Microwave-alkali synergistic preparation method of bamboo-based carbon / silicon composite negative electrode

Abstract

The invention belongs to the technical field of energy storage materials, and particularly relates to a microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode. According to the method, bamboo powder which is a plant waste material is used, a'microwave-alkali etching synergistic activation 'strategy is provided, and the method is a green, rapid and low-energy-consumption manufacturing method. Natural cellulose / lignin in bamboo powder is used as a carbon source, carbonization of a bamboo powder precursor obtained through microwaves is achieved at the temperature of 800 DEG C or below through the coupling effect of microwave local high temperature and sodium hydroxide melting etching, silicon chemical vapor deposition is conducted on the carbonized bamboo powder precursor, and the carbon / silicon composite negative electrode can be obtained. Meanwhile, the hard carbon component and the graphite component are provided, and chemical vapor deposition silicon is matched, so that the conductivity of the material and the cycling stability of the battery are improved, and a new idea is provided for development of a high-energy-density lithium ion battery negative electrode material.

Description

Technical Field

[0001] The invention belongs to the technical field of energy storage materials, and in particular relates to a microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode. Background Art

[0002] With the rapid development of new energy vehicles and smart grids, lithium-ion batteries are increasingly in need of high energy density, low cost and sustainable negative electrode materials. Traditional graphite negative electrodes are difficult to meet the needs of the next generation of batteries due to their low theoretical capacity (372mAh / g) and the risk of lithium dendrites. Although hard carbon materials have high capacity (>500mAh / g) and excellent rate performance, their first efficiency is low (70-85%), the preparation process is complex and the cost of raw materials is high. These problems restrict their industrialization process. In recent years, biomass-derived carbon has become a research hotspot due to its renewability and structural adjustability. Among them, bamboo is regarded as an ideal precursor due to its short growth period and high cellulose content (40-50%). However, the existing bamboo-based hard carbon preparation technology has significant bottlenecks and high process energy consumption: the traditional two-step method (hydrothermal pretreatment + high temperature carbonization) consumes a lot of energy (>1500℃), and the acid / base etching step produces wastewater pollution and high process energy consumption.

[0003] In recent years, researchers have tried to optimize the performance of bamboo-based hard carbon by pretreatment (hydrothermal, steam explosion) or heteroatom doping, but the results have been limited. For example, although hydrothermal pretreatment can remove some impurities, it leads to excessive hydrolysis of cellulose and a decrease in carbon yield of more than 30%. At the same time, microwave-assisted carbonization technology has attracted attention due to its high efficiency and energy-saving characteristics, but it is mostly used in the drying or pre-oxidation stage in the preparation of biomass hard carbon, and fails to form a synergistic effect with alkaline etching. Summary of the invention

[0004] [Technical issues to be solved]

[0005] In response to the above pain points, this patent innovatively proposes the "microwave-alkali etching synergistic activation" strategy, using natural cellulose / lignin in bamboo powder as a carbon source, and through the coupling of microwave local high temperature and sodium hydroxide melt etching, the bamboo powder precursor obtained by microwave is carbonized below 800°C, and then the carbonized bamboo powder precursor is subjected to chemical vapor deposition of silicon to obtain a carbon / silicon composite negative electrode. This process breaks through the limitations of traditional high-temperature graphitization and provides a new path for the large-scale preparation of high-performance, low-cost biomass negative electrodes.

[0006] [Technical solution]

[0007] 1. A microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode, characterized in that it comprises the following steps:

[0008] Step 1, weigh a certain amount of bamboo powder, put it into a corundum crucible, and add sodium hydroxide solution into it;

[0009] Step 2: Place the corundum crucible containing bamboo powder and sodium hydroxide solution in a microwave oven and heat it continuously for an appropriate time, observe the situation in the crucible, and stir it as appropriate to make the water and solid uniform and sufficient. Finally, take out the microwaved solid to obtain solid 1;

[0010] Step 3, placing the solid object 1 in an oven to continue drying, maintaining a fixed temperature and time, so as to completely remove surface moisture, thereby obtaining a bamboo powder carbonization precursor;

[0011] Step 4, placing the bamboo powder carbonization precursor in a tube furnace and heat treating it under the protection of argon gas to obtain a black solid 2;

[0012] Step 5, collect the solid 2 in a beaker, add excess dilute hydrochloric acid solution, and then filter and wash with deionized water until the washing liquid is neutral, and then dry the solid in an oven to obtain solid 3;

[0013] Step 6: Take a certain amount of solid material 3 and perform chemical vapor deposition. The corresponding silicon source is silane. The deposition reaction continues for a period of time to obtain a black solid material, which is a carbon / silicon composite material.

[0014] A further technical solution of the present invention is that in step 1, the mesh size of the bamboo powder is 80 meshes, the mass is 24 g, the volume of the sodium hydroxide solution added is 70 mL, and the concentration is 1 mol / L;

[0015] According to a further technical solution of the present invention, in step 2, the volume of the corundum crucible is 500 mL, the microwave heating power is 560 W, the operating frequency is 2.45 GHz, and the heating time is 5 to 7 minutes, so that the solid surface is basically dry.

[0016] A further technical solution of the present invention is that in step 3, the oven temperature is 50° C. and the drying time is 24 hours;

[0017] A further technical solution of the present invention is that in step 4, a nickel crucible is used to hold the bamboo powder carbonization precursor, and a heating rate of 10°C / min is set in a tube furnace, and the temperature is raised from room temperature to 800°C and maintained for 2 hours; before heating begins, the tube furnace needs to be evacuated and argon gas is introduced at a rate of 100 standard milliliters / minute (sccm);

[0018] A further technical solution of the present invention is that in step 5, the concentration of dilute hydrochloric acid is 1 mol / L; the oven temperature is set to 50° C., and the drying time is 24 hours;

[0019] A further technical solution of the present invention is that in step 6, a total of 100 mg of the black solid 3 is taken, and a nickel crucible is used as a base to flatten the powder; the heating rate in the tubular furnace is set to 10°C / min, and the temperature is increased from room temperature to 500°C; the tubular furnace needs to be evacuated before heating begins, and after the air pressure is lower than 10Pa, a silane-argon mixed gas (5% silane) is introduced at a rate of 200sccm for 25 minutes.

[0020] When the bamboo-based carbon / silicon composite material provided by the present invention is used as a negative electrode material for lithium-ion batteries, it is verified through experiments that it has a high battery specific capacity and cycle efficiency, and is a lithium-ion battery negative electrode material with great application potential. The carbon / silicon composite negative electrode of the prepared lithium-ion battery maintains a capacity of 500-700mAh / g after 100 cycles at a current of 1A / g, and the coulomb efficiency is stable at more than 98%, and the cycle stability is high. It is proved that the special synthesis path of the material effectively alleviates the problem of volume expansion and improves the overall performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Combined images of scanning electron microscopy (a) and energy spectrum (bd) of bamboo-based carbon / silicon composites

[0022] Figure 2 Transmission electron microscopy image of bamboo-based carbon / silicon composites

[0023] Figure 3 Raman spectrum of bamboo-based carbon / silicon composites

[0024] Figure 4 The voltammetric cycling performance of a button-type half-cell with bamboo-based carbon / silicon composite material as the negative electrode DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention, but it should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present disclosure. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0026] Bamboo-based carbon / silicon composites, sodium carboxymethyl cellulose and acetylene black were mixed with an appropriate amount of deionized water in a mass ratio of 8:1:1, and then a uniform slurry was obtained. The slurry was applied to copper foil with a scraper and dried in a vacuum oven at 80°C for 12h. The copper foil was then punched into discs with a diameter of 12mm. These electrode sheets were assembled into semi-button cells in an argon-filled glove box with oxygen and moisture content below 0.1ppm. Lithium foil was used as the counter electrode, glass fiber paper as the separator, lithium hexafluorophosphate was dissolved in ethylene carbonate (EC) at a concentration of 1mol / L, and then mixed with dimethyl carbonate (DMC) in a volume ratio of 1:1, and 5% mass fraction of fluorocarbon ethylene (FEC) was added as the electrolyte. The packaged button half-cell was left for 24h and then subjected to cyclic voltammetry (CV) tests at an internal voltage of 0.01-2.0V.

[0027] The scanning electron microscopy structure of the bamboo-based carbon / silicon composite material obtained in this example is as follows: Figure 1 As shown in (a), it can be observed that there are a large number of holes distributed on the surface of the material, with a diameter of about 1 to 2 microns, and more holes can be seen inside the holes. This structure greatly increases the specific surface area of ​​the material and exposes more active sites. Figure 1 (b), 1(c), and 1(d) are scanning electron energy spectra, showing the distribution of chemical elements in the part shown in 1(a). It can be seen that a large area of ​​carbon elements and the distribution of silicon elements corresponding to the distribution of holes are observed, proving that the chemical vapor deposition process makes the silicon material adhere to the carbon surface and the porous carbon structure.

[0028] The transmission electron microscopic structure 2 of the bamboo-based carbon / silicon composite material obtained in this example shows that the portion with light-colored lattice stripes is silicon, and the portion with darker color is carbon material. It can be seen that the silicon material and the carbon material are effectively combined at the 10-nanometer scale.

[0029] The Raman spectrum of the bamboo-based carbon / silicon composite material obtained in this example is as follows: Figure 3 As shown, it can be seen at 516cm -1 There is an obvious intensity peak at 1348cm -1 and 1589cm -1 Peaks with similar intensities were detected at the two locations, corresponding to the carbon bond vibrations of disordered graphite and crystallized graphite, respectively. The results of the Raman spectrum verified that Figure 1 , Figure 2 The observed carbon-silicon structure and element distribution indicate that both hard carbon components and graphite components are distributed in the carbon materials we synthesized.

[0030] The volt-ampere cycle performance of the button-type half-cell in this embodiment is as follows: Figure 4As shown, the coulombic efficiency of the carbon / silicon composite negative electrode is 67% when the first cycle rate is 0.2A / g; after 100 cycles at a rate of 1A / g, the capacity remains 500-700mAh / g, and the coulombic efficiency is stabilized at about 100%.

[0031] The approach we use has the following advantages:

[0032] 1. Use bamboo powder as raw material, utilize agricultural waste, turn waste into treasure, and the synthesis process is green, fast and low energy consumption;

[0033] 2. Use microwave method for pretreatment to remove impurities in the raw materials, hydrolyze hemicellulose and partially dissolve lignin, promote the establishment of initial pores in the raw materials, and evenly distribute sodium hydroxide in the structure, which is conducive to subsequent high-temperature etching. The local high temperature generated by microwaves also creates conditions for the growth of graphite crystals;

[0034] 3. By depositing silicon through CVD, crystalline silicon is evenly distributed on the porous carbon surface, which increases the elastic modulus of the carbon matrix, alleviates the mechanical stress generated by the negative electrode material during the charge and discharge process, and improves the battery's cycle capacity retention rate.

Claims

1. A microwave-alkali synergistic preparation method for bamboo-based carbon / silicon composite negative electrode, characterized in that: The following steps are involved: Step 1, weigh a certain amount of bamboo powder, put it into a corundum crucible, and add sodium hydroxide solution into it; Step 2, placing the corundum crucible containing bamboo powder and sodium hydroxide solution in a microwave oven and continuously heating it for an appropriate time, observing the situation in the crucible, and stirring it as appropriate to make the water and solid uniform and sufficient; finally, taking out the microwaved solid to obtain solid 1; Step 3, placing the solid object 1 in an oven to continue drying, maintaining a fixed temperature and time, so as to completely remove surface moisture, thereby obtaining a bamboo powder carbonization precursor; Step 4, placing the bamboo powder carbonization precursor in a tube furnace and heat treating it under the protection of argon gas to obtain a black solid 2; Step 5, collect the solid 2 in a beaker, add excess dilute hydrochloric acid solution, and then filter and wash with deionized water until the washing liquid is neutral, and then dry the solid in an oven to obtain solid 3; Step 6: Take a certain amount of solid material 3 and perform chemical vapor deposition. The corresponding silicon source is silane. The deposition reaction continues for a period of time to obtain a black solid material 3, which is a carbon / silicon composite material.

2. The microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode according to claim 1, characterized in that: In the step 1, the mesh size of the bamboo powder is 80 meshes, the mass is 24 g, the volume of the sodium hydroxide solution added is 70 mL, and the concentration is 1 mol / L.

3. The microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode according to claim 1, characterized in that: In the step 2, the volume of the corundum crucible is 500 mL, the microwave heating power is 560 W, the operating frequency is 2.45 GHz, and the heating time is 5 to 7 minutes, which is suitable for making the solid surface basically dry.

4. The microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode according to claim 1, characterized in that: In step 3, the oven temperature is 50° C. and the drying time is 24 h.

5. The microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode according to claim 1, characterized in that: In step 4, a nickel crucible is used to hold the bamboo powder carbonization precursor, and the heating rate is set to 10°C / min in a tube furnace, from room temperature to 800°C, and maintained for 2 hours; before heating begins, the tube furnace needs to be evacuated and argon gas is introduced at a rate of 100 standard milliliters / minute (sccm).

6. The microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode according to claim 1, characterized in that: In step 5, the concentration of dilute hydrochloric acid is 1 mol / L; the oven temperature is set to 50° C., and the drying time is 24 h.

7. The microwave-alkali synergistic preparation method of a bamboo-based carbon / silicon composite negative electrode according to claim 1, characterized in that: In step 6, a total of 100 mg of the black solid 3 was taken and a nickel crucible was used as a base to flatten the powder; the heating rate in the tubular furnace was set to 10°C / min, and the temperature was increased from room temperature to 500°C; the tubular furnace was evacuated before heating, and after the gas pressure was lower than 10 Pa, a silane-argon mixed gas (5% silane) was introduced at a rate of 200sccm and maintained for 25 minutes.

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