Recycling method of waste lithium iron phosphate positive electrode material, positive electrode plate and battery

Abstract

The invention provides a recovery and regeneration method of a waste lithium iron phosphate positive electrode material, a positive electrode plate and a battery, and the regeneration method comprises the following steps: S1, cutting the waste lithium iron phosphate positive electrode plate into small blocks, adding a composite oxidation auxiliary agent into the small blocks, uniformly mixing, calcining for 1.5-4 hours at 450-600 DEG C in an oxidizing atmosphere, shaking, stripping, grinding and screening to obtain a waste lithium iron phosphate positive electrode material; a first calcined product is obtained; and S2, adjusting the molar ratio of Li, Fe and P in the first calcined product to be (1-1.1): (0.95-1.05): (0.95-1.05), then mixing the first calcined product with a composite reduction additive to obtain a mixture, then carrying out wet ball milling and drying on the mixture, and calcining at 600-900 DEG C for 8-14 hours to obtain a second calcined product which is the regenerated lithium iron phosphate positive electrode material. The regenerated lithium iron phosphate positive electrode material obtained by the recovery and regeneration method has relatively high discharge specific capacity and cycling stability.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium-ion battery recycling, and specifically relates to a recycling and regeneration method for waste lithium iron phosphate positive electrode materials, a positive electrode sheet and a battery. Background Art

[0002] With the booming development of electric vehicles and energy storage grids, the output of lithium-ion batteries is increasing day by day, and the price of raw materials is also rising. Recycling and regenerating waste lithium-ion batteries and recycling metal resources are of great significance to solving the shortage of raw materials, environmental pollution, and achieving sustainable and healthy development of the lithium-ion battery industry. At present, there are two main methods for recycling lithium iron phosphate (LiFePO4) positive electrode materials: hydrometallurgy and direct regeneration and repair.

[0003] Hydrometallurgy consumes a large amount of acid and alkali chemical reagents, and involves subsequent complex separation and purification processes. It also produces a large amount of wastewater and its by-products, which not only increases recycling costs but also poses a potential threat to the environment.

[0004] Direct regeneration and repair mainly restores the structure and electrochemical properties of the material by adding the corresponding missing elements in the material. It can retain the elements in the material to the maximum extent, and the process is simple, and the recycling process is more efficient, environmentally friendly and sustainable. However, although the direct regeneration and repair method of waste lithium iron phosphate has many advantages, there are still many problems in practical application: (1) The prepared LiFePO4 material contains impurities such as fluoride, and some Fe 3+ (1) The carbon residues in the positive electrode material are not reduced; (2) The carbon residues from the decomposition of the coating carbon, binder and conductive agent are relatively large and unevenly distributed; (3) The electrochemical performance of the regenerated and repaired positive electrode materials of severely deteriorated waste batteries is poor. Summary of the invention

[0005] In order to solve the problems and deficiencies in the prior art, the present application provides a recycling method for waste lithium iron phosphate positive electrode materials, a positive electrode sheet and a battery. The recycling method can effectively remove residual impurities in waste lithium iron phosphate, and the subsequent addition of a reducing agent can not only fully remove Fe 3+ Reduction to Fe 2+ Moreover, the carbon decomposed by the carbon-based reducing agent in the reducing agent is evenly coated on the surface of the positive electrode material, which effectively improves the conductivity of the material, and the obtained regenerated lithium iron phosphate positive electrode material has a high discharge specific capacity and cycle stability. Therefore, this method can provide a theoretical basis and technical support for the repair, large-scale synthesis and application of waste lithium iron phosphate.

[0006] According to the first aspect of the present application, a method for recycling and regenerating waste lithium iron phosphate positive electrode materials is provided, comprising the following steps: S1. After cutting the waste lithium iron phosphate positive electrode sheets into small pieces, adding a composite oxidation aid thereto and mixing evenly, calcining at 450 to 600° C. for 1.5 to 4 hours in an oxidizing atmosphere, and obtaining a first calcined product after vibration peeling, grinding, and screening; the composite oxidation aid comprises a first oxidation aid and a second oxidation aid, the first oxidation aid comprises a peroxide, and the second oxidation aid comprises a persulfate; S2. adjusting the Li, Fe, and P in the first calcined product The molar ratio of the raw materials is 1-1.1:0.95-1.05:0.95-1.05, and then mixed with a composite reducing aid to obtain a mixture, which is then wet-milled and dried, and then calcined at 600-900°C for 8-14h to obtain a second calcined product, which is a regenerated lithium iron phosphate positive electrode material; the composite reducing aid includes a first reducing aid and a second reducing aid, the first reducing aid includes an S element, and the valence state of the S element is not less than +3 and less than +6; the second reducing aid includes a carbon-based reducing aid, and the carbon-based reducing aid contains an aldehyde group or a ketone group.

[0007] In the recycling and regeneration method of waste lithium iron phosphate positive electrode material provided in the present application, first, a composite oxidation aid is first used to mix with the waste lithium iron phosphate positive electrode sheet cut into small pieces and calcine in oxygen, and the combination of the first oxidation aid peroxide and the second oxidation aid persulfate, compared with the use of a single oxidant, the composite oxidation aid can more effectively destroy the structure of lithium iron phosphate, greatly improve the leaching rate of lithium, which helps to improve the economic benefits of waste battery recycling, and the synergistic effect of the two accelerates the reaction process, so that the leaching process can reach a higher leaching efficiency in a shorter time, and improve the recovery efficiency. In the pre-oxidation process, the peroxide first quickly oxidizes the impurities in the waste lithium iron phosphate, and then the sulfate radicals generated by the decomposition of the persulfate continue to oxidize the remaining impurities, improve the oxidation efficiency, and in this process, an oxidizing atmosphere is further introduced, and the oxidizing agent can further synergize with the composite oxidation aid to promote the oxidation efficiency of the waste lithium iron phosphate. In addition, the calcination temperature in the pre-oxidation process is maintained in the range of 450 to 600°C. This is because too low a temperature will reduce the oxidation effect on waste lithium iron phosphate, while too high a temperature will make the reaction in the oxidation process too violent, which is not conducive to safety.

[0008] Secondly, since the pre-treatment oxidation decomposition impurities also oxidizes the waste lithium iron phosphate, a reducing aid needs to be added for reduction. The choice of reducing aid will also have a certain impact on the reduction effect. The composite reducing aid selected in this application includes a sulfur compound reducing aid containing a positive low-valent state (the first reducing aid) and a carbon-based reducing aid (the second reducing aid). These two reducing aids can reduce Fe3+ to Fe2+ (the iron element of waste lithium iron phosphate (LiFePO4) is in a relatively stable +3 valence state), thereby destroying the lattice structure of lithium iron phosphate. Among them, the sulfur element in the first reducing aid can be oxidized from a positive low-valent state to a +6 valence, and the aldehyde or ketone group in the carbon-based reducing aid is oxidized to a carboxyl group. The reduced Fe2+ is more easily dissolved from the lattice, so that the lithium element can also be leached. At the same time, the carbon formed by the decomposition of the carbon-based reducing aid can also improve the conductivity of the material. Compared with the use of one oxidant alone, the two can play a synergistic role when used in combination, more effectively destroying the lattice structure of lithium iron phosphate, making lithium elements easier to leach, thereby improving the recovery rate of metals. In addition, the cost of the first reduction aid and the second reduction aid is relatively low, and when the two are used in combination in a certain proportion, the amount of each can be reduced while ensuring the reduction effect, thereby further reducing the recovery cost; the product after the reaction is relatively simple and has little effect on the subsequent recovery process, and will not introduce too many impurities, which is conducive to improving the purity of the recovered product, simplifying the subsequent impurity removal steps, and enhancing the process stability; the composite reduction aid is relatively less affected by environmental factors, and can maintain good reduction performance within a wide range of temperature, pH value and other conditions, so that the stability and controllability of the recovery process are enhanced, and it is easier to achieve industrial large-scale production. In addition, during the reduction process, the calcination temperature is maintained in the range of 600 to 900 ° C, which can help improve the reduction efficiency.

[0009] Furthermore, through the pre-oxidation + reduction process in the present application, the uniform distribution of various elements can be promoted at the same time, and it can also be beneficial to improve the structural strength of the regenerated lithium iron phosphate, optimize the particle size distribution of the obtained regenerated lithium iron phosphate, and make it have a smaller particle size, thereby effectively optimizing the electrochemical properties of the regenerated lithium iron phosphate such as conductivity, lithium ion transport performance, discharge specific capacity and cycle stability, etc.

[0010] Preferably, in S1, the mass ratio of the first oxidation aid to the waste lithium iron phosphate positive electrode sheet is 1.5 to 3:1, and the mass ratio of the second oxidation aid to the waste lithium iron phosphate positive electrode sheet is 0.01 to 0.06:1. Controlling the dosage of the first oxidation aid and the second oxidation aid respectively can ensure that the waste lithium iron phosphate can be fully oxidized, so that the lithium element in the waste lithium iron phosphate can be completely released, and the recovery rate of lithium can be improved. It can also avoid excessive oxidation aids, and excessive oxidation of the structure of the lithium iron phosphate positive electrode material will produce more by-products, increase the difficulty of subsequent separation and purification, and make the regenerated lithium iron phosphate positive electrode material contain more impurities, reduce the charge and discharge efficiency, conductivity, cycle life and energy density, etc. At the same time, ensuring the dosage of the first oxidation aid and the second oxidation aid respectively can be more conducive to the synergistic effect of the two oxidation aids, improve the oxidation efficiency, reduce the dosage of the oxidation aid, and reduce adverse side reactions and impurities, thereby optimizing the overall performance of the regenerated lithium iron phosphate.

[0011] Preferably, in S1, the oxidizing atmosphere includes at least one of air and oxygen.

[0012] Preferably, in S2, the amount of the composite reducing aid is 8-16wt% of the mass of the mixture; the molar ratio of the first reducing aid to the second reducing aid is 1:1-4. Similarly, the amount of the composite reducing aid must also be controlled within a certain range, which can ensure that there is enough reducing aid to reduce the material, and that there is not too much reducing aid to over-reduce the material, avoiding more adverse side reactions, etc. At the same time, controlling the molar ratio of the first reducing aid to the second reducing aid within the above range is more conducive to the synergistic effect of the two, reducing the amount of reducing aid, and can also reduce adverse side reactions and impurities, thereby optimizing the material properties of the regenerated lithium iron phosphate.

[0013] Preferably, the first oxidizing aid includes at least one of hydrogen peroxide, sodium peroxide, peracetic acid, and calcium peroxide; the second oxidizing aid includes at least one of ammonium persulfate, sodium persulfate, and potassium persulfate; the first reducing aid includes at least one of sodium sulfite, sodium pyrosulfite, sodium bisulfite, and sodium dithionite; the second reducing aid includes at least one of glucose, fructose, maltose, ribose, lactose, and galactose. The types of the first and second oxidizing aids and the first and second reducing aids can both oxidize and reduce the waste lithium iron phosphate positive electrode material well, and the combination of such oxidizing aids and reducing aids has a good effect, can effectively improve the recovery rate of lithium, and make the final regenerated lithium iron phosphate positive electrode material have good electrochemical properties.

[0014] Preferably, the first oxidizing aid includes hydrogen peroxide; the second oxidizing aid includes ammonium persulfate; the first reducing aid includes sodium sulfite; and the second reducing aid includes glucose. In particular, when the first and second oxidizing aids and the first and second reducing aids are selected from the above materials, the combination of these materials has the best effect, which can not only achieve sufficient oxidation and reduction of the waste lithium iron phosphate positive electrode material, but also minimize the side reactions generated during the oxidation and reduction process, which is more conducive to reducing impurities and optimizing the electrochemical performance of the regenerated lithium iron phosphate positive electrode material.

[0015] Preferably, in S2, the specific operation of wet ball milling is as follows: the mixture obtained by mixing the first calcined product with the lithium source and the composite reduction aid is dispersed in an alcohol solvent, and the mixture is ball milled at 500-800r / min for 3-6h and then taken out. Furthermore, the first calcined product after oxidation is first mixed with the composite reduction aid and then wet-milled under the above process conditions, which can further evenly distribute the elements in the material, improve the reduction efficiency and reduction quality, and is more conducive to improving the overall performance of the regenerated lithium iron phosphate positive electrode material.

[0016] Preferably, in S2, the specific conditions for wet ball milling drying are: drying at 50-70° C. for 10-14 h.

[0017] Preferably, the alcohol solvent includes at least one of methanol and ethanol.

[0018] Preferably, in S2, during the process of adjusting the molar ratio of Li, Fe, and P in the first calcined product to 1-1.1:0.95-1.05:0.95-1.05, a lithium source, an iron source, or a phosphorus source can be added to the first calcined product according to actual conditions. Here, ICP can be used to detect the content of each element of Li, Fe, and P in the first calcined product to determine which raw material needs to be added.

[0019] Preferably, in S2, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium chloride, and lithium nitrate; the iron source includes at least one of ferric hydroxide, ferrous sulfate, and ferrous oxalate; and the phosphorus source includes at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0020] Preferably, in S1, the temperature is increased to 450-600°C at a heating rate of 3-8°C / min; and in S2, the temperature is increased to 600-900°C at a heating rate of 3-8°C / min.

[0021] Preferably, the D50 of the regenerated lithium iron phosphate cathode material is 1 to 3 μm. In the method for recycling and regenerating waste lithium iron phosphate cathode materials provided in the present application, the obtained regenerated lithium iron phosphate cathode material has a smaller D50 and a more uniform particle size distribution, so that the lithium ion transmission capacity can be effectively improved, so that the regenerated lithium iron phosphate cathode material has better electrochemical performance.

[0022] According to a second aspect of the present application, a positive electrode sheet is provided, comprising a regenerated lithium iron phosphate positive electrode material prepared by the above-mentioned method for recycling and regenerating waste lithium iron phosphate positive electrode material.

[0023] According to the third aspect of the present application, a battery is provided, comprising the above-mentioned positive electrode sheet, or comprising the regenerated lithium iron phosphate positive electrode material prepared by the above-mentioned recycling and regeneration method of waste lithium iron phosphate positive electrode material. The battery prepared by the regenerated lithium iron phosphate positive electrode material prepared by the recycling and regeneration method of waste lithium iron phosphate positive electrode material in the present application has good lithium ion transmission performance, and at the same time has good discharge specific capacity, conductivity and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The SEM characterization results of the regenerated lithium iron phosphate positive electrode materials in Comparative Example 6 and Example 1, wherein (a) is the SEM image of the directly repaired regenerated lithium iron phosphate positive electrode material of Comparative Example 6, and (b) is the SEM image of the regenerated lithium iron phosphate positive electrode material after pre-oxidation and reduction in Example 1.

[0025] Figure 2 These are the EDS characterization results of the regenerated lithium iron phosphate positive electrode materials in Comparative Example 6 and Example 1, wherein (a) is the EDS image of the directly repaired regenerated lithium iron phosphate positive electrode material of Comparative Example 6 (first row of images), and (b) is the EDS image of the regenerated lithium iron phosphate positive electrode material that was reduced after pre-oxidation in Example 1 (second row of images).

[0026] Figure 3 TEM characterization results of the regenerated lithium iron phosphate positive electrode materials in Comparative Example 6 and Example 1, wherein (a) is the TEM image of the directly repaired regenerated lithium iron phosphate positive electrode material of Comparative Example 6 (first row of images), and (b) is the TEM image of the regenerated lithium iron phosphate positive electrode material that was pre-oxidized and then reduced in Example 1 (second row of images).

[0027] Figure 4 This is the particle size distribution diagram of the regenerated lithium iron phosphate positive electrode material after pre-oxidation and reduction in Example 1. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present application, rather than all of the embodiments.

[0029] Example 1

[0030] The method for recycling and regenerating waste lithium iron phosphate positive electrode materials in this embodiment comprises the following steps:

[0031] S1. After cutting the waste lithium iron phosphate positive electrode into small pieces, adding the composite oxidation aid thereto and mixing evenly, calcining at 550°C at 5°C / min in an oxidizing atmosphere for 2h, shaking, peeling, grinding, and sieving to obtain a first calcined product; the composite oxidation aid includes a first oxidation aid and a second oxidation aid, the first oxidation aid is hydrogen peroxide (30%, density 1.11g / cm 3 ), the volume mass ratio of hydrogen peroxide to waste lithium iron phosphate positive electrode sheet is 5:1 (5mL:1g, after conversion, the mass ratio of hydrogen peroxide to waste lithium iron phosphate positive electrode sheet is 1.665:1), the second oxidizing aid is ammonium persulfate, and the mass ratio of ammonium persulfate to lithium iron phosphate positive electrode sheet is 0.03:1;

[0032] S2. The molar ratio of Li, Fe and P in the first calcined product is adjusted to 1.03:1:1 (a lithium source, an iron source or a phosphorus source can be added according to actual conditions, the lithium source is lithium carbonate, the iron source is ferrous sulfate, and the phosphorus source is ammonium dihydrogen phosphate), and then mixed with a composite reducing aid to obtain a mixture, and then the mixture is poured into a ball mill, and an appropriate amount of anhydrous ethanol is added as a ball milling medium, and wet ball milled at 600r / min for 4h. After the mixture is taken out, dried at 60°C for 12h, and then heated to 700°C at 5°C / min and calcined for 10h to obtain a second calcined product, which is a regenerated lithium iron phosphate positive electrode material; the composite reducing aid includes a first reducing aid and a second reducing aid, and the amount of the composite reducing aid is 12wt% of the mass of the mixture, the first reducing aid is sodium sulfite; the second reducing aid is glucose; and the molar ratio of sodium sulfite to glucose is 1:2.

[0033] Example 2

[0034] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in embodiment 1 is that the volume mass ratio of the first oxidizing aid hydrogen peroxide to the waste lithium iron phosphate positive electrode sheet is 3:1 (3 mL: 1 g). The rest is the same as in embodiment 1.

[0035] Example 3

[0036] In the method for recycling and regenerating waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in Embodiment 1 is that the molar ratio of the first reducing aid sodium sulfite to the second reducing aid glucose is 1:0.5. The rest is the same as that in Embodiment 1.

[0037] Example 4

[0038] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in Embodiment 1 is that the mass ratio of the second oxidizing aid ammonium persulfate to the waste lithium iron phosphate positive electrode sheet is 0.075:1. The rest is the same as in Embodiment 1.

[0039] Example 5

[0040] In the method for recycling and regenerating waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in Embodiment 1 is that the second oxidizing aid is adjusted to sodium persulfate. The rest is the same as that in Embodiment 1.

[0041] Example 6

[0042] In the method for recycling and regenerating waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in Embodiment 1 is that the first reducing aid is adjusted to sodium bisulfite. The rest is the same as that in Embodiment 1.

[0043] Example 7

[0044] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in embodiment 1 is that the first and second oxidizing aids are adjusted to peracetic acid and potassium persulfate, respectively, and the first and second reducing aids are adjusted to sodium pyrosulfite and ribose, respectively. The rest is the same as in embodiment 1.

[0045] Example 8

[0046] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this embodiment, the difference from that in embodiment 7 is that the first oxidizing agent is adjusted to sodium peroxide, and the second reducing agent is adjusted to fructose. The rest is the same as in embodiment 7.

[0047] Example 9

[0048] The method for recycling and regenerating waste lithium iron phosphate positive electrode materials in this embodiment is different from that in Embodiment 1 in that in S2, the mixed material is directly ground and then heated to 700° C. at a rate of 5° C. / min and calcined for 10 h. The rest is the same as in Embodiment 1.

[0049] Comparative Example 1

[0050] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this comparative example, the difference from Example 1 is that the second oxidizing agent ammonium persulfate is not added, and the total mass of the oxidizing agent remains unchanged. The rest is the same as Example 1.

[0051] Comparative Example 2

[0052] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this comparative example, the difference from Example 1 is that the first reducing aid sodium sulfite is not added, and the total mass of the reducing aid remains unchanged. The rest is the same as Example 1.

[0053] Comparative Example 3

[0054] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this comparative example, the difference from Example 1 is that only one oxidizing agent is added, and the oxidizing agent is sodium hypochlorite, and the total amount of the oxidizing agent remains unchanged. The rest is the same as Example 1.

[0055] Comparative Example 4

[0056] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this comparative example, the difference from Example 1 is that in S1, the calcination temperature is 500° C. The rest is the same as Example 1.

[0057] Comparative Example 5

[0058] In the recycling and regeneration method of waste lithium iron phosphate positive electrode materials in this comparative example, the difference from Example 1 is that in S2, the calcination temperature is 1100° C. The rest is the same as Example 1.

[0059] Comparative Example 6

[0060] The method for recycling and regenerating waste lithium iron phosphate positive electrode materials in this embodiment comprises the following steps:

[0061] S1. After cutting the waste lithium iron phosphate positive electrode sheet into small pieces, the temperature was raised to 550°C at 5°C / min in a nitrogen atmosphere and calcined for 2h, and after vibration peeling, grinding and sieving, a first calcined product was obtained;

[0062] S2. Adjust the molar ratio of Li, Fe and P in the first calcined product to 1.03:1:1 (a lithium source, an iron source or a phosphorus source may be added according to actual conditions, the lithium source is lithium carbonate, the iron source is ferrous sulfate, and the phosphorus source is ammonium dihydrogen phosphate) to obtain a mixture, then pour the mixture into a ball mill, add an appropriate amount of anhydrous ethanol as a ball milling medium, and perform wet ball milling at 600r / min for 4h. After the mixture is taken out, dry it at 60°C for 12h, and then heat it to 700°C at 5°C / min and calcine it for 10h to obtain a second calcined product. The second calcined product is a regenerated lithium iron phosphate positive electrode material.

[0063] Test analysis

[0064] (1) Morphological characterization

[0065] The regenerated lithium iron phosphate positive electrode materials obtained in the above Example 1 and Comparative Example 6 were characterized by SEM, EDS (energy spectrum analysis) and TEM, and the results were as follows: Figure 1 , 2 , as shown in Figure 3.

[0066] Depend on Figure 1 From the SEM images, we can see that Figure 1 (a) is the regenerated lithium iron phosphate positive electrode material directly repaired in Comparative Example 6, Figure 1(b) is a regenerated lithium iron phosphate positive electrode material that is reduced after pre-oxidation. The particle size of the directly repaired regenerated lithium iron phosphate is uneven, and some tiny particles are attached to the surface of the material. In contrast, the surface of the regenerated lithium iron phosphate particles that are reduced after pre-oxidation is smooth, the particles are relatively uniform and the particle size is relatively small, and there are no more fine particles attached to the surface. This is because while impurities such as PVDF, conductive carbon black and CEI membrane are removed, LiFePO4 is also oxidized and decomposed. Smaller particle size can shorten the Li + The diffusion distance of Li + The diffusion rate is good for the electrochemical performance.

[0067] Depend on Figure 2 From the EDS picture, we can see that Figure 1 (a) is the regenerated lithium iron phosphate positive electrode material directly repaired in Comparative Example 6 (first row of pictures), Figure 1 (b) is the regenerated lithium iron phosphate cathode material after pre-oxidation and reduction (second row of pictures). The Fe, P and O elements are distributed relatively evenly in the two recycled materials, but the directly repaired recycled material contains fluoride ions and the carbon distribution is not very uniform. The presence of fluoride impurities and unevenly distributed carbon in the recycled material will affect the electrochemical performance of the cathode material.

[0068] Depend on Figure 3 From the TEM image (mapping), we can see that Figure 1 (a) is the regenerated lithium iron phosphate positive electrode material directly repaired in Comparative Example 6 (first row of pictures), Figure 1 (b) is the regenerated lithium iron phosphate cathode material after pre-oxidation and reduction (second row of pictures). Fe, P and O are evenly distributed in both materials. Carbon shows obvious accumulation in the directly repaired regenerated lithium iron phosphate. The carbon in the regenerated lithium iron phosphate after pre-oxidation and reduction is evenly distributed, which is consistent with the SEM and EDS test results.

[0069] (2) Particle size distribution test

[0070] The particle size distribution of the regenerated lithium iron phosphate positive electrode material obtained in Example 1 is characterized, and the results are as follows: Figure 4 shown. Figure 4 In the figure, the ordinate represents the size of the regenerated lithium iron phosphate particles, and the abscissa represents the sample size, that is, the number of particle sizes.

[0071] Depend on Figure 4 It can be seen from the particle size distribution diagram that the lithium iron phosphate particles are relatively uniform and the particle size is relatively small, with D50 of 1487.084nm. Therefore, it has good lithium ion transmission performance, which makes the lithium iron phosphate positive electrode have better electrochemical performance.

[0072] (3) Battery performance test

[0073] The regenerated lithium iron phosphate positive electrode materials obtained in the above examples and comparative examples were used to prepare positive electrode sheets, and half cells (button cells) were assembled using metallic lithium as the negative electrode.

[0074] The specific operation of preparing the positive electrode sheet is as follows: the regenerated lithium iron phosphate positive electrode material, carbon black (conductive agent) and PVDF (polyvinylidene fluoride, binder) obtained in the above embodiment and comparative example are mixed in a NMP (N-methylpyrrolidone) solvent at a ratio of 8:1:1 (mass ratio), placed in a homogenizer for stirring and dispersion, and then coated on aluminum foil, and then placed in an oven at 85°C for 30 minutes, and punched into sheets with an area of ​​1 cm 2 The disc was weighed and recorded, and then baked in a vacuum oven at 120°C for 8 hours to obtain the positive electrode, which is the working electrode.

[0075] The specific operation of preparing button cells is as follows: CR-2032 coin cells are assembled in a glove box, lithium metal is used as the counter / reference electrode, the separator is Celgard, the electrolyte is 1M LiPF6, and the ratio of dimethyl carbonate (DMC) to ethylene carbonate (EC) is 1:1.1 (mass ratio).

[0076] The obtained battery was then subjected to the following electrochemical performance tests.

[0077] ① Test of discharge capacity at different rates (0.1C, 0.5C, 1C, 2C)

[0078] The batteries prepared by using the regenerated lithium iron phosphate positive electrode materials obtained in the above embodiment and the comparative example were tested for discharge specific capacity at different rates (0.1C, 0.5C, 1C, 2C). The specific test method is as follows: the prepared battery is fully charged with a constant current of 1C (the cut-off voltage is 3.8V), and then discharged with a constant current of 0.1C / 0.5C / 1C / 2C, and the discharge time is recorded at the same time, until the battery voltage reaches the termination voltage of 2.5V and then the discharge is stopped, and then the discharge specific capacity is calculated according to "discharge specific capacity = discharge current * discharge time / battery mass". The above test results are shown in Table 1.

[0079] Table 1 Discharge capacity results of batteries at different rates (0.1C, 0.5C, 1C, 2C)

[0080]

[0081] ② Test of AC impedance and lithium ion diffusion coefficient (DLi+)

[0082] The batteries prepared with the regenerated lithium iron phosphate positive electrode materials obtained in the above-mentioned Example 1 and Comparative Example 6 were respectively subjected to AC impedance and lithium ion diffusion coefficient (DLi+) tests after 5 cycles and 100 cycles. The specific testing method is as follows: an electrochemical workstation is used to perform EIS test on the lithium ion battery. The frequency range is selected from 0.01 Hz to 1 kHz, the voltage signal amplitude is selected to be about 5 to 15 mV, and the current signal is less than 10% of the working current. Impedance data at different frequencies are obtained, impedance spectra are drawn, and impedance data after 5 and 100 cycles are recorded. In the spectrum, the 45° straight line in the low-frequency part corresponds to the lithium ion diffusion impedance, i.e., the Warburg impedance. The Warburg coefficient σ is determined by fitting analysis, and the lithium ion diffusion coefficient is calculated according to the formula DLi+=0.5*(RT)^2 / (A*n^2*F^2*C*σ)^2, where R is the gas constant 8.314J / (mol*K), T is the absolute temperature, A is the electrode surface area, n is the number of transferred electrons per mole of substance participating in the electrode reaction, F is the Faraday constant 96485.33C / mol, and C is the concentration of lithium in the electrode. The lithium ion diffusion coefficient (DLi+) under 5 and 100 cycles can be calculated according to the above method. At the same time, the AC impedance and lithium ion diffusion coefficient (DLi+) of the batteries prepared from the regenerated lithium iron phosphate positive electrode materials obtained in the remaining embodiments and comparative examples are tested for AC impedance and lithium ion diffusion coefficient (DLi+) after 100 cycles.

[0083] The above test results are shown in Table 2.

[0084] Table 2 AC impedance fitting data, test results of lithium ion diffusion coefficient (DLi+) test

[0085]

[0086] As shown in Tables 1 and 2, the battery prepared by the recycled lithium iron phosphate positive electrode material obtained by the recycling and regeneration method of the waste lithium iron phosphate positive electrode material provided by the present invention has a higher discharge specific capacity at different rates, and also has a lower AC impedance and a higher lithium ion diffusion coefficient. Specific reference is made to Examples 1 to 9.

[0087] In Comparative Example 1, the second oxidizing agent ammonium persulfate is not added; in Comparative Example 2, the first reducing agent sodium sulfite is not added; in Comparative Example 3, only one oxidizing agent is added, and the oxidizing agent is sodium hypochlorite; in Comparative Example 4, the calcination temperature in S1 is too low; in Comparative Example 5, the calcination temperature in S2 is too high; in Comparative Example 6, direct repair and regeneration are performed, that is, without the process of pre-oxidation and reduction. All of the above factors lead to a decrease in the rate performance of the regenerated lithium iron phosphate positive electrode material, an increase in AC impedance, and a decrease in the lithium ion diffusion coefficient. This shows that the composite oxidizing agent and the composite reducing agent in the present invention need to be in a specific combination, and in the process of treating the waste lithium iron phosphate positive electrode sheet, a specific calcination temperature is required to be more conducive to the oxidation and reduction of the waste lithium iron phosphate positive electrode sheet, and then more conducive to the regeneration of the waste lithium iron phosphate positive electrode sheet, so as to achieve the purpose of further optimizing the regenerated lithium iron phosphate positive electrode material. In particular, Comparative Example 6 does not have the process of pre-oxidation and reduction, so it cannot fully improve the recovery rate of lithium, and is also not conducive to the electrochemical performance of the regenerated lithium iron phosphate positive electrode material.

[0088] Further comparing Example 1 with Examples 2 to 4, the first oxidizing aid hydrogen peroxide in Example 2 is too little, the molar ratio of the first reducing aid sodium sulfite to the second reducing aid glucose in Example 3 is not in the range of 1:1 to 4, and the second oxidizing aid ammonium persulfate in Example 4 is too much. The above factors make the rate performance, conductivity, and lithium ion transmission performance in Examples 2 to 4 worse than those in Example 1, which shows that the dosage of the oxidizing aid and the reducing aid will have a certain influence on the performance of the regenerated lithium iron phosphate positive electrode material, and ultimately affect various aspects of the battery performance.

[0089] Comparing Example 1 with Examples 5 to 8, the first or second oxidizing aid, or the first or second reducing aid, was changed in Examples 5 to 8, and the result was that the performance of the batteries in Examples 5 to 8 was reduced. This also shows that the type of oxidizing aid and reducing aid will have a certain impact on the regeneration process of the waste lithium iron phosphate positive electrode sheet, thereby affecting the final regenerated lithium iron phosphate positive electrode material and the battery performance prepared therefrom. From the above performance results, it can be seen that the first and second oxidizing aids, as well as the first and second reducing aids, are best matched in Example 1, and the first and second oxidizing aids, as well as the first and second reducing aids, are also relatively well matched in Example 7.

[0090] Comparing Example 1 with Example 9, the operation without ball milling in Example 9 will result in insufficient reduction reaction during the calcination process, which will reduce the performance of the obtained regenerated lithium iron phosphate positive electrode material, thereby causing a decrease in battery performance.

[0091] The above embodiments are only used to illustrate the technical solution of the present application rather than to limit the protection scope of the present application. Although the present application is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solution of the present application can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present application.

Claims

1. A method for recycling waste lithium iron phosphate positive electrode materials, characterized in that: The steps include: S1. After cutting the waste lithium iron phosphate positive electrode sheet into small pieces, adding a composite oxidation aid thereto and mixing evenly, calcining at 450-600°C for 1.5-4h in an oxidizing atmosphere, and obtaining a first calcined product after vibration peeling, grinding and screening; the composite oxidation aid comprises a first oxidation aid and a second oxidation aid, wherein the first oxidation aid comprises a peroxide, and the second oxidation aid comprises a persulfate; S2. Adjust the molar ratio of Li, Fe and P in the first calcined product to 1-1.1:0.95-1.05:0.95-1.05, and then mix it with a composite reducing aid to obtain a mixture. Then, wet-ball mill the mixture, dry it, and calcine it at 600-900°C for 8-14h to obtain a second calcined product, which is a regenerated lithium iron phosphate positive electrode material; the composite reducing aid includes a first reducing aid and a second reducing aid, the first reducing aid includes an S element, and the valence state of the S element is not less than +3 and less than +6; the second reducing aid includes a carbon-based reducing aid, and the carbon-based reducing aid contains an aldehyde group or a ketone group.

2. The method for recycling waste lithium iron phosphate positive electrode materials according to claim 1, characterized in that: In S1, the mass ratio of the first oxidation aid to the waste lithium iron phosphate positive electrode sheet is 1.5-3:1, and the mass ratio of the second oxidation aid to the waste lithium iron phosphate positive electrode sheet is 0.01-0.06:

1.

3. The method for recycling waste lithium iron phosphate positive electrode materials according to claim 1, characterized in that: In S2, the amount of the composite reducing aid is 8-16 wt% of the mass of the mixture; The molar ratio of the first reducing aid to the second reducing aid is 1:1-4.

4. The method for recycling waste lithium iron phosphate positive electrode materials according to claim 1, characterized in that: The first oxidizing agent includes at least one of hydrogen peroxide, sodium peroxide, peracetic acid, and calcium peroxide; The second oxidizing aid includes at least one of ammonium persulfate, sodium persulfate, and potassium persulfate; The first reducing aid comprises at least one of sodium sulfite, sodium pyrosulfite, sodium bisulfite, and sodium dithionite; The second reducing aid includes at least one of glucose, fructose, maltose, ribose, lactose and galactose.

5. The method for recycling waste lithium iron phosphate positive electrode materials as claimed in claim 4, characterized in that: The first oxidizing agent includes hydrogen peroxide; The second oxidizing aid includes ammonium persulfate; The first reducing aid comprises sodium sulfite; The second reducing aid includes glucose.

6. The method for recycling waste lithium iron phosphate positive electrode materials according to claim 1, characterized in that: In S2, the specific operation of wet ball milling is as follows: The mixture obtained by mixing the first calcined product with the lithium source and the composite reduction aid is dispersed in an alcohol solvent, ball-milled at 500-800 r / min for 3-6 hours, taken out, and dried at 50-70° C. for 10-14 hours.

7. The method for recycling waste lithium iron phosphate positive electrode materials according to claim 1, characterized in that: In S1, the temperature is increased to 450-600° C. at a heating rate of 3-8° C. / min; In S2, the temperature is increased to 600-900°C at a heating rate of 3-8°C / min.

8. The method for recycling waste lithium iron phosphate positive electrode materials according to claim 1, characterized in that: The D50 of the regenerated lithium iron phosphate positive electrode material is 1 to 3 μm.

9. A positive electrode sheet, characterized in that: It comprises the regenerated lithium iron phosphate positive electrode material prepared by the method for recycling and regenerating waste lithium iron phosphate positive electrode material as claimed in any one of claims 1 to 8.

10. A battery, characterized in that: It includes the positive electrode sheet as claimed in claim 9, or includes the regenerated lithium iron phosphate positive electrode material prepared by the recycling and regeneration method of waste lithium iron phosphate positive electrode material as claimed in any one of claims 1 to 8.

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