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 a core and a carbon coating layer covering at least a portion of the surface of the core. The core comprises a matrix and an active substance. The matrix has pores, and at least a portion of the active substance is disposed in the pores of the matrix. The negative electrode material is tested by infrared spectroscopy at a wave number of 2896±10 cm ‑1 The area of ​​the stretching vibration peak at wave number 2924±10 cm ‑1 and 2853±10 cm ‑1 The sum of the stretching vibration peak areas at wave number 2960 ± 10 cm ‑1 The area of ​​the stretching vibration peak at is S3; S1, S2, and S3 satisfy the following: #imgabs0#. Anode sheets and secondary batteries based on this anode material have good electrochemical performance.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a negative electrode material, a negative electrode sheet and a secondary battery. Background Art

[0002] With the rapid development of emerging fields such as new energy vehicles, smart homes, and the Internet of Things, the demand for battery performance is increasing. Traditional graphite anode materials offer advantages such as high efficiency and a stable charge-discharge platform. However, their low specific capacity hinders their further application in high-demand applications. Compared to graphite, silicon materials have a higher theoretical specific capacity and have the potential to be used as anode materials. However, silicon anode materials suffer from low conductivity and large volume changes during cycling, which limit their performance in practical applications.

[0003] The composite of silicon and porous carbon materials can inhibit silicon's volume expansion and improve its conductivity. However, during the composite process, silicon particles are often exposed on the surface of the porous carbon material. During the subsequent aqueous homogenization and battery cycling, the silicon particles on the surface will come into contact with the water and electrolyte in the slurry, triggering gas production. In addition, the silicon particles on the surface lack the synergistic effect of the skeleton and coating, and are prone to shedding, which affects the cycle life of the battery. Summary of the Invention

[0004] In view of this, the present application provides a negative electrode material, a negative electrode sheet and a secondary battery to solve at least one of the above problems.

[0005] To achieve the above-mentioned object, the present application provides a negative electrode material, comprising a core and a carbon coating layer covering at least a portion of the surface of the core, wherein the core comprises a matrix and an active substance, the matrix having pores, and at least a portion of the active substance is disposed in the pores of the matrix. The negative electrode material is tested by infrared spectroscopy analysis, and the results show that the negative electrode material has a carbon coating layer at a wave number of 2896±10 cm -1 The area of ​​the stretching vibration peak at wave number 2924±10 cm -1 and 2853±10 cm -1 The sum of the stretching vibration peak areas at wave number 2960±10cm -1 The area of ​​the stretching vibration peak at is S3; S1, S2 and S3 satisfy: .

[0006] In some possible implementations, the substrate has a surface area of ​​500 m 2 / g to 2000 m 2 / g.

[0007] In some possible implementations, the total pore volume of the matrix is ​​0.5 cm 3 / g to 2.0 cm 3 / g.

[0008] In some possible implementations, the average pore size of the matrix is ​​0.1 nm to 8 nm.

[0009] In some possible implementations, the average particle size of the active material is 0.1 nm to 100 nm.

[0010] In some possible implementations, the thickness of the carbon coating layer is 0.1 nm to 2000 nm.

[0011] In some possible implementations, the specific surface area of ​​the negative electrode material is 1 m 2 / g to 10 m 2 / g.

[0012] In some possible implementations, the powder conductivity of the negative electrode material is 0.05 S / m to 5 S / m.

[0013] In some possible implementations, the median particle size D of the negative electrode material is V 50 is 3 μm to 15 μm.

[0014] In some possible implementations, the matrix includes a carbon material, and the carbon material includes one or more of artificial graphite, natural graphite, amorphous carbon, activated carbon, mesophase carbon microbeads, carbon nanotubes, carbon nanofibers, and graphene.

[0015] In some possible implementations, the matrix includes a non-carbon material, and the non-carbon material includes at least one of metal oxides, silicides, silicates, phosphates, titanates and aluminum borates; the metal oxides include one or more of aluminum oxide, zirconium oxide, germanium dioxide and manganese dioxide; the silicides include one or more of silicon carbide and silicon nitride; the silicates include one or more of cordierite, mullite and zeolite; the phosphates include one or more of aluminum phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate and lanthanum phosphate; the titanates include one or more of calcium titanate, iron titanate, lithium titanate and barium titanate.

[0016] In some possible implementations, the active material includes one or more of Si, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P, and Cu.

[0017] In some possible implementations, the carbon material of the carbon coating layer includes one or more of amorphous carbon and graphitized carbon.

[0018] In some possible implementations, the matrix includes a carbon matrix, the active material includes a silicon material, the silicon material includes one or more of amorphous silicon, crystalline silicon, silicon oxide, silicon alloy, and a composite of crystalline silicon and amorphous silicon, and based on the mass of the negative electrode material, the carbon element mass of the negative electrode material accounts for 30% to 70%.

[0019] In some possible implementations, based on the mass of the negative electrode material, the mass proportion of hydrogen element in the negative electrode material is less than or equal to 5%.

[0020] In some possible implementations, based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 30% to 80%.

[0021] The present application also provides a negative electrode sheet, comprising 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 comprises the above-mentioned negative electrode material.

[0022] The present application also provides a secondary battery comprising the above-mentioned negative electrode sheet.

[0023] In this application, the negative electrode material was tested by infrared spectroscopy at a wave number of 2896 ± 10 cm -1 The area of ​​the stretching vibration peak at wave number 2924±10 cm -1 and 2853±10 cm -1 The sum of the stretching vibration peak areas at wave number 2960 ± 10 cm -1 The area of ​​the stretching vibration peak at is S3; said S1, S2 and S3 satisfy: According to the inventor's reasonable speculation, at wave number 2896±10 cm -1 The stretching vibration peak at the wave number of 2924 ± 10 cm -1 and 2853±10 cm -1 The stretching vibration peak at the wave number of 2960±10 cm is the stretching vibration peak of -CH2. -1 The stretching vibration peak at is the stretching vibration peak of -CH3. By arranging methyl, methylene and methine on the carbon coating layer of the negative electrode material, the methyl group helps to improve the stability of the negative electrode material and adjust the wettability of the negative electrode material in the aqueous slurry, and the methylene and methine help to make the negative electrode material have more electron transmission paths. By arranging methyl, methylene and methine on the carbon coating layer of the negative electrode material, the methyl group helps to improve the stability of the negative electrode material and adjust the wettability of the negative electrode material in the aqueous slurry, and the methylene and methine help to make the negative electrode material have more electron transmission paths. Under infrared spectroscopy analysis test, the stretching vibration peak areas S3, S2 and S1 of methyl, methylene and methine meet On the one hand, it can reduce the gas production behavior of the active material in the negative electrode material and alleviate the expansion volume effect of the negative electrode material, which is beneficial to improving the cycle stability of the resulting secondary battery. On the other hand, it can enhance the conductivity of the material, thereby maintaining or improving the electronic conductivity of the negative electrode sheet, which is beneficial to the rapid transport of electrons in the secondary battery to adapt to the rapid charging and discharging process and obtain a higher specific capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic structural diagram of a secondary battery provided in one embodiment of the present application during charging.

[0025] Figure 2 This is a schematic structural diagram of a secondary battery provided in one embodiment of the present application during discharge.

[0026] Figure 3 This is the Gaussian fitting result of the infrared spectrum of the negative electrode material provided in one embodiment of the present application.

[0027] Description of main component symbols

[0028] Electrode assembly 100

[0029] Positive Electrode 101

[0030] Negative electrode 102

[0031] Isolation film 103. DETAILED DESCRIPTION

[0032] The embodiments of the present application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application, and should not be understood as limiting the present application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present application belongs; the embodiments of the present application and the features in the embodiments can be combined with each other unless there is a conflict; many specific details are set forth in the following description to facilitate a full understanding of the present application, and the embodiments described are only part of the embodiments of the present application, not all of the embodiments.

[0033] An embodiment of the present application provides a secondary battery, comprising a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located within the housing.

[0034] The outer shell can be a packaging bag obtained by packaging film (such as aluminum-plastic film), for example, a soft-pack battery. In other embodiments, it can also be a steel shell battery, an aluminum shell battery, etc.

[0035] See also Figure 1 and Figure 2The electrode assembly 100 includes a positive electrode sheet 101, a negative electrode sheet 102 and a separator 103. The separator 103 is provided between the positive electrode sheet 101 and the negative electrode sheet 102. When an electrolyte (not shown) is provided, when charging, refer to Figure 1 , active ions (such as lithium ions) are deintercalated from the lattice of the positive electrode material (such as the lithiated intercalation compound) of the positive electrode sheet 101, pass through the separator 103 through the electrolyte, reach the negative electrode sheet 102 and insert into the lattice of the negative electrode material. Figure 2 Active ions (such as lithium ions) are deintercalated from the lattice of the negative electrode material of the negative electrode sheet 102, pass through the electrolyte through the isolation membrane 103, reach the positive electrode sheet 101 and are embedded in the lattice of the positive electrode material (such as the lithiated intercalation compound), generating electrons that travel from the negative electrode sheet 102 through the external circuit to the positive electrode sheet 101. The reverse movement of the electrons forms an electric current that can be used by electrical appliances.

[0036] In some embodiments, the electrode assembly 100 may be a laminated structure, formed by alternately stacking the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102. In other embodiments, the electrode assembly 100 may be a wound structure, formed by stacking the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 in sequence and then winding them.

[0037] positive electrode

[0038] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of aluminum foil or nickel foil, etc., or it can be any composite current collector disclosed in the prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and the polymer substrate. The positive electrode material active layer contains a positive electrode active material, and the positive electrode active material includes a compound that can reversibly embed and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. The 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 material (NCM), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4) or at least one of lithium iron phosphate (LiFePO4).

[0039] The positive electrode material active layer also includes a binder to bond the positive electrode active material particles to facilitate film formation and improve the bonding between the positive electrode material active layer and the positive electrode current collector. In some embodiments, the binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0040] The positive electrode material active layer may further comprise a conductive material, including but 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 fibers, 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.

[0041] negative electrode

[0042] The negative electrode sheet 102 includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector. It can also be any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode active material layer includes a negative electrode material.

[0043] The negative electrode material includes a core and a carbon coating covering at least a portion of the surface of the core. The core includes a matrix and an active material. The matrix has pores, and at least a portion of the active material is disposed in the pores of the matrix. The negative electrode material is tested by infrared spectroscopy at a wave number of 2896±10 cm -1 The area of ​​the stretching vibration peak at wave number 2924±10 cm -1 and 2853±10cm -1 The sum of the stretching vibration peak areas at wave number 2960 ± 10 cm -1 The area of ​​the stretching vibration peak at is S3; S1, S2 and S3 satisfy: .

[0044] According to the inventor's reasonable speculation, at wave number 2896±10 cm -1 The stretching vibration peak at the wave number of 2924 ± 10 cm -1 and 2853±10 cm -1The stretching vibration peak at the wave number 2960±10cm is the stretching vibration peak of -CH2. -1 The stretching vibration peak at is the stretching vibration peak of -CH3.

[0045] The negative electrode material of the present application has a carbon coating, and the carbon coating is provided with a methyl group. The methyl group is an electron donating group, and its carbon atom has a relatively high electronegativity compared to a hydrogen atom, so the electron cloud in the methyl group will be biased towards its carbon atom, so that the carbon atom of the methyl group carries a partial negative charge. When the methyl group participates in the formation of a carbochain, this negative charge effect will be transmitted along the carbochain, so that the electron cloud density on the carbon atom connected to the methyl group increases, and the electron cloud density on the carbon atom away from the methyl group will be relatively reduced. Thus, dynamic electron cloud density and charge distribution changes are caused in the carbochain, which helps to alleviate the local charge concentration phenomenon on the carbochain, thereby helping to reduce tension and energy instability inside the molecule. In addition, the σ electrons of the C-H bond in the methyl group can overlap with the π electrons of the unsaturated bond on the adjacent carbochain, forming a hyperconjugated effect, reducing internal energy, and making the entire molecule more stable. In addition, the methyl group is a non-polar group, which is not conducive to the formation of hydrogen bonds and has a certain hydrophobicity. The negative electrode slurry system (negative electrode material active substance, conductive agent, binder, etc.) usually uses water as a solvent to form a water-washed slurry. The hydrophobicity of the methyl group can adjust the wettability and dispersibility of the negative electrode material in the aqueous slurry. When the methyl group is coated on the active material attached to the surface of the substrate (such as silicon particles), it helps to reduce the risk of side reactions of the active material in the aqueous slurry (such as gas production of silicon particles). The carbon coating layer of the present application also includes methylene and methine, both of which have relatively higher reactivity than methyl. The difference in reactivity may change the molecular structure or electron transmission path of the material in some chemical reactions. The higher reactivity of methylene and methine can provide more flow paths for the electrons of the negative electrode material, thereby helping to improve the conductivity of the negative electrode material.

[0046] The present application further defines the relationship between the peak areas of the stretching vibration peaks of methyl (-CH3), methylene (-CH2) and methine (-CH) under infrared spectroscopy testing. The above relationship can characterize the content relationship of methyl, methylene and methine on the carbon coating. Specifically, infrared spectroscopy testing is based on the vibration and rotational energy level transitions that occur when molecules absorb infrared light of a specific wavelength. The vibration energy level transitions of molecules refer to the relative movement of atoms in the molecule near the equilibrium position. These vibration modes include normal vibrations (such as stretching vibrations and angular vibrations) and deformation vibrations. During the vibration process, if the dipole moment of the molecule changes, the molecule will absorb infrared light of a specific frequency and produce vibration transitions. Gaussian fitting of the infrared spectrum of the negative electrode material can obtain the stretching vibration peak areas S1, S2 and S3 of methine (-CH), methylene (-CH2) and methyl (-CH3). Please refer to Figure 3, at wave number 2896±10 cm -1 There is a stretching vibration peak of -CH at wave number 2924±10 cm -1 and 2853±10 cm -1 There is a stretching vibration peak of -CH2 at wave number 2960±10 cm -1 There is a stretching vibration peak of -CH3 at the negative electrode material. The stretching vibration peak area of ​​-CH in the negative electrode material is S1, the sum of the stretching vibration peak areas of -CH2 is S2, and the stretching vibration peak area of ​​-CH3 is S3. Indicates the content of H atoms (methyl hydrogen) from -CH3, Indicates the content of H atoms (methylene hydrogen) from -CH2, represents the content of H atoms (methylene hydrogen) from -CH. In this case, the formula The present application found that when the formula satisfies the above numerical range, that is, When the negative electrode material is prepared, the structural stability and hydrophobicity of the methyl group are enhanced, while the reactivity of the methylene or methine groups is enhanced. This can reduce the gas production of the active material in the negative electrode material and alleviate the volume expansion effect of the negative electrode material, thereby improving the cycle stability of the resulting secondary battery. It can also enhance the conductivity of the material, thereby maintaining or improving the electronic conductivity of the negative electrode sheet prepared from the negative electrode material. This is conducive to the rapid transport of electrons in the secondary battery to accommodate rapid charge and discharge processes and achieve a higher specific capacity. Overall, it helps to achieve an optimal balance between stability and conductivity of the negative electrode material.

[0047] It should be noted that when When it is greater than the upper limit, it means that there are excessive methyl groups on the coating layer, or the content of methylene or methine is too low. It is understandable that excessive methyl groups will increase the steric hindrance of the coated matrix (such as carbon matrix), thereby affecting the approach and reaction of other functional groups or molecules, reducing the electron flow path, and generating the risk of reduced overall conductivity of the negative electrode material. This is not conducive to the reaction activity of the negative electrode material, affects the conductivity, and may cause the capacity and first coulomb efficiency of the battery prepared with the negative electrode material to decrease. When the formula When it is less than the lower limit, it means that the methyl content on the coating layer is small, or the methylene or methine content is too high. Too few methyl groups will reduce the contribution of methyl groups to the stability of the carbon material, which is not conducive to the cycle stability of the negative electrode material. At the same time, too many methylene or methine groups will bring about excessive reaction activity, which is not conducive to the chemical stability of the negative electrode material and may cause the cycle stability of the battery prepared with the negative electrode material to decrease.

[0048] In some embodiments, the infrared spectrum of the negative electrode material of the present application is obtained (eg Figure 3) and analytical tests for S1, S2 and S3 include:

[0049] Step 1: Analyze using a Thermo Fisher Nicolet iS50 Fourier transform infrared spectrometer with an infrared light source wavelength of 400 cm -1 to 4000 cm -1 , detector is DTGS KBr, beam splitter is KBr, resolution is 2 cm -1 , and obtain the analysis data.

[0050] Step 2: Use OMNIC software to analyze the analytical data and obtain the preliminary peak positions of hydrocarbon groups.

[0051] Step 3: Use Origin software to analyze the preliminary peak position results to obtain the stretching vibration peak positions of methine (-CH), methylene (-CH2) and methyl (-CH3) as well as the peak areas S1, S2 and S3.

[0052] See also Figure 3 The peak result formed by the first curve relative to the baseline is the preliminary peak position of the hydrocarbon group obtained by the OMNIC software analysis, the peak result formed by the third curve relative to the baseline is the stretching vibration peak position of the methylene (-CH2), the peak result formed by the fourth curve relative to the baseline is the stretching vibration peak position of the methine (-CH), and the peak result formed by the fifth curve relative to the baseline is the stretching vibration peak position of the methyl (-CH3), and the peak area is calculated; wherein the second curve appears simultaneously after the analysis of the third curve, the fourth curve and the fifth curve during the analysis process. The higher the degree of overlap between the second curve and the first curve, the more accurate the analysis results of the third curve, the fourth curve and the fifth curve are. This application Figure 3 , the second curve substantially coincides with the first curve.

[0053] In some embodiments, the infrared spectrum test may specifically include: taking 1 mg to 2 mg of the sample and grinding it appropriately in an agate mortar, then mixing it evenly with dry potassium bromide powder (about 100 mg, 200 mesh particle size), putting it into a mold, and pressing it into tablets on a tablet press for testing. The analysis was performed using a Thermo Fisher Nicolet iS50 Fourier transform infrared spectrometer, and the light source used was an infrared light source (400 cm -1 to 4000 cm -1 ), the detector is DTGS KBr, the beam splitter is KBr, and the resolution is 2 cm -1, then sample and complete the test. Use OMNIC software to preliminarily determine the relevant peak positions. The peak position results obtained are fitted by Origin software. The fitting process is as follows: Import the infrared test data processed by OMNIC into Origin and draw the graph. In the Origin menu bar, click Analysis, Peak and Baseline, Peak Analysis in sequence. After the dialog box opens, select "Peak Fitting" in the pop-up window, select "User Defined" for baseline mode, uncheck "Enable Automatic Search", select "Clear All" for the number of points to be found, and then click Add to add manually. After clicking Add, double-click at both ends of the curve to add points. After adding, click the Finish button in the upper left corner, click Next, select "Interpolation" for connection mode, select "Spline Curve" for interpolation mode, and the points will be connected in the form of a curve. Click Next, select "Automatically Subtract Baseline", flatten the baseline, click Next, uncheck "Allow Automatic Search", select "Add", and double-click to take points at the expected peak position on the curve according to the peak position in the OMNIC software. Click Next, and you can see the automatically generated peak (similar to Figure 3 ), click the fitting control, and you can see the peaks close to the imported peaks (similar to Figure 3 Click "1 iteration" and click repeatedly. During the iteration process, the "automatically generated peaks" will be automatically adjusted to make the "peaks close to the imported peaks" match the imported peaks more closely until "fit convergence" is displayed. The curve obtained after the fit converges ( Figure 3 The second curve in ) and the original data ( Figure 3 The higher the degree of overlap, the better the fitting effect. Click Finish to obtain the fitting curve and fitting data, that is, the stretching vibration peak positions of methine (-CH), methylene (-CH2), and methyl (-CH3) (corresponding to the fourth, third, and fifth curves, respectively), as well as the peak areas S1, S2, and S3.

[0054] In some embodiments, the specific surface area of ​​the substrate is 500 m 2 / g to 2000 m 2 / g. For example, the specific surface area of ​​the matrix can be 500 m 2 / g, 600 m 2 / g, 800 m 2 / g, 1000 m 2 / g, 1200 m 2 / g, 1400 m 2 / g, 1600 m 2 / g、1800m 2 / g, 2000 m 2 / g or any value within the range formed by any two of the above values. Controlling the specific surface area of ​​the substrate within the above range facilitates uniform dispersion and stable adhesion of the active material (such as silicon particles) on the substrate, thereby reducing the risk of the active material falling off the substrate and improving the stability and durability of the active material.

[0055] In some embodiments, the total pore volume of the matrix is ​​0.5 cm 3 / g to 2.0 cm 3 / g. For example, the total pore volume of the matrix can be 0.5 cm 3 / g, 0.8 cm 3 / g, 1 cm 3 / g, 1.2 cm 3 / g, 1.5 cm 3 / g, 1.8 cm 3 / g, 2 cm 3 / g or any value within the range formed by any two of the above values. When the matrix has abundant pores, these pores can accommodate the active material and reserve space for the volume expansion of the accommodated active material. It is understood that when the matrix and the active material form a composite material, at least a portion of the active material is deposited within the pores of the matrix, and therefore the total pore volume of the formed composite material will be smaller than the total pore volume of the initial matrix.

[0056] In some embodiments, the average pore size of the matrix is ​​0.1 nm to 8 nm. For example, the average pore size of the matrix can be 0.1 nm, 0.5 nm, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, or any value within a range formed by any two of the above values. The average pore size of the matrix is ​​controlled within the above range, indicating that the pores of the matrix include a relatively large number of micropores (pores with a pore diameter of 2 nm or less), and also include at least one or more of mesopores (pores with a pore diameter greater than 2 nm and less than 50 nm) and macropores (pores with a pore diameter greater than 50 nm). The micropores in the matrix can reduce the aggregation of active materials in the matrix, increase the content of active materials in the matrix and the uniformity of the distribution of active materials, thereby improving the specific capacity and mechanical properties of the resulting negative electrode material. The relatively large pore size of the mesopores or macropores can reserve sufficient buffer space for the volume expansion of the active material, effectively alleviate the volume expansion of the active material, and reduce the risk of excessive local expansion stress in the negative electrode material due to the uneven volume change of the active material during the cycle, which may lead to the breakage and pulverization of the negative electrode material. This is beneficial to improving the particle structure stability and cycle stability of the negative electrode material.

[0057] In some embodiments, the average particle size of the active material is 0.1 nm to 100 nm. For example, the average particle size of the active material can be 0.1 nm, 0.5 nm, 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 65 nm, 80 nm, 95 nm, 100 nm, or any value within the range formed by any two of the above values. By setting the average particle size of the active material within the above range, the mechanical stress of the active material (such as silicon particles) during volume expansion can be reduced, so that the secondary battery maintains a good battery capacity and reduces irreversible capacity loss. It can also shorten the electron and ion transmission path. At the same time, the size of the active material is reduced, and the gap between adjacent active material particles is increased, which can reserve space for the volume expansion of the active material.

[0058] In some embodiments, the thickness of the carbon coating layer is 0.1 nm to 2000 nm. For example, the thickness of the carbon coating layer can be 0.1 nm, 0.5 nm, 1 nm, 30 nm, 60 nm, 100 nm, 150 nm, 200 nm, 300 nm, 500 nm, 800 nm, 1200 nm, 1700 nm, 2000 nm, or any value within the range formed by any two of the above values. The carbon coating layer can reduce the solubility of the negative electrode material, thereby reducing the amount of gas produced by the reaction of the dissolved active material (such as silicon particles) with the electrolyte. Controlling the thickness of the carbon coating layer within the above range is beneficial to maintaining the stability of the particle structure of the negative electrode material during the cycle, can reduce the exposed active material on the surface of the negative electrode material, reduce the risk of generating a large amount of SEI during the charge and discharge process due to the exposed active material, and improve the specific capacity and electrochemical performance of the negative electrode material.

[0059] In some embodiments, the specific surface area of ​​the negative electrode material is 1 m 2 / g to 10 m 2 / g. For example, the specific surface area of ​​the negative electrode material can be 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g or any value within the range formed by 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 electrical separation between particles, resulting in a decrease in the reversible capacity and coulombic efficiency of the battery. Therefore, the negative electrode material of this embodiment has a small specific surface area, which is conducive to improving the capacity of the battery's first discharge ratio and the first coulombic efficiency.

[0060] In some embodiments, the powder conductivity of the negative electrode material is 0.05 S / m to 5 S / m. For example, the powder conductivity of the negative electrode material can be 0.05 S / cm, 0.1 S / cm, 0.3 S / cm, 0.5 S / cm, 1 S / cm, 1.5 S / cm, 2 S / cm, 3 S / cm, 4 S / cm, 5 S / cm, or any value within a range formed by any two of the foregoing values.

[0061] In some embodiments, the median particle size D of the negative electrode material is V 50 is 3 μm to 15 μm. For example, the median particle size D of the negative electrode material V 50 can be 3 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm or any value within the range formed by 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. Median particle size D V 50 represents the particle size corresponding to when the cumulative particle size distribution percentage reaches 50%.

[0062] In some embodiments, the substrate comprises a carbon material, and the carbon material includes one or more of artificial graphite, natural graphite, amorphous carbon, activated carbon, mesocarbon microbeads, carbon nanotubes, carbon nanofibers, and graphene. When the substrate is a carbon material, the carbon material can provide pore distribution sites for the active material and form a conductive network.

[0063] In other embodiments, the matrix includes a non-carbon material, which includes at least one of metal oxides, silicides, silicates, phosphates, titanates, and aluminum borates; the metal oxides include one or more of aluminum oxide, zirconium oxide, germanium dioxide, and manganese dioxide; the silicides include one or more of silicon carbide and silicon nitride; the silicates include one or more of cordierite, mullite, and zeolites; the phosphates include one or more of aluminum phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate, and lanthanum phosphate; and the titanates include one or more of calcium titanate, iron titanate, lithium titanate, and barium titanate. The selection of the above non-carbon materials is beneficial for optimizing the lithium ion transmission path, increasing the lithium ion diffusion rate and insertion efficiency, and improving the electrochemical performance.

[0064] In some embodiments, the active material includes one or more of Si, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P, and Cu.

[0065] In some embodiments, the carbon material of the carbon coating layer includes one or more of amorphous carbon and graphitized carbon. In some embodiments, the carbon coating layer can be a single-layer carbon coating layer formed by the above-mentioned single material, or a carbon coating layer formed by a combination of the above-mentioned multiple materials, or a single material to form a multi-layer carbon coating layer, or a multi-layer carbon coating layer to form a multi-layer carbon coating layer, etc. The layer structure of the carbon coating layer can be selected according to actual needs. It can be understood that when the carbon coating layer is a multi-layer coating structure, its density is higher. The carbon coating layer located on the surface of the substrate is beneficial to improving the stability and conductivity of the substrate. The carbon coating layer can also coat the active material exposed on the surface of the substrate, reduce the continuous oxidation of the exposed active material during the placement process, and reduce the risk of reduction in the specific capacity and first coulomb efficiency (ICE) of the negative electrode material. The carbon coating layer can also reduce the direct contact between the active material and the electrolyte, improve the stability of the SEI film, and thus improve the first coulomb efficiency of the negative electrode material.

[0066] In some embodiments, the matrix of the negative electrode material includes a carbon matrix, the active material includes a silicon material, and the silicon material includes one or more of amorphous silicon, crystalline silicon, silicon oxide, silicon alloy, and a composite of crystalline silicon and amorphous silicon. When the active material includes a silicon material, the silicon material is used as a component of the negative electrode active material, which can increase the specific capacity of the negative electrode material and thereby increase the energy density of the secondary battery. Preferably, the silicon material includes amorphous silicon. When the active material includes amorphous silicon, amorphous silicon expands isotropically during the lithium insertion process, which can reduce the collapse of pores in the negative electrode material, inhibit the rapid decay of the specific capacity of the negative electrode material, and is more conducive to improving the lithium insertion cycle performance of the negative electrode material.

[0067] In some embodiments, based on the mass of the negative electrode material, the mass proportion of the carbon element of the negative electrode material is 30% to 70%. For example, the mass proportion of the carbon element of the negative electrode material can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or any value within the range formed by any two of the above values. Silicon material and porous carbon material are compounded to obtain a silicon-carbon composite material, which helps to alleviate the volume expansion of the silicon material. Carbon elements include a carbon matrix and a carbon coating layer. When the mass proportion of carbon elements is within this range, a sufficient carbon-based substrate can be established to provide sufficient distribution sites for the active substance, which is conducive to forming an effective conductive network and improving electrical conductivity and cycle stability.

[0068] In some embodiments, the mass percentage of hydrogen in the negative electrode material is less than or equal to 5% based on the mass of the negative electrode material. For example, the mass percentage of hydrogen in the negative electrode material can be 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, or any value within a range formed by any two of the above values. The hydrogen in the negative electrode material mainly comes from the methyl, methylene, and methine groups on the carbon coating layer. Controlling the hydrogen within the above range is conducive to forming an appropriate content of methyl, methylene, and methine groups, thereby improving the chemical stability and reactivity of the negative electrode material as a whole.

[0069] In some embodiments, based on the mass of the negative electrode material, the mass percentage of silicon in the negative electrode material is 30% to 80%. For example, the mass percentage of silicon in the negative electrode material can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any value within a range formed by any two of the above values. When the mass percentage of silicon is within this range, the lithium battery composed of the negative electrode material can store a higher amount of electricity, that is, the initial discharge specific capacity is higher.

[0070] The negative electrode active material layer also includes a binder to bind the negative electrode active material particles to facilitate film formation and improve the bonding between the negative electrode active material layer 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, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0071] The negative electrode active material layer may further include a conductive material, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0072] Isolation film

[0073] The separator 103 comprises a porous membrane layer, 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 non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0074] electrolyte

[0075] The electrolyte conducts ions between the positive and negative electrodes. The electrolyte can be in one or more of a gel, solid, and liquid state. In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution conducts active ions between the positive and negative electrodes. In some embodiments, the electrolyte solution includes a lithium salt and an organic solvent. The lithium salt can 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(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2, tris(trifluoromethylsulfonyl)methyl lithium (LiC(SO2CF3)3), lithium bis(oxalatoborate) (LiBOB) and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it can provide high ionic conductivity and improve cycle characteristics. The organic solvent can be a carbonate compound, a carboxylate compound, an ether compound, a 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), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene 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.

[0076] One embodiment of the present application further provides a method for preparing the above-mentioned negative electrode material, comprising:

[0077] The first step is to provide a substrate, wherein the substrate has holes.

[0078] In some embodiments, providing a substrate includes subjecting a substrate precursor to high-temperature treatment, activation, and acid washing to obtain a porous substrate. For example, a carbon substrate is subjected to high-temperature carbonization of a carbon source precursor to obtain a carbon-based carbonized material. The carbon-based carbonized material is mixed with an activator, and then subjected to high-temperature activation and acid washing to obtain a porous carbonaceous material, i.e., a porous carbon substrate.

[0079] In some embodiments, the carbon source precursor includes one or more of a biomass-based precursor, a resin-based precursor, a coal-based precursor, a petroleum-based precursor, and a polymer-based precursor. The biomass precursor may include one or more of coconut shell, straw, fruit shell, sugar, starch, and bamboo; the resin-based precursor may include one or more of phenolic resin and epoxy resin; the coal-based precursor may include one or more of coal tar, needle coke, and anthracite; the petroleum-based precursor may include one or more of petroleum coke, asphalt, and heavy oil; and the polymer-based precursor may include one or more of polyfurfural, polyimide, polyacrylonitrile, polyvinylidene fluoride, polystyrene, and polyvinyl chloride.

[0080] In some embodiments, the carbonization process is carried out under an inert atmosphere, wherein the oxygen content is maintained at less than or equal to 500 ppm, and the pressure in the furnace chamber of the carbonization process is maintained at 1 torr to 200 torr. Wherein, the inert atmosphere gas includes one or more of nitrogen, helium, neon, argon, krypton and xenon. In some embodiments, the carbonization temperature is 400°C to 2000°C, preferably 600°C to 1200°C. In some embodiments, the carbonization time is 1 h to 48 h, preferably 2 h to 8 h.

[0081] In some embodiments, the activator in the activation process includes one or more of an alkaline activator, a gas activator, a salt activator, and an acidic activator. The alkaline activator may include one or more of potassium hydroxide, sodium hydroxide, sodium carbonate, potassium carbonate, calcium oxide, magnesium oxide, triethylamine, tetraethylamine, pyridine, and imidazole; the gas activator may include one or more of water vapor, oxygen, and carbon dioxide; the salt activator may include one or more of zinc chloride, potassium chloride, and sodium chloride; and the acidic activator may include one or more of sulfuric acid, phosphoric acid, and nitric acid.

[0082] In some embodiments, the activation treatment temperature during the activation process is 600° C. to 1400° C. In some embodiments, the activation treatment time during the activation process is 2 h to 48 h.

[0083] Step 2: Deposit active substances in the matrix to obtain a core body.

[0084] In some embodiments, depositing the active material in the substrate includes placing the substrate in an atmosphere of a gaseous carrier of the active material, causing the gaseous carrier of the active material to pyrolyze at high temperature, and depositing the active material on the substrate. Taking a carbon substrate and silicon particles as an example, since the carbon substrate has pores and silicon particles can be obtained by pyrolysis of a silicon source gas at high temperature, the carbon substrate is placed in an atmosphere of a silicon source gas and deposited at high temperature in the silicon source gas atmosphere, so that some silicon particles are deposited in the pores of the carbon substrate to obtain a silicon-carbon composite material. The silicon source gas may include one or more of monosilane, disilane, isosilane, and trisilane. In some embodiments, the concentration of the silicon source gas is 10% to 60%. In some embodiments, the deposition temperature is 400°C to 700°C, and the deposition time is 1 hour to 48 hours.

[0085] Step 3: Mix the core with a reaction gas and heat treat it. The reaction gas includes a carbon source gas, hydrogen, and an inert gas. The volume percentage of hydrogen in the reaction gas is greater than or equal to 5% and less than or equal to 30%. The heat treatment temperature is greater than or equal to 500°C and less than or equal to 800°C, and the heat treatment time is greater than or equal to 2 hours and less than or equal to 48 hours, thereby obtaining a negative electrode material. For example, the volume percentage of hydrogen in the reaction gas can be 5%, 10%, 15%, 20%, 25%, 30%, or any value within the range of any two of the above values. The heat treatment temperature can be 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or any value within the range of any two of the above values. The heat treatment time can be 2 hours, 6 hours, 12 hours, 18 hours, 24 hours, 30 hours, 36 hours, 42 hours, 48 ​​hours, or any value within the range of any two of the above values.

[0086] Taking silicon-carbon composites as an example, by mixing the composite with a carbon source gas and heat-treating it, a negative electrode material can be obtained in which the surface of the carbon substrate, or the surface of silicon particles attached to the carbon substrate, is coated with a carbon coating. The carbon coating has methyl, methine, and methylene groups. Further heat treatment in an appropriate hydrogen atmosphere can regulate the concentration of atomic or ionic hydrogen in the reaction gas, demonstrating both hydrogenation and dehydrogenation processes. Hydrogenation causes active hydrogen atoms or ions to react with groups on the carbon material's surface, such as the conversion of methine and methylene groups to methyl groups. Dehydrogenation at high temperatures causes methyl groups to gradually lose hydrogen atoms and transform into methine and methylene groups, while the methine and methylene groups also lose hydrogen. By controlling the hydrogen concentration within an appropriate range, the content of methyl, methylene, and methine groups in the carbon coating of the negative electrode material can be adjusted. In this way, the carbon coating layer is coated on the surface of the carbon matrix, or on the surface of the silicon particles attached to the surface of the carbon matrix. The stability and hydrophobicity of the negative electrode material are adjusted by the methyl structure, and the reactivity and conductivity of the negative electrode material are adjusted by the methylene or methine structure, which is beneficial to improve the problems of structural instability and poor conductivity of the silicon-carbon composite negative electrode material.

[0087] Controlling the volume fraction of hydrogen, the heat treatment temperature and the heat treatment time within the above ranges can control the content of methyl, methylene and methine on the carbon coating layer of the obtained negative electrode material, thereby facilitating the acquisition of a negative electrode material whose methyl, methylene and methine contents conform to the preset relationship and range. In the above preparation process, when there is a lack of hydrogen in the atmosphere, the methyl groups in the material mainly come from the cracking of the carbon source, and the overall content is relatively low; when the proportion of hydrogen in the provided atmosphere is too small, the hydrogenation process in the reaction system is weak, and the low hydrogen concentration is conducive to breaking the original thermodynamic equilibrium, which is more conducive to the occurrence of the dehydrogenation process, resulting in a lower methyl content. When the proportion of hydrogen in the provided atmosphere is too large, the methyl ratio in the material increases due to the strong hydrogenation process. In addition, when the heat treatment temperature is too low, or when the heat treatment time is too short, the carbon source gas cracking is not sufficient. Since the carbon source gas cracking reaction fails to fully proceed to the expected stage, intermediates or by-products containing methyl groups are produced. These methyl groups generally originate from the incomplete cracking process of the carbon source gas molecules and are retained in the product under the influence of the reaction conditions, which leads to an increase in the methyl ratio. When the heat treatment temperature is too high, the dehydrogenation process is more obvious, which reduces the methyl content. Excessively high temperature may also cause the internal structure of the carbon material to become too fragile and the stability to decrease.

[0088] In some embodiments, the volume proportion of the carbon source gas in the reaction gas is greater than or equal to 10% and less than or equal to 50%, and the volume proportion of the inert gas in the reaction gas is greater than or equal to 20% and less than or equal to 85%.

[0089] In some embodiments, the carbon source gas includes one or more of methane, acetylene, ethylene, ethane, propane, propylene, benzene, toluene, xylene, ethanol, cyclohexane, methanol, and acetone.

[0090] In some embodiments, the inert gas includes one or more of nitrogen, helium, neon, argon, and krypton.

[0091] The present invention will be explained below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to explain the present invention and are not to be construed as limiting the present invention. Unless otherwise indicated, the reagents, software, and instruments not specifically described in the following examples are all conventional commercially available products or open source.

[0092] Example 1:

[0093] This example provides a negative electrode material and a lithium-ion battery based on the negative electrode material.

[0094] (1) The preparation method of the negative electrode material includes:

[0095] S1. Place a certain amount of fruit shells in a carbonization furnace and carbonize them at 900 °C for 8 h to obtain carbon-based carbonized materials;

[0096] S2, placing the carbon-based carbonized material obtained in S2 in a mixed atmosphere of 15% water vapor and 85% nitrogen for activation treatment at an activation temperature of 900°C and an activation time of 24 h, ultrasonically stirring the activated material in a 40% hydrochloric acid aqueous solution, vacuum filtering, and washing with pure water until the pH is neutral. After drying, a carbon substrate with pores is obtained;

[0097] S3, transferring the porous carbon substrate obtained in S2 to a CVD device, maintaining the silane concentration at 25%, the reaction temperature at 550°C, and the reaction time at 10 h to obtain a silicon-carbon composite material;

[0098] S4. Place the silicon-carbon composite material obtained in step S3 in a reactor, introduce a reaction gas with a composition of nitrogen: acetylene: hydrogen = 70%: 15%: 15%, and perform heat treatment at a temperature of 700°C for 4 hours. Screen and grade the obtained material to obtain a negative electrode material.

[0099] (2) A method for preparing a lithium-ion battery based on the above-mentioned negative electrode material includes:

[0100] The above-mentioned negative electrode materials, conductive carbon black and carboxymethyl cellulose were thoroughly stirred in a mixed solution of deionized water and anhydrous ethanol in a mass ratio of 7:2:1, evenly coated on copper foil, vacuum-dried at 100°C, and cut into circular electrode sheets. Subsequently, lithium metal was used as the counter electrode, assembled into button batteries in a glove box and tested. The separator used was Celgard C2400, and the electrolyte was a 1.0 M LiPF6 solution of EC, PC and DEC (volume ratio of EC:PC:DEC=3:1:6).

[0101] Example 2:

[0102] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=50%:40%:10%.

[0103] Example 3:

[0104] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=50%:20%:30%.

[0105] Example 4:

[0106] The difference from Example 1 is that in S4, the heat treatment temperature is adjusted to 600°C.

[0107] Example 5:

[0108] The difference from Example 1 is that in S4, the heat treatment time is adjusted to 8 h.

[0109] Example 6:

[0110] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:propane:hydrogen=70%:15%:15%.

[0111] Example 7:

[0112] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen: cyclohexane: hydrogen = 70%: 15%: 15%, and the cyclohexane in the mixed gas is connected to the reactor through an external bubbling device; and the heat treatment time is adjusted to 8 h.

[0113] Example 8:

[0114] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=85%:10%:5%.

[0115] Example 9:

[0116] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=30%:40%:30%.

[0117] Comparative Example 1:

[0118] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene=75%:25%.

[0119] Comparative Example 2:

[0120] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=25%:25%:50%.

[0121] Comparative Example 3:

[0122] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=50%:25%:25%, and the heat treatment temperature is adjusted to 400°C.

[0123] Comparative Example 4:

[0124] The difference from Example 1 is that in S4, the composition of the reaction gas is adjusted to nitrogen:acetylene:hydrogen=50%:25%:25%, and the heat treatment time is adjusted to 1 h.

[0125] The performance of the negative electrode materials obtained in Examples 1-9 and Comparative Examples 1-4 was tested using the following method:

[0126] Test method for the total pore volume and proportion of the matrix and the negative electrode material after removing the active substance: The total pore volume of the matrix refers to the total pore volume per unit mass of the matrix, and the total pore volume of the matrix can be measured by gas adsorption measurement. Nitrogen adsorption is a technique that characterizes the porosity and pore size distribution of a material by condensing gas in the pores of a solid. As the pressure increases, the gas first condenses in the pores with the smallest diameter, and the pressure increases until the saturation point is reached, at which all the pores are filled with liquid. The nitrogen pressure is then gradually reduced to evaporate the liquid from the system. Analysis of the adsorption and desorption isotherms makes it possible to determine the pore volume and pore size distribution, as well as the proportion of the respective pore volumes of micropores, mesopores and macropores in the total pore volume. Among them, the method of removing active substances from the negative electrode material (taking silicon material as an example) is as follows: add concentrated nitric acid with a concentration of 1 M to the negative electrode material and soak it for 4 hours, then add 20% mass fraction HF acid solution drop by drop into the negative electrode material, which will produce yellow smoke. Repeat the addition several times until no yellow smoke is produced in the solution; finally, use concentrated nitric acid with a concentration of 1 M to digest the residue, then wash and dry it to obtain the negative electrode material after removing the silicon material.

[0127] The test method of the median particle size of the negative electrode material is as follows: the particle size distribution range of the negative electrode material is tested by Malvern laser particle size analyzer (Mastersizer3000), and the volume basis cumulative particle size distribution of the particle size distribution is measured by laser diffraction method. V 10 represents the particle size corresponding to the cumulative particle size distribution percentage of the powder reaching 10%, D V 50 represents the particle size corresponding to when the cumulative particle size distribution percentage reaches 50%, D V 90 represents the particle size corresponding to when the cumulative particle size distribution percentage reaches 90%.

[0128] Carbon coating thickness: The material was sectioned using a FIB-SEM device. Ten particles were randomly selected in the SEM, and the carbon coating thickness of each particle was measured three times to obtain the average thickness of the carbon coating.

[0129] Powder conductivity test: This method uses the four-probe method to determine the volume resistivity of the sample. Using Mitsubishi Chemical's MCP-PD51 powder conductivity meter, the conductivity of the powder can be measured at five pressure points: 4 kN, 8 kN, 12 kN, 16 kN, and 20 kN. A computer then automatically calculates the powder's resistivity.

[0130] Carbon mass content test method: Using Germany's Bruker's G4 ICARUS HF infrared carbon and sulfur analyzer, the sample is burned in a high-temperature, oxygen-rich state. The carbon element it contains is oxidized to 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.

[0131] Silicon mass content test method: Nanyang Xinyu's SA2-9-17TP box-type atmosphere furnace is used to burn in an oxygen atmosphere to make the silicon in the sample react into silicon dioxide. The carbon is burned and converted into carbon dioxide and discharged. The silicon content is calculated by weighing.

[0132] Hydrogen mass content test method: Using the German Verder ONH 2000 oxygen, nitrogen and hydrogen element analyzer, the method for detecting the mass proportion of hydrogen in the negative electrode material includes heating and melting the negative electrode material and flux (nickel) in a graphite crucible in a pulse electrode furnace under inert gas (He or Ar) to release hydrogen; after the hydrogen passes through rare earth copper oxide, water vapor is generated, and the water infrared detector outputs the water electrical signal, which is compared with the electrical signal of the known hydrogen standard substance / sample to obtain the hydrogen content.

[0133] Specific surface area test method: The specific surface area of ​​the powder sample was tested by gas adsorption method (nitrogen adsorption multi-point BET) at low temperature controlled by liquid nitrogen cooling using Micromeritics TriStar 3000 & 3020 specific surface area and pore size analyzers from the United States.

[0134] Testing method for average particle size of material particles:

[0135] The material particles are observed by field emission scanning electron microscopy or transmission electron microscopy, and the particle sizes of 5-10 material particles are directly measured using a scale. The average value of the particle sizes is taken as the final average particle size of the material particles.

[0136] The present application also conducted a gas production test on the negative electrode materials obtained in Examples 1-9 and Comparative Examples 1-4: (1) 5 g of the negative electrode material was placed in a sealed container, and deionized water was added until the remaining volume above the container was 60 mL; (2) the container was sealed, mixed evenly, and stored at room temperature for one day; (3) after 24 h, the container was shaken again to disperse the precipitate into the liquid; (4) the lid of the container was opened, and the hydrogen concentration was detected with a hydrogen detector and converted to cc / (kg·day).

[0137] The test method for the stretching vibration peak areas S1, S2 and S3 of the negative electrode material is described in the specification and will not be repeated here.

[0138] The present application further conducted electrochemical performance tests on the lithium-ion batteries obtained in Examples 1-9 and Comparative Examples 1-4, mainly including: using the Blue Electric CT2001A battery testing system, the first coulombic efficiency was tested according to the test method described in the silicon-carbon national standard GB / T38823-2020, and the capacity retention rate was tested according to the test method described in the national standard GB / T36276-2018, and the expansion rate of the negative electrode sheets in these lithium-ion batteries was detected after 50 charge and discharge cycles. The expansion rate was calculated by: observing the electrode sheet with a scanning electron microscope, and obtaining the average particle size of the silicon particles before lithium insertion as D1 according to the corresponding scale; disassembling the buckle after the charge and discharge cycle, cleaning the electrode sheet with DEC, and observing the electrode sheet with a scanning electron microscope, and obtaining the average particle size of the silicon particles after lithium insertion as D2 according to the corresponding scale; considering the silicon particles as a spherical structure, the expansion rate is D2 / D1-1.

[0139] Please refer to Table 1 for some of the preparation conditions of Examples 1-9 and Comparative Examples 1-4 of the present application, and please refer to Table 2 for the above test results.

[0140] Table 1. Some preparation conditions of Examples 1-9 and Comparative Examples 1-4 of this application

[0141]

[0142] Table 2. Test results of negative electrode materials and lithium-ion batteries of Examples 1-9 and Comparative Examples 1-4 of the present application

[0143]

[0144] The negative electrode materials of Examples 1-9 of the present application meet the following requirements: , has a relatively low specific surface area and gas production, and expands less during the charge and discharge cycle. The carbon coating layer in these negative electrode materials is provided with a suitable content of methyl, methylene and methine, which helps to improve the stability and conductivity of the carbon matrix and reduce the gas production of silicon particles and side reactions with the electrolyte, so that the lithium ion batteries of Examples 1-9 have a higher capacity and first coulombic efficiency, and have good cycle stability during the charge and discharge cycle.

[0145] Compared with Example 1:

[0146] Comparative Example 1: Due to the lack of hydrogen in the mixed gas during carbon coating, the methyl groups in the coating layer mainly come from the cracking of the carbon source, and the overall content is relatively low, resulting in the negative electrode material <0.1. In the carbon coating layer, the methyl content is too low, which affects the chemical stability of the resulting negative electrode material, significantly increases the gas production of silicon particles, and significantly increases the expansion of the electrode during the charge and discharge cycle, affecting the electrochemical performance of the resulting lithium-ion battery.

[0147] In Comparative Example 2, since the mixed gas contains excessive hydrogen during carbon coating, the hydrogenation effect of hydrogen dominates during the coating process, causing excessive methine and methylene groups to convert into methyl groups, resulting in excessive methyl groups formed on the carbon coating layer, while the content of methylene and methine groups is relatively insufficient. When the molecular weight is greater than 0.5, the reactivity of the group decreases, and the electron transmission path is relatively reduced, resulting in a decrease in the conductivity of the negative electrode material, affecting the electronic conductivity of the obtained lithium-ion battery, thereby significantly reducing the capacity and first coulombic efficiency of the battery prepared with the negative electrode material.

[0148] In Comparative Examples 3 and 4, the heat treatment temperature and time were insufficient during the carbon coating process, resulting in insufficient cracking of the carbon source gas. Since the carbon source gas cracking reaction failed to proceed to the expected stage, intermediates or by-products containing methyl groups were generated and retained in the product under the influence of the reaction conditions, thereby increasing the proportion of methyl groups in the resulting coating layer. When the ratio is greater than 0.5, the reactivity of the group decreases and the electron transmission path is relatively reduced, resulting in a decrease in the conductivity of the negative electrode material, affecting the electronic conductivity of the obtained lithium-ion battery, thereby reducing the capacity and the first coulombic efficiency.

[0149] The above results show that the carbon coating layer of the negative electrode material includes methyl, methylene and methine. The stretching vibration peak areas S3, S2 and S1 of the methyl, methylene and methine on the carbon coating layer of the negative electrode material are quantitatively analyzed by infrared spectroscopy, and the ... This facilitates the structural stability and hydrophobicity of the methyl group, as well as the reactivity of the methylene or methine groups, thereby improving the chemical stability and conductivity of the resulting negative electrode material, and thereby increasing the capacity and initial coulombic efficiency of the resulting secondary battery. This overcomes the prior art issues of unstable negative electrode material matrices and exposure of active materials on the matrix surface, which impact battery cycle stability and conductivity.

[0150] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application should not depart from the spirit and scope of the technical solutions of the present application.

Claims

1. A negative electrode material, characterized in that: The negative electrode material comprises a core and a carbon coating covering at least a portion of the surface of the core, the core comprising a matrix and an active substance, the matrix having pores, at least a portion of the active substance being disposed within the pores of the matrix, the average particle size of the active substance being 0.1 nm to 100 nm, and the specific surface area of ​​the negative electrode material being 1 m 2 / g to 3 m 2 / g; The negative electrode material was tested by infrared spectroscopy at a wave number of 2896±10 cm -1 The stretching vibration peak at 2924 ± 10 cm is the stretching vibration peak of -CH, and the area of ​​the stretching vibration peak of -CH is S1; at the wave number 2924 ± 10 cm -1 and 2853±10 cm -1 The stretching vibration peak at is the stretching vibration peak of -CH2, and the sum of the areas of the stretching vibration peaks of -CH2 is S2; at wave number 2960±10cm -1 The stretching vibration peak at is the stretching vibration peak of -CH3, and the area of ​​the stretching vibration peak of -CH3 is S3; the S1, S2 and S3 satisfy: ; Based on the mass of the negative electrode material, the mass proportion of hydrogen element in the negative electrode material is less than or equal to 1%, the mass proportion of carbon element in the negative electrode material is 30% to 65%, and the mass proportion of silicon element in the negative electrode material is 30% to 65%.

2. The negative electrode material according to claim 1, wherein The substrate satisfies at least one of the following conditions: (1) The specific surface area of ​​the substrate is 500 m 2 / g to 2000 m 2 / g; (2) The total pore volume of the matrix is ​​0.5 cm 3 / g to 2.0 cm 3 / g; (3) The average pore size of the matrix is ​​0.1 nm to 8 nm.

3. The negative electrode material according to claim 1, wherein The thickness of the carbon coating layer is 0.1 nm to 2000 nm.

4. The negative electrode material according to claim 1, wherein The negative electrode material satisfies at least one of the following conditions: (1) The powder conductivity of the negative electrode material is 0.05 S / m to 5 S / m; (2) Median particle size D of the negative electrode material V 50 is 3 μm to 15 μm.

5. The negative electrode material according to claim 1, wherein The matrix includes one or both of carbon material and non-carbon material; The carbon material includes one or more of artificial graphite, natural graphite, amorphous carbon, activated carbon, mesophase carbon microbeads, carbon nanotubes, carbon nanofibers and graphene; The non-carbon material includes at least one of metal oxides, silicides, silicates, phosphates, titanates and aluminum borates; the metal oxides include one or more of aluminum oxide, zirconium oxide, germanium dioxide and manganese dioxide; the silicides include one or more of silicon carbide and silicon nitride; the silicates include one or more of cordierite, mullite and zeolite; the phosphates include one or more of aluminum phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate and lanthanum phosphate; the titanates include one or more of calcium titanate, iron titanate, lithium titanate and barium titanate.

6. The negative electrode material according to claim 1, wherein The active material includes one or more of Si, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P and Cu.

7. The negative electrode material according to claim 1, wherein The carbon material of the carbon coating layer includes one or more of amorphous carbon and graphitized carbon.

8. The negative electrode material according to claim 1, wherein The matrix includes a carbon matrix, the active material includes a silicon material, and the silicon material includes one or more of amorphous silicon, crystalline silicon, silicon oxide, silicon alloy, and a composite of crystalline silicon and amorphous silicon.

9. A negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, characterized in that: The negative electrode active material layer comprises the negative electrode material according to any one of claims 1 to 8.

10. A secondary battery, characterized in that: Comprising the negative electrode sheet as claimed in claim 9.

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