Coaxial electrostatic spinning lithium battery diaphragm with flame-retardant effect as well as preparation method and application of coaxial electrostatic spinning lithium battery diaphragm

Abstract

The invention discloses a coaxial electrostatic spinning lithium battery diaphragm with a flame-retardant effect as well as a preparation method and application thereof. The diaphragm has a coaxial core-shell structure which takes meta-aramid as a core layer and takes a coating layer of polyacrylonitrile / polyvinylidene fluoride / hydroxyapatite nanosheets as a shell layer; in the diaphragm, the mass of the meta-position aramid fiber accounts for 12wt.%-18wt.% of the total mass of the diaphragm; the mass ratio of the polyacrylonitrile to the polyvinylidene fluoride to the hydroxyapatite nanosheets is 1: 1: (0.3-0.35). The diaphragm disclosed by the invention is prepared from a core layer spinning solution containing poly (m-phenylene isophthalamide) and a shell layer spinning solution containing polyacrylonitrile / polyvinylidene fluoride / nanoscale hydroxyapatite by adopting a coaxial electrostatic spinning technology, and the diaphragm is further endowed with the characteristics of flame retardance, high mechanical strength and high ionic conductivity on the basis of high liquid absorption and high infiltration; the method can be applied to new energy automobiles, and the automobile use safety can be greatly improved.

Description

Technical Field

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

[0002] Lithium-ion batteries have a dominant market position in everything from portable electronic products to new energy vehicles, thanks to their high energy density, high power density, long cycle life, low self-discharge rate, and no memory effect. However, in recent years, frequent explosions and fires have not only led to the recall of a large number of lithium-ion batteries, but also brought serious economic problems to related market sectors. The increasingly prominent safety issues of lithium batteries have become the main factor hindering their development.

[0003] At present, the separators of commercial lithium batteries are mainly polyolefin separators. Their good cycle performance, high mechanical strength and low production cost make it difficult for polyolefin separators to be replaced by other materials in a short time. However, the melting points of the most widely used polyethylene and polypropylene materials are only 135℃ and 165℃, which are easy to shrink or even melt at high temperatures, causing battery short circuits, posing great 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 prepared by coaxial electrospinning technology from a core layer spinning solution containing poly(m-phenylene isophthalamide) and a shell layer spinning solution containing polyacrylonitrile / polyvinylidene fluoride / nano-hydroxyapatite. The spinning separator is endowed with 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 electrostatic spinning lithium battery separator with 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 comprises 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.

[0007] Optionally, the shell layer has a thickness of 400 to 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 to mix, 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 h to 24 h, and is 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 min to 10 min to obtain the diaphragm.

[0023] Optionally, in step (1), the solvent includes dimethylformamide; and 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 transmitter 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 electrostatic spinning lithium battery separator with flame retardant effect.

[0026] Implementing the embodiments of the present invention will have the following beneficial effects:

[0027] The diaphragm 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 by coaxial electrospinning technology, wherein poly(m-phenylene isophthalamide) can work for a long time at about 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 of hydroxyapatite to make it as a flame retardant, selects a hydrothermal synthesis method to synthesize thermally stable hydroxyapatite nanosheets, and blends them with spinning solution, combines coaxial electrospinning and hot pressing treatment 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 with the diaphragm prepared by the present invention can be used in new energy vehicles, which can greatly improve the safety of the vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a comparison chart of the flame retardant performance results of the diaphragms prepared in Example 1, Example 3 and Comparative Examples 1-2 of the present invention. DETAILED DESCRIPTION

[0032] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0033] Example 1

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

[0035] The method for preparing the diaphragm of this 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 stir well to mix; take 1.2g of Na2HPO4 and add 100g of deionized water, stir well to mix; slowly add the Na2HPO4 solution to the calcium chloride solution, and use 0.1mol / L sodium hydroxide solution to adjust the pH of the mixture to neutral. Transfer the mixed liquid to a 100mL polytetrafluoroethylene-lined hydrothermal reactor, and the hydrothermal reaction conditions are: react at 180℃ for 24h. After the reaction is completed, wash with anhydrous ethanol. Bake the sample in a 60℃ oven for 12 hours, and grind it in a mortar for 1 hour to obtain hydroxyapatite nanosheets with a particle size of 90nm.

[0037] (2) Preparation of core layer spinning solution: 1.6 g of poly(m-phenylene isophthalamide) fiber was added with 8.4 g of dimethylacetamide, stirred at 80°C for 4 h, allowed to stand for 24 h, and degassed for later use.

[0038] (3) Preparation of shell spinning solution: 7.7 g of dimethylformamide was added with 0.3 g of hydroxyapatite nanosheets, and ultrasonically dispersed for 10 minutes. Then, 1 g of polyvinylidene fluoride and 1 g of polyacrylonitrile were added, and the mixture was stirred at 80° C. for 4 h, allowed to stand for 24 h, and degassed for later 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 of model 19 / 15, connect the two spinning solutions to the core layer and shell layer of the coaxial spinning needle respectively, and keep the injection rate of the core layer spinning solution at 0.002 ml / min; the injection rate of the shell layer spinning solution is also kept at 0.004 / min. The voltage of electrospinning is 20 kV, the speed of the collecting drum is 150 rpm, and the distance between the transmitter and the receiving substrate is 20 to 24 cm. The spinning volume of the core layer is 1.6 ml; the spinning volume of the shell layer is 3.2 ml. The coaxial electrospinning membrane is obtained on the collecting drum.

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

[0044] Example 3

[0045] The 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. Take 1.1g of anhydrous CaCl2 in a beaker, add 100g of a mixture of methanol and water (mass ratio of 3:7), and stir well; take 0.98g of orthophosphoric acid (H3PO4) and add 100g of deionized water, stir well; slowly add the H3PO4 solution to the calcium chloride solution, and adjust the pH of the mixture to neutral. Transfer the mixed liquid to a 100mL polytetrafluoroethylene-lined hydrothermal reactor, and the hydrothermal reaction conditions are: react at 150°C for 24h. After the reaction is completed, wash with anhydrous ethanol. Bake the sample in a 60°C oven for 12 hours, and grind it 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 introduce hydroxyapatite nanosheets.

[0049] Comparative Example 2

[0050] The diaphragm of this comparative example is a lithium ion battery coaxial electrospinning diaphragm with silicon dioxide introduced into the shell layer, which is different from Example 1 in that:

[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 hours, a TEOS silicon solution is obtained.

[0052] Step 3. Preparation of shell spinning solution: Take 7.7g of dimethylformamide and add 0.3g of tetraethoxysilane silicon solution. After ultrasonic dispersion for 10 minutes, add 1g of polyvinylidene fluoride and 1g of polyacrylonitrile, stir at 80°C for 4h, let 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 with 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 later 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 with 0.4 g of hydroxyapatite nanosheets, and ultrasonically dispersed for 10 min. 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 later use.

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

[0061] Test Case

[0062] 1. The flame retardant and mechanical properties tests were performed 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 diaphragm of Comparative Example 1, which does not introduce hydroxyapatite nanosheets, was ignited in an instant and then burned out within 3s; in contrast, the spinning diaphragm introducing hydroxyapatite nanosheets was baked for 3s under open flame. In addition to showing slight blackening and shrinkage, the spinning diaphragm as a whole was not ignited. This is because the dispersion of hydroxyapatite nanosheets in the spinning solution is more uniform after grinding. Under the joint action of the flaky microscopic physical structure and oxygen-containing functional groups such as hydroxyl groups, the cross-linking of hydroxyapatite sheets, polyvinylidene fluoride and polyacrylonitrile is promoted, so that the thermal stability and mechanical properties of the diaphragm are significantly improved. Similarly, the flame retardant properties of the diaphragm introduced in Comparative Example 2 SiO2 are good, but the introduction of SiO2 increases the difficulty of spinning, and the biggest drawback is that the mechanical properties of the diaphragm are poor.

[0063] 2. The diaphragms prepared in Examples 1-3 and Comparative Examples 1-4 were tested for transverse and longitudinal tensile strength. 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 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 38.7% lower than 73.5% in Example 1. This is because polyvinylidene fluoride mainly acts as a binder in the membrane. After hot pressing at 120°C for 10 minutes, polyvinylidene fluoride will melt to a certain extent. This melting can make adjacent spinning adhere to each other, thereby significantly providing mechanical strength. However, the specific gravity of polyvinylidene fluoride in Comparative Example 3 is too high, and it occupies the gap after melting, which reduces the porosity of the membrane. The solid content of Comparative Example 4 is too high, and the dispersion effect of hydroxyapatite nanosheets in the spinning solution is poor, causing the spinning solution to block the spinning needle, making it impossible to spin continuously, and the production efficiency of the membrane is reduced.

[0068] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached 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 diaphragm, the mass of the meta-aramid fiber accounts for 12wt.% to 18wt.% of the total mass of the diaphragm; 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 layer 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 20-30 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 to mix, 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 h to 24 h, and is 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.; the drying time is 12 h to 16 h.

6. A method for preparing a coaxial electrospun lithium battery separator with flame retardant effect as claimed in 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 min to 10 min 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 transmitter 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 as described in any one of claims 1-5, or the coaxial electrospun lithium battery separator with flame retardant effect prepared by the preparation method as described in any one of claims 6-7.

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