A reduced graphene oxide coated iron phosphate composite material prepared based on waste graphite and a preparation method and application thereof

Abstract

The application discloses a kind of preparation of reducing graphene oxide coated iron phosphate composite based on waste graphite and its preparation method and application.The waste lithium iron phosphate battery of the present application is disassembled, and the positive material waste lithium iron phosphate and the negative material waste graphite are separated out;The waste graphite is purified to obtain pure waste graphite;The pure waste graphite is prepared into graphene oxide by Hummers method;Waste lithium iron phosphate is oxidized in the presence of oxidizing agent to obtain iron phosphate;Graphene oxide and iron phosphate are mixed to carry out hydrothermal reaction to obtain reducing graphene oxide coated iron phosphate composite.The reducing graphene oxide coated iron phosphate composite prepared by the present application has excellent lithium storage performance and electrochemical stability, and is suitable for the positive material of lithium ion battery, effectively realizes the resource utilization of waste lithium iron phosphate battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy material resource utilization and environmental protection, and particularly relates to a reduced graphene oxide coated iron phosphate composite material prepared based on waste graphite as well as a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles and large-scale energy storage markets, the demand for lithium ion batteries is also rapidly growing. It is predicted that the total global shipment of lithium ion batteries will reach 5000 GWh by 2030. The service life of power batteries is generally 5-8 years, and with the rapid development of lithium ion batteries, the amount of retired batteries is also rapidly expanding. In 2023, the weight of waste lithium ion batteries in China exceeded 500,000 tons, and how to properly dispose of these waste batteries has become a problem to be solved. The main components in waste lithium ion batteries include positive electrode materials, negative electrode materials, electrolytes, separators, etc., among which the positive and negative electrode materials are the most valuable resources. At present, the positive electrode material of power battery is mainly lithium iron phosphate, and the negative electrode material is mainly graphite. If these resources rich in waste lithium ion batteries are discarded at will, not only will it cause environmental pollution, but also it will be a great waste of resources. Therefore, it is of great significance to disassemble and recycle waste lithium ion batteries and realize the reuse of electrode materials.

[0003] At present, the traditional recycling methods of waste lithium ion battery positive electrode material lithium iron phosphate mainly include pyrometallurgy and hydrometallurgy. Pyrometallurgy has high energy consumption and causes serious pollution; hydrometallurgy has problems such as large consumption of acid and alkali chemicals, serious environmental pollution, etc. In addition, the products recovered by these two methods are all precursor products such as transition metal salts or hydroxides, which need to be further processed before being applied to lithium ion batteries, and the added value is not high. Direct regeneration technology is a kind of mild green regeneration method that has emerged in recent years. It repairs the degraded electrode material by adding lithium source, etc. under relatively mild conditions, so that its performance can be restored and it can be directly used to manufacture new batteries, avoiding the use of high energy consumption and toxic chemicals, and having the advantages of green environmental protection and high added value. However, the existing direct regeneration technology still has unsatisfactory effect on the improvement of the conductivity of the positive electrode material.

[0004] Lithium iron phosphate as an important lithium-ion battery cathode material has the advantages of low cost, good thermal stability, environmental friendliness, etc. However, its electronic conductivity is low, which limits its further application. The structure of graphite negative electrode material is often damaged to some extent after long-term cycling, and the direct recycling and reuse performance is difficult to guarantee. Therefore, how to efficiently extract phosphorus iron and graphite from waste lithium iron phosphate batteries, reasonably modify them and prepare high-performance composite cathode materials is a key problem to be solved. On the other hand, the recycling and utilization of waste lithium-ion battery negative graphite material is relatively less. Graphite has excellent electrical conductivity and can be used as a conductive filler to improve the electrical conductivity of the positive electrode material. However, graphite is prone to agglomeration and has poor mixing uniformity with the positive electrode material. Graphite can be prepared into graphene oxide with better electrical conductivity after suitable treatment. Therefore, it is of great significance to develop a method for preparing graphene oxide from recycled waste graphite and uniformly coating the regenerated positive electrode material, which can improve the performance of the positive electrode material and realize the cascade utilization of waste lithium-ion battery materials. SUMMARY

[0005] To solve the problems of resource waste and low recycling efficiency of waste lithium-ion batteries in the prior art, the present application provides a reduced graphene oxide coated phosphorus iron composite material prepared based on waste graphite and a preparation method and application thereof. The present application regenerates and utilizes the graphite negative electrode and phosphorus iron positive electrode material of the waste lithium iron phosphate battery to prepare a reduced graphene oxide coated phosphorus iron composite material, which is applied to the lithium-ion battery positive electrode material, effectively improving the electronic conductivity and lithium ion diffusion capacity, and thus improving the electrochemical performance and cycle stability.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] A method for preparing a reduced graphene oxide coated phosphorus iron composite material based on waste graphite, comprising the following steps:

[0008] (1) Disassembling the waste lithium iron phosphate battery to separate the positive electrode material waste lithium iron phosphate and the negative electrode material waste graphite;

[0009] (2) Pretreatment of waste graphite: purifying the waste graphite obtained in step (1) to obtain pure waste graphite;

[0010] (3) Preparation of graphene oxide (GO): preparing graphene oxide from the pure waste graphite obtained in step (2) by Hummers method;

[0011] (4) Preparation of phosphorus iron (FePO4): oxidizing the waste lithium iron phosphate obtained in step (1) in the presence of an oxidizing agent to obtain phosphorus iron;

[0012] (5) Reduced graphene oxide coated iron phosphate: the graphene oxide prepared in step (3) is mixed with the iron phosphate prepared in step (4) to perform a hydrothermal reaction to obtain a reduced graphene oxide coated iron phosphate composite material.

[0013] Preferably, the purification of the waste graphite in step (2) is etched by a NaOH solution.

[0014] Further preferably, the concentration of the NaOH solution is 0.01-0.04M. The low concentration of the NaOH solution can remove the impurities on the surface of the graphite while causing less damage to the graphite.

[0015] Further preferably, the etching time is 2-10h.

[0016] Preferably, the specific steps of preparing the graphene oxide by the Hummers method in step (3) include: mixing the pure waste graphite with concentrated sulfuric acid, phosphoric acid and potassium permanganate in a mass ratio of (1-5):(120-700):(10-50):(6-30), reacting for 6-18 hours under ice bath conditions, then adding hydrogen peroxide for post-treatment to obtain a graphene oxide suspension, and then performing filtration, washing and drying to obtain the graphene oxide. This method can efficiently oxidize the graphite into GO, and the preparation process is simple and controllable.

[0017] Further preferably, the mass concentration of the concentrated sulfuric acid is more than 95%; the mass concentration of the phosphoric acid is more than 80%;

[0018] Further preferably, the amount of hydrogen peroxide added is until the color of the solution no longer changes.

[0019] Preferably, the oxidizing agent in step (4) is at least one of hydrogen peroxide (H2O2), sodium persulfate (Na2S2O8) and potassium permanganate (KMnO4), and the molar ratio of the waste lithium iron phosphate to the oxidizing agent is (1-3):(1-2).

[0020] Preferably, the specific steps of preparing the iron phosphate in step (4) are: adding the waste lithium iron phosphate into an aqueous oxidizing agent solution, stirring and reacting at 20-40℃ for 12-48 hours, and then performing filtration, washing and drying to obtain the iron phosphate.

[0021] Further preferably, the concentration of the aqueous oxidizing agent solution is 0.05-0.2mol / L.

[0022] Preferably, the temperature of the hydrothermal reaction in step (5) is 100-150℃, and the hydrothermal reaction time is 1-8 hours.

[0023] Preferably, the mass ratio of the graphene oxide to the iron phosphate in step (5) is (0.01-0.2):(1-2).

[0024] Further preferably, the mass ratio of graphene oxide to iron phosphate is (0.05-0.15):1.

[0025] Preferably, the hydrothermal reaction of graphene oxide and iron phosphate in water in step (5) is carried out at a mass concentration of 0.01-0.1 g / mL of iron phosphate.

[0026] The reduced graphene oxide coated iron phosphate composite material prepared by the above method.

[0027] The reduced graphene oxide coated iron phosphate composite material described above is used as a positive electrode material for lithium ion batteries.

[0028] The reduced graphene oxide coated FePO4 composite material prepared by the present application has excellent electrochemical performance and can be used as a positive electrode material for lithium ion batteries. The composite material not only improves the electrical conductivity of FePO4, but also improves its cycle stability and rate performance.

[0029] Compared with the prior art, the present application has the following advantages:

[0030] (1) The present application uses graphite and lithium iron phosphate in waste lithium ion batteries as raw materials, and prepares a reduced graphene oxide coated iron phosphate composite material through simple chemical treatment and hydrothermal reaction, realizing the recycling of battery materials and meeting the concept of sustainable development.

[0031] (2) The present application removes impurities from waste graphite through pretreatment, and the GO prepared by the Hummers method has high purity and excellent performance, providing a good carrier for the compounding of FePO4.

[0032] (3) The present application uses reduced graphene oxide to coat and modify iron phosphate, constructs a conductive network, improves the electronic conductivity of the material, and at the same time buffers the volume change in the charging and discharging process, improves the structural stability and cycle life of the material.

[0033] (4) The preparation method of the present application is simple, the conditions are mild, the production cost is low, and it is easy to be applied in industrialization, which provides a new idea for the resource utilization of waste lithium ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 XRD patterns of the reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7 and 8, the lithium iron phosphate powder (wLFP) obtained by disassembling step (3) of Example 1, and the FePO4 material obtained in Comparative Example 1.

[0035] Figure 2XPS spectra of reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7, 8 and FePO4material obtained in Comparative Example 1.

[0036] Figure 3 SEM images of reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7, 8 and (b-d), FePO4material (a) obtained in Comparative Example 1 and TEM images of reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Example 1 (j, k, l), graphene oxide powder obtained for step (2) of Example 1 (e, i) and FePO4powder obtained for step (3) of Example 1 (f, g, h).

[0037] Figure 4 Charge-discharge curves of reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7, 8 and FePO4material obtained in Comparative Example 1 at 0.1 C.

[0038] Figure 5 Rate capability curves of reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7, 8 and FePO4material obtained in Comparative Example 1.

[0039] Figure 6 Cycle performance curves of reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7, 8 and FePO4material obtained in Comparative Example 1 at 1 C. DETAILED DESCRIPTION

[0040] The present application is further described in connection with the following examples, but the examples are not intended to limit the application. The examples are merely for the purpose of illustration. Various modifications can be made by those skilled in the art without departing from the scope of the present application. It should be noted that, if there are processes not specifically described below, they can be implemented or understood by those skilled in the art with reference to the prior art. If the reagents or instruments used are not specified by the manufacturer, they are considered to be conventional products that can be obtained by commercial purchase.

[0041] Example 1

[0042] A method for preparing reduced graphene oxide coated iron phosphate based on waste graphite, comprising the following steps:

[0043] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.01M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0044] (2) 1g of the pure waste graphite powder obtained in step (1) is slowly added to a mixed acid solution composed of 120mL of concentrated sulfuric acid (98wt%) and 13.3mL of phosphoric acid (85wt%), and stirred uniformly in an ice bath. 6g of potassium permanganate is slowly added, and the temperature is controlled not to exceed 20°C for 6 hours of reaction. Deionized water is added to dilute the mixture, and 10mL of 30wt% hydrogen peroxide is slowly added dropwise until the solution color no longer changes. Filtration is performed, and the product is washed with 10% HC1 solution and deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0045] (3) 5g of lithium iron phosphate powder disassembled from the positive electrode of the waste lithium iron phosphate battery is added to an aqueous solution containing 0.1mol / L sodium persulfate (Na2S2O8) with a molar ratio of 2:1. It is magnetically stirred at 30°C for 24 hours. After the reaction is completed, filtration is performed, and the product is washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain black FePO4 powder.

[0046] (4) 0.1g of the graphene oxide prepared in step (2) is dispersed in 30mL of deionized water, and ultrasonic treatment is performed for 30 minutes to form a uniform dispersion. 1g of the FePO4 powder prepared in step (3) is added, and ultrasonic treatment is continued for 30 minutes. The mixture is transferred to a 50mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor, and reacted at 120°C for 2 hours. After natural cooling to room temperature, filtration is performed, and the product is washed with deionized water and vacuum dried at 60°C for 12 hours to obtain reduced graphene oxide coated iron phosphate (FePO4@rGO) composite material.

[0047] Example 2

[0048] A method for preparing reduced graphene oxide coated iron phosphate based on waste graphite, comprising the following steps:

[0049] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.02M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0050] (2) Take 1 g of the pure waste graphite powder obtained in step (1) and slowly add it to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), stirring uniformly in an ice bath. Slowly add 6 g of potassium permanganate, controlling the temperature not to exceed 20°C for 6 hours of reaction. Add deionized water to dilute the mixture, and then slowly add 10 mL of 30 wt% hydrogen peroxide dropwise until the solution color no longer changes. Filter, wash with 10% HC1 solution and deionized water until neutral, and vacuum dry at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0051] (3) Take 5 g of lithium iron phosphate powder obtained from the positive electrode of the waste lithium iron phosphate battery, and add it to an aqueous solution containing 0.1 mol / L sodium persulfate (Na2S2O8), with a molar ratio of 2:1. Stir magnetically at 30°C for 24 hours. After the reaction is complete, filter, wash with deionized water until neutral, and vacuum dry at 60°C for 12 hours to obtain black FePO4 powder.

[0052] (4) Take 0.1 g of graphene oxide prepared in step (2) and disperse it in 30 mL of deionized water, ultrasonic treatment for 30 minutes to form a uniform dispersion. Add 1 g of FePO4 powder prepared in step (3) and continue ultrasonic treatment for 30 minutes. Transfer the mixture to a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor, and react at 120°C for 2 hours. After natural cooling to room temperature, filter, wash with deionized water, and vacuum dry at 60°C for 12 hours to obtain reduced graphene oxide-coated iron phosphate composite material.

[0053] Example 3

[0054] A method for preparing reduced graphene oxide-coated iron phosphate based on waste graphite, comprising the following steps:

[0055] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.03M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0056] (2) Take 1 g of the pure waste graphite powder obtained in step (1) and slowly add it to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), stirring uniformly in an ice bath. Slowly add 6 g of potassium permanganate, controlling the temperature not to exceed 20°C for 6 hours of reaction. Add deionized water to dilute the mixture, and then slowly add 10 mL of 30 wt% hydrogen peroxide dropwise until the solution color no longer changes. Filter, wash with 10% HC1 solution and deionized water until neutral, and vacuum dry at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0057] (3) 5 g of lithium iron phosphate powder obtained from the positive electrode of the waste lithium iron phosphate battery was added to an aqueous solution containing 0.1 mol / L sodium persulfate (Na2S2O8) at a molar ratio of 2:1. It was stirred magnetically at 30°C for 24 hours. After the reaction was completed, it was suction filtered, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain black FePO4 powder.

[0058] (4) 0.1 g of graphene oxide prepared in step (2) was dispersed in 30 mL of deionized water, and ultrasonically treated for 30 minutes to form a uniform dispersion. 1 g of FePO4 powder prepared in step (3) was added, and ultrasonically treated for another 30 minutes. The mixture was transferred to a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor, and reacted at 120°C for 2 hours. After natural cooling to room temperature, it was suction filtered, washed with deionized water, and vacuum dried at 60°C for 12 hours to obtain reduced graphene oxide-coated iron phosphate composite material.

[0059] Example 4

[0060] A method for preparing reduced graphene oxide-coated iron phosphate based on waste graphite, comprising the following steps:

[0061] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery was immersed and stirred in a 0.04M NaOH solution for 2 hours. After etching, the graphite was filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0062] (2) 1 g of the pure waste graphite powder obtained in step (1) was slowly added to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), and stirred uniformly in an ice bath. 6 g of potassium permanganate was slowly added, and the temperature was controlled not to exceed 20°C for 6 hours. Deionized water was added to dilute the mixture, and 10 mL of 30 wt% hydrogen peroxide was slowly added dropwise until the solution color no longer changed. It was suction filtered, washed with 10% HC1 solution and deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0063] (3) 5 g of lithium iron phosphate powder obtained from the positive electrode of the waste lithium iron phosphate battery was added to an aqueous solution containing 0.1 mol / L sodium persulfate (Na2S2O8) at a molar ratio of 2:1. It was stirred magnetically at 30°C for 24 hours. After the reaction was completed, it was suction filtered, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain black FePO4 powder.

[0064] (4) Take 0.1 g of graphene oxide prepared in step (2) and disperse it in 30 mL of deionized water, and ultrasonic treatment for 30 minutes to form a uniform dispersion. Add 1 g of FePO4 powder prepared in step (3), and continue to ultrasonic for 30 minutes. The mixture is transferred to a 50 mL polytetrafluoroethylene lined stainless steel hydrothermal reactor, and reacted at 120°C for 2 hours. After natural cooling to room temperature, suction filtration, deionized water washing, and vacuum drying at 60°C for 12 hours, a reduced graphene oxide coated iron phosphate composite material is obtained.

[0065] Example 5

[0066] A method for preparing a reduced graphene oxide coated iron phosphate from waste graphite, comprising the following steps:

[0067] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.01M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0068] (2) Take 1 g of the pure waste graphite powder obtained in step (1) and slowly add it to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), and stir uniformly in an ice bath. Slowly add 6 g of potassium permanganate, control the temperature not to exceed 20°C, and react for 6 hours. Add deionized water to the mixture to dilute it, and slowly add 10 mL of 30 wt% hydrogen peroxide dropwise until the solution color no longer changes. Suction filtration, washing with 10% HC1 solution and deionized water until neutral, and vacuum drying at 60°C for 12 hours, a brown-black graphene oxide (GO) powder is obtained.

[0069] (3) Take 5 g of lithium iron phosphate powder disassembled from the positive electrode of the waste lithium iron phosphate battery, and add it to an aqueous solution containing 0.08 mol / L sodium persulfate (Na2S2O8), with a molar ratio of 2:1. Stir magnetically at 30°C for 24 hours. After the reaction is completed, suction filtration, deionized water washing until neutral, and vacuum drying at 60°C for 12 hours, a black FePO4 powder is obtained.

[0070] (4) Take 0.1 g of graphene oxide prepared in step (2) and disperse it in 30 mL of deionized water, and ultrasonic treatment for 30 minutes to form a uniform dispersion. Add 1 g of FePO4 powder prepared in step (3), and continue to ultrasonic for 30 minutes. The mixture is transferred to a 50 mL polytetrafluoroethylene lined stainless steel hydrothermal reactor, and reacted at 120°C for 2 hours. After natural cooling to room temperature, suction filtration, deionized water washing, and vacuum drying at 60°C for 12 hours, a reduced graphene oxide coated iron phosphate composite material is obtained.

[0071] Example 6

[0072] A method for preparing reduced graphene oxide coated iron phosphate based on waste graphite, comprising the following steps:

[0073] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.01M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0074] (2) 1g of the pure waste graphite powder obtained in step (1) is slowly added to a mixed acid solution composed of 120mL of concentrated sulfuric acid (98wt%) and 13.3mL of phosphoric acid (85wt%), and stirred uniformly in an ice bath. Then 6g of potassium permanganate is slowly added, and the temperature is controlled not to exceed 20°C for 6 hours of reaction. Deionized water is added to dilute the mixture, and then 10mL of 30wt% hydrogen peroxide is slowly added dropwise until the solution color no longer changes. Filtration is performed, and the product is washed with 10% HC1 solution and deionized water until neutral, and then vacuum dried at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0075] (3) 5g of lithium iron phosphate powder disassembled from the positive electrode of the waste lithium iron phosphate battery is added to an aqueous solution containing 0.12mol / L sodium persulfate (Na2S2O8) with a molar ratio of 2:1. It is magnetically stirred at 30°C for 24 hours. After the reaction is completed, filtration is performed, and the product is washed with deionized water until neutral, and then vacuum dried at 60°C for 12 hours to obtain black FePO4 powder.

[0076] (4) 0.1g of graphene oxide prepared in step (2) is dispersed in 30mL of deionized water, and ultrasonic treatment is performed for 30 minutes to form a uniform dispersion. 1g of FePO4 powder prepared in step (3) is added, and ultrasonic treatment is continued for 30 minutes. The mixture is transferred to a 50mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor, and reacted at 120°C for 2 hours. After natural cooling to room temperature, filtration is performed, and the product is washed with deionized water and vacuum dried at 60°C for 12 hours to obtain reduced graphene oxide coated iron phosphate composite material.

[0077] Example 7

[0078] A method for preparing reduced graphene oxide coated iron phosphate based on waste graphite, comprising the following steps:

[0079] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.01M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0080] (2) Take 1 g of the pure waste graphite powder obtained in step (1) and slowly add it to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), stirring uniformly in an ice bath. Slowly add 6 g of potassium permanganate, controlling the temperature not to exceed 20°C for 6 hours of reaction. Add deionized water to dilute the mixture, and then slowly add 10 mL of 30 wt% hydrogen peroxide dropwise until the solution color no longer changes. Filter, wash with 10% HC1 solution and deionized water until neutral, and vacuum dry at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0081] (3) Take 5 g of lithium iron phosphate powder obtained from the positive electrode of the waste lithium iron phosphate battery, and add it to an aqueous solution containing 0.10 mol / L sodium persulfate (Na2S2O8), with a molar ratio of 2:1. Stir magnetically at 30°C for 24 hours. After the reaction is complete, filter, wash with deionized water until neutral, and vacuum dry at 60°C for 12 hours to obtain black FePO4 powder.

[0082] (4) Take 0.05 g of graphene oxide prepared in step (2) and disperse it in 30 mL of deionized water, ultrasonic treatment for 30 minutes to form a uniform dispersion. Add 1 g of FePO4 powder prepared in step (3) and continue ultrasonic treatment for 30 minutes. Transfer the mixture to a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor, and react at 120°C for 2 hours. After natural cooling to room temperature, filter, wash with deionized water, and vacuum dry at 60°C for 12 hours to obtain reduced graphene oxide-coated iron phosphate composite material.

[0083] Example 8

[0084] A method for preparing reduced graphene oxide-coated iron phosphate based on waste graphite, comprising the following steps:

[0085] (1) The graphite negative electrode sheet disassembled from the waste lithium iron phosphate battery is soaked and stirred in a 0.01M NaOH solution for 2 hours. After etching, the graphite is filtered out, washed with deionized water until neutral, and vacuum dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0086] (2) Take 1 g of the pure waste graphite powder obtained in step (1) and slowly add it to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), stirring uniformly in an ice bath. Slowly add 6 g of potassium permanganate, controlling the temperature not to exceed 20°C for 6 hours of reaction. Add deionized water to dilute the mixture, and then slowly add 10 mL of 30 wt% hydrogen peroxide dropwise until the solution color no longer changes. Filter, wash with 10% HC1 solution and deionized water until neutral, and vacuum dry at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0087] (3) 5 g of lithium iron phosphate powder obtained by disassembling the positive electrode of a waste lithium iron phosphate battery was added to an aqueous solution containing 0.10 mol / L sodium persulfate (Na2S2O8) at a molar ratio of 2:1. It was stirred magnetically at 30°C for 24 hours. After the reaction was completed, it was suction-filtered, washed with deionized water until neutral, and vacuum-dried at 60°C for 12 hours to obtain black FePO4 powder.

[0088] (4) 0.15 g of graphene oxide prepared in step (2) was dispersed in 30 mL of deionized water, and ultrasonically treated for 30 minutes to form a uniform dispersion. 1 g of FePO4 powder prepared in step (3) was added, and ultrasonic treatment was continued for 30 minutes. The mixture was transferred to a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reaction kettle, and reacted at 120°C for 2 hours. After natural cooling to room temperature, it was suction-filtered, washed with deionized water, and vacuum-dried at 60°C for 12 hours to obtain reduced graphene oxide-coated iron phosphate composite material.

[0089] Example 9

[0090] A method for preparing reduced graphene oxide-coated iron phosphate based on waste graphite, comprising the following steps:

[0091] (1) The graphite negative electrode sheet disassembled from a waste lithium iron phosphate battery was immersed and stirred in a 0.01M NaOH solution for 2 hours. After etching, the graphite was filtered out, washed with deionized water until neutral, and vacuum-dried at 60°C for 12 hours to obtain pure waste graphite powder.

[0092] (2) 1 g of the pure waste graphite powder obtained in step (1) was slowly added to a mixed acid solution composed of 120 mL of concentrated sulfuric acid (98 wt%) and 13.3 mL of phosphoric acid (85 wt%), and stirred uniformly in an ice bath. 6 g of potassium permanganate was slowly added, and the temperature was controlled not to exceed 20°C for 6 hours. Deionized water was added to dilute the mixture, and 10 mL of 30 wt% hydrogen peroxide was slowly added dropwise until the solution color no longer changed. It was suction-filtered, washed with 10% HC1 solution and deionized water until neutral, and vacuum-dried at 60°C for 12 hours to obtain brown-black graphene oxide (GO) powder.

[0093] (3) 5 g of lithium iron phosphate powder obtained by disassembling the positive electrode of a waste lithium iron phosphate battery was added to an aqueous solution containing 0.10 mol / L sodium persulfate (Na2S2O8) at a molar ratio of 2:1. It was stirred magnetically at 30°C for 24 hours. After the reaction was completed, it was suction-filtered, washed with deionized water until neutral, and vacuum-dried at 60°C for 12 hours to obtain black FePO4 powder.

[0094] (4) Take 0.1 g of graphene oxide prepared in step (2) and disperse it in 30 mL of deionized water, and ultrasonically treat for 30 minutes to form a uniform dispersion. Add 1 g of FePO4 powder prepared in step (3) and continue to ultrasonically treat for 30 minutes. Transfer the mixture to a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor, and react at 100°C for 2 hours. After natural cooling to room temperature, suction filtration, deionized water washing, and vacuum drying at 60°C for 12 hours, a reduced graphene oxide-coated iron phosphate composite material is obtained.

[0095] Comparative Example 1

[0096] Without adding graphene oxide, the rest of the conditions are the same as in Example 1, to prepare uncoated regenerated FePO4 material.

[0097] Data analysis:

[0098] Figure 1 XRD patterns of the reduced graphene oxide-coated iron phosphate (FePO4@rGO) obtained in step (4) of Examples 1, 7, and 8, lithium iron phosphate powder (wLFP) disassembled from step (3) of Example 1, and FePO4 material obtained in Comparative Example 1; it can be seen that the diffraction peaks in wLFP are highly consistent with the diffraction peaks of LFP and FePO4 PDF cards, and LFP and FePO4 impurity phases are found in wLFP after cycling. In line with previous literature analysis, the main reason for the attenuation of LFP cathode material is that the interface between LFP and FePO4 continuously approaches from the surface to the bulk, thereby further hindering the deintercalation of Li + When FePO4 is coated with rGO of different concentrations, the XRD patterns of FePO4@rGO x (x = 0.05, 0.10, and 0.15) are consistent with the positions of standard FePO4 diffraction peaks and clearly visible, and no other impurity peaks and C peaks are found, indicating that the reduction and coating process after the addition of GO does not change the crystal structure of FePO4. After oxidation with Na2S2O8, wLFP is completely converted into FePO4.

[0099] Figure 2 XPS patterns of the reduced graphene oxide-coated iron phosphate (FePO4@rGO) obtained in step (4) of Examples 1, 7, and 8, and the FePO4 material obtained in Comparative Example 1; it can be seen that all samples show typical C1s at ~ 284 eV, O1s at ~ 533 eV, F1s at ~ 686 eV, P 2p and P 2s at ~ 136 and 192 eV, respectively, and Fe 3p and Fe 2p at ~ 56 and 713 eV, respectively.

[0100] Figure 3The SEM images are of the reduced graphene oxide-coated iron phosphate (FePO4@rGO) (bd) obtained in step (4) of Examples 1, 7, and 8, the FePO4 material (a) obtained in Comparative Example 1, and the TEM images are of the reduced graphene oxide-coated iron phosphate (FePO4@rGO) (j, k, l) obtained in step (4) of Example 1, the graphene oxide powder (e, i) obtained in step (2) of Example 1, and the FePO4 powder (f, g, h) obtained in step (3) of Example 1; from Figure 3 As can be seen from 'a', the FePO4 particles have a spherical morphology, uneven size distribution, with the particle size mainly ranging from 100-500 nm and a smooth surface. Figure 3 As can be seen from b, the rGO reduced with 5 wt% GO did not completely coat the FePO4 particles; instead, the rGO particles were cross-linked, and the bond between the FePO4 particles and the rGO was not tight. However, the rGO reduced with 10 wt% GO coated the FePO4 particles... Figure 3 In step c), it can be clearly observed that an ultrathin and tightly bound layer of rGO is uniformly coated on the FePO4 particles, and the size and morphology of the coated FePO4 particles are clearly visible. When 15 wt% GO is used to reduce rGO to coat the FePO4 particles ( Figure 3 In d), a relatively thick layer of rGO is coated on the surface of the FePO4 particles. To further observe the effects of graphene oxide and reduced graphene oxide coating on the morphology and structure of iron phosphate, TEM measurements were performed. Figure 3 As can be seen from point e, the Hummer method successfully prepared graphene oxide with a wrinkled structure from waste graphite. By comparing FePO4 (… Figure 3 f) and FePO4@rGO 0.10 ( Figure 3 The TEM test results of j) further prove that rGO is coated on the surface of FePO4 particles. It can be clearly observed that the (111) crystal plane of FePO4 with a lattice spacing of 0.342 nm is tightly coated with an amorphous carbon layer with a thickness of about 5 nm.

[0101] For electrochemical performance testing, a working electrode was prepared by mixing and grinding 90 wt% active material (reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained in step (4) of Examples 1, 7, and 8 and FePO4 material obtained in Comparative Example 1), 5 wt% acetylene black, and 5 wt% poly(1,1-difluoroethylene) (PVDF) binder; N-methylpyrrolidone (NMP) solvent was used as the blending solvent. The resulting slurry was coated onto a Cu foil and then dried in a vacuum oven at 80°C for 24 hours. The prepared electrode was used as the working electrode, polypropylene (PP) as the separator, a solution of 1M LiPF6 dissolved in ethylene carbonate / diethyl carbonate / methyl ethyl carbonate (volume ratio: 1 / 1 / 1) as the electrolyte, and a lithium metal sheet as the reference electrode in a glove box under high-purity Ar conditions to form a 2032 button cell. This was for subsequent electrochemical performance testing.

[0102] Figure 4 The charge-discharge curves at 0.1C are shown for the reduced graphene oxide-coated iron phosphate (FePO4@rGO) obtained in step (4) of Examples 1, 7, and 8, and the FePO4 material obtained in Comparative Example 1. It can be seen that all samples exhibit a relatively long stable plateau around 3.4V, which is typical charge-discharge behavior of LiFePO4 cathode materials. The initial discharge capacity of FePO4 is 143.7 mAh g. -1 In comparison, FePO4@rGO x (x = 0.05, 0.10, and 0.15) The initial discharge specific capacity was significantly improved, reaching 155.5, 166.8, and 152.7 mAh g, respectively. -1 At the same time, through Figure 4 The magnified view of the charge-discharge curves shows that after coating with different concentrations of rGO, FePO4@rGO... x The overpotentials for (x = 0.05, 0.10, and 0.15) were also reduced to some extent. Compared with the other three materials (FePO4 = 126 mV, FePO4@rGO), the overpotentials were also reduced. 0.05 94mV and FePO4@rGO 0.15 (90mV), FePO4@rGO 0.10 It exhibits a low overpotential of 77mV. Figure 5 The figures show the rate performance curves of the reduced graphene oxide-coated iron phosphate (FePO4@rGO) obtained in step (4) of Examples 1, 7, and 8, and the FePO4 material obtained in Comparative Example 1. It can be seen that at rate ratios of 0.1, 0.2, 0.5, 1, 2, 5, and 10C, FePO4@rGO... 0.10 The average reversible capacities were 166.8, 164.5, 159.6, 153.4, 145.8, 128.3 and 112.6 mAh g, respectively.-1 It is worth noting that FePO4@rGO 0.15 The discharge specific capacity at 10C rate is low, mainly due to the thicker rGO coating hindering Li + transport at high rate. When the rate is decreased from 10C to 0.1C, all FePO4@rGO x (x = 0.05, 0.10 and 0.15) and FePO4 all show strong recoverability, indicating the cyclic stability of the material structure.

[0103] Figure 6 The cycle performance curves of the reduced graphene oxide coated iron phosphate (FePO4@rGO) obtained for step (4) of Examples 1, 7, 8 and FePO4 material obtained in Comparative Example 1 at 1C; it can be seen that the other electrodes (i.e. FePO4 is 125.6 mAh g -1 and 61.9%, FePO4@rGO 0.05 is 144.1 mAh g -1 and 63.2%, FePO4@rGO 0.15 is 143.9 mAh g -1 and 86.7%) compared to FePO4@rGO 0.10 can provide higher reversible initial specific capacity 153.9 mAh g -1 , the initial capacity retention rate after 500 cycles is 93.9% and the average CE of 500 cycles is 100%, again indicating that the FePO4@rGO 0.10 material has excellent cyclic stability.

Claims

1. A method for preparing reduced graphene oxide coated iron phosphate composite material based on waste graphite, characterized by, The method comprises the following steps: (1) disassembling the waste lithium iron phosphate battery to separate the positive material waste lithium iron phosphate and the negative material waste graphite; (2) pretreatment of the waste graphite: purifying the waste graphite obtained in step (1) to obtain pure waste graphite; the waste graphite is etched by a NaOH solution, the concentration of the NaOH solution is 0.01-0.04 M, and the etching time is 2-10 h; (3) preparation of graphene oxide: the pure waste graphite obtained in step (2) is prepared into graphene oxide by the Hummers method; the specific steps of the Hummers method for preparing graphene oxide include: mixing the pure waste graphite with concentrated sulfuric acid, phosphoric acid and potassium permanganate at a mass ratio of (1-5):(120-700):(10-50):(6-30), reacting for 6-18 h under ice bath conditions, then adding hydrogen peroxide for post-treatment, and then performing filtration, washing and drying treatment to obtain graphene oxide; the mass concentration of the concentrated sulfuric acid is higher than 95%; the mass concentration of the phosphoric acid is higher than 80%; the amount of hydrogen peroxide added is until the color of the solution no longer changes; (4) preparation of iron phosphate: the waste lithium iron phosphate obtained in step (1) is added into an aqueous oxidizing agent solution, and stirred and reacted at 20-40 ℃ for 12-48 h, and then filtered, washed and dried to obtain iron phosphate; the oxidizing agent is at least one of hydrogen peroxide, sodium persulfate and potassium permanganate, and the molar ratio of the waste lithium iron phosphate to the oxidizing agent is (1-3):(1-2); (5) reduction of graphene oxide coated iron phosphate: the graphene oxide prepared in step (3) is mixed with the iron phosphate prepared in step (4) to perform hydrothermal reaction to obtain a reduction of graphene oxide coated iron phosphate composite material; the hydrothermal reaction temperature is 100-150 ℃, the hydrothermal reaction time is 1-8 h, and the mass ratio of the graphene oxide to the iron phosphate is (0.01-0.2):(1-2).

2. The method of claim 1, wherein, In step (4), the concentration of the aqueous oxidizing agent solution is 0.05-0.2 mol / L.

3. The method of claim 1, wherein, In step (3), the mass ratio of the graphene oxide to the iron phosphate is (0.05-0.15):

1.

4. The method of claim 1, wherein, In step (5), the graphene oxide and the iron phosphate are dispersed in water to perform hydrothermal reaction, and the mass concentration of the iron phosphate is 0.01-0.1 g / mL.

5. The reduction of graphene oxide coated iron phosphate composite material prepared by the method of any one of claims 1-4.

6. Application of the reduction of graphene oxide coated iron phosphate composite material of claim 5 as a positive material of a lithium ion battery.

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