Lithium manganese iron phosphate positive electrode material and surface lithiation compensation process thereof

Abstract

The application provides a lithium manganese iron phosphate positive electrode material and a surface lithiumation compensation process thereof, and belongs to the technical field of new energy batteries. The compensation process comprises the following steps: providing a lithium manganese iron phosphate base material; constructing an interface stabilization layer on the surface of the base material, wherein the interface stabilization layer is a rare earth oxide layer; and constructing a composite functional layer on the interface stabilization layer, wherein the composite functional layer comprises carbon material and composite fluoride, and the composite functional layer simultaneously provides stress buffering and lithium ion conduction functions. The lithium ion battery prepared from the lithium manganese iron phosphate positive electrode material has a significant improvement in cycle life and rate performance, can meet the demand of new energy vehicles and energy storage for high-performance batteries, and has a wide market application prospect.

Description

Technical Field

[0001] This application relates to the field of new energy battery technology, and in particular to a lithium manganese iron phosphate cathode material and its surface lithiation compensation process. Background Technology

[0002] Lithium manganese iron phosphate (LMFP), as an upgraded material of lithium iron phosphate, has become a research hotspot for next-generation lithium-ion battery cathode materials due to its high operating voltage and energy density (15-20% higher than lithium iron phosphate). However, in practical industrial applications, LMFP materials still face several key technical challenges: First, manganese dissolution occurs during charging and discharging, leading to loss of active material and structural degradation; second, Mn... 3+ The Jahn-Teller effect causes crystal structure distortion, leading to particle breakage and capacity decay during cycling. Finally, lithium vacancy defects are easily generated during high-temperature sintering, resulting in irreversible capacity loss and poor rate performance.

[0003] Existing technologies attempt to address the aforementioned problems through various approaches. Existing technologies publicly employ polydopamine coating technology, utilizing the coordination effect of its phenolic hydroxyl groups with metal ions to inhibit manganese dissolution; or use liquid-phase co-precipitation methods to achieve atomically uniform distribution of manganese and iron, improving the material's bulk structural stability; other studies have used surface iron concentration gradient design to construct an iron-rich layer on the surface of LMFP particles, effectively suppressing the Jahn-Teller effect. However, none of these methods fundamentally solve the problem of lithium vacancy compensation on the material surface, and most are complex processes with high production costs, making it difficult to meet the needs of large-scale industrialization. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a lithium manganese iron phosphate cathode material and a surface lithiation compensation process thereof.

[0005] Specifically, the first aspect of this application provides a lithiation compensation process for the surface of lithium manganese iron phosphate cathode material, including the following steps: Provide lithium iron phosphate substrate; An interface stabilizing layer is constructed on the surface of the substrate, wherein the interface stabilizing layer is a rare earth oxide layer. A composite functional layer is constructed on the interface stabilization layer. The composite functional layer contains carbon materials and composite fluorides, and the composite functional layer simultaneously provides stress buffering and lithium-ion conduction functions. The carbon material is at least one of carbon nanospheres and acetylene black; the composite fluoride is a mixture of LiF and AlF3, with a molar ratio of LiF to AlF3 of 2.5-3.5:1.

[0006] Furthermore, the method for constructing the interface stabilizing layer is as follows: the lithium manganese iron phosphate substrate is dispersed in a solution containing rare earth salts, the pH is controlled to allow rare earth ions to hydrolyze and deposit on the substrate surface, and then a rare earth oxide layer is formed by heat treatment.

[0007] Furthermore, the rare earth salt is at least one of cerium nitrate, yttrium nitrate, and cerium acetate; The heat treatment conditions are as follows: under an inert atmosphere, heat treatment is carried out at 400-500℃ for 3-5 hours.

[0008] Furthermore, the method for constructing the composite functional layer includes: mixing the carbon material and the composite fluoride with a lithium manganese iron phosphate substrate with an interface stabilization layer already constructed; attaching the carbon material and the composite fluoride to the surface of the interface stabilization layer by mechanical fusion; and then performing heat treatment under an inert atmosphere to melt and densify the composite fluoride to form the composite functional layer.

[0009] Furthermore, the mechanical fusion method is a low-speed dry mixing method with a rotation speed of 300-500 rpm and a mixing time of 10-30 minutes.

[0010] Furthermore, the inert atmosphere is argon; the heat treatment conditions are: holding at 450-500℃ for 2-4 hours, with a heating rate of 2-5℃ / min.

[0011] Furthermore, the mass ratio of the carbon material to the composite fluoride is (0.5-2):1, and the total mass of the composite functional layer accounts for 3-8% of the mass of the lithium manganese iron phosphate substrate.

[0012] A second aspect of this application provides a lithium manganese iron phosphate cathode material, wherein the surface of the lithium manganese iron phosphate cathode material particles comprises, from the inside to the outside, the following: Rare earth oxide interface stabilizing layer; A composite functional layer disposed on the interface stabilization layer, the composite functional layer comprising carbon materials and composite fluorides.

[0013] A third aspect of this application provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the aforementioned lithium manganese iron phosphate positive electrode material.

[0014] The present invention has the following beneficial effects: In this invention, the rare earth oxide interface stabilizing layer can significantly inhibit manganese leaching, reducing the loss of active materials and structural degradation. Rare earth elements possess unique electronic structures and chemical properties; their oxide layers can form stable chemical bonds with the surface of lithium manganese iron phosphate substrates, preventing the migration and leaching of manganese ions, thereby improving the structural stability and cycle performance of the material. Simultaneously, the rare earth oxide layer can also effectively mitigate Mn leaching. 3+The Jahn-Teller effect mitigates the distortion of the crystal structure during charge and discharge, reducing the risk of particle breakage. The carbon material in the composite functional layer possesses excellent conductivity and flexibility, enhancing the electron conduction rate of the electrode material and buffering the stress caused by volume changes during cycling, preventing structural cracking. The composite fluoride provides an additional lithium-ion source, compensating for lithium vacancy defects generated during high-temperature sintering or cycling, significantly improving lithium-ion conductivity, thereby enhancing the battery's rate performance and mitigating irreversible capacity loss. This synergistic effect of the dual-layer coating structure optimizes the overall performance of lithium manganese iron phosphate cathode materials from multiple aspects, including suppressing manganese dissolution, alleviating structural stress, compensating for lithium vacancies, and improving ion / electron conduction. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0016] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0017] The first aspect of this application provides a lithiation compensation process for the surface of a lithium iron phosphate cathode material, including the following steps: (1) Provide lithium manganese iron phosphate substrate; (2) An interface stabilizing layer is constructed on the surface of the substrate, wherein the interface stabilizing layer is a rare earth oxide layer; (3) A composite functional layer is constructed on the interface stabilization layer, the composite functional layer comprising carbon materials and composite fluorides, and the composite functional layer simultaneously provides stress buffering and lithium ion conduction functions. The carbon material is at least one of carbon nanospheres and acetylene black; the composite fluoride is a mixture of LiF and AlF3, with a molar ratio of LiF to AlF3 of 2.5-3.5:1.

[0018] In this embodiment, the lithium manganese iron phosphate substrate (LMFP substrate) is LiMn. 0.7 Fe 0.3 PO4 / C, the substrate underwent its first carbon coating.

[0019] In this embodiment, the method for constructing the interface stabilizing layer is as follows: the lithium manganese iron phosphate substrate is dispersed in a solution containing rare earth salts; the rare earth ions are hydrolyzed and deposited on the substrate surface by controlling the pH; and then, under an inert atmosphere, the solution is kept at 400-500°C for 3-5 hours to form a rare earth oxide layer. The rare earth salt is at least one of cerium nitrate, yttrium nitrate, and cerium acetate, preferably cerium nitrate.

[0020] In step (2), 1000 parts of LMFP substrate were dispersed in deionized water (solid-liquid ratio 1:7), 1 wt% citric acid was added, and the mixture was stirred and washed at 60°C for 1 hour. After centrifugation and washing until neutral, the mixture was vacuum dried at 110°C for 6 hours to obtain pre-activated LMFP substrate. Deionized water (solid-liquid ratio 1:8) was added to a reaction vessel, followed by 5-15 parts of cerium nitrate hexahydrate, 20-40 parts of urea, and 5-10 parts of PEG-400. The mixture was stirred until completely dissolved to obtain a deposition solution. The pre-activated LMFP substrate was dispersed in the above deposition solution. The mixture was stirred at 250 rpm for 4 hours in an 85°C water bath. After the reaction, the mixture was centrifuged and washed three times each with deionized water and ethanol. The product was kept at 400-500°C for 3-5 hours under an inert atmosphere to form a rare earth oxide layer, yielding intermediate product A.

[0021] Cerium nitrate forms a dense passivation layer, isolating the electrolyte and buffering reactive oxygen species; the urea slowly decomposes and uniformly increases the pH of the system, promoting the growth of Ce. 3+ Heterogeneous nucleation and growth occur at the active sites on the surface of LMFP particles, forming a rare earth oxide layer; PEG-400 acts as a dispersant to prevent the rare earth particles from agglomerating.

[0022] In this embodiment, step (3) involves constructing a composite functional layer on the interface stabilization layer. The composite functional layer contains carbon materials and composite fluorides, and simultaneously provides stress buffering and lithium-ion conduction functions.

[0023] The composite functional layer comprises the following raw materials: pitch-based carbon nanospheres, pre-carbonized, with a particle size of 50-200 nm; these can also be replaced with Ketjen Black or acetylene black. Composite fluoride: a mixture of LiF and AlF3 in a molar ratio of 3:1. Polyvinylpyrrolidone (PVP) or ammonium carbonate, which decomposes during heat treatment to create pores as a pore-forming agent.

[0024] The preparation method of step (3) is as follows: intermediate product A, pitch-based carbon nanospheres, composite fluoride, and polyvinylpyrrolidone are mixed at a mass ratio of 100:3-5:2-4:0.1-1. Low-speed mechanical fusion is performed at 300-500 rpm for 10-20 minutes to avoid damaging the rare earth layer and to ensure uniform adhesion of the fluoride and carbon nanospheres to the particle surface. A small amount of ethanol or water is added as a binder. Spray drying is then performed at an inlet air temperature of 150-160℃ and an outlet air temperature of 80-90℃ to obtain spherical composite particles. The particles are then heated to 450-500℃ at a rate of 2-5℃ / min and held for 2-4 hours under an inert atmosphere (Ar or N2).

[0025] An embodiment of the second aspect of this application provides a lithium manganese iron phosphate cathode material, wherein the surface of the lithium manganese iron phosphate cathode material particles comprises, from the inside to the outside: Rare earth oxide interface stabilizing layer; A composite functional layer disposed on the interface stabilization layer, the composite functional layer comprising carbon materials and composite fluorides.

[0026] The lithium manganese iron phosphate cathode material is obtained by using LiMn 0.7 Fe 0.3 Using PO4 / C as the substrate for lithium manganese iron phosphate, a dense rare-earth oxide interface stabilizing layer is first formed on its surface through hydrolytic deposition of rare-earth salts (such as cerium nitrate) followed by heat treatment in an air atmosphere. This interface stabilizing layer effectively inhibits manganese dissolution and improves the structural stability of the material. On top of this interface stabilizing layer, a composite functional layer comprising pitch-based carbon nanospheres and LiF / AlF3 composite fluorides is constructed through low-speed mechanical fusion, spray drying, and heat treatment in an inert atmosphere. This composite functional layer not only utilizes the rigidity of the carbon nanospheres and the potential introduction of porous structures after polymer decomposition to provide stress buffering to cope with volume changes during charging and discharging, but also provides a channel for rapid lithium-ion migration due to the high lithium-ion conductivity of the composite fluorides, thus simultaneously achieving the dual functions of stress buffering and lithium-ion conduction. Through this multi-layer coating and functional design, the resulting lithium manganese iron phosphate cathode material exhibits significant improvements in cycle performance and rate performance.

[0027] An embodiment of the third aspect of this application provides a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is a lithium manganese iron phosphate positive electrode material.

[0028] Example The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight. Unless otherwise stated, all reagents used in the examples are available commercially or synthesized using conventional methods and are ready for use without further processing. Unless otherwise stated, all instruments used in the examples are available commercially.

[0029] Example 1 A surface lithiation compensation process for lithium iron phosphate cathode material includes the following steps: (1) Using commercially available carbon-coated LiMn 0.7 Fe 0.3 PO4 (LMFP) powder, D50 = 3.5 μm; (2) 100g of LMFP substrate was dispersed in deionized water (solid-liquid ratio 1:7), 1wt% citric acid was added, and the mixture was stirred and washed at 60℃ for 1 hour. After centrifugation and washing until neutral, the mixture was vacuum dried at 110℃ for 6 hours. 800mL of deionized water (solid-liquid ratio 1:8) was added to the reaction vessel, followed by 8g of cerium nitrate hexahydrate, 25g of urea and 7g of PEG-400. The mixture was stirred until completely dissolved. The pre-activated LMFP substrate was dispersed in the above deposition solution and reacted in an 85℃ water bath at 250 rpm for 4 hours for homogeneous hydrolysis deposition. After the reaction, the mixture was centrifuged and washed three times each with deionized water and ethanol. The product was kept at 450℃ for 4 hours under an inert Ar atmosphere to form a rare earth oxide layer, yielding intermediate product A. (3) 100g of intermediate product A, 3g of pitch-based carbon nanospheres, 2g of composite fluoride (a mixture of LiF and AlF3 in a molar ratio of 3:1), and 0.2g of polyvinylpyrrolidone were mixed and mechanically fused at a low speed of 400 rpm for 15 minutes. Ethanol was added as a binder to form composite particles. Spray drying was performed at an inlet air temperature of 155℃ and an outlet air temperature of 85℃ to obtain spherical composite particles. The particles were then heated to 460℃ at a rate of 3℃ / min and held for 3 hours under an Ar atmosphere.

[0030] Example 2 This embodiment is basically the same as Embodiment 1, except that the amount of pitch carbon nanospheres in the composite functional layer is adjusted from 3g to 5g.

[0031] Example 3 This embodiment is basically the same as Embodiment 1, except that the amount of composite fluoride in the composite functional layer is adjusted from 2g to 4g.

[0032] Example 4 This embodiment is basically the same as embodiment 1, except that in step (3), the temperature is increased to 500°C at 5°C / min and held for 2.5 hours in an Ar atmosphere.

[0033] Example 5 This embodiment is basically the same as Embodiment 1, except that the deposition time of the rare earth oxide layer is adjusted from 4 hours to 2 hours.

[0034] Comparative Example 1 This comparative example is basically the same as Example 1, except that there is no rare earth oxide layer, and the composite functional layer is directly constructed on the LMFP substrate.

[0035] Comparative Example 2 This comparative example is basically the same as Example 1, except that in step (3), no pitch-based carbon nanospheres are added to the composite functional layer.

[0036] Comparative Example 3 This comparative example is basically the same as Example 1, except that in step (3), no composite fluoride is added to the composite functional layer.

[0037] Comparative Example 3 This comparative example is basically the same as Example 1, except that 100g LMFP and 5g sucrose are mixed in deionized water, spray-dried, and then sintered in a nitrogen atmosphere at 650°C for 6 hours to form a single carbon coating layer.

[0038] Experimental Case The lithium manganese iron phosphate cathode materials of Examples 1-5 and Comparative Examples 1-4, Super P, and PVDF were prepared into a slurry in a mass ratio of 92:4:4, coated on aluminum foil, and assembled with lithium sheets to form CR2032 type button batteries for testing.

[0039] Testing equipment: Blue Battery Testing System, Electrochemical Workstation (EIS Testing), ICP-OES (Manganese Leaching Analysis).

[0040] The test results are shown in Table 1.

[0041]

[0042] Examples 1-5 demonstrate optimal overall performance, with each indicator significantly outperforming any single comparative example. This strongly demonstrates a close synergistic effect between the rare earth oxide layer and the composite functional layer, rather than a simple superposition of functions.

[0043] Comparative Example 1 showed the worst performance in terms of cycle performance and manganese leaching suppression. Due to the lack of a rare-earth oxide interfacial stabilizing layer, the amount of manganese ions leached from the LMFP substrate increased significantly during charge and discharge, leading to an increase in the manganese ion concentration in the electrolyte. This not only accelerated the structural degradation of the cathode material but also facilitated deposition on the anode surface. Simultaneously, the interfacial impedance increased rapidly with increasing cycle count, hindering lithium ion migration at the interface and resulting in a significant decrease in rate performance. This clearly demonstrates that the rare-earth oxide layer plays a crucial role in suppressing manganese leaching and stabilizing the material structure.

[0044] Comparative Example 2 showed a significant decrease in capacity retention during the later stages of cycling, and exhibited severe voltage polarization during charge and discharge. This is because the composite functional layer lacks pitch-based carbon nanospheres, preventing the effective stress buffering effect. During the charge-discharge cycle of lithium manganese iron phosphate, the material undergoes a certain volume change. Without the rigid support and porous structure provided by carbon nanospheres to mitigate this volume effect, the particles are prone to cracking, leading to electrolyte intrusion and exacerbating side reactions. Simultaneously, the conductivity of the material decreases due to structural damage, thus affecting the overall cycle stability and rate performance.

[0045] Comparative Example 3 exhibits poor rate performance, especially at high rates (such as 5C and 10C) during charge and discharge, with rapid capacity decay. The LiF and AlF3 mixture in the composite fluoride possesses high lithium-ion conductivity, providing a rapid pathway for lithium-ion migration. When this component is lacking in the composite functional layer, the conduction resistance of lithium ions at the interface and within the particles increases, resulting in insufficient utilization of the active material during high-current charge and discharge, leading to poor rate performance.

[0046] While Comparative Example 4 may have similar initial conductivity to the materials in the examples, its cycling performance and manganese leaching inhibition effect are inferior to those of the examples after long-term cycling. Although a single carbon layer can improve conductivity to some extent, it cannot effectively inhibit manganese leaching like a rare earth oxide layer, nor does it possess the synergistic effect of stress buffering and high lithium-ion conductivity unique to composite functional layers. As cycling progresses, the carbon layer may break down due to changes in material volume, losing effective protection for the substrate and leading to a gradual deterioration in performance.

[0047] In summary, this application successfully solves the problems of severe manganese leaching, structural instability due to volume changes, and insufficient ionic conductivity in traditional lithium manganese iron phosphate materials by constructing a rare earth oxide interface stabilizing layer and a composite functional layer containing carbon materials and composite fluorides on the surface of the lithium manganese iron phosphate substrate and utilizing the synergistic effect between the layers, thereby significantly improving the overall electrochemical performance of the material.

[0048] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A surface lithiation compensation process for lithium iron phosphate cathode material, characterized in that, Includes the following steps: Provide lithium iron phosphate substrate; An interface stabilizing layer is constructed on the surface of the substrate, wherein the interface stabilizing layer is a rare earth oxide layer. A composite functional layer is constructed on the interface stabilization layer. The composite functional layer contains carbon materials and composite fluorides, and the composite functional layer simultaneously provides stress buffering and lithium-ion conduction functions. The carbon material is at least one of carbon nanospheres and acetylene black; the composite fluoride is a mixture of LiF and AlF3, with a molar ratio of LiF to AlF3 of 2.5-3.5:

1.

2. The lithiation compensation process for the surface of lithium iron phosphate cathode material according to claim 1, characterized in that, The method for constructing the interface stabilizing layer is as follows: the lithium manganese iron phosphate substrate is dispersed in a solution containing rare earth salts, and rare earth ions are hydrolyzed and deposited on the substrate surface by controlling the pH, followed by heat treatment to form a rare earth oxide layer.

3. The lithiation compensation process for the surface of lithium iron phosphate cathode material according to claim 2, characterized in that, The rare earth salt is at least one of cerium nitrate, yttrium nitrate, and cerium acetate; The heat treatment conditions are as follows: under an inert atmosphere, heat treatment is carried out at 400-500℃ for 3-5 hours.

4. The lithiation compensation process for the surface of lithium iron phosphate cathode material according to claim 1, characterized in that, The method for constructing the composite functional layer includes: mixing the carbon material and the composite fluoride with a lithium manganese iron phosphate substrate with an interface stabilization layer already constructed; attaching the carbon material and the composite fluoride to the surface of the interface stabilization layer by mechanical fusion; and then performing heat treatment under an inert atmosphere to melt and densify the composite fluoride to form the composite functional layer.

5. The lithiation compensation process for the surface of lithium iron phosphate cathode material according to claim 4, characterized in that, The mechanical fusion method is a low-speed dry mixing process with a rotation speed of 300-500 rpm and a mixing time of 10-30 minutes.

6. The lithiation compensation process for the surface of lithium iron phosphate cathode material according to claim 4, characterized in that, The inert atmosphere is argon; the heat treatment conditions are: holding at 450-500℃ for 2-4 hours, with a heating rate of 2-5℃ / min.

7. The lithiation compensation process for the surface of lithium iron phosphate cathode material according to claim 4, characterized in that, The mass ratio of the carbon material to the composite fluoride is (0.5-2):1, and the total mass of the composite functional layer accounts for 3-8% of the mass of the lithium manganese iron phosphate substrate.

8. A lithium manganese iron phosphate cathode material, characterized in that, Prepared by the compensation process according to any one of claims 1-7, the surface of the lithium manganese iron phosphate cathode material particles, from the inside to the outside, is as follows: Including an interface stabilizing layer of rare earth oxides; A composite functional layer disposed on the interface stabilization layer, the composite functional layer comprising carbon materials and composite fluorides.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode comprises the lithium manganese iron phosphate positive electrode material as described in claim 8.

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