Carbon-coated slurry, carbon-coated copper foil, negative electrode sheet and lithium-ion battery

Abstract

This invention provides a carbon coating slurry, a carbon-coated copper foil, a negative electrode sheet, and a lithium-ion battery. The carbon coating slurry includes a dispersant, a conductive agent, a binder, and a solvent. The binder includes a silane coupling agent and a polyether thiourea; the molecular weight (Mw) of the polyether thiourea is 2w to 20w. In this technical solution, the silane coupling agent in the carbon coating slurry can react with the hydroxyl groups on the surface of the copper foil, and simultaneously react with the amino groups in the polyether thiourea. The carboxyl groups in the negative electrode binder can react with the amino groups in the polyether thiourea to form amide bonds. Through these three reactions, the adhesion between the carbon coating layer and the copper foil, the cohesion of the carbon coating layer, and the adhesion between the carbon coating layer and the negative electrode coating can be enhanced simultaneously.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to a carbon-coated slurry, a carbon-coated copper foil, a negative electrode sheet, and a lithium-ion battery. Background Technology

[0002] Electrode peel strength is a crucial indicator of the adhesion between the active material of the electrode and the current collector in lithium-ion batteries. It significantly impacts battery performance; low peel strength leads to the active material peeling off from the current collector after charging and discharging, resulting in deteriorated cycle performance.

[0003] Due to the demand for higher energy density, the application of silicon-carbon anodes has become a trend. When the silicon content is high, carbon-coated copper foil is often used to improve the peel strength of the electrode. However, even with carbon-coated copper foil, the anode still requires a high amount of binder, which leads to an increase in battery impedance.

[0004] The main reason for the need for a large amount of binder in the negative electrode formulation is the insufficient adhesion between the negative electrode coating and the carbon coating layer. In order to improve the adhesion between the negative electrode coating and the carbon coating layer, there is an urgent need to provide a material that can improve the adhesion between the carbon coating layer and the negative electrode coating, so as to improve the peel strength of the electrode sheet while reducing the amount of negative electrode binder. Summary of the Invention

[0005] To address the above problems, the present invention aims to provide a carbon-coated slurry, a carbon-coated copper foil, a negative electrode sheet, and a lithium-ion battery.

[0006] The present invention achieves the aforementioned technical effect through the following technical solution:

[0007] In a first aspect, the present invention provides a carbon coating slurry, the carbon coating slurry comprising a dispersant, a conductive agent, a binder, and a solvent, wherein the binder comprises a silane coupling agent and a polyether thiourea; the molecular weight Mw of the polyether thiourea is 2w to 20w.

[0008] Preferably, the mass ratio of the silane coupling agent to the polyether thiourea is 1 to 4:1.

[0009] Preferably, the mass percentage of the silane coupling agent is 0.1-0.4% based on the total mass of the carbon coating slurry; and the mass percentage of the polyether thiourea is 0.025-0.4% based on the total mass of the carbon coating slurry.

[0010] Preferably, the specific surface area of ​​the conductive agent is 50-300 m². 2 / g.

[0011] Preferably, the conductive agent is one or more of conductive carbon black, carbon nanotubes, graphene, Ketjen black, and graphite sheets.

[0012] Preferably, the silane coupling agent contains an amino group; the dispersant is one or more of polyvinylpyrrolidone, hydrogenated nitrile butadiene, and polyester dispersant; and the solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, and N,N-dimethylformamide.

[0013] Secondly, the present invention provides a carbonized copper foil, the carbonized copper foil comprising a carbonized layer formed by coating with the above-mentioned carbonized slurry.

[0014] Thirdly, the present invention provides a negative electrode sheet, which includes the above-mentioned carbon-coated copper foil and a negative electrode slurry layer; the negative electrode slurry layer includes a negative electrode binder containing carboxyl functional groups, wherein the mass percentage of the carboxyl-containing negative electrode binder is 2.5~3.5%, and the carboxyl-containing negative electrode binder is one or more of polyacrylic acid, polypropylene sulfonic acid and polystyrene sulfonic acid.

[0015] Fourthly, the present invention provides a lithium-ion battery comprising the aforementioned negative electrode sheet.

[0016] Compared with the prior art, the present invention has the following beneficial effects: the silane coupling agent in the carbon coating slurry can react with the hydroxyl groups on the surface of the copper foil, and at the same time, it can react with the amino groups in the polyether thiourea. The carboxyl groups in the negative electrode binder can react with the amino groups in the polyether thiourea to form amide bonds. Through these three reactions, the adhesion between the carbon coating layer and the copper foil, the cohesion of the carbon coating layer, and the adhesion between the carbon coating layer and the negative electrode coating can be enhanced simultaneously. Detailed Implementation

[0017] To make the technical problems solved, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the embodiments described herein are only some, not all, of the embodiments of this invention, and are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the protection scope of this invention.

[0018] In a first aspect, the present invention provides a carbon coating slurry, the carbon coating slurry comprising a dispersant, a conductive agent, a binder, and a solvent, wherein the binder comprises a silane coupling agent and a polyether thiourea; the molecular weight Mw of the polyether thiourea is 2w to 20w.

[0019] The silane coupling agent in the carbon coating slurry can react with the hydroxyl groups on the surface of the copper foil, and at the same time, it can react with the amino groups in the polyether thiourea. The carboxyl groups in the negative electrode binder can react with the amino groups in the polyether thiourea to form amide bonds. Through these three reactions, the adhesion between the carbon coating layer and the copper foil, the cohesion of the carbon coating layer, and the adhesion between the carbon coating layer and the negative electrode coating can be enhanced simultaneously.

[0020] Specifically, during the heating and baking process after the carbon coating slurry is applied, the Si-OR in the silane coupling agent undergoes a condensation reaction with the amino group (-NH-), ultimately forming a polymer structure with a cross-linked network. This means that the adhesive combination has strong bonding and stability, improving the cohesion of the carbon coating slurry layer. At the same time, the copper foil of the silicon-carbon anode reacts with the surface oxidation and adsorption of water molecules in the air, resulting in hydroxyl groups on the copper foil surface. The siloxy groups in the silane coupling agent react with the hydroxyl groups on the copper foil surface to form a cross-linked network, achieving strong bonding and stability, and improving the adhesion between the carbon coating slurry layer and the copper foil.

[0021] Specifically, the molecular weight Mw of polyether thiourea is set to be between 2w and 20w, so that it has good adhesion properties as well as good processing properties. When the molecular weight Mw of polyether thiourea is less than 2w, its adhesion properties are poor, and when the molecular weight Mw of polyether thiourea is greater than 20w, its processing is more difficult.

[0022] In some specific embodiments, the mass ratio of the silane coupling agent to the polyether thiourea is 1 to 4:1; ensuring that after the amino groups in the polyether thiourea react with the silane coupling agent, there are still enough amino groups to react with the carboxyl groups in the negative electrode binder.

[0023] In some specific embodiments, the mass percentage of the silane coupling agent is 0.1-0.4% based on the total mass of the carbon coating slurry; and the mass percentage of the polyether thiourea is 0.025-0.4% based on the total mass of the carbon coating slurry.

[0024] In some specific embodiments, the silane coupling agent contains amino or sulfhydryl groups; the silane coupling agent is one or more of KH550, KH560, KH570, KH792, and DL602. Furthermore, aminosilanes (such as KH550) react more readily with -NH-. Due to surface oxidation and the reaction with adsorbed water molecules from the air, the copper foil of the silicon-carbon anode has hydroxyl groups on its surface. The siloxy groups in the silane coupling agent react with the hydroxyl groups on the copper foil surface to form a cross-linked network, achieving strong adhesion and stability, and improving the bonding force between the carbon coating slurry layer and the copper foil.

[0025] In some specific embodiments, the dispersant is one or more of polyvinylpyrrolidone, hydrogenated butyronitrile, and polyester dispersants.

[0026] In some specific embodiments, the solvent is one or more of N-methylpyrrolidone (NMP), dimethyl sulfoxide, and N,N-dimethylformamide.

[0027] In some specific embodiments, the conductive agent is one or more of conductive carbon black, carbon nanotubes, graphene, Ketjen black, and graphite sheets.

[0028] In some specific embodiments, the conductive agent is conductive carbon black (abbreviated as SP), and the specific surface area of ​​the conductive carbon black is 50-300 m². 2 / g. Preferably, the specific surface area of ​​the conductive carbon black is 200-300 m² / g. 2 A high specific surface area ( / g) is beneficial for increasing the contact area between the carbon coating layer and the negative electrode, and for improving the peeling force. However, a specific surface area >300m² is also beneficial. 2 Conductive carbon black is difficult to disperse at a specific surface area of ​​200-300 m² / g, making it impossible to process normally. Therefore, the specific surface area of ​​conductive carbon black is typically between 200-300 m² / g. 2 The effect is best when / g.

[0029] Secondly, the present invention provides a carbonized copper foil, the carbonized copper foil comprising a carbonized layer formed by coating with the above-mentioned carbonized slurry.

[0030] Thirdly, the present invention provides a negative electrode sheet, which includes the above-mentioned carbon-coated copper foil and a negative electrode slurry layer.

[0031] In some specific embodiments, the negative electrode slurry layer includes a negative electrode binder containing carboxyl functional groups; the negative electrode binder includes one or more of carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polypropylene sulfonic acid, and polystyrene sulfonic acid. Here, the negative electrode binder containing carboxyl functional groups provides the ability for the carboxyl groups to form amide bonds (-CO-NH-) with the amino groups (-NH- or -NH2) of polyether thiourea in the carbonized slurry through a condensation reaction, thereby improving the adhesion between the carbonized layer and the negative electrode slurry layer.

[0032] In some specific embodiments, the mass percentage of the negative electrode binder is 2.5 to 3.5% based on the total mass of the negative electrode slurry layer.

[0033] In some specific embodiments, the negative electrode binder includes polyacrylic acid; the negative electrode slurry layer also includes an auxiliary binder, which includes styrene-butadiene rubber (SBR) and carboxymethyl cellulose.

[0034] Fourthly, the present invention provides a lithium-ion battery comprising the aforementioned negative electrode sheet.

[0035] The specific embodiments of the present invention will be further explained and described below through examples and comparative examples.

[0036] Unless otherwise specified, all reagents, materials, and instruments used in the following description are conventional reagents, materials, and instruments, all of which are commercially available. The reagents involved can also be synthesized using conventional synthetic methods. Unless otherwise specified, the methods in the examples are conventional methods in the art.

[0037] Synthesis of polyether thiourea: p-phenylenediamine monomer (1 molar equivalent), elemental sulfur (6 molar equivalent), chloroform (6 molar equivalent), and sodium hydroxide (6 molar equivalent) were dissolved in DMSO, and the solid content was adjusted to 15%. The reaction temperature was controlled at 75±5℃, and the reaction was carried out under an anhydrous nitrogen atmosphere with slow stirring at 50 rpm for 1-24 hours. The stirring time was adjusted according to the required molecular weight. After stirring for 1 hour, polyether thiourea with a molecular weight of 1w was obtained; after stirring for 2 hours, polyether thiourea with a molecular weight of 2w was obtained; after stirring for 5 hours, polyether thiourea with a molecular weight of 5w was obtained; and after stirring for 20 hours, polyether thiourea with a molecular weight of 20w was obtained.

[0038] Example 1

[0039] (1) Preparation of carbon coating slurry: By mass percentage, use 6% polyvinylpyrrolidone and 14% SP (with a specific surface area of ​​200 m²). 2 0.2% silane coupling agent KH550 and 0.2% polyether thiourea (with a molecular weight of 5w) were added to a 10L planetary mixer. The mixture was premixed at 20 rpm for 15 min in the planetary mixer. Then, 79.6% NMP was added as a solvent. After adding NMP, the mixture was premixed at 20 rpm for 15 min, then dispersed at 40 rpm and 4000 rpm for 120 min. Finally, the mixture was slowly stirred to defoam for 30 min to obtain a conductive slurry.

[0040] (2) Carbon coating: The conductive paste is coated onto the copper foil using a gravure transfer coating machine, with the thickness controlled at 0.5-1μm and the coating temperature at 50℃. Double-sided coating is performed.

[0041] (3) Electrode preparation: A negative electrode slurry was prepared by mixing 0.4% CMC, 2.5% PAA, 0.6% SBR, 20% silicon carbide and 76.5% graphite by mass percentage, with water as solvent. The slurry had a solid content of 50±1% and a viscosity of 3500±1000cp. Then, it was coated onto the above-mentioned carbon-coated copper foil using a transfer coating machine and dried at 85℃.

[0042] Peel strength test method: Cut a negative electrode sheet 30mm wide and 150mm long and attach it to double-sided tape. Use a universal tensile testing machine to peel it at 180° and peel speed of 50mm / min. Record the peel force and convert it to peel strength in N / m.

[0043] Cell cycle test: The prepared ternary positive electrode soft pack cell was charged to 4.35V with a constant current of 1CC-CV and cut off at 0.05C. After standing for 5 minutes, it was discharged to 2.75V with 1C and cycled until the capacity retention rate was 80%.

[0044] Improvement effects: Compared with conventional copper foil, the negative electrode sheet made using this method has a peel strength that is more than 40% higher. Cell cycle life is improved by more than 20%.

[0045] (4) Preparation of lithium-ion batteries:

[0046] A lithium-ion battery positive electrode, separator, and electrolyte prepared according to conventional methods in the art, together with a negative electrode prepared using this application, are assembled into a lithium-ion battery according to the general process for preparing lithium-ion batteries.

[0047] Example 2

[0048] The carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Example 2 and Example 1 is that: in the preparation process of carbon-coated slurry, the silane coupling agent KH550 is 0.4% by mass percentage, the polyether thiourea (with a molecular weight of 5W) is 0.1%, and the solvent NMP is 79.5%.

[0049] Example 3

[0050] The carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Example 3 and Example 1 is that in the preparation of carbon-coated slurry, silane coupling agent KH550 was replaced with silane coupling agent KH570.

[0051] Example 4

[0052] The carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Example 4 and Example 1 is that in the preparation of the carbon-coated slurry, polyvinylpyrrolidone was replaced with hydrogenated nitrile butadiene.

[0053] Example 5

[0054] The carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Example 5 and Example 1 is that the solvent NMP was replaced with dimethyl sulfoxide in the preparation process of carbon-coated slurry.

[0055] Example 6

[0056] Carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Example 6 and Example 1 is that the molecular weight of polyether thiourea is 2w.

[0057] Example 7

[0058] Carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Example 7 and Example 1 is that the molecular weight of polyether thiourea is 20w.

[0059] Comparative Example 1

[0060] The carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Comparative Example 1 and Example 1 is that the negative electrode slurry was prepared using a mass percentage ratio of 0.4% CMC, 2.3% PAA, 0.8% SBR, 20% silicon-carbon and 76.5% graphite.

[0061] Comparative Example 2

[0062] The carbon-coated slurry, carbon-coated copper foil, negative electrode slurry, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Comparative Example 2 and Example 1 is that the binder in the carbon-coated slurry is silane coupling agent KH550, which has a mass percentage of 0.4%, and polyether thiourea is not used.

[0063] Comparative Example 3

[0064] The negative electrode slurry, carbon-coated copper foil, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Comparative Example 3 and Example 1 is that the binder in the carbon-coated slurry is polyether thiourea (with a molecular weight of 1W) with a mass percentage of 0.4%, and there is no silane coupling agent.

[0065] Comparative Example 4

[0066] The negative electrode slurry, carbon-coated copper foil, negative electrode sheet and lithium-ion battery were prepared according to the method of Example 1. The difference between Comparative Example 4 and Example 1 is that, by mass percentage, the negative electrode slurry binder includes 0.5% CMC and 3% SBR, and contains no polyacrylic acid.

[0067] The peel strength test and cell cycle test results of the above embodiments and comparative examples are summarized in Table 1.

[0068] Table 1 Test Results

[0069] experimental group Peel strength (N / m) Number of cycles (EOL@80%) Example 1 21 2000 Example 2 19.5 1900 Example 3 21.6 2040 Example 4 20.6 1960 Example 5 20.8 1960 Example 6 19 1860 Example 7 25 2400 Comparative Example 1 18 1800 Comparative Example 2 16.2 1660 Comparative Example 3 15 1520 Comparative Example 4 12 1300

[0070] Therefore, the carbon coating slurry provided by the present invention can improve the electrode peel strength, enhance the battery processing performance and safety, and improve the cell cycle performance.

[0071] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A carbon coating paste, characterized in that, The carbon coating slurry includes a dispersant, a conductive agent, a binder, and a solvent. The binder includes a silane coupling agent and a polyether thiourea. The molecular weight (Mw) of the polyether thiourea is 2w to 20w.

2. The carbon coating slurry according to claim 1, characterized in that, The mass ratio of the silane coupling agent to the polyether thiourea is 1~4:

1.

3. The carbon coating slurry according to claim 1, characterized in that, Based on the total mass of the carbon coating slurry, the mass percentage of the silane coupling agent is 0.1-0.4%; based on the total mass of the carbon coating slurry, the mass percentage of the polyether thiourea is 0.025-0.4%.

4. The carbon coating slurry according to claim 1, characterized in that, The conductive agent has a specific surface area of ​​50-300 m². 2 / g.

5. The carbon coating slurry according to claim 4, characterized in that, The conductive agent is one or more of conductive carbon black, carbon nanotubes, graphene, Ketjen black, and graphite sheets.

6. The carbon coating slurry according to any one of claims 1-5, characterized in that, The silane coupling agent contains an amino group; the dispersant is one or more of polyvinylpyrrolidone, hydrogenated butyronitrile, and polyester dispersant; the solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, and N,N-dimethylformamide.

7. A carbon-coated copper foil, characterized in that, The carbonized copper foil comprises a carbonized layer formed by coating with the carbonized slurry according to any one of claims 1-6.

8. A negative electrode sheet, characterized in that, The negative electrode sheet includes the carbon-coated copper foil as described in claim 7 and a negative electrode slurry layer. The negative electrode slurry layer includes a carboxyl-containing negative electrode binder. The mass percentage of the carboxyl-containing negative electrode binder is 2.5-3.5%, and the carboxyl-containing negative electrode binder is one or more of polyacrylic acid, polypropylene sulfonic acid, and polystyrene sulfonic acid.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the negative electrode as described in claim 8.