Anhydrous ferric phosphate and its synthesis method and application

Abstract

The application discloses anhydrous iron phosphate and a synthesis method and application thereof, and belongs to the technical field of battery materials. The synthesis of the anhydrous iron phosphate comprises the following steps: mixing iron phosphate dihydrate and iron oxide to obtain a mixture; dividing the mixture into n sintering-to-be-performed parts, and sintering the n sintering-to-be-performed parts at n temperatures respectively to obtain sintered products with different primary particle sizes; and mixing the sintered products with different primary particle sizes to obtain the anhydrous iron phosphate; wherein n is greater than or equal to 2. The method can synthesize anhydrous iron phosphate particles with grading effect by sintering the mixture at different temperatures to obtain sintered products with different primary particle sizes, and can reduce the sintering temperature of lithium iron phosphate while making the lithium iron phosphate have a higher compaction density and better electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and more specifically, to anhydrous iron phosphate, its synthesis method, and its application. Background Technology

[0002] With technological advancements and economic development, lithium-ion batteries are becoming increasingly widely used. Lithium-ion batteries made from lithium iron phosphate (LFP) materials possess extremely high safety, long cycle life, high reliability, and low cost, making them the mainstream for automotive power batteries and energy storage batteries in my country. Currently, commercially available LFP materials generally have low compaction density, directly resulting in lower energy density in the prepared LFP batteries. Therefore, the compaction density of LFP restricts its further development in the power and energy storage battery fields. Iron phosphate, as a key precursor to LFP, directly affects its performance; however, existing LFP technologies often struggle to achieve a balance between optimal compaction density and electrochemical performance.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to provide anhydrous ferric phosphate, its synthesis method, and its application, so as to solve or improve the above-mentioned technical problems.

[0005] This invention can be implemented as follows: In a first aspect, the present invention provides a method for synthesizing anhydrous ferric phosphate, comprising the following steps: mixing ferric phosphate dihydrate with iron oxide to obtain a mixture; dividing the mixture into n parts to be sintered, and sintering the n parts to be sintered at n temperatures respectively to obtain sintered products with different primary particle sizes; mixing the sintered products with different primary particle sizes to obtain anhydrous ferric phosphate; wherein, n≥2.

[0006] In an optional embodiment, a portion of the mixture is sintered at temperature T1 to obtain a first sintered product; the remaining mixture is sintered at temperature T2 to obtain a second sintered product; the first sintered product and the second sintered product are mixed to obtain anhydrous ferric phosphate.

[0007] In an optional embodiment, T1 is 650℃~750℃, and the sintering time at T1 is 2h~8h; T2 is 550℃~650℃, and the sintering time at T2 is 2h~4h.

[0008] In an optional embodiment, the mass ratio of the first sintered product to the second sintered product is 5:5 to 2:8.

[0009] In an optional embodiment, the ratio of the total molar amount of Fe in iron oxide and Fe in ferric phosphate dihydrate to the molar amount of P in ferric phosphate dihydrate is 0.985:1 to 1:1. And / or, the D of ferric phosphate dihydrate 50 ≤4.2μm.

[0010] In an optional implementation, the purity of the iron oxide is >99.5%.

[0011] In an optional embodiment, the D of iron oxide 50 ≤0.2μm.

[0012] In an optional embodiment, the preparation of ferric phosphate dihydrate includes: adding a second solution containing hydrogen peroxide and monoammonium phosphate to a first solution containing ferrous sulfate and phosphoric acid, followed by adding a third solution containing hydrogen peroxide, aging at a constant temperature, and separating the solid and liquid to obtain ferric phosphate dihydrate.

[0013] In an optional embodiment, the total amount of hydrogen peroxide in the second solution and the third solution is 1.05 to 1.15 times the theoretical amount of hydrogen peroxide used; wherein, the theoretical amount of hydrogen peroxide used is the amount of Fe in ferrous sulfate. 2+ Completely oxidized to Fe 3+ The required dosage; the mass ratio of hydrogen peroxide in the second solution to that in the third solution is 30:70 to 50:50.

[0014] In an optional embodiment, the third solution further contains monoammonium phosphate, and the total phosphorus (P) content of the monoammonium phosphate in the second and third solutions is 1.02 to 1.05 times the theoretical P content, and the P content of the monoammonium phosphate in the second solution is not less than 80% of the theoretical P content; wherein, the theoretical P content is calculated by adding Fe... 3+ The amount required for complete reaction to produce ferric phosphate.

[0015] In an optional embodiment, the concentration of phosphoric acid in the first solution is 7 g / L to 15 g / L, and Fe... 2+ The concentration is 40g / L~80g / L.

[0016] In an optional embodiment, the concentration of P element in the second solution is 40 g / L to 80 g / L; and the concentration of P element in the third solution does not exceed 40 g / L.

[0017] In an optional embodiment, the reaction temperature after the addition of the second and third solutions is 80°C to 100°C. And / or, the second solution is added over a period of 1 to 4 hours; And / or, the third solution is added over a period of 0.5 h to 2 h.

[0018] Secondly, the present invention provides anhydrous iron phosphate, which is synthesized by any of the synthesis methods described in the foregoing embodiments.

[0019] In an optional embodiment, anhydrous ferric phosphate has at least one of the following characteristics: Feature 1: D of anhydrous ferric phosphate 50 The range is 2.5μm to 4.0μm; Feature 2: The molar ratio of Fe to P in anhydrous ferric phosphate is 0.985:1 to 1:1.

[0020] Thirdly, the present invention provides a lithium iron phosphate cathode material, which is prepared from a lithium source and the iron phosphate of the aforementioned embodiments.

[0021] In an optional embodiment, the lithium iron phosphate cathode material has at least one of the following characteristics: Feature 3: The compaction density of lithium iron phosphate cathode material at a sintering temperature of 750℃~790℃ is not less than 2.48g / cc; Feature 4: The lithium iron phosphate cathode material has an initial discharge specific capacity of no less than 142 mAh / g at a 1C rate.

[0022] Fourthly, the present invention provides a battery containing the lithium iron phosphate cathode material of the aforementioned embodiments.

[0023] The beneficial effects of this invention include: The method for synthesizing anhydrous iron phosphate provided by this invention involves sintering a mixture of iron phosphate dihydrate and iron oxide. This process avoids the formation of iron pyrophosphate phase and removes oxygen (O) from the iron oxide during sintering, leaving only Fe. This Fe element is retained in the anhydrous iron phosphate, increasing the Fe to P ratio and thus improving the electrochemical performance of the downstream lithium iron phosphate product. Furthermore, by sintering the mixture at different temperatures to obtain sintered products with varying particle sizes, and then milling them, anhydrous iron phosphate particles with a graded distribution are obtained. The graded anhydrous iron phosphate provided by this invention allows for lowering the sintering temperature of lithium iron phosphate while maintaining high compaction density and superior electrochemical performance in the lithium iron phosphate cathode material. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 Here is a SEM image of the ferric phosphate dihydrate prepared in Example 1; Figure 2 Here is a SEM image of the anhydrous ferric phosphate prepared in Example 2; Figure 3 The XRD patterns of anhydrous ferric phosphate prepared in Example 1 and Comparative Example 2 are shown below. Figure 4 for Figure 3 A magnified view of a portion of the image. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0027] The following is a detailed description of the anhydrous ferric phosphate, its synthesis method, and its applications provided by this invention.

[0028] This invention provides a method for synthesizing anhydrous ferric phosphate, comprising the following steps: mixing ferric phosphate dihydrate with iron oxide to obtain a mixture; dividing the mixture into n parts to be sintered, and sintering the n parts at n temperatures respectively to obtain sintered products with different primary particle sizes; mixing the sintered products with different primary particle sizes to obtain anhydrous ferric phosphate; wherein, n≥2.

[0029] The above method involves sintering the mixture at different temperatures to obtain sintered products with varying particle sizes. After milling, anhydrous iron phosphate particles with a graded distribution are obtained. The anhydrous iron phosphate provided by this invention can achieve higher compaction density and better electrochemical performance in lithium iron phosphate cathode materials while lowering the sintering temperature.

[0030] It should be noted that the molar ratio of Fe to P in ferric phosphate dihydrate is typically low. This invention addresses this by mixing ferric phosphate dihydrate with iron oxide and then sintering the mixture. During sintering, the oxygen (O) in the iron oxide is removed, leaving only the Fe from the iron oxide. This Fe can remain in the anhydrous ferric phosphate, thereby increasing the Fe to P ratio and thus improving the electrochemical performance of the downstream lithium iron phosphate product. Furthermore, the addition of a certain amount of iron oxide during sintering effectively prevents the formation of the ferric pyrophosphate phase.

[0031] In some optional embodiments, the ratio of the total molar amount of Fe in iron oxide and Fe in ferric phosphate dihydrate to the molar amount of P in ferric phosphate dihydrate is 0.985:1 to 1:1, such as 0.985:1, 0.990:1, 0.995:1, or 1:1, or other values ​​within the range of 0.985:1 to 1:1, so that the iron-to-phosphorus ratio of the final anhydrous ferric phosphate is 0.985:1 to 1:1. That is, by controlling the blending ratio of iron oxide, the present invention can effectively regulate the iron-to-phosphorus ratio of anhydrous ferric phosphate, thereby facilitating the optimization of the electrochemical performance of lithium iron phosphate batteries.

[0032] In some alternative implementations, the iron oxide has a purity >99.5% to ensure a low impurity content.

[0033] In some alternative implementations, the D of iron oxide 50 ≤0.2μm. If the D of iron oxide 50 If the concentration is too high, it will be difficult for it to react fully with ferric phosphate dihydrate, resulting in unreacted iron oxide being mixed into the anhydrous ferric phosphate and reducing the purity of the anhydrous ferric phosphate.

[0034] In some alternative implementations, the D of ferric phosphate dihydrate 50 With a particle size of ≤4.2μm, ferric phosphate dihydrate can react fully with iron oxide.

[0035] In this invention, the value of n can be 2, or it can be 3, 4, 5 or more, depending on the actual needs. Each part to be sintered corresponds to a sintering temperature, and n parts to be sintered correspond to n sintering temperatures.

[0036] In some alternative implementations, n is 2. That is, a portion of the mixture is sintered at temperature T1 to obtain a first sintered product; the remaining mixture is sintered at temperature T2 to obtain a second sintered product; the first sintered product and the second sintered product are mixed to obtain anhydrous ferric phosphate.

[0037] Here, T1 and T2 are different; T1 can be greater than T2, or T1 can be less than T2.

[0038] The following example uses T1 > T2: T1 can be 650℃~750℃, such as 650℃, 680℃, 700℃, 720℃ or 750℃, or other values ​​within the range of 650℃~750℃.

[0039] T2 can be 550℃~650℃, such as 550℃, 580℃, 600℃, 620℃ or 650℃, or other values ​​within the range of 550℃~650℃.

[0040] The sintering time at T1 can be 2h to 8h, such as 2h, 3h, 4h, 5h, 6h, 7h or 8h, or other values ​​within the range of 2h to 8h.

[0041] The sintering time under T2 can be 2h to 4h, such as 2h, 2.5h, 3h, 3.5h or 4h, or other values ​​within the range of 2h to 4h.

[0042] The mass ratio of the first sintered product to the second sintered product can be from 5:5 to 2:8, such as 5:5, 4:6, 3:7 or 2:8, or other values ​​within the range of 5:5 to 2:8.

[0043] It should be noted that when T1 < T2, the mass ratio of the second sintered product to the first sintered product is 5:5 to 2:8.

[0044] In other words, a higher sintering temperature results in a larger primary particle size of the sintered product. In this application, when mixing sintered products with different primary particle sizes, the sintered product obtained at a lower sintering temperature is the main component, in order to increase the proportion of small particle sintered products in anhydrous iron phosphate.

[0045] In this invention, the preparation of ferric phosphate dihydrate may include: adding a second solution containing hydrogen peroxide and monoammonium phosphate to a first solution containing ferrous sulfate and phosphoric acid, followed by adding a third solution containing hydrogen peroxide, aging at a constant temperature, and separating the solid and liquid to obtain ferric phosphate dihydrate.

[0046] The third solution mentioned above may also contain monoammonium phosphate.

[0047] During operation, the first solution can be added to the reactor as the base liquid. Then, the reactor temperature is controlled, and the second solution is added dropwise, with the addition time of the second solution controlled. After the second solution is added, the third solution is added dropwise, with the addition time controlled. After the third solution is completely added, the mixture is kept at a constant temperature for aging, then discharged, washed, and filtered to obtain ferric phosphate dihydrate.

[0048] The total amount of hydrogen peroxide in the second and third solutions is 1.05 to 1.15 times the theoretical amount of hydrogen peroxide, such as 1.05, 1.08, 1.1, 1.12, or 1.15 times, or other values ​​within the range of 1.05 to 1.15 times. The aforementioned "theoretical amount of hydrogen peroxide" refers to the amount of Fe in ferrous sulfate. 2+ Completely oxidized to Fe 3+ The required dosage.

[0049] The total phosphorus (P) content of monoammonium phosphate in the second and third solutions is 1.02 to 1.05 times the theoretical P content, such as 1.02, 1.03, 1.04, or 1.05 times, or other values ​​within the range of 1.02 to 1.05. The aforementioned "theoretical P content" refers to the amount of Fe... 3+ The amount of phosphorus required for complete reaction to produce ferric phosphate. Specifically, the amount of phosphorus (P) in the second solution, specifically monoammonium phosphate, shall not be less than 80% of the theoretical amount of phosphorus.

[0050] In some optional embodiments, the mass ratio of hydrogen peroxide in the second solution to that in the third solution can be from 30:70 to 50:50, such as 30:70, 35:65, 40:60, 45:55, or 50:50, or other values ​​within the range of 30:70 to 50:50. That is, the amount of hydrogen peroxide used in the second solution accounts for 30% to 50% of the theoretical amount of hydrogen peroxide, and the amount of hydrogen peroxide used in the third solution accounts for 50% to 70% of the theoretical amount of hydrogen peroxide.

[0051] In some optional embodiments, the mass ratio of monoammonium phosphate in the second solution to that in the third solution can be from 80:20 to 100:0, such as 80:20, 85:15, 90:10, 95:5, or 100:0, or other values ​​within the range of 80:20 to 100:0. That is, the amount of phosphorus (P) in the monoammonium phosphate in the second solution accounts for 80% to 100% of the theoretical amount of P, and the amount of monoammonium phosphate in the third solution accounts for 0% to 20% of the theoretical amount of P.

[0052] In some alternative embodiments, the concentration of phosphoric acid in the first solution can be 7 g / L to 15 g / L, such as 7 g / L, 8 g / L, 9 g / L, 10 g / L, 11 g / L, 12 g / L, 13 g / L, 14 g / L, or 15 g / L, or other values ​​within the range of 7 g / L to 15 g / L. 2+ The concentration can be 40g / L to 80g / L, such as 40g / L, 50g / L, 60g / L, 70g / L or 80g / L, or other values ​​within the range of 40g / L to 80g / L.

[0053] In some optional embodiments, the concentration of phosphorus in the second solution can be between 40 g / L and 80 g / L, such as 40 g / L, 50 g / L, 60 g / L, 70 g / L, or 80 g / L, or other values ​​within the range of 40 g / L to 80 g / L. In the third solution, the concentration of phosphorus does not exceed 40 g / L, for example, it can be 40 g / L, 30 g / L, 20 g / L, 10 g / L, or 0 g / L, or other values ​​within the range not exceeding 40 g / L.

[0054] In some optional embodiments, the reaction temperature after the addition of the second and third solutions is 80°C to 100°C, such as 80°C, 85°C, 90°C, 95°C, or 100°C, or other values ​​within the range of 80°C to 100°C. If the reaction temperature is below 80°C, the reaction between the first, second, and third solutions is difficult to proceed effectively, making it difficult to obtain ferric phosphate dihydrate precipitate.

[0055] In some alternative implementations, the dripping time of the second solution can be 1h to 4h, such as 1h, 2h, 3h or 4h, or other values ​​within the range of 1h to 4h.

[0056] In some alternative implementations, the dripping time of the third solution can be 0.5h to 2h, such as 0.5h, 1h, 1.5h or 2h, or other values ​​within the range of 0.5h to 2h.

[0057] The dropping time of the above solution corresponds to the dropping rate of the solution. If the dropping time is too short, it means that the dropping rate is too fast, and the precipitate that is formed is prone to reverse dissolution, resulting in less iron phosphate dihydrate. If the dropping time is too long, it means that the dropping rate is too slow and the efficiency is low.

[0058] In the preparation process of the above-mentioned ferric phosphate dihydrate, a second solution containing hydrogen peroxide and monoammonium phosphate is added to a first solution containing ferrous sulfate and phosphoric acid. In this system, PO4... 3- Compared to Fe 3+ It is in excess. This process involves reducing the Fe content in ferrous sulfate. 2+ Oxidized to Fe 3+ Meanwhile, PO4 3- with Fe 3+ The reaction produces ferric phosphate dihydrate. After the second solution is added, a third solution containing at least hydrogen peroxide (and possibly monoammonium phosphate) is added to neutralize the excess PO4. 3- The reaction ensures that there is no excessive Fe in the system. 3+ PO4 3- Residue. Continuing from the above, this invention, by adjusting the mixing ratio of hydrogen peroxide and monoammonium phosphate during the synthesis process and controlling the supersaturation in the reactor, can effectively adjust the morphology and particle size of the synthesized ferric phosphate dihydrate, obtaining blocky particles and D... 50 Ferric phosphate dihydrate with a particle size ≤4μm.

[0059] Accordingly, the present invention also provides anhydrous iron phosphate, which is synthesized by the above-described synthesis method.

[0060] In some alternative embodiments, the single crystal particle size of anhydrous ferric phosphate is 0.2 μm to 1.5 μm.

[0061] In some alternative implementations, anhydrous ferric phosphate D 50 The range is 2.5μm to 4.0μm.

[0062] In some alternative embodiments, the molar ratio of Fe to P in anhydrous ferric phosphate is from 0.985:1 to 1:1.

[0063] In addition, the present invention also provides a lithium iron phosphate cathode material, which is prepared from a lithium source and the aforementioned iron phosphate.

[0064] By using the above-mentioned anhydrous iron phosphate to prepare lithium iron phosphate cathode materials, it is possible to reduce the sintering temperature of lithium iron phosphate while enabling the lithium iron phosphate cathode materials to have higher compaction density and better electrochemical performance.

[0065] In some alternative embodiments, the compaction density of the lithium iron phosphate cathode material at a sintering temperature of 750°C to 790°C is not less than 2.48 g / cc.

[0066] In some optional implementations, the lithium iron phosphate cathode material has an initial discharge specific capacity of not less than 142 mAh / g at a 1C rate.

[0067] In addition, the present invention also provides a battery containing the above-mentioned lithium iron phosphate cathode material, which has better electrochemical performance.

[0068] The features and performance of the present invention will be further described in detail below with reference to the embodiments. The morphology of the materials described below was observed by scanning electron microscopy (SEM), and the composition was determined by XRD.

[0069] Example 1 This embodiment provides anhydrous ferric phosphate, the preparation method of which includes: S1: Preparation of raw and auxiliary materials.

[0070] The first solution is prepared by mixing FeSO4·7H2O, phosphoric acid (with a concentration of 85%), and pure water; in this first solution, Fe... 2+ The concentration is 60 g / L, and the phosphoric acid concentration is 12 g / L.

[0071] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 60 g / L.

[0072] The third solution: hydrogen peroxide (the concentration of hydrogen peroxide itself is 27.5%).

[0073] The total amount of hydrogen peroxide in the second solution and the third solution is 1.1 times the theoretical amount of hydrogen peroxide, and the mass ratio of hydrogen peroxide in the second solution to that in the third solution is 40:60; the amount of phosphorus in the monoammonium phosphate in the second solution is 1.03 times the theoretical amount of phosphorus.

[0074] S2: Precipitation reaction.

[0075] The first solution was added to the reactor as the base liquid, and the temperature was raised to 80°C. Under constant temperature and stirring conditions, the second solution was added dropwise at a uniform rate over 2.5 hours. After the addition was complete, the third solution was added dropwise at a uniform rate over 1 hour. After the third solution was completely added, the reactor was aged at 85°C for another hour. The product was then discharged, washed, and filtered to obtain lumpy, agglomerated ferric phosphate dihydrate (its SEM image is shown below). Figure 1 As shown, D 50 (3.5μm).

[0076] S3: Sintering.

[0077] The obtained ferric phosphate dihydrate was reacted with D 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, with the theoretical molar ratio of Fe to P in the mixture controlled at 0.990:1. The mixture was divided into two parts: one part was sintered at 700℃ for 5 h to obtain the first sintered product (high-temperature material); the other part was sintered at 600℃ for 3 h to obtain the second sintered product (low-temperature material). The first and second sintered products were mechanically blended at a mass ratio of 4:6, followed by crushing and sieving (300 mesh) to obtain the final anhydrous iron phosphate product (XRD pattern as shown). Figure 3 and Figure 4 (As shown).

[0078] Example 2 This embodiment provides anhydrous ferric phosphate, the preparation method of which includes: S1: Preparation of raw and auxiliary materials.

[0079] The first solution is prepared by mixing FeSO4·7H2O, phosphoric acid (with a concentration of 85%), and pure water; in this first solution, Fe... 2+ The concentration is 70 g / L, and the phosphoric acid concentration is 15 g / L.

[0080] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 70 g / L.

[0081] The third solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this third solution is 10 g / L.

[0082] The total amount of hydrogen peroxide in the second solution and the third solution is 1.05 times the theoretical amount of hydrogen peroxide, and the mass ratio of hydrogen peroxide in the second solution to the third solution is 45:55. The total amount of phosphorus (P) in the second solution and the third solution is 1.02 times the theoretical amount of P, and the mass ratio of monoammonium phosphate in the second solution to the third solution is 90:10.

[0083] S2: Precipitation reaction.

[0084] The first solution was added to the reactor as the base liquid, and the temperature was raised to 95°C. Under constant temperature and stirring conditions, the second solution was added dropwise at a uniform rate over 3 hours. After the addition was complete, the third solution was added dropwise at a uniform rate over 0.5 hours. After the third solution was completely added, the reactor was aged at 95°C for another 0.5 hours. The product was then discharged, washed, and filtered to obtain lumpy, agglomerated ferric phosphate dihydrate (D...). 50 (3.0μm).

[0085] S3: Sintering.

[0086] The obtained ferric phosphate dihydrate was reacted with D 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, with the theoretical molar ratio of Fe to P in the mixture controlled at 0.995:1. The mixture was divided into two parts: one part was sintered at 730℃ for 3 h to obtain the first sintered product (high-temperature material); the other part was sintered at 620℃ for 2 h to obtain the second sintered product (low-temperature material). The first and second sintered products were mechanically blended at a mass ratio of 3:7, followed by crushing and sieving (300 mesh) to obtain the final anhydrous iron phosphate product (its SEM image is shown below). Figure 2 (As shown).

[0087] Example 3 This embodiment provides anhydrous ferric phosphate, the preparation method of which includes: S1: Preparation of raw and auxiliary materials.

[0088] The first solution is prepared by mixing FeSO4·7H2O, phosphoric acid (with a concentration of 85%), and pure water; in this first solution, Fe... 2+ The concentration is 45 g / L, and the phosphoric acid concentration is 10 g / L.

[0089] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 45 g / L.

[0090] The third solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this third solution is 15 g / L.

[0091] The total amount of hydrogen peroxide in the second solution and the third solution is 1.15 times the theoretical amount of hydrogen peroxide, and the mass ratio of hydrogen peroxide in the second solution to the third solution is 30:70. The total amount of phosphorus (P) in the monoammonium phosphate (MAP) in the second solution and the third solution is 1.05 times the theoretical amount of P, and the mass ratio of MAP in the second solution to the third solution is 80:20.

[0092] S2: Precipitation reaction.

[0093] The first solution was added to the reactor as the base liquid, and the temperature was raised to 88°C. Under constant temperature and stirring conditions, the second solution was added dropwise at a uniform rate over 1.5 hours. After the addition was complete, the third solution was added dropwise at a uniform rate over 2 hours. After the third solution was completely added, the reactor was aged at 88°C for another 2 hours. The product was then discharged, washed, and filtered to obtain lumpy, agglomerated ferric phosphate dihydrate (D...). 50 (3.9μm).

[0094] S3: Sintering.

[0095] The obtained ferric phosphate dihydrate was reacted with D 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, with the molar ratio of Fe to P in the theoretical mixture controlled at 0.985:1. The mixture was divided into two parts: one part was sintered at 660℃ for 8 hours to obtain the first sintered product (high-temperature material); the other part was sintered at 560℃ for 4 hours to obtain the second sintered product (low-temperature material). The first and second sintered products were mechanically blended at a mass ratio of 5:5, followed by crushing and sieving (300 mesh) to obtain the final anhydrous iron phosphate product.

[0096] Example 4 This embodiment provides anhydrous ferric phosphate, the preparation method of which includes: S1: Preparation of raw and auxiliary materials.

[0097] The first solution is prepared by mixing FeSO4·7H2O, phosphoric acid (with a concentration of 85%), and pure water; in this first solution, Fe... 2+ The concentration is 40 g / L, and the phosphoric acid concentration is 7 g / L.

[0098] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 40 g / L.

[0099] The third solution: hydrogen peroxide (the concentration of hydrogen peroxide itself is 27.5%).

[0100] The total amount of hydrogen peroxide in the second solution and the third solution is 1.12 times the theoretical amount of hydrogen peroxide, and the mass ratio of hydrogen peroxide in the second solution to the third solution is 30:70; the amount of phosphorus in the monoammonium phosphate in the second solution is 1.02 times the theoretical amount of phosphorus.

[0101] S2: Precipitation reaction.

[0102] The first solution was added to the reactor as the base liquid, and the temperature was raised to 90°C. Under constant temperature and stirring conditions, the second solution was added dropwise at a uniform rate over 4 hours. After the addition was complete, the third solution was added dropwise at a uniform rate over 2 hours. After the third solution was completely added, the reactor was aged at 90°C for another 2 hours. The product was then discharged, washed, and filtered to obtain lumpy, agglomerated ferric phosphate dihydrate (D...). 50 (4.2 μm).

[0103] S3: Sintering.

[0104] The obtained ferric phosphate dihydrate was reacted with D 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, with the theoretical molar ratio of Fe to P in the mixture controlled at 0.985:1. The mixture was divided into two parts: one part was sintered at 660℃ for 8 hours to obtain the first sintered product (high-temperature material); the other part was sintered at 550℃ for 4 hours to obtain the second sintered product (low-temperature material). The first and second sintered products were mechanically blended at a mass ratio of 5:5, followed by crushing and sieving (300 mesh) to obtain the final anhydrous iron phosphate product.

[0105] Example 5 This embodiment provides anhydrous ferric phosphate, the preparation method of which includes: S1: Preparation of raw and auxiliary materials.

[0106] The first solution is prepared by mixing FeSO4·7H2O, phosphoric acid (with a concentration of 85%), and pure water; in this first solution, Fe... 2+ The concentration is 80 g / L, and the phosphoric acid concentration is 15 g / L.

[0107] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 80 g / L.

[0108] The third solution: hydrogen peroxide (the concentration of hydrogen peroxide itself is 27.5%).

[0109] The total amount of hydrogen peroxide in the second solution and the third solution is 1.12 times the theoretical amount of hydrogen peroxide, and the mass ratio of hydrogen peroxide in the second solution to that in the third solution is 50:50; the amount of phosphorus in the monoammonium phosphate in the second solution is 1.02 times the theoretical amount of phosphorus.

[0110] S2: Precipitation reaction.

[0111] The first solution was added to the reactor as the base liquid, and the temperature was raised to 90°C. Under constant temperature and stirring conditions, the second solution was added dropwise at a uniform rate over 1 hour. After the addition was complete, the third solution was added dropwise at a uniform rate over 0.5 hours. After the third solution was completely added, the reactor was aged at 90°C for another 2 hours. The product was then discharged, washed, and filtered to obtain lumpy, agglomerated ferric phosphate dihydrate (D...). 50 (2.6μm).

[0112] S3: Sintering.

[0113] The obtained ferric phosphate dihydrate was reacted with D 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, with the molar ratio of Fe to P in the theoretical mixture controlled at 1:1. The mixture was divided into two parts: one part was sintered at 750℃ for 2 hours to obtain the first sintered product (high-temperature material); the other part was sintered at 650℃ for 2 hours to obtain the second sintered product (low-temperature material). The first and second sintered products were mechanically blended at a mass ratio of 5:5, followed by crushing and sieving (300 mesh) to obtain the final anhydrous iron phosphate product.

[0114] Example 6 This embodiment provides anhydrous ferric phosphate, the preparation method of which includes: S1: Preparation of raw and auxiliary materials.

[0115] The first solution is prepared by mixing FeSO4·7H2O, phosphoric acid (with a concentration of 85%), and pure water; in this first solution, Fe... 2+ The concentration is 65 g / L, and the phosphoric acid concentration is 12 g / L.

[0116] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 60 g / L.

[0117] The third solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this third solution is 40 g / L.

[0118] The total amount of hydrogen peroxide in the second solution and the third solution is 1.12 times the theoretical amount of hydrogen peroxide, and the mass ratio of hydrogen peroxide in the second solution to the third solution is 40:60. The total amount of phosphorus (P) in the monoammonium phosphate (MAP) in the second solution and the third solution is 1.05 times the theoretical amount of P, and the mass ratio of MAP in the second solution to the third solution is 90:10.

[0119] S2: Precipitation reaction.

[0120] The first solution was added to the reactor as the base liquid, and the temperature was raised to 90°C. Under constant temperature and stirring conditions, the second solution was added dropwise at a uniform rate over 2 hours. After the addition was complete, the third solution was added dropwise at a uniform rate over 1 hour. After the third solution was completely added, the reactor was aged at 90°C for another 2 hours. The product was then discharged, washed, and filtered to obtain lumpy, agglomerated ferric phosphate dihydrate (D...). 50 (3.3μm).

[0121] S3: Sintering.

[0122] The obtained ferric phosphate dihydrate was reacted with D 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, with the theoretical molar ratio of Fe to P in the mixture controlled at 0.992:1. The mixture was divided into two parts: one part was sintered at 650℃ for 4 h to obtain the first sintered product (high-temperature material); the other part was sintered at 580℃ for 3 h to obtain the second sintered product (low-temperature material). The first and second sintered products were mechanically blended at a mass ratio of 4:6, followed by crushing and sieving (300 mesh) to obtain the final anhydrous iron phosphate product.

[0123] Comparative Example 1 This comparative example provides anhydrous ferric phosphate, which is prepared by the following method: S1: Preparation of raw and auxiliary materials.

[0124] The first solution is obtained by mixing FeSO4·7H2O and pure water. In this first solution, Fe... 2+ The concentration is 70 g / L.

[0125] The second solution is prepared by mixing hydrogen peroxide (with a concentration of 27.5%), monoammonium phosphate, and pure water; the concentration of phosphorus in this second solution is 70 g / L.

[0126] The third solution: ammonia water (concentration of 25%).

[0127] S2: Precipitation reaction.

[0128] The first solution was added to the reactor as the base liquid, and the temperature was raised to 50°C. Under constant temperature and stirring conditions, the second and third solutions were added dropwise at a uniform rate over 2 hours. After the addition was complete, the mixture was aged at 50°C for 2 hours. Subsequently, the mixture was discharged, washed, and filtered to obtain spherical small-particle agglomerates of ferric phosphate dihydrate (D...). 50 (7.6μm).

[0129] S3: Sintering.

[0130] The obtained ferric phosphate dihydrate was sintered at 580℃ for 2 hours, and then crushed and sieved (300 mesh) to obtain the final anhydrous ferric phosphate product.

[0131] The anhydrous ferric phosphate obtained in this comparative example is a small particle agglomerate, not a blocky material, and has a low molar ratio of Fe to P.

[0132] Comparative Example 2 The difference between this comparative example and Example 1 is that iron oxide powder was not mixed in S3. That is, the obtained ferric phosphate dihydrate was divided into two parts. One part was sintered at 700°C for 5 hours to obtain the first sintered product (high-temperature material); the other part was sintered at 600°C for 3 hours to obtain the second sintered product (low-temperature material). The first sintered product and the second sintered product were mechanically mixed at a mass ratio of 4:6, and then crushed and sieved (300 mesh) to obtain the final anhydrous ferric phosphate product.

[0133] The anhydrous ferric phosphate product obtained in this comparative example has an impurity phase of ferric pyrophosphate (XRD pattern as shown). Figure 3 and Figure 4 (As shown).

[0134] Comparative Example 3 The difference between this comparative example and Example 1 is that in S3, all the mixtures are sintered at high temperature, that is, the obtained ferric phosphate dihydrate and D are sintered together. 50 0.2 μm iron oxide powder (purity >99.5%) was dry-mixed, and the molar ratio of Fe to P in the theoretical mixture was controlled to be 0.990:1. The mixture was sintered at 700℃ for 5 h, and then crushed and sieved (300 mesh) to obtain the final anhydrous iron phosphate product.

[0135] Comparative Example 4 The difference between this comparative example and Example 1 is that in S1, the first solution is obtained by mixing FeSO4·7H2O and pure water; in this first solution, Fe 2+ The concentration is 70 g / L.

[0136] The anhydrous ferric phosphate obtained in this comparative example contained flaky aggregates.

[0137] Comparative Example 5 The difference between this comparative example and Example 1 is that S1 does not contain a third solution. The amount of hydrogen peroxide in the second solution is 1.1 times the theoretical amount of hydrogen peroxide needed.

[0138] The anhydrous iron phosphate obtained in this comparative example contained plate-like agglomerates, and no single crystal particles were formed after sintering.

[0139] Comparative Example 6 The difference between this comparative example and Example 1 is that in S2, the reaction temperature of the base liquid is 70°C.

[0140] In this comparative example, no precipitate was produced in step S2, indicating that the synthesis of ferric phosphate dihydrate failed.

[0141] Comparative Example 7 The difference between this comparative example and Example 1 is that in S2, the first solution, the second solution, and the third solution are all mixed at once and then heated to 85°C for reaction.

[0142] The comparative example prepared D 50 The agglomerated spheres with a diameter >20μm did not form single crystal particles after sintering.

[0143] Comparative Example 8 The difference between this comparative example and Example 1 is that in S3, the molar ratio of Fe to P in the mixture is 1.02:1.

[0144] The anhydrous ferric phosphate product prepared in this comparative example contained free Fe2O3 impurities. Fe2O3 impurities are magnetic foreign matter, affecting product specifications and thus constituting a substandard product.

[0145] Comparative Example 9 The difference between this comparative example and Example 1 is that in S3, the D of the iron oxide powder... 50 It is 0.4μm.

[0146] The anhydrous ferric phosphate product prepared in this comparative example contained free Fe2O3 impurities. Fe2O3 impurities are magnetic foreign matter, affecting product specifications and thus constituting a substandard product.

[0147] Comparative Example 10 The difference between this comparative example and Example 1 is that in S3, the purity of the iron oxide powder is 98%.

[0148] The anhydrous ferric phosphate product prepared in this comparative example had impurities exceeding the specifications and was therefore an unqualified product.

[0149] Comparative Example 11 The difference between this comparative example and Example 1 is that in S3, the ratio of the first sintered product to the second sintered product is 1:9.

[0150] Comparative Example 12 The difference between this comparative example and Example 1 is that in S3, the ratio of the first sintered product to the second sintered product is 7:3.

[0151] Experimental Example 1 The performance of anhydrous ferric phosphate prepared in Examples 1-6 and Comparative Examples 1-12 was compared, and the results are shown in Table 1.

[0152] A portion of the anhydrous ferric phosphate product was dissolved in dilute sulfuric acid to obtain anhydrous ferric phosphate solution. The iron content in the anhydrous ferric phosphate solution was then determined by potassium dichromate titration. The phosphorus content in the anhydrous ferric phosphate product was determined by quinomolybdate gravimetric method. The molar ratio of Fe to P was calculated from the iron and phosphorus contents.

[0153] The particle size distribution (D) was tested using a laser particle size analyzer. 50 Particle size.

[0154] Table 1 Comparison of properties of anhydrous ferric phosphate

[0155] Experimental Example 2 The anhydrous iron phosphate prepared in Examples 1-6 and Comparative Examples 1-12 were respectively prepared into lithium iron phosphate cathode materials according to the following methods. Then, the compaction density of each lithium iron phosphate cathode material after sintering at 760℃ was tested using a powder compaction density meter. The results are shown in Table 2.

[0156] The preparation method and conditions of lithium iron phosphate cathode material are as follows: anhydrous iron phosphate, lithium carbonate, glucose, superconducting carbon black, and PEG1500 are mixed in a mass ratio of 10:2.52:0.6:0.1:0.6, and finally 25.6 kg of water is added. After the mixture is uniformly mixed, it is synthesized by spraying, sintering, air-flow powdering and other processes to obtain lithium iron phosphate cathode material.

[0157] Since the anhydrous iron phosphate prepared in Comparative Examples 6-10 was a substandard product, only the lithium iron phosphate cathode materials prepared in Examples 1-6, Comparative Examples 1-5, and Comparative Examples 11-12 were assembled into coin cells, and the first discharge specific capacity of each coin cell at a 1C rate (test voltage was 2.0V) was tested. The results are shown in Table 2.

[0158] Table 2 Comparison of performance of lithium iron phosphate cathode materials

[0159] As can be seen from Tables 1 and 2: The schemes provided in Examples 1 to 6 can all stably prepare anhydrous iron phosphate with suitable particle size, ideal morphology, high iron-to-phosphorus ratio and no impurity phase. When preparing lithium iron phosphate cathode material with this anhydrous iron phosphate, the resulting lithium iron phosphate cathode material can obtain a high compaction density under low temperature sintering conditions, which is beneficial to the battery obtained by further preparation having good electrochemical performance.

[0160] As can be seen from Comparative Examples 1-12, compared to the solution provided by this invention, changes in the synthesis method or conditions of anhydrous iron phosphate lead to a deterioration in product performance. Specifically, Comparative Example 11 used less of the first sintering product, resulting in insufficient large particles to form the framework and preventing small particles from growing, ultimately reducing the compaction density of the lithium iron phosphate cathode material to 2.41 g / cc. Comparative Example 12 used too much of the first sintering product, leading to an excessive number of large particles and insufficient filling of small particles, resulting in extremely poor battery electrochemical performance, with an initial discharge specific capacity of only 130 mAh / g.

[0161] In summary, the solution of the present invention has at least the following advantages compared with the prior art: (1) Compared with the traditional ammonium process, the synthesis process provided by the present invention does not require the addition of alkaline substances to adjust the pH of the system, which simplifies the process flow, reduces raw material costs and operational complexity; it does not require a two-stage aging and whitening treatment, which shortens the production cycle and improves production efficiency. (2) By controlling the mixing ratio and dropping method of hydrogen peroxide and monoammonium phosphate, ferric phosphate dihydrate with a blocky aggregate structure was successfully prepared. The D of this ferric phosphate dihydrate was... 50 ≤4μm, uniform particle size, and controllable morphology; (3) A certain amount of iron oxide is mixed in during sintering, which effectively avoids the formation of iron pyrophosphate phase and improves the iron-to-phosphorus ratio of the product, ultimately improving the electrochemical performance of lithium iron phosphate. (4) By combining high-temperature sintering and low-temperature sintering, small-particle iron phosphate with a single crystal particle size of 0.2~1.5μm can be obtained, and the material can be graded and mixed, thereby enabling lithium iron phosphate to achieve a high compaction density at a lower sintering temperature, and also effectively avoiding the appearance of iron phosphate impurities in the material. (5) The lithium iron phosphate material prepared by the present invention has significantly improved energy density and rate performance while maintaining high safety and long cycle life, thus enhancing the market competitiveness of the product in the field of power battery and energy storage.

[0162] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for synthesizing anhydrous ferric phosphate, characterized in that, Includes the following steps: Ferric phosphate dihydrate is mixed with iron oxide to obtain a mixture; the mixture is divided into n parts to be sintered, and the n parts to be sintered are sintered at n temperatures respectively to obtain sintered products with different primary particle sizes; the sintered products with different primary particle sizes are mixed to obtain anhydrous ferric phosphate; wherein, n≥2; A portion of the mixture is sintered at temperature T1 to obtain a first sintered product; the remaining mixture is sintered at temperature T2 to obtain a second sintered product; the first sintered product and the second sintered product are mixed to obtain anhydrous ferric phosphate. The ratio of the total molar amount of Fe in the iron oxide and Fe in ferric phosphate dihydrate to the molar amount of P in the ferric phosphate dihydrate is 0.985:1 to 1:

1. The preparation of the ferric phosphate dihydrate includes: adding a second solution containing hydrogen peroxide and monoammonium phosphate to a first solution containing ferrous sulfate and phosphoric acid, followed by adding a third solution containing hydrogen peroxide, aging at a constant temperature, and separating the solid and liquid to obtain the ferric phosphate dihydrate.

2. The synthesis method according to claim 1, characterized in that, T1 is 650℃~750℃, and the sintering time under T1 is 2h~8h; T2 is 550℃~650℃, and the sintering time under T2 is 2h~4h.

3. The synthesis method according to claim 1, characterized in that, The mass ratio of the first sintered product to the second sintered product is 5:5 to 2:

8.

4. The synthesis method according to claim 1, characterized in that, The D of the ferric phosphate dihydrate 50 ≤4.2μm.

5. The synthesis method according to claim 1, characterized in that, The purity of the iron oxide is >99.5%.

6. The synthesis method according to claim 1, characterized in that, The iron oxide D 50 ≤0.2μm.

7. The synthesis method according to claim 1, characterized in that, The total amount of hydrogen peroxide in the second solution and the third solution is 1.05 to 1.15 times the theoretical amount of hydrogen peroxide required; wherein, the theoretical amount of hydrogen peroxide is the amount of Fe in the ferrous sulfate. 2+ Completely oxidized to Fe 3+ The required amount; the mass ratio of hydrogen peroxide in the second solution to that in the third solution is 30:70 to 50:

50.

8. The synthesis method according to claim 1, characterized in that, The third solution also contains monoammonium phosphate. The total phosphorus (P) content of the monoammonium phosphate in the second and third solutions is 1.02 to 1.05 times the theoretical P content, and the P content of the monoammonium phosphate in the second solution is not less than 80% of the theoretical P content. The theoretical P content is calculated by adding Fe... 3+ The amount required for complete reaction to produce ferric phosphate.

9. The synthesis method according to claim 1, characterized in that, In the first solution, the concentration of phosphoric acid is 7 g / L to 15 g / L, and Fe... 2+ The concentration is 40g / L~80g / L.

10. The synthesis method according to claim 1, characterized in that, In the second solution, the concentration of phosphorus (P) is 40 g / L to 80 g / L; in the third solution, the concentration of P does not exceed 40 g / L.

11. The synthesis method according to claim 1, characterized in that, The reaction temperature after the addition of the second solution and the third solution is 80℃~100℃; And / or, the second solution is added dropwise over a period of 1 to 4 hours; And / or, the third solution is added over a period of 0.5 h to 2 h.

12. An anhydrous ferric phosphate, characterized in that, It is synthesized by the synthesis method described in any one of claims 1 to 11.

13. The anhydrous ferric phosphate according to claim 12, characterized in that, The anhydrous ferric phosphate D 50 The range is 2.5μm to 4.0μm.

14. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared from a lithium source and anhydrous iron phosphate as described in claim 12 or 13.

15. The lithium iron phosphate cathode material according to claim 14, characterized in that, The lithium iron phosphate cathode material has at least one of the following characteristics: Feature 1: The compaction density of the lithium iron phosphate cathode material at a sintering temperature of 750℃~790℃ is not less than 2.48g / cc; Feature 2: The lithium iron phosphate cathode material has an initial discharge specific capacity of not less than 142 mAh / g at a 1C rate.

16. A battery, characterized in that, The battery contains the lithium iron phosphate cathode material as described in claim 14 or 15.

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