Lithium battery diaphragm and preparation method thereof

Abstract

The invention relates to the technical field of lithium battery diaphragms, and particularly discloses a lithium battery diaphragm and a preparation method thereof. The lithium battery diaphragm is prepared by ternary blending modification of high molecular weight polyethylene, polyethylene wax and polybutylene terephthalate. The preparation method comprises the following steps: proportioning the high molecular weight polyethylene, the low molecular weight polyethylene wax and the polybutylene terephthalate according to the weight ratio, and mixing with the white oil; and carrying out melt blending extrusion, film casting, two-way stretching, extraction to remove white oil and heat setting treatment by using a twin-screw extruder to obtain the lithium battery diaphragm. According to the prepared diaphragm, the hole closing temperature of the diaphragm is reduced, the thermal stability, the mechanical strength and the safety performance of the diaphragm are improved, and a double safety protection mechanism is realized.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator technology, specifically to a lithium battery separator and its preparation method. Background Technology

[0002] With the rapid development of the new energy industry, the market demand for lithium batteries, as a high-efficiency and environmentally friendly energy storage device, is constantly growing. The lithium battery separator is a key component of lithium batteries, and its performance directly affects the safety, cycle life, and overall performance of the battery.

[0003] Currently, the commonly used lithium battery separator materials on the market are mainly polyolefin materials such as polyethylene (PE) and polypropylene (PP). However, these traditional polyolefin separator materials have some technical defects: First, their thermal stability is relatively poor, and the separator is prone to thermal shrinkage and damage under abnormal temperature rise or thermal runaway conditions of lithium batteries; second, their mechanical strength is relatively low, and they are prone to cracking during battery assembly and use; third, the pore-closing temperature of lithium battery separators is usually around 135°C, which cannot cut off the ion channels in time during the early stages of abnormal temperature rise in the battery, posing a safety hazard.

[0004] To improve the overall performance of lithium-ion battery separators, researchers have explored various modification methods, including blending modification, surface coating, and filler addition. Modifying polymer materials is a relatively effective approach, leveraging the advantages of added components to achieve synergistic performance enhancement. However, existing modification techniques often suffer from the following problems: poor compatibility between components, complex processing techniques, high costs, and difficulty in simultaneously achieving a balance between low pore-closure temperature and high mechanical strength.

[0005] Therefore, there is an urgent need to develop a lithium battery separator manufacturing technology that can effectively reduce the pore temperature of the separator, improve thermal stability and mechanical strength, and achieve a dual safety protection mechanism to meet the urgent demand for high safety performance of modern lithium batteries, especially large-scale energy storage batteries. Summary of the Invention

[0006] Based on the problems existing in the background technology, the present invention provides a lithium battery separator and its preparation method, which reduces the pore-closing temperature of the separator, improves the thermal stability, mechanical strength and safety performance of the separator, and realizes a dual safety protection mechanism.

[0007] This invention is implemented through the following technical solutions: This invention discloses a method for preparing a lithium battery separator, comprising the following steps: (1) Raw material preparation: High molecular weight polyethylene, low molecular weight polyethylene wax and polybutylene terephthalate are mixed in weight ratio and then mixed with white oil. (2) Blending extrusion: The raw material in step (1) is added to a twin-screw extruder and hot melt blending extrusion is carried out at a set temperature and screw speed to obtain blended granules; (3) Casting film: The blended granules obtained in step (2) are added to the casting machine, cast at the set temperature and pressure, and cooled to obtain a film; (4) Biaxial stretching: The film obtained in step (3) is subjected to longitudinal and transverse biaxial stretching; (5) Heat setting: The stretched film is extracted to remove white oil, and then heat set to obtain lithium battery separator.

[0008] Further, in step (1), the weight ratio of high molecular weight polyethylene, low molecular weight polyethylene wax and polybutylene terephthalate is 65-75:5-10:20-30.

[0009] Further, in step (1), the molecular weight of the high molecular weight polyethylene is 600,000 to 2,000,000, the molecular weight of the low molecular weight polyethylene wax is 8,000 to 12,000, and the molecular weight of the polybutylene terephthalate is 20,000 to 30,000.

[0010] Furthermore, in step (1), the low molecular weight polyethylene wax is oxidized polyethylene wax that has undergone oxidation treatment.

[0011] In practical applications, low molecular weight polyethylene wax is rarely used alone as a membrane material (because it sacrifices mechanical strength). The low molecular weight polyethylene wax described in this invention needs to be oxidized to obtain oxidized polyethylene wax, which improves its compatibility with the main polymer. When used as an additive, it affects the crystallization process of high molecular weight PE, reduces the degree of crystal integrity, and has a significant effect on lowering the melting point and solving the problem of closed cells.

[0012] PBT is a milky white, translucent to opaque, semi-crystalline thermoplastic resin with high heat resistance, capable of withstanding temperatures up to 140℃. Its regular molecular chain structure and high crystallinity contribute to its good mechanical strength. PBT is blended with high-molecular-weight PE and low-molecular-weight polyethylene wax to prepare lithium-ion battery separators. The low-molecular-weight polyethylene significantly reduces the pore-closing temperature of the separator by decreasing crystal integrity and size. While conventional single-molecule high-molecular-weight polyethylene melts and forms pores around 135℃, the blended separator achieves a pore-closing temperature as low as 100℃, with a pore-closing rate of 92% and a 30% reduction in pore-closing response time. This rapid closure of ion channels at lower temperatures prevents the early propagation of thermal runaway reactions, improving the safety performance of lithium-ion batteries. PBT's high heat resistance allows it to reinforce fine and coarse crystalline structures when the operating temperature of a lithium-ion battery is excessively high, ensuring the integrity of the separator and preventing short circuits caused by contact between the positive and negative electrodes. This dual-safety-protection mechanism makes the separator suitable for modern large-scale energy storage batteries.

[0013] Furthermore, in step (2), the temperature of the co-extrusion is 80-250℃ and the screw speed is 30-150 rpm.

[0014] Furthermore, in step (3), the casting temperature is 200-250℃, the pressure is 1-5MPa, the cooling temperature is 8-30℃, and the thickness of the resulting film is 1500-1700μm.

[0015] Further, in step (4), the film is subjected to biaxial stretching of MD and TD at a stretching temperature of 80-130℃ and a stretching ratio of 5-30 times to obtain a diaphragm with a thickness of 5-16μm.

[0016] Furthermore, in step (5), the heat setting temperature is 100-150℃ and the processing speed is 50-90m / min.

[0017] The present invention discloses a lithium battery separator, which is prepared by the above method, wherein the pore-closing temperature of the separator is 100-115℃.

[0018] The beneficial effects of this invention are: 1. This invention employs a blend modification of high molecular weight PE, low molecular weight polyethylene wax, and PBT. By blending polymer materials of different molecular weights, the advantages of each material can be combined. Low molecular weight polyethylene wax has shorter molecular chains and a less perfect crystal structure, resulting in a lower melting point. During battery heating, the low molecular weight polyethylene wax, acting as an additive, melts more quickly, clogging the pores of the separator. This allows it to distribute throughout the base membrane, creating more easily meltable weak points. High molecular weight PE improves the mechanical strength of the separator, while PBT has high heat resistance. When the operating temperature of the lithium battery remains excessively high, PBT strengthens both fine and coarse grain structures, ensuring the integrity of the separator. The combined properties of these three materials result in a separator with low pore-closing temperature, high mechanical strength, and high safety.

[0019] 2. By blending and modifying high molecular weight PE, low molecular weight polyethylene wax, and PBT, the low molecular weight PE polyethylene wax, as a low-closed-cell additive, lowers the membrane's pore-closure temperature to 100°C. This allows for rapid closure of ion channels at lower temperatures, improving the safety performance of lithium batteries. The addition of PBT reduces shrinkage and rupture under sustained high temperatures, further enhancing lithium battery safety and improving the membrane's mechanical strength. This dual safety protection mechanism strengthens the battery's defenses. The preparation method of this invention is simple, low-cost, and suitable for large-scale production. Detailed Implementation

[0020] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0021] Example 1: This example provides a method for preparing a lithium battery separator, the specific steps of which are as follows: (1) Raw material preparation: Select high molecular weight polyethylene (PE) with a molecular weight of 1 million, low molecular weight polyethylene wax with a molecular weight of 10,000 after oxidation treatment, and polybutylene terephthalate (PBT) with a molecular weight of 25,000. Mix them according to the weight ratio of low molecular weight polyethylene wax:PE:PBT=5:75:20 and mix them with white oil to make the solid content 20% and the white oil content 80%.

[0022] (2) Blending extrusion: The mixture from step (1) is added to a twin-screw extruder and hot melt blending extrusion is carried out at a temperature of 180°C and a screw speed of 80 rpm to obtain blended granules.

[0023] (3) Casting film: The blended granules obtained in step (2) are added to the casting machine and cast at a casting temperature of 220°C and a pressure of 3MPa. After cooling with 8°C cooling water, a film with a thickness of 1600μm is obtained.

[0024] (4) Biaxial stretching: The film obtained in step (3) is subjected to biaxial stretching in MD (longitudinal) and TD (transverse) at a stretching temperature of 100℃ and a stretching ratio of 10 times to obtain a membrane substrate with a thickness of 16μm.

[0025] (5) Extract the stretched film to remove white oil; (6) Heat setting: The stretched film is heat set at 120°C at a speed of 70 m / min to obtain the final lithium battery separator with a thickness of 7 μm.

[0026] Example 2: This example provides a lithium battery separator. The difference between this example and Example 1 lies in the change in the amount of PBT added. The raw materials used are low molecular weight polyethylene wax:PE:PBT = 5:70:25.

[0027] Example 3: This example provides a lithium battery separator. The difference between this example and Example 1 lies in the change in the amount of low molecular weight polyethylene wax added after oxidation treatment. The raw material used is low molecular weight polyethylene wax:PE:PBT = 10:70:20.

[0028] Comparative Example 1: This comparative example provides a lithium battery separator. The difference between this comparative example and Example 1 is that the raw materials used are a mixture of single high molecular weight polyethylene and white oil.

[0029] Comparative Example 2: This comparative example provides a lithium battery separator. The difference between this comparative example and Example 1 is that the raw material used is PE + oxidized low molecular weight polyethylene wax (95:5).

[0030] Comparative Example 3: This comparative example provides a lithium battery separator. The difference between this comparative example and Example 1 is that the raw materials used are PE+PBT (75:25).

[0031] The performance of the lithium battery separators prepared in Examples 1-3 and Comparative Examples 1-3 was tested, and the results are shown in Table 1.

[0032] Table 1. Test results of lithium battery separators prepared in Examples 1-3 and Comparative Examples 1-3

[0033] As shown in Table 1, blending high- and low-molecular-weight PE and PBT in appropriate proportions can significantly improve product safety. The addition of a suitable proportion of high-molecular-weight PE as the main additive strengthens the mechanical strength of the separator, ensuring its safety during use. During use, the product exhibits a dual safety protection mechanism. When the internal temperature of the battery rises abnormally, the polyethylene wax additive creates more "weak points" in the separator matrix at 100℃ (far lower than conventional polyolefin separators). These areas can prematurely close pores, blocking ion passage and stopping the chemical reaction early. PBT strengthens the internal crystal structure of the separator, ensuring its integrity at even higher temperatures and preventing further thermal runaway reactions caused by direct contact between the positive and negative electrodes. Compared to ordinary polyolefin separators, these three types of separators made from blended high-molecular-weight materials not only improve battery safety but also possess a dual protection mechanism, preventing thermal runaway and overheating.

[0034] From Examples 1-3 and Comparative Examples 1-3 in Table 1, it can be seen that the addition of polyethylene wax as a low molecular weight PE additive significantly reduces the membrane closure temperature, and the addition of PBT can improve the thermal stability and mechanical strength of the membrane. Moreover, the membrane closure temperature and mechanical strength will change with the amount of polyethylene wax and PBT added. The membrane prepared by the appropriate ratio of the three materials has a dual safety protection mechanism.

[0035] Finally, it should be noted that the above embodiments are merely illustrative of several implementations of the present invention and are not intended to limit the scope of the invention. For those skilled in the art, any modifications, equivalent substitutions, or improvements made without departing from the concept of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a lithium battery separator, characterized in that, Includes the following steps: (1) Raw material preparation: High molecular weight polyethylene, low molecular weight polyethylene wax and polybutylene terephthalate are mixed in weight ratio and then mixed with white oil. (2) Blending extrusion: The raw material in step (1) is added to a twin-screw extruder and hot melt blending extrusion is carried out at a set temperature and screw speed to obtain blended granules; (3) Casting film: The blended granules obtained in step (2) are added to the casting machine, cast at the set temperature and pressure, and cooled to obtain a film; (4) Biaxial stretching: The film obtained in step (3) is subjected to longitudinal and transverse biaxial stretching; (5) Heat setting: The stretched film is extracted to remove white oil, and then heat set to obtain lithium battery separator.

2. The preparation method according to claim 1, characterized in that, In step (1), the weight ratio of high molecular weight polyethylene, low molecular weight polyethylene wax and polybutylene terephthalate is 65-75:5-10:20-30.

3. The preparation method according to claim 1, characterized in that, In step (1), the molecular weight of high molecular weight polyethylene is 600,000 to 2,000,000, the molecular weight of low molecular weight polyethylene wax is 8,000 to 12,000, and the molecular weight of polybutylene terephthalate is 20,000 to 30,000.

4. The preparation method according to claim 1, characterized in that, In step (1), the low molecular weight polyethylene wax is oxidized polyethylene wax that has undergone oxidation treatment.

5. The preparation method according to claim 1, characterized in that, In step (2), the temperature of the co-extrusion is 80-250℃ and the screw speed is 30-150 rpm.

6. The preparation method according to claim 1, characterized in that, In step (3), the casting temperature is 200-250℃, the pressure is 1-5MPa, the cooling temperature is 8-30℃, and the thickness of the resulting film is 1500-1700μm.

7. The preparation method according to claim 1, characterized in that, In step (4), the film is subjected to biaxial stretching of MD and TD at a stretching temperature of 80-130℃ and a stretching ratio of 5-30 times to obtain a membrane with a thickness of 5-16μm.

8. The preparation method according to claim 1, characterized in that, In step (5), the heat setting temperature is 100-150℃ and the processing speed is 50-90m / min.

9. A lithium battery separator, characterized in that, It is prepared by the method described in any one of claims 1-8.

10. The lithium battery separator according to claim 9, characterized in that, The closure temperature of the diaphragm is 100-115℃.

Images