Preparation method of long-life carbon nanotube doped hard carbon negative electrode material

Abstract

The application relates to a preparation method of a long-life carbon nanotube doped hard carbon negative electrode material. It comprises the following steps: FCNTs material preparation, ultrasonic dispersion, material sensitization, FCNTs@Ag material preparation, and preparation of phosphorus and carbon nanotube co-doped sugar hydrogel derived hard carbon material. The hard carbon negative electrode material is prepared by creatively combining a gel carbonization method, a chemical plating method and a doping method. The prepared material has the characteristics of good cycle performance, high rate and small impedance.

Description

Technical Field

[0001] This invention relates to a method for preparing a long-life carbon nanotube-doped hard carbon anode material, belonging to the field of sodium electrochemical technology. Background Technology

[0002] With the urgent need for large-scale energy storage applications and battery safety, sodium-ion batteries are considered one of the most promising alternatives to lithium-ion batteries due to their abundant reserves, resource sustainability, and advantages in technology transfer. Compared to other anode materials, hard carbon is currently the most suitable anode material for sodium-ion batteries due to its mature process and high sodium storage capacity. However, limited by the intrinsic conductivity and large specific surface area of ​​the material, it still suffers from insufficient rate performance and low cycle stability. Biomass feedstocks are the mainstream hard carbon precursors in the industry due to their comprehensive advantages of high efficiency, low cost, and green sustainability. However, biomass precursors often contain a large number of impurity atoms with uneven distribution, and their state and composition are easily affected by factors such as source, batch, and season. At the same time, differences in the state and composition of the precursor can change the microstructure formed by hard carbon, thus affecting the electrochemical performance of sodium-ion batteries. In addition, biomass materials suffer from high energy consumption and low carbon yield during carbonization. Most common biomass materials have a carbon yield of less than 15%, and the carbonization temperature needs to be maintained above 1200 ℃. This poses a challenge to the large-scale production of high-quality hard carbon by directly carbonizing a certain biomass precursor.

[0003] Existing commercially available CFx materials are mostly prepared using traditional natural graphite as the carbon source via high-temperature gas fluorination. When the degree of fluorination is high, the resulting material surface contains a large number of inert C-F2 and C-F3 bonds. Furthermore, with increasing fluorination intensity during high-temperature fluorination, the CF bond type transforms into strongly polar covalent bonds, leading to low electrical conductivity. When low-conductivity CFx materials are used in batteries, the batteries exhibit low electron and ion transport rates during discharge, accompanied by significant voltage hysteresis, making it difficult to achieve high-rate discharge. To date, these batteries have only been applied in certain specialized fields, and due to the inherent limitations of the materials, they cannot meet the high-power discharge requirements of electronic devices in today's society.

[0004] To further improve the conductivity and cycle performance of the material, metallic silver was modified onto the surface of FCNTs using chemical plating to obtain FCNTs@Ag composite cathode material doped with hard carbon anode material. Summary of the Invention

[0005] To address the above problems, this invention provides a method for preparing long-life carbon nanotube-doped hard carbon anode materials.

[0006] This invention relates to a method for preparing a long-life carbon nanotube-doped hard carbon anode material, comprising the following steps: preparation of FCNTs material, ultrasonic dispersion, material sensitization, preparation of FCNTs@Ag material, and preparation of phosphorus and carbon nanotube co-doped sugar hydrogel-derived hard carbon material.

[0007] Preferably, step (1) involves the preparation of FCNTs materials. FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination reactor. F2 / N2 mixed gas was used as the fluorine source to fluorinate and obtain FCNTs materials. Step (2) Ultrasonic dispersion Disperse the FCNTs from step (1) in an ethanol solution, sonicate and then vacuum filter. Add the filtered material back into the ethanol solution for dispersion and stir for a certain period of time. Step (3) Material sensitization Add the redispersed solution from step (2) to the sensitizer, sensitize, wash with ethanol, centrifuge with deionized water, filter and disperse in ethanol for later use. Step (4) Preparation of FCNTs@Ag materials Dissolve AgNO3 in deionized water, add KOH according to a certain molar ratio, and then add ammonia water dropwise until a clear silver ammonia solution is formed; add the turbid liquid obtained in step (3) into the silver ammonia solution, stir in the dark, add reducing agent and continue stirring, and finally wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation; Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials Add N,N-methylenebisacrylamide and acrylamide to a glucose solution. After the solution is clear, add phosphoric acid and the material from step (4) to the glucose solution and add triethanolamine-ammonium persulfate initiator. Remove the bubbles in the solution and place the solution in an oven to wait for the gelation reaction to complete. After the solution is completely gelled, heat the oven and keep it warm. Break the pre-carbonized gel into blocks, place them in a ball mill jar, ball mill them, add ethanol and ball mill them. Place the sieved powder in a graphite crucible, heat and keep it warm in a tube furnace with an argon atmosphere, and finally obtain the hard carbon material.

[0008] Preferably, step (1) involves the preparation of FCNTs materials. FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 9-10 hours with an F2 / N2 mixed gas as the fluorine source and the fluorination temperature was controlled at 400-500℃ to obtain FCNTs materials.

[0009] The fluorination temperature and time are set to ensure that the material has a high F / C ratio, so that the final FCNTs material with both high fluorine content and high electrical conductivity can be prepared.

[0010] The advantages of this invention are that the FCNTs material obtained after high-temperature fluorination retains a high degree of the original CNT structure. The three-dimensional network structure formed by the interwoven carbon nanotubes provides a shorter path for ion diffusion, ensuring a high fluorine content (F / C=1) while also possessing a certain degree of conductivity. When the temperature is too low and the time is short, some unfluorinated or poorly fluorinated graphitized layered structures exist in the FCNTs, which is beneficial for promoting the transfer of ions and electrons. The high porosity and the interwoven three-dimensional network structure ensure rapid ion diffusion during battery discharge. At the same time, the large specific surface area provides more active sites for electrochemical reactions, which can effectively improve the rate performance of the battery. However, when the temperature is too high and the time is long, the original carbon nanotubes expand to a certain extent, resulting in a significant reduction in the volume of these pores.

[0011] Preferably, step (2) involves ultrasonic dispersion. Disperse 1-3 g of FCNTs from step (1) in an ethanol solution, sonicate for 1-3 h, and then vacuum filter. Add the filtered material back into 10-20 mL of ethanol solution for dispersion and stir for 10-20 min.

[0012] Preferably, step (3) involves material sensitization. Add the redispersed solution from step (2) to 80-100 mL of sensitizer, sensitize for 1-2 h, wash with ethanol 3-5 times, and centrifuge with deionized water at 3000-5000 rpm for 3-5 min each time. Then filter and disperse in 10-20 mL of ethanol for later use.

[0013] Preferably, step (4) involves the preparation of FCNTs@Ag materials. Dissolve 5-10g of AgNO3 in 80-100mL of deionized water, add KOH according to the molar ratio of AgNO3:KOH=500:1-500:100, and then add ammonia water dropwise until a clear silver ammonia solution is formed; add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 200-300 rpm for 5-10 min in the dark at 20-30℃, add 20-30 mL of reducing agent and continue stirring for 25-30 min, and finally wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0014] The advantage of using this invention is that modifying the surface of FCNTs with silver nanoparticles can effectively improve the electrical conductivity of the material, and the electrical conductivity of the material increases with the increase of silver content on the surface of the material.

[0015] The advantages of using this invention are that chemical silver plating modification can also improve the energy density of the battery, and silver modification on the surface of FCNTs can improve the diffusion rate of ions in the electrode and also effectively improve the charge transfer rate.

[0016] Preferably, step (5) involves the preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials. Add 0.6-1 g of N,N-methylenebisacrylamide and 10-20% acrylamide to a glucose solution (20-25 g of glucose monohydrate and 60-70 g of deionized water). After the solution becomes clear, add 30-40 g of phosphoric acid and 20-30 g of the material from step (4) to the glucose solution, and further add triethanolamine-ammonium persulfate initiator. Remove the air bubbles in the solution and place the solution in an oven at 60-80 ℃ to wait for the gelation reaction to complete. After the solution has completely gelled, raise the temperature of the oven to 150-200 ℃ at 1-5 ℃ / min and keep it at that temperature for 4-5 hours. Break the pre-carbonized gel into blocks and place them in a ball mill jar. First, ball mill at 400-500 r / min for 20-30 min, then add ethanol at 300-400 r / min. Ball milling at r / min for 8-10 hours, then placing the sieved powder in a graphite crucible and heating it to 700-800 ℃ in a tube furnace under an argon atmosphere at 2-4 ℃ / min and holding it at that temperature for 2-4 hours to finally obtain hard carbon material.

[0017] The advantages of using this invention are that the semi-circular diameter of the phosphorus-doped hard carbon material is reduced in the high-frequency region, the phosphorus doping can effectively reduce the charge transfer resistance, provide less resistance during battery cycling, improve the cycle performance of the battery, enhance the migration speed of sodium ions, and further improve the sodium storage capacity.

[0018] Among these, the graphitization degree of hard carbon materials decreases with increasing phosphorus content. This is because the introduction of phosphorus atoms leads to lattice distortion in the carbon matrix. Phosphorus atoms have a significantly larger covalent radius than carbon, and when they replace carbon lattice positions or embed themselves in interlayer spaces, they force an increase in the distance between carbon layers through steric repulsion. Furthermore, phosphorus doping sites form localized electron-rich regions, weakening the π-π interactions between adjacent carbon layers and reducing interlayer van der Waals forces, thereby promoting interlayer spacing and disrupting the original ordered sp² hybrid structure, thus increasing defect density. Simultaneously, phosphorus exists on the carbon surface or in pores as oxygen-containing functional groups, which disrupt the continuity of carbon layers, introducing additional disordered structures and further increasing defect concentration.

[0019] The addition of phosphoric acid enhances the cross-linking degree of acrylamide to some extent, resulting in a more compact gel system and less carbonaceous gas leakage. Simultaneously, phosphorus doping promotes the formation of mesoporous structures from some micropores. Furthermore, with increasing phosphorus content, excess phosphorus reacts with the carbon matrix to generate polyphosphates or cross-linked COP bonds, effectively improving the mechanical strength of the carbon skeleton and achieving an enhanced degree of cross-linking.

[0020] The advantages of using this invention are that phosphorus-doped hard carbon has a higher reversible capacity. The addition of phosphorus increases the active sites of the hard carbon material, allowing more sodium ions to be stored. The addition of phosphoric acid enhances the crosslinking degree of acrylamide, resulting in a more compact gel system and less carbonaceous gas leakage.

[0021] The advantage of this invention is that the incorporation of phosphorus forms sp³ hybrid bonds (PC bonds) with carbon atoms to a certain extent, and these PC bonds lead to local lattice distortion. The lattice size mismatch results in the generation of an internal strain field, thereby increasing the defect concentration inside the carbon layer. Therefore, this effectively increases the adsorption sites for sodium ions in the material, which is beneficial for improving the material's sodium storage capacity in the slope region.

[0022] During the high-temperature carbonization process, some excess phosphorus exists in the form of phosphate, phosphorus oxide, or PC bonds. The incorporation of phosphorus will form sp³ hybrid bonds (PC bonds) with carbon atoms to a certain extent. As the phosphorus doping amount increases, the graphitization degree of hard carbon materials further decreases, and a P 2p peak appears after doping, proving that phosphorus has been successfully doped into hard carbon materials.

[0023] Preferably, in step (3), the sensitizer consists of 10-20 g / L SnCl2 and 5-8 mL / L HCl, and the solvent is ethanol / water solution (v:v=1-2).

[0024] Preferably, the reducing agent in step (4) is a mixed aqueous solution of 14-15 g / L, which consists of trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is 2-4 times that of silver nitrate.

[0025] When the silver content in the material is low, increasing the silver content can improve the electrochemical performance of the material. At this time, the conductivity improvement brought by silver nanoparticles is dominant. However, when the concentration of silver ions in the plating solution is too high, the silver deposition rate will also increase. At this time, the silver particles on the material surface will exhibit obvious agglomeration. These agglomerated silver particles block the original ion diffusion channels of the material and reduce the contact area between the active material and the electrolyte, resulting in an increase in the reaction barrier during battery discharge. As the discharge rate increases, the electrode polarization becomes more obvious, indicating that the particle deposition rate is faster as the concentration of silver precursor increases. If an appropriate silver ion concentration is selected, agglomeration can be avoided.

[0026] The advantage of using this invention is that silver nanoparticles with high electrical conductivity, which are added again, are loaded onto the surface of FCNTs through a simple chemical plating method.

[0027] In summary, the beneficial effects of this invention are as follows: 1. The advantage of using this invention is that modifying the surface of FCNTs with silver nanoparticles can effectively improve the electrical conductivity of the material, and the electrical conductivity of the material increases with the increase of silver content on the surface of the material.

[0028] 2. The advantage of using this invention is that chemical silver plating modification can also improve the energy density of the battery. Silver modification on the surface of FCNTs can improve the diffusion rate of ions in the electrode and can also effectively improve the charge transfer rate.

[0029] 3. The advantages of using this invention are that the semi-circular diameter of the phosphorus-doped hard carbon material is reduced in the high-frequency region. The phosphorus doping can effectively reduce the charge transfer resistance, provide less resistance during battery cycling, improve the cycle performance of the battery, increase the migration speed of sodium ions, and further improve the sodium storage capacity.

[0030] 4. The advantages of using this invention are that the phosphorus-doped hard carbon has a higher reversible capacity. The addition of phosphorus increases the active sites of the hard carbon material, thus allowing more sodium ions to be stored. The addition of phosphoric acid enhances the crosslinking degree of acrylamide, resulting in a more compact gel system and less carbonaceous gas leakage.

[0031] 5. The advantage of using this invention is that the incorporation of phosphorus to a certain extent forms sp³ hybrid bonds (PC bonds) with carbon atoms, and these PC bonds lead to local lattice distortion. The lattice size mismatch results in the generation of an internal strain field, thereby increasing the defect concentration inside the carbon layer. Therefore, it can effectively increase the adsorption sites for sodium ions in the material, which is beneficial to improving the sodium storage capacity of the material in the slope region. Attached Figure Description

[0032] Figure 1 This is a flowchart illustrating a long-life carbon nanotube-doped hard carbon anode material and its preparation method. Detailed Implementation

[0033] Example 1

[0034] like Figure 1 As shown, the long-life carbon nanotube-doped hard carbon anode material and its preparation method include: Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 9 h with an F2 / N2 mixed gas as the fluorine source and the fluorination temperature was controlled at 400℃ to obtain FCNTs materials.

[0035] Step (2) Ultrasonic dispersion Disperse 1 g of FCNTs from step (1) in an ethanol solution, sonicate for 1 h, and then vacuum filter. Add the filtered material back into 10 mL of ethanol solution for dispersion and stir for 10 min.

[0036] Step (3) Material sensitization Add the redispersed solution from step (2) to 80 mL of sensitizer, sensitize for 1 h, wash 3 times with ethanol, centrifuge 3 times with deionized water at 3000 rpm for 3 min each time, then filter and disperse in 10 mL of ethanol for later use.

[0037] Step (4) Preparation of FCNTs@Ag materials Dissolve 5g of AgNO3 in 80 mL of deionized water, add KOH at a molar ratio of AgNO3:KOH=500:1, and then add ammonia dropwise until a clear silver ammonia solution is formed. Add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 200 rpm for 5 min in the dark at 20°C, add 20 mL of reducing agent and continue stirring for 25 min. Finally, wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0038] Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 0.6 g of N,N-methylenebisacrylamide and 10% acrylamide were added to a glucose solution (20 g glucose monohydrate and 60 g deionized water). After the solution was clear, 30 g of phosphoric acid and 20 g of the material from step (4) were added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 60 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 150 °C at 1 °C / min and kept at that temperature for 4 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 400 r / min for 20 min, and then ethanol was added and ball-milled at 300 r / min for 8 hours. The sieved powder was placed in a graphite crucible and heated to 700 °C at 2 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 2 hours. Finally, hard carbon material was obtained.

[0039] In step (3), the sensitizer consists of 10 g / L SnCl2 and 5 mL / L HCl, and the solvent is ethanol / water solution (v:v=1).

[0040] Step (4) The reducing agent is a 14 g / L mixed aqueous solution, which consists of: trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is twice that of silver nitrate.

[0041] Example 2

[0042] like Figure 1 As shown, the long-lifetime carbon nanotube-doped hard carbon anode material and its preparation method include: step (1) preparation of FCNTs material. FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 9 h with an F2 / N2 mixed gas as the fluorination source and the fluorination temperature was controlled at 440℃ to obtain FCNTs materials.

[0043] Step (2) Ultrasonic dispersion Disperse 2 g of FCNTs from step (1) in an ethanol solution, sonicate for 2 h, and then vacuum filter. Add the filtered material back into 15 mL of ethanol solution for dispersion and stir for 15 min.

[0044] Step (3) Material sensitization Add the redispersed solution from step (2) to 88 mL of sensitizer, sensitize for 1 h, wash 4 times with ethanol, centrifuge 4 times with deionized water at 4000 rpm for 4 min each time, then filter and disperse in 15 mL of ethanol for later use.

[0045] Step (4) Preparation of FCNTs@Ag materials Dissolve 8 g of AgNO3 in 88 mL of deionized water, add KOH according to the molar ratio of AgNO3:KOH=500:65, and then add ammonia water dropwise until a clear silver ammonia solution is formed. Add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 278 rpm for 7 min in the dark at 26 ℃, add 26 mL of reducing agent and continue stirring for 27 min. Finally, wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0046] Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 0.7 g of N,N-methylenebisacrylamide and 15% acrylamide were added to a glucose solution (23 g glucose monohydrate and 65 g deionized water). After the solution was clear, 35 g of phosphoric acid and 26 g of the material from step (4) were added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 72 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 175 °C at 3 °C / min and kept at that temperature for 4 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 470 r / min for 26 min, and then ethanol was added and ball-milled at 350 r / min for 9 hours. The sieved powder was placed in a graphite crucible and heated to 750 °C at 3 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 3 hours. Finally, hard carbon material was obtained.

[0047] In step (3), the sensitizer consists of 15 g / L SnCl2 and 6 mL / L HCl, and the solvent is ethanol / water solution (v:v=1).

[0048] Step (4) The reducing agent is a mixed aqueous solution of 14.3 g / L, which consists of trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is 3 times that of silver nitrate.

[0049] Example 3

[0050] like Figure 1 As shown, the long-life carbon nanotube-doped hard carbon anode material and its preparation method include: Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 10 h with an F2 / N2 mixed gas as the fluorination source and the fluorination temperature was controlled at 480℃ to obtain FCNTs materials.

[0051] Step (2) Ultrasonic dispersion Disperse 3 g of FCNTs from step (1) in an ethanol solution, sonicate for 3 h, and then vacuum filter. Add the filtered material back into 18 mL of ethanol solution for dispersion and stir for 18 min.

[0052] Step (3) Material sensitization Add the redispersed solution from step (2) to 95 mL of sensitizer, sensitize for 2 h, wash 4 times with ethanol, centrifuge 4 times with deionized water at 4500 rpm for 4 min each time, then filter and disperse in 18 mL of ethanol for later use.

[0053] Step (4) Preparation of FCNTs@Ag materials Dissolve 8 g of AgNO3 in 95 mL of deionized water, add KOH according to the molar ratio of AgNO3:KOH=500:80, and then add ammonia water dropwise until a clear silver ammonia solution is formed. Add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 280 rpm for 8 min in the dark at 28℃, add 28 mL of reducing agent and continue stirring for 28 min. Finally, wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0054] Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 0.8 g of N,N-methylenebisacrylamide and 18% acrylamide were added to a glucose solution (24 g glucose monohydrate and 68 g deionized water). After the solution was clear, 38 g of phosphoric acid and 28 g of the material from step (4) were added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 75 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 180 °C at 4 °C / min and kept at that temperature for 5 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 480 r / min for 28 min, and then ethanol was added and ball-milled at 380 r / min for 10 hours. The sieved powder was placed in a graphite crucible and heated to 780 °C at 4 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 4 hours. Finally, hard carbon material was obtained.

[0055] In step (3), the sensitizer consists of 18 g / L SnCl2 and 7 mL / L HCl, and the solvent is ethanol / water solution (v:v=2).

[0056] Step (4) The reducing agent is a mixed aqueous solution of 14.5 g / L, which consists of trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is 4 times that of silver nitrate.

[0057] Example 4

[0058] like Figure 1 As shown, the long-life carbon nanotube-doped hard carbon anode material and its preparation method include: Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 10 h with an F2 / N2 mixed gas as the fluorine source and the fluorination temperature was controlled at 500℃ to obtain FCNTs materials.

[0059] Step (2) Ultrasonic dispersion Disperse 3 g of FCNTs from step (1) in an ethanol solution, sonicate for 3 h, and then vacuum filter. Add the filtered material back into 20 mL of ethanol solution for dispersion and stir for 20 min.

[0060] Step (3) Material sensitization Add the redispersed solution from step (2) to 100 mL of sensitizer, sensitize for 2 h, wash 5 times with ethanol, centrifuge 5 times with deionized water at 5000 rpm for 5 min each time, then filter and disperse in 20 mL of ethanol for later use.

[0061] Step (4) Preparation of FCNTs@Ag materials Dissolve 10g of AgNO3 in 100mL of deionized water, add KOH according to the molar ratio of AgNO3:KOH=500:100, and then add ammonia water dropwise until a clear silver ammonia solution is formed. Add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 300 rpm for 10 min in the dark at 30℃, add 30 mL of reducing agent and continue stirring for 30 min. Finally, wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0062] Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 1 g of N,N-methylenebisacrylamide and 20% acrylamide were added to a glucose solution (25 g glucose monohydrate and 70 g deionized water). After the solution was clear, 40 g of phosphoric acid and 30 g of the material from step (4) were added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 80 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 200 °C at 5 °C / min and kept at that temperature for 5 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 500 r / min for 30 min, and then ethanol was added and ball-milled at 400 r / min for 10 hours. The sieved powder was placed in a graphite crucible and heated to 800 °C at 4 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 4 hours. Finally, hard carbon material was obtained.

[0063] In step (3), the sensitizer consists of 10-20 g / L SnCl2 and 5-8 mL / L HCl, and the solvent is ethanol / water solution (v:v=2).

[0064] Step (4) The reducing agent is a 15 g / L mixed aqueous solution, which consists of: trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is 4 times that of silver nitrate.

[0065] Comparative Example 1

[0066] Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 9 h with an F2 / N2 mixed gas as the fluorine source and the fluorination temperature was controlled at 400℃ to obtain FCNTs materials.

[0067] Step (2) Ultrasonic dispersion Disperse 1 g of FCNTs from step (1) in an ethanol solution, sonicate for 1 h, and then vacuum filter. Add the filtered material back into 10 mL of ethanol solution for dispersion and stir for 10 min.

[0068] Step (3) Material sensitization Add the redispersed solution from step (2) to 80 mL of sensitizer, sensitize for 1 h, wash 3 times with ethanol, centrifuge 3 times with deionized water at 3000 rpm for 3 min each time, then filter and disperse in 10 mL of ethanol for later use.

[0069] Step (4) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 0.6 g of N,N-methylenebisacrylamide and 10% acrylamide were added to a glucose solution (20 g glucose monohydrate and 60 g deionized water). After the solution was clear, 30 g of phosphoric acid and 20 g of the material from step (3) were added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 60 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 150 °C at 1 °C / min and kept at that temperature for 4 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 400 r / min for 20 min, and then ethanol was added and ball-milled at 300 r / min for 8 hours. The sieved powder was placed in a graphite crucible and heated to 700 °C at 2 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 2 hours. Finally, hard carbon material was obtained.

[0070] In step (3), the sensitizer consists of 10 g / L SnCl2 and 5 mL / L HCl, and the solvent is ethanol / water solution (v:v=1).

[0071] Comparative Example 2

[0072] Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 9 h with an F2 / N2 mixed gas as the fluorine source and the fluorination temperature was controlled at 400℃ to obtain FCNTs materials.

[0073] Step (2) Ultrasonic dispersion Disperse 1 g of FCNTs from step (1) in an ethanol solution, sonicate for 1 h, and then vacuum filter. Add the filtered material back into 10 mL of ethanol solution for dispersion and stir for 10 min.

[0074] Step (3) Material sensitization Add the redispersed solution from step (2) to 80 mL of sensitizer, sensitize for 1 h, wash 3 times with ethanol, centrifuge 3 times with deionized water at 3000 rpm for 3 min each time, then filter and disperse in 10 mL of ethanol for later use.

[0075] Step (4) Preparation of FCNTs@Ag materials Dissolve 5g of AgNO3 in 80 mL of deionized water, add KOH at a molar ratio of AgNO3:KOH=500:1, and then add ammonia dropwise until a clear silver ammonia solution is formed. Add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 200 rpm for 5 min in the dark at 20°C, add 20 mL of reducing agent and continue stirring for 25 min. Finally, wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0076] Step (5) Preparation of carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 0.6 g of N,N-methylenebisacrylamide and 10% acrylamide were added to a glucose solution (20 g glucose monohydrate and 60 g deionized water). After the solution was clear, 20 g of the material from step (4) was added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 60 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 150 °C at 1 °C / min and kept at that temperature for 4 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 400 r / min for 20 min, and then ethanol was added and ball-milled at 300 r / min for 8 hours. The sieved powder was placed in a graphite crucible and heated to 700 °C at 2 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 2 hours. Finally, hard carbon material was obtained.

[0077] In step (3), the sensitizer consists of 10 g / L SnCl2 and 5 mL / L HCl, and the solvent is ethanol / water solution (v:v=1).

[0078] Step (4) The reducing agent is a 14 g / L mixed aqueous solution, which consists of: trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is twice that of silver nitrate.

[0079] Comparative Example 3

[0080] Step (1) Ultrasonic dispersion 1 g of CNTs were dispersed in an ethanol solution, sonicated for 1 h, and then vacuum filtered. The filtered material was then added back into 10 mL of ethanol solution for dispersion and stirred for 10 min.

[0081] Step (2) Material sensitization Add the redispersed solution from step (1) to 80 mL of sensitizer, sensitize for 1 h, wash 3 times with ethanol, centrifuge 3 times with deionized water at 3000 rpm for 3 min each time, then filter and disperse in 10 mL of ethanol for later use.

[0082] Step (3) Preparation of CNTs@Ag materials Dissolve 5g of AgNO3 in 80 mL of deionized water, add KOH at a molar ratio of AgNO3:KOH=500:1, and then add ammonia dropwise until a clear silver ammonia solution is formed. Add the turbid liquid obtained in step (2) to the silver ammonia solution, stir at 200 rpm for 5 min in the dark at 20°C, add 20 mL of reducing agent and continue stirring for 25 min. Finally, wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

[0083] Step (4) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials 0.6 g of N,N-methylenebisacrylamide and 10% acrylamide were added to a glucose solution (20 g glucose monohydrate and 60 g deionized water). After the solution was clear, 30 g of phosphoric acid and 20 g of the material from step (3) were added to the glucose solution, and triethanolamine-ammonium persulfate initiator was added. The air bubbles in the solution were removed and the solution was placed in an oven at 60 °C to wait for the gelation reaction to complete. After the solution was completely gelled, the oven was heated to 150 °C at 1 °C / min and kept at that temperature for 4 hours. The pre-carbonized gel was broken into blocks and placed in a ball mill jar. It was first ball-milled at 400 r / min for 20 min, and then ethanol was added and ball-milled at 300 r / min for 8 hours. The sieved powder was placed in a graphite crucible and heated to 700 °C at 2 °C / min in a tube furnace under an argon atmosphere and kept at that temperature for 2 hours. Finally, hard carbon material was obtained.

[0084] In step (2), the sensitizer consists of 10 g / L SnCl2 and 5 mL / L HCl, and the solvent is ethanol / water solution (v:v=1).

[0085] Step (3) The reducing agent is a mixed aqueous solution of 14 g / L, which consists of: trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is twice that of silver nitrate.

[0086] Comparison of detection experiments: The hard carbon materials, FCNTs materials, FCNTs@Ag materials obtained in Examples 1 to 4, and comparative products 1 to 3 were tested using the following specific testing methods: Cyclic voltammetry (CV) test Cyclic voltammetry (CV) was used to systematically analyze hard carbon materials under a voltage window of 0.01–3 V and a scan rate of 0.1 mV / s.

[0087] Electrochemical impedance spectroscopy (EIS) testing AC impedance spectra were plotted using a Shanghai Chenhua CHI660E, with a test frequency range of 0.01-100 kHz.

[0088] Table 1 Cyclic performance test results

[0089] As shown in Table 1, Example 3 is the best, while Comparative Example 3 and the unplated silver FCNTs are worse. After 100 cycles, Example 3 exhibits a specific capacity of 854.12 mAh / g with a capacity retention of 95.6%, and after 500 cycles, the specific capacity is 788.96 mAh / g with a capacity retention of 88.3%. In contrast, Comparative Example 2 shows a capacity retention of 83.4% after 100 cycles and 65.4% after 500 cycles. Compared to the unpowdered hard carbon in Comparative Example 2, the phosphorus-doped hard carbon exhibits a higher capacity. This increase in capacity indicates that the addition of phosphorus increases the active sites in the hard carbon material, allowing for the storage of more sodium ions. When the silver content in the material is low, further increasing the silver content can improve the electrochemical performance of the material. At this point, the conductivity improvement brought by the silver nanoparticles is dominant, indicating that Example 3 has better long-cycle performance.

[0090] Table 2. Discharge specific capacity results at high rates

[0091] As shown in Table 2, Example 3 is the best, while Comparative Example 3 and the unplated silver FCNTs are worse. Silver modification can effectively improve the discharge specific capacity of the battery, and chemical silver plating can also improve the energy density of the battery. When the concentration of silver ions in the plating solution is too high, the silver deposition rate will also increase. At this time, the silver particles on the material surface will exhibit obvious agglomeration. These agglomerated silver particles block the original ion diffusion channels of the material and reduce the contact area between the active material and the electrolyte, resulting in an increase in the reaction barrier during battery discharge. As the discharge rate increases, the electrode polarization becomes more obvious. Therefore, Example 3 is better.

[0092] Table 3 Electrochemical impedance spectroscopy results

[0093] As shown in Table 3, Example 3 exhibits the best performance, while Comparative Example 3 and the unplated FCNTs material perform worse. This indicates that Example 3 possesses the fastest charge transfer rate and the lowest ion diffusion resistance, effectively reducing electrode polarization. On one hand, this demonstrates that silver modification on the FCNTs surface can increase the ion diffusion rate in the electrode, resulting in an increased ion diffusion rate during battery discharge. The conductive carbon generated during the reaction can promote ion transfer. On the other hand, the silver particles on the material surface can effectively increase the charge transfer rate. The semi-circular diameter of the phosphorus-doped hard carbon material is reduced in the high-frequency region. Phosphorus doping effectively reduces the charge transfer resistance, providing less resistance during battery cycling, improving battery cycle performance, enhancing sodium ion migration speed, and further increasing sodium storage capacity.

[0094] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but as long as they are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for preparing a long-life carbon nanotube-doped hard carbon anode material, characterized in that: Preparation of FCNTs materials, ultrasonic dispersion, material sensitization, preparation of FCNTs@Ag materials, preparation of phosphorus and carbon nanotube co-doped sugar hydrogel-derived hard carbon materials.

2. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 1, characterized in that: Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination reactor. F2 / N2 mixed gas was used as the fluorine source to fluorinate and obtain FCNTs materials. Step (2) Ultrasonic dispersion Disperse the FCNTs from step (1) in an ethanol solution, sonicate and then vacuum filter. Add the filtered material back into the ethanol solution for dispersion and stir for a certain period of time. Step (3) Material sensitization Add the redispersed solution from step (2) to the sensitizer, sensitize, wash with ethanol, centrifuge with deionized water, filter and disperse in ethanol for later use. Step (4) Preparation of FCNTs@Ag materials Dissolve AgNO3 in deionized water, add KOH according to a certain molar ratio, and then add ammonia water dropwise until a clear silver ammonia solution is formed; add the turbid liquid obtained in step (3) into the silver ammonia solution, stir in the dark, add reducing agent and continue stirring, and finally wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation; Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials Add N,N-methylenebisacrylamide and acrylamide to a glucose solution. After the solution is clear, add phosphoric acid and the material from step (4) to the glucose solution and add triethanolamine-ammonium persulfate initiator. Remove the bubbles in the solution and place the solution in an oven to wait for the gelation reaction to complete. After the solution is completely gelled, heat the oven and keep it warm. Break the pre-carbonized gel into blocks, place them in a ball mill jar, ball mill them, add ethanol and ball mill them. Place the sieved powder in a graphite crucible, heat and keep it warm in a tube furnace with an argon atmosphere, and finally obtain the hard carbon material.

3. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 2, characterized in that: Step (1) Preparation of FCNTs materials FCNTs materials were prepared using a simple high-temperature gas fluorination process. Commercial CNTs were used as the carbon source and placed in a rotary Monel alloy fluorination furnace. Fluorination was carried out for 9-10 hours with an F2 / N2 mixed gas as the fluorine source and the fluorination temperature was controlled at 400-500℃ to obtain FCNTs materials.

4. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 3, characterized in that: Step (2) Ultrasonic dispersion Disperse 1-3 g of FCNTs from step (1) in an ethanol solution, sonicate for 1-3 h, and then vacuum filter. Add the filtered material back into 10-20 mL of ethanol solution for dispersion and stir for 10-20 min.

5. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 4, characterized in that: Step (3) Material sensitization Add the redispersed solution from step (2) to 80-100 mL of sensitizer, sensitize for 1-2 h, wash with ethanol 3-5 times, and centrifuge with deionized water at 3000-5000 rpm for 3-5 min each time. Then filter and disperse in 10-20 mL of ethanol for later use.

6. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 4, characterized in that: Step (4) Preparation of FCNTs@Ag materials Dissolve 5-10g of AgNO3 in 80-100mL of deionized water, add KOH according to the molar ratio of AgNO3:KOH=500:1-500:100, and then add ammonia water dropwise until a clear silver ammonia solution is formed; add the turbid liquid obtained in step (3) to the silver ammonia solution, stir at 200-300 rpm for 5-10 min in the dark at 20-30℃, add 20-30 mL of reducing agent and continue stirring for 25-30 min, and finally wash the silver-plated material clean and place it in a vacuum drying oven to dry for sample preparation.

7. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 6, characterized in that: Step (5) Preparation of phosphorus and carbon nanotube co-doped saccharide hydrogel-derived hard carbon materials Add 0.6-1 g of N,N-methylenebisacrylamide and 10-20% acrylamide to a glucose solution (20-25 g of glucose monohydrate and 60-70 g of deionized water). After the solution is clear, add 30-40 g of phosphoric acid and 20-30 g of the material from step (4) to the above glucose solution, and further add triethanolamine-ammonium persulfate initiator. Remove the air bubbles in the solution and place the solution in an oven at 60-80 ℃ to wait for the gelation reaction to complete. After the solution is completely gelled, raise the oven temperature to 150-200 ℃ at 1-5 ℃ / min and keep it at that temperature for 4-5 hours. Break the pre-carbonized gel into blocks and place them in a ball mill jar. First, ball mill at 400-500 r / min for 20-30 min, then add ethanol and ball mill at 300-400 r / min for 8-10 hours. Place the sieved powder in a graphite crucible and in a tube furnace under an argon atmosphere at 2-4 ℃. Heat to 700-800℃ at a rate of ℃ / min and hold for 2-4 hours to obtain hard carbon material.

8. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 5, characterized in that: In step (3), the sensitizer consists of 10-20 g / L SnCl2 and 5-8 mL / L HCl, and the solvent is ethanol / water solution (v:v=1-2).

9. The method for preparing a long-life carbon nanotube-doped hard carbon anode material according to claim 6, characterized in that: Step (4) The reducing agent is a mixed aqueous solution of 14-15 g / L, which consists of trisodium citrate dihydrate, silver nitrate and glucose, wherein the molar amount of glucose is 2-4 times that of silver nitrate.

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