Diaphragm, preparation method thereof and sodium ion battery

Abstract

The embodiment of the invention relates to a diaphragm, a preparation method thereof and a sodium ion battery. The diaphragm comprises a base membrane and a first functional layer located on at least one side surface of the base membrane, the first functional layer comprises a coating film and electrolyte replenishing liquid surrounded by the coating film, and the material of the coating film comprises polyethylene glycol terephthalate and / or polymethyl methacrylate. Therefore, the effect of supplementing the electrolyte can be achieved, so that the expansion of the pole piece is improved, and the cycle performance and the safety performance of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a diaphragm and a preparation method thereof, and a sodium ion battery. Background Art

[0002] Currently, the most widely used secondary battery is the lithium-ion battery. However, the lithium content in the earth's crust is low and unevenly distributed. Therefore, the acquisition of lithium resources has become one of the factors limiting the development of lithium batteries.

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

[0004] Since the radius of sodium ions is larger than that of lithium ions, during the battery cycle, the volume expansion of the electrode is more significant during the embedding and de-embedding of sodium ions in the positive and negative electrodes. The volume expansion of the electrode will cause the electrolyte to be continuously consumed. As the cycle continues, the lack of electrolyte will cause the electrode to further expand and the internal temperature of the battery to continue to rise. In addition, sodium-ion batteries are prone to produce moisture during the cycle process, which seriously threatens the cycle performance and safety performance of sodium-ion batteries. Summary of the invention

[0005] In view of this, the embodiments of the present application provide a diaphragm and a method for preparing the same, as well as a sodium ion battery to solve at least one problem existing in the background technology.

[0006] In a first aspect, an embodiment of the present application provides 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 film and an electrolyte supplement solution surrounded by the coating film, and the material of the coating film includes polyethylene terephthalate and / or polymethyl methacrylate.

[0009] In combination with the first aspect of the present application, in an optional embodiment, the first functional layer is located in the middle of 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 accounts for greater than or equal to 40%.

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

[0011] The second functional layer is located on the surface of the base film away from the first functional layer, and the second functional layer includes carbonate; optionally, the second functional layer is located in the middle of 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 accounts for greater than or equal to 40%.

[0012] In combination with the first aspect of the present application, in an optional embodiment, the electrolyte replenisher 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 salts of fluorosulfonic acids, and sodium salts of fluorosulfonyl imides; optionally, the solvent includes a carbonate solvent and / or an ether solvent; further optionally, the solvent includes a carbonate solvent.

[0013] In combination with the first aspect of the present application, in an optional embodiment, the material of the base film includes a polyalkane polymer; optionally, the material of the base film includes polyethylene and / or polypropylene.

[0014] In a second aspect, an embodiment of the present application provides a method for preparing a diaphragm, the method comprising the following steps:

[0015] S1: dissolving polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, adding an electrolyte supplement, and dispersing the mixture evenly to obtain a mixed emulsion;

[0016] S2: evaporating the organic solvent in the mixed emulsion, so that the polyethylene terephthalate and / or polymethyl methacrylate precipitates to form a coating film, so as to surround the electrolyte replenishing solution in the coating film to form a first functional layer;

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

[0018] In conjunction with the second aspect of the present application, in an optional implementation manner, the method further includes:

[0019] Mixing the carbonate and the binder uniformly 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 on 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 in the middle of 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 accounts for greater than or equal to 40%.

[0021] In conjunction with the second aspect of the present application, in an optional implementation manner, 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 accounts for greater than or equal to 40%;

[0025] (4) The electrolyte replenisher comprises an electrolyte and an emulsifier; optionally, the emulsifier comprises gelatin and / or agar; optionally, the electrolyte comprises a solute and a solvent; optionally, the solute comprises at least one of sodium hexafluorophosphate, sodium perchlorate, sodium salt of fluorosulfonic acid, and sodium salt of fluorosulfonimide; optionally, the solvent comprises a carbonate solvent and / or an ether solvent; further optionally, the solvent comprises a carbonate solvent;

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

[0027] In a third aspect, an embodiment of the present application provides a sodium ion battery, comprising the diaphragm described in any one of the first aspect or the diaphragm prepared by the preparation method described in any one of the second aspect.

[0028] In combination with the third aspect of the present application, in an optional embodiment, the sodium ion battery also includes a positive electrode sheet, a negative electrode sheet and a battery electrolyte, and the first functional layer of the diaphragm is arranged close to the negative electrode sheet; optionally, the battery electrolyte is the same as the electrolyte in the electrolyte replenisher.

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

[0030] The diaphragm and its preparation method and sodium ion battery provided in the embodiments of the present application include: a base film and a first functional layer located on at least one side surface of the base film; the first functional layer includes a coating film and an electrolyte replenisher surrounded by the coating film, and the material of the coating film includes polyethylene terephthalate (PET) and / or polymethyl methacrylate (PMMA). In an embodiment of the present application, the material of the coating film of the first functional layer includes PET and / or PMMA. During the cycle of the sodium ion battery, as sodium ions are embedded and de-embedded, the positive and negative electrodes will expand in volume, the electrolyte will be continuously consumed, the temperature inside the battery will continue to rise, and the positive and negative electrodes will further expand. Under the action of high temperature and pressure caused by the expansion of the electrodes, PET and / or PMMA will undergo a hydrolysis reaction with the water produced by the side reactions of the battery, which can not only absorb moisture but also cause the coating film to rupture, thereby releasing the electrolyte replenisher in the first functional layer, thereby replenishing the electrolyte, thereby improving the expansion of the electrodes and improving the cycle performance of the battery, and the hydrolysis product of the coating film can absorb heat to inhibit the thermal runaway reaction of the battery, thereby effectively improving the safety performance of the battery.

[0031] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0033] Figure 1 A schematic flow chart of a method for preparing a diaphragm provided in an embodiment of the present application. DETAILED DESCRIPTION

[0034] In order to make the technical scheme and beneficial effects of the present invention more obvious and easy to understand, the following is described in detail by combining the accompanying drawings and listing specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples without specifying specific conditions are usually based on conventional experimental conditions. The reagents and raw materials used in the present invention are commercially available unless otherwise specified.

[0035] In the following description, a large number of specific details are given to provide a more thorough understanding of the present application. However, it is obvious to those skilled in the art that the present application can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present application, some technical features known in the art are not described; that is, all features of the actual embodiments are not described here, and well-known functions and steps are not described in detail.

[0036] The purpose of the terms used herein is only to describe specific embodiments and is not intended to be limiting of the present application. When used herein, the singular forms "one", "an" and "said / the" are also intended to include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "consisting of" and / or "comprising", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0037] In order to thoroughly understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solution of the present application. The preferred embodiments of the present application are described in detail below, but in addition to these detailed descriptions, the present application may also have other implementation methods.

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

[0039] If no specific techniques or conditions are specified in the following examples, the conventional techniques or conditions described in the literature in the art, or the conditions recommended by the product instructions and the manufacturer are generally used. The numerical ranges in the following examples all include the endpoint values.

[0040] An embodiment of the present application provides a diaphragm, which includes: a base film and a first functional layer located on at least one side surface of the base film; the first functional layer includes a coating film and an electrolyte replenisher surrounded by the coating film, and the material of the coating film includes polyethylene terephthalate and / or polymethyl methacrylate.

[0041] In the embodiment of the present application, the material of the coating film in the first functional layer includes PET and / or PMMA. In the initial state of the battery, there is no water inside. Even if some water is produced by the side reaction of the battery at room temperature, PET and / or PMMA will not undergo hydrolysis reaction when in contact with water. During the cycle of the sodium ion battery, as the sodium ions are embedded and de-embedded, the positive and negative electrodes will expand in volume, the electrolyte will be continuously consumed, the temperature inside the battery will continue to rise, and the positive and negative electrodes will further expand. Under the action of high temperature and the pressure caused by the expansion of the electrode, PET and / or PMMA will undergo a hydrolysis reaction with the water produced by the battery side reaction, which can not only absorb water but also cause the coating film to rupture, thereby releasing the electrolyte replenisher in the first functional layer, so that it can play a role in replenishing the electrolyte, thereby improving the expansion of the electrode and improving the cycle performance of the battery. Moreover, the hydrolysis products of PET and / or PMMA include alcohols, such as the hydrolysis products of PET are polyethylene terephthalate and ethylene glycol, and the hydrolysis products of PMMA are polymethyl methacrylate and methanol. The alcohol products can absorb heat and inhibit the thermal runaway reaction of the battery, thereby effectively improving the safety performance of the battery. In addition, the alcohol products can also play a role in improving the fluidity of the electrolyte, thereby reducing the impedance inside the battery and increasing the output power of the battery.

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

[0043] In some embodiments, the material of the base film may include polyalkane polymers. Such polymers are chemically stable and will not be affected by alkaline conditions and water, and can better ensure the insulation performance of the diaphragm. Further, the material of the base film may include polyethylene (PE) and / or polypropylene (PP).

[0044] Since the volume expansion of the pole piece (also known as swelling) mainly occurs in the middle part of the pole piece during the battery cycle, the pressure generated by the volume expansion of the pole piece on the diaphragm mainly acts on the middle position of the diaphragm. In some specific embodiments, the first functional layer can be located in the middle position of the base film surface. The middle position here can be understood as a partial area extending from the center point of the base film surface to the surrounding areas of the base film surface, and the center point of this area coincides with the center point of the base film surface. In this way, when the volume of the pole piece expands to a certain extent, the pressure caused by the volume expansion of the pole piece can be quickly and accurately applied to the first functional layer of the diaphragm, thereby promoting faster rupture of the coating film and timely release of the electrolyte supplement, thereby better improving the cycle performance and safety performance of the battery.

[0045] When the area of ​​the first functional layer on the surface of the base film where the first functional layer is located is relatively small, the role of the first functional layer in supplementing the electrolyte and inhibiting the thermal runaway reaction of the battery is relatively limited. Therefore, in some specific embodiments, the area of ​​the first functional layer on the surface of the base film where the first functional layer is located may be greater than or equal to 40%. In this way, the cycle performance and safety performance of the battery can be better improved through the first functional layer. Further optionally, the area of ​​the first functional layer on the surface of the base film where the first functional layer is located may be greater than or equal to 60%. In a specific embodiment, the area of ​​the first functional layer on the surface of the base film where the first functional layer is located may be 100%.

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

[0047] When the battery is cycled until the volume of the pole piece expands to a certain extent and the temperature rises to a certain extent, under the high temperature and pressure of the pole piece volume expansion, the coating film of the first functional layer in the embodiment of the present application undergoes a hydrolysis reaction with the water produced by the battery side reaction, resulting in the destruction of the coating film, causing the electrolyte wrapped inside it to leak out, thereby playing a role in replenishing the electrolyte and inhibiting the thermal runaway reaction of the battery. The electrolyte replenisher includes not only the electrolyte but also an emulsifier. The emulsifying and suspending properties of the emulsifier are used to form a uniform emulsion of the electrolyte replenisher, reducing the fluidity of the electrolyte, so that the electrolyte replenisher can be better and more stably enclosed in the coating film.

[0048] Exemplarily, 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 salt of fluorosulfonic acid, and sodium salt of fluorosulfonimide; and the solvent may include a carbonate solvent and / or an ether solvent.

[0049] In a specific embodiment, the solvent may include a carbonate solvent. Because when the internal temperature of the battery is too high, the carbonate solvent will generate carbon dioxide under high temperature conditions, which can better prevent the occurrence of thermal runaway of the battery. Exemplarily, the carbonate solvent may include ethylene carbonate.

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

[0051] In the embodiment of the present application, the carbonate in the second functional layer can be dissolved in the water produced by the side reaction of the battery, and the carbonate ions are hydrolyzed to produce hydroxide ions, thereby creating an alkaline environment, which can further promote the hydrolysis of the coating film (PET and / or PMMA) in the first functional layer, thereby more promptly suppressing the thermal runaway reaction of the battery and better improving the safety performance and cycle performance of the battery.

[0052] In the embodiment of the present application, the first functional layer and the second functional layer may 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 may at least partially overlap. This allows the hydroxide ions generated by the hydrolysis of the second functional layer to be transmitted to the first functional layer side more quickly, thereby playing a role in promoting the hydrolysis of the coating film more quickly. Furthermore, the vertical projections of the first functional layer and the second functional layer in the thickness direction of the base film may overlap.

[0053] In some specific embodiments, the second functional layer may be located in the middle of the surface of the base film. The middle position here can be understood with reference to the middle position in the above embodiment. This makes it easier for the hydroxide ions generated by hydrolysis to be quickly transferred to the first functional layer side, thereby playing a faster and better role in promoting the hydrolysis of the coating film.

[0054] When the area of ​​the second functional layer on the surface of the base film where the second functional layer is located is relatively small, the role that the second functional layer can play is relatively limited. Therefore, in some specific embodiments, the area of ​​the second functional layer on the surface of the base film where the second functional layer is located can be greater than or equal to 40%. If the second functional layer can better promote the hydrolysis of the coating film 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 where the second functional layer is located can be greater than or equal to 60%. In a specific embodiment, the area of ​​the second functional layer on the surface of the base film where the second functional layer is located can be 100%.

[0055] In some embodiments, the second functional layer may further include a binder. The carbonate may be better bonded to the base film through the binder, thereby improving the stability of the connection between the second functional layer and the base film. The type of the binder may be selected according to actual needs and is not limited here.

[0056] The present application also provides a method for preparing a diaphragm. Figure 1 The present invention provides a method for preparing a diaphragm, comprising the following steps:

[0057] S1: dissolving polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, adding an electrolyte supplement, and dispersing the mixture evenly to obtain a mixed emulsion;

[0058] S2: evaporating the organic solvent in the mixed emulsion, so that polyethylene terephthalate and / or polymethyl methacrylate are precipitated to form a coating film, so as to surround the electrolyte supplement solution in the coating film to form a first functional layer;

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

[0060] In the embodiment of the present application, the organic solvent in the mixed emulsion is evaporated by the solvent evaporation method, so that PET and / or PMMA are precipitated to form a coating film to surround the electrolyte replenisher to form a first functional layer, and then the first functional layer is bonded to at least one side surface of the base film to obtain a diaphragm. The preparation method is simple and easy; moreover, the material of the coating film in the first functional layer includes PET and / or PMMA. During the cycle of the sodium ion battery, under the action of high temperature and pressure caused by the expansion of the pole piece, PET and / or PMMA will undergo a hydrolysis reaction with the water generated by the battery side reaction, which can not only absorb moisture but also cause the coating film to rupture, thereby releasing the electrolyte replenisher in the first functional layer, so that the electrolyte can be replenished, thereby improving the expansion of the pole piece and improving the cycle performance of the battery. Moreover, the hydrolysis products (including alcohols) of PET and / or PMMA can absorb heat to inhibit 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 an electrolyte and an emulsifier, stirring and dispersing the mixture evenly to obtain a mixed emulsion.

[0062] In the implementation of the present application, the electrolyte supplement 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] Exemplarily, 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 salt of fluorosulfonic acid, and sodium salt of fluorosulfonyl imide; the solvent may include a carbonate solvent and / or an ether solvent. Further optionally, the solvent may include a carbonate solvent, such as ethylene carbonate, because carbonate solvents generate carbon dioxide under high temperature conditions, which can better prevent the occurrence of thermal runaway of the battery.

[0064] It should be noted that there is no particular restriction on the amount of the organic solvent and the emulsifier used in the embodiment of the present application, and the first functional layer can be prepared by the solvent evaporation method in the subsequent step S2.

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

[0066] In step S3, the first functional layer may be bonded to at least one surface of the base film by a bonding agent, such as polyvinylidene fluoride (PVDF).

[0067] In some specific embodiments, the material of the base film may include polyalkane polymers; such polymers are chemically stable and will not be affected by alkaline conditions and water, and can better ensure the insulation performance of the diaphragm. Further, the material of the base film 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 here can be understood as a partial area extending from the center point of the base film surface to the surrounding areas of the base film surface, and the center point of this area coincides with the center point of the base film surface. In this way, when the volume of the pole piece expands to a certain extent, the pressure caused by the volume expansion of the pole piece can be quickly and accurately applied to the first functional layer of the diaphragm, thereby promoting faster rupture of the coating film, and timely releasing the electrolyte supplement, thereby better improving the cycle performance and safety performance of the battery.

[0069] In the diaphragm prepared in the embodiment of the present application, the area ratio of the first functional layer on the surface of the base film where the first functional layer is located can be greater than or equal to 40%. If the first functional layer can be used to better improve the cycle performance and safety performance of the battery. Further optionally, the area ratio of the first functional layer on the surface of the base film 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 surface of the base film where the first functional layer is located can be 100%.

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

[0071] In the embodiment of the present application, the carbonate in the second functional layer can dissolve in the water produced by the side reaction of the battery, and the carbonate ions are hydrolyzed to produce hydroxide ions, thereby creating an alkaline environment, which can further promote the hydrolysis of the coating film in the first functional layer, thereby more promptly suppressing the thermal runaway reaction of the battery, and better improving the safety performance and cycle performance of the battery.

[0072] In a specific embodiment, the mass ratio of carbonate to binder can be (8.5-9): (1-1.5). In this way, the stability of the connection between the second functional layer and the base film can be taken into consideration while ensuring that the second functional layer can fully play its role.

[0073] In the actual preparation process, for example, carbonate and binder can be added to a proper amount of solvent (such as N-methylpyrrolidone) according to a set ratio, mixed evenly, and then the mixture is coated on the surface of the base film to form the second functional layer.

[0074] In some embodiments, the second functional layer may be located in the middle of the surface of the base film. The middle position here can be understood with reference to the middle position in the above embodiment. This makes it easier for the hydroxide ions generated by hydrolysis to be quickly transferred to the first functional layer side, thereby playing a faster and better role in promoting the hydrolysis of the coating film.

[0075] In some embodiments, the area of ​​the second functional layer on the surface of the base film where the second functional layer is located may be greater than or equal to 40%. If the second functional layer can better promote the hydrolysis of the coating film 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 where the second functional layer is located may be greater than or equal to 60%. In a specific embodiment, the area of ​​the second functional layer on the surface of the base film where the second functional layer is located may be 100%.

[0076] An embodiment of the present application also provides a sodium ion battery, comprising the diaphragm described in any of the above embodiments or the diaphragm prepared by the preparation method described in any of the above embodiments.

[0077] It should be understood that the beneficial effects of the diaphragm in the above embodiment are all applicable to the sodium ion battery. Since the diaphragm in the above embodiment includes a base film and a first functional layer located on at least one side surface of the base film, the electrolyte supplement in the first functional layer can play a role in supplementing the electrolyte, and the hydrolysis product of the coating film in the first functional layer can absorb heat to inhibit the thermal runaway reaction of the battery. Therefore, the sodium ion battery in the embodiment of the present application has higher cycle performance and safety performance.

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

[0079] In an embodiment of the present application, the coating film of the first functional layer in the diaphragm may include PET. The hydrolysis product of PET contains 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 the sodium ion battery. Therefore, arranging the first functional layer of the diaphragm close to the negative electrode sheet can better improve the performance of the battery.

[0080] In some embodiments, the battery electrolyte and the electrolyte in the electrolyte supplement can be the same, so that when the electrolyte supplement in the first functional layer is released, it can be more compatible with the electrolyte of the sodium ion battery itself, thereby better improving the cycle performance of the battery.

[0081] The technical solution of the present application is further described below in combination with a plurality of 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 is heated and melted to form a viscous solution, and then PET particles are obtained by plasticizing and extruding. 2g of PET particles are weighed and dissolved in 20ml of dichloromethane (organic solvent), and then 5ml of electrolyte and 10ml of gelatin solution (emulsifier) ​​with a mass fraction of 3.5% are added, and stirred and dispersed for 30 minutes to obtain a mixed emulsion. The preparation method of the electrolyte is: the solute sodium hexafluorophosphate is dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1 to form an electrolyte with a sodium salt concentration of 1 mol / L.

[0085] Step S201: evaporating the dichloromethane in the mixed emulsion prepared in step S101 to precipitate PET to form a coating film, and coating the electrolyte replenisher composed of the electrolyte and the gelatin solution to form a first functional layer.

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

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

[0088] In this embodiment, the first functional layer and the second functional layer are located on two opposite surfaces of the PP film, and the area 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 embodiment 1, except that:

[0091] The solute of the electrolyte in step S101 is sodium perchlorate.

[0092] Example 3

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

[0094] The first functional layer and the second functional layer only cover the middle part of the surface of the PP film, and the area of ​​the first functional layer and the second functional layer on the surface of the PP film accounts for 40%.

[0095] Example 4

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

[0097] The step S401 in Example 1 is not included, that is, the diaphragm prepared in this example only includes the first functional layer located on one surface of the PP film, and does not include the second functional layer.

[0098] Example 5

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

[0100] The organic solvent in step S101 is methanol.

[0101] Example 6

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

[0103] The PET in step S101 is replaced by PMMA.

[0104] Comparative Example 1

[0105] The separator in this comparative example is a PP film which is the same as the base film in Example 1.

[0106] The diaphragms prepared in the above embodiments and the diaphragm in comparative example 1 were made into sodium ion batteries, and the performance of the batteries was tested. The battery preparation method is: the positive electrode material sodium vanadium phosphate is mixed with conductive carbon black and binder PVDF in a mass ratio of 96:2:2, NMP is added to make a uniform slurry and coated on aluminum foil, and after drying and rolling, a positive electrode sheet of a sodium ion battery is obtained, the diaphragm prepared in the above embodiments and the diaphragm in comparative example 1 are used as the diaphragm, the electrolyte is the same as the electrolyte in the above embodiment 1, the negative electrode is hard carbon, and the soft-pack battery is assembled to obtain the battery to be tested.

[0107] (1) Cycle performance test: The assembled battery was charged to 4V at 0.1C constant current and constant voltage at a constant temperature of 25°C, with a cut-off current of 0.05C; then discharged to 2V at 0.33C to obtain the discharge capacity of the first cycle. The charge and discharge cycle was continued according to the above steps for 1000 cycles to obtain the discharge capacity of the 1000th cycle. The capacity retention rate = (discharge capacity of the 1000th cycle / discharge capacity of the first cycle)*100%.

[0108] (2) Safety performance test: The battery is subjected to a thermal runaway test in a safety laboratory using a 1000W heating plate, and the battery status is observed. When the battery status satisfies both conditions A and C or both conditions B and C, the battery is judged to have thermal runaway, and the time when the battery has thermal runaway is recorded, where condition A is: the battery voltage drops by more than 25% of the initial voltage; condition B is: the temperature at the monitoring point reaches above 60°C; condition C is: the heating rate at the monitoring point is greater than or equal to 1°C / s and lasts for more than 3 seconds. It should be noted that the wound battery has a bent surface area and a flat surface area (also called a large surface), and the monitoring point is located in the middle of the flat surface area.

[0109] The test results of the above tests are shown in Table 1.

[0110] Table 1

[0111] Group 1000th cycle capacity retention rate (%) Time for battery thermal runaway (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] It can be seen from the data in Table 1 that, compared with the battery containing the diaphragm of Comparative Example 1, the 1000th cycle capacity retention rate of the battery containing the diaphragm prepared in Examples 1 to 6 is improved, and the time for the battery to experience thermal runaway is extended. This shows that during the cycle of the sodium ion battery, with the insertion and deinsertion of sodium ions, the volume expansion of the positive and negative electrodes, the continuous consumption of the electrolyte, the continuous increase in the temperature inside the battery, and the further expansion of the positive and negative electrodes, the diaphragm in the present application can effectively improve the cycle performance and safety performance of the battery. Because, the diaphragm in the present application is formed by setting a first functional layer on at least one side surface of the base film. Since the material of the coating film of the first functional layer includes PET and / or PMMA, under the action of high temperature and pressure caused by the expansion of the pole piece, PET and / or PMMA will undergo hydrolysis reaction with the water produced by the side reaction of the battery, which can not only absorb moisture but also cause the coating film to rupture, thereby releasing the electrolyte replenisher in the first functional layer, which can replenish the electrolyte, thereby improving the expansion of the pole piece and improving the cycle performance of the battery. In addition, when the temperature of the battery rises, the hydrolysis products of the coating film, such as alcohols, can absorb part of the heat, slow down the speed of the thermal runaway reaction, and play a role in inhibiting the thermal runaway reaction of the battery, thereby effectively improving the safety performance of the battery.

[0113] From the data comparison of Example 1 and Example 3 in Table 1, it can be seen that as the area ratio of the first functional layer and the second functional layer on the surface of the PP film increases, the 1000th cycle capacity retention rate of the battery increases, that is, the cycle life of the battery increases, and the time for the battery to experience thermal runaway is significantly extended, that is, the occurrence of thermal runaway of the battery is effectively delayed. This shows that the larger the area ratio of the first functional layer and / or the second functional layer on the surface of the base film where the first functional layer and / or the second functional layer is located, the better the effect of improving the battery cycle performance and safety performance.

[0114] From the data comparison of Example 1 and Example 4 in Table 1, it can be seen that since the diaphragm prepared in Example 4 only includes the first functional layer located on one side of the PP film, and does not include the second functional layer, the 1000th cycle capacity retention rate of the battery is reduced compared with Example 1, that is, the cycle life of the battery is reduced, and the time when the battery undergoes thermal runaway is significantly shortened, that is, the time when the battery undergoes thermal runaway is significantly advanced. This shows that in the diaphragm of the present application, the second functional layer can cooperate with the first functional layer to achieve more timely suppression of the battery thermal runaway reaction and better improve the safety performance and cycle performance of the battery.

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

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

Claims

1. A diaphragm, characterized in that: The diaphragm comprises: A base film and a first functional layer located on at least one side surface of the base film; The first functional layer includes a coating film and an electrolyte supplement solution surrounded by the coating film, and the material of the coating film includes polyethylene terephthalate and / or polymethyl methacrylate.

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; optionally, the area of ​​the first functional layer on the surface of the base film where the first functional layer is located accounts for greater than or equal to 40%.

3. The diaphragm according to claim 1, characterized in that The diaphragm further comprises: The second functional layer is located on the surface of the base film away from the first functional layer, and the second functional layer includes carbonate; optionally, the second functional layer is located in the middle of 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 accounts for greater than or equal to 40%.

4. The diaphragm according to any one of claims 1 to 3, characterized in that: The electrolyte replenisher 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 salts of fluorosulfonic acids, and sodium salts of fluorosulfonyl imides; optionally, the solvent includes a carbonate solvent and / or an ether solvent; further optionally, the solvent includes a carbonate solvent.

5. The diaphragm according to any one of claims 1 to 3, characterized in that: The material of the base film includes polyalkane polymers; optionally, the material of the base film includes polyethylene and / or polypropylene.

6. A method for preparing a diaphragm, characterized in that: The method comprises the following steps: S1: dissolving polyethylene terephthalate and / or polymethyl methacrylate in an organic solvent, adding an electrolyte supplement, and dispersing the mixture evenly to obtain a mixed emulsion; S2: evaporating the organic solvent in the mixed emulsion, so that the polyethylene terephthalate and / or polymethyl methacrylate precipitates to form a coating film, so as to surround the electrolyte replenishing solution in the coating film to form a first functional layer; S3: bonding the first functional layer to at least one surface of the base film.

7. The method for preparing a diaphragm according to claim 6, characterized in that: The method further comprises: Mixing the carbonate and the binder uniformly to obtain a mixture; optionally, the mass ratio of the carbonate to the binder is (8.5-9): (1-1.5); The mixture is coated on 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 in the middle of 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 accounts for greater than or equal to 40%.

8. The method for preparing a diaphragm according to claim 6 or 7, characterized in that: The method satisfies at least one of the following features: (1) The organic solvent comprises at least one of dichloromethane, methanol and triethanolamine; (2) The first functional layer is located in the middle of 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 accounts for greater than or equal to 40%; (4) The electrolyte replenisher comprises an electrolyte and an emulsifier; optionally, the emulsifier comprises gelatin and / or agar; optionally, the electrolyte comprises a solute and a solvent; optionally, the solute comprises at least one of sodium hexafluorophosphate, sodium perchlorate, sodium salt of fluorosulfonic acid, and sodium salt of fluorosulfonimide; optionally, the solvent comprises a carbonate solvent and / or an ether solvent; further optionally, the solvent comprises a carbonate solvent; (5) The material of the base film includes polyalkane polymers; optionally, the material of the base film includes polyethylene and / or polypropylene.

9. A sodium ion battery, characterized in that: The invention comprises the diaphragm according to any one of claims 1 to 5 or the diaphragm prepared by the preparation method according to any one of claims 6 to 8.

10. The sodium ion battery according to claim 9, characterized in that: The sodium ion battery also includes a positive electrode sheet, a negative electrode sheet and a battery electrolyte, and the first functional layer of the diaphragm is arranged close to the negative electrode sheet; optionally, the battery electrolyte is the same as the electrolyte in the electrolyte replenisher.

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