Ferric phosphate dihydrate, its preparation method and application

Abstract

This invention discloses a method for preparing ferric phosphate dihydrate and its applications. The preparation method includes the following steps: S1, adding iron powder to a phosphoric acid solution at 55℃~70℃ for reaction I, followed by solid-liquid separation to obtain a ferrous salt solution; S2, heating the ferrous salt solution to 60℃~69℃, then reacting it with an oxidant in a container at 80~100℃ for reaction II to obtain ferric phosphate yellow; S3, heating the ferric phosphate yellow to 85℃~98℃ for reaction, maintaining the temperature, washing, and separating the solid and liquid to obtain ferric phosphate dihydrate. The ferric phosphate dihydrate prepared by this invention can improve the compaction density, rate performance, and discharge specific capacity of lithium iron phosphate materials prepared from this ferric phosphate dihydrate when used as cathode materials. It exhibits no loss in discharge specific capacity when switching from high current to low current, and better reversibility. The battery performance is good and suitable for fast charging.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to an iron phosphate dihydrate, its preparation method, and its application. Background Technology

[0002] Iron phosphate, as a precursor for lithium iron phosphate cathode materials, has a significant impact on the performance of lithium iron phosphate batteries. In terms of manufacturing processes, adjusting the specific surface area, particle size, and iron-to-phosphorus ratio of iron phosphate is an effective way to prepare lithium iron phosphate with high compaction density and excellent electrical performance.

[0003] Currently, there are three main methods for preparing iron phosphate: the ammonium method, the sodium method (phosphate method), and the iron powder method. The iron powder method uses iron powder and phosphoric acid as raw materials. The iron powder reacts with phosphoric acid to produce ferrous hydrogen phosphate, which is then oxidized with hydrogen peroxide to produce iron phosphate. However, the product prepared by the iron powder method has poor quality, resulting in poor electrical performance in lithium iron phosphate batteries.

[0004] Therefore, it is necessary to develop a method for preparing lithium iron phosphate dihydrate that is beneficial for improving the high compaction and high-rate electrical performance of lithium iron phosphate materials. Summary of the Invention

[0005] 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 ferric phosphate dihydrate, which, when used to prepare lithium iron phosphate materials, can effectively improve compaction and rate performance.

[0006] A second aspect of the present invention provides ferric phosphate dihydrate.

[0007] A third aspect of the present invention provides a lithium iron phosphate material.

[0008] A fourth aspect of the present invention provides a secondary battery.

[0009] A method for preparing ferric phosphate dihydrate according to a first aspect of the present invention includes the following steps: S1. Add iron powder to a phosphoric acid solution at a temperature of 55℃~70℃ to carry out the reaction I. Solid-liquid separation to obtain a ferrous salt solution; S2. When the ferrous salt solution is heated to 60°C~69°C, it is then mixed with an oxidant and flowed into a container at 80~100°C to carry out reaction II to obtain ferric phosphate yellow. S3. The iron phosphate yellow material is heated to 85℃~98℃ for reaction, kept warm, washed and separated into solid and liquid to obtain iron phosphate dihydrate.

[0010] According to a preferred embodiment of the present invention, in step S1, the concentration of phosphoric acid in the phosphoric acid solution is 0.1 mol / L to 5 mol / L.

[0011] According to a preferred embodiment of the present invention, in step S1, the concentration of phosphoric acid in the phosphoric acid solution is 1 mol / L to 3 mol / L.

[0012] According to a preferred embodiment of the present invention, in step S2, the addition time of the oxidant is controlled to be 10 min to 20 min.

[0013] According to a preferred embodiment of the present invention, in step S1, the molar ratio of the iron powder to the phosphoric acid in the phosphoric acid solution is 1:(0.2~1).

[0014] According to a preferred embodiment of the present invention, in step S2, the concentration of ferrous ions is calculated to be 0.5 mol / L to 0.8 mol / L based on the molar amount of ferrous ions in the ferrous salt solution.

[0015] According to a preferred embodiment of the present invention, in step S2, the molar ratio of the oxidant to the ferrous ions is (0.55~0.80):1, calculated based on the molar amount of ferrous ions in the ferrous salt solution.

[0016] According to a preferred embodiment of the present invention, in steps S1 and S3, the solid-liquid separation is independently selected from filtration and drying.

[0017] According to a preferred embodiment of the present invention, in step S3, the conductivity of the washing water after washing is 200~400 uS / cm, and the sulfur content of the dried ferric phosphate dihydrate is between 600ppm and 2000ppm.

[0018] According to a preferred embodiment of the present invention, the oxidant comprises hydrogen peroxide.

[0019] According to a preferred embodiment of the present invention, in step S2, the ferrous salt solution is heated to 62°C to 65°C.

[0020] According to a preferred embodiment of the present invention, in step S2, the pH of the ferrous salt solution also needs to be adjusted to 0.55~0.8.

[0021] According to a preferred embodiment of the present invention, the heat preservation time is 30 min to 180 min.

[0022] The preparation method according to embodiments of the present invention has at least the following beneficial effects: This invention involves reacting iron powder with a phosphoric acid solution at 55°C to 70°C to obtain ferrous salt. The ferrous salt solution is then heated to 60°C to 69°C and reacted with an oxidant in a container at 80°C to 100°C. After further heating and holding, ferric phosphate dihydrate is obtained. This process improves the compaction density, rate performance, and discharge specific capacity of lithium iron phosphate material prepared from this ferric phosphate dihydrate when used as a cathode material. Discharge specific capacity remains unaffected from high to low current, exhibiting better reversibility. The battery performs well and is suitable for fast charging.

[0023] Furthermore, the iron powder is reacted in a phosphoric acid solution at 55℃~70℃, which can achieve an iron dissolution rate of 99%, low by-products, and the shortest reaction time.

[0024] According to a second aspect of the present invention, ferric phosphate dihydrate is provided, which is prepared by the method for preparing ferric phosphate dihydrate as described in the first aspect of the present invention.

[0025] A third aspect of the present invention provides a lithium iron phosphate material made from the iron phosphate dihydrate described in the second aspect of the present invention.

[0026] According to a preferred embodiment of the present invention, the lithium iron phosphate material is prepared by the following method: S1. Anhydrous ferric phosphate is obtained by sequentially crushing, grinding and sintering ferric phosphate dihydrate. S2. Mix the iron phosphate and lithium source to obtain the final product.

[0027] According to a preferred embodiment of the present invention, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium acetate.

[0028] According to a preferred embodiment of the present invention, the sintering temperature is 550°C to 650°C. For example, it includes 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, or any sub-range composed of any two of the above values.

[0029] 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

[0030] 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 schematic diagram of the process for preparing ferric phosphate dihydrate according to Example 1 of the present invention; Figure 2 These are SEM images of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Example 1 of this invention; Figure 3 These are SEM images of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Example 2 of this invention; Figure 4 These are SEM images of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Example 3 of this invention; Figure 5 These are the XRD patterns of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Example 1 of the invention; Figure 6 These are the XRD patterns of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Example 2 of the invention; Figure 7 These are the XRD patterns of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Example 3 of the invention. Detailed Implementation

[0031] 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.

[0032] Unless otherwise specified, the reagents, methods and equipment used in this invention are all conventional reagents, methods and equipment in this technical field.

[0033] In some embodiments of the present invention, a method for preparing ferric phosphate dihydrate is provided, comprising the following steps: S1. Add iron powder to a phosphoric acid solution at a temperature of 55℃~70℃ to carry out the reaction I. Solid-liquid separation to obtain a ferrous salt solution; S2. When the ferrous salt solution is heated to 60°C~69°C, it is then mixed with an oxidant and flowed into a container at 80~100°C to carry out reaction II to obtain ferric phosphate yellow. S3. The iron phosphate yellow material is heated to 85℃~98℃ for reaction, kept warm, washed and separated into solid and liquid to obtain iron phosphate dihydrate.

[0034] Understandably, this invention involves reacting iron powder with a phosphoric acid solution at 55°C~70°C to obtain ferrous salt. The ferrous salt solution is then heated to 60°C~69°C and reacted with an oxidant in a parallel flow. After a subsequent heating and holding process, ferric phosphate dihydrate is obtained. This process improves the compaction density, rate performance, and discharge specific capacity of lithium iron phosphate material prepared from this ferric phosphate dihydrate when used as a cathode material. Discharge specific capacity remains unaffected from high to low current, and reversibility is improved. The battery exhibits good performance and is suitable for fast charging.

[0035] In some embodiments of the present invention, in step S1, the concentration of phosphoric acid in the phosphoric acid solution is 0.1~5 mol / L. For example, it includes 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.25 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.25 mol / L, 2.5 mol / L, 2.75 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, or any sub-range consisting of two of the above values.

[0036] In some embodiments of the present invention, in step S1, the concentration of phosphoric acid in the phosphoric acid solution is 1 mol / L to 3 mol / L. For example, it includes sub-ranges such as 1 mol / L, 1.25 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.25 mol / L, 2.5 mol / L, 2.75 mol / L, 3 mol / L, or any two of the above values. Therefore, within the above concentration range, the leaching rate is high.

[0037] In some embodiments of the present invention, in step S2, the addition time of the oxidant is controlled to be 10 min to 20 min. For example, it includes 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, 16 min, 17 min, 18 min, 19 min, 20 min, or any sub-range composed of any two of the above values.

[0038] If the addition time is short: the oxidation reaction rate is fast, the crystal nucleus formation rate is greater than the crystal nucleus growth rate, resulting in smaller iron phosphate particles and a larger specific surface area. If the addition time is long: the crystal nuclei have more time to grow and aggregate, the particle size increases, and the specific surface area decreases. Therefore, the addition time within the scope of this application results in a better particle size range.

[0039] In addition, the appropriate addition time can improve the crystallinity and phase purity of the product. If the time is too long, impurities may be introduced, and if the time is too short, the reaction may be incomplete.

[0040] In some embodiments of the present invention, in step S1, the molar ratio of the iron powder to the phosphoric acid in the phosphoric acid solution is 1:(0.2~1). For example, it includes 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, or any sub-range consisting of any two of the above ratios.

[0041] In some embodiments of the present invention, in step S2, the concentration of ferrous ions is calculated based on the molar amount of ferrous ions in the ferrous salt solution, and is 0.5 mol / L to 0.8 mol / L. For example, it includes 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, or any sub-range consisting of two of the above values.

[0042] In some embodiments of the present invention, in step S2, the molar ratio of the oxidant to the ferrous ions is calculated based on the molar amount of ferrous ions in the ferrous salt solution as (0.55~0.80):1. For example, this includes sub-ranges such as 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, or any two of the above values.

[0043] In some embodiments of the present invention, in steps S1 and S3, the solid-liquid separation is independently selected from filtration and drying.

[0044] In some embodiments of the present invention, in step S3, the conductivity of the washing water after washing is 200~400 uS / cm, and the sulfur content of the dried ferric phosphate dihydrate is between 600~2000 ppm.

[0045] In some embodiments of the present invention, the oxidant includes hydrogen peroxide.

[0046] In some embodiments of the present invention, in step S2, the ferrous salt solution is heated to 62°C to 65°C. For example, this includes 62°C, 63°C, 64°C, 65°C, or any sub-range consisting of two of the above values.

[0047] In some embodiments of the present invention, step S2 further requires adjusting the pH of the ferrous salt solution to 0.55-0.8. For example, this includes sub-ranges such as 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, or any two of the above values.

[0048] In some embodiments of the present invention, the heat preservation time is 30 min to 180 min. For example, it includes any two values ​​of 30 min, 60 min, 90 min, 120 min, 150 min, 160 min, 170 min, and 180 min as sub-ranges.

[0049] In some embodiments of the present invention, ferric phosphate dihydrate is provided, which is prepared by the method for preparing ferric phosphate dihydrate as described in the first aspect of the present invention.

[0050] In some embodiments of the present invention, a lithium iron phosphate material is provided, which is made of iron phosphate dihydrate as described in the second aspect of the present invention.

[0051] In some embodiments of the present invention, the lithium iron phosphate material is prepared by the following method: S1. Anhydrous ferric phosphate is obtained by sequentially crushing, grinding and sintering ferric phosphate dihydrate. S2. Mix the iron phosphate and lithium source to obtain the final product.

[0052] In some embodiments of the present invention, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium acetate.

[0053] In some embodiments of the present invention, the sintering temperature is 550~650°C.

[0054] In some embodiments of the present invention, the anhydrous ferric phosphate has a water content of less than 7000 ppm, a sulfur content of less than 100 ppm, and a specific surface area of ​​5-10 g / m². 2 The iron-to-phosphorus ratio is between 0.965 and 0.975, and the particle size D50 is between 2 and 10 μm.

[0055] Example 1 This example provides a preparation process diagram of ferric phosphate dihydrate, as shown below. Figure 1 As shown, the preparation steps are as follows: S1. 864 g of phosphoric acid with a purity of 85% was diluted to a volume of 2.25 mol / L and heated to 55°C in an oil bath reactor. 171 g of iron powder with a purity of 98% was slowly added to the phosphoric acid solution and stirred for 30 min until the iron powder no longer dissolved. The mixture was then filtered to obtain a mixed solution of ferrous dihydrogen phosphate and ferrous monohydrogen phosphate with a ferrous ion concentration of 1 mol / L. S2. Heat the above mixed solution to 69°C, then add 245 g of hydrogen peroxide with a purity of 27% (the hydrogen peroxide is added over 15 min) and let it flow into an empty reactor at a temperature of 99°C for stirring and reaction to obtain iron phosphate yellow. S3. Heat the ferric phosphate yellow material to 93℃ and keep the temperature constant. After filtration, wash it. The conductivity of the washing liquid is less than 400 uS / cm. Dry it to obtain lumpy ferric phosphate dihydrate.

[0056] Further, the obtained ferric phosphate dihydrate is crushed and ground, and then sintered at 650°C for 240 min to obtain anhydrous ferric phosphate.

[0057] Furthermore, the dihydrate ferric phosphate and anhydrous ferric phosphate prepared in this invention were subjected to SEM analysis, and the results are as follows: Figure 2 As shown, where Figure 2 In this context, 'a' represents ferric phosphate dihydrate. Figure 2In this diagram, 'b' represents anhydrous ferric phosphate. Observation of the generated dihydrate ferric phosphate at 200 nm shows that the primary particles are relatively large, mainly in granular form, with overlapping particles of different sizes. After anhydrous sintering, the surface is smooth and rounded.

[0058] The ferric phosphate dihydrate prepared in Example 1 of this invention was subjected to XRD analysis, and the results are as follows: Figure 5 As shown, it was found that iron phosphate dihydrate has two crystal forms: orthorhombic and monoclinic.

[0059] Example 2 This example provides a method for preparing ferric phosphate dihydrate, and the preparation steps are as follows: S1. 864 g of phosphoric acid with a purity of 85% was diluted to a volume of 2.25 mol / L and heated to 65°C in an oil bath reactor. 171 g of iron powder with a purity of 98% was slowly added to the phosphoric acid solution and stirred for 30 min until the iron powder no longer dissolved. The mixture was then filtered to obtain a mixed solution of ferrous dihydrogen phosphate and ferrous monohydrogen phosphate with a ferrous ion concentration of 1 mol / L. S2. Heat the mixed solution to 69°C, then add 245 g of hydrogen peroxide with a purity of 27% for 15 minutes, and then let it flow into an empty reactor at a temperature of 89°C for stirring and reaction to obtain iron phosphate yellow. S3. Heat the ferric phosphate yellow material to 93℃ and keep the temperature constant. After filtration, wash it. The conductivity of the washing liquid is less than 400 uS / cm. Dry it to obtain ferric phosphate dihydrate.

[0060] Further, the obtained ferric phosphate dihydrate is crushed and ground, and then sintered at 600℃ for 240 min to obtain anhydrous ferric phosphate.

[0061] Furthermore, the dihydrate ferric phosphate and anhydrous ferric phosphate prepared in this invention were subjected to SEM analysis, and the results are as follows: Figure 3 As shown, where Figure 3 In this context, 'a' represents ferric phosphate dihydrate. Figure 3 In this diagram, 'b' represents anhydrous ferric phosphate. The morphology and size of the primary particles were observed at 200 nm. Under these conditions, the primary particles of the ferric phosphate dihydrate were mainly granular and flake-like, with the flakes and granules combined and the particles of different sizes overlapping. After the anhydrous sintering was completed, the particles of different sizes were observed to be distributed in an alternating pattern at 200 nm.

[0062] The ferric phosphate dihydrate prepared in Example 3 of this invention was subjected to XRD analysis, and the results are as follows: Figure 6 As shown, it was found that iron phosphate dihydrate has two crystal forms: orthorhombic and monoclinic.

[0063] Example 3 This example provides a method for preparing ferric phosphate dihydrate, and the preparation steps are as follows: S1. 864 g of 85% pure phosphoric acid was diluted to a volume of 2.25 mol / L phosphoric acid solution, and the phosphoric acid solution was heated to 65°C in a steam reactor. 171 g of 98% pure iron powder was slowly added to the phosphoric acid solution and stirred for 30 min until the iron powder no longer dissolved. After pressure filtration, a mixed solution of ferrous dihydrogen phosphate and ferrous monohydrogen phosphate was obtained with a ferrous ion concentration of 1 mol / L. S2. Heat the above mixed solution to 69°C, then add 245 g of hydrogen peroxide with a purity of 27% (the hydrogen peroxide is added over 15 min) and let it flow into an empty reactor at a temperature of 80°C for stirring and reaction to obtain iron phosphate yellow. S3. Heat the ferric phosphate yellow material to 93℃ and keep the temperature constant. After filtration, wash it. The conductivity of the washing liquid is less than 400 uS / cm. Dry it to obtain ferric phosphate dihydrate.

[0064] Further, the obtained ferric phosphate dihydrate is crushed and ground, and then sintered at 650°C for 240 min to obtain anhydrous ferric phosphate.

[0065] Furthermore, the dihydrate ferric phosphate and anhydrous ferric phosphate prepared in this invention were subjected to SEM analysis, and the results are as follows: Figure 4 As shown, where Figure 4 In this context, 'a' represents ferric phosphate dihydrate. Figure 4 In this context, 'b' represents anhydrous ferric phosphate. The primary particles are relatively large, mainly in granular form, with overlapping particles of different sizes. After anhydrous sintering, the surface is smooth and rounded.

[0066] The ferric phosphate dihydrate prepared in Example 3 of this invention was subjected to XRD analysis, and the results are as follows: Figure 7 As shown, it was found that iron phosphate dihydrate only has a monoclinic crystal system.

[0067] Example 4 This example provides a ferric phosphate dihydrate, whose preparation steps and dosage are the same as in Example 2. The difference is that the hydrogen peroxide is added at a time of 20 minutes.

[0068] Furthermore, the preparation steps for anhydrous ferric phosphate are the same as in Example 2.

[0069] Example 5 This example provides a ferric phosphate dihydrate, whose preparation steps and dosage are the same as in Example 2. The difference is that the hydrogen peroxide is added at a time of 10 minutes.

[0070] Furthermore, the preparation steps for anhydrous ferric phosphate are the same as in Example 2.

[0071] Example 6 This example provides a ferric phosphate dihydrate, whose preparation steps and dosage are the same as in Example 2. The difference is that the hydrogen peroxide is added at a time of 5 minutes.

[0072] Furthermore, the preparation steps for anhydrous ferric phosphate are the same as in Example 2.

[0073] Example 7 This example provides a ferric phosphate dihydrate, whose preparation steps and dosage are the same as in Example 2. The difference is that the hydrogen peroxide is added at a time of 25 min.

[0074] Furthermore, the preparation steps for anhydrous ferric phosphate are the same as in Example 2.

[0075] Comparative Example 1 This example provides a ferric phosphate dihydrate, whose preparation steps and dosage are the same as those in Example 2. The difference is that in step S2, the empty reactor temperature is 25°C.

[0076] Furthermore, the preparation steps for anhydrous ferric phosphate are the same as in Example 2.

[0077] Performance testing The anhydrous iron phosphate prepared in Examples 1-7 and Comparative Example 1 of this invention was further prepared into lithium iron phosphate material, and the steps are as follows: Anhydrous iron phosphate and lithium carbonate provided in this embodiment of the invention are mixed at a molar ratio of 1:1.02, and then ground, dried and sintered to obtain lithium iron phosphate material.

[0078] Further, lithium iron phosphate material was fabricated into a lithium-ion battery according to the following method. Rate performance, compaction density, and charge-discharge performance were tested. Specifically, anhydrous lithium iron phosphate prepared in the embodiments of this invention was used as the lithium source, with 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 material 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.

[0079] 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.

[0080] Table 1

[0081] As shown in Table 1, pure monoclinic phase (such as in Example 4, 2.555 g / cm³) typically exhibits a high compaction density. However, the compaction density can be further increased by introducing an appropriate amount of orthorhombic phase to form a mixed phase.

[0082] Example 2 (32.7:67.3) achieved the highest value of 2.563 g / cm³. This indicates that when the two phases coexist in a specific ratio, a better particle size distribution or morphology may be produced, resulting in more compact particle packing.

[0083] 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 ferric phosphate dihydrate, characterized in that, Includes the following steps: S1. Add iron powder to a phosphoric acid solution at a temperature of 55℃~70℃ to carry out the reaction I. Solid-liquid separation to obtain a ferrous salt solution; S2. When the ferrous salt solution is heated to 60°C~69°C, it is then mixed with an oxidant and flowed into a container at 80~100°C to carry out reaction II to obtain ferric phosphate yellow. S3. The iron phosphate yellow material is heated to 85℃~98℃ for reaction, kept warm, washed and separated into solid and liquid to obtain iron phosphate dihydrate.

2. The preparation method according to claim 1, characterized in that, In step S1, the concentration of phosphoric acid in the phosphoric acid solution is 0.1~5 mol / L.

3. The preparation method according to claim 1, characterized in that, In step S2, the addition time of the oxidant is controlled to be 10~20 min.

4. The preparation method according to claim 1, characterized in that, In step S1, the molar ratio of the iron powder to the phosphoric acid in the phosphoric acid solution is 1:(0.2~1).

5. The preparation method according to claim 1, characterized in that, In step S2, the concentration of ferrous ions is calculated to be 0.5~0.8 mol / L based on the molar amount of ferrous ions in the ferrous salt solution.

6. The preparation method according to claim 1, characterized in that, In step S2, the molar ratio of the oxidant to the ferrous ions is calculated based on the molar amount of ferrous ions in the ferrous salt solution as (0.55~0.80):

1.

7. A type of ferric phosphate dihydrate, characterized in that, It is prepared by the method for preparing ferric phosphate dihydrate according to any one of claims 1 to 6.

8. A lithium iron phosphate material, characterized in that, It is made from the ferric phosphate dihydrate as described in claim 7.

9. The lithium iron phosphate material according to claim 8, characterized in that, The lithium iron phosphate material is prepared by the following method: S1. Anhydrous ferric phosphate is obtained by sequentially crushing, grinding and sintering ferric phosphate dihydrate. S2. Mix the iron phosphate and lithium source to obtain the final product.

10. A secondary battery, comprising a positive electrode, characterized in that, The positive electrode comprises the lithium iron phosphate material as described in claim 8.

Images