Sodium-ion battery separator and its preparation method and sodium-ion battery

Abstract

This invention relates to the field of sodium-ion battery separator technology, and discloses a sodium-ion battery separator comprising a base film and coatings on both sides of the base film. The coatings comprise fluorinated polyimide microspheres, a sodium-ion solid electrolyte, a gelatable polymer, and a plasticizer. The sodium-ion battery separator provided by this invention exhibits good thermal stability, low internal resistance, and good hot-pressing adhesion, effectively improving problems such as poor cycle performance and electrolyte leakage during sodium-ion battery use, and enhancing the electrical safety of sodium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery separator technology, specifically to a sodium-ion battery separator, its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries offer advantages such as low cost, environmental friendliness, long cycle life, and stable performance, playing a prominent role in energy storage, low-speed electric vehicles, and other fields. As a key material affecting the performance of sodium-ion batteries, the sodium-ion battery separator is attracting increasing attention from researchers.

[0003] As a key component of sodium-ion batteries, the separator plays a crucial role in preventing contact between the positive and negative electrodes, preventing short circuits, and transporting sodium ions. It is a significant determinant of battery capacity, cycle life, and safety performance. Currently, sodium-ion battery separators mainly suffer from the following problems:

[0004] (1) Sodium-ion batteries may experience local overheating during use. The sodium-ion battery separator shrinks due to heat, causing the positive and negative electrodes to come into direct contact, which can lead to a short circuit, explosion, and combustion of the sodium-ion battery, posing a significant safety hazard.

[0005] (2) The weak adhesion between the separator and the positive and negative electrode sheets leads to deformation or softening of the sodium-ion battery, which in turn affects the stability and safety of the sodium-ion battery.

[0006] Therefore, researching a sodium-ion battery separator with high heat resistance and high adhesion is of great significance. Summary of the Invention

[0007] The purpose of this invention is to overcome the problems of poor heat resistance and low adhesion of existing sodium-ion battery separators, and to provide a sodium-ion battery separator with good thermal stability and hot-pressing adhesion, thereby further improving the cycle performance of sodium-ion batteries.

[0008] To achieve the above objectives, a first aspect of the present invention provides a sodium-ion battery separator, wherein the separator comprises a base film and coatings on both sides of the base film; the coatings comprise fluorinated polyimide microspheres, a sodium-ion solid electrolyte, a gelatable polymer, and a plasticizer.

[0009] A second aspect of this invention provides a method for preparing a sodium-ion battery separator, comprising the following steps:

[0010] (1) A mixed solution of gelatable polymer and plasticizer is stirred and mixed with a mixed dispersion of sodium ion solid electrolyte and fluorinated polyimide microspheres to obtain a coating slurry;

[0011] (2) The coating slurry is applied to both sides of the base film and dried to obtain the sodium-ion battery separator.

[0012] A third aspect of the present invention provides a sodium-ion battery separator prepared according to the method described in the second aspect.

[0013] A fourth aspect of the present invention provides a sodium-ion battery, wherein the sodium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator is the separator described in the first aspect or the separator described in the third aspect.

[0014] Compared with the prior art, the present invention has the following beneficial effects through the above technical solution:

[0015] (1) The sodium-ion battery separator provided by this invention has a lighter surface density of fluorinated polyimide microspheres in the coating compared to ordinary ceramics, which is beneficial to improving the energy density of the battery. The addition of sodium-ion solid electrolyte can supplement sodium and play a certain role in reducing the internal resistance of the separator. At the same time, the synergistic effect between fluorinated polyimide microspheres and sodium-ion solid electrolyte effectively improves the thermal stability of the base film and improves safety performance.

[0016] (2) The sodium-ion battery separator provided by the present invention has a gelatable polymer coating that enables the prepared separator to absorb electrolyte gelation. The synergistic effect of the plasticizer and fluorinated polyimide microspheres can also have good hot-pressing adhesion performance, which is beneficial to improving the interface cycle performance and leakage risk of sodium-ion batteries.

[0017] (3) The sodium-ion battery separator provided by this invention has the characteristics of good thermal stability and strong hot-pressing adhesion. Specifically, after maintaining a temperature of 150℃ for 0.5 hours, the lateral shrinkage rate is 0-5%, and the longitudinal shrinkage rate is 0-5%. The adhesion performance between the separator and the positive electrode is 10-60 N / m; the adhesion performance between the separator and the negative electrode is also 10-60 N / m. This high adhesion performance prevents deformation of the positive / negative electrode during long-term use of the sodium-ion battery, and the low thermal shrinkage rate of the separator effectively prevents short circuits inside the battery.

[0018] (4) The method for preparing the sodium-ion battery separator provided by the present invention uses a low-boiling-point organic solvent in conjunction with a temperature and humidity drying oven to prepare the coating, which is simpler and easier to operate.

[0019] (5) The sodium-ion battery separator prepared by the method provided in this invention has good thermal stability, low internal resistance and good hot-pressing bonding performance, which can effectively improve the problems of poor cycle performance and electrolyte leakage during the use of sodium-ion batteries, and improve the electrical safety of sodium-ion batteries. Attached Figure Description

[0020] Figure 1 This is a morphology diagram of the fluorinated polyimide microspheres prepared according to the present invention. Detailed Implementation

[0021] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0022] The first aspect of the present invention provides a sodium-ion battery separator, wherein the separator includes a base film and coatings on both sides of the base film; the coatings include fluorinated polyimide microspheres, sodium-ion solid electrolyte, gelatable polymer and plasticizer.

[0023] In this invention, the coating is applied to both sides of the base membrane, retaining the excellent high-temperature pore-closing function of the porous base membrane, which plays a role in preventing battery short circuits in a timely manner. The synergistic effect of the fluorinated polyimide microspheres and sodium ion solid electrolyte in the coating can effectively improve the thermal shrinkage performance of the separator. The fluorinated polyimide microspheres can improve the peel strength of the coating, and both are lighter than ordinary ceramic particles, which is beneficial to the improvement of energy density. The sodium ion solid electrolyte can play a role in sodium replenishment and reduce the internal resistance of the separator. The gelatable polymer improves the ability to absorb and gel the electrolyte and the performance of hot pressing adhesion, which is beneficial to improving the interface performance. The addition of plasticizer can improve the peel strength and plasticity of the coating.

[0024] According to the present invention, the glass transition temperature of the fluorinated polyimide microspheres is 200-350℃, the melting point is >300℃, and the average particle size is 0.05-10μm, preferably 0.7-4μm.

[0025] In this invention, the preparation method of the fluorinated polyimide microspheres is as follows:

[0026] Weigh out sulfone-containing diamine 2,2-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]sulfone and monomer A. Dissolve the sulfone-containing diamine in a polar solvent and add monomer A in batches. Synthesize a polyamic acid solution in an ice-water bath. Prepare polyamide microspheres by electrostatic spraying in an electric field, then place them in a high-temperature furnace for imidization treatment. After natural cooling to room temperature, obtain fluorinated polyimide microspheres.

[0027] Wherein, monomer A is selected from at least one of pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA), and 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA).

[0028] The solvent is selected from at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and acetone.

[0029] The molar ratio of monomer A to sulfone-containing diamine 2,2-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]sulfone is 1.010-1.020, preferably 1.015.

[0030] The solid content of the polyamic acid solution is 5-18%, preferably 15%.

[0031] The electric field strength of the electrostatic spray is 0.5-5kV / cm, preferably 1kV / cm.

[0032] The conditions for the imidization treatment include: a temperature of 180-300℃ and a time of 10-60 min.

[0033] The average particle size of the fluorinated polyimide microspheres is 0.05-10 μm, preferably 0.7-4 μm.

[0034] In this invention, fluorinated polyimide microspheres have a high melting point and glass transition temperature. Therefore, adding fluorinated polyimide microspheres to the coating can improve the heat resistance of the sodium ion exchange membrane and reduce its thermal shrinkage. Simultaneously, the polar functional groups in the fluorinated polyimide microspheres improve the compatibility between the coating and the base film, and also enhance the peel strength, further improving the thermal stability of the sodium ion exchange membrane.

[0035] In this invention, the particle size of the fluorinated polyimide microspheres affects their dispersion effect, which in turn affects the coating effect and thermal stability. Smaller particle sizes result in better dispersion. However, if the particle size is too small, the increased specific surface area leads to system thickening under the same conditions, which also affects the dispersion effect, resulting in poor dispersion. Conversely, excessively large particle sizes will lead to poor dispersion stability and limit the coating thickness, failing to meet the required performance.

[0036] According to the present invention, the mass ratio of the gelatable polymer to the plasticizer is 90-95:5-10.

[0037] According to the present invention, the mass ratio of the fluorinated polyimide microspheres to the sodium ion solid electrolyte is 50-90:10-50.

[0038] According to the present invention, the total mass of the fluorinated polyimide microspheres and the sodium ion solid electrolyte is A, and the total mass of the gelatable polymer and the plasticizer is B, and the mass ratio of A:B is 50-90:10-50.

[0039] The applicant found in the study that when the mass ratio of fluorinated polyimide microspheres, sodium ion solid electrolyte, gelatable polymer and plasticizer in the coating satisfies the above relationship, the thermal stability and hot-pressing adhesion performance of the sodium ion separator are optimal.

[0040] In this invention, the synergistic effect between the fluorinated polyimide microspheres in the coating and the sodium ion solid electrolyte can effectively improve the thermal stability of the base film and enhance its safety performance.

[0041] In this invention, the synergistic combination of gelatable polymers, plasticizers, and fluorinated polyimide microspheres in the coating can simultaneously provide good hot-pressing adhesion, which is beneficial for improving the interfacial cycle performance and leakage risk of sodium-ion batteries.

[0042] According to the present invention, the thickness of the diaphragm is 9-52 μm; the thickness of the base film is 5-40 μm; and the thickness of the coating is 2-6 μm. Preferably, the thickness of the diaphragm is 9-20 μm; the thickness of the base film is 5-12 μm; and the thickness of the coating is 2-4 μm.

[0043] In this invention, the thickness of the separator is the sum of the thickness of one base film and the thickness of two coating layers. The thinner the separator, the more favorable it is for solutes to pass through, thereby increasing energy density and reducing battery impedance. However, as the separator thickness decreases, its mechanical properties deteriorate, increasing the likelihood of electrode debris or dendrites piercing the separator and causing a physical short circuit, potentially leading to a safety accident.

[0044] In this invention, if the coating thickness is too small, it will be difficult to achieve the required performance; if the coating thickness is too thick, it will hinder the passage of solute, reduce energy density, and increase battery impedance. The thinner the coating, the better, while still meeting performance requirements.

[0045] According to the present invention, the diaphragm is maintained at a temperature of 150°C for 0.5 hours, and the lateral shrinkage rate is 0-5% and the longitudinal shrinkage rate is 0-5%.

[0046] In this invention, the method for testing the thermal shrinkage rate of the diaphragm is as follows:

[0047] Cut a 10cm x 10cm square diaphragm. Mark four points at the center of each of the four sides, 1cm from the edge. Measure the distances between two points (TD, horizontal) and two points (MD, vertical) using an optical microscope, and record them as Lh0 and Lz0. Place three sheets of A4 paper on top and bottom of the cut diaphragm and then place it in a forced-air drying oven at 150℃ (±1℃) for 0.5 hours. After heating, remove the diaphragm and allow it to return to room temperature. Measure the distances between the horizontal and vertical points again using an optical microscope, and record them as Lh and Lz. Calculate the horizontal and vertical shrinkage rates of the diaphragm using Formula 1 and Formula 2 below, respectively. Take the average of the three test results as the thermal shrinkage rate of the diaphragm.

[0048] Thermal shrinkage rate of the diaphragm in the longitudinal direction: ΔLz=(Lz0-Lz) / Lz0×100% (Formula 1)

[0049] The lateral thermal shrinkage rate of the diaphragm is: ΔLh=(Lh0-Lh) / Lh0×100% (Formula 2)

[0050] According to the present invention, the adhesion performance between the separator and the positive electrode is 10-60 N / m.

[0051] According to the present invention, the adhesion performance between the diaphragm and the negative electrode sheet is 10-60 N / m.

[0052] In this invention, the method for testing the adhesion performance between the separator and the positive electrode includes cutting the positive electrode and the separator into 14cm × 8.5cm pieces, stacking the base membrane, positive electrode, and coated separator sequentially, and then stacking 6-8 layers. The layers are then hot-pressed for 60s at a temperature of 90℃ and a pressure of 0.3MPa. After hot pressing, the layers are cut into 3cm strips and stretched at a speed of 300mm / min. The obtained data is the adhesion strength between the separator and the positive electrode, expressed in N / m.

[0053] In this invention, the test method for the bonding performance between the separator and the negative electrode is the same as the test method for the bonding performance between the separator and the positive electrode.

[0054] In this invention, the high adhesion between the sodium-ion battery separator and the positive and negative electrodes can effectively prevent the sodium-ion battery from deforming or becoming soft, thereby improving the stability and safety of the sodium-ion battery.

[0055] According to the present invention, the base film has a porous structure, preferably, the average pore size of the porous structure is 30nm-80nm, more preferably 40nm-50nm, and the porosity is 40-80%, more preferably 40-60%.

[0056] In this invention, for a given electrolyte, a separator with high porosity can effectively reduce battery impedance. However, higher porosity is not always better; higher porosity results in poorer mechanical resistance and resistance to pore formation. Submicron-level pore size is extremely important for preventing short circuits between the positive and negative electrodes of the battery. However, the pore size should not be too large, otherwise the separator will not be able to prevent short circuits, affecting the battery's safety performance.

[0057] According to the present invention, the base membrane is selected from at least one of PP membrane, PE membrane, cellulose membrane, PET membrane, PI membrane, PVDF membrane, PVDF-HFP membrane, and PTFE membrane; preferably selected from at least one of PP membrane, PE membrane, cellulose membrane, PET membrane, PI membrane, and PTFE membrane.

[0058] In this invention, the base film has a high-temperature pore-closing function, which can prevent the battery from short-circuiting in time. The separator retains this property of the base film. By coating both sides of the base film, the thermal stability and hot-pressing adhesion of the separator are further improved, the internal resistance of the separator is reduced, and thus the cycle performance of the sodium-ion battery is improved.

[0059] According to the present invention, the sodium ion solid electrolyte is selected from Na3Zr2Si2PO4. 12 At least one of Na-β-Al2O3, Na-β”-Al2O3, Na3SbS4, and NaCl.

[0060] In this invention, the addition of sodium-ion solid electrolyte firstly provides the thermal shrinkage resistance of the separator, similar to conventional inorganic materials; secondly, compared to conventional inorganic materials, solid electrolytes exhibit excellent ionic conductivity, which helps improve the internal resistance of the separator. Therefore, the addition of sodium-ion solid electrolyte is beneficial for improving the battery's safety and cycle performance.

[0061] In this invention, the fluorinated polyimide microspheres in the coating have a lighter surface density than ordinary ceramics, which is beneficial for improving the energy density of the battery. The addition of sodium-ion solid electrolyte can supplement sodium and reduce the internal resistance of the separator. At the same time, the synergistic effect between the fluorinated polyimide microspheres and the sodium-ion solid electrolyte effectively improves the thermal stability of the base film and enhances safety performance.

[0062] According to the present invention, the gelatable polymer is selected from at least one of polyamide, polyimide, meta-aramid, para-aramid, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polymethyl methacrylate and polyacrylonitrile.

[0063] In this invention, the gelatable polymer acts as a binder in the entire formulation system, ensuring a good coating effect between the coating and the base film and preventing powdering. On the other hand, it has the characteristics of gelatinization, liquid absorption, and liquid retention in the electrolyte, which is beneficial to the wetting of the diaphragm and improves the interface characteristics with the positive and negative electrodes and reduces the risk of leakage.

[0064] According to the present invention, the plasticizer is selected from at least one of polyethylene oxide, polyvinyl alcohol, cyanoethyl polyvinyl alcohol, sulfonated polyvinyl alcohol, and acrylonitrile.

[0065] In this invention, the addition of plasticizers can, on the one hand, improve the toughness of the coating to prevent cracking and peeling. On the other hand, the selected plasticizers can improve the pore-forming effect of the coating to a certain extent, improve the air permeability of the coated diaphragm to a certain extent, and reduce the internal resistance of the diaphragm.

[0066] In this invention, the gelatable polymer in the coating enables the prepared separator to absorb electrolyte gelation. The synergistic effect with plasticizer and fluorinated polyimide microspheres can also provide good hot-pressing adhesion, which is beneficial to improving the interfacial cycle performance and leakage risk of sodium-ion batteries.

[0067] A second aspect of this invention provides a method for preparing a sodium-ion battery separator, comprising the following steps:

[0068] (1) A mixed solution of gelatable polymer and plasticizer is stirred and mixed with a mixed dispersion of sodium ion solid electrolyte and fluorinated polyimide microspheres to obtain a coating slurry;

[0069] (2) The coating slurry is applied to both sides of the base film and dried to obtain the sodium-ion battery separator.

[0070] The method for preparing sodium-ion battery separators provided by this invention uses a low-boiling-point organic solvent in conjunction with a temperature and humidity drying oven to prepare the coating, which makes the process simpler and easier to operate.

[0071] According to the present invention, the mass ratio of the gelatable polymer and the plasticizer in step (1) is 90-95:5-10.

[0072] According to the present invention, the mass ratio of the sodium ion solid electrolyte to the fluorinated polyimide microspheres is 50-90:10-50.

[0073] According to the present invention, the total mass of the fluorinated polyimide microspheres and the sodium ion solid electrolyte is A, and the total mass of the gelatable polymer and the plasticizer is B, and the mass ratio of A to B is 50-90:10-50.

[0074] According to the present invention, the stirring and mixing conditions in step (1) include: a stirring speed of 500 r / min to 1500 r / min, a stirring time of 0.5 h to 3 h, and a stirring temperature of 5 °C to 30 °C.

[0075] In this invention, the solid content of the coating slurry is 10-30%, preferably 15-25%. The solid content of the slurry is closely related to its stability. Under the same process and formulation, the higher the solid content of the slurry, the higher the viscosity, and vice versa. The higher the solid content, the shorter the stirring time of the slurry, the less solvent is required, the higher the coating drying efficiency, and the more time is saved. However, high solid content slurries may increase the wear and tear on equipment and increase the difficulty of coating.

[0076] According to the present invention, the coating in step (2) is selected from at least one of dip coating, precision spraying, micro-gravure roller coating, and wire bar coating.

[0077] In this invention, the coating is selected from wire rod coating.

[0078] According to the present invention, the drying is selected as temperature and humidity oven drying, and the drying conditions include: temperature of 40℃-80℃ and humidity of 40%-80%.

[0079] In this invention, the temperature and humidity of the temperature and humidity oven can be controlled to influence the displacement rate of solvent and non-solvent during polymer phase inversion, thereby affecting the morphology of the coating. If the temperature is too high, the solvent evaporates rapidly, forming a sealing layer on the surface that inhibits the evaporation of the underlying solvent; if the temperature is too low, drying performance decreases, the coating fails to dry, and may even sag. Inappropriate humidity affects the displacement efficiency of solvent and non-solvent, influencing the phase inversion process and thus affecting the morphology.

[0080] A third aspect of the present invention provides a sodium-ion battery separator prepared according to the method described in the second aspect.

[0081] A fourth aspect of the present invention provides a sodium-ion battery, wherein the sodium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the separator is the separator described in the first aspect or the separator described in the third aspect.

[0082] In this invention, the positive electrode is a layered oxide sodium iron manganate positive electrode, the negative electrode is a hard carbon negative electrode, and the electrolyte includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and sodium hexafluorophosphate, wherein EC / EMC / DMC is 1:1:1, and the concentration of sodium hexafluorophosphate is 1.0 mol / L.

[0083] The sodium-ion battery separator prepared by the method provided in this invention exhibits good thermal stability, low internal resistance, and excellent hot-pressing adhesion. Using the sodium-ion battery separator provided by this invention can effectively improve problems such as poor cycle performance and electrolyte leakage during the use of sodium-ion batteries, thereby enhancing the electrical safety of sodium-ion batteries.

[0084] The present invention will be described in detail below through examples. In the following examples, the fluorinated polyimide microspheres were prepared in the laboratory, and the preparation method is as follows:

[0085] Weigh out 3,3',4,4'-dimethyl ketone tetracarboxylic dianhydride and sulfone-containing diamine 2,2-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]sulfone at a molar ratio of 1:1.015. Dissolve the sulfone-containing diamine in a polar solvent and add it to the dianhydride in batches. Synthesize a polyamic acid solution in an ice-water bath to prepare a polyamic acid solution with a solid content of 15%. Prepare polyamide microspheres by electrostatic spraying in an electric field of 1 kV / cm. Then place them in a high-temperature furnace and imidize them at 300℃ for 40 min. After natural cooling to room temperature, obtain fluorinated polyimide microspheres. Figure 1 As shown, the average particle size of the fluorinated polyimide microspheres is approximately 0.7 μm to 4 μm.

[0086] Example 1

[0087] 45g of polyvinylidene fluoride (PVDF) and 5g of cyanoethyl polyvinyl alcohol were added to acetone to obtain mixture I; 100g of biphenyl-type fluorinated polyimide microspheres and 100g of sodium ion solid electrolyte Na-β”-Al2O3 were added to acetone to obtain dispersion II; mixture I and dispersion II were mixed and stirred and dispersed at 10℃ and 800rpm for 3 hours to obtain the required coating slurry with a solid content of 25%.

[0088] A 3μm coating layer was applied to both the top and bottom sides of a 9μm PE separator using a wire rod coating process, and then dried in an oven at 80℃ / 80% temperature and humidity to obtain the required 3+9+3 battery separator, which was marked as S1.

[0089] Example 2

[0090] 47.5g of polyvinylidene fluoride (PVDF) and 2.5g of cyanoethyl polyvinyl alcohol were added to acetone to obtain mixture I; 150g of biphenyl-type fluorinated polyimide microspheres and 50g of sodium ion solid electrolyte Na-β”-Al2O3 were added to acetone to obtain dispersion II; mixture I and dispersion II were mixed and stirred and dispersed at 10℃ and 800rpm for 3 hours to obtain the required coating slurry with a solid content of 25%.

[0091] A 3μm coating layer was applied to both the top and bottom sides of a 9μm PE separator using a wire rod coating process, and then dried in an oven at 80℃ / 80% temperature and humidity to obtain the required 3+9+3 battery separator, which was marked as S2.

[0092] Example 3

[0093] 45g of polyvinylidene fluoride (PVDF) and 5g of cyanoethyl polyvinyl alcohol were added to acetone to obtain mixture I; 100g of biphenyl-type fluorinated polyimide microspheres and 100g of sodium ion solid electrolyte Na-β”-Al2O3 were added to acetone to obtain dispersion II; mixture I and dispersion II were mixed and stirred and dispersed at 10℃ and 800rpm for 3 hours to obtain the required coating slurry with a solid content of 25%.

[0094] A 5μm coating layer was applied to both the top and bottom sides of a 9μm PE separator using a wire rod coating process, and then dried in an oven at 80℃ / 80% temperature and humidity to obtain the required 5+9+5 battery separator, which was marked as S3.

[0095] Example 4

[0096] The separator was prepared according to the method in Example 1, except that the sodium-ion solid electrolyte was replaced with Na3SbS4. The resulting battery separator was labeled as S4.

[0097] Example 5

[0098] The separator was prepared according to the method of Example 1, except that the gelatable polymer was replaced with polymethyl methacrylate (PMMA). The resulting battery separator is labeled S5.

[0099] Example 6

[0100] The separator was prepared according to the method of Example 1, except that the plasticizer was replaced with sulfonated polyvinyl alcohol. The resulting battery separator is labeled S6.

[0101] Comparative Example 1

[0102] The separator was prepared according to the method of Example 1, except that fluorinated polyimide microspheres were not added. The resulting battery separator is labeled D1.

[0103] Comparative Example 2

[0104] The separator was prepared according to the method of Example 1, except that no sodium-ion solid electrolyte was added. The resulting battery separator was labeled D2.

[0105] Comparative Example 3

[0106] The separator was prepared according to the method of Example 1, except that no plasticizer was added. The resulting battery separator was labeled D3.

[0107] The membranes prepared in Examples 1-6 and Comparative Examples 1-3 were tested for their air permeability, thermal stability, adhesion, membrane resistance, and capacity retention of the sodium-ion batteries obtained therefrom. The results are shown in Table 1.

[0108] Methods for testing air permeability:

[0109] Referring to GB / T 36363-2018, the test standard for polyolefin separators for lithium-ion batteries, under normal temperature, humidity, and pressure conditions, and with the Wang Yan-style air permeability tester applying a pressure of 1.12 kPa, the area through which 100 mL of air passes is 6.45 cm². 2 The time required for the diaphragm to be in place. The sample size is 100mm×100mm, and the diaphragm is placed in the test head of the air permeability meter within a suitable range for air permeability testing. The average of the three test results is taken as the air permeability of the diaphragm.

[0110] Test method for heat shrinkage rate:

[0111] Cut a 10cm x 10cm square diaphragm. Mark four points at the center of each of the four sides, 1cm from the edge. Measure the distances between two points (TD, horizontal) and two points (MD, vertical) using an optical microscope, and record them as Lh0 and Lz0. Place three sheets of A4 paper on top and bottom of the cut diaphragm and then place it in a forced-air drying oven at 150 (±1) degrees Celsius for 0.5 hours. After heating, remove the diaphragm and allow it to return to room temperature. Measure the distances between the horizontal and vertical points again using an optical microscope, and record them as Lh and Lz. Calculate the horizontal and vertical shrinkage rates of the diaphragm using Formula 1 and Formula 2 below, respectively. Take the average of the three test results as the thermal shrinkage rate of the diaphragm.

[0112] Thermal shrinkage rate of the diaphragm in the longitudinal direction: ΔLz=(Lz0-Lz) / Lz0×100% (Formula 1)

[0113] The lateral thermal shrinkage rate of the diaphragm is: ΔLh=(Lh0-Lh) / Lh0×100% (Formula 2)

[0114] Test method for adhesion between diaphragm and positive / negative electrode plates:

[0115] Cut the positive / negative electrode sheets into 14cm × 8.5cm pieces, and the separator is the same size. Stack the positive / negative electrode and separator coating together, separated by a base film, and stack 6-8 layers. Apply a hot-pressing condition of 0.3MPa force, 90℃, and 60s. After hot pressing, cut the strips into 3cm pieces and stretch them at a speed of 300mm / min. The obtained data is the bonding strength between the separator and the positive / negative electrode, in N / m.

[0116] Test method for diaphragm resistance:

[0117] Four diaphragms matching the resistance testing mold were cut and placed in an electrolyte solution with a concentration of 1.0 mol / L sodium hexafluorophosphate, ethylene carbonate (EC), and dimethyl carbonate (DMC) in a 1:1:1 volume ratio. The solution was kept sealed and soaked for 2 hours. The electrolyte solution was then injected into the resistance testing mold. One layer of diaphragm was placed in each mold, and its AC impedance was measured. Another layer was placed in, and the AC impedance was measured again, until all four layers were placed. The four AC impedances R1, R2, R3, and R4 were measured. During the measurement process, it was ensured that the electrolyte in the resistance testing mold completely submerged the placed diaphragms.

[0118] Plot a curve with the number of membrane layers on the x-axis and membrane resistance on the y-axis. Calculate the slope and linearity of the curve. When the linearity is greater than 0.99, the ionic conductivity of the membrane is calculated using the following formula. When the linearity is less than 0.99, the test needs to be repeated.

[0119] R = k × 1

[0120] In the formula:

[0121] R – The resistance value of the first diaphragm layer, in ohms (Ω).

[0122] k -- The slope of the curve when the goodness of fit is greater than 0.99.

[0123] Test method for battery capacity retention:

[0124] At ambient temperature (25±5℃), the current was constant at 0.2C until the limiting voltage of 3.95V was reached, then constant voltage charging was switched to charging until the cutoff current was 0.01C. The charging was stopped and left to stand for 5 minutes. Then, the current was discharged at 1C until 2.0V as one cycle. This process was repeated 50 times, and the capacity retention rate was measured.

[0125] Table 1

[0126]

[0127]

[0128] Table 1 (continued)

[0129]

[0130] As shown in Table 1, in Examples 1-6, the coating formulations containing fluorinated polyimide microspheres, sodium ion solid electrolyte, gelatable polymer, and plasticizer resulted in coated diaphragms with good thermal stability, hot-pressing adhesion between the positive and negative electrodes, high membrane breakage temperature, and low membrane resistance. In Comparative Examples 1-3, the coating formulations lacked one of the following: fluorinated polyimide microspheres, sodium ion solid electrolyte, or plasticizer. Consequently, the thermal shrinkage performance, air permeability, positive and negative electrode adhesion, or membrane breakage temperature of the prepared diaphragms differed from those in the examples. Comparative Example 1, without fluorinated polyimide microspheres, and Comparative Example 2, without sodium-ion solid electrolyte, showed significantly increased lateral yield and longitudinal shrinkage compared to the original example, but significantly reduced thermal shrinkage performance. The thermal shrinkage performance of the separator was poor when only sodium-ion solid electrolyte or only fluorinated polyimide microspheres were added. Only when both fluorinated polyimide microspheres and sodium-ion solid electrolyte were added simultaneously could the thermal stability of the separator be improved. This indicates a synergistic effect between the fluorinated polyimide microspheres and the sodium-ion solid electrolyte, effectively improving the thermal stability of the base film and resulting in better safety performance. Comparative Example 3, without plasticizer, showed poor thermal stability and hot-press adhesion of the positive and negative electrodes. This suggests that plasticizers can assist other components in improving the thermal stability of the separator and the hot-press adhesion of the positive and negative electrodes, reducing the interfacial cycle performance and leakage risk of sodium-ion batteries.

[0131] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A sodium-ion battery separator, characterized in that, The diaphragm includes a base membrane and coatings on both sides of the base membrane; the coatings include fluorinated polyimide microspheres, sodium ion solid electrolyte, gelatable polymer, and plasticizer; the mass ratio of the fluorinated polyimide microspheres to the sodium ion solid electrolyte is 50-90:10-50; the mass ratio of the gelatable polymer to the plasticizer is 90-95:5-10; the total mass of the fluorinated polyimide microspheres and the sodium ion solid electrolyte is denoted as A, and the total mass of the gelatable polymer and the plasticizer is denoted as B, with a mass ratio of A:B of 50-90:10-50; The diamine monomer used to prepare the fluorinated polyimide microspheres is 2,2-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]sulfone, and the dianhydride monomer is at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride.

2. The diaphragm according to claim 1, wherein, The fluorinated polyimide microspheres have a glass transition temperature of 200-350℃, a melting point >300℃, and an average particle size of 0.05-10μm.

3. The diaphragm according to claim 1, wherein, The average particle size of the fluorinated polyimide microspheres is 0.7-4 μm.

4. The diaphragm according to claim 1, wherein, The thickness of the diaphragm is 9-52 μm; the thickness of the base film is 5-40 μm; and the thickness of the coating is 2-6 μm.

5. The diaphragm according to claim 4, wherein, The thickness of the diaphragm is 9-20 μm; the thickness of the base film is 5-12 μm; and the thickness of the coating is 2-4 μm.

6. The diaphragm according to claim 1, wherein, The diaphragm, when maintained at 150℃ for 0.5 hours, exhibits a transverse shrinkage rate of 0-5% and a longitudinal shrinkage rate of 0-5%. And / or, the adhesion performance between the diaphragm and the positive electrode sheet is 10-60 N / m; And / or, the adhesion performance between the diaphragm and the negative electrode sheet is 10-60 N / m.

7. The diaphragm according to claim 1, wherein, The base membrane has a porous structure.

8. The diaphragm according to claim 7, wherein, The average pore size of the porous structure is 30-80 nm, and the porosity is 40-80%.

9. The diaphragm according to claim 8, wherein, The average pore size of the porous structure is 40-50 nm, and the porosity is 40%-60%.

10. The diaphragm according to claim 1, wherein, The base membrane is selected from at least one of PP membrane, PE membrane, cellulose membrane, PET membrane, PI membrane, PVDF membrane, PVDF-HFP membrane and PTFE membrane.

11. The diaphragm according to claim 10, wherein, The base membrane is selected from at least one of PP membrane, PE membrane, cellulose membrane, PET membrane, PI membrane and PTFE membrane.

12. The diaphragm according to claim 1, wherein, The sodium-ion solid electrolyte is selected from Na3Zr2Si2PO4. 12 At least one of Na-β-Al2O3, Na-β”-Al2O3, Na3SbS4 and NaCl; And / or, the gelatable polymer is selected from at least one of polyamide, polyimide, meta-aramid, para-aramid, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polymethyl methacrylate and polyacrylonitrile; And / or, the plasticizer is selected from at least one of polyethylene oxide, polyvinyl alcohol, cyanoethyl polyvinyl alcohol, sulfonated polyvinyl alcohol, and acrylonitrile.

13. The method for preparing the sodium-ion battery separator according to any one of claims 1-12, characterized in that, Includes the following steps: (1) The mixed solution of gelable polymer and plasticizer is stirred and mixed with the mixed dispersion of sodium ion solid electrolyte and fluorinated polyimide microspheres to obtain the coating slurry; (2) The coating slurry is applied to both sides of the base film and dried to obtain the sodium-ion battery separator.

14. The method according to claim 13, characterized in that, The mixing conditions described in step (1) include: a stirring speed of 500 r / min to 1500 r / min, a stirring time of 0.5 h to 3 h, and a stirring temperature of 5 °C to 30 °C.

15. The method according to claim 13, wherein, The coating method described in step (2) is selected from at least one of dip coating, precision spraying, micro-gravure roller coating, and wire bar coating; And / or, the drying in step (2) is selected as temperature and humidity oven drying, and the drying conditions include: temperature of 40℃-80℃ and humidity of 40%-80%.

16. A sodium-ion battery, characterized in that, The sodium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator is the separator described in any one of claims 1-12.

Images