Membrane and its preparation method and sodium-ion battery

Abstract

This application relates to a separator, its preparation method, and a sodium-ion battery. The separator includes a base membrane and a first functional layer located on at least one surface of the base membrane. The first functional layer includes a coating membrane and an electrolyte replenishment solution surrounded by the coating membrane. The coating membrane is made of polyethylene terephthalate and / or polymethyl methacrylate. This replenishes the electrolyte, thereby improving electrode expansion and enhancing the battery's cycle performance and safety.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a separator, a method for preparing the separator, and a sodium-ion battery. Background Technology

[0002] Currently, lithium-ion batteries are the most widely used rechargeable batteries. However, lithium content in the Earth's crust is low and its distribution is uneven. Therefore, the availability of lithium resources is one of the factors limiting the development of lithium batteries.

[0003] Sodium resources are relatively abundant, easy to obtain, and inexpensive, which makes sodium-ion batteries a potential alternative to lithium-ion batteries.

[0004] Because sodium ions have a larger radius than lithium ions, the volume expansion of the electrodes is more significant during battery cycling as sodium ions intercalate and deintercalate at the positive and negative electrodes. This volume expansion leads to the continuous consumption of electrolyte. As cycling continues, the lack of electrolyte causes the electrodes to expand further and the internal temperature of the battery to rise continuously. Moreover, sodium-ion batteries are prone to generating moisture during cycling, which seriously threatens the cycle performance and safety of sodium-ion batteries. Summary of the Invention

[0005] In view of this, embodiments of this application provide a separator, a method for preparing the separator, and a sodium-ion battery to solve at least one problem existing in the prior art.

[0006] In a first aspect, embodiments of this application provide a diaphragm, the diaphragm comprising:

[0007] A base film and a first functional layer located on at least one side surface of the base film;

[0008] The first functional layer includes a coating membrane and an electrolyte replenishment solution surrounded by the coating membrane, wherein the coating membrane is made of polyethylene terephthalate and / or polymethyl methacrylate.

[0009] In conjunction with the first aspect of this application, in an optional embodiment, the first functional layer is located at the middle position on the surface of the base film; optionally, the area of ​​the first functional layer on the surface of the base film where the first functional layer is located is greater than or equal to 40%.

[0010] In conjunction with the first aspect of this application, in an alternative embodiment, the diaphragm further includes:

[0011] A second functional layer is located on the surface of the base film away from the first functional layer, and the second functional layer comprises carbonates; optionally, the second functional layer is located at the middle position on the surface of the base film; optionally, the area of ​​the second functional layer on the surface of the base film where the second functional layer is located is greater than or equal to 40%.

[0012] In conjunction with the first aspect of this application, in an optional embodiment, the electrolyte replenishment solution includes an electrolyte and an emulsifier; optionally, the emulsifier includes gelatin and / or agar; optionally, the electrolyte includes a solute and a solvent; optionally, the solute includes at least one selected from sodium hexafluorophosphate, sodium perchlorate, sodium fluorosulfonate, and sodium fluorosulfonamide; optionally, the solvent includes carbonate solvents and / or ether solvents; further optionally, the solvent includes carbonate solvents.

[0013] In conjunction with a first aspect of this application, in an optional embodiment, the material of the base film comprises a polyalkane polymer; alternatively, the material of the base film comprises polyethylene and / or polypropylene.

[0014] Secondly, embodiments of this application provide a method for preparing a diaphragm, the method comprising the following steps:

[0015] S1: Dissolve polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, add electrolyte replenishment, and disperse evenly to obtain a mixed emulsion;

[0016] S2: Evaporate the organic solvent in the mixed emulsion to allow the polyethylene terephthalate and / or polymethyl methacrylate to precipitate and form a coating film to surround the electrolyte replenishment liquid within the coating film, forming a first functional layer;

[0017] S3: Adhere the first functional layer to at least one side surface of the base film.

[0018] In conjunction with a second aspect of this application, in an optional embodiment, the method further includes:

[0019] The carbonate and the binder are mixed evenly to obtain a mixture; optionally, the mass ratio of the carbonate to the binder is (8.5-9):(1-1.5);

[0020] The mixture is coated onto the surface of the base film away from the first functional layer to form a second functional layer; optionally, the second functional layer is located at the middle position of the base film surface; optionally, the area of ​​the second functional layer on the base film surface where the second functional layer is located is greater than or equal to 40%.

[0021] In conjunction with the second aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features:

[0022] (1) The organic solvent includes at least one of dichloromethane, methanol, and triethanolamine;

[0023] (2) The first functional layer is located in the middle of the surface of the base film;

[0024] (3) The area of ​​the first functional layer on the surface of the base film where the first functional layer is located is greater than or equal to 40%;

[0025] (4) The electrolyte replenishment solution includes an electrolyte and an emulsifier; optionally, the emulsifier includes gelatin and / or agar; optionally, the electrolyte includes a solute and a solvent; optionally, the solute includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium fluorosulfonate, and sodium fluorosulfonamide; optionally, the solvent includes carbonate solvents and / or ether solvents; further optionally, the solvent includes carbonate solvents.

[0026] (5) The material of the base film includes polyalkane polymers; optionally, the material of the base film includes polyethylene and / or polypropylene.

[0027] Thirdly, embodiments of this application provide a sodium-ion battery, including the separator described in any one of the first aspects or the separator prepared by any one of the preparation methods described in the second aspect.

[0028] In conjunction with a third aspect of this application, in an optional embodiment, the sodium-ion battery further includes a positive electrode, a negative electrode, and a battery electrolyte, wherein the first functional layer of the separator is disposed close to the negative electrode; optionally, the battery electrolyte is the same as the electrolyte in the electrolyte replenishment solution.

[0029] Compared with the prior art, the embodiments of this application have the following beneficial effects:

[0030] The separator and its preparation method, as well as the sodium-ion battery provided in this application embodiment, include: a base membrane and a first functional layer located on at least one side surface of the base membrane; the first functional layer includes a coating membrane and an electrolyte replenishment liquid surrounded by the coating membrane, and the material of the coating membrane includes polyethylene terephthalate (PET) and / or polymethyl methacrylate (PMMA). In this embodiment, the coating material of the first functional layer includes PET and / or PMMA. During the cycling process of a sodium-ion battery, as sodium ions are inserted and extracted, the positive and negative electrodes expand in volume, the electrolyte is continuously consumed, and the internal temperature of the battery continues to rise, causing the positive and negative electrodes to expand further. Under the pressure caused by the high temperature and electrode expansion, PET and / or PMMA undergo a hydrolysis reaction with the water produced by the battery's side reactions. This not only absorbs water but also causes the coating to rupture, thereby releasing the electrolyte replenishment in the first functional layer. This replenishes the electrolyte, thereby improving the expansion of the electrodes and enhancing the battery's cycle performance. Furthermore, the hydrolysis products of the coating can absorb heat and suppress the battery's thermal runaway reaction, thus effectively improving the battery's safety performance.

[0031] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0032] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0033] Figure 1 This is a schematic flowchart illustrating a method for preparing a diaphragm according to an embodiment of this application. Detailed Implementation

[0034] To make the technical solution and beneficial effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional experimental conditions. Unless otherwise specified, all reagents and raw materials used in this invention are commercially available.

[0035] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and steps described in detail.

[0036] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0037] To fully understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solution of this application. Preferred embodiments of this application are described in detail below; however, in addition to these detailed descriptions, this application may have other implementation methods.

[0038] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those in the technical and scientific field to which this application pertains.

[0039] Unless otherwise specified, the techniques or conditions described in the following embodiments are generally performed in accordance with conventional techniques or conditions described in the literature in this field, or in accordance with the product manual and the manufacturer's recommendations. All numerical ranges in the following embodiments include endpoint values.

[0040] This application provides a diaphragm comprising: a base membrane and a first functional layer located on at least one side surface of the base membrane; the first functional layer comprising a coating membrane and an electrolyte replenishment solution surrounded by the coating membrane, wherein the material of the coating membrane comprises polyethylene terephthalate and / or polymethyl methacrylate.

[0041] In this embodiment, the coating material in the first functional layer includes PET and / or PMMA. In the initial state, the battery is waterless, and even if some water is generated by the side reaction of the battery at room temperature, PET and / or PMMA will not undergo hydrolysis when in contact with water. During sodium-ion battery cycling, the positive and negative electrodes expand in volume due to the insertion and extraction of sodium ions, continuously consuming the electrolyte and causing the internal temperature of the battery to rise. This further expands the electrodes. Under the pressure caused by the high temperature and electrode expansion, PET and / or PMMA undergo hydrolysis with water produced by side reactions in the battery. This not only absorbs water but also causes the coating to rupture, releasing the electrolyte replenishment from the first functional layer. This replenishes the electrolyte, improving electrode expansion and enhancing battery cycle performance. Furthermore, the hydrolysis products of PET and / or PMMA include alcohols, such as polyethylene terephthalate (PET) and ethylene glycol, and polymethyl methacrylate (PMMA) and methanol. These alcohols can absorb heat, suppressing thermal runaway reactions and effectively improving battery safety. In addition, alcohols can improve electrolyte flow, reducing internal impedance and increasing output power.

[0042] The first functional layer can be located on one side of the base film or on multiple sides of the base film. In one specific embodiment, the first functional layer can be located on opposite sides of the base film, which can better improve the cycle performance and safety performance of the battery.

[0043] In some embodiments, the base membrane material may include polyalkane polymers. These polymers are chemically stable and unaffected by alkaline conditions or water, thus better ensuring the insulation performance of the membrane. Further, the base membrane material may include polyethylene (PE) and / or polypropylene (PP).

[0044] During battery cycling, electrode volume expansion (also known as bulging) primarily occurs in the middle portion of the electrode. Therefore, the pressure exerted by this expansion on the separator mainly acts at the middle position of the separator. In some specific embodiments, the first functional layer can be located in the middle of the base film surface. This middle position can be understood as a region extending from the center point of the base film surface outwards, with the center point of this region coinciding with the center point of the base film surface. This allows for rapid and accurate application of the pressure caused by electrode volume expansion to the first functional layer of the separator when the electrode expands to a certain extent, thereby promoting faster rupture of the coating film and timely release of the electrolyte replenishment, thus better improving the battery's cycle performance and safety performance.

[0045] When the area of ​​the first functional layer on the surface of the base film where it is located is relatively small, its ability to replenish electrolyte and suppress thermal runaway reactions is limited. Therefore, in some specific embodiments, the area of ​​the first functional layer on the surface of the base film where it is located can be greater than or equal to 40%. This allows the first functional layer to better improve the cycle performance and safety performance of the battery. Further optionally, the area of ​​the first functional layer on the surface of the base film where it is located can be greater than or equal to 60%. In one specific embodiment, the area of ​​the first functional layer on the surface of the base film where it is located can be 100%.

[0046] In some embodiments, the electrolyte replenishment solution may include an electrolyte and an emulsifier.

[0047] When the battery is cycled to a certain extent and the electrode volume expands to a certain degree, and the temperature rises to a certain level, under the pressure of high temperature and electrode volume expansion, the coating film of the first functional layer in this embodiment undergoes a hydrolysis reaction with the water produced by the battery's side reactions. This causes the coating film to break down, allowing the electrolyte encapsulated inside to leak out, thereby replenishing the electrolyte and suppressing the battery's thermal runaway reaction. The electrolyte replenishment solution includes not only the electrolyte but also an emulsifier. Utilizing the emulsifying and suspending properties of the emulsifier, the electrolyte replenishment solution forms a uniform emulsion, reducing the fluidity of the electrolyte. This allows the electrolyte replenishment solution to be better and more stably encapsulated within the coating film.

[0048] For example, the emulsifier may include gelatin and / or agar. The electrolyte may include a solute and a solvent; the solute may include at least one of sodium hexafluorophosphate, sodium perchlorate, sodium fluorosulfonate, and sodium fluorosulfonamide; the solvent may include carbonate solvents and / or ether solvents.

[0049] In one embodiment, the solvent may include carbonate solvents. This is because carbonate solvents generate carbon dioxide under high-temperature conditions when the internal temperature of the battery is too high, thus better preventing thermal runaway. For example, carbonate solvents may include ethylene carbonate.

[0050] In some embodiments, the membrane may further include a second functional layer located on the surface of the base membrane away from the first functional layer, the second functional layer may include carbonate.

[0051] In this embodiment, the carbonate in the second functional layer can dissolve in the water produced by the battery side reaction. The carbonate ions hydrolyze to produce hydroxide ions, creating an alkaline environment. This further promotes the hydrolysis of the coating film (PET and / or PMMA) in the first functional layer, thereby suppressing the battery thermal runaway reaction more promptly and improving the battery's safety and cycle performance.

[0052] In this embodiment, the first functional layer and the second functional layer can be located on opposite sides of the base film. In some specific embodiments, the vertical projections of the first functional layer and the second functional layer in the thickness direction of the base film can at least partially overlap. This allows hydroxide ions generated by the hydrolysis of the second functional layer to be transported to the first functional layer side more quickly, thereby promoting the hydrolysis of the coating membrane more rapidly. Furthermore, the vertical projections of the first functional layer and the second functional layer in the thickness direction of the base film can coincide.

[0053] In some specific embodiments, the second functional layer can be located in the middle of the base film surface. This "middle position" can be understood with reference to the middle position in the embodiments described above. This facilitates the rapid transport of hydroxide ions generated by hydrolysis to the first functional layer side, thereby enabling it to promote the hydrolysis of the coating membrane more quickly and effectively.

[0054] When the area of ​​the second functional layer on the surface of the base film containing the second functional layer is relatively small, the effect of the second functional layer is relatively limited. Therefore, in some specific embodiments, the area of ​​the second functional layer on the surface of the base film containing the second functional layer can be greater than or equal to 40%. If the second functional layer can better promote the hydrolysis of the coating membrane in the first functional layer, the cycle performance and safety performance of the battery can be further improved. Further optionally, the area of ​​the second functional layer on the surface of the base film containing the second functional layer can be greater than or equal to 60%. In one specific embodiment, the area of ​​the second functional layer on the surface of the base film containing the second functional layer can be 100%.

[0055] In some embodiments, the second functional layer may further include an adhesive. The adhesive helps to better bond the carbonate to the base film, improving the stability of the connection between the second functional layer and the base film. The type of adhesive can be selected according to actual needs and is not limited thereto.

[0056] This application also provides a method for preparing a diaphragm. Please refer to... Figure 1 This application provides a method for preparing a diaphragm, including the following steps:

[0057] S1: Dissolve polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, add electrolyte replenishment, and disperse evenly to obtain a mixed emulsion;

[0058] S2: Evaporate the organic solvent in the mixed emulsion to allow polyethylene terephthalate and / or polymethyl methacrylate to precipitate and form a coating film to surround the electrolyte replenishment solution within the coating film, forming the first functional layer;

[0059] S3: Adhere the first functional layer to at least one side surface of the base film.

[0060] In this embodiment, the organic solvent in the mixed emulsion is first evaporated by solvent evaporation, which allows PET and / or PMMA to precipitate and form a coating film to surround the electrolyte replenishment liquid, forming a first functional layer. The first functional layer is then bonded to at least one side surface of the base film to obtain the separator. The preparation method is simple and easy to implement. Moreover, the coating film material in the first functional layer includes PET and / or PMMA. During the cycle of the sodium-ion battery, under the pressure caused by high temperature and electrode expansion, PET and / or PMMA will undergo a hydrolysis reaction with the water produced by the battery side reaction. This not only absorbs water but also causes the coating film to rupture, thereby releasing the electrolyte replenishment liquid in the first functional layer. This can replenish the electrolyte, thereby improving the expansion of the electrode and enhancing the cycle performance of the battery. Furthermore, the hydrolysis products of PET and / or PMMA (including alcohols) can absorb heat and suppress the thermal runaway reaction of the battery, thereby effectively improving the safety performance of the battery.

[0061] Step S1 may specifically include: dissolving polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, adding electrolyte and emulsifier, stirring and dispersing evenly to obtain a mixed emulsion.

[0062] In this application, the electrolyte replenishment solution includes an electrolyte and an emulsifier. By utilizing the emulsifying and suspending properties of the emulsifier, a more uniform mixed emulsion can be prepared, thereby facilitating the formation of a better first functional layer in the subsequent step S2.

[0063] For example, the organic solvent may include at least one of dichloromethane, methanol, and triethanolamine. The emulsifier may include gelatin and / or agar. The electrolyte may include a solute and a solvent; the solute may include at least one of sodium hexafluorophosphate, sodium perchlorate, sodium fluorosulfonate, and sodium fluorosulfonamide; the solvent may include carbonate solvents and / or ether solvents. Further optionally, the solvent may include carbonate solvents, specifically ethylene carbonate, because carbonate solvents generate carbon dioxide under high-temperature conditions, thus better preventing battery thermal runaway.

[0064] It should be noted that there are no particular restrictions on the amount of organic solvent and emulsifier used in the embodiments of this application. The first functional layer can be prepared by solvent evaporation in the subsequent step S2.

[0065] In step S2, the organic solvent in the mixed emulsion can be evaporated by heating. The heating temperature can be reasonably selected by those skilled in the art based on the type of organic solvent.

[0066] In step S3, the first functional layer can be bonded to at least one side surface of the base film using an adhesive. The adhesive may include, for example, polyvinylidene fluoride (PVDF).

[0067] In some specific embodiments, the base membrane material may include polyalkane polymers; these polymers are chemically stable and unaffected by alkaline conditions and water, thus better ensuring the insulation performance of the membrane. Further, the base membrane material may include polyethylene and / or polypropylene.

[0068] In step S3, the first functional layer can be bonded to the middle position of at least one side surface of the base film. The middle position can be understood as a region extending from the center point of the base film surface outwards to the periphery of the base film surface, with the center point of this region coinciding with the center point of the base film surface. This allows for rapid and accurate application of the pressure caused by the electrode's volume expansion to the first functional layer of the separator when the electrode expands to a certain extent, thereby promoting faster rupture of the coating film and timely release of the electrolyte replenishment, thus better improving the battery's cycle performance and safety performance.

[0069] In the separator fabricated in this embodiment, the area ratio of the first functional layer on the base film surface where the first functional layer is located can be greater than or equal to 40%. This allows for better improvement of the battery's cycle performance and safety performance. Further optionally, the area ratio of the first functional layer on the base film surface where the first functional layer is located can be greater than or equal to 60%. In a specific embodiment, the area ratio of the first functional layer on the base film surface where the first functional layer is located can be 100%.

[0070] In some embodiments, the method for preparing the diaphragm may further include: mixing carbonate and binder uniformly to obtain a mixture; coating the mixture onto the surface of the base membrane away from the first functional layer to form a second functional layer.

[0071] In this embodiment, the carbonate in the second functional layer can dissolve in the water produced by the battery side reaction. The carbonate ions hydrolyze to produce hydroxide ions, creating an alkaline environment. This further promotes the hydrolysis of the coating membrane in the first functional layer, thereby suppressing the battery thermal runaway reaction more promptly and improving the battery's safety and cycle performance.

[0072] In one specific embodiment, the mass ratio of carbonate to binder can be (8.5-9):(1-1.5). This ensures that the second functional layer functions fully while also maintaining the stability of the connection between the second functional layer and the base film.

[0073] In actual preparation processes, for example, carbonates and binders can be added to an appropriate amount of solvent (e.g., N-methylpyrrolidone) in a set ratio, mixed evenly, and then the mixture is coated on the surface of the base film to form a second functional layer.

[0074] In some embodiments, the second functional layer may be located at the middle position on the surface of the base film. This middle position can be understood with reference to the middle position in the above embodiments. This facilitates the rapid transport of hydroxide ions generated by hydrolysis to the first functional layer side, thereby enabling it to promote the hydrolysis of the coating membrane more quickly and effectively.

[0075] In some embodiments, the area of ​​the second functional layer on the surface of the base film containing the second functional layer can be greater than or equal to 40%. If the second functional layer can better promote the hydrolysis of the coating membrane in the first functional layer, the cycle performance and safety performance of the battery can be further improved. Further optionally, the area of ​​the second functional layer on the surface of the base film containing the second functional layer can be greater than or equal to 60%. In one specific embodiment, the area of ​​the second functional layer on the surface of the base film containing the second functional layer can be 100%.

[0076] This application also provides a sodium-ion battery, including the separator described in any of the above embodiments or the separator prepared by the preparation method described in any of the above embodiments.

[0077] It should be understood that the beneficial effects of the separator in the above embodiments are all applicable to the sodium-ion battery. Since the separator in the above embodiments includes a base membrane and a first functional layer located on at least one surface of the base membrane, the electrolyte replenishment liquid in the first functional layer can replenish the electrolyte, and the hydrolysis products of the coating membrane in the first functional layer can absorb heat and suppress the thermal runaway reaction of the battery. Therefore, the sodium-ion battery in the embodiments of this application has high cycle performance and safety performance.

[0078] In some embodiments, the sodium-ion battery may further include a positive electrode, a negative electrode, and a battery electrolyte, with the first functional layer of the separator disposed close to the negative electrode.

[0079] In this embodiment, the coating film of the first functional layer in the separator may include PET. The hydrolysis products of PET include polyethylene terephthalate. Polyethylene terephthalate reacts with sodium salt in the electrolyte to generate disodium terephthalate. Disodium terephthalate can be used as the negative electrode material of sodium-ion battery. Therefore, placing the first functional layer of the separator close to the negative electrode can better improve the performance of the battery.

[0080] In some embodiments, the battery electrolyte and the electrolyte in the electrolyte replenishment solution can be the same. This allows the electrolyte replenishment solution in the first functional layer to be more compatible with the electrolyte of the sodium-ion battery itself when released, thereby improving the battery's cycle performance.

[0081] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0082] Example 1

[0083] The preparation of the diaphragm in this embodiment includes the following steps:

[0084] Step S101: Polyethylene terephthalate (PET) is heated and melted to form a viscous solution, which is then extruded through plasticizing to obtain PET granules. 2g of PET granules are weighed and dissolved in 20ml of dichloromethane (organic solvent). Then, 5ml of electrolyte and 10ml of a 3.5% gelatin solution (emulsifier) ​​are added, and the mixture is stirred and dispersed for 30 minutes to obtain a mixed emulsion. The electrolyte is prepared by dissolving sodium hexafluorophosphate in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1 to form an electrolyte with a sodium salt concentration of 1mol / L.

[0085] Step S201: Evaporate the dichloromethane in the mixed emulsion obtained in step S101 to precipitate PET and form a coating film, which then coats the electrolyte supplement solution composed of electrolyte and gelatin solution to form the first functional layer.

[0086] Step S301: Use PVDF to adhere the first functional layer to one side surface of the PP film (base film).

[0087] Step S401: Sodium carbonate and PVDF are added to the solvent NMP at a mass ratio of 9:1, mixed evenly, and then coated onto the surface of the PP film on the side away from the first functional layer to form the second functional layer.

[0088] In this embodiment, the first functional layer and the second functional layer are located on opposite sides of the PP film, and the area ratio of the first functional layer and the second functional layer on the surface of the PP film is 100%.

[0089] Example 2

[0090] The preparation method of the diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0091] Sodium perchlorate is selected as the solute in the electrolyte in step S101.

[0092] Example 3

[0093] The preparation method of the diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0094] The first and second functional layers cover only the middle part of the PP film surface, and the area ratio of the first and second functional layers on the PP film surface is 40%.

[0095] Example 4

[0096] The preparation method of the diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0097] This embodiment does not include step S401 in Example 1, meaning that the diaphragm obtained in this embodiment only contains the first functional layer located on one side surface of the PP film and does not contain the second functional layer.

[0098] Example 5

[0099] The preparation method of the diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0100] Methanol is selected as the organic solvent in step S101.

[0101] Example 6

[0102] The preparation method of the diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0103] Replace PET with PMMA in step S101.

[0104] Comparative Example 1

[0105] The diaphragm in this comparative example is the same PP membrane as the base membrane in Example 1.

[0106] Sodium-ion batteries were fabricated using the separators prepared in the above embodiments and the separator in Comparative Example 1, and the performance of the batteries was tested. The battery preparation method was as follows: sodium vanadium phosphate, conductive carbon black, and PVDF binder were mixed at a mass ratio of 96:2:2, and NMP was added to form a uniform slurry. The slurry was then coated onto aluminum foil, dried, and rolled to obtain the positive electrode sheet of the sodium-ion battery. The separator used was the same as the separator prepared in the above embodiments and the separator in Comparative Example 1. The electrolyte used was the same as the electrolyte in Example 1. The negative electrode was hard carbon. After assembling the soft-pack battery, the battery to be tested was obtained.

[0107] (1) Cyclic performance test: The assembled battery was charged to 4V at a constant current and constant voltage of 0.1C at a constant temperature of 25℃, with a cutoff current of 0.05C; then discharged to 2V at 0.33C to obtain the discharge capacity of the first cycle. The above steps were continuously performed for charge and discharge cycles for 1000 cycles to obtain the discharge capacity of the 1000th cycle. The capacity retention rate was calculated as (discharge capacity of the 1000th cycle / discharge capacity of the first cycle) * 100%.

[0108] (2) Safety Performance Test: The battery was subjected to a thermal runaway test in a safety laboratory using a 1000W heating plate. The battery state was observed. When the battery state simultaneously met conditions A and C, or simultaneously met conditions B and C, thermal runaway was determined to have occurred, and the time of thermal runaway was recorded. Condition A is: the battery voltage drop exceeds 25% of the initial voltage; Condition B is: the temperature at the monitoring point reaches 60℃ or higher; Condition C is: the heating rate at the monitoring point is greater than or equal to 1℃ / s and lasts for more than 3 seconds. It should be noted that the wound battery has a bent surface area and a straight surface area (also called a large surface area), and the monitoring point is located in the middle of the straight surface area.

[0109] The test results are shown in Table 1.

[0110] Table 1

[0111] Group Capacity retention rate (%) after 1000 laps Time to thermal runaway of the battery (min) Example 1 90.9 15 Example 2 89.8 13 Example 3 89.5 11.5 Example 4 89.3 10.5 Example 5 89.4 10.3 Example 6 90.5 14.5 Comparative Example 1 85 8

[0112] As can be seen from the data in Table 1, compared with the battery containing the separator of Comparative Example 1, the battery containing the separators prepared in Examples 1 to 6 showed improved capacity retention after 1000 cycles and extended the time before thermal runaway. This indicates that during the cycling process of a sodium-ion battery, with the insertion and extraction of sodium ions, the positive and negative electrodes expand in volume, the electrolyte is continuously consumed, the internal temperature of the battery continues to rise, and the positive and negative electrodes further expand. Therefore, the separator in this application can effectively improve the cycle performance and safety performance of the battery. Because the separator in this application has a first functional layer formed on at least one side of the base film, and the material of the coating of the first functional layer includes PET and / or PMMA, under the pressure caused by high temperature and electrode expansion, PET and / or PMMA will undergo hydrolysis with the water produced by the battery side reaction. This not only absorbs water but also causes the coating to rupture, thereby releasing the electrolyte replenishment in the first functional layer. This can replenish the electrolyte, thereby improving the expansion of the electrode and enhancing the cycle performance of the battery. In addition, when the battery temperature rises, the hydrolysis products of the coating, such as alcohols, can absorb some heat, slow down the rate of thermal runaway reaction, and suppress the thermal runaway reaction of the battery, thereby effectively improving the safety performance of the battery.

[0113] A comparison of the data from Examples 1 and 3 in Table 1 shows that as the area ratio of the first and second functional layers on the surface of their respective PP films increases, the capacity retention rate after 1000 cycles increases, i.e., the cycle life of the battery increases, and the time before thermal runaway occurs is significantly prolonged, effectively delaying the occurrence of thermal runaway. This indicates that the larger the area ratio of the first and / or second functional layers on the surface of the base film containing the first and / or second functional layers, the better the effect on improving the battery's cycle performance and safety performance.

[0114] A comparison of the data from Examples 1 and 4 in Table 1 shows that, since the separator prepared in Example 4 only contains the first functional layer located on one side of the PP film and does not contain the second functional layer, the capacity retention rate of the battery after 1000 cycles is reduced compared to Example 1, i.e., the cycle life of the battery is reduced, and the time for thermal runaway is significantly shortened, i.e., the time for thermal runaway to occur in the battery is significantly earlier. This indicates that in the separator of this application, the second functional layer can work in conjunction with the first functional layer to more promptly suppress the battery's thermal runaway reaction, thereby better improving the battery's safety and cycle performance.

[0115] It should be noted that the membrane embodiments, membrane preparation method embodiments and sodium-ion battery embodiments provided in this application belong to the same concept; the technical features in the technical solutions described in each embodiment can be arbitrarily combined without conflict.

[0116] It should be understood that the above embodiments are exemplary and not intended to encompass all possible implementations. Various modifications and changes can be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments can be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only illustrate several implementations of the present invention and do not limit the scope of protection of this patent.

Claims

1. A diaphragm, characterized in that, The diaphragm includes: A base film and a first functional layer located on at least one side surface of the base film, and a second functional layer located on a surface of the base film away from the first functional layer; The first functional layer includes a coating membrane and an electrolyte replenishment solution surrounded by the coating membrane, wherein the coating membrane is made of polyethylene terephthalate and / or polymethyl methacrylate. The second functional layer includes carbonates.

2. The diaphragm according to claim 1, characterized in that, The first functional layer is located in the middle of the surface of the base film.

3. The diaphragm according to claim 2, characterized in that, The area of ​​the first functional layer on the surface of the base film where the first functional layer is located is greater than or equal to 40%.

4. The diaphragm according to claim 1, characterized in that, The second functional layer is located in the middle of the surface of the base film.

5. The diaphragm according to claim 4, characterized in that, The area of ​​the second functional layer on the surface of the base film where the second functional layer is located is greater than or equal to 40%.

6. The diaphragm according to any one of claims 1 to 5, characterized in that, The electrolyte replenishment solution includes an electrolyte and an emulsifier.

7. The diaphragm according to claim 6, characterized in that, The emulsifier includes gelatin and / or agar.

8. The diaphragm according to claim 6, characterized in that, The electrolyte includes a solute and a solvent.

9. The diaphragm according to claim 8, characterized in that, The solute includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium fluorosulfonic acid salts, and sodium fluorosulfonamide salts.

10. The diaphragm according to claim 8, characterized in that, The solvents include carbonate solvents and / or ether solvents.

11. The diaphragm according to claim 10, characterized in that, The solvent includes carbonate solvents.

12. The diaphragm according to any one of claims 1 to 5, characterized in that, The base membrane is made of polyalkane polymers.

13. The diaphragm according to claim 12, characterized in that, The base film is made of polyethylene and / or polypropylene.

14. A method for preparing a diaphragm, characterized in that, The method includes the following steps: S1: Dissolve polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, add electrolyte replenishment, and disperse evenly to obtain a mixed emulsion; S2: Evaporate the organic solvent in the mixed emulsion to allow the polyethylene terephthalate and / or polymethyl methacrylate to precipitate and form a coating film to surround the electrolyte replenishment liquid within the coating film, forming a first functional layer; S3: Adhere the first functional layer to at least one side surface of the base film; S4: Mix the carbonate and binder evenly to obtain a mixture; S5: The mixture is coated onto the surface of the base film away from the first functional layer to form a second functional layer.

15. The method for preparing the diaphragm according to claim 14, characterized in that, The mass ratio of the carbonate to the binder is (8.5-9):(1-1.5).

16. The method for preparing the diaphragm according to claim 14, characterized in that, The second functional layer is located in the middle of the surface of the base film.

17. The method for preparing the diaphragm according to claim 16, characterized in that, The area of ​​the second functional layer on the surface of the base film where the second functional layer is located is greater than or equal to 40%.

18. The method for preparing the diaphragm according to any one of claims 14 to 17, characterized in that, The method satisfies at least one of the following characteristics: (1) The organic solvent includes at least one of dichloromethane, methanol, and triethanolamine; (2) The first functional layer is located at the middle position on the surface of the base film; (3) The area of ​​the first functional layer on the surface of the base film where the first functional layer is located is greater than or equal to 40%; (4) The electrolyte replenishment solution includes an electrolyte and an emulsifier; (5) The material of the base film includes polyalkane polymers.

19. The method for preparing the diaphragm according to any one of claims 14 to 17, characterized in that, The electrolyte replenishment solution includes an electrolyte and an emulsifier; the emulsifier includes gelatin and / or agar.

20. The method for preparing the diaphragm according to any one of claims 14 to 17, characterized in that, The electrolyte replenishment solution includes an electrolyte and an emulsifier; the electrolyte includes a solute and a solvent.

21. The method for preparing the diaphragm according to claim 20, characterized in that, The solute includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium fluorosulfonic acid salts, and sodium fluorosulfonamide salts.

22. The method for preparing the diaphragm according to claim 20, characterized in that, The solvents include carbonate solvents and / or ether solvents.

23. The method for preparing the diaphragm according to claim 22, characterized in that, The solvent includes carbonate solvents.

24. The method for preparing the diaphragm according to any one of claims 14 to 17, characterized in that, The base film is made of polyethylene and / or polypropylene.

25. A sodium-ion battery, characterized in that, The membrane includes the membrane according to any one of claims 1 to 13 or the membrane prepared by the preparation method according to any one of claims 14 to 24.

26. The sodium-ion battery according to claim 25, characterized in that, The sodium-ion battery further includes a positive electrode, a negative electrode, and a battery electrolyte, with the first functional layer of the separator disposed close to the negative electrode.

27. The sodium-ion battery according to claim 26, characterized in that, The battery electrolyte is the same as the electrolyte in the electrolyte replenishment solution.

Images