Composite current collector for lithium battery and method for preparing the same

Abstract

The application discloses a composite current collector for lithium batteries and a preparation method thereof. The composite current collector has a layered structure, and comprises a base film located in a middle position, a first metal layer arranged on upper and lower surfaces of the base film, and a second metal layer arranged on a surface of the first metal layer. The composite current collector substrate is stretched to deform, and a deformation amount is released in advance. In a subsequent battery cycle process, the composite current collector with the deformation amount released in advance can resist an acting force caused by volume changes of positive and negative electrode materials, and the degree of deformation of the composite current collector is greatly reduced.

Description

TECHNICAL FIELD

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

[0002] Nowadays, power batteries are developing towards high energy density and high safety, which requires us to optimize the battery from various aspects. As an important component of lithium ion battery, the current collector greatly affects the performance of lithium battery, so the optimization of the current collector is also essential. The current collector is usually aluminum foil and copper foil, and generally, aluminum foil is used at the positive electrode end and copper foil is used at the negative electrode end. The concept of composite current collector is relative to the traditional current collector, which is an optimization of the traditional current collector. The composite current collector is a new type of material with a multi-layer structure, which usually adopts a polymer film as a base film and deposits a metal layer on the surface thereof. The composite current collector promotes the improvement of energy density and safety of lithium ion battery.

[0003] In the application process of the composite current collector, it will be affected by external factors. For example, during the cycle process of lithium battery, the volume change of positive and negative electrode materials will cause the composite current collector to be subjected to force, and the harsh environment inside the lithium battery will cause the metal layer of the composite current collector to have the risk of cracking, and in severe cases, the metal layer and the positive and negative electrode material layer will be detached together, and finally affect the performance of the lithium battery. How to solve this problem is the key to the application of the composite current collector. SUMMARY

[0004] In view of the above problems, the present application provides a composite current collector for lithium battery, which has the characteristics of small mass, small thickness and good electrical conductivity. The present application also provides a preparation method of the composite current collector.

[0005] To solve the technical problems, the present application adopts the following technical solutions:

[0006] The composite current collector for lithium battery has a layered structure, which comprises a base film, a first metal layer and a second metal layer. The base film is located at the middle position of the layered structure, the first metal layer is arranged on the upper and lower surfaces of the base film, and the second metal layer is arranged on the surface of the first metal layer; the composite current collector is formed by stretching the base film with the first metal layer to deform, and then arranging the second metal layer.

[0007] In a further scheme, the base film is a polymer film, and the thickness thereof is 1-100 μm, preferably 1-50 μm, and more preferably 1-20 μm.

[0008] In a further scheme, the material of the base film is one or more of PP (polypropylene), PET (polyethylene terephthalate), PI (polyimide), PE (polyethylene), PEEK (polyether ether ketone) and the like.

[0009] In a further aspect, the first metal layer is an aluminum metal layer or a copper metal layer. When the composite current collector is applied to a positive electrode of a battery, the first metal layer is an aluminum metal layer. When the composite current collector is applied to a negative electrode of a battery, the first metal layer is a copper metal layer. The thickness of the first metal layer is 100-2000 nm, preferably 500-1500 nm, and more preferably 800-1200 nm.

[0010] In a further aspect, the second metal layer is an aluminum metal layer or a copper metal layer, and the material of the second metal layer is consistent with the material of the first metal layer. When the composite current collector is applied to a positive electrode of a battery, the first metal layer is an aluminum metal layer, and the second metal layer is also an aluminum metal layer. When the composite current collector is applied to a negative electrode of a battery, the first metal layer is a copper metal layer, and the second metal layer is also a copper metal layer. The thickness of the second metal layer is 1-1000 nm, preferably 10-200 nm, and more preferably 10-100 nm.

[0011] The application also provides a preparation method of the composite current collector for a lithium battery, comprising the following steps:

[0012] S1: selecting a polymer film as a base film, plating a first metal layer on the upper and lower surfaces of the base film to obtain a composite current collector base sheet;

[0013] S2: stretching the composite current collector base sheet in the length direction to cause a certain degree of deformation of the composite current collector base sheet;

[0014] S3: plating a second metal layer on the surface of the first metal layer of the composite current collector base sheet that has been deformed to obtain a composite current collector.

[0015] In a further aspect, the plating method of the first metal layer and the second metal layer can be one or a combination of several plating methods such as evaporation plating, magnetron sputtering, ion plating, and electroplating. The plating process can be one-time plating or multi-time plating.

[0016] In a further aspect, the deformation amount of the composite current collector base sheet after stretching is 1%-20%, preferably 3%-15%, and more preferably 5%-10%.

[0017] Compared with the prior art, the application has the following beneficial effects:

[0018] The composite current collector with a layered structure provided by the application can reduce the weight of the current collector because the base film is a polymer film. The composite current collector base sheet is stretched to cause deformation, and the deformation amount is released in advance. In the subsequent battery cycle process, the composite current collector that has released the deformation amount in advance can resist the force caused by the volume change of the positive and negative electrode materials, and the degree of deformation is greatly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 This is a schematic diagram of the composite current collector of the present invention. In the figure, the numbers are: 1-base film, 2-first metal layer, 3-second metal layer. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments.

[0021] refer to Figure 1 A composite current collector for lithium batteries has a layered structure, including a base film 1, a first metal layer 2, and a second metal layer 3. The base film 1 is located in the middle of the layered structure, the first metal layer 2 is disposed on the upper and lower surfaces of the base film 1, and the second metal layer 3 is disposed on the surface of the first metal layer 2. The composite current collector is formed by stretching and deforming the base film 1 on which the first metal layer 2 is formed, and then disposing of the second metal layer 3.

[0022] The base film 1, located in the middle, is a polymer film, which can be one or more of the following materials: PP (polypropylene), PET (polyethylene terephthalate), PI (polyimide), PE (polyethylene), and PEEK (polyetheretherketone). Polymer films are characterized by low mass and high strength, and the composite current collectors prepared from them have better processing capabilities at the battery end, thus positively impacting battery energy density. Typically, the thickness of the base film 1 is 1-100 μm, preferably 1-50 μm, and more preferably 1-20 μm.

[0023] A metal layer of a certain thickness is deposited on the surface of base film 1, such as... Figure 1 As shown, this metal layer is the first metal layer 2, which is either an aluminum or copper metal layer, corresponding to the positive and negative electrodes of the battery. When the composite current collector is applied to the positive electrode of the battery, an aluminum metal layer is selected. When the composite current collector is applied to the negative electrode of the battery, a copper metal layer is selected. The thickness of the first metal layer 2 is 100-2000 nm, preferably 500-1500 nm, and more preferably 800-1200 nm.

[0024] Stretching the base film 1 with the first metal layer 2 causes both the first metal layer 2 and the base film 1 to deform to a certain extent simultaneously, but without causing cracks in the metal layer. The deformation caused by stretching should not be too small; otherwise, it will have insufficient resistance to the forces caused by subsequent volume changes in the positive and negative electrode materials. In the harsh environment inside a lithium battery, the composite current collector may still experience cracks in the metal layer. Conversely, the deformation caused by stretching should not be too large either; otherwise, the first metal layer 2 may crack during the stretching process, leading to an excessive increase in the sheet resistance of the composite current collector, which will adversely affect the battery's internal resistance and cycle performance.

[0025] The second metal layer 3 is disposed on the surface of the first metal layer 2, which is an aluminum metal layer or a copper metal layer, and has the same material as the first metal layer 2. When the composite current collector is applied to the positive electrode of the battery, the first metal layer 2 is an aluminum metal layer, and the second metal layer 3 is also an aluminum metal layer. When the composite current collector is applied to the negative electrode of the battery, the first metal layer 2 is a copper metal layer, and the second metal layer 3 is also a copper metal layer. The thickness of the second metal layer 3 is 1-1000 nm, preferably 10-200 nm, and more preferably 10-100 nm.

[0026] For the preparation of each metal layer, one or a combination of several methods such as evaporation, magnetron sputtering, ion plating, and electroplating is usually selected. For example, when plating copper, evaporation or magnetron sputtering can be selected to achieve the desired thickness by one or multiple plating. Alternatively, a thinner layer can be first plated by evaporation or magnetron sputtering, and then thickened by electroplating to achieve the desired thickness.

[0027] Example 1

[0028] A PET film with a thickness of 6 μm was selected, and an aluminum metal layer with a thickness of 950 nm was plated on the upper and lower surfaces of the PET film by vacuum evaporation. The aluminum metal layer was prepared by selecting a one-time plating process during evaporation, and a composite current collector substrate was obtained. The composite current collector substrate was stretched in the length direction to cause a 6% deformation. Then, an aluminum metal layer with a thickness of 50 nm was plated on the surface of the first metal layer 2 of the deformed composite current collector substrate by vacuum evaporation, the preparation of the second metal layer 3 was completed, and a composite current collector was obtained. The tensile strength and square resistance of the composite current collector were tested.

[0029] The above composite current collector was used as the positive electrode current collector, and a 6 μm copper foil was used as the negative electrode current collector to prepare a battery. The ternary active material NCM622, the conductive agent CNTs, and the binder PVDF were added to the organic solvent NMP in a ratio of 98:1:1, and uniformly stirred to obtain a positive electrode slurry. The graphite, the conductive agent SP, the binder CMC, and SBR were added to the deionized water in a ratio of 96.1:0.7:1.5:1.7, and uniformly stirred to obtain a negative electrode slurry. The positive electrode slurry was uniformly coated on the surface of the positive electrode current collector to ensure that the area density was 390 g / m 2 . The negative electrode slurry was uniformly coated on the surface of the negative electrode current collector, and the area density was 220 g / m 2The positive and negative electrode rolls then undergo baking, rolling, slitting, assembly, electrolyte injection, formation, and capacity testing. The positive and negative electrode materials mentioned above are all conventional materials used in lithium-ion battery production, and the processes involved are standard operations employed in lithium-ion battery manufacturing. The materials used and the processes can be adjusted according to actual needs to complete battery fabrication. After battery fabrication, internal resistance and thickness tests are performed, followed by room temperature cycling. After 300 cycles, internal resistance and thickness tests are conducted again.

[0030] Examples 2-7

[0031] For Examples 2-7, only the tensile deformation of the composite current collector substrate in the length direction was changed. The rest of the steps and processes were the same as those in Example 1 for the preparation of the composite current collector and battery. The tensile deformation of the substrate in the length direction for Examples 2-7 were 3%, 9%, 12%, 15%, 20%, and 25%, respectively.

[0032] Comparative Example 1

[0033] A 6μm thick PET film was selected, and a 950nm aluminum metal layer was deposited on both its upper and lower surfaces using vacuum evaporation. The aluminum metal layer was prepared in a single deposition process, without subsequent stretching. Then, a 50nm thick aluminum metal layer was deposited on both the upper and lower surfaces using vacuum evaporation to complete the composite current collector fabrication. All other steps and the battery fabrication process remained consistent with Example 1.

[0034] Table 1 Tensile strength of composite current collectors

[0035] Composite current collector package Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Comparative Example 1 Tensile strain 6％ 3％ 9％ 12％ 15％ 20％ 25％ 0％ Tensile strength, MPa 286.0 288.3 277.6 261.3 260.9 245.5 237.4 287.2 Sheet resistance, mΩ / □ 35.5 35.8 36.0 36.5 37.4 38.8 44.7 35.6

[0036] Table 2 Battery parameters for each embodiment

[0037]

[0038] Table 1 shows the tensile strength and sheet resistance data of the composite current collectors in Examples 1-7 and Comparative Example 1. It can be seen that the tensile strengths of Examples 1-2 and Comparative Example 1 are basically at the same level, all exhibiting high tensile strength. As shown in Examples 3-7, when the tensile deformation of the substrate in the length direction continues to increase, the tensile strength of the prepared composite current collector decreases to a certain extent. When the tensile deformation reaches 25%, the tensile strength value decreases significantly. At this point, fine cracks begin to appear in the metal layer of the composite current collector. This not only affects the tensile strength of the composite current collector but also adversely affects its sheet resistance and the electrical performance of the battery.

[0039] In Examples 1-3, the sheet resistance of the composite current collector remained at the same level as in Comparative Example 1. As the stretching deformation continued to increase, the sheet resistance of the prepared composite current collector gradually increased. When the stretching deformation reached 25%, the sheet resistance value reached 44.7 mΩ / □, showing a significant increase. Excessively high sheet resistance of the composite current collector will affect the electrical performance of the batteries prepared using it.

[0040] The parameters of the batteries in Examples 1-7 and Comparative Example 1 are shown in Table 2. It can be seen that the initial thickness of the batteries in Examples 1-7 and Comparative Example 1 is basically the same. Regarding the initial internal resistance of the battery, when the tensile deformation is below 12%, i.e., the internal resistance of the batteries in Examples 1-4 does not change significantly. As the tensile deformation continues to increase, the internal resistance of the battery begins to gradually increase, which is due to the sheet resistance of the composite current collector itself.

[0041] After 300 battery cycles, the internal resistance and thickness of the battery in Comparative Example 1 increased significantly. This is because the composite current collector used in the battery of Comparative Example 1 was not subjected to tensile deformation and therefore could not resist the forces generated by the volume changes of the positive and negative electrode materials during battery cycling. In Examples 1-7, the battery internal resistance generally showed a "high-low-high" trend as the tensile deformation increased. For example, the tensile deformation corresponding to Example 2 was 3%, at which point the resistance to the forces generated by the volume changes of the positive and negative electrode materials was relatively small. When the tensile deformation reached 6%-12%, the resistance to the forces generated by the volume changes of the positive and negative electrode materials was relatively large. Continuing to increase the tensile deformation, as in Examples 5-7, the battery internal resistance was greatly affected by the sheet resistance of the composite current collector itself, resulting in a larger battery internal resistance value. Regarding battery thickness, the battery thickness increased in both Examples 1-7 and Comparative Example 1. The increase in battery thickness was more significant in Comparative Example 1, while the increase in battery thickness in Examples 1-7 was smaller. This is because the composite current collectors in Examples 1-7 have been stretched to different degrees during preparation, which can resist some of the forces generated by the battery end, thus greatly reducing the degree of deformation of the composite current collectors.

[0042] Based on the performance data of the composite current collector and the battery mentioned above, this invention believes that the composite current collector that has undergone stretching deformation during the preparation process has better performance in application, with Examples 1 and 3, with stretching deformation of 6% and 9% respectively, showing the best results.

[0043] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite current collector for lithium batteries, characterized in that: The composite current collector has a layered structure, including a base film, a first metal layer, and a second metal layer. The base film is located in the middle of the layered structure, the first metal layer is disposed on the upper and lower surfaces of the base film, and the second metal layer is disposed on the surface of the first metal layer. The composite current collector is formed by stretching the base film with the first metal layer in the length direction to deform it, and then disposing of the second metal layer. The deformation due to stretching is 1% to 20%. The base film is a polymer film with a thickness of 1-100 μm; The first metal layer and the second metal layer are made of the same material, which is either an aluminum metal layer or a copper metal layer; when the composite current collector is applied to the positive terminal of the battery, both the first metal layer and the second metal layer are aluminum metal layers; when the composite current collector is applied to the negative terminal of the battery, both the first metal layer and the second metal layer are copper metal layers.

2. The composite current collector for lithium batteries according to claim 1, characterized in that: The base film is made of one or more of the following materials: polypropylene, polyethylene terephthalate, polyimide, polyethylene, and polyetheretherketone.

3. The composite current collector for lithium batteries according to claim 1, characterized in that: The thickness of the first metal layer is 100-2000 nm.

4. The composite current collector for lithium batteries according to claim 1, characterized in that: The thickness of the second metal layer is 1-1000 nm.

5. A method for preparing a composite current collector for lithium batteries according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1: Select a polymer film as the base film, and deposit a first metal layer on the upper and lower surfaces of the base film to obtain a composite current collector substrate; S2: The composite current collector substrate is stretched along its length to cause a certain degree of deformation. S3: A second metal layer is deposited on the surface of the first metal layer of the composite current collector substrate that has already undergone deformation to obtain a composite current collector.

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