Preparation method of spherical iron phosphate for high-compaction lithium iron phosphate

Abstract

The invention relates to a preparation method of spherical ferric phosphate for high-compaction lithium iron phosphate, which comprises the following steps: S1, adding a complexing agent into a trivalent ferric salt aqueous solution in stirring to obtain a ferric salt complexing solution, the trivalent ferric salt aqueous solution comprising a surfactant A; s2, adding the ferric salt complexing solution and a phosphorus source into an aqueous solution containing a surfactant B while stirring to obtain a mixed solution; s3, the mixed solution reacts and is aged under the heating condition, solid-liquid separation is conducted after aging is finished, solids obtained through separation are sintered, and the spherical iron phosphate is obtained. The iron phosphate prepared by the preparation method disclosed by the invention has relatively good spherical morphology, and the high-compaction lithium iron phosphate prepared by using the iron phosphate has excellent application performance.

Description

Technical Field

[0001] This invention relates to the field of new energy materials technology, and in particular to a method for preparing spherical iron phosphate for high-pressure lithium iron phosphate. Background Technology

[0002] Iron phosphate (FePO4) is a core precursor for the preparation of lithium iron phosphate (LFP) cathode materials. Existing research has shown that the morphology and sulfur content of FePO4 significantly influence the application performance of synthesized LFP materials. Using high-quality FePO4 as a precursor can yield high-performance LFP materials. Currently, some drawbacks remain in the preparation of high-quality FePO4, such as electronic conductivity lower than the lithium-ion diffusion rate and low tap density. Controlling the particle size of FePO4 is one of the important means to improve its performance. For LiFePO4 materials, tap density is a limiting factor in its development, and controlling its particle size and morphology has a significant effect on improving the tap density. This is because particle size changes the path length of lithium-ion diffusion; smaller particle sizes result in shorter diffusion paths, thus increasing the lithium-ion diffusion rate and further improving the electrochemical performance of the material. By improving the particle morphology and size of FePO4, the performance of LFP materials can be effectively improved.

[0003] Currently, the main methods for preparing iron phosphate (FePO4) both domestically and internationally include hydrothermal methods, liquid-phase precipitation methods, sol-gel methods, and controlled crystallization methods. Among these, liquid-phase precipitation has become the mainstream production technology for FePO4 producers due to its advantages such as simple operation, short process flow, and ease of industrial implementation. Liquid-phase precipitation uses ferric sulfate and phosphate as raw materials, and precipitates the product (FePO4) and the byproduct sulfate in an aqueous solution under controlled reaction conditions. Studies have shown that the preparation of FePO4 by liquid-phase precipitation mainly involves two stages: ferric phosphate nucleus formation and crystal growth. In the initial stage of the reaction, a large number of fine ferric phosphate nuclei are formed, with small particle size and low sulfur content. As the reaction time increases, the nucleus grows, encapsulating the byproduct sulfate to form FePO4·xMSO4, which increases both particle size and sulfur content, adversely affecting product quality and the performance of the synthesized lithium iron phosphate materials. Currently, desulfurization is often achieved by increasing the amount of washing water and adding crystal ripening processes. The former causes environmental problems such as the difficulty in treating large amounts of washing wastewater, while the latter results in problems such as long process flow and unstable product quality.

[0004] The paper "Research on the Preparation Technology of Spherical Low-Sulfur Ferric Phosphate," published in *Inorganic Salt Industry* in May 2020, describes the following: By adding the morphology aid hexadecyltrimethylammonium bromide (CTAB), the growth of ferric phosphate crystals during the reaction process was controlled, improving the sphericity of the product, and ultimately obtaining spherical low-sulfur FePO4 with low sulfur content and high morphological consistency. However, the addition of CTAB usually causes a large number of bubbles to be generated in the solution, which is not conducive to the control of the production process, and the obtained ferric phosphate still has the problems of irregular morphology and unstable quality.

[0005] Patent CN117800302A discloses a method for preparing spherical iron phosphate and lithium manganese iron phosphate, which involves adding a surfactant premix during the preparation of iron phosphate. In a specific embodiment, anionic surfactant SDBS and cationic surfactant CTAB are mixed at a mass ratio of 1:1 to prepare the surfactant premix. However, those skilled in the art know that cationic and anionic surfactants have poor compatibility, and precipitation easily occurs after mixing, preventing the surfactants from exerting their full effect. Furthermore, the mixing of cationic and anionic surfactants during stirring easily generates a large number of bubbles, which adversely affects the production of iron phosphate and the subsequent control of its morphology. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for preparing spherical iron phosphate.

[0007] In a first aspect, the present invention provides a method for preparing spherical iron phosphate, the method comprising the following steps: S1. A complexing agent is added to a ferric salt aqueous solution under stirring to obtain a ferric salt complexing solution; wherein the ferric salt aqueous solution includes surfactant A; S2. Under stirring conditions, iron salt complex solution and phosphorus source are simultaneously added to an aqueous solution containing surfactant B to obtain a mixed solution. S3. The mixed solution is reacted and aged under heating conditions. After aging, solid-liquid separation is performed, and the separated solid is sintered to obtain the spherical iron phosphate. Among them, surfactant A includes nonionic surfactants, and surfactant B includes nonionic surfactants and acetylenic diol surfactants.

[0008] Furthermore, the nonionic surfactant in surfactants A and B is a polyoxyethylene ether. More preferably, it is an alkyl alcohol polyoxyethylene ether; its HLB is 13-20; more preferably, its HLB is 14-18.

[0009] Preferably, the alkyl alcohol polyoxyethylene ether nonionic surfactant includes isomeric alkyl alcohol polyoxyethylene ether and branched alkyl alcohol polyoxyethylene ether. Preferably, the number of ethylene oxide (EO) groups in the molecular structure of the nonionic surfactant is 20 to 40.

[0010] Furthermore, the active ingredient of surfactant A in the trivalent iron salt aqueous solution in step S1 has a mass percentage of 0.02~1%; preferably 0.1~0.5%.

[0011] Furthermore, the content of the active ingredient of surfactant B in the aqueous solution in step S2 is 0.5-5% by mass, preferably 1.5-3%.

[0012] Furthermore, the mass percentage of the active ingredients in the nonionic surfactant and acetylenic diol surfactant in surfactant B is (1~10):1; preferably (2~5):1.

[0013] Furthermore, the HLB of the acetylenic diol surfactant is 7-9, preferably 7.5-8.5.

[0014] Preferably, in step S1, the complexing agent is added at a constant rate; the stirring speed is 40-100 rpm. The reaction time of the complexation reaction in step S1 is 10-60 min.

[0015] The ferric salt is at least one of ferric nitrate and ferric chloride; the ferric nitrate may be ferric nitrate nonahydrate.

[0016] The complexing agent is at least one of ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; The molar ratio of the ferric salt to the complexing agent is 1:(0.2-0.5); for example, it can be 1:0.3 or 1:0.4.

[0017] The concentration of the iron salt complex solution is 0.1~1 mol / L, preferably 0.2~0.6 mol / L.

[0018] In step S2, the phosphorus source is at least one of phosphoric acid and ammonium dihydrogen phosphate; In step S2, the iron salt complex solution and the phosphorus source are mixed at a molar ratio of iron to phosphorus of 1:(1-1.2). Preferably, the molar ratio of iron to phosphorus is 1:(1.01~1.05).

[0019] The stirring in step S2 includes two stages: low-speed stirring and high-speed stirring. In the early stage of step S2, low-speed stirring is used, and after the raw materials are added, high-speed stirring is used. The low-speed stirring speed is 100~300 rpm; the high-speed stirring speed is 400~1200 rpm, preferably 600~100 rpm.

[0020] The low-speed stirring time in step S2 is 10-60 minutes; the high-speed stirring time is 20-120 minutes.

[0021] The heating reaction conditions in step S3 include: a reaction temperature of 20-100℃, specifically any value or a range of two values ​​among 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 85℃, 90℃, and 95℃; preferably 40~70℃.

[0022] The reaction conditions in step S3 include: a reaction time of 0.5 to 3 hours, specifically any value or a range of two values ​​from 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, to 3 hours; preferably 1.5 to 2.5 hours.

[0023] The reaction conditions in step S3 include: a stirring rate of 200-400 rpm, which can be any value or a range of two values ​​among 200 rpm, 250 rpm, 300 rpm, 350 rpm, and 400 rpm; preferably 150-300 rpm.

[0024] The reaction conditions in step S3 include: the pH value of the reaction solution is 1-3, specifically any value among 1.0, 1.5, 2, 2.5, and 3, or a range consisting of two values; preferably 1.5-2.5.

[0025] The aging conditions in step S3 include: an aging temperature of 70~100℃, specifically any value or a range of two values ​​among 75℃, 80℃, 85℃, 90℃, and 95℃; preferably 80~95℃.

[0026] The aging conditions in step S3 include: a stirring rate of 10-150 rpm, specifically any value or a range of two values ​​from 20 rpm, 30 rpm, 40 rpm, 50 rpm, 60 rpm, 70 rpm, 80 rpm, 90 rpm, 100 rpm, 110 rpm, 120 rpm, 130 rpm, and 140 rpm; preferably 20-60 rpm.

[0027] The aging conditions in step S3 include: the pH value of the reaction solution is 0.8-2, specifically any value or a range of two values ​​from 1.0, 1.2, 1.5, 1.7, and 1.9; preferably 1.0-1.5.

[0028] The aging conditions in step S3 include an aging time of 0.5 to 3 hours. The specific aging time can be any value from 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, and 3 hours, or a range consisting of two values; preferably 1.5 to 2.5 hours.

[0029] The sintering temperature in step S3 is 500~700℃, specifically any value or a range of two values ​​among 500℃, 550℃, 600℃, 650℃, and 700℃; preferably 550~650℃.

[0030] The sintering time in step S3 is 1 to 5 hours, specifically any value from 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 hours or a range consisting of two values; preferably 2 to 4 hours.

[0031] Another object of the present invention is to provide a spherical iron phosphate, which is prepared by the above-described method for preparing iron phosphate.

[0032] Another objective of this invention is to provide a lithium iron phosphate cathode material, the raw material of which includes iron phosphate, which is prepared by the above-described method for preparing iron phosphate.

[0033] Furthermore, in the lithium iron phosphate cathode material, the iron phosphate raw material prepared by the above preparation method comprises 10% to 100% by mass. Specifically, the iron phosphate prepared by the above preparation method comprises any value or a range of two values ​​from 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, to 90%.

[0034] Another object of the present invention is to provide a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps: The iron phosphate material prepared above was dispersed in anhydrous ethanol with lithium carbonate and glucose at a Fe / Li / C molar ratio of 1:(0.9~1.1):(0.05~0.2). The mixture was ball-milled for 2~5 hours until homogeneous. After spray drying, the mixture was heated to 300~500℃ at a heating rate of 5℃ / min under a nitrogen atmosphere and held for 2~5 hours. Then, it was calcined at 600~800℃ for 7~9 hours to obtain the lithium iron phosphate cathode material.

[0035] Beneficial effects: In the preparation method of this invention, a nonionic surfactant A is added during the preparation of the iron phosphate complex solution in step S1. This surfactant A allows the complexing agent to form a stable complex with iron ions under mild conditions, which is beneficial for the subsequent preparation of spherical iron phosphate with more controllable morphology. Furthermore, this invention involves simultaneously adding the iron salt complex solution and the phosphorus source to an aqueous solution containing surfactant B under stirring conditions to obtain a mixed solution. Experimental results demonstrate that this dispersion method more easily forms iron phosphate crystals with controllable morphology, effectively reducing the agglomeration between iron phosphate grains, and thus further facilitating the preparation of spherical iron phosphate with controllable morphology. Moreover, surfactant B also includes acetylenic diol surfactants, which can act as both a dispersant and an antifoaming agent, suppressing bubbles formed during stirring and avoiding the problem of uneven iron phosphate size caused by bubbles. This results in powder with better morphology and uniform particle size, improving the tap density of the lithium iron phosphate cathode material and thereby enhancing its application performance.

[0036] Using the iron phosphate prepared by this invention as raw material, the prepared lithium iron phosphate cathode material has a regular morphology and size, and achieves good gradation between large and small particles. The resulting lithium iron phosphate cathode material has excellent electrical properties and is suitable for industrial mass production. Detailed Implementation

[0037] The following description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the embodiments and comparative examples of the present invention are all commercially available.

[0038] Example 1 This embodiment provides a method for preparing spherical iron phosphate, the preparation method of which is as follows: S1: A 0.5 mol / L ferric salt solution was prepared using Fe(NO3)3·9H2O. Surfactant A was added to the solution and stirred until homogeneous. Surfactant A was Emulsogen LCN 287 (70% active ingredient by mass), with an active ingredient content of 0.2% in the ferric salt solution. Following a Fe(NO3)3·9H2O:EDTA (ethylenediaminetetraacetic acid) molar ratio of 1:0.2, EDTA aqueous solution was added dropwise to the ferric salt solution containing surfactant A under stirring at 60 rpm. After the addition was complete, stirring was continued for 3 minutes to obtain a ferric salt complex solution. The dropwise addition time was 20 minutes.

[0039] S2: Add the nonionic surfactant Emulsogen LCN 287 and the acetylenol surfactant surfynol 104E (active ingredient mass content of 50%) to 500 mL of deionized water and stir until homogeneous to obtain an aqueous solution containing surfactant B. The mass percentage of active ingredient B in the aqueous solution is 2%, and the mass percentage ratio of the nonionic surfactant to the acetylenol surfactant is 4:1. Subsequently, under stirring at 200 rpm, simultaneously and uniformly add 1000 mL of an iron salt complex solution (according to an iron:phosphorus molar ratio of 1:1.03) and a 0.5 mol / L phosphoric acid solution to 500 mL of the aqueous solution containing surfactant B. After 30 min of dropwise addition, increase the stirring speed to 800 rpm and continue stirring for 50 min to obtain a mixed solution.

[0040] S3: The obtained mixed solution was heated to 60℃ under stirring at 200 rpm, and then ammonia was added dropwise to adjust the pH of the mixed solution to 2. The reaction was carried out for 2 hours, and then aged at 85℃ and stirring at 30 rpm for 3 hours, during which the pH was controlled at 1.5~2.0 to obtain a precipitate. The precipitate was filtered, and the filter was washed and transferred to a forced-air drying oven for drying at 80℃ for 4 hours. After drying, the powder was transferred to a muffle furnace for sintering under a nitrogen atmosphere at 600℃ for 4 hours. After natural cooling to room temperature, the iron phosphate material was obtained.

[0041] Example 2 S1: A 0.3 mol / L ferric salt solution was prepared using Fe(NO3)3·9H2O. Surfactant A was added to the solution and stirred until homogeneous. Surfactant A was Emulsogen LCN 407 (70% active ingredient by mass), with an active ingredient content of 0.25% in the ferric salt solution. Following a Fe(NO3)3·9H2O:EDTA (ethylenediaminetetraacetic acid) molar ratio of 1:0.25, EDTA aqueous solution was added dropwise to the ferric salt solution containing surfactant A under stirring at 60 rpm. After the addition was complete, stirring was continued for 3 minutes to obtain a ferric salt complex solution. The dropwise addition time was 15 minutes.

[0042] S2: Add nonionic surfactant Emulsogen LCN 407 and acetylation diol surfactant Dynol 607 (100% active ingredient content) to 500 mL of deionized water and stir until homogeneous to obtain an aqueous solution containing surfactant B. The active ingredient content of surfactant B in the aqueous solution is 2.5% by mass, and the ratio of the active ingredient content of the nonionic surfactant to the acetylation diol surfactant is 3:1. Subsequently, under stirring at 200 rpm, simultaneously and uniformly add 1000 mL of iron salt complex solution (iron:phosphorus molar ratio of 1:1.03) and 0.6 mol / L phosphoric acid solution to 500 mL of the aqueous solution containing surfactant B. After 30 min of dropwise addition, increase the stirring speed to 800 rpm and continue stirring for 60 min to obtain a mixed solution.

[0043] S3: The obtained mixed solution was heated to 65°C with stirring at 200 rpm, and then ammonia was added dropwise to adjust the pH of the mixed solution to 1.5. The reaction was carried out for 2 hours, and then aged at 90°C and stirring at 30 rpm for 4 hours, during which the pH was controlled at 1.5~2.0 to obtain a precipitate. The precipitate was filtered, and the filter was washed and transferred to a forced-air drying oven for drying at 80°C for 4 hours. After drying, the powder was transferred to a muffle furnace for sintering at 650°C for 3.5 hours under a nitrogen atmosphere. After natural cooling to room temperature, the iron phosphate material was obtained.

[0044] Example 3 Based on Example 1, the mass percentage of the active ingredient of surfactant A in the ferric salt solution in step S1 was adjusted to 1%.

[0045] Example 4 Based on Example 1, the mass percentage of the active ingredient of surfactant B in the aqueous solution in step S2 is adjusted to 5%.

[0046] Example 5 Based on Example 1, the mass percentage of the active ingredients of the nonionic surfactant and the acetylide diol surfactant in surfactant B in step S2 is adjusted to 10:1.

[0047] Example 6 Based on Example 1, the mass percentage of the active ingredients of the nonionic surfactant and the acetylide diol surfactant in surfactant B in step S2 is adjusted to 1:1.

[0048] Comparative Example 1 The difference from Example 1 is that surfactant A is not added in step S1.

[0049] Comparative Example 2 The difference from Example 1 is that in step S2, a nonionic surfactant of the same mass as the active ingredient is used instead of an alkynyl diol surfactant.

[0050] Comparative Example 3 The difference from Example 1 is that in step S2, an acetylsadiol surfactant of the same mass as the active ingredient is used instead of a nonionic surfactant.

[0051] Comparative Example 4 The difference from Example 1 is that the surfactant B in step S2 is hexadecyltrimethylammonium bromide of the same mass as the active ingredient.

[0052] Comparative Example 5 The difference from Example 1 is that the surfactant B in step S2 is sodium dodecylbenzenesulfonate of the same mass as the active ingredient.

[0053] Comparative Example 6 The difference from Example 1 is that in step S2, the iron salt complex solution is first added to the aqueous solution containing surfactant B, and then phosphoric acid solution is added dropwise.

[0054] Comparative Example 7 The difference from Example 1 is that in step S2, the phosphoric acid solution is first added to the aqueous solution containing surfactant B, and then an iron salt complex solution is added dropwise.

[0055] Comparative Example 8 The difference from Example 1 is that surfactant B is not added to the aqueous solution in step S2.

[0056] Performance testing The tap density and particle size of the iron phosphate obtained in Examples 1-6 and Comparative Examples 1-8 were tested using laser diffraction. The test results are shown in Table 1.

[0057] As can be seen from the test results in Table 1, compared with the comparative example, the D of the iron phosphate obtained in the example is higher. 50 It has a small particle size and a high tap density.

[0058] Application examples The iron phosphate materials prepared in Examples 16 and Comparative Examples 1-8 were dispersed in anhydrous ethanol with lithium carbonate and glucose at a Fe / Li / C molar ratio of 1:1:0.1. The mixture was ball-milled for 3 hours until homogeneous, spray-dried, and then heated to 400°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. The mixture was then calcined at 700°C for 8 hours to obtain lithium iron phosphate cathode materials, which were labeled as Application Examples 1-6 and Comparative Application Examples 1-8, respectively.

[0059] Compacted density: Using the Sansi cross-sectional equipment, 0.8g of lithium iron phosphate samples from the application example and the control application example were weighed and loaded into the mold. A compaction test was performed with a force of 3N, and the compacted density was recorded.

[0060] Electrochemical performance: The lithium iron phosphate cathode materials of Application Examples 1-6 and Comparative Application Examples 1-8 were formulated into coin cells. The specific steps included: uniformly mixing lithium iron phosphate cathode material, conductive agent acetylene black and adhesive polyvinylidene fluoride in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry, then coating it on aluminum foil and drying it in a vacuum drying oven, then pressing it into a positive electrode sheet using a tablet press, the negative electrode sheet being a lithium metal sheet, the electrolyte being 1 mol / L LiPF6EC:DMC (volume ratio of 1:1), and a polypropylene porous membrane as the separator, and assembling the battery in an argon glove box.

[0061] The electrochemical performance of the above batteries was tested (with the charge and discharge voltage controlled between 2.0 and 3.75V), and the results are shown in Table 2 below.

[0062] Table 2: Electrochemical Performance Test Results As can be seen from the test results in Table 2, compared with the comparative application example, the lithium iron phosphate obtained in the application example has a higher compaction density and a larger rate capacity.

[0063] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing spherical iron phosphate for high-pressure lithium iron phosphate, characterized in that, The preparation method includes the following steps: S1. A complexing agent is added to a ferric salt aqueous solution under stirring to obtain a ferric salt complexing solution; wherein the ferric salt aqueous solution includes surfactant A; S2. Under stirring conditions, iron salt complex solution and phosphorus source are simultaneously added to an aqueous solution containing surfactant B to obtain a mixed solution. S3. The mixed solution is heated and reacted, and then aged. After aging, solid-liquid separation is performed, and the separated solid is sintered to obtain the spherical iron phosphate. Among them, surfactant A includes nonionic surfactants, and surfactant B includes nonionic surfactants and acetylenic diol surfactants.

2. The preparation method according to claim 1, characterized in that, At least one of the following three conditions must be met: (a) The nonionic surfactant in surfactants A and B is a polyoxyethylene ether, preferably with an HLB of 13-20; (b) The active ingredient of surfactant A in the ferric salt aqueous solution in step S1 is 0.02-1% by mass; (c) The active ingredient of surfactant B in the aqueous solution of step S2 is 0.5-5% by mass.

3. The preparation method according to claim 1, characterized in that, Satisfy at least one of the following two conditions: (1) The mass percentage of the active ingredients in the nonionic surfactant and acetylenic diol surfactant in surfactant B is (1~10):1; (2) The HLB value of the acetylide diol surfactant is 7~9.

4. The preparation method according to claim 1, characterized in that, The ferric salt is at least one of ferric nitrate and ferric chloride; the complexing agent is at least one of ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid.

5. The preparation method according to claim 1, characterized in that, The molar ratio of the trivalent iron salt to the complexing agent is 1:(0.2~0.5); the concentration of the iron salt complexing solution is 0.1~1mol / L.

6. The preparation method according to claim 1, characterized in that, In step S2, the phosphorus source is at least one of phosphoric acid and ammonium dihydrogen phosphate.

7. The preparation method according to claim 1, characterized in that, In step S2, the iron salt complex solution and the phosphorus source are mixed according to the molar ratio of iron to phosphorus of 1:(1~1.2).

8. The preparation method according to claim 1, characterized in that, The heating reaction conditions in step S3 include: a reaction temperature of 20~100℃, a reaction time of 0.5~3h, a stirring rate of 200~400rpm, and a pH value of 1~3 for the reaction solution; the aging conditions in step S3 include: a reaction temperature of 70~100℃, a reaction time of 0.5~3h, a stirring rate of 10~150rpm, and a pH value of 1~2 for the reaction solution; the sintering temperature in step S3 is 500~700℃, and the sintering time is 1~5h.

9. A spherical iron phosphate, characterized in that, It is prepared by the preparation method of any one of claims 1 to 8.

10. A lithium iron phosphate cathode material, characterized in that, The raw materials for its preparation include ferric phosphate, wherein the ferric phosphate includes spherical ferric phosphate prepared by the preparation method of any one of claims 1 to 8.

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