Chromium-molybdenum double-doped lithium iron phosphate material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a chromium-molybdenum double-doped lithium iron phosphate material as well as a preparation method and application thereof. The preparation method is used for solving the problems of low capacity, low electronic conductivity and poor cycling stability of the existing lithium iron phosphate material. According to the preparation method, chromium-molybdenum is doped into lithium iron phosphate, and the bond energy of a Cr-O bond and a Mo-O bond is stronger than that of a Fe-O bond, so that the dissolution loss of Fe < 2 + > in long-term circulation is reduced, and the battery capacity and the intrinsic electron conductivity of the material are improved; lithium iron phosphate is coated with nitrogen-doped carbon nanotubes, and nitrogen is doped in graphite crystal lattices of the carbon nanotubes, so that additional free electrons are provided, the intrinsic conductivity is improved, the resistance is reduced, the rate capability is improved, and the cycle life is prolonged; the polyaniline coating layer forms a barrier layer on the surface of the lithium iron phosphate particles, so that the cycle performance and the battery capacity are improved; the three components cooperate to improve the capacity, conductivity, rate capability and cycling stability of the material.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion batteries, in particular to a chromium-molybdenum double-doped lithium iron phosphate material and a preparation method and application thereof. BACKGROUND

[0002] In the technical field of lithium ion batteries, improving charging efficiency, prolonging battery life and enhancing energy storage performance are the goals that researchers are constantly pursuing. Among these goals, the selection and improvement of positive electrode materials are particularly important. At present, lithium iron phosphate, as a stable positive electrode material, has attracted widespread attention due to its good safety and cycle stability. However, lithium iron phosphate materials still have some deficiencies in practical application, such as low capacity, low electronic conductivity and poor cycle stability. Therefore, modifying lithium iron phosphate materials through doping to further improve their performance has become an important research direction in the field of lithium ion batteries.

[0003] Traditional doping methods mainly replace part of lithium or iron ions in the material with a small total amount of metal ions. Although this method can improve certain performance, it may also introduce new problems such as instability of material structure and inconsistency of electrochemical performance. In this context, exploring more effective doping elements and methods is the key to improving the performance of lithium iron phosphate materials. Double doping, that is, doping with two elements at the same time, can better balance the structure and electrochemical performance of these materials, thereby achieving the purpose of improving overall performance.

[0004] Therefore, the development of a chromium-molybdenum double-doped lithium iron phosphate material and a preparation method and application thereof is of great significance for promoting the development of lithium ion battery technology. SUMMARY

[0005] In order to overcome the above technical problems, the purpose of the present application is to provide a chromium-molybdenum double-doped lithium iron phosphate material and a preparation method and application thereof. The problem of low capacity, low electronic conductivity and poor cycle stability of existing lithium iron phosphate is solved.

[0006] The purpose of the present application can be achieved by the following technical solutions: In a first aspect, the present application provides a preparation method of a chromium-molybdenum double-doped lithium iron phosphate material, comprising the following steps: Step a1: iron source and H2O2 solution are added to a three-necked flask equipped with a stirrer and a thermometer, and magnetic stirring is performed for 1-2 h. CrCl3.6H2O, (NH4)6Mo7O 24• 4H2O and NH4H2PO4 were added into a beaker, mixed and stirred for 1 h to obtain a doping solution; the doping solution was added into the above three-necked flask, mixed and stirred for 30 min, then transferred into a wet ball mill tank, a dispersant was added, the ball-to-material ratio was 10:1, the rotation speed was 350-400 r / min, ball milling was performed for 4-6 h, after the ball milling was completed, the balls and the material were separated, the material was placed in a vacuum drying oven at 100 ℃ for drying for 12 h, after the drying was completed, the material was ground in a mortar for 1-2 h, to obtain chromium-molybdenum double-doped iron phosphate.

[0007] Step a2: chromium-molybdenum double-doped iron phosphate and Li2CO3 were added into a grinder and ground for 10-14 h to obtain a ground material, then the ground material, a carbon source and anhydrous ethanol were added into a planetary ball mill, the ball-to-material ratio was 5:1, the rotation speed was 350-450 r / min, ball milling was performed for 2-4 h, the balls and the material were separated, the material was added into a spray dryer for spray drying, the inlet temperature was 200 ℃, the outlet temperature was 100 ℃, the feeding rate was 10 mL / min, to obtain a dry powder, the dry powder was added into a tube furnace for sintering, argon was introduced for protection, the temperature was increased to 350 ℃ at a rate of 5 ℃ / min, and kept for 4 h, then the temperature was continuously increased to 700 ℃, and kept for 10 h, and then naturally cooled to 25 ℃, to obtain modified lithium iron phosphate.

[0008] Step a3: melamine, Ni(NO3)2·6H2O and deionized water were added into a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic treatment was performed for 1 h to obtain a suspension, the modified lithium iron phosphate was added into the above suspension, and stirred at a water bath at 60 ℃ for 1-2 h, and then placed in a tube furnace, and the temperature was increased to 550 ℃ at a rate of 5 ℃ / min under an argon atmosphere, and kept for 30 min, then the temperature was increased to 750-800 ℃ under a mixed gas, and kept for 1-2 h, and then naturally cooled to 25 ℃, and a hydrochloric acid solution was added, and stirred at 60-80 ℃ for 4-6 h, then suction filtration was performed, the filter cake was washed with anhydrous ethanol for 2-3 times, and dried at 80 ℃ for 1-2 h, to obtain nitrogen-doped carbon nanotube-coated lithium iron phosphate.

[0009] Step a4: nitrogen-doped carbon nanotube-coated lithium iron phosphate and an HCl solution were added into a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic dispersion was performed for 30 min, then transferred into an ice water bath at 0-5 ℃, and magnetically stirred for 1-2 h, then aniline was added, and continued to stir for 1 h, to obtain a dispersion solution; ammonium persulfate was added into the above dispersion solution, and reacted in an ice water bath at 0-5 ℃ for 12-14 h, after the reaction was completed, suction filtration was performed, the filter cake was washed with deionized water and anhydrous ethanol for 3-4 times, after the washing was completed, vacuum drying was performed at 60 ℃ for 12-24 h, to obtain chromium-molybdenum double-doped lithium iron phosphate.

[0010] As a preferred embodiment of the present application, the iron source in step a1, the H2O2 solution, the CrCl3·6H2O, the (NH4)6Mo7O 24 The dosage ratio of the FeC2O4·2H2O, the H2O2 solution, the CrCl3·6H2O, the (NH4)6Mo7O

[0011] As a preferred embodiment of the present application, the iron source in step a1 is one of FeC2O4·2H2O and FeSO4·7H2O; the mass fraction of the H2O2 solution is 27.5%.

[0012] As a preferred embodiment of the present application, the dosage ratio of the chromium-molybdenum double-doped iron phosphate, Li2CO3, the carbon source and the anhydrous ethanol in step a2 is 145-155g: 1.05-1.1mol: 70-75g: 200-300mL.

[0013] As a preferred embodiment of the present application, the carbon source in step a2 is one of sucrose and glucose.

[0014] As a preferred embodiment of the present application, the dosage ratio of the melamine, the Ni(NO3)2·6H2O, the deionized water, the modified lithium iron phosphate and the hydrochloric acid solution in step a3 is 1g: 0.1g: 50mL: 10g: 100-200mL.

[0015] As a preferred embodiment of the present application, the mixed gas in step a3 is a mixed gas of H2 and Ar in a volume ratio of 5:95; the molar concentration of the hydrochloric acid solution is 1-2mol / L.

[0016] As a preferred embodiment of the present application, the dosage ratio of the nitrogen-doped carbon nanotube-coated lithium iron phosphate, the HCl solution, the aniline and the ammonium persulfate in step a4 is 1-2g: 100-110mL: 0.1-0.2g: 0.25-0.5g.

[0017] As a preferred embodiment of the present application, the molar concentration of the HCl solution in step a4 is 1mol / L.

[0018] In the second aspect, the present application provides a chromium-molybdenum double-doped lithium iron phosphate material, which is prepared according to the preparation method of the chromium-molybdenum double-doped lithium iron phosphate material described in the first aspect.

[0019] In the third aspect, the present application provides the application of the chromium-molybdenum double-doped lithium iron phosphate material described in the first aspect in a battery.

[0020] The present application has the following beneficial effects: This invention relates to a chromium-molybdenum co-doped lithium iron phosphate material, its preparation method, and its application. The method involves adding an iron source and H₂O₂ solution to a three-necked flask and stirring magnetically. CrCl₃·6H₂O and (NH₄)₆Mo₇O₇ are then added. 24 • 4H2O and NH4H2PO4 were mixed and stirred to obtain a doped solution; the doped solution was added to the above three-necked flask, transferred to a wet ball mill jar for ball milling, drying, and grinding to obtain chromium-molybdenum co-doped iron phosphate; Cr 3+ and Mo 6+ ionic radius and Fe 2+ Unlike other materials, doping into the crystal lattice and substituting Fe sites causes lattice distortion and expands the Li... + Diffusion channels, reducing Li + Migration resistance; high valence state Mo 6+ Doping creates lithium vacancies, which occupy Fe. 2+ When at a site, Li needs to be consumed. + To compensate for the charge, a large number of lithium vacancies are introduced, accelerating ion diffusion kinetics and allowing a large amount of Li to still be present at high rates. + It can participate in the reaction, thus having a higher reversible capacity; Cr 3+ Mo 6+ Fe replacement 2+ At this time, a larger number of charge compensation defects are generated, introducing a large number of electron carriers and greatly improving the intrinsic electronic conductivity of the material; the bond energies of Cr-O and Mo-O bonds are stronger than those of Fe-O bonds, which strengthens the olivine crystal structure of lithium iron phosphate and inhibits the Li-O bond. + The fatigue and damage caused by lattice expansion and contraction during repeated insertion and extraction processes make the structure more robust, and stronger Mo-O bonds reduce Fe degradation during long-term cycling. 2+ To improve battery capacity, chromium-molybdenum co-doped iron phosphate and Li2CO3 were mixed and ground, then a carbon source and anhydrous ethanol were added and ball-milled. The balls and materials were separated, and the materials were dried, sintered, and naturally cooled to obtain modified lithium iron phosphate. The carbon source served as a reducing agent to prevent Fe from dissolving and leaching. 2+Oxidation, ensure the correct phase generation, the first carbon-coated conductive network, greatly improve the electronic conductivity, inhibit the excessive growth of grain, control the particle size, improve the tap density and material processing performance; the melamine, Ni(NO3)2.6H2O and deionized water are mixed to obtain a suspension, and the modified lithium iron phosphate is added to the water bath and stirred, sintered and cooled to obtain nitrogen-doped carbon nanotube coated lithium iron phosphate; Ni(NO3)2.6H2O as catalyst, melamine provides carbon source, nitrogen source, through pyrolysis into nitrogen-doped carbon nanotube to coat lithium iron phosphate, nitrogen atom has one more electron than carbon atom, when nitrogen atom doped into the graphite lattice of carbon nanotube, it will provide additional free electrons, make carbon nanotube from metal to n-type semiconductor, improve intrinsic conductivity, carbon nanotube has one-dimensional tubular structure, extremely high length-diameter ratio and excellent intrinsic conductivity, when nitrogen-doped carbon nanotube is coated on lithium iron phosphate particles, three-dimensional conductive network is formed, which greatly reduces the overall resistance of the electrode, improves the rate performance, introduces defect anchor points, enhances the bonding force, avoids the carbon nanotube from the surface of lithium iron phosphate in the battery cycle process, ensures the long-term stability of the conductive network, thereby improves the cycle life; the nitrogen-doped carbon nanotube coated lithium iron phosphate, HCl solution is ultrasonically dispersed, ice water bath magnetic stirring, adding aniline, continue to stir, adding ammonium persulfate, ice water bath reaction, suction filtration, filter cake washing, drying, to obtain chromium and molybdenum double-doped lithium iron phosphate; aniline in situ polymerization on the surface of lithium iron phosphate, forming a polyaniline coating layer; the polyaniline coating layer forms a dense and stable physical barrier layer on the surface of lithium iron phosphate particles, significantly reducing the direct contact area of active material and electrolyte, effectively inhibiting the corrosion of electrolyte on lithium iron phosphate, thereby reducing the iron dissolution phenomenon, protecting the active material, improving the cycle capacity retention rate, polyaniline itself is an electroactive material, which stores and releases charges through its own oxidation-reduction reaction, this process is called capacitance, thereby improving the battery capacity; the three of bulk doping, carbon coating and polymer interfacial protection synergistically improve the capacity, conductivity, rate performance and cycle stability of the material. BRIEF DESCRIPTION OF DRAWINGS

[0021] The application will be further described below with reference to the drawings.

[0022] Fig. 1 is the capacity test result schematic diagram of lithium ion battery prepared from chromium and molybdenum double-doped lithium iron phosphate material in examples 1-3 and comparative examples 1-3 in the application.

[0023] Fig. 2 is the powder conductivity test result schematic diagram of lithium ion battery prepared from chromium and molybdenum double-doped lithium iron phosphate material in examples 1-3 and comparative examples 1-3 in the application.

[0024] Fig. 3is a schematic diagram of 1C cycle 2000 times capacity retention test results of lithium ion batteries prepared from chromium-molybdenum double-doped lithium iron phosphate materials in examples 1-3 and comparative examples 1-3 of the present application. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0026] Example 1 The present embodiment is a preparation method of chromium-molybdenum double-doped lithium iron phosphate material, comprising the following steps: Step S1: 0.96 mol FeC2O4·2H2O and 100 mL of 27.5% mass fraction H2O2 solution are added to a three-necked flask equipped with a stirrer and a thermometer, and magnetically stirred for 1 h. 0.01 mol CrCl3·6H2O, 3 g (NH4)6Mo7O 24 ·4H2O and 1 mol NH4H2PO4 are added to a beaker, mixed and stirred for 1 h to obtain a doping solution. The doping solution is added to the above three-necked flask, mixed and stirred for 30 min, then transferred to a wet ball mill tank, and a dispersant is added. The ball-to-material ratio is 10:1, and the rotation speed is 350 r / min. Ball milling is performed for 4 h. After ball milling is completed, the balls and the material are separated, the material is placed in a vacuum drying oven at 100℃ for drying for 12 h, and after drying is completed, the material is ground in a mortar for 1 h to obtain chromium-molybdenum double-doped iron phosphate.

[0027] Step S2: 145 g of chromium-molybdenum double-doped iron phosphate and 1.05 mol Li2CO3 are added to a grinder and ground for 10 h to obtain a ground material. Then the ground material, 70 g of sucrose and 200 mL of anhydrous ethanol are added to a planetary ball mill. The ball-to-material ratio is 5:1, and the rotation speed is 350 r / min. Ball milling is performed for 2 h. The balls and the material are separated, and the material is added to a spray dryer for spray drying. The inlet temperature is 200℃, the outlet temperature is 100℃, and the feeding rate is 10 mL / min. Dry powder is obtained. The dry powder is added to a tube furnace for sintering. Argon gas is introduced for protection. The temperature is increased to 350℃ at a rate of 5℃ / min, and the temperature is maintained for 4 h. Then the temperature is continuously increased to 700℃, and the temperature is maintained for 10 h. The temperature is naturally cooled to 25℃ to obtain modified lithium iron phosphate.

[0028] Step S3: 1 g melamine, 0.1 g Ni(NO3)2·6H2O and 50 mL deionized water were added to a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic treatment was performed for 1 h to obtain a suspension, 10 g modified lithium iron phosphate was added to the suspension, and stirring was performed at 60°C for 1 h in a water bath, and then the mixture was placed in a tube furnace, heated to 550°C at a rate of 5°C / min under an argon atmosphere, and kept at 550°C for 30 min, and then heated to 750°C under a mixed gas of H2 and Ar at a volume ratio of 5:95, and kept at 750°C for 1 h, and then naturally cooled to 25°C, 100 mL of a hydrochloric acid solution with a molar concentration of 1 mol / L was added, stirring was performed at 60°C for 4 h, and then suction filtration was performed, the filter cake was washed twice with anhydrous ethanol, and drying was performed at 80°C for 1 h to obtain nitrogen-doped carbon nanotube-coated lithium iron phosphate.

[0029] Step S4: 1 g of the nitrogen-doped carbon nanotube-coated lithium iron phosphate and 100 mL of a hydrochloric acid solution with a molar concentration of 1 mol / L were added to a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic dispersion was performed for 30 min, and then the mixture was transferred to an ice water bath at 0°C and magnetically stirred for 1 h, 0.1 g of aniline was added, and stirring was continued for 1 h to obtain a dispersion solution; 0.25 g of ammonium persulfate was added to the dispersion solution, and reaction was performed in an ice water bath at 0°C for 12 h, and then suction filtration was performed after the reaction was completed, the filter cake was washed three times with deionized water and anhydrous ethanol, and vacuum drying was performed at 60°C for 12 h after the washing was completed to obtain chromium-molybdenum double-doped lithium iron phosphate.

[0030] Example 2 The embodiment is a preparation method of chromium-molybdenum double-doped lithium iron phosphate material, and includes the following steps: Step S1: 0.98 mol of FeC2O4·2H2O, 105 mL of a hydrogen peroxide solution with a mass fraction of 27.5% and 1 mol of NH4H2PO4 were added to a three-necked flask equipped with a stirrer and a thermometer, and magnetically stirred for 1.5 h, 0.02 mol of CrCl3·6H2O, 3 g of (NH4)6Mo7O 24 24H2O and 1 mol of NH4H2PO4 were added to a beaker, mixed and stirred for 1 h to obtain a doping solution; the doping solution was added to the three-necked flask, mixed and stirred for 30 min, and then transferred to a wet ball mill tank, a dispersant was added, the ball-to-material ratio was 10:1, and the rotation speed was 375 r / min for ball milling for 5 h, the ball and the material were separated after the ball milling was completed, the material was placed in a vacuum drying oven at 100°C for drying for 12 h, and grinding was performed in a mortar for 1.5 h after the drying was completed to obtain chromium-molybdenum double-doped iron phosphate.

[0031] Step S2: 150 g of chromium-molybdenum double-doped iron phosphate, 1.08 mol of Li2CO3 were added to a grinder and ground for 12 h to obtain a ground material, and then the ground material, 73 g of sucrose, and 250 mL of anhydrous ethanol were added to a planetary ball mill at a ball-to-material ratio of 5:1 and a rotation speed of 400 r / min, and ball milling was performed for 3 h, and then the material was separated from the balls and added to a spray dryer for spray drying at an inlet temperature of 200 ℃, an outlet temperature of 100 ℃, and a feeding rate of 10 mL / min to obtain a dry powder, and the dry powder was added to a tube furnace for sintering, argon was introduced for protection, and the temperature was increased to 350 ℃ at a rate of 5 ℃ / min, and then the temperature was kept at 350 ℃ for 4 h, and then the temperature was continuously increased to 700 ℃, and the temperature was kept at 700 ℃ for 10 h, and then the temperature was naturally cooled to 25 ℃ to obtain modified lithium iron phosphate.

[0032] Step S3: 1 g of melamine, 0.1 g of Ni(NO3)2·6H2O, and 50 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic dispersion was performed for 1 h to obtain a suspension, 10 g of modified lithium iron phosphate was added to the suspension, and stirring was performed at 60 ℃ for 1.5 h, and then the mixture was placed in a tube furnace, argon was introduced for protection, the temperature was increased to 550 ℃ at a rate of 5 ℃ / min, and then the temperature was kept at 550 ℃ for 30 min, and then the temperature was increased to 775 ℃ under a mixed gas of H2 and Ar at a volume ratio of 5:95, and the temperature was kept at 775 ℃ for 1.5 h, and then the temperature was naturally cooled to 25 ℃, 150 mL of a hydrochloric acid solution with a molar concentration of 1.5 mol / L was added, stirring was performed at 70 ℃ for 5 h, and then suction filtration was performed, the filter cake was washed with anhydrous ethanol for 3 times, and drying was performed at 80 ℃ for 1.5 h to obtain nitrogen-doped carbon nanotube-coated lithium iron phosphate.

[0033] Step S4: 1.5 g of nitrogen-doped carbon nanotube-coated lithium iron phosphate and 105 mL of an HCl solution with a molar concentration of 1 mol / L were added to a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic dispersion was performed for 30 min, and then the mixture was transferred to an ice water bath at 3 ℃, and magnetic stirring was performed for 1.5 h, 0.15 g of aniline was added, and stirring was continuously performed for 1 h to obtain a dispersion solution, 0.35 g of ammonium persulfate was added to the dispersion solution, and reaction was performed in an ice water bath at 3 ℃ for 13 h, suction filtration was performed after the reaction was completed, the filter cake was washed with deionized water and anhydrous ethanol for 4 times, and vacuum drying was performed at 60 ℃ for 18 h after the washing was completed to obtain chromium-molybdenum double-doped lithium iron phosphate.

[0034] Example 3 The embodiment is a preparation method of a chromium-molybdenum double-doped lithium iron phosphate material, and the method comprises the following steps: Step S1: 1 mol of FeC2O4·2H2O and 110 mL of a H2O2 solution with a mass fraction of 27.5% were added to a three-necked flask equipped with a stirrer and a thermometer, and magnetic stirring was performed for 2 h, 0.02 mol of CrCl3·6H2O, 4 g of (NH4)6Mo7O 24• 4H2O and 1 mol NH4H2PO4 were added into a beaker, mixed and stirred for 1 h to obtain a doping solution; the doping solution was added into the three-necked flask above, mixed and stirred for 30 min, then transferred into a wet ball mill tank, a dispersant was added, the ball-to-material ratio was 10:1, the rotation speed was 400 r / min, and ball milling was performed for 6 h; after the ball milling was completed, the balls and the material were separated, the material was placed in a vacuum drying oven at 100 ℃ for drying for 12 h, after the drying was completed, grinding was performed in a mortar for 2 h, and chromium-molybdenum double-doped iron phosphate was obtained.

[0035] Step S2: 155 g of chromium-molybdenum double-doped iron phosphate and 1.1 mol of Li2CO3 were added into a grinder and ground for 14 h to obtain a ground material; then the ground material, 75 g of sucrose, and 300 mL of anhydrous ethanol were added into a planetary ball mill, the ball-to-material ratio was 5:1, the rotation speed was 450 r / min, ball milling was performed for 4 h, the balls and the material were separated, the material was added into a spray dryer for spray drying, the inlet temperature was 200 ℃, the outlet temperature was 100 ℃, and the feeding rate was 10 mL / min, and a dry powder was obtained; the dry powder was added into a tube furnace for sintering, argon was introduced for protection, the temperature was increased to 350 ℃ at a rate of 5 ℃ / min, and the temperature was maintained for 4 h; then the temperature was continuously increased to 700 ℃, and the temperature was maintained for 10 h; and the temperature was naturally cooled to 25 ℃, and modified lithium iron phosphate was obtained.

[0036] Step S3: 1 g of melamine, 0.1 g of Ni(NO3)2·6H2O, and 50 mL of deionized water were added into a three-necked flask provided with a stirrer and a thermometer, and ultrasonic treatment was performed for 1 h to obtain a suspension; 10 g of modified lithium iron phosphate was added into the suspension, stirring was performed under a water bath at 60 ℃ for 2 h, and the mixture was placed in a tube furnace; argon was introduced for protection, the temperature was increased to 550 ℃ at a rate of 5 ℃ / min, and the temperature was maintained for 30 min; then the temperature was increased to 800 ℃ under a mixed gas of H2 and Ar at a volume ratio of 5:95, and the temperature was maintained for 2 h; the temperature was naturally cooled to 25 ℃, 200 mL of a hydrochloric acid solution with a molar concentration of 2 mol / L was added, stirring was performed at 80 ℃ for 6 h, then suction filtration was performed, the filter cake was washed with anhydrous ethanol for 3 times, and drying was performed at 80 ℃ for 2 h, and nitrogen-doped carbon nanotube-coated lithium iron phosphate was obtained.

[0037] Step S4: 2 g of nitrogen-doped carbon nanotube-coated lithium iron phosphate and 110 mL of an HCl solution with a molar concentration of 1 mol / L were added into a three-necked flask provided with a stirrer and a thermometer, ultrasonic dispersion was performed for 30 min, the mixture was transferred into an ice water bath at 5 ℃, and magnetic stirring was performed for 2 h; 0.2 g of aniline was added, and stirring was continuously performed for 1 h to obtain a dispersion solution; 0.5 g of ammonium persulfate was added into the dispersion solution, and reaction was performed in an ice water bath at 5 ℃ for 14 h; after the reaction was completed, suction filtration was performed, the filter cake was washed with deionized water and anhydrous ethanol for 4 times, after the washing was completed, vacuum drying was performed at 60 ℃ for 24 h, and chromium-molybdenum double-doped lithium iron phosphate was obtained.

[0038] Comparative Example 1 The present comparative example is a preparation method of a chromium-molybdenum double-doped lithium iron phosphate material, comprising the following steps: Step S1: 0.98 mol of FeC2O4·2H2O and 105 mL of a 27.5% by mass H2O2 solution were added to a three-necked flask equipped with a stirrer and a thermometer, and magnetically stirred for 1.5 h. 0.02 mol of CrCl3·6H2O, 3 g of (NH4)6Mo7O 24 4H2O, and 1 mol of NH4H2PO4 were added to a beaker, mixed and stirred for 1 h to obtain a doping solution. The doping solution was added to the above three-necked flask, mixed and stirred for 30 min, and then transferred to a wet ball mill tank. A dispersant was added, the ball-to-material ratio was 10:1, and the rotation speed was 375 r / min. Ball milling was performed for 5 h. After ball milling was completed, the balls and the material were separated, the material was placed in a vacuum drying oven at 100°C and dried for 12 h. After drying was completed, the material was ground in a mortar for 1.5 h to obtain chromium-molybdenum double-doped iron phosphate.

[0039] Step S2: 150 g of the chromium-molybdenum double-doped iron phosphate, 1.08 mol of Li2CO3 were added to a grinder and ground for 12 h to obtain a ground material. Then, the ground material, 73 g of sucrose, and 250 mL of anhydrous ethanol were added to a planetary ball mill, the ball-to-material ratio was 5:1, the rotation speed was 400 r / min, and ball milling was performed for 3 h. The balls and the material were separated, and the material was added to a spray dryer for spray drying. The inlet temperature was 200°C, the outlet temperature was 100°C, the feeding rate was 10 mL / min, and dry powder was obtained. The dry powder was added to a tube furnace for sintering. Argon was introduced for protection, the temperature was increased to 350°C at a rate of 5°C / min, and the temperature was maintained for 4 h. Then, the temperature was continuously increased to 700°C, and the temperature was maintained for 10 h. The temperature was naturally cooled to 25°C to obtain chromium-molybdenum double-doped lithium iron phosphate.

[0040] Comparative Example 2 The present comparative example is a preparation method of a chromium-molybdenum double-doped lithium iron phosphate material, comprising the following steps: Step S1: 0.98 mol of FeC2O4·2H2O and 105 mL of a 27.5% by mass H2O2 solution were added to a three-necked flask equipped with a stirrer and a thermometer, and magnetically stirred for 1.5 h. 0.02 mol of CrCl3·6H2O, 3 g of (NH4)6Mo7O 24• 4H2O and 1 mol NH4H2PO4 were added into a beaker, mixed and stirred for 1 h to obtain a doping solution; the doping solution was added into the three-necked flask above, mixed and stirred for 30 min, then transferred into a wet ball mill tank, a dispersant was added, the ball-to-material ratio was 10:1, the rotation speed was 375 r / min, and ball milling was performed for 5 h; after the ball milling was completed, the balls and the material were separated, the material was placed in a vacuum drying oven at 100 ℃ and dried for 12 h; after the drying was completed, the material was ground in a mortar for 1.5 h to obtain chromium-molybdenum double-doped iron phosphate.

[0041] Step S2: 150 g of chromium-molybdenum double-doped iron phosphate and 1.08 mol of Li2CO3 were added into a grinder and ground for 12 h to obtain a ground material; then the ground material, 73 g of sucrose, and 250 mL of anhydrous ethanol were added into a planetary ball mill, the ball-to-material ratio was 5:1, the rotation speed was 400 r / min, and ball milling was performed for 3 h; the material was separated from the balls, added into a spray dryer, and spray dried, the inlet temperature was 200 ℃, the outlet temperature was 100 ℃, and the feeding rate was 10 mL / min to obtain a dry powder; the dry powder was added into a tube furnace and sintered, argon was introduced for protection, the temperature was increased to 350 ℃ at a rate of 5 ℃ / min, and the temperature was maintained for 4 h; then the temperature was continuously increased to 700 ℃, and the temperature was maintained for 10 h; the temperature was naturally cooled to 25 ℃ to obtain modified lithium iron phosphate.

[0042] Step S3: 1 g of melamine, 0.1 g of Ni(NO3)2·6H2O, and 50 mL of deionized water were added into a three-necked flask equipped with a stirrer and a thermometer, and ultrasonic treatment was performed for 1 h to obtain a suspension; 10 g of modified lithium iron phosphate was added into the suspension, and stirring was performed at a water bath temperature of 60 ℃ for 1.5 h; the mixture was placed in a tube furnace, and the temperature was increased to 550 ℃ at a rate of 5 ℃ / min under an argon atmosphere; the temperature was maintained for 30 min; then the temperature was increased to 775 ℃ under a mixed gas of H2 and Ar at a volume ratio of 5:95, and the temperature was maintained for 1.5 h; the temperature was naturally cooled to 25 ℃; 150 mL of a hydrochloric acid solution with a molar concentration of 1.5 mol / L was added, stirring was performed at 70 ℃ for 5 h; then suction filtration was performed, the filter cake was washed with anhydrous ethanol for 3 times, and drying was performed at 80 ℃ for 1.5 h to obtain chromium-molybdenum double-doped lithium iron phosphate.

[0043] Comparative Example 3 The present comparative example is a preparation method of a chromium-molybdenum double-doped lithium iron phosphate material, which comprises the following steps: Step S1: 0.98 mol of FeC2O4·2H2O and 105 mL of a 27.5% H2O2 solution were added into a three-necked flask equipped with a stirrer and a thermometer, and magnetic stirring was performed for 1.5 h; 0.02 mol of CrCl3·6H2O, 3 g of (NH4)6Mo7O 24· 4H2O and 1 mol NH4H2PO4 are added into a beaker, mixed and stirred for 1 h to obtain a doping solution; the doping solution is added into the three-necked flask above, mixed and stirred for 30 min, then transferred into a wet ball mill tank, a dispersant is added, the ball-to-material ratio is 10:1, the rotating speed is 375 r / min, and ball milling is performed for 5 h; after the ball milling is completed, the balls and the material are separated, the material is placed in a vacuum drying box at 100 ℃ and dried for 12 h; after the drying is completed, the material is ground in a mortar for 1.5 h to obtain chromium-molybdenum double-doped iron phosphate.

[0044] Step S2: 150 g of chromium-molybdenum double-doped iron phosphate and 1.08 mol of Li2CO3 are added into a grinder and ground for 12 h to obtain a ground material; then the ground material, 73 g of sucrose, and 250 mL of anhydrous ethanol are added into a planetary ball mill, the ball-to-material ratio is 5:1, the rotating speed is 400 r / min, and ball milling is performed for 3 h; the balls and the material are separated, the material is added into a spray dryer for spray drying, the inlet temperature is 200 ℃, the outlet temperature is 100 ℃, and the feeding rate is 10 mL / min to obtain a dry powder; the dry powder is added into a tube furnace for sintering, argon gas is introduced for protection, the temperature is increased to 350 ℃ at a rate of 5 ℃ / min, and the temperature is maintained for 4 h; then the temperature is continuously increased to 700 ℃, the temperature is maintained for 10 h, and the temperature is naturally cooled to 25 ℃ to obtain modified lithium iron phosphate.

[0045] Step S3: 1 g of carbon nanotube TOB-TNT-M and 50 mL of deionized water are added into a three-necked flask provided with a stirrer and a thermometer, and ultrasonic dispersion is performed for 1 h to obtain a suspension; 10 g of modified lithium iron phosphate is added into the suspension, stirring is performed at a water bath temperature of 60 ℃ for 1.5 h, and the mixture is placed in a tube furnace; argon gas is introduced for protection, the temperature is increased to 550 ℃ at a rate of 5 ℃ / min, the temperature is maintained for 30 min, then the temperature is increased to 650 ℃ under a mixed gas of H2 and Ar with a volume ratio of 5:95, the temperature is maintained for 8 h, and the temperature is naturally cooled to 25 ℃ to obtain carbon nanotube-coated lithium iron phosphate.

[0046] Step S4: 1.5 g of carbon nanotube-coated lithium iron phosphate and 105 mL of an HCl solution with a molar concentration of 1 mol / L are added into a three-necked flask provided with a stirrer and a thermometer, ultrasonic dispersion is performed for 30 min, the mixture is transferred into an ice water bath at 3 ℃, and magnetic stirring is performed for 1.5 h; 0.15 g of aniline is added, and stirring is continuously performed for 1 h to obtain a dispersion solution; 0.35 g of ammonium persulfate is added into the dispersion solution, and reaction is performed in an ice water bath at 3 ℃ for 13 h; after the reaction is completed, suction filtration is performed, the filter cake is washed with deionized water and anhydrous ethanol for 4 times, vacuum drying is performed at 60 ℃ for 18 h after the washing is completed, and chromium-molybdenum double-doped lithium iron phosphate is obtained.

[0047] The chromium-molybdenum double-doped lithium iron phosphate prepared in Examples 1-3 and Comparative Examples 1-3 was used as a positive electrode material, and then the positive electrode material, a binder PVDF DS202, a conductive agent Super P, and N-methyl pyrrolidone were mixed in a mass ratio of 7:1:2:8, and the positive electrode slurry was prepared by fully stirring, and the positive electrode slurry was uniformly coated on both sides of an aluminum foil; the coated aluminum foil was dried in an oven at 100°C, and then the positive electrode sheet was cut by a mold; lithium was used as a negative electrode, and a separator model was TOB-PP-16; 1 mol of LiPF6 was dissolved in 1 L of a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 to obtain a 1 mol / L LiPF6 electrolyte; the above positive electrode sheet, lithium, separator, and electrolyte were prepared into a button cell model CR2032; the battery capacity was tested according to GB / T 18287-2013; the powder conductivity was measured according to the GB / T 30835-2014 standard; the cycle stability was tested according to the GB / T 31484-2015 standard; and the test results are shown in FIGS. 1-3. Figs. 1-3 Comparison of Examples 1-3 and Comparative Examples 1-3: The doping and surface coating amount of Example 1 is relatively low, resulting in insufficient powder conductivity and cycle performance; the doping amount and process parameters of Example 2 are relatively good, forming a complete conductive network, and the capacity, conductivity, and cycle performance are good; the doping amount of Example 3 is too high, which destroys the integrity of the crystal structure of lithium iron phosphate, and thus defects are generated, so the performance is good but slightly lower than that of Example 2; Comparison of Example 2 and Comparative Example 1: Comparative Example 1 only performs bulk doping and basic carbon coating, which can only solve the problem of internal and surface conductivity of the particles, and the particles still rely on point-to-point physical contact, resulting in a huge resistance, low powder conductivity, and short cycle life; Comparison of Example 2 and Comparative Example 2: Comparative Example 2 already contains Cr / Mo doping, carbon coating, and in-situ growth of nitrogen-doped carbon nanotubes, and has constructed an extremely smooth electron conduction path from the inside to the outside. Comparative Example 2 lacks a polyaniline protective layer, and its interface side reaction cannot be effectively inhibited, resulting in poor capacity and cycle performance of Comparative Example 2; Comparison of Example 2 and Comparative Example 3: Comparative Example 3 does not use nitrogen-doped carbon nanotubes, resulting in low intrinsic conductivity.

[0048] In the description of this specification, the description of the terms "one embodiment", "example", "specific example", and the like means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0049] ​The above merely illustrates and describes the present application, and those skilled in the art can make various modifications or supplements to the described specific embodiments or adopt similar ways to replace, as long as the modifications or supplements do not deviate from the present application or exceed the scope defined by the present application, and should belong to the protection scope of the present application.

Claims

1. A method for preparing a chromium-molybdenum co-doped lithium iron phosphate material, characterized in that, Includes the following steps: Step a1: Add the iron source and H2O2 solution to a three-necked flask equipped with a stirrer and thermometer, stir magnetically, and add CrCl3·6H2O and (NH4)6Mo7O. 24 • 4H2O and NH4H2PO4 are added to a beaker and mixed and stirred to obtain a doped solution; the doped solution is added to the above three-necked flask and mixed and stirred, then transferred to a wet ball mill jar for ball milling. After ball milling, the balls and materials are separated, the materials are dried, and after drying, they are ground to obtain chromium-molybdenum dual-doped iron phosphate; Step a2: Chromium-molybdenum co-doped iron phosphate and Li2CO3 are added to a grinder and ground to obtain grinding material. Then, the grinding material, carbon source and anhydrous ethanol are added to a planetary ball mill for ball milling. The balls and materials are separated. The material is spray dried, sintered and cooled to obtain modified lithium iron phosphate. Step a3: Melamine, Ni(NO3)2·6H2O and deionized water are ultrasonically mixed to obtain a suspension, modified lithium iron phosphate is added, stirred under a bath, placed in a tube furnace and sintered under a mixed gas, naturally cooled, hydrochloric acid solution is added, stirred, filtered, the filter cake is washed and dried to obtain nitrogen-doped carbon nanotube coated lithium iron phosphate. Step a4: Nitrogen-doped carbon nanotube-coated lithium iron phosphate and HCl solution are ultrasonically dispersed, transferred to an ice-water bath, magnetically stirred, aniline is added, and stirring is continued to obtain a dispersion solution; ammonium persulfate is added, and the reaction is carried out in an ice-water bath. After the reaction is completed, the mixture is filtered, the filter cake is washed and dried to obtain chromium-molybdenum dual-doped lithium iron phosphate.

2. The method for preparing a chromium-molybdenum dual-doped lithium iron phosphate material according to claim 1, characterized in that, The iron source, H2O2 solution, CrCl3·6H2O, and (NH4)6Mo7O mentioned in step a1 24 The ratio of ·4H2O and NH4H2PO4 is 0.96-1 mol: 100-110 mL: 0.01-0.02 mol: 3-4 g: 1 mol; the mass fraction of the H2O2 solution is 27.5%; the iron source is one of FeC2O4·2H2O and FeSO4·7H2O.

3. The method for preparing a chromium-molybdenum dual-doped lithium iron phosphate material according to claim 1, characterized in that, The ratio of chromium-molybdenum dual-doped iron phosphate, Li2CO3, carbon source, and anhydrous ethanol in step a2 is 145-155g. 1.05-1.1 mol: 70-75 g: 200-300 mL; the carbon source is either sucrose or glucose.

4. The method for preparing a chromium-molybdenum dual-doped lithium iron phosphate material according to claim 1, characterized in that, In step a3, the ratio of melamine, Ni(NO3)2·6H2O, deionized water, modified lithium iron phosphate, and hydrochloric acid solution is 1g:0.1g:50mL:10g:100-200mL; the molar concentration of the hydrochloric acid solution is 1-2mol / L.

5. The method for preparing a chromium-molybdenum dual-doped lithium iron phosphate material according to claim 1, characterized in that, The mixed gas mentioned in step a3 is a mixture of H2 and Ar in a volume ratio of 5:

95.

6. The method for preparing a chromium-molybdenum dual-doped lithium iron phosphate material according to claim 1, characterized in that, In step a4, the ratio of nitrogen-doped carbon nanotubes coated with lithium iron phosphate, HCl solution, aniline, and ammonium persulfate is 1-2g: 100-110mL: 0.1-0.2g: 0.25-0.5g; the molar concentration of the HCl solution is 1mol / L.

7. A chromium-molybdenum dual-doped lithium iron phosphate material, characterized in that, The chromium-molybdenum dual-doped lithium iron phosphate material is prepared according to any one of claims 1-6.

8. The application of a chromium-molybdenum dual-doped lithium iron phosphate material as described in any one of claims 1-6 in lithium-ion batteries.

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