A method for preparing ferric phosphate

Abstract

This article discloses a method for preparing iron phosphate, which belongs to the field of material synthesis technology. The method introduces a pH-sensitive agent chitosan as a modifier in the process of synthesizing iron phosphate by co-precipitation. During the co-precipitation, the hydrated iron phosphate particles are adsorbed and aggregated and precipitated. At the same time, after adjusting the pH to alkaline, a layer of basic iron phosphate salt is further formed on the surface of the particle grains, and the chitosan becomes hydrophobic and precipitates quickly, thereby inhibiting the agglomeration of grains and recrystallization. The prepared particles are small and uniform in size. After sintering, the basic iron phosphate salt and chitosan will give the iron phosphate a partially loose morphology, which is more conducive to improving the lithium deintercalation performance of the lithium iron phosphate finally prepared.

Description

Technical Field

[0001] This article relates to the field of material technology, and in particular to a method for preparing iron phosphate. Background Art

[0002] Iron phosphate is an important precursor for the solid-phase synthesis of lithium iron phosphate, the positive electrode active material for lithium-ion batteries. During the synthesis process, the morphology and structure of iron phosphate will be inherited by the final prepared lithium iron phosphate. Therefore, the quality of iron phosphate has a great influence on the quality of the prepared lithium iron phosphate.

[0003] In the prior art, iron phosphate is primarily prepared by coprecipitation, a process involving three stages: ion saturation, nucleation, and crystal growth. Each stage affects the final morphology of the iron phosphate. The smaller the iron phosphate particle size, the smaller the final lithium iron phosphate particle size will theoretically be, leading to better electrochemical performance. However, conventional coprecipitation processes for preparing iron phosphate generally experience particle agglomeration, which is detrimental to the further formation of secondary particles. Therefore, additives such as surfactants are used to ameliorate this phenomenon. However, such methods require specific processing steps, some of which can take more than twice as long as conventional processes, resulting in a low production cost-effectiveness. Summary of the Invention

[0004] The purpose of this article is to overcome the shortcomings of the above-mentioned prior art and provide a method for preparing iron phosphate. The method introduces the pH-sensitive agent chitosan as a modifier in the process of co-precipitation synthesis of iron phosphate, adsorbs and aggregates hydrated iron phosphate particles and precipitates them during co-precipitation. At the same time, after adjusting the pH to alkaline, a layer of basic iron phosphate salt is further formed on the surface of the particle grains, and the chitosan becomes hydrophobic and precipitates quickly, thereby inhibiting grain agglomeration and recrystallization. The prepared particles are small and uniform in size. After sintering, the basic iron phosphate salt and chitosan will give the iron phosphate a partially loose morphology, which is more conducive to improving the lithium intercalation and deintercalation performance of the lithium iron phosphate finally prepared.

[0005] To achieve the above objectives, the technical solutions adopted in this paper are:

[0006] A method for preparing ferric phosphate comprises the following steps:

[0007] Mix the iron source, phosphate source and chitosan in water, stir and heat to 60-90°C, adjust the pH of the mixture to 1.5-2.2, and age to obtain a suspension;

[0008] The suspension is adjusted to a pH of 5.5-8 and allowed to stand, and the solid is filtered, washed, and dried in sequence, and then kept at 550-750° C. for 3-10 hours in an air atmosphere to obtain the ferric phosphate.

[0009] Chitosan is a pH-sensitive surface stabilizer that is water-soluble under acidic conditions but hydrophobic under weakly acidic or alkaline conditions. In the process of water solubility, it exhibits good particle adsorption. Therefore, in this paper, it is mixed with an iron source and a phosphate source and the reaction solution is first conditioned to be acidic. In this process, hydrated iron phosphate precipitates are formed, and the water-soluble chitosan will be adsorbed together with the hydrated iron phosphate to achieve good distribution uniformity; then the pH of the suspension containing these particles is adjusted to alkaline. This process not only converts the unreacted iron ions in the solution into basic iron phosphate salts in situ on the particle surface, but also converts the chitosan into hydrophobicity under this condition, accelerating the aggregation of the particles. Aggregation and precipitation are carried out to prevent the particles from becoming too large due to Ostwald ripening and recrystallization. The hydrated iron phosphate mixture particles obtained in this process are not only nano-sized in primary particle size, but also spherical in secondary particles with uniform size and morphology. Finally, after sintering, the impurities (ammonium ions) and chitosan in the basic iron phosphate salt that are in situ coated or adsorbed on the surface of the hydrated iron phosphate are removed together with water by high-temperature decomposition, leaving a loose porous structure and a carbon layer-coated composite structure on the surface of the finally formed iron phosphate. These two structures can not only provide high conductivity for the finally prepared lithium iron phosphate material, but also provide more active sites for the lithium deintercalation process, ultimately achieving excellent electrochemical performance.

[0010] In one embodiment, the molecular weight of chitosan is 30,000-50,000 g / mol, and the degree of deacetylation is 70-80%.

[0011] The molecular weight of chitosan has a certain influence on its precipitation and adsorption effects. When the chitosan described in this article selects low molecular weight chitosan of the above molecular weight, its precipitation and adsorption effects are better.

[0012] In one embodiment, the mass ratio of chitosan to iron source is (0.001-0.1):1.

[0013] Furthermore, the mass ratio of chitosan to iron source is (0.02-0.07):1.

[0014] The main function of chitosan is to adsorb the particles formed by co-precipitation and accelerate their precipitation. Therefore, if the amount introduced is low, it will not be able to improve the large size of the particles and the agglomeration phenomenon to a sufficient extent. However, if the amount introduced is high, it may also cause its own agglomeration and precipitation. The performance is better within the above range.

[0015] In one embodiment, the molar ratio of iron atoms in the iron source to phosphorus atoms in the phosphate source is 1:(1-1.1).

[0016] Furthermore, the iron source is at least one of elemental iron, ferric sulfate, ferric chloride, ferric nitrate, and ferric chloride.

[0017] It should be noted that the iron source described herein can also use a divalent iron source. When using a divalent iron source, it is necessary to first use an oxidant to convert the divalent iron ions in the divalent iron source into trivalent iron ions, and then mix and react with a phosphate source.

[0018] Furthermore, the phosphate source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, and sodium hydrogen phosphate.

[0019] In one embodiment, the stirring rate is 400-1200 r / min.

[0020] In one embodiment, the aging time is 10-30 minutes, and the aging temperature is 60-90°C.

[0021] In one embodiment, the pH adjuster for adjusting the pH of the suspension to 5.5-8 is an ammonia solution with a concentration of 1-5 mol / L.

[0022] Furthermore, the ammonia solution adjusts the pH of the suspension to 6-7.

[0023] In one embodiment, after adjusting the pH of the suspension to 5.5-8, the suspension is allowed to stand for 0.5-2 hours, and the drying temperature is 70-120° C. for 6-16 hours.

[0024] Another purpose of this article is to provide the application of the preparation method of iron phosphate in the preparation of lithium iron phosphate material.

[0025] The iron phosphate obtained by the preparation method of iron phosphate described in this article has a nano-sized primary particle size and uniform morphology due to the adsorption and accelerated sedimentation of chitosan. At the same time, there is an obvious loose structure and carbon layer coating structure on the surface of the iron phosphate, excellent conductivity, and abundant lithium deintercalation sites. After further introducing it into a lithium source to prepare lithium iron phosphate, the resulting product has ideal electrochemical activity.

[0026] In one embodiment, the lithium iron phosphate material is prepared by a solid phase method.

[0027] Furthermore, the preparation method of the lithium iron phosphate material comprises the following steps:

[0028] The iron phosphate prepared by the preparation method of the iron phosphate described herein is mixed with a lithium source and a carbon source in water, and then spray-dried to obtain a precursor powder;

[0029] The precursor powder is heated to 200-300° C. under a protective atmosphere and pre-heated for 1-2 hours, and then heated to 700-800° C. and heated for 6-7 hours to obtain the lithium iron phosphate material.

[0030] In one embodiment, the molar ratio of the iron atoms in the iron phosphate, the lithium atoms in the lithium source, and the carbon atoms in the carbon source is 1:(1.02-1.05):(0.06-0.1).

[0031] Compared with the existing technology, the beneficial effects of this article are:

[0032] This article provides a method for preparing iron phosphate. The method introduces the pH-sensitive agent chitosan as a modifier in the process of co-precipitation synthesis of iron phosphate. During the co-precipitation, the hydrated iron phosphate particles are adsorbed and aggregated and precipitated. At the same time, after adjusting the pH to alkaline, a layer of basic iron phosphate salt is further formed on the surface of the particle grains, and the chitosan becomes hydrophobic and precipitates quickly, thereby inhibiting the agglomeration of grains and recrystallization. The prepared particles are small and uniform in size. After sintering, the basic iron phosphate salt and chitosan will give the iron phosphate a partially loose morphology, which is more conducive to improving the lithium intercalation and deintercalation performance of the lithium iron phosphate finally prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a scanning electron microscope image of iron phosphate prepared by the method for preparing iron phosphate described in this article. DETAILED DESCRIPTION

[0034] To better illustrate the purpose, technical solutions and advantages of this article, this article will be further described below with reference to the accompanying drawings and specific embodiments.

[0035] Unless otherwise specified, the materials used in the examples and comparative examples can be obtained through commercial channels.

[0036] Example 1

[0037] An embodiment of the method for preparing ferric phosphate described herein comprises the following steps:

[0038] (1) A 1 mol / L ferric chloride aqueous solution, a 1 mol / L phosphoric acid solution and chitosan were mixed uniformly in a stirred reactor, stirred at 500 r / min and heated to 85°C, and the pH of the mixture was adjusted to 1.8 with 2 mol / L ammonia water. The mixture was kept warm and aged for 15 min to obtain a suspension. The molecular weight of chitosan was 50,000 g / mol, the degree of deacetylation was 75%, and the mass ratio of chitosan to ferric chloride was 0.05:1. The molar ratio of iron ions to phosphate ions in the reactor was 1:1.

[0039] (2) The suspension was adjusted to pH 6.5 with a 4 mol / L ammonia solution and allowed to stand for 0.5 h. The solid was filtered, washed with distilled water, and dried in an 85°C oven for 16 h. The temperature was then raised to 650°C in a muffle furnace under air atmosphere at a rate of 3°C / min and kept at that temperature for 5 h to obtain the iron phosphate. The iron phosphate was observed under a scanning electron microscope. Figure 1As shown, it can be seen that the sample has good uniform dispersion.

[0040] Example 2

[0041] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the mass ratio of chitosan to ferric chloride is 0.02:1.

[0042] Example 3

[0043] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the mass ratio of chitosan to ferric chloride is 0.07:1.

[0044] Example 4

[0045] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the mass ratio of chitosan to ferric chloride is 0.01:1.

[0046] Example 5

[0047] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the mass ratio of chitosan to ferric chloride is 0.1:1.

[0048] Example 6

[0049] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the pH of the suspension in step (2) is adjusted to 5.5.

[0050] Example 7

[0051] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the pH of the suspension in step (2) is adjusted to 7.5.

[0052] Example 8

[0053] An embodiment of the method for preparing ferric phosphate described herein differs from Example 1 only in that the molecular weight of chitosan in step (1) is 100,000 g / mol.

[0054] Comparative Example 1

[0055] A method for preparing ferric phosphate, comprising the following steps:

[0056] (1) A 1 mol / L ferric chloride aqueous solution and a 1 mol / L phosphoric acid solution were mixed uniformly in a stirred reactor, stirred at 500 r / min, heated to 85°C, and the pH of the mixture was adjusted to 1.8 with 2 mol / L ammonia water. The mixture was kept warm and aged for 3 h to obtain a suspension; the molar ratio of iron ions to phosphate ions in the reactor was 1:1.

[0057] (2) The suspension was filtered, washed with distilled water, and dried in an 85° C. drying oven for 16 h. The temperature was then raised to 650° C. in a muffle furnace under air atmosphere at a rate of 3° C. / min and kept at that temperature for 5 h to obtain the ferric phosphate.

[0058] Comparative Example 2

[0059] A method for preparing ferric phosphate, comprising the following steps:

[0060] (1) A 1 mol / L ferric chloride aqueous solution and a 1 mol / L phosphoric acid solution were mixed uniformly in a stirred reactor, stirred at 500 r / min, heated to 85°C, and the pH of the mixture was adjusted to 1.8 with 2 mol / L ammonia water. The mixture was kept warm and aged for 3 h to obtain a suspension; the molar ratio of iron ions to phosphate ions in the reactor was 1:1.

[0061] (2) The suspension was adjusted to pH 6.5 with a 4 mol / L ammonia solution and allowed to stand for 0.5 h. The solid was filtered, washed with distilled water, and dried in an 85° C. drying oven for 16 h. The temperature was then raised to 650° C. in a muffle furnace under air atmosphere at a rate of 3° C. / min and maintained for 5 h to obtain the ferric phosphate.

[0062] Comparative Example 3

[0063] A method for preparing ferric phosphate, comprising the following steps:

[0064] (1) A 1 mol / L ferric chloride aqueous solution, a 1 mol / L phosphoric acid solution and chitosan were mixed uniformly in a stirred reactor, stirred at 500 r / min and heated to 85°C, and the pH of the mixture was adjusted to 1.8 with 2 mol / L ammonia water. The mixture was kept warm and aged for 15 min to obtain a suspension. The molecular weight of chitosan was 50,000 g / mol, the degree of deacetylation was 75%, and the mass ratio of chitosan to ferric chloride was 0.05:1. The molar ratio of iron ions to phosphate ions in the reactor was 1:1.

[0065] (2) The suspension was filtered, washed with distilled water, and dried in an 85° C. drying oven for 16 h. The temperature was then raised to 650° C. in a muffle furnace under air atmosphere at a rate of 3° C. / min and kept at that temperature for 5 h to obtain the ferric phosphate.

[0066] Comparative Example 4

[0067] A method for preparing ferric phosphate, which differs from Example 1 only in that the pH of the suspension in step (2) is adjusted to 8.5.

[0068] Effect Example 1

[0069] In order to verify the performance of the iron phosphate described herein, the product of each embodiment or comparative example, lithium carbonate, and glucose were mixed in water and ball-milled at 600 rpm for 4 h until uniform, and then spray-dried to obtain a precursor powder; the iron atoms in the iron phosphate product, the lithium atoms in the lithium carbonate, and the carbon atoms in the glucose were in a molar ratio of 1:1.03:0.08;

[0070] The precursor powder was heated to 250° C. at 3° C. / min in a nitrogen atmosphere and pre-heated for 1.5 hours, and then heated to 750° C. and heated for 6.5 hours to obtain the lithium iron phosphate material.

[0071] Each lithium iron phosphate material was used as the positive electrode active material. The positive electrode active material, the conductive agent acetylene black, and the binder polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 90:5:5. The mixture was then coated on aluminum foil and dried in a vacuum drying oven. After drying, the battery was assembled in an argon glove box and pressed into positive electrode sheets using a tablet press. The negative electrode was a metal lithium sheet. The electrolyte was 1M LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane was used as the separator. The discharge specific capacity of each battery was tested at different current densities of 0.1C, 1C, and 3C in the range of 2.5-4.2V. The test was repeated five times at different current densities and the last value was taken. The capacity retention rate was also tested after 100 cycles at a 3C rate. The results are shown in Table 1.

[0072] Table 1

[0073]

[0074]

[0075] As can be seen from Table 1, the iron phosphate prepared by the preparation method of iron phosphate described herein has an ideal electrochemical activity after further preparing the lithium iron phosphate material by the solid phase method. Even at a high rate of 3C, its discharge capacity can still reach more than 130mAh / g, and the cycle performance is excellent. After 100 cycles at a rate of 3C, the capacity retention rate reaches more than 96.3%. In contrast, due to the lack of the introduction of chitosan, the product prepared in Comparative Example 1 can still reach more than 150mAh / g at a rate of 0.1C, but the high rate discharge capacity is low, indicating that the introduction of chitosan in the preparation method described herein has a great influence on the morphological modification of iron phosphate. Based on Comparative Example 1, Comparative Example 2 further carried out two pH adjustments. Although some basic ammonium ferric phosphate was formed on the surface of the hydrated iron phosphate, the performance of the prepared product was not significantly improved due to the lack of the rapid sedimentation effect of chitosan. Although chitosan was introduced in Comparative Example 3, the pH sensitivity of chitosan was not utilized, and the second alkaline pH adjustment was not performed, so the particle morphology could not achieve the expected effect. The pH of the product in Comparative Example 4 was set too high during the second pH adjustment process, which affected the quality of the generated ferric phosphate and resulted in a loss of active substances in the final product. The performance comparison of the products in Examples 1-5 shows that the chitosan addition concentration has a certain effect on the morphology and structure of the product. Compared with Examples 4 and 5, the products in Examples 1-3 corresponding to the moderate addition range have better performance.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this article and are not intended to limit the scope of protection of this article. Although this application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of this article may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of this article.

Claims

1. A method for preparing ferric phosphate, characterized in that: The following steps are involved: Mix the iron source, phosphate source and chitosan in water, stir and heat to 60-90°C, adjust the pH of the mixture to 1.5-2.2, and age to obtain a suspension; The suspension is adjusted to a pH of 5.5-8 and allowed to stand, and the solid is filtered, washed, and dried in sequence, and then kept at 550-750° C. for 3-10 hours in an air atmosphere to obtain the ferric phosphate.

2. The method for preparing ferric phosphate according to claim 1, wherein: The molecular weight of chitosan is 30,000-50,000 g / mol, and the degree of deacetylation is 70-80%.

3. The method for preparing ferric phosphate according to claim 1, wherein: The mass ratio of chitosan to iron source is (0.001-0.1):

1.

4. The method for preparing ferric phosphate according to claim 1, wherein: The molar ratio of the iron atoms in the iron source to the phosphorus atoms in the phosphate source is 1:(1-1.1).

5. The method for preparing ferric phosphate according to claim 4, wherein: The iron source is at least one of elemental iron, ferric sulfate, ferric chloride, ferric nitrate, and ferric chloride.

6. The method for preparing ferric phosphate according to claim 4, wherein: The phosphate source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, and sodium hydrogen phosphate.

7. The method for preparing ferric phosphate according to claim 1, wherein: The aging time is 10-30 minutes, and the aging temperature is 60-90°C.

8. The method for preparing ferric phosphate according to claim 1, wherein: The pH regulator for adjusting the pH of the suspension to 5.5-8 is an ammonia solution with a concentration of 1-5 mol / L.

9. The method for preparing ferric phosphate according to claim 1, wherein: The standing time is 0.5-2 hours, the drying temperature is 70-120° C., and the drying time is 6-16 hours.

10. Use of the method for preparing iron phosphate according to any one of claims 1 to 9 in preparing lithium iron phosphate materials.

11. The use according to claim 10, characterized in that The lithium iron phosphate material is prepared by a solid phase method.

12. The use according to claim 11, characterized in that The preparation method of the lithium iron phosphate material comprises the following steps: Mixing the iron phosphate prepared by the method for preparing iron phosphate according to any one of claims 1 to 9, a lithium source, and a carbon source in water, and then spray-drying to obtain a precursor powder; The precursor powder is heated to 200-300° C. under a protective atmosphere and pre-heated for 1-2 hours, and then heated to 700-800° C. and heated for 6-7 hours to obtain the lithium iron phosphate material.

13. The use according to claim 12, characterized in that The molar ratio of the iron atoms in the ferric phosphate, the lithium atoms in the lithium source, and the carbon atoms in the carbon source is 1:(1.02-1.05):(0.06-0.1).

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