Silicon-carbon composite material, preparation method thereof, negative electrode sheet and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, and discloses a silicon-carbon composite material, a preparation method thereof, a negative plate and a lithium ion battery. The silicon-carbon composite material comprises a carbon matrix and a silicon-carbon composite layer coated on the carbon matrix; wherein the content of silicon elements in the silicon-carbon composite layer decreases from inside to outside. The silicon-carbon composite material provided by the application has good conductivity, low expansion rate and high specific capacity, and when the silicon-carbon composite material is used as a negative active material of a lithium ion battery, the lithium ion battery has excellent energy density and cycle performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a silicon-carbon composite material, a preparation method thereof, a negative electrode sheet and a lithium ion battery. BACKGROUND

[0002] The rapid development of the new energy vehicle industry puts forward higher requirements on the performance indexes such as energy density, cycle life and safety performance of the matching power battery. At present, the negative electrode material of the power battery is mostly graphite material, which is difficult to meet the market demand. As a new negative electrode material, silicon material has become the most potential negative electrode material because of its high theoretical capacity (4200 mAh / g), low lithium intercalation platform and rich resources. However, the silicon material will have a large-scale volume expansion (about 300%) during the lithium intercalation process, which will cause the structure of the silicon material to be pulverized and the solid electrolyte interface (SEI) to be unstable, ultimately leading to a decrease in the lithium intercalation capacity, and the low conductivity of silicon will also affect the performance of the silicon material.

[0003] Therefore, it is urgent to develop a silicon negative electrode material with good conductivity, low volume expansion rate and good cycle performance. SUMMARY

[0004] The purpose of the present application is to overcome the problems of poor conductivity, high volume expansion rate and poor cycle performance of the silicon negative electrode material in the prior art, and to provide a silicon-carbon composite material, a preparation method thereof, a negative electrode sheet and a lithium ion battery.

[0005] In order to achieve the above-mentioned purpose, the present application provides a silicon-carbon composite material in a first aspect, wherein the silicon-carbon composite material comprises a carbon matrix and a silicon-carbon composite layer coated outside the carbon matrix.

[0006] The content of silicon element in the silicon-carbon composite layer decreases from inside to outside.

[0007] The present application provides a preparation method of a silicon-carbon composite material in a second aspect, wherein the method comprises: performing gas phase deposition of a silicon source and a carbon source on a carbon matrix, the gas flow rate of the silicon source decreases with time, and the gas flow rate of the carbon source increases with time, so as to form a silicon-carbon composite layer with a decreasing content of silicon element from inside to outside, thereby obtaining a silicon-carbon composite material.

[0008] The present application provides a silicon-carbon composite material prepared by the preparation method of the second aspect in a third aspect.

[0009] The present application provides a negative electrode sheet in a fourth aspect, which comprises the silicon-carbon composite material provided by the present application.

[0010] The present application provides a lithium ion battery in a fifth aspect, which comprises the negative electrode sheet provided by the present application.

[0011] By the technical solutions, the application has the following beneficial technical effects:

[0012] In the silicon-carbon composite material provided by the application, the carbon matrix provides an attachment point for the deposition of silicon atoms and carbon atoms, and the carbon matrix and the carbon in the silicon-carbon composite layer together build an internal conductive network for the silicon-carbon composite material, thereby increasing the conductivity of the silicon-carbon composite material.

[0013] In the silicon-carbon composite material provided by the application, the silicon element is distributed in the silicon-carbon composite layer in a decreasing manner from inside to outside, that is, the silicon element is distributed in the composite layer in a decreasing manner along a direction away from the matrix, which can increase the strength of the silicon-carbon composite layer and slow down the influence of the expansion of the silicon lithium intercalation reaction on the overall particle structure of the silicon-carbon composite material.

[0014] Therefore, the silicon-carbon composite material provided by the application has good conductivity, low expansion rate, and high specific capacity, and when the silicon-carbon composite material is used as an active material of a lithium ion battery negative electrode, the lithium ion battery has excellent energy density and cycle performance. DETAILED DESCRIPTION

[0015] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not critical to the application. Any numeric range recited is intended to include all values from the lower value to the upper value. For values which are not numeric, the recited range is intended to include all values between the recited values. Ranges can be expressed as being inclusive or exclusive of the values that fall within the range. For numeric values, the range can be expressed as being inclusive or exclusive of the endpoints. The endpoints of the ranges and any values are provided as examples only and are not intended to be limiting.

[0016] The first aspect of the application provides a silicon-carbon composite material, which comprises a carbon matrix and a silicon-carbon composite layer coated on the carbon matrix.

[0017] The content of the silicon element in the silicon-carbon composite layer decreases from inside to outside.

[0018] In the application, the carbon matrix provides an attachment point for the deposition of silicon atoms and carbon atoms, and the carbon matrix and the carbon in the silicon-carbon composite layer together build an internal conductive network for the silicon-carbon composite material, thereby increasing the conductivity of the silicon-carbon composite material. The silicon element is distributed in the silicon-carbon composite layer in a decreasing manner from inside to outside, that is, the silicon element is distributed in the composite layer in a decreasing manner along a direction away from the matrix, which increases the lithium intercalation reaction strength of the silicon-carbon composite layer and slows down the influence of the expansion of the silicon on the overall particle structure of the silicon-carbon composite material.

[0019] The silicon element is mainly concentrated on the side of the silicon-carbon composite layer close to the carbon matrix, that is, the content of the silicon element decreases from the side of the silicon-carbon composite layer close to the carbon matrix to the side far away from the carbon matrix, most of the silicon element is bound on the inside, the expansion rate of the lithium intercalation is effectively reduced, and the cycle performance of the silicon-carbon composite material is increased.

[0020] In the present application, the content of the silicon element in the silicon-carbon composite layer decreases from the inside to the outside can be determined by measuring the content of the silicon element at different positions from the inside to the outside of the silicon-carbon composite layer. The silicon-carbon composite layer is divided into five equal parts from the side close to the carbon matrix, and five points A, B, C, D and E are taken in turn at the division points. Based on the total weight of the silicon-carbon composite layer, the content of the silicon element at point A is 56-91wt%, the content at point B is 53-71wt%, the content at point C is 48-51wt%, the content at point D is 31-47wt%, and the content at point E is 11-46wt%.

[0021] The silicon-carbon composite material provided by the present application has good conductivity, low expansion rate and high specific capacity, and when used as a negative active material of a lithium ion battery, the lithium ion battery has excellent energy density and cycle performance.

[0022] According to some embodiments of the present application, the carbon matrix is selected from at least one of carbon nanotubes, graphene, carbon black and porous carbon. The type of the carbon matrix can be used singly or in combination, and a person skilled in the art can select according to actual needs.

[0023] It can be understood that the carbon matrix can be carbon nanotubes, which can provide a one-dimensional conductive network inside the silicon-carbon composite material; the carbon matrix can be graphene, which can provide a two-dimensional conductive network inside the silicon-carbon composite material; the carbon matrix can be carbon black, which can provide a dispersed zero-dimensional conductive network inside the silicon-carbon composite material; the carbon matrix can be porous carbon, which has a high specific surface area and abundant pore structure, and can provide abundant sites and three-dimensional conductive networks for the deposition of silicon source and carbon source. The carbon matrix can also be a structure in which carbon nanotubes, graphene, carbon black and porous carbon are mixed in any ratio. It can be understood that the length of the carbon nanotubes is 10-200nm, and the maximum width of the graphene sheet diameter is 50-200nm.

[0024] According to some preferred embodiments of the present application, the carbon matrix is porous carbon, and the average particle size (D 50) is 2-8 μm, preferably 4-6 μm. If the average particle size of the porous carbon is too small, the silicon deposition process is prone to deposit silicon on the surface of the porous carbon, resulting in silicon-rich surface of the porous carbon, which is not conducive to the dispersion of silicon elements and causes more side reactions on the surface of the material and serious performance degradation. If the size of the porous carbon is too large, the silicon-carbon composite layer deposition process requires a long time, and in addition, it also leads to large particles of the silicon-carbon composite material and long lithium-embedding path, which is not conducive to the use of the silicon-carbon composite material.

[0025] According to some preferred embodiments of the present application, the silicon in the silicon-carbon composite layer comprises amorphous silicon and optional silicon grains. The amorphous silicon is isotropic, and when the amorphous silicon undergoes the lithium-embedding reaction, the generated stress is small, the structure is relatively stable, and the volume expansion rate is low.

[0026] According to some preferred embodiments of the present application, the silicon-carbon composite layer further comprises silicon grains, and the average particle size (D 50 ) of the silicon grains is 8 nm or less, preferably 3 nm or less. In the present application, if the size of the silicon grains is too large, the silicon will expand more seriously during the lithium-embedding process, the negative electrode sheet will expand more, and the cycle performance will be poorer.

[0027] According to some preferred embodiments of the present application, the total content of the amorphous silicon and the optional silicon grains in the silicon-carbon composite layer is 20-40 wt%, for example, 20 wt%, 23 wt%, 25 wt%, 30 wt%, 35 wt%, 37 wt%, 40 wt%, and any value within the range consisting of any two of the above-mentioned values, preferably 25-35 wt%, based on the total amount of the silicon-carbon composite material.

[0028] The content of the carbon element in the silicon-carbon composite layer is distributed in an increasing manner from the inside to the outside.

[0029] According to some embodiments of the present application, the carbon in the silicon-carbon composite layer is amorphous carbon. The carbon lattice in the amorphous carbon is arranged in disorder, which helps to alleviate the lattice change caused by the lithium ion embedding and ensures the structural stability of the carbon.

[0030] According to some preferred embodiments of the present application, the content of the amorphous carbon in the silicon-carbon composite layer is 20-40 wt%, for example, 20 wt%, 23 wt%, 25 wt%, 30 wt%, 35 wt%, 37 wt%, 40 wt%, and any value within the range consisting of any two of the above-mentioned values, preferably 25-35 wt%, based on the total amount of the silicon-carbon composite material.

[0031] Both silicon and carbon can be used as carriers for lithium ion storage to provide energy density for lithium ion batteries. The theoretical specific capacity of silicon is higher, but its conductivity and cycle stability are poor; the specific capacity of carbon is lower than that of silicon, but its conductivity and cycle stability are good. In addition, the presence of carbon can effectively alleviate the problem of easy oxidation of silicon. The content of silicon (amorphous silicon and optional silicon grains) and the content of carbon (amorphous carbon) are controlled within a suitable range in the present application to further optimize the specific capacity and cycle stability of the silicon-carbon composite material.

[0032] According to some preferred embodiments of the present application, the silicon-carbon composite material further comprises a carbon coating layer coated outside the silicon-carbon composite layer, which can prevent the silicon from being exposed on the surface of the material, leading to poor performance.

[0033] According to some preferred embodiments of the present application, the thickness of the carbon coating layer is 5-100 nm, such as 5 nm, 10 nm, 30 nm, 50 nm, 80 nm, 100 nm, and any value within the range of any two of the above values, preferably 10-30 nm. In the present application, if the thickness of the carbon coating layer is too small, it will lead to the risk of uneven and incomplete coating; if the thickness of the carbon coating layer is too large, the preparation reaction time will be too long, which will lead to the risk of increased silicon grain size and increased carbon layer side reactions.

[0034] According to some preferred embodiments of the present application, the average particle size (D 50 ) of the silicon-carbon composite material is 3-15 μm, such as 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, and any value within the range of any two of the above values, preferably 5-10 μm. In the present application, if the average particle size (D 50 ) of the silicon-carbon composite material is too small, the specific surface area of the material is too high, the surface side reactions are too many, and the cycle performance is poor; if the average particle size is too large, the diffusion distance of lithium ions in the silicon-carbon composite material will increase, part of the silicon will not be fully reacted, and the utilization rate will be low; at the same time, the silicon-carbon composite material in this particle size range can also increase the compaction density of the negative electrode sheet, thereby increasing the energy density of the battery.

[0035] In the present application, the content of silicon element in the silicon-carbon composite layer outside the carbon matrix decreases from inside to outside, which can make the performance of lithium ion battery stable during the cycle and storage process. The use of amorphous silicon and amorphous carbon can reduce the expansion rate of the material and improve the cycle performance. By controlling the content of amorphous silicon and optional silicon grains and the content of amorphous carbon, a silicon-carbon composite material with better performance can be obtained.

[0036] The second aspect of the present application provides a method for preparing a silicon-carbon composite material, wherein the method comprises: performing vapor deposition of a silicon source and a carbon source on a carbon base, a gas flow rate of the silicon source decreases over time, and a gas flow rate of the carbon source increases over time, to form a silicon-carbon composite layer with a content of silicon elements decreasing from inside to outside, thereby obtaining the silicon-carbon composite material.

[0037] In the present application, the above method is simple, fast, and easy to operate, and can obtain a silicon-carbon composite material with good structural stability, and the distribution state of silicon elements in the silicon-carbon composite layer is controllable.

[0038] In the present application, the silicon source and the carbon source form a silicon-carbon composite layer on the surface of the carbon base through chemical reaction. In this process, the silicon source and the carbon source diffuse to the surface of the carbon base, and then deposit, crack (or crack first and then deposit), and rearrange atoms to form corresponding silicon nuclei and carbon nuclei, and the nuclei grow and connect with each other to form an amorphous structure.

[0039] According to some preferred embodiments of the present application, the gas flow rate of the silicon source decreases uniformly over time, and the gas flow rate of the carbon source increases uniformly over time, so that the distribution of silicon and carbon elements in the silicon-carbon composite layer is more uniform, and a silicon-carbon composite layer with a content of silicon elements and carbon elements uniformly decreasing from inside to outside can be formed.

[0040] Further, the gas flow rate of the silicon source decreases uniformly over time at a rate of 0.056-0.333 sccm / min, and the gas flow rate of the carbon source increases uniformly over time at a rate of 0.056-0.333 sccm / min.

[0041] The gas flow rate of the silicon source and the carbon source determines the probability of their contact with the carbon base, and in turn determines the number of deposition nucleation sites. The greater the gas flow rate, the greater the probability of contact with the carbon base, the more corresponding deposition nucleation sites are generated, and the more corresponding silicon elements or carbon elements are generated. Understandably, when the gas flow rate of the silicon source is greater than that of the carbon source, the probability of the silicon source diffusing to the surface of the carbon base is greater, and the number of silicon deposition nucleation sites formed on the surface of the carbon base is greater, and in turn the content of silicon elements is greater.

[0042] Therefore, the key point of the present application is to control the gas flow rate of the silicon source and the carbon source, increase the probability of their contact with the carbon base or themselves after cracking, and regulate their distribution state.

[0043] For example, monosilane is decomposed into silicon atoms as a silicon source, and ethylene is decomposed into carbon atoms as a carbon source, and the silicon atoms and the carbon atoms form a silicon-carbon composite layer with a content of silicon elements and carbon elements independently decreasing from inside to outside after deposition on the carbon base.

[0044] According to some preferred embodiments of the present application, the ratio of the gas flow rates of the silicon source and the carbon source changes uniformly from 8-17:1 to 1:8-17 over time. Within this ratio range, the content of silicon elements is beneficially distributed in a decreasing manner from the inside to the outside, reaching the aforementioned case where the silicon content of the silicon-carbon composite layer is measured to be different from the inside to the outside (see the aforementioned five-equal-part measurement). If the ratio of the gas flow rates of the silicon source and the carbon source is too small, the content of silicon elements is not obviously distributed in a decreasing manner from the inside to the outside, the difference in the content of silicon elements between the inside and the outside is small, the effect is not achieved, and the performance is poor. If the ratio of the gas flow rates of the silicon source and the carbon source is too large, the content of silicon elements is obviously distributed in a decreasing manner from the inside to the outside, the difference in the content of silicon elements between the inside and the outside is large, and the stress distribution of the material embedded with lithium is uneven, resulting in unstable material structure.

[0045] According to some embodiments of the present application, the silicon source is selected from at least one of monosilane, disilane, trisilane, silicon chloride, silicon trichloride, hexachlorodisilane, and dichlorosilane.

[0046] According to some preferred embodiments of the present application, the carbon source is selected from at least one of acetylene, methane, ethane, ethylene, carbon monoxide, and carbon dioxide.

[0047] According to some preferred embodiments of the present application, the temperature of the vapor deposition is 300-700°C, for example 300°C, 400°C, 500°C, 600°C, 700°C, and any value within the range between any two of the above-mentioned values, and is preferably 400-600°C. In the present application, if the temperature of the vapor deposition is too high, the silicon grains grow, the crystallinity increases, and the cycle performance of the silicon-carbon material is adversely affected. If the temperature of the vapor deposition is too low, the cracking efficiency of the carbon source is low, and the quality of the product is poor.

[0048] According to some preferred embodiments of the present application, the gas flow rate of the silicon source and the carbon source is independently 1-90 sccm, for example 1 sccm, 5 sccm, 10 sccm, 25 sccm, 40 sccm, 50 sccm, 60 sccm, 75 sccm, 80 sccm, 85 sccm, 90 sccm, and any value within the range between any two of the above-mentioned values, and is preferably 5-85 sccm. In the present application, if the gas flow rate of the silicon source and the carbon source is too small, the deposition speed is slow, and the reaction time is too long. If the gas flow rate of the silicon source and the carbon source is too large, the gas is carried away by the gas flow before it can contact the carbon matrix, and thus the atomic deposition after the gas cracking is insufficient.

[0049] According to some preferred embodiments of the present application, the aeration time of the silicon source and the carbon source is 250-500 min, for example 250 min, 280 min, 320 min, 350 min, 380 min, 410 min, 450 min, 475 min, 500 min, and any value within the range between any two of the above-mentioned values, preferably 300-400 min. In the present application, if the aeration time of the silicon source and the carbon source is too short, the particles generated after deposition are too small; if the aeration time of the silicon source and the carbon source is too long, the particles generated after deposition are too large.

[0050] According to some preferred embodiments of the present application, the silicon source and the carbon source are respectively introduced together with a carrier gas. According to some preferred embodiments of the present application, the ratio of the flow rate of the carrier gas to the flow rate of the gas of the silicon source or the carbon source is 4-5.5:1, for example 4:1, 4.2:1, 4.5:1, 4.8:1, 5:1, 5.2:1, 5.5:1, and any value within the range between any two of the above-mentioned values, preferably 4-5:1. In the present application, if the flow rate of the carrier gas is too small, the deposition speed is slow and the reaction time is too long; if the flow rate of the carrier gas is too large, the gas cracking efficiency is low and the atomic deposition after cracking is insufficient.

[0051] According to some embodiments of the present application, after the silicon source and the carbon source are subjected to vapor deposition on the carbon substrate, vapor carbon coating is performed to form a carbon coating layer.

[0052] According to some preferred embodiments of the present application, the carbon source for vapor carbon coating is selected from at least one of methane, ethylene, acetylene, propane and propylene.

[0053] According to some preferred embodiments of the present application, the temperature for vapor carbon coating is 600-1200℃, for example 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, 1200℃, and any value within the range between any two of the above-mentioned values, preferably 800-1000℃; the time is 50-500 min, for example 50 min, 100 min, 150 min, 200 min, 250 min, 300 min, 350 min, 400 min, 450 min, 500 min, and any value within the range between any two of the above-mentioned values, preferably 100-400 min. In the present application, if the temperature for vapor carbon coating is too low, the carbon source cannot be completely cracked at too low a temperature; if the temperature for vapor carbon coating is too high, the silicon grains grow and the crystallinity increases. If the time for vapor carbon coating is too short, the carbon content is low and there is a risk of incomplete coating; if the time for vapor carbon coating is too long, the thickness of the coated carbon layer increases and the side reactions increase.

[0054] According to some preferred embodiments of the present application, the gas flow rate of the carbon source in the gas phase is 5-60 sccm, for example 5 sccm, 10 sccm, 15 sccm, 20 sccm, 25 sccm, 30 sccm, 35 sccm, 40 sccm, 45 sccm, 50 sccm, 55 sccm, 60 sccm, and any value within the range between any two of the above-mentioned values, preferably 10-50 sccm. In the present application, if the gas flow rate of the carbon source is too small, the carbon content will be low and there will be a risk of incomplete coating, or the coating time will be too long; if the gas flow rate of the carbon source is too large, the carbon atoms after cracking of the carbon source will not be able to effectively deposit, resulting in low synthesis efficiency and waste of raw materials.

[0055] According to some embodiments of the present application, the carbon matrix is selected from at least one of carbon nanotubes, graphene, carbon black and porous carbon, preferably porous carbon.

[0056] According to some preferred embodiments of the present application, the silicon in the silicon-carbon composite layer comprises amorphous silicon and optional silicon grains.

[0057] According to some preferred embodiments of the present application, the average particle size (D 50 ) of the silicon grains is 8 nm or less, preferably 3 nm or less.

[0058] According to some preferred embodiments of the present application, the carbon in the silicon-carbon composite layer is amorphous carbon.

[0059] According to some preferred embodiments of the present application, the thickness of the carbon coating layer is 5-100 nm, preferably 10-30 nm.

[0060] According to some preferred embodiments of the present application, the average particle size of the silicon-carbon composite material is 3-15 μm, preferably 5-10 μm.

[0061] According to some preferred embodiments of the present application, the average particle size (D 50 ) of the porous carbon is 2-8 μm, preferably 4-6 μm.

[0062] According to some preferred embodiments of the present application, the content of the amorphous silicon and optional silicon grains in the silicon-carbon composite layer is 20-40 wt%, preferably 25-35 wt%, based on the total amount of the silicon-carbon composite material.

[0063] According to some preferred embodiments of the present application, the content of the amorphous carbon in the silicon-carbon composite layer is 20-40 wt%, preferably 25-35 wt%, based on the total amount of the silicon-carbon composite material.

[0064] The silicon-carbon composite material prepared by the preparation method comprises a carbon matrix and a silicon-carbon composite layer, and the content of silicon elements in the silicon-carbon composite layer decreases from inside to outside, so that the structure of the lithium ion battery is stable during the cycle and storage processes, and excellent charge-discharge performance and cycle performance are obtained.

[0065] According to a particularly preferred embodiment of the present application, a preparation method of a silicon-carbon composite material comprises: firstly performing gas phase deposition of a silicon source and a carbon source on a carbon matrix, wherein the gas flow rate of the silicon source uniformly decreases over time, and the gas flow rate of the carbon source uniformly increases over time, so as to form a silicon-carbon composite layer with a decreasing content of silicon elements from inside to outside; and secondly performing gas phase carbon coating to form a carbon coating layer, thereby obtaining the silicon-carbon composite material.

[0066] The ratio of the gas flow rates of the silicon source and the carbon source uniformly changes from 8-17:1 to 1:8-17 over time.

[0067] The silicon source is selected from at least one of monosilane, disilane, trisilane, silicon chloride, silicon trichloride, hexachlorodisilane and dichlorosilane; and the carbon source is selected from at least one of acetylene, methane, ethane, ethylene, carbon monoxide and carbon dioxide.

[0068] The temperature of the gas phase deposition is 400-600℃.

[0069] The gas flow rate of the silicon source and the carbon source is independently 5-85sccm; and the gas flow time of the silicon source and the carbon source is 300-400min.

[0070] The silicon source and the carbon source are introduced together with a carrier gas; and the ratio of the flow rate of the carrier to the gas flow rate of the silicon source or the carbon source is 4-5.5:1.

[0071] The carbon source for the gas phase carbon coating is selected from at least one of methane, ethylene, acetylene, propane and propylene; the temperature of the gas phase carbon coating is 800-1000℃; the time is 100-400min; and the gas flow rate of the carbon source for the gas phase carbon coating is 10-50sccm.

[0072] The third aspect of the present application provides a silicon-carbon composite material prepared by the preparation method of the second aspect.

[0073] The fourth aspect of the present application provides a negative electrode sheet, which comprises the silicon-carbon composite material provided by the present application.

[0074] The fifth aspect of the present application provides a lithium ion battery, which comprises the negative electrode sheet provided by the present application.

[0075] In a preferred embodiment of the present application, the lithium ion battery has a coulombic efficiency of 82.5-91.5%, an expansion rate of 25-55% after 50 cycles, and a capacity retention rate of 30-70% after 100 cycles.

[0076] In another preferred embodiment of the present application, the lithium ion battery has a coulombic efficiency of 85.5-89.5%, an expansion rate of 28-48% after 50 cycles, and a capacity retention rate of 45-65% after 100 cycles.

[0077] In a more preferred embodiment of the present application, the lithium ion battery has a coulombic efficiency of 88.5-89.5%, an expansion rate of 28-31% after 50 cycles, and a capacity retention rate of 60-65% after 100 cycles.

[0078] The silicon-carbon composite material of the present application can be used as a negative active material of a lithium ion battery, and the lithium ion battery has excellent charge-discharge performance and cycle performance.

[0079] The present application will be described in detail below with reference to examples.

[0080] In the following examples and comparative examples, the specific conditions not specified are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained by market purchase.

[0081] The test methods for the parameters of the silicon-carbon composite material are as follows:

[0082] The size of the silicon grains: the size of the silicon grains is calculated using the Scherrer formula based on the XRD pattern of the refined silicon-carbon composite material, and the Scherrer formula is: D = Kγ / (Bcosθ), wherein K is the Scherrer constant, D is the grain size, B is the measured half-height width of the sample diffraction peak, θ is the Bragg angle, and γ is the X-ray wavelength.

[0083] The measurement method for the carbon content and the silicon content in the silicon-carbon composite layer is as follows: the carbon content of the carbon matrix is determined by weighing before deposition; the silicon-carbon composite material before carbon coating is oxidized by oxygen at high temperature in a combustion furnace, and carbon dioxide is generated after carbon oxidation. The carbon content is measured by a carbon detection device, and the carbon content measured by the device is the sum of the carbon content in the carbon matrix and the silicon-carbon composite layer. The carbon content in the silicon-carbon composite layer is calculated by subtracting the carbon content of the carbon matrix from the carbon content measured by the device. The silicon-carbon composite material is composed of silicon, carbon and oxygen elements, and the oxygen element content is extremely small (≤2 wt%). In order to simplify, the oxygen element content is ignored, and the silicon content is obtained by subtracting the carbon content from the total amount of the silicon-carbon composite material.

[0084] The thickness of the carbon coating layer: the coating layer of the silicon-carbon composite material is observed by TEM transmission electron microscope, and the thickness of the coating layer is calculated according to the magnification and the scale.

[0085] Particle size: The sample was treated before testing, ultrasonic dispersion in anhydrous ethanol for 5 minutes, ultrasonic frequency 59 kHz, power 45 W, the test was carried out in the dispersion medium industrial ethanol, the laser particle size analyzer irradiation optical path / volume was 25 cm / 200 mL, the average particle size of the silicon-carbon composite material was tested, that is, D50.

[0086] The performance of the button cell was tested, and the test method was as follows:

[0087] Charge-discharge performance test: using LANHE blue cell test equipment, the button cell was discharged at 0.1C current to 0.005V at room temperature, then charged to 1.5V at 0.1C, and the first discharge capacity and the first charge capacity of the battery were recorded. Coulomb efficiency (%) = first charge capacity / first discharge capacity x 100%.

[0088] Expansion test: Before assembling the button cell, the original thickness of each group of sample electrode sheets was measured with a micrometer. After 50 cycles, discharge to 0% SOC, disassemble the battery, take out the negative electrode sheet, and clean it with DMC solution, then dry and test the thickness, and calculate the expansion rate, expansion rate (%) = (thickness of electrode sheet after cycling-thickness of electrode sheet original thickness) x 100%.

[0089] Cycle performance test: the button cell was discharged at 0.1C to 0.005V at room temperature, then charged to 1.5V at 0.1C, and the remaining capacity of the battery was recorded after 100 cycles.

[0090] Example 1

[0091] This example is used to illustrate the preparation of a silicon-carbon composite material.

[0092] (1) The average particle size (D 50 ) of the porous carbon used as the carbon matrix was 4.8 μm, and silane and ethylene were continuously introduced into the vacuum rotary furnace, wherein the ratio of the gas flow rate of silane to the gas flow rate of ethylene gradually and uniformly changed from 12:1 to 1:12. At the beginning, the gas flow rate of silane was 60 sccm and the gas flow rate of ethylene was 5 sccm; at the end, the gas flow rate of silane was 5 sccm and the gas flow rate of ethylene was 60 sccm, and the gas flow time was 360 min; the gas flow rate of silane uniformly decreased with time at a rate of 0.153 sccm / min, and the gas flow rate of ethylene uniformly increased with time at a rate of 0.153 sccm / min. Nitrogen was continuously introduced as a carrier gas during the introduction of silane and ethylene, wherein the flow rate of nitrogen was 5 times the gas flow rate of silane or ethylene;

[0093] (2) the thermal decomposition of silane and ethylene in a vacuum rotary furnace, wherein the silane decomposes silicon atoms and the ethylene decomposes carbon atoms, and the silicon atoms and carbon atoms deposit on the porous carbon to form a silicon-carbon composite layer with a gradient distribution of silicon content from inside to outside; wherein the rotation speed of the vacuum rotary furnace is 8° / min, the temperature in the furnace is 500℃, and the inclination angle of the rotary furnace is 15°;

[0094] (3) gas-phase carbon coating with methane and acetylene (mass ratio 1:1) as carbon sources to form a carbon-coated layer outside the silicon-carbon composite layer, wherein the coating temperature is 900℃, the time is 300min, and the gas flow rate of the carbon source is 20sccm, to obtain a silicon-carbon composite material.

[0095] Example 2

[0096] The silicon-carbon composite material was prepared according to the method of Example 1, except that the ratio of the gas flow rate of silane to the gas flow rate of ethylene gradually and uniformly changed from 16:1 to 1:16. At the beginning, the gas flow rate of silane was 80sccm and the gas flow rate of ethylene was 5sccm; at the end, the gas flow rate of silane was 5sccm and the gas flow rate of ethylene was 80sccm; the gas flow rate of silane uniformly decreased at a rate of 0.208sccm / min over time, and the gas flow rate of ethylene uniformly increased at a rate of 0.208sccm / min over time. A silicon-carbon composite material was obtained.

[0097] Example 3

[0098] The silicon-carbon composite material was prepared according to the method of Example 1, except that the ratio of the gas flow rate of silane to the gas flow rate of ethylene gradually and uniformly changed from 8:1 to 1:8. At the beginning, the gas flow rate of silane was 40sccm and the gas flow rate of ethylene was 5sccm; at the end, the gas flow rate of silane was 5sccm and the gas flow rate of ethylene was 40sccm; the gas flow rate of silane uniformly decreased at a rate of 0.097sccm / min over time, and the gas flow rate of ethylene uniformly increased at a rate of 0.097sccm / min over time. A silicon-carbon composite material was obtained.

[0099] Example 4

[0100] The silicon-carbon composite material was prepared according to the method of Example 1, except that the temperature for gas-phase deposition was different. Specifically, in step (2), the temperature in the furnace was 620℃. A silicon-carbon composite material was obtained.

[0101] Example 5

[0102] A silicon-carbon composite was prepared according to the method of Example 1, except that the temperature of the vapor deposition was varied. Specifically, in step (2), the temperature of the furnace was 380°C. A silicon-carbon composite was obtained.

[0103] Example 6

[0104] A silicon-carbon composite was prepared according to the method of Example 1, except that in step (3), the coating temperature was 1100°C. A silicon-carbon composite was obtained.

[0105] Example 7

[0106] A silicon-carbon composite was prepared according to the method of Example 1, except that in step (3), the coating temperature was 700°C. A silicon-carbon composite was obtained.

[0107] Example 8

[0108] A silicon-carbon composite was prepared according to the method of Example 1, except that the ratio of the flow rate of silane to the flow rate of ethylene was gradually and uniformly changed from 25:1 to 1:25. At the beginning, the flow rate of silane was 125 seem and the flow rate of ethylene was 5 seem; at the end, the flow rate of silane was 5 seem and the flow rate of ethylene was 125 seem; the flow rate of silane was uniformly decreased at a rate of 0.333 seem / min over time and the flow rate of ethylene was uniformly increased at a rate of 0.333 seem / min over time. A silicon-carbon composite was obtained.

[0109] Example 9

[0110] A silicon-carbon composite was prepared according to the method of Example 1, except that the ratio of the flow rate of silane to the flow rate of ethylene was gradually and uniformly changed from 5:1 to 1:5. At the beginning, the flow rate of silane was 25 seem and the flow rate of ethylene was 5 seem; at the end, the flow rate of silane was 5 seem and the flow rate of ethylene was 25 seem; the flow rate of silane was uniformly decreased at a rate of 0.056 seem / min over time and the flow rate of ethylene was uniformly increased at a rate of 0.056 seem / min over time. A silicon-carbon composite was obtained.

[0111] Example 10

[0112] A silicon-carbon composite was prepared according to the method of Example 1, except that the temperature of the vapor deposition was varied. Specifically, in step (2), the temperature of the furnace was 750°C. A silicon-carbon composite was obtained.

[0113] Example 11

[0114] A silicon-carbon composite material was prepared according to the method of Example 1, except that the temperature of vapor deposition was different. Specifically, in step (2), the temperature in the furnace was 250°C. A silicon-carbon composite material was obtained.

[0115] Example 12

[0116] A silicon-carbon composite material was prepared according to the method of Example 1, except that in step (3), the coating temperature was 1300°C. A silicon-carbon composite material was obtained.

[0117] Example 13

[0118] A silicon-carbon composite material was prepared according to the method of Example 1, except that in step (3), the coating temperature was 500°C. A silicon-carbon composite material was obtained.

[0119] Example 14

[0120] A silicon-carbon composite material was prepared according to the method of Example 1, except that step (3) was not performed, i.e., the silicon-carbon composite material was not coated with carbon. A silicon-carbon composite material was obtained.

[0121] Comparative Example 1

[0122] A silicon-carbon composite material was prepared according to the method of Example 14, except that in step (1), the gas flow rate of silane was 60 seem, the gas flow rate of ethylene was 0 seem, and the aeration time was 180 min, and the gas flow rate was not changed during this process; then the gas flow rate was adjusted again, so that the gas flow rate of silane was 0 seem, the gas flow rate of ethylene was 60 seem, and the aeration time was 180 min, and the gas flow rate was not changed during this process. A silicon-carbon composite material was obtained.

[0123] Comparative Example 2

[0124] A silicon-carbon composite material was prepared according to the method of Example 14, except that in step (1), the gas flow rate of silane was 0 seem, the gas flow rate of ethylene was 60 seem, and the aeration time was 180 min, and the gas flow rate was not changed during this process; then the gas flow rate was adjusted again, so that the gas flow rate of silane was 60 seem, the gas flow rate of ethylene was 0 seem, and the aeration time was 180 min, and the gas flow rate was not changed during this process. A silicon-carbon composite material was obtained.

[0125] Comparative Example 3

[0126] A silicon-carbon composite material was prepared according to the method of Example 14, except that the gas flow rates of silane and ethylene were kept constant during the entire vapor deposition process, wherein the gas flow rates of silane and ethylene were both 30 seem, and the aeration time was 360 min. A silicon-carbon composite material was obtained.

[0127] The silicon element and the carbon element in the silicon-carbon composite layer of the silicon-carbon composite material are uniformly distributed.

[0128] Comparative Example 4

[0129] The silicon-carbon composite material was prepared according to the method of Example 14, except that the silane and ethylene were periodically introduced. Specifically, in step (1), the silane was first introduced at a gas flow rate of 60 seem for 10 min, and the gas flow rate was kept constant during the introduction; then the ethylene was introduced at a gas flow rate of 60 seem for 10 min, and the gas flow rate was kept constant during the introduction; the above process was repeated 20 times. The silicon-carbon composite material was obtained.

[0130] After the deposition of silicon atoms and carbon atoms on the porous carbon, the silicon-carbon composite layer was formed, in which the silicon element layer and the carbon element layer were periodically and separately distributed.

[0131] Comparative Example 5

[0132] The silicon-carbon composite material was prepared according to the method of Example 14, except that in step (1), the gas flow rate of the silane was uniformly increased over time, and the gas flow rate of the ethylene was uniformly decreased over time, and the ratio of the gas flow rate of the silane to the gas flow rate of the ethylene gradually and uniformly changed from 1:12 to 12:1. At the beginning, the gas flow rate of the silane was 5 seem, and the gas flow rate of the ethylene was 60 seem; at the end, the gas flow rate of the silane was 60 seem, and the gas flow rate of the ethylene was 5 seem, and the gas flow rate was kept constant during the introduction; the gas flow rate of the silane was uniformly increased at a rate of 0.153 seem / min over time, and the gas flow rate of the ethylene was uniformly decreased at a rate of 0.153 seem / min over time. The silicon-carbon composite material was obtained.

[0133] After the deposition of silicon atoms and carbon atoms on the porous carbon, the silicon-carbon composite layer was formed, in which the content of the silicon element was distributed from inside to outside in an increasing manner, and the content of the carbon element was distributed from inside to outside in a decreasing manner.

[0134] The parameters of the silicon-carbon composite materials obtained in the examples and comparative examples are shown in Table 1.

[0135] Table 1

[0136]

[0137] Note: “-” represents that the parameter does not exist;

[0138] The silicon content is the total content of amorphous silicon and silicon grains;

[0139] The carbon content and the silicon content in the silicon-carbon composite layer are based on the total weight of the silicon-carbon composite material.

[0140] Test Example 1

[0141] The distribution of silicon in the silicon-carbon composite layer of each example and the comparative example was characterized by Energy Dispersive Spectrometer (EDS) testing of the cross-section of the silicon-carbon composite material particles of each example and the comparative example. The testing positions were five equal parts from the carbon matrix side outward, and five points (A, B, C, D and E) were taken in turn at the division points, and the silicon content was tested respectively. The test results are shown in Table 2.

[0142] Table 2

[0143]

[0144] Test Example 2

[0145] The silicon-carbon composite material obtained in each example and the comparative example was mixed with acetylene black and sodium carboxymethyl cellulose at a mass ratio of 8:1:1, and then coated on a copper foil to obtain a negative electrode sheet; a lithium metal sheet was used as a counter electrode, and a PE / PP composite film was used as a separator to make a button cell by a conventional method in the art. The charge-discharge performance, expansion rate and cycle performance of the button cell were tested, and the test results are shown in Table 3.

[0146] Table 3

[0147]

[0148] As can be seen from the results of Table 3, the content of silicon element in the silicon-carbon composite layer of the silicon-carbon composite material prepared in Examples 1-14 decreases from inside to outside, the button cell prepared by using the silicon-carbon composite material prepared in Examples 1-14 as negative active material has higher coulombic efficiency, lower expansion rate and better cycle performance. The distribution of silicon element and carbon element in the silicon-carbon composite layer of the silicon-carbon composite material prepared in Comparative Example 1 is that the inner layer is silicon element layer and the outer layer is carbon element layer; the distribution of silicon element and carbon element in the silicon-carbon composite layer of the silicon-carbon composite material prepared in Comparative Example 2 is that the inner layer is carbon element layer and the outer layer is silicon element layer; the silicon element and carbon element in the silicon-carbon composite layer of the silicon-carbon composite material prepared in Comparative Example 3 are uniformly distributed; the silicon element layer and carbon element layer in the silicon-carbon composite layer of the silicon-carbon composite material prepared in Comparative Example 4 are periodically and intervally distributed; the content of silicon element in the silicon-carbon composite layer of the silicon-carbon composite material prepared in Comparative Example 5 increases from inside to outside, the button cell prepared by using the silicon-carbon composite material prepared in Comparative Examples 1-5 as negative active material has significantly reduced coulombic efficiency and capacity retention rate after 100 cycles, and significantly increased expansion rate. Therefore, when the content of silicon element in the silicon-carbon composite layer of the silicon-carbon composite material decreases from inside to outside, the button cell prepared has excellent charge-discharge performance and cycle performance, and has lower expansion rate. Among them, the button cell prepared by using the silicon-carbon composite material prepared in Examples 1-3 as negative active material has the best performance, the coulombic efficiency is above 88%, the expansion rate is below 38%, and the capacity retention rate after 100 cycles is above 55%.

[0149] In addition, Examples 4, 5, 10 and 11 change the temperature of vapor deposition, Examples 6, 7, 12 and 13 change the temperature of carbon coating, compared with Example 1, the coulombic efficiency and the capacity retention rate after 100 cycles of the button cell prepared are all reduced; the silicon-carbon composite material of Example 14 is not subjected to carbon coating, compared with Example 1, the coulombic efficiency and the capacity retention rate after 100 cycles of the button cell prepared are all reduced, and the expansion rate is increased; Examples 8 and 9 change the ratio of the gas flow rate of silane to the gas flow rate of ethylene, compared with Example 1, the coulombic efficiency and the capacity retention rate after 100 cycles of the button cell prepared are all reduced. Therefore, carbon coating is performed on the silicon-carbon composite material, and the temperature of vapor deposition, the temperature of carbon coating, the ratio of the gas flow rate of silicon source to the gas flow rate of carbon source are selected, so that the button cell prepared from the silicon-carbon composite material has more excellent charge-discharge performance and cycle performance, and lower expansion rate.

[0150] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including that each technical feature is combined in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A silicon-carbon composite material, characterized by, The silicon-carbon composite material comprises a carbon matrix and a silicon-carbon composite layer coated on the carbon matrix. The content of silicon in the silicon-carbon composite layer decreases from inside to outside.

2. The silicon-carbon composite material of claim 1, wherein, The carbon matrix is selected from at least one of carbon nanotubes, graphene, carbon black and porous carbon.

3. The silicon-carbon composite material of claim 2, wherein, The carbon matrix is porous carbon.

4. The silicon-carbon composite material of claim 3, wherein, The average particle size of the porous carbon is 2-8 μm.

5. The silicon-carbon composite material of claim 4, wherein, The average particle size of the porous carbon is 4-6 μm.

6. The silicon-carbon composite material of any one of claims 1-5, wherein, The silicon in the silicon-carbon composite layer comprises amorphous silicon and optional silicon grains.

7. The silicon-carbon composite material of claim 6, wherein, The average particle size of the silicon grains is less than 8 nm. The total content of the amorphous silicon and optional silicon grains in the silicon-carbon composite layer is 20-40 wt% based on the total amount of the silicon-carbon composite material.

8. The silicon-carbon composite material of claim 7, wherein, The average particle size of the silicon grains is less than 3 nm. The total content of the amorphous silicon and optional silicon grains in the silicon-carbon composite layer is 25-35 wt% based on the total amount of the silicon-carbon composite material.

9. The silicon-carbon composite of any one of claims 1-5, wherein, The carbon in the silicon-carbon composite layer is amorphous carbon.

10. The silicon-carbon composite material of claim 9, wherein, The content of the amorphous carbon in the silicon-carbon composite layer is 20-40 wt% based on the total amount of the silicon-carbon composite material.

11. The silicon-carbon composite material of claim 10, wherein, The content of the amorphous carbon in the silicon-carbon composite layer is 25-35 wt% based on the total amount of the silicon-carbon composite material.

12. The silicon-carbon composite material of any one of claims 1-5, wherein, The silicon-carbon composite material further comprises a carbon coating layer coated on the silicon-carbon composite layer. The average particle size of the silicon-carbon composite material is 3-15 μm.

13. The silicon-carbon composite material of claim 12, wherein, The thickness of the carbon coating layer is 5-100 nm. The average particle size of the silicon-carbon composite material is 5-10 μm.

14. The silicon-carbon composite of claim 13, wherein, The thickness of the carbon coating layer is 10-30 nm.

15. A method of making a silicon-carbon composite material, characterized by, The method comprises: performing vapor deposition of a silicon source and a carbon source on a carbon matrix, the gas flow rate of the silicon source decreases over time, and the gas flow rate of the carbon source increases over time, to form a silicon-carbon composite layer in which the content of silicon decreases from inside to outside, thereby obtaining a silicon-carbon composite material.

16. The method of manufacturing according to claim 15, wherein, The gas flow rate of the silicon source decreases uniformly over time, and the gas flow rate of the carbon source increases uniformly over time.

17. The method of making according to claim 16, wherein, During the vapor deposition process, the ratio of the gas flow rates of the silicon source and the carbon source uniformly changes from 8-17:1 to 1:8-17 over time.

18. The method of making according to any one of claims 15-17, wherein, The silicon source is selected from at least one of monosilane, disilane, trisilane, silicon chloride, silicon trichloride, hexachlorodisilane and dichlorosilane; The carbon source is selected from at least one of ethyne, methane, ethane, ethylene, carbon monoxide and carbon dioxide; The temperature of the vapor deposition is 300-700 °C. The gas flow rate of the silicon source and the carbon source is independently 1-90 sccm. The gas flow rate of the silicon source and the carbon source is independently 5-85 sccm. The gas flow rate of the silicon source and the carbon source is independently 5-85 sccm.

19. The method of manufacturing according to claim 18, wherein, The gas flow rate of the silicon source and the carbon source is independently 5-85 sccm. The ratio of the flow rate of the carrier gas to the gas flow rate of the silicon source or the carbon source is 4-5.5:

1. ​ ​ 20. The method of making according to any one of claims 15-17, wherein, The silicon source and the carbon source are vapor-deposited on the carbon substrate, and then a carbon-coated layer is formed by vapor carbon-coating.

21. The method of manufacturing according to claim 20, wherein, The carbon source for the vapor carbon-coating is selected from at least one of methane, ethylene, acetylene, propane and propylene; The temperature for the vapor carbon-coating is 600-1200℃, and the time is 50-500 min. The gas flow rate of the carbon source for the vapor carbon-coating is 5-60 sccm.

22. The method of making according to claim 21, wherein, The temperature for the vapor carbon-coating is 800-1000℃, and the time is 100-400 min. The gas flow rate of the carbon source for the vapor carbon-coating is 10-50 sccm.

23. The method of making according to claim 20, wherein, The carbon substrate is selected from at least one of carbon nanotubes, graphene, carbon black and porous carbon; The silicon in the silicon-carbon composite layer includes amorphous silicon and optional silicon grains; The carbon in the silicon-carbon composite layer is amorphous carbon; The thickness of the carbon-coated layer is 5-100 nm. The average particle size of the silicon-carbon composite material is 3-15 μm.

24. The method of manufacturing according to claim 23, wherein, The carbon substrate is porous carbon. The average particle size of the silicon grains is 8 nm or less. The thickness of the carbon-coated layer is 10-30 nm. The average particle size of the silicon-carbon composite material is 5-10 μm.

25. The method of manufacturing according to claim 24, wherein, The average particle size of the silicon grains is 3 nm or less.

26. The method of making according to any one of claims 23-25, wherein, The average particle size of the porous carbon is 2-8 μm. The content of the amorphous silicon and optional silicon grains in the silicon-carbon composite layer is 20-40 wt% based on the total amount of the silicon-carbon composite material. The content of the amorphous carbon in the silicon-carbon composite layer is 20-40 wt% based on the total amount of the silicon-carbon composite material.

27. The method of manufacturing according to claim 26, wherein, The average particle size of the porous carbon is 4-6 μm. The content of the amorphous silicon and optional silicon grains in the silicon-carbon composite layer is 25-35 wt% based on the total amount of the silicon-carbon composite material. The content of the amorphous carbon in the silicon-carbon composite layer is 25-35 wt% based on the total amount of the silicon-carbon composite material.

28. A silicon-carbon composite material prepared by the preparation method of any one of claims 15-27.

29. A negative electrode sheet, wherein The negative electrode sheet comprises the silicon-carbon composite material of any one of claims 1-14 and 28.

30. A lithium-ion battery, wherein, The lithium ion battery comprises the negative electrode sheet of claim 29.

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