Iron phosphate, method for preparing the same and lithium iron phosphate composite cathode material

Abstract

This application provides iron phosphate, its preparation method, and lithium iron phosphate composite cathode material, relating to the battery field. The preparation method of iron phosphate includes: preparing a ferrous solution; preparing a first phosphate salt solution with a pH of 3.0-4.0 and a second phosphate salt solution with a pH of 6.0-8.0 respectively; simultaneously adding the first and second phosphate salt solutions and hydrogen peroxide to the ferrous solution to react and obtain a reaction slurry; filtering and washing the reaction slurry to obtain a filter cake, then slurrying the filter cake to obtain an amorphous iron phosphate slurry; adding an inorganic acid to the amorphous iron phosphate slurry and aging it to obtain a dihydrate iron phosphate slurry; filtering, washing, drying, and calcining the dihydrate iron phosphate slurry to obtain iron phosphate. This application can obtain iron phosphate with both large and small particles, enabling control over the particle size and proportion of iron phosphate, which is beneficial for improving the compaction density and electrochemical performance of lithium iron phosphate cathode materials.

Description

Technical Field

[0001] This application relates to the field of batteries, and in particular to an iron phosphate, its preparation method, and a lithium iron phosphate composite cathode material. Background Technology

[0002] Lithium iron phosphate (LFP) holds a leading position in energy storage and power applications as a safe, long-life, and low-cost cathode material. Due to its relatively low energy density, industry research focuses on adjusting the particle size distribution of LFP to improve compaction density. Based on the microstructure genetic effect, the particle size distribution of the iron phosphate precursor has a positive effect on the particle size distribution of LFP.

[0003] However, the particle size distribution of iron phosphate prepared by existing methods is poor, resulting in poor electrochemical performance of lithium iron phosphate materials. Summary of the Invention

[0004] The purpose of this application is to provide an iron phosphate, a method for preparing the same, and a lithium iron phosphate composite cathode material to solve the above-mentioned problems.

[0005] To achieve the above objectives, this application adopts the following technical solution: A method for preparing ferric phosphate, comprising: Prepare ferrous solution; A first phosphate salt solution and a second phosphate salt solution are prepared using a phosphorus-containing compound and a pH adjuster, wherein the phosphorus-containing compound includes at least one of ammonium monohydrogen phosphate and ammonium dihydrogen phosphate; the pH of the first phosphate salt solution is 3.0-4.0, and the pH of the second phosphate salt solution is 6.0-8.0. Under stirring, the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide are simultaneously added to the ferrous solution to react and obtain a reaction slurry. The reaction slurry is filtered and washed to obtain a filter cake, which is then pulped to obtain an amorphous ferric phosphate slurry. Inorganic acid is added to the amorphous ferric phosphate slurry, the pH of the slurry is adjusted to 1.0-2.0, and it is aged at a temperature of 80-100℃ to obtain ferric phosphate dihydrate slurry; The ferric phosphate dihydrate slurry is filtered, washed, dried, and calcined to obtain ferric phosphate.

[0006] According to embodiments of this application, the ferrous solution includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride solution; And / or, the concentration of the ferrous solution is 0.4-2.0 mol / L; And / or, the pH of the ferrous solution is 2.0-3.0; And / or, the pH adjuster includes at least one of sulfuric acid solution, sodium hydroxide solution, and ammonia solution; And / or, the method further includes: mixing the ferrous compound with water and an acidic substance to obtain a ferrous solution; wherein the acidic substance includes at least one of sulfuric acid, hydrochloric acid, and nitric acid; And / or, the concentration of the first phosphate salt solution is 0.4-2.0 mol / L; And / or, the concentration of the second phosphate salt solution is 0.4-2.0 mol / L; And / or, the molar ratio of phosphorus in the first phosphate salt solution to that in the second phosphate salt solution is 0.1:0.9 to 0.9:0.1.

[0007] According to an embodiment of this application, the molar ratio of iron in the ferrous solution to the total molar ratio of phosphorus in the first phosphate solution and the second phosphate solution is 1:0.3 to 1:3.

[0008] According to an embodiment of this application, the molar ratio of iron in the ferrous solution to the molar ratio of hydrogen peroxide is 1:0.6 to 1:1.

[0009] According to an embodiment of this application, the feeding time of the first phosphate salt solution, the second phosphate salt solution and hydrogen peroxide is 30-120 min.

[0010] According to an embodiment of this application, the method further includes: filtering and washing the reaction slurry until the conductivity of the wash water is ≤5000μs / cm, then stopping the washing to obtain a filter cake; And / or, the solid content of the amorphous ferric phosphate slurry is 5%~15%.

[0011] According to an embodiment of this application, the aging time is 60-120 minutes; And / or, the inorganic acid includes at least one of sulfuric acid, hydrochloric acid, and nitric acid.

[0012] According to an embodiment of this application, after filtering the ferric phosphate dihydrate slurry, the method further includes: washing the filter residue obtained from filtration until the conductivity of the wash water is ≤600μs / cm; And / or, the drying temperature is 120~200℃, and the drying time is 10-15h; And / or, the calcination temperature is 650-710℃, and the calcination time is 1-2 hours.

[0013] This application also provides a ferric phosphate, which is prepared by the preparation method described above; the ferric phosphate includes a first particle and a second particle, wherein the size of the first particle is 100-150 nm and the size of the second particle is 1-5 μm.

[0014] This application also provides a lithium iron phosphate composite cathode material, the preparation method of which includes: calcining iron phosphate, a lithium source, and an organic carbon source to obtain the lithium iron phosphate composite cathode material; The iron phosphate is either the iron phosphate prepared by the preparation method described above or the iron phosphate described above.

[0015] Compared with the prior art, the beneficial effects of this application include: This application obtains amorphous iron phosphate with varying reactivity by adding first and second phosphate solutions with different pH values ​​during the synthesis stage. The highly reactive amorphous iron phosphate is dissolved and recrystallized to obtain small-particle iron phosphate, while the less reactive amorphous iron phosphate is dissolved and recrystallized to obtain large-particle iron phosphate. This application can prepare iron phosphate with both large and small particles, enabling control over the particle size and proportion of iron phosphate, which is beneficial for improving the compaction density and electrochemical performance of lithium iron phosphate composite cathode materials.

[0016] Moreover, the method described in this application can be directly implemented using existing ammonium process production lines without the need for complex technical modifications, and can be quickly introduced into production lines and put into operation. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0018] Figure 1 Here is a SEM image of the iron phosphate prepared in Example 1; Figure 2 Here is a SEM image of the iron phosphate prepared in Example 2; Figure 3 Here is a SEM image of the iron phosphate prepared in Example 3; Figure 4 SEM image of the iron phosphate prepared in Comparative Example 1; Figure 5 SEM image of the iron phosphate prepared in Comparative Example 2; Figure 6 SEM image of the iron phosphate prepared in Comparative Example 3; Figure 7 SEM image of the iron phosphate prepared in Comparative Example 4; Figure 8 SEM image of the iron phosphate prepared in Comparative Example 5; Figure 9 SEM image of the iron phosphate prepared in Comparative Example 6; Figure 10 This is a SEM cross-sectional image of the iron phosphate prepared in Example 2; Figure 11 SEM cross-sectional view of the iron phosphate prepared in Comparative Example 1; Figure 12 SEM cross-sectional image of the iron phosphate prepared in Comparative Example 2; Figure 13 This is a comparison chart of the specific surface area of ​​the reaction slurry after complete drying in step S3 of Examples 1-3 and Comparative Examples 1-6. Detailed Implementation

[0019] As used in this article: "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0020] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0021] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0022] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0023] "Parts by mass" refers to the basic unit of measurement that expresses the mass ratio of multiple components. One part can represent any unit mass, such as 1g or 2.689g. If we say that component A has "a" parts by mass and component B has "b" parts by mass, it means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, it can mean that the mass of component A is aK and the mass of component B is bK (where K is any number representing a multiplier). It is important to understand that, unlike parts by mass, the sum of the mass parts of all components is not limited to 100 parts.

[0024] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0025] A method for preparing ferric phosphate, comprising: Prepare ferrous solution; A first phosphate salt solution and a second phosphate salt solution are prepared using a phosphorus-containing compound and a pH adjuster, wherein the phosphorus-containing compound includes at least one of ammonium monohydrogen phosphate and ammonium dihydrogen phosphate; the pH of the first phosphate salt solution is 3.0-4.0 (e.g., 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0 or any value between 3.0 and 4.0), and the pH of the second phosphate salt solution is 6.0-8.0 (e.g., 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0 or any value between 6.0 and 8.0). Under stirring, the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide are simultaneously added to the ferrous solution to react and obtain a reaction slurry. The reaction slurry is filtered and washed to obtain a filter cake, which is then pulped to obtain an amorphous ferric phosphate slurry. An inorganic acid is added to the amorphous ferric phosphate slurry, and the pH of the slurry is adjusted to 1.0-2.0 (e.g., 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 or any value between 1.0 and 2.0). The slurry is then aged at a temperature of 80-100℃ (e.g., 80℃, 83℃, 85℃, 87℃, 90℃, 93℃, 95℃, 97℃, 100℃ or any value between 80 and 100℃) to obtain a ferric phosphate dihydrate slurry. The ferric phosphate dihydrate slurry is filtered, washed, dried, and calcined to obtain ferric phosphate.

[0026] The reaction equation for amorphous iron phosphate in this application is: 2Fe 2+ + 2HPO4 2- + H2O2→FePO4 (amorphous)↓+ 2H2O As can be seen from the reaction equation, HPO4 2- The proportion of precipitation in the reaction system determines the rate of precipitation reaction.

[0027] This application incorporates first and second phosphate salt solutions with different pH values ​​during the synthesis stage. Ammonium hydrogen phosphate and ammonium dihydrogen phosphate have buffering properties, reducing the HPO4 content in the first and second phosphate salt solutions at different pH values. 2- The proportions differ. The pH of the second phosphate salt solution (6.0-8.0) is higher than that of the first phosphate salt solution (3.0-4.0). The second phosphate salt solution, with its higher pH, contains more HPO4. 2- The proportion of HPO4 is higher than that in the first phosphate solution with a lower pH. 2- The proportion of Fe, therefore, the second phosphate solution with Fe 2+ The reaction with H2O2 is relatively fast and conducive to the formation of small-sized iron phosphate particles; the first phosphate salt solution reacts with Fe 2+ The relatively slow reaction rate with H₂O₂ favors the formation of larger iron phosphate particles. The particle size distribution in iron phosphate can be controlled by adjusting the ratio of the first and second phosphate salt solutions. The microstructure of iron phosphate exhibits a genetic effect during the subsequent preparation of lithium iron phosphate cathode materials, causing the lithium iron phosphate composite cathode material to also form a similar particle size distribution to iron phosphate, thereby improving the compaction density and electrochemical performance of the lithium iron phosphate composite cathode material.

[0028] According to embodiments of this application, the ferrous solution includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride solution; The concentration of the ferrous solution is 0.4-2.0 mol / L; for example, the concentration of the ferrous solution is 0.4 mol / L, 0.5 mol / L, 0.7 mol / L, 1.0 mol / L, 1.3 mol / L, 1.5 mol / L, 1.8 mol / L, 2.0 mol / L or any value between 0.4-2.0 mol / L.

[0029] The pH of the ferrous solution is 2.0-3.0; for example, the pH of the ferrous solution is 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0 or any value between 2.0 and 3.0.

[0030] The method further includes: mixing a ferrous compound with water and an acidic substance to obtain a ferrous solution; wherein the acidic substance includes at least one of sulfuric acid, hydrochloric acid, and nitric acid; The pH adjuster includes at least one of sulfuric acid solution, sodium hydroxide solution, and ammonia water; The concentration of the first phosphate salt solution is 0.4-2.0 mol / L; for example, the concentration of the first phosphate salt solution is 0.4 mol / L, 0.5 mol / L, 0.7 mol / L, 1.0 mol / L, 1.3 mol / L, 1.5 mol / L, 1.8 mol / L, 2.0 mol / L or any value between 0.4-2.0 mol / L.

[0031] The concentration of the second phosphate salt solution is 0.4-2.0 mol / L; for example, the concentration of the second phosphate salt solution is 0.4 mol / L, 0.5 mol / L, 0.7 mol / L, 1.0 mol / L, 1.3 mol / L, 1.5 mol / L, 1.8 mol / L, 2.0 mol / L or any value between 0.4-2.0 mol / L.

[0032] The molar ratio of phosphorus in the first phosphate salt solution to the second phosphate salt solution is 0.1:0.9 to 0.9:0.1. For example, the molar ratio of phosphorus in the first phosphate salt solution to the second phosphate salt solution is any value between 0.1:0.9, 0.2:0.8, 0.3:0.7, 0.4:0.6, 0.5:0.5, 0.6:0.4, 0.7:0.3, 0.8:0.2, 0.9:0.1, or 0.1:0.9 to 0.9:0.1.

[0033] According to embodiments of this application, the ratio of the molar amount of iron in the ferrous solution to the total molar amount of phosphorus in the first phosphate solution and the second phosphate solution is 1:0.3 to 1:3. For example, the ratio of the molar amount of iron in the ferrous solution to the total molar amount of phosphorus in the first phosphate solution and the second phosphate solution is any value between 1:0.3, 1:0.5, 1:0.8, 1:1, 1:1.3, 1:1.5, 1:1.8, 1:2, 1:2.3, 1:2.5, 1:2.8, 1:3, or 1:0.3 to 1:3.

[0034] According to embodiments of this application, the molar ratio of iron to hydrogen peroxide in the ferrous solution is 1:0.6 to 1:1. For example, the molar ratio of iron to hydrogen peroxide in the ferrous solution is 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, or any value between 1:0.6 and 1:1.

[0035] According to the embodiments of this application, the feeding time of the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide is 30-120 min. If the feeding time of the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide is too short, the reaction will be rapidly exothermic, causing amorphous ferric phosphate to transform into crystals and lose its activity. If the feeding time of the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide is too long, the reaction rate will slow down, the production efficiency will decrease, and the specific surface area of ​​amorphous ferric phosphate will tend to decrease.

[0036] This application adds the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide to the ferrous solution simultaneously, which is beneficial for achieving uniform mixing of different synthetic active matrix samples and plays a positive role in achieving ferric phosphate gradation.

[0037] For example, the feeding time of the first phosphate salt solution, the second phosphate salt solution and hydrogen peroxide is any value between 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, 120 min or 30-120 min.

[0038] According to an embodiment of this application, the method further includes: filtering and washing the reaction slurry until the conductivity of the wash water is ≤5000μs / cm, then stopping the washing to obtain a filter cake; The solid content of the amorphous ferric phosphate slurry is 5% to 15%. For example, the solid content of the amorphous ferric phosphate slurry is 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any value between 5% and 15%.

[0039] In some embodiments, the aging pH is 1.0-2.0, under which amorphous ferric phosphate can undergo a dissolution and recrystallization reaction, and the product is monoclinic ferric phosphate dihydrate. If the aging pH is too low, a large amount of acid will be consumed; if the aging pH is too high, heterogeneous basic ferric phosphate will be generated. According to an embodiment of this application, the aging time is 60-120 min; for example, the aging time is any value between 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, 120 min, or 60-120 min.

[0040] The inorganic acid includes at least one of sulfuric acid, hydrochloric acid, and nitric acid.

[0041] According to an embodiment of this application, after filtering the iron phosphate dihydrate slurry, the method further includes: washing the filtered residue until the conductivity of the wash water is ≤600 μS / cm; the conductivity at the washing endpoint of this application is ≤600 μS / cm. By thoroughly washing the intermediate product, the impurity ion content of the final product can be made extremely low. This method does not rely on residual phosphoric acid or other impurities as densifying agents in the calcination process, and does not require the introduction of sintering fluxes, thereby ensuring the purity of iron phosphate and lithium iron phosphate, which is beneficial to the electrochemical performance.

[0042] The drying temperature is 120~200℃, and the drying time is 10-15h; for example, the drying temperature is 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃ or any value between 120~200℃, and the drying time is 10h, 11h, 12h, 13h, 14h, 15h or any value between 10-15h.

[0043] The calcination temperature is 650-710℃, and the calcination time is 1-2 hours. For example, the calcination temperature is any value between 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, or 650-710℃, and the calcination time is any value between 1 hour, 2 hours, or 1-2 hours.

[0044] This application also provides a ferric phosphate, which is prepared by the preparation method described above; the ferric phosphate includes a first particle and a second particle, wherein the size of the first particle is 100-150 nm (e.g., 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm or any value between 100-150 nm), and the size of the second particle is 1-5 μm (e.g., 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or any value between 1-5 μm).

[0045] This application also provides a lithium iron phosphate composite cathode material, the preparation method of which includes: calcining iron phosphate, a lithium source, and an organic carbon source to obtain the lithium iron phosphate composite cathode material; The iron phosphate is either the iron phosphate prepared by the preparation method described above or the iron phosphate described above.

[0046] In some embodiments, the lithium source includes lithium carbonate; The ratio of the molar amount of lithium in the lithium source to the molar amount of iron phosphate is (1.01-1.05):1, for example, 1.01:1, 1.02:1, 1.03:1, 1.035:1, 1.04:1, 1.05:1 or any value between (1.01-1.05):1.

[0047] The organic carbon source includes glucose; The organic carbon source accounts for 8%-10% of the mass of the iron phosphate, for example, the organic carbon source accounts for 8%, 9%, 10% or any value between 8% and 10% of the mass of the iron phosphate.

[0048] The heating rate of the calcination is 7-9℃ / min, for example, 7℃ / min, 8℃ / min, 9℃ / min or any value between 7-9℃ / min.

[0049] The calcination temperature is 700-800℃, for example, 700℃, 750℃, 800℃ or any value between 700-800℃; the calcination time is 3-5h, for example, 3h, 4h, 5h or any value between 3-5h.

[0050] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0051] Example 1 Example 1 provides a primary granular iron phosphate, the preparation method of which includes the following steps: S1: Add sulfuric acid solution to ferrous sulfate to prepare a 0.4 mol / L ferrous sulfate solution with pH 2.0; S2: Using ammonium dihydrogen phosphate and sulfuric acid solution as raw materials, prepare a first phosphate salt solution with pH 3.0 and a concentration of 0.4 mol / L; using ammonium dihydrogen phosphate and ammonia water, prepare a second phosphate salt solution with pH 6.0 and a concentration of 0.4 mol / L. S3: Using the ferrous solution from step S1 as the base liquid, under stirring, the first phosphate salt solution (pH 3.0), the second phosphate salt solution (pH 6.0), and hydrogen peroxide from step S2 are simultaneously added to the ferrous solution to react and obtain a reaction slurry. The ratio of the molar amount of iron in the ferrous solution to the total molar amount of phosphorus in the first phosphate salt solution (pH 3.0) and the second phosphate salt solution (pH 6.0) is Fe / P = 1:0.6; the ratio of the molar amount of iron in the ferrous solution to the molar amount of hydrogen peroxide is Fe / H2O2 = 1:0.6; the molar ratio of P in the first phosphate salt solution (pH 3.0) to the second phosphate salt solution (pH 6.0) is 0.1:0.9; and the addition time of the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide is 30 minutes.

[0052] S4: The reaction slurry obtained in S3 is filtered and washed until the conductivity of the wash water is 4500 μs / cm, at which point the washing is stopped to obtain a filter cake. Then the filter cake is pulped, and the solid content is controlled at 5% during pulping to obtain an amorphous iron phosphate slurry.

[0053] S5: Add sulfuric acid to the amorphous ferric phosphate slurry, control the pH of the slurry to 1.0, heat to 80℃, keep warm for 60 min, and obtain a light pink crystalline ferric phosphate dihydrate slurry; S6: The light pink crystalline ferric phosphate dihydrate slurry was filtered, and the filter residue was washed until the conductivity of the wash water was 500 μs / cm. The washing was then stopped, and the residue was dried at 120 ℃ for 10 h and calcined at 650 ℃ for 1 h to obtain ferric phosphate with self-graded particles.

[0054] The SEM image of the iron phosphate prepared in Example 1 is shown below. Figure 1 As shown.

[0055] Example 2 Example 2 provides a primary granular iron phosphate, the preparation method of which includes the following steps: S1: Add hydrochloric acid solution to ferrous chloride to prepare a 1.0 mol / L ferrous chloride solution with pH 2.5; S2: Using ammonium dihydrogen phosphate and sulfuric acid solution as raw materials, prepare a first phosphate salt solution with pH 3.5 and a concentration of 1.0 mol / L; using ammonium dihydrogen phosphate and ammonia water as raw materials, prepare a second phosphate salt solution with pH 7 and a concentration of 1.0 mol / L. S3: Using the ferrous solution from step S1 as the base liquid, under stirring, the first phosphate salt solution (pH 3.5), the second phosphate salt solution (pH 7), and hydrogen peroxide from step S2 are simultaneously added to the ferrous solution to react and obtain a reaction slurry. The ratio of the molar amount of iron in the ferrous solution to the total molar amount of phosphorus in the first phosphate salt solution (pH 3.5) and the second phosphate salt solution (pH 7) is Fe / P = 1:1; the ratio of the molar amount of iron in the ferrous solution to the molar amount of hydrogen peroxide is Fe / H2O2 = 1:0.6; the molar ratio of P in the first phosphate salt solution (pH 3.5) to the second phosphate salt solution (pH 7.0) is 0.5:0.5; and the addition time for the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide is 60 min.

[0056] S4: The reaction slurry obtained in S3 is filtered and washed until the conductivity of the wash water is 3000 μs / cm, at which point the washing is stopped to obtain a filter cake. Then the filter cake is pulped, and the solid content is controlled at 10% during pulping to obtain an amorphous iron phosphate slurry.

[0057] S5: Add hydrochloric acid to the amorphous ferric phosphate slurry, control the pH of the slurry to 1.2, heat to 85℃, keep warm for 60 min, and obtain a light pink crystalline ferric phosphate dihydrate slurry. S6: The light pink crystalline ferric phosphate dihydrate slurry was filtered, and the filter residue was washed until the conductivity of the wash water was 400 μs / cm. The washing was then stopped, and the residue was dried at 120 ℃ for 10 h and calcined at 670 ℃ for 2 h to obtain ferric phosphate with self-graded particles.

[0058] SEM image of the iron phosphate prepared in Example 2 is shown below. Figure 2 As shown, the SEM cross-sectional image of the iron phosphate prepared in Example 2 is as follows. Figure 10 As shown. By Figure 10 It can be seen that the iron phosphate in Example 2 has a particle size distribution structure.

[0059] Example 3 Example 3 provides a primary granular iron phosphate, the preparation method of which includes the following steps: S1: Add nitric acid solution to ferrous nitrate to prepare a 2.0 mol / L ferrous nitrate solution with pH 3.0; S2: Using ammonium dihydrogen phosphate and sulfuric acid solution as raw materials, prepare a first phosphate salt solution with pH 4.0 and a concentration of 2.0 mol / L; using ammonium dihydrogen phosphate and sodium hydroxide solution as raw materials, prepare a second phosphate salt solution with pH 8.0 and a concentration of 2.0 mol / L. S3: Using the ferrous solution from step S1 as the base liquid, under stirring, the first phosphate salt solution (pH 4.0), the second phosphate salt solution (pH 8.0), and hydrogen peroxide from step S2 are simultaneously added to the ferrous solution to react and obtain a reaction slurry. The ratio of the molar amount of iron in the ferrous solution to the total molar amount of phosphorus in the first phosphate salt solution (pH 4.0) and the second phosphate salt solution (pH 8.0) is Fe / P = 1:2; the ratio of the molar amount of iron in the ferrous solution to the molar amount of hydrogen peroxide is Fe / H2O2 = 1:0.9; the molar ratio of P in the first phosphate salt solution (pH 4.0) and the second phosphate salt solution (pH 8.0) is 0.7:0.3; and the feeding time for the first phosphate salt solution, the second phosphate source solution, and hydrogen peroxide is 80 min.

[0060] S4: The reaction slurry obtained in S3 is filtered and washed until the conductivity of the wash water is 3000 μs / cm, at which point the washing is stopped to obtain a filter cake. The filter cake is then pulped, with the solid content controlled at 12% during pulping, to obtain an amorphous iron phosphate slurry.

[0061] S5: Add nitric acid to the amorphous ferric phosphate slurry, control the pH of the slurry to 1.3, heat to 88℃, and keep warm for 90 min to obtain a light pink crystalline ferric phosphate dihydrate slurry. S6: The light pink crystalline ferric phosphate dihydrate slurry was filtered, and the filter residue was washed until the conductivity of the wash water was 300 μs / cm. The washing was then stopped, and the slurry was dried at 120 ℃ for 10 h and calcined at 680 ℃ for 2 h to obtain ferric phosphate with self-graded particles.

[0062] The SEM image of the iron phosphate prepared in Example 3 is shown below. Figure 3 As shown.

[0063] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that in step S2, only a 0.4 mol / L second phosphate salt solution with pH 6.0 is prepared; in step S3, the second phosphate salt solution with pH 6.0 is added, but the first phosphate salt solution with pH 3.0 is not added. Everything else is the same as in Example 1.

[0064] The SEM image of iron phosphate obtained in Comparative Example 1 is shown below. Figure 4 As shown, the SEM cross-sectional image of the iron phosphate prepared in Comparative Example 1 is as follows. Figure 11 As shown. By Figure 11 It can be seen that the particle size of the ferric phosphate particles in Comparative Example 1 is relatively small.

[0065] Comparative Example 2 The difference between Comparative Example 2 and Example 3 is that in step S2, only a 2.0 mol / L first phosphate salt solution with pH 4.0 is prepared; in step S3, the first phosphate salt solution with pH 4.0 is added, but the second phosphate salt solution with pH 8.0 is not added. Everything else is the same as in Example 3.

[0066] The SEM image of iron phosphate obtained in Comparative Example 2 is shown below. Figure 5 As shown, the SEM cross-sectional image of the iron phosphate prepared in Comparative Example 2 is as follows. Figure 12 As shown. By Figure 12 It can be seen that the particle size of the iron phosphate particles in Comparative Example 2 is relatively large.

[0067] Comparative Example 3 The difference between Comparative Example 3 and Example 2 is that in step S3 of Comparative Example 3, at the beginning of feeding, a first phosphate salt solution with pH 3.5 and 50% of the total amount of hydrogen peroxide are added first, and after reacting for 20 minutes, a second phosphate salt solution with pH 7 and the remaining hydrogen peroxide are added.

[0068] The SEM image of iron phosphate obtained in Comparative Example 3 is shown below. Figure 6 As shown.

[0069] Comparative Example 4 The difference between Comparative Example 4 and Example 2 is that in step S3 of Comparative Example 4, at the beginning of feeding, a second phosphate salt solution with pH 7 and 60% of the total amount of hydrogen peroxide are added first, and after reacting for 30 minutes, a first phosphate salt solution with pH 3.5 and the remaining hydrogen peroxide are added.

[0070] The SEM image of iron phosphate obtained in Comparative Example 4 is shown below. Figure 7 As shown.

[0071] Comparative Example 5 The difference between Comparative Example 5 and Example 2 is that the pH of the first phosphate solution in Comparative Example 5 is 2.5.

[0072] The SEM image of iron phosphate obtained in Comparative Example 5 is shown below. Figure 8 As shown.

[0073] Comparative Example 6 The difference between Comparative Example 6 and Example 2 is that the pH of the second phosphate salt solution in Comparative Example 6 is 5.

[0074] The SEM image of iron phosphate obtained in Comparative Example 6 is shown below. Figure 9 As shown.

[0075] The iron phosphates prepared in Examples 1-3 and Comparative Examples 1-6 were used to prepare lithium iron phosphate composite cathode materials under the same conditions, and then assembled into coin cells under the same conditions.

[0076] Specifically, lithium carbonate and iron phosphate prepared in the examples or comparative examples were added in an amount calculated according to a lithium element to iron phosphate molar ratio of 1.035:1. Glucose was added at 9% of the iron phosphate mass to prepare a slurry with a certain solid content. Deionized water was used as a solvent, and ball milling was performed using zirconia balls with a diameter of 0.6 mm at a ball-to-material ratio of 5:1. After mixing, the mixture was transferred to a sand mill and milled at 1000-1500 r / min for 3-5 h. The resulting slurry had a solid content of 50%, and was spray-dried at a rate of 300 mL / h to prepare a precursor. The inlet and outlet temperatures of the sprayer were 180 ℃ and 100 ℃, respectively. The nozzle diameter was 1 mm, and the inlet pressure was 0.4 MPa. The precursor was then transferred to a tube furnace and heated at 750 ℃ ​​for 4 h at a heating rate of 8 ℃ / min under a flowing nitrogen protective atmosphere to synthesize the LiFePO4 / C composite material.

[0077] LiFePO4 / C composite material, conductive carbon material, and polyvinylidene chloride binder were mixed in a mass ratio of 90:5:5. N-methylpyrrolidone was added dropwise, and the mixture was ground into a paste. This paste was then coated onto aluminum foil, dried at 120°C, cut into sheets, and seamlessly rolled to obtain the positive electrode test electrode. The active material loading on the positive electrode sheet was controlled at 9–10 mg. The electrode sheet was compacted under 3 tons of pressure, and the compaction density was measured. The coin cell model was CR2032. A lithium metal sheet was used as the counter electrode (reference electrode), and the electrolyte was a product from Shenzhen Capchem Technology Co., Ltd. The coin cells were assembled in a glove box with controlled moisture and oxygen levels.

[0078] The button cells were placed in a 25℃ constant temperature chamber for 24 hours and then connected to a Blue Electric Tester for charge-discharge testing. The button cells were placed in a 25℃ constant temperature chamber for charge-discharge testing. The nominal capacity at 1C was set at 160 mAh / g. Within the range of 2.0–3.75V, the cells were first activated at 0.1C for one week, then charged and discharged at 1.0C. At a constant voltage cutoff current of 0.02C at 3.75V, the charge-discharge curves were observed. Charge-discharge rate performance tests were performed at 0.1C, 0.2C, 0.3C, 0.5C, 1C, and 2C. The test voltage was 2.0–3.8V, and the 1C capacity was set at 160 mAh / g. The charge-discharge performance test was set according to the following steps: constant current charging → constant voltage charging → rest → constant current discharging → rest → cycling → stop. Cyclic performance was tested using 0.5C charging and 1C discharging. The measured electrochemical performance and powder compaction density results are shown in Table 1.

[0079] Table 1 Performance test results of lithium iron phosphate composite cathode material

[0080] As can be seen from Table 1, the powder compaction density of the lithium iron phosphate composite cathode materials in Examples 1-3 is higher than that in Comparative Examples 1-6. The discharge capacity and capacity retention rate of Examples 1-3 at different rates and after 150 cycles are better than those in Comparative Examples 1-6. In other words, the electrochemical performance of Examples 1-3 is better than that of Comparative Examples 1-6. Figure 13 This is a comparison of the specific surface area of ​​the reaction slurry after complete drying in step S3 of Examples 1-3 and Comparative Examples 1-6 (intermediate). The specific surface area of ​​this intermediate can reflect the reactivity of amorphous ferric phosphate. Examples 1-3 successfully prepared mixtures of amorphous ferric phosphate with different activities by adding first and second phosphate solutions of different pH values. Comparative Example 1 used only a high-pH phosphate solution, resulting in a vigorous reaction and the highest specific surface area of ​​the intermediate; Comparative Example 2 used only a low-pH phosphate solution, resulting in a slower reaction and the lowest specific surface area of ​​the intermediate. Comparative Examples 3 and 4 used a stepwise feeding method, which failed to achieve uniform mixing of the two active substances, resulting in intermediates with intermediate specific surface areas in the middle, ultimately leading to poor particle size distribution. Comparative Examples 5 and 6 had intermediates with lower activity due to phosphate pH values ​​exceeding the range of this application, making it difficult to control the ideal particle size distribution.

[0081] Examples 1-3 of this application can precisely control the activity differences and mixing state of amorphous iron phosphate, thereby forming a graded structure containing both 100-150nm small particles and 1-5μm large particles. The lithium iron phosphate composite material prepared from this iron phosphate has excellent electrochemical performance and also has a high lithium iron phosphate compaction density.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0083] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, in the foregoing claims, any of the claimed embodiments can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for preparing ferric phosphate, characterized in that, include: Prepare ferrous solution; A first phosphate salt solution and a second phosphate salt solution are prepared using a phosphorus-containing compound and a pH adjuster, wherein the phosphorus-containing compound includes at least one of ammonium monohydrogen phosphate and ammonium dihydrogen phosphate; the pH of the first phosphate salt solution is 3.0-4.0, and the pH of the second phosphate salt solution is 6.0-8.

0. Under stirring, the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide are simultaneously added to the ferrous solution to react and obtain a reaction slurry. The reaction slurry is filtered and washed to obtain a filter cake, which is then pulped to obtain an amorphous ferric phosphate slurry. Inorganic acid is added to the amorphous ferric phosphate slurry, the pH of the slurry is adjusted to 1.0-2.0, and it is aged at a temperature of 80-100℃ to obtain ferric phosphate dihydrate slurry; The ferric phosphate dihydrate slurry is filtered, washed, dried, and calcined to obtain ferric phosphate.

2. The method for preparing ferric phosphate according to claim 1, characterized in that, The ferrous solution includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride solutions; And / or, the concentration of the ferrous solution is 0.4-2.0 mol / L; And / or, the pH of the ferrous solution is 2.0-3.0; And / or, the pH adjuster includes at least one of sulfuric acid solution, sodium hydroxide solution, and ammonia solution; And / or, the concentration of the first phosphate salt solution is 0.4-2.0 mol / L; And / or, the concentration of the second phosphate salt solution is 0.4-2.0 mol / L; And / or, the molar ratio of phosphorus in the first phosphate salt solution to that in the second phosphate salt solution is 0.1:0.9 to 0.9:0.

1.

3. The method for preparing ferric phosphate according to claim 2, characterized in that, The ratio of the molar amount of iron in the ferrous solution to the total molar amount of phosphorus in the first phosphate solution and the second phosphate solution is 1:0.3 to 1:

3.

4. The method for preparing ferric phosphate according to claim 2, characterized in that, The molar ratio of iron in the ferrous solution to the molar ratio of hydrogen peroxide is 1:0.6 to 1:

1.

5. The method for preparing ferric phosphate according to claim 1, characterized in that, The feeding time for the first phosphate salt solution, the second phosphate salt solution, and hydrogen peroxide is 30-120 min.

6. The method for preparing ferric phosphate according to claim 1, characterized in that, The method further includes: filtering and washing the reaction slurry until the conductivity of the wash water is ≤5000μs / cm, then stopping the washing to obtain a filter cake; And / or, the solid content of the amorphous ferric phosphate slurry is 5%~15%.

7. The method for preparing ferric phosphate according to claim 1, characterized in that, The aging time is 60-120 minutes; And / or, the inorganic acid includes at least one of sulfuric acid, hydrochloric acid, and nitric acid.

8. The method for preparing ferric phosphate according to claim 1, characterized in that, After filtering the ferric phosphate dihydrate slurry, the method further includes washing the filter residue obtained from the filtration until the conductivity of the wash water is ≤600μs / cm. And / or, the drying temperature is 120~200℃, and the drying time is 10-15h; And / or, the calcination temperature is 650-710℃, and the calcination time is 1-2 hours.

9. A type of iron phosphate, characterized in that, The iron phosphate is prepared by the preparation method according to any one of claims 1-8; The iron phosphate comprises a first particle and a second particle, wherein the size of the first particle is 100-150 nm and the size of the second particle is 1-5 μm.

10. A lithium iron phosphate composite cathode material, characterized in that, The preparation method of the lithium iron phosphate composite cathode material includes: calcining iron phosphate, lithium source, and organic carbon source to obtain lithium iron phosphate composite cathode material; The iron phosphate is the iron phosphate prepared by the preparation method according to any one of claims 1-8 or the iron phosphate according to claim 9.

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