High-compaction iron phosphate, preparation method thereof and lithium iron phosphate

Abstract

The invention relates to the field of lithium battery processing, and discloses high-compaction iron phosphate, a preparation method thereof and lithium iron phosphate. The preparation method comprises the following steps: 1) mixing and reacting a phosphoric acid-pyrophosphoric acid mixed solution with iron powder, and then mixing with a ferric iron source to obtain a solution containing ferric ions; wherein in the phosphoric acid-pyrophosphoric acid mixed solution, the concentration of phosphate radical is 7 to 12 mol / L, and the concentration of pyrophosphate radical is 0.01 to 0.1 mol / L; the molar ratio of the iron powder to the pyrophosphate radical is (2-8): 1; and 2) carrying out first filter pressing on the solution containing the ferric ions, reacting the obtained filtrate at 80-120 DEG C for 0.5-6 hours, then reacting at 30-60 DEG C for 0.5-2 hours, and sequentially carrying out second filter pressing, water washing, flash drying and sintering. The preparation method adopts an iron phosphate one-step synthesis route, and is short in flow and simple in process.

Description

Technical Field

[0001] This invention relates to the field of lithium battery processing, specifically to a high-pressure iron phosphate, its preparation method, and lithium iron phosphate. Background Technology

[0002] Iron phosphate is a crucial raw material in the preparation of lithium iron phosphate (LFP) materials. The particle size and morphology of iron phosphate directly determine the compaction density, ion transport efficiency, and processing performance of LFP, ultimately affecting the battery's energy density, rate performance, and cycle life. Currently, commercially available battery-grade iron phosphate often exhibits an irregular particle structure with very small primary particles and large interparticle gaps. Larger particles require special sintering conditions, resulting in low tap density and compaction density in LFP materials prepared from single iron phosphate particles.

[0003] Patent application CN120208175A discloses a method for preparing a dense, high-compact iron phosphate material. This method involves dissolving iron phosphate slag and waste from lithium extraction in a sulfuric acid solution to obtain ferric phosphate liquid. The ferric phosphate liquid is then mixed with a phosphoric acid solution, followed by heating, drying, and calcination to obtain anhydrous iron phosphate. In this method, sulfate ions cannot be effectively removed, directly affecting the crystal structure and electrochemical performance of iron phosphate. Patent application CN121134712A discloses a method for synthesizing iron phosphate from iron oxide red. This method first mixes and dissolves iron oxide red with phosphoric acid, then heats the filtrate obtained after solid-liquid separation and performs further solid-liquid separation. The resulting filter residue is washed with water and dried to obtain iron phosphate. These methods do not address the optimization of iron phosphate particle morphology, internal compactness, and tap density. Summary of the Invention

[0004] To overcome the shortcomings of existing iron phosphates, such as their morphology and performance failing to meet the requirements of high-compact lithium iron phosphate, and the long preparation process of existing iron phosphates being prone to introducing impurities and having low raw material utilization, this invention provides a high-compact iron phosphate, its preparation method, and lithium iron phosphate. This preparation method adopts a one-step synthesis route of iron phosphate, with a short process and simple steps. The prepared iron phosphate has a high tap density and uniform particle size and morphology, laying a high-quality raw material foundation for the preparation of high-compact lithium iron phosphate.

[0005] To achieve the above objectives, the first aspect of the present invention provides a method for preparing high-pressure iron phosphate, the method comprising:

[0006] 1) A phosphoric acid-pyrophosphate mixed solution is mixed with iron powder and reacted, and then mixed with a ferric iron source to obtain a solution containing ferric ions; wherein, in the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate is 7~12 mol / L and the concentration of pyrophosphate is 0.01~0.1 mol / L; the molar ratio of iron powder to pyrophosphate is 2~8:1; 2) The solution containing ferric ions is subjected to a first pressure filtration. The filtrate is reacted at 80~120℃ for 0.5~6h, and then reacted at 30~60℃ for 0.5~2h. Then, a second pressure filtration, water washing, flash drying and sintering are carried out in sequence.

[0007] Preferably, in step 1), the method for preparing the phosphoric acid-pyrophosphate mixed solution includes: reacting a phosphoric acid solution with a concentration of 80~85wt% at a temperature of 100~300℃ for 0.5~6h, and then mixing it with water.

[0008] Preferably, in step 1), the trivalent iron source is at least one of ferric oxide, ferric hydroxide, ferric oxalate, and ferric citrate.

[0009] Preferably, in step 1), the molar ratio of iron to phosphorus in the solution containing ferric ions is 0.8 to 1:1.

[0010] Preferably, step 1) is performed at 100~150°C.

[0011] Preferably, in step 2), the concentration of iron in the filtrate is 2-4 mol / L and the concentration of phosphorus is 2-5 mol / L.

[0012] Preferably, in step 2), the flash drying is carried out in a rotary flash dryer, and the temperature of the rotary flash dryer is 100~200℃.

[0013] Preferably, in step 2), the sintering is carried out in a rotary kiln, and the operating conditions of the rotary kiln include: a temperature of 500~700℃ and a time of 1~8h.

[0014] Preferably, in step 2), the pressure of the first filter press is 0.4~0.8MPa.

[0015] Preferably, in step 2), the pressure of the second filter press is 0.5~1MPa.

[0016] A second aspect of the present invention provides a high-pressure iron phosphate prepared using the above-described preparation method.

[0017] Preferably, the tap density of the high-pressure compacted ferric phosphate is 1.35~1.55 g / cm³. 3 .

[0018] Preferably, the specific surface area of ​​the high-pressure compacted ferric phosphate is 5-9 m² / g. 2 / g.

[0019] Preferably, the particle size D50 of the high-pressure compacted ferric phosphate is 2.5~6.5μm.

[0020] A third aspect of the present invention provides a lithium iron phosphate, wherein the raw materials for preparing the lithium iron phosphate include the above-mentioned high-pressure iron phosphate.

[0021] Compared with the prior art, the present invention has the following advantages: 1) Precise morphology control enables the preparation of high-pressure lithium iron phosphate: This invention synthesizes large-sized ionic groups composed of ferrous ions and pyrophosphate ions in situ, providing stable sites for crystal nucleus growth. On this basis, large crystal nuclei are continuously grown and cultivated, eventually forming dense iron phosphate particles with compact structure and small pores. The product has high tap density and uniform particle size and morphology, laying a high-quality raw material foundation for the preparation of high-pressure lithium iron phosphate.

[0022] 2) Strong controllability of reaction conditions: By precisely controlling the process parameters, this invention can ensure that the large-sized ionic groups formed by the ferrous ions and pyrophosphate ions can exist stably in the solution for a long time, which can efficiently help the nucleation and growth crystallization of iron phosphate particles.

[0023] 3) The process is highly efficient and economical, with significant environmental advantages: This invention adopts a one-step synthesis route of iron phosphate, which has a short process and simple procedures; the raw materials are widely available and inexpensive, and no sulfur elements are introduced throughout the process. There is no need to add an additional water washing and desulfurization process, which greatly reduces wastewater discharge and treatment costs, alleviates environmental pressure, and further enhances the cost advantage.

[0024] 4) High raw material utilization rate and further reduction in cost: This invention constructs special groups by adding trace amounts of iron powder. The amount of iron powder used is small and completely dissolved. Undissolved trivalent iron source residue can enter the next round of dissolution cycle. The iron element utilization rate can reach 100%, which significantly reduces the raw material cost while ensuring the experimental effect. Attached Figure Description

[0025] Figure 1 The image shows the XRD pattern of the iron phosphate prepared in Example 2 of this invention. Figure 2 Here is an electron microscope image of the iron phosphate prepared in Example 2 of this invention; Figure 3 This is an electron microscope image of the iron phosphate prepared in Example 2 of this invention; Figure 4 This is an electron microscope image of the iron phosphate prepared in Example 2 of this invention; Figure 5 This is an electron microscope image of the iron phosphate prepared in Example 2 of this invention; Figure 6 This is an electron microscope image of the iron phosphate prepared in Comparative Example 2 of this invention; Figure 7 This is an electron microscope image of the iron phosphate prepared in Comparative Example 2 of this invention; Figure 8 This is a particle size distribution diagram of the iron phosphate prepared in Example 2 of the present invention; Figure 9 This is a particle size distribution diagram of lithium iron phosphate prepared using the iron phosphate method of Example 2 of the present invention. Detailed Implementation

[0026] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0027] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0028] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating relative importance or implying the number of technical features indicated. Therefore, unless otherwise stated, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature; "multiple" means two or more. The term "comprising" and any variations thereof mean non-exclusive inclusion, where one or more other features, integers, steps, operations, units, components, and / or combinations thereof may be present or added.

[0029] The method for preparing high-pressure iron phosphate according to the present invention includes: 1) A phosphoric acid-pyrophosphate mixed solution is mixed with iron powder and reacted, and then mixed with a ferric iron source to obtain a solution containing ferric ions; wherein, in the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate is 7~12 mol / L and the concentration of pyrophosphate is 0.01~0.1 mol / L; the molar ratio of iron powder to pyrophosphate is 2~8:1; 2) The solution containing ferric ions is subjected to a first pressure filtration. The filtrate is reacted at 80~120℃ for 0.5~6h, and then reacted at 30~60℃ for 0.5~2h. Then, a second pressure filtration, water washing, flash drying and sintering are carried out in sequence.

[0030] In step 1) above, the phosphoric acid-pyrophosphate mixed solution can be prepared by conventional methods in the art, specifically including: reacting an 80-85 wt% phosphoric acid solution at 100-300°C for 0.5-6 hours, followed by mixing with water. This preparation method involves heating the phosphoric acid solution to induce dehydration and condensation to generate pyrophosphate. After the reaction is complete, the phosphoric acid-pyrophosphate mixed solution is pale yellow. Pure water can then be added to adjust the concentration of phosphate to 7-12 mol / L, preferably 8-11 mol / L, and the concentration of pyrophosphate to 0.01-0.1 mol / L, preferably 0.02-0.08 mol / L.

[0031] In step 1) above, the phosphoric acid-pyrophosphate mixed solution is mixed with iron powder. The iron powder is added according to the molar amount of pyrophosphate in the phosphoric acid-pyrophosphate mixed solution; specifically, the molar ratio of iron powder to pyrophosphate is 2-8:1, preferably 3-7:1. After the iron powder dissolves in the phosphoric acid-pyrophosphate mixed solution, ferrous ions combine with pyrophosphate ions to form Fe. 2+ Combined with P2O7 4- The large-sized ionic groups, stable under acidic conditions, have an ionic radius of at least 6 Å. These ionic groups, combined with a high diffusion rate, facilitate mass transport, thereby promoting lattice growth. Furthermore, during crystallization, the high density of active growth sites and low interfacial energy of these ionic groups further enhance their ability to form stable and large-sized crystals.

[0032] In step 1) above, the ferric iron source can be at least one of iron oxide, iron hydroxide, iron oxalate, and iron citrate. The ferric iron source undergoes a conventional chemical dissolution reaction to obtain a solution containing ferric ions. In the solution containing ferric ions, the molar ratio of iron to phosphorus can be 0.8~1:1, preferably 0.9~1:1. In this invention, the amount of ferric iron source added is determined by the molar amounts of iron and phosphorus in the solution after mixing the phosphoric acid-pyrophosphate mixed solution with iron powder. Specifically, the operation can be as follows: the phosphoric acid-pyrophosphate mixed solution is mixed with iron powder and reacted. After the reaction is complete, the ferric iron source is added to the reaction solution. During the addition process, the molar ratio of iron to phosphorus in the solution is monitored in real time. When the molar ratio of iron to phosphorus in the solution is 0.8~1:1, the addition of the ferric iron source is stopped, resulting in a solution containing ferric ions. The method for monitoring the molar ratio of iron to phosphorus in the solution is a conventional method in the art. Specifically, the Fe in the reaction system can be monitored in real time using online ICP-OES / ICP-MS. 2+ / Fe 3+ With PO4 3- Concentration, calculate Fe / P molar ratio.

[0033] To maintain the stability of pyrophosphate, step 1) above is preferably carried out at 100~150°C.

[0034] In step 2) above, the solution containing ferric ions is subjected to a first pressure filtration. The resulting filtrate is a high-concentration iron-phosphorus solution. The concentration of iron in the filtrate can be controlled to be 2-4 mol / L and the concentration of phosphorus to be 2-5 mol / L by adding pure water. The filter residue obtained from the first pressure filtration can be recycled as a source of ferric iron.

[0035] In step 2) above, the pressure of the first filter press is 0.4~0.8MPa.

[0036] In this invention, the filtrate obtained from the first pressure filtration is further reacted at 80-120°C, preferably 90-120°C, for 0.5-6 hours, preferably 1-4 hours, to carry out crystal transformation, which can effectively suppress Fe. 2+ Combined with P2O7 4- The large-sized ionic groups are deactivated by hydrolysis, allowing them to remain stable in the solution for a longer period. During the precipitation process, high-pressure iron phosphate particles are continuously induced to form. As the reaction progresses, the iron phosphate particles grow larger, and their internal fusion becomes more compact and dense. After the crystallization is complete, the reaction continues at 30-60℃ for 0.5-2 hours to hydrolyze and consume the remaining pyrophosphate ions in the solution.

[0037] In step 2) above, the filter residue obtained by the second pressure filtration is a white precipitate. The white precipitate is washed with water until the conductivity of the washing liquid is lower than 600 μs / cm to obtain ferric phosphate dihydrate. The ferric phosphate dihydrate is further subjected to flash drying and sintering to obtain anhydrous ferric phosphate, which is the high-pressure compacted ferric phosphate.

[0038] In step 2) above, the flash drying is carried out in a rotary flash dryer, and the temperature of the rotary flash dryer is 100~200℃.

[0039] In step 2) above, the sintering is carried out in a rotary kiln, and the operating conditions of the rotary kiln include: a temperature of 500~700℃ and a time of 1~8h.

[0040] In step 2), the pressure of the second filter press is 0.5~1MPa.

[0041] In some specific implementations, the method for preparing the high-pressure iron phosphate can be as follows: 1) A phosphoric acid solution with a concentration of 80~85wt% is reacted at a temperature of 100~300℃ for 0.5~6h, and then mixed with pure water to obtain a phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate is 7~12mol / L and the concentration of pyrophosphate is 0.01~0.1mol / L. 2) The phosphoric acid-pyrophosphate mixed solution is mixed with iron powder and reacted, and then a ferric iron source is added to obtain a solution containing ferric ions, wherein the molar ratio of the iron powder to the pyrophosphate is 2~8:1; 3) The solution containing ferric ions is subjected to a first pressure filtration. The filtrate is reacted at 80~120℃ for 0.5~6h, then at 30~60℃ for 0.5~2h, followed by a second pressure filtration. The solid product is washed with water to obtain ferric phosphate dihydrate. 4) The ferric phosphate dihydrate is subjected to flash drying and sintering in sequence to obtain high-pressure compacted ferric phosphate.

[0042] The method of this invention can be used to prepare high-pressure compacted ferric phosphate, which is spherical and has a tap density of 1.35~1.55 g / cm³. 3 Specific surface area is 5~9m² 2 With a particle size D50 of 2.5~6.5μm, it has the advantages of high tap density, uniform particle size distribution, uniform morphology, and few and dense internal pores, which is beneficial for preparing high-compact lithium iron phosphate materials. Moreover, since the purity of the trivalent iron source used is above 96%, the purity of the prepared high-compact lithium iron phosphate can reach above 98%.

[0043] The iron phosphate prepared by this invention has particles of different sizes, including small, medium, and super-large. When the cumulative volume percentage reaches 10%, the corresponding particle size D10 is 0.1~2μm; when the cumulative volume percentage reaches 50%, the corresponding particle size D50 is 2.5~6.5μm; and when the cumulative volume percentage reaches 90%, the corresponding particle size D90 is 20~35μm. The particle size distribution span is 3~8, and the particle size distribution pattern has a trimodal distribution shape. Through the "gradation filling effect" of coarse particles forming a skeleton, medium particles filling secondary voids, and fine particles filling micro voids, the vibratory density and compacted density can be significantly improved.

[0044] In this invention, high-compact lithium iron phosphate is prepared using the above-mentioned high-compact lithium iron phosphate as raw material. The high-compact lithium iron phosphate has high compaction density and good electrochemical performance. The powder compaction density reaches more than 2.6 g / cc, the 0.1C first discharge capacity reaches more than 155 mAh / g, and the 1C discharge capacity reaches more than 140 mAh / g.

[0045] The following examples further illustrate the high-pressure iron phosphate, its preparation method, and lithium iron phosphate of the present invention. These examples are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following examples.

[0046] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following embodiments are commercially available.

[0047] Example 1 1) In a reactor equipped with three layers of stirring paddles, add 8L of 80wt% phosphoric acid solution, heat to 100℃ under normal pressure via an oil bath jacket, and react for 6 hours. Then add pure water to the reactor to obtain 13.5L of phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate is 8mol / L and the concentration of pyrophosphate is 0.02mol / L. 2) Keep the phosphoric acid-pyrophosphate mixed solution in the reactor at 100℃, and add 100g of iron powder. After the iron powder dissolves and reacts, add ferric oxide to the reaction solution until the molar ratio of iron to phosphorus in the reaction solution is 0.9:1, and obtain a solution containing ferric ions. 3) The solution containing ferric ions obtained in step 2) was subjected to a first pressure filtration at 0.6 MPa. Pure water was added to the filtrate to adjust the concentration of iron to 2.7 mol / L and the concentration of phosphorus to 3.0 mol / L. The filtrate was then reacted at 120℃ for 1 h, and then at 60℃ for 0.5 h. The filtrate was then subjected to a second pressure filtration at 0.8 MPa. The resulting white solid product was washed with water until the conductivity of the washing solution was less than 600 μs / cm, thus obtaining ferric phosphate dihydrate. 4) The ferric phosphate dihydrate obtained in step 3) is dried in a rotary flash dryer at 200°C for 4 hours, and then sintered in a rotary furnace at 600°C for 3 hours to obtain high-pressure compacted ferric phosphate.

[0048] Example 2 1) In a reactor equipped with three layers of stirring paddles, 7L of 85wt% phosphoric acid solution was added, heated to 200℃ under normal pressure via an oil bath jacket, and reacted for 2 hours. Then, pure water was added to the reactor to obtain 12.7L of phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate was 8mol / L and the concentration of pyrophosphate was 0.04mol / L. 2) Keep the phosphoric acid-pyrophosphate mixed solution in the reactor at 150℃, and add 145g of iron powder at the same time. After the iron powder dissolves and reacts, add ferric oxide to the reaction solution until the molar ratio of iron to phosphorus in the reaction solution is 1:1, and obtain a solution containing ferric ions. 3) The solution containing ferric ions obtained in step 2) was subjected to a first pressure filtration at 0.8 MPa. Pure water was added to the filtrate to adjust the concentration of iron to 4.0 mol / L and the concentration of phosphorus to 4.0 mol / L. The filtrate was then reacted at 90℃ for 4 h and then at 30℃ for 1 h. The filtrate was then subjected to a second pressure filtration at 1.0 MPa. The white solid product was washed with water until the conductivity of the washing solution was less than 600 μs / cm to obtain ferric phosphate dihydrate. 4) The ferric phosphate dihydrate obtained in step 3) is dried in a rotary flash dryer at 150°C for 2 hours, and then sintered in a rotary furnace at 700°C for 1 hour to obtain high-pressure compacted ferric phosphate.

[0049] Example 3 1) In a reactor equipped with three layers of stirring paddles, add 8L of 80wt% phosphoric acid solution, heat to 280℃ under normal pressure through an oil bath jacket, react for 1h, and then add pure water to the reactor to obtain 9.5L of phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate is 11mol / L and the concentration of pyrophosphate is 0.08mol / L. 2) Keep the phosphoric acid-pyrophosphate mixed solution in the reactor at 150℃, and add 137g of iron powder. After the iron powder dissolves and reacts, add ferric oxide to the reaction solution until the molar ratio of iron to phosphorus in the reaction solution is 0.95:1, and obtain a solution containing ferric ions. 3) The solution containing ferric ions obtained in step 2) was subjected to a first pressure filtration at 0.5 MPa. Pure water was added to the filtrate to adjust the concentration of iron to 4.0 mol / L and the concentration of phosphorus to 4.2 mol / L. The filtrate was then reacted at 100℃ for 1 h and then at 40℃ for 0.5 h. The solution was then subjected to a second pressure filtration at 0.5 MPa. The resulting white solid product was washed with water until the conductivity of the washing solution was less than 600 μs / cm, thus obtaining ferric phosphate dihydrate. 4) The ferric phosphate dihydrate obtained in step 3) is dried in a rotary flash dryer at 100°C for 2 hours, and then sintered in a rotary furnace at 500°C for 8 hours to obtain high-pressure compacted ferric phosphate.

[0050] Example 4 High-pressure iron phosphate was prepared according to the method of Example 3, except that in step 1), 8L of 80wt% phosphoric acid solution was added, heated to 250°C under normal pressure through an oil bath jacket, and reacted for 4 hours. Then, pure water was added to the reactor to obtain 8.7L of phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate was 12mol / L and the concentration of pyrophosphate was 0.1mol / L.

[0051] Example 5 High-pressure iron phosphate was prepared according to the method of Example 1, except that in step 2), the amount of iron powder used was 31g.

[0052] Example 6 High-pressure iron phosphate was prepared according to the method of Example 1, except that in step 2), ferric oxide was added to the reaction solution until the molar ratio of iron to phosphorus in the reaction solution was 0.8:1.

[0053] Example 7 High-pressure iron phosphate was prepared according to the method of Example 1, except that in step 2), the phosphoric acid-pyrophosphate mixed solution in the reactor was kept at 80°C.

[0054] Comparative Example 1 High-pressure ferric phosphate was prepared according to Example 1, except that in step 1), 8L of 80wt% phosphoric acid solution was added, heated to 310°C under normal pressure via an oil bath jacket, and reacted for 8 hours. Then, pure water was added to the reactor to obtain 9.7L of phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate was 10mol / L and the concentration of pyrophosphate was 0.5mol / L.

[0055] Comparative Example 2 High-pressure iron phosphate was prepared according to Example 2, except that in step 1), 8L of 85wt% phosphoric acid solution was added, heated to 80°C under normal pressure through an oil bath jacket, and reacted for 10h to obtain 8.34L of phosphoric acid-pyrophosphate mixed solution. In the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate was 14mol / L and the concentration of pyrophosphate was 0.01mol / L.

[0056] Comparative Example 3 The high-pressure ferric phosphate was prepared according to the method of Example 1, except that in step 3), the filtrate of the first pressure filter after dilution with pure water was reacted at 70°C for 6 hours.

[0057] Comparative Example 4 The high-pressure ferric phosphate was prepared according to the method of Example 1, except that in step 3), the filtrate of the first pressure filter after dilution with pure water was reacted at 130°C for 0.5 h.

[0058] Test Example 1 The morphology and other properties of the iron phosphate prepared in Example 2 and Comparative Example 2 were tested, and the results are as follows: Figure 1-7 As shown.

[0059] Depend on Figure 1 It can be seen that the ferric phosphate prepared in Example 2 has high purity, good crystallinity, and no other impurity peaks. Figures 2-5 It can be seen that the iron phosphate particles prepared in Example 2 are relatively large, with aggregate particle sizes ranging from 4 to 10 μm. The particles are dispersed and uniform in size, and the surface of the aggregate particles is covered with some tiny particles, which is conducive to the formation of graded iron phosphate, thus obtaining high-compact iron phosphate. High-compact lithium iron phosphate is then prepared using this as a raw material. The slice data shows that the iron phosphate aggregate particles are relatively compact internally with few pores, which is beneficial for the formation of high-compact lithium iron phosphate. Figures 6-7 It can be seen that the iron phosphate prepared in Comparative Example 2 has very small agglomerate particle size, with an average particle size of 1~3μm. The primary particles are small and the morphology is quite different from that of the examples.

[0060] The particle size of lithium iron phosphate prepared in Example 2 and using the iron phosphate from Example 2 was measured respectively, and the results are as follows: Figures 8-9 As shown. By Figure 8 It can be seen that the ferric phosphate particle size distribution obtained in Example 2 has a three-peak distribution pattern, with one main peak and two secondary peaks. The tiered distribution of small, medium, and large particles in this ferric phosphate is more significant, while commercially available ferric phosphate mainly exhibits a single-peak or double-peak particle size distribution pattern. Therefore, this ferric phosphate can significantly improve the vibratory density and compacted density through the "gradation filling effect" of large particles forming a skeleton, medium particles filling secondary voids, and small particles filling micro voids.

[0061] Depend on Figure 9 It can be seen that the particle size distribution of lithium iron phosphate prepared using iron phosphate in Example 2 has a certain inheritance from that of the raw material iron phosphate. The particle size distribution curve of this lithium iron phosphate material has an obvious three-peak distribution pattern. This lithium iron phosphate material also has a tiered particle distribution of small particles, medium particles and ultra-large particles. Therefore, this type of lithium iron phosphate material has the application potential of high compaction density.

[0062] Test Example 2 The ferric phosphate prepared in the examples and comparative examples was subjected to performance tests, and the test results are shown in Table 1.

[0063] Table 1

[0064] As shown in Table 1, if the pyrophosphate concentration is too high, iron pyrophosphate impurities are easily generated during the synthesis of iron phosphate, resulting in a lower iron-to-phosphorus ratio, reduced purity of the synthesized iron phosphate, and a smaller specific surface area and lower tap density, which in turn affects the electrochemical performance of lithium iron phosphate. On the other hand, if the phosphate concentration is too high, the solid content will be too high during the synthesis of iron phosphate, resulting in low crystallinity, larger specific surface area, lower tap density, and difficulty in controlling and stabilizing the synthesis process.

[0065] As shown in Table 1, the D50 of the ferric phosphate particles prepared by this invention is 2.5~6.5μm, indicating that the main particle size is between 2.5~6.5μm. This spherical particle size is suitable for controlling the milling parameters and will not lead to problems such as excessively long or short milling time, difficulty in stabilizing the slurry, and low milling efficiency. The D90 is 20~35μm, providing some ultra-large particles, stabilizing the slurry state, and providing a source for the sintering of large particles. The particle size distribution span is 3~8, indicating that the ferric phosphate in this embodiment has a narrower particle size distribution, which is beneficial to improving the compaction density and the consistency of electrochemical performance.

[0066] As shown in Table 1, in step 3), if the heating temperature of the filtrate is too low, the iron phosphate particles will be too small and will not agglomerate, resulting in a large specific surface area; if the temperature is too high, excessive crystallization will occur during the formation of iron phosphate solids, which will easily induce the formation of impurity phases and result in a low specific surface area of ​​iron phosphate.

[0067] Test Example 3 The iron phosphates prepared in the examples and comparative examples were then used to prepare lithium iron phosphate under the same conditions according to conventional preparation methods in the art. The specific preparation methods are as follows: 1) Mix 1 kg of ferric phosphate with 257.2 g of lithium carbonate, 100 g of glucose and 10 g of dopant titanium dioxide, add water and stir to obtain a slurry; 2) The slurry obtained in step 1) is subjected to wet grinding in sequence. The wet grinding controls the particle size D50 of the slurry to be 0.45μm. Then, it is spray dried, and the first sintering is carried out under nitrogen atmosphere at a sintering temperature of 650℃ for 8 hours. Finally, it is subjected to air jet milling to obtain the pulverized first sintered material. 3) Mix the pulverized primary sintering material obtained in step 2) with 200g of glucose, add water and stir to obtain a slurry; 4) The slurry obtained in step 3) is subjected to wet grinding. The wet grinding controls the particle size D50 of the slurry to be 0.15μm. Then, it is spray dried and sintered for the second time under a nitrogen atmosphere at a temperature of 700℃ for 6 hours. Then, it is sieved to obtain lithium iron phosphate material.

[0068] The prepared lithium iron phosphate was tested for its electrical performance under the same conditions using conventional methods in the art. The test results are shown in Table 2 below. Table 2

[0069] As shown in Table 2, the lithium iron phosphate prepared using the iron phosphate prepared in Examples 1-7 as raw materials can all achieve a compaction density of over 2.6 g / cc, a first discharge capacity of over 155 mAh / g at 0.1C, and a discharge capacity of over 140 mAh / g at 1C. It can be seen that the lithium iron phosphate prepared using the iron phosphate of the present invention has higher compaction density and better electrochemical performance.

[0070] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing high-pressure iron phosphate, characterized in that, The preparation method includes: 1) A phosphoric acid-pyrophosphate mixed solution is mixed with iron powder and reacted, and then mixed with a ferric iron source to obtain a solution containing ferric ions; wherein, in the phosphoric acid-pyrophosphate mixed solution, the concentration of phosphate is 7~12 mol / L and the concentration of pyrophosphate is 0.01~0.1 mol / L; the molar ratio of iron powder to pyrophosphate is 2~8:1; 2) The solution containing ferric ions is subjected to a first pressure filtration. The filtrate is reacted at 80~120℃ for 0.5~6h, and then reacted at 30~60℃ for 0.5~2h. Then, a second pressure filtration, water washing, flash drying and sintering are carried out in sequence.

2. The preparation method according to claim 1, characterized in that, In step 1), the preparation method of the phosphoric acid-pyrophosphate mixed solution includes: A phosphoric acid solution with a concentration of 80-85 wt% is reacted at a temperature of 100-300℃ for 0.5-6 hours, and then mixed with water.

3. The preparation method according to claim 1 or 2, characterized in that, In step 1), the ferric source is at least one of ferric oxide, ferric hydroxide, ferric oxalate, and ferric citrate; and / or In the solution containing ferric ions, the molar ratio of iron to phosphorus is 0.8 to 1:

1.

4. The preparation method according to any one of claims 1-3, characterized in that, Step 1) is carried out at 100~150℃.

5. The preparation method according to any one of claims 1-4, characterized in that, In step 2), the concentration of iron in the filtrate is 2-4 mol / L and the concentration of phosphorus is 2-5 mol / L.

6. The preparation method according to any one of claims 1-5, characterized in that, In step 2), the flash drying is carried out in a rotary flash dryer, and the temperature of the rotary flash dryer is 100~200℃.

7. The preparation method according to any one of claims 1-6, characterized in that, In step 2), the sintering is carried out in a rotary kiln, and the operating conditions of the rotary kiln include: a temperature of 500~700℃ and a time of 1~8h.

8. The preparation method according to any one of claims 1-7, characterized in that, In step 2), the pressure of the first filter press is 0.4~0.8 MPa; and / or The pressure of the second filter press is 0.5~1MPa.

9. High-pressure iron phosphate prepared by the preparation method according to any one of claims 1-8; Preferably, the tap density of the high-pressure compacted ferric phosphate is 1.35~1.55 g / cm³. 3 ; Preferably, the specific surface area of ​​the high-pressure compacted ferric phosphate is 5-9 m² / g. 2 / g; Preferably, the particle size D50 of the high-pressure compacted ferric phosphate is 2.5~6.5μm.

10. A lithium iron phosphate, characterized in that, The raw materials for preparing this lithium iron phosphate include the high-pressure iron phosphate as described in claim 9.

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