Composite separator, preparation method therefor, secondary battery and electrical apparatus

Abstract

The present application relates to the field of batteries. Disclosed are a composite separator, a preparation method therefor, a secondary battery, and an electrical apparatus. The secondary battery comprises a positive electrode plate, a composite separator, a negative electrode plate, and an electrolyte, wherein the composite separator comprises a plurality of independent porous film layers stacked and connected to each other, and the porosities of the plurality of porous film layers successively increase in the direction close to the negative electrode plate. The present application can improve the performance of secondary batteries resistant to internal short circuits and widen charging windows of the secondary batteries.

Description

Composite separator and its preparation method, secondary battery and power device

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. 202510314677.0, filed on March 17, 2025, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, specifically to a composite separator and its preparation method, a secondary battery, and an electrical device. Background Technology

[0004] In recent years, with the increasing demand for clean energy, secondary batteries have been widely used in energy storage power 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. During the use of secondary batteries, dendrites will form on the surface of the negative electrode, especially under high-rate charging conditions, where the growth of dendrites becomes more disordered, easily leading to internal short circuits. Summary of the Invention

[0005] 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, which can improve the internal short-circuit resistance of the secondary battery and broaden the charging window of the secondary battery.

[0006] To achieve the above objectives, this application proposes a composite separator and its preparation method, a secondary battery, and an electrical device thereof.

[0007] In a first aspect, embodiments of this application propose a secondary battery, which includes a positive electrode, a composite separator, a negative electrode, and an electrolyte; wherein the composite separator includes multiple independent and interconnected porous membrane layers, and the porosity of the multiple porous membrane layers increases sequentially in the direction approaching the negative electrode.

[0008] Therefore, in the technical solution of this application, the porosity of the multiple porous membrane layers of the composite separator increases sequentially in the direction close to the negative electrode. The multiple porous membrane layers are independently arranged and interconnected. From the positive electrode side to the negative electrode side, the porosity gradient of the multiple porous membrane layers of the composite separator increases, which can widen the ion flow channel, effectively improve local dynamics, improve the uniformity of metal deposition, and inhibit dendrite growth. The multiple porous membrane layers also help prevent dendrites from piercing the composite separator. At the same time, the multiple porous membrane layers of the composite separator are independently arranged and interconnected, which provides a high degree of freedom of selection and facilitates the control of the porosity gradient difference of the multiple porous membrane layers, which is beneficial to inhibiting dendrite growth. Thus, the internal short-term resistance of the secondary battery can be improved, and the charging window of the secondary battery can be widened.

[0009] In any embodiment, the composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in a direction close to the outer layer. The outer layer includes a third porous membrane layer, and the porosity of the first porous membrane layer, the second porous membrane layer, and the third porous membrane layer increases sequentially.

[0010] A first adhesive layer is disposed between the first porous membrane layer and the second porous membrane layer. The first porous membrane layer and the second porous membrane layer are bonded together by the first adhesive layer, which can improve the stability of the composite membrane. Alternatively, the first porous membrane layer and the second porous membrane layer are bonded together by hot pressing. The first porous membrane layer and the second porous membrane layer are bonded together by hot pressing, which can eliminate the need for an adhesive and helps reduce the generation of side reactions; and / or,

[0011] A second adhesive layer is provided between the third porous membrane layer and the second porous membrane layer. The second porous membrane layer and the third porous membrane layer are bonded together by the second adhesive layer, which can improve the stability of the composite membrane. Alternatively, the third porous membrane layer includes a non-woven fabric layer formed on the second porous membrane layer. The non-woven fabric layer can be formed directly on the second porous membrane layer and bonded to the second porous membrane layer by electrostatic adsorption, which helps to reduce the generation of side reactions.

[0012] In any embodiment, the composite separator includes a separator body and an outer layer disposed on one side of the separator body. The separator body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first, second, and third porous membrane layers increases sequentially, wherein the porosity of the third porous membrane layer is 60% to 90%. The porosity of the third porous membrane layer is within the above range, which can improve the uniformity of metal deposition and help suppress dendrite growth. The third porous membrane layer has a large porosity and can also support the metal deposited from the negative electrode, alleviating expansion. At the same time, the first and second porous membrane layers can effectively prevent dendrites from piercing the composite separator, thereby improving the short-term resistance of the secondary battery and widening the charging window of the secondary battery.

[0013] In any embodiment, the porosity of the first porous membrane layer is 20% to 30%. A porosity within this range is beneficial in preventing dendrites from piercing the composite membrane; and / or,

[0014] The porosity of the second porous membrane layer is 35% to 45%. The porosity of the second porous membrane layer falling within this range helps prevent dendrites from piercing the composite membrane.

[0015] In any embodiment, the pore size of the plurality of porous membrane layers increases sequentially in the direction approaching the negative electrode sheet. From the positive electrode side to the negative electrode side, the pore size of the composite separator increases layer by layer, which helps to improve the uniformity of metal deposition, thereby improving the internal short-circuit tolerance of the secondary battery and widening the charging window of the secondary battery.

[0016] In any embodiment, the composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in the direction near the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first, second, and third porous membrane layers increases sequentially, and the pore size of the first, second, and third porous membrane layers increases sequentially, wherein:

[0017] The pore size of the first porous membrane layer is 10 nm to 30 nm. The pore size of the first porous membrane layer falling within this range helps to prevent dendrites from piercing the composite separator; and / or,

[0018] The pore size of the second porous membrane layer is 30 nm to 150 nm. The pore size of the second porous membrane layer falling within this range helps to prevent dendrites from piercing the composite separator; and / or,

[0019] The pore size of the third porous film is 2000 nm to 20000 nm. The pore size of the third porous film falling within this range can improve the uniformity of metal deposition and help suppress dendrite growth.

[0020] In any embodiment, the secondary battery includes a non-negative electrode sodium metal battery. Non-negative electrode sodium metal batteries have good fast-charging performance, and the composite separator can improve the uniformity of sodium deposition, enhance the short-term internal resistance of the non-negative electrode sodium metal battery, and widen the charging window of the non-negative electrode sodium metal battery.

[0021] Secondly, embodiments of this application propose a composite membrane, which includes multiple independent porous membrane layers that are interconnected, wherein the porosity of the multiple porous membrane layers increases sequentially.

[0022] The composite separator is used in a secondary battery. The porous membrane layer with the highest porosity is positioned close to the negative electrode. From the positive electrode side to the negative electrode side, the porosity gradient of the multiple porous membrane layers in the composite separator increases, which widens the ion flow channels, effectively improves local dynamics, enhances the uniformity of metal deposition, and inhibits dendrite growth. The multiple porous membrane layers also help prevent dendrites from piercing the composite separator. Simultaneously, the multiple porous membrane layers of the composite separator are independently arranged and interconnected, offering high flexibility in selection and facilitating the control of the porosity gradient difference among the multiple porous membrane layers, which is beneficial for inhibiting dendrite growth. Therefore, it can improve the internal short-term resistance of the secondary battery and widen the charging window of the secondary battery.

[0023] In any embodiment, the composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in a direction close to the outer layer. The outer layer includes a third porous membrane layer, and the porosity of the first porous membrane layer, the second porous membrane layer, and the third porous membrane layer increases sequentially.

[0024] A first adhesive layer is disposed between the first porous membrane layer and the second porous membrane layer. The first porous membrane layer and the second porous membrane layer are bonded together by the first adhesive layer, which can improve the stability of the composite membrane. Alternatively, the first porous membrane layer and the second porous membrane layer are bonded together by hot pressing. The first porous membrane layer and the second porous membrane layer are bonded together by hot pressing, which can eliminate the need for an adhesive and helps reduce the generation of side reactions; and / or,

[0025] A second adhesive layer is provided between the third porous membrane layer and the second porous membrane layer. The second porous membrane layer and the third porous membrane layer are bonded together by the second adhesive layer, which can improve the stability of the composite membrane. Alternatively, the third porous membrane layer includes a non-woven fabric layer formed on the second porous membrane layer. The non-woven fabric layer can be formed directly on the second porous membrane layer and bonded to the second porous membrane layer by electrostatic adsorption, which helps to reduce the generation of side reactions.

[0026] In any embodiment, the composite separator includes a separator body and an outer layer disposed on one side of the separator body. The separator body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first, second, and third porous membrane layers increases sequentially, wherein the porosity of the third porous membrane layer is 60% to 90%. When used in a secondary battery, placing the third porous membrane layer close to the negative electrode plate, with the porosity of the third porous membrane layer within the aforementioned range, can improve the uniformity of metal deposition and suppress dendrite growth. The third porous membrane layer has a large porosity and can also support metal deposited from the negative electrode, alleviating expansion. Simultaneously, the first and second porous membrane layers can effectively prevent dendrites from piercing the composite separator, thereby improving the internal short-circuit tolerance of the secondary battery and widening its charging window.

[0027] In any embodiment, the porosity of the first porous membrane layer is 20% to 30%. A porosity within this range is beneficial in preventing dendrites from piercing the composite membrane; and / or,

[0028] The porosity of the second porous membrane layer is 35% to 45%. The porosity of the second porous membrane layer falling within this range helps prevent dendrites from piercing the composite membrane.

[0029] In any embodiment, the pore size of the plurality of porous membrane layers increases sequentially along the direction in which the porosity of the plurality of porous membrane layers increases sequentially. When used in a secondary battery, the pore size of the composite separator increases layer by layer from the positive electrode side to the negative electrode side, which helps to improve the uniformity of metal deposition, thereby improving the internal short-circuit tolerance of the secondary battery and widening its charging window.

[0030] Thirdly, this application provides a method for preparing a composite membrane, which includes the following steps: bonding a first porous membrane layer and a second porous membrane layer together with a first adhesive to obtain a membrane body;

[0031] A nonwoven fabric layer is prepared by spinning on the side of the second porous membrane layer opposite to the first porous membrane layer to obtain a composite membrane.

[0032] The porosity of the first porous membrane layer, the second porous membrane layer, and the nonwoven fabric layer increases sequentially.

[0033] The first porous membrane layer and the second porous membrane layer are bonded together by a first adhesive to obtain the membrane body, which can improve the stability of the membrane body. On the side of the second porous membrane layer opposite to the first porous membrane layer, a non-woven fabric layer is prepared by spinning. The non-woven fabric layer and the second porous membrane layer are bonded together by electrostatic adsorption, which helps to reduce the generation of side reactions. In the obtained composite membrane, the first porous membrane layer, the second porous membrane layer and the non-woven fabric are independently and layered with each other, and the porosity increases sequentially, which can improve the internal short-term resistance of the secondary battery and widen the charging window of the secondary battery.

[0034] In any embodiment, the step of bonding the first porous membrane layer and the second porous membrane layer together with a first adhesive to obtain the diaphragm body includes: coating the first porous membrane layer with the first adhesive, drying it, then heating it to melt the first adhesive, then bonding the second porous membrane layer with the melted first adhesive, cooling, and curing to obtain the diaphragm body. Using dry lamination technology to bond the first and second porous membrane layers to obtain the diaphragm body can improve the quality of the diaphragm body and also improve production efficiency.

[0035] In any embodiment, in the step of preparing a nonwoven fabric layer by spinning on the side of the second porous membrane layer opposite to the first porous membrane layer to obtain a composite separator, the spinning includes electrospinning or meltblown spinning. Using the above spinning method helps to ensure the quality of the composite separator.

[0036] Fourthly, embodiments of this application propose an electrical device including a secondary battery as described in the first aspect. Attached Figure Description

[0037] Figure 1 is a schematic diagram of a composite diaphragm according to an embodiment of this application;

[0038] Figure 2 is a schematic diagram of a secondary battery according to an embodiment of this application;

[0039] Figure 3 is an exploded view of the secondary battery according to an embodiment of this application shown in Figure 2;

[0040] Figure 4 is a schematic diagram of a battery module according to an embodiment of this application;

[0041] Figure 5 is a schematic diagram of a battery pack according to an embodiment of this application;

[0042] Figure 6 is an exploded view of the battery pack of one embodiment of this application shown in Figure 5;

[0043] Figure 7 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of this application.

[0044] Explanation of reference numerals in the attached drawings: 10 Composite separator; 11 First porous membrane layer; 12 Second porous membrane layer; 13 Non-woven fabric layer; 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0045] 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.

[0046] 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.

[0047] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0048] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0049] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, optionally 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 method may also include step (c), indicating 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.

[0050] 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.

[0051] In recent years, with the increasing demand for clean energy, secondary batteries have been widely used in energy storage power 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. During the use of secondary batteries, dendrites will form on the surface of the negative electrode, especially under high-rate charging conditions, where the growth of dendrites becomes more disordered, easily leading to internal short circuits.

[0052] Based on this, this application proposes a composite separator and its preparation method, a secondary battery, and an electrical device.

[0053] In a first aspect, embodiments of this application propose a secondary battery, which includes a positive electrode, a composite separator 10, a negative electrode, and an electrolyte; wherein the composite separator 10 includes a plurality of independent and interconnected porous membrane layers, and the porosity of the plurality of porous membrane layers increases sequentially in the direction close to the negative electrode.

[0054] Therefore, in the technical solution of this application, the porosity of the multiple porous membrane layers of the composite separator 10 increases sequentially in the direction close to the negative electrode. The multiple porous membrane layers are independently arranged and interconnected. From the positive electrode side to the negative electrode side, the porosity gradient of the multiple porous membrane layers of the composite separator 10 increases, which can widen the ion flow channel, effectively improve local dynamics, improve the uniformity of metal deposition, and inhibit dendrite growth. The multiple porous membrane layers also help prevent dendrites from piercing the composite separator 10. At the same time, the multiple porous membrane layers of the composite separator 10 are independently arranged and interconnected, which provides a high degree of freedom of selection and facilitates the control of the porosity gradient difference of the multiple porous membrane layers, which is beneficial to inhibiting dendrite growth. Thus, the internal short-term resistance of the secondary battery can be improved and the charging window of the secondary battery can be widened.

[0055] It should be noted that the secondary battery includes, but is not limited to, lithium-ion batteries, sodium-ion batteries, lithium metal batteries, sodium metal batteries, negative electrode-free lithium metal batteries, or negative electrode-free sodium metal batteries; multiple independent porous membrane layers refer to each porous membrane layer being formed separately.

[0056] In any embodiment, the composite membrane 10 includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer 11 and a second porous membrane layer 12 disposed sequentially in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer 11, the second porous membrane layer 12 and the third porous membrane layer increases sequentially. A first adhesive layer is disposed between the first porous membrane layer 11 and the second porous membrane layer 12. The first porous membrane layer 11 and the second porous membrane layer 12 are bonded by the first adhesive layer, which can improve the stability of the composite membrane 10.

[0057] In any embodiment, the first porous membrane layer 11 and the second porous membrane layer 12 are bonded together by hot pressing. This hot pressing bonding eliminates the need for adhesives and helps reduce the generation of side reactions.

[0058] In any embodiment, a second adhesive layer is disposed between the third porous membrane layer and the second porous membrane layer 12. The second porous membrane layer 12 and the third porous membrane layer are bonded together by the second adhesive layer, which can improve the stability of the composite membrane 10. It is understood that the first adhesive and the second adhesive may be the same or different.

[0059] In any embodiment, the third porous membrane layer includes a nonwoven fabric layer 13 formed on the second porous membrane layer 12. The nonwoven fabric layer 13 is formed directly on the second porous membrane layer 12 and is bonded to the second porous membrane layer 12 by electrostatic adsorption, which helps to reduce the generation of side reactions. It is understood that the nonwoven fabric layer 13 can be formed directly on the second porous membrane layer 12 by spinning, wherein the spinning can be electrostatic or meltblown spinning.

[0060] In any embodiment, the composite separator 10 includes a separator body and an outer layer disposed on one side of the separator body. The separator body includes a first porous membrane layer 11 and a second porous membrane layer 12 sequentially disposed in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer 11, the second porous membrane layer 12, and the third porous membrane layer increases sequentially, wherein the porosity of the third porous membrane layer is 60% to 90%. The porosity of the third porous membrane layer within the above range can improve the uniformity of metal deposition and help suppress dendrite growth. The third porous membrane layer has a large porosity and can also support the metal deposited from the negative electrode, alleviating expansion. Simultaneously, the first porous membrane layer 11 and the second porous membrane layer 12 can effectively prevent dendrites from piercing the composite separator 10, thereby improving the short-term resistance of the secondary battery and widening the charging window of the secondary battery. The porosity of the third porous membrane layer can be 60%, 65%, 70%, 75%, 80%, 85%, or 90%.

[0061] In any embodiment, the porosity of the first porous membrane layer 11 is 20% to 30%. A porosity within this range is beneficial in preventing dendrites from piercing the composite diaphragm 10. The porosity of the first porous membrane layer 11 can be 20%, 22.5%, 25%, 27.5%, or 30%.

[0062] In any embodiment, the porosity of the second porous membrane layer 12 is 35% to 45%. A porosity within this range is beneficial in preventing dendrites from piercing the composite diaphragm 10. The porosity of the second porous membrane layer 12 can be 35%, 37.5%, 40%, 42.5%, or 45%.

[0063] In any embodiment, the pore size of the plurality of porous membrane layers increases sequentially in the direction approaching the negative electrode sheet. From the positive electrode side to the negative electrode side, the pore size of the composite separator 10 increases layer by layer, which is beneficial to improving the uniformity of metal deposition, thereby improving the internal short-circuit tolerance of the secondary battery and widening the charging window of the secondary battery.

[0064] In any embodiment, the composite membrane 10 includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer 11 and a second porous membrane layer 12 sequentially disposed in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer 11, the second porous membrane layer 12, and the third porous membrane layer increases sequentially, and the pore size of the first porous membrane layer 11, the second porous membrane layer, and the third porous membrane layer increases sequentially. Specifically, the pore size of the first porous membrane layer 11 is 10 nm to 30 nm. The pore size of the first porous membrane layer 11 being within the above range is beneficial in preventing dendrites from piercing the composite membrane 10. The pore size of the first porous membrane layer 11 can be 10 nm, 15 nm, 20 nm, 25 nm, or 30 nm.

[0065] In any embodiment, the pore size of the second porous membrane layer 12 is 30 nm to 150 nm. Having the pore size of the second porous membrane layer 12 within this range helps prevent dendrites from piercing the composite separator 10. The pore size of the second porous membrane layer 12 can be 30 nm, 50 nm, 70 nm, 90 nm, 110 nm, 130 nm, or 150 nm.

[0066] In any embodiment, the pore size of the third porous film layer is 2000 nm to 20000 nm. The pore size of the third porous film layer falling within this range can improve the uniformity of metal deposition and help suppress dendrite growth. The pore size of the third porous film layer can be 2000 nm, 4000 nm, 6000 nm, 8000 nm, 10000 nm, 12000 nm, 14000 nm, 16000 nm, 18000 nm, or 20000 nm.

[0067] In any embodiment, the secondary battery includes a non-negative electrode sodium metal battery. Non-negative electrode sodium metal batteries have good fast-charging performance, and the composite separator 10 can improve the uniformity of sodium deposition, enhance the short-term internal resistance of the non-negative electrode sodium metal battery, and widen the charging window of the non-negative electrode sodium metal battery.

[0068] Secondly, please refer to Figure 1. This application embodiment proposes a composite membrane 10, which consists of multiple independent and interconnected porous membrane layers, wherein the porosity of the multiple porous membrane layers increases sequentially.

[0069] The composite separator 10 is used in a secondary battery. The porous membrane layer with the highest porosity is positioned close to the negative electrode. From the positive electrode side to the negative electrode side, the porosity gradient of the multiple porous membrane layers in the composite separator 10 increases, which can widen the ion flow channels, effectively improve local dynamics, enhance the uniformity of metal deposition, and suppress dendrite growth. The multiple porous membrane layers also help prevent dendrites from piercing the composite separator 10. Simultaneously, the multiple porous membrane layers of the composite separator 10 are independently arranged and interconnected, offering high flexibility in selection and facilitating the control of the porosity gradient difference among the multiple porous membrane layers, which is beneficial for suppressing dendrite growth. Therefore, it can improve the internal short-circuit tolerance of the secondary battery and widen the charging window of the secondary battery.

[0070] In any embodiment, the composite membrane 10 includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer 11 and a second porous membrane layer 12 disposed sequentially in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer 11, the second porous membrane layer 12 and the third porous membrane layer increases sequentially. A first adhesive layer is disposed between the first porous membrane layer 11 and the second porous membrane layer 12. The first porous membrane layer 11 and the second porous membrane layer 12 are bonded by the first adhesive layer, which can improve the stability of the composite membrane 10.

[0071] In any embodiment, the first porous membrane layer 11 and the second porous membrane layer 12 are bonded together by hot pressing. This hot pressing bonding eliminates the need for adhesives and helps reduce the generation of side reactions.

[0072] In any embodiment, a second adhesive layer is provided between the third porous membrane layer and the second porous membrane layer 12, and the second porous membrane layer 12 and the third porous membrane layer are bonded by the second adhesive layer, which can improve the stability of the composite membrane 10.

[0073] In any embodiment, the third porous membrane layer includes a nonwoven fabric layer 13 formed on the second porous membrane layer 12. The nonwoven fabric layer 13 can be formed directly on the second porous membrane layer 12 and bonded to the second porous membrane layer 12 by electrostatic adsorption, which helps to reduce the generation of side reactions.

[0074] In any embodiment, the composite separator 10 includes a separator body and an outer layer disposed on one side of the separator body. The separator body includes a first porous membrane layer 11 and a second porous membrane layer 12 sequentially disposed in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer 11, the second porous membrane layer 12, and the third porous membrane layer increases sequentially, wherein the porosity of the third porous membrane layer is 60% to 90%. When used in a secondary battery, placing the third porous membrane layer close to the negative electrode plate, with the porosity of the third porous membrane layer within the aforementioned range, can improve the uniformity of metal deposition and help suppress dendrite growth. The third porous membrane layer has a large porosity and can also support the metal deposited from the negative electrode, alleviating expansion. Simultaneously, the first porous membrane layer 11 and the second porous membrane layer 12 can effectively prevent dendrites from piercing the composite separator 10, thereby improving the internal short-circuit tolerance of the secondary battery and widening the charging window of the secondary battery. The porosity of the third porous membrane layer can be 60%, 65%, 70%, 75%, 80%, 85%, or 90%.

[0075] In any embodiment, the porosity of the first porous membrane layer 11 is 20% to 30%. A porosity within this range is beneficial in preventing dendrites from piercing the composite diaphragm 10. The porosity of the first porous membrane layer 11 can be 20%, 22.5%, 25%, 27.5%, or 30%.

[0076] In any embodiment, the porosity of the second porous membrane layer 12 is 35% to 45%. A porosity within this range is beneficial in preventing dendrites from piercing the composite diaphragm 10. The porosity of the second porous membrane layer 12 can be 35%, 37.5%, 40%, 42.5%, or 45%.

[0077] In any embodiment, the pore size of the plurality of porous membrane layers increases sequentially along the direction in which the porosity of the plurality of porous membrane layers increases sequentially. When used in a secondary battery, the pore size of the composite separator 10 increases layer by layer from the positive electrode side to the negative electrode side, which helps to improve the uniformity of metal deposition, thereby improving the internal short-circuit tolerance of the secondary battery and widening its charging window.

[0078] Thirdly, embodiments of this application provide a method for preparing a composite diaphragm 10, comprising the following steps:

[0079] The first porous membrane layer 11 and the second porous membrane layer 12 are bonded together with a first adhesive to obtain the membrane body;

[0080] A nonwoven fabric layer 13 is prepared by spinning on the side of the second porous membrane layer 12 opposite to the first porous membrane layer 11 to obtain a composite diaphragm 10.

[0081] The porosity of the first porous membrane layer 11, the second porous membrane layer 12, and the nonwoven fabric layer 13 increases sequentially.

[0082] The first porous membrane layer 11 and the second porous membrane layer 12 are bonded together by a first adhesive to obtain the separator body, which can improve the stability of the separator body. On the side of the second porous membrane layer 12 opposite to the first porous membrane layer 11, a non-woven fabric layer 13 is prepared by spinning. The non-woven fabric layer 13 is bonded to the second porous membrane layer 12 by electrostatic adsorption, which helps to reduce the generation of side reactions. In the composite separator 10, the first porous membrane layer 11, the second porous membrane layer 12 and the non-woven fabric are independently and layered with each other, and the porosity increases sequentially, which can improve the internal short-circuit resistance of the secondary battery and widen the charging window of the secondary battery.

[0083] In any embodiment, the step of bonding the first porous membrane layer 11 and the second porous membrane layer 12 together with a first adhesive to obtain the diaphragm body includes: coating the first porous membrane layer 11 with the first adhesive, drying it, then heating it to melt the first adhesive, then bonding the second porous membrane layer 12 to the melted first adhesive, cooling, and curing to obtain the diaphragm body. Using dry lamination technology to bond the first porous membrane layer 11 and the second porous membrane layer 12 to obtain the diaphragm body can improve the quality of the diaphragm body and also improve production efficiency. It is understood that the drying can be achieved by using a drying tunnel to dry the solvent of the first adhesive. Using a drying tunnel is beneficial for improving the quality of the diaphragm body and also for improving production efficiency.

[0084] In any embodiment, in the step of preparing a nonwoven layer 13 by spinning on the side of the second porous membrane layer 12 opposite to the first porous membrane layer 11 to obtain the composite separator 10, the spinning includes electrospinning or meltblown spinning. Using the above spinning method helps to ensure the quality of the composite separator 10. It is understood that using meltblown spinning is beneficial for improving production efficiency.

[0085] Fourthly, embodiments of this application propose an electrical device including a secondary battery as described in the first aspect.

[0086] In addition, the secondary battery, battery module, battery pack and power device of this application will be described below with appropriate reference to the accompanying drawings.

[0087] In one embodiment of this application, a secondary battery is provided, wherein the secondary battery is a sodium metal battery without a negative electrode.

[0088] Typically, a sodium metal battery without a negative electrode includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes. The separator is positioned between the positive and negative electrodes, primarily preventing short circuits while allowing ions to pass through. The negative electrode serves as the negative current collector, and the separator is the composite separator 10 described in this application.

[0089] [Positive electrode plate]

[0090] 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, the positive electrode film layer including the positive electrode active material of the first aspect of this application.

[0091] 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.

[0092] 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.).

[0093] In some embodiments, the positive electrode active material may be a positive electrode active material known in the art for use in sodium-metal batteries without a negative electrode. As an example, the positive electrode active material may include at least one of the following materials: sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds. However, 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. These positive electrode active materials may be used alone or in combination of two or more.

[0094] In the sodium transition metal oxide, the transition metal may include at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. A non-limiting example of a sodium transition metal oxide is Na. x MO2, wherein M may include at least one of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0 < x ≤ 1.

[0095] Polyanionic compounds can contain sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds with anionic units. Transition metals may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may include at least one of P, S, and Si; n represents (YO4). n- The valence state. Polyanionic compounds may also include those with sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide anions. Transition metals may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may include at least one of P, S, and Si, where n represents (YO4). n- The valence state; halogens can include at least one of F, Cl, and Br. Polyanionic compounds can also include those with sodium ions, tetrahedral (YO4) valence. n- Anionic unit, polyhedral unit (ZO) y ) m+ And a class of compounds with optional halide anions. Y may include at least one of P, S, and Si, and n represents (YO4). n- The valence state; Z represents a transition metal, which may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m represents (ZO). y ) m+ The valence state; halogens may include at least one of F, Cl, and Br. Polyanionic compounds may include NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2(P2O7), NaM'PO4F, Na3V2(PO4)2F3, and Na3(VO y )2(PO4)2F 3-2y At least one of (0≤y≤1). Wherein, M' in NaM'PO4F may include at least one of V, Fe, Mn and Ni.

[0096] Prussian blue compounds may include those containing sodium ions, transition metal ions, and cyanide ions (CN). - A class of compounds. Transition metals may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. A non-limiting example of Prussian blue compounds is Na. a Me b Me' c (CN)6, wherein Me and Me' can each independently be at least one of Ni, Cu, Fe, Mn, Co and Zn, 0 < a ≤ 2, 0 < b < 1, 0 < c < 1.

[0097] In some embodiments, the positive electrode film layer further includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0098] In some embodiments, the positive electrode film layer further includes 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.

[0099] 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, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode sheet structure, and obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0100] [Negative electrode current collector]

[0101] The surface of the negative electrode current collector does not have a layer of negative electrode active material.

[0102] In some embodiments, the negative electrode current collector used may include at least one of metal foil current collector, metal foam current collector, metal mesh current collector, carbon felt current collector, carbon cloth current collector, and carbon paper current collector.

[0103] In some embodiments, the surface of the negative electrode current collector may be provided with a conductive coating. The conductive coating may include a conductive agent and a binder. The conductive agent may include one or more of hard carbon, conductive carbon black, graphite, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and fullerene. The binder may include one or more of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium polyacrylate, polytetrafluoroethylene, polyimide, and polyurethane.

[0104] [Electrolytes]

[0105] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0106] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0107] In some embodiments, the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium difluorosulfonamide, sodium difluoromethanesulfonamide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium difluorophosphate, sodium perchlorate, and sodium chloride.

[0108] In some embodiments, the solvent may be selected from at least one of cyclic ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and chain-like dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, and 1,3-dioxane.

[0109] 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.

[0110] In some implementations, the positive electrode, negative current collector, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0111] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0112] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0113] 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. For example, Figure 2 shows a square-structured secondary battery 5 as an example.

[0114] In some embodiments, referring to FIG3, the outer packaging may include a housing 51 and a cover plate 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 plate 53 is capable of covering the opening and is configured to close the receiving cavity. A positive electrode, a negative electrode, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within 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 selected by those skilled in the art according to specific practical needs.

[0115] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0116] Figure 4 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be fixed in place using fasteners.

[0117] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0118] 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 one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0119] Figures 5 and 6 show a battery pack 1 as an example. Referring to Figures 5 and 6, 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 box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0120] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided 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 include, but is not limited to, mobile devices (e.g., mobile phones, 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.

[0121] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0122] Figure 7 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0123] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0124] Example

[0125] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0126] Example 1

[0127] A method for preparing a composite membrane includes the following steps:

[0128] An adhesive is coated on one side of the first porous membrane layer. The solvent of the adhesive is dried by passing it through an oven, and then the adhesive is melted by heating. The first porous membrane layer is a polyethylene porous membrane layer with a porosity of 20%, a pore size of 10 nm, and a thickness of 7 μm. The adhesive is polyvinylidene fluoride, and the amount of adhesive used is 1.5 mg / 1540.25 mm. 2 The heating temperature is 60℃;

[0129] The second porous membrane layer is bonded to the molten adhesive, cooled, and cured to obtain the membrane body; wherein, the second porous membrane layer is a polyethylene porous membrane layer, the porosity of the second porous membrane is 35%, the pore size of the second porous membrane is 30nm, and the thickness of the second porous membrane is 7μm.

[0130] A nonwoven fabric layer is prepared by melt-blown spinning on the side of the second porous membrane layer opposite to the first porous membrane layer 11 to obtain a composite diaphragm; wherein, the nonwoven fabric layer is a polypropylene nonwoven fabric layer, the porosity of the nonwoven fabric layer is 80%, the pore size of the nonwoven fabric layer is 1000nm, and the thickness of the nonwoven fabric layer is 25μm.

[0131] Example 2

[0132] Referring to Example 1, the difference is that the porosity of the first porous membrane layer is 25%.

[0133] Example 3

[0134] Referring to Example 1, the difference is that the porosity of the first porous membrane layer is 30%.

[0135] Example 4

[0136] Referring to Example 1, the difference is that the pore size of the first porous membrane layer is 20 nm.

[0137] Example 5

[0138] Referring to Example 1, the difference is that the pore size of the first porous membrane layer is 30 nm.

[0139] Example 6

[0140] Referring to Example 1, the difference is that the porosity of the second porous membrane layer is 40%.

[0141] Example 7

[0142] Referring to Example 1, the difference is that the porosity of the second porous membrane layer is 45%.

[0143] Example 8

[0144] Referring to Example 1, the difference is that the pore size of the second porous membrane layer is 90 nm.

[0145] Example 9

[0146] Referring to Example 1, the difference is that the pore size of the second porous membrane layer is 150 nm.

[0147] Example 10

[0148] Referring to Example 1, the difference is that the porosity of the nonwoven fabric layer is 55%.

[0149] Example 11

[0150] Referring to Example 1, the difference is that the porosity of the nonwoven fabric layer is 60%.

[0151] Example 12

[0152] Referring to Example 1, the difference is that the porosity of the nonwoven fabric layer is 90%.

[0153] Example 13

[0154] Referring to Example 1, the difference is that the pore size of the nonwoven fabric layer is 2000nm.

[0155] Example 14

[0156] Referring to Example 1, the difference is that the pore size of the nonwoven fabric layer is 20000nm.

[0157] Comparative Example 1

[0158] Referring to Example 1, the difference is that the porosity of the second porous membrane layer is 15%.

[0159] Comparative Example 2

[0160] Referring to Example 1, the difference is that the porosity of the first porous membrane layer is 35%, and the porosity of the nonwoven fabric layer is 35%.

[0161] Comparative Example 3

[0162] A method for preparing a composite membrane includes the following steps:

[0163] Solution preparation: Dissolve polypropylene in N,N-dimethylformamide (DMF) to prepare a polymer solution with a mass concentration of 15%. Add polyvinylpyrrolidone (PVP) and stir thoroughly to ensure uniform mixing until a transparent and homogeneous film-forming solution is formed. The mass concentration of polyvinylpyrrolidone in the film-forming solution is 5%.

[0164] Film formation: The film formation solution is uniformly coated on the substrate to a thickness of 50 μm;

[0165] Phase separation: Thermally induced phase separation (TIPS) is used to create a gradient pore size (from small to large, 10 nm to 20000 nm) in the coated film;

[0166] Curing and drying: The phase-separated membrane is taken out from the non-solvent bath, washed with deionized water to remove residual solvent and non-solvent, and then placed in an oven to dry (40℃) to completely remove moisture and cure the membrane structure to obtain a composite membrane.

[0167] The parameters of the composite membranes of Examples 1 to 14 and Comparative Examples 1 to 3 of this application are shown in Table 1.

[0168] Preparation of secondary batteries

[0169] The composite separators of Examples 1 to 14 and Comparative Examples 1 to 3 were respectively used to prepare secondary batteries, and the preparation methods are as follows:

[0170] (1) Preparation of positive electrode sheet

[0171] Sodium iron pyrophosphate (SO4), carbon black (Super P), and polyvinylidene fluoride (PVDF) (batch) were thoroughly mixed in an appropriate amount of N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5 to form a positive electrode slurry. The positive electrode slurry was then coated onto the surface of the positive electrode current collector aluminum foil at a coating weight of 30 mg / cm³. 2 After drying and cold pressing, a positive electrode sheet is obtained.

[0172] (2) Preparation of negative electrode sheet

[0173] Carbon nanotubes (CNTs) and sodium carboxymethyl cellulose (CMC) were thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 50:50 to form an interface modification layer slurry. The interface modification layer slurry was then coated onto the surface of the copper foil of the negative electrode current collector with a thickness of 5 μm to obtain the negative electrode sheet.

[0174] (3) Preparation of electrolyte

[0175] Sodium hexafluorophosphate (NaPF6) was dissolved in diethylene glycol dimethyl ether (DEGDME) to prepare an electrolyte with a concentration of 1 mol / L.

[0176] A button cell is fabricated by stacking the positive electrode, composite separator, and negative electrode in sequence to obtain an electrode assembly, thus obtaining a sodium metal battery without a negative electrode.

[0177] Performance testing:

[0178] Fast charging cycle life test: At 25℃, the secondary battery is charged at a 3C rate and discharged at a 3C rate to perform a 0% to 100% SOC (state of charge) cycle test until the SOH (state of health) of the secondary battery is 80%. The number of cycles is recorded as the fast charging cycle life of the secondary battery.

[0179] The sodium metal batteries without negative electrodes prepared in Examples 1 to 14 and Comparative Examples 1 to 3 were subjected to the above tests, and the results are shown in Table 1.

[0180] As shown in Table 1, compared to Comparative Examples 1 to 3, the fast-charge cycle life of the anode-free sodium metal batteries prepared in Examples 1 to 14 is significantly increased, indicating that this application can improve the internal short-circuit tolerance of the secondary battery and broaden its charging window. Compared to Example 5, the fast-charge cycle life of the anode-free sodium metal batteries prepared in Examples 1 and 4 is significantly increased, indicating that the pore size of the multiple porous film layers increases sequentially in the direction close to the anode sheet, which can further improve the internal short-circuit tolerance of the secondary battery and broaden its charging window.

[0181] 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, wherein, It includes a positive electrode, a composite separator, a negative electrode, and an electrolyte; wherein the composite separator comprises multiple independent and interconnected porous membrane layers, and the porosity of the multiple porous membrane layers increases sequentially in the direction close to the negative electrode.

2. The secondary battery as described in claim 1, wherein, The composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in the direction near the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer, the second porous membrane layer, and the third porous membrane layer increases sequentially. A first adhesive layer is disposed between the first porous membrane layer and the second porous membrane layer, or the first porous membrane layer and the second porous membrane layer are bonded together by a hot-pressing layer; and / or, A second adhesive layer is disposed between the third porous membrane layer and the second porous membrane layer, or the third porous membrane layer includes a nonwoven fabric layer formed on the second porous membrane layer.

3. The secondary battery as described in claim 1 or 2, wherein, The composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer disposed sequentially in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer, the second porous membrane layer and the third porous membrane layer increases sequentially. The porosity of the third porous membrane layer is 60% to 90%.

4. The secondary battery as described in claim 2 or 3, wherein, The porosity of the first porous membrane layer is 20% to 30%; and / or, The porosity of the second porous membrane layer is 35% to 45%.

5. The secondary battery according to any one of claims 1 to 4, wherein, The pore size of the plurality of porous membrane layers increases sequentially in the direction approaching the negative electrode sheet.

6. The secondary battery according to any one of claims 1 to 5, wherein, The composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in the direction near the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first, second, and third porous membrane layers increases sequentially, and the pore size of the first, second, and third porous membrane layers also increases sequentially. The pore size of the first porous membrane layer is 10 nm to 30 nm; and / or, The pore size of the second porous membrane is 30 nm to 150 nm; and / or, The pore size of the third porous membrane layer is 2000nm to 20000nm.

7. The secondary battery according to any one of claims 1 to 6, wherein, The secondary battery includes a sodium metal battery without a negative electrode.

8. A composite diaphragm, wherein, It includes multiple independent porous membrane layers that are layered together, and the porosity of the multiple porous membrane layers increases sequentially.

9. The composite diaphragm as described in claim 8, wherein, The composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer sequentially disposed in the direction near the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer, the second porous membrane layer, and the third porous membrane layer increases sequentially. A first adhesive layer is disposed between the first porous membrane layer and the second porous membrane layer, or the first porous membrane layer and the second porous membrane layer are bonded together by a hot-pressing layer; and / or, A second adhesive layer is disposed between the third porous membrane layer and the second porous membrane layer, or the third porous membrane layer includes a nonwoven fabric layer formed on the second porous membrane layer.

10. The composite diaphragm as described in claim 8 or 9, wherein, The composite membrane includes a membrane body and an outer layer disposed on one side of the membrane body. The membrane body includes a first porous membrane layer and a second porous membrane layer disposed sequentially in the direction close to the outer layer. The outer layer includes a third porous membrane layer. The porosity of the first porous membrane layer, the second porous membrane layer and the third porous membrane layer increases sequentially. The porosity of the third porous membrane layer is 60% to 90%.

11. The composite diaphragm as described in claim 9 or 10, wherein, The porosity of the first porous membrane layer is 20% to 30%; and / or, The porosity of the second porous membrane layer is 35% to 45%.

12. The composite separator according to any one of claims 8 to 11, wherein, Along the direction in which the porosity of the plurality of porous membrane layers increases sequentially, the pore size of the plurality of porous membrane layers increases sequentially.

13. A method for preparing a composite diaphragm, wherein, Includes the following steps: The first porous membrane layer and the second porous membrane layer are bonded together with a first adhesive to obtain the membrane body; A nonwoven fabric layer is prepared by spinning on the side of the second porous membrane layer opposite to the first porous membrane layer to obtain a composite membrane. The porosity of the first porous membrane layer, the second porous membrane layer, and the nonwoven fabric layer increases sequentially.

14. An electrical appliance, wherein, Includes the secondary battery as described in any one of claims 1 to 7.

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