Silicon-carbon composite negative electrode material for lithium ion battery and preparation method of silicon-carbon composite negative electrode material

Abstract

The invention relates to the field of lithium battery negative electrode materials, and particularly discloses a silicon-carbon composite negative electrode material for a lithium ion battery and a preparation method of the silicon-carbon composite negative electrode material. The method comprises the following steps: preparing a porous carbon substrate, and depositing silicon nanoparticles in pores and on the surface of the porous carbon substrate by adopting a fluidized bed chemical vapor deposition process and taking silane as a silicon source to obtain a silicon-carbon core material; continuously carrying out carbon coating on the surface of the silicon-carbon core material in the fluidized bed by taking acetylene as a carbon source to form a carbon coating layer; and leading out the obtained material from the fluidized bed, taking acetylene as a carbon source, and carrying out carbon coating on the surface of the primary carbon coating layer again by adopting the rotary furnace to form a secondary carbon coating layer. According to the preparation method, a porous carbon matrix and double-layer functional carbon coating process is adopted, so that the volume expansion of silicon in the circulation process is remarkably inhibited, the interface side reaction is reduced, and the circulation stability of the material is effectively improved.

Description

Technical Field

[0001] The present application relates to the field of lithium battery negative electrode materials, and more specifically, to a silicon-carbon composite negative electrode material for lithium ion batteries and a preparation method thereof. Background Art

[0002] As an important part of modern energy storage technology, lithium-ion batteries have developed rapidly in recent years. Especially in the fields of new energy vehicles and portable electronic devices, their high energy density and long cycle life make them the mainstream choice. As a key component of lithium-ion batteries, negative electrode materials directly affect the overall performance of the battery. At present, silicon-based negative electrode materials have attracted much attention because their theoretical specific capacity is much higher than that of traditional graphite materials, but they still face many challenges in practical applications. In silicon-carbon composite negative electrode materials, the formation and stability of SEI film are core issues. The high volume expansion rate of silicon causes the SEI film to rupture and regenerate repeatedly, consuming lithium ions and reducing cycle performance.

[0003] At present, in order to solve the volume expansion and interface stability problems of silicon-based negative electrode materials, the technical means commonly used in the industry include ball milling to prepare silicon-carbon composite materials, single-layer carbon coating process, and protective layer formation through surface modification. The ball milling method disperses silicon particles in the carbon matrix through mechanical force, but its dispersion effect is limited; the single-layer carbon coating process forms a carbon film on the surface of silicon particles through chemical vapor deposition or pyrolysis of carbon sources to alleviate volume expansion and isolate the electrolyte; in addition, there is also a technology to form an artificial SEI layer through surface modification, which aims to improve interface stability.

[0004] However, the above methods still have significant defects in practical applications. Specifically, the silicon particles in the silicon-carbon material prepared by the traditional ball milling method are unevenly dispersed, resulting in serious particle breakage and rapid capacity decay during the cycle; the single-layer carbon coating is prone to cracks after long-term cycling and cannot effectively isolate the electrolyte, thereby triggering side reactions; at the same time, the SEI film on the silicon surface repeatedly grows and thickens during the cycle, resulting in increased impedance and loss of active lithium inventory, which seriously affects the cycle stability and energy density of the battery. Summary of the invention

[0005] In order to solve the above technical problems, the present application provides a silicon-carbon composite negative electrode material for a lithium-ion battery and a preparation method thereof.

[0006] This application adopts the following technical solutions: In a first aspect, the present application provides a method for preparing a silicon-carbon composite negative electrode material for a lithium-ion battery, comprising: (1) Preparing a porous carbon substrate, wherein the pore volume of the porous carbon substrate is ≥ 0.8 cm 3 / g, average pore size is 1.8-2.0nm; (2) using a fluidized bed chemical vapor deposition process, using silane as a silicon source, to deposit silicon nanoparticles in the pores and on the surface of the porous carbon matrix to obtain a silicon-carbon core material, and controlling the silicon content in the silicon-carbon core material to be 40-50wt%; (3) Using a fluidized bed chemical vapor deposition process, acetylene is used as a carbon source to carbon coat the surface of the silicon-carbon core material to form a carbon coating layer with a controlled specific surface area of ​​30-50m 2 / g; (4) The obtained material is guided out of the fluidized bed, and acetylene is used as the carbon source, and carbon coating is performed again on the surface of the first carbon coating layer by using a rotary kiln to form a second carbon coating layer, and the specific surface area is controlled to be ≤5m 2 g, after cooling, a silicon-carbon negative electrode material with a double-layer carbon coating is formed.

[0007] By adopting the above technical solution, silane (SiH 4 ) decomposes into Si and H at high temperature 2 , through the gas-solid mass transfer of the fluidized bed, silicon nanoparticles (<150nm) are uniformly deposited in the pores and surface of the porous carbon. The high pore volume and hierarchical porous structure (micropores and mesopores) of the porous carbon matrix provide sufficient deposition space and expansion buffer space for silicon nanoparticles, and limit the agglomeration of silicon nanoparticles, effectively solving the problem of silicon volume expansion and significantly improving the cyclic stability of the material. Subsequently, the fluidized bed CVD process is continued to form a relatively rough carbon coating layer (specific surface area of ​​30-50m 2 / g), the carbon layer covers the active sites on the silicon surface, which, on the one hand, prevents it from being oxidized by air during the transfer from the fluidized bed, and on the other hand, helps to reduce the decomposition side reactions of the electrolyte. In order to further improve the overall conductivity of the material, a dense and uniform carbon layer (specific surface area ≤5m 2 / g), and finally formed a gradient double-layer carbon protective layer with a dense outer layer and a loose inner layer, which further suppressed the volume effect, reduced the interfacial side reactions, and stabilized the SEI film.

[0008] Furthermore, in the above step (2), the flow ratio of silane to carrier gas is 1:5-10, the temperature is 400-500° C., and the treatment time is 3-15 h.

[0009] By adopting the above technical solution, by controlling the flow ratio of silane to carrier gas, temperature and processing time, the deposition amount and distribution uniformity of silicon nanoparticles can be accurately adjusted, so that the silicon content is stabilized in the range of 40-50wt%, avoiding particle agglomeration and rupture, and ensuring the structural stability and electrochemical properties of the material.

[0010] Furthermore, in the above step (2), during the preparation of the silicon-carbon core material, the particle breakage rate is controlled to be ≤5%.

[0011] By adopting the above technical solution, the preparation process of silicon-carbon core materials is further optimized, and the loss of active substances due to particle breakage is avoided to a great extent, thereby improving the conductivity and service life of the final product.

[0012] Furthermore, in the above step (3), the flow ratio of acetylene to carrier gas is 1:4-8, the coating temperature is 530-670°C, and the coating time is 2-6h.

[0013] By adopting the above technical solution, a carbon coating layer can be formed on the surface of the silicon-carbon core material obtained in step (2) very conveniently using a fluidized bed, thereby isolating the active sites on the silicon surface from being oxidized.

[0014] Furthermore, in the above step (4), the flow ratio of acetylene to carrier gas is 1:2-3, the coating temperature is 550-580°C, and the coating time is 2-6h.

[0015] The above technical solution combines the preparation conditions of the porous carbon matrix, silicon nanoparticle deposition and the first carbon coating layer, which can further optimize the uniformity and stability of the second carbon coating layer, effectively reduce the interfacial impedance of the material during the cycle, enhance the isolation ability to the external electrolyte, and reduce the occurrence of side reactions.

[0016] Furthermore, after the coating is completed, the obtained coated material is cooled at a cooling rate of 5-10°C / min under nitrogen protection.

[0017] By adopting the above technical solution, cooling at a cooling rate of 5-10℃ / min after coating is completed can effectively avoid structural damage caused by thermal stress, ensure the integrity and performance stability of the material, and further improve the reversible capacity and coulombic efficiency.

[0018] Furthermore, the mesopores of the porous carbon matrix account for 5-25% and the specific surface area is 1800-2200m 2 / g.

[0019] By adopting the above technical solution, the mesopore ratio of the porous carbon matrix is ​​5-25%, which can effectively improve the dispersion uniformity of silicon nanoparticles and reduce the particle breakage caused by silicon volume expansion, thereby significantly improving the cycle stability of the material. The specific surface area is 1800-2000m 2 / g porous carbon matrix can provide sufficient active sites and expansion buffer space and limit the agglomeration of silicon particles.

[0020] In a second aspect, the present application provides a silicon-carbon composite negative electrode material for a lithium-ion battery prepared by a preparation method.

[0021] Furthermore, the first coulombic efficiency of the above silicon-carbon composite negative electrode material is ≥ 90%, and the reversible capacity is 1750-1850 mAh / g.

[0022] In summary, the present application includes at least one of the following beneficial technical effects: 1. The porous carbon matrix has a high pore volume and moderate pore size, which can evenly disperse silicon nanoparticles, significantly inhibit the volume expansion of silicon during the cycle, and effectively improve the cycle stability of the material; 2. The double-layer functionalized carbon coating structure can effectively alleviate the problem of easy cracking of single-layer carbon coating. The inner carbon coating provides good mechanical support, and the outer carbon coating further enhances the effect of isolating the electrolyte, significantly reducing the occurrence of interfacial side reactions and SEI film thickening.

[0023] 3. By optimizing the deposition process parameters, the silicon content in the silicon-carbon core material is precisely controlled to ensure the high capacity characteristics of the material. At the same time, the growth thickness of the SEI film is controlled to reduce the interface impedance and active lithium loss, thereby greatly improving the long cycle life and energy density of the battery. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 This is the XRD detection diagram of the silicon-carbon composite negative electrode material provided in Example 1 of the present application. DETAILED DESCRIPTION

[0024] The embodiments of the present invention will be described in detail below in conjunction with examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. The specific conditions not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0025] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0026] Example 1

[0027] This embodiment provides a silicon-carbon composite negative electrode material for a lithium ion battery, and a preparation method thereof comprises the following steps: Prepare a porous carbon matrix (commercially available). The porous carbon matrix has a uniform morphology and a particle size distribution satisfying D10=4nm, D50=7.5nm, and D100≤20nm. Other technical indicators are shown in Table 1: Table 1.

[0028] A vertical fluidized bed with a diameter of Φ500 mm and a height-to-diameter ratio of 1:4 (polished inner wall, Ra<0.8μm) was used for chemical vapor deposition process, with silane as the silicon source, nitrogen as the carrier gas, a flow ratio of 1:7, the temperature controlled at 440-460℃, and the air intake time of 8h. Silicon nanoparticles were deposited in the pores and on the surface of the porous carbon matrix to obtain a silicon-carbon core material.

[0029] The EDS surface scanning method was used to measure the silicon distribution uniformity of the silicon-carbon core material, and the silicon content was measured to be 45wt%. The integrity of the carbon skeleton was measured using a laser particle size analyzer, and the particle breakage rate was measured to be 3%.

[0030] Continue to carry out chemical vapor deposition process in the vertical fluidized bed, use acetylene as carbon source, use nitrogen as carrier gas, flow ratio is 1:6, temperature is controlled at 580-600°C, coating time is 6h, carbon coating is carried out on the surface of silicon-carbon core material, and cooling is carried out at a rate of 8°C / min under nitrogen protection to form a carbon coating layer; The specific surface area of ​​the obtained carbon coating material was measured by a static surface analyzer and was 42 m 2 / g.

[0031] The obtained material is guided out of the fluidized bed and placed in a rotary furnace, with acetylene as the carbon source and nitrogen as the carrier gas, the volume ratio is 1:2.5, the temperature is controlled at 570-580°C, the coating time is 5 h, and carbon coating is performed again on the surface of the first carbon coating layer, and cooled at a rate of 8°C / min under nitrogen protection to form a second carbon coating layer, which is a silicon-carbon composite negative electrode material.

[0032] The silicon-carbon composite negative electrode material was characterized by XRD. Figure 1 As shown in the figure, it can be seen that the silicon-carbon composite negative electrode material obtained in this embodiment is amorphous silicon, which meets the battery requirements; at the same time, there is no impurity peak, indicating that the material is of high purity and does not contain impurities such as silicon carbide and silicon oxide.

[0033] The following methods are used to measure the performance parameters of silicon-carbon composite negative electrode materials. (1) Tap density: measured using a tap density meter; (2) Specific surface area: static specific surface analyzer; (3) Powder compaction resistivity: The resistivity is calculated by applying current and detecting the voltage drop using the four-probe method.

[0034] (4) Reversible capacity: The battery test system is charged and discharged cycled to calculate the amount of lithium embedded per unit mass of material.

[0035] (5) Initial efficiency: The battery test system is charged and discharged for the first time, and the ratio of discharge capacity to charge capacity is calculated.

[0036] The results are shown in Table 2: Table 2.

[0037] Example 2

[0038] The difference between this embodiment and embodiment 1 lies in the porous carbon substrate, and its technical indicators are shown in Table 3: Table 3.

[0039] The various performance parameters of the silicon-carbon composite negative electrode materials obtained in the above examples and comparative examples were measured, and the results are shown in Table 4: Table 4.

[0040] It can be seen from Table 4 that the specifications of the porous carbon matrix have a significant impact on the conductivity and cycle stability of the silicon-carbon composite negative electrode material. Specifically, compared with Examples 1-2, Comparative Example 1 uses a porous carbon matrix with a smaller specific surface area and pore volume, and the reversible capacity and first efficiency of the obtained material are reduced, indicating that its cycle performance is very poor, mainly due to the capacity attenuation caused by the volume expansion of silicon.

[0041] Example 3

[0042] The difference between this embodiment and embodiment 1 is that in step (2), the process parameters in the preparation process of the silicon-carbon core material are different: silane is used as the silicon source, nitrogen is used as the carrier gas, the flow ratio is 1:6, the temperature is controlled at 440-460°C, and the air intake time is 14h.

[0043] Comparative Example 2: Silane was used as the silicon source, nitrogen was used as the carrier gas, the flow ratio was 1:4, the temperature was controlled at 440-460°C, and the air intake time was 8 hours.

[0044] Comparative Example 3: Silane was used as the silicon source, nitrogen was used as the carrier gas, the flow ratio was 1:12, the temperature was controlled at 440-460° C., and the air intake time was 8 h.

[0045] The various performance parameters of the silicon-carbon composite negative electrode materials obtained in the above examples and comparative examples were measured, and the results are shown in Table 5: Table 5.

[0046] As can be seen from Table 5, in the silicon deposition step, the flow ratio of silane to nitrogen and the gas intake time have a key impact on the reversible capacity and first efficiency of the silicon-carbon composite negative electrode material. In particular, in terms of reversible capacity, Comparative Examples 2 and 3 have a significant decline.

[0047] Example 4

[0048] The difference between this embodiment and embodiment 1 is that the carbon coating process and parameters are different:

[0049] A carbon coating layer: In a vertical fluidized bed, acetylene was used as the carbon source, nitrogen was used as the carrier gas, the flow ratio was 1:8, the temperature was controlled at 530-550°C, the coating time was 4 h, and the cooling rate was 5°C / min.

[0050] Two carbon coating layers: In a rotary furnace, acetylene is used as the carbon source and nitrogen is used as the carrier gas, the volume ratio is 1:2, the temperature is controlled at 550-560°C, the coating time is 2h, and the cooling rate is 5°C / min.

[0051] Example 5

[0052] The difference between this embodiment and embodiment 1 is that the carbon coating process and parameters are different:

[0053] A carbon coating layer: in a vertical fluidized bed, acetylene is used as the carbon source, nitrogen is used as the carrier gas, the flow ratio is 1:4, the temperature is controlled at 660-670°C, the coating time is 3h, and the cooling rate is 10°C / min.

[0054] Two carbon coating layers: In a rotary furnace, acetylene is used as the carbon source and nitrogen is used as the carrier gas, the volume ratio is 1:3, the temperature is controlled at 550-560°C, the coating time is 4h, and the cooling rate is 10°C / min.

[0055] Comparative Example 4 This comparative example only uses a vertical fluidized bed for primary carbon coating, without secondary carbon coating. The specific parameters are: In a vertical fluidized bed, acetylene was used as the carbon source, nitrogen was used as the carrier gas, the flow ratio was 1:6, the temperature was controlled at 580-600°C, the coating time was 10h, and the cooling rate was 8°C / min.

[0056] Comparative Example 5 The difference between this comparative example and Example 1 is that the coating time of a carbon coating layer is 1 hour.

[0057] Comparative Example 6 The difference between this comparative example and Example 1 is that the coating time of the second carbon coating layer is 1 h.

[0058] Comparative Example 7 The difference between this comparative example and Example 1 is that the coating time of a carbon coating layer is 9 hours.

[0059] The various performance parameters of the silicon-carbon composite negative electrode materials obtained in the above examples and comparative examples were measured, and the results are shown in Table 6: Table 6

[0060] As can be seen from Table 6, the double carbon coating layer of the silicon-carbon composite negative electrode material has a significant impact on the performance of the material. Compared with comparative examples 4-7, Examples 1 and 4-5 of the present application can achieve a reversible capacity greater than 1800 mAh / g, and the first efficiency is greater than 90%. This shows that the carbon coating process of the present application ultimately forms a gradient double-layer carbon protective layer with a dense outer layer and a loose inner layer, which can further suppress the volume effect and improve the conductivity and cycle stability of the material. The coating time affects the thickness of the carbon coating layer. If the coating time is too short or too long, it is not conducive to the performance of the material.

[0061] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A method for preparing a silicon-carbon composite negative electrode material for a lithium-ion battery, characterized in that: It includes: (1) Preparing a porous carbon substrate, wherein the pore volume of the porous carbon substrate is ≥ 0.8 cm 3 / g, average pore size is 1.8-2.0nm; (2) using a fluidized bed chemical vapor deposition process, using silane as a silicon source, to deposit silicon nanoparticles in the pores and on the surface of the porous carbon matrix to obtain a silicon-carbon core material, and controlling the silicon content in the silicon-carbon core material to be 40-50wt%; (3) Using a fluidized bed chemical vapor deposition process, acetylene is used as a carbon source to carbon coat the surface of the silicon-carbon core material to form a carbon coating layer with a controlled specific surface area of ​​30-50m 2 / g; (4) The obtained material is guided out of the fluidized bed, and acetylene is used as the carbon source, and carbon coating is performed again on the surface of the first carbon coating layer by using a rotary kiln to form a second carbon coating layer, and the specific surface area is controlled to be ≤5m 2 / g, and after cooling, a silicon-carbon negative electrode material with a double-layer carbon coating is formed.

2. The method for preparing a silicon-carbon composite negative electrode material for a lithium ion battery according to claim 1, characterized in that: In the step (2), the flow ratio of silane to carrier gas is 1:5-10, the temperature is 400-500° C., and the treatment time is 3-15 hours.

3. The method for preparing a silicon-carbon composite negative electrode material for a lithium ion battery according to claim 2, characterized in that: In the step (2), during the preparation of the silicon-carbon core material, the particle breakage rate is controlled to be ≤5%.

4. The method for preparing a silicon-carbon composite negative electrode material for a lithium ion battery according to claim 1, characterized in that: In the step (3), the flow ratio of acetylene to carrier gas is 1:4-8, the coating temperature is 530-670°C, and the coating time is 2-6h.

5. The method for preparing a silicon-carbon composite negative electrode material for a lithium-ion battery according to claim 1, characterized in that: In the step (4), the flow ratio of acetylene to carrier gas is 1:2-3, the coating temperature is 550-580°C, and the coating time is 2-6h.

6. The method for preparing a silicon-carbon composite negative electrode material for a lithium ion battery according to claim 4 or 5, characterized in that: After the coating is completed, the obtained coated material is cooled at a cooling rate of 5-10°C / min under nitrogen protection.

7. The method for preparing a silicon-carbon composite negative electrode material for a lithium-ion battery according to claim 1, characterized in that: The mesopore ratio of the porous carbon matrix is ​​5-25%, and the specific surface area is 1800-2200m 2 / g.

8. A silicon-carbon composite negative electrode material for lithium-ion batteries prepared according to the preparation method according to any one of claims 1 to 7.

9. The silicon-carbon composite negative electrode material for lithium-ion batteries according to claim 8, characterized in that: The first coulombic efficiency of the silicon-carbon composite negative electrode material is ≥ 90%, and the reversible capacity is 1750-1850 mAh / g.

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