Surface treatment method for copper foil

Abstract

The present application relates to a kind of surface treatment methods of copper foil, comprising the following steps: step one, preparation roughing treatment solution, roughing treatment solution uses deionized water as main solvent, and roughing treatment solution includes mass fraction 0.2wt%-1.0wt% copper trifluoroacetate hydrate and mass fraction 5wt%-10wt% peracetic acid;Step two, copper foil is placed in roughing treatment solution and soaked for 60-300s;Step three, after using deionized water to clean the surface of copper foil, dry processing is carried out.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion secondary battery technology, and more specifically to a surface treatment method for copper foil, which can be used as a current collector for the electrode of a lithium-ion secondary battery. Background Technology

[0002] This section provides background information relevant to this application, which does not necessarily constitute prior art.

[0003] Copper foil is a key auxiliary material for lithium-ion secondary batteries. Lithium-ion secondary batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode is generally formed by coating carbon particles as a layer of negative electrode active material on the surface of a negative electrode current collector made of copper foil with two smooth sides, drying it, and then extruding it.

[0004] The copper foil used as the current collector in the negative electrode of a lithium-ion secondary battery is generally produced by electrolysis and surface-treated. To enhance the adhesion between the copper foil and the negative electrode coating, the surface of the copper foil is roughened. This is typically achieved through multiple electrodepositions in a copper sulfate electrolyte, with the roughness controlled by adjusting the electrolyte concentration and current. However, this method has limited effectiveness in improving adhesion and can easily result in two different morphologies on the two sides of the copper foil: a smooth surface and a rough surface. This asymmetry can lead to asymmetrical contact resistance between the copper foil and the coatings on both sides, resulting in uneven capacity release from the negative electrode. Furthermore, the asymmetry can cause inconsistent adhesion strength of the negative electrode coatings, leading to a severe imbalance in the charge-discharge cycle life of the two coatings and accelerating battery capacity decay.

[0005] If the difference in surface roughness Rz between the two surfaces of copper foil used as the negative electrode current collector in lithium-ion secondary batteries can be reduced, the decrease in the charge and discharge efficiency of lithium-ion secondary batteries can be suppressed.

[0006] For copper foil used as the negative electrode current collector in lithium-ion secondary batteries, the reasonable range for the surface roughness Rz (the height of ten micro-irregularities, measured, for example, using a stylus surface roughness meter) of the copper foil surface is 0.8-2.8 μm, in order to suppress the peeling between the negative electrode coating and the copper foil. If the surface roughness Rz is below 0.8 μm, there will be no suppression effect. Furthermore, even if Rz is above 2.8 μm, the suppression effect will become saturated, and the capacity retention rate will actually deteriorate during charge and discharge. Therefore, forming a negative electrode coating containing an active material layer on a copper foil with a surface roughness Rz of 0.8-2.8 μm can effectively suppress the peeling between the negative electrode coating and the copper foil. Summary of the Invention

[0007] This invention provides a surface treatment method for copper foil, wherein the two surfaces of the treated copper foil satisfy the following conditions: the surface roughness Rz is 0.8-2.8μm and the difference in surface roughness Rz between the two surfaces of the copper foil is not greater than 0.1.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a surface treatment method for copper foil, comprising the following steps:

[0009] Step 1: Prepare a roughening treatment solution. The roughening treatment solution uses deionized water as the main solvent and contains 0.2wt%-1.0wt% copper trifluoroacetate hydrate and 5wt%-10wt% peracetic acid.

[0010] Step 2: Immerse the copper foil in the roughening solution for 60-300 seconds;

[0011] Step 3: Clean the surface of the copper foil with deionized water and then dry it.

[0012] In one or more embodiments, the drying process involves drying the copper foil at an ambient temperature of 80-140°C.

[0013] The working principle of this invention is as follows: The peel strength between the negative electrode coating and the copper foil has a significant impact on the performance of lithium-ion secondary batteries. To improve the adhesion between the copper foil and the negative electrode coating, a roughening treatment solution is used to micro-etch the surface of the copper foil to achieve surface roughening. This method can effectively increase the specific surface area and active sites of the copper foil, thereby facilitating mutual contact and penetration between the copper foil and the coating, forming an intertwined structure to enhance the adhesion of the coating. Furthermore, the similarity of the structures on both sides reduces the reduction in battery performance caused by structural asymmetry.

[0014] The beneficial effects of this invention are as follows: This invention provides a surface treatment method for copper foil, which uses a roughening treatment solution to perform micro-etching on the surface of the copper foil, increasing the active sites and specific surface area of ​​the copper foil, thereby improving the adhesion of the coating to the copper foil, greatly reducing the probability of slurry peeling and the reduction in battery performance caused by the asymmetry of the two-sided structure. The process principle and flow of this method are simple and have great market application prospects.

[0015] The following description is based on specific embodiments. Attached Figure Description

[0016] The accompanying drawings further illustrate the invention, but the embodiments in the drawings do not constitute any limitation on the invention.

[0017] Figure 1 This is a schematic diagram of the microstructure of the copper foil after surface treatment in Embodiment 3 of the present invention. Detailed Implementation

[0018] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0019] It should be noted that the copper foil described in this invention can be used as a current collector for the negative electrode of a lithium-ion secondary battery. The copper foil can be rolled copper foil, electrolytic copper foil, or composite copper foil. The copper foil described in the following embodiments is all electrolytic copper foil, and the electrolytic copper foil satisfies the following requirement: tensile strength of 400 N / mm². 2 The elongation is 4.5%-13%, and the surface roughness Ra (mean arithmetic deviation of the profile) is 0.01-1μm.

[0020] For example, the preparation of the electrolytic copper foil may include:

[0021] (a) Using an aqueous solution of sulfuric acid and copper sulfate as the electrolyte, the electrolyte is supplied between an insoluble anode made of titanium coated with platinum group elements or their oxides and a titanium cathode roller disposed opposite to the anode.

[0022] (b) The cathode roller is rotated at a certain speed, and a direct current is passed between the two electrodes, thereby depositing copper on the surface of the cathode roller.

[0023] (c) The copper deposited is peeled off from the surface of the cathode roller and continuously wound up, and this method is used for manufacturing.

[0024] More specifically, electrolytic copper foil can be manufactured by adding the following leveling agent, brightener, and chloride ions to a sulfuric acid-copper sulfate electrolyte. The leveling agent is one or more additives selected from thiourea or thiourea derivatives. Examples of thiourea or thiourea derivatives include water-soluble thiourea and thiourea derivatives such as thiourea, N,N'-dimethylthiourea, N,N'-diethylthiourea, tetramethylthiourea, aminothiourea, N-allylthiourea, and ethylenethiourea. The brightener is one or more additives selected from animal glue, gelatin, polyethylene glycol, polypropylene glycol, starch, water-soluble cellulose polymers (carboxymethyl cellulose, hydroxyethyl cellulose, etc.) and other polysaccharides, polyethyleneimine, and polyacrylamide.

[0025] Although electrolytic copper foil (untreated copper foil) can be used directly as a current collector, in most cases, it undergoes rust prevention treatment before being used as the negative electrode current collector for lithium-ion secondary batteries. Rust prevention treatments include: (a) inorganic rust prevention treatments such as chromate treatment, Ni or Ni alloy plating, Co or Co alloy plating, Zn or Zn alloy plating, Sn or Sn alloy plating, or chromate treatment following the above electroplating processes; (b) organic rust prevention treatments such as benzotriazole; and (c) silane coupling agent treatments. It should be noted that the surface treatment method provided by this invention is located at the forefront of the rust prevention treatment process.

[0026] It should be noted that the copper foils described in the following examples are all 10 μm thick and were prepared using the following methods:

[0027] Step 1: Elemental copper is added to a copper-dissolving tank containing sulfuric acid. High-temperature air is blown in using a screw blower to dissolve the copper and prepare an acidic copper sulfate main electrolyte. After multi-stage filtration, the main electrolyte is mixed with an additive solution to obtain the final electrolyte. The resulting electrolyte has a copper ion concentration of 120 g / L, a sulfuric acid concentration of 140 g / L, and a chloride ion concentration of 90 mg / L. The composition of the other additives is as follows:

[0028] Ethylenethiourea: 2 mg / L;

[0029] Sodium 3-mercapto-1-propanesulfonate: 2.5 g / L;

[0030] Polyethylene glycol: 25 mg / L

[0031] Polyethyleneimine (number average molecular weight 1200): 12 mg / L;

[0032] Gelatin: 3g / L;

[0033] Hydroxyethyl cellulose: 0.3 g / L;

[0034] Step two: The electrolyte is heated to 45°C via a heat exchanger and then pumped into the electrolytic cell. The cathode of the electrolytic cell is a seamless roller-type titanium roller, and the anode is a titanium electrode coated with noble metal oxides. The operating temperature is 40 A / dm³. 2 Copper foil was prepared by electrolysis at 45°C under a current density of [value missing].

[0035] It should be noted that the CAS number of copper trifluoroacetate hydrate described in the following examples is 123333-88-0, and the CAS number of peracetic acid described in the following examples is 79-21-0.

[0036] It should be noted that the surface roughness Rz described in the following embodiments is a value obtained by measuring with a stylus-type surface roughness meter, that is, the sum of the average value of the five largest profile peak heights and the average value of the five largest profile valley depths within the sampling length of the copper foil surface. Surface roughness Rz belongs to micro-geometric error, and the smaller the surface roughness Rz, the smoother the surface.

[0037] [Example 1]

[0038] This embodiment provides a surface treatment method for copper foil, including the following steps:

[0039] Step 1: Prepare a roughening treatment solution. The roughening treatment solution uses deionized water as the main solvent and contains 0.2 wt% copper trifluoroacetate hydrate and 5 wt% peracetic acid.

[0040] Step 2: Immerse the copper foil in the roughening solution for 300 seconds;

[0041] Step 3: Clean the surface of the copper foil with deionized water and then dry the copper foil at an ambient temperature of 80°C.

[0042] [Example 2]

[0043] This embodiment provides a surface treatment method for copper foil, including the following steps:

[0044] Step 1: Prepare a roughening treatment solution. The roughening treatment solution uses deionized water as the main solvent and contains 1.0 wt% copper trifluoroacetate hydrate and 10 wt% peracetic acid.

[0045] Step 2: Immerse the copper foil in the roughening solution for 60 seconds;

[0046] Step 3: Clean the surface of the copper foil with deionized water and then dry the copper foil at an ambient temperature of 140°C.

[0047] [Example 3]

[0048] This embodiment provides a surface treatment method for copper foil, including the following steps:

[0049] Step 1: Prepare a roughening treatment solution. The roughening treatment solution uses deionized water as the main solvent and contains 0.5 wt% copper trifluoroacetate hydrate and 7.5 wt% peracetic acid.

[0050] Step 2: Immerse the copper foil in the roughening solution for 180 seconds;

[0051] Step 3: Clean the surface of the copper foil with deionized water and then dry the copper foil at an ambient temperature of 120°C.

[0052] The microstructure of the surface of copper foil after surface treatment is as follows: Figure 1 As shown, the copper foil has a rough surface after surface treatment.

[0053] Comparative Examples 1-7 all provide a surface treatment method for copper foil. The only difference between the surface treatment methods provided in Comparative Examples 1-7 and the surface treatment method provided in Example 3 is the use of different roughening solutions. A comparison of the roughening solutions used in Example 3 and those used in Comparative Examples 1-7 is shown in Table 1.

[0054] It should be noted that methoxylated polyethylene glycol and cyclohexanone in Table 1 can be regarded as roughening and homogenizing agents, and 1,4-butanediol and 2-mercapto-1-methylimidazole in Table 1 can be regarded as roughening stabilizers.

[0055] Table 1

[0056] roughening treatment solution Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Comparative Example 7 peracetic acid 7.5wt% 8.0wt% / / 7.5wt% / / / Copper trifluoroacetate hydrate 0.5wt% / 0.5wt% 0.5wt% / / / / sulfuric acid / / / 5.0wt% / / 5.0wt% 3.0wt% hydrochloric acid / / / 2.5wt% / / / 2.0wt% Phosphoric acid / / / / / 2.5wt% / / Formic acid / / 5.0wt% / / 2.5wt% / / Acetic acid / / 2.5wt% / / 2.5wt% / 2.0wt% hydrogen peroxide / / / / / / 2.0wt% / 1,4-Butanediol / / / / / / 0.75wt% 0.25wt% 2-Mercapto-1-methylimidazolium / / / / / 0.25wt% Methoxylated polyethylene glycol / / / / 0.25wt% / 0.25wt% 0.25wt% Cyclohexanone / / / / 0.25wt% / / 0.25wt%

[0057] The following tests were performed on the surface-treated copper foils corresponding to Examples 1-3 and Comparative Examples 1-7:

[0058] [Surface Roughness Rz]

[0059] According to the test method GB / T29847-2013, the surface roughness Rz of the first surface and the surface roughness Rz of the second surface of the copper foil were tested using a TR200 roughness tester manufactured by Beijing Shidai Ruida Technology Co., Ltd. The test results are shown in Table 2.

[0060] [Peel strength test]

[0061] A slurry was prepared using Toyo Graphite IG-110 from Japan as the active material. The slurry comprised 64 wt% of the active material, 16 wt% of acetylene black powder (AB), and 20 wt% of polyamic acid solution. Next, the slurry was coated onto the surface-treated copper foil to form a nearly uniform thin layer. After drying, the coating was compressed using an extruder to adhere the active material layer to the current collector. The electrode was then dried under reduced pressure to form the negative electrode. Finally, the electrode was sintered at 250°C in an argon atmosphere and cut into 75 mm × 30 mm rectangular samples for testing.

[0062] The test sample was patterned using photolithography with straight lines, where the line / spacing was 1 mm / 1 mm. The patterned test sample was fixed to an FR4 board (i.e., fiberglass board) measuring 98 mm (L) × 31 mm (W) using double-sided tape and then mounted on a sliding sample holder. The front portion of the patterned test sample was then manually peeled off and clamped to an upper fixture. The peel strength was tested using a testing machine at a traction speed of 50.8 mm / min, and the peel strength was measured and recorded in kN / m. The average peel strength data for the three test lines are listed in Table 2.

[0063] Based on the results in Table 2, it can be determined that the surface treatment method for copper foil provided by the present invention uses a roughening treatment solution to perform micro-etching on the surface of the copper foil. The two surfaces of the copper foil after surface treatment (i.e., the first surface and the second surface) both satisfy the following: the surface roughness Rz is 0.8-2.8 μm and the difference in surface roughness Rz between the two surfaces of the copper foil is not greater than 0.1. Therefore, it can greatly reduce the probability of slurry detachment and the reduction in battery performance caused by the asymmetry of the two-sided structure.

[0064] Table 2

[0065]

[0066] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0067] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A surface treatment method for copper foil, characterized in that, Includes the following steps: Step 1: Prepare a roughening treatment solution. The roughening treatment solution uses deionized water as the main solvent and contains 0.2wt%-1.0wt% copper trifluoroacetate hydrate and 5wt%-10wt% peracetic acid. Step 2: Immerse the copper foil in the roughening treatment solution for 60-300 seconds; Step 3: Clean the surface of the copper foil with deionized water and then dry it.

2. The surface treatment method according to claim 1, characterized in that, The drying process involves drying the copper foil at an ambient temperature of 80-140°C.

3. The surface treatment method according to claim 1, characterized in that, The roughening solution contains 0.5 wt% copper trifluoroacetate hydrate and 7.5 wt% peracetic acid.

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