Negative plate, preparation method thereof and battery

Abstract

The invention provides a negative electrode plate and a preparation method thereof and a battery, the negative electrode plate comprises a negative electrode material layer, the negative electrode material layer comprises a negative electrode active material, a conductive agent, a first adhesive and a second adhesive, by adding the first adhesive with the molecular weight of 10w-80w, a bonding effect can be achieved, the viscosity of negative electrode slurry can be effectively reduced, and the service life of the negative electrode slurry is prolonged. Meanwhile, by adding the second adhesive, the bonding effect can be enhanced, the viscosity of the negative electrode slurry can be effectively regulated and controlled, and the situation that the coating process is affected due to the fact that the viscosity of the negative electrode slurry is too low is avoided.

Description

Technical Field

[0001] The present application relates to the field of energy storage technology, and in particular to a negative electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] As one of the most widely used battery types in the world, lithium-ion batteries have shown significant growth and wide application. They have not only grown rapidly in the fields of electric vehicles and energy storage, but have also expanded to many fields such as traditional portable devices and small power equipment. The requirements for battery energy density in various fields are also constantly increasing, so the application of high-capacity positive and negative electrode materials is imperative. Summary of the Invention

[0003] In view of this, the purpose of this application is to provide a negative electrode sheet, a preparation method thereof, and a battery.

[0004] Based on the above purpose, the first aspect of the present application provides a negative electrode sheet, including a negative electrode material layer, wherein the negative electrode material layer includes a negative electrode active material, a conductive agent, a first adhesive and a second adhesive, wherein the first adhesive is a high molecular polymer with a molecular weight of 10w to 80w.

[0005] Optionally, the mass ratio of the negative electrode active material, the conductive agent, the first binder and the second binder is (95-97): (0.1-0.5): (1.6-3.0): (0.5-2.7).

[0006] Optionally, the mass ratio of the first adhesive to the second adhesive is (1.1-6):1.

[0007] Optionally, the first adhesive is at least one of polyacrylic acid and polyvinyl alcohol.

[0008] Optionally, the molecular weight of the second adhesive is 100w-400w.

[0009] Optionally, the second adhesive is at least one of polyacrylic acid, polyvinyl alcohol, polystyrene butadiene copolymer and sodium carboxymethyl cellulose.

[0010] Optionally, the negative electrode material layer further includes a plasticizer, and the plasticizer is at least one of hexadecyltrimethylammonium bromide, tetra-n-octylammonium bromide, triethylmethylammonium bromide, tetradecyltrimethylammonium bromide and dodecyldimethylethylammonium bromide.

[0011] Optionally, the mass ratio of the plasticizer to the first adhesive is (0.1-0.5): (1.6-3.0).

[0012] Optionally, the negative electrode active material includes at least a first active material, and the D50 particle size of the first active material is less than or equal to 10 μm.

[0013] Optionally, the negative electrode active material includes a first active material and a second active material, the first active material has a D50 particle size less than or equal to 10 μm, and the second active material has a D50 particle size greater than 10 μm and less than or equal to 15 μm.

[0014] Optionally, the first active material is a silicon-based negative electrode material, and / or the second active material is a carbon negative electrode material.

[0015] A second aspect of the present application provides a method for preparing a negative electrode sheet, comprising:

[0016] adding a solution containing a conductive agent to dry powder of anode active material to prepare a pre-kneaded slurry;

[0017] adding a solution containing a first adhesive to the pre-kneaded slurry to prepare a first kneaded slurry, wherein the molecular weight of the first adhesive is 10w to 80w;

[0018] adding a solution containing a second binder to the first kneaded slurry to prepare a second kneaded slurry;

[0019] Under the dispersion condition, adding a solvent to the second kneaded slurry until the viscosity of the second kneaded slurry meets the preset viscosity condition, thereby obtaining a negative electrode slurry;

[0020] The negative electrode slurry is coated on a negative electrode current collector to obtain a negative electrode sheet.

[0021] Optionally, the solid content of the negative electrode slurry is greater than 48%, and the viscosity is 4000-10000 mPa.s.

[0022] The third aspect of the present application provides a battery, comprising the negative electrode sheet described in any one of the first aspect or the negative electrode sheet prepared by the preparation method described in the second aspect, a separator, and a positive electrode sheet.

[0023] As can be seen from the above, the negative electrode sheet, preparation method thereof, and battery provided in the present application, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, a conductive agent, a first adhesive, and a second adhesive. By adding a first adhesive with a molecular weight of 10w to 80w, it can not only play a bonding role, but also effectively reduce the viscosity of the negative electrode slurry. At the same time, by adding the second adhesive, it can not only enhance the bonding effect, but also effectively regulate the viscosity of the negative electrode slurry, thereby avoiding the viscosity of the negative electrode slurry being too low to affect the coating process. DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present disclosure more clear, the present disclosure is further described in detail below with reference to specific embodiments.

[0025] It should be noted that, unless otherwise defined, the technical terms used in the following examples have the same meanings as commonly understood by those skilled in the art to which this invention belongs. The experimental reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the experimental methods described, unless otherwise specified, are all conventional methods.

[0026] As the most technologically mature high-capacity negative electrode material currently available, silicon negative electrode material has attracted widespread attention. The specific capacity of pure silicon negative electrode material can reach 4200mAh / g, which is even higher than that of metallic lithium negative electrode. It is a very ideal negative electrode material for lithium-ion batteries.

[0027] The volume of silicon negative electrode materials is constantly changing during the charging and discharging process. This strong volume change will cause cracks to appear on the surface of the electrode. At the same time, the continuous cracks will also lead to the breakage of the electrode and the pulverization of silicon particles, eventually causing the active material to separate from the current collector, resulting in the interruption of the conductive network, and the capacity will become lower and lower until the battery completely fails.

[0028] In order to suppress the adverse effects of this volume change of silicon anode materials, the following three mainstream methods are currently available: (1) structural design of silicon anode materials, such as the preparation of silicon nanotubes, silicon nanowires, and silicon nanoshells; (2) preparation of silicon-based composite materials to alleviate volume expansion through synergistic effects, such as silicon-carbon anodes and silicon-carbon oxide anodes; and (3) synthesis of high-performance lithium-ion battery adhesives to suppress the volume expansion effect of silicon. These methods can more or less suppress the volume expansion of silicon anode materials to a certain extent, but these methods will greatly increase the preparation cost of silicon anode materials, which is not conducive to industrial application.

[0029] The inventors found that after reducing the particle size of the silicon negative electrode material to a certain extent, the impact of the silicon volume effect can be relatively reduced. However, reducing the particle size will also bring a series of other problems. For example, when making pulp, because the silicon material particles are too small, more solvent is needed inside the powder particles, so the free solvent will be reduced and the viscosity of the slurry will increase. In order to make the viscosity reach the appropriate coating range, it is necessary to add more solvent to reduce the viscosity, which will eventually lead to a lower solid content in the prepared slurry. The lower solid content will have an adverse effect on the coating process, increase the difficulty of slurry coating, and increase the resistivity of the electrode sheet of the final prepared electrode sheet. The electrode sheet peeling strength is reduced, which is not conducive to actual use.

[0030] Therefore, if the viscosity of the negative electrode slurry can be reduced, it is not necessary to add too much solvent when preparing the negative electrode slurry, and the solid content of the slurry can be increased to meet the requirements of the coating process.

[0031] Based on this, the present application provides a negative electrode sheet, which includes a negative electrode material layer. The negative electrode material layer includes a negative electrode active material, a conductive agent, a first adhesive and a second adhesive. The first adhesive is a high molecular polymer with a molecular weight of 10w to 80w.

[0032] Specifically, both the first adhesive and the second adhesive can play a bonding role to ensure the bonding performance of the negative electrode material layer.

[0033] The reason why the adhesive can play a bonding role is that the adhesive contains functional groups such as amino groups, carboxyl groups, hydroxyl groups and halogen atoms. These functional groups can be connected with some functional groups on the surface of the negative electrode active material through hydrogen bonds, and can promote the formation of the negative electrode solid electrolyte interface membrane (Solid Electrolyte Interface membrane, SEI film), so that a denser film is formed in the electrode sheet, increasing the electrical contact between the negative electrode material layer and the current collector, thereby improving the bonding performance of the negative electrode material layer, and thus improving the cycle performance of the negative electrode sheet.

[0034] In the present application, the first adhesive is a polymer. The molecular weight of a polymer typically ranges from several thousand to several million. Even polymers with the same structural formula can have different properties due to their different molecular weights. For polymers with the same structural formula, molecular weight is positively correlated with their adhesive properties: a larger molecular weight indicates better adhesive properties, while a smaller molecular weight indicates worse adhesive properties.

[0035] In the present application, the molecular weight of the first adhesive is controlled to be 10w~80w. Compared with high molecular weight polymers, the molecular weight of 10w~80w is a small molecular weight. The bonding performance of the first adhesive with a small molecular weight is poor, but its fluidity in the solution is good. Therefore, it is added to the negative electrode material layer, mainly to utilize its good fluidity to reduce the viscosity of the negative electrode slurry, and secondly, it can also play a certain bonding role.

[0036] More importantly, small molecular weight polymers have relatively short molecular chains. Compared with long molecular chains, small molecular chains occupy less space. Therefore, functional groups on the molecular chains, such as amino groups, carboxyl groups, hydroxyl groups and halogen atoms, can more easily form hydrogen bonds with some functional groups (such as hydroxyl groups) on the surface of the negative electrode active material, thereby dispersing the negative electrode active material particles into the solvent, reducing the interaction force between the particles, and evenly dispersing the aggregated particles into the solvent, thereby improving the dispersibility of the negative electrode slurry and reducing the viscosity of the negative electrode slurry.

[0037] Therefore, when the molecular weight of the first binder is 10w-80w, the first binder can not only play a bonding role, but also effectively reduce the viscosity of the negative electrode slurry.

[0038] When the molecular weight of the first binder is less than 10w, its molecular chain is very short. Functional groups on the molecular chain, such as amino groups, carboxyl groups, hydroxyl groups, and halogen atoms, can easily form hydrogen bonds with some functional groups (such as hydroxyl groups) on the surface of the negative electrode active material, thereby dispersing the negative electrode active material particles in the solvent and reducing the viscosity of the negative electrode slurry. However, a first binder with a molecular weight that is too small has poor bonding properties and poor bonding effect, resulting in poor bonding performance and inability to achieve bonding, which in turn leads to poor bonding performance of the negative electrode sheet, low electrode peel strength, and poor cycle retention rate.

[0039] When the molecular weight of the first adhesive is greater than 80w, the bonding performance of the first adhesive is better and can provide effective bonding. However, when the molecular weight is greater than 80w, the molecular chain is longer and the space occupied on the molecular chain is larger. Therefore, functional groups such as amino, carboxyl, hydroxyl and halogen atoms on the molecular chain cannot easily form hydrogen bonds with some functional groups on the surface of the negative electrode active material. Therefore, the viscosity of the negative electrode slurry cannot be effectively reduced, which leads to the viscosity of the negative electrode slurry still being relatively high. When preparing the coating slurry, more solvent needs to be added to reduce the viscosity, which ultimately leads to a lower solid content of the prepared slurry. The lower solid content will have an adverse effect on the coating process and increase the difficulty of slurry coating.

[0040] Illustratively, the molecular weight of the first adhesive may be 10w, 20w, 30w, 40w, 50w, 60w, 70w, 80w, etc.

[0041] Preferably, the molecular weight of the first adhesive may be 40w-50w. In this case, the first adhesive can not only play a good bonding role, but also significantly reduce the viscosity of the negative electrode slurry.

[0042] In addition, due to the limitations of the coating process, the viscosity of the negative electrode slurry cannot be too low, otherwise it will increase the difficulty of the coating process. In addition, the negative electrode material also needs to have good bonding properties, otherwise the negative electrode sheet is very likely to peel off during use, resulting in battery failure. Therefore, in this application, only the first adhesive uses a small molecular weight adhesive, and the second adhesive does not use a small molecular weight adhesive. In this way, the second adhesive can play a good bonding role to ensure the bonding performance of the negative electrode material. At the same time, the molecular weight of the second adhesive is large and the molecular chain is long. Therefore, the groups on the molecular chain cannot form hydrogen bonds with some functional groups on the surface of the negative electrode active material, and therefore cannot effectively reduce the viscosity of the negative electrode slurry. The viscosity of the negative electrode slurry can be reasonably regulated to avoid the viscosity of the negative electrode slurry being reduced too much under the action of the first adhesive, so that the final viscosity is too low.

[0043] In the present application, by adding a first adhesive with a molecular weight of 10w to 80w, it can not only play a bonding role, but also effectively reduce the viscosity of the negative electrode slurry. At the same time, by adding a second adhesive, it can not only enhance the bonding effect, but also effectively regulate the viscosity of the negative electrode slurry, thereby avoiding the viscosity of the negative electrode slurry being too low to affect the coating process.

[0044] In some embodiments, the mass ratio of the negative electrode active material, the conductive agent, the first binder, and the second binder is (95-97): (0.1-0.5): (1.6-3.0): (0.5-2.7).

[0045] Specifically, when the mass ratio of the negative electrode active material, the conductive agent, the first binder, and the second binder is (95-97):(0.1-0.5):(1.6-3.0):(0.5-2.7), the content of each component is appropriate.

[0046] The content of the first adhesive is moderate, so that the first adhesive can not only play an effective bonding role, but also effectively reduce the viscosity of the negative electrode slurry.

[0047] The content of the second adhesive is moderate, so that the second adhesive can play an effective bonding role without affecting the electrochemical properties of the negative electrode sheet. It can also ensure that the viscosity of the negative electrode slurry is within an appropriate range, avoiding the viscosity of the negative electrode slurry being too low to affect the coating process; the content of the negative electrode active material is moderate.

[0048] The contents of negative electrode active materials and conductive agents are moderate to ensure the electrochemical performance of the negative electrode sheet.

[0049] When the mass proportion of the first adhesive is too small, the content of the first adhesive is too small, and the viscosity of the negative electrode slurry cannot be effectively reduced, resulting in a higher viscosity of the final negative electrode slurry, so that too much solvent needs to be added during the coating process, resulting in a lower solid content of the negative electrode slurry. Moreover, the content of the first adhesive is too small to play an effective bonding role.

[0050] When the mass proportion of the first adhesive is too large, the content of the first adhesive is too high, resulting in a large decrease in the viscosity of the negative electrode slurry, which is not conducive to the coating process.

[0051] Preferably, the mass percentage of the first adhesive in the negative electrode material layer can be 2.0% to 2.5%. In this way, the first adhesive can not only play a good bonding role, but also significantly reduce the viscosity of the negative electrode slurry, thereby making the solid content of the negative electrode slurry higher and meeting the requirements of the coating process.

[0052] When the mass proportion of the second adhesive is too small, the amount of the second adhesive added is too small, resulting in poor bonding effect; when the mass proportion of the second adhesive is too large, the amount of the second adhesive added is too much, resulting in too large impedance of the negative electrode material, increased resistivity of the electrode sheet, and reduced electrochemical performance of the negative electrode sheet.

[0053] For example, the mass ratio of the negative electrode active material, the conductive agent, the first binder, and the second binder may be 95:0.5:1.6:0.5, 95:0.5:2.0:0.5, 95:0.5:3.0:0.5, 95:0.5:3.0:2.7, and the like.

[0054] Preferably, the mass ratio of the negative electrode active material, the conductive agent, the first adhesive and the second adhesive is (95-97): (0.1-0.5): (2.0-2.5): (0.5-1.5), so that the first adhesive can not only play a good bonding role, but also significantly reduce the viscosity of the negative electrode slurry. The second adhesive can not only play a good bonding role, but also will not affect the electrochemical properties of the negative electrode sheet, and can also ensure that the viscosity of the negative electrode slurry is within an appropriate range.

[0055] In some embodiments, the mass ratio of the first adhesive to the second adhesive is (1.1-6):1.

[0056] Specifically, when the mass ratio of the first adhesive to the second adhesive is (1.1-6):1, the ratio of the first adhesive to the second adhesive is moderate, and the combined effect of the first adhesive and the second adhesive can not only reduce the viscosity of the negative electrode slurry to a range that meets the requirements, but also play an effective bonding role, thereby improving the bonding performance of the negative electrode material layer and improving the cycle performance of the negative electrode sheet.

[0057] When the mass ratio of the first adhesive to the second adhesive is less than 1.1:1, under the condition that the total content of the first adhesive and the second adhesive is constant, the content of the first adhesive is too little and the content of the second adhesive is too much, resulting in the inability to effectively reduce the viscosity of the negative electrode slurry, which in turn results in a higher viscosity of the final negative electrode slurry, so that too much solvent needs to be added during the coating process, which ultimately leads to a lower solid content of the negative electrode slurry. In addition, the excessive content of the second adhesive increases the impedance of the negative electrode sheet, thereby reducing the electrochemical performance of the negative electrode sheet.

[0058] When the mass ratio of the first adhesive to the second adhesive is greater than 6:1, under the condition that the total content of the first adhesive and the second adhesive is constant, the content of the first adhesive is too much and the content of the second adhesive is too little, resulting in too much reduction in the viscosity of the negative electrode slurry, which is not conducive to the coating process. At the same time, the content of the second adhesive is too little, resulting in a decrease in the bonding performance of the negative electrode sheet, resulting in a decrease in the peel strength of the electrode sheet, and worsening the charge and discharge cycle rate retention rate of the battery.

[0059] Illustratively, the mass ratio of the first adhesive to the second adhesive may be 1.1:1, 2:1, 3:1, 4:1, 5:1, 6:1, etc.

[0060] Preferably, the mass ratio of the first adhesive to the second adhesive can be (2-3):1. In this way, the quality of the first adhesive and the second adhesive is relatively good. The combined effect of the first adhesive and the second adhesive can not only effectively reduce the viscosity of the negative electrode slurry to a range that meets the requirements, but also play a good bonding role, thereby improving the bonding performance of the negative electrode material layer, and further improving the cycle performance of the negative electrode sheet.

[0061] In some embodiments, the first adhesive is at least one of polyacrylic acid and polyvinyl alcohol.

[0062] Specifically, when the first binder is polyacrylic acid and polyvinyl alcohol, the carboxyl group in polyacrylic acid and the hydroxyl group in polyvinyl alcohol can form hydrogen bonds with some functional groups (such as hydroxyl groups) on the surface of the negative electrode active material, thereby dispersing the negative electrode active material particles into the solvent, reducing the interaction force between the particles, and evenly dispersing the aggregated particles into the solvent, thereby improving the dispersibility of the negative electrode material and reducing the viscosity of the negative electrode slurry.

[0063] Illustratively, the first adhesive may be only polyacrylic acid or polyvinyl alcohol, or a mixture of polyacrylic acid and polyvinyl alcohol, which is not specifically limited herein.

[0064] In some embodiments, the molecular weight of the second adhesive is 100w-400w.

[0065] Specifically, when the molecular weight of the second adhesive is 100w~400w, the molecular weight of the second adhesive is relatively large and its bonding performance is also better. Therefore, the second adhesive can not only play a good bonding role to ensure the bonding performance of the negative electrode sheet, but also reasonably regulate the viscosity of the negative electrode slurry to avoid the viscosity of the negative electrode slurry being too low to affect the subsequent coating process.

[0066] When the molecular weight of the second adhesive is less than 100w, the molecular weight of the second adhesive is too small, resulting in an inability to reasonably regulate the viscosity of the negative electrode slurry, which may cause the viscosity of the negative electrode slurry to decrease too much under the action of the first adhesive, ultimately affecting the coating process. In addition, the viscosity of the second adhesive is not large enough, resulting in poor bonding performance and inability to play an effective bonding role.

[0067] When the molecular weight of the second adhesive is greater than 400w, the molecular weight of the second adhesive is too large, its molecular chain is too large, it occupies too much space, and its fluidity in the solution is very poor. As a result, even a small amount of the second adhesive will greatly weaken the effect of the first adhesive on reducing the viscosity of the negative electrode slurry, ultimately resulting in a relatively high viscosity of the negative electrode slurry. In order to achieve a suitable coating range for viscosity, more solvent needs to be added to reduce the viscosity, ultimately resulting in a lower solid content of the prepared slurry. A lower solid content will have a negative impact on the coating process and increase the difficulty of slurry coating. At the same time, a second adhesive with too high a viscosity will also increase the resistivity of the electrode sheet and reduce the peel strength of the electrode sheet, which is not conducive to practical use.

[0068] Illustratively, the molecular weight of the second adhesive may be 100w, 150w, 200w, 250w, 300w, 350w, 400w, etc.

[0069] In some embodiments, the second adhesive is at least one of polyacrylic acid, polyvinyl alcohol, polystyrene butadiene copolymer and sodium carboxymethyl cellulose. These adhesives have poor fluidity in the solution and good bonding properties, which facilitates improving the bonding properties of the negative electrode material and can reasonably regulate the viscosity of the negative electrode slurry.

[0070] In some embodiments, the negative electrode material layer further includes a plasticizer, and the plasticizer is at least one of hexadecyltrimethylammonium bromide, tetra-n-octylammonium bromide, triethylmethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyldimethylethylammonium bromide.

[0071] Specifically, within a certain range, adding a plasticizer can reduce the viscosity of the negative electrode slurry and improve the diffusibility and fluidity of the negative electrode material in the solution. This is because the addition of the plasticizer can increase the mobility of the negative electrode material particles and the polymer segments of the first binder, increase the gaps between particles and chains, reduce the interaction forces between particles and between polymer segments, and better disperse the negative electrode material particles and polymer segments in the solvent, thereby achieving the effect of reducing the overall viscosity of the negative electrode slurry.

[0072] In some embodiments, the mass ratio of the plasticizer to the first adhesive is (0.1-0.5): (1.6-3.0).

[0073] Specifically, when the mass ratio of the plasticizer to the first binder is (0.1-0.5): (1.6-3.0), the amount of the plasticizer added is moderate, and the plasticizer and the first binder are combined to effectively reduce the viscosity of the negative electrode slurry.

[0074] When the amount of plasticizer added is too much, although it can significantly reduce the viscosity of the slurry, it will also weaken the intermolecular force, resulting in a decrease in the adhesion of the adhesive.

[0075] For example, the mass ratio of the plasticizer to the first adhesive may be 0.1:1.6, 0.5:3.0, 0.1:2.0, etc.

[0076] In some embodiments, the negative electrode active material includes at least a first active material, and the D50 particle size of the first active material is less than or equal to 10 μm.

[0077] Specifically, the D50 particle size is also called the median diameter or median particle size, which refers to the particle size corresponding to when the cumulative particle size distribution percentage of the sample reaches 50%.

[0078] When the D50 particle size of the first active material is less than or equal to 10 μm, the first active material has a relatively small particle size, which can effectively reduce the volume expansion effect of the negative electrode material during charge and discharge. However, when the particle size is less than or equal to 10 μm, the particles are too small, requiring more solvent within the powder particles. As a result, the free solvent is reduced, and the viscosity of the slurry increases.

[0079] Therefore, in the present application, the viscosity of the negative electrode slurry is reduced by adding a first binder with a molecular weight of 10w to 80w to the negative electrode material. In this way, during the coating process, the viscosity of the negative electrode slurry can meet the requirements of the coating process without adding too much solvent. At the same time, the solid content of the negative electrode slurry can be guaranteed without adding too much solvent, ensuring that the solid content can also meet the coating requirements.

[0080] Preferably, the D50 particle size of the first active material is less than or equal to 1 μm. In this case, the particle size of the negative electrode material is even smaller, and it is more necessary to add the first binder to reduce the viscosity of the negative electrode slurry.

[0081] For example, the D50 particle size of the first active material may be 10 μm, 8 μm, 6 μm, 4 μm, 2 μm, 1.5 μm, 1 μm, 0.5 μm, etc.

[0082] In some embodiments, the negative electrode active material includes a first active material and a second active material, the first active material has a D50 particle size less than or equal to 10 μm, and the second active material has a D50 particle size greater than 10 μm and less than or equal to 15 μm.

[0083] Specifically, the electrochemical performance of the negative electrode materials can be optimized by using a first negative electrode material with a small particle size and a second negative electrode material with a large particle size in combination.

[0084] Furthermore, the first active material is a silicon-based negative electrode material, and the second active material is a carbon negative electrode material. For example, the silicon-based negative electrode material may be a single-element silicon negative electrode material, an oxidized silicon negative electrode material, or a silicon-carbon negative electrode material. The carbon negative electrode material may be carbon black, graphite, or carbon nanotubes.

[0085] In some embodiments, the mass percentage of the first active material to the second active material is 1% to 40% to ensure the electrochemical performance of the negative electrode sheet.

[0086] The present application also provides a method for preparing a negative electrode sheet, comprising:

[0087] Step S100: adding a solution containing a conductive agent to dry powder of anode active material to prepare a pre-kneaded slurry;

[0088] Step S200, adding a solution containing a first adhesive to the pre-kneaded slurry to prepare a first kneaded slurry, wherein the molecular weight of the first adhesive is 10w to 80w;

[0089] Step S300: adding a solution containing a second adhesive to the first kneaded slurry to prepare a second kneaded slurry;

[0090] Step S400: Under dispersion conditions, adding a solvent to the second kneaded slurry until the viscosity of the second kneaded slurry meets a preset viscosity condition, thereby obtaining a negative electrode slurry;

[0091] Step S500: coating the negative electrode slurry on the negative electrode current collector to obtain a negative electrode sheet.

[0092] Specifically, when preparing the negative electrode sheet, the solution containing the conductive agent is directly added to the dry powder of the negative electrode active material without preparing the negative electrode active material into a slurry in advance, which can simplify the preparation process.

[0093] If the negative electrode active material includes a first negative electrode material and a second negative electrode material, the first negative electrode material and the second negative electrode material are first dry-mixed to obtain a negative electrode active material dry powder, and then a conductive agent is added to prepare a pre-kneaded slurry.

[0094] Then, a solution containing the first adhesive is added to the pre-kneaded slurry to prepare the first kneaded slurry. In this way, the addition of the first adhesive with a molecular weight of 10w to 80w can not only play a bonding role but also effectively reduce the viscosity of the negative electrode slurry.

[0095] Then, a solution containing a second binder is added to the first kneaded slurry to form a second kneaded slurry. The addition of the second binder can not only enhance the bonding effect, but also effectively control the viscosity of the negative electrode slurry, preventing the viscosity of the negative electrode slurry from being too low and affecting the coating process.

[0096] Finally, under dispersion conditions, a solvent is added to the second kneaded slurry until the viscosity of the second kneaded slurry meets a preset viscosity condition to obtain a negative electrode slurry, and the negative electrode slurry is coated on a negative electrode current collector to obtain a negative electrode sheet.

[0097] In specific implementations, the content of the conductive agent in the solution containing the conductive agent can be flexibly controlled according to the actual process. For example, the content of the conductive agent in the solution containing the conductive agent can be 1% to 2% so that the fluidity of the solution containing the conductive agent meets the process requirements.

[0098] The content of the first adhesive in the solution containing the first adhesive and the content of the second adhesive in the solution containing the second adhesive can be flexibly controlled based on the actual process. For example, the content of the first adhesive in the solution containing the first adhesive and the content of the second adhesive in the solution containing the second adhesive can both be 5% to 10% to ensure that the fluidity of the solutions containing the first adhesive and the second adhesive meets the requirements of the preparation process.

[0099] In some embodiments, the solid content of the negative electrode slurry is greater than 48%, and the viscosity is 4000-10000 mPa.s.

[0100] Specifically, the negative electrode slurry prepared by the present preparation method has a solid content greater than 48% and a viscosity of 4000 to 10000 MPa.s, which meets the requirements of the coating process and is conducive to the coating process.

[0101] The present application also provides a battery, comprising the negative electrode sheet described in any of the above embodiments or the negative electrode sheet prepared by the preparation method described in any of the above embodiments, a separator, and a positive electrode sheet.

[0102] Specifically, the positive electrode sheet includes a positive electrode material layer and a positive electrode current collector. The positive electrode material layer includes a positive electrode active material, a conductive agent, and a binder. The positive electrode active material can be lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, etc. The separator can be a polyethylene separator or a polypropylene separator, etc.

[0103] The battery described in this application has the technical effects of the negative electrode sheet described in any of the above embodiments, which will not be described in detail here.

[0104] The present application is further discussed below with reference to specific embodiments.

[0105] Example 1

[0106] A negative electrode sheet comprises a negative electrode material layer, wherein the negative electrode material layer comprises a negative electrode active material, a conductive agent, a first adhesive and a second adhesive, wherein the first adhesive is a high molecular polymer with a molecular weight of 10w to 80w.

[0107] The negative electrode active material includes a first negative electrode material and a second negative electrode material. The first negative electrode material is a silicon-based negative electrode material with a particle size of less than or equal to 1.5 μm, and the second negative electrode material is a carbon negative electrode material. The conductive agent is conductive carbon black. The first binder is polyacrylic acid (PAA) with a molecular weight of 10w to 80w, and the second binder is polyacrylic acid with a molecular weight of 100w to 400w.

[0108] The mass fractions of the negative electrode active material, the conductive agent, the first binder and the second binder are 95 parts, 0.5 parts, 3.0 parts and 1.5 parts respectively.

[0109] A method for preparing the above-mentioned negative electrode sheet is prepared according to the following method based on the above-mentioned mass ratio, and the method comprises:

[0110] (1) adding a solution containing a conductive agent to dry powder of a negative electrode active material to prepare a pre-kneaded slurry;

[0111] (2) adding a solution containing a first binder to the pre-kneaded slurry to prepare a first kneaded slurry;

[0112] (3) adding a solution containing a second binder to the first kneaded slurry to prepare a second kneaded slurry;

[0113] (4) Under dispersion conditions, a solvent is added to the second kneaded slurry until the viscosity of the second kneaded slurry is (7500±100) mPa.s to obtain a negative electrode slurry;

[0114] (5) Coating the negative electrode slurry on the negative electrode current collector to obtain a negative electrode sheet.

[0115] A battery is prepared by assembling the negative electrode sheet, separator and positive electrode sheet prepared above into a battery.

[0116] Example 2

[0117] The difference from Example 1 is that the mass fraction of the first adhesive is 2.5 parts.

[0118] Example 3

[0119] The difference from Example 1 is that the mass fraction of the first adhesive is 1.7 parts.

[0120] Example 4

[0121] The difference from Example 1 is that the mass fraction of the second adhesive is 2.7 parts.

[0122] Example 5

[0123] The difference from Example 1 is that the mass fraction of the second adhesive is 0.5 parts.

[0124] Example 6

[0125] The difference from Example 1 is that the first adhesive is polyvinyl alcohol (PVA) with a molecular weight of 10w to 80w.

[0126] Example 7

[0127] The difference from Example 1 is that when the first adhesive is added, cetyltrimethylammonium bromide is added, and the mass fraction of cetyltrimethylammonium bromide is 0.5 parts.

[0128] Example 8

[0129] The difference from Example 1 is that when the first adhesive is added, hexadecyltrimethylammonium bromide is added, and the mass fraction of hexadecyltrimethylammonium bromide is 0.1 parts.

[0130] Example 9

[0131] The difference from Example 1 is that the second adhesive is polyvinyl alcohol (PVA) with a molecular weight of 100w to 400w.

[0132] Comparative Example 1

[0133] The difference from Example 1 is that the mass fraction of the first adhesive is 1.5 parts.

[0134] Comparative Example 2

[0135] The difference from Example 1 is that the mass fraction of the first adhesive is 3.5 parts.

[0136] Comparative Example 3

[0137] The difference from Example 1 is that the mass fraction of the second adhesive is 0.3 parts.

[0138] Comparative Example 4

[0139] The difference from Example 1 is that the mass fraction of the second adhesive is 3.0 parts.

[0140] Comparative Example 5

[0141] The difference from Example 1 is that the first adhesive is replaced by polyacrylic acid with a molecular weight of 100w to 400w, which is the same as the second adhesive.

[0142] Comparative Example 6

[0143] The difference from Example 1 is that the first adhesive is replaced by polyacrylic acid with a molecular weight of 90w.

[0144] Comparative Example 7

[0145] The difference from Example 1 is that the first adhesive is replaced by polyacrylic acid with a molecular weight of 8w.

[0146] Comparative Example 8

[0147] The difference from Example 1 is that the second adhesive is replaced by polyacrylic acid with a molecular weight of 10w to 80w, which is the same as the first adhesive.

[0148] Comparative Example 9

[0149] The difference from Example 1 is that the second adhesive is replaced by polyacrylic acid with a molecular weight of 90w.

[0150] Comparative Example 10

[0151] The difference from Example 1 is that the second adhesive is replaced by polyacrylic acid with a molecular weight of 500w.

[0152] Comparative Example 11

[0153] The difference from Example 1 is that the second adhesive is added first and then the first adhesive.

[0154] Comparative Example 12

[0155] The difference from Example 1 is that when the first adhesive is added, cetyltrimethylammonium bromide is added, and the mass fraction of cetyltrimethylammonium bromide is 0.6 parts.

[0156] The parameters of the above embodiments and comparative examples are detailed in Table 1 below.

[0157] The performance tests of the negative electrode sheets or batteries prepared in the following examples and comparative examples were performed.

[0158] (1) The solid content and viscosity of the negative electrode slurries obtained in the above embodiments and comparative examples were tested.

[0159] (2) The electrode sheet resistivity and electrode sheet peeling strength of the negative electrode sheets obtained in the above-mentioned embodiments and comparative examples were tested.

[0160] The electrode resistivity test process is as follows: Use an electrode resistance meter to test the electrode resistance, take 20cm 2 The negative electrode disc is placed under the probe, and a physical contact test is performed on the electrode surface to obtain the overall resistivity in the thickness direction of the electrode.

[0161] The electrode peel strength test process is as follows: a 2cm*10cm electrode is attached to a steel plate with double-sided tape, the steel plate is fixed to the equipment, the electrode is clamped with the clamp of the tensile equipment, and the 180-degree horizontal peel strength is tested.

[0162] (3) The cycle capacity retention rate of the batteries obtained in the above embodiments and comparative examples was tested, and the test process was as follows: discharge to 5 mV at 0.01 C, stand for 10 min, charge to 1.5 V at 0.05 C and stand for 10 min; after three cycles, discharge to 5 mV at 0.01 C and stand for 10 min, charge to 1.5 V at 0.1 C and stand for 10 min, and cycle three times; discharge to 5 mV at 0.01 C, stand for 10 min, charge to 1.5 mV at 0.33 C and stand for 10 min, and cycle three times.

[0163] The test results are shown in Table 2.

[0164] Table 1 Experimental parameters

[0165]

[0166]

[0167]

[0168] Table 2 Test results list

[0169]

[0170]

[0171] As can be seen from the above table, the solid content of the negative electrode slurry in each embodiment of the present application is greater than 48%, and the viscosity is 4000~10000mPa.s, which meets the requirements of the actual coating process. The peel strength of the pole piece of each embodiment is greater than that of each comparative example, indicating that the bonding performance of the pole piece of each embodiment is better than that of each comparative example. Moreover, the cycle capacity retention rate of the battery of each embodiment is greater than that of each comparative example, indicating that the electrochemical performance of the battery of each embodiment is better than that of each comparative example. It can be seen that adding an appropriate amount of the first adhesive and the second adhesive can not only ensure that the solid content and viscosity of the slurry meet the requirements of the coating process, but also ensure that the pole piece has good bonding performance and will not affect the electrochemical performance of the battery.

[0172] Comparing Examples 7 to 8 with Examples 1 to 6, it can be seen that the viscosity of the negative electrode slurry is further reduced due to the addition of an appropriate amount of plasticizer in Examples 7 to 8.

[0173] In Comparative Example 1, since the amount of the first adhesive added is too low, the viscosity of the negative electrode slurry cannot be effectively reduced, resulting in a higher viscosity of the negative electrode slurry. In addition, the content of the first adhesive is too low to provide effective bonding.

[0174] In Comparative Example 2, since the amount of the first binder added is too much, the viscosity of the negative electrode slurry is reduced too much, which is not conducive to the coating process.

[0175] In Comparative Example 3, since the amount of the second adhesive added is too low, the bonding effect is poor, resulting in low peel strength of the electrode and poor cycle performance.

[0176] In Comparative Example 4, since the amount of the second binder added is too much, the impedance of the negative electrode material is too large, the resistivity of the electrode sheet increases, and the electrochemical performance of the negative electrode sheet is reduced, resulting in poor cycle performance.

[0177] In Comparative Example 5, the first adhesive is replaced with polyacrylic acid having the same molecular weight of 100w to 400w as the second adhesive. The molecular weight of the first adhesive is too large, and the bonding performance is better, which can provide an effective bonding effect. However, due to its large molecular chain, the groups on the molecular chain cannot produce hydrogen bonds with the groups on the surface of the active material, so that the viscosity of the negative electrode slurry cannot be effectively reduced, resulting in the viscosity of the negative electrode slurry still being relatively high.

[0178] In Comparative Example 6, the first adhesive is replaced with polyacrylic acid with a molecular weight of 90w. The first adhesive has a large molecular weight and good bonding performance, which can provide effective bonding. However, the large molecular weight makes it difficult for the groups on the molecular chain to form hydrogen bonds with the groups on the surface of the active material, so that the viscosity of the negative electrode slurry cannot be effectively reduced, resulting in the viscosity of the negative electrode slurry still being large.

[0179] In comparative example 7, the first adhesive is replaced with polyacrylic acid with a molecular weight of 8w. The molecular weight of the first adhesive is too small, and the groups on the molecular chain are very easy to generate hydrogen bonds with the groups on the surface of the active material, which can effectively reduce the viscosity of the negative electrode slurry. However, the molecular chain of the first adhesive with a molecular weight that is too small is too short, and its bonding effect is poor, so that it cannot play a bonding role, which leads to poor bonding performance of the negative electrode sheet, low electrode sheet peeling strength, and poor cycle retention rate.

[0180] In Comparative Example 8, the second adhesive is replaced with polyacrylic acid with a molecular weight of 10w to 80w, the same as the first adhesive. The molecular weight of the second adhesive is too small, which can also effectively reduce the viscosity of the negative electrode slurry, resulting in the viscosity of the negative electrode slurry being reduced too much under the action of the first adhesive and the second adhesive, ultimately affecting the coating process. Moreover, the molecular weight of the second adhesive is not large enough, resulting in poor bonding performance and inability to play an effective bonding role. Therefore, the peel strength of the electrode is low and the cycle performance is poor.

[0181] In Comparative Example 9, the second adhesive is replaced with polyacrylic acid with a molecular weight of 90w. The molecular weight of the second adhesive is relatively small, and it cannot significantly play a role in regulating the viscosity of the negative electrode slurry, resulting in the viscosity of the negative electrode slurry being reduced too much under the action of the first adhesive, ultimately affecting the coating process. Moreover, the small molecular weight of the second adhesive will lead to its poor bonding performance and cannot play an effective bonding role. Therefore, the peel strength of the electrode is low and the cycle performance is poor.

[0182] In Comparative Example 10, the second adhesive is replaced with polyacrylic acid with a molecular weight of 500w. The molecular weight of the second adhesive is too large, its molecular chain is too large, the space occupied is too large, and its fluidity in the solution is very poor, so that even a small amount of the second adhesive will greatly weaken the effect of the first adhesive on reducing the viscosity of the negative electrode slurry, ultimately resulting in the viscosity of the negative electrode slurry still being relatively high, which is not conducive to the coating process.

[0183] In Comparative Example 11, the second adhesive is added first, and then the first adhesive is added. Since the second adhesive has poor fluidity in the solution, the viscosity of the slurry after the first adhesive is added is relatively high. After the first adhesive is added, the first adhesive cannot effectively reduce the viscosity, resulting in a high viscosity, which is not conducive to the coating process.

[0184] In Comparative Example 12, the amount of plasticizer added is too much. Although it can significantly reduce the viscosity of the slurry, it will also weaken the intermolecular force, resulting in a decrease in the adhesion of the adhesive.

[0185] In summary, the negative electrode sheet, preparation method thereof, and battery provided in the present application, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, a conductive agent, a first adhesive, and a second adhesive. By adding a first adhesive with a molecular weight of 10w to 80w, it can not only play a bonding role, but also effectively reduce the viscosity of the negative electrode slurry. At the same time, by adding the second adhesive, it can not only enhance the bonding effect, but also effectively regulate the viscosity of the negative electrode slurry, thereby avoiding the viscosity of the negative electrode slurry being too low to affect the coating process, and at the same time will not affect the electrochemical properties of the electrode sheet and the battery.

[0186] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present disclosure (including the claims) is limited to these examples. Within the scope of the present disclosure, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the embodiments of the present disclosure as described above, which are not provided in detail for the sake of simplicity.

[0187] The embodiments of the present disclosure are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode material layer includes a negative electrode active material, a conductive agent, a first binder and a second binder. The first binder is a high molecular polymer with a molecular weight of 10w to 80w.

2. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of the negative electrode active material, the conductive agent, the first binder and the second binder is (95-97): (0.1-0.5): (1.6-3.0): (0.5-2.7).

3. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of the first adhesive to the second adhesive is (1.1-6):

1.

4. The negative electrode sheet according to claim 1, characterized in that: The first adhesive is at least one of polyacrylic acid and polyvinyl alcohol.

5. The negative electrode sheet according to claim 1, characterized in that: The molecular weight of the second adhesive is 100w-400w.

6. The negative electrode sheet according to claim 5, characterized in that: The second adhesive is at least one of polyacrylic acid, polyvinyl alcohol, polystyrene butadiene copolymer and sodium carboxymethyl cellulose.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The negative electrode material layer further includes a plasticizer, which is at least one of hexadecyltrimethylammonium bromide, tetra-n-octylammonium bromide, triethylmethylammonium bromide, tetradecyltrimethylammonium bromide and dodecyldimethylethylammonium bromide.

8. The negative electrode sheet according to claim 7, characterized in that: The mass ratio of the plasticizer to the first adhesive is (0.1-0.5):(1.6-3.0).

9. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material includes at least a first active material, and the D50 particle size of the first active material is less than or equal to 10 μm.

10. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material includes a first active material and a second active material. The D50 particle size of the first active material is less than or equal to 10 μm, and the D50 particle size of the second active material is greater than 10 μm and less than or equal to 15 μm.

11. The negative electrode sheet according to any one of claims 9 to 10, characterized in that: The first active material is a silicon-based negative electrode material, and / or the second active material is a carbon negative electrode material.

12. A method for preparing a negative electrode sheet, characterized in that: include: adding a solution containing a conductive agent to dry powder of anode active material to prepare a pre-kneaded slurry; adding a solution containing a first adhesive to the pre-kneaded slurry to prepare a first kneaded slurry, wherein the molecular weight of the first adhesive is 10w to 80w; adding a solution containing a second binder to the first kneaded slurry to prepare a second kneaded slurry; Under the dispersion condition, adding a solvent to the second kneaded slurry until the viscosity of the second kneaded slurry meets the preset viscosity condition, thereby obtaining a negative electrode slurry; The negative electrode slurry is coated on a negative electrode current collector to obtain a negative electrode sheet.

13. A battery, characterized in that: The invention comprises the negative electrode sheet according to any one of claims 1 to 11 or the negative electrode sheet prepared by the preparation method according to claim 12, a separator and a positive electrode sheet.

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