Silicon-carbon composite material with core-shell structure, preparation method thereof and secondary battery

Abstract

The application provides a silicon-carbon composite material with a core-shell structure, a preparation method of the silicon-carbon composite material and a secondary battery. The method comprises the following steps: S1. reacting a phenolic compound, an aldehyde compound and a metal salt compound under the condition of lye and a catalyst, and performing calcination treatment on the product after the reaction to obtain a metal-doped porous carbon material; S2. performing heat treatment on the metal-doped porous carbon material in step S1 and a silane mixed gas to obtain a metal-doped silicon-carbon material; and S3. performing calcination treatment on the metal-doped silicon-carbon material in step S2, a passivation gas and a carbon source to obtain the silicon-carbon composite material with the core-shell structure. The application utilizes phenolic aldehyde reaction to dope metal elements to improve the electronic conductivity of the inner core of the material, and coats the outer shell of the material with carbon nanotubes and amorphous carbon to bind the expansion of silicon.

Description

Technical Field

[0001] This invention belongs to the field of negative electrode material technology, specifically relating to a silicon-carbon composite material with a core-shell structure, its preparation method, and a secondary battery. Background Technology

[0002] Currently, most of the modified silicon-carbon materials disclosed in existing technologies are composed of porous carbon and nano-silicon doped in the pores of porous carbon. Compared with silicon-carbon materials prepared by sand milling, the volume expansion phenomenon of modified silicon-carbon materials is improved to some extent, but the improvement effect is not obvious. At the same time, due to the poor electronic conductivity of silicon materials themselves, their cycle performance is poor and their expansion is large during high-rate charge and discharge. This is mainly because the porous carbon materials prepared have many defects on the surface and a large specific surface area, resulting in a low initial coulombic efficiency of the material.

[0003] In recent years, many technical solutions for improving the first-cycle efficiency and fast-charging performance of silicon-carbon materials have been disclosed in existing technologies. This indicates that it is very necessary to improve the electronic conductivity of silicon material core and porous carbon material and reduce the surface defects of the material in order to improve the rate performance of the battery. At the same time, it is also necessary to ensure that silicon-carbon materials have a small volume expansion rate during cycling.

[0004] Therefore, there is an urgent need in this field to develop a silicon-carbon material to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a silicon-carbon composite material with a core-shell structure, its preparation method, and a secondary battery. Addressing the technical problems of large volume expansion, poor rate performance, and low initial efficiency of silicon-carbon materials at full charge in existing technologies, the present invention utilizes phenolic resin reaction doping with metal elements to improve the electronic conductivity of the material core, and coats its outer shell with carbon nanotubes and amorphous carbon to confine the expansion of silicon.

[0006] To achieve this objective, the present invention employs the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a silicon-carbon composite material having a core-shell structure, the method comprising the following steps:

[0008] S1. Phenolic compounds, aldehyde compounds and metal salt compounds are reacted under alkaline conditions and with a catalyst, and the products after the reaction are calcined to obtain metal-doped porous carbon materials.

[0009] S2. Heat-treat the metal-doped porous carbon material from step S1 with a silane mixed gas to obtain a metal-doped silicon-carbon material;

[0010] S3. The metal-doped silicon-carbon material, passivation gas and carbon source from step S2 are calcined to obtain the silicon-carbon composite material with a core-shell structure.

[0011] First, this invention utilizes a phenolic reaction to prepare metal-doped phenolic resin-based microspheres, which are then calcined to generate metal-doped porous carbon, thereby improving the electronic conductivity and initial efficiency of the silicon-carbon core material. Simultaneously, carbon nanotubes grown under catalytic conditions enhance the electronic conductivity of the silicon-carbon material and mitigate its volume expansion during charge and discharge. Second, this invention introduces heteroatoms from a passivation gas to modify the coating layer, thereby improving the material's electronic conductivity and thus enhancing the battery's rate performance. Furthermore, this invention reduces the volume expansion of the silicon-carbon material by passivating its surface with heteroatoms, thereby improving the battery's cycle performance.

[0012] Preferably, in step S1, the phenolic compound includes any one or a combination of at least two of phenol, p-cresol, m-cresol, or o-cresol.

[0013] Preferably, in step S1, the aldehyde compound includes any one or a combination of at least two of formaldehyde, acetaldehyde, or n-butyraldehyde.

[0014] Preferably, in step S1, the molar ratio of the phenolic compound to the aldehyde compound is 1:(1.05-2), for example, it can be 1:1.05, 1:1.08, 1:1.1, 1:1.2, 1:1.5, 1:1.8, 1:2, etc.

[0015] In this invention, by adjusting the molar ratio of phenolic compounds and aldehyde compounds, a structurally stable phenolic resin is generated. If the molar ratio is too low, the structure of the phenolic resin will be unstable and the cycle performance of the battery will be reduced. Conversely, if the molar ratio is too high, there will be an excess of aldehyde compounds, resulting in a low tap density of the material.

[0016] Preferably, in step S1, the metal salt compound includes any one or a combination of at least two of magnesium chloride, magnesium nitrate, magnesium carbonate, or magnesium sulfate.

[0017] Preferably, the alkaline solution comprises any one or a combination of at least two of sodium hydroxide solution, potassium hydroxide solution, or calcium hydroxide solution.

[0018] Preferably, the mass concentration of the alkaline solution is 1-5 wt%, for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc.

[0019] Preferably, the catalyst comprises at least one of nano-iron, nano-cobalt, or nano-nickel.

[0020] Preferably, the particle size of the catalyst is 100-500 nm, for example, it can be 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, etc.

[0021] Preferably, in step S1, the reaction temperature is 50-100℃, for example, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃, etc.; the time is 1-6h, for example, 1h, 2h, 3h, 4h, 5h, 6h, etc.

[0022] Preferably, in step S1, an organic acid needs to be added before the reaction is terminated.

[0023] Preferably, the organic acid includes at least one of toluenesulfonic acid, oxalic acid, or tartaric acid.

[0024] Preferably, the mass ratio of the combination of phenolic compounds and aldehyde compounds, alkaline solution, metal salt compound, catalyst and organic acid is 100:(500-2000):(1-5):(0.5-2):(10-100), for example, 100:500:1:0.5:10, 100:520:1.2:0.8:15, 100:550:1.5:1:20, 100:600:1.8:1.8:30, 100:800:2:2:50, 100:1000:2.5:1:60, 100:1500:4:1.5:80, 100:2000:5:2:100, etc.

[0025] In this invention, by adjusting the combination of phenolic and aldehyde compounds, the mass ratio of alkaline solution, metal salt compound, catalyst and organic acid, the overall performance of the material, including impedance, cycle performance and tap density, is improved. If the mass ratio is too low, the impedance of the material will be large, and if it is too high, the volume expansion of the material will be large and the cycle performance of the battery will be poor.

[0026] Preferably, in step S1, the specific process of the calcination treatment is as follows: the product after the reaction is heated to 400-600℃ and steam is introduced for calcination for 60-600 minutes. The temperature can be, for example, 400℃, 450℃, 500℃, 550℃, 600℃, etc.; the time can be, for example, 60 minutes, 80 minutes, 100 minutes, 200 minutes, 300 minutes, 400 minutes, 500 minutes, 600 minutes, etc.

[0027] Preferably, in step S2, the silane mixed gas is composed of a mixture of silane and argon.

[0028] Preferably, the volume ratio of silane to argon is 1:(1-2), for example, it can be 1:1, 1:1.1, 1:1.2, 1:1.5, 1:1.8, 1:2, etc.

[0029] Preferably, in step S2, the flow rate of the silane mixed gas is 100-500 ml / min, for example, it can be 100 ml / min, 200 ml / min, 300 ml / min, 400 ml / min, 500 ml / min, etc.; the time is 60-300 min, for example, it can be 60 min, 80 min, 100 min, 200 min, 300 min, etc.

[0030] In this invention, by controlling the flow rate and time of the silane mixed gas, nano-silicon is uniformly deposited in porous carbon, avoiding leakage of nano-silicon. At the same time, if the flow rate and time are too high, silicon particles will agglomerate, resulting in a large volume expansion of the material; if the flow rate and time are too low, the silicon deposition time will be too long, reducing production efficiency.

[0031] Preferably, in step S2, the temperature of the heat treatment is 450-650℃, for example, it can be 450℃, 480℃, 500℃, 550℃, 600℃, 650℃, etc.

[0032] Preferably, in step S3, the passivation gas includes at least one of CF4, C4F8, CH2F2, C2F6, or NF3.

[0033] Preferably, in step S3, the flow rate of the passivating gas is 500-2000 ml / min, for example, 500 ml / min, 800 ml / min, 1000 ml / min, 1200 ml / min, 1500 ml / min, 1800 ml / min, 2000 ml / min, etc.; and the time is 300-1200 min, for example, 300 min, 500 min, 800 min, 1000 min, 1200 min, etc.

[0034] In this invention, by controlling the flow rate and time of the passivation gas, the carbon deposition density and specific capacity on the material surface are made more suitable. If the flow rate and time are too long, the deposition amount will be too large, reducing the specific capacity of the material. If the flow rate and time are too short, the deposition thickness will be low, which will have limited effect on improving the rate performance of the material.

[0035] Preferably, in step S3, the carbon source includes at least one of methane, ethane, ethylene, or acetylene.

[0036] Preferably, in step S3, the flow rate of the carbon source is 100-500 ml / min, for example, 100 ml / min, 200 ml / min, 300 ml / min, 400 ml / min, 500 ml / min, etc.; the time is 30-300 min, for example, 30 min, 60 min, 80 min, 100 min, 200 min, 300 min, etc.

[0037] Preferably, in step S3, the calcination temperature is 700-950℃, for example, it can be 700℃, 750℃, 800℃, 850℃, 900℃, 950℃, etc.

[0038] In a second aspect, the present invention provides a silicon-carbon composite material with a core-shell structure, wherein the silicon-carbon composite material with a core-shell structure is prepared by the method for preparing a silicon-carbon composite material with a core-shell structure according to the first aspect.

[0039] Preferably, the core-shell structured silicon-carbon composite material comprises a metal-doped silicon-carbon material core and an inorganic carbon coating layer covering the surface of the metal-doped silicon-carbon material core.

[0040] Preferably, the thickness of the inorganic carbon coating layer is 50-200nm, more preferably 50-100nm, and for example, it can be 50nm, 60nm, 80nm, 100nm, 120nm, 150nm, 180nm, 200nm, etc.

[0041] In this invention, by adjusting the thickness of the inorganic carbon coating layer, the impedance of the material is reduced, while the leakage of the silicon core material is avoided, thereby improving the high-temperature storage performance of the silicon-carbon composite material.

[0042] Thirdly, the present invention provides a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the material of the negative electrode comprises a silicon-carbon composite material having a core-shell structure as described in the second aspect.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] This invention provides a method for preparing silicon-carbon composite materials with a core-shell structure. First, the invention utilizes a phenolic reaction to prepare metal-doped phenolic resin-based microspheres, which are then calcined to generate metal-doped porous carbon, thereby improving the electronic conductivity and first-pass efficiency of the silicon-carbon core material. Simultaneously, carbon nanotubes grown under catalytic conditions can enhance the electronic conductivity of the silicon-carbon material and mitigate its volume expansion during charge and discharge. Second, this invention introduces heteroatoms from a passivation gas to modify the coating layer, thereby improving the material's electronic conductivity and thus enhancing the battery's rate performance. Furthermore, this invention reduces the volume expansion of the silicon-carbon material by passivating its surface with heteroatoms, thereby improving the battery's cycle performance. Attached Figure Description

[0045] Figure 1 SEM image of the silicon-carbon composite material with a core-shell structure provided by the present invention. Detailed Implementation

[0046] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be considered as specific limitations thereof.

[0047] Example 1

[0048] This embodiment provides a silicon-carbon composite material with a core-shell structure and its preparation method. The silicon-carbon composite material with a core-shell structure includes a metal-doped silicon-carbon core and an inorganic carbon coating layer covering the surface of the metal-doped silicon-carbon core. The thickness of the inorganic carbon coating layer is 100 nm. The preparation method includes the following steps:

[0049] S1. 32.4g (1.08mol) of formaldehyde was added to 1000g of 3wt% sodium hydroxide solution and dispersed evenly. Then, 67.6g (0.72mol) of phenol was added and dissolved. Then, 3g of magnesium chloride and 1g of nano-iron catalyst (particle size of 300nm) were added and dispersed evenly. The reaction was carried out at 80℃ for 3h. Then, 50g of toluenesulfonic acid was added to terminate the reaction. The mixture was filtered and vacuum dried at 80℃ for 24h. Then, it was transferred to a tube furnace and heated to 500℃. Water vapor (flow rate of 500ml / min) was introduced for 300min to form pores, thus obtaining metal-doped porous carbon material.

[0050] S2. Transfer the metal-doped porous carbon material from step S1 to a fluidized bed device, heat it to 500°C, and introduce a silane mixed gas (SiH4:argon = 1:1.5) to suspend the metal-doped porous carbon material. The flow rate is 300 ml / min and the introduction time is 120 min to obtain metal-doped silicon-carbon material.

[0051] S3. Transfer the metal-doped silicon-carbon material from step S2 to a tube furnace, heat to 850°C, and introduce CF4 passivation gas at a flow rate of 500 ml / min for 320 min. Then, switch to methane gas at a flow rate of 300 ml / min for 120 min to obtain the desired product. Figure 1 The silicon-carbon composite material shown has a core-shell structure.

[0052] Depend on Figure 1 As can be seen from the example, the silicon-carbon composite material with a core-shell structure prepared in Example 1 has carbon nanotube fiber structure coated on its surface, with a particle size between 10-15 μm.

[0053] Example 2

[0054] This embodiment provides a silicon-carbon composite material with a core-shell structure and its preparation method. The silicon-carbon composite material with a core-shell structure includes a metal-doped silicon-carbon core and an inorganic carbon coating layer covering the surface of the metal-doped silicon-carbon core. The thickness of the inorganic carbon coating layer is 200 nm. The preparation method includes the following steps:

[0055] S1. 70g (0.64mol) m-cresol was added to 500g of 5wt% potassium hydroxide solution and dispersed evenly. Then, 30g (0.67mol) of acetaldehyde was added and dissolved. Then, 1g of magnesium nitrate and 0.5g of nano-cobalt catalyst (particle size 200nm) were added and dispersed evenly. The reaction was carried out at 50℃ for 6h. Then, 10g of oxalic acid was added to terminate the reaction. The mixture was filtered and vacuum dried at 80℃ for 24h. Then, it was transferred to a tube furnace and heated to 400℃. Water vapor (flow rate 100ml / min) was introduced for 600min to form pores, thus obtaining metal-doped porous carbon material.

[0056] S2. Transfer the metal-doped porous carbon material from step S1 to a fluidized bed device, and introduce a silane mixed gas (SiH4:argon = 1:1) to blow the metal-doped porous carbon material into a suspended state. At the same time, heat it to 450°C, and at a flow rate of 100 ml / min and a reaction time of 300 min, obtain metal-doped silicon-carbon material.

[0057] S3. Transfer the metal-doped silicon-carbon material from step S2 to a tube furnace, heat it to 700°C and introduce C4F8 passivation gas at a flow rate of 500 ml / min for a reaction time of 1200 min. Then, switch to acetylene carbon source gas at a flow rate of 100 ml / min for a reaction time of 300 min to obtain the silicon-carbon composite material with a core-shell structure.

[0058] Example 3

[0059] This embodiment provides a silicon-carbon composite material with a core-shell structure and its preparation method. The silicon-carbon composite material with a core-shell structure includes a metal-doped silicon-carbon core and an inorganic carbon coating layer covering the surface of the metal-doped silicon-carbon core. The thickness of the inorganic carbon coating layer is 50 nm. The preparation method includes the following steps:

[0060] S1. 42.8g of o-cresol (0.396mol) compound solution was added to 2000g of 1wt% calcium hydroxide solution and dispersed evenly. Then, 57.09g (0.793mol) of n-butyraldehyde compound was added and dissolved. Then, 5g of magnesium carbonate and 2g of nano-nickel catalyst (particle size of 200nm) were added and dispersed evenly. The reaction was carried out at 100℃ for 1h. Then, 100g of tartaric acid was added to terminate the reaction. The mixture was filtered and vacuum dried at 80℃ for 24h. Then, it was transferred to a tube furnace and heated to 600℃. Water vapor (flow rate of 1000ml / min) was introduced and reacted for 60min to form pores, thus obtaining metal-doped porous carbon material.

[0061] S2. Transfer the metal-doped porous carbon material from step S1 to a fluidized bed device, introduce a silane mixed gas (SiH4:argon = 1:1) to blow it into a suspended state, and heat it to 650°C for 60 minutes to obtain metal-doped silicon-carbon material.

[0062] S3. Transfer the metal-doped silicon-carbon material from step S2 to a tube furnace, heat it to 950°C, and introduce CH2F2 passivation gas at a flow rate of 2000 ml / min for 300 min. Then, introduce methane gas at a flow rate of 500 ml / min for 30 min to obtain the silicon-carbon composite material with a core-shell structure.

[0063] Example 4

[0064] The difference between this embodiment and Example 1 is that the mass ratio of phenol and formaldehyde, sodium hydroxide solution, magnesium chloride, nano-iron catalyst and toluenesulfonic acid is 100:200:0.5:0.1:5, while all other aspects are the same as in Example 1.

[0065] Example 5

[0066] The difference between this embodiment and Example 1 is that the mass ratio of phenol and formaldehyde, sodium hydroxide solution, magnesium chloride, nano-iron catalyst and toluenesulfonic acid is 100:1000:10:5:200, while all other aspects are the same as in Example 1.

[0067] Example 6

[0068] The difference between this embodiment and Embodiment 1 is that in step S3, the CF4 passivation gas is replaced with argon gas, while everything else is the same as in Embodiment 1.

[0069] Example 7

[0070] The difference between this embodiment and Embodiment 1 is that the flow rate of the CF4 passivation gas is 200 ml / min, while all other aspects are the same as in Embodiment 1.

[0071] Comparative Example 1

[0072] The difference between this comparative example and Example 1 is that magnesium chloride and nano-iron catalyst are not added in step S1, while everything else is the same as in Example 1.

[0073] Comparative Example 2

[0074] The difference between this comparative example and Example 1 is that CF4 passivation gas is not added in step S3, while everything else is the same as in Example 1.

[0075] Test conditions

[0076] The silicon-carbon composite materials with core-shell structures provided in Examples 1 to 7 and Comparative Examples 1 to 2 were tested using the following methods:

[0077] The particle size, tap density, and specific surface area of ​​the silicon-carbon composite materials with core-shell structure provided in Examples 1 to 7 and Comparative Examples 1 to 2 were tested according to the method of national standard GBT-38823-2020 "Silicon-Carbon". At the same time, the powder resistivity of the materials was tested using a four-probe tester.

[0078] The test results are shown in Table 1:

[0079] Table 1

[0080]

[0081] As can be seen from Table 1, the silicon-carbon composite materials with core-shell structure prepared in Examples 1-3 of this invention have a suitable particle size range, high tap density, large specific surface area and low powder resistivity.

[0082] A comparison of Examples 1 and 4-5 shows that the combination of phenolic compounds and aldehyde compounds, the alkali solution, the mass ratio of metal salt compounds, the catalyst, and the organic acid have a certain influence on the overall performance of the material. By adjusting the mass ratio of the above components, the present invention enables the prepared silicon-carbon composite material with a core-shell structure to have better overall performance.

[0083] As can be seen from Examples 1, 6 and Comparative Example 2, the absence of a passivating gas containing heteroatoms increases the resistivity of the powder material.

[0084] As can be seen from Example 1 and Comparative Example 1, without the addition of magnesium chloride and nano-iron catalyst, the electronic conductivity of the material cannot be further improved.

[0085] Button cell battery test:

[0086] The silicon-carbon composite materials with core-shell structures from Examples 1-7 and Comparative Examples 1-2 were used as negative electrode materials for lithium-ion batteries to assemble coin cells. The specific preparation method for the negative electrode material was as follows: a binder, conductive agent, and solvent were added to the composite material, stirred to form a slurry, coated onto copper foil, and then dried and rolled. The binder used was LA132, the conductive agent was Super P, and the solvent was double-distilled water. The negative electrode sheet was prepared according to the ratio of composite material:Super P:LA132:double-distilled water of 90g:4g:6g:220mL. A lithium metal sheet was used as the positive electrode. The electrolyte was LiPF6 / EC+DEC, where LiPF6 was the electrolyte, and a 1:1 volume ratio of EC and DEC was used as the solvent. The electrolyte concentration was 1.3mol / L. The separator was a composite membrane of polyethylene, polypropylene, or polyethylene propylene. The coin cells were assembled in an argon-filled glove box.

[0087] Electrochemical performance tests were conducted using a Wuhan Landian CT2001A battery tester. The charge / discharge voltage range was 0.005V to 2.0V, and the charge / discharge rate was 0.1C. The initial discharge capacity and initial efficiency of the coin cells were tested. Simultaneously, the charging DC resistance (50% SOC) and cycle performance (0.5C / 0.5C, 100 cycles) were tested, and the expansion rate of the negative electrode was measured by dissection of the coin cells at 100% SOC after full charge. The test results are shown in Table 2.

[0088] Table 2

[0089]

[0090]

[0091] As can be seen from Tables 1 and 2, the coin cells prepared in Examples 1-3 of this invention have high initial coulombic efficiency, good cycle performance, low DC resistance, and small full-charge expansion rate.

[0092] As can be seen from the comparison of Examples 1 and 4-5, the combination of phenolic compounds and aldehyde compounds, alkaline solution, metal salt compounds, catalyst and organic acid mass ratio have a certain influence on the overall performance of coin cells. The present invention adjusts the mass ratio of the above components to make the prepared coin cells have better overall performance.

[0093] As can be seen from Examples 1, 6, and Comparative Example 2, the DC resistance and full-charge expansion rate of the coin cell are significantly increased without the introduction of a passivating gas containing heteroatoms. This is because the passivation of the material surface by heteroatoms reduces the electronic conductivity of the material and its expansion, thereby improving the cycle performance of the battery.

[0094] As can be seen from Example 1 and Comparative Example 1, the overall performance of the coin cells prepared without the addition of magnesium chloride and nano-iron catalyst is poor.

[0095] Soft-pack battery test:

[0096] The silicon-carbon composite materials with core-shell structures from Examples 1-7 and Comparative Examples 1-2 were mixed with 90% artificial graphite as the negative electrode, and the negative electrode sheets were prepared by slurry mixing and coating. The negative electrode sheets were made using ternary materials (LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 A pouch cell with a capacity of 2 Ah was prepared using O2 as the positive electrode, LiPF6 (solvent EC+DEC, volume ratio 1:1, LiPF6 electrolyte concentration 1.3 mol / L) as the electrolyte, and Celgard 2400 membrane as the separator.

[0097] HPPC Ratio Performance Test:

[0098] The rate performance of the pouch battery was tested under a charge / discharge voltage range of 2.5–4.2V and a temperature of 25±3.0℃. The resistance was measured under different SOC conditions (90%, 70%, 50%, 30%, 10%, 5%) by charging at 3C and discharging at 1.0C. The results are shown in Table 3.

[0099] Table 3

[0100]

[0101] As shown in Table 3, the impedance of the pouch batteries assembled in Examples 1-3 is significantly lower than that in Comparative Examples 1-2, meaning that the charging time is shorter. The reason for this is that lithium ions need to migrate during the charging process. This invention modifies silicon-carbon materials by controlling the parameters, component types, and mass ratios in the preparation process, thereby improving the electronic conductivity of the materials and improving the rate performance of the pouch batteries.

[0102] Cyclic performance test:

[0103] The obtained soft-pack batteries were subjected to cycle performance testing under the following conditions: charge / discharge current 1C / 1C, voltage range 2.5~4.2V, temperature 25±3℃, and 1000 cycles. The test results are shown in Table 4.

[0104] Table 4

[0105]

[0106] As shown in Table 4, the lithium-ion batteries prepared using the core-shell silicon-carbon composite materials obtained in Examples 1-3 exhibit significantly better cycle performance than the batteries in Examples 4-7 and Comparative Examples 1-2 at all stages. The experimental results demonstrate that, on the one hand, this invention improves the electronic conductivity and initial efficiency of the silicon-carbon core material by doping porous carbon with metal. On the other hand, this invention reduces the volume expansion of the silicon-carbon material by passivating its surface with heteroatoms, thereby improving the battery's cycle performance.

[0107] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a silicon-carbon composite material with a core-shell structure, characterized in that, The method includes the following steps: S1. Phenolic compounds, aldehyde compounds and metal salt compounds are reacted under alkaline conditions and with a catalyst, and the products after the reaction are calcined to obtain metal-doped porous carbon materials. S2. Heat-treat the metal-doped porous carbon material from step S1 with a silane mixed gas to obtain a metal-doped silicon-carbon material; S3. The metal-doped silicon-carbon material, passivation gas and carbon source from step S2 are calcined, and carbon nanotubes are grown under catalytic conditions to obtain the silicon-carbon composite material with a core-shell structure, wherein the passivation gas includes at least one of CF4, C4F8, CH2F2, C2F6 or NF3.

2. The method according to claim 1, characterized in that, In step S1, the phenolic compound includes any one or a combination of at least two of phenol, p-cresol, m-cresol or o-cresol; In step S1, the aldehyde compound includes any one or a combination of at least two of formaldehyde, acetaldehyde, or n-butyraldehyde; In step S1, the molar ratio of the phenolic compound to the aldehyde compound is 1:(1.05-2).

3. The method according to claim 1 or 2, characterized in that, In step S1, the metal salt compound includes any one or a combination of at least two of magnesium chloride, magnesium nitrate, magnesium carbonate, or magnesium sulfate. The alkaline solution includes any one or a combination of at least two of sodium hydroxide solution, potassium hydroxide solution, or calcium hydroxide solution; The mass concentration of the alkaline solution is 1-5 wt%; The catalyst includes at least one of nano-iron, nano-cobalt, or nano-nickel; The catalyst has a particle size of 100-500 nm.

4. The method according to claim 1, characterized in that, In step S1, the reaction temperature is 50-100℃ and the time is 1-6h; In step S1, an organic acid needs to be added before the reaction is terminated.

5. The method according to claim 4, characterized in that, The organic acid includes at least one of toluenesulfonic acid, oxalic acid, or tartaric acid.

6. The method according to claim 4, characterized in that, The mass ratio of the combination of phenolic compounds and aldehyde compounds, alkaline solution, metal salt compound, catalyst and organic acid is 100:(500-2000):(1-5):(0.5-2):(10-100).

7. The method according to claim 1, characterized in that, In step S1, the specific process of the calcination treatment is as follows: the product after the reaction is heated to 400-600℃ and calcined with steam for 60-600 minutes.

8. The method according to claim 1, characterized in that, In step S2, the silane mixed gas is composed of a mixture of silane and argon.

9. The method according to claim 8, characterized in that, The volume ratio of silane to argon is 1:(1-2).

10. The method according to claim 1, characterized in that, In step S2, the flow rate of the silane mixed gas is 100-500 ml / min, and the time is 60-300 min.

11. The method according to claim 1, characterized in that, In step S2, the temperature of the heat treatment is 450-650℃.

12. The method according to claim 1, characterized in that, In step S3, the flow rate of the passivation gas is 500-2000 ml / min, and the time is 300-1200 min; In step S3, the carbon source includes at least one of methane, ethane, ethylene, or acetylene; In step S3, the flow rate of the carbon source is 100-500 ml / min, and the time is 30-300 min; In step S3, the calcination temperature is 700-950℃.

13. A silicon-carbon composite material with a core-shell structure, characterized in that, The silicon-carbon composite material with a core-shell structure is prepared by the method for preparing silicon-carbon composite materials with a core-shell structure according to any one of claims 1-12.

14. The silicon-carbon composite material with a core-shell structure according to claim 13, characterized in that, The core-shell structured silicon-carbon composite material includes a metal-doped silicon-carbon material core and an inorganic carbon coating layer covering the surface of the metal-doped silicon-carbon material core.

15. The silicon-carbon composite material with a core-shell structure according to claim 14, characterized in that, The thickness of the inorganic carbon coating layer is 50-200 nm.

16. The silicon-carbon composite material with a core-shell structure according to claim 15, characterized in that, The thickness of the inorganic carbon coating layer is 50-100 nm.

17. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the material of the negative electrode includes a silicon-carbon composite material with a core-shell structure according to any one of claims 13-16.

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