Lithium iron phosphate material, preparation method thereof, positive electrode and lithium ion battery

Abstract

The application provides a lithium iron phosphate material and a preparation method thereof, a positive electrode and a lithium ion battery. The preparation method comprises the following steps: after raw materials including a waste lithium iron phosphate positive electrode sheet and a stripping agent are reacted, a current collector in the waste lithium iron phosphate positive electrode sheet is separated to obtain a regenerated active component; the regenerated active component is first ground to obtain regenerated lithium iron phosphate; raw materials including a first lithium source, an iron source and a first phosphorus source are mixed and then sequentially subjected to second grinding, first sintering and third grinding to obtain new lithium iron phosphate; the regenerated lithium iron phosphate and the new lithium iron phosphate are subjected to surface treatment by using an organic acid to obtain a composite material; and raw materials including the composite material, a second lithium source, a second phosphorus source, a carbon source and a doped metal source are mixed and then subjected to second sintering to obtain the lithium iron phosphate material. The preparation method solves the problems that the waste lithium iron phosphate regeneration method has poor material performance, low compaction density and is difficult to realize high-value utilization.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a lithium iron phosphate material and its preparation method, a cathode, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and environmental friendliness, have been widely used in electric vehicles, energy storage power stations, and portable electronic devices. Among them, lithium iron phosphate (LiFePO4) cathode material, with its good safety, stable cycle performance, and abundant resources, has become one of the preferred materials for power and energy storage batteries. As early lithium iron phosphate batteries gradually reach the end of their lifespan, their recycling and resource utilization have attracted much attention.

[0003] Currently, the recycling routes for waste lithium iron phosphate cathode materials are mainly divided into pyrometallurgical, hydrometallurgical, and direct recycling. Direct recycling is considered the most economically valuable and environmentally beneficial recycling strategy due to its low energy consumption, short process flow, and ability to preserve the original structure of the material to the greatest extent. However, current direct recycling technologies still face many challenges: 1) Damage to material performance: Traditional recycling processes (such as crushing and acid leaching) often cause irreversible damage to the crystal structure of lithium iron phosphate, making it difficult to restore the electrochemical performance (such as specific capacity and rate performance) of the recycled material to the level of virgin material. Materials after simple recycling often suffer from low ionic conductivity and poor interfacial stability. 2) Simple microstructure: The particle size distribution of materials obtained through direct recycling is difficult to control, often presenting as pulverized fine particles. Simple physical mixing with virgin materials cannot construct an optimized particle size distribution at the microscale, resulting in low electrode compaction density, thus limiting the improvement of battery volumetric energy density. 3) Poor interfacial compatibility between new and old materials: There are obvious physical and chemical interfaces between recycled and virgin materials, or between particles from different sources. Without effective interface fusion treatment, the migration resistance of lithium ions between particles will increase significantly, thus affecting the rate performance and cycle life of the battery. 4) Elemental stoichiometry imbalance: After long-term use, the battery will experience lithium loss, causing the recycled material to deviate from the stoichiometric ratio. If there is a lack of precise elemental compensation during the regeneration process, it will directly lead to unstable capacity performance of the final product. 5) From the perspective of reducing costs, it is desirable to incorporate a higher proportion of recycled material into the recycled material. However, simply increasing the proportion of recycled material will exacerbate the above problems, resulting in the overall performance of the recycled material failing to meet the needs of high-end applications.

[0004] Therefore, there is an urgent need to develop a lithium iron phosphate regeneration method that can balance high performance and low cost, and enable the high-value utilization of recycled materials. Summary of the Invention

[0005] The main objective of this invention is to provide a lithium iron phosphate material and its preparation method, a cathode, and a lithium-ion battery, in order to solve the problems of poor material performance, low compaction density, and difficulty in achieving high-value utilization in existing waste lithium iron phosphate recycling methods.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing lithium iron phosphate material is provided, the method comprising: step S1, reacting raw materials including waste lithium iron phosphate cathode sheets and stripping agents, separating the current collector in the waste lithium iron phosphate cathode sheets to obtain a regenerated active component; subjecting the regenerated active component to a first grinding to obtain regenerated lithium iron phosphate; step S2, mixing raw materials including a first lithium source, an iron source, and a first phosphorus source, and sequentially performing a second grinding, a first sintering, and a third grinding to obtain new lithium iron phosphate; step S3, surface treating the regenerated lithium iron phosphate and the new lithium iron phosphate with an organic acid to obtain a composite material; step S4, mixing raw materials including the composite material, a second lithium source, a second phosphorus source, a carbon source, and a doped metal source, and performing a second sintering to obtain the lithium iron phosphate material; wherein the D50 particle size of the regenerated lithium iron phosphate is ≤500nm, and the D50 particle size of the new lithium iron phosphate is ≥2000nm.

[0007] The method for preparing lithium iron phosphate materials in this application achieves a balance between high actual density and excellent electrical performance, high-value recycling of waste lithium iron phosphate, cost reduction, precise and controllable process, and high product consistency through the synergistic effect of "preparing small-particle lithium iron phosphate from waste lithium iron phosphate", "designed mixing of large and small particles" and "integrated acid etching-carbon coating".

[0008] Further, in step S1 above, the stripping agent includes an alkaline solution and an oxidizing agent; wherein, the alkaline solution is selected from any one or more of sodium hydroxide solution, potassium hydroxide solution and sodium carbonate solution, and the pH value of the alkaline solution is 8~12; the oxidizing agent is hydrogen peroxide and / or ammonium persulfate; the mass content of the oxidizing agent in the stripping agent is 1~3%; and / or, the reaction temperature is 80~100℃, and the reaction time is 1~3h.

[0009] By introducing the aforementioned alkaline solution into the stripping agent to synergistically work with the oxidant, the separation efficiency of the active material layer and the current collector in waste lithium iron phosphate cathode sheets can be improved. Preferably, controlling the reaction temperature and time within the aforementioned range helps to completely strip the current collector, while simultaneously improving the integrity of the lithium iron phosphate crystal structure and reducing lattice damage caused by traditional crushing and strong acid leaching. This, in turn, facilitates the restoration of the electrochemical performance of recycled lithium iron phosphate to the level of virgin material.

[0010] Further, in step S2 above, the first lithium source is calculated based on the lithium element, the iron source is calculated based on the iron element, and the first phosphorus source is calculated based on the phosphorus element; the molar ratio of the first lithium source, iron source, and first phosphorus source is (0.98~1.05):1:(1.00~1.05); the first lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate; the iron source is selected from any one or more of ferrous oxalate, ferrous sulfate, and ferrous oxide; the first phosphorus source is selected from any one or more of iron phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate; and / or, the D50 particle size of the second milled mixture is 400~500 nm; and / or, the holding temperature of the first sintering is 650~700℃, the holding time of the first sintering is 8~15 h, the heating rate of the first sintering is 2~5℃ / min, and the atmosphere of the first sintering is nitrogen and / or argon.

[0011] Preferably controlling the molar ratio and types of the first lithium source, iron source, and first phosphorus source within the above-mentioned ranges helps to synthesize single-phase lithium iron phosphate with high crystallinity, no carbon, and uniform particle size. Preferably controlling the holding temperature, holding time, heating rate, and atmosphere of the first sintering process within the above-mentioned ranges helps to achieve low-temperature, slow crystallization, thereby reducing abnormal particle growth and phase transformation, and thus obtaining single-phase lithium iron phosphate with high crystallinity, few defects, and no carbon coating.

[0012] Further, in step S3 above, the mass ratio of regenerated lithium iron phosphate to new lithium iron phosphate is 1~6:4~9; the D50 particle size of the regenerated lithium iron phosphate is 100~500nm, and the D50 particle size of the new lithium iron phosphate is 2000~3000nm; and / or, the organic acid is selected from any one or more of citric acid, oxalic acid, and ascorbic acid; the mass ratio of the total mass of regenerated lithium iron phosphate and new lithium iron phosphate to the mass of the organic acid is 100:(5~15); the surface treatment time is 1~4h, and the surface treatment method is ultrasonic and / or stirring.

[0013] Precisely controlling the mass ratio of recycled lithium iron phosphate (LFP) to virgin LFP, as well as the D50 particle size, within the aforementioned ranges helps achieve a multi-level microstructure where "large virgin particles form the framework, and small recycled particles fill the voids," thereby improving the compaction density, rate performance, and cycle stability of the LFP material. Using the aforementioned types and amounts of organic acids, and controlling the surface treatment method and time within the aforementioned ranges, helps to gently etch the particle surface, activate the crystal faces, thereby improving the interfacial compatibility between the two, promoting the unblocking of lithium-ion transport channels, reducing interfacial impedance, and improving the battery's rate performance and cycle life.

[0014] Further, in step S4 above, the second lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate; the second phosphorus source is selected from any one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate; the doped metal source is selected from any one or more of manganese oxide, scandium oxide, tin oxide, titanium oxide, indium oxide, antimony oxide, bismuth oxide, and zirconium oxide; the carbon source is selected from any one or more of glucose, sucrose, polyethylene glycol, and polyvinyl alcohol, and the mass ratio of carbon source to composite material is (5~10):100.

[0015] Adding a second lithium source and a second phosphorus source after elemental analysis of the composite material is preferred, as this helps with elemental compensation and thus improves the electrochemical performance of the lithium iron phosphate material. Introducing carbon sources of the aforementioned types and mass contents is preferred, as this helps form a carbon layer on the outer surface of the lithium iron phosphate material, thereby improving its mechanical strength and conductivity. Adding doped metal sources of the aforementioned types is preferred, as this helps to further improve the electrochemical performance of the lithium iron phosphate material.

[0016] Furthermore, the raw materials in step S4 above also include a solvent, which is water; the raw materials in step S4 are ball-milled and then dried, sintered a second time, and pulverized in sequence to obtain lithium iron phosphate material; wherein, the ball milling speed is 100~300 rpm, the ball milling time is 30~90 min; and / or, the holding temperature of the second sintering is 650~750℃, the holding time of the second sintering is 6~12 h, the heating rate of the second sintering is 2~5℃ / min, and the atmosphere of the second sintering is nitrogen and / or argon.

[0017] Controlling the ball milling speed and time within the aforementioned range helps the raw materials to be fully dispersed in the solvent, thereby improving the uniformity of the lithium iron phosphate material. Preferably, the holding temperature, holding time, heating rate, and atmosphere of the second sintering are within the aforementioned range, which helps to promote uniform carbonization of the carbon source to form a continuous conductive network, while simultaneously repairing lattice defects in the recycled material, thereby improving the energy density and cycle stability of the lithium iron phosphate material.

[0018] Furthermore, in step S4 above, the chemical formula of the lithium iron phosphate material is Li x Fe y M m P z O4; where x:y:z:m = (0.98~1.05):1:(1.00~1.05):(0.005~0.01); M is selected from any one or more of manganese, scandium, tin, titanium, indium, antimony, bismuth and zirconium; the outer surface of the lithium iron phosphate material is also coated with a carbon layer.

[0019] Preferred lithium iron phosphate materials with chemical formulas within the aforementioned range and coated with a carbon layer on their outer surface contribute to the synergistic optimization of the material's bulk structure and surface functions. The aforementioned types and molar amounts of element M can enter the crystal lattice, widening lithium-ion diffusion channels, reducing activation energy, and thus improving the battery's rate performance. Simultaneously, element M helps enhance lattice stability, reducing phase transitions and microcrack propagation during cycling. A uniform carbon layer coating on the outer surface facilitates the construction of a three-dimensional electronic conduction network, reducing interfacial resistance.

[0020] According to another aspect of the present invention, a lithium iron phosphate material is provided, which is prepared by the preparation method described above.

[0021] The lithium iron phosphate material prepared by the above method has both high compaction density and high conductivity. When used as a positive electrode active material in lithium-ion batteries, it can effectively improve the initial efficiency, specific capacity, energy density and cycle performance of lithium-ion batteries.

[0022] According to another aspect of the present invention, a positive electrode is provided, comprising a positive electrode material, wherein the positive electrode material is the aforementioned lithium iron phosphate material.

[0023] Cathode sheets containing the aforementioned lithium iron phosphate materials can improve the initial efficiency, specific capacity, energy density, and cycle performance of lithium-ion batteries.

[0024] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the aforementioned positive electrode.

[0025] Lithium-ion batteries, including the aforementioned cathode, have high energy density and good cycle performance.

[0026] By applying the technical solution of this invention, the preparation method of lithium iron phosphate material in this application achieves the following effects through the synergistic effect of "preparing small-particle lithium iron phosphate from waste lithium iron phosphate", "designed mixing of large and small particles" and "integrated acid etching-carbon coating": 1) Achieving a balance between high compaction density and excellent electrical performance. This application creatively constructs a multi-level microstructure with "large-particle fresh lithium iron phosphate without carbon as the structural framework and small-particle regenerated lithium iron phosphate as the interface filler". Large-particle fresh lithium iron phosphate with a D50 particle size within the above range can effectively improve the compaction density of lithium iron phosphate material, thereby enhancing the volumetric energy density of the battery; while the small-particle regenerated lithium iron phosphate with a D50 particle size within the above range filling the gaps not only provides abundant lithium-ion migration channels, but also alleviates its volume change during cycling. More importantly, in step S3, the interface between the new and old particles is activated and fused through surface treatment with organic acid, thereby significantly reducing the interface impedance. Finally, in step S4, through elemental compensation, metal doping and uniform carbon coating, the recycled material maintains high energy density while possessing excellent rate performance. 2) Low cost, achieving high-value recycling of waste lithium iron phosphate. The preparation method of this application successfully combines low-cost recycled materials with specially prepared new materials, significantly reducing raw material costs. Compared with traditional high-temperature solid-state sintering, the new lithium iron phosphate adopts a low-temperature one-time sintering synthesis route without carbon coating. At the same time, compared with the use of organic solvent treatment, the waste lithium iron phosphate electrode sheets of this application adopt efficient aqueous stripping and surface treatment of both, significantly reducing energy consumption and environmental costs. This method elevates waste materials, traditionally regarded as "downgraded recycling," into key components for manufacturing high-performance cathode materials, achieving true green recycling and cost reduction and efficiency improvement. 3) Precise and controllable process, high product consistency. Through the stoichiometric compensation in step S4, the problem of batch performance instability caused by lithium loss in recycled materials is fundamentally solved. Furthermore, the electrochemical performance of lithium iron phosphate materials can be further improved by introducing a doped metal source. In summary, the preparation method of this application is easy to scale up, and the prepared lithium iron phosphate materials have high and stable consistency, thus better meeting the material performance requirements of high-end power batteries and energy storage batteries. Attached Figure Description

[0027] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0028] Figure 1 SEM images of the lithium iron phosphate material in Example 1 of this application are shown;

[0029] Figure 2 SEM images of the lithium iron phosphate material in Comparative Example 1 of this application are shown;

[0030] Figure 3 The SEM image of the lithium iron phosphate material in Comparative Example 2 of this application is shown. Detailed Implementation

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0032] As analyzed in the background section of this application, existing methods for recycling waste lithium iron phosphate have problems such as poor material performance, low compaction density, and difficulty in achieving high-value utilization. In order to solve the above problems, this application provides a lithium iron phosphate material, its preparation method, a cathode, and a lithium-ion battery.

[0033] In a typical embodiment of this application, a method for preparing lithium iron phosphate material is provided. The method includes: step S1, reacting raw materials including waste lithium iron phosphate cathode sheets and stripping agents, separating the current collector from the waste lithium iron phosphate cathode sheets to obtain a regenerated active component; subjecting the regenerated active component to a first grinding to obtain regenerated lithium iron phosphate; step S2, mixing raw materials including a first lithium source, an iron source, and a first phosphorus source, and sequentially performing a second grinding, a first sintering, and a third grinding to obtain new lithium iron phosphate; step S3, surface treating the regenerated lithium iron phosphate and the new lithium iron phosphate with an organic acid to obtain a composite material; step S4, mixing raw materials including the composite material, a second lithium source, a second phosphorus source, a carbon source, and a doped metal source, and performing a second sintering to obtain the lithium iron phosphate material; wherein the D50 particle size of the regenerated lithium iron phosphate is ≤500nm, and the D50 particle size of the new lithium iron phosphate is ≥2000nm.

[0034] The method for preparing lithium iron phosphate material in this application achieves the following effects through the synergistic effect of "preparing small-particle lithium iron phosphate from waste lithium iron phosphate", "designed mixing of large and small particles", and "integrated acid etching and carbon coating": 1) Achieving a balance between high compaction density and excellent electrical performance. This application creatively constructs a multi-level microstructure with "large-particle fresh lithium iron phosphate without carbon as the structural framework and small-particle regenerated lithium iron phosphate as the interface filler". Large-particle fresh lithium iron phosphate with a D50 particle size within the above range can effectively improve the compaction density of lithium iron phosphate material, thereby enhancing the volumetric energy density of the battery; while the small-particle regenerated lithium iron phosphate with a D50 particle size within the above range filling the gaps not only provides abundant lithium-ion migration channels, but also alleviates its volume change during cycling. More importantly, in step S3, surface treatment with organic acid activates and fuses the interface between the old and new particles, thereby significantly reducing the interface impedance. Finally, in step S4, through elemental compensation, metal doping, and uniform carbon coating, the recycled material maintains high energy density while possessing excellent rate performance. 2) Low cost, achieving high-value recycling of waste lithium iron phosphate. The preparation method of this application successfully combines low-cost recycled materials with specially prepared new materials, significantly reducing raw material costs. Compared with traditional high-temperature solid-state sintering, the new lithium iron phosphate adopts a low-temperature one-time sintering synthesis route without carbon coating. At the same time, compared with the use of organic solvent treatment, the waste lithium iron phosphate electrode sheets of this application adopt efficient aqueous stripping and surface treatment of both, significantly reducing energy consumption and environmental costs. This method elevates waste materials, traditionally regarded as "downgraded recycling," into key components for manufacturing high-performance cathode materials, achieving true green recycling and cost reduction and efficiency improvement. 3) Precise and controllable process, high product consistency. Through the stoichiometric compensation in step S4, the problem of batch performance instability caused by lithium loss in recycled materials is fundamentally solved. Furthermore, the electrochemical performance of lithium iron phosphate materials can be further improved by introducing a doped metal source. In summary, the preparation method of this application is easy to scale up, and the prepared lithium iron phosphate materials have high and stable consistency, thus better meeting the material performance requirements of high-end power batteries and energy storage batteries.

[0035] By weight percentage, the positive electrode active layer of waste lithium iron phosphate cathode sheets comprises: 85-95% lithium iron phosphate, 3-8% binder, and 2-8% conductive agent. The binder includes, but is not limited to, polyvinylidene fluoride, and the conductive agent includes, but is not limited to, carbon black, acetylene black, graphite, carbon nanotubes, and graphene.

[0036] In one embodiment of this application, in step S1, the stripping agent includes an alkaline solution and an oxidant; wherein the alkaline solution is selected from any one or more of sodium hydroxide solution, potassium hydroxide solution and sodium carbonate solution, and the pH value of the alkaline solution is 8-12; the oxidant is hydrogen peroxide and / or ammonium persulfate; the mass content of the oxidant in the stripping agent is 1-3%; and / or, the reaction temperature is 80-100°C, and the reaction time is 1-3 hours.

[0037] By introducing an alkaline solution into the stripping agent to synergistically work with the oxidant, the separation efficiency of the active material layer and the current collector (such as aluminum foil) in waste lithium iron phosphate cathode sheets can be improved. Preferably, the alkaline solution of the aforementioned type and pH value helps dissolve the binder (such as PVDF), weakening the adhesion between the active material and the current collector. The aforementioned type and mass content of oxidant helps selectively oxidize residual carbon and organic impurities, reducing their re-coating of active particles during the stripping process, while also reducing iron ion dissolution, thereby improving the recovery rate of the recycled material. Preferably, the above reaction is carried out under heating conditions, and preferably the reaction temperature and time are controlled within the aforementioned range, which helps to completely strip the current collector, improves the integrity of the lithium iron phosphate crystal structure, and reduces lattice damage caused by traditional crushing and strong acid leaching, thus facilitating the restoration of the electrochemical performance of the recycled lithium iron phosphate to the level of virgin material. The recycled active components and solvent are formulated into a slurry, and recycled lithium iron phosphate with a particle size within the aforementioned range is obtained through a first grinding process. The solvent is any one or more of water, ethanol, acetone, and methanol, preferably deionized water.

[0038] In one embodiment of this application, in step S2 above, the first lithium source is calculated based on the elemental lithium, the iron source is calculated based on the elemental iron, and the first phosphorus source is calculated based on the elemental phosphorus; the molar ratio of the first lithium source, the iron source, and the first phosphorus source is (0.98~1.05):1:(1.00~1.05); the first lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate; the iron source is selected from any one or more of ferrous oxalate, ferrous sulfate, and ferrous oxide; the first phosphorus source is selected from any one or more of iron phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate; and / or, the D50 particle size of the second ground mixture is 400~500 nm; and / or, the holding temperature of the first sintering is 650~700℃, the holding time of the first sintering is 8~15 h, the heating rate of the first sintering is 2~5℃ / min, and the atmosphere of the first sintering is nitrogen and / or argon.

[0039] Preferably controlling the molar ratio and type of the first lithium source, iron source, and first phosphorus source within the aforementioned ranges helps in synthesizing highly crystalline, carbon-free, and uniformly sized single-phase lithium iron phosphate. Preferably controlling the D50 particle size of the mixture after the second grinding within the aforementioned range helps in the subsequent formation of single, dense, large-particle new lithium iron phosphate. Preferably controlling the holding temperature, holding time, heating rate, and atmosphere of the first sintering within the aforementioned ranges helps in achieving low-temperature, slow crystallization, thereby reducing abnormal particle growth and phase transitions, resulting in highly crystalline, defect-free, and carbon-free single-phase lithium iron phosphate. The product after the first sintering and the solvent are formulated into a slurry, which is then subjected to a third grinding to obtain new lithium iron phosphate with a particle size within the aforementioned range. The solvent is any one or more of water, ethanol, acetone, and methanol, preferably deionized water. The new lithium iron phosphate with the aforementioned particle size obtained through the third grinding has a regular large particle morphology, providing high-quality components for constructing a "skeleton-filler" multi-level structure, thereby improving the compaction density of the material.

[0040] In one embodiment of this application, in step S3 above, the mass ratio of regenerated lithium iron phosphate to new lithium iron phosphate is 1~6:4~9; the D50 particle size of the regenerated lithium iron phosphate is 100~500nm, and the D50 particle size of the new lithium iron phosphate is 2000~3000nm; and / or, the organic acid is selected from any one or more of citric acid, oxalic acid, and ascorbic acid; the mass ratio of the total mass of regenerated lithium iron phosphate and new lithium iron phosphate to the mass of the organic acid is 100:(5~15); the surface treatment time is 1~4h, and the surface treatment method is ultrasonic and / or stirring.

[0041] Preferentially controlling the mass ratio of recycled lithium iron phosphate (LFP) to virgin LFP and the D50 particle size within the aforementioned ranges helps achieve a multi-level microstructure where "large virgin particles form the framework, and small recycled particles fill the voids," thereby improving the compaction density, rate performance, and cycle stability of the LFP material. Using the aforementioned types of organic acids and controlling the amount added within the aforementioned range helps to gently etch the particle surface, activate the crystal faces, improve the interfacial compatibility between the two, promote the unblocking of lithium-ion transport channels, thereby reducing interfacial impedance and improving the battery's rate performance and cycle life. Preferentially controlling the surface treatment method and time within the aforementioned ranges helps to fully activate and fuse the interface between the old and new particles, further reducing interfacial impedance. After surface treatment, the composite material is obtained through solid-liquid separation and washing. With the total mass being 10, the mass ratio of recycled LFP to virgin LFP can be 1:9, 2:8, 3:7, 4:6, etc.

[0042] The optimal D50 particle size ratio of new lithium iron phosphate and regenerated lithium iron phosphate is 10:0.5~2, which helps to optimize particle gradation and thus improve the compaction density, rate performance and cycle stability of lithium iron phosphate materials.

[0043] In one embodiment of this application, in step S4 above, the second lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate; the second phosphorus source is selected from any one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate; the doped metal source is selected from any one or more of manganese oxide, scandium oxide, tin oxide, titanium oxide, indium oxide, antimony oxide, bismuth oxide, and zirconium oxide; the carbon source is selected from any one or more of glucose, sucrose, polyethylene glycol, and polyvinyl alcohol, and the mass ratio of carbon source to composite material is (5~10):100.

[0044] Adding a second lithium source and a second phosphorus source after elemental analysis of the composite material is preferred, as this helps with elemental compensation and thus improves the electrochemical performance of the lithium iron phosphate material. Introducing carbon sources of the aforementioned types and mass contents is preferred, as this helps form a carbon layer on the outer surface of the lithium iron phosphate material, thereby improving its mechanical strength and conductivity. Adding doped metal sources of the aforementioned types is preferred, as this helps to further improve the electrochemical performance of the lithium iron phosphate material.

[0045] In one embodiment of this application, the raw materials in step S4 above further include a solvent, which is water; the raw materials in step S4 are ball-milled and then dried, sintered a second time, and pulverized in sequence to obtain lithium iron phosphate material; wherein the ball milling speed is 100~300 rpm, the ball milling time is 30~90 min; and / or, the holding temperature of the second sintering is 650~750℃, the holding time of the second sintering is 6~12 h, the heating rate of the second sintering is 2~5℃ / min, and the atmosphere of the second sintering is nitrogen and / or argon.

[0046] Ball milling mixes the raw materials, and controlling the milling speed and time within the aforementioned range helps to fully disperse the raw materials in the solvent, thereby improving the uniformity of the lithium iron phosphate material. Preferably, the holding temperature, holding time, heating rate, and atmosphere of the second sintering are within the aforementioned range. This helps to promote uniform carbonization of the carbon source to form a continuous conductive network, while simultaneously driving the insertion of doped metal ions into the crystal lattice, reducing abnormal grain growth, and repairing lattice defects in the recycled material. This achieves integrated "doping-coating-crystallization," resulting in a lithium iron phosphate material with high crystallinity, a uniform carbon coating layer, and stable ion channels, thereby improving the energy density and cycle stability of the lithium iron phosphate material.

[0047] In one embodiment of this application, in step S4 above, the chemical formula of the lithium iron phosphate material is Li x Fe y M m P zO4; where x:y:z:m = (0.98~1.05):1:(1.00~1.05):(0.005~0.01); M is selected from any one or more of manganese, scandium, tin, titanium, indium, antimony, bismuth and zirconium; the outer surface of the lithium iron phosphate material is also coated with a carbon layer.

[0048] The preferred lithium iron phosphate material has a chemical formula within the above-mentioned range and is coated with a carbon layer on its outer surface, which helps to achieve synergistic optimization of the material's bulk structure and surface function. The M element of the above-mentioned type and molar content can enter the crystal lattice, widening the lithium-ion diffusion channels, reducing the activation energy, and thus improving the battery's rate performance; simultaneously, the M element helps to enhance lattice stability, reducing phase transitions and microcrack propagation during cycling. The uniformly coated carbon layer on the outer surface helps to construct a three-dimensional electronic conduction network, reducing interfacial resistance. The preferred lithium iron phosphate material has a powder compaction density ≥2.43 g / cc, preferably ≥2.57 g / cc.

[0049] In another typical embodiment of this application, a lithium iron phosphate material is provided, which is prepared by the above-described preparation method.

[0050] The lithium iron phosphate material prepared by the above method has both high compaction density and high conductivity. When used as a positive electrode active material in lithium-ion batteries, it can effectively improve the initial efficiency, specific capacity, energy density and cycle performance of lithium-ion batteries.

[0051] In another typical embodiment of this application, a positive electrode is provided, comprising a positive electrode material, which is the aforementioned lithium iron phosphate material.

[0052] Cathode sheets containing the aforementioned lithium iron phosphate materials can improve the initial efficiency, specific capacity, energy density, and cycle performance of lithium-ion batteries.

[0053] In another typical embodiment of this application, a lithium-ion battery is provided, including a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the aforementioned positive electrode.

[0054] Lithium-ion batteries, including the aforementioned cathode, have high energy density and good cycle performance.

[0055] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0056] Example 1

[0057] By weight percentage, waste lithium iron phosphate cathode sheets contain 90% lithium iron phosphate, 5% PVDF binder, 5% Super P conductive agent in the cathode active layer, and aluminum foil as current collector.

[0058] A stripping agent was prepared by forming an alkaline solution (sodium hydroxide aqueous solution) with an oxidant (hydrogen peroxide) content of 2% by mass and a pH value of 10. Waste lithium iron phosphate positive electrode sheets and the stripping agent were placed in a reaction vessel and heated to 90°C for 2 hours. After washing and filtration, the regenerated active component (regenerated lithium iron phosphate powder) was obtained. This powder was mixed with deionized water to form a slurry and subjected to a first grinding process to obtain regenerated lithium iron phosphate with a D50 particle size of 300 nm.

[0059] The first lithium source (lithium carbonate), the iron source (ferrous oxalate), and the phosphorus source (ammonium dihydrogen phosphate) were mixed in a molar ratio of lithium, iron, and phosphorus of 1.03:1:1.02, followed by a second grinding process until the particle size D50 was 450 nm, yielding a second-ground mixture. This second-ground mixture was then sintered at 700 °C for 10 h under a nitrogen atmosphere at a heating rate of 5 °C / min to obtain a first-sintered product (carbon-free lithium iron phosphate). This product was then mixed with deionized water to form a slurry, which was then subjected to a third grinding process to obtain new lithium iron phosphate with a D50 particle size of 2500 nm.

[0060] Regenerated lithium iron phosphate and new lithium iron phosphate were mixed at a dry weight ratio of 1:1, and an organic acid (citric acid) accounting for 10% of the total mass of the regenerated lithium iron phosphate and new lithium iron phosphate was added to the mixture. The surface was treated by a combination of ultrasonic and mechanical methods for 2 hours, followed by solid-liquid separation and washing to obtain the composite material.

[0061] Elemental composition analysis of the composite material was performed. A second lithium source (lithium hydroxide), a second phosphorus source (ammonium dihydrogen phosphate), and a doped metal source (zirconia) were added. A carbon source (glucose) was added at 8% of the dry weight of the composite material. The mixture was ball-milled at 200 rpm for 60 min, dried, and then sintered at 720 °C for 10 h under a nitrogen atmosphere at a heating rate of 3 °C / min. After pulverization, lithium iron phosphate material was obtained. The chemical formula of lithium iron phosphate material is Li. 1.03 FeZr 0.008 P 1.02 O4 has a carbon layer on its outer surface.

[0062] Example 2

[0063] The difference from Example 1 is that the stripping agent is an oxidant (ammonium persulfate) with a mass content of 2% and an alkaline solution (potassium hydroxide aqueous solution) with a pH value of 10, which ultimately yields lithium iron phosphate material.

[0064] Example 3

[0065] The difference from Example 1 is that the D50 particle size of the recycled lithium iron phosphate is 490 nm, and the final lithium iron phosphate material is obtained.

[0066] Example 4

[0067] The difference from Example 1 is that the first lithium source (lithium carbonate), the iron source (ferrous oxide), and the phosphorus source (ammonium dihydrogen phosphate) are mixed in a molar ratio of lithium, iron, and phosphorus of 1.02:1:1.03, followed by a second grinding process until the particle size D50 is 410 nm, resulting in a mixture after the second grinding. The final product is a lithium iron phosphate material with the chemical formula Li. 1.02 FeZr 0.008 P 1.03 O4 has a carbon layer on its outer surface.

[0068] Example 5

[0069] The difference from Example 1 is that the second ground mixture was heated to 650°C at a heating rate of 3°C / min under a nitrogen atmosphere for a first sintering of 15h to obtain the first sintered product, and finally the lithium iron phosphate material was obtained.

[0070] Example 6

[0071] The difference from Example 1 is that the D50 particle size of the new lithium iron phosphate is 2100 nm, and the regenerated lithium iron phosphate and the new lithium iron phosphate are mixed in a mass ratio of 1:9 to finally obtain the lithium iron phosphate material.

[0072] Example 7

[0073] The difference from Example 1 is that the D50 particle size of the new lithium iron phosphate is 2900 nm, and the regenerated lithium iron phosphate and the new lithium iron phosphate are mixed in a mass ratio of 6:4 to finally obtain the lithium iron phosphate material.

[0074] Example 8

[0075] The difference from Example 1 is that the recycled lithium iron phosphate and the new lithium iron phosphate are mixed at a dry weight ratio of 1:1, and an organic acid (oxalic acid) accounting for 10% of the total mass of the recycled lithium iron phosphate and the new lithium iron phosphate is added to the mixture. The mixture is then subjected to surface treatment by ultrasound and mechanical means for 1 hour. After solid-liquid separation and washing, a composite material is obtained, and finally, the lithium iron phosphate material is obtained.

[0076] Example 9

[0077] The difference from Example 1 is that the recycled lithium iron phosphate and the new lithium iron phosphate are mixed at a dry basis mass ratio of 1:1, and an organic acid (oxalic acid) accounting for 8% of the total mass of the recycled lithium iron phosphate and the new lithium iron phosphate is added to the mixture. The mixture is then subjected to surface treatment by ultrasound and mechanical means for 2 hours. After solid-liquid separation and washing, a composite material is obtained, and finally, the lithium iron phosphate material is obtained.

[0078] Example 10

[0079] The difference from Example 1 is that the doping metal source is titanium oxide, and the chemical formula of the final lithium iron phosphate material is Li. 1.03 FeTi 0.008 P 1.02 O4 has a carbon layer on its outer surface.

[0080] Example 11

[0081] The difference from Example 1 is that a carbon source (polyethylene glycol) of 10% of the dry basis weight of the composite material is added, and lithium iron phosphate material is finally obtained.

[0082] Example 12

[0083] The difference from Example 1 is that, under a nitrogen atmosphere, the temperature was increased to 750°C at a heating rate of 5°C / min for a second sintering for 10 hours, and finally lithium iron phosphate material was obtained.

[0084] Example 13

[0085] A stripping agent was prepared by forming an alkaline solution (sodium hydroxide aqueous solution) with an oxidant (hydrogen peroxide) content of 1% and a pH value of 8. Waste lithium iron phosphate positive electrode sheets and the stripping agent were placed in a reaction vessel and heated to 80°C for 3 hours. After washing and filtration, the regenerated active component (regenerated lithium iron phosphate powder) was obtained. This powder was mixed with deionized water to form a slurry and subjected to a first grinding process to obtain regenerated lithium iron phosphate with a D50 particle size of 100 nm.

[0086] The first lithium source (lithium carbonate), the iron source (ferrous oxalate), and the phosphorus source (ammonium dihydrogen phosphate) were mixed in a molar ratio of lithium, iron, and phosphorus of 1.05:1:1.05, followed by a second grinding process to a particle size D50 of 400 nm, yielding a second-ground mixture. This second-ground mixture was then sintered at 650 °C for 15 h under a nitrogen atmosphere at a heating rate of 2 °C / min to obtain a first-sintered product (carbon-free lithium iron phosphate). This product was then mixed with deionized water to form a slurry, which was then subjected to a third grinding process to obtain new lithium iron phosphate with a D50 particle size of 2000 nm.

[0087] Regenerated lithium iron phosphate and new lithium iron phosphate were mixed at a dry weight ratio of 3:7. An organic acid (citric acid) of 5% of the total mass of the regenerated and new lithium iron phosphate was added to the mixture. The surface was treated by a combination of ultrasonic and mechanical methods for 4 hours. After solid-liquid separation and washing, the composite material was obtained.

[0088] Elemental composition analysis of the composite material was performed. A second lithium source (lithium hydroxide), a second phosphorus source (ammonium dihydrogen phosphate), and a doped metal source (tin oxide) were added. A carbon source (glucose) was added at 5% of the dry weight of the composite material. The mixture was ball-milled at 100 rpm for 90 min, dried, and then sintered at 650 °C for 12 h under a nitrogen atmosphere at a heating rate of 2 °C / min. After pulverization, lithium iron phosphate material was obtained. The chemical formula of lithium iron phosphate material is Li. 1.05 FeSn 0.008 P 1.05 O4 has a carbon layer on its outer surface.

[0089] Example 14

[0090] The difference from Example 1 is that the stripping agent is an oxidant (ammonium persulfate) with a mass content of 0.5% and an alkaline solution (potassium hydroxide aqueous solution) with a pH value of 10. The reaction temperature is 70°C and the reaction time is 4 hours, and finally lithium iron phosphate material is obtained.

[0091] Example 15

[0092] The difference from Example 1 is that the second ground mixture was heated to 750°C at a heating rate of 1°C / min under a nitrogen atmosphere for a first sintering of 6 hours to obtain the first sintered product, and finally the lithium iron phosphate material was obtained.

[0093] Example 16

[0094] The difference from Example 1 is that the recycled lithium iron phosphate and the new lithium iron phosphate are mixed at a dry basis mass ratio of 1:1, and an organic acid (oxalic acid) accounting for 3% of the total mass of the recycled lithium iron phosphate and the new lithium iron phosphate is added to the mixture. The surface is treated by ultrasound and mechanical means for 0.5 hours, followed by solid-liquid separation and washing to obtain a composite material, and finally obtain the lithium iron phosphate material.

[0095] Example 17

[0096] The difference from Example 1 is that, under a nitrogen atmosphere, the temperature was increased to 800°C at a heating rate of 1°C / min for a second sintering for 5 hours, and finally lithium iron phosphate material was obtained.

[0097] Example 18

[0098] The difference from Example 1 is that regenerated lithium iron phosphate with a D50 particle size of 150 nm and new lithium iron phosphate with a D50 particle size of 3000 nm are mixed at a dry basis mass ratio of 6:4 to finally obtain lithium iron phosphate material.

[0099] Example 19

[0100] The difference from Example 1 is that regenerated lithium iron phosphate with a D50 particle size of 500 nm and new lithium iron phosphate with a D50 particle size of 2000 nm are mixed at a dry basis mass ratio of 0.5:9.5 to finally obtain lithium iron phosphate material.

[0101] Comparative Example 1

[0102] The difference from Example 1 is that no new lithium iron phosphate is added. Regenerated lithium iron phosphate with a D50 particle size of 300 nm and regenerated lithium iron phosphate with a D50 particle size of 2500 nm are mixed at a mass ratio of 1:1. An organic acid (citric acid) accounting for 10% of the total mass of regenerated lithium iron phosphate and new lithium iron phosphate is added to the mixture. The surface is treated by ultrasound and mechanical means for 2 hours. After solid-liquid separation and washing, a composite material is obtained, and finally, lithium iron phosphate material is obtained.

[0103] Comparative Example 2

[0104] The difference from Example 1 is that only 10% of the mass of the recycled lithium iron phosphate (citric acid) was added to the recycled lithium iron phosphate with a D50 particle size of 300 nm. The surface was treated by a combination of ultrasound and mechanical methods for 2 hours. After solid-liquid separation and washing, a composite material was obtained, and finally, the lithium iron phosphate material was obtained.

[0105] Comparative Example 3

[0106] The difference from Example 1 is that no surface treatment is performed, and the final product is lithium iron phosphate material.

[0107] Comparative Example 4

[0108] The difference from Example 1 is that no doping metal source is added, and the final lithium iron phosphate material has the chemical formula Li. 1.03 FeP 1.02 O4 has a carbon layer on its outer surface.

[0109] Test method:

[0110] Lithium iron phosphate material, SP conductive agent, and PVDF binder were weighed and mixed in a mass ratio of 8:1:1, and coated onto current collector aluminum foil to obtain the positive electrode. The negative electrode was a lithium sheet. LiPF6 was mixed with a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC to DMC volume ratio of 1:1) to prepare a 1 mol / L LiPF6 electrolyte. A CR2032 coin cell was fabricated under an argon protective atmosphere.

[0111] Button cell test: The button cell half-cell was charged and discharged at room temperature (25°C) with a voltage range of 2.5~4.0V.

[0112] First-time efficiency (%) = First discharge capacity / First charge capacity × 100%.

[0113] 0.2C discharge specific capacity: The discharge specific capacity obtained by performing a charge-discharge test under 0.2C conditions again after the first charge-discharge test at 0.2C.

[0114] 1C discharge specific capacity: The discharge specific capacity obtained by performing a charge-discharge test under 1C conditions after a 0.2C charge-discharge test.

[0115] 2C discharge specific capacity: The discharge specific capacity obtained by performing a charge-discharge test under 2C conditions after a 1C charge-discharge test.

[0116] Powder compaction density: According to the method in GB / T24533-2019, the lithium iron phosphate powder compaction density tester was used. A sample of 0.5g was taken and a multi-point compaction test was performed. The pressure was maintained for 10s at 21, 71, 121, 171, 221, 271 and 321 MPa. The data measured at 221 MPa is the powder compaction density value under 3t pressure.

[0117] The test results are shown in Table 1.

[0118] Table 1

[0119]

[0120] It should be noted that the composition of the lithium iron phosphate material in this application is expressed as a chemical formula, not a molecular formula. This is because the material is an inorganic solid solution or a non-stoichiometric compound with a three-dimensional crystal structure, and does not contain independent molecules. The subscripts of each element in the chemical formula represent the statistical average molar ratio obtained through elemental analysis, and non-integer values ​​are allowed. For example, in Example 1, the actual molar ratio of the product, measured by X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma optical emission spectroscopy (ICP-OES), was Li:Fe:Zr:P:O = 1.03:1:0.008:1.02:4. The chemical formula of the lithium iron phosphate material can be represented as Li 1.03 FeZr 0.008 P 1.02 O4, therefore, the chemical formula of the lithium iron phosphate material in the above examples and comparative examples represents the statistical average composition of the metal sites in the crystal, not the molecular formula.

[0121] As can be seen from the above, compared with Example 1, both the large-particle cathode material and the small-particle cathode material in Comparative Example 1 use recycled lithium iron phosphate. On the one hand, recycled lithium iron phosphate has certain defects, and the acid etching and re-sintering repair are relatively limited, resulting in a significant decrease in electrical performance. On the other hand, recycled lithium iron phosphate has its own carbon source coating, which affects the accuracy of particle size testing when grinding to prepare large-particle lithium iron phosphate. That is, the actual particle size of large-particle lithium iron phosphate is smaller than the theoretical value, resulting in poor gradation effect and a significant decrease in compaction density.

[0122] Comparative Example 2 did not incorporate particle size gradation design; it only repaired waste lithium iron phosphate, resulting in a significant decrease in compaction density and electrical performance.

[0123] Comparative Example 3, which did not undergo surface etching treatment, showed varying degrees of defects in both the regenerated lithium iron phosphate and the new lithium iron phosphate. The lack of surface etching and activation made it difficult for supplementary elements to penetrate into the particles, and the poor fusion between the old and new particles made it difficult to form a good gradation, ultimately leading to a significant decrease in electrical performance and compaction density.

[0124] In Comparative Example 4, no doped metal elements were added before the secondary sintering. Since the repair effect of waste lithium iron phosphate is limited, the lack of metal element doping and fast ion conductor coating will lead to a serious decline in electrical performance, especially rate performance.

[0125] in, Figure 1 This is a SEM image of the lithium iron phosphate material in Example 1 of Example 1. Figure 1 As can be seen, lithium iron phosphate materials exhibit a typical multi-level gradation structure of "large particles - small particles". The figure shows relatively large, spherical particles (corresponding to virgin lithium iron phosphate, D50 of 2500 nm), whose surfaces and interstices are densely and uniformly filled with extremely small particles (corresponding to regenerated lithium iron phosphate, D50 of 300 nm). The large particles act as a structural framework supporting the overall packing, while the small particles effectively fill the gaps between the large particles, forming a dense and continuous microstructure. Clear signs of mutual fusion are visible between the particles, with no obvious large pores or loose agglomeration. This optimized "framework-filler" gradation structure is beneficial for achieving high compaction density and constructing abundant lithium-ion transport channels.

[0126] Figure 2 The image shows the SEM image of the lithium iron phosphate material in Comparative Example 1. Figure 2 As can be seen, the lithium iron phosphate material lacks significant size gradation differences, exhibiting a "uniform size" mixed state of "two small particles". The figure mainly consists of particles with smaller diameters (similar to the size of recycled lithium iron phosphate). Although there are a few slightly larger agglomerates, the distinct "large particle skeleton support + small particle filling" differentiated structure seen in Example 1 is not observed. Since all particles in Comparative Example 1 are derived from waste lithium iron phosphate (both large and small particles are recycled materials), on the one hand, the carbon coating of the recycled material leads to a smaller actual particle size after grinding; on the other hand, the particle sizes are similar, making it difficult to form an effective "large-small" packing gradation. This results in a relatively loose particle packing, an excessively high degree of morphological uniformity, and a lack of dense packing units capable of constructing high compaction density.

[0127] Figure 3 The image shows the SEM image of the lithium iron phosphate material in Comparative Example 2. Figure 3As can be seen, lithium iron phosphate materials exhibit a highly aggregated state with an extremely uniform and loose particle size distribution. The figure primarily shows numerous small particles clustered together, resulting in highly uneven particle size distribution, a lack of large, regular particles, and no combination of large and small particles. Because Comparative Example 2 only performed surface treatment and repair on the recycled lithium iron phosphate without any particle size distribution design (i.e., no new lithium iron phosphate was added as a framework), the entire system is composed entirely of fine particles. This uniform particle size distribution results in numerous visible pores and loose regions between particles, leading to extremely low packing density and a loose morphology, making it difficult to achieve the high compaction density required for lithium-ion battery cathode materials.

[0128] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0129] The method for preparing lithium iron phosphate material in this application achieves the following effects through the synergistic effect of "preparing small-particle lithium iron phosphate from waste lithium iron phosphate", "designed mixing of large and small particles", and "integrated acid etching and carbon coating": 1) Achieving a balance between high compaction density and excellent electrical performance. This application creatively constructs a multi-level microstructure with "large-particle fresh lithium iron phosphate without carbon as the structural framework and small-particle regenerated lithium iron phosphate as the interface filler". Large-particle fresh lithium iron phosphate with a D50 particle size within the above range can effectively improve the compaction density of lithium iron phosphate material, thereby enhancing the volumetric energy density of the battery; while the small-particle regenerated lithium iron phosphate with a D50 particle size within the above range filling the gaps not only provides abundant lithium-ion migration channels, but also alleviates its volume change during cycling. More importantly, in step S3, surface treatment with organic acid activates and fuses the interface between the old and new particles, thereby significantly reducing the interface impedance. Finally, in step S4, through elemental compensation, metal doping, and uniform carbon coating, the recycled material maintains high energy density while possessing excellent rate performance. 2) Low cost, achieving high-value recycling of waste lithium iron phosphate. The preparation method of this application successfully combines low-cost recycled materials with specially prepared new materials, significantly reducing raw material costs. Compared with traditional high-temperature solid-state sintering, the new lithium iron phosphate adopts a low-temperature one-time sintering synthesis route without carbon coating. At the same time, compared with the use of organic solvent treatment, the waste lithium iron phosphate electrode sheets of this application adopt efficient aqueous stripping and surface treatment of both, significantly reducing energy consumption and environmental costs. This method elevates waste materials, traditionally regarded as "downgraded recycling," into key components for manufacturing high-performance cathode materials, achieving true green recycling and cost reduction and efficiency improvement. 3) Precise and controllable process, high product consistency. Through the stoichiometric compensation in step S4, the problem of batch performance instability caused by lithium loss in recycled materials is fundamentally solved. Furthermore, the electrochemical performance of lithium iron phosphate materials can be further improved by introducing a doped metal source. In summary, the preparation method of this application is easy to scale up, and the prepared lithium iron phosphate materials have high and stable consistency, thus better meeting the material performance requirements of high-end power batteries and energy storage batteries.

[0130] The above are merely embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing lithium iron phosphate material, characterized in that, The preparation method includes: Step S1: After reacting the raw materials including waste lithium iron phosphate cathode sheets and stripping agent, the current collector in the waste lithium iron phosphate cathode sheets is separated to obtain the regenerated active component; the regenerated active component is subjected to a first grinding to obtain regenerated lithium iron phosphate. Step S2: The raw materials including the first lithium source, iron source and the first phosphorus source are mixed and then subjected to the second grinding, the first sintering and the third grinding in sequence to obtain new lithium iron phosphate; Step S3: The recycled lithium iron phosphate and the new lithium iron phosphate are surface treated with organic acids to obtain a composite material; Step S4: After mixing the raw materials including the composite material, the second lithium source, the second phosphorus source, the carbon source and the doped metal source, a second sintering is performed to obtain lithium iron phosphate material. The D50 particle size of the regenerated lithium iron phosphate is ≤500nm, and the D50 particle size of the new lithium iron phosphate is ≥2000nm.

2. The preparation method according to claim 1, characterized in that, In step S1, the stripping agent comprises an alkaline solution and an oxidizing agent; wherein the alkaline solution is selected from any one or more of sodium hydroxide solution, potassium hydroxide solution, and sodium carbonate solution, and the pH value of the alkaline solution is 8-12; the oxidizing agent is hydrogen peroxide and / or ammonium persulfate; the mass content of the oxidizing agent in the stripping agent is 1-3%; And / or, the reaction temperature is 80~100℃, and the reaction time is 1~3h.

3. The preparation method according to claim 1, characterized in that, In step S2, the first lithium source is calculated based on the lithium element therein, the iron source is calculated based on the iron element therein, and the first phosphorus source is calculated based on the phosphorus element therein. The molar ratio of the first lithium source, the iron source, and the first phosphorus source is (0.98~1.05):1:(1.00~1.05). The first lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate; the iron source is selected from any one or more of ferrous oxalate, ferrous sulfate, and ferrous oxide; the first phosphorus source is selected from any one or more of iron phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate. And / or, the D50 particle size of the mixture after the second grinding is 400~500nm; And / or, the holding temperature of the first sintering is 650~700℃, the holding time of the first sintering is 8~15h, the heating rate of the first sintering is 2~5℃ / min, and the atmosphere of the first sintering is nitrogen and / or argon.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S3, the mass ratio of the regenerated lithium iron phosphate to the new lithium iron phosphate is 1~6:4~9; the D50 particle size of the regenerated lithium iron phosphate is 100~500nm, and the D50 particle size of the new lithium iron phosphate is 2000~3000nm. And / or, the organic acid is selected from any one or more of citric acid, oxalic acid and ascorbic acid; the ratio of the total mass of the regenerated lithium iron phosphate and the new lithium iron phosphate to the mass of the organic acid is 100:(5~15); the surface treatment time is 1~4 hours, and the surface treatment method is ultrasonic and / or stirring.

5. The preparation method according to any one of claims 1 to 3, characterized in that, In step S4, the second lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate; the second phosphorus source is selected from any one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate; and the doped metal source is selected from any one or more of manganese oxide, scandium oxide, tin oxide, titanium oxide, indium oxide, antimony oxide, bismuth oxide, and zirconium oxide. The carbon source is selected from any one or more of glucose, sucrose, polyethylene glycol and polyvinyl alcohol, and the mass ratio of the carbon source to the composite material is (5~10):

100.

6. The preparation method according to any one of claims 1 to 3, characterized in that, The raw materials in step S4 also include a solvent, which is water; the raw materials in step S4 are ball-milled and then dried, sintered in the second step, and pulverized in sequence to obtain the lithium iron phosphate material; The ball milling speed is 100~300 rpm, and the ball milling time is 30~90 min; And / or, the holding temperature of the second sintering is 650~750℃, the holding time of the second sintering is 6~12h, the heating rate of the second sintering is 2~5℃ / min, and the atmosphere of the second sintering is nitrogen and / or argon.

7. The preparation method according to any one of claims 1 to 3, characterized in that, In step S4, the chemical formula of the lithium iron phosphate material is Li x Fe y M m P z O4; in, x： y： z:m = (0.98~1.05):1:(1.00~1.05):(0.005~0.01); M is selected from any one or more of manganese, scandium, tin, titanium, indium, antimony, bismuth and zirconium; the outer surface of the lithium iron phosphate material is also coated with a carbon layer.

8. A lithium iron phosphate material, characterized in that, The lithium iron phosphate material is prepared by the preparation method according to any one of claims 1 to 7.

9. A positive electrode, comprising a positive electrode material, characterized in that, The cathode material is the lithium iron phosphate material as described in claim 8.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The positive electrode is the positive electrode as described in claim 9.

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