Method for preparing fluorine / metal cation synergistically doped iron phosphate by using regenerated iron source and application of fluorine / metal cation synergistically doped iron phosphate in lithium ion battery

Abstract

The invention relates to the technical field of lithium ion battery materials, and discloses a method for preparing fluorine / metal cation synergistically doped iron phosphate by using a regenerated iron source and application of the fluorine / metal cation synergistically doped iron phosphate in a lithium ion battery, and the method comprises the following steps: purifying a positive plate of a waste lithium iron phosphate battery to obtain a high-purity regenerated iron source; then carrying out coprecipitation reaction, and synchronously introducing fluorine ions for doping to obtain a fluorine-containing iron phosphate precursor; introducing metal cations such as Mn, Mg, Ti, V and Al through one of three different paths (B1, B2 and B3) for doping, drying and calcining at high temperature to prepare fluorine / metal cation synergistically doped iron phosphate; and mixing the precursor with a lithium source and a carbon source, and carrying out spray drying and two-step sintering to prepare the high-compaction and high-capacity lithium ion battery positive electrode material. High-value utilization of waste battery resources is achieved, the ionic conductivity and structural stability of the material are remarkably improved through synergistic doping, and the product has excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material preparation technology, specifically a method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source and its application in lithium-ion batteries. Background Technology

[0002] With the rapid development of the new energy industry, lithium iron phosphate batteries have become the mainstream choice for power batteries and energy storage systems due to their advantages such as high safety, low cost, and environmental friendliness. However, traditional iron phosphate materials suffer from problems such as low electronic conductivity and slow ion diffusion rate, which restricts further improvement in their electrochemical performance. Meanwhile, the recycling and disposal of large numbers of retired iron phosphate batteries has become an urgent environmental problem. Furthermore, existing technologies mostly employ single cation doping (such as Ti, V, Mn, etc.), while anion doping (such as F...)... - There is limited research on cation and anion co-doping, and the doping process is complex and lacks uniformity.

[0003] Existing technologies primarily focus on recovering valuable metals from spent lithium iron phosphate batteries, but the reuse of recovered materials is mostly limited to simple regeneration, which is insufficient to meet the requirements of high-performance cathode materials. CN119390033A, "A Method for Recovering PVDF-Containing Cathode Material from Spent Lithium Iron Phosphate Batteries and Preparing Fluorine-Doped Lithium Iron Phosphate," proposes using PVDF from spent batteries as a fluorine source to prepare fluorine-doped lithium iron phosphate, but it does not solve the problems of purifying the regenerated iron source and ensuring the uniformity of fluorine doping. Furthermore, traditional doping processes for manganese (Mn)... 2+ ), magnesium (Mg) 2+ The doping efficiency of metal elements such as fluorine and metal cations is low. Therefore, developing a method for preparing iron phosphate materials based on recycled iron sources, which combines efficient fluorine / metal cation synergistic doping with low cost, is of great significance for promoting the sustainable development of the new energy industry. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source and its application in lithium-ion batteries, as detailed below: A method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source includes the following steps: Precursor preparation: S1. Disassemble the waste lithium iron phosphate batteries and separate the positive electrode sheets. Treat the positive electrode sheets in an alkaline solution at 20-35℃ for 10-50 minutes to separate the aluminum foil from the positive electrode material. Treat the positive electrode material with an organic solvent at 55-70℃ for 1-2 hours to remove the PVDF binder. After filtration and washing, the active material is obtained. The active material is leached with sulfuric acid at 60-80℃ for 1-3 hours, while a reducing agent is added to retain Fe. 2+The state of the leachate was adjusted to pH in stages: first, sodium carbonate was used to adjust the pH to 2.5-3.5 to remove Al. 3+ The pH is then adjusted to 8.0-9.0 to remove other divalent metal ions, and finally, sulfate is removed by barium salt precipitation to obtain a regenerated iron source. This segmented purification technique maximizes the retention of iron while effectively avoiding iron loss caused by co-precipitation. The purified regenerated iron source has high iron utilization and the aluminum content is less than 50 ppm. Other divalent metal impurities such as manganese, copper, nickel, chromium, and lead are reduced to the ppm level. Compared with the traditional process that uses high-purity phosphoric acid and iron source, this invention uses a regenerated iron source, which can reduce raw material costs by 40-50%. Simultaneously prepare monoammonium phosphate solution, ammonia solution, hydrogen peroxide solution, fluoride salt solution, manganese salt solution, magnesium salt solution, titanium salt solution, vanadium salt solution and aluminum salt solution.

[0005] Coprecipitation reaction: S2. Introduce N2 into the reactor to ensure that the oxygen content in the reactor is below 50 ppm. Mix the regenerated iron source and monoammonium phosphate solution evenly at an iron-to-phosphorus molar ratio of 1:1 and add them into the reactor. S3. Heat the mixture to 80°C. Once the temperature is reached, add all the ammonia solution and fluoride solution within 30 minutes. Then add hydrogen peroxide solution. After reacting for 10 minutes, centrifuge, wash and dry the slurry to obtain fluorinated iron phosphate precursor A. S4. Divide the iron phosphate precursor A into three equal parts: One method involves adding water to slurry, dispersing it evenly, then adding phosphoric acid for acidification, and simultaneously adding a metal salt solution. After stirring evenly, the temperature is raised to 90-95℃. Once the slurry turns white, it is kept at 100-140℃. After feeding the material, it is rapidly cooled in an ice-water bath and aged for 20-40 minutes. After filtration to remove the mother liquor from the aged white material, water is added and thoroughly washed. After drying, ferric phosphate dihydrate B1 is obtained. Secondly, add water to slurry, disperse evenly, add phosphoric acid to acidify, stir evenly, heat to 90-95℃, after the slurry turns white, keep it at the temperature for 120 minutes, discharge the material and wash directly, after washing, slurry with pure water, add metal salt solution, stir evenly, heat to 90-95℃, keep at the temperature for 20-40 minutes, discharge the material and spin dry to obtain iron phosphate dihydrate B2; Thirdly, a portion of water is added for slurry preparation, and a metal salt solution is added simultaneously. The temperature is raised to 90-95℃. Phosphoric acid is added to another portion of water to prepare a phosphoric acid solution. The phosphoric acid solution is then heated to 90-95℃. The high-temperature slurry is slowly pumped into the phosphoric acid solution. After pumping the slurry, it is kept at this temperature for 100-140 minutes. The material is then discharged, washed, and centrifuged to obtain iron phosphate dihydrate B3.

[0006] S5. Ferric phosphate dihydrate B1, B2 and B3 were dried in a microwave dryer and then transferred to a muffle furnace for high-temperature calcination to obtain fluoride ion synergistic cation doped ferric phosphate C1, C2 and C3.

[0007] It can be used as a cathode material for lithium-ion batteries: Carbon coating and sintering: S6. Mix ferric phosphate, carbon source (one or more of citric acid, glucose, and sucrose), lithium source (one or more of lithium carbonate, lithium hydroxide, and lithium phosphate), dispersant (one or more of PEG6000, PEG4000, PEG2000, and PEG1000) with a certain amount of pure water, grind the mixture evenly, and then spray dry it to form a yellow intermediate. S7. Place the yellow intermediate in an inert atmosphere (N2 / Ar), pre-calcine at 300-400℃, then calcine at 600-800℃. After calcine is completed, cool down to room temperature to obtain fluorine ion synergistic cation doped iron phosphate composite cathode material. Furthermore, in step S1, the alkaline solution is one of 5-10% NaOH or 5-10% KOH, the organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, and acetone, the sulfuric acid concentration is 2-10 mol / L, the iron-phosphorus molar ratio of the active substance leaching is controlled at 0.995-1:1, the reducing agent is one of iron powder and ascorbic acid, the fluoride salt solution is one of sodium fluoride or sodium fluoride, the manganese salt solution is one of manganese sulfate, manganese nitrate, and manganese chloride, the magnesium salt solution is one of magnesium sulfate, magnesium chloride, magnesium nitrate, and magnesium acetate, the titanium salt solution is one of titanium sulfate, titanium oxysulfate, tetrabutyl titanate, ammonium fluorotitanate, and titanium tetrachloride, the vanadium salt solution is one of ammonium metavanadate and vanadium oxysulfate, and the aluminum salt solution is one of aluminum sulfate, aluminum nitrate, and aluminum chloride.

[0008] Moreover, in step S1, the regenerated iron source mainly consists of ferrous ions and a very small amount of impurity ions.

[0009] Moreover, in step S2, the N2 introduction time includes the entire preparation process of the fluorinated iron phosphate precursor A.

[0010] Moreover, in step S2, the reaction process needs to be carried out under high temperature conditions.

[0011] Furthermore, in step S3, the hydrogen peroxide solution is added after all the ammonia solution and fluoride solution have been added, and the addition time is 25-35 minutes. The pH is 2.0-2.2 at the end of the addition process.

[0012] Moreover, in step S4, the solid content after adding water to the slurry is 8-14%.

[0013] Furthermore, in step S4, the ratio of phosphorus to iron in the phosphoric acid added for acidification is 0.35-0.60:1.

[0014] Moreover, in step S4, there are three ways to incorporate the metal elements (Mn, Mg, Ti, V, Al) into iron phosphate.

[0015] Furthermore, in step S5, the drying in the microwave dryer is a rapid drying process with a drying time of 20 minutes, and the moisture content of the dried ferric phosphate dihydrate is 10-15%.

[0016] Moreover, in step S6, the carbon source accounts for 10-20% of the mass, the lithium source 20-30%, and the dispersant 5-10%.

[0017] Furthermore, in step S6, the required amount of pure water is required to maintain the solid content of the system at 30-50%.

[0018] Moreover, in step S6, the particle size of the slurry after thorough grinding is less than 1 micrometer.

[0019] Furthermore, in step S6, the moisture content of the precursor powder (yellow intermediate) formed by spray drying is less than 3%.

[0020] Furthermore, in step S7, the pre-firing time is 2-4 h, the calcination time is 6-12 h, and the programmed cooling is first reduced to 400℃ at 5℃ / min, and then reduced to room temperature at 3℃ / min.

[0021] Furthermore, in step S7, the compaction of the fluoride ion-co-cation-doped iron phosphate composite cathode material powder is ≥2.55 g / cm³. 3 1C discharge specific capacity > 138mAh / g, specific surface area 11-15m² 2 / g, with a high particle size distribution.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The technical solution provided by this invention employs a regenerated iron source purification technology based on the principle of multi-stage pH adjustment and selective precipitation, successfully solving the key technical problem of high impurity content in recycled iron from waste batteries. Specifically, this technology achieves the step-by-step removal of impurity ions by precisely controlling the pH gradient: firstly, Al is preferentially precipitated in a weakly acidic environment (pH=2.5-3.5). 3+Subsequently, other divalent metal ions are removed in a weakly alkaline environment (pH=8.0-9.0), and finally, the interference of sulfate ions is completely eliminated by barium salt precipitation. This invention adopts a segmented purification technology to retain iron while avoiding iron loss caused by co-precipitation. The purified regenerated iron source has high iron utilization rate, and the aluminum content of impurities is less than 50 ppm. Other divalent metal impurities such as manganese, copper, nickel, chromium, and lead are reduced to the ppm level. This not only solves the problem of high impurity content in regenerated materials affecting electrochemical performance, but also significantly reduces raw material costs. Compared with the traditional process that uses high-purity phosphoric acid and iron source, this invention uses a regenerated iron source, which can significantly reduce raw material costs and realize the recycling of battery materials, which is in line with the concept of sustainable development.

[0023] 2. The technical solution of this invention employs synergistic doping of fluoride ions and cations. Its mechanism mainly manifests in electronic structure regulation and crystal structure optimization. Fluoride ions, with their high electronegativity and suitable ionic radius, can partially replace the oxygen sites (O2) in iron phosphate. 2- The introduction of fluoride ions forms strongly polar PF bonds, effectively widening the three-dimensional diffusion channels of lithium ions, reducing the ion migration energy barrier, and thus reducing electrochemical polarization. Simultaneously, the introduction of fluoride ions stabilizes the crystal structure, preventing lattice collapse during charge and discharge, and improving cycle stability. Cation doping elements (Mn, Mg, Ti, V, Al, etc.) replace iron sites (Fe). 3+ This invention enhances the electronic conductivity of the material by introducing lattice defects. Of particular note is that this invention solves the problem of Mn... 2+ Mg 2+ The present invention addresses the problem of low efficiency in traditional doping processes for metal elements, specifically the difficulty in forming phosphates under conventional ferric phosphate synthesis pH conditions (pH ≤ 2.5), resulting in extremely low doping efficiency. However, the technical solution of this invention enables the ferric phosphate dihydrate precursor to fully adsorb metal ions, achieving atomic-level uniform doping during subsequent calcination, thus significantly improving doping efficiency. The synergistic effect of fluoride ions and cations manifests as follows: the strong electronegativity of fluoride ions enhances the interaction between cations and lattice oxygen (or fluorine), further stabilizing the crystal structure; simultaneously, the lattice distortion caused by cation doping provides more insertion sites for fluoride ions. These two factors mutually promote each other, significantly improving the overall electrochemical performance of the material.

[0024] 3. This invention provides three different doping paths (B1, B2, B3): The B1 path (precursor doping) involves acidification followed by the addition of a metal salt solution, and finally high-temperature conversion. During the crystallization process, the doped metal ions directly enter the crystal lattice, achieving atomic-level uniform doping. The principle is that the acidic environment causes partial dissolution of the precursor surface, and the metal ions are preferentially adsorbed on the active sites by interfacial energy. During the subsequent high-temperature conversion, they gradually diffuse into the interior of the crystal lattice, resulting in better doping uniformity and enabling the material to exhibit excellent rate performance and cycle life.

[0025] The B2 path (surface coating doping) involves forming an iron phosphate matrix followed by surface doping. Utilizing the specific surface area characteristics of iron phosphate dihydrate, doping is achieved through surface adsorption and solid-phase diffusion. This creates a doped layer with a concentration gradient distribution on the material surface, maintaining structural stability while providing abundant active sites at the interface. This helps reduce charge transfer impedance and improve performance.

[0026] The B3 path (liquid-phase mixed doping) achieves instantaneous nucleation and doping by pumping a slurry containing metal ions into a high-temperature phosphoric acid solution. Taking advantage of the rapid nucleation at high temperatures, the dopant element is uniformly encapsulated in the generated iron phosphate grains. The mechanism is that iron phosphate has extremely low solubility at high temperatures, resulting in the instantaneous formation of a large number of crystal nuclei. Metal ions are captured within the growing grains, achieving uniform doping at the molecular level.

[0027] 4. This invention, through synergistic doping of fluoride ions and cations and an optimized preparation process, ultimately yields iron phosphate materials with significantly improved structural and electrochemical properties. The specific surface area is controlled at 8-12 m² / g, and the tap density is ≥1.1 g / cm³, which is highly beneficial for improving the electrode's tap density and volumetric energy density. Regarding ion diffusion capability, F… - Partially replaces O 2- Subsequently, due to differences in ionic radius and changes in bonding mode, the original one-dimensional ion channel is widened into a three-dimensional channel, increasing the lithium-ion diffusion coefficient. The migration barrier of lithium ions in the material is significantly reduced, and electrochemical polarization is decreased, resulting in excellent high-current performance. Experiments have shown that the material has a discharge specific capacity ≥138 mAh / g at 1C rate, which is far higher than that of traditional lithium iron phosphate materials.

[0028] 5. This invention achieves particle-level electron channels through the conductive carbon network formed by the thermal decomposition of carbon sources such as citric acid and glucose; while the use of PEG series dispersants ensures the uniform distribution of the carbon layer and avoids material agglomeration. The two-step sintering method ensures sufficient pyrolysis of the carbon source while avoiding excessive reduction of iron at high temperatures, thus preventing the formation of an electrochemically inert Fe2P phase.

[0029] 6. First, this invention obtains a high-purity regenerated iron source through dismantling, leaching, and multi-stage purification of waste batteries. Second, it employs fluoride ions and various metal cations for doping, and achieves efficient and uniform element doping through three different process pathways, effectively solving the problem of traditional processes affecting Mn. 2+ Mg 2+ The problem of low doping efficiency of elements was solved; finally, by optimizing the process through N2 protected co-precipitation, microwave rapid drying and two-step sintering, a lithium iron phosphate cathode material with uniform particle size distribution, high compaction density and excellent electrochemical performance was finally obtained. Attached Figure Description

[0030] Figure 1-7 SEM images of anhydrous ferric phosphate prepared in Examples 1 to 7 are shown respectively. Figure 8-9 SEM images of anhydrous ferric phosphate prepared in Comparative Examples 1 and 2 are shown below. Figure 10-16 SEM images of the lithium iron phosphate cathode materials prepared in Examples 1 to 7 are shown respectively. Figure 17-18 SEM images of lithium iron phosphate cathode materials prepared in Comparative Examples 1 to 27 are shown below. Figure 19 The 1C rate discharge specific capacity curve of the coin cell prepared in Example 1 is shown. Detailed Implementation

[0031] Example 1 A method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source includes the following steps: (1) Disassemble the waste lithium iron phosphate battery and separate the positive electrode sheet. Treat the positive electrode sheet in 5% sodium hydroxide solution at 20°C for 10 min to separate the aluminum foil and the positive electrode material. Treat the positive electrode material with N-methylpyrrolidone at 55°C for 1 h to remove the binder PVDF. After filtration and washing, the active material is obtained. The active material is leached with sulfuric acid at 60°C for 1 h. The iron-phosphorus molar ratio of the leached material is controlled at 0.995:1. At the same time, iron powder is added to maintain Fe 2+ The state of the leachate was adjusted to pH in stages: first, sodium carbonate was used to adjust the pH to 2.5 to remove Al. 3+ Then adjust the pH to 8.0 to remove other divalent metal ions, and finally remove sulfate ions by barium salt precipitation to obtain a regenerated iron source; (2) Introduce N2 into the reactor to ensure that the oxygen content in the reactor is less than 50 ppm. Mix the regenerated iron source and monoammonium phosphate solution evenly in the reactor at an iron-to-phosphorus molar ratio of 1:1. (3) Heat the mixture to 80°C. After reaching the temperature, add all the ammonia solution and fluoride solution within 20 minutes. Then add hydrogen peroxide solution within 20 minutes. Then centrifuge, wash and dry the slurry to obtain fluorinated iron phosphate precursor A. (4) Add water to the ferric phosphate precursor to slurry, disperse it evenly, add phosphoric acid to acidify it, and add manganese salt solution at the same time. After stirring evenly, heat it to 90°C. After the slurry turns white, keep it warm for 100 minutes. After feeding the material, cool it down quickly with an ice water bath and age it for 20 minutes. After filtering the mother liquor from the aged white material, add water to wash it thoroughly, and spin dry to obtain ferric phosphate dihydrate B1. (5) Dry iron phosphate B1 dihydrate in a microwave dryer and then transfer it to a muffle furnace for high-temperature calcination to obtain fluorine-doped iron phosphate C1.

[0032] Step (1) Obtain the physicochemical properties of the recycled iron source as shown in Table 1.

[0033] Table 1 Physicochemical properties of recycled iron source

[0034] Example 2 A method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source and its application in lithium-ion batteries includes the following steps: (1) Disassemble the waste lithium iron phosphate battery and separate the positive electrode sheet. Treat the positive electrode sheet in a 10% sodium hydroxide solution at 35°C for 50 min to separate the aluminum foil and the positive electrode material. Treat the positive electrode material with N-methylpyrrolidone at 70°C for 2 h to remove the binder PVDF. After filtration and washing, the active material is obtained. The active material is leached with sulfuric acid at 80°C for 3 h. The iron-phosphorus molar ratio of the leached material is controlled at 1:1. At the same time, iron powder is added to maintain Fe 2+ The state of the leachate was adjusted to pH in stages: first, sodium carbonate was used to adjust the pH to 3.5 to remove Al. 3+ Then adjust the pH to 9.0 to remove other divalent metal ions, and finally remove sulfate ions by barium salt precipitation to obtain a regenerated iron source; (2) Introduce N2 into the reactor to ensure that the oxygen content in the reactor is less than 50 ppm. Mix the regenerated iron source and monoammonium phosphate solution evenly in the reactor at an iron-to-phosphorus molar ratio of 1:1. (3) Heat the mixture to 80°C. After reaching the temperature, add all the ammonia solution and fluoride solution within 40 minutes. Then add hydrogen peroxide solution within 40 minutes. Then centrifuge, wash and dry the slurry to obtain fluorinated iron phosphate precursor A. (4) Add water to the ferric phosphate precursor to slurry, disperse it evenly, add phosphoric acid to acidify it, stir evenly, heat it to 95°C, keep it at the temperature for 140 min after the slurry turns white, discharge the material and wash it directly, after washing it, slurry it with pure water, add manganese salt solution, stir evenly, heat it to 95°C, keep it at the temperature for 40 min, discharge the material and spin dry to obtain ferric phosphate dihydrate B2. (5) Dry iron phosphate B2 dihydrate in a microwave dryer and then transfer it to a muffle furnace for high-temperature calcination to obtain fluorine-doped iron phosphate C2.

[0035] Example 3 A method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source includes the following steps: (1) Disassemble the waste lithium iron phosphate battery and separate the positive electrode sheet. Treat the positive electrode sheet in 80% sodium hydroxide solution at 30°C for 30 min to separate the aluminum foil and the positive electrode material. Treat the positive electrode material with N-methylpyrrolidone at 60°C for 1.5 h to remove the binder PVDF. After filtration and washing, the active material is obtained. The active material is leached with sulfuric acid at 70°C for 2 h. The iron-phosphorus molar ratio of the leached material is controlled at 0.998:1. At the same time, iron powder is added to maintain Fe 2+ The state of the leachate was adjusted to pH in stages: first, sodium carbonate was used to adjust the pH to 3 to remove Al. 3 + Then adjust the pH to 8.5 to remove other divalent metal ions, and finally remove sulfate ions by barium salt precipitation to obtain a regenerated iron source; (2) Introduce N2 into the reactor to ensure that the oxygen content in the reactor is less than 50 ppm. Mix the regenerated iron source and monoammonium phosphate solution evenly in the reactor at an iron-to-phosphorus molar ratio of 1:1. (3) Heat the mixture to 80°C. After reaching the temperature, add all the ammonia solution and fluoride solution within 30 minutes. Then add hydrogen peroxide solution within 30 minutes. Then centrifuge, wash and dry the slurry to obtain fluorinated iron phosphate precursor A. (4) Add water to the ferric phosphate precursor to slurry, add manganese salt solution, heat to 93°C, add phosphoric acid to another part of water to prepare phosphoric acid solution, heat the phosphoric acid solution to 93°C, slowly pump the slurry into the phosphoric acid solution, keep the slurry at the temperature for 120 min after pumping, discharge, wash and centrifuge to obtain ferric phosphate dihydrate B3.

[0036] (5) Dry iron phosphate B3 dihydrate in a microwave dryer and then transfer it to a muffle furnace for high-temperature calcination to obtain fluorine-doped iron phosphate C3.

[0037] Examples 4-7 A method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source is similar to the steps in Example 1, except that the added metal salt solutions are magnesium salt solution, titanium salt solution, vanadium salt solution and aluminum salt solution, respectively.

[0038] Example 8 A lithium-ion battery cathode material is prepared using fluorine / metal cation co-doped iron phosphate prepared in Example 1 as a precursor. The method for preparing the lithium-ion battery cathode material includes the following steps: Step 1: Mix and grind fluorine / metal cation co-doped iron phosphate, lithium source, carbon source and dispersant in pure water to form a slurry; Step 2: Spray dry the slurry to obtain a yellow intermediate. Step 3: The yellow intermediate is sintered in an inert atmosphere in two steps to obtain the lithium-ion battery cathode material.

[0039] Further, in step 1, the carbon source is glucose and sucrose; the lithium source is lithium carbonate; and the dispersant is PEG600. The carbon source accounts for 15% of the mass, the lithium source accounts for 25% of the mass, and the dispersant accounts for 8% of the mass. Furthermore, in step 3, the specific method for the two-step sintering is as follows: first, pre-fire at 350°C for 3 hours, and then calcin at 700°C for 10 hours; the inert atmosphere is N2 atmosphere.

[0040] Comparative Example 1 A method for preparing fluorine-doped iron phosphate is similar to the steps in Example 1, except that no metal salt solution is added in step S4 of this comparative example.

[0041] Comparative Example 2 A method for preparing iron phosphate is similar to that in Example 1, except that no fluoride salt solution or metal salt solution is added in the preparation steps of this comparative example.

[0042] The physicochemical properties of the iron phosphate prepared in the examples and comparative examples are analyzed, and the results are shown in Table 2.

[0043] Table 2 Physicochemical properties of ferric phosphate in the examples and comparative examples

[0044] Analysis of the data in the table above shows that the Fe / P ratios in the examples are very stable, concentrated in the range of 96.15% - 96.61%, indicating that the technical solution of the present invention has good reproducibility and stability. At the anion doping level, all examples successfully achieved efficient and uniform fluoride ion doping, with the doping amount controlled between 997-1088 ppm, indicating the high stability of the doping process. A comparison of the data from Comparative Example 1 and the examples shows that the synergistic doping process did not affect the stability of fluoride doping when introducing multiple elements. Regarding cation doping, different metal elements were used in each example. Examples 1, 2, and 3 involved manganese doping with a doping amount of 2076-2114 ppm; Example 4 involved magnesium doping with a doping amount of 2013 ppm; Example 5 involved titanium doping with a doping amount of 2029 ppm; Example 6 involved vanadium doping with a doping amount of 2059 ppm; and Example 7 involved aluminum doping with a doping amount of 2053 ppm. Each embodiment achieves the target doping while controlling the content of other non-target impurity elements to a trace level, which is in stark contrast to the cases in Comparative Examples 1 and 2 where only trace amounts of non-designed impurities exist, demonstrating the significant advantages of the process of the present invention in element-selective doping.

[0045] The preparation method provided by this invention can stably produce iron-phosphorus materials with stable iron-phosphorus ratios and other indicators, and achieve fluoride ion co-doping with different cations. Different embodiments are directionally doped with different elements (Mn, Mg, Ti, V, Al), providing diverse samples for studying their electrochemical performance.

[0046] Electron micrographs of anhydrous ferric phosphate in the examples and comparative examples are shown below. Figure 1-9 As shown in the figure, the iron phosphate material prepared in this embodiment of the invention exhibits excellent particle morphology. Its primary particle size is uniform, and a good gradation structure is formed during the secondary agglomeration process. This structure results in tight particle packing with clear interfaces, no obvious agglomeration, and high particle dispersibility.

[0047] In contrast, the comparative samples exhibited significant uneven particle size distribution and severe agglomeration, leading to blurred interparticle interfaces and the formation of large agglomerates. This directly affects the tap density and electrical performance of the lithium iron phosphate cathode material. A uniform and graded particle structure ensures atomic-level uniform distribution of doped ions within the iron iron phosphate matrix, effectively reducing lattice distortion caused by uneven doping. This directly improves the material's structural stability, electronic conductivity, and lithium-ion insertion / extraction in the cathode material, representing a key precursor characteristic for obtaining high-performance lithium iron phosphate cathode materials.

[0048] Lithium iron phosphate powder was prepared using the iron phosphate prepared in the examples and comparative examples as raw materials. Its compaction density was measured, and it was used to fabricate coin cells. The electrochemical performance was tested, and the results are shown in Table 3 below. SEM images are shown in […]. Figure 10-18 , Figure 19 Example 1: Specific capacity of a coin cell - 1C discharge specific capacity.

[0049] Table 3. Performance indicators of lithium iron phosphate prepared in the examples and comparative examples

[0050] A comprehensive analysis of the electrochemical performance data of lithium iron phosphate shown in Table 2 reveals that the embodiments of the present invention successfully achieved synergistic optimization of material compaction density and rate performance through a fluorine ion-coordinated metal cation doping strategy.

[0051] The compaction density of the material obtained in the examples remained stable in the high range of 2.54-2.61 g / cm³, while maintaining a high capacity retention rate under 1C discharge conditions. This indicates that the preparation method effectively avoids the problem of decreased ion transport rate usually caused by high compaction density, achieving a balance between the two. Figure 19 ).

[0052] Depend on Figure 9-18 SEM analysis revealed that the microstructure of the embodiment exhibited a relatively obvious particle size distribution. In contrast, the comparative sample showed uneven particle size distribution, resulting in disordered interparticle pore structure and thus affecting ion transport efficiency.

[0053] Comparing the electrochemical performance, at a low rate of 0.1C, the discharge specific capacity of Comparative Example 1 and the Example 2 is close, indicating that doping does not impair the intrinsic specific capacity of the active material. However, as the discharge rate increases to 0.5C and 1C, the discharge capacity of the Example 2 is higher than that of Comparative Example 1, with the 1C discharge specific capacity generally being 2-6 mAh / g higher. This demonstrates that the synergistic doping effect of fluoride ions and metal cations has a significant effect on improving the performance of lithium iron phosphate cathode materials. The introduction of fluoride ions helps stabilize the crystal structure and broaden the lithium-ion migration channels, while the doping of metal cations effectively improves the intrinsic electronic conductivity of the material. The synergistic effect of the two significantly enhances the ion diffusion coefficient and electronic conductivity of the material, enabling lithium ions and electrons to migrate efficiently at higher current densities, thereby maintaining a high capacity.

[0054] Furthermore, the performance of Comparative Example 2 was poor in all aspects, with a 1C capacity of only 127.3 mAh / g, further confirming that the physicochemical properties of the precursor have a decisive influence on the final performance of the lithium iron phosphate cathode material.

[0055] The fluorine ion synergistic metal cation doping technology employed in this invention successfully resolves the inherent contradiction between high actual density and high rate performance of lithium iron phosphate materials by optimizing the material's microstructure, thus providing potential for its application in high-power lithium-ion batteries.

Claims

1. A method for preparing fluorine / metal cation co-doped iron phosphate using a recycled iron source, characterized in that, Includes the following steps: S1. Preparation of recycled iron source: The waste lithium iron phosphate battery positive electrode sheet is disassembled and purified to obtain recycled iron source; S2, Coprecipitation and Fluorine Doping: Under an inert atmosphere, the regenerated iron source is mixed with monoammonium phosphate, followed by the addition of fluoride salt solution, ammonia solution, and hydrogen peroxide solution. The pH of the slurry is controlled at 2.0-2.2 to carry out a coprecipitation reaction to obtain fluorinated iron phosphate precursor A. S3, Metal cation doping: The fluorinated iron phosphate precursor A is doped with metal cations through one of the following three pathways to obtain iron phosphate dihydrate B1, iron phosphate dihydrate B2 or iron phosphate dihydrate B3. Pathway B1: Fluorine-containing ferric phosphate precursor A is slurried, then acidified with phosphoric acid and simultaneously added with a metal salt solution, followed by a heated reaction to obtain ferric phosphate dihydrate B1. Pathway B2: Fluorine-containing ferric phosphate precursor A is slurried, acidified with phosphoric acid and heated to react, washed, then slurried again and coated with a metal salt solution to obtain ferric phosphate dihydrate B2. Pathway B3: Fluorine-containing ferric phosphate precursor A is mixed with a metal salt solution and slurried to obtain the slurried material; at the same time, phosphoric acid is added to water to prepare a phosphoric acid solution and heated separately, and then the slurried material is pumped into the heated phosphoric acid solution to react and obtain ferric phosphate dihydrate B3. S4. Drying and calcination: The iron phosphate dihydrate B1, iron phosphate dihydrate B2 or iron phosphate dihydrate B3 are dried and calcined at high temperature to obtain fluorine / metal cation co-doped iron phosphate C1, fluorine / metal cation co-doped iron phosphate C2 or fluorine / metal cation co-doped iron phosphate C3. The metal cation is one or more of Mn, Mg, Ti, V, and Al.

2. The method according to claim 1, characterized in that, In step S1, the specific purification process is as follows: Disassemble the waste lithium iron phosphate battery and separate the positive electrode sheet. After treating the positive electrode sheet in an alkaline solution at 20-35℃ for 10-50 minutes, separate the aluminum foil and the positive electrode material. Take the positive electrode material and treat it with an organic solvent at 55-70℃ for 1-2 hours to remove the PVDF binder. After filtration and washing, obtain the active material. Immerse the active material in sulfuric acid at 60-80℃ for 1-3 hours, while adding a reducing agent to retain Fe. 2+ After adjusting the pH of the leachate in stages, a regenerated iron source is obtained.

3. The method according to claim 2, characterized in that, The alkaline solution is 5-10% NaOH or 5-10% KOH; the organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, and acetone; the concentration of the sulfuric acid is 2-10 mol / L; the iron-phosphorus molar ratio of the active substance leaching is controlled at (0.995-1):1; the reducing agent is iron powder or ascorbic acid. The specific method for segmented pH adjustment is as follows: first, adjust the pH to 2.5-3.5 with sodium carbonate to remove Al. 3+ Then adjust the pH to 8.0-9.0 to remove divalent metal ions, and finally remove sulfate ions by barium salt precipitation to obtain a regenerated iron source; The fluoride solution is a sodium fluoride solution or an ammonium fluoride solution.

4. The method according to claim 1, characterized in that, In step S2, the inert atmosphere is N2 atmosphere, and the oxygen content is less than 50 ppm; the N2 introduction time includes the entire preparation process of fluorinated iron phosphate precursor A.

5. The method according to claim 1, characterized in that, In step S3, the metal salt solution is one of manganese salt solution, titanium salt solution, magnesium salt solution, vanadium salt solution, or aluminum salt solution; The manganese salt solution is one of manganese sulfate, manganese nitrate, and manganese chloride; The magnesium salt solution is one of magnesium sulfate, magnesium chloride, magnesium nitrate, and magnesium acetate; The titanium salt solution is one of titanium sulfate, titanium oxysulfate, tetrabutyl titanate, ammonium fluorotitanate, and titanium tetrachloride. The vanadium salt solution is one of ammonium metavanadate and vanadium oxysulfate; The aluminum salt solution is one of aluminum sulfate, aluminum nitrate, and aluminum chloride.

6. The method according to claim 1, characterized in that, In step S3, In path B1, the molar ratio of phosphorus in phosphoric acid to iron in the recycled iron source is 0.35-0.60:1; the reaction temperature is 90-95℃, and the reaction time is 100-140 min; after the reaction, the mixture is rapidly cooled in an ice-water bath and aged for 20-40 min. In path B2, phosphoric acid is added and the temperature is raised to 90-95℃, with a reaction time of 100-140 min; the surface coating temperature is 90-95℃, and the reaction time is 25-35 min. In path B3, the temperature of the pulped material and the phosphoric acid solution is 90-95℃. The pulped material is pumped into the heated phosphoric acid solution and reacted for 100-140 minutes. The phosphoric acid concentration in the phosphoric acid solution is 7-13 g / L.

7. The method according to claim 1, characterized in that, In step S4, the drying is microwave drying, and the moisture content of the material after drying is 10-15%; the high-temperature calcination is carried out in a muffle furnace.

8. A fluorine / metal cation co-doped iron phosphate, characterized in that, It is prepared by the method according to any one of claims 1 to 7, and its general chemical formula is Fe. 1-x H x PO 4-y F y H is one or more of Mn, Mg, Ti, V, and Al; the specific surface area of ​​the doped iron phosphate is 8-12 m². 2 / g, tap density ≥1.1g / cm³ 3 .

9. A lithium-ion battery cathode material, characterized in that, The fluorine / metal cation co-doped iron phosphate described in claim 8 is used as a precursor, and the specific method is as follows: Step 1: Mix and grind fluorine / metal cation co-doped iron phosphate, lithium source, carbon source and dispersant in pure water to form a slurry; Step 2: Spray dry the slurry to obtain a yellow intermediate. Step 3: The yellow intermediate is sintered in an inert atmosphere in two steps to obtain the lithium-ion battery cathode material.

10. The lithium-ion battery cathode material as described in claim 9, characterized in that, In step 1, the carbon source is one or more of citric acid, glucose, and sucrose; the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium phosphate; and the dispersant is one or more of PEG6000, PEG4000, PEG2000, and PEG1000. The carbon source accounts for 10-20% of the mass, the lithium source accounts for 20-30% of the mass, and the dispersant accounts for 5-10% of the mass. In step 3, the specific method of the two-step sintering is as follows: first, pre-fire at 300-400℃ for 2-4 hours, and then calcin at 600-800℃ for 6-12 hours; the inert atmosphere is N2 / Ar atmosphere.

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