Negative electrode material, negative electrode sheet and secondary battery

Abstract

A negative electrode material, a negative electrode sheet and a secondary battery. The negative electrode material comprises an inner core and a coating layer arranged on at least part of the surface of the inner core, wherein the inner core comprises a matrix and an active material, and at least part of the active material is arranged inside the matrix. The atomic percentage content of oxygen on the surface of the negative electrode material is A1%; the atomic percentage content of oxygen in a surface layer of the negative electrode material is A2%; the atomic percentage content of oxygen in an inner layer of the negative electrode material is A3%; and A1 is greater than A2, and 2≤A3≤10. The negative electrode sheet or the secondary battery that uses the negative electrode material has a relatively high reversible specific capacity and initial Coulombic efficiency, and a good cycling stability.

Description

Negative electrode material, negative electrode sheet and secondary battery Cross-reference to related applications

[0001] The present application claims priority from the Chinese patent application No. 202510063793.X filed on January 14, 2025 and entitled "Negative electrode material, negative electrode sheet and secondary battery". TECHNICAL FIELD

[0002] The present application relates to the field of electrochemical energy storage, in particular to a negative electrode material, a negative electrode sheet and a secondary battery. BACKGROUND

[0003] With the rapid development and wide application of electric vehicles, the market has put forward higher requirements for the energy density and cycle life of lithium ion batteries. The energy density of lithium ion batteries mainly depends on the selection of electrode materials. The traditional negative electrode material mainly includes graphite, but the specific capacity of graphite negative electrode has approached the theoretical limit and it is difficult to further improve. Silicon has a theoretical capacity of about 4200 mAh / g, has the advantages of high capacity, wide source, environmental friendliness, moderate lithium intercalation potential, etc., and is generally considered to be one of the next generation of high specific capacity candidate lithium ion battery negative electrode materials. However, silicon expands significantly during lithium extraction and insertion, which easily leads to the breakage and pulverization of silicon particles, resulting in the destruction of the electrode structure or the repeated growth of the solid electrolyte interface (SEI) film, and if the silicon oxygen content is not controlled, it will lead to the problem of first coulomb reduction, resulting in rapid decay of the electrochemical performance of the silicon negative electrode.

[0004] The existing coating layer in the silicon-carbon composite material has poor protection for the silicon material, which leads to the oxidation reaction of the silicon-based active material inside the coating layer with air, resulting in the generation of irreversible capacity of the battery during the first charge and discharge process. At the same time, the oxygen in the carbon matrix also leads to the decrease of the first coulomb efficiency of the battery. Therefore, how to control the oxygen content of the silicon-carbon negative electrode material is the key to the preparation of the silicon-carbon negative electrode material. SUMMARY

[0005] The present application provides a negative electrode material to solve at least one of the above problems.

[0006] To achieve the above purpose, the present application provides a negative electrode material, which comprises a core and a coating layer arranged on at least part of the surface of the core. The core comprises a matrix and an active material, and at least part of the active material is arranged inside the matrix. The atomic percentage of oxygen elements on the surface of the negative electrode material is A1%, the atomic percentage of oxygen elements in the surface layer of the negative electrode material is A2%, and the atomic percentage of oxygen elements in the inner layer of the negative electrode material is A3%. A1 is greater than A2, and 2≤A3≤10.

[0007] The application also provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode material.

[0008] The application also provides a secondary battery, comprising the negative electrode sheet.

[0009] The application reflects the protection of the coating layer of the negative electrode material on the active substance by limiting the atomic percentage of oxygen elements (A1 and A2) on the surface and in the surface layer of the negative electrode material, thereby characterizing the degree of oxidation reaction of the active substance (such as a silicon-based material or silicon particles) in the negative electrode material in contact with the outside world. The application also reflects the oxidation degree of the matrix by limiting the atomic percentage of oxygen elements (A3) in the inner layer of the negative electrode material, thereby characterizing the oxygen content of the matrix in the negative electrode material.

[0010] In the application, A1 is greater than A2, indicating that the coating layer on the surface of the core of the negative electrode material has high uniformity and density, which provides good protection for the active substance and is conducive to reducing the oxidation of the active substance in the negative electrode material. At the same time, in the application, 2≤A3≤10, indicating that the oxygen content of the active substance or the matrix of the negative electrode material is low, and the low oxygen content of the matrix is conducive to improving the electrical conductivity of the matrix, reducing the side reaction of the negative electrode material with lithium during lithium intercalation, and the low oxygen content of the active substance is conducive to improving the specific capacity of the negative electrode material. The above two aspects are synergistic to improve the conductivity, specific capacity, first coulombic efficiency and cycle performance of the negative electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a schematic structural diagram of a secondary battery provided by an embodiment of the application during charging.

[0012] FIG. 2 is a schematic structural diagram of a secondary battery provided by an embodiment of the application during discharging.

[0013] FIG. 3 is a scanning electron microscope image of a negative electrode material provided by Example 1 of the application.

[0014] MAIN ELEMENT SYMBOL EXPLANATION

[0015] Electrode assembly 100

[0016] Positive electrode sheet 101

[0017] Negative electrode sheet 102

[0018] Separator film 103. DETAILED DESCRIPTION

[0019] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; where there is no conflict, the implementation methods and features of the implementation methods of this application can be combined with each other; many specific details are set forth in the following description to provide a full understanding of this application, and the described implementation methods are only a part of the implementation methods of this application, and not all of the implementation methods.

[0020] To address the issue of silicon particle expansion in silicon anode materials, common strategies include introducing inactive materials to buffer the volume expansion effect of silicon, selecting nanoscale silicon materials that maintain relative structural stability during lithiation, and providing silicon-carbon composite materials. Among these, silicon-carbon composite materials (silicon-carbon anode materials) show great promise due to their advantages such as good stability, small volume change, and excellent conductivity. However, oxygen is inevitably introduced or oxidation reactions occur during or after the preparation of silicon-carbon anode materials. Increased oxygen content consumes active materials during cycling, leading to a decrease in the initial coulombic efficiency of the battery and consequently, a decrease in the battery's cycle stability.

[0021] Silicon-carbon composite materials typically consist of a carbon matrix, silicon-based active materials, and a coating layer on their surface. This application found that the coating layers in some existing silicon-carbon composite materials provide poor protection for the silicon material, leading to easy oxidation of the silicon-based active materials within the coating layer upon contact with air. This results in irreversible capacity loss during the first charge-discharge cycle. Simultaneously, oxygen in the carbon matrix also contributes to a decrease in the initial coulombic efficiency of the battery. Therefore, controlling the oxygen content of the silicon-carbon anode material is crucial for its preparation. Based on this, this application improves the silicon-carbon composite material and its preparation method, which facilitates the formation of a high-quality coating layer on the surface of the silicon-based active materials, thereby reducing the oxidation of exposed silicon-based active materials in the silicon-carbon composite material. Furthermore, it facilitates the acquisition of a low-oxygen carbon matrix, further reducing side reactions between the anode material and the electrolyte during battery cycling and improving the initial coulombic efficiency of the battery, including the anode material.

[0022] Based on this, one embodiment of this application provides a secondary battery, including a casing, an electrode assembly, and an electrolyte. Both the electrode assembly and the electrolyte are located within the casing.

[0023] The outer casing can be a packaging bag encapsulated with a sealing film (such as aluminum-plastic film), for example, a pouch battery. In other embodiments, it can also be a steel-cased battery, an aluminum-cased battery, etc.

[0024] Referring to Figures 1 and 2, the electrode assembly 100 includes a positive electrode 101, a negative electrode 102, and a separator 103, with the separator 103 disposed between the positive electrode 101 and the negative electrode 102. When an electrolyte (not shown) is present, during charging (referring to Figure 1), active ions (such as lithium ions) are extracted from the lattice of the positive electrode material (such as a lithium-ion intercalated compound) of the positive electrode 101, pass through the separator 103 via the electrolyte, reach the negative electrode 102, and are inserted into the lattice of the negative electrode material. During discharging (referring to Figure 2), active ions (such as lithium ions) are extracted from the lattice of the negative electrode material of the negative electrode 102, pass through the separator 103 via the electrolyte, reach the positive electrode 101, and are inserted into the lattice of the positive electrode material (such as a lithium-ion intercalated compound). Electrons are generated and travel from the negative electrode 102 to the positive electrode 101 via an external circuit. The reverse movement of electrons forms an electric current, which can be used by electrical appliances.

[0025] In some embodiments, the electrode assembly 100 may be a stacked structure, which is formed by alternatingly stacking a positive electrode 101, a separator 103, and a negative electrode 102. In other embodiments, the electrode assembly 100 may also be a wound structure, which is formed by sequentially stacking and then winding the positive electrode 101, the separator 103, and the negative electrode 102.

[0026] Positive electrode film

[0027] The positive electrode 101 includes a positive current collector and a positive electrode material active layer disposed on at least one surface of the positive current collector. The positive current collector can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The positive electrode material active layer includes a positive electrode active material, which includes a compound that reversibly inserts and extracts lithium ions (i.e., a lithiation intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. This lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material may include, but is not limited to, lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary materials (NCM), lithium manganese oxide (LiMn2O4), and lithium nickel manganese oxide (LiNi). 0.5 Mn 1.5 At least one of lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4).

[0028] The positive electrode material active layer also includes an adhesive for bonding the positive electrode active material particles to facilitate the formation of the film layer, and also to improve the bonding force between the positive electrode material active layer and the positive electrode current collector. In some embodiments, the adhesive may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0029] The positive electrode material active layer may further include a conductive material, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0030] negative electrode sheet

[0031] The negative electrode 102 includes a negative electrode current collector and an active layer of negative electrode material disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil or 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 polymer substrate.

[0032] The active layer of the negative electrode material includes a negative electrode material, which includes a core and a coating layer disposed on at least a portion of the surface of the core. The core includes a matrix and an active substance, with at least a portion of the active substance disposed within the matrix. The atomic percentage of oxygen on the surface of the negative electrode material is A1%, the atomic percentage of oxygen at a depth of 20 nm from the surface of the negative electrode material is A21%, and the atomic percentage of oxygen at a depth of 160 nm from the surface of the negative electrode material is A31%, where A1 is greater than A21, and 2 ≤ A31 ≤ 10. For example, A31 can be 2, 3, 4, 5, 6, 7, 8, 9, 10, or any value within the range of any two of the above values.

[0033] In this application, A1 is greater than A21, indicating that the coating layer on the core surface of the negative electrode material has high uniformity and density, providing good protection for the active material and reducing its oxidation. Simultaneously, in this application, 2 ≤ A31 ≤ 10, indicating that the oxygen content of the active material or matrix of the negative electrode material is low. Lower oxygen content in the matrix is ​​beneficial for improving the matrix's conductivity and reducing side reactions between the negative electrode material and lithium during lithium intercalation. Lower oxygen content in the active material is beneficial for improving the specific capacity of the negative electrode material. These two aspects synergistically improve the conductivity, specific capacity, initial coulombic efficiency, and cycle performance of the battery.

[0034] In some embodiments, A21 / A ≤ 0.66. Specifically, A21 / A1 can be 0.66, 0.60, 0.56, 0.50, 0.39, 0.28, or any value within the range of any two of the above values. This indicates that the coating layer on the core surface of the negative electrode material has high uniformity and density, providing good protection for the active material and helping to reduce the oxidation of the active material in the negative electrode material.

[0035] In some embodiments, 1 ≤ A21 ≤ 8. For example, A21 can be 1, 2, 3, 4, 5, 6, 7, 8, or any value within the range of any two of the above values. When A1 or A21 is further controlled to meet the above preset range, it is beneficial to further improve the protective effect of the coating layer, thereby further improving the cycle performance of the battery including the negative electrode material.

[0036] In some embodiments, A21 is less than A31. When A21 and A31 are further controlled to satisfy the above-mentioned preset relationship, it indicates that the oxidation degree of the active material on the surface of the negative electrode material is lower than that of the matrix in the inner layer (the oxygen in the matrix mainly comes from the introduction during the preparation process), indicating that the coating layer of the negative electrode material provides a strong protective effect for the active material on the surface of the negative electrode material, which is beneficial to further improve the cycle performance of the negative electrode material.

[0037] The active layer of the negative electrode material includes the negative electrode material, which includes a core and a coating layer disposed on at least a portion of the surface of the core. The core includes a matrix and an active substance, with at least a portion of the active substance disposed within the matrix. The atomic percentage of oxygen on the surface of the negative electrode material is A1%, the atomic percentage of oxygen in the surface layer of the negative electrode material is A2%, and the atomic percentage of oxygen in the inner layer of the negative electrode material is A3%, where A1 is greater than A2, and 2 ≤ A3 ≤ 10. For example, A3 can be 2, 3, 4, 5, 6, 7, 8, 9, 10, or any value within the range of any two of the above values.

[0038] This application characterizes the degree of oxidation reaction of the active material (such as silicon-based material or silicon particles) in the negative electrode material upon contact with the external environment by defining the atomic percentage content of oxygen on the surface and within the outer layer of the negative electrode material, thereby reflecting the protective effect of the coating layer on the active material. This application also characterizes the oxygen content of the matrix in the negative electrode material by defining the atomic percentage content of oxygen in the inner layer of the negative electrode material, reflecting the degree of oxidation of the matrix. In this application, the outer layer of the negative electrode material is defined as a depth of 20 nm to 60 nm from the surface, and the average oxygen content at 20 nm, 40 nm, and 60 nm is used to characterize the oxygen content of the outer layer. The inner layer of the negative electrode material is defined as a depth of 160 nm to 200 nm from the surface, and the average oxygen content at 160 nm, 180 nm, and 200 nm is used to characterize the oxygen content of the inner layer.

[0039] In this application, A1 is greater than A2, indicating that the coating layer on the core surface of the negative electrode material has high uniformity and density, providing good protection for the active material and reducing its oxidation. Simultaneously, in this application, 2 ≤ A3 ≤ 10, indicating that the oxygen content of the active material or matrix of the negative electrode material is low. Lower oxygen content in the matrix is ​​beneficial for improving the matrix's conductivity and reducing side reactions between the negative electrode material and lithium during lithium intercalation. Lower oxygen content in the active material is beneficial for improving the specific capacity of the negative electrode material. These two aspects synergistically improve the conductivity of the negative electrode material, the initial coulombic efficiency of the battery, and its cycle performance.

[0040] Understandably, when A1 is less than or equal to A2, the surface of the negative electrode material is highly oxidized, and the uniformity and density of the coating layer may be insufficient, which is detrimental to the cycle performance of the negative electrode material. Conversely, when A3 is too large, the oxygen atom content in the inner layer of the negative electrode material, i.e., the core, is too high. This indicates that the matrix of the negative electrode material may have introduced too much oxygen during the preparation process, which is detrimental to the conductivity of the matrix. Furthermore, the oxidized matrix may undergo side reactions with lithium, hindering the reversible extraction of lithium during lithium intercalation, resulting in poor conductivity of the negative electrode material and poor reversible specific capacity of the battery.

[0041] Therefore, when A1 and A2 satisfy the above-mentioned preset relationship and A3 satisfies the above-mentioned preset range, the coating layer on the core surface of the negative electrode material has high uniformity and density, which plays a good protective role for the active material and helps to reduce the oxidation of the active material in the negative electrode material. At the same time, the oxygen content of the matrix of the negative electrode material is low, which helps to improve the conductivity of the matrix and reduce the side reaction between the negative electrode material and lithium during lithium intercalation, thereby helping to improve the conductivity of the negative electrode material, the first coulombic efficiency of the battery and the cycle performance.

[0042] In some embodiments, 3 ≤ A1 ≤ 15. For example, A1 can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or any value within the range of any two of the above values.

[0043] In some embodiments, 1 ≤ A2 ≤ 8. For example, A2 can be 1, 2, 3, 4, 5, 6, 7, 8, or any value within the range of any two of the above values. When A1 or A2 is further controlled to meet the above preset range, it is beneficial to further improve the protective effect of the coating layer, thereby further improving the cycle performance of the battery including the negative electrode material.

[0044] In some embodiments, A21 / A1 ≤ 0.65. Specifically, A21 / A1 can be 0.65, 0.60, 0.56, 0.50, 0.39, 0.28, or any value within the range of any two of the above values. This indicates that the coating layer on the core surface of the negative electrode material has high uniformity and density, providing good protection for the active material and helping to reduce the oxidation of the active material in the negative electrode material.

[0045] In some embodiments, A2 is less than A3. When A2 and A3 are further controlled to satisfy the above-mentioned preset relationship, it indicates that the oxidation degree of the active material on the surface of the negative electrode material is lower than that of the matrix in the inner layer (oxygen mainly comes from the introduction during the preparation process), indicating that the coating layer of the negative electrode material provides a strong protective effect for the active material on the surface of the negative electrode material, which is beneficial to further improve the cycle performance of the negative electrode material.

[0046] In some embodiments, the matrix has pores, and at least a portion of the active material is disposed within the pores of the matrix. Disposing the active material within the pores of the matrix helps reduce direct contact between the active material and the electrolyte, thereby reducing the consumption of the active material and improving the specific capacity and cycle performance of the battery fabricated from the negative electrode material.

[0047] In some embodiments, the matrix comprises a carbon matrix, which includes one or more of the following: artificial graphite, natural graphite, amorphous carbon, activated carbon, mesophase carbon microspheres, carbon nanotubes, carbon nanofibers, porous carbon, and graphene. The selection of the above-mentioned materials as the carbon matrix can provide pore distribution sites for the active material and form a conductive network.

[0048] In some embodiments, the active substance includes one or more of Si, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P, and Cu.

[0049] In some embodiments, the active material includes a silicon-based material, which includes one or more of elemental silicon, silicon oxide, and silicon alloy.

[0050] When the silicon-based material includes elemental silicon, the elemental silicon includes one or more of amorphous silicon, crystalline silicon, and composites of crystalline and amorphous silicon. Preferably, the elemental silicon includes amorphous silicon, which expands isotropically during lithium intercalation, reducing the collapse of pores in the matrix, suppressing the rapid decay of the specific capacity of the battery including the negative electrode material, and further improving the lithium intercalation cycle performance of the negative electrode material.

[0051] When silicon-based materials include silicon oxides, the silicon oxides include silicon oxide SiOx, where 0 < x ≤ 2. A silicon oxide is a silicon-oxygen complex containing oxygen atoms and silicon atoms, with a molar ratio of oxygen atoms to silicon atoms of 0 to 2, excluding 0. It can be SiO... 0.2 SiO 0.5 SiO 0.8 SiO, SiO 1.2 SiO 1.5 SiO 1.8 It may be a substance composed of two or more of the following: SiO2, or a compound with the chemical formula SiOx. Of course, it may also be other values ​​within the above range, which are not limited here.

[0052] When silicon-based materials include silicon alloys, the silicon alloys can be silicon-lithium alloys, silicon-magnesium alloys, etc. Of course, it should be noted that in some cases, silicon alloys include elemental silicon particles and alloys.

[0053] In some embodiments, the silicon-based material further includes silicon particles and a silicon oxide layer on the surface of the silicon particles. The silicon oxide layer includes silicon oxide with the general formula SiOx, where 0 < x ≤ 2. Specifically, SiOx may be SiO 0.5 SiO 0.7 SiO 0.9 SiO, SiO 1.2 SiO 1.5 SiO 1.8 SiO 1.9 The terms are not limited here. The silicon particles in this application refer to elemental silicon.

[0054] In some embodiments, the silicon-based material further includes silicon particles and a silicon oxide layer on the surface of the silicon particles. Based on the mass of the silicon-based material, the mass percentage of oxygen atoms in the silicon-based material is between 1% and 18%. Specifically, the mass percentage of oxygen atoms in the silicon-based material can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, or any value within the range of any two of the above values. Controlling the mass percentage of oxygen atoms in the silicon-based material within the above range is beneficial for forming a stable silicon oxide layer on the surface of the silicon particles, reducing direct contact between the silicon particles and the electrolyte, thereby reducing side reactions between the silicon particles and the electrolyte, and improving the cycle stability of the negative electrode material; it can also maintain the stable activity of the silicon-based material and increase the specific capacity of the negative electrode material.

[0055] In some embodiments, the average particle size of the silicon-based material is from 0.1 nm to 500 nm. For example, the average particle size of the silicon-based material can be 0.1 nm, 0.5 nm, 1 nm, 5 nm, 10 nm, 20 nm, 40 nm, 80 nm, 140 nm, 250 nm, 400 nm, 500 nm, or any value within the range of any two of the above values. By setting the average particle size of the silicon-based material within the above range, the mechanical stress during volume expansion of the silicon-based material can be reduced, which is beneficial for maintaining a good battery capacity and reducing irreversible capacity loss. It can also shorten the electron and ion transport paths. At the same time, the smaller size of the silicon-based material and the larger gap between adjacent silicon-based materials can reserve space for volume expansion of the silicon-based material.

[0056] In some embodiments, the morphology of the silicon-based material includes one or more of the following: dot-like, spherical, ellipsoidal, and sheet-like.

[0057] In some embodiments, the purity of the silicon-based material is greater than or equal to 99%, meaning that the mass percentage of silicon in the silicon-based material is greater than or equal to 99%. For example, the purity of the silicon-based material can be 99%, 99.1%, 99.3%, 99.5%, 99.6%, 99.7%, 99.8%, 99.9%, or any value within a range of any two of the above values. High-purity silicon-based materials are more conducive to Li-Si alloying with lithium, improving the cycle performance of lithium-ion batteries.

[0058] In some embodiments, the coating layer material includes one or more of carbon materials, metal oxides, amorphous silicon, conductive polymers, fluorides, phosphates, nitrides, and metal salts. Located on the outer layer of the negative electrode material, the coating layer possesses good conductivity, improving the conductivity of the negative electrode material. Furthermore, it can coat the active material exposed on the matrix surface, reducing the continuous oxidation of the exposed active material during storage and minimizing the decrease in specific capacity and initial coulombic efficiency (ICE) of the negative electrode material. The coating layer also reduces direct contact between the active material and the electrolyte, improving the stability of the SEI film and thus increasing the initial coulombic efficiency of the negative electrode material.

[0059] In some embodiments, the coating layer can be a single-layer coating layer formed from a single material, a coating layer formed from a combination of multiple materials, a multi-layer coating layer formed from a single material, or a multi-layer coating layer formed from multiple materials, etc., and the layer structure of the coating layer can be selected according to actual needs. It is understood that when the coating layer has a multi-layer coating structure, the density is higher.

[0060] In some embodiments, the thickness of the coating layer is from 1 nm to 100 nm. For example, the thickness of the coating layer can be 1 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any value within the range of any two of the above values. The coating layer can reduce the solubility of the negative electrode material, thereby reducing the amount of gas generated by the reaction of the dissolved active material with the electrolyte. Controlling the thickness of the coating layer within the above range is beneficial for maintaining the stability of the particle structure of the negative electrode material during cycling, reducing the amount of exposed active material on the surface of the negative electrode material, reducing the generation of a large amount of SEI during charge and discharge due to exposed active material, and improving the specific capacity and electrochemical performance of the negative electrode material. In some embodiments, the thickness of the coating layer is preferably from 1 nm to 50 nm, and more preferably from 1 nm to 30 nm. This is beneficial for the rapid and reversible insertion and extraction of lithium.

[0061] In some embodiments, the median particle size Dv50 of the negative electrode material particles is less than or equal to 10 μm. For example, the median particle size Dv50 of the negative electrode material particles can be 10 μm, 9 μm, 8 μm, 7 μm, 6 μm, 5 μm, 4 μm, 3 μm, 2 μm, 1 μm, or any value within the range of any two of the above values. A median particle size Dv50 within the above range ensures sufficient time for lithium-ion insertion and disengagement, allowing the negative electrode material to achieve rapid and complete lithium insertion, thereby guaranteeing the charge and discharge performance of the lithium-ion battery.

[0062] In some embodiments, the specific surface area of ​​the negative electrode material is less than or equal to 5 m². 2 / g. For example, the specific surface area of ​​the negative electrode material can be 5m².2 / g、4m 2 / g、3m 2 / g、2m 2 / g、1m 2 / g, 0.5m 2 / g or any value within the range of any two of the above values. When the specific surface area of ​​the negative electrode material is large, the SEI film will consume excessive lithium salt, and the volume effect will easily cause electrodesorption between particles, resulting in a decrease in the reversible capacity and coulombic efficiency of the battery. Therefore, the small specific surface area of ​​this embodiment can effectively improve the capacity and initial coulombic efficiency of the battery during the first discharge.

[0063] In some embodiments, the powder conductivity of the negative electrode material at 20 kN is between 1.2 S / cm and 1.6 S / cm. For example, the powder conductivity can be 1.2 S / cm, 1.25 S / cm, 1.3 S / cm, 1.35 S / cm, 1.4 S / cm, 1.45 S / cm, 1.5 S / cm, 1.55 S / cm, 1.6 S / cm, or any value within the range of any two of the above values.

[0064] In some embodiments, the negative electrode material comprises carbon, with a carbon content of 48% to 51% by mass. For example, the carbon content can be 48%, 48.5%, 49%, 49.5%, 50%, 50.5%, 51%, or any value within the range of any two of the above values. The carbon originates from the carbon matrix and the coating layer. The composite of carbon and silicon-based materials can provide a conductive platform and buffer space for the silicon-based materials, improving the structural instability and poor conductivity of silicon-based materials during cycling. When the carbon content is within this range, a sufficient carbon-based substrate can be established, providing ample distribution sites for the active material, which is beneficial for forming an effective conductive network and improving conductivity and cycle stability.

[0065] In some embodiments, the negative electrode material comprises silicon, with a silicon content of 45% to 47% by mass. For example, the silicon content by mass is 45%, 45.3%, 45.5%, 45.8%, 46%, 46.2%, 46.5%, 46.7%, 47%, or any value within the range of any two of the above values. When the silicon content in the negative electrode material falls within this range, the resulting battery can store a higher amount of electricity, i.e., a higher initial discharge specific capacity.

[0066] The active layer of the negative electrode material also includes a binder to bond the negative electrode active material particles, thereby facilitating the formation of the film layer and improving the bonding force between the active layer of the negative electrode material and the negative electrode current collector. In some embodiments, the binder may include, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0067] The active layer of the negative electrode material may further include a conductive material, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0068] Separating membrane

[0069] The separator 103 includes a membrane layer with a porous structure, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator 103 may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0070] electrolytes

[0071] The electrolyte serves to conduct ions between the positive electrode 101 and the negative electrode 102. The electrolyte can be in one or more states, including gel, solid, and liquid. In some embodiments, the electrolyte is a liquid electrolyte solution. The liquid electrolyte solution serves to conduct active ions between the positive electrode 101 and the negative electrode 102. In some embodiments, the liquid electrolyte solution includes a lithium salt and an organic solvent. The lithium salt may be selected from, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium tris(trifluoromethanesulfonyl)methyllithium (LiC(SO2CF3)3), lithium dioxolaneborate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it provides high ionic conductivity and improves cycling characteristics. The organic solvent may be a carbonate compound, a carboxylic acid ester compound, or an ether. Compounds, nitrile compounds, other organic solvents, or combinations thereof. Examples of carbonate compounds include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0072] Another embodiment of this application also provides a method for preparing a negative electrode material, including:

[0073] Step 1: Heat-treat the porous matrix using a first mixed gas, which includes hydrogen and an inert gas, to obtain the precursor.

[0074] The above steps utilize hydrogen to provide a reducing atmosphere. Under an inert gas atmosphere, the substrate is pretreated to remove oxygen-containing functional groups from the porous substrate. Controlling the volume percentage of hydrogen within the aforementioned range effectively regulates the oxygen content in the porous substrate, reducing its oxygen content and helping to increase active sites, thus facilitating the subsequent deposition of silicon-based materials.

[0075] In some embodiments, the volume percentage of hydrogen in the first mixture is 4% to 40%. For example, the volume percentage of hydrogen in the first mixture is 4%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 40%, or any value within the range of any two of the above values. Further controlling the hydrogen percentage within the above range is beneficial for further reducing the oxygen content of the porous matrix.

[0076] In some embodiments, the inert gas in the first mixture includes argon or nitrogen.

[0077] The second step involves mixing the precursor with a silicon source and a second mixed gas, which includes an inert gas and hydrogen, and then heat-treating the mixture to obtain the core.

[0078] During the deposition of the silicon source onto the precursor, hydrogen provides a reducing atmosphere. In an inert gas atmosphere, hydrogen within the aforementioned content range is beneficial for regulating the oxygen content in the resulting core.

[0079] In some embodiments, the volume ratio of inert gas to hydrogen in the second mixture is from 19:1 to 3:1. For example, the volume ratio of inert gas to hydrogen can be 19:1, 18:1, 17:1, 16:1, 15:1, 14:1, 13:1, 12:1, 11:1, 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, or any value within the range of any two of the above values. Further controlling the volume percentage of hydrogen within the above range is beneficial for further reducing the oxygen content of the porous matrix.

[0080] In some embodiments, the silicon source includes a gas-phase silicon source or a liquid-phase silicon source. Understandably, when the silicon source is a gas-phase silicon source, the silicon source is deposited on the porous substrate by vapor phase chemical deposition; when the silicon source is a liquid-phase silicon source, the silicon source is deposited on the porous substrate by liquid phase chemical deposition.

[0081] In some embodiments, when the silicon source is a gaseous silicon source, the volume percentage of the gaseous silicon source in the gaseous silicon source and the second mixed gas is approximately 15%. Controlling the concentration of the gaseous silicon source has the effect of regulating the silicon source pyrolysis rate, which is beneficial for the gaseous silicon source to pyrolyze at a suitable rate, thereby facilitating the good deposition of silicon-based materials after pyrolysis.

[0082] In some embodiments, the gaseous silicon source includes at least one of silane, dimethylsilane, trimethylsilane, and tetramethylsilane.

[0083] The third step involves mixing the core with a gaseous carbon source and a second mixed gas. In the mixture of the gaseous carbon source and the second mixed gas, the volume percentage of the gaseous carbon source is 10% to 60%. The mixture is then subjected to heat treatment to obtain the negative electrode material.

[0084] During the formation of the coating layer, hydrogen in the second gas mixture provides a reducing atmosphere. Under an inert gas atmosphere, the hydrogen content within the aforementioned range is beneficial for adjusting the oxygen content in the resulting anode material. Simultaneously, the second gas mixture also includes argon, which participates in providing the inert gas atmosphere. Argon also helps improve the penetration of the gaseous carbon source onto the core surface, thereby enhancing the density of the coating layer. A highly dense coating layer provides better protection for the anode material core and helps reduce contact oxidation reactions on the surface of the anode material core.

[0085] In some embodiments, the core is mixed with a first gaseous carbon source and a second mixed gas, wherein the volume percentage of the first gaseous carbon source in the first gaseous carbon source mixture and the second mixed gas mixture is 10% to 20%, and then heat-treated to obtain an intermediate; the intermediate is mixed with a second gaseous carbon source and the second mixed gas, wherein the volume percentage of the second gaseous carbon source in the second gaseous carbon source mixture and the second mixed gas mixture is 40% to 60%, and then heat-treated to obtain a negative electrode material.

[0086] First, a lower concentration of the first gas-phase carbon source is used for coating, which is beneficial for forming sub-nanometer-scale crystal nuclei that are uniformly adsorbed onto the core, improving the coverage of the core surface (such as covering defects and uneven areas). Then, a higher concentration of the second gas-phase carbon source is used for further coating. At this point, the second gas-phase carbon source tends to grow epitaxially along the crystal nucleus, forming a thinner and denser carbon coating layer. Thus, by setting up multi-layer coating of the core, the density and protective strength of the coating layer can be significantly improved.

[0087] In some embodiments, the heat treatment temperature in the first, second, and third steps described above is between 400°C and 1000°C, for example, it can be 400°C, 500°C, 520°C, 600°C, 700°C, 800°C, 900°C, 1000°C, or any value within the range of any two of the above values. Preferably, the temperature in the first step is between 600°C and 1000°C, the temperature in the second step is approximately 400°C to 700°C, and the temperature in the third step is approximately 500°C to 1000°C.

[0088] In some embodiments, the heat treatment time in the first, second, and third steps described above is from 0.5 h to 12 h, for example, it can be 0.5 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, or any value within the range of any two of the above values. Preferably, the first step takes about 2 h, the second step takes about 12 h, and the third step takes about 4 h.

[0089] The present application's solution will be explained below with reference to embodiments. Those skilled in the art will understand that the following examples are for illustrative purposes only and should not be construed as limiting the present application. Unless otherwise stated, reagents, software, and instruments involved in the following embodiments that are not specifically mentioned are all conventional commercially available products or open-source materials.

[0090] Example 1:

[0091] A negative electrode material, the preparation method of which includes:

[0092] S1. The porous carbon matrix is ​​placed in the reaction chamber and heated to 800°C under an argon atmosphere. Then, the porous carbon is pretreated for 2 hours with a first mixed gas consisting of 95% argon and 5% hydrogen by volume to obtain the precursor.

[0093] S2, silane and a second mixed gas are introduced into the reaction chamber and mixed with the precursor. The second mixed gas consists of nitrogen:argon:hydrogen in a volume ratio of 14:5:1. The volume ratio of silane in the silane and carrier gas is controlled to be 15%. The temperature is raised to 520°C and the reaction is carried out for 12 hours to obtain the core.

[0094] S3. After the obtained core is cooled to room temperature, propylene gas and the second mixed gas are continuously introduced, and the volume percentage of propylene in the system is controlled to be 15%. The system is kept at 750°C for 1 hour. Then, the propylene concentration is increased to 48%, and the system is kept at 750°C for another 3 hours to obtain the negative electrode material.

[0095] S4. The resulting negative electrode material is cooled to room temperature and then sieved and graded.

[0096] Example 2: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to 90% nitrogen and 10% hydrogen by volume.

[0097] Example 3: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to 60% nitrogen and 40% hydrogen by volume.

[0098] Example 4: The difference from Example 2 is that in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 13:5:2.

[0099] Example 5: The difference from Example 2 is that in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 10:5:5.

[0100] Example 6: The difference from Example 3 is that in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 10:5:5.

[0101] Example 7: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to 96% nitrogen + 4% hydrogen; in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 14.5:5:0.5.

[0102] Example 8: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to 96% nitrogen + 4% hydrogen.

[0103] Example 9: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to 93% nitrogen + 7% hydrogen; in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 14:5.5:0.5.

[0104] Example 10: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to 94% nitrogen + 6% hydrogen; in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 13:6:1.

[0105] Example 11: The difference from Example 1 is that in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon:hydrogen = 14.8:5:0.2.

[0106] Example 12: The difference from Example 1 is that in S3, after the obtained core is cooled to room temperature, propylene gas and the second mixed gas are continuously introduced, and the volume ratio of propylene in the system is controlled to be 15%; the negative electrode material is obtained by keeping it at 750°C for 4 hours.

[0107] Example 13: The difference from Example 1 is that in S3, after the obtained core is cooled to room temperature, propylene gas and the second mixed gas are continuously introduced, and the volume ratio of propylene in the system is controlled to be 48%; the negative electrode material is obtained by keeping it at 750°C for 4 hours.

[0108] Example 14: The difference from Example 6 is that in S3, after the obtained core is cooled to room temperature, propylene gas and the second mixed gas are continuously introduced, and the volume ratio of propylene in the system is controlled to be 10%; the temperature is maintained at 750°C for 1.5h, and then the propylene concentration is increased to 40%, and the temperature is maintained at 750°C for another 3.5h to obtain the negative electrode material.

[0109] Comparative Example 1: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to nitrogen with a volume percentage of 100%.

[0110] Comparative Example 2: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to nitrogen with a volume ratio of 100%; in S2 and S3, the composition of the second mixed gas is adjusted to a volume ratio of nitrogen:argon = 15:5.

[0111] Comparative Example 3: The difference from Example 1 is that in S1, the composition of the first mixed gas is adjusted to nitrogen with a volume percentage of 100%; in S2 and S3, the composition of the second mixed gas is adjusted to nitrogen with a volume percentage of 100%.

[0112] Taking Example 1 as an example, the obtained negative electrode material was tested using a HITACHI-S4800 scanning electron microscope (SEM). Please refer to Figure 3. It can be seen that the obtained negative electrode material is micron-sized powder or particles.

[0113] This application tests the physical and electrochemical properties of the negative electrode materials obtained in Examples 1-14 and Comparative Examples 1-3 as follows:

[0114] 1. Atomic percentage of oxygen at different depths in the anode material: X-ray photoelectron spectroscopy (XPS) was performed on the anode material using a Thermo Scientific K-Alpha instrument. The entire testing process was conducted in an argon-filled glove box. The X-ray source was monochromatic Al Kα rays (energy 1486.6 eV), and Ar... +Sputtering etching was performed using an ion gun with an ion energy of 1000 eV, a beam current of 2 μA, and an etched area of ​​2 mm × 2 mm. Ta₂O₅ was used as a sputtering standard to calibrate the sputtering rate and depth scale. A uniformly thick thermally oxidized Ta₂O₅ film (300 nm thick) was prepared on a single-crystal Ta substrate. The Ta₂O₅ layer was sputtered off, and the intensity changes of the Ta₂O₅ characteristic peak and the elemental Ta characteristic peak in the Ta₄f peak were monitored to determine the time required for complete Ta₂O₅ removal. The sputtering rate (rate = thickness / sputtering time) was calculated based on the known Ta₂O₅ thickness, and the sputtering depth was calculated from the sputtering rate. Each spectral acquisition requires testing the fine spectra of C, O, and Si. Each argon ion sputtering depth is 20 nm. The specific testing mode is as follows: without any processing, spectral acquisition is performed to obtain the atomic percentage of oxygen on the surface; sputtering etching is performed at a depth of 20 nm+ for spectral acquisition, and this cycle is repeated. During the 1st, 2nd, and 3rd sputtering, the atomic percentage of oxygen at a depth of 20 nm, 40 nm, and 60 nm from the surface is measured. During the 8th, 9th, and 10th sputtering, the atomic percentage of oxygen at a depth of 160 nm, 180 nm, and 200 nm from the surface is measured. The average of 10 test data at the same test depth for the negative electrode material is taken as the atomic percentage of oxygen at that depth. The average of the 10 test results is calculated to obtain the atomic percentage of oxygen at the surface (A1%), the atomic percentages of oxygen at depths of 20nm, 40nm, and 60nm from the surface (A21%, A22%, and A23%), and the atomic percentages of oxygen at depths of 160nm, 180nm, and 200nm from the surface (A31%, A32%, and A33%). The average atomic percentage of oxygen at depths of 20nm, 40nm, and 60nm from the surface is defined as the atomic percentage of oxygen within the surface layer of the negative electrode material (A2), where A2 = (A21 + A22 + A23) / 3. The average atomic percentage of oxygen at depths of 160nm, 180nm, and 200nm from the surface is defined as A3%, and the atomic percentage of oxygen in the inner layer of the negative electrode material is defined as A3, where A3 = (A31 + A32 + A33) / 3.

[0115] 2. Silicon content test of negative electrode material: The sample was burned in an oxygen atmosphere using a box-type atmosphere furnace (brand: Nanyang Xinyu, model: SA2-9-17TP) to make silicon and silicon suboxide in the negative electrode material react to form silicon dioxide, and carbon was burned into carbon dioxide and discharged. The silicon content was calculated by weighing.

[0116] 3. Carbon content test of negative electrode material: Using a Bruker G4 ICARUS HF infrared carbon-sulfur analyzer: The sample is burned in a high temperature and oxygen-rich state, and the carbon and sulfur elements contained therein are oxidized into carbon dioxide and sulfur dioxide, respectively. The generated gas enters the infrared detector with the carrier gas. By quantitatively analyzing the changes in carbon dioxide and sulfur dioxide signals, the carbon and sulfur content can be calculated separately.

[0117] 4. Coating thickness test of negative electrode material: The material can be cross-cut using an ion milling machine. Ten cross-sections of negative electrode material particles are randomly selected, and ten sites are randomly selected from each particle to measure the coating thickness. The average value is then used to calculate the coating thickness.

[0118] 5. Particle size testing of the negative electrode material: The particle size distribution range of the negative electrode material was tested using a Malvern 3000 laser particle size analyzer (Mastersizer 3000). The dispersant (ethanol, pure water, and a low-foaming surfactant) and the test sample were placed in a 50 mL beaker, followed by the addition of a certain amount of pure water. The mixture was stirred thoroughly with a glass rod to ensure uniform dispersion. The pump speed was set to 2400 r / min to 2500 r / min, and the frequency was 19.5 Hz for particle size testing. The median particle size Dv50 refers to the particle size value corresponding to a cumulative distribution percentage of 50% by volume.

[0119] 6. Specific Surface Area Test of Anode Material: A Jingwei Gaobo DX400 material specific surface area tester was used. The sample was loaded into a sample tube, and an isothermal jacket was placed on the sample tube. The filler rod was placed inside the bubble tube, and the retaining ring and O-ring were attached to the bubble tube. The assembled sample bubble tube was then placed in the corresponding analysis station for testing. At constant temperature and low temperature, the amount of gas adsorbed on the solid surface at different relative pressures was measured. Based on the Brownnor-Etter-Taylor adsorption theory and its formula (BET formula), the monolayer adsorption amount of the sample was calculated, thereby determining the specific surface area of ​​the material.

[0120] 7. The testing method for the conductivity of negative electrode material powder includes: using the MCP-PD51 powder resistance testing system from Mitsubishi Chemical (Japan) to test the conductivity at a pressure of 20 kN, and using the four-probe method to determine the volume resistivity of the sample. This instrument can measure the resistance of the powder, and then the computer automatically calculates the conductivity of the powder.

[0121] 8. Electrochemical performance testing:

[0122] (1) The negative electrode materials of each embodiment and comparative example were respectively mixed with sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive graphite (KS-6) and carbon black (SP) in a mass ratio of 92:2:2:2:2 to form a slurry. The slurry was uniformly coated on copper foil and dried to form a negative electrode sheet. A coin cell was assembled in an argon atmosphere glove box. The separator used was a polypropylene microporous membrane. The electrolyte used was 1 mol / L lithium hexafluorophosphate (the solvent was a mixed slurry of ethylene carbonate, methyl ethyl carbonate and dimethyl carbonate). The counter electrode used was a lithium metal sheet.

[0123] (2) At room temperature, each battery was tested for discharge specific capacity on the Blue Electric CT2001A battery testing system. The 25℃ cycle capacity retention rate was as follows: ① Rest for 6 hours; ② 0.1C constant current discharge to 0.01V, constant voltage discharge to 0.05C; ③ Rest for 30 minutes; ④ 0.1C constant current charging to 1.5V; ⑤ Rest for 30 minutes; ⑥ 0.1C constant current discharge to 0.01V, constant voltage discharge to 0.05C; ⑦ Rest at 25℃ for 180 minutes; ⑧ 1C constant current charging to 1.5V; ⑨ Rest for 60 minutes; ⑩ 1C constant current discharge to 0.01V, constant voltage discharge to 0.05C; Rest for 30 minutes. Then, steps ⑧ to ⑩ were repeated 100 times, and the test was stopped. The 100-cycle capacity retention rate = discharge specific capacity after 100 cycles / discharge specific capacity in the first cycle.

[0124] (3) The first coulombic efficiency test was conducted on each battery on the Blue Electric CT2001A battery testing system. The charge and discharge current was 0.05C. The first coulombic efficiency was measured. The measured value was the average value of 3-5 button cells of each material.

[0125] Please refer to Table 1 for some of the preparation conditions of Examples 1-14 and Comparative Examples 1-3 above, and Table 2 for the test results above.

[0126] Table 1. Partial preparation conditions of Examples 1-14 and Comparative Examples 1-3 of this application.

[0127] Table 2 Basic parameters of Examples 1-14 and Comparative Examples 1-3 of this application

[0128] Table 3 Performance test results of Examples 1-14 and Comparative Examples 1-3 of this application

[0129] Under the preparation process conditions of this application, the negative electrode materials of Examples 1-14 of this application form a core composed of a matrix and active material and a coating layer covering the surface of the core, with a preset relationship of A1 and A2, and a preset range of A3. That is, the degree of oxidation of the active material in the negative electrode material upon contact with the outside world is low and the oxygen content of the matrix is ​​low. Thus, a coating layer with high uniformity and density is formed on the surface of the core of the negative electrode material. The battery containing the negative electrode material has high initial coulombic efficiency and cycle performance. At the same time, the degree of additional oxygen introduced into the matrix of the negative electrode material during the preparation process is low. Therefore, the battery has good conductivity and higher reversible specific capacity.

[0130] Among them, based on satisfying the preset relationship between A1 and A2 and the preset range of A3: Example 7 further satisfies the relationship that A2 is less than A3, the oxidation degree of the active material on the surface of the negative electrode material is lower than that of the matrix in the inner layer (oxygen mainly comes from the introduction during the preparation process), and the coating layer of the negative electrode material provides a strong protective effect for the active material on the surface of the negative electrode material, further improving the cycle performance of the battery; Example 9 further satisfies that A2 falls into the range of 1 to 8, which is conducive to further improving the protective effect of the coating layer, thereby further improving the cycle performance of the negative electrode material; Example 8 further satisfies that A2 is less than A3 and A1 falls into the range of 3 to 15, and Example 10 further satisfies that A1 falls into the range of 3 to 15 and A2 falls into the range of 1 to 8, all of which further improve the cycle performance and first coulombic efficiency of the negative electrode material; Examples 1-6 further satisfy that A2 is less than A3, A1 falls into the range of 3 to 15 and A2 falls into the range of 1 to 8, thereby having significantly improved cycle performance and first coulombic efficiency.

[0131] In Examples 12 and 13, a fixed concentration of carbon source was used during the coating process. In contrast, the coating layer of the negative electrode material in the examples using different concentrations of carbon source in multiple coatings has higher density and protective strength. Therefore, the batteries of these negative electrode materials have a further improved capacity retention rate and a further improved cycle performance.

[0132] In Examples 1-3, as the amount of hydrogen used in the matrix treatment process increased sequentially, the A3 of the resulting negative electrode material decreased sequentially, while the initial reversible specific capacity increased sequentially. This indicates that a certain amount of hydrogen providing a reducing atmosphere can simultaneously regulate the oxygen content of the matrix. A lower oxygen content in the matrix is ​​beneficial for improving conductivity, thereby increasing the reversible specific capacity. It also reduces side reactions with lithium during lithium intercalation and is beneficial for improving the initial coulombic efficiency and cycle stability. In Examples 4 and 5, or Examples 3 and 6, under the same matrix treatment conditions, during the active material deposition and coating layer formation process, as the amount of hydrogen used in the second mixed gas increased sequentially, the A1 and A2 of the resulting negative electrode material decreased sequentially, while the initial coulombic efficiency and cycle performance increased sequentially. This indicates that the amount of hydrogen used in the second mixed gas is beneficial for regulating the deposition of the active material and the coating effect of the coating layer.

[0133] Compared to Example 1, Comparative Examples 1-3 did not introduce hydrogen during the matrix treatment process, resulting in an excessively high A3 value in the obtained negative electrode material. This led to an excessively high inherent oxygen content in the matrix, affecting the performance of the negative electrode material. In Comparative Example 2, although argon was used to improve the permeability of the coating layer during its formation, the lack of hydrogen introduction resulted in poor protection of the coating layer. The degree of oxidation of the core of the obtained negative electrode material upon contact with the external environment was high, leading to significantly inferior performance compared to the Example 1. In Comparative Example 3, neither argon nor hydrogen was introduced during the coating layer formation process, resulting in even worse protection of the coating layer. The degree of oxidation of the core of the obtained negative electrode material upon contact with the external environment was exacerbated, severely affecting the electrochemical performance of the obtained negative electrode material.

[0134] In summary, this application provides an improved method for preparing a negative electrode material. Under the process conditions of this method, the obtained negative electrode material has A1 and A2 that conform to a preset relationship and A3 within a preset range. This indicates that the obtained negative electrode material has a highly uniform and dense coating layer, which provides good protection for the active material. Furthermore, the matrix of the negative electrode material introduces less additional oxygen during the preparation process, which can maintain good conductivity and reduce the risk of side reactions with lithium. As a result, the negative electrode material has a high reversible specific capacity, initial coulombic efficiency, and good cycle performance.

[0135] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material includes a core and a coating layer disposed on at least a portion of the surface of the core. The core includes a matrix and an active substance, with at least a portion of the active substance disposed inside the matrix. The atomic percentage of oxygen on the surface of the negative electrode material is A1%, the atomic percentage of oxygen in the surface layer of the negative electrode material is A2%, and the atomic percentage of oxygen in the inner layer of the negative electrode material is A3%, where A1 is greater than A2, and 2 ≤ A3 ≤ 10.

2. The negative electrode material as described in claim 1, characterized in that, The negative electrode material satisfies at least one of the following conditions: (1) A3 is 2, 3, 4, 5, 6, 7, 8, 9, 10 or any value within the range of any two of the above values; (2) 3≤ A1 ≤15； (3) A1 is 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or any value within the range of any two of the above values; (4)1≤A2≤8； (5) A2 is 1, 2, 3, 4, 5, 6, 7, 8 or any value within the range of any two of the above values; (6) A2 is less than A3.

3. A negative electrode material, characterized in that, The negative electrode material includes a core and a coating layer disposed on at least a portion of the surface of the core. The core includes a matrix and an active substance, with at least a portion of the active substance disposed within the matrix. The atomic percentage of oxygen on the surface of the negative electrode material is A1%, the atomic percentage of oxygen at a depth of 20 nm from the surface of the negative electrode material is A21%, and the atomic percentage of oxygen at a depth of 160 nm from the surface of the negative electrode material is A31%, where A1 is greater than A21, and 2 ≤ A31 ≤ 10.

4. The negative electrode material as described in claim 3, characterized in that, The negative electrode material satisfies at least one of the following conditions: (1) A31 is 2, 3, 4, 5, 6, 7, 8, 9, 10 or any value within the range of any two of the above values; (2)3≤A1≤15； (3) A1 is 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or any value within the range of any two of the above values; (4)1≤A21≤8； (5) A21 is 1, 2, 3, 4, 5, 6, 7, 8 or any value within the range of any two of the above values; (6) A21 is less than A31.

5. The negative electrode material as described in claim 1 or 3, characterized in that, The negative electrode material satisfies at least one of the following conditions: (1) The matrix has pores, and at least a portion of the active substance is disposed within the pores of the matrix; (2) The matrix includes a carbon matrix, which includes one or more of artificial graphite, natural graphite, amorphous carbon, activated carbon, mesophase carbon microspheres, carbon nanotubes, carbon nanofibers, porous carbon and graphene. (3) The active substance includes one or more of Si, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P and Cu.

6. The negative electrode material as described in claim 1 or 3, characterized in that, The active material includes a silicon-based material, which satisfies at least one of the following conditions: (1) The silicon-based material includes one or more of elemental silicon, silicon oxide, and silicon alloy; (2) The average particle size of the silicon-based material is 0.1 nm to 500 nm; (3) The morphology of the silicon-based material includes one or more of the following: dot-shaped, spherical, ellipsoidal, and sheet-like. (4) The purity of the silicon-based material is greater than or equal to 99%.

7. The negative electrode material as described in claim 1 or 3, characterized in that, The active material includes a silicon-based material, which comprises silicon particles and a silicon oxide layer on the surface of the silicon particles. The silicon-based material also satisfies at least one of the following conditions: (1) The silicon oxide layer includes silicon oxide, the general formula of which is SiOx, 0 < x ≤ 2; (2) Based on the mass of the silicon-based material, the mass percentage of oxygen atoms in the silicon-based material is 1% to 18%.

8. The negative electrode material as described in claim 1 or 3, characterized in that, The covering layer satisfies at least one of the following conditions: (1) The material of the coating layer includes one or more of carbon materials, metal oxides, amorphous silicon, conductive polymers, fluorides, phosphates, nitrides and metal salts; (2) The thickness of the coating layer is 1 nm to 100 nm.

9. The negative electrode material as described in claim 1 or 3, characterized in that, The median particle size Dv50 of the negative electrode material particles is less than or equal to 10 μm.

10. The negative electrode material as described in claim 1 or 3, characterized in that, The specific surface area of ​​the negative electrode material is less than or equal to 5m². 2 / g.

11. The negative electrode material as described in claim 1 or 3, characterized in that, The negative electrode material has a powder conductivity of 1.2 S / cm to 1.6 S / cm at 20 kN.

12. The negative electrode material as described in claim 1 or 3, characterized in that, The negative electrode material comprises carbon and silicon, and the negative electrode material satisfies at least one of the following conditions: (1) Based on the mass of the negative electrode material, the mass percentage of the carbon element is 48% to 51%; (2) Based on the mass of the negative electrode material, the mass percentage of silicon is 45% to 47%.

13. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, characterized in that, The negative electrode active material layer includes the negative electrode material as described in any one of claims 1-12.

14. A secondary battery, characterized in that, Includes the negative electrode as described in claim 13.

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