Negative plate and preparation method thereof, battery and power utilization device

Abstract

The invention provides a negative electrode plate and a preparation method thereof, a battery and a power utilization device, the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer located on the surface of the negative electrode current collector, and the negative electrode active layer comprises a negative electrode active material, a binder and a conductive agent, the binder comprises a waterborne polyurethane binder and a three-dimensional network polymer binder with an ester group. The negative plate comprises a three-dimensional network polymer binder with an ester group and a waterborne polyurethane binder, so that the binders have high binding power and excellent mechanical strength and flexibility, the problem of volume expansion of an active material can be effectively solved, and the corresponding battery has high capacity, high first efficiency and long cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery manufacturing technology, and more specifically, to a negative electrode sheet and its preparation method, a battery, and an electrical device. Background Technology

[0002] Currently, negative electrode active materials generally exhibit significant volume changes during charge and discharge processes, especially silicon-based materials with ultra-high theoretical specific capacity. This can easily lead to material pulverization, resulting in irreversible capacity decay and poor overall electrical performance of the corresponding battery.

[0003] Based on this, technicians thought of improving the problem of material volume expansion from the binder level. Specifically, commonly used binders can form hydrogen bonds or covalent bonds with active materials, thereby improving the problem of active material volume expansion to a certain extent. However, the improvement effect of these commonly used binders is not good, resulting in the corresponding battery capacity, first efficiency and cycle performance still being unsatisfactory. Summary of the Invention

[0004] The purpose of this application is to provide a negative electrode sheet and its preparation method, battery and power device. The negative electrode sheet includes a three-dimensional network polymer binder with ester groups and an aqueous polyurethane binder, so that the binder has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of active materials, thereby enabling the corresponding battery to have high capacity, high first efficiency and long cycle performance.

[0005] The embodiments of this application are implemented as follows: In a first aspect, embodiments of this application provide a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active layer located on the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent. The binder includes an aqueous polyurethane binder and a three-dimensional network polymer binder with ester groups.

[0006] In the above technical solution, the binder in the negative electrode sheet includes an aqueous polyurethane binder and a three-dimensional network polymer binder with ester groups. The three-dimensional network polymer binder with ester groups has high adhesion, excellent mechanical strength and deformation resistance, while the aqueous polyurethane binder has high adhesion and high flexibility. The composite binder formed by the two binders has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of active materials, thereby enabling the corresponding battery to have high capacity, high initial efficiency and long cycle performance.

[0007] In some alternative embodiments, the negative electrode active material includes a silicon-based material, the waterborne polyurethane binder is a linear waterborne polyurethane binder, and the waterborne polyurethane binder is interspersed in a three-dimensional network polymer binder.

[0008] In the above technical solution, the volume expansion problem of silicon-based materials during charging and discharging is quite serious. Using silicon-based materials in combination with the above-mentioned composite binder can effectively solve the problem of serious volume expansion of silicon-based materials, thereby enabling the battery corresponding to this material system to have high capacity, high first-time efficiency and long cycle performance. In addition, the binder is a linear waterborne polyurethane binder, and the waterborne polyurethane binder is interspersed in the three-dimensional network polymer binder, so that the composite binder forms a semi-interpenetrating network structure. The composite binder with this structural characteristic has higher adhesion, better mechanical strength and flexibility, which helps to further improve the problem of serious volume expansion of silicon-based active materials.

[0009] In some alternative embodiments, the mass ratio of the waterborne polyurethane adhesive to the three-dimensional network polymer adhesive is (1.5~2.5):3, or / and, and the molecular weight of the waterborne polyurethane adhesive is 20,000~100,000.

[0010] In the above technical solution, the mass ratio of waterborne polyurethane adhesive and three-dimensional network polymer adhesive is limited to the above range so that both waterborne polyurethane adhesive and three-dimensional network polymer adhesive have a more suitable mass ratio, thereby making the corresponding adhesive have more suitable mechanical strength and flexibility, so as to better improve the problem of volume expansion of silicon-based materials; the molecular weight of waterborne polyurethane is limited to the above range so that when the mass ratio of waterborne polyurethane adhesive is constant, the composite adhesive can have better flexibility.

[0011] In some alternative embodiments, the three-dimensional network polymer binder includes a first polymer binder and a second polymer binder, wherein the first polymer binder includes at least one of polyacrylic acid, polymethacrylic acid and polyglutamic acid, and the second polymer binder includes at least one of carboxymethyl cellulose, polyvinyl alcohol and sodium alginate.

[0012] In the above technical solution, the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups are selected from the above types, so that the corresponding three-dimensional network polymer binder with ester groups has higher adhesion, better mechanical strength and deformation resistance. At the same time, both polymer binders are applicable to a wide variety of types, which can provide more feasible solutions, thereby facilitating the promotion and application of the technical solutions provided in the embodiments of this application.

[0013] In some alternative embodiments, the first polymer binder is selected from polyacrylic acid, and the molecular weight of the first polymer binder is 100,000 to 300,000; the second polymer binder is selected from carboxymethyl cellulose, and the molecular weight of the second polymer binder is 400,000 to 650,000.

[0014] In the above technical solution, the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups are selected from the above types and their molecular weights are limited to the above range, so that the corresponding three-dimensional network polymer binder with ester groups has higher adhesion, better mechanical strength and deformation resistance.

[0015] In some alternative embodiments, the mass ratio of the first polymeric adhesive to the second polymeric adhesive is (0.8~1.2):1.

[0016] In the above technical solution, the mass ratio of the two polymer adhesives is limited to the above range so that the corresponding three-dimensional network polymer adhesive with ester groups has higher adhesion, better mechanical strength and deformation resistance.

[0017] In some alternative embodiments, the mass ratio of negative electrode active material to binder is (90~95):(4~6), or / and the mass ratio of negative electrode active material to conductive agent is (90~95):(1~2).

[0018] In the above technical solution, the mass ratio of the negative electrode active material and the binder is limited to the above range so that the two have a more suitable mass ratio, which can effectively improve the problem of volume expansion of silicon-based materials; the mass ratio of the negative electrode active material and the conductive agent is limited to the above range so that the two have a more suitable mass ratio, so that the corresponding negative electrode sheet has better conductivity and capacity.

[0019] Secondly, embodiments of this application provide a method for preparing a negative electrode sheet as provided in the first aspect embodiment, comprising the following steps: mixing a negative electrode active material, a first polymer binder having carboxyl groups, a second polymer binder having hydroxyl groups, a linear aqueous polyurethane binder, a conductive agent, and water, wherein the negative electrode active material includes a silicon-based material, to obtain a negative electrode slurry; applying the negative electrode slurry to the surface of a negative electrode current collector to obtain a negative electrode sheet intermediate; and then heat-treating the negative electrode sheet intermediate under vacuum conditions to cause the first polymer binder and the second polymer binder to crosslink in situ to form a three-dimensional network polymer binder, and simultaneously causing the aqueous polyurethane binder to interpenetrate in the formed three-dimensional network polymer binder to obtain a negative electrode sheet.

[0020] In the above technical solution, the anode sheet is prepared according to the above process so that the prepared anode sheet includes a composite binder with a semi-interpenetrating network structure formed by a three-dimensional network polymer binder with ester groups and a linear aqueous polyurethane binder. The composite binder has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of silicon-based materials, thereby enabling the battery corresponding to the anode sheet to have high capacity, high initial efficiency and long cycle performance.

[0021] In some alternative implementations, prior to the heat treatment step, the negative electrode intermediate is further subjected to a drying process at a temperature lower than that of the heat treatment to remove moisture from the negative electrode intermediate.

[0022] In the above technical solution, a low-temperature drying process is performed before the high-temperature heat treatment. The low-temperature drying process allows the moisture to be removed slowly, so as to finally prepare a high-quality negative electrode sheet with a denser structure, higher surface flatness and fewer cracks. At the same time, after the moisture is removed, the two polymer binders are esterified and crosslinked, so that the resulting three-dimensional network polymer binder with ester groups has higher adhesion, better mechanical strength and deformation resistance.

[0023] In some alternative implementations, the drying process is carried out at a temperature of 50°C to 80°C; the heat treatment process is carried out at a temperature of 110°C to 140°C.

[0024] In the above technical solution, limiting the drying temperature within the above range enables the moisture in the negative electrode intermediate to be removed more thoroughly under relatively mild conditions; limiting the heat treatment temperature within the above range enables the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups to be more fully and thoroughly crosslinked to form a three-dimensional network polymer binder with ester groups, while also effectively protecting the remaining functional components in the negative electrode intermediate.

[0025] In some alternative embodiments, the first polymer binder is selected from polyacrylic acid, the second polymer binder is selected from carboxymethyl cellulose, and the mixing step includes: dissolving carboxymethyl cellulose in water to obtain a carboxymethyl cellulose solution; adding polyacrylic acid, a waterborne polyurethane binder, and a conductive agent to the carboxymethyl cellulose solution and mixing to obtain a slurry intermediate; adding a negative electrode active material to the slurry intermediate and mixing to obtain a negative electrode slurry.

[0026] In the above technical solution, carboxymethyl cellulose, which has poor water solubility, is dissolved first, and then the remaining components are added to the carboxymethyl cellulose solution and mixed. This allows each binder component to dissolve efficiently and completely in water. At the same time, it can also improve the mixing uniformity of each raw material component, so as to prepare a high-quality negative electrode sheet with a denser structure, higher surface smoothness and fewer cracks. In addition, it can also enable the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups to crosslink more fully and completely to form a three-dimensional network polymer binder with ester groups.

[0027] Thirdly, embodiments of this application provide a battery including the negative electrode sheet as provided in the first aspect embodiment.

[0028] In the above technical solution, the battery includes a negative electrode sheet as provided in the first aspect embodiment. Since the negative electrode sheet includes a composite binder with a semi-interpenetrating network structure formed by a three-dimensional network polymer binder with ester groups and a linear aqueous polyurethane binder, the composite binder has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of silicon-based materials, thereby enabling the corresponding battery to have high capacity, high initial efficiency and long cycle performance.

[0029] Fourthly, embodiments of this application provide an electrical device including a battery as provided in the third aspect embodiment. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a process flow diagram of a method for preparing a negative electrode sheet provided in an embodiment of this application. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0033] It should be noted that the terms "and / or" in this application, such as "feature 1 and / or feature 2", all refer to the three cases of "feature 1" alone, "feature 2" alone, and "feature 1" plus "feature 2".

[0034] In addition, in the description of this application, unless otherwise stated, "one or more" means two or more; the range of "numerical value a to numerical value b" includes the two endpoints "a" and "b"; and "unit of measurement" in "numerical value a to numerical value b + unit of measurement" represents the "unit of measurement" of both "numerical value a" and "numerical value b".

[0035] The inventors discovered that while existing binders can improve the volume expansion problem of active materials by forming hydrogen bonds or covalent bonds with them, these binders are usually linear or branched. These types of binders are prone to relative sliding with the active material during lithium insertion / extraction. In addition, the binders themselves are difficult to have both high adhesion and excellent mechanical strength, making it difficult to effectively suppress the volume expansion of the active material (manifested as easy cracking of the electrode), especially for silicon-based materials with more severe volume expansion.

[0036] Based on this, the inventors further discovered that polymer binders with carboxyl and hydroxyl groups can be crosslinked through esterification to form a three-dimensional network polymer binder. This type of binder has both high adhesion and excellent mechanical strength, and can effectively suppress the volume expansion of active materials.

[0037] However, the inventors discovered that the three-dimensional network polymer binder lacks flexibility, which also affects its ability to suppress the volume expansion of active materials. Therefore, there is an urgent need to develop a composite binder that combines high adhesion, excellent mechanical strength, and flexibility to better suppress the volume expansion of active materials.

[0038] The following is a detailed description of a negative electrode sheet, its preparation method, battery, and power-consuming device according to embodiments of this application.

[0039] In a first aspect, embodiments of this application provide a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active layer located on the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent. The binder includes an aqueous polyurethane binder and a three-dimensional network polymer binder with ester groups.

[0040] It should be noted that the specific type of waterborne polyurethane is not limited, for example, it can be at least one of carboxyl waterborne polyurethane, sulfonic acid waterborne polyurethane and tertiary amino waterborne polyurethane.

[0041] It should be noted that the three-dimensional network polymer binder with ester groups is formed by crosslinking a first polymer binder with carboxyl groups and a second polymer binder with hydroxyl groups through an esterification reaction.

[0042] It should be noted that waterborne polyurethane adhesives were chosen because they possess the characteristics of high adhesion, good flexibility, non-reaction with first polymer adhesives containing carboxyl groups, non-reaction with second polymer adhesives containing hydroxyl groups, and good water solubility.

[0043] In this application, the binder in the negative electrode sheet includes an aqueous polyurethane binder and a three-dimensional network polymer binder with ester groups. The three-dimensional network polymer binder with ester groups has high adhesion, excellent mechanical strength and deformation resistance, while the aqueous polyurethane binder has high adhesion and high flexibility. The composite binder formed by the two binders has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of active materials, thereby enabling the corresponding battery to have high capacity, high initial efficiency and long cycle performance.

[0044] As an example, the negative electrode active material includes a silicon-based material, the waterborne polyurethane binder is a linear waterborne polyurethane binder, and the waterborne polyurethane binder is interspersed in a three-dimensional network polymer binder.

[0045] In this embodiment, the volume expansion problem of silicon-based materials during charging and discharging is quite serious. Using silicon-based materials in combination with the above-mentioned composite binder can effectively solve the problem of serious volume expansion of silicon-based materials, thereby enabling the battery corresponding to this material system to have high capacity, high initial efficiency, and long cycle performance. In addition, the binder is a linear waterborne polyurethane binder, and the waterborne polyurethane binder is interspersed in the three-dimensional network polymer binder, so that the composite binder forms a semi-interpenetrating network structure. The composite binder with this structural characteristic has higher adhesion, better mechanical strength and flexibility, which helps to further improve the problem of serious volume expansion of silicon-based active materials. As an example, the mass ratio of waterborne polyurethane adhesive to three-dimensional network polymer adhesive is (1.5~2.5):3, for example, but not limited to any one of the mass ratios of 1.5:3, 1.6:3, 1.7:3, 1.8:3, 1.9:3, 2.0:3, 2.1:3, 2.2:3, 2.3:3, 2.4:3 and 2.5:3 or any range of values ​​between the two.

[0046] In this embodiment, the mass ratio of the waterborne polyurethane adhesive and the three-dimensional network polymer adhesive is limited to the above-mentioned range so that both the waterborne polyurethane adhesive and the three-dimensional network polymer adhesive have a more suitable mass ratio, thereby enabling the corresponding adhesive to have more suitable mechanical strength and flexibility, so as to better improve the problem of volume expansion of silicon-based materials.

[0047] As an example, the molecular weight of the waterborne polyurethane adhesive is 20,000 to 100,000, such as, but not limited to, any point value or range between any two of the molecular weights of 20,000, 40,000, 60,000, 80,000 and 100,000.

[0048] In this embodiment, the molecular weight of the waterborne polyurethane adhesive is limited to the above-mentioned range. When the mass percentage of the waterborne polyurethane adhesive is constant, the composite adhesive can have better flexibility.

[0049] As an example, the three-dimensional network polymer binder includes a first polymer binder and a second polymer binder, wherein the first polymer binder includes at least one of polyacrylic acid, polymethacrylic acid and polyglutamic acid, and the second polymer binder includes at least one of carboxymethyl cellulose, polyvinyl alcohol and sodium alginate.

[0050] In this embodiment, the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups are selected from the above-mentioned types, so that the corresponding three-dimensional network polymer binder with ester groups has higher adhesion, better mechanical strength and deformation resistance. At the same time, both polymer binders are applicable to a wide variety of types, which can provide more feasible implementation schemes, thereby facilitating the promotion and application of the technical solutions provided in the embodiments of this application.

[0051] As an example, the first polymeric binder is selected from polyacrylic acid, and the molecular weight of the first polymeric binder is from 100,000 to 300,000, for example, but not limited to any one of the molecular weights of 100,000, 150,000, 200,000, 250,000 and 300,000 or any range between the two; the second polymeric binder is selected from carboxymethyl cellulose, and the molecular weight of the second polymeric binder is from 400,000 to 650,000, for example, but not limited to any one of the molecular weights of 400,000, 450,000, 500,000, 550,000, 600,000 and 650,000 or any range between the two.

[0052] In this embodiment, the first polymer binder having carboxyl groups and the second polymer binder having hydroxyl groups are selected from the above-mentioned types and their molecular weights are limited to the above-mentioned ranges, so that the corresponding three-dimensional network polymer binder having ester groups has higher adhesion, better mechanical strength and deformation resistance.

[0053] As an example, the mass ratio of the first polymeric adhesive to the second polymeric adhesive is (0.8~1.2):1, for example, but not limited to any one of the mass ratios of 0.8:1, 0.9:1, 1:1, 1.1:1 and 1.2:1 or any range between the two.

[0054] In this embodiment, the mass ratio of the two polymer binders is limited to the aforementioned range so that the corresponding ester-based three-dimensional network polymer binder possesses higher adhesion, superior mechanical strength, and resistance to deformation. As an example, the mass ratio of the negative electrode active material to the binder is (90~95):(4~6), for example, but not limited to, any one of the mass ratios of 90:4, 90:5, 90:6, 95:4, 95:5, and 95:6, or any range between the two.

[0055] In this embodiment, the mass ratio of the negative electrode active material and the binder is limited to the above-mentioned range so that the two have a more suitable mass ratio, which can effectively improve the problem of volume expansion of silicon-based materials.

[0056] As an example, the mass ratio of the negative electrode active material to the conductive agent is (90~95):(1~2), for example, but not limited to any one of the mass ratios of 90:1, 90:1.5, 90:2, 95:1, 95:1.5 and 95:2 or any range between the two.

[0057] In this embodiment, the mass ratio of the negative electrode active material and the conductive agent is limited to the above-mentioned range so that the two have a more suitable mass ratio, so that the corresponding negative electrode sheet has better conductivity and capacity.

[0058] It should be noted that functional components in the negative electrode that are not specifically described or limited can be selected and set in accordance with conventional methods in this field.

[0059] As an example, the silicon-based material is selected from at least one of silicon suboxide, silicon nanoparticles, silicon nanowires, and silicon-carbon composites.

[0060] As an example, the conductive agent is selected from at least one of acetylene black, graphene, carbon nanotubes, and conductive carbon fibers.

[0061] As an example, the negative electrode active material also includes graphite material, wherein the mass ratio of silicon-based material to graphite material is (15~25):(65~75).

[0062] As an example, the negative current collector is made of copper foil.

[0063] Secondly, embodiments of this application provide a method for preparing a negative electrode sheet as provided in the first aspect embodiment, comprising the following steps: mixing a negative electrode active material, a first polymer binder having carboxyl groups, a second polymer binder having hydroxyl groups, a linear aqueous polyurethane binder, a conductive agent, and water, wherein the negative electrode active material includes a silicon-based material, to obtain a negative electrode slurry; applying the negative electrode slurry to the surface of a negative electrode current collector to obtain a negative electrode sheet intermediate; and then heat-treating the negative electrode sheet intermediate under vacuum conditions to cause the first polymer binder and the second polymer binder to crosslink in situ to form a three-dimensional network polymer binder, and simultaneously causing the aqueous polyurethane binder to interpenetrate in the formed three-dimensional network polymer binder to obtain a negative electrode sheet.

[0064] In this application, the negative electrode is prepared according to the above process, so that the prepared negative electrode includes a composite binder with a semi-interpenetrating network structure formed by a three-dimensional network polymer binder with ester groups and a linear aqueous polyurethane binder. The composite binder has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of silicon-based materials, thereby enabling the battery corresponding to the negative electrode to have high capacity, high first efficiency and long cycle performance.

[0065] As an example, in the negative electrode slurry, the ratio of the mass of the negative electrode active material to the sum of the masses of the three binders is (90~95):(4~6), for example, but not limited to any one of the mass ratios of 90:4, 90:5, 90:6, 95:4, 95:5 and 95:6 or any range between the two.

[0066] As an example, in the negative electrode slurry, the mass ratio of the waterborne polyurethane binder to the sum of the masses of the other two polymer binders is (1.5~2.5):3, for example, but not limited to any one of the mass ratios of 1.5:3, 1.6:3, 1.7:3, 1.8:3, 1.9:3, 2.0:3, 2.1:3, 2.2:3, 2.3:3, 2.4:3 and 2.5:3 or any range between the two.

[0067] As an example, in the negative electrode slurry, the mass ratio of the first polymer binder and the second polymer binder is (0.8~1.2):1, for example, but not limited to any one of the mass ratios of 0.8:1, 0.9:1, 1:1, 1.1:1 and 1.2:1 or any range between the two.

[0068] In this embodiment, limiting the mass ratio of the first polymer binder and the second polymer binder to the above-mentioned range enables the first polymer binder having carboxyl groups and the second polymer binder having hydroxyl groups to be crosslinked more fully and thoroughly to form a three-dimensional network polymer binder having ester groups.

[0069] As an example, in the negative electrode slurry, the mass ratio of the negative electrode active material to the conductive agent is (90~95):(1~2), for example, but not limited to any one of the mass ratios of 90:1, 90:1.5, 90:2, 95:1, 95:1.5 and 95:2 or any range between the two.

[0070] As an example, the negative electrode active material also includes graphite material, wherein the mass ratio of silicon-based material to graphite material is (15~25):(65~75).

[0071] As an example, prior to the heat treatment step, a drying process is included for the negative electrode intermediate, wherein the drying temperature is lower than the heat treatment temperature, in order to remove moisture from the negative electrode intermediate.

[0072] In this embodiment, a low-temperature drying process is performed before the high-temperature heat treatment. The low-temperature drying process allows the moisture to be removed slowly, so as to finally prepare a high-quality negative electrode sheet with a denser structure, higher surface flatness and fewer cracks. At the same time, after the moisture is removed, the two polymer binders are esterified and crosslinked, so that the resulting three-dimensional network polymer binder with ester groups has higher adhesion, better mechanical strength and deformation resistance.

[0073] As an example, during the drying process, the processing temperature is 50°C to 80°C, for example, but not limited to any one of 50°C, 60°C, 70°C and 80°C or any range between two of them.

[0074] In this embodiment, by limiting the drying temperature within the aforementioned range, the moisture in the negative electrode intermediate can be removed more thoroughly under relatively mild conditions.

[0075] As an example, during the heat treatment process, the treatment temperature is 110℃~140℃, for example, but not limited to any one of 110℃, 120℃, 130℃ and 140℃ or any range between two of them; the treatment time is 1 h~5 h, for example, but not limited to any one of 1 h, 2 h, 3 h, 4 h and 5 h or any range between two of them.

[0076] In this embodiment, limiting the temperature and time of the heat treatment to the above-mentioned ranges enables the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups to crosslink more fully and thoroughly to form a three-dimensional network polymer binder with ester groups. At the same time, it can also effectively protect the remaining functional components in the negative electrode intermediate.

[0077] As an example, the first polymer binder having a carboxyl group includes at least one of polyacrylic acid, polymethacrylic acid, and polyglutamic acid, and the second polymer binder having a hydroxyl group includes at least one of carboxymethyl cellulose, polyvinyl alcohol, and sodium alginate.

[0078] As an example, the first polymer binder is selected from polyacrylic acid, the second polymer binder is selected from carboxymethyl cellulose, and the mixing process includes: dissolving carboxymethyl cellulose in water to obtain a carboxymethyl cellulose solution; adding polyacrylic acid, waterborne polyurethane binder and conductive agent to the carboxymethyl cellulose solution and mixing to obtain a slurry intermediate; adding negative electrode active material to the slurry intermediate and mixing to obtain a negative electrode slurry.

[0079] In this embodiment, carboxymethyl cellulose, which has poor water solubility, is dissolved first, and then the remaining components are added to the carboxymethyl cellulose solution and mixed. This allows each binder component to dissolve efficiently and completely in water, while also improving the mixing uniformity of each raw material component. This results in a high-quality negative electrode sheet with a denser structure, higher surface smoothness, and fewer cracks. Furthermore, it allows the first polymer binder with carboxyl groups and the second polymer binder with hydroxyl groups to crosslink more fully and completely to form a three-dimensional network polymer binder with ester groups.

[0080] As an example, the steps of dissolving carboxymethyl cellulose in water to obtain a carboxymethyl cellulose solution include: dissolving carboxymethyl cellulose in water and stirring at 500 rpm to 2000 rpm for 3 h to 6 h to obtain a carboxymethyl cellulose solution with a mass concentration of 3 wt% to 6 wt%.

[0081] As an example, in the step of adding the negative electrode active material to the slurry intermediate and mixing it, the negative electrode active material is added in batches.

[0082] In this embodiment, the negative electrode active material accounts for a relatively high mass proportion in the negative electrode slurry. By adding it in batches, the dispersion uniformity of the negative electrode active material in the slurry can be improved, thereby giving the prepared negative electrode sheet the advantage of a more uniform distribution of the negative electrode active material.

[0083] It should be noted that there is no limit to the number of times you can add the ingredients in batches; for example, you can add them 2, 3, 4, or 5 times.

[0084] It should be noted that for any processes or steps in the preparation of the negative electrode sheet that are not specifically described or limited, they can be carried out in accordance with conventional processes in this field.

[0085] As an example, a process flow diagram of the negative electrode preparation method is exemplarily provided. Figure 1 .

[0086] Thirdly, embodiments of this application provide a battery including the negative electrode sheet as provided in the first aspect embodiment.

[0087] In this application, the battery includes a negative electrode sheet as provided in the first aspect embodiment. Since the negative electrode sheet includes a composite binder with a semi-interpenetrating network structure formed by a three-dimensional network polymer binder with ester groups and a linear aqueous polyurethane binder, the composite binder has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of silicon-based materials, thereby enabling the corresponding battery to have high capacity, high initial efficiency and long cycle performance.

[0088] It should be noted that functional components in the battery that are not specifically described or limited can be selected and configured in accordance with the conventional methods in this field.

[0089] Fourthly, this application provides an electrical device, which includes the battery provided in the third aspect embodiment. The type of electrical device is not limited, and it can be, for example, a vehicle, an aircraft, a robot, a computer, a mobile phone, or other products.

[0090] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0091] Example 1 This application provides a method for preparing a negative electrode sheet, including the following steps: Raw material preparation: By mass percentage, negative electrode active material: silicon-carbon material (model: Si@C) 20.4%, graphite material 73%; binder: carboxymethyl cellulose (CMC) with a molecular weight of 500,000 1.5%, linear waterborne polyurethane (carboxylated waterborne polyurethane) with a molecular weight of 50,000 2%, polyacrylic acid (PAA) with a molecular weight of 200,000 1.5%; conductive agent: acetylene black 1.5%, carbon nanotubes 0.1%; deionized water as needed.

[0092] CMC was added to deionized water and stirred at 1000 rpm for 3 h to obtain a CMC solution with a mass concentration of 5%. Then, PAA, WPU, acetylene black and carbon nanotubes were added to the CMC solution under stirring to obtain a slurry intermediate. Then, the negative electrode active material was added to the slurry intermediate in two batches under stirring to obtain the negative electrode slurry.

[0093] The negative electrode slurry was coated on both sides of the copper foil to obtain the negative electrode intermediate. Then, the negative electrode intermediate was dried at 60°C for 2 h to remove the moisture. The dried negative electrode intermediate was then rolled to a thickness of 110 μm. Finally, the rolled negative electrode intermediate was heat-treated in a vacuum chamber at 130°C for 3 h to obtain the negative electrode sheet.

[0094] Example 2 This application provides a method for preparing a negative electrode sheet, which differs from Example 1 only in that the mass ratio of WPU is 1.5%.

[0095] Example 3 This application provides a method for preparing a negative electrode sheet, which differs from Example 1 only in that the mass percentage of WPU is 2.5%.

[0096] Example 4 This application provides a method for preparing a negative electrode sheet, which differs from Example 1 only in that the mass percentage of WPU is 1.0%.

[0097] Example 5 This application provides a method for preparing a negative electrode sheet, which differs from Example 1 only in that the mass percentage of WPU is 3.0%.

[0098] Comparative Example 1 This application provides a comparative method for preparing a negative electrode sheet, which differs from Example 1 only in that: no heat treatment is performed, that is, CMC and PAA are not subjected to esterification crosslinking reaction.

[0099] Comparative Example 2 This application provides a comparative method for preparing a negative electrode sheet, which differs from Example 1 only in that it does not contain WPU.

[0100] Comparative Example 3 This application provides a comparative example of a method for preparing a negative electrode sheet, which differs from Example 1 only in that WPU is replaced with styrene-butadiene rubber (SBR).

[0101] Comparative Example 4 This application provides a comparative example of a method for preparing a negative electrode sheet, which differs from Example 1 only in that WPU is replaced with sodium alginate (SA).

[0102] Comparative Example 5 This application provides a comparative example of a method for preparing a negative electrode sheet, which differs from Example 1 only in that WPU is replaced with polyvinyl alcohol (PVA).

[0103] Test case Electrical performance testing Test method: The negative electrode sheets of Examples 1-5 and Comparative Examples 1-5 were assembled into lithium-ion batteries, and the initial discharge capacity, initial efficiency and cycle performance of each battery were tested. The test results are summarized in Table 1.

[0104] The battery assembly steps are as follows: LiNi with a mass ratio of 96.8:2.0:1.2 0.8 Co 0.1 Mn 0.1 O2 (positive electrode active material), conductive carbon black (conductive agent) and polyvinylidene fluoride (binder) are dispersed in N-methyl-2-pyrrolidone to obtain a positive electrode slurry; then, the positive electrode slurry is uniformly coated on both sides of an aluminum foil and then dried and calendered to obtain a positive electrode sheet with a thickness of 125 μm.

[0105] The prepared negative electrode, separator (a ceramic separator with a thickness of 14 μm) and positive electrode are stacked in sequence and wound to prepare a bare cell. Then, the bare cell is injected with electrolyte (the concentration of lithium salt (LiPF6) in the electrolyte is 1 mol / L, and the organic solvent is EC, DMC, EMC and FEC with a volume ratio of 1:1:1:0.1). Then, it is packaged, left to stand, formed, shaped and capacity tested in sequence to obtain a lithium secondary battery.

[0106] The test procedure for initial discharge capacity is as follows: At 25°C, the battery is charged at a constant current of 0.33C to 4.25V, then charged at a constant voltage until the current is less than or equal to 0.05C. After that, it is left to stand for 5 minutes, and then discharged at a constant current of 0.33C to 2.8V. After that, it is left to stand for 5 minutes. This is one charge-discharge cycle. The discharge capacity of this cycle is recorded as the discharge capacity of the battery in the first cycle. Wherein, discharge capacity = discharge capacity / mass of active material.

[0107] The steps for testing the first-effect response are as follows: At 25°C, the battery is charged at a constant current of 0.33C to 4.25V, then charged at a constant voltage until the current is less than or equal to 0.05C. After resting for 5 minutes, it is discharged at a constant current of 0.33C to 2.8V, and then rested for 5 minutes. This is one charge-discharge cycle. The charge-discharge capacity of this cycle is recorded as the first charge-discharge capacity. The first efficiency (first coulombic efficiency) = first discharge capacity / first charge capacity × 100%.

[0108] The test steps for cycle performance are as follows: At 25℃, the battery is charged at a constant current of 1C to 4.25V, then charged at a constant voltage until the current is less than or equal to 0.05C. After that, it is left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.8V. After that, it is left to stand for 5 minutes. This is one charge-discharge cycle. The battery cells are subjected to multiple charge-discharge cycles in the above manner. The discharge capacity of the 1000th cycle is measured, and the capacity retention rate of the battery cells after the cycle is calculated by the following formula: Capacity retention rate (%) after 1000 cycles = (Discharge capacity of the 1000th cycle / Discharge capacity of the 1st cycle) × 100%.

[0109] Table 1

[0110] Referring to Table 1, the test results of Examples 1-5 and Comparative Examples 1-5 show that the negative electrode sheet includes a composite binder with a semi-interpenetrating network structure formed by a three-dimensional network polymer binder with ester groups and a linear aqueous polyurethane binder. This composite binder has high adhesion, excellent mechanical strength and flexibility, which can effectively improve the problem of volume expansion of silicon-based materials, thereby enabling the corresponding battery to have high capacity, high initial efficiency and long cycle performance.

[0111] As can be seen from the test results of Examples 1 to 5, when the ratio of the mass of waterborne polyurethane to the sum of the masses of the other two binders is limited to the range of (1.5 to 2.5):3, the batteries corresponding to the former have better capacity, first-time efficiency and cycle performance compared to those outside this range.

[0112] As can be seen from the test results of Example 1 and Comparative Examples 3-5, when WPU is replaced with other binders, the batteries corresponding to the former have better capacity, first efficiency and cycle performance.

[0113] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent. The binder includes an aqueous polyurethane binder and a three-dimensional network polymer binder with ester groups.

2. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material includes a silicon-based material, and the waterborne polyurethane binder is a linear waterborne polyurethane binder, which is interspersed within the three-dimensional network polymer binder.

3. The negative electrode sheet according to claim 2, characterized in that, The mass ratio of the waterborne polyurethane adhesive to the three-dimensional network polymer adhesive is (1.5~2.5):3, or / and, and the molecular weight of the waterborne polyurethane adhesive is 20,000~100,000.

4. The negative electrode sheet according to claim 2, characterized in that, The three-dimensional network polymer binder includes a first polymer binder and a second polymer binder. The first polymer binder includes at least one of polyacrylic acid, polymethacrylic acid, and polyglutamic acid, and the second polymer binder includes at least one of carboxymethyl cellulose, polyvinyl alcohol, and sodium alginate.

5. The negative electrode sheet according to claim 4, characterized in that, The first polymer binder is selected from the polyacrylic acid, and the molecular weight of the first polymer binder is 100,000 to 300,000; the second polymer binder is selected from the carboxymethyl cellulose, and the molecular weight of the second polymer binder is 400,000 to 650,000.

6. The negative electrode sheet according to claim 4, characterized in that, The mass ratio of the first polymer binder to the second polymer binder is (0.8~1.2):

1.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The mass ratio of the negative electrode active material to the binder is (90~95):(4~6), or / and the mass ratio of the negative electrode active material to the conductive agent is (90~95):(1~2).

8. A method for preparing a negative electrode sheet as described in any one of claims 1 to 7, characterized in that, Includes the following steps: A negative electrode active material, a first polymer binder having carboxyl groups, a second polymer binder having hydroxyl groups, a linear aqueous polyurethane binder, a conductive agent, and water are mixed to obtain a negative electrode slurry. The negative electrode slurry is applied to the surface of the negative electrode current collector to obtain a negative electrode intermediate; then the negative electrode intermediate is heat-treated under vacuum conditions to allow the first polymer binder and the second polymer binder to crosslink in situ to form the three-dimensional network polymer binder, and simultaneously the waterborne polyurethane binder is interwoven in the formed three-dimensional network polymer binder to obtain the negative electrode.

9. The preparation method according to claim 8, characterized in that, Before the heat treatment step, the negative electrode intermediate is further subjected to a drying treatment, wherein the temperature of the drying treatment is lower than the temperature of the heat treatment, so as to remove moisture from the negative electrode intermediate.

10. The preparation method according to claim 9, characterized in that, During the drying process, the processing temperature is 50℃~80℃; during the heat treatment process, the processing temperature is 110℃~140℃.

11. The preparation method according to any one of claims 8 to 10, characterized in that, The first polymer binder is selected from polyacrylic acid, the second polymer binder is selected from carboxymethyl cellulose, and the mixing process includes: The carboxymethyl cellulose was dissolved in water to obtain a carboxymethyl cellulose solution; The polyacrylic acid, the waterborne polyurethane adhesive, and the conductive agent are added to the carboxymethyl cellulose solution and mixed to obtain a slurry intermediate. The negative electrode active material is added to the slurry intermediate and mixed to obtain the negative electrode slurry.

12. A battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1 to 7.

13. An electrical appliance, characterized in that, Includes the battery as described in claim 12.

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