Lithium manganese iron phosphate composite material, preparation method thereof, and secondary battery

Abstract

The present application belongs to the field of battery technology, and in particular relates to a lithium iron phosphate composite material and a preparation method, as well as a secondary battery. Among them, the preparation method of the carbon-coated lithium iron phosphate composite material includes the following steps: respectively obtaining lithium iron phosphate recycled material, conductive agent recycled material and fluorine-containing recycled material; after the lithium iron phosphate recycled material is made into a precursor with lithium iron phosphate and a lithium source; sintering the precursor, and then mixing it with the conductive agent recycled material and the fluorine-containing recycled material and performing a secondary sintering to obtain a carbon-coated lithium iron phosphate composite material with a core-shell structure; including a lithium iron phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; and doped with fluorine element along the radial gradient of the composite material. The method of the present application utilizes recycled materials from waste batteries as raw materials, and can further purify and utilize the recycled materials during the preparation process, simplifying the recycling process and reducing the recycling cost. The prepared carbon-coated lithium iron phosphate composite material has excellent electrochemical properties and structural stability.

Description

Technical Field

[0001] The present application belongs to the field of battery technology, and in particular relates to a lithium manganese iron phosphate composite material and a preparation method thereof, as well as a secondary battery. Background Art

[0002] Compared to lithium iron phosphate (LFP), lithium iron phosphate (LFP) has a higher voltage platform, reaching approximately 4.1V, while LFP's voltage is around 3.4-3.5V. While both have the same theoretical specific capacity, due to its higher voltage, LFP's theoretical energy density is 15-20% higher under the same conditions. Currently, lithium-ion secondary batteries using LFP as the positive electrode material have begun to be widely used in power tools and electric vehicles due to their high energy density and excellent safety performance. At the same time, the number of used lithium-ion batteries is expected to increase annually. Therefore, recycling the LFP material in used batteries has become increasingly necessary to recycle and reuse materials, save costs, and protect the environment.

[0003] In the prior art, the recycling and utilization method of the positive electrode material of the lithium iron manganese phosphate battery includes the following steps: first dissolving the positive electrode material of the lithium iron manganese phosphate battery in an oxidizing acid solution, obtaining an oxidized acidified slurry through an oxidation reaction, and then filtering to obtain a lithium-rich solution and ferromanganese slag, wherein the ferromanganese slag is a mixture of manganese oxide and iron phosphate; removing impurities from the lithium-rich solution to obtain a lithium-rich purified liquid, and precipitating the lithium-rich purified liquid with sodium carbonate to obtain lithium carbonate; roasting the ferromanganese slag with sodium hydroxide, dissolving the obtained roasted material with water to obtain a water-soluble roasted material, and filtering the water-soluble roasted material to obtain a sodium manganate solution and iron phosphate; and adding a reducing agent to the sodium manganate solution to obtain manganese dioxide through an oxidation-reduction reaction.

[0004] Existing recycling methods for lithium manganese iron phosphate battery cathode materials often consider the recovery of elements. Not only are the recycling processes complex and the recycling costs high, but the recovery rate is low, the added value of the recycled products is low, and the utilization effect is poor. Summary of the Invention

[0005] The purpose of this application is to provide a lithium manganese iron phosphate composite material and a preparation method thereof, as well as a secondary battery, aiming to solve to a certain extent the problems of low recovery rate of existing lithium manganese iron phosphate battery positive electrode materials, low added value of recycled products, and poor utilization effect.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0007] In a first aspect, the present application provides a method for preparing a carbon-coated lithium manganese iron phosphate composite material, comprising the following steps:

[0008] Disassemble waste batteries to obtain lithium manganese iron phosphate recycled materials, conductive agent recycled materials and fluorine-containing recycled materials;

[0009] The lithium iron phosphate recovered material is crushed and then mixed with lithium iron phosphate and a lithium source to form a precursor having the lithium iron phosphate and the lithium source as a core and the lithium iron phosphate as a coating layer;

[0010] sintering the precursor to form a core-shell structure composite material;

[0011] The core-shell structure composite material is mixed with the conductive agent recycled material and the fluorine-containing recycled material and then subjected to secondary sintering to obtain a carbon-coated lithium manganese iron phosphate composite material; the carbon-coated lithium manganese iron phosphate composite material is a core-shell structure, including a lithium manganese iron phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; fluorine element is doped along the radial gradient of the carbon-coated lithium manganese iron phosphate composite material.

[0012] In a second aspect, the present application provides a carbon-coated lithium manganese iron phosphate composite material, which has a core-shell structure, including a lithium manganese iron phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; fluorine element is doped along the radial gradient of the carbon-coated lithium manganese iron phosphate composite material.

[0013] In a third aspect, the present application provides a secondary battery, which comprises the carbon-coated lithium manganese iron phosphate composite material prepared by the above method, or the above-mentioned carbon-coated lithium manganese iron phosphate composite material.

[0014] The method for preparing a carbon-coated lithium iron manganese phosphate composite material provided in the first aspect of this application directly utilizes recycled lithium iron manganese phosphate, recycled conductive agent, and recycled fluorine-containing materials obtained from dismantling waste batteries as raw materials for preparing the carbon-coated lithium iron manganese phosphate composite material. During the preparation process, the recycled materials can be further purified and utilized, simplifying the recycling process and reducing recycling costs. The method is suitable for industrial large-scale production and application. Furthermore, the prepared carbon-coated lithium iron manganese phosphate composite material exhibits excellent electrochemical properties and structural stability.

[0015] The carbon-coated lithium iron manganese phosphate composite material provided in the second aspect of the present application comprises a lithium iron manganese phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer, and the carbon-coated lithium iron manganese phosphate composite material is doped with fluorine along the radial gradient. The lithium iron manganese phosphate of the core and the lithium iron phosphate of the middle layer ensure the electrochemical properties of the composite material, such as energy density and discharge efficiency. The carbon outer shell can not only improve the electrical conductivity of the composite material, but also improve the structural stability of the composite material. In addition, the carbon-coated lithium iron manganese phosphate composite material is doped with fluorine along the radial gradient. The doped fluorine can improve the battery cycle performance and rate performance. The fluorine has a strong binding force with the metal ions on the surface of lithium iron manganese phosphate and lithium iron phosphate, which is conducive to further improving the stability of the core-shell structure.

[0016] The third aspect of the present application provides a secondary battery comprising the carbon-coated lithium manganese iron phosphate composite material. The carbon-coated lithium manganese iron phosphate composite material exhibits electrochemical properties such as high energy density, high conductivity, and good hydrophobicity, as well as excellent structural stability. Thus, the secondary battery's cycle stability and service life are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0018] Figure 1 1 is a schematic flow chart of a method for preparing a carbon-coated lithium manganese iron phosphate composite material provided in an embodiment of the present application;

[0019] Figure 2 Schematic diagram of the structure of the carbon-coated lithium manganese iron phosphate composite material provided in the embodiment of the present application. DETAILED DESCRIPTION

[0020] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0021] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0022] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b or c", or "at least one of a, b and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple.

[0023] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0024] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0025] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass in the examples of this application may be μg, mg, g, kg, etc., which are mass units known in the chemical industry.

[0026] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be understood to indicate or imply relative importance or to implicitly specify the quantity of the technical features being referred to. For example, without departing from the scope of the embodiments of this application, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0027] As attached Figure 1 As shown, the first aspect of the embodiment of the present application provides a method for preparing a carbon-coated lithium manganese iron phosphate composite material, comprising the following steps:

[0028] S10. Disassemble the used batteries to obtain lithium iron manganese phosphate recycled materials, conductive agent recycled materials, and fluorine-containing recycled materials;

[0029] S20. The lithium iron manganese phosphate recovered material is crushed and mixed with lithium iron phosphate and a lithium source to form a precursor with lithium iron manganese phosphate and a lithium source as the core and lithium iron phosphate as the coating layer;

[0030] S30. The precursor is sintered to form a core-shell composite material;

[0031] S40. The core-shell structure composite material is mixed with the conductive agent recycled material and the fluorine-containing recycled material and then subjected to secondary sintering to obtain a carbon-coated lithium iron manganese phosphate composite material; the carbon-coated lithium iron manganese phosphate composite material is a core-shell structure, including a lithium iron manganese phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; fluorine element is doped along the radial gradient of the carbon-coated lithium iron manganese phosphate composite material.

[0032] The preparation method of the carbon-coated lithium iron manganese phosphate composite material provided in the first aspect of the embodiment of the present application, the raw material components such as lithium iron manganese phosphate, conductive agent, fluorine-containing material are all recovered from waste batteries, which makes full use of waste resources, is economical and environmentally friendly. The lithium iron manganese phosphate recycled material recovered initially is crushed and mixed with lithium iron phosphate and a lithium source to form a precursor of a core-shell structure, wherein the lithium source is used to supplement the loss of lithium source of the lithium iron manganese phosphate recycled material during battery operation. The precursor is then sintered. On the one hand, it can promote the reaction between the lithium iron manganese phosphate recycled material and the lithium source, so that the material restores the normal lithium content; the core-shell structure composite material of lithium iron phosphate coated with lithium iron manganese phosphate with a stable structure is formed by sintering; on the other hand, the sintering treatment can purify the lithium iron manganese phosphate recycled material, convert the residual organic matter in the lithium iron manganese phosphate recycled material into carbon material, and remove some volatile residual impurity components. The core-shell composite material is then mixed with recycled conductive material and fluorine-containing recycled material and subjected to a secondary sintering process. During the sintering process, the recycled conductive material is purified and forms a carbon coating on the surface of the core-shell composite material. The fluorine-containing recycled material decomposes at high temperatures to produce HF. HF can penetrate the lithium iron phosphate core and lithium iron phosphate intermediate layer of the composite material, forming a gradient fluorine-doped structure, improving the cycling and rate performance of the carbon-coated lithium iron phosphate composite material. HF also has a strong affinity for metal ions on the surfaces of lithium iron phosphate and lithium iron phosphate, enhancing the stability of the composite material's core-shell structure. Furthermore, HF can form fluoride salts with residual metal impurities in the recycled conductive material and fluorine-containing recycled material, doping the carbon coating with fluorinated metal salts. This prevents metal element loss in the carbon-coated lithium iron phosphate composite material, improving its electrochemical performance, and enhancing its hydrophobicity and stability. The method for preparing the carbon-coated lithium iron manganese phosphate composite material provided in the embodiments of the present application directly utilizes recycled lithium iron manganese phosphate, recycled conductive agent, and recycled fluorine-containing materials obtained from the dismantling of waste batteries as raw materials for preparing the carbon-coated lithium iron manganese phosphate composite material. During the preparation process, the recycled materials can be further purified and utilized, simplifying the recycling process and reducing recycling costs. The method is suitable for industrial large-scale production and application. Furthermore, the prepared carbon-coated lithium iron manganese phosphate composite material exhibits excellent electrochemical properties and structural stability.

[0033] In some embodiments, in the above step S10, the steps of disassembling the waste batteries and obtaining lithium iron manganese phosphate recycled materials, conductive agent recycled materials and fluorine-containing recycled materials respectively include: stripping the battery positive electrode material from the positive electrode sheet and dispersing it in an organic solvent, the fluorine-containing binder will dissolve in the organic solvent, while the positive electrode active material and the conductive agent and other materials will not dissolve. After filtering, the filter residue mainly includes lithium iron manganese phosphate positive electrode active material and conductive agent. The lithium iron manganese phosphate recycled material and the conductive agent recycled material can be separated from the filter residue by simple screening. The fluorine-containing binder dissolves in the filtrate, and the fluorine-containing recycled material can be directly recovered from the filtrate by drying. In the embodiment of the present application, lithium iron manganese phosphate recycled materials, conductive agent recycled materials and fluorine-containing recycled materials can be obtained from waste batteries through preliminary disassembly and sorting. These recycled materials are directly used as raw material components for preparing carbon-coated lithium iron manganese phosphate composite materials. The recycled materials can be further purified in the subsequent sintering process to remove and utilize related impurity components. The recycling process is simplified and the recycling cost is reduced.

[0034] In some specific embodiments, the lithium manganese iron phosphate battery is first discharged and then disassembled to obtain the battery shell, negative and positive electrode sheets, and separator. The positive electrode sheet is then soaked in water and then subjected to ultrasound. The cavitation effect of ultrasound in the liquid creates strong mechanical vibrations, which causes the destructive force of the mechanical vibrations in the positive electrode material coating to be greater than the binding force of binders such as polyvinylidene fluoride (PVDF). The positive electrode material is then peeled from the aluminum foil current collector. The peeled positive electrode material is then added to an organic solvent such as N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), or cyclopentyl methyl ether (CP) to dissolve the binders such as PVDF. The stripped positive electrode material is then filtered to obtain a residue of positive electrode powder and conductive carbon. The positive electrode powder and conductive carbon are then separated by screening. Fluorine-containing organic substances such as polyvinylidene fluoride (PVDF) are recovered from the filtrate.

[0035] In some embodiments, the organic solvent includes at least one of N-methylpyrrolidone NMP, N,N-dimethylformamide DMF, and cyclopentyl methyl ether CP. These organic solvents can fully dissolve organic matter such as fluorine-containing binders in the battery positive electrode material, disperse the lithium manganese iron phosphate active material and the conductive agent, and facilitate separation of the two.

[0036] In some embodiments, the fluorine-containing recycled material includes at least one of polytetrafluoroethylene, fluorobenzene, and perfluorocarboxylic acid; these fluorine-containing recycled materials can decompose to produce HF during the preparation process of the carbon-coated lithium manganese iron phosphate composite material, form fluoride salts with metal impurities in the carbon-coated lithium manganese iron phosphate composite material, and combine with the lithium manganese iron phosphate core and lithium iron phosphate intermediate layer in the carbon-coated lithium manganese iron phosphate composite material to improve the structural stability and electrochemical properties of the composite material.

[0037] In some embodiments, the recycled conductive agent includes at least one of carbon nanotubes, conductive carbon black, graphene, and graphite. These carbon material recycled conductive agent materials can be further purified during the sintering process to form a carbon coating on the surface of the core-shell structure composite material. This can not only improve the structural stability of the carbon-coated lithium manganese iron phosphate composite material, but also improve the hydrophobicity of the composite material, making the carbon-coated lithium manganese iron phosphate composite material easy to dry and non-water-absorbing, thereby preventing the impact of environmental moisture on the performance of the composite material and improving the structural and performance stability of the composite material. This facilitates the storage and application of the carbon-coated lithium manganese iron phosphate composite material.

[0038] In some embodiments, in step S20, the recycled lithium iron manganese phosphate is crushed to make it more suitable for coating, thereby facilitating the subsequent composite material. After the recycled lithium iron manganese phosphate is crushed, it is mixed with lithium iron phosphate and a lithium source to form a precursor with the lithium iron manganese phosphate and lithium source as the core and the lithium iron phosphate as the coating layer, facilitating the subsequent preparation of a core-shell structure composite material.

[0039] In some embodiments, the mass ratio of lithium iron manganese phosphate recycled material, lithium iron phosphate and lithium source is (8-16): 1: (0.005-0.01). Since lithium ions in lithium iron manganese phosphate recycled material are lost during battery operation, the embodiment of the present application supplements the lithium ions lost in lithium iron manganese phosphate recycled material by adding a lithium source to ensure the electrochemical properties of the composite material. In addition, lithium iron manganese phosphate recycled material is used as the core and lithium iron phosphate is used as the precursor of the coating layer, wherein the lithium iron phosphate coating layer can significantly improve the conductive properties of the composite material. In some embodiments, the amount of lithium source added is 0.5% to 1% of the mass of lithium iron manganese phosphate recycled material. After the two are fully mixed, lithium iron phosphate is added according to the mass ratio of lithium iron phosphate to lithium iron manganese phosphate recycled material of 1: (8-16), and ball milling is carried out for 1 to 3 hours to form a precursor with lithium iron manganese phosphate and lithium source as the core and lithium iron phosphate as the coating layer.

[0040] In some embodiments, the lithium source is selected from at least one of lithium phosphate, lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium nitrite, lithium acetate, lithium oxide, and lithium oxalate; these lithium sources are mixed with lithium iron manganese phosphate recycled materials as the precursor core, and in the subsequent sintering process, they can better supplement the lithium ions in the lithium iron manganese phosphate recycled materials that are missing due to battery operation.

[0041] In some embodiments, in the above step S30, the conditions for sintering the precursor include: sintering for 5 to 8 hours in an inert atmosphere at a temperature of 700 to 800°C; reacting the lithium manganese iron phosphate in the precursor core with the lithium source, replenishing the lithium ions in the lithium manganese iron phosphate recovered material, and forming a lithium manganese iron phosphate core with stable structural properties. At the same time, the lithium iron phosphate coating forms a tightly bonded and stable shell. In some embodiments, the sintering temperature includes but is not limited to 700 to 720°C, 720 to 750°C, 750 to 780°C, 780 to 800°C, etc., and the sintering time includes but is not limited to 5 to 6 hours, 6 to 7 hours, 7 to 8 hours, etc. The inert atmosphere includes but is not limited to nitrogen, argon, helium, etc., to prevent the oxidation of the raw material components and improve the stability of the reaction and the product.

[0042] In some embodiments, in the above step S40, the core-shell structure composite material is mixed with the conductive agent recycled material and the fluorine-containing recycled material and then subjected to secondary sintering. The conditions for the secondary sintering treatment include: sintering in an inert atmosphere such as nitrogen, argon, helium, etc. at a temperature of 700-800°C for 1-3 hours. During the secondary sintering process, the conductive agent recycled material can be purified and a carbon coating layer can be formed on the surface of the core-shell structure composite material; and the fluorine-containing recycled material can be decomposed at high temperature to produce HF, which penetrates into the lithium iron phosphate core and lithium iron phosphate middle layer of the composite material to form a gradient F-doped structure, thereby improving the cycle performance and rate performance of the carbon-coated lithium iron phosphate composite material. In addition, HF has a strong binding ability with the metal ions on the surface of lithium iron phosphate and lithium iron phosphate, which can improve the stability of the core-shell structure of the composite material. HF can form fluoride salts with the metal impurities remaining in the conductive agent recycled material and the fluorine-containing recycled material, so that the carbon coating layer is doped with fluoride metal salts, which can prevent the loss of metal elements in the carbon-coated lithium manganese iron phosphate composite material and improve the electrochemical properties of the carbon-coated lithium manganese iron phosphate composite material, and can also improve the hydrophobicity of the carbon-coated lithium manganese iron phosphate composite material and improve the stability of the carbon-coated lithium manganese iron phosphate composite material.

[0043] In some embodiments, the mass ratio of the core-shell structure composite material, the conductive agent recycled material and the fluorine-containing recycled material is 100: (1-5): (1-3). The carbon coating layer generated by this ratio can completely include the carbon-coated lithium manganese iron phosphate composite material, which has a good improvement effect on the conductive properties of the carbon-coated lithium manganese iron phosphate composite material. At the same time, it is beneficial to improve the stability of the structure and particle size of the composite material and avoid the aggregation of the composite material. If the ratio of the conductive agent recycled material is too high, the carbon coating layer formed will be too thick, reducing the content of active substances in the composite material and affecting the charge and discharge performance of the composite material; if the ratio of the conductive agent recycled material is too low, it will be difficult to form a complete carbon coating layer, and the effect of improving the structural stability of the composite material will be poor. The ratio of the fluorine-containing recycled material ensures that the amount of fluorine doped in the composite material is beneficial to improving the charge and discharge capacity and cycle performance of the composite material. If the ratio of the fluorine-containing recycled material is too high or too low, it is not conducive to improving the charge and discharge capacity and cycle performance of the composite material.

[0044] The carbon-coated lithium manganese iron phosphate composite material prepared in the embodiment of the present application is a core-shell structure, including a lithium manganese iron phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; fluorine elements are doped along the radial gradient of the carbon-coated lithium manganese iron phosphate composite material, and this structure is beneficial to improving the energy density and conductivity of the composite material. The doped fluorine element exists in the form of fluoride ions, which are anions. After being doped into the lithium manganese iron phosphate core and the lithium iron phosphate middle layer, it can partially replace the phosphate position, improve the structural stability, and thus improve the cycle performance of the composite material. In some embodiments, in the carbon-coated lithium manganese iron phosphate composite material, the molar ratio of fluoride ions to phosphate ions is (0.01 to 0.03): 1; this molar ratio is beneficial to improving the capacity, platform performance and cycle performance of the composite material, and fully ensures that fluorine doping improves the cycle performance and rate performance of the carbon-coated lithium manganese iron phosphate composite material, as well as the stability of the core-shell structure of the composite material.

[0045] In some embodiments, the fluorine doping concentration increases radially from the inside to the outside of the carbon-coated lithium manganese iron phosphate composite material. Fluorine is doped according to this pattern, and the fluorine doped in the core and intermediate layers has a strong ability to bind to the metal ions on the surface of the lithium manganese iron phosphate and lithium iron phosphate, thereby improving the stability of the composite material's core-shell structure and enhancing the battery's cycling performance and rate performance. Fluorine doping in the carbon coating layer can prevent metal element loss in the carbon-coated lithium manganese iron phosphate composite material, increase its utilization rate, and enhance the electrochemical performance of the carbon-coated lithium manganese iron phosphate composite material. It can also enhance the hydrophobicity of the carbon-coated lithium manganese iron phosphate composite material and improve its stability.

[0046] In some embodiments, a fluorine doping method is used to form a fluorine doping with increasing concentration from the inside to the outside in the radial direction of the carbon-coated lithium manganese iron phosphate composite material by a thermally driven diffusion method. The formation principle includes but is not limited to: when the ambient temperature increases, the fluorine atoms in the fluorine-containing recycled material and the core-shell structure composite material vibrate near the equilibrium lattice point, and some of them will obtain sufficient energy to leave the equilibrium lattice point. When the adjacent fluorine atoms or matrix atoms migrate to the vacancy, vacancy diffusion occurs. As the atoms move, vacancies are generated in the core-shell structure composite material, and the fluorine atoms in the fluorine-containing recycled material become atoms in a filling state. The difficulty of thermally driven diffusion of the composite material atoms decreases radially from the inside to the outside, thereby decreasing the difficulty of doping fluorine atoms in the composite material, and forming a fluorine doping with increasing concentration from the inside to the outside in the radial direction of the carbon-coated lithium manganese iron phosphate composite material.

[0047] In some embodiments, the fluorine doping percentage in the lithium iron phosphate core gradually increases from 0.1% to 0.5% from the inside out, i.e., increases progressively; the fluorine doping percentage in the lithium iron phosphate middle layer gradually increases from 0.5% to 1% from the inside out; and the fluorine doping percentage in the carbon outer shell gradually increases from 1% to 3% from the inside out. The above-described doping concentration gradient of fluorine in the lithium iron phosphate core, lithium iron phosphate middle layer, and carbon outer shell of the present application embodiment simultaneously ensures the electrochemical, structural, and performance stability characteristics of the carbon-coated lithium iron phosphate composite material, which is beneficial for improving the capacity, platform performance, and cycle performance of the composite material.

[0048] In other embodiments, the carbon-coated lithium manganese iron phosphate composite material has a doping concentration gradient of fluorine in the lithium manganese iron phosphate core that can be expressed as 2.5*10 -5 mol / nm~0.001mol / nm; the doping concentration gradient of fluorine element in the lithium iron phosphate middle layer can be expressed as 0.001mol / nm~0.005mol / nm; the doping concentration gradient of fluorine element in the carbon outer shell layer can be expressed as 0.05mol / nm~0.105mol / nm.

[0049] In some embodiments, the carbon outer shell layer contains a fluoride salt. In some embodiments, the fluoride salt includes at least one of AlF3, CrF3, CuF2, KF, MnF2, NaF, and NiF2. HF generated by the decomposition of the fluorine-containing recycled material during the sintering process can react with residual Al, Cr, Cu, K, Mn, Na, Ni, and other metal impurities in the conductive agent recycled material and the fluorine-containing recycled material to form fluoride salts such as AlF3, CrF3, CuF2, KF, MnF2, NaF, and NiF2, thereby doping the carbon coating layer with fluoride metal salts. This prevents the loss of metal elements in the carbon-coated lithium manganese iron phosphate composite material, improves the electrochemical performance of the carbon-coated lithium manganese iron phosphate composite material, and improves the hydrophobicity and stability of the carbon-coated lithium manganese iron phosphate composite material.

[0050] In some embodiments, the particle size of the lithium iron phosphate core is 100-200 nm; the thickness of the lithium iron phosphate intermediate layer is 5-25 nm; and the thickness of the carbon outer layer is 1-2 nm. The thickness distribution pattern of the core, intermediate layer, and outer layer in the carbon-coated lithium iron phosphate composite material of the present application embodiment not only helps improve the electrochemical properties of the composite material, such as capacity, platform performance, and cycle performance, but also enables the carbon-coated lithium iron phosphate composite material to have a high specific surface area, increase the reaction interface, provide more diffusion channels, increase the lithium ion diffusion rate, and further enhance the performance of the lithium-ion battery.

[0051] As attached Figure 2 As shown, the second aspect of the embodiment of the present application provides a carbon-coated lithium iron manganese phosphate composite material, which is a core-shell structure, including a lithium iron manganese phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; fluorine element is doped along the radial gradient of the carbon-coated lithium iron manganese phosphate composite material.

[0052] The carbon-coated lithium iron manganese phosphate composite material provided in the second aspect of the embodiment of the present application includes a lithium iron manganese phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer, and the carbon-coated lithium iron manganese phosphate composite material is doped with fluorine elements along the radial gradient. Among them, the lithium iron manganese phosphate core has high energy density but poor conductivity. Using lithium iron manganese phosphate as the core and lithium iron phosphate as the shell is conducive to improving the conductivity of lithium iron manganese phosphate; the synergistic effect of the lithium iron manganese phosphate in the core and the lithium iron phosphate in the middle layer ensures the electrochemical properties of the composite material such as energy density, discharge efficiency, and conductivity. The carbon outer shell can not only improve the conductivity of the composite material, but also improve the structural stability of the composite material. In addition, the carbon-coated lithium iron manganese phosphate composite material is doped with fluorine elements along the radial gradient. The doped fluorine elements can improve the battery cycle performance and rate performance. The fluorine elements have a strong binding force with the metal ions on the surface of lithium iron manganese phosphate and lithium iron phosphate, which is conducive to further improving the stability of the core-shell structure.

[0053] The carbon-coated lithium manganese iron phosphate composite material of the embodiment of the present application can be prepared by the method of the embodiment described above.

[0054] In some embodiments, in the carbon-coated lithium manganese iron phosphate composite material, the molar ratio of fluoride ions to phosphate ions is (0.01-0.03):1.

[0055] In some embodiments, the doping concentration of the fluorine element increases gradually from the inside to the outside along the radial direction of the carbon-coated lithium manganese iron phosphate composite material.

[0056] In some embodiments, the doping mass percentage of fluorine in the lithium iron phosphate core gradually increases from 0.1% to 0.5% from the inside to the outside; the doping mass percentage of fluorine in the lithium iron phosphate middle layer gradually increases from 0.5% to 1% from the inside to the outside; and the doping mass percentage of fluorine in the carbon outer layer gradually increases from 1% to 3% from the inside to the outside. In other embodiments, in the carbon-coated lithium iron phosphate composite material, the doping concentration gradient of fluorine in the lithium iron phosphate core can be expressed as 2.5*10 -5 mol / nm~0.001mol / nm; the doping concentration gradient of fluorine element in the lithium iron phosphate middle layer can be expressed as 0.001mol / nm~0.005mol / nm; the doping concentration gradient of fluorine element in the carbon outer shell layer can be expressed as 0.05mol / nm~0.105mol / nm.

[0057] In some embodiments, the carbon outer shell layer comprises a fluoride salt, and further, the fluoride salt comprises at least one of AlF3, CrF3, CuF2, KF, MnF2, NaF, and NiF2.

[0058] In some embodiments, the particle size of the lithium iron phosphate core is 100-200 nm; the thickness of the lithium iron phosphate middle layer is 5-25 nm; and the thickness of the carbon outer shell is 1-2 nm.

[0059] The beneficial effects of the above technical solutions in the embodiments of the present application have been discussed above and will not be repeated here.

[0060] A third aspect of an embodiment of the present application provides a secondary battery, which includes the carbon-coated lithium manganese iron phosphate composite material prepared by the above method, or the above-mentioned carbon-coated lithium manganese iron phosphate composite material.

[0061] The secondary battery provided in a third aspect of the present invention comprises the carbon-coated lithium manganese iron phosphate composite material. The carbon-coated lithium manganese iron phosphate composite material exhibits electrochemical properties such as high energy density, high conductivity, and good hydrophobicity, as well as excellent structural stability. Thus, the secondary battery's cycle stability and service life are improved.

[0062] In order to enable those skilled in the art to clearly understand the above-mentioned implementation details and operations of the present application, as well as the carbon-coated lithium manganese iron phosphate composite material and its preparation method in the embodiment of the present application, and the significant embodiment of the improved performance of the secondary battery, the above-mentioned technical solution is illustrated by multiple embodiments below.

[0063] Example 1

[0064] A carbon-coated lithium manganese iron phosphate composite material, the preparation of which comprises the following steps:

[0065] 1. The lithium manganese iron phosphate battery is first discharged and then disassembled to obtain the battery shell, negative electrode material, positive electrode material, and separator. The positive electrode material is soaked in water and then ultrasonically stripped from the aluminum foil. The stripped positive electrode material is added with NMP to dissolve the PVDF. The stripped positive electrode material is filtered and separated to obtain lithium manganese iron phosphate recycled material and conductive carbon recycled material. The recycled material is then separated by sieving to obtain lithium manganese iron phosphate recycled material and conductive carbon recycled material. The filtrate is dried to obtain PVDF recycled material.

[0066] 2. Take 50g of the lithium iron manganese phosphate recovered material obtained in step 1 and ball-mill it into a 10μm powder. Add 5g of lithium iron phosphate and 0.5g of lithium carbonate and mix them by ball-milling to obtain a precursor with lithium iron manganese phosphate and lithium carbonate as the core and lithium iron phosphate as the coating shell structure;

[0067] 3. Place the precursor in a tube furnace, introduce nitrogen, sinter at 700°C for 8 hours, cool, and crush to obtain a core-shell structure composite material;

[0068] 4. Take 1g of conductive carbon recycled material and 1g of PVDF recycled material to make a nanoscale slurry, mix it with the core-shell structure composite material obtained in step 3, and place it in a gradient doping reactor. Sinter it at 800℃ for 1h, cool it, and crush it to obtain a carbon-coated lithium manganese iron phosphate composite material, including a lithium manganese iron phosphate core with a particle size of 100nm, a lithium iron phosphate middle layer with a thickness of 12nm, and a carbon outer layer with a thickness of 1nm. The doping concentration gradient of the fluorine element in the core is 2*10 -4 mol / nm~8*10 -4 mol / nm; the doping concentration gradient of fluorine element in the lithium iron phosphate middle layer is 0.002mol / nm~0.004mol / nm from the inside to the outside; the doping concentration gradient of fluorine element in the carbon outer layer is 0.06mol / nm~0.085mol / nm from the inside to the outside.

[0069] Example 2

[0070] A carbon-coated lithium manganese iron phosphate composite material, the preparation of which comprises the following steps:

[0071] 1. The lithium manganese iron phosphate battery is first discharged and then disassembled to obtain the battery shell, negative electrode material, positive electrode material, and separator. The positive electrode material is soaked in water and then ultrasonically stripped from the aluminum foil. The stripped positive electrode material is added with DMF to dissolve the PVDF. The stripped positive electrode material is filtered and separated to obtain lithium manganese iron phosphate recycled material and conductive carbon recycled material. The recycled material is then separated by sieving to obtain lithium manganese iron phosphate recycled material and conductive carbon recycled material. The filtrate is dried to obtain PVDF recycled material.

[0072] 2. Take 100g of the lithium iron manganese phosphate recovered material obtained in step 1 and ball-mill it into a 30μm powder. Add 12.5g of lithium iron phosphate and 0.2g of lithium hydroxide and mix them by ball-milling to obtain a precursor with lithium iron manganese phosphate and lithium carbonate as the core and lithium iron phosphate as the coating shell structure;

[0073] 3. Place the precursor in a tube furnace, introduce nitrogen, sinter at 800°C for 5 hours, cool, and crush to obtain a core-shell structure composite material;

[0074] 4. Take 5g of conductive carbon recycled material and 1g of PVDF recycled material to make a nanoscale slurry, mix it with the core-shell structure composite material obtained in step 3, and place it in a gradient doping reactor. Sinter it at 750℃ for 3h, cool it, and crush it to obtain a carbon-coated lithium manganese iron phosphate composite material, including a lithium manganese iron phosphate core with a particle size of 150nm, a lithium iron phosphate middle layer with a thickness of 18nm, and a carbon outer layer with a thickness of 3nm. The doping concentration gradient of the fluorine element in the core is 2.5*10 -5 mol / nm~5*10 -4 mol / nm; the doping concentration gradient of fluorine element in the lithium iron phosphate middle layer is 0.001mol / nm~0.002mol / nm from the inside to the outside; the doping concentration gradient of fluorine element in the carbon outer layer is 0.05mol / nm~0.075mol / nm from the inside to the outside.

[0075] Example 3

[0076] A carbon-coated lithium manganese iron phosphate composite material, the preparation of which comprises the following steps:

[0077] 1. The lithium manganese iron phosphate battery is first discharged and then disassembled to obtain the battery shell, negative electrode material, positive electrode material, and separator. The positive electrode material is soaked in water and then ultrasonically stripped from the aluminum foil. The stripped positive electrode material is added with NMP, DMF, and CP to dissolve the PVDF. The stripped positive electrode material is filtered and separated to obtain lithium manganese iron phosphate recycled material and conductive carbon recycled material. The recycled material is then separated by sieving to obtain lithium manganese iron phosphate recycled material and conductive carbon recycled material. The filtrate is dried to obtain PVDF recycled material.

[0078] 2. Take 80g of the lithium iron manganese phosphate recovered material obtained in step 1 and ball-mill it into a 20μm powder. Add 6g of lithium iron phosphate and 0.4g of lithium nitrate and mix them by ball-milling to obtain a precursor with lithium iron manganese phosphate and lithium carbonate as the core and lithium iron phosphate as the coating shell structure;

[0079] 3. Place the precursor in a tube furnace, introduce nitrogen, sinter at 750°C for 7 hours, cool, and crush to obtain a core-shell structure composite material;

[0080] 4. Take 3g of conductive carbon recycled material and 2g of PVDF recycled material to make a nanoscale slurry, mix it with the core-shell structure composite material obtained in step 3, and place it in a gradient doping reactor. Sinter it at 700°C for 2h, cool it, and crush it to obtain a carbon-coated lithium manganese iron phosphate composite material, including a lithium manganese iron phosphate core with a particle size of 200nm, a lithium iron phosphate middle layer with a thickness of 12nm, and a carbon outer layer with a thickness of 1.5nm. The doping concentration gradient of the fluorine element in the core is 6*10 -4 mol / nm~0.001mol / nm; the doping concentration gradient of fluorine element in the lithium iron phosphate middle layer is 0.002mol / nm~0.005mol / nm from the inside to the outside; the doping concentration gradient of fluorine element in the carbon outer layer is 0.08mol / nm~0.105mol / nm from the inside to the outside.

[0081] Comparative Example 1

[0082] A method for recycling lithium manganese iron phosphate positive electrode material comprises the following steps:

[0083] The lithium iron phosphate battery cathode material is first dissolved in a sodium persulfate solution, oxidized and acidified to produce an oxidized slurry. This is then filtered to produce a lithium-rich solution and ferromanganese slag, a mixture of manganese oxide and iron phosphate. The lithium-rich solution is then cleaned to produce a lithium-rich purified solution, which is then precipitated with a sodium carbonate solution to produce lithium carbonate. The ferromanganese slag is then calcined with solid sodium hydroxide at 500°C. The resulting calcined material is dissolved in water to produce a water-soluble calcined material, which is then filtered to produce a sodium manganate solution and iron phosphate. The sodium manganate solution is then added with the reducing agent manganese trioxide to produce manganese dioxide through an oxidation-reduction reaction. The lithium iron phosphate battery cathode material is the cathode material separated from the lithium iron phosphate battery. Among them, the impurity removal operation is to use sodium hydroxide to adjust the pH value of the lithium-rich solution to 9 to remove trace manganese ions, and then use sodium hydroxide to adjust the pH value to 11; the amount of sodium persulfate used is 1.1 times the stoichiometric ratio, the amount of sodium hydroxide used is 1.05 times the stoichiometric ratio, and the amount of reducing agent used is 1.2 times the stoichiometric ratio.

[0084] Comparative Example 2

[0085] A carbon-coated lithium manganese iron phosphate composite material, which differs from Example 1 in that:

[0086] In step 4, no PVDF is added, and the obtained carbon-coated lithium manganese iron phosphate composite material is not doped with fluorine.

[0087] Comparative Example 3

[0088] A carbon-coated lithium manganese iron phosphate composite material, which differs from Example 1 in that:

[0089] In step 2, the precursor has lithium manganese iron phosphate and lithium carbonate as the coating shell and lithium iron phosphate as the core;

[0090] The carbon-coated lithium manganese iron phosphate composite material prepared in step 4 includes a lithium iron phosphate core with a particle size of 100 nm, a lithium manganese iron phosphate middle layer with a thickness of 10 nm, and a carbon outer layer with a thickness of 1 nm. The doping concentration gradient of the fluorine element in the core is 2*10 -4 mol / nm~8*10 -4 mol / nm; the doping concentration gradient of fluorine element in the lithium iron phosphate middle layer is 0.002mol / nm~0.004mol / nm from the inside to the outside; the doping concentration gradient of fluorine element in the carbon outer layer is 0.06mol / nm~0.085mol / nm from the inside to the outside.

[0091] Furthermore, in order to verify the progress of the embodiments of the present application, the following performance tests were performed on the above embodiments 1 to 3 and comparative examples 1 to 3 respectively:

[0092] 1. Battery performance test: Using the carbon-coated lithium manganese iron phosphate composite materials provided in the above examples and comparative examples, batteries were assembled as follows:

[0093] Preparation of positive electrode sheet: Carbon-coated lithium manganese iron phosphate composite material, SP (conductive carbon black), PVDF (polyvinylidene fluoride) and NMP (N-methylpyrrolidone) are stirred in a ball mill for 2 hours in a mass ratio of 93.5:2.5:4:100 to obtain a positive electrode slurry; the prepared positive electrode slurry is added to aluminum foil, evenly scraped with a scraper, dried at 130°C and then rolled to obtain a positive electrode sheet.

[0094] Battery assembly process: The prepared positive electrode is attached to the positive electrode metal shell with conductive adhesive, a metal lithium sheet is used as the negative electrode, a Celgard 2400 microporous membrane is used as the separator, and a 1.0 mol / L LiPF6 solution is used as the electrolyte. The solvent of the electrolyte is a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1. The batteries are assembled into button cells in a glove box.

[0095] The electrochemical properties of the button cell, including resistivity and charge / discharge performance, were tested using a LAND electrochemical tester. The charge cut-off voltage was 4.2 V, and the discharge cut-off voltage was 2.0 V. The test results are shown in Table 1 below:

[0096] Table 1

[0097]

[0098] It can be seen from the test results in Table 1 above that the carbon-coated lithium manganese iron phosphate composite materials prepared in Examples 1 to 3 of the present application, because they include a lithium manganese iron phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer, and the radial gradient of the carbon-coated lithium manganese iron phosphate composite material is doped with fluorine elements, all show excellent charge and discharge performance, low resistivity, and good conductivity. In addition, the preparation process is simple and the utilization rate of waste battery materials is high. However, the recycling method of the lithium manganese iron phosphate positive electrode material in Comparative Example 1 is complex in process, has low recycling efficiency, and poor practicality. In addition, the composite material without fluorine doping prepared in Comparative Example 2 and the composite material with lithium iron phosphate as the core and lithium manganese iron phosphate as the middle layer prepared in Comparative Example 3 have significantly reduced charge and discharge performance, high resistivity, and poor conductivity.

[0099] 2. Hydrophobic performance test:

[0100] Take 0.2g of sample, add a certain amount of boric acid, and use a tablet press to press the sample into a thin slice at a pressure of 50MPa. Use the DSA 100 measuring instrument of Kruss Company of Germany to perform the drop method contact angle test on the coal sample slice. The test process is as follows: first place the sample slice on the sample table, observe and adjust the knob with the help of a camera to keep the sample surface level. Then use a microliter syringe to drop a drop of deionized water droplet on the sample surface according to a certain volume, and start recording at the same time. Select the image of the droplet contacting the surface of the coal slice at 0.5s, and use the angle measurement method to calculate the contact angle. Select different positions on the sample slice and perform 2 more tests according to the same method. After error analysis, the average value of multiple measurements is selected as the contact angle measurement result of the sample. The test results are shown in Table 2 below:

[0101] Table 2

[0102]

[0103]

[0104] It can be seen from the test results in Table 2 above that, relative to the composite material not doped with fluorine element prepared in Comparative Example 2, the composite material with lithium iron phosphate as the core and lithium manganese iron phosphate as the intermediate layer prepared in Comparative Example 3, the carbon-coated lithium manganese iron phosphate composite material prepared in Examples 1 to 3 of the present application, because it includes a lithium manganese iron phosphate core, a lithium iron phosphate intermediate layer and a carbon outer shell layer, and the radial gradient of the carbon-coated lithium manganese iron phosphate composite material is doped with fluorine element, has a larger contact angle and better hydrophobicity, so that the prepared carbon-coated lithium manganese iron phosphate composite material is easier to dry, does not absorb water, and improves the storage stability of the composite material.

[0105] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A method for preparing a carbon-coated lithium manganese iron phosphate composite material, characterized in that: The following steps are involved: Disassemble waste batteries to obtain lithium manganese iron phosphate recycled materials, conductive agent recycled materials and fluorine-containing recycled materials; The lithium iron phosphate recovered material is crushed and then mixed with lithium iron phosphate and a lithium source to form a precursor having the lithium iron phosphate and the lithium source as a core and the lithium iron phosphate as a coating layer; sintering the precursor to form a core-shell structure composite material; The core-shell structure composite material is mixed with the conductive agent recycled material and the fluorine-containing recycled material and then subjected to secondary sintering to obtain a carbon-coated lithium manganese iron phosphate composite material; the carbon-coated lithium manganese iron phosphate composite material is a core-shell structure, including a lithium manganese iron phosphate core, a lithium iron phosphate middle layer and a carbon outer shell layer; fluorine element is doped along the radial gradient of the carbon-coated lithium manganese iron phosphate composite material.

2. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 1, wherein: The steps of respectively obtaining lithium manganese iron phosphate recovered material, conductive agent recovered material and fluorine-containing recovered material include: stripping the battery positive electrode material from the positive electrode sheet and dispersing it in an organic solvent, filtering, separating the lithium manganese iron phosphate recovered material and the conductive agent recovered material from the filter residue, and recovering the fluorine-containing recovered material from the filtrate.

3. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, wherein: In the carbon-coated lithium manganese iron phosphate composite material, the molar ratio of fluoride ion to phosphate ion is (0.01-0.03):1; And / or, the doping concentration of the fluorine element increases gradually from the inside to the outside along the radial direction of the carbon-coated lithium manganese iron phosphate composite material; And / or, the carbon outer shell layer contains a fluoride salt.

4. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 3, wherein: The doping mass percentage of the fluorine element in the lithium manganese iron phosphate core gradually increases from 0.1% to 0.5% from the inside to the outside; The doping mass percentage of the fluorine element in the lithium iron phosphate intermediate layer gradually increases from 0.5% to 1% from the inner to the outer layer; The doping mass percentage of the fluorine element in the carbon outer shell gradually increases from 1% to 3% from the inside to the outside.

5. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to any one of claims 1 to 4, wherein: The particle size of the lithium manganese iron phosphate core is 100-200 nm; The thickness of the lithium iron phosphate intermediate layer is 5 to 25 nm; The thickness of the carbon outer layer is 1-2 nm.

6. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to any one of claims 1 to 4, wherein: The mass ratio of the lithium manganese iron phosphate recovered material, the lithium iron phosphate and the lithium source is (8-16):1:(0.005-0.01); And / or, the mass ratio of the core-shell structure composite material, the conductive agent recycled material and the fluorine-containing recycled material is 100: (1-5): (1-3).

7. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 1, wherein: The sintering treatment conditions include: sintering in an inert atmosphere at a temperature of 700-800°C for 5-8 hours; And / or, the conditions of the secondary sintering treatment include: sintering at a temperature of 700-800° C. in an inert atmosphere for 1-3 hours.

8. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 3, wherein: The lithium source is selected from at least one of lithium phosphate, lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium nitrite, lithium acetate, lithium oxide, and lithium oxalate; And / or, the fluorine-containing recycled material includes at least one of polytetrafluoroethylene, fluorobenzene, and perfluorocarboxylic acid; And / or, the conductive agent recycled material includes at least one of carbon nanotubes, conductive carbon black, graphene, and graphite; and / or, the fluoride salt comprises at least one of AlF3, CrF3, CuF2, KF, MnF2, NaF, and NiF2; And / or, the organic solvent includes at least one of N-methylpyrrolidone, N,N-dimethylformamide, and cyclopentyl methyl ether.

9. A secondary battery, characterized in that: The secondary battery comprises a carbon-coated lithium manganese iron phosphate composite material prepared by the method according to any one of claims 1 to 8.

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