A lithium iron phosphate material, a lithium iron phosphate cathode material, their preparation methods and applications

Abstract

This invention belongs to the field of lithium-ion battery material technology, and specifically relates to an iron phosphate material, a lithium iron phosphate cathode material, its preparation method, and its application. The preparation method of the iron phosphate material includes the following steps: mixing N,N-di(3-methylpyridinyl)dicarboxamide benzoyl chloride, m-tetra(1-aminophenyl)porphyrin, a catalyst, and a solvent, stirring to obtain a 1,3,5-tricarboxyamide-linked porphyrin compound; adding an iron source and solvent, heating, and separating to obtain a 1,3,5-tricarboxyamide-linked iron porphyrin solid product; then mixing with N,N',N'-tris(3-methylpyridinyl)trimethylamide, a solvent, a surfactant solution, and a crosslinking agent, separating to prepare a precursor powder; dispersing the precursor powder in a solvent, adding phosphate, mixing, adjusting the pH, heating the mixture under stirring, aging, filtering, and calcining the precipitate to obtain the iron phosphate material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material technology, and specifically relates to an iron phosphate material, a lithium iron phosphate cathode material, its preparation method and application. Background Technology

[0002] The energy density of lithium-ion batteries is closely related to the structure and composition of the cathode material. Among commercially available lithium-ion cathode materials, olivine-type lithium iron phosphate (LiFePO4) boasts high specific capacity, low cost, and excellent safety performance, making it a focus of research in recent years. However, this material suffers from low conductivity and a small lithium-ion diffusion coefficient, leading to poor performance at low temperatures and high charge / discharge rates, severely impacting its applications in various fields. Iron phosphate (FePO4) is a crucial precursor for preparing lithium iron phosphate cathode materials; its structure, particle size, and morphology directly affect the performance of lithium iron phosphate products. Therefore, controlling the nucleation and growth of the iron phosphate precursor and regulating its particle size and specific surface area can improve the electrochemical performance of lithium iron phosphate cathode materials. Currently, nano-sizing, pore formation, and metal ion doping are commonly used to modify iron phosphate to prepare high-performance lithium iron phosphate. Current processes for preparing lithium iron phosphate often result in products with irregular particle morphology, uneven distribution, and severe agglomeration. This leads to a small specific surface area, which prolongs the diffusion path of lithium ions during subsequent reactions with the lithium source. Consequently, lithium iron phosphate materials prepared using these processes suffer from poor charge-discharge performance, low conductivity, and poor processing performance.

[0003] Therefore, developing a simple and reliable method to prepare iron phosphate precursors with regular morphology, uniform particle distribution, and easy lithium-ion diffusion is of great significance for improving the electrochemical performance of lithium iron phosphate cathode materials. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in related technologies. To this end, this invention proposes an iron phosphate material, its preparation method, and its application. This iron phosphate material can effectively improve the conductivity of the cathode material and enhance the overall electrochemical performance of lithium batteries.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0006] A method for preparing an iron phosphate material includes the following steps:

[0007] (1) N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride, m-tetra(1-aminophenyl)porphyrin, catalyst and first solvent were mixed and stirred in the dark under a protective atmosphere to obtain porphyrin compounds linked by 1,3,5-tricarboxyamide.

[0008] (2) The 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) is mixed with an iron source, a second solvent is added, and the mixture is heated under reflux. After separation, the solid product of 1,3,5-tricarboxyamide-linked iron porphyrin is obtained.

[0009] (3) The 1,3,5-tricarboxyamide-linked iron porphyrin solid product and N,N',N'-tris(3-methylpyridinyl)trimethylamide obtained in step (2) are mixed with a third solvent, and then a surfactant solution and a crosslinking agent are added. The mixture is stirred and separated to prepare a cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin.

[0010] (4) Disperse the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin obtained in step (3) in a fourth solvent, add phosphate to mix into a mixture, adjust the pH, heat the mixture under stirring, then let it stand for aging, filter, take the precipitate, and calcine the precipitate to obtain the iron phosphate material.

[0011] In one embodiment, in step (1), the first solvent is chloroform.

[0012] In one embodiment, in step (1), the catalyst is N,N-diisopropylethylamine.

[0013] In one embodiment, in step (1), the molar amount of the added m-tetra(1-aminophenyl)porphyrin is the same as that of the N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride.

[0014] In one embodiment, in step (1), the stirring time in the dark is at least 48 hours.

[0015] In one embodiment, in step (1), the solution obtained after stirring in the dark is evaporated to obtain a solid.

[0016] In one embodiment, in step (1), the solid is washed sequentially with water, ethanol and dichloroethane and then dried to obtain the 1,3,5-tricarboxyamide-linked porphyrin compound.

[0017] In one embodiment, in step (2), the iron source is a soluble ferric salt.

[0018] In one embodiment, the second solvent in step (2) and the third solvent in step (3) are both dimethylformamide.

[0019] In one embodiment, in step (2), the molar ratio of the 1,3,5-tricarboxyamide-linked porphyrin compound to the iron ions in the iron source is 1:(1-3).

[0020] In one embodiment, in step (2), the heating for reflux reaction refers to heating to 100-150°C and then refluxing for 3-8 hours.

[0021] In one embodiment, in step (2), after the reflux reaction is carried out by heating, water is added after cooling, and after standing, solid-liquid separation is performed. The obtained solid phase is then washed and dried to obtain the 1,3,5-tricarboxyamide-linked iron porphyrin solid product.

[0022] In one embodiment, in step (2), the settling time is 12-18 hours.

[0023] In one embodiment, in step (2), the washing refers to washing the solid phase sequentially with anhydrous ethanol and water.

[0024] In one embodiment, in step (2), the drying is performed under vacuum at 50-80°C.

[0025] In one embodiment, in step (3), the surfactant solution is prepared by mixing sorbitan monooleate (Span-80) with n-hexane.

[0026] In one embodiment, in step (3), the crosslinking agent is 1,4-bis(bromomethyl)benzene, and the molar amount of the crosslinking agent added is the same as that of N,N',N'-tris(3-methylpyridinyl)trimethylamide.

[0027] In one embodiment, in step (3), the separation refers to centrifugation to collect the precipitate, washing the precipitate, and drying it to obtain a cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin.

[0028] In one embodiment, in step (3), the washing is performed using dichloromethane, water, and anhydrous ethanol.

[0029] In one embodiment, in step (3), the drying is performed under vacuum at 50-80°C.

[0030] In one embodiment, in step (4), the fourth solvent is water.

[0031] In one embodiment, in step (4), the phosphate is at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and sodium hydrogen phosphate.

[0032] In one embodiment, in step (4), adjusting the pH means adjusting the pH of the reaction to 1.5-2.2.

[0033] In one embodiment, in step (4), the pH is adjusted by adding an alkaline substance, such as ammonia or sodium hydroxide, to the mixture.

[0034] In one embodiment, in step (4), the temperature of the heating reaction is 70-90°C and the heating reaction time is 6-10h.

[0035] In one embodiment, in step (4), the settling and aging time is 3-5 hours.

[0036] In one embodiment, in step (4), the precipitate was washed and dried before calcination.

[0037] In one embodiment, in step (4), the calcination conditions are: heating to 500-750°C and holding for 6-10 hours under a protective atmosphere.

[0038] In one embodiment, in step (4), the molar ratio of phosphorus in the added phosphate to iron in the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin is 1:1.

[0039] A ferric phosphate material is prepared by the preparation method described above, and the ferric phosphate material has a hollow spherical structure.

[0040] A method for preparing a lithium iron phosphate cathode material includes the following steps: mixing the lithium iron phosphate material as described above with a lithium source, and sintering it under a protective atmosphere to obtain the lithium iron phosphate cathode material.

[0041] In one embodiment, the molar ratio of iron in the iron phosphate material to lithium in the lithium source is 1:1.

[0042] In one embodiment, the iron phosphate material is mixed with a lithium source, ball-milled for 2-5 hours, dried, and then sintered.

[0043] In one embodiment, the sintering method is low-temperature pre-sintering and secondary sintering, wherein the low-temperature pre-sintering temperature is 400-550℃ and the holding time is 3-5h; the secondary sintering temperature is 650-750℃ and the holding time is 6-10h.

[0044] In one embodiment, the lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium acetate.

[0045] A lithium iron phosphate cathode material is prepared by the preparation method described above.

[0046] A lithium-ion battery comprising the aforementioned lithium iron phosphate cathode material.

[0047] The application of lithium iron phosphate materials as described above in the preparation of lithium-ion batteries.

[0048] The beneficial effects of this invention are:

[0049] (1) The iron phosphate material prepared by this invention has a hollow spherical structure. The lithium iron phosphate cathode material obtained by this invention has a certain inheritance of the morphology of iron phosphate. Compared with irregular bulk materials, the lithium iron phosphate cathode material with this structure has a larger specific surface area, which increases the contact area between the electrode material and the electrolyte and greatly shortens the Li-E ... + diffusion pathway and reduction of Li + The diffusion resistance is beneficial to improving the utilization rate and rate performance of the active material LiFePO4; in addition, the spherical structure particles are easy to form a tightly packed array, which is beneficial to improving the tap density of the cathode material.

[0050] (2) In the process of synthesizing iron phosphate materials, the cationic hollow spherical polymer framework precursor powder POPs-Fe(ш) loaded with iron porphyrin prepared in this invention can not only serve as a template for the synthesis of spherical iron phosphate, but also form a nitrogen-containing carbon layer doped in the iron phosphate structure after high-temperature pyrolysis and carbonization. This can effectively improve the electronic / ionic conductivity of the cathode material and enhance the transmission and migration of electrons / ions. In addition, the nitrogen-containing carbon layer can also serve as a carbon source for the synthesis of lithium iron phosphate after mixing iron phosphate with lithium source and calcining at high temperature. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the synthesis of the cationic hollow spherical polymer framework precursor powder (POPs-Fe(ш)) loaded with iron porphyrin in Example 1 of the present invention.

[0052] Figure 2 The image shows the XRD pattern of the iron phosphate material prepared in Example 1 of this invention.

[0053] Figure 3 This is a scanning electron microscope image of the iron phosphate material prepared in Example 1 of the present invention;

[0054] Figure 4 This is a transmission electron microscope (TEM) image of the iron phosphate material prepared in Example 1 of the present invention. Detailed Implementation

[0055] The present invention will be further described below with reference to specific embodiments.

[0056] Example 1:

[0057] A method for preparing an iron phosphate material, such as Figure 1 As shown, it includes the following steps:

[0058] (1) 0.02 mol N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride and 0.02 mol m-tetra(1-aminophenyl)porphyrin were dispersed in 100 mL chloroform, mixed and stirred evenly, and then 800 μL N,N-diisopropylethylamine was added. The mixture was stirred in the dark at room temperature under nitrogen atmosphere for 48 h. The solution was then evaporated and dried to obtain a solid. The solid was washed with deionized water, ethanol and dichloroethane, and dried under vacuum to obtain a 1,3,5-tricarboxyamide-linked porphyrin compound (PTTA).

[0059] (2) 0.01 mol of the 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) and 0.02 mol of ferric chloride hexahydrate were placed in a 250 mL three-necked flask, and then 100 mL of dimethylformamide (DMF) was added. The mixture was stirred and mixed evenly, and the mixture was refluxed at 140 °C for 5 h. After the reactants were cooled to room temperature, 100 mL of deionized water was added and the mixture was left to stand overnight. The mixture was then filtered under reduced pressure. The filter cake was washed repeatedly with anhydrous ethanol and deionized water, and then dried in a vacuum oven at 60 °C overnight to obtain the 1,3,5-tricarboxyamide-linked iron porphyrin solid product (PTTA-Fe(ш)).

[0060] (3) 5 mmol of the 1,3,5-tricarboxyamide-linked iron porphyrin solid product obtained in step (2) and 5 mmol of N,N',N'-tris(3-methylpyridinyl)trimethylamide (TMPTA) were placed in 80 mL of dimethylformamide and mixed evenly to obtain a mixed solution; 3-5 drops of sorbitan monooleate (Span-80) were added to 120 mL of n-hexane and mixed. After stirring evenly, the mixture was poured into the above mixed solution. After magnetic stirring for 1 h, 5 mmol of crosslinking agent 1,4-bis(bromomethyl)benzene was added to the mixture. The mixture was stirred at room temperature for 48 h. The precipitate was collected by centrifugation and washed several times with dichloromethane, deionized water and anhydrous ethanol. Then it was dried under vacuum at 60 °C to obtain the cationic hollow spherical polymer framework precursor powder (POPs-Fe(ш)) loaded with iron porphyrin.

[0061] (4) 1.0 g of the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin prepared in step (3) was uniformly dispersed in 100 mL of deionized water. Then, ammonium hydrogen phosphate was added to the mixture according to a Fe / P molar ratio of 1:1. After stirring for 30 min, ammonia water was added dropwise to adjust the pH of the solution to 1.5. The mixture was then heated and stirred continuously. The temperature of the reaction system was controlled at 70 °C and kept at that temperature for 6 h. After the reaction was completed, the mixture was allowed to stand for 3 h. The product was centrifuged and filtered, washed several times with deionized water and anhydrous ethanol, and then vacuum dried at 80 °C to obtain iron phosphate precipitate. Subsequently, the product was placed in a muffle furnace and heated to 500 °C in a nitrogen atmosphere for 6 h to remove the water of crystallization, thus obtaining the nitrogen-containing carbon layer-doped iron phosphate material. The XRD pattern of the prepared iron phosphate material is shown in the figure. Figure 2 As shown, the scanning electron microscope image of the iron phosphate material is as follows: Figure 3 As shown, the transmission electron microscope image of the iron phosphate material is as follows: Figure 4 As shown.

[0062] from Figure 2 It can be seen that the XRD curve of the iron phosphate material prepared in Example 1 has the same characteristic peaks as the FePO4 standard card, indicating good crystallinity.

[0063] from Figure 3 It can be seen that the iron phosphate material prepared in Example 1 has a spherical structure.

[0064] from Figure 4 It can be seen that the iron phosphate material prepared in Example 1 has a hollow spherical structure.

[0065] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0066] The iron phosphate material prepared above was dispersed with lithium carbonate in anhydrous ethanol at a Fe / Li molar ratio of 1:1. The mixture was ball-milled for 3 hours until homogeneous. After spray drying, the mixture was heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. Then, it was calcined at 700°C for 8 hours to obtain the lithium iron phosphate cathode material.

[0067] Example 2:

[0068] A method for preparing an iron phosphate material includes the following steps:

[0069] (1) 0.02 mol N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride and 0.02 mol m-tetra(1-aminophenyl)porphyrin were dispersed in 100 mL chloroform, mixed and stirred evenly, and then 800 μL N,N-diisopropylethylamine was added. The mixture was stirred in the dark at room temperature under nitrogen atmosphere for 48 h. The solution was then evaporated and dried to obtain a solid. The solid was washed with deionized water, ethanol and dichloroethane, and dried under vacuum to obtain a 1,3,5-tricarboxyamide-linked porphyrin compound (PTTA).

[0070] (2) 0.01 mol of the 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) and 0.02 mol of ferric chloride hexahydrate were placed in a 250 mL three-necked flask, and then 100 mL of dimethylformamide (DMF) was added. The mixture was stirred and mixed evenly, and the mixture was refluxed at 140 °C for 5 h. After the reactants were cooled to room temperature, 100 mL of deionized water was added and the mixture was left to stand overnight. The mixture was then filtered under reduced pressure. The filter cake was washed repeatedly with anhydrous ethanol and deionized water, and then dried in a vacuum oven at 60 °C overnight to obtain the 1,3,5-tricarboxyamide-linked iron porphyrin solid product (PTTA-Fe(ш)).

[0071] (3) 5 mmol of the 1,3,5-tricarboxyamide-linked iron porphyrin solid product obtained in step (2) and 5 mmol of N,N',N'-tris(3-methylpyridinyl)trimethylamide (TMPTA) were placed in 80 mL of dimethylformamide and mixed evenly to obtain a mixed solution; 3-5 drops of sorbitan monooleate (Span-80) were added to 120 mL of n-hexane and mixed. After stirring evenly, the mixture was poured into the above mixed solution. After magnetic stirring for 1 h, 5 mmol of crosslinking agent 1,4-bis(bromomethyl)benzene was added to the mixture. The mixture was stirred at room temperature for 48 h. The precipitate was collected by centrifugation and washed several times with dichloromethane, deionized water and anhydrous ethanol. Then it was dried under vacuum at 60 °C to obtain the cationic hollow spherical polymer framework precursor powder (POPs-Fe(ш)) loaded with iron porphyrin.

[0072] (4) 1.0 g of the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin prepared in step (3) was uniformly dispersed in 100 mL of deionized water. Then, ammonium hydrogen phosphate was added to the above mixture according to the Fe / P molar ratio of 1:1. After stirring for 30 min, ammonia water was added dropwise to the mixture to adjust the pH of the solution to 1.8. Then, the mixture was heated and stirred continuously. The temperature of the reaction system was controlled at 80 °C and kept at that temperature for 8 h. After the reaction was completed, the mixture was allowed to stand for 5 h. The product was centrifuged and filtered, washed several times with deionized water and anhydrous ethanol, and then vacuum dried at 80 °C to obtain iron phosphate precipitate. Then, the above product was placed in a muffle furnace and heated to 500 °C in a nitrogen atmosphere for 6 h to remove the water of crystallization, thus obtaining the hollow structure iron phosphate material doped with nitrogen carbon layer.

[0073] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0074] The iron phosphate material prepared above was dispersed with lithium carbonate in anhydrous ethanol at a Fe / Li molar ratio of 1:1. The mixture was ball-milled for 3 hours until homogeneous. After spray drying, the mixture was heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. Then, it was calcined at 700°C for 8 hours to obtain the lithium iron phosphate cathode material.

[0075] Example 3:

[0076] A method for preparing an iron phosphate material includes the following steps:

[0077] (1) 0.02 mol N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride and 0.02 mol m-tetra(1-aminophenyl)porphyrin were dispersed in 100 mL chloroform, mixed and stirred evenly, and then 800 μL N,N-diisopropylethylamine was added. The mixture was stirred in the dark at room temperature under nitrogen atmosphere for 48 h. The solution was then evaporated and dried to obtain a solid. The solid was washed with deionized water, ethanol and dichloroethane, and dried under vacuum to obtain a 1,3,5-tricarboxyamide-linked porphyrin compound (PTTA).

[0078] (2) 0.01 mol of the 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) and 0.02 mol of ferric chloride hexahydrate were placed in a 250 mL three-necked flask, and then 100 mL of dimethylformamide (DMF) was added. The mixture was stirred and mixed evenly, and the mixture was refluxed at 140 °C for 5 h. After the reactants were cooled to room temperature, 100 mL of deionized water was added and the mixture was left to stand overnight. The mixture was then filtered under reduced pressure. The filter cake was washed repeatedly with anhydrous ethanol and deionized water, and then dried in a vacuum oven at 60 °C overnight to obtain the 1,3,5-tricarboxyamide-linked iron porphyrin solid product (PTTA-Fe(ш)).

[0079] (3) 5 mmol of the 1,3,5-tricarboxyamide-linked iron porphyrin solid product obtained in step (2) and 5 mmol of N,N',N'-tris(3-methylpyridinyl)trimethylamide (TMPTA) were placed in 80 mL of dimethylformamide and mixed evenly to obtain a mixed solution; 3-5 drops of sorbitan monooleate (Span-80) were added to 120 mL of n-hexane and mixed. After stirring evenly, the mixture was poured into the above mixed solution. After magnetic stirring for 1 h, 5 mmol of crosslinking agent 1,4-bis(bromomethyl)benzene was added to the mixture. The mixture was stirred at room temperature for 48 h. The precipitate was collected by centrifugation and washed several times with dichloromethane, deionized water and anhydrous ethanol. Then it was dried under vacuum at 60 °C to obtain the cationic hollow spherical polymer framework precursor powder (POPs-Fe(ш)) loaded with iron porphyrin.

[0080] (4) 1.0 g of the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin prepared in step (3) was uniformly dispersed in 100 mL of deionized water. Then, ammonium hydrogen phosphate was added to the above mixture according to the Fe / P molar ratio of 1:1. After stirring for 30 min, ammonia water was added dropwise to the mixture to adjust the pH of the solution to 1.8. Then, the mixture was heated and stirred continuously. The temperature of the reaction system was controlled at 80 °C and kept at that temperature for 8 h. After the reaction was completed, the mixture was allowed to stand for 5 h. The product was centrifuged and filtered, washed several times with deionized water and anhydrous ethanol, and then vacuum dried at 80 °C to obtain iron phosphate precipitate. Subsequently, the above product was placed in a muffle furnace and heated to 650 °C in a nitrogen atmosphere for 8 h to remove the water of crystallization, thus obtaining the hollow structure iron phosphate material doped with nitrogen carbon layer.

[0081] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0082] The iron phosphate material prepared above was dispersed with lithium carbonate in anhydrous ethanol at a Fe / Li molar ratio of 1:1. The mixture was ball-milled for 3 hours until homogeneous. After spray drying, the mixture was heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. Then, it was calcined at 700°C for 8 hours to obtain the lithium iron phosphate cathode material.

[0083] Example 4:

[0084] A method for preparing an iron phosphate material includes the following steps:

[0085] (1) 0.02 mol N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride and 0.02 mol m-tetra(1-aminophenyl)porphyrin were dispersed in 100 mL chloroform, mixed and stirred evenly, and then 800 μL N,N-diisopropylethylamine was added. The mixture was stirred in the dark at room temperature under nitrogen atmosphere for 48 h. The solution was then evaporated and dried to obtain a solid. The solid was washed with deionized water, ethanol and dichloroethane, and dried under vacuum to obtain a 1,3,5-tricarboxyamide-linked porphyrin compound (PTTA).

[0086] (2) 0.01 mol of the 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) and 0.02 mol of ferric chloride hexahydrate were placed in a 250 mL three-necked flask, and then 100 mL of dimethylformamide (DMF) was added. The mixture was stirred and mixed evenly, and the mixture was refluxed at 140 °C for 5 h. After the reactants were cooled to room temperature, 100 mL of deionized water was added and the mixture was left to stand overnight. The mixture was then filtered under reduced pressure. The filter cake was washed repeatedly with anhydrous ethanol and deionized water, and then dried in a vacuum oven at 60 °C overnight to obtain the 1,3,5-tricarboxyamide-linked iron porphyrin solid product (PTTA-Fe(ш)).

[0087] (3) 5 mmol of the 1,3,5-tricarboxyamide-linked iron porphyrin solid product obtained in step (2) and 5 mmol of N,N',N'-tris(3-methylpyridinyl)trimethylamide (TMPTA) were placed in 80 mL of dimethylformamide and mixed evenly to obtain a mixed solution; 3-5 drops of sorbitan monooleate (Span-80) were added to 120 mL of n-hexane and mixed. After stirring evenly, the mixture was poured into the above mixed solution. After magnetic stirring for 1 h, 5 mmol of crosslinking agent 1,4-bis(bromomethyl)benzene was added to the mixture. The mixture was stirred at room temperature for 48 h. The precipitate was collected by centrifugation and washed several times with dichloromethane, deionized water and anhydrous ethanol. Then it was dried under vacuum at 60 °C to obtain the cationic hollow spherical polymer framework precursor powder (POPs-Fe(ш)) loaded with iron porphyrin.

[0088] (4) 1.0 g of the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin prepared in step (3) was uniformly dispersed in 100 mL of deionized water. Then, ammonium hydrogen phosphate was added to the above mixture according to the Fe / P molar ratio of 1:1. After stirring for 30 min, ammonia water was added dropwise to the mixture to adjust the pH of the solution to 2.2. Then, the mixture was heated and stirred continuously. The temperature of the reaction system was controlled at 90 °C and kept at 10 h. After the reaction was completed, it was allowed to stand for 5 h. The product was centrifuged and filtered, washed several times with deionized water and anhydrous ethanol, and dried under vacuum at 80 °C to obtain iron phosphate precipitate. Then, the above product was placed in a muffle furnace and heated to 650 °C in a nitrogen atmosphere for 8 h to remove the water of crystallization, thus obtaining the hollow structure iron phosphate material doped with nitrogen carbon layer.

[0089] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0090] The iron phosphate material prepared above was dispersed with lithium carbonate in anhydrous ethanol at a Fe / Li molar ratio of 1:1. The mixture was ball-milled for 3 hours until homogeneous. After spray drying, the mixture was heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. Then, it was calcined at 700°C for 8 hours to obtain the lithium iron phosphate cathode material.

[0091] Example 5:

[0092] A method for preparing an iron phosphate material includes the following steps:

[0093] (1) 0.02 mol N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride and 0.02 mol m-tetra(1-aminophenyl)porphyrin were dispersed in 100 mL chloroform, mixed and stirred evenly, and then 800 μL N,N-diisopropylethylamine was added. The mixture was stirred in the dark at room temperature under nitrogen atmosphere for 48 h. The solution was then evaporated and dried to obtain a solid. The solid was washed with deionized water, ethanol and dichloroethane, and dried under vacuum to obtain a 1,3,5-tricarboxyamide-linked porphyrin compound (PTTA).

[0094] (2) 0.01 mol of the 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) and 0.02 mol of ferric chloride hexahydrate were placed in a 250 mL three-necked flask, and then 100 mL of dimethylformamide (DMF) was added. The mixture was stirred and mixed evenly, and the mixture was refluxed at 140 °C for 5 h. After the reactants were cooled to room temperature, 100 mL of deionized water was added and the mixture was left to stand overnight. The mixture was then filtered under reduced pressure. The filter cake was washed repeatedly with anhydrous ethanol and deionized water, and then dried in a vacuum oven at 60 °C overnight to obtain the 1,3,5-tricarboxyamide-linked iron porphyrin solid product (PTTA-Fe(ш)).

[0095] (3) 5 mmol of the 1,3,5-tricarboxyamide-linked iron porphyrin solid product obtained in step (2) and 5 mmol of N,N',N'-tris(3-methylpyridinyl)trimethylamide (TMPTA) were placed in 80 mL of dimethylformamide and mixed evenly to obtain a mixed solution; 3-5 drops of sorbitan monooleate (Span-80) were added to 120 mL of n-hexane and mixed. After stirring evenly, the mixture was poured into the above mixed solution. After magnetic stirring for 1 h, 5 mmol of crosslinking agent 1,4-bis(bromomethyl)benzene was added to the mixture. The mixture was stirred at room temperature for 48 h. The precipitate was collected by centrifugation and washed several times with dichloromethane, deionized water and anhydrous ethanol. Then it was dried under vacuum at 60 °C to obtain the cationic hollow spherical polymer framework precursor powder (POPs-Fe(ш)) loaded with iron porphyrin.

[0096] (4) 1.0 g of the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin prepared in step (3) was uniformly dispersed in 100 mL of deionized water. Then, ammonium hydrogen phosphate was added to the above mixture according to the Fe / P molar ratio of 1:1. After stirring for 30 min, ammonia water was added dropwise to the mixture to adjust the pH of the solution to 2.2. Then, the mixture was heated and stirred continuously. The temperature of the reaction system was controlled at 90 °C and kept at 10 h. After the reaction was completed, it was allowed to stand for 5 h. The product was centrifuged and filtered, washed several times with deionized water and anhydrous ethanol, and dried under vacuum at 80 °C to obtain iron phosphate precipitate. Then, the above product was placed in a muffle furnace and heated to 750 °C in a nitrogen atmosphere for 10 h to remove the water of crystallization, thus obtaining the hollow structure iron phosphate material doped with nitrogen carbon layer.

[0097] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0098] The iron phosphate material prepared above was dispersed with lithium carbonate in anhydrous ethanol at a Fe / Li molar ratio of 1:1. The mixture was ball-milled for 3 hours until homogeneous. After spray drying, the mixture was heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. Then, it was calcined at 700°C for 8 hours to obtain the lithium iron phosphate cathode material.

[0099] Comparative Example 1:

[0100] A method for preparing an iron phosphate material includes the following steps:

[0101] 1.0 g of ferric chloride hexahydrate was dispersed in 100 mL of deionized water. Then, ammonium hydrogen phosphate was added to the mixture at a Fe / P molar ratio of 1:1. After stirring for 30 min, ammonia was added dropwise to adjust the pH of the solution to 1.5. The mixture was then heated and stirred continuously at 70 °C for 6 h. After the reaction was completed, the mixture was allowed to stand for 3 h. The product was centrifuged and filtered, washed several times with deionized water and anhydrous ethanol, and then vacuum dried at 80 °C to obtain ferric phosphate precipitate. Subsequently, the product was placed in a muffle furnace and heated to 500 °C in a nitrogen atmosphere for 6 h to remove the water of crystallization, thus obtaining the ferric phosphate material.

[0102] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0103] The iron phosphate material prepared above was dispersed in anhydrous ethanol with lithium carbonate and glucose at a Fe / Li / C molar ratio of 1:1:0.1. The mixture was ball-milled for 3 hours until homogeneous. After spray drying, the mixture was heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. Then, it was calcined at 700°C for 8 hours to obtain the lithium iron phosphate cathode material.

[0104] Experimental example:

[0105] The tap density of the lithium iron phosphate cathode materials in Examples 1-5 and Comparative Example 1 was measured respectively. The lithium iron phosphate cathode materials in Examples 1-5 and Comparative Example 1 were then formulated into coin cells. The specific steps included: uniformly mixing the lithium iron phosphate cathode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry, then coating it onto aluminum foil, and drying it in a vacuum drying oven. The slurry was then pressed into a positive electrode sheet using a tablet press, and the negative electrode sheet was a lithium metal sheet. The electrolyte was 1 mol / L LiPF6-EC:DMC (volume ratio of 1:1), and a polypropylene porous membrane was used as the separator. The battery assembly was carried out in an argon glove box.

[0106] The electrochemical performance of the above batteries was tested (with the charge and discharge voltage controlled between 2.5 and 4.5V), and the results are shown in Table 1 below.

[0107] Table 1: Test Results

[0108]

[0109]

[0110] As shown in Table 1, the tap density of the lithium iron phosphate cathode material prepared by this invention is 1.46 g / cm³. 3The above-mentioned lithium-ion batteries, after assembly, exhibit a 0.1C discharge specific capacity exceeding 157.4 mAh / g and a 1C discharge specific capacity exceeding 149.7 mAh / g, with an initial charge-discharge efficiency exceeding 98.04%. The electrochemical performance of the lithium iron phosphate cathode material prepared in these embodiments is significantly better than that of the comparative examples, especially in Example 4. Compared to the irregular iron phosphate bulk material prepared in Comparative Example 1, the iron phosphate prepared in these embodiments is a spherical hollow structure doped with nitrogen and carbon layers. The lithium iron phosphate cathode material prepared using this precursor has a large specific surface area, high conductivity, short and fast lithium-ion diffusion path, and low insertion / extraction resistance, thus resulting in superior electrochemical performance of the lithium-ion battery.

Claims

1. A method for preparing an iron phosphate material, characterized in that: Includes the following steps: (1) N,N-bis(3-methylpyridinyl)dicarboxamide benzoyl chloride, m-tetra(1-aminophenyl)porphyrin, catalyst and first solvent were mixed and stirred in the dark under a protective atmosphere to obtain porphyrin compounds linked by 1,3,5-tricarboxyamide. (2) The 1,3,5-tricarboxyamide-linked porphyrin compound obtained in step (1) is mixed with an iron source, a second solvent is added, and the mixture is heated under reflux. After separation, the solid product of 1,3,5-tricarboxyamide-linked iron porphyrin is obtained. (3) The 1,3,5-tricarboxyamide-linked iron porphyrin solid product and N,N',N'-tris(3-methylpyridinyl)trimethylamide obtained in step (2) are mixed with a third solvent, and then a surfactant solution and a crosslinking agent are added. The mixture is stirred and separated to prepare a cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin. (4) Disperse the cationic hollow spherical polymer framework precursor powder loaded with iron porphyrin obtained in step (3) in a fourth solvent, add phosphate to mix into a mixture, adjust the pH, heat the mixture under stirring, then let it stand for aging, filter, take the precipitate, and calcine the precipitate to obtain the iron phosphate material.

2. The method for preparing an iron phosphate material according to claim 1, characterized in that: In step (4), adjusting the pH means adjusting the pH of the reaction to 1.5-2.

2.

3. The method for preparing an iron phosphate material according to claim 1, characterized in that: In step (4), the temperature of the heating reaction is 70-90℃ and the heating reaction time is 6-10h.

4. The method for preparing an iron phosphate material according to claim 1, characterized in that: In step (4), the settling and aging time is 3-5 hours.

5. The method for preparing an iron phosphate material according to claim 1, characterized in that: In step (4), the calcination conditions are: heating to 500-750℃ and holding for 6-10 hours under a protective atmosphere.

6. A ferric phosphate material, characterized in that: The material is prepared by the preparation method according to any one of claims 1-5, and the iron phosphate material has a hollow spherical structure.

7. A method for preparing a lithium iron phosphate cathode material, characterized in that: The process includes the following steps: mixing the iron phosphate material as described in claim 6 with a lithium source, and sintering the mixture under a protective atmosphere to obtain the lithium iron phosphate cathode material.

8. The method for preparing a lithium iron phosphate cathode material according to claim 7, characterized in that: The sintering method is low-temperature pre-sintering and secondary sintering, wherein the low-temperature pre-sintering temperature is 400-550℃ and the holding time is 3-5h; the secondary sintering temperature is 650-750℃ and the holding time is 6-10h.

9. A lithium iron phosphate cathode material, characterized in that: It is prepared by the preparation method according to any one of claims 7-8.

10. A lithium-ion battery, characterized in that: Including the lithium iron phosphate cathode material as described in claim 9.

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