Negative electrode material, negative electrode plate comprising same, and electrochemical device

Abstract

The invention discloses a negative electrode material, a negative electrode plate containing the negative electrode material and an electrochemical device, and belongs to the technical field of electrochemical energy storage. The negative electrode material provided by the invention comprises porous carbon and nano silicon, the porous carbon is provided with an inner layer and an outer layer arranged on at least part of the surface of the inner layer, and the nano silicon is arranged in the inner layer; the side reaction between the negative electrode material and the electrolyte during subsequent preparation of the electrochemical device can be effectively reduced, the wettability of the electrolyte to the negative electrode material is improved, and meanwhile, enough buffer capability can be provided for expansion of silicon; furthermore, the electrochemical device prepared subsequently has good cycle performance and rate capability.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage technology, particularly negative electrode materials, including negative electrode sheets and electrochemical devices thereof. Background Technology

[0002] Silicon-based materials have high specific capacity, which can increase battery capacity and make portable electronic devices lighter and faster. Silicon-based materials are beginning to be widely used in everyday products such as mobile phones, computers, automobiles, drones, and electric vehicles. However, silicon-based materials suffer from a high rate of expansion, which significantly reduces battery life. Current technologies, while improving battery capacity, struggle to address the degradation in cycle performance and rate capability caused by silicon expansion. Summary of the Invention

[0003] The purpose of this application is to solve the technical problem that the large expansion rate of the negative electrode material in the prior art makes it difficult to achieve excellent cycle performance and rate performance of the electrochemical device at the same time. The application proposes a negative electrode material with a low expansion rate, a negative electrode sheet containing the material, and an electrochemical device.

[0004] To achieve the above objectives, a first aspect of this application provides a negative electrode material comprising porous carbon and nano-silicon, wherein the porous carbon has an inner layer and an outer layer disposed on at least a portion of the surface of the inner layer; The nano-silicon is disposed in the inner layer.

[0005] As an embodiment of this application, the thickness h of the outer layer and the average particle size d of the negative electrode material satisfy 0.01≤h / d≤0.082.

[0006] As an embodiment of this application, h is 0.10-0.50 μm.

[0007] As an embodiment of this application, d is 5-10 μm.

[0008] As an embodiment of this application, the negative electrode material satisfies 0.8≤y≤7; Where y = c / h; c represents the sphericity of the negative electrode material; h represents the thickness of the outer layer.

[0009] As an embodiment of this application, the sphericity of the negative electrode material is 0.24-1.

[0010] As an embodiment of this application, the mass percentage of the nano-silicon is 47-53% based on the mass of the negative electrode material.

[0011] As an embodiment of this application, the average particle size of the nano-silicon is 1.4-1.9 nm.

[0012] As an embodiment of this application, the surface roughness Ra of the negative electrode material is 0.06-0.3 μm.

[0013] As an embodiment of this application, the pore volume of the outer layer is 0.035-0.143 g / cm³. 3 .

[0014] As an embodiment of this application, based on the total volume of the outer layer pores, the micropore volume accounts for 85-95% and the mesopore volume accounts for 5-15%.

[0015] A second aspect of this application provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material, the negative electrode active material comprising the negative electrode material described in this application.

[0016] In a third aspect, this application provides an electrochemical device, which includes the negative electrode described in this application.

[0017] Compared with the prior art, the beneficial effects of this application are: The negative electrode material provided in this application includes porous carbon and nano-silicon. The porous carbon has an inner layer and an outer layer disposed on at least a portion of the surface of the inner layer, and the nano-silicon is disposed in the inner layer. This can effectively reduce the side reactions between the negative electrode material and the electrolyte during the subsequent fabrication of the electrochemical device, improve the wettability of the electrolyte to the negative electrode material, and provide sufficient buffering capacity for the expansion of silicon. As a result, the electrochemical device subsequently fabricated has good cycle performance and rate performance. Attached Figure Description

[0018] Figure 1 Here is a schematic diagram of the structure of the negative electrode material prepared in Example 1: 1-Inner layer, 2-Outer layer. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The term "polymer" refers to a polymeric compound prepared by polymerizing the same or different types of monomers. The general term "polymer" includes the terms "homopolymer," "copolymer," "trimer," and "interpolymer."

[0021] The term "binder" refers to a substance used to bind inorganic fillers to or to porous substrate materials.

[0022] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0023] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0024] As used herein, the terms “approximately,” “generally,” “substantially,” and “about” are used to describe and indicate small variations. When used in conjunction with an event or situation, the terms may refer to examples in which the event or situation occurred precisely and examples in which the event or situation occurred very approximately. For example, when used in conjunction with numerical values, the terms may refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, if the difference between two values ​​is less than or equal to ±10% of the average of the values ​​(e.g., less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%), then the two values ​​can be considered "substantially" the same.

[0025] Additionally, quantities, ratios, and other numerical values ​​are sometimes presented in range format in this document. It should be understood that such range format is for convenience and brevity and should be interpreted flexibly to include not only the numerical values ​​explicitly specified as range limits, but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0026] In the detailed description and claims, a list of items connected by the terms "one of," "among," "a kind of," or other similar terms may mean any of the listed items. For example, if items A and B are listed, then the phrase "one of A and B" means only A or only B. In another example, if items A, B, and C are listed, then the phrase "one of A, B, and C" means only A; only B; or only C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0027] In the detailed description and claims, the list of items connected by the term "at least one of" can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C.

[0028] In the following description, all figures disclosed in this application are approximate values, regardless of whether the terms "about" or "approximately" are used in conjunction. They may vary by 1%, 2%, 5%, or sometimes 10% to 20%. Whenever a range of values ​​with a lower limit (RL) and an upper limit (RU) is disclosed, any values ​​falling within that range are specifically disclosed. Specifically, the following values ​​within this range are specifically disclosed: R = RL + k * (RU - RL), where k is a variable with a 1% increment from 1% to 100%, i.e., k is 1%, 2%, 3%, 4%, 5%, ..., 50%, 51%, 52%, ..., 95%, 96%, 97%, 98%, 99%, or 100%. Furthermore, any range of values ​​defined by the two R values ​​as defined above are also specifically disclosed.

[0029] Throughout this specification, references to "implementation," "partial implementation," "one implementation," "another implementation," "specific method," or "partial method" mean that at least one implementation or embodiment in this application includes the specific features, structures, materials, or characteristics described in that implementation or embodiment.

[0030] In this application, numerical ranges are involved. Unless otherwise specified, the numerical ranges mentioned above are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form a range not explicitly stated.

[0031] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0032] In one embodiment of this application, a first aspect of this application provides a negative electrode material comprising porous carbon and nano-silicon, wherein the porous carbon has an inner layer and an outer layer disposed on at least a portion of the surface of the inner layer. The nano-silicon is disposed in the inner layer.

[0033] The negative electrode material provided in this application includes porous carbon and nano-silicon. The porous carbon has an inner layer and an outer layer disposed on at least a portion of the surface of the inner layer, and the nano-silicon is disposed in the inner layer. This can effectively reduce the side reactions between the negative electrode material and the electrolyte during the subsequent fabrication of the electrochemical device, improve the wettability of the electrolyte to the negative electrode material, and provide sufficient buffering capacity for the expansion of silicon. As a result, the electrochemical device subsequently fabricated has good cycle performance and rate performance.

[0034] Specifically, in this application, nano-silicon is disposed in the inner layer, which effectively reduces the contact between silicon and the electrolyte, reduces the gas generated by side reactions between silicon and the electrolyte, and improves the kinetics of the electrochemical device. Simultaneously, the introduction of silicon effectively increases the capacity of the electrochemical device. Furthermore, with nano-silicon disposed in the inner layer, the wettability of the subsequent electrolyte is better, and the outer layer can absorb the gas generated by the reaction between the inner nano-silicon and the electrolyte, improving the cycle performance of the electrochemical device. On the other hand, since nano-silicon is disposed in the inner layer instead of being introduced into the outer layer, the outer layer of the SEI film formed by the subsequent electrochemical device is a porous carbon matrix in contact with the electrolyte. Therefore, it can reduce by-products and alleviate volume expansion.

[0035] In one embodiment, the thickness h of the outer layer and the average particle size d of the negative electrode material satisfy 0.01≤h / d≤0.082.

[0036] It should be noted that the test methods for the thickness h of the outer layer and the average particle size d of the negative electrode material are as follows: the thickness of the outer layer can be obtained by measuring the size through SEM images, and the average particle size of the negative electrode material can be obtained by testing with a laser particle size analyzer, so that the ratio between the two can be further calculated.

[0037] For example, h / d can be any point value between 0.01 and 0.082 or a range value between any two points, such as 0.01, 0.012, 0.014, 0.016, 0.018, 0.02, 0.022, 0.024, 0.026, 0.028, 0.03, 0.032, 0.034, 0.036, 0.038, 0.04, 0.042, 0.044, 0.046, 0.048, 0.05, 0.055, 0.06, 0.065, 0.07, 0.075, 0.08, 0.082, etc.

[0038] In one embodiment, the thickness h of the outer layer and the average particle size d of the negative electrode material satisfy 0.05 ≤ h / d ≤ 0.068. For example, it can be 0.05, 0.052, 0.054, 0.056, 0.058, 0.06, 0.062, 0.064, 0.066, 0.068, etc.

[0039] This study found that reducing the ratio of the outer layer thickness *h* to the average particle size *d* of the negative electrode material within a certain range effectively preserves the contribution of silicon to capacity. Conversely, increasing the ratio of the outer layer thickness *h* to the average particle size *d* of the negative electrode material within a certain range effectively buffers the electrolyte and silicon, reducing the impact of gases generated by side reactions in the electrolyte and silicon on the SEI and on volume expansion, thereby improving the cycle performance and rate performance of the electrochemical device. When the ratio of the outer layer thickness *h* to the average particle size *d* of the negative electrode material is further selected to satisfy 0.01 ≤ h / d ≤ 0.07, and especially 0.05 ≤ h / d ≤ 0.068, the resulting electrochemical device exhibits even superior cycle performance and rate performance.

[0040] In one embodiment, h is 0.10-0.50 μm.

[0041] For example, h can be any point value between 0.10 and 0.50 μm or a range value between any two points, such as 0.10 μm, 0.12 μm, 0.14 μm, 0.16 μm, 0.18 μm, 0.2 μm, 0.22 μm, 0.24 μm, 0.26 μm, 0.28 μm, 0.3 μm, 0.32 μm, 0.34 μm, 0.36 μm, 0.38 μm, 0.4 μm, 0.42 μm, 0.44 μm, 0.46 μm, 0.48 μm, 0.50 μm, etc.

[0042] In one embodiment, h is 0.2-0.4 μm. For example, it can be 0.2 μm, 0.21 μm, 0.23 μm, 0.25 μm, 0.27 μm, 0.29 μm, 0.3 μm, 0.33 μm, 0.35 μm, 0.38 μm, 0.4 μm, etc.

[0043] This study found that the thickness h of the outer layer affects the mass content of nano-silicon in the inner layer; it also affects the buffering effect between the electrolyte and silicon, thus affecting the gas production from side reactions, and consequently the cycle performance and rate performance of the electrochemical device. When h is further selected to be 0.10-0.50 μm, especially 0.2-0.35 μm, the overall performance of the electrochemical device prepared is better.

[0044] In one embodiment, d is 5-10 μm.

[0045] For example, d can be any point value between 5 and 10 μm or a range value between any two points, such as 5 μm, 5.2 μm, 5.5 μm, 5.8 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, etc.

[0046] In one embodiment, d is 6-8 μm. For example, it can be 6 μm, 6.5 μm, 7.5 μm, 8 μm, etc.

[0047] This study found that the average particle size of the negative electrode material affects the structural stability of the subsequently prepared negative electrode sheet, as well as its capacity and structural stability; it also affects its specific surface area, thereby affecting the lithium-ion transport efficiency; when d is further selected as 5-10 μm, especially 6-8 μm, the cycle performance and rate performance of the subsequently prepared electrochemical device are better, and the capacity is higher.

[0048] In one embodiment, the negative electrode material satisfies 0.8≤y≤7; Where y = c / h; c represents the sphericity of the negative electrode material; h represents the thickness of the outer layer.

[0049] For example, y can be any point value between 0.8 and 7 or a range value between any two points, such as 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, 5.5, 6, 6.57, etc.

[0050] In one embodiment, the negative electrode material satisfies 1.5 ≤ y ≤ 2.5. For example, it can be 1, 1.5, 2, 2.5, etc.

[0051] This study found that controlling the sphericity of the negative electrode material and the thickness of the outer layer to satisfy 0.8≤c / h≤7, especially 1.5≤y≤2.5, can effectively reduce the subsequent lithium-ion transport resistance and the polarization effect of the electrode, thereby achieving good cycle performance and rate performance of the electrochemical device.

[0052] It should be noted that the sphericity test method for the negative electrode material is as follows: image analysis is used with a Camsizer X2 instrument to statistically analyze the sphericity distribution of a large number of particles.

[0053] In one embodiment, the sphericity of the negative electrode material is 0.24-1.

[0054] For example, the sphericity of the negative electrode material can be any point value between 0.24 and 1 or a range value between any two points, such as 0.24, 0.26, 0.28, 0.3, 0.32, 0.35, 0.38, 0.4, 0.42, 0.45, 0.48, 0.5, 0.52, 0.55, 0.58, 0.6, 0.62, 0.65, 0.68, 0.7, 0.72, 0.75, 0.78, 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1, etc.

[0055] In one embodiment, the sphericity of the negative electrode material is 0.6-1. For example, it can be 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, 0.72, 0.74, 0.76, 0.78, 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1, etc.

[0056] This study found that the sphericity of the negative electrode material affects its fluidity, thereby affecting the tap density and compaction density of the subsequently prepared negative electrode sheet, and consequently the capacity of the electrochemical device. Simultaneously, the sphericity of the negative electrode material also affects its specific surface area, thus influencing the contact area with the electrolyte and the uniformity of current density distribution, thereby affecting the cycle performance and rate performance of the electrochemical device. Furthermore, the sphericity of the negative electrode material also affects its isotropy, thereby affecting the uniformity of lithium-ion insertion and extraction volume changes, and consequently the cycle performance and rate performance of the electrochemical device.

[0057] In one embodiment, the mass percentage of the nano-silicon is 47-53% based on the mass of the negative electrode material.

[0058] For example, based on the mass of the negative electrode material, the mass percentage of the nano-silicon can be any point value or a range between any two points between 47% and 53%, such as 47%, 47.5%, 48%, 48.5%, 49%, 49.5%, 50%, 50.5%, 51%, 51.5%, 52%, 52.5%, 53%, etc.

[0059] This study found that the mass percentage of nano-silicon in the anode material affects the capacity, as well as the degree of subsequent volume expansion and the extent of side reactions with the electrolyte. When the mass percentage of nano-silicon in the anode material is further selected to be 47-53%, the overall performance of the electrochemical device prepared subsequently is better.

[0060] In one embodiment, the average particle size of the nano-silicon is 1.4-1.9 nm.

[0061] It should be noted that the average particle size of the nano-silicon was tested using XRD, followed by calculation using the Scherrer equation. Specifically: Scherrer's formula: B = kλ / (tcosθ) Where B: half-width at half maximum (FWHM) or integral width of the diffraction peak, in radians, reflecting the degree of peak broadening; λ: wavelength of the incident X-ray (usually Å or nm); t: average size of the grain in the direction normal to the diffraction plane (not three-dimensional grain size); θ: Bragg diffraction angle, which determines the diffraction geometry; k: shape factor (usually 0.89-1.39, 0.9 for spherical particles).

[0062] For example, the average particle size of the nano-silicon can be any point value between 1.4 and 1.9 nm or a range between any two points, such as 1.4 nm, 1.45 nm, 1.5 nm, 1.55 nm, 1.6 nm, 1.65 nm, 1.7 nm, 1.75 nm, 1.8 nm, 1.85 nm, 1.9 nm, etc.

[0063] This study found that if the average particle size of the deposited nano-silicon is too small, there will be more silicon-carbon interfaces, more lithium-ion transfer interfaces, greater impedance, and greater polarization of the anode material during use. If the average particle size of the nano-silicon is too large, the uniformity of lithium intercalation inside the silicon will be worse, making it easier to break and expand. Therefore, when the average particle size of the nano-silicon is selected to be 1.4-1.9 nm, the cycle performance and rate performance of the electrochemical device are better.

[0064] In one embodiment, the surface roughness Ra of the negative electrode material is 0.06-0.3 μm.

[0065] It should be noted that the surface roughness test method for the negative electrode material is interferometric measurement. Interferometry is a technique for measuring surface roughness using an interference microscope, combining the principles of interference and microscopic magnification. This method uses optical wave interferometry to magnify and measure microscopic unevenness in the vertical direction, while simultaneously using a microscopic magnification system to measure horizontal parameters. Specifically, the test method is as follows: (1) The sample to be tested is placed on the vertical scanning device; (2) The reference mirror of the interferometer is fixed, and the sample stage (or objective lens) is precisely vertically scanned by piezoelectric ceramic. (3) When the height of a point on the surface being measured is just such that the optical path difference between the measuring light and the reference light is zero, the point will show the interference fringe with the highest contrast (the center of the white light interference fringe). (4) The detector (usually a CCD camera) records the signal of how the light intensity of each pixel changes with the scanning position during the scanning process; (5) Find the scanning position (Z coordinate) corresponding to the maximum value of the light intensity signal of each pixel by using algorithms (such as envelope detection, coherent peak detection); (6) Through this process, the three-dimensional height coordinates of each pixel on the surface can be accurately obtained, thereby constructing a complete three-dimensional surface topography map and obtaining roughness information.

[0066] For example, the surface roughness Ra of the negative electrode material can be any point value or a range between any two points between 0.06 and 0.3 μm, such as 0.06 μm, 0.08 μm, 0.1 μm, 0.12 μm, 0.15 μm, 0.18 μm, 0.2 μm, 0.22 μm, 0.25 μm, 0.28 μm, 0.3 μm, etc.

[0067] In one embodiment, the surface roughness Ra of the negative electrode material is 0.1-0.25 μm. For example, it can be 0.1 μm, 0.12 μm, 0.14 μm, 0.16 μm, 0.18 μm, 0.2 μm, 0.22 μm, 0.24 μm, 0.25 μm, etc.

[0068] This study found that the surface roughness of the negative electrode material affects its ability to bond with other substances, such as providing more anchor points for the conductive agent added to the subsequent negative electrode sheet, thereby affecting the structural stability of the negative electrode sheet; it also affects the ion-electron transport efficiency; when the surface roughness Ra of the negative electrode material is further selected to be 0.06-0.3 μm, especially 0.1-0.25 μm, the cycle performance and rate performance of the obtained electrochemical device are better.

[0069] In one embodiment, the pore volume of the outer layer is 0.035-0.143 cm³. 3 / g.

[0070] It should be noted that the method for testing the pore volume of the outer layer is the gas adsorption method, which infers the pore structure characteristics by measuring the amount of gas (usually nitrogen) adsorbed by the material under different pressures. Low-temperature nitrogen adsorption isotherms are a standard method for pore structure analysis, capable of comprehensively characterizing the material's specific surface area, pore volume, and pore size distribution. This technique can detect pore differences as small as 0.1 nm with extremely high precision. Specifically, the gas adsorption method is as follows: (1) Immerse the pretreated sample tube in a low-temperature bath (liquid nitrogen, 77K). (2) By precisely controlling the valve, the nitrogen pressure in the sample tube is gradually increased (from high vacuum to near saturated vapor pressure). (3) At each pressure equilibrium point, accurately measure the volume (or mass) of nitrogen adsorbed by the sample; this gives the adsorption branch curve. (4) Then, gradually reduce the pressure and measure the volume of desorbed nitrogen to obtain the desorption branch curve; the adsorption branch and the desorption branch together constitute a complete adsorption-desorption isotherm.

[0071] For example, the pore volume of the outer layer can be 0.035-0.143 cm³. 3 Any point value between / g or any range between two points, for example, 0.035cm. 3 / g, 0.045 cm 3 / g, 0.06cm 3 / g, 0.09 cm 3 / g, 0.095cm 3 / g, 0.11 cm 3 / g, 0.12 cm 3 / g, 0.13 cm 3 / g, 0.143 cm 3 / g.

[0072] This study found that the pore volume of the outer layer affects its buffering capacity against silicon volume expansion; furthermore, the pore volume also affects the extent of side reactions. When the pore volume of the outer layer is further selected to be 0.035-0.143 cm³, the results are more significant. 3 When the ratio is / g, the resulting electrochemical device exhibits superior cycle performance and rate performance.

[0073] In one embodiment, the micropore volume accounts for 85-95% of the total outer layer pore volume, and the mesopore volume accounts for 5-15%.

[0074] It should be noted that the test methods for the proportion of micropore volume and mesopore volume, based on the total volume of the outer pores, are as follows: The BET (Brunauer-Emmett-Teller) gas adsorption method: primarily used to measure the specific surface area and pore size distribution of micropores (diameter less than 2 nm). This is achieved by adsorbing nitrogen or argon gas onto the sample at low temperatures, and the results are calculated based on adsorption isotherm data. The BJH (Barrett-Joyner-Halenda) model: analyzes desorption isotherms to deduce the pore size distribution, and is particularly suitable for determining the pore size distribution of mesoporous materials. Here, micropores refer to pores with a diameter ≤ 2 nm, and mesopores refer to pores with a diameter of 2-50 nm.

[0075] For example, based on the total volume of the outer layer pores, the micropore volume ratio can be any point value or any range between two points between 85% and 95%, such as 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, etc.; the mesopore volume ratio can be any point value or any range between two points between 5% and 15%, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0076] This study found that micropores can provide a higher specific surface area and electrochemical active sites, and have a strong adsorption and capillary force on lithium ions in the electrolyte; mesopores have a certain influence on the electrolyte entry and lithium ion migration ability; when the micropore volume ratio and mesopore volume ratio are selected within the above range, the cycle performance and rate performance of the electrochemical device can be better achieved.

[0077] In one embodiment, the method for preparing the negative electrode material includes the following steps: S1. Carbonization: Carbonization is carried out in an inert gas environment using resins as a carbon source. S2, Activation and Pore Formation: Mix the carbonized material from step S1 with the pore-forming agent and react at a constant temperature. S3. Purification: The product after the reaction in step S2 is acid-washed, water-washed until neutral, and then dried; then the dried product is calcined to obtain a porous carbon matrix. S4. Silane deposition: The porous carbon matrix obtained in step S3 is placed in a rotary kiln / fluidized bed, and a mixture of protective gas (nitrogen or rare gas) and silane gas is introduced to carry out the deposition reaction. S5. Etching: A mixture of etching gas and protective gas is introduced into the product after the deposition reaction in step S4 for etching. After etching, the negative electrode material is obtained.

[0078] In some embodiments, in step S1, the carbonization temperature is 600-800°C and the carbonization time is 6-8 hours.

[0079] This study found that excessively low carbonization temperatures lead to incomplete carbonization and an abundance of heteroatoms; excessively high carbonization temperatures result in a porous carbon structure that is loose and has poor structural stability; simultaneously, excessively short carbonization times lead to incomplete carbonization and an abundance of heteroatoms; excessively long carbonization times result in a porous carbon structure that is loose and has poor structural stability. When the carbonization temperature and carbonization time are selected within the above-mentioned ranges, the obtained porous carbon has a stable structure and fewer heteroatoms, resulting in excellent overall performance of the subsequently prepared anode material and electrochemical device.

[0080] In some embodiments, in step S1, the resin includes at least one of phenolic resin, polyester resin, and polyamide resin.

[0081] In some embodiments, the inert gas environment in S1 includes a nitrogen environment and a rare gas environment.

[0082] In some embodiments, a surfactant may also be added in step S1. The surfactant includes at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, fatty alcohol polyoxyethylene ether, and alkyl glycoside. The amount of surfactant added is 2-5% based on the mass of the resin.

[0083] In some embodiments, step S1 further includes a process of mixing and stirring the resin with a surfactant before carbonization, wherein the stirring time is 3-10 hours.

[0084] In some embodiments, S1 further includes an airflow pulverization process prior to carbonization.

[0085] In some embodiments, in step S2, the pore-forming agent comprises an aqueous solution of an alkaline compound; the alkaline compound comprises at least one of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

[0086] In some embodiments, the concentration of the solute in the aqueous solution of the alkaline compound is 1-2 mol / L.

[0087] In some embodiments, in step S2, the temperature of the isothermal reaction is 400-800°C, and the isothermal reaction time is 0.5-2 hours.

[0088] In some embodiments, in step S2, the mass ratio of the carbonized substance to the pore-forming agent is 1:(0.08-0.15).

[0089] In some embodiments, in step S3, the calcination environment is an inert gas environment, the calcination temperature is 800-1300℃, and the calcination time is 2-6 hours.

[0090] This study found that a large number of impurities and heteroatoms are generated during the activation and pore-forming stage in step S2. Acid washing can effectively purify the product. Subsequent calcination can further reduce heteroatoms, such as oxygen atoms, thereby improving the overall performance of the subsequent anode material and electrochemical device.

[0091] In some embodiments, in step S4, the volume percentage of the protective gas in the mixed gas is 10-30%.

[0092] The volume percentage of the protective gas affects production efficiency and safety. When the volume percentage of the protective gas is selected within the above range, the overall effect is better.

[0093] In some embodiments, in step S4, the deposition reaction temperature is 400-500°C and the deposition reaction time is 1-3 hours.

[0094] The deposition reaction temperature affects the degree to which silanes are reduced, and also the degree to which nano-silicon transforms from amorphous silicon to crystalline silicon, thus affecting the anisotropy of expansion and the expansion rate of the material; it may also affect the amount of silicon carbide formed. The deposition reaction time affects the amount of silicon deposited internally and on the surface. If the time is too short, the amount deposited internally will be insufficient; if the time is too long, the amount deposited on the surface will be insufficient, increasing the material expansion. When the deposition reaction temperature and time are further selected within the above-mentioned ranges, the overall performance is better.

[0095] In one embodiment, in step S4, the mixed gas is deposited using a three-stage mixed gas flow rate of low speed-high speed-low speed; wherein the low speed in the first and third stages is 140-160 L / min, and the high speed in the second stage is 190-210 L / min; the deposition times corresponding to the first, second, and third stages are 490-510 min, 580-620 min, and 90-110 min, respectively.

[0096] This study found that if the flow rate of the mixed gas is too high, silicon is easily deposited on the surface; if the flow rate of the mixed gas is too low, the production capacity is low, but silicon can be deposited better inside, and the pores of porous carbon can be blocked, reducing the possibility of silicon overflowing from porous carbon during lithium intercalation and reducing the generation of by-products during the charging and discharging of electrochemical devices.

[0097] In some embodiments, in step S5, the protective gas accounts for 10-30% of the volume of the mixed gas. The protective gas includes at least one of nitrogen and rare gases.

[0098] In some embodiments, in step S5, the etching temperature is 300-350°C and the etching time is 0.1-3 hours.

[0099] In some embodiments, the etching gas in step S5 includes at least one of chlorine and bromine chloride.

[0100] In some embodiments, the flow rate of the etching gas in step S5 is 0.05-0.5 L / min.

[0101] It should be noted that this application can adjust the pore volume of the outer layer, as well as the proportion of micropore volume and mesopore volume, by controlling parameters in the activation pore-forming process, such as the concentration of the alkaline compound aqueous solution, the temperature and time of the pore-forming reaction.

[0102] It should be noted that this application can change the thickness of the outer layer by adjusting the etching gas flow rate and reaction time. For example, increasing the etching gas flow rate and reaction time can increase the thickness of the outer layer. However, extending the etching time will, to some extent, impair the capacity of the electrochemical device.

[0103] It should be noted that this application can change the sphericity of the negative electrode material by controlling the resin synthesis process. For example, increasing the amount of surfactant during resin synthesis can increase the sphericity.

[0104] It should be noted that this application can change the surface roughness of the negative electrode material by adjusting the resin carbonization process, for example, it is related to the amount of surfactant and the stirring time after the surfactant is added; for example, increasing the amount of surfactant during resin carbonization can increase the surface roughness of the negative electrode material.

[0105] It should be noted that this application can change the mass percentage of nano-silicon by adjusting the silane deposition flow rate and deposition time. For example, increasing the deposition time and slowing down the deposition flow rate can increase the mass percentage of nano-silicon.

[0106] It should be noted that this application can change the average particle size of nano-silicon by adjusting the deposition temperature. For example, increasing the deposition temperature can increase the average particle size of nano-silicon.

[0107] In a second aspect, this application provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material, the negative electrode active material comprising the negative electrode material described in this application.

[0108] In one embodiment, the negative electrode active material further includes graphite.

[0109] In one embodiment, the mass content of the negative electrode material in this application is 5-90% based on the mass of the negative electrode active material, and the mass content of the graphite is 10-95%.

[0110] In one embodiment, the negative electrode sheet further includes a negative electrode binder and a negative electrode conductive agent.

[0111] In some embodiments, the negative electrode binder may include at least one of polyacrylic acid, polymethacrylic acid, polyacrylate, polymethacrylate, polyacrylamide, styrene-butadiene rubber, acrylic styrene-butadiene rubber, acrylic acid-acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylonitrile-butadiene rubber, nitrile rubber, acrylonitrile-styrene-butadiene copolymer, acryloyl rubber, butyl rubber, fluororubber, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, polystyrene, latex, acrylic resin, phenolic resin, epoxy resin, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl cellulose, carboxymethyl chitosan, polyester, polyamide, polyether, polyimide, polycarboxylic acid ester, polycarboxylic acid, polyurethane, alginate, fluorinated polymer, chlorinated polymer, polyvinylidene fluoride, and poly(vinylidene fluoride)-hexafluoropropylene. The negative electrode binder of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as battery negative electrode binders.

[0112] In some embodiments, the negative electrode conductive agent may include at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, carbon nanotubes, activated carbon, and mesoporous carbon. The negative electrode conductive agent of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as battery negative electrode conductive agents.

[0113] In a third aspect, this application provides an electrochemical device including the negative electrode described in this application.

[0114] The electrochemical device provided in this application includes any device in which an electrochemical reaction occurs to convert chemical energy into electrical energy and vice versa. Specific, non-limiting examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0115] In one embodiment, the electrochemical device further includes a positive electrode.

[0116] In one embodiment, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least a portion of the surface of the positive current collector, the positive active material layer including a positive active material, a positive binder, and a positive conductive agent.

[0117] This application does not have any special requirements for the selection of the positive electrode active material; conventionally available positive electrode active materials in the art can be used. For example, the positive electrode active material may be selected from LiCoO2, LiNiO2, or LiNi.x Mn y O2, Li 1+z Ni x Mn y Co 1-x- y O2, LiNi x Co y Al z The group consisting of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, Li2MnO3, LiFeO2, LiFePO4, LiMnPO4, and combinations thereof, wherein each x is independently 0.2 to 0.9; each y is independently 0.1 to 0.45; and each z is independently 0 to 0.2. The positive electrode active material of this application is not limited to the above-mentioned materials, but also includes other materials that can be used as positive electrode active materials.

[0118] Alternatively, the positive electrode active material can be selected from LiCoO2, LiNiO2, or LiNi. x Mn y O2, Li 1+z NixMnyCo 1-x-y O2, LiNi x Co y Al z The group consisting of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, LiFeO2, LiFePO4 and combinations thereof, wherein each x is independently 0.4 to 0.6; each y is independently 0.2 to 0.4; and each z is independently 0 to 0.1.

[0119] Or the positive electrode active material is Li 1+x Ni a Mn b Co c Al (1-a-b-c) O2; where -0.2≤x≤0.2, 0≤a<1, 0≤b<1, 0≤c<1 and a+b+c≤1.

[0120] Or the positive electrode active material is Li 1+x Ni a Mn b Co c Al (1-a-b-c)O2, where 0.33≤a≤0.92, 0.33≤a≤0.9, 0.33≤a≤0.8, 0.5≤a≤0.92, 0.5≤a≤0.9, 0.5≤a≤0.8, 0.6≤a≤0.92 or 0.6≤a≤0.9; 0≤b≤0.5, 0≤b≤0.3, 0.1≤b≤0.5, 0.1≤b≤0.4, 0.1≤b≤0.3, 0.1≤b≤0.2 or 0.2≤b≤0.5; 0≤c≤0.5, 0≤c≤0.3, 0.1≤c≤0.5, 0.1≤c≤0.4, 0.1≤c≤0.3, 0.1≤c≤0.2 or 0.2≤c≤0.5.

[0121] In some embodiments, the positive electrode active material is doped with a dopant selected from the group consisting of Fe, Ni, Mn, Al, Mg, Zn, Ti, La, Ce, Sn, Zr, Ru, Si, Ge, and combinations thereof. In some embodiments, the dopant is not Fe, Ni, Mn, Mg, Zn, Ti, La, Ce, Ru, Si, or Ge. In some embodiments, the dopant is not Al, Sn, or Zr.

[0122] In some embodiments, the positive electrode active material may include LiNi. 0.33 Mn 0.33 Co 0.33 O2, LiNiO2, LiNi 0.4 Mn 0.4 Co 0.2 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.7 Mn 0.15 Co 0.15 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2, LiNi 0.92 Mn 0.04 Co 0.04 O2, LiNi 0.8 Co 0.15 Al 0.05 At least one of O2.

[0123] This application does not have specific requirements for the selection of the positive electrode binder; conventionally available positive electrode binders in the art can be used. Exemplarily, the positive electrode binder may be at least one of the following: polyvinylidene fluoride (PVDF), poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, polyacrylic acid, polyacrylonitrile, polyimide, polyurethane, polyvinyl butyral, polyvinylpyrrolidone (PVP), acrylic acid-acrylonitrile-acrylamide copolymer, and acrylic acid-acrylonitrile-acrylate copolymer. The positive electrode binder in this application is not limited to the above materials and also includes other materials that can be used as battery positive electrode binders.

[0124] This application does not have any special requirements for the selection of the positive electrode conductive agent; conventionally available positive electrode conductive agents in the art can be used. For example, the positive electrode conductive agent may be at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon nanotubes, carbon nanotubes, activated carbon, and mesoporous carbon. The positive electrode conductive agent in this application is not limited to the above materials and also includes other materials that can be used as positive electrode conductive agents in batteries.

[0125] In one embodiment, the secondary battery further includes an electrolyte.

[0126] Electrolytes include liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, etc., which can be used in the production of secondary batteries, but are not limited to these.

[0127] Liquid electrolytes (electrolytes) can include organic solvents and lithium salts. The use of organic solvents is unrestricted, as long as they serve as a medium for the movement of ions participating in the battery's electrochemical reactions. Specifically, organic solvents can be ester-based solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether-based solvents, such as dibutyl ether, tetrahydrofuran, etc.; ketone-based solvents, such as cyclohexanone; aromatic hydrocarbon-based solvents, such as benzene, fluorobenzene, etc.; carbonate-based solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; alcohol-based solvents, such as ethanol, isopropanol, etc.; nitriles, such as R-CN (where R is a straight-chain, branched, or cyclic C2 to C20 hydrocarbon group, and may include double-bonded aromatic rings or ether bonds); amide solvents, such as dimethylformamide; dioxolane solvents, such as 1,3-dioxolane; or sulfolane solvents, etc. Preferably, carbonate solvents are used, and more preferably, cyclic carbonates with high ionic conductivity and high dielectric constant, such as ethylene carbonate or propylene carbonate, which can improve the charge and discharge performance of the battery, are used in mixtures with low-viscosity linear carbonate compounds (such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.). In this case, when cyclic carbonates and chain carbonates are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.

[0128] The lithium salt can be any compound that can provide lithium ions for lithium secondary batteries, without any particular limitation. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent electrolyte performance, and lithium ions can move effectively.

[0129] In addition to the electrolyte components mentioned above, additives can be added to the electrolyte, such as halogenated alkylene carbonates like ethylene difluorocarbonate; or pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexaphosphotriamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidinanes, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc., to improve battery life characteristics, inhibit battery capacity decline, and increase battery discharge capacity. The content of the additives can be 0.1 to 5% by weight of the total electrolyte.

[0130] In one embodiment, the electrochemical device further includes a diaphragm.

[0131] The separator separates the negative and positive electrodes and provides a pathway for lithium-ion migration. The use of the separator is not particularly limited, as long as it is a separator commonly used in electrochemical devices. In particular, separators with low resistance to electrolyte ion movement and excellent electrolyte permeability are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes formed from polyolefin-based polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc.) or laminated structures with two or more layers. Alternatively, nonwoven fabrics formed from conventional porous nonwoven fabrics (e.g., glass fibers with high melting points, polyethylene terephthalate fibers, etc.) can be used. Furthermore, coated separators containing ceramic components or polymer materials to ensure heat resistance or mechanical strength can be used, and can optionally be used as single-layer or multi-layer structures.

[0132] Example 1 This application provides a negative electrode material, the preparation method of which includes the following steps: S1. Carbonization: Phenolic resin is used as a carbon source and carbonization is carried out in a nitrogen atmosphere. Sodium dodecylbenzenesulfonate (LAS) is added as a surfactant before carbonization. The mixture is stirred with phenolic resin for 6 hours. After stirring, air jet crushing is carried out (compressed air is used for crushing at a flow rate of 80 L / min for 4 hours). Then carbonization treatment is carried out. The amount of surfactant added is 3.5% of the mass of phenolic resin. The carbonization temperature is 700℃ and the carbonization time is 7 hours to obtain the carbonized material. S2, Activation and Pore Formation: The carbonized material from step S1 is mixed with a pore-forming agent (a 1.5 mol / L potassium hydroxide aqueous solution) at a mass ratio of 1:0.12 and reacted at a constant temperature of 600℃ for 1.2 hours. S3. Purification: The product after the reaction in step S2 is washed with hydrochloric acid, then washed with water until neutral and dried; then the dried product is calcined to obtain a porous carbon matrix. The calcination temperature is 1000℃ and the calcination time is 2h. S4. Silane deposition: The porous carbon matrix obtained in step S3 is placed in a rotary kiln (20 kg size), and a mixture of protective gas (nitrogen) and silane gas (the volume ratio of protective gas in the mixture is 20%) is introduced to carry out a three-stage deposition reaction. The deposition reaction temperature is 450℃, and the flow rates of the mixed gas during the three-stage deposition reaction are 150 L / min, 200 L / min, and 150 L / min, respectively. The deposition reaction times for the three stages are 500 min, 600 min, and 100 min, respectively. S5. Etching: A mixture of etching gas and protective gas (the protective gas accounts for 20% of the volume of the mixture, the etching gas is chlorine, and the flow rate is 0.25 L / min) is introduced into the product after the deposition reaction in step S4 to perform etching (etching temperature is 320℃, etching time is 1.5 h). After etching, the negative electrode material is obtained. The structural schematic diagram of the obtained negative electrode material is shown below. Figure 1 As shown.

[0133] Example 2-3 This application provides a negative electrode material, which differs from the negative electrode material of Example 1 in that... Example 2: The etching time of S5 was extended to 2 hours to change the outer layer thickness, and the amount of surfactant in S1 was adjusted from 3.5% to 4% to change the sphericity of the negative electrode material. Example 3: Adjust the pulverization process parameters of the carbonized S1 material to change the average particle size, and adjust the amount of S1 surfactant from 3.5% to 6% to change the sphericity of the negative electrode material. To achieve the parameters in Table 1-2.

[0134] Examples 4-5 This application provides a negative electrode material, which differs from the negative electrode material of Example 1 in that... Example 4: By adjusting the S5 etching gas flow rate to 0.1 L / min and shortening the reaction time to 1 h, the outer layer thickness was changed; Example 5: Adjusting the S5 etching reaction time to 2 hours achieves a change in the outer layer thickness; To achieve the parameters in Table 1-2.

[0135] Examples 6-7 This application provides a negative electrode material. The difference between the negative electrode material and that in Example 1 is that the pulverization process parameters of the material after S1 carbonization are adjusted to change the average particle size of the negative electrode material, so as to achieve the parameters in Table 1-2.

[0136] Examples 8-9 This application provides a negative electrode material, which differs from the negative electrode material of Example 1 in that... Example 8: The amount of surfactant S1 was adjusted from 3.5% to 1.5% to change the sphericity of the negative electrode material; Example 9: The amount of surfactant S1 was adjusted from 3.5% to 6% to change the sphericity of the negative electrode material; To achieve the parameters in Table 1-2.

[0137] Examples 10-11 This application provides a negative electrode material, which differs from the negative electrode material of Example 1 in that... Example 10: The silane deposition flow rate in S4 was adjusted to 120 L / min, 180 L / min, and 120 L / min, while the deposition time remained unchanged, in order to change the mass percentage of silicon in the negative electrode material; Example 11: The silane deposition flow rate in S4 was adjusted to 170 L / min, 220 L / min, and 170 L / min, with the deposition time remaining constant, in order to change the mass percentage of silicon in the negative electrode material; To achieve the parameters in Table 1-2.

[0138] Examples 12-13 This application provides a negative electrode material, which differs from the negative electrode material of Example 1 in that... Example 12: Adjusting the S4 deposition temperature to 420°C to change the average particle size of nano-silicon; Example 13: Adjusting the S4 deposition temperature to 480°C to change the average particle size of nano-silicon; To achieve the parameters in Table 1-2.

[0139] Example 14 This application provides a negative electrode material. The difference between the negative electrode material and that in Example 1 is that the S5 etching gas flow rate is reduced to 0.12 L / min and the reaction time is shortened to 1.1 h to change the outer layer thickness, thereby achieving the parameters in Table 1-2.

[0140] Examples 15-17 This application provides a negative electrode material, the difference between the negative electrode material and that of Example 1 is that the stirring time of the surfactant and resin in S1 is adjusted; In S1 of Example 15, the mixture was first stirred with phenolic resin for 4 hours. In S1 of Example 16, the mixture was first stirred with phenolic resin for 8 hours. In S1 of Example 17, the mixture was first stirred with phenolic resin for 3 hours. To achieve the parameters in Table 1-2.

[0141] Comparative Example 1 This application provides a negative electrode material in a comparative example, the difference in the preparation method of the negative electrode material in Example 1 being that there is no etching step.

[0142] The h / d, h, d, c, y, mass percentage w of nano-silicon in the negative electrode material, average particle size D, Ra of nano-silicon, pore volume C, micropore volume as a percentage of the total outer pore volume W1, and mesopore volume as a percentage of the total outer pore volume W2 in the negative electrode material prepared in the examples are shown in Table 1-2. Table 1 Table 2 The negative electrode active materials prepared in the examples and comparative examples were used to prepare secondary batteries. The preparation of a secondary battery is as follows: 1) Lithium cobalt oxide (charging cut-off voltage greater than 4.48V), conductive carbon, and binder (polyvinylidene fluoride) are mixed evenly in N-methylpyrrolidone solvent at a mass ratio of 97.6:1.1:1.3 to prepare a positive electrode slurry. Then, it is coated on aluminum foil, dried, cold-pressed, and slit to make a positive electrode sheet. 2) The negative electrode active material (including graphite and the negative electrode material corresponding to this application, with a mass ratio of graphite: the negative electrode material of this application = 80%: 20%), CMC, and SBR are mixed evenly in deionized water at a mass ratio of 97.7: 1.1: 1.2 to prepare a negative electrode slurry. Then, it is coated on copper foil, dried, cold-pressed, and slit to prepare a negative electrode sheet. 3) The above-mentioned positive electrode sheet, conventional separator, and negative electrode sheet are wound into a battery cell. The positive electrode is led out by spot welding with aluminum tabs, and the negative electrode is led out by spot welding with nickel tabs. Then the battery cell is placed in an aluminum-plastic packaging bag, injected with commercial lithium hexafluorophosphate electrolyte, and processed by encapsulation, formation, and capacity testing to produce a secondary battery. Performance tests are as follows: 1. Rate performance test: Charged at a rate of 0.2C with constant current and constant voltage to 4.53V, cutoff rate is 0.025C; Resting time: 5 minutes; constant current discharge at a rate of 0.2C, cutoff voltage 3.0V, marked capacity as C0; Charged at a constant current and constant voltage rate of 0.7C to 4.55V, with a cut-off rate of 0.025C; Discharge to 3.0V at a rate of 2.0C. Read the discharge capacity C1 at each rate. The discharge performance at different rates is equal to C1 / C0.

[0143] 2. Cyclic performance test: 3C constant current charging with a 4.53V cutoff voltage and a 0.02C cutoff current. 1C constant current discharging, 1000cls cycle time.

[0144] 3. Expansion Rate Test: The cell thickness in the initial state (half-charged state, 4.0V) is measured using a PPG cell thickness gauge. The cell thickness is tested every 100 cycles. Cell expansion rate per 100 cycles = cycle node thickness / initial thickness.

[0145] The results are shown in Table 3. Table 3: As can be seen from Table 3, when the technical solution provided in this application is adopted, the obtained negative electrode material has excellent performance. When applied to subsequent electrochemical devices, it has good cycle performance and rate performance, and also has a low volume expansion rate. Specifically, in the rate performance test of the obtained secondary battery, the discharge performance at 2C rate is above 79%, the capacity retention rate before and after 1000 cycles is above 72%, and the volume expansion rate after 100 cycles is below 21%. As can be seen from Examples 1-17 and Comparative Example 1, when etching is not performed, the cycle capacity retention and rate performance of the obtained products decrease significantly, and the volume expansion rate increases significantly.

[0146] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A negative electrode material, characterized by, The negative electrode material comprises porous carbon and nano-silicon, the porous carbon has an inner layer and an outer layer arranged on at least part of the surface of the inner layer; The nano-silicon is arranged in the inner layer.

2. The negative electrode material according to claim 1, characterized in that, The thickness h of the outer layer and the average particle size d of the negative electrode material satisfy 0.01≤h / d≤0.

082.

3. The negative electrode material according to claim 2, characterized in that, The h is 0.10-0.50μm; And / or, the d is 5-10μm.

4. The negative electrode material of claim 1, wherein, The negative electrode material satisfies 0.8≤y≤7; Wherein, y=c / h; c is the sphericity of the negative electrode material; h is the thickness of the outer layer.

5. The negative electrode material according to claim 4, characterized in that, The sphericity of the negative electrode material is 0.24-1.

6. The negative electrode material of claim 1, wherein, The mass percentage of the nano-silicon is 47-53% based on the mass of the negative electrode material; And / or, the average particle size of the nano-silicon is 1.4-1.9nm.

7. The negative electrode material of claim 1, wherein, The surface roughness Ra of the negative electrode material is 0.06-0.3μm.

8. The negative electrode material of claim 1, wherein, The outer layer has a pore volume of 0.035 to 0.143 g / cm3 3 ; And / or, the ratio of the micropore volume to the total pore volume of the outer layer is 85-95%, and the ratio of the mesopore volume to the total pore volume of the outer layer is 5-15%.

9. A negative electrode sheet characterized by The negative electrode sheet comprises the negative electrode material according to any one of claims 1-8.

10. An electrochemical device, characterized by The electrochemical device comprises the negative electrode sheet according to claim 9.

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