Silicon-carbon negative electrode material based on spherical starch porous carbon and preparation method of silicon-carbon negative electrode material

Abstract

The invention belongs to the field of lithium battery electrode materials, and particularly relates to a silicon-carbon negative electrode material based on spherical starch porous carbon and a preparation method of the silicon-carbon negative electrode material. The preparation method comprises the following steps: carrying out esterification reaction on plant starch and an esterifying agent, carbonizing, crushing, activating with potassium hydroxide, pickling, and carrying out a vapor deposition silane technology to further prepare the silicon-carbon negative electrode material. The starch subjected to esterification reaction is used as a precursor, porous carbon is prepared through carbonization, activation and the like, the silicon-carbon negative electrode material is prepared through the vapor deposition silane technology, and the silicon-carbon negative electrode material not only has a spherical porous carbon skeleton, is low in volume effect and good in conductivity, but also is high in first charge and discharge capacity and first efficiency.

Description

Technical Field

[0001] The present invention belongs to the field of lithium battery electrode materials, and particularly relates to a silicon-carbon anode material based on quasi-spherical starch porous carbon and a preparation method thereof. Background Art

[0002] With the pursuit of high-capacity batteries in the energy industry, the insufficient energy density of lithium batteries restricts the further development of the overall energy storage battery. Silicon materials are considered to be one of the most promising anode materials for lithium-ion batteries due to their extremely high theoretical capacity and low lithium potential. However, during the process of lithium deintercalation and intercalation, silicon materials will have a serious volume expansion problem, resulting in a significant reduction in the capacity of the anode, so the cycle life of silicon materials as anode materials is very short. In addition, as a semiconductor, silicon materials have a low electrical conductivity, which makes the output power of the battery low, thus reducing the use efficiency of silicon materials in lithium-ion batteries.

[0003] In order to solve the influence caused by volume expansion of silicon materials in lithium-ion batteries, an effective method is to deposit gaseous silicon on the surface of porous carbon to form a coated silicon-carbon material. Among them, porous carbon is used as a carrier, and the silicon source gas is deposited in its pores to form a silicon-carbon anode material. The pores of porous carbon are mainly micropores and mesopores, and their pore channels are highly ordered, which are widely used in fields such as adsorption purification, energy storage, and catalysis. Porous carbon is the core of preparing silicon-carbon anode materials by CVD, and the morphology, composition, and pore structure of porous carbon have a significant impact on the performance of porous silicon-carbon anode materials.

[0004] Biomass, as a low-cost, environmentally friendly, and sustainable resource, has attracted extensive attention in recent years. Due to the influence of the morphological and structural diversity of biomass, a large number of biomass precursors have been used to manufacture carbon materials, and these materials have been applied in many fields, such as alkali metal ion batteries, supercapacitors, and catalysis. Starch is one of the most abundant renewable resources on the earth, widely existing in various plants. It is a typical polysaccharide with a simple structure and a high carbon content, which makes it an ideal carbon precursor for lithium / sodium ion battery anode materials. In addition, the natural spherical structure of starch itself makes it a very competitive candidate for preparing spherical carbon materials. However, the starch molecular chains connected by glycosidic bonds show poor thermal stability. During the pyrolysis process, the thermal cleavage of glycosidic bonds will release a large amount of volatile products, especially levoglucosan. At the same time, the rapid escape of volatiles will destroy the spherical morphology, resulting in structural melting and bubbles, and a low carbon yield. Therefore, it is of great significance to develop a silicon-carbon anode material that can use starch as a carbon source and effectively overcome the above problems. Summary of the Invention

[0005] In view of the above defects or improvement requirements of the prior art, the present invention provides a silicon-carbon anode material based on spherical starch porous carbon and a preparation method thereof. The silicon-carbon anode material not only has a spherical porous carbon skeleton, low volume effect, good electrical conductivity, but also has high first charge-discharge capacity and first efficiency, and excellent coin cell performance.

[0006] To achieve the above object, the present invention adopts the following technical solutions: The first object of the present invention is to provide a preparation method of a silicon-carbon anode material based on spherical starch porous carbon, comprising the following steps: S1. Add plant starch and an esterifying agent to a high-speed mixer in sequence. After stirring and mixing evenly, raise the temperature to 70°C - 100°C for an esterification reaction, and continue the reaction for 2h - 6h. Plant starch is a polysaccharide formed by connecting glucose units through α-1,4 and α-1,6 glycosidic bonds, and each glucose unit has 3 free hydroxyl groups (at C2, C3, and C6 positions). In this step, through the esterification reaction, these hydroxyl groups undergo nucleophilic substitution reactions or transesterification reactions with the esterifying agent. As the ester groups are gradually formed, the activation energy of the reaction will gradually increase. In this technical solution, an acid anhydride is selected as the esterifying agent. The alcoholysis activity of the acid anhydride is higher than that of the corresponding carboxylic acid, and its alcoholysis temperature is usually lower, which is more suitable for the process requirements of the present invention. Taking succinic anhydride as an example, the reaction formula is as follows: (CH 2 CO) 2 O + Starch-OH → Starch-O-COCH 2 CH 2 COO - + H + , Starch-O-COCH 2 CH 2 COO - + Starch-OH → Starch-O-COCH 2 CH 2 OC-O-Starch + OH - 。

[0007] S2. Place the starch after the S1 esterification reaction in an atmosphere furnace, and raise the temperature to 800°C - 950°C in a nitrogen atmosphere for carbonization. The carbonization of esterified starch is essentially a high-temperature pyrolysis reaction, which converts organic matter into a carbon skeleton through high temperature in an inert atmosphere while retaining or enhancing its specific functions, such as pore structure and electrical conductivity; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then, evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve, let it stand for 24h, and then dry it; S4. Place the carbonized material dried in S3 into a rotary kiln and heat it to 800°C - 950°C for activation. A series of reactions occur during the carbonization process as follows: 2KOH → K 2 O + H 2 O Equation (1-1), H 2 O + C → H 2 + CO Equation (1-2), H 2 O + CO → H 2 + CO 2 Equation (1-3), K 2 O + CO 2 → K 2 CO 3 Equation (1-4), When the activation temperature is higher than 700°C, the K 2 CO 3 formed in Equation 1-4 decomposes into CO 2 and K 2 O, and is further reduced by carbon to produce CO and K. The specific reaction process is as follows: K 2 O + C → 2K + CO Equation (1-5), K 2 CO 3 + 2C → 2K + 3CO Equation (1-6), CO 2 + C → 2CO Equation (1-7).

[0008] K 2 CO 3 Potassium carbonate begins to decompose to produce CO 2 and K 2 O, and decomposes completely at 800°C. At higher temperatures, the physical activation effect of CO 2 and water vapor is significantly enhanced, resulting in a significant improvement in the overall activation effect. In addition, the potassium vapor generated during the reaction of K 2 O with C can carry out intercalation activation, shuttle between the carbon microcrystalline layers, and achieve the effect of pore formation and pore expansion.

[0009] S5. Wash the carbon material activated in S4 with acid 3 - 5 times, then wash it with deionized water until neutral, and then place it in a drying oven at 100 ± 5°C and dry it to constant weight. The obtained carbon material is spherical porous carbon; S6, place the carbon material obtained in S5 in a fluidized bed, introduce nitrogen and heat it to 450℃-550℃, introduce silane gas to deposit silicon, after deposition for 3h-6h, stop the silane gas, introduce acetylene gas, keep for 1h-2h, and cool to obtain silicon-carbon negative electrode material. The porous carbon obtained after esterification and carbonization activation has a large specific surface area and developed porosity, which can be used to convert silane gas SiH 4 Adsorbed in its pores, SiH 4 When heated, Si and H are generated. 2 , thereby forming a silicon-carbon negative electrode material.

[0010] The present invention can effectively prevent the starch from foaming and expanding during the pyrolysis process by first subjecting the starch to an esterification reaction and then to carbonization. Not only is the carbon yield high, but the original spherical morphology can also be effectively maintained. Potassium hydroxide is then used for activation treatment to obtain spherical porous carbon with a high specific surface area. Finally, silicon is adsorbed on the surface and pores of the carbon material through vapor deposition silane technology to further prepare a silicon-carbon negative electrode material. The obtained silicon-carbon negative electrode material has a higher compaction density, good conductivity, and excellent withholding performance.

[0011] Furthermore, in the above technical solution S1, the plant starch is one or more of potato starch, corn starch, and sweet potato starch; the esterifying agent is one or more of succinic anhydride, maleic anhydride, and phthalic anhydride; the amount of the esterifying agent added is 10%-20% of the total mass of the plant starch; and the stirring speed is 100rpm-200rpm. Since the activation energy of the reaction will gradually increase with the gradual generation of ester groups when polybasic organic acids and hydroxyl groups undergo esterification reactions, and the esterification temperature of polybasic carboxylic acids is usually high, the present invention selects anhydrides as esterifying agents. The alcoholysis activity of anhydrides is higher than that of the corresponding carboxylic acids, and their alcoholysis temperature is usually lower, which is more suitable for the process requirements of the present invention. The anhydrides are crushed in advance before use.

[0012] Furthermore, in the above technical solution S1, the plant starch is dried to a moisture content between 4% and 5% before the esterification reaction.

[0013] Furthermore, in the above technical solution S2, the heating rate is 2°C / min-5°C / min, and when the temperature rises to 500°C, it is kept for 2 hours before continuing to heat up for carbonization. In this technical solution, when the temperature rises to 500°C and is kept for 2 hours, the degree of polymerization of the starch carbon material can be increased, the conductivity of the starch carbon can be increased, and the initial effect of the subsequent negative electrode material can be improved.

[0014] Furthermore, in the above technical solution S3, the mass ratio of potassium hydroxide to carbonized material is 1:1-5, and the drying temperature is 80°C-100°C.

[0015] Further, in the above technical solution S5, the acid is 5 mol / L hydrochloric acid. In this technical solution, high-concentration acid is used for rinsing to remove excess potassium hydroxide and potassium carbonate generated by activation.

[0016] Further, in the above technical solution S6, the heating rate is 5 °C / min, the rate of introducing silane gas is 3 L / min, and the rate of introducing acetylene gas is 3 L / min.

[0017] Further, in the above technical solution S6, the silane gas is silane or disilane.

[0018] Further, in the above technical solution S6, the pipeline needs to be purged with high-purity nitrogen before and after introducing silane gas.

[0019] The second object of the present invention is to provide a silicon-carbon anode material prepared by the above preparation method. The silicon-carbon anode material is spherical-like, and the initial Coulombic efficiency is between 88% and 92%.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: By carrying out an esterification reaction on starch, the present invention enables starch not to foam and expand during the carbonization process, which can provide higher conductivity for the silicon-carbon material; at the same time, carbonizing the esterified starch can maintain the spherical-like morphology of starch, providing a larger specific surface area for porous carbon. The obtained silicon-carbon material is also spherical-like, has a higher tap density, initial efficiency, etc., and has excellent coin cell performance.

[0021] The carbon source used in the preparation method of the present invention is rich, wide-ranging, and inexpensive. After pretreatment, it can maintain its original morphology. After activation, it has advantages such as a larger specific surface area, good physical and chemical stability, excellent conductivity, and adjustable pore structure. The obtained silicon-carbon anode material has excellent performance and good market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 SEM image of the porous carbon prepared in Example 1 of the present invention.

[0024] Figure 2 SEM image of the porous carbon prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] Unless otherwise specified, the experimental methods in the following examples are all conventional methods. Unless otherwise specified, the raw materials involved in the following examples are all ordinary commercially available products and can be obtained through market purchase.

[0026] Any of the above technical features of the present invention can be combined with the technical features specifically described below (such as in the examples) to form new or preferred technical solutions.

[0027] The raw materials involved in the embodiments of the present invention are either existing commercially available products or can be prepared according to existing methods, and the testing methods are industry methods.

[0028] The plant starch is dried to a moisture content between 4% and 5% before the esterification reaction.

[0029] Example 1 A preparation method of a silicon-carbon anode material based on spherical starch porous carbon includes the following steps: S1. Add 10 kg of potato starch and 2 kg of succinic anhydride to a high-speed mixer in sequence. After stirring and mixing evenly at a speed of 100 rpm, raise the temperature to 70 °C for an esterification reaction, and continue the reaction for 2 h; S2. Place the starch after the S1 esterification reaction in an atmosphere furnace. Under a nitrogen atmosphere, raise the temperature to 500 °C at a rate of 2 °C / min, hold for 2 h, and then continue to raise the temperature to 800 °C for carbonization; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve (the mass ratio of KOH to the carbonized material is 1:3). After standing for 24 h, dry it in an 80 °C drying oven; S4. Place the carbonized material after drying in S3 in a rotary furnace and raise the temperature to 800 °C for activation; S5. Wash the carbon material after activation in S4 5 times with 5 mol / L hydrochloric acid (to remove excess KOH and K 2 CO 3 generated during activation), then wash it with deionized water until neutral, and then put it in a 100 ± 5 °C drying oven to dry to constant weight to obtain porous carbon; S6. Place 1 kg of the carbon material obtained in S5 in a fluidized bed, introduce nitrogen, raise the temperature to 450 °C at a rate of 5 °C / min, introduce 3 L / min of silane and disilane gases for silicon deposition. After depositing for 3 h, stop the silane gas, introduce nitrogen to evacuate the silane gas, then introduce 3 L / min of acetylene gas, hold for 1 h, and then cool to obtain the silicon-carbon anode material.

[0030] Example 2 A preparation method of a silicon-carbon anode material based on spherical starch porous carbon includes the following steps: S1. Add 10 kg of corn starch and 2 kg of succinic anhydride into a high-speed mixer in sequence. After stirring and mixing evenly at a speed of 150 rpm, raise the temperature to 80 °C for esterification reaction and continue the reaction for 3 h; S2. Place the starch after the S1 esterification reaction in an atmosphere furnace. Under a nitrogen atmosphere, raise the temperature to 500 °C at a rate of 4 °C / min, hold for 2 h, and then continue to raise the temperature to 800 °C for carbonization; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve (the mass ratio of KOH to the carbonized material is 1:4). After standing for 24 h, dry it in a drying oven at 90 °C; S4. Place the carbonized material after drying in S3 in a rotary furnace and raise the temperature to 850 °C for activation; S5. Wash the carbon material after activation in S4 4 times with 5 mol / L hydrochloric acid (to remove excess KOH and K 2 CO 3 generated during activation), then wash it with deionized water until neutral, and then put it in a drying oven at 100 ± 5 °C to dry to constant weight to obtain porous carbon; S6. Place 1 kg of the carbon material obtained in S5 in a fluidized bed, introduce nitrogen, raise the temperature to 500 °C at a rate of 5 °C / min, introduce silane and disilane gases at a rate of 3 L / min for silicon deposition. After depositing for 4 h, stop the silane gas, introduce nitrogen to evacuate the silane gas, then introduce acetylene gas at a rate of 3 L / min, keep it for 1.5 h, and then cool to obtain the silicon-carbon anode material.

[0031] Example 3 A preparation method of a silicon-carbon anode material based on quasi-spherical starch porous carbon, comprising the following steps: S1. Add 10 kg of potato starch, corn starch, sweet potato starch and 2 kg of succinic anhydride, maleic anhydride, phthalic anhydride into a high-speed mixer in sequence. After stirring and mixing evenly at a speed of 200 rpm, raise the temperature to 100 °C for esterification reaction and continue the reaction for 2 h; S2. Place the starch after the S1 esterification reaction in an atmosphere furnace. Under a nitrogen atmosphere, raise the temperature to 500 °C at a rate of 5 °C / min, hold for 2 h, and then continue to raise the temperature to 950 °C for carbonization; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve (the mass ratio of KOH to the carbonized material is 1:5). After standing for 24 h, dry it in a drying oven at 80 °C; S4. Place the carbonized material after drying in S3 in a rotary furnace and raise the temperature to 950 °C for activation; S5. Wash the carbon material after activation in S4 3 times with 5 mol / L hydrochloric acid (to remove excess KOH and K2 CO 3 ), and then washed with deionized water until neutral, and then placed in an oven at 100 ± 5 °C to dry to a constant weight to obtain porous carbon; S6. Place 1 kg of the carbon material obtained in S5 in a fluidized bed, introduce nitrogen, heat it to 550 °C at a rate of 5 °C / min, introduce 3 L / min of silane and disilane gases for silicon deposition. After 6 h of deposition, stop the silane gas, introduce nitrogen to evacuate the silane gas, then introduce 3 L / min of acetylene gas, keep it for 2 h, and then cool to obtain the silicon-carbon negative electrode material.

[0032] Comparative Example 1 A preparation method of a silicon-carbon negative electrode material, comprising the following steps: S1. Add 10 kg of potato starch and 0.5 kg of succinic anhydride to a high-speed mixer in sequence. After stirring and mixing evenly at a speed of 100 rpm, heat it to 70 °C for an esterification reaction and continue the reaction for 2 h; S2. Place the starch after the esterification reaction in S1 in an atmosphere furnace. Under a nitrogen atmosphere, heat it to 500 °C at a speed of 2 °C / min, keep it for 2 h, and then continue to heat it to 800 °C for carbonization; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve (the mass ratio of KOH to the carbonized material is 1:3). After standing for 24 h, dry it in an oven at 80 °C; S4. Place the carbonized material dried in S3 in a rotary furnace and heat it to 800 °C for activation; S5. Wash the carbon material activated in S4 5 times with 5 mol / L hydrochloric acid (to remove the excess KOH and K generated by activation 2 CO 3 ), and then washed with deionized water until neutral, and then placed in an oven at 100 ± 5 °C to dry to a constant weight to obtain porous carbon; S6. Place 1 kg of the carbon material obtained in S5 in a fluidized bed, introduce nitrogen, heat it to 450 °C at a rate of 5 °C / min, introduce 3 L / min of silane and disilane gases for silicon deposition. After 3 h of deposition, stop the silane gas, introduce nitrogen to evacuate the silane gas, then introduce 3 L / min of acetylene gas, keep it for 1 h, and then cool to obtain the silicon-carbon negative electrode material.

[0033] Comparative Example 2 A preparation method of a silicon-carbon negative electrode material, comprising the following steps: S1. Add 10 kg of potato starch and 2.5 kg of succinic anhydride to a high-speed mixer in sequence. After stirring and mixing evenly at a speed of 100 rpm, heat it to 70 °C for an esterification reaction and continue the reaction for 2 h; S2. Place the starch after the S1 esterification reaction in an atmosphere furnace, heat it up to 500 °C at a rate of 2 °C / min under a nitrogen atmosphere, hold for 2 h, and then continue to heat up to 800 °C for carbonization; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then, evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve (the mass ratio of KOH to the carbonized material is 1:3). After standing for 24 h, dry it in an 80 °C drying oven; S4. Place the carbonized material after drying in S3 in a rotary furnace and heat it up to 800 °C for activation; S5. Wash the carbon material after activation in S4 5 times with 5 mol / L hydrochloric acid (to remove excess KOH and K 2 CO 3 generated during activation), then wash it with deionized water until neutral, and then put it in a 100 ± 5 °C drying oven and dry it to a constant weight to obtain porous carbon; S6. Place 1 kg of the carbon material obtained in S5 in a fluidized bed, heat it up to 450 °C at a rate of 5 °C / min by introducing nitrogen, introduce silane and disilane gases at a rate of 3 L / min for silicon deposition. After depositing for 3 h, stop the silane gas, introduce nitrogen to evacuate the silane gas, then introduce acetylene gas at a rate of 3 L / min, hold for 1 h, and then cool to obtain the silicon-carbon negative electrode material.

[0034] Comparative Example 3 A preparation method of a silicon-carbon negative electrode material based on quasi-spherical starch porous carbon, comprising the following steps: S1. Add 10 kg of potato starch and 2 kg of succinic anhydride to a high-speed mixer in sequence. After stirring and mixing evenly at a speed of 100 rpm, heat it up to 70 °C for esterification reaction and continue the reaction for 2 h; S2. Place the starch after the S1 esterification reaction in an atmosphere furnace, heat it up to 800 °C at a rate of 2 °C / min under a nitrogen atmosphere for carbonization; S3. Crush the carbonized material after S2 and pass it through a 325-mesh sieve. Then, evenly spray the potassium hydroxide solution dissolved in water onto the carbonized material under the sieve (the mass ratio of KOH to the carbonized material is 1:3). After standing for 24 h, dry it in an 80 °C drying oven; S4. Place the carbonized material after drying in S3 in a rotary furnace and heat it up to 800 °C for activation; S5. Wash the carbon material after activation in S4 5 times with 5 mol / L hydrochloric acid (to remove excess KOH and K 2 CO 3 generated during activation), then wash it with deionized water until neutral, and then put it in a 100 ± 5 °C drying oven and dry it to a constant weight to obtain porous carbon; S6. Place the carbon material obtained in S5 (1 kg) in a fluidized bed, introduce nitrogen, heat it to 450 °C at a rate of 5 °C / min, introduce silane and disilane gases at a rate of 3 L / min for silicon deposition. After 3 hours of deposition, stop the silane gas, introduce nitrogen to evacuate the silane gas, then introduce acetylene gas at a rate of 3 L / min. After maintaining for 1 hour, cool it to obtain the silicon-carbon negative electrode material.

[0035] Test Example 1. Detect the specific surface area and porosity of the porous carbon obtained during the preparation processes of Examples 1 - 3 and Comparative Examples 1 - 3, and observe the micro-morphology. Use a scanning electron microscope and a specific surface area and porosity analyzer for testing respectively. The results are shown in Table 1. The micro-morphologies of the porous carbon obtained in Example 1 and Comparative Example 1 are as Figure 1 and Figure 2 shown.

[0036] Table 1 Test Results of Specific Surface Area and Porosity of Porous Carbon

[0037] As can be seen from Table 1, the specific surface areas of Examples 1 - 3 are all > 1750 m 2 / g, and the microporosity is > 85%. This indicates that the porous carbon obtained by the preparation method of the present invention already meets the conditions for preparing silicon-carbon by gas-phase deposition of silane. However, the specific surface area and microporosity of Comparative Example 1 are both relatively low. This is because the amount of esterifying agent is too small and the starch esterification is incomplete, resulting in foaming and expansion during the starch carbonization process, generating mesopores and macropores, which will lead to a poor ability for gas-phase deposition of silane; the specific surface area and microporosity in Comparative Example 2 are similar to those of Examples 1 - 3, which is because the amount of esterifying agent is already excessive. Therefore, selecting the amount of esterifying agent of the present invention is sufficient to fully esterify with starch and can avoid the expansion during the starch carbonization process.

[0038] In addition, as can be seen from Figure 1 , the morphology of the porous carbon in Example 1 basically maintains a relatively uniform spherical shape, indicating that the spherical shape of the starch has not been greatly affected during the carbonization and activation processes; while as can be seen from Figure 2 , the spherical shape of the starch carbon has been completely broken, which is likely due to the low content of the esterifying agent resulting in incomplete esterification of the starch, so that the starch foams and expands during the carbonization process, destroying the spherical shape.

[0039] 2. To detect the performance of the silicon-carbon negative electrode materials prepared in Examples 1 - 3 and Comparative Examples 1 - 3 for lithium-ion batteries, use the half-cell test method and 18650 steel shell full-cell.

[0040] (1) The half-cell test method is to prepare a slurry with the silicon-carbon materials obtained from Examples 1 - 3 and Comparative Examples 1 - 3 as the negative electrode active material. The slurry ratio is as follows: active material : CNTs (including dispersant) : CMC : SBR = 89% : 4.5% : 1.5% : 5%. Then, the slurry is coated on a copper foil and vacuum dried for 12 h to make a negative electrode sheet. The electrolyte is commercially purchased, the separator is a PE film, and the lithium sheet is the counter electrode. A half-cell is assembled in a glove box. A constant current charge-discharge experiment is carried out on a LAND battery test system, and the charge-discharge voltage is limited to 0.005 V - 2 V. A computer-controlled charge-discharge cabinet is used for data acquisition and control. The comparison results of the coin cell performance of Examples 1 - 3 and Comparative Examples 1 - 3 are shown in Table 2.

[0041] Table 2 Coin Cell Performance Test Results

[0042] From the results in Table 2, it can be seen that from the first charge and the first discharge, the coin cell tests of Examples 1 - 3 are all relatively excellent. This is mainly because the starch carbon generated after the starch is completely esterified by the esterifying agent has excellent conductivity. At the same time, after being activated into porous carbon, it has a large specific surface area and a high microporosity, so that the capacity of silicon can be fully released after the silane deposition; while the first charge and the first discharge of Comparative Example 1 are poor because the content of the esterifying agent is reduced, and incomplete esterification causes the starch to foam and dissolve during the carbonization process, generating more mesopores and macropores, and at the same time, the conductivity is also poor, unable to fully release the content of silicon; in Comparative Example 2, there is no obvious difference between its first charge and the first discharge and that of Example 1 because the esterifying agent is already in excess, which is sufficient to undergo a sufficient esterification reaction with the starch to avoid the expansion during the starch carbonization process. Therefore, it is further determined that the dosage of the esterifying agent of 10% - 20% of the starch mass can fully esterify the starch, and a higher ratio is not required; in Comparative Example 3, the temperature is directly raised to 800 °C during the activation process, and due to the poor degree of polymerization, the first efficiency is affected.

[0043] In summary, the present invention uses starch as a carbon source. After pretreatment for an esterification reaction, it can effectively overcome the foaming during its carbonization process, which affects the morphology and conductivity. The obtained silicon-carbon negative electrode material not only has a spherical porous carbon skeleton, low volume effect, good electrical conductivity, but also has a high first charge-discharge capacity and a high first efficiency.

[0044] Finally, it should be emphasized that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a silicon-carbon negative electrode material based on spherical starch porous carbon, characterized in that: The steps include: S1. Add plant starch and esterifying agent into a high-speed mixer in sequence, stir and mix evenly, then heat to 70°C-100°C for esterification reaction, and continue the reaction for 2h-6h; S2, placing the starch after esterification reaction in S1 in an atmosphere furnace, heating it to 800°C-950°C under a nitrogen atmosphere for carbonization; S3, crush the carbonized material after carbonization in S2 and pass it through a 325-mesh sieve, then evenly spray a potassium hydroxide solution dissolved in water onto the carbonized material under the sieve, let it stand for 24 hours, and then dry it; S4, placing the carbonized material dried in S3 in a rotary kiln and heating it to 800°C-950°C for activation; S5. Wash the carbon material activated in S4 with acid for 3-5 times, then wash with deionized water until neutral, and then put it into a drying oven at 100±5℃ to constant weight; S6. Place the carbon material obtained in S5 in a fluidized bed, introduce nitrogen gas to raise the temperature to 450°C-550°C, introduce silane gas to deposit silicon. After deposition for 3h-6h, stop the silane gas, introduce acetylene gas, maintain for 1h-2h, and cool to obtain the silicon-carbon negative electrode material.

2. The preparation method according to claim 1, characterized in that: In S1, the plant starch is one or more of potato starch, corn starch, and sweet potato starch; the esterifying agent is one or more of succinic anhydride, maleic anhydride, and phthalic anhydride; the added amount of the esterifying agent is 10%-20% of the total mass of the plant starch; and the stirring speed is 100rpm-200rpm.

3. The preparation method according to claim 1, characterized in that: In S1, the plant starch is dried to a moisture content between 4% and 5% before the esterification reaction.

4. The preparation method according to claim 1, characterized in that: In S2, the heating rate is 2°C / min-5°C / min. When the temperature rises to 500°C, it is kept for 2 hours and then the temperature is continued to be carbonized.

5. The preparation method according to claim 1, characterized in that: In S3, the mass ratio of potassium hydroxide to carbonized material is 1:1-5, and the drying temperature is 80°C-100°C.

6. The preparation method according to claim 1, characterized in that: In S5, the acid is 5 mol / L hydrochloric acid.

7. The preparation method according to claim 1, characterized in that: In S6, the heating rate is 5°C / min, the rate of introducing silane gas is 3 L / min, and the rate of introducing acetylene gas is 3 L / min.

8. The preparation method according to claim 1, characterized in that: In S6, the silane gas is monosilane or disilane.

9. The preparation method according to claim 1, characterized in that: In S6, the pipeline needs to be purged with high-purity nitrogen before and after the silane gas is passed.

10. A silicon-carbon negative electrode material prepared by the preparation method according to any one of claims 1 to 9, characterized in that: The silicon-carbon negative electrode material is spherical in shape, and the first coulombic efficiency is between 88% and 92%.

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