Iron phosphate material and preparation method therefor

Abstract

An iron phosphate material and a preparation method therefor, relating to the technical field of materials. According to the method, a hydrothermal system having high cost performance is still used as a basis; when raw materials are conventionally introduced, urea-intercalated halloysite nanotubes are used as additives for mixing; the urea-intercalated halloysite nanotubes can slowly release urea in a hydrothermal process to maintain a stable pH balance of the system; and the urea in intercalation layers has improved high-temperature stability and is prevented from rapid decomposition due to high temperature and high pressure of the hydrothermal system in an initial phase of a reaction, so that the final prepared iron phosphate material has an ideal structural morphology. The lithium iron phosphate positive electrode material prepared from an iron phosphate precursor obtained by the method has excellent electrochemical performance.

Description

A kind of iron phosphate material and preparation method thereof Technical Field

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

[0002] Lithium iron phosphate (LIFP), a common cathode material used in lithium-ion batteries, is typically prepared by first synthesizing a precursor, iron phosphate, which is then mixed with a lithium source and calcined to produce the final product. Because the structure of FIPO is very similar to that of the final LFP, controlling and optimizing its structure, morphology, and particle size during LFP preparation can effectively improve the electrochemical performance of the resulting LFP.

[0003] Current methods for preparing iron phosphate include hydrothermal, sol-gel, and co-precipitation. The hydrothermal method is more common due to its low production cost and high yield. However, the hydrothermal method is generally carried out in a closed high-pressure reactor, making it difficult to control and adjust the internal parameters during the hydrothermal process, especially the pH of the reaction system. If the pH cannot be maintained stable during the iron phosphate formation process, resulting in too low a pH, the growth efficiency of the iron phosphate crystal will be significantly reduced; and if the pH is too high, it will lead to the formation of iron hydroxide. Both of these will affect the morphology and structure of the iron phosphate.

[0004] Summary of the Invention

[0005] The purpose of this article is to overcome the shortcomings of the above-mentioned existing technologies and provide an iron phosphate material and a preparation method thereof. This method is still based on a cost-effective hydrothermal system. Under the condition of conventional introduction of various raw materials, urea-intercalated halloysite nanotubes are mixed as additives. This material can slowly release urea during the hydrothermal process to maintain a stable pH balance of the system. In addition, the high-temperature stability of the urea in the intercalation is improved, and it will not decompose rapidly in the initial stage of the reaction due to the high temperature and high pressure of the hydrothermal system. The finally prepared iron phosphate material has an ideal structural morphology, and the lithium iron phosphate positive electrode material prepared using the iron phosphate precursor obtained by this method has excellent electrochemical properties.

[0006] To achieve the above objectives, the technical solutions adopted in this paper are:

[0007] A method for preparing an iron phosphate material comprises the following steps:

[0008] Mixing urea and halloysite nanotubes in a mass ratio of (1-5):(1-3) and grinding for 10-40 minutes to obtain a urea-intercalated halloysite nanotube mixture;

[0009] The urea-intercalated halloysite nanotube mixture, an iron source, and a phosphorus source are mixed in water, followed by reaction at 120-180° C. for 1.5-6 hours, filtration, and drying to obtain a dry solid; the molar ratio of the urea to the volume of water is (0.01-0.5) mol: 1 L;

[0010] The dried solid is calcined at 500-700° C. for 2-6 hours under a protective atmosphere and crushed to obtain the iron phosphate material.

[0011] In the traditional hydrothermal process for preparing ferric phosphate, urea is a common pH-adjusting additive. However, this substance is easily hydrolyzed by high temperature and high pressure, and the hydrothermal reaction generally takes a long time. Therefore, in the later stage of the reaction, urea is basically inactivated and can no longer play a role in regulating the pH of the system. The pH gradually decreases, the crystal nucleus content increases, and the crystal growth effect is poor. If a large dose of urea is introduced at the raw material addition stage, although the inactivation time of urea will be delayed, the pH of the reaction system in the initial stage is too high, which will lead to the production of impurities such as iron hydroxide, affecting the quality of the formed ferric phosphate.

[0012] Halloysite nanotubes (HNTs) are tubular nanomaterials composed of silicate minerals. These materials possess a large surface area of ​​both the outer wall and the hollow cavity, and their surface is rich in hydroxyl and silyl groups, resulting in high adsorption. In this study, urea was introduced into the halloysite nanotubes through physical grinding to achieve composite properties. The urea displaced the smaller water molecules present in the halloysite nanotubes and intercalated into their interlayer structure (increasing the interlayer spacing from 7 angstroms to 10 angstroms). This significantly improved the stability of the urea, significantly delaying its deactivation time after introduction into the hydrothermal system without changing the dosage. Furthermore, the intercalated urea, based on the properties of the halloysite nanotubes, slowly released the urea, stabilizing the pH of the system. The pH of the reaction system remained stable throughout the hydrothermal reaction. Furthermore, the tubular structure of the halloysite nanotubes, when incorporated into the final lithium iron phosphate, provides sites for lithium ion intercalation and deintercalation, shortening the lithium ion transport path.

[0013] However, as mentioned above, since halloysite nanotubes fix urea in the form of intercalation, the composite amount of the two needs to be specifically considered. If the amount of halloysite nanotubes added is too small, the probability of successful urea intercalation becomes low, and the degree of improvement in maintaining the pH stability of the hydrothermal system is insufficient. However, if the amount of halloysite nanotubes added is too much, since the halloysite nanotubes themselves have low conductivity and do not participate in the reaction during the hydrothermal process, they may cause agglomeration during the reaction, affecting the uniformity of the product, and also reducing the conductivity of the final prepared lithium iron phosphate.

[0014] In one embodiment, the mass ratio of the urea to the halloysite nanotubes is (2-4):2.

[0015] In one embodiment, the halloysite nanotubes have a length of 100-1500 nm, an outer diameter of 40-70 nm, and an inner diameter of 15-30 nm.

[0016] In one embodiment, the molar ratio of the phosphorus source to urea is 1:(1-10);

[0017] Similar to conventional hydrothermal reaction systems, the total amount of urea added in this article can be adapted to the phosphorus source to ensure that the pH can be maintained in an appropriate range while the urea is slowly released.

[0018] In one embodiment, the ratio of the molar amount of urea to the volume of water is (0.2-0.4) mol:1L;

[0019] When the added concentration of urea is maintained within the above range, the morphology and structure of the iron phosphate prepared by the preparation method described herein is better, and the electrochemical activity of the lithium iron phosphate further prepared is better.

[0020] In one embodiment, the urea and halloysite nanotubes are mixed and ground for 15-25 minutes.

[0021] The milling time is related to the intercalation amount of urea. Under the milling time, the intercalation effect of urea is better and too many halloysite nanotubes are not introduced.

[0022] In one embodiment, the iron source is a ferric iron source;

[0023] Furthermore, the ferric iron source is at least one of ferric chloride hydrate and ferric nitrate;

[0024] Furthermore, the concentration of the iron source in water is 0.01-0.5 mol / L.

[0025] In one embodiment, the phosphorus source is at least one of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate.

[0026] In one embodiment, the protective atmosphere is at least one of nitrogen, helium, and argon.

[0027] Another purpose of this article is to provide the iron phosphate material prepared by the preparation method of iron phosphate.

[0028] In one embodiment, the particle size of the crushed iron phosphate material is 2-6 μm.

[0029] Another purpose of this article is to provide an application of the iron phosphate material in the preparation of lithium iron phosphate.

[0030] As a precursor of lithium iron phosphate, the morphology and structure of iron phosphate will basically be inherited by lithium iron phosphate. Therefore, in the iron phosphate material in this article, not only does it have a more uniform and ideal structural morphology compared to the iron phosphate prepared by the conventional hydrothermal method due to the slow release of urea, but a small amount of halloysite nanotubes is also introduced. This substance can give the iron phosphate material more lithium deintercalation sites after it is prepared into lithium iron phosphate, so that it has higher transmission efficiency when used for lithium deintercalation.

[0031] Another object of this invention is to provide a method for preparing lithium iron phosphate, comprising the following steps: mixing the iron phosphate material described herein with a lithium source and ball-milling the mixture in a solvent, then adding a carbon source and calcining the mixture at 600-800°C for 5-15h under a protective atmosphere to obtain the lithium iron phosphate.

[0032] In one embodiment, the lithium source is at least one of lithium carbonate, lithium acetate, and lithium chloride.

[0033] In one embodiment, the solvent is anhydrous ethanol.

[0034] Compared with the existing technology, the beneficial effects of this article are:

[0035] This article provides an iron phosphate material and a preparation method thereof. The method is still based on a cost-effective hydrothermal system. Under the condition of conventional introduction of various raw materials, urea-intercalated halloysite nanotubes are mixed as additives. This material can slowly release urea during the hydrothermal process to maintain a stable pH balance of the system. In addition, the high-temperature stability of the urea in the intercalation is improved, and it will not decompose rapidly in the initial stage of the reaction due to the high temperature and high pressure of the hydrothermal system. The iron phosphate material finally prepared has an ideal structural morphology. The lithium iron phosphate positive electrode material prepared from the iron phosphate precursor obtained by this method has excellent electrochemical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is a scanning electron microscope image of the iron phosphate described in Example 1 of this invention. DETAILED DESCRIPTION

[0037] 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.

[0038] Unless otherwise specified, the materials used in the examples and comparative examples can be obtained through commercial channels.

[0039] In each embodiment and comparative example, the protective atmosphere is nitrogen.

[0040] Example 1

[0041] An embodiment of the ferric phosphate and its preparation method described herein includes the following steps:

[0042] (1) Urea and halloysite nanotubes were mixed in a mass ratio of 3:2 and ground for 20 min to obtain a urea-intercalated halloysite nanotube mixture;

[0043] (2) The urea-intercalated halloysite nanotube mixture, the iron source ferric chloride hexahydrate, and the phosphorus source phosphoric acid were mixed in water, and then reacted at 160°C for 2 hours, filtered, and dried to obtain a dry solid; the molar ratio of the urea to the volume of water was 0.3 mol:1 L; the molar ratio of the iron source to the phosphorus source was 1:1, and the concentration of the iron source was 0.15 mol / L

[0044] (3) The dried solid was calcined at 600° C. for 3 h under a protective atmosphere and crushed to an average particle size of 5 μm to obtain the iron phosphate material.

[0045] The halloysite nanotubes have an average length of 1000 nm, an average outer diameter of 50 nm, and an average inner diameter of 20 nm.

[0046] Example 2

[0047] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that the grinding time in step (1) is 10 minutes.

[0048] Example 3

[0049] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that the mass ratio of urea to halloysite nanotubes in step (1) is 3:1.

[0050] Example 4

[0051] An embodiment of the iron phosphate and its preparation method described herein differs from Example 1 only in that the grinding time in step (1) is 10 minutes and the mass ratio of urea to halloysite nanotubes is 3:1.

[0052] Example 5

[0053] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that the grinding time in step (1) is 30 minutes.

[0054] Example 6

[0055] An embodiment of the iron phosphate and its preparation method described herein differs from Example 1 only in that the mass ratio of urea to halloysite nanotubes in step (1) is 1:3.

[0056] Example 7

[0057] An embodiment of the iron phosphate and its preparation method described herein differs from Example 1 only in that the grinding time in step (1) is 30 minutes, and the mass ratio of urea to halloysite nanotubes is 1:3.

[0058] Example 8

[0059] An embodiment of the iron phosphate and its preparation method described herein differs from Example 1 only in that the grinding time in step (1) is 40 minutes and the mass ratio of urea to halloysite nanotubes is 3:1.

[0060] Example 9

[0061] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that, in step (2), the ratio of the molar amount of urea to the volume of water is 0.1 mol:1 L.

[0062] Example 10

[0063] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that, in step (2), the ratio of the molar amount of urea to the volume of water is 0.2 mol:1 L.

[0064] Example 11

[0065] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that, in step (2), the ratio of the molar amount of urea to the volume of water is 0.4 mol:1 L.

[0066] Example 12

[0067] An embodiment of the ferric phosphate and its preparation method described herein differs from Example 1 only in that, in step (2), the ratio of the molar amount of urea to the volume of water is 0.5 mol:1 L.

[0068] Comparative Example 1

[0069] A ferric phosphate and a preparation method thereof, which differs from Example 1 only in that it comprises the following steps:

[0070] (1) Urea, ferric chloride hexahydrate as an iron source, and phosphoric acid as a phosphorus source are mixed in water, followed by reaction at 160° C. for 2 h, filtration, and drying to obtain a dry solid; the molar ratio of the urea to the volume of water is 0.3 mol:1 L; the molar ratio of the iron source to the phosphorus source is 1:1, and the concentration of the iron source is 0.15 mol / L;

[0071] (2) The dried solid was calcined at 600° C. for 3 h under a protective atmosphere and crushed to a particle size of 5 μm to obtain the iron phosphate material.

[0072] Comparative Example 2

[0073] A ferric phosphate and a preparation method thereof, which differs from Example 1 only in that it comprises the following steps:

[0074] (1) mixing urea and halloysite nanotubes in a mass ratio of 3:2 to obtain a urea-haloysite nanotube mixture;

[0075] (2) Urea is mixed with a mixture of halloysite nanotubes, an iron source of ferric chloride hexahydrate, and a phosphorus source of phosphoric acid in water, and then reacted at 160°C for 2 hours, filtered, and dried to obtain a dry solid; the molar ratio of the urea to the volume of water is 0.3 mol:1 L; the molar ratio of the iron source to the phosphorus source is 1:1, and the concentration of the iron source is 0.15 mol / L

[0076] (3) The dried solid was calcined at 600° C. for 3 h under a protective atmosphere and crushed to a particle size of 5 μm to obtain the iron phosphate material.

[0077] Comparative Example 3

[0078] A ferric phosphate and a preparation method thereof, which differ from Example 1 only in that the grinding time in step (1) is 60 minutes, and the mass ratio of urea to halloysite nanotubes is 5:0.5.

[0079] Effect Example 1

[0080] In order to verify the performance of the iron phosphate materials prepared in each embodiment and comparative example, the iron phosphate materials prepared in each embodiment and comparative example and lithium carbonate were mixed and placed in a ball mill, followed by adding an appropriate amount of anhydrous ethanol and ball milling at 500 rpm for 6 hours. Subsequently, glucose was added and calcined at 700°C under a nitrogen atmosphere for 8 hours to obtain lithium iron phosphate; the mass ratio of the iron phosphate material, lithium carbonate and glucose was 1:1.05:0.05.

[0081] The prepared lithium iron phosphate was dissolved in N-methylpyrrolidone with acetylene black and PVDF at a weight ratio of 100:4:5, stirred evenly, and then coated on aluminum foil. A positive electrode was then prepared. A button-type lithium-ion battery was assembled using a commercial lithium sheet as the negative electrode. The battery's initial charge and discharge capacity was tested at a voltage of 2-3.7V and a current density of 0.1C, and the coulombic efficiency was calculated. The rate performance was then tested at a voltage of 2-4.2V and a current density of 0.2-10C. Ten cycles were performed at each rate, and the discharge capacity at the best single cycle was recorded. The results are shown in Table 1.

[0082] Table 1

[0083] As can be seen from Table 1, the lithium iron phosphate further synthesized from the iron phosphate prepared by the preparation method described herein has ideal charge and discharge performance and cycle performance. The initial charge specific capacity can reach more than 166 mAh / g, and the coulombic efficiency is as high as more than 99.2%. In the rate test, even if the charge and discharge rate reaches 10C, the product can still maintain a discharge specific capacity of more than 139 mAh / g. This performance is far superior to the comparative example 1 product prepared by the traditional hydrothermal method, indicating that the introduction of urea into the hydrothermal system after intercalation into the halloysite nanotubes is of great help to the synthesis of iron phosphate. The iron phosphate obtained in Example 1 is observed by scanning electron microscopy, as shown in Figure 1. It can be seen that the morphology of the product is uniform and free of impurities. Although the comparative example 2 product also introduces halloysite nanotubes, urea is not intercalated into the halloysite nanotubes. Urea still decomposes rapidly under high temperature and high pressure and cannot play a good pH adjustment role. Although the rate performance of the product is slightly improved under the action of the halloysite nanotubes, the overall performance is similar to that of Example 1. At the same time, since the conductivity of the halloysite nanotubes themselves is not high, and the intercalation of urea has a certain relationship with the grinding time, as shown in Example 1 and Example 2-4, when the grinding time of the urea intercalated halloysite nanotubes is shortened during preparation, or the amount of halloysite nanotubes added is small, the urea loading capacity of the halloysite nanotubes will become less, resulting in a decrease in the sustained release effect of urea, thereby causing the performance of the product to decrease; on the other hand, it can be seen from Example 1 and Example 5-7 that the addition of halloysite nanotubes is not conducive to the overall conductivity of the product, which reduces the rate performance of the product, and the long grinding time will not significantly improve the loading capacity; in addition, according to Example 8, it can be seen that the long grinding time has a limited increase in the amount of intercalation of urea intercalation, and can not further improve the morphology of the product. From Example 1 and Example 9-12, it can be seen that the addition concentration of urea in the preparation process should not be too much or too little, and 0.2-0.4 mol / L is the best. In the comparative example 3, the content of halloysite nanotubes in the urea-intercalated halloysite nanotube mixture used in the preparation of ferric phosphate is too low, and the sustained-release effect of urea is not improved even after long-term grinding. Therefore, the performance of the prepared product is not ideal.

[0084] 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 preparation method of iron phosphate material, characterized in that, it includes the following steps: Mix urea and halloysite nanotubes according to the mass ratio of (1 - 5):(1 - 3) and grind for 10 - 40 min to obtain a urea-intercalated halloysite nanotube mixture; Mix the urea-intercalated halloysite nanotube mixture, an iron source, and a phosphorus source in water, then react at 120 - 180 °C for 1.5 - 6 h, filter, and dry to obtain a dry solid; the molar amount of the urea and the volume ratio of water is (0.01 - 0.5) mol:1 L; Calcine the dry solid at 500 - 700 °C for 2 - 6 h in a protective atmosphere, and crush to obtain the iron phosphate material.

2. The preparation method of the iron phosphate material according to claim 1, characterized in that, the mass ratio of the urea to the halloysite nanotubes is (2 - 4):

2.

3. The preparation method of the iron phosphate material according to claim 1, characterized in that, the molar ratio of the phosphorus source to the urea is 1:(1 - 10).

4. The preparation method of the iron phosphate material according to claim 1, characterized in that, the molar amount of the urea and the volume ratio of water is (0.2 - 0.4) mol:1 L.

5. The preparation method of the iron phosphate material according to claim 1, characterized in that, the mixing and grinding time of the urea and the halloysite nanotubes is 15 - 25 min.

6. The preparation method of the iron phosphate material according to claim 1, characterized in that, the concentration of the iron source in water is 0.01 - 0.5 mol / L.

7. The preparation method of the iron phosphate material according to claim 1, characterized in that, the phosphorus source is at least one of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate; the iron source is a trivalent iron source, and the trivalent iron source is at least one of hydrated ferric chloride and ferric nitrate.

8. An iron phosphate material prepared by the preparation method of iron phosphate according to any one of claims 1 - 7.

9. The application of the iron phosphate material according to claim 8 in the preparation of lithium iron phosphate.

10. A preparation method of lithium iron phosphate, characterized in that, it includes the following steps: Mix the iron phosphate material according to claim 8 with a lithium source and ball-mill and mix in a solvent, then add a carbon source and calcine at 600 - 800 °C for 5 - 15 h in a protective atmosphere to obtain the lithium iron phosphate.

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