Preparation method of pitch-based silicon-carbon negative electrode material for lithium ion battery and product thereof

Abstract

A preparation method of pitch-based silicon-carbon negative electrode material for lithium ion battery, comprising: (1) putting pitch into a reaction kettle and heating to the softening point of pitch; (2) forming nano gas by passing gas into molecular sieve, then passing into the reaction kettle and controlling the inside of the reaction kettle to reach a predetermined pressure, continuing to heat to 50 DEG C above the softening point temperature of pitch, and obtaining porous pitch material after heat preservation and cooling; (3) carrying out carbonization treatment on the porous pitch material to obtain pitch-based porous carbon material; (4) carrying out silicon deposition treatment on the pitch-based porous carbon material. The pitch-based porous carbon material is prepared by a simple and controllable process, the intermediate product has high specific surface area, moderate pore volume and narrow pore size distribution, the silicon deposition amount and silicon deposition rate can be accurately controlled in the subsequent silicon deposition process, the porous carbon material and the silicon material are well combined, and thus the lithium ion battery with high initial efficiency, high capacity and excellent cycle stability is obtained.

Description

Technical Field

[0001] This invention relates to the technical field of electrode materials, and in particular to a method for preparing pitch-based silicon-carbon anode material for lithium-ion batteries and its product. Background Technology

[0002] Lithium-ion batteries, as a new type of rechargeable battery, have advantages such as light weight, large energy storage, high power, no pollution, long life, and low auto-discharge coefficient. As the market demands higher energy density from lithium-ion batteries, the requirements for lithium-ion battery electrode materials are also constantly increasing, with the negative electrode material being a key component of lithium-ion batteries.

[0003] Currently, graphite is the primary anode material for lithium-ion batteries, but its theoretical capacity is relatively low. Silicon, with its high capacity, is considered the future direction for anode materials, but its high coefficient of thermal expansion significantly impacts battery lifespan. Porous carbon, as a typical amorphous carbon material, is considered to have enormous potential in the field of commercial lithium-ion battery electrode materials due to its high cost-effectiveness and environmental friendliness. The introduction of porous carbon increases the conductivity between the current collector and the anode in lithium-ion batteries; furthermore, porous carbon provides space for the volume expansion of the silicon anode, which can greatly improve the long-term cycle stability and safety of the battery.

[0004] Current methods for preparing porous carbon include physical activation, chemical activation, catalytic activation, and template methods. Chemical activation requires the introduction of a strong alkali as an activating agent, followed by multiple acid and water washing processes to remove impurity ions. Template methods involve the introduction and removal of a template, requiring precise control of process parameters. Physical activation involves carbonizing the material in an inert atmosphere, followed by pore-forming with H2O or CO2 at high temperatures; the process is simple and easy to operate. However, existing physical activation methods result in porous carbon materials with a predominantly microporous structure and a small specific surface area, making it difficult to achieve 1500 nm. 2 / g and above severely restrict its subsequent use.

[0005] Chinese patent document CN110697705A discloses a rapid preparation method for pitch-based activated carbon with a hierarchical porous structure. This invention uses coal tar pitch as raw material, incorporates an activator and a pore-forming agent, and controls the temperature rise process to directly prepare pitch-based activated carbon with a hierarchical porous structure through a one-step carbonization-activation method. While this method directly prepares pitch-based activated carbon with a hierarchical porous structure through a one-step carbonization-activation method, it introduces a strong alkali as an activator and a pore-forming agent during the reaction process, requiring multiple washings with hydrochloric acid, filtration, and deionized water rinsing. Furthermore, the pore size formation is highly dependent on the temperature rate, and improper control can easily lead to pore collapse. The experimental process requires extremely precise control, making large-scale production difficult. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention discloses a method for preparing pitch-based silicon-carbon anode materials for lithium-ion batteries. The method uses a simple and controllable process to prepare pitch-based porous carbon materials. These intermediate products have high specific surface area, moderate pore volume, and narrow pore size distribution. This allows for precise control of silicon deposition amount and rate in the subsequent silicon deposition process, resulting in a better combination of porous carbon materials and silicon materials. This leads to lithium-ion batteries with high initial efficiency, high capacity, and excellent cycle stability.

[0007] The specific technical solution is as follows:

[0008] A method for preparing a pitch-based silicon-carbon anode material for lithium-ion batteries includes the following steps:

[0009] (1) Put the asphalt into the reactor, evacuate it, and then heat it to the softening point of the asphalt.

[0010] (2) After the gas is introduced into the molecular sieve, nano gas is formed. The nano gas is introduced into the reaction vessel and the pressure inside the reaction vessel is controlled to reach the predetermined pressure. The temperature is continued to rise to 50°C above the softening point temperature of the asphalt. After the temperature is kept for a period of time, it is cooled to room temperature to obtain porous asphalt material.

[0011] (3) The porous asphalt material is crushed and then carbonized to obtain asphalt-based porous carbon material.

[0012] (4) The asphalt-based porous carbon material is subjected to silicon deposition treatment to obtain the asphalt-based silicon-carbon anode material for lithium-ion batteries.

[0013] The preparation method disclosed in this invention uses readily available asphalt as raw material, forms nano-gas through molecular sieves, and utilizes this nano-gas to form a rich porous structure inside softened asphalt (pre-softened and then heated to a semi-carbonized solidified state), thereby preparing a material with a specific surface area greater than 1600 m². 2 / g, pore volume 0.7~1.0cm 3 / g, pitch-based porous carbon material with concentrated pore size distribution is used to prepare pitch-based silicon-carbon anode material through conventional silicon deposition process. Experiments revealed that the formation of nano-gas is the key to the preparation process of this invention. If the gas is used directly for pore formation without passing through a molecular sieve, the resulting pitch-based carbon material has extremely low specific surface area and pore volume. During subsequent silicon deposition, the deposited layer is all on the surface of the carbon material, leading to cracking during rolling and making it impossible to produce electrode sheets, thus rendering it useless for application.

[0014] In step (1):

[0015] The asphalt is selected from one or more of low-temperature asphalt, medium-temperature asphalt, and high-temperature asphalt;

[0016] The amount of asphalt added accounts for 10-90% of the reactor volume.

[0017] The pore-forming process in step (2) of this invention is a key step in the preparation process. Experiments have shown that the pore size of the molecular sieve, the flow rate of the nano gas, and the control of the internal pressure of the reactor in this step can directly affect the pore volume, pore size, and specific surface area of ​​the prepared pitch-based porous carbon material.

[0018] Preferably, the pore size of the molecular sieve is 4–15 nm. Experiments have shown that when the pore size of the molecular sieve is too small (~2 nm), the specific surface area and pore volume of the prepared porous carbon material are both large, but the average pore size is too small, which is not conducive to the subsequent bonding of the porous carbon material with silicon, affecting the battery capacity and first-time efficiency. When the pore size of the molecular sieve is too large (~50 nm), the specific surface area and pore volume of the prepared pitch-based porous carbon material decrease significantly, and the average pore size is too large, and the pore size distribution is also wide, which is also not conducive to the subsequent bonding of the porous carbon material with silicon, affecting the battery capacity and first-time efficiency.

[0019] Further preferred, the pore size of the molecular sieve is 10-12 nm; experiments have shown that the pitch-based porous carbon material prepared by nano-gas pore formation using the molecular sieve pore size range has a large specific surface area and a moderate pore volume range, a moderate average pore size and a more concentrated pore size distribution.

[0020] Preferably, the flow rate of the nano-gas is 8-22 L / min, the infusion time is 25-80 min, and the internal pressure of the reactor is controlled at 5-12 MPa. Experiments have shown that when the flow rate of the nano-gas is too low or too high, the pore size distribution of the prepared pitch-based porous carbon material will be uneven, resulting in multiple peaks. This situation does not directly lead to a significant change in the specific surface area and pore volume of the prepared pitch-based porous carbon material, but in the subsequent silicon deposition process, the uneven distribution of the internal pore size will make it difficult to control the amount and rate of silicon deposition, and the porous carbon material cannot be well combined with silicon, affecting the capacity and first-time efficiency of the final battery anode material. Furthermore, even with appropriate flow rates, improper control of the internal pressure of the reactor can significantly affect the specific surface area and pore volume of the prepared pitch-based porous carbon material. The reasons for this may be as follows: when the pressure is too high, the mechanical strength of the internal pore walls is insufficient, leading to pore collapse and destruction of the internal pore structure; when the internal pressure is too low, more air bubbles overflow to the softened pitch surface and are difficult to seal inside the pitch, resulting in a reduction in the internal pore structure.

[0021] Further preferably, the flow rate of the nano-gas is 10-20 L / min, the introduction time is 25-40 min, and the internal pressure of the reactor is controlled at 7-10 MPa.

[0022] More preferably, the flow rate of the nano-gas is 12-18 L / min, and the internal pressure of the reactor is controlled at 7-8 MPa.

[0023] With continuous optimization of the molecular sieve pore size, the flow rate of nano-gas, and the internal pressure of the reactor in step (2), the specific surface area of ​​the prepared pitch-based porous carbon materials is all greater than 1600 m². 2 / g, pore volume 0.7~1.0cm 3 / g, with concentrated pore size distribution, and after silicon deposition and carbon coating treatment, the negative electrode material is obtained. The final assembled lithium-ion battery has better electrochemical performance.

[0024] Preferably, the specific surface area of ​​the prepared pitch-based porous carbon material is greater than 1800 m². 2 / g, up to 1965m 2 / g; the pore size range of the prepared pitch-based porous carbon material is 1.5-12nm, more preferably 3-8nm, and even more preferably 3-6nm.

[0025] In step (2):

[0026] The gas is selected from one or more of nitrogen, carbon monoxide, carbon dioxide, water vapor, and inert gases;

[0027] Experiments have shown that when the gas is selected from carbon dioxide and / or water vapor, the prepared asphalt-based porous carbon material has a higher specific surface area and pore volume. This may be because these two gases can continue to react inside the asphalt under high temperature conditions to obtain other small molecule gases such as carbon monoxide or hydrogen. Based on the external introduction of gas, the reaction can continue to form pores inside the asphalt, resulting in a better pore structure and a higher specific surface area.

[0028] In step (2), the temperature is further increased to 50°C above the softening point of the asphalt, which can ensure that the asphalt is in a semi-carbonized solidified state, which is more conducive to the formation of a stable pore structure. Preferably, the temperature is further increased to 50-100°C above the softening point of the asphalt, and the holding time is 1-10 hours. More preferably, the temperature is further increased to 60-100°C above the softening point of the asphalt.

[0029] In step (3):

[0030] The porous asphalt material is pulverized to a particle size of 2–10 μm;

[0031] The carbonization process is carried out under an inert atmosphere at a temperature of 650–950°C.

[0032] In step (4):

[0033] The silicon deposition employs conventional techniques in the art, and the deposition amount is controlled between 45 and 55 wt%.

[0034] Preferably, the asphalt-based porous carbon material undergoes a carbon coating process after silicon deposition treatment; this carbon coating process also employs conventional techniques in the field, with the carbon coating amount controlled at 4-5 wt%.

[0035] The present invention also discloses a pitch-based silicon-carbon anode material for lithium-ion batteries prepared according to the above process. The lithium-ion battery assembled with the anode material has high initial efficiency, high capacity and excellent cycle stability.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] This invention discloses a method that uses inexpensive, high-carbon, and low-ash asphalt as raw material and utilizes a purely physical activation method to form a rich porous structure within the softened asphalt by binding the gas. The number and distribution of pores formed can be controlled by adjusting factors such as the molecular sieve pore size, the flow rate of the nano-gas, and the internal pressure of the reactor. This allows for easy control of the specific surface area, pore volume, pore size, and pore distribution of the asphalt-based porous carbon material according to actual production needs. The pore-forming process avoids strong alkali and complex process control, and requires no post-treatment. The process is simple, environmentally friendly, and highly controllable.

[0038] The pitch-based porous carbon material prepared by this invention has a high specific surface area, moderate pore volume, and narrow pore size distribution. It is then used in a conventional silicon deposition process and / or carbon coating process to prepare a pitch-based silicon-carbon anode material. Due to the use of the pitch-based porous carbon material with the aforementioned advantages, the porous carbon material and silicon material are well integrated, resulting in a lithium-ion battery with high initial efficiency, high capacity, and excellent cycle stability. Its specific capacity can reach 1800 mAh / g or higher, the initial coulombic efficiency is 90% or higher, the capacity retention rate after 100 cycles is 84% ​​or higher, with a maximum of 88%; and the capacity retention rate after 500 cycles is 65% or higher, with a maximum of 77%. Attached Figure Description

[0039] Figure 1 The N2 adsorption-desorption isotherm of the pitch-based porous carbon material prepared in Example 1;

[0040] Figure 2 The pore size distribution curve of the pitch-based porous carbon material prepared in Example 1 is shown.

[0041] Figure 3 TEM image of the pitch-based porous carbon material prepared in Example 1;

[0042] Figure 4Pore ​​size distribution curve of the pitch-based porous carbon material prepared in Comparative Example 4;

[0043] Figure 5 The pore size distribution curve of the pitch-based porous carbon material prepared for Comparative Example 5. Detailed Implementation

[0044] The present invention will be described in further detail below with reference to embodiments and comparative examples, but the implementation of the present invention is not limited thereto.

[0045] Example 1

[0046] (1) Add 60% of the high-temperature asphalt powder in the reactor to the reactor, evacuate the vacuum, and set the program to heat up to 120°C. The high-temperature asphalt powder gradually softens completely.

[0047] (2) Nitrogen gas is introduced into a molecular sieve with a pore size of 10 nm to form nano gas, which is then introduced into the reactor. The flow rate of the nano gas is 15 L / min and the introduction time is 30 min. Nano bubbles are formed inside the softened asphalt, and the pressure inside the reactor is controlled to be 8 MPa.

[0048] (3) The reactor is heated to 200°C and kept at that temperature for 4 hours. After the temperature is kept at that temperature, the reactor is cooled to room temperature and then removed to obtain a blocky porous asphalt material.

[0049] (4) Crush the blocky porous asphalt material so that the particle size of the crushed material is controlled between 2 and 10 μm. Then put the crushed material into a carbonization furnace and heat it to 850°C under a nitrogen atmosphere for 2 hours until it is completely carbonized. After cooling to room temperature, take it out to obtain asphalt-based porous carbon material.

[0050] (5) Place the pitch-based porous carbon material in a CVD furnace. Under a nitrogen atmosphere, introduce silane gas into the deposition furnace at a flow rate of 3 L / min. Keep the temperature of the deposition furnace at 500℃ and continue to circulate the gas until the porous carbon material is continuously nucleated and a silicon deposition layer is formed inside and on the surface. Keep the gas circulation time for 10 hours to obtain silicon / carbon material. Control the amount of silicon deposition to be 50% of the total mass of silicon-carbon material.

[0051] (6) Add silicon / carbon material into CVD furnace, introduce acetylene gas at a flow rate of 1L / min under nitrogen atmosphere, and deposit carbon coating at 900℃ for 2h. The carbon coating amount is 4% (based on product). After dispersing, sieving and demagnetizing, the negative electrode material is obtained.

[0052] Figure 1 The N2 adsorption-desorption curve of the pitch-based porous carbon material prepared in this embodiment is shown below. Figure 1 It can be seen that the curve has the characteristics of a type I isotherm curve, indicating that the pitch-based porous carbon material prepared in this embodiment has micropores and relatively narrow mesopores.

[0053] Figure 2 To prepare the pore size distribution curve of the pitch-based porous carbon material in this embodiment, based on... Figure 2 It can be seen that its pore size is concentrated in the range of 3 to 8 nm, or more specifically, in the range of 3 to 6 nm, and the pore size is uniform.

[0054] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below. Figure 2 The pore size distribution curve, combined with the data provided in this table, demonstrates that the material forms micropores and small mesopores.

[0055] Figure 3 This is a TEM image of the pitch-based porous carbon material prepared in this embodiment. Observing this image confirms that the pitch-based porous carbon material has a porous structure with clear pores.

[0056] Example 2

[0057] The preparation process is basically the same as in Example 1, except that:

[0058] In step (2), the pore size of the molecular sieve is replaced with 12 nm, the flow rate of the introduced nano gas is replaced with 12 L / min, the introduction time is replaced with 35 min, and the pressure inside the reaction is controlled to be 7 MPa.

[0059] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0060] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 3 to 8 nm.

[0061] Example 3

[0062] The preparation process is basically the same as in Example 1, except that the pore size of the molecular sieve in step (2) is replaced with 4 nm.

[0063] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0064] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 1–9 nm.

[0065] Example 4

[0066] The preparation process is basically the same as in Example 1, except that the pore size of the molecular sieve in step (2) is replaced with 15 nm.

[0067] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0068] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 2–12 nm.

[0069] Comparative Example 1

[0070] The preparation process is basically the same as in Example 1, except that the pore size of the molecular sieve in step (2) is replaced with 2 nm.

[0071] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this comparative example are listed in Table 1 below.

[0072] Tests showed that the pore size of the pitch-based porous carbon material prepared in this comparative example was concentrated in the range of 0.5–3.5 nm.

[0073] Comparative Example 2

[0074] The preparation process is basically the same as in Example 1, except that the pore size of the molecular sieve in step (2) is replaced with 50 nm.

[0075] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this comparative example are listed in Table 1 below.

[0076] Tests showed that the pore size of the pitch-based porous carbon material prepared in this comparative example was concentrated in the range of 12–25 nm.

[0077] Comparative Example 3

[0078] The preparation process is basically the same as in Example 1, except that in step (2), nitrogen gas (without passing through a molecular sieve) is directly introduced into the reactor.

[0079] The BET, pore volume, pore size, and pore size distribution data of the pitch-based carbon materials prepared in this comparative example are listed in Table 1 below.

[0080] Example 5

[0081] The preparation process is basically the same as in Example 1, except that:

[0082] In step (2), the flow rate of the introduced nano gas is replaced with 10 L / min, the introduction time is replaced with 40 min, and the pressure inside the reaction is controlled to be 8 MPa.

[0083] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0084] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 2.5 to 8 nm.

[0085] Example 6

[0086] The preparation process is basically the same as in Example 1, except that:

[0087] In step (2), the flow rate of the introduced nano gas is replaced with 20 L / min, the introduction time is replaced with 25 min, and the pressure inside the reaction is controlled to be 8 MPa.

[0088] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0089] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment was concentrated in the range of 3–9 nm.

[0090] Comparative Example 4

[0091] The preparation process is basically the same as in Example 1, except that:

[0092] In step (2), the flow rate of the introduced nano gas is replaced with 30 L / min and the introduction time is replaced with 17 min.

[0093] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this comparative example are listed in Table 1 below.

[0094] Figure 4 The figure shows the pore size distribution of the pitch-based porous carbon material prepared in this comparative example. Observing the figure, it can be found that there are multiple peaks and the pore size distribution is not concentrated.

[0095] Comparative Example 5

[0096] The preparation process is basically the same as in Example 1, except that:

[0097] In step (2), the flow rate of the introduced nano gas is replaced with 5 L / min and the introduction time is replaced with 100 min.

[0098] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this comparative example are listed in Table 1 below.

[0099] Figure 5 The figure shows the pore size distribution of the pitch-based porous carbon material prepared in this comparative example. Observing this figure, it can also be found that there are multiple peaks and the pore size distribution is not concentrated.

[0100] Comparative Example 6

[0101] The preparation process is basically the same as in Example 1, except that:

[0102] In step (2), the flow rate of the introduced nano-gas is changed to 20 L / min, the introduction time is changed to 40 min, and the pressure inside the reaction is controlled at 15 MPa.

[0103] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this comparative example are listed in Table 1 below.

[0104] Tests showed that the pore size of the pitch-based porous carbon material prepared in this comparative example was concentrated in the range of 3–9.5 nm.

[0105] Comparative Example 7

[0106] The preparation process is basically the same as in Example 1, except that:

[0107] In step (2), the flow rate of the introduced nano-gas is 20 L / min, the introduction time is changed to 10 min, and the pressure inside the reaction is controlled at 4 MPa.

[0108] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this comparative example are listed in Table 1 below.

[0109] Tests showed that the pore size of the pitch-based porous carbon material prepared in this comparative example was concentrated in the range of 2.5–7 nm.

[0110] Example 7

[0111] (1) Add 70% of the high-temperature asphalt powder in the reactor to the reactor, evacuate the vacuum, and set the program to heat up to 120°C. The high-temperature asphalt powder gradually softens completely.

[0112] (2) Helium gas is introduced into a molecular sieve with a pore size of 10 nm to form nano gas, which is then introduced into the reactor. The flow rate of the nano gas is 15 L / min and the introduction time is 30 min. Nano bubbles are formed inside the softened asphalt, and the pressure inside the reactor is controlled to be 8 MPa.

[0113] Steps (3) to (6) are exactly the same as in Example 1.

[0114] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0115] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 3 to 8 nm.

[0116] Example 8

[0117] The preparation process is basically the same as in Example 7, except that the helium in step (2) is replaced with water vapor.

[0118] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0119] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment was concentrated in the range of 1.5 to 10 nm.

[0120] Example 9

[0121] The preparation process is basically the same as in Example 7, except that helium in step (2) is replaced with carbon dioxide.

[0122] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0123] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 2 to 9.5 nm.

[0124] Example 10

[0125] (1) Add 60% of the volume of the low-temperature asphalt powder into the reactor, evacuate the vacuum, and set the program to heat up to 75°C. The low-temperature asphalt powder gradually softens completely.

[0126] (2) Nitrogen gas is introduced into a molecular sieve with a pore size of 10 nm to form nano gas, which is then introduced into the reactor. The flow rate of the nano gas is 18 L / min and the introduction time is 25 min. Nano bubbles are formed inside the softened asphalt, and the pressure inside the reactor is controlled to be 8 MPa.

[0127] (3) The reactor is set to heat up to 135°C and the heat is maintained for 8 hours. After the heat is maintained, the pressure is released and nitrogen is introduced. After cooling to room temperature, the material is taken out and a blocky porous asphalt material is obtained.

[0128] Steps (4) to (6) are exactly the same as in Example 1.

[0129] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0130] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 2.5 to 8 nm.

[0131] Example 11

[0132] (1) Add medium-temperature asphalt powder, which accounts for 60% of the volume of the reactor, into the reactor. After evacuating the vacuum, set the program to raise the temperature to 90°C. The medium-temperature asphalt powder gradually softens completely.

[0133] (2) Nitrogen gas is introduced into a molecular sieve with a pore size of 10 nm to form nano gas, which is then introduced into the reactor. The flow rate of the nano gas is 18 L / min and the introduction time is 25 min. Nano bubbles are formed inside the softened asphalt, and the pressure inside the reactor is controlled to be 8 MPa.

[0134] (3) The reactor is programmed to heat up to 190°C and held for 5 hours. After the holding time is completed, the pressure is released and nitrogen is introduced. After cooling to room temperature, the material is taken out to obtain a blocky porous asphalt material.

[0135] Steps (4) to (6) are exactly the same as in Example 1.

[0136] The BET, pore volume, pore size, and pore size distribution data of the pitch-based porous carbon material prepared in this embodiment are listed in Table 1 below.

[0137] Tests showed that the pore size of the pitch-based porous carbon material prepared in this embodiment is concentrated in the range of 3 to 8 nm.

[0138] Table 1

[0139]

[0140]

[0141] As can be seen from Table 1, the pore size of the molecular sieve, the gas flow rate, the internal reaction pressure, and the type of gas introduced can all directly affect the pore volume, pore size, and specific surface area of ​​the material.

[0142] A comparison of the data from Example 1 and Comparative Examples 1-2 shows that when the pore size of the molecular sieve is changed, the pore size inside the porous carbon material also changes, which in turn affects the specific surface area and pore volume of the material. When the pore size of the molecular sieve is too small (~2nm) or too large (~50nm), the specific surface area and pore volume of the prepared porous carbon material are not conducive to the subsequent bonding of carbon material with silicon, thus affecting the battery capacity and first-time efficiency. A comparison of the data from Example 1 and Comparative Example 3 reveals that if the gas is not treated with a molecular sieve, the specific surface area and pore volume of the prepared pitch-based carbon material are extremely low, and it has no application value.

[0143] Comparing the data of Example 1 with those of Comparative Examples 4 and 5, as well as the pore size distribution diagrams of the three, it can be seen that both excessively high and low gas flow rates can lead to uneven pore distribution. Although the final specific surface area and pore volume of the material are not significantly different from those of the examples, the uneven distribution of internal pore size makes it difficult to control the amount and rate of silicon deposition in the subsequent silicon deposition process. As a result, the target carbon material cannot be well bonded with silicon, affecting the capacity and first-time efficiency of the final battery anode material.

[0144] A comparison of the data from Example 1 with Comparative Examples 6 and 7 shows that both excessively high and low internal reaction pressures lead to a significant decrease in the specific surface area and pore volume of the prepared porous carbon material. The reasons for this may be: when the pressure is too high, the mechanical strength of the internal pore walls is insufficient, resulting in pore collapse and damage to the internal pore structure; when the internal pressure is too low, more air bubbles overflow to the surface of the softened asphalt, making it difficult to seal them inside the asphalt, thus reducing the internal pore structure.

[0145] Application examples

[0146] The specific process of assembling lithium-ion batteries is as follows:

[0147] (1) Preparation of positive electrode sheet: The positive electrode active material lithium nickel cobalt manganese oxide (NCM811), conductive agent SuperP, carbon nanotubes, and binder polyvinylidene fluoride (PVDF) are mixed with N-methylpyrrolidone (NMP) in a mass ratio of 97:1:0.5:1.5 to prepare a positive electrode slurry (solid content of 70%). The slurry is coated on both sides of the current collector aluminum foil, dried at 100°C, and then cold-pressed at room temperature at 4MPa. The slurry is then trimmed, cut into strips, slit, and the tabs are welded to form the positive electrode sheet.

[0148] (2) Preparation of negative electrode sheet: Under a nitrogen protective atmosphere, the solvent N-methylpyrrolidone (NMP) and binder PVDF are stirred and mixed, then the conductive agent SuperP is added and stirred and mixed, and then the negative electrode active material is added and stirred and mixed thoroughly to prepare a negative electrode slurry (solid content is 50%).

[0149] Regarding the negative electrode active material, it is obtained by thoroughly mixing the pitch-based silicon-carbon negative electrode material prepared in the above examples and comparative examples with graphite, so that the specific capacity of the prepared negative electrode material is 450 mAh / g.

[0150] The above-mentioned negative electrode slurry is coated on both sides of the current collector copper foil, dried at 100°C, and then cold-pressed at 4MPa at room temperature. After that, the edges are cut, the foil is cut into strips, and the tabs are welded to form the negative electrode sheet.

[0151] (3) Assembly of lithium-ion batteries

[0152] Using a porous PE polymer film as the separator, the prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes, and wound to obtain a bare cell. The bare cell is placed in an aluminum-plastic shell package and dried at 100°C under a relative vacuum pressure of -0.95×10⁵ Pa until the moisture content is below 100 ppm. The electrolyte, composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) (EC:EMC:DEC volume ratio = 1:1:1) and LiPF₆ (1.0M), is injected into the dried bare cell. The process includes encapsulation, settling, formation (0.02C constant current charging for 2 hours, 0.1C constant current charging for 2 hours), shaping, and capacity testing (capacity grading) to produce a soft-pack liquid lithium-ion battery.

[0153] During battery assembly, five batteries were prepared for each test group, and a total of five sets of data were tested. The final performance was taken as the average of the five sets of data.

[0154] Battery cycle performance was tested on Newway equipment, specifically as follows:

[0155] At 25℃, the capacitor was first discharged to 0.005V at 0.1C, then discharged to 0.001V at 0.08C, then to 0.001V at 0.05C, and then to 0.001V at 0.02C, and left to stand for 10 minutes. It was then charged to 1.5V at 0.1C and left to stand for 10 minutes. The charge-discharge capacity after the first cycle was recorded, and the initial coulombic efficiency was calculated. This process was repeated 100 times, and the charge-discharge capacity after 100 cycles was recorded. The capacity retention rate after 100 cycles was calculated. The same method was used to test and calculate the capacity retention rate after 500 cycles. The test results are shown in Table 2 below.

[0156] Table 2

[0157]

[0158]

[0159] As shown in Table 2, the lithium-ion batteries assembled using the silicon-carbon anode material prepared according to this invention achieve a specific capacity of 1800 mAh / g or higher, an initial coulombic efficiency of 90% or higher, a capacity retention rate of 84% or higher after 100 cycles, with a maximum of 88%; and a capacity retention rate of 65% or higher after 500 cycles, with a maximum of 77%. This indicates that the silicon-carbon material obtained by combining the pitch-based porous carbon material prepared by the special method in this invention with silicon can accommodate more silicon atoms, improving the capacity of the electrode material. It also helps buffer the volume changes of silicon material during charging and discharging, maintaining the good stability of the silicon-carbon composite material. In contrast, lithium-ion batteries assembled using silicon-carbon anode materials prepared without molecular sieve treatment suffer from excessively thick deposition layers on the surface of the pitch-based material, leading to rolling cracking and preventing the formation of electrode sheets.

[0160] The above-described embodiments are preferred embodiments, but the scope of protection of the present invention is not limited thereto. Those skilled in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and variations, but as long as they do not depart from the spirit of the present invention, they are all within the scope of protection of the present invention.

Claims

1. A method for preparing a pitch-based silicon-carbon anode material for lithium-ion batteries, characterized in that, Includes the following steps: (1) Add the asphalt into the reactor, evacuate the vacuum, and heat it to the softening point of the asphalt; (2) After the gas is introduced into the molecular sieve, nano gas is formed. The nano gas is introduced into the reactor to form nano bubbles inside the softened asphalt. The pressure inside the reactor is controlled to reach the predetermined pressure. The temperature is continued to rise to 50°C above the softening point temperature of the asphalt. After holding the temperature for a period of time, it is cooled to room temperature to obtain porous asphalt material. The gas is selected from one or more of nitrogen, carbon monoxide, carbon dioxide, water vapor, and inert gases; The molecular sieve has a pore size of 4~15 nm; The flow rate of the nano-gas introduced is 8~22L / min, the introduction time is 25~80min, and the internal pressure of the reactor is controlled at 5~12MPa; (3) The porous asphalt material is crushed and then carbonized to obtain asphalt-based porous carbon material; (4) The asphalt-based porous carbon material is subjected to silicon deposition treatment to obtain the asphalt-based silicon-carbon anode material for lithium-ion batteries.

2. The method for preparing the pitch-based silicon-carbon anode material for lithium-ion batteries according to claim 1, characterized in that, In step (1): The asphalt is selected from one or more of low-temperature asphalt, medium-temperature asphalt, and high-temperature asphalt; The amount of asphalt added accounts for 10-90% of the volume of the reactor.

3. The method for preparing pitch-based silicon-carbon anode material for lithium-ion batteries according to claim 1, characterized in that, In step (2): Continue heating to 50-100℃ above the softening point of asphalt, and keep it warm for 1-10 hours.

4. The method for preparing the pitch-based silicon-carbon anode material for lithium-ion batteries according to claim 1, characterized in that, In step (3): The porous asphalt material is pulverized to a particle size of 2~10μm; The carbonization process is carried out under an inert atmosphere at a temperature of 650~950℃.

5. The method for preparing the pitch-based silicon-carbon anode material for lithium-ion batteries according to claim 1, characterized in that, In step (4): The asphalt-based porous carbon material undergoes a carbon coating process after silicon deposition treatment.

6. The method for preparing the pitch-based silicon-carbon anode material for lithium-ion batteries according to any one of claims 1 to 5, characterized in that: The molecular sieve has a pore size of 10~12 nm; The flow rate of the nano-gas introduced is 10~20L / min, the introduction time is 25~40min, and the internal pressure of the reactor is controlled at 7~10MPa.

7. The method for preparing the pitch-based silicon-carbon anode material for lithium-ion batteries according to claim 6, characterized in that: The flow rate of the nano-gas introduced is 12~18L / min, and the internal pressure of the reactor is controlled at 7~8MPa.

8. The method for preparing the pitch-based silicon-carbon anode material for lithium-ion batteries according to claim 7, characterized in that: Continue heating to 60-100℃ above the softening point of asphalt.

9. A pitch-based silicon-carbon anode material for lithium-ion batteries prepared by the method according to any one of claims 1 to 8.

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