Regenerated lithium iron phosphate composite material and preparation method thereof

Abstract

The invention relates to the technical field of battery material recovery and regeneration, in particular to a regenerated lithium iron phosphate composite material and a preparation method thereof. The method comprises the following steps: mixing the iron phosphate waste residue, a lithium source, an organic carbon source and a phosphoric acid source in the presence of an organic solvent, and heating at 400-500 DEG C in an inert atmosphere to obtain a precursor; and in the presence of a solvent, mixing the precursor and graphene oxide, performing freeze drying, and performing heat treatment at 650-800 DEG C in an inert atmosphere to obtain the regenerated lithium iron phosphate composite material. According to the method, on one hand, the battery material is effectively regenerated, and on the other hand, a multi-layer conductive three-dimensional network structure is formed through two-step coating, so that the rate capability of the regenerated lithium iron phosphate composite material and the long cycle stability under high rate can be greatly improved. According to the method, the complete lithium iron phosphate material regeneration chain can be built, and the economic benefit of lithium iron phosphate hydrometallurgy is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery material recovery and regeneration, and in particular to a regenerated lithium iron phosphate composite material and a preparation method thereof. Background Art

[0002] In recent years, the generation of a large number of discarded lithium-ion batteries has caused serious resource and environmental problems. Among them, batteries based on lithium iron phosphate (LiFePO4) positive electrode materials occupy a large share, and will usher in a larger peak of retirement in recent years. In response to this situation, it is urgent to establish a complete lithium iron phosphate material regeneration chain. At present, the commercial recovery strategy for degraded LiFePO4 materials is mainly to recover the Li element through wet chemical methods (a few companies also extract lithium through electrochemical methods). However, in these large-scale lithium extraction processes, only Li has economic value, while other products are treated at low prices or piled up as by-products, which makes the recovery efficiency of lithium iron phosphate low and also causes environmental pollution. However, as the main by-product, iron phosphate waste slag (mainly FePO4) still has regeneration value.

[0003] Therefore, there is an urgent need to develop a preparation method for regenerating lithium iron phosphate composite materials from iron phosphate waste residue. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the present invention provides a regenerated lithium iron phosphate composite material and a preparation method thereof, which can effectively and fully utilize FePO4 waste slag generated by hydrometallurgy, avoid environmental pollution, and improve the economic benefits of hydrometallurgy.

[0005] To this end, the first aspect of the present invention provides a method for preparing a regenerated lithium iron phosphate composite material, comprising the following steps:

[0006] In the presence of an organic solvent, waste iron phosphate residue, a lithium source, an organic carbon source and a phosphoric acid source are mixed, and then heated at 400-500° C. in an inert atmosphere to obtain a precursor;

[0007] In the presence of a solvent, the precursor and graphene oxide are mixed, freeze-dried, and then heat-treated at 650-800° C. in an inert atmosphere to obtain a regenerated lithium iron phosphate composite material.

[0008] The invention adopts a step-by-step coating and multi-step annealing method, and the step-by-step coating has the advantages that: first, an organic carbon source is added when preparing a lithium iron phosphate precursor, and after heating at 400-500°C, the organic carbon source forms an amorphous carbon layer, which can be well coated on the surface of the lithium iron phosphate precursor, hindering the growth of particles in the subsequent heat treatment process, and at the same time can become a point-to-point conductive node between the lithium iron phosphate precursor particles; then, the reduced graphene oxide formed by the heat treatment can form a point-to-point conductive network in a large range, and the lithium iron phosphate precursor is coated with graphene oxide, and the lithium iron phosphate precursor is attached to the reduced graphene oxide. The formed three-dimensional network structure can greatly increase the contact area between the material and the electrolyte, improve the interface energy, and increase the Li + The active sites for binding and detachment of Li + Diffusion dynamics, reduce the difference in diffusion rates between electrons and ions, and reduce the degree of electrochemical polarization. Multi-step annealing can assist in the formation of a multi-level conductive network to prevent inorganic carbon or organic carbon from agglomerating alone. In addition, the material stress can be eliminated to a certain extent during the annealing cooling and then heating process to avoid structural collapse of the regenerated lithium iron phosphate material during the deintercalation and embedding of lithium ions. After such step-by-step coating and multi-step annealing, the multi-level conductive three-dimensional network structure formed can greatly improve the rate performance of the regenerated lithium iron phosphate composite material and the long-cycle stability at high rates. In addition, the process flow of the present invention is simple and easy to achieve large-scale production.

[0009] In some embodiments of the present invention, the iron phosphate waste residue after lithium extraction in the hydrometallurgical industry can be reduced to obtain lithium iron phosphate; the iron phosphate waste residue includes one or more of lithium iron phosphate, dihydrated iron phosphate, and iron phosphate. The present invention utilizes the industrial lithium extraction waste iron phosphate waste residue to regenerate into regenerated lithium iron phosphate composite materials, effectively solving the current large amount of iron phosphate waste waste in industrial production. Aiming at the technical difficulties such as low efficiency of lithium iron phosphate recovery, it fills the process gap of direct conversion and regeneration of iron phosphate waste residue. The intervention of organic carbon source can convert Fe in iron phosphate into 3+ Reduction to Fe 2+ , avoiding the subsequent consumption of relatively more expensive graphene oxide, and at the same time coating the particles so that they will not continue to grow during the subsequent annealing process.

[0010] In some embodiments of the present invention, the lithium source includes one or more of lithium hydroxide, lithium carbonate, and lithium acetate.

[0011] In some embodiments of the present invention, the organic carbon source includes one or more of glucose, ascorbic acid, citric acid, phenolic resin, polyvinyl pyrrolidone, and polyacrylic acid.

[0012] In some embodiments of the present invention, the phosphoric acid source includes one or more of phosphoric acid and diammonium phosphate.

[0013] In some embodiments of the present invention, the ferric phosphate waste slag is sintered at 400-500° C. for 1-2 hours to obtain pre-oxidized ferric phosphate waste slag, and then the pre-oxidized ferric phosphate waste slag, a lithium source, an organic carbon source and a phosphoric acid source are mixed.

[0014] In some embodiments of the present invention, the mass ratio of the pre-oxidized iron phosphate waste residue, the lithium source, the organic carbon source and the phosphoric acid source is 1:(0.1-1):(0.01-0.5):(0.01-0.5).

[0015] In some embodiments of the present invention, the mass ratio of the precursor to graphene oxide is (5-30):1.

[0016] In some embodiments of the present invention, the freeze drying temperature is -20 to -50°C, the time is 48-72 hours, and the vacuum degree is 100-1000 Pa. Freeze drying helps to maintain the chemical stability of the material and improve the rate performance and cycle performance of the material.

[0017] In some embodiments of the present invention, the mixing of the iron phosphate waste residue, the lithium source, the organic carbon source and the phosphoric acid source is carried out by ball milling; the rotation speed of the ball mill is 400-600 rpm, and the time is 8-10 hours.

[0018] The second aspect of the present invention provides a regenerated lithium iron phosphate composite material obtained by the preparation method of the regenerated lithium iron phosphate composite material.

[0019] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0021] Figure 1 This is a flow chart for preparing the regenerated lithium iron phosphate composite material according to Example 1 of the present invention;

[0022] Figure 2 The X-ray diffraction patterns of the materials prepared in the examples and comparative examples and the waste ferric phosphate residue (D-FP);

[0023] Figure 3 The scanning electron microscope images of the waste iron phosphate before and after pretreatment in the present invention;

[0024] Figure 4 The scanning electron microscope images of the materials prepared in the examples and comparative examples are shown;

[0025] Figure 5Energy dispersive X-ray spectroscopy element distribution diagram of the material prepared in Example 2;

[0026] Figure 6 The first cycle charge and discharge curves of the materials prepared in the examples and comparative examples at 0.1C, 25°C, and a voltage range of 2.5-4.2V;

[0027] Figure 7 The rate performance test curves of the materials prepared in the examples and comparative examples and commercial lithium iron phosphate (C-LFP) at different charging rates, 25°C and a voltage range of 2.5-4.2V;

[0028] Figure 8 The cycle performance test curves of the materials prepared in the examples and comparative examples at 1C, 25°C, and a voltage range of 2.5-4.2V;

[0029] Fig. 9 The cycle performance test curves of the materials prepared in the examples and comparative examples at 5C, 25°C, and a voltage range of 2.5-4.2V;

[0030] Fig.10 The materials prepared in the examples and comparative examples were tested at 0.1 mV / s cyclic voltammetry. DETAILED DESCRIPTION

[0031] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be understood as limiting the present invention.

[0032] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. Further, in the description of the present invention, unless otherwise specified, the meaning of "plurality" is two or more.

[0033] The first aspect of the present invention provides a method for preparing a regenerated lithium iron phosphate composite material, comprising the following steps:

[0034] In the presence of an organic solvent, waste iron phosphate residue, a lithium source, an organic carbon source and a phosphoric acid source are mixed, and then heated at 400-500° C. in an inert atmosphere to obtain a precursor;

[0035] In the presence of a solvent, the precursor and graphene oxide are mixed, freeze-dried, and then heat-treated at 650-800° C. in an inert atmosphere to obtain a regenerated lithium iron phosphate composite material.

[0036] The invention adopts a step-by-step coating and multi-step annealing method, and the step-by-step coating has the advantages that: first, an organic carbon source is added when preparing a lithium iron phosphate precursor, and after heating at 400-500°C, the organic carbon source forms an amorphous carbon layer, which can be well coated on the surface of the lithium iron phosphate precursor, hindering the growth of particles in the subsequent heat treatment process, and at the same time can become a point-to-point conductive node between the lithium iron phosphate precursor particles; then, the reduced graphene oxide formed by the heat treatment can form a point-to-point conductive network in a large range, and the lithium iron phosphate precursor is coated with graphene oxide, and the lithium iron phosphate precursor is attached to the reduced graphene oxide. The formed three-dimensional network structure can greatly increase the contact area between the material and the electrolyte, improve the interface energy, and increase the Li + The active sites for binding and detachment of Li + Diffusion dynamics, reduce the difference in diffusion rates between electrons and ions, and reduce the degree of electrochemical polarization. Multi-step annealing can assist in the formation of a multi-level conductive network to prevent inorganic carbon or organic carbon from agglomerating alone. In addition, the material stress can be eliminated to a certain extent during the annealing cooling and then heating process to avoid structural collapse of the lithium iron phosphate battery during the deintercalation and embedding of lithium ions. After such step-by-step coating and multi-step annealing, the multi-level conductive three-dimensional network structure formed can greatly improve the rate performance of the regenerated lithium iron phosphate composite material and the long-cycle stability at high rates. In addition, the process flow of the present invention is simple and easy to achieve large-scale production.

[0037] In some embodiments of the present invention, the iron phosphate waste residue after lithium extraction in the hydrometallurgical industry can be reduced to obtain lithium iron phosphate; the iron phosphate waste residue includes one or more of lithium iron phosphate, dihydrated iron phosphate, and iron phosphate. The present invention utilizes the industrial lithium extraction waste iron phosphate waste residue to regenerate into regenerated lithium iron phosphate composite materials, effectively solving the current large amount of iron phosphate waste waste in industrial production. Aiming at the technical difficulties such as low efficiency of lithium iron phosphate recovery, it fills the process gap of direct conversion and regeneration of iron phosphate waste residue. The intervention of organic carbon source can convert Fe in iron phosphate into 3+ Reduction to Fe 2+ , avoiding the subsequent consumption of relatively more expensive graphene oxide, and at the same time coating the particles so that they will not continue to grow during the subsequent annealing process.

[0038] In some embodiments of the present invention, the lithium source includes one or more of lithium hydroxide, lithium carbonate, and lithium acetate; further, the lithium source is lithium acetate. Lithium acetate is an organic lithium salt, which will also form a graphitized carbon layer after high-temperature carbonization, which can assist the organic carbon source in coating the particles.

[0039] In some embodiments of the present invention, the organic carbon source includes one or more of glucose, ascorbic acid, citric acid, phenolic resin, polyvinyl pyrrolidone, and polyacrylic acid; further, the organic carbon source is citric acid. Citric acid can complex metal particles, and the particle morphology formed after high-temperature carbonization will be more uniform and have a spatial network structure.

[0040] In some embodiments of the present invention, the phosphate source includes one or more of phosphoric acid and ammonium dihydrogen phosphate; further, the phosphate source is ammonium dihydrogen phosphate. Supplementation of the phosphate source is beneficial to improving the synthesis conversion rate of lithium iron phosphate.

[0041] In some embodiments of the present invention, the ferric phosphate waste slag is sintered at 400-500° C. for 1-2 hours to obtain pre-oxidized ferric phosphate waste slag, and then the pre-oxidized ferric phosphate waste slag, a lithium source, an organic carbon source and a phosphoric acid source are mixed.

[0042] In some embodiments of the present invention, the mass ratio of the pre-oxidized iron phosphate waste residue, lithium source, organic carbon source and phosphoric acid source is 1: (0.1-1): (0.01-0.5): (0.01-0.5); further, the mass ratio of the pre-oxidized iron phosphate waste residue, lithium source, organic carbon source and phosphoric acid source is 1: (0.5-1): (0.1-0.3): (0.01-0.1).

[0043] In some embodiments of the present invention, the mixing time of the iron phosphate waste residue, the lithium source, the organic carbon source and the phosphoric acid source is 8-12 hours.

[0044] In some embodiments of the present invention, the mixing of ferric phosphate waste residue, lithium source, organic carbon source and phosphoric acid source is carried out by mechanical mixing; further, the mechanical mixing is ball milling; the rotation speed of the ball milling is 400-600rpm, and the time is 8-10h.

[0045] In some embodiments of the present invention, the organic solvent includes an alcohol solvent; the alcohol solvent includes one or more of methanol, ethanol, isopropanol, ethylene glycol, glycerol, n-butanol, isopentanol, and benzyl alcohol.

[0046] In some embodiments of the present invention, as an example, the iron phosphate waste residue, the lithium source, the organic carbon source and the phosphoric acid source are mixed and then heated at 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500°C.

[0047] In some embodiments of the present invention, the heating time is 1-3 hours. As an example, the heating time can be 1, 2, or 3 hours.

[0048] In some embodiments of the present invention, graphene oxide is mixed with the precursor in the form of a solution; the concentration of the graphene oxide solution is 1-10 mg / ml. As an example, the concentration of the graphene oxide solution can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 mg / ml.

[0049] In some embodiments of the present invention, the present invention has no special requirements on the source of graphene oxide, and graphene oxide purchased from the market or obtained by conventional methods in the art can be used.

[0050] In some embodiments of the present invention, graphene oxide is prepared by Hummers method, which specifically comprises the following steps:

[0051] Concentrated sulfuric acid, sodium nitrate, flake graphite and potassium permanganate are mixed and reacted at 0-5°C for 60-100 minutes, and then reacted at 40-100°C for 90-200 minutes, and solid-liquid separation is performed to obtain graphene oxide.

[0052] In some embodiments of the present invention, the mass ratio of the precursor to graphene oxide is (5-30): 1. As an example, the mass ratio of the precursor to graphene oxide can be 10:1, 15:1, 20:1, 25:1 or 30:1.

[0053] In some preferred embodiments of the present invention, the mass ratio of the precursor to graphene oxide is (5-15):1.

[0054] In some embodiments of the present invention, the precursor and graphene oxide are mixed for 2-5 hours; further, the mixing is performed under stirring conditions; and the stirring speed is 200-300 rpm.

[0055] In some embodiments of the present invention, the freeze drying temperature is -20 to -50°C, the time is 48-72 hours, and the vacuum degree is 100-1000 Pa. Freeze drying helps to maintain the chemical stability of the material and improve the rate performance and cycle performance of the material.

[0056] In some embodiments of the present invention, as an example, the precursor and graphene oxide are mixed, freeze-dried, and then heat-treated at 550, 600, 650, 700, 750, 800°C.

[0057] In some embodiments of the present invention, the heat treatment time is 4-12 hours. As an example, the heat treatment time is 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 hours.

[0058] In some embodiments of the invention, the solvent comprises water.

[0059] In some embodiments of the present invention, the inert atmosphere includes one or more of nitrogen, argon, helium, neon, krypton, and xenon.

[0060] The second aspect of the present invention provides a regenerated lithium iron phosphate composite material obtained by the preparation method of the regenerated lithium iron phosphate composite material.

[0061] The scheme of the present disclosure will be explained below in conjunction with the examples. It will be appreciated by those skilled in the art that the following examples are only used to illustrate the present disclosure and should not be considered as limiting the scope of the present disclosure. Where specific techniques or conditions are not indicated in the examples, the techniques or conditions described in the literature in this area or the product instructions are used. The reagents or instruments used that do not indicate the manufacturer are all conventional products that can be obtained commercially.

[0062] The ferric phosphate waste residue (D-FP) used in the examples and comparative examples of the present invention mainly contains FePO4.

[0063] The graphene oxide (GO) solution used in the embodiments and comparative examples of the present invention is prepared by the Hummers method, comprising the following steps:

[0064] Weigh 46 ml of concentrated sulfuric acid, add 3.2 g of sodium nitrate (NaNO3) to the concentrated sulfuric acid, wait for it to dissolve, place the beaker containing the reactants in an ice bath and apply continuous magnetic stirring, then add 2 g of flake graphite, continue stirring for 45 minutes, and slowly add KMnO4 (6 g) (0.3 g / 3 min) during this period. Always keep the temperature of the reaction system 5 ° C lower, and the reaction system will become a purple-red oily mixture after adding. After continuing to stir in the ice bath for 60 minutes, transfer the reaction vessel from the ice bath to a 40 ° C oil bath, continue stirring for 90 minutes, and slowly add 92 ml of deionized water to the reaction system. The temperature of the system continues to rise with the addition of distilled water. After adding water, transfer to a 100 ° C oil bath and continue to react for 15 minutes. Take it out and place it. When the temperature drops to about 60 ° C, slowly add 180 ml of distilled water, and the temperature of the system begins to drop. Then slowly add about 30 ml of hydrogen peroxide solution (add until no bubbles are generated in the solution and the reaction solution gradually changes from brown to golden yellow). After the reaction is terminated, centrifuge (4500r / min) after standing for 12 hours to obtain a graphite oxide filter cake. Finally, deionized water is used to prepare a GO solution with a concentration of 3 mg / ml and a pH of 2-3.

[0065] Example 1

[0066] The regenerated lithium iron phosphate composite material of this embodiment is as follows Figure 1 As shown, the preparation method comprises the following steps:

[0067] (1) Pre-oxidation and cleaning: Weigh 20g of waste iron phosphate (D-FP) and sinter it in a muffle furnace at 400℃ for 2h with a heating rate of 5℃ / min. Then, immerse the pre-sintered sample in a beaker filled with alcohol and ultrasonically disperse it for 2h. Pour it into a suction cup for filtration. Finally, put the filtered sample into a blast oven for drying to obtain pre-oxidized iron phosphate (P-FP).

[0068] (2) Ball milling: Weigh 3.016 g of pre-oxidized iron phosphate (P-FP), 2.24 g of lithium acetate, 0.23 g of diammonium phosphate and 0.842 g of citric acid, place in a 250 ml ball mill, add 100 ml of alcohol to the ball mill so that the ball milling medium volume accounts for 2 / 3 of the total volume. Ball mill at 400 rpm on a planetary ball mill for 8 h. Dry to remove the alcohol and obtain a pretreated precursor sample.

[0069] (3) Low-temperature annealing: The pretreated precursor sample was ground and transferred to an alumina magnetic boat, which was placed in a tubular furnace and passed with argon. The sample was heated to 400°C at a heating rate of 5°C / min and kept at this temperature for 3 h to obtain the annealed precursor.

[0070] (4) Freeze drying: Weigh 0.4 g of the annealed precursor sample, add 6.6 ml of 3 mg / ml graphene oxide (GO) solution, stir on a magnetic stirrer for 3 h at a stirring speed of 300 rpm, then cool with liquid nitrogen and place in a freeze dryer for 48 h at a freeze drying temperature of -20°C and a vacuum degree of 100 Pa to obtain a freeze-dried precursor.

[0071] (5) High temperature annealing: The freeze-dried precursor was ground and transferred to an alumina magnetic boat, placed in a tube furnace with argon, and heated at 650°C for 8 h (heating rate of 10°C / min). The sample was taken out to obtain a regenerated lithium iron phosphate composite material with a dual carbon source of 5% GO by mass, marked as LFP-5% GO / CA.

[0072] Example 2

[0073] The regenerated lithium iron phosphate composite material of this embodiment is different from that of Embodiment 1 only in that the amount of GO solution in step (4) is adjusted to 11 ml in this embodiment; the preparation method specifically comprises the following steps:

[0074] (1) Weigh 20g of waste iron phosphate (D-FP) and sinter it in a muffle furnace at 400℃ for 2h with a heating rate of 5℃ / min. Then, immerse the pre-sintered sample in a beaker filled with alcohol and ultrasonically disperse it for 2h. Pour it into a suction filtration cup for suction filtration. Finally, put the filtered sample into a blast oven for drying to obtain pre-oxidized iron phosphate (P-FP).

[0075] (2) Weigh 3.016 g of pre-oxidized iron phosphate (P-FP), 2.24 g of lithium acetate, 0.23 g of ammonium dihydrogen phosphate and 0.842 g of citric acid, and place them in a 250 ml ball mill. Add 100 ml of alcohol to the ball mill so that the ball milling medium volume accounts for 2 / 3 of the total volume. Mill at 400 rpm on a planetary ball mill for 8 h. Dry to remove the alcohol and obtain a pretreated precursor sample.

[0076] (3) Low-temperature annealing: The pretreated precursor sample was ground and transferred to an alumina magnetic boat, which was placed in a tubular furnace and passed argon gas at 400°C for 3 h (heating rate of 5°C / min) to obtain the annealed precursor.

[0077] (4) Weigh 0.4 g of the annealed precursor sample, add 11 ml of 3 mg / ml GO solution, and stir on a magnetic stirrer for 3 h. Then cool with liquid nitrogen and place in a freeze dryer for 48 h. The freeze drying temperature is -20 ° C and the vacuum degree is 100 Pa to obtain a freeze-dried precursor.

[0078] (5) High temperature annealing: The freeze-dried precursor was ground and transferred to an alumina magnetic boat, placed in a tube furnace with argon, and heated at 650°C for 8 h (heating rate of 10°C / min). The sample was taken out to obtain a regenerated lithium iron phosphate composite material with a mass fraction of 8% GO, marked as LFP-8% GO / CA.

[0079] Example 3

[0080] The regenerated lithium iron phosphate composite material of this embodiment is different from that of embodiment 1 only in that the amount of GO solution in step (4) is adjusted to 20 ml in this embodiment; the preparation method specifically comprises the following steps:

[0081] (1) Weigh 20g of waste iron phosphate (D-FP) and sinter it in a muffle furnace at 400℃ for 2h with a heating rate of 5℃ / min. Then, immerse the pre-sintered sample in a beaker filled with alcohol and ultrasonically disperse it for 2h. Pour it into a suction filtration cup for suction filtration. Finally, put the filtered sample into a blast oven for drying to obtain pre-oxidized iron phosphate (P-FP).

[0082] (2) Weigh 3.016 g of pre-oxidized iron phosphate (P-FP), 2.24 g of lithium acetate, 0.23 g of ammonium dihydrogen phosphate and 0.842 g of citric acid, and place them in a 250 ml ball mill. Add 100 ml of alcohol to the ball mill so that the ball milling medium volume accounts for 2 / 3 of the total volume. Mill at 400 rpm on a planetary ball mill for 8 h. Dry to remove the alcohol and obtain a pretreated precursor sample.

[0083] (3) Low-temperature annealing: The pretreated precursor sample was ground and transferred to an alumina magnetic boat, which was placed in a tubular furnace and heated at 400°C for 3 h (heating rate of 5°C / min) to obtain an annealed precursor.

[0084] (4) Weigh 0.4 g of the annealed precursor sample, add 20 ml of 3 mg / ml GO solution, and stir on a magnetic stirrer for 3 h. Then cool with liquid nitrogen and place in a freeze dryer for 48 h. The freeze drying temperature is -20 ° C and the vacuum degree is 100 Pa to obtain a freeze-dried precursor.

[0085] (5) High temperature annealing: The freeze-dried precursor was ground and transferred to an alumina magnetic boat, placed in a tube furnace with argon, and heated at 650°C for 8 h (heating rate of 10°C / min). The sample was taken out to obtain a regenerated lithium iron phosphate composite material with a mass fraction of 15% GO, marked as LFP-15% GO / CA.

[0086] Comparative Example 1

[0087] The regenerated lithium iron phosphate composite material of this comparative example is different from that of Example 2 in that: the citric acid in step (2) is omitted in this comparative example, and the process is changed to single-step annealing; the remaining steps are carried out with reference to the method in Example 2, and the preparation method specifically includes the following steps:

[0088] (1) Weigh 20g of waste iron phosphate (D-FP) and sinter it in a muffle furnace at 400℃ for 2h with a heating rate of 5℃ / min. Then, immerse the pre-sintered sample in a beaker filled with alcohol and ultrasonically disperse it for 2h. Pour it into a suction filtration cup for suction filtration. Finally, put the filtered sample into a blast oven for drying to obtain pre-oxidized iron phosphate (P-FP).

[0089] (2) Weigh 3.016 g of pre-oxidized iron phosphate (P-FP), 2.24 g of lithium acetate, and 0.23 g of ammonium dihydrogen phosphate, and place them in a 250 ml ball mill. Add 100 ml of alcohol to the ball mill so that the ball milling medium volume accounts for 2 / 3 of the total volume. Mill at 400 rpm on a planetary ball mill for 8 h. Dry to remove the alcohol and obtain a pretreated precursor sample.

[0090] (3) Weigh 0.4 g of the pretreated precursor sample, add 11 ml of 3 mg / ml GO solution, and stir on a magnetic stirrer for 3 h. Then cool with liquid nitrogen and place in a freeze dryer for 48 h. The freeze drying temperature is -20 ° C and the vacuum degree is 100 Pa to obtain a freeze-dried precursor.

[0091] (4) High temperature annealing: The freeze-dried precursor was ground and transferred to an alumina magnetic boat in a tube furnace with argon, heated at 400°C for 3 h (heating rate of 5°C / min), and then continued to heat to 650°C for 8 h (heating rate of 10°C / min). The sample was taken out to obtain a lithium iron phosphate material with a mass fraction of 8% GO, marked as LFP-8% GO.

[0092] Comparative Example 2

[0093] The regenerated lithium iron phosphate composite material of this comparative example is different from that of Example 2 in that: this comparative example omits step (3) and step (); and the process is changed to single-step annealing (annealing process refers to comparative example 1), and the remaining steps are carried out according to the method in Example 2. The preparation method specifically includes the following steps:

[0094] (1) Weigh 20g of waste iron phosphate (D-FP) and sinter it in a muffle furnace at 400℃ for 2h with a heating rate of 5℃ / min. Then, immerse the pre-sintered sample in a beaker filled with alcohol and ultrasonically disperse it for 2h. Pour it into a suction filtration cup for suction filtration. Finally, put the filtered sample into a blast oven for drying to obtain pre-oxidized iron phosphate (P-FP).

[0095] (2) Weigh 3.016 g of pre-oxidized iron phosphate (P-FP), 2.24 g of lithium acetate, 0.23 g of ammonium dihydrogen phosphate and 0.842 g of citric acid, and place them in a 250 ml ball mill. Add 100 ml of alcohol to the ball mill so that the ball milling medium volume accounts for 2 / 3 of the total volume. Mill at 400 rpm on a planetary ball mill for 8 h. Dry to remove the alcohol and obtain a pretreated precursor sample.

[0096] (3) High temperature annealing: In an alumina magnetic boat and under argon, heat at 400°C for 3h (heating rate of 5°C / min), then continue to heat to 650°C for 8h (heating rate of 10°C / min). Take out the sample to obtain a 3% carbon-coated lithium iron phosphate material, marked as LFP-CA.

[0097] Test Case

[0098] The following tests were performed using the samples obtained in the examples and comparative examples, iron phosphate waste residue, and commercial lithium iron phosphate (C-LFP, purchased from Shenzhen Kejing Zhida Technology Co., Ltd., model P198-S20) as raw materials.

[0099] 1. X-ray diffraction (XRD) test

[0100] The model used is Smartlab (9), and the test parameters are Cu / Kα radiation. The voltage was 40 kV, the current was 100 mA, the scanning speed was 10° / min, the step size was 0.02°, and the scanning angle was 15°-65°.

[0101] The experimental results are as follows Figure 2 As shown by Figure 2 It can be seen that the XRD spectrum of D-FP is very consistent with the characteristic peaks in the standard PDF card (JCPDS No. 37-0478), which shows that after the wet treatment to remove Li, the main structure is still FP, which also shows that the waste FePO4 residue has a certain recycling value. The XRD patterns of LFP-CA (organic carbon regenerated lithium iron phosphate composite material), LFP-8% GO (inorganic carbon regenerated lithium iron phosphate composite material), LFP-5% GO / CA, LFP-8% GO / CA, LFP-15% GO / CA (dual carbon source regenerated lithium iron phosphate composite material) show similar strong diffraction peaks, and both are basically consistent with the standard PDF card (JCPDS No. 83-2092), which shows that they have high crystallinity and have a typical olivine structure, with an orthorhombic Pmnb space group, and the structure is reduced.

[0102] 2. Scanning electron microscope test

[0103] The scanning electron microscope (SEM) instrument model is: SU8020.

[0104] The experimental results are as follows Figure 3-5 As shown by Figure 3 It can be seen that the D-FP sample mainly shows serious particle agglomeration and large size. This is because D-FP comes from the byproduct of hydrometallurgy in the factory. Since it has not been treated at high temperature, it contains impurities such as PVDF and conductive carbon black, which will cause a large degree of agglomeration between particles. These residual PVDF and conductive carbon black will cause the particles to agglomerate in the subsequent process, thereby increasing the particle size and prolonging the Li + The diffusion rate will have a negative impact on the electrochemical performance of the material. In addition, the residual adhesion between the particles will also hinder the diffusion of the Li element during the subsequent annealing process, which is not conducive to battery regeneration. However, for the sample (P-FP) after high-temperature pre-oxidation, the residual PVDF and conductive carbon black are effectively removed, the particle surface becomes clear, and the particle size is well controlled.

[0105] Depend on Figure 4It can be seen that the typical particle size of the LFP-CA sample is in the range of 100-300nm. Among them, a large number of LiFePO4 particles are connected together by an amorphous carbon layer. The image with a lower magnification confirms that the particle agglomeration phenomenon is effectively improved, and a point-to-point conductive network is formed between particles through the amorphous carbon layer. However, in another sample, the LFP-GO sample, the particle size has increased abnormally, with a typical size of more than 1μm, but it can be clearly observed that rGO is coated on the surface of the particles. The image with a lower magnification also confirms that the particle size of the sample is generally larger. This may be because when only GO is added for coating modification, the sheet diameter of GO (more than 10μm) is larger than that of iron phosphate particles (300nm), which will lead to poor coating effect, and thus the growth of particles will not be hindered by the carbon layer during annealing. However, due to the presence of rGO, the sample forms a three-dimensional multi-level conductive network. Compared with the first two samples, the dual-carbon source regenerated lithium iron phosphate composites (LFP-5% GO / C, LFP-8% GO / CA, LFP-15% GO / CA) show uniform particle distribution, with a typical size between 100-200nm. And rGO can also be observed coated on the surface of the particles. Unlike the LFP-GO samples, this type of sample does not show excessive particle growth. The lower magnification images shown confirm that this type of sample has a more uniform and fine particle size, and also relies on rGO to form a spatial multi-level conductive network. However, at different proportions of GO addition, the morphology is different. Excessive GO content (LFP-15% GO / CA) leads to the presence of a lot of rGO between the particles that is not in contact with the lithium iron phosphate particles, which will undoubtedly cause the final battery tap density to decrease. Therefore, from the SEM morphology, the morphology of the LFP-5% GO / C and LFP-8% GO / CA samples is more prominent. In addition, the reason why the particle sizes of samples from different types of processes differ is that the dense carbon layer formed by the CA added before the first step of annealing in the dual-carbon source multi-step annealing process after carbonization can effectively hinder the continued growth of particles in the subsequent solid-phase reaction process.

[0106] Figure 5 Energy dispersive X-ray spectroscopy (EDS) shows that the P, Fe, and O elements of the LFP-8% GO / CA sample are evenly distributed, which also shows that the addition of GO does not affect the formation of LFP.

[0107] 3. Cyclic voltammetry test

[0108] Chenhua electrochemical workstation, instrument model: CHI600E, scan rate is 0.01mV / s, voltage range is 2.5-4.2V.

[0109] Constant current charge and discharge test

[0110] LAND battery testing system, instrument model: CT2001A, current density is 1C and 5C, voltage range is 2.5-4.2V.

[0111] The experimental results are as follows Figure 6-10 As shown by Figure 6 It can be seen that the 0.1C first cycle discharge specific capacities of LFP-CA, LFP-GO, LFP-5% GO / C, LFP-8% GO / CA, LFP-15% GO / CA and commercial LFP (C-LFP) materials are 163.1, 132.1, 155.0, 163.6, 157.9 and 156.6 mAh g, respectively. -1 , the performance of the sample with only GO added is not very outstanding. This is mainly because adding only GO will not be able to completely coat, resulting in abnormal growth of particles, which ultimately affects the electrochemical performance. However, the samples with dual carbon source multi-step coating all show discharge specific capacity comparable to or even better than commercial materials. This is mainly due to the first step annealing process. The amorphous carbon layer formed after CA carbonization can hinder particle growth. This also verifies the conjecture during morphological characterization. In addition, LFP-8% GO / CA shows the smallest polarization voltage difference (50.4mV) during the cycle process. These results show that LFP-8% GO / CA has a higher available capacity and a smaller degree of polarization.

[0112] Figure 7 The 0.1C-15C rate performance diagram of different types of samples shows that at low rates (0.1C and 0.5C), LFP-CA and the dual-carbon source multi-step coated sample cannot pull apart, but as the rate increases, the performance of the dual-carbon source multi-step coated sample becomes more outstanding, far exceeding commercial materials. Specifically, at 5C, LFP-5% GO / C, LFP-8% GO / CA, and LFP-15% GO / CA exhibit 115.3, 125.6, and 110.4 mA hg -1 The discharge capacity of LFP-8%GO / CA is outstanding, while that of LFP-CA, LFP-GO and C-LFP are 96.2, 68.4 and 99.7 mA h g -1 Even at a high rate of 15C, the LFP-8% GO / CA sample can show a discharge capacity of 93.6 mA hg -1 The discharge specific capacity of the samples is much higher than that of other types of samples. This is mainly due to the multi-level conductive network constructed by the dual-carbon source multi-step coating process.

[0113] Figure 8 and Fig. 9The long cycle performance diagrams of different types of samples at 1C and 5C currents are shown in the figure. As shown in the figure, except for LFP-CA, other samples can achieve a capacity retention rate of >90%. In particular, LFP-8% GO / CA shows excellent stability, with a capacity retention rate of 99.5% after 300 cycles. Next, in order to further test the superiority of the dual-carbon source coating process, five samples were subjected to long-cycle tests at 5C. At such a high rate, the capacity retention rate of LFP-CA dropped significantly after only 300 cycles, and only 19.2% after 600 cycles. For the LFP-GO sample, although it still has a retention rate of 91.8% at about 1000 cycles. However, it is impossible to achieve a high discharge specific capacity. In this environment, the samples with dual-carbon source multi-step coating process showed better capacity retention than other samples, especially the LFP-8% GO / CA sample, which had a capacity retention of 90.2% after 1500 cycles, while the capacity retention of the LFP-5% GO / CA and LFP-15% GO / CA samples also dropped significantly after about 1000 cycles. This is mainly because too little Go is added, and the multi-level conductive network cannot be fully established, and the conductive performance is still a major limitation. Excessive GO content will lead to excessive contact area between the electrolyte and the active material, which will inevitably lead to an increase in the degree of polarization and affect the stability of the battery during cycling.

[0114] Fig.10 For different types of samples at 0.1mV s -1 The cyclic voltammetry (CV) curves with a scan rate of 2.5–4.2 V and a voltage range of 2.5–4.2 V show that the reduction and oxidation peaks at about 3.2 and 3.6 V represent the oxidation of Fe 3+ To Fe 2+ Reduction and removal of Fe 2+ To Fe 3+ oxidation, corresponding to Li + Deintercalation and intercalation. All five samples showed good symmetry, which means that all samples were successfully regenerated. Compared with LFP-CA and LFP-GO samples, LFP-8% GO / CA showed the highest peak intensity, which means that this sample has the best conductivity. The potential intervals between the anode and cathode peaks of LFP-CA, LFP-GO, and LFP-8% GO / CA were 288mV, 335mV, and 241mV, respectively. A smaller interval means a lower degree of electrochemical polarization, which means that the kinetics of the LFP-8% GO / CA sample was well improved after regeneration. It is worth noting that when comparing the CV test results of samples with different dual-carbon source multi-step coating processes, LFP-15% GO / CA showed the highest peak intensity, which was mainly due to the addition of more GO. However, compared with the other two samples, the potential interval between its anode and cathode peaks was large, which showed a greater degree of polarization.

[0115] Table 1 Comparison of electrochemical performance

[0116]

[0117]

[0118] It can be seen from Table 1 that, compared with the comparative example, the discharge specific capacity and 1C capacity retention rate of the regenerated lithium iron phosphate composite materials of Examples 1-3 of the present invention are improved. In addition, Examples 1-3 exhibit discharge specific capacities comparable to or even better than those of commercial materials.

[0119] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", "some implementation schemes" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0120] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a regenerated lithium iron phosphate composite material, characterized in that: The following steps are involved: In the presence of an organic solvent, waste iron phosphate residue, a lithium source, an organic carbon source and a phosphoric acid source are mixed, and then heated at 400-500° C. in an inert atmosphere to obtain a precursor; In the presence of a solvent, the precursor and graphene oxide are mixed, freeze-dried, and then heat-treated at 650-800° C. in an inert atmosphere to obtain a regenerated lithium iron phosphate composite material.

2. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The iron phosphate waste residue includes one or more of lithium iron phosphate, iron phosphate dihydrate, and iron phosphate; And / or, the lithium source includes one or more of lithium hydroxide, lithium carbonate, and lithium acetate.

3. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The organic carbon source includes one or more of glucose, ascorbic acid, citric acid, phenolic resin, polyvinyl pyrrolidone and polyacrylic acid.

4. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The phosphoric acid source includes one or more of phosphoric acid and diammonium phosphate.

5. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The ferric phosphate waste residue is sintered at 400-500° C. for 1-2 hours to obtain pre-oxidized ferric phosphate waste residue, and then the pre-oxidized ferric phosphate waste residue, a lithium source, an organic carbon source and a phosphoric acid source are mixed.

6. The method for preparing the regenerated lithium iron phosphate composite material according to claim 5, characterized in that: The mass ratio of the pre-oxidized iron phosphate waste residue, the lithium source, the organic carbon source and the phosphoric acid source is 1:(0.1-1):(0.01-0.5):(0.01-0.5).

7. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The mass ratio of the precursor to graphene oxide is (5-30):

1.

8. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The freeze drying temperature is -20 to -50°C, the time is 48 to 72 hours, and the vacuum degree is 100 to 1000 Pa.

9. The method for preparing the regenerated lithium iron phosphate composite material according to claim 1, characterized in that: The mixing of the iron phosphate waste residue, the lithium source, the organic carbon source and the phosphoric acid source is carried out by ball milling; The ball milling speed is 400-600 rpm, and the time is 8-10 hours.

10. The regenerated lithium iron phosphate composite material obtained by the preparation method of the regenerated lithium iron phosphate composite material according to any one of claims 1 to 9.

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