Secondary battery and electronic device

Abstract

The invention belongs to the field of electrochemistry, and particularly discloses a secondary battery and an electronic device. The invention provides a secondary battery which comprises electrolyte, a positive plate, a negative plate and a composite film arranged between the positive plate and the negative plate, the composite film comprises a first conductive layer, a first polymer layer, a second polymer layer, a third polymer layer and a second conductive layer which are sequentially stacked, the first conductive layer is in contact with the positive plate, and the second conductive layer is in contact with the negative plate. The second conductive layer is in contact with the negative plate; the first polymer layer and the third polymer layer comprise a class of polymer, and the class of polymer has a porous structure; the second polymer layer includes a class II polymer. The composite membrane has good mechanical flexibility and low internal resistance, the safety and the output power density of the battery can be improved, the composite membrane can automatically bear a positive plate and a negative plate, the composite membrane has the functions of a current collector and a diaphragm, and the light weight of the battery can be realized.

Description

Technical Field

[0001] This application belongs to the field of electrochemistry, and particularly relates to a secondary battery and an electronic device. Background Art

[0002] Secondary batteries such as lithium-ion batteries have occupied the mainstream position in the market due to their advantages of high energy density, good safety, no memory effect, and long service life. The lithium-ion battery components include an electrolyte, a composite film, and positive and negative electrode sheets containing active substances. Generally, the active substances of the positive and negative electrodes are respectively loaded on the metal current collectors (copper foil or aluminum foil) of the positive and negative electrodes to form the positive and negative electrode sheets, and then the positive electrode sheet, the composite film, and the negative electrode sheet are laminated and molded to obtain an electrode. The composite film can separate the positive and negative electrodes and prevent internal short circuit of the battery, but it allows ions to pass through, thereby completing the rapid transmission of lithium ions between the positive and negative electrodes during the electrochemical charge and discharge process of the battery. The performance of the composite film determines the interface structure, internal resistance, structural stability, etc. of the battery, and directly affects the output power density and safety performance of the battery. The current composite films are mostly polyolefin porous films. Currently, the composite films generally have relatively large interfacial impedance and diffusion impedance, resulting in low output power density of the battery, and their flexibility and resistance to external forces are poor. Especially under some special conditions, such as puncture, overcharge, or foreign object extrusion, it is easy to cause damage to such composite films. Once the composite film shrinks, melts, oxidizes, or breaks, it will cause internal short circuit of the battery, resulting in safety accidents such as battery heating, smoking, or even explosion and fire. Therefore, it is still necessary to continue to develop a battery composite film with low impedance, high output power density, and good mechanical flexibility. Summary of the Invention

[0003] Aiming at the above problems existing in the prior art, this application provides a secondary battery and an electronic device to improve the problems of large impedance, low output power density, and poor mechanical flexibility of the composite film.

[0004] In a first aspect, this application provides a secondary battery, including an electrolyte, a positive electrode sheet, a negative electrode sheet, and a composite film disposed between the positive electrode sheet and the negative electrode sheet. The composite film includes a first conductive layer, a first polymer layer, a second polymer layer, a third polymer layer, and a second conductive layer that are sequentially laminated. The first conductive layer is in contact with the positive electrode sheet, and the second conductive layer is in contact with the negative electrode sheet; the first polymer layer and the third polymer layer include a type of polymer, and the type of polymer has a porous structure; the second polymer layer includes a second type of polymer.

[0005] The composite film of the present application is composed of multiple polymer layers and a conductive layer, has good mechanical flexibility and low internal resistance, can improve the safety and output power density of the battery, and can be used as a carrier by itself, with the characteristic of self-support. It can carry the positive electrode sheet and the negative electrode sheet on both sides respectively. The active substances on the positive electrode sheet and the negative electrode sheet are in direct contact with the composite film, so that the composite film and the active substances of the positive and negative electrode sheets form a double electrode unit respectively, shortening the migration path of lithium ions and improving the output power density of the battery. And because the migration path of lithium ions becomes shorter, the concentration polarization of the electrolyte becomes smaller, the concentration of lithium ions can be made uniform, lithium deposition can be reduced, the interfacial impedance and diffusion impedance can be reduced, and the safety and output power density of the battery can be further improved. In addition, compared with traditional batteries, the composite film of the present application carries the positive electrode sheet and the negative electrode sheet by itself, and has the functions of both a separator and a current collector, making the battery lighter in weight and lower in density, realizing the lightweight of the battery, and having an advantage in improving the energy density of the battery.

[0006] In some embodiments, the surface energy of a class of polymers is 15 - 45 mJ / m 2 . Selecting a class of polymers with a surface energy of 15 - 45 mJ / m 2 can further improve the mechanical flexibility of the composite film and the output power density of the secondary battery. Preferably, the surface energy of a class of polymers is 22 - 45 mJ / m 2 .

[0007] In some embodiments, the elastic modulus of a class of polymers is 5 - 600 GPa. Selecting a class of polymers with an elastic modulus of 5 - 600 GPa can further improve the mechanical flexibility of the composite film and the output power density of the secondary battery. Preferably, the elastic modulus of a class of polymers is 5 - 600 GPa.

[0008] In some embodiments, the class of polymers is selected from at least one of polyamide, polyvinylidene fluoride - hexafluoropropylene, polytetrafluoroethylene, polypropylene, polyethylene, polyethersulfone or polyimide. Selecting the above types of a class of polymers can further improve the mechanical flexibility of the composite film and the output power density of the secondary battery.

[0009] In some embodiments, the class of polymers is selected from at least one of aramid fiber, carbon fiber, polyester fiber, polyvinyl alcohol fiber or polyamide fiber. Selecting the above types of a class of polymers can further improve the mechanical flexibility of the composite film and the output power density of the secondary battery.

[0010] In some embodiments, the thickness of the composite film is 10 - 40 μm. Controlling the thickness of the composite film to be 10 - 40 μm can further improve the mechanical flexibility of the composite film and the output power density of the secondary battery.

[0011] In some embodiments, the thickness ratio of the first conductive layer, the first polymer layer, the second polymer layer, the third polymer layer, and the second conductive layer is (0.5 - 2 μm):(2 - 5 μm):(5 - 20 μm):(2 - 5 μm):(0.5 - 2 μm). When the thicknesses of the first conductive layer, the first polymer layer, the second polymer layer, the third polymer layer, and the second conductive layer are controlled to satisfy the above ratio, the mechanical flexibility of the composite film and the output power density of the secondary battery can be further improved.

[0012] In some embodiments, both the first conductive layer and the first polymer layer include conductive carbon. Based on the sum of the mass of conductive carbon in the first polymer layer and the mass of conductive carbon in the first conductive layer, the mass percentage of conductive carbon in the first polymer layer is 5% - 80%. By making the first conductive layer and the first polymer layer satisfy the above settings, the mechanical flexibility of the composite film and the output power density of the secondary battery can be further improved. Preferably, the mass percentage of conductive carbon in the first polymer layer is 47% - 80%.

[0013] In some embodiments, both the second conductive layer and the third polymer layer include conductive carbon. Based on the sum of the mass of conductive carbon in the third polymer layer and the mass of conductive carbon in the second conductive layer, the mass percentage of conductive carbon in the third polymer layer is 5% - 80%. By making the second conductive layer and the third polymer layer satisfy the above settings, the mechanical flexibility of the composite film and the output power density of the secondary battery can be further improved. Preferably, the mass percentage of carbon element of conductive carbon in the third polymer layer is 50% - 80%.

[0014] In some embodiments, the first conductive layer includes at least one of silver nanoparticles, carbon black, carbon nanotubes, or graphene, and the second conductive layer includes at least one of copper nanoparticles, carbon nanotubes, or graphene. By making the first conductive layer and the second conductive layer satisfy the above settings, the mechanical flexibility of the composite film and the output power density of the secondary battery can be further improved.

[0015] In some embodiments, the first conductive layer includes Ag element, and the first polymer layer also includes Ag element. Based on the sum of the mass of Ag element in the first polymer layer and the mass of Ag element in the first conductive layer, the mass percentage of Ag element in the first polymer layer is 10% - 80%. By making the first conductive layer and the first polymer layer satisfy the above settings, the mechanical flexibility of the composite film and the output power density of the secondary battery can be further improved.

[0016] In some embodiments, the second conductive layer includes Cu element, and the third polymer layer also includes Cu element. Based on the sum of the mass of Cu element in the third polymer layer and the mass of Cu element in the second conductive layer, the mass proportion of Cu element in the third polymer layer is 10%-80%. By making the second conductive layer satisfy the above settings, the mechanical flexibility of the composite film and the output power density of the secondary battery can be further improved.

[0017] In some embodiments, the electrolyte includes ethylene carbonate. Based on the mass of the electrolyte, the mass proportion of ethylene carbonate is A%, and the sum of the thickness of the first polymer layer and the thickness of the third polymer layer is H μm. A and H satisfy the following relationship: 80 ≤ A × H ≤ 160.

[0018] In a second aspect, the present application provides an electronic device including the secondary battery.

[0019] Advantages of the present application: The composite film of the present application is composed of multiple polymer layers and conductive layers, has good mechanical flexibility and low internal resistance, can improve the safety and output power density of the battery, and the composite film can carry the positive and negative electrode sheets by itself without using a metal current collector, which can reduce the weight of the battery and realize the lightweight of the battery. Specific embodiments

[0020] To better illustrate the purpose, technical solution and advantages of the present application, the present application will be further described below through specific comparative examples and embodiments.

[0021] To better illustrate the purpose, technical solution and advantages of the present application, the technical solution of the present application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The embodiments described herein are illustrative in nature and are used to provide a basic understanding of the present application. And the embodiments of the present application should not be construed as a limitation to the present application.

[0022] For the sake of brevity, only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, each individually disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a range not explicitly recited.

[0023] In the description of this application, unless otherwise specified, "above" and "below" include the corresponding numbers. Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured by various common measurement methods in the art. In the description of this application, a list of items connected by terms such as "at least one of", "at least one item of", "at least one of", "at least one kind of" or other similar terms may mean any combination of the listed items.

[0024] In order to improve the problems of large impedance, low output power density and poor mechanical flexibility of the composite film, this application provides a secondary battery, including an electrolyte, a positive electrode sheet, a negative electrode sheet, and a composite film disposed between the positive electrode sheet and the negative electrode sheet. The composite film includes a first conductive layer, a first polymer layer, a second polymer layer, a third polymer layer, and a second conductive layer that are sequentially stacked. The first conductive layer is in contact with the positive electrode sheet, and the second conductive layer is in contact with the negative electrode sheet; the first polymer layer and the third polymer layer include a type of polymer, and the type of polymer has a porous structure; the second polymer layer includes a second type of polymer.

[0025] The composite film of this application is composed of multiple polymer layers and conductive layers, has good mechanical flexibility and low internal resistance, can improve the safety and output power density of the battery, and can be used as a carrier by itself, with the characteristic of self-support. It can carry the positive electrode sheet and the negative electrode sheet on both sides respectively. The active substances on the positive electrode sheet and the negative electrode sheet are in direct contact with the composite film, so that the composite film and the active substances of the positive and negative electrode sheets respectively form a double-electrode unit, shortening the migration path of lithium ions and increasing the output power density of the battery; and because the migration path of lithium ions becomes shorter, the concentration polarization of the electrolyte becomes smaller, the concentration of lithium ions can be made uniform, lithium deposition can be reduced, the interface impedance and diffusion impedance can be reduced, and the safety and output power density of the battery can be further improved. In addition, compared with traditional batteries, the composite film of this application can carry the positive electrode sheet and the negative electrode sheet by itself, and can lack a metal current collector, making the battery lighter in weight and lower in density, realizing the lightweight of the battery, which is advantageous for improving the energy density of the battery.

[0026] In some embodiments, the surface energy of the type of polymer is 15 - 45 mJ / m 2 , specifically, it can be 15 mJ / m 2 , 20 mJ / m 2 , 25 mJ / m 2 , 30 mJ / m 2 , 35 mJ / m 2 , 40 mJ / m 2 , 45 mJ / m 2, or a range formed by any two of these values. Thus, a class of polymers has high surface properties, is more likely to deposit and adhere to conductive substances, making it easier for the conductive layer to combine with the class of polymers. While increasing the conductivity of the composite film, it is more likely to form a composite film with a stable structure, improving the stability of the composite film structure. The surface energy can be measured by conventional methods. For example, the contact angle method can be used to test the surface energy of the second polymer.

[0027] In some embodiments, the density of the class of polymers is 0.91 - 2.6 g / cm 3 , specifically, it can be 0.91 g / cm 3 , 1 g / cm 3 , 1.2 g / cm 3 , 1.4 g / cm 3 , 1.6 g / cm 3 , 1.8 g / cm 3 , 2 g / cm 3 , 2.2 g / cm 3 , 2.4 g / cm 3 , 2.6 g / cm 3 , or a range formed by any two of these values.

[0028] In some embodiments, the tensile strength of the class of polymers is 20 - 150 MPa. Specifically, it can be 20 MPa, 40 MPa, 60 MPa, 80 MPa, 100 MPa, 120 MPa, 140 MPa, 150 MPa, or a range formed by any two of these values.

[0029] In some embodiments, the elastic modulus of the class of polymers is 0.5 - 4 GPa. Specifically, it can be 0.5 GPa, 1 GPa, 2 GPa, 3 GPa, 4 GPa, or a range formed by any two of these values. The elastic modulus of the class of polymers is tested by the GB1040 - 79 plastic tensile test method.

[0030] In some embodiments, the coefficient of thermal expansion of the class of polymers is (20 - 180) × 10 -6 K -1 , specifically, it can be 20 × 10 -6 K -1 , 30 × 10 -6 K -1 , 50 × 10 -6 K -1 , 70 × 10 -6 K -1 , 90 × 10 -6 K -1 , 110 × 10 -6 K-1 、 130×10 -6 K -1 、 150×10 -6 K -1 、 170×10 -6 K -1 、 180×10 -6 K -1 or a range formed by any two of these values.

[0031] In some embodiments, the molecular weight of the class of polymers is 10,000 - 2,000,000 g / mol. Specifically, it can be 10,000 g / mol, 500,000 g / mol, 1,000,000 g / mol, 1,500,000 g / mol, 2,000,000 g / mol, or a range formed by any two of these values.

[0032] In some embodiments, the average pore diameter of the class of polymers is 0.01 - 10 μm. Specifically, it can be 0.01 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range formed by any two of these values.

[0033] In some embodiments, the porosity of the class of polymers is 20% - 95%. Specifically, it can be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or a range formed by any two of these values.

[0034] In some embodiments, the average pore diameter of the class of polymers is greater than that of the class of polymers. The pore diameter of the class of polymers is greater than that of the second polymer, such that the depth of penetration of the conductive agent only reaches the class of polymers, preventing the conductive substance in the class of polymers from further penetrating into the class of polymers and causing a short circuit.

[0035] In some embodiments, the class of polymers is selected from at least one of polyamide (PA), polyvinylidene fluoride - hexafluoropropylene (PVDF - HFP), polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene (PE), polyethersulfone (PES), or polyimide (PI).

[0036] The above-mentioned type of polymer has a porous structure with a uniform pore size distribution, is lightweight, has good mechanical strength and flexibility, corrosion resistance, stability and flame retardancy. It can better adsorb the conductive layer, has high electrolyte permeability, and the characteristics of the porous structure can also shorten the transmission path of lithium ions, improve the transmission efficiency of lithium ions, improve the kinetic performance, and further increase the output power density of the battery. The type of polymer can use a commercially available porous polymer membrane, or can polymerize the monomers of the corresponding polymer to form a type of polymer with a porous structure.

[0037] The first polymer layer and the third polymer layer can be attached to the second polymer layer in a conventional manner. For example, it can include coating a solution containing the type of polymer on both sides of the second polymer layer, and then successively undergoing phase separation induction, standing, and annealing to obtain the first polymer layer and the third polymer layer respectively. In some embodiments, the mass percentage content of the type of polymer in the solution containing the type of polymer is 5%-15%. In some embodiments, the solution containing the type of polymer includes at least one solvent of hexafluoroisopropanol (HFIP), N,N-dimethylformamide (DMF), NMP or water. In some embodiments, the second polymer solution includes HFIP and water, and the volume ratio of HFIP to water is (0.5-2):(3-5). In some embodiments, the solution for phase separation induction includes a mixed solution of water and ethanol, the volume of water and ethanol is 1:0.5-2, and in the phase separation induction, the immersion time in the solution for phase separation induction is 5-15 min. In some embodiments, the coating method includes at least one of spraying, spin coating or roll coating. In some embodiments, the spraying distance is 10-20 cm, the spraying rate is 10-20 m / s, and the spraying pressure is 0.2-0.4 MPa. In some embodiments, the standing time is 12-48 h, the annealing temperature is 60-85 °C, and the annealing time is 1-3 h. In addition, the first polymer layer and / or the third polymer layer can also be compounded on the second polymer layer by methods such as melt coating method, electrospinning method, hot pressing method, etc. to form the first polymer layer and / or the third polymer layer.

[0038] In some embodiments, the first type of polymer layer is also treated with oxygen plasma; in some embodiments, the time of the oxygen plasma pretreatment is 1 - 10 min, and the power of the oxygen plasma pretreatment is 20 - 150 W. Through plasma treatment, the surface energy and the degree of surface roughening can be increased, and the bonding strength between the second type of polymer and the first type of polymer can be improved. In some embodiments, the elastic modulus of the second type of polymer is 5 - 600 GPa. Specifically, it can be 5 GPa, 50 GPa, 100 GPa, 200 GPa, 300 GPa, 400 GPa, 500 GPa, 600 GPa, or the range composed of any two of these values.

[0039] The elastic modulus of the second type of polymer is tested by using the plastic tensile test method of GB1040 - 79.

[0040] The second type of polymer having a high elastic modulus can endow the second polymer layer with better mechanical flexibility, improve the ability of the composite film to resist external forces, and improve the situation where cracks or even fracture defects occur in the composite film under the action of external forces, which is beneficial to improving the safety of the battery.

[0041] In some embodiments, the density of the second type of polymer is 0.97 - 1.8 g / cm 3 , specifically, it can be 0.97 g / cm 3 , 1 g / cm 3 , 1.1 g / cm 3 , 1.2 g / cm 3 , 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , or the range composed of any two of these values.

[0042] In some embodiments, the tensile strength of the second type of polymer is 0.05 - 4.5 GPa. Specifically, it can be 0.04 GPa, 0.1 GPa, 0.5 GPa, 1 GPa, 1.5 GPa, 2 GPa, 2.5 GPa, 3 GPa, 3.5 GPa, 34 GPa, 4.5 GPa, or the range composed of any two of these values.

[0043] In some embodiments, the thermal expansion coefficient of the second type of polymer is (0.5 - 180)×10 -6 K -1 , specifically, it can be 0.5×10 -6 K -1 、1×10-6 K -1 、 10×10 -6 K -1 、 30×10 -6 K -1 、 50×10 -6 K -1 、 70×10 -6 K -1 、 90×10 -6 K -1 、 110×10 -6 K -1 、 130×10 -6 K -1 、 150×10 -6 K -1 、 170×10 -6 K -1 、 180×10 -6 K -1 , or a range formed by any two of these values.

[0044] In some embodiments, the second type of polymer is selected from at least one of aramid fiber, carbon fiber, polyester fiber, polyvinyl alcohol fiber, or polyamide fiber.

[0045] Using the above fiber material with a relatively small density and coefficient of expansion, high mechanical strength and aspect ratio, and good resistance to most acids, bases, and organic solvents as the second type of polymer can endow the composite film with good mechanical properties (flexibility, rigidity, and strength), solvent resistance, thermal conductivity, chemical stability, and flame retardancy. It can serve as the base film for the first polymer layer and the third polymer layer, providing the main mechanical properties of the composite film, and forming a complementarity with the first polymer layer and the third polymer layer in terms of chemical stability, thermal stability, and mechanical properties, meeting the requirements of all aspects of the battery composite film.

[0046] In some embodiments, the second type of polymer is also modified with amino groups. For example, the second type of polymer can be impregnated in an ethanol solution containing 0.5 wt% - 5 wt% of 3-aminopropyltriethoxysilane and subjected to an amino group modification reaction at 50°C to 70°C for 1 - 3 h. The second type of polymer modified with amino groups can improve its binding strength with the first type of polymer.

[0047] In some embodiments, the second type of polymer layer is also sequentially treated with acetone and oxygen plasma; in some embodiments, the time of the oxygen plasma pretreatment is 1 - 10 min, and the power of the oxygen plasma pretreatment is 20 - 150 W. After the plasma treatment, the surface energy and the degree of surface roughening can be increased, and the binding strength between the second type of polymer and the first type of polymer can be improved.

[0048] In some embodiments, the average aspect ratio of the second type of polymer is 500 - 5000. Specifically, it can be 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, or a range composed of any two of these values.

[0049] In some embodiments, the specific surface area of the second type of polymer is 0.1 - 5.0 m 2 / g. Specifically, it can be 0.1 m 2 / g, 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, or a range composed of any two of these values.

[0050] In some embodiments, the molecular weight of the second type of polymer is 1000 - 5000000 g / mol. Specifically, it can be 1000 g / mol, 5000 g / mol, 10000 g / mol, 500000 g / mol, 1000000 g / mol, 2000000 g / mol, 3000000 g / mol, 4000000 g / mol, 5000000 g / mol, or a range composed of any two of these values.

[0051] In some embodiments, the porosity of the second type of polymer is 20% - 95%. Specifically, it can be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or a range composed of any two of these values.

[0052] In some embodiments, the thickness of the composite membrane is 10 - 40 μm. Specifically, it can be 10 μm, 12.5 μm, 15 μm, 17.5 μm, 20 μm, 22.5 μm, 25 μm, 27.5 μm, 30 μm, 32.5 μm, 35 μm, 37.5 μm, 40 μm, or a range composed of any two of these values.

[0053] In some embodiments, the thickness of the first conductive layer is 0.5 - 2 μm. Specifically, it can be 0.5 μm, 0.75 μm, 1 μm, 1.25 μm, 1.5 μm, 1.75 μm, 2 μm, or a range composed of any two of these values.

[0054] In some embodiments, the thickness of the second conductive layer is 0.5 - 2 μm. Specifically, it can be 0.5 μm, 0.75 μm, 1 μm, 1.25 μm, 1.5 μm, 1.75 μm, 2 μm, or a range composed of any two of these values.

[0055] In some embodiments, the thickness of the first polymer layer is 2 - 5 μm. Specifically, it can be 2 μm, 3 μm, 4 μm, 5 μm, or a range composed of any two of these values.

[0056] In some embodiments, the thickness of the second polymer layer is 5 - 20 μm. Specifically, it can be 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, or a range composed of any two of these values.

[0057] In some embodiments, the thickness of the third polymer layer is 2 - 5 μm. Specifically, it can be 2 μm, 3 μm, 4 μm, 5 μm, or a range composed of any two of these values.

[0058] In some embodiments, the thickness ratio of the first conductive layer, the first polymer layer, the second polymer layer, the third polymer layer, and the second conductive layer is (0.5 - 2 μm):(2 - 5 μm):(5 - 20 μm):(2 - 5 μm):(0.5 - 2 μm). Specifically, it can be 0.5:2:5:2:0.5, 2:5:20:5:2, 1:2.5:10:2.5:1, 0.75:2.25:7.5:2.25:0.75, 1.75:4.25:10.5:4.25:1.75, or a range composed of any two of these values.

[0059] In some embodiments, the first conductive layer includes at least one of elemental metal, alloy, conductive oxide, and conductive carbon.

[0060] In some embodiments, the first conductive layer includes at least one of silver nanoparticles, carbon black, carbon nanotubes, or graphene.

[0061] In some embodiments, when the first conductive layer includes conductive carbon, the first polymer layer also includes conductive carbon. Based on the sum of the mass of conductive carbon in the first polymer layer and the mass of conductive carbon in the first conductive layer, the mass ratio of conductive carbon in the second polymer layer is 5% - 80%. Specifically, it can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or a range composed of any two of these values. Preferably, the mass ratio of conductive carbon in the first polymer layer is 47% - 80%.

[0062] When the first conductive layer contains a conductive carbon material, the conductive carbon material will penetrate into the pore structure of the first polymer layer. The distribution of the penetrated conductive carbon material mainly depends on the thickness, pore size, and deposition conditions of the first polymer layer. If the content of the deposited conductive carbon material is too low, the conductivity may not meet the requirements of conductivity and internal resistance reduction. If the content of the conductive carbon material is too high, when the penetration of the conductive carbon into the first polymer layer is excessive, it may cause a decrease in the mechanical properties of the first polymer layer, such as a reduction in toughness, and may even affect the stability of the overall structure. When the mass ratio of the conductive carbon material in the first polymer layer ranges from 5% to 80%, the composite film can have good enough conductivity and does not significantly reduce the mechanical properties of the first polymer layer, maintaining good stability of the composite film.

[0063] In some embodiments, the first conductive layer includes Ag element, and the first polymer layer also includes the Ag element. Based on the sum of the mass of the Ag element in the first polymer layer and the mass of the Ag element in the first conductive layer, the mass ratio of the Ag element in the first polymer layer is 10% - 80%. Specifically, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or the range composed of any two of these values.

[0064] When preparing the conductive layer by depositing silver nanoparticles, the Ag element will penetrate into the porous structure of the first polymer layer. The diffusion distance and content of the Ag element are related to the pore size, porosity, pore structure, surface chemical properties of the selected first polymer layer, and the deposition rate of the metal in the conductive layer, and are also related to the deposition conditions. If it is electroplating deposition, the deposition of the conductive layer is related to the current density, electroplating time, electrolyte concentration, reducing agent concentration, temperature, deposition temperature, vacuum degree, deposition time, etc. By adjusting these parameters, a complete surface-deposited conductive layer can be achieved, and the mass of the Ag element in the first polymer can be controlled, enabling the overall composite film to have low internal resistance, good mechanical properties, and low weight. The content of the Ag element can be tested by common methods in the prior art, such as the common ICP test method.

[0065] The first conductive layer can be deposited on the first polymer layer in a conventional manner, such as by at least one of physical vapor deposition, chemical vapor deposition, or electroless plating. In some embodiments, the physical vapor deposition includes at least one of vacuum evaporation, atomic layer deposition, or magnetron sputtering. In some embodiments, the power of the magnetron sputtering is 50W - 100KW, and the base film transmission rate of the magnetron sputtering is 0.1 - 0.5μm / s. When the deposited first conductive layer is metallic aluminum, the temperature of the substrate is controlled to be less than 80°C, so that low-temperature film formation of Al can be achieved, making the aluminum denser and having a strong bonding force with the first polymer layer. In some embodiments, in the atomic deposition method, the deposition temperature is 80°C - 120°C, and the thickness of a single deposition is 0.09nm - 0.15nm.

[0066] In some embodiments, the second conductive layer includes at least one of elemental metals, alloys, conductive oxides, and conductive carbon.

[0067] In some embodiments, the second conductive layer includes at least one of copper nanoparticles, carbon nanotubes, or graphene.

[0068] In some embodiments, when the second conductive layer includes conductive carbon, the third polymer layer also includes conductive carbon. Based on the sum of the mass of conductive carbon in the third polymer layer and the mass of conductive carbon in the second conductive layer, the mass proportion of conductive carbon in the third polymer layer is 5% - 80%. Specifically, it can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or any range composed of any two of these values. Preferably, the mass proportion of carbon element of conductive carbon in the third polymer layer is 50% - 80%.

[0069] In some embodiments, when the second conductive layer includes Cu element, the third polymer layer also includes the Cu element. Based on the sum of the mass of Cu element in the third polymer layer and the mass of Cu element in the second conductive layer, the mass proportion of Cu element in the third polymer layer is 10% - 80%. Specifically, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or any range composed of any two of these values.

[0070] When preparing the conductive layer by depositing copper nanoparticles, Cu elements will penetrate into the porous structure of the third polymer layer. The diffusion distance and content of Cu elements are related to the pore size, porosity, pore structure, surface chemical properties of the selected third polymer layer, and the deposition rate of the metal in the conductive layer, and are also related to the deposition conditions. For example, in electroplating deposition, the deposition of the conductive layer is related to the current density, electroplating time, electrolyte concentration, reducing agent concentration, temperature, deposition temperature, vacuum degree, deposition time, etc. By adjusting these parameters, a complete surface-deposited conductive layer can be achieved, and the mass of Cu elements in the third polymer can be controlled, so that the overall composite film has high conductivity, good mechanical properties and low weight. The content of Cu elements can be tested by common methods in the prior art, such as the common ICP test method, etc.

[0071] The second conductive layer can be deposited on the third polymer layer by conventional methods, such as at least one of physical vapor deposition, chemical vapor deposition or chemical electroplating. In some embodiments, the physical vapor deposition method includes at least one of vacuum evaporation, atomic layer deposition or magnetron sputtering. In some embodiments, the power of the magnetron sputtering is 50W - 100KW, and the substrate transfer rate of the magnetron sputtering is 0.1 - 0.5μm / s. When the deposited second conductive layer is metallic aluminum, the temperature of the substrate is controlled to be less than 80°C, so that Al forms a film at low temperature, which can make the aluminum denser and have a strong bonding force with the second polymer layer. In some embodiments, in the atomic deposition method, the deposition temperature is 80°C - 120°C, and the thickness of a single deposition is 0.09nm - 0.15nm.

[0072] In some embodiments, the first conductive layer includes silver nanoparticles, and the second conductive layer includes copper nanoparticles. In this way, the first conductive layer close to the positive electrode side can withstand high voltage (working range 3.7 - 4.5V) and has good oxidation resistance, and the second conductive layer close to the negative electrode side has good reduction stability and low voltage stability (0 - 1.5V).

[0073] In some embodiments, the electrolyte includes ethylene carbonate (EC). Based on the mass of the electrolyte, the mass ratio of ethylene carbonate is A%. The sum of the thicknesses of the first polymer layer and the third polymer layer is Hμm, and A and H satisfy the following relationship: 80 ≤ A×H ≤ 160. Further, 105 ≤ A×H ≤ 150. The specific value of A×H can be 80, 90, 100, 110, 120, 130, 140, 150, 160, or the range composed of any two of these values. The inventors of the present application found that A×H is related to the electrode impedance of the battery and the output power density of the battery, reflecting the ion transport resistance. A relatively lower A×H is more conducive to improving the output power density of the battery.

[0074] In some embodiments, A is 4 - 8. Specifically, it can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, or a range composed of any two of these values.

[0075] In some embodiments, H is 4 - 8 μm. Specifically, it can be 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, or a range composed of any two of these values.

[0076] When H is larger, the composite film has a relatively thicker thickness, and A needs to be smaller. The electrolyte has a lower viscosity and is easier to wet the composite film. When H is smaller, the composite film is relatively thinner, has a higher porosity and a larger specific surface area, which is beneficial to the rapid penetration and uniform distribution of the electrolyte. At this time, A is larger, and the electrolyte viscosity is relatively higher, which can also wet the composite film well.

[0077] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an additive. The types of the organic solvent and the lithium salt are not specifically limited in this application and can be selected according to actual needs.

[0078] In some embodiments, the organic solvent can be at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4 - butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), or diethyl sulfone (ESE).

[0079] In some embodiments, the organic solvent includes PC, EC, and DEC.

[0080] In some embodiments, the organic solvent includes PC and DEC, and the mass ratio of PC to DEC is 1:(0.5 - 1.5).

[0081] In some embodiments, the lithium salt may be at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorophosphate (LiPO2F2), lithium difluoro(oxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalato)phosphate (LiTFOP), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro(oxalato)borate (LiDFOB), or lithium bis(oxalato)borate (LiBOB).

[0082] In some embodiments, the positive electrode sheet includes a positive electrode active material.

[0083] In some embodiments, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium manganate, lithium manganese iron phosphate, lithium titanate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, or lithium manganese silicate.

[0084] In some embodiments, based on the total mass of the positive electrode sheet, the mass percentage of the positive electrode active material is 50% - 99%.

[0085] In some embodiments, the positive electrode sheet further includes a binder and a conductive agent.

[0086] In some embodiments, the binder includes at least one of styrene-butadiene rubber (SBR), aqueous acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA), or polyvinyl alcohol (PVA). It is not limited to the above types, and the binder can be selected according to actual needs.

[0087] In some embodiments, based on the total mass of the positive electrode sheet, the mass percentage of the binder is less than or equal to 5%.

[0088] In some embodiments, the conductive agent includes at least one of graphite, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, or carbon nanofibers. However, it is not limited to the above several types, and the conductive agent can be selected according to actual needs.

[0089] In some embodiments, based on the total mass of the positive electrode sheet, the mass percentage of the conductive agent is 1% to 3%.

[0090] The positive electrode sheet can be prepared by conventional preparation methods in the art. For example, the preparation method of the positive electrode sheet includes the following steps: mixing a solvent, a conductive agent, a binder, and a positive electrode active material to obtain a positive electrode slurry; coating the positive electrode slurry on one side surface of the composite film, and drying and cold pressing to obtain the positive electrode sheet.

[0091] In some embodiments, the solvent may include N-methylpyrrolidone (NMP), but is not limited thereto.

[0092] In some embodiments, the negative electrode sheet includes a negative electrode active material.

[0093] In some embodiments, the negative electrode active material includes at least one of graphite, mesocarbon microbeads, hard carbon, soft carbon, silicon, silicon-carbon material, silicon-oxygen material, Li alloy, and metallic lithium, but is not limited thereto.

[0094] In some embodiments, based on the total mass of the negative electrode sheet, the mass percentage of the negative electrode active material is 50%-99%.

[0095] In some embodiments, the negative electrode sheet further includes a binder and a conductive agent.

[0096] The negative electrode sheet can be prepared by conventional preparation methods in the art. For example, the preparation method of the negative electrode sheet includes the following steps: mixing a solvent, a conductive agent, a binder, and a negative electrode active material to obtain a negative electrode slurry; coating the negative electrode slurry on one side surface of the composite film, and drying and cold pressing to obtain the negative electrode sheet.

[0097] The secondary battery of the present application can be prepared according to conventional methods in the art; specifically, the preparation method may include the following steps: winding a structure in which a positive electrode sheet, a composite film, and a negative electrode sheet are stacked to obtain an electrode assembly; placing the electrode assembly in a packaging shell, injecting an electrolyte, and sealing to obtain the secondary battery.

[0098] In a second aspect, the present application provides an electronic device including the secondary battery. The electronic device of the present application can be used in various fields such as electronic products, energy storage, power batteries for electric vehicles, etc., such as mobile phones, laptop computers, power tools, video recorders, backup power supplies, electric vehicles, electric motorcycles, game consoles, cameras, and drones.

[0099] The technical solution of the present application will be described below with reference to specific examples and comparative examples. Unless otherwise specified, materials, reagents, equipment, etc. used can be obtained through commercial channels.

[0100] Example 1

[0101] (1) Second polymer layer: A porous aramid-based fiber Kevlar membrane with an average pore size of 200 nm, a porosity of 55%, and a thickness of 10 μm is used as the second polymer. It is treated with O2 plasma with a power of 100 W for 3 minutes during the treatment process to increase the surface energy. Then, it is ultrasonicated in a mixed solution of ethanol and deionized water with a volume ratio of 1:1 for 10 minutes to remove surface impurities, and dried in a vacuum oven at 60 °C for 2 hours. This is used as the second polymer layer, which serves as the base film for the first and third polymer layers.

[0102] (2) First and third polymer layers: PVDF-HFP is dissolved in hexafluoroisopropanol (HFIP), and the concentration of PVDF-HFP is controlled at 12 wt%. Deionized water (with a volume ratio of water to solvent of 1:4) is added and stirred at room temperature for 4 h until a transparent solution is obtained. The second polymer layer is kept flat, and the above homogeneous solution is evenly sprayed on both sides of the second polymer layer by the spraying method, ensuring that the solution completely covers the second polymer layer and controlling the target thickness. Then, it is immersed in a mixed solution bath of water and ethanol (with a volume ratio of 1:1) for 10 minutes for phase separation induction, left standing in a fume hood at 25 °C for 24 h, and then annealed at 80 °C for 1 h. The first and third polymer layers are formed on both sides of the second polymer layer respectively. Then, it is treated with O2 plasma with a power of 100 W for 10 minutes to increase the surface energy, and a composite membrane is obtained.

[0103] (3) First and second conductive layers: Using the magnetron sputtering method, carbon from a carbon target is sputtered onto the surface of the first polymer layer of the composite membrane. The sputtering power is 10 kW, the atmosphere is argon, the working pressure is 0.5 Pa, and the base film transfer rate is 1 m / min. The composite membrane is fixed on a rotatable or tiltable sample stage to improve the uniformity of coverage inside the pores. At the same time, the pore coverage and deposition thickness are monitored by SEM, the thickness is controlled by the sputtering time, and the substrate temperature is kept below the glass transition temperature of the polymer in the composite membrane by water cooling or intermittent sputtering (depositing in multiple short time intervals). After cooling, conductive carbon black is deposited on one side of the composite polymer as the first conductive layer. The composite membrane is flipped, and copper nanoparticles are deposited on the third polymer by magnetron sputtering as the second conductive layer. Among them, the high purity of the copper target is 99.99%, the sputtering power is 20 kW, and the working pressure is 1 Pa. The composite membrane is fixed on a rotatable or tiltable sample stage to improve the uniformity of coverage inside the pores. At the same time, the pore coverage and deposition thickness are monitored by SEM, the thickness is controlled by the sputtering time, and the substrate temperature is kept below the glass transition temperature of the polymer in the composite membrane by water cooling or intermittent sputtering (depositing in multiple short time intervals). After complete cooling, it is taken out to obtain the composite membrane.

[0104] (4) The positive electrode active material lithium cobalt oxide, acetylene black conductive agent, and polyvinylidene fluoride binder are prepared in a weight ratio of 96:2:2, and an appropriate amount of N-methylpyrrolidone (NMP) solvent is added, and then stirred evenly under the action of a vacuum mixer to obtain a positive electrode slurry with a solid content of 70wt%. Artificial graphite, styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed with deionized water in a mass ratio of 96:2:2, stirred evenly, and formulated into a negative electrode slurry with a solid content of 45wt%. The positive electrode slurry is applied to the surface of the first conductive layer of the composite film obtained in the above step (3), and then the negative electrode slurry is applied to the surface of the second conductive layer of the composite film. After drying and cold pressing, an electrode is obtained, and then after cutting and welding of the pole ears, an electrode assembly is obtained by winding.

[0105] (6) In a dry argon atmosphere glove box, lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solution containing propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC). Based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass percentage of ethylene carbonate (EC) was 15%, and the remaining solvents were propylene carbonate (PC) and diethyl carbonate (DEC), and the mass percentage ratio of propylene carbonate (PC) to diethyl carbonate (DEC) was 1:1.

[0106] (7) Placing the electrode assembly in an outer packaging aluminum-plastic film, injecting electrolyte, and encapsulating, and obtaining a lithium-ion battery through a process of formation, degassing, and trimming.

[0107] The information related to the prepared first polymer layer, the second polymer layer, the third polymer layer, the first conductive layer, the second conductive layer, the composite film, and the EC content in the electrolyte are shown in Tables 1 and 2.

[0108] Embodiment 2-5

[0109] Compared with Example 1, Examples 2-5 change the types of the two types of polymers, wherein the two types of polymers in Examples 2-5 are carbon fiber, polyester fiber, polyvinyl alcohol fiber, and polyamide fiber, respectively. The specific preparation process differences are shown in Tables 1 and 2, and the rest is the same as Example 1.

[0110] Examples 6-10

[0111] Compared with Example 1, Examples 6-10 change the type of a class of polymers, wherein the class of polymers in Examples 6-10 are polyamide (PA), polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene (PE), and polyether sulfone (PES), respectively. The specific differences are shown in Tables 1 and 2, and the rest are the same as Example 1.

[0112] Examples 11-12

[0113] Examples 11 - 12 compared with Example 1, the thickness of the second polymer layer was changed. The specific differences are shown in Tables 1 and 2, and the rest is the same as in Example 1.

[0114] Examples 13 - 15

[0115] Examples 13 - 15 compared with Example 1, the total thickness H of the first and third polymer layers and the EC content A in the electrolyte were changed. While changing the EC content, the mass ratios of PC and DEC were adjusted adaptively, and the mass ratio of PC and DEC remained unchanged. The specific differences are shown in Tables 1 and 2, and the rest is the same as in Example 1.

[0116] Examples 16 - 18

[0117] Examples 16 - 18 compared with Example 1, the thickness of the first conductive layer was changed. The specific differences are shown in Tables 1 and 2, and the rest is the same as in Example 1.

[0118] Examples 19 - 21

[0119] Examples 19 - 21 compared with Example 1, the thickness of the second conductive layer was changed. The specific differences are shown in Tables 1 and 2, and the rest is the same as in Example 1.

[0120] Examples 22 - 24

[0121] Examples 22 - 24 compared with Example 1, the conductive carbon in the first conductive layer was replaced with Ag nanoparticles. The specific differences are shown in Tables 1 and 2, and the rest is the same as in Example 1.

[0122] Among them, Ag nanoparticles were deposited by magnetron sputtering. The high purity of the silver target was 99.99%, the sputtering power was 200 W, and the atmosphere was argon. The working pressure was 1 Pa, the substrate transfer rate was 1 m / min. The composite film was fixed on a rotatable or tiltable sample stage to improve the uniformity of coverage inside the pores. At the same time, the pore coverage and deposition thickness were monitored by SEM, the thickness was controlled by sputtering time, and the substrate temperature was kept below the glass transition temperature of the polymer in the composite film by water cooling or intermittent sputtering (depositing in multiple short - time intervals).

[0123] Examples 25 - 28

[0124] Examples 25 - 28 compared with Example 1, the elemental Cu in the second conductive layer was replaced with conductive carbon. The deposition method of the conductive carbon was the same as that of the first conductive layer in Example 1. The specific differences are shown in Tables 1 and 2, and the rest is the same. Test method:

[0125] (1) Areal density of the composite film (g / cm 2):Take the composite membranes (without positive and negative active material layers) of the same volume size prepared above, weigh them, and calculate their areal density. The one with a smaller weight has more advantages for the energy density of the battery.

[0126] (2) Electrode impedance test: Use electrochemical impedance spectroscopy (EIS) to test the impedance of the positive electrode plate (composite membrane and positive active material layer) and the negative electrode plate (composite membrane and negative active material layer), and evaluate the interfacial impedance and diffusion impedance.

[0127] (3) Lithium-ion battery output power density test: Assemble a battery with a traditional PE composite membrane, and the other components of the battery are the same as those in Example 1 to obtain a traditional lithium-ion battery. Perform power density tests on the output power density of this traditional lithium-ion battery and the lithium-ion batteries obtained in the above examples respectively, and calculate the multiple of the increase in the output power density of each of the above examples compared to the output power density of this traditional lithium-ion battery.

[0128] Among them, the power density is tested by the pulse power method (HPPC, Hybrid Pulse Power Characterization), and it specifically includes the following steps: Apply a short-term (such as 10 seconds) large current pulse discharge to the battery, and record the voltage transient response. Calculate the internal resistance (R = Delta V / I) according to the voltage drop (Delta V) and current (I) at the end of the pulse. Maximum power: P = V 2 / (4R)), where V is the open-circuit voltage.

[0129] (4) Composite membrane bending mechanical flexibility test: Fix the composite membrane (without positive and negative active material layers) on a bending tester, apply a certain bending radius, and record the deformation and recovery of the electrode assembly at different bending degrees. In this example, the mechanical flexibility is evaluated by the bending radius. The smaller the bending radius, the better the mechanical flexibility of the composite membrane, and vice versa. Among them, Rmin = D / (2σ max ×t), Rmin is the minimum bending radius, D is the bending stiffness of the composite membrane, σ max is the maximum stress borne by the composite membrane during bending, and t is the total thickness of the composite membrane.

[0130] The test results are shown in Table 3.

[0131] Table 1

[0132]

[0133]

[0134] Table 2

[0135]

[0136]

[0137] Table 3

[0138]

[0139]

[0140] It can be seen from Examples 1-10 that by changing the types of the first polymer and the second polymer, the elastic modulus or surface energy of each polymer is different, which has a certain impact on the conductive agent content in the first polymer layer and the third polymer, the surface density and mechanical flexibility of the composite film. The scheme of the present application can select a variety of types of polymers to achieve the effect of the present application, and obtain a composite film with relatively low density and excellent mechanical flexibility, which meets the actual application requirements of the battery for the composite film.

[0141] It can be seen from Example 1 and Examples 11-12 that increasing the thickness of the second polymer layer increases the surface density of the composite film and reduces the mechanical flexibility. Therefore, selecting a second polymer layer with a lower thickness is more conducive to reducing the surface density of the composite film and improving the mechanical flexibility.

[0142] It can be seen from Examples 13-15 that A×H is related to the electrode impedance of the battery and the output power density of the battery, reflecting the ion transmission resistance. A relatively low A×H is more conducive to improving the output power density of the battery.

[0143] It can be seen from Examples 1 and 16-18 that as the thickness of the first conductive layer increases, the surface density of the composite film gradually increases, the impedance of the positive electrode sheet gradually decreases, the multiple of the output power density increase decreases, and the mechanical flexibility gradually decreases. It can be seen from Examples 1 and 19-21 that as the thickness of the second conductive layer increases, the surface density of the composite film gradually increases, the impedance of the negative electrode sheet gradually decreases, the multiple of the output power density increase decreases, and the mechanical flexibility gradually decreases. Therefore, selecting a relatively low thickness conductive layer is more conducive to improving the output power density and mechanical flexibility of the battery and reducing the impedance of the electrode sheet.

[0144] It can be seen from Example 1 and Examples 16-18, Examples 19-21, and Examples 22-24 that compared with the solution in which conductive carbon is respectively combined with Ag metal and Cu metal to form a conductive layer, the solution in which Ag metal and Cu metal are respectively used as the conductive agents for the first conductive layer and the second conductive layer, the battery has a lower electrode impedance and a better battery output power density.

[0145] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application rather than to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A secondary battery, comprising an electrolyte, a positive electrode sheet, a negative electrode sheet, and a composite membrane disposed between the positive electrode sheet and the negative electrode sheet, characterized in that: The composite film includes a first conductive layer, a first polymer layer, a second polymer layer, a third polymer layer and a second conductive layer stacked in sequence, the first conductive layer is in contact with the positive electrode sheet, and the second conductive layer is in contact with the negative electrode sheet; the first polymer layer and the third polymer layer include a type of polymer, and the type of polymer has a porous structure; the second polymer layer includes two types of polymers.

2. The secondary battery according to claim 1, wherein: Include at least one of the following: The surface energy of the polymer is 15-45 mJ / m 2 ; The elastic modulus of the second type of polymer is 5-600 GPa.

3. The secondary battery according to claim 1, wherein: Include at least one of the following: The polymer is selected from at least one of polyamide, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, polypropylene, polyethylene, polyether sulfone or polyimide; The second type of polymer is selected from at least one of aramid fiber, carbon fiber, polyester fiber, polyvinyl alcohol fiber or polyamide fiber.

4. The secondary battery according to claim 1, wherein: The thickness of the composite film is 10-40 μm.

5. The secondary battery according to claim 1, wherein: The thickness ratio of the first conductive layer, the first polymer layer, the second polymer layer, the third polymer layer and the second conductive layer is (0.5-2 μm):(2-5 μm):(5-20 μm):(2-5 μm):(0.5-2 μm).

6. The secondary battery according to claim 1, wherein: The secondary battery satisfies at least one of the following: (1) The first conductive layer and the first polymer layer both include conductive carbon, and the mass proportion of the conductive carbon in the second polymer layer is 5%-80% based on the sum of the mass of the conductive carbon in the first polymer layer and the mass of the conductive carbon in the first conductive layer; (2) Both the second conductive layer and the third polymer layer include conductive carbon, and based on the sum of the mass of the conductive carbon in the third polymer layer and the mass of the conductive carbon in the second conductive layer, the mass proportion of the conductive carbon in the third polymer layer is 5%-80%.

7. The secondary battery according to claim 1, wherein: The first conductive layer includes at least one of silver nanoparticles, carbon black, carbon nanotubes or graphene, and the second conductive layer includes at least one of copper nanoparticles, carbon nanotubes or graphene.

8. The secondary battery according to claim 7, characterized in that The secondary battery satisfies at least one of the following: (1) Both the first conductive layer and the first polymer layer include Ag elements, and based on the sum of the mass of the Ag element in the first polymer layer and the mass of the Ag element in the first conductive layer, the mass proportion of the Ag element in the first polymer layer is 10%-80%; (2) Both the second conductive layer and the third polymer layer include Cu element. Based on the sum of the mass of the Cu element in the third polymer layer and the mass of the Cu element in the second conductive layer, the mass proportion of the Cu element in the third polymer layer is 10%-80%.

9. The secondary battery according to any one of claims 1 to 8, characterized in that: The electrolyte includes ethylene carbonate, and based on the mass of the electrolyte, the mass proportion of the ethylene carbonate is A%, the sum of the thickness of the first polymer layer and the thickness of the third polymer layer is H μm, and A and H satisfy the following relationship: 80≤A×H≤160.

10. An electronic device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 9.

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