Direct recycling method for waste lithium iron phosphate cathodes based on galvanic cell reaction

Abstract

This application relates to a method for direct recycling of waste lithium iron phosphate cathodes based on a galvanic cell reaction. The method includes the following steps: S1: dissolving a film-removing agent and soluble lithium salt in water to prepare a lithium replenishment regeneration solution; S2: immersing the waste lithium iron phosphate electrode sheets in the lithium replenishment regeneration solution; S3: removing the immersed waste electrode sheets, repeatedly washing them with clean water, and drying them to obtain regenerated cathode material. This recycling method yields regenerated cathode material, which can be further used to prepare regenerated cathode sheets and button batteries. This method requires no additional reducing agent or applied voltage, and has good compatibility with the existing battery recycling industry. The entire reaction can be completed in an electrolyte environment, and the lithiation process consumes no additional energy, effectively solving the problems of cumbersome processes, high energy consumption, and low economic efficiency of traditional recycling methods, demonstrating significant technical and economic advantages.

Description

Technical Field

[0001] This invention relates to a method for the direct recycling of waste lithium iron phosphate cathodes based on galvanic cell reactions, belonging to the field of rechargeable lithium-ion secondary batteries, and can be used in energy, industry, scientific research and other fields. Background Technology

[0002] With the rapid development of new energy vehicles and smart grids, lithium-ion batteries using lithium iron phosphate (LFP) cathode materials have been widely used in power batteries and energy storage systems due to their high safety, long cycle life, and low cost. In recent years, both the installed capacity of power batteries and energy storage systems have shown continuous growth. However, this growth has also brought about the recycling and disposal of a large number of retired batteries. Among waste lithium-ion batteries, cathode materials have become a key focus for recycling due to their high added value. Currently, the recycling methods for cathode materials are mainly divided into two paths: elemental extraction and direct regeneration. In comparison, direct regeneration repair can significantly improve economic efficiency while maintaining the material structure, and has greater application potential. Lithium iron phosphate has a unique olivine crystal structure, which exhibits good stability during cycling. Direct recycling methods for waste lithium iron phosphate mainly include high-temperature calcination and solution immersion. High-temperature calcination regenerates waste materials through high-temperature treatment, but this process is complex, consumes a lot of energy, and involves certain carbon emissions, which is not conducive to achieving green recycling (e.g., patent CN116053631A). Solution immersion is a wet chemical process that uses a reducing agent to reduce iron phosphate in waste materials to lithium iron phosphate. However, existing solution immersion methods have significant limitations: when the reducing agent is weak, heating is usually required to promote the reaction (e.g., patent CN117088347A), further increasing energy consumption; while when the reducing agent is strong, it is often unstable and expensive (e.g., ACSSustainable Chem. Eng. 2021, 9, 48, 16384). These problems limit the industrial application of solution immersion.

[0003] Therefore, current direct recycling methods still suffer from problems such as complex processes, high energy consumption, and high economic costs, failing to meet the needs of industrial applications. Developing an efficient, green, and low-cost direct recycling method for lithium iron phosphate cathodes is a crucial issue that urgently needs to be addressed, as it can improve the reuse efficiency of retired batteries and promote the sustainable development of the waste lithium battery recycling sector. Summary of the Invention

[0004] In view of the deficiencies in the existing technology, the purpose of this invention is to provide a method for direct recycling of waste lithium iron phosphate cathodes based on galvanic cell reaction. This method is a simple, efficient and environmentally friendly method for recycling waste lithium iron phosphate cathode materials.

[0005] The aluminum current collector in waste lithium iron phosphate has strong reducing properties, but due to the extremely high protective effect of the dense oxide film naturally formed on its surface, the actual oxidation potential of metallic aluminum in the electrolyte is much higher than the theoretical oxidation potential. Therefore, it is difficult to directly reduce the iron phosphate phase in waste lithium iron phosphate and regeneration cannot be achieved.

[0006] The inventors discovered through research that halide anions in the electrolyte can act as nucleophiles, reacting with Al atoms in alumina crystals, which have high charge density and small ionic radii. 3+ It takes effect, thereby weakening Al 3+ With O 2- The electrostatic interaction between the electrodes disrupts the local structure of the oxide film, causing it to lose its protective function and exposing the underlying aluminum metal directly to the electrolyte, where it undergoes oxidation. The degree of oxide film failure is closely related to the nucleophilicity of halide anions and the concentration of anions in the solution. Furthermore, the oxidation process of the aluminum foil is also affected by the solution environment; the corrosion potential of aluminum varies significantly in different electrolytes.

[0007] Based on the above characteristics, the inventors have innovatively designed a highly efficient and green method for the spontaneous regeneration of waste lithium iron phosphate cathode materials. This method only requires immersing the waste lithium iron phosphate electrodes in a specially formulated electrolyte to achieve automatic material regeneration. In this electrolyte, the corrosion potential of aluminum can be adjusted to be lower than the reduction potential of iron phosphate in the waste lithium iron phosphate. When the waste electrode is immersed in the electrolyte, halide anions effectively destroy the oxide film on the current collector surface, while the active components in the electrolyte further increase the potential of the aluminum foil surface, thereby activating the strong reducing properties of metallic aluminum. Through this process, metallic aluminum and iron phosphate in the waste lithium iron phosphate spontaneously undergo a redox reaction: metallic aluminum is oxidized to trivalent aluminum ions, while lithium-deficient iron phosphate is reduced to lithium iron phosphate, thus successfully achieving the relithiation of the waste lithium iron phosphate cathode.

[0008] Based on the above objectives and principles, this application provides the following technical solutions:

[0009] One of the technical solutions of this application provides a method for direct recycling of waste lithium iron phosphate cathodes based on galvanic cell reactions. The specific steps of this method are as follows:

[0010] S1: Dissolve the film remover and soluble lithium salt in water to prepare a lithium replenishment and regeneration solution;

[0011] S2: Immerse the waste lithium iron phosphate electrode sheets in the lithium replenishment and regeneration solution;

[0012] S3: Take out the soaked waste electrode sheets, wash them repeatedly with clean water and dry them to obtain the recycled positive electrode material.

[0013] Further, the film-removing agent in step S1 is selected from at least one of soluble metal chlorides, soluble metal bromides, soluble metal iodides, soluble moderately strong acids, and soluble strong bases, preferably soluble metal chlorides; the concentration of the film-removing agent in the lithium replenishment regeneration solution is 0.005 M-0.2 M, preferably 0.01 M-0.05 M.

[0014] Furthermore, the soluble lithium salt in step S1 is selected from at least one of lithium nitrate and lithium sulfate; the total concentration of lithium ions in the lithium replenishment regeneration solution is not less than 0.1 M to ensure the reaction rate.

[0015] Furthermore, the soaking time in step S2 is determined by the concentration of lithium ions in the soaking solution. The lower limit of the soaking time is 20 min / lithium ion concentration (M). Taking a lithium ion concentration of 1 M and a film remover concentration of 0.01 M as an example, the minimum soaking time is 20 min.

[0016] After the inventors removed the oxide film on the surface of the aluminum current collector using a film-removing agent, the aluminum metal and the iron phosphate in the waste lithium iron phosphate formed a galvanic cell, which then underwent a spontaneous self-discharge reaction. The aluminum metal was oxidized, and the ferric iron in the iron phosphate was reduced to ferrous iron. At the same time, lithium ions in the solution spontaneously entered the lattice of the lithium iron phosphate, causing the iron phosphate to be relithiated. Taking LiCl as the film-removing agent and LiNO3 as the lithium source as an example, the reaction of the film removal process is as follows:

[0017] 3LiCl + 2Al₂O₃ = AlCl₃ + 3LiAlO₂;

[0018] The self-discharge recovery reaction of ferric phosphate and Al is as follows:

[0019] 3LiNO3+3FePO4+Al=Al(NO3)3+3LiFePO4.

[0020] The second technical solution of this application provides a regenerated cathode material prepared by the above method.

[0021] The third technical solution of this application provides a regenerated positive electrode sheet, which includes the above-mentioned regenerated positive electrode.

[0022] Furthermore, the method for preparing the regenerated positive electrode sheet includes the following steps:

[0023] A slurry is prepared by regenerating the positive electrode, Super P and PVDF, and then coated onto aluminum foil and dried to obtain the regenerated positive electrode sheet.

[0024] Furthermore, the mass ratio of the regenerated positive electrode, Super P, and PVDF is 90:5:5; the slurry coating is performed on a wet film preparation device using a doctor blade, and the thickness of the slurry is 100-200 micrometers, preferably 150 micrometers.

[0025] Furthermore, the drying process involves placing the product at 60-100°C for 10-15 hours; preferably, it involves placing the product in an oven at 80°C for 12 hours.

[0026] The fourth technical solution of this application provides a button cell battery, wherein the positive electrode of the button cell battery is the aforementioned regenerated positive electrode sheet.

[0027] Furthermore, the preparation steps of the button cell are as follows: the regenerated positive electrode sheet is cut into a disc, lithium metal is used as the counter electrode, and a 2025 type button cell is assembled.

[0028] Furthermore, the diameter of the disc is preferably 12 mm; the electrolyte of the button battery is preferably a commercial electrolyte with a formulation of 1M LiPF6in EC:DEC=1:1.

[0029] Compared with the prior art, this application has at least the following advantages and beneficial effects:

[0030] (1) The method of the present invention is simple to operate, requires no additional reducing agent or external voltage, and has good compatibility with the existing battery recycling industry.

[0031] (2) The entire reaction can be completed in the electrolyte environment. The lithiation process has no additional energy consumption, and it is both highly efficient and environmentally friendly. It effectively solves the problems of cumbersome process, high energy consumption and low economic benefits of traditional recycling methods, and has significant technical and economic advantages. Attached Figure Description

[0032] Figure 1 This is a flowchart of the regenerated positive electrode of this application;

[0033] Figure 2 The LSV curve of aluminum foil in 0.1 M LiCl electrolyte is shown. The working electrode is Al, the counter electrode is Pt, and the reference electrode is calomel electrode.

[0034] Figure 3 The LSV curve of aluminum foil in 0.1 M LiNO3 electrolyte is shown. The working electrode is Al, the counter electrode is Pt, and the reference electrode is calomel electrode.

[0035] Figure 4 The LSV curve of aluminum foil in 0.01 M LiCl electrolyte is shown. The working electrode is Al, the counter electrode is Pt, and the reference electrode is calomel electrode.

[0036] Figure 5 This is a photograph of the aluminum foil separated in Example 1. Detailed Implementation

[0037] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to specific embodiments. It should be noted that the following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be pointed out that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0038] All raw materials used in this invention are not particularly limited in their source; they can be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0039] The flowchart of the regenerated positive electrode in the embodiments of this application is as follows: Figure 1 As shown, the specific steps are as follows:

[0040] S1: Dissolve the film remover and soluble lithium salt in water to prepare a lithium replenishment and regeneration solution;

[0041] S2: Immerse the waste lithium iron phosphate electrode sheets in the lithium replenishment and regeneration solution;

[0042] S3: Take out the soaked waste electrode sheets, wash them repeatedly with clean water and dry them to obtain the recycled positive electrode material.

[0043] Examples 1-5 and Comparative Examples 1-6 of this application are as follows:

[0044] Example 1: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.01 M LiCl + 1 M LiNO3 (lithium replenishment regeneration solution, hereinafter all electrolytes refer to lithium replenishment regeneration solution) for 30 min, washed and dried to obtain the regenerated cathode sample E1.

[0045] Example 2: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.03 M LiCl + 3 M LiNO3 for 10 min, washed and dried to obtain the regenerated cathode sample E2.

[0046] Example 3: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.01 M LiOH + 1 M LiNO3 for 30 min, washed and dried to obtain the regenerated cathode sample E3.

[0047] Example 4: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.005 M LiBr + 2 M LiNO3 for 10 min, washed and dried to obtain the regenerated cathode sample E4.

[0048] Example 5: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.2 M LiCl + 1 M Li2SO4 for 25 min, washed and dried to obtain the regenerated cathode sample E5.

[0049] Comparative Example 1: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.01 M LiOH + 1 M LiNO3 for 10 min, washed and dried to obtain the regenerated cathode sample C1.

[0050] Comparative Example 2: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.001 M LiCl + 2 M LiNO3 for 20 min, washed and dried to obtain the regenerated cathode sample C2.

[0051] Comparative Example 3: The waste lithium iron phosphate cathode was immersed in an electrolyte of 0.3 M LiCl + 2 M LiNO3 for 30 min, washed and dried to obtain the regenerated cathode sample C3.

[0052] Comparative Example 4: The waste lithium iron phosphate cathode was immersed in an electrolyte of 1 M LiCl + 1 M LiNO3 for 10 min, washed and dried to obtain the regenerated cathode sample C4.

[0053] Comparative Example 5: The waste lithium iron phosphate cathode was immersed in 1 M LiNO3 electrolyte for 30 min, washed and dried to obtain the regenerated cathode sample C5.

[0054] Comparative Example 6: Waste lithium iron phosphate cathode was directly used as cathode sample C6.

[0055] The regenerated positive electrode samples from Examples 1-5 and Comparative Examples 1-6 were used to prepare positive electrode sheets and button cells as follows:

[0056] (1) A slurry was prepared with a ratio of regenerated positive electrode sample: Super P: PVDF = 90:5:5, and the slurry was coated on aluminum foil with a thickness of 150 micrometers;

[0057] (2) Place the positive electrode obtained in (1) in an 80 ℃ oven for 12 h;

[0058] (3) Cut the dried positive electrode into a circular piece with a diameter of 12 mm, use lithium metal as the counter electrode, and use commercial electrolyte (formulation 1M LiPF6in EC:DEC=1:1) as the electrolyte, and equip it with a 2025 button cell. Then, conduct electrochemical tests on the button cell at a current of 15 mA / g in the voltage range of [2.5V, 3.8V]. The battery is charged first and then discharged.

[0059] The electrochemical tests include:

[0060] Instrument: Xinwei Battery Tester;

[0061] Method: Galvanostatic method (constant current test method). Using this method, the following data can be directly obtained and recorded on the above instrument.

[0062] (1) First charge specific capacity: The amount of charge that a unit mass or volume of active material can hold during the first charge of the battery, usually expressed in milliampere-hours per gram (mAh / g) or milliampere-hours per cubic centimeter (mAh / cm³).

[0063] (2) First discharge specific capacity: The first discharge specific capacity refers to the amount of charge released by a unit mass or volume of active material during the first discharge of the battery, and is also expressed as (mAh / g) or (mAh / cm³).

[0064] (3) First-time efficiency (%): First-time efficiency (first-time efficiency) refers to the ratio of the discharge specific capacity to the charge specific capacity of the battery during the first charge-discharge cycle, usually expressed as a percentage (%). It reflects the degree of irreversible loss of the battery during the first cycle.

[0065] from Figures 2-4 It can be seen from this that under the condition of chloride ions ( Figure 2 Al foil is oxidized at very low potentials (below 0 V vs. SCE), and the potential is related to the chloride ion concentration. Figure 4 ), while in the environment of lithium nitrate ( Figure 3 Because the oxide film was not removed, it was not oxidized even at 0.8 V vs. SCE, while the potential for reduction of iron phosphate in waste lithium iron phosphate is approximately 0.3 V vs. SCE.

[0066] Table 1 shows that the coulombic efficiency of batteries in Examples 1-5 was greater than 98% in the first charge cycle, and the specific capacity of batteries in the first charge cycle was greater than 155 mAh / g, indicating that they were no longer deficient in lithium; and from Figure 5The results show that the separated aluminum foil was heavily corroded, indicating that Al, as a reducing agent, participated in the re-lithiation of iron phosphate. In Comparative Example 1, due to insufficient soaking time, lithium replenishment was not complete, resulting in a low charge capacity, normal discharge capacity, and an initial efficiency below 100%. In Comparative Example 2, due to the low concentration of the film remover, the oxide film removal was slow, limiting the reaction rate. Therefore, the waste electrode was not completely lithiated within a given time, resulting in a low charge capacity but normal discharge capacity. In Comparative Examples 3 and 4, due to the high concentration of the film remover, the applicant found that the film remover itself also reacted with iron phosphate, causing damage to the olivine structure, thus significantly reducing the charge and discharge capacities. In Comparative Example 5, since no film remover was added, iron phosphate could not form a galvanic cell with metallic aluminum, so its charge and discharge capacity was close to that of the untreated Comparative Example 6.

[0067] Table 1. First-cycle charge / discharge specific capacity and first-cycle coulombic efficiency of the above embodiments and comparative examples.

[0068]

[0069] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for direct recycling of waste lithium iron phosphate cathodes based on galvanic cell reactions, characterized in that, The specific steps of this method are as follows: S1: Dissolve the film-removing agent and soluble lithium salt in water to prepare a lithium replenishment and regeneration solution; the film-removing agent is selected from at least one of soluble metal chlorides, soluble metal bromides, soluble metal iodides, soluble moderately strong acids, and soluble strong bases; the soluble lithium salt is selected from at least one of lithium nitrate and lithium sulfate; the concentration of the film-removing agent in the lithium replenishment and regeneration solution is 0.005 M-0.2 M; the total concentration of lithium ions in the lithium replenishment and regeneration solution is not less than 0.1 M; S2: Immerse the waste lithium iron phosphate electrode sheets in a lithium replenishment and regeneration solution; the minimum immersion time is 20 min / lithium ion concentration, where the lithium ion concentration is M. S3: Take out the soaked waste electrode sheets, wash them repeatedly with clean water and dry them to obtain the recycled positive electrode material.

2. A recycled cathode material, characterized in that, It is prepared by the method for direct recycling of waste lithium iron phosphate cathode based on galvanic cell reaction as described in any one of claims 1.

3. A regenerated positive electrode sheet, characterized in that, The material comprises the regenerated positive electrode material as described in claim 2; the method for preparing the regenerated positive electrode sheet includes the following steps: preparing the regenerated positive electrode, Super P and PVDF into a slurry, coating the slurry onto an aluminum foil and drying it to obtain the regenerated positive electrode sheet.

4. A regenerated positive electrode sheet according to claim 3, characterized in that, In the preparation method, the mass ratio of regenerated positive electrode, Super P and PVDF is 90:5:5; the thickness of the slurry coating is 100~200 micrometers.

5. A regenerated positive electrode sheet according to claim 3, characterized in that, In the preparation method, the drying temperature is 60~100 ℃ and the time is 10~15 h.

6. A button battery, characterized in that, The positive electrode of the button cell is the regenerated positive electrode sheet as described in claim 3; the counter electrode of the button cell is lithium metal; and the electrolyte formula of the button cell is 1M LiPF6 in EC:DEC=1:1.

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