Preparation method of Si-C and graphite composite material for negative electrode of lithium ion battery

Abstract

The invention provides a preparation method of a Si-C and graphite composite material for a negative electrode of a lithium ion battery, which comprises the following steps: mixing phenolic resin, citric acid and water to obtain a mixed solution, adding silicon powder and graphite, mixing again, sanding to obtain slurry, and drying to obtain a sample; and performing first-stage heat treatment and second-stage heat treatment on the sample obtained in the previous step in an inert gas atmosphere, and then introducing mixed gas of inert gas and propane to perform third-stage heat treatment, thereby obtaining the Si-C and graphite composite material. The problem that the quality of a coating layer is difficult to control is solved through a process of combining liquid-phase coating and gas-phase coating, and meanwhile, silicon and graphite are mixed more uniformly through specific process steps, so that the cycle performance of the silicon-graphite composite material serving as a lithium ion battery negative electrode material is improved, and the service life of the silicon-graphite composite material is prolonged. And the problem of cycle performance degradation caused by poor conductivity and expansion when the silicon-based material is used as the negative electrode of the lithium ion battery is relieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of silicon-carbon anodes for lithium batteries, and relates to a method for preparing Si@C and graphite composite materials for the negative electrode of lithium-ion batteries. Background Art

[0002] With the increasingly serious energy consumption, the development and utilization of new energy have gradually attracted attention. Among them, lithium-ion batteries, as the current mainstream energy storage method, have received extensive attention. However, the requirements for the energy density and cycle performance of lithium-ion batteries in various industries are getting higher and higher, and traditional battery materials are difficult to meet the usage requirements. Although the development of new high specific capacity cathodes is relatively fast, the currently used graphite anodes are difficult to match the requirements of new cathode materials because they have approached the theoretical limit. In order to meet the requirements of higher performance lithium-ion batteries, it is urgent to develop high specific capacity anode materials.

[0003] Silicon-based materials are one of the most promising anode materials at present because they have a very high theoretical specific capacity and are very rich in the earth's reserves. However, due to the expansion rate of up to 300% of silicon-based materials during the process of lithium insertion and extraction (i.e., charging and discharging), silicon fragmentation occurs during the cycling process, resulting in poor cycle performance. In addition, the poor electrical conductivity of silicon-based materials is also an important reason affecting the performance.

[0004] Although the above problems can be comprehensively solved by carbon coating, there are still problems in the current coating process, such as the difficulty in controlling the thickness and uniformity of the carbon coating layer, and the uneven composite of silicon and graphite, resulting in the current products being difficult to meet the usage requirements.

[0005] Therefore, how to find a more suitable method to solve the above problems caused by the existing coating process and have better controllability has become one of the urgent problems to be solved by many front-line researchers in the industry. Summary of the Invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a method for preparing Si@C and graphite composite materials for the negative electrode of lithium-ion batteries. The preparation method provided by the present invention can increase the specific capacity per gram and cycle performance of the anode material, and the preparation method is simple, environmentally friendly, and easy to realize the popularization and application of large-scale production.

[0007] The present invention provides a method for preparing Si@C and graphite composite materials for the negative electrode of lithium-ion batteries, comprising the following steps:

[0008] 1) Mix phenolic resin, citric acid and water to obtain a mixed solution, then add silicon powder and graphite and mix again. After sanding, the obtained slurry is dried to obtain a sample;

[0009] 2) Under an inert gas atmosphere, the sample obtained in the above step is subjected to the first-stage heat treatment and the second-stage heat treatment, and then a mixed gas of inert gas and propane is introduced for the third-stage heat treatment to obtain the Si@C and graphite composite material.

[0010] Preferably, the mass ratio of citric acid to phenolic resin is (1-5):(10-20);

[0011] In the mixed solution, the total mass concentration of phenolic resin and citric acid is 10% - 20%.

[0012] Preferably, the silicon powder includes nano-silicon spheres and / or nano-silicon wafers;

[0013] The mass ratio of phenolic resin to silicon powder is (10-20):(40-50).

[0014] Preferably, the primary particle size of the nano-silicon spheres is 150 - 200 nm;

[0015] The primary particle diameter of the nano-silicon wafers is 200 - 500 nm;

[0016] The primary particle thickness of the nano-silicon wafers is 30 - 80 nm.

[0017] Preferably, the mass ratio of phenolic resin to graphite is (10-20):(25-40);

[0018] The way of the re-mixing is to mix by ultrasonic first and then by stirring;

[0019] The time of ultrasonic is 15 - 45 min;

[0020] The time of stirring and mixing is 30 - 60 min.

[0021] Preferably, the time of sanding is 1 - 5 h;

[0022] The sanding is specifically carried out under nitrogen or inert atmosphere;

[0023] The drying method includes freeze-drying.

[0024] Preferably, the temperature of the first-stage heat treatment is 230 - 250 °C;

[0025] The time of the first-stage heat treatment is 1 - 1.5 h;

[0026] The heating rate for entering the first-stage heat treatment is 2 - 8 °C / min.

[0027] Preferably, the temperature of the second-stage heat treatment is 850 - 950 °C;

[0028] The time of the second-stage heat treatment is 1 to 1.5 h;

[0029] The heating rate for entering the second-stage heat treatment is 2 to 8 °C / min.

[0030] Preferably, the flow rate ratio of the inert gas to propane is 200:(10 - 40);

[0031] The temperature of the third-stage heat treatment is 850 to 950 °C;

[0032] The time of the third-stage heat treatment is 20 to 90 min.

[0033] The present invention also provides an application of the Si@C and graphite composite material prepared by the preparation method described in any one of the above technical solutions in a lithium-ion battery.

[0034] The present invention provides a preparation method of a Si@C and graphite composite material for the negative electrode of a lithium-ion battery, including the following steps: First, mix phenolic resin, citric acid and water to obtain a mixed solution, then add silicon powder and graphite and mix again to obtain a slurry after sanding, and then dry to obtain a sample; then, under an inert gas atmosphere, perform the first-stage heat treatment and the second-stage heat treatment on the sample obtained in the above step, and then introduce a mixed gas of inert gas and propane for the third-stage heat treatment to obtain the Si@C and graphite composite material. Compared with the prior art, the present invention can solve the problem of difficult control of the quality of the coating layer through a process combining liquid-phase coating and gas-phase coating. By mixing silicon and graphite and then sanding, the silicon and graphite can be mixed more uniformly, which can greatly improve its cycle performance as a negative electrode material for lithium-ion batteries.

[0035] The present invention makes the carbon source, silicon and graphite mixed uniformly by sanding the raw materials, then performs carbon coating, and further combines segmented heat treatment. The first stage is at a low temperature to cure the phenolic resin, then high-temperature carbonization, and then a certain amount of propane is introduced for chemical vapor deposition, and finally a nano-powder composite of Si@C and graphite is obtained. The present invention can solve the problems of poor conductivity and cycle performance decay caused by expansion when the silicon-based material is used as the negative electrode of a lithium-ion battery. Specific Embodiments

[0036] To further understand the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the invention.

[0037] For all raw materials of the present invention, there is no special limitation on their sources, and they can be purchased on the market or prepared by conventional methods well-known to those skilled in the art.

[0038] For all raw materials of the present invention, there is no particular limitation on their purity. The present invention preferably uses analytical pure or conventional purity prepared from silicon-carbon anode materials.

[0039] For all raw materials of the present invention, their grades and abbreviations belong to the conventional grades and abbreviations in the art. Each grade and abbreviation is clear and definite in the field of its relevant use. Those skilled in the art can obtain them from the market or prepare them by conventional methods according to the grade, abbreviation and corresponding use.

[0040] For all processes of the present invention, their abbreviations belong to the conventional abbreviations in the art. Each abbreviation is clear and definite in the field of its relevant use. Those skilled in the art can understand its conventional process steps according to the abbreviation.

[0041] The present invention provides a method for preparing a Si@C and graphite composite material for the negative electrode of a lithium-ion battery, comprising the following steps:

[0042] 1) After mixing phenolic resin, citric acid and water, a mixed solution is obtained. Then, silicon powder and graphite are added and mixed again. The slurry obtained after sanding is then dried to obtain a sample.

[0043] 2) Under an inert gas atmosphere, the sample obtained in the above step is subjected to a first-stage heat treatment and a second-stage heat treatment, and then a mixed gas of inert gas and propane is introduced for a third-stage heat treatment to obtain a Si@C and graphite composite material.

[0044] The present invention first mixes phenolic resin, citric acid and water to obtain a mixed solution, then adds silicon powder and graphite and mixes again. The slurry obtained after sanding is then dried to obtain a sample.

[0045] In the present invention, the mass ratio of citric acid to phenolic resin is preferably (1-5):(10-20), more preferably (2-4):(12-18), and even more preferably (3-5):(14-16).

[0046] In the present invention, in the mixed solution, the total mass concentration of phenolic resin and citric acid is preferably 10% - 20%, more preferably 12% - 18%, and even more preferably 14% - 16%.

[0047] In the present invention, the silicon powder preferably comprises nano-silicon spheres and / or nano-silicon wafers, more preferably nano-silicon spheres or nano-silicon wafers.

[0048] In the present invention, the mass ratio of phenolic resin to silicon powder is preferably (10-20):(40-50), more preferably (12-18):(42-48), and even more preferably (14-16):(44-46).

[0049] In the present invention, the primary particle size of the nano-silica spheres is preferably 150 - 200 nm, more preferably 160 - 190 nm, and even more preferably 170 - 180 nm.

[0050] In the present invention, the primary particle diameter of the nano-silica flakes is preferably 200 - 500 nm, more preferably 250 - 450 nm, and even more preferably 300 - 400 nm.

[0051] In the present invention, the primary particle thickness of the nano-silica flakes is preferably 30 - 80 nm, more preferably 40 - 70 nm, and even more preferably 50 - 60 nm.

[0052] In the present invention, the mass ratio of the phenolic resin to the graphite is preferably (10 - 20) : (25 - 40), more preferably (12 - 18) : (28 - 37), and even more preferably (14 - 16) : (31 - 34).

[0053] In the present invention, the method of the re-mixing is preferably ultrasonic treatment followed by stirring and mixing.

[0054] In the present invention, the time of the ultrasonic treatment is preferably 15 - 45 min, more preferably 20 - 40 min, and even more preferably 25 - 35 min.

[0055] In the present invention, the time of the stirring and mixing is preferably 30 - 60 min, more preferably 35 - 55 min, and even more preferably 40 - 50 min.

[0056] In the present invention, the time of the sanding is preferably 1 - 5 h, more preferably 1.8 - 4.2 h, and even more preferably 2.6 - 3.4 h.

[0057] In the present invention, the sanding is specifically preferably carried out under nitrogen or an inert atmosphere.

[0058] In the present invention, the drying method preferably includes freeze-drying.

[0059] Finally, in the present invention, under an inert gas atmosphere, the sample obtained in the above steps is subjected to a first-stage heat treatment and a second-stage heat treatment, and then a third-stage heat treatment is carried out by introducing a mixed gas of an inert gas and propane, to obtain the Si@C and graphite composite material.

[0060] In the present invention, the temperature of the first-stage heat treatment is preferably 230 - 250 °C, more preferably 234 - 246 °C, and even more preferably 238 - 242 °C.

[0061] In the present invention, the time of the first-stage heat treatment is preferably 1 - 1.5 h, more preferably 1.1 - 1.4 h, and even more preferably 1.2 - 1.3 h.

[0062] In the present invention, the heating rate for entering the first-stage heat treatment is preferably 2 to 8 °C / min, more preferably 3 to 7 °C / min, and even more preferably 4 to 6 °C / min.

[0063] In the present invention, the temperature of the second-stage heat treatment is preferably 850 to 950 °C, more preferably 870 to 930 °C, and even more preferably 890 to 910 °C.

[0064] In the present invention, the time of the second-stage heat treatment is preferably 1 to 1.5 h, more preferably 1.1 to 1.4 h, and even more preferably 1.2 to 1.3 h.

[0065] In the present invention, the heating rate for entering the second-stage heat treatment is preferably 2 to 8 °C / min, more preferably 3 to 7 °C / min, and even more preferably 4 to 6 °C / min.

[0066] In the present invention, the flow rate ratio of the inert gas to propane is preferably 200:(10 - 40), more preferably 200:(15 - 35), and even more preferably 200:(20 - 30).

[0067] In the present invention, the temperature of the third-stage heat treatment is preferably 850 to 950 °C, more preferably 870 to 930 °C, and even more preferably 890 to 910 °C.

[0068] In the present invention, the time of the third-stage heat treatment is preferably 20 to 90 min, more preferably 30 to 80 min, even more preferably 40 to 70 min, and even more preferably 50 to 60 min.

[0069] In order to complete and refine the overall technical solution of the present invention, better improve the controllability of the coating quality, and further improve the electrochemical performance of the Si@C and graphite composite material, the preparation method of the Si@C and graphite composite material for the negative electrode of a lithium-ion battery can specifically include the following steps:

[0070] A preparation method of a Si@C and graphite composite material for the negative electrode of a lithium-ion battery, comprising the following steps:

[0071] Step 1: Take a certain amount of phenolic resin and citric acid, add them to deionized water, and the total mass concentration of the phenolic resin and citric acid is 10 to 20%. After the phenolic resin is dissolved, add silicon powder and graphite, ultrasonically treat for 30 min, then continue to stir for 30 to 60 min, and then grind for 1 to 5 h. After the grinding is completed, freeze-dry the obtained slurry at -80 °C;

[0072] Step 2: Place the sample obtained in Step 1 into an atmospheric tube furnace and heat-treat it under an inert gas atmosphere. Wait until the temperature rises to 230 - 250 °C and hold for 1 - 1.5 h, then raise the chamber temperature to 850 - 950 °C and hold for 1 - 1.5 h; Pass the gas according to the inert gas: propane flow ratio of 200:10 - 40. After 20 - 90 min, lower the chamber temperature to room temperature to obtain the composite nano-powder of Si@C and graphite.

[0073] Specifically, the mass ratio of citric acid, phenolic resin, silicon powder, and graphite in Step 1 is 1 - 5:10 - 20:40 - 50:25 - 40; The silicon powder is nano-silica spheres with a primary particle size of 150 - 200 nm or nano-silicon wafers with a primary particle diameter of 200 - 500 and a thickness of 30 - 80 nm;

[0074] Specifically, the sanding atmosphere in Step 1 is nitrogen or argon;

[0075] Specifically, the heating rate in Step 2 is 2 - 8 °C / min;

[0076] Specifically, the inert gas in Step 2 is high-purity argon.

[0077] Furthermore,

[0078] A preparation method of a Si@C and graphite composite material for the negative electrode of a lithium-ion battery, comprising the following steps:

[0079] Step 1: Take a certain amount of phenolic resin and citric acid, add them to deionized water, and the total mass concentration of phenolic resin and citric acid is 10 - 20%. After the phenolic resin is dissolved, add silicon powder and graphite, ultrasonicate for 30 min, then continue stirring for 30 - 60 min, and then sand for 1 - 5 h. After sanding, freeze-dry the obtained slurry at -80 °C;

[0080] The mass ratio of citric acid, phenolic resin, silicon powder, and graphite is 1 - 5:10 - 20:40 - 50:25 - 40; The silicon powder is nano-silica spheres with a primary particle size of 150 - 200 nm or nano-silicon wafers with a primary particle diameter of 200 - 500 and a thickness of 30 - 80 nm; The sanding atmosphere is nitrogen or argon;

[0081] Step 2: Place the sample obtained in Step 1 into an atmospheric tube furnace and heat-treat it under an inert gas atmosphere. Wait until the temperature rises to 230 - 250 °C and hold for 1 - 1.5 h, then raise the chamber temperature to 850 - 950 °C and hold for 1 - 1.5 h; Pass the gas according to the inert gas: propane flow ratio of 200:10 - 40. After 20 - 90 min, lower the chamber temperature to room temperature to obtain the composite nano-powder of Si@C and graphite.

[0082] The heating rate is 2 - 8 °C / min; the inert gas is high-purity argon.

[0083] The present invention discloses a preparation method of a Si@C and graphite composite material for the negative electrode of a lithium-ion battery. Phenolic resin and citric acid are dissolved in deionized water, and then uniformly compounded with silicon powder and graphite. The composite is freeze-dried at -80 °C. The dried sample is placed in an atmosphere tube furnace and subjected to segmented heat treatment under an inert gas atmosphere. In the first stage, it is at a low temperature to cure the phenolic resin, then carbonized at a high temperature, and then a certain amount of propane is introduced for chemical vapor deposition to obtain a nano-powder composite of Si@C and graphite. This method can solve the problems of poor conductivity and cyclic performance decay caused by expansion when the silicon-based material is used as the negative electrode of a lithium-ion battery by grinding the raw materials to make the carbon source, silicon, and graphite evenly mixed, and then performing carbon coating.

[0084] The present invention provides an application of the Si@C and graphite composite material prepared by the preparation method described in any one of the above technical solutions 1 - 9 in a lithium-ion battery.

[0085] The present invention provides a preparation method and an application of a Si@C and graphite composite material for the negative electrode of a lithium-ion battery. Through a process combining liquid-phase coating and gas-phase coating, the present invention can solve the problem of difficult control of the quality of the coating layer. By grinding the mixture of silicon and graphite, the silicon and graphite can be more evenly mixed, which can greatly improve the cyclic performance of the material as the negative electrode of a lithium-ion battery.

[0086] The present invention grinds the raw materials to make the carbon source, silicon, and graphite evenly mixed, then performs carbon coating, and further combines segmented heat treatment. In the first stage, it is at a low temperature to cure the phenolic resin, then carbonized at a high temperature, and then a certain amount of propane is introduced for chemical vapor deposition, and finally a nano-powder composite of Si@C and graphite is obtained. The present invention can solve the problems of poor conductivity and cyclic performance decay caused by expansion when the silicon-based material is used as the negative electrode of a lithium-ion battery.

[0087] To further illustrate the present invention, the following examples are used to describe in detail a silicon-carbon composite material and its preparation method provided by the present invention. However, it should be understood that these examples are implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given only to further illustrate the features and advantages of the present invention, rather than a limitation on the claims of the present invention. The protection scope of the present invention is not limited to the following examples.

[0088] Example 1

[0089] Step 1: Take a certain amount of phenolic resin and citric acid, add them into deionized water. The total mass concentration of phenolic resin and citric acid is 10%. After the phenolic resin is dissolved, add silicon powder and graphite. The mass ratio of citric acid, phenolic resin, silicon powder and graphite is 5:20:50:25. The silicon powder is nano-silica spheres with a primary particle diameter distribution of 150 - 200 nm. After ultrasonic treatment for 30 min, continue stirring for 30 min, and then grind for 2 h. The grinding atmosphere is nitrogen. After grinding, freeze-dry the obtained slurry at -80 °C;

[0090] Step 2: Place the sample obtained in Step 1 in an atmospheric tube furnace and heat-treat it in a high-purity argon atmosphere. When the temperature rises to 230 °C, hold for 1 h, then raise the temperature of the cavity to 890 °C and hold for 1.5 h. The heating rate is 8 °C / min in both cases. Pass gas according to the flow ratio of high-purity argon: propane of 200:40. After 30 min, lower the temperature of the cavity to room temperature to obtain nano-powders of Si@C composite with graphite.

[0091] Mix the sample obtained in this example with CMC, SP, Si@C / graphite, and SBR in a ratio and order of 0.5:1:8:0.5 and stir evenly. Coat it on a copper foil. First, bake it in an oven at 70 °C for 1 - 2 h, then put it in a vacuum oven and bake overnight at 50 - 80 °C. After cooling to room temperature, cut it into small round pieces with a diameter of 14 mm using a slicing machine. Assemble a battery in the order of positive electrode case, electrode sheet, electrolyte, separator, lithium sheet, gasket, elastic sheet, and negative electrode case in an inert gas protection glove box. Use a Neware battery detection system, set the voltage to 0.005 - 1.5 V, and set the charge and discharge current to 0.1 C for charge and discharge tests.

[0092] The first charge and discharge efficiency of the composite material obtained in this example is 88.4%, the specific capacity is 2030.6 mAh / g, and the capacity retention rate after 100 cycles of charge and discharge at 0.1 C is 81.5%.

[0093] Example 2

[0094] Step 1: Take a certain amount of phenolic resin and citric acid, add them into deionized water. The total mass concentration of phenolic resin and citric acid is 20%. After the phenolic resin is dissolved, add silicon powder and graphite. The silicon powder is nano-silica sheets with a primary particle diameter of 200 - 500 nm and a thickness of 30 - 80 nm. The mass ratio of citric acid, phenolic resin, silicon powder and graphite is 1:15:50:34. After ultrasonic treatment for 30 min, continue stirring for 60 min, and then grind for 5 h. The grinding atmosphere is argon. After grinding, freeze-dry the obtained slurry at -80 °C;

[0095] Step 2: Place the sample obtained in Step 1 into an atmospheric tube furnace and perform heat treatment under a high-purity argon atmosphere. When the temperature rises to 250 °C, keep it warm for 1 h, then raise the chamber temperature to 850 °C, keep it warm for 1.5 h, and the heating rate is 8 °C / min; Pass the gas according to the flow ratio of high-purity argon: propane of 200:40. After 60 min, lower the chamber temperature to room temperature to obtain the nano powder composite of Si@C and graphite.

[0096] Mix the sample obtained in this example with CMC, SP, Si@C / graphite, and SBR in the ratio and order of 0.5:1:8:0.5 and stir evenly. Coat it on the copper foil. First, bake it in an oven at 70 °C for 1 - 2 h, then put it into a vacuum oven and bake it overnight at 50 - 80 °C. After cooling to room temperature, cut it into small round pieces with a diameter of 14 mm with a slicing machine. Assemble the battery in the order of positive electrode case, electrode sheet, electrolyte, separator, lithium sheet, gasket, elastic sheet, and negative electrode case in an inert gas protection glove box. Use a Neware battery detection system, set the voltage to 0.005 - 1.5 V, and set the charge and discharge current to 0.1C to perform charge and discharge tests.

[0097] The first charge-discharge efficiency of the composite material obtained in this example is 89.2%, the specific capacity is 1864.9 mAh / g, and the capacity retention rate is 83.2% after 100 cycles of charge and discharge at 0.1C.

[0098] Example 3

[0099] Step 1: Take a certain amount of phenolic resin and citric acid and add them to deionized water. The total mass concentration of phenolic resin and citric acid is 20%. After the phenolic resin is dissolved, add silicon powder and graphite. The silicon powder is nano-silicon spheres with a primary particle size of 150 - 200 nm or nano-silicon wafers with a primary particle diameter of 200 - 500 and a thickness of 30 - 80 nm. The mass ratio of citric acid, phenolic resin, silicon powder, and graphite is 3:15:40:25. After ultrasonic treatment for 30 min, continue to stir for 30 min, and then grind for 3 h. The grinding atmosphere is high-purity argon. After grinding, freeze-dry the obtained slurry at -80 °C;

[0100] Step 2: Place the sample obtained in Step 1 into an atmospheric tube furnace and perform heat treatment under an inert gas atmosphere. When the temperature rises to 250 °C, keep it warm for 1 h, then raise the chamber temperature to 900 °C, the heating rate is 2 °C / min, and keep it warm for 1 h; Pass the gas according to the flow ratio of high-purity argon: propane of 200:20. After 40 min, lower the chamber temperature to room temperature to obtain the nano powder composite of Si@C and graphite.

[0101] The samples obtained in this example were stirred and mixed evenly in the ratio and order of CMC, SP, Si@C / graphite, and SBR at 0.5:1:8:0.5, coated on copper foil, first dried in an oven at 70°C for 1 - 2 h, then placed in a vacuum oven and dried overnight at 50 - 80°C. After cooling to room temperature, they were cut into small round pieces with a diameter of 14 mm, and assembled into a battery in the order of positive electrode case, electrode sheet, electrolyte, separator, lithium sheet, gasket, shrapnel, and negative electrode case in an inert gas - protected glove box. Using a Neware battery detection system, the voltage was set to 0.005 - 1.5 V, and the charge - discharge current was set to 0.1C for charge - discharge testing.

[0102] The first charge - discharge efficiency of the composite material obtained in this example was 88.9%, the specific capacity was 1930.6 mAh / g, and the capacity retention rate was 82.9% after 100 charge - discharge cycles at 0.1C.

[0103] The above has introduced in detail the preparation method and application of a silicon - carbon composite material provided by the present invention. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above examples is only used to help understand the method of the present invention and its core idea, including the best mode, and also enables any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. The protection scope of this invention patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to the literal description of the claims, or if they include equivalent structural elements that have no substantial difference from the literal description of the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A preparation method of Si@C and graphite composite material for the negative electrode of a lithium-ion battery, characterized in that, it comprises the following steps: 1) After mixing phenolic resin, citric acid and water, a mixed solution is obtained, then silicon powder and graphite are added and mixed again. The slurry obtained after sanding is then dried to obtain a sample; 2) In an inert gas atmosphere, the sample obtained in the above step is subjected to a first-stage heat treatment and a second-stage heat treatment, and then a mixed gas of inert gas and propane is introduced for a third-stage heat treatment to obtain a Si@C and graphite composite material.

2. The preparation method according to claim 1, characterized in that, the mass ratio of the citric acid to the phenolic resin is (1-5):(10-20); in the mixed solution, the total mass concentration of the phenolic resin and the citric acid is 10% - 20%.

3. The preparation method according to claim 1, characterized in that, the silicon powder includes nano-silicon spheres and / or nano-silicon flakes; the mass ratio of the phenolic resin to the silicon powder is (10-20):(40-50).

4. The preparation method according to claim 3, characterized in that, the primary particle size of the nano-silicon spheres is 150 - 200 nm; the primary particle diameter of the nano-silicon flakes is 200 - 500 nm; the primary particle thickness of the nano-silicon flakes is 30 - 80 nm.

5. The preparation method according to claim 1, characterized in that, the mass ratio of the phenolic resin to the graphite is (10-20):(25-40); the way of the re-mixing is to first perform ultrasonic treatment and then stir and mix; the time of the ultrasonic treatment is 15 - 45 min; the time of the stir and mix is 30 - 60 min.

6. The preparation method according to claim 1, characterized in that, the time of the sanding is 1 - 5 h; the sanding is specifically carried out under nitrogen or an inert atmosphere; the drying method includes freeze-drying.

7. The preparation method according to claim 1, characterized in that, the temperature of the first-stage heat treatment is 230 - 250 °C; the time of the first-stage heat treatment is 1 - 1.5 h; the heating rate for entering the first-stage heat treatment is 2 - 8 °C / min.

8. The preparation method according to claim 1, characterized in that, the temperature of the second-stage heat treatment is 850 - 950 °C; the time of the second-stage heat treatment is 1 - 1.5 h; the heating rate for entering the second-stage heat treatment is 2 - 8 °C / min.

9. The preparation method according to claim 1, characterized in that, the flow rate ratio of the inert gas to the propane is 200:(10-40); the temperature of the third-stage heat treatment is 850 - 950 °C; the time of the third-stage heat treatment is 20 - 90 min.

10. Application of the Si@C and graphite composite material prepared by the preparation method according to any one of claims 1 - 9 in a lithium-ion battery.