Preparation method of lithium iron phosphate

Abstract

The invention discloses a preparation method of lithium iron phosphate, which comprises the following steps: preparing orthorhombic iron phosphate by adopting a two-step oxidation method to accurately control a crystallization process, preparing spherical monoclinic iron phosphate by adopting a hydrothermal method, mixing the orthorhombic iron phosphate and the spherical monoclinic iron phosphate according to a certain proportion, adding a lithium source and a carbon source, and carrying out spray drying to obtain a lithium iron phosphate precursor; and preparing the lithium iron phosphate in a three-section calcining manner. By regulating and controlling the ratio of orthogonal and monoclinic iron phosphate, the two crystal forms are adapted, so that the iron phosphate has high structural stability and compaction density and an excellent ion diffusion path; by controlling different morphologies of particles of two crystal forms, grain composition is realized, and a three-stage atmosphere sintering process is combined to realize synergistic conversion of the crystal forms, so that the compaction density of lithium iron phosphate is improved to 2.585-2.652 g / cm < 3 >, and the cycle performance is improved to 1C 50 times of circulation, and the capacity retention rate is 98.2%; the process is compatible with an existing production line, only a crystal form ratio regulation link needs to be added, and industrial popularization is easy.

Description

Technical Field

[0001] This invention relates to a method for preparing a positive electrode material for lithium-ion batteries, and more particularly to a method for preparing lithium iron phosphate. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has been widely used in power batteries and energy storage due to its advantages such as high safety, long cycle life, and low cost. As a key precursor in the synthesis of lithium iron phosphate, the crystal structure of iron phosphate (FePO4) has a decisive influence on the performance of the final product. In existing technologies, the preparation methods of iron phosphate mainly include the iron method, ammonium method, and sodium method. Under conventional processes, the product is mostly a monoclinic dihydrate (FePO4·2H2O), which suffers from poor structural stability and low compaction density. To improve performance, researchers have developed orthorhombic iron phosphate preparation techniques. For example, invention patent CN202411052041A discloses a method for preparing fully orthorhombic (Pnma) single-crystal iron phosphate, which obtains orthorhombic products with high compaction density by controlling the degree of oxidation and pH value. However, lithium iron phosphate prepared from orthorhombic iron phosphate suffers from insufficient lithium-ion diffusion rate, while monoclinic (P21 / C) has better ion conductivity but insufficient structural stability. Patent CN120736493A discloses a lithium iron phosphate dihydrate, its preparation method, and its application. It utilizes a two-stage aging process to synthesize iron phosphate with two crystal forms simultaneously. However, the aging crystal transformation is affected by multiple factors such as concentration, temperature, and pH, resulting in significant fluctuations and instability in the bicrystalline phase conversion ratio. The monoclinic iron phosphate phase is mostly irregularly plate-like and prone to agglomeration. Patent CN120681738A discloses a production process, equipment, and iron phosphate for preparing iron phosphate based on an oxygen micro-interface reaction gas oxidation method. By improving the oxidation process and equipment in iron phosphate synthesis, it induces the crystal form transformation of iron phosphate, achieving directional control of the iron phosphate crystal structure and generating iron phosphate with orthorhombic and monoclinic structures. However, the transformation from amorphous to monoclinic or orthorhombic crystal forms is uncontrollable, and the proportion of two or three crystal forms of iron phosphate cannot be precisely controlled. The aforementioned iron phosphate has limited effect on improving the performance of lithium iron phosphate. The prepared lithium iron phosphate has poor controllability and stability in terms of the proportion of various crystal forms. The purity and morphology of iron phosphate cannot be precisely controlled. Therefore, the prepared lithium iron phosphate cannot achieve both high solid density and high ion conductivity. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to address the problem that lithium iron phosphate prepared from a single crystal form of iron phosphate cannot simultaneously possess both high solid density and ion conductivity, and to provide a method for preparing lithium iron phosphate.

[0004] Technical solution: The method for preparing lithium iron phosphate according to the present invention includes the following steps:

[0005] (S1) Two-step oxidation to prepare orthorhombic iron phosphate, and hydrothermal method to prepare monoclinic iron phosphate;

[0006] (S2) Mix the two crystal forms of iron phosphate, with the molar ratio of orthorhombic iron phosphate to monoclinic iron phosphate being 1~5:1;

[0007] (S3) Add lithium source and carbon source, mix and dry to prepare lithium iron phosphate precursor, and calcine to obtain lithium iron phosphate.

[0008] Orthorhombic iron phosphate (FeFe) has a structure more similar to lithium iron phosphate (LFP), possessing a one-dimensional lithium-ion diffusion channel, resulting in high specific capacity and good cycle stability. Monoclinic FeFe, on the other hand, contains some water of crystallization, exhibiting poor thermal stability and low compaction density, but boasts a high ion diffusion system. However, its inherent properties lead to poor cycle stability and rapid capacity decay. Due to the differences in structure and performance between the two crystal forms, as well as their different preparation methods, lattice parameter differences can cause interfacial stress-induced microcracks. Furthermore, the complex three-dimensional framework formed by the octahedral distortion of the monoclinic FeFe, when mixed with the three-dimensional framework of the orthorhombic FeFe, easily creates defects, leading to uneven charge / discharge rates, increased battery internal resistance, and decreased rate performance. Therefore, simply mixing them does not improve performance but instead degrades electrochemical performance. This application addresses this issue by precisely controlling the preparation processes and mixing ratios of the two FeFeFe crystal forms, thereby altering interfacial properties and improving their compatibility. This results in a lithium iron phosphate product that combines the properties of both crystal forms, thus simultaneously achieving high compaction density and high ion conductivity.

[0009] Furthermore, the preparation method of orthorhombic iron phosphate in step (S1) includes the following steps:

[0010] Mix the iron source with the oxidant;

[0011] Add a phosphorus source, wherein the molar ratio of the phosphorus source to the iron source is 1:1.05~1.20, adjust the pH to 1.5~2.5, add some oxidant, heat at 50~60℃ for 10~12h, filter and dry to obtain orthorhombic ferric phosphate;

[0012] The iron source used in the orthorhombic ferric phosphate is selected from ferrous sulfate, ferrous chloride, ferrous nitrate, or ferrous oxalate; the phosphorus source is selected from monoammonium phosphate, trisodium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, calcium dihydrogen phosphate, or ammonium dihydrogen phosphate; the oxidant is selected from hydrogen peroxide; the concentration of the iron source is 0.3~1.0 mol / L; the concentration of the phosphorus source is 0.5~1.2 mol / L. The oxidant is added in stages. First, most of the oxidant is added to convert most of the ferrous ions into ferric ions to participate in the reaction with the phosphorus source to generate ferric phosphate. After the initial oxidation, the solution retains 3~5 mol% ferrous ions. A small portion of the oxidant reacts with the ferric ions and phosphorus source to a certain extent. After the mixture turns white, it is added back to the system to promote the complete participation of ferric ions in the reaction to produce the target product. The two-step oxidation method precisely controls the crystallization process and improves the purity of the finished ferric phosphate.

[0013] Furthermore, the preparation method of monoclinic iron phosphate in step (S1) includes the following steps:

[0014] Mix the iron source and the phosphorus source in a molar ratio of 1:1.0~1.10;

[0015] The pH was adjusted to 5-9 with ammonium bicarbonate, and the mixture was subjected to hydrothermal reaction at 150-180℃ for 5-8 hours. After filtration and drying, monoclinic ferric phosphate product was obtained.

[0016] The iron source used for the monoclinic lithium iron phosphate is selected from ferrous sulfate, ferrous chloride, ferrous nitrate, or ferrous oxalate; the phosphorus source is selected from monoammonium phosphate, trisodium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, calcium dihydrogen phosphate, or ammonium dihydrogen phosphate; the iron source concentration is 0.5~1.5 mol / L; the phosphorus source concentration is 0.7~1.7 mol / L. Spherical particles are prepared using a hydrothermal method, and when mixed with orthorhombic lithium iron phosphate, they easily produce gradation, improving the battery's compaction density and capacity.

[0017] Further, in step (S3), lithium carbonate is added at a Li:Fe ratio of 1.01~1.08, and a carbon source, including glucose, sucrose, polyethylene glycol, oxalic acid, or polyvinylpyrrolidone, is added at a mass ratio of 5~15% to iron phosphate. The mixture is finely ground to D50 = 0.3~0.6 nm and spray-dried to obtain a lithium iron phosphate precursor. This precursor is then calcined in an inert atmosphere at a rate of 6℃ / min to 350℃ for 3~4 h, then to 650℃ for 4~5 h, and finally to 820℃ for 7~8 h. After cooling, lithium iron phosphate is obtained. The first calcination stage removes residual organic matter from the material, preventing interference with subsequent crystal form adaptation and ensuring the stability of the anhydrous system. The second calcination stage promotes the adaptation of the two iron phosphate crystal forms, enhances atomic diffusion at the crystal interface, and constructs a lattice adaptation region. The third calcination stage promotes the compactness of the crystal particles, thereby improving the compaction density and battery capacity of the produced lithium iron phosphate battery.

[0018] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: 1. By controlling the preparation process of orthogonal and monoclinic iron phosphate, and adjusting the ratio of the two crystal forms, the two crystal forms are adapted to achieve a combination of high structural stability, compaction density, and excellent ion diffusion pathway; 2. By controlling the different morphologies of the two crystal forms, particle gradation is achieved, and combined with a three-stage atmosphere sintering process, synergistic transformation of crystal forms is realized, thereby increasing the compaction density of lithium iron phosphate to 2.585~2.652 g / cm³. 3 The cycle performance is improved to 98.2% capacity retention after 50 cycles at 1C; 3. The process is compatible with existing production lines, requiring only the addition of a crystal form ratio control step, making it easy to promote industrialization. Attached Figure Description

[0019] Figure 1 SEM image of lithium iron phosphate prepared in this invention;

[0020] Figure 2 The image shows the XRD pattern of lithium iron phosphate prepared according to this invention.

[0021] Figure 3 Cycle curve of a secondary battery made of lithium iron phosphate prepared according to the present invention;

[0022] Figure 4 The diagram shows the rate performance of the secondary battery made from lithium iron phosphate prepared in this invention. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to specific embodiments, and all reagents used are commercially available. In step (S3), lithium carbonate is added at a ratio of Li:Fe = 1.04, with 5% glucose as the preferred carbon source and D50 controlled at 0.4 nm. This example illustrates the influence of the mixing ratio of two different crystal forms of iron phosphate and the calcination process on the performance of the prepared lithium iron phosphate material.

[0024] Example 1

[0025] A method for synthesizing lithium iron phosphate material P based on the orthogonal / monoclinic phosphate ratio regulation 0.75 C 0.25 The preparation method of LFP (a mixture of orthorhombic and monoclinic iron phosphate in a molar ratio of 3:1) includes the following steps:

[0026] (S1) Mix 0.5 mol / L ferrous sulfate solution with a portion of hydrogen peroxide (molar ratio of iron source to total hydrogen peroxide is 1:0.7) to control the initial oxidation and retain 3 mol% ferrous ions; add 0.6 mol / L trisodium phosphate solution (molar ratio 1:1.15) and adjust the pH to 1.8 with phosphoric acid; react at 55℃ until the mixture turns white, then add the remaining amount of hydrogen peroxide until there are no ferrous ions, keep warm for 12 hours, filter and dry to obtain orthorhombic ferric phosphate product;

[0027] (S2) Mix 0.8 mol / L ferric chloride and 1.0 mol / L monoammonium phosphate at Fe / P=1.03, adjust the pH to 7.0 with ammonium bicarbonate; react hydrothermally at 160℃ for 5 h to obtain spherical monoclinic ferric phosphate product;

[0028] (S3) Two types of crystalline iron phosphate were mixed in a molar ratio of 3:1. Lithium carbonate was added at a ratio of Li:Fe = 1.04. Glucose was added at a mass ratio of 5% to iron phosphate. The mixture was finely ground to D50 = 0.4 nm and spray-dried to obtain the lithium iron phosphate precursor. The temperature was increased to 350℃ at 6℃ / min in an inert N2 atmosphere and held for 3 h. The temperature was then increased to 650℃ and held for 4 h in a 95% N2 + 5% CO atmosphere. The temperature was then increased to 820℃ and held for 7 h in a 98% N2 + 2% CO atmosphere. After cooling, the lithium iron phosphate product was obtained.

[0029] The preparation methods of Examples 2-5 are the same as those of Example 1. The specific components and process parameters are detailed in Table 1.

[0030] Table 1. Components and process parameters of Examples 1-5

[0031]

[0032] Comparative Example 1

[0033] Unlike Example 1, only orthorhombic iron phosphate was synthesized.

[0034] Comparative Example 2

[0035] Unlike Example 1, only monoclinic iron phosphate was synthesized.

[0036] Comparative Example 3

[0037] Unlike Example 1, the two crystal forms of iron phosphate were mixed and calcined in two stages to obtain the finished product: the temperature was raised to 650°C at 6°C / min and held for 4 hours in an inert atmosphere of N2 (or Ar), and then raised to 820°C and held for 7 hours.

[0038] Comparative Example 4

[0039] Unlike Example 1, the two crystal forms of iron phosphate were mixed and then calcined in one step to obtain the finished product: the temperature was raised to 820°C at 6°C / min and held for 14 hours in an inert N2 (or Ar) atmosphere.

[0040] like Figure 1 and 2 The image shows the SEM and XRD patterns of the lithium iron phosphate prepared according to this invention. Figure 1 As can be seen, the prepared lithium iron phosphate particles are uniformly distributed in size, forming a gradation that increases the compaction density of the material; from Figure 2 The spectrum shows characteristic peaks of two crystal forms, which are clearly distinguishable and independent of each other. There is no obvious peak overlap or broadening phenomenon, indicating that the two crystal forms of lithium iron phosphate coexist independently and stably, and the crystal forms remain intact. There is no interaction that leads to structural distortion or abnormal crystal form transformation, thus enabling the lithium iron phosphate cathode material to have both high solid density and high ion conductivity.

[0041] The secondary batteries made from the lithium iron phosphate cathode materials prepared in the above examples and comparative examples, with the cathode material made of lithium iron phosphate, were mixed with conductive carbon black (SP) and binder polyvinylidene fluoride (PVDF) in a weight ratio of 90:5:5. The mixture was uniformly mixed in an N-methylpyrrolidone (NMP) solution to obtain a slurry. The slurry was coated on a bright aluminum foil, and then the aluminum foil was placed in an oven to dry at 100°C. After the NMP had completely evaporated, the electrode was cut into electrode sheets with a diameter of 13 mm. The electrode sheets were then placed in an oven to dry overnight at 105°C. After weighing the electrode sheets, they were transferred to a glove box. Using lithium metal foil as the counter electrode, Celgard 2400 as the separator, and 1 mol / L LiPF6 / EC+DEC as the electrolyte, a CR2032 coin cell was assembled. The assembled batteries underwent compaction density and electrochemical performance testing using a Xinwei 4008 constant current test cabinet with a test voltage range of 2.0-4.35V. The 0.1C charge / discharge specific capacity was measured. Detailed test results are shown in Table 2. Figure 3 , Figure 4 .

[0042] Table 2 Performance test results of secondary batteries prepared in different embodiments and comparative examples

[0043]

[0044] Figure 3 and 4Table 2 shows the cycle performance and rate performance of the secondary battery. By mixing orthorhombic and monoclinic lithium iron phosphate in an appropriate ratio, structural stability and ion channel optimization are both optimized. In particular, in Example 1, when the ratio of orthorhombic and monoclinic lithium iron phosphate is controlled at 3:1, the compaction density, charge-discharge performance and cycle stability of the secondary battery are optimal.

[0045] Compared with Example 1 and Comparative Examples 1 and 2, secondary batteries made of lithium iron phosphate with orthorhombic or monoclinic crystal forms alone cannot simultaneously achieve high compaction density and battery capacity, good cycle performance and rate performance.

[0046] Compared with Comparative Examples 1 and 3 and 4, the three-stage calcination method in Example 1 can promote the synergistic transformation of the two crystal forms, inhibit the formation of Fe2P impurities, and form a uniform carbon coating layer, which can further improve the compaction density, cycle stability, and rate performance of the material. The two-stage or one-stage calcination methods in Comparative Examples 3 and 4 cannot effectively remove residual organic matter in the system, affecting the subsequent synergistic transformation of crystal forms and the stability of the system.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, Includes the following steps: (S1) Two-step oxidation to prepare orthorhombic iron phosphate, and hydrothermal method to prepare monoclinic iron phosphate; (S2) Mix the two crystal forms of iron phosphate, with the molar ratio of orthorhombic iron phosphate to monoclinic iron phosphate being 1~5:1; (S3) Add lithium source and carbon source, mix and dry to prepare lithium iron phosphate precursor, and calcine to obtain lithium iron phosphate.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The preparation method of orthorhombic ferric phosphate in step (S1) includes the following steps: Mix the iron source with the oxidant; A phosphorus source was added, the pH was adjusted, and then some oxidant was added and heated to react. After filtration and drying, orthorhombic iron phosphate was obtained.

3. The method for preparing lithium iron phosphate according to claim 2, characterized in that, In step (S1), the iron source for the orthorhombic iron phosphate is selected from ferrous sulfate, ferrous chloride, ferrous nitrate or ferrous oxalate; the phosphorus source is selected from monoammonium phosphate, trisodium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, calcium dihydrogen phosphate or ammonium dihydrogen phosphate; and the oxidant is selected from hydrogen peroxide.

4. The method for preparing lithium iron phosphate according to claim 3, characterized in that, In step (S1), the concentration of the iron source used for orthorhombic iron phosphate is 0.3~1.0 mol / L, and the concentration of the phosphorus source is 0.5~1.2 mol / L; the molar ratio of the iron source to the phosphorus source is 1:1.05~1.

20.

5. The method for preparing lithium iron phosphate according to claim 2, characterized in that, In step (S1), the molar ratio of the total amount of iron source and oxidant used for orthorhombic iron phosphate is 1:0.5~1.5; the oxidant is added in two parts, with part of the oxidant first mixed with the iron source, and the remaining oxidant added after the phosphorus source is added and the pH is adjusted to 1.5~2.

5.

6. The method for preparing lithium iron phosphate according to claim 5, characterized in that, In step (S1), the oxidant used for orthorhombic iron phosphate is mixed with the iron source, and after initial oxidation, 3-5 mol% of ferrous ions are retained in the solution; the remaining oxidant is added after the mixture turns white.

7. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The method for preparing monoclinic iron phosphate in step (S1) includes the following steps: Iron and phosphorus sources are mixed, pH is adjusted, hydrothermal reaction is carried out, and the mixture is filtered and dried to obtain monoclinic ferric phosphate product.

8. The method for preparing lithium iron phosphate according to claim 6, characterized in that, In step (S1), the iron source for monoclinic iron phosphate is selected from ferrous sulfate, ferrous chloride, ferrous nitrate or ferrous oxalate; the phosphorus source is selected from monoammonium phosphate, trisodium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, calcium dihydrogen phosphate or ammonium dihydrogen phosphate.

9. The method for preparing lithium iron phosphate according to claim 8, characterized in that, In step (S1), the concentration of the iron source used for monoclinic iron phosphate is 0.5~1.5 mol / L, and the concentration of the phosphorus source is 0.7~1.7 mol / L; the molar ratio of the iron source to the phosphorus source is 1:1.0~1.10; after mixing the iron source and the phosphorus source, the pH is adjusted to 5~9; and the temperature of the hydrothermal reaction is 150~180℃.

10. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (S3), lithium carbonate is added at a Li:Fe ratio of 1.01 to 1.08, and a carbon source is added at a mass ratio of 5 to 15% with iron phosphate. The mixture is finely ground to D50 = 0.3 to 0.6 nm and spray-dried to obtain a lithium iron phosphate precursor. The precursor is then calcined in an inert atmosphere at a temperature of 6 °C / min to 300 to 400 °C for 3 to 4 h, then to 600 to 700 °C for 4 to 5 h, and finally to 800 to 850 °C for 7 to 8 h. After cooling, lithium iron phosphate is obtained. The carbon source includes glucose, sucrose, polyethylene glycol, oxalic acid, or polyvinylpyrrolidone.

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