Preparation method of battery-grade iron phosphate with particle composite sheet-shaped morphology

Abstract

The invention discloses a preparation method of particle composite sheet-shaped battery-grade iron phosphate, and particularly relates to the field of lithium ion battery positive electrode materials. The method comprises the steps of purification and oxidation of a ferric salt solution, preparation and synthesis of a phosphorus salt solution, aging and separation. The iron phosphate with the particle composite sheet-shaped morphology is low in impurity content, low in specific surface area and small in particle size, and presents a hierarchical structure morphology formed by combining particles and sheets, and the prepared material shows higher specific capacity, higher rate capability and good cycle performance, and has huge application potential in the aspect of improving the performance of a lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery positive electrode materials, and in particular to a method for preparing battery-grade iron phosphate with a particle composite sheet-like morphology. Background Art

[0002] Driven by the rapid development of the new energy vehicle and energy storage industries, demand for lithium iron phosphate (LiFePO4) cathode materials has surged, making its preparation a research hotspot. Currently, both domestic and international research and development efforts are accelerating the development of novel synthesis processes and material modification technologies. The morphological characteristics of LiFePO4 play a crucial role in its electrochemical performance. Different morphological characteristics can lead to different microstructures and crystal structures, significantly impacting the material's electrochemical performance.

[0003] Common iron phosphate morphologies currently available on the market include particles, nanowires, nanorods, and nanosheets. For example, Zhang Zhen and colleagues at Tsinghua University successfully produced quasi-spherical iron phosphate particles with a particle size range of 1 to 20 microns using controlled crystallization, using ferric nitrate and phosphoric acid as raw materials. Gong Fuzhong and colleagues employed a homogeneous precipitation method, using ferric nitrate as the iron source and urea as the homogeneous precipitant, to synthesize iron phosphate with a disc-like structure. Qian et al. also prepared nanoscale mesoporous iron phosphate using electrochemical synthesis techniques.

[0004] Chinese patent CN112456461A discloses a method for producing battery-grade flaky iron phosphate using a cobalt-iron hydrometallurgical leachate. The cobalt-iron leachate is an acidic leachate of cobalt ore or cobalt alloy, characterized by high concentrations of cobalt and iron ions. The resulting battery-grade flaky iron phosphate exhibits a thin morphology with a specific aspect ratio and a thickness of less than 50 nm. However, this patent only produces a thin flaky morphology, resulting in the iron phosphate having only the characteristics of a thin flaky iron phosphate, which has limited potential for improving lithium-ion battery performance.

[0005] Chinese patent CN103274383A, a battery-grade iron phosphate with controllable morphology and its preparation method, the steps include: ① preparation of reaction mother liquor; ② dissolving raw materials in the mother liquor to form a reaction solution; ③ hydrothermal reaction; ④ filtering, washing and drying the product. The process of the present invention is simple, with few control parameters, and can obtain iron phosphate with regular morphology without any forming agent and dispersant. The filtrate in step ④ can be recycled and reused as reaction mother liquor, which is economical and environmentally friendly. The iron phosphate prepared by the method of this patent has high purity and a phosphorus-iron ratio of 1:1. It is composed of iron phosphate particles with regular morphology. The particles are composed of nano-scale flaky or columnar primary particles. The shape of the combined particles is disc-shaped, spherical, spherical or porous spherical. It belongs to the traditional iron phosphate morphology and has limited effect in improving the performance of lithium-ion batteries.

[0006] Therefore, how to prepare battery-grade iron phosphate with a granular composite flake morphology is a difficult problem that needs to be solved urgently. Summary of the Invention

[0007] To this end, the present invention provides a method for preparing battery-grade iron phosphate with a particle composite flake morphology to solve the problems in the prior art.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] According to the present invention, a method for preparing battery-grade iron phosphate with a composite flaky morphology of particles is provided, the method comprising:

[0010] Step 1: Purification and oxidation

[0011] Take the titanium dioxide by-product ferrous sulfate heptahydrate, purify it, add primary phosphoric acid, mix well, and then add excess oxidant to oxidize it to obtain an iron salt solution;

[0012] Step 2: Preparation of phosphate solution

[0013] Take industrial-grade monoammonium phosphate and add water to prepare a solution, then add secondary phosphoric acid and a surfactant to obtain a phosphate solution;

[0014] Step 3: Synthesis, aging, and separation

[0015] After preheating the iron salt solution and the phosphate salt solution respectively, the phosphate salt solution is used as the base material, and the iron salt solution is added while stirring to obtain a mixed slurry; after the mixed slurry is kept warm for a period of time, the temperature is increased for aging reaction until it turns white; after the white slurry is kept warm for a period of time, solid-liquid separation is carried out, the separated solid phase is washed with pure water, and the washed filter cake is dried and calcined to obtain an iron phosphate product with a granular and flake composite morphology.

[0016] Furthermore, in step 1, the purification method includes heating the solution to 60° C., adding ammonia water to adjust the pH to 3-5, removing impurities (mainly Al and Ti) in the ferrous sulfate heptahydrate solution to meet the use standard, and filtering to obtain a ferrous sulfate heptahydrate solution with a low impurity content.

[0017] Furthermore, in step 1, the purpose of adding phosphoric acid is to adjust the pH of the iron salt and supplement the phosphorus source.

[0018] Furthermore, in step 1, hydrogen peroxide is used as the oxidant.

[0019] Furthermore, in the step 1, the amount of hydrogen peroxide added is 1.3 to 1.8 times the amount of iron in the ferrous sulfate heptahydrate solution.

[0020] To accelerate oxidation, the pH of this step is controlled at 2-3.

[0021] Furthermore, in step 1, the concentration of the iron salt solution is 50 g / L to 65 g / L.

[0022] Furthermore, in step 2, the concentration of the phosphate solution is 40g / L to 60g / L, the amount of phosphoric acid added is 1ml to 5ml, and the phosphoric acid concentration is 75% to 85%. The phosphorus source is supplemented and the acidity of the solution is adjusted to prevent the local pH from being too high, which may cause premature hydrolysis of Fe3+.

[0023] Furthermore, in step 2, the surfactant is CTAB or SDBS. The added amount is calculated based on the amount of anhydrous iron phosphate generated: the mass fraction is 0.05% to 0.5%. Directed adsorption is used to regulate the growth of iron phosphate crystals and promote the formation of a flaky structure.

[0024] Furthermore, in step 3, the preheating and insulation reaction temperatures are both 45°C to 60°C; the temperature for the aging reaction is 80°C to 92°C. The heating rate is controlled to increase from 45°C to 60°C to 80°C to 92°C at a rate of 1.5 to 2.5°C / min.

[0025] Furthermore, in step 3, the P / Fe ratio of the added material is controlled at 1.05 to 1.3, the addition time is 30 to 60 minutes, and the stirring speed is 200 to 500 rpm. To control the amount of iron and phosphate salts added, only the concentration of the iron and phosphate salts is known, and the amount of iron and phosphate salts to be added must be calculated according to the molar ratio of phosphorus to iron. Here, different P / Fe ratios will affect the performance of the ferric phosphate product.

[0026] Furthermore, in the step three, the whitening slurry is extended for 60 to 90 minutes.

[0027] Furthermore, in the step three, the drying temperature is 100-110°C, and the calcination temperature is 530°C to 620°C.

[0028] A two-stage calcination (pre-calcination at 300°C for 1 h and main calcination at 600°C for 2 h) with a heating rate of 5°C / min was used to avoid particle sintering and obtain a high-crystallinity LiFePO4 precursor.

[0029] The present invention has the following advantages:

[0030] The iron phosphate with a granular composite flake morphology of the present invention has a low impurity content, a low specific surface area, and a small particle size, and presents a hierarchical structural morphology composed of particles and flakes. The material prepared therefrom exhibits higher specific capacity, higher rate performance and good cycle performance, and has great application potential in improving the performance of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0032] The structures, proportions, sizes, etc. illustrated in this specification are intended only to complement the contents disclosed herein and to facilitate understanding and reading by persons familiar with the art. They are not intended to limit the conditions under which the present invention may be implemented and therefore have no substantive technical significance. Any structural modifications, changes in proportions, or adjustments in sizes, without affecting the efficacy and objectives of the present invention, shall still fall within the scope of the technical contents disclosed herein.

[0033] Figure 1 This is a SEM image of ferric phosphate dihydrate provided in Example 1 of the present invention;

[0034] Figure 2 This is an SEM image of ferric phosphate dihydrate provided in Example 2 of the present invention;

[0035] Figure 3 This is a SEM image of anhydrous ferric phosphate provided in Example 2 of the present invention;

[0036] Figure 4 The XRD pattern of the iron phosphate provided in Example 2 of the present invention;

[0037] Figure 5 This is a SEM image of ferric phosphate dihydrate provided in Example 3 of the present invention;

[0038] Figure 6 This is an SEM image of ferric phosphate dihydrate provided in Example 4 of the present invention;

[0039] Figure 7 This is an SEM image of ferric phosphate dihydrate provided in Comparative Example 2 of the present invention;

[0040] Figure 8 This is an SEM image of ferric phosphate dihydrate provided in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0041] The following describes the implementation of the present invention using specific embodiments. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. Obviously, the embodiments described are only a portion of the present invention, not all of it. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0042] Example 1

[0043] This embodiment provides a method for preparing battery-grade iron phosphate with a composite flaky particle morphology:

[0044] 1. Take the purified ferrous sulfate heptahydrate solution, add primary phosphoric acid and mix well, add 1.5 times excess hydrogen peroxide using a peristaltic pump, and oxidize at room temperature to obtain an iron salt solution with an iron content of 56g / L.

[0045] 2. Prepare a 45 g / L phosphate solution with a P / Fe ratio of 1.2, and add 2 ml of secondary phosphoric acid and 0.05% CTAB.

[0046] 3. Preheat the iron salt solution and the phosphate salt solution to 45°C, use the phosphate salt solution as the base material, and add the iron salt solution dropwise to obtain a mixed slurry. The addition time is 30 minutes and the stirring rate is 300 rpm.

[0047] 4. Keep the mixed slurry at 45°C for 4 hours and then heat it to 80°C (control the heating rate at 2°C / min, from 45°C to 80°C), until the slurry turns white;

[0048] 5. The whitened slurry was heated at 80°C for 60 minutes for solid-liquid separation and washed online with hot water;

[0049] 6. The washed filter cake was dried at 105°C to obtain ferric phosphate dihydrate. The SEM of ferric phosphate dihydrate is as follows: Figure 1 As shown, two-stage calcination (pre-calcination at 300℃ for 1h, main calcination at 600℃ for 2h) with a heating rate of 5℃ / min was used to avoid particle sintering and obtain anhydrous iron phosphate crystals of high crystallinity LiFePO4 precursor with a specific surface area of 5.649m 2 / g.

[0050] The sample is in the form of micro-nanoscale crystal particles. The crystals were monitored by dynamic light scattering (DLS). The morphology was a composite of flakes and granules. The flakes were thinner, with an average thickness of about 45nm and an average size of the longest side of about 800nm. The particles were smaller, with an average particle size of about 50nm. They were distributed on the surface of the flakes, had a relatively flat surface, and had good crystallinity.

[0051] Example 2

[0052] This embodiment provides a method for preparing battery-grade iron phosphate with a composite flaky particle morphology:

[0053] 1. Take the purified ferrous sulfate heptahydrate solution, add primary phosphoric acid and mix well. Use a peristaltic pump to add 1.5 times excess hydrogen peroxide and oxidize at room temperature to obtain an iron salt solution with an iron content of 56g / L.

[0054] 2. Prepare a 50 g / L phosphate solution with a P / Fe ratio of 1.1, and add 2 ml of secondary phosphoric acid and 0.05% CTAB.

[0055] 3. Preheat the iron salt solution and the phosphate salt solution to 60°C, add the iron salt solution dropwise to the phosphate salt solution to obtain a mixed slurry. The addition time is 30 minutes and the stirring rate is 300 rpm.

[0056] 4. Keep the mixed slurry at 60℃ for 4 hours and then heat it to 80℃ (control the heating rate at 2℃ / min, from 60℃ to 80℃, until the slurry turns white;

[0057] 5. The whitened slurry was heated at 80°C for 60 minutes for solid-liquid separation and washed online with hot water; the washed filter cake was dried at 105°C to obtain ferric phosphate dihydrate, as shown in the SEM image. Figure 2 As shown, two-stage calcination (pre-calcination at 300℃ for 1h, main calcination at 600℃ for 2h) with a heating rate of 5℃ / min was used to avoid particle sintering and obtain anhydrous iron phosphate crystals with high crystallinity. The SEM image is shown in Figure 3 As shown, the XRD pattern is Figure 4 As shown, the specific surface area of anhydrous iron phosphate is 6.033m 2 / g.

[0058] Example 3

[0059] This embodiment provides a method for preparing battery-grade iron phosphate with a composite flaky particle morphology:

[0060] 1. Take the purified ferrous sulfate heptahydrate solution, add primary phosphoric acid and mix well. Use a peristaltic pump to add 1.5 times excess hydrogen peroxide and oxidize at room temperature to obtain an iron salt solution with an iron content of 56g / L.

[0061] 2. Prepare a 50 g / L phosphate solution with a P / Fe ratio of 1.1, and add 3 ml of secondary phosphoric acid and 0.1% CTAB.

[0062] 3. Preheat the iron salt solution and the phosphate salt solution to 50°C, use the phosphate salt solution as the base material, and add the iron salt solution dropwise to obtain a mixed slurry. The addition time is 30 minutes and the stirring rate is 300 rpm.

[0063] 4. Keep the mixed slurry at 50℃ for 4 hours and then heat it to 80℃ (control the heating rate at 1.5℃ / min, from 50℃~60℃ to 80℃, until the slurry turns white;

[0064] 5. The whitened slurry was heated at 80°C for 30 minutes for solid-liquid separation and washed online with hot water;

[0065] The washed filter cake was dried at 105℃ to obtain ferric phosphate dihydrate. The SEM of ferric phosphate dihydrate is as follows: Figure 5 Two-stage calcination (pre-calcination at 300℃ for 1h, main calcination at 600℃ for 2h) with a heating rate of 5℃ / min was used to avoid particle sintering and obtain anhydrous iron phosphate with high crystallinity and a specific surface area of 5.644m 2 / g.

[0066] Example 4

[0067] This embodiment provides a method for preparing battery-grade iron phosphate with a composite flaky particle morphology:

[0068] 1. Take the purified ferrous sulfate heptahydrate solution, add primary phosphoric acid and mix well. Use a peristaltic pump to add 1.5 times excess hydrogen peroxide and oxidize at room temperature to obtain an iron salt solution with an iron content of 61g / L.

[0069] 2. Prepare a 50 g / L phosphate solution with a P / Fe ratio of 1.1, and add 2 ml of secondary phosphoric acid and 0.1% SDBS.

[0070] 3. Preheat the iron salt solution and the phosphate salt solution to 60°C, add the iron salt solution dropwise to the phosphate salt solution to obtain a mixed slurry. The addition time is 30 minutes and the stirring rate is 300 rpm.

[0071] 4. Keep the mixed slurry at 60°C for 4 hours and then heat it to 85°C (control the heating rate at 1.5°C / min from 60°C to 85°C until the slurry turns white;

[0072] 5. The whitened slurry was heated at 85°C for 30 minutes for solid-liquid separation and washed online with hot water;

[0073] The washed filter cake was dried at 105℃ to obtain ferric phosphate dihydrate. The SEM of ferric phosphate dihydrate is as follows: Figure 6 As shown in the figure, two-stage calcination (pre-calcination at 300℃ for 1h, main calcination at 600℃ for 2h) with a heating rate of 5℃ / min was used to avoid particle sintering and obtain anhydrous iron phosphate with high crystallinity and a specific surface area of 6.726m 2 / g.

[0074] Comparative Example 1

[0075] This comparative example provides a method for preparing battery-grade iron phosphate with a granular composite flake morphology:

[0076] In this comparative example, the ratio of P / Fe=0.9 is completely consistent with Example 1. The anhydrous ferric phosphate finally obtained, when P / Fe=0.9, is insufficient in the reaction system, and the reaction will not turn white. The test slurry of this group of chambers is yellowish and does not turn white 4 hours after heating, so subsequent experiments cannot be carried out.

[0077] Comparative Example 2

[0078] This comparative example provides a method for preparing battery-grade iron phosphate with a granular composite flake morphology:

[0079] In this comparative example, iron salt is used as the base material, and phosphorus salt is added back to the iron salt. Other aspects are completely consistent with Example 1, and anhydrous ferric phosphate is finally obtained. The specific surface area of the ferric phosphate sample is relatively low, and the specific surface area of ferric phosphate dihydrate is 133.477m 2 / g, the specific surface area of anhydrous iron phosphate after calcination is 13.767m 2 / g. The morphology of iron phosphate dihydrate is small flakes. The SEM scan of iron phosphate dihydrate is as follows Figure 7 .

[0080] Comparative Example 3

[0081] This comparative example provides a method for preparing battery-grade iron phosphate with a granular composite flake morphology:

[0082] In this comparative example, no secondary phosphoric acid and surfactant were added, and the other aspects were completely consistent with Example 1 to obtain anhydrous ferric phosphate.

[0083] The sample has a high specific gravity, and the specific gravity of anhydrous iron phosphate after calcination is 12.563m 2 / g, the morphology of ferric phosphate dihydrate is relatively agglomerated, compact and thick blocks. Figure 8 shown.

[0084] Comparative Example 4

[0085] This comparative example provides a method for preparing battery-grade iron phosphate with a granular composite flake morphology:

[0086] 1. Take the purified ferrous sulfate heptahydrate solution, add phosphoric acid, mix thoroughly, and then add 0.1% CTAB of iron to activate the solution to obtain a surface-activated iron salt solution;

[0087] 2. Add the iron salt solution in step 1 to a stirred reactor, add 0.5 mol of sodium hypochlorite to oxidize the ferrous ions, control the oxidation-reduction potential of the reaction solution to 0.5 V, and keep the temperature in the reactor within 65°C;

[0088] 3. The pH value adjusting agent solution is adjusted to 0.5 by using ammonium bicarbonate solution with a mass concentration of 5%. The reaction solution is aged for 2 hours to obtain iron phosphate slurry. The slurry is separated into solid and liquid, and the battery-grade flaky iron phosphate product is obtained after washing and drying.

[0089] Experimental Example 1

[0090] The anhydrous ferric phosphate prepared in Examples 1-4 and Comparative Examples 2-4 was subjected to physical and chemical analysis, and the specific components are shown in Tables 1 and 2.

[0091] Table 1

[0092]

[0093] Table 2

[0094]

[0095] As can be seen from Table 1 and Table 2, the battery-grade anhydrous ferric phosphate prepared by adding secondary phosphoric acid and a surfactant to the phosphate salt solution in Examples 1-4 has a low impurity content, a low specific surface area, a small particle size, a good crystal structure, and a stable iron-phosphorus ratio.

[0096] Experimental Example 2

[0097] In this experimental example, the anhydrous iron phosphate prepared in Examples 1-4 and Comparative Examples 2-4 was prepared into lithium iron phosphate for testing. The results are shown in Table 3.

[0098] Table 3 Electrochemical performance indicators of lithium iron phosphate prepared in Examples 1-4 and Comparative Examples 2-4

[0099]

[0100] It can be seen that the addition of secondary phosphoric acid and surfactant to the phosphate salt in Examples 1-4 plays an important role in the morphology of iron phosphate, including determining the amount of precipitate initially produced, determining the pH environment of the solution after growth during the heat treatment process, etc. This method adds part of the phosphoric acid and surfactant to the phosphate salt solution to change the reaction environment of the base phosphate salt, and because the addition of the surfactant inhibits or promotes the growth of crystals, it eventually forms a hierarchical structure morphology of iron phosphate composed of particles and sheets. It has low impurity content, low surface area, small particle size, and a relatively stable iron-phosphorus ratio. The lithium iron phosphate battery prepared under the same conditions has higher specific capacity and rate performance than the control example, better cycle performance, and exhibits better electrochemical properties.

[0101] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made based on the present invention. Therefore, such modifications and improvements, which do not depart from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A method for preparing battery-grade iron phosphate with a granular composite flake morphology, characterized in that: The method comprises: Step 1: Purification and oxidation Take the titanium dioxide by-product ferrous sulfate heptahydrate, purify it, add phosphoric acid, mix well, and then add excess oxidant to oxidize it to obtain an iron salt solution; Step 2: Preparation of phosphate solution Take industrial-grade monoammonium phosphate and add water to prepare a solution, then add phosphoric acid and a surfactant to obtain a phosphate solution; Step 3: Synthesis, aging, and separation After preheating the iron salt solution and the phosphate salt solution respectively, the phosphate salt solution is used as the base material, and the iron salt solution is added while stirring to obtain a mixed slurry; after the mixed slurry is kept warm for a certain period of time, the temperature is increased for aging reaction until it turns white; after the white slurry is kept warm for a period of time, solid-liquid separation is carried out, the separated solid phase is washed with pure water, and the washed filter cake is dried and calcined to obtain an iron phosphate product with a granular and flake composite morphology.

2. The method for preparing battery-grade iron phosphate with a composite flaky particle morphology according to claim 1, characterized in that: In the step 1, hydrogen peroxide is selected as the oxidant.

3. The method for preparing battery-grade iron phosphate with a composite flaky morphology of particles according to claim 2, characterized in that: In the step 1, the amount of hydrogen peroxide added is 1.3 to 1.8 times the amount of iron in the ferrous sulfate heptahydrate solution.

4. The method for preparing battery-grade iron phosphate with a particle composite flake morphology according to claim 1, characterized in that: In the step 1, the concentration of the iron salt solution is 50 g / L to 65 g / L.

5. The method for preparing battery-grade iron phosphate with a particle composite flake morphology according to claim 1, characterized in that: In the step 2, the concentration of the phosphate solution is 40 g / L to 60 g / L, and the amount of secondary phosphoric acid added is 1 ml to 5 ml.

6. The method for preparing battery-grade iron phosphate with a composite flaky particle morphology according to claim 1, characterized in that: In the step 2, the surfactant is CTAB or SDBS.

7. The method for preparing battery-grade iron phosphate with a composite flaky particle morphology according to claim 1, characterized in that: In the step 3, the temperature of the preheating and insulation reactions is 45°C to 60°C; the temperature of the heating reaction is 80°C to 92°C.

8. The method for preparing battery-grade iron phosphate with a composite flaky particle morphology according to claim 1, characterized in that: In the step 3, the P / Fe ratio of the added material is controlled at 1.05 to 1.3, the adding time is 30 to 60 minutes, and the stirring speed is 200 to 500 rpm.

9. The method for preparing battery-grade iron phosphate with a composite flaky particle morphology according to claim 1, characterized in that: In the step 3, the whitening slurry is extended for 60 to 90 minutes.

10. The method for preparing battery-grade iron phosphate with a particle composite flake morphology according to claim 1, characterized in that: In the step 3, the drying temperature is 100-110°C, and the calcination temperature is 530-620°C.

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