Preparation method of high-compaction lithium iron phosphate with multi-level gradation

Abstract

A kind of preparation method of high compaction lithium iron phosphate of multistage gradation, including step one: iron source, phosphorus source, lithium source, carbon source, dispersing agent, element dopant and solvent are dispersed according to proportion, slurry after dispersion is divided into A, B, C three slurry according to proportion, three slurry are grinded respectively, and then are respectively carried out spray drying;Step two: three spray materials are respectively pre-sintered at different temperatures, and pre-sintered material is respectively crushed;Step three: the material after pre-sintering is mixed to realize multistage particle gradation.Certain mass ratio of carbon source and pre-crushed material are dry mixed;Step four: after secondary sintering, crushing and magnetic removal process, the multistage gradation high compaction lithium iron phosphate positive material is obtained.The present application realizes particle gradation of once sintered material, secondary carbon coating and sintering strengthening by grading particle size design, and realizes the collaborative improvement of compaction density and specific capacity of lithium iron phosphate positive material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a method for preparing high-pressure lithium iron phosphate with multi-stage gradation. Background Technology

[0002] Lithium-ion batteries, as the core of today's energy storage technology, directly determine the application prospects of electric vehicles, portable electronic devices, and large-scale energy storage systems. Cathode materials are a key component of lithium-ion batteries, and their physicochemical properties have a decisive impact on the battery's energy density, cycle life, safety, and cost. Among many cathode materials, lithium iron phosphate (LiFePO4, LFP) has become one of the preferred materials in the fields of power batteries and energy storage due to its excellent thermal stability, high cycle life (>2000 cycles), environmental friendliness, and low cost. However, lithium iron phosphate has an inherent drawback—a relatively low lithium-ion diffusion coefficient (approximately 10⁻⁻¹). 14 (cm² / s) and electronic conductivity (approximately 10⁻⁻¹) 9 The low compaction density (typically <2.5 g / cm³), which is difficult to improve using traditional processes, severely limits further increases in energy density. With the increasing demand for longer driving range in electric vehicles, developing lithium iron phosphate materials that combine high compaction density and high specific capacity has become a critical technological bottleneck for the industry. Research shows that for every 0.1 g / cm³ increase in compaction density, the volumetric energy density of the battery can increase by approximately 5%-8%. Therefore, achieving tight particle packing through material design and process optimization while maintaining high specific capacity has become the core direction for technological innovation in lithium iron phosphate.

[0003] To improve electrochemical performance, early research focused on nanoscale strategies: synthesizing nanoscale lithium iron phosphate (particle size <200 nm) through ball milling, sol-gel methods, or hydrothermal methods. Nanoparticles can shorten the lithium-ion diffusion path and increase specific capacity (e.g., up to 160 mAh / g at 0.1C), but excessively small particles lead to reduced tap density (<1 g / cm³), and the high surface energy of nanoparticles makes them prone to aggregation, which is detrimental to electrode processing. Furthermore, nanomaterials are prone to breakage during high-pressure compaction, exacerbating interfacial side reactions. To compensate for insufficient conductivity, carbon coating technology has been widely adopted, forming a conductive carbon layer on the particle surface through the high-temperature pyrolysis of carbon sources such as glucose and sucrose. However, excessively thick carbon layers (>5 nm) occupy the volume of the active material, reducing the volumetric energy density; while uneven carbon distribution may lead to increased local polarization, affecting rate performance.

[0004] In recent years, methods to improve compaction density through multi-scale particle gradation have gradually attracted attention. For example, CN114314550 A proposes mixing large-particle precursors (D50 15-30 μm) with small-particle precursors (D50 5-15 μm) in a specific ratio, using small particles to fill the gaps between large particles to increase packing density. However, this method does not consider the differences in sintering behavior of particles of different sizes: if a high-temperature sintering is performed after direct mixing, small particles may grow abnormally due to their high surface energy, destroying the gradation effect. In addition, existing gradation techniques mostly rely on simple physical mixing, making it difficult to achieve chemical bonding between particles, which leads to the collapse of the gradation structure during electrode cycling, and the decrease in compaction density with increasing cycle number.

[0005] The sintering process is a key parameter affecting the crystallinity, particle morphology, and carbon coating quality of lithium iron phosphate (LFP). Traditional processes typically employ a single high-temperature sintering, which, while beneficial for improving crystallinity, can easily lead to excessive particle growth, reducing specific surface area and reactivity. Simultaneously, the carbon coating layer is severely damaged, impacting electrochemical performance. A single high-temperature sintering process cannot simultaneously achieve high compaction and high capacity. Therefore, developing novel sintering processes and improving the carbon coating effect are crucial for upgrading and iterating LFP. Numerous patented technologies propose novel processes combining secondary carbon coating and secondary sintering to significantly improve the energy density of LFP. For example, CN 118373399 A employs a secondary wet carbon coating technology, which involves secondary mixing of pre-sintered materials with a certain proportion of carbon source, followed by secondary milling, secondary spraying, and secondary sintering to prepare high-compact LFP cathode materials. However, this involves complex processes and high production costs. Therefore, optimizing process steps and reducing production costs are aspects that urgently need improvement in the secondary sintering process. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention proposes a multi-stage gradation, secondary dry carbon doping, and gradient sintering synergistic control strategy:

[0007] (1) Grading particle size design and pre-sintering: The precursor slurry is divided into different particle size groups (A: 0.35-0.45 μm, B: 0.45-0.6 μm, C: 0.6-0.75 μm). The porosity and surface characteristics of each group of particles are controlled by different pre-sintering temperatures (A: 700-750℃, B: 650-700℃, C: 600-650℃). After mixing, they form a three-level compact packing structure of "large-medium-small", which improves the compaction density by three-level particle gradation.

[0008] (2) Secondary carbon coating and sintering strengthening: A carbon source is introduced a second time on the surface of graded mixed particles to improve the carbon coating during pre-sintering. A uniform conductive network is formed through high-temperature sintering, while repairing lattice defects that may occur during pre-sintering and improving the overall electrochemical activity of the material. Furthermore, dry carbon doping helps to simplify the production process (simplifying the secondary sand milling and secondary spraying processes) and effectively control production costs.

[0009] Through the above innovations, this invention improves the compaction density to over 2.6 g / cm³ while ensuring high specific capacity, providing an ideal cathode material solution for next-generation high-energy-density lithium-ion batteries.

[0010] The technical solution of the present invention is as follows:

[0011] A method for preparing multi-stage graded high-pressure lithium iron phosphate includes the following steps:

[0012] Step 1: Disperse the iron source, phosphorus source, lithium source, carbon source a, dispersant, elemental dopant and solvent in a certain proportion. Divide the dispersed slurry into three parts A, B and C in a certain proportion. Then grind the three parts of slurry to a certain particle size and spray dry them respectively.

[0013] Step 2: Pre-sinter the three spray-dried materials at different temperatures, and then pulverize the pre-sintered materials to control the particle size D50 to be 0.6-1.1μm;

[0014] Step 3: Mix the pulverized pre-sintered material to achieve multi-stage particle size distribution. Then, dry mix a certain mass ratio of carbon source b with the pre-pulverized material.

[0015] Step 4: After secondary sintering, crushing and demagnetizing of the mixed materials, a multi-stage high-pressure lithium iron phosphate cathode material is obtained.

[0016] Furthermore, in step one, the iron and phosphorus sources are iron phosphate; the lithium source is any one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; carbon source a is any one or more of glucose, sucrose, PVDF, graphene, and carbon nanotubes; the dispersant is any one or more of PEG2000, PEG6000, and PVA2000; the elemental dopant is any one or more of titanium dioxide, ammonium metavanadate, magnesium acetate, aluminum oxide, and niobium oxide; and the solvent is pure water or ethanol. In step three, carbon source b is either glucose or sucrose.

[0017] Furthermore, in step one, the molar ratio of iron phosphate, lithium source, and metal dopant is iron:phosphorus:lithium:metal element in metal dopant = 0.95-1:1-1.1:1-1.1:0.005-0.05, and the mass ratio of iron phosphate, carbon source a, and dispersant is iron phosphate:carbon source:dispersant = 1:0.01-0.1:0.01-0.1; the solid content of the mixed slurry is 38%-45%.

[0018] Furthermore, in step one, the mass ratio of slurry A:B:C is 1:1-2:0.5-2; the grinding particle sizes are controlled as follows: slurry A has a particle size of D50 of 0.35-0.45μm, slurry B has a particle size of D50 of 0.45-0.6μm, and slurry C has a particle size of D50 of 0.6-0.75μm.

[0019] Furthermore, in step one, the inlet air temperature of the spray dryer is 200-240℃, the outlet air temperature of the spray dryer is 75-110℃, and the moisture content of the material obtained by spray drying is less than 3wt%.

[0020] Furthermore, in step two, sintering is carried out under a protective atmosphere of nitrogen or argon. The sintering temperature of material A is 700-750℃, the sintering temperature of material B is 650-700℃, and the sintering temperature of material C is 600-650℃. The sintering time for the three materials is the same, which is 6-10 h.

[0021] Furthermore, in step three, the mass ratio of carbon source b to the material pulverized in the first step is 0.5-3:100.

[0022] Furthermore, in step four, sintering is carried out under the protection of nitrogen or argon, at a temperature of 750-820℃, for a time of 6-10 hours.

[0023] Furthermore, in step four, the particle size D50 of the high-pressure lithium iron phosphate is 0.8-1.4 μm.

[0024] The beneficial effects of this invention are as follows:

[0025] This invention provides a multi-stage gradation method for preparing high-compact lithium iron phosphate. By controlling the porosity and surface characteristics of each group of particles at different pre-calcination temperatures using slurries with different milling particle sizes, a three-stage compacted packing structure of "large-medium-small" particles is formed after mixing, achieving improved compaction density through this three-stage particle gradation. Furthermore, the use of a secondary dry carbon doping and secondary sintering process optimizes the uniformity of carbon coating on the particle surface, repairs the carbon coating layer from the first calcination process, significantly improves the conductivity of lithium iron phosphate, and enhances rate performance. The secondary sintering further optimizes the sphericity of the primary particles, which is beneficial for increasing the packing density of lithium iron phosphate. In addition, the dry carbon doping process uses dry mixing equipment to mix the carbon source and pre-sintered material, simplifying the secondary milling and secondary spraying processes in the wet carbon doping process, significantly reducing production costs. Mass production can be achieved using existing lithium iron phosphate production lines without additional equipment investment, enabling rapid industrialization. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of the preparation method of the present invention.

[0027] Figure 2 This is a SEM image of the lithium iron phosphate cathode material prepared in Example 1.

[0028] Figure 3 This is the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1.

[0029] Figure 4 The first charge-discharge curve of the lithium iron phosphate cathode material prepared in Example 1 at 0.1C is shown.

[0030] Figure 5 This is an SEM image of the pre-calcined A material from Example 1. The upper right corner shows the particle size distribution of the primary particles of the pre-calcined A material.

[0031] Figure 6 This is an SEM image of the pre-calcined B material from Example 1. The upper right corner shows the particle size distribution of the primary particles of the pre-calcined B material.

[0032] Figure 7 This is an SEM image of the pre-calcined C material from Example 1. The upper right corner shows the particle size distribution of the primary particles in the pre-calcined C material. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0034] <Example 1>

[0035] A method for preparing multi-stage graded high-pressure lithium iron phosphate includes the following steps:

[0036] Step 1: Add 1000 g of ferric phosphate, 253.6 g of lithium carbonate, 20 g of sucrose, 90 g of PEG6000, 7.0 g of titanium dioxide, and 2.4 g of ammonium metavanadate to 2 L of pure water. Stir mechanically for 30 min to mix thoroughly, obtaining a uniformly dispersed slurry. Divide the dispersed slurry into three portions, A, B, and C, at a mass ratio of 1:1.33:1. Grind slurry A to a D50 of 0.4 μm, slurry B to a D50 of 0.5 μm, and slurry C to a D50 of 0.65 μm. Then, spray dry each portion separately, controlling the inlet air temperature at 220℃ and the outlet air temperature at 95℃.

[0037] Step 2: Transfer the three spray materials A, B, and C to a box furnace and pre-sinter them at 750, 700, and 650℃ for 10 hours respectively under a nitrogen atmosphere and a heating rate of 3℃ / min. (To more intuitively demonstrate that using graded particle size and differentiated pre-sintering temperatures can achieve "large-medium-small" particle size differences for A, B, and C slurries during subsequent sintering, SEM images of samples A, B, and C after pre-sintering under median specification conditions were analyzed. The results are as follows...) Figure 5-7 As shown, the average primary particle size of pre-fired material A is 570 nm, that of pre-fired material B is 300 nm, and that of pre-fired material C is 180 nm. Therefore, after subsequent mixing processes, a precise "large-medium-small" particle size distribution can be achieved. This further confirms that reducing the grinding particle size can lead to spontaneous particle agglomeration, which easily leads to the formation of "large particles" during subsequent sintering. Therefore, strictly controlling the slurry particle size and pre-fired temperature is crucial. The pre-fired materials are then pulverized using an air jet mill, with the particle size D50 controlled at 0.9 ± 0.2 μm.

[0038] Step 3: Transfer the pulverized pre-sintered materials A, B, and C into a batch mixer for mixing to achieve multi-stage particle size distribution. Add glucose at a mass ratio of 3.6% (relative to the total mass of the pre-sintered material) to the mixing equipment for dry mixing with the pre-sintered material.

[0039] Step 4: Transfer the mixed materials to a box furnace and hold at 795℃ for 10 hours under a nitrogen atmosphere with a heating rate of 3℃ / min. The secondary sintered material is then pulverized using an air jet mill, controlling the particle size D50 to be 1.2±0.1μm. Finally, after a demagnetization process, a multi-graded high-pressure lithium iron phosphate cathode material is obtained.

[0040] <Example 2>

[0041] A method for preparing multi-stage graded high-pressure lithium iron phosphate includes the following steps:

[0042] Step 1: Add 1000 g of ferric phosphate, 253.6 g of lithium carbonate, 30 g of glucose, 90 g of PEG6000, 7.0 g of titanium dioxide, and 6.2 g of magnesium acetate to 2 L of pure water. Stir mechanically for 30 min to mix thoroughly, obtaining a uniformly dispersed slurry. Divide the dispersed slurry into three portions, A, B, and C, at a mass ratio of 1:1.33:1. Grind slurry A to a D50 of 0.4 μm, slurry B to a D50 of 0.5 μm, and slurry C to a D50 of 0.65 μm. Then, spray dry each portion separately, controlling the inlet air temperature at 220℃ and the outlet air temperature at 95℃.

[0043] Step 2: Transfer the three spray materials A, B, and C to a box furnace and pre-sinter them at 720, 680, and 620℃ respectively for 10 h under a nitrogen atmosphere and a heating rate of 3℃ / min. Then, pulverize the pre-sintered materials by air jet milling, controlling the particle size D50 to be 0.9±0.2 μm.

[0044] Step 3: Transfer the pulverized pre-sintered materials A, B, and C into a batch mixer for mixing to achieve multi-stage particle size distribution. Add glucose at a mass ratio of 2.5% (relative to the total mass of the pre-sintered material) to the mixing equipment for dry mixing with the pre-sintered material.

[0045] Step 4: Transfer the mixed materials to a box furnace and hold at 800℃ for 10 h under a nitrogen atmosphere with a heating rate of 3℃ / min. The secondary sintered material is then pulverized using an air jet mill, controlling the particle size D50 to be 1.2±0.1 μm. Finally, after a demagnetization process, a multi-graded high-pressure lithium iron phosphate cathode material is produced.

[0046] <Example 3>

[0047] A method for preparing multi-stage graded high-pressure lithium iron phosphate includes the following steps:

[0048] Step 1: Add 1000 g of ferric phosphate, 253.6 g of lithium carbonate, 20 g of sucrose, 5 g of graphene, 90 g of PEG2000, and 8.9 g of titanium dioxide to 2 L of pure water, and mechanically stir for 30 min to mix evenly, obtaining a uniformly dispersed slurry. Divide the dispersed slurry into three portions, A, B, and C, at a mass ratio of 1:1:0.5. Grind slurry A to a D50 of 0.4 μm, slurry B to a D50 of 0.5 μm, and slurry C to a D50 of 0.65 μm, and then spray dry them separately, controlling the inlet air temperature at 220℃ and the outlet air temperature at 95℃.

[0049] Step 2: Transfer the three spray materials A, B, and C to a box furnace and pre-sinter them at 750, 700, and 650℃ for 10 h respectively under a nitrogen atmosphere and a heating rate of 3℃ / min. Then, pulverize the pre-sintered materials by air jet milling, controlling the particle size D50 to be 0.9±0.2 μm.

[0050] Step 3: Transfer the pulverized pre-sintered materials A, B, and C into a batch mixer for mixing to achieve multi-stage particle size distribution. Add 2% by mass (relative to the total mass of the pre-sintered material) of sucrose to the mixing equipment for dry mixing with the pre-sintered material.

[0051] Step 4: Transfer the mixed materials to a box furnace and hold at 795℃ for 10 h under a nitrogen atmosphere with a heating rate of 3℃ / min. The secondary sintered material is then pulverized using an air jet mill, controlling the particle size D50 to be 1.2±0.1 μm. Finally, after a demagnetization process, a multi-graded high-pressure lithium iron phosphate cathode material is produced.

[0052] <Comparative Example 1>

[0053] The difference between Comparative Example 1 and Example 1 lies only in step two: the three spray materials A, B, and C were transferred to a box furnace and pre-sintered at 700°C for 10 h under a nitrogen atmosphere with a heating rate of 3°C / min.

[0054] <Comparative Example 2>

[0055] The only difference between Comparative Example 2 and Example 1 is step one: grinding slurries AB and C to the same particle size D50: 0.5 μm.

[0056] <Comparative Example 3>

[0057] The only difference between Comparative Example 3 and Example 1 is in step three: glucose is not added for secondary carbon coating, and the pre-sintered materials A, B, and C are directly mixed and then subjected to secondary sintering.

[0058] <Comparative Example 4>

[0059] The difference between Comparative Example 4 and Example 1 is that Comparative Example 4 uses a traditional one-step sintering process. The specific steps are as follows: 1000 g of mixed iron phosphate, 253.2 g of lithium carbonate, 57 g of sucrose, 90 g of PEG6000, 7.0 g of titanium dioxide, and 2.4 g of ammonium metavanadate were added to 2 L of pure water and mechanically stirred for 30 min to obtain a uniformly dispersed slurry. The dispersed slurry was ground to a D50 of 0.4 μm and then spray-dried, with the inlet air temperature controlled at 220℃ and the outlet air temperature at 95℃. Sintering was carried out under a nitrogen atmosphere at a heating rate of 3℃ / min at 810℃ for 10 h. The sintered material was then pulverized using an air jet mill, controlling the particle size D50 to be 1.2 ± 0.1 μm.

[0060] <Test Example 1>

[0061] Compacted density test

[0062] The compaction density of the lithium iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-4 was tested using a compaction density meter at a pressure of 3T. The test results are shown in Table 1.

[0063] <Test Example 2>

[0064] Physical characterization and electrochemical performance testing

[0065] The lithium iron phosphate cathode material prepared in Example 1 was subjected to scanning electron microscopy (SEM) testing. The SEM images are shown below. Figure 2 As shown. By Figure 2 It can be seen that the prepared lithium iron phosphate samples have a uniform distribution of "large-medium-small" particles with a wide particle size distribution and the particles are spherical. When the particles are tightly packed, the synergistic effect of the three-level particle gradation and the spherical primary particles can significantly improve the packing density of the material.

[0066] XRD phase analysis was performed on the lithium iron phosphate cathode material prepared in Example 1, and the results are as follows: Figure 3 As shown. By Figure 3 It can be seen that the prepared lithium iron phosphate sample corresponds to the LiFePO4 standard card PDF#83-2092, indicating that the lithium iron phosphate material with olivine structure was successfully prepared, and the sample has high purity with no other impurity peaks found.

[0067] <Test Example 3>

[0068] Electrochemical performance testing

[0069] The lithium iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to the following operations: The active material, binder PVDF, and conductive additive SP were dispersed in NMP solution at a mass ratio of 95:2.5:2.5. After thorough mixing, a slurry was prepared, coated onto aluminum foil, vacuum dried, rolled, and cut into sheets to obtain the cathode sheet. The mass of the active material in the cathode sheet was approximately 12 mg. The anode was a lithium metal sheet, the separator was a polypropylene porous membrane, and the electrolyte was 1 mol / L LiPF6 / EC+DEC+DMC (where the volume ratio EC:DEC:DMC = 1:1:1). The charge / discharge specific capacity of the coin cell was tested in the voltage range of 2.0 V-3.75 V, and the test results are shown in Table 1.

[0070] The first charge-discharge curve of the lithium iron phosphate cathode material prepared in Example 1 at 0.1C is shown below. Figure 4 As shown.

[0071] Table 1. Compaction density and electrochemical performance test results of Examples 1-3 and Comparative Examples 1-4

[0072]

[0073] The above embodiments are only for illustrating the technical solutions and features of the present invention, and are intended to enable those skilled in the art to implement them better. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention are within the scope of protection of the present invention. The parts not described in detail are prior art.

Claims

1. A process for the preparation of a high compacted lithium iron phosphate of a multi-grade gradation, characterized by, It comprises the following steps: Step one, the iron source, phosphorus source, lithium source, carbon source a, dispersant, element dopant and solvent are dispersed in proportion, the dispersed slurry is divided into A, B, C three parts slurry in proportion, then A, B, C three parts slurry are respectively grinded, and then respectively carried out spray drying; Step two, the three parts of spray dried materials are respectively pre-sintered, the pre-sintered materials are respectively crushed, and the crushing particle size D50 is controlled to be 0.6-1.1 μm; Step three, the crushed pre-sintered materials are mixed to realize multi-stage particle grading, and the carbon source b is dry mixed with the pre-crushed materials; Step four, the mixed materials are sintered again, crushed, and then subjected to magnetic removal process to obtain a multi-stage grading high compaction lithium iron phosphate positive electrode material; In step one, the iron source and the phosphorus source are iron phosphate; the lithium source is any one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate; the carbon source a is any one or more of glucose, sucrose, PVDF, graphene and carbon nanotube; the dispersant is any one or more of PEG2000, PEG6000 and PVA2000; the element dopant is any one or more of titanium dioxide, ammonium metavanadate, magnesium acetate, aluminum oxide and niobium oxide; the solvent is pure water or ethanol; in step three, the carbon source b is one of glucose and sucrose; In step one, the mass ratio of A, B, C three parts of slurry is 1:(1-2):(0.5-2); the sand grinding particle size is controlled to be D50:0.35-0.45 μm for slurry A, D50:0.45-0.6 μm for slurry B and D50:0.6-0.75 μm for slurry C; In step two, the sintering is carried out under the protection of nitrogen or argon atmosphere, the sintering temperature of material A is 700-750 ℃, the sintering temperature of material B is 650-700 ℃, the sintering temperature of material C is 600-650 ℃, and the sintering time of the three materials is the same, which is 6-10 h.

2. The process for preparing a high-density multi-sized graded lithium iron phosphate of claim 1, wherein, In step one, the amount of iron phosphate, lithium source and metal dopant is calculated by molar ratio, which is iron element: phosphorus element: lithium element: metal element in metal dopant = (0.95-1):(1-1.1):(1-1.1):(0.005-0.05); the amount of iron phosphate, carbon source a and dispersant is calculated by mass ratio, which is mixed iron phosphate: carbon source a: dispersant = 1:(0.01-0.1):(0.01-0.1); the solid content of the mixed slurry is 38%-45%.

3. The process for preparing a high-density multi-sized graded lithium iron phosphate of claim 1, wherein, In step one, the inlet air temperature of spray drying is 200-240 ℃, the outlet air temperature of spray drying is 75-110 ℃, and the water content of the material obtained by spray drying is less than 3wt%.

4. The process for preparing a high-density multi-sized graded lithium iron phosphate of claim 1, wherein, In step three, the mass ratio of carbon source b to the first crushed material is (0.5-3):

100.

5. The process for the preparation of a high-density multi-sized graded lithium iron phosphate as claimed in claim 1 wherein, In step four, the sintering is carried out under the protection of nitrogen or argon, the sintering temperature is 750-820 ℃, and the sintering time is 6-10 h.

6. The process for preparing a high-density multi-sized high-density lithium iron phosphate of claim 5, wherein, In step four, the particle size D50 of the high compaction lithium iron phosphate is 0.8-1.4 μm.

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