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 carbon matrix and an active material. The negative electrode material further comprises a boron element and an oxygen element, wherein the mass fraction of the boron element in the negative electrode material is A%, the mass fraction of the oxygen element in the negative electrode material is B%, 0<A<2, and 2A<B<4. The negative electrode material provided by the present application can effectively increase the conductivity, initial coulombic efficiency and specific capacity of the negative electrode material while reducing the gas evolution and expansion rate.

Description

Negative electrode material, negative electrode sheet and secondary battery TECHNICAL FIELD

[0001] The present application relates to the field of new energy materials, in particular to a negative electrode material, a negative electrode sheet and a secondary battery. BACKGROUND

[0002] The negative electrode material is one of the key materials for realizing high capacity and long cycle of a secondary battery (such as a lithium ion battery). Graphite is the most widely used negative electrode material, which has the advantages of high efficiency and stable charge-discharge platform. However, the lower specific capacity hinders the further application of graphite. Compared with graphite, elemental silicon is considered as an ideal negative electrode material that can replace graphite due to its higher theoretical specific capacity and suitable working voltage. However, the large volume expansion in the alloying / de-alloying reaction of elemental silicon and lithium will cause rapid capacity decay of the electrochemical device (for example, a lithium ion battery).

[0003] In the related art, silicon is dispersed in a porous carbon material to prepare a silicon-carbon negative electrode material, so as to alleviate the volume expansion of silicon. However, the above-mentioned silicon-carbon negative electrode material has low relative particle strength and poor electrical conductivity, and still has large volume expansion. In addition, LiPF6 electrolyte will decompose to produce HF during battery cycling, and the existing silicon-carbon negative electrode material will react with HF, resulting in a negative electrode material with a high gas production value. SUMMARY

[0004] The present application provides a negative electrode material to solve at least one of the above technical problems.

[0005] To achieve the above-mentioned purpose, the present application provides a negative electrode material, which comprises a carbon matrix and an active material, and further contains boron and oxygen, wherein the mass fraction of boron in the negative electrode material is A%, the mass fraction of oxygen in the negative electrode material is B%, 0<A<2, and 2A<B<4.

[0006] The present application also provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector, and the negative electrode active material layer comprises the above-mentioned negative electrode material.

[0007] The present application also provides a secondary battery, which comprises the above-mentioned negative electrode sheet.

[0008] Compared with the prior art, the negative electrode material of the application comprises a carbon matrix and specific contents of boron elements and oxygen elements, sp2 hybridization of boron atoms can provide empty electron orbits for the π-π conjugated system of the carbon material, increase the flowability of the conjugated electrons of the carbon matrix, and thus increase the electrical conductivity of the negative electrode material; the content of oxygen elements is controlled because the tetrahedral structure formed by the S and P electron orbits of oxygen atoms can cause the carbon atoms in the carbon matrix to deviate from the π-π conjugated plane, reduce the π-π conjugated carbon atoms, and reduce the electronic conductivity of the negative electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0009] Fig. 1 is a process flow chart of the preparation method of the negative electrode material provided by an embodiment of the application.

[0010] Fig. 2 is a structural schematic diagram of an electrochemical device provided by an embodiment of the application.

[0011] Main element symbol Explanation electrochemical device 100 positive electrode sheet 10 negative electrode sheet 20 separator film 30 electrolyte 40

[0012] The following specific embodiments will further illustrate the application in combination with the above-mentioned drawings. DETAILED DESCRIPTION

[0013] The embodiments of the application are described in detail below. The embodiments described below with reference to the drawings are exemplary and are only used to explain the application and cannot be understood as a limitation on the application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as understood by those skilled in the art in the technical field of the application; the embodiments of the application and the features in the embodiments can be combined with each other without conflict; in the following description, a lot of specific details are set forth in order to fully understand the application, and the described embodiments are only part of the embodiments of the application, not all the embodiments.

[0014] An embodiment of the application provides a negative electrode material comprising a carbon matrix and an active material, and further comprising boron elements and oxygen elements, wherein, calculated in terms of mass percentage, the content of the boron elements in the negative electrode material is A%, the content of the oxygen elements in the negative electrode material is B%, and the contents of the boron elements and the oxygen elements satisfy the following relationship: 0<A<2, 2A<B<4.

[0015] In the related art, the active material of the negative electrode material is a silicon-based active material, which mainly plays a role of high-capacity lithium ion storage, can absorb and release lithium ions, and thus realizes the charging and discharging process of an electrochemical device (such as a battery). The active material is usually filled in a carbon matrix to improve a series of defects of the active material itself. The carbon matrix mainly plays a role of skeleton support, can stabilize the volume expansion and contraction of the active material, and thus reduces the volume expansion of the active material. In addition, since the carbon matrix itself has good electrical conductivity, it can provide an electrical conduction path, which helps to improve the cycle stability and electrochemical performance of the negative electrode material. However, such a negative electrode material still needs to be improved in terms of improving the volume expansion and electrical conductivity. In addition, the relative particle strength of the negative electrode material is low, and the problem of high gas production value exists in the cycle process of the battery prepared by the negative electrode material.

[0016] Since the hybridization modes of boron atoms mainly include sp2 and sp3, the sp2 hybridization refers to the hybridization of one 2s orbital and two 2p orbitals of a boron atom into three equal-energy sp2 hybridization orbitals to form a planar triangular structure, and the sp3 hybridization refers to the hybridization of one 2s orbital and three 2p orbitals of a boron atom into four equal-energy sp3 hybridization orbitals to form a regular tetrahedral structure. Therefore, the sp2 hybridization of boron atoms is formed by a specific content of boron elements, the sp2 hybridization of boron atoms can provide empty electron orbitals for the π-π conjugated system of the carbon matrix, promote electron transmission, improve the electrical conductivity of the negative electrode material, and thus increase the initial coulomb efficiency of the negative electrode material.

[0017] In the present application, the content of boron elements and oxygen elements is also regulated, so that the content of boron elements and oxygen elements satisfies the following relationship: 0

[0018] When the content of boron element is too high (i.e., A≥2), the excess boron atoms are difficult to be incorporated into the carbon matrix of the negative electrode material to form sp2 hybridization, thereby causing an increase in sp3 hybridization, which can destroy the π-π conjugated system of the carbon matrix, hinder the flow of electrons in the conjugated system, and cause the conductivity and the initial coulombic efficiency of the negative electrode material to decrease. In addition, when the content of boron is too high, the amount of boron oxide also increases, and an excessive mass fraction of boron oxide can cause the specific capacity of the negative electrode material to decrease.

[0019] When the content of oxygen element is too high (i.e., B≥4), on the one hand, the tetrahedral structure formed by the S and P electron orbits of the oxygen atoms can cause the carbon atoms in the carbon matrix to deviate from the π-π conjugated plane, resulting in a decrease in the π-π conjugated carbon atoms and a decrease in the electronic conductivity of the negative electrode material. On the other hand, too much oxygen element can cause the boron element in the negative electrode material to exist in the form of boron oxide, and an excessive mass fraction of boron oxide can cause the specific capacity of the negative electrode material to decrease. When the content of oxygen element is too low relative to the content of boron element (i.e., B≤2A), the relative content of the generated boron oxide is low, and it is difficult to form a stable lithium borate conductive layer, which can cause the conductivity and the initial coulombic efficiency of the obtained negative electrode material to decrease.

[0020] In some embodiments, the negative electrode material includes boron carbide. In the negative electrode material of the present application, part of the boron element forms boron carbide with the carbon element, and the boron carbide distributed in the negative electrode material is beneficial to improving the strength of the carbon matrix skeleton, and further improving the relative particle strength and anti-expansion performance of the negative electrode material.

[0021] In some embodiments, the negative electrode material includes boron oxide. In the negative electrode material of the present application, part of the boron element can form boron oxide with the oxygen element, and the generated boron oxide has the following two aspects: on the one hand, during the cycle of the battery prepared by the negative electrode material, the boron oxide can form a stable lithium borate conductive layer on the surface of the negative electrode material, which can promote the transmission of lithium ions and further improve the initial coulombic efficiency of the negative electrode material; on the other hand, the boron oxide on the surface of the negative electrode material can consume HF generated by the decomposition of LiPF6 electrolyte, thereby reducing the corrosion of the active material (such as silicon) in the negative electrode material by HF, reducing the gas production of the negative electrode material, and reducing the expansion of the battery prepared by the negative electrode material. The X-ray photoelectron spectrometer (Thermo Scientific K-Alpha) can be used to test the existence of B-C and / or B-O bonds in the negative electrode material. After the sample is pressed into a tablet, it is attached to the sample disc, and the sample is placed in the sample chamber of the Thermo Scientific K-Alpha XPS instrument. When the pressure in the sample chamber is better than 5x10-7mbar, the sample is sent into the analysis chamber, the spot size is 400μm, the working voltage is 12kV, and the filament current is 6mA; the full-spectrum scanning pass energy is 150eV, and the step is 1eV; the narrow-spectrum scanning pass energy is 50eV, and the step is 0.1eV.

[0022] In some embodiments, the relative particle strength Cx of the negative electrode material is in the range of 100 MPa to 150 MPa. For example, Cx can be 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa, 150 MPa, or any value within the range defined by any two of the above values. The relative particle strength Cx of the negative electrode material is positively correlated with the mass fraction of boron in the negative electrode material within a certain range. Within the above range, the particle structure of the negative electrode material is neither too loose nor too dense. The moderate relative particle strength allows a certain degree of expansion while maintaining the overall structure of the negative electrode material, thereby reducing the damage caused by expansion and improving the anti-expansion performance of the negative electrode material.

[0023] In some embodiments, the median particle size D50 of the negative electrode material is in the range of 6.0 μm to 15.0 μm. Further, the median particle size D50 of the negative electrode material can be in the range of 7.0 μm to 12.0 μm. For example, the median particle size D50 of the negative electrode material can be 6.0 μm, 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, 11.0 μm, 12.0 μm, 13.0 μm, 14.0 μm, 15.0 μm, or any value within the range defined by any two of the above values. Controlling the median particle size of the negative electrode material within the above range helps to improve the consistency of the particle size of the negative electrode material, thereby improving the performance stability of the negative electrode material. It also effectively shortens the diffusion path of lithium ions, ensures the time of lithium ion insertion and extraction, and enables the negative electrode material to achieve a state of rapid and sufficient lithium insertion, thereby improving the charge and discharge performance of the battery.

[0024] In some embodiments, the particle size distribution of the negative electrode material satisfies the following relationship: the particle size distribution satisfies 0.6≤(D90-D10) / D50≤5. Specifically, it can be 0.6, 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 defined by any two of the above values. When the particle size distribution of the negative electrode material is within the above range, the large particles with large particle sizes and the small particles with small particle sizes of the negative electrode material can cooperate with each other, and the small particles can fill the pores between the large particles, thereby improving the tap density of the negative electrode material.

[0025] It should be noted that the volume-based cumulative particle size distribution D10 measured by the laser diffraction method through the Malvern 3000 laser particle size analyzer represents the particle size corresponding to the cumulative particle size distribution percentage of 10%, D50 represents the particle size corresponding to the cumulative particle size distribution percentage of 50%, and D90 represents the particle size corresponding to the cumulative particle size distribution percentage of 90%.

[0026] In some embodiments, the specific surface area of the negative electrode material is 0.5 m 2 / g to 10 m 2 / g. Further, the specific surface area of the negative electrode material can be 0.5 m 2 / g to 5 m 2 / g. Exemplarily, the specific surface area of the negative electrode material can be 0.5 m 2 / g, 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 a range defined by any two of the above values. Understandably, when the specific surface area of the negative electrode material is large, the SEI film consumes excessive lithium salt, and the volume effect easily causes electrical disengagement between the particles, resulting in a decrease in the reversible capacity and coulombic efficiency of the battery prepared by the negative electrode material. Therefore, the specific surface area of the negative electrode material in the embodiments of the present application is small, which can effectively improve the first discharge specific capacity and the first coulombic efficiency of the battery.

[0027] In some embodiments, the specific surface area of the negative electrode material is measured by the ASAP2046 specific surface area and pore size analyzer equipment of the American Micromeritics Company.

[0028] In some embodiments, the tap density of the negative electrode material is 0.8 g / cm 3 to 1.2 g / cm 3 . Exemplarily, the tap density of the negative electrode material can be 0.8 g / cm 3 , 0.9 g / cm 3 , 1.0 g / cm 3 , 1.1 g / cm 3 , 1.2 g / cm 3 , or any value within a range defined by any two of the above values. Within the above tap density range, the negative electrode material has a suitable charge transfer rate and electrolyte contact area, and also has a suitable good structural stability.

[0029] In some embodiments, the CARVER4350.22 powder tap density instrument of the American Micron is used to place a sample of a specified mass in a mold, apply a pressure of 1.0T, maintain the pressure for 30s, then remove the pressure to test the thickness, and calculate the tap density of the negative electrode material.

[0030] In some embodiments, the specific capacity of the negative electrode material is 1600 mAh / g to 2200 mAh / g. Further, the specific capacity of the negative electrode material can be 1800 mAh / g to 2200 mAh / g. Within the above specific capacity range, the negative electrode material has good electrochemical performance and energy storage capability. The specific capacity range generally indicates that the negative electrode material has good absorption and release capability for lithium ions, while maintaining the stability and reliability of the material.

[0031] In some embodiments, the electrical conductivity of the negative electrode material is 0.25 S / cm to 1.21 S / cm. That is, the powder electrical conductivity P value is 0.25 to 1.21. Exemplarily, the powder electrical conductivity of the negative electrode material can be specifically 0.25 S / cm, 0.35 S / cm, 0.45 S / cm, 0.6 S / cm, 0.8 S / cm, 1.0 S / cm, 1.21 S / cm, or any value within the range formed by any two of the above values. It can be understood that the electrical conductivity of the negative electrode material within the above range is conducive to more efficient transmission of lithium ions by the negative electrode material, so that more lithium ions can participate in the reversible charge and discharge reaction during the first charge and discharge process, reducing the irreversible capacity loss, thereby improving the first coulombic efficiency of the negative electrode material.

[0032] In some embodiments, the first coulombic efficiency of the negative electrode material is 87% to 94%. Further, the first coulombic efficiency of the negative electrode material can be 90% to 94%. Within this range, the negative electrode material can achieve a high utilization rate during the first charging process, reducing the loss during charging, thereby improving the energy conversion efficiency of the battery, which means that the negative electrode material has stable electrochemical performance and long-term cycle stability.

[0033] In some embodiments, the gas production value of the negative electrode material is less than or equal to 25.3 cm 3 / kg / d. It can be understood that the gas production value of the negative electrode material within the above range is conducive to the long-term stability and cycle life of the negative electrode material, because less gas production can reduce bubble formation and electrolyte loss, while reducing the safety risk related to gas release.

[0034] In some embodiments, the active material includes a silicon-based active material, and the silicon-based active material includes at least one of crystalline silicon, amorphous silicon, crystalline silicon and amorphous silicon composite particles, and further, the silicon-based active material can be amorphous silicon. Amorphous silicon isotropically expands during lithium intercalation, which can reduce the collapse of the pore structure in the carbon matrix, inhibit the rapid decay of the specific capacity of the negative electrode material, and improve the lithium intercalation cycle performance of the negative electrode material.

[0035] In some embodiments, the active material includes a silicon-based active material, and the silicon element accounts for 30% to 55% of the mass of the negative electrode material. For example, the silicon element accounts for 30%, 35%, 40%, 45%, 50%, 55% of the mass of the negative electrode material, or any value within the range defined by any two of the above values. When the mass of the silicon element is within the range, the lithium battery formed by the negative electrode material has a higher capacity, i.e., a higher initial specific discharge capacity.

[0036] In some embodiments, the method for detecting the mass of the silicon element in the negative electrode material includes using a Nanyang Xiyu box-type atmosphere furnace (model: SA2-9-17TP) to burn the negative electrode material at a high temperature of 1200°C in an oxygen or air environment. In the process, the carbon in the negative electrode material reacts with oxygen to generate CO2 gas, and the silicon reacts with oxygen to generate SiO2. The material is finally completely converted into SiO2. According to the mass of the reaction product SiO2, the mass of the silicon element in the original sample can be calculated, and thus the content of the silicon element in the negative electrode material can be obtained.

[0037] In some embodiments, the average particle size of the silicon-based active material particles is 0.1 nm to 50 nm. Further, the average particle size of the silicon-based active material particles can be 0.1 nm to 5 nm. For example, the average particle size of the silicon-based active material particles can be 0.1 nm, 1 nm, 2 nm, 5 nm, 10 nm, 20 nm, 25 nm, 30 nm, 40 nm, 45 nm, 50 nm, or any value within the range defined by any two of the above values. The mechanical stress of the silicon-based active material when swelling decreases with the decrease of the particle size, and the size reduction can shorten the electron and ion transport path. At the same time, the size of the silicon-based active material particles decreases, and the gap between adjacent silicon-based active material particles increases, which can reserve space for the swelling of the silicon-based active material. It can be understood that the average particle size of the silicon-based active material particles within the above range can ensure the battery capacity of the lithium ion battery and reduce the irreversible capacity loss. In some embodiments, the average particle size of the active material can be measured by a transmission electron microscope. The particle size of 5 to 10 nanometer silicon material is directly measured by a scale, and the average value of the particle size is the final average particle size of the silicon material.

[0038] In some embodiments, the purity of the silicon-based active material can be greater than 99%. It can be understood that high-purity silicon particles are beneficial to Li-Si alloying with lithium and improve the cycle performance of the lithium ion battery.

[0039] In some embodiments, the active material includes at least one of Si, Sn, P, S, Ge, and Pb. The type of the active material can be selected according to actual needs, and can be an element, a compound, an alloy, etc., which is not limited herein.

[0040] In some embodiments, the morphology of the active material can include at least one of a point, a sphere, an ellipsoid, and a sheet, etc. The morphology of the active material can be selected as needed, and is not limited herein. By way of example, the active material can be silicon particles, and the alloying mechanism of silicon enables the silicon particles to have a high capacity advantage, and the prepared negative electrode material has a high capacity characteristic.

[0041] In some embodiments, the carbon matrix includes at least one of amorphous carbon, graphitized carbon, mesocarbon microbeads, and carbon gel, etc. The amorphous carbon can include at least one of hard carbon, soft carbon, and activated carbon, etc.; and the graphitized carbon can include at least one of graphite and graphitized carbon nanotubes, etc. The type of the carbon matrix can be selected as needed, and is not limited herein. It can be understood that the carbon matrix selected from the above materials can stabilize the volume expansion and contraction of the active material, and provide an electrical conduction path, which helps to improve the cycle stability and electrochemical performance of the negative electrode material.

[0042] In some embodiments, the carbon matrix has a pore structure. Further, the pores of the carbon matrix can include mesopores and micropores, and it can be understood that the pores of the carbon matrix can provide an inlaying space and an expansion space for the active material, thereby improving the structural stability of the negative electrode material. At least part of the active material can be distributed in the pores of the carbon matrix, and due to the high surface area and uniform pore structure of the carbon matrix, the deposition of the active material can be effectively stabilized. The compounding of the active material and the carbon matrix can balance the electrical conductivity and specific capacity of the negative electrode material.

[0043] In some embodiments, the mass percentage of carbon in the negative electrode material is 30% to 55% based on the mass of the negative electrode material. Further, the mass percentage of carbon in the negative electrode material can be 40% to 55%. By way of example, the mass percentage of carbon in the negative electrode material can be 30%, 35%, 40%, 42%, 45%, 50%, 52%, or 55%, etc. Carbon as a conductive material can improve the electrical conductivity of the negative electrode material; and an appropriate amount of carbon content can maintain the structural stability of the material and reduce the structural damage caused by the expansion of the active material (such as silicon particles). By adjusting the carbon content, the electrochemical performance and structural stability of the negative electrode material can be optimized. In some embodiments, the detection method of the mass percentage of carbon in the negative electrode material includes converting carbon into carbon monoxide or carbon dioxide by burning in an oxygen stream, carbon monoxide is oxidized into carbon dioxide at high temperature, and the infrared absorption spectrum of carbon dioxide in the oxygen stream is used for measurement.

[0044] In some embodiments, the negative electrode material has a coating layer, and the material of the coating layer includes at least one of metal oxide, carbon material, amorphous silicon, conductive polymer, fluoride, phosphate, and nitride, etc. Understandably, the coating layer located at the outermost layer of the negative electrode material has good electrical conductivity, which can improve the electrical contact performance between the active material and the current collector, accelerate the transmission speed of electrons, and thus improve the charge and discharge performance of the battery; at the same time, the coating layer can reduce the direct contact between the active material and the electrolyte, reduce the occurrence of side reactions between the active material and the electrolyte, and thus improve the initial efficiency and cycle stability of the battery prepared by the negative electrode material.

[0045] In some embodiments, the material of the coating layer includes metal oxide, and the metal oxide contains at least one of Sn, Ge, Fe, Si, Cu, Ti, Na, Mg, Al, Ca, and Zn, etc.

[0046] In some embodiments, the material of the coating layer includes carbon material, and the carbon material includes at least one of amorphous carbon and graphitized carbon, etc.

[0047] In some embodiments, the material of the coating layer includes conductive polymer, and the conductive polymer includes at least one of polyaniline, polyacetylene, polypyrrole, polythiophene, poly-3-hexylthiophene, poly-p-phenylenevinylene, polypyridine, and polyphenylvinylene, etc.

[0048] In some embodiments, the material of the coating layer includes fluoride, and the fluoride includes at least one of polyvinyl fluoride, sodium fluoride, potassium fluoride, fluorocarbon polymer, fluorosilicon polymer, hexafluorobutyl acrylate, fluorinated ethylene-propylene copolymer, perfluoroalkoxy resin, polychlorotrifluoroethylene, ethylene-chlorotrifluoroethylene copolymer, and polyvinylidene fluoride, etc.

[0049] In some embodiments, the material of the coating layer includes phosphate, and the phosphate includes at least one of magnesium phosphate, calcium phosphate, aluminum phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate, and lanthanum phosphate, etc.

[0050] In some embodiments, the material of the coating layer includes nitride, and the nitride includes at least one of titanium nitride, vanadium nitride, cobalt nitride, nickel nitride, and carbon nitride, etc.

[0051] In actual application, the material of the coating layer can be selected according to actual needs, which is not limited herein. The coating layer can be a single-layer coating layer formed by a single material, a coating layer formed by a combination of multiple materials, a multi-layer coating layer formed by a single material, a multi-layer coating layer formed by multiple materials, and the like. For example, the coating layer can be coated with a carbon material, then coated with a conductive polymer, or coated with a polymer, then coated with an oxide, and the like. The layer structure of the coating layer can be selected according to actual needs, which is not limited herein. It can be understood that the coating layer has a higher density when the coating layer has a multi-layer structure.

[0052] In some embodiments, the thickness of the coating layer is 1 nm to 300 nm. Further, the thickness of the coating layer can be 1 nm to 50 nm, or 1 nm to 30 nm. For example, the thickness of the coating layer can be 1 nm, 50 nm, 150 nm, 200 nm, 250 nm, or 300 nm, or the like.

[0053] In some embodiments, the mass fraction of the coating layer in the negative electrode material is less than or equal to 10%. For example, the mass fraction of the coating layer in the negative electrode material can be 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or the like, or other values within the range, which can be selected according to actual needs, which is not limited herein.

[0054] It can be understood that the coating layer with the above thickness and mass fraction has good electrical conductivity, which can improve the electrical contact performance between the active material and the current collector, accelerate the transmission speed of electrons, and thus improve the charge and discharge of the battery. At the same time, the coating layer in the negative electrode material can reduce the direct contact between the active material and the electrolyte in the negative electrode material, reduce the occurrence of side reactions, and thus improve the initial efficiency and cycle stability of the battery.

[0055] Compared with the prior art, the negative electrode material provided by an embodiment of the present application has the following beneficial effects:

[0056] 1. At least part of the boron elements in the negative electrode material form sp2 hybridization of boron atoms, which can provide empty electron orbits for the π-π conjugated system of the carbon matrix, increase the flowability of the conjugated electrons of the carbon matrix, and thus improve the electrical conductivity of the negative electrode material and effectively improve the initial coulomb efficiency of the negative electrode material.

[0057] 2. At least part of the boron elements in the negative electrode material can form boron carbide with the carbon matrix, improve the relative particle strength of the carbon skeleton in the negative electrode material, and thus improve the anti-swelling performance of the negative electrode material.

[0058] 3. At least part of the boron elements in the negative electrode material also exist in the form of boron oxides. These boron oxides form a stable lithium borate conductive layer on the surface of the negative electrode material during the cycling of the battery prepared from the negative electrode material, on the one hand, to promote the transmission of lithium ions and thus improve the initial coulombic efficiency of the negative electrode material; on the other hand, the boron oxides can consume HF generated by the decomposition of LiPF6 electrolyte, thereby reducing the corrosion of the active material in the negative electrode material by HF and thus reducing the gas production of the negative electrode material and the risk of cycle expansion.

[0059] 4. The content of boron elements and oxygen elements in the negative electrode material is limited, which is conducive to the moderate doping of boron elements and the appropriate generation of boron carbide and boron oxides, so that the negative electrode material has excellent electrochemical performance while having a low expansion rate and a low gas production value.

[0060] An embodiment of the present application also provides a preparation method of the negative electrode material. Referring to FIG. 1, the preparation method comprises the following steps:

[0061] Step S1: acid washing, water washing and drying of the carbonaceous material to obtain a first carbon precursor.

[0062] The acid washing usually uses an acid agent to treat the surface of the carbonaceous material to remove soluble sugars, metal salts, silicon-containing impurities and the like in the carbonaceous material, so as to improve the purity of the carbonaceous material. The surface of the carbonaceous material after acid washing may be left with acidic substances, which need to be further removed by water washing. Then, the water is removed by drying, so that the carbonaceous material reaches a state suitable for subsequent processing and use. Specifically, the water washing can be washing the carbonaceous material with deionized water and anhydrous ethanol until the filtrate is neutral.

[0063] In some embodiments, the carbonaceous material includes one of bamboo, charcoal, graphite and activated carbon and the like. The above-mentioned carbonaceous materials have abundant sources, high carbon content, pore structure, certain chemical activity and stability, and are conducive to the preparation of the carbon material required by the present application.

[0064] In some embodiments, the carbonaceous material further includes at least one of sucrose, glucose, polyethylene, polyvinyl alcohol, polyethylene glycol, polyaniline, epoxy resin, phenolic resin, furfural resin, acrylic resin, polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polyvinyl chloride and pitch and the like. These carbonaceous materials can be ground and compounded with the active material. The types of carbonaceous materials can be selected according to actual needs.

[0065] In some embodiments, the acid agent comprises at least one of hydrochloric acid and hydrofluoric acid, and the concentration of the acid agent is 1-5 mol / L. For example, the concentration of the acid agent can be 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L or any value within the range between any two of the above values. Further, the acid washing can be performed by using a single acid agent or a plurality of acid agents in a step-by-step combination, as long as the acid washing effect is achieved, which is not limited herein. For example, the acid washing is performed by using hydrochloric acid and hydrofluoric acid in sequence, and the concentration of each of the hydrochloric acid and the hydrofluoric acid is 1 mol / L, and the cleaning time is 6-12 h.

[0066] In some embodiments, the water cleaning time is 6-12 h. For example, the water cleaning time can be 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h or any value within the range between any two of the above values. Within the above range, the carbonaceous material can be cleaned, the impurities and residual acid agent in the carbonaceous material can be removed, the content of the impurities can be reduced, and the occurrence of surface side reactions can be prevented.

[0067] In some embodiments, the drying comprises at least one of spray drying and oven drying, which is not limited herein.

[0068] In some embodiments, the drying temperature is 60-90℃, and the drying time is 6-18 h. For example, the drying temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃ or any value within the range between any two of the above values. The drying time can be 6 h, 7 h, 8 h, 9 h, 10 h, 12 h, 14 h, 16 h, 18 h or any value within the range between any two of the above values. By controlling the drying temperature and time within the above range, the carbonaceous material can be effectively dried.

[0069] Step S2: After the first carbon precursor is soaked in the water-soluble resin glue, the first carbon precursor is taken out and subjected to hot-pressing treatment to obtain a second carbon precursor.

[0070] The water-soluble resin glue enters the pore structure of the first carbon precursor through dissolution and impregnation, and under a certain temperature and pressure, the water-soluble resin glue forms a uniform glue layer in the pores of the first carbon precursor, which can reduce the number of large pores in the obtained second carbon precursor and reduce the average pore size of the second carbon precursor, which is beneficial to enhancing the overall mechanical strength of the prepared negative electrode material and maintaining and stabilizing the porous structure.

[0071] In some embodiments, the water-soluble resin glue comprises at least one of water-soluble phenolic resin, polyvinyl alcohol, polyacrylic acid and polyacrolein, and the concentration of the water-soluble resin glue is 25% to 35%. Exemplarily, the concentration of the water-soluble resin glue can be 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35% or any value within the range between any two of the above values. The water-soluble resin glue has good solubility and adhesion, and can form a stable colloidal solution in water. Controlling the concentration of the resin glue within the above range is conducive to promoting the resin to fully penetrate and fill the pore structure of the first carbon precursor, while avoiding forming a too thick or uneven coating.

[0072] In some embodiments, the soaking time is 6h to 12h. Exemplarily, the soaking time can be 6h, 7h, 8h, 9h, 10h, 11h, 12h or any value within the range between any two of the above values. Controlling the soaking time within the above range is conducive to the water-soluble resin glue to fully penetrate and impregnate the pore structure of the first carbon precursor, form a uniform glue layer, and improve the uniformity and adhesion strength of the glue layer.

[0073] In some embodiments, the hot-pressing temperature is 130°C to 150°C, the hot-pressing pressure is 2.5MPa to 3.0MPa, and the hot-pressing time is 5min to 10min. Exemplarily, the hot-pressing temperature can be 130°C, 135°C, 140°C, 145°C, 150°C or any value within the range between any two of the above values. The hot-pressing pressure can be 2.5MPa, 2.6MPa, 2.7MPa, 2.8MPa, 2.9MPa, 3.0MPa or any value within the range between any two of the above values. The hot-pressing time can be 5min, 6min, 7min, 8min, 9min, 10min or any value within the range between any two of the above values. Controlling the hot-pressing temperature within the above range, the water-soluble resin glue can be effectively cured without excessive decomposition or loss of adhesion. Controlling the hot-pressing pressure within the above range, the water-soluble resin glue can be promoted to be uniformly distributed and fully filled in the pores of the first carbon precursor. Controlling the hot-pressing time within the above range, the water-soluble resin glue can be fully cured without excessive processing or unnecessary heat generation. Limiting the hot-pressing processing parameters is conducive to optimizing the structure and performance of the second carbon precursor, and is conducive to having excellent electrochemical performance, stable pore structure and good mechanical strength in subsequent preparation.

[0074] Step S3: high-temperature carbonizing the second carbon precursor to obtain a carbon material.

[0075] Specifically, the high-temperature carbonization is to place the second carbon precursor after hot pressing in an inert gas atmosphere (the inert gas can be nitrogen, helium, argon, and the gas flow can be 50 L / h-300 L / h) for high-temperature treatment, so that the second carbon precursor undergoes a carbonization reaction, thereby being converted into the final carbon material. During the high-temperature carbonization process, the carbonaceous material will undergo pyrolysis and carbonization reaction, and the resin glue layer formed after hot pressing is converted into carbon-based material during the high-temperature carbonization process, leaving the pore structure of the carbonaceous material. The final carbon material has high surface area, excellent electrical conductivity, stable pore structure, and good mechanical strength.

[0076] In some embodiments, the temperature of the high-temperature carbonization is 600-950°C, and the time of the high-temperature carbonization is 1-6h. Illustratively, the temperature of the high-temperature carbonization can be 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, or any value within the range consisting of any two of the above values. The time of the high-temperature carbonization can be 1h, 2h, 3h, 4h, 5h, 6h, or any value within the range consisting of any two of the above values. By setting the high-temperature carbonization temperature and time within the above range, the structure and performance of the carbon material can be effectively controlled.

[0077] In some embodiments, the median particle size D50 of the carbon material is 6.0-15.0μm. Illustratively, the median particle size D50 of the carbon material can be 6.0μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15.0μm, or any value within the range consisting of any two of the above values. By controlling the median particle size of the porous carbon material within the above range, the median particle size of the negative electrode material can be further controlled, thereby improving the performance stability of the negative electrode material, increasing the specific surface area of the negative electrode material, etc. It should be noted that the particle size distribution is determined by the Malvern 3000 laser particle size analyzer using the laser diffraction method.

[0078] Step S4: mixing the carbon material with a boron dopant to obtain a mixture, and high-temperature treating the mixture to obtain a carbon matrix.

[0079] In some embodiments, the boron dopant includes an alkyl boronic acid, which can be at least one of ethyl boronic acid, methyl boronic acid, phenyl boronic acid, and 4-tolyl boronic acid. The alkyl boronic acid boron dopant is cracked at high temperature, providing the carbon matrix with boron elements, oxygen elements, and carbon elements to form sp2 hybridization of boron atoms and generate boron carbide and boron oxide.

[0080] Specifically, the carbon material is mixed with the boron dopant ethyl borate and high-temperature doping is performed under an inert gas atmosphere (the inert gas can be nitrogen, helium, argon, and the gas flow rate can be 50 L / h-300 L / h). When boron elements are doped, as the temperature increases, part of the boron elements form high-strength boron carbide in the skeleton of the carbon material and on the pore wall, and another part of the boron elements form boron oxides on the outer surface and inner surface of the carbon material, so that the mass content of boron elements in the carbon material increases. Moreover, the ethyl group of ethyl borate is cracked to generate deposited carbon atoms, which connect the generated boron carbide and boron oxides, increase the firmness of the combination between the boron carbide and the boron oxides, and continuously increase the relative particle strength of the carbon matrix after boron doping. Based on the improvement of the relative particle strength of the carbon matrix, the relative particle strength of the obtained negative electrode material is also greatly improved, and the expansion rate of the cycle is reduced.

[0081] In the process of boron doping, when the amount of doping is appropriate (the mass fraction of the boron dopant in the mixture is 0.5%-10%), most of the boron atoms can provide empty orbitals for the π-π conjugated system of the carbon matrix, increase the flowability of the electrons in the π-π conjugated system of the carbon matrix, increase the electrical conductivity of the carbon matrix and the negative electrode material, and increase the first coulomb efficiency of the silicon-carbon negative electrode material. However, as the amount of doping increases excessively (the mass fraction of the boron dopant in the mixture is greater than 10%), more and more boron atoms are difficult to be doped into the carbon skeleton of the carbon material to form sp2 hybridization, and thus the sp3 hybridization gradually increases, which hinders the flow of electrons in the π-π conjugated system in the carbon skeleton of the carbon material, resulting in a decrease in the electrical conductivity and the first coulomb efficiency of the negative electrode material. In addition, the boron oxides also increase excessively, and the excessive mass ratio of the boron oxides leads to a decrease in the specific capacity of the negative electrode material.

[0082] In some embodiments, the temperature of high-temperature doping is 600°C-950°C. Illustratively, the temperature of high-temperature doping can be 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, or any value within the range consisting of any two of the above-mentioned values. A higher temperature is helpful to promote the uniform distribution and activation of boron in the carbon material, improve the efficiency and uniformity of doping, and promote the structural recombination of the carbon material, so that the negative electrode material has better electrical conductivity and connectivity of the conductive path, which is beneficial to improve the rate and efficiency of the electrochemical reaction of the negative electrode material.

[0083] In some embodiments, the time of high-temperature doping is 1 h-6 h. Illustratively, the time of high-temperature doping can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, or any value within the range consisting of any two of the above-mentioned values. The above-mentioned doping time is helpful to promote the sufficient diffusion and uniform distribution of boron elements in the structure of the carbon material, and improve the stability and cycle life of the material.

[0084] Step S5: filling the carbon matrix with active substance to obtain the negative electrode material.

[0085] The active substance can improve the electrochemical activity of the negative electrode material, such as improving the capacity, conductivity and cycle stability; and also helps to improve the reaction activity of the surface of the negative electrode material, and accelerates the kinetic response speed of the electrochemical reaction.

[0086] In some embodiments, the active substance is filled by a chemical vapor deposition method, and the deposition temperature of the chemical vapor deposition can be 450-750°C. Illustratively, the deposition temperature can be 450°C, 470°C, 490°C, 500°C, 530°C, 550°C, 580°C, 600°C, 610°C, 620°C, 650°C, 720°C, 750°C or any value within the range of any two of the above values.

[0087] In some embodiments, the deposition time of the vapor deposition is 1-10h. Illustratively, the deposition time can be 1h, 2h, 5h, 8h, 10h or any value within the range of any two of the above values.

[0088] It can be understood that the process conditions of the vapor deposition (such as the deposition temperature and the deposition time) will affect the deposition of the active substance in the pores of the carbon matrix. Controlling the temperature and time of the vapor deposition within the above ranges can ensure that the reaction gas does not decompose before entering the pores of the carbon matrix, and quickly decomposes after entering the pores.

[0089] In some embodiments, the reaction gas includes an active substance gas source, and the active substance gas source is a silicon source gas, which can specifically be at least one of monosilane, disilane, monochlorosilane, dichlorosilane, trichlorosilane and tetrachlorosilane. It should be noted that when the raw material of the silicon source gas is monosilane, disilane, monochlorosilane or dichlorosilane, it is gaseous at room temperature; when the raw material of the silicon source gas is trichlorosilane or tetrachlorosilane, it is liquid at room temperature, and the liquid silicon source will be gasified into gaseous silicon source during the vapor deposition process.

[0090] In some embodiments, the flow rate of the silicon source gas is 1-20L / min. The flow rate of the silicon source gas can specifically be 1L / min, 1.5L / min, 2L / min, 5L / min, 6L / min, 8L / min, 10L / min, 12L / min, 15L / min, 18L / min, 20L / min or any value within the range of any two of the above values.

[0091] In some embodiments, the filling of the active material further comprises a grinding method, and the active material used in the grinding method can be a silicon material, which can be nano-silicon, and the average particle size of the nano-silicon can be 0.1 nm to 50 nm. For example, the average particle size can be 0.1 nm, 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, or any value within the range defined by any two of the above values.

[0092] In some embodiments, the grinding process of the above grinding method can add an additive, which can include at least one of an organic acid, an alcohol, and an amino compound, etc. The organic acid includes at least one of lauric acid, acrylic acid, and polyacrylic acid, etc.; the alcohol includes at least one of propylene alcohol, polypropylene alcohol, phenol, resorcinol, and phenolic resin, etc.; the amino compound includes at least one of aniline, m-phenylenediamine, and diphenyl diamine, etc., which are not limited herein. The addition of the above additive is conducive to the uniform dispersion of silicon and the firm compounding with the carbon matrix.

[0093] In some embodiments, the content of the additive in the total mass of the nano-silicon, the carbon matrix, and the additive is greater than 0 and less than or equal to 10%. For example, the content of the additive can be 0.001%, 0.01%, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any value within the range defined by any two of the above values. Therefore, the mass ratio of the nano-silicon, the carbon matrix, and the additive can be 50:40:10, 50:45:5, 50:49:1, 48:45:8, or 48:50:2, etc., which are not limited herein.

[0094] In some embodiments, the grinding includes at least one of ball mill grinding and sand grinding, etc., wherein the time of the ball mill grinding is 2 h to 8 h, and further 4 h to 8 h, for example, 2 h, 4 h, 6 h, 8 h, or any value within the range defined by any two of the above values.

[0095] In some embodiments, the mixture of the carbon matrix and the active material after grinding needs to be subjected to a sintering treatment, and the sintering atmosphere can be an inert atmosphere, which can be at least one of nitrogen, argon, helium, neon, etc., which are not limited herein. The sintering temperature is 600°C to 1000°C. For example, the sintering temperature can be 600°C, 700°C, 800°C, 900°C, 1000°C, or any value within the range defined by any two of the above values. The sintering time is 2 h to 8 h. For example, the sintering time can be 2 h, 4 h, 6 h, 8 h, or any value within the range defined by any two of the above values.

[0096] In some embodiments, the carbon matrix after the active material filling is further subjected to crushing and 500-mesh sieving to obtain a suitable particle size.

[0097] Compared with the prior art, the preparation method of the negative electrode material provided by the embodiment of the application has the following beneficial effects:

[0098] 1. The first carbon precursor is impregnated with a water-soluble resin glue, and under certain temperature and pressure, the water-soluble resin glue forms a uniform glue layer in the pores of the carbon material, which can reduce the content of large pores in the obtained second carbon precursor, reduce the average pore size of the second carbon precursor, and be beneficial to enhancing the overall mechanical strength of the negative electrode material and maintaining and stabilizing the porous structure.

[0099] 2. A proper amount of boron elements is doped into the carbon matrix by a high-temperature doping process, part of the boron elements form high-strength boron carbide, and another part of the boron elements form boron oxides. The sp2 hybridization of boron atoms can provide empty electron orbits for the π-π conjugate system of the carbon material, increase the flowability of the conjugate electrons of the carbon material, and then increase the electrical conductivity of the negative electrode material; the boron oxides form a stable lithium borate conductive layer on the surface of the negative electrode material, improve the lithium ion transmission efficiency, and at the same time consume HF generated by the decomposition of LiPF6 electrolyte, reduce the corrosion of silicon in the silicon-carbon negative electrode, and then reduce the gas production of the negative electrode material and reduce the risk of cycle expansion.

[0100] 3. The preparation method is simple in process and low in cost, is beneficial to large-scale production of the negative electrode material, and has excellent commercialization prospects.

[0101] The embodiment of the application further provides an electrochemical device. Referring to FIG. 2, the electrochemical device 100 includes a positive electrode sheet 10, a negative electrode sheet 20, a separator 30, and an electrolyte 40. The separator 30 is arranged between the positive electrode sheet 10 and the negative electrode sheet 20, and the negative electrode sheet 20 includes a negative electrode active material, which includes the negative electrode material described above. The electrochemical device 100 prepared by using the negative electrode material described above has low expansion rate, low gas production value, and high initial coulomb efficiency.

[0102] In some embodiments, the electrochemical device 100 is a lithium ion battery.

[0103] The negative electrode material, the preparation method of the negative electrode material, and the electrochemical device of the application will be explained below in combination with embodiments. Those skilled in the art will understand that the following examples are only used to explain the application and cannot be understood as a limitation of the application. Unless otherwise indicated, the reagents, software, and instruments not specifically indicated in the following examples are all conventional commercially available products or open source.

[0104] Embodiment 1, a negative electrode material, a preparation method thereof includes:

[0105] Step 1: Bamboo is first subjected to 1 mol / L hydrochloric acid pickling for 12 h, then 1 mol / L hydrofluoric acid pickling for 12 h, then water washing for 12 h, and finally drying at 80℃ for 12 h to obtain a first carbon precursor.

[0106] Step 2: The first carbon precursor is soaked in a water-soluble phenolic resin with a mass concentration of 30% for 12 h, then taken out and hot-pressed at a temperature of 140℃, a pressure of 3.0 MPa, and a time of 10 min to obtain a second carbon precursor.

[0107] Step 3: The second carbon precursor is placed in a kiln and nitrogen gas with a flow rate of 200 L / h is introduced, and carbonization is carried out at 800℃ for 6 h. After carbonization and cooling, the material is taken out and subjected to airflow crushing to obtain a carbon material with a median particle size D50 of 10 μm.

[0108] Step 4: The carbon material and ethyl boronic acid are mixed to obtain a mixture, and the mass fraction of ethyl boronic acid in the mixture is 0.1%. Then the mixture is placed in a nitrogen gas atmosphere and subjected to high-temperature doping at 800℃ for 4 h to obtain a carbon matrix.

[0109] Step 5: The carbon matrix is placed in a kiln and 1 L / min of nitrogen gas and 5 L / min of silane are introduced, and deposition is carried out at 450℃ for 10 h to obtain a negative electrode material.

[0110] Example 2:

[0111] The specific process of the preparation process is referred to Example 1, except that the mass fraction of ethyl boronic acid in the mixture in Step 4 is 0.5%, and the rest of the preparation method of the negative electrode material is basically the same as Example 1, which will not be described in detail here.

[0112] Example 3:

[0113] The specific process of the preparation process is referred to Example 1, except that the mass fraction of ethyl boronic acid in the mixture in Step 4 is 3%, and the rest of the preparation method of the negative electrode material is basically the same as Example 1, which will not be described in detail here.

[0114] Example 4:

[0115] The specific process of the preparation process is referred to Example 1, except that the mass fraction of ethyl boronic acid in the mixture in Step 4 is 6%, and the rest of the preparation method of the negative electrode material is basically the same as Example 1, which will not be described in detail here.

[0116] Example 5:

[0117] The specific process of the preparation process is referred to Example 1, except that the mass fraction of ethyl boronic acid in the mixture in Step 4 is 10%, and the rest of the preparation method of the negative electrode material is basically the same as Example 1, which will not be described in detail here.

[0118] Example 6

[0119] The preparation process is similar to that of Example 3, except that the active material filling method in Step 5 is grinding. The active material is nano-silicon with an average particle size of 5 nm, and the mass ratio of nano-silicon: carbon matrix: phenolic resin additive is 50:48:2. The mixture after grinding for 4 h is sintered at 800°C in a nitrogen atmosphere for 4 h.

[0120] Example 7

[0121] The preparation process is similar to that of Example 3, except that the active material filling method in Step 5 is grinding. The active material is nano-silicon with an average particle size of 5 nm, and the mass ratio of nano-silicon: carbon matrix: phenolic resin additive is 50:45:5. The mixture after grinding for 5 h is sintered at 800°C in a nitrogen atmosphere for 5 h.

[0122] Comparative Example 1

[0123] The preparation process is similar to that of Example 1, except that there is no Step 4, i.e. no boron element is doped. The preparation method of the remaining negative electrode material is basically the same as that of Example 1, and will not be described in detail here.

[0124] Comparative Example 2

[0125] The preparation process is similar to that of Example 1, except that the mass fraction of ethyl borate in the mixture in Step 4 is 12%. The preparation method of the remaining negative electrode material is basically the same as that of Example 1, and will not be described in detail here.

[0126] Comparative Example 3

[0127] The preparation process is similar to that of Example 5, except that the boron dopant used in Step 4 is boric acid. The preparation method of the remaining negative electrode material is basically the same as that of Example 1, and will not be described in detail here.

[0128] Comparative Example 4

[0129] The preparation process is similar to that of Example 5, except that the boron dopant used in Step 4 is naphthyl borate. The preparation method of the remaining negative electrode material is basically the same as that of Example 1, and will not be described in detail here.

[0130] The negative electrode materials obtained in Examples 1-7 and Comparative Examples 1-4 were subjected to the following tests.

[0131] 1. The carbon matrix and negative electrode materials obtained in Examples 1-7 and Comparative Examples 1-4 were subjected to material performance tests. The material performance test results of the carbon matrix are shown in Table 1, and the material performance test results of the negative electrode materials are shown in Table 2.

[0132] (1) The testing method of the content of boron element: using Perkin Elmer's OPTIMA8000 inductively coupled plasma emission spectrometer to test the sample after aqua regia digestion, filtration and constant volume. Under the action of excitation light, the ground state boron atoms of the sample obtain enough energy, and the outer electrons jump from the ground state to the higher energy level of the excited state. The boron atoms in the excited state are unstable, and the lifetime is less than 10-8s, and the outer electrons jump from the high energy level to the lower energy level or the ground state. The excess energy is emitted in the form of electromagnetic radiation to obtain the emission spectrum of boron. The content of boron element is obtained by qualitative and quantitative analysis of the wavelength and intensity of the emission spectrum of boron element.

[0133] (2) The testing method of the content of oxygen element: the sample is melted under the package of fluxing agent in inert atmosphere, and the contained oxygen element is reduced to carbon dioxide by carbon in the graphite crucible, and the generated carbon dioxide enters the German Veld ONH2000 device infrared detector with carrier gas, and the content of oxygen element can be calculated by quantitative statistics of the change of carbon dioxide infrared signal.

[0134] (3) The testing method of the conductivity: using Japan Mitsubishi Chemical powder conductivity tester MCP-PD51, the volume resistivity of the sample is measured by four-probe method to measure the resistance of the powder under 20kN pressure point, and finally the conductivity of the powder is calculated by the instrument program.

[0135] (4) Relative particle strength test: using Shimadzu DUH-211S dynamic ultra-micro hardness tester, under the action of electromagnetic force, the corresponding force on the pressure head gradually increases at a certain speed, and when the particle reaches the breaking point, the particle breaks; at this time, because the resistance of the pressure head disappears, its displacement rapidly decreases. The instrument records the displacement of the pressure head in real time to determine the breaking point (rapidly increasing displacement), and records the pressure on the particle at this time. According to the relationship between pressure (Force), particle size (D) and relative particle strength (Cx) (Cx=2480×Force / (πD^2)), the relative particle strength of boron-doped carbon matrix and negative electrode material is obtained. Randomly test 10 single particles to obtain the relative strength values of 10 particles, and take the average value as the relative particle strength of the sample.

[0136] (5) Electrochemical performance test: the negative electrode material, conductive carbon black (super P) and binder (LA133) were mixed in a mass ratio of 70:15:15 in an aqueous solvent to form a slurry and uniformly coated on a copper foil, and after drying, an electrode sheet was prepared. The dried electrode sheet was rolled under a pressure of 3 MPa to obtain a negative electrode sheet with a certain compactness. The rolled negative electrode sheet was subjected to a button cell test, and the battery was assembled in an argon glove box, with a lithium metal sheet as the negative electrode, an electrolyte of 1 mol / L lithium hexafluorophosphate + ethylene carbonate (EC) + methyl ethyl carbonate (EMC), and a polyethylene / propylene composite microporous membrane as the separator. The button cell was assembled and tested for the first coulombic efficiency and specific capacity on a Blue Electric battery test cabinet M340A, and the charge and discharge voltage was 0.01V-1.5V.

[0137] (6) Gas production value test method: carboxymethyl cellulose was coated at a content of 1.4%, and after uniform dispersion, 20g of the glue solution was mixed with 25g of the negative electrode material in the application, sealed in a 10cm*10cm aluminum plastic film bag, and then the aluminum plastic film bag was immersed in a graduated cylinder containing a certain volume of water. At this time, the liquid reading (unit: mL) in the graduated cylinder was L1, and after 24h, the liquid reading (unit: mL) in the graduated cylinder was L2. The daily gas production of the material was (L2-L1)*1000 / 25 (cm 3 / kg / d), representing the gas production of 1kg of negative electrode material per day.

[0138] (7) Swelling rate test method: the electrode sheet prepared in the electrochemical performance test was measured for a thickness of d0 before being assembled into a button cell, wherein the copper foil thickness was d. The button cell was subjected to repeated 50 times of charge and discharge test at a current density of 1C in the charge and discharge range of 0.01V-5V, and the button cell was disassembled, and the electrode sheet was taken out, washed and dried. The thickness of the electrode sheet was measured again as d1, and the swelling rate was (d1-d0) / (d0-d)*100%.

[0139] The above test results are shown in Table 1 and Table 2.

[0140] Table 1

[0141] Table 2

[0142] With the combination of Examples 1-7 and Tables 1-2, it can be seen that the mass fraction of the added dopant ethyl borate in the mixture is different, the boron content of the carbon matrix formed is also different, and the relative particle strength will also change. Obviously, the higher the boron content, the higher the relative particle strength of the boron-doped carbon matrix. In Examples 1-7, the content of boron element (A%) in the negative electrode material produced after the boron-doped carbon matrix and silicon were combined was maintained within 2%, and the content of oxygen element (B%) was maintained within 2A%-4%. The relative particle strength of the prepared negative electrode material reached as high as 148.4 MPa, which was significantly improved compared with Comparative Examples 1, 3 and 4. However, in Comparative Example 2, because an excessive amount of ethyl borate (12%) was added, the relative particle strength of the negative electrode material was extremely high, the content of boron element in the negative electrode material was too high (i.e. A≥2), and the excess boron atoms were difficult to be doped into the carbon matrix in the negative electrode material to form sp2 hybridization, thereby increasing the sp3 hybridization, which would destroy the π-π conjugated system of the carbon matrix, hinder the flow of electrons in the conjugated system, and result in the decrease of the conductivity and the first coulombic efficiency of the negative electrode material. In addition, when the boron content is too high, the amount of boron oxide will also increase, and the excessive mass fraction of boron oxide will result in the decrease of the specific capacity of the negative electrode material.

[0143] Comparing Example 1 with Comparative Example 1, it can be seen that when no dopant ethyl borate is added, the content of boron element in the negative electrode material is 0, and the content of oxygen element is also low, only 0.01%. The expansion rate of the negative electrode material in Comparative Example 1 is as high as 45.3%, and the gas production value reaches 56.2 cm 3 / kg / d. The first coulombic efficiency, specific capacity and conductivity of the negative electrode material in Comparative Example 1 are significantly lower than those in Example 1. Therefore, the doping of boron can alleviate the expansion effect of the negative electrode material and improve the electrochemical performance of the negative electrode material.

[0144] With the combination of Example 5 and Comparative Example 3, when boric acid is used to replace ethyl borate, more oxygen elements are introduced, so that the content of oxygen element in the negative electrode material exceeds 4%, resulting in excessive increase of boron oxide. The excessive mass fraction of boron oxide leads to the decrease of the specific capacity of the negative electrode material (only 1438 mAh / g), and the decrease of alkyl cleavage leads to the decrease of the amount of deposited carbon, the amount of boron carbide for interface connection is less, and the relative particle strength and anti-expansion property of the negative electrode material are poorer than those of Example 5. At the same time, the sp2-doped boron element is reduced, the degree of participation of boron element in the π-π conjugated system of the carbon matrix is reduced, the p-π conjugated system of the negative electrode material is reduced, the number of flowable electrons in the carbon matrix is reduced, and thus the conductivity of the negative electrode material is reduced. The conductivity of the finally formed negative electrode material is only 0.23 S / cm, the expansion rate reaches 43.5%, and the gas production value reaches 50.4 cm 3 / kg / d.

[0145] In combination with Example 5 and Comparative Example 4, the results show that when naphthyl boronic acid is used to replace ethyl boronic acid, more alkyl cleavage results in too much deposited carbon, which leads to the formation of soft and loose disordered carbon. These disordered carbons hinder sp2 hybridization of boron doping, and the conductivity of the negative electrode material decreases, and the first coulombic efficiency decreases. Moreover, the external soft and loose disordered carbon also affects the close combination of boron carbide and boron oxide with the carbon skeleton, so that the relative particle strength of the negative electrode material is difficult to greatly improve, and the anti-swelling and anti-gas performance is also difficult to improve. The conductivity of the negative electrode material prepared by using naphthyl boronic acid to replace ethyl boronic acid is 0.18 S / cm, and the first coulombic efficiency is 83.1%. The swelling rate is even 44.5%, and the gas production value is about 18 times of Example 5.

[0146] Therefore, it can be known that by doping the negative electrode material with boron and oxygen elements and controlling the content thereof, the conductivity, the first coulombic efficiency and the specific capacity of the negative electrode material can be effectively increased, and the gas production value and the swelling rate can be reduced.

[0147] The above embodiments are only used to illustrate the technical solutions of the present application and not to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application.

Claims

1. A negative electrode material, characterized by, The negative electrode material comprises a carbon matrix and an active material, and further comprises boron and oxygen, the mass fraction of the boron in the negative electrode material is A%, the mass fraction of the oxygen in the negative electrode material is B%, 0 2. The negative electrode material according to claim 1, characterized in that, The negative electrode material satisfies at least one of the following conditions: (1) A is 0.01, 0.05, 0.1, 0.11, 0.2, 0.3, 0.4, 0.5, 0.7, 0.72, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 1.95, or any value within a range defined by any two of the above values; (2) A is 0.01-1.9; (3) A is 0.1-1.

4.

3. The negative electrode material of claim 1, wherein, The negative electrode material comprises at least one of boron carbide and boron oxide.

4. The negative electrode material of claim 1, wherein, The relative particle strength of the negative electrode material is Cx, Cx is 100-150 MPa.

5. The negative electrode material of claim 1, wherein, The median particle size D50 of the negative electrode material is 6.0-15.0 μm.

6. The negative electrode material of claim 1, wherein, The specific surface area of the negative electrode material is 0.5 m 2 / g ~ 10 m 2 / g.

7. The negative electrode material of claim 1, wherein, The compaction density of the negative electrode material is 0.8 g / cm 3 ~ 1.2 g / cm 3 .

8. The negative electrode material of claim 1, wherein, The electrical conductivity of the negative electrode material is 0.25-1.21 S / cm.

9. The negative electrode material of claim 1, wherein, The negative electrode material has a gas generation value of less than or equal to 25.3 cm 3 / kg / d.

10. The negative electrode material of claim 1, wherein, The mass fraction of carbon in the negative electrode material is 30%-55% based on the mass of the negative electrode material.

11. The negative electrode material of claim 1, wherein, The carbon matrix comprises at least one of amorphous carbon, graphitized carbon, mesocarbon microbeads, and carbon gel.

12. The negative electrode material of claim 1, wherein, The carbon matrix has pores, and at least part of the active material is distributed in the pores of the carbon matrix, the pores comprising mesopores and micropores.

13. The negative electrode material of claim 1, wherein, The active material comprises a silicon-based active material, and the silicon-based active material comprises at least one of crystalline silicon, amorphous silicon, composite particles of crystalline silicon and amorphous silicon.

14. The negative electrode material of claim 13, wherein, The mass fraction of silicon in the negative electrode material is 30%-55% based on the mass of the negative electrode material.

15. 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 by The negative electrode active material layer comprises the negative electrode material according to any one of claims 1-14.

16. A secondary battery characterized by comprising: The secondary battery comprises the negative electrode sheet according to claim 15.

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