Polyolefin-based film and its manufacturing method, separator, secondary battery, and power consumption device

Abstract

The present application relates to a polyolefin base film, which has a uniform pore structure by optimizing the tortuosity of the pore structure in the polyolefin base film, thereby improving the conformity of the base film, reducing the performance deviation of the base film, and effectively guaranteeing the air permeability of the base film. At the same time, the thickness of the polyolefin base film of the present application is extremely thin, and the energy density of the battery core can be increased by about 0.7% for every 1 μm reduction in thickness, so that the energy density of the battery core can be improved by controlling the thickness to 7 μm or less. When the polyolefin base film is used as a separator in a battery, it can improve the electrical performance of the battery, including reducing the internal resistance, DC internal resistance, self-discharge rate, short circuit rate, etc. of the battery. The present application further relates to a method for producing the polyolefin base film, a separator, a secondary battery, and a power consumption device.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to international patent application No. PCT / CN2022 / 128110, filed on October 28, 2022, the entire contents of which are incorporated herein by reference. [Technical field]

[0002] The present application relates to the field of battery technology, and in particular to a polyolefin-based film and its manufacturing method, a separator, a secondary battery, and a power consuming device. [Background technology]

[0003] Secondary batteries are currently widely used in fields such as pure electric vehicles, hybrid electric vehicles, and smart grids. In secondary batteries, separators are used to separate the positive and negative electrodes to prevent the positive and negative electrodes from directly contacting each other and causing a short circuit. As people's demand for the use of portable products increases day by day, ultra-thin separators have begun to be widely applied. However, the polyolefin base film for manufacturing conventional ultra-thin separators cannot achieve both low thickness and uniformity of pore structure, and the performance deviation of the base film is relatively large.

[0004] Therefore, there is a strong demand for the development of a polyolefin-based film that can achieve both a low thickness and a uniform pore structure. Summary of the Invention

[0005] In view of the problems existing in the background art, the present application provides a polyolefin-based film that can achieve both a low thickness and uniform pore structure.

[0006] The polyolefin base film according to the first aspect of the present application comprises a pore structure, the polyolefin base film has a thickness of ≦7 μm, and the tortuosity of the pore structure is 7-10.

[0007] In the technical proposal of the embodiment of the present application, the tortuosity of the pore structure is optimized to make the polyolefin base film have a uniform pore structure, thereby improving the conformity of the base film, reducing the performance deviation of the base film, making the ion transport faster, and effectively guaranteeing the air permeability of the base film. At the same time, the thickness of the polyolefin base film of the present application is extremely thin, and the energy density of the battery core can be increased by about 0.7% every time the thickness is reduced by 1 μm, so that the energy density of the battery core can be improved by controlling the thickness to be 7 μm or less. When this polyolefin base film is used as a separator in a battery, it can improve the electrical performance of the battery, including reducing the internal resistance, DC internal resistance, self-discharge rate, short circuit rate, etc. of the battery.

[0008] In some embodiments, according to the first aspect, a first example of the first aspect is provided, wherein the tortuosity of the pore structure is between 7.5 and 9.5, optionally the tortuosity of the pore structure is between 7.5 and 9.

[0009] In some embodiments, according to the first aspect, a first example of the first aspect is provided, wherein the maximum pore size of the pore structure is ≦45 nm, optionally, the maximum pore size of the pore structure is ≦40 nm, and further optionally, the maximum pore size of the pore structure is between 20 nm and 40 nm.

[0010] In some embodiments, according to the first aspect, a first example of the first aspect is provided, wherein the polyolefin base film has a thickness of 2 μm to 6.2 μm, and optionally, the polyolefin base film has a thickness of 4 μm to 6.2 μm.

[0011] In this design, the thickness of the polyolefin base film, the tortuosity of the pore structure and the maximum pore size of the pore structure are optimized, which is favorable for further improving the consistency of the polyolefin base film, reducing the performance deviation of the base film, improving the air permeability of the base film, and improving the energy density of the battery core.

[0012] In some embodiments, according to the first aspect, there is provided a second example of the first aspect, wherein the polyolefin base film comprises a polyolefin having a weight average molecular weight Mw of 500,000 or more, and optionally, the polyolefin base film comprises a polyolefin having a weight average molecular weight Mw of 500,000 to 1,500,000.

[0013] In this design, polyolefin having a weight average molecular weight Mw of 500,000 or more is used as the raw material, which is advantageous in improving the uniformity of pore formation.

[0014] In some embodiments, according to the first aspect, there is provided a third example of the first aspect, wherein the polyolefin has a polydispersity index Mw / Mn≦3.5, and optionally the polyolefin has a polydispersity index Mw / Mn of 1-3.5.

[0015] In this design, it is advantageous to optimize the polydispersity index of the polyolefin, reduce the distribution width of the polyolefin molecular weight, and improve the molecular weight consistency.

[0016] In some embodiments, according to the first aspect, there is provided a fourth example of the first aspect, wherein the polyolefin is one or two of polyethylene and polypropylene.

[0017] This design is advantageous in that it optimizes the type of polyolefin, further improves the conformity of the polyolefin base film, improves the strength and elongation rate of the base film, and reduces the thermal shrinkage rate of the base film.

[0018] In some embodiments, according to the first aspect, there is provided a fifth example of the first aspect, wherein the polyolefin base film has an air permeability of ≦180 s / 100 cc.

[0019] This design is advantageous in optimizing the air permeability of the polyolefin base film and improving the air permeability of the separator.

[0020] In some embodiments, according to the first aspect, a sixth example of the first aspect is provided, in which the longitudinal stretch ratio of the polyolefin base film is ≧60%, optionally the longitudinal stretch ratio of the polyolefin base film is ≧100%, and further optionally the longitudinal stretch ratio of the polyolefin base film is 100%-120%. The transverse stretch ratio of the polyolefin base film is ≧100%, optionally the transverse stretch ratio of the polyolefin base film is ≧110%, and further optionally the transverse stretch ratio of the polyolefin base film is 110%-160%.

[0021] In this design, the increase in the elongation ratio can effectively improve the toughness of the separator. When metal particles (which may come from the operating environment, positive and negative electrode plates, or separators, etc.) occur during the winding process, the separator can effectively cover the particles, prevent the battery core from being destroyed, and improve the manufacturability of the battery core.

[0022] In some embodiments, according to the first aspect, there is provided a seventh example of the first aspect, wherein the polyolefin base film has a longitudinal heat shrinkage of ≦4% and a transverse heat shrinkage of ≦4% at 115° C.

[0023] This design can optimize the thermal shrinkage rate of the polyolefin-based film, effectively improve the thermal stability of the separator, and improve the safety performance of the battery core.

[0024] In some embodiments, according to the first aspect, there is provided an eighth example of the first aspect, wherein the polyolefin base film is formed by subjecting the polyolefin base film to the following conditions (1)-(4): (1) The puncture strength of the polyolefin base film is ≧270 gf; (2) Polyolefin base film longitudinal tensile strength ≧ 2000kgf / cm 2 Transverse tensile strength ≧ 2000kgf / cm 2 and (3) The porosity of the polyolefin base film is 25% to 40%; (4) The surface density of the polyolefin base film is 2 to 5 g / m 2 and the condition that at least one of the following is satisfied.

[0025] In this design, the strength of the polyethylene porous base film is optimized, which is favorable for improving the resistance of the separator and effectively guaranteeing the yield of the separator manufacturing process.The pore structure of the polyethylene porous base film is optimized, which is favorable for ensuring the consistency of the pore size and favorable for achieving effective ion conduction.

[0026] A second aspect of the present application provides a method for producing a polyolefin-based film, the method comprising: mixing a polyolefin with a pore-forming agent and forming the mixture into a film sheet; biaxially stretching the film sheet; Removing the pore-forming agent from the film sheet after biaxial stretching to form voids; and performing tensile fixing and heat fixing on the film sheet after forming the voids to obtain the polyolefin base film. The polyolefin base film includes a pore structure, the thickness of the polyolefin base film is ≦7 μm, and the tortuosity of the pore structure is 7-10.

[0027] In the technical proposal of the embodiment of the present application, a polyolefin having an ultra-high weight average molecular weight is used as a raw material, and at the same time, combined with biaxial tension, an ultra-thin polyolefin-based film having a pore structure with a tortuosity of 7.5-9.5 and a maximum pore size of the pore structure of ≦45 nm can be produced.

[0028] In some embodiments, according to the second aspect, a first example of the second aspect is provided, in which the weight average molecular weight Mw of the polyolefin is 500,000 or more, and optionally, the polyolefin base film includes a polyolefin having a weight average molecular weight Mw of 500,000 to 1,500,000.

[0029] In some embodiments, according to the second aspect, a first example of the second aspect is provided, where the occupancy rate of the polyolefin in the mixture is 20 wt% to 30 wt%.

[0030] In this design, the solid content of polyolefin is optimized to improve the uniformity of the thickness of the film surface, improve the consistency of the pore size of the base film, and achieve the standard requirement of high elongation rate of the base film. If the solid content of polyolefin is too high (>30wt%), the fluctuation of the pressure curve of the extruder is large, the uniformity of the thickness of the film surface is poor, and the consistency of the pore size of the base film is poor. If the solid content of polyolefin is too low (<20wt%), the content of the pore-forming agent such as white oil is too high, the tensile ratio is limited, and the standard requirement of high elongation rate cannot be achieved. By optimizing the solid content of polyolefin, the uniformity of pore formation can be improved while achieving the standard requirement of high elongation rate.

[0031] In some embodiments, according to the second aspect, a second example of the second aspect is provided, in which the biaxial tension includes longitudinal tension and transverse tension, where the longitudinal tension magnification is 5-15 times, the transverse tension magnification is 5-15 times, and the longitudinal tension magnification and the transverse tension magnification are not 5 times at the same time, and optionally the longitudinal tension magnification is 6-15 times, and the transverse tension magnification is 6-15 times.

[0032] In this design, the thickness of the polyolefin base film can be further reduced and the uniformity of hole formation can be improved by optimizing the machine direction tensile ratio and the cross direction tensile ratio.

[0033] A third aspect of the present application provides a separator comprising the polyolefin base film of the first aspect of the present application or the polyolefin base film obtained by the production method of the second aspect of the present application.

[0034] In the technical proposals of the examples of the present application, a polyolefin-based film according to the first aspect of the present application or a polyolefin-based film obtained by the manufacturing method according to the second aspect of the present application is used, so that the separator of the present application has advantages such as being ultra-thin and having a uniform pore structure.

[0035] In some embodiments, according to the third aspect, there is provided a first example of the third aspect, wherein the separator further comprises a coating disposed on at least one surface of the polyolefin base film.

[0036] By applying a coating to the surface of the polyolefin-based film, the electrical and safety performance of the battery core can be improved.

[0037] In some embodiments, according to the third aspect, there is provided a first example of the third aspect, wherein the coating comprises a filler, the filler comprising at least one selected from inorganic particles, organic particles, and organic-inorganic hybrid particles.

[0038] By selecting a filler that has good thermal stability and is not easily decomposed, the heat resistance of the separator can be further improved.

[0039] In some embodiments, according to the third aspect, there is provided a second example of the third aspect, wherein the separator further comprises an adhesive layer. The adhesive layer is disposed on at least a portion of a surface of the coating. The adhesive layer comprises a particulate adhesive, and optionally the particulate adhesive comprises at least one of an acrylate-based monomer homopolymer or copolymer, an acrylic-based monomer homopolymer or copolymer, and a fluorine-containing olefin monomer homopolymer or copolymer.

[0040] The adhesive layer can not only prevent the coating from falling off and improve the safety performance of the secondary battery, but also improve the interface between the separator and the electrode, and improve the cycle performance of the secondary battery.

[0041] A fourth aspect of the present application provides a secondary battery including the separator of the third aspect of the present application.

[0042] In the technical solution of the embodiment of the present application, the polyolefin-based film of the first aspect of the present application or the polyolefin-based film obtained by the manufacturing method of the second aspect of the present application is used, so that the secondary battery of the present application has improved energy density and electrical performance.

[0043] A fifth aspect of the present application provides a power consuming device, the power consuming device including the secondary battery of the fourth aspect of the present application.

[0044] In the technical solution of the embodiment of the present application, since the polyolefin-based film of the first aspect of the present application or the polyolefin-based film obtained by the manufacturing method of the second aspect of the present application is used, the power consumption device of the present application has improved energy density and electrical performance.

[0045] The above description is only an outline of the technical solution of the present application, which can be implemented in accordance with the contents of the specification, in order to make the technical means of the present application more clearly understood; and in order to make the above and other objectives, features and advantages of the present application more clear and understandable, the following particularly cites specific embodiments of the present application to describe them. [Brief description of the drawings]

[0046] [Figure 1] FIG. 2 is a pore size distribution diagram of the polyolefin base film produced in Example 1 of the present application. [Diagram 2] FIG. 2 is a pore size distribution diagram of the polyolefin base film produced in Comparative Example 1. [Diagram 3] FIG. 2 is a pore size distribution diagram of the polyolefin base film produced in Comparative Example 2. [Figure 4] FIG. 1 is a schematic diagram of a battery cell according to an embodiment of the present application. [Diagram 5] FIG. 5 is an exploded view of the battery cell of one embodiment of the present application shown in FIG. 4. [Figure 6]FIG. 1 is a schematic diagram of a battery module according to an embodiment of the present application. [Figure 7] FIG. 1 is a schematic diagram of a battery pack according to an embodiment of the present application. [Figure 8] FIG. 8 is an exploded view of the battery pack shown in FIG. 7 according to an embodiment of the present application. [Figure 9] 1 is a schematic diagram of a power consuming device powered by a secondary battery according to an embodiment of the present application; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Hereinafter, exemplary embodiments of the present disclosure will be described in more detail with reference to the drawings. Although exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure is not limited to the embodiments described herein, but may be realized in various forms. Rather, these embodiments are provided to allow the present disclosure to be more fully understood, and to fully convey the scope of the present disclosure to those skilled in the art.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art of the present application, and the terms used herein are only for describing specific examples and are not intended to limit the present application. The terms "including" and "having" and any variations thereof in the specification and claims of this application and the description of the drawings above are intended to cover non-exclusive inclusions.

[0049] For convenience, only some numerical ranges are explicitly disclosed herein. However, any lower limit and any upper limit can be combined to form a range not expressly described, and any lower limit and another lower limit can be combined to form a range not expressly described, and similarly any upper limit and any other upper limit can be combined to form a range not expressly described. It should be noted that, although not expressly described, each point or single value between the range endpoints is included in this range. Thus, each point or single value can be combined with any other point or single value as its own lower limit or upper limit, or with another lower limit or upper limit to form a range not expressly described.

[0050] In the description of this specification, it should be explained that unless otherwise specified, "more than" and "less than" are inclusive, and "multiple" in "one or more" means two or more (including two).

[0051] Secondary batteries are currently widely used in fields such as pure electric vehicles, hybrid electric vehicles, and smart grids. In secondary batteries, separators are used to separate the positive and negative electrodes to prevent the positive and negative electrodes from directly contacting each other and causing a short circuit. As people's demand for the use of portable products increases day by day, ultra-thin separators have begun to be widely applied. However, the polyolefin-based film used to manufacture conventional ultra-thin separators cannot achieve both low thickness and uniform pore structure, and the performance deviation of the base film is relatively large. Therefore, there is a strong demand for the development of a polyolefin-based film that can achieve both low thickness and uniform pore structure.

[0052] The inventors have studied and designed a polyolefin base film, and optimized the tortuosity of the pore structure of the polyolefin base film and the maximum pore size of the pore structure of the polyolefin base film, so that the polyolefin base film has a uniform pore structure, thereby improving the conformity of the base film, reducing the performance deviation of the base film, and effectively guaranteeing the air permeability of the base film. At the same time, the thickness of the polyolefin base film of the present application is extremely thin, and every 1 μm reduction in thickness can increase the energy density of the battery core by about 0.7%, so that the energy density of the battery core can be improved by controlling the thickness to 7 μm or less. When this polyolefin base film is used as a separator in a battery, it can improve the electrical performance of the battery, including reducing the internal resistance, DC internal resistance, self-discharge rate, short circuit rate, etc. of the battery.

[0053] The technical solutions described in the embodiments of the present application are applicable to polyolefin-based films, and further to a manufacturing process of the polyolefin-based films, separators using the polyolefin-based films, secondary batteries using the separators, and power-consuming devices using the secondary batteries.

[0054] According to some embodiments of the present application according to a first aspect, the present application provides a polyolefin base film, the polyolefin base film comprising a pore structure, the thickness of the polyolefin base film being ≦7 μm, and the tortuosity of the pore structure being 7-10.

[0055] In the technical solution of the embodiment of this application, the polyolefin base film has ultra-thin thickness and uniform pore structure, which can effectively improve the overall performance of the base film. When this base film is wound on the NCM811 system, it can effectively reduce the internal resistance, DC internal resistance and self-discharge rate of the battery, and the mass-produced short circuit yield level is equivalent to that of a general 7 μm thick polyolefin base film.

[0056] In this application, the tortuosity of the pore structure of the polyolefin base film is jointly determined by the air permeability of the base film, the thickness of the base film, the porosity of the base film and the average pore size of the pore structure of the base film. The tortuosity of the pore structure of the polyolefin base film can be calculated by the following formula:

number

[0057] where τ represents the tortuosity of the pore structure, and t gur represents the air permeability of the base film, ε represents the porosity of the base film, d represents the average pore diameter of the pore structure, and L represents the thickness of the base film, and the unit is cm.

[0058] In some embodiments, according to the first aspect, a first example of the first aspect is provided, wherein the tortuosity of the pore structure is between 7.5 and 9.5. Further, the tortuosity of the pore structure is between 7.5 and 9.

[0059] In some embodiments, according to the first aspect, a first example of the first aspect is provided, wherein the maximum pore size of the pore structure is ≦45 nm. Further, the maximum pore size of the pore structure is ≦40 nm. Even further, the maximum pore size of the pore structure is 20 nm to 40 nm.

[0060] In some embodiments, according to the first aspect, a first example of the first aspect is provided, wherein the polyolefin base film has a thickness of 2 μm to 6.2 μm. Further, the polyolefin base film has a thickness of 4 μm to 6.2 μm.

[0061] In this design, the thickness, tortuosity of the pore structure and maximum pore size of the polyolefin base film are optimized, which is favorable for further improving the consistency of the polyolefin base film, reducing the performance deviation of the base film, improving the air permeability of the base film, and improving the energy density of the battery core.

[0062] In some specific examples, the thickness of the polyolefin base film is 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3.0 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4 μm, 4.1 μm, 4.2 μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm, 5.0 μm, 5.1 μm, 5.2 μm, 5.3 μm, 5.4 μm, 5.5 μm, 5.6 μm, 5.7 μm, 5.8 μm, 5.9 μm, 5.8 μm, 5.9 μm, 5.1 μm, 5.2 μm, 5.3 μm, 5.4 μm, 5.5 μm, 5.6 μm, 5.7 ... The thickness of the polyolefin base film may be 4 μm to 5.0 μm, 5.0 μm to 6.2 μm, or 4.5 μm to 5.5 μm.

[0063] In some specific embodiments, the tortuosity of the pore structure may be 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, or 10.0.

[0064] In some specific embodiments, the maximum pore size of the pore structure may be 45 nm, 44 nm, 43 nm, 42 nm, 41 nm, 40 nm, 39.9 nm, 39.8 nm, 39.7 nm, 39.6 nm, 39.5 nm, 39.4 nm, 39.3 nm, 39.2 nm, 39.1 nm, 39 nm, 38 nm, 35 nm, 30 nm, 25 nm, or 20 nm.

[0065] In some embodiments, according to the first aspect, there is provided a second example of the first aspect, wherein the polyolefin base film comprises a polyolefin having a weight average molecular weight Mw of 500,000 or more.

[0066] The use of a polyolefin having a weight average molecular weight Mw of 500,000 or more as a raw material is advantageous in improving the uniformity of pore formation.

[0067] In this application, a single weight average molecular weight should be understood to mean that the weight average molecular weight of the polyolefin is uniquely determined.

[0068] In some specific embodiments, the weight average molecular weight Mw of the polyolefin may be 500,000 to 1,500,000. If the weight average molecular weight is too large, it is difficult to process, the pressure of the extruder is difficult to control, and the consistency of the thickness of the film surface is poor, and if the weight average molecular weight is too small, the molecular chain is relatively short and the crystal region occupancy rate after stretching is relatively small, which reduces the strength of the base film. For example, the weight average molecular weight Mw of the polyolefin may be 500,000, 550,000, 600,000, 650,000, 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, or 1,500,000.

[0069] In some embodiments according to the first aspect, providing a third example of the first aspect, the polyolefin has a polydispersity index Mw / Mn≦3.5.

[0070] In this design, it is advantageous to optimize the polydispersity index of the polyolefin, reduce the distribution width of the polyolefin molecular weight, and improve the molecular weight consistency.

[0071] In this application, Mn is the number average molecular weight of polyolefin, Mw is the weight average molecular weight of polyolefin, and the polydispersity index is the value obtained by dividing the weight average molecular weight by the number average molecular weight, i.e., weight average molecular weight / number average molecular weight (Mw / Mn), which can represent the uniformity of the molecular weight distribution.

[0072] In some specific embodiments, the polyolefin polydispersity index Mw / Mn can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, or 3.5. Optionally, the polyolefin polydispersity index Mw / Mn can be 1-3.5, 1-2, or 2-3.5, etc.

[0073] In some embodiments, according to the first aspect, there is provided a fourth example of the first aspect, wherein the polyolefin is polyethylene, polypropylene, or a mixture of polyethylene and polypropylene.

[0074] This design is advantageous in that it optimizes the type of polyolefin, further improves the conformity of the polyolefin base film, improves the strength and elongation rate of the base film, and reduces the thermal shrinkage rate of the base film.

[0075] In some embodiments, according to the first aspect, there is provided a fifth example of the first aspect, wherein the polyolefin base film has an air permeability of ≦180 s / 100 cc.

[0076] This design is advantageous in optimizing the air permeability of the polyolefin base film and improving the air permeability of the separator.

[0077] In some specific embodiments, the air permeability of the polyolefin base film may be 100 to 180 s / 100 cc. For example, the air permeability of the polyolefin base film may be 100 s / 100 cc, 105 s / 100 cc, 110 s / 100 cc, 115 s / 100 cc, 120 s / 100 cc, 125 s / 100 cc, 130 s / 100 cc, 135 s / 100 cc, 140 s / 100 cc, 145 s / 100 cc, 150 s / 100 cc, 155 s / 100 cc, 160 s / 100 cc, 165 s / 100 cc, 170 s / 100 cc, 175 s / 100 cc, or 180 s / 100 cc. Alternatively, the air permeability of the polyolefin base film may be 140 to 170 s / 100 cc.

[0078] In some embodiments, according to the first aspect, a sixth example of the first aspect is provided, wherein the polyolefin base film has a machine direction (MD) stretch ratio of ≧60%, optionally the machine direction stretch ratio of the polyolefin base film is ≧100%, and further optionally the machine direction stretch ratio of the polyolefin base film is 100%-120%. The polyolefin base film has a transverse direction (TD) stretch ratio of ≧100%, optionally the transverse direction stretch ratio of the polyolefin base film is ≧110%, and further optionally the transverse direction stretch ratio of the polyolefin base film is 110%-160%.

[0079] In this design, the increase in the elongation ratio indicates that the separator has better toughness. When metal particles (which may come from the operating environment, positive and negative electrode plates, or separators, etc.) occur during the winding process, the separator can effectively cover the particles, prevent the battery core from being destroyed, and improve the manufacturability of the battery core.

[0080] In some specific embodiments, the longitudinal stretch ratio of the polyolefin base film may be 60% to 120%, for example, 60%, 65%, 70%, 73%, 75%, 80%, 82%, 85%, 88%, 89%, 90%, 93%, 95%, 96%, 97%, 98%, 99%, 100%, 101%, 104%, 105%, 109%, 110%, 112%, 113%, 114%, 115%, 116%, or 120%.

[0081] In some specific embodiments, the transverse stretch ratio of the polyolefin base film may be 100% to 160%, for example, 100%, 101%, 102%, 105%, 106%, 107%, 108%, 109%, 110%, 111%, 113%, 114%, 115%, 117%, 118%, 120%, 121%, 125%, 130%, 135%, 140%, 145%, 150%, 155%, 160%.

[0082] In some embodiments, according to the first aspect, there is provided a seventh example of the first aspect, wherein the polyolefin base film has a longitudinal heat shrinkage of ≦4% and a transverse heat shrinkage of ≦4% at 115° C.

[0083] In this design, the reduced heat shrinkage rate of the polyolefin-based film indicates the higher thermal stability of the separator, thus improving the safety performance of the battery core.

[0084] In some specific embodiments, the longitudinal heat shrinkage of the polyolefin base film at 115° C. may be 0% to 3%, for example, 0%, 0.5%, 1%, 1.5%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, or 3%. Optionally, the longitudinal heat shrinkage of the polyolefin base film at 115° C. may be 2% to 2.5%.

[0085] In some specific embodiments, the transverse heat shrinkage of the polyolefin base film at 115°C may be 0% to 3%, for example, 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.5%, or 3%. Optionally, the transverse heat shrinkage of the polyolefin base film at 115°C may be 0.5% to 2% or 0.9% to 1.6%.

[0086] In some embodiments, according to the first aspect, there is provided an eighth example of the first aspect, wherein the polyolefin base film is formed by subjecting the polyolefin base film to the following conditions (1)-(4): (1) The puncture strength of the polyolefin base film is ≧270 gf; (2) Polyolefin base film longitudinal tensile strength ≧ 2000kgf / cm 2 Transverse tensile strength ≧ 2000kgf / cm 2 and (3) The porosity of the polyolefin base film is 25% to 40%; (4) The surface density of the polyolefin base film is 2 to 5 g / m 2 and the condition that at least one of the following is satisfied.

[0087] In this design, the polyethylene porous base film has a relatively high puncture strength, which shows that the separator has good resistance and can effectively guarantee the yield of the separator manufacturing process. Optimizing the porosity of the polyethylene porous base film is favorable to guarantee the consistency of the pore size and to achieve effective ion conduction.

[0088] In some specific embodiments, the polyolefin base film may have a puncture strength of 270gf, 275gf, 280gf, 285gf, 290gf, 295gf, 300gf, 305gf, 310gf, 315gf, 320gf, 325gf, 330gf, 340gf, or 350gf, etc. Optionally, the polyolefin base film may have a puncture strength of 270-350gf.

[0089] In some specific embodiments, the polyolefin base film has a longitudinal tensile strength of 2000 kgf / cm 2 , 2100kgf / cm 2 , 2200kgf / cm 2 , 2300kgf / cm 2 , 2400kgf / cm 2 , 2500kgf / cm 2 , 2600kgf / cm 2 , 2700kgf / cm 2 , 2800kgf / cm 2 , 2900kgf / cm 2 or 3000kgf / cm 2 Optionally, the polyolefin base film may have a longitudinal tensile strength of 2400 to 2800 kgf / cm 2 may be also possible.

[0090] In some specific embodiments, the polyolefin base film has a transverse tensile strength of 2000 kgf / cm 2 , 2100kgf / cm 2 , 2200kgf / cm 2 , 2300kgf / cm 2 , 2400kgf / cm 2 , 2500kgf / cm 2 , 2600kgf / cm 2 , 2700kgf / cm 2 , 2800kgf / cm 2 , 2900kgf / cm 2 or 3000kgf / cm 2 Optionally, the transverse tensile strength of the polyolefin base film may be 2100 to 2600 kgf / cm 2may be also possible.

[0091] In some specific embodiments, the porosity of the polyolefin base film may be 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%. Optionally, the porosity of the polyolefin base film may be 30% to 35%.

[0092] In some specific examples, the polyolefin base film has an areal density of 2 g / m 2 , 2.1g / m 2 , 2.2g / m 2 , 2.3g / m 2 , 2.4g / m 2 , 2.5g / m 2 , 2.6g / m 2 , 2.7g / m 2 , 2.8g / m 2 , 2.9g / m 2 , 3.0g / m 2 , 3.1g / m 2 , 3.2g / m 2 , 3.3g / m 2 , 3.4g / m 2 , 3.5g / m 2 , 3.6g / m 2 , 3.7g / m 2 , 3.8g / m 2 , 3.9g / m 2 , 4g / m 2 , 4.5g / m 2 or 5 g / m 2 Optionally, the areal density of the polyolefin base film is 2.5 to 4 g / m 2 may be also possible.

[0093] A second aspect of the present application provides a method for producing a polyolefin-based film, the method comprising: mixing a polyolefin with a pore-forming agent and forming the mixture into a film sheet; biaxially stretching the film sheet; Removing the pore-forming agent from the film sheet after biaxial stretching to form voids; and performing tensile fixing and heat fixing on the film sheet after forming the voids to obtain the polyolefin base film. The polyolefin base film includes a pore structure, the thickness of the polyolefin base film is ≦7 μm, and the tortuosity of the pore structure is 7-10.

[0094] In the technical solution of the embodiment of the present application, polyolefin is used as raw material, and at the same time, combined with biaxial tension, an ultra-thin polyolefin-based film with a pore structure tortuosity of 7-10 and a thickness of ≦7 μm can be produced.

[0095] Furthermore, in the technical solution of this embodiment, the polyolefin-based film produced has the same characteristics and advantages as the polyolefin-based film described above, and will not be further described here.

[0096] In some embodiments, according to the second aspect, a first example of the second aspect is provided, wherein the weight average molecular weight Mw of the polyolefin is 500,000 or more. Further, the polyolefin base film includes a polyolefin having a weight average molecular weight Mw of 500,000 to 1,500,000.

[0097] In some embodiments, according to the second aspect, a first example of the second aspect is provided, where the occupancy rate of the polyolefin in the mixture is 20 wt% to 30 wt%.

[0098] In this design, the occupancy rate of the polyolefin mixture is optimized to improve the uniformity of the thickness of the membrane surface, improve the consistency of the pore size of the base film, and achieve the standard requirements for the high elongation rate of the base film. If the occupancy rate is too large (>30wt%), the extruder pressure curve fluctuates greatly, the uniformity of the thickness of the membrane surface is poor, and the consistency of the pore size of the base film is poor. If the occupancy rate is too small (<20wt%), the content of the pore-forming agent is too high, the tensile ratio is limited, and the standard requirements for the high elongation rate cannot be achieved. By optimizing the occupancy rate of the polyolefin mixture, the uniformity of the pore formation can be improved while achieving the standard requirements for the high elongation rate.

[0099] In some specific embodiments, the percentage of polyolefin in the blend is 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, or 30 wt%.

[0100] In some embodiments, according to the second aspect, a second example of the second aspect is provided, in which the biaxial tension includes longitudinal tension and transverse tension, where the longitudinal tension magnification is 5 to 15 times, the transverse tension magnification is 5 to 15 times, and the longitudinal tension magnification and the transverse tension magnification are not 5 times at the same time. Further, the longitudinal tension magnification is 6 to 15 times, and the transverse tension magnification is 6 to 15 times.

[0101] In this design, the thickness of the polyolefin base film can be further reduced and the uniformity of hole formation can be improved by optimizing the machine direction tensile ratio and the cross direction tensile ratio.

[0102] In some specific embodiments, the longitudinal tensile ratio is 5 to 12 times, for example, 5 times, 6 times, 7 times, 8 times, 8.1 times, 8.2 times, 8.3 times, 8.4 times, 8.5 times, 8.6 times, 8.7 times, 8.8 times, 8.9 times, 9 times, 9.1 times, 9.2 times, 9.3 times, 9.4 times, 9.5 times, 10 times, 11 times, or 12 times. Optionally, the longitudinal tensile ratio may be 8 to 9.5 times. The longitudinal tensile temperature may be 109 to 115°C.

[0103] In some specific embodiments, the lateral tensile ratio is 5 to 12 times, for example, 5 times, 6 times, 7 times, 8 times, 8.1 times, 8.2 times, 8.3 times, 8.4 times, 8.5 times, 8.6 times, 8.7 times, 8.8 times, 8.9 times, 9 times, 9.1 times, 9.2 times, 9.3 times, 9.4 times, 9.5 times, 9.6 times, 9.7 times, 9.8 times, 9.9 times, 10 times, 11 times, or 12 times. Optionally, the lateral tensile ratio may be 8.5 to 10 times. The lateral tensile temperature may be 113 to 119 degrees Celsius.

[0104] The present application is not particularly limited to the pore-forming agent used, as long as it can sufficiently dissolve polyolefin. For example, the pore-forming agent may be one or more of white oil, liquid paraffin, mineral oil, soybean oil, phthalate ester, and aromatic ether, but is not limited thereto. Optionally, the pore-forming agent is white oil. In some specific embodiments, the pore-forming agent is white oil, and a single weight average molecular weight polyolefin can be mixed with the white oil to obtain a liquid mixture. The occupancy rate of the polyolefin in the liquid mixture is the solid content of the polyolefin.

[0105] In some embodiments, according to the second aspect, providing a third example of the second aspect, the thickness of the film sheet may be 1 mm or more, optionally 1 to 5 mm, for example, 1 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm, 2 mm, 2.1 mm, 2.2 mm, 2.3 mm, 2.4 mm, 2.5 mm, 3 mm, 4 mm, or 5 mm.

[0106] In some embodiments, according to the second aspect, there is provided a fourth example of the second aspect, where the biaxial tensioning includes performing longitudinal tension first and then transverse tension, or performing transverse tension first and then longitudinal tension, or performing longitudinal tension and transverse tension synchronously.

[0107] In some embodiments, according to the second aspect, a fifth example of the second aspect is provided, in which removing the pore-forming agent comprises extracting the pore-forming agent in the film sheet using an extractant. The type of the extractant can be selected according to the type of the pore-forming agent, and is mainly used to dissolve the pore-forming agent to form voids in the material. Optionally, the extractant is dichloromethane.

[0108] In some embodiments, according to the second aspect, there is provided a sixth example of the second aspect, in which the tension fixing includes tensioning the film sheet in the transverse direction at a small ratio. The tension ratio may be 1 to 3 times. The tension temperature may be 130 to 133°C.

[0109] In some embodiments, according to the second aspect, there is provided a seventh example of the second aspect, wherein the heat fixing comprises heating the film sheet to fix it. The heating temperature is ≧133° C., optionally 133-135° C. The heating time is ≧20 s, optionally 20-60 s, for example 28-40 s or 30-40 s.

[0110] A third aspect of the present application provides a separator comprising the polyolefin base film of the first aspect of the present application or the polyolefin base film obtained by the production method of the second aspect of the present application.

[0111] In the technical solution of the embodiment of the present application, the polyolefin base film of the first aspect of the present application or the polyolefin base film obtained by the manufacturing method of the second aspect of the present application is used, so that the separator of the present application has the advantages of being ultra-thin and having a uniform pore structure, etc. In addition, the electrical performance and safety performance of the battery core can be improved by applying a coating on the surface of the polyolefin base film.

[0112] In some embodiments, according to the third aspect, there is provided a first example of the third aspect, wherein the separator further comprises a coating disposed on at least one surface of the polyolefin base film.

[0113] In some embodiments, according to the third aspect, there is provided a first example of the third aspect, wherein the coating comprises a filler, the filler comprising at least one selected from inorganic particles, organic particles, and organic-inorganic hybrid particles.

[0114] In some embodiments, optionally, the decomposition temperature of the filler is 200° C. or higher, so that the filler has excellent thermal stability and is not easily decomposed, and the heat resistance of the separator can be further improved.

[0115] In some embodiments, the inorganic particles optionally include at least one of inorganic particles having a dielectric constant of 5 or greater, inorganic particles that are ionically conductive but do not store ions, and inorganic particles capable of undergoing electrochemical reactions.

[0116] Optionally, the inorganic particles having a dielectric constant of 5 or more 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, magnesium lithium silicate, magnesium sodium silicate, bentonite, hectorite, zirconium titanate, barium titanate, Pb(Zr,Ti)O3 (abbreviated as PZT), Pb 1-m La m Zr 1-n Ti n O 33 (Abbreviated as PLZT, 0 <m<1、0<n<1)、Pb(Mg3Nb 2 / 3)O3-PbTiO3 (abbreviated as PMN-PT), and at least one of the 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 (e.g., using a silane coupling agent, a titanate coupling agent, etc.), surfactant modification, polymer graft modification, etc. The physical modification method may be mechanical force dispersion, ultrasonic dispersion, high energy treatment, etc. The modification treatment can reduce the aggregation of the inorganic particles, and by modifying the inorganic particles by selecting a coupling agent, a surface active material, or a polymer having a specific functional group, it contributes to improving the wetting property of the coating with respect to the electrolyte and the adhesion of the coating.

[0117] Alternatively, inorganic particles that are ionically conductive but do not store ions include Li3PO4, lithium titanium phosphate, Li x1 Ti y1 (PO4)3, Lithium titanium aluminum phosphate Li x2 Al y2 Ti z1 (PO4)3, (LiAlTiP) x3 O y3 Glass, Lithium lanthanum 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 S y7 S z3 and P2S5 type glass Li x8 P y8 S z4including at least one of them, where 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. Thereby, the ion transport characteristics of the separator can be further improved.

[0118] Optionally, the inorganic particles capable of undergoing an electrochemical reaction include at least one of lithium-containing transition metal oxides, lithium-containing phosphates, carbon-based materials, silicone-based materials, tin-based materials, and lithium titanium compounds.

[0119] The organic particles have the characteristics of good thermal stability and being difficult to decompose, which can improve the heat resistance of the separator. At the same time, when the internal temperature of the secondary battery reaches the melting point of the organic particles due to overheating and thermal runaway, the organic particles can further melt and be inhaled into the pores of the porous substrate by capillary action to play the role of closing pores and blocking, which is beneficial to ensuring that the secondary battery has high safety performance.

[0120] In some embodiments, the organic particles include, but are not limited to, at least one of polyethylene particles, polypropylene particles, polystyrene particles, cellulose, cellulose modifiers (such as carboxymethyl cellulose), melamine resin particles, phenol resin particles, polyester particles (such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate), silicone resin particles, polyimide particles, polyamideimide particles, polyaramid particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, polyallyletherketone particles, and copolymers of butyl acrylate and ethyl methacrylate (such as cross-linked polymers of butyl acrylate and ethyl methacrylate).

[0121] In some embodiments, the glass transition temperature of the organic particles may be 130° C. or higher. This prevents the organic particles from changing from a glass state to a viscous flow state when the internal temperature of the secondary battery reaches 130° C., and prevents the separator from contracting violently. Further, the organic particles may include, but are not limited to, at least one of melamine formaldehyde resin particles, phenolic resin particles, polyester particles, silicone resin particles, polyimide particles, polyamideimide particles, polyaramid particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, and polyaryletherketone particles.

[0122] Optionally, nanocellulose may be included in the coating. Nanocellulose refers to a general term for cellulose whose size in any dimension is nanoscale (e.g., within 100 nm), and it has the properties of cellulose but also has the properties of nanoparticles. Nanocellulose may be a polymeric nanomaterial extracted from wood, cotton, etc. in nature by one or more means such as chemical, physical, and biological, and has the advantages of a wide supply source, low cost, biodegradability, high elastic modulus, and high specific surface area.

[0123] In some embodiments, the nanocellulose may include at least one of cellulose nanofibers (Cellulose nanofibrils, CNF, also known as nanofibrillated cellulose or microfibrillated cellulose), cellulose nanocrystals (CNC, also known as cellulose nanocrystals or nanocrystalline cellulose), and bacterial nanocellulose (BNC, also known as bacterial cellulose or microbial cellulose).

[0124] In some embodiments, the nanocellulose may include at least one of unmodified nanocellulose (also called hydroxy nanocellulose) and modified nanocellulose, and optionally modified nanocellulose. The nanocellulose may include modifying groups. The modifying groups may include at least one of amine groups, carboxylic acid groups, aldehyde groups, sulfonic acid groups, boric acid groups, and phosphate groups, and optionally at least one of sulfonic acid groups, boric acid groups, and phosphate groups.

[0125] In some examples, the organic-inorganic hybrid particles may be selected from metal-organic framework materials, such as MOFs.

[0126] In some embodiments, the coating may further include other organic compounds, such as polymers for improving heat resistance, dispersants, wetting agents, adhesives, etc. The present application is not particularly limited to the types of the other organic compounds, and any known material having good improving performance may be selected and used.

[0127] In some specific embodiments, the coating thickness is ≦5 μm.

[0128] In some specific embodiments, the method for preparing a separator includes the steps of (1) providing a polyolefin base film, (2) providing a coating slurry, mixing a filler and a solvent in a predetermined ratio to prepare a coating slurry, and (3) applying the coating slurry of step (2) to at least one side of the polyolefin base film of step (1) to form a coating and drying to obtain a separator, where the polyolefin base film comprises a pore structure, the thickness of the polyolefin base film is ≦7 μm, the tortuosity of the pore structure is 7.5-9.5, and the maximum pore size of the pore structure is ≦45 nm.

[0129] In some specific embodiments, the separator further comprises an adhesive layer disposed on at least a portion of a surface of the coating, the adhesive layer comprising a particulate adhesive, and optionally the particulate adhesive comprises at least one of an acrylate-based monomer homopolymer or copolymer, an acrylic-based monomer homopolymer or copolymer, and a fluorine-containing olefin monomer homopolymer or copolymer.

[0130] A fourth aspect of the present application provides a secondary battery including the separator of the third aspect of the present application.

[0131] Since the polyolefin-based film according to the first aspect of the present application or the polyolefin-based film obtained by the manufacturing method according to the second aspect of the present application is used, the secondary battery according to the present application has improved energy density and electrical performance.

[0132] In general, a secondary battery may include a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. During charging and discharging of the battery, active ions shuttle between the positive electrode plate and the negative electrode plate to be absorbed and desorbed. The separator is disposed between the positive electrode plate and the negative electrode plate and serves to isolate the electrodes. The electrolyte serves to conduct ions between the positive electrode plate and the negative electrode plate.

[0133] [Negative electrode plate] The negative electrode plate generally includes a negative electrode current collector and a negative electrode film layer disposed on the negative electrode current collector. The negative electrode film layer includes a negative electrode active material.

[0134] The negative electrode current collector may be a conventional metal foil sheet or a composite current collector (e.g., a metal material may be deposited on a polymeric substrate to form a composite current collector). For example, the negative electrode current collector may be a copper foil.

[0135] The negative electrode active material may be a negative electrode active material for a battery known in the art. For example, the negative electrode active material may include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, a silicone-based material, a tin-based material, and lithium titanate. The silicone-based material may be selected from at least one of a silicone simple substance, a silicone oxide, a silicone carbon composite, a silicone nitrogen composite, and a silicone alloy. The tin-based material may be at least one selected from a tin simple substance, a tin oxide, and a tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery negative electrode active material may be used. These negative electrode active materials may be used alone or in combination of two or more.

[0136] The negative electrode film layer generally further optionally contains an adhesive, a conductive agent and other optional auxiliary agents.

[0137] By way of 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), graphene, and carbon nanofibers.

[0138] By way of 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).

[0139] By way of example, other optional auxiliaries may be thickening and dispersing agents (eg, sodium carboxymethylcellulose CMC-Na), PTC thermistor materials.

[0140] [Positive electrode plate] The positive electrode plate generally includes a positive electrode current collector and a positive electrode film layer disposed on the positive electrode current collector. The positive electrode film layer includes a positive electrode active material.

[0141] The positive electrode current collector may be a conventional metal foil sheet or a composite current collector (metal material may be deposited on a polymeric substrate to form a composite current collector). As an example, the positive electrode current collector may be an aluminum foil.

[0142] The specific type of the positive electrode active material is not limited, and any active material that can be used for a secondary battery positive electrode known in the art may be adopted, and those skilled in the art can select it according to actual needs.

[0143] For example, the positive electrode active material may include, but is not limited to, one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, 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 lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, lithium iron phosphate and carbon composite, lithium manganese phosphate, lithium manganese phosphate and carbon composite, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composite, and their modified compounds. All of these materials can be obtained commercially.

[0144] In some embodiments, the modifying compounds of the materials may provide doping and / or surface coating modifications to the materials.

[0145] The positive electrode layer generally further optionally contains an adhesive, a conductive agent and other optional auxiliary agents.

[0146] By way of 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), graphene, and carbon nanofibers.

[0147] By way of 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), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0148] [Electrolyte] The electrolyte serves to conduct ions between the positive electrode and the negative electrode, and may include an electrolyte salt and a solvent.

[0149] By way of example, the electrolyte salt may 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 difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalato)phosphate (LiDFOP), and lithium tetrafluoro(oxalato)phosphate (LiTFOP).

[0150] By way of example, the solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE).

[0151] In some embodiments, the electrolyte further includes an additive. For example, the additive may include a negative electrode film-forming additive, may include a positive electrode film-forming additive, and may further include an additive that can improve some performance of the battery, such as an additive that improves the overcharge performance of the battery, an additive that improves the high temperature performance of the battery, an additive that improves the low temperature performance of the battery, etc.

[0152] In some embodiments, the positive plates, negative plates and separators can be fabricated into an electrode assembly by a winding process or a lamination process.

[0153] In some embodiments, the battery cell may include an exterior body, which may be used to package the electrode assembly and electrolyte.

[0154] In some embodiments, the battery cell exterior may be a hard case, such as a hard plastic case, an aluminum case, a steel case, etc. The battery cell exterior may be a pouch, such as a bag-shaped pouch. The pouch may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0155] The present application is not particularly limited to the shape of the battery cell, which may be cylindrical, rectangular, or any other shape. For example, FIG. 4 shows a battery cell 5 with a rectangular structure as an example.

[0156] In some embodiments, referring to FIG. 5, the exterior body may include a case 51 and a top cover assembly 53. Here, the case 51 may include a bottom plate and a side plate connected on the bottom plate, and the bottom plate and the side plate surround and form a receiving cavity. The case 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 may cover the opening to seal the receiving cavity. The positive electrode plate, the negative electrode plate and the separator may form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. An electrolyte is infiltrated into the electrode assembly 52. ​​The number of electrode assemblies 52 included in the battery cell 5 may be one or more, and can be selected by those skilled in the art according to specific actual needs.

[0157] In some embodiments, the battery cells may be assembled into a battery module, and the number of battery cells included in the battery module may be one or more, with the specific number being selectable by one skilled in the art depending on the application and capacity of the battery module.

[0158] Fig. 6 shows an example of a battery module 4. Referring to Fig. 6, in the battery module 4, a plurality of battery cells 5 may be arranged in sequence along the longitudinal direction of the battery module 4. Of course, the battery cells 5 may be arranged in any other manner. Furthermore, the plurality of battery cells 5 may be fixed by fasteners.

[0159] Optionally, the battery module 4 may further include a housing having an accommodating space, and the multiple battery cells 5 are accommodated in the accommodating space.

[0160] In some embodiments, the battery modules may be assembled into a battery pack, and the battery pack may include one or more battery modules, the specific number of which may be selected by one skilled in the art according to the application and capacity of the battery pack.

[0161] 7 and 8 show an example of a battery pack 1. Referring to Fig. 7 and Fig. 8, the battery pack 1 may include a battery box and a plurality of battery modules 4 installed in the battery box. The battery box includes an upper housing 2 and a lower housing 3, and the upper housing 2 may be covered by the lower housing 3 to form a sealed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0162] The present application further provides a power consuming device, the power consuming device including a secondary battery according to the present application, the secondary battery including at least one of a battery cell, a battery module, and a battery pack. The secondary battery may be used as a power source for the power consuming device, or may be used as an energy storage unit for the power consuming device. The power consuming device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0163] The power consuming device can be selected as a battery cell, a battery module or a battery pack based on its usage needs.

[0164] 9 shows an example power consuming device, such as a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. A battery pack or battery module may be employed to meet the demand for high power and high energy density of the power source of the power consuming device.

[0165] Another example of the device may be a mobile phone, a tablet computer, a notebook computer, etc. This device is generally required to be thin and may employ a battery cell as a power source.

[0166] The present invention will now be further described in conjunction with examples. It should be understood that these examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention.

[0167] Manufacturing of polyethylene porous base film Example 1 First, the weight average molecular weight is 8.0 × 10 5 , polyethylene with Mw / Mn=3.5 is selected and mixed with white oil. Here, the solid content of polyethylene is 25wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled and molded to obtain a 2mm cast film sheet, and the cast film sheet is stretched in the longitudinal and transverse directions. The longitudinal tensile ratio is 8.2 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 8.7 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film sheet with an area increased by 71 times is obtained, and dichloromethane is used to extract the white oil in the film sheet to form voids, and the dried microporous film is stretched again in the transverse direction at a small ratio of 2 times. The tensile temperature in the transverse direction at a small ratio is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 5.1 μm and a crystallinity of 78.7% can be obtained. The pore size distribution of the polyolefin-based film is shown in FIG.

[0168] Example 2 First, the weight average molecular weight is 7.0 × 10 5, polyethylene with Mw / Mn=3.5 is selected and mixed with white oil. Here, the solid content of polyethylene is 25wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled and molded to obtain a 1.5mm cast film sheet, and the cast film sheet is stretched in the longitudinal and transverse directions. The longitudinal tensile ratio is 9.2 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 9.7 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film sheet with an area increased by 71 times is obtained, and dichloromethane is used to extract the white oil in the film sheet to form voids, and the dried microporous film is stretched again in the transverse direction at a small ratio of 2 times. The tensile temperature in the transverse direction at a small ratio is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 4.0 μm and a crystallinity of 79.6% can be obtained.

[0169] Example 3 First, the weight average molecular weight is 8.0 × 10 5 , polyethylene with Mw / Mn=3.0 is selected and mixed with white oil. Here, the solid content of polyethylene is 25wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled and molded to obtain a 2.1mm cast film sheet, and the cast film sheet is stretched in the longitudinal and transverse directions. The longitudinal tensile ratio is 8.2 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 8.7 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film with an area increased by 71 times is obtained, and dichloromethane is used to extract the white oil in the film to form voids, and the microporous film after drying is stretched again in the transverse direction at a small ratio of 2 times. The tensile temperature at the small ratio in the transverse direction is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 6.2 μm and a crystallinity of 79.3% can be obtained.

[0170] Example 4-23 The method described in Example 1 was followed, with the difference being that the parameters listed in Table 1 below were different from those in Example 1.

[0171] Comparative Example 1 First, the weight average molecular weight is 4.0 × 10 5 , polyethylene with Mw / Mn=3.5 is selected and mixed with white oil. Here, the solid content of polyethylene is 25wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled and molded to obtain a 2mm cast film sheet, and the cast film sheet is stretched in the longitudinal and transverse directions. The longitudinal tensile ratio is 8.2 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 8.7 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film sheet with an area increased by 71 times is obtained, and dichloromethane is used to extract the white oil in the film sheet to form voids, and the dried microporous film is stretched again in the transverse direction at a small ratio of 2 times. The tensile temperature in the transverse direction at a small ratio is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 5.2 μm and a crystallinity of 78.7% can be obtained. The pore size distribution of the polyolefin base film is tested using the method of Example 1. The resulting pore size distribution diagram is shown in Figure 2.

[0172] Comparative Example 2 First, the weight average molecular weight is 8.0 × 10 5, polyethylene with Mw / Mn=4.0 is selected and mixed with white oil. Here, the solid content of polyethylene is 25wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled and molded to obtain a 2mm cast film sheet, and the cast film sheet is stretched in the longitudinal and transverse directions. The longitudinal tensile ratio is 8.2 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 8.7 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film sheet with an area increased by 71 times is obtained, and dichloromethane is used to extract the white oil in the film sheet to form voids, and the dried microporous film is stretched again in the transverse direction at a small ratio of 2 times. The tensile temperature in the transverse direction at a small ratio is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 5.2 μm and a crystallinity of 78.7% can be obtained. The pore size distribution of the polyolefin base film is tested using the method of Example 1. The resulting pore size distribution diagram is shown in FIG.

[0173] Comparative Example 3 First, the weight average molecular weight is 8.0 × 10 5 , polyethylene with Mw / Mn=3.5 is selected and mixed with white oil. Here, the solid content of polyethylene is 25wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled and molded to obtain a 2mm cast film sheet, and the cast film sheet is tensioned in the longitudinal and transverse directions. The longitudinal tensile ratio is 5.0 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 5.0 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film sheet with an area increased by 25 times is obtained, and dichloromethane is used to extract the white oil in the film sheet to form voids, and the dried microporous film is tensioned again in the transverse direction at a small ratio of 2 times. The tension temperature at the small ratio in the transverse direction is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 5.2 μm and a crystallinity of 78.7% can be obtained.

[0174] Comparative Example 4 First, the weight average molecular weight is 8.0 × 10 5 , polyethylene with Mw / Mn=3.5 is selected and mixed with white oil. Here, the solid content of polyethylene is 10wt%. Then, it is heated and melted in an extruder, followed by cooling and casting, and cooled to form a 2mm cast film sheet, and the cast film sheet is stretched in the longitudinal and transverse directions. The longitudinal tensile ratio is 8.2 times, the longitudinal tensile temperature is 110°C, the transverse tensile ratio is 8.7 times, and the transverse tensile temperature is 115°C. After keeping the temperature, a film sheet with an area increased by 71 times is obtained, and dichloromethane is used to extract the white oil in the film sheet to form voids, and the microporous film after drying is stretched again in the transverse direction at a small ratio of 2 times. The tensile temperature in the transverse direction at a small ratio is 132°C, and heat setting is performed at 133°C accordingly. The heat setting time is 25 seconds, and a polyethylene porous base film with a thickness of 5.2 μm and a crystallinity of 78.7% can be obtained.

[0175] Comparative Example 5-6 The method described in Example 1 was followed, with the difference being that the parameters listed in Table 1 below were different from those in Example 1.

[0176] [Table 1]

[0177] Polyethylene-based film related parameter testing 1. Measurement of the thickness of polyolefin-based film: Clean the measurement platform and the measurement head. The measurement platform and the measurement head should be kept clean. The test was carried out using a micrometer. If the data remains stable, the data was recorded in Table 2-4.

[0178] 2. Average pore size and maximum pore size test of the pore structure of polyolefin base film: Direct measurement is performed using PMI's capillary flow porosimeter or mercury porosimeter. (i) The pores of the base film to be measured are completely wetted and filled with liquid, and positive pressure is created in the pores by capillary action. (ii) The base film is placed in a sealed tank and pressurized with gas pressure to push the liquid out of the capillary pores. (iii) Based on the relative relationship between the pressure applied when the liquid in a single pore is completely pushed out of the capillary pore and the diameter of the pore, the average pore size and maximum pore size of the pore structure of the polyolefin base film can be obtained according to the Laplace equation. The test results are shown in Table 2-4 below.

[0179] 3. Air permeability test: Cut the base film sample, and the sample size should be greater than 40*40mm. Place the sample on the sample test platform. The sample should cover the entire sample area, tighten the knob, select the measurement area to ensure that the film sheet can cover the test platform. Press the RESET key first (the screen will display "TIMER SEEKING STARTING MARK" at this time), then release the slide cylinder to start the test. After the measurement is completed, slowly loosen the cock until the slide cylinder descends to the lowest position. Take out the sample and slowly pull up the slide cylinder to prepare for the next measurement. The test results are as shown in Table 2-4 below.

[0180] 4. Tensile strength test: Spline punching: The base film is punched into a base film sheet with a width of 15mm and a length of more than 40mm. Then, the tension machine jig is set to an initial jig interval of 40mm and a speed of 50mm / min, the spline to be tested is placed in the middle of the jig, and the upper and lower ends are clamped by the jig respectively, and the tensile curve is recorded. Each set is five parallel samples. The test results are as shown in Table 2-4 below.

[0181] 5. Elongation test: The base film is punched into a base film sheet with a width of 15 mm and a length of more than 40 mm. Then, the tension machine jig is set to an initial jig interval of 40 mm and a speed of 50 mm / min, the spline to be tested is placed in the middle of the jig, and the length of the base film after tension is recorded. Elongation = length after tension / 40 mm*100%. The test results are shown in Table 2-4 below.

[0182] 6. Thermal shrinkage test: The base film was punched into samples of 100mm*50mm. Before baking, the numbers were identified with a marker and measured in two dimensions, and the baking temperature and time were set. After the oven reached the set temperature, the base film was baked in the oven together with the steel disk, and after the specified baking time was reached, it was taken out and left at room temperature for 10 minutes. The horizontal and vertical dimensions of the base film with the same number after baking were measured respectively. If the edge shrinkage of the sample is not uniform, the position of maximum shrinkage was used as the basis. The test results are shown in Table 2-4 below.

[0183] 7. Puncture strength test: Using Gotech tensile machine, 1mm prototype needle is passed through the base film at a speed of 50mm / min. The maximum force obtained is the puncture strength, each set is 5 parallel samples, and the average value obtained is the puncture strength. The test results are shown in Table 2-4 below.

[0184] 8. Porosity test: The cut base film sample was punched with a 100mm*50mm blade to produce a rectangular sample. The length L and width W of the rectangular sample were measured. Then, the thickness was measured with a 0.1um micrometer, and the thickness values ​​were measured at a total of 5 points, 4 points on the edge and 1 point in the middle, and the average value of the thicknesses at the 5 points was taken as the final thickness, which was marked as T. The weighed rectangular sample was weighed with an analytical balance with an accuracy of 0.0001g, which was marked as M1. Using the measured length L, width W, thickness T, and material density ρ of the base film, the theoretical weight of the base film was calculated, which was marked as M2: M2=L*W*T*ρ. Porosity=(1-M1 / M2)*100%. The test results are shown in Table 2-4 below.

[0185] 9. Surface density test: The cut base film sample was punched with a 100mm*50mm blade to produce a rectangular sample. The length L and width W of the rectangular sample were measured, and the weight of the sheet-like base film was measured and recorded as M. Surface density = M / (L*W). The test results are shown in Table 2-4 below.

[0186] 10. Calculation of tortuosity of pore structure Calculate the tortuosity of the pore structure according to the following formula:

number

[0187] [Table 2]

[0188] [Table 3]

[0189] [Table 4]

[0190] Using the base films of Examples 1-23 and Comparative Examples 1-6, lithium ion batteries are manufactured according to the following general manufacturing method.

[0191] Lithium-ion battery manufacturing (1) Manufacturing of positive electrodes The positive electrode active material LFP, the conductive agent Super P, and the adhesive polyvinylidene fluoride (PVDF) were mixed in a mass ratio of positive electrode active material:Super P:PVDF=8:1:1, added to the solvent N-methylpyrrolidone (NMP), and uniformly stirred under the action of a vacuum stirrer to obtain a positive electrode slurry. The solid content in the positive electrode slurry is 50wt%. The positive electrode slurry was uniformly applied to a positive electrode current collector aluminum foil and dried at 85°C, then cold pressed, trimmed, cut, stripped, and finally dried under vacuum conditions at 85°C for 4h to obtain a positive electrode plate.

[0192] (2) Manufacturing of negative electrodes The negative electrode active material graphite, conductive agent Super P, thickener sodium carboxymethylcellulose (CMC), and adhesive styrene butadiene rubber emulsion (SBR) are mixed in a mass ratio of graphite:Super P:CMC:SBR=80:15:3:2, added to the solvent deionized water, and uniformly stirred under the action of a vacuum stirrer to obtain a negative electrode slurry. The solid content in the negative electrode slurry is 30wt%. The negative electrode slurry is uniformly coated on a negative electrode current collector copper foil and dried at 85℃, then cold pressed, trimmed, cut, stripped, and finally dried under vacuum conditions at 120℃ for 12h to obtain a negative electrode plate.

[0193] (3) Separator manufacturing A base film of Example 1 was provided, and inorganic particles of aluminum oxide (Al2O3), organic particles of vinylidene fluoride-hexafluoropropylene copolymer (number average molecular weight 550,000), and adhesive aqueous polyacrylic acid were uniformly mixed in a mass ratio of 79.1:20:0.9 with an appropriate amount of solvent deionized water to obtain a coating slurry with a solid content of 38% (calculated by weight), which was applied to two surfaces of the base film by a coater, and a separator was obtained through processes such as drying and slitting. Here, the line number of the gravure roll of the coater was 125LPI, the coating speed was 50m / min, the coating line speed ratio was 1.2, the drying temperature was 50℃±5℃, and the drying time was 30s.

[0194] (4) Manufacturing of electrolyte Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 30:70 to obtain an organic solvent, and thoroughly dried electrolyte salt LiPF6 was dissolved in the above mixed solvent to obtain an electrolyte solution having a concentration of 1.0 mol / L.

[0195] (5) The positive electrode plate from step (1), the separator from step (3), and the negative electrode plate from step (2) are stacked in order, and the separator is placed between the positive and negative electrodes to serve as an insulator, then wound into a rectangular bare cell, the tabs are welded, and the bare cell is placed in a foil aluminum plastic film packaging, then baked at 80°C to remove water, and then the electrolyte is injected and sealed. After that, it is left to stand, hot pressed, cold pressed, chemically formed (charged at a constant current of 0.02C to 3.3V, then charged at a constant current of 0.1C to 3.6V), shaped, capacity tested, and other processes, and the finished pouch lithium-ion battery is obtained. Its thickness is 4.0mm, its width is 60mm, and its length is 140mm.

[0196] The secondary batteries of Examples 2-23 and Comparative Examples 1-6 were manufactured using methods similar to that of the secondary battery of Example 1, except that different base films were used.

[0197] Battery performance testing 1. Battery DC internal resistance test At a certain temperature, the lithium-ion battery is charged and discharged at a high current for a short time (30s), and the DC internal resistance can be obtained by calculating the ratio of the voltage difference and current before and after charging and discharging. The test conditions and test results are shown in Table 5 below.

[0198] 2. Battery internal resistance test That is, AC resistance, AC internal resistance equipment: IT5100 series battery internal resistance tester from Itech. Test method: A fixed frequency of 1KHz and a fixed current of 50mA are applied to the test battery core, the voltage is sampled, and the resistance value can be calculated by a rectifier. The test results are shown in Table 5 below.

[0199] 3. Bare cell self-discharge rate test After the battery core is aged and cooled, the test voltage is OCV1, and it is placed in a normal temperature and humidity environment, and after leaving it for 48 hours, the voltage is measured again and recorded as OCVB. Self-discharge rate = (OCV1-OCVB) / 48h, and the test results are shown in Table 5 below.

[0200] 4. Bare cell short circuit test pass rate After winding, the bare cell was subjected to a short circuit rate test under the conditions of 100V, 80℃, 10s, and 5MPa. The test results are shown in Table 5 below.

[0201] [Table 5]

[0202] As can be seen from Table 5, the pore structure of the polyethylene porous base film of the present application has a relatively large tortuosity, and the base film has a uniform pore structure, thereby improving the conformity of the base film, reducing the performance deviation of the base film, and effectively ensuring the air permeability of the base film. When the polyolefin base film of the present application is used as a separator in a battery, it can significantly improve the electrical performance of the battery, including reducing the DC internal resistance, internal resistance, self-discharge rate, and short circuit rate of the battery.

[0203] As can be seen by comparing Example 1 and Comparative Example 1, the weight average molecular weight of the polyethylene used in Comparative Example 1 is not within the scope of the present application, the tortuosity of the pore structure of the polyethylene porous base film produced in Comparative Example 1 is obviously small, and the DC internal resistance, battery internal resistance, self-discharge rate and short circuit rate of the battery corresponding to Comparative Example 1 are obviously large.

[0204] As can be seen by comparing Example 1 and Comparative Example 2, the polydispersity index Mw / Mn of the polyethylene used in Comparative Example 2 is not within the scope of the present application, the tortuosity of the pore structure of the polyethylene porous base film produced in Comparative Example 2 is obviously small, and the DC internal resistance, battery internal resistance, self-discharge rate and short circuit rate of the battery corresponding to Comparative Example 2 are obviously large.

[0205] As can be seen by comparing Example 1, Comparative Example 3 and Comparative Example 6, the longitudinal tensile ratio and transverse tensile ratio of Comparative Examples 3 and 6 are not within the scope of the present application, the tortuosity of the pore structure of the polyethylene porous base film produced in Comparative Examples 3 and 6 is obviously small, and the DC internal resistance, battery internal resistance, self-discharge rate and short circuit rate of the batteries corresponding to Comparative Examples 3 and 6 are all obviously large.

[0206] As can be seen by comparing Example 1 with Comparative Example 4-5, the solid content of the polyethylene in Comparative Example 4-5 is not within the scope of the present application, the tortuosity of the pore structure of the polyethylene porous base film produced in Comparative Example 4-5 is obviously small, and the DC internal resistance, battery internal resistance, self-discharge rate and short circuit rate of the battery corresponding to Comparative Example 4-5 are all obviously large.

[0207] The above are merely comparatively excellent specific embodiments of the present invention, and the scope of protection of the present invention is not limited thereto, and any changes or replacements that a person skilled in the art can easily think of within the technical scope of the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be in accordance with the scope of protection of the claims. [Explanation of symbols]

[0208] 1: battery pack, 2: upper housing, 3: lower housing, 4: battery module, 5: battery cell, 51: case, 52: electrode assembly, 53: top cover assembly.

Claims

1. A polyolefin base film, comprising a pore structure, the thickness of the polyolefin base film being ≦7 μm, and the tortuosity of the pore structure being 7-10.

2. 2. The polyolefin-based film according to claim 1, wherein the tortuosity of the pore structure is 7.5-9.5, and optionally the tortuosity of the pore structure is 7.5-9.

3. The polyolefin base film according to claim 1 or 2, characterized in that the maximum pore diameter of the pore structure is ≦45 nm, optionally, the maximum pore diameter of the pore structure is ≦40 nm, and further optionally, the maximum pore diameter of the pore structure is 20 nm to 40 nm.

4. The polyolefin base film according to any one of claims 1 to 3, characterized in that the thickness of the polyolefin base film is from 2 μm to 6.2 μm, and optionally the thickness of the polyolefin base film is from 4 μm to 6.2 μm.

5. The polyolefin base film according to any one of claims 1 to 4, characterized in that the polyolefin base film contains a polyolefin having a weight average molecular weight Mw of 500,000 or more, and optionally, the polyolefin base film contains a polyolefin having a weight average molecular weight Mw of 500,000 to 1,500,000.

6. 6. The polyolefin base film according to claim 5, wherein the polyolefin has a polydispersity index Mw / Mn≦3.5, and optionally the polyolefin has a polydispersity index Mw / Mn of 1 to 3.

5.

7. 6. The polyolefin-based film of claim 5, wherein the polyolefin is one or two of polyethylene and polypropylene.

8. 8. The polyolefin base film according to claim 1, wherein the polyolefin base film has an air permeability of ≦180 s / 100 cc.

9. The longitudinal stretch ratio of the polyolefin base film is ≧60%, optionally, the longitudinal stretch ratio of the polyolefin base film is ≧100%, and further optionally, the longitudinal stretch ratio of the polyolefin base film is 100% to 120%; The polyolefin base film according to any one of claims 1 to 8, characterized in that the transverse stretch ratio of the polyolefin base film is ≧100%, optionally the transverse stretch ratio of the polyolefin base film is ≧110%, and further optionally the transverse stretch ratio of the polyolefin base film is 110% to 160%.

10. 10. The polyolefin base film according to claim 1, wherein the polyolefin base film has a longitudinal heat shrinkage rate of ≦4% and a transverse heat shrinkage rate of ≦4% at 115°C.

11. The polyolefin base film satisfies the following conditions (1) to (4): (1) The polyolefin base film has a puncture strength of ≧270 gf; (2) The longitudinal tensile strength of the polyolefin base film is ≧2000 kgf / cm 2 and the transverse tensile strength is ≧2000 kgf / cm 2 and (3) The porosity of the polyolefin base film is 25% to 40%; (4) The surface density of the polyolefin base film is 2 to 5 g / m 2 11. The polyolefin base film according to claim 1, wherein the polyolefin base film satisfies at least one of the following conditions:

12. A method for producing a polyolefin base film, comprising the steps of: mixing a polyolefin with a pore-forming agent and forming the mixture into a film sheet; biaxially stretching the film sheet; Removing the pore-forming agent from the film sheet after biaxial stretching to form voids; and performing tensile fixing and heat fixing on the film sheet after forming the voids to obtain the polyolefin base film. The polyolefin base film includes a pore structure, the thickness of the polyolefin base film is ≦7 μm, and the tortuosity of the pore structure is 7-10.

13. The method according to claim 12, characterized in that the weight average molecular weight Mw of the polyolefin is 500,000 or more, and optionally the polyolefin base film contains a polyolefin having a weight average molecular weight Mw of 500,000 to 1,500,000.

14. The method according to claim 12 or 13, characterized in that the occupancy rate of the polyolefin in the mixture is 20 wt % to 30 wt %.

15. The biaxial tension includes longitudinal tension and transverse tension, wherein the longitudinal tension magnification is 5-15 times, the transverse tension magnification is 5-15 times, and the longitudinal tension magnification and the transverse tension magnification are not 5 times at the same time, and optionally, the longitudinal tension magnification is 6-15 times, and the transverse tension magnification is 6-15 times.

16. A separator comprising the polyolefin base film according to any one of claims 1 to 11 or a polyolefin base film produced by the production method according to any one of claims 12 to 15.

17. 17. The separator of claim 16, further comprising a coating disposed on at least one surface of the polyolefin-based film.

18. 18. The separator of claim 17, wherein the coating comprises a filler comprising at least one selected from inorganic particles, organic particles, and organic-inorganic hybrid particles.

19. 19. The separator according to claim 17 or 18, further comprising an adhesive layer, the adhesive layer being disposed on at least a portion of the surface of the coating, the adhesive layer comprising a particulate adhesive, and optionally the particulate adhesive comprising at least one of an acrylate-based monomer homopolymer or copolymer, an acrylic-based monomer homopolymer or copolymer, and a fluorine-containing olefin monomer homopolymer or copolymer.

20. A secondary battery comprising the separator according to any one of claims 16 to 19.

21. A power consuming device comprising the secondary battery of claim 20.

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