Composite current collector and lithium ion battery

Abstract

A composite current collector and a lithium ion battery, relating to the technical field of battery materials. The composite current collector comprises a modified high molecular polymer film, and a conductive layer and a protective layer that are stacked on at least one side surface of the modified high molecular polymer film; the modified high molecular polymer film comprises a high molecular polymer and a modified material; the modified material comprises a carbon nanotube-grafted carbon fiber material. According to the composite current collector, the high molecular polymer film having improved mechanical properties is prepared by compounding the high molecular polymer and the carbon nanotube-grafted carbon fiber material, and the high molecular polymer film and the conductive layer have excellent cohesiveness, so that the mechanical properties of the composite current collector are significantly improved, the defect rate of a product is reduced, and the popularization of the composite current collector at an application end is promoted.

Description

Composite current collector and lithium ion battery Technical Field

[0001] The present application belongs to the technical field of battery materials, and specifically relates to a composite current collector and a lithium-ion battery. Background Art

[0002] At present, composite current collectors based on polymer membranes have received widespread attention and application in the new energy industry. The preparation process of the composite current collector is usually: a layer of metal (aluminum, copper, etc.) material is deposited on a polymer film (such as polypropylene, polyethylene, polyester, etc.) by physical vapor deposition (PVD) method. The prepared surface metallized film with a certain conductivity is the composite current collector. Compared with traditional current collectors, composite current collectors based on polymer membranes have the characteristics of low cost, light weight, and good internal insulation. These characteristics enable the composite current collector to reduce the cost of the battery and improve the energy density and safety of the battery when used in the battery.

[0003] Among the many composite current collectors based on polymer films, those based on polypropylene films are the most common. Currently, the polypropylene films on the market are mainly polypropylene films for capacitors. However, in the process of preparing composite current collectors using traditional polypropylene films for capacitors as the base film, there are problems with poor mechanical properties, namely, relatively low tensile strength (generally less than 200 MPa), which makes it easy to break the film under the PVD system environment (winding system tension, bombardment of metal atoms, and high temperature of the film surface). In addition, in the back-end application process of the composite current collector, the coating and product composite molding processes involved also place relatively high requirements on the tensile strength of the base film.

[0004] Therefore, in order to address the above problems, it is necessary to develop a high molecular polymer membrane with high mechanical properties, so as to reduce the product defect rate of the composite current collector prepared thereby and promote the promotion of the composite current collector in the application end.

[0005] Summary of the Invention

[0006] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0007] In response to the shortcomings of the prior art, the purpose of this application is to provide a composite current collector and a lithium-ion battery. This application combines a polymer with a carbon fiber material grafted with carbon nanotubes, which not only constructs a stable interface and promotes the crystallization of the polymer, thereby preparing a polymer film with improved mechanical properties, but also forms a network structure in the polymer film formed by the carbon fiber material grafted with carbon nanotubes, which can improve the bonding effect between the polymer film and the conductive layer, thereby reducing the product defect rate caused by film breakage in the process of preparing a composite current collector using the modified polymer film as the base film, and improving the mechanical properties of the prepared composite current collector, thereby promoting the promotion of the composite current collector in the application end.

[0008] To achieve this goal, this application adopts the following technical solutions:

[0009] In a first aspect, the present application provides a modified polymer film, wherein the modified polymer film comprises a polymer and a modified material;

[0010] The modified material includes carbon fiber material grafted with carbon nanotubes.

[0011] The present application composites a polymer and a carbon fiber material grafted with carbon nanotubes. Since the carbon fiber material grafted with carbon nanotubes has a low surface energy and the carbon fiber surface has a network structure of carbon nanotubes interwoven, it can promote its good wettability with the polymer, enhance the van der Waals interaction between the carbon nanotube grafted carbon fiber material and the polymer, thereby constructing a stable interface between the two, enhancing the dispersion and orientation of the carbon nanotube grafted carbon fiber material in the polymer, and promoting the crystallization of the polymer, thereby preparing a polymer film with improved mechanical properties. In addition, the network structure formed by the carbon nanotube grafted carbon fiber material in the polymer film can enhance the bonding effect between the polymer film and the conductive layer, ultimately achieving a reduction in the product defect rate caused by film breakage in the process of preparing a composite current collector using the modified polymer film as the base film, and enhancing the structural stability and mechanical properties of the prepared composite current collector, thereby promoting the promotion of the composite current collector at the application end.

[0012] It should be noted that if a polymer is mixed with a carbon fiber material that is not grafted with carbon nanotubes, the wettability between the carbon fiber material and the polymer is poor, and a stable interface cannot be formed, resulting in poor dispersion and orientation in the polymer, and the performance of the prepared polymer film cannot be significantly improved. Grafting carbon nanotubes onto carbon fiber materials can solve the above problems.

[0013] As an optional technical solution of the present application, the average diameter of the modified material is 10-100 nm, for example, it can be 10 nm, 30 nm, 50 nm, 70 nm or 90 nm, and the aspect ratio is 5-50, for example, it can be 5, 10, 20, 30, 40 or 50, etc.

[0014] In the present application, if the diameter and aspect ratio of the modified material are too high, it will be detrimental to its uniform dispersion in the polymer membrane, resulting in the inability to significantly improve the mechanical properties of the polymer membrane; if the diameter and aspect ratio of the modified material are too small, it will easily agglomerate, and the dispersion effect in the polymer membrane will be poor, resulting in the inability to significantly improve the mechanical properties of the polymer membrane.

[0015] In one embodiment, the carbon nanotube content in the carbon nanotube-grafted carbon fiber material is 0.5-50 wt.%, for example, it can be 0.5 wt.%, 1 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.% or 50 wt.%.

[0016] In this application, if the content of carbon nanotubes is too low, the modified material mainly exhibits the surface properties of carbon fibers. When added to the polymer, the interface stability of the polymer is not significantly improved, resulting in no significant improvement in the performance of the polymer film; if the content of carbon nanotubes is too high, the modified material mainly exhibits the surface properties of carbon nanotubes, the interface stability of the polymer is not significantly improved, resulting in no significant improvement in the performance of the polymer film.

[0017] In one embodiment, in the carbon nanotube-grafted carbon fiber material, the carbon nanotubes include any one of general carbon nanotubes, fluorinated carbon nanotubes, sulfur-doped carbon nanotubes, and nitrogen-doped carbon nanotubes, or a combination of at least two thereof.

[0018] It should be noted that general carbon nanotubes refer to ordinary carbon nanotubes that are not doped or modified.

[0019] In this application, compared with general carbon nanotube grafted carbon fiber materials, the introduction of fluorinated carbon nanotube grafted carbon fiber materials can better improve the performance of the modified polymer membrane. This is because the fluorinated carbon nanotube grafted carbon fiber material has a lower surface energy and can form a more stable interface with the polymer, thereby promoting the improvement of the tensile strength of the modified polymer membrane and leading to an increase in the tensile strength of the corresponding composite current collector.

[0020] As an optional technical solution of the present application, the high molecular polymer includes polypropylene.

[0021] In one embodiment, the melt index of the high molecular polymer is 3-4 g / 10 min, for example, it can be 3 g / 10 min, 3.2 g / 10 min, 3.4 g / 10 min, 3.6 g / 10 min, 3.8 g / 10 min or 4 g / 10 min, and the isotacticity is ≥96%, for example, it can be 96%, 97%, 98%, 99% or 99.5%, etc.

[0022] It should be noted that the melt index of polymers is tested at a temperature of 230°C and a load of 2.16 kg. In this application, if the melt index is too low, the molecular weight is too high, resulting in poor film forming properties during film drawing; if the melt index is too high, the molecular weight is too low, resulting in poor film forming properties and poor mechanical properties of the prepared film.

[0023] In the present application, the higher the isotacticity, the higher the regularity of the polymer. The improvement of the regularity of the polymer can improve the orientation and crystallinity of the prepared polymer film, thereby improving the mechanical properties of the prepared modified polymer film.

[0024] In one embodiment, based on the mass of the modified polymer film, the content of the polymer is 95-99.9wt.%, for example, it can be 95wt.%, 96wt.%, 97wt.%, 98wt.%, 99wt.% or 99.5wt.%, etc., and the content of the modified material is 0.1-5wt.%, for example, it can be 0.1wt.%, 0.5wt.%, 1wt.%, 2wt.%, 3wt.%, 4wt.% or 5wt.%, etc.

[0025] In this application, if the content of the modifying material is too low, the mechanical properties of the membrane will not be significantly improved; if the content of the modifying material is too high, agglomeration will easily occur, resulting in poor interface stability of the membrane and poor mechanical properties.

[0026] In one embodiment, the thickness of the modified polymer film is ≥1 μm, for example, it can be 1 μm, 2 μm, 5 μm, 10 μm, 15 μm or 20 μm, and can further be 2-20 μm.

[0027] In this application, considering that the thinner the modified polymer film is, the more it can promote the improvement of the energy density of the composite current collector, while taking into account the production difficulty (the thinner the film, the greater the production difficulty and the lower the yield), a thickness of 2-20 μm can be further selected.

[0028] In a second aspect, the present application provides a method for preparing the modified polymer film as described in the first aspect, the preparation method comprising the following steps:

[0029] Mixing a high molecular polymer and a modified material, performing a melting process to obtain a molten material, then performing sheet casting and stretching on the molten material, and winding to obtain the modified high molecular polymer film;

[0030] The modified material includes carbon fiber material grafted with carbon nanotubes.

[0031] The preparation method provided in this application is simple and easy to implement, and can be easily scaled up.

[0032] As an optional technical solution of the present application, the method of grafting the carbon nanotubes in the modified material onto the carbon fibers includes a chemical grafting method and / or a chemical vapor deposition method.

[0033] It should be noted that the present application does not limit the source of carbon fiber. For example, the carbon fiber material produced by Xianfeng Nano can be used.

[0034] For example, the specific steps of the chemical grafting method include:

[0035] (a) Pretreatment of carbon fiber:

[0036] The carbon fibers are added to the acid solution, stirred and mixed, and ultrasonically treated, followed by adding a pH adjuster to adjust the pH of the solution to 7, followed by centrifugation, filtration, washing, and drying to obtain pretreated carbon fibers;

[0037] (b) Pretreatment of carbon nanotubes;

[0038] Adding carbon nanotubes to an acid solution, stirring and mixing, and performing ultrasonic treatment, then adding a pH adjuster to adjust the pH of the solution to 7, and then centrifuging, filtering, washing and drying to obtain pretreated carbon nanotubes;

[0039] (c) dispersing the pretreated carbon nanotubes in an acetone solution to obtain a dispersion, and then dropwise adding the dispersion to the pretreated carbon fiber under ultrasonic conditions. After the dropwise addition is completed, heat treatment is performed at 150-250° C. (for example, 150° C., 170° C., 200° C., 220° C., or 250° C.), washing, and drying to obtain a carbon nanotube-grafted carbon fiber material.

[0040] As an optional technical solution of the present application, the temperature of the melt treatment is 200-270°C, for example, it can be 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C or 270°C.

[0041] In one embodiment, the stretching treatment is biaxial stretching, which includes synchronous stretching and asynchronous stretching.

[0042] In one embodiment, the simultaneous stretching process includes a preheating zone, a stretching zone, and a heat setting zone.

[0043] In one embodiment, the preheating zone includes a first preheating stage and a second preheating stage. The temperature of the first preheating stage is 120-125°C, for example, 120°C, 121°C, 122°C, 123°C, 124°C or 125°C, and the temperature of the second preheating stage is 126-130°C, for example, 126°C, 127°C, 128°C, 129°C or 130°C.

[0044] In one embodiment, the stretching zone includes one-stage stretching, two-stage stretching and three-stage stretching, the temperature of the one-stage stretching is 130-135°C, for example, it can be 130°C, 131°C, 132°C, 133°C, 134°C or 135°C, the temperature of the two-stage stretching is 136-140°C, for example, it can be 136°C, 137°C, 138°C, 139°C or 140°C, the temperature of the three-stage stretching is 141-150°C, for example, it can be 142°C, 144°C, 146°C, 148°C or 150°C, the longitudinal stretching ratio of the stretching zone is 5-8 times, for example, it can be 5 times, 6 times, 7 times or 8 times, and the transverse stretching ratio is 4-7 times, for example, it can be 4 times, 5 times, 6 times or 7 times, etc.

[0045] In this application, if the longitudinal stretching ratio and the transverse stretching ratio are too low, it will be detrimental to the orientation of the polymer and the modified material, resulting in poor mechanical properties of the prepared modified polymer film; if the longitudinal stretching ratio and the transverse stretching ratio are too high, the film will easily break and defects will occur.

[0046] In one embodiment, the heat setting zone includes a first heat setting stage and a second heat setting stage, wherein the temperature of the first heat setting stage is 155-160°C, for example, 155°C, 156°C, 157°C, 158°C, 159°C or 160°C, and the temperature of the second heat setting stage is 161-165°C, for example, 161°C, 162°C, 163°C, 164°C or 165°C.

[0047] In one embodiment, the asynchronous stretching process includes longitudinal stretching, transverse stretching and heat treatment;

[0048] In one embodiment, the preheating temperature for longitudinal stretching is 110-135°C, for example, 110°C, 115°C, 120°C, 125°C, 130°C or 135°C, etc., the stretching temperature is 136-150°C, for example, 140°C, 145°C or 150°C, etc., and the stretching ratio is 5-8 times, for example, 5 times, 6 times, 7 times or 8 times, etc.

[0049] In one embodiment, the preheating temperature for the transverse stretching is 120-149°C, for example, 120°C, 125°C, 130°C, 135°C, 140°C or 145°C, etc., the stretching temperature is 150-160°C, for example, 150°C, 152°C, 154°C, 156°C, 158°C or 160°C, etc., and the stretching ratio is 4-7 times, for example, 4 times, 5 times, 6 times or 7 times, etc.

[0050] In one embodiment, after the longitudinal stretching, the film is first cooled to room temperature and then transverse stretching is performed.

[0051] In one embodiment, the heat treatment temperature is 120-140°C, for example, 120°C, 125°C, 130°C, 135°C or 140°C.

[0052] In this application, the purpose of heat treatment is to eliminate the internal stress of the membrane and improve the thermal stability of the membrane.

[0053] As an optional technical solution of the present application, the preparation method comprises the following steps:

[0054] (1) The polymer and the carbon fiber material grafted with carbon nanotubes are mixed uniformly in a certain proportion, and then added into a corresponding twin-screw extruder, melted at 200-270° C., and then filtered and extruded through a die head;

[0055] (2) casting the molten material onto a casting roll, and forming the molten material by cooling the casting roll and water cooling, with the cooling temperature being 20-70°C;

[0056] (3) Asynchronous stretching is adopted, and longitudinal stretching is first performed, with a preheating temperature of 110-135°C, a stretching temperature of 136-150°C, and a stretching ratio of 5-8 times. After longitudinal stretching, the film is cooled to room temperature, and then transverse stretching is performed, with a preheating temperature of 120-149°C, a stretching temperature of 150-160°C, a stretching ratio of 4-7 times, and a heat setting temperature of 161-165°C (for example, 161°C, 162°C, 163°C, 164°C, or 165°C, etc.), and finally heat treatment is performed at a heat treatment temperature of 120-140°C;

[0057] (4) The heat-treated film is air-cooled in the platform area and then enters the winding system through the traction system for film sheet winding to obtain the modified high molecular polymer film.

[0058] In a third aspect, the present application provides a composite current collector, comprising the modified high molecular polymer film as described in the first aspect, and a conductive layer and a protective layer stacked on at least one side surface of the modified high molecular polymer film.

[0059] In the present application, the modified high molecular polymer film acts as a carrier of the composite current collector, providing support.

[0060] As an optional technical solution of the present application, the material of the conductive layer includes any one of elemental copper, copper alloy, elemental aluminum, aluminum alloy, elemental nickel, nickel alloy, elemental titanium or elemental silver, or a combination of at least two of them.

[0061] In one embodiment, the thickness of the conductive layer is 500-2000 nm, for example, 500 nm, 1000 nm, 1500 nm or 2000 nm, and can further be 700-1200 nm.

[0062] In one embodiment, the conductive layer is prepared by any one of physical vapor deposition, electroplating, and chemical plating, or a combination of at least two of them.

[0063] It should be noted that the physical vapor deposition method can be resistance heating vacuum evaporation, electron beam heating vacuum evaporation, laser heating vacuum evaporation or magnetron sputtering.

[0064] As an optional technical solution of the present application, the material of the protective layer includes any one of elemental nickel, elemental chromium, nickel-based alloy, copper-based alloy, aluminum oxide, nickel oxide, chromium oxide, cobalt oxide, copper chromate, copper chromite, graphite, carbon black, acetylene black, Ketjen black, carbon nano-quantum dots, carbon nanotubes, carbon nanofibers or graphene, or a combination of at least two of them.

[0065] In one embodiment, the thickness of the protective layer is 5-100 nm, for example, 5 nm, 10 nm, 50 nm, 100 nm or 150 nm, and can further be 10-80 nm.

[0066] In one embodiment, the thickness of the protective layer is less than or equal to one tenth of the thickness of the conductive layer.

[0067] In one embodiment, the protective layer is prepared by any one of physical vapor deposition, chemical vapor deposition, in-situ forming, and coating methods, or a combination of at least two of these methods.

[0068] In one embodiment, the physical vapor deposition method includes vacuum evaporation and / or magnetron sputtering.

[0069] In one embodiment, the chemical vapor deposition method includes atmospheric pressure chemical vapor deposition and / or plasma enhanced chemical vapor deposition.

[0070] In one embodiment, the in-situ forming method is a method of in-situ forming a metal oxide passivation layer on the surface of the conductive layer.

[0071] In one embodiment, the coating method includes any one of die coating, blade coating or extrusion coating.

[0072] In a fourth aspect, the present application provides a lithium-ion battery, comprising an electrode plate, wherein the electrode plate comprises the composite current collector as described in the third aspect.

[0073] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0074] Compared with the prior art, this application has the following beneficial effects:

[0075] (1) The present application combines a polymer and a carbon fiber material grafted with carbon nanotubes. Since the carbon fiber material grafted with carbon nanotubes has a low surface energy and the carbon fiber surface has a network structure of carbon nanotubes interwoven, it can promote its good wettability with the polymer, enhance the van der Waals interaction between the carbon nanotube grafted carbon fiber material and the polymer, thereby constructing a stable interface between the two, enhancing the dispersion and orientation of the carbon nanotube grafted carbon fiber material in the polymer, and promoting the crystallization of the polymer, thereby preparing a polymer film with improved mechanical properties. In addition, the network structure formed by the carbon nanotube grafted carbon fiber material in the polymer film can enhance the bonding effect between the polymer film and the conductive layer, ultimately achieving a reduction in the product defect rate caused by film breakage in the process of preparing a composite current collector using the modified polymer film as the base film, and enhancing the mechanical properties of the prepared composite current collector, thereby promoting the promotion of the composite current collector at the application end.

[0076] (2) The preparation method provided in this application is simple and easy to implement and can be easily scaled up.

[0077] Still other aspects will become apparent upon reading and understanding the detailed description. DETAILED DESCRIPTION

[0078] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0079] It should be noted that the room temperature below refers to 25°C.

[0080] Example 1

[0081] This embodiment provides a modified polymer film, wherein the modified polymer film includes polypropylene and a modified material;

[0082] The modified material is a carbon fiber material grafted with general carbon nanotubes, and the content of general carbon nanotubes is 0.5wt.%;

[0083] The modified material has an average diameter of 10 nm and an aspect ratio of 5;

[0084] The polypropylene has a melt index of 3.3 g / 10 min and an isotacticity of 96%;

[0085] Based on the mass of the modified polymer film, the content of the polypropylene is 99.9 wt.%, and the content of the modified material is 0.1 wt.%;

[0086] The thickness of the modified high molecular polymer film is 4.5 μm.

[0087] This embodiment also provides a method for preparing the modified polymer film, which comprises the following steps:

[0088] (1) The polymer and the carbon fiber material grafted with general carbon nanotubes were mixed uniformly according to the above ratio, and then added into the corresponding twin-screw extruder, melted at 250°C, and then filtered through a filter (10 μm filter mesh), and the molten material was extruded through a die head, and the temperature of the die head was 250°C;

[0089] The preparation steps of carbon nanotube grafted carbon fiber materials generally include:

[0090] (a) Pretreatment of carbon fiber:

[0091] 200 g of carbon fiber (manufacturer: Xianfeng Nano) was added to 8000 mL of an acid solution (comprising nitric acid and sulfuric acid in a volume ratio of 1:3), stirred and mixed, and then placed in an ultrasonic water bath for ultrasonic treatment for 60 min at an ultrasonic power of 200 W. Subsequently, a pH regulator (i.e., 1 mol / L sodium hydroxide) was added to adjust the pH of the solution to 7. The solution was then centrifuged and filtered, and the filtrate was washed three times with pure water. After washing, the filtrate was dried in an oven at 80° C. to obtain pretreated carbon fiber;

[0092] (b) General pretreatment of carbon nanotubes;

[0093] 200 g of carbon nanotubes were added to 8000 mL of an acid solution (comprising nitric acid and sulfuric acid in a volume ratio of 1:3), stirred and mixed, and then placed in an ultrasonic water bath for 60 min at an ultrasonic power of 200 W. A pH adjuster (i.e., 1 mol / L sodium hydroxide) was then added to adjust the pH of the solution to 7. The solution was then centrifuged and filtered, and the filtrate was washed three times with pure water. After washing, the solution was dried in an oven at 80° C. to obtain pretreated general carbon nanotubes.

[0094] (c) taking 0.5 g of the pretreated general carbon nanotubes, dispersing them in 500 mL of acetone solution by ultrasound (ultrasonic power of 200 W) to obtain a dispersion, and then adding the dispersion dropwise to 99.5 g of the pretreated carbon fiber under ultrasonic conditions (ultrasonic power of 100 W), heat-treating the mixture at 200° C. for 5 h after the addition, and then washing the heat-treated product three times with pure water. After washing, drying the mixture at 60° C. to obtain a carbon fiber material grafted with general carbon nanotubes;

[0095] (2) casting the molten material onto a casting roll, and forming the molten material through a cooling treatment of the casting roll and water cooling, with the cooling temperature being 30° C.;

[0096] (3) Asynchronous stretching is adopted, and longitudinal stretching is first performed, with a preheating temperature of 130°C, a stretching temperature of 140°C, and a stretching ratio of 5 times. After longitudinal stretching, the film is cooled to room temperature, and then transverse stretching is performed, with a preheating temperature of 140°C, a stretching temperature of 155°C, a stretching ratio of 4 times, and a heat setting temperature of 163°C. Finally, heat treatment is performed at a heat treatment temperature of 125°C.

[0097] (4) The heat-treated film is air-cooled in the platform area and then enters the winding system through the traction system for film sheet winding to obtain the modified high molecular polymer film.

[0098] This embodiment also provides a composite current collector, which includes the modified high molecular polymer film as described above, and a conductive layer and a protective layer stacked on one surface of the modified high molecular polymer film;

[0099] The conductive layer is made of single-element copper with a thickness of 1040 nm, and the protective layer is made of graphene with a thickness of 10 nm.

[0100] This embodiment also provides a method for preparing the composite current collector, which comprises the following steps:

[0101] Preparation of the conductive layer: The modified polymer film is placed in the cabin of an evaporation machine, and a copper metal wire with a purity of 99.99% is used as the evaporation raw material. The raw material is heated to 1500°C, and a 50nm thick metal layer is deposited on the surface of the modified polymer film to obtain a composite current collector semi-finished product; then, the prepared composite current collector semi-finished product is used as a substrate for electroplating. The combination of the electroplating solution includes: 100g / L copper sulfate, 120g / L sulfuric acid, 65mg / L hydrochloric acid, 1mg / L sodium polydisulfide dipropane sulfonate, 0.5mg / L 2-mercaptopyridine and 200mg / L polyethylene glycol (molecular weight 8000). The temperature of the electroplating solution is 25°C, and the average cathode current density is 2A / dm 2 , the electroplating treatment time is 5 min to obtain a conductive layer;

[0102] Preparation of a protective layer: The above-mentioned composite film with a conductive layer is placed in a coating device, and a graphene solution with a solid content of 0.10 wt.% (the solvent is nitrogen methyl pyrrolidone) is used as the coating liquid. The coating liquid is then evenly coated on the surface of the conductive layer on both sides through a die coating process, and finally dried at 70°C to obtain a protective layer, and finally a composite current collector with a total thickness of 6.6 μm is obtained.

[0103] Example 2

[0104] The difference between this embodiment and embodiment 1 is that, based on the mass of the modified high molecular polymer film, the content of polypropylene is 98 wt.%, and the content of the modified material is 2 wt.%.

[0105] The rest of the preparation methods and parameters remained the same as in Example 1.

[0106] Example 3

[0107] The difference between this embodiment and embodiment 1 is that, based on the mass of the modified high molecular polymer film, the content of polypropylene is 95 wt.%, and the content of the modified material is 5 wt.%.

[0108] The rest of the preparation methods and parameters remained the same as in Example 1.

[0109] Example 4

[0110] The difference between this embodiment and embodiment 1 is that the content of carbon nanotubes in the carbon fiber material grafted with carbon nanotubes is 10 wt.%.

[0111] The rest of the preparation methods and parameters remained the same as in Example 1.

[0112] Example 5

[0113] The difference between this embodiment and embodiment 1 is that the content of carbon nanotubes in the carbon fiber material grafted with carbon nanotubes is 30 wt.%.

[0114] The rest of the preparation methods and parameters remained the same as in Example 1.

[0115] Example 6

[0116] The difference between this embodiment and embodiment 1 is that the content of carbon nanotubes in the carbon fiber material grafted with carbon nanotubes is 50 wt.%.

[0117] The rest of the preparation methods and parameters remained the same as in Example 1.

[0118] Example 7

[0119] The difference between this embodiment and embodiment 1 is that the average diameter of the carbon fiber material grafted with carbon nanotubes is generally 50 nm.

[0120] The rest of the preparation methods and parameters remained the same as in Example 1.

[0121] Example 8

[0122] The difference between this embodiment and embodiment 1 is that the average diameter of the carbon fiber material grafted with carbon nanotubes is generally 100 nm.

[0123] The rest of the preparation methods and parameters remained the same as in Example 1.

[0124] Example 9

[0125] The difference between this embodiment and embodiment 1 is that the aspect ratio of the carbon fiber material grafted with carbon nanotubes is generally 20.

[0126] The rest of the preparation methods and parameters remained the same as in Example 1.

[0127] Example 10

[0128] The difference between this embodiment and embodiment 1 is that the aspect ratio of the carbon fiber material grafted with carbon nanotubes is generally 50.

[0129] The rest of the preparation methods and parameters remained the same as in Example 1.

[0130] Example 11

[0131] The difference between this embodiment and embodiment 1 is that the stretching ratio of the longitudinal stretching in step (3) is 7 times.

[0132] The rest of the preparation methods and parameters remained the same as in Example 1.

[0133] Example 12

[0134] The difference between this embodiment and embodiment 1 is that the stretching ratio of the longitudinal stretching in step (3) is 8 times.

[0135] The rest of the preparation methods and parameters remained the same as in Example 1.

[0136] Example 13

[0137] The difference between this embodiment and embodiment 1 is that in step (1), the carbon fiber material grafted with conventional carbon nanotubes is replaced with the carbon fiber material grafted with fluorinated carbon nanotubes.

[0138] The rest of the preparation methods and parameters remained the same as in Example 1.

[0139] Example 14

[0140] The difference between this embodiment and embodiment 1 is that, based on the mass of the modified high molecular polymer film, the content of the polypropylene is 99.95 wt.%, and the content of the modified material is 0.05 wt.%.

[0141] The rest of the preparation methods and parameters remained the same as in Example 1.

[0142] Example 15

[0143] The difference between this embodiment and embodiment 1 is that, based on the mass of the modified high molecular polymer film, the content of the polypropylene is 94.8 wt.%, and the content of the modified material is 5.2 wt.%.

[0144] The rest of the preparation methods and parameters remained the same as in Example 1.

[0145] Example 16

[0146] The difference between this embodiment and embodiment 1 is that the content of general carbon nanotubes in the carbon fiber material grafted with general carbon nanotubes is 0.3 wt.%.

[0147] The rest of the preparation methods and parameters remained the same as in Example 1.

[0148] Example 17

[0149] The difference between this embodiment and embodiment 1 is that the content of general carbon nanotubes in the carbon fiber material grafted with general carbon nanotubes is 52 wt.%.

[0150] The rest of the preparation methods and parameters remained the same as in Example 1.

[0151] Example 18

[0152] The difference between this embodiment and embodiment 1 is that the average diameter of the modified material is 5 nm and the aspect ratio is 3.

[0153] The rest of the preparation methods and parameters remained the same as in Example 1.

[0154] Comparative Example 1

[0155] The difference between this comparative example and Example 1 is that, based on the mass of the modified high molecular polymer film, the content of the polypropylene is 100 wt.%, and the content of the modified material is 0 wt.%.

[0156] The rest of the preparation methods and parameters remained the same as in Example 1.

[0157] Comparative Example 2

[0158] The difference between this comparative example and Example 1 is that the content of general carbon nanotubes in the carbon fiber material grafted with general carbon nanotubes is 0 wt.%.

[0159] The rest of the preparation methods and parameters remained the same as in Example 1.

[0160] Comparative Example 3

[0161] The difference between this comparative example and Example 1 is that the carbon fiber material grafted with general carbon nanotubes is replaced by a mixture of general carbon nanotubes and carbon fiber materials, and the mass ratio of the two is 1:1.

[0162] The rest of the preparation methods and parameters remained the same as in Example 1.

[0163] Performance Testing

[0164] The tensile strength of the modified polymer films prepared in the above examples and comparative examples, the tensile strength of the composite current collector, the adhesion between the conductive layer and the modified polymer film in the composite current collector, and the defective rate caused by film breakage during the preparation of the composite current collector were tested and evaluated. The specific test methods are as follows:

[0165] ①Tensile strength of modified polymer membrane and composite current collector: The test refers to the national standard GB / T1040.3-2006.

[0166] ② Defective rate: The ratio of the number of unqualified products caused by film breakage during the preparation process to the total number of products. Since the width is consistent, the number is calculated based on the length.

[0167] ③ Adhesion between the modified polymer film and the conductive layer in the composite current collector: a layer of Permacel P-94 double-sided tape was adhered to a 1 mm thick aluminum foil, the composite current collector was adhered on top of the double-sided tape, and a layer of ethylene acrylic acid copolymer film (DuPont Nurcel0903, thickness of 50 μm) was covered on the composite current collector. 5 N / m 2 The film was hot-pressed at 120°C for 10 seconds, cooled to room temperature, and cut into 150 mm x 15 mm strips. The ethylene acrylic acid copolymer film strip was secured to the upper fixture of the tensile testing machine, while the remaining film strip was secured to the lower fixture. Once secured, the two strips were peeled off at an angle of 180° and a speed of 100 mm / min to test the peel force, which measures the adhesion between the modified polymer film and the conductive layer.

[0168] The above test results are shown in Table 1.

[0169] Table 1

[0170] analyze:

[0171] As can be seen from the above table, the present application compounds polymers and carbon fiber materials grafted with carbon nanotubes, which can not only construct a stable interface and promote the crystallization of polymers, thereby preparing polymer films with improved mechanical properties, but also the network structure formed by the carbon fiber materials grafted with carbon nanotubes in the polymer film can improve the bonding effect between the polymer film and the conductive layer, thereby achieving a reduction in the product defective rate caused by film breakage in the process of preparing composite current collectors using modified polymer films as base membranes, and improving the mechanical properties of the prepared composite current collectors, thereby promoting the promotion of composite current collectors at the application end.

[0172] It can be seen from Examples 1-13 and Comparative Example 1 that due to the introduction of carbon nanotube grafted carbon fiber materials, the tensile strength of the prepared modified polymer film is significantly improved, and the defective rate caused by film breakage when preparing a composite current collector using this as the base film is significantly reduced, and the tensile strength and adhesion of the prepared composite current collector are improved.

[0173] From Examples 1-3, Examples 14-15 and Comparative Example 1, it can be seen that as the content of carbon nanotube-grafted carbon fiber material increases, the tensile strength of the prepared modified polymer film first increases and then decreases, and when using this as the base film to prepare a composite current collector, the defects caused by film breakage first decrease and then increase, and the tensile strength and adhesion of the prepared composite current collector first increase and then decrease. This is because as the content of carbon nanotube-grafted carbon fiber increases, more stable interfaces can be formed with the polypropylene polymer, forming stronger van der Waals forces, promoting the crystallization of polypropylene, and forming more network structures in the film, thereby promoting the improvement of the above-mentioned properties. However, when the content of carbon nanotube-grafted carbon fiber is too low, the performance improvement is not obvious, and when the content is too high, agglomeration is likely to occur, causing the formation of defects, resulting in the deterioration of the above-mentioned properties.

[0174] It can be seen from Examples 1, 7-10, and 18 that as the diameter or aspect ratio of the carbon nanotube-grafted carbon fibers increases, the tensile strength of the prepared modified polymer film first increases and then decreases, and when using this as the base film to prepare a composite current collector, the defects caused by film breakage first decrease and then increase, and the tensile strength and adhesion of the prepared composite current collector first increase and then decrease. This is because if the diameter or aspect ratio is too low, agglomeration will easily occur, and the dispersion effect in the modified polymer film is poor, resulting in the inability to significantly improve the mechanical properties of the film; and if the diameter or aspect ratio is too high, it is not conducive to its uniform dispersion in the film, resulting in the inability to significantly improve the mechanical properties of the film. When the aspect ratio is not within the preferred range, the performance improvement of the prepared polypropylene film is not obvious.

[0175] As can be seen from Examples 1, 11, and 12, by increasing the draw ratio of longitudinal stretching during film formation, the tensile strength of the film is improved, and when using this as the base film to prepare a composite current collector, the defect rate caused by film breakage is reduced, and the tensile strength and adhesion of the prepared composite current collector are improved. This is because increasing the draw ratio of longitudinal stretching during film formation can improve the regular arrangement of carbon nanotube-grafted carbon fibers and polypropylene polymers, promote the formation of a more stable interface between the two, promote the crystallization of polypropylene, and form more network structures in the film, thereby promoting the improvement of the above-mentioned properties.

[0176] It can be seen from Example 1 and Example 13 that compared with general carbon nanotube grafted carbon fiber materials, the introduction of fluorinated carbon nanotube grafted carbon fiber materials can better improve the performance of the modified polymer membrane. This is because the fluorinated carbon nanotube grafted carbon fiber material has a lower surface energy and can form a more stable interface with the polymer, thereby promoting the improvement of the tensile strength of the modified polymer membrane and leading to an increase in the tensile strength of the corresponding composite current collector.

[0177] It can be seen from Examples 1, 4-6 and 16-17 that if the content of carbon nanotubes is too low, the modified material mainly exhibits the surface properties of carbon fibers. When added to the polymer, the interfacial stability of the polymer is not significantly improved, resulting in no significant improvement in the performance of the polymer film; if the content of carbon nanotubes is too high, the modified material mainly exhibits the surface properties of carbon nanotubes, the interfacial stability of the polymer is not significantly improved, resulting in no significant improvement in the performance of the polymer film.

[0178] It can be seen from Example 1 and Comparative Example 2 that if the modified material does not contain carbon nanotubes, the wettability of the modified material with the polymer is poor, resulting in poor interface stability between the two, and poor dispersion and orientation of the modified material in the polymer, thereby causing the performance of the prepared polymer film and the composite current collector prepared using the polymer film as the base film to deteriorate.

[0179] It can be seen from Example 1 and Comparative Example 3 that if the carbon fiber material grafted with carbon nanotubes is replaced with a mixture of carbon nanotubes and carbon fiber materials, the wettability of this mixture with the polymer is poor, and a stable interface cannot be constructed. In addition, the dispersion and orientation of the mixture in the polymer deteriorate, resulting in poor mechanical properties of the prepared polymer film. The defective rate of the composite current collector prepared based on this increases, and the mechanical properties deteriorate.

[0180] The applicant declares that while the above-mentioned embodiments are used to illustrate the technical solutions of this application, this application is not limited to these embodiments, and does not imply that this application must rely on these embodiments in order to be implemented. Persons skilled in the art should understand that any improvements to this application, equivalent replacements for the raw materials of the products of this application, addition of auxiliary ingredients, and selection of specific methods, etc., fall within the scope of protection and disclosure of this application.

Claims

1. A composite current collector comprising a modified polymer film, and a conductive layer and a protective layer stacked on at least one surface of the modified polymer film; The modified high molecular polymer film comprises a high molecular polymer and a modified material; The modified material includes carbon fiber material grafted with carbon nanotubes.

2. The composite current collector according to claim 1, wherein The modified material has an average diameter of 10-100 nm and an aspect ratio of 5-50; The carbon nanotube-grafted carbon fiber material has a carbon nanotube content of 0.5-50 wt.%; In the carbon nanotube-grafted carbon fiber material, the carbon nanotubes include any one of general carbon nanotubes, fluorinated carbon nanotubes, sulfur-doped carbon nanotubes, and nitrogen-doped carbon nanotubes, or a combination of at least two of them.

3. The composite current collector according to claim 1 or 2, wherein: The high molecular polymer includes polypropylene; The high molecular weight polymer has a melt index of 3-4 g / 10 min and an isotacticity of ≥96%; Based on the mass of the modified high molecular polymer film, the content of the high molecular polymer is 95-99.9 wt.%, and the content of the modified material is 0.1-5 wt.%; The thickness of the modified high molecular polymer film is ≥1 μm.

4. The composite current collector according to any one of claims 1 to 3, wherein: The preparation method of the modified high molecular polymer film comprises the following steps: Mixing a high molecular polymer and a modified material, performing a melting process to obtain a molten material, then performing sheet casting and stretching on the molten material, and winding to obtain the modified high molecular polymer film; The modified material includes carbon fiber material grafted with carbon nanotubes.

5. The composite current collector according to claim 4, wherein: The temperature of the melt treatment is 200-270°C; The stretching treatment method is biaxial stretching, which includes synchronous stretching and asynchronous stretching; The synchronous stretching process includes a preheating zone, a stretching zone and a heat setting zone; The preheating zone includes a first stage preheating and a second stage preheating, the temperature of the first stage preheating is 120-125°C, and the temperature of the second stage preheating is 126-130°C; The stretching zone includes one-stage stretching, two-stage stretching and three-stage stretching, the temperature of the one-stage stretching is 130-135°C, the temperature of the two-stage stretching is 136-140°C, and the temperature of the three-stage stretching is 141-150°C. The longitudinal stretching ratio of the stretching zone is 5-8 times, and the transverse stretching ratio is 4-7 times; The heat setting zone includes a first heat setting zone and a second heat setting zone, wherein the temperature of the first heat setting zone is 155-160°C, and the temperature of the second heat setting zone is 161-165°C; The asynchronous stretching process includes longitudinal stretching, transverse stretching and heat treatment; The preheating temperature of the longitudinal stretching is 110-135°C, the stretching temperature is 136-150°C, and the stretching ratio is 5-8 times; The preheating temperature of the transverse stretching is 120-149°C, the stretching temperature is 150-160°C, and the stretching ratio is 4-7 times; The temperature of the heat treatment is 120-140°C.

6. The composite current collector according to claim 4 or 5, wherein: The preparation method of the modified high molecular polymer film comprises the following steps: (1) The polymer and the carbon fiber material grafted with carbon nanotubes are mixed uniformly in a certain proportion, and then added into a corresponding twin-screw extruder, melted at 200-270° C., and then filtered and extruded through a die head; (2) casting the molten material onto a casting roll, and forming the molten material by cooling the casting roll and water cooling, with the cooling temperature being 20-70°C; (3) Asynchronous stretching is used, first longitudinal stretching is performed, the preheating temperature is 110-135°C, the stretching temperature is 136-150°C, the stretching ratio is 5-8 times, and after longitudinal stretching, it is cooled to room temperature and then transverse stretching is performed. The preheating temperature is 120-149°C, the stretching temperature is 150-160°C, the stretching ratio is 4-7 times, the heat setting temperature is 161-165°C, and finally the heat treatment is carried out at a temperature of 120-140°C; (4) The heat-treated film is air-cooled in the platform area and then enters the winding system through the traction system for film sheet winding to obtain the modified high molecular polymer film.

7. The composite current collector according to any one of claims 1 to 6, wherein: The material of the conductive layer includes any one of elemental copper, copper alloy, elemental aluminum, aluminum alloy, elemental nickel, nickel alloy, elemental titanium or elemental silver, or a combination of at least two thereof; The thickness of the conductive layer is 500-2000 nm; The preparation method of the conductive layer includes any one of physical vapor deposition, electroplating or chemical plating, or a combination of at least two of them.

8. The composite current collector according to any one of claims 1 to 7, wherein: The material of the protective layer includes any one or a combination of at least two of elemental nickel, elemental chromium, nickel-based alloy, copper-based alloy, aluminum oxide, nickel oxide, chromium oxide, cobalt oxide, copper chromate, copper chromite, graphite, carbon black, acetylene black, Ketjen black, carbon nano-quantum dots, carbon nanotubes, carbon nanofibers or graphene; The thickness of the protective layer is 5-100 nm; The preparation method of the protective layer includes any one of physical vapor deposition, chemical vapor deposition, in-situ forming or coating methods, or a combination of at least two of them.

9. A lithium-ion battery comprising an electrode plate, wherein the electrode plate comprises the composite current collector according to any one of claims 1 to 8.

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