CN108842303B - Boehmite/polyacrylonitrile composite nanofiber separator and its preparation method and application - Google Patents

Abstract

The invention discloses a boehmite/polyacrylonitrile composite nanofiber diaphragm and a preparation method and application thereof. The preparation method is to prepare boehmite sol by controlling the hydrolysis of aluminum alkoxide to form a sol by a sol-gel method, and then stirring and mixing the boehmite sol with a polyacrylonitrile solution to obtain a homogeneous spinning solution. Boehmite/polyacrylonitrile composite nanofiber separators spun into porous three-dimensional network structures. The boehmite/polyacrylonitrile composite nanofiber separator is used as a separator material for lithium-ion batteries. It has excellent lyophilic properties, heat resistance and mechanical properties, and can significantly improve the electrochemical performance of lithium-ion batteries. In addition, the preparation method of the present invention has the advantages of simple process, environmental protection, low cost, good repeatability, and good application prospect.

Description

Boehmite/polyacrylonitrile composite nanofiber separator and its preparation method and application

technical field

The invention relates to the field of preparation technology of lithium battery separators, in particular to a preparation method of a boehmite/polyacrylonitrile composite nanofiber separator.

Background technique

The lithium-ion battery separator is a porous film that blocks the positive and negative electrodes to prevent short circuits inside the battery, but allows the rapid passage of ion currents, thereby completing the rapid transport of lithium ions between the positive and negative electrodes during electrochemical charging and discharging. The traditional commercial separators for lithium batteries are mainly polyolefin separators manufactured by dry or wet methods. Such separators have shortcomings such as lyophilic properties and poor heat resistance, which will affect the electrochemical performance of the battery.

Boehmite is the main component of bauxite and is an important chemical raw material with unique crystal structure. It is widely used in catalysts, adsorbents, inorganic flame retardants and other fields.

Chinese invention patent application 2014108301100 discloses an aqueous multi-layer separator for lithium-ion batteries. The multi-layer diaphragm is composed of a polymer base material, an inorganic particle coating and an organic particle coating. The prepared inorganic or organic slurry is coated on the polymer base material by a coating method. The high temperature resistance and mechanical properties of the diaphragm are improved. However, due to the use of a multi-layer coating process and the simple physical blending of the inorganic particles and the binder, it is easy to cause the problems of poor inter-layer coordination and insufficient adhesion, resulting in overall peeling and "powder drop". " of the hidden dangers. In addition, since the base film is a microporous film, the coating process may cause inorganic particles to block the micropores, resulting in problems such as low porosity, which is not conducive to improving the safety and electrochemical performance of the battery.

Chinese invention patent application 201680020283 discloses a composite separator for lithium secondary batteries and a manufacturing method thereof. The composite separator for a lithium secondary battery includes a porous base layer, a heat-resistant layer and a fusion layer. The process adopts a simultaneous coating method, and uses a multi-layer slot coating mold to simultaneously coat the heat-resistant layer slurry and the fusion layer slurry on one side or both sides of the microporous base film to form a multi-layer composite diaphragm. The thermal stability and service life of the diaphragm prepared by this method are improved. However, the process still uses polyolefin-based microporous film as the base film material, and is modified by introducing inorganic/organic particle coating. Inorganic particles block the micropores, causing the problem of reduced porosity, which is not conducive to the transport of lithium ions, thereby affecting the electrochemical performance of the battery.

SUMMARY OF THE INVENTION

In view of the deficiencies of the existing commercial separators, the present invention provides a method that overcomes the problem of "powder drop" in the existing multi-layer coating technology; the nanofiber separator has a porous network structure, the porosity reaches 70%-85%, and the ionic The invention discloses a boehmite/polyacrylonitrile composite nanofiber separator with an electrical conductivity of 1.5mS cm ^-1 to 3.0mS cm ^-1 and a contact angle of 0° to 15° and a preparation method thereof.

Another object of the present invention is to provide the application of the boehmite/polyacrylonitrile composite nanofiber separator as a lithium ion battery separator.

The separator has a porous three-dimensional network structure, and the composite nanofibers have a high specific surface area, which helps to improve the mechanical properties and lyophilic properties of the separator. The good thermal conductivity and flame retardancy of boehmite also effectively improve the heat resistance of the composite separator. The application of composite separators in lithium-ion batteries greatly improves the safety performance and cycle performance of batteries due to their good lyophilic properties and thermodynamic properties.

Boehmite has the characteristics of high heat resistance temperature and good compatibility with organic substances; its low hardness can reduce mechanical wear and has advantages in cost; at the same time, boehmite is an inorganic flame retardant. The composite of materials can effectively improve the heat resistance of materials. The excellent thermal conductivity of boehmite can improve the thermal conductivity of lithium-ion batteries, and its flame retardancy can prevent large-scale combustion and explosion of the battery. Boehmite is a layered structure. When the material is abnormally heated, the boehmite expands and closes conduction. holes, thereby blocking the current flow. When the temperature drops, the volume of the material shrinks and the current-conducting pores reopen. Therefore, boehmite can effectively improve the safety performance of lithium-ion batteries and provide the possibility for the application of high-energy batteries such as power batteries.

The nanofiber membrane prepared by electrospinning has a spatial network structure, high porosity, and good mechanical properties, and is an ideal lithium battery separator material.

In the invention, the boehmite sol is obtained by the hydrolysis method of aluminum alkoxide, and then mixed with the polyacrylonitrile solution to form a homogeneous spinning solution, and the boehmite/polyacrylonitrile composite nanofiber membrane is prepared by the electrospinning method. The composite membrane prepared by the invention has a porous three-dimensional network structure, has excellent liquid absorption performance, heat resistance performance and mechanical performance, and has good electrochemical performance under the assembled lithium battery.

For realizing the above-mentioned technical purpose, the present invention adopts the following technical solutions:

A preparation method of boehmite/polyacrylonitrile composite nanofiber diaphragm, comprising the following steps:

1) Add aluminum alkoxide into deionized water, stir at 60-80°C for 3-4 hours, when white precipitate appears, add dilute nitric acid solution to adjust the pH of the solution to 2-5, and continue heating at 90-100°C Stir under reflux for 8-12 hours to obtain stable boehmite sol;

2) Add the polyacrylonitrile powder into an organic solvent, first swell at room temperature for 8-12 hours, and then stir at 70-90°C for 5-6 hours to obtain a polyacrylonitrile solution; the organic solvent is N,N-dimethylformaldehyde One or more of methylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and acetone;

3) After the polyacrylonitrile solution is cooled to room temperature, add the boehmite sol obtained in step 1) to the polyacrylonitrile solution, mix and stir at 70 to 90° C. for 5 to 6 hours, naturally cool to room temperature, and keep the solution Set aside for 1 to 2 hours to obtain a boehmite/polyacrylonitrile spinning solution; then obtain a boehmite/polyacrylonitrile composite diaphragm by electrospinning, and vacuum dry at 80 to 160°C to remove organic solvents and moisture; control The mass ratio of boehmite sol and polyacrylonitrile powder is 1-5:10.

In order to further achieve the object of the present invention, preferably, in step 1), the aluminum alkoxide is one or more of aluminum isopropoxide, aluminum sec-butoxide and boehmite.

Preferably, in step 3), the conditions of electrospinning are: the temperature is 15-30°C; the voltage is: 10-25kV; the flow rate is: 0.05-1.5mL/h; the receiving distance is: 14-18cm; : 15～35cm/min; spinning time 8-15h; needle diameter: 0.15～0.25mm.

Preferably, in step 1), the mass ratio of the aluminum alkoxide to deionized water is 1-5:100, and the mass ratio of the aluminum alkoxide to dilute nitric acid is 1:0.1-0.5.

Preferably, in step 1), the first stirring and the second stirring are mechanical stirring or magnetic stirring, and the stirring rate is 400-1000 r/min.

Preferably, in step 2), the mass ratio of the polyacrylonitrile powder and the organic solvent is 0.5-2:10.

Preferably, in step 2), the stirring is mechanical stirring or magnetic stirring, and the stirring rate is 600-1000 r/min.

Preferably, in step 3), the absolute vacuum degree of the vacuum drying is -0.1～-0.05MPa, and the vacuum drying time is 8～15h.

A boehmite/polyacrylonitrile composite nanofiber diaphragm is prepared by the above preparation method. The boehmite/polyacrylonitrile nanofiber composite diaphragm has a porous three-dimensional network structure, and the porosity is 70% to 85%. The average fiber diameter is 250nm～400nm, the thickness is 40μm～70μm, the contact angle is 0°～15°, the shrinkage rate is 0%～10% at 135℃, and the ionic conductivity is 1.5mScm ^-1 ～3.0mS cm ^-1 .

Application of the boehmite/polyacrylonitrile composite nanofiber separator as a lithium ion battery separator.

Compared with the prior art, the present invention has the following advantages:

1) In the present invention, the boehmite sol is compounded with the polyacrylonitrile solution, and the boehmite/polyacrylonitrile composite nanofiber membrane is prepared by the electrospinning method. The separator has a porous three-dimensional network structure, and the composite nanofibers have a high specific surface area, which helps to improve the mechanical properties and lyophilic properties of the separator. The good thermal conductivity and flame retardancy of boehmite also effectively improve the heat resistance of the composite separator. The application of composite separators in lithium-ion batteries greatly improves the safety performance and cycle performance of batteries due to their good lyophilic properties and thermodynamic properties.

2) The preparation process of the invention is simple, the conditions are mild, the operation is convenient, and the cost is low, and the prepared boehmite/polyacrylonitrile nanofiber composite membrane has excellent comprehensive properties such as heat resistance and electrochemical properties.

3) The present invention adopts aluminum alkoxide hydrolysis and sol-gel method to prepare boehmite sol, which is then compounded with polyacrylonitrile solution to obtain a homogeneous spinning solution, and a porous nanofiber composite membrane is prepared by electrospinning technology. Among them, the composite diaphragm obtained by spinning can be directly formed, and the boehmite particles are firmly embedded on the polyacrylonitrile fiber, which overcomes the problems of insufficient adhesion and "powder drop" in the existing multi-layer coating technology.

4) The nanofiber diaphragm prepared by the electrospinning method of the present invention has a porous network structure, which significantly improves the porosity of the diaphragm, which is much higher than that of the diaphragm prepared by the prior art; and the composite diaphragm is directly formed and has good repeatability.

Description of drawings

Fig. 1 is the SEM image of the boehmite/polyacrylonitrile composite nanofiber separator prepared in Example 1, wherein Fig. 1a) and Fig. 1b) are the SEM images at 5000 times and 50000 times respectively;

Fig. 2 is the contact angle test chart of the boehmite/polyacrylonitrile composite nanofiber separator prepared in Example 1 and the Celgard 2400 type single-layer PP separator; wherein Fig. 2a) and Fig. 2b) are boehmite/polyacrylonitrile, respectively The contact angle test diagram of the composite nanofiber separator from 0s to 2s; Figure 2c) and Figure 2d) are the contact angle test diagrams of the Celgard 2400 single-layer PP separator from 0s to 2s, respectively.

Fig. 3 is the DSC diagram of the boehmite/polyacrylonitrile composite nanofiber diaphragm prepared in Example 1 and the PP diaphragm;

4 is a cycle performance diagram of a button battery assembled with the boehmite/polyacrylonitrile composite nanofiber separator prepared in Example 1 and the PP separator respectively.

Detailed ways

For better understanding of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments, but the embodiments of the present invention are not limited thereto.

Example 1

A preparation method of boehmite/polyacrylonitrile composite nanofiber diaphragm, comprising the following steps:

(1) In parts by mass, slowly add 1 part of aluminum sec-butoxide into 100 parts of deionized water, and stir at 80° C. for 4 hours. When white precipitation occurs, add 0.1 part of dilute nitric acid solution to adjust the pH of the solution to 3. The magnetic stirring was continued at 90 °C and 800 r/min for 12 h to obtain a stable boehmite sol;

(2) In parts by mass, add 1 part of polyacrylonitrile powder to 10 parts of N,N-dimethylformamide, first swell at room temperature for 12h, and then magnetically stir at 80°C and 800r/min for 6h, to obtain a polyacrylonitrile solution;

(3) In parts by mass, after the polyacrylonitrile solution was cooled to room temperature, 1 part of boehmite sol was added to 10 parts of polyacrylonitrile solution, and stirred magnetically for 6 hours at 80° C. and 1000 r/min, and mixed uniformly. Naturally cooled to room temperature and stood for 2 hours to obtain a boehmite/polyacrylonitrile spinning solution, which was then electrospinned (temperature: 25°C, voltage: 16kV, receiving distance: 16cm, flow rate: 0.64mL/h, Needle size: 0.21mm, needle back-and-forth speed: 20cm/min, time 8h) to obtain a boehmite/polyacrylonitrile composite septum, vacuum drying at 80°C under -0.01MPa absolute vacuum for 12h to remove excess solvent.

A Celgard 2400 type single-layer PP separator (Celgard, USA) was used as a comparative example with a thickness of 25 μm and a porosity of 41%.

Fig. 1 is the SEM image of the boehmite/polyacrylonitrile composite nanofiber separator prepared in Example 1, wherein Fig. 1a) and Fig. 1b) are the SEM images at 5000 times and 50000 times respectively. It can be seen from Figure 1 that the boehmite/polyacrylonitrile nanofiber composite separator has a porous three-dimensional network structure with a porosity of 85%, an average fiber diameter of 390±50 nm, and a thickness of 45 μm measured by a micrometer.

The contact angle of the separators was tested with an OCA40Micro surface tension surface contact angle tester. In Fig. 2, Fig. 2a) and Fig. 2b) are respectively the contact angle of the boehmite/polyacrylonitrile composite nanofiber separator from 0s to 2s, the contact angle is from 70.1°, and changes to 6° after 2s; in Fig. 2, Fig. 2c) and Fig. 2d) are the contact angle of Celgard 2400 single-layer PP separator from 0 s to 2 s, the contact angle changes from 103.1° to 101.5° after 2 s, indicating that the boehmite/polyacrylonitrile nanofiber composite separator is better than the PP separator. Has more excellent wetting properties. The liquid absorption rate of the composite membrane obtained by wet weight method is as high as 1247% (the single-layer PP membrane of Celgard2400 is only 169%).

The thermal stability of the separator was tested by differential scanning calorimetry (DSC), the temperature range was 100-250°C, and the heating rate was 10°C/min. It can be seen from Figure 3 that when the temperature is 167 °C, the PP separator has a larger endothermic peak, while the boehmite/polyacrylonitrile nanofiber composite separator has no obvious peak, indicating that the composite separator has more excellent thermal properties. stability.

The boehmite/polyacrylonitrile nanofiber composite separator and the comparative PP separator were assembled into a CR2016 coin cell in a glove box, and the charge-discharge cycle was 100 times at a rate of 0.5C. It can be seen from Fig. 4 that the application of boehmite/ The battery with the polyacrylonitrile nanofiber composite separator showed better cycle performance than the PP separator, and the capacity retention rate reached 87.68%, while the PP separator was only 55.73%.

In the prior art, there are problems such as insufficient cohesion of the separator, low porosity, etc., as well as the hidden danger of "dropping powder", which is not conducive to improving the safety and electrochemical performance of the battery. In this example, the boehmite sol is prepared by hydrolysis of aluminum alkoxide and sol-gel method, and then compounded with polyacrylonitrile solution to form a homogeneous spinning solution, and the porous nanofiber composite membrane is obtained by electrospinning. Among them, the composite diaphragm is directly formed by electrospinning, and does not need to be modified by multiple layers of coating, and the boehmite particles are more firmly embedded on the polyacrylonitrile fiber, which overcomes the lack of adhesion in the existing coating technology. It causes the problem of "powder drop"; meanwhile, the nanofiber separator prepared by electrospinning has a porous network structure, which significantly improves the porosity of the separator. Therefore, the composite membrane prepared in this example can effectively overcome the shortcomings of the prior art, and significantly improve the safety performance and electrochemical performance of the battery.

Example 2

A preparation method of boehmite/polyacrylonitrile composite nanofiber diaphragm, comprising the following steps:

(1) In parts by mass, slowly add 2 parts of aluminum isoalkoxide into 100 parts of deionized water, and stir at 80°C for 4 hours. When white precipitation occurs, add 0.2 parts of dilute nitric acid solution, adjust the pH of the solution to 3.5, and then add Continuous magnetic stirring at 90 °C and 900 r/min for 12 h to obtain a stable boehmite sol;

(2) In parts by mass, add 0.5 part of polyacrylonitrile powder to 8 parts of N,N-dimethylformamide and 2 parts of acetone, first swell at room temperature for 12h, and then swell at 80°C and 800r/min Magnetic stirring for 6h to obtain a polyacrylonitrile solution;

(3) In parts by mass, after the polyacrylonitrile solution is cooled to room temperature, add 2 parts of boehmite sol to 10 parts of polyacrylonitrile solution, stir magnetically at 80°C and 1000r/min for 6h, and mix evenly, Naturally cooled to room temperature and stood for 2 hours to obtain a boehmite/polyacrylonitrile spinning solution, which was then electrospinned (temperature: 25°C, voltage: 15kV, receiving distance: 15cm, flow rate: 0.58mL/h, Needle size: 0.21mm, needle back-and-forth speed: 25cm/min, time 8h) to obtain a boehmite/polyacrylonitrile composite diaphragm, 80°C, vacuum drying under -0.01MPa absolute vacuum for 12h to remove excess solvent.

Example 3

A preparation method of boehmite/polyacrylonitrile composite nanofiber diaphragm, comprising the following steps:

(1) In parts by mass, add 1 part of aluminum sec-butoxide and 2 parts of aluminum isopropoxide into 100 parts of deionized water, stir at 80°C for 4 hours, and add 0.3 part of dilute nitric acid solution when white precipitation occurs, The pH of the solution was adjusted to 3, and the magnetic stirring was continued at 90 °C and 800 r/min for 12 h to obtain a stable boehmite sol;

(2) In parts by mass, add 1.5 parts of polyacrylonitrile powder into 10 parts of N,N-dimethylacetamide, first swell at room temperature for 12h, and then magnetically stir at 80°C and 900r/min for 6h, to obtain a polyacrylonitrile solution;

(3) In parts by mass, after the polyacrylonitrile solution is cooled to room temperature, add 3 parts of boehmite sol to 10 parts of polyacrylonitrile solution, stir magnetically for 6 hours at 80°C and 1000 r/min, and mix evenly, Naturally cooled to room temperature and stood for 2 hours to obtain a boehmite/polyacrylonitrile spinning solution, and then electrospinning (temperature: 25°C, voltage: 16.8kV, receiving distance: 16cm, flow rate: 0.73mL/h) , needle size: 0.23mm, needle back and forth speed: 25cm/min, time 12h) to obtain a boehmite/polyacrylonitrile composite diaphragm, 80 ° C, vacuum drying under -0.01MPa absolute vacuum for 12h to remove excess solvent.

Example 4

A preparation method of boehmite/polyacrylonitrile composite nanofiber diaphragm, comprising the following steps:

(1) In parts by mass, add 3 parts of boehmite to 100 parts of deionized water, stir at 80°C for 4h, when white precipitation occurs, add 0.3 part of dilute nitric acid solution, adjust pH=3, and then Continuous magnetic stirring at 90°C and 800r/min for 12h to obtain a stable boehmite sol;

(2) In parts by mass, add 1 part of polyacrylonitrile powder to 10 parts of N-methylpyrrolidone, first swell at room temperature for 12h, and then magnetically stir at 80°C and 800r/min for 6h to obtain a polyacrylonitrile solution ;

(3) In parts by mass, after the polyacrylonitrile solution is cooled to room temperature, add 4 parts of boehmite sol to 10 parts of polyacrylonitrile solution, stir magnetically for 6 hours at 80° C. and 1000 r/min, and mix evenly, Naturally cooled to room temperature and stood for 2 hours to obtain a boehmite/polyacrylonitrile spinning solution, which was then electrospinned (temperature: 30°C, voltage: 17.5kV, receiving distance: 17cm, flow rate: 0.58mL/h , needle size: 0.22mm, needle back and forth speed: 30cm/min, time 10h) to obtain a boehmite/polyacrylonitrile composite diaphragm, 80 ℃, vacuum drying under -0.01MPa absolute vacuum for 12h to remove excess solvent.

For the above examples and PP separators, various physical and electrochemical performance tests are shown in Table 1 below. Among them, the PP diaphragm is Celgard2400 type single-layer diaphragm.

The thickness of the composite diaphragm is measured by a micrometer;

The porosity is measured and calculated by the n-butanol method. After immersing different diaphragms in n-butanol for 1 hour, the weight of the diaphragm before and after liquid absorption is weighed. The equation is:

Porosity (%)=(weight after imbibition-weight before imbibition)×100%/(density of n-butanol×volume of diaphragm);

The measurement of the liquid absorption rate is that after soaking the diaphragm in the electrolyte for 1 hour in the glove box, the weights of the diaphragm before and after liquid absorption are respectively weighed and calculated. The equation is:

Liquid absorption rate (%)=(weight after liquid absorption-weight before liquid absorption)×100%/(weight before liquid absorption);

For the shrinkage test of the diaphragm, the diaphragm was cut into 19mm diameter discs using a punching machine, and the different diaphragms were placed in a blast drying oven, heated at 135 ° C for 1 h, and the area of the diaphragm before and after heating was measured. Calculated, the equation is:

Shrinkage rate (%)=(area of diaphragm before heating-area of diaphragm after heating)×100%/(area of diaphragm before heating);

The ionic conductivity is obtained by assembling different diaphragms into a stainless steel sheet|diaphragm|stainless steel sheet CR 2016 occluded battery, and the impedance of the different diaphragms is measured by the Gamry electrochemical workstation, which is calculated by the equation, the equation is:

Ionic conductivity=diaphragm thickness/(diaphragm volume resistance×diaphragm area);

The different separators were assembled into CR 2016 batteries in a glove box, and the electrochemical properties such as discharge specific capacity, charge-discharge efficiency, cycle rate, etc. of the battery were tested by Xinwei battery comprehensive performance tester.

Table 1

It can be seen from the test results in Table 1 that, compared with the PP separators, Examples 1 to 4 not only have more excellent porosity, liquid absorption rate, and ionic conductivity, but also have low shrinkage rate, good thermal stability, and excellent thermal stability. electrochemical performance.

In the invention, the boehmite sol is prepared by hydrolysis of aluminum alkoxide and sol-gel method, and then compounded with polyacrylonitrile solution to obtain a homogeneous spinning solution, and a porous nanofiber composite membrane is prepared by electrospinning technology. Among them, the composite diaphragm obtained by spinning can be directly formed, and the boehmite particles are firmly embedded on the polyacrylonitrile fiber, which overcomes the problems of insufficient adhesion and "powder drop" in the existing multi-layer coating technology; at the same time, electrospinning The nanofiber diaphragm prepared by the method has a porous network structure, which significantly increases the porosity of the diaphragm, which is much higher than that of the diaphragm prepared by the prior art; and the composite diaphragm is directly formed, the preparation process is relatively simple, and the repeatability is good. Therefore, the nanofiber composite membrane prepared by the present invention can effectively overcome the shortcomings of the prior art, and can significantly improve the safety performance and electrochemical performance of the battery.

To sum up, the boehmite/polyacrylonitrile composite nanofiber separator prepared by the present invention can not only meet the basic requirements of lithium ion battery separator, but also can effectively improve the mechanical properties, lyophilic performance, heat resistance and thermal resistance of the separator. electrochemical performance. The preparation process of the invention is simple, the conditions are mild, the operation is convenient, and the cost is low, and the prepared boehmite/polyacrylonitrile nanofiber composite diaphragm has excellent comprehensive properties such as heat resistance and electrochemical properties, and has a good application prospect.

The above embodiments are not intended to limit the present invention, and any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention should be included within the protection scope of the claims of the present invention.

Claims (9)

1. A preparation method of a boehmite/polyacrylonitrile composite nanofiber membrane is characterized by comprising the following steps:

1) adding aluminum alkoxide into deionized water, stirring for 3-4 h at 60-80 ℃, adding a dilute nitric acid solution when white precipitates appear, adjusting the pH of the solution to 2-5, and continuously heating, refluxing and stirring for 8-12 h at 90-100 ℃ to obtain stable boehmite sol;

2) adding polyacrylonitrile powder into an organic solvent, firstly swelling for 8-12 h at room temperature, and then stirring for 5-6 h at 70-90 ℃ to obtain a polyacrylonitrile solution; the organic solvent is one or more of N, N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone and acetone; the mass ratio of the polyacrylonitrile powder to the organic solvent is 0.5-2: 10;

3) after the polyacrylonitrile solution is cooled to room temperature, adding the boehmite sol obtained in the step 1) into the polyacrylonitrile solution, mixing and stirring for 5-6 hours at 70-90 ℃, naturally cooling to room temperature, and standing for 1-2 hours to obtain a boehmite/polyacrylonitrile spinning solution; then obtaining a boehmite/polyacrylonitrile composite nanofiber membrane by an electrostatic spinning method, and carrying out vacuum drying at 80-160 ℃ to remove an organic solvent and water; controlling the mass ratio of the boehmite sol to the polyacrylonitrile powder to be 1-5: 10;

the boehmite/polyacrylonitrile composite nanofiber membrane has a porous three-dimensional network structure, the porosity is 70-85%, the average fiber diameter is 250-400 nm, the thickness is 40-70 mu m, the contact angle is 0-15 ℃, the shrinkage at 135 ℃ is 0-10%, and the ionic conductivity is 1.5mS cm^-1~3.0mS cm^-1。

2. The preparation method of the boehmite/polyacrylonitrile composite nanofiber membrane according to claim 1, wherein in the step 3), the electrostatic spinning conditions are as follows: the temperature is 15-30 ℃; the voltage is as follows: 10-25 kV; the flow rate is: 0.05-1.5 mL/h; the receiving distance is: 14-18 cm; the needle head moves back and forth: 15-35 cm/min; the spinning time is 8-15 h; the diameter of the needle head is as follows: 0.15-0.25 mm.

3. The preparation method of the boehmite/polyacrylonitrile composite nanofiber membrane according to claim 1, wherein in step 1), the aluminum alkoxide is one or more of aluminum isopropoxide and aluminum sec-butoxide.

4. The preparation method of the boehmite/polyacrylonitrile composite nanofiber membrane according to claim 1, wherein in the step 1), the mass ratio of the aluminum alkoxide to the deionized water is 1-5: 100, and the mass ratio of the aluminum alkoxide to the dilute nitric acid is 1: 0.1-0.5.

5. The preparation method of the boehmite/polyacrylonitrile composite nanofiber membrane according to claim 1, wherein in the step 1), the first stirring and the second stirring are mechanical stirring or magnetic stirring, and the stirring speed is 400-1000 r/min.

6. The preparation method of the boehmite/polyacrylonitrile composite nanofiber membrane according to claim 1, characterized in that in the step 2), the stirring is mechanical stirring or magnetic stirring, and the stirring speed is 600-1000 r/min.

7. The preparation method of the boehmite/polyacrylonitrile composite nanofiber membrane according to claim 1, wherein in the step 3), the absolute vacuum degree of vacuum drying is-0.1 to-0.05 MPa, and the time of vacuum drying is 8 to 15 hours.

8. A boehmite/polyacrylonitrile composite nanofiber membrane prepared by the preparation method of any one of claims 1-7, wherein the boehmite/polyacrylonitrile composite nanofiber membrane has a porous three-dimensional network structure, the porosity is 70% -85%, the average fiber diameter is 250-400 nm, the thickness is 40-70 μm, the contact angle is 0-15%, the shrinkage at 135 ℃ is 0% -10%, and the ionic conductivity is 1.5mS cm^-1~3.0mS cm^-1。

9. The use of the boehmite/polyacrylonitrile composite nanofiber membrane described in claim 8 as a lithium ion battery membrane.

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