A method for preparing iron phosphate dihydrate

Abstract

The application discloses a method for preparing iron phosphate dihydrate, and comprises the following steps: firstly, preparing a mixed solution of iron salt and phosphate according to a preset molar ratio of iron and phosphorus; adding an oxidizing agent into the mixed solution to completely oxidize ferrous ions into ferric ions; adding an alkali solution into the mixed solution for constant temperature reaction, and then obtaining a filter cake through pressure filtration and washing; forming a slurry from the filter cake, and then adding an acid solution, high-phosphorus ammonium sulfate and titanyl sulfate for heat preservation and aging, and finally obtaining the iron phosphate dihydrate through pressure filtration, washing and low-temperature drying. The method is suitable for large-scale production line application, and can control the mass fraction of the basic ammonium phosphate iron in different batches to be within the range of 0.3%-1.0%. Based on the generation of the small amount of basic ammonium phosphate iron, the comprehensive performance of the positive electrode material prepared from the iron phosphate dihydrate can be improved.

Description

Technical Field

[0001] This invention belongs to the field of iron phosphate dihydrate preparation technology. Background Technology

[0002] Iron phosphate, as one of the raw materials for preparing lithium iron phosphate (LiFePO4) cathode materials, plays a decisive role in improving the safety and cycle stability of LiFePO4 batteries. The performance of LiFePO4 batteries is mainly determined by iron phosphate; therefore, optimizing its preparation process and controlling costs are of great significance for improving battery performance and reducing production costs. Iron phosphate dihydrate (FePO4·2H2O), as the core precursor for preparing LiFePO4 cathode materials, directly determines the rate performance, cycle stability, and energy density of subsequent LiFePO4 materials due to its crystal integrity, morphological characteristics, impurity phase content, and doping uniformity, playing a crucial role in the overall performance of power batteries and energy storage batteries.

[0003] In the industrial-scale synthesis of ferric phosphate dihydrate, the presence of the basic ferric ammonium phosphate phase has a dual effect. On the one hand (negative effect), the formation of the basic ferric ammonium phosphate phase causes the iron-to-phosphorus ratio of the final precursor to deviate from the golden range of 0.97-0.99, resulting in impurity phases during the subsequent synthesis of lithium iron phosphate or lithium manganese iron phosphate. Furthermore, a large amount of basic ferric ammonium phosphate phase will generate ammonia and water vapor during the sintering process of lithium iron phosphate or lithium manganese iron phosphate cathode materials, leading to structural collapse and reduced compaction density of the cathode material. On the other hand (positive effect), the moderate and controlled formation of the basic ferric ammonium phosphate phase can induce sphericity during the cathode material preparation process, thereby promoting the formation of excellent spherical morphology in the cathode material.

[0004] However, when using ammonia to adjust the pH or using ammonium dihydrogen phosphate or diammonium hydrogen phosphate as the phosphorus source, the formation and dissolution of basic ferric ammonium phosphate in the iron oxidation precipitation system are crucial throughout the entire nucleation and aging process. Furthermore, its formation is easily affected by factors such as pH fluctuations in the reaction system, residual ammonium ions, iron-phosphorus ratio imbalance, and reaction temperature changes, leading to unstable impurity phase content. For different production processes of ferric phosphate dihydrate, even if the control parameters are kept within the same range for each batch, the uniformity of stirring and the batch addition of raw materials can still cause localized excessively high pH levels, resulting in unstable content or uncontrollable distribution of the basic ferric ammonium phosphate impurity phase, thus causing uncontrollable performance of the final cathode material.

[0005] Based on this, a preparation process for ferric phosphate dihydrate is designed to control the content of the impurity phase of basic ferric ammonium phosphate during the large-scale production of ferric phosphate dihydrate, achieve batch stability, and utilize it as a functional component to help improve the overall performance of the final prepared cathode material. Summary of the Invention

[0006] Purpose of the invention: The technical problem to be solved by the present invention is to provide an industrial-scale production method for iron phosphate dihydrate, which can simultaneously control the content of the impurity phase of basic iron ammonium phosphate to achieve batch stability and utilize it as a functional component to help improve the overall performance of the final prepared cathode material.

[0007] Technical solution: The present invention provides a method for preparing ferric phosphate dihydrate, wherein the ferric phosphate dihydrate contains 0.3%-1.0% by mass of basic ferric ammonium phosphate, calculated based on nitrogen content, and is obtained by the following steps:

[0008] (1) Prepare a mixed solution of iron salt and phosphate according to the preset iron-phosphorus molar ratio, and add an oxidant to the mixed solution to completely oxidize ferrous ions to ferric ions;

[0009] (2) Add alkali solution to the mixed solution in step (1) to adjust the pH value to 1.8-2.7, react at a constant temperature of 55℃-65℃ for 8min-12min, and then obtain filter cake by pressure filtration and washing.

[0010] (3) After the filter cake is formed into a slurry, acid, high-phosphorus ammonium sulfate and titanium oxysulfate are added, and the temperature is raised to 85℃-95℃ and kept for aging for 1h-3h. After pressure filtration, washing and low-temperature drying, ferric phosphate dihydrate is obtained. The molar number of high-phosphorus ammonium sulfate added is based on ammonium ions and is 0.05-0.15 times the molar number of iron ions. The molar number of titanium oxysulfate added is based on titanium and is 0.05%-0.5% of the molar number of iron ions.

[0011] Furthermore, in step (1) of the preparation method, the iron-phosphorus molar ratio in the mixed solution is 1:(1-1.05), the iron salt is selected from ferrous sulfate and / or ferrous chloride, and the phosphate is selected from ammonium dihydrogen phosphate and / or diammonium hydrogen phosphate.

[0012] Furthermore, in step (1) of the preparation method, the initial concentration of iron ions in the mixed solution is 0.4 mol / L-0.8 mol / L.

[0013] Furthermore, in step (1) of the preparation method, the oxidant is selected from hydrogen peroxide or sodium hypochlorite.

[0014] Furthermore, in step (2) of the preparation method, the alkaline solution is an ammonia solution or sodium hydroxide solution with a mass fraction of 5wt%-10wt%.

[0015] Furthermore, in step (3) of the preparation method, the content of ammonium sulfate in the high-phosphorus ammonium sulfate is 60wt%-80wt%, and the content of phosphate is 5wt%-18wt%.

[0016] Furthermore, in step (3) of the preparation method, the acid solution is a dilute sulfuric acid with a mass fraction of 10wt%-20wt%, and the amount added is to maintain the pH of the system at 1.8-2.7.

[0017] Furthermore, in step (3) of the preparation method, the liquid-to-solid ratio of the slurry is (5-10):1.

[0018] Furthermore, in step (3) of the preparation method, the temperature of the low-temperature drying is 80℃-100℃ and the time is 4h-6h.

[0019] Beneficial effects: Compared with the prior art, the significant advantage of the present invention is that the preparation process of ferric phosphate dihydrate is suitable for large-scale production line application, and at the same time, it can control the mass fraction of basic ferric ammonium phosphate between different batches to maintain within the range of 0.3%-1.0%. Based on the formation of this small amount of basic ferric ammonium phosphate phase, it can help improve the overall performance of the cathode material prepared by ferric phosphate dihydrate. Attached Figure Description

[0020] Figure 1 Here is a SEM image of the iron phosphate prepared in Example 1 of this invention;

[0021] Figure 2 Here is a SEM image of the iron phosphate prepared in Example 2 of this invention;

[0022] Figure 3 The XRD pattern of the iron phosphate prepared in Example 2 of this invention;

[0023] Figure 4 Here is a SEM image of the iron phosphate prepared in Example 3 of this invention;

[0024] Figure 5 Here is a SEM image of the iron phosphate prepared in Example 4 of this invention;

[0025] Figure 6 The image shows a SEM image of the iron phosphate prepared in Comparative Example 1 of this invention.

[0026] Figure 7 This is a SEM image of the iron phosphate prepared in Comparative Example 2 of this invention. Detailed Implementation

[0027] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0028] It should be noted that the high-phosphorus ammonium sulfate used in this invention is derived from a byproduct of the production line, and the content of ammonium sulfate in the high-phosphorus ammonium sulfate is 60wt%-80wt%, and the content of phosphate is 5wt%-18wt%. In the examples below, the content of ammonium sulfate in the high-phosphorus ammonium sulfate is 68wt%, and the content of phosphate is 13wt%. The CAS number of the titanium oxysulfate used is 13825-74-6.

[0029] Example 1

[0030] (1) According to the iron-phosphorus molar ratio of 1:1.03, ferrous sulfate and ammonium dihydrogen phosphate are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, and the amount added is such that the ferrous ions are completely oxidized to ferric ions.

[0031] (2) Add 8 wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 1.8 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron dihydrate to obtain slurry.

[0032] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0033] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 8:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.2), high phosphorus ammonium sulfate (0.1 times the number of iron ions based on ammonium ions), and titanium oxysulfate (0.2% of the number of iron ions based on Ti) in sequence, and heat to 90℃ and keep warm for 2 hours.

[0034] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0035] Testing revealed that the target product contained 0.6% (as nitrogen) of impurities in basic ferric ammonium phosphate, with a D50 of 4.5 μm and a BET of 10.09 μm. 2 / g, tap density 1.02g / cm³, Fe / P ratio 97.09%.

[0036] The structure of the ferric phosphate dihydrate product obtained in step (5) was characterized using scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown in the figure, the product prepared in Example 1 is ferric phosphate dihydrate containing basic ammonium ferric phosphate.

[0037] Example 2

[0038] (1) According to the iron-phosphorus molar ratio of 1:1.05, ferrous sulfate and ammonium dihydrogen phosphate are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, and the amount added is such that the ferrous ions are completely oxidized to ferric ions.

[0039] (2) Add 8 wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 2.7 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron phosphate dihydrate to obtain slurry.

[0040] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0041] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 10:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.3), high-phosphorus ammonium sulfate (0.12 times the number of iron ions based on ammonium ions), and titanium oxysulfate (0.4% of the number of iron ions based on Ti) in sequence, and heat to 90℃ and keep warm for 2 hours.

[0042] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0043] Testing revealed that the target product contained 0.8% (as N) of impurities in basic ferric ammonium phosphate, with a D50 of 3.2 μm and a BET of 12.31 μm. 2 / g, tap density 1.28g / cm³, Fe / P ratio 96.89%.

[0044] The ferric phosphate product obtained in step (5) was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 2 As shown in the figure, the ferric phosphate dihydrate containing basic ammonium ferric phosphate (red box) prepared in Example 2 is shown in the figure. Meanwhile, the crystal structure of the ferric phosphate product obtained in step (5) was characterized using X-ray diffraction (XRD), and the results are as follows. Figure 3 As shown, the main peak position matches the standard PDF card of basic ammonium iron phosphate, and the shift to a higher angle indicates that titanium has been doped into the crystal lattice.

[0045] Example 3

[0046] (1) According to the iron-phosphorus molar ratio of 1:1.03, ferrous sulfate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate (mass ratio of ammonium dihydrogen phosphate and diammonium hydrogen phosphate 3:1) are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, the amount of which is added so that the ferrous ions are completely oxidized to ferric ions.

[0047] (2) Add 8 wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 1.8 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron dihydrate to obtain slurry.

[0048] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0049] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 8:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.2), high phosphorus ammonium sulfate (the amount added is 0.08 times the molar amount of iron ions), and titanium oxysulfate (calculated as Ti, which is 0.1% of the molar amount of iron ions), and heat to 90℃ and keep warm for 2 hours.

[0050] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0051] Testing revealed that the target product contained 0.4% (as N) of impurities in basic ferric ammonium phosphate, with a D50 of 5.1 μm and a BET of 9.69 μm. 2 / g, tap density 0.90g / cm³, Fe / P ratio 97.21%.

[0052] The ferric phosphate product obtained in step (5) was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 4 As shown in the figure, the ferric phosphate dihydrate containing basic ammonium ferric phosphate was prepared in Example 3.

[0053] Example 4

[0054] (1) According to the iron-phosphorus molar ratio of 1:1.05, ferrous sulfate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate (mass ratio of ammonium dihydrogen phosphate and diammonium hydrogen phosphate 3:1) are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, the amount of which is added so that the ferrous ions are completely oxidized to ferric ions.

[0055] (2) Add 8wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 2.5 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron dihydrate phosphate to obtain slurry.

[0056] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0057] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 10:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.7), high phosphorus ammonium sulfate (the amount added is 0.14 times the molar amount of iron ions), and titanium oxysulfate (calculated as Ti, which is 0.45% of the molar amount of iron ions) in sequence, and heat to 90℃ and keep warm for 2 hours.

[0058] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0059] Testing revealed that the target product contained 0.9% (as N) of impurities in basic ferric ammonium phosphate, with a D50 of 3.8 μm and a BET of 13.18 μm. 2 / g, tap density 1.08g / cm³, Fe / P ratio 97.01%.

[0060] The ferric phosphate product obtained in step (5) was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 5 As shown in the figure, the ferric phosphate dihydrate containing basic ammonium ferric phosphate was prepared in Example 4.

[0061] Comparative Example 1

[0062] The basic steps are the same as in Example 2, except that high-phosphorus ammonium sulfate is not added. The specific steps are as follows:

[0063] (1) According to the iron-phosphorus molar ratio of 1:1.05, ferrous sulfate and ammonium dihydrogen phosphate are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, and the amount added is such that the ferrous ions are completely oxidized to ferric ions.

[0064] (2) Add 8 wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 2.7 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron phosphate dihydrate to obtain slurry.

[0065] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0066] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 10:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.3) and titanium oxysulfate (calculated as Ti, which is 0.4% of the molar amount of iron ions), and heat to 90℃ and keep warm for 2 hours.

[0067] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0068] Testing revealed that the product contained 1.89% (calculated as nitrogen) of basic ferric ammonium phosphate impurities, which were randomly distributed in an amorphous state within the ferric phosphate dihydrate matrix, accompanied by 1.2% ferric hydroxide impurities; the D50 was 7.2 μm, with a wide particle size distribution (2~11 μm), and the BET was 8.37 μm. 2 / g, tap density 1.32g / cm³ 3 The Fe / P ratio is 96.28%.

[0069] The ferric phosphate product obtained in step (5) was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 6 As shown, Comparative Example 1 verifies the preparation of ferric phosphate dihydrate containing basic ammonium ferric phosphate.

[0070] Comparative Example 2

[0071] The basic steps are the same as in Example 2, except that titanium oxysulfate is not added. The specific steps are as follows:

[0072] (1) According to the iron-phosphorus molar ratio of 1:1.05, ferrous sulfate and ammonium dihydrogen phosphate are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, and the amount added is such that the ferrous ions are completely oxidized to ferric ions.

[0073] (2) Add 8 wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 2.7 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron phosphate dihydrate to obtain slurry.

[0074] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0075] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 10:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.3) and high-phosphorus ammonium sulfate (the amount added is 0.12 times the molar amount of iron ions) in sequence, and heat to 90℃ and keep warm for 2 hours.

[0076] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0077] Testing revealed that the product contained 0.58% (as N) of impurity phase in basic iron ammonium phosphate; D50 reached 8.5μm, exhibiting an irregular blocky morphology with uneven particle size distribution (3~13μm); BET was 7.36㎡ / g; tap density was only 0.78g / cm³; Fe / P was 96.88%; titanium was not detected, and there was no titanium doping effect.

[0078] The ferric phosphate product obtained in step (5) was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 7 As shown. Comparative Example 2 prepared ferric phosphate dihydrate of basic ferric phosphate in an uncontrollable amount.

[0079] Comparative Example 3

[0080] Comparative Example 3 is a different batch of experiments from Comparative Example 2. The basic steps are the same as in Example 2, except that titanium oxysulfate is not added. The specific steps are as follows:

[0081] (1) According to the iron-phosphorus molar ratio of 1:1.05, ferrous sulfate and ammonium dihydrogen phosphate are dissolved in deionized water to prepare a 500mL mixed solution, in which the initial concentration of iron ions is 0.5mol / L; 30wt% hydrogen peroxide solution is added to the mixed solution as an oxidant, and the amount added is such that the ferrous ions are completely oxidized to ferric ions.

[0082] (2) Add 8 wt% ammonia solution to the mixed solution in step (1) to adjust the pH of the system to 2.7 (pH fluctuation ≤ ±0.1), and then react at a constant temperature of 60℃ for 10 min to induce the formation of primary crystal nuclei of iron phosphate dihydrate to obtain slurry.

[0083] (3) Filter the obtained slurry, collect the filter cake and wash it with deionized water until the conductivity of the wash water filtrate is ≤5mS / cm, which meets the impurity control requirements.

[0084] (4) Add the washed filter cake to deionized water and stir and slurry at a liquid-solid ratio of 10:1; add 15wt% dilute sulfuric acid (the amount added is to maintain the pH of the system at 2.3) and high-phosphorus ammonium sulfate (the amount added is 0.12 times the molar amount of iron ions) in sequence, and heat to 90℃ and keep warm for 2 hours.

[0085] (5) After aging, the product is filtered, washed, and dried at 90°C for 5 hours to obtain the target product of ferric phosphate dihydrate.

[0086] Tests showed that the product contained 1.35% (as nitrogen) of impurities in basic ferric ammonium phosphate.

[0087] Electrochemical performance testing of cathode materials

[0088] Using the iron phosphate dihydrate obtained in Examples 1 to 4 and Comparative Examples 1 and 3 as precursors, lithium iron phosphate powder was prepared under the same process conditions. Following conventional methods in the art, each of the lithium iron phosphate powders prepared above was used to prepare coin cells under the same conditions. The coin cell prepared using the lithium iron phosphate dihydrate product of Example 1 as raw material was named #1 coin cell; the coin cell prepared using the lithium iron phosphate dihydrate product of Example 2 as raw material was named #2 coin cell; the coin cell prepared using the lithium iron phosphate dihydrate product of Example 3 as raw material was named #3 coin cell; the coin cell prepared using the lithium iron phosphate dihydrate product of Example 4 as raw material was named #4 coin cell; the coin cell prepared using the lithium iron phosphate dihydrate product of Comparative Example 1 as raw material was named #5 coin cell; and the coin cell prepared using the lithium iron phosphate dihydrate product of Comparative Example 3 as raw material was named #6 coin cell. The coin cell prepared from lithium iron phosphate obtained using pure iron phosphate dihydrate as raw material was named 7# coin cell. Subsequently, the compaction density of the above-mentioned lithium iron phosphate powder and the electrochemical performance of the corresponding coin cell were tested under the same test conditions according to conventional testing methods in the field. The test results are shown in Table 1 below.

[0089] Table 1. Results of compaction density and electrochemical performance tests

[0090]

[0091] Based on the detection data from Examples 1 to 4, this invention optimizes and improves the preparation process of ferric phosphate dihydrate, thereby consistently achieving a basic ferric ammonium phosphate impurity phase content within the range of 0.3%-1.0% in different batches of prepared ferric phosphate dihydrate. Furthermore, a comparison with the 7# coin cell performance in Table 1 shows that within this content range, the basic ferric ammonium phosphate impurity phase can further enhance the overall performance of the cathode material.

[0092] Further analysis of Examples 2 and Comparative Examples 1, 2, and 3 reveals that only by simultaneously introducing high-phosphorus ammonium sulfate and titanium oxysulfate during the aging process of ferric phosphate dihydrate can a stable content of basic ferric ammonium phosphate impurity phase within the range of 0.3%-1.0% be obtained. In contrast to Comparative Examples 2 and 3, while adding only high-phosphorus ammonium sulfate can sometimes maintain the basic ferric ammonium phosphate impurity phase within the range of 0.3%-1.0%, it cannot achieve stable batches, resulting in a high degree of randomness in the final obtained basic ferric ammonium phosphate impurity phase content. Furthermore, comparing the electrical performance of the cathode materials of battery #2 in Example 2, battery #5 in Comparative Example 1, and batteries #6 and #7 in Comparative Example 3 shows that a content of basic ferric ammonium phosphate impurity phase higher than 1% actually has a negative impact on the performance of the cathode material.

[0093] Based on the above experimental conclusions, further reasoning reveals that by simultaneously introducing acid, high-phosphorus ammonium sulfate, and titanium oxysulfate for aging during the slurrying process of the ferric phosphate dihydrate precursor, the acidic conditions provided by the acid will promote the reverse reaction of the formation of basic ferric phosphate, thus forcing the basic ferric phosphate to redissolve. Under these conditions, the excess phosphate or dihydrogen phosphate ions provided by the high-phosphorus ammonium sulfate can ensure that the free trivalent iron can be completely precipitated into ferric phosphate, thereby reducing the formation of impurity phases. At the same time, the tetravalent titanium in titanium oxysulfate has a higher charge density and hydration ability than trivalent iron ions. When titanium oxysulfate hydrolyzes, titanium ions preferentially combine with phosphate ions to form substitution doping. This ion substitution will disturb the basic ferric phosphate from growing into an ordered lattice according to the original required pattern, thus destroying the crystallization ability of basic ferric phosphate and reducing its regeneration ability.

[0094] This invention involves simultaneously adding acid, high-phosphorus ammonium sulfate, and titanium sulfate during the aging process. This forces the metastable basic iron ammonium phosphate to redissolve, promoting the positive formation of iron phosphate while enhancing the ability to prevent the regeneration of basic iron ammonium phosphate. Ultimately, this yields a small amount of basic iron ammonium phosphate with controllable content, achieving stability across different batches on a large scale.

[0095] In addition to the above embodiments, the technical effects claimed by the present invention can be achieved by using the process steps and parameters defined by the present invention, and therefore no further testing and verification are required.

Claims

1. A method for preparing ferric phosphate dihydrate, characterized in that, Based on nitrogen content, this ferric phosphate dihydrate contains 0.3%-1.0% by mass of basic ferric ammonium phosphate, which is prepared by the following steps: (1) Prepare a mixed solution of iron salt and phosphate according to the preset iron-phosphorus molar ratio, and add an oxidant to the mixed solution to completely oxidize ferrous ions to ferric ions; (2) Add alkali solution to the mixed solution in step (1) to adjust the pH value to 1.8-2.7, react at a constant temperature of 55℃-65℃ for 8min-12min, and then obtain filter cake by pressure filtration and washing. (3) After the filter cake is formed into a slurry, acid, high-phosphorus ammonium sulfate and titanium oxysulfate are added, and the temperature is raised to 85℃-95℃ and kept for aging for 1h-3h. After pressure filtration, washing and low-temperature drying, ferric phosphate dihydrate is obtained. The molar number of high-phosphorus ammonium sulfate added is based on ammonium ions and is 0.05-0.15 times the molar number of iron ions. The molar number of titanium oxysulfate added is based on titanium and is 0.05%-0.5% of the molar number of iron ions.

2. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (1), the iron-phosphorus molar ratio in the mixed solution is 1:(1-1.05), the iron salt is selected from ferrous sulfate and / or ferrous chloride, and the phosphate is selected from diammonium dihydrogen phosphate and / or diammonium hydrogen phosphate.

3. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (1), the initial concentration of iron ions in the mixed solution is 0.4 mol / L-0.8 mol / L.

4. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (1), the oxidant is selected from hydrogen peroxide or sodium hypochlorite.

5. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (2), the alkaline solution is an ammonia solution or sodium hydroxide solution with a mass fraction of 5wt%-10wt%.

6. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (3), the ammonium sulfate content in the high-phosphorus ammonium sulfate is 60wt%-80wt%, and the phosphate content is 5wt%-18wt%.

7. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (3), the acid solution is a dilute sulfuric acid with a mass fraction of 10wt%-20wt%, which is added to maintain the pH of the system at 1.8-2.

7.

8. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (3), the liquid-to-solid ratio of the slurry is (5-10):

1.

9. The method for preparing ferric phosphate dihydrate according to claim 1, characterized in that, In step (3), the temperature of the low-temperature drying is 80℃-100℃ and the time is 4h-6h.

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