Process for the preparation of anhydrous iron phosphate

Abstract

The application discloses a preparation method of anhydrous iron phosphate, and comprises the following steps: S1, mixing iron powder and a phosphoric acid solution to react at 60-65 DEG C; solid-liquid separation is performed to obtain a filtrate; S2, mixing the filtrate and an oxidant I to react, then the temperature is increased to an aging temperature for I, and an oxidant II is added for II; then solid-liquid separation and calcination are performed to obtain the anhydrous iron phosphate; the molar amount of the oxidant I is 0.5-0.6 times the molar amount of the iron powder; and the molar amount of the oxidant II is 0.1-0.28 times the molar amount of the iron powder. The anhydrous iron phosphate prepared by the method is used for preparing lithium iron phosphate, and a positive electrode material with high compaction density and good rate performance can be obtained.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate technology, and in particular to a method for preparing anhydrous ferric phosphate. Background Technology

[0002] Iron phosphate is an important basic chemical raw material, widely used in photocatalysis, wastewater treatment, and the synthesis of lithium-ion battery cathode materials. The purity, structure, morphology, and particle size of iron phosphate significantly affect its catalytic and electrochemical performance. In actual production, the Fe / P ratio of iron phosphate has a significant impact on the key performance indicators (compacted density, electrical properties, product consistency, etc.) of the final lithium iron phosphate product.

[0003] Therefore, in order to obtain lithium iron phosphate cathode materials with high compaction density and good rate performance, it is necessary to develop a method for preparing anhydrous iron phosphate. Summary of the Invention

[0004] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the first aspect of the present invention proposes a method for preparing anhydrous iron phosphate, wherein lithium iron phosphate made from the anhydrous iron phosphate obtained by the method has the characteristics of high compaction density and good rate performance.

[0005] According to a first aspect of the present invention, a method for preparing anhydrous ferric phosphate is provided, comprising the following steps: S1. At 60~65℃, iron powder and phosphoric acid solution are mixed and reacted; solid-liquid separation is performed to obtain filtrate; S2. The filtrate and oxidant I are mixed and reacted, then heated to the aging temperature and kept at temperature I, and oxidant II is added and kept at temperature II; then solid-liquid separation and calcination are performed to obtain anhydrous ferric phosphate; Based on the molar amount of the iron powder, the molar amount of oxidant I is 0.5 to 0.6 times the molar amount of iron powder; the molar amount of oxidant II is 0.1 to 0.28 times the molar amount of iron powder.

[0006] According to a preferred embodiment of the present invention, the aging temperature is 90~110℃.

[0007] According to a preferred embodiment of the present invention, the total time for heat preservation I and heat preservation II is 1 to 6 hours.

[0008] According to a preferred embodiment of the present invention, the molar ratio of phosphoric acid and iron powder in the phosphoric acid solution is (1~3):1.

[0009] According to a preferred embodiment of the present invention, the concentration of the phosphoric acid solution is 1~4 mol / L.

[0010] According to a preferred embodiment of the present invention, in step S2, before mixing the filtrate and oxidant I, the filtrate is further diluted to an iron ion concentration of 0.1~0.7 mol / L.

[0011] According to a preferred embodiment of the present invention, in step S2, before the filtrate and oxidant I are mixed, the pH of the filtrate is adjusted to 0.8-3.

[0012] According to a preferred embodiment of the present invention, the oxidant I and oxidant II are independently selected from hydrogen peroxide and / or oxygen.

[0013] According to a preferred embodiment of the present invention, in step S1, the reaction time is 1 h to 3 h.

[0014] According to a preferred embodiment of the present invention, in step S2, the calcination temperature is 500~700℃. According to a preferred embodiment of the present invention, in step S2, the calcination time is 1h to 6h.

[0015] According to a preferred embodiment of the present invention, in step S1 or S2, the solid-liquid separation step includes filtration and / or drying.

[0016] According to a preferred embodiment of the present invention, the drying temperature is 80°C to 100°C.

[0017] The preparation method according to embodiments of the present invention has at least the following beneficial effects: This invention first involves leaching iron powder in a phosphoric acid solution at 60-65°C, which maximizes the iron powder leaching rate and minimizes byproduct formation. The filtrate is then mixed with oxidant I for reaction, followed by heating to the aging temperature. Oxidant II is added while maintaining this temperature, which promotes primary particle growth and further improves the iron-to-phosphorus ratio. The anhydrous iron phosphate obtained through this method is used to prepare lithium iron phosphate, resulting in a cathode material with high compaction density and good rate performance.

[0018] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a SEM image of ferric phosphate dihydrate prepared in Example 5 of this invention; Figure 2This is a SEM image of the anhydrous ferric phosphate prepared in Example 5 of this invention. Detailed Implementation

[0020] The following are specific embodiments of the present invention, and the technical solutions of the present invention will be further described in conjunction with the embodiments, but the present invention is not limited to these embodiments.

[0021] Unless otherwise specified, the reagents, methods and equipment used in this invention are all conventional reagents, methods and equipment in this technical field.

[0022] In some embodiments of the present invention, a method for preparing ferric phosphate is provided, comprising the following steps: S1. At 60~65℃, iron powder and phosphoric acid solution are mixed and reacted; solid-liquid separation is performed to obtain filtrate; S2. The filtrate and oxidant I are mixed and reacted, then heated to the aging temperature and kept at temperature I, and oxidant II is added and kept at temperature II; then solid-liquid separation and calcination are performed to obtain ferric phosphate; Based on the molar amount of the iron powder, the molar amount of oxidant I is 0.5 to 0.6 times the molar amount of iron powder; the molar amount of oxidant II is 0.1 to 0.28 times the molar amount of iron powder.

[0023] Understandably, this invention first leaches iron powder in a phosphoric acid solution at 60-65°C, achieving the highest iron powder leaching rate and minimizing byproduct formation. The filtrate is then mixed with oxidant I for reaction, followed by heating to the aging temperature. Oxidant II is added while maintaining this temperature, which promotes primary particle growth and further improves the iron-to-phosphorus ratio. The iron phosphate obtained through this preparation method is used to prepare lithium iron phosphate, resulting in a cathode material with high compaction density and good rate performance.

[0024] Furthermore, based on the molar amount of the iron powder, the present invention specifies that the molar amount of oxidant I is 0.5 to 0.6 times the molar amount of iron powder, and the molar amount of oxidant II is 0.1 to 0.28 times the molar amount of iron powder. This allows for a further increase in the iron-to-phosphorus ratio, resulting in higher quality iron phosphate.

[0025] In some embodiments of the present invention, the aging temperature is 90~110℃. For example, it includes 90℃, 91℃, 92℃, 93℃, 94℃, 95℃, 96℃, 97℃, 98℃, 99℃, 100℃, 101℃, 103℃, 105℃, 106℃, 107℃, 108℃, 109℃, 110℃, or any sub-range of two of the above values. The aging temperature can adjust the morphology, size, and thickness of the primary particles; when within the above temperature range, it exhibits better grain growth and agglomeration effects.

[0026] In some embodiments of the present invention, the total time for heat preservation I and heat preservation II is 1 to 6 hours. For example, it includes 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, or any sub-range consisting of two of the above values. Therefore, adding oxidant II during the heat preservation time can further increase the iron-to-phosphorus ratio, resulting in higher quality iron phosphate.

[0027] In some embodiments of the present invention, the molar ratio of phosphoric acid to iron powder in the phosphoric acid solution is (1~3):1. For example, it includes 1:1, 1.2:1, 1.5:1, 1.6:1, 1.8:1, 2:1, 2.2:1, 2.4:1, 2.5:1, 2.6:1, 2.8:1, 3:1, or any sub-range consisting of any two of the above ratios.

[0028] In some embodiments of the present invention, the concentration of the phosphoric acid solution is 1-4 mol / L. For example, it includes sub-ranges such as 1 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L, 2.6 mol / L, 2.8 mol / L, 3 mol / L, 3.2 mol / L, 3.4 mol / L, 3.5 mol / L, 3.8 mol / L, 4 mol / L, or any two of the above values. Therefore, a suitable phosphoric acid solution concentration can improve the iron dissolution efficiency.

[0029] In some embodiments of the present invention, in step S2, before mixing the filtrate and oxidant I, the filtrate is further diluted to an iron ion concentration of 0.1~0.7 mol / L. For example, this includes sub-ranges such as 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, or any two of the above values. This allows for adjustment of the particle stacking density and secondary particle size of anhydrous iron phosphate, resulting in lithium iron phosphate with better compaction density and rate performance.

[0030] In some embodiments of the present invention, in step S2, before mixing the filtrate and oxidant I, the pH of the filtrate is adjusted to 0.8-3. For example, this includes sub-ranges such as 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, or any two of the above values. This allows for the adjustment of the particle size of the finished iron phosphate, resulting in lithium iron phosphate with better compaction density and rate performance.

[0031] In some embodiments of the present invention, the oxidant I and oxidant II are independently selected from hydrogen peroxide and / or oxygen.

[0032] In some embodiments of the present invention, the reaction time in step S1 is 1 hour to 3 hours. For example, it may include a sub-range of 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or any two of the above values. This ensures that the iron dissolution process is complete.

[0033] In some embodiments of the present invention, the calcination temperature in step S2 is 500~700℃. For example, it includes 500℃, 550℃, 580℃, 600℃, 620℃, 650℃, 680℃, 700℃, or any sub-range composed of any two of the above values.

[0034] In some embodiments of the present invention, the calcination temperature in step S2 is 550~680℃. For example, it includes 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 680℃, or any sub-range composed of any two of the above values.

[0035] In some embodiments of the present invention, the calcination time in step S2 is 1h to 6h. For example, it includes 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, or any sub-range composed of any two of the above values.

[0036] In some embodiments of the present invention, the calcination time in step S2 is 2h to 4h. For example, it includes 2h, 2.5h, 3h, 3.5h, 4h, or any sub-range consisting of two of the above values.

[0037] In some embodiments of the present invention, in step S1 or S2, the solid-liquid separation step includes filtration and / or drying.

[0038] In some embodiments of the present invention, the drying temperature is 80°C to 100°C. For example, it includes 80°C, 85°C, 90°C, 95°C, 100°C, or any sub-range consisting of two of the above values.

[0039] Example 1 This example provides a method for preparing ferric phosphate, including the following steps: S1. First, dilute phosphoric acid to 1 mol / L and place it in an oil bath reactor and heat it to the reaction temperature of 60°C. Then, add iron powder, with a molar ratio of iron powder to phosphoric acid of 1:2.5. Next, keep it at 60°C for 3 h, filter the suspension, and wash it to obtain the filtrate. S2. The iron ion concentration of the filtrate was 0.9 mol / L, diluted to 0.6 mol / L, and the pH was adjusted to 1.8. The filtrate was poured into an oil bath reactor. Based on the molar amount of iron powder, 0.6 times the amount of hydrogen peroxide was added for oxidation, then the temperature was raised to 95 ℃ and aged for 1.5 h. Then, 0.2 times the amount of hydrogen peroxide was added for complete oxidation and aged for another 1.5 h. After cooling to room temperature, the mixture was filtered, washed, and the filter cake was dried overnight at 100 ℃ to obtain ferric phosphate dihydrate. This ferric phosphate dihydrate was calcined (calcined at 680 ℃ for 4 h) to obtain anhydrous ferric phosphate with an iron-to-phosphorus ratio (Fe / P) of 0.974.

[0040] Example 2 This example provides a method for preparing anhydrous ferric phosphate, which is the same as in Example 1, except that in step S2, based on the molar amount of iron powder, 0.55 times the amount of hydrogen peroxide is added first, and after aging for 1.5 hours, 0.25 times the amount of hydrogen peroxide is added. After calcination at 680 °C for 4 hours, anhydrous ferric phosphate is obtained with an iron-to-phosphorus ratio of 0.978.

[0041] Example 3 This example provides a method for preparing anhydrous ferric phosphate, which is the same as in Example 1, except that in step S2, based on the molar amount of iron powder, the amount of hydrogen peroxide added first is 0.6 times, and after aging for 1.5 hours, 0.1 times the amount of hydrogen peroxide is added.

[0042] Example 4 This example provides a method for preparing anhydrous ferric phosphate, which is the same as in Example 1, except that in step S2, based on the molar amount of iron powder, the amount of hydrogen peroxide added first is 0.5 times, and after aging for 1.5 hours, 0.28 times the amount of hydrogen peroxide is added.

[0043] Example 5 This example provides a method for preparing anhydrous ferric phosphate, which is the same as in Example 1, except that in step S2, based on the molar amount of iron powder, the amount of hydrogen peroxide added first is 0.55 times, and after aging for 1.5 hours, 0.15 times the amount of hydrogen peroxide is added.

[0044] Comparative Example 1 This example provides a method for preparing anhydrous ferric phosphate, which is the same as in Example 1, except that in step S2, based on the molar amount of iron powder, the amount of hydrogen peroxide added first is 0.45 times, and after aging for 1.5 hours, 0.3 times the amount of hydrogen peroxide is added.

[0045] Comparative Example 2 This example provides a method for preparing anhydrous ferric phosphate, which is the same as in Example 1, except that: based on the molar amount of iron powder, 0.7 times the amount of hydrogen peroxide is directly added for oxidation, then the temperature is raised to 95 ℃ for aging for 3 hours, cooled to room temperature, filtered and washed to obtain the filter cake, dried overnight at 100 ℃ to obtain ferric phosphate dihydrate, and calcined at 680 ℃ for 4 hours to obtain anhydrous ferric phosphate.

[0046] Performance testing The iron phosphate dihydrate prepared in Example 5 of this invention was subjected to SEM analysis. The morphology and size of the primary particles were observed at 200 nm, and the size of the secondary aggregates was observed at 20 μm. The results are as follows: Figure 1 As shown, the iron phosphate dihydrate generated under these conditions is in the form of thin flakes with tight interparticle bonds. The secondary aggregates at a size of 20 μm are well-matched in size and have a loose and porous interior.

[0047] Furthermore, the anhydrous ferric phosphate prepared in Example 5 of this invention was subjected to SEM analysis. The primary particle morphology and size were observed at 200 nm, and the secondary aggregate size was observed at 20 μm. The results are as follows: Figure 2 As shown, the primary particles of anhydrous iron phosphate are spherical with a loose and porous interior, and the secondary aggregates are a combination of particles of different sizes.

[0048] Anhydrous lithium iron phosphate prepared in Examples 1-5 and Comparative Examples 1-2 of this invention were used as lithium carbonate as the lithium source, polyethylene glycol and glucose as the carbon source and reducing agent, and titanium dioxide as an additive to synthesize carbon-coated lithium iron phosphate cathode materials using a high-temperature solid-state method. The compaction density of the prepared lithium iron phosphate powder at 3T pressure was measured using a compaction density meter, and the results are shown in Table 1.

[0049] Furthermore, the lithium iron phosphate cathode material prepared above is then used to fabricate a button cell, as follows: Using the lithium iron phosphate cathode material prepared in the above examples and comparative examples as the active material, it was uniformly dispersed in NMP with carbon nanotube conductive liquid, SP carbon black, and PVDF binder at a mass ratio of 90:5:5. The mixture was then coated onto aluminum foil, which was dried in a vacuum drying oven at 100°C for at least 2 hours. The resulting slices were used as the cathode. A coin cell was assembled using lithium metal sheet as the anode and LiPF6 solution as the electrolyte, and constant current charge-discharge was performed with a cutoff voltage of 2.0-3.75V. The charge-discharge specific capacity at 0.1C, 0.5C, and 1C rates was recorded, and the results are shown in Table 1.

[0050] Table 1. Comparison of iron-phosphorus ratio and lithium iron phosphate performance prepared in different embodiments.

[0051] As shown in Table 1, Comparative Example 2, which directly added 0.7 eq of hydrogen peroxide, produced the lowest iron-phosphorus ratio (FPP) of lithium iron phosphate (LiFePO4), resulting in high compaction but poor rate performance. Stepwise addition of hydrogen peroxide improved the PFPP ratio, with higher PFPP ratios achieved through two-step addition of hydrogen peroxide. The lower the amount of oxidant 1 added, the higher the PFPP ratio. Examples 2 (0.978), 4 (0.978), and Comparative Example 1 (0.984) showed low compaction and excellent rate performance. Examples 3 and 5, with a total hydrogen peroxide addition of 0.7 eq, produced moderate PFPP ratios of 0.959 and 0.965, respectively, resulting in high compaction and electrical properties of the LiFePO4. Example 5, with even less oxidant 1 added, exhibited the best performance.

[0052] The present invention has been described in detail above with reference to the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing anhydrous ferric phosphate, characterized in that, Includes the following steps: S1. At 60~65℃, iron powder and phosphoric acid solution are mixed and reacted; solid-liquid separation is performed to obtain filtrate; S2. Mix the filtrate and oxidant I to react, then heat to the aging temperature and keep warm for I, and add oxidant II to continue keeping warm for II; Further solid-liquid separation and calcination yielded anhydrous ferric phosphate; Based on the molar amount of the iron powder, the molar amount of oxidant I is 0.5 to 0.6 times the molar amount of iron powder; the molar amount of oxidant II is 0.1 to 0.28 times the molar amount of iron powder.

2. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, The aging temperature is 90~110℃.

3. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, The total time for heat preservation I and heat preservation II is 1 to 6 hours.

4. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, The molar ratio of phosphoric acid to iron powder in the phosphoric acid solution is (1~3):

1.

5. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, The concentration of the phosphoric acid solution is 1~4 mol / L.

6. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, In step S2, before mixing the filtrate and oxidant I, the filtrate is diluted to an iron ion concentration of 0.1~0.7 mol / L.

7. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, In step S2, before mixing the filtrate and oxidant I, the pH of the filtrate is adjusted to 0.8-3.

8. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, The oxidant I and oxidant II are independently selected from hydrogen peroxide and / or oxygen.

9. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, In step S2, the calcination temperature is 500~700℃.

10. The method for preparing anhydrous ferric phosphate according to claim 1, characterized in that, In step S2, the calcination time is 1 to 6 hours.

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