A method for preparing low-temperature and highly dispersible spherical nano-ferric phosphate

Abstract

The present invention discloses a method for preparing low-temperature and highly dispersible spherical nano-ferric phosphate, which comprises the following steps: dissolving a divalent iron salt as a base liquid, pouring a soluble phosphate or phosphoric acid solution under stirring, and dropping a hydrogen peroxide solution to oxidize the Fe 2+ , warming to 50-70°C, adjusting the pH using sodium hydroxide solution, reacting in an oil bath for 3 hours under continuous stirring, the slurry is filtered while hot to obtain an iron phosphate precipitate, washed with pure water, filtered, dried, and calcined to obtain a white iron phosphate powder. The present invention uses cheap divalent iron salts and soluble phosphates or phosphoric acid as raw materials, saving costs. The prepared nano-scale anhydrous iron phosphate has the characteristics of high dispersibility, uniform particle size distribution, and small particle size. It can be used as a preparation method for a lithium iron phosphate cathode material precursor with high electronic conductivity and ion diffusivity, and has the characteristics of low reaction temperature and low energy consumption. In addition, the preparation method does not introduce additives, is pollution-free, low cost, and simple post-processing.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of ferric phosphate, and particularly relates to a method for preparing low-temperature and highly dispersible spherical nano-ferric phosphate. Background Art

[0002] Lithium-ion batteries offer advantages such as high power density, long cycle life, low self-discharge, excellent cycling performance, and high voltage. The cathode of a lithium-ion battery is crucial to its overall performance. In recent years, the olivine-structured lithium-ion battery cathode material, LiFePO₄, has become one of the most promising commercial materials due to its high theoretical capacity (170 mAh / g), abundant resources, low cost, environmental friendliness, non-toxicity, excellent thermal stability, and long cycle life. However, LiFePO₄ also has disadvantages, including low electronic and ionic conductivity and a low lithium-ion diffusion coefficient. These disadvantages directly affect the material's rate capability and limit its application in charging applications. Therefore, LiFePO₄ requires modification through methods such as ion doping, surface coating, nanomaterial synthesis, and porous material synthesis. After Li is removed from LiFePO4, FePO4 is formed. The volume change before and after the two is not much, and they are very similar. Therefore, FePO4 is an important precursor of LiFePO4. The performance of FePO4 (structure, particle size, morphology) will be carried over to LiFePO4, thereby affecting the electrochemical performance of LiFePO4. Therefore, in order to improve the electrochemical performance of LiFePO4, we usually modify FePO4. The methods include ①Nano-materialization: controlling the morphology and particle size of the product to prepare porous or nanomaterials. Porous materials: have a large specific surface area and abundant pores, which are not only conducive to the large-scale storage of lithium ions, that is, conducive to the large-scale adsorption of lithium ions on the FePO4 surface during the charge and discharge process, but also can reduce the diffusion resistance of lithium ions during the charge and discharge process, thereby increasing the conductivity of the material and improving the electrochemical performance of the material. Nanomaterials: shorten the lithium ion diffusion path, reduce the lithium ion diffusion resistance, and effectively improve the electrochemical properties of LiFePO4 positive electrode materials; ② Surface coating (physical method): cover the surface of FePO4 particles with a layer of highly conductive material to increase the conductivity; ③ Ion doping (chemical method): dope metal ions or highly conductive ions with FePO4 to change the internal structure of FePO4 and increase conductivity.

[0003] As a precursor to lithium iron phosphate (LFP), its morphology, particle size, and dispersibility are transferred to the LFP material and significantly impact its electrochemical performance. Therefore, research into preparing LFP with a regular morphology, small particle size, and high dispersibility holds great application value. Chinese patent CN201010209297.4 discloses a method for preparing LFP using a nonionic surfactant. Specifically, LFP with a particle size of 50-300 nm is synthesized in an aqueous system using phosphate and a ferric salt (ferric nitrate, ferric sulfate, or ferric chloride) in the presence of a nonionic surfactant (polyethylene glycol, polymethyl acrylate). This method produces LFP with high dispersibility, a simple synthesis process, and ease of industrialization. However, the particle size distribution is uneven, affecting the electrochemical performance of the subsequent LFP. Furthermore, the reaction process requires high temperatures (80-100°C), resulting in high energy consumption and the expensive ferric salt used as the raw material. Furthermore, the introduction of additives can cause pollution and complicate post-processing. Chinese patent CN200710059805.3 discloses a method for preparing iron phosphate for lithium iron phosphate materials. Specifically, an anionic surfactant is added to an aqueous solution of a soluble phosphate and a trivalent iron salt to form surfactant-encapsulated micelles of iron ions. The anionic surfactant used is sodium dodecylbenzenesulfonate, sodium olefinsulfonate, or sodium lauryl sulfate. The resulting iron phosphate is spherical and evenly dispersed, and the process is simple. However, the particle size is 12-25 μm, which is large and affects the electrochemical performance of the subsequent lithium iron phosphate. Furthermore, the added additive complicates the post-processing process and causes pollution. Patent application publication number CN10695998A discloses a method for preparing nanoscale iron phosphate. The prepared iron phosphate particles have a size of 10-300 nm. Although nanoscale, the particle size distribution is wide, and the rotating packed bed reactor used is complex to use, making it unsuitable for large-scale production. Chinese patent CN202010006642.8 discloses a nano-iron phosphate material prepared using highly conductive carbon nanomaterials and dispersants (polyethylene glycol, polyvinyl alcohol, citric acid, polyvinyl pyrrolidone). The material has a small particle size (100nm) and high dispersibility. The doped carbon can improve thermal uniformity and enhance the stability and consistency of iron phosphate during high-temperature dehydration. However, the experimental temperature required is high (90-99°C), which consumes a lot of energy. At the same time, the added dispersant complicates the post-processing process and causes pollution. Summary of the Invention

[0004] In view of the problems existing in the prior art, the object of the present invention is to provide a method for preparing low-temperature and highly dispersible spherical nano-ferric phosphate.

[0005] The specific technical solutions are as follows:

[0006] A method for preparing low-temperature, highly dispersible spherical nano-ferric phosphate comprises the following steps:

[0007] 1) Use pure water to dissolve divalent iron salt as the base liquid, pour soluble phosphate or phosphoric acid solution into it while stirring continuously, and use a constant pressure dropping funnel to add hydrogen peroxide solution to oxidize Fe 2+ ;

[0008] 2) Raise the temperature to 50-70°C, adjust the pH to 1.7-2.2 with sodium hydroxide solution, and react in an oil bath with constant stirring for 3 hours;

[0009] 3) Filter the slurry while hot to obtain iron phosphate precipitate, wash with pure water, filter, repeat 5 times, dry the product, and calcine to obtain white iron phosphate powder.

[0010] Furthermore, the divalent iron salt is ferrous nitrate, ferrous chloride or ferrous sulfate, and the soluble phosphate is ammonium phosphate, sodium phosphate or potassium phosphate.

[0011] Furthermore, the concentration of the divalent iron salt solution in step 1) is 165-240 g / L.

[0012] Furthermore, in step 1), the amount of soluble phosphate or phosphoric acid added is 3-10% excess of the theoretical stoichiometric ratio, the stirring speed is 800-1000 r / min, and the stirring time is 2-3 minutes.

[0013] Furthermore, in step 1), the amount of hydrogen peroxide added is 10-20% excess of the theoretical stoichiometric ratio, and the Fe 2+ The method is potassium permanganate titration or o-phenanthroline indicator method, the temperature is room temperature, and the dropping speed is 1d / s.

[0014] Furthermore, in step 2), the stirring speed is 800-1000 r / min, the dropping speed of sodium hydroxide is 1 d / s, and the amount of sodium hydroxide used is 1.3-1.6 of the amount of ferrous sulfate used.

[0015] Furthermore, in step 3), pure water ten times the mass of the ferric phosphate precipitate is used for washing.

[0016] Furthermore, in step 3), the drying temperature is 60-120° C., and the drying time is 6-12 hours.

[0017] Furthermore, the calcination device in step 3) is a tubular furnace or a muffle furnace, the calcination temperature is 500-550°C, the calcination time is 3-4 hours, and the heating rate is 5-10°C / min.

[0018] The beneficial effects of the present invention are:

[0019] The use of inexpensive ferrous sulfate heptahydrate and phosphoric acid as raw materials saves costs and makes the entire experimental process simple to operate. The prepared nano-scale anhydrous ferric phosphate has a small particle size, which shortens the lithium ion diffusion path, reduces the diffusion resistance of lithium ions during the charge and discharge process, increases the conductivity of the material, and improves the electrochemical properties of the material. No additives are introduced, the cost is low, the post-processing is simple, and the interference and contamination of impurities are avoided. The experimental process requires a low temperature, a short reaction time, and low energy consumption. The spherical particles have excellent fluidity and good dispersibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the XRD pattern of the anhydrous ferric phosphate prepared in Example 1 of the present invention;

[0021] Figure 2 This is a scanning electron micrograph of the anhydrous ferric phosphate prepared in Example 1 of the present invention;

[0022] Figure 3 This is a scanning electron micrograph of anhydrous ferric phosphate prepared in Example 2 of the present invention;

[0023] Figure 4 This is a scanning electron micrograph of the anhydrous ferric phosphate prepared in Comparative Example 1 of the present invention;

[0024] Figure 5 This is a scanning electron micrograph of anhydrous ferric phosphate prepared in Comparative Example 2 of the present invention;

[0025] Figure 6 This is a scanning electron microscope image of the anhydrous ferric phosphate prepared in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the protection scope of the present invention is not limited thereto.

[0027] Example 1

[0028] Add 200 ml of pure water to 69.5188 g of FeSO4·7H2O to prepare a solution, which was then poured into a 1000 ml three-necked flask as the base solution. The pH was measured at 24.6°C to be 2.78.

[0029] 2) Turn on the speed to 1000 r / min, then pour 30.0061 g of H3PO4 (85% H3PO4) into the three-necked flask at once. Use a pipette to rinse the beaker containing H3PO4 with 2 ml of pure water and pour it into the flask. Stir for 2 minutes. The pH is 0.80 at 27.2°C.

[0030] 3) Pour 17.0115g H2O2 with a mass concentration of 30% into a 60ml constant pressure dropping funnel, adjust the speed to 500r / min, adjust the dripping rate to 1d / s, and detect Fe 2+ The method was potassium permanganate titration. After the addition was complete, the temperature was 44.7°C and the pH was measured to be 1.10.

[0031] 4) Insert the condenser, rotate at 500 rpm, turn on the oil bath heating button, and raise the temperature from 31°C to 70°C (use the thermometer to measure the liquid temperature in the flask to be 70°C). The actual oil bath display shows 92°C, and the heating time is 30 minutes.

[0032] 5) Pour the prepared 6.66 mol / L NaOH solution into a 60 ml constant pressure dropping funnel at 1000 r / min and a dripping rate of 1 d / s. The pH is measured to be 1.70. The amount of NaOH used is 50 ml. The solution is reacted in the three-necked flask for 3 h.

[0033] 6) After the reaction is complete, filter while hot. After filtration, place the solid in a 1000ml beaker, add 500ml of pure water, and stir with an electric stirrer for 3 minutes at a speed of 200r / min. This is the first time. Then filter and wash again, and repeat this process 5 times.

[0034] 7) After washing, transfer the solids to a large glass Petri dish and place in a forced air drying oven at 60°C for 12 hours. Do not heat the oven until the temperature reaches 60°C.

[0035] 8) Grind, place the sample in a quartz boat, and calcine in a tube furnace at 550°C for 3 h in an air atmosphere at a heating rate of 5°C / min to obtain anhydrous ferric phosphate product.

[0036] like Figure 1 As shown, the XRD pattern shows that the spectrum is similar to the spectrum and diffraction data of the FePO4 standard card (29-0715), indicating hexagonal FePO4 with space group P321 and unit cell parameters of a=5.035, b=5.035, and c=11.245. The peaks are sharp, indicating good crystallinity of the sample, and no other impurity peaks are observed in the spectrum, indicating that the ferric phosphate sample prepared by this method is pure FePO4.

[0037] like Figure 2 As shown, scanning electron microscopy observation shows that the prepared anhydrous ferric phosphate particles are less than 100 nm in size, spherical in shape, uniform in size, and well dispersed.

[0038] Example 2

[0039] Add 200 ml of pure water to 69.5279 g of FeSO4·7H2O to prepare a solution, which was poured into a 1000 ml three-necked flask as the base solution. The pH was measured at 24.7 °C and was 2.49.

[0040] 2) Turn on the speed to 1000 r / min, then pour 29.9888 g of H3PO4 (85% H3PO4) into the three-necked flask at once. Use a pipette to rinse the beaker containing H3PO4 with 2 ml of pure water and pour it into the flask. Stir for 2 minutes. The pH is 0.90 at 26.0°C.

[0041] 3) Pour 17.0532g H2O2 with a mass concentration of 30% into a 60ml constant pressure dropping funnel, adjust the speed to 500r / min, adjust the dripping rate to 1d / s, and detect Fe 2+ The method is potassium permanganate titration. After the addition is completed, the temperature is 47.0℃ and the pH is measured to be 0.75.

[0042] 4) Insert the condenser, rotate at 500 rpm, turn on the oil bath heating button, and raise the temperature from 29°C to 50°C (use the thermometer to measure the liquid temperature in the flask to be 50°C). The actual oil bath display shows 55°C, and the heating time is 17 minutes.

[0043] 5) Pour the prepared 6.66 mol / L NaOH solution into a 60 ml constant pressure dropping funnel at 1000 r / min and a dripping rate of 1 d / s. The pH is measured to be 1.71. The amount of NaOH used is 48 ml. The solution is reacted in the three-necked flask for 3 h.

[0044] 6) After the reaction is complete, filter while hot. After filtration, place the solid in a 1000ml beaker, add 500ml of pure water, and stir with an electric stirrer for 3 minutes at a speed of 200r / min. This is the first time. Then filter and wash again, and repeat this process 5 times.

[0045] 7) After washing, transfer the solids to a large glass Petri dish and place in a forced air drying oven at 60°C for 12 hours. Do not heat the oven until the temperature reaches 60°C.

[0046] 8) Grind, place the sample in a quartz boat, and calcine in a tube furnace at 550°C for 3 h in an air atmosphere at a heating rate of 5°C / min to obtain anhydrous ferric phosphate product.

[0047] Comparative Example 1

[0048] Add 200 ml of pure water to 69.5074 g of FeSO4·7H2O to prepare a solution. Pour it into a 1000 ml three-necked flask as the bottom solution. The pH is 2.90 at 25.4 °C.

[0049] Turn on the speed to 1000 r / min, then pour 29.9804 g of H3PO4 with a mass concentration of 85% into the three-necked flask at once. Use a pipette to transfer 2 ml of pure water to the beaker containing H3PO4 and pour it into the flask. Stir for 2 minutes. The pH is 1.44 at 26.6°C.

[0050] Pour 17.0007g H2O2 with a mass concentration of 30% into a 60ml constant pressure dropping funnel, adjust the speed to 500r / min, adjust the dropping rate to 1d / s, and detect Fe 2+ The method is potassium permanganate titration. After the addition is completed, the temperature is 48°C and the pH is measured to be 1.19;

[0051] Insert the condenser, rotate at 500 r / min, turn on the oil bath heating button, and raise the temperature from 31°C to 70°C (the temperature of the liquid in the flask is 70°C). The actual oil bath display shows 90°C, and the heating time is 28 minutes.

[0052] Pour all the prepared 6.66 mol / L NaOH solution into a 60 ml constant pressure dropping funnel, adjust the speed to 1000 r / min, and the dropping rate to 1 d / s. The pH is measured to be 1.70, and the amount of NaOH is 48 ml. The solution is reacted in the three-necked flask for 4 h.

[0053] After the reaction is completed, filter while hot. After filtration, put the solid into a 1000ml beaker, add 500ml of pure water, and stir with an electric stirrer for 3 minutes at a speed of 200r / min. This is the first time, then filter and wash again, and repeat this 5 times;

[0054] After washing, the solid was transferred to a large Petri dish and placed in a forced air drying oven at 60°C for 12 h without a temperature increase program. The solid was placed in the forced air drying oven when it reached 60°C.

[0055] After grinding, the sample was placed in a quartz boat and calcined in a tube furnace at 550°C in an air atmosphere for 3 h at a heating rate of 5°C / min to obtain anhydrous ferric phosphate product.

[0056] Comparative Example 2

[0057] Add 200 ml of pure water to 69.5038 g of FeSO4·7H2O to prepare a solution, which was then poured into a 1000 ml three-necked flask as the base solution. The pH was measured at 23.4°C to be 3.37.

[0058] Turn on the speed to 1000 r / min, then pour 29.9830 g of H3PO4 with a mass concentration of 85% into the three-necked flask at once. Use a pipette to transfer 2 ml of pure water to the beaker containing H3PO4 and pour it into the flask. Stir for 2 minutes. The pH is 1.11 at 24.6°C.

[0059] Pour 17.0080g H2O2 with a mass concentration of 30% into a 60ml constant pressure dropping funnel, adjust the speed to 500r / min, adjust the dripping rate to 1d / s, and detect Fe 2+ The method was potassium permanganate titration. After the addition was complete, the temperature was 44.7°C and the pH was measured to be 0.82.

[0060] Insert the condenser, rotate at 500 r / min, turn on the oil bath heating button, and raise the temperature from 31°C to 90°C (the temperature of the liquid in the flask is measured to be 90°C by thermometer). The actual oil bath display shows 122°C, and the heating time is 28 minutes.

[0061] Pour all the prepared 6.66 mol / L NaOH solution into a 60 ml constant pressure dropping funnel, adjust the speed to 1000 r / min, and the dropping rate to 1 d / s. The pH is measured to be 1.72, and the amount of NaOH is 47 ml. The solution is reacted in a three-necked flask for 3 h.

[0062] After the reaction is complete, filter while hot. After filtration, put the solid into a 1000ml beaker, add 500ml of pure water, and stir with an electric stirrer for 3 minutes at a speed of 200r / min. This is the first time, then filter and wash again, and repeat this process 5 times.

[0063] After washing, transfer the solids to a large Petri dish and place in a forced air drying oven at 60°C for 12 hours. There is no heating program, so wait until the forced air drying oven reaches 60°C before placing it in.

[0064] After grinding, the sample was placed in a quartz boat and calcined in a tube furnace at 550°C in an air atmosphere for 3 h at a heating rate of 5°C / min to obtain anhydrous ferric phosphate product.

[0065] Comparative Example 3

[0066] Add 200 ml of pure water to 69.5082 g of FeSO4·7H2O to prepare a solution, which was then poured into a 1000 ml three-necked flask as the base solution. The pH was measured at 23.4°C to be 3.04.

[0067] Turn on the speed to 1000 r / min, then pour 29.9848 g of H3PO4 with a mass concentration of 85% into the three-necked flask at once. Use a pipette to transfer 2 ml of pure water to the beaker containing H3PO4 and pour it into the flask. Stir for 2 minutes. The pH is 1.58 at 25.5°C.

[0068] Pour 17.0086g of H2O2 with a mass concentration of 30% into a 60ml constant pressure dropping funnel, adjust the speed to 500r / min, and adjust the dropping rate to 1d / s. After the addition is completed, the temperature is 45℃ and the pH is measured to be 1.28.

[0069] Insert the condenser, rotate at 500 r / min, turn on the oil bath heating button, and raise the temperature from 31°C to 100°C (the temperature of the liquid in the flask is 100°C). The actual oil bath display shows 140°C, and the heating time is 42 minutes.

[0070] Pour all the prepared 6.66 mol / L NaOH solution into a 60 ml constant pressure dropping funnel, adjust the speed to 1000 r / min, and the dropping rate to 1 d / s. The pH value is 1.71, the amount of NaOH is 46 ml, and the reaction is carried out for 3 h.

[0071] After the reaction is complete, filter while hot. After filtration, put the solid into a 1000ml beaker, add 500ml of pure water, and stir with an electric stirrer for 3 minutes at a speed of 200r / min. This is the first time, then filter and wash again, and repeat this process 5 times.

[0072] After washing, transfer the solids to a large Petri dish and place them in a forced air drying oven at 60°C for 12 hours. There is no temperature increase program. Wait until the forced air drying oven reaches 60°C before placing them in.

[0073] After grinding, the sample was placed in a quartz boat and calcined in a tube furnace at 550°C in an air atmosphere for 3 h at a heating rate of 5°C / min to obtain anhydrous ferric phosphate product.

[0074] The main difference between the preparation step 1 of Comparative Example 1 and Example 1 is that the reaction time is increased by 1 hour. The main difference between the preparation steps of Comparative Examples 2 and 3 and Example 1 is that the temperature is increased to 90°C and 100°C. Example 1 is a sample at a temperature of 70°C, and Example 2 is a sample at a temperature of 50°C. Figure 2 and Figure 3 It can be found that the prepared anhydrous iron phosphate particles are less than 100nm in size, spherical in shape, uniform in size, and well dispersed. The samples of comparative examples 1, 2, and 3 were analyzed by scanning electron microscopy. Figures 4 to 6As shown, it can be seen that with the increase of reaction temperature and the extension of reaction time, the aggregation of iron phosphate particles is significantly enhanced (the higher the reaction temperature is, the better. As the temperature continues to rise, the Brownian motion of molecules intensifies, the kinetic energy of molecular motion increases, the chance of collision and contact between particles increases, and small particles aggregate with each other to form aggregates), and the morphology becomes no longer regular and uniform.

Claims

1. A method for preparing low-temperature and highly dispersible spherical nano-ferric phosphate, characterized in that: The steps include: 1) Dissolve divalent iron salt in pure water as the base liquid, pour soluble phosphate or phosphoric acid solution into it under constant stirring, and use a constant pressure dropping funnel to add hydrogen peroxide solution to oxidize Fe 2+ ; 2) Raise the temperature to 50-70°C, adjust the pH to 1.7-2.2 with sodium hydroxide solution, and react in an oil bath with constant stirring for 3 hours; 3) Filter the slurry while hot to obtain an iron phosphate precipitate, wash it with pure water, and filter it, repeat this process five times, dry the product, and calcine it to obtain a white iron phosphate powder; In step 2), the dropping speed of sodium hydroxide is 1d / s, and the amount of sodium hydroxide is 1.3-1.6 of the amount of ferrous sulfate.

2. The preparation method according to claim 1, wherein The divalent iron salt is ferrous nitrate, ferrous chloride or ferrous sulfate, and the soluble phosphate is ammonium phosphate, sodium phosphate or potassium phosphate.

3. The preparation method according to claim 2, wherein The concentration of the ferrous salt solution in step 1) is 165-240 g / L.

4. The preparation method according to claim 2, wherein In step 1), the amount of soluble phosphate or phosphoric acid added is 3-10% excess of the theoretical stoichiometric ratio, the stirring speed is 800-1000 r / min, and the stirring time is 2-3 minutes.

5. The preparation method according to claim 4, wherein In step 1), the amount of hydrogen peroxide added is 10-20% excess of the theoretical stoichiometric ratio. 2+ The method is potassium permanganate titration or o-phenanthroline indicator method, the temperature is room temperature, and the dropping speed is 1d / s.

6. The preparation method according to claim 1, wherein The stirring speed in step 2) is 800-1000 r / min.

7. The preparation method according to claim 1, wherein In step 3), the precipitated iron phosphate is washed with pure water in an amount ten times the mass of the precipitated iron phosphate.

8. The preparation method according to claim 1, wherein In step 3), the drying temperature is 60-120° C. and the drying time is 6-12 hours.

9. The preparation method according to claim 1, wherein The calcination device in step 3) is a tubular furnace or a muffle furnace, the calcination temperature is 500-550° C., the calcination time is 3-4 hours, and the heating rate is 5-10° C. / min.

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