Ag-MWCNTs reinforced cellulose hard carbon negative electrode material and preparation method and application thereof

Abstract

The invention discloses an Ag-MWCNTs reinforced cellulose hard carbon negative electrode material as well as a preparation method and application thereof. The preparation method comprises the following steps: oxidizing carbon nanotubes, loading Ag nanoparticles to obtain Ag-MWCNTs, carrying out molecular crosslinking on the Ag-MWCNTs and renewable cellulose, and carrying out washing, freeze drying, pre-carbonization, crushing and high-temperature carbonization to obtain the Ag-MWCNTs reinforced cellulose hard carbon negative electrode material. According to the prepared Ag-MWCNTs reinforced cellulose hard carbon negative electrode material, the crystallinity of cellulose is adjusted through a molecular crosslinking strategy, so that the hard carbon material has a rich pore structure, the cellulose hard carbon capacity is improved, and meanwhile, the conductivity is improved and the impedance is reduced through the added Ag-MWCNTs.

Description

Technical Field

[0001] This invention belongs to the technical field of anode materials for sodium-ion batteries, specifically relating to an Ag-MWCNTs-reinforced cellulose hard carbon anode material, its preparation method, and its application. Background Technology

[0002] Batteries have become an indispensable part of social life, playing a vital role in power, energy storage, and digital fields. Among them, lithium-ion batteries are the most widely used due to their balanced performance: they have a long cycle life, high energy density, and excellent rate performance. However, lithium-ion batteries also have shortcomings in low-temperature performance and safety, and frequent battery accidents have spurred every battery engineer to continuously develop safer and more convenient batteries. Sodium batteries have better low-temperature performance and safety, but due to their large ionic radius and small interlayer spacing, graphite cannot be used well as a negative electrode material for sodium-ion batteries, thus requiring a more suitable negative electrode material.

[0003] Hard carbon is an amorphous carbon material that is difficult to graphitize. It exhibits short-range order but long-range disorder, contains numerous pseudo-graphitized domains, and possesses a large interlayer spacing and abundant microporous structure, facilitating the insertion and extraction of sodium ions. Simultaneously, hard carbon materials possess low sodium storage potential, high specific capacity, and long cycle life, making them ideal anode materials for sodium-ion batteries. Current hard carbon anode materials can be mainly classified into biomass-based, resin-based, and pitch-based materials. Among these, biomass-based materials have attracted significant attention due to their abundant reserves and environmental friendliness. Cellulose, composed of glucose, is the most widely distributed and abundant polysaccharide in nature. High-temperature carbonization results in high residual carbon content and porosity, making cellulose one of the main raw materials for biomass hard carbon.

[0004] However, the carbonization temperature during the preparation of cellulose hard carbon is relatively low, and many impurities in the material cannot be completely removed, resulting in high impedance and poor conductivity when used as a negative electrode material in practical applications. This invention uses renewable cellulose as a precursor and adjusts the crystallinity of cellulose through a molecular cross-linking strategy to prepare a hard carbon material with a rich pore structure, thereby improving the capacity of the cellulose hard carbon. Simultaneously, carbon nanotubes loaded with Ag nanoparticles are added to enhance its conductivity and reduce impedance. Summary of the Invention

[0005] The purpose of this invention is to provide an Ag-MWCNTs-enhanced cellulose hard carbon anode material.

[0006] Another objective of this invention is to provide a method for preparing Ag-MWCNTs-enhanced cellulose hard carbon anode materials.

[0007] Another objective of this invention is the application of Ag-MWCNTs-enhanced cellulose hard carbon anode materials.

[0008] The preparation method of the Ag-MWCNTs-reinforced cellulose hard carbon anode material of the present invention includes the following steps: Step 1: Oxidize carbon nanotubes with potassium permanganate to obtain carbon nanotubes rich in hydroxyl and carboxyl groups: Carbon nanotubes and 200 mL of 0.5 mol / L sulfuric acid were added sequentially to a 500 mL three-necked flask and ultrasonically dispersed for 30 min. At the same time, the oxidizing agent was dissolved in 200 mL of 0.5 mol / L sulfuric acid. The carbon nanotubes and sulfuric acid dispersion were placed in an oil bath at 120℃-180℃ and the oxidizing agent sulfuric acid solution was added dropwise under stirring to carry out oxidation. The reaction was carried out for 5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was collected. The cake was added to 30 mL of hydrochloric acid to remove byproducts, then washed with water until neutral, and dried at 80℃ for 12 h to obtain oxidized carbon nanotubes. Step 2: Using sodium citrate as a reducing agent and dimethyl sulfoxide as a solvent, silver nitrate is reduced to obtain Ag nanoparticles grown in situ on carbon nanotubes in Step 1. The resulting product is denoted as Ag-MWCNTs. 180 mL of solvent was added to a three-necked flask under a nitrogen atmosphere and placed in a 20 °C water bath. A reducing agent was added dropwise while stirring at 200 r / min. At the same time, carbon oxide nanotubes and 180 mL of solvent were added sequentially to a beaker and ultrasonically dispersed for 30 minutes. Then, the mixture was slowly added to the three-necked flask and stirred until homogeneous. Silver nitrate solution was then added dropwise. After the reaction was completed, the mixture was subjected to a reduction reaction. The mixture was then cooled and filtered, and washed multiple times with ethanol and pure water. Finally, it was dried at 80 °C for 12 h to obtain Ag-MWCNTs. The reduction reaction conditions are as follows: react in a water bath at 10-20℃ for 2 hours, then transfer to a constant temperature water bath at 60-80℃ and continue the reaction for 1 hour; Step 3: Cellulose was dissolved using lithium hydroxide and urea, and the Ag nanoparticle-loaded carbon nanotubes obtained in Step 2 were added. After crosslinking with epichlorohydrin, Ag-MWCNTs / cellulose hydrogel was obtained. Microcrystalline cellulose, LiOH, and urea were dissolved in deionized water and mechanically stirred in an ice-water bath for 30 min. Then Ag-MWCNTs were added and stirred vigorously for 1 h. After that, a crosslinking agent was added and stirred for another 30 min. The mixture was then transferred to a 60°C oven and reacted for 3 h to obtain Ag-MWCNTs / cellulose hydrogel. Step 4: Wash the hydrogel obtained in Step 3 with water until it is neutral, and then freeze-dry it at -40°C to obtain a porous three-dimensional material; Step 5: Place the porous structure material obtained in Step 4 into a box-type atmosphere furnace and pre-carbonize it under a protective atmosphere. After cooling in the furnace, crush it to a particle size Dv50 of 5-9 μm. Then, perform high-temperature carbonization under a protective atmosphere to obtain the Ag-MWCNTs reinforced cellulose hard carbon anode material. The pre-carbonization temperature is 400℃-700℃, the heating rate is 2~5℃ / min, and the holding time is 2h-4h; the high-temperature carbonization temperature is 1100℃-1600℃, the heating rate is 3~6℃ / min, and the carbonization time is 3h-7h.

[0009] The carbon nanotubes mentioned in step 1 of this invention are multi-walled carbon nanotubes, the oxidizing agent is potassium permanganate, and the mass ratio of carbon nanotubes to potassium permanganate is 1:1~3.

[0010] Preferably, the oxidation temperature in step 1 of the present invention is 150°C.

[0011] The solvent in step 2 of this invention is dimethyl sulfoxide, and the mass ratio of carbon nanotubes: silver nitrate: sodium citrate is 1:0.5~2:0.1~2.

[0012] Preferably, the reduction reaction conditions in step 2 of the present invention are: reacting in a water bath at 20°C for 2 hours, then transferring to a constant temperature water bath at 60°C for another 1 hour.

[0013] The cellulose dissolution system described in step 3 of this invention is lithium hydroxide, urea, and Lithium hydroxide: Urea: The mass ratio of cellulose to the solution system is 1:2~5:15~20, and the mass ratio of cellulose to the solution system is 1:10~20; the crosslinking agent is epichlorohydrin, and the amount of crosslinking agent used for cellulose is 0.67 mL / g; the mass ratio of cellulose to Ag-MWCNTs is 10:0.1~1.

[0014] The protective gas for pre-carbonization and high-temperature carbonization in step 5 of this invention is at least one of nitrogen, argon, and helium.

[0015] Preferably, in step 5 of the present invention, the pre-carbonization temperature is 600℃, the heating rate is 3℃ / min, and the holding time is 2h; the high-temperature carbonization temperature is 1300℃, the heating rate is 5℃ / min, and the carbonization time is 4h.

[0016] The preparation method described in this invention yields Ag-MWCNTs-reinforced cellulose hard carbon anode material.

[0017] The present invention relates to the application of Ag-MWCNTs-reinforced cellulose hard carbon material as a negative electrode material for sodium-ion batteries.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses renewable microcrystalline cellulose as a precursor and employs freeze-dried cellulose hydrogel to generate a porous structure in hard carbon materials. Simultaneously, by reducing the crystallinity of cellulose through molecular cross-linking, more disordered layer structures are constructed, promoting the formation of pseudo-graphite domains, thereby achieving the purpose of increasing the capacity of cellulose hard carbon materials.

[0019] 2. This invention oxidizes carbon nanotubes to introduce hydroxyl and carboxyl functional groups on their surface, enhancing their interaction with cellulose and promoting uniform dispersion of carbon nanotubes in cellulose. At the same time, it can participate in the cellulose cross-linking reaction, reduce the crystallinity of cellulose, and generate more disordered layer structures and pseudo-graphite domains in the cellulose hard carbon material, thereby increasing the capacity of the cellulose hard carbon material.

[0020] 3. This invention grows Ag nanoparticles in situ on carbon nanotubes to enhance their radial conductivity and introduces them into cellulose hard carbon materials to reduce the impedance of cellulose hard carbon materials and improve their conductivity, so that the prepared cellulose hard carbon materials have high capacity and low impedance. Attached Figure Description

[0021] Figure 1 TEM image of Ag nanoparticles loaded on carbon oxide nanotubes in Example 1 of this invention; Figure 2 XRD pattern of Ag nanoparticles supported on carbon oxide nanotubes in Example 1 of this invention; Figure 3 SEM image of Ag-MWCNTs-enhanced cellulose hard carbon anode material in Example 1 of this invention; Figure 4 Charge-discharge curves of Ag-MWCNTs-enhanced cellulose hard carbon anode material in Example 1 of this invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described in detail below through specific embodiments.

[0023] Example 1 (1) Preparation of carbon nanotubes: 1 g of carbon nanotubes and 200 mL of 0.5 mol / L sulfuric acid were added to a 500 mL three-necked flask and ultrasonically dispersed for 30 min; at the same time, 2.5 g of potassium permanganate was dissolved in 200 mL of 0.5 mol / L sulfuric acid; the carbon nanotubes and sulfuric acid dispersion were placed in an oil bath at 150 °C, and potassium permanganate sulfuric acid solution was added dropwise under stirring. The reaction was carried out for 5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was collected. The filter cake was added to 30 mL of hydrochloric acid to remove the byproduct manganese dioxide. The mixture was then washed with water until neutral and dried at 80 °C for 12 h to obtain carbon nanotubes.

[0024] (2) Preparation of Ag nanoparticles loaded on carbon nanotubes: 180 mL of dimethyl sulfoxide was added to a three-necked flask under a nitrogen atmosphere and placed in a 20 °C water bath. 6 mL of 0.1176 g / mL sodium citrate was added dropwise while stirring at 200 r / min. At the same time, 0.3 g of carbon nanotubes and 180 mL of dimethyl sulfoxide were added to a beaker and ultrasonically dispersed for 30 minutes. Then, the mixture was slowly added to the three-necked flask and stirred evenly. 24 mL of 0.1 mol / L silver nitrate solution was added dropwise. After the reaction was completed, the mixture was reacted in a 20 °C water bath for 2 h and then transferred to a 60 °C constant temperature water bath for another 1 h. The mixture was then cooled and filtered, and rinsed multiple times with ethanol and pure water. Finally, it was dried at 80 °C for 12 h to obtain carbon nanotubes loaded with Ag nanoparticles, denoted as Ag-MWCNTs.

[0025] (3) Preparation of Ag-MWCNTs / cellulose hydrogel: 7.2g of microcrystalline cellulose, 5.5g of LiOH and 18g of urea were dissolved in 96mL of deionized water and mechanically stirred in an ice-water bath for 30min. Then, 0.36g of carbon nanotubes loaded with Ag nanoparticles were added and stirred vigorously for 1h. After that, 4.8mL of epichlorohydrin was added and stirred for another 30min. Then, the mixture was transferred to an oven at 60℃ and reacted for 3h to obtain Ag-MWCNTs / cellulose hydrogel.

[0026] (4) The hydrogel in step (3) was washed with deionized water until neutral, and then freeze-dried at -40°C to obtain Ag-MWCNTs / cellulose composite material rich in porous structure.

[0027] (5) Preparation of Ag-MWCNTs / cellulose hard carbon anode material: The porous composite material obtained in step (4) is subjected to a two-stage carbonization process in a box-type atmosphere furnace to obtain the hard carbon anode material. The carbonization process is as follows: under a nitrogen protective atmosphere, the temperature is raised to 600℃ at 3℃ / min for 2.0h for pre-carbonization, cooled in the furnace and crushed to Dv50: 5-9μm, and then under a nitrogen protective atmosphere, the temperature is raised to 1300℃ at 5℃ / min for 4.0h for high-temperature carbonization to obtain the hard carbon anode material.

[0028] Example 2 Compared with Example 1, the mass of Ag-MWCNTs added in step (3) was changed to 0.72g, and the remaining steps were the same as in Example 1.

[0029] Example 3 (1) Preparation of carbon nanotubes: 1g of carbon nanotubes and 200mL of 0.5mol / L sulfuric acid were added to a 500mL three-necked flask and ultrasonically dispersed for 30min; at the same time, 1g of potassium permanganate was dissolved in 200mL of 0.5mol / L sulfuric acid; the carbon nanotubes and sulfuric acid dispersion were placed in an oil bath at 120℃, and potassium permanganate sulfuric acid solution was added dropwise under stirring. The reaction was carried out for 5h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was collected. The filter cake was added to 30mL of hydrochloric acid to remove the byproduct manganese dioxide. Then it was washed with water until neutral and dried at 80℃ for 12h to obtain carbon nanotubes.

[0030] (2) Preparation of Ag nanoparticles loaded on carbon nanotubes: 180 mL of dimethyl sulfoxide was added to a three-necked flask under a nitrogen atmosphere and placed in a 20 °C water bath. 6 mL of 0.1176 g / mL sodium citrate was added dropwise while stirring at 200 r / min. At the same time, carbon nanotubes and 180 mL of dimethyl sulfoxide were added sequentially to a beaker and ultrasonically dispersed for 30 minutes. Then, the mixture was slowly added to the three-necked flask and stirred evenly. 24 mL of 0.1 mol / L silver nitrate solution was added dropwise. After the reaction was completed, the mixture was reacted in a 20 °C water bath for 2 h and then transferred to a 40 °C constant temperature water bath for another 1 h. The mixture was then cooled and filtered, and rinsed multiple times with ethanol and pure water. Finally, it was dried at 80 °C for 12 h to obtain carbon nanotubes loaded with Ag nanoparticles, denoted as Ag-MWCNTs.

[0031] (3) Preparation of Ag-MWCNTs / cellulose hydrogel: 7.2g of microcrystalline cellulose, LiOH and urea were prepared in a lithium hydroxide:urea: The Ag-MWCNTs / cellulose hydrogel was dissolved in deionized water at a mass ratio of 1:2:15 and mechanically stirred in an ice-water bath for 30 min. Then, 0.72 g of carbon nanotubes loaded with Ag nanoparticles was added, and the mixture was stirred vigorously for 1 h. Epichlorohydrin was then added, and the mixture was stirred for another 30 min. The mixture was then transferred to an oven at 60 °C and reacted for 3 h to obtain the Ag-MWCNTs / cellulose hydrogel.

[0032] (4) The hydrogel in step (3) was washed with deionized water until neutral, and then freeze-dried at -40°C to obtain Ag-MWCNTs / cellulose composite material rich in porous structure.

[0033] (5) Preparation of Ag-MWCNTs / cellulose hard carbon anode material: The porous composite material obtained in step (5) is subjected to two-stage carbonization in a box-type atmosphere furnace to obtain hard carbon anode material; the carbonization process is as follows: under argon protection atmosphere, the temperature is raised to 400℃ at 2℃ / min for 2.0h for pre-carbonization, cooled with the furnace and crushed to Dv50: 5-9μm, and then under argon protection atmosphere, the temperature is raised to 1100℃ at 3℃ / min for 3.0h for high-temperature carbonization to obtain hard carbon anode material.

[0034] Example 4 (1) Preparation of carbon nanotubes: 1 g of carbon nanotubes and 200 mL of 0.5 mol / L sulfuric acid were added to a 500 mL three-necked flask and ultrasonically dispersed for 30 min; at the same time, 3 g of potassium permanganate was dissolved in 200 mL of 0.5 mol / L sulfuric acid; the carbon nanotubes and sulfuric acid dispersion were placed in an oil bath at 180 °C, and potassium permanganate sulfuric acid solution was added dropwise under stirring. The reaction was carried out for 5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was collected. The filter cake was added to 30 mL of hydrochloric acid to remove the byproduct manganese dioxide. The mixture was then washed with water until neutral and dried at 80 °C for 12 h to obtain carbon nanotubes.

[0035] (2) Preparation of Ag nanoparticles loaded on carbon nanotubes: 180 mL of dimethyl sulfoxide was added to a three-necked flask under a nitrogen atmosphere and placed in a 20 °C water bath. 6 mL of 0.1176 g / mL sodium citrate was added dropwise while stirring at 200 r / min. At the same time, carbon nanotubes and 180 mL of dimethyl sulfoxide were added sequentially to a beaker and ultrasonically dispersed for 30 minutes. Then, the mixture was slowly added to the three-necked flask and stirred evenly. 24 mL of 0.1 mol / L silver nitrate solution was added dropwise. After the reaction was completed, the mixture was reacted in a 20 °C water bath for 2 h and then transferred to an 80 °C constant temperature water bath for another 1 h. The mixture was then cooled and filtered, and rinsed multiple times with ethanol and pure water. Finally, it was dried at 80 °C for 12 h to obtain carbon nanotubes loaded with Ag nanoparticles, denoted as Ag-MWCNTs.

[0036] (3) Preparation of Ag-MWCNTs / cellulose hydrogel: 7.2g of microcrystalline cellulose, LiOH and urea were prepared in a lithium hydroxide:urea: The solution was dissolved in deionized water at a mass ratio of 1:5:20. The mixture was mechanically stirred in an ice-water bath for 30 min. Then, 0.36 g of carbon nanotubes loaded with Ag nanoparticles was added. After vigorous stirring for 1 h, epichlorohydrin was added. After stirring for another 30 min, the mixture was transferred to a 60 °C oven and reacted for 3 h to obtain Ag-MWCNTs / cellulose hydrogel.

[0037] (4) The hydrogel in step (3) was washed with deionized water until neutral, and then freeze-dried at -40°C to obtain Ag-MWCNTs / cellulose composite material rich in porous structure.

[0038] (5) Preparation of Ag-MWCNTs / cellulose hard carbon anode material: The porous composite material obtained in step (4) is subjected to two-stage carbonization in a box-type atmosphere furnace to obtain the hard carbon anode material. The carbonization process is as follows: under helium protection atmosphere, the temperature is raised to 700℃ at 5℃ / min for 4.0h for pre-carbonization, cooled in the furnace and crushed to Dv50: 5-9μm, and then under helium protection atmosphere, the temperature is raised to 1600℃ at 6℃ / min for 7.0h for high-temperature carbonization to obtain the hard carbon anode material.

[0039] Comparative Example 1 (1) Preparation of cellulose hydrogel: 7.2g of microcrystalline cellulose, 5.5g of LiOH and 18g of urea were dissolved in 96mL of deionized water, mechanically stirred in an ice-water bath for 30min, and then stirred vigorously for 1h. After that, 4.8mL of epichlorohydrin was added, and the mixture was stirred for another 30min. Then it was transferred to an oven at 60℃ and reacted for 3h to obtain cellulose hydrogel.

[0040] (2) The hydrogel in step (1) was washed with deionized water until neutral, and then freeze-dried at -40°C to obtain a cellulose three-dimensional material rich in porous structure.

[0041] (3) Preparation of cellulose hard carbon anode material: The three-dimensional porous material obtained in step (2) is subjected to two-stage carbonization in a box-type atmosphere furnace to obtain hard carbon anode material; the carbonization process is as follows: under nitrogen protection atmosphere, the temperature is raised to 600℃ at 3℃ / min for 2.0h for pre-carbonization, cooled in the furnace and crushed to Dv50: 5-9μm, and then under nitrogen protection atmosphere, the temperature is raised to 1300℃ at 5℃ / min for 4.0h for high-temperature carbonization to obtain hard carbon anode material.

[0042] Comparative Example 2 Compared with Example 1, step (2) is omitted, and the carbon nanotubes added in step (3) are the carbon oxide nanotubes obtained in step (1). The remaining steps are the same as in Example 1.

[0043] To verify the effectiveness of the invention, the inventive team conducted a series of experiments, as follows: The hard carbon materials prepared in the above examples and comparative examples were used as negative electrode materials for sodium-ion batteries, and their electrochemical performance was tested using the following methods: The hard carbon anode material, sodium carboxymethyl cellulose, conductive agent, and binder prepared in Examples 1-2 and Comparative Examples 1-2 were weighed in a mass ratio of 95:0.96:1.74:2.3 and added in batches to a mixing tank containing a small amount of water. The mixture was stirred until it became a black paste. The paste was then coated onto a copper foil current collector to serve as a test electrode. A coin cell was assembled using a sodium metal sheet as the counter electrode. The electrolyte was a 1M sodium hexafluorophosphate solution dissolved in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC). 5 wt.% of fluoroethylene carbonate (FEC) was added to the electrolyte as an additive. Glass fiber was used as the separator. Capacity and initial coulombic efficiency tests were conducted at a constant rate of 0.1C within a voltage range of 0.01-3.0V.

[0044] Figure 1 and Figure 2The images show atomic force microscopy (AFM) and X-ray diffraction (XRD) patterns of Ag nanoparticles grown in situ on carbon nanotubes. The AFM image clearly shows many small black dots evenly distributed on the carbon nanotubes. To confirm that the black dots are Ag nanoparticles, X-ray diffraction analysis was performed. The spectrum shows strong diffraction peaks representing Ag crystals, indicating that the present invention has successfully deposited uniform Ag nanoparticles on carbon nanotubes.

[0045] Figure 3 The image shows a field emission scanning electron microscope (FET) image of the Ag-MWCNTs-reinforced cellulose hard carbon anode material prepared in Example 1. It can be seen from the image that the material surface contains pores. Thanks to the cross-linking of cellulose and the addition of carbon nanotubes, the crystallinity of cellulose is reduced, which promotes the formation of disordered layer structure and voids.

[0046] The charge-discharge curves of the Ag-MWCNTs-reinforced cellulose hard carbon anode material prepared in Example 1 are as follows: Figure 4 As can be seen from the figure, the hard carbon anode material prepared in Example 1 of the present invention has a small impedance, which is due to the addition of Ag-MWCNTs, which enhances the conductivity of the hard carbon anode material and improves the kinetic performance of the material.

[0047] Table 1 Electrochemical performance of biomass-based hard carbon composite anode materials ; As shown in Table 1, the Ag-MWCNTs-reinforced cellulose hard carbon anode material prepared in this invention exhibits higher specific capacity and first-pass coulombic efficiency. This is because the addition of Ag-MWCNTs in the examples enhances the interaction between the hydroxyl and carboxyl groups on the oxidized carbon nanotubes and cellulose, promoting the uniform dispersion of carbon nanotubes. Simultaneously, it can participate in the cross-linking of cellulose, reducing its crystallinity and promoting the formation of disordered layers and voids, resulting in superior capacity compared to pure cellulose hard carbon anode material. Furthermore, the addition of carbon nanotubes significantly improves the conductivity of the cellulose hard carbon anode material, reduces impedance, and minimizes side reactions. In contrast, the hard carbon anode material prepared in the comparative examples did not contain carbon nanotubes loaded with Ag nanoparticles; it only exhibited self-interaction and cross-linking of cellulose, resulting in higher impedance and lower specific capacity and first-pass coulombic efficiency.

[0048] Although the present invention has been described in detail above with general descriptions, specific embodiments, and experiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing Ag-MWCNTs-reinforced cellulose hard carbon anode material, characterized in that, Includes the following steps: Step 1: Oxidize carbon nanotubes using potassium permanganate to obtain carbon nanotubes rich in hydroxyl and carboxyl groups: Carbon nanotubes and 200 mL of 0.5 mol / L sulfuric acid were added sequentially to a 500 mL three-necked flask and ultrasonically dispersed for 30 min. At the same time, the oxidizing agent was dissolved in 200 mL of 0.5 mol / L sulfuric acid. The carbon nanotubes and sulfuric acid dispersion were placed in an oil bath at 120℃-180℃ and the oxidizing agent sulfuric acid solution was added dropwise under stirring to carry out oxidation. The reaction was carried out for 5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was collected. The cake was added to 30 mL of hydrochloric acid to remove byproducts, then washed with water until neutral, and dried at 80℃ for 12 h to obtain oxidized carbon nanotubes. Step 2: Using sodium citrate as a reducing agent and dimethyl sulfoxide as a solvent, silver nitrate is reduced to obtain Ag nanoparticles grown in situ on carbon nanotubes in Step 1. The resulting product is denoted as Ag-MWCNTs. 180 mL of solvent was added to a three-necked flask under a nitrogen atmosphere and placed in a 20 °C water bath. A reducing agent was added dropwise while stirring at 200 r / min. At the same time, carbon oxide nanotubes and 180 mL of solvent were added sequentially to a beaker and ultrasonically dispersed for 30 minutes. Then, the mixture was slowly added to the three-necked flask and stirred until homogeneous. Silver nitrate solution was then added dropwise. After the reaction was completed, the mixture was subjected to a reduction reaction. The mixture was then cooled and filtered, and washed multiple times with ethanol and pure water. Finally, it was dried at 80 °C for 12 h to obtain Ag-MWCNTs. The reduction reaction conditions are as follows: react in a water bath at 10-20℃ for 2 hours, then transfer to a constant temperature water bath at 60-80℃ and continue the reaction for 1 hour; Step 3: Cellulose was dissolved using lithium hydroxide and urea, and the Ag nanoparticle-loaded carbon nanotubes obtained in Step 2 were added. After crosslinking with epichlorohydrin, Ag-MWCNTs / cellulose hydrogel was obtained. Microcrystalline cellulose, LiOH, and urea were dissolved in deionized water and mechanically stirred in an ice-water bath for 30 min. Then Ag-MWCNTs were added and stirred vigorously for 1 h. After that, a crosslinking agent was added and stirred for another 30 min. The mixture was then transferred to a 60°C oven and reacted for 3 h to obtain Ag-MWCNTs / cellulose hydrogel. Step 4: Wash the hydrogel obtained in Step 3 with water until it is neutral, and then freeze-dry it at -40°C to obtain a porous three-dimensional material; Step 5: Place the porous structure material obtained in Step 4 into a box-type atmosphere furnace and pre-carbonize it under a protective atmosphere. After cooling in the furnace, crush it to a particle size Dv50 of 5-9 μm. Then, perform high-temperature carbonization under a protective atmosphere to obtain the Ag-MWCNTs reinforced cellulose hard carbon anode material. The pre-carbonization temperature is 400℃-700℃, the heating rate is 2~5℃ / min, and the holding time is 2h-4h; the high-temperature carbonization temperature is 1100℃-1600℃, the heating rate is 3~6℃ / min, and the carbonization time is 3h-7h.

2. The preparation method according to claim 1, characterized in that, The carbon nanotubes mentioned in step 1 are multi-walled carbon nanotubes, and the oxidizing agent is potassium permanganate. The mass ratio of carbon nanotubes to potassium permanganate is 1:1~3.

3. The preparation method according to claim 1, characterized in that, The oxidation temperature described in step 1 is 150°C.

4. The preparation method according to claim 1, characterized in that, The solvent in step 2 is dimethyl sulfoxide, and the mass ratio of carbon nanotubes: silver nitrate: sodium citrate is 1:0.5~2:0.1~2.

5. The preparation method according to claim 1, characterized in that, The reduction reaction conditions described in step 2 are: reacting in a water bath at 20°C for 2 hours, then transferring to a constant temperature water bath at 60°C for another 1 hour.

6. The preparation method according to claim 1, characterized in that, The cellulose dissolution system described in step 3 is lithium hydroxide, urea, and Lithium hydroxide: Urea: The mass ratio of cellulose to the solution system is 1:2~5:15~20, and the mass ratio of cellulose to the solution system is 1:10~20; the crosslinking agent is epichlorohydrin, and the amount of crosslinking agent used for cellulose is 0.67 mL / g; the mass ratio of cellulose to Ag-MWCNTs is 10:0.1~1.

7. The preparation method according to claim 1, characterized in that, The protective gas for pre-carbonization and high-temperature carbonization in step 5 is at least one of nitrogen, argon, and helium.

8. The preparation method according to claim 1, characterized in that, The pre-carbonization temperature in step 5 is 600℃, the heating rate is 3℃ / min, and the holding time is 2h; the high-temperature carbonization temperature is 1300℃, the heating rate is 5℃ / min, and the carbonization time is 4h.

9. An Ag-MWCNTs-reinforced cellulose hard carbon anode material, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. The application of the Ag-MWCNTs-reinforced cellulose hard carbon anode material as described in claim 9 as an anode material for sodium-ion batteries.

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