A low-closure high-broken-membrane temperature separator plate and a preparation method thereof

Abstract

This invention discloses a low-closed-cell, high-breakage-temperature separator and its preparation method. The separator has a TMA breakage temperature ≥180℃, a TMA pore-closing temperature ≤130℃, and the difference between the TMA breakage temperature and the TMA pore-closing temperature is ≥50℃. The method includes the following steps: S1, mixing a polyethylene mixture of high-density polyethylene, high-molecular-weight high-density polyethylene, and ultra-high-molecular-weight polyethylene with paraffin oil, followed by mixing, extrusion, cooling casting, stretching, and extraction to obtain a substrate; S2, mixing a photoinitiator with a solvent, dissolving, and stirring evenly to obtain a modifier solution; S3, fully contacting the modifier solution obtained in step S2 with the substrate obtained in step S1, and drying to obtain an initial separator; S4, subjecting the initial separator to phototreatment to obtain a low-closed-cell, high-breakage-temperature separator. This invention effectively reduces the pore-closing temperature of the separator while increasing the breakage temperature, which is beneficial for improving the safety of lithium batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery separator technology, specifically relating to a separator with low pore size and high membrane breakage temperature and its preparation method. Background Technology

[0002] With social development and economic progress, the general public has increasingly higher demands for living environment and quality of life. Green energy is gradually gaining attention, and the lithium battery industry, as an important component of green energy, plays a pivotal role.

[0003] Lithium-ion batteries consist of electrodes, a separator, and an electrolyte. The separator plays a crucial role in isolating the positive and negative electrodes, preventing short circuits and the risk of explosion. Conventional lithium-ion battery separators use polyethylene as the raw material, which offers advantages such as good chemical stability, high mechanical strength, and low cost. Furthermore, the porous polyethylene separator used in these membranes can close its pores during abnormal temperature increases, thus stopping the battery reaction, preventing further temperature rise, and preventing the separator from melting and rupturing. However, polyethylene has a relatively low melting point, and the pore-closing temperature and rupture temperature of the polyethylene separator are close. If the battery overheats abnormally, the separator may not completely close its pores, leading to melting and rupture, causing a short circuit and posing a serious danger to the user's life. Therefore, lowering the pore-closing temperature and raising the rupture temperature of the separator would improve the safety of lithium batteries.

[0004] To improve battery safety, existing technologies employ the following methods: (1) doping with specially formulated high-melting-point resins. For example, Asahi Kasei Corporation of Japan provides a battery separator comprising a polyolefin porous membrane, which includes polyethylene and polypropylene resins. The membrane rupture temperature can reach 160°C. (2) siloxane-grafted polyethylene modification and crosslinking. For example, LG Chem of South Korea's patents CN 105576172 A, CN 111108627 A, CN 111108628 A and Liyang Yuequan of China's patent CN 111081949 A provide a method for preparing a crosslinked separator with increased rupture temperature. (3) Patent CN 111295285 A provides a multilayer composite membrane that simultaneously reduces the pore-closing temperature (130°C) and increases the rupture temperature (165°C). The above patents have disadvantages such as the raw materials being difficult to obtain, complex processes, and the insignificant increase in the difference between the rupture temperature and the pore-closing temperature. Summary of the Invention

[0005] Purpose of the invention: In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing a separator with low pore closure temperature and high membrane rupture temperature, which effectively reduces the pore closure temperature of the separator and increases the membrane rupture temperature, thereby improving the safety of lithium batteries.

[0006] Technical solution: To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0007] One object of the present invention is to provide a diaphragm with low closed-cell and high rupture temperature, wherein the TMA rupture temperature of the diaphragm is ≥180°C, the TMA closed-cell temperature is ≤130°C, and the difference between the TMA rupture temperature and the TMA closed-cell temperature is ≥50°C.

[0008] In one embodiment of the present invention, the TMA rupture temperature of the low-closed-cell, high-rupture-temperature diaphragm is ≥200°C, the TMA closure temperature is ≤130°C, and the difference between the TMA rupture temperature and the TMA closure temperature is ≥70°C.

[0009] In one embodiment of the present invention, the TMA rupture temperature of the low-closure-cell, high-rupture-temperature diaphragm is 207°C, the TMA closure-cell temperature is 129°C, and the difference between the TMA rupture temperature and the TMA closure-cell temperature is 78°C.

[0010] In one embodiment of the present invention, the membrane thickness of the low closed-cell high rupture temperature diaphragm is 5um-20um, the air permeability is ≤400s / 100cc, the pore size is 20nm-80nm, the needle penetration strength is ≥35gf / um, the porosity is 20%-60%, and the heat shrinkage rate at 130℃ is ≤25%.

[0011] In one embodiment of the present invention, the membrane thickness of the low closed-cell high rupture temperature diaphragm is 5um-10um, the air permeability is ≤200s / 100cc, the pore size is 30nm-50nm, the needle penetration strength is ≥40gf / um, the porosity is 25%-50%, and the heat shrinkage rate at 130℃ is ≤18%.

[0012] In one embodiment of the present invention, the low closed-cell high rupture temperature diaphragm has a film thickness of 8.6 μm, an air permeability of 146 s / 100 cc, a pore size of 38 nm, a needle penetration strength of 45 gf / μm, a porosity of 34%, and a thermal shrinkage rate of ≤18% at 130°C.

[0013] Another object of the present invention is to provide a method for preparing a diaphragm with low closed-cell and high membrane rupture temperature, comprising the following steps:

[0014] S1. A polyethylene mixture formed by mixing high-density polyethylene (molecular weight 300,000-600,000), high molecular weight high-density polyethylene (molecular weight 600,000-1,000,000), and ultra-high molecular weight polyethylene (molecular weight greater than 1,300,000) is mixed with paraffin oil, and then subjected to intensive mixing, extrusion, cooling casting, stretching, and extraction to obtain the base material.

[0015] S2. Mix the photoinitiator with the solvent, dissolve, and stir until homogeneous to obtain a modifier solution;

[0016] S3. The modifier solution prepared in step S2 is brought into full contact with the substrate prepared in step S1, and after drying, the initial diaphragm is obtained.

[0017] S4. The initial diaphragm obtained in step S3 is subjected to light irradiation to obtain a diaphragm with low closed-cell and high membrane rupture temperature.

[0018] High-density polyethylene (HDPE) exhibits good heat and cold resistance, excellent chemical stability, high rigidity and toughness, and good mechanical strength. It also demonstrates good dielectric properties and resistance to environmental stress cracking. Its hardness, tensile strength, and creep resistance are superior to low-density polyethylene; furthermore, it exhibits good abrasion resistance, electrical insulation, toughness, and cold resistance.

[0019] High molecular weight high-density polyethylene (HMWHDPE) is a relatively new type of polyethylene, with an average molecular weight of 200,000-500,000. It can be divided into homopolymers and copolymers. HMWHDPE exhibits superior stress cracking resistance, impact strength, tensile strength, rigidity, abrasion resistance, and chemical stability compared to high-density polyethylene, allowing its products to be used for extended periods in harsh environments.

[0020] Ultra-high molecular weight polyethylene (UHMWPE) is a linear thermoplastic engineering plastic with excellent comprehensive properties. It has superior wear resistance, self-lubricating properties, high strength, stable chemical properties, and strong anti-aging properties.

[0021] The high-density polyethylene has a melting point of 120℃~130℃, and the high molecular weight high-density polyethylene and the ultra-high molecular weight polyethylene each have a melting point above 130℃. Specifically, the high molecular weight high-density polyethylene has a melting point of 130℃~136℃, and the ultra-high molecular weight polyethylene has a melting point of 130℃~136℃. This invention reduces the pore-closure temperature of the diaphragm by introducing low-melting-point high-density polyethylene, but this reduces the strength of the diaphragm product. The introduction of ultra-high molecular weight polyethylene, however, maintains the high strength performance of the diaphragm. Simultaneously, through modification with a photoinitiator, the rupture temperature of the diaphragm is significantly increased. Therefore, the final diaphragm product, while meeting other physical property requirements, also possesses the characteristics of low pore-closure temperature and high rupture temperature, achieving a relatively high temperature difference between the low pore-closure temperature and the high rupture temperature.

[0022] In one embodiment of the present invention, in step S1, the polyethylene mixture is composed of 10% high-density polyethylene, 80% high molecular weight high-density polyethylene, and 10% ultra-high molecular weight polyethylene by mass percentage. The high molecular weight high-density polyethylene is the main component. On the one hand, the introduction of low-melting-point high-density polyethylene can lower the pore-closure temperature of the membrane; on the other hand, the introduction of ultra-high molecular weight polyethylene with a higher melting point can maintain and increase the membrane rupture temperature, achieving a balance between low pore-closure temperature and high rupture temperature among the three polyethylene raw materials.

[0023] In one embodiment of the present invention, in step S1, the polyethylene mixture and paraffin oil are mixed at a mass ratio of 1-5:1.

[0024] In one embodiment of the present invention, in step S1, the polyethylene mixture and paraffin oil are mixed at a mass ratio of 3:1.

[0025] In one embodiment of the present invention, the substrate obtained in step S1 has a TMA breaking temperature ≥152°C and a TMA closing temperature ≤130°C.

[0026] In one embodiment of the present invention, the substrate obtained in step S1 has a TMA breaking temperature of 153°C and a TMA closing temperature of 129°C.

[0027] In one embodiment of the present invention, the substrate obtained in step S1 has a film thickness of 5um-20um, an air permeability value of ≤400s / 100cc, and a pore size of 20nm-80nm.

[0028] In one embodiment of the present invention, the substrate obtained in step S1 has a film thickness of 5um-10um, an air permeability value of ≤200s / 100cc, and a pore size of 30nm-50nm.

[0029] In one embodiment of the present invention, the substrate obtained in step S1 has a film thickness of 9 μm, an air permeability of 142 s / 100 cc, and a pore size of 38 nm.

[0030] In one embodiment of the present invention, the substrate obtained in step S1 has a needle punching strength ≥35gf / um, a porosity of 20%-60%, and a heat shrinkage rate of ≤25% at 130℃.

[0031] In one embodiment of the present invention, the substrate obtained in step S1 has a needle punching strength ≥45gf / um, a porosity of 30%-35%, and a heat shrinkage rate of ≤20% at 130℃.

[0032] In one embodiment of the present invention, in step S2, the photoinitiator is a hydrogen-abstracting photoinitiator, including one or more combinations of benzophenone photoinitiators, thioxanthone photoinitiators, and anthraquinone photoinitiators;

[0033] The solvent includes one or more combinations of methanol, ethanol, dichloromethane, acetone, diethyl ether, petroleum ether, n-hexane, dimethylformamide, dimethylacetamide, and methylpyrrolidone.

[0034] In one embodiment of the present invention, in step S3, the contact method is coating.

[0035] In one embodiment of the present invention, the contact method includes dip coating.

[0036] In one embodiment of the present invention, in step S4, the light source for the light treatment is ultraviolet light with a wavelength of 100-400nm and an illumination time of 0.1s-600s.

[0037] In one embodiment of the present invention, the wavelength is 300-400nm and the illumination time is 1s-300s.

[0038] Another object of the present invention is to provide an application of a separator with low closed-cell and high membrane breakage temperature for use in battery separators.

[0039] Beneficial Effects: Compared with existing technologies, this invention offers at least the following advantages: The present invention provides a separator for lithium-ion and other batteries with low pore-closure temperature and high membrane rupture temperature. The separator preparation process includes preparing the substrate using a traditional wet separator preparation process, dispersing the modifier to various parts of the substrate using a solution coating method, and achieving cross-linking between the polyolefin chains of the substrate through light irradiation. This improves upon the drawback of traditional separators where the pore-closure temperature and membrane rupture temperature are relatively similar. This method is simple, produces a separator with a low pore-closure temperature and a high membrane rupture temperature, can be prepared online or offline, and does not affect other properties of the substrate besides pore-closure temperature, membrane rupture temperature, and gel content. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the pore closure and rupture temperature TMA curves of the diaphragm in an embodiment of the present invention. Detailed Implementation

[0042] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention. However, those skilled in the art will readily understand that the specific material ratios, process conditions, and results described in the embodiments are for illustrative purposes only and should not, and will not, limit the present invention as described in detail in the claims.

[0043] Example 1

[0044] In this embodiment of the invention, the three polyethylene raw materials used are simplified as A, B, and C, respectively, where A represents high-density polyethylene, B represents high molecular weight high-density polyethylene, and C represents ultra-high molecular weight polyethylene.

[0045] A diaphragm with low closed-cell and high rupture temperature, unless otherwise specified, will be referred to as "diaphragm" in this invention. A method for preparing a diaphragm with high rupture temperature includes the following steps:

[0046] S1. High-density polyethylene A, high-density polyethylene B, and ultra-high molecular weight polyethylene C are premixed in a mass ratio of 1:8:1 to obtain a polyethylene mixture. The polyethylene mixture is then mixed with paraffin oil (mass ratio of 3:1) and kneaded at 230°C for 10 minutes. The mixture is then extruded, cooled, and cast into sheets to obtain an oil-containing substrate. The oil-containing substrate is then subjected to biaxial stretching (stretch ratio of 7 times in both directions) and extraction to obtain a base material.

[0047] In this embodiment, the substrate obtained by S1 meets the following conditions: film thickness is 8.7 μm, air permeability is 142 s / 100 cc, pore size is 38 nm, needle penetration strength is 46 gf / μm, porosity is 20%-60%, heat shrinkage rate at 130℃ is ≤18%, TMA rupture temperature is 153℃, TMA pore closing temperature is 129℃, and the difference between the pore closing temperature and rupture temperature of the diaphragm is 24℃.

[0048] S2. Mix the photoinitiator 2-isopropylthioxanthraquinone with dichloromethane at a mass ratio of 0.5:99.5, stir to dissolve, and obtain a modifier solution;

[0049] S3. The substrate obtained in step S1 is dipped into the modifier solution obtained in step S2, and after drying, the initial diaphragm is obtained.

[0050] S4. The initial diaphragm obtained in step S3 is subjected to UV light irradiation for 10 seconds to obtain a diaphragm with low closed-cell and high membrane rupture temperature.

[0051] In this embodiment, the low closed-cell and high rupture temperature diaphragm meets the following conditions: membrane thickness of 9 μm, air permeability of 144 s / 100 cc, pore size of 38 nm, pore tortuosity of 2.22, needle penetration strength ≥45 gf / μm, porosity of 34%, thermal shrinkage rate at 130℃ ≤18%, TMA rupture temperature of 207℃, TMA closed-cell temperature of 129℃, and the difference between the closed-cell temperature and the rupture temperature of the diaphragm is 78℃.

[0052] Comparative Example 1

[0053] Unlike Example 1, in step S1, high-density polyethylene A and high-density polyethylene B are premixed at a mass ratio of 1:9.

[0054] Comparative Example 2

[0055] Unlike Example 1, in step S1, high-density polyethylene B and ultra-high molecular weight polyethylene C are pre-mixed at a mass ratio of 9:1. The mixing temperature during manufacturing is high, and the film is easily broken.

[0056] Comparative Example 3

[0057] High-density polyethylene B and paraffin oil (mass ratio 3:1) are mixed and then kneaded at 230°C for 10 minutes. The mixture is then extruded, cooled, and cast into an oil-containing substrate. The oil-containing substrate is then subjected to biaxial stretching (stretch ratio of 7 times in both directions), extraction, and drying to obtain a diaphragm.

[0058] Comparative Example 4

[0059] Unlike Example 1, no coating or subsequent operations are performed. That is, only step S1 is performed.

[0060] The products obtained in Example 1 and Comparative Examples 1-5 were tested below, and the results are shown in Table 1:

[0061] Table 1. Product performance test results of the examples

[0062]

[0063] The data in the table above shows that the closure temperature of the diaphragm can be reduced by introducing low-melting-point high-density polyethylene, and the rupture temperature of the diaphragm is also significantly improved by modifying it with a photoinitiator.

[0064] By comparing the data from Example 1 and Comparative Example 4, it can be seen that Example 1 of the present invention lowers the closed-cell temperature by changing the formulation (introducing high-density polyethylene A and ultra-high molecular weight polyethylene C) and increases the rupture temperature by introducing a photoinitiator. Under the premise that other properties remain unchanged, the closed-cell temperature of the diaphragm in Example 1 is significantly reduced to 130°C, and the rupture temperature is significantly increased to 207°C. The difference between the closed-cell temperature and the rupture temperature of the diaphragm is significantly increased, from 24°C in Comparative Example 4 to 78°C in Example 1. Compared to Comparative Example 4 undergoing step S1, the porosity, air permeability, needle penetration strength per unit thickness, pore size, and thermal shrinkage at 130°C of the modified diaphragm of the present invention are consistent with the properties of the substrate before modification obtained in step S1, fully demonstrating that the modification method used in the present invention does not change other properties except for the rupture temperature.

[0065] By comparing Example 1 and Comparative Example 1, it can be seen that Comparative Example 1, without the addition of ultra-high molecular weight polyethylene (UHMWPE) C, has a 51°C difference between its closed-cell temperature and rupture temperature. Although the temperature difference is relatively large, its needle penetration strength is only 35.2 gf / µm, which is significantly low, indicating insufficient strength. This demonstrates that UHMWPE C plays a crucial role in the stability and strength of the diaphragm. In Example 1 of this invention, the introduction of UHMWPE C significantly maintains the needle penetration strength of the diaphragm product, thus preserving its stability and strength.

[0066] By comparing Example 1 and Comparative Example 2, it can be seen that Comparative Example 2 did not introduce low-melting-point high-density polyethylene A, and its closed-cell temperature was relatively high, reaching 143°C. This resulted in a small difference between its closed-cell temperature and rupture temperature. Although the needle-punching strength was relatively high, the ultra-high molecular weight polyethylene had extremely poor fluidity, making it difficult to process during manufacturing. Furthermore, the membrane porosity was also low and the pore size was small.

[0067] By comparing Example 1 and Comparative Example 4, Comparative Example 4 did not involve the assistance of a photoinitiator. In Example 1, after treatment with the modified photoinitiator, the closed-cell temperature was not affected, but the membrane rupture temperature increased significantly, indicating that the introduction of the photoinitiator can increase the membrane rupture temperature.

[0068] Figure 1 The graph shows the TMA curves for closed-cell and rupture temperatures, where the troughs represent the closed-cell temperatures and the peaks represent the rupture temperatures. The graph indicates that using only high-molecular-weight high-density polyethylene B as the polyethylene raw material...

[0069] Comparative Example 3 had a pore-closing temperature of 142°C and a film-breaking temperature of 153°C; while Comparative Example 4, which was not modified with a photoinitiator, had a pore-closing temperature of 129°C and a film-breaking temperature of 153°C, with a temperature difference of 24°C; and Example 1, after modification, had a pore-closing temperature of 129°C and a film-breaking temperature of 207°C, with a temperature difference of 78°C, which significantly increased the difference between the pore-closing and film-breaking temperatures.

[0070] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A diaphragm with low closed-cell and high membrane breakage temperature, characterized in that, The TMA rupture temperature of the low-closed-cell, high-rupture-temperature diaphragm is ≥180℃, the TMA rupture temperature is ≤130℃, and the difference between the TMA rupture temperature and the TMA rupture temperature is ≥50℃. The method for preparing the low-closed-pore, high-rupture-temperature diaphragm includes the following steps: S1. A polyethylene mixture formed by mixing high-density polyethylene, high molecular weight polyethylene and ultra-high molecular weight polyethylene is mixed with paraffin oil, and then subjected to intensive mixing, extrusion, cooling casting, stretching and extraction to obtain the base material. S2. Mix the photoinitiator with the solvent, dissolve, and stir until homogeneous to obtain a modifier solution; S3. The modifier solution prepared in step S2 is brought into full contact with the substrate prepared in step S1, and after drying, the initial diaphragm is obtained. S4. The initial diaphragm obtained in step S3 is subjected to light irradiation to obtain a diaphragm with low closed-cell and high membrane rupture temperature. In step S1, the polyethylene mixture is composed of 10% high-density polyethylene, 80% high molecular weight high-density polyethylene, and 10% ultra-high molecular weight polyethylene by mass percentage.

2. The low-closed-cell, high-breakage-temperature diaphragm according to claim 1, characterized in that, The membrane thickness of the low closed-cell, high rupture temperature diaphragm is 5um-20um, the air permeability is ≤400s / 100cc, the pore size is 20nm-80nm, the needle penetration strength is ≥35gf / um, the porosity is 20%-60%, and the heat shrinkage rate at 130℃ is ≤25%.

3. The low-closed-cell, high-breakage-temperature diaphragm according to claim 1, characterized in that, In step S1, the polyethylene mixture and paraffin oil are mixed at a mass ratio of 1-5:1, based on a percentage mass ratio.

4. The low-closed-cell, high-rupture-temperature diaphragm according to claim 1, characterized in that, The substrate obtained in step S1 has a TMA breaking temperature ≥152℃ and a TMA closing temperature ≤130℃.

5. The low-closed-cell, high-breakage-temperature diaphragm according to claim 1, characterized in that, The substrate obtained in step S1 has a film thickness of 5um-20um, an air permeability of ≤400s / 100cc, a pore size of 20nm-80nm, a needle penetration strength of ≥35gf / um, a porosity of 20%-60%, and a heat shrinkage rate of ≤25% at 130℃.

6. The low-closed-cell, high-rupture-temperature diaphragm according to claim 1, characterized in that, In step S2, the photoinitiator is a hydrogen-abstracting photoinitiator, including one or more combinations of benzophenone photoinitiators, thioxanthone photoinitiators, and anthraquinone photoinitiators; the solvent includes one or more combinations of methanol, ethanol, dichloromethane, acetone, diethyl ether, petroleum ether, n-hexane, dimethylformamide, dimethylacetamide, and methylpyrrolidone.

7. The low-closed-cell, high-breakage-temperature diaphragm according to claim 1, characterized in that, In step S3, the contact method is coating, which includes dip coating; in step S4, the light source for the light treatment is ultraviolet light with a wavelength of 100-400nm and an illumination time of 0.1s-600s.

8. The application of the low closed-cell, high rupture temperature diaphragm as described in any one of claims 1 to 7, characterized in that, Used in battery separators.

Images