Iron phosphate, its preparation method and application

Abstract

This disclosure provides ferric phosphate, its preparation method, and its application. The preparation method includes the following steps: (1) mixing ferrous salt, phosphorus source, and solvent to obtain a mixed salt solution, and freezing the mixed solution to obtain a frozen mixed salt solution; (2) placing the frozen mixed salt solution in a reflux device, controlling the reflux of hydrogen peroxide solution between the reflux device and the reaction device, controlling the concentration of hydrogen peroxide in the system to carry out the reaction, until the frozen mixed salt solution is completely dissolved; (3) adjusting the pH in the system to carry out an aging reaction, and separating the solid and liquid to obtain the ferric phosphate. In this disclosure, the solution containing ferrous salt and phosphorus source is frozen in advance, and hydrogen peroxide is controlled to flow through the frozen mixed salt solution. By controlling the concentration and flow rate of hydrogen peroxide, the reaction can be carried out slowly, so that the iron and phosphorus elements in the prepared ferric phosphate precursor are evenly distributed and have few surface defects.

Description

Technical Field

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

[0002] Iron phosphate is a white or grayish-white monoclinic crystalline powder, mainly used in the preparation of lithium iron phosphate cathode materials, catalysts, and ceramics. With the rapid development of lithium iron phosphate batteries, the demand for iron phosphate, as one of the main precursors for lithium iron phosphate cathode materials, has increased significantly, with an estimated annual demand exceeding 50,000 tons.

[0003] The structure of iron phosphate is very similar to that of lithium iron phosphate. Therefore, the performance of lithium iron phosphate can be controlled by controlling the structure, morphology and particle size of iron phosphate. Existing technologies usually use precipitation or solid-phase methods to synthesize lithium iron phosphate.

[0004] CN116443834A discloses a method for continuous synthesis of ferric phosphate using a microreactor, in which the ferric phosphate raw material solution and the oxidizing solution are separately introduced into the microreactor and mixed; the reaction occurs in the dynamic environment of the microreactor to obtain a product slurry; and the product slurry is post-treated to obtain ferric phosphate.

[0005] CN103682341A discloses a method for synthesizing high tap density lithium iron phosphate material. The method involves preparing iron phosphate dihydrate precipitate by precipitation, and then mixing the iron phosphate precursor with lithium source, carbon source, etc. and sintering to obtain carbon-coated lithium iron phosphate cathode material.

[0006] The iron phosphate particles obtained by the above method are relatively large and have a wide particle size distribution. When sulfate is used as a raw material for precipitation, sulfate ions are easily carried in the synthesized iron phosphate, resulting in a high sulfur content in the synthesized iron phosphate, making it difficult to obtain nano-sized iron phosphate particles. Summary of the Invention

[0007] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0008] The purpose of this disclosure is to provide an iron phosphate, its preparation method, and its application. The method described in this disclosure can reduce the byproducts encapsulated during the crystal nucleus growth process, and obtain nano-iron phosphate particles with stable particle size composition, fine and uniform particle size, and low sulfur content. The lithium iron phosphate cathode material prepared using this iron phosphate can exhibit excellent rate performance and cycle performance.

[0009] To achieve this objective, the present disclosure adopts the following technical solution:

[0010] In a first aspect, this disclosure provides a method for preparing ferric phosphate, the method comprising the following steps:

[0011] (1) A mixed salt solution is obtained by mixing ferrous salt, phosphorus source and solvent, and the mixed salt solution is subjected to freezing treatment to obtain a frozen mixed salt solution;

[0012] (2) Place the frozen mixed salt solution in a reflux device, control the hydrogen peroxide solution to reflux between the reflux device and the reaction device, control the concentration of hydrogen peroxide in the system to carry out the reaction, until the frozen mixed salt solution is completely dissolved;

[0013] (3) Adjust the pH of the system to carry out the aging reaction, and separate the solid and liquid to obtain the iron phosphate.

[0014] This disclosure pre-freezes a solution containing ferrous salt and phosphorus source, and controls the flow of hydrogen peroxide through the frozen mixed salt solution. By controlling the concentration and flow rate of hydrogen peroxide, the reaction can proceed slowly, resulting in a uniform distribution of iron and phosphorus elements and fewer surface defects in the prepared ferric phosphate.

[0015] In one embodiment, the ferrous salt in step (1) includes any one or a combination of at least two of ferrous chloride, ferrous sulfate, or ferrous nitrate.

[0016] In one embodiment, the phosphorus source in step (1) includes any one or a combination of at least two of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or ammonium phosphate.

[0017] In one embodiment, the concentration of iron in the mixed salt solution in step (1) is 0.05 to 1 mol / L, for example: 0.05 mol / L, 0.1 mol / L, 0.3 mol / L, 0.5 mol / L or 1 mol / L, etc.

[0018] In one embodiment, the concentration of phosphorus in the mixed salt solution in step (1) is 0.5 to 1 mol / L, for example: 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L or 1 mol / L, etc.

[0019] In one embodiment, the pH of the mixed salt solution in step (1) is 1.8 to 2.5, for example: 1.8, 1.9, 2, 2.2 or 2.5, etc.

[0020] In one embodiment, the freezing temperature in step (1) is -30 to -40°C, for example: -30°C, -32°C, -35°C, -38°C or -40°C, etc.

[0021] In one embodiment, the freezing time in step (1) is 6 to 12 hours, for example: 6 hours, 7 hours, 8 hours, 10 hours or 12 hours.

[0022] In one embodiment, the mass concentration of the hydrogen peroxide solution in step (2) is 5-10%, for example: 5%, 6%, 7%, 8% or 10%, etc.

[0023] In one embodiment, the total molar amount of hydrogen peroxide in the hydrogen peroxide solution in step (2) and the molar ratio of iron in the iron source are (1-2):1, for example: 1:1, 1.2:1, 1.5:1, 1.8:1 or 2:1, etc.

[0024] In one embodiment, the reflux rate of the hydrogen peroxide solution in step (2) is 50 to 100 mL / min, for example: 50 mL / min, 60 mL / min, 70 mL / min, 80 mL / min or 100 mL / min, etc.

[0025] In one embodiment, the hydrogen peroxide mass concentration in the reaction of step (2) is 5-10%, for example: 5%, 6%, 7%, 8% or 10%, etc.

[0026] In one embodiment, the reaction temperature in step (2) is 40 to 60°C, for example: 40°C, 45°C, 50°C, 55°C or 60°C.

[0027] In one embodiment, phosphoric acid is added during the reaction in step (2) to control the pH.

[0028] In one embodiment, step (2) involves controlling the pH to be between 1.8 and 2.5.

[0029] In one embodiment, the pH adjuster in step (3) includes ammonia.

[0030] In one embodiment, the pH in step (3) is 2.5 to 3.5, for example: 2.5, 2.8, 3, 3.2 or 3.5, etc.

[0031] In one embodiment, the aging reaction temperature in step (3) is 40 to 80°C, for example: 40°C, 50°C, 60°C, 70°C or 80°C.

[0032] In one embodiment, the stirring speed of the aging reaction in step (3) is 100 to 200 rpm, for example: 100 rpm, 120 rpm, 150 rpm, 180 rpm or 200 rpm.

[0033] In one embodiment, the aging reaction time in step (3) is 10 to 60 hours, for example: 10 hours, 20 hours, 30 hours, 50 hours or 60 hours.

[0034] In one embodiment, the solid-liquid separation in step (3) is followed by washing and vacuum drying.

[0035] In one embodiment, the detergent used for washing includes deionized water.

[0036] In one embodiment, the temperature of the vacuum drying process is 60 to 100°C, for example: 60°C, 70°C, 80°C, 90°C, or 100°C.

[0037] In a second aspect, this disclosure provides an iron phosphate, which is prepared by the method described in the first aspect.

[0038] In one embodiment, the iron phosphate has a particle size of 100–300 nm, for example: 100 nm, 150 nm, 200 nm, 250 nm or 300 nm.

[0039] Thirdly, this disclosure provides a lithium iron phosphate, which is prepared by sintering a mixture of iron phosphate and a lithium source as described in the second aspect.

[0040] Compared with the prior art, this disclosure has the following beneficial effects:

[0041] (1) The method described in this disclosure can produce nanoscale iron phosphate. The iron phosphate has a stable composition, uniform element distribution and low sulfur content. The lithium iron phosphate cathode material prepared using the iron phosphate exhibits good rate performance and cycle performance.

[0042] (2) The lithium iron phosphate battery prepared by the method described in this disclosure has a 0.1C discharge capacity of more than 161.57 mAh / g, a 0.5C / 0.1C capacity retention rate of more than 97.32%, and a capacity retention rate of more than 98.51% after 100 cycles.

[0043] After reading and understanding the accompanying diagrams and detailed descriptions, the other aspects can be understood. Attached Figure Description

[0044] The accompanying drawings are used to provide a further understanding of the technical solutions in this paper and form part of the specification. They are used together with the embodiments of this application to explain the technical solutions in this paper and do not constitute a limitation on the technical solutions in this paper.

[0045] Figure 1 This is a schematic diagram of the apparatus used in the preparation method of lithium iron phosphate cathode material according to an embodiment of the present disclosure. 1-Frozen mixed salt solution, 2-Reflux device, 3-Pump, 4-Reaction device, 5-Stirring rod, 6-Hydrogen peroxide solution, 7-Feed port.

[0046] Figure 2 This is a SEM image of lithium iron phosphate prepared from iron phosphate as described in Example 1 of this disclosure.

[0047] Figure 3 This is a SEM image of lithium iron phosphate prepared from iron phosphate as described in Comparative Example 1. Detailed Implementation

[0048] The technical solutions of this disclosure will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as specific limitations thereof.

[0049] Example 1

[0050] This embodiment provides a method for preparing ferric phosphate, and the schematic diagram of the apparatus used in the method for preparing the ferric phosphate is shown below. Figure 1 As shown, 1 is a frozen mixed salt solution, 2 is a reflux device, 3 is a pump, 4 is a reaction device, 5 is a stirring rod, 6 is a mixed solution, and 7 is a feed port. The preparation method of the ferric phosphate is as follows:

[0051] (1) A mixed salt solution with ferrous sulfate, phosphoric acid and water was obtained by mixing ferrous sulfate, phosphoric acid and water, with a ferrous sulfate concentration of 0.5 mol / L, a phosphorus concentration of 0.75 mol / L and a pH of 2.1. The solution was then frozen at -40℃ for 8 h to obtain a frozen mixed salt solution.

[0052] (2) Place the frozen mixed salt solution in a reflux device, and control the hydrogen peroxide solution with a mass concentration of 7% to reflux between the reflux device and the reaction device at a rate of 60 mL / min. Control the mass concentration of hydrogen peroxide in the system to remain constant at 7%. Add phosphoric acid dropwise through the feed port to maintain the pH of the solution in the reaction vessel at 2.1. Control the reaction temperature at 50℃ to carry out the reaction until the frozen mixed salt solution is completely dissolved. The molar ratio of the total amount of hydrogen peroxide added during the reaction to the iron source is 1.5:1.

[0053] (3) The pH was adjusted to 3.1 by ammonia water, the reaction temperature was controlled at 50℃, the aging reaction was carried out for 12 hours, and the stirring speed was 100 rpm. After solid-liquid separation, the solid product was washed three times with deionized water. The washed product was vacuum dried at 80℃ for 8 hours to obtain iron phosphate with a median particle size (D50) of 200 nm.

[0054] The SEM image of the positive electrode material made from iron phosphate is shown below. Figure 2 As shown.

[0055] Example 2

[0056] This embodiment provides a method for preparing iron phosphate, which is as follows:

[0057] (1) A mixed salt solution with ferrous chloride, ammonium dihydrogen phosphate and water was obtained by mixing ferrous chloride, ammonium dihydrogen phosphate and water, with ferrous concentration of 0.05 mol / L, phosphorus concentration of 0.5 mol / L and pH of 1.8. The solution was then frozen at -30℃ for 12 h to obtain a frozen mixed salt solution.

[0058] (2) Place the frozen mixed salt solution in a reflux device, and control the hydrogen peroxide solution with a mass concentration of 5% to reflux between the reflux device and the reaction device at a rate of 80 mL / min. Control the mass concentration of hydrogen peroxide in the system to remain constant at 7%. Add phosphoric acid dropwise through the feed port to maintain the pH of the solution in the reaction vessel at 1.8. Control the reaction temperature at 40℃ to carry out the reaction until the frozen mixed salt solution is completely dissolved. The total amount of hydrogen peroxide added during the reaction is in a molar ratio of 1:1 to the iron source.

[0059] (3) The pH was adjusted to 2.5 with ammonia water, the reaction temperature was controlled at 40℃, the aging reaction was carried out for 60h with a stirring speed of 150rpm, the solid product was washed three times with deionized water after solid-liquid separation, and the washed product was vacuum dried at 60℃ for 8h to obtain iron phosphate with a median particle size (D50) of 230nm.

[0060] Example 3

[0061] This embodiment provides a method for preparing iron phosphate, which is as follows:

[0062] (1) A mixed salt solution with ferrous nitrate, ammonium hydrogen phosphate and water was obtained by mixing ferrous nitrate, ammonium hydrogen phosphate and water, with ferrous nitrate concentration of 0.1 mol / L, phosphorus concentration of 0.1 mol / L and pH of 2.5. The solution was then frozen at -35℃ for 8 h to obtain a frozen mixed salt solution.

[0063] (2) Place the frozen mixed salt solution in a reflux device, and control the hydrogen peroxide solution with a mass concentration of 10% to reflux between the reflux device and the reaction device at a rate of 100 mL / min. Control the mass concentration of hydrogen peroxide in the system to remain constant at 10%. Add phosphoric acid dropwise through the feed port to maintain the pH of the solution in the reaction vessel at 2.5. Control the reaction temperature at 60℃ to carry out the reaction until the frozen mixed salt solution is completely dissolved. The total amount of hydrogen peroxide added during the reaction is in a molar ratio of 2:1 to the iron source.

[0064] (3) The pH was adjusted to 3.5 with ammonia water, the reaction temperature was controlled at 60℃, the aging reaction was carried out for 10h with a stirring speed of 150rpm, the solid product was washed three times with deionized water after solid-liquid separation, and the washed product was vacuum dried at 100℃ for 6h to obtain iron phosphate with a median particle size (D50) of 250nm.

[0065] Example 4

[0066] The only difference between this embodiment and Embodiment 1 is that the mass concentration of hydrogen peroxide is 3%, and the mass concentration of hydrogen peroxide in the reaction system is controlled at 3%. All other conditions and parameters are exactly the same as in Embodiment 1.

[0067] Example 5

[0068] The only difference between this embodiment and Embodiment 1 is that the mass concentration of hydrogen peroxide is 12%, and the mass concentration of hydrogen peroxide in the reaction system is also controlled at 12%. All other conditions and parameters are exactly the same as in Embodiment 1.

[0069] Example 6

[0070] The only difference between this embodiment and Example 1 is that the reaction temperature in step (2) is 35°C, while the other conditions and parameters are exactly the same as in Example 1.

[0071] Example 7

[0072] The only difference between this embodiment and Example 1 is that the reaction temperature in step (2) is 65°C, while the other conditions and parameters are exactly the same as in Example 1.

[0073] Comparative Example 1

[0074] This comparative example provides an iron phosphate, the preparation method of which is as follows:

[0075] (1) Ferrous sulfate, phosphoric acid and water were mixed to obtain a mixed salt solution with a ferrous concentration of 0.5 mol / L, a phosphorus concentration of 0.75 mol / L and a pH of 2.1. Hydrogen peroxide with a mass concentration of 7% was added dropwise to the mixed salt solution. The total amount of hydrogen peroxide added was in a molar ratio of 1.5:1 to the iron source.

[0076] (2) After the addition is complete, adjust the pH to 3.1 with ammonia water, control the reaction temperature to 50°C, age the reaction for 12 hours, stir at 100 rpm, transfer the resulting suspension to a solid-liquid separator for solid-liquid separation, wash the solid product three times with deionized water, and dry the washed product under vacuum at 80°C for 8 hours to obtain the iron phosphate.

[0077] The SEM image of the positive electrode material made from iron phosphate is shown below. Figure 3 As shown.

[0078] Performance testing:

[0079] The iron phosphate prepared in the examples and comparative examples was mixed with lithium hydroxide in a molar ratio of Li:Fe = 1.03:1. Glucose of 8% of the theoretical mass of lithium iron phosphate was weighed. The above materials were wet ball-milled, dried, and then placed in a tube furnace. Under a protective atmosphere, the temperature was raised to 750°C at a rate of 3°C / min and held for calcination for 10 hours to obtain carbon-coated lithium iron phosphate cathode material. The lithium iron phosphate cathode material, acetylene black and binder polytetrafluoroethylene were mixed in a mass ratio of 90:10:6. NMP was added and stirred into a slurry. The slurry was then coated onto aluminum foil to form a cathode sheet. Lithium sheet was used as the anode. The electrolyte was 1 mol / L LiPF6 / ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethyl methyl carbonate (EMC). Celgard 2400 was selected as the separator material. The CR2430 coin cell was assembled in a vacuum glove box. Charge-discharge tests were conducted using a 0.1C current, with a voltage range of 2.3–4.2V. The test results are shown in Table 1.

[0080] Table 1

[0081]

[0082]

[0083] As can be seen from Table 1, as obtained from Examples 1-3, the lithium iron phosphate batteries prepared by the method described in this disclosure can achieve a 0.1C discharge capacity of over 161.57 mAh / g, a 0.5C / 0.1C capacity retention rate of over 97.32%, and a capacity retention rate of over 98.51% after 100 cycles.

[0084] A comparison of Examples 1 and 4-5 shows that the concentration of hydrogen peroxide (the concentration of hydrogen peroxide in the reaction system) affects the performance of the prepared ferric phosphate during the preparation process described in this disclosure. Controlling the mass concentration of hydrogen peroxide at 5-10% results in better performance of the prepared ferric phosphate. If the concentration of hydrogen peroxide is too high, the oxidation precipitation process is too fast, and the particles are prone to agglomeration, leading to a decrease in the electrochemical performance of the material. If the concentration of hydrogen peroxide is too low, the reaction rate is too slow, the reaction is not sufficient, and the valence state of the obtained material is inconsistent, affecting the electrochemical performance of the material.

[0085] A comparison of Examples 1 and 6-7 shows that the reaction temperature affects the performance of the iron phosphate produced during the preparation process described in this disclosure. Controlling the reaction temperature between 40 and 60°C results in better performance of the iron phosphate. If the reaction temperature is too high, the reaction rate will be too fast, leading to incomplete reaction. If the reaction temperature is too low, the reaction rate will be too slow, making it difficult for the grains to grow. In the later stages, when the material is completely melted and the temperature is raised, the outer layer particles will be generated too quickly, resulting in a large difference in density between the inner and outer layers, which will affect the stability of the material.

[0086] As can be seen from the comparison between Example 1 and Comparative Example 1, the present disclosure pre-freezes the solution containing ferrous salt and phosphorus source, controls the flow of hydrogen peroxide through the frozen mixed salt solution, and by controlling the concentration and flow rate of hydrogen peroxide, the reaction can proceed slowly, so that the iron and phosphorus elements in the prepared iron phosphate precursor are evenly distributed and have few surface defects.

Claims

1. A method for preparing ferric phosphate, the method comprising the following steps: (1) A mixed salt solution is obtained by mixing ferrous salt, phosphorus source and solvent, and the mixed salt solution is subjected to freezing treatment to obtain a frozen mixed salt solution; (2) Place the frozen mixed salt solution in a reflux device, control the hydrogen peroxide solution to reflux between the reflux device and the reaction device, control the concentration of hydrogen peroxide in the system to carry out the reaction, until the frozen mixed salt solution is completely dissolved; (3) Adjust the pH of the system to carry out the aging reaction, and separate the solid and liquid to obtain the iron phosphate; The reflux rate of the hydrogen peroxide solution in step (2) is 50~100 mL / min; The mass concentration of hydrogen peroxide in the reaction of step (2) is 5~10%.

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

3. The preparation method according to claim 1, wherein, The phosphorus source in step (1) includes any one or a combination of at least two of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or ammonium phosphate.

4. The preparation method according to claim 1, wherein, The concentration of iron in the mixed salt solution in step (1) is 0.05~1 mol / L.

5. The preparation method according to claim 1, wherein, The concentration of phosphorus in the mixed salt solution in step (1) is 0.5~1 mol / L.

6. The preparation method according to claim 1, wherein, The pH of the mixed salt solution in step (1) is 1.8 to 2.

5.

7. The preparation method according to claim 1, wherein, The freezing temperature in step (1) is -30~-40℃.

8. The preparation method according to claim 1, wherein, The freezing process in step (1) takes 6 to 12 hours.

9. The preparation method according to claim 1, wherein, The mass concentration of the hydrogen peroxide solution in step (2) is 5-10%.

10. The preparation method according to claim 1, wherein, In step (2), the total molar amount of hydrogen peroxide in the hydrogen peroxide solution and the molar ratio of iron in the iron source are (1~2):

1.

11. The preparation method according to claim 1, wherein, The reaction temperature in step (2) is 40~60℃.

12. The preparation method according to claim 1, wherein, Phosphoric acid is added during the reaction in step (2) to control the pH.

13. The preparation method according to claim 1, wherein, Step (2) involves controlling the pH to be 1.8 to 2.

5.

14. The preparation method according to claim 1, wherein, The pH adjuster in step (3) includes ammonia.

15. The preparation method according to claim 1, wherein, The pH value in step (3) is 2.5 to 3.

5.

16. The preparation method according to claim 1, wherein, The aging reaction in step (3) is carried out at a temperature of 40~80℃.

17. The preparation method according to claim 1, wherein, The stirring speed for the aging reaction in step (3) is 100~200 rpm.

18. The preparation method according to claim 1, wherein, The aging reaction in step (3) takes 10 to 60 hours.

19. The preparation method according to claim 1, wherein, After solid-liquid separation in step (3), the mixture is washed and vacuum dried.

20. The preparation method according to claim 19, wherein, The detergent used for washing includes deionized water.

21. The preparation method according to claim 19, wherein, The temperature for the vacuum drying process is 60~100℃.

22. An iron phosphate, said iron phosphate being prepared by the method according to any one of claims 1-21.

23. The iron phosphate as described in claim 22, wherein, The particle size of the iron phosphate is 100~300nm.

24. A lithium iron phosphate, wherein the lithium iron phosphate is prepared by sintering a mixture of iron phosphate and a lithium source as described in claim 22 or 23.

Images