A method for repairing lithium iron phosphate by lithium iodide circulation

Abstract

The present invention relates to a method for recycling lithium iron phosphate using lithium iodide, comprising 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 reduced material; washing the reduced material with a cleaning solvent to obtain regenerated lithium iron phosphate powder and an eluate, collecting the eluate, and recrystallizing it to obtain regenerated lithium iodide; the lithium source is lithium iodide and lithium hydroxide in a molar ratio of 1:1 to 5, and the cleaning solvent comprises at least one of water, ethanol, and acetone. The present invention fully utilizes the eutectic system formed by lithium iodide and lithium hydroxide to reduce the reaction temperature. Simultaneously, lithium iodide acts as a reducing agent, and lithium hydroxide provides lithium ions, thereby jointly repairing the lithium iron phosphate and achieving the recycling of lithium iodide.

Description

Technical Field

[0001] The present 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, batteries are the primary energy storage device. Commercial lithium-ion batteries can be divided into two main categories: those with lithium iron phosphate (LFP) as the cathode material, and those with ternary materials (such as NMC). These two types of batteries account for the majority of the market share. LFP batteries, with their long cycle performance and lifespan (average 6-9 years), high safety performance, and low manufacturing costs, are widely used in the market and already account for two-thirds of the domestic lithium battery market.

[0003] The lifespan of lithium iron phosphate batteries, introduced early to the market, is nearing the end of their useful lifespan, necessitating the disposal of a significant amount of scrapped lithium iron phosphate batteries, estimated to reach 3.8 million tons by 2030. There are two primary technical approaches for recycling scrapped lithium iron phosphate batteries: the first involves destroying the batteries and extracting their elements through a series of methods for recycling; the second involves restoration, which restores their original performance by replenishing lost metal elements and repairing damaged crystal structures. The former primarily involves a combination of dry and wet methods to recover the elements in a classified manner. However, this method's energy consumption and chemical reagent input still result in significant secondary pollution and greenhouse gas emissions. Therefore, in the waste battery recycling sector, restoration is a more advantageous option over destruction, particularly for lithium iron phosphate (LFP), a cathode material that contains no precious metals and contains only a minimal proportion of valuable metals. Wet methods for recovering lithium and iron yield limited economic benefits, and regenerating LFP is a more cost-effective option for maximizing economic returns. 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 utilization 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. After multiple cycles, it will cause lattice distortion of the positive electrode material. 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, with lithium atoms and iron atoms located at different positions of the octahedron. In this crystal structure, the positions of Li and Fe have a certain geometric relationship and coordination environment. Since the FeO octahedra form a chain structure by sharing oxygen atoms, the iron atoms in each octahedron cannot be tightly connected, making 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 antisite defects will continue to rise, and the battery capacity will decay rapidly. In addition, due to the long-term lithium deficiency state, and the long-term cycle process under overcharge and high voltage environment, it may undergo redox reaction with organic matter in the electrolyte, causing the divalent iron in lithium iron phosphate to be oxidized 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 during the reset of antisite defects. Therefore, the presence of trivalent iron greatly increases the difficulty of regeneration. Finally, due to cyclic charge and discharge, lithium ions are constantly extracted and embedded in the positive electrode material, which may damage the lithium ion transport channel and cause lattice distortion.

[0005] In response to the above failure problems, the existing technology mainly compensates for the missing active lithium by adding a certain amount of lithium source, successfully embedding the lithium in the lithium-rich phase of the lithium source into the lithium-poor phase of the lithium iron phosphate, and restoring 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 to migrate to the lithium-poor phase but also reduce the activation energy for resetting the antisite defect. At present, the main means are to achieve repair through liquid phase, solid phase, electrochemical and other reaction environments. Currently, the commonly used lithium sources include 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, while the commonly used reducing agents include citric acid, ascorbic acid, malic acid, etc., which are relatively expensive and account for a large part of the repair cost. Therefore, discovering low-cost reducing agents and developing new repair methods have become one of the main research topics at this stage. Summary of the Invention

[0006] Based on this, the purpose of the present invention is to provide a method for recycling 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 production costs. 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 properties of the regenerated lithium iron phosphate.

[0007] The method for recycling lithium iron phosphate using lithium iodide according to 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, wherein 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. According to the molar ratio corresponding to the ratio of the eutectic point, a mixed lithium salt is obtained as the lithium source used. Due to the interaction between lithium iodide and lithium hydroxide, the low-temperature eutectic salt breaks the original ion arrangement. The lattice energy of the new ion arrangement is much lower than the lattice energy of lithium iodide and lithium hydroxide respectively, making the melting point of the eutectic system 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). This is something that cannot be achieved by the solid-phase method and the melting of a single lithium salt. Usually, the solid-phase method requires a temperature of 700-800°C, and the melting of a single lithium salt also requires 500-600°C. Therefore, the use of a 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 larger (relatively weaker binding capacity for outer electrons), so iodine anions also have stronger electron-donating capacity (stronger reducing properties). 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 a reducing atmosphere. In the present invention, this characteristic of iodine anions is utilized 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 phase of the lithium source to the active sites in the lithium iron phosphate. At the same time, iodine anions can also serve as electron-donating groups and have electron-donating capacity. When electron-donating groups are connected to the atomic groups around lithium or iron atoms, the electron cloud density at these positions is increased, making the charge distribution at the lithium-iron anti-site defect more uniform, reducing the energy barrier generated by the charge imbalance caused by the anti-site, thereby facilitating the anti-site atom 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 and reuse of iodine. Existing processes for regenerating lithium iron phosphate often cannot achieve the recycling of reactants. In liquid phase methods, the reactants have no recycling value after the reaction is completed, and in solid phase methods, the reactants are often pyrolyzed due to high temperatures, making them impossible to recycle. However, 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. Therefore, iodine anions with greater reducing power than carbon are introduced to react with the oxidized trivalent iron in the waste lithium iron phosphate to oxidize it into elemental iodine; when the hydroxide content is low, elemental iodine reacts with hydroxide to form hypoiodite ions with strong oxidizing power. Since the reducing power of carbon is stronger than divalent iron, the hypoiodite ions will undergo a carbothermal 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, elemental iodine will quickly react with hydroxide to form iodate with stronger oxidizing power, and then undergo a carbothermal 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 will be too high, which will lead to lithium excessively occupying the active sites of iron, causing iron to precipitate, forming lithium phosphate, and destroying the crystal structure. The entire process demonstrates the catalytic properties of iodine anions, with the main reactants consumed being hydroxide ions and carbon, with no consumption of the precious element iodine. After the heating reaction, an appropriate solvent is selected to clean and filter the repaired lithium iron phosphate. The resulting solvent is then dried in a vacuum oven for recrystallization. This achieves an ion utilization rate exceeding 99% in the process, reducing production costs. The reaction equations involved are 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 further comprises 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 increases from 186°C to 200°C as the number of lithium iodide cycles increases, and the temperature is kept for 2 hours. The main purpose is to fully melt the mixed lithium salt and form a uniform molten system. The lowest eutectic point of lithium iodide and lithium hydroxide is approximately 186°C. As the number of cycles increases, there will be a small amount of lithium iodide 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 kept 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, allowing lithium ions to enter the crystal lattice, further replenishing lithium vacancies, and thus repairing defects caused by lithium deficiency. During the two-stage heating process, high temperature can enable more lithium ions to 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 defect position, such as the lithium atoms and iron atoms in the lithium iron antisite defect, have increased vibration amplitude, 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. 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, since 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 element in a high temperature environment.

[0021] Furthermore, because lithium iron phosphate materials have a specific crystal structure, they contain channels for lithium ion diffusion. In a high-temperature molten salt environment, these channels expand, and the lithium ions become highly active. This allows lithium ions to diffuse along the channels into the lithium iron phosphate particles, allowing them to embed into the lithium iron phosphate cathode structure. In addition to the lithium ions at the active sites, some lithium ions are embedded within the PO tetrahedron and FeO octahedron structures and on the surface of the regenerated lithium iron phosphate. This lithium serves as the raw material for the formation of the SEI film on the surface of the negative electrode material during the battery formation phase, avoiding the consumption of active lithium ions and the resulting battery capacity degradation. Therefore, repairing lithium iron phosphate while pre-lithiating can effectively save production costs and eliminate the need for negative electrode pretreatment during the subsequent preparation of electrode materials. Although current liquid-phase repair methods can provide sufficient lithium salt concentration, their reaction temperature is insufficient, making pre-lithiation difficult. Solid-phase repair methods, while capable of providing higher temperatures, are limited in the lithium salt concentration they can provide, making pre-lithiation also difficult to achieve. As can be seen, pre-lithiation has very high requirements for temperature, holding time, and lithium salt concentration. If the hydroxide concentration is too high or the reaction temperature is too high for too long, iron will precipitate and destroy the crystal structure. Therefore, the present invention avoids the occurrence of side reactions by controlling the hydroxide concentration neutralized by lithium iodide, the reaction temperature, and the reaction time.

[0022] As a preferred embodiment, the waste lithium iron phosphate is tested for its iron, lithium, and phosphorus content before mixing, and the amount of lithium deficiency in the waste lithium iron phosphate is calculated based on the atomic ratio of lithium iron phosphate. The molar ratio of lithium iodide, lithium hydroxide, and the amount of lithium deficiency, calculated as lithium, is 1:1 to 5:1. More preferably, the iron, lithium, and phosphorus content of the waste lithium iron phosphate is tested by ICP, and the amount of lithium deficiency is calculated. This allows for optimizing the mix ratio in the mixture, maximizing raw material utilization, and 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 in a recycling reaction, thereby reducing production costs and waste generated by the reaction.

[0024] As a preferred embodiment, the molar ratio of lithium iodide, lithium hydroxide, and the amount of lithium deficiency is 1:1:1. At this ratio, it can be approximately assumed 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 recrystallization of the eluate. Furthermore, the eluate contains substantially no 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 embodiment, the recrystallization temperature of the eluate is 60-100° C. More preferably, the recrystallization temperature is 90° C. At this temperature, lithium iodide is ensured to be stable and not decomposed during the recrystallization process.

[0027] As a preferred solution, after washing the reducing material, 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 This is a charge and discharge performance cycle diagram of a 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 1 is the XRD pattern of waste lithium iron phosphate and regenerated lithium iron phosphate prepared in Examples 1 to 4;

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

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

[0033] Figure 7 1 is an 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] Figure 9 is a graph showing the mass change of the regenerated lithium iodide recrystallized and precipitated in Examples 1 to 4;

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

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

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

[0039] Figure 13This is the XRD pattern of the regenerated lithium iron phosphate of Comparative Example 2;

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

[0041] Figure 15 This is the XRD pattern 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] The waste lithium iron phosphate battery was placed in a saturated sodium chloride solution for full discharge, the battery was disassembled to separate the positive electrode material, the positive electrode material was placed in a vacuum and heated, and the positive electrode material was separated by mechanical vibration to obtain waste lithium iron phosphate; the iron, lithium and phosphorus contents were obtained by ICP. After calculation, it was found that 0.01g of lithium was 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 at 1000 r / min for 2 min to obtain a mixture.

[0046] The mixture was placed in a tube furnace, heated to 200°C at a rate of 5°C / min in an inert atmosphere, kept warm for 2 hours, and then heated to 550°C at a rate of 5°C / min, kept warm 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 in 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 prepare 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 lithium metal sheet was used as the negative electrode, a 15μm-thick PE separator, and a 1.0M LiPF6 solution as the electrolyte. The solvent was a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a 1:1:1 mass ratio. The positive electrode, negative electrode, separator, and electrolyte were assembled into CR2025 button cells in an argon-filled glove box to test the electrical performance of the lithium-ion battery. The charge and discharge range was 2.8-4.0V. Sequential charge and discharge tests were performed to determine the specific capacity.

[0064] Table 1 Cycling performance at 0.2C rate for 3 times

[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. 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 cyclic 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 antisite defects, restoring its integrity and stability, so that lithium ions can be more smoothly deintercalated and intercalated during the charge and discharge process, thereby enhancing the cycle stability of the battery. Examples 2-4 show 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 antisite 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%, demonstrating cycle stability. However, the capacity retention rate of the waste lithium iron phosphate is only 56.2% after 200 cycles at a rate of 1C.

[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 phase composition and crystal structure of the electrode materials were determined using an X-ray diffractometer (Rigaku Ultima IV, Japan) using the waste lithium iron phosphate prepared in Example 1 and the regenerated lithium iron phosphate prepared in Examples 1-4. The experimental conditions were Cu-Kα radiation, an accelerating voltage of 40 kV, a measurement angle range of 2θ = 15° to 60°, and a scan rate of 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 and the concentration of iron-lithium antisite defects before and after regeneration. 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. The regenerated lithium iron phosphate powder prepared in Example 1 is all lithium iron phosphate phase, 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 antisite defects has been greatly improved. The proportion of iron-lithium antisite defects in the waste lithium iron phosphate powder is 5.41%, and the proportion of iron-lithium antisite 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 prepared in Example 1 and the regenerated lithium iron phosphate were scanned by a field emission scanning electron microscope (SEM, Tescan CLARA, Czech Republic).

[0078] like Figure 7As shown, the left picture shows waste lithium iron phosphate, and the right picture shows regenerated lithium iron phosphate prepared in Example 1. It can be seen that the regenerated lithium iron phosphate has not changed its original 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 recrystallized from Examples 1-4 were analyzed for phase composition and crystal structure using X-ray diffraction (Rigaku Ultima IV, Japan). The experimental conditions were Cu-Kα radiation, an accelerating voltage of 40 kV, a measurement angle range of 2θ = 15° to 60°, and a scan rate of 3° / min.

[0081] like Figure 8 Shown, it can be seen that the regeneration lithium iodide that recrystallization separates out in embodiment 1-4 does not have too much impurity existence, still has higher purity, can well play its original effect, and degree of crystallinity is higher, proves reaction and recrystallization process, does not make lithium iodide crystal structure be destroyed and crystal face lacks, ensures the performance of recrystallized lithium iodide.Although a small amount of crystal water mixes with lithium iodide crystal, does not affect its own performance, in subsequent reaction process, wherein crystal water breaks away from and volatilizes.When crystal water is too much, can be dehydrated by desiccant, 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 measure the mass of the lithium iodide recovered each time.

[0084] like Figure 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] Example 11

[0086] XPS characterization: The lithium iron phosphate powder regenerated 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 Figure 10 As shown, waste lithium iron phosphate in the 2p of iron 3 / 2 Orbitals and 2p 1 / 2 The orbitals contain characteristic peaks of ferric and ferrous iron. Semi-quantitative analysis based on the fitted areas of these peaks shows a ferric / ferrous iron fitting peak area ratio of 2.47, indicating that a large amount of ferric iron is present on the surface of the waste lithium iron phosphate. In the regenerated lithium iron phosphate powder, the two characteristic peaks of the iron 2p orbital show a significant shift toward lower binding energies, while only the characteristic peak of ferrous iron is observed, indicating that the iodine anion in the lithium iodide can act as an electron donor to reduce ferric iron. Moreover, the hypoiodite ion generated by the reaction does not oxidize the ferrous iron to ferric iron, further demonstrating 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 Figure 11 As shown, the 3d orbitals of iodine in the regenerated lithium iodide are all peaks of iodine anions, with no other impurity peaks present. This indicates that the iodine element is successfully recycled in the form of lithium iodide throughout the reaction process, and the hypoiodite in the system is successfully reduced to iodine anions, demonstrating the feasibility of lithium iodide recycling.

[0090] Comparative Example 1

[0091] The waste lithium iron phosphate battery was placed in a saturated sodium chloride solution for full discharge, the battery was disassembled to separate the positive electrode material, the positive electrode material was placed in a vacuum and heated, and the positive electrode material was separated by mechanical vibration to obtain waste lithium iron phosphate; the iron, lithium and phosphorus contents were obtained by ICP. After calculation, it was found that 0.01g of lithium was missing in 1g of waste lithium iron phosphate.

[0092] 10 g of waste lithium iron phosphate and 0.6 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.

[0093] The mixture was placed in a tube furnace, heated to 200°C at a rate of 5°C / min in an inert atmosphere, kept warm for 2 hours, and then heated to 550°C at a rate of 5°C / min, kept warm for 4 hours to obtain a reducing material.

[0094] The reduced 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 Figure 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. This shows that without the addition of 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 reducing property 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 entire reaction process, iodine anions are important repair components. They not only play a reducing role, but also neutralize high concentrations of hydroxide ions to prevent the crystal structure from being destroyed.

[0097] Comparative Example 2

[0098] The waste lithium iron phosphate battery was placed in a saturated sodium chloride solution for full discharge, the battery was disassembled to separate the positive electrode material, the positive electrode material was placed in a vacuum and heated, and the positive electrode material was separated by mechanical vibration to obtain waste lithium iron phosphate; the iron, lithium and phosphorus contents were obtained by ICP. After calculation, it was found that 0.01g of lithium was 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 at 1000 r / min for 2 min to obtain a mixture.

[0100] The mixture was placed in a tube furnace, heated to 200°C at a rate of 5°C / min in an inert atmosphere, kept warm for 2 hours, and then heated to 800°C at a rate of 5°C / min, kept warm for 4 hours to obtain a reducing material.

[0101] The reduced 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 Figure 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. This shows that when other reaction conditions remain unchanged, excessively high temperature 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 iron in the form of single iron. The excess lithium occupies the iron site to form a lithium phosphate phase, and the precipitated iron is easily oxidized to an iron oxide phase.

[0104] Comparative Example 3

[0105] The waste lithium iron phosphate battery was placed in a saturated sodium chloride solution for full discharge, the battery was disassembled to separate the positive electrode material, the positive electrode material was placed in a vacuum and heated, and the positive electrode material was separated by mechanical vibration to obtain waste lithium iron phosphate; the iron, lithium and phosphorus contents were obtained by ICP. After calculation, it was found that 0.01g of lithium was 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 at 1000 r / min for 2 min to obtain a mixture.

[0107] The mixture was placed in a tube furnace, heated to 200°C at a rate of 5°C / min in an inert atmosphere, kept warm for 2 hours, and then heated to 550°C at a rate of 5°C / min, kept warm for 5 hours to obtain a reducing material.

[0108] The reduced 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 Figure 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 shows 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 iron in the form of iron element. The excess lithium occupies the iron site to form a lithium phosphate phase, and the precipitated iron is easily oxidized to an iron oxide phase.

[0111] Comparative Example 4

[0112] The waste lithium iron phosphate battery was placed in a saturated sodium chloride solution for full discharge, the battery was disassembled to separate the positive electrode material, the positive electrode material was placed in a vacuum and heated, and the positive electrode material was separated by mechanical vibration to obtain waste lithium iron phosphate; the iron, lithium and phosphorus contents were obtained by ICP. After calculation, it was found that 0.01g of lithium was 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 mixture was placed in a tube furnace, heated to 200°C at a rate of 5°C / min in an inert atmosphere, kept warm for 2 hours, and then heated to 550°C at a rate of 5°C / min, kept warm for 4 hours to obtain a reducing material.

[0115] The reduced 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 Figure 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. This shows that when other reaction conditions remain unchanged, the excessive concentration of the lithium source will lead to a series of side reactions. For example, the lithium in the lithium source reacts excessively with the lithium iron phosphate, resulting in the precipitation of iron in the form of iron element. The excess lithium occupies the iron site to form a lithium phosphate phase, and the precipitated iron is easily oxidized to an 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 achieve 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: the first stage heating is 200°C, and the heat preservation is 2h; the second stage heating is 550°C, and the heat preservation is 4h. The amount of lithium salt used is twice the amount of lithium deficiency in the waste lithium iron phosphate. Under these process parameters, the lithium iron phosphate regeneration effect 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 composition of the iodine element.

[0120] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. A person skilled in the art would be able to make numerous modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A method for repairing lithium iron phosphate by lithium iodide circulation, characterized in that: The steps include: Detecting the elemental contents of iron, lithium, and phosphorus in the waste lithium iron phosphate, and calculating the lithium deficiency of the waste lithium iron phosphate according to the atomic number ratio of the lithium iron phosphate; The waste lithium iron phosphate is mixed with a lithium source to obtain a mixture, wherein the molar ratio of the lithium iodide, lithium hydroxide and lithium deficiency is 1: (1-5): 1, calculated on the basis of lithium; The mixed material is heated to 186-200° C. in an inert atmosphere to form a eutectic system, and then the temperature is raised to 400-700° C. 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, wherein 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, 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: The regenerated lithium iodide is recycled to partially or completely replace the lithium iodide.

4. The method for recycling lithium iron phosphate with lithium iodide according to claim 3, characterized in that: The molar ratio of the lithium iodide, lithium hydroxide and lithium deficiency is 1:1:

1.

5. The method for recycling lithium iron phosphate with lithium iodide according to claim 4, characterized in that: The cleaning solvent is water.

6. The method for recycling lithium iron phosphate with lithium iodide according to claim 5, characterized in that: The recrystallization temperature of the eluate is 60-100°C.

7. 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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