Method for manufacturing an electrode sheet, energy storage battery

Abstract

The application provides a preparation method of an electrode sheet and an energy storage battery. The preparation method comprises the following steps: S1. coating a plurality of layers of electrode slurry on at least one side of a current collector in sequence, and the content of a binder in each layer of electrode slurry gradually decreases from the side close to the current collector to the side far from the current collector; and S2. baking the wet electrode sheet obtained in S1. At least three baking procedures are set, and the wet electrode sheet is baked at gradually decreasing temperatures and gradually decreasing air speeds in the process of sequentially performing each baking procedure. After cooling and rolling, the electrode sheet is obtained. The preparation method effectively improves the problems of the low content of the binder close to the current collector and the uneven distribution of the binder in the vertical direction caused by the problems such as the upward floating of the binder due to capillary force and the poor baking process, optimizes the stability of the electrode sheet, and further optimizes the performance of the energy storage battery, especially the high-rate performance.

Description

Technical Field

[0001] This application belongs to the field of lithium-ion battery technology, specifically relating to a method for preparing an electrode and an energy storage battery. Background Technology

[0002] With the continuous advancement of technology, the energy density requirements for battery products have long surpassed the level of mass production in workshops. Currently, high energy density designs for lithium-ion batteries can be achieved by increasing the thickness of the electrode sheets. However, conventional lithium batteries typically use aqueous graphite anode sheets (or other carbon-based active materials). Increasing the coating thickness without changing the traditional anode slurry formulation, reducing the peel force between the coating and the current collector, or increasing the electrode resistance is very difficult.

[0003] Patent application number 202410429626.8 discloses a method for improving the peel strength of adhesives in aqueous lithium-ion battery anodes. This method incorporates triazole compounds as additives into the adhesive, without altering the main structure of the adhesive. It exists solely as an additive, requiring only a small dosage, yet significantly improves peel strength, making it an effective method for enhancing the peel force between the coating and the current collector. However, this method is only applicable to electrodes with a coating layer thickness of 30–60 μm after rolling. It is not suitable for electrodes with a thickness exceeding 70 μm. Furthermore, adding insulating organic compounds to traditional anode slurry formulations inevitably increases electrode resistance, affecting the battery's electrochemical performance.

[0004] Patent application number 202410254968.0 discloses a strongly adhesive sodium-ion battery negative electrode sheet. By optimizing the slurry formulation, controlling the coating density, controlling the oven temperature, and using aluminum foil as the current collector, it solves the problems of poor electrode adhesion, stress shrinkage and material leakage in aluminum foil coating, and inability to weld tabs to the negative electrode, which are common issues in current sodium battery negative electrode applications using aluminum foil. Importantly, it improves the peel strength of the negative electrode sheet by 2-3 times, stabilizes the manufacturing process, and effectively reduces production costs by replacing copper foil, thus improving the low peel strength problem. However, this negative electrode sheet is designed for sodium battery manufacturing processes, which differ from the slurry used in conventional lithium battery negative electrode sheets. Furthermore, the current collector is limited to aluminum foil and is not suitable for conventional lithium battery copper foil current collector negative electrode sheets. Summary of the Invention

[0005] To address the problems and shortcomings in the existing technology, this application provides a method for preparing an electrode sheet and an energy storage battery. In this electrode sheet preparation method, by specifically controlling the binder content in each layer of electrode slurry coated on the wet electrode blank, and by baking the wet electrode blank under specific process conditions, the problems of low binder content and uneven vertical distribution of binder on the side near the current collector due to capillary force floating and poor baking process are effectively improved. Therefore, the bonding strength between the active material layer and the current collector is effectively improved, the problem of powder shedding and detachment of the active material layer is reduced, the stability of the electrode sheet is optimized, and the performance of the energy storage battery, especially the high-rate performance, is optimized.

[0006] According to the first aspect of this application, a method for preparing an electrode sheet is provided, characterized by comprising the following steps: S1. Coating a plurality of layers of electrode paste sequentially on at least one side of a current collector, wherein the content of binder in each layer of electrode paste decreases progressively from the side closer to the current collector to the side farther away from the current collector; S2. Then baking the wet electrode blank obtained in S1, setting at least three baking programs, and baking the wet electrode blank at a gradually decreasing temperature and a gradually decreasing air velocity during each baking program; then cooling and rolling to obtain an electrode sheet.

[0007] In battery manufacturing, electrode fabrication is the most crucial step. Electrode fabrication begins with mixing all materials for the slurry according to the formula, resulting in a uniformly distributed slurry. Then, using a coating machine, the slurry is evenly coated onto the current collector according to the specified coating density. The electrode is then placed in a high-temperature oven, where almost all the solvent evaporates from the active material layer. Finally, the electrode is rolled and shaped to make the active material in the active material layer more compact. Throughout the entire electrode fabrication process, besides the binder content in the formula affecting its distribution in the active material layer during slurry preparation, the baking process during coating also has an impact. Figure 1 As shown, before and after electrode baking, with the evaporation of the solvent, the binder, such as SBR, floats to the surface due to capillary forces, resulting in a reduction in the SBR content of the lower layer. Secondly, rolling cannot change the result of a higher binder content on the upper layer and a lower content on the lower layer, which inevitably reduces the peel force between the active material layer and the current collector. Moreover, as the thickness of the active material layer increases, the binder floating phenomenon becomes more pronounced, the lower layer of binder distribution becomes sparser, and the peel force is even lower. This is especially true for the fabrication of thick electrodes, where the binder floating phenomenon is more significant, making the fabrication of thick electrodes more difficult.

[0008] To address the aforementioned issues, this application firstly provides an electrode paste coated with at least two layers on at least one side of the current collector, and controls the binder content to decrease layer by layer in each electrode paste layer coated from the side closest to the current collector to the side furthest away from the current collector. Since the electrode paste closest to the current collector is in direct contact with it, the binder content in this layer directly affects the peel strength between the active material layer formed after the electrode paste dries and the current collector. Furthermore, the closer the binder is to the current collector interface, the more likely it is to migrate and be lost during subsequent baking. This application utilizes this specific design—increasing the binder content in the electrode paste closest to the current collector—to mitigate the decrease in peel strength between the current collector and the coating area caused by binder migration later on. Therefore, this specific design ensures that the binder is more evenly distributed in the vertical direction of the electrode plane after the wet blank is dried. This reduces the problem of "more binder at the top and less at the bottom" in the active material layer, optimizes the uniform distribution of binder in the electrode, and improves the bonding strength between the active material layer and the current collector. This, in turn, optimizes the overall stability of the electrode and the performance of the battery, such as rate performance.

[0009] Secondly, this application incorporates at least three baking processes during the drying of the wet electrode blank, baking the wet electrode blank with progressively decreasing temperatures and air velocities. During the drying process, the binder applied to the current collector migrates from the side closest to the current collector to the other due to solvent loss and capillary effects; however, the migration rate is influenced by the baking temperature and air velocity. This application, by setting a baking process with progressively decreasing temperatures and air velocities, can significantly reduce binder migration and minimize uneven binder distribution after drying. Specifically, initially, the highest temperature and air velocity dry the surface of the wet material area, allowing for rapid film formation and curing. Subsequently, the temperature and air velocity are reduced to further dry the middle layer of the wet material area, gradually slowing the drying speed and removing most of the internal solvent. Then, the temperature and air velocity are further reduced to further dry the bottom layer and the incompletely dried middle layer of the wet material area, slowly removing the solvent near the current collector and other residual solvents. This multi-baking process design not only facilitates the complete removal of solvent but also promotes a lower migration rate of the binder, improving the uniform distribution of the binder in the electrode. At the same time, it prevents other materials in the active material layer formed by the electrode slurry from maintaining a good uniform distribution in the electrode, resulting in a high bonding strength between the active material layer and the current collector without affecting the conductivity and other properties of the electrode. This further optimizes the performance of the electrode and the battery.

[0010] Preferably, the adhesive includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

[0011] Preferably, the adhesive comprises styrene-butadiene rubber.

[0012] Preferably, the thickness of each layer of electrode slurry gradually increases from the side closest to the current collector to the side furthest from the current collector. This thickness design is more conducive to the uniform distribution of the binder in the overall active material layer formed by all electrode slurries, and also to the uniform distribution of other materials in the active material layer, further optimizing the overall performance of the electrode.

[0013] Preferably, the length of the oven gradually increases in each sequential baking process. Initially, the baking temperature and air velocity are higher, and the oven length is kept short to prevent the binder from rising too quickly, which would affect the uniform distribution of the binder. Subsequently, the oven length increases step by step, while the baking temperature and air velocity in later baking processes gradually decrease. This facilitates the drying of the lower and middle layers of wet material, slowly drying out the solvent in the lower and middle layers, while minimizing the rise of the binder and the migration of other materials, which is beneficial for the uniform distribution of the binder and other materials in the electrode.

[0014] Preferably, in each sequential baking process, a baking process with vertical airflow is performed first, followed by a baking process with horizontal airflow. Preferably, there are at least two baking processes with vertical airflow and at least one baking process with horizontal airflow. Airflow direction also has a certain impact on the migration of the binder. Using vertical airflow first facilitates rapid film formation on the surface; continuing with vertical airflow helps remove most of the internal solvent; and further using horizontal airflow helps remove the solvent from the middle and lower layers, even those closest to the current collector interface. This design also helps control the binder migration rate and direction, optimizing its uniform distribution within the electrode.

[0015] Preferably, the coated electrode paste consists of two or three layers. When the coated electrode paste consists of two layers, the layers are a first electrode paste and a second electrode paste, arranged sequentially from the side closest to the current collector to the side furthest away from the current collector. The first electrode paste includes a first binder, the content of which is 'a'. The second electrode paste includes a second binder, the content of which is (1 / 3 to 1)a. When the coated electrode paste consists of three layers, the layers are a first electrode paste, a second electrode paste, and a third electrode paste, arranged sequentially from the side closest to the current collector to the side furthest away from the current collector. The first electrode paste includes a first binder, the content of which is 'a'. The second electrode paste includes a second binder, the content of which is (1 / 2 to 1)a. The third electrode paste includes a third binder, the content of which is (1 / 3 to 1)a. Wherein, 'a' = 0.5 to 3 wt%. In particular, for two or three layers, controlling the content of binder in each layer within the above range is more conducive to obtaining an electrode sheet with uniform binder distribution. Meanwhile, excessively high binder content can increase the brittleness of the electrode and reduce its electrical properties. Therefore, controlling the binder content within the above range is beneficial for obtaining electrodes with good mechanical and electrical properties.

[0016] Preferably, when the coated electrode paste consists of two layers, the coating thickness of the first electrode paste is b, and the coating thickness of the second electrode paste is (1-3)b; when the coated electrode paste consists of three layers, the coating thickness of the first electrode paste is b, the coating thickness of the second electrode paste is (1-2)b, and the coating thickness of the third electrode paste is (1-3)b; wherein b = 5-50 μm. The thickness of each coated electrode paste layer and the thickness relationship satisfying the above range are more conducive to the uniform distribution of binder and other materials in the electrode sheet, while also ensuring that the electrode sheet as a whole possesses good electrical properties and other performance characteristics. Furthermore, the coating thickness here refers to the thickness of the paste before rolling after drying.

[0017] It should be noted that four or more layers of coating can also be applied following the same pattern. However, this would increase the complexity of the process, most notably by requiring higher precision in controlling the thickness of each layer, and increasing the risk of uneven coating density.

[0018] Preferably, the baking process of the electrode wet blank includes a first baking program, a second baking program, and a third baking program set sequentially; in the first baking program, the baking temperature is 90-120℃ and the wind speed is 1-10m / s; in the second baking program, the baking temperature is 80-110℃ and the wind speed is 0.5-5m / s; in the third baking program, the baking temperature is 70-100℃ and the wind speed is 0.1-3m / s.

[0019] Preferably, in the first baking process, the airflow direction is vertical; in the second baking process, the airflow direction is vertical; and in the third baking process, the airflow direction is horizontal; wherein, vertical direction represents the direction perpendicular to the electrode plane; and horizontal direction represents the direction horizontal to the electrode plane.

[0020] Preferably, in the first baking process, the oven length is 1-5m and the moving speed of the wet electrode blank is 2-7m / min; and / or, in the second baking process, the oven length is 2-8m and the moving speed of the wet electrode blank is 2-7m / min; and / or, in the third baking process, the oven length is 5-30m and the moving speed of the wet electrode blank is 2-12m / min.

[0021] Preferably, the moving speed of the electrode wet blank is consistent in each baking process.

[0022] Preferably, the total thickness of the electrode paste coated on one side is 70–170 μm; after rolling, the total thickness of the electrode sheet is 150–220 μm.

[0023] Preferably, the thickness of the current collector is 3–12 μm.

[0024] Preferably, the electrode slurry includes at least one of a positive electrode slurry and a negative electrode slurry.

[0025] Preferably, the electrode slurry is a negative electrode slurry; the negative electrode slurry further includes a negative electrode active material, a conductive agent, and a dispersant; the negative electrode active material includes at least one of natural graphite, artificial graphite, and composite graphite; the conductive agent includes at least one of acetylene black, carbon fiber, conductive carbon black, Ketjen black, graphene, and carbon nanotubes; the dispersant includes at least one of methylcellulose, carboxymethylcellulose, sodium carboxymethylcellulose, and N-methylpyrrolidone; and the binder includes styrene-butadiene rubber.

[0026] Preferably, the mass ratio of the negative electrode active material, conductive agent, dispersant, and binder is 85-99.7:0.1-5:0.1-7:0.1-3.

[0027] Preferably, the styrene-butadiene rubber is derived from styrene-butadiene rubber latex, and the solid content of the styrene-butadiene rubber latex is 10-100%.

[0028] According to another aspect of this application, an energy storage battery is provided, comprising an electrode prepared by the above-described electrode preparation method, wherein the electrode is a negative electrode. The energy storage battery obtained by this invention has high capacity, low resistivity, and good rate performance. Attached Figure Description

[0029] Figure 1 This is a schematic diagram showing the floating of binders such as SBR during the electrode fabrication process. Detailed Implementation

[0030] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments.

[0031] Example 1

[0032] The electrode prepared in this embodiment is a negative electrode. During the preparation of the negative electrode, a first slurry and a second slurry are sequentially coated on both sides of the current collector (copper foil, 6 μm thick), from the side closer to the current collector to the side farther away from the current collector. The specific preparation steps are as follows:

[0033] S1. Prepare the first slurry and the second slurry respectively; in the first slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:0.9:1.8, that is, the mass percentage of styrene-butadiene rubber as the binder in the first slurry is 1.8 wt%; in the second slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:1.5:1.2, that is, the mass percentage of styrene-butadiene rubber as the binder in the second slurry is 1.2 wt%;

[0034] S2. Coat the first slurry and the second slurry sequentially on both sides of the current collector to obtain a wet electrode blank;

[0035] S3. Bake the electrode blank in the first baking program, the second baking program and the third baking program in sequence. After drying, cool and roll press to obtain the electrode.

[0036] In the process of coating the first slurry and the second slurry, the coating thickness is controlled so that the coating thickness of the first slurry and the second slurry are 41μm and 87μm respectively (here the coating thickness of the slurry refers to the thickness of the slurry before rolling after drying), and the thickness of the first active material layer (corresponding to the first slurry) and the second active material layer (corresponding to the second slurry) on both sides of the current collector in the final rolled electrode are 24μm and 51μm respectively.

[0037] The relevant process parameters for the first baking procedure, the second baking procedure, and the third baking procedure, as well as the relevant parameters for the negative electrode preparation process in this embodiment, are shown in Table 1 below:

[0038] Table 1 shows the relevant process parameters for the first, second, and third baking processes, as well as the relevant parameters for the negative electrode preparation process in this embodiment.

[0039]

[0040] Example 2

[0041] The electrode prepared in this embodiment is a negative electrode. During the preparation of the negative electrode, a first slurry, a second slurry, and a third slurry are sequentially coated on both sides of the current collector (copper foil) from the side closer to the current collector to the side farther away from the current collector. The specific preparation steps are as follows:

[0042] S1. Prepare the first slurry, the second slurry, and the third slurry respectively; in the first slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:0.7:2.0, that is, the mass percentage of styrene-butadiene rubber as the binder in the first slurry is 2.0 wt%; in the second slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:1.2:1.5, that is, the mass percentage of styrene-butadiene rubber as the binder in the second slurry is 1.5 wt%; in the third slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:1.7:1.0, that is, the mass percentage of styrene-butadiene rubber as the binder in the third slurry is 1.0 wt%;

[0043] S2. Coat the two sides of the current collector with the first slurry, the second slurry, and the third slurry in sequence to obtain the electrode wet blank;

[0044] S3. Bake the electrode blank in the first baking program, the second baking program and the third baking program in sequence. After drying, cool and roll press to obtain the electrode.

[0045] In the process of coating the first slurry, the second slurry, and the third slurry, the coating thickness is controlled so that the coating thickness of the first slurry, the second slurry, and the third slurry are 26μm, 55μm, and 78μm, respectively (here, the coating thickness of the slurry refers to the thickness of the slurry before rolling after drying). And in the final rolled electrode, the thicknesses of the first active material layer (corresponding to the first slurry), the second active material layer (corresponding to the third slurry), and the third active material layer (corresponding to the third slurry) on both sides of the current collector are 15μm, 28μm, and 48μm, respectively.

[0046] The relevant process parameters for the first baking procedure, the second baking procedure, and the third baking procedure, as well as the relevant parameters for the negative electrode preparation process in this embodiment, are shown in Table 2 below:

[0047] Table 2 shows the relevant process parameters for the first, second, and third baking processes, as well as the relevant parameters for the negative electrode preparation process in this embodiment.

[0048]

[0049] Example 3

[0050] The difference between this embodiment and Embodiment 2 is that, in the process of preparing the negative electrode sheet, the coating thickness of the first slurry and the second slurry is controlled to be consistent in S2, and the coating thickness of the first slurry, the second slurry and the third slurry are 40μm, 40μm and 79μm respectively, so that the final thickness of the first active material layer, the second active material layer and the third active material layer are 21μm, 21μm and 50μm respectively.

[0051] Example 4

[0052] The difference between this embodiment and Embodiment 1 is that, in the process of preparing the negative electrode sheet, the mass ratio of binder in the first slurry and the second slurry in S1 is controlled to be 1.8 wt% and 0.4 wt%, respectively; that is, in the first slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:0.9:1.8; in the second slurry, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber is 96.7:0.6:2.3:0.4; the rest is the same as in Embodiment 1.

[0053] Example 5

[0054] The difference between this embodiment and Embodiment 1 is that, in the process of preparing the negative electrode sheet, the coating thicknesses of the first slurry and the second slurry in S2 are controlled to be 28 μm and 99 μm, respectively, while the thicknesses of the first active material layer and the second active material layer in S3 are 17 μm and 60 μm, respectively; the rest is the same as in Embodiment 1.

[0055] Example 6

[0056] The difference between this embodiment and Embodiment 1 is that, in the process of preparing the negative electrode sheet, the temperatures of the first baking procedure, the second baking procedure, and the third baking procedure in S3 are 130°C, 100°C, and 90°C, respectively; the rest is the same as in Embodiment 1.

[0057] Example 7

[0058] The difference between this embodiment and Embodiment 1 is that, in the process of preparing the negative electrode sheet, the wind speeds in the first baking process, the second baking process, and the third baking process in S3 are 12 m / s, 5 m / s, and 2 m / s, respectively; the rest is the same as in Embodiment 1.

[0059] Example 8

[0060] The difference between this embodiment and Embodiment 1 is that, during the preparation of the negative electrode sheet, the airflow direction in the second baking process in S3 is adjusted to be horizontal; otherwise, it is the same as Embodiment 1.

[0061] Example 9

[0062] The difference between this embodiment and Embodiment 1 is that, in the process of preparing the negative electrode sheet, the coating thicknesses of the first slurry and the second slurry in S2 are controlled to be 62 μm and 123 μm, respectively, while the thicknesses of the first active material layer and the second active material layer in S3 are 40 μm and 82 μm, respectively; the rest is the same as in Embodiment 1.

[0063] Comparative Example 1

[0064] The difference between this comparative example and Example 1 lies in the fact that, during the preparation of the negative electrode, only one layer of slurry is coated on both sides of the current collector (copper foil). The specific preparation process differs as follows: In S1, the mass ratio of graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber in the slurry is 96.7:0.6:1.2:1.5, meaning that the mass percentage of the styrene-butadiene rubber binder in the slurry is 1.5 wt%; in S2, only the above slurry is coated on both sides of the current collector, with a coating thickness of 128 μm; the baking procedure in S3 is the same as in Example 1; and the final thickness of the active material layer is 75 μm. Everything else is the same as in Example 1.

[0065] Comparative Example 2

[0066] The difference between this comparative example and Example 1 is that, in the preparation process of the negative electrode sheet, the baking procedure in S3 is in the order of the third baking procedure, the second baking procedure, and the first baking procedure as in Example 1. The rest is the same as in Example 1.

[0067] Comparative Example 3

[0068] The difference between this comparative example and Example 1 is that, in the preparation process of the negative electrode sheet, the baking temperatures in the first, second, and third baking processes in S3 are 110°C, 110°C, and 110°C, respectively. The rest is the same as in Example 1.

[0069] Comparative Example 4

[0070] The difference between this comparative example and Example 1 is that, in the preparation process of the negative electrode sheet, the baking wind speeds in the first, second, and third baking processes of S3 are 8 m / s, 8 m / s, and 8 m / s, respectively. The rest is the same as in Example 1.

[0071] Test case

[0072] 1. Experimental Construction Method

[0073] (1) Electrode peeling force test

[0074] The negative electrode sheets prepared in all the above embodiments and comparative examples were tested for peel strength and volume resistivity. The specific test methods are as follows:

[0075] Peel strength test: The coated negative electrode sheet is cut using a custom-made cutter, typically 400mm in length and 10-50mm in width. A flat, thin steel plate, approximately 200-300mm long and 40-60mm wide, is taken. A strip of double-sided tape, longer than the sample test length and the same width as the electrode sheet, is applied to the center of the steel plate. The double-sided tape is smoothed out to ensure it adheres tightly to the center of the steel plate. The double-sided tape is peeled off, and the electrode sheet is attached to the tape, ensuring a perfect fit between the electrode sheet and the tape. The steel plate with the attached electrode sheet is inserted into the lower clamp of the tensile testing machine and fixed vertically. The electrode sheet without tape is inserted into the upper clamp and fixed, so that the electrode sheet attached to the tape is at a 180° angle to the electrode sheet fixed in the upper clamp. A certain tensile or shear force is applied to the tensile testing machine for a period of time to simulate the forces experienced by the battery during long-term use. The testing machine automatically records the applied force and displacement data and calculates relevant parameters such as peel strength.

[0076] Volume resistivity test: The rolled negative electrode sheet is cut using a custom-made cutter, typically 150mm in length and 150mm in width. The thickness of the electrode sheet is measured with a micrometer and entered into the computer. The cut electrode sheet is then placed on the test platform of the volume resistivity tester, and the test is started. The test results are recorded after the test is completed.

[0077] (2) Battery performance test

[0078] Batteries were prepared using the negative electrode sheets obtained in all the above embodiments and comparative examples. In the preparation of the positive electrode sheet, the mass ratio of the positive electrode active material (lithium iron phosphate), conductive agent (CNT, carbon nanotubes), and binder (PVDF, polyvinylidene fluoride) was 97.2:1.1:1.7, the thickness of the positive electrode sheet was 214 μm, and the thickness of the current collector (aluminum foil) was 12 μm. The above-mentioned positive electrode sheet, negative electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (v / v = 1:1:1) electrolyte, PE+alumina separator, and shell were assembled into a soft-pack battery using conventional manufacturing processes.

[0079] The prepared battery was tested for capacity and 3C rate discharge capacity retention. The specific test methods are as follows:

[0080] Both capacity testing and 3C rate discharge testing are performed in a pouch battery charge / discharge testing cabinet. Before being placed in the cabinet, the pouch batteries need to be fitted with custom-made clamps to prevent deformation. After placement, the corresponding test steps are entered into the computer, and the start button is clicked. After the test is completed, the data can be exported for analysis.

[0081] Capacity test steps: 1. Let stand for 5 minutes; 2. Discharge at 0.33C to 2.5V; 3. Let stand for 10 minutes; 4. Charge at 0.33C to 3.65V; 5. Let stand for 10 minutes; 6. Repeat steps 2 to 5 thirteen times; 7. End. Record the discharge capacity of the last three steps in step 2, and take the average value as the test capacity.

[0082] 3C rate discharge capacity retention test steps: 1. Let stand for 5 minutes; 2. Discharge at 0.33C to 2.5V; 3. Let stand for 10 minutes; 4. Charge at 0.33C to 3.65V; 5. Let stand for 10 minutes; 6. Repeat steps 2 to 5 thirteen times (take the average discharge capacity of the last three steps in step 2 as Q1); 7. Discharge at 3.0C to 2.5V (record the discharge capacity as Q2); 8. End. Capacity retention rate = Q2 / Q1.

[0083] 2. Experimental Results

[0084] Table 3 shows the relevant performance of the negative electrode sheets and batteries prepared using the negative electrode sheets in all the above embodiments and comparative examples.

[0085] Table 3 shows the relevant performance data of the negative electrode and battery in the examples and comparative examples.

[0086]

[0087]

[0088] It should be noted that the peel strength test is performed on the coated and dried negative electrode sheet, while the volume resistivity test is performed on the negative electrode sheet after rolling.

[0089] As shown in Table 3, the binder content gradually decreases and the coating thickness gradually increases in the first, second, and third slurries used in double-layer coating, and in triple-layer coating. The electrode baking uses a three-stage oven with gradually decreasing baking temperature and gradually increasing oven length. The airflow direction is first vertical and then horizontal, with gradually decreasing airflow speed. This effectively improves the electrode coating peel force and reduces the electrode volume resistivity. Furthermore, although the capacity difference of pouch batteries made with the same positive electrode material and process is not significant, the capacity retention rate at 3C discharge is significantly increased. See Examples 1-9 for details.

[0090] In Comparative Example 1, coating only one layer of electrode paste will result in uneven distribution of the binder, reduced peeling force of the negative electrode, increased volume resistivity of the negative electrode, and reduced capacity retention rate during 3C discharge.

[0091] The baking order in Comparative Example 2 is the opposite of that in Example 1, which also causes uneven distribution of the binder and is not conducive to the uniform distribution of other components in the slurry. As a result, the peeling force of the negative electrode sheet decreases, the volume resistivity increases, and the capacity retention rate of 3C rate discharge decreases.

[0092] In Comparative Examples 3 and 4, the temperature and air speed of the three baking programs were the same, which was not conducive to the drying of the electrode sheet. Therefore, it was also not conducive to the uniform distribution of the binder and other components, and it would also cause the negative electrode sheet peeling force to decrease, the volume resistivity to increase, and the capacity retention rate of 3C rate discharge to decrease.

[0093] Further comparing Examples 2 and 3, in Example 3, the coating thickness of the first and second slurries was consistent, resulting in a decrease in performance of Example 3 compared to Example 2 in various aspects. This indicates that controlling the coating thickness also has a certain impact on the performance of the negative electrode and the battery. Similarly, comparing Examples 1 and 4, in Example 4, the binder content in the first and second slurries did not conform to a specific relationship (e.g., the binder content in the first slurry was 'a'; in the second slurry, the binder content was 0.2a, i.e., it did not conform to the relationship (1 / 3 to 1)a), which also caused a decrease in the performance of the negative electrode and the battery. This indicates that controlling the binder content in different slurries within a specific range has a certain impact on further optimizing the performance of the negative electrode and the battery. Similarly, comparing Examples 1 and 5, in Example 5, the coating thickness of the first and second slurries did not conform to a specific relationship, which also caused a certain degree of deterioration in the performance of the negative electrode and the battery.

[0094] Comparing Examples 1 and 6, 7, 8 and 9, the temperature of the first baking process in Example 6 was too high, the wind speed of the first baking process in Example 7 was too high, the second baking process in Example 8 was horizontal, and the total thickness of the slurry coating on one side in Example 9 was too large. All of the above factors are not conducive to the uniform distribution of binder and other components of slurry during the drying process, thus causing a decrease in the performance of the negative electrode sheet and the battery.

[0095] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application, but such modifications or substitutions are all within the scope of protection of this application.

Claims

1. A method for preparing an electrode sheet, characterized in that, Includes the following steps: S1. On at least one side of the current collector, a plurality of layers of electrode paste are sequentially coated, and from the side closer to the current collector to the side farther away from the current collector, the content of binder in each layer of the electrode paste decreases layer by layer; S2. Next, the wet electrode blank obtained in S1 is baked. At least three baking programs are set, and the wet electrode blank is baked with a gradually decreasing temperature and a gradually decreasing air speed during each baking program. After cooling and rolling, the electrode is obtained. When the coated electrode paste consists of three layers, the layers are, in order from the side closest to the current collector to the side furthest away from the current collector, a first electrode paste, a second electrode paste, and a third electrode paste; the first electrode paste includes a first binder, and the content of the first binder is a; The second electrode slurry includes a second binder, the content of which is (1 / 2~1)a; The third electrode slurry includes a third binder, the content of which is (1 / 3~1)a; wherein a = 0.5~3wt%; The process of baking the electrode wet blank includes a first baking program, a second baking program, and a third baking program set in sequence. In the first baking program, the baking temperature is 90~120℃ and the wind speed is 1~10m / s; In the second baking process, the baking temperature is 80~110℃ and the air velocity is 0.5~5m / s; In the third baking process, the baking temperature is 70~100℃ and the wind speed is 0.1~3m / s; In the first baking process, the airflow direction is vertical; In the second baking process, the airflow direction is vertical; In the third baking process, the airflow direction is horizontal; Wherein, the vertical direction represents the direction perpendicular to the electrode plane; the horizontal direction represents the direction horizontal to the electrode plane; In each of the sequential baking processes, the length of the oven gradually increases; In the first baking process, the length of the oven is 1~5m, and the moving speed of the wet electrode blank is 2~7m / min; And / or, in the second baking process, the length of the oven is 2~8m, and the moving speed of the electrode wet blank is 2~7m / min; And / or, in the third baking process, the length of the oven is 5~30m, and the moving speed of the wet electrode blank is 2~12m / min.

2. The method for preparing the electrode as described in claim 1, characterized in that: The thickness of each layer of electrode paste gradually increases from the side closest to the current collector to the side furthest from the current collector.

3. The method for preparing the electrode as described in claim 1, characterized in that: When the electrode paste being coated consists of three layers, the coating thickness of the first electrode paste is b, the coating thickness of the second electrode paste is (1~2)b, and the coating thickness of the third electrode paste is (1~3)b. Where b = 5~50μm.

4. The method for preparing the electrode as described in claim 1, characterized in that: The total thickness of the electrode paste coated on one side is 70~170μm; After rolling, the total thickness of the electrode sheet is 150~220μm.

5. The method for preparing the electrode sheet according to any one of claims 1 to 4, characterized in that: The electrode slurry is a negative electrode slurry, and the negative electrode slurry further includes a negative electrode active material, a conductive agent, and a dispersant; The negative electrode active material includes at least one of natural graphite, artificial graphite, and composite graphite. The conductive agent includes at least one of acetylene black, carbon fiber, conductive carbon black, Ketjen black, graphene, and carbon nanotubes. The dispersant includes at least one of methylcellulose, carboxymethylcellulose, sodium carboxymethylcellulose, and N-methylpyrrolidone; The adhesive includes styrene-butadiene rubber.

6. An energy storage battery, characterized in that: The electrode is prepared by the method described in any one of claims 1 to 5, wherein the electrode is a negative electrode.

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