Novel silicon-carbon negative electrode material and preparation method thereof

Abstract

The invention belongs to the field of lithium battery electrode materials, and particularly relates to a novel silicon-carbon negative electrode material and a preparation method thereof. The preparation method comprises the following steps: mixing, kneading and aging plant starch, a nitrogen-boron doping agent and nano silicon to obtain a silicon-carbon negative electrode precursor, and then performing carbonization and gas-phase coating to obtain the novel silicon-carbon negative electrode material with the cage structure. Nanometer silicon is uniformly dispersed in a high-strength amorphous carbon shell formed by carbonization of plant starch gel, the conductivity of the nanometer silicon is effectively improved, expansion of the nanometer silicon in the charging and discharging process is inhibited, and through gas phase coating, the specific surface area of the material is remarkably reduced, and the first efficiency and cycle performance are improved.

Description

Technical Field

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

[0002] With the development of new energy vehicles and the low-altitude economy, lithium batteries are continuously developing towards the trend of high energy density. The main path is to use positive and negative active materials with higher specific capacities. However, due to the decline in the safety of the high-specific-capacity cathode, the trend of high energy density of the cathode has encountered certain setbacks. Currently, improving the energy density of lithium batteries through the anode has regained widespread attention.

[0003] 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, silicon materials also have some problems. One is that they will experience serious volume expansion during the process of lithium deintercalation and intercalation, which will lead to material cracking and pulverization. The other is that silicon materials, as semiconductors, have low conductivity, resulting in poor performance of the battery in terms of fast charging and discharging.

[0004] To address the volume expansion problem of silicon materials, an effective method is to nanoscale the silicon materials to shorten the lithium-ion diffusion path, reduce the concentration of internal stress, and inhibit material cracking and pulverization. Although the electrochemical performance of silicon materials has been greatly improved after nanoscale treatment, there are still problems such as large specific surface area, low initial Coulomb efficiency, easy agglomeration, and poor conductivity. Therefore, pure silicon is difficult to be used as an anode material. Currently, silicon-carbon composite materials (Si / C) based on nanoscale silicon materials have the most promising commercial application prospects. Carbon materials can improve the conductivity of silicon-based materials, enhance the rate performance; inhibit the expansion of silicon materials, enhance the cycle performance; isolate the direct contact between the electrolyte and nanoscale silicon, and enhance the initial Coulomb efficiency.

[0005] Biomass carbon materials show important application potential in the field of batteries. Their hierarchical pore structure, high specific surface area, and adjustable surface chemical properties make them a key component of high-performance electrode materials. In lithium-ion batteries, biomass-derived carbon (such as coconut shell carbon and bamboo charcoal) can be used as a buffer skeleton for silicon-based anodes, effectively alleviating the volume expansion of silicon and enhancing conductivity; in sodium / potassium ion batteries, its disordered hard carbon structure provides stable ion insertion sites, supporting low-cost energy storage systems. Starch is one of the most abundant renewable resources on earth, widely existing in various plants. It is a typical polysaccharide with a simple structure and high carbon content, which makes it an ideal carbon precursor for anode materials of lithium / sodium ion batteries. Therefore, it is of great significance to develop a novel silicon-carbon anode material using starch as a precursor. Summary of the Invention

[0006] In view of the above deficiencies or improvement requirements of the existing technology, the present invention provides a novel silicon-carbon anode material and its preparation method. The silicon-carbon anode material not only has a low volume effect and good electrical conductivity, but also has a high first charge-discharge capacity and first efficiency, and excellent electrochemical performance.

[0007] 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 novel silicon-carbon anode material, comprising the following steps: S1. Sequentially add plant starch, nitrogen-boron dopant, and nano-silicon into a double planetary mixer, stir and mix for 2 h to 3 h, then add deionized water, raise the temperature to 55°C to 100°C, and knead for 4 h to 6 h, and leave it to age in air for 12 h to 15 h to obtain a silicon-carbon anode precursor; Starch is a polysaccharide formed by connecting glucose units through α-1,4 and α-1,6 glycosidic bonds. Depending on the type of starch, the gelatinization temperature varies from 55 to 100°C. During the gelatinization of starch in this step, due to the insertion of water molecules, the α-1,4 and α-1,6 glycosidic bonds break, and the viscosity increases sharply. The strong shear force of the kneader can evenly mix each component; during aging, the branched-chain starch chain structure recombines to form a cage structure that wraps the nano-silicon; finally, by introducing nitrogen and boron atoms, the electronic structure of the carbon matrix is changed, the charge carrier concentration is increased, the electrical conductivity of the material can be significantly improved, and active sites are formed, providing additional lithium storage active centers and increasing the reversible capacity.

[0008] S2. Place the silicon-carbon anode precursor in an atmosphere furnace under nitrogen protection, raise the temperature to 260°C, keep it warm for 2 h to 4 h, then raise the temperature to 800°C, keep it warm for 6 h to 8 h, take out the carbonized material, crush it with an air flow crusher, and pass through a 325-mesh sieve to obtain an intermediate material; in this step, starch reacts violently at 260°C to release a large amount of volatile components such as tar and wood vinegar, and an amorphous carbon skeleton can be formed at 800°C without causing the phase transformation of silicon crystals, and the electrochemical performance is excellent.

[0009] S3. Place the intermediate material in a rotary furnace, introduce acetylene gas when the temperature is raised to 600°C under a nitrogen atmosphere, continuously raise the temperature to 800°C, keep it warm for 2 h to 3 h, then close the acetylene gas, keep it warm for 1 h to 2 h, and cool to obtain the novel silicon-carbon anode material.

[0010] In the present invention, the starch is first gelatinized, so that the branched starch is broken and uniformly mixed with nano-silicon and nitrogen-boron dopants. Then, during aging, the starch molecules are recombined to form a cage structure to trap the nano-silicon, which can effectively inhibit the volume effect of nano-silicon during cycling. At the same time, boron doping treatment is carried out to improve the conductivity of the material and increase the reversible capacity. After carbonization, the specific surface area of the material is further reduced by vapor deposition technology to inhibit the side reactions between the material and the electrolyte. The prepared silicon-carbon negative electrode material has excellent electrochemical performance and cycling performance.

[0011] Further, in the above technical solution S1, the plant starch is one or more of potato starch, cassava starch, and corn starch.

[0012] Further, in the above technical solution S1, the nitrogen-boron dopant is one or more of ammonium borate, tetraammonium borate, and pentaammonium borate, and the addition amount is 1% - 2% of the total mass of the plant starch.

[0013] Further, in the above technical solution S1, the addition amount of the nano-silicon is 10% - 15% of the total mass of the plant starch, and the particle size Dv50 is 90nm - 150nm.

[0014] Further, in the above technical solution S1, the rotation speed of stirring and mixing is 80r / min - 120r / min.

[0015] Further, in the above technical solution S1, the addition amount of the deionized water is 100% - 120% of the total mass of the plant starch.

[0016] Further, in the above technical solution S2, the heating rate is 1℃ / min - 5℃ / min.

[0017] Further, in the above technical solution S3, the heating rate is 2℃ / min - 5℃ / min, the nitrogen inlet rate is 8L / min, and the acetylene inlet rate is 2L / min.

[0018] The second object of the present invention is to provide a novel silicon-carbon negative electrode material prepared by the above preparation method, and the initial Coulomb efficiency of the novel silicon-carbon negative electrode material is ≥90%.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: By utilizing the gelatinization characteristics of starch, the branched starch is fully gelatinized and broken, and after being fully mixed with nano-silicon and nitrogen-boron dopants, the aging characteristics of starch are utilized to form a cage structure when the branched starch recombines to wrap the nano-silicon therein. After carbonization, the nano-silicon is uniformly dispersed in the high-strength amorphous carbon shell formed by the carbonization of the plant starch gel, effectively improving the conductivity of the nano-silicon and suppressing the expansion during the charge and discharge process of the nano-silicon. After gas-phase coating, the specific surface area of the material is significantly reduced, and the initial efficiency and cycle performance are improved. The obtained silicon-carbon material has the advantages of high energy density, high initial efficiency, and high cycle stability.

[0020] The carbon source raw material used in the preparation method of the present invention is low in price and sufficient in supply, and the prepared silicon-carbon anode material has excellent performance and high market competitiveness. Brief Description of the Drawings

[0021] 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.

[0022] Figure 1 It is the SEM diagram of the silicon-carbon anode material prepared in Example 1 of the present invention.

[0023] Figure 2 It is the 1C 300-cycle diagram of the silicon-carbon anode material prepared in Example 1 of the present invention. Detailed Embodiments

[0024] The experimental methods in the following embodiments are all conventional methods unless otherwise specified. The raw materials involved in the following embodiments are all ordinary commercially available products unless otherwise specified, and can all be obtained through market purchase.

[0025] All the above technical features of the present invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions.

[0026] The raw materials involved in each embodiment 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.

[0027] Example 1 A preparation method of a novel silicon-carbon anode material includes the following steps: Add 100 g of potato starch, 12 g of nano-silicon with a Dv50 of 100 nm, and 1 g of ammonium borate into a double planetary mixer, mix at a rotation speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 60 °C for 6 h, and let it stand and age in air for 12 h to obtain precursor 1; Place precursor 1 in an atmosphere furnace, under nitrogen protection, heat it to 260 °C at a heating rate of 2 °C / min, hold for 2 h, then heat it to 800 °C at a heating rate of 2 °C / min, hold for 8 h, take it out, crush it with a jet mill and pass through a 325-mesh sieve to obtain precursor 2; Place precursor 2 in a rotary furnace, introduce nitrogen at a flow rate of 8 L / min, heat it to 800 °C at a heating rate of 5 °C / min, introduce acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; After cooling, a novel silicon-carbon anode material is obtained.

[0028] Example 2 A preparation method of a novel silicon-carbon anode material, comprising the following steps: Add 100 g of corn starch, 12 g of nano-silicon with a Dv50 of 100 nm, and 1 g of ammonium tetraborate into a double planetary mixer, mix at a rotation speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 65 °C for 4 h, and let it stand and age in air for 12 h to obtain precursor 1; Place precursor 1 in an atmosphere furnace, under nitrogen protection, heat it to 260 °C at a heating rate of 2 °C / min, hold for 2 h, then heat it to 800 °C at a heating rate of 2 °C / min, hold for 8 h, take it out, crush it with a jet mill and pass through a 325-mesh sieve to obtain precursor 2; Place precursor 2 in a rotary furnace, introduce nitrogen at a flow rate of 8 L / min, heat it to 800 °C at a heating rate of 5 °C / min, introduce acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; After cooling, a novel silicon-carbon anode material is obtained.

[0029] Example 3 A preparation method of a novel silicon-carbon anode material, comprising the following steps: Add 100 g of cassava starch, 12 g of nano-silicon with a Dv50 of 100 nm, and 1 g of ammonium pentaborate into a double planetary mixer, mix at a rotation speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 60 °C for 6 h, and leave to age in air for 12 h to obtain precursor 1; place precursor 1 in an atmosphere furnace, under nitrogen protection, heat up to 260 °C at a heating rate of 2 °C / min, hold for 2 h, then heat up to 800 °C at a heating rate of 2 °C / min, hold for 8 h, take out and crush with a jet mill and pass through a 325-mesh sieve to obtain precursor 2; place precursor 2 in a rotary furnace, introduce nitrogen at a flow rate of 8 L / min, heat up to 800 °C at a heating rate of 5 °C / min, introduce acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; cool to obtain the novel silicon-carbon anode material.

[0030] Example 4 A preparation method of a novel silicon-carbon anode material, comprising the following steps: Add 100 g of potato starch, 12 g of nano-silicon with a Dv50 of 150 nm, and 1 g of ammonium borate into a double planetary mixer, mix at a rotation speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 60 °C for 6 h, and leave to age in air for 12 h to obtain precursor 1; place precursor 1 in an atmosphere furnace, under nitrogen protection, heat up to 260 °C at a heating rate of 2 °C / min, hold for 2 h, then heat up to 800 °C at a heating rate of 2 °C / min, hold for 8 h, take out and crush with a jet mill and pass through a 325-mesh sieve to obtain precursor 2; place precursor 2 in a rotary furnace, introduce nitrogen at a flow rate of 8 L / min, heat up to 800 °C at a heating rate of 5 °C / min, introduce acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; cool to obtain the novel silicon-carbon anode material.

[0031] Comparative Example 1 A preparation method of a novel silicon-carbon anode material, comprising the following steps: Add 100 g of potato starch, 12 g of nano-silicon with a Dv50 of 100 nm, and 1 g of ammonium borate into a double planetary mixer, mix at a rotation speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 60 °C for 6 h to obtain precursor 1; Place precursor 1 in an atmosphere furnace, under nitrogen protection, heat up to 260 °C at a heating rate of 2 °C / min, hold for 2 h, then heat up to 800 °C at a heating rate of 2 °C / min, hold for 8 h, take out and crush with an air flow crusher and pass through a 325-mesh sieve to obtain precursor 2; Place precursor 2 in a rotary furnace, introduce nitrogen at a flow rate of 8 L / min, heat up to 800 °C at a heating rate of 5 °C / min, introduce acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; After cooling, a novel silicon-carbon anode material is obtained.

[0032] Comparative Example 2 A preparation method of a novel silicon-carbon anode material, comprising the following steps: Add 100 g of potato starch, 12 g of nano-silicon with a Dv50 of 100 nm, and 1 g of ammonium borate into a double planetary mixer, mix at a rotation speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 60 °C for 6 h, and leave it to age in air for 12 h to obtain precursor 1; Place precursor 1 in an atmosphere furnace, under nitrogen protection, heat up to 260 °C at a heating rate of 2 °C / min, hold for 2 h, then heat up to 800 °C at a heating rate of 2 °C / min, hold for 8 h, take out and crush with an air flow crusher and pass through a 325-mesh sieve to obtain precursor 2; Place precursor 2 in a rotary furnace, introduce nitrogen at a flow rate of 8 L / min, heat up to 900 °C at a heating rate of 5 °C / min, introduce acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; After cooling, a novel silicon-carbon anode material is obtained.

[0033] Comparative Example 3 A preparation method of a novel silicon-carbon anode material, comprising the following steps: Add 100 g of potato starch and 12 g of nano-silicon with a Dv50 of 100 nm to a double planetary mixer, mix at a speed of 90 r / min for 2.5 h, then add 100 g of deionized water, knead at 60 °C for 6 h, and let it stand and age in air for 12 h to obtain precursor 1; place precursor 1 in an atmosphere furnace, under nitrogen protection, heat it at a heating rate of 2 °C / min to 260 °C, hold for 2 h, then heat it at a heating rate of 2 °C / min to 800 °C, hold for 8 h, take it out, crush it with an air flow crusher and pass through a 325-mesh sieve to obtain precursor 2; place precursor 2 in a rotary furnace, pass nitrogen at a flow rate of 8 L / min, heat it to 800 °C at a heating rate of 5 °C / min, pass acetylene at a rate of 2 L / min for 3 h, then close the acetylene gas, and hold at 800 °C for 1 h; after cooling, a novel silicon-carbon anode material is obtained.

[0034] Experimental Example 1. Detect the novel silicon-carbon anode material obtained during the preparation process of Example 1, and observe its micro-morphology by scanning electron microscope. The results are as Figure 1 shown. It can be seen that the distribution of each material in the silicon-carbon anode material is uniform, the nano-silicon has good dispersion in the material and the coating integrity of the material. At the same time, using high-strength biomass as the substrate can effectively inhibit the expansion of silicon.

[0035] 2. To detect the performance of the silicon-carbon anode materials prepared in Examples 1 - 4 and Comparative Examples 1 - 3 for lithium-ion batteries, a half-cell test method is used.

[0036] The half-cell test method is as follows: Prepare slurries using the silicon-carbon materials prepared in Examples 1 - 4 and Comparative Examples 1 - 3 as the negative electrode active materials. The slurry ratio is: active material: CNTs (including dispersant): CMC: SBR = 89%: 4.5%: 1.5%: 5%, where CMC is a 1.5% aqueous solution. Then coat the slurry on copper foil and vacuum dry it for 12 h to make a negative electrode sheet. The electrolyte is commercially purchased, the separator is a PE membrane, and the lithium sheet is the counter electrode. Assemble a half-cell in a glove box. Conduct constant current charge and discharge experiments on a LAND battery test system. The charge and discharge voltage is limited to 0.005 V - 2 V, and a computer-controlled charge and discharge cabinet is used for data acquisition and control. The test results of the physical and electrochemical properties of the negative electrode materials in Examples 1 - 4 and Comparative Examples 1 - 3 are shown in Table 1 and Figure 2 as follows.

[0037] Table 1 Test Results of Physical and Electrochemical Properties of Negative Electrode Materials

[0038] It can be seen from Table 1 that the specific surface areas of the materials of Examples 1 to 4 are relatively low, indicating that the coated carbon layer is uniform and dense, which improves the conductivity while avoiding direct contact between silicon and the electrolyte, so that the materials have good cycle performance, and the first reversible capacity is greater than 1570 mAh / g; the first coulomb efficiency is greater than 90%; the capacity retention rate after 300 cycles is greater than 91%, which has the advantages of high energy density, high first efficiency, and high cycle stability. While the specific surface area of comparative example 1 is larger, and the first coulombic efficiency and the cycle capacity retention rate are lower. This is because there is no aging step, resulting in the starch molecules still in a broken state and unable to wrap the silicon, increasing the specific surface area, and the exposed nano-silicon has a violent side reaction with the electrolyte, resulting in low first coulombic efficiency and cycle capacity retention rate; the first reversible capacity, first coulombic efficiency and cycle capacity retention rate of comparative example 2 are all reduced, because the carbonization temperature is increased to 900°C, the nano-silicon crystal phase undergoes a transformation, and the electrochemical performance of the silicon-based material part decreases, resulting in a decrease in the first reversible capacity, first coulombic efficiency and cycle capacity retention rate; the first reversible capacity of comparative example 3 is reduced, because comparative example 3 does not add nitrogen and boron dopants, the charge carrier concentration decreases, and the active sites decrease, resulting in a decrease in material conductivity and a decrease in reversible capacity.

[0039] In summary, the present invention uses starch as a carbon precursor and utilizes the gelatinization properties of starch to fully gelatinize and break the branched starch. After fully mixing with nano-silicon and nitrogen-boron dopants, the aging properties of starch are utilized to form a cage structure when the branched starch is reorganized to wrap the nano-silicon therein, which can effectively inhibit the expansion of the nano-silicon. The obtained silicon-carbon material has the advantages of high energy density, high initial efficiency, and high cycle stability.

[0040] Finally, it should be emphasized that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various changes and modifications. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A preparation method of a novel silicon-carbon anode material, characterized in that, It includes the following steps: S1. Add plant starch, nitrogen-boron dopant, and nano-silicon into a double planetary mixer in sequence. After stirring and mixing for 2 h to 3 h, add deionized water, heat up to 55 °C to 100 °C and knead for 4 h to 6 h, and then let it stand and age in air for 12 h to 15 h to obtain a silicon-carbon anode precursor; S2. Place the silicon-carbon anode precursor in an atmosphere furnace under nitrogen protection, heat up to 260 °C, keep the temperature for 2 to 4 h, then heat up to 800 °C, keep the temperature for 6 h to 8 h. After taking out the carbonized material, crush it with a jet mill and pass through a 325-mesh sieve to obtain an intermediate material; S3. Place the intermediate material in a rotary furnace, when the temperature rises to 600 °C in a nitrogen atmosphere, introduce acetylene gas, continuously heat up to 800 °C and keep the temperature for 2 h to 3 h, then turn off the acetylene gas, keep the temperature for 1 h to 2 h, and obtain a novel silicon-carbon anode material after cooling.

2. The preparation method according to claim 1, wherein, In S1, the plant starch is one or more of potato starch, cassava starch, and corn starch.

3. The preparation method according to claim 1, wherein In S1, the nitrogen-boron dopant is one or more of ammonium borate, tetraammonium borate, and pentaammonium borate, and the addition amount is 1% to 2% of the total mass of the plant starch.

4. The preparation method according to claim 1, characterized in that In S1, the addition amount of the nano-silicon is 10% to 15% of the total mass of the plant starch, and the particle size Dv50 is 90 nm to 150 nm.

5. The preparation method according to claim 1, characterized in that, In S1, the rotation speed of stirring and mixing is 80 r / min to 120 r / min.

6. The preparation method according to claim 1, wherein In S1, the addition amount of the deionized water is 100% to 120% of the total mass of the plant starch.

7. The preparation method according to claim 1, characterized in that, In S2, the heating rate is 1 °C / min to 5 °C / min.

8. The preparation method according to claim 1, wherein, In S3, the heating rate is 2 °C / min to 5 °C / min, the nitrogen gas introduction rate is 8 L / min, and the acetylene gas introduction rate is 2 L / min.

9. A novel silicon-carbon anode material prepared by the preparation method according to any one of claims 1-8, characterized in that, The initial Coulombic efficiency of the novel silicon-carbon anode material is ≥90%.

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