Polymer film, and preparation method therefor and use thereof

Abstract

Provided in the present application are a polymer film, and a preparation method therefor and the use thereof. The polymer film comprises a polymer and a whisker material at a mass ratio of (95.0-99.5):(0.5-5). In order to solve the problems of low tensile strength and poor thermal conductivity of a traditional polymer base film of a composite current collector in the related art, the present application provides a polymer film with both high tensile strength and enhanced thermal conductivity, and the polymer film is used as a base film to prepare a composite current collector; therefore, the problem of low yield caused by low tensile strength and the problems of hole defects and poor electrical conductivity caused by poor thermal conductivity can be solved, and a high-strength composite current collector can be prepared, thereby promoting further promotion and application of the composite current collector.

Description

A polymer film and its preparation method and application Technical Field

[0001] The present application relates to the technical field of current collector materials, such as a polymer film and its preparation method and application. Background Art

[0002] At present, the composite current collector prepared based on polymer membranes has received widespread attention and application in the new energy industry. The composite current collector in the related technology is usually prepared by depositing a layer of metal (aluminum, copper, etc.) material on the polymer membrane by the physical vapor deposition (PVD) method, and finally a surface metallized film with a certain conductivity is obtained, which is the composite current collector. Compared with traditional current collectors, the composite current collector based on polymer membranes has the characteristics of low cost, light weight and good internal insulation. The above characteristics enable the composite current collector to reduce the cost of the battery when used in the battery, and can improve the energy density and safety of the battery.

[0003] At present, traditional polymer films used to prepare composite current collectors have the following problems: ① Low tensile strength, which leads to easy film breakage during the preparation of composite current collectors by PVD method, resulting in low product yield, and the tensile strength of the prepared composite current collector is reduced, which is not conducive to the subsequent coating and rolling operations of the battery during the preparation process; ② Poor thermal conductivity. During the preparation of composite current collectors by PVD method using it 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, thereby causing the prepared composite current collector to produce hole defects, and ultimately causing the conductivity of the prepared composite current collector to deteriorate.

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

[0005] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0006] The present application provides a polymer film, a preparation method, and applications thereof. To address the low tensile strength and poor thermal conductivity issues of conventional polymer-based films used in composite current collectors in related technologies, the present application provides a polymer film with both high tensile strength and enhanced thermal conductivity. Composite current collectors prepared using this polymer film as a base film can address the low yield issues associated with low tensile strength, the pore defects and poor electrical conductivity associated with poor thermal conductivity, and can produce high-strength composite current collectors, thereby promoting their further promotion and application.

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

[0008] The present application introduces thermally conductive whisker materials into the polymer film preparation process. The introduction of whisker materials has the following two advantages: on the one hand, because the whiskers have a nano-linear structure, they can be oriented along the stretching direction along with the polymer during the stretching film formation process, thereby being regularly arranged, and can form a network structure with the polymer, thereby improving the tensile strength of the polymer film, and the whisker material can serve as a crystallization nucleating agent for polymer molecules, promoting the crystallinity of the polymer during the film formation process, improving the crystallinity of the polymer film, and further improving the tensile strength of the polymer film. On the other hand, because the whiskers have high thermal conductivity and are regularly arranged in the polymer film, the thermal conductivity of the prepared polymer film can be improved. The present application uses a polymer film with good tensile strength and thermal conductivity as a base film to prepare a composite current collector with high yield, low defect rate and strong conductivity.

[0009] In the present application, the polymer film includes a polymer and a whisker material in a mass percentage of (95.0-99.5):(0.5-5), for example, it can be 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] In one embodiment, the polymer film comprises polymer and whisker material in a mass percentage of (97.0-99.0):(1-3), for example, 97:3, 97.5:2.5, 97.8:2.2, 98:2, 98.5:1.5, 99:1, etc.

[0011] In the present application, by regulating the mass percentage of the polymer and whisker material, the prepared polymer film achieves good comprehensive performance. Too low a mass percentage will result in poor film-forming effect and poor performance of the prepared polymer film. Conversely, the tensile strength and thermal conductivity of the prepared polymer film will not be significantly improved.

[0012] In one embodiment, the polymer includes 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 oxide, polystyrene or polyimide, and can be selected from any one or a combination of at least two of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate.

[0013] In one embodiment, the weight average molecular weight of the polymer is 20,000-100,000, for example, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, etc.

[0014] In the present application, by regulating the weight-average molecular weight of the polymer, the prepared polymer film achieves good comprehensive performance. If the weight-average molecular weight is too low, the tensile strength of the prepared polymer film will be low. Conversely, it will lead to poor film-forming effect and poor performance of the prepared polymer film.

[0015] In one embodiment, the whisker material includes any one or a combination of at least two of 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, and can be a combination of metal whisker material and non-metal nitride whisker material, or metal whisker material and metal nitride whisker material.

[0016] In the present application, in the process of preparing a composite current collector by depositing a metal layer on the surface of a polymer, the above-mentioned whisker material can provide nucleation sites for the deposition of metal grains, promote the uniform deposition of metal grains, and improve the tensile strength of the prepared composite current collector. In addition, the synergistic interface effect between the metal and the nitride can enhance the dispersion uniformity of the whiskers, promote them to form a uniformly dispersed network structure in the polymer film, thereby improving the tensile strength of the polymer film, and further promote the deposition of surface metal, thereby improving the tensile strength of the prepared composite current collector. At the same time, the combination of metal and nitride can provide more conductive channels and can also improve the high temperature resistance and thermal conductivity of the polymer film.

[0017] In one embodiment, 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] In one embodiment, 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 the present application, the polymer film has good conductive properties by regulating the mass ratio of the metal whisker material and the nitride whisker material. If the mass ratio is too high, the distance between the particles may become smaller, resulting in enhanced interaction between the particles, thereby increasing the resistivity of the polymer film and decreasing the conductive properties; if the mass ratio is too low, the synergistic effect between the two cannot be fully exerted, resulting in insufficient conductive properties of the film.

[0020] In one embodiment, the metal whisker material includes any one of Cu whiskers, Al whiskers, Ni whiskers, Zn whiskers, Fe whiskers or Ag whiskers, or a combination of at least two thereof, and Cu whiskers and / or Al whiskers may be selected.

[0021] In one embodiment, the non-metallic whisker material comprises Si whiskers.

[0022] In one embodiment, the metal nitride whisker material comprises AlN whiskers.

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

[0024] In one embodiment, the metal oxide whisker material comprises ZnO whiskers.

[0025] In one embodiment, the non-metallic carbide whisker material comprises SiC whiskers.

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

[0027] In the present application, by regulating the diameter and length of the whisker material, the prepared polymer film can achieve higher tensile strength and thermal conductivity.

[0028] In one embodiment, the whisker material has a diameter of 10-60 nm, for example, 10 nm, 15 nm, 20 nm, 40 nm, 50 nm, 60 nm, etc.; and a length of 80-400 nm, for example, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, etc.

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

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

[0031] In one embodiment, the melt extrusion temperature is 200°C-400°C, for example, 200°C, 250°C, 300°C, 350°C, 400°C, etc.

[0032] In one embodiment, the biaxial stretching pressure is such that the longitudinal stretching ratio is (3-7):1, for example, 3:1, 4:1, 5:1, 6:1, 7:1, etc.; the transverse stretching ratio is (2-4):1, for example, 2:1, 3:1, 4:1, etc.

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

[0034] In one embodiment, the thickness of the support layer is 2-10 μm, for example, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

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

[0036] In one embodiment, the thickness of the metal layer is 500-2000 nm, optionally 700-1200 nm, for example, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 2000 nm, etc.

[0037] In one embodiment, the material of the protective layer includes any one of nickel, chromium, nickel-based alloy, copper-based alloy, copper oxide, aluminum oxide, nickel oxide, chromium oxide, cobalt oxide, copper chromium oxide, graphite, carbon black, acetylene black, Ketjen black, carbon nano-quantum dots, carbon nanotubes, carbon nanofibers or graphene, or a combination of at least two thereof.

[0038] In one embodiment, the thickness of the protective layer is 10-100 nm, optionally 20-80 nm, for example, 10 nm, 15 nm, 20 nm, 40 nm, 50 nm, 70 nm, 80 nm, 100 nm, etc.

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

[0040] Compared with the related art, this application has the following beneficial effects:

[0041] The present application provides a polymer film, which introduces a thermally conductive whisker material during the preparation process of the polymer film. The introduction of the whisker material has the following two advantages: on the one hand, since the whiskers have a nano-linear structure, they can be oriented along the stretching direction along with the polymer during the stretching film formation process, thereby being regularly arranged, and can form a network structure with the polymer, thereby improving the tensile strength of the polymer film, and the whisker material can serve as a crystallization nucleating agent for the polymer molecules, promoting the crystallinity of the polymer during the film formation process, improving the crystallinity of the polymer film, and further improving the tensile strength of the polymer film. On the other hand, since the whiskers have high thermal conductivity and are regularly arranged in the polymer film, the thermal conductivity of the prepared polymer film can be improved. The present application uses a polymer film with good tensile strength and thermal conductivity as a base film to prepare a composite current collector with high yield, low defect rate and strong conductivity.

[0042] Still other aspects will become apparent upon reading and understanding the detailed description. DETAILED DESCRIPTION

[0043] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0044] Example 1

[0045] This embodiment provides a polymer film, which includes polypropylene and Cu whisker materials in a mass percentage of 99.5:0.5; the weight average molecular weight of the polypropylene is 80,000; the diameter of the Cu whisker is 10 nm and the length is 50 nm.

[0046] This embodiment also provides a method for preparing the above polymer film, which comprises the following steps:

[0047] Melt extrusion: The above raw materials were added into the corresponding twin-screw extruder. The mass percentages of polypropylene and Cu whiskers in the raw materials were 99.5% and 0.5%, respectively. The raw materials were melted at 250°C, and then filtered through a filter (10 micron filter mesh). The melt was extruded through a die head at a die head temperature of 260°C.

[0048] Casting: The molten polypropylene material extruded from the die head is cast onto the casting roller, and is formed by the casting roller and water cooling treatment. The cooling temperature is 30℃.

[0049] Biaxial stretching: longitudinal stretching: preheating temperature is 120℃; stretching temperature is 145℃; longitudinal stretching ratio is 7:1, and then cooled to room temperature; transverse stretching: preheating temperature: 140℃; stretching temperature is 150℃, and stretching ratio is 3:1.

[0050] Heat treatment: The heat treatment temperature is 110℃ and the treatment time is 2s.

[0051] Winding: After the heat-treated film is air-cooled in the platform area, it enters the winding system through the traction system for film winding. The winding tension is 30N / m, and a polypropylene film with a thickness of 4.5μm is prepared.

[0052] This embodiment also provides a composite current collector and a preparation method thereof, which comprises the following steps:

[0053] The polymer film prepared above was placed in a magnetron sputtering chamber, and a 70 nm thick metal layer was plated on the composite film prepared in the previous step by magnetron sputtering in the magnetron sputtering chamber using copper metal with a purity of 99.99% as a target and argon as a gas source. Then, the composite film prepared in the above step was electroplated as a substrate to thicken the conductive copper layer. The electroplating process was divided into the following three steps: ① Electroplating to thicken the metal layer: the plating solution was 120 g / L copper sulfate, 100 g / L sulfuric acid, 50 mg / L chloride ion, 2.0 mg / L sodium 3-mercapto-1-propane sulfonate, 0.8 mg / L 2-mercaptopyridine, and 180 mg / L polyethylene glycol 8000. The plating solution temperature was 25°C, and the average cathode current density was 2 A / dm 2 , electroplating for 5 minutes. ② Protective Layer Preparation: After electroplating, the plated film was rinsed in a clean water tank. Then, a surface protective layer was prepared in a protective layer preparation tank containing a 5g / L potassium dichromate aqueous solution at 25°C for 25 seconds, followed by another rinse in a clean water tank. ③ Drying: The washed film was dried in an oven at 65°C to obtain a composite copper current collector with a total thickness of 6.9μm.

[0054] Example 2

[0055] 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. Other aspects are the same as those in embodiment 1.

[0056] Example 3

[0057] 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. Other aspects are the same as those in embodiment 1.

[0058] Example 4

[0059] The difference between this embodiment and embodiment 1 is that the length of the Cu whisker material is 200 nm, and the rest is the same as embodiment 1.

[0060] Example 5

[0061] The difference between this embodiment and embodiment 1 is that the length of the Cu whisker material is 500 nm, and the rest is the same as embodiment 1.

[0062] Example 6

[0063] The difference between this embodiment and embodiment 1 is that the diameter of the Cu whisker material is 50 nm, and the rest is the same as embodiment 1.

[0064] Example 7

[0065] The difference between this embodiment and embodiment 1 is that the diameter of the Cu whisker material is 100 nm, and the rest is the same as embodiment 1.

[0066] Example 8

[0067] The difference of the present embodiment and embodiment 1 is, polypropylene is replaced by polyethylene terephthalate (PET) (available from Sinopec Yizheng Chemical Fiber Co., Ltd., the trade mark is FG611) in the raw material, and its preparation technology is as follows: 1. raw material: selected raw material is polyethylene terephthalate (PET) and Cu whisker material, and wherein the weight-average molecular weight of PET is 30,000, and the diameter of Cu whisker material is 10nm, and length is 50nm. 2. melt extrusion: above-mentioned raw materials are joined in the corresponding twin screw extruder respectively, the mass percent of PET and Cu whisker material is respectively 99.5%, 0.5%, 280 ℃ of following melting, after filtering through strainer (10 micron filter screens), by the die head extrusion melt, die head temperature is 280 ℃ then. 3. slab: the molten material that the die head is extruded is cast on the slab roller, and through slab roller and water-cooled cooling treatment moulding, cooling temperature is 25 ℃. ④ Longitudinal stretching: preheating temperature 90°C, stretching temperature 110°C, longitudinal stretch ratio 3.5:1, and cooling to room temperature after longitudinal stretching; ⑤ Transverse stretching: preheating temperature 90°C, stretching temperature 120°C, transverse stretch ratio 3:1. ⑥ Heat treatment: heat treatment temperature 130°C. ⑦ Winding: The heat-treated film was air-cooled in the platform area and then passed through the traction system to the winding system for film winding. The winding tension was 30 N / m, resulting in a PET film with a thickness of 4.5 μm. All other conditions were the same as in Example 1.

[0068] Example 9

[0069] The difference between this embodiment and embodiment 8 is that the PET in the raw material is replaced by polyethylene naphthalate (purchased from Teijin Co., Ltd. of Japan, brand TN-8050SC), and the rest is the same as embodiment 8.

[0070] Example 10

[0071] The difference of the present embodiment and embodiment 1 is, polypropylene is replaced by polytetrafluoroethylene (PTFE) in the raw material, and its preparation technology is as follows: 1. raw material: selected raw material is PTFE (U.S. Dupont 601A) and Cu whisker material, and wherein the diameter of Cu whisker material is 10nm, and length is 50nm. 2. melt extrusion: above-mentioned raw materials are joined in the corresponding twin screw extruder respectively, the mass percent of PTFE and Cu whisker material is respectively 99.5%, 0.5%, melts down at 350 ℃, after filtering through strainer (10 micron filter screens), extrude melt by the die head, die head temperature is 350 ℃ then. 3. slab: the molten material that the die head is extruded is cast on the slab roller, and through slab roller and water-cooled cooling treatment moulding, cooling temperature is 30 ℃. ④ Longitudinal stretching: preheating temperature 280°C, stretching temperature 300°C, longitudinal stretch ratio 4:1, and cooling to room temperature after longitudinal stretching; ⑤ Transverse stretching: preheating temperature 280°C, stretching temperature 310°C, transverse stretch ratio 3:1. ⑥ Heat treatment: heat treatment temperature 200°C. ⑦ Winding: After the heat-treated membrane is air-cooled in the platform area, it enters the winding system through the traction system for membrane sheet winding. The winding tension is 30 N / m, and a PTFE membrane with a thickness of 4.5 μm is obtained. All other conditions are the same as in Example 1.

[0072] Example 11

[0073] The difference between this embodiment and embodiment 1 is that the Cu whisker material in the raw material is replaced with Al whisker material, and the rest is the same as embodiment 1.

[0074] Example 12

[0075] The difference between this embodiment and embodiment 1 is that the Cu whisker material in the raw material is replaced with BN whisker material, and the rest is the same as embodiment 1.

[0076] Example 13

[0077] The difference between this embodiment and embodiment 1 is that the Cu whisker material in the raw material is replaced with AlN whisker material, and the rest is the same as embodiment 1.

[0078] Example 14

[0079] The difference between this embodiment and embodiment 1 is that the Cu whisker material in the raw material is replaced by a mixture of Cu whisker material and AlN whisker material, and the ratio of the two is 1:1. The other aspects are the same as those in embodiment 1.

[0080] Example 15

[0081] The difference between this embodiment and embodiment 1 is that the Cu whisker material in the raw material is replaced by a mixture of Cu whisker material and BN whisker material, and the ratio of the two is 1:1. The other aspects are the same as those in embodiment 1.

[0082] Comparative Example 1

[0083] The difference between this comparative example and Example 1 is that the raw material contains only polypropylene, and the other aspects are the same as those of Example 1.

[0084] Comparative Example 2

[0085] The difference between this comparative example and Example 1 is that the content of Cu whisker material in the raw material is 0.3%, and the other aspects are the same as those in Example 1.

[0086] Comparative Example 3

[0087] The difference between this comparative example and Example 1 is that the content of Cu whisker material in the raw material is 5.5%, and the other aspects are the same as those in Example 1.

[0088] Test conditions

[0089] The polymer films and composite current collectors provided in Examples 1 to 15 and Comparative Examples 1 to 3 were tested using the following test methods:

[0090] Polymer membrane:

[0091] ① Thermal conductivity: The thermal conductivity of the polymer film prepared above was tested according to the standard ISO 22007-3:2008.

[0092] ②Tensile strength: The tensile strength of the polymer film prepared above was tested according to the standard GB / T 1040.3-2006.

[0093] Composite current collector:

[0094] ① Defective rate caused by broken film during the preparation process: The ratio of the number of unqualified products caused by broken film during the preparation process to the total number of products. Since the width is consistent, the number is calculated based on the length.

[0095] ② Number of hole defects per unit area of ​​composite current collector: The prepared composite current collector sample is placed in a surface quality detection system (micro-visual charge-coupled device CCD), its surface is scanned, and then the optical signal is converted into an electrical signal and transmitted to a computer to count the number of hole defects per unit area of ​​the composite current collector with a pore size less than 100 μm (it is generally required that the finished product cannot have holes larger than 100 μm).

[0096] ③ Square resistance: Place the prepared composite current collector sample on the sample table and use a four-probe square resistance meter to test the square resistance of the sample.

[0097] The test results are shown in Table 1:

[0098] Table 1

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

[0100] (1) It can be seen from Examples 1-3 and Comparative Examples 1-3 that when adding an appropriate amount of Cu whiskers and an appropriate amount of polymer material in the preparation process of the polymer film, the prepared polymer film has the advantages of high tensile strength and high thermal conductivity, which leads to a decrease in the defective rate caused by the breakage of the base film in the process of preparing the composite current collector using the polymer film as the base film, and an increase in the tensile strength of the prepared composite current collector and a decrease in the number of holes. At the same time, due to the decrease in the number of holes, the metal layer becomes more dense, resulting in a decrease in its square resistance. Specifically, as the content of Cu whiskers in the polymer film continues to increase, 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 defective rate caused by the breakage of the polymer base film in the process of preparing the composite current collector, and the corresponding 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, and the corresponding number of holes in the process of preparing the composite current collector first decreases to 0 and then remains unchanged, and the square resistance first decreases and then remains unchanged. When the content of Cu whiskers exceeds the maximum value of the optional range, the film-forming property of the polymer will be poor, resulting in defects in the polymer film prepared by biaxial stretching, thereby reducing the tensile strength and thermal conductivity of the polymer film. In addition, the defect rate caused by film breakage in this application refers to the defect rate caused by film breakage during the preparation process of the polymer film, while the square resistance characterizes the conductivity of the metal layer of the composite current collector and is related to the number of holes in the composite current collector. Although the thermal conductivity of the polymer film prepared in Comparative Example 3 is reduced, it is still relatively high. No hole defects will be generated during the preparation of the composite current collector, so the square resistance has not changed.

[0101] (2) It can be seen from Examples 1, 4, and 5 that: as the length of the Cu whiskers in the polymer film increases, the tensile strength of the prepared polymer film shows a trend of first increasing and then decreasing, resulting in the defect caused by the breakage of the base film in the preparation process 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, resulting in a decrease in the number of hole defects in the corresponding prepared composite current collector and a decrease in square resistance.

[0102] (3) It can be seen from Examples 1, 6, and 7 that: as the diameter of the Cu whiskers in the polymer film increases, the tensile strength of the prepared polymer film shows a trend of first increasing and then decreasing, resulting in the defect caused by the breakage of the base film in the preparation process 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, resulting in a decrease in the number of hole defects in the corresponding prepared composite current collector and a decrease in the square resistance.

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

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

[0105] (6) It can be seen from Examples 1, 14, and 15 that when the polymer film contains a combination of one or more whisker materials selected from metal whisker materials, metal nitride whisker materials, or non-metallic nitride whisker materials, the tensile strength of the composite current collector prepared using the 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-metallic nitride whisker materials are introduced into the polymer film, the interfacial interaction between the metal and the nitride can enhance the uniformity of the dispersion of the particle whiskers, promote the formation of a uniformly dispersed network structure within the polymer film, thereby improving the tensile strength of the polymer film and subsequently improving the tensile strength of the composite current collector based on the polymer film. On the other hand, in the process of depositing a metal layer on the polymer surface to prepare the composite current collector, the above-mentioned whisker material can provide more nucleation sites for the deposition of metal grains, promote the uniform deposition of metal grains, and thereby improve the tensile strength of the prepared composite current collector.

[0106] The applicant declares that while the above-mentioned embodiments are used to illustrate the process of the present application, the present application is not limited to the above-mentioned process steps, which does not mean that the present application must rely on the above-mentioned process steps in order to be implemented. Those skilled in the art should understand that any improvements to the present application, equivalent replacements for the raw materials used in the present application, addition of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present application.

Claims

1. A polymer film comprising a polymer and a whisker material in a mass percentage of (95.0-99.5):(0.5-5).

2. The polymer film according to claim 1, wherein The polymer film comprises a polymer and a whisker material in a mass percentage of (97.0-99.0):(1-3).

3. The polymer film according to claim 1 or 2, wherein The polymer includes 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 can be selected from any one or a combination of at least two of polypropylene, polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate.

4. The polymer film according to any one of claims 1 to 3, wherein The weight average molecular weight of the polymer is 20,000-100,000.

5. The polymer film according to any one of claims 1 to 4, wherein The whisker material includes any one of 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, or a combination of at least two of the materials. The material may be a combination of metal whisker material and non-metal nitride whisker material, or a metal whisker material and a metal nitride whisker material.

6. The polymer film according to claim 5, wherein The mass ratio of the metal whisker material to the non-metal nitride whisker material is (0.5-2):

1.

7. The polymer film according to claim 5 or 6, wherein The mass ratio of the metal whisker material to the metal nitride whisker material is (0.5-2):

1.

8. The polymer film according to any one of claims 5 to 7, wherein The metal whisker material includes any one of Cu whisker, Al whisker, Ni whisker, Zn whisker, Fe whisker or Ag whisker or a combination of at least two thereof, and may be Cu whisker and / or Al whisker; Optionally, the non-metallic whisker material includes Si whiskers; Optionally, the metal nitride whisker material comprises AlN whiskers; Optionally, the non-metallic nitride whisker material includes Si3N4 whiskers and / or BN whiskers; Optionally, the metal oxide whisker material comprises ZnO whiskers; Optionally, the non-metallic carbide whisker material includes SiC whiskers.

9. The polymer film according to any one of claims 1 to 8, wherein The whisker material has a diameter of 10-100 nm and a length of 50-500 nm; Optionally, the whisker material has a diameter of 10-60 nm and a length of 80-400 nm.

10. A method for preparing a polymer film according to any one of claims 1 to 9, comprising the following steps: The polymer and the whisker material are melt-extruded and biaxially stretched to obtain the polymer film.

11. The method according to claim 10, wherein: The temperature of the melt extrusion is 200°C-400°C.

12. The method according to claim 10 or 11, wherein: In the biaxial stretching, the longitudinal stretching ratio is (3-7):1, and the transverse stretching ratio is (2-4):

1. 13 . A composite current collector, comprising a support layer and a metal layer and a protective layer sequentially stacked on both sides of the support layer, wherein the support layer comprises the polymer film according to claim 1 .

14. The composite current collector according to claim 13, wherein: The thickness of the support layer is 2-10 μm; Optionally, the material of the metal layer includes any one of copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium or silver, or a combination of at least two thereof; Optionally, the thickness of the metal layer is 500-2000nm, optionally 700-1200nm; Optionally, the material of the protective layer includes any one of nickel, chromium, nickel-based alloy, copper-based alloy, copper oxide, aluminum oxide, nickel oxide, chromium oxide, cobalt oxide, copper-chromium oxide, graphite, carbon black, acetylene black, Ketjen black, carbon nano-quantum dots, carbon nanotubes, carbon nanofibers or graphene, or a combination of at least two thereof; Optionally, the thickness of the protective layer is 10-100 nm, and optionally 20-80 nm. 15 . 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 the composite current collector according to claim 13 or 14.

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