Base film for composite current collector, preparation method of base film and composite current collector

Abstract

The invention provides a base film for a composite current collector, a preparation method of the base film and the composite current collector, and belongs to the technical field of composite current collector manufacturing. The base film for the composite current collector comprises a three-layer co-extruded core layer and surface layers located on the two sides of the core layer, the core layer is made of polypropylene, the surface layers are made of polypropylene and polyethyleneimine, and in the surface layers, the mass of the polypropylene is not smaller than that of the polyethyleneimine. The base film has excellent heat resistance and has excellent bonding strength with the metal layer, and meanwhile, the preparation method has the advantages of being environmentally friendly, easy to industrialize, simple in process step, low in manufacturing cost and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of composite current collector manufacturing, in particular to a base film for composite current collector, a preparation method thereof and a composite current collector. BACKGROUND

[0002] As a key material for improving the energy density and safety of secondary batteries, the composite current collector is usually composed of a polymer substrate and metal layers on both sides of the polymer substrate. Among them, polypropylene has become one of the mainstream materials for the polymer substrate due to its strong chemical stability, good safety and low cost.

[0003] However, polypropylene also has some defects: (1) polypropylene is a non-polar polymer material, which has poor adhesion with the metal layer, resulting in easy peeling of the metal layer of the corresponding composite current collector. (2) The upper limit of the use temperature of the traditional polypropylene base film is usually around 100℃, which leads to defects in high temperature environment, for example, when the battery is in thermal runaway state (its temperature is usually as high as 150℃), the polypropylene base film will seriously shrink and deform, resulting in wrinkles and fracture of the metal layer, and loss of conductivity; for another example, the polypropylene film is prone to shrinkage, deformation and melting during high temperature processing, which seriously restricts the production efficiency and yield, resulting in a significant increase in production cost.

[0004] At present, for defect (1), the current mainstream methods include chemical etching, adhesive coating and plasma treatment. However, the current technical means all have their own defects. Specifically, chemical etching can damage the mechanical properties of the substrate; adhesive coating has poor environmental friendliness, and increases the process flow and easily introduces impurities; the time effectiveness of plasma treatment is short, it is difficult to ensure uniformity when treating a large area, and the equipment cost is high. For defect (2), the current main technical means include adding inorganic fillers to the substrate raw material, coating a heat-resistant coating on the surface of the base film, and UV crosslinking treatment of the base film. However, these methods all have their own defects. Specifically, adding inorganic fillers can easily reduce the processing fluidity of the substrate raw material, thereby making it difficult to form the base film and affecting the mechanical properties of the base film; coating a heat-resistant coating on the surface of the base film has the problem that the bonding strength between the heat-resistant coating and the base film is low and the heat-resistant coating can easily fall off later, and it also affects the bonding strength between the base film and the metal layer; UV crosslinking treatment needs to add a photoinitiator in the base film in advance, which can introduce impurities and reduce the mechanical properties of the base film, and the UV energy is weak, easy to decay and only acts on the surface of the base film (i.e. the heat resistance improvement is limited). SUMMARY

[0005] The application aims to provide a base film for a composite current collector, a preparation method of the base film and a composite current collector, the base film has excellent heat resistance and excellent bonding strength with a metal layer, and the preparation method has the advantages of green environmental protection, easy industrialization, simple process steps and low manufacturing cost.

[0006] Embodiments of the application are implemented as follows: In a first aspect, the embodiments of the application provide a base film for a composite current collector, comprising a three-layer co-extruded core layer and surface layers located on both sides of the core layer, wherein the material of the core layer comprises polypropylene, the material of the surface layers comprises polypropylene and polyethylene imine, and in the surface layers, the mass of polypropylene is not less than the mass of polyethylene imine.

[0007] In the above technical solution, the base film has a three-layer co-extruded structure, specifically comprising a core layer in the middle and surface layers located on both sides of the core layer, wherein the material of the surface layers is mainly polypropylene and supplemented by polyethylene imine, so that polyethylene imine is attached to the surface of polypropylene and the two are combined firmly. On the one hand, the presence of polyethylene imine can increase the surface polarity of polypropylene and enhance the wettability of polypropylene with the surface of the metal layer. At the same time, polyethylene imine can also form hydrogen bonds or coordination bonds with the polar groups (such as hydroxyl groups) on the surface of the metal layer, so that the base film and the metal layer have excellent bonding strength. On the other hand, polyethylene imine has a highly cross-linked dendritic structure and stable imine bonds, so it has excellent heat resistance and excellent chemical stability, thereby making the corresponding base film have excellent heat resistance and also enabling the corresponding base film to have excellent stability in the electrolyte. That is, the base film provided by the embodiments of the application has excellent heat resistance and excellent bonding strength with the metal layer.

[0008] In some optional embodiments, in the surface layers, the mass ratio of polypropylene to polyethylene imine is (85-99):(1-15).

[0009] In the above technical solution, the mass ratio of polypropylene to polyethylene imine in the surface layers is limited within the above range, so that there is a relatively appropriate amount of polyethylene imine in the surface layers, which can effectively improve the heat resistance of the base film and the bonding strength of the base film and the metal layer while maintaining the inherent properties (such as mechanical properties) of polypropylene.

[0010] In some optional embodiments, in the surface layers, the mass ratio of polypropylene to polyethylene imine is (85-95):(3-10).

[0011] In the technical solution, the mass ratio of the polypropylene and the polyethylene imine in the surface layer is limited in the range, so that the surface layer has a more appropriate amount of the polyethylene imine, and the heat resistance of the base film and the bonding strength between the base film and the metal layer can be effectively improved while the inherent properties (for example, mechanical properties) of the polypropylene are maintained.

[0012] In some optional embodiments, the surface layer further includes maleic anhydride grafted polypropylene.

[0013] In the technical solution, the surface layer further includes maleic anhydride grafted polypropylene, which can improve the interfacial compatibility between the polypropylene and the polyethylene imine in the surface layer, help to reduce the risk of phase separation of the two, thereby improving the bonding strength of the two and the structural uniformity of the surface layer; at the same time, since the maleic anhydride grafted polypropylene also has a polar group, it also helps to improve the bonding strength between the base material and the metal layer.

[0014] In some optional embodiments, in the surface layer, the mass of the maleic anhydride grafted polypropylene is m1, the sum of the masses of the polypropylene and the polyethylene imine is m2, and the ratio of m1 and m2 is (3-5):(95-97).

[0015] In the technical solution, the mass ratio of the polypropylene and the polyethylene imine in the surface layer is limited in the range, so that the surface layer has a more appropriate amount of the polyethylene imine, and the heat resistance of the base film and the bonding strength between the base film and the metal layer can be effectively improved while the inherent properties (for example, mechanical properties) of the polypropylene are maintained.

[0016] In a second aspect, the embodiments of the present application provide a preparation method of a base film for a composite current collector, including the following steps: melt granulating a surface layer raw material, wherein the surface layer raw material includes polypropylene and polyethylene imine, and in the surface layer raw material, the mass of the polypropylene is not less than the mass of the polyethylene imine, to obtain surface layer granules; adding two portions of the surface layer granules and one portion of core layer granules into three extruders for melt plasticization, wherein the material of the core layer granules includes polypropylene; then performing co-extrusion casting through a three-layer co-extrusion die, wherein the core layer is located between the two surface layers, to obtain a base film precursor with a three-layer structure; and performing stretching treatment on the base film precursor to obtain a base film.

[0017] In the technical scheme, the surface layer raw material containing polypropylene and polyethylene imine is melt granulated (i.e. after being heated and melted, the polypropylene and polyethylene imine are granulated again), so that the polyethylene imine is firmly attached to the surface of the polypropylene and connected with the polypropylene body. On the one hand, the presence of the polyethylene imine can increase the surface polarity of the polypropylene and enhance the wettability of the polypropylene and the surface of the metal layer. Meanwhile, the polyethylene imine can form hydrogen bonds or coordination bonds with the polar groups (such as hydroxyl groups) on the surface of the metal layer, so that the prepared base film has excellent bonding strength with the metal layer. On the other hand, the presence of the polyethylene imine can also make the corresponding base film have excellent heat resistance and chemical stability. In addition, the preparation method has the advantages of green environmental protection, easy industrialization, simple process steps and low manufacturing cost.

[0018] In some optional embodiments, the surface layer raw material further includes maleic anhydride grafted polypropylene.

[0019] In the technical scheme, the surface layer raw material further includes maleic anhydride grafted polypropylene, which can act as a compatibilizer during the melt granulation process. Specifically, the maleic anhydride groups in the maleic anhydride grafted polypropylene can react with the polar groups in the polyethylene imine, so that the polypropylene and the polyethylene imine are better combined together, which helps to reduce the risk of phase separation after the combination of the two, thereby improving the structural uniformity of the surface layer.

[0020] In some optional embodiments, the processing temperature in the melt granulation step is 185°C-230°C.

[0021] In the technical scheme, the processing temperature in the melt granulation step is limited within the above range, so that the various components can be efficiently and thoroughly mixed, and the maleic anhydride grafted polypropylene and the polyethylene imine can be fully reacted, thereby making the polypropylene and the polyethylene imine better combined together.

[0022] In some optional embodiments, after the stretching treatment, the base film precursor is sequentially subjected to supercritical carbon dioxide treatment and gamma-ray irradiation cross-linking treatment.

[0023] In the technical scheme, after the stretching treatment, the supercritical carbon dioxide treatment and the gamma-ray irradiation cross-linking treatment are sequentially performed. The supercritical carbon dioxide treatment can instantaneously penetrate and uniformly plasticize the entire base film, so that the molecular chains of the polypropylene are relaxed. During the subsequent gamma-ray irradiation cross-linking treatment, the uniformity of the irradiation cross-linking (i.e. the improvement of the heat resistance is more obvious) can be promoted, and the dose required for the irradiation cross-linking can be reduced, so that the base film has improved heat resistance while maintaining excellent mechanical properties.

[0024] In some optional embodiments, in the step of supercritical carbon dioxide treatment, the treatment pressure is 10 MPa to 30 MPa, the treatment temperature is 40°C to 60°C, and the treatment time is 5 min to 30 min; or / and, in the step of gamma-ray irradiation crosslinking treatment, the irradiation dose is 5 kGy to 20 kGy.

[0025] In the above technical solution, the treatment pressure, the treatment temperature and the treatment time in the step of supercritical carbon dioxide treatment are respectively limited in the above ranges, so that the treated polypropylene has a more suitable relaxation degree, which helps to promote the uniformity of irradiation crosslinking and effectively reduce the dose required for irradiation crosslinking; the dose of irradiation crosslinking is limited in the above range, which can more effectively improve the heat resistance of the base film while maintaining the mechanical properties of the base film.

[0026] In a third aspect, the embodiments of the present application provide a composite current collector, comprising a polymer substrate layer and a metal layer located on at least one side surface of the polymer substrate layer, and the polymer substrate layer is the base film provided in the first aspect or the base film prepared by the preparation method provided in the second aspect.

[0027] In the above technical solution, the composite current collector comprises a polymer substrate layer and a metal layer located on at least one side surface of the polymer substrate layer, and the polymer substrate layer is a three-layer co-extrusion structure, and the surface of the polypropylene in the surface layer is attached with polyethylene imine, so that the polymer substrate layer has excellent heat resistance and excellent bonding strength with the metal layer. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows, and it should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0029] Figure 1 A structural schematic diagram of a base film provided in the embodiments of the present application; Figure 2 A structural schematic diagram of a preparation method of a base film provided in the embodiments of the present application.

[0030] Figure legend: 10-base film; 100-core layer; 200-surface layer. DETAILED DESCRIPTION

[0031] In order to make the purposes, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Unless otherwise specified in the embodiments, the implementation is performed according to conventional conditions or the conditions recommended by the manufacturer. Unless otherwise specified, the reagents or instruments used are conventional products that can be purchased on the market.

[0032] It should be noted that, in the present application, “and / or”, such as “feature 1 and / or feature 2”, refers to “feature 1” alone, “feature 2” alone, or “feature 1” plus “feature 2”.

[0033] In addition, in the description of the present application, “multiple” in “one or more” means two or more; the range of “value a~value b” includes both end values “a” and “b”, and “unit of measurement” in “value a~value b+unit of measurement” represents the “unit of measurement” of both “value a” and “value b”.

[0034] The composite current collector base film, the preparation method thereof, and the composite current collector according to an embodiment of the present application will be described in detail below.

[0035] In a first aspect, the present application provides a composite current collector base film, which comprises a three-layer co-extruded core layer and surface layers located on both sides of the core layer, wherein the material of the core layer comprises polypropylene, and the material of the surface layers comprises polypropylene and polyethylene imine, and in the surface layers, the mass of polypropylene is not less than the mass of polyethylene imine.

[0036] In the present application, the base film has a three-layer co-extruded structure, specifically comprising a core layer in the middle and surface layers located on both sides of the core layer, wherein the material of the surface layers mainly comprises polypropylene and is supplemented by polyethylene imine, so that polyethylene imine is attached to the surface of polypropylene and the two are combined relatively firmly. On the one hand, the presence of polyethylene imine can increase the surface polarity of polypropylene and enhance the wettability of polypropylene with the surface of the metal layer; at the same time, polyethylene imine can also form hydrogen bonds or coordination bonds with the polar groups (such as hydroxyl groups) on the surface of the metal layer, so that the base film and the metal layer have relatively excellent bonding strength. On the other hand, polyethylene imine has a highly cross-linked dendritic structure and stable imine bonds, so that it has relatively excellent heat resistance and relatively excellent chemical stability, thereby making the corresponding base film have relatively excellent heat resistance and also enabling the corresponding base film to have relatively excellent stability in the electrolyte. That is, the base film provided in the present application has relatively excellent heat resistance and relatively excellent bonding strength with the metal layer.

[0037] In order to better understand the structure of the base film, the structure of a base film is used as an aid for illustration here. Referring to Figure 1The base film 10 comprises a core layer 100 and a surface layer 200 on both sides of the core layer 100.

[0038] As an example, the mass ratio of polypropylene to polyethylene imine in the surface layer is (85-99):(1-15), such as but not limited to any one of the point values or the range values between any two of 85:15, 90:10, 95:5 and 99:1.

[0039] In this embodiment, the mass ratio of polypropylene to polyethylene imine in the surface layer is limited in the above range, so that the surface layer has a suitable amount of polyethylene imine, which can effectively improve the heat resistance of the base film and the bonding strength between the base film and the metal layer while maintaining the inherent properties of polypropylene (such as mechanical properties).

[0040] As an example, the mass ratio of polypropylene to polyethylene imine in the surface layer is (85-95):(3-10), such as but not limited to any one of the point values or the range values between any two of 85:10, 90:8, 92:6 and 95:3.

[0041] In this embodiment, the mass ratio of polypropylene to polyethylene imine in the surface layer is limited in the above range, so that the surface layer has a more suitable amount of polyethylene imine, which can more effectively improve the heat resistance of the base film and the bonding strength between the base film and the metal layer while maintaining the inherent properties of polypropylene (such as mechanical properties).

[0042] As an example, the surface layer further comprises maleic anhydride grafted polypropylene.

[0043] In this embodiment, the surface layer further contains maleic anhydride grafted polypropylene, which can improve the interfacial compatibility between polypropylene and polyethylene imine in the surface layer, help to reduce the risk of phase separation of the two, thereby improving the bonding strength of the two and the structural uniformity of the surface layer; at the same time, since the maleic anhydride grafted polypropylene also has a polar group, it also helps to improve the bonding strength between the base material and the metal layer.

[0044] As an example, the mass of the maleic anhydride grafted polypropylene in the surface layer is m1, the sum of the masses of polypropylene and polyethylene imine is m2, and the ratio of m1 to m2 is (3-5):(95-97), such as but not limited to any one of the point values or the range values between any two of 3:97, 4:96 and 5:95.

[0045] In this embodiment, the mass ratio of the maleic anhydride grafted polypropylene in the surface layer is limited in the above range, so that the surface layer has a suitable amount of maleic anhydride grafted polypropylene, which can effectively improve the interfacial compatibility between polypropylene and polyethylene imine.

[0046] As an example, the ratio of the thickness of the core layer to the thickness of the single surface layer is (6~8):(1~2), such as but not limited to any one of 6:1, 6:2, 7:1, 7:2, 8:1 and 8:2 point values or a range value between any two of them.

[0047] In this embodiment, the ratio of the thickness of the core layer to the thickness of the single surface layer is within the above range, which can improve the bonding strength between the substrate and the metal layer while better maintaining the inherent properties of polypropylene (such as chemical stability and mechanical properties, etc.).

[0048] It should be noted that the thickness of the base film is not limited, and can be adaptively adjusted according to actual needs.

[0049] As an example, the thickness of the base film is 2 μm~10 μm, such as but not limited to any one of 2 μm, 4 μm, 6 μm, 8 μm and 10 μm point values or a range value between any two of them.

[0050] As an example, the heat shrinkage rate of the base film after being heated at 150℃ for 30 min is 1.2%~2.5%, such as but not limited to any one of 1.2%, 1.5%, 2% and 2.5% point values or a range value between any two of them.

[0051] In this embodiment, the heat shrinkage rate of the base film after being heated at 150℃ for 30 min is within the above range, i.e., the base film has excellent heat resistance.

[0052] It should be noted that the upper limit of the use temperature of the conventional polypropylene base film is usually 105℃, which leads to defects in high-temperature environments, such as when the battery is in a thermal runaway state (its temperature is usually as high as 150℃), which makes the polypropylene base film severely shrink and deform in the thermal runaway state, causing the metal layer to wrinkle and break, losing the ability to conduct electricity; for example, the polypropylene film is prone to shrinkage, deformation and melting during high-temperature processing, which seriously restricts production efficiency and yield, resulting in a significant increase in production cost. However, the base film in the embodiments of the present application has excellent heat resistance, thereby effectively improving the problem that the polypropylene base film is prone to defects in high-temperature environments.

[0053] As an example, the longitudinal tensile strength of the base film is 255 MPa~285 MPa, and the transverse tensile strength of the base film is 200 MPa~230 MPa.

[0054] In this embodiment, the longitudinal tensile strength and the transverse tensile strength of the base film are within the above ranges, respectively, indicating that the base film has excellent mechanical properties.

[0055] It should be noted that the structures not specially mentioned or limited in the base film can be set according to the conventional selection in the field.

[0056] In a second aspect, the embodiments of the present application provide a preparation method of a base film for a composite current collector, comprising the following steps: melt granulating a surface layer raw material, wherein the surface layer raw material comprises polypropylene and polyethylene imine, and the mass of the polypropylene in the surface layer raw material is not less than the mass of the polyethylene imine, to obtain surface layer granules; adding two portions of the surface layer granules and one portion of core layer granules into three extruders for melt plasticization, wherein the material of the core layer granules comprises polypropylene; then performing co-extrusion casting through a three-layer co-extrusion die, wherein the core layer is located between the two surface layers, to obtain a base film precursor with a three-layer structure; and performing stretching treatment on the base film precursor to obtain a base film.

[0057] In the present application, the surface layer raw material containing polypropylene and polyethylene imine is melt granulated (i.e., after heating and melting the polypropylene and the polyethylene imine, they are re-granulated), which can make the polyethylene imine firmly adhere to the surface of the polypropylene and be connected with the polypropylene body. On the one hand, the presence of the polyethylene imine can increase the surface polarity of the polypropylene and enhance the wettability of the polypropylene with the surface of the metal layer. At the same time, the polyethylene imine can also form hydrogen bonds or coordination bonds with the polar groups (such as hydroxyl groups) on the surface of the metal layer, so that the base film prepared finally has excellent bonding strength with the metal layer. On the other hand, the presence of the polyethylene imine can also make the corresponding base film have excellent heat resistance and chemical stability.

[0058] In addition, the preparation method also has the advantages of green environmental protection, easy industrialization, simple process steps, and low manufacturing cost. Specifically, compared with chemical etching, the above preparation method has less influence on the mechanical properties of the polypropylene material; compared with adhesive coating, the above preparation method has the advantages of green environmental protection and simple process; compared with plasma treatment, the above preparation method has the advantages of long time effectiveness (i.e., the base film and the metal layer have excellent bonding strength for a long time), easy industrialization (conventional three-layer co-extrusion equipment and stretching equipment can be used), and low manufacturing cost.

[0059] As an example, in the surface layer raw material, the mass ratio of the polypropylene to the polyethylene imine is (85-99) : (1-15), such as but not limited to any one point value or a range value between any two of 85:15, 90:10, 95:5, and 99:1.

[0060] In this embodiment, the mass ratio of the polypropylene and the polyethylene imine in the surface layer raw material is limited in the above range, so that the surface layer raw material has a more appropriate amount of the polyethylene imine, and the heat resistance of the base film and the bonding strength between the base film and the metal layer can be more effectively improved while maintaining the inherent properties (for example, mechanical properties) of the polypropylene.

[0061] As an example, the mass ratio of the polypropylene and the polyethylene imine in the surface layer raw material is (85-95):(3-10), for example, but not limited to, any one of the point values of 85:10, 90:8, 92:6 and 95:3 or a range value between any two of them.

[0062] In this embodiment, the mass ratio of the polypropylene and the polyethylene imine in the surface layer raw material is limited in the above range, so that the surface layer raw material has a more appropriate amount of the polyethylene imine, and the heat resistance of the base film and the bonding strength between the base film and the metal layer can be more effectively improved while maintaining the inherent properties (for example, mechanical properties) of the polypropylene.

[0063] As an example, the surface layer raw material further includes maleic anhydride grafted polypropylene.

[0064] In this embodiment, the surface layer raw material further includes maleic anhydride grafted polypropylene, which can act as a compatibilizer during the melt granulation process. Specifically, the maleic anhydride groups in the maleic anhydride grafted polypropylene react with the polar groups in the polyethylene imine, so that the polypropylene and the polyethylene imine are better combined together, which helps to reduce the risk of phase separation after the combination of the two, thereby improving the structural uniformity of the surface layer.

[0065] As an example, the mass of the maleic anhydride grafted polypropylene in the surface layer raw material is m1, the sum of the masses of the polypropylene and the polyethylene imine is m2, and the ratio of m1 and m2 is (3-5):(95-97), for example, but not limited to, any one of the point values of 3:97, 4:96 and 5:95 or a range value between any two of them.

[0066] In this embodiment, the mass ratio of the maleic anhydride grafted polypropylene in the surface layer raw material is limited in the above range, so that the surface layer raw material has a more appropriate amount of the maleic anhydride grafted polypropylene, and the interfacial compatibility between the polypropylene and the polyethylene imine can be more effectively improved.

[0067] As an example, in the step of melt granulation, the processing temperature is 185°C-230°C, for example, but not limited to, any one of the point values of 185°C, 190°C, 200°C, 210°C, 220°C and 230°C or a range value between any two of them.

[0068] In this embodiment, the processing temperature in the melt granulation step is limited in the above range, which can make the various components melt and mix more efficiently and thoroughly, and at the same time, can make the maleic anhydride grafted polypropylene and the polyethylene imine fully react, and further make the polypropylene and the polyethylene imine better combined together.

[0069] As an example, the polypropylene in the core layer and the surface layer is selected from at least one of homopolymer polypropylene or / and copolymer polypropylene, and the melt index (230℃, 2.16 kg) of the polypropylene is 2 g / 10 min~10 g / 10 min, for example but not limited to any one point value or a range value between any two of 2 g / 10 min, 4 g / 10 min, 6 g / 10 min, 8 g / 10 min and 10 g / 10 min.

[0070] In this embodiment, the polypropylene with the above characteristics is selected, which has good processing performance and can make the prepared base film have excellent tensile resistance.

[0071] It should be noted that the stretching treatment mode is not limited, which can be unidirectional stretching or bidirectional stretching, and the specific mode can be adaptively selected according to actual needs.

[0072] As an example, the stretching treatment mode is bidirectional stretching, which includes: first performing longitudinal stretching on the base film precursor, wherein the processing temperature is 120℃~140℃ (for example but not limited to any one point value or a range value between any two of 120℃, 130℃ and 140℃), and the stretching ratio is 3 times~5 times (for example but not limited to any one point value or a range value between any two of 3 times, 4 times and 5 times); and then performing transverse stretching, wherein the processing temperature is 150℃~170℃ (for example but not limited to any one point value or a range value between any two of 150℃, 160℃ and 170℃), and the stretching ratio is 3 times~10 times (for example but not limited to any one point value or a range value between any two of 3 times, 4 times, 6 times, 8 times and 10 times), to obtain the base film.

[0073] In this embodiment, the bidirectional stretching is adopted and the above parameters are used, so that the prepared base film has excellent mechanical properties.

[0074] As an example, before the step of bidirectional stretching, a step of preheating the base film precursor is further included, wherein the preheating temperature is 90-110°C (for example, but not limited to, any one point value or a range value between any two of 90°C, 100°C and 110°C), and the preheating time is 3-5 min (for example, but not limited to, any one point value or a range value between any two of 3 min, 4 min and 5 min).

[0075] In this embodiment, the preheating of the base film precursor according to the above parameter conditions can reduce stress concentration in the subsequent bidirectional stretching process, thereby preparing a base film with more excellent mechanical properties.

[0076] As an example, after the stretching treatment, a step of sequentially performing supercritical carbon dioxide treatment and gamma-ray irradiation crosslinking treatment on the base film precursor is further included.

[0077] In this embodiment, the supercritical carbon dioxide treatment and gamma-ray irradiation crosslinking treatment are sequentially performed after the stretching treatment, wherein the supercritical carbon dioxide treatment can instantaneously penetrate and uniformly plasticize the entire base film, so that the molecular chains of polypropylene are relaxed, and during the subsequent gamma-ray irradiation crosslinking treatment (gamma-ray irradiation crosslinking can cause the molecular chains of polypropylene to undergo a bulk crosslinking reaction to form a three-dimensional network structure, thereby limiting the thermal motion of the molecular chains to improve the heat resistance), the uniformity of irradiation crosslinking (i.e., the improvement of heat resistance is more obvious) can be promoted, and at the same time, the dose required for irradiation crosslinking can be reduced, so that the base film improves the heat resistance while maintaining excellent mechanical properties.

[0078] It should be noted that, at present, in order to improve the heat resistance of the base film, the main technical means include adding inorganic fillers to the base material, coating a heat-resistant coating on the surface of the base film, and performing UV crosslinking treatment on the base film. However, these means all have their own defects. Specifically, adding inorganic fillers can easily reduce the processing flowability of the base material, thereby making it difficult to form the base film and affecting the mechanical properties of the base film; coating a heat-resistant coating on the surface of the base film has the problem that the bonding strength between the heat-resistant coating and the base film is low and the heat-resistant coating is easy to fall off subsequently, and at the same time, it can affect the bonding strength between the base film and the metal layer; the UV crosslinking treatment needs to add a photoinitiator in the base film in advance, which can introduce impurities to reduce the mechanical properties of the base film, and the UV energy is weak, easy to decay and only acts on the surface of the base film (i.e., the improvement of heat resistance is limited). However, in the present application, polyethyleneimine is selected as an auxiliary additive material for polypropylene, and the base film precursor is sequentially subjected to supercritical carbon dioxide treatment and gamma-ray irradiation crosslinking treatment, which can not only effectively improve the heat resistance of the base film, but also overcome the defects of the above-mentioned many existing technical means.

[0079] It should be noted that the supercritical carbon dioxide treatment of the base film precursor in turn can also produce micropores on the surface of the base film, increase its specific surface area, and to some extent, improve the bonding strength of the base film and the metal layer.

[0080] As an example, in the step of supercritical carbon dioxide treatment, the treatment pressure is 10 MPa to 30 MPa (for example, but not limited to, any one of the point values of 10 MPa, 20 MPa and 30 MPa or the range value between any two of them), the treatment temperature is 40°C to 60°C (for example, but not limited to, any one of the point values of 40°C, 50°C and 60°C or the range value between any two of them), and the treatment time is 5 min to 30 min (for example, but not limited to, any one of the point values of 5 min, 10 min, 20 min and 30 min or the range value between any two of them).

[0081] In this embodiment, the treatment pressure, the treatment temperature and the treatment time in the step of supercritical carbon dioxide treatment are respectively limited in the above ranges, so that the polypropylene after treatment has a more appropriate relaxation, which helps to promote the uniformity of irradiation crosslinking and effectively reduce the dose required for irradiation crosslinking.

[0082] As an example, the thickness of the base film precursor is 4 μm to 6 μm, and in the step of supercritical carbon dioxide treatment, the treatment pressure is 25 MPa to 30 MPa, the treatment temperature is 55°C to 60°C, and the treatment time is 5 min to 15 min.

[0083] In this embodiment, when the thickness of the base film precursor is in the above range, the supercritical carbon dioxide treatment is carried out according to the above parameter conditions, so that the polypropylene after treatment has a more appropriate relaxation, which helps to promote the uniformity of irradiation crosslinking and effectively reduce the dose required for irradiation crosslinking.

[0084] As an example, in the step of γ-ray irradiation crosslinking treatment, the irradiation dose is 5 kGy to 20 kGy, for example, but not limited to, any one of the point values of 5 kGy, 10 kGy, 15 kGy and 20 kGy or the range value between any two of them.

[0085] In this embodiment, the dose of irradiation crosslinking is limited in the above range, which can more effectively improve the heat resistance of the base film while maintaining the mechanical properties of the base film.

[0086] It should be noted that the processes or steps not specially mentioned or limited in the preparation of the base film can be selected according to the conventional selection in the art.

[0087] As an example, a process flow chart of the preparation method of the base film is exemplarily shown in FIG. 1. Figure 2 .

[0088] In a third aspect, the embodiments of the present application provide a composite current collector, comprising a polymer substrate layer and a metal layer located on at least one side surface of the polymer substrate layer, wherein the polymer substrate layer is the base film provided in the first aspect or the base film prepared by the preparation method provided in the second aspect.

[0089] In the present application, the composite current collector comprises a polymer substrate layer and a metal layer located on at least one side surface of the polymer substrate layer, wherein the polymer substrate layer is a three-layer co-extrusion structure, and the surface of the polypropylene in the surface layer is attached with polyethylene imine, so that the polymer substrate layer and the metal layer have excellent bonding strength.

[0090] It should be noted that the material of the metal layer in the composite current collector is not limited, for example, it can be a copper foil, and then the composite current collector is a negative electrode current collector, or it can be an aluminum foil, and then the composite current collector is a positive electrode current collector.

[0091] It should be noted that since the core of the present application is the structural design and preparation method of the base film, the specific type of the composite current collector can be set according to the conventional form in the art, which is not described in detail in the embodiments of the present application.

[0092] The features and performances of the present application are further described in detail below in combination with embodiments.

[0093] Embodiment 1 The embodiments of the present application provide a preparation method of a composite current collector, comprising the following steps: (1) Polypropylene (purchased from North European Chemical, brand HC300BF, melt index 3.3 g / 10min, ash content ≤20ppm, isotactic index 98%), polyethylene imine (molecular weight 1800) and maleic anhydride grafted polypropylene are mixed in a mass ratio of 94:3:3, and then melt granulation is performed by a double screw extruder (screw rotation speed 200 rpm, extrusion temperature 220°C) to obtain surface layer granules; at the same time, polypropylene granules (purchased from North European Chemical, brand HC300BF, melt index 3.3 g / 10min, ash content ≤20ppm, isotactic index 98%) are dried at 90°C for 3 h to obtain core layer granules.

[0094] (2) The two portions of the surface layer granules and one portion of the core layer granules are respectively added to three extruders for melt plasticization (wherein the melt temperature is 220°C), and then co-extrusion casting is performed by a three-layer co-extrusion die, wherein the temperature of the cooling roller after the three-layer co-extrusion die is 25°C, the core layer is located between the two surface layers, and the thickness ratio of the core layer to a single surface layer is 70:15, to obtain a base film precursor with a three-layer structure.

[0095] (3) preheating the base film precursor, wherein the preheating temperature is 100°C and the preheating time is 4 min.

[0096] (4) longitudinally stretching the base film precursor after the preheating, wherein the processing temperature is 130°C and the stretching ratio is 5 times; then transversely stretching the base film precursor, wherein the processing temperature is 160°C and the stretching ratio is 5 times, so that the thickness of the base film precursor is 4.5 μm.

[0097] (5) sequentially performing supercritical carbon dioxide treatment and γ-ray irradiation cross-linking treatment on the base film precursor, wherein the supercritical carbon dioxide treatment comprises: after the biaxially stretched film enters a supercritical CO2 online treatment module, performing supercritical CO2 treatment on the film; wherein the processing pressure is 28 MPa, the temperature is 58°C, and the time is 12 min; in the step of the γ-ray irradiation cross-linking treatment, the irradiation dose is 10 kGy, and the base film is obtained.

[0098] (6) forming a copper foil on both sides of the base film by using a magnetron sputtering coating technology, and obtaining a composite current collector.

[0099] Embodiment 2 The embodiment of the present application provides a preparation method of a composite current collector, comprising the following steps: (1) mixing polypropylene (purchased from North European Chemical, brand HC300BF, melt index 3.3 g / 10min, ash content ≤20ppm, isotacticity 98%), polyethylene imine (molecular weight 1800) and maleic anhydride grafted polypropylene according to a mass ratio of 91:6:3, and then performing melt granulation by using a double-screw extruder (screw rotation speed 200 rpm, extrusion temperature 220°C) to obtain surface layer granules; meanwhile, drying polypropylene granules (purchased from North European Chemical, brand HC300BF, melt index 3.3 g / 10min, ash content ≤20ppm, isotacticity 98%) at 90°C for 3 h to obtain core layer granules.

[0100] (2) adding the two portions of surface layer granules and one portion of core layer granules into three extruders for melt plasticization (wherein the melt temperature is 220°C), and then performing co-extrusion by using a three-layer co-extrusion die, wherein the temperature of the cooling roller after the three-layer co-extrusion die is 25°C, the core layer is located between the two surface layers, and the thickness ratio of the core layer and a single surface layer is 70:15, to obtain a base film precursor with a three-layer structure.

[0101] (3) preheating the base film precursor, wherein the preheating temperature is 100°C and the preheating time is 4 min.

[0102] (4) The base film precursor after preheating treatment is first stretched longitudinally, wherein the treatment temperature is 130°C and the stretching ratio is 5 times; then it is stretched transversely, wherein the treatment temperature is 160°C and the stretching ratio is 5 times, so that the thickness of the base film precursor is 4.5 μm.

[0103] (5) The base film precursor is subjected to supercritical carbon dioxide treatment and γ-ray irradiation crosslinking treatment in sequence. In the step of supercritical carbon dioxide treatment, the treatment pressure is 30 MPa, the temperature is 60°C, and the time is 15 min; in the step of γ-ray irradiation crosslinking treatment, the irradiation dose is 15 kGy, to obtain the base film.

[0104] (6) A copper foil is formed on both sides of the base film by using a magnetron sputtering coating technology, to obtain the composite current collector.

[0105] Example 3 The embodiment of the present application provides a preparation method of a composite current collector, comprising the following steps: (1) Polypropylene (purchased from North European Chemical, brand HC300BF, melt index 3.3 g / 10 min, ash content ≤20 ppm, isotacticity 98%), polyethyleneimine (molecular weight 1800) and maleic anhydride grafted polypropylene are mixed according to the mass ratio of 88:9:3, and then melt granulation is performed by using a double-screw extruder (screw rotation speed 200 rpm, extrusion temperature 220°C) to obtain surface layer granules; at the same time, polypropylene granules (purchased from North European Chemical, brand HC300BF, melt index 3.3 g / 10 min, ash content ≤20 ppm, isotacticity 98%) are dried at 90°C for 3 h to obtain core layer granules.

[0106] (2) The two portions of surface layer granules and one portion of core layer granules are respectively added into three extruders for melt plasticization (wherein the melt temperature is 220°C), and then co-extrusion casting is performed by using a three-layer co-extrusion die, wherein the temperature of the cooling roller after the three-layer co-extrusion die is 25°C, the core layer is located between the two surface layers, and the thickness ratio of the core layer and a single surface layer is 70:15, to obtain a base film precursor with a three-layer structure.

[0107] (3) The base film precursor is subjected to preheating treatment, wherein the preheating temperature is 100°C and the preheating time is 4 min.

[0108] (4) The base film precursor after preheating treatment is first stretched longitudinally, wherein the treatment temperature is 125°C and the stretching ratio is 3.5 times; then it is stretched transversely, wherein the treatment temperature is 155°C and the stretching ratio is 3.5 times, so that the thickness of the base film precursor is 4.5 μm.

[0109] (5) The base film precursor is subjected to supercritical carbon dioxide treatment and gamma-ray irradiation crosslinking treatment in sequence, in the step of supercritical carbon dioxide treatment: the treatment pressure is 26 MPa, the temperature is 56℃, and the time is 13 min; in the step of gamma-ray irradiation crosslinking treatment, the irradiation dose is 12 kGy, to obtain the base film.

[0110] (6) The copper foil is formed on both sides of the base film by using a magnetron sputtering film forming technology, to obtain the composite current collector.

[0111] Example 4 The example of the present application provides a preparation method of a composite current collector, which is only different from the example 3 in that the surface layer raw material does not contain maleic anhydride grafted polypropylene.

[0112] Example 5 The example of the present application provides a preparation method of a composite current collector, which is only different from the example 3 in that the supercritical carbon dioxide treatment is not performed.

[0113] Comparative Example 1 The comparative example of the present application provides a preparation method of a composite current collector, which is only different from the example 3 in that the surface layer raw material does not contain polyethylene imine.

[0114] Comparative Example 2 The comparative example of the present application provides a preparation method of a composite current collector, which is only different from the example 3 in that the surface layer raw material does not contain polyethylene imine, and the supercritical carbon dioxide treatment and the gamma-ray irradiation crosslinking treatment are not performed.

[0115] Comparative Example 3 The comparative example of the present application provides a preparation method of a composite current collector, which is only different from the example 3 in that the polyethylene imine is completely replaced by maleic anhydride.

[0116] Test Example (1) Peeling strength test Test method: the composite current collectors prepared by the examples 1-5 and the comparative examples 1-3 are respectively taken as samples, and then the peeling strength of the base film and the metal layer is tested, wherein the test steps are as follows: 3M681 adhesive tape is adhered to the surface of the sample, a rubber roller is rolled back and forth twice under the condition of only self-weight, then a blade is used to cut along the edge of the adhesive tape, and then a universal tensile testing machine is used for 180° peeling test, wherein the adhesive tape is 20 mm wide, the gauge length is 100 mm, the roller self-weight is 2 kg, and the tensile speed is 50 mm / min, the peeling strength is recorded, and the test results are summarized in Table 1.

[0117] (2) Water contact angle test Test method: the base film prepared from examples 1-5 and comparative examples 1-3 was taken as sample, and then the water contact angle was tested, wherein the test procedure was that the sample was laid on the upper surface of a glass slide, and then the water drop angle tester was tested under the pinhole, the liquid used was distilled water, the drop amount was 4uL, then the drop shape was collected to calculate the contact angle size, and the test results were summarized in table 1.

[0118] (3) Thermal shrinkage test Test method: the base film prepared from examples 1-5 and comparative examples 1-3 was taken as sample, and then the thermal shrinkage rate after treatment at 150℃ for 30 min was tested, wherein the test procedure was that the sample was cut by a sampler and put into an oven for baking, the sample size was 10mm long and 10mm wide, then the sample size after heat treatment (test temperature 150℃, treatment time 30min) was measured, then the length size change rate and width size change rate of the sample were calculated respectively, and the average of the two results was obtained to obtain the thermal shrinkage rate, and then the test results were summarized in table 1.

[0119] (4) Tensile strength test Test method: the base film prepared from examples 1-5 and comparative examples 1-3 was taken as sample, and then the transverse tensile strength and longitudinal tensile strength were tested, wherein the test conditions were that the sample size was 15mm wide, 10cm long, 100mm gauge length, and the tensile speed was 50mm / min, and then the test results were summarized in table 1.

[0120] Table 1

[0121] Referring to table 1, according to the test results of examples 1-5 and comparative examples 1-3, the base film provided by the application has excellent heat resistance and tensile strength, and also has excellent bonding strength with the metal layer.

[0122] According to the test results of examples 3 and 4, the addition of maleic anhydride grafted polypropylene in the surface layer makes the corresponding base film have more excellent heat resistance and tensile strength, and also has more excellent bonding strength with the metal layer.

[0123] According to the test results of examples 3 and 5, the supercritical carbon dioxide treatment after stretching treatment makes the corresponding base film have more excellent heat resistance and tensile strength, and also has more excellent bonding strength with the metal layer.

[0124] The embodiments described above are only part of the embodiments of the present application, rather than all the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

Claims

1. A base membrane for composite current collectors, characterized in that, The material comprises a three-layer co-extruded core layer and surface layers located on both sides of the core layer. The core layer is made of polypropylene, and the surface layers are made of polypropylene and polyethyleneimine. In the surface layer, the mass of polypropylene is not less than the mass of polyethyleneimine.

2. The composite current collector base film according to claim 1, characterized in that, In the surface layer, the mass ratio of polypropylene to polyethyleneimine is (85~99):(1~15). Optionally, in the surface layer, the mass ratio of polypropylene to polyethyleneimine is (85~95):(3~10).

3. The composite current collector base film according to claim 1 or 2, characterized in that, The surface layer also includes maleic anhydride-grafted polypropylene.

4. The composite current collector base film according to claim 3, characterized in that, In the surface layer, the mass of the maleic anhydride-grafted polypropylene is m1, the sum of the masses of polypropylene and polyethyleneimine is m2, and the ratio of m1 to m2 is (3~5):(95~97).

5. A method for preparing a base film for composite current collectors, characterized in that, Includes the following steps: The surface material is melt-granulated, wherein the surface material includes polypropylene and polyethyleneimine, and the mass of polypropylene in the surface material is not less than the mass of polyethyleneimine, to obtain surface granules. Two portions of the surface layer granules and one portion of the core layer granules are respectively added to three extruders for melt plasticization, wherein the core layer granules are made of polypropylene; then, they are co-extruded and cast through a three-layer co-extrusion die, wherein the core layer is located between the two surface layers, to obtain a three-layer structured base film precursor; The base film precursor is stretched to obtain the base film.

6. The method for preparing the base film for composite current collectors according to claim 5, characterized in that, The surface material also includes maleic anhydride-grafted polypropylene. Optionally, in the melt granulation step, the processing temperature is 185℃~230℃.

7. The method for preparing the base film for composite current collectors according to claim 5 or 6, characterized in that, Following the stretching treatment, the base film precursor is further subjected to supercritical carbon dioxide treatment and γ-ray irradiation crosslinking treatment in sequence.

8. The method for preparing the composite current collector base film according to claim 7, characterized in that, In the supercritical carbon dioxide treatment step, the treatment pressure is 10 MPa~30 MPa, the treatment temperature is 40℃~60℃, and the treatment time is 5 min~30 min; Or / and, in the step of the γ-ray irradiation crosslinking treatment, the irradiation dose is 5 kGy to 20 kGy.

9. A composite current collector, characterized in that, It includes a polymer substrate layer and a metal layer located on at least one side surface of the polymer substrate layer, wherein the polymer substrate layer is a base film as described in any one of claims 1 to 4 or a base film prepared by any one of claims 5 to 8.

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