Method for cyclically repairing lithium iron phosphate through lithium iodide

Abstract

The invention relates to a method for cyclically repairing lithium iron phosphate by lithium iodide, which comprises the following steps: mixing waste lithium iron phosphate with a lithium source to obtain a mixture; heating the mixture in an inert atmosphere to obtain a reduction material; cleaning the reduction material with a cleaning solvent to obtain regenerated lithium iron phosphate powder and eluate, collecting the eluate, and recrystallizing to obtain regenerated lithium iodide; the lithium source is lithium iodide and lithium hydroxide in a molar ratio of 1: (1-5), and the cleaning solvent comprises at least one of water, ethanol and acetone. The lithium iodide and the lithium hydroxide are fully utilized to form a co-melting system, the reaction temperature is reduced, meanwhile, the lithium iodide serves as a reducing agent, the lithium hydroxide provides lithium ions, lithium iron phosphate is repaired together, and cyclic utilization of the lithium iodide is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of recycling and cyclic utilization of waste lithium-ion batteries, and in particular to a method for cyclically repairing lithium iron phosphate using lithium iodide. Background Art

[0002] In the field of new energy power equipment, the main energy storage device is the battery. Commercial lithium-ion batteries can be mainly divided into two categories: one is the battery with lithium iron phosphate (LFP) as the positive electrode material; the other is the battery with ternary materials (NMC, etc.). These two types of batteries occupy the majority of the market share. Among them, LFP batteries are widely used in the market for their longer cycle performance and life (average 6-9 years), higher safety performance and lower manufacturing cost, and have occupied 2 / 3 of the domestic lithium battery industry market.

[0003] At present, the "lifespan" of lithium iron phosphate batteries that were put on the market early is coming to an end, and a large number of waste lithium iron phosphate batteries are facing disposal, which is expected to reach 3.8 million tons by 2030. There are two main technical routes for the recycling of waste lithium iron phosphate batteries: the first is to destroy the waste lithium iron phosphate batteries, extract the elements in them through a series of means, and achieve recycling; the second is a repair recovery method, which restores its original performance by replenishing the lost metal elements and repairing the damaged crystal structure. The former is mainly a combination of dry and wet methods to classify and recycle the elements, but the energy consumption and chemical reagents of this method will still bring about greater secondary pollution and greenhouse gases. Therefore, compared with the field of waste battery recycling, considering energy consumption and pollution factors, repair recovery is a better choice than destruction recovery, especially for lithium iron phosphate (LFP) cathode materials, which does not contain precious metal components and has a very small proportion of valuable metals. If the wet method is used to recover lithium and iron, the economic benefits generated are quite limited. From the perspective of maximizing economic benefits, the solution of regenerating lithium iron phosphate (LFP) is more suitable. By adopting this method, on the one hand, we can achieve more substantial recycling benefits and improve the economic benefits of the entire recycling process; on the other hand, it can also promote more full and efficient use of resources, reduce resource waste, and conform to the concept of sustainable development.

[0004] There are several main reasons for the failure of lithium iron phosphate positive electrodes. The most important one is that the active lithium in the positive electrode material may be lost due to some side reactions. For example, under high temperature conditions, the positive electrode material may react with the electrolyte to generate some substances that are not electrochemically active, thereby reducing the amount of active substances that can participate in the electrochemical reaction. Lithium iron phosphate forms a lithium-poor phase due to the lack of active lithium, resulting in battery capacity decay. Secondly, there is a phase transition between lithium iron phosphate and iron phosphate during the charge and discharge process. There is a certain volume difference between the two phases, which leads to internal stress in the positive electrode material, which will cause lattice distortion of the positive electrode material after multiple cycles. Lithium iron phosphate belongs to the orthorhombic crystal system with a space group of Pnmb. The crystal is composed of PO tetrahedrons and FeO octahedrons, and lithium atoms and iron atoms are located at different positions of the octahedron. In this crystal structure, the positions of Li and Fe have certain geometric relationships and coordination environments. Since the FeO octahedra form a chain structure by sharing oxygen atoms, the iron atoms in each octahedron cannot be tightly connected, which makes the crystal structure open and variable, providing a structural basis for the interchange of the positions of Li and Fe atoms. Under certain conditions, Fe atoms may enter the position of Li, or Li atoms may enter the position of Fe. Since lithium iron phosphate is an orthorhombic crystal system, active lithium ions can only be transported on the crystal plane

[010] . If Fe occupies the active site of Li, it will block the transportation of all active lithium ions in the one-dimensional channel, resulting in battery capacity decay. During repeated charge and discharge, the concentration of Fe / Li anti-site defects will continue to rise, and the battery capacity will decay rapidly. In addition, due to the long-term lithium deficiency state, and the overcharge and high voltage environment during the long-term cycle process, it may undergo redox reactions with organic matter in the electrolyte, resulting in the oxidation of divalent iron in lithium iron phosphate to trivalent iron. Trivalent iron not only has a stronger repulsion force on lithium ions, making it difficult to replenish active lithium during the regeneration process, but also has a higher activation energy for resetting the anti-site defects. Therefore, the presence of trivalent iron will greatly increase the difficulty of regeneration. Finally, due to the cyclic charge and discharge, lithium ions are constantly removed and embedded in the positive electrode material, which may also damage the lithium ion transport channel and cause lattice distortion.

[0005] In response to the above failure problems, the existing technology mainly adds a certain amount of lithium source to make up for the missing part of active lithium, successfully embeds the lithium in the lithium-rich phase of the lithium source into the lithium-poor phase in the lithium iron phosphate, and restores its electrochemical activity. In addition, a reducing agent is added to reduce the oxidized trivalent iron and create a reducing reaction environment, which can not only reduce the energy barrier for lithium migration to the lithium-poor phase but also reduce the activation energy of the anti-site defect reset. At this stage, the main means are to achieve repair through liquid phase, solid phase, electrochemical and other reaction environments. Currently, the commonly used lithium sources are lithium sulfate, lithium nitrate, lithium hydroxide, lithium acetate and multifunctional composite lithium salts. These lithium sources can supplement the missing active lithium to a certain extent, and the current common reducing agents are citric acid, ascorbic acid, malic acid, etc., which are relatively expensive and occupy a large repair cost. Therefore, discovering low-cost reducing agents and developing new repair methods have become one of the main topics of research at this stage. Summary of the invention

[0006] Based on this, the purpose of the present invention is to provide a method for cyclically repairing lithium iron phosphate with lithium iodide, using lithium hydroxide as a lithium source and lithium iodide as a reducing agent. The reducing agent can be recycled, which greatly reduces the production cost. At the same time, it can also reduce the energy consumption of the reaction process and increase the tap density of the repaired lithium iron phosphate (reduce the carbon content), thereby improving the electrochemical performance of the regenerated lithium iron phosphate.

[0007] The method for recycling lithium iron phosphate with lithium iodide of the present invention comprises the following steps:

[0008] Mixing waste lithium iron phosphate with a lithium source to obtain a mixture, wherein the lithium source is lithium iodide and lithium hydroxide in a molar ratio of 1:1 to 5;

[0009] heating the mixed material in an inert atmosphere to obtain a reducing material;

[0010] The reducing material is washed with a washing solvent to obtain regenerated lithium iron phosphate powder and an eluate, and the eluate is collected and recrystallized to obtain regenerated lithium iodide. The washing solvent includes at least one of water, ethanol and acetone.

[0011] The mixed lithium salt of lithium iodide and lithium hydroxide used in the present invention can form a low-temperature eutectic salt, and the mixed lithium salt is obtained as the lithium source according to the molar ratio corresponding to the ratio of the eutectic point. Due to the interaction between lithium iodide and lithium hydroxide, the low-temperature eutectic salt breaks the original ion arrangement mode, and the lattice energy of the new ion arrangement mode is much lower than the lattice energy of lithium iodide and lithium hydroxide respectively, so that the melting point of the eutectic system is lower than the melting point of a single lithium salt. When the molar ratio of lithium iodide to lithium hydroxide is 1:1, it is only about 186°C (the melting point of lithium iodide is 443°C, and the melting point of lithium hydroxide is 471°C), which is impossible to achieve with the solid phase method and the melting of a single lithium salt. Usually, the solid phase method needs to reach 700-800°C, and the melting of a single lithium salt also needs 500-600°C, so the use of eutectic salt can greatly reduce energy consumption and improve economic benefits.

[0012] In addition, iodine anions themselves have reducing properties, and their ion half-valence is relatively large (relatively weaker binding capacity for outer electrons), so iodine anions also have strong electron-donating capacity (stronger reducing property). This is something that lithium hydroxide, lithium sulfate, lithium nitrate, and lithium acetate cannot do when used alone as lithium sources. Some multifunctional composite lithium salts with reducing properties also need to be pyrolyzed at 600 to 800 ° C to generate reducing atmospheres. In the present invention, the iodine anion is used to reduce the oxidized trivalent iron in waste lithium iron phosphate batteries to divalent iron, thereby reducing the free energy of lithium ions migrating from the lithium-rich lithium phase of the lithium source to the active sites in the lithium iron phosphate. At the same time, iodine anions can also be used as electron-donating groups, with electron-donating capacity. When the electron-donating groups are connected to the atomic groups around lithium or iron atoms, the electron cloud density at these positions will be increased, making the charge distribution at the lithium-iron anti-site defect more uniform, reducing the energy barrier caused by the charge imbalance caused by the anti-site, thereby facilitating the anti-site atoms to return to the correct lattice position, reducing the activation energy of reset.

[0013] Furthermore, the present invention makes full use of the reaction between iodine anions, hydroxide ions and carbon to achieve the recycling of iodine. The existing process of regenerating lithium iron phosphate often cannot achieve the recycling of reactants. The reactants in the liquid phase method have no recycling value after the reaction is completed, and the reactants in the solid phase method are often pyrolyzed due to high temperature, and both cannot be recycled. The present invention utilizes the composite reaction of iodine anions, hydroxide ions and carbon to successfully recycle the precious element iodine. It can also reduce the carbon content in waste lithium iron phosphate and increase the tap density. The reaction process of iodine is as follows: First, the eutectic temperature of lithium iodide and lithium hydroxide is relatively low, and carbon cannot reduce trivalent iron to divalent iron at this temperature, so iodine anions with a reducing power greater than carbon are introduced to react with the oxidized trivalent iron in the waste lithium iron phosphate to oxidize into elemental iodine; when the hydroxide content is low, the iodine reacts with hydroxide to form hypoiodite ions with strong oxidizing power. Since the reducing power of carbon is stronger than that of divalent iron, the hypoiodite ions will undergo a carbon thermal reduction reaction with the carbon in the waste lithium iron phosphate and be reduced to iodine anions without oxidizing divalent iron; if the hydroxide content is high, the iodine reacts quickly with hydroxide to generate iodate with stronger oxidizing power, and then undergoes a carbon thermal reaction with carbon at high temperature, and will also be reduced to iodine anions; if the hydroxide content is too high, the lithium ion content and alkali concentration in the molten system are too high, which leads to excessive occupation of the active sites of iron by lithium, causing iron to precipitate, forming lithium phosphate, and destroying the crystal structure. The whole process reflects the catalytic properties of iodine anions, in which the main reactants consumed are hydroxide ions and carbon, and no iodine, a precious element, is consumed. After the heating reaction, an appropriate solvent is selected to clean and filter the repaired lithium iron phosphate, and the solvent obtained by cleaning is dried in a vacuum oven for recrystallization. The ion utilization rate in the process exceeds 99%, reducing production costs. The reaction equation involved is as follows:

[0014] I - +Fe 3+ = = I2 + Fe 2+

[0015] (low hydroxide content) I2+2OH - ==IO - +I - +H2O

[0016] 2IO - +C==2I - +CO2

[0017] (High in hydroxide content) 3I2+6OH - ==5I - +IO3 - +3H2O

[0018] 2IO3 - +3C==2I -+3CO2

[0019] As a preferred embodiment, the heating of the mixture also includes the following steps: placing the mixture in an inert atmosphere, heating it to 186-200°C at a rate of 1-5°C / min, keeping it warm for 0.5-4h, then heating it to 400-700°C at a rate of 1-5°C / min, keeping it warm for 2-4h, to obtain the reducing material.

[0020] The present invention adopts two-stage heating. More preferably, the first stage heating temperature gradually rises from 186°C to 200°C with the number of cycles of lithium iodide, and is kept warm for 2 hours. The main purpose is to fully melt the mixed lithium salt and form a uniform melting system. The lowest eutectic point of lithium iodide and lithium hydroxide is about 186°C. With the increase of the number of cycles, lithium iodide will have a small loss, and the eutectic point will gradually increase. In the embodiment of the present invention, in order to facilitate the experimental process and avoid frequent temperature changes, 200°C is uniformly used; the second stage heating temperature is 550°C, and the temperature is kept warm for 4 hours. The purpose is that high temperature will promote the reaction of lithium ions in the lithium source with lithium iron phosphate crystals, so that lithium ions enter the lattice, further supplement lithium vacancies, and thus repair the defects caused by lithium deficiency. During the two-stage heating process, high temperature can make more lithium ions cross the energy barrier and embed into other crystal structures of lithium iron phosphate. In addition, under high temperature conditions, the atoms in the lithium iron phosphate crystal obtain enough energy, and the thermal motion intensifies. The atoms originally in the defective position, such as the lithium atoms and iron atoms in the lithium-iron antisite defect, have their vibration amplitude increased, which can overcome certain energy barriers and "jump" from the wrong lattice position to the correct position, thereby repairing the defect. Furthermore, under high temperature conditions, the crystal lattice will expand and relax to a certain extent, so that the internal stress is released, and the thermal motion and rearrangement of atoms also help to reconstruct the lattice, making the crystal structure more regular, reducing defects caused by lattice distortion, restoring the integrity and periodicity of the crystal, reducing the energy of the system, and improving the stability of the crystal. At the same time, because iodine is easy to volatilize at high temperature, the high temperature environment can also make iodine react quickly with hydroxide, avoiding the loss of iodine in a high temperature environment.

[0021] Furthermore, since the lithium iron phosphate material has a certain crystal structure, there are channels for lithium ions to diffuse inside it. In a high-temperature molten salt environment, these channels will expand and the lithium ions will also have high activity, which allows the lithium ions to diffuse along the channels into the lithium iron phosphate particles, so that the lithium ions are embedded in the lithium iron phosphate positive electrode structure. In addition to the lithium ions at the active sites, some lithium ions will be embedded in the PO tetrahedron and FeO octahedron structures and the surface of the regenerated lithium iron phosphate. These lithiums can serve as raw materials for the formation of SEI film on the surface of the negative electrode material during the battery formation stage, avoiding the consumption of active lithium ions and causing the battery capacity to decay. Therefore, repairing lithium iron phosphate and pre-lithiation at the same time can effectively save production costs, and in the subsequent preparation of electrode materials, the negative electrode pretreatment process can be eliminated. At present, although the liquid phase repair method can provide sufficient lithium salt concentration, it is difficult to pre-lithiate due to its insufficient reaction temperature; although the solid phase repair method can provide a higher temperature, the lithium salt concentration that can be provided is limited, and it is also difficult to achieve pre-lithiation. It can be seen that pre-lithiation has very high requirements for temperature, holding time and lithium salt concentration. When the hydroxide concentration is too high, the reaction temperature is too high and the reaction time is too long, iron will be precipitated, causing the destruction of the crystal structure. Therefore, the present invention avoids the occurrence of side reactions by controlling the concentration of hydroxide neutralized by lithium iodide, the reaction temperature and the reaction time.

[0022] As a preferred solution, the waste lithium iron phosphate is first tested for the elemental content of iron, lithium and phosphorus before mixing, and the lithium deficiency of the waste lithium iron phosphate is calculated according to the atomic number ratio of lithium iron phosphate; in terms of lithium, the molar ratio of lithium iodide, lithium hydroxide and lithium deficiency is 1:1 to 5:1. More preferably, by detecting the elemental content of iron, lithium and phosphorus in waste lithium iron phosphate by ICP and calculating the lithium deficiency, the proportion in the mixture can be optimized to ensure maximum utilization of raw materials while reducing production costs.

[0023] As a preferred solution, the regenerated lithium iodide is recycled to partially or completely replace the lithium iodide. The lithium iodide generated by recrystallization is used for a cyclic reaction, thereby reducing production costs and waste generated by the reaction.

[0024] As a preferred solution, the molar ratio of lithium iodide, lithium hydroxide and lithium deficiency is 1:1:1. Under this ratio, it can be approximately considered that the lithium added to the lithium iron phosphate comes entirely from the lithium in the lithium hydroxide, ensuring that only lithium iodide is precipitated after the eluate is recrystallized; at the same time, the eluate basically does not contain lithium hydroxide, and most of the lithium iodide can be directly recovered by recrystallization.

[0025] As a preferred embodiment, the cleaning solvent is water. Lithium iodide can be easily separated by recrystallization in water.

[0026] As a preferred solution, the recrystallization temperature of the eluate is 60-100° C. More preferably, the recrystallization temperature is 90° C. At this temperature, lithium iodide is guaranteed to be stable and not decomposed during the recrystallization process.

[0027] As a preferred solution, after the reducing material is washed, the remaining solid is dried at 50-70° C. for 6-12 hours to obtain the regenerated lithium iron phosphate powder. The dried regenerated lithium iron phosphate powder can be directly used to prepare electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 - is a process flow chart of a method for recycling lithium iron phosphate using lithium iodide;

[0029] Figure 2-3 The charge and discharge performance cycle diagram of the battery prepared from waste lithium iron phosphate and the regenerated lithium iron phosphate prepared in Examples 1 to 4 ( Figure 2 The cycle rate is 1C, Figure 3 The cycle rate is high rate 5C);

[0030] Figure 4 is the XRD diagram of waste lithium iron phosphate and the regenerated lithium iron phosphate prepared in Examples 1 to 4;

[0031] Figure 5 is an XRD refinement diagram of waste lithium iron phosphate and regenerated lithium iron phosphate prepared in Example 1;

[0032] Figure 6 is a diagram of element molar ratios of waste lithium iron phosphate and regenerated lithium iron phosphate prepared in Examples 1 to 4;

[0033] Figure 7 is a SEM image of waste lithium iron phosphate and regenerated lithium iron phosphate prepared in Example 1;

[0034] Figure 8 is the XRD pattern of the regenerated lithium iodide precipitated by recrystallization in Examples 1 to 4;

[0035] Fig. 9 is a graph showing the mass change of the regenerated lithium iodide precipitated by recrystallization in Examples 1 to 4;

[0036] Fig.10 is an XPS graph of waste lithium iron phosphate and regenerated lithium iron phosphate prepared in Example 1;

[0037] Fig.11 is the XPS graph of the regenerated lithium iodide prepared in Example 1;

[0038] Fig.12 is the XRD pattern of the regenerated lithium iron phosphate of Comparative Example 1;

[0039] Fig.13This is the XRD diagram of the regenerated lithium iron phosphate of Comparative Example 2;

[0040] Fig.14 is the XRD pattern of the regenerated lithium iron phosphate of Comparative Example 3;

[0041] Fig.15 This is the XRD diagram of the regenerated lithium iron phosphate of Comparative Example 4. DETAILED DESCRIPTION

[0042] Example 1

[0043] like Figure 1 As shown, a method for recycling lithium iron phosphate with lithium iodide comprises the following steps:

[0044] Place the waste lithium iron phosphate battery in a saturated sodium chloride solution for 10 minutes to fully discharge, disassemble the battery to separate the positive electrode material, heat the positive electrode material in a vacuum, separate the positive electrode material by mechanical vibration, and obtain waste lithium iron phosphate; obtain the iron, lithium and phosphorus contents by ICP, and calculate that 0.01g of lithium is missing in 1g of waste lithium iron phosphate.

[0045] 10 g of waste lithium iron phosphate, 0.96 g of lithium iodide and 0.3 g of lithium hydroxide monohydrate were weighed, put into a powder grinder, and mixed for 2 min at 1000 r / min to obtain a mixture.

[0046] The mixed material is placed in a tubular furnace, and in an inert atmosphere, the temperature is increased to 200°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then the temperature is increased to 550°C at a rate of 5°C / min, and kept at this temperature for 4 hours to obtain a reducing material.

[0047] The reducing material was washed with deionized water, and the remaining solid was dried at 70° C. for 12 h to obtain regenerated lithium iron phosphate powder. At the same time, the eluate was placed in a vacuum drying oven at 90° C. for recrystallization to obtain regenerated lithium iodide.

[0048] Example 2

[0049] like Figure 1 As shown, a method for recycling lithium iron phosphate with lithium iodide comprises the following steps:

[0050] 10 g of the waste lithium iron phosphate prepared in Example 1, 0.3 g of lithium hydroxide monohydrate and all of the regenerated lithium iodide prepared in Example 1 were weighed, put into a powder grinder, and mixed at 1000 r / min for 2 min to obtain a mixture.

[0051] The remaining steps are exactly the same as those in Example 1.

[0052] Example 3

[0053] like Figure 1As shown, a method for recycling lithium iron phosphate with lithium iodide comprises the following steps:

[0054] 10 g of the waste lithium iron phosphate prepared in Example 1, 0.3 g of lithium hydroxide monohydrate and all of the regenerated lithium iodide prepared in Example 2 were weighed, put into a powder grinder, and mixed at 1000 r / min for 2 min to obtain a mixture.

[0055] The remaining steps are exactly the same as those in Example 1.

[0056] Example 4

[0057] like Figure 1 As shown, a method for recycling lithium iron phosphate with lithium iodide comprises the following steps:

[0058] 10 g of the waste lithium iron phosphate prepared in Example 1, 0.3 g of lithium hydroxide monohydrate and all of the regenerated lithium iodide prepared in Example 3 were weighed, put into a powder grinder, and mixed at 1000 r / min for 2 min to obtain a mixture.

[0059] The remaining steps are exactly the same as those in Example 1.

[0060] Example 5

[0061] Battery charge and discharge performance test: The waste lithium iron phosphate prepared in Example 1 and the regenerated lithium iron phosphate prepared in Examples 1 to 4 were assembled into button batteries.

[0062] Lithium iron phosphate, conductive carbon (SuperP) and polyvinylidene fluoride were dissolved in NMP solution at a mass ratio of 8:1:1, and stirred in a planetary mixer for 3 hours. The solid content of the slurry was controlled to 50% to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on a clean aluminum foil, and then placed in a vacuum drying oven at 100°C for 12 hours. It was then pressed, weighed, cut, dried, and punched into discs with a diameter of 14 mm as positive electrode sheets.

[0063] A metal lithium sheet was used as the negative electrode sheet, a 15μm thick PE diaphragm, and a 1.0M LiPF6 solution as the electrolyte. The solvent of the electrolyte was prepared by mixing ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) in a mass ratio of 1:1:1. The positive electrode sheet, negative electrode sheet, diaphragm and electrolyte were assembled into a CR2025 button cell in an argon-filled glove box to test the electrical properties of the lithium-ion battery. During the test, the charge and discharge range was 2.8-4.0V. The charge and discharge tests were carried out in sequence to determine its specific capacity.

[0064] Table 1 0.2C rate 3 cycle performance

[0065]

[0066] As shown in Table 1, the regenerated lithium iron phosphate prepared in Examples 1 to 4 of the present invention all have good battery performance, from which it can be concluded that the regenerated lithium iodide obtained after three cycles can still play a good repair effect when used to repair waste lithium iron phosphate, which further proves that the regenerated lithium iodide can be well recycled during the cleaning and recrystallization process, and the regenerated lithium iodide can still play a very good repair and reduction effect in the next repair process, thereby realizing multiple cycles of regeneration of the lithium iodide reducing agent.

[0067] like Figure 2 As shown, the battery performance obtained by the regenerated lithium iron phosphate prepared in Examples 1-4 is good in cycle performance. In the process of repairing lithium iron phosphate, the supplemented lithium source diffuses into the lithium vacancies, reconstructs the crystal structure, reduces the oxidized trivalent iron, and reduces the concentration of anti-site defects, restores its integrity and stability, so that lithium ions can be more smoothly deintercalated and embedded during the charge and discharge process, and enhances the cycle stability of the battery. It can be seen from Examples 2-4 that the regenerated lithium iodide can still maintain the reducing effect of the reducing agent and the ability to reduce the activation energy of the anti-site defect reset. After 200 cycles at a rate of 1C, Example 1 can achieve a capacity retention rate of 95.5%, Example 2 can achieve a capacity retention rate of 95.0%, Example 3 can achieve a capacity retention rate of 96.8%, and Example 4 can achieve a capacity retention rate of 97.4%, proving the cycle stability. However, at a rate of 1C, the capacity retention rate of waste lithium iron phosphate is only 56.2% for 200 cycles.

[0068] like Figure 3 As shown, the regenerated lithium iron phosphate prepared in Examples 1-4 can still have good cycle stability under high rate cycles. At a rate of 5C, after 300 cycles, Example 1 can achieve a capacity retention rate of 87.2%, Example 2 can achieve a capacity retention rate of 91.3%, Example 3 can achieve a capacity retention rate of 87.8%, and Example 4 can achieve a capacity retention rate of 90.8%. It can be seen that the lithium iron phosphate obtained by the lithium iron phosphate environmentally friendly repair and regeneration process of the present invention has excellent stability even at high rates.

[0069] Example 6

[0070] XRD characterization: The waste lithium iron phosphate prepared in Example 1 and the regenerated lithium iron phosphate prepared in Examples 1 to 4 were tested by X-ray diffractometer (XRD, Rigaku Ultima IV, Japan) to detect the phase composition and crystal structure of the electrode materials. The test conditions were Cu-Kα radiation, acceleration voltage 40 kV, measurement angle range 2θ = 15° to 60°, and scanning rate 3° / min.

[0071] like Figure 4As shown, the regenerated lithium iron phosphate prepared in Examples 1-4 is successfully converted from the iron phosphate phase to the lithium iron phosphate phase compared to the waste lithium iron phosphate powder, and the crystallinity of Examples 1-4 is greatly improved compared to the waste lithium iron phosphate. The prepared products are all pure lithium iron phosphate phases, the missing crystal planes in the waste lithium iron phosphate are successfully repaired, and more crystal planes

[101] and

[200] are grown, which is more conducive to the diffusion of lithium ions in the

[010] direction. It can be seen that no matter how many times the regenerated lithium iodide is used, the regenerated lithium iron phosphate obtained by the reaction can maintain excellent physical properties.

[0072] like Figure 5 As shown, the data obtained by XRD characterization was refined using GSASⅡ to obtain the proportion of iron phosphate phase before and after regeneration and the concentration of iron-lithium anti-site defects. Compared with the waste lithium iron phosphate powder, the regenerated lithium iron phosphate powder prepared in Example 1 has an iron phosphate phase ratio of 8.3% in the waste lithium iron phosphate powder, and all the regenerated lithium iron phosphate powders prepared in Example 1 are lithium iron phosphate phases, proving that the melting process successfully supplements the lithium of the lithium-rich phase in the lithium source into the lithium-poor phase in the lithium iron phosphate. At the same time, the concentration of iron-lithium anti-site defects has been greatly improved. The proportion of iron-lithium anti-site defects in the waste lithium iron phosphate powder is 5.41%, and the proportion of iron-lithium anti-site defects in the powder obtained in Example 1 is reduced to 2.1%. This is more conducive to the diffusion of lithium ions during the charge and discharge process, reduces the amount of lithium ions that cannot diffuse due to iron ion obstruction, and improves the charge and discharge capacity of the battery.

[0073] Example 7

[0074] ICP characterization: The lithium, iron and phosphorus elements in the waste lithium iron phosphate prepared in Example 1 and the regenerated lithium iron phosphate prepared in Examples 1 to 4 were determined using an inductively coupled plasma emission spectrometer (ICP, Thermo Fisher, USA).

[0075] like Figure 6 As shown, the regenerated lithium iron phosphate prepared in Examples 1-4 effectively replenishes active lithium into active sites and repairs missing lithium vacancies; the molar ratio of lithium, iron and phosphorus is controlled at about 1:1:1, and the regenerated lithium iron phosphate is pre-lithiated to improve performance. When prepared into a battery, it is embedded in the structure of the lithium iron phosphate positive electrode. These excess lithium raw materials are transformed into raw materials for forming the negative electrode SEI film during the first cycle of charging of the battery, thereby preventing the loss of active lithium due to battery formation and thus reducing the battery capacity.

[0076] Example 8

[0077] SEM characterization: The waste lithium iron phosphate and the regenerated lithium iron phosphate prepared in Example 1 were scanned by a field emission scanning electron microscope (SEM, Tescan CLARA, Czech Republic).

[0078] like Figure 7As shown, the left picture is waste lithium iron phosphate, and the right picture is regenerated lithium iron phosphate prepared in Example 1. It can be seen that the regenerated lithium iron phosphate does not change its own crystal morphology characteristics, and the particle size of the regenerated lithium iron phosphate is smaller than that of the waste lithium iron phosphate, which is more conducive to the diffusion of lithium ions, improves the disadvantage of poor conductivity of lithium iron phosphate itself, and improves the cycle stability at high rates.

[0079] Example 9

[0080] XRD characterization: The regenerated lithium iodide crystals precipitated by recrystallization in Example 1-4 were subjected to X-ray diffraction (XRD, Rigaku Ultima IV, Japan) to detect the phase composition and crystal structure. The experimental conditions were Cu-Kα radiation, acceleration voltage 40 kV, measurement angle range 2θ = 15° to 60°, and scanning rate 3° / min.

[0081] like Figure 8 As shown, it can be seen that the regenerated lithium iodide recrystallized and precipitated in embodiment 1-4 does not have too much impurity, still has higher purity, can play its original role well, and crystallinity is higher, proves that in reaction and recrystallization process, lithium iodide crystal structure is not destroyed and crystal face is missing, and the performance of recrystallized lithium iodide is guaranteed. Although a small amount of crystal water mixes with lithium iodide crystal, it does not affect its own performance, and in subsequent reaction process, crystal water breaks away from and volatilizes. When crystal water is too much, it can be removed by desiccant to improve purity.

[0082] Example 10

[0083] Lithium iodide recovery test: The regenerated lithium iodide precipitated by recrystallization in Examples 1 to 4 was weighed by an electronic balance to weigh the mass of the lithium iodide recovered each time.

[0084] like Fig. 9 As shown, the masses of the regenerated lithium iodide precipitated by recrystallization in Examples 1 to 4 are 0.9521g, 0.9462g, 0.9388g and 0.9312g, respectively. It can be seen that the mass loss of the regenerated lithium iodide precipitated by recrystallization in Examples 1-4 is less than 1% compared to the lithium iodide added in the reaction, and the trace loss can be ignored and does not affect the performance of subsequent recycling. Since the amount of reducing agent used in the laboratory reaction is also small, the loss amount may be relatively large, but the amount used in industrial production is usually large, and the relative loss amount will be reduced, so the trace loss of lithium iodide will not have much effect on the performance of lithium iron phosphate during the recycling process.

[0085] Embodiment 11

[0086] XPS characterization: The regenerated lithium iron phosphate powder from the waste lithium iron phosphate prepared in Example 1 was subjected to X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA) to determine the valence state of the surface iron element.

[0087] like Fig.10 As shown, waste lithium iron phosphate has a 2p 3 / 2 Orbitals and 2p 1 / 2 There are characteristic peaks of ferric iron and ferrous iron on the orbital. Semi-quantitative analysis based on the fitting area of ​​these peaks shows that the ferric iron / ferrous iron fitting peak area ratio is 2.47, indicating that there is a large amount of ferric iron on the surface of waste lithium iron phosphate. In the regenerated lithium iron phosphate powder, the two characteristic peaks of the iron 2p orbital show a significant shift to a lower binding energy, and only the characteristic peak of ferrous iron is observed, indicating that the iodine anion in lithium iodide can act as an electron donor to reduce ferric iron. And the hypoiodite ions generated by the reaction do not oxidize ferrous iron to ferric iron, which further proves the feasibility and effectiveness of the entire process condition design.

[0088] The regenerated lithium iodide prepared in Example 1 was subjected to X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA) to determine the valence state of the surface iodine element.

[0089] like Fig.11 As shown in the figure, the 3d orbital of iodine in the regenerated lithium iodide is full of iodine anion peaks, and there are no other impurity peaks. This shows that the iodine element is successfully recycled in the form of lithium iodide during the entire reaction process, and the hypoiodite in the system is successfully reduced to iodine anion, proving the feasibility of recycling lithium iodide.

[0090] Comparative Example 1

[0091] Place the waste lithium iron phosphate battery in a saturated sodium chloride solution for 10 minutes to fully discharge, disassemble the battery to separate the positive electrode material, heat the positive electrode material in a vacuum, separate the positive electrode material by mechanical vibration, and obtain waste lithium iron phosphate; obtain the iron, lithium and phosphorus contents by ICP, and calculate that 0.01g of lithium is missing in 1g of waste lithium iron phosphate.

[0092] Weigh 10 g of waste lithium iron phosphate and 0.6 g of lithium hydroxide monohydrate, put them into a powder grinder, and mix them at 1000 r / min for 2 min to obtain a mixture.

[0093] The mixed material is placed in a tubular furnace, and in an inert atmosphere, the temperature is increased to 200°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then the temperature is increased to 550°C at a rate of 5°C / min, and kept at this temperature for 4 hours to obtain a reducing material.

[0094] The reducing material was washed with deionized water, and the remaining solid was dried at 70° C. for 12 h to obtain regenerated lithium iron phosphate powder.

[0095] XRD characterization: The phase composition and crystal structure of the regenerated lithium iron phosphate powder prepared in Comparative Example 1 were detected by X-ray diffraction (XRD, Rigaku Ultima IV, Japan).

[0096] like Fig.12 As shown, the regenerated lithium iron phosphate prepared in Comparative Example 1 contains obvious lithium phosphate phase, iron oxide phase and iron phosphate phase compared with the standard PDF card, which indicates that without adding iodine anions as a reducing agent, the oxidized trivalent iron in the waste lithium iron phosphate cannot be reduced to divalent iron by relying solely on the reducibility of carbon at high temperature; and in the molten state, when the hydroxide concentration is high and the alkalinity is strong, it will cause the precipitation of iron atoms to form lithium phosphate, destroying the crystal structure, and the precipitated iron is easily oxidized to the iron oxide phase. It can be seen that in the whole reaction process, iodine anions are important repair components, which can not only play a reducing role, but also neutralize high-concentration hydroxide ions to prevent the crystal structure from being destroyed.

[0097] Comparative Example 2

[0098] Place the waste lithium iron phosphate battery in a saturated sodium chloride solution for 10 minutes to fully discharge, disassemble the battery to separate the positive electrode material, heat the positive electrode material in a vacuum, separate the positive electrode material by mechanical vibration, and obtain waste lithium iron phosphate; obtain the iron, lithium and phosphorus contents by ICP, and calculate that 0.01g of lithium is missing in 1g of waste lithium iron phosphate.

[0099] 10 g of waste lithium iron phosphate, 0.96 g of lithium iodide and 0.3 g of lithium hydroxide monohydrate were weighed, put into a powder grinder, and mixed for 2 min at 1000 r / min to obtain a mixture.

[0100] The mixed material is placed in a tubular furnace, and in an inert atmosphere, the temperature is increased to 200°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then the temperature is increased to 800°C at a rate of 5°C / min, and kept at this temperature for 4 hours to obtain a reducing material.

[0101] The reducing material was washed with deionized water, and the remaining solid was dried at 70° C. for 12 h to obtain regenerated lithium iron phosphate powder.

[0102] XRD characterization: The phase composition and crystal structure of the regenerated lithium iron phosphate powder prepared in Comparative Example 2 were detected by X-ray diffraction (XRD, Rigaku Ultima IV, Japan).

[0103] like Fig.13As shown, the regenerated lithium iron phosphate prepared in Comparative Example 2 contains obvious lithium phosphate phase, iron oxide phase and iron phosphate phase compared with the standard PDF card, which indicates that when other reaction conditions remain unchanged, excessively high temperature will lead to a series of side reactions, such as excessive reaction of lithium in the lithium source with lithium iron phosphate, resulting in the precipitation of iron element in the form of single iron, and excess lithium occupies the iron site to generate a lithium phosphate phase, while the precipitated iron is easily oxidized to an iron oxide phase.

[0104] Comparative Example 3

[0105] Place the waste lithium iron phosphate battery in a saturated sodium chloride solution for 10 minutes to fully discharge, disassemble the battery to separate the positive electrode material, heat the positive electrode material in a vacuum, separate the positive electrode material by mechanical vibration, and obtain waste lithium iron phosphate; obtain the iron, lithium and phosphorus contents by ICP, and calculate that 0.01g of lithium is missing in 1g of waste lithium iron phosphate.

[0106] 10 g of waste lithium iron phosphate, 0.96 g of lithium iodide and 0.3 g of lithium hydroxide monohydrate were weighed, put into a powder grinder, and mixed for 2 min at 1000 r / min to obtain a mixture.

[0107] The mixed material is placed in a tubular furnace, and in an inert atmosphere, the temperature is increased to 200°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then the temperature is increased to 550°C at a rate of 5°C / min, and kept at this temperature for 5 hours to obtain a reducing material.

[0108] The reducing material was washed with deionized water, and the remaining solid was dried at 70° C. for 12 h to obtain regenerated lithium iron phosphate powder.

[0109] XRD characterization: The phase composition and crystal structure of the regenerated lithium iron phosphate powder prepared in Comparative Example 3 were detected by X-ray diffraction (XRD, Rigaku Ultima IV, Japan).

[0110] like Fig.14 As shown, the regenerated lithium iron phosphate prepared in Comparative Example 3 contains obvious lithium phosphate phase, iron oxide phase and iron phosphate phase compared with the standard PDF card. This indicates that when other reaction conditions remain unchanged, too high a reaction time will lead to a series of side reactions. For example, the lithium in the lithium source over-reacts with the lithium iron phosphate, resulting in the precipitation of the iron element in the form of single iron. Excess lithium occupies the iron site to generate a lithium phosphate phase, and the precipitated iron is easily oxidized to an iron oxide phase.

[0111] Comparative Example 4

[0112] Place the waste lithium iron phosphate battery in a saturated sodium chloride solution for 10 minutes to fully discharge, disassemble the battery to separate the positive electrode material, heat the positive electrode material in a vacuum, separate the positive electrode material by mechanical vibration, and obtain waste lithium iron phosphate; obtain the iron, lithium and phosphorus contents by ICP, and calculate that 0.01g of lithium is missing in 1g of waste lithium iron phosphate.

[0113] 10 g of waste lithium iron phosphate, 5.76 g of lithium iodide and 1.8 g of lithium hydroxide monohydrate were weighed, put into a powder grinder, and mixed at 1000 r / min for 2 min to obtain a mixture.

[0114] The mixed material is placed in a tubular furnace, and in an inert atmosphere, the temperature is increased to 200°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then the temperature is increased to 550°C at a rate of 5°C / min, and kept at this temperature for 4 hours to obtain a reducing material.

[0115] The reducing material was washed with deionized water, and the remaining solid was dried at 70° C. for 12 h to obtain regenerated lithium iron phosphate powder.

[0116] XRD characterization: The phase composition and crystal structure of the regenerated lithium iron phosphate powder prepared in Comparative Example 1 were detected by X-ray diffraction (XRD, Rigaku Ultima IV, Japan).

[0117] like Fig.15 As shown, the regenerated lithium iron phosphate prepared in Comparative Example 4 contains obvious lithium phosphate phase, iron oxide phase and iron phosphate phase compared with the standard PDF card, which indicates that when other reaction conditions remain unchanged, too high concentration of lithium source will lead to a series of side reactions, such as excessive reaction of lithium in the lithium source with lithium iron phosphate, resulting in precipitation of iron element in the form of single iron, excess lithium occupies the iron site to generate lithium phosphate phase, and the precipitated iron is easily oxidized to iron oxide phase.

[0118] It can be seen from Comparative Examples 1 to 4 that changes in reaction temperature, reaction time and lithium salt concentration will affect the phase composition and crystallization effect of regenerated lithium iron phosphate, and adjusting any one of the parameters alone cannot directly lead to the best regeneration effect.

[0119] Therefore, the embodiment of the present invention achieves the best regeneration effect of lithium iron phosphate by controlling the three parameters of reaction temperature, reaction time and lithium salt concentration. The three parameters affect each other. The present invention takes into account factors such as cost and energy consumption, and designs the best molten salt process parameters for lithium iodide and lithium hydroxide as eutectic lithium salts. The process parameters are that the first stage is heated to 200°C and kept warm for 2h; the second stage is heated to 550°C and kept warm for 4h, and the amount of lithium salt is twice the amount of lithium deficiency in the waste lithium iron phosphate. Under these process parameters, the regeneration effect of lithium iron phosphate can be optimized, and the iodine element can be completely precipitated in the form of lithium iodide, achieving a lithium iodide recovery rate of more than 99%, and realizing the recycling of lithium iodide. And there is almost no side reaction that affects the lithium iron phosphate and lithium iodide crystals and the valence state composition of the iodine element.

[0120] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. For ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for repairing lithium iron phosphate by lithium iodide circulation, characterized in that: The steps include: Mixing waste lithium iron phosphate with a lithium source to obtain a mixture, wherein the lithium source is lithium iodide and lithium hydroxide in a molar ratio of 1:1 to 5; heating the mixed material in an inert atmosphere to obtain a reducing material; The reducing material is washed with a washing solvent to obtain regenerated lithium iron phosphate powder and an eluate, and the eluate is collected and recrystallized to obtain regenerated lithium iodide. The washing solvent includes at least one of water, ethanol and acetone.

2. The method for recycling lithium iron phosphate with lithium iodide according to claim 1, characterized in that: The heating of the mixture also includes the following steps: placing the mixture in an inert atmosphere, heating it to 186-200°C at a rate of 1-5°C / min, keeping it warm for 0.5-4h, and then heating it to 400-700°C at a rate of 1-5°C / min, keeping it warm for 2-4h to obtain the reducing material.

3. The method for recycling lithium iron phosphate with lithium iodide according to claim 1, characterized in that: Before mixing, the waste lithium iron phosphate is first tested for the element contents of iron, lithium and phosphorus, and the lithium deficiency of the waste lithium iron phosphate is calculated according to the atomic number ratio of lithium iron phosphate; in terms of lithium, the molar ratio of lithium iodide, lithium hydroxide and lithium deficiency is 1:1 to 5:

1.

4. The method for cyclically repairing lithium iron phosphate with lithium iodide according to claim 3, characterized in that: The regenerated lithium iodide is recycled to partially or completely replace the lithium iodide.

5. The method for recycling lithium iron phosphate with lithium iodide according to claim 4, characterized in that: The molar ratio of the lithium iodide, lithium hydroxide and lithium deficiency is 1:1:

1.

6. The method for recycling lithium iron phosphate with lithium iodide according to claim 5, characterized in that: The cleaning solvent is water.

7. The method for recycling lithium iron phosphate with lithium iodide according to claim 6, characterized in that: The recrystallization temperature of the eluate is 60-100°C.

8. The method for recycling lithium iron phosphate with lithium iodide according to claim 1, characterized in that: After the reducing material is washed, the remaining solid is dried at 50-70° C. for 6-12 hours to obtain the regenerated lithium iron phosphate powder.

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