A rare earth doped gradient-coated lithium iron manganese phosphate positive electrode material and a preparation method thereof
By modifying lithium manganese iron phosphate materials with rare earth doping and gradient carbon coating, the problems of low electronic conductivity and manganese ion dissolution at high temperatures were solved, and the high conductivity and long life performance of the materials were achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANXI TEWASHI ENERGY TECHNOLOGY CO LTD
- Filing Date
- 2026-04-29
- Publication Date
- 2026-07-21
AI Technical Summary
Existing lithium manganese iron phosphate materials exhibit low electronic conductivity and severe manganese ion dissolution at high temperatures, leading to a decline in cycle life. Current modification methods are insufficient to simultaneously improve conductivity and stability.
Rare earth element doping is used to form a gradient carbon-coated structure, including a dense carbon layer, a porous carbon layer and a metal oxide protective layer, which synergistically improves the material's electronic conductivity, ion transport performance and interface stability.
The material's cycle stability and high-temperature performance have been significantly improved. By stabilizing the crystal lattice through rare earth doping, optimizing conductivity through gradient carbon layers, and enhancing interface protection through metal oxide layers, the material has achieved a long lifespan under high-temperature environments.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of lithium battery technology, specifically to a rare earth-doped gradient-coated lithium manganese iron phosphate cathode material and its preparation method. Background Technology
[0002] Lithium manganese iron phosphate (LiFe) 1-x-y Mn x MnO4 has gained widespread attention in the fields of power batteries and energy storage batteries in recent years due to its relatively high voltage platform (approximately 4.1V) and superior energy density compared to lithium iron phosphate. However, this material still faces two major technical bottlenecks in practical applications: firstly, its intrinsic electronic conductivity is low, resulting in poor rate performance; secondly, during charge-discharge cycles, MnO4... 3+ Due to the Jan Taylor effect, lattice distortion occurs, which easily dissolves from the lattice and deposits on the negative electrode side, disrupting the integrity of the solid electrolyte interfacial film and causing a severe reduction in battery cycle life. These problems are even more pronounced under high-temperature operating environments, becoming a key obstacle to the commercialization of lithium manganese iron phosphate materials.
[0003] In existing technologies, researchers mainly modify lithium manganese iron phosphate (LFP) through carbon coating, metal ion doping, and surface modification. For example, carbon coating layers are constructed on the material surface to improve electronic conductivity, or elements such as magnesium, aluminum, and titanium are doped to stabilize the crystal structure. However, single modification methods often fail to simultaneously improve conductivity and suppress manganese dissolution. While a single carbon coating layer can improve conductivity, the carbon layer is easily penetrated by the electrolyte during long-term cycling, failing to effectively prevent the dissolution and migration of manganese ions. While single metal ion doping stabilizes the crystal lattice to some extent, its protective effect at the interface between the material surface and the electrolyte is limited. In recent years, researchers have attempted to use multilayer coating structures, but there is still room for improvement in the effect of suppressing manganese dissolution under high-temperature, long-cycle conditions. Therefore, there is an urgent need to develop a novel LFP cathode material that combines strong structural stability with excellent interface protection capabilities. Summary of the Invention
[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a rare earth-doped gradient coated lithium manganese iron phosphate cathode material and its preparation method. The rare earth doping inhibits manganese dissolution, the gradient carbon layer takes into account both electronic conductivity and ion transport, and the metal oxide layer improves the interface stability. The three factors work together to significantly improve the cycle stability and high-temperature performance of the material.
[0005] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a rare earth-doped gradient-coated lithium manganese iron phosphate cathode material, comprising the following steps: S1: According to LiFe 1-x-y Mnx M y The manganese source, iron source, lithium source, phosphorus source and M source are mixed uniformly in PO4 stoichiometry, wherein 0.4≤x≤0.7, 0.01≤y≤0.05, and M is La or Gd; the mixture is sintered at 600-750℃ for 6-12h under an inert atmosphere to obtain rare earth element doped lithium manganese iron phosphate material. S2: Mix rare earth element-doped lithium manganese iron phosphate material with carbon source precursor evenly, and sinter at 500-650℃ for 4-8 hours under an inert atmosphere to form a dense carbon coating layer on the material surface. S3: The material obtained in step S2 is placed in an atmosphere containing carbon gas for chemical vapor deposition to form a porous carbon coating layer on the surface of the dense carbon coating layer. S4: Mix the material obtained in step S3 with the metal oxide precursor solution, dry it, and sinter it at 300-500℃ for 2-6 hours under an inert atmosphere to form a metal oxide protective layer on the surface of the material; wherein the metal oxide precursor is a nitrate, chloride or alkoxide of La or Gd.
[0006] Preferably, in step S1, the lithium source is lithium carbonate, the iron source is ferrous oxalate, the manganese source is manganese carbonate, the phosphorus source is ammonium dihydrogen phosphate, and the M source is lanthanum nitrate or gadolinium nitrate.
[0007] Preferably, in step S2, the carbon source precursor is glucose, sucrose, citric acid, or polyethylene glycol.
[0008] Preferably, in step S2, the mass ratio of rare earth element-doped lithium manganese iron phosphate material to carbon source precursor is 100:(5-8).
[0009] Preferably, in step S3, the carbon-containing gas is methane, acetylene, ethylene, or benzene vapor.
[0010] Preferably, in step S3, the chemical vapor deposition temperature is 600-750℃, the processing time is 30-120 min, and the flow rate of the carbon-containing gas is 50-200 sccm.
[0011] On the other hand, the present invention provides a rare earth-doped gradient-coated lithium manganese iron phosphate cathode material, which is prepared by the above-mentioned method for preparing rare earth-doped gradient-coated lithium manganese iron phosphate cathode material.
[0012] Compared with the prior art, the present invention has the following advantages: (1) This invention effectively suppresses the dissolution of manganese ions through rare earth element doping. After rare earth element M is doped into the lithium manganese iron phosphate lattice, due to its large ionic radius (such as La), the dissolution of manganese ions is effectively suppressed. 3+ Its radius is approximately 1.06 Å, larger than that of Fe. 2+ and Mn 2+The high bond energy of rare earth elements (with their reduced ionic radius) can moderately increase the lattice spacing and optimize lithium-ion diffusion channels. More importantly, the high bond energy rare earth bonds formed between rare earth elements and oxygen can significantly enhance the bonding strength of the Mn-O bond, fundamentally suppressing the diffusion of Mn-O atoms. 3+ The lattice distortion and manganese ion dissolution caused by the Ginger-Taylor effect.
[0013] (2) The present invention achieves both conductivity and ion transport performance through a gradient carbon coating structure. The dense carbon layer is closely bonded to the lithium manganese iron phosphate core, forming good interfacial electronic contact and significantly reducing interfacial impedance; the porous carbon layer provides abundant lithium ion diffusion channels, promoting the rapid insertion and extraction of lithium ions, and achieving the best balance between electronic conductivity and ion conductivity.
[0014] (3) This invention further enhances the interfacial stability of the material through the outer coating of a metal oxide layer. As the outermost layer, the metal oxide protective layer blocks direct contact between the electrolyte and the carbon coating layer and the internal materials, effectively inhibiting the erosion of the material by corrosive substances such as hydrofluoric acid, while further preventing the migration of a small amount of dissolved manganese ions. Under high-temperature cycling conditions, the chemical stability of the metal oxide protective layer is significantly better than that of the conventional carbon coating layer, greatly improving the cycle life of the battery under high-temperature conditions.
[0015] (4) The rare earth doping, gradient carbon coating, and metal oxide outer coating work synergistically. The combination of the three layers is not a simple superposition, but forms a synergistic protection system of "bulk doping enhances lattice stability → dense carbon layer reduces interfacial resistance → porous carbon layer promotes ion transport → metal oxide layer provides chemical barrier". Compared with single modification methods, the cycling stability and high-temperature performance of the material of this invention are significantly improved. Detailed Implementation
[0016] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0017] Example 1 The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material in this embodiment includes the following steps: Preparation of S1 rare earth element La-doped lithium manganese iron phosphate core: Lithium carbonate, ferrous oxalate (FeC2O4·2H2O), manganese carbonate (MnCO3), ammonium dihydrogen phosphate (NH4H2PO4), and lanthanum nitrate (La(NO3)3·6H2O) were weighed according to the stoichiometric ratio Li:Fe:Mn:La:P = 1:0.4:0.55:0.05:1. The above raw materials were added to a planetary ball mill and ball-milled for 4 hours with anhydrous ethanol as the medium at a speed of 300 rpm and a ball-to-material ratio of 5:1. The slurry after ball milling was vacuum-dried at 80℃ for 12 hours to obtain precursor powder. The precursor powder was placed in a tube furnace and heated to 700℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, held for 8 hours, and then naturally cooled to room temperature. After grinding and sieving, La-doped lithium manganese iron phosphate powder with the chemical composition LiFe was obtained. 0.4 Mn 0.55 La 0.05 PO4.
[0018] Formation of S2 dense carbon coating: La-doped lithium manganese iron phosphate powder and glucose were mixed at a mass ratio of 100:5. Deionized water was added and stirred to form a slurry. After the water was evaporated at 80°C, the slurry was placed in a tube furnace and heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere. The sintering was then held for 6 hours to form a dense carbon coating on the material surface.
[0019] Formation of the porous carbon coating layer in step S3: The material obtained in step S2 is placed in a chemical vapor deposition tube furnace and heated to 650°C under nitrogen protection. Acetylene gas (flow rate 100 sccm) and nitrogen gas (flow rate 200 sccm) are introduced and deposited for 60 min to form a porous carbon coating layer on the surface of the dense carbon layer.
[0020] Formation of the S4 metal oxide protective layer: The material obtained in step S3 is dispersed in an ethanol solution of lanthanum nitrate (lanthanum nitrate concentration 0.1 mol / L), stirred for 1 h, filtered and dried, and then sintered in a tube furnace at 450 °C under a nitrogen atmosphere for 3 h to form a lanthanum oxide protective layer on the surface of the material, thus obtaining the lithium manganese iron phosphate cathode material.
[0021] Example 2 The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material in this embodiment includes the following steps: Preparation of S1 rare earth element La-doped lithium manganese iron phosphate core: Lithium carbonate, ferrous oxalate (FeC2O4·2H2O), manganese carbonate (MnCO3), ammonium dihydrogen phosphate (NH4H2PO4), and gadolinium nitrate (Gd(NO3)3·6H2O) were weighed according to the stoichiometric ratio Li:Fe:Mn:La:P = 1:0.4:0.55:0.05:1. The above raw materials were added to a planetary ball mill and ball-milled for 4 hours with anhydrous ethanol as the medium at a speed of 300 rpm and a ball-to-material ratio of 5:1. The slurry after ball milling was vacuum-dried at 80℃ for 12 hours to obtain precursor powder. The precursor powder was placed in a tube furnace and heated to 600℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, held for sintering for 12 hours, and then naturally cooled to room temperature. After grinding and sieving, La-doped lithium manganese iron phosphate powder with the chemical composition LiFe was obtained. 0.4 Mn 0.55 La 0.05 PO4.
[0022] Formation of S2 dense carbon coating: La-doped lithium manganese iron phosphate powder is mixed with sucrose at a mass ratio of 100:6, deionized water is added and stirred to form a slurry, the water is evaporated at 80°C and then placed in a tube furnace, heated to 500°C at a heating rate of 5°C / min under a nitrogen atmosphere, and sintered at that temperature for 8 hours to form a dense carbon coating on the material surface.
[0023] Formation of the porous carbon coating layer in step S3: The material obtained in step S2 is placed in a chemical vapor deposition tube furnace and heated to 600°C under nitrogen protection. Methane gas (flow rate 50 sccm) and nitrogen gas (flow rate 200 sccm) are introduced and deposited for 120 min to form a porous carbon coating layer on the surface of the dense carbon layer.
[0024] Formation of the S4 metal oxide protective layer: The material obtained in step S3 is dispersed in an ethanol solution of gadolinium nitrate (lanthanum nitrate concentration 0.1 mol / L), stirred for 1 h, filtered and dried, and then sintered in a tube furnace at 300 °C under a nitrogen atmosphere for 6 h to form a lanthanum oxide protective layer on the surface of the material, thus obtaining the lithium manganese iron phosphate cathode material.
[0025] Example 3 The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material in this embodiment includes the following steps: Preparation of S1 rare earth element La-doped lithium manganese iron phosphate core: Lithium carbonate, ferrous oxalate (FeC2O4·2H2O), manganese carbonate (MnCO3), ammonium dihydrogen phosphate (NH4H2PO4), and lanthanum nitrate (La(NO3)3·6H2O) were weighed according to the stoichiometric ratio Li:Fe:Mn:La:P = 1:0.43:0.55:0.02:1. The above raw materials were added to a planetary ball mill and ball-milled for 4 hours with anhydrous ethanol as the medium at a speed of 300 rpm and a ball-to-material ratio of 5:1. The slurry after ball milling was vacuum-dried at 80℃ for 12 hours to obtain precursor powder. The precursor powder was placed in a tube furnace and heated to 750℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, held for 6 hours, and then naturally cooled to room temperature. After grinding and sieving, La-doped lithium manganese iron phosphate powder with the chemical composition LiFe was obtained. 0.43 Mn 0.55 La 0.02 PO4.
[0026] Formation of S2 dense carbon coating: La-doped lithium manganese iron phosphate powder and citric acid are mixed at a mass ratio of 100:8. Deionized water is added and stirred to form a slurry. After the water is evaporated at 80°C, the slurry is placed in a tube furnace and heated to 650°C at a heating rate of 5°C / min under a nitrogen atmosphere. The sintering is then held for 4 hours to form a dense carbon coating on the material surface.
[0027] Formation of the porous carbon coating layer in step S3: The material obtained in step S2 is placed in a chemical vapor deposition tube furnace and heated to 750°C under nitrogen protection. Ethylene gas (flow rate 200 sccm) and nitrogen gas (flow rate 200 sccm) are introduced and deposited for 30 min to form a porous carbon coating layer on the surface of the dense carbon layer.
[0028] Formation of the S4 metal oxide protective layer: The material obtained in step S3 is dispersed in an ethanol solution of lanthanum nitrate (lanthanum nitrate concentration 0.1 mol / L), stirred for 1 h, filtered and dried, and then sintered in a tube furnace at 500 °C under a nitrogen atmosphere for 2 h to form a lanthanum oxide protective layer on the surface of the material, thus obtaining the lithium manganese iron phosphate cathode material.
[0029] Comparative Example 1 The difference from Example 1 is that, in step S1, lanthanum nitrate is not added. Lithium carbonate, ferrous oxalate (FeC2O4·2H2O), manganese carbonate (MnCO3), and ammonium dihydrogen phosphate (NH4H2PO4) are weighed according to the stoichiometric ratio Li:Fe:Mn:P = 1:0.45:0.55:1. The resulting lithium manganese iron phosphate core has a chemical composition of LiFe. 0.45 Mn 0.55 PO4.
[0030] Comparative Example 2 The difference from Example 1 is that step S4 is not performed.
[0031] Comparative Example 3 The difference from Example 1 is that steps S3 and S4 are not performed.
[0032] The cathode materials prepared in the above embodiments and comparative examples were assembled into coin cells, and their electrochemical performance was tested.
[0033] Preparation of positive electrode sheet: The positive electrode material, conductive carbon black and polyvinylidene fluoride are mixed in a mass ratio of 90:5:5, N-methylpyrrolidone is added and stirred evenly, coated on aluminum foil, vacuum dried at 120℃ for 12h, and punched into round sheets with a diameter of 12mm.
[0034] Battery assembly: Using lithium metal sheets as the negative electrode, Celgard 2400 as the separator, and a 1 mol / L LiPF6 EC / DMC / EMC (volume ratio 1:1:1) solution as the electrolyte, CR2032 coin cells were assembled in an argon-filled glove box.
[0035] (1) Cyclic performance test A constant current charge-discharge cycle test was conducted at a current density of 0.5C within a voltage range of 2.5-4.3V at a high temperature of 55℃. After 500 cycles, the capacity retention rate was recorded. The results are shown in Table 1. Table 1 Cycle performance test results of assembled batteries in each embodiment and comparative example As can be seen from Table 1, in Comparative Example 1, no rare earth elements were added in step S1, i.e., no rare earth doping was performed. Therefore, there are no high-energy rare earth-oxygen bonds in the crystal lattice to enhance the bonding strength of the Mn-O bonds. 3+The lattice distortion and manganese ion dissolution caused by the Jan Taylor effect cannot be effectively suppressed, leading to decreased structural stability and accelerated capacity decay during cycling. Comparative Example 2 did not perform step S4, i.e., it lacked a metal oxide protective layer (lanthanum oxide or gadolinium oxide). Without this outer layer, corrosive substances in the electrolyte (such as HF) will directly erode the carbon coating layer and internal materials. At the same time, the small amount of dissolved manganese ions also lack a physical barrier, easily migrating to the negative electrode and damaging the SEI film, resulting in a decrease in cycle life. Comparative Example 3 lacked both the porous carbon coating layer (step S3) and the metal oxide protective layer (step S4). This resulted in: 1) the lack of a porous carbon layer, reducing lithium ion diffusion channels and decreasing ion transport efficiency; 2) the lack of a metal oxide layer, resulting in poor interfacial chemical stability and severe electrolyte erosion; 3) although the bulk phase had rare earth doping, the surface protection was insufficient, making it difficult to achieve the synergistic effect mechanism of "bulk phase doping + gradient carbon coating + metal oxide layer", ultimately resulting in the lowest capacity retention rate among all samples in Comparative Example 3. Examples 1-3 exhibited a capacity retention rate of 92.5-94.2% after 500 cycles, significantly higher than Comparative Examples 1-3, fully demonstrating the significant improvement in cycle stability resulting from the synergistic effect of rare earth doping, gradient carbon coating, and metal oxide outer coating.
[0036] (2) Manganese ion dissolution test The batteries from Examples 1 and 1-3 were disassembled after the cyclic testing was completed, and the electrolyte was taken for inductively coupled plasma atomic emission spectrometry (ICP-AES) to detect Mn in the electrolyte. 2+ The dissolution concentrations are shown in Table 2: Table 2. Mn content of the battery electrolyte after cycle testing in Example 1 and Comparative Examples 1-3 2+ Dissolution concentration test results The results show that Comparative Example 1, without rare earth doping, lacks high-energy rare earth-oxygen bonds in its lattice, resulting in insufficient Mn-O bond strength. 3+ During charging and discharging, manganese ions are more easily dissolved from the crystal lattice, resulting in a significant increase in the concentration of manganese ions in the electrolyte. Comparative Example 2 lacks a metal oxide protective layer. Although it contains rare earth doping and carbon coating, the absence of the outermost layer means that the small amount of dissolved manganese ions have no physical barrier to prevent them from easily entering the electrolyte. Simultaneously, substances such as HF in the electrolyte more easily corrode the material surface, further exacerbating manganese dissolution. Comparative Example 3 lacks both a porous carbon layer and a metal oxide layer. Although it retains rare earth doping, surface protection is severely inadequate. While the dense carbon layer is conductive, it cannot effectively prevent electrolyte penetration and manganese ion migration, leading to the most severe manganese dissolution. This also indirectly verifies the crucial role of the "chemical barrier + ion channel" design of the porous carbon layer + metal oxide layer in suppressing manganese dissolution. Mn in the electrolyte of Example 1 2+The lowest leaching concentration (8.2 ppm) was only about 22% of that in Comparative Example 1 and 19% of that in Comparative Example 3, which fully demonstrates that the present invention effectively suppresses the leaching of manganese ions through the synergistic effect of rare earth element doping and metal oxide outer coating.
Claims
1. A method for preparing a rare-earth-doped gradient-coated lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1: According to LiFe 1-x-y Mn x M y The manganese source, iron source, lithium source, phosphorus source and M source are mixed uniformly in PO4 stoichiometry, wherein 0.4≤x≤0.7, 0.01≤y≤0.05, and M is La or Gd; the mixture is sintered at 600-750℃ for 6-12h under an inert atmosphere to obtain rare earth element doped lithium manganese iron phosphate material. S2: Mix rare earth element-doped lithium manganese iron phosphate material with carbon source precursor evenly, and sinter at 500-650℃ for 4-8 hours under an inert atmosphere to form a dense carbon coating layer on the material surface. S3: The material obtained in step S2 is placed in an atmosphere containing carbon gas for chemical vapor deposition to form a porous carbon coating layer on the surface of the dense carbon coating layer. S4: Mix the material obtained in step S3 with the metal oxide precursor solution, dry it, and sinter it at 300-500℃ for 2-6 hours under an inert atmosphere to form a metal oxide protective layer on the surface of the material; wherein the metal oxide precursor is a nitrate, chloride or alkoxide of La or Gd.
2. The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S1, the lithium source is lithium carbonate, the iron source is ferrous oxalate, the manganese source is manganese carbonate, the phosphorus source is ammonium dihydrogen phosphate, and the M source is lanthanum nitrate or gadolinium nitrate.
3. The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the carbon source precursor is glucose, sucrose, citric acid, or polyethylene glycol.
4. The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the mass ratio of rare earth element-doped lithium manganese iron phosphate material to carbon source precursor is 100:(5-8).
5. The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S3, the carbon-containing gas is methane, acetylene, ethylene, or benzene vapor.
6. The preparation method of the rare earth-doped gradient-coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S3, the chemical vapor deposition temperature is 600-750℃, the processing time is 30-120 min, and the flow rate of carbon-containing gas is 50-200 sccm.
7. A rare-earth-doped gradient-coated lithium manganese iron phosphate cathode material, characterized in that, The rare earth-doped gradient coated lithium manganese iron phosphate cathode material was prepared by the preparation method described in any one of claims 1-6.