Functional separator for improving cycle life of sodium metal anode-free battery and preparation method thereof

Abstract

The present application relates to the technical field of electrochemical energy storage, and particularly relates to a functional separator for improving the cycle life of a sodium metal anode-free battery and a preparation method thereof, which comprises a base film and a functional coating coated on the surface of the base film, the functional coating comprises an A-side coating and a B-side coating distributed on both sides of the base film, the A-side coating comprises a laminar fast sodium ion material and a solvent, and the B-side coating comprises a laminar fast sodium ion material, a polymer and a solvent. The present application builds the coatings with different functions on both sides of the separator, cooperatively improves the bulk phase migration rate and the interface desolvation rate of sodium ions, thereby inhibiting the growth of sodium dendrites and improving the coulombic efficiency and the cycle life of the sodium metal anode-free battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemical energy storage, in particular to a functional separator for improving the cycle life of a sodium metal anode-free battery and a preparation method thereof. BACKGROUND

[0002] Sodium metal anode-free batteries are considered as a strong contender for the next generation of energy storage technology due to their high theoretical energy density and low cost. However, there are two problems: first, the sodium ion migration kinetics is slow, on the positive side, the migration speed of sodium ions in the electrolyte and the separator limits the fast charging performance of the battery, and leads to uneven sodium deposition under large current density; second, the high energy barrier of sodium ion desolvation and serious side reactions, on the negative side, the sodium ions solvated from the electrolyte need to overcome a high energy barrier to remove the solvation shell, and then be reduced to metallic sodium. Slow desolvation dynamics will lead to the accumulation of sodium ions on the negative electrode surface, causing sodium dendrite growth and the formation of dead sodium. At the same time, the solvent molecules will have side reactions with the highly active sodium metal, continuously consuming active sodium and electrolyte, leading to low coulombic efficiency and rapid cycle life decay.

[0003] Commercial polyolefin separators (such as PE separators) have excellent electrochemical stability and mechanical strength, but they are inert and cannot solve the above problems. Currently, researchers try to coat inorganic coatings (such as silicon dioxide, aluminum oxide) or polymer coatings on the separator to improve its affinity with the electrolyte. However, a single improvement method cannot solve the two coupled problems of ion migration and desolvation. Sodium-based montmorillonite is a natural two-dimensional layered clay mineral, and the sodium ions in its sheet structure can migrate quickly. However, traditional bulk montmorillonite is tightly stacked in the separator coating, with a long ion migration path, limiting its performance. In addition, a pure ion conductor coating cannot regulate the desolvation process of sodium ions.

[0004] Therefore, it is of great significance to develop a new type of functional separator that can simultaneously optimize the sodium ion migration and desolvation kinetics for promoting the practical application of sodium metal anode-free batteries. SUMMARY

[0005] The purpose of the present application is to provide a functional separator for improving the cycle life of a sodium metal anode-free battery and a preparation method thereof, by constructing coatings with different functions on both sides of the separator, synergistically improving the bulk phase migration rate and interface desolvation rate of sodium ions, thereby inhibiting sodium dendrite growth and improving the coulombic efficiency and cycle life of the sodium metal anode-free battery.

[0006] To achieve the above object, the application provides a functional diaphragm for improving the cycle life of a sodium metal anode-free battery, comprising a base film and a functional coating layer coated on the surface of the base film, the functional coating layer comprising an A-side coating layer and a B-side coating layer distributed on both sides of the base film, the A-side coating layer comprising a laminar fast sodium ion material and a solvent, and the B-side coating layer comprising a laminar fast sodium ion material, a high molecular polymer and a solvent.

[0007] Preferably, the thickness of the base film is 9-12 m, the thickness of the functional coating layer is 1-4 m, and the base film is one of PE, PP and PI films.

[0008] Preferably, the laminar fast sodium ion material comprises one of a laminar sodium-based montmorillonite, a vermiculite and a hydroxyapatite.

[0009] Preferably, the solvent is one of nitrogen methyl pyrrolidone and anhydrous ethanol, the solid content of the laminar fast sodium ion material in the solvent in the A-side coating layer is 10%-20%, and the solid content of the laminar fast sodium ion material in the solvent in the B-side coating layer is 10%-20%.

[0010] Preferably, the mass ratio of the laminar fast sodium ion material to the high molecular polymer in the B-side coating layer is 1:1-3:1, and the high molecular polymer is polybutyl acrylate.

[0011] The above method for preparing the functional diaphragm for improving the cycle life of a sodium metal anode-free battery comprises the following steps:

[0012] S1, preparing a laminar fast sodium ion material powder: uniformly dispersing the laminar fast sodium ion material in deionized water, stripping into a laminar shape through high-speed mechanical stirring and ice-water bath ultrasonic, and drying after centrifugation to obtain the laminar fast sodium ion material powder;

[0013] S2, preparing a coating slurry of the A-side coating layer: dispersing the laminar fast sodium ion material powder in a solvent, and forming an A-side coating slurry through high-speed mechanical stirring and ice-water bath ultrasonic;

[0014] S3, preparing a coating slurry of the B-side coating layer: dispersing the laminar fast sodium ion material powder in a solvent, adding a high molecular polymer, stirring at a low speed until the high molecular polymer is completely dissolved, and then forming a B-side coating slurry through high-speed mechanical stirring and ice-water bath ultrasonic;

[0015] S4, an asymmetric coating and drying step: coating the A-side coating slurry on one side of the surface of the base film, preliminarily drying in a vacuum oven after coating is completed, coating the B-side coating slurry on the other side of the surface of the base film, and continuing to dry in the vacuum oven after coating is completed to obtain an asymmetric functional diaphragm.

[0016] Preferably, in S1, the solid content of the sheet-shaped fast sodium ion material in deionized water is 5% to 15%, the speed of high-speed mechanical stirring is 1000 to 3000 rpm, the stirring time is 2 to 8 hours, the power of ice water bath ultrasonic is 500 to 1000 W, the ultrasonic working time is 2 to 5 seconds, the intermittent time is 1 to 3 seconds, the total ultrasonic time is 5 to 10 hours, the centrifugal speed is 8000 to 10000 r per minute, the centrifugal treatment time is 1 to 3 minutes, and the drying temperature is 60 to 80 degrees Celsius.

[0017] Preferably, in S2, the speed of high-speed mechanical stirring is 800 to 1200 rpm, the stirring time is 1.5 to 2.5 hours, the power of ice water bath ultrasonic is 550 to 650 W, the ultrasonic working time is 2 seconds, the intermittent time is 3 seconds, and the total ultrasonic time is 30 minutes.

[0018] Preferably, in S3, the speed of low-speed stirring is 200 to 500 rpm, the stirring time is 1 to 2 hours, the speed of high-speed mechanical stirring is 800 to 1200 rpm, the stirring time is 3.5 to 4.5 hours, the power of ice water bath ultrasonic is 550 to 650 W, the ultrasonic working time is 2 seconds, the intermittent time is 3 seconds, and the total ultrasonic time is 30 minutes.

[0019] Preferably, in S4, the coating is performed using a doctor blade coater, the preliminary drying temperature is 75 to 85 degrees Celsius, the drying time is 1.5 to 2.5 hours, the continued drying temperature is 55 to 65 degrees Celsius, and the drying time is 23 to 24 hours.

[0020] Therefore, the functional separator for improving the cycle life of a sodium metal anode-free battery and the preparation method thereof have the following beneficial effects:

[0021] (1) The functional separator can accelerate sodium ion transmission, improve ion migration efficiency, reduce desolvation energy barrier, inhibit sodium dendrite and side reactions, realize transmission and deposition optimization, significantly improve the electrochemical performance of the battery, and prolong the cycle life.

[0022] (2) The functional separator has good mechanical strength and flexibility, can inhibit coating swelling, maintain structural integrity, prevent the coating from falling off or cracking in long-term cycling, ensure the stability of the separator function, and further prolong the service life of the battery.

[0023] (3) The preparation method is simple and controllable, does not require special equipment, has low raw material cost and wide sources, has strong compatibility, has a wide application range, can be quickly applied to sodium metal anode-free batteries in different scenarios such as large-scale energy storage and portable electronic devices, and has commercial application potential.

[0024] The technical solutions of the present application are described in further detail below with reference to the accompanying drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a structural schematic diagram of the functional separator of the present application;

[0026] Figure 2 is a scanning electron microscope image of the montmorillonite before the exfoliation treatment of the present application;

[0027] Figure 3 is a scanning electron microscope image of the montmorillonite after the exfoliation treatment of the present application;

[0028] Figure 4 is a stress-strain curve test diagram of the functional separator prepared in Example 2, Comparative Example 1, and Comparative Example 2 of the present application;

[0029] Figure 5 is a schematic diagram of the coulomb efficiency and cycle life of the functional separator prepared in Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present application in a button cell. DETAILED DESCRIPTION

[0030] The present application is further described below with reference to the accompanying drawings and examples. Unless otherwise defined, the technical terms or scientific terms used in the present application should be understood as the usual meaning understood by a person having ordinary skills in the art to which the present application belongs. The features mentioned above or the features mentioned in the specific examples can be combined in any manner, and these specific examples are only used to illustrate the present application and not to limit the scope of the present application.

[0031] The present application provides a functional separator for improving the cycle life of a sodium metal anode-free battery, comprising a base film and a functional coating layer coated on the surface of the base film, the functional coating layer comprising an A-side coating layer and a B-side coating layer distributed on both sides of the base film, the A-side coating layer comprising an exfoliated fast sodium ion material and a solvent, and the B-side coating layer comprising an exfoliated fast sodium ion material, a high molecular polymer, and a solvent.

[0032] In the present application, the base film provides mechanical support and ensures the mechanical strength of the functional separator. The A-side coating layer directly contacts the positive electrode of the battery and provides a high-speed sodium ion migration channel. The B-side coating layer directly contacts the negative electrode of the battery, provides an ion migration channel, preferentially coordinates with sodium ions, reduces the desolvation energy barrier of sodium ions, promotes rapid and uniform sodium deposition, thereby inhibiting the growth of sodium dendrites and improving the coulomb efficiency and cycle life of the sodium metal anode-free battery.

[0033] Preferably, the thickness of the base film is 9-12 m, and the thickness of the functional coating layer is 1-4 m, and the base film is one of PE, PP, and PI film.

[0034] Preferably, the sheet layered fast sodium ion material comprises one of sheet layered sodium-based montmorillonite, vermiculite and hydroxyapatite.

[0035] Preferably, the solvent is one of nitrogen methyl pyrrolidone and anhydrous ethanol; the solid content of the sheet layered fast sodium ion material in the solvent in the A surface coating layer is 10% to 20%, and the solid content of the sheet layered fast sodium ion material in the solvent in the B surface coating layer is 10% to 20%.

[0036] Preferably, the mass ratio of the sheet layered fast sodium ion material to the high molecular polymer in the B surface coating layer is 1:1 to 3:1, and the high molecular polymer is polybutyl acrylate.

[0037] The preparation method of the functional diaphragm for improving the cycle life of the sodium metal anode-free battery comprises the following steps:

[0038] S1, preparing sheet layered fast sodium ion material powder: uniformly dispersing the sheet layered fast sodium ion material in deionized water, stripping into sheet layers through high-speed mechanical stirring and ice water bath ultrasonic, and drying after centrifugation to obtain sheet layered fast sodium ion material powder;

[0039] S2, preparing coating slurry of the A surface coating layer: dispersing the sheet layered fast sodium ion material powder in the solvent, and forming the A surface coating slurry through high-speed mechanical stirring and ice water bath ultrasonic;

[0040] S3, preparing coating slurry of the B surface coating layer: dispersing the sheet layered fast sodium ion material powder in the solvent, adding the high molecular polymer, stirring at low speed until the high molecular polymer is completely dissolved, and then forming the B surface coating slurry through high-speed mechanical stirring and ice water bath ultrasonic;

[0041] S4, asymmetric coating and drying step: coating the A surface coating slurry on one side of the base film surface, preliminarily drying in a vacuum oven after coating is completed, coating the B surface coating slurry on the other side of the base film surface, and continuing to dry in the vacuum oven after coating is completed to obtain an asymmetric functional diaphragm.

[0042] In the present application, the sheet layered fast sodium ion material is stripped into thinner nanosheet layers through sheet layering treatment, which greatly increases the specific surface area and exposes more ion exchange sites, providing more and shorter two-dimensional in-plane migration paths for sodium ions and significantly reducing the resistance of ion migration.

[0043] The interlayer domain of the sheet layered fast sodium ion material in the A surface coating layer forms a nano-confined space, in which the diffusion behavior of hydrated sodium ions is changed, and the migration energy barrier is lower than in the bulk electrolyte, thereby realizing ultrafast ion conduction, improving the number of sodium ion migrations, effectively solving the problem of slow ion transport on the positive electrode side, and laying a foundation for the fast charging performance and large current cycle stability of the battery.

[0044] The carbonyl (C=O) on the molecular chain of the polybutyl acrylate (PBA) in the B-side coating of the present application is a stronger Lewis base than the conventional carbonate solvent (such as EC, DEC). When the solvated sodium ion (Na + + approaches, the C=O group will preferentially form a stronger coordination with Na + , partially or completely replacing the original solvation shell; this pre-desolvation effect weakens the binding force between the sodium ion and the original solvent molecules, and when this PBA-modified sodium ion reaches the negative electrode surface for electrochemical reduction, it needs to remove (or partially remove) the PBA coordination layer with weak binding, rather than the tightly bound original solvation shell, thereby greatly reducing the energy required for the desolvation step. The lamellar fast sodium ion material dispersed in PBA not only ensures the ionic conductivity of the coating, but also enhances the skeleton, preventing the pure polymer coating from excessive swelling in the electrolyte and maintaining the structural integrity of the functional separator.

[0045] The high-speed ion channels of the A-side coating and the desolvation regulation of the B-side coating in the present application form a synergistic effect, ensuring the rapid transmission of sodium ions from the positive electrode to the separator, and ensuring efficient and uniform deposition of sodium ions at the negative electrode, avoiding the limitations of single coating that can only optimize a certain link, and comprehensively improving the energy density and cycle stability of the battery.

[0046] Preferably, in S1, the solid content of the lamellar fast sodium ion material in deionized water is 5% to 15%, the rate of high-speed mechanical stirring is 1000 to 3000 rpm, and the stirring time is 2 to 8 hours, ensuring the preliminary dispersion of the raw materials; the power of the ice water bath ultrasonic is 500 to 1000 W, the ultrasonic working time is 2 to 5 seconds, the intermittent time is 1 to 3 seconds, the total ultrasonic time is 5 to 10 hours, avoiding the destruction of the layered structure of the material due to temperature rise during ultrasonic process; the centrifugation is at a speed of 8000 to 10000 r min, and the centrifugation time is 1 to 3 min, removing the unpeeled blocky particles; the drying is at 60 to 80 °C for 6 to 12 hours.

[0047] Preferably, in S2, the rate of high-speed mechanical stirring is 800 to 1200 rpm, the stirring time is 1.5 to 2.5 hours, the power of the ice water bath ultrasonic is 550 to 650 W, the ultrasonic working time is 2 seconds, the intermittent time is 3 seconds, and the total ultrasonic time is 30 minutes. The lamellar fast sodium ion material powder is uniformly dispersed to form a stable A-side coating slurry.

[0048] Preferably, in S3, the low-speed stirring rate is 200-500 rpm, the stirring time is 1-2 h, the high-speed mechanical stirring rate is 800-1200 rpm, the stirring time is 3.5-4.5 h, the ice water bath ultrasonic power is 550-650 W, the ultrasonic working time is 2 s, the intermittent time is 3 s, and the total ultrasonic time is 30 min. The lamellar sodium ion material powder is uniformly dispersed to form a stable B-side coating slurry.

[0049] Preferably, in S4, the coating is performed using a doctor blade coater, the preliminary drying temperature is 75-85 , the drying time is 1.5-2.5 h, more than 90% of the solvent is removed to avoid slurry penetration during subsequent coating; the continued drying temperature is 55-65 , the drying time is 23-24 h, and the solvent is completely volatilized.

[0050] Raw materials used in the present application:

[0051] Base film: commercial 12 m PE separator (Enjie Stock, tensile strength 200 MPa, air permeability 100 s 100 mL);

[0052] Fast sodium ion material: sodium-based montmorillonite (purity > 98%, particle size 5-10 μm);

[0053] Polymer: polybutyl acrylate (PBA, Mw=200000, solid content 30 wt%);

[0054] Solvent: N-methyl pyrrolidone (NMP, analytical pure, water content <0.05%).

[0055] Example 1

[0056] The present application provides a functional separator for improving the cycle life of sodium metal anode-free batteries, which is prepared by the following method, comprising:

[0057] S1, preparing a lamellar fast sodium ion material powder: 5 g of lamellar sodium-based montmorillonite is uniformly dispersed in 100 mL of deionized water, and is subjected to high-speed mechanical stirring at 1000 rpm for 2 h, and then is subjected to ice water bath ultrasonic treatment at a power of 500 W, with an ultrasonic working time of 2 s and an intermittent time of 1 s, for a total ultrasonic time of 5 h, so as to be peeled into a lamellar shape. Then, the solution is centrifuged at a speed of 8000 r min for 1 min, and the obtained precipitate is dried at 60 for 6 h to obtain a lamellar fast sodium ion material powder.

[0058] S2, preparation of the coating slurry of the A-side coating layer: 1.0 g of the flaky fast sodium ion material powder was dispersed in 10 mL of a nitrogen methyl pyrrolidone solvent, and was subjected to high-speed mechanical stirring at 800 rpm for 1.5 h, and then was subjected to ice water bath ultrasonic treatment at a power of 550 W, with an ultrasonic working time of 2 s and an intermittent time of 3 s, for a total ultrasonic time of 30 min, so as to uniformly disperse the same and form a stable A-side coating slurry.

[0059] S3, preparation of the coating slurry of the B-side coating layer: 1.0 g of the flaky fast sodium ion material powder was dispersed in 10 mL of a nitrogen methyl pyrrolidone solvent, and 1.0 g of polybutyl acrylate was added, and was subjected to low-speed stirring at 200 rpm for 1 h until the polymer was completely dissolved, and then was subjected to high-speed mechanical stirring at 800 rpm for 3.5 h, and then was subjected to ice water bath ultrasonic treatment at a power of 550 W, with an ultrasonic working time of 2 s and an intermittent time of 3 s, for a total ultrasonic time of 30 min, so as to uniformly disperse the same and form a stable B-side coating slurry.

[0060] S4, asymmetric coating and drying step: on a 9 m thick PE diaphragm, the A-side coating slurry was first coated on one side of the surface of the PE diaphragm using a doctor blade with a 2 m gap, and after the coating was completed, the PE diaphragm was placed in a vacuum oven and was dried at 75 C for 1.5 h, the PE diaphragm was turned over, and the B-side coating slurry was coated on the other side of the surface of the PE diaphragm using a doctor blade with a 2 m gap, and after the coating was completed, the PE diaphragm was transferred to a vacuum oven and was dried at 55 C for 23 h, and an asymmetric functional diaphragm was obtained.

[0061] Example 2

[0062] The present application provides a functional diaphragm for improving the cycle life of a sodium metal anode-free battery, which is prepared by the following method, comprising:

[0063] S1, preparation of the flaky fast sodium ion material powder: 10 g of flaky sodium-based montmorillonite was uniformly dispersed in 100 mL of deionized water, and was subjected to high-speed mechanical stirring at 3000 rpm for 8 h, and then was subjected to ice water bath ultrasonic treatment at a power of 1000 W, with an ultrasonic working time of 5 s and an intermittent time of 1 s, for a total ultrasonic time of 5 h, so as to exfoliate the same into a flaky shape. The solution was then subjected to centrifugal treatment at a speed of 10000 r min for 3 min, and the obtained precipitate was oven dried at 80 C for 12 h, to obtain the flaky fast sodium ion material powder.

[0064] S2, preparation of the coating slurry of the A-side coating layer: 1.5 g of the flaky layered fast sodium ion material powder was weighed and dispersed in 10 mL of a nitrogen methyl pyrrolidone solvent, and was subjected to high-speed mechanical stirring at 1000 rpm for 2 h, followed by ice water bath ultrasonic treatment at a power of 600 W, with an ultrasonic working time of 2 s and an intermittent time of 3 s, and a total ultrasonic time of 30 min, so as to uniformly disperse the material and form a stable A-side coating slurry.

[0065] S3, preparation of the coating slurry of the B-side coating layer: 1.5 g of the flaky layered fast sodium ion material powder was weighed and dispersed in 10 mL of a nitrogen methyl pyrrolidone solvent, and 0.5 g of polybutyl acrylate was added, and was subjected to low-speed stirring at 400 rpm for 1.5 h until the polymer was completely dissolved, and then was subjected to high-speed mechanical stirring at 1000 rpm for 4 h, followed by ice water bath ultrasonic treatment at a power of 600 W, with an ultrasonic working time of 2 s and an intermittent time of 3 s, and a total ultrasonic time of 30 min, so as to uniformly disperse the material and form a stable B-side coating slurry.

[0066] S4, asymmetric coating and drying step: on a 12 μm-thick PE diaphragm, the A-side coating slurry was first coated on one side of the PE diaphragm using a doctor blade with a 2 μm gap, and after coating, the PE diaphragm was placed in a vacuum oven and was dried at 80 ° C for 2 h, and then the PE diaphragm was turned over, and the B-side coating slurry was coated on the other side of the PE diaphragm using a doctor blade with a 2 μm gap, and after coating, the PE diaphragm was transferred to a vacuum oven and was dried at 60 ° C for 24 h, to obtain an asymmetric functional diaphragm, as shown in FIG. 2. Figure 1

[0067] Example 3

[0068] The present application provides a functional diaphragm for improving the cycle life of a sodium metal anode-free battery, which is prepared by the following method, comprising:

[0069] S1, preparation of the flaky layered fast sodium ion material powder: 15 g of flaky layered sodium-based montmorillonite was uniformly dispersed in 100 mL of deionized water, and was subjected to high-speed mechanical stirring at 2000 rpm for 5 h, followed by ice water bath ultrasonic treatment at a power of 750 W, with an ultrasonic working time of 4 s and an intermittent time of 2 s, and a total ultrasonic time of 8 h, so as to exfoliate the material into a flaky layered structure. Then, the solution was subjected to centrifugal treatment at a speed of 9000 r / min for 2 min, and the obtained precipitate was oven dried at 70 ° C for 9 h, to obtain the flaky layered fast sodium ion material powder.

[0070] ​S2, preparation of the coating slurry of the A-side coating layer: 2.0 g of the flaky fast sodium ion material powder was dispersed in 10 mL of the flaky sodium-based montmorillonite solvent, and was mechanically stirred at 1000 rpm for 2.0 h, and then was ultrasonically treated in an ice water bath at a power of 650 W, with an ultrasonic working time of 2 s and an intermittent time of 3 s, for a total ultrasonic time of 30 min, so as to uniformly disperse the same and form a stable A-side coating slurry.

[0071] S3, preparation of the coating slurry of the B-side coating layer: 2.0 g of the flaky fast sodium ion material powder was dispersed in 10 mL of the N-methyl pyrrolidone solvent, and 1.0 g of the polybutyl acrylate was added, and was first stirred at 500 rpm for 2 h until the polymer was completely dissolved, and then was mechanically stirred at 800-1200 rpm for 4.5 h, and then was ultrasonically treated in an ice water bath at a power of 650 W, with an ultrasonic working time of 2 s and an intermittent time of 3 s, for a total ultrasonic time of 30 min, so as to uniformly disperse the same and form a B-side coating slurry.

[0072] S4, asymmetric coating and drying step: on a 12 m-thick PE diaphragm, the A-side coating slurry was first coated on one side of the surface of the PE diaphragm using a doctor blade with a gap of 2 m, and after the coating was completed, the PE diaphragm was placed in a vacuum oven and was dried at 85 for 2.5 h, the PE diaphragm was turned over, and the B-side coating slurry was coated on the other side of the surface of the PE diaphragm using a doctor blade with a gap of 2 m, and after the coating was completed, the PE diaphragm was transferred to a vacuum oven and was dried at 65 for 24 h, to obtain an asymmetric functional diaphragm.

[0073] Comparative Example 1

[0074] A commercially available 12 m-thick PE diaphragm was directly used without any treatment.

[0075] Comparative Example 2

[0076] Based on Example 2, the difference from Example 2 was that step S3 was not performed, and in step S4, only the A-side coating slurry was coated on one side of a 12 m-thick PE diaphragm, and the other side was not coated, and the rest was the same as Example 2.

[0077] Comparative Example 3

[0078] Based on Example 2, the difference from Example 2 was that step S2 was not performed, and in step S4, only the B-side coating slurry was coated on one side of a 12 m-thick PE diaphragm, and the other side was not coated, and the rest was the same as Example 2.

[0079] Performance Test

[0080] The sodium-based montmorillonite before the sheet layer treatment and the sodium-based montmorillonite after the sheet layer treatment (the sheet layer sodium ion material powder obtained in step S1 in Example 2) were respectively scanned by a scanning electron microscope, and the results are shown in Figures 1 and 2. Figures 2-3 As shown in the figures, the montmorillonite material before the sheet layer treatment has a block agglomerated structure, which causes the ion migration path to be lengthened, the ion transmission resistance to be large, and the sodium ions to be not conducive to fast migration in the separator coating; the montmorillonite after the sheet layer treatment has a transparent nanosheet layer structure, the sheet layer length is about 700-800 nm, and there is no obvious agglomeration phenomenon, so that the specific surface area of the material is improved, more ion exchange sites are exposed, and sufficient reaction interfaces are provided for sodium ion transmission.

[0081] With sodium metal sheet as a counter electrode, Cu foil as a working electrode (for simulating the negative electrode side of a negative electrode-free battery), 1M NaPF6 in EC DEC (1:1v v) as an electrolyte, the separators prepared from Example 2, Comparative Examples 1-3 were respectively used to assemble CR2032 button cells, and the battery electrochemical performance was tested.

[0082] Sodium ion migration number test: The test was performed by direct current polarization combined with alternating current impedance method. The results are shown in Table 1.

[0083] Table 1: Sodium ion migration number of different separators

[0084]

[0085] Coulomb efficiency test: At a current density of 0.5 mAcm -2 , 1 mAhcm -2 of sodium was deposited on the Cu foil each time, and then it was completely stripped at the same current density, the data of the first 100 cycles were recorded, and the average coulomb efficiency was calculated. The results are shown in Table 1.

[0086] Constant current cycle test, the change of battery capacity retention rate and coulomb efficiency with time was recorded, and the results are shown in Figure 5 .

[0087] Result analysis:

[0088] As can be seen from Table 1 and Figure 5 , the functional separator of Example 2 of the present application performs best in all performance indicators. The high sodium ion migration number (0.65) is due to the high-speed ion channel of the A-side coating sheet layer fast sodium ion material. The extremely high average coulomb efficiency (99.2%) and the super-long cycle life prove the key role of the B-side coating polybutyl acrylate in promoting desolvation, inhibiting side reactions and dendrite growth.

[0089] Although Comparative Example 2 has a high ion transport number, it lacks regulation of desolvation, resulting in unsatisfactory coulombic efficiency and lifetime. Comparative Example 3 has a low ion transport number, indicating that improvements in desolvation regulation have limited impact on the overall battery performance, and synergistic improvements in sodium ion transport number are needed to enhance the battery's electrochemical performance.

[0090] The functional diaphragms of Examples 2 and Comparative Examples 1-2 were cut into strips 50 mm long and 10 mm wide. A universal testing machine (such as an Instron 5967) equipped with a 5 kN load cell was used, and the tensile rate was set to 5 mm. min, ambient temperature 25 1 The relative humidity is <30%. The stress (MPa) and strain rate (%) of the specimen are recorded in real time during the tensile process until the specimen breaks, and a complete stress-strain curve is obtained.

[0091] like Figure 4 As shown, the tensile strength and elongation at break of Example 2 are significantly higher than those of Comparative Example 1 and Comparative Example 2, indicating that the functional separator of the present invention has better mechanical properties and superior flexibility. This is due to the toughening effect of the asymmetric coating. The high elasticity of PBA in the B-side coating and the rigid skeleton of the lamellar fast sodium ion material form a rigid-flexible composite structure, which allows the functional separator to disperse stress through micro-deformation inside the coating when under stress, thereby improving the elongation at break. This enhances the adaptability of the functional separator to changes in electrode volume during battery cycling (such as volume expansion during sodium metal deposition / stripping), preventing the functional separator from delaminating from the electrode or being pierced by dendrites. This ensures the mechanical stability of the functional separator during electrode cutting and winding, and provides a guarantee for the structural integrity of the battery during long-term cycling, ultimately contributing to the improvement of battery cycle life.

[0092] Therefore, the present invention employs the above-mentioned functional separator and its preparation method for improving the cycle life of sodium metal anode-free batteries. The asymmetric design of the A-side coating and the B-side coating produces a significant synergistic effect, and both are indispensable, jointly achieving a breakthrough improvement in the cycle life of sodium metal anode-free batteries.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A functional separator for improving the cycle life of sodium metal anode-free batteries, characterized in that: It includes a base film and a functional coating coated on the surface of the base film. The functional coating includes an A-side coating and a B-side coating distributed on both sides of the base film. The A-side coating includes a sheet-like fast sodium ion material and a solvent, and the B-side coating includes a sheet-like fast sodium ion material, a polymer and a solvent. The lamellar fast sodium ion material includes one of lamellar sodium-based montmorillonite, vermiculite, and hydroxyapatite; the polymer is polybutyl acrylate. The method for preparing the above-mentioned functional separator for improving the cycle life of sodium metal anode-free batteries includes the following steps: S1. Preparation of sheet-like fast sodium ion material powder: The sheet-like fast sodium ion material is uniformly dispersed in deionized water, and then exfoliated into sheet-like form by high-speed mechanical stirring and ice-water bath ultrasonic peeling. After centrifugation and drying, the sheet-like fast sodium ion material powder is obtained. S2. Preparation of coating slurry for surface A: Disperse the lamellar fast sodium ion material powder in a solvent, and form surface A coating slurry by high-speed mechanical stirring and ice-water bath ultrasonication; S3. Preparation of coating slurry for B-side coating: Disperse the lamellar fast sodium ion material powder in a solvent, add a polymer, stir at low speed until the polymer is completely dissolved, then stir at high speed and sonicate in an ice-water bath to form B-side coating slurry. S4. Asymmetric Coating and Drying Steps: Coat one side of the base film surface with the A-side coating slurry. After coating, place it in a vacuum oven for preliminary drying. Coat the other side of the base film surface with the B-side coating slurry. After coating, transfer it to a vacuum oven for further drying to obtain an asymmetric functional diaphragm.

2. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: The thickness of the base film is 9-12μm, the thickness of the functional coating is 1-4μm, and the base film is one of PE, PP, or PI film.

3. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: The solvent is one of N-methylpyrrolidone and anhydrous ethanol; the solid content of the lamellar fast sodium ion material in the A-side coating is 10%~20% in the solvent, and the solid content of the lamellar fast sodium ion material in the B-side coating is 10%~20% in the solvent.

4. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: The mass ratio of lamellar fast sodium ion material to polymer in the B-side coating is 1:1 to 3:

1.

5. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: In S1, the solid content of the lamellar fast sodium ion material in deionized water is 5%~15%, the high-speed mechanical stirring rate is 1000~3000rpm, the stirring time is 2~8h, the power of ice-water bath ultrasound is 500~1000W, the ultrasound working time is 2~5s, the interval time is 1~3s, the total ultrasound duration is 5~10h, the centrifugation is performed at 8000~10000r / min for 1~3min, and the drying is performed at 60~80℃ for 6~12h.

6. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: In S2, the speed of high-speed mechanical stirring is 800~1200rpm, the stirring time is 1.5~2.5h, the power of ice water bath ultrasound is 550~650W, the ultrasound working time is 2s, the interval time is 3s, and the total ultrasound duration is 30min.

7. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: In S3, the low-speed stirring rate is 200~500 rpm, the stirring time is 1~2 h, the high-speed mechanical stirring rate is 800~1200 rpm, the stirring time is 3.5~4.5 h, the power of the ice-water bath ultrasound is 550~650 W, the ultrasound working time is 2 s, the interval time is 3 s, and the total ultrasound duration is 30 min.

8. The functional separator for improving the cycle life of sodium metal anode-free batteries according to claim 1, characterized in that: In S4, the coating is applied using a doctor blade coater. The initial drying temperature is 75-85℃, and the drying time is 1.5-2.5 hours. The subsequent drying temperature is 55-65℃, and the drying time is 23-24 hours.

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