Negative plate of lithium ion battery, preparation method of negative plate and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a negative plate of a lithium ion battery, which comprises a negative current collector and a negative coating, the negative electrode coating comprises a negative electrode coating material coated on the surface of at least one side of the negative electrode current collector, and the negative electrode coating material comprises a negative electrode active substance; the negative electrode active substance comprises a graphite material and a silicon-based material, and the graphite material accounts for 70-95 wt% of the negative electrode active substance in percentage by mass; the mass percentage of the silicon-based material in the negative electrode active material is 3-25 wt%; wherein the surface of the negative electrode active material is provided with a carbon layer, the carbon layer is doped with a nitrogen element, and in an energy dispersion X-ray spectrogram of the carbon layer, WN is more than 1 and less than 7, and WN / WC is more than 0.01 and less than 0.1. Therefore, the lithium ion migration rate and the electron transmission capability can be effectively improved, and electrolyte permeation and volume expansion are inhibited, so that the interface stability is enhanced, and the fast charging performance and the cycling stability of the lithium ion battery are further improved. The invention further discloses a preparation method of the negative plate and a lithium ion battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a negative electrode sheet of a lithium-ion battery and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] With the widespread application of lithium-ion batteries in new energy vehicles, energy storage systems, and portable electronic devices, the requirements for their fast charging performance, low-temperature adaptability, and cycle life continue to increase. The anode, a key carrier for lithium ion storage and release in the battery structure, is the primary choice for lithium-ion battery anodes. Graphite-based anode materials are the primary choice for lithium-ion battery anodes due to their low lithium insertion potential, high reversible capacity, and good conductivity.

[0003] In related art, the lithium ion diffusivity of graphite anodes affects ion / electron transfer efficiency, while their interfacial chemical stability affects cycling stability. To address this, related art provides a negative electrode material comprising an active material and a coating layer, the coating layer being disposed on at least a portion of the surface of the active material. The active material comprises a silicon-based material and a carbon material, and the negative electrode material has pores.

[0004] During the implementation of the embodiments of the present disclosure, it was found that at least the following problems exist in the related art:

[0005] The above methods can improve battery performance to a certain extent. However, when the battery is fast-charged, the rate of lithium ion insertion between the graphite layers does not match the electron transfer efficiency, which may lead to concentration polarization and the risk of lithium dendrites. At the same time, the increase in electrolyte viscosity at low temperatures may lead to an increase in the impedance of the solid electrolyte interface (SEI) on the graphite surface, causing lithium ion deintercalation hysteresis. In addition, the disordered structure of the coating layer will also hinder the transmission of lithium ions across the interface, thereby affecting its fast-charge performance and cycle stability.

[0006] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to ordinary technicians in this field. Summary of the Invention

[0007] In order to provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. The summary is not an extensive review, nor is it intended to identify key / critical elements or delineate the scope of protection of these embodiments, but rather serves as a prelude to the detailed description that follows.

[0008] The embodiments of the present disclosure provide a negative electrode sheet for a lithium-ion battery, a preparation method thereof, and a lithium-ion battery, which can effectively improve the lithium ion migration rate and electron transmission capacity, inhibit electrolyte penetration and volume expansion, thereby enhancing interface stability, and further improving its fast charging performance and cycle stability.

[0009] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode coating. The negative electrode coating includes a negative electrode coating material coated on at least one side of the negative electrode current collector, and the negative electrode coating material includes a negative electrode active material; the negative electrode active material includes a graphite material and a silicon-based material, and the mass percentage of the graphite material in the negative electrode active material is 70wt% to 95wt%; the mass percentage of the silicon-based material in the negative electrode active material is 3wt% to 25wt%;

[0010] The surface of the negative electrode active material is provided with a carbon layer, the carbon layer is doped with nitrogen, and in its energy dispersive X-ray spectrum, the following conditions are met:

[0011] 1<W N <7、0.01<W N / W C <0.1;

[0012] Among them, W N W is the mass percentage of nitrogen obtained after normalization of the energy dispersive X-ray spectrum of the carbon layer of the negative electrode active material; C is the mass percentage of the carbon element obtained after normalization of the energy dispersive X-ray spectrum of the carbon layer of the negative electrode active material.

[0013] Optionally, the graphite material includes natural graphite and / or artificial graphite; the silicon-based material includes silicon-oxygen material and / or silicon-carbon material.

[0014] Optionally, the negative electrode active material is prepared by heat-treating graphitized particles, asphalt and nitrogen element precursor, and the mass ratio of the graphitized particles, asphalt and nitrogen element precursor is 100:7:1 to 100:7:3.

[0015] Optionally, the compaction density of the negative electrode sheet is 1.6 g / cm 3 In the case of hole impedance Z≤8Ωcm 2 .

[0016] Optionally, the average particle size D of the graphite material 50 The average particle size D of silicon-based materials is 8μm to 18μm. 50 4μm~10μm.

[0017] Optionally, the negative electrode coating further includes a negative electrode conductive agent, which includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon black; the negative electrode coating further includes a negative electrode binder, which includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylonitrile, polystyrene-acrylic acid and styrene-butadiene rubber.

[0018] In some embodiments, the method for preparing the negative electrode sheet comprises the following steps:

[0019] Preparation of negative electrode coating material: Preparation of negative electrode coating material: needle coke and pitch are mixed according to a preset mass ratio, and after air flow grinding, original particles are obtained; the original particles are heated and stirred to obtain graphitized particles; the graphitized particles, pitch and nitrogen precursor are mixed according to a preset mass ratio and heated and stirred in a nitrogen environment to obtain a negative electrode active material; then the negative electrode active material, a negative electrode binder and a negative electrode conductive agent are mixed according to a preset mass ratio to obtain a negative electrode coating material in which the surface carbon layer of the negative electrode active material is doped with nitrogen element;

[0020] Preparation of negative electrode sheet: coating the negative electrode coating material on at least one side of the negative electrode current collector, drying and cold pressing to obtain the negative electrode sheet.

[0021] Optionally, the original particles are heated and stirred to obtain graphitized particles, including:

[0022] The original particles are heated and stirred under a nitrogen environment to obtain an intermediate material;

[0023] The intermediate material meeting the conditions is heated at a temperature of 2500° C. to 3000° C. to obtain graphitized particles.

[0024] In some embodiments, the lithium-ion battery comprises a positive electrode sheet and a negative electrode sheet as described in the above embodiments; the positive electrode sheet comprises a positive electrode current collector and a positive electrode coating coated with at least one layer containing a positive electrode active material; the positive electrode active material comprises lithium nickel cobalt manganese oxide (Li1Ni x Co y Mn z M b O2) and lithium iron phosphate; wherein, 0.70≤x≤0.95, 0.05≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1; the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd.

[0025] Optionally, the positive electrode sheet further includes: an electrolyte, including a lithium salt, a solvent and an additive; the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; the solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate; the additive includes one or more of fluoroethylene carbonate, bis(fluoroethylene carbonate), vinyl sulfate, vinyl sulfite, vinyl carbonate and vinyl carbonate.

[0026] Optionally, after 500 cycles at 2C / 10C at 25°C, the capacity retention rate Q R >85%.

[0027] The negative electrode sheet of a lithium-ion battery and its preparation method, as well as the lithium-ion battery provided in the embodiments of the present disclosure, can achieve the following technical effects:

[0028] Since nitrogen atoms have a greater electronegativity than carbon atoms, that is, they have a stronger ability to attract electrons. Therefore, the present application dopes nitrogen into the surface carbon layer of the negative electrode active material and optimizes the mass percentages of nitrogen and carbon. In this way, the electron cloud distribution on the graphite surface can be changed, and more defect sites can be introduced, thereby improving the lithium ion migration rate and electron transport capacity, accelerating the charge transfer efficiency between the electrode and the electrolyte, and thus improving the charging rate of the battery.

[0029] On this basis, the nitrogen-doped carbon layer can also form a strong chemical bond (CNC bond) with the graphite substrate, which can inhibit electrolyte penetration and volume expansion, enhance its interfacial stability, and thus improve its cycling stability. On the other hand, the nitrogen-doped carbon layer can also serve as a protective layer, enhancing the oxidation resistance and corrosion resistance of the graphite material, and resisting the erosion of the internal structure of the graphite by the external environment, thereby further improving battery performance.

[0030] The above general description and the following description are exemplary and explanatory only and are not intended to limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] One or more embodiments are exemplarily described by corresponding drawings. These exemplary descriptions and drawings do not limit the embodiments. Elements with the same reference numerals in the drawings are shown as similar elements. The drawings do not constitute a scale limitation. In addition,

[0032] Figure 1 This is a schematic flow chart of a method for preparing a negative electrode sheet provided by an embodiment of the present disclosure;

[0033] Figure 2 is a schematic structural diagram of a lithium-ion battery provided by an embodiment of the present disclosure;

[0034] Figure 3 is a schematic structural diagram of a lithium-ion battery provided by an embodiment of the present disclosure;

[0035] Figure 4 Schematic diagram of an expanded lithium-ion battery provided by an embodiment of the present disclosure.

[0036] Reference numerals:

[0037] 1-positive terminal; 10-battery cell; 11-positive column; 12-negative terminal; 2-shell; 3-negative electrode; 4-diaphragm; 5-positive electrode. DETAILED DESCRIPTION

[0038] In order to be able to understand the features and technical content of the embodiments of the present disclosure in more detail, the implementation of the embodiments of the present disclosure is described in detail below in conjunction with the accompanying drawings. The accompanying drawings are for reference only and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of convenience of explanation, a full understanding of the disclosed embodiments is provided through multiple details. However, one or more embodiments can still be implemented without these details. In other cases, to simplify the drawings, well-known structures and devices can be simplified for display.

[0039] In the description and claims of the embodiments of the present disclosure, as well as in the accompanying drawings, the terms "first," "second," and the like are used to distinguish similar items and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate to describe the embodiments of the present disclosure herein. In addition, the terms "including," "having," and any variations thereof are intended to cover non-exclusive inclusions.

[0040] In the embodiments of the present disclosure, the terms "upper", "lower", "inside", "middle", "outside", "front", "back" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. These terms are mainly intended to better describe the embodiments of the present disclosure and their embodiments, and are not intended to limit the indicated devices, elements or components to having a specific direction, or to be constructed and operated in a specific direction. Moreover, in addition to being used to indicate directions or positional relationships, some of the above terms may also be used to indicate other meanings. For example, the term "upper" may also be used to indicate a certain dependency or connection relationship in certain circumstances. For those of ordinary skill in the art, the specific meanings of these terms in the embodiments of the present disclosure can be understood according to specific circumstances.

[0041] Furthermore, the terms "disposed," "connected," and "fixed" should be interpreted broadly. For example, "connected" can mean a fixed connection, a removable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediary, or an internal connection between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in the embodiments of this disclosure based on the specific circumstances.

[0042] Unless otherwise stated, the term "plurality" means two or more.

[0043] In the embodiment of the present disclosure, the character " / " indicates that the preceding and following objects are in an "or" relationship. For example, A / B means: A or B.

[0044] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0045] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present disclosure can be combined with each other.

[0046] The present disclosure provides a negative electrode sheet for a lithium-ion battery, comprising a negative electrode current collector and a negative electrode coating. The negative electrode coating comprises a negative electrode coating material applied to at least one surface of the negative electrode current collector, wherein the negative electrode coating material comprises a negative electrode active material; the negative electrode active material comprises a graphite material and a silicon-based material, wherein the mass percentage of the graphite material in the negative electrode active material is 70wt% to 95wt%; and the mass percentage of the silicon-based material in the negative electrode active material is 3wt% to 25wt%.

[0047] The surface of the negative electrode active material is provided with a carbon layer, the carbon layer is doped with nitrogen, and in its energy dispersive X-ray spectrum, the following conditions are met:

[0048] 1<W N <7、0.01<W N / W C <0.1;

[0049] Among them, W N W is the mass percentage of nitrogen obtained after normalization of the energy dispersive X-ray spectrum of the carbon layer of the negative electrode active material; C is the mass percentage of the carbon element obtained after normalization of the energy dispersive X-ray spectrum of the carbon layer of the negative electrode active material.

[0050] The negative electrode sheet of the lithium-ion battery provided by the embodiment of the present disclosure has a greater electronegativity of nitrogen atoms than that of carbon atoms, that is, a stronger ability to attract electrons. Therefore, the present application dopes nitrogen into the surface carbon layer of the negative electrode active material and optimizes the mass percentages of nitrogen and carbon elements. In this way, the electron cloud distribution on the graphite surface can be changed, and more defect sites can be introduced, thereby improving the lithium ion migration rate and electron transport capacity, accelerating the charge transfer efficiency between the electrode and the electrolyte, and thus improving the battery charging rate.

[0051] On this basis, the nitrogen-doped carbon layer can also form a strong chemical bond (CNC bond) with the graphite substrate, which can inhibit electrolyte penetration and volume expansion, enhance its interfacial stability, and thus improve its cycling stability. On the other hand, the nitrogen-doped carbon layer can also serve as a protective layer, enhancing the oxidation resistance and corrosion resistance of the graphite material, and resisting the erosion of the internal structure of the graphite by the external environment, thereby further improving battery performance.

[0052] Optionally, the negative electrode active material is prepared by heat-treating graphitized particles, asphalt and nitrogen element precursor, and the mass ratio of the graphitized particles, asphalt and nitrogen element precursor is 100:7:1 to 100:7:3.

[0053] Optionally, the graphite material includes natural graphite and / or artificial graphite; the silicon-based material includes silicon-oxygen material and / or silicon-carbon material.

[0054] Optionally, the average particle size D of the graphite material 50 The average particle size D of silicon-based materials is 8μm to 18μm. 50 4μm~10μm.

[0055] Optionally, the negative electrode coating further includes a negative electrode conductive agent, and the negative electrode conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon black.

[0056] Optionally, the negative electrode coating further includes a negative electrode binder, and the negative electrode binder includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylonitrile, polystyrene-acrylic acid, and styrene-butadiene rubber.

[0057] Optionally, the compaction density of the negative electrode sheet is 1.6 g / cm 3 In the case of hole impedance Z≤8Ωcm 2 .

[0058] Combine Figure 1 As shown, the embodiment of the present disclosure also provides a method for preparing a negative electrode sheet, comprising the following steps:

[0059] Preparation of negative electrode coating materials:

[0060] S101, mixing needle coke and asphalt according to a preset mass ratio, and jet milling the mixture to obtain raw particles;

[0061] In the embodiment, the graphite raw material is needle coke, which is mixed with pitch, and then the mixed raw material is ground into particles of a set particle size, thereby completing the pretreatment of the raw material.

[0062] S102, heating and stirring the original particles to obtain graphitized particles;

[0063] In the embodiments, graphitized particles refer to particles having a layered carbon structure. During the processing of the original particles, the original particles are converted into an intermediate material with a particle size of 10 mm to 20 mm, which is then ground into a secondary material with a particle size of 6 μm to 10 μm, and screened by a screening machine; the screened secondary material is graphitized to obtain graphitized particles.

[0064] S103, mixing the graphitized particles, the pitch, and the nitrogen precursor according to a preset mass ratio, and heating and stirring the mixture in a nitrogen environment to obtain a negative electrode active material;

[0065] In this embodiment, after obtaining graphitized particles, they are mixed with pitch and a nitrogen precursor, wherein the nitrogen precursor refers to a substance capable of introducing nitrogen, so that the negative electrode active material can have a carbon layer doped with nitrogen.

[0066] S104, then mixing the negative electrode active material with the negative electrode binder and the negative electrode conductive agent according to a preset mass ratio to obtain a negative electrode coating material in which the surface carbon layer of the negative electrode active material is doped with nitrogen;

[0067] S105, preparing a negative electrode sheet: coating a negative electrode coating material on at least one surface of a negative electrode current collector, drying and cold pressing to obtain a negative electrode sheet.

[0068] Optionally, the original particles are heated and stirred to obtain graphitized particles, including:

[0069] The original particles are heated and stirred under a nitrogen environment to obtain an intermediate material;

[0070] In the embodiment, in order to ensure the purity of the original particles, heating and stirring need to be performed in a nitrogen environment or other inert gas environment.

[0071] The intermediate material meeting the conditions is heated at a temperature of 2500° C. to 3000° C. to obtain graphitized particles.

[0072] The embodiments of the present disclosure further provide a lithium-ion battery, comprising a positive electrode sheet and a negative electrode sheet as described in the above embodiments.

[0073] The specific structure of the negative electrode sheet refers to the above embodiments. Since the lithium-ion battery adopts all the technical solutions of all the above embodiments, it has at least all the technical effects brought by the technical solutions of the above embodiments, which will not be described one by one here.

[0074] Optionally, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating coated with at least one layer containing a positive electrode active material; the positive electrode active material includes lithium nickel cobalt manganese oxide (Li1Ni x Co y Mn z M b O2) and lithium iron phosphate; wherein, 0.70≤x≤0.95, 0.05≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1; the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd.

[0075] Optionally, the positive electrode sheet further includes: an electrolyte, including a lithium salt, a solvent and an additive; the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; the solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate; the additive includes one or more of fluoroethylene carbonate, bis(fluoroethylene carbonate), vinyl sulfate, vinyl sulfite, vinyl carbonate and vinyl carbonate.

[0076] Optionally, after 500 cycles at 2C / 10C at 25°C, the capacity retention rate Q R >85%.

[0077] In the embodiment of the present disclosure, the lithium-ion battery further includes a shell, and the positive electrode sheet and the negative electrode sheet are encapsulated in the shell. Figures 2 to 4 As shown, the lithium-ion battery specifically includes a cylindrical shell 2, the interior of which is used to accommodate the battery cell, wherein the top is the positive terminal 1, the bottom is the negative terminal 12, and the positive terminal 1 is provided with a positive column 11. Specifically, Figure 3 shows a schematic structural diagram of the lithium-ion battery in this application, Figure 4 FIG1 shows a schematic diagram of the lithium-ion battery in the present application. Figure 4 As shown in the stacking, then winding to form Figure 3The cylindrical battery cell 10 shown has the electrode end at the cylinder axis at the beginning of winding and at the outer surface of the cylinder at the end of winding. The positive electrode sheet 5 comprises a strip of positive electrode foil, a positive electrode coating applied to the surface of the positive electrode foil strip, and a first hollow foil region. The negative electrode sheet 3 comprises a strip of negative electrode foil, a negative electrode coating applied to the surface of the negative electrode foil strip, and a second hollow foil region. The first and second hollow foil regions are perpendicular to the winding direction and are formed by flattening or cutting and stacking to form the top and bottom end faces of the lithium-ion battery.

[0078] On this basis, an embodiment of the present disclosure provides a method for preparing a lithium-ion battery, comprising:

[0079] Preparation of positive electrode sheet: Mix the positive electrode coating materials, apply them on both sides of the aluminum foil, dry and cold press to obtain the positive electrode sheet;

[0080] Preparation of negative electrode sheet: Mix the negative electrode coating material, apply it on both sides of the copper foil, dry it and cold press it to obtain the negative electrode sheet;

[0081] Preparation of battery cells: The positive and negative electrode sheets are rolled and slit, and then wound together with the separator to obtain battery cells;

[0082] Assembling lithium-ion batteries: Welding the tabs of the lithium-ion battery to the electrical connectors, placing them into the battery case, and performing the electrolyte injection, sealing, and formation processes to obtain a lithium-ion battery.

[0083] In addition, an embodiment of the present disclosure provides an electrical device, comprising a lithium-ion battery for providing power as described in the present application.

[0084] The present invention is further explained below with reference to the following examples.

[0085] Example 1

[0086] This embodiment 1 provides a method for preparing a lithium ion battery as follows:

[0087] Preparation of positive electrode sheet: Select positive electrode active material (Li1Ni 0.9 Co 0.05 Mn 0.05 O2), conductive carbon black and polyvinylidene fluoride (PVDF) are thoroughly stirred and mixed in an N-methylpyrrolidone solvent system in a mass ratio of 96:1:3 to form a stable positive electrode slurry with a solid content of 70%. The positive electrode slurry is then coated on a 12μm thick aluminum foil, dried and cold pressed to obtain a positive electrode sheet.

[0088] Preparation of negative electrode sheet: including preparation of negative electrode coating material, specifically including:

[0089] After needle coke and pitch are mixed in a preset mass ratio of 10:1, the raw and auxiliary materials with a particle size of 5mm to 10mm are ground into original particles with a particle size of 3μm to 10μm, and the original particles are collected by a cyclone dust collector;

[0090] The original particles are placed in a reactor and nitrogen is introduced, heated to 300°C under a certain pressure and stirred for 2 hours; the mixture is further heated to 400°C and stirred continuously to form a material with a particle size of 10mm to 20mm, and the intermediate material is obtained after cooling;

[0091] The intermediate material is ground into particles of 6 μm to 10 μm by mechanical ball milling, and then screened by a screening machine, wherein the screen selected for screening is 1250 mesh to 2500 mesh;

[0092] The screened intermediate material is heated at a temperature of 2500° C. to 3000° C. to obtain graphitized particles with a stable layered structure;

[0093] Graphitized particles, asphalt, and urea are mixed in a preset mass ratio; urea is a nitrogen precursor, and the mass ratio of graphitized particles, asphalt, and urea is 100:7:1; the mixture is placed in a reactor and nitrogen is introduced, heated to 350°C under a certain pressure and stirred for 5 hours; further heated to 700°C and stirred for 12 hours, and after cooling, a negative electrode active material with a surface carbon layer doped with nitrogen is obtained;

[0094] A negative electrode active material, silicon-carbon material, carbon nanotubes, sodium carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) were selected and mixed in a mass ratio of 86:10:1.5:1:1.5, and stirred in deionized water to form a uniformly mixed and stable negative electrode slurry, wherein the solid content of the negative electrode slurry was 40%; then the negative electrode coating material was coated on an 8μm thick copper foil, dried and cold pressed to form a negative electrode sheet, wherein the compaction density was 1.6g / cm 3 .

[0095] Preparation of electrolyte: lithium salt lithium hexafluorophosphate (LiPF6), organic solvent ethylene carbonate (EC), dimethyl carbonate (DMC), first-class additive fluoroethylene carbonate (FEC), second-class additive vinyl sulfate (DTD) and third-class additive vinylene carbonate (VC) are mixed in a preset mass ratio of 10:22:53:3:7:5 to obtain an electrolyte.

[0096] Preparation of the diaphragm: A high-porosity diaphragm is selected, in which the thickness of the base film polyethylene (PE) is 9 μm, the thickness of the ceramic coating on both sides of the base film is 1 μm, and the thickness of the polyvinylidene fluoride (PVDF) coating is 1 μm.

[0097] Assembling the lithium-ion battery: The positive and negative electrode sheets are rolled and slit separately, then wound together with the separator to form a cylindrical battery core. This core is then welded to the electrical connector and assembled into the battery casing. After completing the injection, sealing, and formation processes, the lithium-ion battery of this example is obtained. The casing is cylindrical, with dimensions of 21.0 mm in diameter and 70.0 mm in length.

[0098] Example 2

[0099] Example 2 provides a lithium-ion battery. This example differs from Example 1 in that the mass ratio of graphite, asphalt, and urea in the graphitized particles is 100:7:1.5. All other aspects are the same as Example 1.

[0100] Example 3

[0101] Example 3 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of graphite, asphalt, and urea in the graphitized particles is 100:7:2. All other aspects are the same as Example 1.

[0102] Example 4

[0103] Example 4 provides a lithium-ion battery. This example differs from Example 1 in that the mass ratio of graphite, asphalt, and urea in the graphitized particles is 100:7:2.5. All other aspects are the same as Example 1.

[0104] Example 5

[0105] Example 5 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of graphite, asphalt, and urea in the graphitized particles is 100:7:3. All other aspects are the same as Example 1.

[0106] Comparative Example 1

[0107] Comparative Example 1 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that the surface carbon layer of the negative electrode active material is not doped with nitrogen. Other aspects are the same as Example 1.

[0108] Comparative Example 2

[0109] Comparative Example 2 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that the mass ratio of graphite, asphalt and urea in the graphitized particles is 100:7:4, and the other components are the same as Example 1.

[0110] The lithium ion batteries of Examples 1 to 5, and Comparative Examples 1 and 2 were subjected to corresponding tests.

[0111] This embodiment provides an active material particle D 50 The testing method comprises the following steps:

[0112] Discharge the lithium-ion battery at a constant current to 2.5V. Ensure the battery is in a safe state to reduce the risk of short circuits or thermal runaway during disassembly. Disassemble the battery and remove the negative electrode from the cylindrical cell in a glove box under a pure argon or other inert atmosphere. Use tweezers or a suitable tool to peel the electrode away, avoiding damage to the active material layer.

[0113] Cut the removed negative electrode sheet into appropriate sizes and soak it in an anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove residual electrolyte and possible byproducts. Remove the electrode and wipe the surface with a dust-free wipe. Then replace the anhydrous dimethyl carbonate (DMC) solution and repeat the soaking and wiping process three times to ensure that there are no residual contaminants on the electrode surface.

[0114] Rinse the electrode with anhydrous ethanol and wipe it again to further remove the solvent and impurities.

[0115] After cleaning, the negative electrode sheet is placed in a glove box for 48 hours to ensure complete drying to prevent interference from residual solvent in subsequent testing. The dried negative electrode sheet is scraped with a plastic scraper or blade to remove the negative electrode active material, ensuring that the collected powder is free of contamination. The scraped powder is transferred to a centrifuge tube containing anhydrous ethanol and ultrasonically dispersed in an ultrasonic cleaner for 30 minutes to further remove any residual electrolyte, binder, and impurities.

[0116] After ultrasonic treatment, the sample was centrifuged at 5000 rpm for 2 min, the supernatant was removed, and the powder was re-dispersed with anhydrous ethanol. Ultrasonic dispersion was performed again using an ultrasonic cleaner for 10 min, and then centrifuged again. This process was repeated three times to ensure the purity of the powder sample.

[0117] The precipitate was collected and transferred to a vacuum drying oven and dried at 80 °C for 12 h to ensure that the residual solvent was completely removed. The dried powder was placed in a sealed bag or sealed sample box and immediately taken out of the glove box. The sample was quickly tested for active substance particles D 50 test.

[0118] Take a small amount of sample powder and spread it evenly on the conductive tape;

[0119] Take clear particle images of at least five different areas under a scanning electron microscope (SEM) and import the particle images using ImageJ software or Nano Measure software;

[0120] After calibrating the scale, measure the projected diameters of more than 200 particles manually or automatically, arrange the data in ascending order and draw a cumulative distribution curve, and take the particle size value corresponding to 50% of the cumulative percentage as D 50 .

[0121] Among them, the interference data of obvious aggregates need to be excluded to ensure accuracy.

[0122] This example provides a method for testing sample surface elements. Specifically, an SU-8010 energy dispersive spectrometer was used to analyze the sample surface elements. All samples were gold-sputtered before testing. The sputtering current was 20 mA, and the sputtering process lasted 60 seconds. The test was performed in surface scanning mode with an accelerating voltage of 15 kV.

[0123] This embodiment also provides a method for testing the compaction density of a negative electrode sheet, comprising the following steps:

[0124] The negative electrode sheet rinsed with dimethyl carbonate and vacuum dried was cut into 6 standard-sized square samples with a length of 3 cm and a width of 3 cm.

[0125] The active material on the front and back of three square samples was wiped off, and the samples were rinsed with ethanol, then dried, weighed, and the average mass M1 was calculated. The average thickness L1 of the samples was measured using a micrometer.

[0126] Weigh the other three square samples and calculate the average mass M2. Use a micrometer to measure the average thickness L2 of the samples.

[0127] By formula Calculate the compacted density of the pole piece.

[0128] This embodiment also provides a method for testing the rate performance of a lithium-ion battery. The lithium-ion battery is discharged to 2.5V and placed in a constant temperature box at 25°C for 6 hours. The test is performed according to the following steps:

[0129] Under the condition of 1C charging rate, constant current and constant voltage charging to 4.2V, the cut-off current is 0.1C, and it is left to stand for 30 minutes. The capacity of constant current charging to 4.2V is Q1;

[0130] At a discharge rate of 1C, discharge at a constant current until the voltage reaches 2.5V, with a cutoff current of 0.1C, and then let it stand for 30 minutes.

[0131] Under the condition of 10 charging rate, constant current and constant voltage charging to 4.2V, cut-off current is 0.1C, and it is left to stand for 30min. The capacity of constant current charging to 4.2V is Q10 ;

[0132] Under the condition of 1C discharge rate, constant current discharge is carried out until 2.5V is cut off, the cut-off current is 0.1C, and it is left to stand for 30 minutes.

[0133] Here, through Q1 and Q 10 Calculate the capacity retention rate Q of lithium-ion batteries R , where Q R =Q1 / Q 10 ×100%.

[0134] This embodiment also provides a method for testing the cycling performance of a lithium-ion battery. The lithium-ion battery is placed in a constant temperature box at 25° C. for 6 hours and tested according to the following steps:

[0135] First cycle charging steps: At a charge rate of 0.1C, charge at constant current to 4.2V, then switch to constant voltage charging until the current drops to 0.01C; let it rest for 30 minutes after charging is completed.

[0136] First cycle discharge steps: discharge to 2.5V at a discharge rate of 0.1C.

[0137] Cyclic charge and discharge process: At a 2C charge rate, charge at a constant current to 4.2V; then let it rest for 30 minutes. At a 10C discharge rate, discharge at a constant current to 2.5V.

[0138] The charge and discharge process is repeated for a total of 500 cycles. The discharge capacity of the battery after 1 cycle and 500 cycles is Q1 and Q 500 Calculate the capacity retention rate Q of lithium-ion batteries R , Q R =Q 500 / Q1×100%.

[0139] After Examples 1 to 5, and Comparative Examples 1 and 2 were subjected to the above tests, the corresponding 10C charge capacity retention rate and the capacity retention rate after 500 cycles of 2C / 10C were obtained. The relevant data are shown in Table 1 below:

[0140]

[0141] Table 1

[0142] In Table 1, by comparing Examples 1 to 5, it can be seen that as W N / W C The value gradually increases to 0.044, W NWhen the value of gradually increases to 3.97, the lithium-ion battery under 10C high rate charging conditions, 10C charging capacity retention rate and 2C / 10C capacity retention rate after 500 cycles gradually improve. N / W C The value is greater than 0.044, W N When the value is greater than 3.97, the 10C charging capacity retention rate and the capacity retention rate after 500 cycles of 2C / 10C of lithium-ion batteries under 10C high-rate charging conditions show a downward trend. Therefore, doping the surface carbon layer of the negative electrode active material with nitrogen and optimizing the mass percentage of nitrogen and carbon can effectively improve the charging rate, rate performance and cycle stability.

[0143] Table 1 shows that, by comparing Example 1 and Comparative Example 1, the lithium-ion battery in which the surface carbon layer of the negative electrode active material is doped with nitrogen exhibits improved 10C charge capacity retention and capacity retention after 500 cycles of 2C / 10C compared to the lithium-ion battery without nitrogen doping. This is primarily due to the fact that nitrogen doping significantly improves the reaction kinetics on the electrode surface, accelerating the charge transfer efficiency between the electrode and the electrolyte, thereby increasing the battery's charge rate. Furthermore, the nitrogen-doped carbon layer inhibits electrolyte penetration and volume expansion, enhancing interfacial stability and thus improving cycling stability.

[0144] In Table 1, by comparing Examples 1 to 5 and Comparative Example 2, it can be seen that when W N / W C The value is greater than 0.1, W N When the value is greater than 7, the 10C charging capacity retention rate and the capacity retention rate after 500 cycles of the lithium-ion battery decrease significantly after 500 cycles of 2C / 10C.

[0145] Therefore, by comparing the above embodiments, it can be seen that when the surface carbon layer of the negative electrode active material is doped with nitrogen, the charging rate, rate performance and cycle stability can be effectively improved. However, when the mass percentages of nitrogen and carbon exceed a reasonable range, excessive doping will cause serious distortion of the lattice, destroy the stability of the surface coating structure of the material, and aggravate the occurrence of side reactions. These negative effects will cause serious damage to the electrochemical properties of the material, resulting in a decrease in the battery rate performance and a significant shortening of the cycle life. This application optimizes and adjusts the mass percentages of nitrogen and carbon to satisfy 1<W N <7、0.01<W N / W C <0.1, which can further improve the charging rate, rate performance and cycle stability.

[0146] The above description and the accompanying drawings sufficiently illustrate the embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Unless expressly required, individual components and functions are optional, and the order of operations may vary. Portions and features of some embodiments may be included in or replace portions and features of other embodiments. The embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A negative electrode sheet for a lithium-ion battery, characterized in that: include: negative electrode current collector; A negative electrode coating, comprising a negative electrode coating material coated on at least one side of the negative electrode current collector, wherein the negative electrode coating material comprises a negative electrode active material; the negative electrode active material comprises a graphite material and a silicon-based material, wherein the mass percentage of the graphite material in the negative electrode active material is 70wt% to 95wt%; and the mass percentage of the silicon-based material in the negative electrode active material is 3wt% to 25wt%; The surface of the negative electrode active material is provided with a carbon layer, the carbon layer is doped with nitrogen, and in its energy dispersive X-ray spectrum, the following conditions are met: 1<W N <7、0.01<W N / IN C <0.1; Among them, W N W is the mass percentage of nitrogen obtained after normalization of the energy dispersive X-ray spectrum of the carbon layer of the negative electrode active material; C is the mass percentage of the carbon element obtained after normalization of the energy dispersive X-ray spectrum of the carbon layer of the negative electrode active material.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material is prepared by heat treatment of graphitized particles, asphalt and nitrogen element precursor, and the mass ratio of the graphitized particles, asphalt and nitrogen element precursor is 100:7:1 to 100:7:

3.

3. The negative electrode sheet according to claim 1, characterized in that: The compaction density of the negative electrode sheet is 1.6 g / cm 3 In the case of hole impedance Z≤8Ωcm 2 .

4. The negative electrode sheet according to claim 1, characterized in that: The average particle size D of the graphite material 50 The average particle size D of silicon-based materials is 8μm to 18μm. 50 4μm~10μm.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The negative electrode coating also includes a negative electrode conductor and a binder, wherein the negative electrode conductor includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon black; the negative electrode coating also includes a negative electrode binder, wherein the negative electrode binder includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylonitrile, polystyrene-acrylic acid and styrene-butadiene rubber.

6. A method for preparing a negative electrode sheet according to any one of claims 1 to 5, characterized in that: The following steps are involved: Preparation of negative electrode coating material: needle coke and pitch are mixed in a preset mass ratio, and after air flow grinding, raw particles are obtained; the raw particles are heated and stirred to obtain graphitized particles; the graphitized particles, pitch and nitrogen precursor are mixed in a preset mass ratio and heated and stirred in a nitrogen environment to obtain a negative electrode active material; then the negative electrode active material, a negative electrode binder and a negative electrode conductive agent are mixed in a preset mass ratio to obtain a negative electrode coating material in which the surface carbon layer of the negative electrode active material is doped with nitrogen; Preparation of negative electrode sheet: coating the negative electrode coating material on at least one side of the negative electrode current collector, drying and cold pressing to obtain the negative electrode sheet.

7. The preparation method according to claim 6, characterized in that The original particles are heated and stirred to obtain graphitized particles, including: The original particles are heated and stirred under a nitrogen environment to obtain an intermediate material; The intermediate material meeting the conditions is heated at a temperature of 2500° C. to 3000° C. to obtain graphitized particles.

8. A lithium-ion battery, characterized in that: The invention comprises a positive electrode sheet and a negative electrode sheet according to any one of claims 1 to 5, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode coating coated with at least one layer containing a positive electrode active material; the positive electrode active material comprises lithium nickel cobalt manganese oxide (Li1Ni x Co y Mn z M b O2) and lithium iron phosphate; wherein, 0.70≤x≤0.95, 0.05≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1; the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd.

9. The lithium-ion battery according to claim 8, characterized in that Also includes: electrolyte, including lithium salts, solvents, and additives; The lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; The solvent comprises one or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate; The additive includes one or more of fluoroethylene carbonate, bisfluoroethylene carbonate, vinyl sulfate, vinyl sulfite, vinylene carbonate and vinyl carbonate.

10. The lithium-ion battery according to claim 8, characterized in that After 500 cycles at 2C / 10C at 25°C, the capacity retention rate Q R >85%.

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