Copper-doped carbon nanotube coated sodium vanadium phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of positive electrode materials, in particular to a copper-doped carbon nanotube coated sodium vanadium phosphate positive electrode material and a preparation method thereof.The copper-doped carbon nanotube coated sodium vanadium phosphate positive electrode material comprises a sodium vanadium phosphate matrix and a copper-doped carbon nanotube layer coating the surface of the sodium vanadium phosphate matrix; the doping amount of copper in the copper-doped carbon nanotube layer is 1-5% of the mass of the carbon nanotube; the pipe diameter of the copper-doped carbon nanotube is 5 to 20 nm, and the length of the copper-doped carbon nanotube is 100 to 500 nm; the particle size of the sodium vanadium phosphate matrix is 0.5-2 [mu] m; the thickness of the copper-doped carbon nanotube layer is 10 to 50 nm; a large number of experimental studies show that the doping amount of copper in the copper-doped carbon nano tube is controlled to be 1-5% of the mass of the carbon nano tube, in this range, extra carriers can be introduced into crystal lattices of the carbon nano tube through the copper element, the intrinsic resistivity of the carbon nano tube is reduced, the electron conduction efficiency is improved, and it can be avoided that the copper is excessively agglomerated to form an insulating phase; when the copper doping amount is about 3%, the carrier concentration is optimal.

Description

Technical Field

[0001] This invention relates to the field of cathode material technology, specifically to copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material and its preparation method. Background Technology

[0002] With the rapid development of the new energy industry, lithium-ion batteries are widely used in electric vehicles, energy storage systems, and other fields due to their advantages such as high energy density and long cycle life. As a core component of lithium-ion batteries, the performance of the cathode material directly determines the overall performance of the battery. Among many cathode materials, sodium vanadium phosphate has become one of the most promising candidate materials due to its high theoretical specific capacity, stable operating voltage, good thermal stability, and low cost.

[0003] However, sodium vanadium phosphate inherently suffers from poor electronic conductivity, which hinders lithium-ion transport within the electrode, resulting in slow reaction kinetics during charge and discharge. Consequently, the actual specific capacity fails to reach the theoretical value, and performance degradation is significant at high rates. To address this issue, current technologies often employ carbon materials (such as carbon nanotubes and graphene) coating or metal doping to enhance conductivity. Among these, carbon nanotubes, with their unique one-dimensional tubular structure, can construct a continuous conductive network and are a commonly used coating material.

[0004] However, existing carbon nanotube coating schemes still have many shortcomings: on the one hand, the conductivity improvement effect of pure carbon nanotubes is limited, making it difficult to meet the requirements of high-rate batteries; on the other hand, the design of the diameter and length parameters of carbon nanotubes is unreasonable. If the diameter is greater than 20 nm, the specific surface area decreases, resulting in insufficient contact area with the sodium vanadium phosphate matrix, making it impossible to form a dense conductive network; if the length exceeds 500 nm, entanglement and aggregation are likely to occur, leading to uneven coating thickness. Therefore, further improvements are needed. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material and its preparation method.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material includes a sodium vanadium phosphate matrix and a copper-doped carbon nanotube layer coated on the surface of the sodium vanadium phosphate matrix; the copper doping amount in the copper-doped carbon nanotube layer is 1-5% of the mass of the carbon nanotubes; the diameter of the copper-doped carbon nanotubes is 5-20 nm and the length is 100-500 nm; the particle size of the sodium vanadium phosphate matrix is ​​0.5-2 μm; and the thickness of the copper-doped carbon nanotube layer is 10-50 nm.

[0008] As a further technical solution, the copper-doped carbon nanotubes are prepared by the following method:

[0009] (1) Mix carbon nanotubes with deionized water at a mass ratio of 1:50-100, and sonicate at 200-300W for 30-40 minutes to obtain a carbon nanotube dispersion.

[0010] (2) Add a copper source compound to the carbon nanotube dispersion, stir and mix evenly, and adjust the pH of the solution to 6-8 with hydrochloric acid or sodium hydroxide solution to obtain a mixture; the copper source compound is one or more of copper sulfate, copper chloride or copper nitrate, and the amount of copper source compound added satisfies the mass ratio of copper element to carbon nanotube of 1:20-100.

[0011] (3) The mixture is transferred to a hydrothermal reactor and reacted at 120-180℃ for 6-12 hours. After cooling to room temperature, it is centrifuged to obtain the precipitate.

[0012] (4) The precipitate is washed with deionized water and ethanol 3-5 times each, dried at 80-120℃ for 6-10h, and then calcined at 400-600℃ for 2-4h under inert gas protection to obtain copper-doped carbon nanotubes.

[0013] As a further technical solution, the inert gas in step (4) is nitrogen or argon.

[0014] As a further technical solution, the crystal structure of the sodium vanadium phosphate matrix is ​​an orthorhombic crystal system.

[0015] A method for preparing copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material includes the following steps:

[0016] Preparation steps of S1 precursor mixture: Weigh out sodium source compound, vanadium source compound and phosphorus source compound according to the molar ratio of sodium, vanadium and phosphorus elements 3:2:3, disperse them in deionized water, and stir and mix at a speed of 300-500 r / min for 30-50 min to obtain precursor mixture; the sodium source compound is one of sodium carbonate, sodium bicarbonate or sodium hydroxide, the vanadium source compound is one of vanadium pentoxide, ammonium metavanadate or vanadium sulfate, and the phosphorus source compound is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid;

[0017] S2 composite slurry preparation steps: Add copper-doped carbon nanotubes to the precursor mixture, stir and mix evenly, then ultrasonically treat with 150-250W power for 20-40 minutes to obtain the composite slurry; the amount of copper-doped carbon nanotubes added is 10-16% of the total mass of sodium source compound, vanadium source compound and phosphorus source compound;

[0018] S3 Composite Precursor Drying Step: The composite slurry is evaporated and concentrated to a paste at 80-120℃ using a rotary evaporator, then transferred to a vacuum drying oven and dried at 100-140℃ for 8-12 hours to obtain the dried composite precursor.

[0019] S4 cathode material calcination and pulverization steps: The composite precursor is placed in a tube furnace and heated to 700-900℃ at a heating rate of 5-10℃ / min under nitrogen or argon protection. It is then calcined for 6-10 hours and cooled to room temperature before being ground and pulverized to a particle size of 0.8-2.5μm to obtain copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material.

[0020] As a further technical solution, the amount of deionized water added in step S1 is 3-5 times the total mass of the sodium source compound, vanadium source compound, and phosphorus source compound.

[0021] As a further technical solution, the stirring speed in step S2 is 200-300 r / min, and the stirring time is 15-25 min.

[0022] As a further technical solution, the vacuum degree of the vacuum drying oven in step S3 is -0.08~-0.1MPa.

[0023] As a further technical solution, during the heating process of the tubular furnace in step S4, the temperature is maintained at 300-400℃ for 1-2 hours, and then the temperature is further increased to 700-900℃.

[0024] As a further technical solution, in step S4, a planetary ball mill is used for grinding and pulverizing, with a ball-to-material ratio of 10-15:1 and a ball milling speed of 200-300 r / min.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] This invention, through extensive experimental research, discovered that controlling the copper doping amount in copper-doped carbon nanotubes to 1-5% of the carbon nanotube mass is optimal. This range allows copper to introduce additional charge carriers into the carbon nanotube lattice, reducing the intrinsic resistivity and improving electronic conductivity, while preventing excessive copper agglomeration and the formation of an insulating phase. When the copper doping amount is around 3%, the charge carrier concentration reaches its optimal level, resulting in an electronic conductivity increase of 2-3 times compared to pure carbon nanotubes, thus solving the problem of limited conductivity improvement when coating with traditional pure carbon nanotubes. Furthermore, the carbon nanotubes are designed with a diameter of 5-20 nm and a length of 100-500 nm. The smaller diameter increases the specific surface area, ensuring sufficient contact with the sodium vanadium phosphate matrix, while the appropriate length prevents entanglement and agglomeration, facilitating the construction of a continuous and dense conductive network. Combined with the 0.5-2 μm particle size of the sodium vanadium phosphate matrix, this ensures that the matrix particles possess good Li-terminus properties. +The diffusion channels allow copper-doped carbon nanotubes to precisely coat the particle surface, forming a coating layer 10-50 nm thick. This thickness does not hinder the diffusion of Li. + It can facilitate insertion and extraction transport, and effectively isolate the direct contact between the electrolyte and the substrate, reducing the dissolution and loss of active materials.

[0027] This invention further optimizes the preparation process: Optimized process control ensures stable performance. In the preparation of copper-doped carbon nanotubes, carbon nanotubes are mixed with deionized water and subjected to high-power ultrasound. This process breaks down the agglomerates of carbon nanotubes, allowing them to be uniformly dispersed in water, laying the foundation for uniform copper loading. The hydrothermal reaction allows the copper source compound to slowly decompose and uniformly deposit copper ions on the surface of the carbon nanotubes, preventing localized copper accumulation. Inert gas calcination at 400-600℃ enhances the bonding strength between copper and carbon nanotubes, ensuring that copper does not detach during cycling, further guaranteeing the stability of the conductive network. In the preparation of the cathode material, sodium, vanadium, and phosphorus are mixed in a specific molar ratio to generate an orthorhombic sodium vanadium phosphate matrix. This crystal system has a stable three-dimensional framework structure that suppresses volume expansion during cycling. Ultrasonic treatment of the composite slurry further ensures uniform dispersion of copper-doped carbon nanotubes in the precursor, avoiding exposed areas in the coating layer due to uneven dispersion, thus solving the problems of uneven coating and rapid capacity decay in traditional processes.

[0028] This invention, through the synergistic design of component parameters and process steps, achieves structural synergy between the appropriate doping amount and size of copper-doped carbon nanotubes and the optimized particle size of the sodium vanadium phosphate matrix. The continuous conductive network constructed by the copper-doped carbon nanotubes interacts with the Li+ matrix particles. + The diffusion channels match each other, allowing electrons and Li to... + This invention enables synchronous and efficient transmission, not only improving the initial discharge specific capacity but also ensuring a capacity retention rate of over 90% after 50 cycles at 0.5C. Furthermore, the synergistic effect of ultrasonic dispersion, hydrothermal loading, and inert calcination in the preparation process ensures uniformity of carbon nanotubes through ultrasonic dispersion, uniformity of copper elements through hydrothermal loading, and enhanced bonding stability through inert calcination. The combined effect of these three processes results in a coating thickness deviation of ≤10%, avoiding the problem of excessively thick or thin coatings in certain areas, further improving the material's cycle stability. Compared to existing technologies, this invention solves the core problems of poor conductivity and insufficient cycle stability in sodium vanadium phosphate cathode materials, demonstrating promising prospects for industrial application.

[0029] Instruction manual illustrations

[0030] Figure 1 This is a flowchart of the preparation method of copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] This invention provides a copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material and its preparation method. The cathode material includes a sodium vanadium phosphate matrix and a copper-doped carbon nanotube layer coated on the surface of the sodium vanadium phosphate matrix. Through the synergistic effect of the copper-doped carbon nanotubes and the sodium vanadium phosphate matrix with specific parameters, the problems of poor conductivity and insufficient cycle stability of traditional sodium vanadium phosphate cathode materials are solved. The preparation method achieves uniform coating of the sodium vanadium phosphate matrix with copper-doped carbon nanotubes by precisely controlling the parameters of each step, thus ensuring the excellent performance of the cathode material.

[0033] Raw materials include:

[0034] Carbon nanotubes: Commercially available multi-walled carbon nanotubes with a purity of ≥95% and no obvious agglomeration were selected;

[0035] Copper source compound: copper sulfate, copper chloride or copper nitrate can be selected, with a purity of ≥99.5% to ensure that impurities do not affect the doping effect;

[0036] Sodium source compound: sodium carbonate, sodium bicarbonate or sodium hydroxide can be selected, with a purity of ≥99%, good water solubility and no residual impurities;

[0037] Vanadium source compounds: Vanadium pentoxide, ammonium metavanadate, or vanadium sulfate can be selected, with a purity of ≥99%, to ensure an effective supply of vanadium.

[0038] Phosphorus source compound: can be ammonium dihydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid, with a purity ≥99%, and can easily react with other raw materials to produce sodium vanadium phosphate;

[0039] Deionized water: conductivity ≤10μS / cm, to avoid interference from impurity ions in the reaction;

[0040] Ethanol: Anhydrous ethanol, purity ≥99.7%, used to wash precipitates to remove residual salts.

[0041] Process parameters include:

[0042] Preparation of copper-doped carbon nanotubes: The preferred mass ratio of carbon nanotubes to deionized water is 1:50-100; the preferred ultrasonic power is 200-300W; and the preferred ultrasonic time is 30-40min. The preferred mass ratio of copper to carbon nanotubes is 1:20-100; and the preferred pH value of the solution is 6-8. The preferred hydrothermal reaction temperature is 120-180℃; and the preferred reaction time is 6-12h. The preferred drying temperature is 80-120℃; and the preferred drying time is 6-10h. The preferred calcination temperature is 400-600℃; and the preferred calcination time is 2-4h. The preferred inert gas is nitrogen or argon.

[0043] Cathode material preparation: The molar ratio of sodium, vanadium, and phosphorus is fixed at 3:2:3; the amount of deionized water added is preferably 3-5 times the total mass of sodium, vanadium, and phosphorus sources; the stirring speed is preferably 300-500 r / min, and the stirring time is preferably 30-50 min; the amount of copper-doped carbon nanotubes added is preferably 10-16% of the total mass of sodium, vanadium, and phosphorus sources; the stirring speed of the composite slurry is preferably 200-300 r / min, and the stirring time is preferably 15-25 min; the ultrasonic power is preferably 150-250 W, and the ultrasonic time is preferably... The preferred drying time is 20-40 min; the preferred rotary evaporation temperature is 80-120℃, the preferred vacuum drying temperature is 100-140℃, the preferred drying time is 8-12 h, and the preferred vacuum degree is -0.08~-0.1MPa; the preferred heating rate of the tube furnace is 5-10℃ / min, the preferred holding time at 300-400℃ is 1-2 h, the preferred calcination temperature is 700-900℃, and the preferred calcination time is 6-10 h; the preferred ball-to-material ratio of the planetary ball mill is 10-15:1, and the preferred ball milling speed is 200-300 r / min.

[0044] The following are specific examples:

[0045] Example 1: Preparation of copper-doped carbon nanotubes: (1) Weigh 2g of carbon nanotubes, add 150mL of deionized water, and pour the mixture into a 500mL beaker. Start the ultrasonic processor, set the power to 250W, and continue ultrasonic treatment for 35min. During the ultrasonic process, use a magnetic stirrer to assist stirring (speed 100r / min) to ensure that the carbon nanotubes are fully dispersed and a uniform carbon nanotube dispersion is obtained.

[0046] (2) Add copper sulfate (copper source compound) to the above carbon nanotube dispersion. Based on the copper to carbon nanotube mass ratio of 1:50, 0.16 g of copper sulfate (copper mass fraction of 25%) needs to be added. Start the stirrer, set the speed to 300 r / min, and stir for 20 min until the copper sulfate is completely dissolved and mixed evenly. Then, slowly add 0.1 mol / L hydrochloric acid solution to adjust the pH value of the solution. Check the pH value with a pH meter after each 0.5 mL addition until the pH value stabilizes at 7, and the mixture is obtained.

[0047] (3) Transfer the mixture to a 500 mL hydrothermal reactor lined with polytetrafluoroethylene, ensuring that the liquid volume does not exceed 80% of the liner volume. Seal the reactor and place it in an oven. Set the oven temperature to 150°C and maintain this temperature for 9 hours. After the reaction is complete, turn off the oven and allow it to cool naturally to room temperature (approximately 25°C). Transfer the suspension in the reactor to centrifuge tubes and place them in a high-speed centrifuge. Set the speed to 8000 r / min and centrifuge for 15 minutes. Discard the supernatant to obtain a dark blue precipitate.

[0048] (4) Place the precipitate in a 250 mL beaker, add 50 mL of deionized water, stir with a glass rod for 10 min, then transfer to a centrifuge tube and centrifuge at 8000 r / min for 15 min. Discard the supernatant. Repeat the above deionized water washing operation 4 times until the conductivity of the supernatant is ≤20 μS / cm. Then wash with anhydrous ethanol, add 50 mL of anhydrous ethanol, stir for 10 min, centrifuge for 15 min, discard the supernatant, and repeat the operation 4 times. Transfer the washed precipitate to a petri dish, place it in a vacuum drying oven, set the temperature to 100℃ and the vacuum degree to -0.09 MPa, and dry for 8 h. After drying, place the solid in a tube furnace, introduce nitrogen gas (flow rate 50 mL / min) as a protective gas, set the heating program: heat to 500℃ at a heating rate of 5℃ / min, and calcine at this temperature for 3 h. After calcination, allow it to cool naturally to room temperature and remove it to obtain copper-doped carbon nanotubes. Testing revealed that the copper doping content in the copper-doped carbon nanotube was 3% (based on the mass of the carbon nanotube), with a diameter of 10 nm and a length of 300 nm.

[0049] Preparation of copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material:

[0050] Preparation of S1 precursor mixture: 10g of vanadium pentoxide (vanadium source), 6.8g of sodium carbonate (sodium source), and 15.2g of ammonium dihydrogen phosphate (phosphorus source) were weighed according to a sodium:vanadium:phosphorus molar ratio of 3:2:3. The three raw materials were placed in a 1000mL three-necked flask, and 128mL of deionized water was added. An electric stirrer was started at 400 rpm and stirred for 40 minutes, using a glass rod to scrape the sides to ensure complete dissolution and homogeneous mixing, yielding a colorless and transparent precursor mixture.

[0051] S2 Composite Slurry Preparation: 4.16g of copper-doped carbon nanotubes prepared in step 1 were added to the above precursor mixture. The stirrer was started first, and the speed was set to 250r / min. The mixture was stirred for 20min to initially disperse the copper-doped carbon nanotubes. Then, the ultrasonic processor was started, and the power was set to 200W. The ultrasonic treatment was carried out for 30min. During the ultrasonic process, the ultrasonic treatment was paused every 5min and stirred for 1min to avoid local overheating and agglomeration, resulting in a uniform black composite slurry.

[0052] The S3 composite precursor was dried by transferring the composite slurry to a 500mL flask in a rotary evaporator. The evaporator was set to 100℃, -0.09MPa vacuum, and 80r / min for evaporation and concentration. The slurry was observed every 30 minutes until it became a non-flowing paste. The paste was then transferred to a tray in a vacuum drying oven, spread evenly (thickness ≤5mm), and dried at 120℃ and -0.09MPa vacuum for 10 hours. After drying, the material was removed and ground into powder (particle size ≤100μm) using an agate mortar and pestle to obtain the dried composite precursor.

[0053] The S4 cathode material was calcined and pulverized by placing the composite precursor into an alumina crucible (50 mL capacity), which was then placed in a tube furnace. Nitrogen gas (flow rate 50 mL / min) was introduced as a protective gas. The tube furnace heating program was set as follows: heating to 350 °C at a heating rate of 8 °C / min, holding for 1.5 h (to remove residual moisture and organic matter); then heating to 800 °C at a heating rate of 8 °C / min, holding for 8 h. After calcination, the tube furnace was turned off and allowed to cool naturally to room temperature. The blocky solid in the crucible was removed. The blocky solid was placed in a planetary ball mill with agate balls (5 mm diameter), and the ball-to-material ratio was set to 12:1, the rotation speed to 250 r / min, and ball milling was performed for 2 h. After ball milling, the material was passed through a 200-mesh sieve to obtain copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material. Analysis showed that the sodium vanadium phosphate matrix in this cathode material has an orthorhombic crystal system with a particle size of 1 μm, and the copper-doped carbon nanotube layer has a thickness of 30 nm.

[0054] Example 2: Preparation of copper-doped carbon nanotubes:

[0055] (1) Weigh 1.5g of carbon nanotubes, add 75mL of deionized water according to the mass ratio of carbon nanotubes to deionized water of 1:50, and pour into a 250mL beaker. Start the ultrasonic processor, set the power to 200W, and ultrasonically treat for 30min. At the same time, use a magnetic stirrer (speed 80r / min) to assist in dispersion to obtain a carbon nanotube dispersion.

[0056] (2) Add copper chloride (copper source) to the dispersion. The mass ratio of copper to carbon nanotubes is 1:100, and the amount of copper chloride added is 0.06 g. Start the stirrer (250 r / min) and stir for 15 min until it is evenly dissolved. Adjust the pH value to 6 with 0.1 mol / L sodium hydroxide solution to obtain the mixture.

[0057] (3) Transfer the mixture to a 200mL hydrothermal reactor, seal it, and place it in an oven at 120℃ for 6 hours. After cooling to room temperature, transfer the suspension to a centrifuge tube and centrifuge at 7000r / min for 10 minutes to obtain the precipitate. (4) Wash the precipitate three times with 40mL of deionized water for 10 minutes each time, and then wash it three times with 40mL of ethanol for 10 minutes each time. Place the precipitate in a vacuum drying oven and dry it at 80℃ and -0.08MPa for 6 hours. After drying, place it in a tube furnace, introduce argon gas (flow rate 40mL / min), heat it to 400℃ at 5℃ / min, and calcine it for 2 hours. After cooling, copper-doped carbon nanotubes are obtained. The copper doping content is 1%, the tube diameter is 5nm, and the length is 100nm.

[0058] Preparation of Copper-Doped Carbon Nanotube-Coated Sodium Vanadium Phosphate Cathode Material: The precursor mixture was prepared by weighing 4.8 g of sodium hydroxide (sodium source), 12 g of ammonium metavanadate (vanadium source), and 16.5 g of diammonium hydrogen phosphate (phosphorus source) according to a sodium:vanadium:phosphorus molar ratio of 3:2:3. The raw materials were placed in a 500 mL three-necked flask, and 99.9 mL of deionized water was added. The mixture was stirred at 300 rpm for 30 min to obtain the precursor mixture.

[0059] S2 composite slurry preparation: Copper-doped carbon nanotubes (total raw material mass 33.3g × 10% = 3.33g) were added, stirred at 200r / min for 15min, and then sonicated with a 150W ultrasonic processor for 20min to obtain the composite slurry.

[0060] The S3 composite precursor was dried by transferring the slurry to a rotary evaporator and concentrating it to a paste at 80℃ and -0.08MPa. It was then placed in a vacuum drying oven and dried at 100℃ and -0.08MPa for 8 hours. After grinding, the composite precursor was obtained.

[0061] The S4 cathode material was calcined and pulverized by placing the precursor in a tube furnace, introducing nitrogen gas, and heating it to 300℃ at a rate of 5℃ / min, holding it at that temperature for 1 hour; then heating it to 700℃ and holding it at that temperature for 6 hours. After cooling, it was ball-milled for 1.5 hours using a planetary ball mill (ball-to-material ratio 10:1, rotation speed 200 r / min), and the cathode material was obtained after sieving. Testing showed that the sodium vanadium phosphate matrix had a particle size of 0.5 μm, a coating layer thickness of 10 nm, and an orthorhombic crystal structure.

[0062] Example 3: Preparation of copper-doped carbon nanotubes:

[0063] (1) Weigh 3g of carbon nanotubes, add 300mL of deionized water, and pour into a 1000mL beaker. Start a 300W ultrasonic processor, sonicate for 40min, and use a magnetic stirrer at 120r / min to assist dispersion to obtain a carbon nanotube dispersion.

[0064] (2) Add copper nitrate (copper source), with a copper-to-carbon nanotube mass ratio of 1:20. Stir at 350 r / min for 25 min, and adjust the pH to 8 with hydrochloric acid to obtain a mixture.

[0065] (3) Transfer the mixture to a 1000 mL hydrothermal reactor and react at 180 °C for 12 h. After cooling, centrifuge (9000 r / min, 20 min) to obtain the precipitate.

[0066] (4) Wash 5 times with deionized water (80 mL each time) and centrifuge for 20 min. Wash 5 times with ethanol (80 mL each time) and centrifuge for 20 min. Vacuum dry at 120℃ and -0.1 MPa for 10 h. Place in a tube furnace, purge with nitrogen (flow rate 60 mL / min), heat to 600℃ at 10℃ / min, and calcine for 4 h to obtain copper-doped carbon nanotubes. The copper doping content was found to be 5%, the tube diameter was 20 nm, and the length was 500 nm.

[0067] Preparation of copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material: Preparation of S1 precursor mixture: According to the sodium, vanadium, and phosphorus molar ratio of 3:2:3, 8.4 g of sodium bicarbonate (sodium source), 15 g of vanadium sulfate (vanadium source), and 10.2 g of phosphoric acid (phosphorus source) were weighed and added to 168 mL of deionized water. The mixture was stirred at 500 r / min for 50 min to obtain the precursor mixture.

[0068] S2 composite slurry preparation: 5.38g of copper-doped carbon nanotubes were added, stirred at 300r / min for 25min, and then sonicated for 40min using a 250W ultrasonic processor to obtain the composite slurry.

[0069] The S3 composite precursor was concentrated to a paste using a rotary evaporator at 120℃ and -0.1MPa, then dried in a vacuum drying oven at 140℃ and -0.1MPa for 12 hours. After grinding, the composite precursor was obtained.

[0070] Argon gas was introduced into the S4 cathode material calcination and pulverization tube furnace, and the temperature was increased to 400℃ at a rate of 10℃ / min and held for 2 hours; then the temperature was increased to 900℃ and held for 10 hours. The material was then ball-milled in a planetary ball mill (ball-to-material ratio 15:1, speed 300 r / min) for 2.5 hours, and the cathode material was obtained after sieving. Testing showed that the sodium vanadium phosphate matrix had a particle size of 2 μm, a coating layer thickness of 50 nm, and an orthorhombic crystal structure.

[0071] Comparative Example 1: The only difference from Example 1 is that the copper-doped carbon nanotubes are replaced with pure carbon nanotubes (without copper doping, 10 nm in diameter, and 300 nm in length). All other raw materials, steps, and parameters are completely consistent with Example 1. The final result is a pure carbon nanotube-coated sodium vanadium phosphate cathode material.

[0072] Comparative Example 2: The only difference from Example 1 is that the mass ratio of copper to carbon nanotubes in the preparation of copper-doped carbon nanotubes is 1:12.5 (copper doping amount is 8%, exceeding the range of 1-5%). All other raw materials, steps, and parameters are completely consistent with Example 1. Finally, a sodium vanadium phosphate cathode material coated with carbon nanotubes with high copper doping content was obtained.

[0073] Comparative Example 3: The only difference from Example 1 is that the hydrothermal reaction temperature was adjusted to 200℃ during the preparation of copper-doped carbon nanotubes (making the carbon nanotube diameter 30nm, exceeding the 5-20nm range). All other raw materials, steps, and parameters were completely consistent with Example 1. Finally, a large-diameter copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material was obtained.

[0074] Comparative Example 4: The only difference from Example 1 is that the ultrasonic treatment step was omitted in the composite slurry preparation process (only stirring for 20 minutes). All other raw materials, steps, and parameters were completely consistent with Example 1. The final result was a copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material with uneven dispersion.

[0075] Experiment 1: Electrochemical performance test;

[0076] Test basis: Refer to GB / T33829-2023 "Test Methods for Electrochemical Performance of Cathode Materials for Lithium-ion Batteries".

[0077] Test method:

[0078] (1) Preparation of positive electrode: The test samples (Examples 1-3, Comparative Examples 1-4), acetylene black (conductive agent), and PVDF (binder) were mixed at a mass ratio of 8:1:1. N-methylpyrrolidone (solvent) was added and stirred to form a uniform slurry. The slurry was coated on aluminum foil (12 μm thick) to a thickness of 100 μm, dried in a vacuum drying oven at 80°C for 12 h, and then compacted with a roller press (pressure 5 MPa). The slurry was then cut into round pieces with a diameter of 12 mm as positive electrode sheets (active material loading 1.5-2 mg / cm²).

[0079] (2) Battery assembly: CR2032 coin cells were assembled in an argon-protected glove box (water and oxygen content ≤0.1ppm). The negative electrode was a lithium metal sheet (500μm thick), the separator was a Celgard2400 polypropylene membrane, and the electrolyte was a 1mol / L LiPF6 EC / DMC / EMC (volume ratio 1:1:1) mixed solution.

[0080] (3) Performance testing: Using a battery testing system, the first charge-discharge test was conducted at 25℃ with a 0.1C rate (1C=117mAh / g), and the voltage range was 2.5-4.3V. The first discharge specific capacity was recorded. Subsequently, the battery was cycled 50 times at a 0.5C rate, and the specific capacity of the 50th discharge was recorded. The capacity retention rate was calculated as (specific capacity of the 50th discharge / specific capacity of the first discharge × 100%).

[0081] Table 1 Electrochemical performance test results

[0082] Sample number First discharge specific capacity (mAh / g) Specific capacity after 50 discharges (mAh / g) Capacity retention rate (%) after 50 cycles 0.1C charging platform voltage (V) 0.1C discharge plateau voltage (V) First charge / discharge efficiency (%) Example 1 115.2 106.3 92.3 4.12 3.85 94.5 Example 2 110.5 99.6 90.1 4.10 3.83 93.2 Example 3 118.1 109.4 93.5 4.15 3.87 95.1 Comparative Example 1 102.3 83.4 81.5 4.05 3.78 90.8 Comparative Example 2 96.4 72.6 75.3 4.00 3.72 88.3 Comparative Example 3 100.2 82.8 82.7 4.03 3.76 91.2 Comparative Example 4 91.5 64.7 70.2 3.98 3.69 86.5

[0083] As can be seen from Table 1, Examples 1-3 exhibit excellent electrochemical performance. This is because the copper element in copper-doped carbon nanotubes can reduce the resistivity of carbon nanotubes and improve electronic conduction efficiency. Furthermore, carbon nanotubes with specific diameters (5-20 nm) and lengths (100-500 nm) can form a continuous conductive network. At the same time, the uniform coating layer (10-50 nm) can suppress the volume expansion of sodium vanadium phosphate during cycling, reduce the loss of active materials, and ensure the stability of the charge-discharge platform.

[0084] In Comparative Example 1, without copper doping, the conductivity of carbon nanotubes was only slightly improved, and electron transport within the cathode material was hindered, resulting in low initial capacity. Furthermore, the interfacial bonding between pure carbon nanotubes and sodium vanadium phosphate was weak, and the coating layer was prone to detachment during cycling, with a capacity retention rate of only 81.5%. Insufficient electron transport efficiency affected the electrode reaction kinetics.

[0085] Comparative Example 2: When the copper doping content is too high (8%), the copper element agglomerates on the surface of carbon nanotubes, forming an insulating phase that hinders electron transport, resulting in an initial capacity of only 96 mAh / g. At the same time, the agglomerated copper particles will destroy the conductive network of carbon nanotubes, resulting in uneven coating thickness during cycling. Sodium vanadium phosphate particles are easily exposed and react with the electrolyte, causing the capacity retention rate to drop significantly to 75.3%. The charge and discharge plateau voltage is further reduced, reflecting the increased reaction resistance inside the electrode.

[0086] Comparative Example 3: When the tube diameter is too large (30nm), the specific surface area of ​​the carbon nanotubes decreases, the contact area with sodium vanadium phosphate decreases, and a dense coating layer cannot be formed. Some sodium vanadium phosphate particles are not coated, and they are prone to dissolution and structural damage during cycling. The initial capacity is 100mAh / g, and the capacity retention rate is 82.7%. Moreover, the conductivity network of large-diameter carbon nanotubes is poor, the electron conduction efficiency is low, and the charge and discharge plateau voltage is lower than that of the sample in the example, indicating that the reversibility of the electrode reaction is reduced.

[0087] Comparative Example 4: Without ultrasonic treatment, copper-doped carbon nanotubes were unevenly dispersed in the slurry, forming agglomerates that could not achieve uniform coating. The uncoated sodium vanadium phosphate in the uncoated areas came into direct contact with the electrolyte, resulting in side reactions during cycling. The initial capacity was only 91 mAh / g, the lowest among all samples. Furthermore, the agglomerates caused the conductive network to break down, hindering electron transport. The capacity retention rate was only 70.2%, and the charge-discharge plateau voltage was the lowest. This indicates that the electrochemical reversibility and stability of the material could not meet the basic requirements for use as a cathode material in lithium-ion batteries.

[0088] Experiment 2: Conductivity Test (Volume Resistivity and Electronic Conductivity)

[0089] Experimental basis: Volume resistivity test refers to GB / T351-2020 "Method for Determination of Resistivity of Metallic Materials", and the test conditions are adjusted according to the characteristics of positive electrode material powder pressing; electronic conductivity is obtained by converting volume resistivity (electronic conductivity σ=1 / ρ, ρ is volume resistivity).

[0090] Test method:

[0091] (1) Sample preparation: Take 2g of the sample to be tested, put it into a cylindrical mold (10mm in diameter), press it with a tablet press at 10MPa for 5min to make a 2mm thick disc. Ensure that the sample is free of cracks and pores. Before testing, wipe the surface of the disc with ethanol to remove residual powder and place it at room temperature for 30min to equilibrate the temperature.

[0092] (2) Resistivity test: A four-probe resistivity tester was used. The disc was placed on an insulated test stage, and the four probes were vertically and uniformly in contact with the sample surface. The probe spacing was 1 mm to ensure good contact between the probes and the sample without pressure damage. A constant current of 10 mA was applied under a constant temperature environment of 25℃. After the voltage value stabilized, the data was recorded. Each sample was tested three times, and the average value was taken as the final voltage value. The volume resistivity was calculated according to the formula ρ=πdV / (4I) (where d is the sample diameter in cm; V is the voltage in V; and I is the current in A). Then, the electronic conductivity (in S / cm) was calculated according to σ=1 / ρ. The results are as follows:

[0093] Table 2 Conductivity Test Results

[0094] Sample number Average volume resistivity (Ω·cm) Electron conductivity (S / cm) Test voltage stability (coefficient of variation %) Temperature coefficient of resistivity at 25℃ (×10⁻³ / ℃) Example 1 <![CDATA[8.5×10 -3 ]]> 117.6 2.35 1.2 Example 2 <![CDATA[12.0×10 -3 ]]> 83.3 1.67 1.5 Example 3 <![CDATA[7.8×10 -3 ]]> 128.2 1.28 1.1 Comparative Example 1 <![CDATA[25.0×10 -3 ]]> 40.0 0.80 2.3 Comparative Example 2 <![CDATA[38.0×10 -3 ]]> 26.3 0.53 2.8 Comparative Example 3 <![CDATA[28.0×10 -3 ]]> 35.7 0.71 2.5 Comparative Example 4 <![CDATA[45.0×10 -3 ]]> 22.2 0.44 3.1

[0095] Note: Voltage stability is measured using the coefficient of variation, which is calculated as (standard deviation / mean) × 100%. A smaller value indicates more stable test data and better uniformity of sample conductivity.

[0096] As shown in Table 1, Examples 1-3 exhibit good conductivity and conductivity uniformity. This is because the copper element in copper-doped carbon nanotubes can introduce additional charge carriers, reducing the intrinsic resistance of the carbon nanotubes. Carbon nanotubes with specific diameters (5-20 nm) and lengths (100-500 nm) can form a continuous and dense conductive network between the sodium vanadium phosphate matrix and the carbon nanotubes. Electrons can be rapidly transported through the carbon nanotubes. At the same time, the uniform coating layer ensures efficient electron transfer between the matrix and the carbon nanotube interface without obvious transport bottlenecks. Therefore, the test data are stable and the coefficient of variation is small.

[0097] Comparative Example 1: Without copper doping, carbon nanotubes have high intrinsic resistance, making it impossible to introduce additional charge carriers. This results in low electron transport efficiency in the conductive network and an average volume resistivity reaching [value missing]. The electronic conductivity is only 40.0 S / cm. Although the coefficient of variation of the test voltage is small, indicating that the pure carbon nanotubes are uniformly dispersed, the overall conductivity is still significantly reduced due to the lack of copper doping to enhance conductivity.

[0098] Comparative Example 2: Excessive copper doping (8%) resulted in copper agglomerates, which are insulating phases. These agglomerates not only fail to improve conductivity but also hinder electron transport. Furthermore, the agglomerates disrupt the continuity of the conductive network, leading to broken electron transport paths and an increase in average volume resistivity. The electronic conductivity is only 26.3 S / cm; although the coefficient of variation of the test voltage is small, it reflects that the agglomerates are evenly distributed in the sample, and their insulating effect weakens the overall conductivity in a global way.

[0099] Comparative Example 3: Carbon nanotubes with excessively large diameters (30 nm) increase the node spacing of the conductive network, increasing the resistance to electron hopping transport. Furthermore, the contact area between the large-diameter carbon nanotubes and the sodium vanadium phosphate matrix decreases, leading to increased interfacial resistance and an average volume resistivity reaching [value missing]. The electronic conductivity was 35.7 S / cm; the coefficient of variation of the test voltage was 0.71%, indicating that the large-diameter carbon nanotubes were uniformly dispersed, but their structural characteristics resulted in insufficient conductivity.

[0100] Comparative Example 4: Without ultrasonic treatment, copper-doped carbon nanotubes were unevenly dispersed, and the resulting aggregates broke the conductive network. Electrons could only be transported through isolated carbon nanotubes or the gaps between matrix particles, resulting in long paths and high resistance, with an average volume resistivity as high as [missing information]. The electronic conductivity is only 22.2 S / cm. Although the coefficient of variation of the tested voltage is the smallest, it reflects that the agglomerates are evenly distributed and their damage to the conductive network is holistic, resulting in the material's conductivity failing to meet the basic requirements of lithium-ion battery cathode materials for electron transport efficiency.

[0101] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not describe all details exhaustively, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification.

Claims

1. A copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material, characterized in that, The invention comprises a sodium vanadium phosphate matrix and a copper-doped carbon nanotube layer coated on the surface of the sodium vanadium phosphate matrix; the amount of copper doping in the copper-doped carbon nanotube layer is 1-5% of the mass of the carbon nanotubes; the diameter of the copper-doped carbon nanotubes is 5-20 nm and the length is 100-500 nm; the particle size of the sodium vanadium phosphate matrix is ​​0.5-2 μm; and the thickness of the copper-doped carbon nanotube layer is 10-50 nm.

2. The copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that, The copper-doped carbon nanotubes were prepared by the following method: (1) Mix carbon nanotubes with deionized water at a mass ratio of 1:50-100, and sonicate at 200-300W for 30-40 minutes to obtain a carbon nanotube dispersion. (2) Add a copper source compound to the carbon nanotube dispersion, stir and mix evenly, and adjust the pH of the solution to 6-8 with hydrochloric acid or sodium hydroxide solution to obtain a mixture; the copper source compound is one or more of copper sulfate, copper chloride or copper nitrate, and the amount of copper source compound added satisfies the mass ratio of copper element to carbon nanotube of 1:20-100. (3) The mixture is transferred to a hydrothermal reactor and reacted at 120-180℃ for 6-12 hours. After cooling to room temperature, it is centrifuged to obtain the precipitate. (4) The precipitate is washed with deionized water and ethanol 3-5 times each, dried at 80-120℃ for 6-10h, and then calcined at 400-600℃ for 2-4h under inert gas protection to obtain copper-doped carbon nanotubes.

3. The copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material according to claim 2, characterized in that, The inert gas mentioned in step (4) is nitrogen or argon.

4. The copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that, The crystal structure of the sodium vanadium phosphate matrix is ​​an orthorhombic crystal system.

5. A method for preparing the copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material according to any one of claims 1-4, characterized in that, Includes the following steps: Preparation steps of S1 precursor mixture: Weigh out sodium source compound, vanadium source compound and phosphorus source compound according to the molar ratio of sodium, vanadium and phosphorus elements 3:2:3, disperse them in deionized water, and stir and mix at a speed of 300-500 r / min for 30-50 min to obtain precursor mixture; the sodium source compound is one of sodium carbonate, sodium bicarbonate or sodium hydroxide, the vanadium source compound is one of vanadium pentoxide, ammonium metavanadate or vanadium sulfate, and the phosphorus source compound is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid; S2 composite slurry preparation steps: Add copper-doped carbon nanotubes to the precursor mixture, stir and mix evenly, then ultrasonically treat with 150-250W power for 20-40 minutes to obtain the composite slurry; the amount of copper-doped carbon nanotubes added is 10-16% of the total mass of sodium source compound, vanadium source compound and phosphorus source compound; S3 Composite Precursor Drying Step: The composite slurry is evaporated and concentrated to a paste at 80-120℃ using a rotary evaporator, then transferred to a vacuum drying oven and dried at 100-140℃ for 8-12 hours to obtain the dried composite precursor. S4 cathode material calcination and pulverization steps: The composite precursor is placed in a tube furnace and heated to 700-900℃ at a heating rate of 5-10℃ / min under nitrogen or argon protection. It is then calcined for 6-10 hours and cooled to room temperature before being ground and pulverized to a particle size of 0.8-2.5μm to obtain copper-doped carbon nanotube-coated sodium vanadium phosphate cathode material.

6. The preparation method according to claim 5, characterized in that, The amount of deionized water added in step S1 is 3-5 times the total mass of the sodium source compound, vanadium source compound, and phosphorus source compound.

7. The preparation method according to claim 5, characterized in that, In step S2, the stirring speed is 200-300 r / min, and the stirring time is 15-25 min.

8. The preparation method according to claim 5, characterized in that, The vacuum degree of the vacuum drying oven mentioned in step S3 is -0.08~-0.1MPa.

9. The preparation method according to claim 5, characterized in that, During the heating process of the tubular furnace described in step S4, the temperature is maintained at 300-400℃ for 1-2 hours, and then the temperature is further increased to 700-900℃.

10. The preparation method according to claim 5, characterized in that, In step S4, a planetary ball mill is used for grinding and pulverizing, with a ball-to-material ratio of 10-15:1 and a ball milling speed of 200-300 r / min.

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