Negative electrode material, negative electrode sheet, and secondary battery

Abstract

A kind of negative material, negative pole piece and secondary battery, negative material contains nitrogen element, including carbon matrix and active substance, carbon matrix is equipped with hole, at least part active substance is arranged in the hole of carbon matrix, define the region of depth less than or equal to 200nm from the outer surface of negative material as first area, the region of depth greater than or equal to 1000nm from the outer surface of negative material as second area, the atomic percentage content of nitrogen element in first area is α, the atomic percentage content of nitrogen element in second area is β, α < β;Define the deposition parameter of active substance as γ, γ ≥ 0.86.The above-mentioned negative material, relatively large part of active substance is distributed in the hole of carbon matrix, reduces the gas generation behavior of negative material and is favorable to the conductive performance of carbon matrix to play, so that the secondary battery using the negative material has higher discharge specific capacity, first coulomb efficiency and better cycle stability.

Description

Negative electrode materials, negative electrode sheets, and secondary batteries Cross-references to related applications

[0001] This application claims priority to Chinese patent application filed on January 7, 2025, with application number 202510040372.5 and entitled "Anode Material, Anode Sheet and Secondary Battery". Technical Field

[0002] This application relates to the field of electrochemical energy storage, specifically to a negative electrode material, a negative electrode sheet, and a secondary battery. Background Technology

[0003] With the rapid development and widespread application of electric vehicles, the market has placed higher demands on the energy density and cycle life of secondary batteries (such as lithium-ion batteries). The energy density of lithium-ion batteries mainly depends on the choice of electrode materials. Traditional mainstream anode materials mainly include graphite, but the specific capacity of graphite anodes has approached its theoretical upper limit, making further improvement almost impossible. Silicon, with a theoretical capacity of around 4200 mAh / g, possesses advantages such as high capacity, wide availability, environmental friendliness, and moderate lithium intercalation potential, and is widely considered one of the next-generation high-energy-density candidate anode materials for lithium-ion batteries.

[0004] However, silicon undergoes significant volume expansion during lithium insertion / extraction, which can easily lead to silicon particle breakage and pulverization, resulting in electrode structure damage or repeated growth of the solid electrolyte interphase (SEI) film, causing rapid degradation of the electrochemical performance of the silicon anode. Summary of the Invention

[0005] This application provides an anode material to solve at least one of the above-mentioned problems. To achieve the above objective, this application provides an anode material containing nitrogen, comprising a carbon matrix and an active material. The carbon matrix has pores, and at least a portion of the active material is disposed within the pores of the carbon matrix. A first region is defined as a region with a depth of less than or equal to 200 nm from the outer surface of the anode material, and a second region is defined as a region with a depth of greater than or equal to 1000 nm from the outer surface of the anode material. The atomic percentage of nitrogen in the first region is α%, and the atomic percentage of nitrogen in the second region is β%, where α < β. The deposition parameter of the active material is defined as γ. γ≥0.86; where ρ is the theoretical density of the active material; the mass percentage of the active material in the negative electrode material is a1, and the specific pore volume of the negative electrode material is p1; a negative electrode material with a mass of m1 is placed in a sufficient amount of acid solution and stirred, washed and dried, and the mass of the remaining material is measured to be m2, the mass percentage of the active material in the remaining material is a2, and the specific pore volume of the remaining material is p2.

[0006] This application also provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer includes the aforementioned negative electrode material.

[0007] This application also provides a secondary battery, including the aforementioned negative electrode.

[0008] This application research found that when the negative electrode material contains nitrogen, nitrogen atoms can cause changes in the local surface chemical activity of the carbon matrix, potentially affecting the contact between the carbon matrix and other substances. This application further found that when the negative electrode material is divided into a first region and a second region, the second region is closer to the interior of the carbon matrix than the first region. More nitrogen in the negative electrode material is distributed inside the carbon matrix rather than on the surface. Taking vapor-phase deposited active materials as an example, this distribution promotes the decomposition and deposition of more vapor-phase active material sources (such as vapor-phase silanes) within the pores of the carbon matrix, thereby promoting the deposition of more active materials within the pores of the carbon matrix. This results in an active material deposition parameter γ greater than or equal to 0.86, reducing the proportion of active materials distributed on the surface of the carbon matrix. This, in turn, reduces side reactions between the negative electrode material and the electrolyte, reduces gas generation behavior of the negative electrode material, and the nitrogen distribution within the carbon matrix is ​​beneficial to the conductivity of the carbon matrix. Secondary batteries including the negative electrode material of this application can have higher discharge specific capacity, initial coulombic efficiency, and better cycle stability. Attached Figure Description

[0009] Figure 1 is a schematic diagram of the structure of a secondary battery provided in one embodiment of this application during charging.

[0010] Figure 2 is a schematic diagram of the structure of a secondary battery provided in one embodiment of this application during discharge.

[0011] Figure 3 is a morphology test image of the negative electrode material provided in Embodiment 1 of this application.

[0012] Figure 4 shows the XRD pattern of the negative electrode material provided in Example 1 of this application.

[0013] Figure 5 is a schematic diagram of the target area in the mass percentage test of the coating layer according to the embodiments of this application.

[0014] Explanation of main component symbols

[0015] Electrode assembly 100

[0016] Positive electrode 101

[0017] Negative electrode 102

[0018] Separator 103. Detailed Implementation

[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 problem caused by silicon particle expansion in silicon anode materials, the lithium-ion battery field has seen the emergence of innovative silicon-carbon anode materials. These materials combine the high specific capacity of silicon with the stability of carbon, aiming to improve the energy density and cycle life of batteries. To further leverage the advantages of silicon-carbon anode materials and enhance their capacity, this application seeks methods to increase the silicon loading from a structural design perspective. This application finds that during the composite process of silicon and carbon materials, a large amount of silicon material is easily exposed on the surface of the carbon material. This causes a significant volume effect during the lithium insertion / extraction process in the silicon-carbon anode material, affecting the overall conductivity of the anode material. Furthermore, the exposed silicon material may exhibit significant gas generation behavior in aqueous slurries or electrolyte systems, posing safety hazards and hindering the capacity development of the silicon-carbon anode material, leading to a decrease in the initial coulombic efficiency of secondary batteries containing silicon-carbon anode materials.

[0021] Based on this, this application improves the preparation method of silicon-carbon anode material, and reduces the content of silicon material exposed on the surface of carbon material while having the same amount of silicon deposition, thereby achieving the purpose of improving the capacity, first coulombic efficiency and cycle stability of silicon-carbon 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 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 these 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 the negative electrode material containing nitrogen. The negative electrode material comprises a carbon matrix and an active substance. The carbon matrix has pores, and at least a portion of the active substance is disposed within these pores. A region with a depth of less than or equal to 200 nm from the outer surface of the negative electrode material is defined as the first region, and a region with a depth of greater than or equal to 1000 nm from the outer surface of the negative electrode material is defined as the second region. The atomic percentage of nitrogen in the first region is α%, and the atomic percentage of nitrogen in the second region is β%, where α < β. The deposition parameter of the active substance is defined as γ. γ≥0.86; where ρ is the density of the active material. The mass percentage of the active material in the negative electrode material is a1, and the specific pore volume of the negative electrode material is p1. A negative electrode material with a mass of m1 is placed in a sufficient amount of acid solution, stirred, washed, and dried. The mass of the remaining material is measured to be m2, the mass percentage of the active material in the remaining material is a2, and the specific pore volume of the remaining material is p2.

[0033] Specifically, γ can be 0.86, 0.87, 0.88, 0.89, 0.90, 0.92, 0.94, 0.97, 0.98, 0.99, or any value within the range of any two of the above values.

[0034] This application research found that when the negative electrode material contains nitrogen, nitrogen atoms can cause changes in the local surface chemical activity of the carbon matrix, potentially affecting the contact between the carbon matrix and other substances. This application further found that when the negative electrode material is divided into a first region and a second region, the second region is closer to the interior of the carbon matrix than the first region. More nitrogen in the negative electrode material is distributed inside the carbon matrix rather than on the surface. Taking vapor-phase deposited active materials as an example, this distribution promotes the decomposition and deposition of more vapor-phase active material sources (such as vapor-phase silanes) within the pores of the carbon matrix, thereby promoting the deposition of more active materials within the pores of the carbon matrix. This results in an active material deposition parameter γ greater than or equal to 0.86, reducing the proportion of active materials distributed on the surface of the carbon matrix. This, in turn, reduces side reactions between the negative electrode material and the electrolyte, reduces gas generation behavior of the negative electrode material, and the nitrogen distribution within the carbon matrix is ​​beneficial to the conductivity of the carbon matrix. Secondary batteries including the negative electrode material of this application can have higher discharge specific capacity, initial coulombic efficiency, and better cycle stability.

[0035] Understandably, when α is greater than β, nitrogen atoms are distributed in the more superficial regions of the carbon matrix, while the content of nitrogen atoms distributed inside the carbon matrix is ​​relatively small. The role of nitrogen atoms in enhancing the adsorption capacity of the carbon matrix pores for active materials is weaker, causing the active materials to tend to deposit on the surface of the carbon matrix. This leads to an increase in side reactions between the negative electrode material and water or electrolyte, and intensifies the gas generation behavior of the negative electrode material, which is not conducive to the utilization of the secondary battery capacity and results in a decrease in the initial coulombic efficiency and cycle stability of the secondary battery.

[0036] In the above formula for calculating γ, the polynomial m2×p2-m1×p1 represents the difference between the pore volume of the remaining material of mass m2 and the pore volume of the negative electrode material of mass m1, used to characterize the volume of the active material disposed within the pores of the carbon matrix in the negative electrode material of mass m1 of this application; the polynomial m1×a1-m2×a2 represents the difference between the mass of the active material in the negative electrode material of mass m1 and the mass of the active material in the remaining material of mass m2 of this application, used to characterize the total mass of the active material removed by acid etching of the negative electrode material of mass m1 of this application (including all active materials disposed within the pores of the carbon matrix and in other locations on the carbon matrix), and the ratio of the polynomial m1×a1-m2×a2 to the density ρ of the active material corresponds to characterizing the total volume of the active material etched away by acid etching of the negative electrode material of mass m1 of this application. Therefore, the formula for calculating γ is used to characterize the proportion of the active material disposed within the pores of the carbon matrix in the negative electrode material to the total active material.

[0037] In some embodiments, removing active substances from the negative electrode material includes: mixing the negative electrode material with an acid solution. The negative electrode material is mixed with an acid solution of sufficient concentration and stirred thoroughly. The acid solution is used to etch the negative electrode material, which can remove active substances from the pores and surface of the carbon matrix to a certain extent. The etched negative electrode material is then cleaned and dried to complete the removal of active substances.

[0038] In some embodiments, the acid solution includes one or more of hydrochloric acid, nitric acid, and hydrofluoric acid.

[0039] In some embodiments, taking silicon material as the active material in the negative electrode material as an example, the above-mentioned "removal of active material from the negative electrode material" includes: preparing an acid solution using 70% hydrochloric acid and 50% hydrofluoric acid at a volume ratio of 2:1, placing the negative electrode material in the acid solution and stirring for at least 10 hours, and then cleaning and drying the negative electrode material. In the formula for calculating γ, ρ can be taken as the theoretical density value of silicon material, 2.34, to calculate the deposition parameter γ of the silicon material, which can further characterize the proportion of silicon material deposited in the pores of the carbon matrix.

[0040] In related technologies, taking silicon as an active material as an example, when the silicon content reaches a certain level, some silicon will begin to adhere to the surface of the carbon matrix, thereby significantly increasing the specific surface area of ​​the negative electrode material. This leads to increased contact between the negative electrode material and the electrolyte during charging and discharging, resulting in more side reactions and making it difficult to maintain a stable SEI film. Furthermore, silicon particles attached to the carbon matrix surface are prone to hydrolysis during slurry preparation and battery use, generating a large amount of gas, which leads to decreased slurry stability and potential battery safety hazards. In addition, with a fixed silicon content in the negative electrode material, a higher silicon content on the carbon matrix surface means a lower silicon content inside the carbon matrix. This may result in excess porosity within the particles, leading to a decrease in the strength of the negative electrode material particles. This application characterizes the proportion of active material located within the pores of the carbon matrix in the negative electrode material by measuring the changes in pore volume and mass of the negative electrode material before and after the removal of active material, i.e., the deposition parameter γ. In this application, when the active material deposition parameter γ is greater than or equal to 0.86, it indicates that most of the active material is distributed within the pores of the carbon matrix. This can reduce the side reactions between the negative electrode material and the electrolyte during charging and discharging, improve the cycle performance of the battery, and reduce the hydrolysis of active material during slurry preparation and battery use, i.e., reduce gas generation and improve the safety performance of the battery. In addition, when the silicon content in the negative electrode material is constant, increasing the silicon content within the pores of the carbon matrix can also improve the particle strength of the negative electrode material. In other embodiments, when the deposition parameter γ is less than 0.86, the specific pore volume, specific surface area, micropore volume (pores with a diameter of less than or equal to 2 nm) and gas production of the resulting negative electrode material increase significantly, which is not conducive to obtaining good electrochemical performance. This also shows that the present application has important practical application value in obtaining a negative electrode material with relatively more active materials located in the pores of the carbon matrix by setting the deposition parameter γ to be greater than or equal to 0.86.

[0041] Anode materials that meet the above requirements for α, β, and γ have excellent filling effect of active materials. More active materials are distributed in the pores of the carbon matrix, which reduces the gas generation behavior of the anode material and is conducive to the conductivity of the carbon matrix. As a result, secondary batteries using this anode material have higher discharge specific capacity, first coulombic efficiency, and better cycle stability.

[0042] In some embodiments, 0.5 ≤ α < β ≤ 10. For example, α can be independently exemplified as 0.5, 1.0, 1.5, 1.8, 1.9, 2.2, 2.7, 2.9, 3, 4, 5, 6, 7, 8, 9, or any value within the range of any two of the above values; or β can be independently exemplified as 1, 2, 3, 4, 4.7, 4.8, 5.2, 5.6, 7.8, 8.9, 9.5, 9.8, 10, or any value within the range of any two of the above values; or α and β can be 1.5 and 5.6, 2.1 and 10, 1.0 and 4.8, 1.9 and 8.9, 0.5 and 4.7, 2.9 and 9.8, 2.7 and 9.5, 2.2 and 7.8, 1.8 and 5.2, or any other numerical combination that satisfies the above relationships. Understandably, controlling the magnitudes of α and β can regulate the nitrogen content in the carbon matrix. When α and β satisfy the above relationship and range, it is beneficial for sufficient nitrogen doping to form in the carbon matrix, thereby better leveraging the role of nitrogen atoms in enhancing the adsorption capacity of the carbon matrix pores for active materials. This promotes the distribution of relatively more active materials within the pores of the carbon matrix, further reducing the side reaction between the negative electrode material and the electrolyte, further limiting the gas generation behavior of the negative electrode material, improving the capacity of the secondary battery, and enhancing the initial coulombic efficiency and cycle stability of the secondary battery.

[0043] In some embodiments, 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 carbon matrix and an active material. The coating layer is made of one or more of carbon materials, metal oxides, amorphous silicon, conductive polymers, fluorides, phosphates, and nitrides. The coating layer can reduce the solubility of the core portion of the negative electrode material in the electrolyte, thereby reducing the amount of gas generated by the reaction of the dissolved active material (such as silicon material) with the electrolyte. The carbon material may include one or more of graphene, soft carbon, and hard carbon. The carbon coating layer located on the outer layer of the negative electrode material has good conductivity, which can improve the conductivity of the negative electrode material.

[0044] 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. When the coating layer has a multi-layer coating structure, the density is higher.

[0045] In some embodiments, the thickness of the coating layer is from 5 nm to 200 nm. For example, the thickness of the coating layer can be 5 nm, 10 nm, 20 nm, 30 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, or any value within the range of any two of the above values. Controlling the thickness of the coating layer within the above range ensures the strength of the coating layer without affecting the energy density. This is beneficial for maintaining the stability of the particle structure of the anode material during cycling, reducing the exposed active material on the surface of the anode material, reducing the risk of generating a large amount of SEI during charge and discharge due to exposed active material, and simultaneously maintaining a good interfacial resistance of the anode material, thereby improving the specific capacity and electrochemical performance of the anode material.

[0046] In some embodiments, the coating layer accounts for less than or equal to 10% of the mass of the negative electrode material. For example, the mass percentage of the coating layer can be 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or any value within the range of any two of the above values. A mass percentage of the coating layer within the above range allows for sufficient intercalation of active ions into the electrode, thereby enabling the battery to have a higher charge / discharge capacity and helping to reduce the risk of conductivity degradation.

[0047] In some embodiments, the specific surface area of ​​the carbon matrix is ​​800 m². 2 / g to 3000m 2 / g. For example, the specific surface area of ​​a carbon matrix can be 800m². 2 / g, 1000m 2 / g、1200m 2 / g, 1400m 2 / g, 1600m 2 / g、1800m 2 / g、2000m 2 / g、2300m 2 / g、2500m 2 / g、2800m 2 / g、3000m 2 / g or any value within the range of any two of the above values. Controlling the specific surface area of ​​the carbon matrix within the above range is beneficial for the uniform dispersion and stable adhesion of the active material on the carbon matrix, thereby reducing the risk of the active material detaching from the carbon matrix and improving the stability of the active material.

[0048] In some embodiments, the specific pore volume of the carbon matrix is ​​0.5 cm³. 3 / g to 2.0cm 3 / g. For example, the total pore volume of the carbon matrix can be 0.5 cm³. 3 / g, 0.8cm 3 / g, 1cm 3 / g, 1.2cm 3 / g, 1.5cm 3 / g, 1.8cm 3 / g, 2.0cm 3 / g or any value within the range of any two of the above values. The carbon matrix has abundant pores, which can accommodate active materials and reserve space for the volume expansion of the accommodated active materials, which is beneficial to improving the capacity and cycle stability of batteries made of anode materials.

[0049] In some embodiments, the porosity of the carbon matrix is ​​40% to 80%. For example, the porosity of the carbon matrix can be 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any value within a range of any two of the above values. Porosity refers to the percentage of pore volume in a material to the total volume of the material in its natural state. Controlling the porosity of the carbon matrix within the above range is beneficial for the good deposition of active materials, thereby reducing the volume expansion effect of the anode material.

[0050] In some embodiments, the carbon matrix includes one or more of hard carbon, soft carbon, natural graphite, artificial graphite, carbon nanotubes, carbon fibers, and graphene. The selection of these materials as the carbon matrix can provide distribution sites for active substances and form a conductive network.

[0051] In some embodiments, the active material includes one or more of Li, Na, K, Sn, Ge, Si, Fe, Mg, SiO, Ti, Zn, Al, P, and Cu. Combining the carbon matrix with the aforementioned metallic materials allows the metallic materials to induce lithium (or sodium, potassium) to precipitate within the pores in an alloy form, thereby increasing the lithium (or sodium, potassium) storage space of the carbon matrix. Combining the carbon matrix with phosphorus can improve conductivity and simultaneously reduce the volume change of the negative electrode material during discharge / charge.

[0052] In some embodiments, the active material includes silicon material, which comprises one or more of amorphous silicon, crystalline silicon, and composites of crystalline and amorphous silicon. When the active material includes silicon material, the silicon material serves as a component of the negative electrode active material, which can improve the specific capacity of the negative electrode material, thereby increasing the energy density of the secondary battery. Preferably, the silicon material includes amorphous silicon. When the active material further includes amorphous silicon, the amorphous silicon expands isotropically during lithium intercalation, which can reduce the collapse of pores in the material, suppress the rapid decay of the specific capacity of the negative electrode material, and further improve the lithium intercalation cycle performance of the negative electrode material.

[0053] In some embodiments, the silicon material includes one or more of silicon particles, silicon oxide, and silicon alloys. In some embodiments, the silicon material includes silicon particles and a silicon oxide layer located on the surface of the silicon particles, wherein the silicon oxide layer includes silicon oxide.

[0054] In some embodiments, the average particle size of the silicon material is from 0.1 nm to 500 nm. For example, the average particle size of the silicon material can be 0.1 nm, 0.5 nm, 1 nm, 5 nm, 10 nm, 20 nm, 30 nm, 50 nm, 80 nm, 100 nm, 200 nm, 300 nm, 400 nm, 450 nm, 500 nm, or any value within the range of any two of the above values. For example, the silicon material can be silicon particles. By setting the average particle size of the silicon particles within the above range, the mechanical stress when the silicon particles expand in volume can be reduced, allowing the secondary battery to maintain a good battery capacity and reducing irreversible capacity loss. It can also shorten the electron and ion transport path. At the same time, the smaller the size of the silicon particles, the larger the gap between adjacent silicon particles, which can reserve space for the volume expansion of the silicon particles.

[0055] In some embodiments, the coating layer comprises a carbon material, wherein the mass percentage of carbon in the negative electrode material is 40% to 60% based on the mass of the negative electrode material. For example, the mass percentage of carbon can be 40%, 43%, 45%, 50%, 53%, 55%, 58%, 60%, or any value within the range of any two of the above values. More preferably, it is 40% to 55%. The carbon material is derived from the carbon matrix and the coating layer. The composite of the carbon material and the silicon material can provide a conductive platform and buffer space for the silicon material, improving the structural instability and poor conductivity of the negative electrode material during cycling.

[0056] In some embodiments, the active material includes silicon material, and the mass percentage of silicon in the negative electrode material is 30% to 65% based on the mass of the negative electrode material. For example, the mass percentage of silicon in the negative electrode material can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or any value within the range of any two of the above values. More preferably, it is 45% to 60%. Since the main elemental component of the silicon material is silicon, the negative electrode material composed of a carbon matrix and silicon material can effectively solve the expansion problem of silicon material during cycling, maintain the stability of the battery material, and improve the volumetric capacity of the material.

[0057] In some embodiments, the specific surface area of ​​the negative electrode material is 0.5 m². 2 / g to 10m 2 / g. For example, the specific surface area of ​​the negative electrode material can be 0.5m². 2 / g, 1m 2 / g、2m 2 / g、3m2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 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 on the surface of the negative electrode material will consume excessive lithium salt, and the volume effect will easily cause electro-deposition between particles, resulting in a decrease in the discharge specific capacity and coulombic efficiency of the battery. Therefore, the negative electrode material in this embodiment has a smaller specific surface area, which is beneficial to improving the initial discharge specific capacity and initial coulombic efficiency of the battery. Further, the specific surface area of ​​the negative electrode material can preferably be 0.5m². 2 / g to 2.5m 2 / g. Further controlling the specific surface area of ​​the negative electrode material within the above range is beneficial for further improving the capacity of the battery's first discharge ratio and the first coulombic efficiency.

[0058] In some embodiments, the powder conductivity of the negative electrode material is from 0.5 S / cm to 3 S / cm. For example, the powder conductivity of the negative electrode material can be 0.5 S / cm, 1 S / cm, 1.5 S / cm, 2 S / cm, 2.5 S / cm, 3 S / cm, or any value within the range of any two of the above values. The powder conductivity of the negative electrode material reflects its electrical conductivity, and good electrical conductivity is beneficial to the capacity utilization of the battery. Further, the powder conductivity of the negative electrode material is preferably from 1.5 S / cm to 3 S / cm. Further controlling the powder conductivity of the negative electrode material within the above range is beneficial to further improving the utilization of lithium in lithium-ion batteries.

[0059] In some embodiments, the 24-hour gas production value of the negative electrode material is less than or equal to 4 cc / (kg·day). For example, the 24-hour gas production value of the negative electrode material can be 0.5 cc / (kg·day), 0.6 cc / (kg·day), 0.8 cc / (kg·day), 0.9 cc / (kg·day), 1 cc / (kg·day), 1.1 cc / (kg·day), 1.2 cc / (kg·day), 1.3 cc / (kg·day), 2 cc / (kg·day), 2.5 cc / (kg·day), 3 cc / (kg·day), 4 cc / (kg·day), or any value within the range of any two of the above values. A lower gas production value of the negative electrode material is beneficial for reducing the consumption of active materials and improving the cycle stability of the battery. Further, the 24-hour gas production value of the negative electrode material is preferably less than 2.7 cc / (kg·day). Further controlling the gas production value of the negative electrode material within the above range is beneficial for further reducing the gas production behavior of the negative electrode material and improving cycle stability.

[0060] In some embodiments, the median particle size D50 of the negative electrode material is between 5 μm and 20 μm. For example, the median particle size D50 of the negative electrode material can be 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 17 μm, 20 μm, or any value within the range of any two of the above values. Controlling the median particle size of the negative electrode material within the above range is beneficial to improving the cycle performance of the negative electrode material.

[0061] In some embodiments, the particle size distribution (D90-D10) / D50 of the negative electrode material is 0.9 to 5. For example, the particle size distribution (D90-D10) / D50 of the negative electrode material can be 0.9, 1, 1.2, 1.5, 2, 2.6, 3, 3.5, 4, 4.3, 4.6, 4.8, 5, or any value within the range of any two of the above values. When the particle size distribution of the negative electrode material is within the above range, the larger particles and smaller particles in the negative electrode material can cooperate with each other, and the smaller particles fill the pores between the larger particles, which can improve the tap density of the negative electrode material.

[0062] Among them, the measured cumulative particle size distribution based on volume is defined as follows: D10 represents the particle size corresponding to a cumulative particle size distribution percentage of 10%, D50 represents the particle size corresponding to a cumulative particle size distribution percentage of 50%, and D90 represents the particle size corresponding to a cumulative particle size distribution percentage of 90%.

[0063] In some embodiments, the compaction density of the negative electrode material under 1T pressure is 0.8 g / cm³. 3 Up to 1.3 g / cm 3 For example, the compaction density can be 0.8 g / cm³.3 0.9g / cm 3 1.0g / cm 3 1.1g / cm 3 1.2g / cm 3 1.3g / cm 3 Or any value within the range of any two of the above values. Controlling the compaction density within the above range helps reduce the diffusion path of lithium ions in the negative electrode material, thereby improving the rate performance of the battery.

[0064] In some embodiments, the tap density of the negative electrode material after 3000 vibrations is 0.5 g / cm³. 3 Up to 1.5g / cm 3 For example, the tap density can be 0.5 g / cm³. 3 0.7g / cm 3 0.9g / cm 3 1.1g / cm 3 1.3g / cm 3 1.5g / cm 3 Or any value within the range of any two of the above values. Controlling the tap density within the above range is beneficial for the formation of a suitable compactness of the internal structure of the negative electrode material, thereby improving lithium-ion transport and electron conduction, increasing battery energy density, extending cycle life, and improving safety performance.

[0065] 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.

[0066] 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.

[0067] Separating membrane

[0068] 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.

[0069] electrolytes

[0070] 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.

[0071] Another embodiment of this application also provides a method for preparing a negative electrode material, including:

[0072] Step 1: Mix the carbon matrix, zinc salt and organic surfactant, which includes one or more of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP) and poloxamer (Pluronic F127), and heat in a reducing gas atmosphere to reduce the zinc ions in the zinc salt and attach them to the carbon matrix to obtain the first precursor.

[0073] The aforementioned organic surfactants have amino and hydroxyl groups, and can be nonionic or anionic surfactants, which can promote the reduction of zinc salts and promote the adhesion of reduced elemental zinc to the surface and pores of the carbon matrix.

[0074] In some embodiments, the zinc salt includes one or more of zinc chloride, zinc nitrate, zinc acetate, and zinc sulfate.

[0075] In some embodiments, the reducing gas includes hydrogen. It is understood that the reducing gas atmosphere may further include an inert gas, for example, the reducing gas atmosphere may be hydrogen, a mixture of hydrogen and helium, a mixture of hydrogen and nitrogen, etc.

[0076] In some embodiments, the heating temperature is 600°C to 800°C, and the heating time is 2 hours to 6 hours. For example, the heating temperature can be 600°C, 620°C, 650°C, 700°C, 750°C, 780°C, 800°C, or any value within the range of any two of the above values. The heating time can be 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, or any value within the range of any two of the above values.

[0077] Step 2: Mix and dissolve the first precursor with the acid solution to obtain the second precursor.

[0078] The acid solution can act on the elemental zinc attached to the surface of the carbon matrix, reducing the amount of elemental zinc on the surface of the carbon matrix, thereby obtaining a carbon matrix precursor with elemental zinc attached inside the pores, i.e., the second precursor.

[0079] In some embodiments, the acid solution includes one or two of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid.

[0080] In some embodiments, the concentration of the acid solution is from 0.5 mol / L to 2 mol / L. For example, the concentration of the acid solution can be 0.5 mol / L, 0.7 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, or any value within the range of any two of the above values.

[0081] In some embodiments, the dissolution time of the acid solution is from 1 hour to 20 hours. For example, the dissolution time can be 1 hour, 3 hours, 5 hours, 8 hours, 10 hours, 12 hours, 15 hours, 17 hours, 19 hours, 20 hours, or any value within the range of any two of the above values. Controlling the concentration of the acid solution and the dissolution time within the above range is beneficial for the acid solution to remove elemental zinc from the surface of the carbon matrix, and at the same time, it is beneficial to reduce the reaction between the acid solution and elemental zinc in the pores of the carbon matrix.

[0082] Step 3: A mixture of ammonia and protective gas in a certain volume ratio is reacted with the second precursor under high temperature conditions. The concentration of ammonia is 1% to 50%, the reaction temperature is 400℃ to 1000℃, the reaction time is 0.5h to 10h, and the reaction pressure is 1Pa to 100kPa to obtain the third precursor.

[0083] For example, the concentration of ammonia can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 45%, 50%, or any value within the range of any two of the above values. The reaction temperature can be 400℃, 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, 950℃, 1000℃, or any value within the range of any two of the above values. The reaction time can be 0.5h, 1h, 2h, 4h, 5h, 7h, 8h, 9h, 10h, or any value within the range of any two of the above values. The reaction pressure can be 1Pa, 50Pa, 500Pa, 1kPa, 10kPa, 20kPa, 40kPa, 60kPa, 80kPa, 100kPa, or any value within the range of any two of the above values.

[0084] Under negative pressure, allowing ammonia gas molecules to fully enter the pores of the carbon matrix facilitates nitrogen doping within these pores. Nitrogen doping enhances the surface chemical activity of the carbon material, thereby increasing its adsorption capacity for gaseous silane molecules and improving deposition efficiency. Controlling the ammonia concentration within the specified range during this process ensures sufficient nitrogen doping in the negative electrode material to achieve the desired doping effect and promotes a nitrogen atom distribution conforming to a predetermined trend. Controlling the reaction temperature and time within the specified range further promotes the reaction between ammonia and the second precursor, facilitating sufficient nitrogen doping in the negative electrode material and promoting a nitrogen atom distribution conforming to a predetermined trend.

[0085] In some embodiments, the protective gas includes at least one of nitrogen, helium, neon, argon, and krypton.

[0086] Step 4: Mix the third precursor with the active material precursor and perform vapor deposition to obtain the negative electrode material.

[0087] During the vapor deposition process, under the catalytic action of elemental zinc, the active material is deposited in the pores of the carbon matrix, so that the active material in the prepared negative electrode material is mainly distributed in the pores of the carbon matrix. This reduces the reaction between the active material adhering to or dissolving on the surface of the negative electrode material and the electrolyte, effectively reducing the gas generation value of the negative electrode material and increasing the powder conductivity of the negative electrode material, thereby improving the various performance characteristics of the battery prepared by the negative electrode material.

[0088] In some embodiments, when the active material is a silicon material, the active material precursor includes a silicon-containing gas, which includes one or more of silane, disilane, trisilane, and tetrasilane.

[0089] In some embodiments, the concentration of the silicon-containing gas is from 10% to 80%. For example, the concentration of the silicon-containing gas can be 10%, 15%, 20%, 30%, 50%, 70%, 80%, or any value within the range of any two of the above values. Further, the gas used to dilute the silicon-containing gas can be selected from one or both of inert gases and hydrogen.

[0090] In some embodiments, the temperature of vapor deposition is from 300°C to 800°C. For example, the temperature of vapor deposition can be 300°C, 400°C, 450°C, 500°C, 520°C, 550°C, 600°C, 700°C, 800°C, or any value within the range of any two of the above values.

[0091] In some embodiments, the vapor deposition time is from 0.5 h to 15 h. For example, the vapor deposition time can be 0.5 h, 1 h, 2 h, 5 h, 8 h, 10 h, 12 h, 15 h, or any value within the range of any two of the above values.

[0092] The process conditions of vapor deposition (such as deposition temperature and deposition time) will affect the deposition of silicon material in the pores of carbon material. Controlling the temperature and time of vapor deposition within the above range is beneficial to reduce the decomposition and deposition of reactive gases before they enter the pores of carbon material and promote the decomposition and deposition of reactive gases after they enter the pores.

[0093] In some embodiments, after performing the above-described vapor deposition, the resulting product (i.e., the core) is further subjected to a coating process. The coating includes at least one of carbon coating, oxide coating, metal salt coating, and polymer coating. More preferably, multiple coating methods are combined, such as carbon coating followed by oxide coating, or carbon coating followed by metal salt coating.

[0094] In some embodiments, when the coating includes carbon coating, the coating material comprises a carbon material derived from at least one of gaseous carbon, liquid carbon, and solid carbon. Gaseous carbon sources include at least one of methane, acetylene, ethylene, ethane, propane, propylene, gaseous benzene, gaseous toluene, gaseous xylene, gaseous ethanol, gaseous cyclohexane, gaseous methanol, and gaseous acetone. Liquid carbon sources include at least one of n-hexane, toluene, benzene, xylene, methanol, ethanol, propanol, butanol, pentanol, acetone, butanone, 2-pentanone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, amyl acetate, and glycerol. Solid carbon sources include at least one of polyvinyl chloride, polyvinyl butyral, polyacrylonitrile, polyacrylic acid, polyethylene glycol, polypyrrole, polyaniline, sucrose, glucose, maltose, citric acid, asphalt, furfural resin, epoxy resin, and phenolic resin.

[0095] In some embodiments, the coating process includes mixing the coating material with the core and then performing heat treatment.

[0096] 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.

[0097] Example 1:

[0098] A negative electrode material, the preparation method of which includes:

[0099] S1, weigh 2kg of zinc chloride and 400g of Pluronic F127 and dissolve them in 20L of water. Then add 5kg of activated carbon, stir for 24h, filter and dry, and place in a rotary furnace to calcine at 900℃ under an argon atmosphere for 4h to obtain the first precursor.

[0100] S2, the first precursor was placed in 20 L of 1 mol / L dilute sulfuric acid and stirred for 4 h. The sample was then washed with pure water and dried for later use to obtain the second precursor.

[0101] S3, the second precursor is placed in the reaction chamber and heated to 800°C under a nitrogen atmosphere. The pressure inside the chamber is maintained at 100 kPa. Then, the second precursor is reacted with a mixture of 90% nitrogen and 10% ammonia for 2 hours to obtain the third precursor.

[0102] S4. Adjust the chamber temperature to 520℃, introduce silane and carrier gas into the chamber. The carrier gas is a mixture of nitrogen, argon and hydrogen. Control the silane concentration to 15%, react for 10 hours, and obtain the core material.

[0103] S5. Adjust the chamber temperature to 700℃, introduce methane and carrier gas (a mixture of nitrogen, argon, and hydrogen) into the chamber, control the methane concentration at 15%, and maintain this for 4 hours. Then, turn off the methane gas, introduce nitrogen, and cool to room temperature.

[0104] S6. The obtained sample is sieved and graded to obtain the negative electrode material.

[0105] Example 2:

[0106] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 80% nitrogen and 20% ammonia.

[0107] Example 3:

[0108] The difference from Example 1 is that in S3, the reaction temperature is adjusted to 600°C.

[0109] Example 4:

[0110] The difference from Example 1 is that in S3, the reaction temperature is adjusted to 1000°C.

[0111] Example 5:

[0112] The difference from Example 1 is that in S3, the reaction time is adjusted to 0.5h.

[0113] Example 6:

[0114] The difference from Example 1 is that in S3, the reaction time is adjusted to 4 hours.

[0115] Example 7:

[0116] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 80% nitrogen and 20% ammonia, the reaction temperature is adjusted to 700°C, the reaction pressure is adjusted to 50 kPa, and the reaction time is adjusted to 1 h.

[0117] Example 8:

[0118] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 50% nitrogen and 50% ammonia, the reaction pressure is adjusted to 50 kPa, and the reaction time is adjusted to 0.5 h.

[0119] Example 9:

[0120] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 95% nitrogen and 5% ammonia, the reaction temperature is adjusted to 700°C, and the reaction time is adjusted to 10h.

[0121] Example 10:

[0122] The difference from Example 1 is that S5 is not performed.

[0123] Example 11:

[0124] The difference from Example 1 is that in S3, the treatment is carried out under normal pressure.

[0125] Example 12:

[0126] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 99% nitrogen and 1% ammonia.

[0127] Example 13:

[0128] The difference from Example 1 is that in S3, the reaction temperature is adjusted to 400°C.

[0129] Example 14:

[0130] The difference from Example 1 is that in S3, the reaction time is adjusted to 10 min.

[0131] Comparative Example 1:

[0132] The difference from Example 1 is that S3 is not performed.

[0133] Comparative Example 2:

[0134] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 20% nitrogen and 80% ammonia.

[0135] Comparative Example 3:

[0136] The difference from Example 1 is that in S3, the reaction temperature is adjusted to 1800°C.

[0137] Comparative Example 4:

[0138] The difference from Example 1 is that in S3, the reaction time is adjusted to 20h.

[0139] Taking Example 1 as an example, the negative electrode material obtained in Example 1 was tested using a scanning electron microscope (Hitachi S4800) with a voltage of 3kV and a current of 10μA. The image was focused and astigmatism was eliminated. Please refer to Figure 3. The negative electrode material prepared in this application example is uniformly distributed and there is no obvious agglomeration phenomenon, indicating that the preparation method of this application can achieve uniform distribution of the coating on the graphite material.

[0140] Taking Example 1 as an example, the negative electrode material obtained in Example 1 was tested using an X-ray diffraction analyzer (Rigaku Ultima lv., Japan). The X-ray source was copper (Cu) Kα line, the incident angle range was set to 10° to 90°, and the step size was set to 0.02°. Please refer to Figure 4. A characteristic peak appeared at 2θ = 28.4°, indicating that the silicon in the negative electrode material is mainly amorphous silicon.

[0141] The negative electrode materials obtained in Examples 1-14 and Comparative Examples 1-4 were subjected to the following physical and electrochemical performance tests:

[0142] 1. α and β testing of the anode material: X-ray photoelectron spectroscopy (XPS) of the anode material was performed using a Thermo Scientific K-Alpha instrument. The entire testing process was conducted in an argon-filled glove box. Each spectral sampling required testing of four nitrogen atoms: C, O, Si, and N. The argon ion sputtering depth was 50 nm each time. The specific testing mode was: spectral sampling + etching at 50 nm. After 4 sputterings, the nitrogen atom percentage at a depth of 200 nm was measured, and after 20 sputterings, the nitrogen atom percentage at a depth of 1000 nm was measured. Ten random samples were taken at the same depth for testing, and the average value was used to obtain α3 and β3. It can be understood that as long as the sputtering depth meets the requirements of the first and second regions, it is sufficient. For example, 2 sputterings can yield α1, 3 sputterings can yield α2, 25 sputterings can yield β1, and 30 sputterings can yield β2. α = (α1 + α2 + α3) / 3, β = (β1 + β2 + β3) / 3.

[0143] 2. Deposition parameter γ of silicon material in anode material:

[0144] Step 1: Take a negative electrode material with a mass of m1, measure the specific pore volume p1 using a Microfinance ASAP2460 microporous surface area and pore size analyzer, and burn it in an oxygen atmosphere using a Nanyang Xinyu SA2-9-17TP box-type atmosphere furnace to make silicon and silicon suboxide in the sample react to form silicon dioxide, and carbon is burned into carbon dioxide and discharged. Weigh and calculate the silicon element content percentage a1.

[0145] Step 2: Using an acid solution prepared with 70% HCl and 50% HF in a volume ratio of 2:1, a negative electrode material of mass m1 is placed into a sufficient amount of this solution and stirred for more than 10 hours. After washing and drying, the mass of the remaining material is measured to be m2.

[0146] Step 3: Take the remaining material with mass m2 obtained in Step 2, and use the same test method as in Step 1 to test the specific pore volume p2 and the silicon content percentage a2 of the remaining material.

[0147] The density of silicon material is taken as the theoretical value of 2.34 g / cm3. The deposition parameter γ of silicon material in the negative electrode material is calculated using the following formula:

[0148]

[0149] 3. Testing of specific surface area, specific pore volume, and porosity of the anode material and carbon matrix:

[0150] a) Taking silicon as the active material as an example, under stirring, 150 mL of 20% HF acid solution is added dropwise to 10 g of negative electrode material. This will generate SiF4 and H2 gas and release heat. After no more gas is generated, the supernatant acid solution is removed by centrifugation. Another 150 mL of 20% HF acid solution is added to the negative electrode material. After stirring for 12 h, the supernatant acid solution is removed by centrifugation again. Then, the negative electrode material is washed with pure water until neutral and dried to obtain the negative electrode material after removing the silicon material, i.e., the carbon matrix.

[0151] b) The specific surface area of ​​the material was measured using the American Micro ASAP2460 microporous specific surface area and pore size analyzer;

[0152] c) The pore volume of a material can be measured through gas adsorption, while nitrogen adsorption is a technique that characterizes the porosity and pore size distribution of a material by condensing a gas within the pores of a solid. As pressure increases, the gas first condenses in the pores with the smallest diameter, and the pressure continues to rise until a saturation point is reached, at which point all pores are filled with liquid. The nitrogen pressure is then gradually decreased to allow the liquid to evaporate from the system. Analysis of the adsorption and desorption isotherms allows for the determination of pore volume and pore size distribution, as well as the proportion of micropores, mesopores, and macropores in the total pore volume.

[0153] 4. Powder conductivity testing of negative electrode materials: The conductivity at a pressure of 20 kN was tested using the MCP-PD51 powder resistance testing system from Mitsubishi Chemical Corporation of Japan. The volume resistivity of the sample was determined using the four-probe method. This instrument can measure the resistance of the powder, and then the computer automatically calculates the conductivity and resistivity of the powder.

[0154] 5. Gas generation test of negative electrode material:

[0155] a) Put 5g of negative electrode material into a sealed container and add deionized water until the remaining volume at the top of the container is 60mL.

[0156] b) Seal the container, mix thoroughly, and store at room temperature;

[0157] c) After 24 hours, shake the container thoroughly again to disperse the precipitate into the liquid.

[0158] Open the container lid, use a hydrogen detector to measure the hydrogen concentration, and convert it to cc / (kg·day). cc / (kg·day) is the volume of gas produced by each kilogram (kg) of negative electrode material in 24 hours (day).

[0159] 6. Particle size testing of negative electrode materials: The testing method refers to GB / T 19077-2016. D50 was measured using a Malvern laser particle size analyzer (Mastersizer 3000), which exhibits a symmetrical distribution resembling a normal distribution. In its volumetric reference distribution, the cumulative 50% diameter is D50, and so on, with the cumulative 90% diameter being D90 and the cumulative 10% diameter being D10. Therefore, the particle size distribution of the material can also be obtained as (D90-D10) / D50.

[0160] 7. Compacted density test of negative electrode material: A CARVER 4350.22 powder compaction density tester from the United States was used. A sample with a specified mass m was placed in a mold and a pressure of 1.0T was applied. After holding the pressure for 30 seconds, the pressure was released and the thickness was measured. The compacted density was then calculated.

[0161] 8. Tap density test of negative electrode material: Using a CANTA DAT-6-220 tap density meter, place the sample of specified mass in a graduated cylinder and vibrate it for the specified number of times (3000 times for routine testing). Read the volume of the graduated cylinder after vibration and calculate the tap density.

[0162] 9. 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 element contained therein is oxidized into carbon dioxide. The generated gas enters the infrared detector with the carrier gas. The carbon content can be calculated by quantitatively analyzing the changes in the carbon dioxide signal.

[0163] 10. Coating thickness test: The material is cross-sectioned using FIB-SEM equipment. Ten particles are randomly selected from the SEM, and the coating thickness of each particle is measured three times. The coating thickness is then calculated.

[0164] 11. Mass ratio of the coating layer: Cross-sections of the negative electrode material particles can be prepared using dual-beam focused ion microscopy (DIB-SEM). Ten cross-sections of negative electrode material particles are randomly selected and used as the target region, as shown in Figure 5. The intersection of the longest diameter (or longest diagonal) and the shortest diameter (or shortest diagonal) in the target region is defined as point O. Two straight lines are drawn from point O and intersect the edge of the target region (the surface of the negative electrode material), with intersection points M and N, respectively. The intersection point of line OM with the edge of the core in the negative electrode material is M', and the intersection point of line ON with the edge of the core in the negative electrode material is N'. MM'N'N encloses region A1, with an area of ​​S1. M'N'O encloses region A2, with an area of ​​S2. By scanning the target area using EDS, the mass content of carbon elements in target area A1 (c1) and target area A2 (c2) are obtained. The mass percentage of carbon material coating layer is calculated as [S1×c1 / (S1×c1+S2×c2)]×the mass content of carbon elements in the negative electrode material.

[0165] 12. Electrochemical performance testing:

[0166] The negative electrode materials prepared in the above examples and comparative examples were mixed with sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive graphite (KS-6), and carbon black (SP) in a ratio of 92:2:2:2:2 to form a slurry. The slurry was uniformly coated onto copper foil and dried to form a negative electrode sheet. The negative electrode sheet was then assembled into a coin cell in an argon atmosphere glove box. The separator used was a polypropylene microporous membrane, the electrolyte was 1 mol / L lithium hexafluorophosphate (the solvent was a mixed slurry of ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate), and the counter electrode was a lithium metal sheet.

[0167] The negative electrode materials prepared in the above embodiments and comparative examples were used to prepare batteries, and the discharge specific capacity was tested on the Blue Electric CT2001A battery testing system. The ratio of the discharge capacity to the battery capacity in 1 hour is the discharge specific capacity.

[0168] Batteries were fabricated using the negative electrode materials prepared in the above embodiments and comparative examples. The initial coulombic efficiency was tested using a Blue Electric CT2001A battery testing system with a charge / discharge current of 0.05C. The measured values ​​were the average values ​​of 3 to 5 coin cells for each material.

[0169] Please refer to Table 1 for some of the preparation conditions of Examples 1-14 and Comparative Examples 1-4 above, and please refer to Table 2 for the test results above.

[0170] Table 1. Partial preparation conditions of Examples 1-14 and Comparative Examples 1-4 of this application.

[0171] Table 2 Performance test results of Examples 1-14 and Comparative Examples 1-4 of this application

[0172] Under the preparation process conditions of this application, the negative electrode materials of Examples 1-14 contain nitrogen and form a structure with a carbon matrix core with silicon material deposition and a coating layer on the core surface. Nitrogen has a predetermined content and distribution trend in the negative electrode material, which enhances the adsorption capacity of the carbon matrix pores for gaseous silicon sources. This helps to improve the value of the deposition parameter γ to a certain extent, ensuring that the silicon deposition parameter γ meets the predetermined range. This relatively reduces silicon material deposition on the carbon matrix surface, reduces side reactions between the negative electrode material surface and the electrolyte, and effectively reduces the gas generation behavior of the negative electrode material. The above-mentioned negative electrode material has excellent silicon material filling effect and low gas generation value. Secondary batteries using it have correspondingly higher discharge specific capacity, initial coulombic efficiency, and better cycle stability.

[0173] In Examples 1-9 and 11-14, carbon coating was also performed, resulting in anode materials with superior overall performance. Examples 1-11 and 13 further satisfied the preset ranges for α and β, thus further enhancing the overall performance of the resulting anode materials. During the nitrogen doping process on the carbon matrix in Examples 1 and 2, 3 and 4, and 5 and 6, the ammonia dosage, reaction temperature, and reaction time were sequentially increased within appropriate ranges, resulting in a corresponding increase in the values ​​of α and β in the obtained anode materials while maintaining α < β.

[0174] Compared to Example 1, Comparative Example 1 did not perform nitrogen doping on the carbon matrix, resulting in a lower detection rate of nitrogen atoms in the negative electrode material, which can be considered as the background value of nitrogen in the negative electrode material. The nitrogen element is evenly distributed in this negative electrode material, so α and β are basically equal. However, due to the adsorption capacity of the carbon matrix of Comparative Example 1 on the gaseous silicon source, the silicon material filling effect of the obtained negative electrode material is not ideal (γ is less than that of Example 1). The proportion of silicon material distributed on the surface of the carbon matrix is ​​relatively large, and the gas generation behavior is more intense, which affects the conductivity of the carbon matrix and also leads to an increase in the specific surface area of ​​the negative electrode material. As a result, the discharge specific capacity and initial coulombic efficiency of the obtained secondary battery are not as good as those of Example 1.

[0175] Compared to Example 1, Comparative Example 2 used excessive ammonia during nitrogen doping of the carbon matrix, leading to excessive nitrogen doping and hindering control of the α-β relationship, resulting in α > β. Comparative Example 3 used excessively high temperatures during nitrogen doping of the carbon matrix, potentially causing rapid ammonia decomposition, disordered nitrogen atom attachment, and possible collapse of the carbon matrix structure, further hindering control of the α-β relationship and leading to α > β. The excessively high temperatures also exacerbated the graphitization of the carbon matrix, hindering silicon deposition. Comparative Example 4 used excessively long reaction times during nitrogen doping of the carbon matrix, potentially causing subsequently decomposed nitrogen to easily dope in the more superficial regions of the carbon matrix, resulting in α > β. In the above negative electrode materials, the proportion of silicon material distributed on the carbon matrix surface is relatively large, resulting in vigorous gas generation, affecting the conductivity of the carbon matrix, and increasing the specific surface area of ​​the negative electrode material, thus limiting the discharge specific capacity and initial coulombic efficiency of the resulting secondary battery.

[0176] 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 α, β, and γ within a preset range, and α and β conform to a preset relationship. This results in excellent filling effect of the active material on the carbon matrix in the obtained negative electrode material, reduces the gas generation behavior of the negative electrode material, and is beneficial to the conductivity of the carbon matrix. As a result, the secondary battery using this negative electrode material has a higher discharge specific capacity, initial coulombic efficiency, and better cycle stability.

[0177] 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 contains nitrogen and includes a carbon matrix and an active material. The carbon matrix has pores, and at least a portion of the active material is disposed within the pores of the carbon matrix. A region with a depth of less than or equal to 200 nm from the outer surface of the negative electrode material is defined as a first region, and a region with a depth of greater than or equal to 1000 nm from the outer surface of the negative electrode material is defined as a second region. The atomic percentage of nitrogen in the first region is α%, and the atomic percentage of nitrogen in the second region is β%, where α < β. The deposition parameter of the active material is defined as γ. Wherein, ρ is the theoretical density of the active substance; The mass percentage of the active material in the negative electrode material is a1, and the specific pore volume of the negative electrode material is p1; The negative electrode material with a mass of m1 was placed in a sufficient amount of acid solution and stirred. After washing and drying, the mass of the remaining material was measured to be m2, the mass percentage of the active substance in the remaining material was a2, and the specific pore volume of the remaining material was p2.

2. The negative electrode material as described in claim 1, characterized in that, γ is any value within the range of 0.86, 0.87, 0.88, 0.89, 0.90, 0.92, 0.94, 0.97, 0.98, 0.99, or any two of the above values.

3. The negative electrode material as described in claim 1, characterized in that, 0.5≤α<β≤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) α is 0.5, 1.0, 1.5, 1.8, 1.9, 2.2, 2.7, 2.9, 3, 4, 5, 6, 7, 8, 9 or any value within the range of any two of the above values; (2) β is 1, 2, 3, 4, 4.7, 4.8, 5.2, 5.6, 7.8, 8.9, 9.5, 9.8, 10 or any value within the range of any two of the above values.

5. The negative electrode material as described in claim 1, 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 the carbon matrix and the active material. The coating 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 and nitrides; (2) The thickness of the coating layer is 5 nm to 200 nm; (3) The mass percentage of the coating layer in the negative electrode material is less than or equal to 10%.

6. The negative electrode material as described in claim 1, characterized in that, The carbon matrix satisfies at least one of the following conditions: (1) The specific surface area of ​​the carbon matrix is ​​800 m². 2 / g to 3000m 2 / g; (2) The specific pore volume of the carbon matrix is ​​0.5 cm³. 3 / g to 2.0cm 3 / g; (3) The porosity of the carbon matrix is ​​40% to 80%; (4) The carbon matrix includes one or more of hard carbon, soft carbon, natural graphite, artificial graphite, carbon nanotubes, carbon fibers and graphene.

7. The negative electrode material as described in claim 1, characterized in that, The active substance satisfies at least one of the following conditions: (1) The active substance includes one or more of Li, Na, K, Sn, Ge, Si, Fe, Mg, SiO, Ti, Zn, Al, P and Cu; (2) The active material includes silicon material, which includes one or more of amorphous silicon, crystalline silicon and a composite of crystalline silicon and amorphous silicon; (3) The active material includes silicon material, which includes one or more of elemental silicon, silicon oxide, and silicon alloy; (4) The active material includes silicon material, which includes silicon particles and a silicon oxide layer on the surface of the silicon particles, and the silicon oxide layer includes silicon oxide. (5) The active material includes silicon material, wherein the average particle size of the silicon material is 0.1 nm to 500 nm.

8. The negative electrode material as described in claim 5, characterized in that, The active material includes silicon, the coating layer includes carbon, 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 carbon in the negative electrode material is 40% to 60%; (2) Based on the mass of the negative electrode material, the mass percentage of silicon in the negative electrode material is 30% to 65%; (3) The average particle size of the silicon material is 0.1 nm to 500 nm.

9. The negative electrode material as described in claim 1, characterized in that, The negative electrode material also satisfies at least one of the following conditions: (1) The specific surface area of ​​the negative electrode material is 0.5 m². 2 / g to 10m 2 / g; (2) The powder conductivity of the negative electrode material is 0.5 S / cm to 3 S / cm; (3) The 24-hour gas production value of the negative electrode material is less than or equal to 4 cc / (kg·day).

10. The negative electrode material as described in claim 1, characterized in that, The negative electrode material also satisfies at least one of the following conditions: (1) The median particle size of the negative electrode material is 5 μm to 20 μm; (2) The particle size distribution (D90-D10) / D50 of the negative electrode material is 0.9 to 5; (3) The compaction density of the negative electrode material under 1T pressure is 0.8 g / cm³. 3 Up to 1.3 g / cm 3 ; (4) The tap density of the negative electrode material after 3000 vibrations is 0.5 g / cm³. 3 Up to 1.5g / cm 3 .

11. 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 1-10.

12. A secondary battery, characterized in that, Includes the negative electrode as described in claim 11.

Images