A polymer film, its preparation method and use

Abstract

The application provides a polymer film and a preparation method and application thereof. The polymer film comprises a polymer and a whisker material in a mass percentage of (95.0-99.5):(0.5-5). In order to solve the problems of low tensile strength and poor thermal conductivity of the conventional polymer base film of the composite current collector in the prior art, the application provides a polymer film with high tensile strength and enhanced thermal conductivity, and a composite current collector is prepared by taking the polymer film as a base film, so that the problems of low yield rate caused by low tensile strength, hole defects and poor conductivity caused by poor thermal conductivity can be solved, and a composite current collector with high strength can be prepared, thereby promoting further popularization and application of the composite current collector.

Description

Technical Field

[0001] This invention belongs to the field of current collector materials technology, specifically relating to a polymer membrane, its preparation method, and its application. Background Technology

[0002] Currently, composite current collectors based on polymer films are receiving widespread attention and application in the new energy industry. Existing composite current collectors are typically prepared by physical vapor deposition (PVD) to deposit a layer of metal (aluminum, copper, etc.) on a polymer film, ultimately resulting in a surface-metallized thin film with a certain level of conductivity – the composite current collector. Compared to traditional current collectors, polymer film-based composite current collectors offer advantages such as lower cost, lighter weight, and better internal insulation. These characteristics enable composite current collectors to reduce battery costs and improve battery energy density and safety when used in batteries.

[0003] Currently, traditional polymer films used to prepare composite current collectors have the following problems: ① Low tensile strength, which makes the film prone to breakage during the PVD preparation of composite current collectors, resulting in a low product yield. Furthermore, the tensile strength of the prepared composite current collector is reduced, which is not conducive to the subsequent coating and rolling processes in the battery preparation process; ② Poor thermal conductivity. During the PVD preparation of composite current collectors using this as the base film, the heat generated by atomic deposition cannot be quickly conducted to the cooling surface of the main roller, that is, rapid heat exchange between the polymer film and the main roller cannot be achieved. This causes pore defects in the prepared composite current collector, ultimately resulting in poor conductivity of the prepared composite current collector.

[0004] Therefore, there is an urgent need in this field to develop a composite current collector to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a polymer film, its preparation method, and its applications. To solve the problems of low tensile strength and poor thermal conductivity in traditional polymer-based films used in composite current collectors, the present invention provides a polymer film that combines high tensile strength and enhanced thermal conductivity. Using this polymer film as a base film to prepare composite current collectors solves the problems of low yield due to low tensile strength, and porosity and poor electrical conductivity due to poor thermal conductivity. Furthermore, it enables the preparation of high-strength composite current collectors, thereby promoting the further popularization and application of composite current collectors.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a polymer film comprising a polymer and whisker material in a mass percentage ratio of (95.0-99.5):(0.5-5).

[0008] This invention introduces thermally conductive whisker materials during the polymer film preparation process. The introduction of whisker materials offers two main advantages: Firstly, due to their nano-linear structure, whiskers can orient themselves along the stretching direction with the polymer during the stretching process, resulting in a regular arrangement and the formation of a network structure with the polymer. This enhances the tensile strength of the polymer film. Furthermore, whisker materials act as nucleating agents for polymer molecules, promoting the crystallinity of the polymer during film formation and further increasing the tensile strength of the polymer film. Secondly, whiskers possess high thermal conductivity and are regularly arranged within the polymer film, thus improving the thermal conductivity of the prepared polymer film. This invention utilizes a polymer film with both good tensile strength and thermal conductivity as the base film to prepare composite current collectors with high yield, low defect rate, and strong electrical conductivity.

[0009] In this invention, the polymer film comprises a polymer and whisker material in a mass percentage ratio of (95.0-99.5):(0.5-5), such as 95:5, 95.5:4.5, 96:4, 96.5:3.5, 97:3, 97.5:2.5, 98:2, 98.5:1.5, 99:1, 99.5:0.5, etc.

[0010] Preferably, the polymer film comprises a polymer and whisker material in a mass percentage ratio of (97.0-99.0):(1-3), such as 97:3, 97.5:2.5, 97.8:2.2, 98:2, 98.5:1.5, 99:1, etc.

[0011] In this invention, by controlling the mass percentage of polymer and whisker materials, the prepared polymer film achieves good overall performance. Too low a mass percentage will result in poor film formation and poor performance of the prepared polymer film, while too high a mass percentage will result in no significant improvement in the tensile strength and thermal conductivity of the prepared polymer film.

[0012] Preferably, the polymer comprises any one or a combination of at least two of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene, polypropylene, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, polyphenylene sulfide, polyphenylene ether, polystyrene, or polyimide, and more preferably any one or a combination of at least two of polypropylene, polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

[0013] Preferably, the weight-average molecular weight of the polymer is 20,000 to 100,000, for example, it can be 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, etc.

[0014] In this invention, by controlling the weight-average molecular weight of the polymer, the prepared polymer film achieves good overall performance. If the weight-average molecular weight is too low, the tensile strength of the prepared polymer film will be low, and vice versa, the film-forming effect will be poor, resulting in poor performance of the prepared polymer film.

[0015] Preferably, the whisker material includes any one or a combination of at least two of the following: metal whisker material, non-metal whisker material, metal nitride whisker material, non-metal nitride whisker material, metal oxide whisker material, or non-metal carbide whisker material. More preferably, it is a combination of metal whisker material and non-metal nitride whisker material, or a combination of metal whisker material and metal nitride whisker material.

[0016] In this invention, during the preparation of a composite current collector by depositing a metal layer on the polymer surface, the aforementioned whisker material provides nucleation sites for the deposition of metal grains, promoting uniform deposition of the metal grains and improving the tensile strength of the prepared composite current collector. Furthermore, the synergistic interfacial interaction between the metal and nitride enhances the dispersion uniformity of the whiskers, promoting the formation of a uniformly dispersed network structure within the polymer film, thereby increasing the tensile strength of the polymer film and further promoting surface metal deposition, thus enhancing the tensile strength of the prepared composite current collector. Simultaneously, the combination of metal and nitride provides more conductive channels and also improves the high-temperature resistance and thermal conductivity of the polymer film.

[0017] Preferably, the mass ratio of the metal whisker material to the non-metal nitride whisker material is (0.5-2):1, for example, it can be 0.5:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, etc.

[0018] Preferably, the mass ratio of the metal whisker material to the metal nitride whisker material is (0.5-2):1, for example, it can be 0.5:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, etc.

[0019] In this invention, by adjusting the mass ratio of metal whisker material and nitride whisker material, the polymer film can have good conductivity. If the mass ratio is too high, the distance between the particles may become smaller, resulting in enhanced interaction between the particles, which in turn increases the resistivity of the polymer film and reduces its conductivity. If the mass ratio is too low, the synergistic effect between the two will not be fully utilized, resulting in insufficient conductivity of the film.

[0020] Preferably, the metal whisker material includes any one or a combination of at least two of Cu whiskers, Al whiskers, Ni whiskers, Zn whiskers, Fe whiskers or Ag whiskers, and is preferably Cu whiskers and / or Al whiskers.

[0021] Preferably, the non-metallic whisker material includes Si whiskers.

[0022] Preferably, the metal nitride whisker material includes AlN whiskers.

[0023] Preferably, the non-metallic nitride whisker material includes Si3N4 whiskers and / or BN whiskers.

[0024] Preferably, the metal oxide whisker material includes ZnO whiskers.

[0025] Preferably, the non-metallic carbide whisker material includes SiC whiskers.

[0026] Preferably, the diameter of the whisker material is 10-100nm, for example, it can be 10nm, 15nm, 20nm, 40nm, 50nm, 60nm, 80nm, 100nm, etc.; and the length is 50-500nm, for example, it can be 50nm, 80nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, etc.

[0027] In this invention, by controlling the diameter and length of the whisker material, the prepared polymer film achieves high tensile strength and thermal conductivity.

[0028] Preferably, the diameter of the whisker material is 10-60nm, for example, it can be 10nm, 15nm, 20nm, 40nm, 50nm, 60nm, etc.; the length is 80-400nm, for example, it can be 80nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, etc.

[0029] In a second aspect, the present invention provides a method for preparing a polymer film according to the first aspect, the method comprising the following steps:

[0030] The polymer and whisker material are melt-extruded and then biaxially stretched to obtain the polymer film.

[0031] Preferably, the temperature of the melt extrusion is 200℃-400℃, for example, 200℃, 250℃, 300℃, 350℃, 400℃, etc.

[0032] Preferably, the biaxial stretching pressure has a longitudinal stretching ratio of (3-7):1, such as 3:1, 4:1, 5:1, 6:1, 7:1, etc.; and a transverse stretching ratio of (2-4):1, such as 2:1, 3:1, 4:1, etc.

[0033] Thirdly, the present invention provides a composite current collector, the composite current collector comprising a support layer and a metal layer and a protective layer sequentially stacked on both sides of the support layer, the support layer comprising the polymer film according to the first aspect.

[0034] Preferably, the thickness of the support layer is 2-10 μm, for example, it can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0035] Preferably, the material of the metal layer includes any one or a combination of at least two of copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, or silver.

[0036] Preferably, the thickness of the metal layer is 500-2000nm, more preferably 700-1200nm, and for example, it can be 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1500nm, 1600nm, 1700nm, 1800nm, 2000nm, etc.

[0037] Preferably, the material of the protective layer includes any one or a combination of at least two of the following: nickel, chromium, nickel-based alloys, copper-based alloys, copper oxide, aluminum oxide, nickel oxide, chromium oxide, cobalt oxide, copper chromium oxide, graphite, carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon nanofibers, or graphene.

[0038] Preferably, the thickness of the protective layer is 10-100nm, more preferably 20-80nm, and can be, for example, 10nm, 15nm, 20nm, 40nm, 50nm, 70nm, 80nm, 100nm, etc.

[0039] Fourthly, the present invention provides a secondary battery comprising an electrode, an electrolyte, and a separator, wherein the electrode comprises a current collector and an active material layer disposed on at least one side of the current collector, and the current collector comprises a composite current collector according to the third aspect.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] This invention provides a polymer film by introducing a thermally conductive whisker material during the polymer film preparation process. The introduction of the whisker material offers two advantages: Firstly, due to their nano-linear structure, the whiskers can orient themselves along the stretching direction with the polymer during the stretching process, resulting in a regular arrangement and the formation of a network structure with the polymer. This enhances the tensile strength of the polymer film. Furthermore, the whisker material acts as a nucleating agent for polymer molecules, promoting the crystallinity of the polymer during film formation and further increasing the tensile strength of the polymer film. Secondly, the whiskers possess high thermal conductivity and are regularly arranged within the polymer film, thus improving the thermal conductivity of the prepared polymer film. This invention utilizes a polymer film with both good tensile strength and thermal conductivity as the base film to prepare composite current collectors with high yield, low defect rate, and strong electrical conductivity. Detailed Implementation

[0042] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0043] Example 1

[0044] This embodiment provides a polymer film comprising polypropylene and Cu whisker materials in a mass percentage ratio of 99.5:0.5; wherein the weight-average molecular weight of the polypropylene is 80,000; and the diameter of the Cu whiskers is 10 nm and the length is 50 nm.

[0045] This embodiment also provides a method for preparing the above-mentioned polymer film, which includes the following steps:

[0046] Melt extrusion: The above raw materials are added to the corresponding twin-screw extruder. The mass percentages of polypropylene and Cu whiskers in the raw materials are 99.5% and 0.5%, respectively. The mixture is melted at 250°C, filtered through a filter (10-micron filter screen), and then extruded through a die at a die temperature of 260°C.

[0047] Casting: The molten polypropylene material extruded from the die is cast onto the casting roller, and then shaped by the casting roller and water cooling treatment at a temperature of 30°C.

[0048] Biaxial stretching: Longitudinal stretching: Preheating temperature 120℃; stretching temperature 145℃; longitudinal stretching ratio 7:1, then cool to room temperature; Transverse stretching: Preheating temperature 140℃; stretching temperature 150℃; stretching ratio 3:1.

[0049] Heat treatment: The heat treatment temperature is 110℃ and the treatment time is 2s.

[0050] Winding: After heat treatment, the membrane is cooled by air in the platform area and then enters the winding system through the traction system to wind the membrane. The winding tension is 30 N / m, and a polypropylene membrane with a thickness of 4.5 μm is prepared.

[0051] This embodiment also provides a composite current collector and its preparation method, which includes the following steps:

[0052] The polymer film prepared above was placed in a magnetron sputtering chamber. Using 99.99% pure copper metal as the target and argon gas as the gas source, a 70nm thick metal layer was deposited on the composite film prepared in the previous step using magnetron sputtering. Then, the composite film was used as a substrate for electroplating to thicken the conductive copper layer. The electroplating process consisted of three steps: ① Electroplating to thicken the metal layer: The electroplating solution consisted of 120g / L copper sulfate, 100g / L sulfuric acid, 50mg / L chloride ions, 2.0mg / L sodium 3-mercapto-1-propanesulfonate, 0.8mg / L 2-mercaptopyridine, and 180mg / L polyethylene glycol 8000. The electroplating solution temperature was 25℃, and the average cathode current density was 2A / dm³. 2 ① Electroplating treatment for 5 minutes. ② Protective layer preparation: After electroplating, the plated film is cleaned in a water bath, and then a surface protective layer is prepared in a protective layer preparation tank containing a 5 g / L potassium dichromate aqueous solution at 25℃ for 25 seconds. Finally, it is cleaned again in a water bath. ③ Drying: The cleaned film is dried in an oven at 65℃ to obtain a composite copper current collector with a total thickness of 6.9 μm.

[0053] Example 2

[0054] The difference between this embodiment and Embodiment 1 is that the mass percentages of polypropylene and Cu whisker material in the raw materials are 98.0% and 2.0%, respectively, while all other aspects are the same as in Embodiment 1.

[0055] Example 3

[0056] The difference between this embodiment and Embodiment 1 is that the mass percentages of polypropylene and Cu whisker material in the raw materials are 95.0% and 5.0%, respectively, while all other aspects are the same as in Embodiment 1.

[0057] Example 4

[0058] The difference between this embodiment and Embodiment 1 is that the length of the Cu whisker material is 200 nm, while all other aspects are the same as in Embodiment 1.

[0059] Example 5

[0060] The difference between this embodiment and Embodiment 1 is that the length of the Cu whisker material is 500 nm, while all other aspects are the same as in Embodiment 1.

[0061] Example 6

[0062] The difference between this embodiment and Embodiment 1 is that the diameter of the Cu whisker material is 50 nm, while all other aspects are the same as in Embodiment 1.

[0063] Example 7

[0064] The difference between this embodiment and Embodiment 1 is that the diameter of the Cu whisker material is 100 nm, while all other aspects are the same as in Embodiment 1.

[0065] Example 8

[0066] The difference between this embodiment and Example 1 is that the polypropylene in the raw materials is replaced with polyethylene terephthalate (PET) (purchased from Sinopec Yizheng Chemical Fiber Co., Ltd., grade FG611). The preparation process is as follows: ① Raw materials: The selected raw materials are polyethylene terephthalate (PET) and Cu whisker material, wherein the weight average molecular weight of PET is 30,000, and the diameter of Cu whisker material is 10 nm and the length is 50 nm. ② Melt extrusion: The above raw materials are added to the corresponding twin-screw extruders, with the mass percentages of PET and Cu whisker material being 99.5% and 0.5%, respectively. The mixture is melted at 280°C, filtered through a filter (10-micron filter screen), and then extruded through a die at a die temperature of 280°C. ③ Casting: The molten material extruded from the die is cast onto a casting roller, and then shaped by cooling with water at a temperature of 25°C. ④ Longitudinal stretching: Preheating temperature is 90℃, stretching temperature is 110℃, longitudinal stretching ratio is 3.5:1, and the film is cooled to room temperature after longitudinal stretching. ⑤ Transverse stretching: Preheating temperature is 90℃, stretching temperature is 120℃, and transverse stretching ratio is 3:1. ⑥ Heat treatment: Heat treatment temperature is 130℃. ⑦ Winding: After heat treatment, the film is cooled by air in the platform area and then entered the winding system through the traction system for film winding. The winding tension is 30N / m, resulting in a PET film with a thickness of 4.5μm. All other steps are the same as in Example 1.

[0067] Example 9

[0068] The difference between this embodiment and Example 8 is that PET in the raw materials is replaced with polyethylene naphthalate (purchased from Teijin Corporation, Japan, brand name TN-8050SC), while all other aspects are the same as in Example 8.

[0069] Example 10

[0070] The difference between this embodiment and Embodiment 1 is that polypropylene is replaced with polytetrafluoroethylene (PTFE) in the raw materials. The preparation process is as follows: ① Raw materials: The selected raw materials are PTFE (DuPont 601A) and Cu whisker material, wherein the diameter of the Cu whisker material is 10 nm and the length is 50 nm. ② Melt extrusion: The above raw materials are added to the corresponding twin-screw extruders, with the mass percentages of PTFE and Cu whisker material being 99.5% and 0.5%, respectively. The mixture is melted at 350°C, filtered through a filter (10-micron filter screen), and then extruded through a die at a die temperature of 350°C. ③ Casting: The molten material extruded from the die is cast onto a casting roller, and then shaped by cooling with water at a temperature of 30°C. ④ Longitudinal stretching: Preheating temperature is 280℃, stretching temperature is 300℃, longitudinal stretching ratio is 4:1, and the film is cooled to room temperature after longitudinal stretching. ⑤ Transverse stretching: Preheating temperature is 280℃, stretching temperature is 310℃, and transverse stretching ratio is 3:1. ⑥ Heat treatment: Heat treatment temperature is 200℃. ⑦ Winding: After heat treatment, the film is cooled by air in the platform area and then entered the winding system through the traction system for film winding. The winding tension is 30 N / m, resulting in a PTFE film with a thickness of 4.5 μm. All other steps are the same as in Example 1.

[0071] Example 11

[0072] The difference between this embodiment and Embodiment 1 is that Cu whisker material in the raw materials is replaced with Al whisker material, while all other aspects are the same as in Embodiment 1.

[0073] Example 12

[0074] The difference between this embodiment and Embodiment 1 is that Cu whisker material is replaced with BN whisker material in the raw materials, while everything else is the same as in Embodiment 1.

[0075] Example 13

[0076] The difference between this embodiment and Embodiment 1 is that Cu whisker material in the raw materials is replaced with AlN whisker material, while all other aspects are the same as in Embodiment 1.

[0077] Example 14

[0078] The difference between this embodiment and Embodiment 1 is that the Cu whisker material in the raw materials is replaced with a mixture of Cu whisker material and AlN whisker material in a ratio of 1:1. All other aspects are the same as in Embodiment 1.

[0079] Example 15

[0080] The difference between this embodiment and Embodiment 1 is that the Cu whisker material in the raw materials is replaced with a mixture of Cu whisker material and BN whisker material in a ratio of 1:1. All other aspects are the same as in Embodiment 1.

[0081] Comparative Example 1

[0082] The difference between this comparative example and Example 1 is that the raw materials contain only polypropylene, while everything else is the same as in Example 1.

[0083] Comparative Example 2

[0084] The difference between this comparative example and Example 1 is that the content of Cu whisker material in the raw materials is 0.3%, while all other aspects are the same as in Example 1.

[0085] Comparative Example 3

[0086] The difference between this comparative example and Example 1 is that the content of Cu whisker material in the raw materials is 5.5%, while all other aspects are the same as in Example 1.

[0087] Test conditions

[0088] The polymer films and composite current collectors provided in Examples 1 to 15 and Comparative Examples 1 to 3 were tested using the following methods:

[0089] Polymer film:

[0090] ① Thermal conductivity: The thermal conductivity of the polymer film prepared above was tested according to standard ISO 22007-3:2008.

[0091] ②Tensile strength: The tensile strength of the polymer film prepared above was tested according to standard GB / T 1040.3-2006.

[0092] Composite current collector:

[0093] ① Defect rate caused by film breakage during preparation: The proportion of defective products caused by film breakage during preparation to the total number of products. Since the width is consistent, the quantity is calculated by length.

[0094] ② Number of pore defects per unit area of ​​composite current collector: The prepared composite current collector sample is placed in a surface quality detection system (micro-vision charge-coupled device CCD) to scan its surface. Then, the optical signal is converted into an electrical signal and transmitted to a computer to count the number of pore defects per unit area of ​​composite current collector with a pore diameter of less than 100μm (generally, the finished product should not have pores larger than 100μm).

[0095] ③ Sheet resistance: The prepared composite current collector sample is placed on the sample stage, and the sheet resistance of the sample is tested using a four-probe sheet resistance meter.

[0096] The test results are shown in Table 1:

[0097] Table 1

[0098]

[0099] As can be seen from Table 1, the following conclusions can be drawn:

[0100] (1) As can be seen from Examples 1-3 and Comparative Examples 1-3, when an appropriate amount of Cu whiskers and a suitable amount of polymer material are added during the preparation of the polymer film, the prepared polymer film has the advantages of both high tensile strength and high thermal conductivity. This leads to a reduction in the defect rate caused by the breakage of the base film during the preparation of the composite current collector using this polymer film as the base film, an increase in the tensile strength of the prepared composite current collector, and a reduction in the number of pores. At the same time, due to the reduction in the number of pores, the metal layer becomes denser, resulting in a reduction in its sheet resistance. Specifically, as the content of Cu whiskers in the polymer film continuously increases, the tensile strength of the prepared polymer film shows a trend of first increasing and then decreasing, resulting in a trend of first decreasing and then increasing the defect rate caused by the breakage of the polymer base film during the preparation of the composite current collector. Correspondingly, the tensile strength of the prepared composite current collector shows a trend of first increasing and then decreasing. The thermal conductivity of the prepared polymer film shows a trend of continuously increasing. Correspondingly, the number of pores in the preparation of the composite current collector first decreases to 0 and then remains unchanged, and the sheet resistance first decreases and then remains unchanged. When the Cu whisker content exceeds the maximum value within the preferred range, it leads to poor film-forming properties of the polymer, resulting in defects in the biaxially stretched polymer film and consequently reducing the tensile strength and thermal conductivity of the polymer film. Furthermore, the defect rate caused by film breakage in this invention refers to the defect rate occurring during polymer film breakage during preparation, while sheet resistance characterizes the conductivity of the composite current collector's metal layer and is related to the number of pores in the composite current collector. Although the thermal conductivity of the polymer film prepared in Comparative Example 3 decreased, it remained relatively high, and no pore defects were generated during the preparation of the composite current collector, so the sheet resistance did not change.

[0101] (2) As can be seen from Examples 1, 4 and 5, as the length of Cu whiskers in the polymer film increases, the tensile strength of the prepared polymer film shows a trend of first increasing and then decreasing. This leads to the defects caused by the base film breakage during the preparation of the corresponding composite current collector first decreasing and then increasing, and the tensile strength of the corresponding prepared composite current collector shows a trend of first increasing and then decreasing. The thermal conductivity of the prepared polymer film continues to increase, which leads to a decrease in the number of pore defects and a decrease in sheet resistance of the corresponding prepared composite current collector.

[0102] (3) As can be seen from Examples 1, 6 and 7, as the diameter of Cu whiskers in the polymer film increases, the tensile strength of the prepared polymer film shows a trend of first increasing and then decreasing. This leads to the defects caused by the base film breakage during the preparation of the corresponding composite current collector first decreasing and then increasing, and the tensile strength of the corresponding prepared composite current collector shows a trend of first increasing and then decreasing. The thermal conductivity of the prepared polymer film continues to increase, which leads to a decrease in the number of pore defects and a decrease in sheet resistance of the corresponding prepared composite current collector.

[0103] (4) As can be seen from Examples 1 and 8-10, replacing the polypropylene in the polymer film raw material with other preferred polymer types can still achieve good overall results.

[0104] (5) As can be seen from Examples 1 and 11-15, replacing the Cu whiskers in the polymer film raw materials with other whisker materials still yields good overall results. In particular, the combination of Cu whiskers with AlN whiskers (i.e., the combination of metal whisker materials and metal nitride whisker materials) and Cu whiskers with BN whiskers (i.e., the combination of metal whisker materials and non-metal nitride whiskers) can significantly improve the tensile strength of the prepared polymer films. Among them, the polymer materials used in Examples 12 and 13 are both polypropylene, only the whisker materials are different. The reason for the large difference in tensile strength between the two is mainly that AlN whisker materials have a better crystallization promoting effect on polypropylene than BN whisker materials, resulting in a higher crystallinity of the polypropylene film prepared in Example 13, thereby improving the tensile strength of the polypropylene film.

[0105] (6) As can be seen from Examples 1, 14, and 15, when the polymer film contains a combination of one or more whisker materials, such as metal whisker materials, metal nitride whisker materials, or non-metal nitride whisker materials, the tensile strength of the composite current collector prepared using this polymer film as the base film is relatively high. This is due to two reasons. On the one hand, when metal whisker materials, metal nitride whisker materials, or non-metal nitride whisker materials are introduced into the polymer film, the interfacial interaction between the metal and the nitride can enhance the dispersion uniformity of the whisker particles and promote the formation of a uniformly dispersed network structure within the polymer film, thereby increasing the tensile strength of the polymer film and consequently increasing the tensile strength of the composite current collector based on this polymer film. On the other hand, during the process of depositing a metal layer on the polymer surface to prepare the composite current collector, the aforementioned whisker materials can provide more nucleation sites for the deposition of metal grains, promoting the uniform deposition of metal grains and thus improving the tensile strength of the prepared composite current collector.

[0106] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A polymer film, characterized in that, The polymer film comprises a polymer and whisker material in a mass percentage ratio of (97.0-99.0):(1-3); The whisker material includes a combination of metal whisker material and non-metal nitride whisker material, or a combination of metal whisker material and metal nitride whisker material; The metal whisker material includes Cu whiskers and / or Al whiskers; The metal nitride whisker material includes AlN whiskers; The non-metallic nitride whisker material includes BN whiskers; The mass ratio of the metal whisker material to the non-metal nitride whisker material is (0.5-2):1; The mass ratio of the metal whisker material to the metal nitride whisker material is (0.5-2):

1.

2. The polymer film according to claim 1, characterized in that, The polymer includes any one or a combination of at least two of the following: polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene, polypropylene, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, polyphenylene sulfide, polyphenylene ether, polystyrene, or polyimide.

3. The polymer film according to claim 2, characterized in that, The polymer includes any one or a combination of at least two of polypropylene, polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

4. The polymer film according to claim 1, characterized in that, The polymer has a weight-average molecular weight of 20,000 to 100,000.

5. The polymer film according to claim 1, characterized in that, The metal nitride whisker material includes AlN whiskers.

6. The polymer film according to claim 1, characterized in that, The whisker material has a diameter of 10-100 nm and a length of 50-500 nm.

7. The polymer film according to claim 6, characterized in that, The whisker material has a diameter of 10-60 nm and a length of 80-400 nm.

8. A method for preparing a polymer film according to any one of claims 1-7, characterized in that, The method includes the following steps: The polymer and whisker material are melt-extruded and then biaxially stretched to obtain the polymer film.

9. The method according to claim 8, characterized in that, The temperature of the melt extrusion is 200℃-400℃.

10. The method according to claim 8, characterized in that, In the biaxial stretching, the longitudinal stretching ratio is (3-7):1, and the transverse stretching ratio is (2-4):

1.

11. A composite current collector, characterized in that, The composite current collector includes a support layer and a metal layer and a protective layer sequentially stacked on both sides of the support layer, wherein the support layer includes a polymer film according to any one of claims 1-7.

12. The composite current collector according to claim 11, characterized in that, The thickness of the support layer is 2-10 μm.

13. The composite current collector according to claim 11, characterized in that, The material of the metal layer includes any one or a combination of at least two of copper, copper alloys, aluminum, aluminum alloys, nickel, nickel alloys, titanium, or silver.

14. The composite current collector according to claim 11, characterized in that, The thickness of the metal layer is 500-2000 nm.

15. The composite current collector according to claim 14, characterized in that, The thickness of the metal layer is 700-1200 nm.

16. The composite current collector according to claim 11, characterized in that, The material of the protective layer includes any one or a combination of at least two of the following: nickel, chromium, nickel-based alloys, copper-based alloys, copper oxide, aluminum oxide, nickel oxide, chromium oxide, cobalt oxide, copper chromium oxide, graphite, carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon nanofibers, or graphene.

17. The composite current collector according to claim 11, characterized in that, The thickness of the protective layer is 10-100 nm.

18. The composite current collector according to claim 17, characterized in that, The thickness of the protective layer is 20-80 nm.

19. A secondary battery, characterized in that, The secondary battery includes electrodes, an electrolyte, and a separator. The electrodes include a current collector and an active material layer disposed on at least one side of the current collector. The current collector includes a composite current collector according to any one of claims 11 to 18.

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