A high-rate lithium iron phosphate / carbon nanotube composite cathode material and a preparation process thereof

Abstract

The application relates to the field of positive electrode materials, in particular to a high-rate lithium iron phosphate / carbon nanotube composite positive electrode material and a preparation process thereof, which are used for solving the problems of poor conductivity, slow ion transmission and insufficient cycle stability of traditional lithium iron phosphate, etc.; the preparation method is that aniline-carbazole copolymer / carbon nanotube composite dispersion liquid and Mg and N doped lithium iron phosphate are compounded to obtain lithium iron phosphate coated by aniline-carbazole copolymer and carbon nanotubes; the positive electrode material obtained by the preparation method has excellent performance, can significantly improve electronic conductivity, improve rate performance, accelerate lithium ion transmission, reduce polarization loss, enhance structural stability and prolong cycle life.

Description

Technical Field

[0001] This invention relates to the field of cathode materials, specifically to a high-rate lithium iron phosphate / carbon nanotube composite cathode material and its preparation process. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become the mainstream cathode material in power batteries and large-scale energy storage due to its advantages such as high theoretical capacity, good safety, long cycle life, and low cost. However, its intrinsic electronic conductivity is extremely low and its lithium-ion diffusion coefficient is small, resulting in poor high-rate charge and discharge performance, which makes it difficult to meet the requirements of fast charging of new energy vehicles and high power output of energy storage systems.

[0003] To improve the electrochemical performance of lithium iron phosphate (LFP), current technologies mainly employ carbon coating, elemental doping, and nano-sizing. While carbon coating can improve electronic conductivity, the traditional carbon layer structure is loose, easily increasing the resistance to lithium-ion diffusion. Single-element doping can optimize the crystal structure, but it is difficult to simultaneously solve the problems of electron transport and ion migration. Although nano-sizing can shorten the ion diffusion path, it reduces the material compaction density, leading to a decrease in energy density. In addition, existing composite conductive agents mostly use a simple mixture of carbon black and carbon nanotubes, which suffers from uneven dispersion and discontinuous conductive networks, restricting further improvements in high-rate performance.

[0004] Carbon nanotubes are ideal conductive additives, with conductivity more than 10 times that of copper, and can form a three-dimensional conductive network to reduce electrode internal resistance. However, there are problems such as the inability to uniformly combine carbon nanotubes with lithium iron phosphate and the lack of synergistic effects when using carbon nanotubes alone compared to other modification methods. Summary of the Invention

[0005] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a high-rate lithium iron phosphate / carbon nanotube composite cathode material and its preparation process.

[0006] The objective of this invention can be achieved through the following technical solutions: In a first aspect, this application provides a high-rate lithium iron phosphate / carbon nanotube composite cathode material, comprising the following components by weight: 50-52 parts of lithium iron phosphate doped with Mg and N, and 400-430 parts of aniline-carbazole copolymer / carbon nanotube composite dispersion; The aniline-carbazole copolymer / carbon nanotube composite dispersion is prepared by the following steps: Step a1: Add aniline, 3-amino-9-ethylcarbazole and methanol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel, and purge with nitrogen for protection. Stir the reaction at 25-30℃ and 300-500 r / min for 10-15 min. Then, add sodium persulfate-hydrochloric acid solution dropwise while stirring, controlling the dropping rate to 1-2 drops / s. After the addition is complete, continue stirring for 24-26 h. After the reaction is complete, filter under reduced pressure, wash with deionized water 5-7 times, and then place in a vacuum drying oven and dry at 55-60℃ for 12-13 h to obtain aniline-carbazole copolymer. Step a2: Add carbon nanotubes, concentrated nitric acid, and concentrated sulfuric acid to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Stir the reaction at 75-80℃ and a stirring rate of 250-300 r / min for 6-7 h. After the reaction is complete, cool the reaction product to room temperature, then filter it. Wash the filter cake with deionized water 3-5 times, then place it in a vacuum drying oven and dry it at 55-60℃ for 12-14 h to obtain acidified carbon nanotubes. Step a3: Add the aniline-carbazole copolymer to methanol and ultrasonically disperse it for 30-35 minutes at a speed of 400-450 r / min and a power of 100-150 W to form a dispersion. Then add the acidified carbon nanotubes, increase the speed to 450-500 r / min and ultrasonically disperse it for 40-45 minutes at a power of 200-250 W to obtain the aniline-carbazole copolymer / carbon nanotube composite dispersion.

[0007] In a preferred embodiment of the present invention, the ratio of aniline, 3-amino-9-ethylcarbazole, methanol and sodium persulfate-hydrochloric acid solution used in step a1 is 0.93-0.95g: 2.1-2.5g: 120-150mL: 60-66mL.

[0008] In a preferred embodiment of the present invention, the sodium persulfate-hydrochloric acid solution in step a1 is a solution prepared by mixing sodium persulfate, hydrochloric acid, and deionized water in a ratio of 4.5g:2.5mL:57.5mL; the concentration of the hydrochloric acid is 12mol / L.

[0009] In a preferred embodiment of the present invention, the ratio of carbon nanotubes, concentrated nitric acid and concentrated sulfuric acid used in step a2 is 1-2g: 25-50mL: 75-150mL.

[0010] In a preferred embodiment of the present invention, the carbon nanotubes in step a2 are multi-walled carbon nanotubes with a diameter of 15-20 nm; the mass fraction of the concentrated nitric acid is 65%; and the mass fraction of the concentrated sulfuric acid is 98%.

[0011] In a preferred embodiment of the present invention, the ratio of the aniline-carbazole copolymer, methanol, and acidified carbon nanotubes in step a3 is 4-4.8g: 400-480mL: 1-1.2g.

[0012] The Mg and N-doped lithium iron phosphate is prepared by the following steps: Step b1: FePO4, Li2CO3, Mg(NO3)2·6H2O, urea, and glucose are poured into a planetary ball mill. Anhydrous ethanol is added as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 are used. The milling is carried out at a speed of 250-300 r / min for 4-5 hours to form a slurry. The slurry is transferred to an evaporating dish, and the ethanol is evaporated in a water bath at a temperature of 55-60℃ to obtain the precursor powder. The precursor powder is then... The material was placed in an alumina crucible and then in a tube furnace. High-purity nitrogen was introduced at a rate of 50 mL / min for 30 min. The temperature was then programmed to rise to 350°C at a rate of 5°C / min and held for 2 h. The temperature was then increased to 700-750°C at a rate of 3°C / min and held for 6-8 h. The material was then allowed to cool naturally to room temperature to obtain a black, blocky sintered body. The sintered body was then transferred to an agate mortar and ground for 30-35 min before being passed through a 200-mesh sieve to obtain Mg and N-doped lithium iron phosphate.

[0013] In a preferred embodiment of the present invention, the ratio of FePO4, Li2CO3, Mg(NO3)2·6H2O, urea, glucose and anhydrous ethanol in step b1 is 91.8-99g: 23.4-25g: 3.2-3.4g: 54.8-58g: 2.6-2.8g: 150-180mL.

[0014] Secondly, this application provides a preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, comprising the following steps: Step 1: Weigh out the Mg and N-doped lithium iron phosphate and aniline-carbazole copolymer / carbon nanotube composite dispersion according to the specified weight parts, and set aside. Step 2: Add Mg and N-doped lithium iron phosphate to the aniline-carbazole copolymer / carbon nanotube composite dispersion and ultrasonically disperse for 1-1.5 h at a speed of 550-600 r / min and a power of 150-200 W. Transfer to a rotary evaporator and evaporate under reduced pressure at a temperature of 35-40℃ and a speed of 45-50 r / min. Then place in a vacuum drying oven and dry at a temperature of 55-60℃ for 8-9 h. Grind the solid and pass it through a 200-mesh sieve to obtain high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0015] The beneficial effects of this invention are: This invention discloses a high-rate lithium iron phosphate / carbon nanotube composite cathode material and its preparation process. The method involves combining an aniline-carbazole copolymer / carbon nanotube composite dispersion with Mg and N-doped lithium iron phosphate to obtain lithium iron phosphate co-coated with aniline-carbazole copolymer and carbon nanotubes. The cathode material obtained by this preparation method exhibits excellent performance, significantly improving electronic conductivity and rate performance; accelerating lithium-ion transport and reducing polarization loss; enhancing structural stability and extending cycle life. This solves the problems of poor conductivity, slow ion transport, and insufficient cycle stability inherent in traditional lithium iron phosphate cathodes.

[0016] High-rate lithium iron phosphate / carbon nanotube composite cathode materials can significantly improve electronic conductivity and rate performance. Magnesium doping into the LiFePO4 lattice can replace part of the Li. + or Fe 2+ Nitrogen doping introduces free electrons into the lattice, further enhancing bulk electronic conductivity. Aniline-carbazole copolymers are conjugated conductive polymers with high conductivity. The coating can tightly wrap around the surface of lithium iron phosphate particles, forming a continuous "electron transport channel" to prevent interruption of electron transport between particles. Carbon nanotubes have extremely high electron mobility and a one-dimensional nanostructure, which can be interwoven into the material to form a "three-dimensional conductive framework," complementing the polymer coating and opening up electron transport paths between particles.

[0017] High-rate lithium iron phosphate / carbon nanotube composite cathode materials can accelerate lithium-ion transport and reduce polarization loss. Mg 2+ The ionic radius of Li + The close proximity and doping can expand the migration channels of lithium ions and reduce the migration resistance of lithium ions in the crystal lattice; the aniline-carbazole copolymer has a certain affinity for electrolytes, and the coating can increase the contact area between lithium iron phosphate and electrolyte, promote the uniform wetting of electrolyte on the particle surface, and avoid the dead zone of ion transport caused by the agglomeration of traditional carbon coatings; the interpenetration of carbon nanotubes can form micron-scale porous channels inside the material, which facilitates electrolyte penetration and lithium ion diffusion, and further reduces interfacial polarization.

[0018] High-rate lithium iron phosphate / carbon nanotube composite cathode materials can enhance structural stability and extend cycle life. 2+ Doping with [a substance] can enhance the rigidity of the LiFePO4 lattice and reduce the [resistance] of Li during charging and discharging. + Lattice volume fluctuations caused by insertion / extraction; N doping optimizes the bonding forces between lattice atoms, suppressing particle agglomeration and breakage; the aniline-carbazole copolymer coating tightly coats the surface of lithium iron phosphate particles, isolating the cathode material from direct contact with the electrolyte and reducing Fe... 2+ It can dissolve and inhibit the shedding of active substances during cycling; carbon nanotubes have excellent mechanical strength and flexibility, and can form support inside the material to relieve the volume stress of particles during cycling and avoid structural cracking. Attached Figure Description

[0019] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0020] Figure 1 This diagram illustrates the capacity retention test results of the high-rate lithium iron phosphate / carbon nanotube composite cathode materials in Examples 1-3 and Comparative Examples 1-3 of the present invention.

[0021] Figure 2 This is a schematic diagram showing the discharge specific capacity test results of the high-rate lithium iron phosphate / carbon nanotube composite cathode materials in Examples 1-3 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: This embodiment describes a preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, including the following steps: Step s1: 0.93g aniline, 2.1g 3-amino-9-ethylcarbazole and 120mL methanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. The mixture was stirred for 10min at 25℃ and a stirring rate of 300r / min. Then, 60mL sodium persulfate-hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the mixture was stirred and reacted for 24h. After the reaction was completed, the mixture was filtered under reduced pressure and washed 5 times with deionized water. Then it was placed in a vacuum drying oven and dried at 55℃ for 12h to obtain the aniline-carbazole copolymer. Step s2: 1g of carbon nanotubes (multi-walled carbon nanotubes with a diameter of 15nm), 25mL of 65% concentrated nitric acid and 75mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 75℃ and 250r / min for 6h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried at 55℃ for 12h to obtain acidified carbon nanotubes. Step s3: Add 4g of aniline-carbazole copolymer to 400mL of methanol and ultrasonically disperse for 30min at 400r / min and 100W to form a dispersion. Then add 1g of acidified carbon nanotubes, increase the speed to 450r / min and ultrasonically disperse for 40min at 200W to obtain an aniline-carbazole copolymer / carbon nanotube composite dispersion. Step s4: Add 91.8g FePO4, 23.4g Li2CO3, and 3.2g... Mg(NO3)2·6H2O, 54.8g of urea, and 2.6g of glucose were added to a planetary ball mill, along with 150mL of anhydrous ethanol as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 were used. The mill was run at 250r / min for 4 hours to form a slurry. The slurry was transferred to an evaporating dish, and the ethanol was evaporated in a water bath at 55℃ to obtain a precursor powder. The precursor powder was placed in an alumina crucible and placed in a tube furnace. High-purity nitrogen was introduced at 50mL / min for 30 minutes. Then, the temperature was programmed: first, the temperature was increased to 350℃ at 5℃ / min and held for 2 hours; then, the temperature was increased to 700℃ at 3℃ / min and held for 6 hours. The temperature was then allowed to cool naturally to room temperature to obtain a black blocky sintered body. The sintered body was transferred to an agate mortar and ground for 30 minutes. After passing through a 200-mesh sieve, Mg and N-doped lithium iron phosphate was obtained. Step s5: Weigh out 50 parts by weight of Mg and N-doped lithium iron phosphate and 400 parts by weight of aniline-carbazole copolymer / carbon nanotube composite dispersion, and set aside. Step s6: Add Mg and N-doped lithium iron phosphate to the aniline-carbazole copolymer / carbon nanotube composite dispersion and ultrasonically disperse for 1 hour at a speed of 550 r / min and a power of 150 W. Transfer to a rotary evaporator and evaporate under reduced pressure at a temperature of 35℃ and a speed of 45 r / min. Then place in a vacuum drying oven and dry at a temperature of 55℃ for 8 hours. Grind the solid and pass it through a 200-mesh sieve to obtain a high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0024] Example 2: This embodiment describes a preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, including the following steps: Step s1: 0.94 g aniline, 2.3 g 3-amino-9-ethylcarbazole and 135 mL methanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. The mixture was stirred for 12 min at 27 °C and a stirring rate of 400 r / min. Then, 63 mL sodium persulfate-hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the mixture was stirred and reacted for 25 h. After the reaction was completed, the mixture was filtered under reduced pressure and washed 6 times with deionized water. Then it was placed in a vacuum drying oven and dried at 57 °C for 12.5 h to obtain the aniline-carbazole copolymer. Step s2: 1.5g of carbon nanotubes (multi-walled carbon nanotubes with a diameter of 17nm), 37mL of 65% concentrated nitric acid and 112mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 77℃ and 270r / min for 6.5h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed 4 times with deionized water and then placed in a vacuum drying oven and dried at 57℃ for 13h to obtain acidified carbon nanotubes. Step s3: Add 4.4g of aniline-carbazole copolymer to 440mL of methanol and ultrasonically disperse for 33min at a speed of 425r / min and a power of 125W to form a dispersion. Then add 1.1g of acidified carbon nanotubes, increase the speed to 470r / min and ultrasonically disperse for 43min at a power of 230W to obtain an aniline-carbazole copolymer / carbon nanotube composite dispersion. Step s4: Add 96g FePO4, 24.5g Li2CO3, and 3.3g... Mg(NO3)2·6H2O, 56g of urea, and 2.7g of glucose were added to a planetary ball mill, along with 165mL of anhydrous ethanol as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 were used. The mill was run at 270r / min for 4.5h to form a slurry. The slurry was transferred to an evaporating dish, and the ethanol was evaporated in a water bath at 57℃ to obtain a precursor powder. The precursor powder was placed in an alumina crucible and placed in a tube furnace. High-purity nitrogen was introduced at 50mL / min for 30min. Then, the temperature was programmed: first, it was increased to 350℃ at 5℃ / min and held for 2h; then, it was increased to 730℃ at 3℃ / min and held for 7h. After natural cooling to room temperature, a black blocky sintered body was obtained. The sintered body was transferred to an agate mortar and ground for 33min. After passing through a 200-mesh sieve, Mg and N-doped lithium iron phosphate was obtained. Step s5: Weigh out 51 parts by weight of lithium iron phosphate doped with Mg and N and 415 parts by weight of aniline-carbazole copolymer / carbon nanotube composite dispersion, and set aside. Step s6: Add Mg and N-doped lithium iron phosphate to the aniline-carbazole copolymer / carbon nanotube composite dispersion and ultrasonically disperse for 1.3 h at a speed of 570 r / min and a power of 170 W. Transfer to a rotary evaporator and evaporate under reduced pressure at a temperature of 37 °C and a speed of 47 r / min. Then place it in a vacuum drying oven and dry at a temperature of 57 °C for 8.5 h. Grind the solid and pass it through a 200-mesh sieve to obtain a high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0025] Example 3: This embodiment describes a preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, including the following steps: Step s1: 0.95g aniline, 2.5g 3-amino-9-ethylcarbazole and 150mL methanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. The mixture was stirred for 15min at 30℃ and a stirring rate of 500r / min. Then, 66mL sodium persulfate-hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 2 drops / s. After the addition was completed, the mixture was stirred and reacted for 26h. After the reaction was completed, the mixture was filtered under reduced pressure and washed 7 times with deionized water. Then it was placed in a vacuum drying oven and dried at 60℃ for 13h to obtain the aniline-carbazole copolymer. Step s2: Add 2g of carbon nanotubes (multi-walled carbon nanotubes with a diameter of 20nm), 50mL of 65% concentrated nitric acid and 150mL of 98% concentrated sulfuric acid to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Stir the reaction at 80℃ and 300r / min for 7h. After the reaction is completed, cool the reaction product to room temperature, filter it, wash the filter cake with deionized water 5 times, and then place it in a vacuum drying oven and dry it at 60℃ for 14h to obtain acidified carbon nanotubes. Step s3: Add 4.8g of aniline-carbazole copolymer to 480mL of methanol and ultrasonically disperse for 35min at 450r / min and 150W to form a dispersion. Then add 1.2g of acidified carbon nanotubes, increase the speed to 500r / min and ultrasonically disperse for 45min at 250W to obtain an aniline-carbazole copolymer / carbon nanotube composite dispersion. Step s4: Add 99g FePO4, 25g Li2CO3, and 3.4g... Mg(NO3)2·6H2O, 58g of urea, and 2.8g of glucose were added to a planetary ball mill, along with 180mL of anhydrous ethanol as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 were used. The mill was run at 300r / min for 5 hours to form a slurry. The slurry was transferred to an evaporating dish, and the ethanol was evaporated in a water bath at 60℃ to obtain a precursor powder. The precursor powder was placed in an alumina crucible and placed in a tube furnace. High-purity nitrogen was introduced at 50mL / min for 30 minutes. Then, the temperature was programmed: first, the temperature was increased to 350℃ at 5℃ / min and held for 2 hours; then, the temperature was increased to 750℃ at 3℃ / min and held for 8 hours. The mixture was then allowed to cool naturally to room temperature to obtain a black blocky sintered body. The sintered body was transferred to an agate mortar and ground for 35 minutes. After passing through a 200-mesh sieve, Mg and N-doped lithium iron phosphate was obtained. Step s5: Weigh out 52 parts by weight of lithium iron phosphate doped with Mg and N and 430 parts by weight of aniline-carbazole copolymer / carbon nanotube composite dispersion, and set aside. Step s6: Add Mg and N-doped lithium iron phosphate to the aniline-carbazole copolymer / carbon nanotube composite dispersion and ultrasonically disperse for 1.5 h at a speed of 600 r / min and a power of 200 W. Transfer to a rotary evaporator and evaporate under reduced pressure at a temperature of 40 °C and a speed of 50 r / min. Then place it in a vacuum drying oven and dry at a temperature of 60 °C for 9 h. Grind the solid and pass it through a 200-mesh sieve to obtain a high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0026] Comparative Example 1: This comparative example illustrates a preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, including the following steps: Step s1: 1g of carbon nanotubes (multi-walled carbon nanotubes with a diameter of 15nm), 25mL of 65% concentrated nitric acid and 75mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 75℃ and 250r / min for 6h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried at 55℃ for 12h to obtain acidified carbon nanotubes. Step s2: Add 91.8g FePO4, 23.4g Li2CO3, and 3.2g... Mg(NO3)2·6H2O, 54.8g of urea, and 2.6g of glucose were added to a planetary ball mill, along with 150mL of anhydrous ethanol as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 were used. The mill was run at 250r / min for 4 hours to form a slurry. The slurry was transferred to an evaporating dish, and the ethanol was evaporated in a water bath at 55℃ to obtain a precursor powder. The precursor powder was placed in an alumina crucible and placed in a tube furnace. High-purity nitrogen was introduced at 50mL / min for 30 minutes. Then, the temperature was programmed: first, the temperature was increased to 350℃ at 5℃ / min and held for 2 hours; then, the temperature was increased to 700℃ at 3℃ / min and held for 6 hours. The temperature was then allowed to cool naturally to room temperature to obtain a black blocky sintered body. The sintered body was transferred to an agate mortar and ground for 30 minutes. After passing through a 200-mesh sieve, Mg and N-doped lithium iron phosphate was obtained. Step s3: Weigh out 50 parts of Mg and N-doped lithium iron phosphate, 1 part of acidified carbon nanotubes, and 250 mL of N,N-dimethylformamide according to the weight ratio, and set aside. Step s4: Add Mg and N-doped lithium iron phosphate and acidified carbon nanotubes to N,N-dimethylformamide and ultrasonically disperse for 1 hour at a speed of 550 r / min and a power of 150 W. Transfer to a rotary evaporator and evaporate under reduced pressure at a temperature of 35℃ and a speed of 45 r / min. Then place in a vacuum drying oven and dry at a temperature of 55℃ for 8 hours. Grind the solid and pass it through a 200-mesh sieve to obtain a high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0027] Comparative Example 2: This comparative example illustrates a preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, including the following steps: Step s1: 0.93g aniline, 2.1g 3-amino-9-ethylcarbazole and 120mL methanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. The mixture was stirred for 10min at 25℃ and a stirring rate of 300r / min. Then, 60mL sodium persulfate-hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the mixture was stirred and reacted for 24h. After the reaction was completed, the mixture was filtered under reduced pressure and washed 5 times with deionized water. Then it was placed in a vacuum drying oven and dried at 55℃ for 12h to obtain the aniline-carbazole copolymer. Step s2: 1g of carbon nanotubes (multi-walled carbon nanotubes with a diameter of 15nm), 25mL of 65% concentrated nitric acid and 75mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 75℃ and 250r / min for 6h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried at 55℃ for 12h to obtain acidified carbon nanotubes. Step s3: Add 4g of aniline-carbazole copolymer to 400mL of methanol and ultrasonically disperse for 30min at 400r / min and 100W to form a dispersion. Then add 1g of acidified carbon nanotubes, increase the speed to 450r / min and ultrasonically disperse for 40min at 200W to obtain an aniline-carbazole copolymer / carbon nanotube composite dispersion. Step s4: 91.8g FePO4, 23.4g Li2CO3 and 2.6g glucose were poured into a planetary ball mill, and 150mL of anhydrous ethanol was added as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 were selected and milled at 250r / min for 4h to form a slurry. The slurry was transferred to an evaporating dish, and the ethanol was evaporated in a water bath at 55℃ to obtain precursor powder. The precursor powder was placed in an alumina crucible and placed in a tube furnace. High-purity nitrogen was introduced at 50mL / min for 30min. Then the temperature was programmed: first, it was increased to 350℃ at 5℃ / min and held for 2h; then it was increased to 700℃ at 3℃ / min and held for 6h. It was then naturally cooled to room temperature to obtain a black blocky sintered body. The sintered body was transferred to an agate mortar and ground for 30min. After passing through a 200-mesh sieve, lithium iron phosphate was obtained. Step s5: Weigh out 50 parts by weight of lithium iron phosphate and 400 parts by weight of aniline-carbazole copolymer / carbon nanotube composite dispersion, and set aside. Step s6: Lithium iron phosphate was added to the aniline-carbazole copolymer / carbon nanotube composite dispersion and ultrasonically dispersed for 1 hour at a speed of 550 r / min and a power of 150 W. The dispersion was then transferred to a rotary evaporator and evaporated under reduced pressure at a temperature of 35℃ and a speed of 45 r / min. After that, the dispersion was placed in a vacuum drying oven and dried at a temperature of 55℃ for 8 hours. The solid was then ground and passed through a 200-mesh sieve to obtain a high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0028] Comparative Example 3: This example illustrates a fabrication process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, including the following steps: Step s1: 1g of carbon nanotubes (multi-walled carbon nanotubes with a diameter of 15nm), 25mL of 65% concentrated nitric acid and 75mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 75℃ and 250r / min for 6h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried at 55℃ for 12h to obtain acidified carbon nanotubes. Step s2: 91.8g FePO4, 23.4g Li2CO3 and 2.6g glucose were poured into a planetary ball mill, and 150mL of anhydrous ethanol was added as the milling medium. Zirconia balls with a ball-to-material ratio of 10:1 were selected and milled at 250r / min for 4h to form a slurry. The slurry was transferred to an evaporating dish, and the ethanol was evaporated in a water bath at 55℃ to obtain precursor powder. The precursor powder was placed in an alumina crucible and placed in a tube furnace. High-purity nitrogen was introduced at 50mL / min for 30min. Then the temperature was programmed: first, it was increased to 350℃ at 5℃ / min and held for 2h; then it was increased to 700℃ at 3℃ / min and held for 6h. It was then naturally cooled to room temperature to obtain a black blocky sintered body. The sintered body was transferred to an agate mortar and ground for 30min. After passing through a 200-mesh sieve, lithium iron phosphate was obtained. Step s3: Weigh out 50 parts by weight of lithium iron phosphate, 1 part by weight of acidified carbon nanotubes and 250 mL of N,N-dimethylformamide, and set aside. Step s4: Add lithium iron phosphate and acidified carbon nanotubes to N,N-dimethylformamide and ultrasonically disperse for 1 hour at a speed of 550 r / min and a power of 150 W. Transfer to a rotary evaporator and evaporate under reduced pressure at a temperature of 35℃ and a speed of 45 r / min. Then place in a vacuum drying oven and dry at a temperature of 55℃ for 8 hours. Grind the solid and pass it through a 200-mesh sieve to obtain a high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

[0029] Positive electrode preparation: 5g of polyvinylidene fluoride (CAS No.: 24937-79-9), 2g of conductive carbon black (model: Super P), 93g of lithium manganese iron phosphate material, and 300mL of N-methylpyrrolidone were placed in a beaker and stirred planetarily at 300r / min for 2h to form a slurry. The slurry was coated onto aluminum foil with a single-sided thickness of 100μm using a doctor blade coater. The foil was dried in a vacuum drying oven at 120℃ for 12h. The dried electrode was then compacted using a roller press at a pressure controlled at 6MPa and cut into electrode sheets with a diameter of 14mm to obtain the positive electrode sheet.

[0030] Battery manufacturing: Using lithium foil as the negative electrode and Celgard 2400 polypropylene microporous membrane as the separator, the electrolyte was 1 mol / L LiPF6 (EC:DEC=1:1, v / v). The positive electrode, negative electrode, separator and electrolyte were assembled in an argon glove box with water and oxygen content of less than 1 ppm. After assembly, the battery was left to stand for 24 hours to obtain the battery.

[0031] Performance testing: The preparation process of a high-rate lithium iron phosphate / carbon nanotube composite cathode material in Examples 1-3 and Comparative Examples 1-3 was tested under charge / discharge cutoff voltages of 2.5-4.2V. Capacity retention was measured after 1000 cycles at 1C rate and after 500 cycles at 5C rate. The test results are as follows: Figure 1 As shown; the specific capacity was tested at 1C, 5C, and 10C rates, and the test results are as follows. Figure 2 As shown.

[0032] See Figure 1-2 As shown, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that the high-rate lithium iron phosphate / carbon nanotube composite cathode material can significantly improve electronic conductivity and rate performance; accelerate lithium-ion transport and reduce polarization loss; enhance structural stability and extend cycle life.

[0033] Based on the comparison between Example 1 and Comparative Example 1, it can be seen that the cathode material obtained by combining aniline-carbazole copolymer / carbon nanotube composite with lithium iron phosphate doped with Mg and N has better performance than the cathode material obtained by combining carbon nanotube with lithium iron phosphate doped with Mg and N. Based on the comparison between Example 1 and Comparative Example 2, it can be seen that the cathode material obtained by combining aniline-carbazole copolymer / carbon nanotube composite with lithium iron phosphate doped with Mg and N has better performance than the cathode material obtained by combining aniline-carbazole copolymer / carbon nanotube composite with lithium iron phosphate. Based on the comparison between Example 1 and Comparative Example 3, it can be seen that the cathode material obtained by combining aniline-carbazole copolymer / carbon nanotube composite with lithium iron phosphate doped with Mg and N has better performance than the cathode material obtained by combining carbon nanotube with lithium iron phosphate.

[0034] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0035] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A high-rate lithium iron phosphate / carbon nanotube composite cathode material, characterized in that, Includes the following components by weight: 50-52 parts of lithium iron phosphate doped with Mg and N, and 400-430 parts of aniline-carbazole copolymer / carbon nanotube composite dispersion; The aniline-carbazole copolymer / carbon nanotube composite dispersion is prepared by the following steps: Step a1: Add aniline, 3-amino-9-ethylcarbazole and methanol to a three-necked flask, stir and react, add sodium persulfate-hydrochloric acid solution, continue stirring and reacting, filter under reduced pressure, wash and dry to obtain aniline-carbazole copolymer; Step a2: Add carbon nanotubes, concentrated nitric acid and concentrated sulfuric acid to a three-necked flask, stir to react, cool and filter, wash and dry to obtain acidified carbon nanotubes. Step a3: Add aniline-carbazole copolymer to methanol and disperse ultrasonically, then add acidified carbon nanotubes and disperse ultrasonically to obtain aniline-carbazole copolymer / carbon nanotube composite dispersion; The Mg and N-doped lithium iron phosphate is prepared by the following steps: Step b1: FePO4, Li2CO3, Mg(NO3)2·6H2O, urea and glucose are poured into a planetary ball mill, anhydrous ethanol is added, the mixture is ball-milled, transferred to an evaporating dish, the ethanol is evaporated in a water bath, the mixture is placed in an alumina crucible, placed in a tube furnace, and high-purity nitrogen is introduced. Then the temperature is programmed: first, it is increased to 350℃ at 5℃ / min and held for 2h, then increased to 700-750℃ at 3℃ / min and held for 6-8h, then cooled naturally, transferred to an agate mortar, ground and sieved to obtain Mg and N doped lithium iron phosphate; By combining an aniline-carbazole copolymer / carbon nanotube composite dispersion with lithium iron phosphate doped with Mg and N, lithium iron phosphate co-coated with aniline-carbazole copolymer and carbon nanotubes is obtained.

2. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The ratio of aniline, 3-amino-9-ethylcarbazole, methanol, and sodium persulfate-hydrochloric acid solution used in step a1 is 0.93-0.95g: 2.1-2.5g: 120-150mL: 60-66mL.

3. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The sodium persulfate-hydrochloric acid solution in step a1 is a solution prepared by mixing sodium persulfate, hydrochloric acid, and deionized water in a ratio of 4.5g:2.5mL:57.5mL; the concentration of the hydrochloric acid is 12mol / L.

4. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The ratio of carbon nanotubes, concentrated nitric acid, and concentrated sulfuric acid used in step a2 is 1-2g: 25-50mL: 75-150mL.

5. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The carbon nanotubes mentioned in step a2 are multi-walled carbon nanotubes with a diameter of 15-20 nm.

6. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The concentrated nitric acid has a mass fraction of 65%; the concentrated sulfuric acid has a mass fraction of 98%.

7. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The ratio of the aniline-carbazole copolymer, methanol, and acidified carbon nanotubes used in step a3 is 4-4.8g: 400-480mL: 1-1.2g.

8. The high-rate lithium iron phosphate / carbon nanotube composite cathode material according to claim 1, characterized in that, The ratio of FePO4, Li2CO3, Mg(NO3)2·6H2O, urea, glucose and anhydrous ethanol used in step b1 is 91.8-99g: 23.4-25g: 3.2-3.4g: 54.8-58g: 2.6-2.8g: 150-180mL.

9. A preparation process for a high-rate lithium iron phosphate / carbon nanotube composite cathode material, characterized in that, The preparation of the high-rate lithium iron phosphate / carbon nanotube composite cathode material as described in any one of claims 1-8 includes the following steps: Step 1: Weigh out the Mg and N-doped lithium iron phosphate and aniline-carbazole copolymer / carbon nanotube composite dispersion according to the specified weight parts, and set aside. Step 2: Add Mg and N-doped lithium iron phosphate to the aniline-carbazole copolymer / carbon nanotube composite dispersion and ultrasonically disperse. Transfer to a rotary evaporator, evaporate under reduced pressure, dry, grind the solid and pass it through a 200-mesh sieve to obtain high-ratio lithium iron phosphate / carbon nanotube composite cathode material.

Images