Tear-resistant diaphragm for electrochemical device as well as preparation method and application of tear-resistant diaphragm

Abstract

The invention discloses a tear-resistant diaphragm for an electrochemical device, which is characterized in that long-range molecular chain entanglement of a polyolefin film is regulated and controlled by blending long-branch-chain component polyethylene and short-branch-chain high-molecular-weight high-density polyethylene and controlling the overall viscosity-average molecular weight, and after heat treatment and secondary directional stretching forming, the tear-resistant diaphragm is prepared. It is found that the puncture resistance in the direction perpendicular to the surface of the film is excellent, the tear resistance of the film breadth is also synchronously improved, and cracks and rapid tearing caused by increase of the production speed and increase of the tension of rim charges can be avoided; the diaphragm provided by the invention can better cope with the generation and rapid expansion of cracks caused by external impact, sharp object impact and tension fluctuation.

Description

Technical Field

[0001] The present invention relates to the technical field of diaphragms for electrochemical devices, and more particularly, to a tear-resistant diaphragm for electrochemical devices, a preparation method thereof, and an application in electrochemical devices. Background Art

[0002] The diaphragms for electrochemical devices in the prior art are mainly polyolefin porous diaphragms. A polyolefin porous diaphragm is a three-dimensional structure with a large number of tiny and interconnected pores inside and a polyolefin framework supporting a large number of pores. Although these materials have good isolation performance, there are still some limitations in terms of tear resistance. Moreover, due to the thin thickness of the diaphragms for electrochemical devices, the diaphragms are prone to crack under the action of external impact or tension, and the cracks are likely to spread rapidly. In the actual production and use environments of electrochemical devices represented by lithium-ion batteries, there are situations such as resistance to tearing, external impact, impact by sharp objects, and tension fluctuation in the following scenarios, for example:

[0003] 1. The assembly process of electrochemical devices: During the battery assembly process, the diaphragm may be subjected to mechanical forces, such as the extrusion force during manual assembly or automatic assembly, which may cause cracks in the diaphragm.

[0004] 2. The transportation and handling of electrochemical devices: During the transportation and handling of the battery, the battery diaphragm may be subjected to external impact or impact by sharp objects, such as collision, dropping, etc.

[0005] 3. The use environment of electrochemical devices: During the actual use of electrochemical devices, various external impacts and impacts by sharp objects may be encountered, such as collision in mobile devices, jolting in automobiles, etc.

[0006] 4. The charge and discharge cycle of electrochemical devices: During the charge and discharge cycle of electrochemical devices, the pressure and tension inside the battery will fluctuate, which may affect the tear resistance of the diaphragm.

[0007] In these scenarios, if the tear resistance of the battery diaphragm is insufficient, cracks are likely to occur and spread rapidly, resulting in the rapid tearing of the diaphragm. Therefore, in order to ensure the performance and safety of electrochemical devices, there are important requirements for the tear resistance of the diaphragm.

[0008] Moreover, in order to improve the puncture resistance safety of traditional wet-formed porous membranes, the puncture resistance performance is usually improved by increasing the diaphragm thickness or enhancing the tensile strength. However, during this process, as the diaphragm tension increases, the tolerance of the diaphragm to abnormal factors such as tension fluctuations gradually decreases. When there are small-amplitude tension fluctuations or external forces, especially when the production speed increases and the tension of the edge material rises, the diaphragm is extremely prone to cracking and rapid tearing, resulting in unnecessary production losses. Summary of the Invention

[0009] The purpose of the present invention is to provide a tear-resistant separator for an electrochemical device. By improving the tear-resistant effect, the production stability can be effectively improved, and during the application process, the resistance of the separator to being pierced by foreign objects can also be enhanced, and the separator can effectively resist the rupture caused by external force impact.

[0010] In addition, the present invention also provides a preparation method of the tear-resistant separator for an electrochemical device and the application of the torn separator for an electrochemical device in an electrochemical device.

[0011] The present invention is realized as follows:

[0012] The present invention first provides a tear-resistant separator for an electrochemical device, which is made of at least a polyethylene microporous membrane, and is characterized in that the polyethylene microporous membrane contains at least polyethylene containing long-chain components and short-chain high-molecular-weight high-density polyethylene;

[0013] Based on the weight percentage of the polyethylene microporous membrane, the content of the polyethylene containing long-chain components is 5-85 wt%, and the content of the short-chain high-molecular-weight high-density polyethylene is 15-95 wt%;

[0014] The viscosity-average molecular weight of the polyethylene microporous membrane is not less than 400,000 g / mol;

[0015] The transverse crack propagation speed of the tear-resistant separator for an electrochemical device is ≤200 m / s;

[0016] The transverse crack propagation speed is measured by the following method:

[0017] ① Cut the separator into a rolled sample with a width of 1000 mm;

[0018] ② Cut a 2000-mm-long separator, flatten it with a fixed tension, and apply a force of 100 N in the width direction;

[0019] ③ Use a cutter to vertically cut the edge of the separator at a speed of 5 m / s, with a cutting depth of 10 mm ± 1 mm, record whether the crack propagates, and the propagation speed of the crack along the width direction of the separator is defined as the transverse crack propagation speed;

[0020] Further, the polyethylene containing long-chain branched components is a copolymer of ethylene and an α-olefin with a carbon chain of at least 4, and the content of the α-olefin with a carbon chain of at least 4 in the copolymer composition is 0.1-5 wt%;

[0021] Further, the short-chain branched high molecular weight high density polyethylene is a polyethylene homopolymer or a copolymer of ethylene and propylene. The content of ethylene in the short-chain branched high molecular weight high density polyethylene is not less than 90%, and the viscosity-average molecular weight of the short-chain branched high molecular weight high density polyethylene is 450,000 g / mol to 1,200,000 g / mol;

[0022] Further, the microporous membrane further contains a pore-forming agent and an antioxidant. The content of the pore-forming agent is 10-50 wt%, and the content of the antioxidant is 0-0.5 wt%;

[0023] Further, the tear-resistant separator for electrochemical devices is prepared according to the following process: Step 1, a polyethylene resin raw material containing at least polyethylene containing long-chain branched components and short-chain branched high molecular weight high density polyethylene is dispersed, mixed and plasticized at high temperature by a twin-screw extruder to form a uniform melt, which is extruded through a die to form a cast film and then cooled and formed;

[0024] The cast film is subjected to the first stretching. During the stretching process, by controlling the temperature field, the temperature of the cast film itself is within the range of 100-135 °C to obtain an oil film;

[0025] Step 2, the oil film is extracted, and then subjected to the second stretching and heat setting, wherein the temperature of the heat setting is 110-145 °C;

[0026] Step 3, the shaped separator is wound up to obtain the tear-resistant separator for electrochemical devices.

[0027] Further, in Step 1, the temperature of the melt is 150-260 °C;

[0028] Further, in Step 1, the cast film is rapidly cooled and formed on a metal roller surface at 5-35 °C;

[0029] Further, in the first stretching of Step 1, the stretching ratio in the MD direction is 3-15 times, and the stretching ratio in the TD direction is 3-15 times.

[0030] Further, in the second stretching of Step 2, the stretching ratio in the MD direction is 1.0-2.5 times, and the stretching ratio in the TD direction is 1.1-2.5 times.

[0031] The present invention also provides a method for preparing a tear-resistant separator for electrochemical devices, specifically the following steps:

[0032] Step 1: Feed a polyethylene resin raw material containing at least polyethylene with long-chain branches and short-chain high molecular weight high-density polyethylene into a twin-screw extruder for high-temperature dispersion, mixing, and plasticization to form a homogeneous melt. Extrude the melt through a die to form a cast film, and then cool and shape it. Perform the first stretching on the cast film. During the stretching process, control the temperature field to keep the temperature of the cast film within the range of 100 - 135 °C to obtain an oil film. Based on the weight percentage of the polyethylene microporous membrane, the content of the polyethylene with long-chain branches is 5 - 85 wt%, and the content of the short-chain high molecular weight high-density polyethylene is 15 - 95 wt%.

[0033] Step 2: Extract the oil film, and then perform the second stretching and heat setting. The temperature for heat setting is 110 - 145 °C.

[0034] Step 3: Wind up the shaped separator to obtain the tear-resistant separator for electrochemical devices.

[0035] Further, in Step 1, the polyethylene with long-chain branches is a copolymer of ethylene and an α-olefin with a carbon chain of at least 4, and the content of the α-olefin with a carbon chain of at least 4 in the copolymer composition is 0.1 - 5 wt%.

[0036] Further, in Step 1, the short-chain high molecular weight high-density polyethylene is a polyethylene homopolymer or a copolymer of ethylene and propylene. The content of ethylene in the short-chain high molecular weight high-density polyethylene is not less than 90%, and the viscosity-average molecular weight of the short-chain high molecular weight high-density polyethylene is 450,000 g / mol to 1,200,000 g / mol.

[0037] Further, in Step 1, the polyethylene resin raw material also contains a pore-forming agent and an antioxidant. Based on the total mass of the polyethylene resin raw material, the content of the pore-forming agent is 40 - 95 wt%, and the content of the antioxidant is 0 - 0.5 wt%.

[0038] Further, in Step 1, the melt temperature is 150 - 260 °C.

[0039] Further, in Step 1, the cast film is rapidly cooled and shaped on a metal roller surface at 5 - 35 °C.

[0040] Further, in the first stretching in Step 1, the stretching ratio in the MD direction is 3 - 15 times, and the stretching ratio in the TD direction is 3 - 15 times.

[0041] Further, in the second stretching in Step 2, the stretching ratio in the MD direction is 1.0 - 2.5 times, and the stretching ratio in the TD direction is 1.1 - 2.5 times.

[0042] The present invention also provides an application of a tear-resistant separator for an electrochemical device, wherein the tear-resistant separator for an electrochemical device is applied to an electrochemical device.

[0043] The present invention also provides an electrochemical device, wherein the electrochemical device contains the tear-resistant separator for an electrochemical device of the present invention.

[0044] Technical effects of the present invention:

[0045] By blending a long-chain branched component polyethylene and a short-chain high-molecular-weight high-density polyethylene and controlling the overall viscosity-average molecular weight, the present invention regulates the long-range molecular chain entanglement of the polyolefin membrane. After heat treatment and secondary orientation stretching molding, it is found that not only the puncture resistance in the direction perpendicular to the membrane surface is excellent, but also the tear resistance in the membrane width direction is improved synchronously. It can avoid the generation of cracks and rapid tearing caused by the increase in production speed and the increase in the tension of the edge material, indicating that the separator of the present invention can better cope with the cracks and rapid expansion caused by external impacts, sharp object impacts, and tension fluctuations. Description of the Drawings

[0046] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0047] Figure 1 It is a SEM micrograph of the separator prepared in Example 1 of the present invention at a magnification of 20,000 times. Detailed Embodiments

[0048] [Polyethylene microporous membrane]

[0049] The polyethylene microporous membrane of the present invention is a membrane mainly formed of polyethylene, having a large number of fine pores inside and a structure in which these fine pores are connected, and gas or liquid can pass from one surface to the other surface. From the viewpoint of obtaining an appropriate membrane resistance and shut-off function, the porosity of the porous substrate is 15-70%, preferably 20%-60%. The average pore diameter of the polyethylene microporous membrane is preferably 0.01 μm - 0.1 μm, particularly preferably 0.02 - 0.08 μm. When the average pore diameter is above 0.01 μm, better permeability can be obtained and the permeability of the electrolyte becomes good. However, when the average pore diameter of the separator exceeds 0.1 μm, the growth of dendritic crystals cannot be suppressed and short-circuiting of the electrodes is likely to occur. As described above, the average pore diameter depends on the type of porous substrate material, the content of the pore-forming agent, and conditions such as the stretching treatment and temperature treatment during production. The viscosity-average molecular weight of the entire polyethylene microporous membrane is not less than 400,000 g / mol, preferably not less than 450,000 g / mol, and further preferably not less than 480,000 g / mol. The researchers found that when the viscosity-average molecular weight of the selected polyethylene microporous membrane as a whole is relatively large, it is a factor promoting the tear resistance of the separator, but too high a molecular weight will result in too high a melt strength and poor flowability, which is not conducive to the winding and pore formation of the separator.

[0050] Based on the design of the battery size and taking into account the comprehensive consideration of the internal resistance and thermal runaway performance, the thickness of the polyethylene microporous membrane is 2 - 30 μm, preferably 4 - 15 μm, and further preferably 4 - 13 μm.

[0051] The polyethylene microporous membrane contains at least polyethylene containing a long-chain branched component and short-chain high-molecular-weight high-density polyethylene. The content of the polyethylene containing a long-chain branched component is 5 - 85 wt%, and the content of the short-chain high-molecular-weight high-density polyethylene is 15 - 95 wt%. Preferably, the content of the polyethylene containing a long-chain branched component is 10 - 80 wt%, and the content of the short-chain high-molecular-weight high-density polyethylene is 20 - 90 wt%. Further preferably, the content of the polyethylene containing a long-chain branched component is 15 - 70 wt%, and the content of the short-chain high-molecular-weight high-density polyethylene is 30 - 85 wt%.

[0052] The polyethylene containing long-chain branch components is a copolymer of ethylene and an α-olefin with a carbon chain of at least 4, such as a copolymer of ethylene and 1-butene, a copolymer of ethylene and 1-pentene, a copolymer of ethylene and 1-hexene, a copolymer of ethylene and 1-heptene, a copolymer of ethylene and 1-octene, a copolymer of ethylene and 1-nonene, a copolymer of ethylene and 1-decene, preferably a copolymer of ethylene and 1-butene or a copolymer of ethylene and 1-pentene. The content of the α-olefin with a carbon chain of at least 4 in the copolymer composition is 0.1-5 wt%, preferably 0.2-4 wt%, and more preferably 0.5-3 wt%. It can be speculated here that by introducing an appropriate amount of long-chain branches, the degree of branching of the polyethylene molecular chain can be increased, making it more difficult for the chain to slip and disengage when stressed, thereby improving the tear resistance and puncture resistance of the material. However, when the introduced long-chain branches are excessive, it may lead to excessive dispersion and cross-linking of the polyethylene molecular chain, thereby reducing the mechanical strength of the material. At the same time, too high a long-chain branch content may cause the material to decompose, deform or lose its physical properties in a high-temperature environment, thereby affecting the stability and safety of the battery separator.

[0053] The short-chain branched high molecular weight high density polyethylene is a polyethylene homopolymer or a copolymer of ethylene and propylene. The content of ethylene in the short-chain branched high molecular weight high density polyethylene is not less than 90%, preferably not less than 92%, and more preferably not less than 95%. And the viscosity-average molecular weight of the short-chain branched high molecular weight high density polyethylene is 450,000 g / mol to 1,200,000 g / mol, preferably 500,000 g / mol to 1,000,000 g / mol, and more preferably 550,000 g / mol - 900,000 g / mol. Thus, the short-chain branched high molecular weight high density polyethylene is usually preferably high molecular weight high density polyethylene (HMW-HDPE), with a density between 0.94 g / cm 3 and 0.96 g / cm 3 . By preferably adding a small amount of propylene monomer short-chain structure, the high molecular weight HDPE can have a higher density. Setting the short-chain branched high molecular weight high density polyethylene in the polyethylene microporous membrane in the above ratio helps to improve the puncture performance and mechanical properties of the separator, enabling it to better withstand internal pressure and external impact during battery operation.

[0054] [Auxiliary agent]

[0055] The microporous membrane further contains a pore-forming agent and an antioxidant. The pore-forming agent includes, but is not limited to, one or more liquid alkanes such as pentane, hexane, n-heptane, octane, nonane, isodecane, n-decane, and paraffin oil, and paraffin oil is preferably used. Through extraction, these pore-forming agents can form pores in the microporous membrane, increasing the permeability and porosity of the membrane. Based on the total mass of the polyethylene microporous membrane, the content of the pore-forming agent is 40-95 wt%, preferably 50-90 wt%, and further preferably 60-85 wt%.

[0056] The antioxidant includes, but is not limited to, one or more of antioxidant Irg1010, antioxidant 1076, antioxidant CA, and antioxidant P168. These antioxidants can be used in battery separators and have functions such as antioxidant, anti-aging, and inhibiting battery self-discharge, which can improve the stability and lifespan of the battery. Based on the total mass of the polyethylene microporous membrane, the content of the antioxidant is 0-0.5 wt%, preferably 0.05-0.4 wt%, and further preferably 0.1-0.3 wt%.

[0057] [Molding Process and Heat Treatment]

[0058] A method for preparing a tear-resistant separator for an electrochemical device disclosed by the present invention specifically comprises the following steps:

[0059] Step 1: A polyethylene resin raw material containing at least polyethylene containing a long-chain branched component and a short-chain high-molecular-weight high-density polyethylene and other additives are subjected to high-temperature dispersion, mixing, and plasticization by a twin-screw extruder to form a uniform melt. At this time, the melt temperature is 150-260 °C, preferably 160-220 °C, and further preferably 165-200 °C. The melt is extruded through a die to form a cast film, and then rapidly cooled and formed on a metal roller surface at 5-35 °C. The sheet thickness is controlled at 0.5-3 mm. A higher melt temperature is beneficial to improving the polymer fluidity and chain segment arrangement, thereby enhancing the tear resistance of the separator. However, too high a melt temperature causes partial fracture or decomposition of polyethylene molecules, thereby reducing its molecular weight and material properties.

[0060] The cast film is subjected to the first stretching. During the stretching process, by controlling the temperature field, the temperature of the cast film itself is within the range of 100 - 135 °C, preferably within 105 - 130 °C, and further preferably within 110 - 125 °C, so as to obtain an oil film. The control of the temperature field can be achieved by setting temperature sensors on both sides of the cast film to monitor its surface temperature in real time. Common heating methods include setting heating wires, hot air heating, and heating rollers, etc. According to the monitored temperature data, the control algorithm can make judgments and controls based on the set temperature range. If the temperature is lower than 100 °C, the control algorithm can start the heating device; if the temperature exceeds 135 °C, the control algorithm can stop the heating device. When the temperature is between 100 - 135 °C, the control algorithm can make fine adjustments as needed.

[0061] In the first stretching, the stretching ratio in the MD direction is 3 - 15 times, and the stretching ratio in the TD direction is 3 - 15 times. Preferably, the stretching ratio in the MD direction is 5 - 12 times, and the stretching ratio in the TD direction is 5 - 12 times. Further preferably, the stretching ratio in the MD direction is 7 - 10 times, and the stretching ratio in the TD direction is 7 - 10 times. By performing a large - scale stretching before extraction, on the one hand, the molecular structure of the cast film is rearranged and stretched in the stretching direction, thereby improving its mechanical properties; on the other hand, the molecular arrangement of the cast film becomes more uniform and compact, which can reduce the voids and defects between molecules, improve its dimensional stability, and avoid the emergence of stress concentration points during the oil film extraction process, thus preventing stress concentration when subjected to impact or tearing. However, the stretching amplitude should not exceed 15 times to avoid the fracture or breakage of the cast film and the deformation of the micropores.

[0062] Step 2: The oil film is extracted, and then subjected to the second stretching and heat setting. For paraffin - based porogens, alcohol solvents such as methanol, ethanol, or isopropanol, ether solvents such as petroleum ether, ketone solvents such as acetone, and halogenated alkane solvents such as dichloromethane are usually selected as extraction agents. Preferably, dichloromethane is used for extraction. Through extraction, the pore structure of the cast film is formed. Before the second stretching and heat setting, the extraction agent needs to be completely removed from the cast film, and usually methods such as evaporation drying are used to volatilize the solvent.

[0063] In the second stretching, the stretching ratio in the MD direction is 1.0 - 2.5 times, and the stretching ratio in the TD direction is 1.1 - 2.5 times. Preferably, the stretching ratio in the MD direction is 1.0 - 2.0 times, and the stretching ratio in the TD direction is 1.0 - 2.0 times. Further preferably, the stretching ratio in the MD direction is 1.2 - 1.8 times, and the stretching ratio in the TD direction is 1.2 - 1.8 times. When the second stretching is within the above - mentioned ratio range, the flatness and pore distribution of the diaphragm can be further improved, making the micropores have a more uniform distribution. The uniform distribution of the micropores is also beneficial to the tear resistance of the polyethylene microporous membrane.

[0064] After the second stretching, the polyethylene microporous membrane is heat-set under high-speed traction. For the polyethylene microporous membrane, the heat-setting temperature is 110 - 145 °C, preferably 120 - 140 °C, and more preferably 125 - 135 °C. During the heat-setting process, the polyethylene containing long-chain branched components and the short-chain high-molecular-weight polyethylene will undergo molecular rearrangement, making the molecular structure short-range ordered, eliminating part of the stress caused by stretching, thereby improving the mechanical strength and tear resistance of the material.

[0065] Step 3: Wind up the heat-set separator to obtain the tear-resistant separator for the electrochemical device; a single-axis winder, a double-axis winder, a full-automatic winder, a coating winder, etc. can be used for winding.

[0066] [Testing Method]

[0067] 1 - Crack transverse propagation speed

[0068] The crack transverse propagation speed of the present invention is measured by the following method:

[0069] ① Cut the separator into a wound sample with a width of 1000 mm.

[0070] ② Cut a 2000-mm-long separator, flatten it with a fixed pulling force, and apply a force of 100 N in the width direction.

[0071] ③ Use a cutter to vertically cut the edge of the separator at a speed of 5 m / s, with a cutting depth of 10 mm ± 1 mm, record whether the crack propagates, and the propagation speed of the crack along the width direction of the separator is defined as the crack transverse propagation speed.

[0072] The purpose of measuring the crack transverse propagation speed is to evaluate the resistance of the separator material to the transverse crack propagation. By applying a fixed pulling force and cutting the edge of the sample, the stress concentration and physical damage that may occur during actual use are simulated, so as to measure the crack propagation speed. This method can help evaluate the durability and reliability of the separator material and provide a basis for product design and optimization.

[0073] 2 - Puncture strength

[0074] Prepare a 5-cm-wide sheet-shaped polyethylene microporous membrane sample, fix it under the test fixture, use a high-speed tensile machine and a needle-punching fixture, use a 1-mm-diameter thorn needle on the puncture tester, and perform puncture at a speed of 50 mm / min. After the measured data is stable, the top thorn force F is obtained, and then the puncture strength (unit: gf) is calculated as F / 9.8 * 1000.

[0075] 3 - Gurley air permeability value

[0076] Cut a 100mm×100mm sheet-like polyethylene microporous membrane sample, and use the American Gurley 4110N air permeability tester to test in the 100cc test gas mode. Record the time when all the test gas passes through the separator membrane sample with the porous membrane, which is the Gurley value.

[0077] The features and properties of the present invention will be further described in detail below in conjunction with examples and comparative examples.

[0078] Example 1

[0079] Use 21.4 parts by weight of ultra-high molecular weight polyethylene powder (GUR4116, Ticona Celanese, viscosity-average molecular weight of 600,000 g / mol, density of 0.95 g / cm 3 )), the polyethylene of the long-chain branched component is a copolymer of ethylene and 1-butene, in which the content of 1-butene in the polyethylene of the long-chain branched component is 3 wt%, the content of the polyethylene of the long-chain branched component is 8.6 parts by weight, 69.7 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), add 0.2 parts by weight of antioxidant (Irg1010 (Ciba Specialty Chemicals BASF)) and 0.1 parts by weight of antioxidant (P168 (manufactured by Ciba Specialty Chemicals Corporation)), and mix them in sequence. Put the ultra-high molecular weight polyethylene material after mixing the additives and paraffin oil into a twin-screw extruder for kneading to finally form a mixture melt gel.

[0080] Use a T-die to extrude the gel, control the extrusion melt temperature at 185°C, and then quickly cool it on a constant-temperature metal pair of rollers with a surface temperature of 15°C and roll it into a sheet with a sheet thickness of 1.5 mm.

[0081] Traction and the first stretching are carried out on the obtained oil film sheet. At 120°C, stretching is carried out at 8 times in both the longitudinal direction (MD) and the transverse direction (TD). After stretching, an oil film is obtained. The oil film is led into extraction, and the paraffin oil in the oil film is extracted with dichloromethane solvent. Then, the residue in the diaphragm is carried out by hot air at a constant temperature of 40°C. After the extraction is completed, the second stretching is carried out, and stretching is carried out at 1.5 times in both the longitudinal direction (MD) and the transverse direction (TD). Then, heat setting treatment is carried out at 130°C to finally obtain a tear-resistant diaphragm. Its SEM micrograph is as Figure 1 shown.

[0082] Example 2

[0083] Compared with Example 1, use ultra-high molecular weight polyethylene powder (GUR4116, Ticona Celanese, viscosity-average molecular weight of 600,000 g / mol, density of 0.95 g / cm3 ) 9.3 parts by weight, the polyethylene of the long-chain branched component is a copolymer of ethylene and 1-butene, in which the content of 1-butene in the polyethylene of the long-chain branched component is 3 parts by weight, the content of the polyethylene of the long-chain branched component is 25.7 parts by weight, 64.7 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), 0.2 parts by weight of antioxidant (Irg1010 (manufactured by Ciba Specialty Chemicals BASF)), 0.1 parts by weight of antioxidant (P168 (manufactured by Ciba Specialty Chemicals Corporation)), and they are mixed in sequence.

[0084] The rest remains unchanged.

[0085] Example 3

[0086] Compared with Example 1, the polyethylene of the long-chain branched component is a copolymer of ethylene and 1-octene, in which the content of 1-octene in the polyethylene of the long-chain branched component is 4.5 wt%, and the content of the polyethylene of the long-chain branched component is 8.6 parts by weight.

[0087] The rest remains unchanged.

[0088] Example 4

[0089] Compared with Example 1, the ultra-high molecular weight polyethylene powder (GUR4116, Ticona Celanese, with a viscosity-average molecular weight of 600,000 g / mol and a density of 0.95 g / cm 3 ) is replaced with an ethylene-propylene copolymer powder with a viscosity-average molecular weight of 850,000 g / mol.

[0090] The rest remains unchanged.

[0091] Example 5

[0092] Compared with Example 1, the obtained oil film sheet is pulled and subjected to the first stretching. At 120 °C, the longitudinal (MD) and transverse (TD) are both stretched by 5 times.

[0093] The rest remains unchanged.

[0094] Example 6

[0095] Compared with Example 1, the obtained oil film sheet is pulled and subjected to the first stretching. At 120 °C, the longitudinal (MD) and transverse (TD) are both stretched by 14 times.

[0096] The rest remains unchanged.

[0097] Example 7

[0098] Compared with Example 1, using a T-die head, the gel was extruded, the extrusion melt temperature was controlled at 150 °C, then it was rapidly cooled on a constant-temperature metal pair of rollers with a surface temperature of 15 °C, and rolled into a sheet with a sheet thickness of 1.5 mm.

[0099] The obtained oil film sheet was drawn and subjected to the first stretching. At 100 °C, stretching was carried out at a ratio of 8 times in both the longitudinal direction (MD) and the transverse direction (TD).

[0100] The rest remained unchanged.

[0101] Example 8

[0102] Compared with Example 1, using a T-die head, the gel was extruded, the extrusion melt temperature was controlled at 250 °C, then it was rapidly cooled on a constant-temperature metal pair of rollers with a surface temperature of 15 °C, and rolled into a sheet with a sheet thickness of 1.5 mm.

[0103] The obtained oil film sheet was drawn and subjected to the first stretching. At 130 °C, stretching was carried out at a ratio of 8 times in both the longitudinal direction (MD) and the transverse direction (TD).

[0104] The rest remained unchanged.

[0105] Example 9

[0106] Compared with Example 1, using a T-die head, the gel was extruded, the extrusion melt temperature was controlled at 185 °C, then it was rapidly cooled on a constant-temperature metal pair of rollers with a surface temperature of 32 °C, and rolled into a sheet with a sheet thickness of 1.5 mm.

[0107] The rest remained unchanged.

[0108] Example 10

[0109] Compared with Example 1, after the extraction was completed, the second stretching was carried out, and stretching was carried out at a ratio of 2 times in both the longitudinal direction (MD) and the transverse direction (TD).

[0110] The rest remained unchanged.

[0111] Example 11

[0112] Compared with Example 1, after the extraction was completed, the second stretching was carried out, and stretching was carried out at a ratio of 1.5 times in both the longitudinal direction (MD) and the transverse direction (TD). Then, it was subjected to a heat setting treatment at 140 °C, and finally a tear-resistant diaphragm was obtained.

[0113] The rest remained unchanged.

[0114] Example 12

[0115] Compared with Example 1, after the extraction is completed, a second stretching is carried out. The second stretching only performs transverse (TD) stretching, and the transverse stretching ratio is 1.1 times. Then, it is subjected to heat setting treatment at 130 °C, and finally a tear-resistant diaphragm is obtained.

[0116] Example 13

[0117] Compared with Example 1, after the extraction is completed, a second stretching is carried out, in which both longitudinal (MD) and transverse (TD) are stretched at a ratio of 1.1 times. Then, it is subjected to heat setting treatment at 130 °C, and finally a tear-resistant diaphragm is obtained.

[0118] Comparative Example 1

[0119] Compared with Example 1, polyethylene without long-chain branch components, ultra-high molecular weight polyethylene powder (GUR4116, Ticona Celanese, with a viscosity-average molecular weight of 600,000 g / mol and a density of 0.95 g / cm 3 ) is 30 parts by weight.

[0120] The rest remains unchanged.

[0121] Comparative Example 2

[0122] Compared with Example 1, 4.1 parts by weight of ultra-high molecular weight polyethylene powder (GUR4116, Ticona Celanese, with a viscosity-average molecular weight of 600,000 g / mol and a density of 0.95 g / cm 3 ) is used. The polyethylene with long-chain branch components is a copolymer of ethylene and 1-butene, in which the content of 1-butene in the polyethylene with long-chain branch components is 3 wt%, the content of the polyethylene with long-chain branch components is 30.9 parts by weight, 64.7 parts by weight of paraffin oil (70#, Zhejiang Zhengxin) is added, and 0.2 wt% of antioxidant (Irg1010 (BASF, Ciba Specialty Chemicals)) and 0.1 wt% of antioxidant (P168 (manufactured by Ciba Specialty Chemicals Co., Ltd.)) are added, and they are mixed in sequence.

[0123] The rest remains unchanged.

[0124] Comparative Example 3

[0125] Compared with Example 1, a T-shaped die is used to extrude the gel. The extrusion melt temperature is controlled at 185 °C, and then it is cooled at a rate of 10 °C / min, and rolled into a sheet with a sheet thickness of 1.5 mm.

[0126] The rest remains unchanged.

[0127] Comparative Example 4

[0128] Compared with Example 1, the ultra-high molecular weight polyethylene powder (GUR4116, Ticona Celanese, with a viscosity-average molecular weight of 600,000 g / mol and a density of 0.95 g / cm 3 ) was replaced with LLDPE (Lanzhou Petrochemical, LL101AA), with a viscosity-average molecular weight of 200,000 g / mol and a density of 0.925 g / cm 3 .

[0129] The rest remained unchanged.

[0130] Comparative Example 5

[0131] Compared with Example 1, the obtained oil film sheet was drawn and subjected to the first stretching. At 130 °C, stretching was carried out at a ratio of 2.5 times in both the longitudinal direction (MD) and the transverse direction (TD).

[0132] The rest remained unchanged.

[0133] Comparative Example 6

[0134] Compared with Example 1, the second stretching was not carried out, and heat setting was directly carried out after extraction.

[0135] Examples 1-13 and Comparative Examples 1-6 were tested as shown in the following table:

[0136]

[0137]

[0138] It can be seen that compared with Comparative Example 1 without long-chain branched component polyethylene, Comparative Example 2 with the long-chain branched component polyethylene ratio not within the defined range, Comparative Example 3 without quenching and cooling in the process, Comparative Example 4 with the molecular weight of the polyolefin raw material used not within the defined range, Comparative Example 5 with the first stretching not within the defined range, and Comparative Example 6 without the second stretching, Examples 1-13 have better crack transverse propagation speed, puncture strength, and Gurley value reflecting ion transport performance.

[0139] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A tear-resistant separator for an electrochemical device, which is made of at least a polyethylene microporous membrane, Characterized in that, The polyethylene microporous membrane contains at least polyethylene containing a long-chain branch component and short-chain high molecular weight high-density polyethylene; Based on the weight percentage of the polyethylene microporous membrane, the content of polyethylene containing a long-chain branch component is 5-85 wt%, and the content of short-chain high molecular weight high-density polyethylene is 15-95 wt%; The viscosity-average molecular weight of the polyethylene microporous membrane is not less than 400,000 g / mol; The lateral crack propagation speed of the tear-resistant separator for an electrochemical device is ≤200 m / s; Wherein the lateral crack propagation speed is measured by the following method: ① Cut the separator into a rolled sample with a width of 1000 mm; ② Cut a 5000 mm long separator, flatten it with a fixed tensile force, and apply a force of 100 N in the width direction; ③ Use a cutter to vertically cut the edge of the separator at a speed of 5 m / s, with a cutting depth of 10 mm ± 1 mm, record whether the crack propagates, and the propagation speed of the crack along the width direction of the separator is defined as the lateral crack propagation speed.

2. The tear-resistant separator for an electrochemical device according to claim 1, Characterized in that, The polyethylene containing a long-chain branch component is a copolymer of ethylene and an α-olefin with a carbon chain of not less than 4, and the content of the α-olefin with a carbon chain of not less than 4 in the copolymer composition is 0.1-5 wt%.

3. The tear-resistant separator for an electrochemical device according to claim 1, Characterized in that, The short-chain high molecular weight high-density polyethylene is a polyethylene homopolymer or a copolymer of ethylene and propylene. The content of ethylene in the short-chain high molecular weight high-density polyethylene is not less than 90%, and the viscosity-average molecular weight of the short-chain high molecular weight high-density polyethylene is 450,000 g / mol to 1,200,000 g / mol.

4. The tear-resistant separator for an electrochemical device according to claim 1, Characterized in that, The separator is prepared according to the following process: Step 1: Plasticize a polyethylene resin raw material containing at least polyethylene containing a long-chain branch component and short-chain high molecular weight high-density polyethylene through a twin-screw high-temperature dispersion and mixing to form a uniform melt, extrude it through a die to form a cast film, and cool and form it; Perform the first stretching on the cast film. During the stretching process, control the temperature field so that the temperature of the cast film itself is within the range of 100-135 °C to obtain an oil film; Step 2: Extract the oil film, and then perform the second stretching and heat setting, where the temperature of the heat setting is 110-145 °C; Step 3: Wind up the shaped separator to obtain the tear-resistant separator for an electrochemical device.

5. The tear-resistant separator for an electrochemical device according to claim 4, Characterized in that, In step 1, the melt temperature is 150-260 °C.

6. The tear-resistant separator for an electrochemical device according to claim 4, Characterized in that, In step 1, the cast film is rapidly cooled and formed on a metal roller surface at 5-35 °C.

7. The tear-resistant separator for an electrochemical device according to claim 4, Characterized in that, In the first stretching in Step 1, the stretching ratio in the MD direction is 3 - 15 times, and the stretching ratio in the TD direction is 3 - 15 times.

8. The tear-resistant separator for an electrochemical device according to claim 4, characterized in that in the second stretching in Step 2, the stretching ratio in the MD direction is 1.0 - 2.5 times, and the stretching ratio in the TD direction is 1.1 - 2.5 times.

9. The method for preparing a tear-resistant separator for an electrochemical device according to claim 1, characterized in that it specifically comprises the following steps: Step 1: A polyethylene resin raw material containing at least polyethylene containing a long-chain branch component and a short-chain high molecular weight high-density polyethylene is subjected to high-temperature dispersion mixing and plasticization by a twin-screw extruder to form a uniform melt, which is extruded through a die to form a cast film and then cooled and shaped; The cast film is subjected to the first stretching. During the stretching process, by controlling the temperature field, the temperature of the cast film itself is within the range of 100 - 135 °C, thereby obtaining an oil film; wherein, based on the weight percentage of the polyethylene microporous membrane, the content of the polyethylene containing a long-chain branch component is 5 - 85 wt%, and the content of the short-chain high molecular weight high-density polyethylene is 15 - 95 wt%; Step 2: The oil film is extracted, and then subjected to the second stretching and heat setting, wherein the heat setting temperature is 110 - 145 °C; Step 3: The shaped separator is wound up to obtain the tear-resistant separator for an electrochemical device.

10. The preparation method according to claim 9, characterized in that in Step 1, the polyethylene containing a long-chain branch component is a copolymer of ethylene and an α-olefin with a carbon chain of at least 4, and the content of the α-olefin with a carbon chain of at least 4 in the copolymer composition is 0.1 - 5 wt%.

11. The preparation method according to claim 9, characterized in that in Step 1, the short-chain high molecular weight high-density polyethylene is a polyethylene homopolymer or a copolymer of ethylene and propylene. The content of ethylene in the short-chain high molecular weight high-density polyethylene is not less than 90%, and the viscosity-average molecular weight of the short-chain high molecular weight high-density polyethylene is 450,000 g / mol to 1,200,000 g / mol.

12. The preparation method according to claim 9, characterized in that in Step 1, the melt temperature is 150 - 260 °C.

13. The preparation method according to claim 9, characterized in that in Step 1, the cast film is rapidly cooled and shaped on a metal roller surface at 5 - 35 °C.

14. The preparation method according to claim 9, characterized in that in the first stretching in Step 1, the stretching ratio in the MD direction is 3 - 15 times, and the stretching ratio in the TD direction is 3 - 15 times.

15. The preparation method according to claim 9, characterized in that in the second stretching in Step 2, the stretching ratio in the MD direction is 1.0 - 2.5 times, and the stretching ratio in the TD direction is 1.1 - 2.5 times.

16. The application of a tear-resistant separator for an electrochemical device, wherein the tear-resistant separator for an electrochemical device according to claims 1 - 8 is applied to an electrochemical device.

17. An electrochemical device, wherein the electrochemical device contains the tear-resistant separator for an electrochemical device according to claims 1-8.

Images