A lithium manganese iron phosphate positive electrode material, a preparation method and application thereof

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a lithium iron manganese phosphate positive electrode material and a preparation method and application thereof; the preparation method comprises the following steps: S1, acidolysis is carried out on recovered lithium ion battery positive electrode material black powder to obtain an aluminum ion-containing leaching solution; S2, hydrothermal reaction is carried out on the leaching solution to obtain a lithium iron manganese phosphate precursor; S3, the lithium iron manganese phosphate precursor is mixed with a lithium source and a carbon source, and heat treatment is carried out to obtain the lithium iron manganese phosphate positive electrode material; wherein, aluminum ions in the leaching solution in step S1 are used to perform body phase doping on the lithium iron manganese phosphate in the heat treatment step. Compared with the prior art, the application directly uses recovered black powder as the source of iron manganese lithium, and uses impurity aluminum ions in the black powder as a body phase dopant, so that the structural stability and the cycle life of the lithium iron manganese phosphate positive electrode material are improved while the cost is reduced.

Description

Technical Field

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

[0002] Lithium-ion batteries, as one of the most universal and attractive energy storage systems, have become the preferred choice for electric vehicles and grid energy storage, and are widely used in communications, portable electronic devices, electric vehicles, and smart grids. With the continuous development of the new energy industry, my country's lithium-ion battery production has achieved a leading position globally. Data shows that from January to July 2025, China's lithium-ion battery production reached approximately 940 GWh, a year-on-year increase of about 68%, accounting for more than 80% of global production. However, the lifespan of lithium-ion batteries is typically only 5-10 years. As a large number of batteries gradually reach the end of their service life, a massive amount of waste lithium-ion batteries will be generated in the future. Therefore, how to efficiently utilize the residual value of waste batteries has become an important issue that urgently needs to be addressed. Summary of the Invention

[0003] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a lithium manganese iron phosphate cathode material, its preparation method, and its application. The precursor is synthesized by directly hydrothermal reaction after acid hydrolysis of recycled black powder. During sintering, the material is bulk doped with aluminum ions inherent in the leachate, and simultaneously coated with carbon. This simplifies the process, reduces production costs, and significantly improves the electrochemical performance of the lithium manganese iron phosphate cathode material.

[0004] To solve the above-mentioned technical problems, the present invention is implemented as follows: According to one aspect of the present invention, a method for preparing a lithium manganese iron phosphate cathode material is provided, comprising the following steps: S1. The recycled lithium-ion battery cathode material black powder is acid-hydrolyzed to obtain a leachate containing aluminum ions. S2. The leachate obtained in step S1 is subjected to a hydrothermal reaction to obtain lithium manganese iron phosphate precursor. S3. Mix the lithium manganese iron phosphate precursor obtained in step S2 with the lithium source and carbon source, and perform heat treatment to obtain the lithium manganese iron phosphate cathode material. In this process, aluminum ions from the leaching solution in step S1 are used to bulk dope the lithium manganese iron phosphate cathode material during the heat treatment step.

[0005] In some embodiments of the present invention, in step S1: the recovered lithium-ion battery cathode material black powder includes at least two of lithium manganese oxide, lithium iron manganese phosphate, and lithium iron phosphate.

[0006] In some embodiments of the present invention, the aluminum ions in the leaching solution containing aluminum ions are derived from the recovered lithium-ion battery cathode material black powder.

[0007] In some embodiments of the present invention, the concentration of the acid solution used in the acidolysis operation is 1.5 mol / L to 3 mol / L.

[0008] In some embodiments of the present invention, the leachate further includes lithium ions, manganese ions, and iron ions.

[0009] In some embodiments of the present invention, in step S1: the molar ratio of manganese ions to iron ions in the leachate is 1:1 to 4:1.

[0010] In some embodiments of the present invention, in step S2: before the hydrothermal reaction, a reducing agent is added to the leachate obtained in step S1, and a reduction reaction is carried out under an inert atmosphere.

[0011] In some embodiments of the present invention, in step S2: the reducing agent is iron powder, and after adding iron powder, the molar ratio of manganese to iron in the leachate is 5~7:5~3.

[0012] In some embodiments of the present invention, in step S2, the temperature of the hydrothermal reaction is 120°C to 180°C.

[0013] In some embodiments of the present invention, the hydrothermal reaction time is 8h to 12h.

[0014] In some embodiments of the present invention, in step S3: the lithium source includes at least one of LiOH, Li3PO4, and Li2CO3.

[0015] In some embodiments of the present invention, the carbon source includes at least one selected from glucose, chitosan, polyethylene glycol, sucrose, and starch.

[0016] In some embodiments of the present invention, the temperature of the heat treatment is 600°C to 800°C.

[0017] In some embodiments of the present invention, the heat treatment time is 8h to 12h.

[0018] In some embodiments of the present invention, the heat treatment is performed in an inert atmosphere.

[0019] In some embodiments of the present invention, in step S3: the carbon source includes glucose and chitosan, and the mass ratio of glucose to chitosan is 90~99:10~1.

[0020] In some embodiments of the present invention, the amount of aluminum doped in the lithium manganese iron phosphate cathode material does not exceed 1 wt%.

[0021] According to another aspect of the present invention, a lithium manganese iron phosphate cathode material is provided, which is prepared by the preparation method described in the above technical solution.

[0022] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode sheet, wherein the positive electrode sheet comprises lithium manganese iron phosphate positive electrode material prepared by the preparation method described above or the lithium manganese iron phosphate positive electrode material described above.

[0023] Implementing the technical solution of the present invention has at least the following beneficial effects: 1. This invention synthesizes precursors in the liquid phase via a hydrothermal method, achieving uniform mixing of lithium, iron, and manganese at the atomic level. This effectively overcomes the problem of uneven mixing in traditional solid-phase methods, laying the foundation for obtaining a final product with a uniform structure. Simultaneously, it creatively transforms aluminum ions, impurities in black powder, into bulk dopants, allowing them to uniformly enter the LMFP bulk phase during sintering. The aluminum ions stabilize the crystal structure and suppress manganese dissolution during cycling, thus eliminating the tedious deep impurity removal steps and significantly reducing the cost of recycling. Furthermore, the synergistic carbon coating layer further constructs a uniform and stronger conductive network, significantly improving electronic conductivity and suppressing excessive growth and aggregation of nanoparticles during sintering.

[0024] 2. This invention uses recycled lithium iron phosphate (LFP), lithium manganese oxide (LMO), and lithium manganese iron phosphate (LMFP) black powder as sources of iron, manganese, and lithium, thereby increasing the recycling value of lithium-ion battery cathode materials, eliminating dependence on high-purity and expensive raw materials, and significantly reducing raw material costs.

[0025] 3. This invention retains and measures Al. 3+ Using concentration as a doping benchmark, impurity Al is converted into bulk dopant in situ and quantitatively, allowing impurity elements to uniformly enter the lithium manganese iron phosphate lattice, thereby reducing costs while improving the structural stability and cycle life of the material.

[0026] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0027] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0028] 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 or 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.

[0029] In the description of this application, "same chemical composition" should be interpreted broadly, that is, the main components of the two have the same chemical composition, or the two have substantially the same chemical composition, but may have errors or impurities within the acceptable range that can be understood by those skilled in the art.

[0030] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0031] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0032] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0033] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0034] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0035] Lithium-ion batteries, as one of the most universal and attractive energy storage systems, have become the preferred choice for electric vehicles and grid energy storage, and are widely used in communications, portable electronic devices, electric vehicles, and smart grids. With the continuous development of the new energy industry, my country's lithium-ion battery production has achieved a leading position globally. Data shows that from January to July 2025, China's lithium-ion battery production reached approximately 940 GWh, a year-on-year increase of about 68%, accounting for more than 80% of global production. However, the lifespan of lithium-ion batteries is typically only 5-10 years. As a large number of batteries gradually reach the end of their service life, a massive amount of waste lithium-ion batteries will be generated in the future. Therefore, how to efficiently utilize the residual value of waste batteries has become an important issue that urgently needs to be addressed.

[0036] Cathode materials constitute the largest portion of a battery's cost (approximately 40% to 50% of the total material cost) and are rich in key metals such as lithium, cobalt, nickel, and manganese. Recycling and utilizing these materials can enhance my country's resource self-sufficiency and supply chain resilience in the new energy industry, while also holding significant strategic value for ensuring resource security, protecting the environment, reducing costs, and promoting sustainable industrial development.

[0037] Among the widely used cathode materials, lithium iron phosphate (LFP) has the highest market share, but its capacity and compaction density are approaching their theoretical limits, making it difficult to significantly improve energy density. Lithium manganese iron phosphate (LMFP) has long been considered an upgrade to LFP, offering a higher voltage platform and an energy density approximately 10% to 20% higher than LFP. However, the large-scale application of LFP still faces the following core challenges: ① High cost: High-purity manganese sources and modifying additives result in high raw material costs; complex preparation processes lead to increased equipment investment and energy consumption; and the current level of industrial scale is insufficient, resulting in an overall cost significantly higher than mature lithium iron phosphate (approximately 40% higher). ② Difficult processing: The synthesis process requires precise control to prevent manganese from causing structural instability; simultaneously, to improve its inherently poor conductivity, additional modification steps such as carbon coating and doping are necessary, posing significant challenges to process complexity and consistency. ③ Performance limitations: Poor conductivity affects fast charging and rate performance. Insufficient cycle life, with manganese leaching leading to rapid capacity decay at high temperatures and long cycles.

[0038] While LMFP (Lithium Manganese Iron Phosphate) has theoretical potential, its commercialization still requires overcoming limitations in cost, technology, and performance. Therefore, recycling cathode materials from spent batteries and using them to prepare lithium manganese iron phosphate not only achieves resource recycling of key metals but also helps promote the low-cost application of next-generation cathode materials, possessing broad development prospects and value in the future.

[0039] Building upon this foundation, this invention utilizes an innovative integrated process of "recycling-doping-controllable synthesis-composite coating" to prepare high-performance lithium manganese iron phosphate (LMFP), achieving a balance between low cost and high performance. Specifically, an acid-leaching solution containing lithium, iron, manganese, and aluminum ions is directly subjected to a hydrothermal reaction to synthesize the LMFP precursor in situ. In subsequent heat treatment, aluminum ions derived from recycled black powder in the leaching solution are used for bulk doping of the LMFP. Simultaneously, a carbon coating process is combined to produce the LMFP cathode material. This invention avoids lengthy deep impurity removal steps, achieving atomic-level uniform mixing of Fe, Mn, and P elements using a hydrothermal method. Furthermore, aluminum, previously a difficult-to-remove impurity, is transformed into a stable dopant that can penetrate the crystal lattice, thereby improving the rate performance and structural stability of the LMFP material.

[0040] Specifically, the present invention adopts the following technical solution: According to one aspect of the present invention, a lithium manganese iron phosphate cathode material is provided, comprising the following steps: S1. The recycled lithium-ion battery cathode material black powder is acid-hydrolyzed to obtain a leachate containing aluminum ions. S2. The leachate obtained in step S1 is subjected to a hydrothermal reaction to obtain lithium manganese iron phosphate precursor. S3. Mix the lithium manganese iron phosphate precursor obtained in step S2 with the lithium source and carbon source, and perform heat treatment to obtain the lithium manganese iron phosphate cathode material. In this process, aluminum ions from the leaching solution in step S1 are used to bulk dope lithium manganese iron phosphate in the heat treatment step.

[0041] The recycled lithium-ion battery cathode material black powder was acid-hydrolyzed to obtain a leachate containing aluminum ions.

[0042] In a specific embodiment of the present invention, in step S1: the recovered lithium-ion battery cathode material black powder includes at least two of lithium manganese oxide, lithium manganese iron phosphate, and lithium iron phosphate, specifically a mixture of lithium manganese oxide and lithium manganese iron phosphate, a mixture of lithium manganese oxide and lithium iron phosphate, or a mixture of lithium manganese iron phosphate and lithium iron phosphate. The present invention directly uses the recovered black powder as a source of iron, manganese, and lithium, which can fully utilize the huge stock of waste battery materials in the market, broaden the raw material channels, and significantly reduce dependence on primary mineral resources.

[0043] In a specific embodiment of the present invention, in step S1: the aluminum ions in the leaching solution containing aluminum ions originate from the recovered lithium-ion battery cathode material black powder, typically aluminum foil current collector debris mixed into the lithium-ion battery cathode material black powder. Those skilled in the art will understand that the black powder may contain other trace metallic impurities, but aluminum is one of the most common and relatively abundant impurity elements, and is the focus of this method.

[0044] In a specific embodiment of the present invention, in step S1: the acid solution used in the acidolysis operation may include, but is not limited to, inorganic acids such as hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3), or organic acids. The concentration of the acid solution used in the acidolysis operation is 1.5 mol / L to 3 mol / L, specifically 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, and any value between the two mentioned above; the present invention selects the above-mentioned concentration of acid solution to ensure efficient leaching of the target metal.

[0045] In a specific embodiment of the present invention, in step S1: the acid hydrolysis process involves reacting an acid solution with compounds such as metal oxides and phosphates in the recovered black powder, causing lithium, manganese, iron, and aluminum to enter the solution in ionic form. The leachate also includes lithium ions, manganese ions, and iron ions. The molar ratio of manganese ions to iron ions in the leachate is 1:1 to 4:1, specifically 1:1, 2:1, 3:1, 4:1, or any value between these two ratios.

[0046] After obtaining a leachate containing aluminum ions, the leachate is subjected to a hydrothermal reaction to obtain a lithium manganese iron phosphate precursor.

[0047] In a specific embodiment of the present invention, in step S2: before the hydrothermal reaction, a reducing agent is added to the leachate obtained in step S1, and a reduction reaction is carried out under an inert atmosphere to reduce the Fe in the solution. 3+ The Fe is reduced. The inert atmosphere includes at least one of nitrogen, argon, and helium. The inert atmosphere is created to prevent the reduced Fe from being reduced. 2+ In subsequent operations, it is re-oxidized by oxygen in the air, ensuring the stability of the reduction effect.

[0048] In a specific embodiment of the present invention, in step S2: the reducing agent is preferably iron powder. After adding the iron powder, the molar ratio of manganese to iron in the leachate is 5~7:5~3, specifically 5:5, 6:4, 7:3, or any value between the two mentioned above. The present invention uses the above-mentioned reducing agent to avoid introducing new impurity cations, thus preventing interference with subsequent reactions. It can also serve as an additional iron source to fine-tune the manganese-iron ratio in the leachate.

[0049] In a specific embodiment of the present invention, in step S2: the present invention uses a hydrothermal method to uniformly synthesize the precursor at the molecular level in the liquid phase, which ensures the atomic-level uniform mixing of Fe, Mn and P elements, overcomes the inherent problems of traditional mechanical mixing, and fundamentally lays the foundation for the formation of LMFP crystals with uniform composition.

[0050] In a specific embodiment of the present invention, in step S2: the temperature of the hydrothermal reaction is 120℃~180℃, specifically 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, and any value between any two of the above; the time of the hydrothermal reaction is 8h~12h, specifically 8h, 9h, 10h, 11h, 12h, and any value between any two of the above. The parameters of the hydrothermal reaction selected in the present invention are beneficial for forming lithium manganese iron phosphate precursors with moderate crystallinity, uniform particle size, and nanoscale composition.

[0051] After obtaining the lithium manganese iron phosphate precursor, the precursor is mixed with a lithium source and a carbon source, and then subjected to heat treatment to obtain the lithium manganese iron phosphate cathode material. Specifically, aluminum ions from the leachate are used to bulk dope the lithium manganese iron phosphate cathode material during the heat treatment step.

[0052] In a specific embodiment of the present invention, in step S3: the lithium source includes at least one of LiOH, Li3PO4, and Li2CO3, preferably Li2CO3. The addition of the lithium source is to supplement the lithium that may be insufficient in the lithium manganese iron phosphate precursor and to ensure the stoichiometry of the final product.

[0053] In a specific embodiment of the present invention, the carbon source includes at least one of glucose, chitosan, polyethylene glycol, sucrose, and starch. The role of the carbon source is to decompose during heat treatment, forming a continuous conductive carbon network on the surface of LMFP particles, and potentially penetrating into the interparticle spaces to improve the electronic conductivity of the material. The carbon source preferably includes glucose and chitosan, with a mass ratio of glucose to chitosan of 90-99:10-1. A large amount of glucose serves as the main carbon skeleton, ensuring conductive continuity; a small amount of chitosan acts as a functional additive, and the nitrogen-doped carbon produced by its pyrolysis enhances the bonding force between the carbon layer and the material surface and improves the intrinsic conductivity. The two work synergistically to construct a more robust and efficient conductive network, solving the problems of uneven carbon coating and weak bonding force.

[0054] In a specific embodiment of the present invention, the heat treatment temperature is 600℃~800℃, specifically 600℃, 650℃, 700℃, 750℃, 800℃, and any value between two of the above; the heat treatment time is 8h~12h, specifically 8h, 9h, 10h, 11h, 12h, and any value between two of the above. The present invention uses the above heat treatment parameters to achieve complete crystallization of the lithium manganese iron phosphate precursor, while simultaneously realizing carbon coating and crucial aluminum ion bulk phase doping.

[0055] In a specific embodiment of the present invention, the heat treatment is carried out in an inert atmosphere to prevent the lithium manganese iron phosphate precursor from being oxidized at high temperatures and the carbon source from being burned off during carbonization.

[0056] In a specific embodiment of this invention, aluminum ions in the leaching solution are used to bulk dope the lithium manganese iron phosphate cathode material during the heat treatment step. Traditional recycling processes require deep removal of residual metals, which is costly. This invention utilizes residual aluminum and other impurities in the black powder that are difficult to remove completely, converting them into bulk dopants. By precisely controlling acid hydrolysis and subsequent processes, aluminum impurities are uniformly incorporated into the lithium manganese iron phosphate lattice, reducing costs while improving the structural stability and cycle life of the lithium manganese iron phosphate cathode material. Furthermore, this process occurs in situ, eliminating the need for additional aluminum salts as dopants.

[0057] In a specific embodiment of the present invention, in step S3: In the specific embodiment of the present invention, the aluminum doping amount in the lithium manganese iron phosphate cathode material does not exceed 1 wt%. Appropriate aluminum doping can effectively stabilize the crystal lattice and inhibit manganese dissolution, while excessive aluminum may block the lithium-ion diffusion channels, which may have a negative impact on performance.

[0058] This invention uses recycled lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP), and lithium manganese oxide (LMO) black powder as sources of iron, manganese, and lithium, eliminating dependence on high-purity and expensive raw materials, significantly reducing raw material costs, and increasing the recycling value of materials. It utilizes residual aluminum and other impurities in the black powder, which are difficult to completely remove, to transform them into bulk dopants. This allows impurities to uniformly enter the lithium manganese iron phosphate lattice, avoiding the lengthy and energy-intensive steps of traditional recycling processes that separate and purify valuable metals to elemental or simple compounds before using them as chemical raw materials for synthesis. This reduces costs while improving the structural stability and cycle life of the material. Furthermore, it directly synthesizes nanoscale precursors via a hydrothermal method and combines this with a novel coating carbonization technology to construct a uniform and more robust conductive network, significantly improving electronic conductivity and suppressing excessive growth and agglomeration of nanoparticles during sintering.

[0059] According to one aspect of the present invention, a lithium manganese iron phosphate cathode material is provided, which is prepared by the preparation method described above.

[0060] According to one aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a lithium manganese iron phosphate positive electrode material prepared by the preparation method described above or the lithium manganese iron phosphate positive electrode material described above.

[0061] In a specific embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector along the thickness direction; wherein, the positive current collector has two surfaces opposite to each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0062] In a specific embodiment of the present invention, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder; wherein, the positive electrode active material includes lithium manganese iron phosphate positive electrode material; the positive electrode conductive agent includes, but is not limited to, one or more of conductive carbon, conductive carbon black (SP), carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the positive electrode binder includes, but is not limited to, one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The present invention does not impose any special restrictions on the source of the positive electrode active material, positive electrode conductive agent, and positive electrode binder; commercially available products or self-made products well known to those skilled in the art can be used.

[0063] In a specific embodiment of the present invention, the preparation method of the positive electrode sheet adopts a method well known to those skilled in the art: firstly, the raw materials are mixed in a solvent in a certain proportion to form a slurry, and then the positive electrode slurry is coated on a positive electrode current collector, and dried and rolled to prepare the positive electrode sheet. The solvent is preferably N-methylpyrrolidone (NMP).

[0064] In a specific embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector along the thickness direction; wherein, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0065] In a specific embodiment of the present invention, the negative electrode active material layer comprises a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder; wherein, the negative electrode active material includes, but is not limited to, one or more of natural graphite, artificial graphite, hard carbon, soft carbon, silicon-based materials, and silicon-carbon composite materials. The negative electrode conductive agent includes, but is not limited to, one or more of conductive carbon, conductive carbon black (SP), carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The negative electrode binder includes, but is not limited to, one or more of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The present invention does not impose any special restrictions on the source of the negative electrode active material, negative electrode conductive agent, and negative electrode binder; commercially available products or self-made products well known to those skilled in the art can be used.

[0066] In a specific embodiment of the present invention, the preparation method of the negative electrode sheet adopts a method well known to those skilled in the art: firstly, the raw materials are mixed in a solvent in a certain proportion to form a slurry, and then the negative electrode slurry is coated on a negative electrode current collector, and dried and rolled to prepare the negative electrode sheet. The solvent is preferably deionized water.

[0067] In a specific embodiment of the present invention, the separator comprises a porous polymer membrane, specifically a polypropylene (PP) membrane, a polyethylene (PE) membrane, a PP / PE / PP composite membrane, etc. The present invention selects the above-mentioned separator, which provides sufficient mechanical strength to prevent internal short circuits without excessively increasing ion transport resistance, and exhibits good wettability and compatibility with the electrolyte system.

[0068] In a specific embodiment of the present invention, the lithium-ion battery is preferably assembled in the order of negative electrode, separator, and positive electrode. In a preferred embodiment of the present invention, the battery preparation process includes: sequentially assembling the negative electrode, separator, and positive electrode, then impregnating them with an electrolyte, and finally processing them through formation and other processes to form a lithium-ion battery. The specific conditions and parameters for each step in the above preparation process can be achieved using battery preparation techniques well-known to those skilled in the art, and the present invention does not impose any special limitations on them.

[0069] The following detailed description of this application is based on specific embodiments, but the implementation and protection of this invention are not limited thereto. The following embodiments are only some embodiments of this application and are not intended to limit this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0070] Example 1 S1, Acid hydrolysis of black powder: 10g of recycled lithium-ion battery cathode material powder (6.32g lithium manganese oxide, 3.68g lithium iron phosphate, manganese-iron molar ratio 3:1) was slowly added to hydrochloric acid with a concentration of 2mol / L. The temperature was controlled at 60℃, and the mixture was stirred at this constant temperature for 1 hour to ensure complete dissolution without bubbles. The Al in the solution was retained. 3+ Its concentration, determined by ICP-OES, was 0.05 mol / L. Al 3+ The content of aluminum ions was 0.0025 mol, resulting in a leachate containing aluminum ions.

[0071] S2, hydrothermal synthesis precursor: Under continuous argon gas flow, 1.4 grams of iron powder were added to the leachate in small, repeated amounts to reduce the Fe content in the solution. 3+ The metal was reduced, at which point the molar ratio of manganese to iron in the leachate was 6:4. After stirring thoroughly, the solid residue was filtered out, and the filtrate was transferred to a high-pressure reactor and hydrothermally reacted at 180°C in an oven for 12 hours. After the reaction, the product was washed multiple times with water and ethanol, and then vacuum dried at 80°C for 12 hours to obtain lithium manganese iron phosphate precursor powder.

[0072] S3, Precursor sintering: Approximately 8.76 g of precursor powder was weighed and 2.3 g of Li₂CO₃ was added. The mixture was then subjected to high-speed wet milling (2500 r / min, 2 h, deionized water medium) to form the first slurry. Next, 9 wt% glucose and 1 wt% chitosan were added and mixed to form the second slurry. Finally, the mixture was dried and held at 700°C for 10 h in an argon-protected tube furnace to obtain the lithium manganese iron phosphate cathode material, denoted as LiMn. 0.6 Fe 0.4 PO4 / C.

[0073] Example 2 Example 2 differs from Example 1 in that, in step S1, the recovered lithium-ion battery cathode material black powder includes 3.54g of lithium manganese oxide and 5g of lithium iron manganese phosphate, with a manganese-iron molar ratio of 3:1; the Al in the solution... 3+ The content is 0.0025 mol.

[0074] Example 3 The difference between Example 3 and Example 1 is that in step S3, starch is used instead of chitosan.

[0075] Example 4 The difference between Example 4 and Example 1 is that the sintering temperature in step S3 is 600°C.

[0076] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that in step S1, extraction separation is used to remove Al. 3+ .

[0077] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that in step S2, the hydrothermal reaction is carried out at a temperature of 120°C for 8 hours.

[0078] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that in step S3, the carbon source in the second slurry is 5 wt% glucose.

[0079] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the concentration of hydrochloric acid in step S1 is 1 mol / L.

[0080] Performance testing: The lithium manganese iron phosphate cathode materials prepared in Examples 1-5 and Comparative Examples 1-3 were used to manufacture cathode sheets, assembled into batteries, and tested. The battery manufacturing method and performance testing were based on GB / T 42161-2022 "Electrochemical Performance Testing of Lithium Iron Phosphate: First Discharge Specific Capacity and First Charge-Discharge Efficiency Test Method".

[0081] The test results are shown in Table 1 below.

[0082] Table 1 Performance test data for each embodiment and comparative example Referring to Table 1, it can be seen from Example 1 and Comparative Example 4 that using a lower concentration of acid to digest the black powder results in incomplete digestion, leading to lower actual amounts of Mn, Fe, and Li obtained in the solution compared to theoretical calculations. Undissolved residues or abnormal nucleation due to ion concentration fluctuations result in a wide particle size distribution of the hydrothermal products (including nanoparticles, micron-sized particles, and even the residue itself). Large particles have long lithium-ion diffusion paths and high polarization, leading to poor rate performance. Furthermore, carbon coating is difficult to form a uniform conductive layer on the smooth surface of large particles, further limiting capacity utilization.

[0083] As can be seen from Examples 1, 3, and Comparative Example 3, changing the type of carbon source or reducing the carbon source content will worsen the coating effect, making it impossible to effectively construct a continuous and efficient conductive network, and significantly reducing the specific capacity. Meanwhile, during cycling, the chitosan-derived nitrogen-doped carbon layer has a stronger bond with the surface of the active material particles, forming a denser and more continuous protective layer. This effectively buffers volume changes during charge and discharge, protects the particles from pulverization, and effectively inhibits manganese dissolution. In contrast, the starch-derived carbon layer, due to its uneven coating and brittle texture, is more prone to breakage during long-term cycling, leading to increased side reactions with the electrolyte and accelerated capacity decay.

[0084] As can be seen from Example 1 and Comparative Example 1, removing aluminum from the solution leads to obstructed electron / ion transport, insufficient capacity utilization and a decrease in specific capacity; at the same time, due to the inability to suppress the Jahn-Teller effect and manganese dissolution, the crystal structure is more easily destroyed during cycling, and the capacity retention rate is significantly reduced.

[0085] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0086] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0087] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1. The recycled lithium-ion battery cathode material black powder is acid-hydrolyzed to obtain a leachate containing aluminum ions. S2. The leachate obtained in step S1 is subjected to a hydrothermal reaction to obtain lithium manganese iron phosphate precursor. S3. Mix the lithium manganese iron phosphate precursor obtained in step S2 with the lithium source and carbon source, and perform heat treatment to obtain the lithium manganese iron phosphate cathode material. In this process, aluminum ions from the leaching solution in step S1 are used to bulk dope the lithium manganese iron phosphate cathode material during the heat treatment step.

2. The preparation method according to claim 1, characterized in that, In step S1: the recovered lithium-ion battery cathode material black powder includes at least two of lithium manganese oxide, lithium iron manganese phosphate, and lithium iron phosphate; And / or, the aluminum ions in the leachate containing aluminum ions are derived from the recovered lithium-ion battery cathode material black powder. And / or, the concentration of the acid solution used in the acidolysis operation is 1.5 mol / L to 3 mol / L; And / or, the leachate may also contain lithium ions, manganese ions, and iron ions.

3. The preparation method according to claim 2, characterized in that, In step S1: the molar ratio of manganese ions to iron ions in the leachate is 1:1 to 4:

1.

4. The preparation method according to claim 1, characterized in that, In step S2: Before the hydrothermal reaction, a reducing agent is added to the leachate obtained in step S1, and a reduction reaction is carried out under an inert atmosphere.

5. The preparation method according to claim 4, characterized in that, In step S2: the reducing agent is iron powder. After adding iron powder, the molar ratio of manganese to iron in the leachate is 5~7:5~3.

6. The preparation method according to claim 1, characterized in that, In step S2: the temperature of the hydrothermal reaction is 120℃~180℃; And / or, the hydrothermal reaction time is 8h~12h.

7. The preparation method according to claim 1, characterized in that, In step S3: the lithium source includes at least one of LiOH, Li3PO4, and Li2CO3; And / or, the carbon source includes at least one of glucose, chitosan, polyethylene glycol, sucrose, and starch; And / or, the temperature of the heat treatment is 600℃~800℃; And / or, the heat treatment time is 8h~12h; And / or, the heat treatment is carried out in an inert atmosphere.

8. The preparation method according to claim 7, characterized in that, In step S3: the carbon source includes glucose and chitosan, and the mass ratio of glucose to chitosan is 90~99:10~1; And / or, in the lithium manganese iron phosphate cathode material, the amount of aluminum doping does not exceed 1 wt%.

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

10. A lithium-ion battery, characterized in that, The cathode includes a positive electrode sheet, wherein the positive electrode sheet comprises a lithium manganese iron phosphate cathode material prepared by the preparation method according to any one of claims 1 to 8 or a lithium manganese iron phosphate cathode material according to claim 9.

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