Negative active material as well as preparation method and application thereof

Abstract

The invention provides a negative electrode active material and a preparation method and application thereof. The negative electrode active material comprises silicon particles; the graphite material layer coats at least part of the surfaces of the silicon particles, a metal compound is doped in the graphite material layer, and metal elements in the metal compound comprise one or a combination of at least two of copper, cerium, magnesium, chromium, manganese or nickel; in a plurality of sites of the graphite material layer, the variance of the content of the metal compound in the graphite material layer is less than 2.5 * 10 <-5 >. According to the negative electrode active material as well as the preparation method and the application thereof, expansion of silicon particles can be inhibited, the ion transmission performance of the material can be enhanced, and the cycle and rate performance of a battery can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a negative electrode active material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries have many advantages such as high voltage, high capacity, long cycle life, low self-discharge efficiency, and good safety performance, and are widely used in fields such as 3C / digital products, electric vehicles, and energy storage. The negative electrode sheet is an important part of a lithium-ion battery and has a decisive influence on the electrochemical performance of the lithium-ion battery. As an important part of a lithium-ion battery, the negative electrode active material has a particularly significant influence on the performance of the lithium-ion battery. Compared with graphite negative electrode materials, the theoretical specific capacity of silicon negative electrodes is as high as 4200 mAh / g, but the volume change of silicon during charge and discharge exceeds 120% and may even reach 300%, resulting in easy pulverization of silicon particles during charge and discharge, leading to rapid capacity decay.

[0003] By using silicon-oxygen negative electrodes (SiO ,

[0009] , 0 < x < 2) or silicon-carbon materials, the above problems can be improved, but during the cycling process, Li will react with SiO x to form nano-silicon, Li2O, and lithium silicate. This process will cause a relatively high irreversible loss of active lithium, resulting in a low first-cycle efficiency of the silicon-oxygen material. The first efficiency of silicon-carbon materials has a certain improvement compared with silicon-oxygen materials, but the improvement of the volume change of silicon by carbon is small. Therefore, the cycling performance of silicon-carbon negative electrodes is slightly worse than that of silicon-oxygen negative electrodes. Summary of the Invention

[0004] The present invention provides a negative electrode active material, a preparation method thereof, and an application thereof, which can inhibit the expansion of silicon particles, enhance the ion transport performance of the material, and improve the cycling performance and rate performance of the battery.

[0005] To solve the above technical problems, the present invention is realized through the following technical solutions.

[0006] The present invention provides a negative electrode active material, comprising:

[0007] Silicon particles;

[0008] A graphite material layer coated on at least part of the surface of the silicon particles. The graphite material layer is doped with a metal compound, and the metal elements in the metal compound include one or at least two combinations of copper, cerium, magnesium, chromium, manganese, or nickel; among multiple sites of the graphite material layer, the variance of the content of the metal compound in the graphite material layer is less than 2.5×10 -5 .

[0009] In one embodiment of the present invention, the metal compound in the graphite material layer includes cerium trioxide and cerium dioxide.

[0010] In one embodiment of the present invention, in the graphite material layer, the molar ratio of cerium trioxide to cerium dioxide is (1-1.3):1.

[0011] In one embodiment of the present invention, the median particle size of the silicon particles is 0.5 μm-4 μm.

[0012] In one embodiment of the present invention, the mass ratio of the silicon particles to the graphite material layer is 1:0.6-1:0.8; the content of the metal compound in the negative electrode active material is 0.2wt%-0.6wt%.

[0013] The present invention also provides a method for preparing a negative electrode active material, used for preparing the above-mentioned negative electrode active material, characterized in that it includes:

[0014] The carbon source, silicon particles, and first solvent are mixed evenly according to the mass ratio to obtain the first slurry;

[0015] The first slurry was freeze-dried, then washed and dried with anhydrous ethanol to obtain the first product.

[0016] The first product is subjected to a first calcination process under a stable atmosphere to obtain the second product;

[0017] The second product is added to a metal salt solution of a preset concentration and mixed evenly to obtain a second slurry;

[0018] The second slurry was freeze-dried, then washed and dried with anhydrous ethanol to obtain the third product;

[0019] The third product is subjected to a second calcination process under a stable atmosphere to obtain a negative electrode active material.

[0020] In one embodiment of the present invention, the carbon source is selected from one or more of citric acid, sodium alginate, sucrose, glucose, cellulose, lactose, polyvinyl alcohol, or phenolic resin.

[0021] And / or, the metal salt in the metal salt solution is selected from one or more of copper nitrate, cerium nitrate, magnesium nitrate, chromium nitrate, manganese sulfate, manganese oxalate, and nickel nitrate;

[0022] And / or, the preset concentration is 0.5wt%-1.5wt%.

[0023] In one embodiment of the present invention, the metal salt in the metal salt solution is selected from one or more of cerium nitrate or copper nitrate;

[0024] And / or, the mass ratio of the carbon source to the silicon particles is 0.5:1-3:1;

[0025] And / or, the mass ratio of the silicon particles to the first solvent is 1:20-1:60, and the first solvent includes deionized water;

[0026] And / or, the mass ratio of the second product to the metal salt solution is 1:3 to 1:10.

[0027] In one embodiment of the present invention, the freeze-drying temperature is -40°C and the drying pressure is 100 Pa;

[0028] And / or, the temperature of the first calcination process is 200℃-500℃, and the time is 1h-3h;

[0029] And / or, the temperature of the second calcination process is 500℃-900℃, and the time is 2h-5h.

[0030] The present invention also provides a lithium-ion battery, comprising:

[0031] A negative electrode sheet, comprising a negative electrode active material, wherein the negative electrode active material is selected from the negative electrode active material described above or a negative electrode active material obtained by the preparation method described above;

[0032] Positive electrode sheet;

[0033] A diaphragm is disposed between the positive electrode and the negative electrode.

[0034] In summary, this invention proposes a negative electrode active material, its preparation method, and its application. After silicon particles are coated with a graphite material layer, a metal compound is uniformly added to the graphite material layer. The metal compound is more stable and less prone to side reactions with the electrolyte. Furthermore, the metal oxide has high ionic conductivity, which does not affect lithium-ion transport, thereby enhancing the mechanical toughness and conductivity of the graphite material layer and effectively improving the first-cycle efficiency, thus improving the battery's first-charge efficiency and cycle stability. Simultaneously, metal compound doping can reduce the thickness of the solid electrolyte interface film, improving the first-cycle coulombic efficiency. Moreover, the uniform distribution of the metal compound in the graphite material layer improves the consistency of conductivity in the negative electrode active material, increases the strength of the graphite material layer, better suppresses silicon particle expansion, enhances the material's ion transport performance, and improves the battery's cycle and rate performance. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 The image shows the X-ray photoelectron spectrum of the negative electrode active material obtained in Example 1.

[0037] Figure 2 The image shows the energy spectrum of the negative electrode active material obtained in Example 1. Detailed Implementation

[0038] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0039] It should be understood that the invention can be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0040] The technical solution of the present invention will be further described in detail below with reference to several embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] This invention proposes a negative electrode active material, comprising silicon particles and a graphite material layer, wherein the graphite material layer coats at least a portion of the silicon particle surface, and the graphite material layer is doped with a metal compound, wherein the metal element in the metal compound includes one or at least two combinations of copper, cerium, magnesium, chromium, manganese, or nickel; and the variance of the metal compound content in the graphite material layer at multiple sites is less than 2.5 × 10⁻⁶. -5 After silicon particles are coated with a graphite material layer, doped metals are uniformly added to the graphite material layer, which improves the toughness and conductivity of the graphite material layer. At the same time, metal doping can also reduce the thickness of the solid electrolyte interface film and improve the first coulombic efficiency.

[0042] In one embodiment of the present invention, the median particle size D50 of the silicon particles is, for example, 0.5 μm-4 μm, or more specifically, 1 μm-2.5 μm, 2.5 μm-3 μm, or 3 μm-3.5 μm. The median particle size D50 refers to the particle size value corresponding to a cumulative volume distribution percentage of 50% in the volume distribution curve. By controlling the particle size of the silicon particles, the expansion space can be adapted, the interfacial bonding with the graphite material layer can be enhanced, the lithium-ion diffusion path can be shortened, and the rate performance can be improved.

[0043] In one embodiment of the present invention, the metal compound is, for example, a metal oxide, or, for example, one or at least a combination of two oxides of copper, cerium, magnesium, chromium, manganese, or nickel. By doping with metal oxides, compared to elemental metals, metal oxides are more stable, less prone to side reactions with the electrolyte, and have higher ionic conductivity, thus not affecting lithium-ion transport. This enhances the mechanical toughness and conductivity of the graphite material layer, effectively improves the first-cycle efficiency, and consequently improves the battery's first-charge efficiency and cycle stability.

[0044] In one embodiment of the present invention, the metal compound includes, for example, cerium trioxide (Ce₂O₃) and cerium dioxide (CeO₂), and the molar ratio of cerium trioxide to cerium dioxide in the graphite material layer is, for example, (1-1.3):1. The +3 valence Ce is beneficial for improving electronic and ionic conductivity, promoting electron and lithium-ion transport, while the +4 valence Ce is beneficial for suppressing silicon expansion, maintaining structural stability, and improving cycle life. Specifically, CeO₂ can enhance the toughness of the carbon layer, reduce particle cracking, decrease the active lithium consumed in SEI film formation, and improve cycle performance; Ce₂O₃ is beneficial for improving the ionic conductivity of the composite material and improving rate performance. Through the combined doping of Ce₂O₃ and CeO₂, the cycling stability and kinetics of the material are comprehensively improved.

[0045] In one embodiment of the present invention, the metal compound is uniformly distributed in the graphite material layer. Specifically, the content of the metal compound is tested at multiple sites in the graphite material layer to obtain a variance of less than 2.5 × 10⁻⁶ for the content of the metal compound. -5To calculate the variance of the metal compound content, the surface of the silicon particle is first evenly divided into multiple sites along its perimeter. Then, the graphite layer at each site is sampled and analyzed. The metal compound content per unit area of ​​the graphite layer at different sites is measured, and the average content of the metal compound at all sites is calculated. The variance of the metal compound content in the graphite layer is then obtained through calculation. The site shape is, for example, rectangular or circular, and the number of sites is, for example, 15-25. In this embodiment, the site shape is, for example, rectangular, and the number of sites is, for example, 20. The dimensions of the rectangular sites are, for example, 200nm*150nm, 200nm*100nm, 200nm*50nm, 100nm*100nm, 100nm*50nm, 300nm*150nm, 300nm*100nm, or 300nm*50nm, etc. The uniform distribution of metal compounds in the graphite material layer can improve the consistency of conductivity in the negative electrode active material, increase the strength of the graphite material layer, better suppress the expansion of silicon particles, enhance the ion transport performance of the material, and improve the cycle and rate performance of the battery.

[0046] In one embodiment of the present invention, the mass ratio of silicon particles to graphite material layer in the negative electrode active material is, for example, 1:0.6-1:0.8, and the content of metal compound in the negative electrode active material is, for example, 0.2wt%-0.6wt%. If the content of graphite material layer is too low, the amount of graphite coating on the surface of silicon particles will decrease, reducing the ability to suppress silicon expansion and having little effect on improving expansion suppression. If the content of graphite material layer is too high, it will affect the overall specific capacity of the negative electrode active material. If the doping amount of metal compound is too low, it will not have the effect of suppressing expansion and increasing conductivity. If the doping amount of metal compound is too high, it will destroy the original structure of the graphite material layer. Therefore, controlling the content of graphite material layer and metal compound ensures the capacity of the negative electrode active material while suppressing silicon expansion, and simultaneously increases the conductivity of the negative electrode active material.

[0047] This invention also proposes a method for preparing a negative electrode active material. The method includes: mixing a carbon source, silicon particles, and a first solvent in a mass ratio to obtain a first slurry; freezing the first slurry, washing it with anhydrous ethanol, and drying it to obtain a first product; subjecting the first product to a first calcination process under a stable atmosphere to obtain a second product; adding the second product to a metal salt solution of a preset concentration and mixing it evenly to obtain a second slurry; freezing the second slurry, washing it with anhydrous ethanol, and drying it to obtain a third product; and subjecting the third product to a second calcination process under a stable atmosphere to obtain the negative electrode active material.

[0048] In one embodiment of the present invention, the carbon source is selected from one or more of citric acid, sodium alginate, sucrose, glucose, cellulose, lactose, polyvinyl alcohol, or phenolic resin, etc. The first solvent includes, for example, an aqueous solvent such as deionized water, and the mass ratio of carbon source to silicon particles is 0.5:1-3:1, or for example, 1:1-2:1, and the mass ratio of silicon particles to the first solvent is 1:20-1:60. The carbon source, silicon particles, and first solvent are mixed, for example, by stirring, for a stirring time of, for example, 10-15 hours. The mass ratio of carbon source to silicon particles is controlled by the mass ratio of silicon particles to graphite material layers in the negative electrode active material to ensure capacity and expansion suppression performance. The amount of the first solvent added is controlled to ensure uniform mixing of the carbon source and silicon particles.

[0049] In one embodiment of the present invention, after uniform mixing, the product is dried by freeze-drying, wherein the freezing temperature is, for example, -40°C and the drying pressure is, for example, 100 Pa. After drying, the dried product is collected and placed in a Buchner funnel lined with filter paper. The funnel is connected to a vacuum filtration flask, which is connected to a low-pressure pump with a pressure of 40,000 Pa to 50,000 Pa. After washing with anhydrous ethanol, the product is dried, for example, in a vacuum drying oven at 40°C to 50°C to obtain the first product. Freeze-drying allows the carbon source to be uniformly attached to the silicon particles, and vacuum filtration with anhydrous ethanol accelerates the cleaning process.

[0050] In one embodiment of the present invention, after obtaining the first product, the first product is subjected to a first calcination process under a stable atmosphere to obtain a second product. The stable gas is, for example, nitrogen or argon. The temperature of the first calcination process is, for example, 200℃-500℃, or more specifically, 300℃-350℃, and the time is, for example, 1 hour-3 hours, preferably 2 hours. The equipment for the first calcination process is, for example, a tube furnace or an atmosphere-protected furnace. Through the first calcination process, by controlling the calcination temperature and time, the carbon source first partially decomposes on the surface of the silicon particles, forming carbon-coated silicon particles.

[0051] In one embodiment of the present invention, the metal salt in the metal salt solution is selected from one or more of, for example, copper nitrate, cerium nitrate, magnesium nitrate, chromium nitrate, manganese sulfate, manganese oxalate, or nickel nitrate, preferably one or more of cerium nitrate or copper nitrate. The solvent for the metal salt solution is, for example, deionized water, and the preset concentration of the metal salt is, for example, 0.5wt%-1.5wt%, or more specifically, 0.75wt%-1.1wt%. The mass ratio of the second product to the metal salt solution is 1:3-1:10, or more specifically, 1:5. By immersing the second product in the metal salt solution, the uniformity of metal doping can be improved. Simultaneously, by controlling the concentration of the metal salt solution and the mass ratio of the second product to the metal salt solution, the doping amount of the metal compound can be controlled.

[0052] In one embodiment of the present invention, the second slurry is dried by freeze-drying, wherein the freezing temperature is, for example, -40°C and the drying pressure is, for example, 100 Pa. After drying, the dried product is collected and placed in a Buchner funnel lined with filter paper. The funnel is connected to a vacuum filtration flask, which is connected to a low-pressure pump with a pressure of 40,000 Pa to 50,000 Pa. After washing with anhydrous ethanol, the product is dried, for example, in a vacuum drying oven at 40°C to 50°C to obtain the third product. Freeze-drying enables the metal salts to be evenly distributed, and vacuum filtration with anhydrous ethanol accelerates the cleaning process.

[0053] In one embodiment of the present invention, after obtaining the third product, the third product is subjected to a second calcination process under a stable atmosphere to obtain a negative electrode active material. The stable gas is, for example, nitrogen or argon. The temperature of the second calcination process is, for example, 500℃-900℃, or more specifically, 750℃-850℃, and the time is, for example, 2h-5h, preferably 3h. The equipment for the second calcination process is, for example, a tube furnace or an atmosphere-protected furnace. Through the second calcination process, by controlling the calcination temperature and time, the metal salt reacts with the incompletely decomposed carbon source on the surface of the silicon particles to form metal oxides, thereby enhancing the strength of the graphite material layer.

[0054] This invention also proposes a lithium-ion battery, comprising a casing and a bare cell disposed within the casing. The bare cell includes a positive electrode, a separator, and a negative electrode. The separator is placed between the positive and negative electrodes to prevent short circuits and allow lithium ions to pass through. The negative electrode includes a negative active material selected from the above-mentioned negative active materials or those obtained by the above preparation method. The positive electrode, separator, and negative electrode are sequentially stacked to ensure that a separator is provided between any positive and negative electrode. A multi-layer stack is obtained by winding or folding, and this stack is then placed into the battery casing as the bare cell. Finally, electrolyte is injected into the casing once or in multiple stages to completely immerse the bare cell in the electrolyte, which conducts ions between the positive and negative electrodes. In one embodiment of this invention, the lithium-ion battery is, for example, a secondary battery, which may be, for example, a pouch battery, a prismatic battery, or a cylindrical battery. This invention does not specifically limit the type of lithium-ion battery.

[0055] In one embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active layer coated on at least one surface of the positive current collector. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, or non-woven fabric. The thickness of the positive current collector is, for example, 8 μm-15 μm. In this embodiment, the positive current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm.

[0056] In one embodiment of the present invention, the positive electrode active layer includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent. The positive electrode active material includes, for example, any one or a combination of at least two of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide. The positive electrode binder is selected from, for example, any one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinylidene fluoride-tetrafluoroethylene-propylene terpolymer (ETFE), ethylene-vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer (TFE-HFP-VDF), or tetrafluoroethylene-hexafluoropropylene copolymer (FEP). The positive electrode conductive agent is selected from, for example, any one or more of conductive carbon black (SuperP), acetylene black, or Ketjen black. The mass ratio of positive electrode active material, positive electrode conductive agent and positive electrode binder in the positive electrode active layer is, for example, (90-98):(1-5):(1-5).

[0057] In one embodiment of the present invention, the positive electrode active material is, for example, LiNi. 0.9 Mn 0.05 Co 0.05 O2, a positive electrode conductive agent (e.g., acetylene black), and a positive electrode binder (e.g., polyvinylidene fluoride) are mixed in a mass ratio of 95:3:2. An organic solvent is then added, and the mixture is thoroughly stirred to obtain a positive electrode slurry. The organic solvent is, for example, N-methylpyrrolidone (NMP). The positive electrode slurry is uniformly coated onto aluminum foil, then air-dried at room temperature before being transferred to an oven for drying. The positive electrode sheet is then obtained through cold pressing, edge trimming, cutting, and slitting processes. In other embodiments, the positive electrode sheet can also be obtained using any other method of forming a positive electrode sheet.

[0058] In one embodiment of the present invention, the negative electrode sheet includes, for example, a negative current collector and a negative active layer coated at least on one surface of the negative current collector. The negative current collector is selected from, for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, or a stainless steel current collector, and the thickness of the negative current collector is, for example, 6 μm-15 μm. In this embodiment, the negative current collector is, for example, copper foil, and the thickness of the copper foil is, for example, 8 μm.

[0059] In one embodiment of the present invention, the negative electrode active layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode active material is selected from the aforementioned negative electrode active materials. The negative electrode binder is selected, for example, from at least one of PVDF, polymerized styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylic acid (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), or carboxymethyl chitosan (CMCS). The negative electrode conductive agent is selected, for example, from any one or more of conductive carbon black, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. The mass ratio of negative electrode active material, negative electrode conductive agent, and negative electrode binder in the negative electrode active layer is, for example, (80-98):(1-10):(1-10).

[0060] In one embodiment of the present invention, the negative electrode active material is selected from the above-mentioned negative electrode active material, the negative electrode conductive agent is selected from conductive carbon black, and the negative electrode binder is selected from PVDF. The negative electrode active material, negative electrode conductive agent, and negative electrode binder are mixed, for example, at a mass ratio of 8:1:1, NMP is added, and the mixture is thoroughly stirred to obtain a negative electrode slurry. The negative electrode slurry is coated onto a copper foil, then air-dried at room temperature and transferred to an oven for drying. After cold pressing, edge trimming, cutting, and slitting, a negative electrode sheet is obtained. In other embodiments, the negative electrode sheet can also be obtained by any other method of forming a negative electrode sheet.

[0061] In one embodiment of the present invention, the electrolyte includes, for example, an organic solvent, a lithium salt, and additives. The organic solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl acetate (EA), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC). The lithium salt is selected from, for example, one or more of lithium bis(fluorosulfonyl)imide (LiFSi), lithium difluorophosphate (LiPO2F2), lithium hexafluorophosphate (LiPF6), or lithium tetrafluoroborate (LiBF4). The additives are selected from one or more of vinylene carbonate (VC), lithium difluorooxalate borate (LiDFOB), or 1,3-propanesultone (1,3-PS). In this embodiment, in an argon atmosphere glove box with a water content of less than 10 ppm, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate are mixed, for example, at a mass ratio of 3:5:2. Thoroughly dried LiPF6, VC, and LiDFOB are dissolved in the mixed organic solvent, and after thorough mixing, an electrolyte is obtained, wherein the content of LiPF6 is 12.5 wt%, the content of VC is 3 wt%, and the content of LiDFOB is 0.5 wt%.

[0062] In one embodiment of the present invention, the separator is, for example, a polyethylene membrane, a polypropylene membrane, a glass fiber membrane, or a composite membrane, and the thickness of the separator is, for example, 9 μm to 15 μm. In another embodiment of the present invention, the separator is, for example, an 8 μm to 10 μm polyethylene film as the base film, and a 2 μm to 4 μm thick nano-alumina coating is coated on at least one side of the base film. The above-mentioned positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrode to act as a separator, and the stacked or wound layers are used to obtain a bare cell. The bare cell is installed in a casing, baked at 80°C-100°C to remove water, injected with the above-mentioned electrolyte, and sealed. Afterwards, it undergoes processes such as settling, hot and cold pressing, formation, clamping, and capacity testing to obtain a lithium-ion battery.

[0063] The present invention will be explained in more detail below by referring to embodiments, which should not be construed as limiting. Appropriate modifications can be made within the scope of the present invention, and all such modifications fall within the technical scope of the present invention.

[0064] Example 1

[0065] Preparation of negative electrode active material: Sodium alginate and silicon particles were mixed at a mass ratio of 1:1 and placed in deionized water. The median particle size of the silicon particles was 2 μm, and the mass ratio of silicon particles to deionized water was 1:40. After stirring for 12 hours to mix evenly, a first slurry was obtained. The first slurry was dried at -40℃ and a drying pressure of 100 Pa. The dried product was collected and placed in a Buchner funnel lined with filter paper. The funnel was connected to a vacuum filtration flask, which was connected to a low-pressure pump at a pressure of 45000 Pa. After washing with anhydrous ethanol, the product was dried in a vacuum drying oven at 45℃ to obtain the first product. The first product was placed in a tube furnace and calcined at 300℃ for 2 hours under a nitrogen atmosphere to obtain the second product. The second product was added to a 1% concentration cerium nitrate solution at a mass ratio of 1:5 and stirred for 12 hours to mix evenly, resulting in the second slurry. The second slurry was dried at -40℃ and 100Pa. The dried product was collected and placed in a Buchner funnel lined with filter paper. The funnel was connected to a vacuum filtration flask, which was connected to a low-pressure pump at 45000Pa. After washing with anhydrous ethanol, the product was dried in a vacuum drying oven at 45℃ to obtain the third product. The third product was placed in a tube furnace and calcined at 800℃ for 3 hours under a nitrogen atmosphere to obtain the negative electrode active material.

[0066] Preparation of negative electrode sheet: The above-mentioned negative electrode active material, conductive carbon black and PVDF are mixed in a mass ratio of 8:1:1, NMP is added, and the mixture is stirred and mixed evenly to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil, and then dried at room temperature and transferred to an oven for drying. After cold pressing, edge trimming, sheet cutting and slitting processes, the negative electrode sheet is obtained.

[0067] Preparation of positive electrode: LiNi 0.9 Mn 0.05 Co 0.05 O2, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:3:2, and NMP was added. The mixture was stirred thoroughly to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto aluminum foil, dried at room temperature, and then transferred to an oven for drying. After cold pressing, edge trimming, cutting, and slitting, the positive electrode sheet was obtained.

[0068] Membrane selection: 9μm polyethylene was selected as the base membrane, and a 3μm thick nano-alumina coating was applied to both sides of the base membrane.

[0069] Preparation of electrolyte: In an argon atmosphere glove box with a water content of less than 10 ppm, ethylene carbonate, diethyl carbonate and fluoroethylene carbonate are mixed, for example, in a mass ratio of 3:5:2. Thoroughly dried LiPF6, VC and LiDFOB are dissolved in the mixed organic solvent and mixed evenly to obtain the electrolyte, wherein the content of LiPF6 is 12.5 wt%, the content of VC is 3 wt%, and the content of LiDFOB is 0.5 wt%.

[0070] Preparation of lithium-ion batteries: The above-mentioned positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The stacked or wound electrodes yield a bare cell. The bare cell is then installed in a casing, baked at 80°C to remove moisture, injected with the above-mentioned electrolyte, and sealed. Following this, it undergoes standing, hot and cold pressing, formation, clamping, and capacity testing processes to obtain a lithium-ion battery.

[0071] Fabrication of the test coin cell: Assemble the negative electrode sheet with a diameter of 12mm, the separator with a diameter of 15mm, the lithium sheet with a diameter of 12mm, and the gasket with a thickness of 0.5mm in sequence, add the above electrolyte, and the gasket is a stainless steel sheet with a diameter of 12mm to obtain the CR2016 model lithium copper coin cell.

[0072] Example 2

[0073] In preparing the negative electrode active material, magnesium nitrate was used as the metal salt solution, and the other steps were the same as in Example 1.

[0074] Example 3

[0075] In preparing the negative electrode active material, manganese nitrate was used as the metal salt solution, and the other steps were the same as in Example 1.

[0076] Example 4

[0077] In preparing the negative electrode active material, chromium nitrate solution was used as the metal salt solution, and the other steps were the same as in Example 1.

[0078] Example 5

[0079] In the preparation of the negative electrode active material, the calcination temperature of the third product is 600°C, and the other steps are the same as in Example 1.

[0080] Example 6

[0081] In the preparation of the negative electrode active material, the calcination temperature of the third product is 700°C, and the other steps are the same as in Example 1.

[0082] Example 7

[0083] In the preparation of the negative electrode active material, the calcination temperature of the third product is 500°C, and the other steps are the same as in Example 1.

[0084] Example 8

[0085] In the preparation of the negative electrode active material, the calcination temperature of the third product is 900°C, and the other steps are the same as in Example 1.

[0086] Example 9

[0087] In preparing the negative electrode active material, silicon particles with a median particle size of 0.5 μm were used, and the other steps were the same as in Example 1.

[0088] Example 10

[0089] When preparing the negative electrode active material, silicon particles with a median particle size of 4 μm were used, and the other steps were the same as in Example 1.

[0090] Example 11

[0091] In preparing the negative electrode active material, silicon particles with a median particle size of 5 μm were used, and the other steps were the same as in Example 1.

[0092] Example 12

[0093] In preparing the negative electrode active material, sodium alginate and silicon particles were mixed at a mass ratio of 0.6:1, and the other steps were the same as in Example 1.

[0094] Example 13

[0095] In preparing the negative electrode active material, sodium alginate and silicon particles were mixed at a mass ratio of 0.8:1, and the other steps were the same as in Example 1.

[0096] Example 14

[0097] In preparing the negative electrode active material, sodium alginate and silicon particles were mixed at a mass ratio of 1.2:1, and the other steps were the same as in Example 1.

[0098] Example 15

[0099] In preparing the negative electrode active material, sodium alginate and silicon particles were mixed at a mass ratio of 1.5:1, and the other steps were the same as in Example 1.

[0100] Example 16

[0101] In the preparation of the negative electrode active material, the mass ratio of the third product to the 1% concentration cerium nitrate solution is 1:3, and the other steps are the same as in Example 1.

[0102] Example 17

[0103] In the preparation of the negative electrode active material, the mass ratio of the third product to the 1% concentration cerium nitrate solution was 1:4, and the other steps were the same as in Example 1.

[0104] Example 18

[0105] In the preparation of the negative electrode active material, the mass ratio of the third product to the 1% cerium nitrate solution was 1:7, and the other steps were the same as in Example 1.

[0106] Example 19

[0107] In the preparation of the negative electrode active material, the mass ratio of the third product to the 1% cerium nitrate solution was 1:9, and the other steps were the same as in Example 1.

[0108] Comparative Example 1

[0109] When preparing the negative electrode active material, magnesium nitrate was used as the metal salt solution, and the stirring time for the first slurry was 8 hours. Other steps were the same as in Example 1.

[0110] Comparative Example 2

[0111] In preparing the negative electrode active material, ferric nitrate was used as the metal salt solution, and the other steps were the same as in Example 1.

[0112] Comparative Example 3

[0113] In preparing the negative electrode active material, deionized water was used when making the second slurry, and the other steps were the same as in Example 1.

[0114] Comparative Example 4

[0115] In preparing the negative electrode active material, no carbon source was added when making the first slurry, and deionized water was used when making the second slurry. Other steps were the same as in Example 1.

[0116] In this invention, different negative electrode active materials were obtained in Examples 1-19 and Comparative Examples 1-4, and lithium-ion batteries were prepared and coin cells were tested. The performance of the coin cells was tested, and the test results are shown in Table 2.

[0117] Table 1. Partial characteristics of the negative electrode active materials in Examples 1 to 19 and Comparative Examples 1 to 4

[0118]

[0119] In one embodiment of the present invention, X-ray photoelectron spectroscopy (XPS) testing uses a [AlKα] ray source, and the binding energy is calibrated using the C1s characteristic peak (284.8 eV). The negative electrode active material powder from the examples and comparative examples is uniformly coated onto copper conductive adhesive. The powder sample is flattened using a tablet press at 15 MPa for 15 seconds, and excess powder is blown away with a syringe to obtain the test sample. The test sample is placed on a sample stage, the sample stage is placed in a sample chamber, and the test is performed after vacuuming.

[0120] In one embodiment of the present invention, the content of silicon and metal compounds in the negative electrode active material can be determined by inductively coupled plasma (ICP) testing, thereby obtaining the mass ratio of silicon particles to graphite material layers and the content of metal compounds.

[0121] In one embodiment of the present invention, when testing the initial coulombic efficiency, the test coin cells obtained in the examples and comparative examples are charged and discharged within a voltage range of 0.01V to 1.5V at a charge / discharge rate of 0.1C (1C = 1800mA / g), and the initial discharge capacity and charge capacity are recorded. Initial coulombic efficiency = first discharge capacity / first charge capacity.

[0122] In one embodiment of the present invention, when testing the cycle performance of the battery, the test coin cells prepared in the examples and comparative examples are operated at a voltage range of 0.01V to 1.5V, and at a charge / discharge rate of 0.1C / 0.1C (1C = 1800mA / g). The discharge capacity of the battery in the first cycle is recorded as the 0.1C discharge capacity. After multiple cycles, the discharge capacity and discharge capacity in the 200th cycle are measured. The cycle capacity retention rate (%) = (200-cycle discharge capacity / 1-cycle discharge capacity) × 100%.

[0123] Table 2 shows the performance of the coin cells tested in Examples 1 to 19 and Comparative Examples 1 to 4.

[0124]

[0125] Please refer to Table 1-2. Comparing Examples 1-4 and Comparative Examples 3-4, it can be seen that when a graphite material layer is coated on the surface of silicon particles and a metal compound is doped into the graphite material layer, the initial coulombic efficiency, initial discharge specific capacity, discharge specific capacity after 200 cycles, and capacity retention after 200 cycles of the battery can be improved. This indicates that after silicon particles are coated with a graphite material layer, the uniform addition of doped metals into the graphite material layer improves the toughness and conductivity of the graphite material layer, achieving the effects of suppressing expansion and increasing conductivity. At the same time, metal doping can also reduce the thickness of the SEI film and improve the initial coulombic efficiency.

[0126] Please refer to Table 1-2. Comparing Examples 1-4, it can be seen that the improvement effect on battery performance varies when different metal compounds are doped into the graphite material layer. When cerium is selected, +3 valence Ce is beneficial for improving electronic and ionic conductivity, promoting electron and lithium-ion transport, while +4 valence Ce is beneficial for suppressing silicon expansion, maintaining structural stability, and improving cycle life, resulting in the best performance improvement. Comparing Examples 2 and Comparative Example 1, it can be seen that when the doped metal compound is consistent, the smaller the variance of the metal compound content, the better the performance improvement. This indicates that the high uniformity of the metal compound distribution in the graphite material layer can improve the consistency of conductivity in the negative electrode active material, increase the strength of the graphite material layer, better suppress silicon particle expansion, enhance the ion transport performance of the material, and improve battery performance. Comparing Examples 1-4 and Comparative Example 2, it can be seen that when the doped metal is not within the range of copper, cerium, magnesium, chromium, manganese, or nickel, the improvement on battery performance is limited.

[0127] Please refer to Table 1-2. Comparing Examples 1 and 5-8, it can be seen that when the molar ratio of Ce₂O₃ to CeO₂ increases, the proportion of Ce₂O₃ increases, and the initial coulombic efficiency and initial discharge specific capacity of the battery increase. The capacity retention rate after cycling shows a trend of first increasing and then decreasing. This is because +3 valence Ce is beneficial for improving electronic and ionic conductivity, promoting the transport of electrons and lithium ions, while +4 valence Ce is beneficial for suppressing silicon expansion, maintaining structural stability, and improving cycle life. When the CeO₂ content decreases, the suppression of expansion weakens, and the cycle performance deteriorates. Therefore, controlling the molar ratio of Ce₂O₃ to CeO₂ is crucial to improving the overall performance of the battery.

[0128] Please refer to Table 1-2. Comparing Examples 1 and 9-11, it can be seen that as the median particle size D50 of silicon particles increases, the initial coulombic efficiency, initial discharge specific capacity, discharge specific capacity after 200 cycles, and capacity after 200 cycles all initially increase and then decrease. This indicates that silicon particles of suitable size can adapt to the expansion space, enhance the interfacial bonding with the graphite material layer, shorten the lithium-ion diffusion path, and improve rate performance.

[0129] Please refer to Tables 1-2. Comparing Examples 1 and 12-15, it can be seen that when the mass ratio of silicon particles to graphite material layers increases, the proportion of graphite material increases, the initial coulombic efficiency of the battery increases, the initial discharge specific capacity decreases, and the capacity retention rate after cycling generally shows a trend of first increasing and then decreasing. This is because when the content of graphite material layers is too low, it reduces the ability to suppress silicon expansion, and the improvement effect on suppressing expansion is not significant. When the content of graphite material layers is too high, it affects the overall specific capacity of the negative electrode active material. Therefore, controlling the mass ratio of silicon particles to graphite material layers can improve capacity while suppressing expansion, thereby improving the cycle and rate performance of the battery.

[0130] Please refer to Table 1-2. Comparing Examples 1 and 16-19, it can be seen that when the content of metal compounds in the negative electrode active material increases, the initial coulombic efficiency, initial discharge specific capacity, discharge specific capacity after 200 cycles, and capacity after 200 cycles of the battery initially increase and then decrease. This is because when the doping amount of metal compounds is too low, it cannot suppress expansion and increase conductivity; when the doping amount of metal compounds is too high, it will destroy the original structure of the graphite material layer and affect the effect of suppressing expansion. Therefore, controlling the content of metal compounds can suppress silicon expansion while simultaneously increasing the conductivity of the negative electrode active material.

[0131] Please see Figure 1 As shown in Example 1, when the doped metal element is Ce, XPS tests reveal that Ce exists primarily in the graphite layer as +3 and +4 valence oxides (Ce₂O₃ and CeO₂). Based on the ratio of their peak intensities, the molar ratio of Ce₂O₃ to CeO₂ is between 1 and 1.3. CeO₂ enhances the toughness of the carbon layer, reduces particle cracking, decreases the active lithium consumed in SEI formation, and improves cycle performance. Ce₂O₃ improves the ionic conductivity of the composite material, enhancing rate performance. The combined doping of Ce₂O₃ and CeO₂ comprehensively improves the cycle stability and kinetics of the negative electrode active material.

[0132] To obtain the distribution of metal compounds in the graphite material layer, the composition of the negative electrode active material was analyzed using scanning electron microscopy (SEM). Silicon was used as a characteristic element to identify silicon particles, and metal and carbon elements were used as characteristic elements to identify the graphite material layer. The surface of the silicon particles was evenly divided into 20 square sites with a perimeter of 100 nm * 100 nm. Each square site included part of the surface of the silicon particle and the graphite material layer on its surface. Then, energy dispersive X-ray spectroscopy (EDS) was used to test the graphite material layer in each site to measure the metal element content per unit area on the surface of the silicon particles in different sites.

[0133] Table 3. Elemental percentages at different sites in the negative electrode active material in Example 1

[0134]

[0135] Please refer to Table 3 and Figure 2 As shown, in the negative electrode active material, the silicon material layer is uniformly coated on the surface of the silicon particles, and the metal compound is uniformly distributed, thereby improving the strength of the coating layer, better suppressing the expansion of silicon particles, enhancing the ion transport performance of the material, and improving the cycle and rate performance of the battery.

[0136] This invention also provides an electronic device comprising at least one of the aforementioned lithium-ion batteries, which provides electrical energy. The electronic device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, or power tool, etc. In one embodiment of this invention, the vehicle is, for example, a new energy vehicle, which can be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. The electronic device includes the aforementioned lithium-ion battery, and therefore the advantages of including the aforementioned lithium-ion battery are not elaborated here.

[0137] In summary, this invention proposes a negative electrode active material, its preparation method, and its application. After silicon particles are coated with a graphite material layer, a metal compound is uniformly added to the graphite material layer. The metal oxide is more stable and less prone to side reactions with the electrolyte. Furthermore, the metal oxide has high ionic conductivity, which does not affect lithium-ion transport, thereby enhancing the mechanical toughness and conductivity of the graphite material layer and effectively improving the initial efficiency, thus improving the battery's initial charging efficiency and cycle stability. Simultaneously, metal compound doping can reduce the thickness of the solid electrolyte interface film, improving the initial coulombic efficiency. The uniform distribution of the metal compound in the graphite material layer improves the consistency of conductivity in the negative electrode active material, increases the strength of the graphite material layer, better suppresses silicon particle expansion, enhances the material's ion transport performance, and improves the battery's cycle and rate performance. By controlling the particle size of the silicon particles, the expansion space can be adapted, enhancing the interfacial bonding with the graphite material layer, shortening the lithium-ion diffusion path, and improving rate performance. By forming a graphite material layer with high coverage, the performance degradation problem caused by volume changes of silicon particles during charging and discharging is significantly improved. During the preparation process, stepwise calcination and two freeze-drying processes enable the carbon source to be uniformly attached to the silicon particles, thereby improving the uniformity of the graphite material layer and the metal compound.

[0138] The above description is merely a preferred embodiment of this application and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. For example, technical solutions formed by replacing the above-mentioned features with technical features with similar functions disclosed in this application (but not limited to) each other.

[0139] Apart from the technical features described in the specification, the other technical features are known to those skilled in the art. To highlight the innovative features of this invention, the other technical features will not be described in detail here.

Claims

1. A negative electrode active material, characterized by, The silicon particles comprise: In the graphite material layer, the metal compound comprises cerium sesquioxide and cerium dioxide. a layer of graphite material coated on at least part of the surface of the silicon particles, the layer of graphite material doped with a metal compound, the metal element in the metal compound comprising one or a combination of at least two of copper, cerium, magnesium, chromium, manganese or nickel; and a variance of the content of the metal compound in the layer of graphite material is less than 2.5 x 10 -5 .

2. The negative electrode active material according to claim 1, characterized by In the graphite material layer, the molar ratio of the cerium sesquioxide and the cerium dioxide is (1-1.3):

1.

3. The negative electrode active material according to claim 2, characterized by The median particle size of the silicon particles is 0.5-4 μm.

4. The negative electrode active material according to claim 1, characterized by The mass ratio of the silicon particles and the graphite material layer is 1:0.6-1:0.8; and the content of the metal compound in the negative electrode active material is 0.2-0.6 wt%.

5. The negative electrode active material according to claim 1, characterized by The silicon particles comprise:

6. A method for producing a negative electrode active material, for producing the negative electrode active material according to any one of claims 1 to 5, characterized by, The carbon source, the silicon particles and the first solvent are mixed uniformly according to a mass ratio to obtain a first slurry; The first slurry is subjected to freeze-drying, and then washed with anhydrous ethanol and dried to obtain a first product; The first product is subjected to a first calcination process in a stable atmosphere to obtain a second product; The second product is added into a metal salt solution of a preset concentration and mixed uniformly to obtain a second slurry; The second slurry is subjected to freeze-drying, and then washed with anhydrous ethanol and dried to obtain a third product; The third product is subjected to a second calcination process in a stable atmosphere to obtain a negative electrode active material. The carbon source is selected from one or more of citric acid, sodium alginate, sucrose, glucose, cellulose, lactose, polyvinyl alcohol or phenolic resin; 7. The method of producing a negative electrode active material according to claim 6, characterized by, And / or, the metal salt in the metal salt solution is selected from one or more of copper nitrate, cerium nitrate, magnesium nitrate, chromium nitrate, manganese sulfate, manganese oxalate and nickel nitrate; And / or, the preset concentration is 0.5-1.5 wt%. The metal salt in the metal salt solution is selected from one or more of cerium nitrate or copper nitrate; 8. The method of producing a negative electrode active material according to claim 6, characterized by, And / or, the mass ratio of the carbon source and the silicon particles is 0.5:1-3:1; And / or, the mass ratio of the silicon particles and the first solvent is 1:20-1:60, and the first solvent comprises deionized water; And / or, the mass ratio of the second product and the metal salt solution is 1:3-1:

10. The temperature of the freeze-drying is -40°C, and the pressure during the drying is 100 Pa; 9. The method of producing a negative electrode active material according to claim 6, characterized by, And / or, the temperature of the first calcination process is 200-500°C, and the time is 1-3 h; And / or, the temperature of the second calcination process is 500-900°C, and the time is 2-5 h. The silicon particles comprise:

10. A lithium-ion battery, characterized by, The negative electrode active material is selected from the negative electrode active material according to any one of claims 1-5 or obtained by the preparation method according to any one of claims 6-9; The positive electrode tab; The separator is arranged between the positive electrode tab and the negative electrode tab. ​

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