Recycling method of waste lithium iron phosphate positive electrode material and lithium iron phosphate material

Abstract

The invention discloses a waste lithium iron phosphate positive electrode material recycling method and a lithium iron phosphate material, and the method comprises the following steps: calcining a waste lithium iron phosphate positive electrode material to obtain a lithium iron phosphate / carbon powder material; the element content of the lithium iron phosphate / carbon powder is obtained, and the amount of lithium, iron and phosphorus needing to be supplemented is determined; lithium, iron and phosphorus are supplemented to the lithium iron phosphate / carbon powder, then a reducing agent is added and mixed, a mixture is obtained, and the reducing agent comprises at least one of a non-amino acid reducing agent and an amino acid reducing agent; and calcining the mixture to obtain the recycled and regenerated lithium iron phosphate material. According to the method, elements missing in the waste lithium iron phosphate material are analyzed and supplemented, and the type of the reducing agent is adjusted, so that efficient repair of the waste lithium iron phosphate material is realized. The repairing efficiency of the waste lithium iron phosphate material is high, and the battery assembled by the repaired lithium iron phosphate material has excellent rate capability and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery material recycling, and in particular to a method for recycling waste lithium iron phosphate positive electrode materials and lithium iron phosphate materials. Background Art

[0002] Lithium-ion batteries are widely used in many fields due to their excellent performance characteristics, including high energy density, long life, low self-discharge rate and low pollution. The positive electrode material is the most important component of lithium-ion batteries, accounting for 30% of the battery cost structure. Lithium iron phosphate is considered to be an ideal new generation of lithium-ion battery positive electrode material due to its low price, environmental friendliness, good cycle performance and high safety performance. However, as the number of cycles increases, the lithium iron phosphate positive electrode material will continue to age, causing the battery performance to decrease. It can be foreseen that in the near future, the waste lithium iron phosphate batteries will show explosive growth.

[0003] The waste lithium iron phosphate positive electrode material has a low content of valuable components, which is not suitable for recycling and reuse by high-cost hydrometallurgical and pyrometallurgical technologies, and the wet dissolution or pyrometallurgical melting process will cause secondary damage to the crystal structure. As a very promising solution, repair and regeneration technology can not only protect the environment and save resources, but also simplify the recycling process and improve economic benefits. There are also some methods for regenerating waste lithium iron phosphate and achieving certain results. For example, CN116789096A discloses a regenerated lithium iron phosphate positive electrode material and a preparation method thereof, which includes analyzing the amount of lithium supplementation, phosphorus supplementation, iron supplementation and carbon supplementation after high-temperature treatment of waste lithium iron phosphate, and calcining the material after supplementing the corresponding elements. This method can prepare carbon-coated regenerated lithium iron phosphate positive electrode materials with good electrochemical properties. However, the obtained regenerated material was only tested at a rate lower than 0.2C, and the cycle effect at a higher rate was unpredictable. At the same time, there are also some studies using reducing agents such as glucose and citric acid to repair waste lithium iron phosphate. The reducing agent not only reduces the oxidized iron, but also acts as a uniform carbon layer to enhance the conductivity of the material.

[0004] However, the effect of using reducing agents for repair is not ideal. It is necessary to construct a method for recycling waste lithium iron phosphate positive electrodes into high-performance positive electrode materials. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a method for recycling and regenerating waste lithium iron phosphate positive electrode materials, comprising: Calcining the waste lithium iron phosphate positive electrode material to obtain lithium iron phosphate / carbon powder; Obtaining the element content of the lithium iron phosphate / carbon powder material to determine the amount of lithium, iron and phosphorus that needs to be supplemented; Adding lithium, iron and phosphorus to the lithium iron phosphate / carbon powder material and then adding a reducing agent to mix to obtain a mixture, wherein the reducing agent includes at least one of a non-amino acid reducing agent or an amino acid reducing agent; The mixture is calcined to obtain recycled lithium iron phosphate material.

[0006] Furthermore, the non-amino acid reducing agent includes at least one of acetic acid, citric acid, glucose and sucrose; The amino acid reducing agent includes at least one of glycine, alanine, valine, leucine, proline, tryptophan, serine, and histidine. The molar ratio of the non-amino acid reducing agent to the amino acid reducing agent is 1-20:1.

[0007] Furthermore, the molar ratio of iron in the lithium iron phosphate / carbon powder material after replenishing lithium, iron and phosphorus to the added reducing agent is 5-20:1.

[0008] Furthermore, the molar ratio of lithium, iron and phosphorus in the lithium iron phosphate / carbon powder material after the lithium, iron and phosphorus are supplemented is 1-1.05:1:1.

[0009] Furthermore, the calcination of the waste lithium iron phosphate positive electrode material is continued for 3-5 hours in an inert gas atmosphere or vacuum at a temperature of 400-600°C.

[0010] Furthermore, the mixture is calcined at a temperature of 700-900° C. in an inert gas atmosphere or in a vacuum for 4-6 hours.

[0011] Furthermore, the mixture is obtained by wet ball milling with ethanol as a dispersion medium and then drying.

[0012] Furthermore, the wet ball milling is continued for 2-4 hours at a rotation speed of 400-600 rpm.

[0013] It should be noted that the waste lithium iron phosphate positive electrode material of the present invention is obtained by disassembling a lithium battery and peeling off the aluminum foil, and is obtained according to a conventional processing process. Since it also includes conductive carbon black and an organic binder, lithium iron phosphate / carbon powder can be obtained after calcination.

[0014] It should be noted that the supplements of lithium, iron and phosphorus in the present invention do not need to be strictly limited. For example, lithium can be provided by at least one of lithium carbonate, lithium oxalate, lithium acetate, lithium phosphate, lithium hydroxide, etc.; iron can be provided by at least one of ferrous oxalate, ferric phosphate, ferric chloride, ferric nitrate, ferrous sulfate, etc.; phosphorus can be provided by at least one of diammonium phosphate, diammonium hydrogen phosphate, etc.

[0015] The main reason for the decline in performance of lithium iron phosphate batteries is Li + The lack of Fe2+ Oxidized to Fe 3+ , and induces the migration of iron ions to produce Fe-Li antisite defects. Although it has been charged and discharged for a long time many times, the lattice and morphology of the waste lithium iron phosphate are maintained, which provides the possibility for the repair and regeneration of waste lithium iron phosphate positive electrode materials. The use of a reducing agent to reduce the oxidized iron can repair the waste lithium iron phosphate material. In addition, amino acids, as natural biomass, are one of the renewable resources with the lowest cost and the most abundant resources, and compared with other polymer biomass resources, amino acids have the characteristics of natural degradation and harmless degradation. In addition, even if amino acids are carbonized at a higher temperature, their products can still maintain a high nitrogen content, and the nitrogen element can be doped into the product very evenly. The present invention uses amino acids and other types of reducing agents to repair waste lithium iron phosphate, which can form a stable nitrogen-doped carbon coating layer, thereby achieving efficient repair of waste lithium iron phosphate positive electrode materials.

[0016] The present invention also provides a lithium iron phosphate material prepared by the above method.

[0017] The present invention also provides application of the above-mentioned lithium iron phosphate material in lithium ion battery materials.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The present invention realizes efficient repair of waste lithium iron phosphate materials by analyzing and supplementing the missing elements in waste lithium iron phosphate materials, and adjusting the type of reducing agent. Overall, the present invention introduces fewer types of production reagents and high-demand process steps, and has high repair efficiency for waste lithium iron phosphate materials. Batteries assembled with the repaired lithium iron phosphate materials have excellent rate performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1a , Figure 1b , Figure 1c The scanning electron microscope images of the lithium iron phosphate materials of Example 1, Example 2 and Comparative Example 1 are shown respectively; Figure 2 The XRD patterns of lithium iron phosphate materials of Example 1, Example 2 and Comparative Example 1 are shown; Figure 3 The cyclic voltammetry curves of the half-cell assembled with the lithium iron phosphate material of Example 1 at different scan rates are shown; Figure 4 The cyclic voltammetry curves of the half-cell assembled with the lithium iron phosphate material of Example 2 at different scan rates are shown; Figure 5 The cyclic voltammetry curves of the half-cell assembled with the lithium iron phosphate material of Comparative Example 1 at different scan rates are shown; Figure 6 The first cycle charge and discharge curves of the half-cells assembled with the lithium iron phosphate materials of Examples 1 to 4 and Comparative Example 1 at a rate of 1C are shown; Figure 7 A comparison chart of the discharge specific capacities of the half-cells assembled in Examples 1-3 and the comparative example at 0.5C, 1C, 2C, 3C and 5C is shown; Figure 8 A cycle comparison diagram of half cells assembled in Examples 1-4 and Comparative Examples 1-2 at a 1C rate is shown; Fig. 9 A cycle comparison diagram of half cells assembled in Examples 1, 5, 6 and Comparative Example 1 at a 1C rate is shown. DETAILED DESCRIPTION

[0021] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0022] The following will be combined with the specific embodiments of the present invention and the drawings of the specification to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] Example 1 A method for recycling waste lithium iron phosphate positive electrode materials, the steps are as follows: (1) The waste lithium iron phosphate cathode material to be repaired after stripping the aluminum foil is used as a raw material, pushed into a tube furnace, and calcined at 500°C for 2 hours in an argon atmosphere to obtain a lithium iron phosphate / carbon powder material; (2) An inductively coupled plasma spectrometer (ICP) was used to detect the content of each element in the lithium iron phosphate / carbon powder. According to the test results, the corresponding elements were supplemented with a molar ratio of lithium, iron and phosphorus of 1.03:1:1. The ICP results showed that the molar ratio of lithium, iron and phosphorus was 0.7:1:1, so lithium supplementation was required; (3) 5 g of lithium iron phosphate / carbon powder was placed in a ball mill, and then supplemented with lithium: 8.5 mmol lithium carbonate, reducing agent: 2.5 mmol citric acid and 1 mmol proline (molar ratio of 2.5:1) were added, and wet ball milling was performed at a speed of 500 rpm for 3 h using 30 mL of anhydrous ethanol as a dispersion medium to obtain a slurry. The slurry was placed in an 80°C forced air drying oven for 12 h to fully dry it to obtain a powder; (4) The powder is pushed into a tubular furnace and calcined at 800° C. for 4 h in an argon atmosphere to obtain recycled lithium iron phosphate material.

[0024] Example 2 The process is basically the same as Example 1, except that the reducing agent in step (3) is 3.5 mmol citric acid.

[0025] Example 3 The process is basically the same as Example 1, except that the reducing agent in step (3) is 3.5 mmol glucose.

[0026] Example 4 The process is basically the same as Example 1, except that the reducing agent in step (3) is 3.5 mmol proline.

[0027] Example 5 The method is basically the same as Example 1, except that the reducing agent in step (3) is obtained by mixing citric acid and proline in a molar ratio of 10:1.

[0028] Example 6 The method is basically the same as Example 1, except that the reducing agent in step (3) is obtained by mixing citric acid and proline in a molar ratio of 1:1.

[0029] Embodiment 7-13 The method is basically the same as Example 1, except that: the reducing agent in step (3) is replaced by glycine, alanine, valine, leucine, tryptophan, serine, and histidine instead of proline.

[0030] Comparative Example 1 Waste lithium iron phosphate positive electrode material to be repaired after stripping the aluminum foil.

[0031] Comparative Example 2 A method for recycling waste lithium iron phosphate positive electrode materials, the steps are as follows: (1) The waste lithium iron phosphate cathode material to be repaired after stripping the aluminum foil is used as a raw material, pushed into a tube furnace, and calcined at 500°C for 2 hours in an argon atmosphere to obtain a lithium iron phosphate / carbon powder material; (2) An inductively coupled plasma spectrometer (ICP) was used to detect the content of each element in the lithium iron phosphate / carbon powder. According to the test results, the corresponding elements were supplemented with a molar ratio of lithium, iron and phosphorus of 1.03:1:1. The ICP results showed that the molar ratio of lithium, iron and phosphorus was 0.7:1:1, so lithium supplementation was required; (3) 5 g of lithium iron phosphate / carbon powder was placed in a ball mill, and then supplemented with lithium: 8.5 mmol of lithium carbonate was added. The mixture was wet-milled at 500 rpm for 3 h using 30 mL of anhydrous ethanol as a dispersion medium to obtain a slurry. The slurry was placed in an 80°C forced air drying oven for 12 h to fully dry the slurry to obtain a powder. (4) The powder is pushed into a tubular furnace and calcined at 800° C. for 4 h in an argon atmosphere to obtain recycled lithium iron phosphate material.

[0032] Test Case The microscopic morphology of the materials of Example 1, Example 2 and Comparative Example 1 was observed by scanning electron microscopy, and the results were as follows: Figure 1a , Figure 1b , Figure 1c As shown, it can be seen that compared with the untreated comparative example 1, there are obviously fewer impurities on the surfaces of Example 1 and Example 2.

[0033] The XRD results of the materials of Example 1, Example 2 and Comparative Example 1 are as follows: Figure 2 As shown, it can be seen that the diffraction peaks of these materials are in similar positions, with obvious sharp peaks matching the lithium iron phosphate standard card, which indicates that the crystal form of the material has no obvious change after the recycling and regeneration of the present invention.

[0034] The lithium iron phosphate materials of the embodiments of the present invention and the comparative examples were respectively assembled into batteries as positive electrode active materials for electrochemical testing. The specific process is: weigh 400 mg of positive electrode material and 50 mg of conductive carbon black, grind them evenly in a mortar, then add 50 mg of polyvinylidene fluoride (PVDF) into a vial containing 2 mL of N-methylpyrrolidone, stir evenly, add the positive electrode material and conductive carbon black, stir again to form a well-dispersed slurry, and then evenly coat it on aluminum foil, put it in an 80°C blast oven for 12 hours to dry, and then cut it into a positive electrode sheet with a diameter of 10 mm by a cutting machine; use a high-purity lithium sheet as the negative electrode, a Celgard 2500 diaphragm as the diaphragm, and an electrolyte containing 1.0 mol / L lithium hexafluorophosphate, 1.0% vinylene carbonate, a mixture of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate in a volume ratio of 1:1:1, and assemble it into a half-battery.

[0035] Figure 3 , Figure 4 and Figure 5The cyclic voltammetry curves of the half-cells assembled in Example 1, Example 2 and Comparative Example 1 at different scan rates respectively show that the cyclic voltammetry curve of the unrepaired waste lithium iron phosphate in Comparative Example 1 is disordered, while the oxidation peak and reduction peak of the cyclic voltammetry curves of Examples 1 and 2 are more symmetrical, and the difference between the oxidation potential and the reduction potential is smaller.

[0036] The assembled half-cell was cycled at a test voltage of 2.5-4.2V and a rate of 1C. Figure 6 The first cycle charge and discharge curves of the half-cells assembled in Examples 1 to 4 and Comparative Example 1 at a rate of 1C are shown, and it can be seen that the charge and discharge specific capacity of the waste lithium iron phosphate increases significantly after regeneration and repair by the method of the present invention. Table 1 also summarizes the cycle test results of the half-cells assembled in the Examples and Comparative Examples.

[0037] Table 1 Battery cycle performance test results

[0038] From the test results in Table 1, it can be seen that Example 1 has the highest first-cycle discharge specific capacity of 120.1 mAh / g, and the capacity retention rate after 50 cycles is 99.6%, which is significantly better than other examples and comparative examples. Among them, Example 2, Example 3, and Example 4 only use citric acid, glucose, and proline as reducing agents, respectively; compared with Example 1, the molar ratio of citric acid and proline in the reducing agent in Example 5 and Example 6 is different; compared with Example 1, Comparative Example 2 does not add any reducing agent. These results show that the addition of a reducing agent during calcination can form a uniform carbon layer to improve the conductivity of the material, thereby improving the electrical properties and stability of the material; the introduction of an amino acid reducing agent can introduce nitrogen doping into the uniform carbon layer, thereby further improving the performance of the repaired lithium iron phosphate material; the results of Example 1, Example 5, and Example 6 further illustrate that optimizing the ratio of amino acid reducing agents to non-amino acid reducing agents can further optimize the performance of the repaired lithium iron phosphate.

[0039] The discharge performance of the battery at a higher rate was also tested and the results are shown in Table 2.

[0040] Table 2 Battery rate performance test results

[0041] It can be seen from the results in Table 2 that the half-cell assembled in Example 1 has the best rate performance.

[0042] Figure 7The discharge specific capacities of the half-cells assembled in Examples 1-3 and the comparative examples at 0.5C, 1C, 2C, 3C and 5C, and the discharge capacity when the rate is restored to 0.5C are shown. Among them, Example 1 exhibits excellent discharge specific capacities of 135, 124, 112, 105 and 94 mAh / g at 0.5C, 1C, 2C, 3C and 5C, respectively. When the rate is restored to 0.5C, the reversible capacity is restored to 135 mAh / g, also showing excellent reversible specific capacity.

[0043] Figure 8 and Fig. 9 A cycle comparison diagram of half-cells assembled from different embodiments and comparative examples at a 1C rate is shown, and it can be seen that embodiment 1 has the best cycle performance.

[0044] The recycled lithium iron phosphate materials prepared in Examples 7-13 were also assembled into half-cells, and their performance was close to that of Example 1.

[0045] In summary, the present invention achieves efficient repair of waste lithium iron phosphate materials by analyzing the missing elements in the waste lithium iron phosphate materials and supplementing them, and adjusting the type of reducing agent. Overall, the present invention introduces fewer types of production reagents and high-demand process steps, and at the same time has high repair efficiency for waste lithium iron phosphate materials, and the battery assembled with the repaired material has excellent rate performance.

[0046] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for recycling waste lithium iron phosphate positive electrode materials, characterized in that: include, Calcining the waste lithium iron phosphate positive electrode material to obtain lithium iron phosphate / carbon powder; Obtaining the element content of the lithium iron phosphate / carbon powder material to determine the amount of lithium, iron and phosphorus that needs to be supplemented; Adding lithium, iron and phosphorus to the lithium iron phosphate / carbon powder material and then adding a reducing agent to mix to obtain a mixture, wherein the reducing agent includes at least one of a non-amino acid reducing agent or an amino acid reducing agent; The mixture is calcined to obtain recycled lithium iron phosphate material.

2. The method according to claim 1, characterized in that The non-amino acid reducing agent includes at least one of acetic acid, citric acid, glucose and sucrose; The amino acid reducing agent includes at least one of glycine, alanine, valine, leucine, proline, tryptophan, serine, and histidine; The molar ratio of the non-amino acid reducing agent to the amino acid reducing agent is 1-20:

1.

3. The method according to claim 1, characterized in that The molar ratio of the reducing agent to the iron in the lithium iron phosphate / carbon powder material after the lithium, iron and phosphorus are supplemented is 5-20:

1.

4. The method according to claim 1, characterized in that: The molar ratio of lithium, iron and phosphorus in the lithium iron phosphate / carbon powder material after the lithium, iron and phosphorus are supplemented is 1-1.05:1:

1.

5. The method according to claim 1, characterized in that The calcination of the waste lithium iron phosphate positive electrode material is carried out in an inert gas atmosphere or vacuum at a temperature of 400-600° C. for 3-5 hours.

6. The method according to claim 1, characterized in that The mixture is calcined at a temperature of 700-900° C. in an inert gas atmosphere or in a vacuum for 4-6 hours.

7. The method according to any one of claims 1 to 6, characterized in that: The mixture is obtained by wet ball milling with ethanol as a dispersion medium and then drying.

8. The method according to claim 7, characterized in that The wet ball milling is continued for 2-4 hours at a rotation speed of 400-600 rpm.

9. A lithium iron phosphate material, characterized in that: The method is prepared by any one of claims 1 to 8.

10. Use of the lithium iron phosphate material according to claim 9 in lithium ion battery materials.

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