Preparation method of composite diaphragm, secondary battery and electric device

Abstract

The invention provides a preparation method of a composite diaphragm, a secondary battery and an electric device. The composite diaphragm comprises a base membrane and an adhesive coating, wherein the adhesive coating comprises a plurality of gel points which are arranged at preset intervals. And the gel point is subjected to volume swelling after absorbing the electrolyte, so that a space is effectively expanded between the base film and the negative pole piece, a space is reserved for metal deposition in advance, volume expansion caused by metal deposition on the negative pole piece in the charging and discharging process of the secondary battery is relieved to a certain extent, and the capacity and the cycle life of the battery are effectively improved.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, specifically relating to a composite separator and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Rechargeable batteries, also known as secondary batteries, not only provide a powerful and long-lasting power source for various new energy vehicles, but also offer an efficient energy management solution for energy storage systems. However, obvious problems also limit the development of secondary batteries. During the charge and discharge cycles of secondary batteries, metals tend to deposit on the negative electrode, causing the battery's volume to expand. If too much metal is deposited on the negative electrode, it can squeeze or even puncture the separator, causing a connection between the positive and negative electrodes and resulting in an internal short circuit. An internal short circuit in a secondary battery will affect its capacity and cycle life. Summary of the Invention

[0003] This application is made in view of the above-mentioned problems, and its purpose is to provide a composite separator and its preparation method, a secondary battery and an electrical device, so as to solve the problem that the volume expansion caused by metal deposition during the charge and discharge cycle of the secondary battery affects the capacity and cycle life of the secondary battery.

[0004] This application provides a secondary battery comprising a composite separator, the composite separator comprising a base film and an adhesive coating, the adhesive coating comprising a plurality of gel dots arranged at predetermined intervals. The composite separator, comprising an adhesive coating consisting of a plurality of gel dots arranged at predetermined intervals, allows the gel dots to swell in volume after absorbing electrolyte, effectively creating space between the base film and the negative electrode, pre-reserving space for metal deposition, and effectively preventing volume expansion caused by metal deposition on the negative electrode during the charging and discharging process of the secondary battery. This provides a solution for improving the capacity and cycle life of the secondary battery.

[0005] In any embodiment, the gel point comprises a polymer, which may be at least one of polyether and polyester. Polyether and polyester polymers do not exhibit significant gas generation or discoloration when added to the electrolyte system, demonstrating better compatibility with electrolyte systems compared to other polymers. Furthermore, polyether and polyester are high-molecular-weight polymers with good wetting and liquid absorption properties in the electrolyte. Small molecules in the electrolyte solvent can more easily penetrate into the interior of the polyether and polyester, thereby expanding the molecular chains and causing the polyether and polyester to swell, effectively creating space between the base film and the negative electrode.

[0006] In any embodiment, the polyether comprises at least one of polyethylene oxide, polypropylene ether, polyetherketone, and polymethyl ether; the polyester comprises at least one of polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, and trimethylolpropane triacrylate. The polyethers or polyesters mentioned above have superior liquid absorption and wetting capabilities compared to their counterparts. After absorbing electrolyte, they can create a larger space between the base film and the negative electrode, facilitating sufficient space for metal deposition and mitigating the volume expansion of the secondary battery during charge-discharge cycles.

[0007] In any embodiment, the composite separator further includes a heat-resistant coating, which is attached to at least one surface of the base membrane; the heat-resistant coating comprises at least one of inorganic or organic particles. The heat-resistant coating improves the heat resistance of the separator, reduces thermal shrinkage, and allows the separator to maintain good dimensional structure even at ultra-high temperatures, preventing internal short circuits caused by contact between the positive and negative electrodes.

[0008] In any embodiment, the coverage of the gel points on the base film or heat-resistant coating is 20%-60%.

[0009] In any embodiment, the spacing between the gel dots is 100um-400um.

[0010] In any embodiment, the average diameter of the gel dots is 2µm-6µm.

[0011] The spacing between gel dots and the average diameter affect the coverage of gel dots on the heat-resistant coating of the resulting composite membrane. The coverage of gel dots on the heat-resistant coating affects the size of the space effectively expanded after the polymer swells. By controlling the coverage of gel dots on the heat-resistant coating, the spacing between gel dots, and the average diameter of gel dots within the above range, the polymer can support sufficient space and obtain a better support effect after swelling.

[0012] In any embodiment, the thickness of the gel point is 2-20 μm. The thickness of the polymer gel layer determines the effective space created after the polymer swells. Maintaining the thickness of the polymer gel layer at 2-20 μm ensures that sufficient space is created between the separator and the negative electrode after the polymer swells, providing enough space for metal deposition and mitigating the volume expansion of the secondary battery during charge-discharge cycles.

[0013] In any embodiment, the adhesive coating is attached to one or both sides of the heat-resistant coating.

[0014] In any embodiment, the adhesive coating is attached to the heat-resistant coating with its side facing the negative electrode. During the charge-discharge cycle of the secondary battery, the negative electrode is the receiving end for active ions. During charging, active ions migrate from the positive electrode to the negative electrode and undergo a reduction reaction at the negative electrode, i.e., embedding into the negative electrode material or undergoing electrodeposition. The positive electrode is the releasing end for active ions. During discharging, active ions escape from the positive electrode, enter the electrolyte, and then migrate to the negative electrode. Typically, the positive electrode material is structurally stable during charge-discharge; it provides active ions but does not undergo significant deposition, while the negative electrode will have metal deposition. Having the polymer gel layer attached to the heat-resistant coating with its side facing the negative electrode can more effectively reduce the volume expansion of the secondary battery, improving its capacity and cycle life.

[0015] In any embodiment, the secondary battery includes a negative electrode-free metal battery, optionally a negative electrode-free sodium metal battery or a negative electrode-free lithium metal battery. During charging and discharging, the metal in a negative electrode-free metal battery deposits and dissolves on the negative electrode current collector, resulting in more pronounced volume expansion and contraction. The composite separator described in this application, used in negative electrode-free metal batteries, effectively mitigates the volume expansion problem.

[0016] The second aspect of this application provides a method for preparing a composite membrane, comprising the following steps: providing a base membrane, coating a heat-resistant coating on one or both sides of the base membrane; taking a polymer and water, mixing them to form a slurry; and coating the slurry formed by mixing onto one or both sides of the heat-resistant coating in a dot matrix coating manner.

[0017] In any embodiment, the mass ratio of the polymer to water is 20-40:60-80. This allows the viscosity of the resulting slurry to be maintained within a suitable range, which is beneficial for achieving stable coating.

[0018] In any embodiment, the coating speed of the dot matrix coating is 60 m / min to 120 m / min. Controlling the coating speed within the above range is beneficial for balancing production efficiency and the quality of gel point distribution.

[0019] A third aspect of this application provides an electrical device that includes the secondary battery described in the first aspect. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of one embodiment of the composite diaphragm of this application;

[0022] Figure 2 This is a schematic diagram of one embodiment of the secondary battery of this application;

[0023] Figure 3 This is an exploded view of one embodiment of the secondary battery of this application;

[0024] Figure 4 This is a schematic diagram of one embodiment of the battery module of this application;

[0025] Figure 5 This is a schematic diagram of one embodiment of the battery pack of this application;

[0026] Figure 6 yes Figure 5 An exploded view of an embodiment of the battery pack shown;

[0027] Figure 7 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0028] The accompanying drawings are not necessarily drawn to scale. The reference numerals are as follows: 1 Battery pack, 2 Upper casing, 3 Lower casing, 4 Battery module, 5 Secondary battery, 51 Housing, 52 Electrode assembly, 53 Cover plate. Detailed Implementation

[0029] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the composite separator, its preparation method, secondary battery, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0030] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0031] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0032] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0033] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0034] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0035] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0036] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0037] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0038] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0039] Unless otherwise specified, in this application, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions.

[0040] Rechargeable batteries, also known as secondary batteries, are widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields. With the development of rechargeable batteries, the demand for their capacity and cycle life is increasing. However, a significant problem also limits their development: during the charge-discharge cycle of a rechargeable battery, metal tends to deposit on the negative electrode, causing the battery's volume to expand. If too much metal is deposited on the negative electrode, it can compress or even puncture the separator, causing a connection between the positive and negative electrodes and resulting in an internal short circuit. An internal short circuit in a rechargeable battery will affect its capacity and cycle life.

[0041] Based on this, this application provides a method for preparing a composite separator, a secondary battery, and an electrical device.

[0042] In a first aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte. During the charging and discharging process of the secondary battery, active ions repeatedly insert and extract between the positive and negative electrode. The electrolyte serves to conduct active ions between the positive and negative electrode. The composite separator is disposed between the positive and negative electrode to isolate them. When the secondary battery is a sodium-ion battery, the negative electrode, positive electrode, separator, and electrolyte can be prepared as follows:

[0043] diaphragm

[0044] The separator is a composite separator prepared by the method described in this application. The composite separator includes a base film and an adhesive coating. The adhesive coating includes multiple gel dots arranged at predetermined intervals. The gel dots have strong wetting and absorption capabilities for the electrolyte, easily absorbing the electrolyte and swelling in volume. After absorbing the electrolyte and swelling, the gel dots effectively create space between the base film and the negative electrode, reserving space for metal deposition in advance. This effectively prevents volume expansion caused by metal deposition during the charging and discharging process of the secondary battery, providing a solution for improving the capacity and cycle life of the secondary battery.

[0045] Figure 1 This is a schematic diagram of one embodiment of the composite diaphragm prepared in this application, as shown below. Figure 1 The composite membrane shown includes a base membrane and an adhesive coating, wherein the polymer gel layer includes a plurality of gel dots arranged at a predetermined interval.

[0046] In this article, the term "base membrane" refers to the basic material layer that constitutes the separator. It is the main body of the separator and has a porous structure. It is usually made of materials with certain mechanical strength, chemical stability, and insulation properties, such as polyolefins (e.g., PP, PE). The base membrane plays a supporting and protective role in the separator, providing the necessary physical and chemical properties to ensure its stability and safety in applications such as batteries.

[0047] In this article, the term "adhesive coating" refers to a functional coating used to improve the adhesion between the separator and other parts of the battery cell (such as electrodes), thereby enhancing the stability and reliability of the battery structure. Materials such as PVDF (polyvinylidene fluoride) are typically used as the adhesive coating for the separator. The adhesive properties of PVDF can tightly bond the electrodes to the separator, eliminate air from internal gaps, increase cell rigidity, and maintain cell thickness consistency.

[0048] In this paper, the term "gel point" refers to the dotted pattern formed on a base film by a polymer-based gel.

[0049] To facilitate understanding, the principle behind the aforementioned beneficial effects of the composite separator in the secondary battery of this application will be explained below:

[0050] This application addresses the volume expansion problem of the negative electrode caused by metal deposition by dot-coating a gel onto a base film. The prepared composite separator, positive electrode, and negative electrode are assembled into a battery cell and injected with electrolyte. The composite separator is immersed in the electrolyte within the battery cell. Because the gel dots on the composite separator are composed of high-molecular-weight polymers, small molecules in the electrolyte solvent easily penetrate into the polymer, expanding the molecular chains and causing the polymer to swell. The swollen gel dots compress the separator, effectively increasing the distance between the separator and the electrode. Furthermore, to ensure a uniform and consistent adhesive layer on the base film, this application employs a dot-matrix coating method to apply the gel to the base film. The gel dots are coated onto the base film using a dot matrix coating method. The resulting composite separator has a uniform and regular distribution of gel dots. After absorbing electrolyte, the gel dots swell and compress the separator. The separator is subjected to uniform and consistent stress after compression, which can uniformly create space between the electrode and the base film, providing sufficient space for metal deposition, mitigating the volume expansion of the secondary battery, and effectively improving the capacity and cycle life of the secondary battery. If the adhesive layer of the composite separator does not use the gel mentioned in this application, but uses a common binder (such as PVDF), the volume swelling rate of the binder after absorbing electrolyte is smaller, and the space created by compressing the separator is smaller under the same conditions. If the adhesive layer is coated using a method other than the dot matrix coating mentioned in this application (such as spraying), the adhesive layer cannot be made uniform and regular. After the adhesive layer swells and absorbs electrolyte, it cannot uniformly create space between the separator and the negative electrode. Therefore, coating the gel on the base film using a dot matrix coating method is superior. After the gel absorbs the electrolyte and swells, it creates a space between the base film and the negative electrode, providing sufficient space for the deposition of metal on the negative electrode. This greatly alleviates the volume expansion of the secondary battery and improves its cycle life.

[0051] In some embodiments, the gel point comprises a polymer, which includes at least one of polyether and polyester.

[0052] In this article, the term "polyether" refers to a class of organic compounds, which are macromolecules mainly composed of ether bonds (COC). They are typically composed of oxygen atoms linked to several alkyl or aromatic groups.

[0053] In this article, the term "polyester" refers to a class of polymeric compounds formed by the condensation polymerization of polyols and polyacids through esterification reactions, possessing an ester group. (The ester group is the functional group of esters in carboxylic acid derivatives, with the structural formula -COOR, where R is generally an alkyl group or other non-H group.)

[0054] Compared to other polymers, polyethers and polyesters exhibit better compatibility in electrolyte systems, showing no significant gas generation or discoloration upon addition. Furthermore, polyethers and polyesters demonstrate superior wetting and liquid absorption in electrolytes compared to other polymers. Small molecules in the electrolyte solvent more easily penetrate the interior of the polyether or polyester, thereby expanding their molecular chains and causing volume swelling, effectively creating space between the base film and the negative electrode.

[0055] In some embodiments, the polyether includes at least one of polyethylene oxide, polypropylene ether, polyetherketone, and polymethyl methacrylate; the polyester includes at least one of polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, and trimethylolpropane triacrylate. The polyethers or polyesters mentioned above have superior liquid absorption and wetting capabilities compared to their counterparts, and can create a larger space between the base film and the negative electrode after absorbing the electrolyte.

[0056] In some embodiments, the composite membrane further includes a heat-resistant coating, which is attached to at least one side surface of the base membrane; the heat-resistant coating comprises at least one of inorganic particles or organic particles.

[0057] In this article, the term "heat-resistant coating" refers to a coating material that can improve the heat resistance of the base film. The heat-resistant coating can reduce the thermal shrinkage of the separator, allowing the separator to maintain a good dimensional structure at ultra-high temperatures and improving the safety performance of the battery.

[0058] The heat-resistant coating comprises at least one of inorganic or organic particles. Inorganic particles include boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon oxides, tin dioxide, titanium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, hafnium dioxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride; organic particles include polymethyl methacrylate (PMMA), polyimide (PI), and polyphenylene sulfide (PPS).

[0059] In any embodiment, the coverage of gel dots on the base film or heat-resistant coating is 20%-60%.

[0060] In any embodiment, the coverage of the gel points on the base film or heat-resistant coating can be selected as 20%, 30%, 40%, 50%, 60%, or any value range between the two.

[0061] In any embodiment, the spacing between gel dots is 100um-400um.

[0062] In any embodiment, the spacing between gel dots can be selected as 100um, 200um, 300um, 400um or any value range between the two.

[0063] In any embodiment, the average diameter of the gel dots is 2µm-6µm.

[0064] In any embodiment, the average diameter of the gel points can be selected as 2 μm, 3 μm, 4 μm, 5 μm, 6 μm or any range between the two.

[0065] The spacing between gel dots and the average diameter of the gel dots affect the coverage of the gel dots on the base film or heat-resistant coating of the resulting composite membrane. The coverage of the gel dots on the base film or heat-resistant coating affects the size of the space effectively expanded after the polymer swells. By controlling the coverage of the gel dots on the base film or heat-resistant coating, the spacing between the gel dots, and the average diameter of the gel dots within the above range, the polymer can support sufficient space and obtain a better support effect after swelling.

[0066] In some embodiments, the thickness of the gel points is 2-20 μm.

[0067] In some embodiments, the thickness of the gel point can be selected from 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, or any value between two of these. The thickness of the gel point determines the effective space created after the polymer swells. Maintaining the thickness of the gel point between 2-20μm ensures that the space created between the separator and the negative electrode after the polymer swells is 10-30μm, providing sufficient space for metal deposition.

[0068] In some embodiments, the adhesive coating is attached to one or both sides of the heat-resistant coating.

[0069] In some embodiments, the adhesive coating is attached to one side of the heat-resistant coating and faces the negative electrode.

[0070] During the charge-discharge cycle of a secondary battery, the negative electrode is the receiving end for active ions. During charging, active ions migrate from the positive electrode to the negative electrode and undergo a reduction reaction there, either embedding into the negative electrode material or undergoing electrodeposition. The positive electrode is the releasing end for active ions. During discharging, active ions escape from the positive electrode, enter the electrolyte, and then migrate to the negative electrode. Typically, the positive electrode material is structurally stable during charge-discharge; it provides active ions but does not undergo significant deposition, while the negative electrode shows metal deposition. When the polymer gel layer is attached to the base film or heat-resistant coating and faces the negative electrode, it can more effectively reduce the volume expansion of the secondary battery, improving its capacity and cycle life.

[0071] In some embodiments, the material of the separator may include one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0072] Positive electrode sheet

[0073] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector and including a positive active material. As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0074] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0075] In some embodiments, the positive electrode active material of the sodium-ion battery may be a known positive electrode active material for sodium-ion batteries. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc., but this application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries may also be used. For example, as an optional technical solution in this application, the transition metal in the sodium transition metal oxide may be at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide may be, for example, Na. x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, 0 <x≤1。

[0076] As an optional technical approach in this application, the polyanionic compound can be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents (YO4). n-The valence state. Polyanionic compounds can also have sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si, and n represents (YO4). n- The valence state; the halogen can be at least one of F, Cl, and Br. Polyanionic compounds can also have sodium ions, tetrahedral (YO4) valence states. n- Anionic unit, polyhedral unit (ZO) y ) m+ And a class of compounds with optional halide anions. Y can be at least one of P, S, and Si, and n represents (YO4). n- Valence state: Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; m represents (ZO) y ) m+ The valence state; the halogen can be at least one of F, Cl, and Br. Examples of polyanionic compounds include NaFePO4, Na3V2(PO4)3, NaM'PO4F (M' is one or more of V, Fe, Mn, and Ni), and Na3(VO4)2(PO4)3. y )2(PO4)2F 3-2y At least one of (0≤y≤1). Prussian blue compounds can contain sodium ions, transition metal ions, and cyanide ions (CN). - A class of compounds. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Prussian blue compounds include, for example, Na. a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 <a≤2,0<b<1,0<c<1。

[0077] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0078] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0079] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, additives, conductive agents, binders and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto a positive current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0080] Negative electrode sheet

[0081] The negative electrode includes a negative current collector and a negative active layer disposed on at least one surface of the negative current collector.

[0082] The negative electrode current collector may be a metal foil or a metal / polymer composite current collector, and the negative electrode active layer includes a negative electrode active material. As a non-limiting example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0083] The negative electrode active material can be any negative electrode active material known in the art for use in batteries. As a non-limiting example, the negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0084] In some embodiments, the negative electrode active layer may optionally include a binder. The binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0085] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0086] In some embodiments, the negative electrode active layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0087] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (a non-limiting example of a solvent is deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto at least one surface of the negative electrode current collector, and then obtaining the negative electrode sheet after processes such as drying and cold pressing. The surface of the negative electrode current collector coated with the negative electrode slurry can be a single surface of the negative electrode current collector or both surfaces of the negative electrode current collector.

[0088] electrolytes

[0089] Electrolytes function to conduct ions between the positive and negative electrode plates. This application does not impose any particular restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or completely solid.

[0090] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0091] In some embodiments, the electrolyte salt may include one or more of sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium perchlorate (NaClO4), sodium hexafluoroarsenate (NaAsF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaTFS), sodium difluorophosphate (NaPO2F2), sodium difluorooxalate borate (NaDFOB), sodium dioxalate borate (NaBOB), sodium difluorodioxalate phosphate (NaDFOP), and sodium tetrafluorooxalate phosphate (NaTFOP).

[0092] In some embodiments, the solvent may include one or more of ethylene carbonate, propylene carbonate, ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butenyl carbonate, fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0093] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0094] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0095] This application does not impose any particular limitation on the type of secondary battery. For example, the secondary battery can be a lithium-ion battery, a sodium-ion battery, a lithium-sulfur battery, etc. Typically, a secondary battery includes a positive electrode, a negative electrode, a composite separator, and an electrolyte. During the charging and discharging process of the secondary battery, active ions repeatedly insert and extract between the positive and negative electrode. The electrolyte acts as a conductor for the active ions between the positive and negative electrode. The composite separator is disposed between the positive and negative electrode to isolate them.

[0096] In some embodiments, the secondary battery may be a metal-free battery, such as a sodium-free metal battery or a lithium-free metal battery.

[0097] In this paper, "negative electrode-free metal battery" refers to a battery that does not add negative electrode active materials during the manufacturing process, and only uses a negative electrode current collector as the nominal negative electrode. However, the negative electrode current collector itself does not have the function of a negative electrode. Only after the first charge is completed, the metal in the positive electrode material migrates to the surface of the negative electrode current collector and forms a metal layer on it, thus becoming a true negative electrode. The innovation of this battery technology lies in its simplification of battery structure, improvement of battery energy density, and potential reduction of battery production costs. At the same time, due to the elimination of negative electrode active materials, this battery may also exhibit certain advantages in terms of safety. During the charging and discharging process, the negative electrode-free metal battery will experience greater volume expansion due to the deposition and dissolution of metals and the characteristics of the negative electrode-free structure. The composite separator of this application, used in negative electrode-free metal batteries, can effectively improve its volume expansion.

[0098] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 2 This is an example of a square-structured secondary battery 5.

[0099] In some embodiments, such as Figure 3As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0100] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0101] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0102] Figure 4 This is a schematic diagram of battery module 4 as an example. Figure 4 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0103] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0104] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0105] Figure 5 and Figure 6 This is a schematic diagram of battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0106] The second aspect of this application provides a method for preparing a composite diaphragm, comprising the following steps: providing a base membrane, coating a heat-resistant coating on one or both sides of the base membrane, then taking a polymer and water, mixing them to form a slurry, and coating the slurry formed by the mixture onto one or both sides of the heat-resistant coating in a dot matrix coating manner.

[0107] In this paper, the term "matrix coating" refers to the process of spraying coating material into tiny particles through a high-pressure nozzle, forming a matrix-like coating pattern. Compared to other coating processes, dot coating allows the operator to precisely place the slurry in dots at specific locations on the substrate film, avoiding the unevenness that may result from traditional large-area coating. Furthermore, because dot coating involves precise placement of the slurry, unnecessary slurry usage is reduced, thus saving slurry and preventing waste. Through dot coating, the slurry can be uniformly coated on the substrate film surface, and after the polymer absorbs the electrolyte and swells, it can uniformly create space between the substrate film and the negative electrode.

[0108] In some embodiments, the mass ratio of polymer to water is 20-40:60-80.

[0109] In some embodiments, the mass content of the polymer in the slurry may be selected as 20%, 30%, 40%, or any value between the two.

[0110] In some embodiments, the water content in the slurry can be selected as 60%, 70%, 80%, or any value range between the two.

[0111] The slurry formed by mixing polymer and water has stronger viscosity than the polymer itself, resulting in greater adhesion to the base film. It is less prone to peeling or detachment after coating, leading to greater durability and stability. When polymer and water are mixed in the aforementioned mass ratio to form a slurry, the viscosity can be maintained within a suitable range (1000–3000 mPa·s). This facilitates stable coating, preventing both insufficient viscosity from affecting polymer particle formation and excessive viscosity from clogging the nozzle during coating.

[0112] In some embodiments, the coating speed of the dot matrix coating is 60 m / min to 120 m / min. Controlling the coating speed within the above range is beneficial for balancing production efficiency and the quality of gel point distribution.

[0113] In any embodiment, the coating speed for coating the polymer gel layer onto the base film or heat-resistant layer can be selected as 60 m / min, 70 m / min, 80 m / min, 90 m / min, 100 m / min, 110 m / min, 120 m / min or any value range between the two.

[0114] This application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may be, but is not limited to, mobile devices (e.g., mobile phones, tablets, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0115] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0116] Figure 7 This is a schematic diagram of an example electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0117] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0118] Example

[0119] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0120] (1) Preparation of composite membrane

[0121] Example 1

[0122] A heat-resistant coating is applied to one or both sides of a polyethylene film as the base film.

[0123] Take polyethylene oxide and water, and mix them in a mass ratio of 3:7 to form a slurry;

[0124] The spacing between the holes on the engraving roller was controlled to be 200 μm, and the hole depth was 7 μm. The mixed slurry was coated onto the base film using the engraving roller at a coating speed of 90 m / min. The thickness of the polymer gel points on the resulting composite membrane was 10 μm.

[0125] The average diameter of the gel dots is 3 μm, and the coverage of the gel dots on the base film is 60%.

[0126] Example 2

[0127] The preparation method of Example 2 is basically the same as that of Example 1, except that the type of polymer used is changed, as shown in Table 1.

[0128] Examples 3-4

[0129] The preparation methods of Examples 3-4 are basically the same as those of Example 1. The difference is that the thickness of the polymer gel points is changed by controlling the coating speed and the depth of the holes on the engraving roller, as shown in Table 1.

[0130] Examples 5-6

[0131] The preparation methods of Examples 5-6 are basically the same as those of Example 1. The difference is that the coverage of the polymer gel dots on the base film is changed by changing the spacing between gel dots and the average diameter of the gel dots, as shown in Table 1.

[0132] Examples 7-8

[0133] The preparation methods of Examples 7-8 are basically the same as those of Example 1, except that the spacing between gel dots and the average diameter of gel dots are changed, thereby changing the coverage of the polymer gel dots on the base film, as shown in Table 1.

[0134] Examples 9-10

[0135] The preparation methods of Examples 9-10 are basically the same as those of Example 1, except that the thickness of the polymer gel points is changed by controlling the coating speed and the depth of the holes on the engraving roller, as shown in Table 1.

[0136] Comparative Example 1

[0137] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the coating slurry method is changed, as shown in Table 1.

[0138] Comparative Example 2

[0139] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that the type of polymer is changed, as shown in Table 1.

[0140] Comparative Example 3

[0141] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that the coating slurry method and the type of polymer used are changed, as shown in Table 1.

[0142] (2) Preparation of secondary batteries

[0143] (a) Positive electrode plate

[0144] The positive electrode active material sodium iron pyrophosphate, the conductive agent carbon nanotubes, and the binder metahexafluorophosphate are thoroughly mixed in N-methylpyrrolidone (NMP) at a mass ratio of 95:2:3 to form a uniform positive electrode slurry. The positive electrode slurry is coated on the surface of the positive electrode current collector aluminum foil, and after drying, cold pressing, and die cutting, the positive electrode sheet is obtained.

[0145] (b) Negative electrode plate

[0146] 10 wt% conductive carbon black and an appropriate amount of deionized water are mixed to form a slurry, and then a conductive coating with a thickness of 2 μm is formed on the surface of copper foil by extrusion coating.

[0147] (c) Separating membrane

[0148] The composite membrane as described above or the composite membrane prepared according to the above method.

[0149] (d) Electrolyte

[0150] In an argon-filled glove box with a water content of <1ppm, diethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 1:3, and sodium hexafluorophosphate (NaPF6) with a concentration of 1.0mol / L was added. After stirring evenly, an electrolyte was obtained.

[0151] (e) Assembling the battery

[0152] The positive electrode, separator, and copper foil prepared in the above steps are stacked in sequence, so that the separator is between the positive electrode and the copper foil and can isolate the positive electrode and the copper foil. Then, the stacked components are stacked to obtain an electrode assembly. The electrode assembly is placed in the housing, dried, and then injected with electrolyte. After formation, settling and other processes, a sodium metal battery without a negative electrode is obtained.

[0153] Performance testing

[0154] (1) Polymer swelling test

[0155] Swelling tests are a method for evaluating the swelling behavior of polymer materials in a specific solvent. The steps are as follows:

[0156] 1. Sample preparation: Take polymer material, wipe the disc press, and then use the disc press to mold the polymer material into small discs with a thickness of 1mm, ensuring that the surface of the obtained sample is clean and free of impurities.

[0157] 2. Solvent selection: Dimethyl ether (DME) was selected as the solvent.

[0158] 3. Swelling test: Place the sample in the selected solvent and immerse it at 25°C for 24 hours. Check the swelling and dimensional changes of the sample periodically.

[0159] 4. Data Analysis: Measure the dimensions of the sample using a micrometer and record the changes in sample dimensions during the swelling process. Calculate the swelling rate, typically using the following formula: Swelling Rate = (Swelled Size - Original Size) / Original Size × 100%.

[0160] (2) Test of effective space expansion after polymer swelling

[0161] Effective support space test method: 1. Prepare a layer of positive electrode / diaphragm coated with adhesive layer / negative electrode stacked together, and measure the thickness H1 of the sample with a micrometer under a surface pressure of 1 MPa;

[0162] 2. After immersing the above sample in DME solvent for 48 hours, remove it and measure the thickness H2 of the sample with a micrometer under a surface pressure of 1 MPa.

[0163] 3. Effective support space: (H2-H1) / H1

[0164] 4. Effectively supports spatial accounting:

[0165] ① Dot matrix coating: 80% of the thickness after swelling

[0166] ② Spray coating: Thickness after swelling × 50%

[0167] (3) Test of average diameter and spacing of gel dots

[0168] 1. Sample preparation: Cut a 5cm*5cm diaphragm sample and coat the sample surface with a conductive film to reduce the impact of charge accumulation on imaging quality;

[0169] 2. Instrument Operation: After sample preparation, install the sample in the SEM instrument. By adjusting the instrument parameters, such as voltage, current, and magnification, select the appropriate working mode and imaging mode to obtain a clear image.

[0170] 3. The obtained images were analyzed and processed in reverse, and the diameter and spacing of the gel dots in the matrix were measured.

[0171] (4) Secondary battery single-cycle expansion force test

[0172] Take three steel clamps, including a first clamp and a second clamp. Clamp the secondary battery to be tested between the first clamp and the second clamp. Place a sensor between the clamp and the secondary battery. Then charge the battery cell at room temperature at a rate of 0.33C until the voltage is equal to 3.65V. Record the expansion force F1 of the sensor. Then discharge it at a rate of 0.33C until the voltage is equal to 1.5V. Record the expansion force F2 of the sensor. The difference between F1 and F2 is the expansion force of the secondary battery in one cycle.

[0173] (5) Secondary battery cycle capability test

[0174] Each battery cell is charged at room temperature at a rate of 0.33C until the voltage equals 3.65V, and then discharged at a rate of 0.33C until the voltage equals 1.5V. The reversible capacity, C0, is measured using a charge / discharge machine. This charging and discharging process is repeated until the discharge capacity Cn / C0 = 80% in a certain cycle. The total number of cycles is denoted as X-Cycle, where Cn is the reversible capacity at the nth cycle.

[0175] Table 1

[0176]

[0177]

[0178]

[0179] Table 2

[0180]

[0181]

[0182] As can be seen from Example 1 and Comparative Examples 1-3, when gel is dotted on the base membrane, the space effectively supported by the gel after it absorbs electrolyte and swells is larger. When the composite separator prepared in the above manner is used in a secondary battery, the expansion force of the secondary battery during cycle is smaller, and the cycle capacity of the secondary battery is better.

[0183] As can be seen from Examples 1 and 3-4, the greater the thickness of the gel point, the larger the space supported after the gel absorbs electrolyte and swells, resulting in better cycle performance of the composite separator used in secondary batteries.

[0184] As can be seen from Examples 1 and 5-6, the greater the coverage of gel points on the base film, the larger the space effectively supported after the gel absorbs electrolyte and swells, resulting in better cycle performance of the composite separator used in secondary batteries.

[0185] As can be seen from Examples 1 and 5-8, when the coverage of the gel points on the base film is controlled at 20%-60%, the gel swells after absorbing the electrolyte and effectively supports a larger space, resulting in a composite separator with better cycle capacity for secondary batteries.

[0186] As can be seen from Examples 1 and 3-4, 9-10, when the thickness of the gel point is controlled at 2-20 μm, the gel swells after absorbing the electrolyte and effectively supports a larger space, resulting in a composite separator with better cycle performance for secondary batteries.

[0187] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a composite separator, which includes a base film and an adhesive coating. The adhesive coating includes a plurality of gel dots arranged at a predetermined interval.

2. The secondary battery according to claim 1, characterized in that, The gel point includes a polymer, which may be at least one of polyether and polyester.

3. The secondary battery according to claim 2, characterized in that, The polyether includes at least one of polyethylene oxide, polypropylene ether, polyetherketone, and polymethyl ether; and / or The polyester includes at least one of polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, and trimethylolpropane triacrylate.

4. The secondary battery according to claim 1, characterized in that, The composite membrane further includes a heat-resistant coating, which is attached to one or both sides of the base membrane; and / or The heat-resistant coating includes at least one of inorganic particles or organic particles.

5. The secondary battery according to any one of claims 1-4, characterized in that, The gel points cover 20%-60% of the heat-resistant coating.

6. The secondary battery according to any one of claims 1-5, characterized in that, The spacing between the gel dots is 100um-400um.

7. The secondary battery according to any one of claims 1-6, characterized in that, The average diameter of the gel points is 2µm-6µm.

8. The secondary battery according to claim 1, characterized in that, The thickness of the gel point is 2-20 μm.

9. The secondary battery according to claim 1, characterized in that, The adhesive coating is attached to one or both sides of the heat-resistant coating.

10. The secondary battery according to claim 1, characterized in that, The secondary battery also includes a negative electrode sheet, and the adhesive coating is attached to one side of the heat-resistant coating, facing the negative electrode sheet.

11. The secondary battery according to claim 1, wherein the secondary battery comprises a negative electrode-free metal battery, optionally a negative electrode-free sodium metal battery or a negative electrode-free lithium metal battery.

12. A method for preparing a composite diaphragm, characterized in that, Includes the following steps: Provide a base film, and coat one or both sides of the base film with a heat-resistant coating; Take the polymer and water, and mix them to form a slurry; The mixed slurry is applied to one or both sides of the heat-resistant coating using a dot matrix coating method.

13. The method for preparing the composite diaphragm according to claim 12, characterized in that, The polymer includes at least one of polyether or polyester.

14. The method for preparing the composite diaphragm according to claim 12, characterized in that, The mass ratio of the polymer to water is 20-40:60-80.

15. The method for preparing the composite diaphragm according to any one of claims 12-14, characterized in that, The coating speed of the dot matrix coating is 60m / min-120m / min.

16. An electrical appliance, characterized in that, Includes the secondary battery as described in claims 1-11 or the composite separator prepared by the composite separator preparation method described in claims 12-15.

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