Microporous diaphragm, preparation method thereof and electrochemical device

Abstract

The invention provides a microporous diaphragm and a preparation method thereof, the microporous diaphragm at least comprises a polyolefin microporous layer, the polyolefin microporous layer comprises a polyolefin component and an antioxidant and meets the following conditions: Mo / Me is greater than or equal to 0.1% and less than or equal to 0.5%, Mo / Ve is greater than or equal to 0.0015 g / cm < 3 > and less than or equal to 0.006 g / cm < 3 >, Mo represents the mass of the antioxidant in the unit area of the microporous diaphragm, and Ve represents the mass of the polyolefin component in the unit area of the microporous diaphragm. Me represents the mass of the polyolefin component in the unit area of the microporous diaphragm, and ve represents the total volume of micropores in the unit area of the polyolefin microporous layer. The microporous diaphragm has higher oxidation resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separators, and in particular to a microporous separator, a preparation method thereof, and an electrochemical device. Background Art

[0002] The separator is one of the key materials of lithium-ion batteries, which directly affects the electrical performance and safety performance of the batteries. Currently, the commonly used separator is a polyolefin separator, and a ceramic-coated separator or a glue-coated separator obtained by coating an inorganic material or a polymer material on the surface of the polyolefin separator to improve the heat shrinkage or adhesion performance, etc. However, the above-mentioned separators all have problems such as short storage time and low separator strength caused by the aging of polyolefin materials. Moreover, the use environment of lithium-ion batteries, such as electric fields, heat, mechanical stress, etc., will accelerate the aging of polyolefin materials. Therefore, good oxidation resistance is of great significance for extending the storage time of the separator or its service life in lithium-ion batteries, especially for extending the service life in high electrochemical potential battery systems.

[0003] In view of this, the present invention is specifically proposed. Summary of the Invention

[0004] In a first aspect, one of the purposes of the present invention is to provide a microporous separator with higher oxidation resistance.

[0005] To achieve the above purpose, the present invention provides a microporous separator, which at least includes one layer of polyolefin microporous layer. The polyolefin microporous layer includes a polyolefin component and an antioxidant, and satisfies the following conditions: 1% ≤ Mo / Me ≤ 5‰, 0.0015 g / cm 3 ≤ Mo / Ve ≤ 0.006 g / cm 3 , where Mo represents the mass of the antioxidant per unit area of the microporous separator, Me represents the mass of the polyolefin component per unit area of the microporous separator, and Ve represents the total volume of the micropores per unit area of the polyolefin microporous layer.

[0006] In a second aspect, another purpose of the present invention is to provide a preparation method of the microporous separator.

[0007] To achieve the above purpose, the present invention provides a preparation method of the microporous separator, which is to mix an antioxidant, a polyolefin, and paraffin oil, roll them into sheets, stretch, extract, and heat-set to obtain the battery separator. The stretching ratio is greater than 5 times, preferably > 7 times, and more preferably > 10 times.

[0008] In a third aspect, another purpose of the present invention is to provide an electrochemical device, which includes a positive electrode, a negative electrode, and the microporous separator described above.

[0009] Advantageous Effects:

[0010] The microporous separator provided by the present invention controls the relationship between the mass of the antioxidant and the mass of the polyolefin component, and the pore volume of the polyolefin microporous layer simultaneously, so as to further delay the performance degradation of the battery after the separator is assembled into the battery while improving the oxidation resistance of the separator. Detailed Embodiments

[0011] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work shall fall within the protection scope of the present invention.

[0012] A microporous separator comprises at least one polyolefin microporous layer, the polyolefin microporous layer comprising a polyolefin component and an antioxidant, and satisfying the following conditions: 1%o ≤ Mo / Me ≤ 5%o, 0.0015 g / cm 3 ≤ Mo / Ve ≤ 0.006 g / cm 3 , where Mo represents the mass of the antioxidant per unit area of the microporous separator, Me represents the mass of the polyolefin component per unit area of the microporous separator, and Ve represents the total volume of the micropores per unit area of the polyolefin microporous layer.

[0013] The antioxidant can be an existing commercial antioxidant, such as one or several of chemical substances with the function of delaying or inhibiting the oxidation process of polymers, such as amine-based, phenolic antioxidants, thioester-based antioxidants, phosphite-based antioxidants, etc. 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, and the antioxidant components can be one or several of amine-based antioxidant groups, hindered phenolic antioxidant groups, thioester-based antioxidant groups and phosphite-based antioxidant groups. Preferably, it is a hindered phenolic antioxidant group.

[0014] The hindered phenolic antioxidant group preferably has the following structure:

[0015]

[0016] where 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.

[0017] The value of Mo can be obtained by measuring the raw materials, or after extracting the antioxidant components from the separator, it can be measured by using precision microanalysis equipment such as gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and high-performance liquid chromatography (HPLC). Me can be calculated from the areal density and volume of the polyolefin microporous layer, Me = areal density * unit area sample volume - Mo. Ve can be measured by the water pressure method. In the present invention, preferably, the Mo is 0.0004 to 0.075 g / m 2 , specifically it can be 0.0004 g / m 2 , 0.001 g / m 2 , 0.005 g / m 2 , 0.01 g / m 2 , 0.02 g / m 2 , 0.03 g / m 2 , 0.04 g / m 2 , 0.05 g / m 2 , 0.06 g / m 2 , 0.07 g / m 2 , 0.075 g / m 2 , Ve is 0.2 to 12 cm 3 / m 2 , specifically it can be 0.2 cm 3 / m 2 , 1 cm 3 / m 2 , 3 cm 3 / m 2 , 5 cm 3 / m 2 , 8 cm 3 / m 2 , 10 cm 3 / m 2 , 12 cm 3 / m 2 , Me is 0.4 to 14 g / m 2 , specifically it can be 0.4 g / m 2 , 1 g / m 2 , 3 g / m 2 , 5 g / m 2 , 7 g / m 2 , 9 g / m 2 , 11 g / m 2 , 14 g / m 2 .

[0018] In the present invention, 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. 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 specifically be 40 wt%, 30 wt%, 20 wt%, 10 wt%, 5 wt%.

[0019] 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.

[0020] In the present invention, the antioxidant 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 difficult 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. Too wide a particle size distribution will lead to film surface defects such as poor plasticization and gel points. 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.

[0021] In the present invention, the antioxidant has no mass change after being washed with solvent for 2 h. 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.

[0022] In the present invention, the oxidation induction time of the microporous separator ≥ 30 min. For the battery cell made with this microporous separator, after 500 high-temperature cycles at 45°C, the attenuation rate of the puncture strength of the separator ≤ 15%, and the attenuation rate of the breakdown voltage ≤ 15%.

[0023] A method for preparing a microporous separator, comprising mixing an antioxidant, a polyolefin component, and paraffin oil, rolling them into a sheet, stretching, extracting, and heat-setting to obtain the battery separator. The stretching ratio is greater than 5 times, preferably >7 times, more preferably >10 times. Specifically, the stretching ratio can be 5 times, 8 times, 10 times, 15 times, 20 times, 25 times, or 30 times.

[0024] In the present invention, the crystallinity of the battery separator after heat setting is ≥75%, preferably ≥80%, more preferably ≥85%.

[0025] An electrochemical device, comprising a positive electrode, a negative electrode, and the microporous separator. The arrangements of the positive electrode and the negative electrode are conventional techniques in the art, and the present invention does not make specific limitations.

[0026] Example 1

[0027] Preparation and evaluation of the antioxidant:

[0028] Dissolve potassium permanganate in 85% concentrated phosphoric acid and 50% concentrated nitric acid to prepare a 10% potassium permanganate treatment solution; mix polyethylene powder (hydroxyl content >7 mol%) and the potassium permanganate treatment solution in 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 dried polyethylene powder in 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%.

[0029] Example 2

[0030] 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), 0.044 parts by weight of the antioxidant prepared in Example 1, and 0.1 parts by weight of antioxidant (P168 (manufactured by Ciba Specialty Chemicals)), and then put them into a twin-screw extruder for kneading to finally form a mixture melt gel.

[0031] Use a T-shaped die to extrude the gel, control the extrusion melt temperature below 200 °C, then quickly cool 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.

[0032] The sheet (oil film) is drawn and stretched. At 115 °C, stretching is carried out at a ratio of 7 times both in 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 after heat setting treatment at 130 °C, a microporous diaphragm is finally obtained. The mass Mo of the antioxidant on the microporous diaphragm is 0.0111 g, and the total volume Ve of the micropores per unit area is 5.187 cm 3 / m 2 ..

[0033] Example 3

[0034] 25 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%), 75 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), 0.025 parts by weight of the antioxidant prepared in Example 1, and 0.1 parts by weight of antioxidant (P168 (manufactured by Ciba Specialty Chemicals)) are mixed, and then put into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0035] Using a T-die, the gel is extruded, the extrusion melt temperature is controlled below 200 °C, and then it is rapidly 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.

[0036] The sheet (oil film) is drawn and stretched. At 115 °C, stretching is carried out at a ratio of 9 times both in 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 after heat setting treatment at 130 °C, a microporous diaphragm is finally obtained. The mass Mo of the antioxidant on the microporous diaphragm is 0.0056 g, and the total volume Ve of the micropores per unit area is 3.131 cm 3 / m 2 .

[0037] Example 4

[0038] 25 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%), 75 parts by weight of paraffin oil (70#, Zhejiang Zhengxin), 0.028 parts by weight of the antioxidant prepared in Example 1, and 0.1 parts by weight of antioxidant (P168 (manufactured by Ciba Specialty Chemicals)) are mixed, and then put into a twin-screw extruder for kneading to finally obtain a mixture melt gel.

[0039] Using a T-shaped die head, 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 sheet thickness of 1.5 mm.

[0040] Traction and stretching are carried out on the sheet (oil film). At 115 °C, stretching is carried out at a magnification of 12 times both longitudinally (MD) and transversely (TD). After stretching, an oil film is obtained. The oil film is led into extraction to extract the paraffin oil in the oil film. Then, the residual dichloromethane in the diaphragm is carried out by hot air at a constant temperature of 40 °C, and then through heat setting treatment at 130 °C, and finally a microporous diaphragm is obtained. The mass Mo of the antioxidant-resistant agent on the microporous diaphragm is 0.0084 g, and the total volume Ve of the micropores per unit area is 5.374 cm 3 / m 2 。

[0041] Comparative Example 1

[0042] 30 parts by weight of high-density polyethylene powder (GUR4116, Ticona Celanese) with a molecular weight of 750,000, 77 parts by weight of paraffin oil (No. 70, Zhejiang Zhengxin), 0.0265 parts by weight of the antioxidant prepared in Example 1, and 0.1 parts by weight of antioxidant (P 168 (manufactured by Ciba Specialty Chemicals Co., Ltd.)) are mixed in sequence, and then put into a twin-screw extruder for kneading to finally make a mixture melt gel.

[0043] Using a T-shaped die head, 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 sheet thickness of 1.5 mm.

[0044] Traction and stretching are carried out on the sheet (oil film). At 115 °C, stretching is carried out at a magnification of 7 times both longitudinally (MD) and transversely (TD). After stretching, an oil film is obtained. The oil film is led into extraction to extract the paraffin oil in the oil film. Then, the residual dichloromethane in the diaphragm is carried out by hot air at a constant temperature of 40 °C, and then through heat setting treatment at 130 °C, and a diaphragm is obtained. The mass Mo of the antioxidant-resistant agent on the diaphragm is 0.0055 g, and the total volume Ve of the micropores per unit area is 5.187 cm 3 / m 2 。。

[0045] Performance test:

[0046] Crystallinity:

[0047] This patent measures the crystallinity by referring to GB / T 19466.3 or ISO 11357-3. Weigh about 5 mg - 10 mg of the separator and lay it flat in an aluminum crucible. Set the heating rate to 10 °C / min and heat it up to 200 °C to obtain the first heating curve. Analyze the curve to get the enthalpy of fusion required for melting, and compare it with 293 J / g to obtain the crystallinity.

[0048] Areal density:

[0049] Test the areal density per unit thickness of the separator according to the standard FZ / T60003.

[0050] Oxidation induction period:

[0051] Fold the porous membrane into several layers and then cut it into a 5-mm circle. Put the disc into an aluminum crucible and weigh 10 mg to 15 mg. Lay the disc flat in the crucible, close to the bottom plane. Use a steel needle to pierce two small holes in the crucible lid, with a hole diameter ≥ 0.5 mm and a distance between the two small holes > 1.5 mm. Press the crucible lid tightly. Under a nitrogen atmosphere, heat it from room temperature to 200 °C at a heating rate of 20 °C / min, keep it at a constant temperature for 3 min, and then introduce high-purity oxygen for measurement, with an oxygen flow rate of 40 ml / min. The time difference between the time at the intersection of the tangents on both sides of the inflection point of the starting exothermic curve and the time of introducing oxygen is the duration value of the oxidation induction period.

[0052] Needle punching strength attenuation rate:

[0053] Assemble the battery with the following materials: use LiNixCoyMn1-x-yO2 for the positive electrode; use artificial graphite for the negative electrode; the electrolyte is 1M LiPF6, and the solvent composition is EC∶EMC∶DEC = 1∶1∶1.

[0054] The test steps are as follows: ① Put the lithium-ion battery into a temperature chamber and keep it in a constant temperature environment of 45 °C, discharge it at 1C to the cut-off voltage of 2.75 V; ② Leave it for 30 min; ③ Charge it at 1C to 4.2 V with a cut-off current of 0.05C; ④ Leave it for 30 min; ⑤ Discharge it at 1C to the cut-off voltage of 2.75 V; ⑥ Repeat steps ② - ⑤ 500 times.

[0055] Method for obtaining the separator after high-temperature cycling: Disassemble the lithium-ion battery after high-temperature cycling in a low-humidity environment with a dew point of -40 °C, soak and clean the obtained separator in absolute ethanol, and dry it at room temperature.

[0056] Diaphragm needle puncture strength test method: Referring to the requirements of ASTM D4833-00e1, using a pressure tester with a resolution of 0.01 gf, a puncture needle with a diameter of 1.0 mm, flatten the diaphragm in the fixture and clamp it tightly, and perform puncture at a puncture speed of 0.1 cm / s. The obtained data is the diaphragm needle puncture strength data. Cut a 15 cm * 15 cm block-shaped separator membrane, measure the needle puncture strength every 2 cm according to the above method, and take the average value of the five measurement results. The needle puncture strength of the diaphragm before assembling the battery is recorded as F0, and the needle puncture strength of the diaphragm after high-temperature cycling is recorded as F1; the calculation method of the diaphragm needle puncture strength attenuation rate is: needle puncture strength attenuation rate = (F0 - F1) / F0 * 100%.

[0057] Breakdown voltage attenuation rate:

[0058] Use a single-layer diaphragm for testing. Cut a circular diaphragm sample with a diameter of 60 mm and place it on a square aluminum plate with a side length of 150 mm. Place a cylindrical electrode made of brass with a diameter of 50 mm, a height of 30 mm, and a weight of 500 g on it, and connect the TOS5051A dielectric breakdown characteristic tester manufactured by Kikusui Electronics Industry Co., Ltd. Apply an AC voltage at a rising speed of 0.15 KV / second, set the current to 2 mA, the test voltage to 1.5 KV, and read the voltage when the diaphragm sample breaks down. The measurement of the breakdown voltage is carried out 15 times respectively to obtain the average value. The breakdown voltage of the diaphragm before assembling the battery is recorded as U0, and the breakdown voltage of the diaphragm after high-temperature cycling is recorded as U1. The calculation method of the diaphragm breakdown voltage attenuation rate is: breakdown voltage attenuation rate = (U0 - U1) / U0 * 100%;

[0059]

[0060] Certainly, the present invention may 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 microporous separator, comprising at least one polyolefin microporous layer, characterized in that, The polyolefin microporous layer comprises a polyolefin component and an antioxidant, and satisfies the following conditions: 1‰≤Mo / Me≤5‰, 0.0015 g / cm 3 ≤Mo / Ve≤0.006 g / cm 3 , where Mo represents the mass of the antioxidant per unit area of the microporous separator, Me represents the mass of the polyolefin component per unit area of the microporous separator, and Ve represents the total volume of the micropores per unit area of the polyolefin microporous layer.

2. The microporous separator according to claim 1, wherein, The Mo is 0.0004 to 0.075 g / m 2 , the Ve is 0.2 to 12 cm 3 / m 2 , the Me is 0.4 to 14 g / m 2 .

3. The microporous separator according to claim 1, characterized in that, 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%.

4. The microporous separator according to claim 3, characterized in that, The polyolefin raw material with a weight-average molecular weight higher than 800,000 g / mol is a polyethylene raw material.

5. The microporous separator according to claim 1, characterized in that, The antioxidant is one or more of amine antioxidants, phenolic antioxidants, thioester antioxidants, and phosphite antioxidants.

6. The microporous separator according to claim 1, wherein The antioxidant is polyethylene grafted with an antioxidant component, and the antioxidant component is one or more of amine antioxidant groups, hindered phenolic antioxidant groups, thioester antioxidant groups, and phosphite antioxidant groups.

7. The microporous separator according to claim 6, wherein, The antioxidant component is a hindered phenolic antioxidant group.

8. The microporous separator according to claim 6, characterized in that, 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.

9. The microporous separator according to claim 1, characterized in that, The antioxidant is in powder form, 50μm ≤ D50 ≤ 200μm, 1.5 < D90 / D10 < 4.0, with an average molecular weight of 1 million - 2 million g / mol, preferably 1 million - 600,000 g / mol, and a melting point of 132°C - 140°C.

10. The microporous separator according to claim 1, wherein After being leached with a solvent for 2 h, the antioxidant has no mass change. After infrared testing, the attenuation of the characteristic peak intensity of the antioxidant component is < 5%; the solvent is one or more of acetone, dichloromethane, chloroform, hexane, tetrahydrofuran, ethanol, heptane, and petroleum ether.

11. The microporous separator according to claim 1, characterized in that, The oxidation induction time of the microporous separator is ≥ 30 min. For the battery cell made with this microporous separator, after 500 high-temperature cycles at 45°C, the attenuation rate of the diaphragm puncture strength is ≤ 15%, and the attenuation rate of the breakdown voltage is ≤ 15%.

12. The method for preparing the microporous separator according to any one of claims 1 to 11, characterized in that, The battery separator is prepared by mixing the antioxidant, polyolefin component, and paraffin oil, rolling them into a sheet, stretching, extracting, and heat-setting. The stretching ratio is greater than 5 times, preferably > 7 times, and more preferably > 10 times.

13. The method for preparing the microporous separator according to claim 12, wherein, After heat-setting, the crystallinity of the battery separator is ≥ 75%, preferably ≥ 80%, and more preferably ≥ 85%.

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

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