NANO silicon-carbon composite material, preparation method therefor, and use thereof

Abstract

Disclosed are a nano silicon-carbon composite material, a preparation method therefor, and a use thereof. The nano silicon-carbon composite material comprises co-agglomerated nano silicon particles and nano carbon particles. The mass ratio of the nano silicon particles to the nano carbon particles is (45-60):(40-55). The nano silicon particles have an average particle size of 1-50 nm and a crystallite grain size of 1-10 nm. The nano silicon particles have varied crystal orientations and are freely combined with the nano carbon particles. In the nano silicon-carbon composite material, the nano silicon particles and the nano carbon particles are uniformly dispersed, and the particle size of the nano silicon particles is small. Moreover, due to the dispersion of the nano carbon, the phenomenon of agglomeration between the nano silicon particles can be reduced. When the composite material is applied to negative electrode materials and batteries, the phenomenon of electrochemical sintering can be effectively mitigated.

Description

A nano-silicon-carbon composite material and its preparation method and application Technical Field

[0001] The invention relates to a nano silicon-carbon composite material and a preparation method and application thereof, belonging to the technical field of silicon-carbon composite materials. Background Art

[0002] The greenhouse effect and climate change caused by the long-term use of fossil fuels have affected human survival and development. Therefore, carbon neutrality has gradually become a global consensus, and more and more countries are planning to gradually phase out the production of fuel vehicles in the near future. However, current lithium-ion battery technology cannot meet the rapidly growing demand for power batteries and energy storage batteries, and there is an urgent need to develop lithium-ion batteries with high energy density and power density. Si materials are alloying materials with the highest theoretical lithium storage capacity, and silicon-carbon negative electrode materials composed of nano-silicon and carbon materials are also considered to be the most promising next-generation negative electrode materials and have always been a research hotspot. However, their application in commercial lithium-ion batteries is still very slow. One of the important reasons is that the size of nano-silicon particles is still relatively large, and their drastic volume expansion / contraction brings a series of side effects. Secondly, the agglomeration of silicon particles affects its long-term cycle performance.

[0003] There are currently two main methods for preparing nano-silicon: the first is the traditional sand grinding method, which grinds the silicon material to 100nm and then compounds it with various carbon materials to form a silicon-carbon material. This method is easy to prepare in large quantities, but because the size of silicon is still large, the expansion of the silicon-carbon material is still large, and the long-term cycle performance is also poor. The second is the latest silane cracking method, which uses high temperature to crack silane into small-sized nano-silicon particles (<50nm), which are then deposited into porous carbon materials. The nano-silicon particles are separated by the carbon skeleton to form a material with uniform distribution of silicon particles and carbon materials.

[0004] However, in the silicon-carbon material prepared by the above method, the silicon-silicon bonds in the silicon material will break during the lithium ion embedding process and form a silicon-lithium alloy with lithium; during the lithium ion extraction process, the silicon-lithium bonds will break and the silicon-silicon bonds will be reconstructed. During this process, the silicon atoms in the contact part of adjacent silicon particles will also form silicon-silicon bonds under the action of pressure, causing the two silicon particles to become a single larger silicon particle (this phenomenon is called electrochemical sintering), which will cause the silicon particles to grow, be more prone to breakage, and lead to poor electrochemical performance.

[0005] Therefore, there is an urgent need to provide a nano-silicon-carbon composite material that is small in size, does not agglomerate, and can effectively alleviate the electrochemical sintering phenomenon.

[0006] Summary of the Invention

[0007] The object of the present invention is to provide a nano-silicon-carbon composite material, a preparation method and application thereof, in which nano-silicon particles and nano-carbon particles are evenly dispersed, and the nano-silicon particles have a small particle size. Due to the dispersion of nano-carbon, the agglomeration phenomenon between nano-silicon particles can be reduced. When applied to negative electrode materials and batteries, it can effectively alleviate the electrochemical sintering phenomenon.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A nano-silicon-carbon composite material comprises co-agglomerated nano-silicon particles and nano-carbon particles; wherein the mass ratio of the nano-silicon particles to the nano-carbon particles is (45-60):(40-55); the average particle size of the nano-silicon particles is 1-50 nm, preferably 10-20 nm; the grain size is 1-10 nm, preferably 1-5 nm; the nano-silicon particles have different crystal orientations and are freely combined with the nano-carbon particles.

[0010] Preferably, the nano-silicon particles are further doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum.

[0011] Preferably, the average particle size of the nanocarbon is 1-50 nm, preferably 5-10 nm.

[0012] The method for preparing any of the above-mentioned nano-silicon-carbon composite materials is to use a carbon source material and a silicon source material as electrodes, and connect them to the positive and negative electrodes of a pulse power supply respectively, use an inert gas as a working medium, apply power and collect the generated particles, and the resulting particles are the nano-silicon-carbon composite materials; specifically, the steps include:

[0013] In the first step, the bulk silicon material is used as the workpiece electrode and the carbon tube is used as the tool electrode, which are connected to the two poles of the pulse power supply respectively. Inert gas is used as the working medium, and there is a certain gap between the two electrodes.

[0014] Step 2: During the discharge process, as the positive and negative charges of the two electrodes accumulate, an electric field is formed. When the electric field strength is higher than the tolerance limit of the working medium, impact ionization occurs between the tool electrode and the workpiece electrode, forming an ion channel, thereby forming a discharge channel; the passage of current causes the medium in the channel to ionize, generating high-temperature plasma, causing thermal radiation to the surfaces of the workpiece electrode and the tool electrode. The high temperature of thousands of degrees Celsius will cause the silicon electrode and carbon electrode to melt and vaporize.

[0015] In the third step, after the discharge is completed, the vaporized silicon atom clusters and carbon atom clusters are carried out of the discharge area by the working medium, rapidly cooled, and agglomerated, self-assembling into precursor silicon-carbon particles, namely nano-silicon-carbon composite materials.

[0016] Preferably, the carbon source material is any one of soft carbon, hard carbon, and graphite; the carbon source material is made into a cylindrical electrode with a diameter of 0.1-10 mm, preferably 0.5-3 mm;

[0017] The silicon source material is intrinsic silicon, or doped silicon doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum; the silicon source material is a block electrode with a thickness of 1-50 mm, preferably 5-10 mm;

[0018] The pulse width of the electric pulse of the pulse power supply is 50ns-500μs, preferably 50-200ns;

[0019] The inert gas is one or a combination of at least two of high-purity argon, helium, neon, krypton and xenon.

[0020] A method for preparing a negative electrode material, comprising mixing any of the above-mentioned nano-silicon-carbon composite materials and an organic carbon source, mechanically fusing and granulating, and then carbonizing the mixture; wherein the mixing mass ratio of the nano-silicon-carbon composite material to the organic carbon source is (1-20):1, preferably (5-10):1; or,

[0021] The nano-silicon-carbon composite material according to any one of claims 1 to 3 is mixed with an organic carbon source and a graphite material and then coated; wherein the mass ratio of the nano-silicon-carbon composite material, the organic carbon source and the graphite material is (30-80):(1-10):(20-70).

[0022] Preferably, the organic carbon source is one or a combination of at least two of coal tar, coal pitch, petroleum asphalt, epoxy resin, phenolic resin, acrylic resin, furfural resin, polyvinyl chloride, polyacrylonitrile and polyvinylidene fluoride; the median particle size of the organic carbon source is 0.01-100 μm, preferably 0.01-10 μm.

[0023] The graphite material is one or a combination of two of artificial graphite, natural spherical graphite, natural flake graphite and mesophase carbon microspheres. The median particle size of the graphite material is 1-50 μm, preferably 5-10 μm.

[0024] Preferably, the mixing conditions are: a rotation speed of 500-1500 rpm, preferably 1000-1500 rpm; and a mixing time of 0.5-10 h, preferably 3-5 h.

[0025] Preferably, the conditions for mechanical fusion granulation are: the gap between the tool and the cavity wall is 10-1000 mm, preferably 10-100 mm; the rotation speed is 500-1800 rpm, preferably 1200-1800 rpm; and the fusion time is 0.2-10 h, preferably 0.5-3 h.

[0026] Preferably, the carbonization conditions are: under a protective gas atmosphere, a heating rate of 1-20°C / min, preferably 5-15°C / min; a carbonization temperature of 500-1250°C, preferably 800-1200°C; and a carbonization time of 1-20h, preferably 5-15h.

[0027] Preferably, the protective gas is one or a combination of at least two of nitrogen, argon, helium, neon, krypton and xenon.

[0028] Preferably, the coating conditions are: in the coating equipment, the spindle speed is 100-1000 rpm, preferably 150-500 rpm; the heating rate is 1-20°C / min, preferably 3-10°C / min; the coating temperature is 100-1100°C, preferably 300-850°C; the coating time is 0.5-30h, preferably 8-20h.

[0029] A negative electrode sheet comprises any of the above-mentioned nano-silicon-carbon composite materials or a negative electrode material prepared by any of the above-mentioned methods.

[0030] A battery comprises the above-mentioned negative electrode sheet.

[0031] The beneficial effects of the present invention are:

[0032] 1. The nano-silicon-carbon composite material separates the silicon particles through nano-carbon particles, forming a structure in which nano-silicon particles and nano-carbon particles are evenly dispersed, avoiding the agglomeration of silicon materials to form larger silicon particles, and the size of the internal silicon particles is small; and because of the addition of a large amount of carbon material, the direct agglomeration between nano-silicon particles is greatly reduced, which can effectively alleviate the electrochemical sintering phenomenon.

[0033] 2. Nano-silicon-carbon composite materials utilize the principle of pulse discharge, with silicon source materials as the workpiece electrode, carbon source materials as the tool electrode, and high-purity inert gas as the working medium. During the discharge process, the positive charge on the workpiece electrode and the negative charge on the tool electrode will continue to accumulate, leaving a tiny gap between the two, thus forming an electric field. When the electric field strength exceeds the limit, it will break through the inter-electrode medium, forming a discharge channel. The electrons in the medium rush toward the positive electrode at high speed, and the positive ions rush to the negative electrode. This process generates a large amount of heat energy, causing both the workpiece electrode and the tool electrode to melt and vaporize to a certain extent. The vaporized silicon atom clusters and form nano-silicon particles during the cooling process. At the same time, the vaporized carbon atom clusters will also agglomerate to form nano-carbon particles. The two will combine into larger particles as they leave the working area, further releasing energy. At this time, since the discharge process has long ended and the temperature has dropped rapidly, the material formed is mainly agglomerated particles of nano-silicon and nano-carbon particles, and very little silicon carbide is formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of the preparation of nano-silicon-carbon composite materials;

[0035] FIG2 is a SEM image of the nano-silicon-carbon composite material obtained in Example 1;

[0036] FIG3 is a SEM image of the nano-silicon carbon negative electrode material obtained in Example 1;

[0037] FIG4 is a SEM image of the nano-silicon carbon negative electrode material obtained in Comparative Example 1;

[0038] FIG5 is a SEM image of the nano-silicon carbon negative electrode material obtained in Comparative Example 2;

[0039] FIG6 is a SEM image of the nano-silicon carbon negative electrode material obtained in Example 2;

[0040] FIG7 is a SEM image of the nano-silicon carbon negative electrode material obtained in Comparative Example 3. DETAILED DESCRIPTION

[0041] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0042] The nano-silicon-carbon composite material provided by the present invention is self-assembled from nano-silicon particles and nano-carbon particles. The self-assembly process specifically refers to the process in which nano-silicon particles and nano-carbon particles that are condensed and vaporized at high temperature are freely combined to form the nano-silicon-carbon composite material. The specific preparation method is to utilize the principle of pulse discharge, with a silicon wafer as the workpiece electrode, a carbon material as the tool electrode, and a high-purity inert gas as the working medium. During the discharge process, the positive charge on the workpiece electrode and the negative charge on the tool electrode will continue to accumulate, with a small gap between the two, thereby forming an electric field. When the electric field strength exceeds the limit, it will break through the inter-electrode medium to form a discharge channel, in which electrons rush to the positive electrode at high speed and positive ions rush to the negative electrode. This process generates a large amount of heat energy, causing both the workpiece electrode and the tool electrode to melt and vaporize to a certain extent. The vaporized silicon atom clusters and form nano-silicon particles during the cooling process. At the same time, the vaporized carbon atom clusters will also agglomerate to form nano-carbon particles. The two will combine with each other to form larger particles when leaving the working area, further releasing energy. A schematic diagram is shown in Figure 1. At this time, since the discharge process has ended long ago and the temperature has dropped rapidly, the material formed is mainly agglomerated particles of nano-silicon and nano-carbon particles, and very little silicon carbide is formed.

[0043] Example 1

[0044] Preparation process of high specific capacity nano silicon carbon material:

[0045] In the first step, a 5 mm thick boron-doped bulk silicon material (resistivity 0.01 Ω·cm) was selected as the workpiece electrode, a 1 mm diameter soft carbon was used as the tool electrode, and high-purity argon was used as the working medium.

[0046] In the second step, a rectangular pulse voltage with a pulse width of 50 ns, a duty cycle of 1:4, and an open circuit voltage of 160 V generated by a pulse power supply is applied between the workpiece electrode and the tool electrode to ionize and break down the insulating working medium to form a plasma discharge channel. The high-temperature etching material is generated, and the resulting precursor silicon carbon particles are recorded as Si / C-Q1 (SEM image shown in Figure 2).

[0047] In the third step, 2 kg of precursor silicon carbon particles and 0.4 kg of coal tar were weighed and put into a mixing device, and mixed at 1200 rpm for 3 hours; then put into a mechanical fusion device and fused at 1000 rpm for 1 hour; finally, put into a high-temperature kiln and sintered at 900 ° C for 3 hours under the protection of high-purity nitrogen; after screening and demagnetization, a nano-silicon carbon negative electrode material with an average particle size of 12 μm was obtained, which was recorded as Si / C-1 (SEM image shown in Figure 3).

[0048] Comparative Example 1

[0049] The difference from Example 1 is that in step 2, the pulse power supply parameters are: discharge pulse width of 500 μs, duty cycle of 1:4, and open circuit voltage of 160 V. In step 3, the precursor silicon-carbon particles obtained are denoted as Si / C-Q2; the nano-silicon-carbon negative electrode material with an average particle size of 12 μm obtained in step 3 is denoted as Si / C-2 (SEM image shown in Figure 4).

[0050] Comparative Example 2

[0051] The difference from Example 1 is that in step 3, the mixed materials are directly subjected to a carbonization process without mechanical fusion granulation treatment, and the obtained nano-silicon carbon negative electrode material is recorded as Si / C-3 (SEM image shown in Figure 5).

[0052] Example 2

[0053] Preparation process of low specific capacity nano silicon carbon material:

[0054] The difference from Example 1 is that in step 3, the precursor silicon carbon particles are attached to the surface of the finished graphite material by high-temperature coating. 0.5 kg of precursor silicon carbon particles Si / C-Q1, 4.4 kg of finished artificial graphite (particle size 10 μm), and 0.1 kg of coal tar were weighed and placed in a mixing device and mixed at 1200 rpm for 3 hours; then transferred to a high-temperature coating device with a spindle speed of 300 rpm, a coating temperature of 850°C, a heating rate of 10°C / min, and a coating time of 8 hours. After screening and demagnetization, the nano-silicon carbon negative electrode material was obtained and recorded as Si / C-4 (SEM image shown in Figure 6).

[0055] Comparative Example 3

[0056] The difference from Example 2 is that the precursor silicon carbon particles Si / C-Q2 prepared with a pulse width of 500 μs are used, and the final obtained nano silicon carbon negative electrode material is recorded as Si / C-5 (SEM image is shown in Figure 7).

[0057] Table 1 is the test results of the materials obtained in Examples 1-2 and Comparative Examples 1-3.

[0058] Table 1:

[0059] As can be seen from Table 1, the median particle size of silicon particles and the average grain size inside nano-silicon of Si / C-Q1 are smaller than those of Si / C-Q2. The reason is that the smaller discharge pulse width means a shorter discharge time, a shorter duration of high temperature, and a smaller heat-affected zone. As a result, the size of the agglomerated silicon particles formed under high temperature conditions will be smaller. In addition, the faster cooling rate can inhibit the growth of the grain size inside the silicon. At the same time, a large number of nano-carbon particles will agglomerate with the nano-silicon particles, further reducing the agglomeration of pure silicon. Nano-carbon particles not only improve the conductivity of the material, but also act as a barrier between silicon materials, effectively inhibiting the electrochemical sintering behavior of silicon between different silicon particles due to phase change during the charge and discharge process, thereby avoiding the growth of silicon particles and effectively improving its cycle performance.

[0060] Table 2 shows the main electrochemical test results of the electrode materials prepared in Examples 1-2 and Comparative Examples 1-3 after being made into batteries.

[0061] Table 2:

[0062] In Table 2, the Si / C-1, Si / C-2, and Si / C-3 samples in the electrochemical performance of the low specific capacity silicon-carbon negative electrode half-cell and the Si / C-1, Si / C-2, and Si / C-3 samples in the low specific capacity silicon-carbon negative electrode full-cell performance are obtained by mixing the corresponding high specific capacity samples (capacity > 1400mAh / g) with finished graphite in a mass ratio of 1:9.

[0063] Combining the corresponding data of Si / C-1 (Example 1) and Si / C-2 (Comparative Example 1) in Tables 1 and 2, it can be seen that after the discharge pulse width increases, the size of the nano-silicon and the internal grain size increase. After the same process is prepared into a high specific capacity silicon-carbon material, its capacity and first cycle efficiency change little, but the impedance increases, and the cycle performance is much worse than that of Si / C-1. After 100 cycles, the capacity retention rate is only 68.3%, which is much lower than 85.7% of Si / C-1. After mixing the same proportion of graphite material to form a low specific capacity silicon-carbon negative electrode, the pole piece of Si / C-2 expands more, and the cycle performance in both half-cell and full-cell is significantly worse than that of Si / C-1, indicating that the larger the size of the nano-silicon, the greater the expansion, which not only increases the impedance but also has a fatal effect on the cycle performance.

[0064] As shown in Table 2, the corresponding data of Si / C-1 (Example 1) and Si / C-3 (Comparative Example 2) show that the mixed material is directly subjected to carbonization process without mechanical fusion granulation. The obtained nano silicon carbon negative electrode material Si / C-3 (Figure 5) has a loose structure, many defects, and a large impedance. Although it still has a higher reversible capacity, the first coulombic efficiency is only 86.7%, which is lower than 90.3% of Si / C-1. After mixing the same proportion of graphite materials to form a low specific capacity silicon carbon negative electrode, the capacity retention rate after 900 cycles of the full battery is 78.2%, which is lower than 85.4% of Si / C-1, but still higher than 63.7% of Si / C-2. This shows that the internal silicon particles are small, and the cycle performance of the material can be well improved by combining an excellent granulation process. It is also explained that the silicon particle size is large, and even if the granulation process is excellent, its cycle performance is difficult to improve to the level of small-particle silicon particles.

[0065] The precursor silicon-carbon particles Si / C-Q1 and Si / C-Q2 prepared in Example 1 and Comparative Example 1, respectively, were bonded to the surface of the finished graphite using an asphalt material. The low-capacity silicon-carbon negative electrode materials Si / C-4 (Example 2) and Si / C-5 (Comparative Example 3) with a capacity of 425 mAh / g were prepared by heat coating. Compared with Example 1 (Si / C-1) and Comparative Examples 1 and 2 (Si / C-2 and -3), the silicon-carbon material was more evenly distributed, and the expansion caused by the silicon particles was also evenly distributed throughout the negative electrode sheet, thereby effectively reducing the overall expansion of the sheet. The expansion of the Si / C-4 sheet was only 10.6%, lower than the 12.5% ​​of Si / C-1; the expansion of the Si / C-5 sheet was only 13.1%, lower than the 16.2% of Si / C-1. At the same time, the cycling performance of the full battery was also improved to a certain extent.

Claims

1. A nano silicon-carbon composite material, characterized in that: The invention comprises co-agglomerated nano-silicon particles and nano-carbon particles; wherein the mass ratio of the nano-silicon particles to the nano-carbon particles is: (45-60): (40-55); The average particle size of the nano silicon particles is 1-50 nm, and the grain size is 1-10 nm.

2. The nano-silicon-carbon composite material according to claim 1, characterized in that: The nano silicon particles are also doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum.

3. The nano-silicon-carbon composite material according to claim 1, characterized in that: The average particle size of the nanocarbon is 1-50nm.

4. The method for preparing the nano-silicon-carbon composite material according to any one of claims 1 to 3, characterized in that: The carbon source material and the silicon source material are used as electrodes and connected to the positive and negative electrodes of a pulse power supply respectively. An inert gas is used as a working medium. Electricity is applied and the generated particles are collected. The obtained particles are nano-silicon-carbon composite materials.

5. The method for preparing the nano-silicon-carbon composite material according to claim 4, characterized in that: The carbon source material is any one of soft carbon, hard carbon and graphite; The silicon source material is intrinsic silicon, or doped silicon doped with one or two of phosphorus, nitrogen, arsenic, boron, indium and aluminum elements; The pulse width of the electric pulse of the pulse power supply is 50ns-500μs.

6. A method for preparing a negative electrode material, characterized in that: The nano-silicon-carbon composite material according to any one of claims 1 to 3 and an organic carbon source are mixed, mechanically fused and granulated, and then carbonized; wherein the mixing mass ratio of the nano-silicon-carbon composite material to the organic carbon source is (1-20):1; or, The nano silicon-carbon composite material according to any one of claims 1 to 3 is mixed with an organic carbon source and a graphite material and then coated; wherein the mass ratio of the nano silicon-carbon composite material, the organic carbon source and the graphite material is (30-80):(1-10):(20-70).

7. The method for preparing the negative electrode material according to claim 6, characterized in that: The organic carbon source is one or a combination of at least two of coal tar, coal pitch, petroleum pitch, epoxy resin, phenolic resin, acrylic resin, furfural resin, polyvinyl chloride, polyacrylonitrile and polyvinylidene fluoride; The graphite material is one or a combination of two of artificial graphite, natural spherical graphite, natural flake graphite and mesophase carbon microspheres.

8. The method for preparing the negative electrode material according to claim 7, characterized in that: The carbonization conditions are: under a protective gas atmosphere, a heating rate of 1-20°C / min, a carbonization temperature of 500-1250°C, and a carbonization time of 1-20h; The coating conditions are: heating rate 1-20°C / min, coating temperature 100-1100°C, coating time 0.5-30h.

9. A negative electrode sheet, characterized in that: The negative electrode material comprises the nano silicon-carbon composite material described in any one of claims 1 to 3 or the negative electrode material prepared by the method described in any one of claims 6 to 8.

10. A battery, characterized in that: Including the negative electrode sheet as claimed in claim 9.

Images