Negative electrode plate and preparation method therefor, lithium ion battery, and vehicle

Abstract

Disclosed are a negative electrode plate and a preparation method therefor, a lithium ion battery, and a vehicle. The negative electrode plate comprises a current collector, a first active material layer disposed on the current collector, and a second active material layer disposed on the first active material layer. The first active material layer comprises first graphite, and the second active material layer comprises second graphite. The convexity and volume median diameter of the first graphite, and the convexity and volume median diameter of the second graphite satisfy: 1≤(T1×Dv502) / (T2×Dv501)≤4.

Description

Negative electrode sheet and preparation method thereof, lithium-ion battery and automobile

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on January 9, 2024, with application number 2024100392213 and invention name "A negative electrode sheet and its preparation method, lithium-ion battery and automobile", the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The embodiments of the present invention relate to, but are not limited to, the field of lithium battery technology, and specifically, to a negative electrode sheet and a preparation method thereof, a lithium-ion battery, and a vehicle. Background Art

[0003] Lithium-ion batteries, with their advantages of high energy density, long cycle life, high voltage, and low self-discharge, have been widely used in consumer electronics, electric vehicles, and energy storage systems. In recent years, the rapid development of large-scale energy storage and electric vehicles has placed higher demands on the energy density of lithium-ion batteries.

[0004] Currently, the research and development of high-energy-density lithium-ion batteries primarily encompasses three areas: the development of high-voltage cathode materials, thick electrode architectures, and high-performance auxiliary materials. Thick electrode architecture development involves increasing electrode thickness and reducing current collector volume without changing the electrochemical system, thereby increasing the active material loading and proportion. Thick electrode design has become an effective method for achieving higher energy density and reducing costs.

[0005] However, using thick electrodes lengthens the electron and lithium-ion transmission path, increases battery impedance, and can also lead to poor electrolyte wetting. Research has shown that increasing electrode thickness leads to uneven distribution of materials within the electrode and uneven electrode chemical reactions. Thick electrodes are also prone to cracking, active material flaking, and foil leakage during the drying process. Consequently, lithium secondary batteries using high-load electrodes face challenges such as poor rate performance and sluggish reaction kinetics. Furthermore, increasing battery energy density at the expense of kinetic performance often yields more problems than benefits. Summary of the Invention

[0006] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0007] An embodiment of the present invention discloses a negative electrode sheet, comprising a current collector, a first active material layer disposed on the current collector, and a second active material layer disposed on the first active material layer, wherein the first active material layer comprises a first graphite, the second active material layer comprises a second graphite, and the convexity and volume median particle size of the first graphite and the convexity and volume median particle size of the second graphite satisfy the following relationship: 1≤(T1×Dv502) / (T2×Dv501)≤4;

[0008] Wherein, T1 is the convexity of the first graphite, Dv501 is the volume median particle size of the first graphite, T2 is the convexity of the second graphite, and Dv502 is the volume median particle size of the second graphite.

[0009] Optionally, the convexity of the first graphite is 0.75 to 1.0, and the convexity of the second graphite is 0.25 to 0.75.

[0010] Optionally, the volume median particle size of the first graphite is larger than the volume median particle size of the second graphite.

[0011] Optionally, the surface density of the negative electrode sheet is greater than 190g / m 2 and less than or equal to 320g / m 2 .

[0012] Optionally, a mass ratio of the first active material layer to the second active material layer is 3:7 to 7:3.

[0013] Optionally, the first active material layer further includes a first conductive agent, a second conductive agent, a first thickener and a first binder, and the second active material layer further includes a third conductive agent, a second thickener and a second binder.

[0014] Optionally, the weight ratio of the first graphite to the weight ratio of the first active material layer is 91.6% to 96.2%, the weight ratio of the first conductive agent to the weight ratio of the first active material layer is 0.7% to 1.5%, the weight ratio of the second conductive agent to the weight ratio of the first active material layer is 0.7% to 1.5%, the weight ratio of the first binder to the weight ratio of the first active material layer is 1.4% to 3.6%, and the weight ratio of the first thickener to the weight ratio of the first active material layer is 1.0% to 1.8%; and / or,

[0015] The weight ratio of the second graphite to the weight ratio of the second active material layer is 93.8% to 97.4%, the weight ratio of the third conductive agent to the weight ratio of the second active material layer is 0.6% to 2.0%, the weight ratio of the second binder to the weight ratio of the second active material layer is 1.0% to 2.4%, and the weight ratio of the second thickener to the weight ratio of the second active material layer is 1.0% to 1.8%.

[0016] The present invention also discloses a method for preparing a negative electrode sheet, which is used to prepare the negative electrode sheet as described above, comprising:

[0017] A first graphite, a first conductive agent, a second conductive agent, a first thickener, a first binder and a first solvent are mixed to obtain a first active material layer slurry; a second graphite, a third conductive agent, a second thickener, a second binder and a second solvent are mixed to obtain a second active material layer slurry; the relationship between the convexity and volume median particle size of the first graphite and the convexity and volume median particle size of the second graphite is: 1≤(T1×Dv502) / (T2×Dv501)≤4, wherein T1 is the convexity of the first graphite, Dv501 is the volume median particle size of the first graphite, T2 is the convexity of the second graphite, and Dv502 is the volume median particle size of the second graphite;

[0018] The first active material layer slurry and the second active material layer slurry are coated on a current collector and dried, the first active material layer slurry forms a first active material layer, and the second active material layer slurry forms a second active material layer, thereby obtaining a negative electrode sheet.

[0019] An embodiment of the present invention further discloses a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet, and the negative electrode sheet is the negative electrode sheet described above.

[0020] An embodiment of the present invention further discloses a car comprising the lithium-ion battery described above.

[0021] The negative electrode sheet and preparation method thereof, lithium-ion battery and automobile of the embodiments of the present invention have the following beneficial effects: graphite particles with specific convexity and volume median particle size are screened from the raw material end as active materials for the first active material layer and the second active material layer of the thick electrode sheet, and (T1×Dv502) / (T2×Dv501) is controlled between 1 and 4 by jointly regulating the convexity and volume median particle size of the first graphite and the second graphite, which helps to achieve excellent electrolyte infiltration and increase the utilization rate of active materials, so that the lithium-ion battery using the thick electrode sheet has high energy density and good rate performance. In addition, the method of regulating the convexity of the graphite active material can reduce the material-side design control links and process control links and shorten the material verification cycle compared to regulating parameters such as the specific surface area, carbon coating amount, coating layer thickness, pore volume and electrode porosity of the graphite material.

[0022] Still other aspects will become apparent upon reading and understanding the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG1 is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present invention;

[0024] FIG2 is a schematic diagram of the convex hull perimeter and the actual perimeter of the material in an embodiment of the present invention.

[0025] Explanation of reference numerals: 1 - current collector; 2 - first active material layer; 3 - second active material layer. DETAILED DESCRIPTION

[0026] In order to make the above-mentioned objects, features and advantages of the embodiments of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0027] Thick electrode design can increase the active material loading by increasing the thickness of the electrode sheet, thereby increasing the battery energy density. It is an effective way to achieve higher energy density and reduce costs. However, there are problems such as poor electrolyte infiltration, low utilization of electrode active materials, and large charge transfer impedance, which lead to poor rate performance and slow reaction kinetics of lithium secondary batteries. In addition, an optional method is to increase the energy density of single lithium-ion batteries by increasing the surface density of positive and negative electrodes, but the problem of poor reaction kinetics has not been effectively solved. Based on the current higher requirements of lithium-ion batteries for high-loaded electrodes, it is very necessary to further study the preparation of high-loaded negative electrodes that take into account both high energy density and rate performance.

[0028] In the current electrochemical system of lithium-ion batteries, graphite has become the most commonly used and reliable negative electrode material due to its advantages such as good cycle stability, low polarization and safety issues during charge and discharge, abundant resources, and low cost. The embedding of lithium ions into graphite generally requires the following stages: (1) the exfoliation of the solvated sheath of lithium ions; (2) the diffusion of lithium ions into the graphite through the SEI membrane; and (3) the diffusion of lithium ions into the graphite.

[0029] The surface of graphite particles is anisotropic, and the higher the degree of anisotropy of graphite, the fewer lithium insertion sites on its surface, which is not conducive to meeting the current demand for fast charging capabilities of lithium-ion batteries. In addition, in theory, lithium ions can only be embedded in graphite from the end faces of graphite. The diffusion rate of lithium ions in graphite is less than the diffusion rate in the electrolyte, and the closer to the collector side, the slower the lithium insertion speed of the internal graphite. This leads to poor kinetic performance of thick electrodes. Severe electrochemical polarization even causes lithium ions to be plated on the graphite surface to form lithium dendrites, which not only leads to low utilization of active materials, but also to the risk of lithium dendrites piercing the diaphragm and causing short circuits. It can be seen that the microstructure of graphite particles has a dominant effect on lithium insertion. In view of this, the inventors considered starting with optimizing the microscopic characteristics of raw graphite in order to achieve an improvement in its kinetic performance, and optimizing the thick electrode architecture in order to achieve a balance between energy density and rate performance. Therefore, the embodiments of the present invention provide a high-load negative electrode sheet and a preparation method thereof, as well as a high-energy-density lithium-ion battery to meet the increasing high energy density and dynamics requirements of power batteries or energy storage products.

[0030] In order to make the above-mentioned objects, features and advantages of the embodiments of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0031] Referring to FIG. 1 , a negative electrode sheet according to an embodiment of the present invention includes a current collector 1, a first active material layer 2 disposed on the current collector 1, and a second active material layer 3 disposed on the first active material layer 2. The first active material layer 2 includes a first graphite, and the second active material layer 3 includes a second graphite. The convexity and volume median particle size of the first graphite and the convexity and volume median particle size of the second graphite satisfy the following relationship: 1≤(T1×Dv502) / (T2×Dv501)≤4;

[0032] Wherein, T1 is the convexity of the first graphite, Dv501 is the volume median particle size of the first graphite, T2 is the convexity of the second graphite, and Dv502 is the volume median particle size of the second graphite.

[0033] The inventors found in their research that the sphericity parameter - convexity has a great influence on the fast charging performance of graphite particles. The so-called convexity is defined as the ratio of the convex hull perimeter L1 of the outer contour of the particle to the actual perimeter L2. The outer contour of the particle is obtained by photographing the granular material to obtain an image and performing image processing on the image. The convex hull refers to the smallest convex polygon that contains all the points inside the material, that is, the smallest circumscribed polygon of the particle image, which contains all the points inside the material and has no concave parts. Correspondingly, the convex hull perimeter is the perimeter of the smallest convex polygon that surrounds all the points inside the material, that is, the perimeter of the smallest circumscribed polygon of the particle image. As shown in Figure 2, the length of the dotted frame of the ellipse is the convex hull perimeter of the material in the figure. The actual perimeter of the material is the true perimeter of the material surface, that is, the perimeter of the outer boundary of the material. The ratio of the convex hull perimeter of the material to its actual perimeter is defined as convexity, which can be used to judge the degree of concavity and convexity and roughness of the material particle contour. It can be understood that the smaller the convexity of a material, the more uneven the surface profile of the material and the greater the degree of surface defects. In other words, the greater the degree of convexity, the greater the roughness, and the more uneven and uneven the material surface. Conversely, the smoother the material surface, the smaller the roughness, and the fewer surface defects. Therefore, for graphite materials with smaller convexity, their particle surfaces are rougher, and rough surfaces lead to more defects and irregular structures. At the same time, the surface area of ​​the particles also increases, all of which lead to more lithium insertion active sites, which helps to increase the insertion and deinsertion rate of lithium ions, thereby improving the fast charging performance of the battery. The greater the convexity of the graphite particles, the lower the surface roughness, which may lead to fewer lithium insertion active sites and a lower ion transfer rate.

[0034] The volume median particle size Dv50 of graphite active material refers to the particle size at which the volume accumulation reaches 50% from the small particle size side in the volume-based particle size distribution of the material, that is, the volume of material smaller than this particle size accounts for 50% of the total volume of the material. It can be simply understood as the particle size corresponding to 50% in the volume distribution. Studies have found that the volume median particle size Dv50 of graphite has a significant impact on the energy density and fast charging performance of the battery. A smaller Dv50 may lead to an increase in the specific surface area of ​​the graphite particles, which helps to increase the insertion and deinsertion rate of lithium ions, thereby improving the fast charging performance of the battery. In addition, smaller particles may increase the filling density of graphite, thereby increasing the energy density of the battery. However, a too small Dv50 may also lead to an increase in the contact area between particles, increase the internal resistance of the battery, and reduce the fast charging performance and energy density of the battery.

[0035] Starting from the thermodynamic theory of lithium ion embedding in graphite, the inventors took the control of active material particle size, convexity and surface defects as the starting point to design a high-load thick electrode structure. Through a large number of experiments and with the help of statistical methods, they studied and analyzed the sphericity parameters of graphite - convexity and particle size. They found that graphite particles with specific convexity and volume median particle size were screened from the raw material end as active materials for the first active material layer and the second active material layer of the thick electrode. The convexity and volume median particle size of the first graphite and the second graphite were jointly regulated, and (T1×Dv502) / (T2×Dv501) was controlled between 1 and 4, which helped to achieve excellent electrolyte infiltration and increased active material utilization, thereby making the lithium-ion battery using thick electrodes have high energy density and good rate performance. On the one hand, the surface convexity of graphite particles will affect their surface roughness and specific surface area. By screening graphite with specific convexity, the electrolyte wetting performance in the negative electrode material can be improved, which helps to increase the contact area between the electrode material and the electrolyte, thereby promoting the diffusion and transmission of lithium ions in the electrode material; and the appropriate surface convexity can increase the density of lithium-intercalated active sites of graphite particles, improve the utilization rate of active materials, and increase the lithium ion transmission rate; and graphite particles with appropriate convexity help to improve the electrode material's ability to embed and release lithium ions, avoiding irreversible electrochemical reactions of lithium ions on the electrode surface, such as lithium plating before lithium ions reach the storage site, thereby reducing lithium precipitation.

[0036] In summary, this embodiment achieves a lithium-ion battery with high energy density and good rate performance from the material design end by precisely grasping the microscopic characteristics of the active material. It balances the battery's energy density and fast-charging performance by regulating the interaction of material particle size, sphericity, and surface defect control technology. For example, under the condition of the same graphite particle size and content, regulating the sphericity parameter of the graphite active material - convexity - can significantly improve the electrolyte infiltration effect of the active material and increase the lithium ion transmission rate. There is no need to regulate parameters such as the specific surface area, carbon coating amount, coating thickness, pore volume, and electrode porosity of the graphite material, thus reducing the material design control links and process control links, shortening the material verification cycle.

[0037] In some embodiments, the convexity T1 of the first graphite is 0.75 to 1.0, and the convexity T2 of the second graphite is 0.25 to 0.75.

[0038] In this embodiment, the first active material layer is located near the current collector. This first active material layer utilizes a graphite active material with a relatively flat surface, providing a larger effective contact area and more binding points and surfaces, thereby improving the binding between the active material and the current collector. Furthermore, the surface with a relatively low roughness can reduce the uneven stress distribution between the active material and the current collector. This helps reduce the risk of active material loss and shedding, improves the battery's cycling stability, and extends its service life. Specifically, with the same material dosage, the battery can provide more charge and discharge cycles, thereby increasing its energy density. The second active material layer is located near the electrolyte. This second active material layer utilizes a graphite active material with a relatively low convexity. Its rougher surface provides more active sites for lithium insertion, facilitating lithium ion diffusion and thus improving battery kinetics. This overall improvement in battery kinetics helps the battery accept charge more quickly during fast charging and maintains performance more stably during charging. Furthermore, the graphite material with a relatively low convexity improves electrolyte wetting and lithium ion transfer rates, preventing lithium plating before the ions reach the graphite storage sites, thereby enhancing the safety of the lithium battery.

[0039] In some embodiments, the volume median particle size Dv501 of the first graphite is greater than the volume median particle size Dv502 of the second graphite.

[0040] In this embodiment, the volume median particle size of the graphite particles in the first active material layer close to the current collector side is larger, so that the graphite particles can achieve a higher compaction density in the first active material layer, thereby improving the battery energy density. In addition, the volume median particle size Dv50 of the graphite is large, which means that the particle size of the graphite particles is relatively large. Larger particles can provide more active material, thereby increasing the energy storage capacity of the battery. Therefore, by designing the second graphite to have a faster charging capability than the first graphite and designing the use of the first graphite with a higher energy density, a battery with excellent overall performance is obtained.

[0041] In some embodiments, the first graphite is a single particle or a secondary particle of artificial graphite, and the second graphite is a secondary particle of artificial graphite;

[0042] The gram capacity of the first graphite is 355 mAh / g to 365 mAh / g, and the ultimate powder compaction density of the first graphite is 1.65 g / cm 3 to 1.80g / cm 3 ;

[0043] The gram capacity of the second graphite is 340 mAh / g to 355 mAh / g, and the ultimate powder compaction density of the second graphite is 1.50 g / cm 3 to 1.65g / cm 3 .

[0044] A single graphite particle is the most basic unit of graphite material, while a secondary graphite particle is a particle structure composed of multiple single particles. The gram capacity of a graphite active material indicates the amount of charge that a unit mass of the graphite active material can release or absorb during the charge and discharge process. For example, a gram capacity of primary graphite of 355 mAh / g to 365 mAh / g indicates that each gram of graphite material can embed or release approximately 355 mAh to 365 mAh of charge. The ultimate powder compaction density of graphite generally refers to the highest density that the graphite powder can achieve during compaction under ideal conditions.

[0045] In this embodiment, the first graphite has a higher gram capacity and ultimate powder compaction density, which is beneficial to improving the energy density and cycle performance of the battery. The second graphite has a lower ultimate powder compaction density, which helps to improve the charge and discharge performance and charge transfer rate of the electrode. By selecting the above materials, the energy density and charge and discharge performance of the battery can be balanced, thereby obtaining better battery performance. Specifically, since the surface of the first graphite is relatively regular (due to its larger convexity), and its ultimate powder compaction density is higher, the graphite sheet is larger, and therefore a higher lithium insertion capacity can be provided. The surface of the second graphite is relatively rough (due to its smaller convexity), and its concave and convex structure is more, thereby improving the overall insertion rate of lithium ions, and the ultimate powder compaction density is slightly lower than that of the first graphite, so it helps to fully infiltrate the electrolyte and improve the liquid retention performance of the electrode material, thereby avoiding many problems such as poor electrolyte infiltration, low material utilization, and excessive electrochemical polarization when thick pole pieces are used.

[0046] In some embodiments, the first active material layer further includes a first conductive agent, a second conductive agent, a first thickener, and a first binder, and the second active material layer further includes a third conductive agent, a second thickener, and a second binder.

[0047] In this embodiment, a composite conductive agent (a composite of a first conductive agent and a second conductive agent) is used in the first active material layer near the current collector to improve overall electronic conductivity and reduce the battery's internal resistance. Specifically, the composite conductive agent can form a conductive network in the electrode material, increasing electron transport channels. The composite conductive agent itself has good electrical conductivity. Adding it to the electrode material can improve the efficiency of electron transport within the electrode material, thereby increasing overall electronic conductivity and reducing the battery's internal resistance.

[0048] In some embodiments, the first conductive agent includes one of conductive carbon black, acetylene black, Ketjen black and conductive graphite, the second conductive agent includes one of multi-walled carbon nanotubes, carbon nanofibers and graphene, the first binder includes polyacrylic acid, and the first thickener includes sodium carboxymethyl cellulose; the third conductive agent includes one of conductive carbon black, acetylene black and conductive graphite, the second binder includes styrene-butadiene rubber, and the second thickener includes sodium carboxymethyl cellulose.

[0049] In this embodiment, the performance of the battery is comprehensively improved by arranging different active materials and binders inside the electrode. The first active material layer close to the current collector side uses a polyacrylic acid binder with stronger bonding force, which can better fix the active material, improve the bonding strength between the active material and the current collector, and reduce the loss and shedding of the active material, thereby improving the cycle stability and safety of the battery. In combination with the first graphite with high energy density, the energy density and cycle stability of the battery are comprehensively improved. The second active material layer close to the electrolyte side uses a styrene-butadiene rubber binder that is more compatible with the electrolyte. Since the SBR binder is more compatible with the electrolyte, it reduces the internal resistance of the battery, improves the charging speed and charging efficiency of the battery, and in combination with the second graphite with excellent fast charging performance, the energy density and cycle performance of the battery are comprehensively improved.

[0050] In some embodiments, the weight ratio of the first graphite to the weight ratio of the first active material layer is 91.6% to 96.2%; the weight ratio of the first conductive agent to the weight ratio of the first active material layer is 0.7% to 1.5%; the weight ratio of the second conductive agent to the weight ratio of the first active material layer is 0.7% to 1.5%; the weight ratio of the first binder to the weight ratio of the first active material layer is 1.4% to 3.6%; the weight ratio of the first thickener to the weight ratio of the first active material layer is 1.0% to 1.8%;

[0051] The weight ratio of the second graphite to the weight ratio of the second active material layer is 93.8% to 97.4%; the weight ratio of the third conductive agent to the weight ratio of the second active material layer is 0.6% to 2.0%; the weight ratio of the second binder to the weight ratio of the second active material layer is 1.0% to 2.4%; and the weight ratio of the second thickener to the weight ratio of the second active material layer is 1.0% to 1.8%.

[0052] The weight percentage of graphite active materials directly affects the battery's energy density. A high weight of graphite active materials can increase the battery's total capacity, thereby increasing its energy density. However, excessive weight can increase the battery's internal resistance, hindering charge transfer and reducing its actual energy density. Conductive agents can improve the electrode's charge transfer performance and enhance the battery's charge and discharge rates. Binders are used to strengthen the bonding between the graphite active material and the conductive agent. An appropriate amount of binder helps the graphite active material and conductive agent disperse evenly within the electrode, creating a suitable pore structure that facilitates electrolyte penetration and ion transport, thereby affecting the electrode's charge and discharge performance. However, excessive binder can increase the electrode's internal resistance, hindering charge transfer and reducing the battery's energy density and charge and discharge performance. Thickeners adjust the electrode's viscosity, affecting its coating properties and porosity, thereby affecting its charge and discharge performance. Adding appropriate amounts of conductive and thickeners can improve the electrode's conductivity and mechanical stability, reduce active material shedding and structural damage, and thus improve the battery's cycle life and stability. However, excessive amounts of conductive and thickeners can occupy space within the active material, reducing the battery's capacity and energy density. Taking the above into consideration, by designing the weight of each component in the first active material layer and the second active material layer to be within the above range, the battery can have higher comprehensive performance.

[0053] In addition, this embodiment utilizes high-energy-density graphite in the first active material layer and optimizes the binder composition and content to enhance binder strength, improve active material utilization, and prevent peeling and cracking of thick electrode sheets during baking, maximizing the balance between energy density and battery performance. Furthermore, the first active material layer near the current collector has a slightly higher first binder content than the second active material layer near the electrolyte, promoting adhesion between the membrane layer and the current collector layer. A composite conductive agent is used to enhance overall electronic conductivity and reduce battery internal resistance. The second active material layer has a slightly lower second binder content, facilitating rapid lithium ion diffusion and thereby improving the kinetic performance of lithium-ion batteries using thick electrode sheets.

[0054] In some embodiments, the thickness of the first active material layer is 50 μm to 110 μm, the thickness of the second active material layer is 50 μm to 110 μm, and the mass ratio of the first active material layer to the second active material layer is 3:7 to 7:3.

[0055] The thickness and mass ratio of the active material layer directly affect the energy density, cycle stability and charge and discharge performance of the battery. Generally speaking, a thicker active material layer can accommodate more active materials, thereby increasing the total capacity of the battery and increasing the energy density of the battery. However, excessive thickness will also cause the charge transfer path to be too long, thereby affecting the charge and discharge performance. An overly thick active material layer or an unreasonable mass ratio will also increase the internal stress of the electrode, causing the active material to fall off or the structure to be destroyed. This embodiment adopts the parameters within the above range to achieve a higher energy density of the lithium-ion battery while also ensuring higher charge and discharge performance.

[0056] In some embodiments, the surface density of the negative electrode sheet is greater than 190 g / m 2 and less than or equal to 320g / m 2 Areal density refers to the mass of electrode active material per unit area, which directly affects the energy density of the battery. Increasing the lithium ion areal density can improve the energy density of lithium-ion batteries. It should be understood that during the preparation of the negative electrode sheet, the active material is coated on one or both sides of the current collector. Areal density refers to the ratio of the mass of the coated active material to the surface area of ​​the current collector. For single-sided coating, it is the ratio of the single-sided coating amount to the single-sided surface area; for double-sided coating, it is the ratio of the double-sided coating amount to the double-sided surface area.

[0057] In some embodiments, the current collector is a copper current collector or a coated copper current collector.

[0058] Another embodiment of the present invention provides a method for preparing a negative electrode sheet, which is used to prepare the above-mentioned negative electrode sheet, comprising:

[0059] A first graphite, a first conductive agent, a second conductive agent, a first thickener, a first binder and a first solvent are mixed to obtain a first active material layer slurry; a second graphite, a third conductive agent, a second thickener, a second binder and a second solvent are mixed to obtain a second active material layer slurry; the relationship between the convexity and volume median particle size of the first graphite and the convexity and volume median particle size of the second graphite is: 1≤(T1×Dv502) / (T2×Dv501)≤4, wherein T1 is the convexity of the first graphite, Dv501 is the volume median particle size of the first graphite, T2 is the convexity of the second graphite, and Dv502 is the volume median particle size of the second graphite;

[0060] The first active material layer slurry and the second active material layer slurry are coated on a current collector and dried, the first active material layer slurry forms a first active material layer, and the second active material layer slurry forms a second active material layer, thereby obtaining a negative electrode sheet.

[0061] In at least one embodiment, a double-layer coating machine is used to uniformly coat the first active material layer slurry and the second active material layer slurry on the current collector at a certain mass ratio. The slurry is then placed in a coating oven for drying. The oven temperature is set at 110°C. After coating one side of the current collector (denoted as side A), the other side of the current collector (denoted as side B) is switched and the above steps are repeated to complete double-sided coating. After coating, roller pressing and die cutting are performed to obtain a negative electrode sheet with a high load capacity.

[0062] This embodiment uses a double-layer coating technique to coat the current collector with a first active material layer and a second active material layer, respectively. The active material near the current collector and the active material near the electrolyte adopt more appropriate formulations, respectively. By utilizing the interaction of the particle size, convexity, and surface defect control of the two layers of graphite particles, the battery's energy density and fast-charging performance can be balanced, thereby enabling the negative electrode sheet to achieve both high energy density and high fast-charging performance. Furthermore, compared to single-layer coating, double-layer coating has a higher utilization rate of active material, and compared to multi-layer coating (>2 layers), the preparation process of double-layer coating is simpler and more controllable.

[0063] An embodiment of the present invention further provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet, and the negative electrode sheet is the aforementioned negative electrode sheet. The battery of this embodiment can achieve a high energy density while also having good rate performance and cycle performance.

[0064] The embodiment of the present invention further provides a car, comprising the aforementioned lithium-ion battery. It should be understood that the car of this embodiment includes an electric car.

[0065] The present invention is described in detail below through specific examples.

[0066] Example 1

[0067] This embodiment provides a method for preparing a negative electrode sheet and a method for preparing a lithium-ion battery.

[0068] 1.1 Preparation of negative electrode

[0069] The first graphite particles (convexity of 0.75, Dv501 of 14.6), the first conductive agent (conductive carbon black), the second conductive agent (multi-walled carbon nanotubes), the first binder (polyacrylic acid), and the first thickener (sodium carboxymethyl cellulose) were added to a homogenizer in a mass ratio of 93.6:1.2:1.2:2.8:1.2 and dry-stirred for 30 minutes. Then, a solvent (deionized water) was added and kneaded and stirred for 180 minutes to obtain a first active material layer slurry. The slurry had a solid content of 49.8% and a viscosity of 5200 MPa·s.

[0070] The second graphite particles (convexity of 0.25, Dv502 of 10.6), the third conductive agent (acetylene black), the second binder (styrene-butadiene rubber), and the second thickener (sodium carboxymethyl cellulose) were added to a homogenizer in a mass ratio of 95.8:1.4:1.6:1.2 and dry-stirred for 30 minutes. Then, a solvent (deionized water) was added and kneaded and stirred for 180 minutes to obtain a second active material layer slurry. The slurry had a solid content of 54.5% and a viscosity of 7500 mPa·s.

[0071] A double-layer coating machine was used to coat the first active material layer slurry and the second active material layer slurry at an area density of 260 g / m 2 The first active material layer and the second active material layer were uniformly coated on the current collector at a mass ratio of 6:4. After coating side A, the electrode was switched to side B and the above steps were repeated. After coating, the electrode was rolled and die-cut, with a compaction density of 1.60 g / cc. The resulting electrode thickness was 170.5 mm.

[0072] The convexity and volume median particle size of the graphite particle powder are measured as follows:

[0073] Convexity T: The convex perimeter and actual perimeter of graphite particle powder are measured using a particle size and shape analyzer (Mastersizer R-3000). The powder sample is evenly dispersed on the surface of an optical glass plate and photographed using a telecentric zoom lens. The captured images are statistically analyzed for particle morphology and particle count. The length of the solid line of the outer contour is the actual perimeter, while the length of the dashed line of the outer contour is the convex hull perimeter. Convexity = convex hull perimeter / actual perimeter.

[0074] It should be noted that the convexity is obtained through statistical analysis, not by comparing the convex hull perimeter of a specific cross-section of the material with the actual perimeter, but by comparing the sum of the convex hull perimeters of N cross-sections in the material that can reflect its overall morphological characteristics with the sum of the actual perimeters of N cross-sections.

[0075] Volume Median Particle Size (Dv50): The Dv50 of graphite particle powder is determined using a laser diffraction particle size analyzer (MS3000). Sample particles are dispersed in a liquid medium at a concentration of 200 mg / L to 300 mg / L, and a monochromatic light beam is passed through the medium. Light is scattered by the particles and distributed at different angles. A multi-element detector receives the corresponding scattering patterns at these angles and records these values ​​for analysis. Using the Rayleigh scattering formula, the scattered values ​​are calculated to determine the ratio of the volume of each particle size class to the total volume, thereby determining the volume distribution of the particle size.

[0076] 1.2 Preparation of lithium-ion batteries

[0077] (1) Preparation of positive electrode sheet

[0078] The positive electrode active material lithium iron phosphate LFP, the conductive agent (multi-walled carbon nanotubes), and the binder (polyvinylidene fluoride) are mixed uniformly in an appropriate amount of solvent NMP according to a mass ratio of 96:2:2 to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on both sides of the positive electrode current collector aluminum foil (thickness 15μm), and the positive electrode surface density is calculated according to the excess ratio N / P = 1.115 (i.e., the negative electrode sheet double-sided surface density is 260g / m 2 Under these conditions, the positive electrode surface density is converted to 550g / m 2 ), the designed compaction density is 2.50g / cm 3 , and then roll-pressed and die-cut to obtain the positive electrode sheet.

[0079] (2) Preparation of electrolyte

[0080] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a concentration of 1 mol / L to obtain an electrolyte after full dissolution.

[0081] (3) Preparation of isolation membrane

[0082] A polyethylene film with a ceramic coating was selected as the separator with a thickness of 12 μm.

[0083] (4) Preparation of lithium-ion soft-pack batteries

[0084] The positive electrode sheet, separator, and negative electrode sheet are stacked in order with the separator between the positive and negative electrode sheets, and then wound to obtain a bare cell. The bare cell is encapsulated in an aluminum-plastic shell, and after drying, the electrolyte is injected. After vacuum packaging, standing, formation, degassing, shaping and other processes, a lithium-ion soft-pack battery is obtained.

[0085] Example 2-17

[0086] The differences between Examples 2 to 17 and Example 1 are shown in Tables 1 and 2, and the other aspects are the same. Among them, the difference between Examples 1 to 3 is that the convexity of the first graphite is different. From Example 1 to Example 3, the convexity of the first graphite gradually increases, and accordingly, the value of the index (T1×Dv502) / (T2×Dv501) also gradually increases, but the upper limit does not exceed 4. The difference between Examples 4 to 6 is that the convexity of the second graphite is different. From Example 4 to Example 6, the convexity of the second graphite gradually increases, and accordingly, the value of the index (T1×Dv502) / (T2×Dv501) gradually decreases, but the lower limit is not less than 1. The difference between Example 7 and Example 8 is that the volume median particle size Dv501 of the first graphite is different. The difference between Example 9 and Example 10 is that the volume median particle size Dv502 of the second graphite is different. The graphite particle characteristics (convexity and volume median particle size) of Examples 11 to 17 are the same, and the differences lie in the negative electrode sheet density, the mass ratio of the first active material layer to the second active material layer, the formula of the first active material layer, and the formula of the second active material layer.

[0087] Comparative Examples 1-7

[0088] The differences between Comparative Examples 1 to 7 and Example 1 are shown in Tables 1 and 2, and the other aspects are the same. Among them, the graphite particle characteristics (convexity and volume median particle size) of Comparative Examples 1 and 7 are the same as those of Examples 11-17, and the main difference is that the surface density of the negative electrode sheet is different. The index (T1×Dv502) / (T2×Dv501) of Comparative Examples 2 to 5 exceeds the upper and lower limits. Comparative Example 5 does not adopt a double-layer coating technology, and only one active material layer is coated on the surface of the current collector. Although Comparative Example 6 meets 1≤(T1×Dv502) / (T2×Dv501)≤4, it does not fully meet the convexity value range of the first graphite and the second graphite and the volume median particle size relationship of the first graphite and the second graphite.

[0089] Table 1:

[0090] Table 2:

[0091] The performance of the lithium-ion batteries obtained in Examples 1-17 and Comparative Examples 1-7 was tested, and the results are shown in Table 3.

[0092] Among them, the performance test method of lithium-ion batteries is as follows:

[0093] Energy density test: At 25°C, use a balance to test the mass of the soft-pack battery cell after final sealing, record it as m1, unit / kg; use a blue battery tester to charge the battery cell at 0.33C to a voltage of 4.25V, let it stand for 30 minutes, and discharge it at 0.33C to 2.5V. Record the 0.33C discharge energy of the soft-pack battery cell as Q1, unit Wh; then the energy density of the soft-pack battery cell = Q1 / m1, unit Wh / kg.

[0094] Rate charge test: At 25°C, the lithium-ion batteries prepared in the examples and comparative examples were charged at a constant current and constant voltage of 0.33C to 4.25V, and the charge capacity was recorded as C1. They were then discharged at a constant current of 0.33C to 2.5V; then charged at a constant current of 2C to 4.25V, and the charge capacity was recorded as C2. The rate charge capability = 2C charge capacity C2 / 0.33C charge capacity C1.

[0095] DCIR test: Test the DCIR of lithium-ion soft-pack batteries at 50% SOC (state of charge or charge). Stand at 25°C for 1 hour and record the terminal static voltage as V0. Discharge at a constant current of 1C for 40 seconds and record the sample voltage V1. Calculate the transient discharge battery DC internal resistance DCR = (V0-V1) / 1ln1, where 1ln1 is the discharge current value at 1C.

[0096] Peeling and powdering test: At 25°C, cycle 1C charge / 1C discharge for 500cls and then disassemble to observe the peeling and powdering of the membrane layer and the current collector at the disassembly interface. If the separation or powdering area of ​​the negative electrode interface membrane layer and the current collector is 1% to 10%, it is considered a minor defect. If the separation or powdering area is 10% to 30%, it is considered a moderate defect. If the peeling area of ​​the separation or powdering area is greater than 30%, it is considered a severe defect.

[0097] Table 3:

[0098] It can be seen from Table 1 that the convexity and volume median particle size of the first graphite and the second graphite of the negative electrode sheets of Examples 1 to 17 satisfy 1≤(T1×Dv502) / (T2×Dv501)≤4. According to the test results in Table 3, it can be seen that the energy density of the battery is greater than 195wh / kg, the rate charging performance (2C charging capacity / 0.33C charging capacity) is greater than 90%, and the discharge DCR at 50% SOC is lower than 84mΩ. After disassembly, it was observed that the interface of the negative electrode sheet was good, thereby realizing the preparation of high energy density and high rate performance batteries assembled with high load and thick electrode sheets. This is mainly due to the fact that the first graphite material provided in Examples 1 to 17 has a large convexity and a slightly higher particle size, which can provide a higher capacity for lithium insertion; the second graphite material has a small convexity and a slightly lower particle size, and its surface roughness is large. The concave-convex structural characteristics of the surface are conducive to the full infiltration of the electrolyte and the rapid diffusion of lithium ions. At the same time, by optimizing the formulation of the two active layers, the conductivity and bonding properties of the membrane layer close to the current collector are improved, the charge transfer impedance is reduced, the utilization rate of the active material is increased, the polarization effect of the thick electrode is reduced, and a good interface condition is maintained after multiple cycles. Test results show that lithium-ion soft-pack batteries equipped with this high-load thick electrode have a high energy density, a significantly improved charging performance at a 2C rate, and a cycling performance that meets the stringent requirements of current energy storage batteries.

[0099] Compared with Example 13, the graphite particle characteristics (convexity and volume median particle size) of Comparative Example 1 are the same, the formula of the first active material layer and the second active material layer are the same, and the mass ratio of the first active material layer to the second active material layer is in the range of 3:7 to 7:3. The main difference lies in the difference in the negative electrode sheet density. The negative electrode sheet density of Comparative Example 1 is not 190 g / m 2 Up to 320g / m 2 Compared with Example 17, the graphite particle characteristics (convexity and volume median particle size) of Comparative Example 1 are the same, the mass ratio of the first active material layer to the second active material layer is the same, the formula of the first active material layer is the same, and the formula of the second active material layer is within the weight proportion range of each component. The main difference lies in the different negative electrode sheet density. The negative electrode sheet density of Comparative Example 1 is not 190g / m 2 Up to 320g / m 2 By comparing Example 1 with Example 13 or Example 17, it can be seen that increasing the surface density of the negative electrode sheet can significantly increase the material energy density. For example, in Example 1, the surface density of the negative electrode sheet is 190g / m 2 , much lower than 260g / m2 of Example 13 2 According to the test results in Table 3, it can be seen that the energy density of Comparative Example 1 is particularly low, only 170Wh / kg, while the energy density of Example 13 is 216Wh / kg.

[0100] Compared with Example 13, the particle characteristics (convexity and volume median particle size) of the graphite in Comparative Example 7 are the same. The formulas of the first active material layer and the second active material layer both meet the weight ratio range of their respective components. The mass ratio of the first active material layer to the second active material layer is in the range of 3:7 to 7:3. The main difference lies in the different surface densities of the negative electrode sheets. The surface density of Comparative Example 7 is greater than 340 g / m 2 , exceeding the required 190g / m 2 Up to 320g / m 2 The upper limit of the battery. According to the test results in Table 3, although the battery has a high energy density, its rate charging performance (2C charging capacity / 0.33C charging capacity) is low, and the discharge DCR at 50% SOC is as high as 96.96mΩ. After disassembly, the negative electrode interface peeling and powder loss are more serious. It can be seen that setting the surface density of the negative electrode sheet to 190g / m 2 Up to 320g / m 2 (excluding 190g / m 2 ), which can ensure that the lithium-ion battery has high energy density and rate performance, as well as good interface appearance and low DCR.

[0101] By comparing Comparative Example 2 with Examples 1-10, it can be seen that when the index (T1×Dv502) / (T2×Dv501) exceeds the upper limit of 4 due to the larger volume median particle size of the second graphite particles, the battery's 2C rate charging performance and disassembly interface remain at a good level, but the battery energy density is slightly lower than that of Examples 1-10. This is mainly because when the volume median particle size of the first graphite particles near the current collector is smaller than the volume median particle size of the second graphite particles, it is difficult for the first graphite particles to achieve a high compaction density internally, resulting in a decrease in the battery energy density.

[0102] By comparing Comparative Examples 3-4 with Example 6, it can be seen that when the index (T1×Dv502) / (T2×Dv501) exceeds the lower limit, the rate charging performance of the lithium-ion battery is slightly poor, and when the pole piece is disassembled, powdering is found, and even the membrane layer and the current collector are peeled off. Analysis of the reasons may be that the second graphite particles in the second active material layer have a large convexity (Comparative Example 4), and their surface morphology features are regularized, resulting in a less than ideal improvement in rate performance. Alternatively, the first graphite particles are too large (Comparative Example 3), resulting in an index below the lower limit, a longer lithium ion diffusion path, and a lower diffusion rate, resulting in lower rate performance.

[0103] In addition, through Comparative Example 4, it was found that if the convexity of the first graphite is smaller and the convexity of the second graphite is larger, the energy density of the lithium-ion battery is lower and the rate charging performance is poor. Powdering and peeling phenomena occur on the disassembly interface covering more than 30% of the electrode area. This indicates that if graphite with a larger convexity is used on the electrolyte side, the polarization effect during charging and discharging causes its active material utilization and fast charging performance to deteriorate.

[0104] Comparative Example 5, in which only the first active material layer is applied to the current collector surface, demonstrates a higher energy density compared to the double-layer coating of Examples 1-17, as the double-layer coating provides a more uniform and stable protective layer, helping to reduce internal polarization. Single-layer coating, however, is more susceptible to severe peeling or powder loss due to uneven surface coating and insufficient adhesion. The discharge DCR at 50% SOC is as high as 108.69%.

[0105] Compared with Examples 1 to 10, Comparative Example 6 has the same areal density of the negative electrode sheet, the formulations of the first and second active material layers, and the mass ratio of the first active material layer to the second active material layer. However, the characteristics of the graphite particles (convexity and volume median particle size) are different. In Comparative Example 6, the volume median particle size of the first graphite is smaller than that of the second graphite, and the volume median particle size of the graphite particles closer to the current collector is smaller. This makes it difficult for the graphite particles to achieve a high compaction density in the first active material layer, resulting in a battery energy density slightly lower than that of Examples 1 to 10. In addition, since the first graphite has a large convexity, a small particle size is not conducive to improving the lithium insertion capacity. The second graphite has a small convexity and a large surface roughness. If the particle size is large, it is not conducive to sufficient electrolyte infiltration and rapid diffusion of lithium ions, resulting in a higher internal resistance of the battery. Therefore, the battery of Comparative Example 6 exhibits low unrate charge performance, a high DCR, and slight powdering on the electrode surface.

[0106] In summary, the embodiments of the present invention can effectively improve the battery energy density while having better rate charging performance and cycle performance by designing a double-layer coating for the negative electrode sheet and limiting the volume median particle size and convexity of the two types of graphite in the double-layer coating to an appropriate range.

[0107] Although the embodiments of the present invention are disclosed above, the protection scope of the embodiments of the present invention is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention, and these changes and modifications will fall within the protection scope of the embodiments of the present invention.

Claims

1. A negative electrode sheet, comprising a current collector (1), a first active material layer (2) disposed on the current collector (1), and a second active material layer (3) disposed on the first active material layer (2). The first active material layer (2) comprises a first graphite, and the second active material layer (3) comprises a second graphite. The convexity and volume median diameter of the first graphite and the convexity and volume median diameter of the second graphite satisfy the following relationship: 1 ≤ (T1 × Dv502) / (T2 × Dv501) ≤ 4; Among them, T1 is the convexity of the first graphite, Dv501 is the volume median diameter of the first graphite, T2 is the convexity of the second graphite, and Dv502 is the volume median diameter of the second graphite.

2. The negative electrode sheet according to claim 1, wherein, The convexity of the first graphite is from 0.75 to 1.0, and the convexity of the second graphite is from 0.25 to 0.

75.

3. The negative electrode sheet according to claim 1, wherein, The volume median diameter of the first graphite is greater than the volume median diameter of the second graphite.

4. The negative electrode sheet according to claim 1, wherein, The areal density of the negative electrode sheet is greater than 190 g / m 2 and less than or equal to 320 g / m 2 .

5. The negative electrode sheet according to claim 1, wherein The mass ratio of the first active material layer (2) to the second active material layer (3) is from 3:7 to 7:

3.

6. The negative electrode sheet according to claim 1, wherein, The first active material layer (2) further comprises a first conductive agent, a second conductive agent, a first thickening agent, and a first binder, and the second active material layer (3) further comprises a third conductive agent, a second thickening agent, and a second binder.

7. The negative electrode sheet according to claim 6, wherein, The weight ratio of the first graphite in the first active material layer (2) is from 91.6% to 96.2%, the weight ratio of the first conductive agent in the first active material layer (2) is from 0.7% to 1.5%, the weight ratio of the second conductive agent in the first active material layer (2) is from 0.7% to 1.5%, the weight ratio of the first binder in the first active material layer (2) is from 1.4% to 3.6%, and the weight ratio of the first thickening agent in the first active material layer (2) is from 1.0% to 1.8%; and / or, The weight ratio of the second graphite in the second active material layer (3) is from 93.8% to 97.4%, the weight ratio of the third conductive agent in the second active material layer (3) is from 0.6% to 2.0%, the weight ratio of the second binder in the second active material layer (3) is from 1.0% to 2.4%, and the weight ratio of the second thickening agent in the second active material layer (3) is from 1.0% to 1.8%.

8. A method for preparing a negative electrode sheet for preparing the negative electrode sheet according to any one of claims 1-7, comprising: Mixing the first graphite, the first conductive agent, the second conductive agent, the first thickening agent, the first binder with a first solvent to obtain a first active material layer slurry; Mix the second graphite, the third conductive agent, the second thickening agent, the second binder and the second solvent to obtain a second active material layer slurry; the relationship between the convexity and volume median diameter of the first graphite and those of the second graphite is: 1 ≤ (T1 × Dv502) / (T2 × Dv501) ≤ 4, where T1 is the convexity of the first graphite, Dv501 is the volume median diameter of the first graphite, T2 is the convexity of the second graphite, and Dv502 is the volume median diameter of the second graphite; Coat the first active material layer slurry and the second active material layer slurry on the current collector (1) and perform a drying treatment. The first active material layer slurry forms a first active material layer (2), and the second active material layer slurry forms a second active material layer (3) to obtain a negative electrode sheet.

9. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and the negative electrode sheet is the negative electrode sheet according to claim 8.

10. An automobile, comprising the lithium-ion battery according to claim 9.

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