Lithium iron phosphate material, preparation method and application thereof

Abstract

The application relates to the field of lithium ion batteries, in particular to a lithium iron phosphate material and a preparation method and application thereof, which are used to solve the problems that the existing lithium iron phosphate material has a relatively low discharge specific capacity, a poor cycle performance and a serious influence on the overall performance of a lithium ion battery; the main raw material of the lithium iron phosphate material is double-layer coated silver-doped lithium iron phosphate; the double-layer coated silver-doped lithium iron phosphate is composed of a polypyrrole containing a nitro group and a fluorine element, a carbon layer double-layer coating and silver-doped lithium iron phosphate; the double-layer coated silver-doped lithium iron phosphate can effectively improve the electronic conductivity of the lithium iron phosphate material, promote the rapid transmission of electrolyte ions, significantly improve the cycle stability of the lithium iron phosphate material, further enhance the electrochemical performance of the overall material, especially the cycle stability and the rate performance, and greatly improve the comprehensive performance and service life of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a lithium iron phosphate material and a preparation method and application thereof. BACKGROUND

[0002] In the prior art, lithium ion batteries, as an important energy storage device, have been widely used in the fields of electronic devices, electric vehicles and large-scale energy storage systems. Lithium iron phosphate, as an important positive electrode material of lithium ion batteries, has been widely concerned due to its non-toxicity, environmental protection and low cost. However, the traditional lithium iron phosphate material has some problems: firstly, its discharge specific capacity is relatively low, which limits the energy density of the battery; secondly, the cycle stability of the lithium iron phosphate material needs to be improved, and the lithium iron phosphate material will change in volume during the charging and discharging process, which leads to poor cycle performance and affects the service life of the battery. Therefore, developing a lithium iron phosphate material and a preparation method and application thereof has important practical significance and application value for improving the overall performance of lithium ion batteries. SUMMARY

[0003] In order to overcome the above technical problems, the purpose of the present application is to provide a lithium iron phosphate material and a preparation method and application thereof, which solves the problem of low discharge specific capacity and poor cycle performance of the existing lithium iron phosphate material, which seriously affects the overall performance of lithium ion batteries.

[0004] The purpose of the present application can be achieved by the following technical solutions:

[0005] In a first aspect, the present application provides a lithium iron phosphate material, comprising the following components by weight:

[0006] Double-layer coated silver-doped lithium iron phosphate 8-10 parts, binder 1-1.2 parts, conductive agent 1-1.5 parts and solvent 15-17 parts;

[0007] The double-layer coated silver-doped lithium iron phosphate is prepared by the following steps:

[0008] Step s1: lithium carbonate, iron nitrate nonahydrate, ammonium dihydrogen phosphate and citric acid solution are added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, nitrogen is introduced for protection, and stirring is carried out at a temperature of 25-30℃ and a stirring rate of 200-300r / min for 20-30min, then silver nitrate is added and stirring is continued for 20-30min, then ammonia water is added to adjust the pH to 8.5-9.5, then stirring is continued for 2-4h, after the reaction is completed, the reaction product is left to stand for 10-15h, then it is placed in a vacuum drying oven and dried at a temperature of 80-85℃ for 2-3h to obtain a silver-doped lithium iron phosphate precursor;

[0009] Step s2: the silver-doped lithium iron phosphate precursor, glucose and deionized water are added into a three-necked flask provided with a stirrer, a thermometer and a gas inlet pipe, nitrogen is introduced for protection, stirring is conducted at a temperature of 25-30 DEG C and a stirring speed of 200-300 r / min for 1-2 h, then spray drying is conducted to obtain a particle size of less than or equal to 2 microns, then the product is placed in a tube furnace, argon is introduced for protection, and sintering is conducted at a temperature of 350-360 DEG C with a temperature rising rate of 8-10 DEG C / min for 3-4 h, then sintering is conducted at a temperature of 650-750 DEG C for 6-8 h, and then the furnace is cooled to obtain carbon-coated silver-doped lithium iron phosphate;

[0010] Step s3: pyrrole, potassium tert-butoxide, 5-chloro-2-nitrobenzotrifluoride and anhydrous ether are added into a three-necked flask provided with a stirrer, a thermometer, a gas inlet pipe and a reflux condenser, nitrogen is introduced for protection, stirring is conducted at a temperature of 25-30 DEG C and a stirring speed of 200-300 r / min for 30-40 min, then the temperature is raised to the refluxing temperature and stirring is continued for 7-8 h, after the reaction is completed, the reaction product is cooled to room temperature, then poured into ice water, the organic phase is removed by rotary evaporation, then placed in a vacuum drying oven and dried at a temperature of 50-55 DEG C for 3-4 h, and then purified by silica gel column chromatography with an eluent to obtain modified pyrrole;

[0011] Step s4: anhydrous ferric chloride and ethanol solution are added into a three-necked flask provided with a stirrer and a thermometer, stirring is conducted at a temperature of 0-5 DEG C and a stirring speed of 200-300 r / min for 30-50 min, then carbon-coated silver-doped lithium iron phosphate is added and stirring is continued for 20-30 min, then modified pyrrole is added and the temperature is raised to 25-30 DEG C and stirring is continued for 15-20 h, after the reaction is completed, the reaction product is centrifuged, the precipitate is washed with anhydrous methanol and distilled water for 2-3 times, then placed in a vacuum drying oven and dried at a temperature of 60-65 DEG C for 4-5 h to obtain double-layer coated silver-doped lithium iron phosphate.

[0012] As a further scheme of the present application: the amount ratio of the lithium carbonate, ferric nitrate nonahydrate, ammonium dihydrogen phosphate, citric acid solution and silver nitrate in step s1 is 11-13 mmol: 10 mmol: 10 mmol: 50-60 mL: 0.1-0.5 g.

[0013] As a further scheme of the present application: the mass fraction of the citric acid solution in step s1 is 3-5%; and the mass fraction of the ammonia water is 20-25%.

[0014] As a further scheme of the present application: the amount ratio of the silver-doped lithium iron phosphate precursor, glucose and deionized water in step s2 is 10 g: 1-7 g: 90-100 mL.

[0015] As a further scheme of the present application: the use amount ratio of the pyrrole, potassium tert-butoxide, 5-chloro-2-nitrobenzotrifluoride and anhydrous ether in step s3 is 10 mmol: 10 mmol: 10 mmol: 60-70 mL

[0016] As a further scheme of the present application: the eluent in step s3 is a mixture of petroleum ether and ethyl acetate in a volume ratio of 10-15:3.

[0017] As a further scheme of the present application: the use amount ratio of the anhydrous ferric chloride, ethanol solution, carbon-coated silver-doped lithium iron phosphate and modified pyrrole in step s4 is 2.5-4.5 g: 70-80 mL: 5 g: 0.3-0.9 g.

[0018] As a further scheme of the present application: the volume fraction of the ethanol solution in step s4 is 50-60%.

[0019] In a second aspect, the present application provides a preparation method of a lithium iron phosphate material, comprising the following steps:

[0020] Step one: according to weight parts, 8-10 parts of double-layer coated silver-doped lithium iron phosphate, 1-1.2 parts of a binder, 1-1.5 parts of a conductive agent and 15-17 parts of a solvent are weighed and prepared;

[0021] Step two: the double-layer coated silver-doped lithium iron phosphate, the binder, the conductive agent and the solvent are added to a ball mill, and ball milling is carried out at a ball milling speed of 200-300 r / min for 3-5 h to obtain the lithium iron phosphate material.

[0022] As a further scheme of the present application: the binder is Kynar Flex PVDF 2850-07.

[0023] As a further scheme of the present application: the conductive agent is conductive carbon black Super P.

[0024] As a further scheme of the present application: the solvent is N-methyl pyrrolidone.

[0025] In a third aspect, the present application provides an application of the lithium iron phosphate material prepared by the preparation method of the lithium iron phosphate material.

[0026] The present application has the following beneficial effects:

[0027] The application discloses a lithium iron phosphate material and a preparation method and application thereof, and the lithium iron phosphate material is prepared by ball milling double-layer coated silver-doped lithium iron phosphate, a binder, a conductive agent and a solvent; the main raw material of the lithium iron phosphate material is the double-layer coated silver-doped lithium iron phosphate, which is composed of poly-pyrrole containing nitro and fluorine elements, a carbon layer and silver-doped lithium iron phosphate; the lithium iron phosphate material can effectively improve the electronic conductivity of the lithium iron phosphate material, promote the rapid transmission of electrolyte ions, and significantly improve the cycle stability of the lithium iron phosphate material, thereby enhancing the electrochemical performance of the overall material, especially the cycle stability and rate performance, and greatly improving the comprehensive performance and service life of the battery.

[0028] In the preparation process of the lithium iron phosphate material, a double-layer coated silver-doped lithium iron phosphate is first prepared; silver-doped lithium iron phosphate precursors are prepared by taking lithium carbonate, iron nitrate nonahydrate and ammonium dihydrogen phosphate as raw materials and silver nitrate as a doping substance; the silver-doped lithium iron phosphate precursors are then treated by using glucose as a carbon source, and a carbon layer is formed by sintering to realize primary coating, thereby obtaining carbon-coated silver-doped lithium iron phosphate; modified pyrrole containing nitro and fluorine elements is obtained by the reaction of pyrrole and 5-chloro-2-nitro trifluorotoluene; the modified pyrrole is then polymerized to form poly-pyrrole coated on the surface of the carbon-coated silver-doped lithium iron phosphate, thereby realizing secondary coating and obtaining double-layer coated silver-doped lithium iron phosphate; silver doping can improve the electronic conductivity inside the lithium iron phosphate material; after the carbon layer coating, a continuous conductive network is formed on the surface of the lithium iron phosphate material, the active particles are connected, the contact resistance between the particles is effectively reduced, the electronic conductivity is further enhanced, the volume change of the lithium iron phosphate particles is buffered during the charging and discharging process, the particles are prevented from breaking and pulverizing, and the cycle stability of the lithium iron phosphate material is improved; meanwhile, the pyrrole ring in the poly-pyrrole endows the poly-pyrrole with excellent conductive performance; the nitrogen atom in the nitro group has strong electron-accepting ability; during the charging and discharging process, the nitro group can realize the rapid transmission of electrons through reversible redox reaction, further enhance the conductive performance of the poly-pyrrole, and the introduction of the fluorine atom in the molecular structure of the poly-pyrrole can improve the chemical stability of the poly-pyrrole, so that the poly-pyrrole can resist the erosion and oxidation of the electrolyte, and the conductive performance and cycle stability of the lithium iron phosphate material are further significantly improved, and the lithium iron phosphate material is endowed with excellent electrochemical performance. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments of the application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0030] Embodiment 1:

[0031] The embodiment is a preparation method of a lithium iron phosphate material, comprising the following steps:

[0032] Step A1: 11 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate and 50 mL of a 3% by mass citric acid solution are added into a three-neck flask provided with a stirrer, a thermometer and a gas inlet tube, nitrogen is introduced for protection, and the reaction is stirred at a temperature of 25°C and a stirring speed of 200 r / min for 20 min, then 0.1 g of silver nitrate is added and the reaction is continuously stirred for 20 min, then 20% by mass ammonia water is used to adjust the pH to 8.5, and the reaction is continuously stirred for 2 h, after the reaction is completed, the reaction product is left to stand for aging for 10 h, and then is placed in a vacuum drying box and dried at a temperature of 80°C for 2 h, to obtain a silver-doped lithium iron phosphate precursor;

[0033] Step A2: 10 g of the silver-doped lithium iron phosphate precursor, 1 g of glucose and 90 mL of deionized water are added into a three-neck flask provided with a stirrer, a thermometer and a gas inlet tube, nitrogen is introduced for protection, and the reaction is stirred at a temperature of 25°C and a stirring speed of 200 r / min for 1 h, then is spray dried to a particle size of ≤2 μm, then is placed in a tube furnace, argon is introduced for protection, and the temperature is raised to 350°C at a temperature raising rate of 8°C / min, and sintering is performed for 3 h, then the temperature is raised to 650°C, and sintering is performed for 6 h, and then the furnace is cooled, to obtain carbon-coated silver-doped lithium iron phosphate;

[0034] Step A3: 10 mmol of pyrrole, 10 mmol of potassium tert-butoxide, 10 mmol of 5-chloro-2-nitrobenzotrifluoride and 60 mL of anhydrous ether are added into a three-neck flask provided with a stirrer, a thermometer, a gas inlet tube and a reflux condenser, nitrogen is introduced for protection, and the reaction is stirred at a temperature of 25°C and a stirring speed of 200 r / min for 30 min, then the temperature is raised to the refluxing temperature, and the reaction is continuously stirred for 7 h, after the reaction is completed, the reaction product is cooled to room temperature, then is poured into ice water, the organic phase is rotary evaporated to remove the solvent, then is placed in a vacuum drying box and dried at a temperature of 50°C for 3 h, then is purified by silica gel column chromatography using an eluent prepared by mixing petroleum ether and ethyl acetate at a volume ratio of 10:3, to obtain modified pyrrole;

[0035] Step A4: 2.5 g of anhydrous ferric chloride, 70 mL of an ethanol solution with a volume fraction of 50% were added to a three-necked flask equipped with a stirrer, a thermometer, and stirred at a temperature of 0°C and a stirring rate of 200 r / min for 30 min, then 5 g of carbon-coated silver-doped lithium iron phosphate was added and the reaction was continued for 20 min, then 0.3 g of modified pyrrole was added and the temperature was raised to 25°C and the reaction was continued for 15 h. After the reaction was completed, the reaction product was centrifuged, the precipitate was washed with anhydrous methanol and distilled water twice, and then placed in a vacuum drying oven and dried at a temperature of 60°C for 4 h to obtain double-layer coated silver-doped lithium iron phosphate;

[0036] Step A5: Double-layer coated silver-doped lithium iron phosphate 8 parts, binder 1 part, conductive agent 1 part, and solvent 15 parts were weighed according to weight parts and prepared for use. The binder was Kynar Flex PVDF 2850-07, the conductive agent was conductive carbon black Super P, and the solvent was N-methyl pyrrolidone.

[0037] Step A6: Double-layer coated silver-doped lithium iron phosphate, binder, conductive agent, and solvent were added to a ball mill and ball milled at a ball milling speed of 200 r / min for 3 h to obtain lithium iron phosphate material.

[0038] Example 2:

[0039] The present embodiment is a preparation method of lithium iron phosphate material, comprising the following steps:

[0040] Step A1: 12 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate, and 55 mL of a citric acid solution with a mass fraction of 4% were added to a three-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and protected by nitrogen gas, stirred at a temperature of 28°C and a stirring rate of 250 r / min for 25 min, then 0.3 g of silver nitrate was added and the reaction was continued for 25 min, then the pH was adjusted to 9 with 22% ammonia water, and the reaction was continued for 3 h. After the reaction was completed, the reaction product was left to stand for 12 h, and then placed in a vacuum drying oven and dried at a temperature of 82°C for 2.5 h to obtain silver-doped lithium iron phosphate precursor.

[0041] Step A2: 10 g of silver-doped lithium iron phosphate precursor, 4 g of glucose, and 95 mL of deionized water were added to a three-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and protected by nitrogen, stirred at a temperature of 28℃ and a stirring rate of 250 r / min for 1.5 h, then spray dried to a particle size of ≤2 μm, and then placed in a tube furnace, protected by argon, and sintered at a temperature of 355℃ at a heating rate of 9℃ / min for 3.5 h, and then sintered at a temperature of 700℃ for 7 h, and then cooled in the furnace, to obtain carbon-coated silver-doped lithium iron phosphate;

[0042] Step A3: 10 mmol of pyrrole, 10 mmol of potassium tert-butoxide, 10 mmol of 5-chloro-2-nitrobenzotrifluoride, and 65 mL of anhydrous ether were added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube, and a reflux condenser, and protected by nitrogen, stirred at a temperature of 28℃ and a stirring rate of 250 r / min for 35 min, and then continued to stir at a reflux temperature for 7.5 h, and after the reaction was completed, the reaction product was cooled to room temperature, then poured into ice water, the organic phase was rotary evaporated to remove the solvent, then placed in a vacuum drying oven at a temperature of 52℃ for 3.5 h, and then purified by silica gel column chromatography with an eluent of petroleum ether and ethyl acetate mixed at a volume ratio of 12:3, to obtain modified pyrrole;

[0043] Step A4: 3.5 g of anhydrous iron chloride and 75 mL of an ethanol solution with a volume fraction of 55% were added to a three-necked flask equipped with a stirrer and a thermometer, and stirred at a temperature of 3℃ and a stirring rate of 250 r / min for 40 min, then 5 g of carbon-coated silver-doped lithium iron phosphate was added and stirred for 25 min, then 0.6 g of modified pyrrole was added and the temperature was raised to 28℃ and continued to stir for 18 h, and after the reaction was completed, the reaction product was centrifuged, the precipitate was washed twice with anhydrous methanol and distilled water, and then placed in a vacuum drying oven at a temperature of 62℃ for 4.5 h, to obtain double-layer coated silver-doped lithium iron phosphate;

[0044] Step A5: double-layer coated silver-doped lithium iron phosphate 9 parts, binder 1.1 parts, conductive agent 1.3 parts, and solvent 16 parts were weighed according to weight parts, and reserved; the binder was Kynar Flex PVDF 2850-07; the conductive agent was conductive carbon black Super P; and the solvent was N-methyl pyrrolidone;

[0045] Step A6: double-layer coated silver-doped lithium iron phosphate, binder, conductive agent, and solvent were added to a ball mill, and ball milled at a ball milling rate of 250 r / min for 4 h, to obtain lithium iron phosphate material.

[0046] Example 3:

[0047] The embodiment is a preparation method of a lithium iron phosphate material, including the following steps:

[0048] Step A1: 13 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate and 60 mL of a 5% citric acid solution were added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, and nitrogen was introduced for protection, and the reaction was stirred at a temperature of 30°C and a stirring rate of 300 r / min for 30 min, then 0.5 g of silver nitrate was added and the reaction was continued to be stirred for 30 min, then 25% ammonia water was used to adjust the pH to 9.5, and then the reaction was continued to be stirred for 4 h, after the reaction was completed, the reaction product was left to stand for 15 h, and then was placed in a vacuum drying oven and dried at a temperature of 85°C for 3 h to obtain a silver-doped lithium iron phosphate precursor;

[0049] Step A2: 10 g of the silver-doped lithium iron phosphate precursor, 7 g of glucose and 100 mL of deionized water were added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, and nitrogen was introduced for protection, and the reaction was stirred at a temperature of 30°C and a stirring rate of 300 r / min for 2 h, then was spray dried to a particle size of ≤2 μm, then was placed in a tube furnace, argon was introduced for protection, and was sintered at a temperature of 360°C at a temperature increasing rate of 10°C / min for 4 h, then was sintered at a temperature of 750°C for 8 h, and then was cooled in the furnace to obtain carbon-coated silver-doped lithium iron phosphate;

[0050] Step A3: 10 mmol of pyrrole, 10 mmol of potassium tert-butoxide, 10 mmol of 5-chloro-2-nitrobenzotrifluoride and 70 mL of anhydrous ether were added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a reflux condenser, and nitrogen was introduced for protection, and the reaction was stirred at a temperature of 30°C and a stirring rate of 300 r / min for 40 min, then was continued to be stirred at a reflux temperature for 8 h, after the reaction was completed, the reaction product was cooled to room temperature, then was poured into ice water, the organic phase was rotary evaporated to remove the solvent, then was placed in a vacuum drying oven and dried at a temperature of 55°C for 4 h, then was purified by silica gel column chromatography using an eluent prepared by mixing petroleum ether and ethyl acetate at a volume ratio of 15:3 to obtain modified pyrrole;

[0051] Step A4: 4.5 g of anhydrous ferric chloride, 80 mL of an ethanol solution with a volume fraction of 60% were added to a three-necked flask equipped with a stirrer, a thermometer, and stirred at a temperature of 5 ℃ and a stirring rate of 300 r / min for 50 min, then 5 g of carbon-coated lithium iron phosphate doped with silver was added and the reaction was continued for 30 min, then 0.9 g of modified pyrrole was added and the temperature was raised to 30 ℃ and the reaction was continued for 20 h, after the reaction was completed, the reaction product was centrifuged, the precipitate was washed with anhydrous methanol and distilled water three times in turn, and then was placed in a vacuum drying oven and dried at a temperature of 65 ℃ for 5 h to obtain double-layer coated lithium iron phosphate doped with silver;

[0052] Step A5: double-layer coated lithium iron phosphate doped with silver 10 parts, binder 1.2 parts, conductive agent 1.5 parts, and solvent 17 parts were weighed according to weight parts and prepared for use; the binder was Kynar Flex PVDF 2850-07; the conductive agent was conductive carbon black Super P; and the solvent was N-methyl pyrrolidone;

[0053] Step A6: double-layer coated lithium iron phosphate doped with silver, binder, conductive agent, and solvent were added to a ball mill and ball milled at a ball milling rate of 300 r / min for 5 h to obtain lithium iron phosphate material.

[0054] Comparative Example 1

[0055] This comparative example is a preparation method of lithium iron phosphate material, comprising the following steps:

[0056] Step A1: 13 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate, and 60 mL of a 5% by mass citric acid solution were added to a three-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and protected by nitrogen, and stirred at a temperature of 30 ℃ and a stirring rate of 300 r / min for 30 min, then adjusted to pH 9.5 with 25% by mass ammonia water, and continued to stir for 4 h, after the reaction was completed, the reaction product was left to stand for 15 h, and then was placed in a vacuum drying oven and dried at a temperature of 85 ℃ for 3 h to obtain lithium iron phosphate precursor;

[0057] Step A2: 10 g of lithium iron phosphate precursor and 100 mL of deionized water were added to a three-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and protected by nitrogen, and stirred at a temperature of 30 ℃ and a stirring rate of 300 r / min for 2 h, then spray dried to a particle size of ≤2 μm, then placed in a tube furnace, protected by argon, and sintered at a temperature of 360 ℃ for 4 h at a temperature increasing rate of 10 ℃ / min, then sintered at a temperature of 750 ℃ for 8 h, and then cooled in the furnace to obtain lithium iron phosphate;

[0058] Step A3: 10 parts of lithium iron phosphate, 1.2 parts of binder, 1.5 parts of conductive agent and 17 parts of solvent were weighed according to weight parts for standby; the binder was Kynar Flex PVDF 2850-07; the conductive agent was conductive carbon black Super P; the solvent was N-methyl pyrrolidone;

[0059] Step A4: lithium iron phosphate, binder, conductive agent and solvent were added to the ball mill, and ball milling was carried out at a ball milling speed of 300 r / min for 5 h to obtain a lithium iron phosphate material.

[0060] Comparative Example 2:

[0061] The present comparative example is a preparation method of a lithium iron phosphate material, comprising the following steps:

[0062] Step A1: 13 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate and 60 mL of 5% citric acid solution were added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, and protected by nitrogen, and stirred at a temperature of 30°C and a stirring speed of 300 r / min for 30 min, then adjusted to pH 9.5 with 25% ammonia water, and then continued to stir for 4 h. After the reaction was completed, the reaction product was left to stand for 15 h, and then placed in a vacuum drying oven and dried at a temperature of 85°C for 3 h to obtain a lithium iron phosphate precursor;

[0063] Step A2: 10 g of lithium iron phosphate precursor, 7 g of glucose and 100 mL of deionized water were added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, and protected by nitrogen, and stirred at a temperature of 30°C and a stirring speed of 300 r / min for 2 h, then spray dried to a particle size of ≤2 μm, then placed in a tube furnace, protected by argon, and sintered at a temperature of 360°C at a heating rate of 10°C / min for 4 h, then sintered at a temperature of 750°C for 8 h, and then cooled in the furnace to obtain carbon-coated lithium iron phosphate;

[0064] Step A3: 10 parts of carbon-coated lithium iron phosphate, 1.2 parts of binder, 1.5 parts of conductive agent and 17 parts of solvent were weighed according to weight parts for standby; the binder was Kynar Flex PVDF 2850-07; the conductive agent was conductive carbon black Super P; the solvent was N-methyl pyrrolidone;

[0065] Step A4: carbon-coated lithium iron phosphate, binder, conductive agent and solvent were added to the ball mill, and ball milling was carried out at a ball milling speed of 300 r / min for 5 h to obtain a lithium iron phosphate material.

[0066] Comparative Example 3:

[0067] The present comparative example is a preparation method of a lithium iron phosphate material, comprising the following steps:

[0068] Step A1: 13 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate, and 60 mL of a 5% by mass citric acid solution were added to a three-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and protected by nitrogen. The reaction was stirred at a temperature of 30°C and a stirring rate of 300 r / min for 30 min, then 0.5 g of silver nitrate was added and the reaction was stirred for another 30 min. Then, 25% by mass ammonia water was added to adjust the pH to 9.5, and the reaction was stirred for another 4 h. After the reaction was completed, the reaction product was left to stand for 15 h, and then was placed in a vacuum drying oven and dried at a temperature of 85°C for 3 h to obtain a silver-doped lithium iron phosphate precursor;

[0069] Step A2: 10 g of the silver-doped lithium iron phosphate precursor, 7 g of glucose, and 100 mL of deionized water were added to a three-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and protected by nitrogen. The reaction was stirred at a temperature of 30°C and a stirring rate of 300 r / min for 2 h, and then was spray dried to a particle size of ≤2 μm. Then, the product was placed in a tube furnace, protected by argon, and sintered at a temperature of 360°C at a temperature increasing rate of 10°C / min for 4 h, and then sintered at a temperature of 750°C for 8 h. After that, the product was cooled in the furnace to obtain carbon-coated silver-doped lithium iron phosphate;

[0070] Step A3: The carbon-coated silver-doped lithium iron phosphate, a binder, a conductive agent, and a solvent were weighed according to the proportions of 10 parts, 1.2 parts, 1.5 parts, and 17 parts, respectively, and were reserved for use. The binder was Kynar Flex PVDF 2850-07, the conductive agent was conductive carbon black Super P, and the solvent was N-methyl pyrrolidone;

[0071] Step A4: The carbon-coated silver-doped lithium iron phosphate, the binder, the conductive agent, and the solvent were added to a ball mill, and were ball milled at a ball milling rate of 300 r / min for 5 h to obtain a lithium iron phosphate material.

[0072] Comparative Example 4:

[0073] The present comparative example is a preparation method of a lithium iron phosphate material, comprising the following steps:

[0074] Step A1: 13 mmol of lithium carbonate, 10 mmol of iron nitrate nonahydrate, 10 mmol of ammonium dihydrogen phosphate and 60 mL of 5% citric acid solution were added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, protected by nitrogen, stirred at 30°C and 300 r / min for 30 min, then 0.5 g of silver nitrate was added and stirred for another 30 min, then adjusted to pH 9.5 with 25% ammonia water, and then stirred for another 4 h. After the reaction was completed, the reaction product was aged for 15 h, and then dried in a vacuum drying oven at 85°C for 3 h to obtain a silver-doped lithium iron phosphate precursor;

[0075] Step A2: 10 g of silver-doped lithium iron phosphate precursor, 7 g of glucose and 100 mL of deionized water were added to a three-necked flask equipped with a stirrer, a thermometer and a gas inlet tube, protected by nitrogen, stirred at 30°C and 300 r / min for 2 h, then spray dried to a particle size of ≤2 μm, then placed in a tube furnace, protected by argon, and sintered at a temperature of 360°C at a heating rate of 10°C / min for 4 h, then sintered at a temperature of 750°C for 8 h, and then cooled in the furnace to obtain carbon-coated silver-doped lithium iron phosphate;

[0076] Step A3: 4.5 g of anhydrous iron chloride and 80 mL of 60% ethanol solution were added to a three-necked flask equipped with a stirrer and a thermometer, stirred at 5°C and 300 r / min for 50 min, then 5 g of carbon-coated silver-doped lithium iron phosphate was added and stirred for another 30 min, then 0.9 g of pyrrole was added and the temperature was raised to 30°C, and the stirring was continued for another 20 h. After the reaction was completed, the reaction product was centrifuged, the precipitate was washed with anhydrous methanol and distilled water for 3 times, and then placed in a vacuum drying oven at 65°C for 5 h to obtain double-layer coated silver-doped lithium iron phosphate;

[0077] Step A4: double-layer coated silver-doped lithium iron phosphate 10 parts, binder 1.2 parts, conductive agent 1.5 parts and solvent 17 parts were weighed according to weight parts, and used as prepared; the binder was Kynar Flex PVDF 2850-07; the conductive agent was conductive carbon black Super P; and the solvent was N-methyl pyrrolidone;

[0078] Step A5: double-layer coated silver-doped lithium iron phosphate, binder, conductive agent and solvent were added to a ball mill, and ball milled at a ball milling speed of 300 r / min for 5 h to obtain lithium iron phosphate material.

[0079] Performance test

[0080] The lithium iron phosphate materials of Examples 1-3 and Comparative Examples 1-4 were coated on aluminum foils, and then the coated aluminum foils were cut into circular pieces with a diameter of 14 mm, and then placed in a vacuum drying oven and dried at a temperature of 120°C for 12 h to obtain positive electrode pieces; and CR2025 button cells were assembled with lithium pieces as negative electrode pieces, Celgard 2400 as separators, and 1 mol / L LiPF6 (EC:DMC = 1:1, v / v) as electrolyte, and left to stand for 12 h to test their performance.

[0081] The test results are shown in Table 1:

[0082] Table 1: Test results of discharge specific capacity and cycle stability of lithium iron phosphate materials

[0083]

[0084] Referring to the data in the above table, according to the comparison between Examples 1-3 and Comparative Examples 1-4, it can be seen that the lithium iron phosphate material of the present application has high discharge specific capacity and excellent cycle stability, and exhibits good electrochemical performance.

[0085] Among them, according to the comparison between Example 3 and Comparative Example 1, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is doped with silver, coated with a carbon layer and coated with a double layer of modified polypyrrole.

[0086] Among them, according to the comparison between Example 3 and Comparative Example 2, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is doped with silver and coated with modified polypyrrole; according to the comparison between Comparative Example 1 and Comparative Example 2, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is coated with a carbon layer.

[0087] Among them, according to the comparison between Example 3 and Comparative Example 3, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is coated with modified polypyrrole; according to the comparison between Comparative Example 1 and Comparative Example 3, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is doped with silver and coated with a carbon layer.

[0088] Among them, according to the comparison between Example 3 and Comparative Example 4, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is coated with modified polypyrrole; according to the comparison between Comparative Example 1 and Comparative Example 4, it can be seen that the discharge specific capacity and cycle stability of the lithium iron phosphate material can be significantly improved after the lithium iron phosphate material is doped with silver, coated with a carbon layer and coated with a double layer of polypyrrole.

[0089] In the description of the specification, reference to terms "one embodiment", "an example", "a specific example" and so on is intended to indicate that a particular feature, structure, material or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the application. Descriptive expressions of the above terms in the specification do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics can be combined in any one or more embodiments or examples in a suitable manner.

[0090] The above is only an example and illustration of the application, and those skilled in the art can make various modifications or supplements to the described specific embodiments or replace them with similar ways, as long as they do not deviate from the application or exceed the scope defined in the application.

Claims

1. A lithium iron phosphate material, characterized in that, Includes the following components by weight: The mixture consists of 8-10 parts of silver-doped lithium iron phosphate double-coated compound, 1-1.2 parts of binder, 1-1.5 parts of conductive agent, and 15-17 parts of solvent. The double-layer coated silver-doped lithium iron phosphate is prepared by the following steps: Step s1: Lithium carbonate, ferric nitrate nonahydrate, ammonium dihydrogen phosphate and citric acid solution are stirred and reacted. Then silver nitrate is added, and the pH is adjusted with ammonia. The reaction is continued to be stirred. After the reaction is completed, the reaction product is allowed to stand and age, and then dried to obtain the silver-doped lithium iron phosphate precursor. Step s2: The silver-doped lithium iron phosphate precursor, glucose and deionized water are stirred and reacted, then spray dried, then sintered and cooled in the furnace to obtain carbon-coated silver-doped lithium iron phosphate. Step s3: Pyrrole, potassium tert-butoxide, 5-chloro-2-nitrotrifluorotoluene and anhydrous diethyl ether were stirred and reacted. After the reaction was completed, the reaction product was cooled and then poured into ice water. The organic phase was evaporated by rotary evaporation, dried and purified by silica gel column chromatography to obtain modified pyrrole. Step s4: Anhydrous ferric chloride and ethanol solution are stirred and reacted, then carbon-coated silver-doped lithium iron phosphate and modified pyrrole are added and the reaction is continued. After the reaction is completed, the reaction product is centrifuged, and the precipitate is washed and dried to obtain double-layer coated silver-doped lithium iron phosphate.

2. The lithium iron phosphate material according to claim 1, characterized in that, In step s1, the ratio of lithium carbonate, ferric nitrate nonahydrate, ammonium dihydrogen phosphate, citric acid solution, and silver nitrate is 11-13 mmol: 10 mmol: 10 mmol: 50-60 mL: 0.1-0.5 g; the mass fraction of the citric acid solution is 3-5%.

3. The lithium iron phosphate material according to claim 1, characterized in that, The mass fraction of the ammonia water in step s1 is 20-25%.

4. The lithium iron phosphate material according to claim 1, characterized in that, In step s2, the ratio of the silver-doped lithium iron phosphate precursor, glucose, and deionized water is 10g:1-7g:90-100mL.

5. A lithium iron phosphate material according to claim 1, characterized in that, The ratio of pyrrole, potassium tert-butoxide, 5-chloro-2-nitrotrifluorotoluene, and anhydrous diethyl ether in step s3 is 10 mmol: 10 mmol: 10 mmol: 60-70 mL.

6. The lithium iron phosphate material according to claim 1, characterized in that, The ratio of anhydrous ferric chloride, ethanol solution, carbon-coated silver-doped lithium iron phosphate, and modified pyrrole in step s4 is 2.5-4.5g: 70-80mL: 5g: 0.3-0.9g.

7. The lithium iron phosphate material according to claim 1, characterized in that, The volume fraction of the ethanol solution in step s4 is 50-60%.

8. A method for preparing lithium iron phosphate material, characterized in that, Includes the following steps: Step 1: Weigh out 8-10 parts by weight of the double-layer coated silver-doped lithium iron phosphate, 1-1.2 parts by weight of the binder, 1-1.5 parts by weight of the conductive agent, and 15-17 parts by weight of the solvent as described in claim 1, and set aside. Step 2: Add the double-layer coated silver-doped lithium iron phosphate, binder, conductive agent and solvent into a ball mill, and ball mill for 3-5 hours at a ball milling rate of 200-300 r / min to obtain lithium iron phosphate material.

9. The method for preparing a lithium iron phosphate material according to claim 8, characterized in that, The solvent is N-methylpyrrolidone.

10. The application of lithium iron phosphate material prepared by the method of any one of claims 8-9 in the positive electrode of a lithium-ion battery.

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