High-temperature-aging-resistant lithium ion battery diaphragm, preparation method thereof and electrochemical device

Abstract

The invention provides a high-temperature-aging-resistant lithium ion battery diaphragm, a preparation method thereof and an electrochemical device. The diaphragm comprises at least one polyolefin porous layer, the strength attenuation rate P of the diaphragm is less than or equal to 20%, the attenuation rate P is equal to (1-100)% * a ' / a, a is the ratio of the needling strength to the surface density of the diaphragm under the condition of 20 DEG C, and a' is the ratio of the needling strength to the surface density of the diaphragm under the condition that the diaphragm is subjected to heat treatment at 130 DEG C for 36 hours and then is cooled to 20 DEG C within 1 hour. The diaphragm provided by the invention is low in strength attenuation rate, the stability of the diaphragm at high temperature of the battery is greatly improved, and the safety of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery separators, and particularly relates to a high-temperature aging-resistant lithium-ion battery separator, a preparation method thereof, and an electrochemical device. Background Art

[0002] Currently, in the field of wet separator production, existing lithium-ion battery separators are prone to rupture in a long-term high-temperature environment, which may further lead to short circuit due to the contact between the positive and negative electrodes, causing battery safety problems.

[0003] In order to improve the high-temperature safety of traditional wet-formed porous membranes, the temperature resistance performance and safety at high temperatures are usually improved by coating; however, the coated separator still faces the problem of rapid strength attenuation at temperatures above 100 °C. Summary of the Invention

[0004] It can be found by heating the separator that after the ordinary separator is heat-treated at 130 °C for 100 h, the strength of the separator is less than 30% of the original, and it can be found that the separator itself will be somewhat broken. The purpose of the present invention is to provide a high-temperature resistant lithium-ion battery separator and a preparation method thereof to solve the problems of excessive strength decline and easy breakage of lithium-ion battery separators in a high-temperature environment in the prior art.

[0005] In a first aspect, the present invention provides a high-temperature aging-resistant lithium-ion battery separator, the separator includes at least one polyolefin porous layer, the strength attenuation rate P of the separator ≤ 20%, and the attenuation rate P = 1 - 100% * a' / a, where a is the ratio of the puncture strength to the areal density of the separator at 20 °C, and a' is the ratio of the puncture strength to the areal density of the separator after the separator is heat-treated at 130 °C for 36 h and then cooled to 20 °C within 1 h.

[0006] In a second aspect, the present invention provides a preparation method of a high-temperature aging-resistant lithium-ion battery separator. A polyolefin component, an auxiliary agent, and a lubricant are mixed, dried, rolled into a sheet, stretched, extracted, and heat-set to obtain the battery separator, and the stretching ratio is greater than 5 times.

[0007] In a third aspect, the present invention further provides an electrochemical device, including a positive electrode, a negative electrode, and the separator.

[0008] Advantages of the Present Invention

[0009] The separator provided by the present invention has a low strength attenuation rate, greatly improves the stability of the separator at high battery temperatures, and improves battery safety. Description of the Drawings

[0010] Figure 1 It is a picture of the separator in Example 2 after the attenuation rate test;

[0011] Figure 2 It is a picture of the diaphragm of Comparative Example 2 after the attenuation rate test. Specific Embodiments

[0012] The present invention will be specifically described below through embodiments. It is necessary to point out here that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the protection scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention according to the above content of the present invention.

[0013] A high-temperature aging-resistant lithium-ion battery diaphragm, the diaphragm includes at least one polyolefin porous layer, the strength attenuation rate P of the diaphragm is ≤20%, and the attenuation rate P = 1 - 100% * a' / a, where a is the ratio of the puncture strength to the surface density of the diaphragm at 20°C, and a' is the ratio of the puncture strength to the surface density of the diaphragm after heat treatment of the diaphragm at 130°C for 36 hours and then cooling the diaphragm to 20°C within 1 hour.

[0014] In the present invention, the ratio of the puncture strength to the thickness of the diaphragm before heat treatment at 20°C > 30 gf / μm, and the ratio of the puncture strength to the surface density > 50 gf·m 2 / g.

[0015] In the present invention, the surface density of the diaphragm before heat treatment at 20°C is > 1.5 g / m 2 .

[0016] In the present invention, the polyolefin porous layer of the diaphragm contains a polyolefin component and an additive. The polyolefin component is composed of one or more polyolefin raw materials with a molecular weight distribution of less than 5. At least one polyolefin raw material with a weight average molecular weight higher than 800,000 g / mol is included in the polyolefin component. For example, the weight average molecular weight can be 800,000 g / mol, 1,000,000 g / mol, 1,500,000 g / mol, 2,000,000 g / mol, 3,000,000 g / mol, 4,000,000 g / mol, 5,000,000 g / mol, 6,000,000 g / mol. The component with a molecular weight less than 100,000 g / mol in the polyolefin raw material with a weight average molecular weight higher than 800,000 g / mol is less than 40 wt%. For example, it can be specifically 40 wt%, 30 wt%, 20 wt%, 10 wt%, 5 wt%.

[0017] In the present invention, the polyolefin raw material with a weight average molecular weight higher than 800,000 g / mol is a polyethylene raw material.

[0018] In the present invention, the auxiliary agent is an antioxidant, and the antioxidant can be an existing commercial antioxidant, such as one or several of amine-based, phenolic antioxidants, thioester antioxidants, phosphite antioxidants and other chemical substances that can delay or inhibit the oxidation process of polymers. Preferably, it is a hindered phenolic antioxidant, and particularly preferably antioxidant 1010. It can also be a self-made modified antioxidant, such as polyethylene grafted with antioxidant components. The antioxidant components can be one or several of amine-based antioxidant groups, hindered phenolic antioxidant groups, thioester antioxidant groups and phosphite antioxidant groups. Preferably, it is a hindered phenolic antioxidant group.

[0019] The hindered phenolic antioxidant group preferably has the following structure:

[0020] Wherein R1 and R2 can be the same or different and are alkyl groups with carbon atoms greater than or equal to 3; R3 is an alkyl group with carbon atoms greater than or equal to 2.

[0021] In the present invention, the auxiliary agent is in powder form, 50μm ≤ D50 ≤ 200μm. For example, D50 can specifically be 50μm, 70μm, 90μm, 120μm, 150μm, 180μm, 200μm. When the particle size is too large, it will cause difficulty in plasticization and defects such as poor plasticization on the film surface; 1.5 < D90 / D10 < 4.0. For example, D90 / D10 can specifically be 1.5, 2.0, 3.0, 4.0. If the particle size distribution is too wide, it will lead to film surface defects such as poor plasticization and gel point. The average molecular weight is 100,000 - 2,000,000 g / mol, preferably 100,000 - 600,000 g / mol. Specifically, it can be 100,000 g / mol, 300,000 g / mol, 500,000 g / mol, 800,000 g / mol, 1,000,000 g / mol, 1,500,000 g / mol, 2,000,000 g / mol. The melting point is 132°C - 140°C. Specifically, it can be 132°C, 135°C, 140°C. If the melting point is too low, it will cause an abnormal increase in the air permeability of the separator and the failure of the separator; if the melting point is too high, it will cause defects such as poor plasticization of the separator.

[0022] In the present invention, after the auxiliary agent is washed with a solvent for 2 hours, there is no change in mass. After infrared testing, the attenuation of the characteristic peak intensity of the antioxidant component < 5%; the solvent is one or several of acetone, dichloromethane, chloroform, hexane, tetrahydrofuran, ethanol, heptane and petroleum ether.

[0023] In the present invention, the mass ratio of the polyolefin component to the auxiliary agent is 1000:1 - 100:1. For example, it can be 1000:1, 800:1, 500:1, 300:1, 100:1.

[0024] A preparation method of a high-temperature aging-resistant lithium-ion battery separator, which mixes a polyolefin component, an additive, and paraffin oil, rolls them into a sheet after drying, stretches, extracts, and heat-sets to obtain the battery separator. The stretching ratio is greater than 5 times, preferably > 7 times, more preferably > 10 times. The specific stretching ratio can be 5 times, 8 times, 10 times, 15 times, 20 times, 25 times, 30 times.

[0025] In the present invention, an independent drying hopper is used to feed the mixed antioxidant, polyolefin component, and paraffin oil, and the materials are dried in the drying hopper. The drying conditions are as follows: drying is carried out using hot air with a cleanliness level of 10,000, the drying temperature is 45 - 105°C, and the specific drying temperature can be 45°C, 60°C, 80°C, 105°C. The air supply volume is not less than 1 kgPE / 100L air / min, such as 1 kgPE / 100L air / min, 1.5 kgPE / 100L air / min, 2 kgPE / 100L air / min, 2.5 kgPE / 100L air / min, 3 kgPE / 100L air / min, 3.5 kgPE / 100L air / min.

[0026] An electrochemical device includes a positive electrode, a negative electrode, and the separator. The settings of the positive electrode and the negative electrode are both conventional techniques in the art, and the present invention does not make specific limitations.

[0027] Example 1

[0028] Preparation and evaluation of the additive:

[0029] Dissolve potassium permanganate with 85% concentrated phosphoric acid and 50% concentrated nitric acid to prepare a 10% concentration potassium permanganate treatment solution; mix polyethylene powder (hydroxyl content > 7 mol%) and the potassium permanganate treatment solution at a mass ratio of 1:1 to form a suspension, treat for 24 h, then soak in deionized water for 24 h, wash and dry; then redisperse the above dried polyethylene powder with dioxane solution to obtain a polyethylene dispersion. Add 3,5-methyl ester to the polyethylene dispersion, and the addition amount of 3,5-methyl ester is 1% of the mass of polyethylene in the polyethylene dispersion, and react at 60 h for 24 h; then wash and dry with absolute ethanol to obtain a modified antioxidant. Its average molecular weight is 400,000 g / mol, D50 is 60 μm, D90 / D10 is 2, the melting point is 136.5°C, and the mass does not change after being rinsed with acetone for 2 h. After rinsing, the characteristic peak intensity of the hindered phenol group decays by 3%.

[0030] Example 2

[0031] Mix 25 parts by weight of polyethylene powder (molecular weight distribution is 4, weight-average molecular weight is 1.5 million g / mol, and the component with a molecular weight less than 100,000 g / mol is 10 wt%), 75 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), and 0.8 parts by weight of the additive prepared in Example 1. Dry and feed the mixed material through a drying barrel. Use hot air with a cleanliness level of 10,000 in the drying barrel for drying. The air supply volume is 1 kgPE / 100L air / min, and the drying temperature is 90°C. Feed it into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0032] Use a T-shaped die to extrude the gel. Control the extrusion melt temperature below 200°C, then quickly cool it on a constant-temperature metal pair of rollers with a surface temperature of 10°C, and roll it into a sheet with a thickness of 1.5 mm.

[0033] Traction and stretching of the sheet (oil film) are carried out. At 115°C, stretch it 7 times both longitudinally (MD) and transversely (TD). An oil film is obtained after stretching. The oil film is led into extraction to extract the paraffin oil in the oil film. Then, use constant-temperature hot air at 40°C to carry out the residual dichloromethane in the diaphragm, and then carry out heat setting treatment at 130°C to finally obtain a high-temperature resistant diaphragm.

[0034] Example 3

[0035] Mix 30 parts by weight of polyethylene powder (molecular weight distribution is 3, weight-average molecular weight is 1.5 million g / mol, and the component with a molecular weight less than 100,000 g / mol is 5 wt%), 70 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), and 0.8 parts by weight of the additive prepared in Example 1. Dry and feed the mixed material through a drying barrel. Use hot air with a cleanliness level of 10,000 in the drying barrel for drying. The air supply volume is 1.5 kgPE / 100L air / min, and the drying temperature is 85°C. Feed it into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0036] Use a T-shaped die to extrude the gel. Control the extrusion melt temperature below 200°C, then quickly cool it on a constant-temperature metal pair of rollers with a surface temperature of 10°C, and roll it into a sheet with a thickness of 1.5 mm.

[0037] Traction and stretching of the sheet (oil film) are carried out. At 115°C, stretch it 9 times both longitudinally (MD) and transversely (TD). An oil film is obtained after stretching. The oil film is led into extraction to extract the paraffin oil in the oil film. Then, use constant-temperature hot air at 40°C to carry out the residual dichloromethane in the diaphragm, and then carry out heat setting treatment at 130°C to finally obtain a high-temperature resistant diaphragm.

[0038] Example 4

[0039] Mix 15 parts by weight of polyethylene powder (molecular weight distribution is 4, weight-average molecular weight is 900,000 g / mol, and the component with a molecular weight less than 100,000 g / mol is 10 wt%), 85 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), and 1 part by weight of the additive prepared in Example 1. Dry and feed the mixed material through a drying hopper. Use hot air with a cleanliness level of 10,000 in the drying hopper for drying. The air supply volume is 2 kgPE / 100L air / min, and the drying temperature is 75°C. Feed it into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0040] Use a T-die to extrude the gel. Control the extrusion melt temperature below 200°C, then quickly cool it on a constant-temperature metal pair of rollers with a surface temperature of 10°C, and roll it into a sheet with a thickness of 1.5 mm.

[0041] Traction and stretching of the sheet (oil film). At 115°C, perform stretching at a ratio of 12 times in both the longitudinal direction (MD) and the transverse direction (TD). After stretching, an oil film is obtained. Pull the oil film into extraction to extract the paraffin oil in the oil film. Then, use hot air at a constant temperature of 40°C to carry out the residual dichloromethane in the diaphragm, and then perform heat setting treatment at 133°C to finally obtain a high-temperature resistant diaphragm.

[0042] Comparative Example 1

[0043] Mix 30 parts by weight of polyethylene powder (molecular weight distribution is 3, weight-average molecular weight is 1,500,000 g / mol, and the component with a molecular weight less than 100,000 g / mol is 5 wt%), 70 parts of paraffin oil (70#, Zhejiang Zhengxin), 0.3 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)). Dry and feed the mixed material through a drying hopper. Use hot air with a cleanliness level of 10,000 in the drying hopper for drying. The air supply volume is 2 kgPE / 100L air / min, and the drying temperature is 75°C. Feed it into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0044] Use a T-die to extrude the gel. Control the extrusion melt temperature below 200°C, then quickly cool it on a constant-temperature metal pair of rollers with a surface temperature of 10°C, and roll it into a sheet with a thickness of 1.5 mm.

[0045] The sheet (oil film) is drawn and stretched. At 115 °C, stretching is carried out at a ratio of 4.5 times in both the longitudinal direction (MD) and the transverse direction (TD). After stretching, an oil film is obtained. The oil film is drawn into extraction to extract the paraffin oil in the oil film. Then, the residual dichloromethane in the diaphragm is removed by hot air at a constant temperature of 40 °C, and then heat setting treatment is carried out at 130 °C to obtain a common diaphragm.

[0046] Comparative Example 2

[0047] 40 parts by weight of high-density polyethylene powder with a molecular weight of 400,000 (VH095, KPIC, Korea Petrochemical), 60 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), 0.3 parts by weight of antioxidant (Irg1010 (BASF, Ciba Specialty Chemicals)) and 0.1 parts by weight of antioxidant (P168 (manufactured by Ciba Specialty Chemicals Co., Ltd.)) are mixed. The mixed material is dried and fed through a drying hopper. Hot air with a cleanliness level of 10,000 is used for drying in the drying hopper. The air supply volume is 2 kgPE / 100L air / min, and the drying temperature is 75 °C. It is put into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0048] Using a T-die, the gel is extruded. The extrusion melt temperature is controlled below 200 °C, and then it is quickly cooled on a constant-temperature metal pair of rollers with a surface temperature of 10 °C and rolled into a sheet with a thickness of 1.5 mm.

[0049] The sheet (oil film) is drawn and stretched. At 115 °C, stretching is carried out at a ratio of 4.9 times in both the longitudinal direction (MD) and the transverse direction (TD). After stretching, an oil film is obtained. The oil film is drawn into extraction to extract the paraffin oil in the oil film. Then, the residual dichloromethane in the diaphragm is removed by hot air at a constant temperature of 40 °C, and then heat setting treatment is carried out at 131 °C to obtain a common diaphragm.

[0050] Performance Test

[0051] Crystallinity:

[0052] The crystallinity of this patent is tested with reference to GB / T 19466.3 or ISO 11357-3. Weigh about 5 mg - 10 mg of the diaphragm, lay it flat in an aluminum crucible, set the heating rate to 10 °C / min and heat it to 200 °C to obtain a first heating curve. Analyze the curve to obtain the melting enthalpy required for melting, and compare it with 293 J / g to obtain the crystallinity.

[0053] Needle punching strength:

[0054] Test according to the requirements of ASTM D4833-00el or GB / T 10004-2008. Cut a specimen with a width of about 50 mm along the TD direction of the film roll, place it under the fixed fixture of the sample stage, and conduct the test; the shape of the needle head is a hemisphere with Φ = 1.0 mm, and the running speed of the needle head is 1 mm / s. Measure the puncture strength of the base film and the coated film, and the difference between the two is the puncture strength of the coating. The average value of all the puncture strengths of the coating is recorded as the average puncture strength of the coating, and the ratio of the average puncture strength of the coating to the average coating thickness is recorded as the puncture strength per unit thickness of the coating.

[0055] Areal density:

[0056] Test the areal density per unit thickness of the separator according to the standard FZ / T60003.

[0057] Attenuation rate: Cut the prepared separator into square samples with an area of 10 cm × 10 cm, measure its puncture strength and areal density at 20 °C to obtain the ratio a of the puncture strength to the areal density. After heat treatment at 130 °C for 36 hours respectively, cool it to room temperature (20 °C) within 1 h, and observe the crack situation of the separator sample. When cracks appear in the separator but the separator can be picked up as a whole, measure its puncture strength and areal density again to obtain the ratio a' of the puncture strength to the areal density, and calculate the attenuation rate P = 1 - 100% * a' / a to obtain the puncture performance attenuation value. When the separator is pulverized and broken and cannot be picked up, its puncture strength is considered to be zero.

[0058]

[0059] Of course, the present invention can also have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and deformations according to the present invention, but these corresponding changes and deformations should all fall within the protection scope of the claims of the present invention.

Claims

1. A high-temperature aging-resistant lithium-ion battery separator, characterized in that, The separator includes at least one layer of polyolefin porous layer, the strength attenuation rate P of the separator is ≤ 20%, and the attenuation rate P = 1 - 100% * a' / a, where a is the ratio of the puncture strength to the surface density of the separator at 20°C, and a' is the ratio of the puncture strength to the surface density of the separator after heat treatment at 130°C for 36 hours and then cooling the separator to 20°C within 1 hour.

2. The high-temperature aging-resistant lithium-ion battery separator according to claim 1, wherein The ratio of the puncture strength to the thickness of the diaphragm before heat treatment at 20°C is > 30 gf / μm, and the ratio of the puncture strength to the areal density is > 50 gf*m 2 / g.

3. The high-temperature aging-resistant lithium-ion battery separator according to claim 1, characterized in that, The areal density of the diaphragm before heat treatment at 20 °C is > 1.5 g / m 2 .

4. The high-temperature aging-resistant lithium-ion battery separator according to claim 1, characterized in that, The polyolefin porous layer of the separator contains a polyolefin component and an additive. The polyolefin component is composed of one or more polyolefin raw materials with a molecular weight distribution less than 5. At least one polyolefin raw material in the polyolefin component has a weight-average molecular weight higher than 800,000 g / mol, and the component with a molecular weight less than 100,000 g / mol in the polyolefin raw material with a weight-average molecular weight higher than 800,000 g / mol is less than 40 wt%.

5. The high-temperature aging-resistant lithium-ion battery separator according to claim 4, characterized in that, The polyolefin raw material with a weight-average molecular weight higher than 800,000 g / mol is a polyethylene raw material.

6. The high-temperature aging-resistant lithium-ion battery separator according to claim 4, wherein, The additive is one or several of amine antioxidants, phenolic antioxidants, thioester antioxidants, and phosphite antioxidants.

7. The high-temperature aging-resistant lithium-ion battery separator according to claim 4, wherein The additive is polyethylene grafted with an antioxidant component, and the antioxidant component is one or several of amine antioxidant groups, hindered phenolic antioxidant groups, thioester antioxidant groups, and phosphite antioxidant groups.

8. The high-temperature aging-resistant lithium-ion battery separator according to claim 7, characterized in that The antioxidant component is a hindered phenolic antioxidant group.

9. The high-temperature aging resistant lithium-ion battery separator according to claim 7, wherein The hindered phenol antioxidant group has the following structure: Wherein R1 and R2 can be the same or different and are alkyl groups with a carbon atom number greater than or equal to 3; R3 is an alkyl group with a carbon atom number greater than or equal to 2.

10. The high-temperature aging-resistant lithium-ion battery separator according to claim 7, wherein The additive is in powder form, 50μm ≤ D50 ≤ 200μm, 1.5 < D90 / D10 < 4.0, and the average molecular weight is 100,000 - 600,000 g / mol.

11. The high-temperature aging resistant lithium-ion battery separator according to claim 7, characterized in that, After the additive is washed with a solvent for 2 hours, there is no change in mass, and the attenuation of the characteristic peak intensity of the antioxidant component by infrared test is < 5%; the solvent is one or several of acetone, dichloromethane, chloroform, hexane, tetrahydrofuran, ethanol, heptane, and petroleum ether.

12. The high-temperature aging-resistant lithium-ion battery separator according to claim 4, wherein, The mass ratio of the polyolefin component to the additive is 1000∶1 - 100∶1.

13. The preparation method of the high-temperature aging-resistant lithium-ion battery separator according to any one of claims 1-12, characterized in that, The polyolefin component, the additive, and paraffin oil are mixed, dried, rolled into sheets, stretched, extracted, and heat-set to obtain the battery separator. The stretching ratio is greater than 5 times, preferably > 7 times, and more preferably > 10 times.

14. The preparation method of the high-temperature aging resistant lithium-ion battery separator according to claim 13, wherein, An independent drying hopper is used to feed the mixed additive, polyolefin component, and paraffin oil, and the materials are dried in the drying hopper. The drying conditions are: drying with hot air of ten-thousand-class cleanliness, the drying temperature is not lower than 45°C and not higher than 105°C, and the air supply volume is not lower than 1 kg PE / 100 L air / min.

15. The preparation method of the high-temperature aging-resistant lithium-ion battery separator according to claim 13, characterized in that, After heat setting, the crystallinity is ≥ 75%, preferably ≥ 80%, and more preferably ≥ 85%.

16. An electrochemical device, characterized in that, It includes a positive electrode, a negative electrode, and the separator as described in claim 1.

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