A lithium iron manganese phosphate / carbon fiber composite positive electrode material and a preparation method thereof

Abstract

The application relates to the field of positive electrode materials, in particular to a lithium iron manganese phosphate / carbon fiber composite positive electrode material and a preparation method thereof, which are used for solving the problems of poor electronic conductivity of lithium iron manganese phosphate and the like; the preparation method is characterized in that the carbon fiber is first subjected to graft modification and ZrO2 film coating, then the modified carbon fiber is compounded with lithium iron manganese phosphate to obtain the lithium iron manganese phosphate / carbon fiber composite positive electrode material; the lithium iron manganese phosphate / carbon fiber composite positive electrode material can greatly improve the electronic conductivity, improve the rate performance, inhibit the dissolution of active substances and the side reaction, improve the cycle stability, optimize the lithium ion diffusion channel, improve the ion conduction efficiency, enhance the mechanical property of the electrode, and improve the structural reliability. Meanwhile, the advantages of the components are exerted, and the electrochemical performance and the structural stability of the composite positive electrode are finally significantly improved.

Description

Technical Field

[0001] This invention relates to the field of cathode materials, specifically to a lithium manganese iron phosphate / carbon fiber composite cathode material and its preparation method. Background Technology

[0002] While traditional lithium manganese iron phosphate batteries offer high safety and long cycle life, there is still room for improvement in energy density and charge / discharge efficiency. The relatively poor conductivity of lithium manganese iron phosphate itself also affects the overall performance of the battery.

[0003] This invention provides a method that can significantly improve the energy density and extend the cycle life of a battery while maintaining the original excellent properties of the material. This gives it broad application potential and market prospects in various fields. Summary of the Invention

[0004] In order to overcome the above-mentioned technical problems, the present invention aims to provide a lithium manganese iron phosphate / carbon fiber composite cathode material and its preparation method.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] In a first aspect, this application provides a lithium manganese iron phosphate / carbon fiber composite cathode material, comprising the following components by weight:

[0007] Li2CO3 9-10 parts, Mn(CH3COO)2·4H2O 30-32 parts, Fe(CH3COO)2·4H2O 30-32 parts, NH4H2PO4 28-30 parts, deionized water 726-800 parts, citric acid 47-51 parts, modified ZrO2 coated carbon fiber 2-3 parts, N-methylpyrrolidone 50-55 parts, and glucose 5-6 parts;

[0008] The modified ZrO2-coated carbon fibers are prepared by the following steps:

[0009] Step a1: Add terephthalaldehyde, 2-nitrobenzeneacetonitrile, and anhydrous methanol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection and stir the reaction at 30-35℃ and a stirring rate of 300-400 r / min for 10-15 min. Then, add piperidine dropwise while stirring, controlling the dropping rate to 1-2 drops / s. After the addition is complete, raise the temperature to 65-70℃ and continue stirring for 24-26 h. After the reaction is complete, cool the reaction product to room temperature, then filter under vacuum. Wash the filter cake 3-5 times with anhydrous ethanol, and then place it in a vacuum drying oven and dry it at 60-65℃ for 2-3 h to obtain the first intermediate.

[0010]

[0011] Step a2: Add tin powder, the first intermediate, and anhydrous ethanol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection and stir the reaction at 0-5℃ and a stirring rate of 300-400 r / min for 10-15 min. Then, while stirring, add hydrochloric acid solution dropwise at a rate of 1-2 drops / s. After the addition is complete, raise the temperature to 25-30℃ and continue stirring for 24-26 h. After the reaction is complete, cool the reaction product to room temperature, adjust the pH to 7 with saturated sodium bicarbonate solution, and extract with dichloromethane 3-4 times. Combine the organic phases and dry with anhydrous magnesium sulfate to remove the solvent. Then elute with dichloromethane-petroleum ether solution to obtain the carbon fiber modifier.

[0012]

[0013] Step a3: Add carbon fiber, concentrated nitric acid and concentrated hydrochloric acid to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Stir the reaction at 55-60℃ and 300-400 r / min for 3-4 h. After the reaction is completed, cool the reaction product to room temperature, then filter it under vacuum. Wash the filter cake with deionized water 3-5 times, then place it in a vacuum drying oven and dry it at 75-80℃ for 10-12 h to obtain surface-activated carbon fiber.

[0014] Step a4: Add zirconium nitrate and ethylene glycol to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Stir the reaction at 25-30℃ and 200-300 r / min for 30-35 min. Then adjust the pH to 4-4.5 with ammonia and continue stirring for 1-2 h to obtain ZrO2 sol. Add the surface-activated carbon fibers to the ZrO2 sol and sonicate at 150-200 W for 40-45 min. Remove the sol and hang it vertically to stand at room temperature for 30 min. Then place it in a vacuum drying oven and dry at 60-65℃ for 2-3 h. Transfer the sol to a tube furnace and heat it to 550℃ at 3℃ / min in air atmosphere. Hold the temperature for 2 h and cool it naturally to room temperature to obtain ZrO2-coated carbon fibers.

[0015] Step a5: Weigh ZrO2-coated carbon fibers and disperse them in N,N-dimethylformamide. Sonicate the dispersion for 25-30 minutes at 250-300W. Then transfer the dispersion to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Add 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, purge with nitrogen for protection, and stir the reaction at 25-30℃ and a stirring rate of 300-400 r / min for 1-2 hours. Afterward, add carbon fibers dropwise while stirring. The fiber modifier, N,N-dimethylformamide, was added at a controlled dropping rate of 1-2 drops / s. After the addition was complete, the temperature was raised to 50-55℃ and the reaction was stirred for 8-10 hours. After the reaction was completed, the reaction product was cooled to room temperature and centrifuged at 7000-8000 r / min for 10-12 minutes. The solid product was collected, washed 5-6 times with deionized water, and then placed in a vacuum drying oven and dried at 60-65℃ for 12-13 hours to obtain modified ZrO2-coated carbon fibers.

[0016] In a preferred embodiment of the present invention, the ratio of terephthalaldehyde, 2-nitrophenylacetonitrile, anhydrous methanol and piperidine in step a1 is 1.34-1.50g: 3.24-3.70g: 15-20mL: 1.0-1.3mL.

[0017] In a preferred embodiment of the present invention, the ratio of tin powder, first intermediate, anhydrous ethanol and hydrochloric acid solution in step a2 is 1.19-1.30g: 2.11-2.50g: 20-25mL: 2.0-2.3mL.

[0018] In a preferred embodiment of the present invention, the hydrochloric acid solution in step a2 has a mass fraction of 37%; the dichloromethane-petroleum ether solution is a solution of dichloromethane and petroleum ether mixed in a volume ratio of 8:1.

[0019] In a preferred embodiment of the present invention, the ratio of carbon fiber, concentrated nitric acid and concentrated hydrochloric acid used in step a3 is 10-12g: 30-45mL: 10-15mL.

[0020] In a preferred embodiment of the present invention, the carbon fiber in step a3 is a PAN-based carbon fiber filament with a diameter of 7-10 μm and a length of 1 cm; the concentrated nitric acid has a mass fraction of 68%; and the concentrated hydrochloric acid has a mass fraction of 37%.

[0021] In a preferred embodiment of the present invention, the ratio of zirconium nitrate, ethylene glycol and surface-activated carbon fiber in step a4 is 5-7g: 50-70mL: 10-12g.

[0022] In a preferred embodiment of the present invention, the mass fraction of the ammonia water in step a4 is 25%.

[0023] In a preferred embodiment of the present invention, the ratio of ZrO2-coated carbon fiber, N,N-dimethylformamide, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and carbon fiber modifier -N,N-dimethylformamide in step a5 is 10-12g:100-120mL:0.5-0.7g:20-24mL.

[0024] In a preferred embodiment of the present invention, the carbon fiber modifier-N,N-dimethylformamide in step a5 is a solution of carbon fiber modifier and N,N-dimethylformamide mixed in a ratio of 0.5g:20mL.

[0025] Secondly, this application provides a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, comprising the following steps:

[0026] Step 1: Weigh Li2CO3, Mn(CH3COO)2·4H2O, Fe(CH3COO)2·4H2O, and NH4H2PO4, dissolve them in deionized water, add citric acid, and stir at 55-60℃ for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; ultrasonically disperse modified ZrO2-coated carbon fibers and N-methylpyrrolidone at 250-300W for 30-35 minutes, add the lithium manganese iron phosphate precursor sol and glucose to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser, and stir at 25-30℃ and 900-1000 r / min for 2-3 hours; transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 200-250 r / min for 2-3 hours to form a slurry;

[0027] Step 2: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0028] The beneficial effects of this invention are:

[0029] This invention discloses a lithium manganese iron phosphate / carbon fiber composite cathode material and its preparation method. First, carbon fibers are grafted and modified and coated with a ZrO2 film. Then, the modified carbon fibers are composited with lithium manganese iron phosphate to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material. This preparation method solves the inherent defects of lithium manganese iron phosphate (LMPF) cathode material, while giving full play to the advantages of each component, and ultimately significantly improves the electrochemical performance and structural stability of the composite cathode.

[0030] Carbon fiber (CF) has excellent electronic conductivity and a one-dimensional network structure, which can build continuous electronic conduction pathways between LMPF particles, significantly reduce electrode internal resistance, greatly improve electronic conductivity, and improve rate performance.

[0031] ZrO2 is an oxide with extremely high chemical stability. When coated on the surface of CF, it can form a physical barrier between LMPF particles and the electrolyte, preventing Mn from entering the electrolyte. 2+ The coating of ZrO2 can inhibit the volume expansion / contraction of LMPF during cycling, reduce electrode cracking and active material detachment, ensure the integrity of the electrode structure after long-term cycling, suppress the dissolution of active material and side reactions, and improve cycling stability.

[0032] After grafting with carbon fiber modifiers, the polar groups such as amino groups in the molecules can form hydrogen bonds or coordination with the hydroxyl groups on the surface of LMPF, enhancing interfacial bonding. The one-dimensional structure of carbon fiber and the nanoscale coating layer of ZrO2 can jointly form a porous microstructure of "CF-ZrO2-LMPF", providing a continuous diffusion channel for lithium ions, avoiding ion transport obstruction caused by particle agglomeration, optimizing lithium ion diffusion channels, and improving ion conduction efficiency.

[0033] Carbon fiber has extremely high tensile strength and flexibility, and can serve as the "mechanical skeleton" of the electrode to disperse the stress of the electrode during cycling and suppress electrode deformation. Grafting of carbon fiber modifiers further enhances the interfacial bonding force between CF and LMPF and binder, reduces the shedding of active material during the winding and charging / discharging process of the electrode, improves the mechanical integrity and long-term service reliability of the electrode, enhances the mechanical properties of the electrode, and improves the structural reliability.

[0034] The reaction principle of carbon fiber modifier is as follows: The first step is the Knoevenagel condensation reaction, in which a nucleophilic attack occurs. Piperidine abstracts the α-H of 2-nitrophenylacetonitrile to form a carbanion, which attacks the carbonyl carbon of terephthalaldehyde. Dehydration forms a double bond, and the addition product undergoes proton transfer and dehydration to form a carbon-carbon double bond. Terephthalaldehyde contains two aldehyde groups, which condense with two 2-nitrophenylacetonitrile molecules respectively to construct a conjugated double bond structure, generating the first intermediate. The second step is the nitro reduction reaction, in which Sn reacts with HCl to generate SnCl2 reducing agent, which gradually reduces the nitro group to nitroso and hydroxylamine, finally yielding an amino group, thus generating the carbon fiber modifier. Attached Figure Description

[0035] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0036] Figure 1 This is a schematic diagram showing the capacity retention test results of carbon-coated lithium iron phosphate cathode materials in Examples 1-3 and Comparative Examples 1-3 of the present invention.

[0037] Figure 2 This is a schematic diagram showing the discharge specific capacity test results of carbon-coated lithium iron phosphate cathode materials in Examples 1-3 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0038] 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.

[0039] Example 1:

[0040] This embodiment describes a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, including the following steps:

[0041] Step s1: 1.34 g of terephthalaldehyde, 3.24 g of 2-nitrobenzeneacetonitrile and 15 mL of anhydrous 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 10 min at 30 °C and a stirring rate of 300 r / min. Then, 1.0 mL of piperidine was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 65 °C and the reaction was stirred for 24 h. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed three times with anhydrous ethanol and then placed in a vacuum drying oven and dried at 60 °C for 2 h to obtain the first intermediate.

[0042] Step s2: 1.19 g of tin powder, 1.19 g of the first intermediate, and 20 mL of anhydrous ethanol 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, and the mixture was stirred for 10 min at 0 °C and a stirring rate of 300 r / min. Then, 2.0 mL of hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 25 °C and the reaction was stirred for 24 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the pH was adjusted to 7 with saturated sodium bicarbonate solution. The product was then extracted three times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate to remove the solvent. The product was then eluted with dichloromethane-petroleum ether solution to obtain the carbon fiber modifier.

[0043] Step s3: 10g of carbon fiber (PAN-based carbon fiber filament, 7μm in diameter and 1cm in length), 30mL of concentrated nitric acid and 10mL of concentrated hydrochloric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred and reacted for 3h at 55℃ and 300r / min. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried for 10h at 75℃ to obtain surface-activated carbon fiber.

[0044] Step s4: Add 5g of zirconium nitrate and 50mL of ethylene glycol to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Stir the reaction at 25℃ and 200r / min for 30min. Then adjust the pH to 4 with ammonia and continue stirring for 1h to obtain ZrO2 sol. Add 10g of surface-activated carbon fiber to the ZrO2 sol and sonicate at 150W for 40min. Remove and hang vertically, let stand at room temperature for 30min, then place in a vacuum drying oven and dry at 60℃ for 2h. Transfer to a tube furnace and heat to 550℃ at 3℃ / min in air atmosphere, hold for 2h, and cool naturally to room temperature to obtain ZrO2-coated carbon fiber.

[0045] Step s5: Weigh 10g of ZrO2-coated carbon fibers and disperse them in 100mL of N,N-dimethylformamide. Sonicate the dispersion for 25min at 250W. Then transfer the dispersion to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Add 0.5g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Purge with nitrogen for protection and stir for 1h at 25℃ and a stirring rate of 300r / min. 20 mL of carbon fiber modifier - N,N-dimethylformamide was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was complete, the temperature was raised to 50℃ and the reaction was stirred for 8 h. After the reaction was completed, the reaction product was cooled to room temperature and centrifuged at 7000 r / min for 10 min. The solid product was collected, washed 5 times with deionized water, and then placed in a vacuum drying oven and dried at 60℃ for 12 h to obtain modified ZrO2 coated carbon fiber.

[0046] Step s6: Weigh 9 parts Li2CO3, 30 parts Mn(CH3COO)2·4H2O, 30 parts FeFe(CH3COO)2·4H2O, and 28 parts NH4H2PO4, dissolve them in 726 parts deionized water, add 47 parts citric acid, and stir at 55°C for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; add 2 parts modified ZrO2-coated carbon fibers and 50 parts N-methylpyrrolidone to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser, and stir at 25°C and a stirring rate of 900 r / min for 2 hours; transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 200 r / min for 2 hours to form a slurry;

[0047] Step s7: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0048] Example 2:

[0049] This embodiment describes a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, including the following steps:

[0050] Step s1: 1.40 g of terephthalaldehyde, 3.50 g of 2-nitrobenzeneacetonitrile and 17 mL of anhydrous 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 at 33 °C and 350 r / min for 13 min. Then, 1.2 mL of piperidine was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 67 °C and the reaction was stirred for 25 h. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed 4 times with anhydrous ethanol and then placed in a vacuum drying oven and dried at 63 °C for 2 h to obtain the first intermediate.

[0051] Step s2: 1.25g of tin powder, 1.25g of the first intermediate, and 23mL of anhydrous ethanol 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, and the mixture was stirred for 12min at 3℃ and a stirring rate of 350r / min. Then, 2.2mL of hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 27℃ and the reaction was stirred for 25h. After the reaction was completed, the reaction product was cooled to room temperature, and then the pH was adjusted to 7 with saturated sodium bicarbonate solution. The product was then extracted three times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate to remove the solvent. The product was then eluted with dichloromethane-petroleum ether solution to obtain the carbon fiber modifier.

[0052] Step s3: 11g of carbon fiber (PAN-based carbon fiber filament, 8μm in diameter and 1cm in length), 38mL of concentrated nitric acid and 13mL of concentrated hydrochloric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred and reacted at 57℃ and 350r / min for 30h. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed 4 times with deionized water and then placed in a vacuum drying oven and dried at 77℃ for 11h to obtain surface-activated carbon fiber.

[0053] Step s4: Add 6g of zirconium nitrate and 60mL of ethylene glycol to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Stir the reaction at 27℃ and 250r / min for 33min. Then adjust the pH to 4.2 with ammonia and continue stirring for 1.5h to obtain ZrO2 sol. Add 11g of surface-activated carbon fibers to the ZrO2 sol and sonicate at 170W for 43min. Remove and hang vertically, let stand at room temperature for 30min, then place in a vacuum drying oven and dry at 63℃ for 2.5h. Transfer to a tube furnace and heat to 550℃ at 3℃ / min in air atmosphere, hold for 2h, and cool naturally to room temperature to obtain ZrO2-coated carbon fibers.

[0054] Step s5: Weigh 11g of ZrO2-coated carbon fibers and disperse them in 110mL of N,N-dimethylformamide. Sonicate the dispersion for 27min at 270W. Then transfer the dispersion to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Add 0.6g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Purge with nitrogen for protection and stir the reaction at 27℃ and 350r / min for 1.5h. While stirring, 22 mL of carbon fiber modifier - N,N-dimethylformamide was added dropwise at a rate of 1 drop / s. After the addition was complete, the temperature was raised to 53°C and the reaction was stirred for 9 h. After the reaction was completed, the reaction product was cooled to room temperature and centrifuged at 7500 r / min for 11 min. The solid product was collected, washed 5 times with deionized water, and then placed in a vacuum drying oven and dried at 63°C for 12 h to obtain modified ZrO2 coated carbon fibers.

[0055] Step s6: Weigh 9.5 parts Li2CO3, 31 parts Mn(CH3COO)2·4H2O, 31 parts Fe(CH3COO)2·4H2O, and 29 parts NH4H2PO4, dissolve them in 770 parts deionized water, add 49 parts citric acid, and stir at 57°C for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; add 2.5 parts modified ZrO2-coated carbon fibers and 53 parts N-methylpyrrolidone to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser, and stir at 27°C and a stirring rate of 950 r / min for 2.5 hours; transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 230 r / min for 2.5 hours to form a slurry;

[0056] Step s7: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0057] Example 3:

[0058] This embodiment describes a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, including the following steps:

[0059] Step s1: 1.50 g of terephthalaldehyde, 3.70 g of 2-nitrobenzeneacetonitrile and 20 mL of anhydrous 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 at 35 °C and a stirring rate of 400 r / min for 15 min. Then, 1.3 mL of piperidine was added dropwise while stirring, with the dropping rate controlled at 2 drops / s. After the addition was completed, the temperature was raised to 70 °C and the reaction was stirred for 26 h. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed 5 times with anhydrous ethanol and then placed in a vacuum drying oven and dried at 65 °C for 3 h to obtain the first intermediate.

[0060] Step s2: 1.30 g of tin powder, 1.30 g of the first intermediate, and 25 mL of anhydrous ethanol 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, and the mixture was stirred for 15 min at 5 °C and a stirring rate of 400 r / min. Then, 2.3 mL of hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 2 drops / s. After the addition was completed, the temperature was raised to 30 °C and the reaction was continued for 26 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the pH was adjusted to 7 with saturated sodium bicarbonate solution. The product was then extracted four times with dichloromethane, the organic phases were combined and dried with anhydrous magnesium sulfate to remove the solvent. The product was then eluted with dichloromethane-petroleum ether solution to obtain the carbon fiber modifier.

[0061] Step s3: 12g of carbon fiber (PAN-based carbon fiber filament, 10μm in diameter and 1cm in length), 45mL of concentrated nitric acid and 15mL of concentrated hydrochloric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred and reacted for 4h at 60℃ and 400r / min. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed 5 times with deionized water and then placed in a vacuum drying oven and dried for 12h at 80℃ to obtain surface-activated carbon fiber.

[0062] Step s4: Add 7g of zirconium nitrate and 70mL of ethylene glycol to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Stir the reaction at 30℃ and 300r / min for 35min. Then adjust the pH to 4.5 with ammonia and continue stirring for 2h to obtain ZrO2 sol. Add 12g of surface-activated carbon fiber to the ZrO2 sol and sonicate at 200W for 45min. Remove and hang vertically, let stand at room temperature for 30min, then place in a vacuum drying oven and dry at 65℃ for 3h. Transfer to a tube furnace and heat to 550℃ at 3℃ / min in air atmosphere, hold for 2h, and cool naturally to room temperature to obtain ZrO2-coated carbon fiber.

[0063] Step s5: Weigh 12g of ZrO2-coated carbon fibers and disperse them in 120mL of N,N-dimethylformamide. Sonicate the dispersion for 30min at 300W. Then transfer the dispersion to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Add 0.7g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Purge with nitrogen for protection and stir for 2h at 30℃ and a stirring rate of 400r / min. Afterward, continue stirring. 24 mL of carbon fiber modifier - N,N-dimethylformamide was added dropwise while stirring, with the dropping rate controlled at 2 drops / s. After the addition was complete, the temperature was raised to 55℃ and the reaction was stirred for 10 h. After the reaction was completed, the reaction product was cooled to room temperature and centrifuged at 8000 r / min for 12 min. The solid product was collected, washed 6 times with deionized water, and then placed in a vacuum drying oven and dried at 65℃ for 13 h to obtain modified ZrO2 coated carbon fiber.

[0064] Step s6: Weigh 10 parts Li2CO3, 32 parts Mn(CH3COO)2·4H2O, 32 parts Fe(CH3COO)2·4H2O, and 30 parts NH4H2PO4, dissolve them in 800 parts deionized water, add 51 parts citric acid, and stir and react at 60℃ for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; add 3 parts modified ZrO2-coated carbon fiber and 55 parts N-methylpyrrolidone to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser, and stir and react at 30℃ and a stirring rate of 1000 r / min for 3 hours; transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 250 r / min for 3 hours to form a slurry;

[0065] Step s7: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0066] Comparative Example 1:

[0067] This comparative example illustrates a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, comprising the following steps:

[0068] Step s1: 1.34 g of terephthalaldehyde, 3.24 g of 2-nitrobenzeneacetonitrile and 15 mL of anhydrous 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 10 min at 30 °C and a stirring rate of 300 r / min. Then, 1.0 mL of piperidine was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 65 °C and the reaction was stirred for 24 h. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed three times with anhydrous ethanol and then placed in a vacuum drying oven and dried at 60 °C for 2 h to obtain the first intermediate.

[0069] Step s2: 1.19 g of tin powder, 1.19 g of the first intermediate, and 20 mL of anhydrous ethanol 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, and the mixture was stirred for 10 min at 0 °C and a stirring rate of 300 r / min. Then, 2.0 mL of hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 25 °C and the reaction was stirred for 24 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the pH was adjusted to 7 with saturated sodium bicarbonate solution. The product was then extracted three times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate to remove the solvent. The product was then eluted with dichloromethane-petroleum ether solution to obtain the carbon fiber modifier.

[0070] Step s3: 10g of carbon fiber (PAN-based carbon fiber filament, 7μm in diameter and 1cm in length), 30mL of concentrated nitric acid and 10mL of concentrated hydrochloric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred and reacted for 3h at 55℃ and 300r / min. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried for 10h at 75℃ to obtain surface-activated carbon fiber.

[0071] Step s4: Weigh 10g of surface-activated carbon fiber and disperse it in 100mL of N,N-dimethylformamide. Sonicate the dispersion for 25min at 250W. Then transfer it to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Add 0.5g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and purge with nitrogen for protection. Stir and react for 1h at 25℃ and 300r / min. Then add 20mL of carbon fiber modifier -N,N-dimethylformamide dropwise while stirring, controlling the dropping rate to 1 drop / s. After the addition is complete, raise the temperature to 50℃ and continue stirring for 8h. After the reaction is complete, cool the reaction product to room temperature and centrifuge at 7000r / min for 10min. Collect the solid product and wash it 5 times with deionized water. Then place it in a vacuum drying oven and dry it at 60℃ for 12h to obtain modified carbon fiber.

[0072] Step s5: Weigh 9 parts Li2CO3, 30 parts Mn(CH3COO)2·4H2O, 30 parts Fe(CH3COO)2·4H2O, and 28 parts NH4H2PO4, dissolve them in 726 parts deionized water, add 47 parts citric acid, and stir at 55℃ for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; ultrasonically disperse 2 parts modified carbon fiber and 50 parts N-methylpyrrolidone at 250W for 30 minutes, add the above lithium manganese iron phosphate precursor sol and 5 parts glucose to a three-necked flask equipped with a stirrer, thermometer and reflux condenser, and stir at 25℃ and 900 r / min for 2 hours; transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 200 r / min for 2 hours to form a slurry;

[0073] Step s6: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0074] Comparative Example 2:

[0075] This comparative example illustrates a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, comprising the following steps:

[0076] Step s1: 10g of carbon fiber (PAN-based carbon fiber filament, 7μm in diameter and 1cm in length), 30mL of concentrated nitric acid and 10mL of concentrated hydrochloric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred and reacted for 3h at 55℃ and 300r / min. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried for 10h at 75℃ to obtain surface-activated carbon fiber.

[0077] Step s2: Add 5g of zirconium nitrate and 50mL of ethylene glycol to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Stir the reaction at 25℃ and 200r / min for 30min. Then adjust the pH to 4 with ammonia and continue stirring for 1h to obtain ZrO2 sol. Add 10g of surface-activated carbon fiber to the ZrO2 sol and sonicate at 150W for 40min. Remove and hang vertically, let stand at room temperature for 30min, then place in a vacuum drying oven and dry at 60℃ for 2h. Transfer to a tube furnace and heat to 550℃ at 3℃ / min in air atmosphere, hold for 2h, and cool naturally to room temperature to obtain ZrO2-coated carbon fiber.

[0078] Step s3: Weigh 9 parts Li2CO3, 30 parts Mn(CH3COO)2·4H2O, 30 parts Fe(CH3COO)2·4H2O, and 28 parts NH4H2PO4, dissolve them in 726 parts deionized water, add 47 parts citric acid, and stir at 55°C for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; ultrasonically disperse 2 parts ZrO2-coated carbon fibers and 50 parts N-methylpyrrolidone at 250W for 30 minutes, add the above lithium manganese iron phosphate precursor sol and 5 parts glucose into a three-necked flask equipped with a stirrer, thermometer and reflux condenser, and stir at 25°C and 900 r / min for 2 hours; transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 200 r / min for 2 hours to form a slurry;

[0079] Step s4: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0080] Comparative Example 3:

[0081] This comparative example illustrates a method for preparing a lithium manganese iron phosphate / carbon fiber composite cathode material, comprising the following steps:

[0082] Step s1: 10g of carbon fiber (PAN-based carbon fiber filament, 7μm in diameter and 1cm in length), 30mL of concentrated nitric acid and 10mL of concentrated hydrochloric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred and reacted for 3h at 55℃ and 300r / min. After the reaction was completed, the reaction product was cooled to room temperature and then vacuum filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried for 10h at 75℃ to obtain surface-activated carbon fiber.

[0083] Step s2: Weigh 9 parts Li2CO3, 30 parts Mn(CH3COO)2·4H2O, 30 parts Fe(CH3COO)2·4H2O, and 28 parts NH4H2PO4, dissolve them in 726 parts deionized water, add 47 parts citric acid, and stir at 55°C for 2 hours to form a uniform lithium manganese iron phosphate precursor sol; disperse 2 parts surface-activated carbon fibers and 50 parts N-methylpyrrolidone ultrasonically at 250W for 30 minutes, add the above lithium manganese iron phosphate precursor sol and 5 parts glucose to a three-necked flask equipped with a stirrer, thermometer and reflux condenser, stir at 25°C and 900 r / min for 2 hours, transfer to a planetary ball mill, add agate balls with a ball-to-material ratio of 3:1, and ball mill at 200 r / min for 2 hours to form a slurry;

[0084] Step s3: The slurry is treated by a spray dryer. Under nitrogen protection, the inlet temperature is set to 180℃, the outlet temperature to 80℃, and the feed rate to 5mL / min. The powder is collected and placed in a tube furnace. The temperature in the tube furnace is increased to 780℃ at 5℃ / min and held at this temperature for 4 hours under an argon atmosphere. The powder is then cooled to room temperature with the furnace to obtain the lithium manganese iron phosphate / carbon fiber composite cathode material.

[0085] Preparation of the positive electrode sheet:

[0086] 5g of polyvinylidene fluoride (CAS No.: 24937-79-9), 10g of conductive carbon black (model: Super P), 85g of lithium manganese iron phosphate / carbon fiber composite cathode material, and 150mL of N-methylpyrrolidone were placed in a beaker and mechanically stirred for 4 hours at 500r / min to form a slurry. The slurry was coated onto aluminum foil with a single-sided thickness of 80μm using a doctor blade coater. The foil was pre-dried in a 60℃ forced-air drying oven for 30min, and then transferred to a 120℃ vacuum drying oven for 12h. The dried electrode was compacted using a roller press with a pressure controlled at 150MPa and cut into electrode sheets with a diameter of 14mm to obtain the cathode electrode.

[0087] Battery manufacturing:

[0088] 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.

[0089] Performance testing:

[0090] The lithium manganese iron phosphate / carbon fiber composite cathode materials and their preparation methods from Examples 1-3 and Comparative Examples 1-3 were tested for capacity retention after 1000 cycles at 1C and 5C charge-discharge rates. The test results are as follows: Figure 1 As shown; the discharge specific capacity at 1C and 5C rates was tested, and the test results are as follows. Figure 2 As shown.

[0091] 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 lithium manganese iron phosphate / carbon fiber composite cathode material can significantly improve electronic conductivity, improve rate performance, suppress the dissolution of active materials and side reactions, and improve cycle stability.

[0092] Based on the comparison between Example 1 and Comparative Example 1, it can be seen that the cathode material obtained by combining ZrO2-coated and carbon fiber modified with lithium manganese iron phosphate has better performance than the cathode material obtained by combining carbon fiber modified with lithium manganese iron phosphate with only carbon fiber modified with carbon fiber modified with lithium manganese iron phosphate.

[0093] Based on the comparison between Example 1 and Comparative Example 2, it can be seen that the cathode material obtained by combining ZrO2-coated and ZrO2-modified carbon fibers with lithium manganese iron phosphate has better performance than the cathode material obtained by combining ZrO2-coated carbon fibers with lithium manganese iron phosphate.

[0094] Based on the comparison between Example 1 and Comparative Example 3, it can be seen that the cathode material obtained by combining ZrO2-coated and carbon fiber modified with lithium manganese iron phosphate has better performance than the cathode material obtained by combining ordinary carbon fiber with lithium manganese iron phosphate.

[0095] 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.

[0096] 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 lithium iron manganese phosphate / carbon fiber composite cathode material, characterized in that, Raw materials comprising the following components by weight: Li2CO3 9-10 parts, Mn(CH3COO2)·4H2O 30-32 parts, Fe(CH3COO)2·4H2O 30-32 parts, NH4H2PO4 28-30 parts, modified ZrO2 coated carbon fiber 2-3 parts, and glucose 5-6 parts; The modified ZrO2 coated carbon fiber is prepared by the following steps: Step a1: stirring and reacting p-xylylformaldehyde, 2-nitrophenylacetonitrile and anhydrous methanol, then adding piperidine and continuing to stir and react while heating, cooling, suction filtering, washing and drying to obtain a first intermediate; Step a2: stirring and reacting tin powder, the first intermediate and anhydrous ethanol, then adding a hydrochloric acid solution and continuing to stir and react while heating, cooling, adjusting pH, extracting, combining and drying the organic phase, and eluting to obtain a carbon fiber modifier; Step a3: stirring and reacting carbon fiber, concentrated nitric acid and concentrated hydrochloric acid, cooling, suction filtering, washing and drying to obtain surface-activated carbon fiber; Step a4: stirring and reacting zirconium nitrate and ethylene glycol, adjusting pH, and continuing to stir to obtain a ZrO2 sol, adding the surface-activated carbon fiber to the ZrO2 sol, ultrasonic treatment, standing, drying, sintering, and cooling to obtain ZrO2 coated carbon fiber; Step a5: weighing the ZrO2 coated carbon fiber, dispersing it in N,N-dimethylformamide, ultrasonic dispersing, adding 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, stirring and reacting, then adding the carbon fiber modifier-N,N-dimethylformamide, continuing to stir and react while heating, cooling, centrifuging, washing and drying to obtain the modified ZrO2 coated carbon fiber. 2.The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, characterized in that, The amount ratio of the p-xylylformaldehyde, 2-nitrophenylacetonitrile, anhydrous methanol and piperidine in step a1 is 1.34-1.50 g:3.24-3.70 g:15-20 mL:1.0-1.3 mL. 3.The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, characterized in that, The amount ratio of the tin powder, the first intermediate, anhydrous ethanol and the hydrochloric acid solution in step a2 is 1.19-1.30 g:2.11-2.50 g:20-25 mL:2.0-2.3 mL.

4. The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, wherein, The amount ratio of the carbon fiber, concentrated nitric acid and concentrated hydrochloric acid in step a3 is 10-12 g:30-45 mL:10-15 mL.

5. The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, wherein the carbon fiber is a carbon nanofiber. The amount ratio of the zirconium nitrate, ethylene glycol and surface-activated carbon fiber in step a4 is 5-7 g:50-70 mL:10-12 g.

6. The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, wherein, The amount ratio of the ZrO2 coated carbon fiber, N,N-dimethylformamide, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and the carbon fiber modifier-N,N-dimethylformamide in step a5 is 10-12 g:100-120 mL:0.5-0.7 g:20-24 mL.

7. The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, wherein the carbon fiber is a carbon nanofiber. The mass fraction of the hydrochloric acid solution in step a2 is 37%.

8. The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, wherein, The carbon fiber in step a3 is PAN-based carbon fiber filament with a diameter of 7-10 μm and a length of 1 cm; the mass fraction of the concentrated nitric acid is 68%; and the mass fraction of the concentrated hydrochloric acid is 37%.

9. The lithium iron manganese phosphate / carbon fiber composite cathode material of claim 1, wherein, The carbon fiber modifier - N, N-dimethylformamide in step a5 is a solution of carbon fiber modifier, N, N-dimethylformamide mixed in a ratio of 0.5 g: 20 mL.

10. A method for preparing a lithium iron manganese phosphate / carbon fiber composite cathode material, characterized in that, The method for preparing the lithium iron manganese phosphate / carbon fiber composite positive electrode material as claimed in any one of claims 1-9 comprises the following steps: Step one: weigh Li2CO3, Mn(CH3COO)2·4H2O, Fe(CH3COO)2·4H2O and NH4H2PO4, dissolve in deionized water, add citric acid, stir for 2h to form a uniform lithium manganese iron phosphate precursor sol; ultrasonic dispersion of modified ZrO2 coated carbon fiber and N-methyl pyrrolidone, then add lithium manganese iron phosphate precursor sol and glucose, stir and react, transfer to a planetary ball mill, ball mill for 2-3h to form a slurry; Step two: the slurry is treated by a spray dryer, the powder is collected, and the powder is placed in a tube furnace, heated for 4h in the tube furnace, and cooled to room temperature with the furnace, to obtain a lithium iron manganese phosphate / carbon fiber composite positive electrode material.

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