Method for recovering PVDF-containing positive electrode material from waste lithium iron phosphate battery and preparing fluorine-doped lithium iron phosphate

Abstract

The invention discloses a method for recovering a PVDF (Polyvinylidene Fluoride)-containing positive electrode material from a waste lithium iron phosphate battery and preparing fluorine-doped lithium iron phosphate, and belongs to the technical field of waste battery recovery. The method comprises the following steps: discharging and disassembling waste lithium iron phosphate batteries to separate out positive plates; soaking the positive plate in hot water, and separating to obtain an invalid lithium iron phosphate positive electrode material containing PVDF and an aluminum foil; the preparation method comprises the following steps: drying and grinding the PVDF-containing invalid lithium iron phosphate positive electrode material into powder, uniformly mixing the powder with LiOH, and calcining in a non-oxidizing atmosphere to obtain the fluorine-doped lithium iron phosphate. According to the method, PVDF in the failed lithium iron phosphate material is fully utilized, the failed lithium iron phosphate positive electrode material can be efficiently repaired, the fluorine-doped lithium iron phosphate positive electrode material with better electrochemical performance is obtained, and the method is simple in process, convenient to operate and efficient.

Description

Technical Field

[0001] The invention relates to a method for recycling waste lithium iron phosphate batteries, in particular to a method for recycling PVDF-containing positive electrode materials from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate, belonging to the technical field of recycling waste lithium iron phosphate batteries. Background Art

[0002] With the development of science and technology, various power tools have shown explosive growth, and people's demand for secondary batteries has increased dramatically. Lithium-ion batteries are environmentally friendly, have high specific energy, high voltage platform, long cycle life, good rate performance, low self-discharge, and no memory effect. They are far superior to lead-acid, Ni-Cd, and Ni-MH batteries, and have been widely used in military and civilian fields. At present, lithium-ion secondary batteries with lithium iron phosphate as the positive electrode material have begun to be widely used in power tools and electric vehicles due to their low cost and good safety performance. With the development of electric vehicles, power batteries with lithium iron phosphate as the positive electrode material have been widely used, and the demand for waste lithium-ion batteries has increased year by year. In order to recycle and reuse materials, save costs and protect the environment, it is imperative to recycle lithium iron phosphate waste.

[0003] In the prior art, Chinese patents CN101359756A, CN102280673A and CN101394015A involve methods for treating lithium iron phosphate waste in industrial production processes, but the methods are not suitable for the recycling of waste lithium iron phosphate batteries containing PVDF. Chinese patent CN102285673A discloses a method for recovering lithium and iron from lithium iron phosphate power batteries of electric vehicles, which mainly recovers lithium, iron and aluminum by hydrometallurgical methods, but in the recycling process, the recovery of valuable metals is incomplete, the recovery efficiency is low, and there is a certain degree of damage to the environment, and the economic benefits are not good. Chinese patent CN101916889A discloses a method for recovering and preparing lithium iron phosphate from waste lithium-ion power batteries of water-based systems, which only recycles waste batteries of water-based viscose systems, and does not involve the recycling of waste batteries of oil-based viscose systems. Chinese patent CN102017276A discloses a method for recycling lithium iron phosphate power batteries, which simply separates and cleans the positive electrode sheets and can directly use them as positive electrode materials. However, since the positive electrode sheets of the original lithium iron phosphate batteries have expired, there are no steps to process and repair the separated lithium iron phosphate positive electrode materials, so the battery performance made by this method will be defective.

[0004] For the PVDF binder in the waste lithium iron phosphate battery, the prior art mainly recovers it by dissolving it with an organic solvent. For example, Chinese patent CN110416652A discloses the use of an organic solvent to selectively dissolve the PVDF in the tailings of the waste lithium iron phosphate positive electrode powder acid leaching, and obtains a PVDF solution after liquid-solid separation; the PVDF solution is then heated to volatilize the organic solvent, so as to separate the PVDF from the organic solvent and obtain a regenerated PVDF product. This method consumes a large amount of organic solvent, and the energy consumption of the recovery process is high, and the recovery value is not high. At the same time, there is also a method of directly decomposing the PVDF binder by pyrolysis. For example, Chinese patent CN112225191A discloses the complete degradation of the PVDF binder by two calcinations. This method is difficult to avoid the corrosion of the lithium iron phosphate particles by hydrofluoric acid produced by the pyrolysis of PVDF, and does not achieve the resource utilization of the PVDF binder. Summary of the invention

[0005] In view of the defects of the prior art in that the recovery rate of lithium iron phosphate is low, the recovery is incomplete, and it is difficult to achieve low-cost recovery and resource utilization of PVDF, the purpose of the present invention is to provide a method for recovering PVDF-containing positive electrode materials from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate. The method not only makes full use of the PVDF binder in the failed lithium iron phosphate material, but also can achieve efficient repair of the lithium iron phosphate positive electrode material to obtain fluorine-doped lithium iron phosphate positive electrode material with better electrochemical performance. The method has a simple process, is easy to operate and is highly efficient. The obtained fluorine-doped lithium iron phosphate positive electrode material has improved battery performance and better cycle performance than the lithium iron phosphate positive electrode material.

[0006] In order to achieve the above technical objectives, the present invention provides a method for recovering PVDF-containing positive electrode materials from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate, the method comprising the following steps:

[0007] 1) Discharge and disassemble the waste lithium iron phosphate battery to separate the positive electrode sheet;

[0008] 2) Soaking the positive electrode sheet in hot water to separate the spent lithium iron phosphate positive electrode material containing PVDF and the aluminum foil;

[0009] 3) The spent lithium iron phosphate positive electrode material containing PVDF is dried and ground into powder, and the powder is mixed with a lithium source including LiOH, and then calcined in a non-oxidizing atmosphere to obtain fluorine-doped lithium iron phosphate.

[0010] The key to the technical solution of the present invention is that the PVDF in the failed lithium iron phosphate positive electrode material is not subjected to thermal decomposition or organic solvent dissolution treatment, but directly uses strongly alkaline LiOH as a lithium source to perform high-temperature repair on the failed lithium iron phosphate positive electrode material. On the one hand, the strong alkalinity of LiOH can promote the degradation of PVDF. On the other hand, LiOH can react with hydrogen fluoride produced by thermal decomposition of PVDF to form lithium fluoride, and lithium fluoride can dope and modify lithium iron phosphate. On the three aspects, LiOH can be used as a lithium source to compensate for the loss of lithium source in the charging and discharging process of lithium iron phosphate.

[0011] As a preferred solution, the mass of the lithium source including LiOH accounts for 5-10% of the mass of the spent lithium iron phosphate positive electrode material powder containing PVDF, and the mass of LiOH accounts for more than 50% of the mass of the lithium source. The lithium source also includes Li2CO 3、 At least one of LiAc and Li2O. The lithium source can be a conventional lithium source in the prior art, but it must contain an appropriate amount of lithium hydroxide with a strong alkalinity, which can promote the breaking of the CF bond in PVDF and capture hydrofluoric acid, converting it into lithium fluoride to dope the lithium iron phosphate as a fluorine source.

[0012] As a preferred solution, the calcination conditions are: first, heating to 300-400°C at a heating rate of 5-15°C / min, and keeping warm for 2-5h; then heating to 600-900°C at a heating rate of 5-15°C / min, and keeping warm for 3-15h; then cooling with the furnace. During the calcination process, the temperature is first kept at 300-400°C. The function and purpose is to use a strong base to promote the degradation of PVDF at a relatively low temperature, avoid the pyrolysis and carbonization of PVDF, and convert LiOH into LiF at this stage; the function and purpose of keeping warm at 600-900°C is to ensure fluorine doping and lithium supplementation in defective lithium iron phosphate under liquid phase conditions, and to ensure the density of the positive electrode material.

[0013] As a preferred solution, the non-oxidizing atmosphere is an inert atmosphere and / or nitrogen. The inert atmosphere is argon, argon-hydrogen mixed gas, etc.

[0014] As a preferred solution, the soaking treatment conditions are: hot water temperature is 60-80° C., and the soaking time is 20-30 minutes. The soaking treatment is mainly to separate the spent lithium iron phosphate positive electrode material containing PVDF from the aluminum foil, thereby separating the aluminum foil.

[0015] As a preferred solution, the drying conditions are: drying at a temperature of 70 to 90° C. for 8 to 24 hours.

[0016] The method provided by the present invention for recovering PVDF-containing positive electrode material from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate mainly comprises the following steps:

[0017] 1) Discharge the residual power of the waste lithium iron phosphate battery, disassemble the battery, separate the positive electrode sheet, negative electrode sheet and diaphragm, and wash the positive electrode sheet;

[0018] 2) Soak the cleaned positive electrode sheet in hot water at 60-80°C for 20-30 minutes to separate the lithium iron phosphate and aluminum foil in the positive electrode sheet, and the aluminum foil can be recycled separately;

[0019] 3) placing the waste lithium iron phosphate positive electrode active material after separating the aluminum foil in an oven at 70-90° C. overnight, grinding to obtain waste lithium iron phosphate positive electrode powder, adding about 5-10 wt % of LiOH or a mixture thereof to the waste lithium iron phosphate solid powder to obtain a premix;

[0020] 4) After the premix is ​​ground and mixed uniformly, the temperature is raised to 300-400° C. at 5-15° C. / min in a non-oxidizing atmosphere and kept warm for 2-5 hours; then the temperature is raised to 600-900° C. at 5-15° C. / min and kept warm for 3-15 hours, and then cooled in the furnace to obtain a fluorine-doped lithium iron phosphate battery positive electrode material.

[0021] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0022] The present invention can realize the repair and regeneration of lithium iron phosphate positive electrode materials, and fluorine doping is performed during the repair process. Compared with lithium iron phosphate materials, the specific energy and cycle performance of fluorine-doped lithium iron phosphate materials are improved. For example, the first discharge capacity at 0.1C reaches 146.3mAh / g, and the capacity after 50 cycles is 140.1mAh / g.

[0023] The present invention can realize resource utilization of PVDF in failed lithium iron phosphate positive electrode materials, and utilizes it as a fluorine source to perform doping modification on lithium iron phosphate.

[0024] The method of the invention has simple process, is environmentally friendly, and has a high recovery rate of lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The XRD spectra of fluorine-doped lithium iron phosphate and lithium iron phosphate, the products of Example 1 of the present invention.

[0026] Figure 2 The discharge cycle curves of fluorine-doped lithium iron phosphate and lithium iron phosphate, the products of Example 1 of the present invention.

[0027] Figure 3 This is a SEM image of the fluorine-doped lithium iron phosphate product of Example 1 of the present invention.

[0028] Figure 4 This is the EDS (fluorine element) image of the fluorine-doped lithium iron phosphate, the product of Example 1 of the present invention.

[0029] Figure 5 These are EIS graphs of fluorine-doped lithium iron phosphate and lithium iron phosphate, the products of Example 1 of the present invention. DETAILED DESCRIPTION

[0030] The following specific examples are intended to further illustrate the present invention, rather than to limit the scope of protection of the claims of the present invention.

[0031] The charge and discharge experiments of the assembled batteries were carried out using the Newware CT2001A tester produced by Shenzhen Newware Electronics Co., Ltd.

[0032] The assembled battery was tested by AC impedance spectroscopy using Gamry Instrument's Interface 1010E electrochemical workstation.

[0033] In the following specific examples, waste lithium iron phosphate positive electrode powder is prepared by the following method:

[0034] 1) Discharge and disassemble the waste lithium iron phosphate battery to separate the positive electrode sheet;

[0035] 2) Soaking the positive electrode sheet in hot water at 70° C. for 25 minutes to separate the spent lithium iron phosphate positive electrode material containing PVDF and the aluminum foil;

[0036] 3) The spent lithium iron phosphate cathode material containing PVDF was placed in an oven at 80° C. overnight and then ground into powder.

[0037] Example 1

[0038] (1) Weigh 2.0 g of waste lithium iron phosphate positive electrode powder, add 0.15 g of LiOH, and raise the temperature to 300°C at 5°C / min in an Ar atmosphere and keep it for 2 h; then raise the temperature to 750°C at 5°C / min and keep it for 3 h. Cool the furnace to obtain fluorine-doped lithium iron phosphate positive electrode material. The EDS image confirms that fluorine is doped in the lithium iron phosphate.

[0039] (2) Fluorine-doped lithium iron phosphate was used as the positive electrode material of the battery, mixed with conductive black (Super-P) and polyvinylidene fluoride (PVDF) in a ratio of 8:1:1, N-methylpyrrolidone (NMP) was added dropwise, ground into a slurry with a mortar, and the slurry was evenly applied to aluminum foil and dried in a vacuum oven for 12 hours. A button cell was assembled with aluminum foil as the positive electrode and lithium sheet as the negative electrode, and the performance of the battery was tested.

[0040] (3) The results show that the first discharge capacity of the fluorine-doped lithium iron phosphate cathode material at 0.1C reaches 146.3mAh / g, and the capacity after 50 cycles is 140.1mAh / g, and its impedance is reduced by nearly 25Ω. Therefore, the performance of the fluorine-doped lithium iron phosphate material has been improved.

[0041] Example 2

[0042] (1) Weigh 2.0 g of waste lithium iron phosphate positive electrode powder, add 0.10 g of LiOH, and in an Ar atmosphere, increase the temperature to 350°C at 5°C / min and keep it for 2 h; then increase the temperature to 800°C at 5°C / min and keep it for 3 h, and cool it in the furnace to obtain a fluorine-doped lithium iron phosphate positive electrode material.

[0043] (2) Fluorine-doped lithium iron phosphate was used as the positive electrode material of the battery, mixed with conductive black (Super-P) and polyvinylidene fluoride (PVDF) in a ratio of 8:1:1, N-methylpyrrolidone (NMP) was added dropwise, ground into a slurry with a mortar, and the slurry was evenly applied to aluminum foil and dried in a vacuum oven for 12 hours. A button cell was assembled with aluminum foil as the positive electrode and lithium sheet as the negative electrode, and the performance of the battery was tested.

[0044] (3) The results show that the first discharge capacity of the fluorine-doped lithium iron phosphate positive electrode material at 0.1C reaches 143.2mAh / g, and the capacity is 136.7mAh / g after 50 cycles, and its impedance is also reduced by nearly 25Ω, so the performance of the fluorine-doped lithium iron phosphate material is improved.

[0045] Example 3 (Comparative Example)

[0046] (1) Weigh 2.0 g of waste lithium iron phosphate positive electrode powder, add 0.04 g of LiOH and 0.06 g of Li2CO3, and raise the temperature to 300°C at 5°C / min in an Ar atmosphere and keep it for 2 h; then raise the temperature to 750°C at 5°C / min and keep it for 3 h. Then cool it in the furnace to obtain a fluorine-doped lithium iron phosphate positive electrode material.

[0047] (2) Fluorine-doped lithium iron phosphate was used as the positive electrode material of the battery, mixed with conductive black (Super-P) and polyvinylidene fluoride (PVDF) at a ratio of 8:1:1, N-methylpyrrolidone (NMP) was added dropwise, and ground into a slurry with a mortar. The slurry was evenly applied to aluminum foil and dried in a vacuum oven for 12 hours. A button cell was assembled with aluminum foil as the positive electrode and lithium sheet as the negative electrode, and the performance of the battery was tested.

[0048] (3) The results show that the impedance of the battery assembled with fluorine-doped lithium iron phosphate positive electrode material is nearly 30Ω greater than that of the battery assembled with pure lithium iron phosphate positive electrode material, indicating that trace amounts of fluorine doping will lead to poor performance of the prepared positive electrode material.

[0049] Example 4 (Comparative Example)

[0050] (1) Weigh 2.0 g of waste lithium iron phosphate positive electrode powder, add 0.10 g of Li2CO3, and raise the temperature to 300°C at 5°C / min in an Ar atmosphere and keep it for 2 h; then raise the temperature to 750°C at 5°C / min and keep it for 3 h, and cool it in the furnace to obtain a lithium iron phosphate positive electrode material.

[0051] (2) The lithium iron phosphate prepared above was used as the positive electrode material of the battery, and was mixed with conductive black (Super-P) and polyvinylidene fluoride (PVDF) at a ratio of 8:1:1. N-methylpyrrolidone (NMP) was added dropwise thereto, and the mixture was ground into a slurry using a mortar. The slurry was evenly applied to an aluminum foil and dried in a vacuum oven for 12 hours. A button cell was assembled with the aluminum foil as the positive electrode and the lithium sheet as the negative electrode, and the performance of the cell was tested.

[0052] (3) The results show that the XRD pattern of the lithium iron phosphate positive electrode material recycled and prepared by this method is the same as that of pure lithium iron phosphate, and the impedance of the button battery assembled with it is almost equal to that of the button battery assembled with pure lithium iron phosphate, indicating that PVDF does not degrade under this condition and no fluorine doping reaction occurs.

[0053] Example 5 (Comparative Example)

[0054] (1) Weigh 2.0 g of waste lithium iron phosphate positive electrode powder, add 0.10 g of LiOH, and directly increase the temperature to 750° C. at 5° C. / min in an Ar atmosphere, maintain for 3 h, and cool in the furnace to obtain a lithium iron phosphate positive electrode material.

[0055] (2) The lithium iron phosphate prepared above was used as the positive electrode material of the battery, and was mixed with conductive black (Super-P) and polyvinylidene fluoride (PVDF) at a ratio of 8:1:1. N-methylpyrrolidone (NMP) was added dropwise thereto, and the mixture was ground into a slurry using a mortar. The slurry was evenly applied to an aluminum foil and dried in a vacuum oven for 12 hours. A button cell was assembled with the aluminum foil as the positive electrode and the lithium sheet as the negative electrode, and the performance of the cell was tested.

[0056] (3) The results show that the XRD pattern of the lithium iron phosphate positive electrode material recycled and prepared by this method is the same as that of pure lithium iron phosphate, and the impedance of the button cell assembled with it is almost equal to that of the button cell assembled with pure lithium iron phosphate, indicating that no fluorine doping reaction occurs in this preparation process. This means that direct calcination at high temperature will make it difficult to convert PVDF into lithium fluoride to achieve fluorine doping of lithium iron phosphate.

Claims

1. A method for recovering PVDF-containing positive electrode materials from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate, characterized in that: The following steps are involved: 1) Discharge and disassemble the waste lithium iron phosphate battery to separate the positive electrode sheet; 2) Soaking the positive electrode sheet in hot water to separate the spent lithium iron phosphate positive electrode material containing PVDF and the aluminum foil; 3) The spent lithium iron phosphate positive electrode material containing PVDF is dried and ground into powder, and the powder is mixed with a lithium source including LiOH, and then calcined in a non-oxidizing atmosphere to obtain fluorine-doped lithium iron phosphate.

2. The method of recovering PVDF-containing positive electrode material from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate according to claim 1, characterized in that: The mass of the lithium source including LiOH accounts for 5-10% of the mass of the spent lithium iron phosphate positive electrode material powder containing PVDF, and the mass of the LiOH accounts for more than 50% of the mass of the lithium source.

3. The method of recovering PVDF-containing positive electrode material from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate according to claim 2, characterized in that: The lithium source also includes Li2CO 3、 At least one of LiAc and Li2O.

4. A method for recovering PVDF-containing positive electrode materials from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate according to claim 1, 2 or 3, characterized in that: The calcination conditions are: firstly heating to 300-400°C at a heating rate of 5-15°C / min, and keeping the temperature for 2-5h; then heating to 600-900°C at a heating rate of 5-15°C / min, and keeping the temperature for 3-15h; and then cooling with the furnace.

5. The method of recovering PVDF-containing positive electrode material from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate according to claim 1, characterized in that: The non-oxidizing atmosphere is an inert atmosphere and / or nitrogen.

6. The method of recovering PVDF-containing positive electrode material from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate according to claim 1, characterized in that: The soaking treatment conditions are: hot water temperature is 60-80° C., and the soaking time is 20-30 minutes.

7. The method of recovering PVDF-containing positive electrode material from waste lithium iron phosphate batteries and preparing fluorine-doped lithium iron phosphate according to claim 1, characterized in that: The drying conditions are: drying at a temperature of 70 to 90° C. for 8 to 24 hours.

Images