Ultra-thin biaxially oriented polypropylene film for composite current collector and manufacturing method of ultra-thin biaxially oriented polypropylene film

Abstract

The invention relates to an ultra-thin biaxially oriented polypropylene film for a composite current collector and a manufacturing method of the ultra-thin biaxially oriented polypropylene film, and belongs to the technical field of current collector films, the film comprises a three-layer composite structure, and the three-layer composite structure is obtained by melt extrusion of a composite melt through a three-layer composite die head; the middle layer is homo-polypropylene, and the surface layer is of a sandwich structure of grafted polypropylene containing polar groups; wherein the film is prepared from the following raw materials in percentage by weight: 80%-95% of homo-polypropylene and 5%-20% of polypropylene random copolymer containing grafted polar groups. Different polar group-containing grafted polypropylene side chain grafted monomers are adopted, so that the influence on physical property test data of a sample is relatively large, and when the polar group-containing grafted polypropylene side chain grafted monomers are maleic acid, the performance of the film sample in various aspects is relatively good; and the addition of the wear-resistant layer has obvious influence on the physical properties of the film, so that the wear-resistant layer can improve the tensile property and the fracture resistance of a film sample.

Description

Technical Field

[0001] The invention belongs to the technical field of current collector films, and in particular relates to an ultra-thin biaxially oriented polypropylene film for a composite current collector and a manufacturing method thereof. Background Art

[0002] In lithium-ion batteries, the current collector is one of the indispensable electrode materials, and its functions include carrying active substances and collecting microcurrents. Traditional current collector materials are mainly metal foils, among which aluminum foil is used as the positive current collector and copper foil is used as the negative current collector. This choice is based on the excellent conductivity of aluminum foil and copper foil, the stability of chemical and electrochemical properties, and their ability to tightly combine with active substances, ensuring that the battery can work efficiently and safely during the charging and discharging process.

[0003] However, the traditional pure metal foil current collector material requires a certain thickness to achieve good physical properties, which invariably increases the weight of the battery, increases the transportation cost of the battery, and reduces the scope of use of the battery. In addition, the physical properties of the traditional pure metal foil current collector material are inherent in the metal itself. When in use, some metal materials do not have good tensile properties, fracture resistance, bending strength and rebound rate. When the battery is in use, it is easily affected by the weather and expands and contracts due to heat, causing the battery to become larger in size, affecting other internal structures, and making the battery unable to continue to reach its expected life. Summary of the invention

[0004] The object of the present invention is to provide an ultra-thin biaxially oriented polypropylene film for a composite current collector and a method for producing the same in order to solve the above problems.

[0005] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0006] The present invention provides an ultra-thin biaxially oriented polypropylene film for a composite current collector, the film comprising a three-layer composite structure, the three-layer composite structure being obtained by melt extrusion of a composite melt through a three-layer composite die head;

[0007] The middle layer is homopolymer polypropylene, and the surface layer is a sandwich structure of grafted polypropylene containing polar groups;

[0008] Wherein, the raw materials for preparing the film include, by weight percentage: 80%-95% homopolymer polypropylene and 5%-20% random copolymer polypropylene containing grafted polar groups.

[0009] As a further optimization scheme of the present invention, the grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 90%-98% or random copolymerization, an ash content of less than 100 ppm, a melt index of 4g / 10min-5g / 10min, a polar group grafting rate of 1%-8%, and diisopropylbenzene peroxide as an initiator.

[0010] As a further optimization scheme of the present invention, the grafted polypropylene side chain grafting monomer containing polar groups is one of maleic acid, methacrylic acid, and hexanediol diacrylate.

[0011] The present invention also provides a process for preparing an ultra-thin biaxially oriented polypropylene film for a composite current collector. The preparation process of the three-layer composite structure comprises the following steps:

[0012] Preparing ingredients, mixing homopolymer polypropylene and random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0013] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 210-280°C, and the extrusion temperature of the surface layer is 190-250°C;

[0014] Casting sheet forming, placing the melt on a cooling roller at a temperature of 10°C-85°C and a casting air knife gas temperature of 25°C-95°C to cool the casting sheet forming, and obtain a thick sheet;

[0015] Biaxial stretching: the thick sheet is longitudinally stretched at 125°C-145°C and a longitudinal stretching ratio of 5.0-8.0 times, and then transversely stretched at 157°C-162°C and a transverse stretching ratio of 6-8 times to obtain a roughened film;

[0016] Corona treatment: 15-20KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 15-40KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure.

[0017] As a further optimization scheme of the present invention, the film also includes a wear-resistant layer on the outside of the three-layer composite structure. The raw materials for preparing the wear-resistant layer include, by weight percentage: 30-45 parts of tetrafluoroethylene, 5-10 parts of tartaric acid, 2-6 parts of pentaerythritol and 1-3 parts of a catalyst.

[0018] As a further optimization scheme of the present invention, the preparation process of the wear-resistant layer is as follows: polytetrafluoroethylene, tartaric acid, pentaerythritol and catalyst are put into a mixer according to a weight-to-mass ratio, the stirring rate is set to 1450-1550rpm, and the reaction is stirred for 60-90min at a temperature of 80-110°C to obtain a mixture, and the mixture is injected into a spraying device, and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer is adjusted to: 0.5-1.5μm, and the spraying time is 3-5s to obtain a thin film.

[0019] As a further optimization scheme of the present invention, the catalyst is sodium isopropoxide.

[0020] The beneficial effects of the present invention are as follows: the different grafted monomers of the grafted polypropylene side chains containing polar groups used in the present invention have a greater impact on the physical property test data of the sample, and it is concluded that when the grafted monomer of the grafted polypropylene side chains containing polar groups is maleic acid, the film sample has better performance in all aspects; and the addition of the wear-resistant layer also has a more obvious impact on the physical properties of the film, and it can be seen that the wear-resistant layer can increase the tensile properties and fracture resistance of the film sample; after metal plating, the film sample is used as a current collector material with a lighter weight. When used in chemical batteries such as lithium and vanadium batteries, the weight of the battery can be reduced, and the transportation and use of the battery are convenient. In addition, the current collector material prepared by the film sample obtained by the method has better bending strength and resilience. When used in batteries, it can effectively increase the strength of the battery surface and improve the use range of the battery. DETAILED DESCRIPTION

[0021] The present application is described in further detail below. It is necessary to point out that the following specific implementation methods are only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Technicians in this field can make some non-essential improvements and adjustments to the present application based on the above application content.

[0022] Material

[0023] Unless otherwise specified, the methods used in the present invention are conventional methods known to those skilled in the art, and the reagents and other materials used are commercially available products unless otherwise specified.

[0024] Example 1

[0025] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 90% or random copolymerization, an ash content of less than 100ppm, a melt index of 4g / 10min, a polar group grafting rate of 2%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is maleic acid);

[0026] Prepare ingredients, mix 80% homopolymer polypropylene and 5% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0027] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 210°C, and the extrusion temperature of the surface layer is 190°C;

[0028] Casting sheet forming, placing the melt on a cooling roller at a temperature of 10°C and a casting air knife gas temperature of 25°C to cool the casting sheet forming, to obtain a thick sheet;

[0029] Biaxial stretching: the thick sheet is longitudinally stretched at 125°C and a longitudinal stretching ratio of 5.0 times, and then transversely stretched at 157°C and a transverse stretching ratio of 6 times to obtain a roughened film;

[0030] Corona treatment: applying 15KV high voltage electricity to one or both sides of the biaxially stretched film, with a discharge power of 15KW, to perform corona treatment on the roughened film to obtain a three-layer composite structure;

[0031] To prepare a wear-resistant layer, 30 parts of polytetrafluoroethylene, 5 parts of tartaric acid, 2 parts of pentaerythritol and 1 part of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1450 rpm, and the mixture was stirred and reacted at 80°C for 60 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 0.5 μm, and the spraying time was 3 seconds to obtain a thin film.

[0032] Example 2

[0033] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 94% or random copolymerization, an ash content of less than 100ppm, a melt index of 5g / 10min, a polar group grafting rate of 5%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is maleic acid);

[0034] Prepare ingredients, mix 90% homopolymer polypropylene and 15% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0035] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 250°C, and the extrusion temperature of the surface layer is 220°C;

[0036] The melt is placed on a cooling roller at a temperature of 60°C and a casting air knife at a gas temperature of 68°C to cool the casting sheet and obtain a thick sheet.

[0037] Biaxial stretching: the thick sheet is longitudinally stretched at 130°C and a longitudinal stretching ratio of 6.0 times, and then transversely stretched at 160°C and a transverse stretching ratio of 6-8 times to obtain a roughened film;

[0038] Corona treatment: 18KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 30KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure;

[0039] To prepare a wear-resistant layer, 40 parts of polytetrafluoroethylene, 8 parts of tartaric acid, 5 parts of pentaerythritol and 2 parts of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1500 rpm, and the mixture was stirred and reacted at 95°C for 80 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 1 μm, and the spraying time was 4 seconds to obtain a thin film.

[0040] Example 3

[0041] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 98% or random copolymerization, an ash content of less than 100ppm, a melt index of 5g / 10min, a polar group grafting rate of 8%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is maleic acid);

[0042] Preparing ingredients, mixing 95% homopolymer polypropylene and 20% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0043] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 280°C, and the extrusion temperature of the surface layer is 250°C;

[0044] Casting sheet forming, placing the melt on a cooling roller at a temperature of 85°C and a casting air knife gas temperature of 95°C to cool the casting sheet forming, to obtain a thick sheet;

[0045] Biaxial stretching: the thick sheet is longitudinally stretched at 145°C and a longitudinal stretching ratio of 8.0 times, and then transversely stretched at 162°C and a transverse stretching ratio of 8 times to obtain a roughened film;

[0046] Corona treatment: applying 20KV high voltage electricity to one or both sides of the biaxially stretched film, with a discharge power of 40KW, to corona treat the roughened film to obtain a three-layer composite structure;

[0047] To prepare a wear-resistant layer, 45 parts of polytetrafluoroethylene, 10 parts of tartaric acid, 6 parts of pentaerythritol and 3 parts of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1550 rpm, and the reaction was stirred at 110°C for 90 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 1.5 μm, and the spraying time was 5 seconds to obtain a thin film.

[0048] Example 4

[0049] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 94% or random copolymerization, an ash content of less than 100ppm, a melt index of 5g / 10min, a polar group grafting rate of 5%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is methacrylic acid);

[0050] Prepare ingredients, mix 90% homopolymer polypropylene and 15% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0051] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 250°C, and the extrusion temperature of the surface layer is 220°C;

[0052] The melt is placed on a cooling roller at a temperature of 60°C and a casting air knife at a gas temperature of 68°C to cool the casting sheet and obtain a thick sheet.

[0053] Biaxial stretching: the thick sheet is longitudinally stretched at 130°C and a longitudinal stretching ratio of 6.0 times, and then transversely stretched at 160°C and a transverse stretching ratio of 6-8 times to obtain a roughened film;

[0054] Corona treatment: 18KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 30KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure;

[0055] To prepare a wear-resistant layer, 40 parts of polytetrafluoroethylene, 8 parts of tartaric acid, 5 parts of pentaerythritol and 2 parts of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1500 rpm, and the mixture was stirred and reacted at 95°C for 80 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 1 μm, and the spraying time was 4 seconds to obtain a thin film.

[0056] Example 5

[0057] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 94% or random copolymerization, an ash content of less than 100ppm, a melt index of 5g / 10min, a polar group grafting rate of 5%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is hexanediol diacrylate);

[0058] Prepare ingredients, mix 90% homopolymer polypropylene and 15% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0059] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 250°C, and the extrusion temperature of the surface layer is 220°C;

[0060] The melt is placed on a cooling roller at a temperature of 60°C and a casting air knife at a gas temperature of 68°C to cool the casting sheet and obtain a thick sheet.

[0061] Biaxial stretching: the thick sheet is longitudinally stretched at 130°C and a longitudinal stretching ratio of 6.0 times, and then transversely stretched at 160°C and a transverse stretching ratio of 6-8 times to obtain a roughened film;

[0062] Corona treatment: 18KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 30KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure;

[0063] To prepare a wear-resistant layer, 40 parts of polytetrafluoroethylene, 8 parts of tartaric acid, 5 parts of pentaerythritol and 2 parts of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1500 rpm, and the mixture was stirred and reacted at 95°C for 80 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 1 μm, and the spraying time was 4 seconds to obtain a thin film.

[0064] Comparative Example 1

[0065] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 94% or random copolymerization, an ash content of less than 100ppm, a melt index of 5g / 10min, a polar group grafting rate of 5%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is maleic acid);

[0066] Prepare ingredients, mix 90% homopolymer polypropylene and 15% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0067] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 250°C, and the extrusion temperature of the surface layer is 220°C;

[0068] The melt is placed on a cooling roller at a temperature of 60°C and a casting air knife at a gas temperature of 68°C to cool the casting sheet and obtain a thick sheet.

[0069] Biaxial stretching: the thick sheet is longitudinally stretched at 130°C and a longitudinal stretching ratio of 6.0 times, and then transversely stretched at 160°C and a transverse stretching ratio of 6-8 times to obtain a roughened film;

[0070] Corona treatment: 18KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 30KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure;

[0071] To prepare a wear-resistant layer, 40 parts of polytetrafluoroethylene, 13 parts of pentaerythritol and 2 parts of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1500 rpm, and the mixture was stirred and reacted at 95°C for 80 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 1 μm, and the spraying time was 4 seconds to obtain a thin film.

[0072] Comparative Example 2

[0073] The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 94% or random copolymerization, an ash content of less than 100ppm, a melt index of 5g / 10min, a polar group grafting rate of 5%, and diisopropylbenzene peroxide as an initiator (the side chain grafting monomer of the grafted polypropylene containing polar groups is maleic acid);

[0074] Prepare ingredients, mix 90% homopolymer polypropylene and 15% random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material;

[0075] Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 250°C, and the extrusion temperature of the surface layer is 220°C;

[0076] The melt is placed on a cooling roller at a temperature of 60°C and a casting air knife at a gas temperature of 68°C to cool the casting sheet and obtain a thick sheet.

[0077] Biaxial stretching: the thick sheet is longitudinally stretched at 130°C and a longitudinal stretching ratio of 6.0 times, and then transversely stretched at 160°C and a transverse stretching ratio of 6-8 times to obtain a roughened film;

[0078] Corona treatment: 18KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 30KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure;

[0079] To prepare a wear-resistant layer, 40 parts of polytetrafluoroethylene, 13 parts of tartaric acid and 2 parts of sodium isopropoxide were put into a mixer according to a weight-to-mass ratio, the stirring rate was set to 1500 rpm, and the reaction was stirred at 95°C for 80 minutes to obtain a mixture, which was injected into a spraying device and sprayed on the front and back sides of the three-layer composite structure. The diameter of the water mist sprayed by the sprayer was adjusted to 1 μm, and the spraying time was 4 seconds to obtain a thin film.

[0080] Performance Testing

[0081] 1.1. Observe the cross-section of the thin film samples prepared by the methods of Examples 1-5 and Comparative Examples 1-2 using an electron microscope to determine their thickness;

[0082] 1.2. Using a tensile testing machine, the film samples prepared by the methods of Examples 1-5 and Comparative Examples 1-2 were subjected to tensile property testing;

[0083] 1.3. The film samples prepared by the methods of Examples 1-5 and Comparative Examples 1-2 were tested for fracture resistance performance using an impact testing machine.

[0084] The test results are shown in the following table

[0085]

[0086]

[0087] The above test results show that among the film samples prepared by the methods of Examples 1-5 and Comparative Examples 1-2, the film sample obtained by Example 2 has better tensile properties and fracture resistance performance and has better physical properties;

[0088] Among them, by comparing Examples 1-5, it can be seen that the different grafted monomers containing polar groups on the grafted polypropylene side chains used in the scheme have a greater impact on the physical property test data of the samples. It is concluded that when the grafted monomer containing polar groups on the grafted polypropylene side chains is maleic acid, the performance of the film sample in all aspects is better;

[0089] By comparing Example 2 with Comparative Examples 1-2, it can be found that the addition of the wear-resistant layer also has a significant effect on the physical properties of the film, and it can be seen that the wear-resistant layer can increase the tensile properties and fracture resistance of the film sample.

[0090] 2.1. The film sample of the present invention is used as a support for metal electroplating. The film is subjected to single-sided or double-sided metal electroplating. The finished product can be used as a current collector material. The finished product and the traditional pure metal foil current collector material (recorded as the blank group) are weighed, tested for bending strength and tested for rebound, and the bending resistance is recorded.

[0091] The experimental conditions are as follows:

[0092] The thin film samples prepared by the methods of Example 2 and Comparative Examples 1-2 are used for single-sided or double-sided metal plating, and any metal such as aluminum, copper, nickel, silver, gold, etc. is plated on both sides of the film;

[0093] When single-sided metal plating is performed, a one-step metal plating method is used, and a metal layer with a thickness of 1000 nanometers on a single side is formed to complete a single-sided metal plating product;

[0094] When double-sided metal plating is performed, a multi-step metal plating method is adopted. First, an 80-nanometer metal layer is plated on each side of the film, and then the metal layer is thickened to 1000 nanometers through electroplating to complete the double-sided metal plating product.

[0095]

[0096] It can be seen from the above test results that the thin film sample obtained by the method of Example 2 of the present invention, after metal plating, is used as a current collector material and has a lighter weight. When used in chemical batteries such as lithium and vanadium batteries, the weight of the battery can be reduced, which is convenient for transportation and use of the battery. In addition, the current collector material prepared by the thin film sample obtained by this method has good bending strength and rebound rate. When used in batteries, it can effectively increase the strength of the battery surface and improve the range of use of the battery.

[0097] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. An ultra-thin biaxially oriented polypropylene film for a composite current collector, characterized in that: The film comprises a three-layer composite structure, and the three-layer composite structure is obtained by melt extrusion of a composite melt through a three-layer composite die head; The middle layer is homopolymer polypropylene, and the surface layer is a sandwich structure of grafted polypropylene containing polar groups; Wherein, the raw materials for preparing the film include, by weight percentage: 80%-95% homopolymer polypropylene and 5%-20% random copolymer polypropylene containing grafted polar groups.

2. The ultra-thin biaxially oriented polypropylene film for a composite current collector according to claim 1, characterized in that: The grafted polypropylene containing polar groups is a grafted polypropylene resin material with an isotacticity of 90%-98% or random copolymerization, an ash content of less than 100ppm, a melt index of 4g / 10min-5g / 10min, a polar group grafting rate of 1%-8%, and dicumyl peroxide as an initiator.

3. The ultra-thin biaxially oriented polypropylene film for a composite current collector according to claim 2, characterized in that: The grafted polypropylene side chain grafting monomer containing polar groups is one of maleic acid, methacrylic acid and hexanediol diacrylate.

4. A method for preparing an ultra-thin biaxially oriented polypropylene film for a composite current collector according to any one of claims 1 to 3, characterized in that: The preparation process of the three-layer composite structure comprises the following steps: Preparing ingredients, mixing homopolymer polypropylene and random copolymer polypropylene containing grafted polar groups according to weight percentage to obtain a prefabricated material; Melt extrusion, the middle layer and the surface layer are simultaneously melted by the main and auxiliary double extruders and then extruded through the auxiliary-main-auxiliary three-layer composite die to form a melt, wherein the extrusion temperature of the middle layer is 210-280°C, and the extrusion temperature of the surface layer is 190-250°C; Casting sheet forming, placing the melt on a cooling roller at a temperature of 10°C-85°C and a casting air knife gas temperature of 25°C-95°C to cool the casting sheet forming, and obtain a thick sheet; Biaxial stretching: the thick sheet is longitudinally stretched at 125°C-145°C and a longitudinal stretching ratio of 5.0-8.0 times, and then transversely stretched at 157°C-162°C and a transverse stretching ratio of 6-8 times to obtain a roughened film; Corona treatment: 15-20KV high voltage electricity is applied to one or both sides of the biaxially stretched film, and the discharge power is 15-40KW high voltage discharge, and the roughened film is corona treated to obtain a three-layer composite structure.

5. The method for preparing an ultra-thin biaxially oriented polypropylene film for a composite current collector according to claim 4, characterized in that: The film also includes a wear-resistant layer on the outside of the three-layer composite structure. The raw materials for preparing the wear-resistant layer include, by weight percentage: 30-45 parts of tetrafluoroethylene, 5-10 parts of tartaric acid, 2-6 parts of pentaerythritol and 1-3 parts of a catalyst.

6. The method for preparing an ultra-thin biaxially oriented polypropylene film for a composite current collector according to claim 5, characterized in that: The preparation process of the wear-resistant layer is as follows: polytetrafluoroethylene, tartaric acid, pentaerythritol and a catalyst are put into a stirrer according to a weight-to-mass ratio, the stirring rate is set to 1450-1550rpm, and the mixture is stirred and reacted for 60-90min at a temperature of 80-110°C to obtain a mixture, and the mixture is injected into a spraying device, and sprayed on the front and back sides of the three-layer composite structure, the diameter of the water mist sprayed by the sprayer is adjusted to: 0.5-1.5μm, and the spraying time is 3-5s to obtain a thin film.

7. The method for preparing an ultra-thin biaxially oriented polypropylene film for a composite current collector according to claim 6, characterized in that: The catalyst is sodium isopropoxide.

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