Secondary battery and method for manufacturing the same

Abstract

The present invention provides a secondary battery and a method for preparing the same. The secondary battery includes an electrode assembly, and the electrode assembly includes a positive electrode, a negative electrode, a separator, and an electrolyte. Among them, the negative electrode includes a negative electrode material coating, and the negative electrode material coating is obtained from a negative electrode slurry. By weight, the negative electrode slurry includes 96 to 98 parts of a negative electrode active material, 0.6 to 1.2 parts of a conductive agent, 1 to 2.5 parts of a binder, 0.5 to 1.5 parts of a dispersant, 80 to 100 parts of a solvent, and 1.5 to 3 parts of a thickening agent. Among them, the solvent includes a first solvent (water) and a second solvent (organic solvent), and the boiling point and surface tension of the second solvent are 110 to 210 °C and 20 to 50 dyn / cm respectively; the mass ratio of the two is 1:10 to 1:2. The thickening agent added in the present invention ensures the viscosity of the slurry while reducing the amount of the low surface tension solvent, thereby playing a significant role in eliminating the "thick edge" during coating.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and more particularly, to a secondary battery and a method for manufacturing the same. Background Art

[0002] The manufacture of electrode sheets is crucial for the performance of lithium-ion batteries. Among them, the coating process is the core step to ensure the quality of electrode sheets, and it has a direct impact on the success or failure of subsequent processes such as rolling and die-cutting. Coating is the process of evenly applying a uniformly mixed slurry onto a current collector and then drying it. The final quality of the coating is jointly determined by various factors such as slurry characteristics, drying conditions, and coating rate. During the coating process, various defects may occur. Among them, edge thickening (i.e., "thick edge") is a common problem (see Figure 1 ), which occurs when the negative electrode graphite aqueous slurry of the electrode sheet is coated and dried (see Figure 2 ). At the edge, due to a larger area-to-volume ratio, the water evaporation rate is faster. Under the capillary action, the slurry particles move towards the edge to form a "thick edge". After drying, the thickness of the edge part of the electrode sheet increases significantly. The edge coating is several to more than ten micrometers thicker than the normal area. This causes a larger bulge to accumulate when the electrode sheet is wound up during coating, resulting in the edge of the wound electrode bulging. In extreme cases, it may lead to the fracture of the electrode sheet. In addition, the thick edge phenomenon also causes the edge of the electrode sheet to bear higher pressure during rolling, resulting in uneven lateral density of the electrode sheet, further affecting the overall performance and safety of the battery.

[0003] It can be seen that there is an urgent need to explore a more ideal method for eliminating the "thick edge" at the coating edge. Summary of the Invention

[0004] The main object of the present invention is to provide a secondary battery and a method for manufacturing the same, so as to solve the problem in the prior art that it is difficult to balance the electrical performance and safety performance of the battery cell when solving the "thick edge" of the coating during the formation of the electrode sheet.

[0005] To achieve the above object, according to one aspect of the present invention, a secondary battery is provided, which includes an electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, a separator, and an electrolyte. Among them, the negative electrode includes a negative current collector and a negative electrode material coating laminated on the surface of the negative current collector. The negative electrode material coating is obtained by drying and compressing the negative electrode slurry after coating it on the surface of the negative current collector. By weight, the negative electrode slurry includes 96-98 parts of negative electrode active material, 0.6-1.2 parts of conductive agent, 1-2.5 parts of binder, 0.5-1.5 parts of dispersant, 80-100 parts of solvent, and 1.5-3 parts of thickener; among them, the solvent includes a first solvent and a second solvent. The first solvent is water, and the second solvent is an organic solvent. The boiling point of the second solvent is 110-210 °C, and the surface tension of the second solvent is 20-50 dyn / cm; the mass ratio of the second solvent to the first solvent is 1:10-1:2.

[0006] Further, the boiling point of the second solvent is 10-110 °C higher than the boiling point of water, and / or the surface tension of the second solvent is 15-50 dyn / cm lower than the surface tension of water.

[0007] Further, the second solvent is 15-30 parts by weight, and / or the second solvent is selected from any one or more of ethylene glycol, ethylenediamine, butanol, acetic acid, propylene glycol, and methylformamide.

[0008] Further, the mass ratio of the thickener to the second solvent is 1:20-1:5; and / or the thickener is selected from any one or more of polyethylene glycol, polyacrylate, and hydroxyethyl cellulose, and / or the molecular weight of polyethylene glycol is less than 600; and / or the viscosity of the negative electrode slurry is 6000-9000 。

[0009] Further, the negative electrode active material is selected from any one or more of graphite, silicon material, silicon-carbon material, and hard carbon; and / or the conductive agent is selected from any one or more of SP conductive agent, carbon black, carbon nanotube, and graphene; and / or the binder is SBR binder and / or PAA binder; and / or the dispersant is CMC dispersant.

[0010] Further, the negative electrode slurry includes 96-97 parts by weight of graphite, 0.8-1.0 parts by weight of SP conductive agent, 2-2.5 parts by weight of SBR binder, 1.2-1.5 parts by weight of CMC dispersant, 15-30 parts by weight of ethylene glycol, 60-75 parts by weight of deionized water, and 2-3 parts by weight of polyethylene glycol.

[0011] Further, along the width direction of the above-mentioned negative electrode material coating, the negative electrode material coating includes a middle region and edge transition regions located on both sides of the middle region. The thickness of the edge transition region gradually decreases in the direction away from the middle region. The thickness of the middle region is H1, and in the direction away from the middle region, the width of the edge transition region is L2, where L2 / H1 is 0.004 to 0.01:1.

[0012] Further, the above-mentioned L2 is 1 to 2 mm, and H1 is 200 to 230 mm.

[0013] According to another aspect of the present invention, there is provided a method for preparing the above-mentioned secondary battery, including: preparing a negative electrode slurry, coating the negative electrode slurry on a negative electrode current collector and then drying and compressing it in sequence to obtain a negative electrode; preparing a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector and then drying and compressing it in sequence to obtain a positive electrode; assembling each unit including the positive electrode, negative electrode, separator, and electrolyte to obtain a secondary battery; the preparation steps of the negative electrode slurry include: Step S1, dry-mixing the negative electrode active material, conductive agent, and dispersant to obtain a dry mixture; Step S2, kneading the dry mixture with a part of the first solvent to obtain a kneaded product; Step S3, performing a first wet mixing on the kneaded product, the remaining part of the first solvent, and the second solvent to obtain a wet mixture; Step S4, performing a second wet mixing on the wet mixture, thickening agent, and binder to obtain a negative electrode slurry; wherein the total amount of the part of the first solvent and the remaining part of the first solvent is the total amount of the first solvent.

[0014] Further, in the above-mentioned Step S1, the stirring parameters during the dry mixing are: the revolution speed is 15 to 25 rpm, the stirring speed is 500 to 800 rpm, and the stirring time is 30 to 60 min; and / or, in Step S2, the kneading time is 60 to 80 min, and the stirring parameters during the kneading are: the revolution speed is 20 to 25 rpm, the stirring speed is 300 to 500 rpm; and / or, in Step S3, the stirring parameters during the first wet mixing are: the revolution speed is 22 to 25 rpm, the stirring speed is 1000 to 1500 rpm, and the stirring time is 90 to 120 min; and / or, in Step S4, the stirring parameters during the second wet mixing are: the revolution speed is 22 to 25 rpm, the stirring speed is 300 to 600 rpm, and the stirring time is 30 to 40 min.

[0015] Applying the technical solution of the present application, the problem of "thick edges" of the coating is solved by the co-mixed solvent method, that is, an organic solvent with a low surface tension and a high boiling point relative to water is introduced and co-mixed with water to prepare the negative electrode slurry. Starting from the internal mechanism of the formation of "thick edges" at the coating edge, the co-mixed solvent method first co-mixes a solvent with a low surface tension and a high boiling point relative to water with water. During the baking process of the dual-solvent slurry, due to the high-boiling solvent, the concentration of the second solvent at the edge is higher than that at the middle position during the drying process. Thus, a surface tension gradient is formed from the inside to the edge of the coating, inducing the movement of the slurry particles at the edge towards the inner layer, which cancels out the capillary action effect of forming "thick edges", that is, cancels out the edge aggregation effect brought by capillary action, thereby solving the problem of "thick edges" of the coating. However, the introduced solvent with a low surface tension will result in a lower viscosity of the slurry, thus causing a decrease in the coatability of the slurry and also being not conducive to the elimination of "thick edges". Therefore, in view of the above problems, the present invention adds a thickening agent on the basis of the co-mixed solvent, thereby ensuring the viscosity of the slurry while reducing the dosage of the low-surface-tension solvent. Thus, under the dual action above, it plays a significant role in eliminating the "thick edges" of the coating. Compared with the thinning method, the method for eliminating thick edges in the present application is not only simple in operation, but also can improve the energy density of the battery cell and reduce the risk of lithium deposition at the edge of the electrode sheet, thereby significantly improving the stability and safety performance of the battery cell. In addition, since the first solvent and the second solvent of the above types are a mixed solvent of an organic solvent and water, it is more suitable as the solvent of the negative electrode slurry, so that each component in the negative electrode slurry is more fully dispersed therein to obtain a uniform negative electrode slurry. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The schematic diagrams in the specification drawings forming a part of the present application are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0017] Figure 1 It shows a schematic diagram of the edge of a coating with an overly thick edge formed during the coating process of an electrode sheet in the prior art;

[0018] Figure 2 It shows Figure 1 A schematic diagram of the drying principle of the negative electrode coating shown;

[0019] Figure 3 It shows a schematic diagram of the edge of a coating obtained after thinning the edge of an electrode sheet during coating in the prior art;

[0020] Figure 4 It shows a schematic diagram of the edge of a coating formed during the coating process of an electrode sheet provided in Embodiment 1 of the present invention;

[0021] Figure 5 It shows Figure 4Schematic diagram of the drying principle of the negative electrode coating shown

[0022] Figure 6 The process flow chart of preparing a negative electrode paste provided in Embodiment 1 of the present invention is shown

[0023] Figure 7 The distribution diagram of the thickness measurement results of #1 to #5 of the present invention is shown

[0024] Figure 8 The distribution diagram of the measurement results of the coating thickness of the negative electrode sheets in Embodiments 1-7 and Comparative Examples 1 to 5 of the present invention is shown Specific embodiments

[0025] It should be noted that, without conflict, the embodiments and features in the embodiments in the present application may be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments

[0026] Currently, in order to solve the problem of "thick edges" of the coating during the coating process, the strategy commonly adopted in the industry is to thin the edges of the electrode sheet during coating (see Figure 3 ) This method reduces the coating amount in the edge area of the coater to form a thinned area, so as to reduce the problem of excessive thickness of the coating edge after drying. Such treatment can effectively avoid the drum edge phenomenon during the winding of the electrode roll, and the problem of inconsistent transverse density caused by uneven thickness during the rolling process. However, for the coating thinning method, although it can eliminate the problem of thick edges of the electrode sheet, reduce the protrusion of the electrode sheet edge, and reduce the risk of electrode sheet breakage. However, the reduction of the thickness in the edge area of the coating results in a reduction in the amount of active material for the positive electrode sheet, thus causing a reduction in the capacity of the battery cell. At the same time, the thinning degree is difficult to control, the requirements for equipment capabilities are high, and the fluctuation of the thinning size is relatively large during the production process. And for the negative electrode sheet edge thinning, it may cause a decrease in the N / P ratio, resulting in lithium deposition at the edge, directly affecting the stability and safety of the battery cell. In addition, the thinned area will also cause a certain gap between the electrode sheets of the wound core, and the electrode sheets are not closely attached, resulting in an increase in the local internal resistance during the charge and discharge process of the battery and lithium deposition phenomenon, affecting the battery performance. Generally speaking, the thinning of the electrode sheet is a compromise of design to manufacturing, and it is necessary to minimize the thinning width as much as possible while satisfying the elimination of "thick edges". Therefore, for the problem of "thick edges" of the coating, the industry pays more attention to how to more precisely control the thinning size, hoping to minimize the thinning size while eliminating the "thick edges"

[0027] As described above, in the prior art, it is difficult to balance the electrical performance and safety performance of the battery cell when solving the "thick edges" of the coating during the formation of the electrode sheet. To solve this problem, the present application provides a secondary battery and a method for preparing the same

[0028] In a typical embodiment of the present application, a secondary battery is provided, including an electrode assembly, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. Among them, the negative electrode includes a negative current collector and a negative electrode material coating laminated on the surface of the negative current collector. The negative electrode material coating is obtained by coating the negative electrode slurry on the surface of the negative current collector and then drying and compressing it in sequence. It is characterized in that, by weight, the negative electrode slurry includes: 96 to 98 parts of negative electrode active material, 0.6 to 1.2 parts of conductive agent, 1 to 2.5 parts of binder, 0.5 to 1.5 parts of dispersant, 80 to 100 parts of solvent, and 1.5 to 3 parts of thickener; wherein, the solvent includes a first solvent and a second solvent, the first solvent is water, the second solvent is an organic solvent, the boiling point of the second solvent is 110 to 210 °C, and the surface tension of the second solvent is 20 to 50 dyn / cm; the mass ratio of the second solvent to the first solvent is 1:10 to 1:2.

[0029] The co - blending solvent method is adopted to solve the problem of "thick edge" of the coating, that is, an organic solvent with a low surface tension and a high boiling point relative to water is introduced and blended with water to prepare the negative electrode slurry. Starting from the internal mechanism of the formation of "thick edge" at the coating edge, first, a solvent with a low surface tension and a high boiling point relative to water is blended with water. During the baking process of the dual - solvent slurry, due to the high - boiling - point solvent, the concentration of the second solvent at the edge is higher than that at the middle position during the drying process. Thus, a surface tension gradient is formed from the inside to the edge of the coating, inducing the movement of the slurry particles at the edge towards the inner layer, which counteracts the capillary action effect of forming "thick edge", that is, counteracts the edge aggregation effect brought by capillary action, thereby solving the problem of "thick edge" of the coating. However, the introduced solvent with a low surface tension will result in a lower viscosity of the slurry, thus causing a decrease in the coatability of the slurry and also being unfavorable for the elimination of "thick edge". Therefore, in view of the above problems, the present invention adds a thickener on the basis of the co - blending solvent, thereby ensuring the viscosity of the slurry while reducing the amount of the low - surface - tension solvent. Thus, under the above dual effects, it plays a significant role in eliminating the "thick edge" of the coating. Compared with the thinning method, the method for eliminating the thick edge in the present application is not only simple in operation, but also can improve the energy density of the electrode assembly and reduce the risk of lithium deposition at the edge of the electrode sheet, thereby significantly improving the stability and safety performance of the electrode assembly. In addition, since the above - mentioned types of the first solvent and the second solvent are a mixed solvent of an organic solvent and water, it is more suitable as the solvent of the negative electrode slurry, so that each component in the negative electrode slurry is more fully dispersed therein to obtain a uniform negative electrode slurry.

[0030] In an embodiment of the present application, the boiling point of the above - mentioned second solvent is 10 to 110 °C higher than the boiling point of water, and / or, the surface tension of the second solvent is 15 to 50 dyn / cm lower than the surface tension of water.

[0031] Preferably, the second solvent having the above-mentioned boiling point difference and surface tension value with the first solvent helps to minimize the negative impact of the reduction in slurry viscosity caused by the second solvent while achieving the purpose of inducing the edge slurry particles to move towards the inner layer. Specifically, if the boiling point of the second solvent is too high compared to that of water, the solvent is difficult to volatilize. If the boiling point of the second solvent is too low compared to that of water, the viscosity of the negative electrode slurry is too low, making it difficult to coat on the current collector. Therefore, it is preferred that the boiling point of the second solvent is 50 - 100 °C higher than that of water. For example, the boiling point of the second solvent is 50 °C, 60 °C, 70 °C, 80 °C, 90 °C or 100 °C higher than that of water, but not limited to the listed values. Other unlisted values within the range of 10 - 110 °C are also applicable. If the surface tension of the second solvent is higher than that of water, the problem of "thick edges" of the negative electrode coating cannot be solved. If the surface tension of the second solvent is too low compared to that of water, the viscosity of the negative electrode slurry is too low, making it difficult to apply the negative electrode slurry. Therefore, it is preferred that the surface tension of the second solvent is 20 - 45 dyn / cm lower than that of water. For example, the surface tension of the second solvent is 20 dyn / cm, 25 dyn / cm, 30 dyn / cm, 35 dyn / cm, 40 dyn / cm or 45 dyn / cm lower than that of water, but not limited to the listed values. Other unlisted values within the range of 15 - 50 dyn / cm are also applicable.

[0032] In one embodiment of the present application, the above-mentioned second solvent is 15 - 30 parts by weight, and / or the second solvent is selected from any one or more of ethylene glycol, ethylenediamine, butanol, acetic acid, propylene glycol and methylformamide.

[0033] When the proportion of the second solvent is too low, the surface tension gradient between the edge and the middle slurry during the coating baking process is too small, and the coating is prone to the "thick edge" problem. When the proportion of the second solvent is too high, the required baking temperature during the coating process is relatively high, which is likely to cause problems such as coating cracking and increase costs. It is preferred to select the type and content of the above-mentioned second solvent. Preferably, the second solvent is 20 - 30 parts by weight. For example, the second solvent is 20 parts by weight, 21 parts by weight, 22 parts by weight, 23 parts by weight, 24 parts by weight, 25 parts by weight, 26 parts by weight, 27 parts by weight, 28 parts by weight, 29 parts by weight or 30 parts by weight, but not limited to the listed values. Other unlisted values within the range of 15 - 30 parts by weight are also applicable. Further preferably, the second solvent is selected from any one or more of ethylene glycol, propylene glycol and methylformamide, so as to minimize the amount of the second solvent used on the basis of being able to induce the edge slurry particles to move towards the inner layer, thereby alleviating its effect of reducing the slurry viscosity.

[0034] In one embodiment of the present application, the mass ratio of the above thickener to the second solvent is 1:20 to 1:5; and / or, the thickener is selected from any one or more of polyethylene glycol, polyacrylate, and hydroxyethyl cellulose, and / or, the molecular weight of polyethylene glycol is less than 600; and / or, the viscosity of the negative electrode slurry is 6000 to 9000 .

[0035] Preferably, the mass ratio of the thickener to the second solvent is within the above range, which not only helps to form a surface tension gradient from the inside to the edge of the coating by using the second solvent, inducing the movement of the edge slurry particles towards the inner layer, but also helps to make up for the deficiency of the reduced viscosity of the slurry caused by the second solvent through the thickener, so as to be able to more fully take into account the above two aspects of the role, making the effect of the second solvent play to the best, and thus playing a significant role in eliminating the "thick edge" of the coating. Among them, the preferred mass ratio of the thickener to the second solvent is 1:15 to 1:10. For example, the mass ratio of the thickener to the second solvent is 1:15, 1:14, 1:13, 1:12, 1:11 or 1:10, but not limited to the listed values. Other unlisted values within the numerical range of 1:20 to 1:5 are also applicable. Preferably, the molecular weight of polyethylene glycol is less than or equal to 400. For example, the molecular weight of polyethylene glycol is about 200, 300 or 400, but not limited to the listed values. Other unlisted values within the numerical range of the molecular weight of polyethylene glycol less than 600 are also applicable. Preferably, the viscosity of the negative electrode slurry is 6500 to 8500 , for example, the viscosity of the negative electrode slurry is 6500 , 7000 , 7500 , 8000 or 8500 , but not limited to the listed values. Other unlisted values within the numerical range of 6000 to 9000 are also applicable.

[0036] In one embodiment of the present application, the above negative electrode active material is selected from any one or more of graphite, silicon material, silicon-carbon material, and hard carbon; and / or the conductive agent is selected from any one or more of SP conductive agent, carbon black, carbon nanotube, and graphene; and / or the binder is SBR binder and / or PAA binder; and / or the dispersant is CMC dispersant.

[0037] Preferably, the above negative electrode active material, conductive agent, binder and dispersant are more helpful for their cooperation with the above types of solvents, so as to reduce the risk of "thick edge" in the coating process of the whole negative electrode slurry.

[0038] Further, to improve the synergistic cooperation effect among the components in the negative electrode slurry, in an embodiment of the present application, it is preferred that the above-mentioned negative electrode slurry includes 96 to 97 parts by weight of graphite, 0.8 to 1.0 part by weight of SP conductive agent, 2 to 2.5 parts by weight of SBR binder, 1.2 to 1.5 parts by weight of CMC dispersant, 15 to 30 parts by weight of ethylene glycol, 60 to 75 parts by weight of deionized water, and 2 to 3 parts by weight of polyethylene glycol. For example, the negative electrode slurry contains 96 parts by weight, 96.5 parts by weight, or 97 parts by weight of graphite; the SP conductive agent is 0.8 part by weight, 0.9 part by weight, or 1.0 part by weight; the SBR binder is 2.1 parts by weight, 2.2 parts by weight, 2.3 parts by weight, 2.3 parts by weight, or 2.5 parts by weight; the CMC dispersant is 1.2 parts by weight, 1.3 parts by weight, 1.4 parts by weight, or 1.5 parts by weight; preferably, the ethylene glycol is 20 to 30 parts by weight, for example, the ethylene glycol is 20 parts by weight, 21 parts by weight, 22 parts by weight, 23 parts by weight, 24 parts by weight, 25 parts by weight, 26 parts by weight, 27 parts by weight, 28 parts by weight, 29 parts by weight, or 30 parts by weight; preferably, the deionized water is 65 to 75 parts by weight, for example, the deionized water is 65 parts by weight, 66 parts by weight, 67 parts by weight, 68 parts by weight, 69 parts by weight, 70 parts by weight, 71 parts by weight, 72 parts by weight, 73 parts by weight, 74 parts by weight, or 75 parts by weight; the polyethylene glycol is 2 parts by weight, 2.5 parts by weight, or 3 parts by weight; however, it is not limited to the listed values, and other unlisted values within the above ranges of each component are equally applicable.

[0039] In an embodiment of the present application, along the width direction of the above-mentioned negative electrode material coating, the negative electrode material coating includes a middle region and edge transition regions located on both sides of the middle region. The thickness of the edge transition region gradually thins along the direction away from the middle region. The thickness of the middle region is H1, and along the direction away from the middle region, the width of the edge transition region is L2, where L2 / H1 is 0.004 to 0.01:1.

[0040] Preferably, the range of L2 / H1 is 0.004 to 0.01:1. For example, 0.004:1, 0.005:1, 0.006:1, 0.007:1, 0.008:1, 0.009:1, or 0.01:1. However, it is not limited to the listed values, and other unlisted values within the above range are equally applicable. Compared with the thinning process in the prior art, the preferred value of L2 / H1 helps to make the battery cell have higher safety and energy density on the basis of avoiding the problem of "thick edges" in coating.

[0041] Further, to improve the adaptability of the actual battery cell to the above scheme, so that the battery cell has excellent electrical performance, in an embodiment of the present application, it is preferred that the above-mentioned L2 is 1 to 2 mm and H1 is 200 to 230 mm.

[0042] Further, to improve the adaptability of the actual battery cell in the above solution and enable the battery cell to have excellent electrical performance, in an embodiment of the present application, it is preferred that the above L2 is 1 to 2 mm. For example, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm or 2 mm, but not limited to the listed values, and other unlisted values within the above range are equally applicable. H1 is 200 to 230 mm. For example, 210 mm, 215 mm, 220 mm, 225 mm or 230 mm, but not limited to the listed values, and other unlisted values within the above range are equally applicable.

[0043] In addition, by changing the types of the above first solvent and second solvent and through their combination, it can also be suitable as the solvent of the positive electrode paste, so that each component in the positive electrode paste is more fully dispersed therein to obtain a uniform positive electrode paste. Further, the problem of "thick edge" of the coating during the coating process of the positive electrode sheet is solved.

[0044] In another typical embodiment of the present application, a method for preparing the aforementioned secondary battery is provided, including: preparing a negative electrode paste, coating the negative electrode paste on a negative electrode current collector and then drying and compressing it in sequence to obtain a negative electrode; preparing a positive electrode paste, coating the positive electrode paste on a positive electrode current collector and then drying and compressing it in sequence to obtain a positive electrode; assembling each unit including the positive electrode, negative electrode, separator and electrolyte to obtain a secondary battery; wherein, the preparation steps of the negative electrode paste include: step S1, dry-mixing the negative electrode active material, conductive agent and dispersant to obtain a dry mixture; step S2, kneading the dry mixture with a part of the first solvent to obtain a kneaded product; step S3, performing a first wet mixing on the kneaded product, the remaining part of the first solvent and the second solvent to obtain a wet mixture; step S4, performing a second wet mixing on the wet mixture, thickening agent and binder to obtain a negative electrode paste; wherein, the total amount of the part of the first solvent and the remaining part of the first solvent is the total amount of the first solvent.

[0045] The preferred above step-by-step mixing process makes the components in the negative electrode paste mix more evenly, so that the coating formed after coating the paste is more uniform, and the elimination effect of the problem of "thick edge" of the coating is more significant, and further enables the battery cell to have excellent electrical performance.

[0046] In an embodiment of the present application, in the above step S1, the stirring parameters during dry mixing are as follows: the revolution speed is 15 - 25 rpm, the stirring speed is 500 - 800 rpm, and the stirring time is 30 - 60 min; and / or, in step S2, the kneading time is 60 - 80 min, and the stirring parameters during kneading are as follows: the revolution speed is 20 - 25 rpm, and the stirring speed is 300 - 500 rpm; and / or, in step S3, the stirring parameters during the first wet mixing are as follows: the revolution speed is 22 - 25 rpm, the stirring speed is 1000 - 1500 rpm, and the stirring time is 90 - 120 min; and / or, in step S4, the stirring parameters during the second wet mixing are as follows: the revolution speed is 22 - 25 rpm, the stirring speed is 300 - 600 rpm, and the stirring time is 30 - 40 min.

[0047] Preferably, the control of the stirring parameters in each of the above mixing steps enables each component to obtain a better mixing effect with adaptable stirring parameters after being added, thereby obtaining a uniform anode slurry. Furthermore, it can ensure the uniformity of the rolling process of the electrode sheet, reduce the stress concentration caused by uneven rolling of the electrode sheet, and effectively reduce the problems such as wavy edges of the electrode sheet during rolling. At the same time, the coating thickness obtained by this method is uniform, and for the subsequent assembly process, the degree of fit between the electrode sheets is greatly improved, significantly enhancing the performance of the battery cell. Among them, in the above step S1, the stirring parameters during dry mixing are as follows: the revolution speed is 15 rpm, 20 rpm, or 25 rpm, the stirring speed is 500 rpm, 600 rpm, 700 rpm, or 800 rpm, and the stirring time is 30 min, 40 min, 50 min, or 60 min; in step S2, the kneading time is 60 min, 70 min, or 80 min, and the stirring parameters during kneading are as follows: the revolution speed is 20 rpm, 21 rpm, 22 rpm, 23 rpm, 24 rpm, or 25 rpm, and the stirring speed is 300 rpm, 400 rpm, or 500 rpm; in step S3, the stirring parameters during the first wet mixing are as follows: the revolution speed is 22 rpm, 23 rpm, 24 rpm, or 25 rpm, the stirring speed is 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, or 1500 rpm, and the stirring time is 90 min, 100 min, 110 min, or 120 min; in step S4, the stirring parameters during the second wet mixing are as follows: the revolution speed is 22 rpm, 23 rpm, 24 rpm, or 25 rpm, the stirring speed is 300 rpm, 400 rpm, 500 rpm, or 500 rpm, and the stirring time is 30 min, 35 min, or 40 min. However, it is not limited to the listed values, and other unlisted values within the numerical ranges of the above conditions are equally applicable.

[0048] The beneficial effects of the present application will be described below in conjunction with specific embodiments and comparative examples.

[0049] To explore the preferred ratio of the first solvent to the second solvent, as shown in Table 1 specifically.

[0050] Table 1

[0051]

[0052] After mixing the components in #1 to #5 according to the following mixing process, the negative electrode slurries of #1 to #5 are obtained.

[0053] Put the above amounts of graphite powder, conductive agent, and dispersant into a stirring tank and dry mix for 30 minutes, with the stirring revolution speed being 15 rpm and the dispersion speed being 500 rpm. Then add 50 parts of deionized water and knead and stir for 60 min, with the stirring revolution speed being 20 rpm and the dispersion speed being 300 rpm. After that, add the second solvent and the remaining deionized water for high-speed dispersion, with the stirring revolution speed being 22 rpm and the dispersion speed being 1000 rpm. Finally, add the binder and continue to stir for 30 min, with the stirring revolution speed being 22 rpm and the dispersion speed being 500 rpm, and finally obtain the negative electrode slurries of #1 to #5.

[0054] Coating: According to the requirements of the cell design, coat the negative electrode slurries of #1 to #5 on the copper foil respectively to obtain the negative electrode sheets of #1 to #5. The thickness measurement result distribution diagrams of the negative electrode sheets of #1 to #5 are as Figure 7 shown, and the thicknesses of the negative electrode sheets of #1 to #5 at different positions are shown in Table 2.

[0055] Table 2

[0056]

[0057]

[0058]

[0059] Combined with Table 1 and Figure 7 , it can be seen that the preferred mass ratio of ethylene glycol to deionized water is 1:10 to 1:2.

[0060] On the basis of the preferred mass ratio of ethylene glycol to deionized water being 1:10 to 1:2, explore the influence of the thickener on the coating thickness of the negative electrode sheet, as shown in Table 3 specifically.

[0061] Table 3

[0062]

[0063] After mixing the components in Examples 1 to 4 according to the following mixing process, the negative electrode slurries in Examples 1 to 4 were obtained.

[0064] Put the above amounts of graphite powder, conductive agent, and dispersant into a stirring tank and dry mix for 30 minutes. The stirring revolution speed is 15 rpm, and the dispersion speed is 500 rpm. Then add 50 parts of deionized water and knead and stir for 60 min. The stirring revolution speed is 20 rpm, and the dispersion speed is 300 rpm. Then add the second solvent and the remaining deionized water for high-speed dispersion. The stirring revolution speed is 22 rpm, and the dispersion speed is 1000 rpm. Finally, add the thickening agent and the binder and continue stirring for 30 min. The stirring revolution speed is 22 rpm, and the dispersion speed is 500 rpm. Finally, the negative electrode slurries in Examples 1 to 4 were obtained.

[0065] Coating: According to the requirements of the cell design, the negative electrode slurries of Examples 1 to 4 were respectively coated on copper foils to obtain the negative electrode sheets of Examples 1 to 4.

[0066] Example 5

[0067] The difference from Example 2 is that the second solvent is N-methylformamide, which has a boiling point 82.5 °C higher than that of deionized water and a surface tension 13.62 dyn / cm lower than that of water. Finally, a negative electrode sheet was obtained.

[0068] Example 6

[0069] The difference from Example 2 is that the above amounts of graphite powder, conductive agent, and dispersant were put into a stirring tank and dry mixed for 60 minutes. The stirring revolution speed is 25 rpm, and the dispersion speed is 800 rpm. Then add 50 parts of deionized water and knead and stir for 80 min. The stirring revolution speed is 25 rpm, and the dispersion speed is 500 rpm. Then add the second solvent and the remaining deionized water for high-speed dispersion for 120 min. The stirring revolution speed is 25 rpm, and the dispersion speed is 1500 rpm. Finally, add the thickening agent and the binder and continue stirring for 40 min. The stirring revolution speed is 25 rpm, and the dispersion speed is 300 rpm to obtain the negative electrode slurry of Example 6. Finally, a negative electrode sheet was obtained.

[0070] Example 7

[0071] The difference from Example 2 is that there are 96 parts of silicon-carbon material (negative electrode active material), 0.8 part of SP (conductive agent), 2 parts of SBR (binder), 1.2 parts of CMC (dispersant), 25.71 parts of ethylene glycol (second solvent), 64.29 parts of deionized water, and 1.5 parts of polyethylene glycol (thickening agent). Finally, a negative electrode sheet was obtained.

[0072] Comparative Example 1

[0073] Preparation of negative electrode slurry:

[0074] Weigh the negative electrode slurry raw materials according to the following parts by weight: 96 parts of graphite powder (negative electrode active material), 0.8 parts of SP (conductive agent), 2 parts of SBR (binder), 1.2 parts of CMC (dispersant), and 90 parts of deionized water.

[0075] Put the above amounts of graphite powder, conductive agent, and dispersant into the stirring tank for dry mixing for 30 minutes. The stirring revolution speed is 15 rpm, and the dispersion speed is 500 rpm. Then add 50 parts of deionized water for kneading and stirring for 60 min. The stirring revolution speed is 20 rpm, and the dispersion speed is 300 rpm. After that, add the remaining deionized water for high-speed dispersion. The stirring revolution speed is 22 rpm, and the dispersion speed is 1000 rpm. Finally, add the binder and continue stirring for 30 min. The stirring revolution speed is 22 rpm, and the dispersion speed is 500 rpm, and finally obtain the negative electrode slurry of Comparative Example 1.

[0076] Coating:

[0077] Coat the copper foil with the negative electrode slurry of Comparative Example 1 according to the cell design requirements without thinning treatment to obtain the negative electrode sheet of Comparative Example 1.

[0078] Comparative Example 2

[0079] Preparation of negative electrode slurry:

[0080] Prepare the negative electrode slurry of Comparative Example 2 according to the slurry preparation process of Comparative Example 1

[0081] Coating:

[0082] Coat the copper foil with the negative electrode slurry of Comparative Example 2 according to the cell design requirements, and thin the edge during the coating process to obtain the negative electrode sheet of Comparative Example 2.

[0083] Comparative Example 3

[0084] The difference from Example 2 is that the total weight parts of ethylene glycol (second solvent) and deionized water are 90 parts, and the mass ratio of ethylene glycol to deionized water is 3:1, and finally obtain the negative electrode sheet.

[0085] Comparative Example 4

[0086] The difference from Example 2 is that the total weight parts of ethylene glycol (second solvent) and deionized water are 90 parts, and the mass ratio of deionized water to ethylene glycol is 11:1, and finally obtain the negative electrode sheet.

[0087] Comparative Example 5

[0088] The difference from Example 1 is that the second solvent is ethanol (boiling point 78.5 °C, surface tension 24.05 dyn / cm), and finally a negative electrode sheet is obtained.

[0089] Distribution diagrams of the thickness measurement results of the negative electrode sheets of Examples 1 to 7 and Comparative Examples 1 to 5 are as Figure 8 shown. The thicknesses of the negative electrode sheets of Examples 1 to 6 at different positions are shown in Table 4, and the thicknesses of the negative electrode sheets of Example 7 and Comparative Examples 1 to 5 at different positions are shown in Table 5.

[0090] Table 4

[0091]

[0092]

[0093] Table 5

[0094]

[0095]

[0096]

[0097] Performance test:

[0098] Thickness test of the electrode sheet in the TD direction: Along the TD direction of the coated electrode sheet, the thickness of the electrode sheet was measured every 5 mm using a micrometer. Three values were measured at each point and their average was calculated, and the thicknesses of all points were summarized and plotted.

[0099] The viscosity of the negative electrode slurry was tested by a rotational viscometer, and the test results are shown in Table 6.

[0100] Tests of L1, L2, H1 and H2: Measured using a micrometer, and the ratios of L1 to H1 (L1 / H1) and L2 to H1 (L2 / H1) were calculated. The results are shown in Table 1, where L1 represents the width of the thick-edge area of the negative electrode coating, and H2 represents the maximum thickness of the thick-edge area of the negative electrode coating. See Figure 1 for details.

[0101] The sodium iron phosphate positive electrode, the separator and the negative electrode sheets obtained in the above examples and comparative examples were stacked in sequence, and then assembled into an electric core by winding or laminating. An electrolyte (lithium hexafluorophosphate + solvent, the solvent is ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 1:1:1) was injected into the electric core. The electric core was placed in an aluminum plastic film, the tabs were welded or laser welded, and then sealed and formed to obtain a soft-pack lithium-ion battery with a nominal capacity of 2.3 Ah. The discharge specific capacity of the lithium-ion battery was tested at 0.5C, and the test results are listed in Table 6.

[0102] Table 6

[0103]

[0104] As can be seen from the data in Table 6, the methods of Examples 1 to 7 not only solve the problem of "thick edges" of the coating, but also ensure that the lithium-ion battery has a high discharge capacity. However, in Comparative Example 2, there is a serious risk of lithium plating, which greatly reduces the cycle performance and safety performance of the lithium-ion battery. In Comparative Example 3, due to the high content of the second solvent, the baking temperature of the negative electrode sheet is relatively high, resulting in serious cracking on the surface of the negative electrode sheet. Comparative Examples 1, 4, and 5 do not solve the problem of "thick edges" of the coating.

[0105] From Figure 1 Compared with Figure 4 , it can be seen that this application solves the problem of "thick edges" of the negative electrode coating, that is, Figure 1 the thick edges corresponding to L1 in disappear in the negative electrode coating of this application.

[0106] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0107] The method of using a blended solvent is adopted to solve the problem of "thick edges" of the coating, that is, an organic solvent with a low surface tension and a high boiling point relative to water is introduced and blended with water to prepare the negative electrode slurry. Starting from the internal mechanism of the formation of "thick edges" at the coating edge, first, a solvent with a low surface tension and a high boiling point relative to water is blended with water. During the baking process of the dual-solvent slurry, due to the high-boiling solvent, the concentration of the second solvent at the edge is higher than that at the middle position during the drying process. At this point, a surface tension gradient is formed from the inside to the edge of the coating, inducing the movement of the slurry particles at the edge towards the inner layer, which cancels out the capillary action effect of forming "thick edges", that is, cancels out the edge aggregation effect brought by capillary action, thus solving the problem of "thick edges" of the coating. However, the introduced solvent with a low surface tension will cause the viscosity of the slurry to be relatively low, resulting in a decrease in the coatability of the slurry, and it is also not conducive to the elimination of "thick edges". Therefore, in view of the above problems, the present invention adds a thickening agent on the basis of the blended solvent, thereby ensuring the viscosity of the slurry while reducing the dosage of the low surface tension solvent. Thus, under the above dual effects, it plays a significant role in eliminating the "thick edges" of the coating. Compared with the thinning method, the method of eliminating thick edges in this application is not only simple in operation, but also can improve the energy density of the battery cell and reduce the risk of lithium plating at the edge of the electrode sheet, thereby significantly improving the stability and safety performance of the battery cell. In addition, since the above types of the first solvent and the second solvent are a mixed solvent of an organic solvent and water, it is more suitable as the solvent for the negative electrode slurry, so that each component in the negative electrode slurry is more fully dispersed therein to obtain a uniform negative electrode slurry.

[0108] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein, The negative electrode includes a negative current collector and a negative electrode material coating laminated on the surface of the negative current collector. The negative electrode material coating is obtained by coating the negative electrode slurry on the surface of the negative current collector and then drying and compressing it in sequence. It is characterized in that, by weight, the negative electrode slurry includes: 96 to 98 parts of negative electrode active material; 0.6 to 1.2 parts of conductive agent; 1 to 2.5 parts of binder; 0.5 to 1.5 parts of dispersant; 80 to 100 parts of solvent; 1.5 to 3 parts of thickener; Wherein, the solvent includes a first solvent and a second solvent. The first solvent is water, and the second solvent is an organic solvent. The boiling point of the second solvent is 110 to 210 °C, and the surface tension of the second solvent is 20 to 50 dyn / cm; the mass ratio of the second solvent to the first solvent is 1:10 to 1:2; The thickener is selected from any one or more of polyethylene glycol, polyacrylate, and hydroxyethyl cellulose; The mass ratio of the thickener to the second solvent is 1:20 to 1:

5.

2. The secondary battery according to claim 1, characterized in that, The boiling point of the second solvent is 10 to 110 °C higher than the boiling point of water, and / or the surface tension of the second solvent is 15 to 50 dyn / cm lower than the surface tension of water.

3. The secondary battery according to claim 1 or 2, characterized in that, The second solvent is 15 to 30 parts by weight, and / or the second solvent is selected from any one or more of ethylene glycol, ethylenediamine, butanol, acetic acid, propylene glycol, and methylformamide.

4. The secondary battery according to claim 3, characterized in that, The molecular weight of the polyethylene glycol is less than 600; and / or the viscosity of the negative electrode slurry is 6000 to 9000 Pa∙s.

5. The secondary battery according to claim 1 or 2, characterized in that, The negative electrode active material is selected from any one or more of graphite, silicon material, silicon-carbon material, and hard carbon; and / or the conductive agent is selected from any one or more of carbon black, carbon nanotubes, and graphene; the carbon black is SP conductive agent; and / or the binder is SBR binder and / or PAA binder; and / or the dispersant is CMC dispersant.

6. The secondary battery according to claim 5, characterized in that, The negative electrode slurry includes 96 to 97 parts by weight of the graphite, 0.8 to 1.0 parts by weight of the SP conductive agent, 2 to 2.5 parts by weight of the SBR binder, 1.2 to 1.5 parts by weight of the CMC dispersant, 15 to 30 parts by weight of ethylene glycol, 60 to 75 parts by weight of deionized water, and 2 to 3 parts by weight of polyethylene glycol.

7. The secondary battery according to claim 1 or 2, characterized in that, Along the width direction of the negative electrode material coating, the negative electrode material coating includes a middle region and edge transition regions located on both sides of the middle region. The thickness of the edge transition region gradually decreases in the direction away from the middle region. The thickness of the middle region is H1. In the direction away from the middle region, the width of the edge transition region is L2, where L2 / H1 is 0.004 to 0.01:

1.

8. The secondary battery according to claim 7, wherein, The L2 is 1 to 2 mm, and the H1 is 200 to 230 mm.

9. A method for preparing a secondary battery according to any one of claims 1 to 8, comprising: Preparing a negative electrode slurry, coating the negative electrode slurry on a negative current collector, and then drying and compressing it in sequence to obtain a negative electrode; Prepare a positive electrode paste, coat the positive electrode paste on a positive electrode current collector, and then successively dry and compact it to obtain a positive electrode; Assemble each unit including the positive electrode, the negative electrode, a separator, and an electrolyte to obtain a secondary battery; It is characterized in that the preparation steps of the negative electrode paste include: Step S1: Dry-mix a negative electrode active material, a conductive agent, and a dispersant to obtain a dry mixture; Step S2: Knead the dry mixture with a part of a first solvent to obtain a kneaded product; Step S3: Perform a first wet-mix on the kneaded product, the remaining part of the first solvent, and a second solvent to obtain a wet mixture; Step S4: Perform a second wet-mix on the wet mixture, a thickener, and a binder to obtain a negative electrode paste; Wherein, the total amount of the part of the first solvent and the remaining part of the first solvent is the total amount of the first solvent.

10. The preparation method according to claim 9, characterized in that, In the step S1, the stirring parameters during the dry-mixing: the revolution speed is 15-25 rpm, the stirring speed is 500-800 rpm, and the stirring time is 30-60 min; and / or, in the step S2, the kneading time is 60-80 min, and the stirring parameters during the kneading: the revolution speed is 20-25 rpm, the stirring speed is 300-500 rpm; and / or, in the step S3, the stirring parameters during the first wet-mixing: the revolution speed is 22-25 rpm, the stirring speed is 1000-1500 rpm, and the stirring time is 90-120 min; and / or, in the step S4, the stirring parameters during the second wet-mixing: the revolution speed is 22-25 rpm, the stirring speed is 300-600 rpm, and the stirring time is 30-40 min.

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