Iron phosphate material and method for its preparation
By using urea-intercalated halloysite nanotubes in a hydrothermal method, the problem of pH instability during the preparation of lithium iron phosphate was solved, and lithium iron phosphate materials with ideal morphology and high electrochemical performance were prepared, thus improving the electrochemical performance of lithium-ion batteries.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- GUANGDONG BRUNP RECYCLING TECH CO LTD
- Filing Date
- 2023-11-27
- Publication Date
- 2026-07-10
AI Technical Summary
In the existing hydrothermal method for preparing lithium iron phosphate, it is difficult to maintain the pH stability of the reaction system, which leads to low crystal growth efficiency of lithium iron phosphate or the generation of ferric hydroxide impurities, affecting the electrochemical performance of lithium iron phosphate.
Using urea-intercalated halloysite nanotubes as an additive, urea is inserted into the interlayer of halloysite nanotubes through physical grinding, and urea is slowly released to maintain the pH stability of the hydrothermal reaction system. Combined with the improvement of high-temperature stability, iron phosphate materials with ideal structural morphology are prepared.
The prepared iron phosphate material exhibits excellent electrochemical performance, and the electrochemical activity and lithium-ion transport efficiency of the lithium iron phosphate cathode material are improved.
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Figure CN117858845B_ABST
Abstract
Description
Technical Field
[0001] This article relates to the field of materials technology, and in particular to an iron phosphate material and its preparation method. Background Technology
[0002] The common preparation method for lithium iron phosphate (LFP) cathode materials used in lithium-ion batteries is to first synthesize the precursor iron phosphate, and then mix it with a lithium source and calcine it to obtain the final product. Since iron phosphate has a very similar structure to the final LFP, controlling and optimizing the structure, morphology, and particle size of iron phosphate during LFP preparation can effectively improve the electrochemical performance of the final LFP.
[0003] Current methods for preparing ferric phosphate include hydrothermal methods, sol-gel methods, and co-precipitation methods. Among these, 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 kept stable during the formation of ferric phosphate, resulting in a pH that is too low, the growth efficiency of the ferric phosphate crystal form will be significantly reduced. On the other hand, if the pH is too high, it will lead to the formation of ferric hydroxide. Both of these factors will affect the morphology and structure of ferric phosphate. Summary of the Invention
[0004] The purpose of this paper is to overcome the shortcomings of the existing technology and provide an iron phosphate material and its preparation method. The method is still based on a cost-effective hydrothermal system. Under the condition of conventionally introducing various raw materials, urea-intercalated halloysite nanotubes are used as additives for mixing. This substance can slowly release urea during the hydrothermal process to maintain the stable pH balance of the system. Moreover, the high-temperature stability of the intercalated urea is improved, and it will not decompose rapidly due to the high temperature and pressure of the hydrothermal system in the initial stage of the reaction. The iron phosphate material prepared in the end has an ideal structural morphology. The lithium iron phosphate cathode material prepared using the iron phosphate precursor obtained by this method has excellent electrochemical performance.
[0005] To achieve the above objectives, the technical solution adopted in this paper is as follows:
[0006] A method for preparing an iron phosphate material includes the following steps:
[0007] Urea and halloysite nanotubes were mixed at a mass ratio of (1-5):(1-3) and ground for 10-40 min to obtain a urea-intercalated halloysite nanotube mixture.
[0008] A mixture of urea-intercalated halloysite nanotubes, an iron source, and a phosphorus source was mixed in water and then reacted at 120-180℃ for 1.5-6 hours. The mixture was then filtered and dried to obtain a dry solid. The molar ratio of urea to water was (0.01-0.5) mol: 1 L.
[0009] The dried solid material is calcined at 500-700℃ for 2-6 hours under a protective atmosphere and then crushed to obtain the iron phosphate material.
[0010] In the traditional hydrothermal process for preparing ferric phosphate, urea is a common pH-adjusting additive. However, urea is easily hydrolyzed by high temperature and pressure, and the hydrothermal reaction generally takes a long time. Therefore, urea is basically deactivated in the later stage of the reaction and can no longer play a role in adjusting the pH of the system. The pH gradually decreases, the crystal nucleus content increases, and the crystal growth effect is poor. If a large amount of urea is introduced at the raw material addition stage, although the deactivation time of urea will be delayed, the pH of the reaction system in the initial stage will be too high, which will lead to the generation of ferric hydroxide, an impurity, and affect the quality of the ferric phosphate formed.
[0011] Halloysite nanotubes (HNTs) are tubular nanomaterials composed of silicate minerals. These materials possess a large outer wall and hollow tube specific surface area, and their surface is rich in hydroxyl and silanol groups, thus exhibiting high adsorption capacity. In this paper, urea was introduced into halloysite nanotubes through physical grinding to achieve composite formation. Urea replaced the smaller water molecules originally present in the halloysite nanotubes and intercalated into their interlayer structure (increasing the interlayer spacing from 7 Å to 10 Å). The stability of urea was significantly improved; the deactivation time after introduction into the hydrothermal system was significantly delayed without changing the addition amount. Furthermore, based on the properties of halloysite nanotubes, urea in the intercalation can achieve a slow release effect, stabilizing the pH of the system. The pH of the reaction system remained stable throughout the hydrothermal reaction process. In addition, the tubular structure of the halloysite nanotubes, inherited by the final prepared lithium iron phosphate, can provide sites for lithium ion insertion / extraction, shortening the lithium ion transport path.
[0012] However, as mentioned above, since halloysite nanotubes are used to fix urea in the form of intercalation, the amount of the two combined needs to be carefully considered. If too little halloysite nanotubes are added, the probability of successful urea intercalation will be lower, and the improvement in maintaining the pH stability of the hydrothermal system will be insufficient. However, if too much halloysite nanotubes are added, 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 reducing the conductivity of the final lithium iron phosphate.
[0013] In one embodiment, the mass ratio of urea to halloysite nanotubes is (2-4):2.
[0014] In one embodiment, the halloysite nanotube has a length of 100-1500 nm, an outer diameter of 40-70 nm, and an inner diameter of 15-30 nm.
[0015] In one embodiment, the molar ratio of the phosphorus source to urea is 1:(1-10);
[0016] Similar to conventional hydrothermal reaction systems, the total amount of urea added in this study can be matched with the phosphorus source to ensure that the pH can be maintained within a suitable range while the urea is released slowly.
[0017] In one embodiment, the molar ratio of urea to water volume is (0.2-0.4) mol: 1 L;
[0018] When the concentration of added urea is maintained within the above range, the morphology and structure of the iron phosphate prepared by the preparation method described in this article are better, and the electrochemical activity of the further prepared lithium iron phosphate is better.
[0019] In one embodiment, the urea and halloysite nanotubes are mixed and ground for 15-25 minutes.
[0020] The grinding time is related to the amount of urea intercalation. At the specified grinding time, the urea intercalation effect is better and no excessive halloysite nanotubes are introduced.
[0021] In one embodiment, the iron source is a trivalent iron source;
[0022] Furthermore, the trivalent iron source is at least one of hydrated ferric chloride and ferric nitrate;
[0023] Furthermore, the concentration of the iron source in the water is 0.01-0.5 mol / L.
[0024] In one embodiment, the phosphorus source is at least one selected from phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate.
[0025] In one embodiment, the protective atmosphere is at least one of nitrogen, helium, and argon.
[0026] Another objective of this article is to provide the iron phosphate material prepared by the aforementioned method.
[0027] In one embodiment, the particle size of the crushed iron phosphate material is 2-6 μm.
[0028] Another objective of this article is to provide the application of the aforementioned iron phosphate material in the preparation of lithium iron phosphate.
[0029] As a precursor to lithium iron phosphate, iron phosphate's morphology and structure are largely inherited by lithium iron phosphate. Therefore, in the iron phosphate material presented in this paper, not only does it exhibit a more uniform and ideal structural morphology compared to iron phosphate prepared by the conventional hydrothermal method due to the slow release of urea, but a small amount of halloysite nanotubes are also introduced. This material can endow the iron phosphate material with more lithium intercalation / deintercalation sites after it is prepared into lithium iron phosphate, thus enabling it to have higher transport efficiency when applied to lithium intercalation / deintercalation.
[0030] Another objective of this paper 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 at 600-800°C for 5-15 hours under a protective atmosphere to obtain the lithium iron phosphate.
[0031] In one embodiment, the lithium source is at least one of lithium carbonate, lithium acetate, and lithium chloride.
[0032] In one embodiment, the solvent is anhydrous ethanol.
[0033] Compared to existing technologies, the advantages of this paper are:
[0034] This paper presents an iron phosphate material and its preparation method. The method is still based on a cost-effective hydrothermal system. Under the condition of conventionally introducing various raw materials, urea-intercalated halloysite nanotubes are used as additives for mixing. This substance can slowly release urea during the hydrothermal process to maintain the stable pH balance of the system. Moreover, the high-temperature stability of the intercalated urea is improved, and it will not decompose rapidly due to the high temperature and pressure of the hydrothermal system in the initial stage of the reaction. The final prepared iron phosphate material has an ideal structural morphology. The lithium iron phosphate cathode material prepared using the iron phosphate precursor obtained by this method has excellent electrochemical performance. Attached Figure Description
[0035] Figure 1 This is a scanning electron microscope image of the iron phosphate described in Example 1 of this article. Detailed Implementation
[0036] To better illustrate the purpose, technical solutions, and advantages of this document, the following description will be provided in conjunction with the accompanying drawings and specific embodiments.
[0037] Unless otherwise specified, all materials used in the embodiments and comparative examples are commercially available.
[0038] In all embodiments and comparative examples, the protective atmosphere is nitrogen.
[0039] Example 1
[0040] An embodiment of the iron phosphate and its preparation method described herein includes the following steps:
[0041] (1) Mix urea and halloysite nanotubes at a mass ratio of 3:2 and grind for 20 min to obtain a urea-intercalated halloysite nanotube mixture.
[0042] (2) A mixture of urea-intercalated halloysite nanotubes, ferric chloride hexahydrate (iron source), and phosphoric acid (phosphoric acid source) was mixed in water, and then reacted at 160°C for 2 hours. The mixture was filtered, dried, and a dry solid was obtained. The molar ratio of urea to water was 0.3 mol: 1 L; the molar ratio of iron source to phosphoric acid source was 1:1, and the concentration of iron source was 0.15 mol / L.
[0043] (3) The dried solids were calcined at 600°C for 3 hours under a protective atmosphere and crushed to an average particle size of 5 μm to obtain the iron phosphate material.
[0044] 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.
[0045] Example 2
[0046] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the grinding time in step (1) is 10 min.
[0047] Example 3
[0048] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the mass ratio of urea to halloysite nanotubes in step (1) is 3:1.
[0049] Example 4
[0050] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the grinding time in step (1) is 10 min and the mass ratio of urea to halloysite nanotubes is 3:1.
[0051] Example 5
[0052] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the grinding time in step (1) is 30 min.
[0053] Example 6
[0054] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the mass ratio of urea to halloysite nanotubes in step (1) is 1:3.
[0055] Example 7
[0056] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the grinding time in step (1) is 30 min and the mass ratio of urea to halloysite nanotubes is 1:3.
[0057] Example 8
[0058] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that the grinding time in step (1) is 40 min and the mass ratio of urea to halloysite nanotubes is 3:1.
[0059] Example 9
[0060] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that, in step (2), the molar ratio of urea to water volume is 0.1 mol: 1 L.
[0061] Example 10
[0062] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that, in step (2), the molar ratio of urea to water volume is 0.2 mol: 1 L.
[0063] Example 11
[0064] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that, in step (2), the molar ratio of urea to water volume is 0.4 mol: 1 L.
[0065] Example 12
[0066] An embodiment of the iron phosphate and its preparation method described in this article differs from Example 1 only in that, in step (2), the molar ratio of urea to water volume is 0.5 mol: 1 L.
[0067] Comparative Example 1
[0068] A method for preparing ferric phosphate differs from Example 1 only in that it includes the following steps:
[0069] (1) Urea, iron source ferric chloride hexahydrate and phosphoric acid source are mixed in water, and then reacted at 160℃ for 2h. After filtration and drying, dry solids are obtained. The molar ratio of urea to water volume is 0.3mol:1L. The molar ratio of iron source to phosphoric acid source is 1:1 and the concentration of iron source is 0.15mol / L.
[0070] (2) The dried solid was calcined at 600°C for 3 hours under a protective atmosphere and then crushed to a particle size of 5 μm to obtain the iron phosphate material.
[0071] Comparative Example 2
[0072] A method for preparing ferric phosphate differs from Example 1 only in that it includes the following steps:
[0073] (1) Mix urea and halloysite nanotubes at a mass ratio of 3:2 to obtain a mixture of urea and halloysite nanotubes;
[0074] (2) A mixture of urea and halloysite nanotubes, ferric chloride hexahydrate (iron source), and phosphoric acid (phosphoric acid source) was mixed in water, and then reacted at 160°C for 2 hours. The mixture was filtered, dried, and a dry solid was obtained. The molar ratio of urea to water was 0.3 mol:1 L; the molar ratio of iron source to phosphoric acid source was 1:1, and the concentration of iron source was 0.15 mol / L.
[0075] (3) The dried solid was calcined at 600°C for 3 hours under a protective atmosphere and then crushed to a particle size of 5 μm to obtain the iron phosphate material.
[0076] Comparative Example 3
[0077] A method for preparing iron phosphate is described, which differs from Example 1 only in that the grinding time in step (1) is 60 min and the mass ratio of urea to halloysite nanotubes is 5:0.5.
[0078] Example 1
[0079] 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 were mixed with lithium carbonate and placed in a ball mill. Then, an appropriate amount of anhydrous ethanol was added, and the mixture was ball-milled at 500 rpm for 6 hours. Subsequently, glucose was added, and the mixture was calcined at 700°C for 8 hours under a nitrogen atmosphere to obtain lithium iron phosphate. The mass ratio of the iron phosphate material, lithium carbonate, and glucose was 1:1.05:0.05.
[0080] The prepared lithium iron phosphate was dissolved in N-methylpyrrolidone with acetylene black and PVDF at a weight ratio of 100:4:5. After stirring evenly, the solution was coated onto aluminum foil, and a positive electrode sheet was prepared. A coin cell lithium-ion battery was assembled using a commercial lithium sheet as the negative electrode. The initial charge-discharge capacity of the battery was tested at 2-3.7V and 0.1C current density, and the coulombic efficiency was calculated. The rate performance was tested at 2-4.2V and 0.2-10C current density, with 10 cycles at each rate. The discharge specific capacity of the best cycle at each rate was recorded. The results are shown in Table 1.
[0081] Table 1
[0082]
[0083] As shown in Table 1, the lithium iron phosphate synthesized from the iron phosphate prepared by the method described in this paper exhibits ideal charge-discharge and cycle performance. The initial charge specific capacity can reach over 166 mAh / g, and the coulombic efficiency is over 99.2%. In the rate test, even at a charge-discharge rate of 10C, the product can still maintain a discharge specific capacity of over 139 mAh / g. This performance is far superior to the product of Comparative Example 1 prepared by the traditional hydrothermal method, indicating that introducing urea intercalation into halloysite nanotubes into the hydrothermal system greatly aids in the synthesis of iron phosphate. Scanning electron microscopy observation of the iron phosphate obtained in Example 1 further demonstrates… Figure 1 As shown, the product has a uniform morphology and is free of impurities. In contrast, although halloysite nanotubes were also introduced into the product of Comparative Example 2, the urea was not intercalated in the halloysite nanotubes. The urea still decomposed rapidly under high temperature and pressure, and could not play a good role in pH adjustment. Although the rate performance of the product was slightly improved under the action of halloysite nanotubes, the overall performance was similar to that of Example 1. Meanwhile, since halloysite nanotubes themselves have low conductivity, and the intercalation of urea is related to the grinding time, as shown in the comparison between Examples 1 and Examples 2-4, when the grinding time of the halloysite nanotubes intercalated with urea is shorter or the amount of halloysite nanotubes added is less, the urea loading of the halloysite nanotubes will decrease, resulting in a decrease in the sustained-release effect of urea and thus a decrease in product performance. On the other hand, as can be seen from Examples 1 and Examples 5-7, adding a large amount of halloysite nanotubes is not conducive to the overall conductivity of the product, resulting in a decrease in the rate performance of the product, while excessively long grinding time does not significantly improve the loading. In addition, as shown in Example 8, excessively long grinding time has a limited effect on the increase in the amount of urea intercalation and cannot further improve the morphology of the product. As can be seen from the comparison between Examples 1 and Examples 9-12, the concentration of urea added during the preparation process should not be too high or too low, with 0.2-0.4 mol / L being optimal. In Comparative Example 3, the urea-intercalated halloysite nanotube mixture used in the preparation of ferric phosphate contained too little halloysite nanotubes. Even after long-term grinding, the sustained-release effect of urea was not improved, and the performance of the prepared product was 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this article without departing from the essence and scope of the technical solutions of this article.
Claims
1. A method for preparing an iron phosphate material, characterized in that, Includes the following steps: Urea and halloysite nanotubes were mixed at a mass ratio of (1-5):(1-3) and ground for 10-40 min to obtain a urea-intercalated halloysite nanotube mixture. A mixture of urea-intercalated halloysite nanotubes, an iron source, and a phosphorus source was mixed in water and then reacted at 120-180℃ for 1.5-6 hours. The mixture was then filtered and dried to obtain a dry solid. The molar ratio of urea to water was (0.2-0.4) mol:1 L, and the molar ratio of phosphorus source to urea was 1:(1-10). The dried solid material is calcined at 500-700℃ for 2-6 hours under a protective atmosphere and then crushed to obtain the iron phosphate material.
2. The method for preparing the iron phosphate material as described in claim 1, characterized in that, The mass ratio of urea to halloysite nanotubes is (2-4):
2.
3. The method for preparing the iron phosphate material as described in claim 1, characterized in that, The urea and halloysite nanotubes are mixed and ground for 15-25 minutes.
4. The method for preparing the iron phosphate material as described in claim 1, characterized in that, The concentration of the iron source in the water is 0.01-0.5 mol / L.
5. The method for preparing the iron phosphate material as described in 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, which is at least one of hydrated ferric chloride and ferric nitrate.
6. The iron phosphate material prepared by the method of any one of claims 1-5.
7. The application of the iron phosphate material as described in claim 6 in the preparation of lithium iron phosphate.
8. A method for preparing lithium iron phosphate, characterized in that, The process includes the following steps: mixing the iron phosphate material of claim 6 with a lithium source and ball milling the mixture in a solvent, then adding a carbon source and calcining at 600-800°C for 5-15 hours under a protective atmosphere to obtain the lithium iron phosphate.