Negative electrode sheet, secondary battery, and manufacturing method for negative electrode sheet

Abstract

The present application relates to the technical field of new energy, and provides a negative electrode sheet, a secondary battery, and a manufacturing method for a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and a negative electrode coating; the negative electrode coating is applied to the negative electrode current collector; the negative electrode coating comprises a solid-state mixture; the solid-state mixture comprises a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickening agent; the composition ratios of the negative electrode coating satisfy the relational expression: 2.6%<pm1 / 100+2m2+m3≤10%, wherein m1 is the mass fraction of the thickening agent in the solid-state mixture, m2 is the mass fraction of the dispersant in the solid-state mixture, m3 is the mass fraction of the binder in the solid-state mixture, and p is the degree of polymerization of the thickening agent. The negative electrode sheet provided by the present application can solve the problems of increased internal resistance, reduced rate capability and shortened cycle service life of a battery caused by the increase in the energy density of the battery, thereby simultaneously improving the energy density, rate capability and cycle service life of the battery.

Description

A negative electrode sheet, a secondary battery, and a method for preparing the negative electrode sheet. This application claims priority to Chinese Patent Application No. 2024115697360, filed on November 5, 2024, entitled “A negative electrode sheet, a secondary battery and a method for preparing a negative electrode sheet”, the entire contents of which are incorporated herein by reference. Technical Field

[0001] This application relates to the field of new energy technology, and in particular to a negative electrode sheet, a secondary battery, and a method for preparing the negative electrode sheet. Background Technology

[0002] Lithium-ion rechargeable batteries have been widely used in electric passenger vehicles, commercial energy storage, and communication energy storage in recent years, which have placed higher demands on the energy density of lithium rechargeable batteries. Among these applications, increasing the proportion of active material in the negative electrode is an important technical means to improve battery energy density.

[0003] In related technologies, increasing the proportion of active material in the negative electrode sheet comes at the cost of reducing the amount of other solid materials in the negative electrode formulation besides the active material. However, although the energy density of the battery is improved after adopting this formulation of the negative electrode sheet, it is also accompanied by an increase in the internal resistance of the battery and a decrease in rate performance, which leads to a shortened cycle life of the battery. Summary of the Invention

[0004] This application provides a negative electrode sheet, a secondary battery, and a method for preparing the negative electrode sheet, in order to solve the problems of increased internal resistance, decreased rate performance, and shortened cycle life caused by the increase in battery energy density, thereby achieving a simultaneous improvement in battery energy density, rate performance, and cycle life.

[0005] In a first aspect, this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode coating; the negative electrode coating is coated on the negative electrode current collector, and the negative electrode coating comprises a solid mixture, which includes a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickener, and the proportions of the negative electrode coating satisfy the following relationship: Where m1 is the mass fraction of thickener in the solid mixture, m2 is the mass fraction of dispersant in the solid mixture, m3 is the mass fraction of binder in the solid mixture, and p is the degree of polymerization of thickener.

[0006] The negative electrode sheet provided in the first aspect above, by satisfying the aforementioned relationship in the ratio of the negative electrode coating, allows the mass fractions of thickener, dispersant, and binder to be controlled within a small range. This increases the mass fractions of the negative electrode active material and conductive agent. When the amount of conductive agent required for different batteries is met, the amount of negative electrode active material layer can be increased, thereby increasing the battery's energy density. Simultaneously, the negative electrode coating has a high viscosity in the slurry state, making the negative electrode slurry less prone to sedimentation and unevenness. Furthermore, after the negative electrode slurry is coated onto the negative electrode current collector, its adhesion to the current collector is good. During battery cycling, the negative electrode coating is less likely to detach from the current collector. Therefore, batteries using the aforementioned negative electrode sheet also exhibit high rate performance and long cycle life. It is evident that, satisfying the aforementioned relationship, batteries using the aforementioned negative electrode sheet can achieve a balance of high energy density, excellent rate performance, and long cycle life.

[0007] In one possible design, the thickener comprises at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and sodium alginate, and the mass fraction of the thickener in the solid mixture satisfies 0%. <m1≤2%。

[0008] Through the above scheme, when the negative electrode sheet is in a slurry state, the viscosity of the negative electrode slurry can be appropriately increased, its fluidity reduced, and its uniformity increased by using a certain amount of thickener. This makes it easier to coat and process, and the negative electrode coating formed after the negative electrode slurry dries has good adhesion to the negative electrode current collector, preventing the battery from swelling. By controlling the mass fraction of the thickener within the above range, the impact of the thickener's addition on the mass fraction of other substances in the solid mixture can be reduced, enabling the battery to achieve a balance of high energy density, excellent cycle performance, and long cycle life.

[0009] In one possible design, the degree of polymerization of the thickener satisfies 100 ≤ p ≤ 5000.

[0010] The above method can control the viscosity and toughness of the negative electrode slurry within a certain range, thereby improving the processability of the negative electrode slurry.

[0011] In one possible design, the degree of polymerization of the thickener satisfies 100 ≤ p ≤ 2000.

[0012] The above scheme further improves the processability of the negative electrode slurry.

[0013] In one possible design, the dispersant includes at least one of polyacrylic acid, carboxymethyl chitosan, polyvinyl alcohol, and modified materials thereof, and the mass fraction of the dispersant in the solid mixture satisfies 0% ≤ m2 ≤ 2%.

[0014] By using a certain amount of dispersant in the solid mixture, when the negative electrode sheet is in slurry state, the components of the solid mixture can be more evenly dispersed in the solvent. After the negative electrode slurry is dried on the surface of the negative electrode current collector to become a negative electrode coating, the composition of each part of the negative electrode coating will also be more uniform, which is beneficial to improving the battery capacity and cycle performance.

[0015] In one possible design, the adhesive includes at least one of styrene-butadiene rubber, styrene-acrylate, polyacrylonitrile, polymethyl methacrylate, and modified materials thereof, and the mass fraction of the adhesive in the solid mixture satisfies 0% ≤ m3 ≤ 2%.

[0016] By using a certain amount of binder in the solid mixture, the viscosity of the negative electrode slurry can be increased when the negative electrode sheet is in a slurry state, thereby improving the processing performance of the negative electrode slurry. After the negative electrode slurry dries to become a negative electrode coating on the surface of the negative electrode current collector, the negative electrode coating will also be denser, and the adhesion between the negative electrode coating and the negative electrode current collector will be good and not easy to detach, reducing the expansion of the negative electrode sheet during use.

[0017] In one possible design, the proportions of the solid mixture satisfy the following relationship:

[0018] Through the above scheme, the energy density of the battery using this negative electrode sheet is further increased, the rate performance is better, and the cycle life is longer.

[0019] In one possible design, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphite sheets, and graphene; and / or, the negative electrode active material includes at least one of graphite, silicon suboxide, silicon, silicon carbide, lithium titanate, and modified materials thereof.

[0020] Through the above scheme, in the negative electrode coating containing any of the above-mentioned conductive agents and any of the above-mentioned negative electrode active materials, as long as the solid mixture satisfies the above-mentioned relationship, the energy density, rate performance and cycle life of the battery can be improved simultaneously.

[0021] Secondly, this application provides a secondary battery, including the negative electrode sheet in any of the above embodiments.

[0022] Through the above scheme, the secondary battery has high energy density, excellent rate performance and long cycle life.

[0023] Thirdly, this application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0024] A solid mixture is formed by mixing the negative electrode active material, conductive agent, binder, dispersant, and thickener. The proportions of the solid mixture satisfy the following relationship: Where m1 is the mass fraction of thickener in the solid mixture, m2 is the mass fraction of dispersant in the solid mixture, m3 is the mass fraction of binder in the solid mixture, and p is the degree of polymerization of thickener.

[0025] The negative electrode slurry is obtained by mixing the solid mixture with a solvent.

[0026] The negative electrode slurry is coated on both sides of the negative electrode current collector.

[0027] The negative electrode current collector coated with negative electrode slurry is dried, cold-pressed, and cut to obtain the negative electrode sheet.

[0028] The above method involves first preparing a solid mixture, ensuring that all components are uniformly mixed, and then adding a solvent to prepare the negative electrode slurry, resulting in a more uniform texture. Furthermore, by ensuring the solid mixture satisfies the aforementioned relationship, the prepared negative electrode sheet enables the battery to exhibit high energy density, excellent rate performance, and a long cycle life during use.

[0029] The secondary battery and its beneficial effects provided in the second aspect and its various possible designs described above can be found in the first aspect and the beneficial effects of its various possible embodiments, and will not be repeated here. The method for preparing the negative electrode sheet provided in the third aspect, and the negative electrode sheet prepared therefrom, can be found in the various embodiments of the negative electrode sheet of the first aspect and their beneficial effects, and will not be repeated here either. Attached Figure Description

[0030] Figure 1 is a flowchart of a method for preparing a negative electrode sheet according to an embodiment of this application. Detailed Implementation

[0031] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments and comparative examples. It should be understood that the embodiments described in this specification are merely for explaining the invention and are not intended to limit the invention. The formulations, proportions, etc., of the embodiments can be selected according to local conditions without substantially affecting the results.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims and drawings of this application are intended to cover non-exclusive inclusion.

[0033] In the description of this application, unless otherwise stated, "multiple" means two or more (including two), and similarly, "multiple groups" means two or more (including two groups).

[0034] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of the phrase "embodiment" in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0035] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can serve as its own lower or upper limit and be combined with any other point or individual value, or with other lower or upper limits, to form an undefined range.

[0036] The above description of the invention is not intended to describe every disclosed embodiment or implementation of the invention. Instead, the following description provides more specific examples of exemplary embodiments. Throughout this application, guidance is provided through a series of embodiments that can be used in various combinations. In each embodiment, the listed combinations are merely representative and should not be construed as exhaustive.

[0037] Increasing the proportion of active material in the negative electrode of a battery is an important technical means to improve the energy density of the battery. Increasing the proportion of active material in conventional negative electrode sheets will inevitably reduce the amount of other solid materials in the negative electrode formula. Although the energy density of the battery is improved after adopting this formula, it is also accompanied by an increase in the internal resistance of the battery, a decrease in rate performance, and a shortening of the cycle life of the battery.

[0038] The inventors discovered through research that the cause of the above problems is that, apart from the active material, other solid substances in the negative electrode formulation usually increase the viscosity, stability, dispersion uniformity, and conductivity of the negative electrode slurry (the state before the negative electrode coating dries). Reducing these substances leads to poor stability of the negative electrode slurry, easy sedimentation, and consequently, poor coating uniformity when the negative electrode slurry is coated onto the negative electrode current collector, as well as poor adhesion between the negative electrode coating and the current collector. Furthermore, it also increases the internal resistance of the battery. All of these factors ultimately lead to problems such as rapid expansion during battery charging and discharging, poor rate performance, and short cycle life.

[0039] Therefore, those skilled in the art have been seeking a method that can balance the energy density, rate performance, and cycle life of a battery.

[0040] In view of this, this application provides a negative electrode sheet and a secondary battery, which achieves the goal of increasing the proportion of active material while effectively utilizing the functions of each component, thus ensuring the energy density, rate performance, and cycle life of the battery, by rationally combining the components of the solid mixture in the negative electrode coating according to certain requirements and proportions. The details are described below.

[0041] In a first aspect, this application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode coating; the negative electrode coating is coated on the surface of the negative electrode current collector, and the negative electrode coating includes a solid mixture comprising a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickener, and the proportions of the negative electrode coating satisfy the following relationship: Where m1 is the mass fraction of thickener in the solid mixture, m2 is the mass fraction of dispersant in the solid mixture, m3 is the mass fraction of binder in the solid mixture, and p is the degree of polymerization of thickener.

[0042] The negative electrode current collector is an indispensable component of the negative electrode sheet. It not only carries the negative electrode coating, but also collects and outputs the negative electrode current. The negative electrode current collector can be any material suitable for use as a negative electrode current collector in lithium-ion batteries. For example, the negative electrode current collector can be, but is not limited to, metal foil, and more specifically, copper foil or carbon-coated copper foil.

[0043] The negative electrode coating is a coating applied to the surface of the negative electrode current collector to conduct electrons under the wetting of the electrolyte. The negative electrode coating is formed by drying the negative electrode slurry, which is a viscous substance formed by mixing a solid mixture with a solvent. The negative electrode slurry is coated on the surface of the negative electrode current collector. After drying, the solvent is evaporated, and the remaining solid mixture adheres to the surface of the negative electrode current collector as the negative electrode coating.

[0044] The solid mixture in this embodiment includes a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickener, and the total mass fraction of the solid mixture is 100%.

[0045] In some embodiments, the negative electrode active material includes at least one of graphite, silicon suboxide, silicon, silicon carbide, lithium titanate, and modified materials thereof.

[0046] In some embodiments, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphite sheets, and graphene.

[0047] In some embodiments, the adhesive includes at least one of styrene-butadiene rubber, styrene-acrylate, polyacrylonitrile, polymethyl methacrylate, and modified materials thereof, and the mass fraction of the adhesive in the solid mixture satisfies 0% ≤ m3 ≤ 2%.

[0048] Specifically, the mass fraction of the adhesive can be 0%, 0.6%, 1.2%, 1.5%, 1.8%, 2%, etc.

[0049] When the negative electrode sheet is in a slurry state, the binder can increase the viscosity of the negative electrode slurry, thereby improving the processing performance of the negative electrode slurry. After the negative electrode slurry dries on the surface of the negative electrode current collector to become a negative electrode coating, the negative electrode coating will also be more dense, and the adhesion between the negative electrode coating and the negative electrode current collector is good and not easy to detach, reducing the expansion of the negative electrode sheet during use and improving the charge and discharge performance of the battery.

[0050] When the mass fraction of the adhesive is within the above range, by reasonably combining the mass fraction m1 of the thickener, the degree of polymerization P of the thickener, and the mass fraction m2 of the dispersant, the following conditions can be met: Both methods enable the negative electrode slurry to have good processing performance, while the battery using the negative electrode sheet can achieve high energy density, excellent cycle performance and long cycle life.

[0051] In some embodiments, the dispersant includes at least one of polyacrylic acid, carboxymethyl chitosan, polyvinyl alcohol, and modified materials thereof, and the mass fraction of the dispersant in the solid mixture satisfies 0% ≤ m2 ≤ 2%.

[0052] Specifically, the mass fraction of the dispersant can be 0, 0.6%, 1.2%, 2%, etc.

[0053] When the negative electrode sheet is in the slurry state, the dispersant can make the components of the solid mixture disperse more evenly in the solvent. After the negative electrode slurry is dried on the surface of the negative electrode current collector to become a negative electrode coating, the composition of each part of the negative electrode coating will also be more uniform, which is beneficial to improving the battery capacity and cycle performance.

[0054] When the mass fraction of the dispersant is within the above range, the following can be achieved by rationally combining the mass fraction m1 of the thickener, the degree of polymerization P of the thickener, and the mass fraction m1 of the binder: Both methods enable the negative electrode slurry to have good processing performance, while the battery using the negative electrode sheet can also achieve high energy density, excellent cycle performance and long cycle life.

[0055] In some embodiments, the thickener comprises at least one selected from sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and sodium alginate, and the mass fraction of the thickener in the solid mixture is 0%. <m1≤2%。

[0056] Specifically, the mass fraction of the thickener can be 0.1%, 0.3%, 0.5%, 0.8%, 1.5%, 2%, etc.

[0057] Thickeners can appropriately increase the viscosity of negative electrode slurry, reduce its fluidity, increase its uniformity, and make it easier to coat and process. Furthermore, the negative electrode coating formed after the negative electrode slurry dries has good adhesion to the negative electrode current collector, making the battery less prone to swelling.

[0058] By controlling the mass fraction of the thickener within the above range, the impact of the addition of the thickener on the mass fraction of other substances in the solid mixture can be reduced, which is beneficial for the battery to achieve high energy density, excellent cycle performance and long cycle life.

[0059] In the above embodiments, the mass fractions of binder, dispersant, and thickener were controlled to ensure the mass fractions of conductive agent and negative electrode active material in the solid mixture. This avoids excessive reduction of conductive agent and negative electrode active material due to the increase of binder, dispersant, and thickener, thus ensuring the high energy density and low resistivity of the battery.

[0060] The degree of polymerization (DP) is an indicator of the molecular size of a polymer, representing the number of repeating units (or chain segments) in the polymer macromolecular chain. The degree of polymerization has a significant impact on the physical and chemical properties of polymers, such as molecular weight, solubility, and mechanical strength. Generally, a higher degree of polymerization in a thickener indicates a longer linear molecular structure, resulting in better viscosity and toughness of the negative electrode slurry for a given mass fraction of thickener. Therefore, this application incorporates the degree of polymerization of the thickener as a parameter into the above formula.

[0061] In some embodiments, the degree of polymerization of the thickener satisfies 100 ≤ p ≤ 5000.

[0062] Specifically, the degree of polymerization P of the thickener can be 100, 200, 400, 1000, 2000, 4000, 5000, etc.

[0063] The above method can control the viscosity and toughness of the negative electrode slurry within a certain range, thereby improving the processability of the negative electrode slurry.

[0064] It is understandable that, when the relational expression is satisfied... Meanwhile, when the polymer content of the thickener is too high, the mass fraction of the thickener, dispersant and binder in the solid mixture will be affected and reduced. If the mass fraction of each component is reduced significantly, it will affect the performance of the battery.

[0065] Therefore, in some embodiments, the degree of polymerization of the thickener further satisfies 100 ≤ p ≤ 2000. When the degree of polymerization of the thickener is within this range, in order to satisfy the above formula, the mass fractions of the binder, dispersant, and thickener can be set more reasonably, thereby improving the final electrochemical performance of the negative electrode sheet while improving the processability of the negative electrode slurry.

[0066] It is understandable that in the same negative electrode sheet, the mass fractions of the binder, dispersant, and thickener, as well as the degree of polymerization of the thickener, can all satisfy the ranges defined in the above embodiments. Alternatively, only the mass fractions of one or two components can satisfy the corresponding ranges, as long as the requirements are met. All of these can increase the energy density of the battery to a certain extent, improve the processability of the negative electrode sheet, and increase the rate performance and cycle life of the battery.

[0067] This application also provides a method for preparing a negative electrode sheet, as shown in Figure 1. The method for preparing the negative electrode sheet in this embodiment includes the following steps:

[0068] S100 is a solid mixture composed of a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickener. The proportions of the solid mixture satisfy the following relationship: Where m1 is the mass fraction of thickener in the solid mixture, m2 is the mass fraction of dispersant in the solid mixture, m3 is the mass fraction of binder in the solid mixture, and p is the degree of polymerization of thickener.

[0069] In the above steps, the negative electrode active material, conductive agent, binder, dispersant, and thickener in the solid mixture can be reasonably combined as needed, based on satisfying the numerical range of the formulas listed above. The calculated values ​​can be 2.7%, 3%, 3.3%, 3.6%, 3.8%, 4.2%, 4.3%, 4.4%, 5%, 9%, etc.

[0070] S200 is a process of mixing a solid mixture with a solvent to obtain a negative electrode slurry.

[0071] S300 involves coating the negative electrode slurry onto both sides of the negative electrode current collector.

[0072] S400 involves drying, cold pressing, and cutting a negative electrode current collector coated with negative electrode slurry to obtain a negative electrode sheet.

[0073] It is understandable that in the above S400, after the negative current collector coated with negative electrode slurry is dried, the solvent in the negative electrode slurry evaporates, and the negative electrode slurry becomes a negative electrode coating. The solid mixture in the negative electrode coating has the same composition as the solid mixture in the negative electrode slurry.

[0074] In the above scheme, a solid mixture is first prepared to ensure that all components are uniformly mixed before a solvent is added to prepare the negative electrode slurry, resulting in a more uniform texture. Simultaneously, by ensuring the solid mixture satisfies the aforementioned relationship, the prepared negative electrode sheet enables the battery to exhibit high energy density, excellent rate performance, and a long cycle life during use.

[0075] This application also provides a secondary battery, including the negative electrode sheet in any of the above embodiments.

[0076] Because of the use of the negative electrode sheet in the above embodiments, the secondary battery has high energy density, excellent rate performance and long cycle life.

[0077] The following examples and comparative examples illustrate the effects of different ratios of the above solid mixture on the properties of the negative electrode sheet.

[0078] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all mass fractions, percentages, and ratios involved in the following examples are based on weight, and all reagents used in the examples are commercially available and ready for use without further processing.

[0079] Example 1

[0080] (1) Preparation of positive electrode sheet

[0081] Lithium iron phosphate (positive electrode active material), carbon black (conductive agent), and polyvinylidene fluoride (binder) are mixed with N-methylpyrrolidone (positive electrode solvent) in a mass ratio of 96.5%:1.5%:2% to obtain a positive electrode slurry. The solid content of the positive electrode slurry accounts for 58% of the mass fraction of the positive electrode slurry. The positive electrode slurry is coated on both sides of aluminum foil (positive electrode current collector), dried, cold-pressed, and cut to obtain a positive electrode sheet.

[0082] (2) Preparation of negative electrode sheet

[0083] A negative electrode slurry is prepared by mixing graphite (negative electrode active material), carbon black SP (conductive agent), sodium carboxymethyl cellulose (thickener), polyacrylic acid (dispersant), and styrene-butadiene rubber (adhesive) with water in a mass ratio of 96.9%:0.5%:0.8%:1.2%:0.6%. The solid content of the negative electrode slurry accounts for 50% of the mass fraction of the negative electrode slurry. The negative electrode slurry is coated on both sides of copper foil (negative electrode current collector), dried, cold-pressed, and cut to obtain the negative electrode sheet.

[0084] (3) Preparation of the diaphragm

[0085] A polypropylene diaphragm with a thickness of 16 μm is used.

[0086] (4) Preparation of electrolyte

[0087] Dimethyl carbonate, ethylene carbonate, and diethyl carbonate were mixed in a 1:1:1 ratio, and lithium hexafluorophosphate was added at a concentration of 1.1 mol / L to obtain the electrolyte.

[0088] (5) Assembly

[0089] The above-mentioned positive electrode, negative electrode, separator, electrolyte, and aluminum-plastic film, along with other components, are assembled into a soft-pack lithium-ion battery.

[0090] Examples 2-15 and Comparative Examples 1-5

[0091] The preparation processes of the positive electrode, separator, electrolyte, and assembly processes in Examples 2-15 and Comparative Examples 1-5 are the same as those in Example 1. The difference from Example 1 is that the mass ratios of graphite (negative electrode active material), carbon black SP (conductive agent), sodium carboxymethyl cellulose (thickener), polyacrylic acid (dispersant), and styrene-butadiene rubber (adhesive) in the preparation process of the negative electrode in Example 1 are changed to the ratios listed in Tables 1 and 2 to prepare different negative electrode sheets. The above negative electrode sheets are then assembled with other components such as positive electrode, electrolyte, separator, and aluminum-plastic film to form different soft-pack lithium-ion batteries.

[0092] The negative electrode sheets or batteries prepared in Examples 1-15 and Comparative Examples 1-5 were tested as follows.

[0093] Testing process

[0094] (1) Viscosity test of negative electrode slurry

[0095] Tested using a viscometer.

[0096] (2) Adhesion test between negative electrode coating and negative electrode current collector

[0097] Peel strength was quantitatively determined using the 180° peel method. The free end of the negative electrode sheet was folded 180°, and the free end and the test end were clamped onto upper and lower clamps respectively. Continuous peeling was performed using a tensile testing machine in the same environment until the negative electrode current collector and the negative electrode coating were completely separated, yielding the peel force value. The peel force value measures the adhesion strength between the negative electrode coating and the negative electrode current collector; a higher peel force value indicates greater adhesion strength, and vice versa.

[0098] (3) DC internal resistance (DCR) test

[0099] Adjust the battery to 50% SOC (State of Charge) at 25℃, let it rest for 1 hour, record OCV1, discharge at 3C for 10 seconds, record OCV2, and then calculate DCR according to (OCV1-OCV2) / 3C×1000.

[0100] (4) Cyclic life test

[0101] Charge the battery at 25°C with a constant current and constant voltage of 1C to 3.65V, cut off the current at 0.05C, let it rest for 30 minutes, discharge it with a constant current of 1C, cut off the current at 2.0V, let it rest for 30 minutes after discharging, and repeat the charge and discharge steps until the capacity decays to 80% of the capacity of the first cycle, and record the number of cycles.

[0102] The data obtained from the above tests are recorded in Tables 1 and 2 below. Table 1 Table 2

[0103] It should be noted that in Tables 1 and 2 above, the energy density of the battery is directly proportional to the percentage of graphite (the negative electrode active material). The higher the percentage of graphite, the higher the energy density of the battery. Generally, the higher the energy density of the battery, the better.

[0104] Slurry viscosity is used to measure the processability of negative electrode slurries. Generally, excessively high viscosity can lead to problems such as difficulty in uniform mixing, uneven coating, and precise application, resulting in poor processability. Conversely, excessively low viscosity can cause excessive fluidity and sedimentation of solid mixtures, similarly reducing processability and causing uneven coating. Therefore, a moderate viscosity is preferable for negative electrode slurries. In this viscosity test, the easily processable viscosity range for the negative electrode slurry was 3000-16000 mPa·s. It is understandable that the suitable viscosity range for processing may vary depending on the test conditions.

[0105] The peel force value reflects the ease with which the negative electrode coating and the negative electrode current collector can peel off, and is used to measure the adhesion strength between them. A higher peel force value indicates greater adhesion strength, and vice versa. In this test, a peel force within the range of 0.35-0.7 N was optimal. Understandably, the optimal range for the peel force value may differ depending on the test conditions.

[0106] As can be seen from Examples 1-15, when the mass fractions of binder, dispersant, and thickener in the negative electrode coating, as well as the degree of polymerization of the thickener, are within a suitable range and conform to the relationship listed in this invention, the energy density of the battery can be improved by reasonably matching the amount of each material and the degree of polymerization of the thickener. At the same time, the processability of the negative electrode slurry in the manufacturing process is improved, ensuring that the battery has a small DC internal resistance, excellent cycle performance, and a long cycle life.

[0107] As shown in Examples 1-8 of Table 1, when the overall amount of inactive substance is reduced, the relationships and parameter ranges listed in this invention can be satisfied by adjusting the amount of one of the materials or the degree of polymerization P. For example, in Examples 2 and 3, only the amount of thickener m1 is adjusted; in Examples 6 and 7, only the amount of dispersant m2 is adjusted; in Examples 3-5, only the degree of polymerization p of the thickener is adjusted; and in Examples 7 and 8, only the amount of adhesive m3 is adjusted. The amounts of several materials and the degree of polymerization P of the thickener can also be adjusted to satisfy the relationships and parameter ranges listed in this invention. All of the above methods can achieve the satisfaction of the relationships and parameter ranges listed in this invention, and... The goal is to increase the proportion of graphite in the active material, thereby improving the battery's energy density. Simultaneously, it ensures the viscosity of the negative electrode slurry remains within a processable range, maintaining processing capability. This results in stable adhesion between the prepared electrode active coating and the current collector, and also reduces the battery's internal resistance and increases cycle life. Furthermore, during the preparation of the negative electrode, it was found that the coating weight of the negative electrode slurry per unit area of ​​the negative electrode current collector is more uniform, leading to superior charge / discharge performance and rate performance of the battery.

[0108] As can be seen from Examples 9-15 in Table 2, when the amount of inactive substance is reduced and the amount of dispersant m2 or binder m3 is at extreme values, the relationship listed in this application can still be satisfied by reasonably matching the design and amount of other materials. The required ranges for each parameter are as follows: in Examples 9 and 10, the amount of dispersant m2 is adjusted when the amount of adhesive m3 is 0; in Examples 11-14, the amount of thickener m1, the degree of polymerization p of the thickener, and the amount of adhesive m3 are adjusted when the amount of dispersant m2 is 0. All of the above methods can satisfy the relationships listed in this application. It can also increase the proportion of active material graphite and ensure that the viscosity of the negative electrode slurry is within the processable range, resulting in a battery with high energy density and long cycle life.

[0109] As can be seen from Comparative Examples 1-5 in Tables 1 and 2, if the relationships listed in this invention are not satisfied, the performance of the battery will be adversely affected regardless of whether the composition and design of each material are within the range.

[0110] For example, as can be seen from Comparative Examples 1 and 5, the listed The result is greater than the relationship listed in this application, which will lead to problems such as higher viscosity of negative electrode slurry, greater adhesion between negative electrode coating and current collector, excessively high internal resistance of battery, and shortened cycle life of battery.

[0111] As can be seen from Comparative Examples 3 and 4, the listed If the result is less than the relationship listed in this application, it will lead to a lower viscosity of the negative electrode slurry, poor slurry processability, solid precipitation, and poor adhesion between the negative electrode coating and the current collector, which will affect the cycle life of the battery. This may be related to the fact that in the later stages of battery use, the negative electrode coating of the negative electrode sheet detaches from the negative electrode current collector, and electron transport is blocked.

[0112] As can be seen from Comparative Example 2, when the amount of the selected adhesive m3 is not within the range, the adhesion between the negative electrode coating and the current collector is too large, resulting in excessive internal resistance of the battery and a shorter cycle life of the battery.

[0113] Furthermore, in Examples 1-15 above, the slurry viscosity in Example 5 is significantly higher than that in other examples while still meeting the requirements. The cycle life of the battery in Example 15 is slightly lower than that of the batteries in other examples while still meeting the requirements. Therefore, to achieve a battery that simultaneously possesses high energy density, excellent rate performance, and long cycle life, and further enhances these three aspects, in some examples, the mass fractions of the thickener (m1), dispersant (m2), and binder (m3) in the solid mixture, and the degree of polymerization (p) of the thickener can be further optimized to meet the following requirements:

[0114] In summary, the embodiments of this application achieve the desired results by ensuring that the ratio of the negative electrode coating meets the following requirements. This allows the total mass fraction of thickener, dispersant, and binder to be controlled within a small range, thereby increasing the mass fraction of negative electrode active material and conductive agent. When the amount of conductive agent required for different batteries is met, the amount of negative electrode active material layer can be increased, thus increasing the battery's energy density. Simultaneously, the negative electrode coating has a high viscosity in the slurry state, making the negative electrode slurry less prone to sedimentation and unevenness. Furthermore, after being coated onto the negative electrode current collector, the negative electrode slurry exhibits good adhesion to the current collector, preventing the negative electrode coating from easily detaching during battery cycling. Batteries using the aforementioned negative electrode sheet also exhibit higher rate performance and longer cycle life. In summary, under the conditions of satisfying the above relationships, batteries using the aforementioned negative electrode sheet can achieve a balance of high energy density, excellent rate performance, and long cycle life.

Claims

1. A negative electrode sheet, characterized in that, include: Negative electrode current collector; A negative electrode coating is applied to the negative electrode current collector. The negative electrode coating comprises a solid mixture including a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickener. The proportions of the negative electrode coating satisfy the following relationship: Wherein, m1 is the mass fraction of the thickener in the solid mixture, m2 is the mass fraction of the dispersant in the solid mixture, m3 is the mass fraction of the binder in the solid mixture, and p is the degree of polymerization of the thickener.

2. The negative electrode sheet according to claim 1, characterized in that, The thickener comprises at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and sodium alginate, and the mass fraction of the thickener in the solid mixture satisfies 0%. <m1≤2%。 3. The negative electrode sheet according to claim 1, characterized in that, The degree of polymerization of the thickener satisfies 100 ≤ p ≤ 5000.

4. The negative electrode sheet according to claim 3, characterized in that, The degree of polymerization of the thickener satisfies 100 ≤ p ≤ 2000.

5. The negative electrode sheet according to claim 1, characterized in that, The dispersant includes at least one of polyacrylic acid, carboxymethyl chitosan, polyvinyl alcohol, and modified materials thereof, and the mass fraction of the dispersant in the solid mixture satisfies 0% ≤ m2 ≤ 2%.

6. The negative electrode sheet according to claim 1, characterized in that, The adhesive includes at least one of styrene-butadiene rubber, styrene-acrylate, polyacrylonitrile, polymethyl methacrylate, and modified materials thereof, and the mass fraction of the adhesive in the solid mixture satisfies 0% ≤ m3 ≤ 2%.

7. The negative electrode sheet according to claim 1, characterized in that, The proportions of the solid mixture satisfy the following relationship:

8. The negative electrode sheet according to any one of claims 1 to 7, characterized in that, The conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphite sheets, and graphene; and / or the negative electrode active material includes at least one of graphite, silicon suboxide, silicon, silicon carbide, lithium titanate, and their modified materials.

9. A secondary battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1 to 8.

10. A method for preparing a negative electrode sheet, characterized in that, include: A solid mixture is formed by mixing a negative electrode active material, a conductive agent, a binder, a dispersant, and a thickener, wherein the proportions of the solid mixture satisfy the following relationship: Wherein, m1 is the mass fraction of the thickener in the solid mixture, m2 is the mass fraction of the dispersant in the solid mixture, m3 is the mass fraction of the binder in the solid mixture, and p is the degree of polymerization of the thickener; The solid mixture is mixed with a solvent to obtain a negative electrode slurry; The negative electrode slurry is coated on both sides of the negative electrode current collector; The negative electrode current collector coated with the negative electrode slurry is dried, cold-pressed, and cut to obtain the negative electrode sheet.

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