Isolating membrane and preparation method thereof, secondary battery and electric device

Abstract

The isolating membrane (10) comprises a first base membrane (11) and a second base membrane (12), the melting point of the second base membrane (12) is higher than that of the first base membrane (11), and the creep flexibility of the first base membrane (11) is larger than that of the second base membrane (12). The creep flexibility of the first base membrane (11) and the creep flexibility of the second base membrane (12) are different, so that the deformation capacities of the two base membranes are different under the action of stress, the damage of dendritic crystals to the diaphragm is jointly inhibited through stress buffering of one layer of base membrane and strength supporting of the other layer of base membrane, the risk of short circuit caused by the fact that the dendritic crystals penetrate through the diaphragm is reduced, and the service life of the diaphragm is prolonged. Therefore, the reliability and cycle life of the battery are improved.

Description

Separator and preparation method thereof, secondary battery and electric device Technical Field

[0001] The present application belongs to the technical field of secondary batteries, and specifically relates to an isolation membrane and a preparation method thereof, a secondary battery, and an electrical device. Background Art

[0002] Secondary batteries are widely used in various consumer electronic products and electric vehicles due to their outstanding features of light weight, no pollution and no memory effect.

[0003] With the continuous development of the new energy industry, users have placed higher demands on secondary batteries. However, lithium deposition is prone to occur during battery use, posing a risk of electrode short-circuit failure. This poses challenges to the reliability of secondary batteries.

[0004] Summary of the Invention

[0005] In view of the technical problems existing in the background technology, the present application provides an isolation membrane and a preparation method thereof, a secondary battery and an electrical device, aiming to improve the reliability of the isolation membrane and the secondary battery.

[0006] In order to achieve the above-mentioned purpose, the first aspect of the present application provides an isolation film, which includes a first base film and a second base film, the melting point of the second base film is higher than the melting point of the first base film, and the creep compliance of the first base film is greater than the creep compliance of the second base film.

[0007] Compared with the prior art, the present application includes at least the following beneficial effects: by setting the creep compliance of the first base film and the second base film to be different, and through the mutual cooperation of the two-layer base film structure, the deformation capabilities of the two base films under stress are different, and at least one side of the base film can have a larger deformation capacity to buffer the damage to the isolation film caused by lithium dendrites, etc., and can also play a role in inhibiting the growth of lithium dendrites, reducing the risk of electrode short circuit due to dendrites penetrating the isolation film, thereby improving the reliability of the battery over the long life cycle.

[0008] In any embodiment of the present application, the ratio of the creep compliance of the first base film to the creep compliance of the second base film is 1.1-3.0, and optionally 1.3-2.0. When the ratio of the creep compliance of the first base film to the creep compliance of the second base film meets the above conditions, the first base film's ability to buffer deformation can be further enhanced while also increasing the strength of the second base film, thereby improving the overall strength of the separator and enhancing its ability to resist dendrites, thereby improving the reliability of the battery over the long term.

[0009] In any embodiment of the present application, the creep compliance of the first base film is 0.0013 MPa -1 -0.0050MPa -1 ; Optional: 0.0013MPa-1 -0.0032MPa -1 and / or the creep compliance of the second base film is 0.0010MPa -1 -0.0023MPa -1 . Optional 0.0010MPa -1 -0.0020MPa -1 When the ratio of the creep compliance of the first base film to the creep compliance of the second base film meets the above range, the first base film's cushioning effect against deformation can be further enhanced, while ensuring the strength of the second base film, reducing the risk of short circuits caused by dendrites penetrating the isolation film, thereby improving the reliability of the battery over its long life.

[0010] In any embodiment of the present application, the ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film is greater than or equal to 1.05, and can be optionally 1.2-10. By limiting the ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film to the above range, the first base film and the second base film can be made sufficiently flexible while having sufficient strength.

[0011] In any embodiment of the present application, the relative molecular mass of the material of the first base film is 300,000-2.5 million, optionally 500,000-2 million; and / or the relative molecular mass of the material of the second base film is 10,000-2 million, optionally 10,000-1.2 million. Limiting the relative molecular mass of the material of the first base film and the relative molecular mass of the material of the second base film to the above ranges can further improve the creep compliance of the first base film, thereby enhancing its ability to cope with deformation, while also increasing the support strength of the second base film, thereby reducing the probability of lithium dendrite penetration, thereby improving the reliability of the battery over the long term.

[0012] In any embodiment of the present application, the crystallinity of the first base film is greater than that of the second base film; optionally, the ratio of the crystallinity of the first base film to the crystallinity of the second base film is 1.1-5.0, or optionally 1.5-3.0. By limiting the ratio of the crystallinity of the first base film to the crystallinity of the second base film to the above range, the first base film and the second base film can be made sufficiently flexible while having sufficient strength.

[0013] In any embodiment of the present application, the crystallinity of the first base film is 40%-90%, optionally 75%-85%; and / or the crystallinity of the second base film is 20%-70%, optionally 30%-45%. By limiting the crystallinity of the first base film and the crystallinity of the second base film to the above ranges, the first base film and the second base film can be made sufficiently flexible while having sufficient strength.

[0014] In any embodiment of the present application, the melting point of the first base film is lower than the melting point of the second base film, and the ratio of the melting point of the first base film to the melting point of the second base film is 0.3-0.85, and optionally 0.35-0.65. Limiting the ratio of the melting point of the first base film to the melting point of the second base film to the given range can ensure that the first base film and the second base film have good heat resistance and sufficient flexibility, thereby improving the reliability of the battery over the long term.

[0015] In any embodiment of the present application, the melting point of the first base film is 120° C.-280° C., optionally 130° C.-265° C., and / or the melting point of the second base film is 150° C.-360° C., optionally 160° C.-350° C. Limiting the melting points of the first base film and the second base film to the above ranges can ensure that the first base film and the second base film have good heat resistance and creep compliance, thereby improving the reliability of the battery over the long life cycle.

[0016] In any embodiment of the present application, the first base film and the second base film are each independently selected from at least one of polyolefins and their derivatives, halogenated polyolefins and their derivatives, polyethers and their derivatives, polyetheretherketones and their derivatives, polyesters and their derivatives, polyimides and their derivatives, polyvinyl alcohols and their derivatives, polytetrafluoroethylenes and their derivatives, polyvinyl fluoride and their derivatives, polyvinylidene fluoride and their derivatives, and polyethylene terephthalate and its derivatives. Using at least one of the above materials as the first and second base films can ensure that the first and second base films have good chemical stability, while also providing the separator with high flexibility and strength.

[0017] In any embodiment of the present application, an adhesive layer is further provided between the first base film and the second base film. The adhesive layer includes an adhesive; optionally, the adhesive layer includes an adhesive and a filler. When an adhesive layer is provided between the first base film and the second base film, it not only compensates for process defects during the overheating and pressing lamination process but also further improves the physical stability of the separator, thereby enhancing the reliability of the secondary battery.

[0018] In any embodiment of the present application, the adhesive includes one or more of polyacrylate, polyacrylic acid, polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-trichloroethylene copolymer, polyvinyl pyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, polyacrylonitrile, polyvinyl alcohol, polyethylene, polypropylene, starch, and cyanoethyl pullulan.

[0019] In any embodiment of the present application, the filler includes at least one of inorganic particles, organic particles, and organic-metal framework materials.

[0020] In any embodiment of the present application, the thickness of the adhesive layer is less than or equal to 4 μm, and can be optionally 0.5-2 μm. When the thickness of the adhesive layer is within the given range, the reliability of the battery over a long life cycle can be improved.

[0021] In any embodiment of the present application, the transverse heat shrinkage rate of the isolation film at 250° C. for 1 hour is less than or equal to 0.4%, and optionally less than or equal to 0.25%.

[0022] In any embodiment of the present application, the longitudinal thermal shrinkage rate of the isolation film at 250° C. for 1 hour is less than or equal to 0.4%, and can be optionally less than or equal to 0.25%.

[0023] In any embodiment of the present application, the transverse tensile strength of the separator is greater than or equal to 2500 kg / cm 2 , optional 3000kg / cm 2 -4000kg / cm 2 .

[0024] In any embodiment of the present application, the longitudinal tensile strength of the separator is greater than or equal to 2500 kg / cm 2 , optional 3000kg / cm 2 -4000kg / cm 2 .

[0025] When at least one of the above items of the separator satisfies the given range, the separator has good physical properties, thereby improving the reliability of the secondary battery.

[0026] A second aspect of the present application provides a method for preparing an isolation membrane. The method comprises providing a first base membrane and a second base membrane, wherein the second base membrane has a higher melting point than the first base membrane and the first base membrane has a greater creep compliance than the second base membrane; and compounding the first base membrane and the second base membrane to obtain the isolation membrane according to any embodiment of the first aspect.

[0027] In any embodiment of the present application, the preparation method further includes preparing an adhesive layer slurry, wherein the adhesive layer slurry includes an adhesive, and the adhesive layer slurry is applied to the first base film and / or the second base film before lamination. Optionally, the adhesive layer slurry includes an adhesive and a filler. When the adhesive layer slurry also includes a filler, the physical properties of the separator can be further improved, thereby enhancing the reliability of the secondary battery.

[0028] The third aspect of the present application provides a secondary battery comprising the separator of the first aspect of the present application or the separator prepared according to the method of the second aspect of the present application. When the secondary battery uses the separator, the reliability of the secondary battery can be improved.

[0029] In any embodiment of the present application, it also includes a positive electrode sheet and a negative electrode sheet, and the isolation membrane is arranged between the positive electrode sheet and the negative electrode sheet. The first base membrane faces the negative electrode sheet. The first base membrane has good creep flexibility and faces the negative electrode, which can enhance the anti-dendritic puncture effect and improve the reliability of the battery during the long life. At the same time, the second base membrane facing the positive electrode has a high melting point and can enhance heat resistance.

[0030] The fourth aspect of the present application provides an electric device, which includes the secondary battery of the second aspect of the present application. When the secondary battery of the electric device uses the given separator, the reliability of the electric device can be improved.

[0031] Since the device of the present application includes the secondary battery provided by the present application, it has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without inventive effort.

[0033] FIG1 is a schematic structural diagram of an isolation membrane according to an embodiment of the present application.

[0034] FIG2 is a schematic structural diagram of an isolation membrane according to an embodiment of the present application.

[0035] FIG3 is a schematic flow chart of an embodiment of a method for preparing an isolation film according to the present application.

[0036] FIG. 4 is a schematic diagram of an embodiment of a secondary battery.

[0037] FIG. 5 is an exploded view of FIG. 4 .

[0038] FIG. 6 is a schematic diagram of an embodiment of a battery module.

[0039] FIG. 7 is a schematic diagram of an embodiment of a battery pack.

[0040] FIG8 is an exploded view of FIG7 .

[0041] FIG. 9 is a schematic diagram of an embodiment of a device using a secondary battery as a power source. DETAILED DESCRIPTION

[0042] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0043] For the sake of clarity, only some numerical ranges are specifically disclosed herein. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0044] In the description herein, unless otherwise indicated, the term "or" is inclusive. That is, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0045] In the description of this article, it should be noted that, unless otherwise specified, “above” and “below” include the number itself, and “several” in “one or several” means two or more.

[0046] Unless otherwise specified, the terms used in this application have the commonly understood 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 using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0047] secondary batteries

[0048] A secondary battery is a battery that can be recharged to activate the active materials after discharge and continue to be used.

[0049] Typically, a secondary battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The separator is placed between the positive and negative electrodes to provide isolation. The electrolyte conducts ions between the positive and negative electrodes.

[0050] [Isolation film]

[0051] 1 , an embodiment of the present application provides an isolation film 10 , including a first base film 11 and a second base film 12 . The melting point of the second base film is higher than that of the first base film, and the creep compliance of the first base film is greater than that of the second base film.

[0052] Without wishing to be bound by any theory, the inventors have discovered through extensive research that, in the specific separator structure of this application, by providing different creep compliances for the first and second base membranes, the two base membranes exhibit different deformation capacities under stress. One base membrane has a greater deformation capacity. When dendrites grow and impact the separator, the base membrane with greater creep compliance deforms and wraps around the dendrites, buffering them from damage. Meanwhile, the base membrane with less creep compliance deforms less, providing sufficient support. The stress buffering provided by one base membrane and the support provided by the other jointly suppress dendrite damage to the separator, reducing the risk of dendrites penetrating the separator and causing a short circuit, thereby improving the battery's reliability and cycle life over the long term.

[0053] After in-depth research, the inventors of the present application have found that when the separator of the present application satisfies the above conditions and optionally satisfies one or more of the following conditions, the performance of the secondary battery can be further improved.

[0054] In any embodiment of the present application, the ratio of the creep compliance of the first basement membrane to the creep compliance of the second basement membrane is 1.1-3.0, and may be 1.3-2.0. In some embodiments, the ratio of the creep compliance of the first basement membrane to the creep compliance of the second basement membrane may be 1.4-1.8. In other embodiments, the ratio of the creep compliance of the first basement membrane to the creep compliance of the second basement membrane may be 1.1, 1.2, 1.4, 1.6, 1.7, 1.9, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, etc., or a range consisting of any two of the above values. For example, the ratio of the creep compliance of the first base film to the creep compliance of the second base film can be selected as 1.10-1.22, 1.22-1.40, 1.31-1.79, 1.40-1.70, 1.56-1.90, 1.70-2.39, 2.13-2.78, 2.35-2.78, 1.2-1.9, 1.6-2.6, 1.9-2.4, 2.6-2.8, etc.

[0055] In this embodiment, when the ratio of the creep flexibility of the first base film to the creep flexibility of the second base film meets the above conditions, the flexibility and strength of the isolation membrane can be well balanced; the flexibility of the first base film can be used to improve the ability of the isolation membrane to cope with external forces, and has a certain buffering effect on the deformation caused by external forces; the strength of the second base film can be used to improve the strength of the isolation membrane against external forces, and reduce the risk of short circuit caused by foreign matter (such as lithium dendrites) penetrating the isolation membrane, thereby improving the reliability and cycle life of the battery in the long-term cycle life.

[0056] In any embodiment of the present application, the creep compliance of the first base film is 0.0013 MPa -1 -0.0050MPa -1In some optional embodiments, the creep compliance of the first base film is 0.0013 MPa -1 -0.0032MPa -1 The creep compliance of the first base film can be 0.0013 MPa. -1 , 0.0015MPa -1 , 0.0017MPa -1 , 0.0019MPa -1 , 0.0022MPa -1 , 0.0025MPa -1 , 0.0028MPa -1 , 0.0031MPa -1 , 0.0034MPa -1 , 0.0039MPa -1 , 0.0043MPa -1 , 0.0048MPa -1 , 0.0050MPa -1 etc. Or a range consisting of any two of the above values. For example, the creep compliance of the first base film is 0.0013 MPa -1 -0.0015MPa -1 , 0.0013MPa -1 -0.0019MPa -1 , 0.0017MPa -1 -0.0022MPa -1 , 0.0022MPa -1 -0.0028MPa -1 , 0.0025MPa -1 -0.0032MPa -1 , 0.0028MPa -1 -0.0039MPa -1 , 0.0015MPa -1 -0.0017MPa -1 , 0.0019MPa -1 -0.0022MPa -1 , 0.0025MPa -1 -0.0028MPa -1 , 0.0031MPa -1 -0.0039MPa -1 , 0.0043MPa -1 -0.0048MPa -1 wait.

[0057] In some optional embodiments, the creep compliance of the second base film is 0.0010 MPa -1 -0.0023MPa-1 In some optional embodiments, the creep compliance of the second base film is 0.0010 MPa -1 -0.0020MPa -1 The creep compliance of the second base film can be 0.0010 MPa. -1 , 0.0012MPa -1 , 0.0014MPa -1 , 0.0015MPa -1 , 0.0016MPa -1 , 0.0017MPa -1 , 0.0018MPa -1 , 0.0019MPa -1 , 0.0020MPa -1 , 0.0021MPa -1 , 0.0022MPa -1 , 0.0023MPa -1 etc. Or a range consisting of any two of the above values. For example, the creep compliance of the second base film is 0.0010 MPa -1 -0.0013MPa -1 , 0.0013MPa -1 -0.0016MPa -1 , 0.0014MPa -1 -0.0020MPa -1 , 0.0016MPa -1 -0.0018MPa -1 , 0.0020MPa -1 -0.0023MPa -1 , 0.0010MPa -1 -0.0015MPa -1 , 0.0015MPa -1 -0.0020MPa -1 , 0.0015MPa -1 -0.0016MPa -1 , 0.0017MPa -1 -0.0019MPa -1 , 0.0018MPa -1 -0.0021MPa -1 , 0.0020MPa -1 -0.0021MPa -1 wait.

[0058] When the creep flexibility of the first base film and the creep flexibility of the second base film meet the above range, the flexibility and strength of the isolation membrane can be well balanced; the flexibility of the first base film can be used to improve the isolation membrane's ability to cope with external forces, and has a certain buffering effect on the deformation caused by external forces. The strength of the second base film can be used to improve the isolation membrane's strength against external forces, and reduce the risk of short circuits caused by foreign matter (such as lithium dendrites) penetrating the isolation membrane, thereby improving the reliability and cycle life of the battery over a long cycle life.

[0059] According to some embodiments, the creep compliance of the first and second base films can be tested using equipment and methods known in the art. For example, the following steps can be used: cut the sample to be tested into a rectangular strip with a length of 50 mm and a width of 4 mm. The strip is clamped on a dynamic mechanical analyzer. The test mode is set to creep mode, the temperature is 25°C, and the test time is 220 minutes. The value at the plateau of the curve is the creep compliance of the sample.

[0060] Those skilled in the art will appreciate that the creep compliance of the base film can be adjusted by adjusting the intrinsic parameters of the base film (e.g., one or more of the crystallinity and relative molecular weight of the base film material) and the base film preparation process parameters (e.g., one or more of the stretch ratio and stretching temperature). For example, under the condition that other conditions remain unchanged, the smaller the relative molecular weight of the base film material, the greater the creep compliance; and the greater the crystallinity, the smaller the creep compliance. Those skilled in the art can adjust the creep compliance of the base film by known methods within the parameter ranges given in this application, such as adjusting the production process of the base film (controlling the crystallinity, molecular weight, etc.), and obtain a base film with the desired creep compliance through a limited number of experiments.

[0061] In any embodiment of the present application, the ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film is greater than or equal to 1.05, and can be selected from 1.2 to 10. In some optional embodiments, the ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film is 2.2 to 8. In some embodiments, the ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film is 1.05, 1.1, 1.3, 1.5, 1.7, 2.0, 2.4, 2.8, 3.2, 3.6, 4.5, 4.9, 5.3, 5.6, 6.2, 6.7, 7.5, 7.9, 8.7, 9.4, 10, etc. Or a range consisting of any two of the above values. For example, the ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film is 1.05-1.3, 1.2-1.5, 1.1-1.7, 2.4-2.8, 2.2-2.6, 3.6-4.9, 4.5-5.3, 5.6-6.2, 6.7-7.9, 8.7-9.4, etc.

[0062] In any embodiment of the present application, the relative molecular mass of the material of the first base film is 300,000-2.5 million, and can be 500,000-2 million. In some optional embodiments, the relative molecular mass of the material of the first base film is 600,000-1.8 million. In some embodiments, the relative molecular mass of the material of the first base film can be 300,000, 340,000, 420,000, 500,000, 750,000, 900,000, 1.2 million, 1.36 million, 1.5 million, 1.7 million, 1.82 million, 1.9 million, 1.95 million, 2 million, 2.2 million, 2.5 million, etc., or a range consisting of any two of the above values. For example, the relative molecular mass of the material of the first base film is 300,000-450,000, 300,000-500,000, 500,000-750,000, 750,000-1.2 million, 350,000-500,000, 750,000-900,000, 1.2 million-1.36 million, 1.5 million-1.7 million, 1.82 million-2.1 million, 2.3 million-2.43 million, etc.

[0063] In some embodiments, the relative molecular mass of the material of the second base film is 10,000-2,000,000, optionally 10,000-1,200,000. In some embodiments, the relative molecular mass of the material of the second base film is 400,000-800,000. The relative molecular mass of the material of the second base film can be 10,000, 25,000, 100,000, 200,000, 350,000, 190,000, 280,000, 370,000, 510,000, 650,000, 800,000, 1,050,000, 1,130,000, 1,300,000, 1,440,000, 1,630,000, 1,770,000, 1,800,000, 1,930,000, 2,000,000, or the like, or a range consisting of any two of the above values. For example, the relative molecular mass of the material of the second base film is 10,000-25,000, 25,000-70,000, 80,000-150,000, 25,000-150,000, 130,000-180,000, 150,000-250,000, 180,000-400,000, 300,000-400,000, 500,000-650,000, 190,000-280,000, 370,000-510,000, 650,000-800,000, 1.05 million-1.13 million, 1.3 million-1.44 million, 1.67 million-1.77 million, 1.8 million-1.93 million, etc.

[0064] The relative molecular mass of the base film material has a well-known meaning in the art and can be measured using equipment and methods known in the art, for example, using a high-temperature GPC test (differential refractive index detector).

[0065] The greater the relative molecular weight of the base film material, the longer its molecular chains are likely to be. This means the chains are likely to be more tightly entangled, making them less susceptible to deformation and, in turn, reducing creep compliance. Correspondingly, the smaller the relative molecular weight of the base film material, the greater its creep compliance. Furthermore, the greater the relative molecular weight of the base film material, the greater its corresponding strength. Therefore, when selecting the relative molecular weight of the base film material, it is necessary to balance the two properties of material compliance and strength. To increase the strength of the base film, a larger relative molecular weight can be selected; however, this will reduce creep compliance.

[0066] In any embodiment of the present application, the crystallinity of the first base film is greater than the crystallinity of the second base film; optionally, the ratio of the crystallinity of the first base film to the crystallinity of the second base film is 1.1-5.0. In some optional embodiments, the ratio of the crystallinity of the first base film to the crystallinity of the second base film is 1.5-3.0. The ratio of the crystallinity of the first base film to the crystallinity of the second base film can be 1.1, 1.3, 1.8, 2.2, 2.5, 2.9, 3.4, 3.7, 4.0, 4.5, 4.8, 5.0, etc., or a range consisting of any two of the above values. For example, the ratio of the crystallinity of the first base film to the crystallinity of the second base film can be selected as 1.1-1.3, 1.3-1.5, 1.5-1.8, 1.3-1.8, 1.8-2.5, 2.2-2.9, 2.9-3.4, 3.4-3.7, 3.7-4.0, 3.9-4.4, 4.4-5.0, etc.

[0067] In any embodiment of the present application, the crystallinity of the first base film is 40%-90%, optionally 75%-85%. The crystallinity of the first base film can be 40%, 45%, 50%, 59%, 64%, 70%, 75%, 78%, 80%, 82%, 86%, 88%, 90%, etc. Or it can be a range consisting of any two of the above values. For example, the crystallinity of the first base film is 40%-45%, 45%-50%, 50%-59%, 60%-65%, 54%-62%, 62%-68%, 60%-71%, 72%-80%, 59%-80%, 64%-82%, 80%-86%, 82%-88%, etc.

[0068] In some embodiments, the crystallinity of the second base film is 20%-70%, optionally 30%-45%. The crystallinity of the second base film can be 20%, 24%, 29%, 38%, 44%, 59%, 67%, 69%, 70%, etc., or a range consisting of any two of the above values. For example, the crystallinity of the second base film is 20%-23%, 24%-29%, 29%-44%, 29%-38%, 38%-42%, 42%-47%, 48%-55%, 55%-59%, 60%-65%, 38%-59%, 59%-67%, 67%-70%, etc.

[0069] According to some embodiments, crystallinity can be tested using equipment and methods known in the art. Specifically, 4-6 mg of the sample to be tested is placed in the sample chamber of a differential scanning calorimeter and heated from 25-350°C at a rate of 10°C / min to obtain a melting endothermic curve. The crystallinity can be determined by calculating the peak area of ​​the curve relative to a reference value of 100% crystalline polyolefin.

[0070] Crystallinity indicates the proportion of crystalline regions within the base film material. Crystallinity is the orderly arrangement of molecular chains, forming a molecular chain. Generally, the higher the crystallinity, the more regular the molecular chain arrangement.

[0071] Crystallinity also has a certain impact on the creep compliance of the basement membrane. Generally, the greater the crystallinity, the lower the creep compliance. Increased crystallinity can improve the strength of the basement membrane, such as by increasing puncture resistance. Therefore, in addition to the relative molecular weight of the basement membrane material, a balance must also be struck between crystallinity to ensure the separator has sufficient strength and flexibility. Therefore, in this application, the relative molecular weight of the first basement membrane material is greater than that of the second basement membrane material, and the crystallinity of the first basement membrane is greater than that of the second basement membrane, thereby improving the strength and creep compliance of the first basement membrane. In other words, by balancing the relative molecular weight and crystallinity of the first basement membrane, the first basement membrane can have both higher strength and higher creep compliance. Correspondingly, the second basement membrane can be made of a material with a more complex molecular chain, and the creep compliance can be balanced by reducing the crystallinity. Furthermore, the degree of branching of the first basement membrane material can be made lower than that of the second basement membrane material to improve creep compliance. In other words, by balancing the relative molecular weight, degree of branching, and crystallinity of the first basement membrane, the first basement membrane can have both higher strength and higher creep compliance. Therefore, by limiting the crystallinity, relative molecular mass of the material of the first base film and the crystallinity, relative molecular mass and their ratio of the material of the second base film to the above range, the creep flexibility of the first base film can be greater than the creep flexibility of the second base film; at the same time, the isolation film has a certain strength.

[0072] In any embodiment of the present application, the melting point of the first base film is lower than the melting point of the second base film. The ratio of the melting point of the first base film to the melting point of the second base film is 0.3-0.85, and can be selected as 0.35-0.65. For example, in some embodiments, the ratio of the melting point of the first base film to the melting point of the second base film can be selected as 0.3, 0.35, 0.4, 0.5, 0.6, 0.65, 0.7, 0.8, 0.85, etc. Or it can be a range consisting of any two of the above values. For example, the ratio of the melting point of the first base film to the melting point of the second base film can be selected as 0.3-0.35, 0.35-0.4, 0.4-0.5, 0.5-0.6, 0.6-0.65, 0.5-0.7, 0.7-0.8, 0.8-0.85, etc.

[0073] In any embodiment of the present application, the melting point of the first base film is 120°C-280°C, optionally 130°C-265°C. In some embodiments, the melting point of the first base film is 120°C, 125°C, 130°C, 146°C, 150°C, 152°C, 165°C, 180°C, 196°C, 204°C, 220°C, 246°C, 258°C, 265°C, 270°C, 280°C, etc., or a range consisting of any two of the above values. For example, the melting point of the first base film is 120℃-125℃, 125℃-130℃, 130℃-146℃, 146℃-152℃, 155℃-180℃, 180℃-206℃, 210℃-230℃, 231℃-240℃, 245℃-254℃, 255℃-265℃, and 265℃-280℃.

[0074] In some embodiments, the melting point of the second base film is 150°C-360°C, optionally 160°C-350°C. For example, in some embodiments, the melting point of the second base film is 150°C, 158°C, 165°C, 170°C, 185°C, 200°C, 210°C, 240°C, 255°C, 272°C, 296°C, 320°C, 335°C, 360°C, etc., or a range consisting of any two of the above values. For example, the melting point of the second base film is 150°C-165°C, 165°C-170°C, 170°C-190°C, 190°C-210°C, 210°C-240°C, 240°C-272°C, 272°C-320°C, 320°C-340°C, or 340°C-360°C.

[0075] Both excessively high and low melting points can affect the physical properties of the base film material. For example, after exceeding the predetermined melting point, even if the material has high creep compliance, its strength is too low to inhibit dendrite penetration. By limiting the melting points of the first and second base films, as well as their ratio, within the above range, the second base film with a higher melting point complements the heat resistance, while the first base film with a lower melting point complements the flexibility, resulting in a separator with good heat resistance and flexibility. This can improve the physical properties of both the first and second base films, thereby enhancing the reliability of the secondary battery.

[0076] According to some embodiments, the melting points of the first and second base films have meanings well known in the art and can be measured using methods known in the art. For example, differential scanning calorimetry can be used for measurement. For details, reference can be made to standard GB / T 19466.3-2004. As an example, the measurement can be performed as follows: 4-6 mg of a sample to be tested is placed in the sample chamber of a differential scanning calorimeter. The temperature is raised from 25°C to 400°C at a rate of 10°C / min. The melting endothermic curve of the sample is obtained, and the temperature corresponding to the peak of the curve is the melting point of the sample.

[0077] In any embodiment of the present application, the first base film and the second base film are each independently selected from at least one of polyolefins and their derivatives, halogenated polyolefins and their derivatives, polyethers and their derivatives, polyetheretherketones and their derivatives, polyesters and their derivatives, polyimides and their derivatives, polyvinyl alcohol and their derivatives, polytetrafluoroethylene and their derivatives, polyvinyl fluoride and their derivatives, polyvinylidene fluoride and their derivatives, and polyethylene terephthalate and their derivatives. For example, the first base film can be made of polyolefins and their derivatives or halogenated polyolefins and their derivatives. The first base film can also be made of polyethers and their derivatives, polyetheretherketones and their derivatives, polyesters and their derivatives, etc. The first base film and the second base film can also be made of polyvinylidene fluoride and its derivatives or polyethylene terephthalate, etc. Among them, derivatives generally refer to products derived from the replacement of hydrogen atoms or atomic groups in a compound by other atoms or atomic groups. At least one of the above materials as the first base film and the second base film can improve the chemical stability of the first base film and the second base film, while making the isolation film have higher flexibility and higher strength.

[0078] Referring to FIG. 2 , an embodiment of the present application provides a separator 10 comprising a first base film 11 and a second base film 12 . In any embodiment of the present application, an adhesive layer 13 is provided between the first base film 11 and the second base film 12 . The adhesive layer comprises an adhesive. Optionally, the adhesive layer comprises an adhesive and a filler.

[0079] When an adhesive layer is provided between the first and second base films, and the adhesive layer includes an adhesive, it not only compensates for process defects during the hot-pressing lamination of the base films, but also further improves the physical properties of the separator (e.g., tensile strength, puncture strength, heat resistance, etc.), thereby enhancing the reliability of the secondary battery. The filler is located between the first and second base films, thereby reducing the risk of powder loss.

[0080] In any embodiment of the present application, the adhesive includes one or more of polyacrylate, polyacrylic acid, polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-trichloroethylene copolymer, polyvinyl pyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, polyacrylonitrile, polyvinyl alcohol, polyethylene, polypropylene, starch, and cyanoethyl pullulan.

[0081] In any embodiment of the present application, the filler includes at least one of inorganic particles, organic particles, and organic-metal framework materials.

[0082] Optionally, the inorganic particles include one or more of inorganic particles having a dielectric constant of 5 or greater, inorganic particles having ion conductivity but not storing ions, and inorganic particles capable of undergoing electrochemical reactions.

[0083] Alternatively, the inorganic particles having a dielectric constant of 5 or more may include boehmite, aluminum oxide, zinc oxide, silicon oxide, titanium oxide, zirconium oxide, barium oxide, calcium oxide, magnesium oxide, nickel oxide, tin oxide, cerium oxide, yttrium oxide, hafnium oxide, aluminum hydroxide, magnesium hydroxide, silicon carbide, boron carbide, aluminum nitride, silicon nitride, boron nitride, magnesium fluoride, calcium fluoride, barium fluoride, barium sulfate, magnesium aluminum silicate, lithium magnesium silicate, sodium magnesium silicate, bentonite, hectorite, zirconium titanate, barium titanate, Pb(Zr,Ti)O3 (abbreviated as PZT), Pb1-mLamZr1-nTinO3 (abbreviated as PLZT, 0<m<1, 0<n<1), Pb(Mg3Nb 2 / 3)O3-PbTiO3 (abbreviated as PMN-PT), and at least one of their respective modified inorganic particles. Optionally, the modification method of each inorganic particle may be chemical modification and / or physical modification. The chemical modification method includes coupling agent modification (for example, using silane coupling agent, titanate coupling agent, etc.), surfactant modification, polymer grafting modification, etc. The physical modification method may be mechanical force dispersion, ultrasonic dispersion, high energy treatment, etc. The modification treatment can reduce the agglomeration of inorganic particles, thereby enabling the adhesive layer to have a more stable and uniform structure; in addition, by selecting a coupling agent, surfactant or polymer with a specific functional group to modify the inorganic particles, it is also helpful to improve the wetting and retention properties of the adhesive layer to the electrolyte, and improve the adhesion of the adhesive layer to the first base film and the second base film.

[0084] Alternatively, the inorganic particles having ion conductivity but not storing ions may include Li3PO4, lithium titanium phosphate Li x1 Ti y1 (PO4)3, lithium aluminum titanium phosphate Li x2 Al y2 Ti z1 (PO4)3, (LiAlTiP) x3 O y3 Type glass, lanthanum lithium titanate Li x4 La y4 TiO3, lithium germanium thiophosphate Li x5 Ge y5 P z2 S w , lithium nitride Li x6 N y6 、SiS2 type glass Li x7 Si y7 S z3 and P2S5 glass Li x8 P y8 S z4 At least one of the following: 0<x1<2, 0<y1<3, 0<x2<2, 0<y2<1, 0<z1<3, 0<x3<4, 0<y3<13, 0<x4<2, 0<y4<3, 0<x5<4, 0<y5<1, 0<z2<1, 0<w<5, 0<x6<4, 0<y6<2, 0<x7<3, 0<y7<2, 0<z3<4, 0<x8<3, 0<y8<3, 0<z4<7. This can further improve the ion conductivity of the isolation membrane.

[0085] Alternatively, the inorganic particles capable of undergoing electrochemical reaction may include at least one of lithium-containing transition metal oxides, lithium-containing phosphates, carbon-based materials, silicon-based materials, tin-based materials, and lithium-titanium compounds.

[0086] Alternatively, the organic particles may include one or more of polycarbonate, polythiophene, polypyridine, polystyrene, polyacrylic wax, polyethylene, polypropylene, cellulose, a cellulose modifier (e.g., carboxymethyl cellulose), a melamine resin, a phenolic resin, a polyester (e.g., polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate), a silicone resin, a polyimide, a polyamideimide, a polyaromatic amide, polyphenylene sulfide, a polysulfone, a polyethersulfone, a polyetheretherketone, a polyaryletherketone, a copolymer of butyl acrylate and ethyl methacrylate (e.g., a cross-linked polymer of butyl acrylate and ethyl methacrylate).

[0087] Optionally, the organic-metal framework material may include one or more of a nitrogen-containing heterocyclic ligand building structure, an organic carboxylic acid ligand building structure, and a nitrogen-oxygen mixed ligand building structure.

[0088] In some embodiments, the content of the adhesive may be greater than or equal to 10%, optionally 10%-30%, based on the total weight of the adhesive layer.

[0089] In some embodiments, the content of the filler may be less than or equal to 90%, optionally 40%-90%, 60%-80%, based on the total weight of the adhesive layer.

[0090] In some embodiments, the adhesive layer may further include a dispersant, such as carboxymethyl cellulose, to adjust the viscosity of the adhesive layer slurry and improve the quality and uniformity of the adhesive layer.

[0091] In some embodiments, the content of the dispersant may be less than or equal to 25%, optionally less than or equal to 20%, based on the total weight of the adhesive layer.

[0092] The thickness of the adhesive layer is less than or equal to 4 μm, and can be optionally 0.5-2 μm.

[0093] According to some embodiments, the adhesive layer thickness can be tested using equipment and methods known in the art. Specifically, a scanning electron microscope (SEM) image of a cross-section of the separator can be obtained using a scanning electron microscope (e.g., a ZEISS Sigma 300) in accordance with JY / T010-1996. As an example, the following method can be used to test the thickness of the coating: randomly select multiple areas on the cross-section of the separator and measure the coating thickness at least five times at a specific magnification. The measured values ​​for the different areas are then tallied and the average value is taken as the adhesive layer thickness.

[0094] In any embodiment of the present application, the transverse heat shrinkage rate of the isolation film at 250°C and 1 hour is less than or equal to 0.4%, optionally less than or equal to 0.25%; and / or the longitudinal heat shrinkage rate of the isolation film at 250°C and 1 hour is less than or equal to 0.4%, optionally less than or equal to 0.25%; and / or the transverse tensile strength of the isolation film is greater than or equal to 2500 kg / cm 2 , optional 3000kg / cm 2 -4000kg / cm 2 ; and / or the longitudinal tensile strength of the separator is greater than or equal to 2500kg / cm 2 , optional 3000kg / cm 2 -4000kg / cm 2 When at least one of the above items of the separator satisfies the given range, the separator has good heat resistance and physical properties, thereby improving the reliability of the secondary battery.

[0095] According to some embodiments, the transverse tensile strength, longitudinal tensile strength, transverse heat shrinkage, and longitudinal heat shrinkage of the first and second base films have well-known meanings in the art and can be tested using equipment and methods known in the art. For example, the testing can be performed in accordance with standard GB / T 36363-2018.

[0096] [Method for preparing isolation film]

[0097] Referring to FIG3 , an embodiment of the present application provides a method for preparing an isolation membrane. The method comprises the following steps:

[0098] S10: providing a first base film and a second base film, wherein the melting point of the second base film is higher than the melting point of the first base film, and the creep compliance of the first base film is greater than the creep compliance of the second base film.

[0099] S20: Compounding the first base film and the second base film to obtain an isolation film.

[0100] The first base film and the second base film can be directly laminated by hot pressing. During the hot pressing process, if the temperature is too high, the porosity will be small and the air permeability will be poor; if the temperature is too low, the first base film and the second base film will not be firmly bonded. Therefore, it is necessary to adjust the hot pressing temperature appropriately. Optionally, the hot pressing temperature is 20°C-50°C.

[0101] In any embodiment of the present application, the method for preparing the isolation membrane further comprises:

[0102] An adhesive layer slurry is provided, the adhesive layer slurry includes an adhesive, and the adhesive layer slurry is coated on the first base film and / or the second base film before compounding.

[0103] Optionally, the adhesive layer slurry includes an adhesive and a filler. In this way, an adhesive layer can be provided between the first base film and the second base film to improve the physical properties of the separator, thereby improving the reliability of the secondary battery.

[0104] Unless otherwise specified, all raw materials used in the method for preparing the separator (eg, the first base film, the second base film, the adhesive, the filler, etc.) can be purchased commercially.

[0105] [Positive electrode]

[0106] In a secondary battery, a positive electrode sheet generally includes a positive electrode current collector and a positive electrode film layer disposed on the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0107] The positive electrode current collector may be a conventional metal foil or a composite current collector (a metal material may be disposed on a polymer substrate to form a composite current collector). As an example, the positive electrode current collector may be an aluminum foil.

[0108] There is no limitation on the specific type of the positive electrode active material. Active materials known in the art that can be used for the positive electrode of a secondary battery can be used. Those skilled in the art can select according to actual needs.

[0109] As an example, the positive electrode active material may include, but is not limited to, one or more of lithium transition metal oxides, olivine-structured lithium-containing phosphates, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their modified compounds. These materials can all be obtained through commercial channels.

[0110] The modified compounds of the above materials may be doping-modified and / or surface-coated modified materials.

[0111] The positive electrode film layer usually optionally includes a binder, a conductive agent and other optional additives.

[0112] As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene, and carbon nanofibers.

[0113] As an example, the adhesive can be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

[0114] [Negative electrode]

[0115] In a secondary battery, a negative electrode plate generally includes a negative electrode current collector and a negative electrode film layer disposed on the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0116] The negative electrode current collector may be a conventional metal foil or a composite current collector (for example, a metal material may be disposed on a polymer substrate to form a composite current collector). As an example, the negative electrode current collector may be a copper foil.

[0117] The specific type of negative electrode active material is not limited, and active materials that can be used for the negative electrode of a secondary battery known in the art can be used, and those skilled in the art can make a selection according to actual needs. As an example, the negative electrode active material may include, but is not limited to, one or more of artificial graphite, natural graphite, hard carbon, soft carbon, silicon-based materials and tin-based materials. Silicon-based materials can be selected from one or more of elemental silicon, silicon oxides (such as silicon monoxide), silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials can be selected from one or more of elemental tin, tin oxides, and tin alloys. These materials can all be obtained commercially.

[0118] In some embodiments, to further increase the energy density of the battery, the negative electrode active material may include a silicon-based material.

[0119] The negative electrode film layer usually optionally includes a binder, a conductive agent and other optional additives.

[0120] As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0121] As an example, the adhesive may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

[0122] As an example, other optional auxiliary agents may be thickening and dispersing agents (such as sodium carboxymethyl cellulose CMC-Na), and PTC thermistor materials.

[0123] [Electrolyte]

[0124] Secondary batteries may include an electrolyte that conducts ions between a positive electrode and a negative electrode. The electrolyte may include an electrolyte salt and a solvent.

[0125] As an example, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0126] As an example, the solvent may be selected from ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl darbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), n-propyl propionate (NP), propylene glycol (PEG), propylene glycol (PP), propylene glycol (C1), propylene glycol (C2), propylene glycol (C3), propylene glycol (C4), propylene glycol (C5), propylene glycol (C6), propylene glycol (C7), propylene glycol (C8), propylene glycol (C9), propylene glycol (D1), propylene glycol (D2), propylene glycol (D3), propylene glycol (D4), propylene glycol (D5), propylene glycol (D6), propylene glycol (D7), propylene glycol (D8), propylene glycol (D9), propylene glycol (D10), propylene glycol (D11), propylene glycol (D12), propylene glycol (D13), propylene glycol (D14), propylene glycol (D15), propylene glycol (D16), propylene glycol (D17), propylene glycol (D18), propylene glycol (D19), propylene glycol (D20), propylene glycol (D21), propylene glycol (D22), propylene glycol (D23), propylene glycol (D24), propylene glycol (D25), propylene glycol (D26), propylene glycol (D27), propylene glycol (D28), propylene glycol (D29), propylene glycol (D31), propylene glycol (D32), propylene glycol (D33), propylene glycol (D34), propylene glycol (D35), propylene glycol (D36), propylene glycol (D37), propylene glycol (D38), propylene glycol (D39), propylene One or more of the group consisting of propionate (PP), methyl butyrate (Methyl Butyrate MB), ethyl butyrate (Ethyl Butyrate, EB), 1,4-butyrolactone (1,4-Butyrolactone, GBL), tetramethylene sulfone (SF), dimethyl sulfone (Methyl Sulfone, MSM), methyl ethyl sulfone (Methyl Ethyl Sulfone, EMS) and diethyl sulfone (Diethyl Sulfone, ESE).

[0127] In some embodiments, the electrolyte further includes additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature performance.

[0128] In some embodiments, the secondary battery may be a lithium-ion secondary battery. In any embodiment of the present application, the secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet, and the separator may be any of the separators in the above embodiments.

[0129] In some embodiments, in a secondary battery, the first base film of the separator faces the negative electrode. That is, the base film on the side with greater creep compliance faces the negative electrode. When lithium dendrites precipitate on the negative electrode side, the separator can be more resistant to the impact of the lithium dendrites on the separator, reducing the risk of the separator being punctured and causing a short circuit between the positive and negative electrodes. At the same time, the melting point of the second base film facing the positive electrode is higher than that of the first base film, and it can withstand higher temperatures, reducing the risk of the separator being thermally broken down and causing a short circuit between the positive and negative electrodes. This arrangement can improve the reliability of the secondary battery.

[0130] The embodiment of the present application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other shape. FIG4 shows a secondary battery 5 with a square structure as an example.

[0131] In some embodiments, the secondary battery may include an outer packaging for encapsulating a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0132] 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 package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0133] In some embodiments, referring to FIG5 , the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, which together form a receiving cavity. The housing 51 may have an opening communicating with the receiving cavity, and the cover 53 may be positioned over the opening to seal the receiving cavity.

[0134] The preparation method of the secondary battery of the present application is well known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly through a winding process and / or a lamination process, and the electrode assembly is placed in an outer package, dried and injected with electrolyte, and then vacuum packaged, left to stand, formed, and shaped to obtain a battery cell. Multiple battery cells can also be further connected in series, in parallel, or mixed to form a battery module. Multiple battery modules can also be connected in series, in parallel, or mixed to form a battery pack. In some embodiments, multiple battery cells can also directly form a battery pack.

[0135] Figure 6 shows an example battery module 4. Referring to Figure 6 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured using fasteners.

[0136] The battery module 4 may further include a housing having a housing space, and the plurality of secondary batteries 5 may be housed in the housing space.

[0137] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0138] Figures 7 and 8 illustrate an example battery pack 1. Referring to Figures 7 and 8 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0139] Device

[0140] The present application also provides an electrical device, which includes the secondary battery of the present application. The battery cell, battery module or battery pack can be used as a power source for the device, or as an energy storage unit for the device. The device can be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck), an electric train, a ship and a satellite, and an energy storage system.

[0141] The device can select battery cells, battery modules or battery packs according to its usage requirements.

[0142] Figure 9 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0143] As another example, an electric device may be a mobile phone, a tablet computer, or a laptop computer. Such an electric device is generally required to be lightweight and thin, and may use a battery cell as a power source.

[0144] The beneficial effects of the present application are further illustrated below with reference to the examples.

[0145] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present application clearer, the following will be further described in detail with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0146] 1. Preparation of batteries

[0147] Example 1

[0148] 1. Preparation of positive electrode sheet

[0149] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive agent carbon black (SuperP), and adhesive polyvinylidene fluoride (PVDF) are mixed evenly in an appropriate amount of solvent N-methylpyrrolidone (NMP) in a mass ratio of 96.2:2.7:1.1 to obtain positive electrode slurry, which is then coated on the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained through drying, cold pressing, slitting, and cutting processes.

[0150] 2. Preparation of negative electrode sheet

[0151] The negative electrode active material artificial graphite, conductive agent carbon black (SuperP), binder styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC-Na) were mixed uniformly in an appropriate amount of solvent deionized water at a mass ratio of 96.4:0.7:1.8:1.1 to obtain a negative electrode slurry. The negative electrode slurry was coated on the negative electrode current collector copper foil, and the negative electrode sheet was obtained through the processes of drying, cold pressing, slitting and cutting.

[0152] 3. Preparation of isolation membrane

[0153] (1) Providing a first base film: the material includes polyethylene (PE), the melting point is 135 ° C, and the creep compliance is 0.0013 MPa -1 , crystallinity is 88%, molecular weight is 500,000;

[0154] Provide a second base film: the material includes polypropylene (PP), the melting point is 165 ° C, the creep compliance is 0.0012 MPa -1 , crystallinity is 24%, molecular weight is 370,000;

[0155] (2) Preparation of bonding layer slurry: boehmite, polyacrylate and carboxymethyl cellulose are mixed evenly in a proper amount of deionized water as solvent in a ratio of 4:1:1 to prepare bonding layer slurry.

[0156] (3) The adhesive layer slurry described in step (2) is applied to the PE base film to form an adhesive layer, and the PP and PE base films are hot-pressed so that the adhesive layer is between the PP base film and the PE base film to prepare an isolation film.

[0157] 4. Preparation of electrolyte

[0158] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 30:70 to obtain an organic solvent. Fully dried electrolyte salt LiPF6 is dissolved in the above mixed solvent. The concentration of the electrolyte salt is 1.0 mol / L. After mixing evenly, an electrolyte solution is obtained.

[0159] 5. Preparation of secondary batteries

[0160] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to serve as an isolation, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer package, and the prepared electrolyte is injected into the dried shell. After vacuum packaging, standing, formation, and shaping processes, a secondary battery is obtained.

[0161] The secondary batteries of Examples 2-14 and Comparative Examples 1-4 were prepared in a similar manner to the secondary battery of Example 1, except that different separators were used. See Table 1 for details.

[0162] Unless otherwise specified, all raw materials used in the method for preparing the separator (eg, the first base film, the second base film, the adhesive, the filler, etc.) can be purchased commercially.

[0163] 2. Performance Testing

[0164] (1) Test of creep compliance of isolation membrane

[0165] Step 1: Take 6 sets of parallel samples of polymer film layers, the size of which is a rectangular spline with a width of 4 mm and a length of 50 mm;

[0166] Step 2: The specimen is clamped on the fixture of the dynamic thermomechanical analyzer, wherein the test mode is the creep mode, the experimental temperature is room temperature, and the time is 0-220 min.

[0167] (2) Battery cycle life (25°C)

[0168] The secondary batteries prepared in the Examples and Comparative Examples were charged and discharged for the first time at 25°C. The process was as follows: After standing at 25°C for 30 minutes, the secondary batteries were charged at a constant current of 1C to 4.35V. Constant voltage charging was continued until the current was ≤0.05C. The batteries were then discharged at a constant current of 1C to 2.8V. This constituted one charge and discharge cycle, and the discharge capacity at this point was recorded as the discharge capacity of the first cycle. This charge and discharge cycle was repeated, and the number of cycles until the capacity dropped to 80% was recorded.

[0169]

[0170] As can be seen from Table 1, in Examples 1-14, the melting point of the first base film is lower than that of the second base film; at the same time, the creep compliance of the first base film is greater than that of the second base film; the resulting batteries have low transverse heat shrinkage, high transverse tensile strength, and long cycle life; whereas in Comparative Examples 1-4, the separators do not simultaneously meet the requirements of having the melting point of the first base film lower than that of the second base film and having the creep compliance of the first base film greater than that of the second base film; the resulting batteries have high transverse heat shrinkage and short cycle life. Therefore, by using the separators defined in this application with good creep compliance, and under the premise of the same cycle conditions and battery performance standards, the batteries prepared using the separators provided in this application have good cycle performance. That is, under the same battery performance standards, the batteries prepared using the separators provided in this application can cycle more charge and discharge cycles and have higher reliability.

[0171] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present application, and such modifications or substitutions should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. An isolation film, comprising a first base film and a second base film; the melting point of the second base film is higher than the melting point of the first base film, and the creep compliance of the first base film is greater than the creep compliance of the second base film.

2. The isolation film according to claim 1, wherein: The ratio of the creep compliance of the first base film to the creep compliance of the second base film is 1.1-3.0, and can be optionally 1.3-2.

0.

3. The isolation film according to claim 1 or 2, wherein: The creep compliance of the first base film is 0.0013 MPa -1 -0.0050MPa -1 , optional 0.0013MPa -1 -0.0032MPa -1 ; and / or The creep compliance of the second base film is 0.0010 MPa -1 -0.0023MPa -1 , optional 0.0010MPa -1 -0.0020MPa -1 .

4. The isolation film according to any one of claims 1 to 3, wherein: The ratio of the crystallinity of the first base film to the crystallinity of the second base film is 1.1-5.0, and can be optionally 1.5-3.

0.

5. The isolation film according to any one of claims 1 to 4, wherein: The crystallinity of the first base film is 40%-90%, and optionally 75%-85%; and / or The crystallinity of the second base film is 20%-70%, and optionally 30%-45%.

6. The isolation film according to any one of claims 1 to 5, wherein: The ratio of the relative molecular mass of the material of the first base film to the relative molecular mass of the material of the second base film is greater than or equal to 1.05, and can be selected to be 1.2-10.

7. The isolation film according to any one of claims 1 to 6, wherein: The relative molecular weight of the material of the first base film is 300,000-2.5 million, and can be 500,000-2 million; and / or, The relative molecular weight of the material of the second base film is 10,000-2,000,000, and can be optionally 10,000-1,200,000.

8. The isolation film according to any one of claims 1 to 7, wherein: The ratio of the melting point of the first base film to the melting point of the second base film is 0.3-0.85, and can be optionally 0.35-0.

65.

9. The isolation film according to any one of claims 1 to 8, wherein: The melting point of the first base film is 120°C-280°C, optionally 130°C-265°C, and / or The melting point of the second base film is 150°C-360°C, and can be optionally 160°C-350°C.

10. The isolation film according to any one of claims 1 to 9, wherein: The first base film and the second base film are each independently selected from at least one of polyolefins and their derivatives, halogenated polyolefins and their derivatives, polyethers and their derivatives, polyetheretherketones and their derivatives, polyesters and their derivatives, polyimides and their derivatives, polyvinyl alcohols and their derivatives, polytetrafluoroethylene and their derivatives, polyvinyl fluoride and their derivatives, polyvinylidene fluoride and their derivatives, and polyethylene terephthalate and its derivatives.

11. The isolation film according to any one of claims 1 to 10, wherein: An adhesive layer is further provided between the first base film and the second base film, and the adhesive layer includes an adhesive; optionally, the adhesive layer includes an adhesive and a filler.

12. The isolation film according to claim 11, wherein: The adhesive includes one or more of polyacrylate, polyacrylic acid, polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-trichloroethylene copolymer, polyvinyl pyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, polyacrylonitrile, polyvinyl alcohol, polyethylene, polypropylene, starch, and cyanoethyl pullulan.

13. The isolation film according to claim 11 or 12, wherein: The filler includes at least one of inorganic particles, organic particles, and organic-metal framework materials.

14. The isolation film according to any one of claims 1 to 13, wherein: The thickness of the adhesive layer is less than or equal to 3 μm, and can be selected to be 0.5-2 μm.

15. The isolation film according to any one of claims 1 to 14, wherein: The transverse heat shrinkage rate of the isolation film at 250° C. for 1 hour is less than 0.4%, and can be optionally less than or equal to 0.25%; and / or The longitudinal heat shrinkage rate of the isolation film at 250° C. for 1 hour is less than 0.4%, and can be optionally less than or equal to 0.25%; and / or The transverse tensile strength of the isolation film is greater than or equal to 2500 kg / cm 2 , optional 3000kg / cm 2 -4000kg / cm 2 ; and / or The longitudinal tensile strength of the isolation film is greater than or equal to 2500 kg / cm 2 , optional 3000kg / cm 2 -4000kg / cm 2 .

16. A method for preparing an isolation film, comprising: Providing a first base film and a second base film, wherein the melting point of the second base film is higher than the melting point of the first base film, and the creep compliance of the first base film is greater than the creep compliance of the second base film; The first base film and the second base film are compounded to obtain the isolation film.

17. The method for preparing an isolation film according to claim 16, further comprising: preparing an adhesive layer slurry, wherein the adhesive layer slurry includes an adhesive, and coating the adhesive layer slurry on the first base film and / or the second base film before compounding; Optionally, the bonding layer slurry includes an adhesive and a filler. 18 . A secondary battery comprising the separator according to claim 1 , or the separator prepared according to the method of claim 16 or 17 .

19. The secondary battery according to claim 18, wherein It also includes a positive electrode sheet and a negative electrode sheet, the isolation film is arranged between the positive electrode sheet and the negative electrode sheet, and the first base film faces the negative electrode sheet.

20. An electrical device comprising the secondary battery according to claim 18 or 19.

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