CN113072052B

CN113072052B - Waste lithium iron phosphate lithium supplement repair method and application

Info

Publication number: CN113072052B
Application number: CN202110332731.6A
Authority: CN
Inventors: 孙静; 江镇宇; 王文龙; 贾平山; 于冠群; 宋占龙; 毛岩鹏; 赵希强
Original assignee: Shandong University
Current assignee: Shandong University
Priority date: 2021-03-29
Filing date: 2021-03-29
Publication date: 2022-06-28
Anticipated expiration: 2041-03-29
Also published as: CN113072052A

Abstract

The invention relates to the technical field of recycling and cyclic utilization of waste power lithium ion batteries, in particular to a waste lithium iron phosphate lithium supplement repairing method and application. The method can effectively overcome the defects of slow process, low recovery efficiency, complex flow, high cost, secondary pollution and the like in the traditional pyrogenic process or wet process separation and resynthesis process.

Description

Waste lithium iron phosphate lithium supplement repair method and application

Technical Field

The invention relates to the technical field of recycling and cyclic utilization of waste power lithium ion batteries, in particular to a waste lithium iron phosphate lithium supplement repairing method and application.

Background

The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and is not necessarily to be construed as an admission or any form of suggestion that this information forms the prior art that is already known to a person of ordinary skill in the art.

In recent years, in order to meet the energy storage requirements of emerging technology systems such as new energy power automobiles, clean energy power grid systems, 5G base stations and the like, researchers are not keen on developing high-performance lithium ion battery electrode materials. However, the sustainability of commercial li-ion batteries and next generation rechargeable batteries is not highly regarded. In consideration of the importance of realizing sustainable development of economy, energy, environment and resources, an effective waste battery resource recycling strategy is developed, so that the method is beneficial to recycling valuable substances in waste batteries, reduces the resource and energy consumption investment of primary production, reduces the pollution of waste battery disposal on the environment, promotes the sustainable development of the lithium battery industry, and is particularly suitable for downstream links. Meanwhile, the worldwide recycling market for lithium ion batteries is expected to reach $ 237.2 billion by 2030. Therefore, recycling of lithium ion batteries presents a huge opportunity, but at the same time it is a very challenging problem.

Particularly in the field of new energy power equipment, commercial lithium ion batteries mainly have two types: one is that the anode material is lithium iron phosphate (LiFePO)₄LFP for short) battery; another class is the ternary materials (LiNi)_xMn_yCo_1-x-yO₂NMC for short, etc.). LFP batteries have already occupied nearly 1/3 market throughout the lithium battery industry with their long cycle performance and life (5-8 years on average), high safety and thermal stability, low production cost, but lower energy density has become a bottleneck limiting their development. Under the demand of future users for high voltage and high energy density, the LFP system is gradually replaced by a high-end ternary system power lithium battery which is mature.

At present, the technical routes for recycling the lithium ion battery electrode materials at home and abroad are mainly divided into two types: firstly, destructive recovery in a subtraction mode, namely recovery of metal substances or electrode raw materials is realized by a metal separation means; and the other is 'addition' type repairability recovery, namely, the direct regeneration of the material is realized by a repairing modification means. The former is usually an effective combination of pyrometallurgical and hydrometallurgical processes and has been commercially implemented for Co, Ni, Li recovery, which often involve disassembly, smelting, acid leaching, chemical precipitation, extraction separation, etc., and finally recovered as simple compounds and used in the production of new cathode materials. For example, although the cost of the recovery process is not low, the recovery process of chinese patent CN108899601A and chinese patent CN106910959A can indeed recover the transition metal with high value (for example, Co can be sold to 30$/kg), thereby obtaining a certain benefit. However, the energy consumption and the investment of chemical reagents still bring about large greenhouse gas emission and secondary pollution. Moreover, most of the positive electrode materials are mainly valued in their unusual material structure composition in the primary production, and their original structural characteristics are completely lost by this destructive recovery process. Therefore, remediation recovery with less energy consumption and less pollution is more desirable than destruction recovery, especially for LFP, which itself does not contain precious metals, is more desirable. The LFP contains less valuable metals, and if a scheme of recovering Li and Fe by a wet method is adopted, the economic value is lower, and the method is more suitable for regenerating the LFP. The method has higher recovery benefit and higher comprehensive utilization rate of resources.

One of the obvious manifestations of lithium ion battery failure is the reduction in the capacity of the positive electrode material. During the process of lithium ion charging and discharging, lithium ions are continuously inserted into and removed from the anode material, the structure of the anode material is changed to a certain degree, and part of the lithium ions can not return to the original structural position of the material, so that the loss of Li and the reduction of capacity of the anode material are caused. In particular, the charge capacity loss of the waste LFP is large, but the morphology and bulk crystalline structure remain intact. Vacancy defects of Li and inversion defects of Fe are the main causes of deterioration of LFP performance. The vacancy of Li not only results in Fe²⁺Oxidation to Fe³⁺Also results in part of Fe²⁺Migrate to Li vacancy to form inversion defect, and block Li⁺Normal migration. The repairability method is generally to add Li source into LFP waste material, and the method of solid phase repair (conventional calcination or microwave heating) and hydrothermal repair can be used for directly repairing and regenerating the anode material, but the method is generally suitable for the anode material with low impurity content in the same batchExtreme waste, such as CN 102280673A. However, the conventional calcination or microwave direct heat treatment adopted during repair has the same limitations, and is easy to cause local burning or burning loss of a sintering material, so that the performance of the repaired regenerated lithium ion battery anode material is reduced, the consistency of the product is poor, and if the carbon-coated material is modified at the same time, the conventional calcination has the defects of higher reaction temperature requirement, low graphitization degree, poor thermal stability and chemical stability; however, the conventional hydrothermal method solves the problem of local sintering caused by non-uniform heating temperature, but has long reaction time, relatively high requirements on reaction conditions, and needs a large amount of reducing agent and Li source.

Disclosure of Invention

In order to solve the problems in the prior art, the disclosure provides a method for repairing the waste lithium iron phosphate by lithium supplement and an application thereof, and the method can effectively overcome the defects of slow process, low recovery efficiency, complex process, high cost, secondary pollution and the like in the traditional pyrogenic process or wet separation and resynthesis process.

Specifically, the technical scheme of the present disclosure is as follows:

in a first aspect of the disclosure, a method for repairing lithium in waste lithium iron phosphate is characterized in that regenerated lithium iron phosphate is prepared by a microwave hydrothermal method.

In a second aspect of the disclosure, a method for repairing waste lithium iron phosphate by lithium supplementation is characterized in that a microwave hydrothermal method is adopted to repair lithium iron phosphate and simultaneously reduce and oxidize graphene for coating modification, so as to obtain regenerated lithium iron phosphate/reduced and oxidized graphene.

In a third aspect of the disclosure, an application of the waste lithium iron phosphate lithium supplement repairing method in the field of lithium ion battery electrode materials comprises small-sized power equipment, a lithium battery car and medical equipment.

One or more technical schemes in the disclosure have the following beneficial effects:

(1) the method realizes the harmless treatment of the waste lithium ion battery anode material LFP (waste lithium iron phosphate), and realizes the direct reutilization of secondary resources and the synchronous carbon composite modification.

(2) The microwave hydrothermal reduction repair method adopted by the method is based on the unique heat effect of microwaves, so that the reduction repair reaction is promoted, and the dosage of a reagent and the investment of energy consumption, time and labor cost are reduced to the great extent.

(3) The present disclosure does not cause leakage and contamination of harmful gas in a closed hydrothermal reactor.

(4) The LFP regenerated material obtained by the combination method of any group of processes provided by the disclosure can be directly applied to the field of lithium ion battery electrode materials, in particular to small-sized power equipment, lithium battery cars, medical mechanical equipment and the like.

(5) The method disclosed by the invention is simple in operation method, low in cost, high in added value of products, universal and easy for large-scale production.

Drawings

The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure and are not to limit the disclosure.

Embodiments of the present disclosure are described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 is a scanning electron micrograph of the recovered spent LFP;

FIG. 2 is a flow chart of examples 1 and 2;

FIG. 3 is a scanning electron micrograph of a regenerated LFP for microwave hydrothermal reduction lithium replenishment repair in example 1;

Fig. 4 is a scanning electron microscope image of the regenerated modified LFP in which lithium is repaired by microwave hydrothermal reduction and graphene is composited simultaneously in example 2;

FIG. 5 is a 0.1C rate cycle 50 cycle electrochemical performance plot of the regenerated and spent LFPs of example 1;

FIG. 6 is a graph of electrochemical performance of the products of examples 1, 2, 3 with 0.2C rate cycles of 100 cycles, the weight ratio of graphene oxide to LFP is 0.01-0.05:1, and when the ratio is 0.01:1, 1% is obtained; at a ratio of 0.05:1, 5% was obtained.

Detailed Description

The disclosure is further illustrated with reference to specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. The experimental procedures, in which specific conditions are not noted in the following examples, are generally carried out according to conventional conditions or according to conditions recommended by the manufacturers.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The reagents or starting materials used in the present invention can be purchased from conventional sources, and unless otherwise specified, the reagents or starting materials used in the present invention can be used in a conventional manner in the art or in accordance with the product specifications. In addition, any methods and materials similar or equivalent to those described herein can be used in the methods of the present invention. The preferred embodiments and materials described herein are intended to be exemplary only.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, and it should also be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of the features, steps, operations and/or combinations thereof.

At present, for repairing waste lithium iron phosphate, conventional calcination or microwave direct heat treatment is adopted, the same limitations exist, a sintered material is easily burned or damaged locally, the performance of a repaired regenerated lithium ion battery anode material is reduced, the consistency of a product is poor, and if carbon coating material modification is carried out at the same time, the conventional calcination has the defects of higher reaction temperature requirement, low graphitization degree, poor thermal stability and chemical stability; however, the conventional hydrothermal method solves the problem of local sintering caused by non-uniform heating temperature, but has long reaction time, relatively high requirements on reaction conditions, and needs a large amount of reducing agent and Li source. In order to solve the problems, the disclosure provides a waste lithium iron phosphate lithium supplement repairing method and application.

In an embodiment of the disclosure, a method for repairing lithium in waste lithium iron phosphate is characterized in that regenerated lithium iron phosphate is prepared by a microwave hydrothermal method, so that local burning loss of materials can be effectively avoided, and the performance of products is greatly improved.

In one embodiment of the disclosure, the microwave hydrothermal method includes dissolving a lithium source and a reducing agent into a waste lithium iron phosphate suspension, transferring the waste lithium iron phosphate suspension into a reaction kettle, and performing microwave heating.

The microwave hydrothermal method can realize the accurate repair of Li vacancy defects and Fe inversion defects, i.e. reduce Fe³⁺Re-supplement of Li⁺. Due to high valence Fe during ion migration³⁺In the presence of strong electrostatic resistance, Fe³⁺Migration back to original Fe²⁺The position of (2) requires a high activation energy of 1.4 eV. Theoretical research shows that the reduction atmosphere can reduce the obstruction of activation, thereby accelerating Fe³⁺Transfer, and many reducing agents can be on Fe³⁺And (4) carrying out reduction. Natural organic reducing agents (such as citric acid CA, ascorbic acid AA) have suitable redox potentials and are ideal reducing agents for assisting in the regeneration of LFP. In the experiment, Li⁺The reducing agent in the aqueous solution provides electrons to reduce Fe³⁺Reduction of electrostatic resistance, i.e. reduction of Fe²⁺Threshold for migration from Li vacancies back to their own position, while also promoting Li ⁺Is added into the vacancy through the reaction process.

In addition, the waste lithium iron phosphate lithium supplement and repair method further comprises the working procedures of disassembling and pretreating the waste lithium iron phosphate. Further, firstly, the waste lithium iron phosphate battery is placed in a NaCl solution to be soaked for a period of time and then disassembled; further, the positive plate is calcined for 1-5h at the temperature of 400 ℃ under the air atmosphere in a muffle furnace at 300 ℃ to realize the separation of the aluminum foil and the LEP material, and the LFP material is peeled and put into a ball mill to be ball-milled to obtain the LFP material with a certain particle size. Wherein, the grain diameter of the LFP material obtained in the pretreatment procedure is 100-200 meshes, and the grain diameter range can effectively ensure the uniform repair of the LFP.

In an embodiment of the present disclosure, in the waste lithium iron phosphate suspension, a solid-to-liquid ratio of the waste lithium iron phosphate to deionized water is 2: (80-100) in g: mL; further, the molar ratio of the waste lithium iron phosphate to the lithium source is 0.5-1.5: 1-1.5, preferably, 1.5: 1; further, the molar ratio of the waste lithium iron phosphate to the reducing agent is 0.5-1.5: 0.3-0.8, preferably 1: 0.5. The waste lithium iron phosphate is repaired by a microwave hydrothermal method, the using amounts of a lithium source and a reducing agent can be greatly reduced, the waste LFP is uniformly repaired, and the quality of the regenerated LFP is greatly improved.

In one embodiment of the present disclosure, the lithium source is selected from lithium hydroxide, lithium carbonate, lithium sulfate, lithium nitrate, preferably, lithium hydroxide; further, the reducing agent is selected from citric acid, vitamin C, reducing sugars (glucose, fructose, sucrose), hydrogen rich water, preferably vitamin C; further, the microwave hydrothermal reaction time is 1-1.5h, preferably 1 h; further, the microwave power is 200-400W, preferably 300W; further, the temperature of the microwave hydrothermal reaction is controlled at 120 ℃ of 100-; further, the mixed solution after the microwave hydrothermal reaction is washed and dried to obtain regenerated lithium iron phosphate. The whole repair time is greatly reduced through microwave hydrothermal reaction, and green and efficient repair and regeneration of the waste LFP positive electrode material are realized only by a low-concentration lithium source, a green and cheap reducing agent, water and low-power microwave energy.

In an embodiment of the disclosure, a method for repairing waste lithium iron phosphate by lithium supplementation is characterized in that a microwave hydrothermal method is adopted to repair lithium iron phosphate and simultaneously reduce and oxidize graphene for coating modification, so as to obtain regenerated lithium iron phosphate/reduced and oxidized graphene.

And (3) compounding the LFP material with reduced graphene oxide (graphene oxide is simultaneously reduced in the process of microwave hydrothermal reduction lithium supplement reaction to obtain thermochemically reduced graphene). The carbon content of the composite reduced graphene oxide can reach 80-95%, and the composite reduced graphene oxide has good conductivity, uniform distribution of a coated carbon source and good quality stability.

The microwave hydrothermal method overcomes the problems that sintering adhesion of materials can be caused by directly heating the materials by microwaves and the heating efficiency of the conventional hydrothermal method is low, combines the advantages of efficient selective heating and hydrothermal treatment of the microwaves, greatly promotes the simultaneous occurrence of reduction lithium supplement reaction and reduction graphene oxide coating reaction by utilizing the good wave absorption characteristics of LFP materials and graphene oxide and the mechanism of microwave selective action wave absorption medium, and improves the efficiency of waste LFP repair and regeneration.

In one embodiment of the present disclosure, the microwave hydrothermal method includes mixing the first solution and the second solution, and transferring the mixture into a reaction kettle to perform a microwave hydrothermal reaction; the first solution comprises waste lithium iron phosphate, a lithium source and a reducing agent; the second solution includes graphene oxide. By compounding the reduced graphene oxide with the LFP material, the conductivity and the quality of the material are greatly improved. Wherein the microwave hydrothermal reaction time is 1-1.5 hours, the microwave power is 200-400W, and the reaction temperature is controlled at 100-120 ℃. In order to further reduce energy consumption, the microwave power is 300W, the structure of the LFP cannot be damaged by the reduced reaction temperature, the quality of the regenerated LFP is improved, and the obtained regenerated LFP/RGO can be directly used as a battery electrode material.

Further, the molar ratio of the waste lithium iron phosphate to the lithium source is 0.5-1.5: 1-1.5, preferably, 1.5: 1; the molar ratio of the waste lithium iron phosphate to the reducing agent is 0.5-1.5: 0.3 to 0.8, preferably, 1: 0.5. the lithium source and the reducing agent are low in dosage, the uniformity of reaction with the graphene oxide is improved by controlling the proportion of each component, and the graphene oxide is compounded with the lithium source and the reducing agent in the optimal proportion.

In an embodiment of the present disclosure, the graphene oxide is obtained by processing graphite obtained by peeling off negative electrode sheets of waste lithium iron phosphate by an improved Hummers method; further, the graphene oxide is dispersed in deionized water or an aqueous solution of ethylene glycol. Wherein the weight ratio of the graphene oxide to the LFP is 0.01-0.05:1, preferably 0.05: 1; the weight ratio of the graphene oxide to deionized water is 0.05-0.1:40-50, the graphene oxide is in the solution, the dispersion degree of the graphene oxide is good, and particularly, in order to further improve the dispersion degree of the graphene oxide, the graphene oxide can be dissolved in an aqueous solution of ethylene glycol. Graphene oxide obtained by improving a Hummers method from graphite recovered from a negative electrode is added into a reduction lithium supplement process, so that the coating modification of the reduced graphene oxide can be realized while LFP is repaired, and the cycle performance of regenerated LFP is further improved. The LFP particles repaired and regenerated by the method have high purity, uniform appearance, good dispersibility, simple and convenient method and low cost, and the energy consumption in the process can be relatively reduced by 80-90 percent and the emission of greenhouse gases is reduced by 70 percent.

In one embodiment of the present disclosure, the regenerated lithium iron phosphate/reduced graphene oxide is further heat treated in a microwave fluidized bed; the microwave fluidized bed is under the protection of inert atmosphere; further, the microwave power is 900-; further, the microwave irradiation time is 2-5min, preferably 3 min.

In one embodiment of the present disclosure, the inert gas may be replaced with a reducing atmosphere to improve the repair degree of LFP inversion defects and the reduction degree of graphene.

In an embodiment of the disclosure, the application of the waste lithium iron phosphate lithium supplement repairing method in the field of lithium ion battery electrode materials is characterized in that the application comprises small-sized power equipment, a lithium battery car and medical equipment.

In order to make the technical solutions of the present disclosure more clearly understood by those skilled in the art, the technical solutions of the present disclosure will be described in detail below with reference to specific embodiments.

Example 1

A method for repairing waste lithium iron phosphate by lithium supplement comprises the following steps:

(1) waste LFP (as shown in figure 1) batteries are placed in NaCl solution (10 wt%) to be soaked for 24 hours for discharging, and then manual disassembly, classification and recycling are carried out. Roasting the positive plate for 2 hours at 400 ℃ in a muffle furnace under the air atmosphere, separating the aluminum foil from the LFP material, peeling the LFP material, putting the peeled LFP material into a ball mill for ball milling for 6 hours, and screening to obtain the LFP material with the expected particle size; and stripping the negative plate, recovering the copper foil and the graphite, removing impurities from the graphite, and treating by an improved Hummers method to obtain the graphene oxide material.

(2) Pouring waste LFP powder into a beaker filled with deionized water, and performing ultrasonic dispersion to form a suspension, wherein the solid-to-liquid ratio of the waste LFP powder to the deionized water is 2: 100; and adding lithium hydroxide and citric acid into the suspension, and performing magnetic stirring at room temperature for 60min to fully dissolve the lithium hydroxide and the citric acid in the solution, wherein the molar ratio of the LFP to the Li source is 1:1, and the molar ratio of the LFP to the reducing agent is 1: 0.5.

(3) Dividing the mixed solution prepared in the step (2) into a plurality of equal parts, and uniformly pouring the equal parts into a polytetrafluoroethylene reaction tank for microwave heating, wherein the amount of the mixed solution accounts for 1/2 of the volume of the reaction tank. The microwave hydrothermal reaction time is 1 hour, the microwave power is 300W, and the reaction temperature is controlled at 120 ℃.

(4) And (4) centrifuging and washing the mixed solution obtained in the step (3) for several times, wherein the centrifugal speed is 4000rpm, the centrifugal time is 6min, and the detergent is deionized water. And then carrying out conventional drying and ball milling on the obtained precipitate to obtain the regenerated LFP.

The flow chart of example 1 is shown in fig. 2, and the scanning electron micrograph of the obtained regenerated LFP is shown in fig. 3, it can be seen that, compared with the discarded LFP particles that are not uniformly and irregularly agglomerated in fig. 1, the regenerated LFP particles still have the adhesion phenomenon after the microwave hydrothermal treatment, but the agglomeration is greatly reduced, and the particles become finer and more uniform. This is due to the microwave thermal energy in the microwave hydrothermal process leading to structural remodeling and particle fusion. The result shows that the shape of the material after the microwave hydrothermal treatment obtains certain remodeling and changing effects, and the obtained material has more uniform shape and more concentrated particle size distribution.

Example 2:

a waste lithium iron phosphate lithium supplement and repair method comprises the following steps:

(1) and (3) placing the waste LFP battery into a NaCl solution (10 wt%) to be soaked for 24h for discharging, and then manually disassembling, classifying and recycling. Roasting the positive plate for 2 hours at 400 ℃ in a muffle furnace under the air atmosphere, separating the aluminum foil from the LFP material, peeling the LFP material, putting the LFP material into a ball mill for ball milling for 6 hours, and screening to obtain the LFP material with the expected particle size; and stripping the negative plate, recovering the copper foil and the graphite, removing impurities from the graphite, and then treating by an improved Hummers method to obtain the graphene oxide material.

(2) Pouring waste LFP powder into a beaker filled with deionized water, and performing ultrasonic dispersion to form a suspension, wherein the solid-to-liquid ratio of the waste LFP powder to the deionized water is 2: 50; adding lithium hydroxide and glucose into the suspension, and performing magnetic stirring at room temperature for 30min to fully dissolve the lithium hydroxide and the glucose in the solution, wherein the molar ratio of the LFP to the Li source is 1:1.05, and the molar ratio of the LFP to the reducing agent is 1: 0.6.

(3) Ultrasonically dispersing the graphene oxide obtained in the step 1 in an aqueous solution (5:95vol) of ethylene glycol to form a dispersion liquid, wherein the weight ratio of the graphene oxide to LFP is 0.01:1, the weight ratio of the graphene oxide to water is 0.1:50, and the dispersion time is 60 min. Then the dispersion was slowly added to the mixture in 2 and magnetic stirring was continued for 60 min.

(4) The mixed solution prepared in 3 was divided into several equal parts and poured uniformly into a teflon reaction tank for microwave heating, and the amount of the mixed solution was 1/2 of the volume of the reaction tank. The microwave hydrothermal reaction time is 1.5 hours, the microwave power is 300W, and the reaction temperature is controlled at 110 ℃.

(5) And (4) centrifuging and washing the mixed solution obtained in the step (4) for several times, wherein the centrifugal rotating speed is 8000rpm, the centrifugal time is 8min, and the detergent is alcohol and deionized water. The obtained precipitate is then subjected to conventional drying and ball milling to obtain regenerated LFP/RGO. The specific flow chart of example 2 is shown in fig. 2, and the morphology of the graphene oxide composite material obtained by the regeneration of the negative electrode is also shown in fig. 4, in which the LFP crystal grains are approximately spherical and have a uniform size distribution, and the reduced graphene oxide shows a clear layered structure.

Example 3

(1) the waste LFP battery is placed in NaCl solution (10 wt%) to be soaked for 24h for discharging, and then manual disassembly, classification and recycling are carried out. Roasting the positive plate for 2 hours at 400 ℃ in a muffle furnace under the air atmosphere, separating the aluminum foil from the LFP material, peeling the LFP material, putting the peeled LFP material into a ball mill for ball milling for 6 hours, and screening to obtain the LFP material with the expected particle size; and stripping the negative plate, recovering the copper foil and the graphite, removing impurities from the graphite, and treating by an improved Hummers method to obtain the graphene oxide material.

(2) Pouring waste LFP powder into a beaker filled with deionized water, and performing ultrasonic dispersion to form a suspension, wherein the solid-to-liquid ratio of the waste LFP powder to the deionized water is 2: 50; adding lithium hydroxide and glucose into the suspension, and performing magnetic stirring at room temperature for 30min to fully dissolve the lithium hydroxide and the glucose in the solution, wherein the molar ratio of LFP to Li source is 1:1.05, and the molar ratio of LFP to reducing agent is 1: 0.6.

(3) Ultrasonically dispersing the graphene oxide obtained in the step 1 in an aqueous solution (5:95vol) of ethylene glycol to form a dispersion liquid, wherein the weight ratio of the graphene oxide to the LFP is 0.05:1, the weight ratio of the graphene oxide to the LFP to water is 0.1:50, and the dispersion time is 60 min. Then the dispersion was slowly added to the mixture in 2 and magnetic stirring was continued for 60 min.

(5) And (5) centrifuging and washing the mixed solution obtained in the step (4) for several times, wherein the centrifugal rotating speed is 8000rpm, the centrifuging time is 8min, and the washing agent is alcohol and deionized water. And then carrying out conventional drying and ball milling on the obtained precipitate to obtain the regenerated LFP/RGO. The specific flow chart of example 2 is shown in fig. 2, and as shown in fig. 4, a composite structure of graphene and regenerated LFP is clearly observed.

Example 4:

(1) and (3) placing the waste LFP battery into a NaCl solution (10 wt%) to be soaked for 24h for discharging, and then manually disassembling, classifying and recycling. Roasting the positive plate for 2 hours at 400 ℃ in a muffle furnace under the air atmosphere, separating the aluminum foil from the LFP material, peeling the LFP material, putting the peeled LFP material into a ball mill for ball milling for 6 hours, and screening to obtain the LFP material with the expected particle size; and stripping the negative plate, recovering the copper foil and the graphite, removing impurities from the graphite, and treating by an improved Hummers method to obtain the graphene oxide material.

(2) Pouring waste LFP powder into a beaker filled with deionized water, and performing ultrasonic dispersion to form a suspension, wherein the solid-to-liquid ratio of the waste LFP powder to the deionized water is 2: 50; adding lithium hydroxide and vitamin C into the suspension, and performing magnetic stirring at room temperature for 30min to fully dissolve the lithium hydroxide and the vitamin C in the solution, wherein the molar ratio of LFP to Li source is 1:1.05, and the molar ratio of LFP to reducing agent is 1: 0.6.

(3) Ultrasonically dispersing the graphene oxide obtained in the step 1 in an aqueous solution (5:95vol) of ethylene glycol to form a dispersion liquid, wherein the weight ratio of the graphene oxide to LFP is 0.05:1, the weight ratio of the graphene oxide to water is 0.1:50, and the dispersion time is 60 min. Then the dispersion was slowly added to the mixture in 2 and magnetic stirring was continued for 60 min.

(5) And (4) centrifuging and washing the mixed solution obtained in the step (4) for several times, wherein the centrifugal rotating speed is 8000rpm, the centrifugal time is 8min, and the detergent is alcohol and deionized water. The obtained precipitate was then subjected to conventional drying and ball milling.

(6) The powder obtained in 5 was further heat treated in a microwave fluidized bed in a reducing atmosphere of argon/hydrogen (90: 10). The microwave power is 1000W, the radiation time is 3min, and the regenerated LFP/G is obtained. The specific procedure of example 3 is shown in FIG. 2.

And (3) electrochemical performance testing:

the electrochemical performance test is carried out on the waste LFP and the products of the embodiment 1, the embodiment 2 and the embodiment 3, and the test steps are as follows:

preparing a pole piece: the aluminum foil was wiped with absolute ethanol and air-dried. The method comprises the steps of mixing a regenerated positive electrode material, a binder (PVDF) and a conductive agent (Super P) according to a mass ratio of 8:1:1, grinding the materials in an agate mortar for about 10min until the materials are mixed uniformly, adding a plurality of drops of a dispersing agent (N-methylpyrrolidone solvent, NMP for short) according to a solid-liquid ratio of the binder to the dispersing agent of 5:95, and grinding the materials for 10min to obtain uniformly mixed slurry. The slurry was uniformly coated on the matte side of the aluminum foil using a coater, and then placed in a vacuum oven at 80 ℃ for 12 hours. The aluminum foil coated with the positive electrode material was compacted by a roll press, and a circular positive electrode plate (diameter 15mm) was cut out by a sheet punch.

Assembling the battery: in an inert atmosphere glove box, the positive plate is placed in a CR2025 positive shell, 30-50uL of electrolyte is uniformly dripped, then a diaphragm, a metal lithium plate, a steel sheet, a gasket, a negative shell and the like are sequentially placed, and finally a button cell sealing machine is used for pressing and sealing. Wherein the electrolyte is 1 mol.L^-1LiPF₆(the solvent is EC and DMC mixed solution with the volume ratio of 1: 1), and the diaphragm is a polyethylene porous composite membrane (Celgard 2400). And (4) flatly placing and standing the assembled battery for 12 hours, and then carrying out electrochemical performance test.

And (4) performance testing: and (3) carrying out constant-current charge and discharge tests on the button cell by adopting a blue battery test system (CT3001A), and recording a cyclic charge and discharge capacity curve. The test voltage range is 2.5-4.2V, the test temperature is room temperature, and the current multiplying power is 0.1-0.2C. The constant current charging and discharging steps are as follows: standing for 30min, then charging to 4.2V with constant current, then discharging to 2.5V with constant current, and alternately charging and discharging.

FIG. 5 shows that during the 0.1C constant current charge-discharge cycle, the initial charge capacity was very small, only about 95mAh g, due to the loss of lithium ions from the spent LFP^-1However, the amount of lithium ions supplied from the lithium metal sheet was significantly increased in 50 cycles, and it was found that the LFP was electrochemically replenished with lithium by the lithium ions supplied from the lithium metal sheet after several charges and discharges, and the charging capacity of the LFP was significantly increased. Further, the key to regenerating LFP is to replenish lithium ions. LFP regenerated by microwave hydrothermal shows good charging capacity, 50-circle charging and discharging circulation capacity is kept at 150- ^-1The level of (c).

As shown in fig. 6, during the 0.2C constant current charge/discharge cycle, the waste LFP increased in capacity due to electrochemical lithium replenishment in 50 cycles, but the capacity decreased significantly in the following cycles due to incomplete defect repair and poor conductivity. LFP regenerated by microwave hydrothermal can repair the defects to the maximum extent and show good charging capacity, and the 100-circle charging and discharging circulation capacity is kept at 130-140mAh g^-1The level of (c). By synchronisationThe conductivity is improved by compounding 1% or 5% of reduced graphene oxide, and the capacity can be improved to 150-160 mAh.g^-1The level of (c).

Although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for repairing waste lithium iron phosphate by lithium supplement is characterized in that regenerated lithium iron phosphate is prepared by a microwave hydrothermal method; the microwave hydrothermal method comprises the steps of dissolving a lithium source and a reducing agent into a waste lithium iron phosphate suspension, transferring the suspension into a reaction kettle, and carrying out microwave heating;

In the waste lithium iron phosphate suspension, the solid-to-liquid ratio of the waste lithium iron phosphate to the deionized water is 2: 80-100, the unit is g: mol;

the molar ratio of the waste lithium iron phosphate to the lithium source is 0.5-1.5: 1-1.5;

the molar ratio of the waste lithium iron phosphate to the reducing agent is 0.5-1.5: 0.3-0.8;

the microwave hydrothermal reaction time is 1-1.5 h; the microwave hydrothermal reaction has the microwave power of 200-400W; the temperature of the microwave hydrothermal reaction is controlled at 100-120 ℃.

2. The method for repairing lithium in waste lithium iron phosphate according to claim 1, wherein the molar ratio of the waste lithium iron phosphate to the lithium source is 1.5: 1.

3. The method for repairing lithium by using waste lithium iron phosphate as claimed in claim 1, wherein the molar ratio of the waste lithium iron phosphate to the reducing agent is 1: 0.5.

4. The method for repairing lithium in waste lithium iron phosphate according to claim 1, wherein the lithium source is selected from lithium hydroxide, lithium carbonate, lithium sulfate, lithium nitrate; the reducing agent is selected from citric acid, vitamin C, reducing sugar and hydrogen-rich water.

5. The method for repairing lithium in waste lithium iron phosphate according to claim 4, wherein the lithium source is lithium hydroxide; the reducing agent is vitamin C.

6. The method for repairing lithium to be supplemented by waste lithium iron phosphate according to claim 1, wherein the microwave hydrothermal reaction time is 1 h; the microwave power is 300W, and the temperature of the microwave hydrothermal reaction is controlled to be 120 ℃.

7. The method for repairing lithium by using waste lithium iron phosphate as claimed in claim 1, wherein the mixed solution after the microwave hydrothermal reaction is washed and dried to obtain regenerated lithium iron phosphate.

8. A waste lithium iron phosphate lithium supplementing repairing method is characterized in that a microwave hydrothermal method is adopted to repair lithium iron phosphate, and simultaneously reduced graphene oxide is coated and modified to obtain regenerated lithium iron phosphate/reduced graphene oxide; the microwave hydrothermal method comprises the steps of mixing the first solution and the second solution, and transferring the mixture into a reaction kettle for microwave hydrothermal reaction; the first solution comprises waste lithium iron phosphate, a lithium source and a reducing agent; the second solution comprises graphene oxide;

the graphene oxide is obtained by processing graphite stripped from negative plates of waste lithium iron phosphate by an improved Hummers method;

the regenerated lithium iron phosphate/reduced graphene oxide is further heated in a microwave fluidized bed, and the microwave fluidized bed is under the protection of inert atmosphere;

The microwave power is 900-1200W; the microwave radiation time is 2-5 min.

9. The method for repairing lithium with waste lithium iron phosphate according to claim 8, wherein the graphene oxide is dispersed in deionized water or an aqueous solution of ethylene glycol.

10. The method for repairing lithium in waste lithium iron phosphate according to claim 8, wherein the microwave power is 1000W; the microwave irradiation time is 3 min.

11. The method for repairing lithium in waste lithium iron phosphate according to claim 8, wherein the inert gas is replaced by a reducing atmosphere.

12. The application of the method for repairing lithium in waste lithium iron phosphate in the field of lithium ion battery electrode materials as claimed in any one of claims 1 to 11, wherein the application comprises small power equipment, lithium battery cars and medical equipment.