat-set at 100℃ and then wound up to obtain the final product.

[0046] Example 4

[0047] A method for preparing a high-strength lithium-ion battery separator, wherein the raw material is a blend of high-density polyethylene with a molecular weight of 800,000 and polypropylene with a molecular weight of 500,000, with a solid content of 50%; the additive is polychlorotrifluoroethylene resin and its copolymers with a particle size range of about 80 micrometers and a fluorine content of 40%.

[0048] Raw materials, additives, and No. 10 white oil are mixed and then plasticized and melted using a twin-screw extruder. The extrusion temperature is 200℃, and the screw speed is 40 rpm. The melt, after being cast and extruded from the die under the control of a metering pump, is cooled by a 10℃ chiller roller to obtain a 0.5mm thick cast sheet. The cast sheet is then biaxially stretched into a film using a synchronous biaxial stretching machine at 130℃ with a biaxial stretching ratio of 10 x 10. The film is then extracted and degreased using dichloromethane at 20℃, dried, and then subjected to a second stretching using a cross-stretching machine at 120℃ with a cross-stretching ratio of 1.2. Finally, it is heat-set at 25℃ and then wound up to obtain the final product.

[0049] Example 5

[0050] A method for preparing a high-strength lithium-ion battery separator, wherein the raw material is a blend of high-density polyethylene with a molecular weight of 500,000 and polypropylene with a molecular weight of 700,000, with a solid content of 45%; the additives are polyvinyl fluoride resin and copolymers with a particle size range of about 50 micrometers and a fluorine content of 50%.

[0051] Raw materials, additives, and 70# white oil are mixed and then plasticized and melted using a twin-screw extruder. The extrusion temperature is 140℃, and the screw speed is 80 rpm. The melt, after being cast and extruded from the die under the control of a metering pump, is cooled by a 30℃ chiller roller to obtain a 0.5mm thick cast sheet. The cast sheet is then biaxially stretched into a film using a synchronous biaxial stretching machine at 130℃ with a biaxial stretching ratio of 10*10. The film is then extracted and degreased using dichloromethane at 25℃, dried, and then subjected to a second stretching using a cross-stretching machine at 150℃ with a cross-stretching ratio of 1.8. Finally, the film is heat-set at 50℃ and then wound up to obtain the final product.

[0052] The properties of the high-strength lithium-ion battery separator obtained are shown in Table 1.

[0053] Table 1 Performance of the high-strength lithium-ion battery separators prepared in Examples 1-5

[0054]

[0055]

[0056] As shown in Table 1, the high-strength lithium-ion battery separator of the present invention has a thickness of 3-25 μm, a porosity of 40-50%, an average pore size of 30-40 nm, a puncture strength ≥320 MPa, a biaxial tensile strength ≥220 MPa, and an air permeability ≤260 s / 100 cc.

[0057] In the description of this specification, the references to terms such as "one embodiment," "one example," "some embodiments," "preferred embodiment," and "specific embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0058] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a high-strength lithium-ion battery separator, comprising the following steps: S1: High-density polyethylene or a polyethylene-polypropylene blend system, additives, and white oil are mixed and then plasticized and melt-extruded using a twin-screw extruder; the solid content of the high-density polyethylene or polyethylene-polypropylene blend system is 30-50%; the molecular weight of the polyethylene is 100,000-1,000,000, and the molecular weight of the polypropylene is 500,000-2,000,000; the grade of the white oil is 10#-70#; the additives are fluorinated polymers with a particle size range of 50-200 micrometers and a fluorine content of 40-80%. S2: The melt after S1 is extruded is cooled by a chiller roller to obtain a casting sheet; S3: The cast sheet obtained in S2 is biaxially stretched into a thin film using a synchronous or asynchronous biaxial stretching machine. S4: Extract and remove oil from the film obtained in S3; S5: After drying the degreased film obtained in S4, it is then stretched a second time using a cross-stretching machine; the cross-stretching ratio is 1.2-1.8 times, and the cross-stretching temperature is 120-150℃. S6: The film obtained in S5 is heat-set and then wound up to obtain the final product.

2. The preparation method according to claim 1, characterized in that, S1 satisfies one or more of the following conditions: a. The solid content is 35-45%; b. The molecular weight of the polyethylene is 300,000-800,000; the molecular weight of the polypropylene is 700,000-1,500,000. c. The grade of the white oil is 15#-50#.

3. The preparation method according to claim 1, characterized in that, In S1, the additive is a polyperfluoroalkoxy resin and its copolymer, a polyvinylidene fluoride resin and its copolymer, a polychlorotrifluoroethylene resin and its copolymer, or a polyvinylidene fluoride resin and its copolymer.

4. The preparation method according to claim 3, characterized in that, The additive is polyvinylidene fluoride resin and its copolymers.

5. The preparation method according to claim 1, characterized in that, S1 also satisfies one or more of the following conditions: d. The extrusion temperature of the twin-screw extruder is 130-230℃; e. The screw speed of the twin-screw extruder is 30-150 rpm.

6. The preparation method according to claim 1, characterized in that, In S2, the cooling temperature is 10-80℃, and the thickness of the casting sheet is 0.4-5mm.

7. The preparation method according to claim 1, characterized in that, In S3, the stretching temperature is 105-130℃, and the biaxial stretching ratio is 5*5-13*13 times.

8. The preparation method according to claim 1, characterized in that, In S4, the extraction is carried out at room temperature; the extractant for the extraction is dichloromethane.

9. The preparation method according to claim 1, characterized in that, In S6, the heat setting temperature is 25-100℃.

10. A high-strength lithium-ion battery separator, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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