Solvothermal reaction regeneration and repair method and application of waste lithium iron phosphate positive electrode material

Abstract

The invention belongs to the technical field of electrode material recovery, and provides a solvothermal reaction regeneration and repair method and application of a waste lithium iron phosphate positive electrode material. The regeneration and repair method comprises the following steps: stirring and mixing waste lithium iron phosphate and N-methyl pyrrolidone, and sequentially drying and sieving to obtain a positive active material; mixing the positive electrode active material, a lithium source, a reducing agent, organic acid and a solvent, and performing solvothermal reaction to obtain a lithium-supplemented lithium iron phosphate positive electrode active material; and carrying out annealing treatment on the lithium-supplemented lithium iron phosphate positive electrode active material in a reducing gas atmosphere to obtain the structure-repaired lithium iron phosphate material. According to the technical scheme, the electrochemical performance of the waste lithium iron phosphate is effectively improved, the operation is simple, the energy consumption is low, the efficiency is high, a new thought is provided for regeneration and repair of the positive electrode material of the waste lithium iron phosphate battery, and the application prospect is relatively great.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of electrode material recycling, and in particular to a solvothermal reaction regeneration and repair method for waste lithium iron phosphate positive electrode material and application. BACKGROUND

[0002] Lithium iron phosphate batteries have advantages of high energy density and high safety. With the large-scale application of lithium iron phosphate batteries in the energy storage field and electric vehicles, the contradiction between battery design and service life is increasingly prominent, and the recycling demand caused by the retirement of the batteries is rapidly rising. The internal factors causing performance degradation mainly include: vacancy effect caused by lithium ion deintercalation, which causes Fe 2+ migration to lithium sites, causing local collapse of olivine structure and irreversible phase change. When there are lithium vacancies in the deintercalation process, lithium-iron anti-site defects will be generated due to the similar ionic radius of Fe2 + and Li + . This will hinder the re-embedding of Li+. In addition, the inherent anisotropic property of the phase change of lithium iron phosphate during the cycle process will cause mechanical stress. Although the volume change is negligible, LFP particles will undergo spatial anisotropic phase change during charge and discharge cycles. The local non-uniform phase change behavior of this multi-phase material will affect the interface dynamics and the migration of Li + ions. This not only causes the cycle capacity of the material to decay, but also produces micro-cracks on the surface of the waste lithium iron phosphate material; the continuous growth of the negative electrode interface SEI film not only reduces the initial coulombic efficiency, but also thickens in the long-term charge and discharge, hindering the diffusion of lithium ions and consuming active lithium. Factors such as external extreme temperature change and external stress extrusion will also affect the battery life.

[0003] If a large number of retired batteries are accumulated and improperly treated, not only will the metal resources in the batteries be wasted, but also the economic benefits will be reduced; the heavy metal elements contained in the positive electrode material and the toxic gases such as hydrogen fluoride generated by the electrolyte will pose a double threat to human health and the soil. The current mainstream battery recycling methods are wet recycling and fire recycling. The products obtained by wet recycling have high purity, but the steps are complex, a large amount of waste liquid containing strong acid and strong base is generated in the treatment process, and the pollution control cost is high. Although fire recycling is suitable for mixed waste batteries, it has high energy consumption and produces harmful gases. Although the traditional battery recycling method can recycle the metal elements in the material, it also destroys the structure of the material itself. In the face of fluctuations in the price of raw materials for batteries and bottlenecks in recycling technology, developing a green and efficient and economically feasible regeneration process has become a new trend. SUMMARY

[0004] Therefore, the application provides a solvothermal reaction regeneration and repair method for waste lithium iron phosphate positive electrode material and application, and aims to solve the problems of high energy consumption and environmental pollution in the existing recycling method of waste lithium iron phosphate positive electrode material.

[0005] In order to achieve the above-mentioned application purposes, the application provides the following technical solutions.

[0006] The application provides a solvothermal reaction regeneration and repair method for waste lithium iron phosphate positive electrode material, which comprises the following steps.

[0007] S1, waste lithium iron phosphate and N-methyl pyrrolidone are stirred and mixed, then dried and sieved in sequence to obtain positive electrode active material;

[0008] S2, the positive electrode active material, lithium source, reducing agent, organic acid and solvent are mixed and subjected to solvothermal reaction to obtain lithium-supplemented lithium iron phosphate positive electrode active material;

[0009] S3, the lithium-supplemented lithium iron phosphate positive electrode active material is subjected to annealing treatment in a reducing gas atmosphere to obtain structure-repaired lithium iron phosphate material.

[0010] Further, in the step S1, the solid-liquid ratio of waste lithium iron phosphate and N-methyl pyrrolidone is 5-15 g: 25-75 mL.

[0011] Further, in the step S1, the temperature for stirring and mixing is 20-30 DEG C, and the time for stirring and mixing is 20-30 h.

[0012] Further, in the step S1, the temperature for drying is 80-100 DEG C, and the time for drying is 8-10 h; the mesh size of the sieve used for sieving is greater than or equal to 100 mesh.

[0013] Further, in the step S2, the lithium source comprises lithium acetate; the reducing agent comprises thioacetamide; the organic acid comprises citric acid or ascorbic acid; and the solvent comprises ethylene glycol aqueous solution, and the volume concentration of the ethylene glycol aqueous solution is 40-60%.

[0014] Further, in the step S2, the amount ratio of the positive electrode active material, lithium source, reducing agent, organic acid and solvent is 500 mg: 50-250 mg: 150-500 mg: 50 mg: 50 mL.

[0015] Further, in the step S2, the temperature for solvothermal reaction is 140-200 DEG C, and the time for solvothermal reaction is 3-18 h.

[0016] Further, in the step S3, the reducing gas is 10% hydrogen-argon gas; and in the annealing treatment, the holding temperature is 500-800 DEG C, and the holding time is 1-4 h.

[0017] The application provides a structure-repaired lithium iron phosphate material obtained through the solvent thermal reaction regeneration repair method of the above waste lithium iron phosphate positive electrode material.

[0018] The application further provides a lithium ion battery comprising the above structure-repaired lithium iron phosphate material.

[0019] Compared with the prior art, the application has the following beneficial effects through the above technical scheme:

[0020] 1. The regeneration repair method provided by the application has simple steps and easily available materials, has greater economic benefits compared with wet recovery and fire recovery, can significantly reduce energy consumption and greenhouse gas emissions, and has great environmental benefits.

[0021] 2. In the process of repairing and regenerating the lithium iron phosphate crystal lattice, nitrogen elements are doped in the lithium iron phosphate carbon coating layer to form carbon-nitrogen conjugated structure groups, which greatly improves the electronic conductivity of the coating layer and the lithium iron phosphate particles, thereby improving the overall performance of the regenerated material, and the process flow is simple and the cost-effectiveness is high. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 XRD patterns of waste lithium iron phosphate, commercial lithium iron phosphate and the lithium iron phosphate positive electrode material repaired by the application;

[0023] Figure 2 A comparison chart of charge-discharge cycle performance test of the button cell assembled by the waste lithium iron phosphate and the regenerated lithium iron phosphate positive electrode material prepared in Example 1;

[0024] Figure 3 A comparison chart of charge-discharge rate performance test of the button cell assembled by the waste lithium iron phosphate and the regenerated lithium iron phosphate positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0025] The application provides a solvent thermal reaction regeneration repair method of waste lithium iron phosphate positive electrode material, comprising the following steps:

[0026] S1, stirring and mixing the waste lithium iron phosphate and N-methyl pyrrolidone, then sequentially drying and sieving to obtain a positive electrode active material;

[0027] S2, mixing the positive electrode active material, a lithium source, a reducing agent, an organic acid and a solvent to perform solvent thermal reaction to obtain a lithium-supplemented lithium iron phosphate positive electrode active material;

[0028] S3, annealing the lithium-supplemented lithium iron phosphate positive electrode active material in a reducing gas atmosphere to obtain a structure-repaired lithium iron phosphate material.

[0029] The regeneration and repair method provided by this invention can regenerate and repair waste lithium iron phosphate cathode materials without damaging their structure. Ethylene glycol / water solution provides a more homogeneous liquid environment and a moderately polar environment, promoting the dissolution of lithium salts (lithium acetate) and organic additives. The volume ratio of ethylene glycol to water balances viscosity—pure ethylene glycol has too high a viscosity, affecting mass transfer, while adding too much water reduces reducibility. Organic acids provide an environment conducive to proton exchange, assisting lithium acetate in replenishing the missing lithium element in lithium iron phosphate cathode materials. For example, the tricarboxylic acid structure provides a proton gradient, accelerating the Li-to-Li exchange process in lithium acetate. + It migrates to vacancy sites in the LFP lattice. Simultaneously, citric acid can remove Fe from the FePO4 generated during lithium delithiation. 3+ Reduced to Fe 2+ This invention repairs Li-Fe antisite defects. Thioacetamide and other sulfur- and nitrogen-containing organic reducing agents help to directly and precisely repair lattice defects in lithium iron phosphate, thus performing preliminary repair on the lithium iron phosphate material. The primary amino group of thioacetamide not only helps to bind the preliminarily repaired cathode material but also acts as a sulfur and nitrogen source, cross-linking on the cathode material surface to form a more uniform coating layer. Simultaneously, organic acids such as citric acid act as carbon sources, synergistically coating the lithium iron phosphate surface with thioacetamide. The nitrogen-doped carbon coating forms a good continuous electronic conductivity layer, improving the electrochemical performance of the material. The method of this invention can effectively control the particle size and morphology of lithium iron phosphate, enabling the regenerated lithium iron phosphate cathode material to obtain a more uniform chemical composition and phase structure.

[0030] In this invention, in step S1, the solid-liquid ratio of waste lithium iron phosphate and N-methylpyrrolidone is 5-15g:25-75mL, preferably 10g:50mL.

[0031] In this invention, in step S1, the stirring and mixing temperature is 20-30°C, preferably 25°C; the stirring and mixing time is 20-30h, preferably 22-28h, and more preferably 24-26h.

[0032] In this invention, in step S1, the drying temperature is 80-100℃, preferably 85-95℃, and more preferably 90℃; the drying time is 8-10h, preferably 9h; and the mesh size of the sieve used for sieving is ≥100 mesh.

[0033] In this invention, in step S2, the lithium source includes lithium acetate; the reducing agent includes thioacetamide; the organic acid includes citric acid or ascorbic acid; and the solvent includes an aqueous ethylene glycol solution, wherein the volume concentration of the aqueous ethylene glycol solution is 40-60%, preferably 50%.

[0034] In the present application, in the step S2, the amount ratio of the positive active material, the lithium source, the reducing agent, the organic acid and the solvent is 500mg: 50-250mg: 150-500mg: 50mg: 50mL, preferably 500mg: 150mg: 300mg: 50mg: 50mL.

[0035] In the present application, in the step S2, the temperature of the solvothermal reaction is 140-200℃, preferably 160-180℃, and the time of the solvothermal reaction is 3-18h, preferably 5-10h, and further preferably 6-8h.

[0036] In the present application, in the step S3, the reducing gas is 10% hydrogen argon gas; in the annealing process, the holding temperature is 500-800℃, preferably 550-750℃, and further preferably 600℃; and the holding time is 1-4h, preferably 2h.

[0037] The present application provides a structure-repaired lithium iron phosphate material obtained by the solvothermal reaction regeneration and repair method of the above waste lithium iron phosphate positive electrode material.

[0038] The present application also provides a lithium ion battery comprising the above structure-repaired lithium iron phosphate material.

[0039] The technical solutions provided by the present application will be described in detail below in conjunction with examples, but they should not be understood as limiting the scope of protection of the present application.

[0040] Example 1

[0041] Take 10g of positive electrode sheet obtained by disassembling a retired lithium iron phosphate battery, and after peeling off the aluminum foil, obtain waste lithium iron phosphate powder containing carbon impurities. Add the powder and 50mL of N-methyl pyrrolidone into a three-necked flask, and magnetically stir in a 25℃ constant temperature water bath for 24h to achieve sufficient dissolution of the binder PVDF. Transfer the slurry to a vacuum drying oven, dry at 80℃ for 8h to constant weight, and then mechanically sieve using a 100 mesh stainless steel screen, and collect the undersize as the positive active material.

[0042] Weigh 500mg of positive active material, 50mg of lithium acetate, 150mg of thioacetamide and 50mg of citric acid into a polytetrafluoroethylene liner, add 50mL of ethylene glycol aqueous solution (ethylene glycol: deionized water = 1:1, v / v), magnetically stir for 30min to form a uniform suspension, and then put the liner into a stainless steel high-pressure reaction kettle, and perform solvothermal reaction at 160℃ for 6h. After the reaction is completed, naturally cool to room temperature, centrifuge the precipitate, wash with ethanol three times, and then vacuum dry at 80℃ for 6h to obtain lithium-supplemented lithium iron phosphate positive active material.

[0043] The lithium-supplemented lithium iron phosphate positive electrode active material was loaded into an alumina crucible and placed in the constant temperature zone of a tube furnace. Vacuum was extracted and 10% H2 / Ar mixed gas was introduced repeatedly three times. The material was heated to 600°C in a reducing atmosphere (flow rate 100 mL / min), and after 2h of heat preservation, it was cooled to below 100°C with the furnace and taken out, obtaining the structure-repaired lithium iron phosphate material.

[0044] Example 2

[0045] 10 g of positive electrode sheet obtained by disassembling a retired lithium iron phosphate battery was taken, and after the aluminum foil was peeled off, waste lithium iron phosphate powder containing carbon impurities was obtained. The powder and 50 mL of N-methyl pyrrolidone were added to a three-necked flask, and magnetic stirring was carried out in a 25°C constant temperature water bath for 24h to achieve sufficient dissolution of the binder PVDF. The slurry was transferred to a vacuum drying oven and dried at 80°C for 8h to constant weight, and then mechanically sieved using a 100 mesh stainless steel screen, and the undersize was collected as the positive electrode active material.

[0046] 500 mg of positive electrode active material, 50 mg of lithium acetate, 150 mg of thioacetamide, and 50 mg of ascorbic acid were weighed into a polytetrafluoroethylene liner, 50 mL of ethylene glycol aqueous solution (ethylene glycol: deionized water = 1:1, v / v) was added, and magnetic stirring was carried out for 30 min to form a uniform suspension. The liner was loaded into a stainless steel high-pressure reaction kettle, and solvothermal reaction was carried out at 160°C for 6h. After the reaction was completed, it was naturally cooled to room temperature, and the precipitate was separated by centrifugation, washed with ethanol three times, and then vacuum dried at 80°C for 6h to obtain the lithium-supplemented lithium iron phosphate positive electrode active material.

[0047] The lithium-supplemented lithium iron phosphate positive electrode active material was loaded into an alumina crucible and placed in the constant temperature zone of a tube furnace. Vacuum was extracted and 10% H2 / Ar mixed gas was introduced repeatedly three times. The material was heated to 600°C in a reducing atmosphere (flow rate 100 mL / min), and after 2h of heat preservation, it was cooled to below 100°C with the furnace and taken out, obtaining the structure-repaired lithium iron phosphate material.

[0048] Comparative Example 1

[0049] No organic acid was added, and the rest was the same as in Example 1.

[0050] Comparative Example 2

[0051] No reducing agent was added, and the rest was the same as in Example 1.

[0052] Button battery: Using NMP as a solvent, a slurry with a solid content of 70% is prepared by mixing lithium iron phosphate material (repaired structure) with Ketjen black and PVDF in a ratio of 8:1:1 (mass ratio) and uniformly coated onto a foil to form the positive electrode. The negative electrode uses a 14mm diameter lithium metal sheet, and the electrolyte is 1mol LiFP6 (EC:EMC = 3:7, volume ratio). The battery is packaged in the following order: negative electrode shell - spring sheet - gasket - lithium sheet - electrolyte - separator - positive electrode sheet - gasket - positive electrode shell. The entire process is completed in a glove box filled with hydrogen.

[0053] Test conditions: Test voltage range 2.5V-4.3V, at 0.1C (1C = 170mAh g) -1 After activating for 3 cycles, the charge-discharge capacity and cycle performance of the assembled button cell were tested at a current density of 1C. The rate performance of the assembled button cell was tested at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C. The performance test results are shown in Table 1.

[0054] Table 1. Test Results of Button Battery Rate Performance

[0055]

[0056] As shown in Table 1, the electrochemical performance of the regenerated lithium iron phosphate in Comparative Example 1 without the addition of organic acids is poor because it cannot provide a protonation environment to promote lithium replenishment. The electrochemical performance of the regenerated lithium iron phosphate in Comparative Example 2 without the addition of thioacetamide is also poor because it cannot provide nitrogen and sulfur sources to form a more uniform nitrogen source surface coating layer, thus failing to better repair the structural defects of lithium iron phosphate.

[0057] Figure 1 The XRD patterns of waste lithium iron phosphate, commercial lithium iron phosphate, and the lithium iron phosphate cathode material repaired according to this invention are shown below. Figure 1 As shown, waste lithium iron phosphate contains both lithium iron phosphate and iron phosphate phases, while regenerated lithium iron phosphate effectively removes impurities such as iron phosphate and iron oxide. The XRD results are similar to those of commercial lithium iron phosphate, indicating that this method effectively repairs lithium iron phosphate.

[0058] Figure 2 A comparison of the charge-discharge cycle performance of coin batteries assembled from waste lithium iron phosphate and the recycled lithium iron phosphate cathode material prepared in Example 1, as shown in the graph. Figure 2 The comparison of the discharge specific capacity of the two methods proves that the waste lithium iron phosphate has been effectively regenerated and repaired.

[0059] Figure 3 A comparison chart showing the charge / discharge rate performance of coin cells assembled from waste lithium iron phosphate and the recycled lithium iron phosphate cathode material prepared in Example 1. Figure 3This indicates that the electrochemical performance of the regenerated lithium iron phosphate repaired by this method is superior to that of the waste lithium iron phosphate.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials, characterized in that, Includes the following steps: S1. Waste lithium iron phosphate and N-methylpyrrolidone are stirred and mixed, then dried and sieved in sequence to obtain positive electrode active material; S2. After mixing the positive electrode active material, lithium source, reducing agent, organic acid and solvent, a solvothermal reaction is carried out to obtain lithium-replenished lithium iron phosphate positive electrode active material; S3. The lithium iron phosphate cathode active material after lithium replenishment is annealed in a reducing gas atmosphere to obtain the structurally repaired lithium iron phosphate material.

2. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to claim 1, characterized in that, In step S1, the solid-liquid ratio of waste lithium iron phosphate and N-methylpyrrolidone is 5-15g: 25-75mL.

3. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to claim 2, characterized in that, In step S1, the mixing temperature is 20-30℃ and the mixing time is 20-30h.

4. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to any one of claims 1 to 3, characterized in that, In step S1, the drying temperature is 80-100℃ and the drying time is 8-10 hours; the mesh size of the sieve used for sieving is ≥100 mesh.

5. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to claim 4, characterized in that, In step S2, the lithium source includes lithium acetate; the reducing agent includes thioacetamide; the organic acid includes citric acid or ascorbic acid; and the solvent includes an aqueous solution of ethylene glycol, wherein the volume concentration of the aqueous solution of ethylene glycol is 40-60%.

6. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to claim 5, characterized in that, In step S2, the ratio of positive electrode active material, lithium source, reducing agent, organic acid and solvent is 500mg: 50-250mg: 150-500mg: 50mg: 50mL.

7. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to claim 3 or 6, characterized in that, In step S2, the temperature of the solvothermal reaction is 140–200°C, and the reaction time is 3–18 h.

8. The solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials according to claim 7, characterized in that, In step S3, the reducing gas is 10% hydrogen argon; in the annealing treatment, the holding temperature is 500-800℃ and the holding time is 1-4h.

9. The structurally repaired lithium iron phosphate material obtained by the solvothermal reaction regeneration and repair method of waste lithium iron phosphate cathode material according to any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, Including the lithium iron phosphate material with structural repair as described in claim 9.

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