A polymer separator, its preparation method, application, and battery

Abstract

This invention discloses a polymer separator, its preparation method, applications, and batteries. The preparation method of the polymer separator includes the following steps: etching a polymer separator A containing magnetic nanorods under an acidic vapor atmosphere until the magnetic nanorods in polymer separator A are etched, thereby obtaining the polymer separator; wherein the angle between the axial direction of the magnetic nanorods and the membrane surface direction of the polymer separator is 0°–90° but not 0°; wherein the acidic vapor atmosphere is obtained by heating an acid solution with an acid mass fraction of 0.08%–0.12% at 70°C–100°C. The polymer film prepared by this invention has an ordered pore structure, which can effectively prevent the formation of lithium dendrites. Furthermore, when this film is applied to batteries, it exhibits excellent ionic conductivity, high specific capacity, and good safety.

Description

Technical Field

[0001] This invention relates to a polymer separator, its preparation method, applications, and batteries. Background Technology

[0002] In recent years, with the rapid development of 5G mobile phones and electric vehicles, the energy density of lithium batteries has faced greater challenges. Replacing traditional graphite with high-capacity (3680 mAh / g) lithium metal as the anode can significantly improve battery energy density, making it a hot research topic. However, problems such as lithium dendrite growth and irreversible lithium loss during battery cycling hinder the practical application of lithium metal anodes.

[0003] Extensive research has been invested in promoting the practical application of lithium metal anodes. Among current strategies for suppressing lithium dendrites, solid-state electrolytes (SSEIs) and artificial SEI films are the two most widely used. High-modulus SSEIs provide a mechanical barrier, effectively preventing the formation and penetration of lithium dendrites and avoiding battery short circuits. Compared to liquid electrolytes, they are less flammable and have fewer side effects. However, their lithium-ion conductivity is low, at least 2-3 orders of magnitude lower than that of liquid electrolytes, a significant limitation on the use of SSEIs in batteries at room temperature. For artificial SEI films, preparation methods can be divided into in-situ and ex-situ formation. In-situ artificial SEI films can largely solve the interfacial impedance problem, but the preparation conditions are demanding, requiring an argon atmosphere. Ex-situ artificial SEI films typically use polymer materials as a substrate, improving their mechanical properties by filling with inorganic materials or crosslinking and copolymerizing with polymers, but achieving a balance between ionic conductivity and mechanical properties is difficult. Therefore, developing lithium metal anode protective films with high ionic conductivity and ease of preparation remains a challenge.

[0004] Chinese patent CN106654369A discloses a gel polymer electrolyte membrane with magnetic nanorods perpendicular to the membrane surface. It is composed of a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymer and magnetic Fe3O4 nanorods perpendicular to the membrane surface. The mass fraction of PVDF-HFP is 95-99%, the mass fraction of Fe3O4 nanorods is 1-5%, the membrane thickness is 70-100 μm, and the pore size is 1-5 μm. The molecular weight of the PVDF-HFP copolymer is 350,000-450,000 g / mol, and the mass ratio of PVDF to HFP is 88:1. 2 Fe3O4 nanorods have a diameter of 80-100 nm and a length of 600-800 nm. While they improve the ionic conductivity of the electrolyte membrane to some extent, they do not consider whether lithium ions are deposited uniformly, leading to significant capacity loss. Summary of the Invention

[0005] The technical problem this invention aims to solve is to overcome the shortcomings of existing electrolyte membranes that cannot simultaneously satisfy both high ionic conductivity and high specific capacity. This invention provides a polymer separator, its preparation method, applications, and batteries. The polymer film prepared by this invention has an ordered pore structure, which can effectively prevent the formation of lithium dendrites. Furthermore, when applied to batteries, this film exhibits excellent ionic conductivity, high specific capacity, and good safety.

[0006] The present invention solves the above-mentioned technical problems through the following technical solutions.

[0007] This invention provides a method for preparing a polymer separator, comprising the following steps:

[0008] The polymer membrane A containing magnetic nanorods is etched in an acid vapor atmosphere until the magnetic nanorods in the polymer membrane A are etched to obtain the polymer membrane.

[0009] Wherein, the angle between the axial direction of the magnetic nanorod and the membrane surface direction of the polymer membrane is 0°-90° but not 0°;

[0010] The acid vapor atmosphere is obtained by heating an acid solution with an acid mass fraction of 0.08%-0.12% at 70℃-100℃.

[0011] In this invention, the directional arrangement is preferably achieved by applying a magnetic field.

[0012] Preferably, the direction of the applied magnetic field is perpendicular to the surface of the polymer diaphragm A.

[0013] In this invention, the magnetic nanorods can be of conventional types in the art, such as Fe3O4 or Mn3O4.

[0014] In this invention, the diameter of the magnetic nanorod can be 40-80 nm, preferably 50-70 nm.

[0015] In this invention, the length of the magnetic nanorod can be 1-10 μm, preferably 2-8 μm.

[0016] In this invention, the magnetic nanorods can be in the form of 3-6 parts by mass.

[0017] In this invention, the angle between the axial direction of the magnetic nanorod and the membrane surface direction of the polymer membrane is preferably 25°-90°, for example 30°, 60° or 90°.

[0018] In this invention, the preparation method of the polymer membrane A preferably includes the following steps: mixing a polymer solution and magnetic nanorods to obtain a slurry, and coating the slurry onto a substrate;

[0019] A magnetic field is applied to the slurry; the magnetic field is at an angle of 0° to 90° but not 0° to the horizontal plane of the substrate.

[0020] The method for preparing the polymer solution may include the following steps: mixing the polymer and a solvent.

[0021] The polymer can be a thin film-forming polymer used in the battery field, preferably a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinylidene fluoride (PVDF), or polyimide (PI).

[0022] In the PVDF-HFP, the mass ratio of PVDF to HFP can be 88:12.

[0023] The molecular weight of the PVDF-HFP can be 350,000-450,000 g / mol, for example, 400,000 g / mol.

[0024] The polymer can be in the form of 100-200 parts by mass.

[0025] The solvent may be a conventional solvent for dissolving polymers, such as N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0026] The solvent may be in parts by mass of 1000.

[0027] The mixing temperature can be 60-90°C, for example 80°C.

[0028] The mixing process also includes a stirring step. The stirring time can be 1-3 hours, for example, 2 hours.

[0029] When the polymer is PVDF-HFP, the preparation method of the PVDF-HFP solution may include the following steps: stirring the PVDF-HFP and N-methylpyrrolidone at 80°C for 2 hours.

[0030] The mass ratio of the polymer to the magnetic nanorod can be 2%-8%, preferably 3%-6%.

[0031] Preferably, when the polymer is PVDF-HFP, the mass ratio of PVDF-HFP to the magnetic nanorod can be 2%-8%, more preferably 3%-6%.

[0032] The mixing process can be conventional in the art. For example, an ultrasonic vibrator can be used for mixing.

[0033] The substrate can be conventional in the art, such as a glass plate.

[0034] The coating device can be conventional in the art, such as a coating applicator.

[0035] The strength of the magnetic field can be 0.3-0.5T, for example 0.4T.

[0036] The angle between the magnetic field and the horizontal plane of the substrate is preferably 25°-90°, for example 30°, 60° or 90°.

[0037] The process of applying the magnetic field also includes a drying step.

[0038] The drying process can be divided into a first drying and a second drying. The first drying is preferably carried out naturally in air. The preferred temperature of the air is 20-35°C. The first drying time can be 6 hours. The second drying is preferably carried out in a vacuum drying oven. The temperature of the second drying can be 80-120°C, for example, 100°C. The second drying time can be 6 hours.

[0039] In this invention, the acid solution can be a conventional volatile acid solution in the art, such as hydrochloric acid solution, nitric acid solution or hydrofluoric acid solution.

[0040] In this invention, the mass fraction of acid in the acid solution is preferably 0.08%-0.12%, for example, 0.1%. When the mass fraction of acid in the acid solution is high (e.g., 1%), the acid solution will corrode the polymer separator A, resulting in excessively large pores in the obtained polymer separator. This allows solvent macromolecules to pass through, leading to side reactions. Consequently, the ionic conductivity and electrochemical performance of the obtained polymer separator in the battery are inferior to those obtained when the mass fraction of acid in the acid solution is 0.08%-0.12%. When the mass fraction of acid in the acid solution is low (e.g., 0.01%), the etching effect of the acid solution on the polymer separator A is weak, resulting in incomplete etching. This leads to fewer pores in the polymer separator and fewer channels for lithium ion migration, resulting in a lower specific capacity in the obtained polymer separator in the battery compared to those obtained when the mass fraction of acid in the acid solution is 0.08%-0.12%.

[0041] In this invention, the temperature at which the acid solution is heated is preferably 70°C-90°C, for example, 80°C.

[0042] In this invention, the etching time is preferably 0.5-2 hours, for example, 1 hour.

[0043] In this invention, the etching under an acidic vapor atmosphere preferably includes the following steps: fixing the polymer diaphragm A onto a plate with a central cutout; placing the plate at the opening of a container containing an acid solution; and heating the acid solution in a water bath at 80°C for 1 hour. The plate can be a glass plate. The container can be a beaker.

[0044] In this invention, after the etching process, the magnetic nanorods in the polymer membrane A are preferably completely etched, and the polymer membrane A can become colorless.

[0045] In this invention, after the etching is completed, the process also includes washing and drying steps, which are conventional in the art.

[0046] Preferably, the washing is performed with anhydrous ethanol. The washing is preferably performed three times.

[0047] Preferably, the drying equipment is a vacuum drying oven. The drying temperature is preferably 80°C.

[0048] The present invention also provides a polymer membrane prepared by the method described above.

[0049] The present invention also provides a polymer membrane, wherein the thickness of the polymer membrane is 20μm-50μm; the pore size of the polymer membrane is 50-100nm; and the angle between the axial direction of the pores in the polymer membrane and the direction of the membrane surface is 0°-90° but not 0°.

[0050] In this invention, the angle between the axial direction of the pores in the polymer membrane and the direction of the membrane surface is preferably 25°-90°, for example 30°, 60° or 90°.

[0051] In this invention, the thickness of the polymer membrane is preferably 25 μm.

[0052] In this invention, the pore size of the polymer membrane is preferably 70 nm.

[0053] The present invention also provides a battery comprising the polymer separator as described above.

[0054] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0055] The reagents and raw materials used in this invention are all commercially available.

[0056] The positive and progressive effects of this invention are as follows:

[0057] (1) The purpose of choosing rod-shaped magnetic nanorods instead of particulate materials is to form a magnetic nanoarray to ensure the design of ordered nanopores, thereby promoting uniform lithium ion flux and uniform lithium deposition, and effectively suppressing the generation of lithium dendrites.

[0058] (2) Orienting magnetic nanorods within a polymer membrane can form an oriented magnetic nanoarray. For example, magnetic nanorods can be oriented within a polymer membrane prepared using PVDF-HFP. PVDF-HFP replaces traditional polyolefin materials in the preparation of polymer membranes. The high dielectric constant of PVDF-HFP facilitates the dissociation of lithium salts and increases carrier concentration. Furthermore, PVDF-HFP contains polar fluorine atoms, and the interaction between fluorine atoms and lithium ions effectively promotes lithium ion transport. All of these factors effectively improve the ionic conductivity of the polymer membrane.

[0059] (3) This invention is the first to develop a method for preparing a polymer membrane with ordered channels by etching a magnetic nanoarray. The resulting polymer membrane has high polarity and an ordered pore structure, which improves the wettability of the polymer membrane to the liquid electrolyte and effectively promotes the wetting and absorption of the electrolyte. The method of etching a magnetic nanoarray to construct ordered channels in this invention can also be used in other polymer or other technical fields. Attached Figure Description

[0060] Figure 1 This is a scanning electron microscope image of the lithium sheet surface after 50 cycles of the battery made from the polymer separator prepared in Example 1.

[0061] Figure 2 The image shows a scanning electron microscope (SEM) image of the lithium sheet surface after 50 cycles of a battery made with a conventional polypropylene separator, as shown in Comparative Example 1.

[0062] Figure 3 This is a schematic diagram of the polymer membranes prepared in Examples 1-5.

[0063] Explanation of reference numerals in the attached figures:

[0064] 1. Magnetic nanorods or channels; 2. Polymer membrane; 3. Thickness of polymer membrane; 4. Surface orientation of polymer membrane. Detailed Implementation

[0065] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0066] All raw materials used in the following examples and comparative examples are commercially available. PVDF-HFP was purchased from Sigma-Aldrich, USA, with a PVDF:HFP mass ratio of 88:12; the magnetic nanorods Fe3O4 were purchased from Nanjing Dongna Biotechnology Co., Ltd.

[0067] Schematic diagrams of the polymer membranes obtained in Examples 1-5 are shown below. Figure 3 As shown.

[0068] Example 1

[0069] 10g of PVDF-HFP was dissolved in 100mL of NMP solvent and stirred at 80℃ for 2h to form a solution. 0.3g of Fe3O4 magnetic nanorods (50nm in diameter and 8μm in length) were added to the solution, and the nanorods were uniformly dispersed in the solution using an ultrasonic vibrator to form a stable slurry. The slurry was uniformly coated onto a flat glass plate using a coating applicator, with a directional magnetic field of 0.4T applied perpendicular to the horizontal plane of the glass during the coating process. The slurry was allowed to air dry naturally for 6h, and then vacuum dried in a 100℃ oven for 6h to obtain polymer membrane A. A 0.1% hydrochloric acid solution was prepared in a beaker, and the membrane was fixed to the mouth of the beaker. The hydrochloric acid solution was heated at 80℃ to etch the magnetic nanorods in the membrane for 1h. The membrane was removed, washed three times with anhydrous ethanol, and then vacuum dried in an 80℃ oven to obtain the polymer membrane.

[0070] Example 2

[0071] The difference from Example 1 is that the acid solution is a nitric acid solution.

[0072] Example 3

[0073] The difference from Example 1 is that the Fe3O4 magnetic nanorods have a mass of 0.6g.

[0074] Example 4

[0075] The difference from Example 1 is that the magnetic nanorods have a diameter of 70 nm and a length of 2 μm.

[0076] Example 5

[0077] The difference from Example 1 is that the mass fraction of acid in the acid solution is 0.08%.

[0078] Example 6

[0079] The difference from Example 1 is that the magnetic nanorods have a mass of 0.1g.

[0080] Example 7

[0081] The difference from Example 1 is that during the coating process, a directional magnetic field with a strength of 0.4T is applied in a direction that forms a 30° angle with the horizontal plane of the glass.

[0082] Example 8

[0083] The difference from Example 1 is that during the coating process, a directional magnetic field with a strength of 0.4T is applied in a direction that forms a 60° angle with the horizontal plane of the glass.

[0084] Comparative Example 1

[0085] Commercially available traditional polypropylene separators are named Celgard.

[0086] Comparative Example 2

[0087] 10g of PVDF-HFP was dissolved in 100mL of NMP solvent and stirred at 80℃ for 2h to form a solution. The slurry was then uniformly coated onto a flat glass plate using a coating applicator and dried in a 100℃ oven for 6h to obtain the PVDF-HFP polymer membrane.

[0088] Comparative Example 3

[0089] 10g of PVDF-HFP was dissolved in 100mL of NMP solvent and stirred at 80℃ for 2h to form a solution. 0.3g of Fe3O4 magnetic nanorods were added to the solution, and the nanorods were uniformly dispersed in the solution using an ultrasonic vibrator to form a stable slurry. The slurry was uniformly coated onto a flat glass plate using a coating applicator, with a directional magnetic field of 0.4T applied perpendicular to the glass plane during the coating process. The slurry was allowed to air dry naturally for 6h, and then vacuum dried in a 100℃ oven for 6h. The membrane was removed, washed three times with anhydrous ethanol, and then vacuum dried in an 80℃ oven to obtain the polymer membrane.

[0090] Comparative Example 4

[0091] The difference from Example 1 is that the mass fraction of acid in the acid solution is 1%.

[0092] Example 1

[0093] The lithium sheet surface of the battery prepared by the polymer separator in Example 1 was observed by scanning electron microscopy after 50 cycles. The results are as follows: Figure 1 As shown.

[0094] The lithium-ion cell surface of the battery prepared with the conventional polypropylene separator in Comparative Example 1 was observed by scanning electron microscopy after 50 cycles. The results are as follows: Figure 2 As shown.

[0095] Depend on Figure 1It can be seen that after 50 cycles, the lithium sheet surface of the battery made with the polymer separator in Example 1 is smooth and free of foreign matter.

[0096] Depend on Figure 2 It can be seen that after 50 cycles, the lithium sheet surface of the battery made with the conventional polypropylene separator in Comparative Example 1 is covered with dendritic lithium dendrites.

[0097] Example 2

[0098] The polymer membranes obtained in Examples 1-8 and Comparative Examples 1-4 were subjected to ionic conductivity tests. The test method was as follows: the polymer membrane was sandwiched between two stainless steel sheets to assemble a button cell, and its AC impedance at room temperature was measured. The frequency range was 0.1Hz-100kHz, and the amplitude was 10mV. The ionic conductivity was calculated from the resistance obtained by the test according to the following calculation method. The test results are shown in Table 1.

[0099] Ionic conductivity (σ) = d / (Rs×S); where d is the membrane thickness (cm); Rs is the membrane impedance (ohms); and S is the effective electrode area (cm²). 2 ).

[0100] Example 3

[0101] The polymer membranes obtained in Examples 1-8 and Comparative Examples 1-4 were used to prepare full cells according to the following method:

[0102] (1) LiFePO4 (LFP) powder, carbon black, and PVDF were mixed in a mass ratio of 8:1:1. An appropriate amount of NMP solvent (PVDF:NMP mass ratio of 3%) was added, and the mixture was stirred at low speed for 12 hours at room temperature to prepare a slurry. After coating, the slurry was vacuum dried in a 100℃ oven for 12 hours. Finally, it was cut into circular slices with a diameter of 12 mm to serve as the positive electrode in the full cell.

[0103] (2) Using the prepared electrode as the positive electrode and the lithium sheet as the negative electrode, a Li / LFP full cell was prepared in a glove box. After the battery was assembled, it was allowed to stand for 12 hours before performance tests were performed. The electrolyte used was 1 mol / L LiPF6 / EC:DEC = 1:1 (volume ratio).

[0104] The electrochemical performance of the prepared full cells was tested. The test method was as follows: a battery cycling system was used to perform charge-discharge cycle tests on the Li / LFP cells equipped with polymer separators. The rates were set to 0.1C and 1.0C, and the specific capacity of the cells was recorded. The test data are shown in Table 1.

[0105] Table 1

[0106]

[0107]

[0108] As shown in Table 1, the polymer membranes of Comparative Examples 1-4 have larger pore sizes (greater than 100 nm) compared to Examples 1-8.

Claims

1. A method for preparing a polymer separator, characterized in that, It includes the following steps: The polymer membrane A containing magnetic nanorods is etched in an acid vapor atmosphere until the magnetic nanorods in the polymer membrane A are completely etched to obtain the polymer membrane. The angle between the axial direction of the magnetic nanorod and the surface direction of the polymer membrane is 25°-90°. The acid vapor atmosphere is obtained by heating an acid solution with an acid mass fraction of 0.08%-0.12% at 70℃-100℃. The diameter of the magnetic nanorods is 40-80 nm; The magnetic nanorods have a length of 1-10 μm; The preparation method of the polymer membrane A includes the following steps: mixing a polymer solution and magnetic nanorods to obtain a slurry, and coating the slurry onto a substrate; A magnetic field is applied to the slurry; the angle between the magnetic field and the horizontal plane of the substrate is 25°-90°. The method for preparing the polymer solution includes the following steps: mixing the polymer and a solvent; The polymer is a polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride, or polyimide.

2. The method for preparing the polymer separator as described in claim 1, characterized in that, The acid solution is a hydrochloric acid solution, a nitric acid solution, or a hydrofluoric acid solution; And / or, the mass fraction of acid in the acid solution is 0.1%; And / or, the acid solution is heated to a temperature of 70°C-90°C; And / or, the etching time is 0.5-2 hours; And / or, the angle between the axial direction of the magnetic nanorod and the surface direction of the polymer membrane is 30°, 60° or 90°.

3. The method for preparing the polymer separator as described in claim 2, characterized in that, The acid solution is heated to 80°C; And / or, the etching time is 1 hour.

4. The method for preparing the polymer separator as described in claim 1, characterized in that, The etching under an acid vapor atmosphere includes the following steps: fixing the polymer diaphragm A onto a plate with a hollow center, placing the plate at the opening of a container containing an acid solution, and heating the acid solution in a water bath at 80°C for 1 hour.

5. The method for preparing the polymer separator according to claim 1, characterized in that, The angle between the magnetic field and the horizontal plane of the substrate is 30°, 60° or 90°. The strength of the magnetic field is 0.3-0.5T.

6. The method for preparing the polymer separator as described in claim 5, characterized in that, The strength of the magnetic field is 0.4T.

7. The method for preparing the polymer separator according to claim 1, characterized in that, The solvent is N-methylpyrrolidone or N,N-dimethylformamide.

8. The method for preparing the polymer separator according to any one of claims 1-7, characterized in that, The magnetic nanorods are of the Fe3O4 or Mn3O4 type; And / or, the diameter of the magnetic nanorod is 50-70 nm; And / or, the length of the magnetic nanorod is 2-8 μm; And / or, the mass ratio of the polymer to the magnetic nanorod is 2%-8%.

9. The method for preparing the polymer separator as described in claim 8, characterized in that, The mass ratio of the polymer to the magnetic nanorod is 3%-6%.

10. A polymer separator, characterized in that, It is prepared by the method of any one of claims 1-9 for the preparation of polymer membranes.

11. A polymer separator as described in claim 10, characterized in that, The thickness of the polymer membrane is 20μm-50μm; the pore size of the polymer membrane is 50-100nm; the angle between the axial direction of the pores in the polymer membrane and the direction of the membrane surface is 25°-90°. The polymer in the polymer diaphragm is a polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride, or polyimide.

12. The polymer separator as claimed in claim 11, characterized in that, The angle between the axial direction of the pores in the polymer diaphragm and the direction of the polymer diaphragm surface is 30°, 60° or 90°; And / or, the thickness of the polymer membrane is 25 μm; And / or, the pore size of the polymer membrane is 70 nm.

13. A battery, characterized in that, It includes the polymer membrane as described in any one of claims 10-12.

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