Electrostatic spinning diaphragm, preparation method thereof and sodium-sulfur battery

Abstract

The invention provides an electrostatic spinning diaphragm, a preparation method thereof and a sodium-sulfur battery, and belongs to the technical field of sodium-sulfur batteries. Polyvinylidene fluoride-hexafluoropropylene and a perfluorosulfonic acid polymer are used as main raw materials, and the electrostatic spinning diaphragm is prepared through an electrostatic spinning technology. A sulfurized polyacrylonitrile positive electrode, a metal sodium negative electrode, an electrostatic spinning diaphragm and electrolyte are assembled into the sodium-sulfur battery. The electrostatic spinning diaphragm prepared by the invention can effectively inhibit shuttling of polysulfide, improve the transference number of sodium ions, and also can effectively enhance the conductivity and stability of the material. And the assembled sodium-sulfur battery has excellent electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-sulfur batteries, and particularly to an electrospun separator, a preparation method thereof, and a sodium-sulfur battery. Background Art

[0002] Room temperature sodium-sulfur (RTNa / S) batteries are becoming promising candidates for stationary energy storage systems due to their high energy density, abundant resources, and environmental friendliness. Their development can be traced back nearly 20 years ago, and since then, their development has been accelerating. Similar to Li-S, some theoretical and technical problems have greatly hindered the performance of RT Na / S batteries. For example: 1) The poor conductivity of S and the discharge products of Na2S2 and Na2S result in slow reaction kinetics of the S positive electrode; 2) The large volume change involved in the conversion process from S to Na2S leads to structural deformation of the S positive electrode, dissolution of NaPSs from the S positive electrode into the electrolyte, and severe capacity attenuation; 3) NaPSs shuttle back and forth on the separator to consume the Na metal negative electrode. This will result in rapid loss of active S in the positive electrode, poor cycle stability, and the instability of the Na metal negative electrode also causes related problems, including unstable SEI film, loss of active Na, and Na dendrite growth; 4) During the plating / stripping process, the host-free nature of Na metal will cause large volume changes, resulting in deformation and rupture of the SEI film; 5) The ruptured SEI layer causes Na metal to be exposed to the electrolyte and form a new SEI film, accompanied by the consumption of Na metal and electrolyte.

[0003] To solve the above problems, researchers have done a lot of research work in aspects such as positive electrode structure design, modification of sodium metal negative electrode, design of new separators, and electrolyte engineering. The separator is an indispensable component of rechargeable batteries, which isolates the negative electrode from the positive electrode to prevent internal short circuit. The separator of RTNa / S batteries is expected to inhibit the shuttle of polysulfides and the growth of Na dendrites. Therefore, in terms of improving the cycle performance of RTNa / S batteries, various modified separators with the functions of blocking, absorbing, and converting polysulfides have been designed and developed. For example, Kaskel et al. reported a PP separator modified with a thin layer of cation-selective Nafion for RTNa / S batteries, which can significantly reduce the shuttle of polysulfides without hindering the ionic conductivity of sodium ions. Similarly, an Al2O3-Nafion composite modified GF separator for RT-Na / S batteries was reported. The results show that Al2O3 nanoparticles can effectively adsorb and capture polysulfides, and the Nafion film has a good selectivity for Na +It has cation-selective permeability, while polysulfides are repelled by the negatively charged sulfonic acid groups on the positive electrode side. Therefore, the RTNa / S battery with a modified separator has a high discharge capacity and enhanced cycling performance. Reviewing the literature shows that the research on functional separators is mainly based on coating an additional layer on commercial separators, which may block NaPSs at the cost of reducing porosity, reducing Na + conductivity, and increasing the separator weight. Therefore, it is proposed that a free-standing separator composed of a mechanically stable polymer and functional nanoparticles in a polymer matrix can combine the advantages of both without sacrificing the inherent porosity and ionic conductivity, which will be a promising alternative for future research. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrospun separator, a preparation method thereof, and a sodium-sulfur battery, so as to solve the problems that the separator of the sodium-sulfur battery in the prior art cannot inhibit the shuttle of polysulfides and has a low sodium ion transference number.

[0005] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0006] The present invention provides an electrospun separator, and the raw materials of the electrospun separator include polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer; the mass ratio of polyvinylidene fluoride-hexafluoropropylene to perfluorosulfonic acid polymer is 1-20:1.

[0007] The present invention also provides a preparation method of the above-mentioned electrospun separator, which includes the following steps:

[0008] (1) Mix polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer in a solvent to obtain an electrospinning solution;

[0009] (2) Perform electrospinning and drying on the electrospinning solution to obtain an electrospun separator.

[0010] Preferably, the solvent is N,N-dimethylformamide; the mass ratio of polyvinylidene fluoride-hexafluoropropylene to the solvent is 1-20:80-120.

[0011] Preferably, the process parameters of the electrospinning are: the voltage of the electrospinning is 10-20 kV, the flow rate is 0.5-1.0 mL / h, the receiving distance is 15-25 cm, and the spinning time is 10-15 h.

[0012] The present invention also provides a sodium-sulfur battery, which includes a polyacrylonitrile sulfide positive electrode, a metallic sodium negative electrode, the above-mentioned electrospun separator, and an electrolyte.

[0013] Preferably, the raw materials of the sulfurized polyacrylonitrile cathode include sulfurized polyacrylonitrile powder, a conductive agent, and a binder; the raw materials of the electrolyte include sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, 1,3-dioxolane (DOL), and fluoroethylene carbonate (FEC).

[0014] Preferably, the mass ratio of the sulfurized polyacrylonitrile powder, the conductive agent, and the binder is 75-95:2.5-12.5:2.5-12.5.

[0015] Preferably, the preparation method of the sulfurized polyacrylonitrile powder is as follows: sulfur powder and polyacrylonitrile are mixed, and calcined and ground under a nitrogen atmosphere to obtain the sulfurized polyacrylonitrile powder.

[0016] Preferably, the mass ratio of the polyacrylonitrile to the sulfur powder is 1:2-6.

[0017] Preferably, the calcination temperature is 300-500 °C, and the calcination time is 1-3 h.

[0018] Advantages of the present invention:

[0019] (1) The perfluorosulfonic acid polymer has cation selectivity and can accelerate the migration of sodium ions. Polyvinylidene fluoride-hexafluoropropylene has a high dielectric constant and strong charge storage ability. By mixing the perfluorosulfonic acid polymer and polyvinylidene fluoride-hexafluoropropylene to prepare an electrospinning solution and then making an electrospun separator through electrospinning, the shuttling of polysulfides can be effectively inhibited, the sodium ion transference number can be increased, and the conductivity and stability of the material can also be effectively enhanced.

[0020] (2) The electrospun separator prepared in the present invention is uniform and dense, can promote the uniform deposition of sodium metal, and inhibit the growth of sodium dendrites.

[0021] (3) The cathode of the sodium-sulfur battery of the present invention is a sulfurized polyacrylonitrile cathode, and the separator prepared by electrospinning using the perfluorosulfonic acid polymer and polyvinylidene fluoride-hexafluoropropylene as raw materials has excellent electrochemical performance and has important application prospects in the field of sodium-sulfur batteries. Description of the drawings

[0022] Figure 1 is the electrochemical impedance test result of a stainless steel-stainless steel battery;

[0023] Figure 2 is the sodium deposition / stripping polarization curve graph of a sodium metal symmetric battery assembled with the electrospun separator prepared in Example 1;

[0024] Figure 3 is the scanning electron microscope image of the electrospun separator of Example 1 and the electrospun separator after DOL polymerization;

[0025] Figure 4 It is a scanning electron microscope image of metallic sodium deposition on carbon-coated aluminum foil in the Na||Al / C battery;

[0026] Figure 5 It is the cyclic stability test result of the sodium-sulfur battery of Example 1;

[0027] Figure 6 It is the linear sweep voltammetry test result of the Na||Al / C battery;

[0028] Figure 7 It is the sodium ion transference number test result of the sodium metal symmetric battery assembled with the electrospun separator of Example 1;

[0029] Figure 8 It is the sodium deposition / stripping polarization curve graph of the sodium-sulfur batteries assembled with the electrospun separators of Examples 1 to 3 and the electrospun separator prepared in Comparative Example 1, where 0% Nafion corresponds to Comparative Example 1, 5% Nafion corresponds to Example 3, 10% Nafion corresponds to Example 1, and 20% Nafion corresponds to Example 2. Detailed implementation manners

[0030] The present invention provides an electrospun separator, and the raw materials of the electrospun separator include polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer; the mass ratio of the polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) to the perfluorosulfonic acid polymer (Nafion) is 1 to 20:1.

[0031] In the present invention, the mass ratio of the polyvinylidene fluoride-hexafluoropropylene to the perfluorosulfonic acid polymer is preferably 5 to 15:1, and more preferably 10:1.

[0032] The framework of the electrospun separator of the present invention is polyvinylidene fluoride-hexafluoropropylene. By making a spinning solution from polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer (Nafion) and performing electrospinning, Nafion can be uniformly distributed inside the spun membrane, and Nafion can serve as an initiator to achieve the purpose of controlled polymerization.

[0033] The present invention also provides a preparation method of the above-mentioned electrospun separator, including the following steps:

[0034] (1) Mix polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer in a solvent to obtain an electrospinning solution;

[0035] (2) Perform electrospinning and drying on the electrospinning solution to obtain an electrospun separator.

[0036] In the present invention, the solvent is N,N-dimethylformamide (DMF); the mass ratio of polyvinylidene fluoride-hexafluoropropylene to the solvent is 1 to 20:80 to 120, preferably 5 to 15:90 to 110, and more preferably 10:100.

[0037] DMF is a polar organic solvent with high dissolving ability and chemical stability, which can dissolve many polar and non-polar compounds, and is particularly suitable for dissolving certain polymer materials and inorganic salts, and is widely used in chemical reactions.

[0038] PVDF-HFP is a polymer material with excellent performance, which is widely used in membrane materials, batteries, coatings, packaging and other fields. In lithium batteries and supercapacitors, PVDF-HFP, as an electrolyte membrane material, can provide excellent electrical properties.

[0039] Nafion is a high-performance fluorinated polymer with great application potential, which is widely used in fuel cells, electrolyzers, sensors and other electrochemical devices. Its excellent electrochemical performance, chemical corrosion resistance and good mechanical properties make it one of the key materials for many advanced technologies.

[0040] In the present invention, the process parameters of the electrospinning are as follows: the voltage of the electrospinning is 10 to 20 kV, preferably 12 to 18 kV, and more preferably 15 kV; the flow rate is 0.5 to 1.0 mL / h, preferably 0.6 to 0.9 mL / h, and more preferably 0.7 to 0.8 mL / h; the receiving distance is 15 to 25 cm, preferably 18 to 22 cm, and more preferably 20 cm; the spinning time is 10 to 15 h, preferably 11 to 14 h, and more preferably 12.5 to 13 h.

[0041] The present invention also provides a sodium-sulfur battery, which includes a sulfurized polyacrylonitrile positive electrode, a metallic sodium negative electrode, the electrospun separator described above, and an electrolyte.

[0042] The main function of the separator is to ensure the guidance of sodium ions and inhibit the shuttle of polysulfides, prevent the migration of polysulfides to the negative electrode, and generate inactive substances on the surface of the negative electrode, which will affect the cycle performance of the battery. However, polypropylene separators or glass fiber membranes cannot simultaneously meet the requirements of facilitating sodium ion transfer and inhibiting polysulfide shuttle. In order to improve the specific capacity and cycle stability, the separator is modified, and a fibrous separator is prepared by electrospinning, and the initiator Nafion is incorporated into it to form a uniform functionalized separator. At the same time, Nafion has cation selectivity, allowing sodium ions to migrate, and repelling polysulfide anions due to the negatively charged sulfonic acid groups. Compared with a solid electrolyte in which the initiator is directly added to the electrolyte to initiate polymerization, using this functionalized separator has more advantages.

[0043] In the present invention, the raw materials of the sulfurized polyacrylonitrile positive electrode include sulfurized polyacrylonitrile powder, a conductive agent, and a binder; the raw materials of the electrolyte include sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, 1,3-dioxolane (DOL), and fluoroethylene carbonate (FEC).

[0044] In the present invention, the mass ratio of the sulfurized polyacrylonitrile powder, the conductive agent, and the binder is 75-95:2.5-12.5:2.5-12.5, preferably 78-90:5-10:5-10, and more preferably 80:10:10.

[0045] In the present invention, the conductive agent includes one or more of SuperP, acetylene black (AB), conductive carbon black, carbon nanotubes, and graphene; the binder includes one or more of styrene-butadiene rubber (SBR) emulsion, polytetrafluoroethylene, carboxymethyl cellulose (CMC), and polyvinylidene fluoride.

[0046] In the present invention, the preparation method of the sulfurized polyacrylonitrile powder is: mixing sulfur powder and polyacrylonitrile, and performing calcination and grinding under a nitrogen atmosphere to obtain the sulfurized polyacrylonitrile powder.

[0047] In the present invention, the mass ratio of the polyacrylonitrile and the sulfur powder is 1:2-6, preferably 1:3-5, and more preferably 1:4.

[0048] In the present invention, the temperature of the calcination is 300-500 °C, preferably 350-450 °C, and more preferably 400 °C; the time of the calcination is 1-3 h, preferably 2 h.

[0049] In the present invention, the particle size of the sulfurized polyacrylonitrile powder is preferably 100-150 nm.

[0050] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0051] Example 1

[0052] Mix 10 g of PVDF-HFP, 1 g of Nafion, and 100 g of DMF to obtain an electrospinning solution, and then perform electrospinning under the conditions of a spinning voltage of 15 kV, a flow rate of 0.8 mL / h, and a receiving distance of 20 cm. The electrospinning time is 12.5 h. Finally, place it in a vacuum oven and dry it for 10 h, and cut it into a 19-mm round piece to obtain an electrospun separator. The addition amount of Nafion relative to PVDF-HFP is 10 wt.%, and reserve it for use.

[0053] In a glove box filled with nitrogen, use the DOL solvent and the FEC solvent Molecular sieve dehydration treatment: Keep NaFSI and NaTFSI at 80 °C and dry overnight. Then take 242.5 mg of NaTFSI and 40.6 mg of NaFSI, dissolve them in 900 μL of DOL solvent, and then add 100 μL of FEC solvent, mix evenly to obtain the electrolyte.

[0054] Take 4 g of sulfur powder and 1 g of polyacrylonitrile, mix evenly, then calcine at 450 °C for 3 h under a nitrogen atmosphere, and grind to obtain polyacrylonitrile sulfide powder with a particle size of 100 - 150 nm; mix the polyacrylonitrile sulfide powder, Super P, CMC, and SBR according to a mass ratio of 80:10:5:5 to obtain the positive electrode slurry. Coating the positive electrode slurry evenly on the carbon-coated aluminum foil with a coating thickness of 40 μm, drying at 80 °C for 10 h under vacuum conditions to obtain the polyacrylonitrile sulfide positive electrode sheet, and finally punching it into a positive electrode sheet with a diameter of 6 mm using a punching machine, and transferring it to a glove box.

[0055] Assemble the above-mentioned static spinning separator, electrolyte, and positive electrode sheet, using sodium metal as the negative electrode to form a sodium-sulfur battery, and conduct electrochemical performance tests at room temperature. The test results are as Figure 5 shown.

[0056] Example 2

[0057] The difference from Example 1 is that the amount of PVDF-HFP used is 5 g, and the addition amount of Nafion relative to PVDF-HFP is 20 wt.%, and other conditions are the same.

[0058] Example 3

[0059] The difference from Example 1 is that the amount of PVDF-HFP used is 20 g, and the addition amount of Nafion relative to PVDF-HFP is 5 wt.%, and other conditions are the same.

[0060] Comparative Example 1

[0061] The difference from Example 1 is that the Nafion component is not added, and the addition amount of Nafion relative to PVDF-HFP is 0 wt.%, and other conditions are the same.

[0062] Figure 3 SEM images of the electrospun separator before and after polymerization. After DOL polymerization, it can be observed that the fibers are covered by polymerization, forming a dense surface.

[0063] Performance verification

[0064] The electrospun separator and electrolyte prepared in Example 1 were assembled into a stainless steel-stainless steel battery, and electrochemical impedance testing was carried out at room temperature. The testing conditions were: temperature 25 °C, frequency range 100000 - 0.1 Hz. The test results are shown in Figure 1 , from Figure 1 it can be seen that the separator has a high ionic conductivity.

[0065] The electrospun separator and electrolyte prepared in Example 1 were assembled into a sodium metal symmetric battery and electrochemical performance testing was carried out at room temperature. The testing conditions were: temperature 25 °C, current density 0.2 mA·cm -2 , deposition capacity 0.2 mAh·cm -2 , and the test results are shown in Figure 2 , from Figure 2 it can be seen that the sodium metal symmetric battery has good cycle stability. The sodium ion transference number of the sodium metal symmetric battery is as shown in Figure 7 . Using this separator as the polymer gel electrolyte, the sodium ion transference number reaches 0.65, which is superior to that of the liquid electrolyte.

[0066] The electrospun separator and electrolyte prepared in Example 1, with the carbon-coated aluminum foil as the working electrode and sodium as the counter electrode, were assembled into a Na||Al / C battery and sodium deposition testing was carried out at room temperature. The testing conditions were: current density 0.5 mA·cm -2 , deposition amount 2 mAh·cm -2 , and sodium metal was deposited onto the carbon-coated aluminum foil. The scanning electron microscope image of the carbon-coated aluminum foil after deposition is as shown in Figure 4 , from Figure 4 it can be observed that the deposition of sodium metal is relatively uniform, and the deposition thickness is 31.3 μm, indicating that the prepared separator can achieve reversible deposition and stripping of sodium metal. The LSV curve of the Na||Al / C battery is as shown in Figure 6 , and the current increases significantly at 5.27 V, indicating good electrochemical stability of the polymer gel electrolyte initiated by Nafion.

[0067] The electrospun separators prepared in Examples 1 - 3 or the electrospun separator prepared in Comparative Example 1 and the electrolyte were assembled into a sodium metal symmetric battery and electrochemical performance testing was carried out at room temperature. The testing conditions were: temperature 25 °C, current density 0.5 mA·cm -2 , deposition capacity 0.5 mAh·cm -2 , and the test results are shown in Figure 8 , from Figure 8 it can be seen that when the Nafion addition amount is 5 wt.% or 10 wt.% respectively, the assembled sodium metal symmetric battery has good cycle stability.

[0068] The cyclic stability test results of the sodium-sulfur battery prepared in Example 1 are as follows: Figure 5 As shown, at a current density of 0.2 Ag -1 , the initial reversible specific capacity of the sulfurized polyacrylonitrile cathode is approximately 341 mAh·g -1 . After 100 cycles, the capacity remains stable.

[0069] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. An electrospinning membrane, characterized in that: The raw materials of the electrostatic spinning membrane include polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer; the mass ratio of the polyvinylidene fluoride-hexafluoropropylene to the perfluorosulfonic acid polymer is 1 to 20:

1.

2. The method for preparing the electrospinning membrane according to claim 1, characterized in that: The steps include: (1) mixing polyvinylidene fluoride-hexafluoropropylene and perfluorosulfonic acid polymer in a solvent to obtain an electrospinning solution; (2) Electrospinning and drying the electrospinning solution to obtain an electrospinning membrane.

3. The method for preparing an electrospinning membrane according to claim 2, characterized in that: The solvent is N,N-dimethylformamide; the mass ratio of the polyvinylidene fluoride-hexafluoropropylene to the solvent is 1-20:80-120.

4. The method for preparing an electrospinning membrane according to claim 2 or 3, characterized in that: The process parameters of the electrospinning are as follows: the electrospinning voltage is 10-20 kV, the flow rate is 0.5-1.0 mL / h, the receiving distance is 15-25 cm, and the spinning time is 10-15 h.

5. A sodium-sulfur battery, characterized in that: The sodium-sulfur battery comprises a sulfurized polyacrylonitrile positive electrode, a metallic sodium negative electrode, the electrospinning membrane according to claim 1 and an electrolyte.

6. The sodium-sulfur battery according to claim 5, characterized in that: The raw materials of the polyacrylonitrile sulfide positive electrode include polyacrylonitrile sulfide powder, a conductive agent and a binder; the raw materials of the electrolyte include sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, 1,3-dioxolane and fluoroethylene carbonate.

7. The sodium-sulfur battery according to claim 6, characterized in that: The mass ratio of the vulcanized polyacrylonitrile powder, the conductive agent and the binder is 75-95:2.5-12.5:2.5-12.

5.

8. The sodium-sulfur battery according to claim 6 or 7, characterized in that: The preparation method of the sulfurized polyacrylonitrile powder is as follows: sulfur powder and polyacrylonitrile are mixed, and the mixture is calcined and ground under a nitrogen atmosphere to obtain the sulfurized polyacrylonitrile powder.

9. The sodium-sulfur battery according to claim 8, characterized in that: The mass ratio of the polyacrylonitrile to the sulfur powder is 1:2-6.

10. The sodium-sulfur battery according to claim 9, characterized in that: The calcination temperature is 300-500° C., and the calcination time is 1-3 hours.

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