Coating composition, current collector and preparation method and application of current collector

Abstract

The invention relates to a coating composition, a current collector and a preparation method and application of the current collector. The coating composition comprises a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, and the mass ratio of the polydopamine to the polyvinylpyrrolidone to the conductive material is 1: (0.6-1): (1.8-5). The coating composition can be applied to the surface of a current collector substrate, the conductivity between a current collector and active substance slurry is ensured while the adhesive force between the current collector and the active substance slurry is improved, and the battery performance is further improved.

Description

Technical Field

[0001] The present application relates to the field of battery materials, and particularly to a coating composition, a current collector, a preparation method thereof, and an application thereof. Background Art

[0002] In recent years, the battery market has developed rapidly. Compared with primary batteries, secondary batteries such as lithium-ion batteries have been widely used in various aspects of human society, such as children's toys, smart phones, electric vehicles, etc., due to advantages such as rechargeability, long life, and low self-discharge rate.

[0003] A secondary battery includes main materials such as a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. Among them, the electrode sheet is obtained by uniformly coating a positive electrode paste or a negative electrode paste on a current collector. However, there is still a problem of poor adhesion between the current active material paste and the current collector, and in the subsequent slitting process of the electrode sheet, phenomena such as foil leakage are likely to occur, affecting the performance of the battery. Summary of the Invention

[0004] Based on this, some embodiments of the present application provide a coating composition that can be applied to the surface of a current collector matrix to improve the adhesion between the current collector and the active material paste, thereby improving the battery performance.

[0005] In addition, some other embodiments of the present application also provide a current collector, a preparation method thereof, and an application thereof.

[0006] A coating composition includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of the polydopamine, the polyvinylpyrrolidone, and the conductive material is 1:(0.6 - 1):(1.8 - 5).

[0007] In some of these embodiments, the preparation steps of the copolymer of polydopamine and polyvinylpyrrolidone are as follows:

[0008] Copolymerize the polydopamine and the polyvinylpyrrolidone at 40°C - 60°C for 8h - 15h.

[0009] In some of these embodiments, the step of copolymerizing the polydopamine and the polyvinylpyrrolidone at 40°C - 60°C for 5h - 15h includes:

[0010] Mix the polydopamine and the polyvinylpyrrolidone, and react at 40°C - 60°C for 5h - 15h under the conditions of heating and stirring.

[0011] In some of these embodiments, the number-average molecular weight of the polyvinylpyrrolidone is 30,000 - 350,000.

[0012] In some of these embodiments, the polyvinylpyrrolidone includes one or more of PVP-K30, PVP-K25, PVP-K45, PVP-K60, and PVP-K70.

[0013] In some of these embodiments, the polydopamine is prepared by a method including the following steps:

[0014] React dopamine in a buffer solution with a pH of 8.5 to 9 for 4 h to 6 h, and then perform solid-liquid separation and drying to prepare the polydopamine.

[0015] In some of these embodiments, the conductive material includes one or more of conductive carbon black, graphite, graphene, and carbon nanotubes.

[0016] In some of these embodiments, the coating composition further includes a binder, and the binder includes one or more of polyacrylic acid, polyvinylidene fluoride, carboxymethyl cellulose, and styrene-butadiene rubber.

[0017] In some of these embodiments, the mass ratio of the binder to the polydopamine is (6 to 20):1.

[0018] An application of a coating composition in preparing a secondary battery, the coating composition includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of the polydopamine, the polyvinylpyrrolidone, and the conductive material is 1:(0.6 to 1):(1.8 to 5).

[0019] A current collector includes a current collector substrate and a conductive coating provided on at least one surface of the current collector substrate, and the material of the conductive coating includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of the polydopamine, the polyvinylpyrrolidone, and the conductive material is 1:(0.6 to 1):(1.8 to 5).

[0020] In some of these embodiments, the thickness of the conductive coating is 0.5 μm to 6 μm.

[0021] In some of these embodiments, the material of the conductive coating further includes a binder, and the binder includes one or more of polyacrylic acid, polyvinylidene fluoride, carboxymethyl cellulose, and styrene-butadiene rubber.

[0022] In some of these embodiments, the mass ratio of the binder to the polydopamine is (6 to 20):1.

[0023] In some of these embodiments, the preparation steps of the copolymer of polydopamine and polyvinylpyrrolidone are as follows:

[0024] React the polydopamine and the polyvinylpyrrolidone at 40°C to 60°C for 8 h to 15 h to copolymerize the polydopamine and the polyvinylpyrrolidone.

[0025] In some embodiments, the step of reacting the polydopamine and the polyvinylpyrrolidone at 40°C to 60°C for 5 h to 15 h includes:

[0026] Mix the polydopamine and the polyvinylpyrrolidone, and react at 40°C to 60°C for 5 h to 15 h under the conditions of heating and stirring.

[0027] In some embodiments, the number-average molecular weight of the polyvinylpyrrolidone is 30,000 to 350,000.

[0028] In some embodiments, the polyvinylpyrrolidone includes one or more of PVP-K30, PVP-K25, PVP-K45, PVP-K60, and PVP-K70.

[0029] In some embodiments, the polydopamine is prepared by a method including the following steps:

[0030] React dopamine in a buffer solution with a pH of 8.5 to 9 for 4 h to 6 h, and then perform solid-liquid separation and drying to prepare the polydopamine.

[0031] In some embodiments, the conductive material includes one or more of conductive carbon black, graphite, graphene, and carbon nanotubes.

[0032] A method for preparing a current collector as described above includes the following steps:

[0033] Form a conductive coating on at least one surface of the current collector substrate to prepare the current collector;

[0034] Wherein, the material of the conductive coating includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, and wherein, the mass ratio of the polydopamine, the polyvinylpyrrolidone, and the conductive material is 1:(0.6 to 1):(1.8 to 5).

[0035] In some embodiments, the step of forming a conductive coating on at least one surface of the current collector substrate includes:

[0036] Obtain a coating composition, the coating composition including a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material;

[0037] Spray the coating composition on at least one surface of the current collector substrate in the form of a solution, and dry and cure to prepare the current collector.

[0038] A pole piece comprises a current collector and an active material layer arranged on at least one side surface of the current collector, wherein the current collector is as described above.

[0039] A secondary battery includes a positive electrode sheet and a negative electrode sheet, wherein at least one of the positive electrode sheet and the negative electrode sheet is as described above.

[0040] The inventors of the present application have found that the polyvinyl pyrrolidone molecule contains a pyrrole ring structure, which interacts with the active groups on the surface of polydopamine, which may involve non-covalent interactions such as hydrogen bonds and van der Waals forces. At the same time, a cross-linking reaction may occur between polydopamine and polyvinyl pyrrolidone to form a more stable composite material. The polymer PDA@PVP formed by copolymerizing polydopamine and polyvinyl pyrrolidone (PVP) has extremely strong adhesion. When combined with conductive materials and applied to the surface of the current collector matrix, it can enhance the adhesion between the current collector and the active material slurry, improve the peel strength of the pole piece, and improve the leakage of foil and other phenomena in the subsequent slitting process, thereby improving the performance of the battery, while ensuring the conductivity between the current collector and the active material slurry. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0042] Figure 1 The peeling force data of the electrode sheets prepared in the examples and comparative examples of the present application;

[0043] Figure 2 These are the cycle curves of the electrodes prepared in Example 1, Comparative Example 1 and Comparative Example 2 of the present application when used in button batteries. DETAILED DESCRIPTION

[0044] To facilitate understanding of the present application, the present application will be described more fully below in conjunction with the specific embodiments. Preferred embodiments of the present application are provided in the specific embodiments. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this application belongs. The terms used in the description of this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0046] Unless otherwise stated or there is a contradiction, the terms or phrases used in this application have the following meanings:

[0047] In this application, "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one of such features.

[0048] In the description of this application, "a plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0049] In this application, "one or several" means any one, any two, or any two or more of the listed items. Among them, "several" means any two or more.

[0050] In this application, the percentage concentration involved, unless otherwise specified, refers to the final concentration. The final concentration refers to the proportion of the added component in the system after adding the component.

[0051] In this text, terms such as "further", "even further", "especially", "for example", "such as", "example", "exemplification" are used for descriptive purposes, indicating that there is an association in the covered content between the different technical solutions before and after, but should not be construed as a limitation on the previous technical solution, nor as a limitation on the protection scope of this text. In this text, unless otherwise stated, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0052] In this text, "optionally", "optional", "option" mean having or not having, that is, any one of the two parallel options of "having" or "not having". If "optional" appears multiple times in a technical solution, unless otherwise specified, and there is no contradiction or mutual restriction relationship, each "optional" is independent. In this application, descriptions such as "optionally contain", "optionally include" mean "contain or not contain". "Optional component X" means that component X exists or does not exist, or means containing or not containing this component X.

[0053] When a numerical range is disclosed in the present application, the above range is considered continuous and includes the minimum and maximum values of the range, as well as each value therebetween. Further, when the range refers to integers, it includes each integer between the minimum and maximum values of the range. In addition, when multiple ranges are provided to describe features or characteristics, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed in the present application should be understood to include any and all sub-ranges subsumed therein.

[0054] In the present application, among the technical features described in an open-ended manner, it includes a closed technical solution composed of the recited features, and also includes an open technical solution containing the recited features.

[0055] The terms "comprising" and "having" and any variations thereof in the embodiments of the present application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally further includes steps or units not listed, or optionally further includes other steps or components inherent to these processes, methods, products or devices.

[0056] Referring to "embodiment" in the present application means that a specific feature, structure or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The phrase appearing at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments.

[0057] In the manufacturing process of secondary batteries such as lithium-ion batteries, coating is one of the important processes in battery production. The purpose is to evenly apply the positive and negative electrode slurries on the foil. Generally, it is required to meet technical requirements such as a smooth and scratch-free surface of the electrode sheet, no material dropping, no edge curling, and no exposed foil. However, there may be a phenomenon of exposed foil in the subsequent slitting process. Exposed foil will affect the performance of the subsequent lithium-ion battery. In severe cases, it will pierce the separator to cause a short circuit, resulting in combustion or even explosion.

[0058] One of the reasons for exposed foil is the poor adhesion between the current collector and the positive and negative electrode slurries. Although in the process of preparing the slurry, a binder is usually added to adhere the active material to the current collector to maintain the mechanical structure integrity of the electrode sheet during battery production and use. However, there is still a problem of poor adhesion between the current collector and the positive and negative electrode slurries, which is prone to exposed foil.

[0059] Based on this, the inventors of this application have found through extensive research that applying a copolymer of polydopamine and polyvinyl pyrrolidone in combination with a conductive material on the surface of the current collector can improve the adhesion between the current collector and the positive and negative electrode slurries, and at the same time, is beneficial to improving the cycle performance of the battery.

[0060] Specifically, the first aspect of the present application provides a coating composition comprising a copolymer of polydopamine and polyvinyl pyrrolidone and a conductive material, wherein the mass ratio of polydopamine, polyvinyl pyrrolidone and the conductive material is 1:(0.6~1):(1.8~5).

[0061] The inventors of this application have discovered that polyvinyl pyrrolidone molecules contain pyrrole ring structures, which interact with the active groups on the surface of polydopamine, possibly involving non-covalent interactions such as hydrogen bonds and van der Waals forces. At the same time, a cross-linking reaction may occur between polydopamine and polyvinyl pyrrolidone to form a more stable composite material. The new polymer PDA@PVP formed by copolymerizing polydopamine with polyvinyl pyrrolidone (PVP) has extremely strong adhesion. When combined with conductive materials and applied to the surface of the current collector matrix, it can enhance the adhesion between the current collector and the active material slurry, improve the peel strength of the electrode, and ensure the conductivity between the current collector and the active material slurry.

[0062] In addition, the amide bond of PVP is beneficial to improving the battery's energy storage performance, and also improves the capacity retention rate of the battery cycle in the later stage.

[0063] Conventional technology discloses a conductive material modified by copolymerization of polydopamine and polyethyleneimine. This material is applied to copper foil to enhance adhesion to the active material and alleviate the problem of separation between the negative electrode and the current collector caused by expansion during charge and discharge, thereby improving the cycle life and energy density of silicon-based batteries. However, the slurry prepared from this material is prone to uneven distribution and agglomeration. The inventors have found that this is because polyethyleneimine (PEI) is generally a positively charged substance and has an electrostatic attraction with the negatively charged PDA. This easily leads to uneven distribution of the carbon material in the slurry, causing agglomeration. After coating on the current collector, the conductivity distribution is uneven and it cannot effectively provide static conductive properties.

[0064] The coating composition provided by this application is that dopamine undergoes an oxidative self-polymerization reaction in a weakly alkaline solution to form polydopamine molecules, which aggregate into nanoparticles in the solution or adhere to the surface of objects in the solution and the inner surface of the container due to its extremely strong adhesion to form a polydopamine coating. This application utilizes the super strong adhesion of polydopamine and the highly viscous PVP molecules to form a new polymer PDA@PVP, which is combined with a conductive material and applied to the current collector matrix. While enhancing the adhesion between the current collector and the slurry, it ensures the uniform distribution of the conductive material in the slurry, is not easy to agglomerate, and is beneficial to improving the conductivity of the battery. At the same time, the amide bond of PVP also has excellent power storage performance, and the capacity retention rate of the battery cycle is also improved in the later stage.

[0065] Illustratively, the mass ratio of polydopamine to polyvinylpyrrolidone can be, but is not limited to, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, or a range consisting of any two of these values.

[0066] In some embodiments, the molecular weight of polyvinyl pyrrolidone is 30,000 to 350,000. For example, the molecular weight of polyvinyl pyrrolidone can be, but is not limited to, 30,000, 50,000, 80,000, 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, or a range consisting of any two of these values.

[0067] In some of the embodiments, in the copolymer of polydopamine and polyvinylpyrrolidone, the polyvinylpyrrolidone includes one or more of PVP-K30, PVP-K25, PVP-K45, PVP-K60 and PVP-K70. In one example, the K value of polyvinylpyrrolidone is less than 90. The present application found that a K value of polyvinylpyrrolidone that is too large will have certain negative effects, which will cause the interaction between PVP and polydopamine to deteriorate. The reason is that when the K value is too large, PVP becomes more granular, and the interaction and chemical reaction with polydopamine in the solution become weaker. Therefore, in some embodiments of the present application, the K value of polyvinylpyrrolidone is less than 90.

[0068] In some embodiments, the preparation steps of the polydopamine and polyvinylpyrrolidone copolymer are as follows:

[0069] Polydopamine and polyvinyl pyrrolidone are reacted at 40° C. to 60° C. for 8 h to 15 h to copolymerize polydopamine and polyvinyl pyrrolidone.

[0070] For example, the temperature for reacting polydopamine and polyvinylpyrrolidone can be, but is not limited to, 40°C, 42°C, 45°C, 48°C, 50°C, 52°C, 55°C, 58°C, 60°C, or the range formed by any two of these values. The time for reacting polydopamine and polyvinylpyrrolidone can be, but is not limited to, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, or the range formed by any two of these values.

[0071] Specifically, the steps for reacting polydopamine and polyvinylpyrrolidone at 40°C to 60°C for 8 h to 15 h are as follows: Mix polydopamine and polyvinylpyrrolidone, and react at 40°C to 60°C for 8 h to 15 h under the conditions of heating and stirring.

[0072] Due to the presence of amines, polyvinylpyrrolidone can react spontaneously with polydopamine to form a copolymer, further improving the adhesion.

[0073] In some embodiments, polydopamine is prepared by a method including the following steps:

[0074] React dopamine in a buffer solution with a pH of 8.5 to 9 for 4 h to 6 h, then perform solid-liquid separation and drying to prepare polydopamine.

[0075] Specifically, the preparation steps of polydopamine are as follows: Dissolve dopamine in a Tris solution (buffer solution), adjust the pH value to 8.5 to 9.0, stir for 4 h to 6 h to obtain a black solution. Perform solid-liquid separation on the black solution and dry it to prepare polydopamine. In a specific example, in the drying step, the temperature is 100°C to 110°C. Dopamine undergoes an oxidative self-polymerization reaction in a weakly alkaline solution to form polydopamine molecules, which aggregate into nanoparticles in the solution. It can be understood that the above only gives a common method for preparing polydopamine, but is not limited thereto, and other common methods in the art can also be used.

[0076] Exemplarily, the mass ratio of polydopamine to the conductive material can be, but is not limited to, 1:1.8, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, or the range formed by any two of these values.

[0077] In some embodiments, the conductive material includes one or more of conductive carbon black, graphite, graphene, and carbon nanotubes.

[0078] In some embodiments, the coating composition further includes a binder, and the binder includes one or more of polyacrylic acid, polyvinylidene fluoride, carboxymethyl cellulose, and styrene-butadiene rubber. Adding a binder to the coating composition can further improve the bonding effect.

[0079] Specifically, the mass ratio of the binder to polydopamine is (6 - 20):1. For example, the mass ratio of the binder to polydopamine can be, but is not limited to, 6:1, 8:1, 10:1, 12:1, 14:1, 15:1, 16:1, 18:1, 20:1, or the range composed of any two of these values.

[0080] In some embodiments, in the coating composition, the mass ratio of polydopamine, polyvinylpyrrolidone, conductive material, and binder is 1:(0.6 - 1):(1.8 - 5):(6 - 20).

[0081] The second aspect of the present application provides an application of a coating composition in the preparation of a secondary battery. The coating composition includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of polydopamine, polyvinylpyrrolidone, and conductive material is 1:(0.6 - 1):(1.8 - 5).

[0082] The specific coating composition is as described in the first aspect above and will not be elaborated here.

[0083] The third aspect of the present application provides a current collector, including a current collector substrate and a conductive coating provided on at least one surface of the current collector substrate. The material of the conductive coating includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of polydopamine, polyvinylpyrrolidone, and conductive material is 1:(0.6 - 1):(1.8 - 5).

[0084] Specifically, the conductive coating is obtained by drying and curing the coating composition provided in the first aspect above in the form of a solution.

[0085] In some embodiments, the thickness of the conductive coating is 0.5 μm - 6 μm. For example, the thickness of the conductive coating can be, but is not limited to, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 4 μm, 5 μm, 6 μm, or the range composed of any two of these values. Optionally, the thickness of the conductive coating is 0.5 μm - 3 μm.

[0086] In some embodiments, the material of the conductive coating further includes a binder, and the binder includes one or more of polyacrylic acid, polyvinylidene fluoride, carboxymethyl cellulose, and styrene-butadiene rubber. Optionally, the mass ratio of the binder to polydopamine is (6 - 20):1.

[0087] The specific materials are the same as those in the first aspect described above and will not be elaborated.

[0088] In some embodiments, the current collector substrate can be a positive current collector substrate, such as aluminum foil, etc., or a negative current collector substrate, such as copper foil, etc.

[0089] In a specific example, the thickness of the current collector matrix may be, but is not limited to, 20 μm.

[0090] It can be understood that the conductive coating can be provided on one side surface of the current collector substrate, or on two opposite sides of the current collector substrate.

[0091] A fourth aspect of the present application provides a method for preparing a current collector, comprising the following steps:

[0092] forming a conductive coating on at least one side of a current collector substrate to prepare a current collector;

[0093] The conductive coating material includes a copolymer of polydopamine and polyvinyl pyrrolidone and a conductive material, wherein the mass ratio of polydopamine, polyvinyl pyrrolidone and the conductive material is 1: (0.6~1): (1.8~5).

[0094] In some embodiments, the step of forming a conductive coating on at least one surface of the current collector substrate includes: obtaining a coating composition, the coating composition including a copolymer of polydopamine and polyvinyl pyrrolidone and a conductive material;

[0095] The coating composition is applied in the form of a solution on at least one surface of a current collector substrate, dried and solidified to prepare a current collector.

[0096] Specifically, in the step of applying the coating composition in the form of a solution to at least one surface of the current collector substrate, the solvent may be, but is not limited to, water. For example, the solution containing the coating composition is applied to at least one surface of the current collector substrate by spraying. In actual practice, the spraying process ensures uniform coating without sagging. For example, an electrospinning device is used for spraying, and the flow rate and voltage are adjusted to ensure uniform coating on the current collector substrate.

[0097] Dopamine undergoes an oxidative self-polymerization reaction in a weakly alkaline solution to form polydopamine molecules, which aggregate into nanoparticles in the solution or adhere to the surface of objects in the solution and the inner surface of the container due to its extremely strong adhesion to form a polydopamine coating. This application utilizes the super strong adhesion of polydopamine and the highly viscous PVP molecules to form a new polymer PDA@PVP. After being evenly mixed with the conductive material, it is coated on the surface of the empty aluminum foil, which can enhance the adhesion between the current collector and the slurry.

[0098] The fifth aspect of the present application provides a pole piece, comprising a current collector and an active material layer arranged on at least one side surface of the current collector, wherein the current collector is as described in the third aspect above.

[0099] It can be understood that the electrode can be a positive electrode or a negative electrode.

[0100] In some embodiments, the electrode is a positive electrode, and the active material layer includes a positive active material, a positive conductive agent, and a positive binder. In some of these embodiments, the positive active material can be, but is not limited to, lithium iron phosphate (LFP), and can also be, for example, a ternary material, etc. The positive conductive agent can be, but is not limited to, conductive carbon black (SP), graphite, graphene, carbon nanotubes, etc. The positive binder can be, but is not limited to, polyvinylidene fluoride (PVDF).

[0101] In other embodiments, the electrode is a negative electrode, and the active material layer includes a negative active material, a negative conductive agent, and a negative binder, etc. In some of these embodiments, the negative active material includes, but is not limited to, graphite, silicon-based materials, etc. The negative binder can be, but is not limited to, sodium carboxymethyl cellulose, styrene-butadiene rubber, etc.

[0102] In some embodiments, the electrode is prepared by coating an active material slurry on at least one surface of a current collector substrate and drying. In some of these embodiments, when the electrode is a positive electrode, the solvent used in the active material slurry is N-methylpyrrolidone (NMP). In other embodiments, when the electrode is a negative electrode, the solvent used in the active material slurry is water.

[0103] The sixth aspect of the present application provides a secondary battery, including a positive electrode and a negative electrode, and at least one of the positive electrode and the negative electrode is as described in the fifth aspect above.

[0104] The specific compositions of the positive electrode and the negative electrode are as described above and will not be elaborated here.

[0105] It can be understood that the secondary battery further includes a separator and an electrolyte.

[0106] Specifically, in the present application, there is no particular limitation on the separator, and it can be commonly used in the art, such as a polyethylene separator, a polypropylene separator, etc.

[0107] The electrolyte is regarded as the "blood" for internal material transport in a lithium battery and plays a crucial role in the energy density, power density, cycle life, safety performance, etc. of the battery. In the present application, there is no particular limitation on the composition of the electrolyte, and it can be commonly used in the art. Specifically, the electrolyte includes a lithium salt and a solvent. The lithium salt is, for example, lithium hexafluorophosphate (LiPF6), and the solvent can be, but is not limited to, one or more of ethylene carbonate (EC) and dimethyl carbonate (DMC).

[0108] In some embodiments, the secondary battery can be, but is not limited to, a lithium-ion battery.

[0109] In order to make the purpose and advantages of the present application clearer, the current collector of the present application and its effects are further described in detail below with reference to specific examples. It should be understood that the specific embodiments described herein are only used to explain the present application and shall not be used to limit the present application. The following examples do not include other components except unavoidable impurities unless otherwise specified. The drugs and instruments used in the examples are conventional choices in the art unless otherwise specified. The experimental methods for which specific conditions are not specified in the examples are implemented according to conventional conditions, such as the conditions described in the literature, books, or methods recommended by the manufacturer.

[0110] Example 1

[0111] This embodiment provides a current collector, including a current collector substrate aluminum foil and a conductive coating coated on the aluminum foil. The preparation steps are as follows:

[0112] (1) Dopamine powder was weighed and dissolved in Tris solution to prepare a 3 mg / mL dopamine solution. The pH of the dopamine solution was adjusted to 8.5 and magnetic stirring was continued for 4 h to obtain a black solution. After stirring, the black solution was centrifuged and washed. The resulting solid was dried in an oven at 100°C to obtain a polydopamine powder material.

[0113] (2) Obtain polydopamine powder, PVP-K30, conductive carbon black and polyacrylic acid in a mass ratio of 1:0.8:3:10.

[0114] (3) PVP30 was prepared into a uniform and stable PVP solution with a concentration of 0.02 g / mL. Polydopamine powder was added to the PVP solution, heated in a water bath at 40°C, and magnetically stirred for 10 h to copolymerize PVP and polydopamine to form PDA@PVP. The solution containing conductive carbon black powder and polyacrylic acid was fully stirred, and the copolymer PDA@PVP was added and stirred for another 10 h to obtain a coating composition.

[0115] (4) Clean the empty aluminum foil to be coated to remove oil and impurities on the surface. Spray the coating composition on the surface of the empty aluminum foil to ensure that the coating is uniform and does not sag.

[0116] (5) The sprayed aluminum foil was placed in a high-temperature oven and dried at 200°C for 10 minutes until the polyacrylic acid solidified to form a stable conductive coating with a thickness of 1 μm, thereby obtaining the current collector of this embodiment.

[0117] This embodiment also provides a positive electrode sheet, and the preparation steps are as follows:

[0118] (1) Lithium iron phosphate (LFP) material, conductive carbon black (SP) and binder (PVDF) were dispersed in solvent NMP at a mass ratio of 8:1:1 and mechanically stirred in vacuum at room temperature for 24 h to obtain a uniformly mixed positive electrode slurry.

[0119] (2) The mixed positive electrode slurry is evenly coated on the above-prepared current collector with a manual scraper and dried in a vacuum oven at 90°C for 12 hours. The coating thickness is about 100 μm. The positive electrode coating after coating and drying is relatively loose. A small tablet press is used for tableting, and the pressure range is set to 80-120 kg / cm 2 After pressing, the electrode is cut into regular positive electrode sheets using a mechanical cutting machine.

[0120] Example 2

[0121] This embodiment provides a current collector comprising an aluminum foil as a current collector substrate and a conductive coating applied to the aluminum foil. The preparation steps are similar to those of Example 1, except that, in step (2), the raw material ratio of the coating composition is different. In this embodiment, the raw material ratio is as follows: polydopamine powder, PVP30, conductive carbon black, and polyacrylic acid in a mass ratio of 1:0.6:1.8:6. The other steps are the same as those of Example 1 and are not further described.

[0122] This embodiment also provides a positive electrode sheet, which has similar preparation steps to the positive electrode sheet of Example 1, except that the current collector is different. Other steps are similar to those of Example 1 and will not be repeated here.

[0123] Example 3

[0124] This embodiment provides a current collector comprising an aluminum foil as a current collector substrate and a conductive coating applied to the aluminum foil. The preparation steps are similar to those of Example 1, except that, in step (2), the raw material ratio of the coating composition is different. In this embodiment, the raw material ratio is as follows: polydopamine powder, PVP30, conductive carbon black, and polyacrylic acid in a mass ratio of 1:1:5:20. The other steps are the same as those of Example 1 and are not further described.

[0125] This embodiment also provides a positive electrode sheet, which is prepared in a similar manner to the positive electrode sheet of embodiment 1, except that the current collector is different. Other steps are similar to those of embodiment 1 and will not be repeated here.

[0126] Example 4

[0127] This embodiment provides a current collector, including a current collector substrate copper foil and a conductive coating coated on the copper foil. The preparation steps are similar to those in Example 1, except that the current collector substrate is different. This embodiment uses copper foil instead of the aluminum foil in Example 1. The other steps are the same as those in Example 1 and will not be repeated here.

[0128] This embodiment also provides a negative electrode sheet, and the preparation steps are as follows: (1) Graphite material, conductive carbon black (SP), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are dispersed in deionized water at a mass ratio of 8:1:1, and mechanically stirred under vacuum at room temperature for 24 h to obtain a uniformly mixed negative electrode slurry.

[0129] (2) The mixed negative electrode slurry is evenly coated on the current collector prepared above with a manual scraper, dried in a vacuum oven at 80 °C for 8 h, and the coating thickness is about 120 μm. The negative electrode sheet coating after coating and drying is relatively loose. Pressing treatment is carried out using a small tablet press, and the pressure range is set to 60-100 kg / cm 2 for pressing. After the pressing is completed, a mechanical cutting machine is used to cut the electrode sheet to obtain a regular negative electrode sheet.

[0130] Example 5

[0131] This embodiment provides a current collector, including a current collector matrix aluminum foil and a conductive coating coated on the aluminum foil. The preparation steps are similar to those in Example 1, except that the polyvinylpyrrolidone used is different. In this embodiment, the polyvinylpyrrolidone used is PVP-K90. Other steps are the same as those in Example 1 and will not be repeated.

[0132] This embodiment also provides a positive electrode sheet, which is similar to the preparation steps of the positive electrode sheet in Example 1, except that the current collector is different, and other steps are similar to those in Example 1 and will not be repeated.

[0133] Comparative Example 1

[0134] Comparative Example 1 provides a current collector that does not contain a coating and only contains empty aluminum foil.

[0135] Comparative Example 1 also provides a positive electrode sheet, which is similar to the preparation steps of the positive electrode sheet in Example 1, except that empty aluminum foil is used as the current collector in the comparative example. Other steps are the same as those in Example 1 and will not be repeated.

[0136] Comparative Example 2

[0137] Comparative Example 2 provides a current collector, including a current collector matrix aluminum foil and a conductive coating provided on the aluminum foil. The preparation steps are as follows:

[0138] (1) Weigh dopamine powder, dissolve it in Tris solution, and prepare a dopamine solution with a concentration of 3 mg / mL. Adjust the pH value of the dopamine solution to 8.5, and continuously stir magnetically for 4 h to obtain a black solution. After the stirring is completed, the black solution is centrifuged and washed, and the obtained solid is dried in an oven at 100 °C to obtain a polydopamine powder material.

[0139] (2) Obtain polydopamine powder, conductive carbon black, and polyacrylic acid with a mass ratio of 1:3:10.

[0140] (3) Stir the solution containing carbon black powder and polyacrylic acid thoroughly, add polydopamine powder, and continue stirring for 10 h to obtain a coating composition.

[0141] (4) Clean the empty aluminum foil to be coated to remove oil and impurities on the surface. Spray the coating composition on the surface of the empty aluminum foil to ensure that the coating is uniform and there is no sagging phenomenon.

[0142] (5) Put the sprayed aluminum foil into a high-temperature oven and dry it at 200 °C for 10 min until the polyacrylic acid is cured to form a stable conductive coating with a thickness of 1 μm, obtaining the current collector of Comparative Example 2.

[0143] Comparative Example 3

[0144] Comparative Example 3 provides a current collector that does not contain a coating and only contains empty copper foil.

[0145] Comparative Example 3 also provides a negative electrode sheet, which is similar to the preparation steps of the negative electrode sheet in Example 4, except that in Comparative Example 3, empty copper foil is used as the current collector. Other steps are the same as those in Example 4 and will not be elaborated here.

[0146] The following is the test part:

[0147] 1. Test of the peeling force of the electrode sheet

[0148] Cut the electrode sheets prepared in the examples and comparative examples into regular sizes, and use a battery sheet tensile testing machine for testing. Stick double-sided tape on the electrode sheet and roll it back and forth. After turning the other side of the electrode sheet over by 90 °C, clamp it on the machine, and continuously peel the electrode sheet at a tensile speed of 15 mm / min until the electrode sheet and the coating are completely separated. Measure 10 times in parallel to obtain the peeling force data. The peeling force data of the electrode sheets in each example and comparative example are shown in Table 1 below. Among them, the peeling force data of the electrode sheets in each example and Comparative Example 1 are as Figure 1 shown.

[0149] Table 1

[0150]

[0151] It can be seen from Table 1 and Figure 1 that after adding PDA@PVP, the peeling force of the electrode sheet is significantly improved, indicating that the adhesion between the current collector and the active material is significantly enhanced.

[0152] 2. Test of the electrochemical performance of the battery

[0153] Place the positive electrode sheets prepared above on a punching machine and slice them respectively to make circular electrode sheets with a diameter of 10 - 15 mm, and dry them for standby.

[0154] The CR2025 type simulated button cell was assembled with the electrode sheet in a glove box filled with high-purity argon. The counter electrode was a lithium metal sheet (produced in Beijing, 99.95%), the electrolyte was 1 mol / L LiPF6, the solvent was a mixed solvent of EC and DMC with a volume ratio of 1:2 (produced in Beijing, battery grade), and the separator was Celgard 2400.

[0155] The assembled button cell was placed on the die of a battery sealer for pressing and encapsulation, and then taken out of the glove box and placed horizontally for 6 - 12 h. After the electrolyte fully infiltrated the inside of the cell, electrochemical performance test and analysis were carried out.

[0156] The button half-cell was subjected to a cycle test. The charge-discharge mode was constant current charge (CC) - constant current discharge (DC), the voltage range was 2.5 - 4.2 V, and the test environment was room temperature.

[0157] Figure 2 Figure 1 is the cycle curve of the button cell prepared with the positive electrode sheets of Example 1, Comparative Example 1, and Comparative Example 2. In Figure 2 it, the abscissa is the number of cycles (Cycle NO), the ordinate is the capacity retention rate (abbreviated as Cap Retention), and 0.5P - 100%DOD Cycle Life represents the cycle life of the cell under the condition of discharging from 0.5 times the rated power to 100% depth of discharge. As can be seen from Figure 2 Figure 1, the capacity retention rate of the group with the polymer PDA@PVP-coated carbon aluminum foil added in Example 1 was increased by about 2.2% compared with the group with the empty aluminum foil in Comparative Example 1, and the capacity retention rate of the group with the polymer PDA@PVP-coated carbon aluminum foil added in Example 1 was increased by about 0.8% compared with the group with the carbon aluminum foil with only PDA added in Comparative Example 2.

[0158] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, 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, it should be considered to be within the scope described in this specification.

[0159] The above-described embodiments merely represent several implementation manners of the present application, which are convenient for understanding the technical solution of the present application specifically and in detail, but should not be construed as a limitation on the protection scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all fall within the protection scope of the present application. It should be understood that the technical solutions obtained by those skilled in the art through logical analysis, reasoning or limited experiments based on the technical solution provided by the present application are all within the protection scope of the appended claims of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the content of the appended claims, and the description and drawings can be used to explain the content of the claims.

Claims

1. A coating composition, characterized in that, A copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of the polydopamine, the polyvinylpyrrolidone and the conductive material is 1:(0.6 - 1):(1.8 - 5).

2. The coating composition according to claim 1, wherein The copolymer of polydopamine and polyvinylpyrrolidone satisfies one or more of the following conditions: (1) The preparation steps of the copolymer of polydopamine and polyvinylpyrrolidone are as follows: Copolymerize the polydopamine and the polyvinylpyrrolidone by reacting them at 40°C to 60°C for 5 h to 15 h; Optionally, the step of reacting the polydopamine and the polyvinylpyrrolidone at 40°C to 60°C for 5 h to 15 h includes: Mix the polydopamine and the polyvinylpyrrolidone, and react them at 40°C to 60°C for 5 h to 15 h under the conditions of heating and stirring; (2) The number-average molecular weight of the polyvinylpyrrolidone is 30,000 to 350,000; Optionally, the polyvinylpyrrolidone includes one or more of PVP-K30, PVP-K25, PVP-K45, PVP-K60 and PVP-K70; (3) The polydopamine is prepared by a method including the following steps: React dopamine in a buffer solution with a pH of 8.5 to 9 for 4 h to 6 h, and then perform solid-liquid separation and drying to prepare the polydopamine.

3. The coating composition according to claim 1 or 2, characterized in that, The coating composition satisfies one or more of the following conditions: (1) The conductive material includes one or more of conductive carbon black, graphite, graphene and carbon nanotubes; (2) The coating composition further includes a binder, and the binder includes one or more of polyacrylic acid, polyvinylidene fluoride, carboxymethyl cellulose and styrene-butadiene rubber; Optionally, the mass ratio of the binder to the polydopamine is (6 - 20):

1.

4. Use of a coating composition in the preparation of a secondary battery, characterized in that, The coating composition includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of the polydopamine, the polyvinylpyrrolidone and the conductive material is 1:(0.6 - 1):(1.8 - 5).

5. A current collector, characterized in that, It includes a current collector substrate and a conductive coating provided on at least one surface of the current collector substrate, and the material of the conductive coating includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, wherein the mass ratio of the polydopamine, the polyvinylpyrrolidone and the conductive material is 1:(0.6 - 1):(1.8 - 5).

6. The current collector according to claim 5, characterized in that, The copolymer of polydopamine and polyvinylpyrrolidone satisfies one or more of the following conditions: (1) The preparation steps of the copolymer of polydopamine and polyvinylpyrrolidone are as follows: Copolymerize the polydopamine and the polyvinylpyrrolidone by reacting them at 40°C to 60°C for 5 h to 15 h; Optionally, the step of reacting the polydopamine and the polyvinylpyrrolidone at 40°C to 60°C for 5 h to 15 h includes: Mix the polydopamine and the polyvinylpyrrolidone, and react them at 40°C to 60°C for 5 h to 15 h under the conditions of heating and stirring; (2) The number average molecular weight of the polyvinylpyrrolidone is 30,000 to 350,000; Optionally, the polyvinylpyrrolidone includes one or more of PVP-K30, PVP-K25, PVP-K45, PVP-K60, and PVP-K70; (3) The polydopamine is prepared by a method including the following steps: React dopamine in a buffer solution with a pH of 8.5 to 9 for 4 h to 6 h, then perform solid-liquid separation and drying to prepare the polydopamine.

7. The current collector according to claim 5 or 6, characterized in that, The conductive coating satisfies one or more of the following conditions: (1) The thickness of the conductive coating is 0.5 μm to 6 μm; (2) The material of the conductive coating further includes a binder, and the binder includes one or more of polyacrylic acid, polyvinylidene fluoride, carboxymethyl cellulose, and styrene-butadiene rubber; Optionally, the mass ratio of the binder to the polydopamine is (6 to 20):1; (3) The conductive material includes one or more of conductive carbon black, graphite, graphene, and carbon nanotubes.

8. A method for preparing a current collector according to any one of claims 5 to 7, characterized in that, It includes the following steps: Form a conductive coating on at least one surface of the current collector substrate to prepare the current collector; Among them, the material of the conductive coating includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material, and the mass ratio of the polydopamine, the polyvinylpyrrolidone, and the conductive material is 1:(0.6 to 1):(1.8 to 5); Optionally, the step of forming a conductive coating on at least one surface of the current collector substrate includes: Obtain a coating composition, and the coating composition includes a copolymer of polydopamine and polyvinylpyrrolidone and a conductive material; Spray the coating composition on at least one surface of the current collector substrate in the form of a solution, and dry and cure to prepare the current collector.

9. A pole piece, characterized in that, It includes a current collector and an active material layer provided on at least one surface of the current collector, and the current collector is as described in any one of claims 5 to 7.

10. A secondary battery, characterized in that, It includes a positive electrode sheet and a negative electrode sheet, and at least one of the positive electrode sheet and the negative electrode sheet is as described in claim 9.

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