Diaphragm and preparation method and application thereof

Abstract

The invention discloses a diaphragm as well as a preparation method and application thereof, and belongs to the technical field of battery diaphragms. Aiming at the technical problems in the prior art that a battery diaphragm is insufficient in mechanical strength, easy to pierce, high in layering or diaphragm breaking risk and difficult to realize ultrahigh-rate stretching preparation in high-speed winding and electrochemical use processes, the invention provides the diaphragm with the thickness of 1m-7m; the needling strength is 100 gf to 950 gf; the ratio of the needling strength to the thickness is larger than or equal to 70 gf / m; the diaphragm is formed by compounding multiple layers of oil films or is prepared by folding and compounding the oil films, the number of the compounded layers is larger than or equal to 2, interlayer interfaces are tightly combined without layering, high mechanical strength is kept, meanwhile, good electrolyte wettability and ionic conduction capacity are achieved, and both electrochemical performance and safety performance can be achieved; and the thin-film solar cell is suitable for the design requirements of high-energy density batteries on thinness and high strength. The invention also provides a preparation method of the diaphragm.

Description

Technical Field

[0001] This invention belongs to the field of battery separator technology, and more specifically, relates to separators, their preparation methods, and applications. Background Technology

[0002] As one of the four key components of a lithium-ion battery, the separator primarily functions as both a physical barrier between the positive and negative electrodes and facilitates ion conduction. The mechanical properties, pore structure, and thermal stability of the separator have a decisive impact on the battery's safety, consistency, and cycle life.

[0003] In the prior art, membranes are usually made of polyolefin materials (such as polyethylene PE and polypropylene PP), and common processes include dry biaxial stretching or wet solvent extraction.

[0004] However, with the rapid development of high-energy-density batteries, separators face the following technical challenges in manufacturing and use: 1) Mechanical load during winding and assembly: During high-speed winding or stacking, the separator needs to withstand high tension and friction. If the tensile strength of the separator is insufficient or delamination defects exist, it is very easy for the membrane to break or wrinkle, resulting in a decrease in production yield. 2) Puncture risk during use: During charge-discharge cycles, metal foreign objects or lithium dendrites may puncture the separator. Insufficient puncture strength will lead to separator failure, thereby causing internal short circuits and thermal runaway risks. 3) Polymer orientation and cooling challenges: To improve separator strength, the industry usually increases the stretching ratio or uses ultra-high molecular weight polyethylene (UHMWPE) raw materials to enhance molecular chain orientation. However, UHMWPE melt has extremely high viscosity and extremely poor thermal conductivity, making it difficult to cool sufficiently during casting and cooling, resulting in uneven casting thickness or insufficient local crystallization, thus limiting the improvement of stretching ratio and final mechanical properties.

[0005] Therefore, how to significantly improve the stretching ratio and molecular chain orientation while maintaining the uniformity of the diaphragm, and prepare an ultra-high stretching ratio diaphragm with high needle penetration strength, no delamination and thinner thickness, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] 1. The problem to be solved To address the technical problems of existing lithium-ion battery separators during high-speed winding and electrochemical use, such as insufficient mechanical strength, susceptibility to puncture, high risk of delamination or breakage, and difficulty in achieving ultra-high rate stretching preparation, this application provides a separator; furthermore, this application also provides a method for preparing the separator; and this application also provides the application of the separator.

[0007] 2. Technical Solution The technical solution adopted in this invention is as follows: Based on the objectives of this invention, a first aspect of this invention provides a diaphragm having a thickness of 1 µm to 7 µm; a needle penetration strength of 100 gf to 950 gf; and a needle penetration strength to thickness ratio ≥ 70 gf / µm. The diaphragm is composed of multiple layers of oil film composite, or is made by folding and compositing oil films.

[0008] Preferably, the thickness of the diaphragm can be 1 µm, 2 µm, 3 µm, 4 µm, 5 µm, 6 µm, or 7 µm; more preferably, the diaphragm has a thickness of 3 µm to 5 µm. The needle penetration strength can be 100 gf, 150 gf, 200 gf, 250 gf, 300 gf, 350 gf, 400 gf, 500 gf, 600 gf, 700 gf, 800 gf, 850 gf, 900 gf, or 950 gf. The ratio of needle penetration strength to thickness can be 70 gf / µm to 160 gf / µm, for example, 70 gf / µm, 75 gf / µm, 78 gf / µm, 80 gf / µm, 85 gf / µm, 90 gf / µm, 95 gf / µm, 100 gf / µm, or 105 gf / µm. gf / µm, 110 gf / µm, 115gf / µm, 120 gf / µm, 125 gf / µm, 130 gf / µm, 135 gf / µm, 140 gf / µm, 145 gf / µm, 150 gf / µm, 155 gf / µm, 160 gf / µm.

[0009] Preferably, the diaphragm has a bidirectionally oriented uniform microporous structure with consistent pore shape distribution on both sides and no obvious elongated pores or delamination defects. The "uniform microporous structure" is supported by SEM statistics; the "no obvious elongated pores or delamination defects" can be supported by the embodiment diagrams or SEM images (not absolutely none, but no obvious defects).

[0010] According to any embodiment of the first aspect of the present invention, the diaphragm has ≥2 composite layers (for example, it may be 2, 3, 4, 5, 6, 7, 8, 9, or 10 layers), and the interlayer interfaces are tightly bonded and without delamination.

[0011] According to any embodiment of the first aspect of the invention, the diaphragm has a porosity of 15% to 80%.

[0012] Preferably, the porosity can be 15%, 17%, 18%, 20%, 30%, 40%, 50%, 60%, 70%, or 80%; more preferably, the diaphragm has a porosity of 30% to 60%; further preferably, the diaphragm has a porosity of 30% to 40%.

[0013] Based on the objectives of this invention, a second aspect of this invention provides a method for preparing a diaphragm, comprising the following steps: S1. Raw material preparation: Polyethylene powder with a viscosity-average molecular weight of 600,000 to 10,000,000 is melt-plasticized and mixed with a plasticizer to obtain a melt. For example, the molecular weight can be 600,000, 650,000, 700,000, 800,000, 900,000, 1,000,000, 1,050,000, 1,100,000, 1,200,000, 1,300,000, 1,500,000, 1,800,000, 2,000,000, 2,200,000, 2,400,000, 2,800,000, 3,000,000, 4,000,000, 5,000,000, 5,500,000, 6,000,000, 7,000,000, 8,000,000, 9,000,000, or 10,000,000.

[0014] S2. Casting: The melt is extruded through a die to form a cast sheet. The thickness of the cast sheet is 0.2 mm to 2 mm, for example, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm, or 2.0 mm.

[0015] Preferably, in step S2, the melt is extruded into castings through at least two (e.g., two, three, four, five, six, seven, or eight) dies; or the melt is extruded into at least two (e.g., two, three, four, five, six, seven, or eight) castings in multiple passes through one die.

[0016] S3. First stretching: The cast sheet is subjected to biaxial stretching with a stretching ratio of MD×TD = 2×2 to 15×15 to obtain a pre-oriented film. For example, the stretching ratios are MD×TD = 2×2, 2×3, 2×4, 2×5, 2×6, 2×7, 2×8, 2×9, 2×10, 2×11, 2×12, 2×13, 2×14, 2×15, 3×2, 3×3, 3×4, 3×5, 3×6, 3×7, 3×8, 3×9, 3×10, 3×11, 3×12, 3×13, 3×14, 3×15. 4×2, 4×3, 4×4, 4×5, 4×6, 4×7, 4×8, 4×9, 4×10, 4×11, 4×12, 4×13, 4×14, 4×15, 5×2, 5×3, 5×4, 5×5, 5×6, 5×7, 5×8, 5×9, 5×10, 5×11, 5×12, 5×13, 5×14, 5×15, 6×2, 6×3, 6×4, 6× 5, 6×6, 6×7, 6×8, 6×9, 6×10, 6×11, 6×12, 6×13, 6×14, 6×15, 7×2, 7×3, 7×4, 7×5, 7×6, 7×8, 7×9, 7×10, 7×11, 7×12, 7×14, 8×4, 8×5, 8×6, 8×8, 8×9, 8×10, 8×11, 8×12, 9×5, 9×6 The metric values ​​are 9×8, 9×9, 9×10, 9×11, 10×5, 10×6, 10×8, 10×9, 10×10, 11×6, 11×8, 11×9, 12×6, 12×8, 13×6, 13×7, 14×4, 14×7, 15×6 or 15×5; more preferably, MD×TD = 2×2 to 8×8, and even more preferably, MD×TD = 2×2 to 6×6.

[0017] Preferably, in step S3, the area stretching ratio is 4 to 100 times, for example, 4 times, 6 times, 8 times, 10 times, 12 times, 14 times, 16 times, 18 times, 20 times, 22 times, 24 times, 26 times, 28 times, 30 times, 32 times, 34 times, 36 times, 38 times, 40 times, 42 times, 44 times, 46 times, 48 ​​times, 50 times, 52 times, 54 times, 56 times, 58 times, 60 times, 62 times, 64 times, 66 times, 68 times, 70 times, 72 times, 74 times, 76 times, 78 times, 80 times, 80 times, 82 times, 84 times, 86 times, 88 times, 90 times, 92 times, 94 times, 96 times, 98 times, 100 times, preferably 4 to 64 times, and more preferably 4 to 36 times.

[0018] In the preparation of multilayer composite diaphragms subjected to ultra-high ratio stretching, the initial casting thickness and the stretching ratio during the first stretching stage play a decisive role in the integrity (whether delamination occurs) and mechanical strength of the subsequent structure. If these two parameters are not properly controlled, the diaphragm may delaminate or bubble during subsequent high-ratio stretching, leading to a decrease in strength.

[0019] S4. Composite and Folding: The at least two (e.g., two, three, four, five, six, seven or eight) initially oriented films are laminated together to obtain a composite film.

[0020] Preferably, in step S4, each of the initial orientation films is folded at least once independently, and the folded initial orientation films are then laminated together to obtain a composite film.

[0021] Preferably, in step S4, each of the initial orientation films is folded independently 1 to 8 times (for example, 1, 2, 3, 4, 5, 6, 7, or 8 times); more preferably, it is folded 1 to 4 times; and even more preferably, it is folded 1 to 2 times.

[0022] Preferably, in step S4, each of the initial orientation films is folded 0 times independently, and the melt in step S2 is extruded into a casting by at least four (e.g., four, five, six, seven or eight) dies to prepare a composite film with ≥4 composite layers; or in step S2, the melt is extruded into at least four (e.g., four, five, six, seven or eight) castings by one die in multiple times to prepare a composite film with ≥4 composite layers.

[0023] Preferably, in step S4, the number of composite layers can be 4 to 256; more preferably 4 to 32; even more preferably 4 to 16; and still more preferably 4 to 8.

[0024] S5. Second stretching: The composite film is subjected to bidirectional secondary stretching with a stretching ratio of MD×TD=1.5×1.5~15×15 to form a bidirectional oriented film.

[0025] Preferably, in step S5, the stretching ratio is MD×TD = 1×2, 1×3, 1×4, 1×5, 1×6, 1×7, 1×8, 1×9, 1×10, 1×11, 1×12, 1×13, 1×14, 1×15, 2×2, 2×3, 2×4, 2×5, 2×6, 2×7, 2×8, 2×9, 2×10, 2×11, 2×12, 2×13, 2×14, 2×15, 3×2, 3×3, 3×4, 3×5, 3×6, 3×7, 3×8, 3×9, 3×10, 3×11, 3×12, 3×13, 3×14, 3×15, 4×2, 4×3, 4×4, 4×5, 4×6, 4×7, 4×8, 4×9, 4×10, 4×11, 4×12, 5×2, 5×3, 5×4, 5×5, 5×6, 5×7, 5×8, 5×9, 5×10, 6×2, 6×3, 6×4, 6×5, 6×6, 6×7, 6×8, 7×2, 7×3, 7×4, 7×5, 7×6, 8×4, 8×5, 8×6, 9×5, 10×5, 11×5, 12×4, 13×3, 14×3 or 15×3; more preferably MD×TD = 1×1 to 7×7; even more preferably MD×TD = 2×2 to 6×6.

[0026] Preferably, in step S5, the area stretching ratio is 4 to 50 times, for example, 4 times, 6 times, 8 times, 10 times, 12 times, 14 times, 16 times, 18 times, 20 times, 22 times, 24 times, 26 times, 28 times, 30 times, 32 times, 34 times, 36 times, 38 times, 40 times, 42 times, 44 times, 46 times, 48 ​​times, 50 times; more preferably, it is 4 to 49 times; even more preferably, it is 4 to 36 times.

[0027] S6. Extraction and drying: The bidirectional oriented membrane is extracted and dried to obtain the extracted membrane.

[0028] S7. Heat setting treatment: The extracted membrane is heat set to obtain the diaphragm.

[0029] Preferably, in step S1, the plasticizer is white oil, and may further be paraffin oil.

[0030] According to any embodiment of the second aspect of the present invention, in the method for preparing a diaphragm, in step S3, the temperature during the biaxial stretching process is 110 ℃ to 135 ℃. For example, 110 ℃, 111 ℃, 112 ℃, 113 ℃, 115 ℃, 120 ℃, 130 ℃, 135 ℃.

[0031] Preferably, the temperature difference between different points in the temperature field during the biaxial stretching process is ≤0.5 ℃ (for example, it can be 0.3 ℃, 0.4 ℃ or 0.5 ℃).

[0032] According to any embodiment of the second aspect of the present invention, in the method for preparing a diaphragm, in step S5, the temperature of the second stretching is 125 ℃ to 140 ℃, for example 125 ℃, 126 ℃, 128 ℃, 130 ℃, 131 ℃, 132 ℃, 133 ℃, 134 ℃, 135 ℃, 136 ℃, 137 ℃, 138 ℃, 139 ℃, or 140 ℃. In step S7, the heat setting temperature is 125 ℃~150 ℃, for example 125 ℃, 126 ℃, 127 ℃, 128 ℃, 129 ℃, 130 ℃, 131 ℃, 132 ℃, 133 ℃, 134 ℃, 135 ℃, 136 ℃, 137 ℃, 138 ℃, 139 ℃, 140 ℃, 145 ℃, 150 ℃.

[0033] Preferably, the temperature difference between the second stretching temperature and the first stretching temperature is 16℃~20℃ (e.g., 16℃, 17℃, 18℃, 19℃, 20℃); the temperature difference between the heat setting temperature and the first stretching temperature is 18℃~25℃ (e.g., 18℃, 19℃, 20℃, 21℃, 22℃, 23℃, 24℃, or 25℃); and the temperature difference between the heat setting temperature and the second stretching temperature is 4℃~5℃.

[0034] According to any embodiment of the second aspect of the present invention, in the method for preparing a diaphragm, in step S1, the raw material further includes paraffin oil; the mass ratio of the polyethylene powder to the paraffin oil is (8%–35%):(65%–92%). For example, 8%:92%, 9%:91%, 10%:90%, 12%:88%, 14%:86%, 16%:84%, 18%:82%, 20%:80%, 22%:78%, 24%:76%, 26%:74%, 28%:72%, 30%:70%, 32%:68%, 34%:66%, 35%:65%.

[0035] According to any embodiment of the second aspect of the present invention, in the method for preparing a diaphragm, in step S1, the raw materials further include an antioxidant; the antioxidant is 0 to 0.5 parts per 100 parts by weight of the polyethylene powder and the paraffin oil. For example, 0 parts, 0.1 parts, 0.2 parts, 0.3 parts, 0.4 parts, or 0.5 parts.

[0036] Preferably, the antioxidant includes a primary antioxidant and a secondary antioxidant; the primary antioxidant includes any one or more of Irganox 1010, Irganox 1076, Irganox 1098 or Irganox 1330; and the secondary antioxidant includes any one or more of Irgafos 168, Irgafos 38 or Irgafos 12.

[0037] Based on the purpose of this invention, a third aspect of this invention provides the application of the separator, which is used in the preparation of lithium batteries.

[0038] In view of the purpose of this invention, a fourth aspect of this invention provides a battery including the aforementioned separator.

[0039] Any embodiment of any aspect of the present invention can be combined with other embodiments without contradiction. Furthermore, in any embodiment of any aspect of the present invention, any technical feature can be applied to the same technical feature in other embodiments without contradiction.

[0040] Without causing contradictions, any technical feature of any aspect or embodiment of the present invention is equally applicable to any other embodiment or embodiment of any other aspect. Of course, when applicable to each other, appropriate modifications may be made to the corresponding features as necessary. The various aspects and features of the present invention are further described below.

[0041] 3. Beneficial effects Firstly, the separator provided by this invention has an ultra-thin thickness of 1 µm to 7 µm and a high needle penetration strength of 100 gf to 950 gf, with a needle penetration strength to thickness ratio ≥70 gf / µm, achieving excellent mechanical strength and structural stability even in an extremely thin state. Through a multi-layer composite structure design, the separator is tightly bonded without delamination, ensuring dimensional stability and integrity during high-rate charge / discharge or winding processes. This separator has a suitable porosity (17%–80%), balancing ion transport efficiency and mechanical support performance, thus exhibiting excellent electrochemical compatibility and safety performance in high-energy-density batteries.

[0042] Secondly, the membrane preparation method provided by this invention forms a multilayer composite membrane structure through steps such as raw material plasticizing and mixing, casting, biaxial stretching, oil film folding and compounding, and secondary orientation. This process route can effectively improve the problems of difficulty in reducing the thickness of single-layer membranes and poor interlayer bonding, achieving the preparation of composite membranes with uniform thickness, stable interfaces, and no delamination. The two biaxial stretching processes employed make the microporous structure more uniform and the orientation more complete, balancing mechanical strength and electrolyte wettability. The overall process parameters are controllable and have good repeatability, making it suitable for continuous industrial production and enabling the stable acquisition of multilayer membrane products with uniform thickness, high strength, and dense structure.

[0043] Thirdly, the separator of this invention can be applied in lithium battery manufacturing, maintaining good dimensional stability and mechanical integrity under high energy density and high-rate charge-discharge conditions, preventing the separator from cracking or shrinking under extreme environments. This separator maintains stable pore structure even under high temperature and long-cycle conditions, significantly improving battery safety and reliability. Due to its high strength, uniform micropores, and stable multilayer bonding, it can be adapted to different electrolyte systems and positive and negative electrode materials, making it particularly suitable for high-power and high-energy-density lithium-ion batteries. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the cross-sectional morphology of the diaphragm under ultra-high ratio stretching conditions in an embodiment of the present invention. It can be seen that the diaphragm layers are tightly bonded and there is no delamination.

[0045] Figure 2 This is a schematic diagram of the cross-sectional morphology of the diaphragm in an embodiment of the present invention, showing that the diaphragm has a uniform multilayer composite structure and good interfacial bonding.

[0046] Figure 3 This is a schematic diagram of the surface morphology of a comparative diaphragm, showing the situation where the diaphragm surface morphology deteriorates due to excessive casting thickness.

[0047] Figure 4 This is a schematic diagram of the cross-sectional morphology of a comparative diaphragm, showing the elongated pore structure formed under improper process conditions.

[0048] Figure 5 This is a schematic diagram of the surface morphology of the diaphragm under ultra-high stretching conditions in an embodiment of the present invention, showing that the obtained diaphragm has a smooth surface and a uniform pore structure.

[0049] Figure 6 This is a schematic diagram of the cross-sectional morphology of a comparative diaphragm, showing the delamination phenomenon caused by poor composite bonding. Detailed Implementation

[0050] All references cited in this invention are incorporated herein by reference in their entirety, and in the event of any inconsistency between the meanings expressed in these references and those expressed herein, the meanings expressed herein shall prevail. Furthermore, the various terms and phrases used in this invention have their general meanings known to those skilled in the art; however, this invention still seeks to provide a more detailed explanation and interpretation of these terms and phrases, and in the event of any inconsistency between the mentioned terms and their known meanings and those expressed herein, the meanings expressed herein shall prevail.

[0051] When an item is described using the combined terms “...and / or ...", the description should be understood to include any one of the listed items and all combinations thereof.

[0052] Generally, the use of the term "about" indicates an approximation that can vary depending on the desired characteristics obtained from the disclosed subject matter and will be interpreted in a context-dependent manner based on function. Therefore, those skilled in the art will be able to interpret a degree of difference on a case-by-case basis. In some cases, the number of significant figures used when expressing a particular value can be a representative technique for determining the difference allowed by the term "about." In other cases, a gradient within a range of values ​​can be used to determine the range of differences allowed by the term "about." Furthermore, all ranges in this disclosure are inclusive and composable, and references to values ​​described within a range include every value within that range.

[0053] In this invention, the term "comprising" or "containing" indicates that various ingredients may be used together in the composition of this invention. Therefore, the terms "consistent with..." and "composed of..." are included in the term "comprising" or "containing".

[0054] Unless otherwise defined, "molecular weight" as used in this article refers to average molecular weight.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terms used herein and / or include any and all combinations of one or more of the associated listed items.

[0056] Unless otherwise stated, any feature disclosed in this specification may be replaced by other equivalent or similar features. Unless otherwise stated, each feature is merely one example of a series of equivalent or similar features. The descriptions are merely to aid in understanding the invention and should not be construed as limiting the invention. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0057] Separator thickness test method: The test shall be conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0058] Five square diaphragm samples were cut along the TD direction using a 10 cm × 10 cm mold and tested. If the TD direction sample was less than 10 cm, a 10 cm sample was cut along the MD direction; in this case, the sample was not square. The four corners and the center point of the sample were measured using a Mahr thickness gauge (C1202), and the average value of these five points was taken as the thickness of a single sample. The average value of the five samples was taken as the thickness of the diaphragm.

[0059] Needle penetration strength test method: The test shall be conducted in accordance with the method specified in GB / T 36363-2018. A diaphragm with a size of 50 mm × 100 mm shall be cut along the TD direction. The diaphragm shall be fixed on the sample fixture of the puncture test machine (model: KES-GNDG5, KNC Technology Co., Ltd.). A steel needle with a diameter of 1.0 mm shall be used to puncture the diaphragm at a speed of 0.1 cm / sec. The maximum load of the steel needle penetrating the diaphragm shall be read. The test shall be performed more than 5 times and the arithmetic mean shall be taken.

[0060] The ratio of needle penetration intensity to thickness is the same as the ratio of the aforementioned needle penetration intensity to the aforementioned thickness.

[0061] Porosity testing method: Cut a 10 cm × 10 cm sample and measure its thickness (Mahr thickness gauge, C1202) and mass (electronic balance, Mettler Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02). Calculate the surface density (unit: g / cm³) 2 ), where m represents the mass of the sample (in g), L represents the length of the sample (in cm), and b represents the width of the sample (in cm). According to 00 Calculate the porosity, where p is the porosity of the sample (in %) and d is the thickness of the sample (in cm).

[0062] The following describes specific embodiments and appendices. Figure 1-6 This application will be further described.

[0063] Example 1 The method for preparing the diaphragm described in this embodiment includes the following steps: S1. Raw material preparation The mixture consisted of 12 parts by weight of polyethylene powder (GUR4022S, KPIC, molecular weight approximately 5 million) and 88 parts by weight of paraffin oil (60#, Zhejiang Zhengxin). To a total of 100 parts by weight of polyethylene powder and paraffin oil, 0.3 parts by weight of antioxidant Irg1010 (Ciba Specialty Chemicals) and 0.1 parts by weight of antioxidant P168 (Ciba Specialty Chemicals) were added.

[0064] After mixing the above raw materials in sequence, they are fed into a twin-screw extruder for kneading, so that the polyethylene is fully plasticized and forms a uniform gel-like melt with the paraffin oil.

[0065] S2. Casting film The system uses two dies to extrude the mixture into a gel melt. The extrusion temperature is controlled below 220°C to mitigate polyethylene decomposition.

[0066] After the melt is extruded through a die with a die opening of 1.5 mm, it is immediately cooled rapidly on a chilling roller with a surface linear speed of 9 m / min and a surface temperature of 15 ℃ to obtain a flat sheet with a final sheet thickness of 1.1 mm.

[0067] S3. First stretch After the obtained sheets are drawn, they are each independently subjected to biaxial stretching at 115 °C at a stretching speed of 100 m / min. The stretching ratio in both the longitudinal (MD) and transverse (TD) directions is 6 times, and the area stretching ratio is 36 times.

[0068] During the stretching process, the temperature field is precisely controlled by a multi-point temperature control device to ensure that the temperature difference within the stretching area is less than 0.3℃, thereby guaranteeing the consistency of the orientation structure and pore shape and obtaining the initial orientation film.

[0069] S4. Composites and Folds The pre-aligned films are each stretched separately and folded twice independently, then flattened using a bending roller. The two folded pre-aligned films are then laminated together again. In total, the process involves four folds, resulting in an eight-layer composite film.

[0070] The composite layers are tightly bonded, without bubbles or wrinkles, forming a multilayer film with a uniform structure.

[0071] S5. Second stretch The composite oil film was subjected to biaxial stretching again at 135 °C, with a stretching ratio of 5 times in the MD direction and a stretching ratio of 5 times in the TD direction, to obtain a biaxially oriented composite film.

[0072] S6. Extraction and Drying The resulting bidirectional oriented composite membrane was subjected to extraction to remove paraffin oil. After extraction, the membrane was dried with constant-temperature hot air at 40 °C to remove residual dichloromethane solvent.

[0073] S7. Heat setting treatment The extracted membrane was heat-set at 140 °C to obtain the final diaphragm.

[0074] The properties of the diaphragm prepared in this embodiment are shown in Table 1.

[0075] Example 2 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S1: the molecular weight of the polyethylene powder is approximately 3 million.

[0076] In step S3: the stretching ratio in both the longitudinal (MD) and transverse (TD) directions is 5 times, and the area stretching ratio is 25 times.

[0077] In step S4: the folding is performed once to prepare a 4-layer composite membrane.

[0078] In step S5: the composite oil film is subjected to biaxial stretching again at 134 ℃, with a stretching ratio of 4 times in the MD direction and a stretching ratio of 4 times in the TD direction.

[0079] In step S7: the dried film is heat-set at 138 °C.

[0080] The properties of the diaphragm prepared in this embodiment are shown in Table 1.

[0081] Example 3 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S1: the molecular weight of the polyethylene powder is approximately 1.5 million.

[0082] In step S2: there are 4 die heads and the sheet thickness is 1.2 mm.

[0083] In step S3: the stretch ratio in both the longitudinal (MD) and transverse (TD) directions is 4 times, and the area stretch ratio is 16 times.

[0084] In step S4: the folding number is 0 times, and a 4-layer composite membrane is prepared.

[0085] In step S5: the composite oil film is subjected to biaxial stretching again at 132 ℃, with a stretching ratio of 4.5 times in the MD direction and 4.5 times in the TD direction.

[0086] In step S7: The dried film is heat-set at 136 °C. The properties of the diaphragm prepared in this embodiment are shown in Table 1.

[0087] Example 4 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S1: the molecular weight of the polyethylene powder is approximately 1 million.

[0088] In step S2: there are 4 die heads and the sheet thickness is 1.2 mm.

[0089] In step S3: the stretch ratio in both the longitudinal (MD) and transverse (TD) directions is 4 times, and the area stretch ratio is 16 times.

[0090] In step S4: the folding number is 0 times, and a 4-layer composite membrane is prepared.

[0091] In step S5: the composite oil film is subjected to biaxial stretching again at 131 ℃, with a stretching ratio of 4.5 times in the MD direction and a stretching ratio of 4.5 times in the TD direction.

[0092] In step S7: The dried film is heat-set at 135 °C. The properties of the diaphragm prepared in this embodiment are shown in Table 1.

[0093] Example 5 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S1: the molecular weight of the polyethylene powder is approximately 1 million.

[0094] In step S2: there are 4 die heads and the sheet thickness is 1.2 mm.

[0095] In step S3: the stretching ratio in both the longitudinal (MD) and transverse (TD) directions is 4 times, and the area stretching ratio is 16 times. The stretching temperature is 118 ℃.

[0096] In step S4: the folding number is 0 times, and a 4-layer composite membrane is prepared.

[0097] In step S5: the composite oil film is subjected to biaxial stretching again at 135 ℃, with a stretching ratio of 4.5 times in the MD direction and a stretching ratio of 4.5 times in the TD direction.

[0098] In step S7: The dried film is heat-set at 136 °C. The properties of the diaphragm prepared in this embodiment are shown in Table 1.

[0099] Comparative Example 1 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S2: there is 1 mold head.

[0100] In step S4: 0 folds, no composite layer.

[0101] In step S5: Biaxial stretching was not performed again.

[0102] In step S7: the dried film is heat-set at 130 °C. The properties of the diaphragm prepared in this comparative example are shown in Table 1.

[0103] Comparative Example 2 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S2: the opening of a single die head is adjusted to 2.1 mm, the surface linear velocity of the chilling roller is 4.6 m / min, and the thickness of the resulting casting is 2 mm.

[0104] In step S3: the stretch ratio in both the longitudinal (MD) and transverse (TD) directions is 15 times.

[0105] In step S4: 0 folds, no composite layer.

[0106] In step S5: Biaxial stretching was not performed again.

[0107] In step S7: The dried film is heat-set at 135 °C. The properties of the diaphragm prepared in this comparative example are shown in Table 1.

[0108] Comparative Example 3 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S1: the molecular weight of the polyethylene powder is approximately 1 million.

[0109] In step S2: there are 4 die heads and the sheet thickness is 1.2 mm.

[0110] In step S3: the stretching temperature is 118 ℃.

[0111] In step S4: the folding number is 0 times, and a 4-layer composite membrane is prepared.

[0112] In step S5: the composite oil film is subjected to biaxial stretching again at 135 ℃, with a stretching ratio of 3 times in the MD direction and a stretching ratio of 3 times in the TD direction.

[0113] In step S7: The dried film is heat-set at 136 °C. The properties of the diaphragm prepared in this comparative example are shown in Table 1.

[0114] Comparative Example 4 The method for preparing the diaphragm in this embodiment is basically the same as in Example 1, except that: In step S1: the molecular weight of the polyethylene powder is approximately 1 million.

[0115] In step S2: there are 4 die heads and the sheet thickness is 1.2 mm.

[0116] In step S3: the stretching ratio in both the longitudinal (MD) and transverse (TD) directions is 4 times, and the area stretching ratio is 16 times. The stretching temperature is 118 ℃.

[0117] In step S4: the folding number is 0 times, and a 4-layer composite membrane is prepared.

[0118] In step S5: the composite oil film is subjected to biaxial stretching again at 120 ℃, with a stretching ratio of 4.5 times in the MD direction and a stretching ratio of 4.5 times in the TD direction.

[0119] In step S7: The dried film is heat-set at 136 °C. The properties of the diaphragm prepared in this comparative example are shown in Table 1.

[0120] Comparative Example 5 The preparation method of the diaphragm in this comparative example is basically the same as that in Example 2, except that the order of steps S3 and S4 is reversed. First, folding is performed, followed by stretching in the longitudinal (MD) and transverse (TD) directions. The properties of the diaphragm prepared in this comparative example are shown in Table 1.

[0121] Table 1

[0122] As can be seen from the examples and comparative examples: (1) The effect of casting thickness on the uniformity and delamination of the diaphragm structure: As shown in the comparison between Example 1 and Comparative Example 2, when the thickness of the cast sheet increased from 1.1 mm to 2.0 mm, the cooling rate decreased significantly, leading to uneven internal crystallization and non-uniform microporous structure, ultimately manifesting as elongated pores and deteriorated surface morphology (see Example 2). Figure 3 Therefore, controlling the thickness of the casting within the range of 1.0 mm to 1.2 mm helps to obtain a dense and uniform initial structure, which is a prerequisite for achieving ultra-high magnification stretching and preventing delamination.

[0123] (2) Effect of single stretching ratio on molecular orientation and interlayer bonding: A comparison of Example 5 and Comparative Example 3 shows that when the initial stretching ratio is too low (3×3), molecular orientation is insufficient, resulting in low mechanical strength of the oil film. In subsequent lamination and secondary stretching, interlayer stress cannot be uniformly transferred, leading to localized delamination (see...). Figure 6 When the stretching ratio is controlled at 5×5 to 6×6, the molecular chains are initially oriented and still maintain ductility, which can maintain good interlayer bonding and structural integrity in subsequent compounding and re-stretching.

[0124] (3) The influence of the number of composite layers and folding process on interlayer bonding and overall strength: A comparison of Example 1 (8-layer composite) and Comparative Example 1 (single-layer membrane) shows that the multi-layer composite structure effectively improves the puncture resistance of the membrane. The composite layers form a microscopic interlocking structure, which can disperse external impacts and block crack propagation paths, thus achieving a needle penetration strength of 465 gf even with a membrane thickness of only 3 μm. Conversely, the strength of the uncomposite single-layer membrane is significantly reduced under the same thickness conditions.

[0125] In addition, too many composite layers or poor bonding can also cause interlayer delamination (Comparative Example 4). Therefore, forming a composite structure of 4 to 8 layers through 0 to 2 folds is a better balance.

[0126] (4) Effects of secondary tensile temperature and magnification on pore structure and strength: In Examples 1 and 2, when the secondary stretching temperature was controlled at 130°C to 135°C and the expansion ratio was 4×4 to 5×5, the stress release within the film was uniform, and the pore structure was fine and well-oriented. If the temperature was too low (such as 120°C in Comparative Example 4), the polyethylene segments were not sufficiently oriented, leading to an increase in the proportion of elongated pores; if the temperature was too high, local melting or delamination was likely to occur. Therefore, the optimal secondary stretching temperature window of the present invention is 130°C to 135°C.

[0127] (5) The effect of polyethylene molecular weight on orientation and mechanical properties: Compared to Examples 2-4 (1-3 million), Example 1 (molecular weight 5 million) shows that as the molecular weight increases, the molecular chain length and entanglement density increase, and the stretch-induced orientation becomes more significant, thus significantly improving the needle-punching strength. Specifically, when the molecular weight is ≥3 million, a needle-punching strength of 465 gf can be achieved with a thickness of 3 μm, demonstrating excellent molecular orientation effect and microcrystalline strengthening effect. However, if the molecular weight is too high and the cooling and stretching conditions are not optimized, it will lead to extrusion difficulties or uneven film thickness (requiring coordinated control through multi-die head and rapid cooling).

[0128] (6) Overall performance and application value: As shown in Table 1, the diaphragms of the embodiments of the present invention, with a thickness of 3 μm to 5 μm, exhibit a needle penetration strength of 350 gf to 465 gf, a needle penetration strength to thickness ratio ≥100 gf / μm, and show no delamination or elongated pores, with a porosity maintained at 30% to 40%, thus balancing mechanical strength and electrolyte wettability. In contrast, the comparative samples either have excessive thickness, uneven pore shape, or delamination.

[0129] (7) The effect of the ratio of setting temperature to stretching temperature on mechanical properties: As can be seen from Examples 1 to 5, when the temperature difference between the second stretching temperature and the first stretching temperature is in the range of 16 ℃ to 20 ℃, the temperature difference between the heat setting temperature and the first stretching temperature is in the range of 18 ℃ to 25 ℃, and the temperature difference between the heat setting temperature and the second stretching temperature is in the range of 4 ℃ to 5 ℃, the stress release in the film is uniform, the pore structure is fine and well oriented, and the polyethylene chain segments are fully oriented, thereby significantly improving the needle punching strength.

[0130] This proves that by rationally designing the thickness of the cast film, the primary and secondary stretching ratios and temperatures, and the matching relationship between the number of composite layers and the molecular weight, it is possible to achieve high needle penetration strength and high structural stability while ensuring extremely thin film thickness, thus achieving a triple performance balance of "high orientation - high strength - non-delamination" that is difficult to achieve with traditional diaphragms.

[0131] The "strength" of a membrane does not depend solely on its thickness, but rather on: the orientation degree of the molecular chains; the ratio of crystalline to amorphous regions; and the coherence and defect density of the crystalline regions. At high stretch ratios: the polyethylene chains are forced to straighten along the stretching direction, strengthening intermolecular van der Waals forces; stretch-induced crystallization increases significantly; and the amorphous regions are "thinned," resulting in more uniform stress distribution. Therefore, both longitudinal (MD) strength and needle-punch strength are greatly improved. Thus, despite a thinner membrane, the molecular chain arrangement per unit area is more regular, the stress distribution is more uniform, and the fracture strain capacity is stronger. The result is—the overall mechanical strength and puncture resistance of the membrane are actually higher.

[0132] This application utilizes an ultra-high ratio biaxial stretching process to highly oriented polyethylene molecular chain segments along the stretching direction, forming ordered microcrystalline regions and a continuous oriented phase structure. This significantly improves the needle-punching strength and overall mechanical properties of the diaphragm while maintaining a relatively thin film thickness, preventing delamination or rupture of the diaphragm under high-ratio stretching.

[0133] Furthermore, based on the separators of Embodiments 1-5 provided by the present invention, any of the separators can be applied to lithium batteries.

Claims

1. A diaphragm, characterized in that: The diaphragm has a thickness of 1 µm to 7 µm; a needle penetration strength of 100 gf to 950 gf; and a needle penetration strength to thickness ratio ≥ 70 gf / µm. The diaphragm is composed of multiple layers of oil film composite, or is made by folding and compositing oil films.

2. The diaphragm according to claim 1, characterized in that: The diaphragm has ≥2 composite layers, and the interlayer interfaces are tightly bonded and without delamination.

3. The diaphragm according to claim 1, characterized in that: The diaphragm has a porosity of 15% to 80%.

4. The method for preparing the diaphragm according to any one of claims 1-3, characterized in that: Includes the following steps: S1. Raw material preparation: Polyethylene powder with a viscosity-average molecular weight of 600,000 to 10,000,000 is melt-plasticized and mixed with plasticizer to obtain a melt; S2. Casting: The melt is extruded through a die to form a cast sheet with a thickness of 0.2 mm to 2.0 mm; S3. First stretching: The casting is subjected to biaxial stretching with a stretching ratio of MD×TD=2×2~15×15 to obtain an initial orientation film; S4. Lamination and folding: The at least two initially oriented films are laminated together to obtain a composite film; or the initially oriented films are each folded at least once independently, and then laminated together to obtain a composite film. S5. Second stretching: The composite film is subjected to bidirectional secondary stretching with a stretching ratio of MD×TD=1.5×1.5~15×15 to form a bidirectional oriented film; S6. Extraction and drying: The bidirectionally oriented membrane is extracted and dried to obtain the extracted membrane; S7. Heat setting treatment: The extracted membrane is heat set to obtain the diaphragm.

5. The method for preparing the diaphragm according to claim 4, characterized in that: In step S3, the temperature during the biaxial stretching process is 110 ℃~135 ℃.

6. The method for preparing the diaphragm according to claim 4, characterized in that: In step S5, the temperature of the second stretching is 125 ℃~140 ℃; in step S7, the temperature of the heat setting is 125 ℃~150 ℃.

7. The method for preparing the diaphragm according to claim 4, characterized in that: In step S1, the raw materials also include paraffin oil.

8. The method for preparing the diaphragm according to claim 7, characterized in that: In step S1, the mass ratio of the polyethylene powder to the paraffin oil is (8%~35%):(65%~92%).

9. The application of the diaphragm, characterized in that: The separator according to any one of claims 1-3 is applied in the preparation of lithium batteries.

10. A battery, characterized in that: It includes the diaphragm according to any one of claims 1-3.

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