A high-pressure lithium manganese iron phosphate material and a method for preparing the material from waste lithium iron phosphate.

Abstract

This invention relates to the field of battery recycling, specifically disclosing a high-compact lithium manganese iron phosphate material and a method for preparing the material from waste lithium iron phosphate. The method involves simultaneous carbon removal and synthesis of lithium iron manganese titanium oxide precursors via aerobic roasting, resulting in atomic-level mixing. Following grinding, drying, and two stages of anaerobic roasting, combined with nitrogen-doped carbon coating, the high-compact lithium manganese iron phosphate material is finally obtained. The resulting product has a compaction density >2.50 g / cm³. When applied to lithium-ion batteries, the finished product exhibits a 0.1C discharge capacity >156 mAh / g, a 1C discharge capacity >140 mAh / g, and a capacity retention rate exceeding 92% after 400 cycles, demonstrating excellent performance. This preparation method can upgrade and repair waste materials into high-value-added cathode materials, suitable for high-end lithium battery applications, and opens up new directions for the recycling of waste battery materials.

Description

Technical Field

[0001] This invention relates to the field of battery recycling, and more particularly to a high-pressure lithium manganese iron phosphate material and a method for preparing the material from waste lithium iron phosphate. Background Technology

[0002] During lithium battery production, a certain proportion of waste batteries are generated due to defects in electrode manufacturing, inadequate sealing of battery casings, and process abnormalities such as cell transport damage, electrode wrinkling, and substandard tab welding. These waste batteries account for 3-8% of the total output of battery manufacturers, posing a significant cost burden. Currently, the mainstream treatment methods for these waste batteries mainly include wet recycling and short-process repair.

[0003] The wet recycling route first obtains Cu, Al, graphite and lithium iron phosphate through electrode sorting and stripping. Then, lithium iron phosphate is leached using chemical raw materials such as acid, hydrogen peroxide, sodium carbonate and hydrogen peroxide, and lithium carbonate and iron phosphate are recovered by element. Overall, this route is quite costly, produces a lot of waste, and has limited profits, making it unsuitable for large-scale promotion.

[0004] Since waste lithium iron phosphate is not recycled, its internal crystal structure, elemental composition, and surface coating are relatively intact, giving it a natural advantage for direct remediation. In existing technologies, short-process remediation routes directly repair lithium iron phosphate based on material separation, without altering its original structure and elemental composition, thus restoring its electrochemical performance as much as possible. This approach is highly favored in terms of cost and technological advancement.

[0005] Nevertheless, short-process repair materials still struggle to reach the level of their original commercial counterparts. The binders and conductive carbon black carried within are difficult to remove completely, and the carbon residue generated by pyrolysis increases the carbon content of the product and damages the coated carbon layer. Furthermore, the HF in the pyrolysis atmosphere can cause some lattice damage to the internal lithium iron phosphate. Ultimately, these factors result in repaired LFP products being inferior to the original commercial materials in terms of processing performance, compaction density, charge-discharge performance, and long-cycle performance. They can only be used in some low-end product manufacturing scenarios, and their prices are significantly lower than those of commercial materials.

[0006] Therefore, there is an urgent need to develop a process that can upgrade and repair waste lithium iron phosphate batteries, so that the materials recycled from waste batteries can be used in some high-end application scenarios. Summary of the Invention

[0007] To address the technical problems existing in the background art, this invention proposes a method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate. In the precursor preparation stage, waste lithium iron phosphate, a manganese source, a lithium source, and a dopant source are added. Air calcination is used to remove binders, conductive agents, and coated carbon from the waste material, while simultaneously forming a solid solution precursor I and increasing the porosity of the deeper material in contact with oxygen. The method includes the following steps: S1. Mix the waste lithium iron phosphate, manganese source, lithium source and doping source M to obtain a mixture, and then calcine the mixture with oxygen to obtain precursor I. S2. Precursor I is then ground with phosphorus source, carbon source, nitrogen source and water to obtain slurry. After drying the slurry, precursor II is obtained. S3. After oxygen-free calcination of precursor II, the finished lithium manganese iron phosphate material is obtained through post-processing. M is selected from at least one of Mg and Ti.

[0008] In this invention, the oxidation roasting process is cleverly utilized to simultaneously achieve the oxidation and decarbonization of plant waste materials and the synthesis of lithium iron manganese oxide precursors, and to achieve atomic-level mixing of metal elements. At the same time, the gases generated by the decomposition of manganese and lithium sources are used to increase porosity and improve the permeability of deep materials to oxygen, which is conducive to the thorough removal of residual carbon from raw materials and ensures the stability of subsequent synthesized materials. Finally, due to the two high-temperature calcinations and the doping of internal crystals with titanium and nitrogen elements and the surface carbon coating, the prepared lithium iron manganese phosphate has a high compaction density (>2.5 g / cm3), high discharge capacity (>156 mAh / g at 0.1C), good rate performance (>140 mAh / g at 1C), and good cycle performance (more than 90% capacity retention after 400 cycles).

[0009] Preferably, the waste lithium iron phosphate is obtained by cutting the lithium iron phosphate positive electrode sheet into 1-3cm fragments and then separating the lithium iron phosphate and aluminum sheet by dry mechanical peeling, and then obtaining the lithium iron phosphate material by air classification and sieving; for small batches, the material can also be peeled by soaking in water and supplementing with ultrasound, and then the raw material is obtained by drying and crushing. In this invention, the stripping method not only achieves high purity and low cost, but also ensures controllable dust pollution during the stripping process and avoids wastewater and exhaust gas emissions. After air classification, the material obtained by this stripping method also contains small amounts of impurities such as binders and conductive agents in the resulting waste lithium iron phosphate. The specific components are as follows: Li: 3.9-4.3%, Fe: 32-35%, P: 18-20%, C: 3-5%, key impurities Cu <10ppm, Al <300ppm, magnetic substances <1000ppb; Preferably, the precursor 1 has the following main components: A solid solution containing Fe in a trivalent state and manganese in an intermediate state between trivalent and tetravalent states, with a carbon content of less than 0.1%. The molar ratio of each metal element is Li:Fe:Mn:M = (1.00-1.03):(0.10-0.90):(0.20-0.80):(0.001-0.020). To ensure that the molar ratios of the elements in the mixture are close to the above proportions, the following ratios can be used as a reference when adding each raw material: More preferably, the manganese dosage is 0.112-0.678 gMn / gFe, calculated based on the actual manganese content percentage in the manganese source. More preferably, the manganese source includes at least one of manganese carbonate, manganese oxalate, manganese oxide, manganese dioxide, manganese hydroxide, manganese phosphate, manganese monoxide, manganese trioxide, manganese tetroxide, and electrolytic manganese. More preferably, based on the fact that the doping source ions are mainly magnesium or titanium ions, the actual titanium (magnesium) content percentage is calculated as follows: the titanium dosage is 0.0015-0.0098 g Ti / g Fe, or the magnesium dosage is 0.0010-0.006 g Mg / g Fe. More preferably, the doping source includes at least one of titanium dioxide, titanium oxalate, magnesium carbonate, and magnesium oxide; More preferably, the molar amount of lithium in the final mixture is calculated based on the actual percentage of lithium content in the lithium source, ensuring that it is 1.03-1.05 times the total molar amount of iron and manganese. Since the waste lithium iron phosphate itself contains a certain amount of lithium, the actual lithium input is calculated by subtracting the original molar amount of lithium in the precursor from the molar amount of (Fe+Mn) in the precursor, which is 1.03-1.05 times the molar amount of (Fe+Mn) in the precursor. More preferably, the lithium source includes at least one of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium hydroxide; In this invention, the aerobic calcination stage can be carried out in air or in a pure oxygen atmosphere. The manganese source, titanium source, lithium iron phosphate, and lithium source are added and calcined in advance, serving four purposes: (i) Use an oxygen-rich atmosphere to thoroughly remove the coating carbon, binder, and conductive agent from the lithium iron phosphate raw material; (ii) Oxidation of lithium iron phosphate to form The solid solution is shown in formula (1), and the manganese source is decomposed to produce manganese oxides, as shown in formulas (2)-(7). At high temperature, the newly generated metal oxides are easy to produce a solid solution with uniform element distribution. (iii) This causes the manganese and lithium sources to release a large amount of substances during the decomposition process. The gas can be utilized on the one hand. The gas reacts with the carbon in the raw material, reducing its density. At the same time, the overflow of the gas will create a large number of loose pores, preventing the raw material from caking during calcination and providing a channel for oxygen to enter the deeper layers of the raw material, which is conducive to the complete removal of carbon impurities in the raw material. (iv) The lithium iron manganese oxide precursor is generated in advance and the Mn and Fe elements in the high oxidation state in the precursor are stabilized. The reaction equation is shown in (8).

[0010] Preferably, the mixture further includes a crushing process before aerobic roasting, and the D50 of the mixture is <30μm; In this invention, the crushing process includes, but is not limited to, mechanical crushing and air jet milling.

[0011] Preferably, the conditions for the aerobic roasting are: temperature 500-800℃, time 2-10h.

[0012] In this invention, a slight positive pressure needs to be maintained during the aerobic roasting process to facilitate exhaust gas discharge.

[0013] Preferably, the solid content during grinding is 30-40 wt%, and the grinding process includes ball milling and sand milling; More preferably, the ball milling time is not less than 1 hour, and the sand milling time is not less than 1.5 hours. More preferably, the slurry D50 < 5.0 μm.

[0014] In this invention, the precursor I needs to be cooled to below 50°C and then initially crushed using crushing equipment.

[0015] Preferably, the slurry is characterized in that the molar ratio of each element is Li:Fe:Mn:Ti(Mg):P:N = (1.00-1.03):(0.20-0.90):(0.10-0.80):(0.001-0.020):1:(0.0001-0.005).

[0016] To ensure that the molar ratios of all elements in the final product are close to those described in the slurry, the following proportions can be used as a reference when adding each raw material: Preferably, the amount of phosphorus added should be equal to 1.01-1.05 times the molar amount of (Fe + Mn) in precursor I minus the original molar amount of phosphorus in the precursor; More preferably, the phosphorus source includes at least one of lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, and phosphoric acid; Preferably, the carbon source can be at least one of glucose, sucrose, and polyethylene glycol; More preferably, the amount of glucose added is 5-15% of the mass of precursor I, and the amount of polyethylene glycol added is 1-5% of the mass of precursor I; More preferably, the molecular weight of the polyethylene glycol is 2000 or 4000; More preferably, the combination of the carbon source is glucose and polyethylene glycol 2000; Preferably, the anaerobic calcination includes a first stage of anaerobic calcination and a second stage of anaerobic calcination.

[0017] More preferably, the first stage anaerobic calcination conditions are: temperature 380-430℃, time 2-6h; the second stage anaerobic calcination conditions are: temperature 650-800℃, time 4-10h. Preferably, the amount of nitrogen source added is 0.5-1.0% of the mass of precursor I; More preferably, the nitrogen source includes at least one of melamine, chitosan, and amino acids; More preferably, the heating rates during the first stage of anaerobic calcination and the second stage of anaerobic calcination are independently 1-5℃ / min.

[0018] In this invention, during the repair process of lithium iron phosphate, numerous Fe-Li antisite defects easily appear in its crystal lattice, affecting the material's cycle performance. By doping a certain amount of nitrogen into the carbon coating on the surface, the Fe-N bonds can fix iron atoms, inhibiting their entry into the lithium-ion diffusion channels, thereby improving the stability of the repaired material.

[0019] During the first stage of anaerobic roasting, the carbon source coats the material surface at this temperature and initially generates lithium manganese iron phosphate crystal nuclei; while during the second stage of anaerobic roasting, the lithium manganese iron phosphate material is formed and the crystals are dense at this temperature.

[0020] Since atomic-level mixing of Li-Mn-Fe was achieved during the preparation of precursor 1, lithium manganese iron phosphate material with uniform metal element mixing can be produced during oxygen-free calcination, taking reaction formula (9) as an example:

[0021] The present invention also proposes a high-pressure lithium manganese iron phosphate material, which is obtained by the above preparation method.

[0022] Preferably, the high-pressure lithium manganese iron phosphate material comprises lithium manganese iron phosphate crystal nuclei and a carbon layer.

[0023] Preferably, the high-compact lithium manganese iron phosphate material is spherical or ellipsoidal in diameter with a diameter of 1.0-2.0 μm, has a particle size of 50-200 nm, a carbon content of about 1-1.5%, and a compaction density greater than 2.5 g / cm³.3 ; Preferably, the stoichiometric formula of the lithium manganese iron phosphate crystal nucleus is: Where x takes values ​​of 1.03-1.05, y+z+u takes values ​​of 0.97-0.99, and y takes values ​​of 0.20-0.90, z takes values ​​of 0.10-0.80, u takes values ​​of 0.001-0.020, and M represents Ti or Mg; More preferably, considering cost, the value of y is 0.50-0.90, and the value of z is 0.10-0.50; The reason why it is necessary to further optimize the ratio of iron and manganese in this invention is that, considering both cost and performance, the ratio of manganese iron phosphate synthesized from waste materials should not be too high. Otherwise, the cost of adding manganese, lithium and phosphorus will increase significantly, resulting in an excessively high cost of the final product, and the cost reduction advantage brought by waste materials cannot be fully utilized.

[0024] Preferably, the post-processing includes cooling, air jet milling, demagnetization, and packaging.

[0025] Beneficial effects of this invention: (1) This invention upgrades and repairs waste lithium iron phosphate into high-pressure lithium manganese iron phosphate. When the finished product is used in lithium-ion batteries, the voltage platform is increased from about 3.4V of lithium iron phosphate to more than 4.1V, and the theoretical energy density can be increased by 15%-20%. Its energy density extreme value can reach 230Wh / kg, that is, it can bring a longer driving range under the same volume. (2) The upgraded and repaired products have high profit margins and can be applied to higher-end scenarios, rather than the traditional homogenization and low-end products, making them more competitive in the market. (3) By adjusting the proportion of each element in the finished product, production costs and finished product performance can be further balanced. Detailed Implementation

[0026] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0028] The technical solution of the present invention will now be described more clearly and completely with reference to specific embodiments and comparative examples.

[0029] Example 1

[0030] This embodiment proposes a method for preparing the material from high-pressure lithium manganese iron phosphate and waste lithium iron phosphate from industrial plants. The preparation method includes the following steps: (1) Lithium iron phosphate cathode sheets were manually screened from the factory waste and sliced ​​into small pieces with a size of 1-3 cm using a slicer. The lithium iron phosphate and aluminum sheets were then separated using a stripper to obtain coarse materials of lithium iron phosphate and aluminum sheets. The mixture was then air-classified and screened to obtain lithium iron phosphate material. Lithium iron phosphate material was selected as the remediation material. The remediation material was tested and found to contain Li: 4.24%, Fe: 33.28%, P: 19.01%, C: 3.92%, Al: 0.0239%, Cu: 0.0009%. (2) Weigh 1000g of lithium iron phosphate, 437g of anhydrous manganese carbonate, 145g of lithium carbonate and 3.9g of titanium dioxide. Use a blade crusher to crush the materials for 60s to obtain a mixture. At this time, the molar ratio of each element in the mixture is close to Li:Fe:Mn:Ti=1.03:0.60:0.38:0.005. Take out the mixture and place it in a sagger. Roast it in air at 700℃ for 3h. After cooling to room temperature, roasted material I is obtained. Roasted material I is then crushed for 30s using a blade crusher. The result is recorded as precursor I. (3) Weigh 447g of anhydrous ammonium dihydrogen phosphate as phosphorus source, 115g of glucose and 15g of polyethylene glycol 2000 as carbon source, and 13.7g of lysine as nitrogen source. Using water as medium, add them together with precursor 1 into the ball mill jar and grind for 1 hour. Then transfer them to the sand mill for fine grinding for 2 hours. Spray dry the final slurry to obtain precursor II. (4) Place precursor II in a sagger and heat it to 400°C in a box furnace under nitrogen atmosphere protection at a heating rate of 3°C / min, and hold for 4 hours; continue to heat to 720°C at a heating rate of 3°C / min, and hold for 6 hours. After that, cool the material to room temperature to obtain calcined material II. (5) The roasted material II is successively passed through airflow pulverization and strong magnet demagnetization to obtain the finished lithium manganese iron phosphate material.

[0031] Example 2

[0032] (1) Lithium iron phosphate cathode sheets were manually screened from the factory waste and sliced ​​into small pieces with a size of 1-3 cm using a slicer. The lithium iron phosphate and aluminum sheets were then separated by ultrasonication and soaking to obtain wet lithium iron phosphate material. After drying, the material was crushed by a pulverizer to obtain the repair material. The repair material contained Li: 4.35%, Fe: 33.63%, P: 19.21%, C: 3.93%, Al: 0.0237%, Cu: 0.0008%. (2) Weigh 1000 g of lithium iron phosphate, 316.5 g of manganese tetroxide, 163.2 g of lithium carbonate and 9.0 g of titanium dioxide. Use a blade crusher to crush the materials for 60s to obtain a mixture. At this time, the molar ratio of each element in the mixture is close to Li:Fe:Mn:Ti=1.03:0.58:0.40:0.009. Take out the mixture and put it in a sagger. Roast it in air at 675 ℃ for 3h. After cooling to room temperature, roasted material I is obtained. Roasted material I is then crushed by a blade crusher for 60s. The result is recorded as precursor I. (3) Weigh 482.0g of phosphoric acid (85wt%) as the phosphorus source, 120g of glucose and 15g of polyethylene glycol 2000 as the carbon source, and 18.5g of lysine as the nitrogen source. Use water as the medium and add it together with precursor 1 into a ball mill for coarse grinding for 2h, then transfer it to a sand mill for fine grinding for 2h. Spray dry the final slurry to obtain precursor II. (4) Place precursor II in a sagger and heat it to 420°C in a box furnace under nitrogen atmosphere protection at a heating rate of 3°C / min, and hold for 4 hours; continue to heat to 700°C at a heating rate of 3°C / min, and hold for 8 hours. After that, cool the material to room temperature to obtain calcined material II. (5) The roasted material II is successively passed through airflow pulverization and strong magnet demagnetization to obtain the finished lithium manganese iron phosphate material.

[0033] Example 3 (1) Lithium iron phosphate cathode sheets were manually sorted from the factory waste and sliced ​​into small pieces with a size of 1-3 cm using a slicer. The lithium iron phosphate and aluminum sheets were then separated by ultrasonication and soaking to obtain wet lithium iron phosphate material. After drying, the material was crushed by a pulverizer to obtain the repair material. The repair material contained Li: 4.40%, Fe: 33.77%, P: 19.42%, C: 3.88%, Al: 0.0142%, Cu: 0.0005%. (2) Weigh 100g of lithium iron phosphate, 66.76g of anhydrous manganese carbonate, 22.17g of lithium carbonate and 0.80g of titanium dioxide. Use a blade crusher to crush the materials for 60s to obtain a mixture. At this time, the molar ratio of each element in the mixture is close to Li:Fe:Mn:Ti=1.03:0.50:0.48:0.008. After taking out the mixture, place it in a sagger and calcine it in air at 725℃ for 3 hours. After cooling to room temperature, calcined material I is obtained. Calcined material I is then crushed for 60s using a blade crusher. The result is recorded as precursor I. (3) Weigh 67.3g of phosphoric acid (85wt%) as the phosphorus source, 13g of glucose and 1.0g of polyethylene glycol 2000 as the carbon source, and 1.5g of lysine as the nitrogen source. Using water as the medium, add them together with precursor 1 into a sand mill and grind them for 3 hours. Spray dry the final slurry to obtain precursor II. (4) Place precursor II in a sagger and heat it to 400°C in a box furnace under nitrogen atmosphere protection at a heating rate of 3°C / min, and hold for 4 hours; continue to heat to 720°C at a heating rate of 3°C / min, and hold for 8 hours. After that, cool the material to room temperature to obtain calcined material II. (5) The roasted material II is successively passed through airflow pulverization and strong magnet demagnetization to obtain the finished lithium manganese iron phosphate material.

[0034] Example 4 This embodiment is the same as the other steps in embodiment 3, except that the molar ratio of each element in the mixture in step (2) is adjusted to Li:Fe:Mn:Ti=1.03:0.60:0.36:0.010, and the calcination temperature is adjusted to 750℃.

[0035] Example 5 This embodiment is the same as the other steps in embodiment 3, except that the phosphorus source in step (3) is replaced with 67.15g of ammonium dihydrogen phosphate, the nitrogen source is replaced with 2.0g of chitosan, and the calcination temperature is adjusted to 750℃.

[0036] Example 6 This embodiment is the same as other steps in embodiment 5, except that "continue to heat up to 720°C at 3°C / min and keep warm for 8 hours" in step (4) is changed to "continue to heat up to 720°C at 3°C / min and keep warm for 10 hours".

[0037] Comparative Example 1 Steps (1), (4), and (5) of this comparative example are the same as those of Example (3). The remaining steps, such as steps (2) and (3), are adjusted as follows: (2) Weigh 100g of lithium iron phosphate, 66.76g of anhydrous manganese carbonate, 22.17g of lithium carbonate and 0.80g of titanium dioxide, and crush the materials for 60s using a blade crusher to obtain a mixture; (3) Weigh 67.3g of phosphoric acid (85wt%) as the phosphorus source, 13g of glucose and 1.0g of polyethylene glycol 2000 as the carbon source, and 1.5g of lysine as the nitrogen source. Use water as the medium and add the mixture together to a sand mill for fine grinding for 3 hours. Spray dry the final slurry to obtain precursor II.

[0038] Comparative Example 2 Steps (1), (4), and (5) of this comparative example are the same as those of Example (3). The remaining steps, such as steps (2) and (3), are adjusted as follows: (2) Weigh 100g of lithium iron phosphate, 22.17g of lithium carbonate, and 0.80g of titanium dioxide. Use a blade crusher to crush the materials for 60s to obtain a mixture. At this time, the molar ratio of each element in the mixture is close to Li:Fe:Ti=1.03:0.50:0.008. After taking out the mixture, place it in a sagger and calcine it in an air atmosphere at 725℃ for 3 hours. After cooling to room temperature, obtain calcined material I. The obtained calcined material I is then crushed for 60s using a blade crusher. The result is recorded as precursor I. (3) Weigh 67.3 g of phosphoric acid (85 wt%) as the phosphorus source, 13 g of glucose and 1.0 g of polyethylene glycol 2000 as the carbon source, 1.5 g of lysine as the nitrogen source, and 66.76 g of anhydrous manganese carbonate as the manganese source. Using water as the medium, add the precursor 1 together to a sand mill and grind for 3 hours. Spray dry the final slurry to obtain precursor II.

[0039] Comparative Example 3 Steps (1), (4), and (5) of this comparative example are the same as those of Example (3). The remaining steps, such as steps (2) and (3), are adjusted as follows: (2) Weigh 100g of lithium iron phosphate, 66.76g of anhydrous manganese carbonate and 0.80g of titanium dioxide. Use a blade crusher to crush the materials for 60s to obtain a mixture. Take out the mixture and place it in a sagger. Roast it in air at 725℃ for 3 hours. After cooling to room temperature, obtain roasted material I. Roasted material I is then crushed by a blade crusher for 60s. The result is recorded as precursor I. (3) Weigh 67.3g of phosphoric acid (85wt%) as the phosphorus source, 13g of glucose and 1.0g of polyethylene glycol 2000 as the carbon source, 1.5g of lysine as the nitrogen source, and 22.17g of lithium carbonate as the lithium source. Using water as the medium, add them together with precursor 1 into a sand mill and grind for 3 hours. Spray dry the final slurry to obtain precursor II.

[0040] Comparative Example 4 This comparative example is the same as Example (3), except that the "calcination at 725°C for 3 h" in step (2) is changed to "calcination at 400°C for 3 h".

[0041] That is, in Comparative Example 4, the oxidation and calcination temperature during the preparation of precursor I in step (2) was only 400℃.

[0042] Comparative Example 5 This comparative example is the same as Example (3), except that the "calcination at 725°C for 3 h" in step (2) is changed to "calcination at 900°C for 3 h".

[0043] That is, in Comparative Example 5, the oxidation and calcination temperature reached 900℃ during the preparation of precursor I in step (2).

[0044] Comparative Example 6 This comparative example is the same as Example (3), except that "1.5g lysine as nitrogen source" in step (3) is changed to "0g lysine".

[0045] That is, in the preparation of precursor II in step (3) of Comparative Example 6, no nitrogen source is added as a surface carbon dopant.

[0046] The finished lithium manganese iron phosphate materials obtained in the above embodiments were subjected to physicochemical analysis, and then the finished lithium manganese iron phosphate was assembled into a button cell for testing. The test results are detailed in Table 1. Physicochemical analysis ICP elemental detection: Inductively coupled plasma mass spectrometry; Compacted density test: Accurately weigh 15g of finished lithium manganese iron phosphate material, put it into a measuring cylinder, fix the measuring cylinder on the support, and measure the corresponding volume after 3000 vibrations. Compacted density = mass after compaction / volume after compaction. D50 test: Malvern laser particle size analyzer test; Button battery assembly: The finished lithium manganese iron phosphate material prepared in each embodiment was used as the positive electrode material, the negative electrode was a lithium metal sheet, the separator was a Celgard 2500 separator, and the electrolyte was fosai LB-002 electrolyte from Suzhou Fosai New Materials Co., Ltd. The CR2032 button cell was assembled according to the existing technical method. The battery was assembled in a dry glove box filled with argon gas. After the assembly was completed, the battery was subjected to electrochemical testing. Electrochemical testing: At 25℃, charge at a constant current of 1C to 4.25V, then maintain a constant voltage of 4.25V to 0.05C, and then discharge at 0.1C to the lower limit voltage of 2.0V. Record the discharge capacity of this cycle as the 0.1C discharge capacity. Repeat the above charging process, then discharge at 1C to the lower limit voltage of 2.0V, and record the discharge capacity of this cycle as the 1C discharge capacity. Repeat the 1C charge-discharge cycle, and record the discharge capacity at the 400th cycle. Calculate the capacity retention rate after 400 cycles using the formula: Capacity retention rate after 400 cycles = (Discharge capacity at the 400th cycle) / (Discharge capacity at the first cycle) 100%; Table 1 Test results of finished lithium manganese iron phosphate materials in each embodiment

[0047] As shown in Table 1, when this material is used in lithium-ion batteries, the discharge capacity is greater than 156 mAh / g at 0.1C rate and greater than 145 mAh / g at 1C rate, demonstrating superior performance that is comparable to that of the original raw material.

[0048] As can be seen from Comparative Example 1, it is necessary to undergo the process of precursor 1 to remove carbon and form a solid solution; otherwise, the carbon content and compaction density of the finished product will increase significantly, and the electrical properties of the material will deteriorate significantly. As shown in Comparative Example 2, it is necessary to add manganese source and co-roast lithium iron phosphate waste in advance. This can form Fe-Mn solid solution in advance and fully, which is conducive to the uniform distribution of elements in subsequent lithium iron manganese materials and has an important promoting effect on the electrical performance of materials. As shown in Comparative Example 3, it is necessary to add a lithium source to the precursor in advance. Although the effect is not as obvious as that of a manganese source, it can still slightly improve the electrical performance of the finished product by forming the Li-Fe-Mn solid solution in advance. As can be seen from Comparative Example 4, excessively low temperatures are not conducive to sufficient oxidation and decarbonization and the formation of a sufficient solid solution, which in turn makes it difficult to achieve the desired carbon content, compaction density, and electrical properties of the finished product. As can be seen from Comparative Example 5, excessively high temperatures can also cause changes in the properties of solid solutions, thereby affecting the electrical properties of the finished product. As shown in Comparative Example 6, incorporating a certain amount of nitrogen into the carbon coating of lithium manganese iron phosphate is beneficial to improving the overall cycle performance of the material. The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate, characterized in that, Includes the following steps: S1. Mix the waste lithium iron phosphate, manganese source, lithium source and doping source M to obtain a mixture, and then calcine the mixture with oxygen to obtain precursor I. S2. Precursor I is ground with phosphorus source, carbon source, nitrogen source and water to obtain slurry, and the slurry is dried to obtain precursor II; S3. After oxygen-free calcination of precursor II, the finished lithium manganese iron phosphate material is obtained through post-processing. M is selected from at least one of Mg and Ti.

2. The method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate according to claim 1, characterized in that, The precursor 1 mainly consists of A solid solution containing Fe in a trivalent state and manganese in an intermediate state between trivalent and tetravalent states, with a carbon content of less than 0.1%. The molar ratio of each metal element is Li:Fe:Mn:M = (1.00-1.03):(0.10-0.90):(0.20-0.80):(0.001-0.020). Preferably, the manganese source includes at least one of manganese carbonate, manganese oxalate, manganese oxide, manganese dioxide, manganese hydroxide, manganese phosphate, manganese monoxide, manganese trioxide, manganese tetroxide, and electrolytic manganese.

3. The method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate according to claim 1 or 2, characterized in that, The conditions for the aerobic roasting are: temperature 500-900℃, time 2-10h.

4. The method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate according to any one of claims 1-3, characterized in that, In the slurry, the molar ratio of each element is Li:Fe:Mn:M:P:N = (1.00-1.03):(0.20-0.90):(0.10-0.80):(0.001-0.020):1:(0.0001-0.005).

5. The method for preparing high-pressure compaction lithium manganese iron phosphate material from waste lithium iron phosphate according to any one of claims 1-4, wherein the slurry D50 < 5.0 μm.

6. The method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate according to any one of claims 1-5, characterized in that, The anaerobic roasting includes a first stage of anaerobic roasting and a second stage of anaerobic roasting.

7. The method for preparing high-pressure lithium manganese iron phosphate material from waste lithium iron phosphate according to claim 6, characterized in that, The first stage of anaerobic roasting conditions are: temperature 380-430℃, time 2-6h; the second stage of anaerobic roasting conditions are: temperature 650-800℃, time 4-10h.

8. A high-pressure lithium manganese iron phosphate material, characterized in that, It is obtained by the preparation method described in claims 1-7.

9. The high-pressure lithium manganese iron phosphate material according to claim 8, characterized in that, The high-pressure lithium manganese iron phosphate material includes lithium manganese iron phosphate crystal nuclei and a nitrogen-doped carbon layer.

10. The high-pressure lithium manganese iron phosphate material according to claim 9, characterized in that, The stoichiometric formula of the lithium manganese iron phosphate crystal nucleus is: , where x takes values ​​of 1.03-1.05, y+z+u takes values ​​of 0.97-0.99, and y takes values ​​of 0.20-0.90, z takes values ​​of 0.10-0.80, and u takes values ​​of 0.001-0.020.

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