Preparation method of heat-resistant lithium battery diaphragm

Abstract

The application discloses a preparation method of a lithium battery diaphragm with high heat resistance, and relates to the technical field of lithium battery diaphragms. The preparation method comprises the following raw materials in mass fractions: 60-75 parts of polyimide, 10-20 parts of aluminum oxide, 3-8 parts of a heat-resistant modifier, 8-12 parts of PVDF-HFP, 5-8 parts of liquid paraffin, and 0.5-1.5 parts of polyacrylammonium. The application performs pretreatment and modification on boron nitride powder in the preparation process of the lithium battery diaphragm, so that the diaphragm forms an effective heat conduction path, promotes the rapid transmission and diffusion of heat, and improves the overall heat resistance of the diaphragm. Meanwhile, the pretreatment and modification of the boron nitride powder solve the problem that the original boron nitride sheet layer is prone to agglomeration under the action of van der Waals force and hydrogen bonding, and is difficult to uniformly disperse in the polymer matrix. In the preparation process, a segmented drying method is adopted to realize resource recycling, and the method is different from traditional acetone soaking and dissolution liquid paraffin, so that the swelling of the base film is avoided.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium battery separators, in particular to a preparation method of a lithium battery separator with high heat resistance. BACKGROUND

[0002] The lithium battery separator is a high polymer film with a microporous structure, which is arranged between the positive electrode and the negative electrode of the battery to prevent the positive electrode and the negative electrode from directly contacting each other and causing short circuit, thereby guaranteeing the safety and stability of the battery, and serving as a channel for the transmission of electrolyte ions to ensure the smooth migration of lithium ions between the positive electrode and the negative electrode and complete the charging and discharging process of the battery.

[0003] However, the traditional polyolefin lithium battery separators, such as polyethylene and polypropylene separators, have the advantages of low cost and good film-forming property, but have poor heat resistance and are prone to shrinkage and deformation at high temperatures, and cannot effectively prevent the positive electrode and the negative electrode from contacting each other. In order to improve the heat resistance of the separator, in the industry, inorganic particles such as aluminum oxide and silicon dioxide are often coated to improve the thermal stability, but such methods are prone to problems such as uneven dispersion of inorganic particles and poor compatibility with the polymer matrix, which can easily lead to a decrease in the mechanical properties of the separator or blockage of the pores. In addition, in the preparation process of the existing separator, the removal of the solvent and the pore-forming agent is usually carried out by soaking and high-temperature baking, which not only has high energy consumption and low efficiency, but also easily causes the swelling and deformation of the base film, and the solvent is difficult to recover, increasing the production cost and the risk of environmental pollution, and there are problems of low practicability and functionality.

[0004] At present, no effective solution has been proposed for the problems in the related art. SUMMARY

[0005] In view of the problems in the related art, the application provides a preparation method of a lithium battery separator with high heat resistance to overcome the above technical problems existing in the prior art.

[0006] The specific technical scheme adopted by the application is as follows:

[0007] The lithium battery separator with high heat resistance comprises the following raw materials in parts by mass: 60-75 parts of polyimide, 10-20 parts of aluminum oxide, 3-8 parts of a heat-resistant modifier, 8-12 parts of PVDF-HFP, 5-8 parts of liquid paraffin, and 0.5-1.5 parts of polyacrylammonium.

[0008] The heat-resistant modifier is prepared by the following steps:

[0009] Step 1: add boron nitride powder to an ethanol / water mixture, and treat it with a 500W probe-type ultrasonic device for 30 minutes to obtain a pretreated intermediate A;

[0010] Step 2, add KH-550 to the pretreated intermediate A, centrifuge after refluxing at 80℃ for 4 hours, remove unreacted substances by washing with ethanol for 3 times, then vacuum dry at 80℃ for 12 hours, and obtain the heat-resistant modified body by air flow crushing.

[0011] Preferably, the mass ratio of ethanol to water in the ethanol / water mixture used in step 1 is 1:1, and the mass ratio of boron nitride powder to the ethanol / water mixture is 1:9.

[0012] Preferably, the mass ratio of boron nitride powder to KH-550 is 1:0.05, the centrifugal speed is 3000 rpm, and the centrifugal time is 10 minutes, and the air flow crushing D50 is 400 nm.

[0013] The preparation method of the lithium battery separator with strong heat resistance comprises the following preparation steps:

[0014] S1, the following raw materials are weighed according to the mass fraction: 60-75 parts of polyimide, 10-20 parts of aluminum oxide, 3-8 parts of heat-resistant modified body, 8-12 parts of PVDF-HFP, 5-8 parts of liquid paraffin, and 0.5-1.5 parts of polyacrylamide;

[0015] S2, the polyimide is added to NMP and stirred at low speed until it is completely dissolved, the liquid paraffin is added and stirred at 20 rpm for 1 hour, filtered through a filter core with a pore size of 5 μm, and vacuum degassing to obtain a prepared solution A;

[0016] S3, first, the PVDF-HFP is pre-dissolved in NMP to obtain a PVDF-HFP pre-solution, the heat-resistant modified body, aluminum oxide, and polyacrylamide are added to NMP and dispersed at high speed, and then grinded in a sand mill at a linear speed of 10 m / s for 2 hours, with the fineness controlled to be ≤0.8 μm, then the PVDF-HFP pre-solution and the prepared solution A are added, and stirred at 15 rpm for 30 min, with the viscosity controlled to be 4000±200 cP, to obtain a slurry;

[0017] S4, pour the slurry into the trough of the coating machine, adjust the screen roller to control the wet film thickness to be 35±2 μm, uniformly coat and control the closed loop tension to ensure that the substrate is flat and wrinkle-free, to obtain a wet film, and then dry the wet film in stages by passing through a drying oven, wherein the first stage is dried by passing through 60℃ pure hot air for 2 minutes, the second stage is dried by injecting 70℃ acetone vapor with a partial pressure of 15 kPa for 5 minutes, and the third stage is dried by passing through 100℃ pure hot air for 4 minutes, while the dissolved paraffin-acetone mixture is pumped to a condensation recovery tank under negative pressure, to obtain a base film;

[0018] S5, heat press the base film by a double-steel-belt continuous heat press machine, cool and shape the heat pressed base film by using a water-cooled roller, and then roll up the heat pressed base film after drying in a vacuum oven for 12 hours, to obtain a lithium battery separator.

[0019] Preferably, the temperature of the low-speed stirring in S2 is 60 DEG C, the stirring rate is 10 rpm, the stirring time is 2 hours, the vacuum degree of the vacuum defoaming to obtain the prepared solution A is -0.09 MPa, the duration is 1 hour, and the mass ratio of the polyimide to NMP is 1:4.

[0020] Preferably, the mass ratio of the total mass of the heat-resistant modifier, the alumina and the ammonium polyacrylate to NMP in S3 is 1:1.86, the stirring rate of the high-speed dispersion is 2000 rpm, the duration is 20 minutes, the zirconium oxide bead in the sand mill is 0.3 mm, and the outlet temperature of the sand mill is <45 DEG C. Preferably, the mass ratio of the total mass of the heat-resistant modifier, the alumina and the ammonium polyacrylate to NMP in S3 is 1:1.86, the stirring rate of the high-speed dispersion is 2000 rpm, the duration is 20 minutes, the zirconium oxide bead in the sand mill is 0.3 mm, and the outlet temperature of the sand mill is <45 DEG C.

[0021] Preferably, the parameters of the anilox roller in S4 are 100 lines / cm, the parameters of the uniform coating are 5 m / min, the closed-loop tension is 15 N, and the parameters of the negative pressure suction are -0.05 kPa.

[0022] Preferably, the parameters of the double-steel-belt continuous hot press in S5 are 120 DEG C, 8 MPa, and the speed is 0.5 m / min, the temperature of the water-cooled roller is 20 DEG C, the drying temperature of the vacuum box is 80 DEG C, and the winding tension is 15 N.

[0023] Preferably, the mass ratio of PVDF-HFP to NMP in the PVDF-HFP pre-solution in S3 is 1:9.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] (1) The present application obtains a heat-resistant modifier by pretreating and modifying boron nitride powder in the process of preparing a lithium battery diaphragm, so that the diaphragm forms an effective heat conduction path, promotes the rapid transfer and diffusion of heat, and improves the overall heat resistance of the diaphragm. Meanwhile, by pre-modifying the boron nitride powder, the problem of easy agglomeration of the original boron nitride sheet under the action of van der Waals force and hydrogen bonding, which leads to difficult uniform dispersion in the polymer matrix, is solved.

[0026] (2) The present application provides basic mechanical properties and certain heat resistance by using polyimide as the main component, and cooperates with raw materials such as alumina and PVDF-HFP, further improves the thermal stability and mechanical strength of the diaphragm through alumina, and improves the flexibility and electrolyte affinity of the diaphragm through PVDF-HFP, so that the lithium battery diaphragm prepared achieves a good balance in heat resistance, mechanical properties and electrochemical properties.

[0027] (3) The present application can effectively remove the solvent and liquid paraffin in the diaphragm by using different temperature hot air and acetone vapor in the preparation process, using gaseous acetone to quickly fill the pores and dissolve paraffin, and negative pressure suction and recycling the dissolved paraffin acetone mixture during the drying process, which not only ensures the performance of the diaphragm, but also realizes resource recycling and reduces production cost. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a flow chart of the preparation method of the heat-resistant lithium battery diaphragm of the present application. DETAILED DESCRIPTION

[0029] The present application will be further described in conjunction with examples.

[0030] The preparation method of the heat-resistant lithium battery diaphragm comprises the following steps:

[0031] S1, the following raw materials are weighed by mass fraction: 60-75 parts of polyimide, 10-20 parts of aluminum oxide, 3-8 parts of heat-resistant modifier, 8-12 parts of PVDF-HFP, 5-8 parts of liquid paraffin, 0.5-1.5 parts of polyacrylamide;

[0032] S2, polyimide is added to NMP and stirred at low speed until completely dissolved, liquid paraffin is added and stirred at 20 rpm for 1 hour, filtered through a filter core with a pore size of 5 μm, and vacuum degassing to obtain a prepared liquid A;

[0033] S3, first, PVDF-HFP is pre-dissolved in NMP to obtain a PVDF-HFP pre-solution, heat-resistant modifier, aluminum oxide, and polyacrylamide are added to NMP and dispersed at high speed, and ground in a sand mill at a linear speed of 10 m / s for 2 hours, with a fineness of ≤0.8 μm, then PVDF-HFP pre-solution and prepared liquid A are added, and stirred at 15 rpm for 30 min, with a viscosity of 4000±200 cP, to obtain a slurry;

[0034] S4, pour the slurry into the trough of the coating machine, adjust the screen roller to control the wet film thickness to be 35±2 μm, coat at a constant speed and control the closed loop tension to ensure that the substrate is flat and wrinkle-free, obtain a wet film, and dry the wet film in a drying oven in stages, wherein the first stage is dried with 60℃ pure hot air for 2 minutes, the second stage is dried with acetone vapor with a partial pressure of 15 kPa at 70℃ for 5 minutes, and the third stage is dried with 100℃ pure hot air for 4 minutes, while the dissolved paraffin acetone mixture is negative pressure suctioned into a condensation recovery tank, to obtain a base film;

[0035] S5, heat pressing the base film through a double steel belt continuous heat press, cooling and shaping using a water-cooled roller after heat pressing, placing the heat-pressed base film into a vacuum oven for drying for 12 hours, and then winding up to obtain the lithium battery separator.

[0036] The heat-resistant modifier is prepared by the following steps:

[0037] Step 1, add boron nitride powder into ethanol / water mixture, and obtain a pretreated intermediate A by 500W probe ultrasonic treatment for 30 minutes, wherein the mass ratio of ethanol to water in the ethanol / water mixture is 1:1, and the mass ratio of boron nitride powder to the ethanol / water mixture is 1:9;

[0038] Step 2, add KH-550 into the pretreated intermediate A, centrifuge at 3000 rpm for 10 minutes after refluxing at 80℃ for 4 hours, remove unreacted substances by ethanol washing for 3 times, then vacuum dry at 80℃ for 12 hours, airflow crushing D50=400nm, and obtain the heat-resistant modifier. The mass ratio of boron nitride powder to KH-550 is 1:0.05.

[0039] Example 1

[0040] The preparation method of the strong heat-resistant lithium battery separator is as follows:

[0041] Firstly, the following raw materials are weighed by mass fraction: 60 parts of polyimide, 10 parts of aluminum oxide, 3 parts of heat-resistant modifier, 8 parts of PVDF-HFP, 5 parts of liquid paraffin, and 0.5 parts of polyacrylammonium;

[0042] Secondly, 60 parts of polyimide are slowly added into NMP, and stirred at a low speed of 10 rpm at 60℃ for 2 hours until completely dissolved, wherein the mass ratio of polyimide to NMP is 1:4, then 5 parts of liquid paraffin is added and stirred at 20 rpm for 1 hour, filtered through a filter core with a pore size of 5μm, and vacuum degassed at-0.09MPa for 1 hour to obtain a preparation liquid A;

[0043] Thirdly, 8 parts of PVDF-HFP is pre-dissolved in NMP to obtain a PVDF-HFP pre-solution, wherein the mass ratio of PVDF-HFP to NMP is 1:9, 3 parts of heat-resistant modifier, 10 parts of aluminum oxide, and 0.5 parts of polyacrylammonium are added into NMP and dispersed at 2000 rpm for 20 minutes, wherein the mass ratio of the total mass of heat-resistant modifier, aluminum oxide, and polyacrylammonium to NMP is 1:1.86, and the zirconium oxide beads in the sand mill are 0.3mm, which are ground in the sand mill at a linear speed of 10m / s for 2 hours, and the outlet temperature is controlled to be less than 45℃ and the fineness is controlled to be less than or equal to 0.8μm, then the PVDF-HFP pre-solution and the preparation liquid A are added, and stirred at a low speed of 15 rpm for 30 minutes to control the viscosity to be 4100cP, and a slurry is obtained; ​

[0044] The fourth step involves pouring the slurry into the coating machine's trough, adjusting the anilox roller to 100 lines / cm, controlling the wet film thickness to 35μm, coating at a uniform speed of 5m / min, and controlling the closed-loop tension to 15N to ensure the substrate is flat and wrinkle-free, thus obtaining a wet film. The wet film is then dried in stages through a drying oven. The first stage involves drying with 60℃ pure hot air for 2 minutes, the second stage involves injecting acetone vapor at 70℃ and a partial pressure of 15kPa for 5 minutes, and the third stage involves drying with 100℃ pure hot air for 4 minutes. Simultaneously, the dissolved paraffin-acetone mixture is drawn into a condensation recovery tank under negative pressure at -0.05kPa to obtain the base film.

[0045] The fifth step involves using a double steel belt continuous hot press to hot press the base film at 120℃ and 8MPa pressure at a speed of 0.5m / min. After hot pressing, the base film is cooled and shaped using a 20℃ water-cooled roller. The hot-pressed base film is then placed in an 80℃ vacuum oven to dry for 12 hours and then wound up under a tension of 15N to obtain the lithium battery separator.

[0046] Example 2

[0047] The specific steps for preparing the heat-resistant lithium battery separator are as follows:

[0048] The first step is to weigh the following raw materials according to the following mass percentages: 75 parts polyimide, 20 parts alumina, 8 parts heat-resistant modifier, 12 parts PVDF-HFP, 8 parts liquid paraffin, and 1.5 parts ammonium polyacrylate.

[0049] The second step involves slowly adding 75 parts of polyimide to NMP and stirring at 10 rpm at 60°C for 2 hours until completely dissolved. The mass ratio of polyimide to NMP is 1:4. Then, 8 parts of liquid paraffin are added and stirred at 20 rpm for 1 hour. The mixture is then filtered through a 5 μm pore size filter and degassed under vacuum at -0.09 MPa for 1 hour to obtain preparative solution A.

[0050] The third step involves pre-dissolving 12 parts of PVDF-HFP in NMP to obtain a PVDF-HFP pre-solution, with a PVDF-HFP to NMP mass ratio of 1:9. Then, 8 parts of the heat-resistant modifier, 20 parts of alumina, and 1.5 parts of ammonium polyacrylate are added to NMP and dispersed at 2000 rpm for 20 minutes. The total mass ratio of the heat-resistant modifier, alumina, and ammonium polyacrylate to NMP is 1:1.86. (The solution is then added to zirconia beads.) Grind in a 0.3mm sand mill at a linear speed of 10m / s for 2 hours, controlling the outlet temperature to <45℃ and the fineness to ≤0.8μm. Then add PVDF-HFP pre-solution and preparative liquid A, stir at 15rpm for 30min, and control the viscosity to 4100cP to obtain a slurry.

[0051] Fourthly, pour the slurry into the trough of the coating machine, adjust the line speed of the screen roller to 100 lines / cm, control the wet film thickness to be 35 μm, uniformly coat at a speed of 5 m / min and control the closed loop tension to be 15 N to ensure that the substrate is flat and wrinkle-free, obtain the wet film, and dry the wet film in stages by passing it through a drying oven, wherein the first stage is dried by passing 60 ℃ pure hot air for 2 minutes, the second stage is dried by injecting 70 ℃ acetone vapor at a partial pressure of 15 kPa for 5 minutes, and the third stage is dried by passing 100 ℃ pure hot air for 4 minutes, while the dissolved paraffin acetone mixture is pumped to a condensation recovery tank under negative pressure of -0.05 kPa, and the base film is obtained;

[0052] Fifthly, heat press the base film by a double steel belt continuous heat press at 120 ℃ and a pressure of 8 MPa at a speed of 0.5 m / min, cool and shape the heat pressed base film by using a 20 ℃ water cooling roller, dry the heat pressed base film in a 80 ℃ vacuum oven for 12 hours, and then wind it up under a tension of 15 N, and the lithium battery separator is obtained.

[0053] Comparative Example 1

[0054] The preparation method of the strong heat-resistant lithium battery separator is as follows:

[0055] Firstly, the following raw materials are weighed according to the mass fraction: 60 parts of polyimide, 10 parts of aluminum oxide, 3 parts of heat-resistant modifier, 8 parts of PVDF-HFP, 5 parts of liquid paraffin, and 0.5 parts of polyacrylamide;

[0056] Secondly, 60 parts of polyimide is slowly added to NMP and stirred at a low speed of 10 rpm at 60 ℃ for 2 hours until completely dissolved, wherein the mass ratio of polyimide to NMP is 1:4, then 5 parts of liquid paraffin is added and stirred at 20 rpm for 1 hour, filtered through a filter with a pore size of 5 μm, and vacuum degassed at -0.09 MPa for 1 hour to obtain a prepared solution A;

[0057] Thirdly, 8 parts of PVDF-HFP is pre-dissolved in NMP to obtain a PVDF-HFP pre-solution, wherein the mass ratio of PVDF-HFP to NMP is 1:9, 3 parts of heat-resistant modifier, 10 parts of aluminum oxide, and 0.5 parts of polyacrylamide are added to NMP and dispersed at a high speed of 2000 rpm for 20 minutes, wherein the mass ratio of the total mass of the heat-resistant modifier, aluminum oxide, and polyacrylamide to NMP is 1:1.86, and the zirconium oxide beads in the sand mill are 0.3 mm, which are ground in the sand mill at a linear speed of 10 m / s for 2 hours, with the outlet temperature controlled at <45 ℃ and the fineness controlled at ≤0.8 μm, then the PVDF-HFP pre-solution and the prepared solution A are added and stirred at a low speed of 15 rpm for 30 min, and the viscosity is controlled at 4100 cP to obtain a slurry;

[0058] ​Fourthly, pour the slurry into the trough of the coating machine, adjust the line speed of the screen roller to 100 lines / cm, control the wet film thickness to be 35 μm, uniformly coat at a speed of 5 m / min and control the closed loop tension to be 15 N to ensure that the substrate is flat and wrinkle-free, obtain the wet film, and dry the wet film in stages, wherein the first stage is drying at 60 ℃ for 2 minutes, the second stage is drying at 90 ℃ for 3 minutes, and the third stage is drying at 120 ℃ for 5 minutes, to obtain the base film;

[0059] Fifthly, immerse the dried base film in 30 ℃ acetone, accelerate the dissolution of the liquid paraffin with the aid of 200 W ultrasonic for 20 minutes, after draining, rinse with deionized water for 3 times to remove residual acetone and impurities, heat press the base film in a double steel belt continuous heat press at 120 ℃ and a pressure of 8 MPa at a speed of 0.5 m / min, cool and shape the heat pressed base film with a water cooling roller at 20 ℃, and dry the heat pressed base film in a vacuum oven at 80 ℃ for 12 hours, control the tension to be 15 N when winding, to obtain the lithium battery separator.

[0060] Comparative Example 2

[0061] The preparation method of the strong heat-resistant lithium battery separator is specifically as follows:

[0062] Firstly, the following raw materials are weighed according to the mass fraction: 75 parts of polyimide, 20 parts of aluminum oxide, 8 parts of heat-resistant modifier, 12 parts of PVDF-HFP, 8 parts of liquid paraffin, and 1.5 parts of polyacrylamide;

[0063] Secondly, 75 parts of polyimide is slowly added into NMP, stirred at a low speed of 10 rpm at 60 ℃ for 2 hours until completely dissolved, wherein the mass ratio of polyimide to NMP is 1:4, then 8 parts of liquid paraffin is added and stirred at 20 rpm for 1 hour, filtered through a filter core with a pore size of 5 μm, and vacuum degassed at-0.09 MPa for 1 hour to obtain a prepared liquid A;

[0064] Thirdly, 12 parts of PVDF-HFP is pre-dissolved in NMP to obtain a PVDF-HFP pre-solution, wherein the mass ratio of PVDF-HFP to NMP is 1:9, 8 parts of heat-resistant modifier, 20 parts of aluminum oxide, and 1.5 parts of polyacrylamide are added into NMP and dispersed at a high speed of 2000 rpm for 20 minutes, wherein the mass ratio of the total mass of the heat-resistant modifier, aluminum oxide, and polyacrylamide to NMP is 1:1.86, and the zirconium oxide beads in the sand mill are 0.3 mm in size, and the grinding is carried out at a linear speed of 10 m / s for 2 hours, the outlet temperature is controlled to be <45 ℃, and the fineness is ≤0.8 μm, then the PVDF-HFP pre-solution and the prepared liquid A are added, and stirred at a low speed of 15 rpm for 30 min, and the viscosity is controlled to be 4100 cP to obtain a slurry;

[0065] ​Fourthly, the slurry was poured into the trough of the coating machine, the line speed of the engraved roller was adjusted to 100 line / cm, the wet film thickness was controlled to 35 pm, the film was coated at a uniform speed of 5 m / min and the closed loop tension was controlled to 15 N to ensure that the substrate was flat and wrinkle-free, and a wet film was obtained. The wet film was subjected to segmented drying, wherein the first stage was drying at 60 °C for 2 min, the second stage was drying at 90 °C for 3 min, and the third stage was drying at 120 °C for 5 min, and a base film was obtained;

[0066] Fifthly, the dried base film was soaked in 30 °C acetone, and ultrasonic assistance was performed at 200 W for 20 min to accelerate the dissolution of liquid paraffin. After draining, the base film was rinsed with deionized water for 3 times to remove residual acetone and impurities. The base film was hot-pressed by a double-steel-belt continuous hot press at 120 °C and a pressure of 8 MPa at a speed of 0.5 m / min. After hot-pressing, the hot-pressed base film was cooled and shaped by using a water-cooled roller at 20 °C. The hot-pressed base film was placed in a vacuum oven at 80 °C for drying for 12 hours. The tension was controlled to 15 N during winding, and a lithium battery separator was obtained.

[0067] Experimental Example 1

[0068] The lithium battery separators obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were subjected to performance tests, including porosity test, acetone residue test, paraffin residue test, contact angle test, and thermal shrinkage rate test. The test results are shown in Table 1.

[0069] Table 1: Performance test table of lithium battery separator

[0070]

[0071] In the porosity test, the lithium battery separators obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were cut into 1 × 1 cm 2 samples, and vacuum dried at 200 °C for 2 hours. The volume of macropores with a size of > 10 pm was measured at a pressure of 0.5-30 psi, and the volume of micropores with a size of 0.003-10 pm was measured at a pressure of 30-30,000 psi. The porosity was calculated as follows:

[0072]

[0073] In the porosity test, the lithium battery separators obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were cut into 1 × 1 cm p samples, and vacuum dried at 200 °C for 2 hours. The volume of macropores with a size of > 10 pm was measured at a pressure of 0.5-30 psi, and the volume of micropores with a size of 0.003-10 pm was measured at a pressure of 30-30,000 psi. The porosity was calculated as follows:

[0074] It can be seen that the porosities of Examples 1 and 2 are significantly higher than those of Comparative Examples 1 and 2, which indicates that the method of passing pure hot air and acetone vapor in the segmented drying of Examples 1 and 2 helps to form more pore structures in the separator, which is beneficial to the absorption of electrolyte and ion conduction.

[0075] Acetone residue test: 0.5 g of the lithium battery separator obtained in Example 1, 2 and Comparative Example 1, 2 was cut into pieces, 5 mL of methanol was added, ultrasonic extraction was performed for 1 hour, and the residual acetone was detected by gas chromatography-mass spectrometry. The average value of five repeated experiments in each group was taken as the acetone residue;

[0076] Paraffin residue test: 1 g of the lithium battery separator obtained in Example 1, 2 and Comparative Example 1, 2 was added with 10 mL of n-hexane, and reflux extraction was performed at 60°C for 2 hours. The residual paraffin was detected by an evaporative light scattering detector. The average value of five repeated experiments in each group was taken as the paraffin residue;

[0077] Contact angle test: the lithium battery separator obtained in Example 1, 2 and Comparative Example 1, 2 was cut into 2x5 cm 2 , and was fixed flat on the sample stage. A 3 μL droplet of electrolyte EC:DMC = 1:1 was placed on the surface, and the contact angle change within 10 s was recorded by a high-speed camera. The average value of five repeated experiments in each group was taken as the contact angle;

[0078] Thermal shrinkage test: the lithium battery separator obtained in Example 1, 2 and Comparative Example 1, 2 was cut into 10x10 cm 2 samples, and a cross-shaped mark was laser scribed on the surface with a line width of 0.1 mm. The sample was preheated in an oven at 200°C for 30 min, and then was heated for 60 min. After cooling to room temperature, the longitudinal / lateral shrinkage distance of the sample was measured by an image instrument to calculate the shrinkage rate. The average value of five repeated experiments in each group was taken as the thermal shrinkage rate.

[0079] It can be seen that, by using the steam extraction method, the pore collapse is avoided, the micropore connectivity in Example 1, 2 is better, the steam extraction and hot pressing are combined to make the boron nitride directional arrangement, the thermal stability is better, the dissolved paraffin acetone mixture is pumped to the condensation recovery under negative pressure during the drying process, which can more effectively remove the acetone in the separator, avoid the adverse effects of acetone residue on the stability of the battery, and the low residue can improve the surface energy of the separator and reduce the contact angle. In Comparative Example 1, 2, the residual paraffin easily forms local hot spots, resulting in an increase in thermal shrinkage rate.

[0080] In summary, the present application obtains a heat-resistant modified body by pretreating and modifying boron nitride powder in the preparation process of lithium battery separator, so that the separator forms an effective heat conduction path, promotes the rapid transfer and diffusion of heat, and improves the overall heat resistance of the separator. At the same time, by pre-modifying the boron nitride powder, the problem of easy agglomeration of the original boron nitride sheet under the action of van der Waals force and hydrogen bond, which leads to difficult uniform dispersion in the polymer matrix, is solved. By using polyimide as the main component to provide basic mechanical properties and certain heat resistance, and by synergistic effect with alumina, PVDF-HFP and other raw materials, the heat stability and mechanical strength of the separator are further improved by alumina, and the flexibility and electrolyte affinity of the separator are improved by PVDF-HFP, so that the lithium battery separator prepared has a good balance of heat resistance, mechanical properties and electrochemical properties.

[0081] By using a segmented drying method in the preparation process, using hot air and acetone vapor at different temperatures, and using gaseous acetone to quickly fill the pores and dissolve paraffin, the solvent and liquid paraffin in the separator can be effectively removed. The dissolved paraffin-acetone mixture is pumped and recovered under negative pressure during the drying process, which not only ensures the performance of the separator, but also realizes resource recycling and reduces production costs. At the same time, unlike the traditional method of soaking and dissolving liquid paraffin in acetone, the swelling of the base film is avoided, and the risk of damage to the base film is reduced.

Claims

1. A heat-resistant lithium battery separator, characterized by, 60-75 parts of polyimide, 10-20 parts of alumina, 3-8 parts of heat-resistant modifier, 8-12 parts of PVDF-HFP, 5-8 parts of liquid paraffin, 0.5-1.5 parts of ammonium polyacrylate; The preparation method of the strong heat-resistant lithium battery separator is as follows: S1, the following raw materials are weighed by mass fraction: 60-75 parts of polyimide, 10-20 parts of alumina, 3-8 parts of heat-resistant modifier, 8-12 parts of PVDF-HFP, 5-8 parts of liquid paraffin, 0.5-1.5 parts of ammonium polyacrylate; The heat-resistant modifier is made by the following steps: Step 1, add boron nitride powder to ethanol / water mixture, treat with 500W probe type ultrasonic for 30 minutes, obtain pretreated intermediate A; Step 2, add KH-550 to pretreated intermediate A, reflux at 80℃ for 4 hours, centrifuge, remove unreacted substances by washing with ethanol for 3 times, then vacuum dry at 80℃ for 12 hours, airflow crushing, obtain heat-resistant modifier; S2, add polyimide to NMP, stir at low speed until completely dissolved, add liquid paraffin, stir at 20rpm for 1 hour, filter through 5μm pore size filter core, vacuum degassing to obtain preparation liquid A, the mass ratio of polyimide to NMP is 1:4; S3, first, PVDF-HFP is pre-dissolved in NMP to obtain PVDF-HFP pre-solution, add heat-resistant modifier, alumina, ammonium polyacrylate to NMP, disperse at high speed, grind in sand mill at linear speed of 10m / s for 2 hours, control fineness≤0.8μm, then add PVDF-HFP pre-solution and preparation liquid A, stir at 15rpm for 30min, control viscosity at 4000±200cP, obtain slurry, the mass ratio of total mass of heat-resistant modifier, alumina, ammonium polyacrylate to NMP is 1:1.86, the mass ratio of PVDF-HFP in PVDF-HFP pre-solution to NMP is 1:9; S4, pour the slurry into the trough of the coating machine, adjust the screen roller to control the wet film thickness to be 35±2μm, uniform speed coating and control closed loop tension to ensure that the substrate is flat and wrinkle-free, obtain wet film, dry the wet film by drying oven in stages, the first stage is dried by 60℃ pure hot air for 2 minutes, the second stage is dried by acetone vapor with 70℃ steam partial pressure of 15kPa for 5 minutes, the third stage is dried by 100℃ pure hot air for 4 minutes, at the same time, the dissolved paraffin acetone mixture is pumped to the condensation recovery tank, obtain base film; S5, heat press the base film by double steel belt continuous heat press, cool and shape by water-cooled roller after heat pressing, put the heat pressed base film into vacuum oven for drying for 12 hours, then roll up, obtain lithium battery separator.

2. The heat-resistant lithium battery separator of claim 1, wherein, The mass ratio of ethanol to water in the ethanol / water mixture used in step 1 is 1:1, the mass ratio of boron nitride powder to ethanol / water mixture is 1:

9.

3. The heat-resistant lithium battery separator of claim 1, wherein, The mass ratio of boron nitride powder to KH-550 is 1:0.05, the centrifugal speed is 3000rpm, the centrifugal time is 10 minutes, the airflow crushing D50=400nm.

4. The lithium battery separator of claim 1, wherein, The temperature of the low-speed stirring in S2 is 60℃, the stirring rate is 10 rpm, the stirring time is 2 hours, the vacuum degree of the prepared liquid A obtained by vacuum degassing is -0.09 MPa, and the duration is 1 hour.

5. The lithium battery separator of claim 1, wherein, The stirring rate of the high-speed dispersion in S3 is 2000 rpm, the duration is 20 minutes, the zirconium oxide bead φ in the sand mill is 0.3 mm, and the outlet temperature of the sand mill is <45℃.

6. The heat-resistant lithium battery separator according to claim 1, characterized in that, The parameters of the anilox roller in S4 are 100 lines / cm, the parameters of the uniform coating are 5 m / min, the closed-loop tension is 15 N, and the parameters of the negative pressure suction are -0.05 kPa.

7. The heat-resistant lithium battery separator according to claim 1, characterized in that, The parameters of the double-steel-belt continuous hot press in S5 are 120℃, 8 MPa, the speed is 0.5 m / min, the temperature of the water-cooled roller is 20℃, the vacuum box drying temperature is 80℃, and the winding tension is 15 N.

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