Double-gradient nano-copper current collector and preparation method and application thereof

Abstract

The invention discloses a double-gradient nano-copper current collector and a preparation method and application thereof, and belongs to the technical field of electrode materials. The preparation method provided by the invention comprises the following steps: sequentially carrying out annealing treatment and cleaning treatment on electrolytic copper foil; the upper surface and the lower surface of the cleaned electrolytic copper foil are respectively subjected to surface ultrasonic rolling treatment, and the double-gradient nano-copper current collector with a structure of a nano-crystal layer, an ultra-fine crystal layer, a micro-crystal layer, an ultra-fine crystal layer and a nano-crystal layer is obtained by regulating and controlling parameters in the surface ultrasonic rolling treatment process. And the total thickness of the nanocrystalline layer and the ultra-fine grain layer is 20-50% of the thickness of the double-gradient nano-copper current collector. The tensile strength of the prepared double-gradient nano-copper current collector can reach 600 MPa or above, the elongation rate can reach 10% or above, oxidation and discoloration do not exist even if the current collector is subjected to heat preservation for 30 min in the high-temperature air environment of 200 DEG C, and good high-temperature stability is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of electrode materials, and in particular, to a dual-gradient nano-copper current collector and a preparation method and application thereof. Background Art

[0002] The information disclosed in the background art of the present invention is only intended to increase the understanding of the overall background of the present invention, and is not necessarily regarded as an admission or an indication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] As the current mainstream portable energy storage device, lithium-ion batteries have been widely used in the fields of electric vehicles, mobile communications, energy storage, etc. As a key structure of lithium-ion batteries, the performance of the current collector is also crucial for the performance of the entire lithium-ion battery. The current collectors of traditional lithium-ion batteries usually use copper foil as the negative electrode material and aluminum foil as the positive electrode material. Although these materials have certain electrical conductivity and mechanical strength, in high-power applications, large-current charging and discharging, and long-term cyclic use, higher requirements are also placed on the electrical conductivity, mechanical strength, and thermal stability of the current collector.

[0004] To solve the above problems, researchers have been exploring new current collector materials. In recent years, nanostructured materials have received extensive attention due to their unique physical and chemical properties. The patent with the authorization announcement number CN114908386B uses a three-stage direct current electrodeposition method to prepare a copper foil with an adjustable microstructure distribution and a fine grain layer-nano twin layer-fine grain layer structure. The internal twin lamellae can hinder the movement of dislocations, making the copper foil have good strength. However, the preparation method of this patent is relatively cumbersome, and the preparation process using the three-stage direct current electrodeposition method will contact air, resulting in interfaces inside the prepared copper foil, and thus the overall mechanical properties are not good. In addition, the microstructure of the copper foil prepared by this method is not continuously transitional, and different microstructures cannot effectively coordinate with each other, resulting in poor overall mechanical properties and thermal stability of the copper foil.

[0005] Therefore, how to provide a copper foil with strong plastic matching and high thermal stability and its efficient preparation process is an urgent problem to be solved. Summary of the Invention

[0006] In view of this, the present invention provides a dual-gradient nano-copper current collector and a preparation method and application thereof. The present invention induces severe plastic deformation of the material through high-energy mechanical action, thereby realizing grain refinement to the nanoscale and forming a gradient distribution in which the grain size gradually increases from the surface layer to the core. The obtained copper foil has both high tensile strength and high elongation rate, and has good high-temperature oxidation resistance.

[0007] In the first aspect, the present invention provides a preparation method of a dual-gradient nano-copper current collector, including the following steps:

[0008] The electrolytic copper foil is annealed and cleaned in sequence.

[0009] The upper and lower surfaces of the electrolytic copper foil after cleaning treatment are respectively subjected to surface ultrasonic rolling treatment. By regulating the parameters of the surface ultrasonic rolling treatment process, a double-gradient nano copper current collector with a structure of "nanocrystalline layer - ultrafine-grained layer - microcrystalline layer - ultrafine-grained layer - nanocrystalline layer" is obtained, and the total thickness of the nanocrystalline layer and the ultrafine-grained layer is 20 - 50% of the thickness of the double-gradient nano copper current collector.

[0010] In a second aspect, the present invention provides a double-gradient nano copper current collector prepared by the above preparation method.

[0011] In a third aspect, the present invention provides the application of the above double-gradient nano copper current collector in a lithium-ion battery.

[0012] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0013] (1) The present invention first anneals the electrolytic copper foil, aiming to make the copper foil grains grow to form a homogeneous microcrystalline copper foil, and at the same time eliminate the residual stress generated during the electro-deposition process; then the present invention uses the high-energy mechanical action of surface ultrasonic rolling treatment to induce severe plastic deformation on the surface of the copper foil, thereby realizing the grain refinement to the nanoscale and forming a gradient distribution with gradually increasing grain size from the surface layer to the core. The method for constructing the gradient structure of the double-gradient nano copper current collector of the present invention is simple and suitable for large-scale production.

[0014] (2) In the double-gradient nano copper current collector of the present invention, the surface nanocrystals have a higher specific surface area and electron conduction ability, which is beneficial to the rapid transmission of electrons, thereby improving the conductivity of the current collector. The microcrystalline layer provides good plastic deformation ability, and the nanocrystalline layer and the ultrafine-grained layer together enhance the hardness and strength of the material, making the overall current collector have higher mechanical strength. At the same time, the continuous transition of the microscopic crystal structure helps to disperse heat and improve the thermal stability of the material, enabling the current collector to still maintain good performance in a high-temperature environment. It is measured that the tensile strength of the double-gradient nano copper current collector prepared by the present invention can reach more than 600 MPa, the elongation rate can reach more than 10%, and there is no oxidation and discoloration when kept in a high-temperature air environment at 200 °C for 30 min, having good high-temperature stability. Description of the Drawings

[0015] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments and descriptions thereof of the invention are used to explain the invention and do not unduly limit the invention. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0016] Figure 1 It is a schematic structural diagram of the copper current collector in Embodiments 1 to 4 of the present invention. Detailed Description of the Invention

[0017] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0018] As described in the background art, in the prior art, when preparing a gradient copper foil current collector, a multi-layer electroplating method is adopted, which has cumbersome steps, low tensile strength and elongation, and the formed multi-grain size distribution is discontinuous, and it is prone to oxidation and color change under high-temperature environments. Therefore, the present invention provides a method for preparing a double-gradient nano-copper current collector, which includes the following steps:

[0019] Anneal and clean the electrolytic copper foil in sequence.

[0020] Perform surface ultrasonic rolling on the upper and lower surfaces of the electrolytic copper foil after cleaning treatment, and obtain a double-gradient nano-copper current collector with a structure of "nanocrystalline layer - ultrafine crystalline layer - microcrystalline layer - ultrafine crystalline layer - nanocrystalline layer" by regulating the parameters of the surface ultrasonic rolling process, and the total thickness of the nanocrystalline layer and the ultrafine crystalline layer is 20-50% of the thickness of the double-gradient nano-copper current collector.

[0021] The present invention first anneals the electrolytic copper foil. This operation can cause the grains of the copper foil to grow and form a homogeneous microcrystalline copper foil, further improving the ductility of the copper current collector and facilitating the subsequent uniform refinement treatment of the copper foil surface. Moreover, the annealing treatment can also eliminate residual stress, thereby optimizing electron transport and stabilizing the electrical conductivity. Then, the present invention removes the oil stains and oxides on the surface of the electrolytic copper foil through a cleaning treatment, making the surface of the copper foil clean and flat, so that the degree of deformation of each part of the copper foil is more uniform during the subsequent plastic deformation process, improving the dimensional accuracy and shape consistency of the copper foil. Then, the present invention uses the high-energy mechanical action of surface ultrasonic rolling treatment to induce severe plastic deformation on the surface of the copper foil, thereby achieving grain refinement to the nanoscale and forming a gradient distribution in which the grain size gradually increases from the surface layer to the core. Among them, the nanocrystals in the surface layer have a higher specific surface area and electron conduction ability, which is conducive to the rapid transmission of electrons, thereby improving the electrical conductivity of the current collector. The microcrystalline layer provides good plastic deformation ability, and the nanocrystalline layer and the ultrafine crystalline layer together enhance the hardness and strength of the material, making the overall current collector have higher mechanical strength. At the same time, the continuously distributed microcrystalline structure helps to disperse heat and improve the thermal stability of the material, enabling the current collector to still maintain good performance in a high-temperature environment. The method for constructing the gradient structure of the double-gradient nanocrystalline copper current collector of the present invention is simple and suitable for large-scale production.

[0022] The present invention limits the total thickness of the nanocrystalline layer and the ultrafine crystalline layer to 20-50% of the thickness of the double-gradient nanocrystalline copper current collector by regulating the parameters of the surface ultrasonic rolling treatment process. The copper foil obtained within this range exhibits excellent tensile strength, elongation rate, and high-temperature stability. If the thickness of the deformed layer (nanocrystalline layer + ultrafine crystalline layer) is too thin, it is not conducive to improving the strength and hardness of the copper foil; if the thickness of the deformed layer is too thick, the elongation rate of the copper foil will be significantly reduced.

[0023] In the present invention, the thickness of the electrolytic copper foil is 10-16 μm; the purity of the electrolytic copper foil is above 99.0%. The present invention does not impose special restrictions on the source of the electrolytic copper foil, which can be prepared by electrolysis or purchased.

[0024] In the present invention, the temperature of the annealing treatment is 200-400 °C, the time of the annealing treatment is 50-120 min, and the annealing treatment is carried out in an argon atmosphere. The annealing treatment time should not be too short, otherwise it is difficult to form microcrystals, which is not conducive to subsequent operations and has little improvement in performance; nor should it be too long, otherwise the grains will grow into coarse crystals, which is not conducive to forming a continuously transitional crystal structure.

[0025] The present invention does not impose special restrictions on the cleaning treatment method, as long as it can remove the oil stains and oxides on the surface of the electrolytic copper foil after annealing. The cleaning treatment method of the present invention is preferably pickling followed by water washing.

[0026] In the present invention, the grain size of the nanocrystals is 10 - 100 nm; the grain size of the ultrafine crystals is 100 - 1000 nm; and the grain size of the microcrystals is 1 - 5 μm. Preferably, the thickness of the nanocrystal layer accounts for 5 - 20% of the thickness of the double-gradient nanocopper current collector.

[0027] In the present invention, the parameters for surface ultrasonic rolling treatment of the upper and lower surfaces of the electrolytic copper foil after cleaning treatment are preferably exactly the same to construct a symmetric gradient structure.

[0028] In the present invention, the downward pressure of the rolling cutter head of the surface ultrasonic rolling treatment equipment is 0.1 - 0.3 μm per pass, and the number of rolling passes is 8 - 12 passes. The downward pressure of the rolling cutter head and the number of rolling passes will affect the thickness of the deformed layer (ultrafine crystal layer + nanocrystal layer). If the downward pressure is too small or the number of rolling passes is too small, the thickness of the deformed layer will be too thin, which is not conducive to improving the strength and hardness of the copper current collector; if the downward pressure is too large or the number of rolling passes is too large, the thickness of the deformed layer will be too thick, which is not conducive to improving the elongation rate of the copper current collector.

[0029] In the present invention, the ultrasonic frequency of the surface ultrasonic rolling treatment is 10 - 30 kHz, and the ultrasonic amplitude is 5 - 20 μm. The vibration of the ultrasonic wave can enable copper atoms to obtain additional energy, enhancing the activity and diffusion ability of the atoms; during the rolling process, copper atoms are more likely to overcome the resistance between the crystal lattices and undergo slip and rearrangement, thereby reducing the deformation resistance of the material and enabling the copper foil to undergo plastic deformation under a smaller external force, which is conducive to the progress of the rolling process. At the same time, the high-frequency vibration of the ultrasonic wave can generate a large number of dislocations and grain boundaries inside the copper foil, and these defects become new crystal nucleus growth points, prompting the grains to continuously refine during the rolling process, thereby improving the hardness and strength of the copper current collector.

[0030] In the present invention, during the surface ultrasonic rolling treatment process, the feed speed of the rolling cutter head is 4000 - 6000 mm / min.

[0031] In the present invention, during the surface ultrasonic rolling treatment process, high-frequency oil spraying treatment is performed on the upper and lower surfaces of the electrolytic copper foil after cleaning treatment, and the high-frequency oil spraying rate is 0.5 - 1 L / min. High-frequency oil spraying is to reduce the friction between the equipment and the electrolytic copper foil, reduce surface wear, and can also timely remove the heat generated by the friction between the equipment and the electrolytic copper foil, and play a certain role in stabilizing the temperature.

[0032] After the ultrasonic surface rolling treatment step of the present invention, it further includes a cleaning and degreasing step, and the present invention does not impose special restrictions on this step, and the commonly used cleaning and degreasing steps in the art can be adopted.

[0033] The present invention also provides a dual-gradient nano copper current collector prepared by the above preparation method.

[0034] The present invention also provides the application of the above dual-gradient nano copper current collector in a lithium-ion battery. The present invention does not impose special restrictions on the specific application method, and the application method of the copper current collector commonly used by those skilled in the art can be adopted.

[0035] The technical solution of the present invention will be further described below in conjunction with specific embodiments. The size of the copper foil in the following examples and comparative examples is 100mm×100mm.

[0036] Example 1

[0037] This example provides a preparation method of a dual-gradient nano copper current collector.

[0038] (1) Select a high-purity electrolytic copper foil with a purity of 99.9% as the raw material, and the thickness of the copper foil is 12μm.

[0039] (2) Place the high-purity electrolytic copper foil sample in an annealing device for annealing treatment. The annealing temperature is 300°C, and the holding time is 60min. The annealing process is carried out in an argon atmosphere.

[0040] (3) Perform surface cleaning treatment on the annealed copper foil. After pickling, wash it with water to remove the oil and oxides on the surface, and fix the cleaned copper foil on the operating table.

[0041] (4) Use a surface ultrasonic rolling treatment device to process the upper surface and the lower surface of the cleaned copper foil respectively. Set the feeding speed of the rolling tool head to 5000mm / min, the spraying rate of high-frequency oil to 0.5L / min, the downward pressure of the rolling tool head to 0.2μm per pass, the number of rolling passes to 10 passes, the ultrasonic frequency to 20kHz, and the amplitude to 10μm; so that a gradient structure layer composed of nanocrystals, ultrafine crystals, and microcrystals with different scales is formed from the upper surface layer and the lower surface layer to the core of the copper foil.

[0042] (5) Wash and degrease the sample after the surface ultrasonic rolling treatment to obtain a dual-gradient nano copper current collector.

[0043] Example 2

[0044] This example provides a preparation method of a dual-gradient nano copper current collector.

[0045] (1) Select a high-purity electrolytic copper foil with a purity of 99.9% as the raw material, and the thickness of the copper foil is 16μm.

[0046] (2) Place the high-purity electrolytic copper foil sample in an annealing equipment for annealing treatment. The annealing temperature is 300 °C, and the holding time is 60 min. The annealing process is carried out under an argon atmosphere.

[0047] (3) Conduct surface cleaning treatment on the annealed copper foil. After pickling, wash it with water to remove the oil and oxides on the surface, and fix the cleaned copper foil on the operating table.

[0048] (4) Use a surface ultrasonic rolling treatment equipment to treat the upper and lower surfaces of the cleaned copper foil respectively. Set the feed speed of the rolling tool head to 5000 mm / min, the rate of high-frequency oil injection to 0.5 L / min, the downward pressure of the rolling tool head to 0.3 μm per pass, the number of rolling passes to 10 passes, the ultrasonic frequency to 20 kHz, and the amplitude to 10 μm; so that a gradient structure layer composed of nanocrystals, ultrafine crystals, and microcrystals with different scales is formed from the upper and lower surfaces to the core of the copper foil.

[0049] (5) Clean and degrease the sample after the surface ultrasonic rolling treatment to obtain a dual-gradient nano copper current collector.

[0050] Example 3

[0051] This example provides a method for preparing a dual-gradient nano copper current collector.

[0052] (1) Select a high-purity electrolytic copper foil with a purity of 99.9% as the raw material, and the thickness of the copper foil is 16 μm.

[0053] (2) Place the high-purity electrolytic copper foil sample in an annealing equipment for annealing treatment. The annealing temperature is 300 °C, and the holding time is 60 min. The annealing process is carried out under an argon atmosphere.

[0054] (3) Conduct surface cleaning treatment on the annealed copper foil. After pickling, wash it with water to remove the oil and oxides on the surface, and fix the cleaned copper foil on the operating table.

[0055] (4) Use a surface ultrasonic rolling treatment equipment to treat the upper and lower surfaces of the cleaned copper foil respectively. Set the feed speed of the rolling tool head to 5000 mm / min, the rate of high-frequency oil injection to 0.5 L / min, the downward pressure of the rolling tool head to 0.2 μm per pass, the number of rolling passes to 10 passes, the ultrasonic frequency to 20 kHz, and the amplitude to 10 μm; so that a gradient structure layer composed of nanocrystals, ultrafine crystals, and microcrystals with different scales is formed from the upper and lower surfaces to the core of the copper foil.

[0056] (5) Clean and degrease the sample after the surface ultrasonic rolling treatment to obtain a dual-gradient nano copper current collector.

[0057] Example 4

[0058] This example provides a method for preparing a dual-gradient nano copper current collector.

[0059] (1) Select a high-purity electrolytic copper foil with a purity of 99.9% as the raw material, and the thickness of the copper foil is 10 μm.

[0060] (2) Place the high-purity electrolytic copper foil sample in an annealing device for annealing treatment. The annealing temperature is 300 °C, and the holding time is 60 min. The annealing process is carried out in an argon atmosphere.

[0061] (3) Perform surface cleaning treatment on the annealed copper foil. After pickling, wash it with water to remove the oil and oxides on the surface, and fix the cleaned copper foil on the operating table.

[0062] (4) Use a surface ultrasonic rolling treatment device to process the upper and lower surfaces of the cleaned copper foil respectively. Set the feed speed of the rolling tool head to 5000 mm / min, the rate of high-frequency oil injection to 0.5 L / min, the downward pressure of the rolling tool head to 0.15 μm per pass, the number of rolling passes to 10 passes, the ultrasonic frequency to 20 kHz, and the amplitude to 10 μm; so that a gradient structure layer composed of nanocrystals, ultrafine crystals, and microcrystals with different scales is formed from the upper surface layer and the lower surface layer to the core of the copper foil.

[0063] (5) Clean and degrease the sample after the surface ultrasonic rolling treatment to obtain a dual-gradient nano copper current collector.

[0064] Comparative Example 1

[0065] Compared with Example 1, the difference in this comparative example is that only the upper surface of the copper foil is subjected to surface ultrasonic rolling treatment, and the other steps are the same.

[0066] Comparative Example 2

[0067] Compared with Example 1, the difference in this comparative example is that this comparative example does not perform annealing treatment.

[0068] Comparative Example 3

[0069] Compared with Example 1, the difference in this comparative example is that the annealing time in this comparative example is 3 h.

[0070] Comparative Example 4

[0071] Compared with Example 1, the difference in this comparative example is that the downward pressure of the rolling tool head is set to 0.5 μm per pass in step (4) of this comparative example.

[0072] Comparative Example 5

[0073] This comparative example is different from Example 1 in that steps (4) and (5) are not carried out in this comparative example.

[0074] Comparative Example 6

[0075] This comparative example is different from Example 1 in that the high-purity electrolytic copper foil with a purity of 99.9% is not annealed and surface ultrasonic rolling treatment is not carried out, and only cleaning treatment is carried out on it.

[0076] Test Example

[0077] 1. Determination of structural parameters:

[0078] The grain size, thickness of each layer and other parameters of the copper current collectors of Examples 1 to 4 and Comparative Examples 1 to 7 are measured. The grain size is measured by transmission electron microscope images (TEM), and the thickness of each layer is measured by SEM; the test results are shown in Table 1, and the structural schematic diagrams of the copper current collectors of Examples 1 to 4 are as Figure 1 shown.

[0079] Table 1 Structural parameter data of the copper current collectors of Examples 1 to 4 and Comparative Examples 1 to 7

[0080]

[0081]

[0082] Note *: After surface ultrasonic rolling treatment, the copper foil will become thinner.

[0083] 2. Performance determination:

[0084] The specific test methods are as follows:

[0085] (1) Tensile property experiment: Cut the copper current collectors of Examples 1 to 4 and Comparative Examples 1 to 6 into "dog bone"-shaped specimens, where the length and width of the gauge section are 5 mm and 2 mm respectively, the width of the clamping section is 4 mm, and the total length of the specimen is 17 mm. Use a Shimadzu AG-IS / 1KN universal testing machine in Japan to conduct tensile property tests to obtain the tensile strength and elongation rate.

[0086] (2) Conductivity test: It is determined according to the method specified in GB / T 32791-2016 "Eddy Current Testing Method for Electrical Conductivity of Copper and Copper Alloys".

[0087] (3) High-temperature oxidation property experiment: Cut the copper current collectors of the examples and comparative examples with a size of 5 mm × 5 mm, place the copper current collector specimens in a tube furnace at a temperature of 200 °C (air atmosphere), take them out after 30 min, and observe the color change of the upper and lower surfaces of the specimens.

[0088] The test results are summarized in Table 2.

[0089] Table 2 Performance test data of copper current collectors in Examples 1-4 and Comparative Examples 1-6

[0090]

[0091] Combined with Table 1 and Table 2, it can be seen that the copper current collectors in Examples 1-4 of the present invention have a double-gradient structure, and the thickness of their deformation layer (nanocrystalline + ultrafine crystalline) is moderate. Therefore, they have both excellent tensile strength and elongation rate, and the nanocrystals on the surface improve the overall electron conduction and antioxidant properties of the copper current collector. In Comparative Example 1, only one side of the copper foil was subjected to surface ultrasonic rolling treatment, resulting in a smaller proportion of the nanocrystalline layer and lower conductivity. And on the side that was not subjected to surface ultrasonic rolling treatment during the tensile process, tensile cracks occurred first, resulting in the overall tensile strength and elongation rate being smaller than those in Example 1. In Comparative Example 2, no annealing treatment was carried out, the initial grain size of the copper foil was smaller, and the grains were further refined after surface ultrasonic rolling treatment, and the nanocrystalline layer and ultrafine crystalline layer were thicker, resulting in the proportion of the micron-sized crystalline layer being less than 50%, making the conductivity and tensile strength slightly higher, and the elongation rate decreased significantly. In Comparative Example 3, the annealing time was too long, resulting in the grains growing into coarse grains. After surface ultrasonic rolling treatment, the core was a coarse-grained layer, which could not cooperate well with the nanocrystalline layer and ultrafine crystalline layer on the surface, and thus the tensile strength and elongation rate were lower. In Comparative Example 4, the downward pressure of the rolling tool head each time was too large, resulting in a lower proportion of micron-sized crystals, obvious refinement of nanocrystals and ultrafine crystals, and a higher proportion of nanocrystals. As a result, the overall tensile strength and conductivity of the copper foil were high, but the elongation rate was low. In Comparative Example 5, the grains of the copper foil grew into micron-sized crystals after annealing treatment, resulting in a high overall elongation rate, and a decrease in tensile strength and conductivity. Comparative Example 6 was a homogeneous micron-sized crystalline copper foil, and the grain size was larger than that in Example 1. Therefore, the tensile strength, elongation rate, and conductivity were lower.

[0092] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a dual-gradient nano-copper current collector, characterized in that: The steps include: The electrolytic copper foil is sequentially subjected to annealing treatment and cleaning treatment; The upper surface and the lower surface of the electrolytic copper foil after cleaning are subjected to surface ultrasonic rolling treatment respectively, and a double-gradient nano-copper current collector with a "nanocrystalline layer-ultrafine crystal layer-micron crystal layer-ultrafine crystal layer-nanocrystalline layer" structure is obtained by adjusting the parameters of the surface ultrasonic rolling treatment process, and the total thickness of the nanocrystalline layer and the ultrafine crystal layer is 20-50% of the thickness of the double-gradient nano-copper current collector.

2. The preparation method according to claim 1, characterized in that The thickness of the electrolytic copper foil is 10-16 μm; the purity of the electrolytic copper foil is above 99.0%.

3. The preparation method according to claim 1, characterized in that: The annealing treatment is performed at a temperature of 200 to 400° C., for a time of 50 to 120 minutes, and in an argon atmosphere.

4. The preparation method according to claim 1, characterized in that: The grain size of the nanocrystal is 10-100 nm; the grain size of the ultrafine crystal is 100-1000 nm; and the grain size of the micron crystal is 1-5 μm.

5. The preparation method according to claim 1, characterized in that: The pressing amount of the rolling blade of the surface ultrasonic rolling treatment equipment is 0.1 to 0.3 μm / pass, and the number of rolling passes is 8 to 12 passes.

6. The preparation method according to claim 5, characterized in that: During the surface ultrasonic rolling treatment process, the feeding speed of the rolling cutter head is 4000-6000 mm / min.

7. The preparation method according to claim 1, characterized in that: The ultrasonic frequency of the surface ultrasonic rolling treatment is 10-30 kHz, and the ultrasonic amplitude is 20-50 μm.

8. The preparation method according to claim 1, characterized in that: During the surface ultrasonic rolling treatment, the upper and lower surfaces of the cleaned electrolytic copper foil are subjected to high-frequency oil spraying treatment, and the rate of the high-frequency oil spraying is 0.5-1 L / min. 9 . The double-gradient nano-copper current collector prepared by the preparation method according to claim 1 .

10. Use of the dual-gradient nano-copper current collector as claimed in claim 9 in a lithium-ion battery.

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