A solvothermal reaction regeneration and repair method for waste lithium iron phosphate positive electrode material and application thereof

Abstract

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

Description

Technical Field

[0001] This invention relates to the field of electrode material recycling technology, and in particular to a solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials and its application. Background Technology

[0002] Lithium iron phosphate (LFP) batteries offer advantages such as high energy density and high safety. However, with their large-scale application in energy storage and electric vehicles, the contradiction of battery design lifespan has become increasingly prominent, leading to a surge in recycling demand due to their retirement. The intrinsic factors causing performance degradation mainly include: the vacancy effect caused by lithium-ion intercalation / deintercalation, which reduces Fe... 2+ Migrating to lithium sites causes localized collapse of the olivine structure and an irreversible phase transition. When lithium vacancies exist during delithiation, due to Fe2+... + With Li + The similar ionic radii of the lithium iron phosphate (LFP) particles lead to lithium-iron antisite defects, which hinder Li+ reintercalation. Furthermore, the inherent anisotropic nature of the LFP phase transition during cycling induces mechanical stress. Although the volume change is negligible, LFP particles undergo spatially anisotropic phase transitions during charge-discharge cycles. This localized non-uniform phase transition behavior in multiphase materials affects interfacial dynamics and Li+. + Ion migration not only causes the cycle capacity of the material to decay, but also creates microcracks on the surface of waste lithium iron phosphate materials; the continuous growth of the SEI film at the negative electrode interface not only reduces the initial coulombic efficiency, but also thickens during long-term charge and discharge, hindering lithium-ion diffusion and consuming active lithium. External extreme temperature changes and external stress compression factors also affect battery life.

[0003] If retired batteries are stockpiled and improperly disposed of, not only will their metal resources be wasted and economic efficiency reduced, but the heavy metals in their cathode materials and the toxic gases such as hydrogen fluoride produced by the electrolyte will pose a dual threat to human health and soil erosion. Currently, the mainstream battery recycling methods are wet recycling and pyrometallurgical recycling. Wet recycling yields products with high purity, but the process is complex and generates large amounts of waste liquid containing strong acids and alkalis, resulting in high pollution control costs. While pyrometallurgical recycling is suitable for mixed waste batteries, it is energy-intensive and produces harmful gases. Traditional battery recycling methods, although able to recover metal elements from materials, simultaneously damage the material's structure. Faced with fluctuating battery raw material prices and technological bottlenecks in recycling, developing recycling processes that are both green, efficient, and economically feasible has become a new trend. Summary of the Invention

[0004] In view of this, the present invention provides a solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials and its application, the purpose of which is to solve the problems of high energy consumption and environmental pollution caused by existing recycling methods for waste lithium iron phosphate cathode battery materials.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials, comprising the following steps:

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

[0008] 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;

[0009] 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.

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

[0011] Furthermore, in step S1, the mixing temperature is 20-30°C, and the mixing time is 20-30 hours.

[0012] Furthermore, in step S1, the drying temperature is 80-100℃, the drying time is 8-10h, and the mesh size of the sieve used for sieving is ≥100 mesh.

[0013] Furthermore, 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%.

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

[0015] Furthermore, in step S2, the temperature of the solvothermal reaction is 140–200°C, and the reaction time is 3–18 h.

[0016] Furthermore, 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.

[0017] This invention provides a structurally repaired lithium iron phosphate material obtained by the solvothermal reaction regeneration and repair method for the above-mentioned waste lithium iron phosphate cathode material.

[0018] The present invention also provides a lithium-ion battery comprising the above-described structurally repaired lithium iron phosphate material.

[0019] As can be seen from the above technical solution, compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] 1. The regeneration and remediation method provided by this invention has simple steps and readily available materials. Compared with wet and pyrometallurgical recycling, it has greater economic benefits; it can significantly reduce energy consumption and greenhouse gas emissions, and has significant environmental benefits.

[0021] 2. In the process of repairing and regenerating lithium iron phosphate lattices, nitrogen elements are doped into the carbon coating layer of lithium iron phosphate to form carbon-nitrogen conjugated structural groups, which greatly improves the electronic conductivity of the coating layer and lithium iron phosphate particles, thereby improving the overall performance of the recycled material. The process is simple and cost-effective. Attached Figure Description

[0022] Figure 1 XRD patterns of waste lithium iron phosphate, commercial lithium iron phosphate, and lithium iron phosphate cathode material repaired according to the present invention;

[0023] Figure 2 Comparison of charge-discharge cycle performance of coin batteries assembled from waste lithium iron phosphate and recycled lithium iron phosphate cathode material prepared in Example 1;

[0024] Figure 3 Comparison of charge / discharge rate performance of coin cells assembled from waste lithium iron phosphate and recycled lithium iron phosphate cathode material prepared in Example 1. Detailed Implementation

[0025] This invention provides a solvothermal reaction regeneration and repair method for waste lithium iron phosphate cathode materials, comprising the following steps:

[0026] S1. Waste lithium iron phosphate and N-methylpyrrolidone are stirred and mixed, then dried and sieved in sequence to obtain positive electrode active material;

[0027] 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;

[0028] 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.

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

[0035] In this invention, in step S2, the temperature of the solvothermal reaction is 140-200°C, preferably 160-180°C, and the time of the solvothermal reaction is 3-18 hours, preferably 5-10 hours, and more preferably 6-8 hours.

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

[0037] This invention provides a structurally repaired lithium iron phosphate material obtained by the solvothermal reaction regeneration and repair method for the above-mentioned waste lithium iron phosphate cathode material.

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

[0039] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0040] Example 1

[0041] 10g of positive electrode sheet obtained from the dismantling of retired lithium iron phosphate batteries was taken, and after peeling off the aluminum foil, waste lithium iron phosphate powder containing carbon impurities was obtained. The powder was added to 50mL of N-methylpyrrolidone in a three-necked flask and magnetically stirred in a constant temperature water bath at 25℃ for 24h to fully dissolve the binder PVDF. The slurry was transferred to a vacuum drying oven and dried at 80℃ for 8h to constant weight. Then, it was mechanically sieved through a 100-mesh stainless steel sieve, and the undersize material was collected as the positive electrode active material.

[0042] Weigh 500 mg of positive electrode active material, 50 mg of lithium acetate, 150 mg of thioacetamide, and 50 mg of citric acid and place them in a polytetrafluoroethylene liner. Add 50 mL of ethylene glycol aqueous solution (ethylene glycol: deionized water = 1:1, v / v), and stir magnetically for 30 min to form a uniform suspension. Place the liner into a stainless steel high-pressure reactor and solvothermal react at 160 °C for 6 h. After the reaction is completed, allow it to cool naturally to room temperature, centrifuge to separate the precipitate, wash it three times with ethanol, and vacuum dry it at 80 °C for 6 h to obtain the lithium iron phosphate positive electrode active material after lithium replenishment.

[0043] The lithium iron phosphate cathode active material after lithium replenishment was placed in an alumina crucible and placed in the constant temperature zone of a tube furnace. The furnace was evacuated and a 10% H2 / Ar mixed gas was introduced and repeated three times to raise the material to 600℃ in a reducing atmosphere (flow rate 100mL / min). After holding at this temperature for 2 hours, the material was cooled to below 100℃ in the furnace and removed to obtain the structurally repaired lithium iron phosphate material.

[0044] Example 2

[0045] 10g of positive electrode sheet obtained from the dismantling of retired lithium iron phosphate batteries was taken, and after peeling off the aluminum foil, waste lithium iron phosphate powder containing carbon impurities was obtained. The powder was added to 50mL of N-methylpyrrolidone in a three-necked flask and magnetically stirred in a constant temperature water bath at 25℃ for 24h to fully dissolve the binder PVDF. The slurry was transferred to a vacuum drying oven and dried at 80℃ for 8h to constant weight. Then, it was mechanically sieved through a 100-mesh stainless steel sieve, and the undersize material was collected as the positive electrode active material.

[0046] Weigh 500 mg of positive electrode active material, 50 mg of lithium acetate, 150 mg of thioacetamide, and 50 mg of ascorbic acid and place them in a polytetrafluoroethylene liner. Add 50 mL of ethylene glycol aqueous solution (ethylene glycol: deionized water = 1:1, v / v), and stir magnetically for 30 min to form a uniform suspension. Place the liner into a stainless steel high-pressure reactor and solvothermal react at 160 °C for 6 h. After the reaction is completed, allow it to cool naturally to room temperature, centrifuge to separate the precipitate, wash it three times with ethanol, and vacuum dry it at 80 °C for 6 h to obtain the lithium iron phosphate positive electrode active material after lithium replenishment.

[0047] The lithium iron phosphate cathode active material after lithium replenishment was placed in an alumina crucible and placed in the constant temperature zone of a tube furnace. The furnace was evacuated and a 10% H2 / Ar mixed gas was introduced and repeated three times to raise the material to 600℃ in a reducing atmosphere (flow rate 100mL / min). After holding at this temperature for 2 hours, the material was cooled to below 100℃ in the furnace and removed to obtain the structurally repaired lithium iron phosphate material.

[0048] Comparative Example 1

[0049] No organic acids were added; otherwise, the process was the same as in Example 1.

[0050] Comparative Example 2

[0051] No reducing agent was added; otherwise, it 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 was 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 positive electrode active material after lithium replenishment is annealed in a reducing gas atmosphere to obtain the structurally repaired lithium iron phosphate material. In step S1, the drying temperature is 80~100℃, and the drying time is 8~10h; the mesh size of the sieve used for sieving is ≥100 mesh. 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%. 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; In step S2, the temperature of the solvothermal reaction is 140~200℃, and the time of the solvothermal reaction is 3~18h; In step S3, the reducing gas is 10% hydrogen argon; the annealing temperature is 500~800℃, and the annealing time is 1~4h.

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 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 3.

5. A lithium-ion battery, characterized in that, Including the lithium iron phosphate material with structural repair as described in claim 4.

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