Method for removing PVDF and aluminum from lithium iron phosphate positive electrode material

Abstract

The present invention provides a method for removing PVDF and aluminum from a lithium iron phosphate positive electrode material, comprising the following steps: S1: performing mixing and ball-milling on lithium iron phosphate powder, a first ionic liquid, and ethanol, and performing solid-liquid separation to obtain PVDF-removed lithium iron phosphate powder; and S2: mixing and heating alcohol, a second ionic liquid, and the PVDF-removed lithium iron phosphate powder prepared in step S1, and separating alcohol and aluminum alkoxide to obtain PVDF-removed and aluminum-removed lithium iron phosphate powder, wherein the first ionic liquid and the second ionic liquid in steps S1 and S2 are independently selected from a fluorine-containing ionic liquid.

Description

A method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a method for removing PVDF and aluminum from a lithium iron phosphate positive electrode material. Background Art

[0002] Lithium-ion batteries, due to their advantages such as high operating voltage, long cycle life, and lack of memory effect, are widely used in pure and hybrid electric vehicles and various electronic devices. Lithium iron phosphate (LFP) batteries, as a simple and safe power source with high cycle performance and the absence of precious metals, hold a large market share in power batteries, and their demand is exploding in sectors such as new energy and base station energy storage. However, the lifespan of lithium-ion batteries is generally only 5 to 7 years, and a large amount of waste LFP batteries will be generated in the coming years. On the one hand, waste batteries contain a large number of valuable components such as iron, copper, aluminum, lithium, and phosphorus, and recycling these valuable components can prevent the waste of these resources. On the other hand, the organic electrolyte in the batteries can cause serious water and soil pollution. Therefore, the recycling of spent LFP batteries is imperative.

[0003] LFP battery recycling methods are generally categorized into three types: direct recycling, pyrometallurgy, and hydrometallurgy. Direct recycling offers the advantage of a shorter and more efficient recycling process compared to pyrometallurgy and hydrometallurgy. While a short and efficient process, direct recycling can be used to recover residual waste, it presents challenges when processing materials with high impurity content, or materials from different sources or types.

[0004] Removing PVDF from waste lithium iron phosphate cathode materials typically relies on high-temperature cracking of PVDF to convert it into hydrogen fluoride and fluorocarbon organic compounds. However, the hydrogen fluoride generated in this process corrodes the lithium iron phosphate material, transforming it into lithium fluoride, which degrades the material's performance. Furthermore, lithium fluoride, as an impurity, is extremely difficult to remove. Furthermore, alkaline solutions are commonly used to remove aluminum impurities from waste lithium iron phosphate cathode materials, but this method inevitably introduces sodium impurities and also damages the lithium iron phosphate material to a certain extent. Therefore, efficiently removing PVDF and aluminum impurities from waste lithium iron phosphate materials while maintaining the structural integrity of the lithium iron phosphate presents significant challenges.

[0005] Summary of the Invention

[0006] The present disclosure aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present disclosure proposes a method for removing PVDF and aluminum from a lithium iron phosphate positive electrode material.

[0007] According to a first aspect of the present disclosure, a method for removing PVDF and aluminum from a lithium iron phosphate cathode material is proposed, comprising the following steps:

[0008] S1: mixing lithium iron phosphate powder, a first ionic liquid, and ethanol, ball-milling the mixture, and performing solid-liquid separation to obtain lithium iron phosphate powder without PVDF;

[0009] S2: mixing the alcohol, the second ionic liquid and the lithium iron phosphate powder obtained in step S1 except for PVDF, and heating to separate the alcohol and aluminum alkoxide to obtain lithium iron phosphate powder except for PVDF and aluminum;

[0010] Wherein, in steps S1 and S2, the first ionic liquid and the second ionic liquid are independently selected from fluorine-containing ionic liquids.

[0011] In some embodiments, in step S1, the lithium iron phosphate powder is obtained by discharging, disassembling and crushing waste lithium iron phosphate batteries.

[0012] In some embodiments, in steps S1 and S2, the ionic liquid is at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (CAS: 174899-82-2), 1-ethyl-3-methylimidazolium tetrafluoroborate (CAS: 143314-16-3), 1-ethyl-3-methylimidazolium hexafluorophosphate (CAS: 155371-19-0), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (CAS: 235789-75-0), 1-ethyl-3-methylimidazolium trifluoroacetate (CAS: 174899-65-1), or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (CAS: 145022-44-2).

[0013] In some embodiments, in step S1, the mass of the ionic liquid is 5-20% of the mass of the lithium iron phosphate powder.

[0014] In some embodiments, in step S1, the liquid-to-solid ratio of the ethanol to the lithium iron phosphate powder is 2-5 mL / g.

[0015] In some embodiments, in step S1, the process parameters of the ball milling are: ball-to-material ratio of (4-10):1; rotation speed of 300-400 rpm; and time of 120-480 min.

[0016] In some embodiments, in step S1, the solid-liquid separation includes washing the ball-milled mixture and separating the washed liquid to obtain lithium iron phosphate powder without PVDF, wherein the washing liquid can be the same as the alcohol added during ball milling.

[0017] In some embodiments, in step S1, the lithium iron phosphate powder except the PVDF is further dried; the drying temperature is 50 to 80° C., and the drying time is 4 to 24 hours.

[0018] In some embodiments, in step S1, the liquid phase after solid-liquid separation is subjected to rotary evaporation, and the ionic liquid and PVDF are obtained after solid-liquid separation. The filtrate obtained after solid-liquid separation after washing the lithium iron phosphate mixture with ethanol is subjected to rotary evaporation to recover ethanol.

[0019] In some embodiments, the rotary evaporation temperature is 50-80° C., and the vacuum degree is about -0.1 MPa.

[0020] In some embodiments, in step S2, the alcohol is at least one of n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, or tert-butanol.

[0021] In some embodiments, in step S2, the liquid-to-solid ratio of the alcohol to the lithium iron phosphate mixture is 5-10 mL / g.

[0022] In some embodiments, in step S2, the mass of the second ionic liquid is 5-20% of the mass of the lithium iron phosphate mixture.

[0023] In some embodiments, in step S2, the heating separation of the alcohol and the aluminum alkoxide includes: first heating to form the aluminum alkoxide, and then heating again to separate the alcohol and the aluminum alkoxide in sequence. Under the heating conditions, the ionic liquid acts as a catalyst to react the aluminum and the alcohol to form the aluminum alkoxide.

[0024] In some embodiments, in step S2, the primary heating temperature is 80-120°C for 60-180 minutes, the secondary heating temperature is 80-180°C for 60-120 minutes. The secondary heating can evaporate aluminum alkoxides, thereby removing aluminum impurities in the lithium iron phosphate.

[0025] According to a second aspect of the present disclosure, a method for recycling lithium-ion batteries is proposed, comprising the steps of the method for removing PVDF and aluminum from the lithium iron phosphate positive electrode material described in the first aspect of the present disclosure.

[0026] According to the third aspect of the present disclosure, the application of the method for removing PVDF and aluminum from the lithium iron phosphate positive electrode material described in the first aspect of the present disclosure in the recycling of lithium ion batteries is proposed.

[0027] According to one embodiment of the present disclosure, there are at least the following beneficial effects:

[0028] (1) The present invention removes PVDF from lithium iron phosphate powder by adding a fluorine-containing ionic liquid to the lithium iron phosphate material to react with PVDF at a certain temperature to form hydrogen bonds. Simultaneously, the fluorine-containing ionic liquid acts as a catalyst to catalyze the reaction between aluminum and alcohol, converting the aluminum impurity into aluminum alkoxide and removing it from the lithium iron phosphate. This method is simple to operate and removes PVDF and aluminum impurities from the lithium iron phosphate cathode material in two steps. Furthermore, the removal of PVDF and aluminum ensures that the structure of the lithium iron phosphate is not destroyed and that no impurities are introduced.

[0029] (2) The present invention separates PVDF from the lithium iron phosphate cathode material by forming hydrogen bonds between the ionic liquid and PVDF, and the hydrogen bonds are easily broken under heating conditions, so that the ionic liquid and PVDF can be recovered and reused.

[0030] (3) The present invention effectively catalyzes the reaction between aluminum and alcohol by forming hydrogen bonds between ionic liquids and alcohols, so that aluminum impurities are converted into aluminum alkoxides for removal, and the alcohol used can also be recycled, forming an economical, green and environmentally friendly closed-loop recycling route.

[0031] (4) Since the disclosed process for removing PVDF and aluminum impurities and regenerating lithium iron phosphate is simple and easy to operate, it can be applied on a large scale in industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present disclosure is further described below with reference to the accompanying drawings and embodiments, wherein:

[0033] FIG1 is a flow chart of removing PVDF and aluminum impurities from the lithium iron phosphate positive electrode material and regenerating lithium iron phosphate in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the concept and technical effects of the present disclosure in conjunction with embodiments to fully understand the purpose, features and effects of the present disclosure.

[0035] Example 1

[0036] A method for removing PVDF and aluminum from a lithium iron phosphate cathode material, as shown in FIG1 , comprises the following steps:

[0037] (1) Discharging, disassembling and crushing the waste lithium iron phosphate batteries to obtain waste lithium iron phosphate positive electrode materials.

[0038] (2) The lithium iron phosphate powder obtained in step 1, 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide (5 wt% of the lithium iron phosphate powder), and ethanol (liquid-to-solid ratio = 2 mL / g) were placed in a ball mill, and zirconium balls with a ball-to-material ratio of 4:1 were added to the ball mill. The mixture was placed on a planetary ball mill and ball milled at 400 rpm for 120 min to obtain a lithium iron phosphate mixture.

[0039] (3) The lithium iron phosphate mixture obtained in step 2 was washed with ethanol having a mass ratio of 5 times that of lithium iron phosphate and then filtered, and the process was repeated 3 times.

[0040] (4) The filter residue obtained in step 3 was placed in an oven and dried at 50° C. for 24 h to obtain lithium iron phosphate powder after PVDF removal and regeneration.

[0041] (5) The filtrate obtained in step 3 was subjected to rotary evaporation at 50° C. to recover ethanol at a vacuum degree of −0.1 MPa to obtain a mixture of 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide and PVDF.

[0042] (6) The mixture obtained in step 5 is filtered to recover 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide and PVDF.

[0043] (7) The lithium iron phosphate powder obtained in step 4 with the PVDF removed, isopropyl alcohol (liquid-to-solid ratio = 5 mL / g), and 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide as a catalyst with a mass of 5 wt% of the lithium iron phosphate powder were added to a flask and heated under reflux at 85° C. in an oil bath for 1 h to convert the aluminum into aluminum isopropylate as completely as possible.

[0044] (8) After the aluminum is converted into aluminum isopropoxide, the mixture is heated at 90°C for 30 minutes to evaporate the isopropanol solvent and recover it. The temperature is then raised to 140°C and heated for 50 minutes to evaporate the aluminum isopropoxide to obtain a mixture of lithium iron phosphate powder and ionic liquid with the PVDF and aluminum removed.

[0045] (9) The mixture obtained in step 8 is filtered to obtain purified lithium iron phosphate powder and 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide is recovered.

[0046] A method for recycling lithium-ion batteries comprises the steps of the above-mentioned method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials.

[0047] Example 2

[0048] A method for removing PVDF and aluminum from a lithium iron phosphate cathode material comprises the following steps:

[0049] (1) Discharging, disassembling and crushing the waste lithium iron phosphate batteries to obtain waste lithium iron phosphate positive electrode materials.

[0050] (2) The lithium iron phosphate powder obtained in step 1, 1-ethyl-3-methylimidazolium tetrafluoroborate with a mass of 10 wt% of the lithium iron phosphate powder, and ethanol (liquid-to-solid ratio = 4 mL / g) were placed in a ball mill, and zirconium balls with a ball-to-material ratio of 6:1 were added to the ball mill. The ball mill was placed on a planetary ball mill and ball milled at 400 rpm for 240 min to obtain a lithium iron phosphate mixture.

[0051] (3) The lithium iron phosphate mixture obtained in step 2 was washed with ethanol having a mass ratio of 5 times that of lithium iron phosphate and then filtered, and the process was repeated 3 times.

[0052] (4) The filter residue obtained in step 3 was placed in an oven and dried at 60° C. for 12 h to obtain lithium iron phosphate powder after PVDF removal and regeneration.

[0053] (5) The filtrate obtained in step 3 was subjected to rotary evaporation at 70° C. with a vacuum degree of −0.1 MPa, and ethanol was recovered to obtain a mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and PVDF.

[0054] (6) The mixture obtained in step 5 is filtered to recover 1-ethyl-3-methylimidazolium tetrafluoroborate and PVDF.

[0055] (7) The lithium iron phosphate powder obtained in step 4 with the PVDF removed, tert-butanol (liquid-to-solid ratio = 8 mL / g), and 1-ethyl-3-methylimidazolium tetrafluoroborate as a catalyst with a mass of 10 wt% of the lithium iron phosphate powder were added to a flask and heated under reflux at 85° C. in an oil bath for 2 h to convert the aluminum into aluminum tert-butoxide as completely as possible.

[0056] (8) After aluminum is converted into aluminum tert-butoxide, continue heating at 85°C for 30 minutes to evaporate the tert-butanol solvent and recover it. Then, raise the temperature to 160°C and heat for 60 minutes to evaporate the aluminum tert-butoxide to obtain lithium iron phosphate powder without PVDF or aluminum.

[0057] (9) The mixture obtained in step 8 is filtered to obtain purified lithium iron phosphate powder and 1-ethyl-3-methylimidazolium tetrafluoroborate is recovered.

[0058] A method for recycling lithium-ion batteries comprises the steps of the above-mentioned method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials.

[0059] Example 3

[0060] A method for removing PVDF and aluminum from a lithium iron phosphate cathode material comprises the following steps:

[0061] (1) Discharging, disassembling and crushing the waste lithium iron phosphate batteries to obtain waste lithium iron phosphate positive electrode materials.

[0062] (2) The lithium iron phosphate powder obtained in step 1, 1-ethyl-3-methylimidazolium hexafluorophosphate with a mass of 15 wt% of the lithium iron phosphate powder, and ethanol (liquid-to-solid ratio = 5 mL / g) were placed in a ball mill, and zirconium balls with a ball-to-material ratio of 10:1 were added to the ball mill. The ball mill was placed on a planetary ball mill and ball milled at 400 rpm for 480 min to obtain a lithium iron phosphate mixture.

[0063] (3) The lithium iron phosphate mixture obtained in step 2 was washed with ethanol having a mass ratio of 5 times that of lithium iron phosphate and then filtered, and the process was repeated 3 times.

[0064] (4) The filter residue obtained in step 3 was placed in an oven and dried at 80° C. for 4 h to obtain lithium iron phosphate powder after PVDF removal and regeneration.

[0065] (5) The filtrate obtained in step 3 was subjected to rotary evaporation at 80° C. and a vacuum degree of −0.1 MPa to recover ethanol, thereby obtaining a mixture of 1-ethyl-3-methylimidazolium hexafluorophosphate and PVDF.

[0066] (6) The mixture obtained in step 5 is filtered to recover 1-ethyl-3-methylimidazolium hexafluorophosphate and PVDF.

[0067] (7) The lithium iron phosphate powder obtained in step 4 with the PVDF removed, n-propanol (liquid-to-solid ratio = 10 mL / g), and 1-ethyl-3-methylimidazolium hexafluorophosphate (15 wt % of the lithium iron phosphate powder) as a catalyst were added to a flask and heated under reflux at 100° C. in an oil bath for 3 h to convert the aluminum into aluminum n-propoxide as completely as possible.

[0068] (8) After the aluminum is converted into aluminum n-propoxide, the mixture is heated at 100° C. for 40 min to evaporate the n-propanol and recover it. The temperature is then raised to 170° C. and heated for 40 min to evaporate the aluminum n-propoxide to obtain lithium iron phosphate powder without PVDF or aluminum.

[0069] (9) The mixture obtained in step 8 is filtered to obtain purified lithium iron phosphate powder and 1-ethyl-3-methylimidazolium hexafluorophosphate is recovered.

[0070] A method for recycling lithium-ion batteries comprises the steps of the above-mentioned method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials.

[0071] Comparative Example 1

[0072] The only difference from Example 1 is that in step 2, 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide in an amount of 2 wt % of lithium iron phosphate powder is added.

[0073] Comparative Example 2

[0074] The only difference from Example 1 is that in step 7, the liquid-to-solid ratio of isopropyl alcohol to lithium iron phosphate powder except PVDF is 3 mL / g.

[0075] Comparative Example 3

[0076] The only difference from Example 1 is that the ionic liquid in step 2 and step 7 is replaced by 1-ethyl-3-methylimidazolium chloride.

[0077] Test example

[0078] Table 1 shows the test data for the Fe, Li, and Al contents (specific data obtained by ICP-AES testing), P content (specific data obtained by titration), and F content (specific data obtained by potentiometry testing) of waste lithium iron phosphate materials, lithium iron phosphate products prepared in Examples 1, 2, and 3, and Comparative Examples 1, 2, and 3. As can be seen from Table 1, the Fe, P, and Li contents in the lithium iron phosphate prepared in the Examples are normal, while the F and Al contents are effectively reduced, meeting the industrial production standards for lithium iron phosphate batteries. The Al content in the lithium iron phosphate prepared in Example 1 was reduced from 2.1% to 0.04%, and the F content was reduced from 0.2% to 0.002%. In Comparative Example 1, compared with Example 1, less 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide was added in step 2, and more PVDF remained in the prepared lithium iron phosphate powder, with an F content of 0.12%; in Comparative Example 2, compared with Example 1, less isopropanol was added in step 7, and more Al remained in the prepared lithium iron phosphate powder, with an Al content of 1.3%; in Comparative Example 3, compared with Example 1, a fluorine-free ionic liquid was used, and the F and Al contents in the prepared lithium iron phosphate powder were substantially unchanged, with an F content of 0.2% and an Al content of 2.08%.

[0079] Table 1 Element content test data table

[0080] Electrochemical performance:

[0081] Table 2 shows the electrochemical performance of lithium iron phosphate batteries prepared from Examples 1, 2, and 3 using raw materials. The specific data were obtained using an electrochemical workstation and other equipment. As shown in Table 2, the electrochemical performance of the lithium iron phosphate products obtained in Examples after PVDF repair and regeneration was improved compared to the raw materials. In Example 1, the charge capacity reached 158.6 mAh / g, and the discharge capacity reached 152.6 mAh / g. The calculated initial charge-discharge efficiency = discharge capacity / charge capacity = 96.2%.

[0082] Table 2 Electrochemical test data

Claims

1. A method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials, characterized in that: The following steps are involved: S1: mixing lithium iron phosphate powder, the first ionic liquid and ethanol and ball milling them, and performing solid-liquid separation to obtain lithium iron phosphate powder except for PVDF; S2: mixing and heating the alcohol, the second ionic liquid and the lithium iron phosphate powder except PVDF obtained in step S1, separating the alcohol and aluminum alkoxide, and obtaining lithium iron phosphate powder except PVDF and aluminum; Wherein, in steps S1 and S2, the first ionic liquid and the second ionic liquid are independently selected from fluorine-containing ionic liquids.

2. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In step S1, the lithium iron phosphate powder is obtained by discharging, disassembling and crushing waste lithium iron phosphate batteries.

3. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In steps S1 and S2, the first ionic liquid and the second ionic liquid are independently selected from at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1-ethyl-3-methylimidazolium trifluoroacetate or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate.

4. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In step S1, the mass of the first ionic liquid is 5-20% of the mass of the lithium iron phosphate powder.

5. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1 or 4, characterized in that: In step S1, the liquid-to-solid ratio of the ethanol to the lithium iron phosphate powder is 2-5 mL / g.

6. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In step S1, the process parameters of ball milling are: ball-to-material ratio is (4-10):1; rotation speed is 300-400 rpm; and time is 120-480 min.

7. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In step S1, the solid-liquid separation includes: washing the ball-milled mixture, separating the washed liquid, and obtaining lithium iron phosphate powder except PVDF.

8. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1 or 7, characterized in that: In step S1, the liquid phase after solid-liquid separation is subjected to rotary evaporation, and the ionic liquid and PVDF are obtained after solid-liquid separation again.

9. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In step S2, the alcohol is at least one of n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol or tert-butanol.

10. The method for removing PVDF and aluminum from lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S2, the liquid-to-solid ratio of the alcohol to the lithium iron phosphate powder except for PVDF is 5-10 mL / g.

11. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1 or 10, characterized in that: In step S2, the mass of the second ionic liquid is 5-20% of the mass of the lithium iron phosphate powder except for PVDF.

12. The method for removing PVDF and aluminum from lithium iron phosphate cathode material according to claim 1, characterized in that: In step S2, the heating to separate alcohol and aluminum alkoxide includes: firstly performing a heating to generate aluminum alkoxide, and then performing a second heating to sequentially separate alcohol and aluminum alkoxide.

13. The method for removing PVDF and aluminum from lithium iron phosphate positive electrode material according to claim 12, characterized in that: The temperature of the first heating is 80-120° C.; the time of the first heating is 60-180 min; the temperature of the second heating is 80-180° C.; the time of the second heating is 60-120 min.

14. A method for recycling lithium-ion batteries, comprising the steps of the method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials as described in any one of claims 1-13.

15. Use of the method for removing PVDF and aluminum from lithium iron phosphate positive electrode materials according to any one of claims 1 to 13 in lithium ion battery recycling.

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