In-situ stripping and repairing method of retired lithium iron phosphate positive electrode material

Abstract

The invention relates to an in-situ stripping and repairing method of a decommissioned lithium iron phosphate positive electrode material, which comprises the following steps: immersing a decommissioned lithium iron phosphate positive electrode plate into a neutral lithiation solution containing a reducing agent and a lithium salt for reaction, sorting out aluminum foil after the reaction, and carrying out solid-liquid separation and drying on residues to obtain a lithium-supplemented regenerated lithium iron phosphate coating. According to the invention, liquid-phase chemical lithium supplement can be carried out at normal temperature and normal pressure, and structural damage of lithium iron phosphate and corrosion of aluminum foil are reduced.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery recycling technology, specifically a method for in-situ stripping and repair of retired lithium iron phosphate cathode materials. Background Technology

[0002] In recent years, the rapid development of new energy vehicles has driven the large-scale application of lithium-ion batteries, among which lithium iron phosphate (LiFePO4) batteries occupy an important market share due to their outstanding safety and cost advantages. With the arrival of the battery retirement cycle, how to achieve efficient and green recycling has become a focus of industry attention. Among various recycling technologies, direct remediation is considered a promising sustainable development path due to its low energy consumption, low pollution, and ability to effectively restore the electrochemical performance of cathode materials. However, this method still faces a key technical challenge in practical implementation: how to achieve efficient and complete separation between the degraded cathode material and the current collector without damaging the structure of the degraded cathode material.

[0003] From a resource value perspective, cathode materials account for approximately 36% of the cost of retired batteries and are rich in key elements such as lithium, iron, and phosphorus. Current mainstream recycling processes primarily rely on pyrometallurgy and hydrometallurgy. The former depends on high-temperature smelting to extract metals; while the process is simple, it suffers from high energy consumption, gas emissions, and low product purity. The latter obtains high-purity products through chemical leaching and multi-step purification, but is hampered by complex processes, large reagent consumption, and heavy waste treatment burdens. More importantly, both methods damage the microstructure of the cathode material, making it difficult to achieve high-value regeneration.

[0004] Against this backdrop, direct recycling strategies aimed at maintaining the structural integrity of materials have gradually become a research hotspot. A typical approach involves collecting waste active powder from electrode sheets, uniformly mixing it with a lithium source, and then achieving re-lithiation through solid-state sintering, thereby directly regenerating it into cathode materials usable in battery manufacturing. This type of method has a short process path and can produce market-competitive recycled materials, helping to promote the formation of a closed-loop battery industry chain.

[0005] It is worth noting that the effectiveness of direct recycling processes is highly dependent on the upstream separation process. An ideal separation method should remove impurities while maximizing the protection of the cathode material structure, avoiding secondary damage or contamination. However, existing separation methods still have many limitations: high-temperature treatment or strong alkali dissolution can easily lead to phase changes in the active material and corrosion of the aluminum foil; organic solvents or novel molten salt technologies often face constraints such as toxicity, cost, or process complexity. Therefore, developing a mild, efficient, and environmentally friendly electrode dissociation technology has become a key breakthrough for the large-scale application of direct recycling. Summary of the Invention

[0006] The present invention aims to provide an in-situ stripping and repair method for retired lithium iron phosphate cathode materials, which performs liquid-phase chemical lithium replenishment at room temperature and pressure, and reduces structural damage to lithium iron phosphate and corrosion of aluminum foil.

[0007] To solve the above technical problems, the specific solution adopted by the present invention is as follows: an in-situ stripping and repair method for retired lithium iron phosphate cathode materials, wherein the retired lithium iron phosphate cathode sheet is immersed in a neutral lithiumization solution containing a reducing agent and lithium salt for reaction, the aluminum foil is sorted out after reaction, and the remaining material is subjected to solid-liquid separation and drying to obtain a lithium iron phosphate coating for lithium replenishment and regeneration.

[0008] Preferably, the lithium iron phosphate coating that has been regenerated by lithium replenishment is ground and then calcined under an inert atmosphere to obtain regenerated lithium iron phosphate.

[0009] Preferably, the lithium iron phosphate coating regenerated by lithium replenishment is ground to D50≤10μm, and the inert atmosphere is nitrogen, argon or a mixture thereof; the low-temperature calcination temperature is 250-450℃, and the calcination time is 1-3h.

[0010] Preferably, the method for preparing a neutral lithium solution is as follows: using deionized water as a solvent, a reducing agent and a lithium salt are mixed, and then the pH is measured. If the pH is not 7, it is adjusted to neutral using a pH adjuster.

[0011] Preferably, the pH adjuster is selected from at least one of sulfuric acid, nitric acid, hydrochloric acid, and sodium hydroxide solution, and its concentration is 0.1-1 mol / L. Preferably, the reaction temperature is 30-60℃, the reaction time is 5-30min, and the solid-liquid ratio of the decommissioned lithium iron phosphate cathode to the neutral lithiation solution is 5-60g / L.

[0012] Preferably, the reducing agent is at least one of hydrazine hydrate, hydroxylamine, hydroxylamine hydrochloride, phenylhydroxylamine, hydroxylamine-O-sulfonic acid, hydroxylamine nitrate, hydroxylamine phosphate, hydroxylamine sulfate, hydrogen peroxide, and oxalic acid, and its concentration in the mixed lithium solution is 0.05-0.5 mol / L.

[0013] Preferably, the lithium salt is at least one of lithium hydroxide, lithium acetate, lithium chloride, lithium nitrate, and lithium sulfate, and its concentration in the mixed lithium solution is 0.05-0.5 mol / L.

[0014] Preferably, after solid-liquid separation, the residue is washed 2-3 times with deionized water for 5-10 minutes each time, and then dried at a temperature of 80-120℃ for 2-4 hours.

[0015] Beneficial effects 1. This invention utilizes a neutral lithium-ion solution for lithium replenishment and repair under low temperature and normal pressure conditions. The lithium salt and reducing agent reduce the ferric iron in the lithium iron phosphate to ferrous iron. Taking hydroxylamine as the reducing agent and lithium sulfate as the lithium salt as an example, the main reaction is as follows: FePO4+1 / 2Li2SO4+NH2OH→LiFePO4+1 / 2N2+H2O+1 / 2H2SO4 In the above reduction reaction, nitrogen gas is also produced simultaneously. The generated nitrogen gas can form bubbles at the interface between lithium iron phosphate and aluminum foil, achieving efficient and near-non-destructive separation of the two through physical peeling.

[0016] 2. Although a small amount of acidic substances are generated during the above reaction, reducing agents such as hydroxylamine can simultaneously undergo a disproportionation reaction to generate ammonia, thereby neutralizing the hydrogen ions produced in the system and automatically maintaining the reaction system in a near-neutral state. This mechanism has the following advantages: firstly, it avoids aluminum foil corrosion: aluminum is an amphoteric metal, easily corroded by both strong acids and strong alkalis, and a neutral environment can effectively protect the aluminum foil structure; secondly, it protects the lithium iron phosphate structure: acidic conditions will dissolve lithium iron phosphate, and under alkaline conditions, iron ions are easily precipitated, leading to structural damage, while a neutral system maintains the integrity of the material's crystal structure to the greatest extent.

[0017] 3. Lithification reaction refers to the reaction of vacant lithium iron phosphate (LiFePO4). x FePO4, x<1) is obtained by intercalation of lithium ions accompanied by Fe 3 ⁺ Reduced to Fe 2 ⁺, restoring LiFePO4 to its complete stoichiometry. During the repair process, FePO4 (trivalent iron phase) in the damaged phase reacts with the reducing agent, simultaneously generating gas, thus achieving synergistic structural repair and gas generation stripping.

[0018] 4. After lithium replenishment, low-temperature calcination is performed to pyrolyze the residual binder into a carbon layer, achieving in-situ fluorine doping. This process simultaneously enhances the material's conductivity and electrochemical performance, and eliminates the need for separate binder removal, simplifying the process flow.

[0019] 5. This invention integrates stripping, lithium replenishment, structural repair, carbon coating and doping into a continuous process, achieving efficient and high-value direct regeneration of retired lithium iron phosphate batteries under mild conditions, with comprehensive advantages of low energy consumption, low pollution and complete structural protection. Attached Figure Description

[0020] Figure 1 This is an experimental flowchart of the in-situ stripping and direct repair of retired lithium iron phosphate cathode materials in this invention. Figure 2 The image shows the XRD pattern of lithium iron phosphate lithium replenishment and repair in Example 1. Figure 3The image shows the XRD pattern of the aluminum foil that was non-destructively peeled off in Example 1. Figure 4 This is a comparison of the leaching rate of aluminum foil in lithium solutions with different pH values ​​during the separation process in Example 1, Comparative Example 1, and Comparative Example 2. Figure 5 The diagram shows the cycle performance of retired lithium iron phosphate, repaired lithium iron phosphate, and regenerated lithium iron phosphate in Example 1. Detailed Implementation

[0021] like Figure 1 As shown, this invention discloses a method for in-situ stripping and repair of decommissioned lithium iron phosphate cathode materials. The technical solution of this invention is illustrated below through several embodiments: Example 1 Includes the following steps: (1) Cut the disassembled retired lithium iron phosphate positive electrode sheet to 1 cm × 1 cm and immerse it in a mixed lithiation solution containing a reducing agent and a lithium salt. The reducing agent used is hydroxylamine hydrochloride with a concentration of 0.5 mol / L. The lithium salt is lithium hydroxide with a concentration of 0.5 mol / L. Therefore, hydroxylamine hydrochloride is an acidic solution and lithium hydroxide is an alkaline solution. Since the concentrations of the two are the same, the mixed lithiation solution is neutral and there is no need to adjust the acidity or alkalinity.

[0022] (2) The liquid phase reaction temperature is 50℃ and the reaction time is 5 min. The solid-liquid ratio of the positive electrode of the retired lithium iron phosphate battery to the lithium solution is 20 g / L.

[0023] (3) After the reaction is completed, the complete aluminum foil current collector is obtained by sorting, and the reaction solution is filtered, washed and dried to separate the lithium iron phosphate coating after lithium replenishment and repair.

[0024] (4) After the repaired lithium iron phosphate coating is thoroughly ground (D50≤10μm), it is calcined at 350℃ for 3h under argon atmosphere protection to obtain regenerated lithium iron phosphate, and its electrochemical performance is tested for comparison.

[0025] Comparative Example 1 The only difference from Example 1 is that the pH of the lithium solution is adjusted to 6 using a pH adjuster.

[0026] Comparative Example 2 The only difference from Example 1 is that the pH of the lithium solution is adjusted to 8 using a pH adjuster.

[0027] Figure 2 The image shows the XRD pattern of the in-situ stripping and repair of the decommissioned lithium iron phosphate cathode sheet in this embodiment. Figure 2The data shows that the characteristic peaks of the repaired lithium iron phosphate are completely consistent with those of its standard card (PDF# 81-1173), with no other impurity peaks, indicating that the lithium iron phosphate has been successfully repaired and has good crystallinity and phase purity. Similarly, Figure 3 The separated aluminum foil has the same characteristic peaks as commercial aluminum foil, and the peaks are sharp and free of impurities.

[0028] Figure 4 In this embodiment, the lithium iron phosphate and the current collector are separated by gas bubbling generated by the lithiation reaction. To highlight the degree of corrosion of the aluminum foil during the separation process, the leaching rate of aluminum foil in lithiation solutions with different pH values ​​(6, 7, 8) under the same conditions is compared. It is clear from the figure that the leaching rate of aluminum foil in the neutral solution of Example 1 is lower than that in the acidic (pH=6) solution of Comparative Example 2 and the alkaline (pH=8) solution of Comparative Example 3. The aluminum foil is almost separated without damage.

[0029] Figure 5 The graph shows the cycle performance of retired lithium iron phosphate, repaired lithium iron phosphate, and regenerated lithium iron phosphate. It can be seen from the graph that at a 1C rate, the initial discharge specific capacity of regenerated lithium iron phosphate is 140 mA hg. -1 It is significantly higher than the repair of lithium iron phosphate at 132 mA hg. -1 and retired lithium iron phosphate 99 mA hg -1 After 300 cycles, the capacity retention rate of regenerated lithium iron phosphate was 95.2%, which was significantly higher than that of repaired lithium iron phosphate and retired lithium iron phosphate, highlighting the material's good cycle stability.

[0030] Example 2 (1) Cut the disassembled retired lithium iron phosphate positive electrode sheet to 1 cm × 1 cm and immerse it in a mixed lithiation solution containing a reducing agent and lithium salt. The reducing agent used is hydroxylamine with a concentration of 0.5 mol / L. The lithium salt is lithium hydroxide with a concentration of 0.5 mol / L. Adjust the lithiation mixed solution to neutral by diluting it with 1 mol / L hydrochloric acid to avoid damage to the lithium iron phosphate structure and corrosion of the aluminum foil current collector. (2) The liquid phase reaction temperature is 40℃ and the reaction time is 10 min. The solid-liquid ratio of the retired lithium iron phosphate battery positive electrode sheet to the lithiation solution is 10 g / L.

[0031] (3) After the reaction is completed, the complete aluminum foil current collector is obtained by sorting, and the reaction solution is filtered, washed and dried to separate the lithium iron phosphate coating after lithium replenishment and repair. (4) After the repaired lithium iron phosphate coating is fully ground (D50≤10μm), it is calcined at 450℃ for 3h under argon atmosphere protection to obtain regenerated lithium iron phosphate.

[0032] Example 3 (1) Cut the disassembled retired lithium iron phosphate positive electrode sheet to 1cm×1cm and immerse it in a mixed lithiation solution containing a reducing agent and lithium salt. The reducing agent used is hydroxylamine sulfate with a concentration of 0.25mol / L; the lithium salt is lithium hydroxide with a concentration of 0.5mol / L. Therefore, hydroxylamine sulfate is an acidic solution and lithium hydroxide is an alkaline solution. The concentrations of hydrogen ions and hydroxide ions in both are the same, so the mixed lithiation solution is neutral and there is no need to adjust the acidity or alkalinity.

[0033] (2) The liquid phase reaction temperature is 60℃ and the reaction time is 1 min. The solid-liquid ratio of the positive electrode of the retired lithium iron phosphate battery to the lithium solution is 30 g / L.

[0034] (3) After the reaction is completed, the complete aluminum foil current collector is obtained by sorting, and the reaction solution is filtered, washed and dried to separate the lithium iron phosphate coating after lithium replenishment and repair. (4) After the repaired lithium iron phosphate coating is fully ground (D50≤10μm), it is calcined at 350℃ for 2h under nitrogen atmosphere protection to obtain regenerated lithium iron phosphate.

[0035] Example 4 The reducing agent used was hydrazine hydrate at a concentration of 0.1 mol / L; the lithium salt was lithium chloride at a concentration of 0.2 mol / L. Decommissioned lithium iron phosphate positive electrode sheets were cut into 1 cm × 1 cm pieces and immersed in a mixed lithiation solution with a solid-liquid ratio of 40 g / L. The reaction temperature was set at 30℃, and the reaction time was 25 min. After the reaction, the aluminum foil was sorted out, filtered, and washed three times with deionized water for 8 min each time. It was then dried at 100℃ for 3 h to obtain a lithium iron phosphate coating for regeneration. The coating was ground to a D50 ≤ 10 μm and calcined at 300℃ for 2.5 h under an argon atmosphere to obtain regenerated lithium iron phosphate.

[0036] Example 5 Phenylhydroxylamine was selected as the reducing agent at a concentration of 0.05 mol / L; lithium nitrate was used as the lithium salt at a concentration of 0.15 mol / L. The pH of the mixed solution was adjusted to 7 using 0.5 mol / L sodium hydroxide solution. The solid-liquid ratio of the electrode to the solution was 50 g / L, and the reaction was carried out at 55 °C for 20 min. After the reaction, the aluminum foil was separated, and after solid-liquid separation, it was washed twice with deionized water for 10 min each time, and dried at 90 °C for 4 h to obtain a lithium-replenishing coating. After grinding, the coating was placed in a nitrogen atmosphere and calcined at 400 °C for 1.5 h to obtain regenerated lithium iron phosphate.

[0037] Example 6 Hydroxylamine-O-sulfonic acid was used as the reducing agent at a concentration of 0.3 mol / L; lithium sulfate was used as the lithium salt at a concentration of 0.4 mol / L. The solution was adjusted to neutral with 0.1 mol / L hydrochloric acid. The solid-liquid ratio was controlled at 5 g / L, the reaction temperature was 35℃, and the reaction time was extended to 30 min. After the reaction was completed, the intact aluminum foil was removed, filtered, washed (washed three times with deionized water, 5 min each time), and dried at 110℃ for 2.5 h to obtain the regenerated coating. After grinding, it was calcined at 250℃ for 3 h under argon protection to obtain the final product.

[0038] Example 7 Hydrogen peroxide was used as the reducing agent at a concentration of 0.5 mol / L; lithium acetate was used as the lithium salt at a concentration of 0.1 mol / L. The pH was adjusted to 7 with 0.3 mol / L sulfuric acid. The solid-liquid ratio was 60 g / L, and the reaction was carried out at 60 °C for 15 min. After the reaction, the aluminum foil was sorted, and solid-liquid separation was performed. The foil was washed twice with deionized water for 6 min each time, and then dried at 120 °C for 2 h to obtain a lithium-replenishing coating. After grinding, the coating was placed in a nitrogen / argon mixed atmosphere and calcined at 450 °C for 1 h to obtain regenerated lithium iron phosphate.

[0039] Example 8 The reducing agent was oxalic acid at a concentration of 0.2 mol / L; the lithium salt was lithium hydroxide at a concentration of 0.25 mol / L. The pH was adjusted to 7 using 0.1 mol / L nitric acid and 0.1 mol / L sodium hydroxide in a synergistic manner. The solid-liquid ratio was 15 g / L, the reaction temperature was 45℃, and the reaction time was 5 min. After the reaction, the aluminum foil was separated, filtered, washed (three times with deionized water, 7 min each time), and dried at 80℃ for 4 h to obtain the coating material. After grinding, it was calcined at 350℃ for 2 h under a pure nitrogen atmosphere to obtain regenerated lithium iron phosphate.

[0040] Example 9 Hydroxylamine nitrate was used as the reducing agent at a concentration of 0.12 mol / L; the lithium salt was a mixture of lithium chloride and lithium sulfate (molar ratio 1:1) with a total concentration of 0.3 mol / L. The solution was adjusted to pH=7 with 0.2 mol / L hydrochloric acid. The solid-liquid ratio was 25 g / L, the reaction temperature was 50℃, and the reaction time was 8 min. After sorting the aluminum foil, solid-liquid separation and washing (twice with deionized water, 10 min each time) were performed, followed by drying at 105℃ for 3 h to obtain a lithium-replenishing coating. After grinding, the coating was placed in an argon atmosphere and calcined at 380℃ for 2.2 h to obtain the regenerated product.

[0041] Example 10 The reducing agent was hydroxylamine phosphate at a concentration of 0.18 mol / L; the lithium salt was a mixture of lithium nitrate and lithium acetate (molar ratio 2:1) with a total concentration of 0.22 mol / L. The pH was adjusted to neutral using 0.5 mol / L sodium hydroxide. The solid-liquid ratio was 55 g / L, the reaction temperature was 33℃, and the reaction time was 28 min. After the reaction, the aluminum foil was completely removed, and after solid-liquid separation and washing (three times with deionized water, 5 min each time), it was dried at 95℃ for 3.5 h to obtain the coating. After grinding, it was calcined at 420℃ for 1.8 h under nitrogen protection to obtain regenerated lithium iron phosphate.

Claims

1. A method for in-situ stripping and repair of decommissioned lithium iron phosphate cathode materials, characterized in that: The retired lithium iron phosphate positive electrode sheet is immersed in a neutral lithiation solution containing a reducing agent and lithium salt for reaction. After the reaction, the aluminum foil is sorted out, and the residue is separated by solid-liquid separation and drying to obtain the lithium iron phosphate coating for lithium replenishment and regeneration.

2. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 1, characterized in that: The lithium iron phosphate coating that has been regenerated by lithium replenishment is ground and then calcined under an inert atmosphere to obtain regenerated lithium iron phosphate.

3. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 2, characterized in that: The lithium iron phosphate coating regenerated by lithium replenishment is ground to D50≤10μm, and the inert atmosphere is nitrogen, argon or a mixture thereof; the low-temperature calcination temperature is 250-450℃, and the calcination time is 1-3h.

4. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 1, characterized in that: The method for preparing a neutral lithium solution is as follows: use deionized water as a solvent, mix the reducing agent and lithium salt, and then test its pH. If its pH is not 7, adjust it to neutral using a pH adjuster.

5. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 4, characterized in that: The pH adjuster is selected from at least one of sulfuric acid, nitric acid, hydrochloric acid, and sodium hydroxide solution, with a concentration of 0.1-1 mol / L. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode material as described in claim 1 is characterized in that: the reaction temperature is 30-60℃, the reaction time is 5-30min, and the solid-liquid ratio of the decommissioned lithium iron phosphate cathode to the neutral lithiation solution is 5-60g / L.

6. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 1, characterized in that: The reducing agent is at least one of hydrazine hydrate, hydroxylamine, hydroxylamine hydrochloride, phenylhydroxylamine, hydroxylamine-O-sulfonic acid, hydroxylamine nitrate, hydroxylamine phosphate, hydroxylamine sulfate, hydrogen peroxide, and oxalic acid, and its concentration in the mixed lithium solution is 0.05-0.5 mol / L.

7. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 1, characterized in that: The lithium salt is at least one of lithium hydroxide, lithium acetate, lithium chloride, lithium nitrate, and lithium sulfate, and its concentration in the mixed lithium solution is 0.05-0.5 mol / L.

8. The in-situ stripping and repair method for decommissioned lithium iron phosphate cathode materials as described in claim 1, characterized in that: After solid-liquid separation, the residue is washed 2-3 times with deionized water for 5-10 minutes each time, and then dried at a temperature of 80-120℃ for 2-4 hours.

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