Neodymium-doped regenerated waste lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of lithium battery positive electrode materials, in particular to a neodymium-doped regenerated waste lithium iron phosphate positive electrode material and a preparation method thereof, and the positive electrode material is LiFe < 1-x > Nd < x > PO4; by doping lithium iron phosphate with a neodymium source, the lattice spacing is widened, the lithium ion transmission rate is increased, and the surfaces of doped material particles have conical flaky particle morphology; according to the method, plasma ball milling is cooperated with traditional solid phase sintering, operation is simple, the process is short, industrial application is easy, and meanwhile the regenerated positive electrode material has smaller particle size and better contact between particles. The discharge specific capacity of the obtained doped material under the condition of 1C reaches 150mAh / g, and the problem that the electrochemical performance is not ideal after neodymium is doped at present is solved to a great extent.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery positive electrode materials, and in particular to a neodymium-doped regenerated waste lithium iron phosphate positive electrode material and a preparation method thereof. Background Art

[0002] The market for used lithium iron phosphate batteries has developed rapidly with the widespread application of new energy vehicles and energy storage systems. Due to the advantages of lithium iron phosphate batteries (LFP) in safety, cycle life and cost-effectiveness, they have become an important part of electric vehicles and stationary energy storage. As these batteries gradually reach their designed life, a large number of used lithium iron phosphate batteries begin to enter the recycling market.

[0003] At present, the recycling of waste lithium iron phosphate batteries mainly involves direct regeneration and indirect regeneration. Direct regeneration refers to regeneration and repair on the basis of retaining the original structure of the material. It mainly includes direct regeneration by solid phase method, direct regeneration by hydrothermal method, direct regeneration by electrochemical lithiation, etc. Indirect regeneration mainly refers to the release of valuable metal elements and conversion into other products with high added value through hydrometallurgical methods. Its general steps mainly include the dissolution of valuable metals (acidic or alkaline solution), extraction, chemical precipitation, electrochemical deposition, etc. The valuable metals are extracted and converted into secondary products through different combinations to achieve metal recycling and reuse. Indirect regeneration can be divided into traditional hydrometallurgical recovery and selective lithium extraction recovery according to whether the elements are dissolved in the solution. Selective lithium extraction recovery can extract and recover lithium on the basis of retaining the original olivine structure of the material. This method is conducive to converting iron and lithium into iron phosphate (orthorhombic system) and lithium carbonate with higher economic added value. However, this method is more demanding on the control of reaction conditions, and there is no large-scale application at present. At present, the lithium and iron in industrial production recycling are still recovered by full dissolution. The recovered iron still exists in the form of iron phosphate, but the crystal structure is different (monoclinic system). Although iron phosphate is currently used in agriculture, medicine, preservatives, etc., it is mainly used in the synthesis of lithium iron phosphate positive electrode materials. However, the direct synthesis of positive electrode materials from waste iron phosphate obtained by direct recycling often has problems such as poor electrochemical performance and limited application range. In order to solve these problems, researchers usually modify materials by methods such as ion doping. Traditional ion doping often changes the diffusion migration channel of lithium ions by introducing doping atoms to achieve the purpose of improving the electrochemical performance of materials by increasing the diffusion migration rate of lithium ions. However, the particle morphology of lithium iron phosphate positive electrode materials is also one of the main reasons affecting the electrochemical performance of materials. On the one hand, good particle morphology helps to improve the electronic conductivity of the material and shorten the migration path of lithium ions, thereby showing better electrochemical performance; on the other hand, it helps to improve the tap density of the material, thereby achieving a larger volume energy density in actual application scenarios.

[0004] The existing Chinese patent document CN201210380658.0 discloses a method for preparing neodymium-doped lithium iron phosphate positive electrode materials by a liquid phase method, comprising the following steps: adding deionized water into a stirring tank, adding Nd(Ac)3, FeSO4·7H2O, and H3PO4 at a molar ratio of Nd:Fe:P=0.01:0.99:1.01 into the stirring tank at a stirring frequency of 4-5 Hz, and controlling the input mass ratio of FeSO4·7H2O to the input mass ratio of deionized water to be 3:2-4; when all the materials are added into the stirring tank, increasing the stirring frequency to 25-35 Hz and stirring for more than 20 minutes; then, slowly dripping H2O2 into the stirring tank, and controlling the material temperature in the stirring tank to be between 50-55°C, until Fe 2+ Oxidation is complete; when Fe 2+ After complete oxidation, add a 20-25% NaOH solution to the stirring tank, and control the temperature of the reactants to be between 90-95°C during the whole process until the pH of the reactants reaches between 2.1-2.3; then stop stirring in the stirring tank and let it stand for more than 1 hour; finally, filter, wash and dry the reactants to obtain a neodymium-doped precursor Fe 0.99 Nd 0.01 PO4·2H2O. Then the neodymium-doped precursor is sintered at 650-680°C for 9-10 hours to remove the crystal water and form Fe 0.99 Nd 0.01 PO4. Then Fe 0.99 Nd 0.01 PO4 and Li2CO3 are added to the ultrafine grinder in a molar ratio of Li:Fe=1.03:1, and then deionized water is added. The mass of deionized water added is equal to the mass of Fe 0.99 Nd 0.01 The total mass ratio of PO4 and Li2CO3 is 1:1; then Fe 0.99 Nd 0.01 2% of the mass of PO4 is added to refined white sugar and ultra-finely ground for 7 to 9 hours; the ground slurry is then spray-dried using a spray drying device to obtain a mixed powder; the spray drying feed inlet temperature is controlled to be less than 180°C and the discharge port temperature is controlled to be less than 60°C; finally, the dried mixed powder is placed in a graphite crucible and sintered at 720 to 740°C for 9 to 10 hours under a nitrogen protective atmosphere to obtain LiFe 0.99 Nd 0.01 PO4 / C products. This method uses Fe 2+ When using liquid phase method to prepare Nd-doped precursor as raw material, it is difficult to avoid Fe 2+The oxidation step involves precise potential and pH control, which places high demands on equipment. At the same time, after the precursor is prepared, it needs to be sintered, ball milled, spray dried, and other processes, which leads to cumbersome processing steps. The processing cost of subsequent steps is increased. The product LiFe 0.99 Nd 0.01 The discharge specific capacity of PO4 / C at 0.5C is only 142 mAh / g, and its electrochemical performance is not ideal.

[0005] The present invention uses plasma ball milling in conjunction with traditional solid-phase sintering, which is not only simple to operate and has a short process, but also easy to apply industrially. The regenerated positive electrode material has a smaller particle size and better inter-particle contact. The obtained doped material has a discharge capacity of 150mAh / g under 1C conditions, which largely solves the problem of unsatisfactory electrochemical performance after neodymium doping. The present invention uses a neodymium source to dope in the process of synthesizing lithium iron phosphate from waste iron phosphate, and introduces a plasma ball milling process in the process. In Nd 3+ The synergistic effect of high-frequency pulse current during plasma milling makes the obtained LiFe 1-x Nd x PO4 material not only achieves the widening of lattice spacing at the microscopic level, but also significantly improves the problem of unsatisfactory electrochemical performance of lithium ion one-dimensional channel diffusion in LiFePO4 material itself. 3+ The large number of defects and vacancies caused by the synergistic effect also significantly affect the formation of primary particles of the material. After doping, the material particles show a conical flake particle morphology with smaller primary particles and better conductive network. The simultaneous improvement of microscopic crystal structure and macroscopic particle morphology is the key to significantly improve electrochemical performance. Summary of the invention

[0006] The purpose of the present invention is to provide a neodymium-doped recycled waste lithium iron phosphate positive electrode material and a preparation method thereof. The plasma ball milling process not only makes the neodymium doping more uniform, but also 3+ The resulting local charge is not conserved and vacancies and defects are easily formed on the surface of the material particles under the continuous impact of high-frequency current. In the subsequent high-temperature solid-phase sintering process, the high-density vacancies contribute to the appearance of more nucleation sites, resulting in smaller particle size and better contact between particles. It helps to improve the electronic conductivity of the material. At the same time, the larger ion radius of neodymium is also more conducive to the expansion of the one-dimensional channel of lithium ions inside the LiFePO4 positive electrode material, thereby increasing the diffusion and migration rate of lithium ions. In Nd 3+ Under the synergistic effect of high-frequency pulse current, a regenerated lithium iron phosphate positive electrode material with excellent electrochemical properties is obtained.

[0007] In order to achieve the above technical objectives and the above technical effects, the present invention is implemented through the following technical solutions:

[0008] A neodymium-doped recycled waste lithium iron phosphate positive electrode material, wherein the positive electrode material is LiFe 1-x Nd x PO4, where x = 0.02-0.05; compared with the value of x = 0.01 in the existing Chinese patent document CN201210380658.0, sufficient Nd 3+ It can cause the local charge of the material to not be conserved, thus causing vacancy defects in the structure. The formation of a large number of vacancies and defects can provide additional diffusion paths for atoms, accelerate atomic migration, and lead to improved particle morphology. The regenerated material exhibits a conical flaky particle morphology with smaller particle size and better contact. In addition, the formation of vacancies and dislocations under the continuous impact of high-frequency pulse current during the high-energy plasma ball milling process also contributes to a better diffusion process and molding process during the subsequent high-temperature sintering of the material. It helps to obtain a regenerated lithium iron phosphate positive electrode material with a better conductive network and higher volume energy density.

[0009] On the other hand, the present invention provides a method for preparing a neodymium-doped regenerated lithium iron phosphate positive electrode material, comprising the following steps:

[0010] S1: After deep discharge, the waste lithium-ion battery is disassembled to separate the positive electrode sheet, which is placed in a beaker for ultrasonic treatment; after the ultrasonic treatment, the lithium iron phosphate positive electrode material falls off the aluminum foil, and is dried in an oven to obtain the waste lithium iron phosphate positive electrode powder;

[0011] S2: The waste lithium iron phosphate cathode powder is placed in an argon atmosphere tube furnace for pretreatment to initially remove the residual binder, SEI film and electrolyte;

[0012] S3: The obtained waste electrode powder is placed in an aqueous solution according to a certain liquid-to-solid ratio, and H2SO4 is added dropwise to adjust the pH for dissolution.

[0013] S4: Add NH3·H2O dropwise to the solution obtained in S3 to adjust the pH and precipitate the precursor.

[0014] S5: The obtained precipitated residue is placed in an oven for drying after being aged.

[0015] S6: calcining the slag after drying S5.

[0016] S7: The precursor obtained after calcination is weighed according to the molar ratio of lithium source, iron source, neodymium source and phosphorus source of 1.03:1-x:x:1.00, and then transferred to a plasma ball mill for processing.

[0017] S8: placing the doped lithium iron phosphate positive electrode material in a tubular furnace for two-stage calcination to obtain a regenerated lithium iron phosphate positive electrode material.

[0018] Furthermore, in step S1, the ultrasonic temperature is 20-60° C. and the time is 60-180 min.

[0019] Furthermore, in step S2, the pretreatment temperature of the tubular furnace is 350-600° C., and the treatment time is 3-5 hours.

[0020] Furthermore, in step S3, the pH of the solution is controlled between -2 and 2.

[0021] Furthermore, in step S4, the pH of the precursor precipitation is controlled between 2 and 5.

[0022] Furthermore, in step S5, the aging time is controlled at 12 to 36 hours, and the drying temperature is controlled at 60 to 120°C.

[0023] Furthermore, in step S6, the calcination temperature is controlled at 400-600° C., the calcination time is controlled at 3-6 hours, the atmosphere is an oxygen atmosphere, and the gas flow rate is 100-400 sccm.

[0024] Further, the lithium source in step S7 is one or more of lithium carbonate, lithium hydroxide, and lithium acetate; the iron source is one or more of iron phosphate, iron nitrate, and iron chloride; the phosphorus source is one or more of lithium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the neodymium source is one of neodymium acetate, neodymium hydroxide, neodymium nitrate hexahydrate, neodymium trichloride, and neodymium fluoride;

[0025] Furthermore, the molar ratio of the lithium source, the iron source, the neodymium source and the phosphorus source in step S7 is 1.03:1-x:x:1.00, wherein x=0.001-0.02.

[0026] Furthermore, in step S7, the speed of the plasma ball mill is controlled at 800-1200 rpm, the ball milling time is 10-60 min, the ball-to-material ratio is 10-20:1, the discharge frequency is 10-15 kHz, and the gas atmosphere is nitrogen.

[0027] Furthermore, in step S8, argon or nitrogen is used as an inert gas in the tubular furnace for calcination.

[0028] Furthermore, in step S8, the air flow rate of the tubular furnace roasting is 30-300 sccm, the heating rate is 5-10°C / min, the first roasting temperature is 200-350°C, and the processing time is 2-4h. The second roasting temperature is 400-500°C, and the processing time is 2-4h. The third roasting temperature is 600-800°C, and the processing time is 6-8h.

[0029] Beneficial effects of the present invention:

[0030] The high-frequency plasma ball milling process not only provides the possibility for uniform doping of neodymium elements, but also the high-frequency pulse current is easy to generate a large number of free electrons and ions to impact the surface of the material under the action of nitrogen medium, forming local vacancies and defects. Vacancies can be used as part of the diffusion path to promote the migration of atoms or ions inside the material. This may cause the growth of particles in a specific direction to accelerate, thereby affecting the overall morphology. At the same time, the presence of vacancies may increase the surface energy of the material. This makes the material more inclined to form a specific morphology to minimize the total surface energy, so that the material particles develop in a smaller direction. In addition, during the high-temperature sintering process of the material, vacancies may aggregate to form larger defects, such as dislocation lines or stacking faults. These aggregated defects can also affect the microstructure of the material, thereby affecting the morphology of the particles. At the same time, the larger ionic radius of the neodymium element helps to broaden the diffusion and migration channel of lithium ions after being doped into the lithium iron phosphate lattice. The XRD spectrum results of Example 1-2 show that the recycled materials obtained by the three-stage roasting method proposed by us and the existing traditional two-stage roasting method do not introduce new impurities and do not destroy the original olivine structure. The regenerated positive electrode material exhibits characteristics that are closer to the standard diffraction peak of olivine-type lithium iron phosphate crystals. In addition, in the small-angle magnification image of XRD, it can be found that the peak of the 211 crystal plane of the doped material shifts to a low angle, indicating that the expansion of the lattice spacing after doping is more conducive to the exposure of the 211 plane, thereby improving the lithium ion migration rate.

[0031] Figure 3 SEM image analysis shows that the particles of the neodymium-doped regenerated lithium iron phosphate cathode material obtained by the three-stage sintering method are more evenly distributed, and the agglomeration phenomenon is significantly reduced. Compared with the particle morphology of the recycled material obtained by the traditional two-stage sintering method, the particle morphology has changed significantly, from the original ellipsoidal spherical shape to the latter's conical flaky particle morphology, and the average particle size of the particles has also been significantly reduced. Uniform and fine particles are conducive to shortening the transmission path of lithium ions and reducing the resistance during lithium ion migration. At the same time, the transformation from ellipsoidal spherical particles to flaky particle morphology helps to improve the entire conductive network and show a better electron diffusion rate. On the other hand, neodymium doping can effectively stabilize the crystal structure, significantly enhance the charge and discharge capacity and cycle stability of the material, and make the cathode material show good long-term cycle stability. Figure 4 and Fig. 9 The cycling performance of the recycled material at 1C current density is demonstrated. Compared with the recycled material obtained by the two-stage sintering method, the first-cycle discharge specific capacity of the recycled material obtained by the three-stage sintering method is significantly improved (Example 1). The cycling performance test in Example 2 also shows the same obvious capacity improvement. Figure 5) further proved that the recycled materials obtained by the three-stage sintering method have better electrochemical performance at different rates. The use of neodymium source to achieve iron doping can widen the lattice spacing of lithium iron phosphate, accelerate the lithium ion transmission rate, and reduce the polarization phenomenon of lithium iron phosphate positive electrode materials.

[0032] The present invention uses plasma ball milling to cooperate with neodymium source doping lithium iron phosphate positive electrode material, and the widened lattice spacing significantly improves the problem of low ion diffusion rate of one-dimensional conductive path in the lithium iron phosphate crystal structure. 3+ The local defects and vacancies caused by the high-frequency pulse process of plasma ball milling also significantly improved the particle morphology of the material during the subsequent sintering process, making the material have a more complete conductive network, thus solving the problems of slow ion transmission rate and low electronic conductivity of lithium iron phosphate materials. 1-x Nd x PO4 material has better charge and discharge capability and cycle stability.

[0033] Of course, any product implementing the present invention does not necessarily need to achieve all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0035] Figure 1 This is the XRD spectrum of the precursor iron phosphate prepared by leaching, precipitation and sintering of the waste lithium iron phosphate positive electrode material in Example 1.

[0036] Figure 2 The recycled material LiFe obtained by the conventional two-stage sintering in Example 1 0.95 Nd 0.05 Recycled LiFe obtained by PO4-two-stage and three-stage sintering methods 0.95 Nd 0.05 Comparison chart of XRD standard cards of PO4-three segments.

[0037] Figure 3 a, 3b are respectively the recycled material LiFe obtained by the conventional two-stage sintering method in Example 1 0.95 Nd 0.05 Recycled material LiFe obtained by PO4-second and third stage sintering 0.95 Nd 0.05 SEM spectrum of PO4-three segments.

[0038] Figure 4 The recycled material LiFe obtained by the conventional two-stage sintering in Example 1 0.95 Nd 0.05 Recycled LiFe obtained by PO4-two-stage and three-stage sintering methods 0.95 Nd 0.05 PO4-half-cell cycle performance diagram for three stages.

[0039] Figure 5 The recycled material LiFe obtained by the conventional two-stage sintering in Example 1 0.95 Nd 0.05 Recycled LiFe obtained by PO4-two-stage and three-stage sintering methods 0.95 Nd 0.05 PO4- three-stage rate performance diagram;

[0040] Figure 6 This is the XRD spectrum of the precursor iron phosphate prepared by leaching, precipitation and sintering of waste lithium iron phosphate positive electrode material in Example 2.

[0041] Figure 7 The recycled material LiFe obtained by the conventional two-stage sintering in Example 2 0.98 Nd 0.02 Recycled LiFe obtained by PO4-two-stage and three-stage sintering methods 0.98 Nd 0.02 Comparison chart of XRD standard cards of PO4-three segments.

[0042] Figure 8 a, 8b are the recycled materials LiFe obtained by the conventional two-stage sintering method in Example 2 0.98 Nd 0.02 Recycled material LiFe obtained by PO4-second and third stage sintering 0.98 Nd 0.02 SEM images of the three segments of PO4-.

[0043] Fig. 9 The recycled material LiFe obtained by the conventional two-stage sintering in 2 is 0.98 Nd 0.02 Recycled LiFe obtained by PO4-two-stage and three-stage sintering methods 0.98 Nd 0.02 PO4-half-cell cycle performance diagram for three stages.

[0044] Fig.10 The recycled material LiFe obtained by the conventional two-stage sintering in Example 2 0.98 Nd 0.02 Recycled LiFe obtained by PO4-two-stage and three-stage sintering methods 0.98 Nd0.02 PO4- three-stage rate performance diagram; DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] Example 1

[0047] The method for preparing the neodymium-doped regenerated lithium iron phosphate positive electrode material described in this embodiment comprises the following steps:

[0048] S1: After deep discharge, the waste lithium-ion battery is disassembled and the positive electrode sheet is separated. The obtained positive electrode sheet is cut into 5×5 cm size and placed in a beaker for ultrasonic treatment; the ultrasonic temperature is 30°C, the ultrasonic time is 180 min, and the ultrasonic frequency is 100 khz. After the ultrasonic treatment, the lithium iron phosphate positive electrode material falls off the aluminum foil, and the waste lithium iron phosphate positive electrode powder is obtained after drying in an oven;

[0049] S2: 10 g of waste lithium iron phosphate cathode powder was placed in an argon atmosphere tube furnace at 550 ° C for 3 h for pretreatment to preliminarily remove residual binder, SEI film and electrolyte;

[0050] S3: The positive electrode powder obtained after pretreatment is placed in a deionized water solution at a liquid-to-solid ratio of 5:1, and concentrated H2SO4 is added dropwise to adjust the pH to -1 for dissolution.

[0051] S4: Add concentrated NH3·H2O dropwise to the solution obtained in S3 to adjust the pH to 3 to precipitate the precursor.

[0052] S5: The obtained precursor precipitate is aged for 24 hours and then placed in an oven at 100° C. for drying.

[0053] S6: The precursor after drying S5 is placed in a tube furnace for calcination in an oxygen atmosphere. The calcination temperature is 600° C., the calcination time is 3 h, and the ventilation volume is 300 sccm.

[0054] S7: The precursor obtained after calcination was weighed according to the molar ratio of lithium hydroxide, iron phosphate slag, neodymium acetate monohydrate, and glucose of 1.03:0.95:0.05:0.15, and then transferred to a plasma ball mill with a ball-to-material ratio of 10:1 under a nitrogen atmosphere, a discharge frequency of 15 kHz, and a rotation speed of 1000 rpm for 30 min;

[0055] S8: The treated material is placed in a tube furnace for three-stage sintering. The first stage sintering temperature is 300°C and the sintering time is 3h; the second stage sintering temperature is 480°C and the sintering time is 4h; the third stage sintering temperature is 650°C and the sintering time is 8h. The heating rate is 5°C / min. Then, a neodymium-doped lithium iron phosphate positive electrode material is obtained.

[0056] Figure 1 It is FePO4 obtained after leaching, precipitation and sintering of waste lithium iron phosphate positive electrode materials. As shown in the figure, its basic crystal structure corresponds well to the standard card PDF#050-1635 of FePO4, which shows that the impurities in the preparation process of the precursor are well removed and no obvious impurity phase appears.

[0057] Figure 2 The XRD spectrum of the regenerated lithium iron phosphate positive electrode material and the regenerated lithium iron phosphate positive electrode material obtained by traditional two-stage calcination under the same conditions. It can be seen from the figure that the regenerated samples obtained by different sintering methods all show the crystal structure of lithium iron phosphate with standard olivine structure, without the appearance of obvious miscellaneous peaks. They all correspond well to the lithium iron phosphate standard card PDF#83-2092. It shows that the two different sintering methods will not significantly change the basic crystal structure of the material under certain conditions, nor will they lead to the formation of impurity phases.

[0058] Figure 3 The SEM images of the regenerated lithium iron phosphate positive electrode material and the regenerated lithium iron phosphate positive electrode material obtained by conventional two-stage calcination under the same conditions. Figure 3 a is the recycled material obtained by the traditional two-stage sintering method. It can be seen from the figure that the particle size of the recycled material is about 0.5um, and the particle size difference between the particles is large. The particles are not uniform and agglomeration occurs. Uneven particle size and poor contact will significantly affect the diffusion process of lithium ions and thus affect the electrochemical performance. Figure 3 b is the recycled material obtained by three-stage sintering. The insulation process of about 300℃ in the three-stage sintering is conducive to the initial conversion of the introduced neodymium source such as neodymium acetate, involving the conversion of anionic groups such as acetate. The initial conversion process can achieve Nd 3+ On the other hand, it can also reduce and avoid the occurrence of side reactions in subsequent reactions. 3+ The local vacancies and defects caused by the high-frequency pulse process make the material more inclined to form a minimum total surface energy. A large number of vacancies and defects and a good diffusion conversion process achieve the refinement and homogenization of material particles. Figure 3In b, it can be seen that the particles of the recycled material obtained after three-stage calcination are clearer, the agglomeration phenomenon is significantly reduced, and the distribution is more uniform. At the same time, the basic morphology of the particles is transformed from the original irregular spherical shape to conical flake particles with better contact between particles. The particle size is about 0.15-0.2um. A smaller primary particle size is more conducive to the diffusion of lithium and the improvement of electronic conductivity to achieve better electrochemical performance.

[0059] Figure 4 The cycle performance diagram of the half-cell assembled with the regenerated lithium iron phosphate positive electrode material and the regenerated lithium iron phosphate positive electrode material obtained by the traditional two-stage sintering under the same conditions at a current density of 1C. As can be seen from the figure, the first cycle discharge capacity of the regenerated material obtained by the traditional two-stage sintering is only 128.2mAh g -1 , while the material regenerated by three-stage sintering has 153.6 mAh g -1 The improvement of discharge capacity is mainly attributed to the good Nd 3 + The improvement of particle morphology caused by the diffusion and conversion process achieves higher lithium ion diffusion migration rate and electronic conductivity.

[0060] Figure 5 The cycle performance diagram of the regenerated lithium iron phosphate cathode material and the regenerated lithium iron phosphate cathode material obtained by traditional two-stage sintering under the same conditions at different rates. The regenerated material obtained by traditional two-stage sintering showed poor rate performance and decayed rapidly in the following cycles. When the current density was 5C, the discharge capacity of the sample was only 42.2mAh g -1 The doped regenerated material showed even better electrochemical performance, with 156.7, 161.3, 162.8, 156.2, 143.1 and 70.7 mAh g at 0.1C, 0.2C, 0.5C, 1C, 2C and 5C, respectively. -1 The discharge capacity.

[0061] Example 2

[0062] The method for preparing the neodymium-doped regenerated lithium iron phosphate positive electrode material described in this embodiment comprises the following steps:

[0063] S1: After deep discharge, the waste lithium-ion battery is disassembled to separate the positive electrode sheet, and the obtained positive electrode sheet is cut into 10×10 cm size and placed in a beaker for ultrasonic treatment; the ultrasonic temperature is 60°C, the ultrasonic time is 90 min, and the ultrasonic frequency is 50 khz. After the ultrasonic treatment, the lithium iron phosphate positive electrode material falls off the aluminum foil, and the waste lithium iron phosphate positive electrode powder is obtained after drying in an oven;

[0064] S2: 10 g of waste lithium iron phosphate cathode powder was placed in an argon atmosphere tube furnace at 600 ° C for 2 h for pretreatment to preliminarily remove residual binder, SEI film and electrolyte;

[0065] S3: The positive electrode powder obtained after pretreatment is placed in a deionized water solution at a liquid-to-solid ratio of 10:1, and concentrated H2SO4 is added dropwise to adjust the pH to 0 for dissolution.

[0066] S4: Add concentrated NH3·H2O dropwise to the solution obtained in S3 to adjust the pH to 2 for the precipitation of the precursor.

[0067] S5: The obtained precursor precipitate was aged for 36 hours and then placed in an oven to dry at 80°C.

[0068] S6: The precursor after drying S5 is placed in a tube furnace for calcination in an oxygen atmosphere. The calcination temperature is 500° C., the calcination time is 5 h, and the ventilation volume is 400 sccm.

[0069] S7: The precursor obtained after calcination was weighed according to the molar ratio of lithium nitrate, iron phosphate, neodymium oxide and glucose of 1.03:0.98:0.01:0.15, and then transferred to a plasma ball mill with a ball-to-material ratio of 20:1 under a nitrogen atmosphere at a discharge frequency of 12 kHz and a rotation speed of 1200 rpm for 20 min;

[0070] S8: The plasma-treated material is placed in a tube furnace for two-stage sintering treatment. The first stage sintering temperature is 350°C and the sintering time is 2h; the second stage sintering temperature is 400°C and the sintering time is 3h; the third stage sintering temperature is 700°C and the sintering time is 7h to obtain the neodymium-doped lithium iron phosphate positive electrode material.

[0071] Figure 6 It is FePO4 obtained after leaching, precipitation and sintering of waste lithium iron phosphate positive electrode materials. As shown in the figure, its basic crystal structure corresponds well to the standard card PDF#050-1635 of FePO4, which shows that the impurities in the preparation process of the precursor are well removed and no obvious impurity phase appears.

[0072] Figure 7 The XRD patterns of the regenerated lithium iron phosphate cathode material and the regenerated lithium iron phosphate cathode material obtained by the traditional two-stage calcination under the same conditions. It can be seen from the figure that the regenerated materials obtained by the two different sintering methods both show the crystalline structure of lithium iron phosphate with olivine structure, and no obvious miscellaneous peaks appear. They all correspond well to the lithium iron phosphate standard card PDF#83-2092. It shows that the two sintering methods will not affect the structure of the material, nor will they lead to the formation of impurity phases.

[0073] Figure 8The SEM images of the regenerated lithium iron phosphate positive electrode material and the regenerated lithium iron phosphate positive electrode material obtained by conventional two-stage calcination under the same conditions. Figure 8 a is the recycled material obtained by the traditional two-stage sintering method. It can be seen from the figure that the recycled material obtained under this condition has a large difference in particle size. The particles are not uniform and agglomeration occurs. Uneven particle size and poor contact will significantly affect the diffusion process of lithium ions and thus affect the electrochemical performance. Figure 8 b is the recycled material obtained by three-stage sintering. The insulation process of about 400℃ in the three-stage sintering is beneficial to the conversion process of the anion groups in the introduced neodymium oxide and the conversion of Nd 3+ A good conversion process can better reduce and avoid the occurrence of intermediate reactions and side reactions in the subsequent reaction process. 3+ On the one hand, good diffusion in the formed vacancies and defects helps to minimize the total surface energy of the material, thereby promoting the refinement and homogenization of the material particles. 3+ Good diffusion in existing vacancies and defects is the basis for good doping. Figure 8 In b, we can see that the material particles are clearer, the agglomeration phenomenon is significantly reduced, and the distribution is more uniform. At the same time, the basic morphology of the particles has changed from the original irregular spherical shape to conical flake particles with better contact between particles. Smaller primary particle size is more conducive to the diffusion of lithium and the improvement of electronic conductivity to achieve better electrochemical performance.

[0074] Fig. 9 The cycle performance diagram of the half-cell assembled with the precursor regenerated lithium iron phosphate positive electrode material and the neodymium doped regenerated lithium iron phosphate positive electrode material under the same conditions at a current density of 1C. As can be seen from the figure, the first cycle discharge capacity of the undoped material is only 126.1 mAh g -1 , while the doped material has 151.6 mAh g -1 The improvement in discharge capacity is mainly attributed to the increase in microscopic lattice spacing and the improvement in macroscopic particle morphology.

[0075] Fig.10 The cycle performance of the precursor regenerated lithium iron phosphate cathode material and the neodymium doped regenerated lithium iron phosphate cathode material under the same conditions at different rates. The undoped material showed poor rate performance and decayed rapidly in the following cycles. When the current density was 5C, the discharge capacity of the sample was only 43.1 mAh g -1 The doped regenerated material showed even better electrochemical performance, with 160.1, 162.0, 160.3, 154.1, 137.3 and 61.3 mAh g at 0.1C, 0.2C, 0.5C, 1C, 2C and 5C, respectively.-1 The discharge capacity.

[0076] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A neodymium-doped recycled waste lithium iron phosphate positive electrode material, characterized in that: The positive electrode material is LiFe 1- x Nd x PO4, wherein x=0.02-0.05; the lattice spacing of lithium iron phosphate is widened by doping the neodymium source, the lithium ion transmission rate is accelerated, and the surface of the doped material particles has a conical flaky particle morphology.

2. The method for preparing the positive electrode material according to claim 1, characterized in that: The following steps are involved: S1: After deep discharge, the waste lithium-ion battery is disassembled to separate the positive electrode sheet, which is placed in a beaker for ultrasonic treatment; after the ultrasonic treatment, the lithium iron phosphate positive electrode material falls off the aluminum foil, and is dried in an oven to obtain the waste lithium iron phosphate positive electrode powder; S2: The waste lithium iron phosphate cathode powder is placed in an argon atmosphere tube furnace for pretreatment to initially remove the residual binder, SEI film and electrolyte; S3: placing the obtained waste electrode powder in an aqueous solution according to a certain liquid-to-solid ratio, and adding H2SO4 to adjust the pH to dissolve; S4: adding NH3·H2O dropwise to the solution obtained in S3 to adjust the pH to precipitate the precursor; S5: placing the obtained precipitated residue in an oven for drying after aging; S6: calcining the slag after drying S5; S7: weighing the precursor obtained after calcination according to the molar ratio of lithium source, iron source, neodymium source and phosphorus source of 1.03:1-x:x:1.00, and then transferring it to a plasma ball mill for processing; S8: placing the doped lithium iron phosphate positive electrode material in a tubular furnace for three-stage sintering to obtain a regenerated lithium iron phosphate positive electrode material.

3. The preparation method according to claim 2, characterized in that: In step S1, the ultrasonic temperature is 20-60° C. and the time is 60-180 min.

4. The preparation method according to claim 2, characterized in that: In step S2, the pretreatment temperature of the tubular furnace is 350-600° C., and the treatment time is 3-5 hours.

5. The preparation method according to claim 2, characterized in that: In step S3, the pH of the solution is controlled between -2 and 2.

6. The preparation method according to claim 2, characterized in that: In step S4, the pH of the precursor precipitation is controlled between 2 and 5.

7. The preparation method according to claim 2, characterized in that: In step S5, the aging time is controlled at 12 to 36 hours, and the drying temperature is controlled at 60 to 120°C.

8. The preparation method according to claim 2, characterized in that: In the step S6, the calcination temperature is controlled at 400-600° C., the calcination time is controlled at 3-6 hours, the atmosphere is an oxygen atmosphere, and the gas flow rate is 100-400 sccm.

9. The preparation method according to claim 2, characterized in that: The lithium source in step S7 is one or more of lithium carbonate, lithium hydroxide, and lithium acetate; the iron source is one or more of iron phosphate, iron nitrate, and iron chloride; the phosphorus source is one or more of lithium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the neodymium source is one of neodymium acetate, neodymium hydroxide, neodymium nitrate hexahydrate, neodymium trichloride, and neodymium fluoride; The molar ratio of the lithium source, the iron source, the neodymium source, and the phosphorus source in step S7 is 1.03:1-x:x:1.00, wherein x=0.02-0.05; In step S7, the speed of the plasma ball mill is controlled at 800-1200 rpm, the ball milling time is 10-60 min, the ball-to-material ratio is 10-20:1, the discharge frequency is 10-15 kHz; and the gas atmosphere is nitrogen.

10. The preparation method according to claim 2, characterized in that: The step S8 tube furnace roasting uses argon or nitrogen as an inert gas; the step S8 tube furnace roasting gas flow rate is 30-300sccm, the heating rate is 5-10°C / min, the first stage roasting temperature is 200-350°C, and the processing time is 2-4h. The second stage roasting temperature is 400-500°C, and the processing time is 2-4h. The third stage roasting temperature is 600-800°C, and the processing time is 6-8h.

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