Preparation method and application of LiFePO4 / C composite material

Abstract

The application discloses a preparation method and application of LiFePO4 / C composite material, and the preparation method comprises the following steps: adding sodium hydrosulfite into a titanium white by-product ferrous sulfate heptahydrate aqueous solution for reduction, then adding citric acid for complexation, carrying out a hydrothermal reaction on the solution after the complexation to obtain a LiFePO4 precursor, and calcining the precursor to obtain the LiFePO4 / C composite material. Through multifunctional design of raw materials, the traditional multi-step process is integrated into a reduction-complexation-hydrothermal synthesis integrated process. The improvement not only shortens the process route, but also significantly reduces the use amount of auxiliary materials and energy, thereby greatly reducing the cost of preparing the high-performance LiFePO4 / C composite material by using the ferrous sulfate of the titanium white by-product, and realizing efficient utilization of materials and energy.

Description

Technical Field

[0001] This invention relates to the field of lithium battery cathode material technology, specifically to a method for preparing and applying a LiFePO4 / C composite material. Background Technology

[0002] Ferrous sulfate heptahydrate, a byproduct of titanium dioxide production via the sulfuric acid process, is produced in large quantities and at a low price. However, due to its numerous impurities, it cannot be used directly and is mostly discarded as waste, wasting iron resources and severely polluting the environment. Currently, converting inexpensive ferrous sulfate into high-value-added LiFePO4 / C composite materials is an ideal path to achieve the resource utilization and high-value-added application of titanium dioxide byproducts.

[0003] LiFePO4 / C composite material, as a novel cathode material for lithium-ion batteries, possesses advantages such as high specific capacity, long cycle life, good thermal stability, and safety, and has broad application prospects in electric vehicles, portable electronic devices, and other fields. Currently, the traditional preparation process for LiFePO4 / C composite material using ferrous sulfate, a byproduct of titanium dioxide production, as raw material involves purification, preparation of iron phosphate, and high-temperature solid-state sintering.

[0004] However, this traditional route faces two major challenges: First, the raw material purification process presents a contradiction between "insufficient impurity removal" and "iron loss," making it difficult to deeply remove impurities such as manganese and copper, which are extremely harmful to battery performance. Furthermore, the presence of insoluble ferric iron results in a low iron recovery rate. Second, the synthesis route presents a contradiction between "complex processes" and "performance limitations." The multi-step conversion process leads to high energy consumption, and the uneven carbon coating of materials synthesized by the solid-phase method makes it difficult to further improve the rate performance and cycle life of the product.

[0005] Therefore, developing a new method for preparing LiFePO4 / C composite materials to simultaneously achieve efficient purification of raw materials, significant simplification of the process, and a marked improvement in the performance of the final product is of great practical significance. Summary of the Invention

[0006] To overcome the shortcomings of the existing technology, the present invention aims to provide a method for preparing LiFePO4 / C composite materials and their application. By utilizing the multiple functions of the core reagents, a short-process, high-quality conversion from industrial by-products to high-performance battery materials is achieved.

[0007] This invention is achieved through the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a LiFePO4 / C composite material, comprising:

[0009] Sodium dithionite was added to the aqueous solution of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, for reduction treatment. The solution was then filtered to remove the precipitate and obtain the filtrate, which yielded the reduced solution. The pH of the reduced solution was adjusted to acidic and heated. The solution was then filtered again to remove the precipitate and obtain the coarse purified filtrate.

[0010] Citric acid was added to the coarse purified filtrate for complexation treatment to obtain a solution containing Fe. 2+ A solution of a citric acid complex; containing Fe 2+ - The pH of the citric acid complex solution was adjusted to weak alkalinity and then filtered to remove the precipitate and obtain the filtrate, thus obtaining the secondary purified filtrate.

[0011] The secondary purified filtrate is subjected to resin adsorption, the effluent is collected, and a lithium source and a phosphorus source are added to the effluent for hydrothermal reaction to obtain a LiFePO4 precursor. The LiFePO4 precursor is then calcined to obtain the LiFePO4 / C composite material.

[0012] Furthermore, the pH of the reduction treatment is 2.5-3.5;

[0013] And / or, the temperature of the reduction treatment is 40-50°C.

[0014] Furthermore, the molar ratio of sodium dithionite to ferrous sulfate heptahydrate is 2-4:100.

[0015] Furthermore, after adding citric acid to complex the reduced solution, the process also includes adjusting the pH of the solution to 8.0-8.5 to precipitate and separate alkaline earth metal impurities.

[0016] Furthermore, before subjecting the complexed solution to a hydrothermal reaction, the process includes passing the solution through an ion exchange column packed with chelating resin to remove manganese ions at a deeper level.

[0017] Furthermore, the chelating resin is a cation exchange resin.

[0018] Preferably, the chelating resin is an acrylic cation exchange resin.

[0019] More preferably, the chelating resin is a weakly acidic acrylic cation exchange resin.

[0020] Furthermore, the molar ratio of ferrous ions to citric acid in the reduced solution is 1:1-1.5.

[0021] Furthermore, the hydrothermal reaction temperature is 180-200℃, and the reaction time is 8-12 hours.

[0022] Preferably, the temperature is first increased to 100°C at a rate of 2°C / min, then increased to 180-200°C at a rate of 1°C / min, and then the hydrothermal reaction is carried out at a temperature of 180-200°C.

[0023] Furthermore, when lithium and phosphorus sources are added to the effluent for hydrothermal reaction, the molar ratio of lithium ions, ferrous ions, and phosphorus atoms is 2.8-3.2:1:1.

[0024] Furthermore, the lithium source is any one of lithium hydroxide monohydrate, lithium carbonate, lithium acetate, and lithium nitrate.

[0025] Preferably, the lithium source is lithium hydroxide monohydrate.

[0026] Furthermore, the phosphorus source is any one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0027] Preferably, the phosphorus source is ammonium dihydrogen phosphate.

[0028] Secondly, the present invention provides a LiFePO4 / C composite material, which is prepared by the above-described method.

[0029] Thirdly, the present invention provides an application of the above-mentioned LiFePO4 / C composite material in the preparation of batteries.

[0030] Fourthly, the present invention provides a battery comprising the above-mentioned LiFePO4 / C composite material.

[0031] The present invention has the following beneficial effects:

[0032] This invention utilizes the multifunctional design of raw materials to integrate traditional multi-step processes into a single "reduction-complexation-hydrothermal synthesis" process. This optimization not only shortens the process path but also significantly reduces the consumption of auxiliary materials and energy, thereby substantially reducing the cost of preparing high-performance LiFePO4 / C composite materials using ferrous sulfate, a byproduct of titanium dioxide production, achieving highly efficient use of materials and energy.

[0033] The key innovation of this invention lies in the multifunctional design of the core raw material. Specifically, sodium dithionite not only acts as a reducing agent to reduce Fe... 3+ Reduced to Fe 2+ Furthermore, it utilizes displacement reactions to deeply remove heavy metal impurities and constructs a reducing environment within the system, achieving a synergistic effect of "activation-impurity removal-protection." Simultaneously, citric acid acts as a selective complexing agent in the purification process, reacting with Fe... 2+It generates stable and soluble complexes, achieving efficient separation of iron and alkaline earth metal impurities; in the synthesis process, it also acts as a molecularly dispersed carbon precursor, forming a uniformly coated conductive carbon network through a hydrothermal-calcination process, thus completing the role transformation from "separation medium" to "performance enhancer".

[0034] This invention constructs a synergistic system that enables efficient utilization of raw materials and precise functional design, successfully avoiding the complexities of introducing multiple additives in traditional processes and maximizing reagent utilization. Attached Figure Description

[0035] Figure 1 The image shows the XRD pattern of the LiFePO4 / C composite material obtained in Example 1 of this invention.

[0036] Figure 2 This is one of the SEM images of the LiFePO4 / C composite material obtained in Example 1 of the present invention;

[0037] Figure 3 This is the second SEM image of the LiFePO4 / C composite material obtained in Example 1 of the present invention;

[0038] Figure 4 This is the third SEM image of the LiFePO4 / C composite material obtained in Example 1 of the present invention;

[0039] Figure 5 This is one of the SEM images of the LiFePO4 / C composite material obtained in Example 2 of the present invention;

[0040] Figure 6 This is the second SEM image of the LiFePO4 / C composite material obtained in Example 2 of the present invention. Detailed Implementation

[0041] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0042] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0043] Example 1

[0044] 100g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide production, was dissolved in 500mL of 65℃ deionized water that had been bubbled with nitrogen for 15 minutes to remove dissolved oxygen. After stirring and dissolving, the solution was filtered through a 0.1μm microporous membrane at a flow rate of 1m / s, a pressure of 0.1MPa, and a temperature of 40℃. The initial filtrate was collected.

[0045] Nitrogen gas was continuously bubbled through the primary filtrate (flow rate 0.8 L / min) to remove dissolved oxygen for 5-10 minutes. Under nitrogen protection and continuous stirring, the pH of the system was adjusted to 3.0 ± 0.1 with 10% (w / w) dilute sulfuric acid. 1.5 g of sodium dithionite was slowly added, and the reaction was carried out at 45 °C for 30 minutes. After the reaction was completed, the solution was filtered through a 0.1 μm microporous membrane under nitrogen protection at a flow rate of 1 m / s, a pressure of 0.1 MPa, and a temperature of 40 °C. The second filtrate was collected.

[0046] The second filtrate was heated to 95°C in an oil bath. While stirring vigorously, a 10% sodium hydroxide solution was slowly added dropwise, and the pH was precisely adjusted to 4.0 ± 0.1 using a precision pH meter for online monitoring. The solution was then incubated at this temperature for 1.5 hours. The third filtrate was collected by suction filtration using a Buchner funnel and a 0.45 μm mixed cellulose ester membrane under incubation.

[0047] After the third filtrate has cooled naturally to below 40°C, at which point the system is acidic, add 6.0g of citric acid with a purity ≥99.5% (molar ratio approximately 1.2:1), and stir until completely dissolved to form stable Fe. 2+ - Citric acid complex solution. While stirring, add a 10% sodium hydroxide solution dropwise to precisely adjust the pH to 8.2. A small amount of white or light-colored precipitate will appear in the solution. After standing for 30 minutes, filter and collect the fourth filtrate.

[0048] The fourth filtrate was passed through an ion exchange column packed with pretreated D113 macroporous weakly acidic acrylic cation exchange resin at a flow rate of 3 BV / h, and the effluent was collected.

[0049] The potassium dichromate method was used to accurately determine the Fe in the above effluent. 2+Using this solution as the iron source, lithium hydroxide monohydrate and ammonium dihydrogen phosphate were added at a molar ratio of Li:Fe:P = 3:1:1, and the mixture was vigorously stirred for 30 minutes to ensure thorough mixing. The mixture was transferred to a stainless steel hydrothermal reactor lined with polytetrafluoroethylene, with the filling degree controlled at 80%. The temperature was first increased to 100℃ at 2℃ / min, then increased to 190℃ at 1℃ / min, and the reaction was carried out at this temperature for 10 hours. After the reaction was completed, the reactor was allowed to cool naturally to room temperature. The reactor was opened, and the product was filtered, washed three times each with deionized water at 60℃ and anhydrous ethanol, and then dried in a vacuum drying oven at 60℃ for 12 hours to obtain the LiFePO4 precursor. The dried LiFePO4 precursor powder was placed in a tube furnace under a high-purity Ar atmosphere (purity ≥99.999%), heated to 650℃ at a rate of 5℃ / min, and calcined at this temperature for 4 hours. After calcination, the furnace is cooled to room temperature and removed under Ar atmosphere protection to obtain the final LiFePO4 / C composite material.

[0050] The XRD pattern of the LiFePO4 / C composite material obtained in this embodiment is as follows: Figure 1 As shown in the electron microscope image... Figure 2-4 As shown. According to Figure 1 It can be seen that the X-ray diffraction pattern of the LiFePO4 / C composite material prepared in this embodiment is consistent with the standard spectrum of orthorhombic olivine structure LiFePO4 (JCPDS 40-1499). Figure 2-3 It can be seen that the LiFePO4 / C composite material obtained in this embodiment has high sphericity and a particle size distribution range of 1-5 μm.

[0051] Example 2

[0052] 100g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide production, was dissolved in 500mL of 60℃ deionized water that had been bubbled with nitrogen for 15 minutes to remove dissolved oxygen. After stirring and dissolving, the solution was filtered through a 0.1μm microporous membrane at a flow rate of 1m / s, a pressure of 0.1MPa, and a temperature of 40℃. The initial filtrate was collected.

[0053] Nitrogen gas was continuously bubbled through the primary filtrate (flow rate 0.8 L / min) to remove dissolved oxygen for 5-10 minutes. Under nitrogen protection and continuous stirring, the pH of the system was adjusted to 3.2 ± 0.1 with 10% (w / w) dilute sulfuric acid. 1.8 g of sodium dithionite was slowly added, and the reaction was carried out at 50 °C for 30 minutes. After the reaction was completed, the solution was filtered through a 0.1 μm microporous membrane under nitrogen protection at a flow rate of 1 m / s, a pressure of 0.1 MPa, and a temperature of 40 °C. The second filtrate was collected.

[0054] The second filtrate was heated to 92°C in an oil bath. While stirring vigorously, a 10% sodium hydroxide solution was slowly added dropwise, and the pH was precisely adjusted to 4.0 ± 0.1 using a precision pH meter for online monitoring. The mixture was then aged at this temperature for 2 hours. The third filtrate was collected by suction filtration using a Buchner funnel and a 0.45 μm mixed cellulose ester membrane under incubation.

[0055] After the third filtrate has cooled naturally to below 45°C, add 5.5g of citric acid with a purity ≥99.5% (molar ratio approximately 1.1:1), and stir until completely dissolved to form stable Fe. 2+ - Citric acid complex solution. While stirring, add a 10% sodium hydroxide solution dropwise to precisely adjust the pH to 8.5. A small amount of white or light-colored precipitate will appear in the solution. After standing for 30 minutes, filter and collect the fourth filtrate.

[0056] The fourth filtrate was passed through an ion exchange column packed with pretreated D113 macroporous weakly acidic acrylic cation exchange resin at a flow rate of 2 BV / h, and the effluent was collected.

[0057] The potassium dichromate method was used to accurately determine the Fe in the above effluent. 2+ Using this solution as the iron source, lithium hydroxide monohydrate and ammonium dihydrogen phosphate were added at a molar ratio of Li:Fe:P = 3:1:1, and the mixture was vigorously stirred for 30 minutes to ensure thorough mixing. The mixture was transferred to a stainless steel hydrothermal reactor lined with polytetrafluoroethylene, with the filling degree controlled at 75%. The temperature was first increased to 100℃ at 2℃ / min, then increased to 185℃ at 1℃ / min, and the reaction was carried out at this temperature for 12 hours. After the reaction, the reactor was allowed to cool naturally to room temperature. The reactor was opened, and the product was filtered, washed three times each with deionized water at 60℃ and anhydrous ethanol, and then dried in a vacuum drying oven at 60℃ for 12 hours to obtain the LiFePO4 precursor. The dried LiFePO4 precursor powder was placed in a tube furnace under a high-purity Ar atmosphere (purity ≥99.999%), heated to 650℃ at a rate of 5℃ / min, and calcined at this temperature for 4 hours. After calcination, the furnace is cooled to room temperature and removed under Ar atmosphere protection to obtain the final LiFePO4 / C composite material.

[0058] The electron microscopy results of the LiFePO4 / C composite material obtained in this embodiment are shown in the figure below. Figure 5-6 As shown, according to Figure 5-6 It can be seen that the LiFePO4 / C composite material obtained in this embodiment has high sphericity and a particle size distribution range of 3-6 μm.

[0059] Comparative Example 1

[0060] Compared with Example 1, the only difference is that the reduction activation and heavy metal deep removal steps are completely omitted, that is, sodium dithionite is not added, and water purification is carried out directly after the initial filtration.

[0061] Comparative Example 2

[0062] The only difference compared to Example 1 is that the addition of citric acid in the complexation protection and precipitation separation steps is completely omitted.

[0063] Comparative Example 3

[0064] The only difference compared to Example 1 is that the deep manganese removal step is completely omitted, i.e., no ion exchange operation is performed.

[0065] Comparative Example 4

[0066] The only difference compared to Example 1 is that, in the reduction activation and heavy metal deep removal steps, an equimolar amount of reduced iron powder is used instead of sodium dithionite.

[0067] Comparative Example 5

[0068] The only difference from Example 1 is that oxalic acid is used instead of citric acid in the complexation protection and precipitation separation steps.

[0069] Comparative Example 6

[0070] The only difference from Example 1 is that, in the complexation protection and precipitation separation steps, an equimolar amount of EDTA-disodium is used instead of citric acid.

[0071] Comparative Example 7

[0072] The only difference compared to Example 1 is that the pH conditions in the water purification step are adjusted from 4.0±0.1 to 5.0±0.1.

[0073] Comparative Example 8

[0074] The only difference compared to Example 1 is that the pH conditions for precipitating impurities after complexation protection were adjusted from 8.2 to 7.2.

[0075] Comparative Example 9

[0076] The only difference compared to Example 1 is that the reaction temperature of sodium dithionite was increased from 45°C to 70°C.

[0077] Comparative Example 10

[0078] The only difference compared to Example 1 is that the molar ratio of citric acid to ferrous acid was increased from 1.2:1 to 2:1.

[0079] Experimental Example 1

[0080] The products obtained in Examples 1-2 and Comparative Examples 1-10 were subjected to performance tests.

[0081] Iron yield: Accurately weigh the initial raw material mass of ferrous sulfate heptahydrate and the final product mass, determine the actual iron content of the product by ICP, and then calculate the total iron yield.

[0082] Mn content: Approximately 50 mg of sample was completely digested with aqua regia in a microwave digester, and after being brought to a final volume, it was determined using inductively coupled plasma mass spectrometry (ICP-MS) with external standard method for quantification.

[0083] Cu content: Approximately 50 mg of sample was completely digested with nitric acid and hydrofluoric acid using a microwave digester and then brought to a final volume. The copper content was determined using an inductively coupled plasma mass spectrometer equipped with a helium collision cell.

[0084] Carbon content: Approximately 0.1g of sample was placed in an oxygen stream in a high-frequency induction furnace and burned to convert carbon into CO2, which was then quantified by an infrared detector.

[0085] Carbon layer quality: A 532nm laser was used as the excitation source, within the range of 100-2000 cm⁻¹. -1 Multiple points of the sample were scanned within the wavenumber range, and the D peak (~1350 cm⁻¹) was selected. -1 ) and G peak (~1580 cm) -1 The average strength ratio of ).

[0086] Electronic conductivity test (four-probe method): Sample powder is pressed into a dense disc in a standard mold under a fixed pressure (10 MPa). Using a four-probe tester, four equally spaced metal probes are pressed vertically onto the sample surface. A constant, minute current is passed through the two outer probes, and the voltage drop between the two inner probes is measured. The resistivity is calculated based on the sample thickness, and its reciprocal is the electronic conductivity.

[0087] The test results are shown in Table 1-3.

[0088] Table 1

[0089]

[0090] Table 2

[0091]

[0092] Table 3

[0093]

[0094] Note: Comparative Example 2 (citric acid omitted), Comparative Example 5 (oxalic acid substituted), and Comparative Example 6 (EDTA substituted) were all interrupted in the early stage of synthesis, and the final LiFePO4 / C material could not be obtained. Therefore, subsequent testing and characterization could not be carried out. Thus, all performance data listed in Tables 1-3 are "unavailable".

[0095] Tables 1 to 3 show that the reduction process using sodium dithionite plays a crucial and indispensable role in improving iron yield and thoroughly removing impurities. Omitting sodium dithionite leads to a sharp decrease in iron yield and the presence of heavy metal impurities, resulting in a comprehensive deterioration in product performance (Comparative Example 1). Replacing it with iron powder results in incomplete activation of the raw material and poor carbon coating quality, leading to overall inferior product performance (Comparative Example 4).

[0096] If citric acid is omitted, iron and impurities will co-precipitate in an alkaline environment, completely interrupting the purification process (Comparative Example 2); when oxalic acid is used to replace citric acid, insoluble precipitates are produced, making liquid-phase selective separation impossible and causing the process to stop (Comparative Example 5); when EDTA is used to replace citric acid, the complex is too stable, hindering hydrothermal synthesis and lacking carbon source function, resulting in synthesis failure (Comparative Example 6); excessive citric acid causes the iron-carbon layer to be too thick, resulting in low graphitization of the excessively thick carbon layer, and the loose and disordered structure hinders lithium-ion diffusion and reduces the tap density of the material (Comparative Example 10).

[0097] Regarding the experimental environment, the increased pH during water purification caused iron loss, resulting in a significant decrease in product capacity (Comparative Example 7); insufficient pH during alkaline precipitation resulted in incomplete removal of manganese impurities, impairing cycle stability (Comparative Example 8); excessively high reduction temperatures introduced sulfur impurities, greatly damaging product capacity and cycle life (Comparative Example 9).

[0098] Furthermore, omitting deep manganese removal will result in severe manganese doping in the product. Although the initial capacity is still acceptable, the cycle life will rapidly decline (Comparative Example 3).

[0099] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing a LiFePO4 / C composite material, characterized in that, include: Sodium dithionite was added to an aqueous solution of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, for reduction treatment. The solution was then filtered to remove the precipitate and obtain the filtrate, thus obtaining the reduced solution. The pH of the reduced solution is adjusted to acidic and heated. The solution is then filtered again to remove the precipitate and obtain a coarse purified filtrate. The pH of the reduction treatment is 2.5-3.

5. The temperature of the reduction treatment is 40-50℃. The molar ratio of sodium dithionite to ferrous sulfate heptahydrate is 2-4:

100. Citric acid was added to the coarse purified filtrate for complexation treatment to obtain a solution containing Fe. 2+ - A solution of citric acid complex; The Fe-containing 2+ - The solution of citric acid complex is adjusted to a weakly alkaline pH and then filtered to remove the precipitate and obtain the filtrate, which is a secondary purified filtrate; the pH of the solution is adjusted to 8.0-8.5 to precipitate and separate alkaline earth metal impurities. The secondary purified filtrate is subjected to resin adsorption, the effluent is collected, and lithium source and phosphorus source are added to the effluent for hydrothermal reaction to obtain LiFePO4 precursor. The LiFePO4 precursor is then calcined to obtain the LiFePO4 / C composite material. In the reduced solution, the molar ratio of ferrous ions to citric acid is 1:1-1.

5.

2. The method for preparing the LiFePO4 / C composite material according to claim 1, characterized in that, Before the complexed solution undergoes a hydrothermal reaction, the process also includes passing the solution through an ion exchange column packed with chelating resin to remove manganese ions at a deeper level.

3. The method for preparing the LiFePO4 / C composite material according to claim 1, characterized in that, The hydrothermal reaction is carried out at a temperature of 180-200℃ for 8-12 hours.

4. A LiFePO4 / C composite material, characterized in that, Prepared by the method described in any one of claims 1-3.

5. The application of the LiFePO4 / C composite material according to claim 4 in the preparation of batteries.

6. A battery, characterized in that, It includes the LiFePO4 / C composite material as described in claim 4.

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