A high-compacted, high-capacity lithium iron manganese phosphate cathode material for constructing a dual-scale particle

Abstract

This invention discloses a high-compactment, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles, prepared by the following method: a spherical precursor is obtained by uniformly mixing a manganese source, an iron source, a lithium source, a composite phosphorus source, a carbon source, and water; the composite phosphorus source includes an inorganic phosphorus source, an organic phosphorus source, and a lithium-based phosphorus source; under an inert atmosphere, the temperature is first raised to 300≤T≤450℃ and held, then raised to 450≤T≤650℃ and held, and finally raised to 700≤T≤800℃ and held, to obtain a lithium manganese iron phosphate cathode material with dual-scale particles and no phosphide impurities. The obtained lithium manganese iron phosphate exhibits a particle size distribution of both large and small crystallites, and the compaction density is increased to 2.60 g / cm³. 3 The material's discharge specific capacity and long-cycle stability are simultaneously improved, with a discharge specific capacity of 153.2 mAh / g or higher at 0.1C and a capacity retention rate of 92.4% or higher after 500 cycles at 1C.

Description

Technical Field

[0001] This invention belongs to the technical field of lithium manganese iron phosphate cathode materials, and particularly relates to a high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles. Background Technology

[0002] Lithium manganese iron phosphate (LMFP), as an upgraded material for lithium iron phosphate cathodes, possesses a higher discharge voltage platform and energy density, while also offering advantages in high safety and low cost, making it suitable for the needs of power batteries and energy storage cells. However, conventional solid-state sintering produces LFP powders with strong synchronous grain growth, uniform particle size, and large packing gaps, resulting in a material compaction density generally below 2.30 g / cm³ and insufficient volumetric energy density, which has become a key bottleneck restricting its industrialization.

[0003] Furthermore, various modification processes in the industry cannot simultaneously achieve both compaction density and discharge capacity, as there is an inherent trade-off between the two: high-temperature densification sintering can improve compaction, but it will cause grain coarsening and high-temperature volatilization of phosphorus, resulting in a significant capacity decay; adding pore-forming agents to prepare porous particles can ensure electrochemical capacity, but permanent pores will further reduce the powder packing density and easily introduce impurities.

[0004] Based on this, a lithium manganese iron phosphate cathode material is provided, which improves compaction by constructing dual-scale particles, and simultaneously enhances discharge capacity and improves electrochemical performance. Summary of the Invention

[0005] Purpose of the invention: The technical problem to be solved by the present invention is to provide a lithium manganese iron phosphate cathode material that improves compaction by constructing dual-scale particles, while simultaneously increasing discharge capacity and improving electrochemical performance, thereby fundamentally solving the technical bottleneck of the mutual constraint between compaction density and discharge capacity of lithium manganese iron phosphate cathode material.

[0006] Technical solution: This invention constructs a high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles, which is obtained by the following steps:

[0007] (1) Manganese source, iron source, lithium source, composite phosphorus source, carbon source and water are mixed evenly to form a slurry; the composite phosphorus source includes inorganic phosphorus source, organic phosphorus source and lithium-based phosphorus source;

[0008] (2) The slurry is milled, sprayed, dried and granulated to obtain spherical precursors;

[0009] (3) Under an inert atmosphere, the temperature is first raised to 300≤T≤450℃ and held for 1-3h to carry out the phosphorus release nucleation reaction of inorganic phosphorus source; then the temperature is raised to 450≤T≤650℃ and held for 5-7h to carry out the slow release phosphorus reaction of organic phosphorus source, and bimodal grains are constructed in situ; finally, the temperature is raised to 700≤T≤800℃ and held for 8-12h to carry out the steady-state release phosphorus source reaction of lithium-based phosphorus source, and a lithium manganese iron phosphate cathode material with dual-scale particles and no phosphide impurity phase is obtained.

[0010] Furthermore, in step (1) of the preparation of the cathode material of the present invention, the inorganic phosphorus source in the composite phosphorus source is selected from ammonium hydrogen phosphate, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate; the organic phosphorus source is selected from triethyl phosphate, methylphosphonic acid, dimethyl phosphonate, or diethyl phosphonate; and the lithium-based phosphorus source is selected from lithium dihydrogen phosphate or lithium phosphate. Even further, the inorganic phosphorus source accounts for 35-40% of the mass of the composite phosphorus source, the organic phosphorus source accounts for 40-45% of the mass of the composite phosphorus source, and the lithium-based phosphorus source accounts for 18-22% of the mass of the composite phosphorus source.

[0011] Furthermore, in step (1) of the cathode material of the present invention, the molar ratio of P:(Mn+Fe):Li in the composite phosphorus source, manganese source, iron source and lithium source is (1.01-1.06):1:(1.02-1.09), and the molar ratio of Mn:Fe is X:(1-X), 0.5≤X≤0.7.

[0012] Furthermore, in step (1) of the cathode material of the present invention, the carbon source is selected from at least one of sucrose, glucose or citric acid, and its added mass is 10-15% of the total mass of the composite phosphorus source, manganese source, iron source and lithium source with different thermal decomposition temperatures.

[0013] Furthermore, the manganese source used in the cathode material of this invention is selected from at least one of manganese carbonate, manganese oxalate, or manganese tetroxide;

[0014] The iron source is selected from at least one of ferrous carbonate, ferrous oxalate, or iron(III) oxide.

[0015] The lithium source is selected from one or more of lithium carbonate, lithium oxalate, or lithium oxide.

[0016] Furthermore, in step (3) of the cathode material of the present invention, the heating rate is 1-5℃ / min.

[0017] Furthermore, in step (1) of the cathode material of the present invention, the solid content of the slurry is 40-55 wt%.

[0018] Furthermore, in step (2) of the cathode material of the present invention, the specific steps of sand milling, spraying, drying and granulation are as follows: the slurry is first sand milled until the particle size D50 reaches 0.3-0.4μm, then stirred for 2-4h to eliminate slurry agglomeration and element segregation, and finally centrifugally spray-dried and granulated under the conditions of inlet air temperature of 180-220℃ and outlet air temperature of 80-110℃ to obtain spherical precursor.

[0019] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are:

[0020] (1) This lithium manganese iron phosphate cathode material achieves time-division gradient slow release of phosphorus by using a three-temperature zone composite functional phosphorus source with mismatched decomposition temperature zones, dispersing the phosphorus source gas generation over a wide temperature range, avoiding the large number of internal through-pores caused by the concentrated gas generation of a single phosphorus source, and reducing particle defects; the low-temperature phosphorus release component preferentially constructs olivine crystal nuclei, the medium- and high-temperature organic phosphorus provides a continuous and stable supply of phosphorus, and the high-temperature lithium-based phosphorus source ensures sufficient crystal growth. The continuous and uniform phosphorus supply effectively suppresses the generation of phosphide impurities, improves phase purity and crystal uniformity, and at the same time, by controlling the phosphorus source ratio and the step sintering system, primary and secondary nucleation can be triggered successively, constructing a bimodal particle size distribution in situ, with small particles filling the gaps between large particles, further improving the compaction density of lithium manganese iron phosphate, which is increased to 2.60 g / cm³. 3 and above.

[0021] (2) The multi-graded particle structure of the lithium manganese iron phosphate cathode material can shorten the lithium ion transport path, improve the lithium ion diffusion efficiency, and simultaneously improve the material's discharge specific capacity and long-cycle stability. The discharge specific capacity at 0.1C reaches 153.2mAh / g or above, and the capacity retention rate at 1C cycle is as high as 92.4% or above after 500 cycles. Attached Figure Description

[0022] Figure 1 This is an electron microscope image of the lithium manganese iron phosphate cathode material of Example 1;

[0023] Figure 2 This is a SEM particle size distribution of the lithium manganese iron phosphate cathode material from Example 1.

[0024] Figure 3 This is a particle size distribution diagram of the lithium iron phosphate cathode material in Example 1.

[0025] Figure 4 This is an electron microscope image of the lithium iron phosphate cathode material of Comparative Example 1;

[0026] Figure 5 This is a SEM particle size distribution of the lithium iron phosphate cathode material in Comparative Example 1.

[0027] Figure 6This is a particle size distribution diagram of the lithium iron phosphate cathode material in Comparative Example 1.

[0028] Figure 7 This is an electron microscope image of the lithium manganese iron phosphate cathode material from Example 2;

[0029] Figure 8 This is a SEM particle size distribution of the lithium iron phosphate cathode material from Example 2.

[0030] Figure 9 This is a particle size distribution diagram of the lithium iron phosphate cathode material in Example 2.

[0031] Figure 10 This is an electron microscope image of the lithium iron phosphate cathode material in Comparative Example 2;

[0032] Figure 11 The SEM particle size distribution of the lithium iron phosphate cathode material in Comparative Example 2 is shown below.

[0033] Figure 12 This is a particle size distribution diagram of the lithium iron phosphate cathode material in Comparative Example 2.

[0034] Figure 13 Here is an electron microscope image of the lithium manganese iron phosphate cathode material of Example 3;

[0035] Figure 14 This is a SEM particle size distribution of the lithium manganese iron phosphate cathode material from Example 3.

[0036] Figure 15 This is a particle size distribution diagram of the lithium iron phosphate cathode material in Example 3;

[0037] Figure 16 This is an electron microscope image of the lithium iron phosphate cathode material in Comparative Example 3;

[0038] Figure 17 The SEM particle size distribution of the lithium iron phosphate cathode material in Comparative Example 3 is shown in Figure 3.

[0039] Figure 18 The particle size distribution diagram is for the lithium manganese iron phosphate cathode material of Comparative Example 3. Detailed Implementation

[0040] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that all raw materials used in the present invention are commercially available.

[0041] It should be noted that the carbon source used in this invention is added at 10-15% of the total mass of the raw materials, and the raw materials are a composite phosphorus source, manganese source, iron source, and lithium source. In the examples and comparative examples, after the raw materials and carbon source are placed in deionized water and stirred evenly, the solid content of the resulting slurry is 40 wt%.

[0042] Example 1

[0043] The lithium iron phosphate cathode material of Example 1 was prepared by the following steps:

[0044] (1) Manganese carbonate, ferrous oxalate, lithium carbonate, and a composite phosphorus source were weighed as raw materials according to the molar ratios Mn:Fe=6:4, (Mn+Fe):P=1:1.03, and Li:(Mn+Fe)=1.05. The composite phosphorus source included 40% ammonium hydrogen phosphate, 40% triethyl phosphate, and 20% lithium dihydrogen phosphate by mass. Sucrose, accounting for 10% of the total mass of the raw materials, was weighed as a carbon source. The raw materials and carbon source were placed in deionized water and stirred evenly to obtain a slurry.

[0045] (2) The mixed slurry was fed into a horizontal sand mill and milled at once. The slurry particle size D50 reached 0.35μm and was discharged. It was then granulated using a centrifugal spray dryer with an inlet air temperature of 200℃, an outlet air temperature of 95℃, and a constant feed rate of 25mL / min using a peristaltic pump to prepare a spherical precursor with a sphericity ≥0.92 and uniform internal components.

[0046] (3) The precursor was evenly spread in a graphite sagger and placed in a box-type atmosphere sintering furnace. Under nitrogen atmosphere, the temperature was raised to 375℃ at a uniform rate of 3℃ / min and held at a constant temperature for 2 hours. Then, the temperature was raised to 550℃ at a uniform rate of 1℃ / min and held at a constant temperature for 6 hours. Finally, the temperature was raised to 750℃ at a uniform rate of 3℃ / min and held at a constant temperature for 10 hours. After sintering, the temperature was controlled by a program at 5℃ / min and the furnace was cooled to room temperature. The entire process was carried out without opening the furnace to break the vacuum or changing the atmosphere, ensuring the continuity of the sintering reaction. The sintered product was broken up by airflow to remove soft agglomerates and then passed through a 200-mesh standard sieve to finally obtain the target lithium manganese iron phosphate cathode material.

[0047] Example 2

[0048] The lithium iron phosphate cathode material of Example 2 was prepared by the following steps:

[0049] (1) Manganese carbonate, ferrous oxalate, lithium carbonate, and a composite phosphorus source were weighed as raw materials according to the molar ratios Mn:Fe=5:5, (Mn+Fe):P=1:1.01, and Li:(Mn+Fe)=1.02. The composite phosphorus source included 38% ammonium dihydrogen phosphate, 42% methylphosphonic acid, and 20% lithium phosphate by mass. Citric acid, accounting for 12% of the total mass of the raw materials, was weighed as a carbon source. The raw materials and carbon source were placed in deionized water and stirred evenly to obtain a slurry.

[0050] (2) The mixed slurry was fed into a horizontal sand mill and milled at once. The slurry particle size D50 reached 0.32μm and was discharged. It was then granulated using a centrifugal spray dryer with an inlet air temperature of 200℃, an outlet air temperature of 95℃, and a constant feed rate of 25mL / min using a peristaltic pump to prepare a spherical precursor with a sphericity ≥0.92 and uniform internal components.

[0051] (3) The precursor was evenly spread in a graphite sagger and placed in a box-type atmosphere sintering furnace. Under nitrogen atmosphere, the temperature was raised to 300℃ at a uniform rate of 3℃ / min and held at a constant temperature for 3h. Then, the temperature was raised to 450℃ at a uniform rate of 1℃ / min and held at a constant temperature for 7h. Finally, the temperature was raised to 700℃ at a uniform rate of 3℃ / min and held at a constant temperature for 12h. After sintering, the temperature was controlled by a program at 5℃ / min and the furnace was cooled to room temperature. The entire process was carried out without opening the furnace to break the vacuum or changing the atmosphere, ensuring the continuity of the sintering reaction. The sintered product was broken up by airflow to remove soft agglomerates and then passed through a 200-mesh standard sieve to finally obtain the target lithium manganese iron phosphate cathode material.

[0052] Example 3

[0053] The lithium iron phosphate cathode material of Example 3 was prepared by the following steps:

[0054] (1) Manganese carbonate, ferrous oxalate, lithium carbonate, and a composite phosphorus source were weighed as raw materials according to the molar ratios Mn:Fe=7:3, (Mn+Fe):P=1:1.05, and Li:(Mn+Fe)=1.09. The composite phosphorus source included 35% diammonium hydrogen phosphate, 45% diethyl phosphonate, and 20% lithium dihydrogen phosphate by mass. 15% of the total mass of the raw materials was weighed as a carbon source. The raw materials and carbon source were placed in deionized water and stirred evenly to obtain a slurry.

[0055] (2) The mixed slurry was fed into a horizontal sand mill and milled at once. The slurry particle size D50 reached 0.36μm and was discharged. It was then granulated using a centrifugal spray dryer with an inlet air temperature of 200℃, an outlet air temperature of 95℃, and a constant feed rate of 25mL / min using a peristaltic pump to prepare a spherical precursor with a sphericity ≥0.92 and uniform internal components.

[0056] (3) The precursor was evenly spread in a graphite sagger and placed in a box-type atmosphere sintering furnace. Under nitrogen atmosphere, the temperature was raised to 420°C at a uniform rate of 3°C / min and held at a constant temperature for 1 hour. Then, the temperature was raised to 650°C at a uniform rate of 1°C / min and held at a constant temperature for 5 hours. Finally, the temperature was raised to 800°C at a uniform rate of 4°C / min and held at a constant temperature for 10 hours. After sintering, the temperature was controlled by a program at 5°C / min and the furnace was cooled to room temperature. The entire process was carried out without opening the furnace to break the vacuum or changing the atmosphere, ensuring the continuity of the sintering reaction. The sintered product was broken up by airflow to remove soft agglomerates and then passed through a 200-mesh standard sieve to finally obtain the target lithium manganese iron phosphate cathode material.

[0057] Comparative Example 1

[0058] The preparation method of lithium manganese iron phosphate in Comparative Example 1 is basically the same as that in Example 1. The main difference is that the phosphorus source in step (1) is a single lithium dihydrogen phosphate, and the three independent isothermal platforms in step (3) are cancelled, and a conventional two-stage sintering process is adopted, as follows:

[0059] (1) Manganese carbonate, ferrous oxalate, lithium carbonate, and lithium dihydrogen phosphate were weighed as raw materials according to the molar ratios Mn:Fe=6:4, (Mn+Fe):P=1:1.03, and Li:(Mn+Fe)=1.05. Sucrose, accounting for 10% of the total mass of the raw materials, was weighed as a carbon source. The raw materials and carbon source were placed in deionized water and stirred evenly to obtain a slurry.

[0060] (2) The mixed slurry was fed into a horizontal sand mill and milled at once. The slurry particle size D50 reached 0.35μm and was discharged. It was then granulated using a centrifugal spray dryer with an inlet air temperature of 200℃, an outlet air temperature of 95℃, and a constant feed rate of 25mL / min using a peristaltic pump to prepare a spherical precursor with a sphericity ≥0.92 and uniform internal components.

[0061] (3) The precursor was evenly spread in a graphite sagger and placed in a box-type atmosphere sintering furnace. Under nitrogen atmosphere, the temperature was raised to 430°C at a uniform rate of 3°C / min and kept at a constant temperature for 6 hours. Then, the temperature was raised to 750°C at a uniform rate of 3°C / min and kept at a constant temperature for 10 hours. After sintering, the temperature was controlled by a program of 5°C / min and the furnace was cooled to room temperature. There was no opening of the furnace to break the vacuum or switching of atmospheres throughout the process, which ensured the continuity of the sintering reaction. The sintered product was broken up by airflow to remove soft agglomerates and then passed through a 200-mesh standard sieve to finally obtain the target lithium manganese iron phosphate cathode material.

[0062] Comparative Example 2

[0063] The preparation method of lithium manganese iron phosphate in Comparative Example 2 is basically the same as that in Example 1. The difference is that the proportion of inorganic phosphorus source is increased and the proportion of organic phosphorus source is reduced in the composite phosphorus source. Specifically, the composite phosphorus source is 50% hydrogen phosphate, 30% triethyl phosphate and 20% lithium dihydrogen phosphate by mass fraction. In the preparation process during sintering, the three independent isothermal intervals are cancelled and only two heat preservation platforms are set. Specifically, the temperature is raised to 375℃ and held for 2 hours, and then raised to 750℃ and held for 10 hours. The other process parameters are the same as those of the test system and Example 1. Finally, lithium manganese iron phosphate cathode material is obtained.

[0064] Comparative Example 3

[0065] The preparation method of lithium manganese iron phosphate in Comparative Example 3 is basically the same as that in Example 1. The difference is that only inorganic phosphorus source and lithium-based phosphorus source are used in the composite phosphorus source. That is, the composite phosphorus source includes 80% ammonium hydrogen phosphate and 20% lithium dihydrogen phosphate by mass. In the sintering process, the three independent isothermal intervals are cancelled and only two heat preservation platforms are set. Specifically, the temperature is raised to 375℃ and held for 2 hours, and then raised to 750℃ and held for 10 hours. The other process parameters are the same as those of the test system and Example 1. Finally, lithium manganese iron phosphate cathode material is obtained.

[0066] Performance Testing - Electrochemical Performance

[0067] The compaction density of lithium manganese iron phosphate cathode material was tested according to GBT30835-2014 using a PRCD1100 powder compaction testing device; the results are shown in Table 1.

[0068] The lithium manganese iron phosphate cathode materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were used to prepare coin cells and their electrochemical performance was evaluated. The lithium manganese iron phosphate cathode material, Super P (conductive carbon black), and PVDF were uniformly mixed in an NMP solution at a ratio of 90:5:5 to obtain a slurry. The slurry was applied to bright aluminum foil by hand and then dried at 100°C. After the NMP had completely evaporated, the electrode was cut into electrode sheets with a diameter of 13 mm. The electrode sheets were then dried overnight at 105°C in a vacuum oven. After weighing, the electrode sheets were quickly transferred to a glove box. Lithium metal was used as the counter electrode, Celgard 2400 as the separator, and the electrolyte was 1 mol / L. The assembled battery was prepared by dissolving LiPF6 in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1). Electrochemical performance was tested using the Xinwei testing system, employing constant current-constant voltage charging (CC-CV) and constant current discharging (DC) methods to analyze the charge and discharge behavior of the coin cell. The test voltage range was 2.0-4.35 V.

[0069] Table 1. Cathode Material Performance Data

[0070]

[0071] Performance Testing - Morphology Analysis

[0072] The lithium manganese iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-3 were characterized by SEM, the primary particle size was statistically analyzed, and the particle size distribution of the lithium manganese iron phosphate cathode materials was detected.

[0073] SEM image of the cathode material in Example 1 ( Figure 1 It can be seen that the two grain sizes are uniformly intercalated and there is no melting coarsening. Figure 2The primary grain size distribution chart shows a standard bimodal distribution with small and large grains. The large primary grains are in the 200-250 nm range, while the small primary grains are in the 90-120 nm range. Figure 3 The secondary powder exhibits an ordered bimodal particle size distribution, with small-diameter secondary spheres effectively filling the gaps between large particles and reducing the packing porosity. Furthermore, combined with the performance data in Table 1, this gradation structure results in a material compaction density of 2.60 g / cm³. 3 The nanocrystals shorten the lithium-ion diffusion path, ensuring a discharge specific capacity of 153.2 mAh / g at 0.1C. The monolithic impurity-free lattice and bidirectional grain support work synergistically to maintain a capacity retention rate of up to 92.4% after 500 cycles at 1C.

[0074] SEM images of the cathode materials in Examples 2 and 3 ( Figure 7 and 13 As shown in the figure, both sizes of grains are uniformly intercalated without melting coarsening; and the primary grain size distribution chart shows a standard bimodal distribution between small and large grains. Figure 9 and 15 As shown); the SEM image clearly shows two peak curves for particle size distribution: large grains and small grains. Figure 8 and 14 (As shown).

[0075] Based on this result, further analysis of the mechanism reveals that this invention employs a composite of three phosphorus sources: a low-temperature gas-generating inorganic phosphorus source, a medium-temperature slow-release organic phosphorus source, and a high-temperature steady-state phosphorus-supplementing lithium-based phosphorus source. The thermal decomposition windows of the composite phosphorus sources do not overlap, and the three isothermal plateaus are precisely matched with the phosphorus release range of the composite phosphorus sources, achieving the following:

[0076] (1) When kept at a low temperature of 300≤T<450℃ for 1-3h, only the inorganic phosphorus source is thermally decomposed to produce gas, which builds through pores inside the particles to prepare porous mother crystal nuclei (avoiding crystal nuclei agglomeration). The phosphorus release temperature remains stable throughout the process for organic phosphorus sources at 450≤T≤650℃ and lithium-based phosphorus sources at 700≤T≤800℃, with no premature phosphorus release interfering with the reaction.

[0077] (2) The porous mother crystal nucleus continues to grow under the medium temperature range of 450≤T≤650℃ for 5-7h, and the organic phosphorus source steadily releases active phosphate. The phosphorus release temperature of the high temperature stable lithium-based phosphorus source at 700≤T≤800℃ remains stable throughout the process. Metal ions rely on the pre-made crystal nucleus to form low-barrier heterogeneous nucleation, and free P ions grow in situ on the inner wall of the pores to form ultrafine grains of 30~80nm.

[0078] (3) Keep at high temperature of 700≤T≤800℃ for 8-12h, and continuously replenish phosphorus and lithium from high temperature lithium-based phosphorus source to compensate for high temperature phosphorus volatilization loss, repair lattice defects, and inhibit Oswald ripening to stabilize dual-scale morphology.

[0079] In contrast, Comparative Example 1 used a single phosphorus source combined with a two-stage sintering process. Therefore, theoretical analysis showed that it was impossible to pre-fabricate porous large crystal nuclei at low temperatures, and the concentrated release of phosphorus at high temperatures caused ion supersaturation, resulting in only a single ultrafine grain. The results were as expected. Figures 4 to 6 As shown, SEM, primary particle size, and laser particle size distribution images all show a single-peak narrow distribution with primary particle sizes of 90-120 nm. The powder is loosely packed, with a compaction density of only 2.27 g / cm³. 3 The lack of a large-grain stress buffer framework leads to the easy pulverization and breakage of the conductive network during cycling. Although the 0.1C capacity of 151.8mAh / g is close to that of Example 1, the capacity retention rate is only 76.5% after 500 cycles at 1C.

[0080] Comparative Example 2 used a composite phosphorus source with an increased proportion of inorganic phosphorus source and a decreased proportion of organic phosphorus source, and lacked three-stage independent isothermal insulation. Theoretical analysis showed that the synchronous release of phosphorus throughout the heating process triggered continuous ion supersaturation in the system, resulting in disordered synchronous grain growth and polarization in grain size. The results are as follows: Figures 10 to 12 As shown. Figure 10 The SEM images show a large number of abnormally coarse grains and grains melting and adhering together. Figure 11 The primary particle size distribution is a broad single peak with a long tail, and the primary particle size is 150-200 nm. Figure 12 The medium-sized laser particles exhibit extensive coarse particle tailing. The disordered microstructure results in low powder filling efficiency, with a compaction density of only 2.31 g / cm³. 3 The coarse grains extend the lithium diffusion distance, and the trace impurities and lattice defects derived from disordered phosphorus release significantly increase the interfacial impedance. Ultimately, the 0.1C capacity drops to 138.6mAh / g, and the capacity retention rate is only 68.1% after 500 cycles at 1C.

[0081] In Comparative Example 3, only inorganic phosphorus source and lithium-based phosphorus source were used in the composite phosphorus source. The three independent isothermal zones were eliminated, and only two insulation platforms were set. During the low-temperature stage, a large amount of phosphorus source was released, forming lithium manganese iron phosphate crystal nuclei. After high-temperature sintering, the particles exhibited severe melting and irregularity, resulting in poor particle sphericity, which was detrimental to improving compaction. The results are as follows: Figure 16-18 As shown, Figure 16 The SEM images show a large number of irregularly shaped particles and severe grain adhesion. Figure 17 The primary particle size distribution is a broad single peak with a long tail, and the primary particle diameter is 170-200 nm. Figure 18 The medium-sized laser particles exhibit a large range of coarse particle tails.

[0082] Comparing the morphology, particle size images, and electrochemical performance differences of the cathode materials in the above embodiments and comparative examples, it can be seen that by relying on the time-decoupled phosphorus supply and segmented crystal control mechanism to construct ordered dual-scale symbiotic grains, and matching the phosphorus source thermal decomposition kinetics design with a three-stage independent isothermal sintering process, the sintering temperature and phosphorus source release behavior are precisely coupled. The entire process of grain nucleation, growth, and lattice ripening is isolated in a time-separated manner, effectively suppressing the problems of simultaneous grain coarsening, ion segregation, and impurity phase precipitation. It can precisely and controllably construct a native dual-scale particle structure with embedded nanocrystals in a large particle matrix. This special gradation structure can use nanocrystals to fill the gaps between large particles, significantly reducing the powder porosity and significantly improving the compaction density. At the same time, the multi-level particle structure can shorten the lithium-ion transport path, and the segmented temperature-preserving sintering process completely preserves the ordered crystal structure of olivine, ensuring smooth ion transport channels and simultaneously improving the material's discharge specific capacity and long-cycle stability.

[0083] In addition to the above embodiments, it should be noted that the process and parameter conditions defined by the present invention can achieve the technical effects claimed above, and therefore no further experimental verification is required.

Claims

1. A high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles, characterized in that, It is prepared by the following steps: (1) Manganese source, iron source, lithium source, composite phosphorus source, carbon source and water are mixed evenly to form a slurry; the composite phosphorus source includes inorganic phosphorus source, organic phosphorus source and lithium-based phosphorus source; (2) The slurry is milled, sprayed, dried and granulated to obtain spherical precursors; (3) Under an inert atmosphere, first heat to 300≤T<450℃ and keep for 1-3h to carry out the phosphorus release nucleation reaction of inorganic phosphorus source; then heat to 450≤T≤650℃ and keep for 5-7h to carry out the slow release reaction of organic phosphorus source, and construct bimodal grains in situ. Finally, the temperature is raised to 700≤T≤800℃ and held for 8-12 hours to carry out a steady-state release reaction of the lithium-based phosphorus source, thereby obtaining a lithium manganese iron phosphate cathode material with dual-scale particles and no phosphide impurities.

2. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1, characterized in that, In step (1), the inorganic phosphorus source in the composite phosphorus source is selected from ammonium hydrogen phosphate, ammonium dihydrogen phosphate or diammonium hydrogen phosphate; the organic phosphorus source is selected from triethyl phosphate, methylphosphonic acid, dimethyl phosphonate or diethyl phosphonate; and the lithium-based phosphorus source is selected from lithium dihydrogen phosphate or lithium phosphate.

3. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1 or 2, characterized in that, The inorganic phosphorus source comprises 35-40% of the mass of the composite phosphorus source, the organic phosphorus source comprises 40-45% of the mass of the composite phosphorus source, and the lithium-based phosphorus source comprises 18-22% of the mass of the composite phosphorus source.

4. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1, characterized in that, In step (1), the molar ratio of P:(Mn+Fe):Li in the composite phosphorus source, manganese source, iron source and lithium source is (1.01-1.06):1:(1.02-1.09), and the molar ratio of Mn:Fe is X:(1-X), 0.5≤X≤0.

7.

5. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1, characterized in that, In step (1), the carbon source is selected from at least one of sucrose, glucose or citric acid, and its added mass is 10-15% of the total mass of the composite phosphorus source, manganese source, iron source and lithium source with different thermal decomposition temperatures.

6. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1 or 4, characterized in that, The manganese source is selected from at least one of manganese carbonate, manganese oxalate, or manganese tetroxide. The iron source is selected from at least one of ferrous carbonate, ferrous oxalate, or iron(III) oxide. The lithium source is selected from one or more of lithium carbonate, lithium oxalate, or lithium oxide.

7. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1, characterized in that, In step (3), the heating rate is 1-5℃ / min.

8. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1, characterized in that, In step (1), the solid content of the slurry is 40-55 wt%.

9. The high-compact, high-capacity lithium manganese iron phosphate cathode material with dual-scale particles as described in claim 1, characterized in that, In step (2), the specific steps of sand milling, spraying, drying and granulation are as follows: the slurry is first sand milled until the particle size D50 reaches 0.3-0.4μm, then stirred for 2-4h, and finally centrifugally spray-dried and granulated under the conditions of inlet air temperature of 180-220℃ and outlet air temperature of 80-110℃ to obtain spherical precursor.

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