A puncture-resistant battery separator and method of making the same

Abstract

The application discloses a kind of puncture-resistant battery separators and preparation method thereof.The preparation method of the application mainly includes four steps of extrusion, stretching, extraction drying and heat setting;Among them, the branching degree, crystallinity and component content of the selected branched high molecular weight polyethylene are selected, that is, by using the short branch of branched high molecular weight polyethylene, a perfect entanglement crosslinking network is formed, and the diffusion and relaxation of polyethylene segment are inhibited.The application uses branched high molecular weight at the same time, and further improves the puncture resistance by using the method of temperature elution to screen low crystallinity and low melting point raw materials, while keeping its permeability and porosity basically unchanged.

Description

Technical Field

[0001] This invention relates to the field of polymer materials, specifically to a puncture-resistant battery separator and its preparation method. Background Technology

[0002] Lithium-ion batteries have experienced rapid development due to their superior characteristics, including high operating voltage, high energy density, long cycle life, light weight, small size, and zero pollution. They have become the main power source for various electronic products and are currently widely used in mobile phones, laptops, cameras, and other electronic products. They are also used as power sources for electric vehicles and hybrid vehicles, and the development of electric vehicles will further drive the demand for lithium-ion batteries. A lithium-ion battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. The battery separator, a polymer membrane between the positive and negative electrodes, is the most critical part of a lithium-ion battery, directly affecting its safety and cost. In the structure of a lithium battery, the separator is one of the key internal components. Its performance directly affects the battery's capacity, cycle performance, and safety characteristics. A high-performance separator plays a vital role in improving the overall performance of the battery.

[0003] The main functions of a battery separator are twofold: first, to isolate the positive and negative electrodes and prevent electrons from freely passing through; and second, to allow ions in the electrolyte to freely pass between the positive and negative electrodes. The ion conductivity of the battery separator directly affects the overall performance of the lithium-ion battery. Its function of isolating the positive and negative electrodes limits the increase in current under overcharging or temperature rise conditions, preventing short circuits and explosions. Its micropore self-sealing function provides safety protection for battery users and equipment.

[0004] Polyolefin microporous membranes have become the most important separator material for lithium-ion batteries due to their high porosity, low electrical resistance, high tear strength, good acid and alkali resistance, good elasticity, and retention of aprotic solvents. Among them, polyethylene microporous membranes prepared by thermally induced phase separation (TIPS) have a very high market share because their pore size and porosity are easily controlled and the resulting products have uniform pore size. TPS, also known as the wet process, is a method for preparing porous membranes developed in recent years. It involves mixing small molecules with polyolefin resin, heating and melting them to form a homogeneous mixture, then cooling to perform phase separation, pressing the mixture into a membrane, and then heating it for biaxial stretching. Residual solvents are then washed away with volatile substances, resulting in interconnected microporous materials. The pore size range of the separator prepared by this method is on the order of the size of the phase micro-interface, which is relatively small and uniform. The stretch ratio range is large, so the separator performance is isotropic and has high strength. It will not cause perforation in the normal process flow, and the product can be made very thin, which can meet the trend of increasingly thinner lithium-ion battery separators. However, the production cost is high.

[0005] With the increasing application of lithium-ion batteries and the trend towards smaller size and lighter weight, the required thickness of lithium battery separators is also decreasing, and safety is becoming increasingly important. One major challenge in separator development is preventing separator punctures that could lead to short circuits, causing the internal battery temperature to rise to the melting point of lithium or the ignition point of the electrolyte, thus triggering a fire. During the charging and discharging process of lithium batteries, dendrites easily grow on the electrodes. These dendrites can easily puncture the separator, causing a short circuit. The heat generated by the short circuit raises the internal temperature of the battery. When the separator temperature reaches the pore-closing temperature, the micropores close, blocking current flow. However, thermal inertia can cause the temperature to rise further, potentially reaching the melting and rupture temperature, leading to separator rupture and continuous heating, potentially resulting in combustion or explosion. Therefore, lithium battery separators require high puncture strength, a large pore-closing temperature, and a large difference between the melting and rupture temperatures.

[0006] Because the molecular chains are stretched and oriented to a high degree, the elongation at break in biaxial tensile strength is often unevenly distributed, making it prone to stress concentration under stress. This results in low elongation at break, low puncture resistance, and small deformation after puncture, making it susceptible to direct damage. Improving the elongation at break in all directions, as well as increasing puncture strength and puncture deformation rate, is a crucial direction for enhancing the safety and performance of lithium-ion battery separators. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing materials and technologies, and to provide a puncture-resistant battery separator and its preparation method. The puncture-resistant battery separator is prepared using branched high molecular weight polyethylene.

[0008] This invention first provides a method for preparing a puncture-resistant battery separator, the method comprising the following steps:

[0009] (1) Extrusion: Branched high molecular weight polyethylene, solvent and antioxidant are added to a swelling tank to swell and prepare a uniform swelling solution. Then the swelling solution is added to a twin-screw extruder and uniformly extruded into a gel film. The working temperature of the twin-screw extruder is 160 to 260°C.

[0010] (2) Stretching: The gel film extruded in step (1) is stretched synchronously or in steps in both directions; the stretching temperature is 100-130℃, and the longitudinal stretching ratio and the transverse stretching ratio are each 2-15 times independently.

[0011] (3) Extraction and drying: The stretched gel membrane is placed in the extraction tank and the solvent in the gel membrane is extracted using an extraction solution at 20-60℃. The extracted gel membrane is then dried at a drying temperature of 50-100℃ and is stretched laterally during the drying process with a stretching ratio of 1.01-1.4.

[0012] (4) Heat setting: The dried gel film is heat set to obtain a puncture-resistant battery separator.

[0013] As a preferred embodiment of the present invention, in step (1), the weight parts of branched high molecular weight polyethylene, solvent and antioxidant are as follows: 100 parts by weight of branched high molecular weight polyethylene, 200 to 3000 parts by weight of solvent and 0 to 20 parts by weight of antioxidant.

[0014] The branched high molecular weight polyethylene has a viscosity-average molecular weight of 1.0 × 10⁻⁶. 5 ~7.0×10 6 g / mol, branching degree of 1–40 branches / 1000 carbon atoms, density of 0.93–0.98 g / cm³ 3 ;

[0015] The content of the component obtained by the heated washing and grading of the branched high molecular weight polyethylene at 80°C is 5.0% to 50 wt%, preferably 10% to 20 wt%.

[0016] The branched high molecular weight polyethylene has a crystallinity of 45% to 60%, preferably 50% to 55%, and a melting point of 135 to 139°C, preferably 135 to 136°C.

[0017] As a preferred embodiment of the present invention, the branched high molecular weight polyethylene in step (1) has a viscosity-average molecular weight of 3.0 × 10⁻⁶. 5 ~1.0×10 6 g / mol, preferably 3.5 × 10 g / mol. 5 ~8.0×10 5 g / mol, more preferably 4.5 × 10 g / mol 5 ~6.5×10 5 g / mol.

[0018] As a preferred embodiment of the present invention, the solvent in step (1) is selected from at least one of decahydronaphthalene, paraffin oil, paraffin wax, kerosene, and white oil.

[0019] As a preferred embodiment of the present invention, the antioxidant mentioned in step (1) is at least one of antioxidant 264, antioxidant 1010, antioxidant 1076, antioxidant B225 and antioxidant B215.

[0020] As a preferred embodiment of the present invention, the extract in step (3) is either heptane or hexane.

[0021] The present invention also provides a puncture-resistant battery separator prepared by the method described above.

[0022] As a preferred embodiment of the present invention, by controlling the setting of the extrusion conditions in step (1) and the setting of the stretching parameters in step (2), the thickness of the puncture-resistant battery separator is 2 to 30 μm and the micropore diameter is 5 to 500 nm.

[0023] As a preferred embodiment of the present invention, the porosity of the puncture-resistant battery separator is 40-50%, and the difference between the pore-closing temperature and the membrane-breaking temperature is 10-50°C.

[0024] As a preferred embodiment of the present invention, the puncture-resistant battery separator has an MD shrinkage of less than 4% and a TD shrinkage of less than 3.5% under the condition of 120℃ / 1h, and a puncture strength of 300~600g for a 5μm membrane.

[0025] Compared with existing products, the present invention has the following beneficial effects:

[0026] This invention provides a membrane material with improved puncture resistance. It utilizes the short branches of branched high-molecular-weight polyethylene to form a complete entangled cross-linked network, while simultaneously inhibiting the diffusion and relaxation of polyethylene chain segments. Polyethylene is a typical semi-crystalline polymer. When the temperature rises, the crystalline phase segments melt, and without the inhibition of entanglement, the deformation and relaxation of the chain segments become more significant, affecting puncture resistance. This invention, while using branched high-molecular-weight polyethylene, further improves puncture resistance by screening low-crystallinity, low-melting-point raw materials through heated rinsing, while maintaining its permeability and porosity essentially unchanged. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] Preparation Example

[0029] The high molecular weight polyethylene used in the examples and comparative examples was prepared according to the following steps: the molecular weight was controlled by adjusting the partial pressure of hydrogen; the aggregated structure of the high molecular weight polyethylene was controlled by adjusting the polymerization time, polymerization temperature, and screw speed; and the degree of branching was controlled by adjusting the content of comonomers and the type of catalyst. High molecular weight polyethylenes were obtained under different experimental conditions, denoted as A1-A8. The correspondence between the experimental conditions and products A1-A8 is shown in Table 1, and the performance parameters are shown in Table 2.

[0030] High molecular weight polyethylene A1-A8 were all prepared according to the following steps:

[0031] (1) The polymerization experiment was carried out in a 1L high-pressure autoclave reactor. Before the experiment, the reactor was vacuum-sealed at 100℃ for 2 hours, and purged with nitrogen every 30 minutes to remove moisture and impurities. After the autoclave was sealed, 350g of n-heptane (solvent) was added to the reactor, and the mixture was stirred at 500rpm for 5 minutes. Then, 0.2MPa of hydrogen, 0.5g of triethylaluminum co-catalyst, and 10mg-20mg of Ziegler-Natta catalyst or metallocene catalyst with a Ti content of 5% were introduced. During the reaction, ethylene and different amounts of butene were continuously introduced to carry out the polymerization reaction. The reaction temperature was 60-80℃, the pressure was maintained at 0.7MPa, and the stirring speed was maintained at 500rpm.

[0032] Table 1

[0033]

[0034]

[0035] Table 2

[0036]

[0037] The method for preparing the puncture-resistant battery separator of the present invention can be carried out according to the following steps:

[0038] (1) Extrusion: Branched high molecular weight polyethylene, solvent and antioxidant are added to a swelling tank to swell and prepare a uniform swelling solution. Then the swelling solution is added to a twin-screw extruder and uniformly extruded into a gel film. The working temperature of the twin-screw extruder is 160 to 260°C.

[0039] (2) Stretching: The gel film extruded in step (1) is stretched synchronously or in steps in both directions; the stretching temperature is 100-130℃, and the longitudinal stretching ratio and the transverse stretching ratio are each 2-15 times independently.

[0040] (3) Extraction and drying: The stretched gel membrane is placed in an extraction tank and the solvent in the gel membrane is extracted using an extraction solution at 20-60℃. The extracted gel membrane is then dried at a temperature of 50-100℃ and is stretched laterally during the drying process with a stretching ratio of 1.01-1.4.

[0041] (4) Heat setting: The dried gel film is heat set to obtain a puncture-resistant battery separator.

[0042] In the above method, the solvent serves as the medium for the mixing process and also acts as a swelling agent. The solvent is required to dissolve or disperse the raw materials well. Preferably, the solvent is at least one of decahydronaphthalene, paraffin oil, paraffin wax, kerosene, and white oil. The restrictions on the amount of solvent used are generally quite lenient; however, to form a film with better performance during the extraction process, the preferred amount is 200–3000 parts by weight of solvent for 100 parts by weight of branched high molecular weight polyethylene.

[0043] In the above method, the twin-screw extruder is a commonly used extrusion molding equipment in the art, and its extrusion operation will not be described in detail. Preferably, the operating temperature of the twin-screw extruder is controlled at 160–260°C.

[0044] Similarly, biaxial stretching is a common operation in the field, but the stretching operation parameters have a significant impact on the membrane performance. Therefore, it is necessary to limit the biaxial stretching operation parameters to ensure the performance of the subsequent battery separator. This invention requires a stretching temperature of 100–130°C, and longitudinal stretching ratio and transverse stretching ratio of 2–15 times, each independently.

[0045] In the above method, extracting the solvent in the gel membrane using an extraction solution is a necessary step. The extraction solution can be at least one of heptane and hexane.

[0046] In the above method, the antioxidant is an optional additive component of the present invention. The antioxidant is at least one selected from antioxidant 264, antioxidant 1010, antioxidant 1076, antioxidant B225, and antioxidant B215. The amount of antioxidant added is 0-20 parts by weight of antioxidant per 100 parts by weight of branched high molecular weight polyethylene.

[0047] In the above methods, drying and heat setting are conventional operations in the field, and the present invention does not limit the specific operation methods of drying and heat setting.

[0048] In the above method, the choice of branched high molecular weight polyethylene has a significant impact on the performance of the battery separator. This invention requires that the branched high molecular weight polyethylene has a viscosity-average molecular weight of 1.0 × 10⁻⁶. 5 ~7.0×10 6 g / mol, preferably, the viscosity-average molecular weight of the branched high molecular weight polyethylene is 3.0 × 10 g / mol. 5 ~1.0×10 6 g / mol, preferably 3.5 × 10 g / mol. 5 ~8.0×10 5 g / mol, more preferably 4.5 × 10 g / mol 5 ~6.5×10 5g / mol. Branching degree is 1–40 branches / 1000 carbon atoms, density is 0.93–0.98 g / cm³. 3 ;

[0049] The content of the component obtained by the heated washing and grading of the branched high molecular weight polyethylene at 80°C is 5.0% to 50 wt%, preferably 10% to 20 wt%.

[0050] The branched high molecular weight polyethylene has a crystallinity of 45% to 60%, preferably 50% to 55%, and a melting point of 135 to 139°C, preferably 135 to 136°C.

[0051] Using the method described above, the battery separator obtained by this invention has a thickness of 2–30 μm and a micropore size of 5–500 nm. More preferably, the porosity of the battery separator is 40–50%, and the difference between the pore-closing temperature and the membrane rupture temperature is 10–50 °C. More preferably, the battery separator exhibits MD shrinkage of less than 4% and TD shrinkage of less than 3.5% under conditions of 120 °C / 1h, and a puncture strength of 300–600 g for a 5 μm membrane. The battery separator obtained by this invention possesses puncture resistance, improving the safety and performance of lithium-ion battery separators.

[0052] Example 1

[0053] (1) Extrusion: 100 parts of branched high molecular weight polyethylene A1 as shown in Table 2, 500 parts of white oil solvent, and 5 parts of antioxidant 1010 are added to a swelling tank to swell and prepare a uniform swelling solution. Then, the swelling solution is added to a twin-screw extruder and uniformly extruded into a gel film. The working temperature of the twin-screw extruder is 220℃.

[0054] (2) Stretching: The gel film extruded in step (1) is subjected to simultaneous or stepwise bidirectional stretching; the stretching temperature is 120℃, and the longitudinal stretching ratio and the transverse stretching ratio are each 8 times independently;

[0055] (3) Extraction and drying: The stretched gel membrane is put into the extraction tank and the solvent in the membrane is extracted by the 30℃ heptane extraction solution. The extracted gel membrane is dried at 60℃ and then stretched laterally by 1.1 times.

[0056] (4) Heat setting: The dried gel film is heat set at 130°C for 20 seconds. After heat setting, the puncture-resistant battery separator B1 is obtained. The specific performance parameters are shown in Table 3.

[0057] Examples 2-4

[0058] Based on Example 1, branched high molecular weight polyethylene A2-A4 was used to replace A1, while other conditions remained the same as in Example 1, to obtain puncture-resistant battery separators B2-B4. The properties are shown in Table 3.

[0059] Comparative Examples 1-4

[0060] Based on Example 1, the branched high molecular weight polyethylene A1 was replaced with A5-A8 as shown in Table 2, while other conditions remained unchanged, to obtain products B5-B8. The properties are shown in Table 3.

[0061] Characterization methods for lithium battery separators and polymers:

[0062] (1) Thickness: determined according to GB / T6672-2001 method.

[0063] (2) Puncture strength: determined according to GB / T 2679.7.

[0064] (3) Aperture: Determined according to GB / T 38949-2020.

[0065] (4) Melting point and crystallinity: determined by differential scanning calorimetry (DSC).

[0066] (5) Porosity: determined according to GB / T 33052-2016.

[0067] (6) Shrinkage rate: The distance L0 between two points on the diaphragm is measured under normal temperature (23℃) test environment. The sample is placed in a stainless steel oven at 120℃±1℃ and kept at the temperature for 1 hour. After the diaphragm is cooled to the normal temperature test environment, the distance L1 between two points on the diaphragm is measured. The shrinkage rate S is calculated according to the following formula: S=(L0-L1) / L0×100%.

[0068] (7) Closed-cell membrane rupture test: Perform DSC test and read the closed-cell temperature and membrane rupture temperature directly from the temperature rise curve on the DSC curve.

[0069] Table 3

[0070]

[0071] As shown in Table 3, the membrane thickness, pore size, and porosity are less affected by the degree of branching. However, the membrane rupture temperature and puncture strength of Examples 1-4 are significantly higher than those of Comparative Examples 1-4, while the transverse and longitudinal shrinkage rates are lower, indicating that the polyethylene selected in this invention can improve puncture strength. A comparison of Examples 1-4 and Comparative Examples 1-4 shows that satisfactory puncture resistance is achieved only when the branching degree, crystallinity, and the content of the component used in the 80°C rinsing process are simultaneously satisfied: 1–40 branches / 1000 carbon atoms, 45%–60%, and 5.0%–50%. Comparing Examples 1 and 4 shows that, under the same conditions, the membrane prepared with low-crystallinity polyethylene has better puncture strength. Comparing Examples 1 and 3 shows that appropriately increasing the content of the component used in the 80°C rinsing process can also increase the membrane rupture temperature and puncture strength, while reducing the shrinkage rate.

[0072] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a puncture-resistant battery separator, characterized in that, The preparation method includes the following steps: (1) Extrusion: Branched high molecular weight polyethylene, solvent and antioxidant are added to a swelling tank to swell and prepare a uniform swelling solution. Then the swelling solution is added to a twin-screw extruder and uniformly extruded into a gel film. The working temperature of the twin-screw extruder is 160~260℃. The weight proportions of branched high molecular weight polyethylene, solvent, and antioxidant are as follows: 100 parts by weight of branched high molecular weight polyethylene, 200-3000 parts by weight of solvent, and 0-20 parts by weight of antioxidant. The branched high molecular weight polyethylene has a viscosity-average molecular weight of 1.0 × 10⁻⁶. 5 ~7.0×10 6 g / mol, branching degree of 1~40 branches / 1000 carbon atoms, density of 0.93~0.98 g / cm³ 3 ; The component content at 80°C obtained by rinsing and classifying the branched high molecular weight polyethylene is 5.0%~50wt%. The branched high molecular weight polyethylene has a crystallinity of 45%~60% and a melting point of 135~139℃; (2) Stretching: The gel film extruded in step (1) is stretched in both directions simultaneously or in stages; the stretching temperature is 100~130℃, and the longitudinal stretching ratio and the transverse stretching ratio are each 2~15 times independently; (3) Extraction and drying: The stretched gel membrane is placed in the extraction tank and the solvent in the gel membrane is extracted using an extraction solution at 20-60℃. The extracted gel membrane is then dried at a temperature of 50-100℃ and stretched laterally during the drying process. The stretching ratio is 1.01-1.

4. (4) Heat setting: The dried gel film is heat set to obtain a puncture-resistant battery separator.

2. The method for preparing the puncture-resistant battery separator according to claim 1, characterized in that, The branched high molecular weight polyethylene mentioned in step (1) has a viscosity-average molecular weight of 3.0 × 10⁻⁶. 5 ~1.0×10 6 g / mol.

3. The method for preparing the puncture-resistant battery separator according to claim 1, characterized in that, The solvent mentioned in step (1) is selected from at least one of decahydronaphthalene, paraffin oil, paraffin, kerosene, and white oil.

4. The method for preparing the puncture-resistant battery separator according to claim 1, characterized in that, The antioxidant mentioned in step (1) is at least one of antioxidant 264, antioxidant 1010, antioxidant 1076, antioxidant B225 and antioxidant B215.

5. The method for preparing the puncture-resistant battery separator according to claim 1, characterized in that, The extractant in step (3) is either heptane or hexane.

6. The puncture-resistant battery separator prepared by the method according to any one of claims 1-5.

7. The puncture-resistant battery separator according to claim 6, characterized in that, The puncture-resistant battery separator has a thickness of 2~30μm and a micropore size of 5~500nm.

8. The puncture-resistant battery separator according to claim 6, characterized in that, The porosity of the puncture-resistant battery separator is 40-50%, and the difference between the pore-closing temperature and the membrane rupture temperature is 10-50℃.

9. The puncture-resistant battery separator according to claim 6, characterized in that, The puncture-resistant battery separator exhibits MD shrinkage of less than 4% and TD shrinkage of less than 3.5% at 120℃ / 1h, with a puncture strength of 300~600g for a 5μm membrane.

Images