Lithium iron manganese phosphate composite cathode material, preparation method thereof and lithium ion battery

Abstract

The application provides a lithium iron manganese phosphate composite positive electrode material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: preparing a first slurry by mixing a lithium source, an iron source, a manganese source, a phosphorus source, a doping element source and a first organic carbon source, and obtaining a first product by calcining the first slurry; preparing a second slurry by mixing the first product and a one-dimensional carbon material, and obtaining a second product by drying the second slurry; forming a SiO2 coating layer on the surface of the second product by means of atomic layer deposition, and obtaining a third product; preparing a third slurry by mixing the third product and a second organic carbon source, and obtaining a fourth product by calcining the third slurry; and obtaining the lithium iron manganese phosphate composite positive electrode material by immersing the fourth product in an etching solution. The application combines the methods of multiple carbon coating, setting an intermediate coating layer and etching treatment to form a void layer, and performs microstructure design on the positive electrode material, so that the transmission rate of electrons and lithium ions in the structure is ensured.

Description

Technical Field

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

[0002] The energy density of lithium iron phosphate (LFP) is already close to its theoretical ceiling. Introducing manganese iron phosphate (Mn) into LFP can significantly increase the discharge voltage to 4.1V, thus raising the energy density of the cathode material and potentially further improving the energy density of the corresponding battery cell. However, the corresponding lithium manganese iron phosphate (LFP) material is not simply a physical mixture of LiFePO4 and LiMnPO4. Currently, by utilizing Fe... 2+ With Mn 2+ The very small difference in ionic radii allows LiFePO4 and LiMnPO4 to form a stable and homogeneous solid solution, namely LiMnPO4, through a synergistic effect. x Fe 1-x LiFePO4 combines the stable electrochemical properties of LiFePO4 with the high potential of LiMnPO4. Therefore, lithium manganese iron phosphate has become an important direction in the development of cathode material technology. Meanwhile, in LiMn... x Fe 1-x Based on PO4 materials, further doping with other elements can further improve its performance.

[0003] In LiMn x Fe 1-x In PO4 materials, x represents the manganese doping ratio, which can take any value between 0 and 1. Due to the high voltage plateau of Mn and the good conductivity of Fe, different manganese-to-iron ratios result in performance differences in lithium manganese iron phosphate. When the manganese content is too low, the voltage boost is limited, and the energy density advantage over lithium iron phosphate is not significant. When the manganese content is too high, the battery voltage and energy density are significantly improved, but the Jahn-Teller effect is prone to occur, leading to Mn dissolution and rapid degradation of cycle performance; in addition, there is also a significant voltage decay.

[0004] In particular, existing lithium iron phosphate cathode materials suffer from significant manganese leaching during cycling, resulting in a substantial amount of manganese (Mn) leaching. 3+ Leaching will cause a disproportionation reaction on the surface of the cathode material to generate Mn. 2+ Mn 4+ Mn 2+ It will react with F in HF (LiPF6 + H2O → POF3 + HF + LiF). － Combining and promoting Mn 2+Dissolved in the electrolyte, it migrates to the negative electrode surface, is reduced and deposited, which in turn damages the SEI film. This leads to problems such as continuous growth and thickening of the SEI film, consumption of active lithium, increased electrode impedance, and gas generation at the negative electrode. Ultimately, this results in continuous capacity decay and voltage plateau collapse, severely affecting battery performance.

[0005] Therefore, how to provide a preparation method that can optimize the performance of lithium manganese iron phosphate cathode materials, based on the elemental composition and microstructure design, so that the resulting cathode material has superior electrochemical performance, is one of the important technical problems that need to be solved in this field. Summary of the Invention

[0006] The main objective of this invention is to provide a lithium manganese iron phosphate composite cathode material, its preparation method, and a lithium-ion battery, so as to solve the problems of poor electrochemical performance, especially poor cycle stability, of lithium-ion battery cathode materials in the prior art.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a lithium manganese iron phosphate composite cathode material, comprising: step S1, preparing a first slurry by mixing a lithium source, an iron source, a manganese source, a phosphorus source, a dopant source, and a first organic carbon source, and obtaining a first product after a first calcination of the first slurry; step S2, preparing a second slurry by mixing the first product with a one-dimensional carbon material, and obtaining a second product after drying the second slurry; step S3, forming a SiO2 coating layer on the surface of the second product by atomic layer deposition, thereby obtaining a third product; step S4, preparing a third slurry by mixing the third product with a second organic carbon source, and obtaining a fourth product after a second calcination of the third slurry; and step S5, placing the fourth product in an etching solution, and obtaining the lithium manganese iron phosphate composite cathode material after immersion; wherein the etching solution is a strong acid solution or a strong alkali solution.

[0008] This invention provides a preparation method that achieves the formation of an electrolyte barrier on the surface of the cathode material, thereby mitigating the potential Mn ion dissolution problem during application and resulting in superior electrochemical performance. The preparation method utilizes multi-step material microstructure design and processing to construct a lithium manganese iron phosphate composite cathode material composed of a doped mixed phosphate core, a first carbon material framework, a first carbon layer, a nanoporous layer, and an outer second carbon layer. The unique microstructure designed and prepared effectively solves the common problems of manganese ion dissolution and voltage decay during lithium-ion battery cathode material cycling. The final prepared lithium manganese iron phosphate composite cathode material exhibits higher energy density, longer cycle life, and superior charge-discharge performance.

[0009] Further, in step S1, the molar ratio of lithium source, iron source, manganese source, phosphorus source and dopant source is 1:(0.1~0.5):(0.5~0.9):1:(0~0.08); and / or, based on the total weight of lithium source, iron source, manganese source, phosphorus source and dopant source as 100%, the amount of first organic carbon source is 8%~10%; and / or, the D50 of the first product is 3μm~6μm; and / or, the dopant source is selected from one or more of Mg, Sc, Ti, V, Co and Mo; and / or, the temperature of the first calcination is 680℃~730℃.

[0010] By optimizing the above-mentioned preparation conditions during the formation of the first product, a first product with high capacity and high stability, namely a mixed phosphate core with a first carbon layer, can be prepared more effectively, thus providing a material basis for the formation of a lithium manganese iron phosphate composite cathode material with excellent cycle performance.

[0011] Further, in step S2, the amount of one-dimensional carbon material used is 0.5% to 1.0% based on the total weight of the first product as 100%; and / or, the one-dimensional carbon material is selected from one or more of carbon nanotubes, carbon nanowires and carbon fibers, preferably carbon nanotubes.

[0012] By further optimizing the amount and type of one-dimensional carbon material, especially carbon nanotubes, a highly efficient and structurally superior electrical conductivity network can be constructed on the surface of the first product, enhancing the mechanical and chemical stability of the resulting lithium manganese iron phosphate composite cathode material, and ultimately achieving a comprehensive improvement in its performance, including capacity retention and cycle performance.

[0013] Furthermore, in order to comprehensively improve the intrinsic electrochemical performance, structural stability, and processing performance of the obtained second product, so as to obtain a lithium manganese iron phosphate composite cathode material with higher cycle stability, step S2 preferably includes: step S2-1, adding the first product and one-dimensional carbon material into a solvent to obtain a dispersion; ultrasonically dispersing the dispersion at 10kHz to 50kHz for 0.5h to 2.0h to obtain a second slurry; step S2-2, drying the second slurry at 100℃ to 150℃ to obtain the second product.

[0014] Furthermore, in step S3, the thickness of the SiO2 coating layer is 30 nm to 50 nm. This thickness range of SiO2 coating layer not only effectively forms an optimized nanoporous structure but also protects the mixed phosphate core and the first carbon layer on its surface from hydrofluoric acid corrosion, thereby improving the overall electrochemical performance of the resulting lithium manganese iron phosphate composite cathode material.

[0015] Preferably, atomic layer deposition includes: placing the second product in an atomic layer deposition reaction chamber, using tris(dimethylamino)silane and O3 as precursors and N2 as a purification gas, performing surface deposition at 200±20°C, and forming a SiO2 coating layer on the surface of the second product. These atomic layer deposition conditions enable more uniform deposition of the SiO2 coating layer, reduce defects, and thus form a more structurally stable lithium manganese iron phosphate composite cathode material after etching.

[0016] Further, in step S4, based on the total weight of the third product as 100%, the amount of the second organic carbon source is 3.0% to 5.0%; and / or, the second calcination temperature is 600℃ to 700℃, and the time is 2h to 4h. During the formation of the second carbon layer, in order to achieve a denser coating so that the etching solution can more uniformly penetrate into the interior during subsequent etching to etch the SiO2 coating layer, a more stable and conductive lithium manganese iron phosphate composite cathode material is obtained.

[0017] Furthermore, in step S5, the etching solution is a hydrofluoric acid solution or a sodium hydroxide solution, and the concentration of the etching solution is 0.05 mol / L to 0.5 mol / L; and / or, the immersion time is 1 min to 30 min. Through the optimized concentration, type, and immersion time of the etching solution described above, a more ideal nanoporous structure can be formed, thereby significantly reducing the contact between the obtained lithium manganese iron phosphate composite cathode material and the electrolyte during application, further improving its electrochemical performance, especially its cycle stability.

[0018] In particular, the preferred etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L; and / or, the immersion time is 5 min to 15 min. The hydrofluoric acid solution within the above concentration range, combined with etching within this time range, can effectively remove the SiO2 coating layer while significantly suppressing excessive corrosion of the material surface, and ultimately improving the long-cycle stability of the obtained lithium manganese iron phosphate composite cathode material.

[0019] Furthermore, the lithium source, iron source, manganese source, and dopant source are each independently added in one or more forms selected from nitrates, phosphates, sulfates, carbon source salts, acetates, and oxides; and / or, the phosphorus source is selected from one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid; and / or, the first organic carbon source and the second organic carbon source are each independently selected from one or more of glucose, quinoline-soluble substances in coal tar pitch, polyethylene glycol, polyvinyl alcohol, citric acid, and dopamine. The lithium manganese iron phosphate composite cathode material prepared from the above raw materials has a more stable structure and superior overall electrochemical performance.

[0020] A second aspect of the present invention provides a lithium manganese iron phosphate composite cathode material, which is prepared by the above-described method for preparing lithium manganese iron phosphate composite cathode materials. The lithium manganese iron phosphate composite cathode material includes a mixed phosphate core, and the molecular formula of the mixed phosphate core is LiMn. x Fe 1-x-y T y PO4, where x is 0.5–0.9, y is 0.001–0.08, and T is selected from one or more of Mg, Sc, Ti, V, Co, and Mo. The resulting lithium manganese iron phosphate composite cathode material has three special coating layers and a framework interspersed within them. Therefore, during application, it can effectively avoid possible manganese leaching and ultimately enable the lithium manganese iron phosphate composite cathode material to exhibit particularly excellent electrochemical performance, especially cycle stability.

[0021] A third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode, a positive electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises the aforementioned lithium manganese iron phosphate composite positive electrode material. Because the positive electrode material provided by the present invention possesses excellent stability and electrochemical performance, the corresponding lithium-ion battery exhibits superior performance.

[0022] This invention provides a method for preparing a highly stable lithium manganese iron phosphate composite cathode material. By combining multiple carbon coatings, the creation of an intermediate coating layer, and etching to form a void layer, the microstructure of the prepared lithium manganese iron phosphate composite cathode material is designed, ensuring the electron and lithium ion transport channels and rates within its structure. Simultaneously, the barrier formed by coating and etching during the preparation process effectively isolates the electrolyte, mitigating the dissolution of Mn ions caused by electrolyte corrosion of the cathode material, achieving high-stability cycling, and suppressing voltage decay during cycling. The final prepared lithium manganese iron phosphate composite cathode material exhibits higher energy density, longer cycle life, and superior charge-discharge performance. Detailed Implementation

[0023] 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 embodiments.

[0024] As described in the background section, existing lithium-ion battery cathode materials suffer from poor electrochemical performance, particularly poor cycle stability. To address these issues, a first aspect of this invention provides a method for preparing a lithium manganese iron phosphate composite cathode material, comprising: step S1, preparing a first slurry from a lithium source, an iron source, a manganese source, a phosphorus source, a dopant element source, and a first organic carbon source; subjecting the first slurry to a first calcination to obtain a first product; step S2, preparing a second slurry from the first product and a one-dimensional carbon material; subjecting the second slurry to drying to obtain a second product; step S3, forming a SiO2 coating layer on the surface of the second product by atomic layer deposition to obtain a third product; step S4, preparing a third slurry from the third product and a second organic carbon source; subjecting the third slurry to a second calcination to obtain a fourth product; and step S5, placing the fourth product in an etching solution and immersing it to obtain the lithium manganese iron phosphate composite cathode material; wherein the etching solution is a strong acid solution or a strong alkali solution.

[0025] This invention achieves a barrier against electrolyte formation on the surface of a mixed phosphate core formed from lithium, iron, manganese, phosphorus, and doped element sources through multiple carbon coating processes, the addition of an intermediate coating layer, and etching to form a void layer. This mitigates the potential Mn ion dissolution problem during application of the resulting cathode material, resulting in superior electrochemical performance. Specifically, in the above preparation method:

[0026] In step S1, a doped mixed phosphate core is first formed to obtain a carbon-coated core material with good electrochemical performance and a stable structure, namely the first product, laying the foundation for subsequent microstructure design. Then, in step S2, a one-dimensional carbon material is deposited on the outer carbon layer surface of the first product, thereby constructing a one-dimensional carbon material conductive network, resulting in a second product with a one-dimensional carbon material attached to its surface. The introduction of the one-dimensional carbon material not only constructs a high-speed conductive bridge, enabling more efficient contact between the phosphate core and the subsequently formed second carbon layer to improve the overall conductivity of the material, but more importantly, it enables the formation of a stable, framework-like structure, thus providing support for the void layer obtained by subsequent etching. In step S3, an atomic layer deposition technique is used to pre-deposit a SiO2 coating layer on the surface of the second product; this layer serves as the precursor for the subsequently formed insulating void layer.

[0027] Furthermore, a carbon coating layer is constructed a second time on its surface through step S4. This not only increases the conductivity of the final cathode material but, more importantly, acts as another barrier to further protect the cathode material from direct erosion by the electrolyte. It also works synergistically with the one-dimensional carbon material introduced in step S2 to improve the transport rate of lithium ions and electrons. At this point, the structure of the fourth product includes a doped mixed phosphate core, and a first carbon layer, a SiO2 coating layer, and a second carbon layer sequentially coated on the surface of this core. The most crucial step is step S6, in which the fourth product is immersed in an etching solution, causing the etching solution to etch and dissolve the SiO2 coating layer, thereby forming a void layer between the first and second carbon layers. Of course, as mentioned above, this void layer can exist stably without collapsing because the one-dimensional carbon material provides support within it. The presence of this void layer, in conjunction with the first and second carbon layers, significantly isolates the phosphate core of the obtained material from contact with the electrolyte during application, thereby reducing the dissolution of manganese ions and ultimately exhibiting excellent electrochemical stability.

[0028] In particular, this invention uses strong acid or strong alkali solutions as etching solutions to etch the SiO2 coating layer because strong acids and strong alkali solutions have high reactivity, enabling more efficient and selective etching of the SiO2 coating layer. This allows for precise etching of the target SiO2 layer without damaging the two carbon coating layers, thus achieving fine control over the structure of the resulting cathode material. Of course, besides strong acid or strong alkali solutions, other types of compounds with etching properties exist in the art, such as CF4 and SF6. However, these gases are highly toxic and easily decompose, posing irritation and potential explosion risks in enclosed environments, and are therefore not suitable for the preparation method provided in this invention.

[0029] In summary, the preparation method provided by this invention, through multi-step material microstructure design and processing, constructs a lithium manganese iron phosphate composite cathode material composed of a doped mixed phosphate core, a first carbon material framework, a first carbon layer, a nanoporous layer, and an outer second carbon layer. The unique microstructure designed and prepared effectively solves the common problems of manganese ion dissolution and voltage decay during the cycling process of lithium-ion battery cathode materials. The final prepared lithium manganese iron phosphate composite cathode material exhibits higher energy density, longer cycle life, and superior charge-discharge performance.

[0030] In some typical embodiments, in order to obtain a mixed phosphate core with a more ideal crystal structure, and thus to achieve subsequent steps and obtain a cathode material with excellent electrochemical performance, the molar ratio of lithium source, iron source, manganese source, phosphorus source and dopant source in step S1 is preferably 1:(0.1~0.5):(0.5~0.9):1:(0~0.08). Furthermore, in order to form a more uniform first carbon layer, and thus to more effectively achieve the composite of the obtained first product and the one-dimensional carbon material, resulting in a more stable structure and longer service life of the lithium manganese iron phosphate composite cathode material, the amount of the first organic carbon source is preferably 8%~10% based on the total weight of the lithium source, iron source, manganese source, phosphorus source and dopant source being 100%.

[0031] Furthermore, to facilitate the subsequent adhesion of one-dimensional carbon materials to its surface and shorten the lithium-ion diffusion path, thereby comprehensively improving the electrochemical performance of the resulting lithium manganese iron phosphate composite cathode material, the D50 of the first product is preferably 3 μm to 6 μm. The preferred doping element source is selected from one or more of Mg, Sc, Ti, V, Co, and Mo, which can specifically improve the electronic structure and electrochemical performance of the resulting mixed phosphate core, reduce structural changes during cycling, suppress Mn ion dissolution and voltage decay, and ultimately result in the resulting lithium manganese iron phosphate composite cathode material exhibiting higher energy density and longer cycle life. Additionally, during the formation of the first product, to form a more perfect crystal structure and reduce impurities introduced by side reactions, thereby improving the purity and cycle life of the resulting lithium manganese iron phosphate composite cathode material, the preferred first calcination temperature is 680℃ to 730℃.

[0032] Regarding the selection of dopant elements, in some more typical implementations, in order to make the dopant elements more compatible with the crystal structure of the mixed phosphate, and thus enable the cathode material to achieve better electrochemical performance, the inventors, through extensive experiments, preferred Mg and Ti as the dopant element sources, with a molar ratio of Mg to Ti of (1.0 to 1.5):1; or, Mg and Co as the dopant element sources, with a molar ratio of Mg to Co of (1.0 to 1.5):1.

[0033] In some typical embodiments, in order to form a more stable conductive network, enhance the mechanical properties of the resulting second product, and reduce the low content of electrochemically active substances due to excessive one-dimensional carbon material, it is preferable that in step S2, the amount of one-dimensional carbon material is 0.5% to 1.0% based on 100% of the total weight of the first product. One-dimensional carbon material within this range can adhere more uniformly and efficiently to the surface of the first product and provide more stable support for the subsequently formed nanoporous layer, thereby promoting the acquisition of a lithium manganese iron phosphate composite cathode material with higher cycle stability. Regarding the type of one-dimensional carbon material, it is preferably selected from one or more of carbon nanotubes, carbon nanowires, and carbon fibers. And to more effectively alleviate the stress caused by volume changes during application cycling of the resulting lithium manganese iron phosphate composite cathode material, thereby improving its cycle life, carbon nanotubes are more preferably used.

[0034] Furthermore, regarding the process of one-dimensional carbon material adhering to the surface of the first product, the inventors, through extensive experimentation, optimized step S2 to include: Step S2-1, adding the first product and one-dimensional carbon material to a solvent to obtain a dispersion; ultrasonically dispersing the dispersion at 10kHz to 50kHz for 0.5h to 2.0h to obtain a second slurry; Step S2-2, drying the second slurry by forced air drying at 100℃ to 150℃ to obtain the second product. Preferably, the ultrasonic frequency for ultrasonic dispersion is 20±2kHz; and / or, the temperature for forced air drying is 120±5℃.

[0035] The preferred and even more preferred ultrasonic frequencies described above can more effectively break up the agglomeration of the first product and one-dimensional carbon material in the dispersion system, promoting the uniform distribution of the one-dimensional carbon material on the surface of the first product. This results in the formation of a more stable conductive network with stronger electron transport capabilities, providing more significant support for the subsequently formed nanoporous layer, and ultimately improving the cycle stability of the lithium manganese iron phosphate composite cathode material. Furthermore, the preferred and even more preferred forced-air drying temperatures described above can effectively remove residual solvent while improving the processing performance of the resulting second product, enabling it to achieve good composite with the SiO2 coating layer during subsequent atomic layer deposition, reducing structural defects, and ultimately yielding a lithium manganese iron phosphate composite cathode material with superior electrochemical performance.

[0036] In some typical embodiments, in order to effectively form an optimized nanoporous structure while protecting the mixed phosphate core and the first carbon layer on its surface from hydrofluoric acid corrosion, thereby improving the overall electrochemical performance of the resulting lithium manganese iron phosphate composite cathode material, in step S3, the thickness of the SiO2 coating layer deposited by atomic layer deposition is preferably 30 nm to 50 nm. During the atomic layer deposition process, in order to deposit a more uniform SiO2 coating layer with fewer defects, so as to form a more stable nanoporous layer during subsequent etching and improve the cycle stability of the lithium manganese iron phosphate composite cathode material, the atomic layer deposition preferably includes: placing the second product in an atomic layer deposition reaction chamber, using tris(dimethylamino)silane and O3 as precursors, and N2 as a purification gas, performing surface deposition at 200±20°C, and forming a SiO2 coating layer on the surface of the second product.

[0037] In step S4, the second organic carbon source is first coated onto the surface of the third product, and then a second conductive carbon layer is formed on its surface through a second calcination. To improve the material's conductivity while more effectively stabilizing the structure and synergistically forming a nanoporous layer with the one-dimensional carbon material and the first carbon layer to improve electrolyte isolation, the amount of the second organic carbon source is preferably 3.0% to 5.0% based on 100% of the total weight of the third product. Regarding the second calcination, the preferred temperature is 600°C to 700°C, and the time is 2 to 4 hours. These conditions promote more complete carbonization of the second organic carbon source, forming a dense and uniform carbon coating layer, facilitating uniform subsequent etching. Simultaneously, the above-mentioned conditions and their optimization reduce material structure damage or excessive graphitization of the carbon layer caused by excessively high temperatures, ultimately effectively improving the overall performance of the obtained lithium manganese iron phosphate composite cathode material.

[0038] In the etching process of step S5, the etching solution is preferably a hydrofluoric acid solution or a sodium hydroxide solution, and the concentration of the etching solution is 0.05 mol / L to 0.5 mol / L. This allows for the selective removal of most of the SiO2 layer, forming a nanoporous layer. Especially, a concentration of 0.05 mol / L to 0.5 mol / L allows for more precise control of the etching rate and extent on the SiO2 coating layer, thereby optimizing the structure of the formed nanoporous layer. This enables it to more effectively isolate the electrolyte from the mixed phosphate core, improving the long-cycle performance of the lithium manganese iron phosphate composite cathode material. Furthermore, to improve the structural integrity and electrochemical performance of the resulting lithium manganese iron phosphate composite cathode material, the immersion time is preferably 1 min to 30 min.

[0039] Regarding the selection of etching solution and etching conditions, the inventors, through extensive experiments, have found that in some more typical embodiments, the preferred etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L; and / or, the immersion time is 5 min to 15 min. Hydrofluoric acid solutions within the above concentration range, combined with etching within this time range, compared to other types of etching solutions such as sodium hydroxide (which reacts with SiO2 to form sodium silicate, leading to byproducts that are difficult to remove and ultimately failing to form a structurally stable and impurity-free nanoporous layer), can more effectively etch the SiO2 coating layer while protecting the core material, forming a structurally stable and well-insulating nanoporous layer. More importantly, the etching conditions mentioned above, including specific etching solutions and etching times, can effectively remove the SiO2 coating layer while significantly inhibiting excessive corrosion of the material surface, reducing the introduction of impurities, and ultimately improving the long-cycle stability of the obtained lithium manganese iron phosphate composite cathode material.

[0040] Furthermore, the lithium source, iron source, manganese source, and dopant source are each independently added in one or more forms selected from nitrates, phosphates, sulfates, carbon source salts, acetates, and oxides; and / or, the phosphorus source is selected from one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid; and / or, the first organic carbon source and the second organic carbon source are each independently selected from one or more of glucose, quinoline-soluble substances in coal tar pitch, polyethylene glycol, polyvinyl alcohol, citric acid, and dopamine. Theoretically, the raw materials involved in the above preparation method can all be of types commonly used in the art. However, the specific types of raw materials selected by the inventors through extensive experiments can more effectively synergize, thereby obtaining a more structurally stable lithium manganese iron phosphate composite cathode material with superior electrochemical performance.

[0041] A second aspect of the present invention provides a lithium manganese iron phosphate composite cathode material, which is prepared by the above-described method for preparing lithium manganese iron phosphate composite cathode materials. The lithium manganese iron phosphate composite cathode material includes a mixed phosphate core, and the molecular formula of the mixed phosphate core is LiMn. x Fe 1-x-y T yPO4, where x is 0.5–0.9, y is 0.001–0.08, and T is selected from one or more of Mg, Sc, Ti, V, Co, and Mo. Furthermore, the structure of the obtained lithium manganese iron phosphate composite cathode material also includes a first carbon layer coating the surface of the mixed phosphate core, a nanoporous layer formed by etching the SiO2 coating layer, a second carbon layer formed by a second organic carbon source, and a one-dimensional carbon material interspersed between the three layers to provide support. When applied, the three coating layers and the interspersed framework can significantly isolate the central mixed phosphate core from the electrolyte without affecting electron transport and mass transfer, thereby effectively avoiding possible manganese dissolution and ultimately resulting in the lithium manganese iron phosphate composite cathode material exhibiting particularly excellent electrochemical performance, especially cycle stability.

[0042] It should be noted that, due to the special nature of the materials field and the limitations of existing testing and characterization methods, it is difficult to perform a comprehensive quantitative characterization of the complex microstructure of the lithium manganese iron phosphate composite cathode material obtained above. However, the performance test results show that the lithium manganese iron phosphate composite cathode material obtained in this application has better electrochemical performance and can significantly improve the various performance characteristics of the lithium-ion battery in which it is located.

[0043] A third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode, a positive electrode sheet, and an electrolyte, wherein the positive electrode comprises the aforementioned lithium manganese iron phosphate composite positive electrode material. Because the positive electrode material provided by the present invention possesses excellent stability and electrochemical performance, when applied to a lithium-ion battery, the resulting lithium-ion battery exhibits higher specific capacity and superior long-cycle stability.

[0044] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0045] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0046] Example 1

[0047] A method for preparing a lithium iron phosphate composite cathode material:

[0048] (1) Lithium carbonate (lithium source), iron phosphate (iron source), manganese carbonate (manganese source), lithium dihydrogen phosphate (phosphorus source), magnesium oxide, and titanium dioxide (doping element source) were weighed according to the molar ratio of Li:Fe:Mn:P:Mg:Ti = 1:0.4:0.55:1:0.03:0.02. Then, glucose was weighed as the first organic carbon, accounting for 8% of the total weight of the above raw materials. The above materials were mixed and ground using a bead mill to control the slurry particle size D50 = 0.4 μm. Afterwards, pelletizing was performed by spray drying, followed by a first calcination under a nitrogen atmosphere at 700℃ for 9 hours. Finally, a doped lithium manganese iron phosphate material with a D50 = 5 μm was obtained, which is the first product.

[0049] (2) According to the weight ratio of the obtained first product powder to carbon nanotubes (CNTs) = 100:1 (i.e., the amount of one-dimensional carbon material is 1.0% based on the total weight of the first product being 100%), CNTs were weighed as one-dimensional carbon material. The first product powder and CNTs were mixed using ethanol as a solvent to obtain a dispersion. The dispersion was ultrasonically dispersed at 20 kHz for 1 h to obtain a homogeneous second slurry. The dispersed second slurry was then dried in a forced-air oven at 120 °C to obtain the second product.

[0050] (3) By using atomic layer deposition technology, that is: the obtained second product is placed into the atomic layer deposition reaction chamber, and tris(dimethylamino)silane and O3 are used as precursors and N2 is used as purification gas. SiO2 is deposited on the surface at 200°C to obtain a 40nm thick SiO2 coating layer on the surface of the second product, and then the third product is obtained.

[0051] (4) Using quinoline-soluble substances in coal tar pitch as the second organic carbon source and ethanol as the solvent, the third product powder with the SiO2 coating and the second organic carbon source were mixed and dispersed in ethanol at a weight ratio of third product:second organic carbon source = 100:3 (i.e., the amount of second organic carbon source is 3.0% based on the total weight of the third product as 100%) to obtain a third slurry. The third slurry was heated until the solvent was completely evaporated, and the resulting powder was subjected to a second calcination at 650°C under a nitrogen atmosphere for 3 hours to obtain the fourth product.

[0052] (5) A 0.1 mol / L hydrofluoric acid solution was used as the etching solution, and the obtained fourth product was immersed in it for 15 minutes. During this process, the SiO2 coating layer formed in step (3) was etched by HF and became a nano-pore layer. After centrifugation, washing, filtration and drying, the lithium manganese iron phosphate composite cathode material was obtained.

[0053] The resulting material includes a mixed phosphate core, namely lithium iron manganese phosphate doped with Mg and Ti, with the molecular formula LiMn.0.55 Fe 0.4 Mg 0.03 Ti 0.02 PO4. Meanwhile, the structure of the obtained lithium manganese iron phosphate composite cathode material also includes a first carbon layer coated on the surface of the mixed phosphate core, a nanopore layer formed by etching the SiO2 coating layer, a second carbon layer formed by the second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers and playing a supporting role.

[0054] Example 2

[0055] A method for preparing a lithium iron phosphate composite cathode material:

[0056] (1) Lithium carbonate (lithium source), iron phosphate (iron source), manganese carbonate (manganese source), lithium dihydrogen phosphate (phosphorus source), magnesium oxide, and titanium dioxide (doping element source) were weighed according to the molar ratio of Li:Fe:Mn:P:Mg:Ti = 1:0.4:0.58:1:0.01:0.01. Then, glucose was weighed as the first organic carbon, accounting for 9% of the total weight of the above raw materials. The above materials were mixed and ground using a bead mill to control the slurry particle size D50 = 0.3 μm. Afterwards, pelletizing was performed by spray drying, followed by a first calcination under a nitrogen atmosphere at 680℃ for 10 h. Finally, a doped lithium manganese iron phosphate material with D50 = 3 μm was obtained, which is the first product.

[0057] (2) According to the weight ratio of the obtained first product powder to carbon nanotubes (CNTs) = 100:0.5 (i.e., the amount of one-dimensional carbon material is 0.5% based on the total weight of the first product as 100%), CNTs were weighed as one-dimensional carbon material. The first product powder and CNTs were mixed with ethanol as solvent to obtain a dispersion. The dispersion was ultrasonically dispersed at 20 kHz for 1 h to obtain a homogeneous second slurry. The dispersed second slurry was then dried in a forced-air oven at 120 °C to obtain the second product.

[0058] (3) By using atomic layer deposition technology, the obtained second product is placed into an atomic layer deposition reaction chamber, and tris(dimethylamino)silane and O3 are used as precursors and N2 is used as purification gas. SiO2 is deposited on the surface at 200°C to obtain a 50nm thick SiO2 coating layer on the surface of the second product, and then the third product is obtained.

[0059] (4) Using quinoline-soluble substances in coal tar pitch as the second organic carbon source and ethanol as the solvent, the third product powder with the SiO2 coating and the second organic carbon source were mixed and dispersed in ethanol at a weight ratio of third product:second organic carbon source = 100:3 (i.e., the amount of second organic carbon source is 3.0% based on the total weight of the third product as 100%) to obtain a third slurry. The third slurry was heated until the solvent was completely evaporated, and the resulting powder was subjected to a second calcination at 700°C under a nitrogen atmosphere for 2 hours to obtain the fourth product.

[0060] (5) A 0.1 mol / L hydrofluoric acid solution was used as the etching solution, and the obtained fourth product was immersed in it for 15 minutes. During this process, the SiO2 coating layer formed in step (3) was etched by HF and became a nano-pore layer. After centrifugation, washing, filtration and drying, the lithium manganese iron phosphate composite cathode material was obtained.

[0061] The resulting lithium iron phosphate composite cathode material includes a mixed phosphate core, namely lithium iron phosphate doped with Mg and Ti, with the molecular formula LiMn. 0.58 Fe 0.4 Mg 0.01 Ti 0.01 PO4. Meanwhile, the structure of the obtained lithium manganese iron phosphate composite cathode material also includes a first carbon layer coated on the surface of the mixed phosphate core, a nanopore layer formed by etching the SiO2 coating layer, a second carbon layer formed by the second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers and playing a supporting role.

[0062] Example 3

[0063] A method for preparing a lithium iron phosphate composite cathode material:

[0064] (1) Lithium carbonate (lithium source), iron phosphate (iron source), manganese carbonate (manganese source), lithium dihydrogen phosphate (phosphorus source), magnesium oxide, and cobalt nitrate (doping element source) were weighed according to the molar ratio of Li:Fe:Mn:P:Mg:Co = 1:0.4:0.55:1:0.03:0.02. Then, glucose was weighed as the first organic carbon, accounting for 10% of the total weight of the above raw materials. The above materials were mixed and ground using a bead mill to control the slurry particle size D50 = 0.3 μm. Afterwards, pelletizing was performed by spray drying, followed by a first calcination under a nitrogen atmosphere at 730℃ for 6 hours. Finally, a doped lithium manganese iron phosphate material with a D50 = 6 μm was obtained, which is the first product.

[0065] (2) According to the weight ratio of the obtained first product powder to carbon nanotubes (CNTs) = 100:0.8 (i.e., the amount of one-dimensional carbon material is 0.8% based on the total weight of the first product as 100%), CNTs were weighed as one-dimensional carbon material. The first product powder and CNTs were mixed with ethanol as solvent to obtain a dispersion. The dispersion was ultrasonically dispersed at 20 kHz for 1 h to obtain a homogeneous second slurry. The dispersed second slurry was then dried in a forced-air oven at 120 °C to obtain the second product.

[0066] (3) By using atomic layer deposition technology, the obtained second product is placed in an atomic layer deposition reaction chamber, and tris(dimethylamino)silane and O3 are used as precursors and N2 is used as purification gas. SiO2 is deposited on the surface at 200°C to obtain a 30nm thick SiO2 coating layer on the surface of the second product, and then the third product is obtained.

[0067] (4) Using quinoline-soluble substances in coal tar pitch as the second organic carbon source and ethanol as the solvent, the third product powder with the SiO2 coating and the second organic carbon source were mixed and dispersed in ethanol at a weight ratio of third product:second organic carbon source = 100:4 (i.e., the amount of second organic carbon source is 4.0% based on the total weight of the third product as 100%) to obtain a third slurry. The third slurry was heated until the solvent was completely evaporated, and the resulting powder was subjected to a second calcination at 600°C under a nitrogen atmosphere for 4 hours to obtain the fourth product.

[0068] (5) A 0.2 mol / L hydrofluoric acid solution was used as the etching solution, and the obtained fourth product was immersed in it for 5 minutes. During this process, the SiO2 coating layer formed in step (3) was etched by HF and became a nano-pore layer. After centrifugation, washing, filtration and drying, the lithium manganese iron phosphate composite cathode material was obtained.

[0069] The resulting lithium iron phosphate composite cathode material includes a mixed phosphate core, namely lithium iron phosphate doped with Mg and Co, with the molecular formula LiMn. 0.55 Fe 0.4 Mg 0.03 Co 0.02 PO4. Meanwhile, the structure of the obtained lithium manganese iron phosphate composite cathode material also includes a first carbon layer coated on the surface of the mixed phosphate core, a nanopore layer formed by etching the SiO2 coating layer, a second carbon layer formed by the second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers and playing a supporting role.

[0070] Example 4

[0071] A method for preparing a lithium iron phosphate composite cathode material:

[0072] The only difference between this embodiment and Embodiment 1 is that, in step (4), quinoline-soluble matter from coal tar pitch is added according to a weight ratio of third product: second organic carbon source = 100:5. That is, the amount of second organic carbon source is changed to 5.0% based on the total weight of the third product being 100%.

[0073] Example 5

[0074] A method for preparing a lithium iron phosphate composite cathode material:

[0075] The only difference between this embodiment and Embodiment 1 is that, in step (5), the soaking time of the fourth product in the hydrofluoric acid solution is changed to 7 minutes.

[0076] Example 6

[0077] A method for preparing a lithium iron phosphate composite cathode material:

[0078] The only difference between this embodiment and Embodiment 1 is that, in step (1), the temperature of the first calcination is changed to 600°C, and the D50 of the resulting first product is 2μm.

[0079] Example 7

[0080] A method for preparing a lithium iron phosphate composite cathode material:

[0081] The only difference between this embodiment and Embodiment 1 is that, in step (1), the temperature of the first calcination is changed to 750°C, and the D50 of the obtained first product is changed to 8μm.

[0082] Example 8

[0083] A method for preparing a lithium iron phosphate composite cathode material:

[0084] The only difference between this embodiment and Embodiment 1 is that, in step (1), CNTs are weighed as one-dimensional carbon materials according to the weight ratio of the obtained first product powder to carbon nanotubes (CNTs) = 100:0.2.

[0085] That is, based on the total weight of the first product being 100%, the amount of one-dimensional carbon material used is changed to 0.2%.

[0086] Example 9

[0087] A method for preparing a lithium iron phosphate composite cathode material:

[0088] The only difference between this embodiment and Embodiment 1 is that, in step (1), CNTs are weighed as one-dimensional carbon materials according to the weight ratio of the obtained first product powder to carbon nanotubes (CNTs) = 100:1.5.

[0089] That is, based on the total weight of the first product being 100%, the amount of one-dimensional carbon material used is changed to 1.5%.

[0090] Example 10

[0091] A method for preparing a lithium iron phosphate composite cathode material:

[0092] The only difference between this embodiment and embodiment 1 is that the ultrasonic dispersion frequency in step (2) is changed to 10kHz and the time is changed to 2h.

[0093] Example 11

[0094] A method for preparing a lithium iron phosphate composite cathode material:

[0095] The only difference between this embodiment and Embodiment 1 is that the temperature of the blower drying in step (2) is changed to 150°C.

[0096] Example 12

[0097] A method for preparing a lithium iron phosphate composite cathode material:

[0098] The only difference between this embodiment and Embodiment 1 is that, in step (3), the thickness of the SiO2 coating obtained by atomic layer deposition is changed to 20 nm.

[0099] Example 13

[0100] A method for preparing a lithium iron phosphate composite cathode material:

[0101] The only difference between this embodiment and Embodiment 1 is that, in step (3), the thickness of the SiO2 coating obtained by atomic layer deposition is changed to 60 nm.

[0102] Example 14

[0103] A method for preparing a lithium iron phosphate composite cathode material:

[0104] The only difference between this embodiment and embodiment 1 is step (4), as follows.

[0105] (4) Using quinoline-soluble substances in coal tar pitch as the second organic carbon source and ethanol as the solvent, the third product powder with the SiO2 coating and the second organic carbon source were mixed and dispersed in ethanol at a weight ratio of third product:second organic carbon source = 100:1 (i.e., the amount of second organic carbon source is 1.0% based on the total weight of the third product being 100%) to obtain a third slurry. The third slurry was heated until the solvent was completely evaporated, and the resulting powder was subjected to a second calcination at 500°C under a nitrogen atmosphere for 3 hours to obtain the fourth product.

[0106] Example 15

[0107] A method for preparing a lithium iron phosphate composite cathode material:

[0108] The only difference between this embodiment and embodiment 1 is step (4), as follows.

[0109] (4) Using quinoline-soluble substances in coal tar pitch as the second organic carbon source and ethanol as the solvent, the third product powder with the SiO2 coating and the second organic carbon source were mixed and dispersed in ethanol at a weight ratio of third product:second organic carbon source = 100:10 (i.e., the amount of second organic carbon source is 10.0% based on the total weight of the third product being 100%) to obtain a third slurry. The third slurry was heated until the solvent was completely evaporated, and the resulting powder was subjected to a second calcination at 800°C under a nitrogen atmosphere for 3 hours to obtain the fourth product.

[0110] Example 16

[0111] A method for preparing a lithium iron phosphate composite cathode material:

[0112] The only difference between this embodiment and Embodiment 1 is that the concentration of the hydrofluoric acid solution used in step (5) is changed to 0.05 mol / L, and the soaking time is changed to 30 min.

[0113] Example 17

[0114] A method for preparing a lithium iron phosphate composite cathode material:

[0115] The only difference between this embodiment and Embodiment 1 is that the concentration of the hydrofluoric acid solution used in step (5) is changed to 0.5 mol / L, and the soaking time is changed to 1 min.

[0116] Example 18

[0117] A method for preparing a lithium iron phosphate composite cathode material:

[0118] The only difference between this embodiment and Embodiment 1 is that a sodium hydroxide solution of equal concentration is used instead of a hydrofluoric acid solution as the etching solution.

[0119] Comparative Example 1

[0120] A method for preparing a lithium iron phosphate composite cathode material:

[0121] The only difference between this comparative example and Example 1 is that steps (2) to (5) were not performed, and the first product obtained in step (1) was directly used as the final lithium manganese iron phosphate composite cathode material.

[0122] Comparative Example 2

[0123] A method for preparing a lithium iron phosphate composite cathode material:

[0124] The only difference between this comparative example and Example 1 is that, in step (2), an equal weight of citric acid is used instead of the one-dimensional carbon material CNT.

[0125] Comparative Example 3

[0126] A method for preparing a lithium iron phosphate composite cathode material:

[0127] The only difference between this comparative example and Example 1 is that, in step (3), a TiO2 coating layer of equal thickness is formed to replace the SiO2 coating layer by atomic layer deposition technology.

[0128] Comparative Example 4

[0129] A method for preparing a lithium iron phosphate composite cathode material:

[0130] The only difference between this comparative example and Example 1 is that step (4) was not performed, but the third product obtained in step (3) was immersed in hydrofluoric acid solution to etch the outermost SiO2 coating layer.

[0131] Comparative Example 5

[0132] A method for preparing a lithium iron phosphate composite cathode material:

[0133] The only difference between this comparative example and Example 1 is that step (5) was not performed, and the fourth product obtained in step (4) was directly used as the final lithium manganese iron phosphate composite cathode material.

[0134] That is, no etching was performed, and the nanoporous layer formed by etching the SiO2 coating layer failed to form. The resulting composite material includes a mixed phosphate core, on which a first carbon layer formed by a first organic carbon source, a SiO2 coating layer, a second carbon layer formed by a second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers are sequentially coated.

[0135] Battery Sample Preparation and Testing Methods

[0136] Battery Sample Preparation: The positive electrode materials obtained in each example and comparative example were mixed evenly in an agate mortar according to the amounts of active material (80 wt.%), carbon black (Super-P, 10 wt.%), and polyvinylidene fluoride (PDVF, 10 wt.%). The resulting mixture was dispersed in N-methylpyrrolidone (NMP) to form a uniform slurry. Subsequently, the slurry was uniformly coated onto aluminum foil using a coating machine, and the coated aluminum foil was dried in a vacuum oven at 120°C for 12 hours. The dried electrode sheets were cut into small round pieces with a diameter of 14 mm, which were then used as positive electrode sheets. Afterward, the battery was assembled and sealed in the following order: negative electrode shell, lithium sheet, electrolyte, separator, electrolyte, positive electrode sheet, and positive electrode shell.

[0137] Battery performance testing: Charge-discharge performance tests were conducted on the battery samples obtained above at 25℃, with a voltage range of 2.0V to 4.5V. This yielded the following results: initial discharge specific capacity at 0.2C; initial discharge specific capacity at 1C; average discharge voltage during the first discharge at 1C (obtained by the Blue Electric testing system, representing the average voltage of a battery sample throughout the discharge process); discharge specific capacity after 200 cycles at 1C; average discharge voltage after 200 cycles at 1C; and capacity retention rate after 200 cycles at 1C. The average voltage decay rate was calculated as follows: (1 - V... 循环200圈后 / V 首圈 ()×100%, the lower this value, the better the cycle stability of the battery sample.

[0138] The results of the above tests are shown in Table 1.

[0139] Table 1

[0140]

[0141]

[0142] As can be seen from the above description, the embodiments of the present invention have achieved the preparation of a composite material with excellent electrochemical performance. When the obtained composite material is used as a positive electrode material in a lithium-ion battery, the corresponding lithium-ion battery exhibits excellent electrical performance, especially higher discharge specific capacity and cycle stability. Specifically:

[0143] By comparing Examples 1 to 5 with Examples 6 and 7, it can be seen that the preferred first calcination temperature and the D50 of the first product can form a more perfect crystal form, reduce impurities introduced by side reactions, facilitate the subsequent adhesion of one-dimensional carbon materials on its surface, shorten the lithium ion diffusion path, and ultimately further improve the purity and cycle life of the obtained lithium manganese iron phosphate composite cathode material.

[0144] By comparing Examples 1 to 5 with Examples 8 and 9, it can be seen that the preferred amount of one-dimensional carbon material is 0.5% to 1.0%, which enables it to adhere more uniformly and efficiently to the surface of the first product and provides a more stable support for the nano-pore layer formed subsequently, thereby obtaining a lithium manganese iron phosphate composite cathode material with higher cycle stability.

[0145] By comparing Examples 1 to 5 with Examples 10 and 11, it can be seen that by optimizing the conditions for the attachment of one-dimensional carbon material to the surface of the first product, the one-dimensional carbon material can be uniformly distributed on the surface of the first product, while reducing structural defects, forming a conductive network with better performance, and further improving the cycle stability of the corresponding battery samples.

[0146] By comparing Examples 1 to 5 with Examples 12 and 13, it can be seen that when the thickness of the SiO2 coating layer formed in the intermediate step is preferably 30 nm to 50 nm, it can effectively form an optimized nanoporous structure while better protecting the mixed phosphate core and the first carbon layer on its surface from hydrofluoric acid corrosion, thereby improving the overall electrochemical performance of the obtained lithium manganese iron phosphate composite cathode material.

[0147] By comparing Examples 1 to 5 with Examples 14 and 15, it can be seen that by optimizing the amount of the second organic carbon source and the temperature conditions of the second calcination, the second organic carbon source can be more fully carbonized, forming a dense and uniform carbon coating layer, which facilitates the uniform occurrence of the subsequent etching process and ultimately effectively improves the electrochemical performance of the obtained lithium manganese iron phosphate composite cathode material.

[0148] Comparing Examples 1 to 5 with Examples 16 and 17, it is evident that, regarding the etching process, when the etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L, and the immersion time is 5 min to 15 min, it can effectively remove the SiO2 coating layer while significantly inhibiting excessive corrosion of the material surface and reducing the introduction of impurities, thereby more effectively improving the electrochemical performance of the obtained lithium manganese iron phosphate composite cathode material. Example 18 demonstrates that, compared to other types of etching solutions such as sodium hydroxide, a suitable concentration of hydrofluoric acid solution can more effectively etch the SiO2 coating layer while protecting the core material, forming a structurally stable and well-insulating nanoporous layer, and ultimately significantly improving the long-cycle stability of the obtained lithium manganese iron phosphate composite cathode material.

[0149] Based on this, Comparative Examples 1 to 5 show that the preparation method provided by the present invention, through multi-step material microstructure design and processing, constructs a lithium manganese iron phosphate composite cathode material composed of a doped mixed phosphate core, a first carbon material framework, a first carbon layer, a nano-void layer, and an outer second carbon layer. This material can effectively solve the common problems of manganese ion dissolution and voltage decay during the cycling process of lithium-ion battery cathode materials. The final prepared lithium manganese iron phosphate composite cathode material has higher energy density, longer cycle life, and better charge and discharge performance. In particular, after 200 cycles, the voltage decay is significantly improved.

[0150] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0151] The above description is merely a preferred embodiment of the present invention and is 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 a lithium iron manganese phosphate composite cathode material, characterized in that, The preparation method of the lithium iron manganese phosphate composite positive electrode material comprises the following steps: In step S1, a lithium source, an iron source, a manganese source, a phosphorus source, a doping element source and a first organic carbon source are prepared into a first slurry, and the first slurry is calcined at a temperature of 680-730 DEG C to obtain a first product; In step S2, the first product and a one-dimensional carbon material are prepared into a second slurry, and the second slurry is dried to obtain a second product; the amount of the one-dimensional carbon material is 0.5-1.0% based on the total weight of the first product; In step S3, a SiO2 coating layer is formed on the surface of the second product by atomic layer deposition to obtain a third product; the thickness of the SiO2 coating layer is 30-50 nm; In step S4, the third product and a second organic carbon source are prepared into a third slurry, and the third slurry is calcined at a temperature of 600-700 DEG C for 2-4 hours to obtain a fourth product; the amount of the second organic carbon source is 3.0-5.0% based on the total weight of the third product; In step S5, the fourth product is placed in an etching solution to obtain the lithium iron manganese phosphate composite positive electrode material after soaking; The etching solution is a strong acid solution or a strong alkali solution.

2. The method for preparing the lithium manganese iron phosphate composite cathode material according to claim 1, characterized in that, In step S1, The molar ratio of the lithium source, the iron source, the manganese source, the phosphorus source and the doping element source is 1:(0.1-0.5):(0.5-0.9):1:(0-0.08); and / or The amount of the first organic carbon source is 8-10% based on the total weight of the lithium source, the iron source, the manganese source, the phosphorus source and the doping element source; and / or The D50 of the first product is 3-6 microns; and / or The doping element source is selected from one or more of Mg, Sc, Ti, V, Co and Mo.

3. The method for preparing the lithium iron phosphate composite cathode material according to claim 2, characterized in that, In step S2, The one-dimensional carbon material is selected from one or more of carbon nanotubes, carbon nanowires and carbon fibers.

4. The method for preparing the lithium iron phosphate composite cathode material according to claim 3, characterized in that, In step S2, the one-dimensional carbon material is carbon nanotubes.

5. The method of producing a lithium iron manganese phosphate composite cathode material according to any one of claims 1 to 4, characterized in that, In step S2, In step S2-1, the first product and the one-dimensional carbon material are added to a solvent to obtain a dispersion liquid, and the dispersion liquid is ultrasonically dispersed at 10-50 kHz for 0.5-2.0 hours to obtain the second slurry; In step S2-2, the second slurry is subjected to air blowing drying at 100-150 DEG C to obtain the second product.

6. The method of producing a lithium iron manganese phosphate composite cathode material according to any one of claims 1 to 4, characterized in that, In step S3, the atomic layer deposition comprises: placing the second product in an atomic layer deposition reaction chamber, using tris(dimethylamino)silane and O3 as precursors, N2 as a purging gas, performing surface deposition at 200±20 DEG C, and forming the SiO2 coating layer on the surface of the second product.

7. The method of producing a lithium iron manganese phosphate composite cathode material according to any one of claims 1 to 4, characterized in that, In step S5, the etching solution is a hydrofluoric acid solution or a sodium hydroxide solution, and the concentration of the etching solution is 0.05-0.5 mol / L; and / or the soaking time is 1-30 minutes.

8. The method of claim 7, wherein the lithium iron manganese phosphate composite cathode material is prepared by the steps of: mixing a lithium source, a manganese source, an iron source, and a phosphorus source; and heating the mixture to a temperature of 600-800°C for 6-24 hours. The etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L; and / or the soaking time is 5 min to 15 min. 9.The method of any one of claims 1 to 4, wherein, the lithium source, the iron source, the manganese source, and the doping element source are each independently added in the form of one or more of a nitrate, a phosphate, a sulfate, an acetate, and an oxide; and / or the phosphorus source is selected from one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid; and / or the first organic carbon source and the second organic carbon source are each independently selected from one or more of glucose, quinoline-soluble substances in coal tar pitch, polyethylene glycol, polyvinyl alcohol, citric acid, and dopamine.

10. A lithium iron manganese phosphate composite cathode material, characterized in that, The lithium manganese iron phosphate composite cathode material is prepared by the method for preparing the lithium manganese iron phosphate composite cathode material according to any one of claims 1 to 9, the lithium manganese iron phosphate composite cathode material comprises a mixed phosphate inner core, and a molecular formula of the mixed phosphate inner core is LiMn x Fe 1-x-y T y PO4, wherein x is 0.5 to 0.9, y is 0.001 to 0.08, and T is selected from one or more of Mg, Sc, Ti, V, Co and Mo.

11. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, characterized by The positive electrode plate comprises the lithium iron manganese phosphate composite positive electrode material of claim 10.

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