Composite diaphragm, preparation method thereof and secondary battery

Abstract

The invention relates to a composite diaphragm, a preparation method thereof and a secondary battery. The composite diaphragm comprises a base membrane and a first coating arranged on one side surface of the base membrane in the thickness direction, the first coating comprises a modified MOF material, the modified MOF material comprises a first MOF material and a coating layer coating the first MOF material, and the coating layer comprises a functional group capable of generating electrostatic interaction and / or coordination interaction with active ions. Therefore, the cycle performance of the battery can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a composite separator, its preparation method, and a secondary battery. Background Technology

[0002] Lithium-ion batteries, as a mature electrochemical energy storage system, have been widely used in consumer electronics due to their excellent cycle stability and safety performance. However, with the increasing demands for energy density and battery life, traditional lithium-ion battery systems are struggling to meet the needs of emerging application scenarios. Lithium metal battery systems, which use lithium metal anodes (theoretical capacity up to 3860 mAh / g) matched with high-nickel cathode materials, have become a research hotspot due to their significantly improved theoretical energy density.

[0003] However, in lithium metal battery systems, side reactions occur between the highly active lithium metal anode and the liquid organic electrolyte, and the high-nickel cathode material itself has structural instability, making it prone to the precipitation of transition metal ions. These problems can easily lead to uneven lithium deposition on the battery anode, which in turn can cause lithium dendrite growth and the formation of "dead lithium," ultimately affecting the battery's cycle performance.

[0004] As a key component of batteries, the separator not only achieves physical isolation between the positive and negative electrodes, but also plays multiple roles such as regulating ion transport, providing mechanical support, and enhancing thermal stability. Therefore, developing novel separators is expected to be an important way to improve the cycle performance of rechargeable batteries. Summary of the Invention

[0005] In view of this, the present application provides a composite separator, a method for preparing the same, and a secondary battery to solve at least one problem existing in the prior art.

[0006] In a first aspect, embodiments of this application provide a composite membrane, including a base membrane and a first coating disposed on one side surface of the base membrane along the thickness direction;

[0007] The first coating includes a modified MOF material, which includes a first MOF material and a coating layer covering the first MOF material. The coating layer includes functional groups capable of generating electrostatic interactions and / or coordination interactions with active ions.

[0008] In conjunction with the first aspect of this application, in an alternative embodiment, the first coating satisfies at least one of the following features:

[0009] (1) The first MOF material includes at least one of ZIF-8, UIO-66, and MIL-100;

[0010] (2) The specific surface area of ​​the first MOF material is 1000 m². 2 / g or more;

[0011] (3) The porosity of the first MOF material is 50% to 80%;

[0012] (4) The coating layer comprises a polymer; optionally, the polymer comprises at least one of polydopamine, polyaniline, polystyrene sulfonic acid, polymethacrylic acid, and polymaleic anhydride;

[0013] (5) The functional group includes at least one of hydroxyl, carboxyl, amino, and sulfonic acid groups;

[0014] (6) The thickness of the coating layer is 1 μm to 3 μm;

[0015] (7) The thickness of the first coating is 2μm to 5μm;

[0016] (8) The first coating further includes first ceramic particles; optionally, the mass ratio of the modified MOF material to the first ceramic particles is 1:9 to 9:1; optionally, the first ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0017] In conjunction with the first aspect of this application, in an optional embodiment, the composite membrane further includes a second coating located on a surface of the base membrane remote from the first coating; the second coating includes a second MOF material.

[0018] In conjunction with the first aspect of this application, in an alternative embodiment, the second coating satisfies at least one of the following characteristics:

[0019] (1) The second MOF material includes at least one of ZIF-8, UIO-66, and MIL-100;

[0020] (2) The specific surface area of ​​the second MOF material is 1000 m². 2 / g or more;

[0021] (3) The porosity of the second MOF material is 50% to 80%;

[0022] (4) The thickness of the second coating is 2μm to 5μm;

[0023] (5) The second coating further includes second ceramic particles; optionally, the mass ratio of the second MOF material to the second ceramic particles is 1:9 to 9:1; optionally, the second ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0024] Secondly, embodiments of this application provide a method for preparing a composite separator, the method comprising the following steps:

[0025] S1: The modified MOF material and the first binder are added to the first solvent and mixed to obtain the first slurry; the modified MOF material includes the first MOF material and a coating layer covering the first MOF material, the coating layer including functional groups that can generate electrostatic interactions and / or coordination interactions with active ions;

[0026] S2: The first slurry is coated on one side surface of the base film along the thickness direction, and after drying, a first coating is formed.

[0027] In conjunction with the second aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features:

[0028] (1) The first adhesive includes at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and polyacrylic acid;

[0029] (2) The first solvent includes deionized water;

[0030] (3) The thickness of the first coating is 2μm to 5μm;

[0031] (4) In the process of preparing the first slurry, first ceramic particles are also added; optionally, the mass ratio of the modified MOF material to the first ceramic particles is 1:9 to 9:1; optionally, the first ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0032] In conjunction with a second aspect of this application, in an optional embodiment, the method for preparing the modified MOF material includes: coating the first MOF material to form the coating layer, thereby obtaining the modified MOF material; the method satisfies at least one of the following characteristics:

[0033] (1) The first MOF material includes at least one of ZIF-8, UIO-66, and MIL-100;

[0034] (2) The specific surface area of ​​the first MOF material is 1000 m². 2 / g or more;

[0035] (3) The porosity of the first MOF material is 50% to 80%;

[0036] (4) The coating layer comprises a polymer, and optionally, the polymer comprises at least one of polydopamine, polyaniline, polystyrene sulfonic acid, polymethacrylic acid, and polymaleic anhydride;

[0037] (5) The functional group includes at least one of hydroxyl, carboxyl, amino, and sulfonic acid groups;

[0038] (6) The thickness of the coating layer is 1 μm to 3 μm;

[0039] (7) The coating process is carried out by liquid phase coating.

[0040] In conjunction with a second aspect of this application, in an optional embodiment, the method further includes:

[0041] The second MOF material and the second binder are added to the second solvent and mixed to obtain the second slurry;

[0042] The second slurry is applied to the surface of the base film away from the first coating, and after drying, a second coating is formed.

[0043] In conjunction with the second aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features:

[0044] (1) The second MOF material includes at least one of ZIF-8, UIO-66, and MIL-100;

[0045] (2) The specific surface area of ​​the second MOF material is 1000 m². 2 / g or more;

[0046] (3) The porosity of the second MOF material is 50% to 80%;

[0047] (4) The second adhesive includes at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and polyacrylic acid;

[0048] (5) The second solvent includes deionized water;

[0049] (6) The thickness of the second coating is 2μm to 5μm;

[0050] (7) In the process of preparing the second slurry, second ceramic particles are also added; optionally, the mass ratio of the second MOF material to the second ceramic particles is 1:9 to 9:1; optionally, the second ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0051] Thirdly, embodiments of this application provide a secondary battery, including a positive electrode, a negative electrode, and a composite separator located between the positive electrode and the negative electrode. The composite separator is the composite separator described in any of the first aspects or a composite separator prepared by any of the methods described in the second aspect; wherein the first coating of the composite separator is disposed facing the negative electrode.

[0052] Compared with the prior art, the embodiments of this application have the following beneficial effects:

[0053] The composite separator provided in this application includes a base film and a first coating disposed on one side surface of the base film along its thickness direction. The first coating includes a modified MOF material, which comprises a first MOF material and a coating layer covering the first MOF material. The coating layer includes functional groups capable of generating electrostatic interactions and / or coordination interactions with active ions (such as lithium ions or sodium ions). Thus, by aligning the first coating of the composite separator towards the negative electrode, the coating layer in the modified MOF material can adjust the lithium-ion solvation structure, reduce the barrier to active ion desolvation, and increase the active ion flux at the negative electrode interface. This facilitates the uniform deposition of active ions at the negative electrode interface, inhibits the growth of lithium dendrites or sodium dendrites, and the first MOF material itself has a porous structure, which can promote electrolyte wetting and active ion transport, thereby effectively improving the cycle performance of the battery.

[0054] 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

[0055] 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:

[0056] Figure 1 This is a schematic flowchart illustrating a method for preparing a composite diaphragm according to an embodiment of this application. Detailed Implementation

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] This application provides a composite membrane, which includes a base membrane and a first coating disposed on one side surface of the base membrane along the thickness direction; the first coating includes a modified MOF material, the modified MOF material includes a first MOF material and a coating layer covering the first MOF material, the coating layer including functional groups capable of generating electrostatic interactions and / or coordination interactions with active ions.

[0064] In this embodiment, the coating layer in the modified MOF material includes functional groups capable of generating electrostatic and / or coordination interactions with active ions. The first coating of the composite separator is positioned towards the negative electrode, thereby adjusting the lithium-ion solvation structure through the coating layer, reducing the barrier to active ion desolvation, and increasing the flux of active ions at the negative electrode interface. This facilitates the uniform deposition of active ions at the negative electrode interface, inhibits the growth of lithium dendrites or sodium dendrites, and the first MOF material itself has a porous structure, which can promote electrolyte wetting and active ion transport, thereby effectively improving the cycle performance of the battery.

[0065] The embodiments of this application do not have special requirements for the base film; any base film type commonly used in the art can be used. For example, the base film may include polypropylene (PP) film, polyethylene (PE) film, etc.

[0066] In some embodiments, the first MOF material may include at least one of ZIF-8, UIO-66, and MIL-100.

[0067] In some embodiments, the specific surface area of ​​the first MOF material can be above 1000 m² / g.

[0068] In some embodiments, the porosity of the first MOF material is 50% to 80%.

[0069] The first MOF material has a high specific surface area and abundant pore structure, which is beneficial for better promoting electrolyte wetting and active ion transport.

[0070] In some embodiments, the functional groups in the coating layer may include at least one of hydroxyl, carboxyl, amino, and sulfonic acid groups. These types of functional groups are capable of generating strong electrostatic and / or coordination interactions with active ions.

[0071] In some embodiments, the coating layer comprises a polymer. Optionally, the polymer comprises at least one selected from polydopamine, polyaniline, polystyrene sulfonic acid, polymethacrylic acid, and polymaleic anhydride.

[0072] In this embodiment, a polymer with self-healing ability is used as the coating layer. On the one hand, the functional groups in the coating layer can generate electrostatic and / or coordination interactions with active ions, thereby reducing the barrier to solvent removal of active ions and promoting the uniform deposition of active ions at the negative electrode interface. On the other hand, the polymer can achieve self-healing through dynamic covalent or non-covalent bonds within the molecule. The self-healing property of functional polymer materials can enhance the mechanical strength of the composite separator under extreme conditions (such as a large number of "dead lithium" and lithium dendrite formation in the later stages of cycling), thereby improving the cycle performance and safety of the battery.

[0073] It is understandable that if the coating layer is too thin, the coating may be uneven and the functional group content may be low; if the coating layer is too thick, on the one hand, it will reduce the overall mechanical strength of the first coating layer, and on the other hand, an excessively thick coating layer will increase the resistance to charge transfer and also lead to a decrease in the specific surface area and porosity of the first MOF material, thereby affecting the cycle performance of the battery. Therefore, in some embodiments, the thickness of the coating layer can be 1 μm to 3 μm, for example, it can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or any value between any two of the above ranges. This is beneficial for balancing the mechanical strength and electrochemical performance of the composite separator.

[0074] It is understandable that if the first coating is too thick, the overall thickness of the separator increases excessively, significantly increasing the migration distance of active ions, increasing the internal resistance of the battery, and thus affecting the active ion transport efficiency, leading to increased battery polarization and poor cycle performance. Conversely, if the first coating is too thin, the mechanical strength of the composite separator is affected, reducing safety. Therefore, in some embodiments, the thickness of the first coating can be 2μm to 5μm, for example, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, or any value between any two of the above ranges. This is beneficial for balancing the battery's cycle performance and safety performance.

[0075] In some embodiments, the first coating further includes first ceramic particles. This is beneficial for improving the mechanical strength and thermal stability of the composite separator, thereby enhancing its safety performance. Moreover, compared to existing separators with an alumina coating on the base membrane, the composite separator in this application can reduce battery weight and improve the overall electrochemical performance and safety of the battery.

[0076] In some specific embodiments, the mass ratio of the modified MOF material to the first ceramic particles can be 1:9 to 9:1, for example, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or any other ratio within the above range. This is beneficial for improving battery safety performance while enhancing battery cycle performance through the composite separator.

[0077] Optionally, the first ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0078] In some embodiments, the composite membrane may further include a second coating located on the surface of the base membrane away from the first coating; the second coating includes a second MOF material.

[0079] When composite separators are applied to batteries, the second coating is positioned towards the positive electrode. The second MOF material can capture transition metal ions (e.g., through coordination, chemical adsorption, and pore size sieving), preventing transition metal ions from diffusing to the negative electrode and inducing failure. This enhances the interfacial stability between the electrode material and the electrolyte, improves battery safety performance, and simultaneously improves the wettability of the composite separator to the electrolyte and increases the battery's liquid retention coefficient.

[0080] In the embodiments of this application, the first coating and the second coating are located on opposite sides of the base film along the thickness direction, that is, the coatings located on opposite sides of the base film in the composite separator are different. Therefore, this composite separator can also be called an asymmetric separator.

[0081] In some embodiments, the second MOF material includes at least one of ZIF-8, UIO-66, and MIL-100.

[0082] In some embodiments, the specific surface area of ​​the second MOF material is 1000 m². 2 / g or more.

[0083] In some embodiments, the porosity of the second MOF material is 50% to 80%.

[0084] The second MOF material has a high specific surface area and abundant pore structure, which is beneficial for better capturing transition metal ions and for better improving the wettability of the composite membrane to the electrolyte.

[0085] In some embodiments, the thickness of the second coating can be 2 μm to 5 μm, for example, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or any value between any two of the above ranges. This allows for effective improvement of the overall performance of the composite membrane through the second coating while avoiding the problem of excessively elongated active ion transport paths due to an excessively thick second coating.

[0086] In some embodiments, the second coating may further include second ceramic particles. This is beneficial for improving the mechanical strength and thermal stability of the composite diaphragm, thereby enhancing its safety performance.

[0087] In some specific embodiments, the mass ratio of the second MOF material to the second ceramic particles can be 1:9 to 9:1, for example, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or any other ratio within the above range. This is beneficial for improving battery cycle performance while simultaneously enhancing battery safety through the composite separator.

[0088] Optionally, the second ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0089] This application also provides a method for preparing a composite diaphragm. Please refer to... Figure 1 The method for preparing the composite diaphragm provided in this application includes the following steps:

[0090] S1: The modified MOF material and the first binder are added to the first solvent and mixed to obtain the first slurry; the modified MOF material includes the first MOF material and a coating layer covering the first MOF material, the coating layer including functional groups that can generate electrostatic interactions and / or coordination interactions with active ions;

[0091] S2: The first slurry is coated on one side of the base film along the thickness direction, and after drying, a first coating is formed.

[0092] In this embodiment, a first coating is formed on one side surface of the base film. The first coating includes a modified MOF material, and the coating layer in the modified MOF material includes functional groups capable of generating electrostatic interactions and / or coordination interactions with active ions. Thus, by aligning the first coating of the composite separator towards the negative electrode, the coating layer in the modified MOF material can adjust the lithium-ion solvation structure, reduce the barrier to active ion desolvation, and increase the flux of active ions at the negative electrode interface. This facilitates the uniform deposition of active ions at the negative electrode interface, inhibits the growth of lithium dendrites or sodium dendrites, and the first MOF material itself has a porous structure, which can promote electrolyte wetting and active ion transport, thereby effectively improving the cycle performance of the battery.

[0093] The method for preparing the modified MOF material in step S1 includes: coating the first MOF material to form a coating layer, thereby obtaining the modified MOF material.

[0094] In some embodiments, the functional groups in the coating layer include at least one of hydroxyl, carboxyl, amino, and sulfonic acid groups. These types of functional groups are capable of generating strong electrostatic and / or coordination interactions with active ions.

[0095] In some embodiments, the coating layer comprises a polymer, optionally including at least one selected from polydopamine, polyaniline, polystyrene sulfonic acid, polymethacrylic acid, and polymaleic anhydride. In the embodiments of this application, using a polymer with self-healing capabilities as the coating layer can better improve the cycle performance and safety of the battery.

[0096] The first MOF material used in step S1 may include at least one of ZIF-8, UIO-66, and MIL-100.

[0097] Furthermore, the specific surface area of ​​the first MOF material is above 1000 m² / g.

[0098] Furthermore, the porosity of the first MOF material is 50%–80%.

[0099] The first MOF material has a high specific surface area and abundant pore structure, which is beneficial for better promoting electrolyte wetting and active ion transport.

[0100] In actual preparation processes, for example, liquid-phase coating can be used to coat the first MOF material. Liquid-phase coating may include: first, adding the first MOF material to deionized water and dispersing it evenly; then, adding the polymer, continuing to stir, and after centrifugation and drying, obtaining the modified MOF material.

[0101] Here, taking polydopamine as an example, we will further explain the process of preparing modified MOF materials by liquid phase coating.

[0102] First, the first MOF material was added to deionized water and sonicated thoroughly to disperse the solution evenly. Then, dopamine hydrochloride was added under magnetic stirring and stirred continuously. Next, a buffer was added to adjust the pH of the solution to weakly alkaline (pH = 8-9) and stirred continuously. After centrifugation and drying, the modified MOF material was obtained.

[0103] The buffer may include, for example, at least one of tris(hydroxymethyl)aminomethane, 3-morpholinopropanesulfonic acid, and N-tris(hydroxymethyl)methyl-4-aminobutyric acid; the mass ratio of the first MOF material to the polymer may be 1:1.

[0104] It should be noted that the addition of a buffer to adjust the solution pH to weakly alkaline in the above steps only applies to the case where the polymer is polydopamine. This is because, under weakly alkaline conditions, dopamine hydrochloride monomers readily polymerize to form polydopamine (PDA), thereby forming a polymer coating layer (polydopamine coating layer) on the surface of the first MOF material. When the polymer is polyaniline, polystyrene sulfonic acid, polymethacrylic acid, or polymaleic anhydride, the polymer can be added directly to the solution without the need for a buffer to adjust the solution pH, and a polymer coating layer can be formed on the surface of the first MOF material.

[0105] In actual preparation, coating layers of different thicknesses can be obtained by controlling the reaction time and feed ratio. In some embodiments, the thickness of the prepared coating layer can be 1 μm to 3 μm, for example, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or any value between any two of the above ranges. This is beneficial for balancing the mechanical strength and electrochemical performance of the composite membrane.

[0106] The first adhesive in step S1 may include, for example, at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and polyacrylic acid. The first solvent may include, for example, deionized water.

[0107] The thickness of the first coating prepared in step S1 can be 2μm to 5μm, for example, it can be 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm or any value between any two of the above ranges. This is beneficial for balancing the cycle performance and safety performance of the battery.

[0108] In some embodiments, first ceramic particles are added during the preparation of the first slurry. This helps to improve the mechanical strength and thermal stability of the final composite membrane, thereby enhancing its safety performance.

[0109] Optionally, the mass ratio of the modified MOF material to the first ceramic particles is 1:9 to 9:1, for example, it can be 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or any other ratio within the above range. This is beneficial for improving battery cycle performance while simultaneously enhancing battery safety through the composite separator.

[0110] Optionally, the first ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0111] In step S2, the process of coating the first slurry onto one side surface of the base film along the thickness direction can be any coating process known to those skilled in the art, and will not be described in detail here.

[0112] In some embodiments, the method for preparing the composite membrane may further include: adding a second MOF material and a second binder to a second solvent, mixing them to obtain a second slurry; coating the second slurry onto the surface of the base membrane away from the first coating, and drying it to form a second coating.

[0113] In this embodiment, a second coating is formed on the other side of the base film. When the composite separator is applied to the battery, the second coating is positioned towards the positive electrode. The second MOF material can capture transition metal ions (e.g., by at least one of coordination, chemical adsorption, and pore size sieving), preventing transition metal ions from diffusing to the negative electrode and inducing failure, enhancing the interfacial stability between the electrode material and the electrolyte, improving battery safety performance, and improving the wettability of the composite separator to the electrolyte and increasing the battery liquid retention coefficient.

[0114] In some embodiments, the second MOF material includes at least one of ZIF-8, UIO-66, and MIL-100.

[0115] In some embodiments, the specific surface area of ​​the second MOF material is 1000 m². 2 / g or more.

[0116] In some embodiments, the porosity of the second MOF material is 50% to 80%.

[0117] The second MOF material has a high specific surface area and abundant pore structure, which is beneficial for better capturing transition metal ions and for better improving the wettability of the composite membrane to the electrolyte.

[0118] For example, the second adhesive includes at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and polyacrylic acid. The second solvent includes deionized water.

[0119] In some embodiments, the thickness of the prepared second coating can be 2 μm to 5 μm, for example, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or any value between any two of the above ranges. This allows for effective improvement of the overall performance of the composite membrane through the second coating while avoiding the problem of excessively elongated active ion transport paths due to an excessively thick second coating.

[0120] In some embodiments, second ceramic particles are added during the preparation of the second slurry. This helps to improve the mechanical strength and thermal stability of the composite diaphragm, thereby enhancing its safety performance.

[0121] Optionally, the mass ratio of the second MOF material to the second ceramic particles is 1:9 to 9:1, for example, it can be 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or any other ratio within the above range. This is beneficial for improving battery cycle performance and battery safety performance through composite separators.

[0122] Optionally, the second ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

[0123] This application also provides a secondary battery, which includes a positive electrode, a negative electrode, and a composite separator located between the positive electrode and the negative electrode. The composite separator is the composite separator described in any of the above embodiments or a composite separator prepared by the method described in any of the above embodiments; wherein, the first coating of the composite separator is disposed facing the negative electrode.

[0124] It is understood that the beneficial effects of the composite separator described in any of the above embodiments are also applicable to the secondary battery in the embodiments of this application. Therefore, the secondary battery in the embodiments of this application has superior cycle performance.

[0125] In some embodiments, the secondary battery may be, for example, a lithium-ion battery or a sodium-ion battery. In a specific embodiment, the secondary battery is a lithium-ion battery. Typically, a secondary battery includes a positive electrode, a negative electrode, an electrolyte, and the aforementioned composite separator. During battery charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor for active ions between the positive and negative electrodes. The composite separator is disposed between the positive and negative electrodes, primarily to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through.

[0126] Further, the negative electrode sheet includes a negative electrode current collector (specifically, for example, a copper foil) and a negative electrode active material layer coated on the negative electrode current collector. Specifically, the negative electrode active material layer may include a negative electrode active material, a conductive agent, and a binder, wherein the negative electrode active material may include at least one of graphite, hard carbon, soft carbon, silicon-based materials, and lithium metal; the conductive agent may include at least one of conductive carbon black, carbon nanotubes, and acetylene black; and the binder may include, for example, polyvinylidene fluoride (PVDF). The positive electrode sheet includes a positive electrode current collector (specifically, for example, an aluminum foil) and a positive electrode active material layer coated on the positive electrode current collector. Specifically, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder, wherein the positive electrode active material may include at least one of lithium iron phosphate, lithium manganese iron phosphate, and high-nickel ternary materials; the conductive agent may include at least one of conductive carbon black, carbon nanotubes, and acetylene black; and the binder may include, for example, polyvinylidene fluoride (PVDF).

[0127] It should be noted that, in the case where the composite separator includes a first coating and a second coating, in the secondary battery, the first coating of the composite separator is positioned facing the negative electrode, and the second coating is positioned facing the positive electrode. Thus, the first coating of the composite separator can regulate the solvation structure of active ions, helping to improve the transport efficiency of active ions and the reaction kinetics of the electrode, enhancing the uniformity of lithium deposition at the negative electrode interface, and suppressing the expansion of the lithium negative electrode. Simultaneously, the second coating can adsorb certain free impurities in the battery, capture transition metal ions, and increase the liquid retention capacity of the composite separator, thereby significantly improving the cycle performance and safety performance of the battery.

[0128] In one specific embodiment, the positive electrode active material includes a high-nickel ternary material, and the negative electrode sheet is lithium metal.

[0129] High-nickel ternary materials inherently possess structural instability and are prone to the precipitation of transition metal ions. The second coating of the composite separator described in the above embodiments, oriented towards the positive electrode, effectively captures the transition metal ions released from the high-nickel ternary material through the second MOF material in the second coating, preventing these ions from diffusing to the negative electrode and inducing failure. This enhances the interfacial stability between the electrode material and the electrolyte, while also increasing the liquid retention capacity of the composite separator. The first coating, oriented towards the negative electrode, allows the modified MOF material in the first coating to adjust the lithium-ion solvation structure, lowering the barrier for active ion desolvation and increasing the flux of active ions at the negative electrode interface. This facilitates uniform deposition of active ions at the negative electrode interface, inhibits lithium dendrite growth, and significantly improves the battery's cycle performance and safety. In other words, the composite separator provided in this application can effectively improve the cycle performance of lithium metal battery systems with lithium metal negative electrodes matched with high-nickel positive electrodes.

[0130] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0131] Example 1

[0132] The method for preparing the composite diaphragm in this embodiment includes the following steps:

[0133] Step S101: Take 10g of ZIF-8 (first MOF material) and add it to 200ml of deionized water. Sonicate for 1h and stir for 1h to fully disperse ZIF-8. Then add tris(hydroxymethyl)aminomethane (buffer) to adjust the pH of the solution to weakly alkaline (pH about 8.5). Next, add 10g of dopamine hydrochloride monomer and stir at room temperature for 8h to form a polydopamine coating layer on the surface of ZIF-8. Then wash the reaction product three times with ethanol and deionized water respectively. After freeze-drying for 24h, ZIF-8@PDA (modified MOF material) is obtained, with a coating layer thickness of 2μm.

[0134] Step S102: Add carboxymethyl cellulose (first binder) to deionized water, stir to disperse fully, then add ZIF-8@PDA and alumina (first ceramic particles) prepared in the above steps in sequence, stir to obtain a uniform slurry, namely the first slurry; wherein, the mass ratio of ZIF-8@PDA to alumina is 2:8, and the mass percentage of carboxymethyl cellulose in the first slurry is 3%;

[0135] Step S103: Add carboxymethyl cellulose (second binder) to deionized water, stir to disperse fully, then add ZIF-8 (second MOF material) and alumina (second ceramic particles) in sequence, stir to obtain a uniform slurry, i.e., the second slurry; wherein, the mass ratio of ZIF-8 to alumina is 2:8, and the mass percentage of carboxymethyl cellulose in the second slurry is 3%;

[0136] Step S104: The first slurry prepared in the above steps is coated on one side of the PP base film along the thickness direction, and after drying, a first coating is formed; the second slurry prepared in the above steps is coated on the other side of the PP base film along the thickness direction, and after drying, a second coating is formed. The thickness of the first coating and the second coating is 3μm, thereby obtaining a composite membrane.

[0137] Example 2

[0138] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0139] In step S103, the preparation method of the second slurry is the same as that of the first slurry in step S102; that is, the second slurry contains modified MOF material ZIF-8@PDA, instead of ordinary MOF material ZIF-8.

[0140] Example 3

[0141] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0142] In step S102, the mass ratio of ZIF-8@PDA to alumina is adjusted to 1:9.

[0143] Example 4

[0144] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0145] In step S102, the mass ratio of ZIF-8@PDA to alumina is adjusted to 9:1.

[0146] Example 5

[0147] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0148] In step S101, after adding dopamine hydrochloride monomer, the stirring time at room temperature is adjusted to 24 hours, and the thickness of the coating layer is 5 μm.

[0149] Example 6

[0150] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0151] In step S101, after adding dopamine hydrochloride monomer, the stirring time at room temperature is adjusted to 12 hours, and the thickness of the resulting coating layer is 3 μm.

[0152] Example 7

[0153] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0154] ZIF-8 was not added in step S103.

[0155] Example 8

[0156] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0157] The thickness of the first coating and the second coating formed in step S104 is 5 μm.

[0158] Comparative Example 1

[0159] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that:

[0160] Step S101 was omitted, meaning ZIF-8@PDA was not prepared, and ZIF-8@PDA in step S102 was changed to ZIF-8.

[0161] The composite separator batteries prepared using the above embodiments and comparative examples are assembled using the following method for preparing the positive electrode: High-nickel ternary material NCM623, PVDF, and conductive carbon black are added to N-methylpyrrolidone in a mass ratio of 9:0.5:0.5, and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry is coated onto aluminum foil, dried, and then cut into circular pieces with a diameter of 14 mm for later use. The active material mass of the positive electrode is controlled at 2.2 mg / cm³. 2 The negative electrode uses a lithium metal disc with a thickness of 100 μm and a diameter of 16 mm; the electrolyte solvent is a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEDC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1 with 10 vol% fluoroethylene carbonate (FEC); the lithium salt uses LITFSI with a concentration of 1 M; the positive electrode, negative electrode, and composite separator are assembled to obtain a button cell, wherein the first coating of the composite separator faces the negative electrode and the second coating faces the positive electrode; 100 μl of electrolyte is added to each button cell.

[0162] The cycle performance of the fabricated battery was tested:

[0163] Capacity retention rate after 200 cycles: The test temperature was 25℃, and the test voltage range was 2.8V~4.3V. The battery was connected to the charging cabinet and first charged and discharged at 0.1C for two cycles to activate the battery. Then, it was charged and discharged at 0.5C / 1C for two cycles. 1C=0.609mA. The resting time between charging and discharging was set to 10min. The discharge capacity of the first cycle and the discharge capacity of the 200th cycle were recorded. The capacity retention rate after 200 cycles = (discharge capacity of the 200th cycle / discharge capacity of the 1st cycle)*100%.

[0164] The test results are shown in Table 1.

[0165] Table 1

[0166] Capacity retention rate after 200 cycles Example 1 89.5％ Example 2 89.1％ Example 3 88.5％ Example 4 83.2％ Example 5 83.8％ Example 6 87.6％ Example 7 82.4％ Example 8 84.7％ Comparative Example 1 80.4％

[0167] In Comparative Example 1, the first coating of the composite separator was prepared using ordinary MOF material ZIF-8. However, in Examples 1 to 8, a modified MOF material (ZIF-8@PDA) was added. As shown in Table 1, the battery containing the composite separators of Examples 1 to 8 exhibits a significantly improved capacity retention rate after 200 cycles compared to the battery containing the composite separator of Comparative Example 1. This indicates that without adding a modified MOF material to the first coating, it is difficult to improve the battery's cycle performance. In this application, the first coating of the composite separator includes a modified MOF material. The modified MOF material coating layer can adjust the lithium-ion solvation structure, reduce the barrier to active ion desolvation, and increase the active ion flux at the negative electrode interface. This facilitates the uniform deposition of active ions at the negative electrode interface, inhibits the growth of lithium dendrites or sodium dendrites, and the first MOF material itself has a porous structure, which promotes electrolyte wetting and active ion transport, thereby effectively improving the battery's cycle performance.

[0168] In Examples 1, 5, and 6, the thicknesses of the coating layer in the modified MOF material were 2 μm, 5 μm, and 3 μm, respectively. As can be seen from the data in Table 1, the capacity retention rate of the battery after 200 cycles decreased with the increase of the coating layer thickness. When the coating layer thickness increased to 5 μm, the capacity retention rate of the battery after 200 cycles dropped to below 84%. This indicates that a coating layer thickness of 1 μm to 3 μm in the modified MOF material is a better technical solution.

[0169] In Example 7, no MOF material was added during the preparation of the second coating of the composite separator, and the improvement in the capacity retention rate of the corresponding battery after 200 cycles was relatively small compared to that of the battery in Comparative Example 1. Therefore, it can be concluded that having both the first and second coatings of the composite separator containing modified MOF material is a superior technical solution.

[0170] It should be noted that the composite membrane embodiments, composite membrane preparation method embodiments, and secondary 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.

[0171] 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 composite diaphragm, characterized in that, It includes a base film and a first coating disposed on one side surface of the base film along the thickness direction; The first coating includes a modified MOF material, which includes a first MOF material and a coating layer covering the first MOF material. The coating layer includes functional groups capable of generating electrostatic interactions and / or coordination interactions with active ions.

2. The composite diaphragm according to claim 1, characterized in that, The first coating satisfies at least one of the following characteristics: (1) The first MOF material includes at least one of ZIF-8, UIO-66, and MIL-100; (2) The specific surface area of ​​the first MOF material is 1000 m². 2 / g or more; (3) The porosity of the first MOF material is 50% to 80%; (4) The coating layer comprises a polymer; optionally, the polymer comprises at least one of polydopamine, polyaniline, polystyrene sulfonic acid, polymethacrylic acid, and polymaleic anhydride; (5) The functional group includes at least one of hydroxyl, carboxyl, amino, and sulfonic acid groups; (6) The thickness of the coating layer is 1 μm to 3 μm; (7) The thickness of the first coating is 2μm to 5μm; (8) The first coating further includes first ceramic particles; optionally, the mass ratio of the modified MOF material to the first ceramic particles is 1:9 to 9:1; optionally, the first ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

3. The composite diaphragm according to claim 1 or 2, characterized in that, The composite membrane further includes a second coating located on the surface of the base membrane away from the first coating; the second coating includes a second MOF material.

4. The composite diaphragm according to claim 3, characterized in that, The second coating satisfies at least one of the following characteristics: (1) The second MOF material includes at least one of ZIF-8, UIO-66, and MIL-100; (2) The specific surface area of ​​the second MOF material is 1000 m². 2 / g or more; (3) The porosity of the second MOF material is 50% to 80%; (4) The thickness of the second coating is 2μm to 5μm; (5) The second coating further includes second ceramic particles; optionally, the mass ratio of the second MOF material to the second ceramic particles is 1:9 to 9:1; optionally, the second ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

5. A method for preparing a composite diaphragm, characterized in that, The method includes the following steps: S1: The modified MOF material and the first binder are added to the first solvent and mixed to obtain the first slurry; the modified MOF material includes the first MOF material and a coating layer covering the first MOF material, the coating layer including functional groups that can generate electrostatic interactions and / or coordination interactions with active ions; S2: The first slurry is coated on one side surface of the base film along the thickness direction, and after drying, a first coating is formed.

6. The method for preparing the composite diaphragm according to claim 5, characterized in that, The method satisfies at least one of the following characteristics: (1) The first adhesive includes at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and polyacrylic acid; (2) The first solvent includes deionized water; (3) The thickness of the first coating is 2μm to 5μm; (4) In the process of preparing the first slurry, first ceramic particles are also added; optionally, the mass ratio of the modified MOF material to the first ceramic particles is 1:9 to 9:1; optionally, the first ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

7. The method for preparing the composite diaphragm according to claim 5, characterized in that, The method for preparing the modified MOF material includes: coating the first MOF material to form the coating layer, thereby obtaining the modified MOF material; the method satisfies at least one of the following characteristics: (1) The first MOF material includes at least one of ZIF-8, UIO-66, and MIL-100; (2) The specific surface area of ​​the first MOF material is 1000 m². 2 / g or more; (3) The porosity of the first MOF material is 50% to 80%; (4) The coating layer comprises a polymer, and optionally, the polymer comprises at least one of polydopamine, polyaniline, polystyrene sulfonic acid, polymethacrylic acid, and polymaleic anhydride; (5) The functional group includes at least one of hydroxyl, carboxyl, amino, and sulfonic acid groups; (6) The thickness of the coating layer is 1 μm to 3 μm; (7) The coating process is carried out by liquid phase coating.

8. The method for preparing the composite diaphragm according to any one of claims 5 to 7, characterized in that, The method further includes: The second MOF material and the second binder are added to the second solvent and mixed to obtain the second slurry; The second slurry is applied to the surface of the base film away from the first coating, and after drying, a second coating is formed.

9. The method for preparing the composite diaphragm according to claim 8, characterized in that, The method satisfies at least one of the following characteristics: (1) The second MOF material includes at least one of ZIF-8, UIO-66, and MIL-100; (2) The specific surface area of ​​the second MOF material is 1000 m². 2 / g or more; (3) The porosity of the second MOF material is 50% to 80%; (4) The second adhesive includes at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, styrene-butadiene rubber, and polyacrylic acid; (5) The second solvent includes deionized water; (6) The thickness of the second coating is 2μm to 5μm; (7) In the process of preparing the second slurry, second ceramic particles are also added; optionally, the mass ratio of the second MOF material to the second ceramic particles is 1:9 to 9:1; optionally, the second ceramic particles include one or more of alumina, boehmite, zirconium oxide, silicon oxide, magnesium oxide, titanium oxide, zinc oxide, and calcium oxide.

10. A secondary battery, characterized in that, The invention includes a positive electrode, a negative electrode, and a composite separator located between the positive electrode and the negative electrode, wherein the composite separator is the composite separator according to any one of claims 1 to 4 or the composite separator prepared by any one of claims 5 to 9; wherein the first coating of the composite separator is disposed facing the negative electrode.

Images