Separator and preparation method therefor, secondary battery, and electric device

Abstract

A separator and a preparation method therefor, a secondary battery, and an electric device. The separator comprises a porous base membrane; the porous base membrane comprises a matrix phase and a filling phase dispersed in the matrix phase, the test curve of a differential scanning calorimeter of the porous base membrane comprises both a melting peak of the matrix phase and a melting peak of the filling phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase. The separator can improve the reliability of a secondary battery.

Description

Separator and preparation method thereof, secondary battery and electric device

[0001] This application claims priority to Chinese patent application No. 2023108893672 filed on July 19, 2023, entitled “Isolation membrane and preparation method thereof, secondary battery and electrical device”, which is incorporated herein by reference in its entirety. Technical Field

[0002] The present application relates to the technical field of secondary batteries, and in particular to an isolation membrane and a preparation method thereof, a secondary battery, and an electrical device. Background Art

[0003] Secondary batteries are widely used in various consumer electronics and electric vehicles due to their lightweight, pollution-free, and memory-free features. With the continuous development of the new energy industry, users are placing higher demands on the reliability and performance of secondary batteries. However, when the temperature of a secondary battery rises abnormally, existing separators cannot effectively prevent the battery from overheating.

[0004] Summary of the Invention

[0005] In view of the technical problems existing in the background technology, the present application provides an isolation film and a preparation method thereof, a secondary battery and an electrical device, aiming to solve the termination of the secondary battery due to abnormal temperature conditions.

[0006] In order to solve the above technical problems, the first technical solution adopted in this application is: to provide an isolation membrane, which includes a porous base membrane, the porous base membrane includes a matrix phase and a filling phase dispersed in the matrix phase, and the test curve of the differential scanning calorimeter of the porous base membrane includes both the melting peak of the matrix phase and the melting peak of the filling phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase.

[0007] In the technical solution of the embodiment of the present application, the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase, that is, the matrix phase has better thermal stability than the filling phase. Therefore, when the internal temperature of the secondary battery is high, the filling phase melts first due to its poor thermal stability, and then promptly closes the pores of the isolation membrane, terminates the electrochemical reaction, prevents the internal temperature of the secondary battery from continuing to rise, and improves the reliability of the secondary battery.

[0008] In any embodiment of the present application, the ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is greater than 1, and can be selected from 1.2 to 2.0.

[0009] In the technical solution of the embodiment of the present application, the ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is within the above range, which can enable sufficient filling phase crystals to be formed in the isolation membrane, which is conducive to the realization of closed-cell performance; and makes the finished isolation membrane have good pore-forming performance and high strength.

[0010] In any embodiment of the present application, in the X-ray diffraction pattern of the porous base film, the porous base film has a diffraction peak in the range of 15°<2θ<17°.

[0011] In the technical solution of the embodiment of the present application, the porous base membrane has a diffraction peak in the range of 15°<2θ<17°. The diffraction peak here is the diffraction peak of the β crystal in the porous base membrane. The higher the β crystal content in the porous base membrane, the higher the overall strength of the porous base membrane.

[0012] In any embodiment of the present application, in the X-ray diffraction pattern of the porous base film, the porous base film has a diffraction peak in the range of 23°<2θ<25°.

[0013] In the technical solution of the embodiment of the present application, the porous base membrane has a diffraction peak in the range of 23°<2θ<25°, where the diffraction peak is the diffraction peak of the α crystal in the porous base membrane. The α crystal in the porous base membrane is conducive to the realization of the thermal closure function.

[0014] In any embodiment of the present application, in the X-ray diffraction pattern of the porous base membrane, the porous base membrane includes a first diffraction peak in the range of 15°<2θ<17°, and the porous base membrane includes a second diffraction peak in the range of 23°<2θ<25°, and the diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak.

[0015] In the technical solution of the embodiment of the present application, the porous base membrane includes a first diffraction peak in the range of 15°<2θ<17°, and the porous base membrane includes a second diffraction peak in the range of 23°<2θ<25°. The diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak. The intensity of the first diffraction peak is greater than that of the second diffraction peak, indicating that the β crystal content accounts for a larger proportion than the α crystal content. Therefore, under the premise that the overall strength of the isolation membrane is high, the thermal closure function of the isolation membrane can be realized, thereby improving the reliability of the isolation membrane.

[0016] In any embodiment of the present application, the filling phase includes α-crystal grains; and the base film phase includes β-crystal grains.

[0017] In the technical solution of the embodiment of the present application, the filling phase of the isolation membrane includes α-crystal grains, and the matrix phase includes β-crystal grains, so that the thermal closure function of the isolation membrane can be achieved while the overall strength of the isolation membrane is high, thereby improving the reliability of the isolation membrane.

[0018] In any embodiment of the present application, the grain size of the filling phase is 0.1 μm-2 μm, and can be optionally 0.1 μm-0.5 μm.

[0019] In the technical solution of the embodiment of the present application, the grain size of the filling phase is controlled within the above-mentioned range. When the internal temperature of the secondary battery is high, the filling phase melts first due to its low melting point, thereby promptly sealing the pores of the isolation membrane, achieving rapid shutdown, terminating the electrochemical reaction, and preventing the internal temperature of the secondary battery from continuing to rise. At the same time, the isolation membrane has better strength and elongation at break, so that the reliability of the secondary battery is improved.

[0020] In any embodiment of the present application, the mass proportion of the filling phase in the substrate is 10%-40%, and optionally 15%-25%.

[0021] In the technical solution of the embodiment of the present application, the mass proportion of the filling phase is controlled within the above range. When the internal temperature of the secondary battery is high, a sufficient amount of the filling phase melts to seal the pores of the isolation membrane, thereby achieving thermal shutdown. At the same time, the matrix phase and the filling phase can balance the physical strength of the isolation membrane and improve the reliability of the secondary battery.

[0022] In any embodiment of the present application, the relative molecular mass of the material of the filling phase is less than or equal to 1.2 million, and can be selected as 400,000-600,000; the relative molecular mass of the material of the matrix phase is greater than or equal to 300,000, and can be selected as 300,000-2.5 million.

[0023] In the technical solution of the embodiment of the present application, the relative molecular weights of the filling phase and the matrix phase are limited to the above range. The matrix phase and the filling phase can balance the physical strength of the isolation membrane and improve the reliability of the secondary battery.

[0024] In any embodiment of the present application, the temperature of the melting peak of the matrix phase is 160°C-350°C, optionally 160°C-180°C; the temperature of the melting peak of the filling phase is 60°C-180°C, optionally 80°C-130°C.

[0025] In the technical solution of the embodiment of the present application, the temperature of the melting peak of the matrix phase and the temperature of the melting peak of the filling phase are limited to the above-mentioned range, which can balance the physical strength of the isolation membrane while timely closing the pores of the isolation membrane when the internal temperature of the secondary battery is high, thereby improving the reliability of the secondary battery.

[0026] In any embodiment of the present application, the material of the matrix phase includes at least one of polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene terephthalate, polyetheretherketone, polyurethane, and polyester; the material of the filling phase includes at least one of polyethylene and polypropylene.

[0027] In the technical solution of the embodiment of the present application, the material of the matrix phase and the filling phase are limited to the above range, which can meet the material requirements of the isolation membrane. When the internal temperature of the secondary battery is high, the filling phase can promptly seal the pores of the isolation membrane, thereby improving the reliability of the secondary battery.

[0028] In any embodiment of the present application, the pore size of the porous base membrane is less than or equal to 0.5 μm, and can be optionally 0.02 μm-0.1 μm.

[0029] In the technical solution of the embodiment of the present application, the pore size of the porous base membrane is less than or equal to 0.5 μm, which can effectively block the pores after the filling phase melts, achieve thermal shutdown, and improve the reliability of the secondary battery.

[0030] In any embodiment of the present application, the isolation film satisfies at least one of the following (1)-(9):

[0031] (1) The closed-cell temperature of the isolation membrane is 120°C-170°C, and can be optionally 120°C-130°C;

[0032] (2) The closing time of the isolation membrane is less than or equal to 10s, and can be selected as 5s-10s;

[0033] (3) The transverse elongation at break of the separator is ≥100%, and can be selected from 100% to 120%;

[0034] (4) The longitudinal elongation at break of the separator is ≥60%, and can be selected from 60% to 80%;

[0035] (5) Transverse tensile strength of the separator ≥1500kgf / cm 2 , optional 2000kgf / cm 2 -4000kgf / cm 2 ;

[0036] (6) The longitudinal tensile strength of the isolation film is ≥2000kgf / cm 2 , optional 2000gf / cm 2 -4000kgf / cm 2 ;

[0037] (7) The puncture strength of the isolation film is ≥60gf, and can be selected from 120gf to 420gf;

[0038] (8) The porosity of the isolation membrane is 30%-90%, and can be optionally 30%-50%;

[0039] (9) The air permeability of the isolation membrane is less than or equal to 300 sec / 100 cc, and can be selected from 100 sec / 100 cc to 300 sec / 100 cc.

[0040] In the technical solution of the embodiment of the present application, the isolation film satisfies at least one of the above conditions.

[0041] It can effectively improve the ductility of the isolation membrane, enhance the puncture performance of the isolation membrane, and quickly respond to thermal anomalies to achieve thermal shutdown, thereby further improving the reliability of the secondary battery.

[0042] The second technical solution adopted in the present application is to provide a method for preparing the isolation membrane as described above. The preparation method includes: mixing the raw materials for forming the matrix phase, the raw materials for forming the filling phase and the nucleating agent to form a precursor, wherein the crystallization temperature of the raw materials for forming the filling phase is lower than the crystallization temperature of the raw materials for forming the matrix phase; extruding the precursor to form a first intermediate product; performing a sheet casting process on the first intermediate product to form a second intermediate product, wherein the sheet casting process includes a first temperature stage and a second temperature stage, and the sheet casting temperature of the first temperature stage is higher than the sheet casting temperature of the second temperature stage; and stretching the second intermediate product to obtain a porous base membrane. The porous base membrane includes a matrix phase and a filling phase dispersed in the matrix phase, and the test curve of the differential scanning calorimeter of the porous base membrane includes both the melting peak of the matrix phase and the melting peak of the filling phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase.

[0043] In the technical solution of the embodiment of the present application, the raw materials for forming the matrix phase, the raw materials for forming the filling phase and the nucleating agent are mixed, wherein the crystallization temperature of the raw materials for the filling phase is lower than the crystallization temperature of the raw materials for forming the matrix phase. In the casting process, a casting process is adopted in which the casting temperature of the first temperature stage is higher than the casting temperature of the second temperature stage, so that the raw materials for the matrix phase crystallize first and the raw materials for the filling phase crystallize later. The filling phase is dispersed in the matrix phase and then subjected to stretching treatment to form a porous isolation membrane. wherein the temperature of the melting peak of the matrix phase of the isolation membrane is higher than the temperature of the melting peak of the filling phase. The matrix phase has better thermal stability than the filling phase. Therefore, when the internal temperature of the secondary battery is high, the filling phase melts first due to its poor thermal stability, thereby timely sealing the pores of the isolation membrane, terminating the electrochemical reaction, and preventing the internal temperature of the secondary battery from continuing to rise, thereby improving the reliability of the secondary battery.

[0044] In any embodiment of the present application, the melt index of the filling phase raw material is greater than the melt index of the matrix phase raw material; optionally, the ratio of the melt index of the filling phase raw material to the melt index of the matrix phase raw material is equal to 1.1-10.1, and can be optionally 1.1-2.1.

[0045] In the technical solution of the embodiment of the present application, the melting index of the filling phase raw material is greater than the melting index of the matrix phase raw material, and the ratio of the two is limited to the above range. The matrix phase has better thermal stability than the filling phase. When the internal temperature of the secondary battery is high, the filling phase melts first, and then the pores of the isolation membrane are sealed in time, terminating the electrochemical reaction, preventing the internal temperature of the secondary battery from continuing to rise, and the reliability of the secondary battery is improved.

[0046] In any embodiment of the present application, the crystallization temperature of the filling phase raw material is 80°C-120°C, optionally 80°C-100°C; and / or the crystallization temperature of the matrix phase raw material is 125°C-135°C, optionally 125°C-130°C.

[0047] In the technical solution of the embodiment of the present application, the crystallization temperature of the filling phase raw material and the crystallization temperature of the matrix phase raw material are limited to the above-mentioned range, so that the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase. When the internal temperature of the secondary battery is high, the filling phase melts first, and then the pores of the isolation membrane are sealed in time, the electrochemical reaction is terminated, and the internal temperature of the secondary battery is prevented from continuing to rise, thereby improving the reliability of the secondary battery.

[0048] In any embodiment of the present application, the casting temperature of the first temperature stage is 125℃-135℃, optionally 125℃-130℃; and / or, the casting temperature of the second temperature stage is 105℃-125℃, optionally 105℃-110℃; and / or, the difference between the casting temperature of the first temperature stage and the casting temperature of the second temperature stage is 10℃-20℃.

[0049] In the technical solution of the embodiment of the present application, limiting the casting temperature to the above range can enable the isolation film to fully crystallize to form a filling phase and a matrix phase.

[0050] In any embodiment of the present application, in the step of stretching the second intermediate product, the transverse stretching temperature is 110°C-120°C, optionally 110°C-115°C; and / or, the longitudinal stretching temperature is 85°C-120°C, optionally 110°C-120°C.

[0051] In the technical solution of the embodiment of the present application, the transverse stretching temperature and the longitudinal stretching temperature are limited to the above ranges, so that the formed isolation film has better physical and chemical properties.

[0052] The third technical solution adopted in this application is to provide a secondary battery comprising the separator described above or a separator prepared by the separator preparation method described above. The secondary battery comprises a positive electrode sheet and a negative electrode sheet, with the separator disposed between the positive electrode sheet and the negative electrode sheet.

[0053] The fourth technical solution adopted in this application is: to provide an electrical device including the secondary battery as described above.

[0054] In the technical solution of the embodiment of the present application, the electrical device includes the above-mentioned secondary battery, and thus has the same advantage of good fast charging performance.

[0055] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] 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.

[0057] FIG1 is a SEM image of an isolation film according to an embodiment of the present application.

[0058] FIG. 2 is a melting curve diagram of the separator shown in FIG. 1 .

[0059] FIG3 is a schematic diagram of an embodiment of a secondary battery provided in the present application.

[0060] FIG. 4 is an exploded view of FIG. 3 .

[0061] FIG5 is a schematic diagram of an embodiment of a battery module provided in the present application.

[0062] FIG6 is a schematic diagram of an embodiment of a battery pack provided in the present application.

[0063] FIG. 7 is an exploded view of FIG. 6 .

[0064] FIG8 is a schematic diagram of an embodiment of a device using a secondary battery provided by the present application as a power source. DETAILED DESCRIPTION

[0065] 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.

[0066] 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.

[0067] 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).

[0068] 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.

[0069] 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).

[0070] [Secondary battery]

[0071] A secondary battery is a battery that can be recharged after discharge to activate the active materials and continue to be used. Typically, a secondary battery consists of a battery cell (positive electrode, negative electrode, separator), electrolyte, and casing. During the battery's charge and discharge process, active ions are embedded and released back and forth between the positive and negative electrodes. The separator is placed between the positive and negative electrodes to isolate them. The electrolyte conducts ions between the positive and negative electrodes.

[0072] [Isolation film]

[0073] As shown in Figures 1 and 2, Figure 1 is a SEM image of an embodiment of a separator, and Figure 2 is a melting curve diagram of the separator shown in Figure 1. The separator provided in the embodiments of the present application includes a porous base membrane, which includes a matrix phase and a filler phase dispersed in the matrix phase. The test curve of the porous base membrane on a differential scanning calorimeter includes both a melting peak of the matrix phase and a melting peak of the filler phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filler phase.

[0074] Differential Scanning Calorimetry (DSC) can be used to measure the curing reaction temperature and thermal effects of polymer materials, the phase transition temperature and thermal effects of materials, the crystallization and melting temperatures of polymer materials, and the glass transition temperature of polymer materials. The melting peak reflects the melting process of a substance—the process by which a solid substance melts into a liquid when the temperature rises to a certain level, releasing a certain amount of heat. The melting peak temperature can be used to assess the thermal stability of a material. Thermal stability refers to the stability of a material at high temperatures. A high melting peak temperature indicates that the material does not melt easily at high temperatures and has good thermal stability. A low melting peak temperature indicates that the material melts easily at high temperatures and has poor thermal stability. In other words, the melting peak temperature can also be understood as the melting point, meaning that the melting point of the matrix phase is higher than that of the filler phase. In the embodiments of the present application, the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase, that is, the matrix phase has better thermal stability than the filling phase. Therefore, when the internal temperature of the secondary battery is high, the filling phase melts first due to its poor thermal stability, and then promptly closes the pores of the isolation membrane, terminates the electrochemical reaction, prevents the internal temperature of the secondary battery from continuing to rise, and improves the reliability of the secondary battery.

[0075] In some embodiments, the ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is greater than 1, and can be selected from 1.2 to 2.0. For example, the ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase can be 1, 1.1, 1.2, 1.3, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 4, or a range consisting of any two of the above values, for example, 1-1.2, 1.1-1.4, 1.3-1.8, 1.6-2.2, 2.2-2.8, 2.8-4, etc. The melting peak area of ​​the X-ray diffraction pattern indicates the crystalline content; the larger the area, the higher the crystalline content. The peak area ratio indicates the proportion of different crystals in the isolation membrane. The ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is within the above range, which can form enough filling phase crystals in the isolation membrane, which is conducive to the realization of closed-cell performance; and makes the finished isolation membrane have good pore-forming performance and high strength.

[0076] In some embodiments, in the X-ray diffraction pattern of the porous base membrane, the porous base membrane has a diffraction peak in the range of 15°<2θ<17°. The physical phenomenon of coherent scattering of X-rays irradiated on a crystal is called diffraction. The diffraction angle of light refers to the angle at which the light path changes when the light is diffracted. For example, in an X-ray diffractometer, 2θ is used as the diffraction angle, and 2θ is the angle at which the direction of the X-ray changes after diffraction. The diffraction peak of the porous base membrane in the range of 15°<2θ<17° is the diffraction peak of the β crystal form, and the β crystal form is the matrix phase crystal. The β crystal form refers to a type of crystal structure of a substance in the solid state. The β crystal form refers to the face-centered cubic crystal system, which belongs to the hexagonal crystal system. The higher the β crystal content, the higher the overall strength of the porous base membrane. The porous base membrane has a diffraction peak in the range of 15°<2θ<17°, which can make the isolation membrane have sufficient strength.

[0077] In some embodiments, in the X-ray diffraction pattern of the porous base membrane, the porous base membrane has a diffraction peak in the range of 23°<2θ<25°. The diffraction peak of the porous base membrane in the range of 23°<2θ<25° is the diffraction peak of the α crystal form, and the α crystal form is the filling phase crystallization. The α crystal form refers to another type of crystal structure of a substance in the solid state. The α crystal form generally refers to the body-centered cubic crystal system, which belongs to the monoclinic crystal system. The higher the α crystal content, the more conducive the filling phase crystallization is to realize the thermal closure function of the isolation membrane. The porous base membrane has a diffraction peak in the range of 23°<2θ<25°, which is conducive to the realization of the thermal closure function of the isolation membrane.

[0078] In some embodiments, in the X-ray diffraction pattern of the porous base membrane, the porous base membrane includes a first diffraction peak in the range of 15°<2θ<17°, and the porous base membrane includes a second diffraction peak in the range of 23°<2θ<25°, and the diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak. The intensity of the diffraction peak depends on the properties such as the type, content, and relative position of the atoms; the position and intensity of the diffraction peak can reflect the characteristics of the crystal structure, thus becoming the basis for distinguishing the phases. The diffraction line intensity of each phase increases with the increase of its phase content. As mentioned above, the diffraction peak of the porous base membrane in the range of 15°<2θ<17° is the diffraction peak of the β crystal form. The higher the β crystal content, the higher the overall strength of the porous base membrane. The diffraction peak of the porous base membrane in the range of 23°<2θ<25° is the diffraction peak of the α crystal form. The α crystal form is the filling phase crystallization, so that the filling phase crystallization is conducive to achieving the thermal closure function of the isolation membrane. The porous base membrane includes a first diffraction peak in the range of 15°<2θ<17°, and the porous base membrane includes a second diffraction peak in the range of 23°<2θ<25°. The diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak, indicating that the β-crystal content of the matrix phase is greater than the α-crystal content of the filling phase. Therefore, under the premise of high overall strength of the isolation membrane, the thermal closure function of the isolation membrane can be achieved, thereby improving the reliability of the isolation membrane.

[0079] In some embodiments, the filler phase comprises α-crystalline grains, and the matrix phase comprises β-crystalline grains. As previously mentioned, a higher α-crystalline content allows the filler phase to crystallize, which facilitates the thermal pore-closing function of the separator. A higher β-crystalline content increases the overall strength of the porous base membrane. Therefore, a filler phase comprising α-crystalline grains and a matrix phase comprising β-crystalline grains allows the separator to achieve its thermal pore-closing function while maintaining overall strength, thereby improving the reliability of the separator.

[0080] In some embodiments, the grain size of the filling phase is 0.1 μm-2 μm, optionally 0.1 μm-0.5 μm. For example, the grain size may be 0.1 μm, 0.15 μm, 0.25 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.7 μm, 0.9 μm, 1.2 μm, 1.4 μm, 1.7 μm, 2 μm, or a range formed by any two of the above values, such as 0.1 μm-0.15 μm, 0.15 μm-0.3 μm, 0.25 μm-0.4 μm, 0.4 μm-0.5 μm, 0.5 μm-0.9 μm, 0.7 μm-1.4 μm, 1.4 μm-2 μm, or the like. Generally speaking, the larger the grain size of the polymer crystals, the higher the melting point. Therefore, controlling the grain size of the filling phase within the above-mentioned range can control the melting point of the filling phase. When the internal temperature of the secondary battery is high, the filling phase melts first due to its low melting point, achieving rapid shutdown, terminating the electrochemical reaction, and preventing the internal temperature of the secondary battery from continuing to rise. At the same time, the isolation membrane has better strength and elongation at break, so that the reliability of the secondary battery is improved.

[0081] In some embodiments, the filler phase accounts for 10%-40% by weight of the substrate, and optionally 15%-25% by weight. For example, the filler phase accounts for 10%, 12%, 15%, 18%, 20%, 22%, 24%, 25%, 28%, 30%, 35%, 38%, 40%, or a range consisting of any two of the foregoing values, such as 10%-15%, 12%-18%, 15%-22%, 18%-25%, 25%-30%, 35%-38%, 38%-40%, or the like. If the filler phase accounts for too little by weight, when the internal temperature of the secondary battery is high, the amount of melted filler phase is insufficient to seal the pores of the separator. If the filler phase accounts for too much by weight, the separator cannot maintain its physical strength. Therefore, the mass proportion of the filling phase is controlled within the above range. When the internal temperature of the secondary battery is high, a sufficient amount of the filling phase melts and closes the pores of the isolation membrane to achieve thermal shutdown. At the same time, the matrix phase and the filling phase can balance the physical strength of the isolation membrane, thereby improving the reliability of the secondary battery.

[0082] In some embodiments, the relative molecular mass of the material of the filler phase is less than or equal to 1.2 million, and may be 400,000-600,000; the relative molecular mass of the material of the matrix phase is greater than or equal to 300,000, and may be 300,000-2.5 million. The relative molecular mass of the material of the filler phase may be 200,000, 300,000, 400,000, 450,000, 480,000, 500,000, 550,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, or a range consisting of any two of the above values, such as 200,000-300,000, 300,000-450,000, 450,000-500,000, 500,000-600,000, 600,000-800,000, 800,000-1,000,000, or 1,000,000-1,200,000. The relative molecular mass of the material of the matrix phase can be 300,000, 500,000, 600,000, 750,000, 860,000, 1,000,000, 1,200,000, 1,400,000, 1,650,000, 1,900,000, 2,200,000, 2,500,000, 3,000,000, 4,000,000, etc., or a range formed by any two of the above values, for example, 300,000-600,000, 500,000-750,000, 600,000-1,000,000, 860,000-1,400,000, 1,200,000-1,900,000, 1,650,000-2,200,000, 1,900,000-2,500,000, 2,500,000-4,000,000, etc.

[0083] The relative molecular mass of the isolation membrane has a well-known meaning in the art and can be tested using equipment and methods known in the art, for example, using a high-temperature GPC test (differential refractive index detector).

[0084] The filling phase with a small relative molecular mass has a stronger crystallization ability and is easier to be stably dispersed in the matrix phase with a large relative molecular mass, thereby reducing the sensitivity of the filling phase to high temperature. That is, after the filling phase is heated, it takes a certain amount of time to conduct heat to the inside of the matrix phase. The larger the relative molecular mass of the matrix phase is, the more heat-resistant it is, and the slower the heat transfer is. A higher temperature (i.e., the closed-pore temperature) is required to melt the filling phase and close the pores. The closed-pore temperature refers to the temperature at which the pores of the isolation membrane are closed. A filling phase with a small relative molecular mass can make the closed-pore temperature of the isolation membrane higher. In other words, when the temperature of the secondary battery rises abnormally to a certain temperature, the filling phase with a small relative molecular mass melts and closes the pores of the isolation membrane. By limiting the relative molecular masses of the filling phase and the matrix phase to the above range, the matrix phase and the filling phase can balance the physical strength of the isolation membrane and improve the reliability of the secondary battery.

[0085] In some embodiments, the temperature of the melting peak of the matrix phase is 160°C-350°C, optionally 160°C-180°C; the temperature of the melting peak of the filling phase is 60°C-180°C, optionally 80°C-130°C. The temperature of the melting peak of the matrix phase can be 160°C, 165°C, 170°C, 175°C, 180°C, 200°C, 250°C, 300°C, 350°C, or a range consisting of any two of the above values, for example, 160°C-170°C, 170°C-175°C, 175°C-200°C, 200°C-300°C, 250°C-350°C, or the like. The temperature of the melting peak of the filler phase can be 60°C, 65°C, 70°C, 75°C, 80°C, 100°C, 110°C, 120°C, 140°C, 160°C, 180°C, or a range consisting of any two of the foregoing values, such as 60°C-70°C, 70°C-80°C, 75°C-110°C, 100°C-140°C, 120°C-160°C, or 140°C-180°C. Limiting the temperatures of the matrix phase and the filling phase melting peaks within the foregoing ranges balances the physical strength of the separator while promptly sealing the separator pores when the internal temperature of the secondary battery is high, thereby improving the reliability of the secondary battery.

[0086] In some embodiments, the material of the matrix phase includes at least one of polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene terephthalate, polyetheretherketone, polyurethane, and polyester; and the material of the filling phase includes at least one of polyethylene and polypropylene. For example, in some embodiments, the material of the matrix phase includes polytetrafluoroethylene, and the material of the filling phase includes polyethylene. The porous base membrane includes a first diffraction peak and a second diffraction peak within the range of 15° < 2θ < 17°, the first diffraction peak corresponding to polytetrafluoroethylene, the second diffraction peak corresponding to polyethylene, and the diffraction peak intensity of the first diffraction peak greater than the diffraction peak intensity of the second diffraction peak. For example, in some embodiments, the material of the matrix phase includes polypropylene, and the material of the filling phase includes polyethylene. The porous base membrane includes a first diffraction peak within the range of 15° < 2θ < 17°, and the porous base membrane includes a second diffraction peak within the range of 23° < 2θ < 25°, the first diffraction peak corresponding to polypropylene, and the second diffraction peak corresponding to polyethylene. The diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak. Limiting the materials of the matrix phase and the filling phase to the above range can meet the material requirements of the isolation membrane. When the internal temperature of the secondary battery is high, the filling phase can promptly seal the pores of the isolation membrane, thereby improving the reliability of the secondary battery.

[0087] In some embodiments, the pore size of the porous base membrane is less than or equal to 0.5 μm, and can be selected from 0.02 μm to 0.1 μm. For example, the pore size of the porous base membrane can be 0.01 μm, 0.02 μm, 0.04 μm, 0.06 μm, 0.08 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.5 μm, etc., or a range consisting of any two of the above values, for example, 0.01 μm to 0.04 μm, 0.02 μm to 0.06 μm, 0.04 μm to 0.1 μm, 0.08 μm to 0.3 μm, 0.1 μm to 0.35 μm, 0.3 μm to 0.5 μm, etc. When the pore size of the porous base membrane is too large, it is easy to cause pore blockage when the active material particles pass through. The pore size of the porous base membrane is less than or equal to 0.5 μm, which can effectively block the pores after the filling phase is melted, achieve thermal shutdown, and improve the reliability of the secondary battery.

[0088] In some embodiments, the isolation film satisfies at least one of the following (1)-(9):

[0089] (1) The closed-cell temperature of the isolation membrane is 120°C-170°C, and can be optionally 120°C-130°C;

[0090] (2) The closing time of the isolation membrane is less than or equal to 10s, and can be selected as 5s-10s;

[0091] (3) The transverse elongation at break of the separator is ≥100%, and can be selected from 100% to 120%;

[0092] (4) The longitudinal elongation at break of the separator is ≥60%, and can be selected from 60% to 80%;

[0093] (5) Transverse tensile strength of the separator ≥1500kgf / cm 2 , optional 2000kgf / cm 2 -4000kgf / cm 2 ;

[0094] (6) The longitudinal tensile strength of the isolation film is ≥2000kgf / cm 2 , optional 2000gf / cm 2 -4000kgf / cm 2 ;

[0095] (7) The puncture strength of the isolation film is ≥60gf, and can be selected from 120gf to 420gf;

[0096] (8) The porosity of the isolation membrane is 30%-90%, and can be optionally 30%-50%;

[0097] (9) The air permeability of the isolation membrane is less than or equal to 300 sec / 100 cc, and can be selected from 100 sec / 100 cc to 300 sec / 100 cc.

[0098] In some embodiments, the isolation membrane has a closed-cell temperature of 120°C-170°C, preferably 120°C-130°C; and a closed-cell time of the isolation membrane is less than or equal to 10 seconds, preferably 5 seconds-10 seconds. For example, the closed-cell temperature of the isolation membrane may be 120°C, 124°C, 130°C, 132°C, 140°C, 146°C, 150°C, 158°C, 163°C, 170°C, or a range consisting of any two of the above values, such as 120°C-130°C, 124°C-140°C, 140°C-150°C, 146°C-158°C, or 150°C-170°C. The closed-cell time of the isolation membrane may be 3 seconds, 4 seconds, 5 seconds, 6 seconds, 7 seconds, 8 seconds, 9 seconds, or 10 seconds, or a range consisting of any two of the above values, such as 3 seconds-5 seconds, 4 seconds-7 seconds, 5 seconds-8 seconds, or 7 seconds-10 seconds. If the separator's pore closure temperature is too high or the pore closure time is too long, the filler phase in the separator needs to reach a certain temperature and time to seal the pores. If the pore closure temperature is too low or the pore closure time is too short, the filler phase in the separator will close immediately at a lower temperature. When the pore closure temperature and pore closure time of the separator are within the above range, the separator can quickly respond to thermal anomalies and achieve thermal shutdown, improving the reliability of the secondary battery.

[0099] The transverse tensile strength (MD), longitudinal tensile strength (TD), transverse elongation at break, and longitudinal elongation at break of a separator have well-known meanings in the art and can be measured using methods known in the art. For example, they can be tested in accordance with standard GB / T36363-2018.

[0100] The puncture strength of the isolation membrane has a meaning well known in the art and can be tested using equipment and methods known in the art. For example, the puncture strength can be tested according to the standard: GB / T 10004-2008. Specifically, the sample to be tested can be cut into strips, wherein the width of the strip sample is 100 mm. A test piece with a width of 100 mm is mounted on the sample membrane fixing clamp ring, and then a steel needle with a diameter of 1.0 mm and a radius of 0.5 mm at the top is used to puncture at a speed of (50±5) mm / min, and the maximum load of the steel needle penetrating the test piece is read. Usually, 5 parallel test samples can be taken at the same time, 3 points are measured for each test sample, and the arithmetic average is taken as the final puncture strength.

[0101] According to some embodiments, porosity has a well-known meaning in the art and can be tested using known equipment and methods. For example, the test can be performed with reference to GB / T 24586-2009. The test method is as follows: punch the isolation membrane or substrate into small disc samples with a diameter of 14 mm, test the thickness, and calculate the apparent volume V1 of the isolation membrane or substrate according to the cylinder volume calculation formula; refer to GB / T 24586-2009, use an inert gas such as helium or nitrogen as the medium, adopt the gas replacement method, and use a true density tester to measure the true volume V2 of the isolation membrane or substrate. Then the porosity of the isolation membrane or substrate = (V1-V2) / V1×100%. The testing instrument can be the AccuPyc II 1340 fully automatic true density tester from Micromeritics, USA.

[0102] According to some embodiments, the air permeability of the isolation membrane can be tested using equipment and methods known in the art. For example, the air permeability of the isolation membrane can be measured using the ASTM (Standard Test Method for Resistance of Nonporous Paper to Passage of Air) method. The porosity can also be tested using equipment and methods known in the art.

[0103] The separator satisfies at least one of the above (1)-(7), which can effectively improve the ductility of the separator and enhance the puncture performance of the separator, thereby further improving the reliability of the secondary battery.

[0104] The second technical solution adopted in this application is to provide a method for preparing the isolation membrane as described above. The preparation method comprises the following steps:

[0105] S100: mixing a raw material for forming a matrix phase, a raw material for forming a filling phase, and a nucleating agent to form a precursor, wherein the crystallization temperature of the raw material for forming the filling phase is lower than the crystallization temperature of the raw material for forming the matrix phase.

[0106] The matrix phase is made of at least one of polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene terephthalate, polyetheretherketone, polyurethane, and polyester; the filler phase is made of at least one of polyethylene and polypropylene. For example, in some embodiments, the matrix phase is made of polypropylene and the filler phase is made of polyethylene.

[0107] The nucleating agent may include an α-crystal nucleating agent. There are no particular limitations on the α-crystal nucleating agent, and various known α-crystal nucleating agents can be used alone or in combination of two or more.

[0108] The α-crystal nucleating agents include inorganic α-crystal nucleating agents and organic α-crystal nucleating agents.

[0109] Inorganic α-crystal nucleating agents mainly include talc, calcium oxide, carbon black, calcium carbonate, mica, inorganic pigments, kaolin and catalyst residues.

[0110] The organic α-crystal nucleating agent may include at least one of an aromatic acid metal soap α-nucleating agent, a sorbitol α-nucleating agent, a rosin α-nucleating agent, and an organic phosphate α-nucleating agent.

[0111] Aromatic acid metal soap α-nucleating agents may include at least one of the following metal salts of aromatic carboxylic acids: sodium benzoate, aluminum benzoate, and aluminum p-tert-butylbenzoate. Sorbitol α-nucleating agents may include at least one of dibenzylidene sorbitol, substituted dibenzylidene sorbitol, 1,3:2,4-di-p-methylbenzylidene sorbitol, and di-(3,4-dimethyldibenzylidene) sorbitol. Rosin α-nucleating agents may include at least one of dehydroabietic acid, abietic acid salts, a mixture of abietic acid and its salts, and abietic amides. Organophosphate α-nucleating agents may include at least one of organophosphate esters, organophosphate basic metal salts, and combinations thereof.

[0112] The nucleating agent may include a β-crystal nucleating agent. There are no particular limitations on the β-crystal nucleating agent, and various known β-crystal nucleating agents can be used alone or in combination of two or more.

[0113] β-crystal nucleating agents include inorganic oxide β-nucleating agents, inorganic salt β-nucleating agents, condensed ring aromatic hydrocarbon β-nucleating agents, organic carboxylic acid and its salt β-nucleating agents, aromatic amide β-nucleating agents and rare earth β-nucleating agents.

[0114] Inorganic oxide β-nucleating agents may include at least one of an oxide of a Group IIA metal of the Periodic Table and aluminum oxide. Inorganic salt β-nucleating agents may include at least one of calcium silicate, calcium carbonate, and calcium sulfate. Condensed-ring aromatic hydrocarbon β-nucleating agents may include at least one of quinacridonequinone, triphendithiazine, and disazo yellow. Organic carboxylic acid β-nucleating agents may include at least one of dibenzoylhydrazide adipic acid and suberic acid. Organic carboxylate β-nucleating agents may include at least one of calcium tetrahydrophthalate, calcium suberate, calcium pimelate, a calcium salt of a polycarboxylic acid, and a zinc salt of a polycarboxylic acid. Aromatic amide β-nucleating agents may include at least one of 2,6-benzenedicarboxylic acid cyclohexylamide, 2,6-naphthalenedicarboxylic acid cyclohexylamide, and an aryldicarbonamide. Rare earth β-nucleating agents may include at least one of a rare earth lanthanide mononuclear metal compound, a rare earth multi-component complex, or a binuclear complex formed between a rare earth and a Group IIA metal.

[0115] Nucleating agents are used to form crystals in the matrix phase. Higher crystallization temperatures increase the likelihood of crystallization. When the crystallization temperature of the filler phase is lower than that of the matrix phase, the filler phase crystallizes before the matrix phase.

[0116] S200: Extruding the precursor to form a first intermediate product.

[0117] S300: performing a slab casting process on the first intermediate product to form a second intermediate product.

[0118] The casting process includes a first temperature stage and a second temperature stage, and the casting temperature in the first temperature stage is higher than the casting temperature in the second temperature stage.

[0119] S400: The second intermediate product is subjected to a stretching process.

[0120] In the technical solution of the embodiment of the present application, the raw materials for forming the matrix phase, the raw materials for forming the filling phase and the nucleating agent are mixed, wherein the crystallization temperature of the raw materials for the filling phase is lower than the crystallization temperature of the raw materials for forming the matrix phase. In the usual casting process, only one temperature treatment will form homogeneous crystals, thereby showing only one melting peak. In the casting process provided in the embodiment of the present application, a casting process is adopted in which the casting temperature of the first temperature stage is higher than the casting temperature of the second temperature stage, so that the raw materials of the matrix phase crystallize first and the raw materials of the filling phase crystallize later, and the filling phase is dispersed in the matrix phase, thereby showing two melting peaks. After stretching, a porous isolation membrane is formed, wherein the temperature of the melting peak of the matrix phase of the isolation membrane is higher than the temperature of the melting peak of the filling phase. The matrix phase has better thermal stability than the filling phase. Therefore, when the internal temperature of the secondary battery is high, the filling phase melts first due to its poor thermal stability, thereby timely closing the pores of the isolation membrane, terminating the electrochemical reaction, preventing the internal temperature of the secondary battery from continuing to rise, and improving the reliability of the secondary battery.

[0121] In some embodiments, the melt index of the filler phase raw material is greater than the melt index of the matrix phase raw material, and the ratio of the two is 1.1-10.1, and optionally 1.1-2.1. For example, the ratio of the melt index of the filler phase raw material to the melt index of the matrix phase raw material is 1.1, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, etc., or a range consisting of any two of the above values, such as 1.1-1.5, 1.4-1.6, 1.5-1.8, 1.7-2.0, 1.9-2.1, etc. Melt index is a numerical value that indicates the fluidity of plastic materials during processing. It was developed by the American Institute of Measurement and Testing (ASTM) based on the method commonly used by DuPont to identify plastic properties. The test method is: first melt the plastic pellets into plastic fluid, and then measure the number of grams (g) of the plastic fluid flowing out through a 2.1mm diameter tube within a certain time (10 minutes) at a certain temperature and pressure (different standards for various materials). The larger the value, the better the processing fluidity of the plastic material, and vice versa. The melt index of the filler phase raw material is greater than the melt index of the matrix phase raw material, and the ratio of the two is limited to the above range. The matrix phase has better thermal stability than the filler phase. When the internal temperature of the secondary battery is high, the filler phase melts first, and then promptly closes the pores of the isolation membrane, terminates the electrochemical reaction, prevents the internal temperature of the secondary battery from continuing to rise, and improves the reliability of the secondary battery.

[0122] In some embodiments, the crystallization temperature of the filler phase raw material is 80°C-120°C, optionally 80°C-100°C; the crystallization temperature of the matrix phase raw material is 125°C-135°C, optionally 125°C-130°C. By limiting the crystallization temperatures of the filler phase raw material and the matrix phase raw material to the aforementioned ranges, the melting peak temperature of the matrix phase is higher than that of the filler phase. When the internal temperature of the secondary battery is high, the filler phase melts first, thereby promptly sealing the pores of the separator, terminating the electrochemical reaction, and preventing the internal temperature of the secondary battery from further increasing, thereby improving the reliability of the secondary battery.

[0123] In some embodiments, the casting temperature in the first temperature stage is 125°C-135°C, optionally 125°C-130°C; the casting temperature in the second temperature stage is 105°C-125°C, optionally 105°C-110°C; optionally, the difference between the casting temperature in the first temperature stage and the casting temperature in the second temperature stage is 10°C-20°C. For example, in some embodiments, the casting temperature in the first temperature stage can be 125°C, 126°C, 128°C, 130°C, 131°C, 133°C, 135°C, etc., or a range consisting of any of the above values, such as 125°C-126°C, 126°C-130°C, 130°C-133°C, 133°C-135°C, etc. In some embodiments, the casting temperature in the second temperature stage can be 105°C, 108°C, 110°C, 115°C, 118°C, 120°C, 122°C, 125°C, or any range thereof, such as 105°C-110°C, 108°C-115°C, 115°C-120°C, or 120°C-125°C. In some embodiments, the difference between the casting temperature in the first temperature stage and the casting temperature in the second temperature stage is 10°C, 12°C, 14°C, 16°C, 18°C, or 20°C, or any range thereof, such as 10°C-12°C, 12°C-16°C, or 16°C-20°C. The filling phase has a higher crystallization temperature, and in the first temperature stage, it preferentially crystallizes and separates from the matrix phase, while in the second temperature stage, the matrix phase crystallizes at the highest rate, resulting in full crystallization. Limiting the casting temperature to the above range allows the separator to fully crystallize into the filling phase and matrix phase.

[0124] In some embodiments, during the step of stretching the second intermediate product, the transverse stretching temperature is 110° C.-120° C., and may be 110° C.-115° C.; the longitudinal stretching temperature is 85° C.-120° C., and may be 110° C.-120° C. For example, in some embodiments, the transverse stretching temperature may be 110° C., 112° C., 114° C., 115° C., 116° C., 118° C., 120° C., or a range thereof, such as 110° C.-114° C., 112° C.-115° C., 115° C.-118° C., 116° C.-120° C., or the like. In some embodiments, the longitudinal stretching temperature may be 85°C, 90°C, 95°C, 100°C, 104°C, 110°C, 115°C, 120°C, or any range thereof, such as 85°C-90°C, 90°C-104°C, 100°C-115°C, or 115°C-120°C. In the step of stretching the second intermediate product, limiting the transverse stretching temperature and the longitudinal stretching temperature to within the aforementioned ranges can ensure that the resulting separator has better performance.

[0125] [Positive electrode]

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] The modified compounds of the above materials may be doping-modified and / or surface-coated modified materials.

[0131] The positive electrode film layer usually optionally includes a binder, a conductive agent and other optional additives.

[0132] As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, conductive carbon black (Super P, SP), graphene, and carbon nanofibers.

[0133] As an example, the binder can be one or more of styrene butadiene rubber (SBR), water-based acrylic resin, polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

[0134] [Negative electrode]

[0135] 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.

[0136] 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.

[0137] 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.

[0138] In some embodiments, to further increase the energy density of the battery, the negative electrode active material may include a silicon-based material.

[0139] The negative electrode film layer usually optionally includes a binder, a conductive agent and other optional auxiliary agents.

[0140] 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.

[0141] As an example, the binder 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).

[0142] As an example, other optional auxiliary agents may be thickening and dispersing agents (such as carboxymethylcellulose sodium, CMC-Na), and PTC thermistor materials.

[0143] [Electrolyte]

[0144] 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.

[0145] 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).

[0146] 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).

[0147] 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.

[0148] In some embodiments, the secondary battery may be a lithium-ion secondary battery. When the porous coating layer 13 is located between the first porous base membrane 11 and the second porous base membrane 12, the binder 14 not only serves to bond the first porous base membrane 11 and the second porous base membrane 12, but also, at least a portion of the filler particles 15 are embedded in the first porous base membrane 11 and / or the second porous base membrane 12 to a depth of greater than or equal to 1 μm, thereby increasing the bonding strength between the porous coating layer 13 and the first porous base membrane 11 and the second porous base membrane 12, thereby improving the reliability of the secondary battery.

[0149] 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.

[0150] In some embodiments, the secondary battery may include an outer packaging for encapsulating the positive electrode sheet, the negative electrode sheet, and the electrolyte.

[0151] 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).

[0152] 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.

[0153] The preparation method of the secondary battery of the present application is well known. In some embodiments, the positive electrode sheet, the separator 10, the negative electrode sheet and the electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, the separator 10, 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, allowed 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.

[0154] 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.

[0155] The battery module 4 may further include a housing having an accommodation space, and the plurality of secondary batteries 5 may be accommodated in the accommodation space.

[0156] 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.

[0157] 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.

[0158] [Installation]

[0159] 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.

[0160] The device can select battery cells, battery modules or battery packs according to its usage requirements.

[0161] Figure 7 shows an example of an electric device 6. The electric device 6 can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the electric device 6's requirements for high power and high energy density of secondary batteries, a battery pack or battery module can be used.

[0162] As another example, the electric device 6 may be a mobile phone, a tablet computer, or a laptop computer. Such an electric device is usually required to be light and thin, and may use a battery cell as a power source.

[0163] The beneficial effects of the present application are further illustrated below with reference to the examples.

[0164] 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.

[0165] 1. Preparation of isolation membrane

[0166] Example 1

[0167] A polyethylene raw material with a crystallization temperature of 127°C, a polypropylene raw material with a crystallization temperature of 114°C, and a nucleating agent of calcium carbonate are provided, and the mixture is mixed to form a precursor. The precursor is extruded and then subjected to sheet casting treatment, with the temperature value of the first temperature stage being 135°C, the temperature value of the second temperature stage being 120°C, and a stretching treatment is performed.

[0168] The preparation processes of Examples 2-16 and Comparative Examples 1-2 are similar, except that the raw material composition, nucleating agent, and casting sheet are adjusted. Please refer to Table 1.

[0169] 2. Isolation membrane performance test, please refer to Table 2.

[0170] (1) Closed-cell temperature

[0171] 1. Sample preparation: Cut the isolation film samples into 50mm*100mm shapes, with at least 20 pieces per group.

[0172] 2. Test: Place the sample in an oven and heat it from room temperature to 110°C, then take it out for air permeability test. Perform air permeability test every time the temperature rises by 1°C, and record the curve of air permeability changing with temperature. The temperature point where the air permeability changes suddenly is recorded as the closed-cell temperature.

[0173] (2) Closure time

[0174] 1. Sample preparation: Cut the isolation film samples into 50mm*100mm shapes, with at least 20 pieces per group.

[0175] 2. Test: Place the sample in an oven and heat it from room temperature to the closed-cell temperature. Take it out at regular intervals for air permeability testing. Record the curve of air permeability change over time. The time point when the air permeability suddenly changes is recorded as the closed-cell time.

[0176] As can be seen from Table 1, compared with Comparative Example 1-2, Example 1-16 meets the requirement that the porous base membrane includes a matrix phase and a filling phase dispersed in the matrix phase, and the test curve of the differential scanning calorimeter of the porous base membrane includes both the melting peak of the matrix phase and the melting peak of the filling phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase; therefore, the performance data of Example 1-16 needs to be better than those of Comparative Example 1-2.

[0177] Comparison of Example 1-2 with Example 3-9 shows that when the ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is greater than 1, the performance data of Example 3-9 should be better than those of Example 1-2.

[0178] Comparing Examples 1-4 and 9 with Examples 5-8, it is shown that when the ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is 1.2-2.0, the performance data of Examples 1-4 and 9 need to be better than those of Examples 5-8.

[0179] Comparison of Examples 6, 7, and 12 with Examples 10 and 11 shows that when the porous base membrane has a diffraction peak in the range of 15°<2θ<17°, the performance data of Examples 6, 7, and 12 need to be better than those of Examples 10 and 11.

[0180] Comparison of Examples 6, 7, 10, and 12 with Example 11 shows that when the porous base membrane has a diffraction peak in the range of 23°<2θ<25°, the performance data of Examples 6, 7, 10, and 12 need to be better than that of Example 11.

[0181] Compared with Examples 10-12, Examples 6 and 7 show that the porous base membrane includes a first diffraction peak in the range of 15°<2θ<17°, and the porous base membrane includes a second diffraction peak in the range of 23°<2θ<25°. When the diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak, the performance data of Examples 6 and 7 need to be better than those of Examples 10-12.

[0182] In summary, the isolation membrane provided by the embodiment of the present application includes a porous base membrane, and the porous base membrane includes a matrix phase and a filling phase dispersed in the matrix phase. The test curve of the differential scanning calorimeter of the porous base membrane includes both the melting peak of the matrix phase and the melting peak of the filling phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase. The temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase, that is, the matrix phase has better thermal stability than the filling phase. Therefore, when the internal temperature of the secondary battery is high, the filling phase melts first due to its poorer thermal stability, thereby timely sealing the pores of the isolation membrane, terminating the electrochemical reaction, and preventing the internal temperature of the secondary battery from continuing to rise, thereby improving the reliability of the secondary battery.

[0183] The above description is only an implementation method of the present application and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the description and drawings of this application, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. An isolation membrane, comprising a porous base membrane, wherein the porous base membrane comprises a matrix phase and a filling phase dispersed in the matrix phase, wherein a test curve of a differential scanning calorimeter of the porous base membrane includes both a melting peak of the matrix phase and a melting peak of the filling phase, and a temperature of the melting peak of the matrix phase is higher than a temperature of the melting peak of the filling phase.

2. The isolation film according to claim 1, wherein The ratio of the peak area of ​​the melting peak of the matrix phase to the peak area of ​​the melting peak of the filling phase is greater than 1, and can be selected to be 1.2-2.

0.

3. The isolation film according to claim 1 or 2, wherein: In the X-ray diffraction spectrum of the porous base film, the porous base film has a diffraction peak in the range of 15°<2θ<17°.

4. The isolation film according to any one of claims 1 to 3, wherein: In the X-ray diffraction spectrum of the porous base film, the porous base film has a diffraction peak in the range of 23°<2θ<25°.

5. The isolation film according to any one of claims 1 to 4, wherein: In the X-ray diffraction pattern of the porous base membrane, the porous base membrane includes a first diffraction peak in the range of 15°<2θ<17°, and the porous base membrane includes a second diffraction peak in the range of 23°<2θ<25°, and the diffraction peak intensity of the first diffraction peak is greater than the diffraction peak intensity of the second diffraction peak.

6. The isolation film according to any one of claims 1 to 5, wherein: The filling phase includes α-crystalline grains; and / or, The base film phase includes β-crystalline grains.

7. The isolation film according to any one of claims 1 to 6, wherein: The grain size of the filling phase is 0.1 μm-2 μm, and can be optionally 0.1 μm-0.5 μm.

8. The isolation film according to any one of claims 1 to 7, wherein: The mass proportion of the filling phase in the substrate is 10%-40%, and can be optionally 15%-25%.

9. The isolation film according to any one of claims 1 to 8, wherein: The relative molecular mass of the material of the filling phase is less than or equal to 1.2 million, and can be selected to be 400,000-600,000; and / or, The relative molecular mass of the material of the matrix phase is greater than or equal to 300,000, and can be selected from 300,000 to 2.5 million.

10. The isolation film according to any one of claims 1 to 9, wherein: The temperature of the melting peak of the matrix phase is 160°C-350°C, optionally 160°C-180°C; and / or, The temperature of the melting peak of the filling phase is 60°C-180°C, and can be optionally 80°C-130°C.

11. The isolation film according to any one of claims 1 to 10, wherein: The material of the matrix phase includes at least one of polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene terephthalate, polyetheretherketone, polyurethane, and polyester; and / or, The material of the filling phase includes at least one of polyethylene and polypropylene.

12. The isolation film according to any one of claims 1 to 11, wherein: The pore size of the porous base membrane is less than or equal to 0.5 μm, and can be selected to be 0.02 μm-0.1 μm.

13. The isolation film according to any one of claims 1 to 12, wherein: The isolation film satisfies at least one of the following (1) to (9): (1) The closed-cell temperature of the isolation film is 120°C-170°C, and can be 120°C-130°C; (2) The closure time of the isolation film is less than or equal to 10 seconds, and can be selected to be 5 seconds to 10 seconds; (3) The transverse elongation at break of the isolation film is ≥ 100%, and can be selected to be 100%-120%; (4) The longitudinal elongation at break of the isolation film is ≥ 60%, which can be 60%-80%; (5) The transverse tensile strength of the isolation film is ≥1500kgf / cm 2 , optional 2000kgf / cm 2 -4000kgf / cm 2 ; (6) The longitudinal tensile strength of the isolation film is ≥ 2000 kgf / cm 2 , optional 2000gf / cm 2 -4000kgf / cm 2 ; (7) The puncture strength of the isolation film is ≥ 60 gf, which can be selected from 120 gf to 420 gf; (8) The porosity of the isolation membrane is 30%-90%, and can be optionally 30%-50%; (9) The air permeability of the isolation membrane is less than or equal to 300 sec / 100 cc, and can be selected from 100 sec / 100 cc to 300 sec / 100 cc.

14. A method for preparing an isolation film according to any one of claims 1 to 13, wherein: include: Mixing a raw material for forming a matrix phase, a raw material for forming a filling phase, and a nucleating agent to form a precursor, wherein the crystallization temperature of the raw material for forming the filling phase is lower than the crystallization temperature of the raw material for forming the matrix phase; Extruding the precursor to form a first intermediate product; Performing a casting process on the first intermediate product to form a second intermediate product, wherein the casting process includes a first temperature stage and a second temperature stage, and the casting temperature of the first temperature stage is higher than the casting temperature of the second temperature stage; and Performing a stretching treatment on the second intermediate product to obtain a porous base film; The porous base membrane includes a matrix phase and a filling phase dispersed in the matrix phase. The test curve of the differential scanning calorimeter of the porous base membrane includes both the melting peak of the matrix phase and the melting peak of the filling phase, and the temperature of the melting peak of the matrix phase is higher than the temperature of the melting peak of the filling phase.

15. The method for preparing a separator according to claim 14, wherein: The melting index of the filling phase raw material is greater than the melting index of the matrix phase raw material; optionally, the ratio of the melting index of the filling phase raw material to the melting index of the matrix phase raw material is equal to 1.1-10.1, and can be optionally 1.1-2.

1.

16. The method for preparing a separator according to claim 14 or 15, wherein: The crystallization temperature of the filling phase raw material is 80°C-120°C, and can be optionally 80°C-100°C; And / or, the crystallization temperature of the matrix phase raw material is 125°C-135°C, optionally 125°C-130°C.

17. The method for preparing a separator according to any one of claims 14 to 16, wherein: The casting temperature in the first temperature stage is 125°C-135°C, and can be 125°C-130°C; and / or, The casting temperature in the second temperature stage is 105°C-125°C, and can be 105°C-110°C; and / or, The difference between the temperature of the casting piece in the first temperature stage and the temperature of the casting piece in the second temperature stage is 10°C-20°C.

18. The method for preparing a separator according to any one of claims 14 to 17, wherein: In the step of stretching the second intermediate product, the transverse stretching temperature is 110°C-120°C, optionally 110°C-115°C; and / or the longitudinal stretching temperature is 85°C-120°C, optionally 110°C-120°C.

19. A secondary battery, comprising an isolation membrane as described in any one of claims 1 to 13 or an isolation membrane prepared by the method for preparing an isolation membrane as described in any one of claims 14 to 18, wherein the secondary battery comprises 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.

20. An electric device comprising the secondary battery according to claim 19.

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