Secondary battery, method for preparing same, energy storage system, and electrical equipment

Abstract

This application relates to the field of energy storage, and provides a secondary battery, a preparation method thereof, an energy storage system, and an electrical device. The preparation method includes: adding a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive to an initial slurry and mixing them to prepare a negative electrode slurry, wherein the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive form an azeotropic mixture with a boiling point lower than 100 °C with water molecules, the boiling point of the first azeotropic additive is 50 °C to 65 °C, the boiling point of the second azeotropic additive is 65 °C to 85 °C, the boiling point of the third azeotropic additive is higher than the boiling point of the first azeotropic additive, the boiling point of the third azeotropic additive is lower than 100 °C, and the third azeotropic additive forms a hydrogen bond with water molecules. Coating the negative electrode slurry on the surface of a negative electrode current collector and performing a drying treatment at a temperature lower than 100 °C to obtain a negative electrode sheet with a small residual moisture content.

Description

Technical Field

[0001] This application relates to the field of energy storage, and particularly to a secondary battery, a preparation method thereof, an energy storage system, and an electrical device. Background Art

[0002] As one of the core components of a lithium-ion battery, the structure design, material selection, and preparation process of the negative electrode sheet directly determine the comprehensive performance of the battery. During the manufacturing process of the negative electrode sheet, the coating process, as a key process, has a decisive impact on the battery performance.

[0003] The coating process is a process of uniformly coating the negative electrode active material slurry on an aluminum foil or copper foil current collector, and forming a porous coating structure after drying. The quality control indicators of this process, including the coating thickness uniformity, surface density distribution consistency, surface defect control, and moisture residue after drying, will significantly affect important indicators such as the battery consistency, cycle life, energy density, and safety performance.

[0004] Therefore, there is an urgent need for a preparation method of a secondary battery to solve the problem of high moisture residue in the negative electrode sheet to meet the complex requirements of the energy storage field. Summary of the Invention

[0005] Embodiments of this application provide a secondary battery, a preparation method thereof, an energy storage system, and an electrical device, which are at least beneficial to solving the problem of relatively high moisture residue in the negative electrode sheet.

[0006] A first aspect of the embodiments of this application provides a preparation method of a secondary battery, including: preparing a negative electrode sheet, and the preparation steps of the negative electrode sheet include: configuring an initial slurry containing a negative electrode active material; adding a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive to the initial slurry and mixing to configure a negative electrode slurry, wherein the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive form an azeotropic mixture with a boiling point less than 100 °C with water molecules in the negative electrode slurry, the boiling point of the first azeotropic additive is 50 °C to 65 °C, the boiling point of the second azeotropic additive is 65 °C to 85 °C, the boiling point of the third azeotropic additive is greater than the boiling point of the first azeotropic additive, the boiling point of the third azeotropic additive is less than 100 °C, and the third azeotropic additive forms a hydrogen bond with the water molecules; coating the negative electrode slurry on the surface of a negative electrode current collector and performing a drying treatment at a temperature less than 100 °C to obtain a negative electrode sheet; providing a positive electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet and then placing them in a housing, and injecting an electrolyte into the housing to form the secondary battery.

[0007] According to some embodiments of the present application, at least one of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive is a polar solvent.

[0008] According to some embodiments of the present application, the boiling point of the first azeotropic additive is 50°C to 55°C, and the boiling point of the second azeotropic additive is 80°C to 85°C.

[0009] According to some embodiments of the present application, in the negative electrode paste, the sum of the masses of the first azeotropic additive and the second azeotropic additive is 0.2% to 0.5% of the mass of the negative electrode active material, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3 to 1:7, and the mass of the third azeotropic additive is 0.1% to 0.3% of the mass of the negative electrode active material.

[0010] According to some embodiments of the present application, the first azeotropic additive includes methyl acetate; the second azeotropic additive includes at least one of tetrahydrofuran, ethanol, isopropanol, and tert-butanol; the third azeotropic additive includes at least one of acetonitrile and methyl ethyl ketone.

[0011] According to some embodiments of the present application, the negative electrode active material includes graphite, the first azeotropic additive includes the methyl acetate, the second azeotropic additive includes the isopropanol, and the third azeotropic additive includes the acetonitrile.

[0012] According to some embodiments of the present application, the negative electrode active material includes graphite; the first azeotropic additive includes the methyl acetate; the second azeotropic additive includes a mixture of the tetrahydrofuran, the ethanol, and the tert-butanol, and the mass ratio of the tetrahydrofuran, the ethanol, and the tert-butanol is (1 to 2):(1 to 2):(1 to 2); the third azeotropic additive includes the acetonitrile.

[0013] According to some embodiments of the present application, the negative electrode active material includes graphite, the first azeotropic additive includes the methyl acetate, the second azeotropic additive includes a mixture of the ethanol and the tert-butanol, and the mass ratio of the tert-butanol to the ethanol is 1:1 to 1:3; the third azeotropic additive includes the acetonitrile.

[0014] According to some embodiments of the present application, the drying treatment includes: providing a multi-section oven, and the negative electrode current collector coated with the negative electrode paste sequentially enters each section of the multi-section oven for drying; wherein, the multi-section oven includes n sections of ovens, the temperature of the first section of oven is 60°C to 70°C, and in the moving direction of the negative electrode current collector, the temperature of the n sections of ovens in the spaced-apart ovens gradually increases, and the temperature of the nth section of oven is 80°C to 90°C.

[0015] In a second aspect of the embodiments of the present application, a secondary battery is provided. The secondary battery can be prepared by using the preparation method of the secondary battery provided in any of the above embodiments, and includes: a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the active material layer is disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer is obtained by drying a negative electrode paste, and the moisture residue of the negative electrode sheet is less than 922 ppm; a housing, a positive electrode sheet, and a separator, and after the positive electrode sheet, the separator, and the negative electrode sheet are wound or laminated, they are placed in the housing, and an electrolyte is injected into the housing.

[0016] In a third aspect of the embodiments of the present application, an energy storage system is further provided, including the secondary battery provided in any of the above embodiments.

[0017] In a fourth aspect of the embodiments of the present application, an electrical device is further provided, including the secondary battery provided in any of the above embodiments and a load, and the secondary battery is used to supply power to the load.

[0018] The technical solutions provided in the embodiments of the present application have at least the following advantages:

[0019] In the technical solution of the preparation method of the secondary battery provided in the embodiments of the present application, the negative electrode paste contains three functional components, namely a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive. These additives can form an azeotropic mixture with a boiling point lower than 100 °C with water molecules. The boiling point of the first azeotropic additive is 50 °C to 65 °C, and it quickly forms a low-boiling azeotrope with water at the initial stage of drying, and quickly evaporates and takes away the moisture. The boiling point of the second azeotropic additive is 65 °C to 85 °C, and it continues to azeotrope with water molecules in the middle stage, evaporates and takes away the moisture. The boiling point of the third azeotropic additive is greater than the boiling point of the first azeotropic additive and less than 100 °C. The third azeotropic additive combines with water molecules through hydrogen bonds, destroys the hydrogen bond network of water molecules, reduces the evaporation enthalpy, and more thoroughly removes the residual moisture, especially for trace water that is difficult to evaporate. Therefore, the negative electrode paste provided in the embodiments of the present application includes azeotropic additives with different boiling points. Through this multi-component synergistic action system, gradient evaporation of additives with different boiling points can be achieved during the drying process after coating, thereby constructing a hierarchical pore structure, significantly improving the evaporation efficiency of moisture in the negative electrode sheet, and reducing the moisture residue in the negative electrode sheet; and, since the boiling point of the third azeotropic additive is greater than the boiling point of the first azeotropic additive, and the third azeotropic additive can form hydrogen bonds with water molecules, after the first azeotropic additive carries away some water molecules by evaporation, the third azeotropic additive can weaken the hydrogen bond interaction between water molecules due to its ability to form hydrogen bonds with water molecules, and inhibit the agglomeration phenomenon caused by local gelation, thereby improving the drying uniformity. Therefore, the embodiments of the present application are beneficial to improving the performance of the formed secondary battery. Description of the Drawings

[0020] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplary illustrations do not constitute a limitation on the embodiments. Unless otherwise stated, the figures in the drawings do not constitute a scale limitation. To more clearly illustrate the technical solutions in the embodiments of the present application or in the prior art, the drawings required to be used in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0021] Figure 1 It is a schematic structural diagram of a negative electrode sheet;

[0022] Figure 2 It is a schematic flow diagram of a method for preparing a secondary battery provided by an embodiment of the present application;

[0023] Figure 3 It is a schematic structural diagram of a negative electrode sheet provided by an embodiment of the present application;

[0024] Figure 4 It is a schematic structural diagram of another negative electrode sheet provided by an embodiment of the present application;

[0025] Figure 5 It is a top view of a negative electrode sheet provided by an embodiment of the present application.

[0026] Explanation of reference numerals:

[0027] 1: Negative electrode substrate; 2: Negative electrode active layer; 11: Negative electrode current collector; 12: Negative electrode active material layer. Detailed implementation manners

[0028] As can be seen from the background art, there is an urgent need for a method for preparing a secondary battery to solve the problem of high water residue in the negative electrode sheet and meet the complex requirements in the energy storage field.

[0029] Taking the negative electrode sheet as an example, the manufacturing steps of the negative electrode sheet include coating and drying treatment. Due to the challenges in controlling the drying process parameters, especially the oven temperature is usually controlled within a range lower than the boiling point of water (100 °C), which results in a problem of high water residue in the coated negative electrode sheet. This water residue brings multiple negative impacts: First, the test result of the weight loss rate of the negative electrode sheet is high, indicating that the water content exceeds the standard; Second, during the charge and discharge process of the battery, the residual water will cause additional energy loss; More importantly, water molecules may react chemically with the lithium salt (such as LiPF6) in the electrolyte to generate strongly corrosive hydrofluoric acid (HF). These side reactions will not only damage the internal structure of the battery, accelerate the capacity decay, but also may cause serious safety hazards such as battery swelling and leakage, ultimately affecting the overall performance and reliability of the battery.

[0030] In addition, although the water content in the negative electrode sheet can be reduced by adjusting the oven parameters, such as increasing the oven temperature, increasing the air frequency, and increasing the drying time, it may cause cracks in the negative electrode sheet during the coating process, and the energy consumption of coating will be higher, which will affect the manufacturing efficiency of the negative electrode sheet and is not conducive to the control of time and economic costs in the battery manufacturing process. In addition, the generation of cracks in the negative electrode sheet will have an adverse impact on the safety performance and cycling performance of the battery.

[0031] Figure 1 It is a schematic structural diagram of a negative electrode sheet.

[0032] For reference Figure 1 As shown, the negative electrode sheet includes a stacked negative electrode substrate 1 and a negative electrode active layer 2, and the negative electrode active layer 2 is disposed on at least one surface of the negative electrode substrate 1. After the negative electrode slurry is coated on the negative electrode substrate 1, it is dried in an oven to form the negative electrode active layer 2 on the surface of the negative electrode substrate 1. Drying in the oven mainly achieves the drying effect through three heat transfer methods: heat conduction, heat convection, and heat radiation. These heat transfer methods interact with each other and complement each other, and can efficiently promote the evaporation of water in the negative electrode active layer 2, thereby achieving the drying effect. However, currently, the main liquid component in the negative electrode slurry is water, whose boiling point is 100 °C, and it is difficult for the drying conditions in the oven to completely and effectively reduce the water residue in the negative electrode active layer 2.

[0033] The inventors found in practice that during the drying process, since the boiling point of the additive in the negative electrode slurry is lower than the boiling point of water (100 °C), it will evaporate out in the oven relatively quickly, and at the same time, it will carry the water away. In addition, due to the relatively high vapor pressure of the additive, its evaporation rate is also relatively fast, which can meet the requirements of rapid drying. And such additives can generally be miscible with water, which means they can be well mixed together. During the evaporation process, the additive molecules can interact with the water molecules to reduce the surface tension of the water, making it easier for the water molecules to escape from the surface of the negative electrode slurry, thereby accelerating the evaporation of the water molecules.

[0034] The embodiment of the present application provides a method for preparing a secondary battery. In the step of preparing the negative electrode slurry, a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive are added to the initial slurry and mixed. The first azeotropic additive, the second azeotropic additive, and the third azeotropic additive form an azeotropic mixture with azeotropic point less than 100 °C with water molecules. The boiling point of the first azeotropic additive is 50 °C to 65 °C, the boiling point of the second azeotropic additive is 65 °C to 85 °C, the boiling point of the third azeotropic additive is greater than the boiling point of the first azeotropic additive, and the boiling point of the third azeotropic additive is less than 100 °C. In the embodiment of the present application, the negative electrode slurry contains multiple azeotropic additives, which is beneficial to accelerating the evaporation of water molecules during the drying process, thereby reducing the water residue in the negative electrode sheet.

[0035] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity, specific order or primary-secondary relationship of the indicated technical features.

[0036] The following will elaborate on the embodiments of the present application in detail. However, those of ordinary skill in the art can understand that in the embodiments of the present application, many technical details are provided to help readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0037] Figure 2 It is a schematic flow chart of preparing a negative electrode sheet in a method for preparing a secondary battery provided by an embodiment of the present application.

[0038] In a first aspect, an embodiment of the present application provides a method for preparing a secondary battery, and the preparation method includes: preparing a negative electrode sheet. Among them, referring to Figure 2 , the preparation steps of the negative electrode sheet include:

[0039] Step 100: Prepare an initial slurry containing a negative electrode active material.

[0040] Step 200: Add a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive to the initial slurry and mix them to prepare a negative electrode slurry.

[0041] Among them, the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive form an azeotropic mixture with a boiling point lower than 100 °C with water molecules in the negative electrode slurry. The boiling point of the first azeotropic additive is 50 °C to 65 °C, the boiling point of the second azeotropic additive is 65 °C to 85 °C, the boiling point of the third azeotropic additive is higher than that of the first azeotropic additive, the boiling point of the third azeotropic additive is lower than 100 °C, and the third azeotropic additive forms a hydrogen bond with water molecules.

[0042] Step 300: Coat the negative electrode slurry on the surface of the negative electrode current collector and perform a drying treatment at a temperature lower than 100 °C to obtain a negative electrode sheet.

[0043] The method for preparing a secondary battery further includes: providing a positive electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet, and then placing them in a housing, and injecting an electrolyte into the housing to form a secondary battery.

[0044] The negative electrode paste provided by the embodiments of the present application contains three functional components, namely a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive. These additives can form an azeotropic mixture with water molecules with an azeotropic point lower than 100°C. On the one hand, since the boiling points of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive are all lower than the boiling point of water molecules, therefore, under the same temperature conditions, the vapor pressures of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive are all higher than that of water molecules. Thus, the evaporation rates of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive are all faster than that of water molecules, which can meet the requirements of rapid drying. The first azeotropic additive, the second azeotropic additive, and the third azeotropic additive form an azeotropic mixture with water molecules. During the evaporation process, the surface tension of water molecules in the negative electrode paste is reduced, making it easier for water molecules to escape from the surface of the negative electrode paste, which is conducive to the evaporation of water molecules. On the other hand, the negative electrode paste provided by the embodiments of the present application contains azeotropic additives with different boiling points. Through this multi-component synergistic action system, gradient evaporation of azeotropic additives with different boiling points can be achieved during the drying process after coating, thereby constructing a multi-level pore structure and significantly improving the evaporation efficiency of water in the negative electrode sheet. For example, the azeotropic mixture composed of the first azeotropic additive and the second azeotropic additive with increasing boiling points and water molecules evaporates in a gradient manner, and the boiling point of the third azeotropic additive is higher than that of the first azeotropic additive, and the third azeotropic additive forms a hydrogen bond with water molecules. This hydrogen bond binding is conducive to the uniform dispersion of water in the negative electrode paste, thus helping to avoid the problem of local aggregation of the paste after the evaporation of the first azeotropic additive, and further avoiding the problem of difficult removal of water caused by local aggregation.

[0045] It can be understood that the "azeotropy" described in the embodiments of the present application means that the additive can carry water molecules to evaporate together during the evaporation period, so there is an azeotropic relationship between the additive and water molecules.

[0046] Specifically, the boiling point of the first azeotropic additive is 50°C~65°C, and it quickly forms a low-boiling-point azeotrope with water in the early stage of drying, so that the low-boiling-point azeotrope can evaporate quickly in the early stage and take away the moisture in the negative electrode slurry. The boiling point of the second azeotropic additive is 65°C~85°C, and it continues to azeotropize with water in the mid-term stage, evaporating and taking away the moisture in the negative electrode slurry, avoiding interruption of the drying process or sudden temperature rise, which may cause material stress problems in the subsequently formed negative electrode sheet. The boiling point of the third azeotropic additive is greater than the boiling point of the first azeotropic additive, and the boiling point of the third azeotropic additive is less than 100°C. The third azeotropic additive combines with water molecules through hydrogen bonds, destroying the hydrogen bond network of water, reducing the evaporation enthalpy, and also helping to improve the dispersion uniformity of water molecules to avoid local aggregation of the negative electrode slurry, which is conducive to more thorough removal of residual moisture, especially for trace water that is difficult to evaporate. Therefore, the embodiment of the present application realizes an efficient, low-temperature and uniform drying process through the synergistic effect of the multi-component azeotropic system, taking into account both material protection and production efficiency, while destroying the hydrogen bond network of water to further optimize the dehydration effect, which can be used for the preparation of high-performance secondary batteries.

[0047] In the embodiments of the present application, the boiling point range of each additive (i.e., the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive) is controlled to achieve controllable regulation of water migration and phase separation behavior. For example, by controlling the boiling point of the first azeotropic additive within the range of 50°C to 65°C, including but not limited to discrete values such as 50°C, 52°C, 55°C, 60°C, or 65°C, or a continuous range between any two of the above values, the low-temperature evaporation component can quickly form micron-scale channels in the initial drying stage. By controlling the boiling point of the second azeotropic additive within the range of 65°C to 85°C, including but not limited to discrete values such as 65°C, 70°C, 75°C, 80°C, or 85°C, or a continuous range between any two of the above values, the intermediate transition component can form submicron-scale transition channels. The boiling point of the third azeotropic additive is greater than that of the first azeotropic additive and less than 100°C. The third azeotropic additive can form hydrogen bonds with water molecules, effectively regulating the migration path of water molecules and the phase separation dynamics, thereby preventing water agglomeration in the slurry and improving water evaporation efficiency.

[0048] Figure 3 This is a schematic cross-sectional structure diagram of a negative electrode sheet in a secondary battery provided in an embodiment of the present application. Figure 4 Another cross-sectional structural diagram of a negative electrode sheet in a secondary battery is provided in an embodiment of the present application. Figure 5 This is a schematic top view of the structure of a negative electrode sheet in a secondary battery provided in an embodiment of the present application.

[0049] like Figure 3 As shown in FIG. 1 , in some embodiments, the negative electrode slurry is coated on one surface of the negative electrode current collector 11 and dried to form the negative electrode active material layer 12. Figure 4As shown, in some other embodiments, the negative electrode paste is coated on two opposite surfaces of the negative electrode current collector 11, and after drying treatment, negative electrode active material layers 12 are respectively formed. Additionally, as Figure 5 shown, in some embodiments, the negative electrode paste may be coated only on the middle region of the negative electrode current collector 11 to form the negative electrode active material layer 12. The embodiments of the present application do not limit the coating region of the negative electrode paste. In some other embodiments, the negative electrode paste may also be coated on the entire region of the negative electrode current collector 11.

[0050] Figures 3 to 5 The porous structure in the negative electrode active material layer is schematically shown in a circular manner in

[0051] Please refer to Figures 3 to 5 shown. During the drying treatment after coating, the multi-stage azeotropic additive system composed of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive can form a stable porous structure through the gradient evaporation mechanism at the interface, cooperate with the surface tension regulation to optimize the path of water migration, and finally achieve the efficient removal of water molecules and the uniform stability of the negative electrode sheet after coating during the drying process. This not only significantly improves the drying efficiency but also forms an optimized porous structure in the negative electrode sheet, which is beneficial to electrolyte penetration, reduces the internal resistance of the negative electrode sheet, and reduces defects. Experiments show that adopting this technical solution can significantly reduce the water residue amount and the resistance of the negative electrode sheet in the secondary battery, and improve the rate performance and cycle stability of the secondary battery.

[0052] In some embodiments, the boiling point of the third azeotropic additive may be greater than the boiling point of the second azeotropic additive. In this way, during the evaporation of water carried by the second azeotropic additive, the third azeotropic additive has not evaporated yet. Therefore, the third azeotropic additive forms a hydrogen bond with water molecules. Thus, the presence of the third azeotropic additive can still make the water in the negative electrode paste evenly dispersed to prevent local aggregation, which is beneficial to further improving the water evaporation effect.

[0053] For example, the second azeotropic additive may be composed of at least two additives, and by reasonably setting the amounts of the additives, the boiling point of the second azeotropic additive is made less than the boiling point of the third azeotropic additive. Or, the second azeotropic additive is composed of one additive, and the boiling point of this additive is less than the boiling point of the third azeotropic additive.

[0054] It can be understood that in some other embodiments, the boiling point of the third azeotropic additive may also be less than the boiling point of the second azeotropic additive.

[0055] In some embodiments, at least one of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive is a polar solvent.

[0056] Since at least one of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive is a polar solvent, and water molecules are polar molecules, and polar molecules are easily soluble in polar solvents. Thus, the polar solvent and water molecules can be fully miscible, which can significantly improve the dispersibility and stability of the negative electrode slurry, enhance the interaction between the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive and water molecules, and thus more efficiently regulate the evaporation behavior during the drying process.

[0057] Specifically, the selection of the polar solvent can be based on its boiling point to match the corresponding azeotropic additive category. For the first azeotropic additive, a polar solvent with a low boiling point, such as a boiling point between 50°C and 65°C, can be selected to promote rapid evaporation in the initial stage of drying and form an initial pore structure, such as micron-sized pores. For the second azeotropic additive, a polar solvent with a high boiling point, such as a boiling point between 65°C and 85°C, can be used to maintain the evaporation gradient in the middle stage of drying and optimize the pore connectivity, such as further forming sub-micron-sized transition pores. For the third azeotropic additive, a polar solvent with a boiling point less than 100°C and higher than the boiling point of the first azeotropic additive can be selected, and this polar solvent forms a hydrogen bond with water molecules, delays the migration of water molecules through hydrogen bond interaction, improves the uniform dispersion effect of water, avoids aggregation and reduces the drying stress, and avoids cracking of the negative electrode sheet.

[0058] For example, any one of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive is a polar solvent. Or, any two of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive are polar solvents. Or, all three of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive are polar solvents.

[0059] The initial slurry also contains a conductive agent and a binder. When the first azeotropic additive, the second azeotropic additive, or the third azeotropic additive in the embodiments of the present application is a polar solvent, it can enhance the compatibility of the negative electrode active material, the conductive agent, and the binder, reduce agglomeration, and improve the uniformity and dispersibility of the negative electrode slurry. The multi-level pores formed by gradient evaporation are beneficial to electrolyte penetration, reduce the internal resistance of the negative electrode sheet, and reduce defects. The hydrogen bond interaction of the polar solvent can reduce the drying stress and inhibit the curling or cracking of the negative electrode sheet.

[0060] In some embodiments, the negative electrode active material, the conductive agent, the dispersant, the binder, and deionized water are mixed to form an initial slurry. In the embodiments of the present application, the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive can inhibit the gelation of the negative electrode slurry and regulate the rheological properties to inhibit the agglomeration of the negative electrode active material.

[0061] In the embodiments of the present application, there are no specific restrictions on the types of conductive agents, dispersants, and binders, which can be selected according to actual needs. Exemplarily, the conductive agent can be at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots (carbon quantum dots, CQDs), carbon nanotubes, graphene, carbon nanofibers, etc. The dispersant includes at least one of polyethylene glycol, hydroxypropyl methylcellulose, polyvinylpyrrolidone, sodium dodecyl sulfate, carboxymethyl starch, polyethylenebenzyltrimethylammonium salt, polyethylene glycol octylphenyl ether, polystyrene sulfonic acid, polypropylene maleic acid, polyethyleneimine, N-methylpyrrolidone, carboxymethyl cellulose (CMC), and carboxymethyl cellulose salts (such as sodium carboxymethyl cellulose Na-CMC). The binder can be at least one of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), water-based acrylic resin, and carboxymethyl cellulose.

[0062] In some embodiments, the boiling point of the first azeotropic additive is 50°C to 55°C, including but not limited to discrete values such as 50°C, 51°C, 52°C, 53°C, 54°C, or 55°C, or continuous range values between any two of the above. This low-temperature evaporation component can rapidly form micron-sized pores at the initial stage of drying. The boiling point of the second azeotropic additive is 80°C - 85°C, including but not limited to discrete values such as 80°C, 81°C, 82°C, 83°C, 84°C, or 85°C, or continuous range values between any two of the above. By precisely controlling the boiling point ranges of the first azeotropic additive and the second azeotropic additive, the evaporation kinetics and pore structure evolution during drying can be optimized.

[0063] In some embodiments, in the negative electrode slurry, the sum of the masses of the first azeotropic additive and the second azeotropic additive is 0.2% to 0.5% of the mass of the negative electrode active material, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3 to 1:7, and the mass of the third azeotropic additive is 0.1% to 0.3% of the mass of the negative electrode active material.

[0064] The sum of the masses of the first azeotropic additive and the second azeotropic additive accounts for 0.2% to 0.5% of the mass of the negative electrode active material, including but not limited to discrete values such as 0.2%, 0.3%, 0.4%, or 0.5%, or continuous range values between any two of the above. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3 to 1:7, including but not limited to discrete values such as 1:3, 1:4, 1:5, 1:6, or 1:7, or continuous range values between any two of the above.

[0065] The mass ratio of the first azeotropic additive to the second azeotropic additive should not be too small or too large. If the mass ratio of the first azeotropic additive to the second azeotropic additive is too small, the amount of the first azeotropic additive is relatively small, and the time for the first azeotropic additive to carry water molecules to evaporate is too short, and the time for establishing micron-scale initial pores is too short. If the mass ratio of the first azeotropic additive to the second azeotropic additive is too large, the amount of the second azeotropic additive is relatively small, and the time for the second azeotropic additive to carry water molecules to evaporate is too short, and the time for establishing submicron-scale pores is also relatively short, making it difficult to achieve the best effect of water molecule evaporation. Therefore, in some embodiments, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3 to 1:7, so as to balance the mass of the first azeotropic additive and the mass of the second azeotropic additive, so that both the first azeotropic additive and the second azeotropic additive can play a good promoting role in the evaporation effect of water molecules.

[0066] The mass of the third azeotropic additive accounts for 0.1% to 0.3% of the mass of the negative electrode active material, including but not limited to discrete values such as 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.25% or 0.3%, or continuous range values between any two of the above.

[0067] In some embodiments, the first azeotropic additive includes methyl acetate. This solvent has a low boiling point, high volatility and moderate polarity, can form an azeotropic system with water, quickly evaporate water and construct initial pores, thereby significantly optimizing the drying kinetics of the negative electrode sheet. The low boiling point characteristic of methyl acetate (boiling point is 56.9 °C) can preferentially evaporate in the range of 50 °C to 60 °C to form micron-scale pores, providing a fast channel for subsequent water dissipation.

[0068] In some embodiments, the second azeotropic additive is selected from at least one of tetrahydrofuran (boiling point is 66 °C), ethanol (boiling point is 78.4 °C), isopropanol (boiling point is 82.4 °C) or tert-butanol (boiling point is 82.5 °C). These solvents have a moderate boiling point range, can form a synergistic evaporation system with the first azeotropic additive, and significantly optimize the drying process and pore structure of the electrode sheet. Tetrahydrofuran (boiling point is 66 °C) as a medium-boiling transition solvent can form a continuous evaporation gradient with the first azeotropic additive (50 °C to 65 °C) to construct a through-hole network. Similarly, ethanol, isopropanol and tert-butanol can ensure stable evaporation in the range of 80 °C to 85 °C to form uniform submicron pores to optimize evaporation kinetics.

[0069] In some embodiments, the third azeotropic additive is preferably at least one of acetonitrile (boiling point: 81.6 °C) or methyl ethyl ketone (boiling point: 79.6 °C). The cyano group (-C≡N) in acetonitrile can form intermolecular hydrogen bonds with the H of water molecules, and the carbonyl group (-C=O) in methyl ethyl ketone can form intermolecular hydrogen bonds with the H of water molecules. By means of hydrogen bonding, the migration of water molecules is delayed, which is conducive to the uniform dispersion of water in the negative electrode paste to avoid local aggregation, and reduces the drying stress to avoid cracking of the negative electrode sheet. In addition, the boiling points of both acetonitrile and methyl ethyl ketone are higher than the boiling point of the first azeotropic additive, which is 50 °C to 65 °C. Acetonitrile or methyl ethyl ketone can form a continuous evaporation gradient with the first azeotropic additive to construct a through-hole network.

[0070] Acetonitrile is an organic compound with the chemical formula CH3CN or C2H3N. It is a colorless and transparent liquid with excellent solvent properties. It can dissolve a variety of organic, inorganic and gaseous substances and is infinitely miscible with water and alcohol. The boiling point of acetonitrile is usually between 81 °C and 82 °C, which is lower than the boiling point of water. As the third azeotropic additive, acetonitrile can evaporate prior to water during the drying process after coating. And due to its high vapor pressure, it can promote the evaporation of water and reduce the water residue in the negative electrode sheet. Acetonitrile can also form an azeotrope with water with a low boiling point, thus significantly reducing the water removal temperature. At the same time, its strong polarity and miscibility can ensure the uniform dispersion of the solvent; by reducing the surface tension of the negative electrode paste, an evaporation gradient is established, optimizing the water migration path and forming a porous structure to accelerate evaporation; in addition, acetonitrile can inhibit the gelation of the negative electrode paste, reduce the agglomeration of active substances by adjusting the rheological properties, and interfere with the hydrogen bond network of water molecules, reducing the risk of water retention, so as to achieve efficient and uniform drying.

[0071] Methyl ethyl ketone is a low-boiling polar additive with a low density and a high vapor pressure. It can dissolve in solvents such as water, ethanol, and ether, and its chemical formula is CH3COCH2CH3, with a boiling point of 79.6 °C. Similarly, as the third azeotropic additive, methyl ethyl ketone will evaporate prior to water during the drying process after coating and carry away the residual water in the negative electrode sheet. The molecular structure of methyl ethyl ketone contains a carbonyl group (C=O) and two alkyl groups (methyl and ethyl), which makes it highly polar and can form hydrogen bonds with water molecules to uniformly disperse water and inhibit local aggregation; at the same time, methyl ethyl ketone can form an azeotropic mixture with water and can optimize the rheological properties of the negative electrode paste, reducing the micropore closure phenomenon caused by the rapid evaporation of the solvent during the drying process and forming a through-type evaporation channel.

[0072] The chemical formula of tert-butanol is C4H 10O, with the structural formula of (CH3)3COH, is usually a colorless transparent liquid with a boiling point of 82.4 °C and has a certain solubility in water. Due to its high vapor pressure, it can accelerate the desorption of moisture during the drying stage after the negative electrode sheet is coated. At the same time, its low surface tension promotes the directional migration of moisture along the three-dimensional channels. During the drying process, its crystallization characteristics are utilized to form a stable ice crystal skeleton, achieving low-temperature and high-efficiency sublimation and suppressing structural stress, effectively controlling the shrinkage rate of the negative electrode sheet. In addition, its molecular structure can not only improve the fluidity of the negative electrode slurry to enhance the coating uniformity but also refine the pore structure through interface wetting regulation, ultimately synergistically optimizing the dehydration efficiency and the structural integrity of the negative electrode sheet, avoiding warping and deformation.

[0073] Methyl acetate has the chemical formula of CH3COOCH3 and a boiling point of 56.8 °C. Due to its low boiling point and its own characteristics, it can also remove the moisture in the negative electrode slurry during the drying process after coating. Its mechanism is mainly based on the synergistic effect of physical and chemistry: relying on its low boiling point characteristics, it evaporates first and absorbs heat. At the same time, it forms an azeotropic mixture with water, significantly reducing the moisture removal temperature. Its surface tension is greatly reduced compared with water, promoting the migration of moisture to the surface of the negative electrode slurry and reducing the residue. Moreover, the porous structure formed by rapid evaporation establishes a channel for subsequent moisture escape. In addition, methyl acetate weakens the hydrogen bond interaction between water molecules by adjusting the solvent polarity, inhibiting the agglomeration phenomenon caused by local gelation, thereby improving the drying uniformity.

[0074] In a specific example, the negative electrode active material can be graphite, the first azeotropic additive can be methyl acetate, the second azeotropic additive can be isopropyl alcohol, and the third azeotropic additive can be acetonitrile. During the drying treatment process after coating, a continuous evaporation curve is formed according to the sequential gradient evaporation trend of methyl acetate (boiling point 56.9 °C) → acetonitrile (81.6 °C) → isopropyl alcohol (82.4 °C), adjusting the evaporation temperature window to the range of 56.9 °C to 83 °C, reducing energy consumption. In addition, through the synergistic effect of the rapid pore-forming effect of methyl acetate, the hydrogen bond regulation function of acetonitrile, and the surface modification effect of isopropyl alcohol, a continuous evaporation gradient is formed, constructing a through-hole network, which is beneficial to the evaporation of water molecules.

[0075] In some other specific examples, the negative electrode active material includes graphite, the first azeotropic additive includes methyl acetate, the second azeotropic additive includes a mixture of tetrahydrofuran and tert-butanol, and the mass ratio of tetrahydrofuran to tert-butanol is 1:1 to 3:1; the third azeotropic additive includes acetonitrile. Among them, the boiling point of tetrahydrofuran is 66 °C, and the boiling point of ethanol is 78.4 °C.

[0076] For example, the first azeotropic additive is methyl acetate, the second azeotropic additive is a mixture of tetrahydrofuran and tert-butanol, and the mass ratio of tetrahydrofuran to tert-butanol is 1:1 to 3:1, and the third azeotropic additive is acetonitrile.

[0077] In a specific example, the negative electrode active material includes graphite; the first azeotropic additive includes methyl acetate; the second azeotropic additive includes a mixture of tetrahydrofuran, ethanol, and tert-butanol, and the mass ratio of tetrahydrofuran, ethanol, and tert-butanol is (1 to 2):(1 to 2):(1 to 2); the third azeotropic additive includes acetonitrile. Thus, a three-stage gradient evaporation system can be formed during the drying process after coating, including: methyl acetate (56.9 °C) rapidly evaporates at 50 °C to 60 °C to form main pores with a size of 5 μm to 8 μm. Tetrahydrofuran (66 °C) evaporates at 60 °C to 75 °C to construct transition pores with a size of 1 μm to 2 μm. Ethanol (78.4 °C) is beneficial for extending the low-temperature evaporation window and forming transition pores with a size of 1 μm to 3 μm. Tert-butanol (82.5 °C) and acetonitrile (81.6 °C) cooperate to form pores with a size of 300 nm to 800 nm at 75 °C to 85 °C.

[0078] In some other specific examples, the negative electrode active material includes graphite, the first azeotropic additive includes a mixture of methyl acetate and ethanol, the second azeotropic additive includes isopropyl alcohol, the third azeotropic additive includes acetonitrile, and the mass ratio of methyl acetate and ethanol is 1:1 to 3:1. Among them, the mass ratio of methyl acetate and ethanol can be 1:1 to 3:1, so the first azeotropic additive can form a binary low-boiling azeotropic system. During the drying process after coating, methyl acetate can rapidly evaporate to construct main pores with a size of 5 μm to 10 μm, and ethanol is beneficial for extending the low-temperature evaporation window and forming transition pores with a size of 1 μm to 3 μm.

[0079] For example, the first azeotropic additive is a mixture of methyl acetate and ethanol, the second azeotropic additive is isopropyl alcohol, the third azeotropic additive is acetonitrile, and the mass ratio of methyl acetate and ethanol is 1:1 to 3:1.

[0080] In still some other specific examples, the negative electrode active material includes graphite, the first azeotropic additive includes methyl acetate, the second azeotropic additive includes ethanol and tert-butanol, and the mass ratio of tert-butanol and ethanol is 1:1 to 1:3, and the third azeotropic additive includes acetonitrile. For example, the first azeotropic additive is methyl acetate, the second azeotropic additive is ethanol and tert-butanol, and the third azeotropic additive is acetonitrile.

[0081] The coating method includes at least one of microgravure coating, spraying, and synchronous coating methods.

[0082] In some embodiments, during the drying process, it includes: providing a multi-section oven, and the negative electrode current collector coated with the negative electrode slurry sequentially enters each section of the multi-section oven for drying; wherein, the multi-section oven includes n sections of ovens, the temperature of the first section of the oven is 60 °C to 70 °C, and in the moving direction of the negative electrode current collector, the temperature of the ovens at intervals in the n sections of ovens gradually increases, and the temperature of the nth section of the oven is 80 °C to 90 °C. n can be a positive integer greater than or equal to 10.

[0083] In the drying step of the embodiments of the present application, there is no need to increase the oven temperature, increase the air frequency, or increase the drying time. Thus, the problem that the negative electrode sheet is prone to crack during the drying process after coating is reduced, which is beneficial to improving the safety performance and cycling performance of the negative electrode sheet, can reduce energy consumption, and improve the efficiency of manufacturing the negative electrode sheet, which is beneficial to controlling the time and economic costs during the manufacturing process of secondary batteries. Specifically, the temperature of the first section of the oven is 60°C to 70°C, and the temperature of the alternately spaced ovens in the n sections of the oven gradually increases. Finally, the temperature of the nth section of the oven reaches 80°C to 90°C. By increasing the temperature in sequence, it can avoid the rapid increase in drying temperature from affecting the material stress. The temperature of the nth oven is 80°C to 90°C, which can ensure the evaporation of the second azeotropic additive and the third azeotropic additive, thus forming a gradient evaporation.

[0084] In some embodiments, the multi-section oven further includes b sections of ovens, which are located at the workstations after the nth section of the oven, and b is a positive integer greater than or equal to 2. Define two consecutive sections of ovens as a unit. The n ovens include m units. The temperature between adjacent units in the first unit to the mth unit increases and the temperature difference is the same, and the temperature difference is 5°C to 10°C. m is a positive integer greater than or equal to 6. The temperature of the (m + 1)th unit (the unit formed by b sections of ovens) is 70°C to 80°C. Exemplarily, the temperature of the first unit is 65°C, the temperature of the second unit is 70°C, the temperature of the third unit is 75°C, the temperature of the fourth unit is 80°C, the temperature of the fifth unit is 85°C, and the temperature of the sixth unit is 75°C. Specifically, the drying time of each oven is 6 s. Methyl acetate will evaporate rapidly in the first unit, tetrahydrofuran will evaporate in the second unit, ethanol will evaporate in the third unit, and tert-butanol and acetonitrile will evaporate in the fifth unit. The processing time of each unit is 12 s (for example, each section of the oven is dried for 6 s). Processing for 12 s at 65°C in the first unit is sufficient for methyl acetate to evaporate; processing for 12 s at 70°C in the second unit allows tetrahydrofuran to evaporate; and so on. In this way, the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive have sufficient time to evaporate at the corresponding temperatures. Through the gradient heating and unitized design, this oven system realizes the efficient staged evaporation of multiple solvents, and the overall drying time does not need to be extended.

[0085] In the second aspect, the embodiments of the present application provide a secondary battery prepared by the preparation method of the above-mentioned secondary battery. It should be noted that the relevant descriptions of the secondary battery in the above embodiments also apply to the following secondary battery solutions. To avoid repetition, the same or corresponding parts as those in the foregoing embodiments will not be described hereinafter, and reference can be made to the descriptions of the foregoing embodiments. Please refer to Figures 3 to 5 As shown, the secondary battery includes:

[0086] The negative electrode sheet, the negative electrode sheet includes a negative electrode current collector 11 and a negative electrode active material layer 12, the negative electrode active material layer 12 is disposed on at least one surface of the negative electrode current collector 11, the negative electrode active material layer 12 is obtained by drying a negative electrode paste, and the moisture residue amount of the negative electrode sheet is less than 922 ppm. A housing, a positive electrode sheet, and a separator, and after the positive electrode sheet, the separator, and the negative electrode sheet are wound or laminated, they are placed in the housing, and an electrolyte is injected into the housing.

[0087] In some embodiments, the material forming the housing includes at least one of nickel-plated steel, aluminum alloy, and aluminum plastic film. In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, and the positive electrode current collector can use aluminum foil.

[0088] The negative electrode current collector 11 can use materials such as metal foil or porous metal plate. For example, it can be a foil or porous plate formed of metals such as copper, nickel, titanium, iron, or their alloys.

[0089] In some embodiments, the material forming the negative electrode active material layer 12 includes a negative electrode active material. The types of negative electrode active materials are not specifically limited in the embodiments of the present application and can be selected according to actual needs. Exemplarily: the negative electrode active material can be at least one of natural graphite, artificial graphite, mesophase microcarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, metallic lithium, etc.

[0090] The material of the separator can be resin, glass fiber, or inorganic substance, etc. In some embodiments, there is no limitation on the above-mentioned separator, and any publicly known porous structure separator with electrochemical stability and chemical stability can also be selected, such as a single-layer or multi-layer film of at least one of glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF).

[0091] Taking the secondary battery as a lithium-ion battery as an example, the electrolyte includes a solvent, a lithium salt, and an additive. The solvent of the electrolyte in the embodiments of the present application can be any solvent known in the related art that can be used as an electrolyte. In addition, the electrolyte according to the present application can include an additive, and the additive can be any additive known in the related art that can be used as an electrolyte additive. As an example, the above-mentioned lithium salt can be at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium bis(oxalate) borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro bis(oxalate) phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate), etc.

[0092] The above secondary battery may be, but is not limited to, alkali metal secondary batteries such as lithium-ion secondary batteries, sodium-ion secondary batteries, and aluminum-ion secondary batteries. The above secondary battery may be, but is not limited to, a wound battery or a laminated battery, etc.

[0093] In a third aspect, an embodiment of the present application provides an energy storage system including the above secondary battery.

[0094] Due to the use of the secondary battery provided in the third aspect of the embodiment of the present application, the energy storage system can have a relatively high energy density. In some embodiments of the present application, the energy storage system includes an energy storage device and a power converter connected electrically. The energy storage device includes a receiving cavity and a secondary battery disposed in the receiving cavity. The power converter is configured to perform power conversion processing on voltage and / or current, and input the changed voltage and / or current into the energy storage device, so that the energy storage device can meet the power consumption requirements of the electrical equipment.

[0095] In a fourth aspect, an embodiment of the present application provides an electrical equipment including the above secondary battery and a load, and the secondary battery is used to supply power to the load.

[0096] Due to the use of the above secondary battery for power supply, the electrical equipment has strong market competitiveness. In some embodiments of the present application, the above electrical equipment includes, but is not limited to, 3C electronic products, such as mobile phones, laptop computers, tablet computers, drones, wearable electronic devices, etc. The electrical equipment may also include power vehicles, such as new energy vehicles, electric bicycles, etc.

[0097] In order to describe in more detail the preparation method of the secondary battery provided in the embodiment of the present application, the following will provide a variety of specific embodiments and comparative examples for more detailed description.

[0098] Example 1

[0099] Dry mix according to the ratio of 95.2 parts of negative electrode active material, 1.6 parts of conductive agent, and 1.4 parts of dispersant; then add deionized water as a solvent and continue to homogenize to obtain an initial slurry; subsequently, add a mixed solution of tert-butanol / methyl acetate (methyl acetate: tert-butanol = 1:3) with a mass ratio of 0.2% of the mass of graphite to the initial slurry; continue to add the remaining deionized water solvent to the initial slurry and homogenize, and add 1.8 parts of binder and stir to obtain a slurry. Finally, add methyl ethyl ketone and acetonitrile with a mass ratio of 1:3 and a mass ratio of 0.1% of the mass of graphite to the initial slurry, stir evenly to obtain a negative electrode slurry, and coat the negative electrode slurry on a copper foil at a coating speed of 15 m / min, and obtain a negative electrode sheet after drying.

[0100] Among them, methyl acetate is the first additive, tert-butanol is the second azeotropic additive, and methyl ethyl ketone and acetonitrile are the third azeotropic additives. The negative electrode active material is graphite, the conductive agent is carbon black, the binder is sodium carboxymethyl cellulose, and the dispersant is polyvinylpyrrolidone. The mass of deionized water in the initial slurry is 60% of the total mass of the initial slurry.

[0101] The drying process uses a multi-section oven with different temperature gradients for drying. There are a total of 12 sections in the oven. Among them, the temperatures of the first 10 sections of the oven gradually increase. The passing time of the negative electrode sheet in each section of the oven is 6 s. The temperatures of the first and second sections of the oven are 65 °C, the temperatures of the third and fourth sections of the oven are 70 °C, the temperatures of the fifth and sixth sections of the oven are 75 °C, the temperatures of the seventh and eighth sections of the oven are 80 °C, and the temperatures of the ninth and tenth sections of the oven are 85 °C. That is, n is 10. And the multi-section oven also includes the eleventh section of the oven and the twelfth section of the oven, and the temperatures of the eleventh section of the oven and the twelfth section of the oven are 75 °C.

[0102] Taking Example 1 as a reference, the types and ratios of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive in the negative electrode slurry are changed, and other steps remain unchanged. The types and ratios of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive in the negative electrode slurry in Examples 2 to 22 are as follows.

[0103] Example 2

[0104] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3, and the ratio between the mass of the third azeotropic additive and the mass of graphite is 0.1%. The negative electrode active material includes graphite, the first azeotropic additive is methyl acetate, the second azeotropic additive is isopropanol, and the third azeotropic additive is methyl ethyl ketone.

[0105] Example 3

[0106] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3, and the ratio between the mass of the third azeotropic additive and the mass of graphite is 0.1%. The negative electrode active material includes graphite, the first azeotropic additive is methyl acetate, the second azeotropic additive is tert-butanol and tetrahydrofuran, the mass ratio of tert-butanol to tetrahydrofuran is 1:3, and the third azeotropic additive is acetonitrile.

[0107] Example 4

[0108] The sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3, and the ratio of the mass of the third azeotropic additive to the mass of graphite is 0.1%. The negative electrode active material includes graphite, the first azeotropic additive is methyl acetate, the second azeotropic additive is tert-butanol and ethanol, and the mass ratio of tert-butanol to ethanol is 1:3. The third azeotropic additive is acetonitrile.

[0109] Example 5

[0110] The sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3, and the ratio of the mass of the third azeotropic additive to the mass of graphite is 0.1%. The negative electrode active material includes graphite, the first azeotropic additive is methyl acetate, the second azeotropic additive is tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 1:1:2. The third azeotropic additive is acetonitrile.

[0111] Example 6

[0112] The sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3, and the ratio of the mass of the third azeotropic additive to the mass of graphite is 0.1%. The negative electrode active material includes graphite, the first azeotropic additive is methyl acetate, the second azeotropic additive is tetrahydrofuran, isopropanol and ethanol, and the mass ratio of tetrahydrofuran, isopropanol and ethanol is 1:1:2. The third azeotropic additive is acetonitrile.

[0113] Example 7

[0114] The sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3, and the ratio of the mass of the third azeotropic additive to the mass of graphite is 0.1%. The negative electrode active material includes graphite, the first azeotropic additive is methyl acetate, the second azeotropic additive is tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 1:2:1. The third azeotropic additive includes acetonitrile.

[0115] Example 8

[0116] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.1%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, isopropyl alcohol and ethanol, and the mass ratio of tetrahydrofuran, isopropyl alcohol and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0117] Example 9

[0118] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.1%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butyl alcohol and ethanol, and the mass ratio of tetrahydrofuran, tert-butyl alcohol and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0119] Example 10

[0120] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.5%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.1%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butyl alcohol and ethanol, and the mass ratio of tetrahydrofuran, tert-butyl alcohol and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0121] Example 11

[0122] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.4%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.1%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butyl alcohol and ethanol, and the mass ratio of tetrahydrofuran, tert-butyl alcohol and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0123] Example 12

[0124] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.3%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol, and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol, and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0125] Example 13

[0126] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.1%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.3%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol, and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol, and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0127] Example 14

[0128] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.3%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol, and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol, and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0129] Example 15

[0130] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.2%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:7. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.3%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol, and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol, and ethanol is 1:2:1. The third azeotropic additive is acetonitrile.

[0131] Example 16

[0132] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additive is acetonitrile.

[0133] Example 17

[0134] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additive is methyl ethyl ketone.

[0135] Example 18

[0136] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additives are acetonitrile and methyl ethyl ketone, and the mass ratio of acetonitrile to methyl ethyl ketone is 3:1.

[0137] Example 19

[0138] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additives are acetonitrile and methyl ethyl ketone, and the mass ratio of acetonitrile to methyl ethyl ketone is 2:1.

[0139] Example 20

[0140] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additives are acetonitrile and methyl ethyl ketone, and the mass ratio of acetonitrile to methyl ethyl ketone is 1:1.

[0141] Example 21

[0142] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additives are acetonitrile and methyl ethyl ketone, and the mass ratio of acetonitrile to methyl ethyl ketone is 1:2.

[0143] Example 22

[0144] The ratio between the sum of the masses of the first azeotropic additive and the second azeotropic additive and the mass of graphite is 0.3%. The mass ratio of the first azeotropic additive to the second azeotropic additive is 1:5. The ratio between the mass of the third azeotropic additive and the mass of graphite is 0.2%. The negative electrode active material includes graphite. The first azeotropic additive is methyl acetate. The second azeotropic additives are tetrahydrofuran, tert-butanol and ethanol, and the mass ratio of tetrahydrofuran, tert-butanol and ethanol is 2:1:1. The third azeotropic additives are acetonitrile and methyl ethyl ketone, and the mass ratio of acetonitrile to methyl ethyl ketone is 1:3.

[0145] Comparative Example 1: The initial slurry was obtained by homogenizing according to the proportions of each substance (graphite, conductive agent, dispersant and binder) in Example 1 without adding the first azeotropic additive, the second azeotropic additive and the third azeotropic additive. This initial slurry was used as the negative electrode slurry, and the negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, a negative electrode sheet was obtained.

[0146] Comparative Example 2: The initial slurry was obtained by mixing according to the proportions of each substance (graphite, conductive agent, dispersant and binder) in Example 1. Subsequently, acetonitrile with a mass proportion of 0.3% of the mass of graphite was added to the initial slurry, and after stirring evenly, a negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, a negative electrode sheet was obtained.

[0147] Comparative Example 3: The initial slurry was obtained by mixing according to the same proportions of each substance (graphite, conductive agent, dispersant, and binder) as in Example 1. Finally, tert-butanol with a mass ratio of 0.3% of the mass of graphite was added to the initial slurry, and after stirring evenly, the negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min, and after drying, a negative electrode sheet was obtained.

[0148] Comparative Example 4: The initial slurry was obtained by mixing according to the same proportions of each substance (graphite, conductive agent, dispersant, and binder) as in Example 1. Finally, methyl acetate with a mass ratio of 0.3% of the mass of graphite was added to the initial slurry, and after stirring evenly, the negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min, and after drying, a negative electrode sheet was obtained.

[0149] Comparative Example 5: The initial slurry was obtained by mixing according to the same proportions of each substance (graphite, conductive agent, dispersant, and binder) as in Example 1. Finally, methyl ethyl ketone with a mass ratio of 0.3% of the mass of graphite was added to the initial slurry, and after stirring evenly, the negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min, and after drying, a negative electrode sheet was obtained.

[0150] Comparative Example 6: The initial slurry was obtained by mixing according to the same proportions of each substance (graphite, conductive agent, dispersant, and binder) as in Example 1. Then, methyl acetate with a mass ratio of 0.3% of the mass of graphite and methyl ethyl ketone with a mass ratio of 0.2% of the mass of graphite were added to the initial slurry, and after stirring evenly, the negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min, and after drying, a negative electrode sheet was obtained.

[0151] Comparative Example 7: The initial slurry was obtained by mixing according to the same proportions of each substance (graphite, conductive agent, dispersant, and binder) as in Example 1. Then, methyl acetate with a mass ratio of 0.3% of the mass of graphite and tert-butanol (mass ratio 1:3) were added to the initial slurry, and after stirring evenly, the negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min, and after drying, a negative electrode sheet was obtained.

[0152] Comparative Example 8: The initial slurry was obtained by mixing according to the same proportions of each substance (graphite, conductive agent, dispersant, and binder) as in Example 1. Then, tert-butanol with a mass ratio of 0.3% of the mass of graphite and acetonitrile with a mass ratio of 0.2% of the mass of graphite were added to the initial slurry, and after stirring evenly, the negative electrode slurry was obtained. The negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min, and after drying, a negative electrode sheet was obtained.

[0153] Comparative Example 9: The initial slurry was obtained by mixing the substances (graphite, conductive agent, dispersant, and binder) in the same proportions as in Example 1. Then, isopropyl alcohol and tert-butanol with a mass ratio of 1:1 and each accounting for 0.3% of the mass of graphite, and acetonitrile accounting for 0.2% of the mass of graphite were added to the initial slurry. After stirring evenly, the negative electrode slurry was obtained, and the negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, the negative electrode sheet was obtained.

[0154] Comparative Example 10: The initial slurry was obtained by mixing the substances (graphite, conductive agent, dispersant, and binder) in the same proportions as in Example 1. Then, isopropyl alcohol and tert-butanol each accounting for 0.3% of the mass of graphite, and acetonitrile accounting for 0.2% of the mass of graphite were added to the initial slurry. After stirring evenly, the negative electrode slurry was obtained, and the negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, the negative electrode sheet was obtained.

[0155] Comparative Example 11: The initial slurry was obtained by mixing the substances (graphite, conductive agent, dispersant, and binder) in the same proportions as in Example 1. Then, isopropyl alcohol and tert-butanol each accounting for 0.3% of the mass of graphite, and methyl ethyl ketone accounting for 0.2% of the mass of graphite were added to the initial slurry. After stirring evenly, the negative electrode slurry was obtained, and the negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, the negative electrode sheet was obtained.

[0156] Comparative Example 12: The initial slurry was obtained by mixing the substances (graphite, conductive agent, dispersant, and binder) in the same proportions as in Example 1. Then, a mixture of tetrahydrofuran, isopropyl alcohol, and ethanol with a mass ratio of 1:1:2 and each accounting for 0.3% of the mass of graphite, and acetonitrile accounting for 0.2% of the mass of graphite were added to the initial slurry. After stirring evenly, the negative electrode slurry was obtained, and the negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, the negative electrode sheet was obtained.

[0157] Comparative Example 13: The initial slurry was obtained by mixing the substances (graphite, conductive agent, dispersant, and binder) in the same proportions as in Example 1. Then, a mixture of methyl acetate, tetrahydrofuran, isopropyl alcohol, and ethanol with the mass ratio of methyl acetate to the sum of the masses of tetrahydrofuran, isopropyl alcohol, and ethanol being 1:3, and the mass ratio of tetrahydrofuran, isopropyl alcohol, and ethanol being 1:1:2, and each accounting for 0.3% of the mass of graphite, and acetonitrile accounting for 0.2% of the mass of graphite were added to the initial slurry. After stirring evenly, the negative electrode slurry was obtained, and the negative electrode slurry was coated on a copper foil at a coating speed of 15 m / min. After drying, the negative electrode sheet was obtained.

[0158] Preparation of Lithium-Ion Battery

[0159] Preparation of the positive electrode sheet: The positive electrode active material (specifically lithium phosphate), the conductive agent (specifically superp), and the positive electrode binder (specifically polyvinylidene fluoride) were added to the solvent (specifically N-methylpyrrolidone) according to a mass ratio of 96.5:1.0:2.5, and stirred evenly to obtain the positive electrode slurry. The above positive electrode slurry was coated on the opposite two side surfaces of the positive electrode current collector (specifically aluminum foil) by using a coating device, and after processes such as drying, cold pressing, and welding the electrode tabs, the positive electrode sheet was prepared.

[0160] Preparation of the negative electrode sheet: The negative electrode sheet was prepared according to the preparation steps of the negative electrode sheet provided in the above embodiment.

[0161] Preparation of the separator: An antioxidant layer (specifically containing ceramics and boehmite) was formed on the base film (specifically containing PP) with a binder (specifically polyvinylidene fluoride).

[0162] Preparation of the battery: The above positive electrode sheet, the separator, and the battery negative electrode sheets prepared in each example and comparative example were subjected to winding treatment or lamination treatment and then placed in a housing, and the battery separator was placed between the positive electrode sheet and the negative electrode sheet, and an electrolyte was injected into the housing; the lithium-ion secondary batteries of each example and comparative example were obtained. Among them, the electrolyte was a 1 mol / L LiPF6 solution, and the electrolyte solvent was ethylene carbonate, diethyl carbonate, and dimethyl carbonate with a volume ratio of 1:1:1.

[0163] Testing method

[0164] (1) Viscosity test

[0165] The viscosity of the stirred negative electrode slurry was tested by a rotational viscometer (the specific model of the viscometer can be IKA ROTAVISClo-vi Complete) at 25 ± 2 °C (4# rotor, rotation speed of 12 revolutions per minute), and the termination condition was that the test time reached 3 min.

[0166] (2) Residual moisture content of the negative electrode sheet

[0167] The Karl Fischer method was used to determine the moisture in the negative electrode sheet: The moisture meter was used in combination with a KF furnace to enable rapid and accurate testing of the moisture content of the battery electrode sheet. Sampling process of the solid sample: About 1 g of the cut battery electrode sheet of the negative electrode sheet was weighed by an electronic balance and placed in a sample bottle, and sealed with a capping device to keep the moisture content of the solid sample unchanged. The gas in the encapsulation environment was taken into the sample bottle and sealed with a capping device as the test blank bottle. The test of the moisture content of the battery electrode sheet was a solid sample, and the heating furnace needed to be turned on. At the end of the experiment, the ventilation system could be stopped only after the temperature dropped to room temperature to prevent the Karl Fischer reagent from being sucked back into the heating device. Test instrument parameter settings: Carrier gas flow rate was 50 mL / min, heating temperature was 120 °C, and termination time was 300 s.

[0168] (3)Negative electrode sheet weight loss rate test:

[0169] Use a Sartorius drying loss test device to measure the weight loss rate of the negative electrode sheet after coating and drying. The test rate is 0.1% / 24 s, and the termination temperature is 130°C.

[0170] (4)Negative electrode sheet film resistance test

[0171] Use a pole piece resistance meter to test the film resistance of the negative electrode sheet after coating, drying, and rolling. Each time, test 5 different positions of the negative electrode sheet, and take the average value as the test result.

[0172] (5)Battery performance test

[0173] Perform a nuclear capacity test on the lithium-ion batteries prepared in each example and comparative example at 0.2C at 25°C, then discharge them at a constant current until 2.5V, and let them stand for 30 min.

[0174] Then, after leaving the lithium-ion battery at 55°C for 4 h, charge it at a constant current and constant voltage of 0.5C until 3.65V / 0.05C cut-off. After standing for 30 min, discharge it at a constant current of 0.5C until 2.5V, and let it stand for 30 min as one cycle. After 500 cycles, record the capacity retention rate of the battery. Among them, the first-cycle Coulomb efficiency = the first charging specific capacity of the battery / the discharging specific capacity × 100%, and the capacity retention rate = the capacity of the battery after 500 cycles / the capacity of the first cycle × 100%.

[0175] Result analysis

[0176] The test results of the relevant performance indicators of the negative electrode slurry, negative electrode sheet, and lithium-ion battery are shown in Table 1:

[0177] Table 1

[0178]

[0179] From the above test results, it can be seen that after introducing the first, second, and third azeotropic additives into the negative electrode slurry system, the viscosity of the system shows a controllable increase, and its viscosity value is distributed in the range of 8500 mPa·s to 9050 mPa·s, which is within 10% higher than that of Comparative Example 1 without adding the azeotropic agent (viscosity value is 8100 mPa·s). This is mainly because the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive can form an azeotropic mixture with water molecules, and the third azeotropic additive can form hydrogen bonds with water molecules to avoid local aggregation during the evaporation process. However, since the mass ratio of the total amount of the three types of azeotropic additives to the mass of graphite is controlled within 0.8%, the viscosity increase range is not large and does not cause adverse effects on the subsequent coating process.

[0180] Performance test data on the moisture residue of the negative electrode sheet shows that by introducing a ternary co-azeotropic dehydration system formed by specific ratios of the first, second, and third azeotropic additives, the water content of the negative electrode sheet can be reduced. Specifically, the moisture residue of the negative electrode sheets in Examples 1 to 22 is controlled within the range of 602 ppm to 922 ppm. Compared with Comparative Example 1 where no components were added (the moisture residue of the negative electrode sheet was 1598 ppm), the moisture residue in the examples of this application decreased by 54.3% to 62.3%; compared with the single-component system (Comparative Examples 2 to 5: the moisture residue of the negative electrode sheet was 1323 ppm to 1426 ppm) and the two-component system (Comparative Examples 6 to 8: the moisture residue of the negative electrode sheet was 1134 ppm to 1299 ppm), the optimization improvements of 35.2% to 57.8% and 20.5% to 53.4% were respectively achieved. In addition, even when compared with Comparative Examples 9 to 13 where three or more components were added (the moisture residue of the negative electrode sheet was 989 ppm to 1139 ppm), it still showed better dehydration performance. Its highest moisture residue (922 ppm) was further reduced by 6.8% compared with the lowest value (989 ppm) of the comparative examples, fully verifying the unique advantages of the ternary azeotropic system for dehydration.

[0181] In addition, the three azeotropic additives construct a multi-stage dehydration network through the boiling point gradient distribution and polarity complementary characteristics: the first azeotropic additive (boiling point of 50°C to 65°C) preferentially forms a low-boiling azeotrope with free water, the second azeotropic additive (boiling point of 65°C to 85°C) combines with water for deep dehydration, and the third azeotropic additive (the boiling point of the third azeotropic additive is less than 100°C) inhibits water re-adsorption through intermolecular forces. And the composite additive system of methyl acetate / tetrahydrofuran + tert-butanol + ethanol / acetonitrile in Example 20 had the best overall performance. The second azeotropic additive includes tetrahydrofuran + tert-butanol + ethanol, which can further evaporate water molecules and can minimize the water molecules in the negative electrode sheet to the greatest extent.

[0182] Furthermore, to verify the above experimental results, after the negative electrodes in Examples 1 to 22 were dried, the weight loss rate of the negative electrodes was measured. The results showed that the ternary synergistic dehydration system formed by introducing the first, second, and third azeotropic additives with specific ratios could reduce the weight loss rate of the negative electrodes. Specifically, the weight loss rate of the negative electrodes in Examples 1 to 22 was 0.25% to 0.39%, which was the lowest compared with Comparative Example 1 (the weight loss rate of the negative electrode was 0.66%) without adding any components. The reduction in the moisture residue amount in the examples of this application reached 40.9% to 62.1%. Compared with the single-component system (Comparative Examples 2 to 5: the weight loss rate of the negative electrode was 0.47% to 0.59%) and the two-component system (Comparative Examples 6 to 8: the weight loss rate of the negative electrode was 0.47% to 0.54%), the optimization improvements of 17.0% to 57.6% and 17.0% to 53.7% were respectively achieved. Among them, even compared with Comparative Examples 9 to 13 (the weight loss rate of the negative electrode was 0.41% to 0.47%) with three or more components added, Examples 1 to 22 still showed a better weight loss rate of the negative electrode. The weight loss rate of the negative electrode (the weight loss rate of the negative electrode was 0.39%) was further reduced by 5.1% compared with the lowest value of the weight loss rate of the negative electrode in all comparative examples (the weight loss rate of the negative electrode was 0.41%), further verifying the excellent dehydration effect of Examples 1 to 22 with the multi-component synergistic formulation.

[0183] Based on the advantage of better dehydration in Examples 1 to 22 of this application, the negative electrodes in Examples 1 to 22 were assembled into lithium-ion secondary batteries to verify the electrical cycling performance. The results showed that the film resistance of the negative electrodes in Examples 1 to 22 was 3.919047 Ω·mm to 5.529465 Ω·mm. Compared with the film resistance of the negative electrodes in Comparative Examples 1 to 13, which was 5.120549 Ω·mm to 7.590324 Ω·mm, the film resistance of the electrode sheets decreased. The first-cycle Coulombic efficiency of Examples 1 to 22 was 94.11% to 94.57%. The first-cycle Coulombic efficiency of the lithium-ion batteries all exceeded 94%. Compared with the first-cycle Coulombic efficiency (91.49% to 93.97%) of the negative electrode film sheets in Comparative Examples 1 to 13, the first-cycle Coulombic efficiency of the lithium-ion secondary batteries was improved. The capacity retention rate after 500 cycles of Examples 1 to 22 was 95.11% to 95.88%. The capacity retention rate after 500 cycles exceeded 95%. Compared with the capacity retention rate (91.22% to 94.13%) after 500 cycles in Comparative Examples 1 to 13, the cycling performance of the lithium-ion secondary batteries in the examples of this application was improved. This was because the reduction of water molecules in the negative electrode reduced the occurrence of various side reactions, reduced the decomposition of the electrolyte and the charge transfer resistance, thus improving the cycling performance of the lithium-ion secondary batteries.

[0184] In summary, the presence of water molecules in the negative electrode sheet will disrupt the conductive network formed by the conductive agent, making the electron transport path in the negative electrode sheet more complex and tortuous, thereby having an adverse impact on the electrical cycling performance of the lithium-ion secondary battery. By introducing the first, second, and third azeotropic additives into the negative electrode slurry system in the embodiments of the present application, it is possible to significantly reduce the water residue and film resistance in the negative electrode sheet without changing the process conditions such as coating and drying, and improve the first-cycle Coulomb efficiency and cycling performance of the lithium-ion battery, and to a certain extent improve the adverse impact of the water residue in the negative electrode sheet on the cycling performance of the lithium-ion secondary battery.

[0185] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments for implementing the present application, and in actual applications, various changes can be made in form and details without departing from the spirit and scope of the present application. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be subject to the scope defined by the claims.

Claims

1. A method for preparing a secondary battery, characterized in that, Comprising: Preparing a negative electrode sheet, the preparation steps of the negative electrode sheet comprising: Preparing an initial slurry containing a negative electrode active material; Adding a first azeotropic additive, a second azeotropic additive, and a third azeotropic additive to the initial slurry and mixing to prepare a negative electrode slurry, wherein the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive form an azeotropic mixture with azeotropic point less than 100 °C with water molecules in the negative electrode slurry, the boiling point of the first azeotropic additive is 50 °C to 65 °C, the boiling point of the second azeotropic additive is 65 °C to 85 °C, the boiling point of the third azeotropic additive is greater than the boiling point of the first azeotropic additive, the boiling point of the third azeotropic additive is less than 100 °C, and the third azeotropic additive forms a hydrogen bond with the water molecules; wherein, the first azeotropic additive comprises methyl acetate; the second azeotropic additive comprises at least one of tetrahydrofuran, ethanol, isopropanol, and tert-butanol; the third azeotropic additive comprises at least one of acetonitrile and methyl ethyl ketone; Coating the negative electrode slurry on the surface of a negative electrode current collector and performing a drying treatment at a temperature less than 100 °C to obtain a negative electrode sheet; Providing a positive electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet and then placing them in a housing, and injecting an electrolyte into the housing to form the secondary battery.

2. The method for preparing a secondary battery according to claim 1, wherein At least one of the first azeotropic additive, the second azeotropic additive, and the third azeotropic additive is a polar solvent.

3. The method for preparing a secondary battery according to claim 1, wherein The boiling point of the first azeotropic additive is 50 °C to 55 °C, and the boiling point of the second azeotropic additive is 80 °C to 85 °C.

4. The method for preparing a secondary battery according to claim 1, wherein, In the negative electrode slurry, the sum of the masses of the first azeotropic additive and the second azeotropic additive is 0.2% to 0.5% of the mass of the negative electrode active material, the mass ratio of the first azeotropic additive to the second azeotropic additive is 1:3 to 1:7, and the mass of the third azeotropic additive is 0.1% to 0.3% of the mass of the negative electrode active material.

5. The method for preparing a secondary battery according to claim 1, wherein The negative electrode active material comprises graphite, the first azeotropic additive comprises the methyl acetate, the second azeotropic additive comprises the isopropanol, and the third azeotropic additive comprises the acetonitrile.

6. The method for preparing a secondary battery according to claim 1, wherein The negative electrode active material comprises graphite; the first azeotropic additive comprises the methyl acetate; the second azeotropic additive comprises a mixture of the tetrahydrofuran, the ethanol, and the tert-butanol, and the mass ratio of the tetrahydrofuran, the ethanol, and the tert-butanol is (1 to 2):(1 to 2):(1 to 2); the third azeotropic additive comprises the acetonitrile.

7. The method for preparing a secondary battery according to claim 1, characterized in that, The negative electrode active material comprises graphite, the first azeotropic additive comprises the methyl acetate, the second azeotropic additive comprises a mixture of the ethanol and the tert-butanol, and the mass ratio of the tert-butanol to the ethanol is 1:1 to 1:3; the third azeotropic additive comprises the acetonitrile.

8. The method for preparing a secondary battery according to claim 1, wherein The drying treatment comprises: Providing a multi-section oven, and the negative electrode current collector coated with the negative electrode slurry sequentially enters each section of the multi-section oven for drying; Among them, the multi-section oven includes n sections of ovens. The temperature of the first section of the oven is 60°C to 70°C, and in the moving direction of the negative current collector, the temperature of the alternately spaced ovens among the n sections of ovens gradually increases, and the temperature of the nth section of the oven is 80°C to 90°C.

9. A secondary battery prepared by using the preparation method of the secondary battery according to any one of claims 1 to 8, characterized in that, Comprising: A negative electrode sheet, the negative electrode sheet includes a negative current collector and a negative active material layer, the negative active material layer is disposed on at least one surface of the negative current collector, the negative active material layer is obtained by drying a negative electrode paste, and the water residue amount of the negative electrode sheet is less than 922 ppm; A housing, a positive electrode sheet, and a separator, and the positive electrode sheet, the separator, and the negative electrode sheet are put into the housing after being wound or laminated, and an electrolyte is injected into the housing.

10. A energy storage system, characterized in that, Comprising the secondary battery according to claim 9.

11. An electrical device, characterized in that, Comprising the secondary battery according to claim 9 and a load, and the secondary battery is used to supply power to the load.

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