A method for recycling and regenerating waste lithium iron phosphate positive electrode material, a positive electrode sheet, and a battery

Abstract

The application provides a recycling method of waste lithium iron phosphate positive electrode material, a positive electrode sheet and a battery. The recycling method comprises the following steps: S1. After the waste lithium iron phosphate positive electrode sheet is cut into small pieces, a composite oxidation aid is added and uniformly mixed, and then the mixture is calcined at 450-600 DEG C for 1.5-4h in an oxidizing atmosphere. After oscillation stripping, grinding and screening, a first calcined product is obtained; S2. The molar ratio of Li, Fe and P in the first calcined product is adjusted to 1-1.1:0.95-1.05:0.95-1.05, then the first calcined product is mixed with a composite reduction aid to obtain a mixture, and then the mixture is subjected to wet ball milling, dried and calcined at 600-900 DEG C for 8-14h to obtain a second calcined product, wherein the second calcined product is a regenerated lithium iron phosphate positive electrode material. The regenerated lithium iron phosphate positive electrode material obtained by the recycling method has high discharge specific capacity and cycle stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery recycling, and particularly relates to a recycling and regeneration method of waste lithium iron phosphate positive material, a positive plate and a battery. BACKGROUND

[0002] With the vigorous development of electric vehicles and energy storage power grids, the production of lithium ion batteries is increasing day by day, and the prices of raw materials are rising. Recycling and regenerating waste lithium ion batteries for recycling of metal resources is of great significance to solve the shortage of raw materials, environmental pollution and realize the sustainable and healthy development of the lithium ion battery industry. At present, there are two methods for recycling lithium iron phosphate (LiFePO4) positive material, namely wet metallurgy and direct regeneration repair.

[0003] Wet metallurgy needs to consume a large amount of acid and alkali chemical reagents, and involves complex separation and purification processes, which not only increases the recycling cost, but also poses a potential threat to the environment.

[0004] Direct regeneration repair mainly restores the structure and electrochemical performance of the material by adding the corresponding missing elements in the material, which can maximize the retention of elements in the material, and the process is simple, efficient, environmentally friendly and sustainable. However, although the direct regeneration repair method of waste lithium iron phosphate has many advantages, there are still many problems in practical application: (1) the LiFePO4 material prepared contains impurities such as fluorides, and part of Fe 3+ is not reduced; (2) the residual carbon from the decomposition of carbon, binder and conductive agent in the positive material is more and unevenly distributed; (3) the electrochemical performance of the regenerated positive material of severely deteriorated waste batteries is poor. SUMMARY

[0005] To solve the problems and deficiencies in the prior art, the application provides a recycling and regeneration method of waste lithium iron phosphate positive material, a positive plate and a battery. The recycling and regeneration method can effectively remove the residual impurities in the waste lithium iron phosphate, and the addition of the reduction aid not only fully reduces Fe 3+ to Fe 2+ , but also uniformly coats the carbon from the decomposition of the carbon-based reduction aid on the surface of the positive material, effectively improving the conductivity of the material, and the regenerated lithium iron phosphate positive material has high discharge specific capacity and cycle stability. Therefore, the 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 a first aspect of the present application, a recycling method of waste lithium iron phosphate positive electrode material is provided, comprising the following steps: S1. After the waste lithium iron phosphate positive electrode sheet is cut into small pieces, a composite oxidation aid is added and mixed uniformly, and then calcination is performed under an oxidizing atmosphere at 450-600℃ for 1.5-4h, followed by shock stripping, grinding, and sieving to obtain a first calcination product; 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. The molar ratio of Li, Fe, and P in the first calcination product is adjusted to 1-1.1:0.95-1.05:0.95-1.05, and then mixed with a composite reduction aid to obtain a mixture, followed by wet ball milling of the mixture, drying, and calcination at 600-900℃ for 8-14h to obtain a second calcination product, which is a regenerated lithium iron phosphate positive electrode material; the composite reduction aid comprises a first reduction aid and a second reduction aid, the first reduction aid comprises S element, and the valence of the S element is not lower than +3 and lower than +6; the second reduction aid comprises a carbon-based reduction aid containing an aldehyde group or a ketone group.

[0007] In the recycling method of waste lithium iron phosphate positive electrode material provided in the present application, first, the composite oxidation aid is mixed with the waste lithium iron phosphate positive electrode sheet cut into small pieces and calcined in oxygen, the combination of the first oxidation aid peroxide and the second oxidation aid persulfate can more effectively destroy the structure of lithium iron phosphate than using a single oxidizing agent, greatly improving 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 reaches a high leaching efficiency in a short time, improving the recycling efficiency. In the pre-oxidation process, the peroxide first rapidly oxidizes the impurities in the waste lithium iron phosphate, and then the sulfate radicals generated by the decomposition of the persulfate continuously oxidize the remaining impurities, improving the oxidation efficiency, and in this process, an oxidizing atmosphere is further introduced, which 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 kept in the range of 450-600℃, because too low temperature will reduce the oxidation effect of the waste lithium iron phosphate, and too high temperature will make the reaction in the oxidation process too violent, which is not conducive to safety.

[0008] Secondly, since the pre-treatment oxidizes and decomposes impurities, it also oxidizes the waste lithium iron phosphate, and a reduction aid needs to be added for reduction. The selection of the reduction aid also has a certain influence on the reduction effect. The composite reduction aid selected in the present application includes a low-valence sulfur compound reduction aid (first reduction aid) and a carbon-based reduction aid (second reduction aid). These two reduction 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 crystal lattice structure of lithium iron phosphate. Among them, the sulfur element in the first reduction aid can be oxidized from a low positive valence to +6 valence, and the aldehyde group or ketone group in the carbon-based reduction aid is oxidized to a carboxyl group. The reduced Fe2+ is more easily dissolved from the crystal lattice, so that the lithium element can also be leached out, and the carbon formed by the decomposition of the carbon-based reduction aid can also improve the electrical conductivity of the material. Compared with using a single oxidizing agent, the two can play a synergistic effect when used together, more effectively destroying the crystal lattice structure of lithium iron phosphate, making the lithium element more easily leached out, thereby improving the metal recovery rate. And the cost of the above-mentioned first reduction aid and second reduction aid is relatively low, and when the two are used in a certain proportion, the use 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 less impact 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 step, and enhancing the process stability; the composite reduction aid is relatively less affected by environmental factors, and can maintain good reduction performance within a relatively wide range of temperature, pH value, etc., enhancing the stability and controllability of the recovery process, and making it easier to achieve large-scale industrial production. In addition, during the reduction process, the calcination temperature is maintained within the range of 600-900°C, which can help improve the reduction efficiency.

[0009] Furthermore, through the pre-oxidation + reduction process in the present application, the uniform distribution of each element can be promoted, the structural strength of the regenerated lithium iron phosphate can be improved, the particle size distribution of the regenerated lithium iron phosphate obtained can be optimized, and the particle size can be smaller, so that the electrochemical performance of the regenerated lithium iron phosphate such as electrical conductivity, lithium ion transport performance, discharge specific capacity, and cycle stability, etc. can be effectively optimized.

[0010] Preferably, in S1, the mass ratio of the first oxidizing aid to the waste old lithium iron phosphate positive electrode sheet is 1.5-3:1, and the mass ratio of the second oxidizing aid to the waste old lithium iron phosphate positive electrode sheet is 0.01-0.06:1. Controlling the amount of the first oxidizing aid and the second oxidizing aid respectively can not only ensure that the waste old lithium iron phosphate is fully oxidized, so that the lithium element in the waste old lithium iron phosphate can be completely released, but also improve the lithium recovery rate. At the same time, it can avoid excessive oxidizing aid, so as to excessively oxidize the structure of the lithium iron phosphate positive electrode material and produce more by-products, increase the difficulty of subsequent separation and purification, and make the impurities in the regenerated lithium iron phosphate positive electrode material more, thereby reducing the charge-discharge efficiency, conductivity, cycle life and energy density, etc. At the same time, controlling the amount of the first oxidizing aid and the second oxidizing aid respectively can be more conducive to the synergistic effect of the two oxidizing aids, improve the oxidation efficiency, reduce the amount of oxidizing aid, reduce adverse side reactions, reduce impurities, and further optimize 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, and 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 also needs to be controlled within a certain range, which can not only ensure that there is enough reducing aid to reduce the material, but also ensure that there is not too much reducing aid to excessively reduce the material and avoid 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, reduces the amount of reducing aid, reduces adverse side reactions, reduces impurities, and further optimizes the material performance of the regenerated lithium iron phosphate.

[0013] Preferably, the first oxidizing aid includes at least one of hydrogen peroxide, sodium peroxide, peroxyacetic 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 metabisulfite, sodium bisulfite, and sodium dithionite; and the second reducing aid includes at least one of glucose, fructose, maltose, ribose, lactose, and galactose. Selecting the above types of first and second oxidizing aids and first and second reducing aids can better oxidize and reduce the waste old lithium iron phosphate positive electrode material, and such a combination of oxidizing aids and reducing aids has a good effect, which can effectively improve the lithium recovery rate and make the regenerated lithium iron phosphate positive electrode material have good electrochemical performance.

[0014] Preferably, the first oxidation aid comprises hydrogen peroxide; the second oxidation aid comprises ammonium persulfate; the first reduction aid comprises sodium sulfite; and the second reduction aid comprises glucose. In particular, when the first and second oxidation aids and the first and second reduction aids are selected from the above materials, the combination of these materials has the best effect, which not only can achieve sufficient oxidation and reduction of the waste lithium iron phosphate positive material, but also has the least side reactions during the oxidation and reduction processes, and is more conducive to reducing impurities and optimizing the electrochemical performance of the regenerated lithium iron phosphate positive 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 then taken out after ball milling at 500-800 r / min for 3-6 h. Further, wet grinding of the oxidized first calcined product after mixing with the composite reduction aid under the above process conditions can further uniformly distribute the elements in the material, improve the reduction efficiency and reduction quality, and be more conducive to improving the overall performance of the regenerated lithium iron phosphate positive material.

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

[0017] Preferably, the alcohol solvent comprises 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, lithium source or iron source or phosphorus source can be added to the first calcined product according to the actual situation. Here, ICP can be used to detect the content of each element Li, Fe and P in the first calcined product to determine which raw material is suitable to add.

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

[0020] Preferably, in S1, the temperature is raised to 450-600℃ at a temperature raising rate of 3-8℃ / min; and in S2, the temperature is raised to 600-900℃ at a temperature raising rate of 3-8℃ / min.

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

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

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

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

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

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

[0027] Figure 4 Particle size distribution diagram of the regenerated lithium iron phosphate positive electrode material reduced after pre-oxidation 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 described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, not all.

[0029] Example 1

[0030] The recycling and regeneration method of the waste lithium iron phosphate positive electrode material in the present embodiment comprises the following steps:

[0031] S1. After the waste lithium iron phosphate positive electrode sheet is cut into small pieces, a composite oxidation aid is added thereto and uniformly mixed, and then calcined at 550℃ for 2h under an oxidizing atmosphere at a temperature rising rate of 5℃ / min. After oscillation stripping, grinding and sieving, a first calcined product is obtained. The composite oxidation aid comprises a first oxidation aid and a second oxidation aid. The first oxidation aid is hydrogen peroxide (30%, density 1.11 g / cm 3 ), and the volume-mass ratio of hydrogen peroxide to the waste lithium iron phosphate positive electrode sheet is 5:1 (5mL:1g, the mass ratio of hydrogen peroxide to the waste lithium iron phosphate positive electrode sheet is 1.665:1 after conversion), and the second oxidation aid is ammonium persulfate, and the mass ratio of ammonium persulfate to the 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 (lithium source, iron source or phosphorus source can be added according to actual conditions, lithium source is lithium carbonate, iron source is ferrous sulfate, and phosphorus source is ammonium dihydrogen phosphate), and then mixed with a composite reduction aid to obtain a mixture. Then the mixture is poured into a ball mill tank, and an appropriate amount of anhydrous ethanol is added as a ball milling medium, and wet ball milling is carried out at 600r / min for 4h. After the end, it is taken out and dried at 60℃ for 12h, and then calcined at 700℃ for 10h at a temperature rising rate of 5℃ / min to obtain a second calcined product, which is a regenerated lithium iron phosphate positive electrode material. The composite reduction aid comprises a first reduction aid and a second reduction aid, and the amount of the composite reduction aid is 12wt% of the mass of the mixture. The first reduction aid is sodium sulfite, and the second reduction aid is glucose. The molar ratio of sodium sulfite to glucose is 1:2.

[0033] Example 2

[0034] In the recycling and regeneration method of the waste lithium iron phosphate positive electrode material in this example, the difference from example 1 is that the volume-mass ratio of the first oxidation aid hydrogen peroxide to the waste lithium iron phosphate positive electrode sheet is 3:1 (3mL:1g). The rest is consistent with example 1.

[0035] Example 3

[0036] In the recycling and regeneration method of the waste lithium iron phosphate positive electrode material in this example, the difference from example 1 is that the molar ratio of the first reduction aid sodium sulfite to the second reduction aid glucose is 1:0.5. The rest is consistent with example 1.

[0037] Example 4

[0038] In the recycling and regeneration method of the waste lithium iron phosphate positive electrode material in this example, the difference from example 1 is that the mass ratio of the second oxidation aid ammonium persulfate to the waste lithium iron phosphate positive electrode sheet is 0.075:1. The rest is consistent with example 1.

[0039] Example 5

[0040] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 1 is that the second oxidation aid is adjusted to sodium persulfate. The rest is consistent with Example 1.

[0041] Example 6

[0042] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 1 is that the first reduction aid is adjusted to sodium bisulfite. The rest is consistent with Example 1.

[0043] Example 7

[0044] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 1 is that the first and second oxidation aids are adjusted to peroxoacetic acid and potassium persulfate, respectively, and the first and second reduction aids are adjusted to sodium metabisulfite and ribose, respectively. The rest is consistent with Example 1.

[0045] Example 8

[0046] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 7 is that the first oxidation aid is adjusted to sodium peroxide, and the second reduction aid is adjusted to fructose. The rest is consistent with Example 7.

[0047] Example 9

[0048] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 1 is that in S2, the mixture is directly ground and then calcined at 700°C for 10h at a temperature rise of 5°C / min. The rest is consistent with Example 1.

[0049] Comparative Example 1

[0050] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 1 is that the second oxidation aid ammonium persulfate is not added, and the total mass of the oxidation aid is unchanged. The rest is consistent with Example 1.

[0051] Comparative Example 2

[0052] In the recycling method of the waste lithium iron phosphate positive electrode material in this example, the difference from Example 1 is that the first reduction aid sodium sulfite is not added, and the total mass of the reduction aid is unchanged. The rest is consistent with Example 1.

[0053] Comparative Example 3

[0054] The difference between the recycling method of the waste lithium iron phosphate positive electrode material in the present comparative example and example 1 is that only one oxidation aid is added, and the oxidation aid is sodium hypochlorite, and the total amount of the oxidation aid is unchanged. The rest is consistent with example 1.

[0055] Comparative example 4

[0056] The difference between the recycling method of the waste lithium iron phosphate positive electrode material in the present comparative example and example 1 is that in S1, the calcination temperature is 500°C. The rest is consistent with example 1.

[0057] Comparative example 5

[0058] The difference between the recycling method of the waste lithium iron phosphate positive electrode material in the present comparative example and example 1 is that in S2, the calcination temperature is 1100°C. The rest is consistent with example 1.

[0059] Comparative example 6

[0060] The recycling method of the waste lithium iron phosphate positive electrode material in the present example includes the following steps:

[0061] S1. After the waste lithium iron phosphate positive electrode sheet is cut into small pieces, it is calcined at 550°C for 2h under a nitrogen atmosphere, and then shaken, ground and sieved to obtain a first calcined product;

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

[0063] Test analysis

[0064] (1) Morphology characterization

[0065] The regenerated lithium iron phosphate positive electrode materials obtained in example 1 and comparative example 6 above were respectively characterized by SEM, EDS (energy spectrum analysis) and TEM, and the results are shown in Figure 1 、 2 , 3.

[0066] From the SEM pictures of Figure 1 , it can be seen that Figure 1 (a) is the regenerated lithium iron phosphate positive electrode material directly repaired in comparative example 6, Figure 1(b) Regenerated lithium iron phosphate cathode material after pre-oxidation and reduction. Directly repaired regenerated lithium iron phosphate has uneven particle size, with some tiny particles adhering to the material surface. In contrast, the regenerated lithium iron phosphate particles after pre-oxidation and reduction have smooth surfaces, relatively uniform particle size, and no longer have tiny particles adhering to the surface. This is because impurities such as PVDF, conductive carbon black, and CEI film are removed, while LiFePO4 is also oxidized and decomposed. Smaller particle size can shorten the LiFePO4 production time. + Increase the diffusion distance of Li + The diffusion rate is beneficial to the performance of electrochemical properties.

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

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

[0069] (2) Particle size distribution test

[0070] The particle size distribution of the recycled lithium iron phosphate cathode material obtained in Example 1 above was characterized, and the results are as follows: Figure 4 As shown. Figure 4 In the figure, the vertical axis represents the particle size of the regenerated lithium iron phosphate, and the horizontal axis represents the sample size, i.e., the number of particles.

[0071] Depend on Figure 4 As can be seen from the particle size distribution diagram, the lithium iron phosphate particles are relatively uniform and have a relatively small particle size, with a D50 of 1487.084 nm. Therefore, they have good lithium-ion transport performance, which makes the lithium iron phosphate cathode 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 respectively prepared into positive electrode sheets, and a half-cell (button cell) was assembled with lithium metal as the negative electrode.

[0074] The specific operation for preparing the positive electrode sheet was as follows: the regenerated lithium iron phosphate positive electrode material obtained in the above examples and comparative examples, carbon black (conductive agent) and PVDF (polyvinylidene fluoride, binder) were mixed in a ratio of 8:1:1 (mass ratio) in an NMP (N-methyl pyrrolidone) solvent, stirred and dispersed in a homogenizer, coated on an aluminum foil, then placed in an 85°C oven for baking for 30 min, punched into a round sheet with an area of 1 cm 2 , weighed and recorded, and then baked in a vacuum oven at 120°C for 8 h to obtain the positive electrode sheet, which was the working electrode.

[0075] The specific operation for preparing the button cell was as follows: CR-2032 coin cell assembly was performed in a glove box, lithium metal was used as the counter / reference electrode, Celgard was used as the separator, and 1M LiPF6, 1:1.1 (mass ratio) dimethyl carbonate (DMC) and ethylene carbonate (EC) were used as the electrolyte.

[0076] Subsequently, the obtained battery was tested for the following electrochemical performance.

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

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

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

[0080]

[0081] ② Test of AC impedance and lithium ion diffusion coefficient (D

[0082] The batteries prepared from the regenerated lithium iron phosphate positive electrode materials obtained in Example 1 and Comparative Example 6 above were subjected to AC impedance and lithium ion diffusion coefficient (DLi+) tests after 5 cycles and 100 cycles, respectively, according to the following specific test method: an electrochemical workstation was used to test the lithium ion battery by EIS, the frequency range was selected to be 0.01 Hz to 1 kHz, the voltage signal was selected to be about 5-15 mV in amplitude, the current signal was selected to be less than 10% of the working current, impedance data at different frequencies were obtained, an impedance spectrum was plotted, and impedance data at 5 cycles and 100 cycles were 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, wherein R is the gas constant 8.314 J / (mol*K), T is the absolute temperature, A is the electrode surface area, n is the number of transfer electrons per mole of substance participating in the electrode reaction, F is the Faraday constant 96485.33 C / mol, and C is the concentration of lithium in the electrode. The lithium ion diffusion coefficient (DLi+) at 5 cycles and 100 cycles can be calculated according to the above method. At the same time, the batteries prepared from the regenerated lithium iron phosphate positive electrode materials obtained in the remaining examples and comparative examples were subjected to AC impedance and lithium ion diffusion coefficient (DLi+) tests after 100 cycles according to the above AC impedance and lithium ion diffusion coefficient (DLi+) method.

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

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

[0085]

[0086] As can be seen from Tables 1 and 2, the regenerated lithium iron phosphate positive electrode material obtained by the waste lithium iron phosphate positive electrode material recycling method provided by the present application has a higher discharge specific capacity at different rates, and also has a lower AC impedance and a higher lithium ion diffusion coefficient. For specific reference, see Examples 1-9.

[0087] The second oxidation aid, ammonium persulfate, is not added in Comparative Example 1; the first reduction aid, sodium sulfite, is not added in Comparative Example 2; only one oxidation aid, sodium hypochlorite, is added in Comparative Example 3; the calcination temperature in S1 is too low in Comparative Example 4; the calcination temperature in S2 is too high in Comparative Example 5; and the waste lithium iron phosphate positive electrode sheet is directly repaired and regenerated without the pre-oxidation and reduction processes in Comparative Example 6. These factors all result in the decrease of the rate performance, the increase of the alternating current impedance, and the decrease of the lithium ion diffusion coefficient of the regenerated lithium iron phosphate positive electrode material, which indicates that the composite oxidation aid and the composite reduction aid in the present application need to be used in a specific combination, and a specific calcination temperature is needed in the process of treating the waste lithium iron phosphate positive electrode sheet, which is more conducive to the oxidation and reduction of the waste lithium iron phosphate positive electrode sheet, and thus is more conducive to the regeneration of the waste lithium iron phosphate positive electrode sheet, so as to further optimize the regenerated lithium iron phosphate positive electrode material. In particular, the process of pre-oxidation and reduction is not included in Comparative Example 6, so the lithium recovery rate cannot be sufficiently improved, and the electrochemical performance of the regenerated lithium iron phosphate positive electrode material is also not conducive.

[0088] Further comparison between Example 1 and Examples 2-4 shows that the amount of the first oxidation aid, hydrogen peroxide, is too small in Example 2, the molar ratio of the first reduction aid, sodium sulfite, to the second reduction aid, glucose, is not within the range of 1:1-4 in Example 3, and the amount of the second oxidation aid, ammonium persulfate, is too large in Example 4. The above factors make the rate performance, the conductivity, and the lithium ion transmission performance in Examples 2-4 worse than those in Example 1, which indicates that the amount of the oxidation aid and the reduction aid has a certain influence on the performance of the regenerated lithium iron phosphate positive electrode material, and finally affects the performance of the battery.

[0089] Comparison between Example 1 and Examples 5-8 shows that the performance of the battery in Examples 5-8 is decreased due to the change of the type of the first or second oxidation aid or the first or second reduction aid, which also indicates that the type combination of the oxidation aid and the reduction aid has a certain influence on the regeneration process of the waste lithium iron phosphate positive electrode sheet, and thus affects the performance of the regenerated lithium iron phosphate positive electrode material and the battery prepared therefrom. As can be seen from the above performance results, the type combination of the first and second oxidation aids and the first and second reduction aids in Example 1 is the best, and the type combination of the first and second oxidation aids and the first and second reduction aids in Example 7 is relatively good.

[0090] Comparison between Example 1 and Example 9 shows that the performance of the regenerated lithium iron phosphate positive electrode material and the battery is decreased due to the insufficient reduction reaction in the calcination process without the ball milling operation in Example 9.

[0091] The above examples are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application. Although the present application is described in detail with reference to the above examples, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently, and these modifications or replacements are within the protection scope of the present application.

Claims

1. A method for recycling and regenerating waste lithium iron phosphate cathode materials, characterized in that, Includes the following steps: S1. After cutting the waste lithium iron phosphate cathode sheet into small pieces, a composite oxidizing agent is added and mixed evenly. Then, the mixture is calcined at 450-600°C for 1.5-4 hours under an oxidizing atmosphere. After shaking, peeling, grinding, and sieving, the first calcined product is obtained. The composite oxidizing agent includes a first oxidizing agent and a second oxidizing agent. The first oxidizing agent includes peroxide, and the second oxidizing agent includes 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, then mix it with the composite reducing agent to obtain a mixture. Subsequently, the mixture is wet-milled, dried, and calcined at 600–900°C for 8–14 hours to obtain a second calcined product. The second calcined product is a regenerated lithium iron phosphate cathode material. The composite reducing agent includes a first reducing agent and a second reducing agent. The first reducing agent includes sulfur (S) with a valence state not lower than +3 and lower than +6. The second reducing agent includes a carbon-based reducing agent containing aldehyde or ketone groups.

2. The method for recycling and regenerating waste lithium iron phosphate cathode material as described in claim 1, characterized in that: In step S1, the mass ratio of the first oxidizing agent to the waste lithium iron phosphate cathode sheet is 1.5 to 3:1, and the mass ratio of the second oxidizing agent to the waste lithium iron phosphate cathode sheet is 0.01 to 0.06:

1.

3. The method for recycling and regenerating waste lithium iron phosphate cathode material as described in claim 1, characterized in that: In step S2, the amount of the composite reducing agent is 8-16 wt% of the mass of the mixture; The molar ratio of the first reducing agent to the second reducing agent is 1:1 to 4.

4. The method for recycling and regenerating waste lithium iron phosphate cathode material as described in 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 agent includes at least one of ammonium persulfate, sodium persulfate, and potassium persulfate; The first reducing agent includes at least one of sodium sulfite, sodium metabisulfite, sodium bisulfite, and sodium dithionite; The second reducing agent includes at least one of glucose, fructose, maltose, ribose, lactose, and galactose.

5. The method for recycling and regenerating waste lithium iron phosphate cathode material as described in claim 4, characterized in that: The first oxidizing agent includes hydrogen peroxide; The second oxidizing agent includes ammonium persulfate; The first reducing agent includes sodium sulfite; The second reducing agent includes glucose.

6. The method for recycling and regenerating waste lithium iron phosphate cathode material as described in claim 1, characterized in that: In step 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 reducing agent is dispersed in an alcohol solvent, ball-milled at 500-800 r / min for 3-6 h, and then dried at 50-70 °C for 10-14 h.

7. The method for recycling and regenerating waste lithium iron phosphate cathode material as described in 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 and regenerating waste lithium iron phosphate cathode material as described in claim 1, characterized in that: The D50 of the regenerated lithium iron phosphate cathode material is 1–3 μm.

9. A positive electrode plate, characterized in that: This includes the regenerated lithium iron phosphate cathode material prepared by the recycling and regeneration method of waste lithium iron phosphate cathode material as described in any one of claims 1 to 8.

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

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