Negative electrode material and lithium ion battery

Abstract

The present application relates to a negative electrode and a lithium ion battery. The negative electrode material comprises secondary particles, the secondary particles comprise at least one primary particle and a first coating layer distributed on at least part of the surface of the primary particle, and the first coating layer is provided with pores; and the negative electrode material further comprises a second coating layer, at least part of the second coating layer is distributed on the surface of the first coating layer, and at least part of the second coating layer is embedded in the pores. At least part of the second coating layer in the negative electrode material of the present application is embedded in the pores of the first coating layer, so that the contact area of the first coating layer and the second coating layer is relatively large, which is conducive to improving the bonding stability of the secondary particles and the second coating layer, then improving the structural stability of the negative electrode material, relieving the volume expansion of the negative electrode material during charge and discharge, and improving the processing performance and the electrochemical stability of the negative electrode material.

Description

[0001] This application claims priority to Chinese patent application No. 2023118251268, filed December 28, 2023. The entire text of the aforementioned Chinese patent application is incorporated herein by reference. Technical Field This application relates to the field of anode material technology, and more particularly to an anode material and a lithium-ion battery. Background: Silicon-based anode materials offer advantages such as high specific capacity, low voltage plateau, environmental friendliness, and abundant resources, making them promising alternatives to graphite anodes for next-generation high-energy-density lithium-ion batteries. However, silicon undergoes significant volume changes during the deintercalation / intercalation process, which can easily lead to particle pulverization and subsequent detachment from the current collector. Furthermore, the repeated volume changes of silicon-based active materials during electrochemical cycling cause the SEI film formed on the surface of the silicon-based active material to continuously break down and regenerate, resulting in continuous consumption of lithium ions and ultimately rapid capacity decay. Silicon anode materials are typically improved through processes such as nano-scaling, carbon coating, and polymer coating, which can inhibit silicon volume expansion to a certain extent. Carbon coating is considered one of the best solutions for improving the electrochemical performance and processing performance of silicon anode materials. Carbon coating is typically achieved through vapor deposition of a gaseous carbon source or solid-phase mixing and high-temperature carbonization of a solid carbon source. The resulting carbon coating layer has a relatively loose structure and low density, resulting in deterioration of the anode material's initial efficiency, cycle performance, and processing performance. Furthermore, researchers applied an inorganic coating to the surface of the carbon coating. Existing inorganic coatings are primarily prepared through stirring, VC mixing, and ball milling. The inorganic coating and silicon anode material are simply physically bonded, and the inorganic coating thickness is only approximately 3 to 5 nm, resulting in poor mechanical stability. During the lithium stripping / insertion process, the significant volume effect of the silicon anode material can easily lead to contact failure between the inorganic coating and the silicon-based material, or even breakage. This leads to failure of the inorganic coating, resulting in degradation of the anode material's capacity, cycling performance, and expansion performance. Therefore, there is an urgent need to provide a silicon anode material with high density, high cycling performance, and excellent processability. Application Content: This application provides an anode material and lithium-ion battery that can improve the anode material's processability, capacity, expansion performance, and cycle performance. In a first aspect, embodiments of the present application provide a negative electrode material comprising secondary particles, each comprising a plurality of primary particles and a first coating layer distributed on at least a portion of the surface of the primary particles, the first coating layer having pores. The negative electrode material further comprises a second coating layer, at least a portion of the second coating layer being distributed on the surface of the first coating layer and at least a portion of the second coating layer being embedded in the pores. In a second aspect, embodiments of the present application provide a lithium-ion battery comprising the negative electrode material described in the first aspect.The technical solution of this application has at least the following beneficial effects: The negative electrode material of this application comprises two coating layers: a first coating layer and a second coating layer. The first coating layer has pores, and the second coating layer is distributed both on the surface of the first coating layer and embedded within the pores. This results in a larger contact area between the first coating layer and the second coating layer, which helps improve the bonding stability between the secondary particles and the second coating layer, thereby enhancing the structural stability of the negative electrode material, mitigating volume expansion of the negative electrode material during charge and discharge, and improving the processing performance and electrochemical stability of the negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS The present application is further described below with reference to the accompanying drawings and examples. Figure 1 is a schematic structural diagram of the negative electrode material of this application; Figure 2 is an XPS spectrum of the negative electrode material of this application; Figure 3 is an enlarged view of the Ols peak in the XPS spectrum of Figure 2; Figure 4 is a flow chart for preparing the negative electrode material of this application; Figure 5 is a schematic diagram of the discharge state of a secondary battery according to one embodiment of this application; Figure 6 is a SEM image of the secondary particles prepared in Example 1; and Figure 7 is a SEM image of the negative electrode material prepared in Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To better understand the technical solutions of this application, embodiments of this application are described in detail below with reference to the accompanying drawings. It should be understood that the described embodiments represent only a portion of the embodiments of this application, and are not exhaustive. Based on the embodiments of this application, all other embodiments devised by persons of ordinary skill in the art without inventive effort are within the scope of protection of this application. The terms used in the embodiments of this application are intended solely to describe specific embodiments and are not intended to limit this application. The singular forms "a," "an," "the," and "the" used in the embodiments of this application and the appended claims are intended to include the plural forms, unless the context clearly indicates otherwise. It should be understood that the term "and / or" as used herein is merely a description of an association between associated objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A alone, A and B simultaneously, or B alone. Furthermore, the character " / " generally indicates an "or" relationship between the associated objects. An embodiment of the present application provides a negative electrode material. As shown in FIG1 , which is a schematic structural diagram of the negative electrode material of the present application, the negative electrode material includes a secondary particle 1, wherein the secondary particle 1 includes at least one primary particle 11 and a first coating layer 12 distributed on at least a portion of the surface of the primary particle 11, and the first coating layer 12 has pores 121. The negative electrode material also includes a second coating layer 2, at least a portion of the second coating layer 2 is distributed on the surface of the first coating layer 12, and at least a portion of the second coating layer 2 is embedded in the pores 121.In some embodiments, the negative electrode material includes secondary particles 1, each of which includes multiple primary particles 11 and a first coating layer 12 distributed on at least a portion of the surface of the primary particle 11. In some embodiments, the negative electrode material includes secondary particles 1, each of which includes a single primary particle 11 and a first coating layer 12 distributed on at least a portion of the surface of the primary particle 11. It should be understood that, in this specification, the term "primary particle" refers to the primary structure of a single particle, and the term "secondary particle" refers to an aggregate of primary particles formed by physical or chemical bonding between primary particles, i.e., a secondary structure. In this application, the primary particle 11 is a single particle. The term "secondary particle" refers to a secondary structure formed by the aggregation of multiple primary particles 11. In the above solution, the negative electrode material of the present application has two coating layers: a first coating layer 12 and a second coating layer 2. The first coating layer 12 has pores 121, and the second coating layer 2 is distributed on the surface of the first coating layer 12 and embedded in the pores 121. This allows for a larger contact area between the first coating layer 12 and the second coating layer 2, which is beneficial for improving the bonding stability between the secondary particles 1 and the second coating layer 2, thereby improving the structural stability of the negative electrode material, alleviating the volume expansion of the negative electrode material during the charge and discharge process, and improving the processing performance and electrochemical stability of the negative electrode material. Furthermore, the first coating layer 12 and the second coating layer 2 form a mortise and tenon structure. A mortise and tenon structure refers to two components (the components in this application refer to the coating layers) connected by a concave-convex bonding method. That is, in this application, the first coating layer 12 and the second coating layer 2 are connected by a concave-convex bonding method, so that the secondary particles 1 and the second coating layer 2 are tightly bonded. When the negative electrode material is subjected to an external force, a combined force of static friction and dynamic friction exists between the secondary particles 1 and the second coating layer 2, thereby reducing the problem of the coating layer on the surface of the negative electrode material being broken, thereby reducing the problem of coating layer failure caused by the crushing. In some embodiments, at least a portion of the second coating layer 2 is in contact with the primary particles 11. It will be understood that the first coating layer 12 has pores 121, and the second coating layer 2 is embedded in the pores 121, so that a portion of the second coating layer 2 is embedded in the pores 121 and in contact with the primary particles 11, forming a "tie structure". This reduces the problem of contact failure or even breakage between the second coating layer and the primary particles, and between the second coating layer and the first coating layer during the lithium stripping / insertion process of the negative electrode material, and solves the problem of the second coating layer being easily failed on the surface of the negative electrode material, thereby improving the comprehensive electrochemical stability and processing performance of the negative electrode material.In some embodiments, the average pore diameter of pores 121 is between 10 nm and 50 nm, specifically 10 nm, 20 nm, 30 nm, 40 nm, or 50 nm, and may also be other values ​​within the aforementioned range, which is not limited herein. The larger pore diameter of the present invention allows for a tight bond between the first coating layer 12 and the second coating layer 2, reduces the specific surface area of ​​the negative electrode material, increases the tap density and compaction density of the negative electrode material, and facilitates improved processing performance of the negative electrode material. In some embodiments, the density of the second coating layer is significantly higher than that of the first coating layer, and the total pore volume of the negative electrode material is significantly smaller than that of the first coating layer. Thus, the second coating layer 2 has a higher density than the first coating layer 12. The high density of the second coating layer 2 effectively isolates the electrolyte from the secondary particles, inhibiting side reactions between the primary particles and the electrolyte and reducing electrolyte consumption. Furthermore, the presence of the M oxide (described below) reduces electrolyte corrosion of the negative electrode material, thereby improving the negative electrode material's initial coulombic efficiency, expansion performance, capacity retention, and cycle performance. Furthermore, the dense second coating layer effectively reduces the specific surface area of ​​the negative electrode material, increasing its tap density and compaction density, which facilitates improved processing performance. By providing a highly dense second coating layer on the surface of the secondary particles, the present application can reduce the specific surface area of ​​the negative electrode material. This reduces side reactions between the negative electrode material and the electrolyte during the initial charge and discharge process, reduces gas production, and improves the initial efficiency, expansion performance, and cycle performance of the negative electrode material. In some embodiments, the material of the first coating layer 12 comprises amorphous carbon. In some embodiments, the first coating layer 12 is an amorphous carbon layer. By selecting an amorphous carbon layer formed from amorphous carbon, due to its excellent fluidity, amorphous carbon can be coated on the surface of the primary particles 11, facilitating the formation of a coating layer. In some embodiments, the material of the second coating layer 2 includes M oxide, where M in the M oxide includes a metal element and / or a non-metal element. Furthermore, in some embodiments, the metal element includes at least one of lithium, magnesium, calcium, titanium, palladium, aluminum, molybdenum, and chromium, and the non-metal element includes boron or phosphorus. It should be noted that the above materials have stable physical and chemical properties and strong corrosion resistance. In some embodiments, the M oxide includes at least one of lithium oxide, magnesium oxide, calcium oxide, titanium oxide, palladium oxide, aluminum oxide, molybdenum oxide, chromium oxide, boron oxide, and phosphorus oxide.It can be understood that the second coating layer 2 is an M oxide layer. Compared to the amorphous carbon layer, the M oxide layer is denser. On the one hand, the dense M oxide layer can effectively isolate the electrolyte from the secondary particles 1, inhibit the occurrence of side reactions between the primary particles 11 and the electrolyte, reduce electrolyte consumption, and improve the initial coulombic efficiency, capacity retention, and cycle performance of the negative electrode material. On the other hand, the dense M oxide layer can effectively reduce the specific surface area of ​​the negative electrode material, increase the tap density and compaction density, and facilitate the processing performance of the negative electrode material. Moreover, the M oxide layer has superior chemical stability and corrosion resistance, effectively resisting corrosion from the electrolyte, thereby ensuring the structural stability and reliability of the battery prepared from the negative electrode material. In some embodiments, the mass percentage of the metal element in the negative electrode material is 0.005 wt% to 2 wt%, specifically 0.005 wt%, 0.01 wt%, 0.05 wt%, 0.1 wt%, 1 wt%, 1.5 wt%, or 2 wt%. Of course, other values ​​within the above range are also possible, and this application is not limited thereto. It is understood that the metal element is primarily present in the second coating layer 2. Selecting a mass percentage of the metal element within the above range helps improve the battery's capacity and initial efficiency. In some embodiments, the primary particles 11 include a silicon-based active material. In some embodiments, the silicon-based active material includes at least one of amorphous silicon, crystalline silicon, or a composite of crystalline silicon and amorphous silicon. In some embodiments, the silicon-based active material includes at least one of silicon oxide and a silicon alloy. The chemical formula of silicon oxide is SiO. x, where 0 < x ≤ 2, the silicon oxide is a silicon-oxygen composite containing oxygen atoms and silicon atoms, and the molar ratio of oxygen atoms to silicon atoms is 0 to 2 and does not include 0. It can be a substance composed of two or more of SiOo.2, SiO0.5, SiOo.8, SiO, SiOi.2, SiOi.5, SiOi.8, or SiO2, etc., or a compound with the chemical formula SiOx. Of course, it can also be other values within the above range, and this application does not limit it here. The silicon alloy can be a silicon-lithium alloy, a silicon-magnesium alloy, etc. Of course, it should be noted that in some cases, the silicon alloy includes elemental silicon particles and an alloy. In some embodiments, the average particle size Dx of the primary particles 11 satisfies: 10 nm ≤ Dx ≤ 150 nm. Specifically, Dx can be 10 nm, 50 nm, 80 nm, 100 nm, 120 nm, or 150 nm, etc. Of course, it can also be other values within the above range, and this application does not limit it here. Within the above-defined range, the primary particles 11 have an appropriate particle size, enabling the anode material to achieve a balance between processing performance and electrochemical performance. If the average particle size of the primary particles 11 is less than 10 nm, it is difficult for the primary particles 11 to be evenly dispersed, resulting in poor processing performance of the anode material. If the average particle size of the primary particles 11 is greater than 150 nm, the volume effect of the anode material is relatively large, and the anode material particles are easily broken, leading to deterioration of the electrochemical performance of the anode material. Preferably, 50 nm ≤ Dx ≤ 100 nm. In some embodiments, the average particle size Dy of the secondary particles 1 satisfies: 2 μm ≤ Dy ≤ 20 μm. Specifically, it can be 2 μm, 5 μm, 10 μm, 12 μm, 15 μm, 18 μm, or 20 μm, etc. Of course, it can also be other values within the above range, and this application does not limit it here. Within the above-defined range, it is beneficial for the preparation of the anode material into an anode electrode sheet during the slurry mixing and coating processes, improving the areal density and structural stability of the anode electrode sheet. If the average particle size of the secondary particles 1 is less than 2 μm, agglomeration will occur during the slurry mixing process of the anode material, resulting in uneven slurry and affecting the processing performance and electrochemical performance of the anode material. If the average particle size of the secondary particles 1 is greater than 20 μm, the surface of the anode material coated on the current collector and compacted to prepare the anode electrode sheet is rough, and the current collector is easily damaged during the compaction process of preparing the anode material into the anode electrode sheet, leading to deterioration of the electrochemical performance and even posing a safety risk. Preferably, the average particle size Dy of the secondary particles 1 satisfies: 5 μm ≤ Dy ≤ 10 μm.In some embodiments, the thickness of the first coating layer 12 is 100 nm to 200 nm, specifically 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 180 nm, 190 nm, or 200 nm, etc. Of course, other values ​​within the aforementioned range are also possible, and this application is not limiting here. It will be understood that since the first coating layer 12 encapsulates multiple primary particles 11, selecting a thickness within the aforementioned range allows for encapsulation of multiple primary particles 11 while maintaining strong structural stability. In some embodiments, the thickness of the second coating layer 2 is 10 nm to 100 nm, specifically 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, or 100 nm, etc. Of course, other values ​​within the aforementioned range are also possible, and this application is not limiting here. Within the above range, a balance can be achieved between the structural stability and capacity performance of the negative electrode material, improving the overall electrochemical performance of the negative electrode material. Preferably, the thickness of the second coating layer 2 is 30 nm to 50 nm. It will be understood that, since at least a portion of the second coating layer 2 is embedded within the pores 121 of the first coating layer 12, the thickness of the second coating layer 2 refers to the shortest distance between the first coating layer 12 and the second coating layer 2. In some embodiments, the mass fraction of the second coating layer 2 in the negative electrode material is 0.01 wt% to 3 wt%, specifically 0.01 wt%, 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, or 3 wt%. Of course, other values ​​within the above range are possible and are not limited herein. Within the above range, a second coating layer 2 of an appropriate thickness is formed, which can achieve a balance between the structural stability and capacity performance of the negative electrode material, improving the overall electrochemical performance of the negative electrode material. Preferably, the mass fraction of the second coating layer 2 in the negative electrode material is 0.1 wt% to 1 wt%. In some embodiments, the mass percentage of the first coating layer 12 in the negative electrode material is 5 wt% to 40 wt%, specifically 5 wt%, 10 wt%, 20 wt%, 25 wt%, 30 wt%, or 40 wt%. Other values ​​within this range are possible and are not limited herein. By selecting the mass percentage of the first coating layer 12 within the above range, multiple primary particles 11 can be coated while maintaining strong structural stability.In some embodiments, chemical bonds are formed between the secondary particles 1 and the second coating layer 2 of the present application. This chemical bond allows for a tighter connection between the second coating layer 2 and the secondary particles 1, improving the structural stability of the negative electrode material and reducing the risk of contact failure or even breakage between the second coating layer 2 and the secondary particles 1 during lithium extraction and insertion. It is understood that, as shown in Figures 2 and 3 , at least one of a Si—O—M bond and a C—O—M bond is formed between the secondary particles 1 and the second coating layer 2, establishing a connection between the two. The C—O—M bond refers to the chemical bond formed between the first coating layer 12 and the second coating layer 2, while the Si—O—M bond refers to the chemical bond formed between the primary particle 11 and the second coating layer 2. In some embodiments, the negative electrode material has a spherical and / or quasi-spherical structure. The closer the negative electrode material particles are to a spherical shape, the greater their isotropy. During the charge and discharge process, the more isotropic the negative electrode material can shrink and expand relatively uniformly along the radial direction, reducing stress concentration caused by the volume expansion of the active material in the negative electrode material. This helps maintain the stability of the negative electrode material's particle structure and reduces pulverization of the negative electrode material particles. In some embodiments, the median particle size D50 of the negative electrode material is between 5 μm and 20 μm, specifically 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, or 20 μm, and can also be other values ​​within this range, which is not limited herein. It should be noted that D50 represents the particle size corresponding to the cumulative particle size volume distribution percentage reaching 50%. Selecting a median particle size D50 within this range facilitates the subsequent slurry preparation process of the negative electrode material. In some embodiments, the negative electrode material has a specific surface area of ​​2 m2. 2 / g ~10 m 2 / g, specifically 2 m 2 / g> 3 m 2 / g> 4 m 2 / g> 5 m 2 / g> 6 m 2 / g> 7 m 2 / g> 8 m 2 / g> 9 m 2 / g or 10 m2 / g, etc., of course, other values ​​within the above range are also possible and are not limited herein. Selecting the above range is beneficial for improving the initial efficiency of the battery. It is understandable that an excessively large specific surface area can easily lead to SEI film formation, consuming excessive amounts of irreversible lithium salt, and reducing the initial coulombic efficiency of the battery. In some embodiments, the tap density of the negative electrode material powder is 0.8 g / cm3 to 1.5 g / cm3, specifically 0.8 g / cm3, 0.9 g / cm3, 10 g / cm3, 1.1 g / cm3, 1.2 g / cm3, etc. 3 > 1.3 g / cm 3 > 1.4 g / cm 3 or 1.5 g / cm 3 The tap density of the negative electrode material is preferably 0.9 g / cm³ to 1.5 g / cm³, and may be other values ​​within the above range, which is not limited by this application. Selecting a tap density within the above range can improve battery energy density and capacity. In some embodiments, the tap density of the negative electrode material is 0.9 g / cm³ to 1.5 g / cm³, specifically 0.9 g / cm³, 1.0 g / cm³, 1.1 g / cm³, 1.2 g / cm³, 1.3 g / cm³, or 1.5 g / cm³. 3 > 1.4 g / cm 3 or 1.5 g / cm 3The above range indicates that the negative electrode material of the present application has an appropriate compaction density, which is beneficial for improving the capacity and energy density of the negative electrode material. Selecting a compaction density within the above range helps improve battery energy density and capacity. A compaction density that is too low can easily lead to high porosity in the final electrode sheet, reducing battery energy density. A compaction density that is too high can result in low porosity, hindering electrolyte access and resulting in reduced capacity. In some embodiments, the mass percentage of oxygen in the negative electrode material is 5 wt% to 15 wt% based on the weight of the negative electrode material, specifically 5 wt%, 7 wt%, 10 wt%, 13 wt%, or 15 wt%. Other values ​​within the above range are also possible and are not limited by this application. Selecting an oxygen content within the above range helps improve initial efficiency and battery capacity. In some embodiments, based on the mass of the negative electrode material, the mass percentage of carbon in the negative electrode material is 5 wt% to 50 wt%, specifically 5 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, or 50 wt%, and other values ​​within this range are possible, and this application is not limiting. Selecting a carbon content within this range helps improve the conductivity of the negative electrode material. In some embodiments, the particle strength of the negative electrode material is 5 MPa to 20 MPa, specifically 5 MPa, 8 MPa, 10 MPa, 12 MPa, 14 MPa, 18 MPa, or 20 MPa, and other values ​​within this range are possible, and this application is not limiting. This application also provides a method for preparing the above-mentioned negative electrode material. As shown in FIG4 , this is a flow chart for preparing the negative electrode material of this application, comprising the following steps:

[0002] S100: mixing primary particles with a first coating material and performing a first heat treatment to obtain a precursor, wherein the precursor includes secondary particles, and the secondary particles include a plurality of primary particles and a first coating layer distributed on at least a portion of the surface of the primary particles, wherein the first coating layer has pores;

[0003] S200: In a liquid environment, mixing the precursor, the second coating material, and the precipitant, and then centrifuging to obtain a precipitate; and

[0004] S300: The precipitate is subjected to a second heat treatment to obtain a negative electrode material. In the above scheme, the present application pre-mixes primary particles with a first coating material and performs a first heat treatment to form a precursor (i.e., secondary particles). The precursor comprises primary particles and a first coating layer distributed on the surface of the primary particles. The first heat treatment carbonizes the first coating material, which undergoes structural destruction during carbonization, forming pores. The precursor, second coating material, and precipitant are mixed in a liquid environment and then centrifuged. The second coating material adheres to the pores and surface of the first coating layer through electrostatic adsorption in the liquid environment. Simultaneously, the precipitant and the second coating material react to produce a precipitate. The reaction system is then centrifuged to drain the liquid. Finally, the second heat treatment dehydrates the precipitate to form the corresponding M oxide, forming a second coating layer on the surface of the precursor. At least a portion of the second coating layer is distributed on the surface of the first coating layer, and at least a portion of the second coating layer is embedded in the pores of the first coating layer. Furthermore, the second coating layer formed by the M oxide has stable chemical and physical properties, strong corrosion resistance, and high density. It can effectively isolate the electrolyte from the secondary particles, inhibit the occurrence of side reactions between the primary particles and the electrolyte, reduce electrolyte consumption, and resist electrolyte corrosion, thereby improving the initial coulombic efficiency, capacity retention, and cycling performance of the negative electrode material. Furthermore, the second coating layer of the present application is distributed both on the surface of the first coating layer and embedded within the pores of the first coating layer, resulting in a large contact area between the first and second coating layers and a tight bond between the first and second coating layers. This improves the structural stability of the negative electrode material and mitigates the volume expansion of the negative electrode material during charge and discharge, which is beneficial for improving the processing performance and electrochemical stability of the negative electrode material. The preparation method of the present application is described in detail below with reference to examples: Step S100: Primary particles are mixed with a first coating material and subjected to a first heat treatment to obtain a precursor. The precursor includes secondary particles. The secondary particles include at least one primary particle and a first coating layer distributed over at least a portion of the surface of the primary particle, wherein the first coating layer has pores. In some embodiments, the secondary particles include multiple primary particles and a first coating layer distributed over at least a portion of the surface of the primary particle. In some embodiments, the secondary particles include one primary particle and a first coating layer distributed over at least a portion of the surface of the primary particle. In some embodiments, the primary particles are prepared by the following method: Plasma-treating a silicon-based raw material to obtain the primary particles. In some embodiments, the silicon-based raw material includes elemental silicon. In some embodiments, the plasma treatment includes at least one of a DC arc plasma treatment and a radio frequency plasma treatment. In some embodiments, the plasma treatment is performed in a protective gas atmosphere.In some embodiments, the protective gas includes at least one of nitrogen, ammonia, argon, oxygen, hydrogen, and argon. In some embodiments, the flow rate of the protective gas is 0.01 m / s. 3 / h~10m 3 / h, specifically 0.01m 3 / h,0.1m 3 / h,

[0005] 0.5m 3 / h> lm 3 / h> 3m 3 / h> 5m 3 / h> 8m 3 / h or 10m' / h, etc., of course, other values ​​within the above range can also be used, and this application is not limited here. In some embodiments, an auxiliary carrier gas is also added during the plasma treatment process. The addition of the auxiliary carrier gas prevents the ingress of oxygen during the plasma treatment process. In some embodiments, the auxiliary carrier gas includes at least one of nitrogen, ammonia, argon, oxygen, hydrogen, and argon. In some embodiments, the flow rate of the auxiliary carrier gas is 0.01 L / h to 10 L / h, specifically 0.01 L / h, 0.1 L / h,

[0006] 0.5 L / h, 1 L / h, 3 L / h, 5 L / h, 7 L / h or 10 L / h, etc., of course, it can also be other values ​​within the above range, and this application is not limited here. In some embodiments, the power of the plasma treatment is 15KW-100KW, specifically 15KW, 30KW,

[0007] 50 KW, 65 KW, 80 KW, or 100 KW, etc., and of course other values ​​within the above range are also possible, and this application is not limited here. In some embodiments, primary particles can also be prepared by vapor deposition. Those skilled in the art can prepare them using micron silicon according to specific processes, which are not described in detail here. It is understood that primary particles can be directly purchased as commercial products. In some embodiments, the first coating material includes a vapor-phase carbon source, which is coated by vapor deposition to obtain a precursor. The precursor includes secondary particles, which include multiple primary particles and a first coating layer (carbon layer). The carbon layer has a loose, porous structure that easily absorbs and stores liquids. In some embodiments, the vapor-phase carbon source includes at least one of liquefied petroleum gas, methane, toluene, acetylene, ethylene, ethane, and cyclohexane. In some embodiments, the flow rate of the gaseous carbon source is 0.01 L / min to 10 L / min. Specifically, the flow rate of the gaseous carbon source is 0.01 L / min, 0.1 L / min, 0.5 L / min, 1 L / min, 3 L / min, 5 L / min, 8 L / min, and 10 L / min, etc. Other values ​​within the above range are possible and are not limited herein. In some embodiments, the first coating material may also be a solid-phase carbon source, i.e., a solid-phase mixing method may be used for coating to obtain a precursor. In some embodiments, the solid-phase carbon source includes at least one of asphalt, glucose, sucrose, cellulose, glycine, alanine, and phenylalanine. In some embodiments, the mass ratio of the solid carbon source to the primary particles is 1:(0.1-1), specifically 1:0.1, 1:0.3, 1:0.5, 1:0.8, or 1:1, etc., and can also be other values ​​within the aforementioned range, which is not limited herein. In some embodiments, the temperature of the first heat treatment is 300°C to 1100°C, specifically 300°C, 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C, and 1100°C, etc., and can also be other values ​​within the aforementioned range, which is not limited herein. In some embodiments, the heating rate of the first heat treatment is 5°C / min to 30°C / min, specifically 5°C / min, 10°C / min, 15°C / min, 20°C / min, 25°C / min, or 30°C / min, etc. Of course, other values ​​within the above range may also be used, and this application is not limited thereto.Preferably, the heating rate of the first heat treatment is 5°C / min to 10°C / min. In some embodiments, the holding time of the first heat treatment is 4 to 36 hours. Specifically, the holding time of the first heat treatment can be, for example, 4 hours, 8 hours, 15 hours, 20 hours, 30 hours, and 36 hours, and can also be other values ​​within the above range, which is not limited herein. Step S200: In a liquid environment, a precursor, a second coating material, and a precipitant are mixed and centrifuged to obtain a precipitate. In some embodiments, mixing the precursor, the second coating material, and the precipitant in a liquid environment includes: Step D1: dispersing the precursor in a solvent to obtain a first dispersion; Step D2: performing a first mixing of the first dispersion and the second coating material to obtain a second dispersion; and Step D3: performing a second mixing of the second dispersion and the precipitant. In some embodiments, in Step D1, the precursor is dispersed in the solvent by ultrasonic treatment to improve the uniformity of the precursor dispersion. In some embodiments, before mixing the second coating material and the first dispersed material, the second coating material is further prepared into a saturated aqueous solution. This saturated aqueous solution can provide a sufficient amount of the second coating material, thereby enhancing the coating effect of the second coating material. In this application, a dispersion containing a precursor, a saturated aqueous solution of the second coating material, and a precipitant are mixed, so that the precursor (i.e., secondary particles) is dispersed in the saturated aqueous solution of an inorganic salt. The second coating material adheres to the surface and pores of the first coating layer through electrostatic adsorption. A soluble precipitant is then added to convert the second coating material into a corresponding precipitate, which adheres to the surface and pores of the first coating layer. In some embodiments, the second mixing of the second dispersed material and the precipitant is performed under stirring to ensure thorough mixing and reaction between the second coating material and the precipitant in the second dispersed material. In some embodiments, the temperature of the second mixing is between 50°C and 100°C, specifically 50°C, 60°C, 70°C, 80°C, and 90°C. (2 or 100°C, etc., and of course other values ​​within the above range may also be used, and this application does not limit this. In some embodiments, the stirring method includes magnetic stirring. In some embodiments, the median particle size of the precursor is 2 pm to 20 pm, specifically 2 pm, 5 pm, 10 pm, 12 pm, 15 pm, 18 pm or 20 pm, etc., and of course other values ​​within the above range may also be used, and this application does not limit this. It can be understood that controlling the median particle size of the precursor within the above range is beneficial to improving the cycle performance of the negative electrode material.Preferably, the median particle size of the precursor is 5 μm to 10 μm. In some embodiments, the solvent includes an alcohol solvent, and the alcohol solvent includes at least one of anhydrous ethanol, n-propanol, and isopropanol. In some embodiments, the second coating material includes an M compound, which is a metal compound and / or non-metal compound capable of reacting with a precipitant to form a precipitate. In some embodiments, the second coating material includes an inorganic salt compound, including but not limited to at least one of boron salts, magnesium salts, aluminum salts, calcium salts, titanium salts, molybdenum salts, chromium salts, iron salts, cobalt salts, arsenic salts, zirconium salts, and selenium salts. Specifically, it includes but is not limited to at least one of lithium nitrate, lithium chloride, magnesium nitrate, magnesium chloride, boric acid, calcium chloride, calcium nitrate, titanyl sulfate, palladium chloride, aluminum nitrate, aluminum chloride, and molybdenum nitrate. In some embodiments, the precipitant includes at least one of a salt and an organic substance. The salt includes a strong base and a weak acid salt. Specifically, the salt includes at least one of sodium carbonate, sodium acetate, sodium phosphate, sodium bicarbonate, potassium carbonate, potassium acetate, potassium phosphate, potassium bicarbonate, calcium carbonate, calcium acetate, calcium phosphate, calcium bicarbonate, molybdenum carbonate, molybdenum acetate, molybdenum phosphate, and molybdenum bicarbonate. The organic substance includes at least one of p-nitroaniline, azobisisobutylnitrile, pyrrolidone, triethylamine, 2-naphthylamine, benzidine, triamine, urea, o-aminoazotoluene, ethyleneimine, pyrrolidine, and nitrogen mustard. In some embodiments, the mass ratio of the precursor, the second coating material, and the precipitant is 1:(0.1-0.3):(0.1-0.3), specifically 1:0.1:0.1, 1:0.1:0.3, 1:0.2:0.1, or 1:0.2:0.2, etc. Of course, other values ​​within the above range are possible and are not limited herein. Within the above range, since the second coating material of the present application is an inorganic substance, the amount of precipitation on the surface and pores of the precursor can be controlled by adjusting the amount of the second coating material and precipitant added, thereby controlling the thickness of the coating layer on the precursor surface. This effectively isolates the electrolyte from the precursor, inhibits side reactions between the precursor and the electrolyte, and improves the initial coulombic efficiency of the negative electrode material. In some embodiments, after centrifugation, the supernatant is filtered and the resulting hydroxide precipitate is dried. Step S300: The precipitate is subjected to a second heat treatment to obtain the negative electrode material.In this step, the precipitate is subjected to a second heat treatment to dehydrate the precipitate to form the corresponding M oxide, which is the second coating layer. The second coating layer is distributed on the surface of the first coating layer and in the pores of the first coating layer, so that the second coating layer and the first coating layer form a "mortise-and-tenon structure". On the one hand, the presence of the "mortise-and-tenon structure" enables the negative electrode material to have a thicker coating layer with structural stability and mechanical strength. The material of the second coating layer is an inorganic oxide, which is beneficial to improving the structural stability and corrosion resistance of the negative electrode material. The presence of the "mortise-and-tenon structure" enables the negative electrode material to generate static friction and dynamic friction during the tensile failure process, thereby improving the tensile resistance of the negative electrode material and preventing the coating layer of the negative electrode material particles from breaking during the charge and discharge process, which may lead to failure. Furthermore, the presence of the "single-element structure" increases the contact area between the first and second coating layers, resulting in a larger number of "Si-O-M" and "C-O-M" bonds between the secondary particles and the second coating layer. This increases the energy barrier to surface damage or even failure of the negative electrode material, and helps enhance the stability of the bond between the second coating layer and the secondary particles. In some embodiments, the temperature of the second heat treatment is between 200°C and 800°C, specifically 200°C, 350°C, 400°C, 500°C, 600°C, 700°C, or 800°C. Other values ​​within this range are possible and are not limited herein. Within this range, the precipitate can be dehydrated to form the corresponding oxide, which then adheres to the surface of the secondary particles to form a coating layer. By adjusting the temperature of the second heat treatment, the degree of precipitate dehydration can be controlled, thereby adjusting the thickness and density of the corresponding oxide coating layer. In some embodiments, the heating rate of the second heat treatment is 2°C / min to 10°C / min, specifically 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, or 10°C / min, etc., and of course other values ​​within the aforementioned range are also possible, and this application is not limited thereto. In some embodiments, the holding time of the second heat treatment is 2 hours to 8 hours, specifically 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours, etc., and of course other values ​​within the aforementioned range are also possible, and this application is not limited thereto. In some embodiments, the second heat treatment is conducted in a protective gas atmosphere, and the protective gas includes at least one of nitrogen, ammonia, argon, oxygen, hydrogen, and argon. In some embodiments, the second heat treatment further comprises the steps of pulverizing, demagnetizing, and screening the material obtained from the second heat treatment.In a third aspect, one embodiment of the present application provides a secondary battery (such as a lithium-ion battery or a sodium-ion battery) comprising a housing, an electrode assembly, and an electrolyte. The electrode assembly and electrolyte are both located within the housing. The housing can be a packaging bag encapsulated with a packaging film (such as an aluminum-plastic film), e.g., a soft-pack secondary battery. In other embodiments, the secondary battery can also be a steel-cased battery, an aluminum-cased battery, etc. FIG5 shows a schematic diagram of a battery in a discharged state, i.e., during operation. As shown in FIG3 , the electrode assembly includes a positive electrode sheet 110, a negative electrode sheet 120, and a separator 130, with the separator being disposed between the positive and negative electrode sheets. The electrode assembly can be a laminated structure, formed by alternating positive electrode sheets, separators, and negative electrode sheets. In other embodiments, the electrode assembly can be a wound structure, formed by stacking positive electrode sheets, separators, and negative electrode sheets in sequence and then winding them. The positive electrode sheet 110 includes a positive electrode current collector 111 and a positive electrode active layer 112 disposed on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of aluminum foil or galvanized steel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive electrode active layer contains a positive electrode active material, which includes a compound that reversibly intercalates and deintercalates metal ions. In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide or a sodium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, arsenic, and arsenic. In some embodiments, the positive electrode active material may include, but is not limited to, at least one of lithium cobalt oxide (LiCoCl2), lithium arsenic cobalt ternary material (NCM), lithium manganese oxide (LiMn2O4), lithium manganese oxide (LiNio.5Mm.5O4), or lithium iron phosphate (LiFePO4). The negative electrode sheet 120 includes a negative electrode current collector 121 and a negative electrode active material layer 122 disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of at least one of copper foil, ferrite foil, stainless steel foil, titanium foil, or a carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode active material layer includes the negative electrode material. During battery operation, that is, when the battery is in a discharged state, metal ions 140 (e.g., lithium ions) in the negative electrode are released from the lattice of the negative electrode material, passed through the separator 130 via the electrolyte, and embedded in the lattice of the positive electrode material.Conversely, when the battery is charged by applying an external circuit, oxidation of the positive electrode material causes metal ions (e.g., lithium ions) in the positive electrode to escape from the positive electrode material's lattice. These ions then migrate through the electrolyte / electrolyte, across the separator, and to the negative electrode. Simultaneously, a reduction reaction occurs in the negative electrode material, causing the metal ions to embed into the negative electrode material's lattice. With the metal ions moving back and forth between the positive and negative electrodes, the battery can achieve thousands of discharge and charge cycles. The following detailed description of the embodiments of the present application is provided below in conjunction with specific examples. However, those skilled in the art will appreciate that the following examples are intended only to illustrate the present application and should not be construed as limiting its scope. Where specific conditions are not specified in the examples, the experiments were conducted under conventional conditions or those recommended by the manufacturer. Reagents or instruments used, for which the manufacturer is not indicated, are commercially available conventional products. Example 1.

[0008] (1) Silicon particles are placed in a plasma furnace chamber for treatment to obtain primary particles. The plasma furnace chamber is filled with oxygen, the working gas flow rate is 1 m' / h, the auxiliary carrier gas flow rate is 0.5 L / h, the median particle size of the silicon particles is 110 nm, and the plasma system power is 35KW.

[0009] (2) The primary particles were placed in a rotary atmosphere furnace and heated to 850°C at a heating rate of 5°C / min under nitrogen atmosphere. After holding the temperature for 15 minutes, high-purity acetylene gas was introduced at a gas flow rate of 1L / min so that the volume ratio of nitrogen to oxygen in the rotary atmosphere furnace chamber was 8:2. The temperature was kept for 5 hours and the precursor was obtained after natural cooling, i.e., the secondary particles. The SEM morphology of the precursor is shown in Figure 6.

[0010] (3) A saturated aqueous solution of magnesium nitrate was prepared. The solubility of magnesium nitrate in water was 420 g / L (room temperature). 420 g of magnesium nitrate was dissolved in 1 L of water to obtain a saturated aqueous solution of magnesium nitrate. Secondary particles with a median particle size of 15 μm were selected and first dispersed in anhydrous ethanol. The particles were then uniformly dispersed in the saturated aqueous solution of magnesium nitrate by ultrasonic dispersion to obtain a mixed solution A. The mixed solution A was placed in a water bath with magnetic stirring at a temperature of 70°C. (2) 100 g of trimeramine was added to the mixed solution. The trimeramine was hydrolyzed to generate precipitated ions OH. The precipitated ions OH reacted with Mg2+ to form magnesium hydroxide (Mg(OH)2) precipitate. The magnesium hydroxide (Mg(OH)2) precipitate adhered to the surface of the secondary particles under the action of intermolecular forces to form a mixed solution B. The mixed solution B was centrifuged and the resulting precipitate was placed in a vacuum drying oven and vacuum dried at 110° C. for 6 hours. The supernatant was recovered and recycled after being supplemented with an appropriate amount of precursor.

[0011] (4) The material obtained in step (3) was placed in a box furnace, oxygen was used as a protective gas, and the gas was exhausted for 1 hour. Then, the temperature was raised to 350°C at a heating rate of 5°C / min, and the first stage was calcined for 2 hours. After the box furnace temperature dropped to room temperature, a preliminary negative electrode material was obtained. 350°C was selected because this temperature is the dehydration temperature of magnesium hydroxide. Selecting the lowest possible first stage calcination temperature can further reduce the preparation cost. The preliminary negative electrode material was then crushed, pulverized, demagnetized, and sieved to obtain a finished negative electrode material with a median particle size of 15 μm. The negative electrode material prepared in Example 1 includes secondary particles, the secondary particles including a plurality of silicon primary particles and a carbon coating layer distributed on the surface of the silicon primary particles, the carbon coating layer having pores, and the negative electrode material further including a magnesium oxide coating layer, the magnesium oxide coating layer being distributed in the pores and on the surface of the carbon coating layer. After testing, the median particle size of the negative electrode material was 15 μm, and the specific surface area was 4 m 2 / g, the tap density of the powder is 0.95 g / cm 3 , compacted density is 1.05 g / cm 3 , oxygen content is 6 wt%, carbon content is 30 wt%, magnesium oxide coating thickness is 35nm, magnesium content in negative electrode material is 1.2wt% o The SEM image of the negative electrode material prepared in Example 1 is shown in FIG7 . As can be seen from FIG6 and FIG7 , the negative electrode material of the present application has a more complete morphology than the secondary particles, and is closer to a spherical shape. The surface of the negative electrode material is dense and smooth, with very few defects. Other parameters are shown in Table 10 Example 2

[0012] (1) Silicon particles were placed in a plasma furnace chamber for treatment to obtain primary particles. The plasma furnace chamber was filled with oxygen. The working gas flow rate was 1 m3 / h, the auxiliary carrier gas flow rate was 0.5 L / h, the median particle size of the silicon particles was 150 nm, and the plasma system power was 40 KWo.

[0013] (2) The primary particles were placed in a rotary atmosphere furnace and heated to 900°C at a heating rate of 4°C / min under the protection of an oxygen atmosphere. After keeping the temperature for 30 minutes, high-purity methane gas was introduced at a gas flow rate of 0.8 L / min so that the volume ratio of oxygen to methane in the rotary atmosphere furnace chamber was 9:1. The temperature was kept for 8 hours and the precursor was obtained after natural cooling, which was the secondary particles.

[0014] (3) A saturated aqueous solution of molybdenum nitrate is prepared. The mass ratio of molybdenum nitrate in the aqueous solution reaches 9%, which is called a saturated aqueous solution of molybdenum nitrate. Secondary particles with a median particle size of 18 μm are selected and first dispersed in anhydrous ethanol. Then, they are uniformly dispersed in a saturated aqueous solution of molybdenum nitrate by ultrasonic dispersion to obtain a mixed solution A. The mixed solution A is placed in a water bath with magnetic stirring at a water bath temperature of 75°C. 15 g of ethyleneimine is added to the mixed solution to obtain a mixed solution B. Ethyleneimine is hydrolyzed to produce precipitated ions OH. The precipitated ions OH react with Ba?+ to form a magnesium hydroxide (Ba(0H)2) ​​layer on the surface of the secondary particles. The second mixed solution after the reaction is centrifuged by centrifugation, and the obtained precipitate is placed in a vacuum drying oven and vacuum dried at a temperature of 110°C for 6 hours.

[0015] (4) The material obtained in step (3) was placed in a box furnace, oxygen was used as a protective gas, and the gas was exhausted for 1 hour. Then, the temperature was raised to 650°C at a rate of ICTC / min and kept at this temperature for 2 hours. After the temperature of the box furnace dropped to room temperature, a preliminary product of the negative electrode material was obtained. The preliminary product was crushed, pulverized, demagnetized and sieved to obtain a finished negative electrode material with a median particle size of 18 μm. The negative electrode material prepared in Example 2 includes secondary particles, the secondary particles include a plurality of silicon primary particles and a carbon coating layer distributed on the surface of the silicon primary particles, the carbon coating layer having pores, and the negative electrode material further includes a molybdenum oxide coating layer, the molybdenum oxide coating layer being distributed in the pores and on the surface of the carbon coating layer. After testing, the median particle size of the finished negative electrode material is 18 μm, and the specific surface area is 5 m 2 / g, the tap density of the powder is 1.0 g / cm 3, compacted density is 1.1 g / cm2, oxygen content is 8 wt%, carbon content is 30 wt%, thickness of molybdenum oxide coating layer is 40 nm, and molybdenum content in negative electrode material is 1.6 wt%. Other parameters are shown in Table 1. Example 3

[0016] (1) Silicon particles were placed in a plasma furnace chamber for treatment to obtain primary particles. The plasma furnace chamber was filled with oxygen. The working gas flow rate was 1 m3 / h, the auxiliary carrier gas flow rate was 0.5 L / h, the median particle size of the silicon particles was 130 nm, and the plasma system power was 40 KWo.

[0017] (2) The primary particles were placed in a rotary atmosphere furnace and heated to 900°C at a heating rate of 4°C / min under the protection of an oxygen atmosphere. After keeping the temperature for 30 minutes, high-purity liquefied petroleum gas was introduced at a gas flow rate of 0.5 L / min so that the volume ratio of oxygen to liquefied petroleum gas in the rotary atmosphere furnace chamber was 9:1. The temperature was kept for 8 hours and the secondary particles were obtained after natural cooling.

[0018] (3) A saturated aqueous solution of calcium nitrate was prepared, and secondary particles with a median particle size of 18 μm were selected. The particles were first dispersed in anhydrous ethanol, and then uniformly dispersed in the saturated aqueous solution of calcium nitrate by ultrasonic dispersion to obtain a mixed solution A. The mixed solution A was placed in a water bath with magnetic stirring at a water bath temperature of 80°C. 25 g of ethyleneimine was added to the mixed solution to obtain a mixed solution B. Ethyleneimine was hydrolyzed to produce precipitated ions OH. The precipitated ions OH reacted with Ca2+ to form a calcium hydroxide (Ca(OH)2) layer on the surface of the secondary particles. The second mixed solution B after the reaction was treated by spray drying.

[0019] (4) The material obtained in step (3) is placed in a box furnace, oxygen is used as a protective gas, the gas is exhausted for 1 hour, and then the temperature is raised to 750°C at a rate of ICTC / min. The temperature is kept at this temperature for 2 hours. After the box furnace temperature drops to room temperature, a primary product of the negative electrode material is obtained. The primary product is crushed, pulverized, demagnetized and sieved to obtain a negative electrode material with a median particle size of 20 μm. The negative electrode material prepared in Example 3 includes secondary particles, the secondary particles include a plurality of silicon primary particles and a carbon coating layer distributed on the surface of the silicon primary particles, the carbon coating layer having pores, and the negative electrode material further includes a calcium oxide coating layer, the calcium oxide coating layer being distributed in the pores and on the surface of the carbon coating layer. After testing, the median particle size of the finished negative electrode material is 18 μm, and the specific surface area is 5 m 2 / g, the tap density of the powder is 1.0 g / cm 3, compacted density is 1.1 g / cm2, oxygen content is 8 wt%, carbon content is 30 wt%, thickness of calcium oxide coating layer is 40 nm, calcium content in negative electrode material is 1.6 wt%o Other parameters are shown in Table 1. Example 4

[0020] (1) Silicon particles are placed in a plasma furnace chamber for treatment to obtain primary particles. The plasma furnace chamber is filled with oxygen. The working gas flow rate is 1 m3 / h, the auxiliary carrier gas flow rate is 0.5 L / h, the median particle size of the micron silicon particles is 130 nm, and the plasma system power is 40KWo.

[0021] (2) The primary particles were placed in a rotary atmosphere furnace and heated to 900°C at a heating rate of 4°C / min under oxygen atmosphere protection. After keeping the temperature for 30 minutes, high-purity liquefied petroleum gas was introduced at a gas flow rate of 0.5 L / min so that the volume ratio of oxygen to the liquefied petroleum gas in the rotary atmosphere furnace chamber was 9:1. The temperature was kept for 8 hours and the secondary particles were obtained after natural cooling.

[0022] (3) A saturated aqueous solution of aluminum nitrate was prepared, and secondary particles with a median particle size of 20 μm were selected. The particles were first dispersed in anhydrous ethanol, and then uniformly dispersed in the saturated aqueous solution of aluminum nitrate by ultrasonic dispersion to obtain a mixed solution Ao. The mixed solution A was placed in a water bath with magnetic stirring at a water bath temperature of 80°C. 18 g of ethyleneimine was added to the mixed solution to obtain a mixed solution Bo. Ethyleneimine was hydrolyzed to produce precipitated ions OH. The precipitated ions OH reacted with Al 3+ to form an aluminum hydroxide (Al(OH)3) layer on the surface of the secondary particles. The second mixed solution B after the reaction was treated by spray drying.

[0023] (4) The material obtained in step (3) was placed in a box furnace, oxygen was used as a protective gas, and the gas was exhausted for 1 hour. Then, the temperature was raised to 250°C at a rate of ICTC / min and kept at this temperature for 2 hours. After the box furnace temperature dropped to room temperature, the primary product of the composite silicon negative electrode material was obtained. The primary product was crushed, pulverized, demagnetized and sieved to obtain a negative electrode material with a median particle size of 20 μm. The negative electrode material prepared in Example 4 includes secondary particles, the secondary particles include a plurality of silicon primary particles and a carbon coating layer distributed on the surface of the silicon primary particles, the carbon coating layer having pores, and the negative electrode material further includes an aluminum oxide coating layer, the aluminum oxide coating layer being distributed in the pores and on the surface of the carbon coating layer. After testing, the median particle size of the finished negative electrode material is 20 μm, and the specific surface area is 5 m 2 / g, the tap density of the powder is 0.95 g / cm3 , compacted density is 1.1 g / cm 3 , the oxygen content is 9 wt%, the carbon content is 30 wt%, the thickness of the alumina coating layer is 50 nm, and the aluminum content in the negative electrode material is 2.0 wt%. The remaining parameters are shown in Table 1. Example 5 is different from Example 1 in that the solute in the saturated aqueous solution of magnesium nitrate in step (3) is 210 g, and the solvent water is 1.0 L. Example 6 is different from Example 1 in that the solute in the saturated aqueous solution of magnesium nitrate in step (3) is 105 g, and the solvent water is 1.0 L. Example 7 is different from Example 1 in that the solute in the saturated aqueous solution of magnesium nitrate in step (3) is 52.5 g, and the solvent water is 1.0 L. Example 8: Different from Example 1, the heat treatment temperature in the box furnace in step (4) is 200°C. Example 9: Different from Example 1, the heat treatment temperature in the box furnace in step (4) is 500°C. Example 10: Different from Example 1, only step (1) is performed. The negative electrode material prepared in Example 10 includes multiple silicon primary particles. The various test parameters of the negative electrode material are shown in Table 10. Example 11: Different from Example 1, only step (1) and step (2) are performed. The negative electrode material prepared in Example 11 includes secondary particles, and the secondary particles include multiple silicon primary particles and a carbon coating layer distributed on the surface of the silicon primary particles. The various test parameters of the negative electrode material are shown in Table 10. Example 12

[0024] (1) Silicon particles are placed in a plasma furnace chamber for treatment to obtain primary particles. The plasma furnace chamber is filled with oxygen. The working gas flow rate is 1 m' / h, the auxiliary carrier gas flow rate is 0.5 L / h, the median particle size of the silicon particles is 110 pm, and the plasma system power is 35 KWo.

[0025] (2) The primary particles were placed in a rotary atmosphere furnace and heated to 850°C at a heating rate of 5°C / min under nitrogen atmosphere. After keeping the temperature for 15 minutes, high-purity ethylene gas was introduced at a gas flow rate of 1L / min so that the volume ratio of nitrogen to oxygen in the rotary atmosphere furnace chamber was 8:2. The temperature was kept for 5 hours and the secondary particles were obtained after natural cooling.

[0026] (3) A dense carbon coating layer was prepared on the surface of the secondary particles using atomic layer deposition technology (ALD). Using a powder layer deposition system, the secondary particles were placed in a heating chamber, the deposition temperature was adjusted to 300°C, and high-purity ethylene gas was introduced at a gas flow rate of 0.5 L / min for deposition. The deposition time was 8 hours. After the deposition was completed, the equipment was naturally cooled to obtain the negative electrode material. The negative electrode material prepared in Example 12 includes secondary particles, and the secondary particles include multiple silicon primary particles, a first carbon coating layer and a second carbon coating layer. The first carbon coating layer is distributed on the surface of the silicon primary particles, and the second carbon coating layer is distributed on the surface of the first carbon coating layer. The test parameters of the negative electrode material are shown in Table 1. Performance test

[0027] (1) Use BET pore distribution to measure the pore volume of the negative electrode material.

[0028] (2) The carbon content of the negative electrode material was measured using the national silicon-carbon standard GB / T38823-2020. Specifically, a G4 ICARUS HF infrared carbon-sulfur analyzer from Bruker, Germany, was used. The sample was burned at high temperature in an oxygen-rich environment, and the carbon contained in it was oxidized to carbon dioxide. The generated gas entered the infrared detector along with the auxiliary carrier gas. The carbon content was calculated by quantitatively analyzing the changes in the carbon dioxide signal.

[0029] (3) The tap density of the negative electrode material is tested using the national silicon carbon standard GB / T38823-2020. Specifically, the tap density T is tested using the Quantachrome tap density analyzer Dual Autotap from Anton Paar (Shanghai) Trading Co., Ltd. The tap density T is the value after 3000 vibrations, and the unit is g / mL.

[0030] (4) The compaction density of the negative electrode material is obtained by testing using the national standard for silicon-carbon GB / T38823-2020. Specifically, the powder compaction density is tested using a CARVER powder compactor, where powder compaction density = mass of the test sample / volume of the test sample.

[0031] (5) The mass percentage of the metal elements in the negative electrode material is obtained by testing in accordance with the national standard for silicon carbon GB / T38823-2020. Specifically, in accordance with the standards and procedures specified in Appendix H of GB / T 24533, inductively coupled plasma emission spectroscopy is used to test the lithium, magnesium, calcium, titanium, palladium, aluminum and molybdenum elements in the sample. The reagent used is aqua regia (concentrated nitric acid: concentrated hydrochloric acid volume ratio of 1:3). The measurement results are rounded to two decimal places in accordance with GB / T 8170.

[0032] (6) The oxygen content of the negative electrode material was measured by Fourier transform infrared spectrometer using Nicolet 10 infrared spectrometer from Thermo Fisher Scientific, USA.

[0033] (7) Average particle size test of primary and secondary particles in negative electrode materials: The primary and secondary particles can be prepared into cross-section test samples using electron beam / ion beam cutting technology. The energy spectrum and backscattering mode of field emission scanning electron microscope and the energy spectrum mode of projection electron microscope can clearly identify the clear morphology of primary and secondary particles in the product particles. The particle size of primary and secondary particles can be identified by the ruler.

[0034] (8) Second coating thickness test: The negative electrode material can be prepared into cross-section test samples using electron beam / ion beam cutting technology. Due to the different compositions, the specific thickness of the second coating layer can be clearly identified by the energy spectrum and backscattering mode of the field emission scanning electron microscope. The specific thickness value can then be identified using a ruler. The average thickness of the second coating layer is determined by measuring at least 100 negative electrode materials, which is the thickness of the second coating layer.

[0035] (9) The median particle size of the negative electrode material was measured using a laser particle size analyzer. Specifically, the test was conducted in accordance with the test conditions specified in GB / T 24533 and with reference to the test methods and procedures of GB / T 19077. The volume particle size distribution of the sample was measured using a Malvern 3000 laser particle size analyzer, and the median particle size D50 was read. The measurement results were rounded to one decimal place in accordance with GB / T 8170.

[0036] (10) Test of the strength of negative electrode material particles: Under the action of electromagnetic force, the corresponding force on the pressure head gradually increases at a certain speed. When the particle breaks, the particle breaks. At this time, the resistance to the pressure head disappears, causing its displacement to drop rapidly. The instrument records the displacement of the pressure head in real time to determine its breaking point (displacement increases rapidly) and records the pressure on the particle at this time. According to the relationship between pressure, particle size, and particle strength (Cs=(2480xForce) / (n(Dia) A 2)), Cs refers to particle strength, Force refers to pressure, and Dia refers to particle size, thereby obtaining the particle strength of the negative electrode material.

[0037] (11) Test method for the average pore size of the first coating layer: Using the national standard GB / T 19857, use an acid to dissolve the second coating layer of the particles, and then measure the pore size by electron scanning electron microscopy. For one negative electrode material particle, measure the pore size of at least 10 pores and take the average value. At least 100 negative electrode material particles are measured and the average value is taken to obtain the average pore size of the pores in the first coating layer.

[0038] (12) Method for testing the thickness of the first coating layer: The negative electrode material can be prepared into a cross-section test sample using electron beam / ion beam cutting technology. Due to the different compositions, the specific thickness of the first coating layer can be clearly identified by the energy spectrum and backscattering mode of the field emission scanning electron microscope. The specific thickness value can then be identified by a ruler. The average thickness of the first coating layer is determined by measuring at least 100 negative electrode materials, which is the thickness of the first coating layer.

[0039] (14) Detection method of Si—O—M bond and C—O—M bond: Si—O—M bond and C—O—M bond were tested using the test method of GB / T 33502-2017. The test was carried out with reference to the test method and test steps of GB / T 33502-2017. Electrochemical performance test

[0040] (1) First coulombic efficiency test method of negative electrode material: The prepared negative electrode material, conductive agent and binder are mixed in a solvent at a mass ratio of 94:1:5 to form a slurry (solid content of 50%), coated on a copper foil current collector, and vacuum dried to obtain a negative electrode plate; then, the negative electrode plate, a ternary positive electrode plate prepared by a traditional mature process, a 1 mol / L LiPF6 / ethylene carbonate + dimethyl carbonate + ethyl methyl carbonate (v / v = 1:1:1) electrolyte, a Celgard2400 separator and a battery shell are assembled into an 18650 cylindrical single cell using a conventional production process. The charge and discharge test of the cylindrical battery is carried out on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd. under the following conditions: constant current charge and discharge at room temperature T 0.2 C, and the charge and discharge voltage is limited to 2.75~4.2V. The first cycle charge capacity and first cycle discharge capacity are obtained. First coulombic efficiency = first cycle discharge capacity / first cycle charge capacity.

[0041] (2) Test method for the first coulombic efficiency: The test is carried out using the national standard GB / T38823-2020 for silicon-carbon. Specifically, according to the standards and steps specified in Appendix D of GB / T 24533, the first coulombic efficiency of the sample is tested using a battery tester. The measurement results are rounded to one decimal place according to GB / T 8170.

[0042] (3) Capacity test method: The test was performed using the national silicon-carbon standard GB / T38823-2020. Specifically, the capacity of the sample was tested using a battery tester in accordance with the specifications and steps specified in Appendix D of GB / T 24533. The test results were rounded to one decimal place in accordance with GB / T 8170. The test results are shown in Table 1. Examples 1 to 12 are denoted as S1 to S12. Table 1. Performance test of the negative electrode materials of various examples As shown in Table 1, the negative electrode materials prepared in Examples 1 to 9 and Example 12 of the present application include secondary particles, the secondary particles including a plurality of silicon primary particles and a first coating layer, a carbon coating layer, distributed on the surface of the silicon primary particles. The carbon coating layer has pores. The negative electrode material also includes a second coating layer, such as aluminum oxide, molybdenum oxide, magnesium oxide, or a material coating layer, distributed on the surface of the secondary particles. The second coating layer is distributed within the pores and on the surface of the first coating layer. The second coating layer covers the surface of the secondary particles and forms a mortise-and-tenon structure with the first coating layer. The second coating layer is in close contact with the secondary particles, which can improve the tap density, compacted density, and structural stability of the negative electrode material, alleviate volume expansion of the negative electrode material during charge and discharge, and facilitate improved processing performance and electrochemical stability of the negative electrode material. The difference between Example 12 and Example 1 is that the second coating layer in Example 12 is a carbon material, while the second coating layer in Example 1 is made of magnesium oxide (magnesium oxide). In Example 12, the negative electrode material was prepared using atomic layer deposition. Although atomic layer deposition of a gaseous carbon source produced a two-layer negative electrode material, and the second coating layer provided a good coating effect, the mechanical properties, mechanical properties, and chemical corrosion resistance of the negative electrode material were far inferior to those of Example 1 of the present application, resulting in lower expansion performance. Furthermore, the primary drawbacks of surface modification and optimization using atomic layer deposition are high cost, low production capacity, difficulty in mass production, and impracticality for large-scale application. The negative electrode material in Example 10 consisted solely of silicon primary particles, which have a large specific surface area. Direct contact between the silicon primary particles and the electrolyte resulted in numerous side reactions between the negative electrode material and the electrolyte, reducing the initial coulombic efficiency and cycle performance of the negative electrode material. Furthermore, the large specific surface area of ​​the silicon primary particles also degraded the processing performance of the negative electrode material. The negative electrode material in Example 11 includes secondary particles, which comprise multiple silicon primary particles and a carbon layer coating the silicon primary particles. Due to the loose structure and low density of the carbon layer, the electrolyte has a high permeability into the negative electrode material, making it unable to suppress side reactions between the negative electrode secondary material and the electrolyte. Furthermore, the loose carbon layer has a high water absorption rate, making it prone to adsorption and storage of moisture, leading to denaturation of the negative electrode material and degradation of electrochemical performance. The foregoing description is merely a preferred embodiment of the present application and is not intended to limit the present application. Persons skilled in the art will readily appreciate that various modifications and variations of the present application are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application are intended to be included within the scope of protection of the present application.

Claims

Claims 1. A negative electrode material, wherein, The negative electrode material includes secondary particles, the secondary particles include at least one primary particle and a first coating layer distributed on at least a part of the surface of the primary particle, the first coating layer has pores; the negative electrode material further includes a second coating layer, at least a part of the second coating layer is distributed on the surface of the first coating layer, and at least a part of the second coating layer is embedded in the pores.

2. The negative electrode material according to claim 1, wherein The secondary particles include a plurality of the primary particles and a first coating layer distributed on at least a part of the surface of the primary particle.

3. The negative electrode material according to claim 1, wherein The negative electrode material includes at least one of the following features (1) to (3): (1) The material of the second coating layer includes M oxide; (2) The material of the second coating layer includes M oxide, and M in the M oxide includes a metal element and / or a non-metal element; (3) The material of the second coating layer includes M oxide, M in the M oxide includes a metal element, and the mass percentage content of the metal element in the negative electrode material is 0.005 wt% to 2 wt%.

4. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (3): (1) The material of the second coating layer includes M oxide, M in the M oxide includes a metal element, and the metal element includes at least one of lithium, magnesium, calcium, titanium, vanadium, aluminum, molybdenum, and chromium; (2) The material of the second coating layer includes M oxide, M in the M oxide includes a non-metal element, and the non-metal element includes boron or phosphorus; (3) The material of the second coating layer includes M oxide, and the M oxide includes at least one of lithium oxide, magnesium oxide, calcium oxide, titanium oxide, vanadium oxide, aluminum oxide, molybdenum oxide, chromium oxide, boron oxide, and phosphorus oxide.

5. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (3): (1) At least a part of the second coating layer passes through the pores and contacts the primary particle; (2) At least one of Si—O—M bond and C—O—M bond is formed between the secondary particle and the second coating layer; (3) The average pore diameter of the pores is 10 nm to 50 nm.

6. The negative electrode material according to claim 1, wherein, The material of the first coating layer includes amorphous carbon.

7. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (3): 22 At least one: (1) The primary particle includes a silicon-based active material; (2) The primary particle includes a silicon-based active material, and the silicon-based active material includes at least one of amorphous silicon, crystalline silicon, and a composite of crystalline silicon and amorphous silicon; (3) The primary particle includes a silicon-based active material, and the silicon-based active material includes at least one of silicon oxide and silicon alloy.

8. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (2): (1) The average particle size Dx of the primary particle satisfies: 10 nm ≤ Dx ≤ 150 nm; (2) The average particle size Dy of the secondary particles satisfies: 2 μm ≤ Dy ≤ 20 μm.

9. The negative electrode material according to claim 1, wherein The negative electrode material includes at least one of the following features (1) to (2): (1) The thickness of the first coating layer is 100 nm to 200 nm; (2) The thickness of the second coating layer is 10 nm to 100 nm.

10. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (2): (1) The mass percentage of the first coating layer in the negative electrode material is 5 wt% to 40 wt%; (2) The mass percentage of the second coating layer in the negative electrode material is 0.01 wt% to 3 wt%.

11. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (2): (1) Based on the mass of the negative electrode material, the mass percentage of oxygen element in the negative electrode material is 5 wt% to 15 wt%; (2) Based on the mass of the negative electrode material, the mass percentage of carbon element in the negative electrode material is 5 wt% to 50 wt%.

12. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (2): (1) The negative electrode material has a spherical structure and / or a quasi-spherical structure; (2) The median particle size of the negative electrode material is 5 μm to 20 μm.

13. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (2): (1) The specific surface area of the negative electrode material is 2 m2 / g to 10 m 2 / g; (2) The particle strength of the negative electrode material is 5 MPa to 20 MPa.

14. The negative electrode material according to claim 1, wherein, The negative electrode material includes at least one of the following features (1) to (2): (1) The tapped density of the powder of the negative electrode material is 0.8 g / cm 3 ~1.5 g / cm 3 ; (2) The tap density of the negative electrode material is 0.9 g / cm 3 ~1.5 g / cm 3 ³ 15. - A lithium-ion battery, wherein, The lithium-ion battery includes the negative electrode material according to any one of claims 1 to 14.

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