Lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of recycling of waste positive electrode materials, and discloses a lithium iron phosphate positive electrode material as well as a preparation method and application thereof. The lithium iron phosphate positive electrode material is of a core-shell structure, a core layer is lithium iron phosphate, and a shell layer is carbon-doped lithium fluoride. The method comprises the following steps: (1) mixing lithium iron phosphate and polyvinylidene fluoride, and carrying out primary ball milling in an inert atmosphere to obtain a primary ball-milled material; (2) mixing the waste positive pole piece stripped from the current collector with the primary ball-milling material, and performing secondary ball-milling in an inert atmosphere to obtain a secondary ball-milling material; and (3) carrying out annealing treatment on the secondary ball-milled material in an inert atmosphere. The lithium iron phosphate positive electrode material has high rate capability and excellent cycle performance, and meanwhile, waste positive plates without liquid injection are recycled by the preparation method, so that the cost is reduced, and the maximum utilization of resources is realized.

Description

Technical Field

[0001] This invention relates to the field of waste cathode material recycling technology, specifically to a lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Driven by the rapid development of the new energy industry, lithium iron phosphate (LFP) and other cathode materials have seen a continuous increase in market demand due to their core advantages of high safety and low cost. However, the LFP battery production process generates a large amount of unfilled cathode waste, mainly including cutting scraps, products that do not meet performance and size requirements, and process prototypes. Statistics show that approximately 200 tons of this type of unfilled cathode waste are generated for every 1 GWh of battery produced. The LFP particle structure within the cathode coating of this waste remains intact, and because it is free from impurities due to the lack of electrolyte injection, it possesses extremely high reuse value. However, due to the lack of targeted high-value recycling technology, most companies directly discard these unfilled LFP cathodes as solid waste or sell them at low prices to grinding companies, which not only causes serious waste of raw materials but also increases environmental pressure.

[0003] On the other hand, to improve the cycle stability and rate performance of cathode materials, coating modification is often used—that is, forming a coating layer on the surface of cathode material particles to create a stable physical barrier, suppressing interfacial side reactions between the electrolyte and the cathode, and reducing lithium-ion migration resistance. However, current coating processes have significant drawbacks: firstly, the raw material costs for coating are high, such as high-purity lithium fluoride materials; secondly, uneven mixing and dispersion of cathode particles and coating agents during the coating process can lead to uncontrolled coating thickness and / or local agglomeration, making it difficult to guarantee the stability of the modification effect and restricting its industrial application.

[0004] In summary, the industry currently faces two major technical challenges: First, there is a lack of high-value recycling pathways for unfilled waste cathode sheets, including cathode active materials and binders and other additives, resulting in significant resource waste and environmental pressure; second, traditional coating and modification processes have high raw material costs and unstable coating effects. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of resource waste caused by the lack of high-value recycling pathways for unfilled waste positive electrode sheets in the existing technology, as well as the high cost of raw materials and unstable coating effect of traditional lithium battery coating modification processes. This invention provides a lithium iron phosphate positive electrode material and its preparation method. The lithium iron phosphate positive electrode material has high rate performance and excellent cycle performance. At the same time, the preparation method can recycle unfilled waste positive electrode sheets, reduce costs and maximize resource utilization.

[0006] To achieve the above objectives, the present invention provides a lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material has a core-shell structure, wherein the core layer is lithium iron phosphate and the shell layer is carbon-doped lithium fluoride.

[0007] Preferably, based on the lithium iron phosphate cathode material, the content of the core layer is 97-99.5 wt%, and the content of the shell layer is 0.5-3 wt%.

[0008] Preferably, the average thickness of the shell is 2-6 nm.

[0009] Preferably, the porosity of the shell is 3-5%, and the average pore size is 3-8 nm.

[0010] A second aspect of the present invention provides a method for preparing lithium iron phosphate cathode material, the method comprising the following steps: (1) Mix lithium iron phosphate and polyvinylidene fluoride and ball mill them once under an inert atmosphere to obtain ball-milled material; (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed and ball milled again under an inert atmosphere to obtain the secondary ball milling material; (3) The secondary ball milled material is annealed under an inert atmosphere.

[0011] Preferably, in step (1), the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:0.5-2.5.

[0012] Preferably, in step (1), the conditions for the first ball milling include: a temperature of -40 to -30°C, a rotation speed of 200-300 r / min, and a time of 0.5-2.5 h.

[0013] Preferably, the particle size of the material from the primary ball milling is ≤30μm.

[0014] Preferably, in step (2), the weight ratio of the waste positive electrode sheet stripped from the current collector to the amount of the primary ball milling material is 1:1-10.

[0015] Preferably, in step (2), the conditions for the secondary ball milling include: a temperature of -40 to -30°C, a rotation speed of 350-450 r / min, and a time of 2-3.5 h.

[0016] Preferably, the particle size of the secondary ball-milled material is 3-10 μm.

[0017] Preferably, in step (3), the annealing conditions include: a heating temperature of 150-400℃, a heating time of 0.5-6h, followed by cooling.

[0018] A third aspect of the present invention provides a lithium iron phosphate cathode material prepared by the above method.

[0019] A fourth aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising the above-mentioned lithium iron phosphate positive electrode material.

[0020] Compared with the prior art, the technical solution of the present invention has the following advantages: (1) This invention can efficiently recover the cathode material and PVDF component in the unfilled waste cathode sheet, which can be recycled as cathode raw material and can also be used as a coating agent, realizing the high economic value and resource utilization of the unfilled waste cathode sheet, and opening up a new path for the high-value recycling of unfilled LFP cathode sheets. (2) The lithium iron phosphate cathode material described in this invention has a core-shell structure, with the shell being a carbon-doped lithium fluoride coating layer, which gives it high rate performance and excellent cycle performance. At the same time, the entire preparation process does not require the addition of special coating agents, which can significantly reduce costs compared with traditional coating processes. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1 of the present invention; Figure 2 This is a TEM image of the lithium iron phosphate cathode material prepared in Example 1 of the present invention. Detailed Implementation

[0022] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0023] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0024] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating relative importance or implying the number of technical features indicated. Therefore, unless otherwise stated, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature; "multiple" means two or more. The term "comprising" and any variations thereof mean non-exclusive inclusion, where one or more other features, integers, steps, operations, units, components, and / or combinations thereof may be present or added.

[0025] The present invention provides a lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material has a core-shell structure, wherein the core layer is lithium iron phosphate and the shell layer is carbon-doped lithium fluoride.

[0026] In this invention, the lithium iron phosphate cathode material has a core-shell structure, wherein the core layer is lithium iron phosphate and the shell layer is carbon-doped lithium fluoride. Based on this, the carbon-doped lithium fluoride serves as a coating layer to coat the lithium iron phosphate. This coating layer can effectively protect and optimize ion transport, and its performance is stable. It can suppress interfacial side reactions between the electrolyte and the cathode, and reduce the migration resistance of lithium ions in the core layer, thereby enabling the lithium iron phosphate cathode material to have high rate performance and excellent cycle performance.

[0027] In a preferred embodiment, based on the lithium iron phosphate cathode material, the content of the core layer is 97-99.5 wt%, and the content of the shell layer is 0.5-3 wt%; preferably, based on the lithium iron phosphate cathode material, the content of the core layer is 98-99 wt%, and the content of the shell layer is 1-2 wt%. Based on this, the coating function of the shell layer can be better realized, and the content of lithium iron phosphate in the core layer gives it good rate performance.

[0028] In a preferred embodiment, the average thickness of the shell layer is 2-6 nm; based on this, the shell layer has a stable coating effect and can further reduce the migration resistance of lithium ions in the core layer.

[0029] In a preferred embodiment, the porosity of the shell layer is 3-5%, and the average pore size is 3-8 nm. Based on this, the shell layer is a continuous and dense coating barrier, which can effectively isolate the electrolyte from direct contact with the substrate, while ensuring smooth lithium ion transport, achieving a balance between the protective function of the coating layer and the ion transport performance, and improving the electrochemical performance and cycle stability of the material.

[0030] A second aspect of the present invention provides a method for preparing lithium iron phosphate cathode material, the method comprising the following steps: (1) Mix lithium iron phosphate and polyvinylidene fluoride and ball mill them once under an inert atmosphere to obtain ball-milled material; (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed and ball milled again under an inert atmosphere to obtain the secondary ball milling material; (3) The secondary ball milled material is annealed under an inert atmosphere.

[0031] In the method described in this invention, in step (1), lithium iron phosphate and polyvinylidene fluoride (PVDF) are ball-milled once under an inert atmosphere to achieve homogenization and premixing of the materials while preventing chemical oxidation reactions, so that PVDF forms a preliminary coating on lithium iron phosphate; in step (2), the waste positive electrode sheet of the stripped current collector and the material from the first ball milling are ball-milled a second time under an inert atmosphere to further achieve homogenization of the materials while refining the particle size of the materials and preventing chemical oxidation reactions. Based on the second ball milling, after the second ball milling is completed, the waste positive electrode sheet of the stripped current collector and the lithium iron phosphate and PVDF are ball-milled. The premixed materials are thoroughly mixed and the particle size of the materials is further controlled to obtain the secondary ball-milled material; in step (3), the secondary ball-milled material is annealed under an inert atmosphere, so that PVDF reacts with lithium iron phosphate to form lithium fluoride, and finally a continuous and dense coating layer of carbon-doped lithium fluoride is formed on the surface of lithium iron phosphate. This coating layer can play a good role in protecting and optimizing ion transport, and the coating layer has stable performance. It can suppress the side reaction at the interface between the electrolyte and the cathode, and reduce the migration impedance of lithium ions in the core layer, thereby enabling the lithium iron phosphate cathode material to have high rate performance and excellent cycle performance.

[0032] In this invention, the "annealing treatment" refers to the entire process of heating and naturally cooling the material. Natural cooling simply means placing the heated material in a heating device to cool it naturally.

[0033] In the method described in this invention, "waste positive electrode sheet after stripping current collector" refers to the electrode material remaining after stripping the current collector (such as aluminum foil), i.e., "waste positive electrode sheet after stripping current collector" contains substances such as PVDF, lithium iron phosphate, and conductive agent (mainly carbon); in a preferred embodiment, the effective active component of PVDF in "waste positive electrode sheet after stripping current collector" is ≥95% to ensure that it is fully utilized in the annealing process, while the residual amount of aluminum foil current collector needs to be controlled to ≤200ppm and the moisture content of "waste positive electrode sheet after stripping current collector" is ≤0.3%.

[0034] In the method described in this invention, there are no special restrictions on the source of the waste positive electrode sheet. Any waste positive electrode sheet generated from lithium iron phosphate positive electrode sheets commonly used in the art can be used. Depending on the source, the content of PVDF, lithium iron phosphate, and carbon materials contained in the "waste positive electrode sheet with stripped current collector" will vary.

[0035] In the method described in this invention, the gas providing the inert atmosphere is selected from one or more of argon, helium, neon, nitrogen, and hydrogen; in a preferred embodiment, the gas providing the inert atmosphere is selected from one or more of argon, nitrogen, and hydrogen.

[0036] In the method described in this invention, the equipment used for primary ball milling and secondary ball milling can be purged with the gas that provides the inert atmosphere. The equipment used for primary ball milling and secondary ball milling includes, but is not limited to, vertical ball mills, horizontal ball mills, or air-jet ball mills.

[0037] In the method described in this invention, the equipment for annealing can be supplied with the gas that provides the inert atmosphere, and the equipment for annealing includes, but is not limited to, box furnace, tube furnace, rotary kiln, pusher kiln and roller kiln.

[0038] In a preferred embodiment, in order to further improve the rate performance and cycle performance of the lithium iron phosphate cathode material, in step (1), the weight ratio of the lithium iron phosphate to the polyvinylidene fluoride is 100:0.5-2.5; specifically, it can be 100:0.5, 100:1, 100:2 or 100:2.5.

[0039] In a preferred embodiment, in step (1), the conditions for the first ball milling include: a temperature of -40 to -30°C, a rotation speed of 200-300 r / min, and a time of 0.5-2.5 h; based on this, the homogenization of lithium iron phosphate and PVDF materials is better achieved, making their mixing more uniform.

[0040] In a preferred embodiment, the particle size of the material from the primary ball milling is ≤30μm.

[0041] In a preferred embodiment, in order to further improve the rate performance and cycle stability of the lithium iron phosphate cathode material, in step (2), the weight ratio of the waste cathode sheet stripped from the current collector to the amount of the primary ball milling material is 1:1-10; specifically, it can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10.

[0042] In a preferred embodiment, in order to further improve the coating stability of the material, the conditions for the secondary ball milling in step (2) include: temperature of -40 to -30℃, rotation speed of 350-450r / min, and time of 2-3.5h; based on this, the homogenization of the materials lithium iron phosphate, PVDF and the waste positive electrode sheet of the stripped current collector is better realized, making the mixing more uniform, while further controlling the particle size of the material.

[0043] In a preferred embodiment, the particle size of the secondary ball-milled material is 3-10 μm.

[0044] In a preferred embodiment, in step (3), the annealing conditions include: a heating temperature of 150-400°C, a heating time of 0.5-6 hours, followed by cooling.

[0045] In a more preferred embodiment, step (3) specifically includes: heating to the target temperature of 150-400°C at a rate of 3-10°C / min for a duration of 0.5-6h, followed by natural cooling; based on this, PVDF reacts with lithium to generate lithium fluoride, and finally forms a more stable carbon-doped lithium fluoride coating layer on the surface of lithium iron phosphate particles, so that the lithium iron phosphate cathode material has higher rate performance and better cycle performance.

[0046] A third aspect of the present invention provides a lithium iron phosphate cathode material prepared by the above method.

[0047] In this invention, a lithium iron phosphate cathode material with a core-shell structure is obtained through the above method. The lithium iron phosphate cathode material has lithium iron phosphate as the core layer and carbon-doped lithium fluoride as the shell layer.

[0048] A fourth aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising the above-mentioned lithium iron phosphate positive electrode material.

[0049] The following examples further illustrate the lithium iron phosphate cathode material, its preparation method, and its application according to the present invention. These examples are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following examples.

[0050] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following embodiments are commercially available; wherein, the waste positive electrode sheet is a defective product generated during the battery electrode sheet production process, and the waste positive electrode sheet with stripped current collector contains 95wt% lithium iron phosphate positive electrode material, 2.5wt% PVDF, and 2.5wt% carbon.

[0051] Example 1 (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:1, and the mixture is ball-milled once at a rate of 200 r / min for 2 h at -30 °C in the presence of nitrogen gas to obtain ball-milled material with a particle size ≤30 μm. (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed, wherein the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:1. The secondary ball milling is carried out at a rate of 450 r / min for 3 h in the presence of nitrogen gas at -30℃ to obtain the secondary ball milling material with a particle size of 3-10 μm. (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 350°C at a rate of 5°C / min, sintered for 4 hours, and then cooled naturally.

[0052] Example 2 (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:2.5, and the mixture is ball-milled once at a rate of 200 r / min for 2 h at -40 °C in the presence of nitrogen gas to obtain ball-milled material with a particle size ≤30 μm. (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed, wherein the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:10. The mixture is ball milled at a rate of 450 r / min for 3 h at -40 °C in the presence of nitrogen gas to obtain the secondary ball milling material with a particle size of 3-10 μm. (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 350°C at a rate of 5°C / min, sintered for 4 hours, and then cooled naturally.

[0053] Example 3 The implementation is carried out in accordance with Example 1, except that in step (1), the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:0.1, that is: (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:0.1, and the mixture is ball-milled once at a rate of 200 r / min for 2 h in the presence of nitrogen gas at -30℃ to obtain ball-milled material with a particle size ≤30 μm. (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed, wherein the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:1. The secondary ball milling is carried out at a rate of 450 r / min for 3 h in the presence of nitrogen gas at -30℃ to obtain the secondary ball milling material with a particle size of 3-10 μm. (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 350°C at a rate of 5°C / min, sintered for 4 hours, and then cooled naturally.

[0054] Example 4 The implementation follows the method of Example 1, except that in step (2), the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:15, that is: (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:1, and the mixture is ball-milled once at a rate of 200 r / min for 2 h at -30 °C in the presence of nitrogen gas to obtain ball-milled material with a particle size ≤30 μm. (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed, wherein the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:15. The mixture is ball milled at a rate of 450 r / min for 3 h at -30 °C in the presence of nitrogen gas to obtain the secondary ball milling material with a particle size of 3-10 μm. (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 350°C at a rate of 5°C / min, sintered for 4 hours, and then cooled naturally.

[0055] Example 5 The process is carried out in accordance with Example 1, except that in step (2), the secondary ball milling rate is 300 r / min and the time is 1 h, that is: (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:1, and the mixture is ball-milled once at a rate of 200 r / min for 2 h at -30 °C in the presence of nitrogen gas to obtain ball-milled material with a particle size ≤30 μm. (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed, wherein the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:1. The secondary ball milling is carried out at a rate of 300 r / min for 1 h in the presence of nitrogen gas at -30℃ to obtain the secondary ball milling material with a particle size of 15-20 μm. (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 350°C at a rate of 5°C / min, sintered for 4 hours, and then cooled naturally.

[0056] Example 6 The implementation is carried out in accordance with Example 1, except that in step (3), the annealing temperature is 140°C, that is: (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:1, and the mixture is ball-milled once at a rate of 200 r / min for 2 h at -30 °C in the presence of nitrogen gas to obtain ball-milled material with a particle size ≤30 μm. (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed, wherein the weight ratio of the waste positive electrode sheet of the stripped current collector to the primary ball milling material is 1:1. The secondary ball milling is carried out at a rate of 450 r / min for 3 h in the presence of nitrogen gas at -30℃ to obtain the secondary ball milling material with a particle size of 3-10 μm. (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 120°C at a rate of 5°C / min, sintered for 4 hours, and then naturally cooled.

[0057] Comparative Example 1 The implementation is carried out in accordance with Example 1, except that in step (2), an equal amount of a mixture of commercially available PVDF, lithium iron phosphate cathode material, and carbon is used to replace the waste cathode sheet from which the current collector has been stripped, i.e.: (1) Lithium iron phosphate and polyvinylidene fluoride are mixed, wherein the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:1, and the mixture is ball-milled once at a rate of 200 r / min for 2 h at -30 °C in the presence of nitrogen gas to obtain ball-milled material with a particle size ≤30 μm. (2) The mixture (PVDF content is 95wt%, lithium iron phosphate cathode material content is 2.5wt%, carbon content is 2.5wt%) and the primary ball milling material are mixed, wherein the weight ratio of the mixture to the primary ball milling material is 1:1, and the mixture is ball milled again at a rate of 450r / min for 3h in the presence of nitrogen gas at -30℃ to obtain the secondary ball milling material with a particle size of 3-10μm; (3) In the presence of nitrogen gas, the secondary ball milled material is placed in a tubular atmosphere furnace, and then the tubular atmosphere furnace is heated to the target temperature of 350°C at a rate of 5°C / min, sintered for 4 hours, and then cooled naturally.

[0058] Test case (1) The XRD pattern analysis of the lithium iron phosphate cathode material prepared in Example 1 was performed, and the results are as follows: Figure 1 As shown; Depend on Figure 1 It can be seen that: through Figure 1 The XRD pattern clearly shows the characteristic diffraction peak of lithium fluoride (LiF) at 685.40 eV.

[0059] (2) The average thickness of the lithium iron phosphate cathode materials prepared in the examples and comparative examples was characterized, and the test results are shown in Table 1. Figure 2 This is a TEM image of the lithium iron phosphate cathode material prepared in Example 1; Depend on Figure 2 It can be seen that the average thickness of the shell layer of the lithium iron phosphate cathode material prepared in Example 1 is about 3 nm.

[0060] (3) The lithium iron phosphate cathode materials prepared in the examples and comparative examples were assembled into 88125 soft-pack cells. Their cycle performance and rate performance were tested at room temperature under a voltage platform of 2.0-3.75V. The results are shown in Table 2. Table 1

[0061] Table 2

[0062] As can be seen from the results in Table 2, the lithium iron phosphate cathode material described in this invention has performance comparable to that of cathode materials prepared from commercially available products, exhibiting high rate performance and cycle stability. Furthermore, the method described in this invention can also recycle waste cathode sheets, realizing their value recovery and maximizing resource utilization.

[0063] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material has a core-shell structure, wherein the core layer is lithium iron phosphate and the shell layer is carbon-doped lithium fluoride.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, Based on the lithium iron phosphate cathode material, the content of the core layer is 97-99.5 wt%, and the content of the shell layer is 0.5-3 wt%.

3. The lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The shell has an average thickness of 2-6 nm; and / or The porosity of the shell is 3-5%, and the average pore size is 3-8 nm.

4. A method for preparing lithium iron phosphate cathode material, characterized in that, The method includes the following steps: (1) Mix lithium iron phosphate and polyvinylidene fluoride and ball mill them once under an inert atmosphere to obtain ball-milled material; (2) The waste positive electrode sheet of the stripped current collector and the primary ball milling material are mixed and ball milled again under an inert atmosphere to obtain the secondary ball milling material; (3) The secondary ball milled material is annealed under an inert atmosphere.

5. The method according to claim 4, characterized in that, In step (1), the weight ratio of lithium iron phosphate to polyvinylidene fluoride is 100:0.5-2.

5.

6. The method according to claim 4 or 5, characterized in that, In step (1), the conditions for the first ball milling include: a temperature of -40 to -30°C, a rotation speed of 200-300 r / min, and a time of 0.5-2.5 h; and / or The particle size of the material from the first ball milling is ≤30μm.

7. The method according to any one of claims 4-6, characterized in that, In step (2), the weight ratio of the waste positive electrode sheet stripped from the current collector to the amount of the primary ball milling material is 1:1-10.

8. The method according to any one of claims 4-7, characterized in that, In step (2), the conditions for the secondary ball milling include: a temperature of -40 to -30°C, a rotation speed of 350-450 r / min, and a time of 2-3.5 h; and / or The particle size of the material from the secondary ball milling is 3-10 μm.

9. The method according to any one of claims 4-8, characterized in that, In step (3), the annealing conditions include: heating temperature of 150-400℃, heating time of 0.5-6h, followed by cooling.

10. A lithium iron phosphate cathode material prepared by the method according to any one of claims 4-9.

11. A positive electrode plate, characterized in that, The positive electrode includes the lithium iron phosphate positive electrode material according to any one of claims 1-3 and 10.

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