A double-coated layer, doped synergistically modified lithium iron manganese phosphate positive electrode material and a preparation method and application thereof

Abstract

The application provides a double-coated lithium iron manganese phosphate positive electrode material with doping synergistic modification and a preparation method and application thereof. A lithium source, an iron source, a manganese source, a phosphorus source, sucrose and rare earth oxide are ball-milled and sintered to form a precursor; then the precursor is mixed with hexachlorocyclotriphosphazene and p-phenylenediamine, and is calcined by rapid heating to obtain a lithium iron manganese phosphate positive electrode. Rare earth elements are introduced into the lithium iron manganese phosphate crystal structure to partially replace Mn sites, and a carbon core layer formed by sucrose pyrolysis and a cross-linked polyphosphazene shell layer formed by in-situ polycondensation and carbonization of hexachlorocyclotriphosphazene and p-phenylenediamine are sequentially constructed outside the primary particles, and a rapid heating strategy in the secondary calcination process is adopted to realize crystal structure regulation, interface stability improvement and electronic conductive network optimization, so that a lithium iron manganese phosphate positive electrode material with high rate, long cycle and high tap density is obtained. The stable olivine structure and the dense double-coated layer on the surface significantly improve the conductivity and interface stability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a double-coated, doped and synergistically modified lithium manganese iron phosphate cathode material, its preparation method and application. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) has attracted widespread attention in power batteries and energy storage systems due to its advantages such as olivine crystal structure, good thermal stability, environmental friendliness, and low cost. However, existing LMFP cathode materials still have the following shortcomings: On the one hand, with the increase of manganese content, its Jahn-Teller distortion effect becomes significant, which easily causes local crystal instability, resulting in significant capacity decay during cycling and making it difficult to meet the requirements of high rate and long cycle life; on the other hand, LMFP materials have low intrinsic electronic conductivity, and traditional single carbon source coating (such as sucrose, polyacrylonitrile, or small organic molecules) is difficult to form a continuous, highly conductive carbon network, resulting in limited interfacial electron transfer capacity and thus limiting the electrochemical performance of the material at high rates. In addition, existing LMFP modification methods usually only use single techniques, such as single-element doping or single-layer carbon coating, which are difficult to balance crystal structure optimization, interfacial stability, and conductivity. This leads to active interfacial side reactions at high voltages, easy dissolution of manganese, and overall unsatisfactory rate performance and cycle stability. Summary of the Invention

[0003] The present invention aims to provide a double-coated, doped and synergistically modified lithium manganese iron phosphate cathode material, its preparation method and application. The material has a stable olivine crystal structure and excellent electronic conductivity. At the same time, the dense double coating on its surface significantly enhances the interface stability, effectively solving the problems of rapid cycle decay and poor rate performance in the prior art.

[0004] This invention is achieved through the following technical solution: In a first aspect, the present invention provides a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material, wherein the lithium manganese iron phosphate cathode material has an olivine crystal structure and the general chemical formula is: LiMn 0.6-x-y Fe 0.4 M 1 x M 2 y PO4, where: 0.01≤x+y≤0.07, x≥0, y≥0; when exactly one of x and y is greater than 0, M 1 Or M 2 M is a rare earth element; when x > 0 and y > 0, 1 and M 2These are two different rare earth elements.

[0005] Secondly, the present invention provides a method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material, comprising the following steps: Step 1, according to Li:Mn:Fe:M 1 M 2 Weigh out Li2CO3, MnCO3, FeC2O4·2H2O, rare earth element oxides and NH4H2PO4 in a molar ratio of P = 1:0.6-xy:0.4:x:y:1, and add sucrose at a mass of 3%~20% of the total mass of the above raw materials. Place the mixture in a ball mill and ball mill it together. Then calcine it at 200℃~450℃ under an argon atmosphere to form a precursor. The rare earth element oxides are selectively added according to the values ​​of x and y, satisfying 0.01≤x+y≤0.07, x≥0, y≥0; when only one of x and y is greater than 0, M is introduced. 1 Or M 2 The oxides corresponding to the non-zero components; when x > 0 and y > 0, two different rare earth elements M are introduced simultaneously. 1 and M 2 Oxides; Step 2: Mix the precursor from Step 1 with hexachlorocyclotriphosphazene (HCCP) and p-phenylenediamine (p-PDA), place them in a ball mill and ball mill them evenly. Then, place them in an argon protective atmosphere and calcine them at a heating rate of 8℃ / min~12℃ / min to 600℃~900℃ to finally synthesize lithium manganese iron phosphate cathode material. The hexachlorocyclotriphosphazene (HCCP) and p-phenylenediamine (p-PDA) are 3% to 10% and 6% to 20% of the precursor mass, respectively.

[0006] Furthermore, the rare earth element oxides mentioned in step 1 are Y2O3, Sc2O3, Yb2O3, Pr2O3 and Lu2O3, and when x>0 and y>0, the rare earth elements contained in the two rare earth element oxides added are different from each other.

[0007] Furthermore, the ball milling speed in step 1 is 400 r / min to 700 r / min, and the ball milling time is 5 h to 10 h.

[0008] Furthermore, in step 1, the calcination temperature is increased to the calcination temperature at a heating rate of 3℃ / min to 5℃ / min, and then held at the calcination temperature for 2h to 5h.

[0009] Furthermore, the ball milling in steps 1 and 2 is wet ball milling, and the amount of wet milling additive added is 10% to 30% of the solid content. The wet milling additive is at least one solvent among anhydrous ethanol, acetone, ethylene glycol, diethylene glycol dimethyl ether, and isopropanol.

[0010] Furthermore, in step 2, the ball milling speed is 200 r / min to 500 r / min, and the ball milling time is 2 h to 5 h.

[0011] Furthermore, the calcination time in step 2 is 8 h to 12 h.

[0012] Thirdly, the present invention provides an application of a double-coated, doped and synergistically modified lithium manganese iron phosphate cathode material in the preparation of lithium-ion battery cathodes or lithium-ion batteries.

[0013] Compared with the prior art, the present invention has the following beneficial effects: This invention introduces rare earth elements into the crystal structure of lithium manganese iron phosphate to partially replace Mn sites. Simultaneously, a carbon core layer formed by sucrose pyrolysis and a cross-linked polyphosphazene shell layer formed by in-situ polycondensation and carbonization of hexachlorocyclotriphosphazene (HCCP) and p-phenylenediamine (p-PDA) are sequentially constructed on the outside of the primary particles. Combined with a rapid heating strategy in the secondary calcination process, crystal structure regulation, interface stability improvement and electronic conductivity network optimization are achieved, thereby obtaining a lithium manganese iron phosphate cathode material with high rate capability, long cycle life and high tap density.

[0014] This invention constructs a double-coating structure consisting of a carbon core layer and a cross-linked polyphosphonic nitrile shell on the outer surface of primary particles. During low-temperature sintering, sucrose forms a disordered carbon-dominated conductive layer, while hexachlorocyclotriphosphazene (HCCP) and p-phenylenediamine (p-PDA) undergo in-situ condensation during high-temperature sintering to form a highly cross-linked cyclic polyphosphonic nitrile three-dimensional network framework (containing P and N). The inner carbon core layer establishes a continuous conductive network on the particle surface, refines the primary particles, and increases the compactness between particles. The outer cross-linked polyphosphonic nitrile shell, relying on its highly cross-linked framework and the P / N functionalized carbon layer formed by high-temperature carbonization, further improves the compactness, interfacial stability, and surface electron transport capability of the coating layer. The synergistic effect of the double coating layer simultaneously enhances electron conduction, suppresses interfacial side reactions, maintains particle integrity, and reduces polarization, thereby improving the rate performance, cycle stability, and tap density of lithium manganese iron phosphate cathode materials.

[0015] This invention involves doping rare earth elements at manganese sites in the crystal structure. Rare earth elements possess unique electronic structures and photoelectric effects, as well as wide ionic radii, enabling them to form performance-enhancing composite materials with LMFPs. Specifically, large-radius rare earth elements are beneficial for expanding lithium-ion diffusion channels but easily introduce impurity phases; medium-radius rare earth elements achieve a balance between structural stability and electrochemical performance; while small-radius rare earth elements may lead to lattice shrinkage and diffusion obstruction. Regarding doping concentration, low doping concentrations help maintain structural integrity but have limited modification effects; medium doping concentrations achieve optimal matching between rate performance and cycle stability; while high doping concentrations lead to capacity reduction due to reduced active sites and impurity phase formation. This invention selects suitable rare earth elements for manganese site doping, utilizing the significant differences in electronic structure and ionic radius between rare earth elements and manganese to modify the olivine structure of LMFPs. This effectively adjusts crystal structure parameters, suppresses Jahn-Teller distortion, and optimizes defect structures, thereby improving the material's structural stability and electrochemical performance.

[0016] This invention employs a rapid heating strategy during secondary calcination. Traditional calcination processes use a slow heating rate of 3°C / min to 5°C / min, which leads to prolonged solid-solid contact between lithium carbonate and other raw materials. This results in uneven lithium distribution within the particles and excessive stress concentration between particles, making the cathode particles prone to breakage during charging and discharging. Since the melting of lithium carbonate mainly occurs during secondary calcination, the rapid heating rate significantly accelerates the melting of lithium carbonate, promoting close liquid-solid contact between raw materials and achieving uniform nucleation within the particles. This microstructure effectively reduces the chemical and mechanical stress within and between particles, thus significantly alleviating material degradation problems during long-cycle charging.

[0017] The lithium manganese iron phosphate cathode material prepared by this invention has a primary particle size of approximately 100-200 nm and a tap density of 0.95 g / cm³. 3 It exhibits excellent long-term cycling stability with a discharge specific capacity of ≥120mAh / g within a voltage range of 2~4.5V and a 2C rate, and a capacity retention rate of ≥98% after 300 cycles. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the process flow of the present invention; Figure 2 Here is a SEM image of the lithium manganese iron phosphate cathode material prepared in Example 1 of this invention; Figure 3 This is a cycle performance diagram of the half-cell of lithium manganese iron phosphate cathode material prepared in Example 1 and Comparative Example 1 of the present invention, in the charge-discharge range of 2.0~4.5 V, at a 2 C rate, and at 25 ℃ for 300 cycles. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0020] Example 1: A double-coated, Y-doped synergistically modified lithium manganese iron phosphate cathode material, with the chemical formula LiMn 0.55 Fe 0.4 Y 0.05 PO4 includes the following steps: Step 1: Weigh Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Y₂O₃, and NH₄H₂PO₄ according to the molar ratio of Li:Mn:Fe:Y:P = 1:0.55:0.4:0.05:1, and add sucrose at 10% of the total mass of the above raw materials; add anhydrous ethanol as a wet milling additive to the mixture at 20% of the total solid mass, place the mixture in a ball mill jar, and wet mill at 500 r / min for 7 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 350 °C under an argon atmosphere at a heating rate of 4 °C / min, and hold for 2 h to form a precursor with an initial carbon layer coating on the surface; Step 2: Weigh the precursor obtained in Step 1, and add hexachlorocyclotriphosphazene (HCCP) at 6% of the precursor's mass and p-phenylenediamine (p-PDA) at 12% of the precursor's mass. Add anhydrous ethanol as a wet milling additive to the mixture, at a mass of 15% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 400 r / min for 2 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it to 700℃ under argon protection at a heating rate of 10℃ / min, hold for 10 h, and naturally cool to room temperature to obtain a double-coated, Y-doped synergistically modified lithium manganese iron phosphate cathode material.

[0021] Example 2: This example is the same as Example 1, except that: Li2CO3, MnCO3, FeC2O4·2H2O, rare earth element oxides and NH4H2PO4 are weighed according to the molar ratio of Li:Mn:Fe:Y:P = 1:0.59:0.4:0.01:1.

[0022] Example 3: This example is the same as Example 1, except that: Li2CO3, MnCO3, FeC2O4·2H2O, rare earth element oxides and NH4H2PO4 are weighed according to the molar ratio of Li:Mn:Fe:Y:P = 1:0.57:0.4:0.03:1.

[0023] Example 4: This example is the same as Example 1, except that: Li2CO3, MnCO3, FeC2O4·2H2O, rare earth element oxides and NH4H2PO4 are weighed according to the molar ratio of Li:Mn:Fe:Y:P = 1:0.53:0.4:0.07:1.

[0024] Example 5: A double-coated, Sc-doped synergistically modified lithium manganese iron phosphate cathode material, with the chemical formula LiMn 0.55 Fe 0.4 Sc 0.05 PO4 includes the following steps: Step 1: Weigh Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Sc₂O₃, and NH₄H₂PO₄ according to the molar ratio of Li:Mn:Fe:Sc:P = 1:0.55:0.4:0.05:1, and add sucrose at 8% of the total mass of the above raw materials. Add a mixture of isopropanol and ethylene glycol as a wet milling additive to the mixture, at an amount of 18% of the total solid mass, wherein the mass ratio of isopropanol to ethylene glycol is 4:1. Place the mixture in a ball mill jar and wet mill at 550 r / min for 6 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 320 °C at a heating rate of 5 °C / min under an argon atmosphere, and hold for 3 h to form a precursor with an initial carbon layer coating on the surface. Step 2: Weigh the precursor obtained in Step 1, and add 5% (by mass) of hexachlorocyclotriphosphazene (HCCP) and 15% (by mass) of p-phenylenediamine (p-PDA) to the mixture. Add acetone as a wet milling additive to the mixture at 12% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 350 r / min for 3 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it at 720℃ under argon protection at a heating rate of 9℃ / min, hold for 9 h, and naturally cool to room temperature to obtain a double-coated, Sc-doped synergistically modified lithium manganese iron phosphate cathode material.

[0025] Example 6: This example is the same as Example 5, except that Li2CO3, MnCO3, FeC2O4·2H2O, Sc2O3 and NH4H2PO4 are weighed according to the molar ratio of Li:Mn:Fe:Sc:P = 1:0.59:0.4:0.01:1.

[0026] Example 7: This example is the same as Example 5, except that Li2CO3, MnCO3, FeC2O4·2H2O, Sc2O3 and NH4H2PO4 are weighed according to the molar ratio of Li:Mn:Fe:Sc:P = 1:0.57:0.4:0.03:1.

[0027] Example 8: This example is the same as Example 5, except that Li2CO3, MnCO3, FeC2O4·2H2O, Sc2O3 and NH4H2PO4 are weighed according to the molar ratio of Li:Mn:Fe:Sc:P = 1:0.53:0.4:0.07:1.

[0028] Example 9: A double-coated, Pr-doped synergistically modified lithium manganese iron phosphate cathode material, with the chemical formula LiMn 0.57 Fe 0.4 Pr 0.03 PO4 includes the following steps: Step 1: Weigh Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Pr₂O₃, and NH₄H₂PO₄ according to the molar ratio of Li:Mn:Fe:Pr:P = 1:0.57:0.4:0.03:1, and add sucrose at 12% of the total mass of the above raw materials; add diethylene glycol dimethyl ether as a wet milling additive to the mixture at 25% of the total solid mass, place the mixture in a ball mill jar, and wet mill at 600 r / min for 8 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 380 °C under an argon atmosphere at a heating rate of 4 °C / min, and hold for 2.5 h to form a precursor with an initial carbon layer coating on the surface; Step 2: Weigh the precursor obtained in Step 1, and add 8% (by mass) of hexachlorocyclotriphosphazene (HCCP) and 18% (by mass) of p-phenylenediamine (p-PDA) to the mixture. Add anhydrous ethanol as a wet milling additive to the mixture, at a rate of 20% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 450 r / min for 2.5 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it at a heating rate of 12 °C / min under argon protection, holding for 8 h, and then naturally cool to room temperature to obtain a double-coated, Pr-doped synergistically modified lithium manganese iron phosphate cathode material.

[0029] Example 10: A double-coated, Yb-doped synergistically modified lithium manganese iron phosphate cathode material, with the chemical formula LiMn 0.56 Fe 0.4 Yb 0.04PO4 includes the following steps: Step 1: Weigh Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Yb₂O₃, and NH₄H₂PO₄ according to the molar ratio of Li:Mn:Fe:Yb:P = 1:0.56:0.4:0.04:1, and add sucrose at 9% of the total mass of the above raw materials; add isopropanol as a wet milling additive to the mixture at 16% of the total solid mass, place the mixture in a ball mill jar, and wet mill at 520 r / min for 7.5 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 340 °C under an argon atmosphere at a heating rate of 4 °C / min, and hold for 2 h to form a precursor with an initial carbon layer coating on the surface; Step 2: Weigh the precursor obtained in Step 1, and add hexachlorocyclotriphosphazene (HCCP) at 7% of the precursor's mass and p-phenylenediamine (p-PDA) at 17% of the precursor's mass. Add anhydrous ethanol as a wet milling additive to the mixture, at a mass of 15% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 380 r / min for 2.5 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it at 710 °C under argon protection at a heating rate of 11 °C / min, hold for 10 h, and naturally cool to room temperature to obtain a double-coated, Yb-doped synergistically modified lithium manganese iron phosphate cathode material.

[0030] Example 11: A double-coated, Y / Yb composite doped lithium iron phosphate cathode material with synergistic modification, its chemical formula is LiMn 0.56 Fe 0.4 Y 0.02 Yb 0.02 PO4 includes the following steps: Step 1: Weigh out Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Y₂O₃, Yb₂O₃, and NH₄H₂PO₄ according to the molar ratio of Li:Mn:Fe:Y:Yb:P = 1:0.56:0.4:0.02:0.02:1, and add sucrose at 10% of the total mass of the above raw materials. Add anhydrous ethanol as a wet milling additive to the mixture at 22% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 580 r / min for 7 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 350 °C under an argon atmosphere at a heating rate of 4 °C / min for 2.5 h to form a precursor with an initial carbon layer coating on the surface. Step 2: Weigh the precursor obtained in Step 1, and add 7.5% (by mass) of hexachlorocyclotriphosphazene (HCCP) and 20% (by mass) of p-phenylenediamine (p-PDA) to the mixture. Add anhydrous ethanol as a wet milling additive to the mixture, at a rate of 18% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 420 r / min for 2 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it at a heating rate of 10 °C / min under argon protection, holding for 9 h, and then naturally cool to room temperature to obtain a double-coated, Y / Yb composite doped synergistic modified lithium manganese iron phosphate cathode material.

[0031] Example 12: A double-coated, Lu-doped synergistically modified lithium manganese iron phosphate cathode material, with the chemical formula LiMn 0.57 Fe 0.4 Lu 0.03 PO4 includes the following steps: Step 1: Weigh Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Lu₂O₃, and NH₄H₂PO₄ according to the molar ratio Li:Mn:Fe:Lu:P = 1:0.57:0.4:0.03:1, and add sucrose at 3% of the total mass of the above raw materials. Add anhydrous ethanol as a wet milling additive to the mixture at 30% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 400 r / min for 10 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 450 °C under an argon atmosphere at a heating rate of 3 °C / min for 4 h to form a precursor with an initial carbon layer coating on the surface. Step 2: Weigh the precursor obtained in Step 1, and add hexachlorocyclotriphosphazene (HCCP) at 3% of the precursor's mass and p-phenylenediamine (p-PDA) at 6% of the precursor's mass. Add ethylene glycol as a wet milling additive to the mixture at 10% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 200 r / min for 5 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it at 900℃ under argon protection at a heating rate of 8℃ / min, hold for 8 h, and naturally cool to room temperature to obtain a double-coated, Lu-doped synergistically modified lithium manganese iron phosphate cathode material.

[0032] Example 13: A double-coated, Y-doped synergistically modified lithium manganese iron phosphate cathode material, with the chemical formula LiMn 0.55 Fe 0.4 Y 0.05 PO4 includes the following steps: Step 1: Weigh Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, Y₂O₃, and NH₄H₂PO₄ according to the molar ratio of Li:Mn:Fe:Y:P = 1:0.55:0.4:0.05:1, and add sucrose at 20% of the total mass of the above raw materials; add anhydrous ethanol as a wet milling additive to the mixture at 10% of the total solid mass, place the mixture in a ball mill jar, and wet mill at 700 r / min for 5 h. After ball milling, dry the slurry at 80 °C for 10 h to obtain a premixed precursor powder. Place the premixed precursor powder in a tube furnace and calcine it at 200 °C under an argon atmosphere at a heating rate of 5 °C / min, and hold for 5 h to form a precursor with an initial carbon layer coating on the surface; Step 2: Weigh the precursor obtained in Step 1, and add hexachlorocyclotriphosphazene (HCCP) at 10% of the precursor's mass and p-phenylenediamine (p-PDA) at 20% of the precursor's mass. Add isopropanol as a wet milling additive to the mixture, at a mass of 30% of the total solid mass. Place the mixture in a ball mill jar and wet mill at 500 r / min for 2 h. After ball milling, dry to obtain a secondary coated mixture. Place the secondary coated mixture in a tube furnace and calcine it at 600℃ under argon protection at a heating rate of 12℃ / min, hold for 12 h, and naturally cool to room temperature to obtain a double-coated, Y-doped synergistically modified lithium manganese iron phosphate cathode material.

[0033] Comparative Example 1 Comparative Example 1 provides a lithium manganese iron phosphate cathode material without rare earth element doping and without double coating synergistic modification, with the chemical formula LiMn. 0.6 Fe 0.4 PO4, the specific steps are as follows: Li₂CO₃, MnCO₃, FeC₂O₄·2H₂O, and NH₄H₂PO₄ were weighed according to the molar ratio of Li:Mn:Fe:P = 1:0.6:0.4:1, and sucrose was added. The sucrose content was 10% of the total raw material mass. Anhydrous ethanol was added as a wet milling additive, with an addition amount of 20% of the total solid mass. The mixture was wet-milled at 500 r / min for 7 h. After drying, the mixture was heated to 350 °C at 4 °C / min under an argon atmosphere and held at that temperature for 2 h. Then, the temperature was increased to 700 °C at 4 °C / min and held at that temperature for 10 h to obtain the sample.

[0034] Comparative Example 2 Comparative Example 2 provides a lithium iron phosphate cathode material with Y doping but without double coating synergistic modification, and its chemical formula is LiMn. 0.55 Fe 0.4 Y 0.05 PO4, the specific steps are as follows: Using the same raw material ratio and first wet milling and pre-calcination process as in Example 1, after obtaining the precursor, HCCP and p-PDA were not added, and a second wet milling coating was not performed. Instead, the sample was directly prepared by heating to 700°C at 4°C / min under an argon atmosphere and holding for 10 hours.

[0035] Comparative Example 3 Comparative Example 3 provides a lithium manganese iron phosphate cathode material with Y doping and dual-coating synergistic modification, but without a rapid heating strategy during secondary calcination. Its chemical formula is LiMn. 0.55 Fe 0.4 Y 0.05 PO4.

[0036] The difference between Comparative Example 3 and Example 1 is that the heating rate during the second calcination is 4°C / min.

[0037] Comparative Example 4 Comparative Example 4 provides a lithium manganese iron phosphate cathode material with Y doping and synergistic modification of double coating layers, but with a doping concentration exceeding the range of this invention. Its chemical formula is LiMn. 0.51 Fe 0.4 Y 0.09 PO4.

[0038] The difference between Comparative Example 4 and Example 1 is that Li2CO3, MnCO3, FeC2O4·2H2O, Y2O3 and NH4H2PO4 were weighed according to the molar ratio of Li:Mn:Fe:Y:P = 1:0.51:0.4:0.09:1. Figure 1 The preparation process of the present invention is shown, based on which the synergistic regulation of rare earth element doping and double coating layer on lithium manganese iron phosphate cathode is achieved.

[0039] Figure 2 The SEM image of the lithium manganese iron phosphate cathode prepared in Example 1 is shown. It can be seen that the primary particles of the obtained cathode have uniform particle size and regular morphology, and the particle size is concentrated in the range of 100~200nm.

[0040] The positive electrode materials prepared in Examples 1, 5, 9, 10, 11 and Comparative Examples 1-4 were weighed respectively. The positive electrode materials, along with conductive agents, binders, etc., were fabricated into electrode sheets using conventional processes. These were then assembled into coin cell half-cells, using lithium metal sheets as the negative electrode, and employing a polypropylene separator and electrolyte. Charge-discharge cycle tests were conducted within a voltage range of 2.0-4.5V, at a 2C rate, and at 25°C. Figure 3The graph shows a comparison of the cycle performance of the LMFP cathode materials prepared in Example 1 and Comparative Example 1, demonstrating that the process of the present invention significantly improves the capacity retention and cycle life of the materials. Table 1 lists the initial specific capacity and capacity retention after 300 cycles of the coin cells prepared in each example and comparative example, used to evaluate the effect of the synergistic modification of the double coating layer and rare earth element doping on the electrochemical performance of the lithium manganese iron phosphate cathode material.

[0041] Table 1. Electrochemical performance of coin half-cells prepared in each embodiment and comparative example. As shown in Table 1, the electrochemical performance of all LMFP materials prepared in the examples with different rare earth element doping and double coating synergistic modification was tested at a rate of 2C. The data indicates that the synergistic effect of Y doping and the double coating in Example 1 significantly improves the discharge specific capacity and cycle stability of the material. Its discharge specific capacity reaches 122.9 mAh / g, and the capacity retention rate after 300 cycles is as high as 98.8%. Example 10, with Yb doping, is second best, with a specific capacity of 121.1 mAh / g and a capacity retention rate of 96.1%, exhibiting excellent long-term cycle stability. Although the specific capacities of Pr-doped Example 9, Sc-doped Example 5, and Y / Yb composite-doped Example 11 are slightly lower, at 121.7, 119.9, and 117.9 mAh / g respectively, they are still significantly better than the unmodified Comparative Example 1.

[0042] All comparative data visually demonstrate the weakening effect of various process defects on electrochemical performance. Comparative Example 1, undoped and without double-coating control, has a discharge specific capacity of only 112.1 mAh / g, with a capacity retention rate as low as 67.9% after 300 cycles, exhibiting poor material structural stability and the most severe capacity decay. Comparative Example 2, with only Y doping and no double-coating control, sees its discharge specific capacity drop to 109.4 mAh / g, with a retention rate of 72.1%, proving that the double coating is crucial for improving material conductivity, buffering volume deformation, and mitigating capacity decay. Comparative Example 3, using a slow heating rate during secondary calcination, has a discharge specific capacity of only 100.8 mAh / g and a capacity retention rate of 75.6%, but still higher than Comparative Example 1's 67.9%, indicating that rapid heating during secondary calcination helps alleviate material degradation during cycling. Comparative Example 4, with excessive Y doping, has a specific capacity of 106.6 mAh / g and a capacity retention rate of 78.1%, indicating that there is an optimal concentration of rare earth doping; excessive doping reduces the proportion of active material, disrupts lattice order, and lowers electrochemical performance.

[0043] In summary, Comparative Example 1, as a control group of lithium manganese iron phosphate cathode material that has not undergone any modification methods involved in this invention, exhibits significantly lower long-cycle stability than Example 1. The initial specific capacity of Comparative Example 1 at 2C (112.1 mAh / g) is lower than that of Example 1 (122.9 mAh / g), and its capacity retention after 300 cycles is only 67.9%, which is 30.9 percentage points lower than the 98.8% of Example 1.

[0044] This invention modifies lattice parameters through rare earth element doping, suppressing crystal structure distortion; a dense double-coating layer enhances conductivity, suppresses interfacial side reactions and manganese dissolution, and reduces polarization; rapid heating during secondary calcination promotes liquid-solid contact between raw materials, effectively reducing mechanical stress within and between particles, thereby significantly alleviating material degradation during long-term cycling. Example 1 prepared according to this invention exhibits superior structural stability and electrochemical reversibility during long-term charge-discharge cycling, significantly improving the material's discharge specific capacity and long-term cycling performance.

Claims

1. A double-coated, doped synergistically modified lithium manganese iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material has an olivine crystal structure and the general chemical formula: LiMn 0.6-x-y Fe 0.4 M 1 x M 2 y PO4, where: 0.01≤x+y≤0.07, x≥0, y≥0; when exactly one of x and y is greater than 0, M 1 Or M 2 M is a rare earth element; when x > 0 and y > 0, 1 and M 2 These are two different rare earth elements.

2. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, Includes the following steps: Step 1, according to Li:Mn:Fe:M 1 M 2 Weigh out Li2CO3, MnCO3, FeC2O4·2H2O, rare earth element oxides and NH4H2PO4 in a molar ratio of P = 1:0.6-xy:0.4:x:y:1, and add sucrose at a mass of 3%~20% of the total mass of the above raw materials. Place the mixture in a ball mill and ball mill it together. Then calcine it at 200℃~450℃ under an argon atmosphere to form a precursor. The rare earth element oxides are selectively added according to the values ​​of x and y, satisfying 0.01≤x+y≤0.07, x≥0, y≥0; when only one of x and y is greater than 0, M is introduced. 1 Or M 2 The oxides corresponding to the non-zero components; when x > 0 and y > 0, two different rare earth elements M are introduced simultaneously. 1 and M 2 Oxides; Step 2: Mix the precursor from Step 1 with hexachlorocyclotriphosphazene (HCCP) and p-phenylenediamine (p-PDA), place them in a ball mill and ball mill them evenly. Then, place them in an argon protective atmosphere and calcine them at a heating rate of 8℃ / min~12℃ / min to 600℃~900℃ to finally synthesize lithium manganese iron phosphate cathode material. The hexachlorocyclotriphosphazene (HCCP) and p-phenylenediamine (p-PDA) are 3% to 10% and 6% to 20% of the precursor mass, respectively.

3. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The rare earth element oxides mentioned in step 1 are Y2O3, Sc2O3, Yb2O3, Pr2O3 and Lu2O3, and when x>0 and y>0, the rare earth elements contained in the two rare earth element oxides added are different from each other.

4. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The ball milling speed in step 1 is 400 r / min to 700 r / min, and the milling time is 5 h to 10 h.

5. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, In step 1, the calcination temperature is increased to the calcination temperature at a heating rate of 3℃ / min to 5℃ / min, and then held at the calcination temperature for 2h to 5h.

6. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The ball milling in steps 1 and 2 is wet ball milling, and the amount of wet milling additive added is 10% to 30% of the solid content. The wet milling additive is at least one solvent among anhydrous ethanol, acetone, ethylene glycol, diethylene glycol dimethyl ether, and isopropanol.

7. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The ball milling speed in step 2 is 200 r / min to 500 r / min, and the milling time is 2 h to 5 h.

8. The method for preparing a double-coated, doped synergistically modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The calcination time in step 2 is 8h~12h.

9. The application of the double-coated, doped and synergistically modified lithium manganese iron phosphate cathode material as described in claim 1 in the preparation of lithium-ion battery cathodes or lithium-ion batteries.

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