Composite current collector and preparation method thereof

Abstract

The composite current collector comprises a metal surface layer, a polymer layer and a metal surface layer which are stacked in sequence, the polymer layer is formed by coating the surface of the metal surface layer with a low-moisture-absorption coating liquid and carrying out thermocuring, and the low-moisture-absorption coating liquid comprises a soluble liquid crystal polymer, epoxy resin, a coupling agent, a leveling agent and a solvent; the soluble liquid crystal polymer contains carboxyl. The polymer layer obtained by compounding the epoxy resin and the soluble liquid crystal polymer is low and stable in crosslinking density and expansion coefficient, and excellent in tensile strength and peel strength; and the coupling agent is compounded to optimize the adhesive force of the polymer layer and the metal surface layer.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a composite current collector and its preparation method. Background Technology

[0002] Current collectors are a key component in electrochemical devices such as lithium-ion batteries, fuel cells, and supercapacitors. Recent emerging technologies include "sandwich" structure aluminum / polymer / aluminum or copper / polymer / copper composite foils. These utilize an extremely thin metal layer combined with a high-molecular polymer, offering advantages such as high safety and light weight (increasing energy density).

[0003] The most widely used polymers currently are polyester film (PET film) and polyimide film (PI film). PET film's advantage lies in its low cost, but its disadvantages are mainly concentrated in its heat resistance (easily melts at high temperatures) and chemical stability (easily hydrolyzes), negatively impacting battery safety and lifespan. While PI film is heat-resistant, it has a high dielectric constant and loss factor, high moisture absorption, and an uncontrollable coefficient of thermal expansion, resulting in unstable product performance. Furthermore, it suffers from severe high-frequency transmission loss and poor structural characteristics.

[0004] Liquid crystal polymers, also known as liquid crystal polymers, are superior to PI and PET films due to their low moisture absorption, low dielectric constant, and low dielectric loss. However, their high orientation and large molecular weight make them difficult to process into films, and they cannot be processed into films smaller than 7 μm using blown film and casting methods.

[0005] Patent CN110499026A discloses a modified liquid crystal polymer film, its preparation method and application. By adding appropriate crosslinking agents and coupling agents, the problem of liquid crystal polymer processing and film formation is solved. The produced PBT film has excellent properties such as low moisture absorption, low dielectric constant, low dielectric loss and high temperature resistance. It is suitable for co-extrusion by casting method, but it is difficult to produce films smaller than 7μm.

[0006] Patent CN113427880A discloses a heat treatment process for LCP thin films and its application. The LCP thin film heat treatment process includes two steps: hot roller shaping and oven shaping of the nascent film, thereby fixing the thickness tolerance, coefficient of thermal expansion, and dielectric constant of the nascent film. The nascent film is obtained by extruding LCP raw material through an extruder and then blowing the film, preventing damage to the metal foil layer and the central LCP film layer from the hot roller pressing. Its thickness is 100 mm. Therefore, this blown film method still struggles to produce films smaller than 7 μm and with a smooth surface.

[0007] Therefore, it is necessary to improve the processing of liquid crystal polymer films in the existing technology. Summary of the Invention

[0008] One of the objectives of this invention is to overcome the deficiencies in the prior art and provide a composite current collector, which is a polymer layer obtained by compounding epoxy resin and soluble liquid crystal polymer, with low and stable crosslinking density and expansion coefficient, while exhibiting excellent tensile strength and peel strength; combined with a coupling agent, the adhesion between the polymer layer and the metal surface is optimized.

[0009] To achieve the above-mentioned process effects, the technical solution of the present invention is as follows: a composite current collector, comprising a metal surface layer, a polymer layer and a metal surface layer stacked sequentially, wherein the polymer layer is coated onto the surface of the metal surface layer by a low moisture absorption coating liquid and thermosetting, wherein the low moisture absorption coating liquid comprises a soluble liquid crystal polymer, an epoxy resin, a coupling agent, a leveling agent and a solvent; The soluble liquid crystal polymer contains carboxyl groups.

[0010] A preferred technical solution is that, by weight, the low-hygroscopic coating liquid comprises 85-95 parts of soluble liquid crystal polymer, 5-15 parts of epoxy resin, 1-3 parts of coupling agent, 0.5-2 parts of leveling agent, and 700-1100 parts of solvent.

[0011] A preferred technical solution is that the coupling agent is γ-glycidoxypropyltrimethoxysilane or γ-aminopropyltriethoxysilane.

[0012] A preferred technical solution is that the solvent is N-methylpyrrolidone and / or N,N-dimethylacetamide.

[0013] The preferred technical solution is that the epoxy resin is a bisphenol A type epoxy resin.

[0014] A preferred technical solution is that the low-hygroscopic coating liquid further includes modified silica, wherein the modified silica is modified by polyurethane containing terminal isocyanate groups, and the polyurethane containing terminal isocyanate groups is obtained by catalytic polymerization of isophorone diisocyanate and polyether diol, wherein the molecular weight of the polyether diol is 1500~2000.

[0015] The preferred technical solution is that the amount of modified silica added is 0.5~2% of the low moisture absorption coating liquid; the molar ratio of isophorone diisocyanate and polyether diol in the polyurethane containing terminal isocyanate groups is (1~1.1):1; and the mass ratio of the polyurethane containing terminal isocyanate groups to silica is (10~20):1.

[0016] A preferred technical solution is that the soluble liquid crystal polymer is one or a combination of SUMITOMO chemical VR300 and KG300-280DS.

[0017] The second objective of this invention is to overcome the deficiencies in the prior art and provide a method for preparing a composite current collector. Based on the above-mentioned composite current collector, the polymer layer includes at least a first unit layer and a second unit layer, and includes the following steps: S1: Prepare a metal surface coating and a low-hygroscopic coating liquid; S2: Apply the low moisture absorption coating liquid to one surface of the metal surface layer and heat cure it to obtain a flow-forming body having a first unit layer or a second unit layer. S3: The first unit layer of one of the two current collectors and the second unit layer of the other are bonded together and heat-sealed to obtain a composite current collector with a polymer layer. The thickness of the metal surface layer is 1~6μm, and the thickness of the first unit layer and / or the second unit layer is 0.5~3μm.

[0018] The preferred technical solution is that, in step S2, the temperature of the heat curing is set in zones, and the continuous temperature is set sequentially to 160±5℃, 220±5℃, 280±5℃, and 320±5℃; in step S3, the heat sealing temperature is 280~380℃.

[0019] The advantages and beneficial effects of this invention are as follows: In this case, the polymer layer in the composite current collector is made by compounding epoxy resin and soluble liquid crystal polymer, chemically crosslinking to increase the polymerization density, ensuring a low and stable coefficient of expansion while improving tensile strength and peel strength; combined with coupling agent to improve the adhesion between the polymer layer and the metal surface. Detailed Implementation

[0020] The specific embodiments of the present invention will be further described below with reference to examples. These examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0021] Low moisture absorption coating liquid The low-hygroscopic coating solution comprises 85-95 parts of soluble liquid crystal polymer, 5-15 parts of epoxy resin, 1-3 parts of coupling agent, 0.5-2 parts of leveling agent, and 700-1100 parts of solvent.

[0022] The soluble liquid crystal polymer is one or a combination of SUMITOMO chemical VR300 and KG300-280DS. Both of these soluble liquid crystal polymers are in particulate form. If a liquid soluble liquid crystal polymer is used, its proportion in the low-hygroscopic coating solution is determined according to its solid content.

[0023] The epoxy resin is a bisphenol A type epoxy resin. Furthermore, the epoxy equivalent of the bisphenol A type epoxy resin is 180~195 g / eq. It crosslinks with the carboxyl groups in the soluble liquid crystal polymer, improving the mechanical strength, chemical resistance, and high-temperature resistance of the polymer layer in the composite current collector.

[0024] The coupling agent is γ-glycidoxypropyltrimethoxysilane or γ-aminopropyltriethoxysilane. The epoxy group of γ-glycidoxypropyltrimethoxysilane and the amino group of γ-aminopropyltriethoxysilane can react with the carboxyl group in the soluble liquid crystal polymer. The methoxy or ethoxy group at the inorganic end of the coupling agent will undergo hydrolysis with water to generate highly reactive silanol groups, which will chemically crosslink with the surface hydroxyl groups (-OH) of the metal surface, thereby improving the adhesion between the polymer layer and the metal surface.

[0025] The solvents are N-methylpyrrolidone and / or N,N-dimethylacetamide. N-methylpyrrolidone is highly hygroscopic, miscible with water in any proportion, and soluble in ether, acetone, esters, halogenated hydrocarbons, aromatic hydrocarbons, and various other organic solvents. It possesses characteristics such as high boiling point, strong polarity, low viscosity, strong solubility, good chemical stability, and biodegradability. N-methylpyrrolidone absorbs moisture from the air and reacts with coupling agents, thereby optimizing the adhesion between the polymer layer and the metal surface. N,N-dimethylacetamide is commonly used as an aprotic polar solvent, miscible with water, alcohols, ethers, esters, benzene, and aromatic compounds in any proportion, and can activate compound molecules; therefore, it is widely used as a solvent and catalyst.

[0026] Modified silica Modified silica is obtained by modifying silica with polyurethane containing terminal isocyanate groups. The polyurethane containing terminal isocyanate groups is prepared by catalytic polymerization of isophorone diisocyanate and polyether diol. The molecular weight of the polyether diol is 1500~2000.

[0027] Isophorone diisocyanate (IPDI) possesses two different types of -NCO groups with varying reactivity, allowing for control of reaction conditions. Polyether diols selectively react with the aliphatic primary -NCO groups to form -NCO-terminated polyurethanes, also known as polyether-based polyurethanes. The selective formation of linear polyurethanes by the polyether diol reduces steric hindrance and allows for greater grafting onto the silica surface. Long-chain polyurethanes with terminal isocyanate groups also improve the dispersibility of silica in the coating solution. The -NCO group at one end reacts with silica, while the hydroxyl group at the other end can crosslink with the carboxyl groups in soluble liquid crystal polymers, thereby optimizing the toughness of the polymer layer. Polyether-based polyurethanes typically have a low glass transition temperature (Tg), thus maintaining good flexibility and elasticity at low temperatures. They also provide good tensile and tear strength. Blending with epoxy resins balances the toughness and strength of the polymer layer.

[0028] Preparation process: Dissolve 10g of silica in 150ml of acetone, add 100-200g of polyurethane containing isocyanate-terminated groups and dibutyltin disilicate catalyst. The amount of catalyst added is 0.5% of the total mass of the reactants (silica and polyurethane containing isocyanate-terminated groups). React at 45-50℃ to obtain polyurethane-modified silica. After the reaction, wash and finally dry. Specifically, the polyurethane containing isocyanate-terminated groups is prepared by mixing isophorone diisocyanate and polyether diol in a molar ratio of (1-1.1):1, dissolving in acetone, and then adding dibutyltin disilicate catalyst. The amount of catalyst added is 1% of the total mass of the reactants (isophorone diisocyanate and polyether diol). The temperature is raised to 80℃ and the reaction is carried out for 3 hours.

[0029] Example 1

[0030] The composite current collector comprises a metal surface layer, a polymer layer, and another metal surface layer stacked sequentially. The polymer layer is formed by applying a low-hygroscopic coating liquid to the surface of the metal surface layer and then thermally curing it. By mass, the low-hygroscopic coating liquid comprises 90 parts of soluble liquid crystal polymer, 10 parts of epoxy resin, 2 parts of coupling agent, 1.2 parts of leveling agent, and 900 parts of solvent.

[0031] The coupling agent is Shin-Etsu KBM-403 from Japan, the solvent is N-methylpyrrolidone, the epoxy resin is JER828, and the soluble liquid crystal polymer is SUMITOMO chemical VR300.

[0032] The preparation method of the composite current collector includes the following steps: Based on the above-mentioned composite current collector, the polymer layer includes at least a first unit layer and a second unit layer; S1: Prepare a metal surface coating and a low-hygroscopic coating liquid; S2: Apply the low moisture absorption coating liquid to one surface of the metal surface, set the temperature of the heat curing in zones, and set the continuous temperature to 160±5℃, 220±5℃, 280±5℃ and 320±5℃ in sequence, and then cool naturally to obtain a flow preform with the first unit layer. S3: The first unit layer of one of the two current collectors and the second unit layer of the other are bonded together and heat-sealed to obtain a composite current collector with a polymer layer. The heat-sealing temperature is 320℃. The metal surface layer is made of copper foil and has a thickness of 3 μm. The thickness of the first unit layer and the second unit layer is 1.5 μm.

[0033] Example 2

[0034] Example 2 is based on Example 1, except that the low-hygroscopic coating solution also includes modified silica. The modified silica is prepared as follows: 10g of silica is dissolved in 150ml of acetone, and 200g of polyurethane containing isocyanate-terminated groups and dibutyltin disilicate catalyst are added. The amount of catalyst added is 0.5% of the total mass of the reactants (silica and polyurethane containing isocyanate-terminated groups). The reaction is carried out at 48℃ to obtain polyurethane-modified silica. After the reaction, the silica is washed and finally dried. Specifically, the polyurethane containing isocyanate-terminated groups is prepared as follows: isophorone diisocyanate and polyether diol (Mn=2000) are mixed in a molar ratio of 1.03:1, dissolved in acetone, and then dibutyltin disilicate catalyst is added. The amount of catalyst added is 1% of the total mass of the reactants (isophorone diisocyanate and polyether diol). The temperature is raised to 80℃, and the reaction is carried out for 3 hours.

[0035] By weight, the low-absorbency coating solution comprises 90 parts of soluble liquid crystal polymer, 10 parts of epoxy resin, 2 parts of coupling agent, 1.2 parts of leveling agent, 10 parts of modified silica, and 900 parts of solvent.

[0036] The processing technology remains unchanged.

[0037] Example 3

[0038] Example 3 is based on Example 1, except that the soluble liquid crystal polymer is KG300-280DS from Shanghai Prit Composite Materials Co., Ltd.

[0039] Example 4

[0040] Example 4 is based on Example 1, except that, by weight, the low-hygroscopic coating liquid comprises 85 parts of soluble liquid crystal polymer, 15 parts of epoxy resin, 2 parts of coupling agent, 1.2 parts of leveling agent, and 900 parts of solvent. The processing technology remains unchanged.

[0041] Example 5

[0042] Example 5 is based on Example 1, except that, by weight, the low-hygroscopic coating liquid comprises 95 parts of soluble liquid crystal polymer, 5 parts of epoxy resin, 2 parts of coupling agent, 1.2 parts of leveling agent, and 900 parts of solvent. The processing technology remains unchanged.

[0043] Example 6

[0044] Example 6 is based on Example 1, except that, by weight, the low-hygroscopic coating solution comprises 90 parts of soluble liquid crystal polymer, 10 parts of epoxy resin, 2 parts of coupling agent, 1.2 parts of leveling agent, and 200 parts of solvent. The thickness of the metal surface layer remains unchanged, and the thickness of both the first unit layer and the second unit layer is 10 μm.

[0045] Comparative Example 1 Comparative Example 1 is based on Example 1, except that no epoxy resin was added to the low-hygroscopic coating liquid, while the other components and processing technology remained unchanged.

[0046] Comparative Example 2 Comparative Example 2 is based on Example 1, except that no coupling agent was added to the low-hygroscopic coating liquid, while the other components and processing technology remained unchanged.

[0047] Comparative Example 3 Comparative Example 3 is based on Example 2, except that no coupling agent was added to the low-hygroscopic coating liquid, but it included polyurethane-modified silica with terminal isocyanate groups. Other components and processing technology remained unchanged.

[0048] Performance testing of composite current collector samples prepared in the examples and comparative examples: 1. Coefficient of thermal expansion: determined according to ISO 11359-2; 2. Tensile strength: Determined according to GB / T 5230-2020; 3. Elongation: Measured according to GB / T 5230-2020; 4. Peel strength: Measured according to ASTM D3330; 5. Adhesion: Draw a vertical line every 1mm x 1mm on the first unit layer of the substrate, 6 horizontal lines and 6 vertical lines. Then, apply 3M tape to the marked areas, pull up the tape, and observe the changes in the first unit layer. The adhesion is graded as follows: Grades range from 0 to 4. Grade 0 indicates small patches of paint peeling at the cut and intersection, with actual damage within the marked area less than 5%; Grade 1 indicates 5-15% peeling area at the cut and intersection edges; Grade 2 indicates partial or large-area peeling at the cut and intersection edges, with a peeled area of ​​16-35%; Grade 3 indicates partial peeling of squares, with a peeled area of ​​36-65%; Grade 4 indicates a peeled area greater than 65%.

[0049] 6. Sheet resistance: Measured using the four-probe method.

[0050] The performance test results of the examples and comparative examples are as follows:

[0051] The sheet resistance of the metal surface layer in the samples of Examples 1-6 and Comparative Examples 1-3 is all between 0.050 and 0.055 Ω / □.

[0052] Compared to Example 1, Example 2, with the addition of isocyanate-terminated polyurethane modified silica, not only improved the tensile strength and toughness of the polymer layer, but also further reduced the coefficient of thermal expansion.

[0053] Example 3 has comparable performance to Example 1.

[0054] Compared to Example 1, Example 4 shows that as the content of soluble liquid crystal polymer decreases and the amount of epoxy resin added increases, the degree of cross-linking of the polymer layer increases, the toughness decreases, and cracks are more likely to occur during the use of the current collector; it also has a negative impact on the coefficient of thermal expansion and peel strength.

[0055] Compared to Example 1, Example 5 shows that as the content of soluble liquid crystal polymer increases, the amount of epoxy resin added decreases, and the degree of crosslinking of the polymer layer decreases, which has a negative impact on tensile strength, peel strength and adhesion.

[0056] Compared to Example 1, Example 6 shows that an excessively thick polymer layer can lead to incomplete curing. Increasing the polymer layer thickness requires reducing the amount of solvent, which can reduce the leveling properties of the low-hygroscopic coating liquid, negatively impacting tensile strength, peel strength, and adhesion. Furthermore, while maintaining a constant current collector thickness, increasing the polymer layer thickness corresponds to a decrease in the thickness of the metal surface layer, leading to an increase in the sheet resistance of the metal surface layer and negatively affecting the electrical performance of the electrode.

[0057] Compared to Example 2, Comparative Example 3, although no coupling agent was added, showed excellent dispersibility of modified silica, with less fluctuation in the tensile strength and coefficient of thermal expansion of the polymer layer, but poor adhesion between the polymer layer and the metal surface layer.

[0058] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A composite current collector, comprising a metal surface layer, a polymer layer, and a metal surface layer stacked sequentially, characterized in that, The polymer layer is formed by applying a low-moisture-absorbing coating liquid to the surface of the metal layer and then thermally curing it. The low-moisture-absorbing coating liquid includes a soluble liquid crystal polymer, epoxy resin, coupling agent, leveling agent, and solvent. The soluble liquid crystal polymer contains carboxyl groups.

2. The composite current collector according to claim 1, characterized in that, The low-absorbency coating liquid comprises, by weight, 85-95 parts of soluble liquid crystal polymer, 5-15 parts of epoxy resin, 1-3 parts of coupling agent, 0.5-2 parts of leveling agent, and 700-1100 parts of solvent.

3. The composite current collector according to claim 1 or 2, characterized in that, The coupling agent is γ-glycidoxypropyltrimethoxysilane or γ-aminopropyltriethoxysilane.

4. The composite current collector according to claim 1 or 2, characterized in that, The solvent is N-methylpyrrolidone and / or N,N-dimethylacetamide.

5. The composite current collector according to claim 1 or 2, characterized in that, The epoxy resin is a bisphenol A type epoxy resin.

6. The composite current collector according to claim 1 or 2, characterized in that, The low-hygroscopic coating liquid also includes modified silica, which is modified by polyurethane containing terminal isocyanate groups. The polyurethane containing terminal isocyanate groups is obtained by catalytic polymerization of isophorone diisocyanate and polyether diol, and the molecular weight of the polyether diol is 1500~2000.

7. The composite current collector according to claim 6, characterized in that, The amount of modified silica added is 0.5~2% of the low-hygroscopic coating liquid; the molar ratio of isophorone diisocyanate and polyether diol in the polyurethane containing terminal isocyanate groups is (1~1.1):1, and the mass ratio of the polyurethane containing terminal isocyanate groups to silica is (10~20):

1.

8. The composite current collector according to claim 1, characterized in that, The soluble liquid crystal polymer is one or a combination of SUMITOMOchemical VR300 and KG300-280DS.

9. A method for preparing a composite current collector, characterized in that, Based on the composite current collector according to any one of claims 1 to 8, the polymer layer comprises at least a first unit layer and a second unit layer, and includes the following steps: S1: Prepare a metal surface coating and a low-hygroscopic coating liquid; S2: Apply the low moisture absorption coating liquid to one surface of the metal surface layer and heat cure it to obtain a flow-forming body having a first unit layer or a second unit layer. S3: The first unit layer of one of the two current collectors and the second unit layer of the other are bonded together and heat-sealed to obtain a composite current collector with a polymer layer. The thickness of the metal surface layer is 1~6μm, and the thickness of the first unit layer and / or the second unit layer is 0.5~3μm.

10. The method for preparing the composite current collector according to claim 9, characterized in that, In S2, the temperature zones for heat curing are set, with the continuous temperatures set sequentially to 160±5℃, 220±5℃, 280±5℃, and 320±5℃; in S3, the heat sealing temperature is 280~380℃.

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