A composite current collector and its preparation method

Abstract

This application provides a composite current collector and its preparation method, belonging to the field of composite current collector manufacturing technology. The preparation method of the composite current collector includes the following steps: maleic anhydride-grafted polypropylene particles are sequentially subjected to melt extrusion and stretching treatments to obtain a modified polypropylene base film; the modified polypropylene base film and two metal layers are simultaneously conveyed to a hot press roller for hot pressing and bonding treatment, wherein the modified polypropylene base film is located between the two metal layers, and in the hot pressing and bonding step, the pressure is 0.3 MPa~0.5 MPa, the hot pressing temperature is 180℃~220℃, and the film conveying speed is 5 m / min~50 m / min, so that both sides of the modified polypropylene base film are bonded to a metal layer respectively, thus obtaining a composite current collector; the composite current collector prepared by this method has the advantages of high interfacial bonding force between the polypropylene base film and the metal layer and a relatively smooth film surface (i.e., fewer wrinkles on the film surface).

Description

Technical Field

[0001] This application relates to the field of composite current collector manufacturing technology, and more specifically, to a composite current collector and its preparation method. Background Technology

[0002] Polypropylene (PP) has become one of the mainstream choices for preparing polymer-based films due to its advantages such as low cost, low density, light weight, excellent mechanical properties, and good process compatibility. However, due to its low polarity, polypropylene-based films have a problem of low interfacial bonding force with metal layers, which limits their promotion and application.

[0003] To address the aforementioned technical challenges, engineers considered plasma or corona treatment of the polypropylene base film to graft polar groups onto its surface, followed by magnetron sputtering deposition to prepare a metal layer. However, since the thickness of the polypropylene base film is typically only a few micrometers (usually not exceeding 10 μm), its voltage tolerance is limited (excessive voltage can cause physical damage to the polypropylene base film, affecting its basic physicochemical properties). Consequently, the number of polar groups that can be grafted onto its surface is also limited, thus the improvement in adhesion remains relatively limited. Therefore, there is an urgent need to design new preparation processes to enhance the interfacial adhesion between the polypropylene base film and the metal layer. Summary of the Invention

[0004] The purpose of this application is to provide a composite current collector and its preparation method. The composite current collector prepared by this method has the advantages of high interfacial bonding force between the polypropylene base film and the metal layer and a relatively smooth film surface (i.e. fewer wrinkles on the film surface).

[0005] The embodiments of this application are implemented as follows: In a first aspect, embodiments of this application provide a method for preparing a composite current collector, comprising the following steps: sequentially subjecting maleic anhydride-grafted polypropylene particles to melt extrusion and stretching to obtain a modified polypropylene base film; synchronously conveying the modified polypropylene base film and two metal layers to a hot-pressing roller for hot-pressing bonding, wherein the modified polypropylene base film is located between the two metal layers, and in the hot-pressing bonding step, the pressure is 0.3 MPa~0.5 MPa, the hot-pressing temperature is 180℃~220℃, and the film conveying speed is 5 m / min~50 m / min, so that both sides of the modified polypropylene base film are respectively bonded to a metal layer to obtain a composite current collector.

[0006] In the above technical solution, maleic anhydride-grafted polypropylene particles are used as the initial raw material and modified polypropylene base film is obtained through melt extrusion and stretching. Specifically, maleic anhydride is used to directly graft and modify polypropylene particles at the source, and the film material is obtained by stretching. Compared with plasma treatment of the formed base film surface, the former can significantly increase the number of polar sites on the surface of the polypropylene base film while maintaining the inherent physicochemical properties of the base film. Then, the modified polypropylene base film and two metal layers are simultaneously transported to the hot press rollers and hot-pressed according to the above positional relationship and parameter conditions. The above treatment temperature can soften the modified polypropylene base film and cause the modified sites to migrate to the surface. The above treatment pressure can make the softened area with modified sites on the surface tightly fill the microscopic surface of the corresponding area of ​​the metal layer. In addition, maleic anhydride graft modification can significantly improve the polarity of the polypropylene base film, so that the final composite current collector has the advantages of high interfacial bonding force between the polypropylene base film and the metal layer and relatively smooth film surface (i.e., fewer film wrinkles).

[0007] In some alternative implementations, the hot-pressing bonding process involves a pressure of 0.35 MPa to 0.45 MPa, a hot-pressing temperature of 190°C to 210°C, and a film-feeding speed of 5 m / min to 20 m / min.

[0008] In the above technical solution, the processing pressure and processing temperature in the hot pressing bonding process are further limited to the above range, which can achieve a better hot pressing bonding effect while maintaining a relatively excellent surface flatness of the film material.

[0009] In some alternative embodiments, the grafting unit in the maleic anhydride-grafted polypropylene particles is selected from polymaleic anhydride segments.

[0010] In the above technical solution, the grafting unit in the maleic anhydride-grafted polypropylene particles is selected from polymaleic anhydride segments. Compared with selecting maleic anhydride monomers as grafting units, polymaleic anhydride segments contain a large number of anhydride groups and have stronger polarity. As grafting units, they can enable the modified polypropylene base film and the metal layer to have a better interfacial bonding strength.

[0011] In some alternative embodiments, the grafting rate of maleic anhydride-grafted polypropylene particles is 0.3% to 0.5%.

[0012] In the above technical solution, when the polymaleic anhydride segment is used as the grafting unit, the grafting rate of the maleic anhydride-grafted polypropylene particles is limited to the above range. This grafting rate range can achieve a reliable bond between the modified polypropylene base film and the metal layer. At the same time, this grafting rate range can better maintain the good processing fluidity of the polypropylene particles, and also enable the formed polypropylene base film to better maintain its inherent physical and chemical properties.

[0013] In some alternative embodiments, the number average molecular weight of the polymaleic anhydride segments is 30,000 to 50,000, and the number average molecular weight of the maleic anhydride-grafted polypropylene particles is 380,000 to 430,000.

[0014] In the above technical solution, the number-average molecular weight of both the polymaleic anhydride segments and the maleic anhydride-grafted polypropylene particles is simultaneously limited to the above range, which can effectively improve the interfacial bonding strength between the polypropylene base film and the metal layer while maintaining the inherent physicochemical properties of the polypropylene base film itself.

[0015] In some alternative embodiments, the stretching process includes: stretching the film obtained by melt extrusion treatment longitudinally and then stretching it transversely; wherein, in the longitudinal stretching step, the stretching temperature is 130°C to 150°C and the stretching ratio is 3 to 5 times; and / or, in the transverse stretching step, the stretching temperature is 150°C to 170°C and the stretching ratio is 5 to 7 times.

[0016] In the above technical solution, the biaxial stretching process is adopted. Compared with the uniaxial stretching process, the base film prepared by biaxial stretching has the advantages of superior mechanical properties and more uniform thickness. On this basis, by limiting the processing temperature and stretching ratio in the longitudinal and transverse stretching steps to the above range, it is possible to achieve a better biaxial stretching effect while improving the problem of polar sites migrating from the surface of the membrane to the membrane body during the stretching process (the more polar sites are embedded after migration, the fewer modification sites on the membrane surface are used to improve the bonding force). This effectively improves the interfacial bonding strength between the subsequently prepared polypropylene base film and the metal layer and better maintains the inherent physicochemical properties of the polypropylene base film itself.

[0017] In some optional embodiments, the step of simultaneously conveying the modified polypropylene base film and two metal layers to a hot press roller for hot pressing and bonding includes: providing a polymer substrate, the polymer substrate including a polymer base layer and a release layer located on one side surface of the polymer base layer, the thickness of the polymer base layer being not less than 20 μm; forming a metal underlayer on the side surface of the release layer opposite to the polymer base layer using magnetron sputtering coating technology, and then thickening the metal underlayer to form a metal layer of a preset thickness to obtain a transfer film; simultaneously conveying the modified polypropylene base film and two transfer film materials to a hot press roller for hot pressing and bonding, wherein the metal layers in both transfer film materials are located on the side closer to the modified polypropylene base film, so that each side of the modified polypropylene base film is bonded to a metal layer; and then removing the release layer and polymer base layer on both sides of the modified polypropylene base film.

[0018] In the above technical solution, the step of simultaneously conveying the modified polypropylene base film and two metal layers to the hot press rollers for hot pressing and bonding is carried out according to the above process. That is, the metal layer is formed on a polymer substrate with a thickness of not less than 20 μm and a release layer using magnetron sputtering coating technology and thickening technology, which can efficiently form a high-quality ultrathin metal layer. Then, the metal layer and the polypropylene base film are hot-pressed and bonded together, and the excess release layer and polymer base layer on both sides of the modified polypropylene base film are removed. The composite current collector prepared by this process can well meet the current demand for ultrathin composite current collectors. At the same time, it also makes the formed composite current collector have the advantages of a more reliable bond between the polypropylene base film and the metal layer and a smoother film surface.

[0019] In some alternative embodiments, the thickness of the polymer substrate layer is 25 μm to 100 μm, and / or the thickness of the individual metal layer is 1 μm to 1.5 μm.

[0020] In the above technical solution, the thickness of the polymer substrate layer is limited to the above range to provide a more suitable thickness, which can effectively improve the efficiency of the magnetron sputtering coating process and the quality of the prepared metal layer (including but not limited to structural integrity, density, and uniform thickness); the thickness of a single metal layer is limited to the above range, and the metal layer within this range can better meet the needs of ultra-thin composite current collectors.

[0021] In some alternative implementations, the release layer has a peel force of 5 g / 25 mm to 15 g / 25 mm.

[0022] In the above technical solution, the peeling force of the release layer is limited to the above range. After the hot-press bonding process (i.e. after the modified polypropylene base film is bonded to the metal layer), the release layer and the metal layer can be peeled off relatively easily and without damage.

[0023] In some alternative embodiments, the release layer is made of at least one of silicone oils, fluorosilicones, polypropylene resins, and polymethylpentene resins, and the thickness of the release layer is 5 μm to 10 μm.

[0024] In the above technical solution, by matching the material and thickness of the release layer, it is relatively easy to make the peel force of the release layer within the range of 5 g / 25mm to 15 g / 25mm, which helps to improve process compatibility.

[0025] Secondly, embodiments of this application provide a composite current collector, which is prepared by the preparation method provided in the first aspect embodiment, wherein the peel strength between the modified polypropylene base film and the metal layer is not less than 2.0 N / cm.

[0026] In the above technical solution, the composite current collector is prepared by the preparation method provided in the first aspect embodiment, such that the peel strength between the modified polypropylene base film and the metal layer in the composite current collector is not less than 2.0 N / cm. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A process flow diagram of a method for preparing a composite current collector provided in this application embodiment. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0030] It should be noted that the terms "and / or" in this application, such as "feature 1 and / or feature 2", all refer to the three cases of "feature 1" alone, "feature 2" alone, and "feature 1" plus "feature 2".

[0031] In addition, in the description of this application, unless otherwise stated, "one or more" means two or more; the range of "numerical value a to numerical value b" includes the two endpoints "a" and "b"; and "unit of measurement" in "numerical value a to numerical value b + unit of measurement" represents the "unit of measurement" of both "numerical value a" and "numerical value b".

[0032] Currently, in order to improve the interfacial bonding strength between polypropylene base film and metal layer, the surface of polypropylene base film is usually subjected to plasma treatment or corona treatment to graft polar groups onto its surface, and then a metal layer is prepared on its surface using magnetron sputtering coating technology. However, this modification method still has limited effect on improving the bonding strength.

[0033] Based on this, the inventors conceived of directly grafting maleic anhydride onto polypropylene particles at the source and then stretching the resulting membrane. Compared to plasma treatment of the formed base membrane surface, this method can significantly increase the number of polar sites on the polypropylene base membrane surface while maintaining the inherent physicochemical properties of the base membrane. Theoretically, preparing composite current collectors based on this modified polypropylene base membrane should significantly improve the interfacial bonding strength between the polypropylene base membrane and the metal layer; however, researchers found that the interfacial bonding strength between the polypropylene base membrane and the metal layer did not improve significantly as expected.

[0034] The inventors discovered that after preparing composite current collectors using modified polypropylene base films combined with magnetron sputtering coating and thickening technology, the interfacial bonding strength between the polypropylene base film and the metal layer did not improve significantly as expected. The main reason is that during the stretching process after melt extrusion, the polar sites on the surface of the maleic anhydride-grafted polypropylene particles tend to migrate inward and be covered by non-polar polypropylene segments, reducing the number of polar sites on the surface of the film. Consequently, the interfacial bonding strength between the metal layer and the polypropylene base film prepared by magnetron sputtering coating and thickening technology is difficult to improve to the expected level.

[0035] Further research by the inventors revealed that, instead of directly forming a metal layer on the surface of a modified polypropylene base film using magnetron sputtering coating technology and thickening treatment technology, they innovatively adopted a hot-press bonding technology. This involves directly hot-pressing the modified polypropylene base film and the metal layer together. (High temperature softens the modified polypropylene base film and causes the modified sites to migrate to the surface, meaning that the polar sites that migrated inward during the stretching stage can migrate back to the film surface. High pressure allows the softened areas with modified sites to tightly fill the corresponding microscopic surface of the metal layer. In addition, maleic anhydride grafting modification can significantly improve the polarity of the polypropylene base film.) This results in a composite current collector with a high interfacial bonding force between the polypropylene base film and the metal layer. At the same time, the composite current collector also has the advantage of a relatively smooth film surface (i.e., fewer wrinkles).

[0036] The following is a detailed description of a composite current collector and its preparation method according to an embodiment of this application.

[0037] In a first aspect, embodiments of this application provide a method for preparing a composite current collector, comprising the following steps: sequentially subjecting maleic anhydride-grafted polypropylene particles to melt extrusion and stretching processes to obtain a modified polypropylene base film; synchronously conveying the modified polypropylene base film and two metal layers to a hot-pressing roller for hot-pressing bonding, wherein the modified polypropylene base film is located between the two metal layers, and in the hot-pressing bonding step, the pressure is 0.3 MPa to 0.5 MPa (e.g., but not limited to any one of 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, and 0.5 MPa, or a range between any two), the hot-pressing temperature is 180℃ to 220℃ (e.g., but not limited to any one of 180℃, 185℃, 190℃, 195℃, 200℃, 205℃, 210℃, 215℃, and 220℃, or a range between any two), and the film-feeding speed is 5 m / min to 50 m / min. m / min (e.g., but not limited to any one of 5 m / min, 10 m / min, 20 m / min, 30 m / min, 40 m / min and 50 m / min or any range between two), so that a metal layer is bonded to each side of the modified polypropylene base film to obtain a composite current collector.

[0038] In this application, maleic anhydride-grafted polypropylene particles are used as the initial raw material and modified polypropylene base film is obtained through melt extrusion and stretching. Specifically, maleic anhydride is used to directly graft and modify polypropylene particles at the source, and the film material is obtained by stretching. Compared with plasma treatment of the formed base film surface, the former can significantly increase the number of polar sites on the surface of the polypropylene base film while maintaining the inherent physicochemical properties of the base film. Then, the modified polypropylene base film and two metal layers are simultaneously transported to a hot press roller and hot-pressed according to the above positional relationship and parameter conditions. The above treatment temperature can soften the modified polypropylene base film and cause the modified sites to migrate to the surface. The above treatment pressure can make the softened area with modified sites on the surface tightly fill the microscopic surface of the corresponding area of ​​the metal layer. In addition, maleic anhydride graft modification can significantly improve the polarity of the polypropylene base film, so that the final composite current collector has the advantages of high interfacial bonding between the polypropylene base film and the metal layer and a relatively smooth film surface (i.e., fewer film wrinkles).

[0039] It is important to emphasize that strict control of pressure, temperature, and film-feeding speed in the hot-pressing process is necessary because unreasonable design of these core parameters makes it difficult to achieve both a reliable bond between the modified polypropylene base film and the metal layer, as well as a high degree of film surface flatness. Furthermore, improving bonding strength and film surface flatness are contradictory. For example, if the pressure is too low, the film surface flatness is satisfied, but the bonding strength is insufficient; if the pressure is too high, the bonding strength is satisfied, but the film surface flatness is difficult to achieve.

[0040] As an example, in the hot-press bonding process, the pressure is 0.35 MPa to 0.45 MPa (e.g., but not limited to any one of 0.35 MPa, 0.38 MPa, 0.40 MPa, 0.42 MPa and 0.45 MPa or any range between two), the hot-press temperature is 190℃ to 210℃ (e.g., but not limited to any one of 190℃, 195℃, 200℃, 205℃ and 210℃ or any range between two), and the film-moving speed is 5 m / min to 20 m / min (e.g., but not limited to any one of 5 m / min, 10 m / min, 15 m / min and 20 m / min or any range between two).

[0041] In this embodiment, the processing pressure and temperature in the hot-pressing bonding process are further limited to the above-mentioned range, which can achieve a better hot-pressing bonding effect while maintaining a relatively excellent surface flatness of the film material.

[0042] It should be noted that the melt extrusion process is not limited and can be carried out according to conventional processes in this field.

[0043] As an example, in the melt extrusion process, the melt temperature is 240°C to 270°C (e.g., but not limited to any one of 240°C, 250°C, 260°C, and 270°C, or a range between any two), and the temperature of the cooling roller is 20°C to 50°C.

[0044] It should be noted that the form of maleic anhydride in maleic anhydride-grafted polypropylene particles is not limited; for example, it can be maleic anhydride monomer or polymaleic anhydride segment.

[0045] As an example, in maleic anhydride-grafted polypropylene particles, the grafting unit is selected from polymaleic anhydride segments.

[0046] In this embodiment, the grafting unit in the maleic anhydride-grafted polypropylene particles is selected from polymaleic anhydride segments. Compared with selecting maleic anhydride monomers as grafting units, polymaleic anhydride segments contain a large number of anhydride groups and have stronger polarity. As grafting units, they can enable the modified polypropylene base film and the metal layer to have a better interfacial bonding strength.

[0047] As an example, the grafting rate of maleic anhydride-grafted polypropylene particles is 0.3% to 0.5%, for example, but not limited to any one of 0.3%, 0.35%, 0.4%, 0.45% and 0.5%, or any range between two.

[0048] In this embodiment, when the polymaleic anhydride segment is used as the grafting unit, the grafting rate of the maleic anhydride-grafted polypropylene particles is limited to the above-mentioned range. This grafting rate range can achieve a reliable bond between the modified polypropylene base film and the metal layer. At the same time, this grafting rate range can better maintain the good processing fluidity of the polypropylene particles, and also enable the formed polypropylene base film to better maintain its inherent physical and chemical properties.

[0049] As an example, the number average molecular weight of the polymaleic anhydride segments is 30,000 to 50,000 (e.g., but not limited to any one of 30,000, 35,000, 40,000, 45,000 and 50,000 or any range between the two), and the number average molecular weight of the maleic anhydride-grafted polypropylene particles is 380,000 to 430,000 (e.g., but not limited to any one of 380,000, 390,000, 400,000, 410,000, 420,000 and 430,000 or any range between the two).

[0050] In this embodiment, by simultaneously limiting the number-average molecular weight of the polymaleic anhydride segments and the maleic anhydride-grafted polypropylene particles to the above-mentioned range, it is possible to effectively improve the interfacial bonding strength between the polypropylene base film and the metal layer while maintaining the inherent physicochemical properties of the polypropylene base film itself.

[0051] As an example, the stretching process includes: first stretching the film material obtained by melt extrusion in the longitudinal direction and then stretching it in the transverse direction; wherein, in the longitudinal stretching step, the stretching temperature is 130°C to 150°C (e.g., but not limited to any one of 130°C, 135°C, 140°C, 145°C and 150°C or any range between any two), and the stretching ratio is 3 to 5 times (e.g., but not limited to any one of 3 times, 4 times and 5 times or any range between any two); and / or, in the transverse stretching step, the stretching temperature is 150°C to 170°C (e.g., but not limited to any one of 150°C, 155°C, 160°C, 165°C and 170°C or any range between any two), and the stretching ratio is 5 to 7 times (e.g., but not limited to any one of 5 times, 6 times and 7 times or any range between any two).

[0052] It is understandable that longitudinal refers to the direction along which the membrane material is conveyed, and transverse refers to the direction perpendicular to the direction of membrane material conveying.

[0053] In this embodiment, a biaxial stretching process is used. Compared with uniaxial stretching, the base film prepared by biaxial stretching has the advantages of superior mechanical properties and more uniform thickness. On this basis, the processing temperature and stretching ratio in the longitudinal and transverse stretching steps are limited to the above range. This can improve the problem of polar sites migrating from the surface of the membrane to the membrane body during the stretching process while achieving a better biaxial stretching effect (the more polar sites that migrate inward and are embedded, the fewer modification sites on the membrane surface are available to improve the bonding force). This effectively improves the interfacial bonding strength between the subsequently prepared polypropylene base film and the metal layer and better maintains the inherent physicochemical properties of the polypropylene base film itself.

[0054] It should be noted that the thickness of the modified polypropylene base film is not limited and can be set in accordance with conventional methods in this field.

[0055] As an example, the thickness of the modified polypropylene base film is 2 μm to 4.5 μm, for example, but not limited to any point value or range between any two of the thicknesses of 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm and 4.5 μm.

[0056] It should be noted that the material of the metal layer is not limited; for example, it can be copper or silver.

[0057] It should be noted that, in addition to pursuing high adhesion between the polymer base film and the metal layer, ultrathinness is also a current research hotspot and trend in composite current collectors. Currently, the fabrication process for ultrathin composite current collectors typically involves directly preparing a metal underlayer on the surface of the polymer base film using magnetron sputtering, followed by thickening treatment to obtain an ultrathin metal layer. However, since the thickness of the polymer base film is usually only a few micrometers (typically not exceeding 10 μm), it is prone to pinholes and film burning during magnetron sputtering, severely affecting the quality and efficiency of the formed metal layer. Furthermore, the polymer base film also requires a long film-running process during magnetron sputtering, resulting in a relatively low surface flatness in the formed composite current collector.

[0058] As an example, the steps of simultaneously conveying a modified polypropylene base film and two metal layers to a hot press roller for hot pressing and bonding include: providing a polymer substrate, the polymer substrate including a polymer base layer and a release layer located on one side surface of the polymer base layer, the thickness of the polymer base layer being not less than 20 μm; forming a metal underlayer on the side surface of the release layer opposite to the polymer base layer using magnetron sputtering coating technology, and then thickening the metal underlayer to form a metal layer of a preset thickness to obtain a transfer film; simultaneously conveying the modified polypropylene base film and two transfer film materials to a hot press roller for hot pressing and bonding, wherein the metal layers in both transfer film materials are located on the side closer to the modified polypropylene base film, so that each side of the modified polypropylene base film is bonded to a metal layer; and then removing the release layer and polymer base layer on both sides of the modified polypropylene base film.

[0059] It should be emphasized that the minimum thickness of the polymer substrate layer is 20 μm, which is greater than the thickness of conventional polymer base films (usually no more than 10 μm). The metal underlayer is formed directly on the thick film by magnetron sputtering, which has the advantages of high coating efficiency and quality.

[0060] It should be noted that after the hot-press bonding process achieves a reliable bond between the modified polypropylene base film and the metal layer, the entire polymer substrate can be peeled off and removed by separating the release layer from the metal layer. In other words, the modified polypropylene base film in the actual composite current collector does not undergo the film-laying process of magnetron sputtering coating, which also helps to improve the flatness of the film surface of the composite current collector.

[0061] It is understandable that the hot-pressing bonding of a high-quality, highly flat metal layer with a modified polypropylene base film also helps to improve the bonding strength between the polypropylene base film and the metal layer.

[0062] In this embodiment, the step of simultaneously conveying the modified polypropylene base film and two metal layers to the hot press rollers for hot pressing and bonding is carried out according to the above process. That is, the metal layer is formed on a polymer substrate with a thickness of not less than 20 μm and a release layer using magnetron sputtering coating technology and thickening technology, which can efficiently form a high-quality ultrathin metal layer. Then, the metal layer and the polypropylene base film are hot-pressed and bonded together, and the excess release layer and polymer base layer on both sides of the modified polypropylene base film are removed. The composite current collector prepared by this process can well meet the current demand for ultrathin composite current collectors. At the same time, it also makes the formed composite current collector have the advantages of a more reliable bond between the polypropylene base film and the metal layer and a smoother film surface.

[0063] In other possible implementations, the metal layer can also be a purchased, thicker finished product.

[0064] It should be noted that the method of synchronously transporting the modified polypropylene base film and the two intermediate transfer films is not limited and can be carried out in accordance with conventional processes in this field.

[0065] As an example, a modified polypropylene base film and two intermediate film materials are respectively fixed on an unwinding roller, and then transported by a three-station unwinding system to simultaneously convey the modified polypropylene base film and the two intermediate film materials to the hot pressing roller for hot pressing and bonding. The unwinding tension of the modified polypropylene base film is 50 N~90 N, the unwinding tension of the intermediate film materials is 180 N~220 N, the pressure is 0.3 MPa~0.5 MPa, the hot pressing temperature is 180℃~220℃, and the film conveying speed is 20 m / min~50 m / min.

[0066] It should be noted that the preparation process for the metal underlayer is not limited and can be carried out according to conventional processes in this field.

[0067] As an example, a polymer substrate is fixed on an unwinding roller and fed into a magnetron sputtering chamber for magnetron sputtering coating, and then wound up to obtain a rolled polymer substrate with a metal underlayer; wherein the thickness of the metal underlayer is 80 nm to 120 nm, the unwinding tension is 130 N to 170 N, the winding tension is 160 N to 200 N, the belt speed is 5 m / min to 20 m / min, and the vacuum degree in the vacuum chamber is 10. -3 The sputtering power is 4 KW.

[0068] It should be noted that the thickening process is not limited and can be carried out according to conventional processes in this field.

[0069] As an example, a polymer substrate with a metal underlayer is fixed on an unwinding roller and conveyed to an aqueous plating tank for thickening treatment, and then wound up to obtain a roll of transfer film material. The thickness of the single metal layer is 1 μm to 1.5 μm. The unwinding tension is 200 N to 240 N, the tension of the conveying roller in the aqueous plating tank is 220 N to 260 N, the winding tension is 260 N to 300 N, the belt speed is 5 m / min to 10 m / min, and the composition of the reagents in the aqueous plating tank is: copper sulfate concentration 180 g / L to 240 g / L, sulfuric acid concentration 60 g / L to 100 g / L, chloride ion concentration 40 ppm to 80 ppm, brightener concentration 4 ml / L to 8 ml / L, and current density 1 A / dm³. 2 ~4A / dm 2 The processing temperature is 20℃~30℃.

[0070] It should be noted that the material of the polymer substrate is not limited, as long as it can be used as a substrate for magnetron sputtering coating, such as biaxially oriented polyethylene terephthalate (BOPET) film.

[0071] As an example, the thickness of the polymer substrate is 25 μm to 100 μm, such as, but not limited to, any point value or range between any two of the thicknesses of 25 μm, 50 μm, 75 μm and 100 μm.

[0072] In this embodiment, the thickness of the polymer substrate layer is limited to the above-mentioned range to provide a more suitable thickness, which can effectively improve the efficiency of the magnetron sputtering coating process and the quality of the prepared metal layer (including but not limited to structural integrity, density, and uniform thickness).

[0073] As an example, the thickness of a single metal layer is 1 μm to 1.5 μm, for example, but not limited to, a point value or a range between any two of the following: 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, and 1.5 μm.

[0074] In this embodiment, the thickness of a single metal layer is limited to the above-mentioned range, and the metal layer within this range can better meet the requirements of ultra-thin composite current collectors.

[0075] As an example, the release layer peel force is 5 g / 25 mm to 15 g / 25 mm, for example, but not limited to, a point value or a range between any two of the peel forces of 5 g / 25 mm, 7 g / 25 mm, 10 g / 25 mm, 12 g / 25 mm and 15 g / 25 mm.

[0076] In this embodiment, the peeling force of the release layer is limited to the above-mentioned range, and after the hot-press bonding process (i.e. after the modified polypropylene base film is bonded to the metal layer), the release layer and the metal layer can be peeled off relatively easily and without damage.

[0077] As an example, the material of the release layer is selected from at least one of silicone oils, fluorosilicones, polypropylene resins and polymethylpentene resins, and the thickness of the release layer is 5 μm to 10 μm, for example, but not limited to any one of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm and 10 μm or any range between two of them.

[0078] In this embodiment, by matching the material and thickness of the release layer, it is relatively easy to make the peel force of the release layer within the range of 5 g / 25mm to 15 g / 25mm, which helps to improve process compatibility.

[0079] It should be noted that the source of the polymer substrate with the release layer is not limited; for example, it can be purchased directly or prepared by oneself. To better understand the technical solution, a preparation process of the release layer is used as an auxiliary explanation here.

[0080] As an example, a BOPET polymer base layer is fixed on an unwinding roller, and a silicone release agent is coated on one side of the polymer base layer using a continuous coating process. Then, the solvent is dried and the layer is wound up to obtain a rolled polymer base (i.e., a polymer base layer with a release layer). The unwinding speed is 80 m / min to 100 m / min, the unwinding tension is 180 N to 220 N, the winding tension is 200 N to 240 N, the oven temperature is 80℃ to 120℃, and the air frequency in the oven is 25 Hz to 40 Hz.

[0081] It should be noted that, unless otherwise specified or limited, the processes or steps in the preparation of composite current collectors can be carried out in accordance with conventional processes in this field.

[0082] As an example, a process flow diagram of the preparation method of the composite current collector is exemplarily shown below. Figure 1 .

[0083] Secondly, embodiments of this application provide a composite current collector, which is prepared by the preparation method provided in the first aspect embodiment, wherein the peel strength between the modified polypropylene base film and the metal layer is not less than 2.0 N / cm.

[0084] In this application, the composite current collector is prepared by the preparation method provided in the first aspect embodiment, such that the peel strength between the modified polypropylene base film and the metal layer in the composite current collector is not less than 2.0 N / cm.

[0085] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0086] Example 1 This application provides a method for preparing a composite current collector, comprising the following steps: Preparation of S1 intermediate membrane material with metal layer: (1) A 30 μm thick BOPET polymer substrate layer is fixed on an unwinding roller. A silicone release agent (Yongzhuo 8660 type silicone release agent) is coated on one side of the polymer substrate layer using a continuous coating process. Then the solvent is dried and the substrate is wound up to obtain a rolled polymer substrate. The unwinding speed is 90 m / min, the unwinding tension is 200 N, the winding tension is 220 N, the oven temperature is 100℃, and the air frequency in the oven is 30 Hz. The thickness of the release layer is 8 μm and the peeling force is 10 g / 25 mm.

[0087] (2) The polymer substrate is fixed on the unwinding roller and fed into the magnetron sputtering chamber for magnetron sputtering coating, and then wound up to obtain a rolled polymer substrate with a metal underlayer; wherein the unwinding tension is 150 N, the winding tension is 180 N, the belt speed is 10 m / min, and the vacuum degree in the vacuum chamber is 10 -3 The sputtering power is 4 KW; the thickness of the metal substrate is 100 nm.

[0088] (3) The polymer substrate with a metal underlayer is fixed on the unwinding roller and conveyed to the water plating tank for thickening treatment. Then, it is wound up to obtain a roll of transfer film material. The unwinding tension is 220 N, the tension of the conveying roller in the water plating tank is 240 N, the winding tension is 280 N, the belt speed is 6 m / min, and the composition of the reagents in the water plating tank is: copper sulfate concentration 210 g / L, sulfuric acid concentration 80 g / L, chloride ion concentration 60 ppm, brightener concentration 6 ml / L, and current density 2 A / dm. 2 The processing temperature is 25℃; the thickness of the single metal layer is 1 μm.

[0089] Preparation of S2 modified polypropylene base film Maleic anhydride-grafted polypropylene particles were subjected to melt extrusion and stretching processes sequentially. The grafting unit consisted of polymaleic anhydride segments with a number average molecular weight of 40,000 and a grafting rate of 0.4%. The number average molecular weight of the maleic anhydride-grafted polypropylene particles was 400,000. The melt extrusion temperature was 250°C and the temperature of the cooling roller at the rear end was 30°C. The stretching process was biaxial stretching: the longitudinal stretching temperature was 130°C and the stretching ratio was 4 times, the transverse stretching temperature was 150°C and the stretching ratio was 6 times, the film stretching speed was uniformly 300 m / min, and the film stretching tension was uniformly 200 N, resulting in a modified polypropylene base film with a thickness of 4 μm.

[0090] S3 hot-press bonding forms a composite current collector The modified polypropylene base film and two intermediate film materials are respectively fixed on an unwinding roller, and then fed by a three-station unwinding system to simultaneously transport the modified polypropylene base film and the two intermediate film materials to the hot press roller for hot pressing and bonding. The unwinding tension of the modified polypropylene base film is 70 N, the unwinding tension of the intermediate film materials is 200 N, the pressure is 0.4 MPa, the hot pressing temperature is 200℃, and the film feeding speed is 30 m / min. Then, the release layer and polymer base layer on both sides of the modified polymer base film are peeled off to obtain the composite current collector.

[0091] Example 2 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.4 MPa and the hot-pressing temperature is 180°C.

[0092] Example 3 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.5 MPa and the hot-pressing temperature is 180°C.

[0093] Example 4 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.5 MPa and the hot pressing temperature is 200℃.

[0094] Example 5 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.4 MPa and the hot-pressing temperature is 190°C.

[0095] Comparative Example 1 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.4 MPa and the hot-pressing temperature is 160°C.

[0096] Comparative Example 2 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.5 MPa and the hot-pressing temperature is 160°C.

[0097] Comparative Example 3 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.6 MPa and the hot-pressing temperature is 160°C.

[0098] Comparative Example 4 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.6 MPa and the hot-pressing temperature is 180°C.

[0099] Comparative Example 5 This application provides a method for preparing a composite current collector, which differs from Example 1 only in that: in S3, the pressure is 0.6 MPa and the hot-pressing temperature is 200℃.

[0100] Comparative Example 6 This application provides a comparative example of a method for preparing a composite current collector, comprising the following steps: (1) Unmodified polypropylene particles were sequentially subjected to melt extrusion and biaxial stretching to obtain a polypropylene base film with a thickness of 4 μm; then the polypropylene base film was subjected to plasma treatment to graft polar groups onto the surface.

[0101] (2) The polypropylene base film from (1) is fixed on the unwinding roller and fed into the magnetron sputtering chamber for magnetron sputtering coating, and then wound up to obtain a polymer base film with metal underlayers on both sides; wherein the unwinding tension is 150 N, the winding tension is 180 N, the belt speed is 10 m / min, and the vacuum degree in the vacuum chamber is 10 -3 The sputtering power is 4KW; the thickness of a single metal substrate is 100 nm.

[0102] (3) Fix the polypropylene base film with metal underlayers on both sides onto the unwinding roller and transport it to the water plating tank for thickening treatment until the thickness of the metal underlayer is increased to 1 μm (i.e., the thickness of a single metal layer is 1 μm). Then, rewind to obtain a coiled composite current collector. The unwinding tension is 220 N, the tension of the conveying roller in the water plating tank is 240 N, the rewinding tension is 280 N, the belt speed is 6 m / min, and the composition of the reagents in the water plating tank is: copper sulfate concentration 210 g / L, sulfuric acid concentration 80 g / L, chloride ion concentration 60 ppm, brightener concentration 6 ml / L, and current density 2 A / dm. 2 The processing temperature is 25℃.

[0103] Comparative Example 7 This application provides a comparative example of a method for preparing a composite current collector, comprising the following steps: (1) Maleic anhydride-grafted polypropylene particles were subjected to melt extrusion and stretching treatment in sequence. The grafting unit was a polymaleic anhydride segment with a number average molecular weight of 40,000 and a grafting rate of 0.4%. The number average molecular weight of the maleic anhydride-grafted polypropylene particles was 400,000. The melt extrusion temperature was 250℃ and the temperature of the cooling roller at the rear end was 30℃. The stretching treatment was biaxial stretching: the longitudinal stretching temperature was 130℃ and the stretching ratio was 4 times. The transverse stretching temperature was 150℃ and the stretching ratio was 6 times. The film stretching speed was uniformly 300 m / min and the film stretching tension was uniformly 200 N. A modified polypropylene base film with a thickness of 4 μm was obtained.

[0104] (2) The modified polypropylene base film is fixed on the unwinding roller and fed into the magnetron sputtering chamber for magnetron sputtering coating, and then wound up to obtain a polymer base film with metal underlayers on both sides; wherein the unwinding tension is 150 N, the winding tension is 180 N, the belt speed is 10 m / min, and the vacuum degree in the vacuum chamber is 10 -3 The sputtering power is 4 KW; the thickness of a single metal substrate is 100 nm.

[0105] (3) The modified polypropylene base film with metal underlayers on both sides is fixed on the unwinding roller and conveyed to the water plating tank for thickening treatment until the thickness of the underlayer metal layer is increased to 1 μm (i.e., the thickness of a single metal layer is 1 μm). Then, it is wound up to obtain a coiled composite current collector. The unwinding tension is 220 N, the tension of the conveying roller in the water plating tank is 240 N, the winding tension is 280 N, the belt speed is 6 m / min, and the composition of the reagents in the water plating tank is: copper sulfate concentration 210 g / L, sulfuric acid concentration 80 g / L, chloride ion concentration 60 ppm, brightener concentration 6 ml / L, and current density 2 A / dm. 2 The processing temperature is 25℃.

[0106] To better understand the process differences between the corresponding embodiments and comparative examples, a summary explanation is provided here in tabular form, as detailed in Table 1.

[0107] Table 1

[0108] Test case (1) The composite current collectors prepared in Example 1, Comparative Example 6 and Comparative Example 7 were used as test samples. The peel strength between the polypropylene base film and the metal layer in each sample was then tested. The specific test parameters were as follows: 15 mm wide 3M681 tape was used to adhere to the composite copper foil, and 10 N rubber roller was used to roll back and forth 3 times. The 3M681 tape and the composite copper foil were peeled off at 180 degrees using a tensile testing machine. The test results were then summarized in Table 2.

[0109] Table 2

[0110] As can be seen from Table 2, the composite current collector prepared using the preparation process provided in the embodiments of this application has the advantage of more reliable bonding and significantly improved bonding strength compared to directly modifying the surface of the polypropylene base film to form a metal layer or directly forming a metal layer on the surface of the modified polypropylene base film using magnetron sputtering coating and thickening technology.

[0111] (2) The composite current collectors prepared in Examples 1-5 and Comparative Examples 1-5 were used as test samples. The peel strength between the polypropylene base film and the metal layer and the flatness of the composite current collector were tested in each sample. The test steps for the flatness of the film are as follows: the sample is laid flat and unfolded without tension. The number of wrinkles on the film surface is observed directly by the naked eye. The test results are then summarized in Table 3.

[0112] Table 3

[0113] It should be noted that “—” in Comparative Examples 1 to 3 indicates that peel strength testing was not performed. This is because the processing temperature was low, and the modified polypropylene base film and the metal layer were not completely bonded, resulting in many delamination areas, so the test was not performed.

[0114] Referring to Table 3, the test results of Examples 1-5 and Comparative Examples 1-5 show that when the pressure and temperature during the hot-pressing process are simultaneously limited to a specific range, the interfacial bonding strength between the modified polypropylene base film and the metal layer can be significantly improved. At the same time, the composite current collector also has the advantage of high film surface flatness.

[0115] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A method for preparing a composite current collector, characterized in that, Includes the following steps: Maleic anhydride-grafted polypropylene particles were sequentially subjected to melt extrusion and stretching treatments to obtain a modified polypropylene base film. The modified polypropylene base film and two metal layers are simultaneously conveyed to a hot press roller for hot pressing and bonding. The modified polypropylene base film is located between the two metal layers. In the hot pressing and bonding process, the pressure is 0.3 MPa to 0.5 MPa, the hot pressing temperature is 180℃ to 220℃, and the film feeding speed is 5 m / min to 50 m / min, so that each side of the modified polypropylene base film is bonded to one of the metal layers to obtain a composite current collector.

2. The method for preparing the composite current collector according to claim 1, characterized in that, In the hot-pressing bonding process, the pressure is 0.35 MPa to 0.45 MPa, the hot-pressing temperature is 190℃ to 210℃, and the film speed is 5 m / min to 20 m / min.

3. The method for preparing the composite current collector according to claim 1, characterized in that, In the maleic anhydride-grafted polypropylene particles, the grafting unit is selected from polymaleic anhydride segments. Optionally, the grafting rate of the maleic anhydride-grafted polypropylene particles is 0.3% to 0.5%.

4. The method for preparing the composite current collector according to claim 3, characterized in that, The number-average molecular weight of the polymaleic anhydride segments is 30,000 to 50,000, and the number-average molecular weight of the maleic anhydride-grafted polypropylene particles is 380,000 to 430,000.

5. The method for preparing the composite current collector according to any one of claims 1 to 4, characterized in that, The stretching process includes: first stretching the film material obtained by melt extrusion treatment longitudinally and then stretching it transversely; In the longitudinal stretching step, the stretching temperature is 130℃~150℃ and the stretching ratio is 3 to 5 times; and / or, in the transverse stretching step, the stretching temperature is 150℃~170℃ and the stretching ratio is 5 to 7 times.

6. The method for preparing the composite current collector according to any one of claims 1 to 4, characterized in that, The step of simultaneously conveying the modified polypropylene base film and the two metal layers to the hot press rollers for hot pressing and bonding includes: A polymer substrate is provided, the polymer substrate comprising a polymer substrate layer and a release layer located on one side surface of the polymer substrate layer, the thickness of the polymer substrate layer being not less than 20 μm; a metal underlayer is formed on the side surface of the release layer opposite to the polymer substrate layer using magnetron sputtering coating technology, and then the metal underlayer is thickened to form a metal layer of a preset thickness, thereby obtaining a transfer film material; The modified polypropylene base film and the two intermediate film materials are simultaneously transported to the hot press rollers for hot pressing and bonding. The metal layers in the two intermediate film materials are located on the side close to the modified polypropylene base film, so that the two sides of the modified polypropylene base film are respectively bonded to a metal layer. Then, the release layer and the polymer base layer on both sides of the modified polypropylene base film are removed.

7. The method for preparing the composite current collector according to claim 6, characterized in that, The thickness of the polymer substrate layer is 25 μm to 100 μm, and / or the thickness of a single metal layer is 1 μm to 1.5 μm.

8. The method for preparing the composite current collector according to claim 6, characterized in that, The peel strength of the release layer is 5 g / 25mm to 15 g / 25mm.

9. The method for preparing the composite current collector according to claim 8, characterized in that, The release layer is made of at least one of silicone oil, fluorosilicone, polypropylene resin and polymethylpentene resin, and the thickness of the release layer is 5 μm to 10 μm.

10. A composite current collector, characterized in that, The modified polypropylene base film is prepared by any one of the preparation methods described in claims 1 to 9, wherein the peel strength between the modified polypropylene base film and the metal layer is not less than 2.0 N / cm.

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