Recycling method of lithium iron phosphate

Abstract

The invention belongs to the field of lithium ion batteries, and particularly discloses a recovery method of lithium iron phosphate. According to the method disclosed by the invention, the lithium iron phosphate is recycled for the positive electrode material in the waste battery, and the recycled lithium iron phosphate can keep good electrical properties on the premise of being simple in process, low in cost and environment-friendly.

Description

Technical Field

[0001] The invention belongs to the field of lithium ion batteries, and in particular relates to a method for recovering lithium iron phosphate. Background Art

[0002] In recent years, lithium-ion batteries, as an efficient and reliable energy storage device, have been widely used in portable electronic devices, electric vehicles, and renewable energy storage systems. However, with the widespread application and rapid popularization of lithium-ion batteries, the number of discarded lithium-ion batteries has also shown a rapid growth trend. Among them, lithium iron phosphate (LFP), as an important positive electrode material, has attracted much attention due to its high energy density, good cycle life and high safety, and has been widely used in lithium-ion batteries. The LFP contained in discarded lithium-ion batteries not only pollutes the environment, but also contains a relatively large amount of LFP, which easily causes waste of resources.

[0003] People can achieve the goal of reducing environmental burden and resource waste by recycling LFP in discarded lithium-ion batteries. The current mainstream recycling technologies mainly include pyrometallurgy and hydrometallurgy. The technical path of pyrometallurgy is already quite complete, but this method consumes a lot of energy, is accompanied by a large amount of energy emissions, and has a low recovery rate and is difficult for lithium metal elements. Hydrometallurgical technology can achieve efficient metal recovery with relatively low energy consumption and cost expenditure, but it will lead to the generation of a large amount of wastewater, resulting in secondary pollution. The existing technology for LFP recovery has the following obvious defects: (1) A large amount of waste liquid or waste residue will be generated during the recovery process, which will create a new source of pollution and violate the original intention of LFP recovery; (2) The existing recovery methods mainly focus on the recovery of metal elements, and the recovered metal elements need to be separated and re-prepared to obtain LFP, resulting in a complex overall LFP production process and high cost; (3) The electrical properties of the recovered LFP are poor, resulting in poor performance of the lithium battery produced. Summary of the invention

[0004] In view of the problems involved in the above-mentioned prior art, such as the easy generation of secondary pollution, complex process, high cost, and poor electrical properties of the recycled LFP, the present invention will provide a method for recovering lithium iron phosphate.

[0005] To achieve the above purpose, the following technical solutions are specifically included:

[0006] A method for recovering lithium iron phosphate comprises the following steps:

[0007] (1) immersing the lithium iron phosphate positive electrode sheet in the waste lithium iron phosphate battery in water at 90-100° C. to separate the positive electrode coating on the lithium iron phosphate positive electrode sheet from the current collector, and collecting the waste containing lithium iron phosphate;

[0008] (2) mixing the waste material containing lithium iron phosphate with N-methylpyrrolidone, subjecting the mixture to ultrasonic treatment at 80-100° C., and then filtering and vacuum drying the mixture to obtain a filtrate;

[0009] (3) The filtered product is mixed with N-methylpyrrolidone, and subjected to ultrasonic treatment at 80-100° C., and then filtered and vacuum dried in sequence to obtain lithium iron phosphate.

[0010] The method of the present invention is to recycle lithium iron phosphate from the positive electrode material in discarded batteries. Under the premise of simple process, low cost and environmental friendliness, the recovered lithium iron phosphate is similar to the initial powder morphology, and the recovered lithium iron phosphate is used as the positive electrode material to make a battery. The obtained battery performance is similar to that of a battery made of new lithium iron phosphate.

[0011] In the method of the present invention, the lithium iron phosphate positive electrode sheet in the waste lithium iron phosphate battery is first immersed in 90-100°C water, so that the positive electrode coating on the lithium iron phosphate positive electrode sheet is fully separated from the current collector, and the positive electrode coating and the current collector are both insoluble in water, and the current collector and the waste containing lithium iron phosphate can be recycled respectively. The used relatively high temperature water can also be reused, and the waste liquid in the recycling process can be recycled to reduce environmental pollution. If the temperature during water immersion is too low, the coating of the positive electrode sheet and the current collector cannot be separated, reducing the purity of the recovered current collector and the recovery rate of lithium iron phosphate; if the temperature during water immersion is too high, it is easy to decompose the material on the positive electrode coating, resulting in more residual impurities in the recovered lithium iron phosphate, thereby reducing its electrical properties. After water immersion, ultrasonic treatment is performed twice in N-methylpyrrolidone (NMP) solvent, impurities such as polyvinylidene fluoride (PVDF) contained in the lithium iron phosphate can be effectively removed, and the purity and electrical properties of the recovered lithium iron phosphate can be improved. If the temperature of the ultrasonic treatment process is too low, the organic impurities will not be fully dissolved, and the powder crushing and dispersion effect will be poor, which will reduce the battery performance of the recovered lithium iron phosphate; if the temperature of the ultrasonic treatment process is too high, the organic impurities will decompose, resulting in impurities remaining in the recovered lithium iron phosphate, reducing its battery capacity.

[0012] Preferably, in step (1), the immersion time is 10-60 min.

[0013] Preferably, in step (2), the mass ratio of the waste material containing lithium iron phosphate to N-methylpyrrolidone is 1:(1-3).

[0014] Preferably, in step (2), the power of the ultrasonic treatment is 200-300 W, and the time of the ultrasonic treatment is 0.5-2 h.

[0015] Preferably, in steps (2) and (3), the vacuum drying temperature is 60-85° C., the vacuum drying pressure is -0.1 to -0.3 MPa, and the vacuum drying time is 2-6 h.

[0016] Preferably, in step (3), the mass ratio of the filtrate to N-methylpyrrolidone is 1:(1-3).

[0017] Preferably, in step (3), the power of the ultrasonic treatment is 200-300 W, and the time of the ultrasonic treatment is 0.5-2 h.

[0018] Preferably, in step (3), the N-methylpyrrolidone may be the filtrate recovered during the filtration process in step (2).

[0019] When the lithium iron phosphate obtained in the lithium iron phosphate recovery method of the present invention is used in the preparation of lithium ion batteries, the performance of the obtained battery is similar to that of a battery made with new lithium iron phosphate.

[0020] Compared with the prior art, the present invention has the following beneficial effects: the method for recovering lithium iron phosphate of the present invention can make the recovered lithium iron phosphate similar to the initial powder morphology and maintain good electrical properties under the premise of simple process, low cost and environmental friendliness, and then use the recovered lithium iron phosphate as the positive electrode material to make a battery, the performance of which is similar to that of a battery made of new lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 SEM images of the original LFP powder (a) and the LFP powder recovered in Example 1 (b).

[0022] Figure 2 Charge and discharge curves of batteries assembled with original LFP and recycled LFP. DETAILED DESCRIPTION

[0023] In order to better illustrate the purpose, technical scheme and advantages of the present invention, the present invention will be further described below through specific examples. The test methods used in the examples and / or comparative examples are conventional methods unless otherwise specified; the materials, reagents, etc. used can be obtained from commercial channels unless otherwise specified.

[0024] The recycling method of the present invention is applicable to conventional lithium iron phosphate batteries, including large cylindrical, small cylindrical, soft-pack, and square aluminum shell lithium iron phosphate batteries. The following example uses a 280Ah square aluminum shell waste lithium iron phosphate battery.

[0025] Example 1

[0026] A method for recovering lithium iron phosphate comprises the following steps:

[0027] (1) Disassembling waste lithium iron phosphate batteries to obtain positive electrode sheets, the positive electrode sheets contain aluminum foil current collectors and LFP coatings coated on the aluminum foil current collectors, the LFP coatings contain polyvinylidene fluoride (PVDF) binders, lithium iron phosphate, and conductive agents. This method mainly removes the PVDF binder, the presence of the conductive agent does not affect the subsequent use of the LFP, and the content of the conductive agent is extremely low;

[0028] (2) immersing the positive electrode sheet in water and heating it to 95° C. After immersion for 30 minutes, the LFP coating on the positive electrode sheet is separated from the aluminum foil current collector. Both the positive electrode coating and the current collector are insoluble in water. The lithium iron phosphate waste and the current collector are separated, and the lithium iron phosphate waste is dried for later use. In this step, the high-temperature water after separation can be reused, and the obtained aluminum foil current collector can be recycled;

[0029] (3) placing the waste lithium iron phosphate obtained in step (2) in a large sealed glass bottle, adding an organic solvent N-methylpyrrolidone (NMP) to the sealed glass, wherein the ratio of the mass of the added NMP to the mass of the waste lithium iron phosphate is 1:1; subjecting the obtained mixture to ultrasonic crushing at 90° C., with an ultrasonic power of 250 W and an ultrasonic time of 1 h;

[0030] (4) After the ultrasonic treatment, the filtration was performed, wherein the pore size of the filter membrane was 0.22 μm. In this step, a filtrate was obtained in the filtration funnel, and a NMP solution containing PVDF was obtained in the filtration bottle. The NMP solution was recovered into a large sealed glass bottle for subsequent reuse; the filtrate obtained by filtration was then vacuum dried in a vacuum oven, wherein the oven temperature was 80° C., the oven pressure was -0.1 MPa, and the baking time was 4 h;

[0031] (5) adding the dried material obtained in step (4) to the large sealed glass bottle containing the recovered NMP solution in step (4), wherein the mass ratio of the recovered NMP solution to the waste lithium iron phosphate is 1:1; subjecting the obtained mixture to ultrasonic crushing at 90° C., with an ultrasonic power of 250 W and an ultrasonic time of 1 h;

[0032] (6) After the ultrasonic treatment, the filtration was performed, wherein the pore size of the filter membrane was 0.22 μm. In this step, a filtrate was obtained in the filtration funnel, and a NMP solution containing PVDF was obtained in the filtration bottle. The NMP solution was recovered into a large sealed glass bottle for subsequent reuse. The filtrate obtained by filtration was then vacuum dried in a vacuum oven, wherein the oven temperature was 80° C., the oven pressure was -0.1 MPa, and the baking time was 4 h to obtain recovered LFP.

[0033] Example 2

[0034] The difference between this embodiment and embodiment 1 is that the temperature during ultrasound in steps (3) and (5) of this embodiment is 80° C., and the rest is the same.

[0035] Example 3

[0036] The difference between this embodiment and embodiment 1 is that the temperature during ultrasound in steps (3) and (5) of this embodiment is 100° C., and the rest is the same.

[0037] Example 4

[0038] The difference between this embodiment and embodiment 1 is that in steps (3) and (5) of this embodiment, the mass ratio of the NMP solution or the recovered NMP solution to the waste lithium iron phosphate is 1:3, and the rest is the same.

[0039] Comparative Example 1

[0040] The difference between this comparative example and Example 1 is that hot water immersion is not used in step (2), but powder scraping treatment is used:

[0041] (1) disassembling a waste lithium iron phosphate battery to obtain a positive electrode sheet (the positive electrode sheet is the same as that in Example 1), wherein the positive electrode sheet comprises an aluminum foil current collector and an LFP coating coated on the aluminum foil current collector, wherein the LFP coating comprises polyvinylidene fluoride (PVDF) and lithium iron phosphate;

[0042] (2) The positive electrode plate is subjected to a powder scraping treatment, and the powder on the plate is hung off and recovered using a tool to obtain lithium iron phosphate waste and a separated aluminum foil current collector;

[0043] (3) placing the waste lithium iron phosphate obtained in step (2) in a large sealed glass bottle, adding an organic solvent N-methylpyrrolidone (NMP) to the sealed glass, wherein the ratio of the mass of the added NMP to the mass of the waste lithium iron phosphate is 1:1; subjecting the obtained mixture to ultrasonic crushing at 90° C., with an ultrasonic power of 250 W and an ultrasonic time of 1 h;

[0044] (4) After the ultrasonic treatment, the filtration was performed, wherein the pore size of the filter membrane was 0.22 μm. In this step, a filtrate was obtained in the filtration funnel, and a NMP solution containing PVDF was obtained in the filtration bottle. The NMP solution was recovered into a large sealed glass bottle for subsequent reuse; the filtrate obtained by filtration was then vacuum dried in a vacuum oven, wherein the oven temperature was 80° C., the oven pressure was -0.1 MPa, and the baking time was 4 h;

[0045] (5) adding the dried material obtained in step (4) to the large sealed glass bottle containing the recovered NMP solution in step (4), wherein the mass ratio of the recovered NMP solution to the waste lithium iron phosphate is 1:1; subjecting the obtained mixture to ultrasonic crushing at 90° C., with an ultrasonic power of 250 W and an ultrasonic time of 1 h;

[0046] (6) After the ultrasonic treatment, the filtration was performed, wherein the pore size of the filter membrane was 0.22 μm. In this step, a filtrate was obtained in the filtration funnel, and a NMP solution containing PVDF was obtained in the filtration bottle. The NMP solution was recovered into a large sealed glass bottle for subsequent reuse. The filtrate obtained by filtration was then vacuum dried in a vacuum oven, wherein the oven temperature was 80° C., the oven pressure was -0.1 MPa, and the baking time was 4 h to obtain recovered LFP.

[0047] Comparative Example 2

[0048] The difference between this comparative example and Example 1 is that in step (2) of this comparative example, the temperature of the water immersion treatment of the positive electrode plate is 70° C., and the rest is the same.

[0049] Comparative Example 3

[0050] The difference between this comparative example and Example 1 is that this comparative example does not adopt the method of steps (3) to (6), and the waste lithium iron phosphate obtained in step (2) is ground in a mortar to directly obtain powder to obtain the recovered LFP:

[0051] (1) disassembling a waste lithium iron phosphate battery to obtain a positive electrode sheet (the positive electrode sheet is the same as that in Example 1), wherein the positive electrode sheet comprises an aluminum foil current collector and an LFP coating coated on the aluminum foil current collector, wherein the LFP coating comprises polyvinylidene fluoride (PVDF) and lithium iron phosphate;

[0052] (2) immersing the positive electrode sheet in water and heating it to 95° C. After immersion for 30 minutes, the LFP coating on the positive electrode sheet is separated from the aluminum foil current collector. Both the positive electrode coating and the current collector are insoluble in water. The waste lithium iron phosphate and the current collector are separated respectively. In this step, the high-temperature water after separation can be reused, and the obtained aluminum foil current collector can be recycled;

[0053] (3) Drying the waste lithium iron phosphate obtained in step (2) and grinding it in a mortar to directly obtain powder, thereby obtaining recovered LFP.

[0054] Comparative Example 4

[0055] The difference between this comparative example and Example 1 is that the temperature during ultrasonic treatment in steps (3) and (5) of this comparative example is 70° C., and the rest is the same.

[0056] A method for assembling a 5.3Ah lithium-ion battery comprises the following steps:

[0057] (1) Preparation of positive electrode sheet: lithium salt (LFP), conductive agent (SP), and binder (PVDF) are made into slurry in a ratio of 96:2:2, and the slurry is coated on aluminum foil;

[0058] (2) Preparation of negative electrode sheet: Graphite, conductive agent (SP), and binder (PVDF) are prepared into a slurry in a ratio of 96:2:2, and the slurry is coated on the copper foil;

[0059] (3) Alternately stacking the positive and negative electrodes and adding a separator to form a battery core;

[0060] (4) placing the battery core into an aluminum-plastic film bag and encapsulating it into a soft-pack battery through a sealing process;

[0061] (5) Injecting electrolyte during the packaging process so that the electrolyte contacts the positive and negative electrodes and forms a battery reaction;

[0062] (6) Packaging of soft-pack lithium batteries: Soft-pack batteries are generally packaged using a heat-sealing process. First, the battery core is placed in an aluminum-plastic film bag, and the aluminum-plastic film bag is sealed by preheating. The sealed battery core is then insulated and waterproofed.

[0063] The LFP recovered from the above-mentioned embodiments and comparative examples and the original LFP were made into batteries according to the above-mentioned battery assembly method. Except that the lithium salt (LFP) used in each battery corresponds to different embodiments, comparative examples and original LFP, the conditions and parameters such as the injected electrolyte and conductive agent are the same.

[0064] Note: The "original LFP" referred to herein is the LFP raw material used in the original preparation of the waste lithium iron phosphate batteries involved in this embodiment and comparative example.

[0065] Testing method for assembled lithium-ion batteries: Use a battery testing device to perform 1.5A cycle charge and discharge on the assembled lithium-ion batteries 4 times, record the fourth charge and discharge curve and the discharge capacity of the next three times, calculate the average value based on the discharge capacity of the next three times to obtain the actual capacity of the battery, and calculate the LFP gram capacity based on the actual capacity of the battery and the mass of LFP used to make the battery.

[0066] Recycled aluminum foil mass ratio (purity): First weigh the mass of the recycled aluminum foil (the aluminum foil in the aluminum foil current collector), then wash the aluminum foil with NMP and water in turn, then wipe the impurities on the surface of the aluminum foil with clean dust-free paper, and finally weigh the aluminum foil. The recycling purity is obtained based on the ratio of the final clean aluminum foil mass to the initial recycled aluminum foil mass.

[0067] The LFP recovered from Example 1 and the original LFP were observed using a scanning electron microscope (SEM) to observe the microscopic morphology of the powders. Figure 1 As shown. Figure 1 It can be seen that the LFP powder recovered in Example 1 of the present invention is well dispersed, and its microscopic morphology, particle size and distribution are similar to those of the original LFP.

[0068] A 5.3Ah battery was assembled using the original LFP and the LFP recovered from Example 1. The cycle charge and discharge curve of the battery is shown in the attached figure. Figure 2 As shown, the capacity of the battery assembled from the LFP recycled in the embodiment and the comparative example and the original LFP and the mass ratio of the aluminum foil current collector that can be recovered in the recycling process are shown in Table 1.

[0069] Table 1

[0070] Group Gram capacity (mAh / g) Recycled aluminum foil mass ratio (purity) Example 1 145 99.98％ Example 2 146 99.97％ Example 3 145 99.97％ Example 4 146 99.96％ Comparative Example 1 126 91％ Comparative Example 2 140 95％ Comparative Example 3 134 99.95％ Comparative Example 4 138 99.96％

[0071] By the attached Figure 2 It can be seen from the battery's cyclic charge and discharge curves that the charge and discharge curves of the battery assembled using the original LFP and the LFP recovered in Example 1 are very close, indicating that the electrical performance of the LFP obtained by the recovery method of the present invention is equivalent to that of the original LFP. The method of the present invention can effectively recycle LPF, and the recovered LFP can be reused as a positive electrode material for lithium batteries.

[0072] It can be seen from Example 1 and Comparative Examples 1-4 that, compared with the conventional scraping separation of lithium iron phosphate-containing waste and aluminum foil current collector and the situation of not using ultrasonic treatment in N-methylpyrrolidone (NMP) solvent, in the method of the present invention, the positive electrode plate is first separated from the lithium iron phosphate-containing waste and the aluminum foil current collector by water immersion, and then ultrasonic treatment is carried out twice in N-methylpyrrolidone (NMP) solvent, which can significantly improve the capacity of the recovered lithium iron phosphate and the purity of the recovered aluminum foil current collector. In addition, the temperature of the water immersion and ultrasonic treatment process will also affect the capacity of the recovered lithium iron phosphate and the purity of the recovered aluminum foil current collector. If the water immersion temperature is too low, the coating and the current collector of the positive electrode plate cannot be fully separated; if the water immersion temperature is too high, the material on the positive electrode coating is easily decomposed, resulting in more impurities in the recovered lithium iron phosphate, thereby reducing its electrical properties. If the temperature of the ultrasonic treatment process is too low, the organic impurities will not be fully dissolved, the powder crushing and dispersion effect will be poor, and the battery capacity of the recovered lithium iron phosphate will be reduced; if the temperature of the ultrasonic treatment process is too high, the organic impurities will decompose, resulting in impurities being left in the recovered lithium iron phosphate, reducing its battery capacity. Therefore, it is preferred that the temperature during water immersion is 90-100°C and the temperature during ultrasonic treatment is 80-100°C. At the above temperature, the battery capacity of the recovered lithium iron phosphate and the purity of the aluminum foil current collector can be made higher.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for recovering lithium iron phosphate, characterized in that: The steps include: (1) immersing the lithium iron phosphate positive electrode sheet in the waste lithium iron phosphate battery in water at 90-100° C. to separate the positive electrode coating on the lithium iron phosphate positive electrode sheet from the current collector, and collecting the waste containing lithium iron phosphate; (2) mixing the waste material containing lithium iron phosphate with N-methylpyrrolidone, subjecting the mixture to ultrasonic treatment at 80-100° C., and then filtering and vacuum drying the mixture to obtain a filtrate; (3) The filtered product is mixed with N-methylpyrrolidone, and subjected to ultrasonic treatment at 80-100° C., and then filtered and vacuum dried in sequence to obtain lithium iron phosphate.

2. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In step (1), the immersion time is 10-60 minutes.

3. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In step (2), the mass ratio of the waste containing lithium iron phosphate to N-methylpyrrolidone is 1:(1-3).

4. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In step (2), the power of the ultrasonic treatment is 200-300W, and the time of the ultrasonic treatment is 0.5-2h.

5. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In steps (2) and (3), the pore size of the filtering membrane is less than or equal to 0.22 μm.

6. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In steps (2) and (3), the vacuum drying temperature is 60-85°C, the vacuum drying pressure is -0.1 to -0.3Mpa, and the vacuum drying time is 2-6h.

7. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In step (3), the mass ratio of the filtrate to N-methylpyrrolidone is 1:(1-3).

8. The method for recovering lithium iron phosphate according to claim 1, characterized in that: In step (3), the power of the ultrasonic treatment is 200-300W, and the time of the ultrasonic treatment is 0.5-2h.

9. Use of the lithium iron phosphate obtained by the method for recovering lithium iron phosphate according to any one of claims 1 to 8 in preparing lithium ion batteries.

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