A method for surface impurity removal and regeneration of the cathode material of waste lithium-ion batteries

Abstract

The present invention discloses a method for surface impurity removal and regeneration of the positive electrode material of waste lithium-ion batteries, which relates to the technical field of battery regeneration and aims to solve problems such as excessive defects in the existing lithium-ion battery recycling methods. It includes pre-treating waste lithium-ion batteries to obtain a positive electrode sheet containing a current collector; assembling the positive electrode sheet containing the current collector into an electrolytic cell as a working electrode, applying a voltage to carry out an electrolytic reaction using the electrolytic cell, and during the electrolytic reaction, the positive electrode sheet containing the current collector undergoes surface reconstruction to obtain a reconstructed positive electrode sheet; removing polyvinylidene fluoride on the reconstructed positive electrode sheet to obtain a surface impurity-removed positive electrode material. The present invention uses an electrochemical method to solve the impurity problem on the surface of actual waste lithium-ion batteries and promotes the adaptation of the direct recycling method to the industrial recycling goal of actual waste lithium-ion batteries.

Description

Technical Field

[0001] The present invention relates to a method for surface impurity removal and regeneration of waste lithium-ion battery cathode materials, belonging to the technical field of battery regeneration. Background Art

[0002] Currently, the main battery recycling methods include pyrometallurgical recycling, hydrometallurgical recycling, and direct recycling.

[0003] Regarding pyrometallurgical recycling, although the reaction temperature of recycling has been continuously improved, the pyrometallurgical recycling process fundamentally relies on material decomposition and high-temperature melting of elements to form alloys. After pyrometallurgical recycling, mixed slag (usually containing Li, Al, Ca, and other elements) will be generated. Therefore, this process cannot achieve complete recovery of metal elements, ultimately resulting in inevitable lithium loss. To avoid waste of important lithium elements, an acid leaching step for the slag is added to separate metal elements and extract them step by step, but this obviously increases the process steps of the entire recycling process. High electricity and natural gas, etc. are required as inputs throughout the process to provide high-temperature conditions. The input of these components represents an increase in the carbon footprint, and graphite and additional carbon sources need to be added to act as reducing agents to improve efficiency, resulting in a cumbersome and energy-consuming pyrometallurgical recycling process.

[0004] Regarding hydrometallurgical recycling, firstly, this process usually requires the use of a large amount of chemical reagents, which not only increases the cost but also may cause environmental pollution problems. Secondly, hydrometallurgical recycling has high energy consumption, and the waste liquid and waste gas generated during the process need to be treated additionally, further increasing the environmental protection pressure. In addition, this method has high requirements for equipment, large investment costs, and there may be problems such as low recovery rate and difficulty in completely removing impurities when extracting valuable metals, affecting the purity and quality of the final product. Therefore, although hydrometallurgical recycling is technically relatively mature, its economy and environmental protection still face challenges and need to be further optimized and improved.

[0005] Regarding the direct recycling method, for actual waste lithium-ion battery cathodes, the impurities on the surface after cycling will greatly affect the effect of direct recycling. During the solid-phase regeneration process, fluoride impurities on the surface will cause fluorine elements to be doped into the cathode material, having an unpredictable impact on the cathode material. Existing research has doped LCO (lithium cobalt oxide) with ammonium fluoride as a fluorinating agent and found that 0.2 - 0.8% of subsurface fluorine doping is beneficial to the lithium-ion diffusion of the cathode material, but doping above 1% will result in insufficient lithium-ion supplementation, leading to the appearance of a lithium-deficient spinel phase. Because the actual residual fluorine impurity content of waste lithium-ion batteries cannot be predicted, direct recycling of waste lithium-ion batteries will cause unpredictable effects.

[0006] Therefore, to solve the problem of such large differences in regeneration in the direct recycling method and to remove the impurities generated during the recycling of these waste lithium-ion batteries and regenerate a normalized cathode material, it is very necessary for the sensitive direct recycling process. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies in the prior art and provide a method for surface impurity removal and regeneration of the cathode material of waste lithium-ion batteries, using an electrochemical method to solve the impurity problem on the surface of actual waste lithium-ion batteries and promoting the direct recycling method to meet the industrial recycling goal of actual waste lithium-ion batteries.

[0008] To achieve the above object, the present invention is implemented by the following technical solutions:

[0009] On the one hand, the present invention provides a method for surface impurity removal of the cathode material of waste lithium-ion batteries, including:

[0010] Pre-treat the waste lithium-ion battery to obtain a cathode pole piece containing a current collector;

[0011] Assemble the cathode pole piece containing the current collector as a working electrode into an electrolytic cell, apply a voltage and use the electrolytic cell to carry out an electrolysis reaction. During the electrolysis reaction, the surface of the cathode pole piece containing the current collector is reconstructed to obtain a reconstructed cathode pole piece;

[0012] Remove the polyvinylidene fluoride on the reconstructed cathode pole piece to obtain a cathode material with surface impurity removal.

[0013] Further, the waste lithium-ion battery is one or more of a lithium cobalt oxide battery, a lithium iron phosphate battery, a lithium manganese oxide battery, a ternary nickel cobalt manganese oxide battery, a ternary nickel cobalt aluminum oxide battery, and a quaternary nickel cobalt manganese aluminum battery;

[0014] The surface impurities of the waste lithium-ion battery include lithium fluoride and transition metal fluorides, and the transition metal oxides include one or more of cobalt fluoride, nickel fluoride, manganese fluoride, iron fluoride, and aluminum fluoride.

[0015] Further, the pre-treatment of the waste lithium-ion battery to obtain a cathode pole piece containing a current collector includes:

[0016] Immerse the waste lithium-ion battery in a salt solution for discharging until the discharge termination voltage is 0.5 - 1.5V;

[0017] Disassemble the discharged waste lithium-ion battery and cut its cathode to obtain a cathode pole piece containing a current collector;

[0018] Among them, the salt solution uses a sodium chloride solution with a concentration range of 4 - 6wt%.

[0019] Further, the counter electrode in the electrolytic cell is one of carbon, graphite, stainless steel, titanium, platinum, gold, silver, lead, conductive glass, etc., and the electrolyte solution is one of electrolyte solutions without alkali metal ions.

[0020] Further, the electrolyte solution is an ammonium salt solution, and the ammonium salt solution is one or more of ammonium salts with strong acid root ions (such as ammonium chloride, ammonium nitrate, ammonium sulfate, etc.), ammonium salts with weak acid root ions (such as ammonium bicarbonate, etc.), and ammonium salts with organic acid root ions (such as ammonium formate, ammonium acetate, ammonium citrate, etc.).

[0021] Further, the voltage is greater than 1V vs. RHE.

[0022] Further, it also includes adding a precipitant to the electrolyte solution after the electrolysis reaction to obtain a lithium salt precipitate, and the precipitant is a carbonate (such as sodium carbonate, ammonium carbonate, etc.);

[0023] Further, the method for removing impurities on the surface of the positive electrode plate after reconstruction to obtain the positive electrode material with surface impurities removed includes: mechanically pulverizing the reconstructed positive electrode plate, collecting the positive electrode material powder, or placing it in an organic solvent, washing, drying, and then obtaining the positive electrode material with surface impurities removed;

[0024] Among them, the organic solvent is one of N,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide, and dimethyl sulfoxide.

[0025] On the other hand, the present invention also provides a method for regenerating the positive electrode material of a waste lithium-ion battery, which includes the method for removing surface impurities of the positive electrode material of a waste lithium-ion battery described in any one of the above, and regenerating the positive electrode material with surface impurities removed to obtain a regenerated positive electrode material, and the regeneration adopts a solid-phase regeneration method or a hydrothermal solid-phase regeneration method;

[0026] The solid-phase regeneration method includes:

[0027] Mixing the positive electrode material with surface impurities removed with a lithium source, grinding evenly, and then calcining to obtain a regenerated positive electrode material;

[0028] The hydrothermal solid-phase regeneration method includes:

[0029] Putting the positive electrode material with surface impurities removed into an aqueous solution of lithium hydroxide for hydrothermal treatment, washing until neutral, and then drying to obtain the positive electrode material after hydrothermal treatment;

[0030] Mixing the positive electrode material after hydrothermal treatment with a lithium source, grinding until uniform, and then calcining to obtain a regenerated positive electrode material;

[0031] Among them, the lithium source is lithium hydroxide or lithium carbonate.

[0032] Further, the concentration of the lithium hydroxide aqueous solution is 4 mol / L; and / or, the hydrothermal treatment includes hydrothermal treatment at 160-220°C for a certain period of time and / or, the calcination includes calcination at 700-1000°C for 6-24 h.

[0033] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0034] The present invention provides an adaptive direct recycling strategy for waste lithium-ion batteries, which induces surface reconstruction of the cathode material at a weak oxygen evolution potential, promotes the self-removal of fluorine impurities, and avoids the phenomenon of uneven batch regeneration effects caused by impurity-induced fluorine doping in traditional direct recycling methods and the various performance impacts of impurities on the recycled cathode material.

[0035] During the recycling process of the present invention, there is no need to use strong acids, strong alkalis, and a large amount of water. The operation process is simple, and it is applicable to batch recycling and regeneration among different batches of the same manufacturer and different manufacturers, and can achieve uniform regenerated electrochemical performance, promoting the industrialization process of direct recycling. At the same time, due to the complete removal of fluorine elements, fluorine elements can be completely enriched and precipitated in a short time, avoiding the damage of fluorine to water bodies during industrial recycling, and realizing the sustainable recycling of waste lithium-ion batteries.

[0036] The present invention can avoid, to the greatest extent, the occurrence of surface fluorine impurity residues and bulk fluorine doping phenomena on the cathode material caused by the electrolyte remaining on the surface of the waste electrode material and the PVDF not completely removed in the process steps, and avoid the poor regeneration effect caused by impurities and the phenomenon that the spinel impurity phase cannot be completely repaired. Brief Description of the Drawings

[0037] Figure 1 It is a schematic diagram showing the change of the leaching rate of fluorine element in the surface impurities of waste lithium cobaltate with time in Example 1 of the present invention;

[0038] Figure 2 It is a SEM schematic diagram of the waste lithium cobaltate after surface reconstruction-induced impurity removal in Example 1 of the present invention;

[0039] Figure 3 It is a SEM schematic diagram of the waste lithium cobaltate after hydrothermal solid-phase regeneration in Example 1 of the present invention;

[0040] Figure 4 It is a TEM schematic diagram of the waste lithium cobaltate after hydrothermal solid-phase regeneration in Example 1 of the present invention;

[0041] Figure 5 It is a schematic diagram comparing the charge-discharge curves of the waste lithium cobaltate after hydrothermal solid-phase regeneration in Example 1 of the present invention with commercial lithium cobaltate;

[0042] Figure 6Schematic diagram of the change in the leaching rate of fluorine element in the surface impurities of waste lithium manganese oxide over time in Example 2 of the present invention;

[0043] Figure 7 SEM schematic diagram of the waste lithium manganese oxide after surface reconstruction-induced impurity removal in Example 2 of the present invention;

[0044] Figure 8 Schematic diagram of the comparison of charge-discharge curves between the waste lithium manganese oxide after solid-phase regeneration and commercial lithium manganese oxide in Example 2 of the present invention;

[0045] Figure 9 Schematic diagram of the change in the leaching rate of fluorine element in the surface impurities of waste lithium iron phosphate over time in Example 3 of the present invention;

[0046] Figure 10 SEM schematic diagram of the waste lithium iron phosphate after surface reconstruction-induced impurity removal in Example 3 of the present invention;

[0047] Figure 11 Schematic diagram of the comparison of charge-discharge curves between the waste lithium iron phosphate after solid-phase regeneration and commercial lithium iron phosphate in Example 3 of the present invention;

[0048] Figure 12 Schematic diagram of the change in the leaching rate of fluorine element in the surface impurities of waste NMC811 over time in Example 4 of the present invention;

[0049] Figure 13 SEM schematic diagram of the waste NMC811 after surface reconstruction-induced impurity removal in Example 4 of the present invention;

[0050] Figure 14 Schematic diagram of the comparison of charge-discharge curves between the waste NMC811 after solid-phase regeneration and commercial NMC811 in Example 4 of the present invention;

[0051] Figure 15 Schematic diagram of the change in the leaching rate of fluorine element in the surface impurities of waste NMC532 over time in Example 5 of the present invention;

[0052] Figure 16 SEM schematic diagram of the waste NMC532 after surface reconstruction-induced impurity removal in Example 5 of the present invention;

[0053] Figure 17 Schematic diagram of the comparison of charge-discharge curves between the waste NMC532 after solid-phase regeneration and commercial NMC532 in Example 5 of the present invention;

[0054] Figure 18 Schematic diagram of the comparison of fluorine content in the electrode materials obtained by the regeneration methods of Example 1 and Comparative Example 1 of the present invention;

[0055] Figure 19 Schematic diagram of the comparative regeneration electrochemistry performance of the electrode materials obtained by the regeneration methods of Example 1 and Comparative Example 1 of the present invention with electrode materials of different batches from the same manufacturer and different manufacturers;

[0056] Figure 20 Schematic diagram of the process comparison between the surface impurity removal and regeneration method of the spent lithium-ion battery cathode material in an embodiment of the present invention and the traditional direct recycling method. Detailed implementation manners

[0057] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention. Example 1

[0058] The embodiment of the present invention provides a method for removing impurities on the surface of spent lithium cobalt oxide batteries and regeneration through surface reconstruction, which specifically includes:

[0059] (1) Immerse the spent lithium cobalt oxide battery in a 5wt% NaCl solution until the discharge cut-off voltage is 1V, and disassemble it to obtain a lithium cobalt oxide positive electrode sheet containing a current collector;

[0060] (2) Use the lithium cobalt oxide positive electrode sheet as the working electrode and a stainless steel mesh as the counter electrode. Add 50 mL of 0.1 mol / L ammonium sulfate aqueous solution to the electrolytic cell to assemble the electrolytic cell, apply a voltage of 1.3V vs. RHE to the electrolytic cell, and react at room temperature for 5h.

[0061] (3) After the electrolysis reaction, take out the lithium cobalt oxide positive electrode sheet, place it in an N-methylpyrrolidone (NMP) solution and ultrasonicate it at 60°C for 6h, collect the bottom powder, wash it by centrifugation and then dry it;

[0062] (4) Place the powder obtained in step (3) in a hydrothermal reaction kettle, add 4 mol / L lithium hydroxide solution, react at 200°C for 6h, collect the bottom powder, centrifuge and wash it until neutral, and then dry it.

[0063] (5) Take a certain amount of the powder obtained in step (4) in a mortar, add 5% by mass of lithium hydroxide and grind it evenly, and place it in a muffle furnace and calcine it at 900°C for 6h.

[0064] Characterize the leaching amount of fluorine of the impurities on the surface of the lithium cobalt oxide positive electrode sheet by ion chromatography, characterize the reconstruction phenomenon of the surface impurities by SEM, and conduct scanning electron microscopy (SEM), transmission electron microscopy (TEM) characterization and electrochemical performance testing on the regenerated positive electrode material.

[0065] The fluorine leaching efficiency curve of the impurities of the lithium cobalt oxide positive electrode sheet electrolyzed in ammonium sulfate solution for 5h is asFigure 1 As shown, it shows that after 5 h of reaction, the leaching amount of fluoride ions reaches the maximum, and at this time, fluorine has been completely leached out.

[0066] During the reaction, the SEM of the reconstructed lithium cobaltate is as Figure 2 shown, indicating the formation of flaky hydroxides on its surface, demonstrating the occurrence of the reconstruction phenomenon.

[0067] The SEM of the positive electrode material regenerated after the reaction is as Figure 3 shown, proving that the surface of the regenerated positive electrode material is smooth and there is no residual impurity phase.

[0068] The TEM of the positive electrode material regenerated after the reaction is as Figure 4 shown, proving that the regenerated positive electrode material has good crystallinity of the layered peak, and the lattice fringes are regular without the presence of impurity phases.

[0069] Taking the regenerated positive electrode material and the commercially available commercial lithium cobaltate as the positive electrode materials respectively, lithium metal as the negative electrode, and selecting a carbonate electrolyte with a concentration of 1 M as the electrolyte, the solvent is ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) with a mass ratio of 1:1:1, and the solute is LiPF6. A button-type lithium-ion battery is assembled in a glove box filled with an argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm). A Wuhan Blue Electric battery test system is used for constant current charge and discharge. The voltage range for charge and discharge is 3.0 - 4.2 V, and the test temperature is a constant temperature of 25 °C. The mass of the active material of the battery used for calculating the specific capacity is calculated based on the mass of the lithium cobaltate positive electrode material. The charge-discharge curve of the regenerated material coincides with that of the commercial positive electrode material as Figure 5 shown, proving its successful regeneration and avoiding the influence of impurities on the electrochemical performance. Example 2

[0070] The embodiment of the present invention provides a method for removing impurities on the surface of waste lithium manganese oxide batteries and regeneration through surface reconstruction, specifically including:

[0071] (1) Immerse the waste lithium manganese oxide battery in a 5 wt% NaCl solution until the discharge cut-off voltage is 1 V, and disassemble it to obtain a lithium manganese oxide positive electrode sheet containing a current collector.

[0072] (2) Using the lithium manganese oxide positive electrode sheet as the working electrode and a stainless steel mesh as the counter electrode, add 50 mL of 0.1 mol / L ammonium sulfate aqueous solution to the electrolytic cell to assemble an electrolytic cell, apply a voltage of 1.3 V vs. RHE to the electrolytic cell, and react at room temperature for 5 h.

[0073] (3) After the electrolytic reaction, take out the lithium manganese oxide positive electrode sheet, place it in an NMP solution, ultrasonicate it at 60 °C for 6 h, collect the bottom powder, wash, centrifuge and dry it;

[0074] (4) Take a certain amount of the powder obtained in step (3) and mix it with lithium hydroxide in an equimolar ratio in a mortar. Additionally, add 10% by mass of lithium hydroxide and grind and mix evenly. Place it in a muffle furnace and calcine at 800 °C for 6 h.

[0075] The leaching amount of fluorine in the surface impurities of lithium manganate is characterized by ion chromatography, the reconstruction phenomenon of the surface impurities is characterized by SEM, and the electrochemical performance of the regenerated cathode material is tested.

[0076] The fluorine leaching efficiency curve of the impurities in the lithium manganate cathode electrolyzed in ammonium sulfate solution for 5 h is as Figure 6 shown, indicating that after 5 h of reaction, the leaching amount of fluoride ions reaches the maximum, and at this time, fluorine has been completely leached.

[0077] The SEM of the reconstructed cathode material during the reaction is as Figure 7 shown, proving that the cathode material has been reconstructed into small particles and there are no impurities on the surface.

[0078] Take the regenerated cathode material and commercially available lithium manganate as the cathode materials respectively, lithium metal as the anode, and select a carbonate-based electrolyte with a concentration of 1 M. The solvent is ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) with a mass ratio of 1:1:1, and the solute is LiPF6. Assemble them into a button-type lithium-ion battery in a glove box filled with argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm). Use the Wuhan Blue Electric battery test system for constant current charge and discharge. The charge and discharge voltage range is 3.5 - 4.3 V, and the test temperature is a constant temperature of 25 °C. The mass of the active material of the battery used for calculating the specific capacity is calculated based on the mass of the lithium manganate cathode material. The charge and discharge curves of the regenerated material coincide with those of the commercial cathode material as Figure 8 shown, proving its successful regeneration and avoiding the influence of impurities on the electrochemical performance. Example 3

[0079] The embodiment of the present invention provides a method for removing surface impurities and regenerating waste lithium iron phosphate lithium-ion batteries through surface reconstruction, specifically including:

[0080] (1) Immerse the waste lithium iron phosphate lithium-ion battery in a 5 wt% NaCl solution until the discharge cut-off voltage is 1 V, and disassemble it to obtain a lithium iron phosphate cathode electrode sheet containing a current collector;

[0081] (2) Take the lithium iron phosphate cathode electrode sheet as the working electrode, a stainless steel mesh as the counter electrode, add 50 mL of 0.1 mol / L ammonium sulfate aqueous solution to the electrolytic cell to assemble an electrolytic cell, apply a voltage of 1.3 V vs. RHE to the electrolytic cell, and react at room temperature for 5 h.

[0082] After the electrolysis reaction, the lithium iron phosphate positive electrode plate is taken out, placed in an NMP solution, and ultrasonically treated at 60 °C for 6 h. The powder at the bottom is collected, washed, centrifuged, and then dried.

[0083] (4) Take a certain amount of the powder obtained in step (3) and mix it with lithium hydroxide in an equimolar ratio in a mortar. Additionally, add 10% by mass of lithium hydroxide and 10% by mass of glucose, and grind and mix them evenly. Then place it in a tube furnace and calcine it at 700 °C for 10 h under an argon atmosphere.

[0084] The leaching amount of fluorine, which is a surface impurity of lithium iron phosphate, is characterized by ion chromatography. The reconstruction phenomenon of the surface impurities is characterized by SEM. The electrochemical performance of the regenerated positive electrode material is tested.

[0085] The fluorine leaching efficiency curve of the impurities in the lithium iron phosphate positive electrode during electrolysis in an ammonium sulfate solution for 5 h is as Figure 9 shown, indicating that after 5 h of reaction, the leaching amount of fluoride ions reaches the maximum, and at this time, fluorine has been completely leached.

[0086] The SEM of the reconstructed lithium iron phosphate during the reaction is as Figure 10 shown, indicating that there are broken particles on its surface, demonstrating the occurrence of the reconstruction phenomenon.

[0087] Take the regenerated positive electrode material and commercially available lithium iron phosphate as the positive electrode materials respectively, lithium metal as the negative electrode, and select a carbonate-based electrolyte with a concentration of 1 M as the electrolyte. The solvent is ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) with a mass ratio of 1:1:1, and the solute is LiPF6. Assemble them into a button-type lithium-ion battery in a glove box filled with an argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm). Use a Wuhan Blue Electric battery test system for constant current charge and discharge. The charge and discharge voltage range is 2.5 - 4 V, and the test temperature is a constant temperature of 25 °C. When calculating the specific capacity, the mass of the active material of the battery used is calculated based on the mass of the lithium iron phosphate positive electrode material. The charge and discharge curves of the regenerated material coincide with those of the commercial positive electrode material as Figure 11 shown, proving its successful regeneration and avoiding the influence of impurities on the electrochemical performance. Example 4

[0088] Taking the recycling of NMC811 batteries and NMC532 batteries as examples for ternary and quaternary lithium-ion batteries, this example provides a method for removing surface impurities and regenerating waste NMC811 batteries through surface reconstruction, specifically including:

[0089] (1) Immerse the waste NMC811 battery in a 5wt% NaCl solution until the discharge cut-off voltage reaches 1V, and disassemble it to obtain the NMC811 positive electrode sheet containing the current collector.

[0090] (2) Use the NMC811 positive electrode sheet as the working electrode and a stainless steel mesh as the counter electrode. Add 50 mL of 0.1 mol / L ammonium sulfate aqueous solution to the electrolytic cell to assemble an electrolytic cell. Apply a voltage of 1.3V vs. RHE to the electrolytic cell and react at room temperature for 5h.

[0091] (3) After the electrolysis reaction, take out the NMC811 positive electrode sheet, place it in an NMP solution, and ultrasonicate it at 60°C for 6h. Collect the powder at the bottom, wash, centrifuge, and then dry it.

[0092] (4) Take a certain amount of the powder obtained in step (3) and mix it with lithium hydroxide in an equimolar ratio in a mortar. Additionally, add 5% by mass of lithium hydroxide and grind it evenly. Place it in a tubular furnace and calcine it at 400°C for 4h in an oxygen atmosphere, and then roast it at 800°C for 6h.

[0093] Characterize the leaching amount of fluorine in the surface impurities of NMC811 by ion chromatography, characterize the reconstruction phenomenon of the surface impurities by SEM, and test the electrochemical performance of the regenerated positive electrode material.

[0094] The fluorine leaching efficiency curve of the impurities in the NMC811 positive electrode during electrolysis in ammonium sulfate solution for 10h is as Figure 12 shown, indicating that after 10h of reaction, the leaching amount of fluoride ions reaches the maximum, and at this time, fluorine has been completely leached. The SEM of the reconstructed NMC811 during the reaction process is as Figure 13 shown, indicating the formation of small particles on its surface, demonstrating the occurrence of the reconstruction phenomenon.

[0095] Use the regenerated positive electrode material and commercially available NMC811 as the positive electrode materials respectively, lithium metal as the negative electrode, and select a carbonate-based electrolyte with a concentration of 1 M. The solvent is ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) with a mass ratio of 1:1:1, and the solute is LiPF6. Assemble it into a button-type lithium-ion battery in a glove box filled with argon gas (O2 < 0.01 ppm, H2O < 0.01 ppm). Use a Wuhan Blue Electric battery test system for constant current charge and discharge. The charge and discharge voltage range is 3.0 - 4.2 V, and the test temperature is a constant temperature of 25°C. When calculating the specific capacity, the mass of the active material of the battery used is calculated based on the mass of the nickel-cobalt-manganese ternary positive electrode material. The charge and discharge curves of the regenerated material coincide with those of the commercial positive electrode material as Figure 14 , proving its successful regeneration and avoiding the influence of impurities on the electrochemical performance. Example 5

[0096] This embodiment provides a method for removing surface impurities and regenerating waste NMC532 batteries through surface reconstruction, specifically including:

[0097] (1) Immerse the waste NMC532 battery in a 5wt% NaCl solution until the discharge termination voltage is 1V, and disassemble it to obtain the NMC532 positive electrode sheet containing the current collector;

[0098] (2) Place the NMC532 positive electrode sheet in an electrolytic cell as the working electrode, use a stainless steel mesh as the counter electrode, add 50 mL of 0.1 mol / L ammonium sulfate aqueous solution to the electrolytic cell to assemble the electrolytic cell, apply a voltage of 1.3V vs. RHE to the electrolytic cell, and react at room temperature for 5h.

[0099] (3) After the electrolytic reaction, take out the NMC532 positive electrode sheet, place it in an NMP solution and ultrasonicate it at 60°C for 6h, collect the bottom powder, wash, centrifuge and then dry it.

[0100] (4) Mix a certain amount of the powder obtained in step (3) with lithium hydroxide in an equimolar ratio in a mortar, and additionally add 5% by mass of lithium hydroxide and grind it evenly. Place it in a tubular furnace and calcine it at 400°C for 4h in an oxygen atmosphere, and then roast it at 800°C for 6h in an oxygen atmosphere.

[0101] The leaching amount of fluorine in the surface impurities of NMC532 is characterized by ion chromatography, the reconstruction phenomenon of the surface impurities is characterized by SEM, and the electrochemical performance of the regenerated positive electrode material is tested.

[0102] The fluorine leaching efficiency curve of the impurities in the NMC532 positive electrode during electrolysis in ammonium sulfate solution for 10h is as Figure 15 shown, indicating that after reacting for 10h, the leaching amount of fluoride ions reaches the maximum, and at this time, fluorine has been completely leached.

[0103] The SEM of the reconstructed NMC532 during the reaction process is as Figure 16As shown, it shows that a large number of small particles are generated on its surface, indicating the occurrence of reconstruction phenomenon; the regenerated positive electrode material and the commercial NMC532 are used as positive electrode materials, lithium metal is used as the negative electrode, the electrolyte is a carbonate electrolyte with a concentration of 1 M, the solvent is ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) with a mass ratio of 1:1:1, and the solute is LiPF6. It is assembled into a button-type lithium-ion battery in a glove box filled with argon atmosphere (O2 <0.01 ppm, H2O <0.01 ppm). The Wuhan Blue Electric Battery Test System is used for constant current charging and discharging. The voltage range of charging and discharging is 3.0 ~4.2 V, and the test temperature is a constant temperature of 25 °C. The mass of active materials of the battery used to calculate the specific capacity is calculated based on the mass of the nickel-cobalt-manganese ternary positive electrode material. The charge and discharge curves of the recycled material are consistent with the charge and discharge curves of the commercial positive electrode material. Figure 17 , proving its successful regeneration and avoiding the influence of impurities on electrochemical performance. Comparative Example 1

[0104] In order to compare the fluorine doping phenomenon produced by the conventional direct regeneration method, the difference between Comparative Example 1 and Example 1 is only that the electrolysis step of step (2) is omitted.

[0105] The hydrothermal solid phase regeneration powder of this comparative example 1 and the hydrothermal solid phase regeneration powder of Example 1 were leached with hydrochloric acid at 50°C for 12 hours and then subjected to ion chromatography test. The results are as follows: Figure 18 As shown, in this comparative example 1, a fluorine element response is clearly observed, while in Example 1, no fluorine element response is observed, which proves that no fluorine doping occurs in Example 1.

[0106] In order to compare the batch regeneration effects of different recycling methods for waste lithium-ion batteries from different manufacturers, the batteries obtained by the direct recycling method of this comparative example 1 and the batteries obtained by the recycling method of Example 1 were compared in multiple batches for electrochemical performance. Figure 19 As shown, Figure 19 In order to compare the capacities of multiple batches of regenerated batteries, three batches of batteries from the same manufacturer with different production times were selected for regeneration comparison with batteries from four different manufacturers. It can be clearly shown that in the presence of fluorine impurities, the traditional method of comparative example 1 will greatly lead to poor performance or uneven performance of the regenerated batteries, while Example 1 is close to the standard capacity and has good batch-to-batch variability. In view of the industrial recycling environment of waste lithium-ion batteries, it is particularly necessary to remove fluorine impurities, which further highlights the importance of the present invention.

[0107] In summary, the principle of removing impurities on the surface of waste lithium-ion batteries by surface reconstruction in the present invention is as follows:

[0108] First, it is clear that the surface impurities of waste lithium-ion batteries include lithium fluoride, transition metal fluorides (including cobalt fluoride, nickel fluoride, manganese fluoride, etc.), fluorine-containing binders such as PVDF, and current collectors. Among them, the role of fluorine-containing binders such as PVDF in lithium-ion batteries is to bond the current collector and the positive electrode plate; lithium fluoride and transition metal fluorides are all caused by the decomposition of the electrolyte on the surface of the positive electrode material during the cycling of waste lithium-ion batteries.

[0109] Regarding lithium fluoride impurities, under the surface reconstruction reaction driven by voltage of the positive electrode material, due to the hydrogen-lithium exchange reaction on the surface of the positive electrode material, the weakly bound lithium fluoride impurities are removed in ionic form.

[0110] Regarding transition metal fluoride impurities such as cobalt fluoride, due to the high polarity of the transition metal-fluorine bond, during the reconstruction process of the positive electrode material, the transition metal-fluorine bond will break rapidly, forming transition metal ions and fluoride ions. The transition metal ions will quickly combine with -OH in the solution and precipitate in-situ on the surface of the positive electrode material, that is, the transition metal fluoride is reconstructed into the corresponding transition metal hydroxide during the reconstruction process of the positive electrode material. This not only avoids the loss of transition metal elements during the recycling process but also facilitates subsequent regeneration.

[0111] Regarding fluorine-containing binders such as PVDF and current collectors, the volume expansion phenomenon of fluorine-containing binders such as PVDF due to the reconstruction of the positive electrode material reduces the adhesion force, which facilitates subsequent separation from the positive electrode material, thereby avoiding the fluorine doping phenomenon caused by PVDF residue. At the same time, based on the reconstruction process induced by the electrochemical oxygen evolution reaction, due to the oxygen evolution on the surface of the aluminum foil current collector, the intermolecular force between PVDF and the surface of the current collector is weakened, causing the separation of the current collector and the electrode sheet layer, avoiding the aluminum doping phenomenon in subsequent regeneration steps caused by aluminum residue.

[0112] Combined Figure 20 , this method utilizes the surface reconstruction phenomenon that occurs in the positive electrode material of waste lithium-ion batteries under voltage control to promote the self-removal of impurities on the surface of the positive electrode material, avoiding the phenomenon of uneven batch regeneration effects caused by impurity-induced fluorine doping in traditional direct recycling methods. During the process, there is no need to use strong acids, strong alkalis, and a large amount of water. The operation process is simple, and it is applicable to batch recycling and regeneration among different batches of the same manufacturer and different manufacturers, and can achieve uniform regenerated electrochemical performance.

[0113] The method of the present invention applies the phenomenon of surface reconstruction of the cathode material promoted by voltage to the removal of harmful impurities, avoiding the influence of impurities on various properties of the regenerated cathode material. This method is applicable to almost all commercial cathode materials, such as layered LCO, spinel LMO, olivine phosphate LFP, and ternary and quaternary electrode materials, and the recovery rates of lithium and transition metals are close to 100%. For example, for the layered LCO cathode material, the method of the present invention can successfully eliminate lithium fluoride impurities on the surface of the cathode material, convert possible cobalt fluoride impurities into hydroxides, and the electrochemical properties of the regenerated cathode material are similar to those of commercial cathode materials and significantly better than those regenerated by traditional recovery methods. In comparing the regeneration effects of waste lithium-ion batteries from different batches of the same manufacturer and different manufacturers, the traditional recovery method shows large inter-batch regeneration differences and unstable capacity, while the regenerated cathode materials obtained by the method of the present invention have small inter-batch differences and good regenerated electrochemical properties.

[0114] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for removing impurities from the surface of waste lithium-ion battery positive electrode materials, characterized in that: include: Pre-treating waste lithium-ion batteries to obtain a positive electrode sheet containing a current collector; Assembling the positive electrode sheet containing the current collector as a working electrode into an electrolytic cell, applying voltage to the electrolytic cell for electrolysis reaction, wherein the electrolyte solution is an ammonium salt solution, wherein the ammonium salt is one or more of a strong acid radical ion ammonium salt, a weak acid radical ion ammonium salt, and an organic acid radical ion ammonium salt, and during the electrolysis reaction, the positive electrode sheet containing the current collector undergoes surface reconstruction to obtain a reconstructed positive electrode sheet; Mechanically crushing the reconstructed positive electrode sheet, collecting the positive electrode material powder or washing and drying it in an organic solvent to obtain a positive electrode material with surface impurities removed; The waste lithium-ion battery is one or more of a cobalt-acid lithium-ion battery, a lithium iron phosphate lithium-ion battery, a lithium manganate lithium-ion battery, a ternary nickel-cobalt-manganate lithium-ion battery, a ternary nickel-cobalt-aluminum lithium-ion battery, and a quaternary nickel-cobalt-manganese-aluminum lithium-ion battery; The surface impurities of the waste lithium-ion battery include lithium fluoride and transition metal fluoride; The transition metal fluoride includes one or more of cobalt fluoride, nickel fluoride, manganese fluoride, aluminum fluoride, and iron fluoride.

2. The method for removing impurities from the surface of the cathode material of waste lithium-ion batteries according to claim 1, characterized in that: The method of obtaining a positive electrode sheet containing a current collector by pre-treating waste lithium-ion batteries comprises: Soak the used lithium-ion battery in a salt solution and discharge it until the discharge termination voltage is 0.5~1.5V; Dismantle the discharged waste lithium-ion batteries, and cut the positive electrodes thereof to obtain positive electrode sheets containing current collectors; Wherein, the salt solution adopts a sodium chloride solution with a concentration range of 4-6wt%.

3. The method for removing impurities from the surface of the cathode material of waste lithium-ion batteries according to claim 1, characterized in that: The counter electrode in the electrolytic cell is one of carbon, graphite, stainless steel, titanium, platinum, gold, silver, lead and conductive glass.

4. The method for removing impurities from the surface of the cathode material of waste lithium-ion batteries according to claim 1, characterized in that: The voltage is greater than 1 V vs. RHE.

5. The method for removing impurities from the surface of the cathode material of waste lithium-ion batteries according to claim 1, characterized in that: The method also includes adding a precipitant to the electrolyte solution after the electrolysis reaction to obtain lithium salt precipitation, wherein the precipitant is a carbonate.

6. The method for removing impurities from the surface of the cathode material of waste lithium-ion batteries according to claim 1, characterized in that: The organic solvent is one of N,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide and dimethyl sulfoxide.

7. A method for regenerating waste lithium-ion battery positive electrode materials, characterized in that: The method comprises the surface impurity removal method of waste lithium-ion battery positive electrode materials according to any one of claims 1 to 6, and regenerating the positive electrode materials from which the surface impurities have been removed to obtain regenerated positive electrode materials, wherein the regeneration adopts a solid phase regeneration method or a hydrothermal solid phase regeneration method; The solid phase regeneration method comprises: The positive electrode material with the surface impurities removed is mixed with a lithium source, ground evenly and then calcined to obtain a regenerated positive electrode material; The hydrothermal solid phase regeneration method comprises: The positive electrode material after surface impurities removal is placed in a lithium hydroxide aqueous solution for hydrothermal treatment, washed to neutrality and then dried to obtain a hydrothermal positive electrode material; The hydrothermal positive electrode material is mixed with the lithium source and ground until uniform, and then calcined to obtain a regenerated positive electrode material; Wherein, the lithium source is lithium hydroxide or lithium carbonate.

8. The method for regenerating the positive electrode material of waste lithium-ion batteries according to claim 7, characterized in that: The concentration of the lithium hydroxide aqueous solution is 4 mol / L; And / or, the hydrothermal treatment includes hydrothermal treatment at 160-220° C. for a certain period of time; And / or, the calcination comprises calcining at 700-1000° C. for 6-24 h.

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