High-performance long-cycle lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a high-performance long-cycle lithium iron phosphate positive electrode material and a preparation method thereof, and solves the technical problem of poor long cycle performance of existing lithium iron phosphate due to non-uniform and unstable carbon layer network. The preparation method comprises the following steps: S1, mixing and ball-milling iron phosphate A and iron phosphate B with different specific surface areas, lithium carbonate, a carbon source, an organic polymer dispersant and a metal ion doping agent to form precursor slurry; s2, drying the precursor slurry to obtain a precursor yellow material; s3, performing primary calcination on the precursor yellow material in an inert gas atmosphere to obtain lithium iron phosphate powder, and crushing the lithium iron phosphate powder; s4, uniformly mixing the crushed lithium iron phosphate with a carbon source, a nitrogen-containing organic matter and a metal ion doping agent, and carrying out spray drying after ball milling; and S5, carrying out secondary calcination on the spray-dried material in an inert gas atmosphere to obtain the lithium iron phosphate with the nitrogen atom-doped carbon coating layer. The lithium iron phosphate positive electrode material has the advantages of stable structure, good long cycle performance and the like.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode material technology, specifically to a high-performance, long-cycle lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] In the lithium iron phosphate preparation process, glucose is a typical carbon coating agent with advantages such as low cost and easy pyrolysis into carbon. However, it also has the disadvantage of insufficient carbon layer density. The carbon layer generated by glucose pyrolysis is usually amorphous carbon with low graphitization degree and poor conductivity. The loose or uneven distribution of the carbon coating layer structure can lead to the breakage of the electron conduction network and the shedding of active material.

[0003] In high-performance, long-cycle lithium iron phosphate (LFP) batteries, a stable and uniform carbon layer network is required to maintain the structural stability of the cathode during cycling. Stable carbon coating also physically isolates the active material from the electrolyte, reducing side reactions and suppressing iron dissolution. Appropriate particle size and distribution of LFP also significantly impact electrical performance; excessively large particles are prone to breakage during charge and discharge, while excessively small particles affect energy density and are easily corroded by the electrolyte.

[0004] Based on the above issues, this patent focuses on the carbon coating, particle size, and particle size distribution of lithium iron phosphate, aiming to improve the long-cycle performance of the product by optimizing the cathode material, lithium iron phosphate. Summary of the Invention

[0005] The technical problem to be solved by this invention is that the existing lithium iron phosphate cathode material has poor long-cycle performance due to the uneven and unstable carbon layer network, which needs to be improved.

[0006] This invention is achieved through the following technical solution: A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material includes the following steps: S1. Two types of iron phosphate A and iron phosphate B with different specific surface areas, lithium carbonate, carbon source, organic polymer dispersant and metal ion dopant are ball-milled in a liquid medium to form a uniform precursor slurry. S2. Dry the precursor slurry obtained in step S1 to obtain precursor yellow material; S3. The lithium iron phosphate precursor yellow material obtained in step S2 is calcined once in an inert gas atmosphere to obtain lithium iron phosphate powder with good gradation, and then the lithium iron phosphate powder is crushed. S4. The pulverized lithium iron phosphate obtained in step S3 is mixed evenly with carbon source, nitrogen-containing organic matter and metal ion dopant in liquid medium, and then ball-milled and spray-dried. S5. The spray-dried material obtained in step S4 is calcined a second time in an inert gas atmosphere to obtain lithium iron phosphate powder with a nitrogen atom doped carbon coating.

[0007] This invention utilizes a two-stage carbon coating process. In the carbon coating process of lithium iron phosphate (LFP), the first carbon coating is generally uneven, mainly due to the presence of free carbon or incomplete coverage. The second carbon coating replenishes carbon and introduces nitrogen-containing organic matter to achieve a nitrogen-doped carbon coating layer, improving the uniformity and continuity of the coating layer. During the second sintering, the active intermediates generated by the pyrolysis of the nitrogen-containing organic matter undergo a gas-solid reaction with the free carbon from the first coating, ultimately repairing the carbon layer and doping with nitrogen while reducing free carbon agglomeration. The newly generated carbon coating layer covers the uncoated particle areas from the first coating through vapor-phase deposition, such as hydrocarbon pyrolysis, forming a continuous carbon layer. Nitrogen-containing functional groups combine with the carbon on the LFP surface to form -NC bonds, enhancing the stability of the carbon coating layer. Furthermore, nitrogen doping itself can alter the electronic structure of the carbon layer, further improving electronic conductivity. By using precursors with different specific surface areas, the particle size and gradation distribution of the LFP product are effectively controlled, improving energy density. Appropriate particle size maintains structural stability during cycling.

[0008] This invention preferably discloses a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein in step S1, the specific surface area of ​​iron phosphate A is 8-12 m². 2 / g, the specific surface area of ​​ferric phosphate B is 12~16m². 2 / g, the mass ratio of ferric phosphate A to ferric phosphate B is 5:2.5~0.5.

[0009] Preferably, the amount of lithium source and iron source added is in a molar ratio of Li:Fe = 1.01~1.05:1.

[0010] This invention achieves a good particle size distribution by calcining iron phosphate A and iron phosphate B with different specific surface areas, which provides a good size distribution for the second sintering, making more effective use of the physical space of the positive electrode sheet, thereby improving the overall performance of the battery and helping to increase the compaction density of lithium iron phosphate.

[0011] The present invention preferably provides a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein in step S1, the amount of carbon source added is 8% to 15% of the amount of lithium iron phosphate added, and in step S4, the amount of carbon source added is 0.1% to 1% of the mass of lithium iron phosphate.

[0012] Preferably, the carbon source is one of glucose, sucrose or starch.

[0013] The present invention preferably provides a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein in step S1, the organic polymeric dispersant is at least one of polyethylene glycol and polyvinyl alcohol, and the amount added is 1.2% to 3.3% of the amount of lithium iron phosphate added.

[0014] Preferably, the metal ion dopant mentioned in S1 is one or more of titanium dioxide, vanadium pentoxide, and niobium pentoxide, and the doping amount is controlled between 500 and 3000 ppm. Preferably, the ball milling time in S1 is 60~120 min; Preferably, the inlet air temperature for the spray drying in S2 is 160–260°C; The present invention preferably provides a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein in step S3, the temperature of the first calcination is 680–800°C and the calcination time is 4–10 hours.

[0015] Preferably, in S3, the standard for pulverization is: the particle size D50 of lithium iron phosphate powder is between 0.8 and 1.5 μm.

[0016] The present invention preferably provides a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein the calcination time for a single calcination is 6-10 hours.

[0017] The present invention preferably provides a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein in step S4, the nitrogen-containing organic compound is at least one of urea, polyaniline, melamine, and polyvinylpyrrolidone, and the amount added is 4 to 10% of the mass of lithium iron phosphate.

[0018] Preferably, the metal ion dopant in S4 is one or more of titanium dioxide, vanadium pentoxide, and niobium pentoxide, and the doping amount is controlled between 500 and 1500 ppm. Preferably, the ball milling time in S4 is 30~60 min; Preferably, the inlet air temperature for spray drying in S4 is 160–260°C.

[0019] The present invention preferably provides a method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, wherein in step S5, the secondary calcination temperature is 680–780°C and the secondary calcination time is 6–10 hours.

[0020] The present invention uses a specific temperature and holding time during the first calcination to prevent the carbon layer from densifying too early and hindering the second step of nitrogen doping; the second calcination provides a sufficiently high temperature to ensure that nitrogen atoms are incorporated into the carbon coating layer.

[0021] The present invention preferably provides a method for preparing high-performance, long-cycle lithium iron phosphate cathode material, wherein in the secondary calcination, ≤5% hydrogen is introduced into an inert gas atmosphere.

[0022] In this invention, nitrogen and / or argon are used for protection during the first calcination, and a small amount of hydrogen is introduced during the second calcination to enhance the reducing power and nitrogen doping efficiency.

[0023] A high-performance, long-cycle lithium iron phosphate cathode material is prepared by the above method.

[0024] The present invention has the following advantages and beneficial effects: 1. The present invention controls the particle size distribution of lithium iron phosphate by mixing iron phosphate with different specific surface areas to form a good particle size distribution at the raw material end. This not only improves the compaction density of the material, but also makes more effective use of the physical space of the positive electrode sheet, thereby improving the overall performance of the battery. 2. In the preparation of precursor slurry by primary calcination, the addition of organic polymeric dispersant is beneficial to the formation of uniformly dispersed particle size distribution during primary calcination, thereby improving the material pressure density of secondary carbon coating and nitrogen doping as well as secondary calcination, and ultimately improving the comprehensive performance of the material. 3. This invention employs a two-stage carbon coating process, fully considering the impact of the first carbon coating on the second carbon coating and nitrogen doping. A carbon source, metal ion dispersant, and nitrogen-containing organic matter are directly added to the lithium iron phosphate particles for secondary sintering. Free carbon reflows at high temperatures, covering the surface of the lithium iron phosphate particles not coated in the first sintering. Furthermore, the interaction between the functional groups of the carbon source and lithium iron phosphate, as well as with the nitrogen-containing organic matter, forms a continuous nitrogen-doped carbon film. This increases the graphitization degree of the carbon coating layer, making the carbon layer denser and smoother, reducing carbon particle agglomeration, and ensuring uniform nitrogen doping. This results in a more stable and resilient coating layer, effectively mitigating carbon layer tearing loss during charge-discharge processes and improving the material's cycle life. Heteroatoms can also act as electron donors and generate numerous defects, further enhancing electrochemical activity. Attached Figure Description

[0025] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings: Figure 1 This is a process flow diagram of the present invention; Figure 2 XRD diffraction of lithium iron phosphate with nitrogen-doped carbon coating in Example 1; Figure 3 The images show the microstructures of lithium iron phosphate with nitrogen-doped carbon coating in Example 1 and lithium iron phosphate prepared by one-pot method in Comparative Example 1; where (a) is the microstructure of Example 1 and (b) is the microstructure of Comparative Example 1. Figure 4 The image shows the microstructure of lithium iron phosphate after a single calcination and pulverization in Example 1. Figure 5The graph shows the rate discharge performance of coin cells made of lithium iron phosphate with nitrogen-doped carbon coating in Example 1 and lithium iron phosphate prepared by one-step sintering in Comparative Example 1. Figure 6 This is a comparison chart of the all-electric long-cycle performance of lithium iron phosphate with nitrogen-doped carbon coating in Example 1 and lithium iron phosphate without nitrogen atoms in Comparative Example 2. Figure 7 The images show the microstructure of the cathode material electrodes made from nitrogen-doped carbon-coated lithium iron phosphate in Examples 1 and Comparative Example 2 before and after cycling. (a) is the microstructure of the cathode material electrode made from nitrogen-doped carbon-coated lithium iron phosphate in Example 1 before cycling; (b) is the microstructure of the cathode material electrode made from undoped lithium iron phosphate in Comparative Example 2 before cycling; (c) is the electrode morphology of the cathode material in Example 1 after 500 cycles; and (d) is the electrode morphology of the cathode material prepared from undoped lithium iron phosphate in Comparative Example 2 after 500 cycles. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention. Example

[0027] A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, such as... Figure 1 As shown, it includes the following steps: S1, with a specific surface area of ​​8~12m² 2 / g of ferric phosphate A and 12~16m 2 / g of iron phosphate B, lithium carbonate, carbon source, organic polymer dispersant and metal ion dopant are ball-milled in liquid medium to form a uniform precursor slurry; S2. Dry the precursor slurry obtained in step S1 to obtain precursor yellow material; S3. The lithium iron phosphate precursor yellow material obtained in step S2 is calcined once in an inert gas atmosphere to obtain lithium iron phosphate powder with good gradation, and then the lithium iron phosphate powder is crushed. S4. The pulverized lithium iron phosphate obtained in step S3 is mixed evenly with carbon source, nitrogen-containing organic matter and metal ion dopant in liquid medium, and then ball-milled and spray-dried. S5. The spray-dried material obtained in step S4 is calcined a second time in an inert gas atmosphere to obtain lithium iron phosphate powder with nitrogen atom doped carbon coating, and then crushed. Example 1

[0028] With a specific surface area of ​​8m 2 / g of ferric phosphate A and a specific surface area of ​​15m 2 The mass ratio of ferric phosphate B to lithium carbonate (LiFe) was 5:1.5. Lithium carbonate was dispersed in deionized water at a lithium-iron molar ratio of 1.02:1. 9 wt% glucose (FeFe), 2 wt% polyethylene glycol 2000 (FeFe), and 2000 ppm titanium dioxide were added. After ball milling for 90 minutes, the mixture was spray-dried at an inlet air temperature of 180℃ to obtain a precursor yellow material. The precursor yellow material was then heated from room temperature to 790℃ under a nitrogen atmosphere and held at 790℃ for 6 hours to complete one calcination. After cooling, it was removed and pulverized to a D50 of 0.8-1.5. The lithium iron phosphate powder with good gradation obtained in step S3 is then mixed with 0.15 wt% glucose, 8 wt% polyaniline, and 1000 ppm titanium dioxide in deionized water and ball-milled for 20 min. After spray drying, the feed temperature of the spray drying is 180℃. The mixture is then calcined twice under a nitrogen atmosphere at a sintering temperature of 740℃ for 8 hours. After cooling to room temperature, the mixture is pulverized to a D50 of 0.8-1.5 μm to obtain a nitrogen-doped carbon-coated lithium iron phosphate. Example 2

[0029] The difference between this embodiment and Embodiment 1 is that 3% hydrogen is added in a nitrogen atmosphere during the secondary calcination. Example 3

[0030] The difference between this embodiment and Embodiment 1 is that, in one calcination, the surface areas of ferric phosphate A and ferric phosphate B are 10m² and 10m² respectively. 2 / g and 13m 2 / g. Example 4

[0031] The difference between this embodiment and Embodiment 1 is that the calcination time for one calcination is 8 hours. Example 5

[0032] The difference between this embodiment and Embodiment 1 is that the calcination time is 4 hours. Example 6

[0033] The difference between this embodiment and Embodiment 1 is that the organic polymeric dispersant is PEG6000, and the amount added is 2 wt% of ferric phosphate. Example 7

[0034] The difference between this embodiment and Embodiment 1 is that the organic polymeric dispersant is PEG2000, and the amount added is 3 wt% of the mass of iron phosphate. Example 8

[0035] The difference between this embodiment and Embodiment 1 is that the organic polymeric dispersant is PEG2000, and the amount added is 1.5 wt% of the mass of iron phosphate. Example 9

[0036] The difference between this embodiment and Embodiment 1 is that the nitrogen-containing organic compound added during the secondary calcination is different; specifically, it is melamine. Example 10

[0037] The difference between this embodiment and Embodiment 1 is that the nitrogen-containing organic compound added during the secondary calcination is different; specifically, it is polyvinylpyrrolidone. Example 11

[0038] The difference between this embodiment and Embodiment 1 is that the metal additives used in the first calcination are different, specifically titanium dioxide with a doping amount of 3000 ppm and vanadium pentoxide with a doping amount of 1000 ppm. Example 12

[0039] The difference between this embodiment and Embodiment 1 is that the metal additives used in the first calcination are different, specifically titanium dioxide with a doping amount of 3000 ppm and niobium pentoxide with a doping amount of 1000 ppm.

[0040] Comparative Example 1 The difference between this comparative example and Example 1 is that a one-step sintering process is used, specifically: The mass ratio of iron phosphate A to iron phosphate B was 5:1.5, and lithium carbonate was dispersed in deionized water at a lithium-iron molar ratio of 1.02:1. 9.15 wt% glucose, 8 wt% polyaniline, 2 wt% polyethylene glycol 2000, and 3000 ppm titanium dioxide were added. After ball milling for 90 minutes, the mixture was spray-dried at an inlet air temperature of 180°C. The precursor was heated from room temperature to 790°C under a nitrogen atmosphere and maintained at 790°C for 8 hours. After cooling, the mixture was removed and pulverized to obtain nitrogen-doped lithium iron phosphate powder.

[0041] Comparative Example 2 The difference between this comparative example and Example 1 is that it does not contain nitrogen.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that ferric phosphate A and B use the same type of ferric phosphate with a specific surface area of ​​10 m². 2 / The ferric phosphate in g is a single raw material and no particle size distribution is performed.

[0043] Comparative Example 4 The difference between this comparative example and Example 1 is that the calcination temperature is lower, at 640°C.

[0044] Comparative Example 5 The difference between this comparative example and Example 1 is that no carbon source is added during the secondary sintering.

[0045] Comparative Example 6 The difference between this comparative example and Example 1 is that twice the amount of carbon as in Example 1 was added during the second sintering.

[0046] The results of testing different embodiments and comparative examples are shown in Tables 1, 2 and 3 below.

[0047] Table 1. Detection results of different embodiments and comparative examples

[0048] Table 2. Results of testing on the particle size, compaction, and electrical properties of lithium iron phosphate with different specific surface areas.

[0049] Table 3. Test results of compaction, electrical properties and iron dissolution of lithium iron phosphate in different embodiments and comparative examples

[0050] The nitrogen-doped carbon-coated lithium iron phosphate powder obtained in Example 1 was subjected to XRD diffraction, and the obtained XRD diffraction pattern is shown in the figure. Figure 2 As shown, the XRD diffraction pattern was compared with the standard card of lithium iron phosphate, proving that the prepared material was lithium iron phosphate, and no impurities were generated after the addition of metal ion dopants and nitrogen-doped carbon coating layer.

[0051] Figure 3 These are microscopic morphology images comparing the nitrogen-doped carbon-coated lithium iron phosphate in Example 1 with the one-pot prepared lithium iron phosphate in Comparative Example 1. In Example 1, glucose and a metal ion dispersant were added to the lithium iron phosphate particles and sintered again to form a continuous carbon film, making the carbon layer denser and smoother, reducing carbon particle agglomeration, and resulting in a more stable and tougher coating. In Comparative Example 1, the lithium iron phosphate had more free carbon, resulting in an unstable, loose carbon layer with a rough surface.

[0052] Figure 4 The image shows the morphology of lithium iron phosphate particles obtained after one calcination. Analysis of the particle size in the image reveals that small particles (100-400nm) account for approximately 50%, medium particles (400-700nm) account for approximately 30%, and large particles (700-1000nm) account for approximately 20%. This provides a good gradation for the second sintering, making more effective use of the physical space of the positive electrode sheet, thereby improving the overall performance of the battery.

[0053] Figure 5The graph shows the rate discharge performance of coin cells made from lithium iron phosphate with nitrogen-doped carbon coating in Example 1 and lithium iron phosphate prepared by one-step sintering in Comparative Example 1. It shows that the battery performance of lithium iron phosphate is significantly improved after secondary sintering. As the discharge rate and cycle number increase, the advantage of Example 1 becomes more obvious. Its capacity is around 130 mAh / g at 10C, while the discharge capacity of Comparative Example 1 is around 100 mAh / g, which is significantly lower.

[0054] Figure 6 This is a comparison chart of the long-cycle performance of nitrogen-doped carbon-coated lithium iron phosphate in Example 1 and undoped lithium iron phosphate in Comparative Example 2 at high 3C rate. It shows that the nitrogen-doped carbon-coated lithium iron phosphate has a better cycle performance. In Example 1, the capacity retention rate is above 93% after 500 cycles, while in Comparative Example 2, the capacity retention rate drops significantly around the 100th cycle, and the capacity retention rate is only 80% after 500 cycles.

[0055] Figure 7 (a) is a microscopic morphology image of the cathode material electrode made of nitrogen-doped carbon-coated lithium iron phosphate in Example 1 before cycling. Figure 7 (b) is a microstructure of the cathode material electrode made from lithium iron phosphate without nitrogen atom doping in Comparative Example 2 before cycling. Figure 7 (c) is an image of the electrode morphology of the cathode material in Example 1 after 500 cycles. A comparison of the morphology before and after shows that the morphology is normal after cycling and shows no significant change compared to before cycling. Figure 7 (d) is an electrode image of the cathode material prepared from nitrogen-free lithium iron phosphate in Comparative Example 2 after 500 cycles. Cracks have appeared in the particles, as shown in the circled area in the image. The formation of cracks disrupts the electrical contact balance between particles and between particles and the conductive agent, resulting in capacity loss and deterioration in cycle performance.

[0056] In addition, Comparative Examples 5 and 6 were subjected to long-cycle performance tests under the same conditions as Example 1, wherein Comparative Example 5 had a capacity retention rate of 75% after 500 cycles, while Comparative Example 6 had a capacity retention rate of 85% after 500 cycles.

[0057] The following conclusions can be drawn from the comparison of different embodiments and comparative examples in the table above: 1. Compared with Example 1, Example 2 shows that by introducing a small amount of hydrogen during sintering, the compaction density remains basically unchanged, while the electrical performance is slightly improved. This is mainly because the addition of hydrogen improves the efficiency of nitrogen doping, which is beneficial to the improvement of electrical performance.

[0058] 2. Comparison of Examples 1, 3, and Comparative Example 3 in Table 2 shows that when using ferric phosphate with a single specific surface area, the particle size distribution is relatively large, failing to form a good gradation. This results in a slight decrease in compaction density and a significant increase in resistivity, leading to a reduction in electrical properties. Example 3 demonstrates that the blending of ferric phosphate with different specific surface areas requires careful selection and matching of these areas; the greater the difference, the more pronounced the particle gradation and the better the effect.

[0059] 3. A comparison of Examples 4 and 5 with Example 1 shows that if the sintering time is too long, it is not conducive to improving the electrical performance and may lead to particle size growth. Although the compaction density is slightly improved, the electrical performance will decrease. On the other hand, if the sintering time is too short, the carbon coating is incomplete and the nitrogen is not fully doped into the carbon layer, which will result in a slight decrease in electrical performance. It can be seen that the sintering time affects the particle size distribution, carbon coating and nitrogen doping. The combination of these factors needs to be fully considered to obtain lithium iron phosphate cathode materials with higher comprehensive performance.

[0060] 4. A comparison between Example 6 and Example 1 shows that the electrical performance decreased after replacing PEG2000 with PEG6000. This is mainly because PEG2000, due to its smaller molecular weight, lower viscosity, and better thermal decomposition behavior, can form a more uniform, denser, and more conductive carbon coating layer during the carbon coating process of lithium iron phosphate in this invention. In addition, the steric hindrance effect of PEG2000 is lower than that of PEG6000, making it easier for nitrogen doping to combine with carbon on the surface of lithium iron phosphate to form Fe-NC bonds, thereby strengthening the adhesion between the carbon coating layer and lithium iron phosphate particles and improving nitrogen doping efficiency.

[0061] 5. Comparison of Examples 7 and 8 with Example 1 shows that the amount of PEG2000 added affects the compaction density and electrical properties. Within the range of 1.5%-3.0%, it performs well in terms of compaction density and electrical properties.

[0062] 6. As can be seen from Table 3, Example 1 and Comparative Example 1, the iron leaching of the lithium iron phosphate product without a second sintering is the highest. The iron leaching of the product after the second sintering is significantly reduced, indicating that the second sintering can greatly improve the carbon coating and reduce the iron leaching phenomenon. After doping the carbon layer with nitrogen atoms, the iron leaching can also be reduced, indicating that after doping the carbon coating layer with nitrogen atoms, the uniformity and continuity of the carbon layer are further optimized.

[0063] 7. Comparing Examples 9 and 10 with Example 1, it can be seen that the addition of polyaniline exhibits better electrical performance. This is mainly due to the structural advantages of polyaniline. Polyaniline (PANI) has precursor advantages; it is a conductive polymer with an extended conjugated π-bond structure. During heat treatment carbonization, this structure can serve as a "template," making it easier to graphitize and form a highly ordered, highly conductive sp² carbon framework. The nitrogen-doped carbon coating layer formed after polyaniline carbonization is continuous, dense, and highly conductive. This not only provides a good electronic conduction pathway but also effectively connects lithium iron phosphate particles, reducing the overall impedance of the electrode. The nitrogen atoms generated by the pyrolysis of polyaniline are more easily integrated into the carbon lattice, forming electrocatalytically active pyridine nitrogen (N-6) and graphitic nitrogen (NQ), which are key to improving electrochemical performance.

[0064] 8. Comparison of Examples 1, 2, 3, and 4 with Example 1 shows that using only one sintering, or without nitrogen doping, using only iron phosphate, or using a lower first sintering temperature all lead to a decrease in compaction density and electrical properties. A lower first sintering temperature may be due to small particle size and a large specific surface area. Since the carbon produced by the pyrolysis of the carbon source needs to cover the surface of all particles, the excessively large specific surface area causes the carbon layer to be "diluted," making it difficult to form a continuous, uniform, and dense coating layer. The end result may be that the carbon layer is too thin or incompletely covering the surface, and this cannot be salvaged even in a second sintering, thus leading to a decrease in electrical properties.

[0065] 9. Comparison of Comparative Examples 5 and 6 with Example 1 shows that an appropriate amount of carbon needs to be added during the second sintering. Comparative Example 5 did not add carbon, and the electrical performance declined significantly because the carbon layer formed in the first sintering could not be repaired, the internal conductive network was incomplete, the particle size was uneven, and the surface properties were unstable. Comparative Example 6 added excessive carbon, which reduced the compaction density and was not conducive to the overall energy density of the battery.

[0066] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1. Two types of iron phosphate A and iron phosphate B with different specific surface areas, lithium carbonate, carbon source, organic polymer dispersant and metal ion dopant are ball-milled in a liquid medium to form a uniform precursor slurry. S2. Dry the precursor slurry obtained in step S1 to obtain the precursor yellow material; S3. The lithium iron phosphate precursor yellow material obtained in step S2 is calcined once in an inert gas atmosphere to obtain lithium iron phosphate powder with good gradation, and then the lithium iron phosphate powder is crushed. S4. The lithium iron phosphate pulverized in step S3 is mixed evenly with carbon source, nitrogen-containing organic matter and metal ion dopant in liquid medium, and then ball-milled and spray-dried. S5. The spray-dried material obtained in step S4 is calcined a second time in an inert gas atmosphere to obtain lithium iron phosphate powder with a nitrogen atom doped carbon coating.

2. The method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1, characterized in that, In S1, the specific surface area of ​​iron phosphate A is 8~12 m². 2 / g, the specific surface area of ​​ferric phosphate B is 12~16m². 2 / g, the mass ratio of ferric phosphate A to ferric phosphate B is 5: 2.5~0.

5.

3. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In step S1, the amount of carbon source added is 8% to 15% of the amount of iron phosphate added, and in step S4, the amount of carbon source added is 0.1% to 1% of the mass of lithium iron phosphate.

4. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In S1, the organic polymeric dispersant is at least one of polyethylene glycol and polyvinyl alcohol, and the amount added is 1.2% to 3.3% of the amount of ferric phosphate added.

5. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In S3, the temperature of the first calcination is 680-800℃, and the calcination time is 4-10 hours.

6. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The calcination time for one calcination is 6-8 hours.

7. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In S4, the nitrogen-containing organic compound is at least one of urea, polyaniline, melamine, and polyvinylpyrrolidone, and the amount added is 4 to 10% of the mass of lithium iron phosphate.

8. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In S5, the temperature of the secondary calcination is 680-780℃, and the secondary calcination time is 6-10 hours.

9. A method for preparing a high-performance, long-cycle lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In the secondary calcination, ≤5% hydrogen is introduced into an inert gas atmosphere.

10. A high-performance, long-cycle lithium iron phosphate cathode material, characterized in that, Prepared by the method described in any one of claims 1-9.

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