A high-purity homogeneous titanium-doped iron phosphate and its low-cost preparation method

Abstract

This application relates to the field of iron phosphate material preparation technology, specifically disclosing a high-purity homogeneous titanium-doped iron phosphate and its low-cost preparation method. The method includes: S1 dissolving ferrous sulfate heptahydrate, a byproduct of titanium dioxide production, adding titanium-containing substances and sulfuric acid, and filtering to obtain a titanium-containing ferrous solution; S2 co-precipitating the titanium-containing ferrous solution with a phosphorus source, oxidant, and ammonia to generate an amorphous titanium-doped iron phosphate slurry; S3 washing the slurry, redispersing it, adding additives, and aging and crystallizing it to obtain titanium-doped iron phosphate dihydrate; S4 drying and calcining to obtain the final product. This invention achieves flexible control of the doping amount by adding an external titanium source and utilizes additives to assist aging, deeply purifying the product while utilizing industrial byproducts, reducing the content of impurities such as aluminum, magnesium, and manganese, and allowing for an adjustable iron-to-phosphorus ratio. The titanium-doped iron phosphate prepared by this method has high purity and uniform doping, and lithium iron phosphate prepared using it as a precursor exhibits excellent electrochemical performance, especially high-rate capacity and cycle stability.

Description

Technical Field

[0001] This invention relates to the field of iron phosphate material preparation technology, specifically to a high-purity homogeneous titanium-doped iron phosphate and its low-cost preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) has become an important cathode material in the fields of power batteries and energy storage due to its high safety, long cycle life, and low cost. However, its intrinsically low ionic and electronic conductivity limits its high-rate performance. There are three main methods to improve the conductivity of LFP: particle nanosizing, conductive material coating, and metal ion doping. Among these, particle nanosizing limits the improvement of product compaction density; conductive material coating has limited improvement on the conductivity inside the particles; elemental doping is an effective means to improve its electrochemical performance, and titanium doping has attracted much attention due to its ionic radius compatibility and oxidation state stability.

[0003] In existing technologies, using ferrous sulfate, a byproduct of titanium dioxide production, as both an iron and titanium source for doping is a cost-effective approach. For example, Chinese patent CN120172374A discloses a method that directly uses titanium-containing ferrous sulfate powder to prepare titanium-doped iron phosphate via a two-step precipitation process. While this method simplifies the process and utilizes titanium from the byproduct, it has significant drawbacks: 1. The titanium doping amount is limited by the raw material, making control inflexible: The titanium doping amount depends entirely on the titanium content (2000-3000 ppm) in the titanium dioxide byproduct, making it impossible to flexibly adjust the doping concentration according to product performance requirements. If a higher titanium doping amount, such as greater than 4000 ppm, is required, an additional titanium source must be added; otherwise, it cannot be achieved. 2. Poor batch stability of raw materials, leading to significant product quality fluctuations: The titanium content in the titanium dioxide byproduct may vary between batches, resulting in inconsistent titanium doping amounts across different batches, affecting product performance stability. 3. Fluctuations in product purity and consistency: This method mainly relies on the physical washing of multi-stage slurry to remove impurities. Although the washing and sedimentation steps can reduce some impurities, the removal depth for impurities that are easily trapped or adsorbed, such as aluminum, is limited, which may affect the purity and consistency of the final product.

[0004] Therefore, developing a method for preparing lithium iron phosphate that can flexibly control the amount of titanium doping, achieve deep impurity purification, ensure uniform titanium doping, and maintain low cost is of great significance for improving the performance and market competitiveness of lithium iron phosphate cathode materials. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the problems existing in the preparation of titanium-doped iron phosphate using titanium dioxide by-products, such as uncontrollable titanium doping amount, product purity being greatly affected by raw material impurities, and uneven doping caused by easy hydrolysis of titanium ions. The invention provides a high-purity homogeneous titanium-doped iron phosphate with flexible and adjustable titanium doping amount, high product purity, uniform phase, and low cost, as well as its preparation method.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: On the one hand, the present invention provides a low-cost method for preparing high-purity homogeneous titanium-doped iron phosphate, comprising the following steps: S1. Preparation of titanium-containing ferrous solution: Dissolve ferrous sulfate heptahydrate, a byproduct of titanium dioxide, in pure water, add titanium-containing substances and sulfuric acid, stir to dissolve, and then filter directly to obtain titanium-containing ferrous solution; wherein, the titanium-containing substances are one or more of titanium citrate, titanium malate, titanium tartrate, titanium ascorbate, titanium oxysulfate, metatitanic acid, and titanium dioxide. S2, Coprecipitation reaction: Under stirring conditions, the titanium-containing ferrous solution, phosphorus source solution, oxidant and ammonia obtained in step S1 are mixed and reacted to generate amorphous titanium-doped ferric phosphate slurry; S3. Aging and Crystallization: After solid-liquid separation and washing, the amorphous titanium-doped iron phosphate slurry obtained in step S2 is redispersed in pure water, and additives are added. The aging and crystallization reaction is carried out under heating conditions to obtain titanium-doped iron phosphate dihydrate. The additives are at least one of ferrous phosphate, pure ferrous sulfate heptahydrate, and ferrous dihydrogen phosphate. S4. Heat treatment: The titanium-doped iron phosphate dihydrate obtained in step S3 is dried and calcined at high temperature to obtain a high-purity homogeneous titanium-doped iron phosphate product.

[0007] Furthermore, in S1, sulfuric acid is added to adjust the pH of the system to 1.0-2.5. The main purpose of adding sulfuric acid is to provide a strongly acidic environment, effectively inhibiting the hydrolysis of titanium ions during storage and subsequent feeding to form precipitates, ensuring that titanium exists stably in the solution in ionic form, and laying the foundation for subsequent uniform doping.

[0008] Furthermore, in S1, the amount of titanium-containing material added is such that the titanium doping content in the final iron phosphate product is in the range of 1000ppm to 10000ppm.

[0009] Furthermore, in S2, the oxidant is one or more of hydrogen peroxide, air, oxygen, ozone, and sodium peroxide.

[0010] Furthermore, in S2, the phosphorus source is one or more of phosphoric acid, monoammonium phosphate, and diammonium phosphate; the reaction temperature is 30-60℃, and the final pH value is 1.5-3.0.

[0011] Furthermore, in S2, the molar ratio of iron, phosphorus, oxidant and ammonia in the titanium-containing ferrous liquid, phosphorus source solution, oxidant and ammonia water is 1:1.05-1.10:0.6-0.75:0.05-0.2.

[0012] Furthermore, in S3, the aging and crystallization reaction is carried out at a temperature of 70-98°C for 2-8 hours.

[0013] Furthermore, in step S3, the amount of additive added, calculated as iron, accounts for 0.5% to 5.0% of the total mass of iron in the titanium dioxide by-product ferrous sulfate heptahydrate raw material used in step S1. In this scheme, the addition of the additive plays a crucial role: 1. It replenishes the iron source or phosphate ions, precisely controlling the iron-phosphorus molar ratio of the final product; 2. During crystal growth, the effective components in the additive can further remove residual aluminum, magnesium, manganese, and other impurity ions from the slurry from the crystal lattice or adsorb them onto the crystal surface through competitive crystallization or adsorption displacement, making them easier to remove in subsequent washing, thereby achieving deep purification.

[0014] Furthermore, in S4, the drying temperature is 100-180℃; the high-temperature calcination temperature is 500-700℃, and the time is 2-6 hours.

[0015] On the other hand, the present invention provides a high-purity homogeneous titanium-doped iron phosphate, which is prepared by the above preparation method. The titanium doping amount of the iron phosphate is 1000-10000ppm, and the iron-phosphorus molar ratio is 0.915-0.965. The content of key impurities aluminum (Al) in the product is less than 30ppm, and the total amount of magnesium (Mg), manganese (Mn), potassium (K) and sodium (Na) is less than 100ppm.

[0016] The beneficial effects of this invention are as follows: 1. Flexible and controllable titanium doping level, resulting in excellent product homogeneity: By adding additional titanium-containing substances, the dependence on the inherent titanium content of titanium dioxide byproducts is completely eliminated. The titanium doping level can be precisely designed and achieved within a wide range (1000-10000ppm) according to the performance requirements of downstream lithium iron phosphate. Simultaneously, the initial acid stabilization of titanium ions and the liquid-phase co-precipitation combined with aging and crystal transformation processes ensure that titanium ions can uniformly enter the iron phosphate lattice, resulting in a homogeneous doped product.

[0017] 2. High product purity and adjustable iron-phosphorus ratio: Specific additives, such as ferrous phosphate, are introduced during the aging and crystallization stage. This step is not only a simple crystallization process but also a purification and ratio adjustment process. The additives can effectively displace or repel impurity ions, significantly reducing the content of harmful impurities such as Al and Mg in the finished product; at the same time, they can supplement iron or phosphorus, allowing for arbitrary adjustment of the iron-phosphorus ratio, thereby improving the final electrochemical performance of lithium iron phosphate.

[0018] 3. Simplified process and significant cost advantage: This invention directly uses ferrous sulfate heptahydrate, a byproduct of titanium dioxide that has not undergone complex impurity removal pretreatment. Through an embedded impurity removal strategy of stabilizing titanium at the source (adding acid) and purifying the process (aging additives), it replaces the traditional multi-stage deep washing or pre-purity removal process. While ensuring high purity, it simplifies the process and reduces energy and material consumption, resulting in a significant cost advantage.

[0019] 4. Comprehensive utilization of resources and green environmental protection: It fully utilizes ferrous sulfate, a byproduct of the titanium dioxide industry, and achieves efficient utilization of both titanium and iron resources, which is in line with the development direction of green manufacturing and circular economy. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] According to a first aspect of the present invention, a method for preparing high-pressure lithium iron phosphate based on organic additives is provided.

[0023] Example 1: A method for preparing high-purity homogeneous titanium-doped iron phosphate, with a target titanium doping amount of 4000 ppm.

[0024] S1: Weigh 100 kg of ferrous sulfate heptahydrate (Fe content 20.5%, Ti content 2500 ppm), a byproduct of titanium dioxide production, and dissolve it in 500 L of pure water by stirring. Add 0.4 kg of titanium oxysulfate (calculated as TiO2) and adjust the pH of the system to 1.8 with concentrated sulfuric acid. After stirring for 30 minutes, filter to obtain a clear solution containing ferrous sulfate and titanium.

[0025] S2: The above-mentioned titanium-containing ferrous sulfate solution and a phosphoric acid solution diluted to 1.5 mol / L were simultaneously added to a reactor containing bottom water using metering pumps. Ammonia gas was introduced concurrently, and hydrogen peroxide was added dropwise. The reaction temperature was controlled at 50℃, the final pH was 2.2, and the reaction time was 2 hours to obtain an amorphous titanium-doped ferric phosphate slurry.

[0026] S3: Press and filter the slurry, then wash the filter cake with pure water until the conductivity is <1000μS / cm. Re-slurry the washed filter cake in pure water, adjusting the solid content to 15%. Add ferrous dihydrogen phosphate crystals (2% of the total iron content in the titanium dioxide by-product ferrous sulfate heptahydrate) as an additive. Aging and crystallization at 90℃ for 4 hours.

[0027] S4: After aging, the slurry is filtered, washed, and dried (110℃, 12h), and then calcined at 600℃ for 4 hours under a nitrogen atmosphere to obtain titanium-doped iron phosphate product.

[0028] Comparative Example 1 The difference from Example 1 is that: no additional titanium oxysulfate is added in S1, and only titanium from the by-product is used. The additive added in S3 is pure ferrous sulfate heptahydrate (accounting for 3% of the total mass of iron in the titanium dioxide by-product ferrous sulfate heptahydrate raw material).

[0029] Comparative Example 2 The method was carried out according to Example 3 of the patent with publication number CN120172374A. Ferrous sulfate, a byproduct of titanium dioxide from the same batch, was used, but without the addition of titanium and sulfuric acid. Titanium-doped ferric phosphate was prepared according to its two-step precipitation and two-washing process.

[0030] Comparative Example 3 The difference from Example 1 is that no additives are added in the S3 aging and crystallization step.

[0031] Performance testing and characterization 1. Composition analysis: The titanium doping content and impurity content of the iron phosphate products obtained in each example and comparative example were detected by ICP-OES.

[0032] 2. Electrochemical Performance: The above-mentioned iron phosphate was mixed with lithium carbonate and glucose in a stoichiometric ratio, milled, spray-dried, and sintered at 720℃ for 20 hours under a nitrogen atmosphere to prepare lithium iron phosphate cathode material. Using this material as the positive electrode active material and lithium metal sheets as the negative electrode, CR2032 coin cell half-cells were assembled in an argon-protected glove box. The first-cycle charge-discharge performance and discharge performance at different rates (0.1C, 0.5C, 1C, 2C, 5C) were tested using a Blue Electric testing system within a voltage range of 2.5-4.2V.

[0033] Table 1. Product component analysis results (unit: ppm) sample Titanium doping Al Mg Mn Iron-to-phosphorus ratio (molar ratio) Example 1 3980 18 22 35 0.956 Comparative Example 1 2480 25 28 40 0.967 Comparative Example 2 2550 115 152 148 0.958 Comparative Example 3 4010 125 160 158 0.923 As shown in Table 1: 1. Controllability and Purity of Doping: Example 1 achieved precise titanium doping (3980 ppm) by adding an external titanium source, and the contents of key impurities such as Al and Mg were significantly lower than those in Comparative Examples 2 and 3. Although Comparative Example 1 did not add titanium, its impurity content was also lower than that of Comparative Examples 2 and 3 due to the use of additive aging, verifying the internal purification effect of the additive. Comparative Example 3 did not add any additives and had the highest impurity content, indicating that the purification effect is limited without additives in the aging step.

[0034] 2. Iron-phosphorus ratio control: Under the condition of adding about 4000 ppm Ti ions, the iron-phosphorus ratio in Example 1 is closest to the theoretical value of 0.960, indicating that the additive can effectively supplement and fine-tune the stoichiometry of the system.

[0035] Table 2 Electrochemical performance of lithium iron phosphate half-cells in the first cycle (0.1C rate) sample First charge capacity (mAh / g) First discharge capacity (mAh / g) Coulomb efficiency (%) Discharge median voltage (V) Example 1 162.5 159.8 98.3 3.376 Comparative Example 1 160.8 158.1 98.3 3.372 Comparative Example 2 158.3 155.0 97.9 3.352 Comparative Example 3 158.6 154.9 97.7 3.348 As shown in Table 2: The lithium iron phosphate half-cell synthesized using the method of this invention exhibits significantly higher first-cycle discharge capacity, coulombic efficiency, and discharge voltage compared to the comparative sample. This is mainly attributed to its higher purity, more ideal iron-phosphorus ratio, and more uniform titanium doping, which collectively enhance the utilization rate of the active material and the reversibility of the reaction.

[0036] Table 3 Rate performance of lithium iron phosphate half-cells (discharge capacity at different rates, mAh / g) sample 0.1C 0.5C 1C 2C 5C 5C Capacity Retention Example 1 159.8 154.2 148.5 138.7 121.3 75.9% Comparative Example 1 158.1 152.1 145.8 135.2 115.8 73.2% Comparative Example 2 155.0 145.1 135.4 119.6 95.2 61.4% Comparative Example 3 154.9 144.3 133.9 117.1 92.8 59.9% Note: 5C capacity retention rate = (5C discharge capacity / 0.1C discharge capacity) × 100% As shown in Table 3: The lithium iron phosphate material prepared by this invention exhibits excellent rate performance.

[0037] 1. High rate capacity: At a high rate of 5C, the discharge capacity of Example 1 is still as high as 121.3 mAh / g, while the capacity of Comparative Examples 2 and 3 is less than 100 mAh / g.

[0038] 2. Excellent capacity retention: The 5C capacity retention of Example 1 reached 75.9%, significantly higher than that of the comparative example (~60%). This indicates that the material of the present invention has faster lithium-ion diffusion kinetics.

[0039] The superior rate performance of Example 1 is attributed to three key characteristics of its precursor: 1. High purity: low impurities reduce lattice defects and side reactions; 2. Precise iron-phosphorus ratio: ensures the stability of the intrinsic structure and uniform titanium doping; 3. Uniformly distributed titanium ions in the lattice stabilize the crystal structure and may introduce an appropriate amount of charge carriers, synergistically improving the ionic and electronic conductivity of the material. In contrast, Comparative Example 2, with slightly higher impurities and potentially insufficient uniformity of titanium doping, and Comparative Example 3, with the highest impurity content, both severely limit lithium-ion transport, leading to a sharp decline in performance at high rates.

[0040] In summary, the method provided by this invention successfully achieves the low-cost preparation of high-purity, homogeneous, and controllable-doping titanium-doped iron phosphate. Lithium iron phosphate prepared using this as a precursor exhibits significant advantages in specific capacity, efficiency, voltage plateau, and especially high-rate performance, demonstrating superior overall performance compared to existing technologies.

[0041] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A low-cost method for preparing high-purity homogeneous titanium-doped iron phosphate, characterized in that: Includes the following steps: S1. Preparation of titanium-containing ferrous solution: Dissolve ferrous sulfate heptahydrate, a byproduct of titanium dioxide, in pure water, add titanium-containing substances and sulfuric acid, stir to dissolve, and then filter directly to obtain titanium-containing ferrous solution; wherein, the titanium-containing substances are one or more of titanium citrate, titanium malate, titanium tartrate, titanium ascorbate, titanium oxysulfate, metatitanic acid, and titanium dioxide. S2, Coprecipitation reaction: Under stirring conditions, the titanium-containing ferrous solution, phosphorus source solution, oxidant and ammonia obtained in step S1 are mixed and reacted to generate amorphous titanium-doped ferric phosphate slurry; S3. Aging and Crystallization: After solid-liquid separation and washing, the amorphous titanium-doped iron phosphate slurry obtained in step S2 is redispersed in pure water, and additives are added. The aging and crystallization reaction is carried out under heating conditions to obtain titanium-doped iron phosphate dihydrate. The additives are at least one of ferrous phosphate, pure ferrous sulfate heptahydrate, and ferrous dihydrogen phosphate. S4. Heat treatment: The titanium-doped iron phosphate dihydrate obtained in step S3 is dried and calcined at high temperature to obtain a high-purity homogeneous titanium-doped iron phosphate product.

2. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S1, sulfuric acid is added to adjust the pH of the system to 1.0-2.

5.

3. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S1, the amount of titanium-containing material added is such that the titanium doping content in the final iron phosphate product is in the range of 1000ppm to 10000ppm.

4. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S2, the oxidant is one or more of hydrogen peroxide, air, oxygen, ozone, and sodium peroxide.

5. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S2, the phosphorus source is one or more of phosphoric acid, monoammonium phosphate, and diammonium phosphate; the reaction temperature is 30-60℃, and the final pH value is 1.5-3.

0.

6. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S2, the molar ratio of iron, phosphorus, oxidant and ammonia in the titanium-containing ferrous liquid, phosphorus source solution, oxidant and ammonia water is 1:1.05-1.10:0.6-0.75:0.05-0.

2.

7. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S3, the aging and crystallization reaction is carried out at a temperature of 70-98°C for 2-8 hours.

8. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In step S3, the amount of the additive added, calculated as iron, accounts for 0.5% to 5.0% of the total mass of iron in the titanium dioxide by-product ferrous sulfate heptahydrate raw material used in step S1.

9. The low-cost preparation method of high-purity homogeneous titanium-doped iron phosphate according to claim 1, characterized in that: In S4, the drying temperature is 100-180℃; the high-temperature calcination temperature is 500-700℃, and the time is 2-6 hours.

10. A high-purity homogeneous titanium-doped iron phosphate prepared by the method according to any one of claims 1 to 9, characterized in that: The titanium doping content of the iron phosphate is 1000-10000ppm, and the iron-phosphorus molar ratio is 0.915-0.965; the aluminum content in the iron phosphate is less than 30ppm, and the total content of magnesium, manganese, potassium and sodium is less than 100ppm.

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