A coaxial electrostatic spinning lithium battery diaphragm with flame retardant effect and a preparation method and application thereof

Abstract

The application discloses a coaxial electrostatic spinning lithium battery diaphragm with a flame-retardant effect and a preparation method and application thereof, and the diaphragm has a coaxial core-shell structure with meta-aramid as a core layer and a polyacrylonitrile / polyvinylidene fluoride / hydroxyapatite nanosheet coating layer as a shell layer; in the diaphragm, the mass of the meta-aramid accounts for 12wt.%-18wt.% of the total mass of the diaphragm; and the mass ratio of the polyacrylonitrile, the polyvinylidene fluoride and the hydroxyapatite nanosheet is 1:1:0.3-0.35.The diaphragm is prepared by using a coaxial electrostatic spinning technology with a core layer spinning solution containing poly-m-phenylene isophthalamide and a shell layer spinning solution containing polyacrylonitrile / polyvinylidene fluoride / nanoscale hydroxyapatite, and on the basis of high liquid absorption and high infiltration, the diaphragm is further endowed with the characteristics of flame retardancy, high mechanical strength and high ionic conductivity, and can be used in new energy vehicles, and can greatly improve the driving safety.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and more specifically, to a coaxial electrospun lithium battery separator with flame retardant effect, and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries, with their advantages of high energy density, high power density, long cycle life, low self-discharge, and lack of memory effect, dominate the market for everything from portable electronics to new energy vehicles. However, in recent years, frequent explosions and fires have not only led to a large number of lithium-ion battery recalls, but also caused serious economic problems for related market sectors. The increasingly prominent safety issues of lithium batteries have become a major obstacle to their development.

[0003] Currently, commercial lithium-ion battery separators are primarily polyolefins. Their excellent cycling performance, high mechanical strength, and low production costs make them difficult to replace with other materials in the short term. However, the melting points of the most widely used polyethylene and polypropylene materials are only 135°C and 165°C, respectively. They are prone to shrinking or even melting at high temperatures, causing battery short circuits and posing significant safety risks. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a coaxial electrospun lithium battery separator with flame retardant effect and its preparation method and application. The separator of the present invention is a coaxial core-shell structure, which is made of a core layer spinning solution containing poly(m-phenylene isophthalamide) and a shell layer spinning solution containing polyacrylonitrile / polyvinylidene fluoride / nano-hydroxyapatite using coaxial electrospinning technology. The spinning separator is given flame retardant properties by introducing hydroxyapatite nanosheet powder into the shell layer; the hot pressing temperature and time are controlled to improve the mechanical strength of the separator. The lithium battery assembled with the separator prepared by the present invention can be used in new energy vehicles, which can greatly improve the safety of the vehicle.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] A coaxial electrospun lithium battery separator with a flame retardant effect, the separator having a coaxial core-shell structure with meta-aramid as a core layer and a coating layer containing hydroxyapatite nanosheets as a shell layer; in the separator, the mass of the meta-aramid accounts for 12wt.% to 18wt.% of the total mass of the separator; the coating layer containing hydroxyapatite nanosheets includes the following components in mass percentage: 10wt.% to 15wt.% of polyacrylonitrile, 10wt.% to 15wt.% of polyvinylidene fluoride and 3wt.% of hydroxyapatite nanosheets; wherein the mass ratio of the polyacrylonitrile, the polyvinylidene fluoride and the hydroxyapatite nanosheets is 1:1:0.3-0.35.

[0007] Optionally, the thickness of the shell layer is 400-700 nm.

[0008] Optionally, the diameter of the core layer is 200-250 nm.

[0009] Optionally, the porosity of the diaphragm is 70% to 75%.

[0010] Optionally, the thickness of the diaphragm is 20-30 μm.

[0011] Optionally, the mass ratio of the polyacrylonitrile, the polyvinylidene fluoride and the hydroxyapatite nanosheets is 1:1:0.3.

[0012] Optionally, the hydroxyapatite nanosheets are synthesized by a hydrothermal method, comprising the following steps:

[0013] Mixing anhydrous calcium chloride with a solvent to obtain a calcium chloride solution with a mass concentration of 1% to 1.5%;

[0014] Slowly adding a 0.01 mol / L Na2HPO4 solution or a H3PO4 solution to the calcium chloride solution and mixing, and adjusting the pH to neutral with a 0.1 mol / L sodium hydroxide solution to obtain a mixed solution;

[0015] The mixed solution is subjected to a hydrothermal reaction at 180° C. for 18 to 24 hours, and is then washed, dried, and ground to obtain the hydroxyapatite nanosheets. The hydroxyapatite nanosheets have a sheet thickness of 30 to 50 nm and an average length of 50 to 100 nm.

[0016] Optionally, the solvent is an aqueous solution containing methanol, and the mass concentration of methanol in the aqueous solution containing methanol is 20% to 30%.

[0017] Optionally, the drying temperature is 60° C. to 80° C.; and the drying time is 12 h to 16 h.

[0018] The present invention also discloses a method for preparing the above-mentioned coaxial electrospun lithium battery separator with flame retardant effect, comprising the following steps:

[0019] (1) dissolving poly(m-phenylene isophthalamide) fiber in a solvent to obtain a core layer spinning solution;

[0020] (2) dissolving polyacrylonitrile, polyvinylidene fluoride and hydroxyapatite nanosheets in a solvent to obtain a shell spinning solution;

[0021] (3) using the core layer spinning solution and the shell layer spinning solution as electrospinning solutions, and adopting a coaxial electrospinning method to prepare a polymer electrospinning membrane with a thickness of 20 μm to 30 μm;

[0022] (4) hot pressing the polymer electrospun membrane at 120° C. and 10 MPa for 5 to 10 minutes to obtain the diaphragm.

[0023] Optionally, in step (1), the solvent includes dimethylformamide; in step (2), the solvent is dimethylformamide.

[0024] Optionally, in step (3), the voltage of coaxial electrospinning is 15kV~20kV, the rotation speed is 100rpm~150rpm, the distance between the emitter and the receiving substrate is 20cm~24cm, the flow rate of the core layer spinning solution is 0.002mL / min~0.005mL / min, and the flow rate of the shell layer spinning solution is 0.004mL / min~0.01mL / min.

[0025] The present invention also discloses a lithium-ion battery, which comprises the above-mentioned coaxial electrostatically spun lithium battery separator with flame retardant effect.

[0026] The implementation of the present invention will have the following beneficial effects:

[0027] The diaphragm of the present invention has a coaxial core-shell structure and is made of a core spinning solution containing poly(m-phenylene isophthalamide) and a shell spinning solution containing polyacrylonitrile / polyvinylidene fluoride / nano-hydroxyapatite using coaxial electrospinning technology. Poly(m-phenylene isophthalamide) can work for a long time at around 250°C, the melting temperature of polyacrylonitrile is 322°C, and the melting temperature of polyvinylidene fluoride is 350°C. On this basis, hydroxyapatite nanosheets with adjustable micro-nanostructure and high phosphorus content are introduced to greatly enhance the heat resistance and flame retardancy of the diaphragm, which can withstand temperatures above 1500°C. In addition, the liquid absorption rate of polyacrylonitrile and polyvinylidene fluoride is extremely high, which can effectively improve the electrolyte affinity of poly(m-phenylene isophthalamide), improve the overall liquid absorption rate of the diaphragm, and increase the ionic conductivity of the diaphragm. At the same time, the hot pressing treatment of the spinning membrane will melt the outermost layer of polyvinylidene fluoride. Controlling the hot pressing temperature and time will produce strong cross-linking points in the outer fibers of the membrane, which can significantly increase the mechanical strength of the electrospinning membrane.

[0028] The present invention utilizes the high thermal stability and electrical insulation properties of hydroxyapatite to make it as a flame retardant. A hydrothermal synthesis method is used to synthesize thermally stable hydroxyapatite nanosheets, which are then blended with a spinning solution and combined with coaxial electrospinning and hot pressing to prepare lithium battery separators, which can significantly improve the safety of the separator.

[0029] The diaphragm prepared by the present invention is further endowed with the characteristics of flame retardancy, high mechanical strength and high ionic conductivity on the basis of high liquid absorption and high wettability.

[0030] The lithium battery assembled by the prepared diaphragm can be used in new energy vehicles, and the vehicle safety can be greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 A comparison chart of the flame-retardant performance results of the diaphragms prepared for Example 1, Example 3 and Comparative Examples 1-2 of the present application. DETAILED DESCRIPTION

[0032] The present application will be further described in conjunction with specific examples, but in no way limits the present application.

[0033] Example 1

[0034] The coaxial electrospun lithium battery diaphragm with flame-retardant effect of the present embodiment has a coaxial core-shell structure with meta-aramid as the core layer and polyacrylonitrile / polyvinylidene fluoride / nanoscale hydroxyapatite as the shell layer; the thickness of the shell layer is 600 nm; the diameter of the core layer is 180 nm; and the porosity of the diaphragm is 73.5%.

[0035] The preparation method of the diaphragm of the present embodiment comprises the following steps:

[0036] (1) Take 1.1g of anhydrous CaCl2 in a beaker, add 100g of a mixture of methanol and water (mass ratio of 3:7) and mix thoroughly; take 1.2g of Na2HPO4 and add 100g of deionized water and mix thoroughly; slowly add the Na2HPO4 solution to the calcium chloride solution and adjust the pH of the mixture to neutral with 0.1mol / L sodium hydroxide solution. Transfer the mixture to a 100mL polytetrafluoroethylene lined hydrothermal reactor, and the hydrothermal reaction conditions are: 180℃ for 24h. After the reaction is completed, wash with anhydrous ethanol. Put the sample into a 60℃ oven and bake for 12 hours, and grind thoroughly with a mortar for 1 hour to obtain hydroxyapatite nanosheets with a particle size of 90nm.

[0037] (2) Preparation of core layer spinning solution: take 1.6g of poly-m-phenylene isophthalamide fiber and add 8.4g of dimethylacetamide, stir at 80℃ for 4h, stand for 24h, and degas for use.

[0038] (3) Preparation of shell layer spinning solution: take 7.7g of dimethylformamide and add 0.3g of hydroxyapatite nanosheets, ultrasonically disperse for 10 minutes with an ultrasonic machine, then add 1g of polyvinylidene fluoride and 1g of polyacrylonitrile, stir at 80℃ for 4h, stand for 24h, and degas for use.

[0039] (4) Take 5 ml of the spinning solution from step 1 and add it to a 5 ml syringe; take 5 ml of the spinning solution from step 2 and add it to a 5 ml syringe; select a coaxial needle with a model of 19 / 15, and connect the two spinning solutions to the core layer and shell layer of the coaxial spinning needle respectively. The injection rate of the core layer spinning solution is maintained at 0.002 ml / min; the injection rate of the shell layer spinning solution is also maintained at 0.004 / min. The electrospinning voltage is 20 kV, the collection drum speed is 150 rpm, and the distance between the emitter and the receiving substrate is 20 to 24 cm. The core layer spinning volume is 1.6 ml; the shell layer spinning volume is 3.2 ml. The coaxial electrospun membrane is obtained on the collection drum.

[0040] (5) The electrospun membrane obtained in step 3 was hot pressed at 120°C and 10 MPa for 10 minutes to obtain a coaxial electrospun lithium battery separator with flame retardant effect. The final thickness of the obtained spinning separator was 10 μm.

[0041] Example 2

[0042] The difference between this embodiment and embodiment 1 is that the hot pressing temperature is different. The details are as follows:

[0043] In step (5), the electrospun membrane obtained in step 4 is cut into four 5cm*5cm membranes, and hot pressed at different temperatures of 80°C, 100°C, 140°C, and 160°C with a pressure of 10 MPa for 10 minutes to obtain four coaxial electrospun membranes with a thickness of 10 μm.

[0044] Example 3

[0045] The only difference between this embodiment and embodiment 1 is that the preparation method of hydroxyapatite nanosheets is different. The details are as follows:

[0046] Step 1. Place 1.1g of anhydrous CaCl2 in a beaker, add 100g of a mixture of methanol and water (3:7 by mass), and stir thoroughly. Add 0.98g of orthophosphoric acid (H3PO4) to 100g of deionized water and stir thoroughly. Slowly add the H3PO4 solution to the calcium chloride solution, adjusting the pH of the mixture to neutral. Transfer the mixture to a 100mL polytetrafluoroethylene-lined hydrothermal reactor and heat at 150°C for 24 hours. After completion, rinse with anhydrous ethanol. Bake the sample in a 60°C oven for 12 hours and grind thoroughly in a mortar for 1 hour to obtain hydroxyapatite nanosheets with a particle size of 100nm.

[0047] Comparative Example 1.

[0048] The only difference between this comparative example and Example 1 is that the diaphragm does not include hydroxyapatite nanosheets.

[0049] Comparative Example 2

[0050] The diaphragm of this comparative example is a coaxial electrospun lithium ion battery diaphragm with silicon dioxide introduced into the shell layer. The difference between it and Example 1 is:

[0051] Step 1. Tetraethoxysilane (TEOS) is selected as a precursor of silicon dioxide. 1 g of TEOS, 9 g of anhydrous ethanol, and 0.01 g of phosphoric acid are taken and slowly stirred to polycondense TEOS. After stirring at room temperature for 6 h, a TEOS silicon solution is obtained.

[0052] Step 3. Preparation of shell spinning solution: Take 7.7g dimethylformamide and add 0.3g tetraethoxysilane silicon solution. After ultrasonic dispersion for 10 minutes, add 1g polyvinylidene fluoride and 1g polyacrylonitrile, stir at 80℃ for 4h, let it stand for 24h, degas and set aside.

[0053] Step 5. The electrospun membrane obtained in step 3 was hot-pressed at 120°C and 10 MPa for 10 min to obtain a 10 μm diaphragm. During the preparation process, it was found that after the introduction of silica, the viscosity of the spinning solution increased and the spinning speed decreased.

[0054] Comparative Example 3

[0055] Compared with Example 1, this comparative example differs only in that:

[0056] (3) Preparation of shell spinning solution: 6.7 g of dimethylformamide was added to 0.3 g of hydroxyapatite nanosheets, and ultrasonically dispersed for 10 minutes. Then, 1 g of polyacrylonitrile and 2 g of polyvinylidene fluoride (mass concentration ratio of 1:2:0.3) were added, stirred at 80 °C for 4 h, allowed to stand for 24 h, and degassed for use.

[0057] Comparative Example 4

[0058] Compared with Example 1, this comparative example differs only in that:

[0059] (3) Preparation of shell spinning solution: 7.7 g of dimethylformamide was added to 0.4 g of hydroxyapatite nanosheets, and ultrasonically dispersed for 10 minutes. Then, 1 g of polyacrylonitrile and 1 g of polyvinylidene fluoride (mass concentration ratio of 1:1:0.4) were added, stirred at 80 °C for 4 h, allowed to stand for 24 h, and degassed for use.

[0060] In this comparative example, the solid content of the shell spinning solution is slightly higher, resulting in a decrease in spinning continuity.

[0061] Test Case

[0062] 1. The flame retardant and mechanical properties tests were carried out on the diaphragms prepared in Example 1, Example 3 and Comparative Examples 1-2. The results are as follows: Figure 1 It can be seen that: under open flame baking, the spinning membrane of Comparative Example 1 without introducing hydroxyapatite nanosheets was ignited in an instant and then burned out within 3s; in contrast, the spinning membrane of the introduction of hydroxyapatite nanosheets was baked for 3s after open flame, except for showing slight blackening and shrinkage, the spinning membrane as a whole was not ignited. This is because the dispersion of hydroxyapatite nanosheets in the spinning solution is more uniform after grinding, and under the joint action of oxygen-containing functional groups such as flaky microphysical structure and hydroxyl groups, the cross-linking of hydroxyapatite sheets, polyvinylidene fluoride and polyacrylonitrile is promoted, so that the thermal stability and mechanical properties of the membrane are significantly improved. Similarly, the flame retardant properties of the membrane of Comparative Example 2 introducing SiO2 are good, but the introduction of SiO2 increases the difficulty of spinning, and the biggest drawback is that the mechanical properties of the membrane are poor.

[0063] 2. The diaphragms prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to transverse and longitudinal tensile strength tests. The results are shown in Table 1.

[0064] Table 1 Transverse and longitudinal tensile strength of diaphragms at different hot pressing temperatures

[0065]

[0066] According to the results in Table 1, when the hot pressing temperature is 80°C, 100°C, 140°C and 160°C, the transverse and longitudinal tensile strengths of the spinning membrane are all lower than those in Example 1. Therefore, 120°C is the optimal hot pressing temperature, and the transverse and longitudinal tensile strengths of the membrane are the highest.

[0067] The porosity of the spinning membrane obtained in Comparative Example 2 was 34.8% after hot pressing at 120°C and 10MPa pressure, which was a decrease of 38.7% compared to 73.5% in Example 1. This is because polyvinylidene fluoride mainly acts as a binder in this membrane. After hot pressing at 120°C for 10 minutes, polyvinylidene fluoride will melt to a certain extent. This melting can make adjacent spinning stick to each other, thereby significantly providing mechanical strength. However, the specific gravity of the polyvinylidene fluoride in Comparative Example 3 is too high. After melting, it occupies the gap and reduces the porosity of the membrane. The solid content of Comparative Example 4 is too high, and the dispersion effect of the hydroxyapatite nanosheets in the spinning solution is poor, causing the spinning solution to clog the spinning needle, making it impossible to spin continuously, and reducing the production efficiency of the membrane.

[0068] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A coaxial electrospun lithium battery separator with flame retardant effect, characterized in that: The diaphragm has a coaxial core-shell structure with meta-aramid as the core layer and a coating layer containing hydroxyapatite nanosheets as the shell layer; In the separator, the mass of the meta-aramid fiber accounts for 12 wt.% to 18 wt.% of the total mass of the separator; The coating layer containing hydroxyapatite nanosheets includes the following components in mass percentage: 10wt.% to 15wt.% of polyacrylonitrile, 10wt.% to 15wt.% of polyvinylidene fluoride and 3wt.% of hydroxyapatite nanosheets; wherein the mass ratio of the polyacrylonitrile, the polyvinylidene fluoride and the hydroxyapatite nanosheets is 1:1:0.3 to 0.

35.

2. The coaxial electrospun lithium battery separator with flame retardant effect according to claim 1, characterized in that: The thickness of the shell is 400 to 700 nm; The diameter of the core layer is 200-250 nm.

3. The coaxial electrospun lithium battery separator with flame retardant effect according to claim 1, characterized in that: The porosity of the diaphragm is 70% to 75%; The thickness of the separator is 20 to 30 μm.

4. The coaxial electrospun lithium battery separator with flame retardant effect according to claim 1, characterized in that: The hydroxyapatite nanosheets are synthesized by a hydrothermal method, comprising the following steps: Mixing anhydrous calcium chloride with a solvent to obtain a calcium chloride solution with a mass concentration of 1% to 1.5%; Slowly adding a 0.01 mol / L Na2HPO4 solution or a H3PO4 solution to the calcium chloride solution and mixing, and adjusting the pH to neutral with a 0.1 mol / L sodium hydroxide solution to obtain a mixed solution; The mixed solution is subjected to a hydrothermal reaction at 180° C. for 18 to 24 hours, and is then washed, dried, and ground to obtain the hydroxyapatite nanosheets. The hydroxyapatite nanosheets have a sheet thickness of 30 to 50 nm and an average length of 50 to 100 nm.

5. The coaxial electrospun lithium battery separator with flame retardant effect according to claim 4, characterized in that: The solvent is an aqueous solution containing methanol; The drying temperature is 60° C. to 80° C.; and the drying time is 12 hours to 16 hours.

6. A method for preparing a flame-retardant coaxial electrospun lithium battery separator according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) dissolving poly(m-phenylene isophthalamide) fiber in a solvent to obtain a core layer spinning solution; (2) dissolving polyacrylonitrile, polyvinylidene fluoride and hydroxyapatite nanosheets in a solvent to obtain a shell spinning solution; (3) using the core layer spinning solution and the shell layer spinning solution as electrospinning solutions, and adopting a coaxial electrospinning method to prepare a polymer electrospinning membrane with a thickness of 20 μm to 30 μm; (4) hot pressing the polymer electrospun membrane at 120° C. and 10 MPa for 5 to 10 minutes to obtain the diaphragm.

7. The preparation method according to claim 6, characterized in that In step (3), the voltage of coaxial electrospinning is 15kV~20kV, the rotation speed is 100rpm~150rpm, the distance between the emitter and the receiving substrate is 20cm~24cm, the flow rate of the core layer spinning solution is 0.002mL / min~0.005mL / min, and the flow rate of the shell layer spinning solution is 0.004mL / min~0.01mL / min.

8. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the coaxial electrospun lithium battery separator with flame retardant effect described in any one of claims 1-5, or the coaxial electrospun lithium battery separator with flame retardant effect prepared by the preparation method described in any one of claims 6-7.

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