Composite iron phosphate, and preparation method and application thereof

Abstract

The application provides a composite iron phosphate and a preparation method and application thereof, and the preparation method comprises the following steps: (1) dissolving chitin powder in an acid solution, adding a ferrous salt, stirring to obtain Fe(II)-chitin fiber hydrogel, freezing and drying, and performing pyrolysis treatment in a nitrogen-oxygen mixed gas to obtain a porous Fe2O3 / CFN material; (2) mixing the porous Fe2O3 / CFN material with urea and a solvent, adding a phosphoric acid solution, and performing heat treatment to obtain an iron phosphate / nitrogen-containing carbon fiber composite material, and performing sintering treatment to obtain the composite iron phosphate. The application uses chitin as a raw material, and through simple sol-gel + annealing sintering and other methods, a porous nitrogen-containing carbon fiber is embedded in the interior and macroscopic surface layer of the iron phosphate material to form a conductive network structure connected to each other, and the conductive grid structure can not only enhance the conductivity of the material, but also enhance the structure and cycle stability of the material.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, and relates to a composite iron phosphate, its preparation method and application. Background Technology

[0002] Rechargeable lithium-ion batteries are considered a promising electrochemical energy storage system due to their high energy density, high cycle stability, and safety and environmental friendliness. The structure and composition of the cathode material are crucial factors affecting battery energy density. Lithium iron phosphate (LiFePO4) is currently a promising candidate cathode material; however, its inherent low conductivity and small lithium-ion diffusion coefficient result in poor rate performance, severely limiting its potential for improving electrochemical performance and practical applications.

[0003] Iron phosphate (FePO4) is an important precursor for the preparation of lithium iron phosphate materials. The volume changes of the two before and after delithiation are not significant, and their morphologies are also very similar. Therefore, the structure, morphology and dispersibility of the precursor FePO4 will be carried over to the LiFePO4 cathode material and affect its electrical performance.

[0004] CN117693487A discloses a modified iron phosphate, its preparation method and application. The preparation method includes the following steps: (1) mixing zirconium salt, hydrogen phosphate and tungstate with a solvent to obtain a mixed salt solution, and heating the solution to obtain zirconium phosphotungstate; (2) using the zirconium phosphotungstate as a seed crystal, mixing it with iron salt, phosphate, phosphoric acid and oxidant to conduct a synthesis reaction to obtain a modified material; (3) sintering the modified material and grinding it to obtain the modified iron phosphate.

[0005] CN117566711A discloses a nickel-carbon co-doped iron phosphate and lithium iron phosphate material and their preparation method, belonging to the field of lithium-ion battery cathode materials. The preparation method of the iron phosphate includes: preparing iron salt and phosphate solutions separately, adjusting pH and reacting at a constant temperature to prepare iron phosphate dihydrate; reacting the iron phosphate dihydrate with an ionic liquid and a nickel-containing organometallic complex Ni-MOF in an alcohol solvent using a microwave-assisted solvothermal reaction to achieve the goal of dehydrating the iron phosphate dihydrate and coating and supporting the iron phosphate structure with the ionic liquid-Ni-MOF complex; and subjecting the obtained nickel-carbon co-doped iron phosphate precursor to low-temperature ionothermal carbonization treatment to finally obtain the nickel-carbon co-doped iron phosphate material.

[0006] The methods described above suffer from limited performance improvement, polarization phenomena, and complex processes that hinder industrial production. Therefore, developing a simple and convenient method to prepare a lithium iron phosphate precursor with high conductivity and favorable lithium-ion diffusion is crucial for obtaining high-performance lithium iron phosphate cathode materials and promoting the further large-scale application of lithium batteries. Summary of the Invention

[0007] The purpose of this invention is to provide a composite iron phosphate, its preparation method, and its application. This invention uses chitin as a raw material and embeds porous nitrogen-containing carbon fibers into the interior of the iron phosphate material and forms an interconnected conductive network structure with the macroscopic surface through simple sol-gel + annealing sintering methods. The conductive network structure can not only enhance the conductivity of the material, but also enhance the structure and cycle stability of the material.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing composite iron phosphate, the method comprising the following steps:

[0010] (1) Chitin powder was dissolved in acetic acid solution and then ferrous salt was added. The mixture was stirred to obtain Fe(II)-chitin fiber hydrogel. After freezing and drying, the porous Fe2O3 / CFN material was obtained by pyrolysis in a nitrogen-oxygen mixture.

[0011] (2) The porous Fe2O3 / CFN material is mixed with urea and solvent, and then a phosphoric acid solution is added and heat-treated to obtain iron phosphate / nitrogen-containing carbon fiber composite material. After sintering, the composite iron phosphate is obtained.

[0012] This invention first mixes ferrous salt with a chitin solution, utilizing the coordination complexation between ferrous ions and chitin hydroxyl groups to assemble the separated chitin molecular chains via a sol-gel method, forming a cross-linked Fe(II)-chitin fiber hydrogel. This hydrogel is then freeze-dried to obtain a porous Fe(II)-chitin nanofiber composite material. Subsequently, this composite material is subjected to high-temperature annealing in a nitrogen-oxygen mixed atmosphere to obtain a Fe2O3 / nitrogen-containing carbon fiber precursor material, namely Fe2O3 / CFN material (where " / " indicates composite). This material is then mixed with phosphoric acid, and after heat treatment and high-temperature calcination, a FePO4 / CFN composite material is prepared. In this composite iron phosphate, the embedded porous nitrogen-containing carbon fiber network structure provides a large number of active sites and electron / ion transport channels, enhancing the conductivity of the cathode material and improving the overall electrochemical performance of the lithium battery.

[0013] Preferably, the mass percentage concentration of the acetic acid solution in step (1) is 5% to 20%, for example: 5%, 8%, 10%, 15% or 20%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0014] Preferably, the solid-liquid ratio of the chitin powder and acetic acid solution in step (1) is 0.1 to 1 g: 100 mL, for example: 0.1 g: 100 mL, 0.2 g: 100 mL, 0.5 g: 100 mL, 0.8 g: 100 mL or 1 g: 100 mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0015] Preferably, the ferrous salt in step (1) includes any one or a combination of at least two of ferrous sulfate, ferrous chloride, or ferrous nitrate. Typical but non-limiting combinations include combinations of ferrous sulfate and ferrous nitrate, ferrous sulfate and ferrous chloride, or ferrous chloride and ferrous nitrate. The ferrous salt raw material used in this invention can also be a hydrate of the above-mentioned ferrous salts.

[0016] Preferably, the mass ratio of ferrous salt to chitin powder in step (1) is (8-12):1, for example: 8:1, 9:1, 10:1, 11:1 or 12:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0017] Preferably, the stirring time in step (1) is 2 to 3 hours, for example: 2 hours, 2.2 hours, 2.5 hours, 2.8 hours or 3 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0018] Preferably, the freezing method in step (1) includes liquid nitrogen freezing.

[0019] Preferably, the drying in step (1) includes freeze-vacuum drying.

[0020] Preferably, the drying temperature in step (1) is -45 to -60°C, for example: -45°C, -48°C, -50°C, -55°C or -60°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] Preferably, the drying time in step (1) is 12 to 24 hours, for example: 12 hours, 16 hours, 18 hours, 20 hours or 24 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] Preferably, the volume percentage of nitrogen in the nitrogen-oxygen mixture in step (1) is 99% to 99.8%, for example: 99%, 99.2%, 99.4%, 99.6% or 99.8%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] Preferably, the volume percentage of oxygen in the nitrogen-oxygen mixture in step (1) is 0.2% to 1%, for example: 0.2%, 0.4%, 0.6%, 0.8%, or 1%, etc., and is not limited to the listed values; other unlisted values ​​within this range are also applicable. Preferably, the temperature of the pyrolysis treatment in step (1) is 450 to 550°C, for example: 450°C, 460°C, 480°C, 490°C, or 500°C, etc., and is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0024] Preferably, the pyrolysis treatment time in step (1) is 4 to 6 hours, for example: 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the mass ratio of the porous Fe2O3 / CFN material to urea in step (2) is 1:(1.5~2.5), for example: 1:1.5, 1:1.8, 1:2, 1:2.2 or 1:2.5, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Preferably, the solvent in step (2) includes water.

[0027] Preferably, the molar ratio of phosphorus in the phosphoric acid solution to iron in the Fe2O3 / CFN material in step (2) is 1:(0.97~1.02), for example: 1:0.97, 1:0.98, 1:0.99, 1:1, 1:1.01 or 1:1.02, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] Preferably, the heat treatment temperature in step (2) is 120 to 150°C, for example: 120°C, 125°C, 130°C, 140°C or 150°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0029] Preferably, the heat treatment time in step (2) is 12 to 24 hours, for example: 12 hours, 16 hours, 18 hours, 20 hours or 24 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] Preferably, after the heat treatment in step (2), the product is subjected to centrifugation, washing, and vacuum drying.

[0031] Preferably, the sintering temperature in step (2) is 500 to 750°C, for example: 500°C, 550°C, 600°C, 700°C or 750°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, the sintering time in step (2) is 4 to 10 hours, for example: 4 hours, 5 hours, 6 hours, 8 hours or 10 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] In a second aspect, the present invention provides a composite iron phosphate, which is prepared by the method described in the first aspect.

[0034] The method described in this invention produces a composite iron phosphate material in which porous nitrogen-containing carbon fibers with abundant surface area are embedded inside the iron phosphate material, forming an interconnected conductive network structure with the macroscopic surface layer. This provides a large number of electron / ion transport channels, thereby effectively enhancing the conductivity of the cathode material. Furthermore, nitrogen doping induces surface defects and changes in the electronic structure of the carbon fiber-like fibers, which provides more active sites for the growth of lithium iron phosphate crystals and the subsequent lithium-ion insertion / extraction process, resulting in superior conductivity.

[0035] Thirdly, the present invention provides a lithium iron phosphate cathode material, which is prepared by sintering a composite iron phosphate and a lithium source as described in the second aspect.

[0036] Fourthly, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material as described in the third aspect.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) This invention uses chitin as raw material and embeds porous nitrogen-containing carbon fibers into the interior of iron phosphate material and forms an interconnected conductive network structure with the macroscopic surface through simple sol-gel + annealing sintering methods. The conductive network structure can not only enhance the conductivity of the material, but also enhance the structure and cycle stability of the material.

[0039] (2) In the composite iron phosphate of the present invention, the network structure formed by nitrogen-containing carbon fibers can generate internal void spaces between iron phosphate particles, which can not only inhibit the agglomeration of particles during the synthesis process, but also alleviate the volume expansion and particle shedding of the cathode material during the charging and discharging process, thereby enhancing the structure and cycle stability of the material; in addition, the nitrogen-containing carbon fibers can also be used as the carbon source for the synthesis of lithium iron phosphate after iron phosphate and lithium source are mixed in an inert atmosphere and calcined at high temperature.

[0040] (3) The composite iron phosphate material prepared by the present invention has good performance indicators. The lithium iron phosphate cathode material prepared by it also exhibits excellent electrochemical performance after being assembled into a lithium battery. Its charge and discharge capacity at 0.1C and 1C can reach up to 161.1mAh / g and 154.4mAh / g, respectively; and its first charge and discharge efficiency can reach up to 99.67%. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the process for preparing composite iron phosphate according to an embodiment of the present invention.

[0042] Figure 2 This is a SEM image of the composite iron phosphate prepared in Example 1 of the present invention.

[0043] Figure 3 This is the XRD pattern of the composite iron phosphate prepared in Example 1 of the present invention. Detailed Implementation

[0044] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0045] Example 1

[0046] This embodiment provides a composite ferric phosphate, and the preparation process of the composite ferric phosphate is shown in the schematic diagram below. Figure 1 As shown, the preparation method of the composite iron phosphate is as follows:

[0047] (1) Dissolve 0.5g of chitin powder in 100mL of acetic acid aqueous solution (10%), mix evenly, add 5g of ferrous sulfate, stir magnetically for 3h, then rapidly freeze the obtained Fe(II)-chitin fiber hydrogel solution with liquid nitrogen, and freeze-dry it under vacuum at -55℃ for 12h. Then grind the product in a mortar to obtain Fe(II)-chitin nanofiber powder, and then pyrolyze the obtained powder at 500℃ for 4h in an oxygen-containing nitrogen atmosphere (nitrogen volume ratio is 99.5%, the rest is oxygen) to form porous Fe2O3 / CFN material.

[0048] (2) 0.8 g of Fe2O3 / CFN material was ultrasonically dispersed in 60 mL of deionized water, and then 1.6 g of urea was added. After stirring evenly, phosphoric acid solution (0.8 mol / L) was added according to the Fe / P molar ratio of 1:1. The mixture was stirred at 25 °C for 30 min, and then transferred to a 100 mL stainless steel autoclave and heated in an oven at 120 °C for 16 h. After cooling to room temperature, the product was collected by centrifugation, washed repeatedly with anhydrous ethanol and deionized water, and dried in a vacuum oven at 60 °C to obtain iron phosphate / nitrogen-containing carbon fiber composite material. Subsequently, the above product was placed in a muffle furnace and heated to 500 °C for 10 h to remove the water of crystallization, thus obtaining the composite iron phosphate material.

[0049] The SEM image of the composite ferric phosphate is shown below. Figure 2 As shown, by Figure 2 It can be seen that CFN and FePO4 are tightly bound together, and CFN is interconnected between FePO4 particles to form a 3D network structure.

[0050] The XRD pattern of the composite iron phosphate is as follows: Figure 3 As shown, by Figure 3 It can be seen that the main phase of CFN is amorphous carbon, with no obvious diffraction peaks; while the FePO4 / CFN composite material has the same characteristic peaks as the iron phosphate standard card, indicating better crystallinity.

[0051] Example 2

[0052] This embodiment provides a composite ferric phosphate, and the preparation process of the composite ferric phosphate is shown in the schematic diagram below. Figure 1 As shown, the preparation method of the composite iron phosphate is as follows:

[0053] (1) Dissolve 1g of chitin powder in 100mL of acetic acid aqueous solution (5%), mix evenly, add 8g of ferrous sulfate, stir magnetically for 2.5h, then rapidly freeze the obtained Fe(II)-chitin fiber hydrogel solution with liquid nitrogen, and then freeze-dry under vacuum at -45℃ for 24h. Then grind the product in a mortar to obtain Fe(II)-chitin nanofiber powder, and then pyrolyze the obtained powder at 450℃ for 6h in an oxygen-containing nitrogen atmosphere (nitrogen volume ratio is 99%, the rest is oxygen) to form porous Fe2O3 / CFN material.

[0054] (2) 1 g of Fe2O3 / CFN material was ultrasonically dispersed in 80 mL of deionized water, and then 2.5 g of urea was added. After stirring evenly, phosphoric acid solution (0.8 mol / L) was added according to the Fe / P molar ratio of 1:1. The mixture was stirred at 25 °C for 30 min, and then transferred to a 100 mL stainless steel autoclave and heated in an oven at 140 °C for 24 h. After cooling to room temperature, the product was collected by centrifugation, washed repeatedly with anhydrous ethanol and deionized water, and dried in a vacuum oven at 60 °C to obtain the iron phosphate / nitrogen-containing carbon fiber composite material. Subsequently, the above product was placed in a muffle furnace and heated to 650 °C for 7 h to remove the water of crystallization, thus obtaining the composite iron phosphate material.

[0055] Example 3

[0056] This embodiment provides a composite ferric phosphate, and the preparation process of the composite ferric phosphate is shown in the schematic diagram below. Figure 1 As shown, the preparation method of the composite iron phosphate is as follows:

[0057] (1) Dissolve 0.1g of chitin powder in 100mL of acetic acid aqueous solution (20%), mix evenly, add 1.2g of ferrous sulfate, stir magnetically for 2h, and then rapidly freeze the obtained Fe(II)-chitin fiber hydrogel solution with liquid nitrogen, and then freeze-dry it under vacuum at -60℃ for 12h. Then grind the product in a mortar to obtain Fe(II)-chitin nanofiber powder, and then pyrolyze the obtained powder at 550℃ for 4h in an oxygen-containing nitrogen atmosphere (nitrogen volume ratio is 99.8%, the rest is oxygen) to form porous Fe2O3 / CFN material.

[0058] (2) 1 g of Fe2O3 / CFN material was ultrasonically dispersed in 80 mL of deionized water, and then 1.5 g of urea was added. After stirring evenly, phosphoric acid solution (0.8 mol / L) was added according to the Fe / P molar ratio of 1:1. The mixture was stirred at 25 °C for 30 min, and then transferred to a 100 mL stainless steel autoclave and heated in an oven at 150 °C for 12 h. After cooling to room temperature, the product was collected by centrifugation, washed repeatedly with anhydrous ethanol and deionized water, and dried in a vacuum oven at 60 °C to obtain the iron phosphate / nitrogen-containing carbon fiber composite material. Subsequently, the above product was placed in a muffle furnace and heated to 750 °C for 10 h to remove the water of crystallization, thus obtaining the composite iron phosphate material.

[0059] Example 4

[0060] The only difference between this embodiment and embodiment 1 is that the mass ratio of ferrous salt and chitin powder in step (1) is 5:1, while the other conditions and parameters are exactly the same as in embodiment 1.

[0061] Example 5

[0062] The only difference between this embodiment and embodiment 1 is that the mass ratio of ferrous salt and chitin powder in step (1) is 15:1, while the other conditions and parameters are exactly the same as in embodiment 1.

[0063] Example 6

[0064] The only difference between this embodiment and embodiment 1 is that the temperature of the pyrolysis treatment in step (1) is 400°C, while the other conditions and parameters are exactly the same as in embodiment 1.

[0065] Example 7

[0066] The only difference between this embodiment and embodiment 1 is that the temperature of the pyrolysis treatment in step (1) is 600°C, while the other conditions and parameters are exactly the same as in embodiment 1.

[0067] Example 8

[0068] The only difference between this embodiment and embodiment 1 is that the mass ratio of the porous Fe2O3 / CFN material to urea in step (2) is 1:1, while the other conditions and parameters are exactly the same as in embodiment 1.

[0069] Example 9

[0070] The only difference between this embodiment and embodiment 1 is that the mass ratio of the porous Fe2O3 / CFN material to urea in step (2) is 1:3. All other conditions and parameters are exactly the same as in embodiment 1.

[0071] Comparative Example 1

[0072] This comparative example provides a composite iron phosphate, the preparation method of which is as follows:

[0073] 0.8 g of Fe₂O₃ was ultrasonically dispersed in 60 mL of deionized water, followed by the addition of 1.6 g of urea. After stirring until homogeneous, phosphoric acid solution (0.8 mol / L) was added at a Fe / P molar ratio of 1:1. The mixture was stirred at room temperature for 30 min, then transferred to a 100 mL stainless steel autoclave and heated in an oven at 120 °C for 12 h. After cooling to room temperature, the product was collected by centrifugation, washed repeatedly with anhydrous ethanol and deionized water, and dried in a vacuum oven at 60 °C to obtain iron phosphate material. Subsequently, the product was placed in a muffle furnace and heated to 500 °C for 4 h to remove the water of crystallization, thus obtaining anhydrous FePO₄ composite material.

[0074] Comparative Example 2

[0075] The only difference between this comparative example and Example 1 is that the chitin powder is replaced with cellulose to prepare an iron phosphate / carbon fiber-like composite material. All other conditions and parameters are exactly the same as in Example 1.

[0076] Performance testing:

[0077] Lithium carbonate and the composite iron phosphate prepared in the examples and comparative examples were dispersed in anhydrous ethanol at a lithium source to iron source molar ratio of 1:1. The mixture was ball-milled for 3 hours until homogeneous, and then spray-dried to obtain precursor powder. Subsequently, the precursor powder was heated to 500°C at a heating rate of 5°C / min and held for 5 hours under a high-purity argon atmosphere, and then calcined at 750°C for 10 hours to obtain lithium iron phosphate cathode material. The obtained lithium iron phosphate cathode material was formulated into a coin cell. 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, and then pressing it into a positive electrode sheet using a tablet press. 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.

[0078] The electrochemical performance of the above batteries was tested (with the charge / discharge voltage controlled between 2.5 and 4.5 V), and the test results are shown in Table 1.

[0079] Table 1

[0080]

[0081] As can be seen from Table 1, and from Examples 1-3, the 0.1C discharge specific capacity of the coin cell made from the composite iron phosphate of the present invention as lithium iron phosphate cathode material can reach more than 159.3 mAh / g, the 1C discharge specific capacity can reach more than 151.5 mAh / g, and the first charge efficiency can reach more than 99.10%.

[0082] Comparing Examples 1 and 4-5, it can be seen that in the preparation process of the composite iron phosphate of the present invention, the mass ratio of ferrous salt and chitin powder in step (1) affects its performance. When the mass ratio of ferrous salt and chitin powder is controlled at 8-12:1, the composite iron phosphate has better performance. If the amount of chitin powder added is too large, the carbon content in the prepared lithium iron phosphate material will be too high, which will affect its capacity and lifespan. If the amount of chitin powder added is too small, the carbon fiber network structure content in the composite material will be low and uneven, which will reduce the conductivity and structural stability of the prepared lithium iron phosphate material.

[0083] Comparing Examples 1 and 6-7, it can be seen that the temperature of the pyrolysis treatment in step (1) of the preparation process of the composite iron phosphate of the present invention will affect its performance. The composite iron phosphate with better performance is obtained by controlling the temperature of the pyrolysis treatment at 450-550℃. If the temperature of the pyrolysis treatment is too high or too low, it will affect the degree of carbonization and strength of the nitrogen-containing carbon fibers, and thus affect the electrochemical performance of the composite iron phosphate material.

[0084] Comparing Examples 1 and 8-9, it can be seen that in the preparation process of the composite iron phosphate of the present invention, the mass ratio of the porous Fe2O3 / CFN material to urea in step (2) affects its performance. When the mass ratio of the porous Fe2O3 / CFN material to urea is controlled at 1:(1.5~2.5), the composite iron phosphate has better performance. If the amount of urea added is too large or too small, the pH value of the precursor solution will fluctuate greatly, and impurities will be mixed in the prepared iron phosphate, thereby affecting the electrical performance of the lithium iron phosphate material.

[0085] As can be seen from the comparison between Example 1 and Comparative Examples 1-2, the composite iron phosphate with iron phosphate / nitrogen-containing carbon fibers prepared by the present invention has higher conductivity and more active sites provided by surface defects. The lithium iron phosphate prepared from it has the advantages of fast ion / electron transport and low resistance to lithium ion mass transfer process. Therefore, the prepared lithium ion battery has excellent electrochemical performance.

[0086] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing composite ferric phosphate, characterized in that, The preparation method includes the following steps: (1) Chitin powder is dissolved in acetic acid solution and then ferrous salt is added. The mixture is stirred to obtain ferrous-chitin fiber hydrogel. After freezing and drying, the hydrogel is subjected to pyrolysis in a nitrogen-oxygen mixture to obtain porous Fe2O3 / nitrogen-containing carbon fiber precursor material. The mass ratio of ferrous salt to chitin powder is (8~12):

1. The temperature of the pyrolysis treatment is 450~550℃. (2) The porous Fe2O3 / nitrogen-containing carbon fiber precursor material is mixed with urea and solvent, and then a phosphoric acid solution is added and heat-treated to obtain iron phosphate / nitrogen-containing carbon fiber composite material. After sintering, the composite iron phosphate is obtained; the mass ratio of the porous Fe2O3 / nitrogen-containing carbon fiber precursor material to urea is 1:(1.5~2.5).

2. The preparation method according to claim 1, characterized in that, The acetic acid solution in step (1) has a mass percentage concentration of 5% to 20%.

3. The preparation method according to claim 1, characterized in that, In step (1), the solid-liquid ratio of the chitin powder and the acetic acid solution is 0.1~1g:100mL.

4. The preparation method according to claim 1, characterized in that, The ferrous salt in step (1) includes any one or a combination of at least two of ferrous sulfate, ferrous chloride, or ferrous nitrate.

5. The preparation method according to claim 1, characterized in that, The stirring time in step (1) is 2-3 hours.

6. The preparation method according to claim 1, characterized in that, The freezing method described in step (1) includes liquid nitrogen freezing.

7. The preparation method according to claim 1, characterized in that, The drying in step (1) includes freeze-vacuum drying.

8. The preparation method according to claim 1, characterized in that, The drying temperature in step (1) is -45~-60℃.

9. The preparation method according to claim 1, characterized in that, The drying time in step (1) is 12~24h.

10. The preparation method according to claim 1, characterized in that, In step (1), the nitrogen volume ratio in the nitrogen-oxygen mixture is 99%~99.8%.

11. The preparation method according to claim 1, characterized in that, The volume percentage of oxygen in the nitrogen-oxygen mixture in step (1) is 0.2% to 1%.

12. The preparation method according to claim 1, characterized in that, The pyrolysis treatment in step (1) takes 4 to 6 hours.

13. The preparation method according to claim 1, characterized in that, The solvent in step (2) includes water.

14. The preparation method according to claim 1, characterized in that, The molar ratio of phosphorus in the phosphoric acid solution to iron in the Fe2O3 / nitrogen-containing carbon fiber precursor material in step (2) is 1:(0.97~1.02).

15. The preparation method according to claim 1, characterized in that, The heat treatment temperature in step (2) is 120~150℃.

16. The preparation method according to claim 1, characterized in that, The heat treatment time in step (2) is 12~24h.

17. The preparation method according to claim 1, characterized in that, After the heat treatment in step (2), the product is centrifuged, washed and vacuum dried.

18. The preparation method according to claim 1, characterized in that, The sintering temperature in step (2) is 500~750℃.

19. The preparation method according to claim 1, characterized in that, The sintering process in step (2) takes 4 to 10 hours.

20. A composite iron phosphate, characterized in that, The composite ferric phosphate is prepared by the method described in any one of claims 1-19.

21. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by sintering the composite iron phosphate and lithium source as described in claim 20.

22. A lithium-ion battery, characterized in that, The lithium-ion battery comprises the lithium iron phosphate cathode material as described in claim 21.

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