Negative active material, negative electrode sheet, and battery

Abstract

The application relates to the technical field of batteries, in particular to a negative electrode active material, a negative electrode sheet comprising the negative electrode active material and a battery. The negative electrode active material comprises silicon-carbon composite particles; the silicon-carbon composite particles have closed pores, and the volume proportion of the closed pores is 4%-50%; the silicon-carbon composite particles have a core-shell structure, and the shell thickness of the core-shell structure is t, 0 < t <= 10 nm; the silicon-carbon composite particles have a first region and a second region on a cross section, the content of silicon elements in the first region is c1, the content of silicon elements in the second region is c2, and 0.15 <= c2 / c1 <= 1.4; wherein the dimension of the perpendicular line of the tangent line of any point on the cross section edge on the cross section is L, and on the perpendicular line, the region with a distance of 0.001L-0.1L from the cross section edge constitutes the first region, and the region with a distance of 0.1L-0.5L from the cross section edge constitutes the second region. The battery of the application can balance the high energy density, the first coulomb efficiency, the rate performance and the cycle stability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a negative electrode active material, a negative electrode sheet including the negative electrode active material, and a battery including the negative electrode active material. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage power stations due to their small size, light weight, and lack of memory effect. Currently, graphite is the primary anode material in commercially available lithium-ion batteries. However, existing graphite materials have relatively low specific capacity (approximately 360 mAh / g) and are approaching their theoretical limit (372 mAh / g), limiting their potential for further development. Silicon-based materials, on the other hand, possess extremely high theoretical specific capacity (4200 mAh / g), making them one of the ideal materials for further improving the energy density of lithium-ion batteries. However, existing silicon-based materials exhibit significant volume expansion during battery charging and discharging, easily leading to rapid battery degradation and large volume change rates. Furthermore, the low lithium-ion diffusion coefficient of silicon-based materials limits the fast-charging capability of silicon-containing lithium-ion batteries. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems of silicon-based materials in the prior art, and to provide a negative electrode active material, a negative electrode sheet including the negative electrode active material, and a battery including the negative electrode active material. The negative electrode active material of this invention, while ensuring high specific capacity, exhibits good ionic and electronic conductivity, and also possesses high structural stability. Batteries including the negative electrode active material of this invention can achieve a balance of high energy density, initial coulombic efficiency, rate performance, and cycle stability.

[0004] In related technologies, silicon-based materials suffer from significant volume expansion and low lithium-ion diffusion coefficients. Through extensive research, the inventors of this invention discovered that by creating a composite material from silicon and carbon, and by controlling the silicon content in specific regions of the silicon-carbon composite particles, the volume ratio of closed pores, and the thickness of the outer shell, the negative electrode active material can achieve high specific capacity while possessing high ionic and electronic conductivity, as well as good structural stability, resulting in minimal volume expansion during battery cycling. The reason for this may be:

[0005] First, since the formation of closed-pore structures in silicon-carbon composite particles is related to silicon deposition during their preparation, the volume of these closed pores is correlated with the silicon content within the particles. The silicon content directly affects the volume expansion of these particles; within a certain range, higher silicon content leads to greater volume expansion. Furthermore, the volume of closed pores provides a buffer for silicon expansion; therefore, controlling the proportion of closed pores can improve the volume expansion of silicon-carbon composite particles.

[0006] Second, setting a carbon-containing shell on the surface of the inner core of the silicon-carbon composite particles can significantly improve the electronic conductivity and ionic conductivity of the silicon-carbon composite particles. However, since the theoretical specific capacity of carbon materials is lower than that of silicon materials, it is necessary to control the thickness of the shell so that the silicon-carbon composite particles can take into account the specific capacity, ionic conductivity, and electronic conductivity.

[0007] Third, only controlling the volume ratio of the closed pores and the thickness of the shell has limited improvement on the performance of the silicon-carbon composite particles. This is because setting a coating layer on the surface of the inner core of the silicon-carbon composite particles has a better improvement effect on the outer electronic conductivity and ionic conductivity of the silicon-carbon composite particles than on the inner side; this will cause the transmission rates of electrons and ions on the outer and inner sides of the silicon-carbon composite particles to be mismatched, thereby affecting the overall ionic conductivity, electronic conductivity, and capacity performance of the negative electrode active material, and at the same time will also have an adverse effect on the volume expansion of the negative electrode active material. Therefore, on the premise of a specific volume ratio of closed pores and a thickness of the shell, it is necessary to coordinately control the silicon content relationship in specific regions of the silicon-carbon composite particles; so that the transmission rates of electrons and ions in each region of the silicon-carbon composite particles are matched, and the volume expansion is balanced. Thereby improving the overall specific capacity performance, ionic conductivity, electronic conductivity, and structural stability of the negative electrode active material. Based on this, the inventors of the present invention have proposed the following solutions:

[0008] In the first aspect of the present invention, a negative electrode active material is provided, and the negative electrode active material includes silicon-carbon composite particles; the silicon-carbon composite particles have closed pores, and the volume ratio of the closed pores is 4%-50%; the silicon-carbon composite particles have a core-shell structure, and the shell of the core-shell structure includes carbon elements, and the thickness of the shell is t, 0 < t ≤ 10 nm; the cross-section of the silicon-carbon composite particles has a first region and a second region, the content of silicon elements in the first region is c1, and the content of silicon elements in the second region is c2, 0.15 ≤ c2 / c1 ≤ 1.4; wherein, the perpendicular line of the tangent at any point on the edge of the cross-section has a size of L on the cross-section, on the perpendicular line, the region at a distance of 0.001L - 0.1L from the edge of the cross-section constitutes the first region, and the region at a distance of 0.1L - 0.5L from the edge of the cross-section constitutes the second region.

[0009] In the second aspect of the present invention, a negative electrode sheet is provided, and the negative electrode sheet includes the negative electrode active material described in the first aspect of the present invention.

[0010] In the third aspect of the present invention, a battery is provided, and the battery includes the negative electrode active material described in the first aspect of the present invention and / or the negative electrode sheet described in the second aspect of the present invention.

[0011] Through the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0012] (1) The negative electrode active material of the present invention has good ionic conductivity and electronic conductivity while ensuring high specific capacity;

[0013] (2) The negative electrode active material of the present invention has good structural stability and can maintain low volume expansion during battery charge and discharge cycles;

[0014] (3) The battery of the present invention can achieve a balance of high energy density, first coulomb efficiency, rate performance and cycle stability.

[0015] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0016] Figure 1 The figure shown is a cross-sectional schematic diagram of silicon-carbon composite particles in an example of the present invention.

[0017] Figure 2 The image shown is a transmission electron microscope (TEM) image of silicon-carbon composite particles in an example of the present invention. Detailed Implementation

[0018] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0019] In the first aspect of the present invention, a negative electrode active material is provided. The negative electrode active material may include silicon-carbon composite particles. The silicon-carbon composite particles may have closed pores, and the volume ratio of the closed pores may be 4%-50%, such as 4%, 5%, 10%, 20%, 30%, 40% or 50%. The silicon-carbon composite particles may have a core-shell structure. The outer shell of the core-shell structure may include carbon elements. The thickness of the outer shell is t, 0 < t ≤ 10 nm, such as 0.1 nm, 0.5 nm, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm. The cross-section of the silicon-carbon composite particles has a first region and a second region. The content of silicon element in the first region is c1, and the content of silicon element in the second region is c2, 0.15 ≤ c2 / c1 ≤ 1.4, such as 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3 or 1.4. Among them, the dimension of the perpendicular line of the tangent at any point on the cross-section edge on the cross-section is L. On the perpendicular line, the region from 0.001L - 0.1L away from the cross-section edge constitutes the first region, and the region from 0.1L - 0.5L away from the cross-section edge constitutes the second region. On the perpendicular line, the region 0.1L away from the cross-section edge belongs to the first region.

[0020] As Figure 1 Shown is a schematic cross-sectional view of silicon-carbon composite particles in an example of the present invention. It can be seen from the figure that the dimension of the perpendicular line of the tangent at any point on the cross-section edge on the cross-section is L. On the perpendicular line, the region from 0.001L - 0.1L away from the cross-section edge 1 constitutes the first region 2, and the region from 0.1L - 0.5L away from the cross-section edge 1 constitutes the second region 3.

[0021] In one example, 0.15 < c2 / c1 ≤ 1.25.

[0022] In one example, 0.23 < c2 / c1 ≤ 0.76.

[0023] When c2 / c1 is greater than 1.4, the silicon content in the second region located relatively inside the silicon-carbon composite particles is higher than that in the first region, which will cause the expansion stress of silicon to be mainly concentrated inside the silicon-carbon composite particles, not conducive to the buffering of the volume expansion of silicon by the closed pores, resulting in the fragmentation of the negative electrode active material during the process of lithium deintercalation and intercalation, thereby reducing the cycle capacity retention rate of the battery.

[0024] In this invention, the silicon content in the first region and the silicon content in the second region can be obtained by conventional methods in the art, such as using an energy dispersive spectroscopy (EDS). Specifically, the silicon-carbon composite particles are mixed evenly with a binder and a solvent and then coated onto the surface of a metal sheet (e.g., copper foil). After drying (or the metal sheet containing the silicon-carbon composite particles is processed using an argon ion cutter (e.g., a NEC IB-19530CP argon ion section polisher), the cut sample is quickly transferred to the scanning electron microscope sample chamber for observation. Using the EDS spot scanning mode, at least 5 points on the first and second regions are measured respectively to obtain the silicon content at each point, and the average value is calculated.

[0025] In one example, the volume percentage of the closed pore is 5%-49%.

[0026] In one example, the volume percentage of the closed pore is 11%-35%.

[0027] When the volume ratio of closed pores is small (e.g., less than 4%), the closed pore volume in the silicon-carbon composite particles is small, making it difficult to effectively buffer the volume expansion of silicon, resulting in a high thickness expansion rate of the battery during cycling. When the volume ratio of closed pores is large (e.g., greater than 50%), the closed pore volume in the silicon-carbon composite particles is large, which leads to a lower energy density of the battery.

[0028] In this invention, the volume of the closed pore refers to the ratio of the volume of the closed pore to the volume of the silicon-carbon composite particles. The volume of the closed pore can be calculated using the following formula: Volume of closed pore = 1 - g / (2.33 - 0.02 × c), where g is the true density of the silicon-carbon composite particles, in g / cm³. 3 c represents the mass content of silicon in the silicon-carbon composite particles.

[0029] In one instance, 0.2nm ≤ t ≤ 10nm.

[0030] In one instance, 1nm ≤ t ≤ 8nm.

[0031] A carbon-containing shell can improve the electronic conductivity of silicon-carbon composite particles. However, when the shell is too thick (e.g., greater than 10 nm), the shell accounts for a large proportion of the mass, which will lead to a lower specific capacity of the silicon-carbon composite particles, resulting in a lower energy density of the battery.

[0032] In this invention, the thickness of the outer shell can be tested using conventional methods in the art, such as TEM, specifically as follows: using a JEM-F200 field emission transmission electron microscope from Japan Electron, at least 10 sites are selected on the surface of the silicon-carbon composite particles, the thickness of the outer shell at each site is measured, and the average value is taken.

[0033] In this invention, the silicon content c1 in the first region can be 30%-75%, for example, 30%, 30%, 40%, 50%, 60%, 70% or 75%.

[0034] In one example, the silicon content c1 in the first region is 31%-71%.

[0035] In one example, the silicon content c1 in the first region is 38%-66%.

[0036] In this invention, the silicon content c2 in the second region can be 5%-60%, for example, 5%, 10%, 20%, 30%, 40%, 50%, 55% or 60%.

[0037] In one example, the silicon content (c2) in the second region is 6%-55%.

[0038] In one example, the silicon content (c2) in the second region is 9%-50%.

[0039] In this invention, the mass content of silicon in the silicon-carbon composite particles can be 20%-65%, for example, 20%, 30%, 40%, 50% or 60%.

[0040] In one example, the silicon-carbon composite particles contain 21%-64% silicon by mass.

[0041] In one example, the silicon content in the silicon-carbon composite particles is 25%-60% by mass.

[0042] When the mass content of silicon in silicon-carbon composite particles is low (e.g., less than 20%), the specific capacity of silicon-carbon composite particles is low, which leads to a low energy density of the battery. When the mass content of silicon in silicon-carbon composite particles is high (e.g., greater than 65%), the volume of closed pores cannot effectively alleviate the volume expansion of silicon, resulting in poor structural stability of silicon-carbon composite particles, which in turn affects the cycle capacity retention rate and thickness expansion rate of the battery.

[0043] In this invention, the mass content of silicon in the silicon-carbon composite particles can be obtained by conventional methods in the art, such as thermogravimetric analysis (TGA), specifically as follows: A Shimadzu DTG-60 thermogravimetric analyzer is used for testing under the following conditions: sample volume 5 mg, air atmosphere, heating rate 10 °C / min, heating from room temperature to 900 °C and holding at that temperature for 40 min. The mass content of silicon in the silicon-carbon composite particles is c = 7z / 15, where z is the final weight percentage remaining after testing.

[0044] like Figure 2The image shown is a TEM image of silicon-carbon composite particles in an example of the present invention. As can be seen from the image, both the core and the surface of the composite particles are amorphous, and the thickness of the surface layer is about 3nm-5nm.

[0045] In this invention, the core of the core-shell structure may include carbon and silicon.

[0046] In one example, the core comprises porous carbon and silicon material located within the pores of the porous carbon.

[0047] In one instance, the shell comprises amorphous carbon.

[0048] In this invention, the silicon-carbon composite particles may include at least one of nitrogen, phosphorus and sulfur.

[0049] Because nitrogen, phosphorus, and sulfur are all more electronegative than carbon, they react more readily with lithium. Introducing at least one of nitrogen, phosphorus, and sulfur into silicon-carbon composite particles can reduce the electronegativity of lithium. + The reaction energy barrier embedded in the silicon-carbon composite particles makes it easier for lithium ions to migrate within the particles, which is beneficial for improving the ionic conductivity of the negative electrode active material.

[0050] In one example, the silicon-carbon composite particles include nitrogen, phosphorus, and sulfur.

[0051] In this invention, the sum of the contents of nitrogen, phosphorus and sulfur in the silicon-carbon composite particles can be 15ppm-1000ppm, for example, 15ppm, 20ppm, 30ppm, 40ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm or 1000ppm.

[0052] In one example, the combined content of nitrogen, phosphorus, and sulfur in the silicon-carbon composite particles is 30 ppm to 500 ppm.

[0053] When the sum of nitrogen, phosphorus, and sulfur content in the silicon-carbon composite particles is too low (e.g., less than 15 ppm), it cannot effectively improve the ionic conductivity of the negative electrode active material. Conversely, when the sum of nitrogen, phosphorus, and sulfur content in the silicon-carbon composite particles is too high (e.g., greater than 1000 ppm), it reduces the specific capacity of the negative electrode active material, leading to a decrease in the battery's initial coulombic efficiency and energy density. When the sum of nitrogen, phosphorus, and sulfur content in the silicon-carbon composite particles is within a specific range, the negative electrode active material exhibits a high lithium-ion diffusion coefficient, which can improve the battery's rapid discharge capability.

[0054] In this invention, the contents of nitrogen, sulfur, and nitrogen in the silicon-carbon composite particles can be determined by methods conventional in the art. For example, the contents of nitrogen and sulfur can be determined using an elemental analyzer. The contents of phosphorus can be determined, for example, using inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0055] In this invention, the particle sizes Dv90 and Dv10 of the silicon-carbon composite particles satisfy 5μm≤Dv90-Dv10≤25μm, for example, 5μm, 10μm, 15μm, 20μm or 25μm.

[0056] In one instance, 7μm≤Dv90-Dv10≤24μm.

[0057] In one instance, 7μm≤Dv90-Dv10≤18μm.

[0058] The difference between Dv90 and Dv10 reflects the particle size distribution concentration of silicon-carbon composite particles. A small difference (e.g., less than 5 μm) indicates that most particles in the silicon-carbon composite are of similar size, reducing the packing density and thus the volumetric energy density of the battery. A large difference (e.g., greater than 25 μm) indicates a higher proportion of smaller particles, resulting in a larger specific surface area and more side reactions between the negative electrode active material and the electrolyte during the first charge-discharge cycle, thus reducing the initial coulombic efficiency. Conversely, a large difference indicates a higher proportion of larger particles, affecting the processing performance of the negative electrode active material. Larger particles are more easily removed during battery manufacturing, leading to a lower volumetric energy density. A difference within a specific range is beneficial for improving both the initial coulombic efficiency and volumetric energy density of the battery.

[0059] In this invention, the particle sizes Dv10 and Dv90 of the silicon-carbon composite particles can be obtained by conventional methods in the art, such as laser particle size analysis, specifically as follows: the particle size is measured using a Malvern particle size analyzer, and the testing steps are as follows: the silicon-carbon composite particles are dispersed in deionized water containing a dispersant (e.g., nonylphenol polyoxyethylene ether) (the mass content of the dispersant is 0.02%-0.03%), sonicated for 2 minutes, and then placed in the Malvern particle size analyzer for testing.

[0060] In this invention, Dv10 can be 1μm-6μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm or 6μm.

[0061] In one instance, Dv10 is 2μm-6μm.

[0062] In one instance, Dv10 is 2μm-5μm.

[0063] In this invention, Dv90 can be 10μm-30μm, for example, 10μm, 15μm, 20μm, 25μm or 30μm.

[0064] In one instance, Dv90 is 12μm-30μm.

[0065] In one instance, Dv90 is 12μm-20μm.

[0066] In this invention, the oil absorption value of the silicon-carbon composite particles can be 10ml / 100g-100ml / 100g, for example, 10ml / 100g, 20ml / 100g, 30ml / 100g, 40ml / 100g, 50ml / 100g, 60ml / 100g, 70ml / 100g, 80ml / 100g, 90ml / 100g or 100ml / 100g.

[0067] In one example, the oil absorption value of the silicon-carbon composite particles is 31 ml / 100g-79 ml / 100g.

[0068] The oil absorption value reflects the volume of the stacked pores formed after the silicon-carbon composite particles are piled up. When the oil absorption value of the silicon-carbon composite particles is low (e.g., less than 10 ml / 100 g), the volume of pores formed after the silicon-carbon composite particles are piled up is small, indicating that the negative electrode containing silicon-carbon composite particles has poor electrolyte retention capacity, which will reduce the cycle capacity retention rate of the battery. Conversely, when the oil absorption value of the silicon-carbon composite particles is high (e.g., greater than 100 ml / 100 g), the volume of pores formed after the silicon-carbon composite particles are piled up is large, indicating that the porosity of the negative electrode containing silicon-carbon composite particles is high, which will reduce the volumetric energy density of the battery. When the oil absorption value of the silicon-carbon composite particles is within a certain range, the battery can achieve both high cycle capacity retention rate and high volumetric energy density.

[0069] In this invention, the oil absorption value of the silicon-carbon composite particles can be obtained by the following method: weigh a clean beaker and a glass rod as m1, add 5g of silicon-carbon composite particles to them and record the total mass as m2, add dioctyl phthalate (DOP) dropwise with a titration bottle, and weigh the total weight of the beaker at this time as m3. The oil absorption value of the silicon-carbon composite particles = (m3-m2) / (m2-m1)×100.

[0070] In this invention, the silicon-carbon composite particles may also include lithium.

[0071] During the production or use of silicon-carbon composite particles, they may come into contact with air or water, resulting in the inclusion of oxygen in the particles. If the silicon-carbon composite particles also contain lithium, the lithium will combine with the oxygen in the particles, preventing the oxygen from reacting with lithium in the electrolyte during the first charge and discharge cycle of the battery. This improves the battery's initial coulombic efficiency.

[0072] In this invention, the lithium content in the silicon-carbon composite particles can be 0.1%-20%; for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%.

[0073] In one example, the lithium content in the silicon-carbon composite particles is 1%-5%.

[0074] When the lithium content in silicon-carbon composite particles is high (e.g., greater than 20%), lithium that is not combined with oxygen is likely to appear in the silicon-carbon composite particles. This portion of lithium is active and is prone to react with the solvent in the slurry during the electrode preparation process and generate gas. Therefore, it will deteriorate the processing performance of the negative electrode active material and is not conducive to the energy density and cycle retention rate of the battery.

[0075] In this invention, the lithium content in the silicon-carbon composite particles can be obtained by methods conventional in the art, such as using ICP-AES.

[0076] In this invention, the thermogravimetric curve of the silicon-carbon composite particles in air atmosphere shows a weight gain peak at 600℃-800℃.

[0077] The weight changes of silicon-carbon composite particles in different temperature ranges during thermogravimetric analysis correspond to different thermal behaviors. For example, weight loss in the <400℃ range is usually due to the evaporation of moisture in the material during heating, while weight loss in the >550℃ range is usually due to the reaction of carbon materials with oxygen in the air to generate CO2. The weight gain in the >600℃ range is due to the reaction of silicon in the silicon-carbon composite particles with oxygen in the air to generate SiO2. The presence of a weight gain peak between 600℃ and 800℃ indicates that the outer shell of the silicon-carbon composite particles completely encapsulates the core, and that the silicon within is active, resulting in a high initial coulombic efficiency for the battery.

[0078] In one example, the thermogravimetric curve of the silicon-carbon composite particles in an air atmosphere shows a weight gain of 0.1%-40% at 600°C-800°C, for example, 0.1%, 0.5%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, or 40%.

[0079] In one example, the thermogravimetric analysis (TGA) curve of the silicon-carbon composite particles in air showed a weight gain of 0.5%-24.8% at 600℃-800℃.

[0080] Since SiO2 reacts with lithium during battery charging and discharging, resulting in irreversible capacity loss, when the weight gain rate is within a specific range of 600℃-800℃, it indicates that the active silicon in the silicon-carbon composite particles is well protected by the outer shell and is not oxidized to SiO2. Furthermore, the thickness of the outer shell is not too thick, which allows the battery to have a high initial coulombic efficiency and energy density.

[0081] In this invention, the specific steps for the thermogravimetric analysis of the silicon-carbon composite particles are as follows: a Shimadzu DTG-60 thermogravimetric analyzer is used, the sample amount is 5mg, the test atmosphere is air, the heating rate is 10℃ / min, and the test range is room temperature to 900℃.

[0082] In this invention, the true density of the silicon-carbon composite particles can be 1.4 g / cm³. 3 -2.2g / cm 3 For example, 1.4 g / cm³ 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 31.8g / cm 3 1.9g / cm 3 2g / cm 3 2.1g / cm 3 Or 2.2g / cm 3 .

[0083] When the true density of silicon-carbon composite particles is within a specific range, the silicon-carbon composite particles exhibit a suitable closed-cell volume. When the true density is relatively low (e.g., less than 1.4 g / cm³), the closed-cell volume is suitable. 3 When the true density is high (e.g., greater than 2.2 g / cm³), excessive closed-cell content in the silicon-carbon composite particles can lead to a lower volumetric energy density in the battery; conversely, when the true density is high (e.g., greater than 2.2 g / cm³), the volumetric energy density of the battery is lower. 3 When the closed-cell content of silicon-carbon composite particles is too low, it is difficult to buffer the volume expansion of silicon, thus affecting the thickness expansion rate of the battery.

[0084] In this invention, the true density of the silicon-carbon composite particles can be obtained by conventional methods in the art, such as the gas volume displacement method, as follows: the test is performed using a JW-M100A fully automatic true density tester, the test gas is helium, and the test environment temperature is 25℃±2℃.

[0085] In this invention, the particle size Dv50 of the silicon-carbon composite particles can be 6μm-15μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm or 15μm.

[0086] In one example, the particle size Dv50 of the silicon-carbon composite particles is 7 μm-12 μm.

[0087] When the particle size Dv50 of silicon-carbon composite particles is small (e.g., less than 6 μm), the average particle size of silicon-carbon composite particles is small and the specific surface area is large. This leads to an increase in side reactions between the negative electrode active material and the electrolyte during battery charge-discharge cycles, which reduces the battery's initial coulombic efficiency and volumetric energy density. Conversely, when the particle size Dv50 of silicon-carbon composite particles is large (e.g., greater than 15 μm), the average particle size of silicon-carbon composite particles is large, and the migration distance of lithium ions in the negative electrode active material becomes longer, affecting the battery's rapid discharge capability.

[0088] In this invention, the particle size Dv50 of the silicon-carbon composite particles can be obtained by conventional methods in the art, such as laser particle size analysis, specifically as follows: the particle size is measured using a Malvern particle size analyzer, and the testing steps are as follows: the silicon-carbon composite particles are dispersed in deionized water containing a dispersant (e.g., nonylphenol polyoxyethylene ether) (the mass content of the dispersant is 0.02%-0.03%), sonicated for 2 minutes, and then placed in the Malvern particle size analyzer for testing.

[0089] In this invention, the specific surface area of ​​the silicon-carbon composite particles can be 0.2 m². 2 / g-20m 2 / g, for example, 0.2m 2 / g, 0.5m 2 / g、1m 2 / g、2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15m 2 / g, 16m 2 / g、17m 2 / g、18m 2 / g、19m 2 / g or 20m 2 / g.

[0090] In one example, the specific surface area of ​​the silicon-carbon composite particles is 0.5 m². 2 / g-10.3m 2 / g.

[0091] When the specific surface area of ​​silicon-carbon composite particles is small (e.g., less than 0.2 m²), 2 When the specific surface area of ​​silicon-carbon composite particles is too small (e.g., greater than 20 μm²), the amount of binder that can be adsorbed on the surface of the negative electrode active material during electrode preparation is relatively small. This makes the negative electrode active material prone to detaching from the negative electrode during battery cycling, resulting in battery capacity decay. Conversely, when the specific surface area of ​​silicon-carbon composite particles is large (e.g., greater than 20 μm²), the negative electrode active material is easily detached from the negative electrode during battery cycling. 2 When the surface area of ​​silicon-carbon composite particles is too large (e.g.), there are more side reactions between the negative electrode active material and the electrolyte, which will reduce the initial coulombic efficiency and volumetric energy density of the battery.

[0092] In this invention, the specific surface area of ​​the silicon-carbon composite particles can be obtained by methods conventional in the art, such as measuring it using a Tri Star II specific surface area analyzer.

[0093] In this invention, the negative electrode active material may further include a carbon-based material. The carbon-based material may include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, and hard carbon.

[0094] In this invention, the mass ratio of the silicon-carbon composite particles to the carbon-based material can be 1:(0.01-50), for example 1:0.01, 1:0.05, 1:0.1, 1:0.5, 1:1, 1:5, 1:10, 1:20, 1:30, 1:40 or 1:50.

[0095] In one example, the mass ratio of the silicon-carbon composite particles to the carbon-based material is 1:(0.1-19).

[0096] The present invention also provides a method for preparing the silicon-carbon composite particles, the method comprising at least the following steps:

[0097] The porous carbon material is placed in a vapor deposition apparatus, N2 is introduced, and the temperature is raised to 300℃-500℃; silane gas is introduced for the first time, and then silane gas is introduced for the second time; the silane gas is stopped, the temperature is raised to 450℃-650℃, and acetylene gas is introduced.

[0098] In this invention, the flow rate of N2 introduced is 100 sccm-300 sccm.

[0099] In this invention, the first flow rate is 10 sccm-200 sccm; the first flow time is 20 min-300 min.

[0100] In this invention, the flow rate of the second inlet is 50 sccm-300 sccm; the time of the first inlet is 20 min-300 min.

[0101] In this invention, the flow rate of the acetylene gas introduced is 50 sccm-200 sccm; the time for introducing the acetylene gas is 5 min-240 min.

[0102] In this invention, the preparation method further includes pretreating the porous carbon material and then placing it in the vapor deposition equipment.

[0103] In this invention, the pretreatment may include soaking the porous carbon material in a solvent. The solvent may include at least one of nitric acid, sulfuric acid, and phosphoric acid. The concentration of the solvent is, for example, 0.001 mol / L to 0.05 mol / L. The soaking temperature may be 90°C to 100°C. The soaking time may be 3 hours to 60 hours.

[0104] In this invention, the preparation method further includes, after introducing acetylene gas, soaking and drying with a lithium-based solvent. The lithium-based solvent includes, for example, a tetrahydrofuran solution of lithium biphenyl. The soaking time is, for example, 1 min to 60 min.

[0105] A second aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising the negative electrode active material described in the first aspect of the present invention.

[0106] In this invention, the negative electrode sheet may include a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector. The negative electrode active material layer may include the negative electrode active material.

[0107] In this invention, the negative electrode active material layer may further include a negative electrode conductive agent and a negative electrode binder. The negative electrode conductive agent may include at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes (including at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon fibers. The negative electrode binder may include at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose, styrene-butadiene rubber, polytetrafluoroethylene, polyethylene oxide, polyacrylic acid, and derivatives of the above substances.

[0108] In this invention, based on the total mass of the negative electrode active material layer, the content of the negative electrode active material can be 80-99.8% by weight (e.g., 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 99, or 99.8% by weight), the content of the negative electrode conductive agent can be 0.1-10% by weight (e.g., 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.5, or 0.1% by weight), and the content of the negative electrode binder can be 0.1-10% by weight (e.g., 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.5, or 0.1% by weight).

[0109] A third aspect of the present invention provides a battery, which may include the negative electrode active material described in the first aspect of the present invention and / or the negative electrode sheet described in the second aspect of the present invention.

[0110] In this invention, the components of the battery other than the negative electrode (e.g., the positive electrode, the separator, and the electrolyte) can all be conventional choices in the art.

[0111] In one example, the battery includes a lithium-ion battery.

[0112] In one example, the battery comprises a lithium-ion secondary battery.

[0113] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0114] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0115] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0116] The following preparation examples are used to prepare the silicon-carbon composite particles of the present invention.

[0117] Preparation Example 1

[0118] Silicon-carbon composite particles were prepared using the following method:

[0119] (1) Pretreatment of porous carbon: The porous carbon material (Dv10 is 4μm, Dv50 is 9μm, Dv90 is 15μm) was placed in a mixed solution of nitric acid, sulfuric acid and phosphoric acid at a ratio of 1g / 100mL (the total concentration of nitric acid, sulfuric acid and phosphoric acid is 0.01mol / L), heated to 100℃ in a sealed container, soaked for 24h, and then taken out, washed and dried.

[0120] (2) Place it in a vapor deposition furnace, introduce 200 sccm of N2, and raise the temperature to 500°C. Then, introduce silane gas at flow rates of 100 sccm and 200 sccm for 2 hours each. After that, stop introducing silane, raise the temperature to 650°C, introduce acetylene gas at a flow rate of 100 sccm, and maintain for 1 hour.

[0121] (3) Soak in a 0.5 mol / L lithium tetrahydrofuran solution for 5 min and then dry.

[0122] Preparation Example 2

[0123] (1) Pretreatment of porous carbon: The porous carbon material (Dv10 is 2μm, Dv50 is 12μm, Dv90 is 20μm) was placed in a mixed solution of nitric acid, sulfuric acid and phosphoric acid at a ratio of 1g / 100mL (the total concentration of nitric acid, sulfuric acid and phosphoric acid is 0.01mol / L), heated to 100℃ in a sealed container, soaked for 10h, and then taken out, washed and dried.

[0124] (2) Place it in a vapor deposition furnace, introduce 200 sccm of N2, and simultaneously raise the temperature to 500°C. Then, introduce silane gas at a flow rate of 100 sccm for 30 min and 200 sccm for 80 min in sequence. After that, stop introducing silane, raise the temperature to 650°C, introduce acetylene gas at a flow rate of 100 sccm, and maintain for 20 min.

[0125] (3) Soak in a 0.5 mol / L tetrahydrofuran solution of biphenyl lithium for 3 min, then dry.

[0126] Preparation Example 3

[0127] (1) Pretreatment of porous carbon: The porous carbon material (Dv10 is 5μm, Dv50 is 7μm, Dv90 is 12μm) was placed in a mixed solution of nitric acid, sulfuric acid and phosphoric acid at a ratio of 1g / 100mL (the total concentration of nitric acid, sulfuric acid and phosphoric acid is 0.01mol / L), heated to 100℃ in a sealed container, soaked for 36h, and then taken out, washed and dried.

[0128] (2) Place it in a vapor deposition furnace, introduce 200 sccm of N2, and simultaneously raise the temperature to 500°C. Then, introduce silane gas at flow rates of 100 sccm and 200 sccm for 150 min each. After that, stop introducing silane, raise the temperature to 650°C, introduce acetylene gas at a flow rate of 100 sccm, and maintain for 120 min.

[0129] (3) Soak in a 0.5 mol / L lithium tetrahydrofuran solution for 10 min and then dry.

[0130] Preparation Example 4

[0131] The examples in this group are used to verify the effect of changes in the "volume percentage of closed pores".

[0132] This set of examples follows the same procedure as Preparation Example 1, except that the volume ratio of closed pores is controlled by adjusting the duration of silane gas introduction, as detailed below:

[0133] Preparation Example 4a: Silicane gas with flow rates of 100 sccm and 200 sccm was sequentially introduced for 180 min each;

[0134] Preparation Example 4b: Silane gas at a flow rate of 100 sccm was introduced sequentially for 25 min and 200 sccm for 60 min.

[0135] Preparation Example 5

[0136] The examples in this group are used to verify the effect of changing the thickness t of the outer shell.

[0137] This set of embodiments is based on Preparation Example 1, except that the thickness t of the outer shell is controlled by adjusting the holding time of the acetylene gas, as detailed below:

[0138] Preparation Example 5a: Acetylene gas was introduced at a flow rate of 100 sccm and maintained for 5 min;

[0139] Preparation Example 5b: Acetylene gas was introduced at a flow rate of 100 sccm and maintained for 200 min.

[0140] Preparation Example 6

[0141] The examples prepared in this group are used to verify the effects of changes in "c2 / c1".

[0142] This set of examples follows the same procedure as Preparation Example 1, except that the c2 / c1 ratio is controlled by adjusting the duration of the silane gas introduction, as detailed below:

[0143] Preparation Example 6a: Silane gas at a flow rate of 100 sccm was introduced sequentially for 150 min and Silane gas at a flow rate of 200 sccm for 90 min.

[0144] Preparation Example 6b: silane gas at a flow rate of 100 sccm was sequentially introduced for 30 min and silane gas at a flow rate of 200 sccm for 150 min.

[0145] Preparation Example 7

[0146] The preparation examples in this group are used to verify the effects of changes in nitrogen, phosphorus and sulfur elements in silicon-carbon composite particles.

[0147] The embodiments in this group are based on Preparation Example 1, except that the nitrogen, phosphorus and sulfur elements in the silicon-carbon composite particles are controlled by adjusting the pretreatment of porous carbon in step (1), as follows:

[0148] Preparation Example 7a: The mixed solution was replaced with a sulfuric acid solution with a concentration of 0.01 mol / L;

[0149] In preparation example 7b, the porous carbon was not pretreated and proceeded directly to step (2).

[0150] Preparation Example 8

[0151] The examples in this group are used to verify the effects of changes in the sum of the contents of nitrogen, phosphorus and sulfur.

[0152] This set of examples is based on Preparation Example 1, except that the total content of nitrogen, phosphorus and sulfur is controlled by adjusting the soaking time in step (1), as follows:

[0153] Preparation Example 8a, the soaking time in step (1) is 3 hours;

[0154] Preparation Example 8b, the soaking time in step (1) is 60 h.

[0155] Preparation Example 9

[0156] This was used to verify the impact of changes to the "silicon-carbon composite particles Dv90-Dv10".

[0157] The preparation was carried out in accordance with Example 1, except that the particle size of the silicon-carbon composite particles was controlled by adjusting the particle size of the porous carbon. Specifically, the particle size of the porous carbon material was: 6 μm for Dv10, 15 μm for Dv50, and 30 μm for Dv90.

[0158] Preparation Example 10

[0159] The preparation examples in this group are used to verify the effect of changing the "lithium content in silicon-carbon composite particles".

[0160] The preparation examples in this group were carried out in accordance with Preparation Example 1, except that the lithium content in the silicon-carbon composite particles was controlled by adjusting step (3), as follows:

[0161] Example 10a: step (3) is not performed, that is, the material obtained in step (2) is silicon-carbon composite particles;

[0162] In Example 10b, the soaking time in step (3) is 1 min;

[0163] In Example 10c, the soaking time in step (3) is 60 min.

[0164] The shell of the silicon-carbon composite particles prepared in the above preparation examples includes amorphous carbon.

[0165] Comparative Preparation Example 1

[0166] The preparation was carried out in accordance with Preparation Example 1, except that the temperature, flow rate and time of the silane were changed in step (2). Specifically, before the silane was introduced, the temperature was adjusted to 450°C, and then silane gas with a flow rate of 25 sccm was introduced for 16 h and silane gas with a flow rate of 50 sccm was introduced for 4 h.

[0167] Comparative Preparation Example 2

[0168] The preparation was carried out in accordance with Preparation Example 1, except that the temperature, flow rate and time of the silane were changed in step (2). Specifically, before the silane was introduced, the temperature was adjusted to 550°C, and then silane gas with a flow rate of 200 sccm was introduced for 1 h and silane gas with a flow rate of 400 sccm for 0.5 h.

[0169] Comparative preparation example 3

[0170] The preparation was carried out in accordance with Preparation Example 1, except that in step (2), after stopping the introduction of silane, acetylene gas was not introduced, and step (3) was carried out directly.

[0171] Comparative preparation example 4

[0172] The preparation was carried out in accordance with Preparation Example 1, except that the holding time of the acetylene gas was changed. Specifically, acetylene gas with a flow rate of 100 sccm was introduced and held for 300 min, wherein the thickness t of the outer shell was 15 nm.

[0173] Comparative preparation example 5

[0174] The preparation was carried out in accordance with Preparation Example 1, except that the time of introducing silane was changed. Specifically, silane gas at a flow rate of 100 sccm was introduced for 200 min and silane gas at a flow rate of 200 sccm for 80 min in sequence; wherein c1 was 40%, c2 was 60%, and c2 / c1 was 1.5.

[0175] Comparative preparation example 6

[0176] The preparation was carried out in accordance with Preparation Example 1, except that the time of introducing silane was changed. Specifically, silane gas at a flow rate of 100 sccm was introduced for 20 min and silane gas at a flow rate of 200 sccm for 80 min in sequence; wherein c1 was 40%, c2 was 5%, and c2 / c1 was 0.125.

[0177] The following examples illustrate the battery of the present invention.

[0178] Example 1

[0179] Prepared according to the following method:

[0180] (1) Preparation of negative electrode sheet

[0181] Artificial graphite, silicon-carbon composite particles prepared in Preparation Example 1, sodium carboxymethyl cellulose, styrene-butadiene rubber, and SuperP were mixed in a mass ratio of 87.7:8.8:1.6:1.6:0.3. Deionized water was added, and a negative electrode slurry was obtained under vacuum stirring. The negative electrode slurry was uniformly coated on both sides of a copper foil and dried in an oven at 80°C for 12 hours. After rolling and slitting, a negative electrode sheet was obtained.

[0182] (2) Preparation of positive electrode sheet

[0183] Lithium cobalt oxide, polyvinylidene fluoride, acetylene black, and carbon nanotubes were mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until a uniform positive electrode slurry was formed. The positive electrode slurry was uniformly coated on both sides of an aluminum foil, baked in an oven, and then dried in a 120°C oven for 8 hours. After rolling and slitting, the positive electrode sheet was obtained.

[0184] (3) Preparation of lithium-ion batteries

[0185] The negative electrode sheet, separator (polyethylene film with a thickness of 8 μm) prepared in step (1) and the positive electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte (a mixed solution of lithium hexafluorophosphate dissolved in ethylene carbonate / dimethyl carbonate (volume ratio 1:1) and 5 vol.% fluoroethylene carbonate, wherein the concentration of lithium hexafluorophosphate is 1 mol / L) is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained.

[0186] (4) Fabrication of button cells

[0187] The silicon-carbon composite particles, SuperP, sodium carboxymethyl cellulose, and styrene-butadiene rubber prepared in Example 1 were mixed in a mass ratio of 96.5:1.6:1.6:0.3. Deionized water was added, and the mixture was stirred evenly under vacuum to obtain a button cell negative electrode slurry. The button cell negative electrode slurry was coated onto copper foil, dried in an oven at 80°C, and then dried in a vacuum oven at 100°C for 12 hours to obtain an areal density of approximately 3 mg / cm³. 2 The negative electrode sheet is formed into a 12mm diameter disc using a stamping machine under dry conditions. In a glove box, the negative electrode disc is used as the working electrode, a lithium metal sheet is used as the counter electrode, and a 20μm thick polyethylene membrane is used as the separator. An electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of ethylene carbonate / dimethyl carbonate (volume ratio 1:1) and 5 vol.% fluoroethylene carbonate, wherein the concentration of lithium hexafluorophosphate is 1 mol / L) is added to assemble a coin cell.

[0188] Examples 2-10 and Comparative Examples 1-6 were prepared in accordance with Example 1, except that silicon-carbon composite particles were prepared instead of those prepared in Example 1, as detailed in Table 4.

[0189] Example 11 group

[0190] This set of examples is used to verify the impact of changing the "mass ratio of silicon-carbon composite particles to carbon-based materials".

[0191] This set of embodiments is based on Embodiment 1, except that the mass ratio of silicon-carbon composite particles to artificial graphite is changed, as follows:

[0192] In Example 11a, artificial graphite, silicon-carbon composite particles prepared in Preparation Example 1, sodium carboxymethyl cellulose, styrene-butadiene rubber, and SuperP were mixed in a mass ratio of 8.8:87.7:1.6:1.6:0.3.

[0193] In Example 11b, artificial graphite, silicon-carbon composite particles prepared in Preparation Example 1, sodium carboxymethyl cellulose, styrene-butadiene rubber, and SuperP were mixed in a mass ratio of 91.7:4.8:1.6:1.6:0.3.

[0194] In Example 11c, artificial graphite, silicon-carbon composite particles prepared in Preparation Example 1, sodium carboxymethyl cellulose, styrene-butadiene rubber, and SuperP were mixed in a mass ratio of 94.6:1.9:1.6:1.6:0.3.

[0195] In Example 11d, the silicon-carbon composite particles prepared in Preparation Example 1, sodium carboxymethyl cellulose, styrene-butadiene rubber and SuperP were mixed in a mass ratio of 96.5:1.6:1.6:0.3.

[0196] Test Case I

[0197] (1) Determination of silicon content

[0198] The silicon content of the silicon-carbon composite particles prepared in the preparation examples and comparative preparation examples was determined. First, the silicon-carbon composite particles were mixed with 6% polyacrylic acid solution at a mass ratio of 80:10 and coated on the surface of copper foil. After drying, the mixture was processed by an argon ion cutter to obtain cross-sectional samples of the silicon-carbon composite particles. Then, the silicon content c1 in the first region and the silicon content c2 in the second region of the cross-section of the silicon-carbon composite particles were determined by EDS. The mass content of silicon in the silicon-carbon composite particles was tested by thermogravimetric analysis. The results are recorded in Table 1.

[0199] (2) Determination of the volume ratio of closed pores

[0200] The closed-pore volume of the silicon-carbon composite particles prepared in the preparation examples and comparative preparation examples was measured. The true density of the silicon-carbon composite particles was obtained by the gas volume displacement method, and the closed-pore volume was calculated. The results are recorded in Table 1.

[0201] (3) Measurement of shell thickness

[0202] The shell thickness of the silicon-carbon composite particles prepared in the preparation examples and comparative preparation examples was measured, and the results are recorded in Table 1.

[0203] (4) Element content determination

[0204] The silicon-carbon composite particles prepared in the preparation example were subjected to elemental content testing. The nitrogen and sulfur contents were determined using an elemental analyzer, and the phosphorus and lithium contents were determined using ICP-AES. The results are recorded in Table 2.

[0205] (5) Particle size determination

[0206] The particle size of the silicon-carbon composite particles prepared in the preparation example was measured, and the results were rounded and recorded in Table 2.

[0207] (6) Oil absorption value determination

[0208] The oil absorption value of the silicon-carbon composite particles prepared in the preparation example was measured, and the results are recorded in Table 3.

[0209] (7) Specific surface area measurement

[0210] The specific surface area of ​​the silicon-carbon composite particles prepared in the preparation example was measured, and the results are recorded in Table 3.

[0211] (8) Thermogravimetric test

[0212] The silicon-carbon composite particles prepared in the preparation example were subjected to thermogravimetric analysis, and the results are recorded in Table 3.

[0213] (9) True density determination

[0214] The true density of the silicon-carbon composite particles prepared in the preparation example was measured. The results showed that the true density of the silicon-carbon composite particles prepared in both the preparation example and the comparative preparation example was 1.4 g / cm³. 3 -2.2g / cm 3 .

[0215] Table 1

[0216]

[0217]

[0218] Table 2

[0219]

[0220] Table 3

[0221] Oil absorption value (ml / 100g) <![CDATA[Specific surface area (m 2 / g)]]> Weight gain rate (%) at 600℃-800℃ Preparation Example 1 31 1.7 6.3 Preparation Example 2 79 10.3 0.5 Preparation Example 3 53 0.5 24.8 Preparation Example 4a 31 1.7 6.2 Preparation Example 4b 31 1.7 6.5 Preparation Example 5a 28 1.7 6 Preparation Example 5b 31 1.7 6.2 Preparation Example 6a 33 1.7 6.3 Preparation Example 6b 32 1.7 6.1 Preparation Example 7a 30 1.7 6 Preparation Example 7b 32 1.7 6.2 Preparation Example 8a 30 1.7 6.2 Preparation Example 8b 33 1.7 6.2 Preparation Example 9 20 1.1 6 Preparation Example 10a 31 1.7 6.2 Preparation Example 10b 30 1.7 6 Preparation Example 10c 32 1.7 6.3

[0222] Test Case II

[0223] (1) Loop test

[0224] The lithium-ion batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows:

[0225] The battery was charged at a constant current density of 2C to 4.5V, then charged at a constant voltage of 4.5V with a cutoff current of 0.05C. After resting for 10 minutes, the battery thickness at this point was recorded as the initial thickness. The battery was then discharged at a current density of 1.5C to 3.0V, and then rested for 10 minutes. The discharge capacity at this point was recorded as the initial capacity. This charging and discharging process was repeated until the 500th cycle of constant voltage charging was completed. After resting for 10 minutes, the battery thickness was measured and recorded as the post-cycle thickness. The battery was then discharged at a current density of 1.5C to 3.0V again, and then rested for 10 minutes. The discharge capacity at this point was recorded as the post-cycle capacity. The cycle capacity retention rate = post-cycle capacity × 100% / initial capacity, and the thickness expansion rate = (post-cycle thickness - initial thickness) × 100% / initial thickness. The cycle capacity retention rate and thickness expansion rate are recorded in Table 4.

[0226] (2) Energy density test

[0227] The energy density of the lithium-ion batteries prepared in the examples and comparative examples was tested using the following specific methods:

[0228] The battery was charged to 4.5V at a constant current density of 0.2C, then charged at a constant voltage of 4.5V with a cutoff current of 0.02C. After resting for 10 minutes, it was discharged to 3.0V at a constant current density of 0.2C and rested for 10 minutes. The first discharge capacity and the first discharge energy of the battery were recorded. Subsequently, the battery was charged at a constant current density of 0.2C, with the cutoff condition being that the charging capacity reached half of the first discharge capacity. After the cutoff, the battery was removed, and its thickness, length, and width were measured. The energy density of the battery is calculated as: first discharge energy / (length × width × thickness). The results are recorded in Table 4.

[0229] (3) Rapid discharge capability test

[0230] The lithium-ion batteries prepared in the examples and comparative examples were tested for their rapid discharge capability. The specific test methods are as follows:

[0231] The battery was charged to 4.5V at a constant current density of 0.2C, and then charged at a constant voltage of 4.5V with a cutoff current of 0.02C. After standing for 10 minutes, it was discharged to 3.0V at current densities of 0.2C and 1C respectively. The ratio of the capacity discharged at 1C to the capacity discharged at 0.2C is the battery's rapid discharge capability. The results are recorded in Table 4.

[0232] (4) First Coulomb efficiency test

[0233] The coin cells prepared in Examples 1-10 and the comparative examples were subjected to the first coulombic efficiency test. The specific test method is as follows:

[0234] After the battery was left to stand for 2 hours, it was discharged at a constant current density of 0.1C to 5mV. After standing for 10 minutes, it was discharged again at a current density of 0.01C to 5mV. After standing for another 10 minutes, it was charged at a constant current density of 0.05C to 1.5V. The initial coulombic efficiency is calculated as: charging capacity × 100% / discharging capacity. The results are recorded in Table 4.

[0235] (5) Ion diffusion coefficient test

[0236] After allowing the battery to rest for 2 hours, discharge it to 5mV using a constant current density of 0.1C. After resting for 10 minutes, discharge it again to 5mV using a current density of 0.01C. After resting for another 10 minutes, charge it to 1.5V using a constant current density of 0.05C. After resting for another 10 minutes, repeat the process of discharging at a constant current density of 0.1C for 30 minutes and then resting for 30 minutes until the battery voltage drops below 0.1V. After resting for another 30 minutes, record the voltage E1 at this point. Then discharge it at a constant current density of 0.1C for 30 minutes, recording the voltage E2 at 10 seconds and the voltage E3 at the discharge cutoff. After resting for 60 minutes, record the voltage E4 after 60 minutes. Then, according to the formula...

[0237]

[0238] Silicon-carbon composite particles can be obtained at 0.1V (vs Li) + Lithium-ion diffusion coefficient at potential / Li (unit: cm⁻¹) 2 / s), where τ is the relaxation time, i.e., τ = 3600s; n is the number of moles of silicon-carbon composite particles, V is the molar volume of silicon-carbon composite particles, and n×V is the volume of silicon-carbon composite particles used in the test, which can be calculated based on the mass w of the coating in the coin cell negative electrode and the true density ρ of the silicon-carbon composite particles, n×V = 0.965×w / ρ; S is the geometric area of ​​the coin cell negative electrode, i.e., S = 1.13cm². 2 The results are recorded in Table 4.

[0239] Table 4

[0240]

[0241]

[0242] Note: Since the initial coulombic efficiency and lithium-ion diffusion coefficient were tested using coin cells, and the coin cells in Example 11 were the same as those in Example 1, the initial coulombic efficiency and lithium-ion diffusion coefficient were not tested in Example 11. The “ / ” in Table 4 indicates that the tests were not conducted.

[0243] As can be seen from Table 4, the battery prepared by the negative electrode active material of the present invention can achieve higher energy density, first coulombic efficiency, rate performance and cycle stability compared with the comparative example.

[0244] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A negative electrode active material, characterized in that, The negative electrode active material includes silicon-carbon composite particles; The silicon-carbon composite particles have closed pores, and the volume proportion of the closed pores is 4% - 50%; The silicon-carbon composite particles have a core-shell structure. The shell of the core-shell structure includes carbon elements, and the thickness of the shell is t, where 0 < t ≤ 10 nm; The core of the core-shell structure includes carbon elements and silicon elements, and the core is composed of porous carbon and silicon materials located in the pores of the porous carbon; On the cross-section of the silicon-carbon composite particles, there are a first region and a second region. The mass content of silicon elements in the first region is c1, and the mass content of silicon elements in the second region is c2, where 0.23 < c2 / c1 ≤ 0.76; wherein, the dimension of the perpendicular line of the tangent at any point on the edge of the cross-section on the cross-section is L. On the perpendicular line, the region at a distance of 0.001L - 0.1L from the edge of the cross-section constitutes the first region, and the region at a distance of 0.1L - 0.5L from the edge of the cross-section constitutes the second region.

2. The negative electrode active material according to claim 1, wherein the volume proportion of the closed pores is 5% - 49%; and / or, 0.2 nm ≤ t ≤ 10 nm.

3. The negative electrode active material according to claim 2, wherein, The volume proportion of the closed pores is 11% - 35%; and / or, 1 nm ≤ t ≤ 8 nm.

4. The negative electrode active material according to claim 1 or 2, wherein, The mass content of silicon elements in the silicon-carbon composite particles is 20% - 65%; and / or, the mass content c1 of silicon elements in the first region is 30% - 75%; and / or, the mass content c2 of silicon elements in the second region is 5% - 60%.

5. The negative electrode active material according to claim 4, wherein, The mass content of silicon elements in the silicon-carbon composite particles is 21% - 64%; and / or, the mass content c1 of silicon elements in the first region is 31% - 71%; and / or, the mass content c2 of silicon elements in the second region is 6% - 55%.

6. The negative electrode active material according to claim 5, wherein, The mass content of silicon elements in the silicon-carbon composite particles is 25% - 60%; and / or, the mass content c1 of silicon elements in the first region is 38% - 66%; and / or, the mass content c2 of silicon elements in the second region is 9% - 50%.

7. The negative electrode active material according to claim 1 or 2, wherein, The shell includes amorphous carbon.

8. The negative electrode active material according to claim 1 or 2, wherein, The silicon-carbon composite particles include at least one of nitrogen element, phosphorus element and sulfur element.

9. The negative electrode active material according to claim 8, wherein, The silicon-carbon composite particles include nitrogen element, phosphorus element and sulfur element.

10. The negative electrode active material according to claim 9, wherein, The sum of the mass contents of nitrogen element, phosphorus element and sulfur element in the silicon-carbon composite particles is 15 ppm - 1000 ppm.

11. The negative electrode active material according to claim 10, wherein, The sum of the mass contents of nitrogen element, phosphorus element and sulfur element in the silicon-carbon composite particles is 30 ppm - 500 ppm.

12. The negative electrode active material according to claim 1 or 2, wherein, The particle size Dv90 and Dv10 of the silicon-carbon composite particles satisfy 5 μm ≤ Dv90 - Dv10 ≤ 25 μm; and / or, Dv10 is 1 μm - 6 μm; and / or, Dv90 is 10 μm - 30 μm.

13. The negative electrode active material according to claim 12, wherein 7 μm ≤ Dv90 - Dv10 ≤ 24 μm; and / or, Dv10 is 2 μm - 5 μm; and / or, Dv90 is 12 μm - 20 μm.

14. The negative electrode active material according to claim 13, wherein, 7 μm ≤ Dv90 - Dv10 ≤ 18 μm.

15. The negative electrode active material according to claim 1 or 2, wherein, The oil absorption value of the silicon-carbon composite particles is 10 ml / 100 g - 100 ml / 100 g.

16. The negative electrode active material according to claim 15, wherein, The oil absorption value of the silicon-carbon composite particles is 31ml / 100g-79ml / 100g.

17. The negative electrode active material according to claim 1 or 2, wherein, The silicon-carbon composite particles include lithium.

18. The negative electrode active material according to claim 17, wherein, The lithium content in the silicon-carbon composite particles is 0.1%-20% by mass.

19. The negative electrode active material according to claim 18, wherein, The lithium content in the silicon-carbon composite particles is 1%-5% by mass.

20. The negative electrode active material according to claim 1 or 2, wherein, The thermogravimetric curve of the silicon-carbon composite particles in air atmosphere shows a weight gain peak at 600℃-800℃.

21. The negative electrode active material according to claim 20, wherein, The thermogravimetric curves of the silicon-carbon composite particles in air show a weight gain rate of 0.1%-40% at 600℃-800℃.

22. The negative electrode active material according to claim 21, wherein, The thermogravimetric curves of the silicon-carbon composite particles in air show a weight gain rate of 0.5%-24.8% at 600℃-800℃.

23. The negative electrode active material according to claim 1 or 2, wherein, The true density of the silicon-carbon composite particles is 1.4 g / cm³. 3 -2.2g / cm 3 ; And / or, the particle size Dv50 of the silicon-carbon composite particles is 6μm-15μm; And / or, the specific surface area of ​​the silicon-carbon composite particles is 0.2 m². 2 / g-20m 2 / g.

24. The negative electrode active material according to claim 23, wherein, The particle size Dv50 of the silicon-carbon composite particles is 7μm-12μm; And / or, the specific surface area of ​​the silicon-carbon composite particles is 0.5 m². 2 / g-10.3m 2 / g.

25. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode active material according to any one of claims 1-24.

26. A battery, characterized in that, The battery comprises the negative electrode active material according to any one of claims 1-24 and / or the negative electrode sheet according to claim 25.

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