High-compaction battery positive electrode material lithium iron phosphate as well as preparation method and application thereof

Abstract

The invention discloses a high-compaction battery cathode material lithium iron phosphate and a preparation method and application thereof, the preparation method comprises the following steps: mixing and grinding a phosphorus source, an iron source, a lithium source, a titanium source, a dispersant and water until D50 is less than or equal to 800nm, and controlling the content of the titanium source to obtain a high-titanium slurry and a low-titanium slurry; carrying out spray drying granulation on the two kinds of slurry to obtain a high-titanium intermediate and a low-titanium intermediate; and mixing the two intermediates in proportion, and performing reduction and carbon coating in a fluidized bed in two stages to obtain the lithium iron phosphate material. Through fluidized bed dynamic reaction coupling particle size control, the problems of non-uniform carbon coating and difficult particle size control in a traditional process are solved, and the obtained material has high compaction density (greater than or equal to 2.5 g / cm < 3 >) and excellent cycling stability (the capacity retention ratio is greater than or equal to 90% after 500 times of circulation of 1C), and is suitable for the field of energy storage batteries.

Description

Technical Field

[0001] The present invention relates to a technology for preparing positive electrode materials for lithium-ion batteries, and specifically to a high-density lithium iron phosphate (LiFePO4) positive electrode material for high-density batteries and a preparation method and application thereof, which is particularly suitable for positive electrode materials for energy storage batteries with high compaction density and long cycle life. Background Art

[0002] Lithium iron phosphate has become the mainstream positive electrode material due to its high safety and low cost, but the traditional solid-phase method has the following defects: 1) Uneven carbon coating: Mechanical mixing of carbon sources leads to incomplete surface coverage of particles and poor electronic conductivity; 2) Complex process: Multiple high-temperature sintering is required, which has high energy consumption (>800℃) and is prone to introducing impurities; 3) Uncontrollable particle morphology: Particles are prone to agglomeration during solid-phase reactions, affecting the lithium ion diffusion rate and compaction density.

[0003] Existing technologies (such as CN112310353A) attempt to use a fluidized bed for material modification, but none of them solve the problem of coordinated optimization of precursor preparation and carbon coating process.

[0004] Therefore, there is an urgent need for an efficient and low-cost method for preparing lithium iron phosphate. Summary of the Invention

[0005] The present invention provides a method for synthesizing a high-compacted-density lithium iron phosphate cathode material, which solves the problems of low compacted density and poor cycle performance in traditional processes through the synergistic effect of titanium gradient design and fluidized bed dynamic coating process.

[0006] Another object of the present invention is to provide such a high compaction density lithium iron phosphate positive electrode material.

[0007] Another object of the present invention is to provide an application of the high compaction density lithium iron phosphate positive electrode material.

[0008] In order to achieve the above-mentioned object of the invention, the specific technical solutions of the present invention are as follows:

[0009] A method for preparing lithium iron phosphate as a positive electrode material for a high-density battery, characterized by comprising the following steps:

[0010] S1, mixing a phosphorus source, an iron source, a lithium source, a titanium source, a dispersant, and water into a slurry and grinding the mixture, respectively controlling the amount of the titanium source added to obtain a high-titanium slurry and a low-titanium slurry, spray drying the ground slurry, and granulating the mixture to obtain a high-titanium intermediate and a low-titanium intermediate;

[0011] S2. Mixing the high-titanium intermediate and the low-titanium intermediate in proportion to obtain a lithium iron phosphate intermediate, and performing reduction and carbon coating in a fluidized bed to obtain a lithium iron phosphate material.

[0012] In some specific embodiments, the amount of the titanium source (calculated as TiO2) added to the high-titanium slurry in step S1 is 0.5-0.8 wt% of the total mass of the phosphorus source, the iron source, and the lithium source, and the amount of the titanium source (calculated as TiO2) added to the low-titanium slurry is 0.05-0.5 wt% of the total mass of the phosphorus source, the iron source, and the lithium source; wherein the titanium source content in the low-titanium slurry is lower than the titanium source content in the high-titanium slurry;

[0013] In some preferred embodiments, the titanium source content in the low titanium slurry differs from the titanium source content in the high titanium slurry by more than 50%;

[0014] In some preferred embodiments, the lithium source, phosphorus source and iron source are added according to a stoichiometric ratio of Li:P:Fe=1.02-1.05:1:0.95-0.98;

[0015] In some preferred embodiments, the dispersant is added in an amount of 0.5 to 3% of the total mass of the phosphorus source, the iron source, and the lithium source.

[0016] In some specific embodiments, the particle size of the slurry after grinding in step S1 is D50≤800nm, preferably D50 is 400-600nm, the inlet temperature of the spray drying is 180-220°C, and the outlet temperature is 90-100°C;

[0017] In some preferred embodiments, a spray dryer is used for granulation, and the particle size of the obtained spherical high-titanium intermediate or low-titanium intermediate is 10 to 30 μm.

[0018] In some specific embodiments, the mixing mass ratio of the high-titanium intermediate to the low-titanium intermediate in step S2 is 5:5 to 8:2, preferably 6:4 to 7:3.

[0019] In some specific embodiments, the fluidized bed in step S2 is a vibrating fluidized bed or a gas-solid circulating fluidized bed, the reaction temperature is 400-800° C., and the reaction time is 4-10 h.

[0020] In some specific embodiments, the reduction and carbon coating in the fluidized bed in step S2 are performed in two stages:

[0021] The first stage: introducing H2 / N2 mixed gas, in which H2 accounts for 2% to 10% by volume, and reducing at 400-500℃ for 2-4h;

[0022] The second stage: switch to high-purity N2 inert atmosphere, add carbon source and perform carbon coating at 650-800°C for 3-5h.

[0023] In some specific embodiments, the carbon source introduced into the carbon coating stage of the fluidized bed in step S2 is one or more of glucose, sucrose, polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP) or phenolic resin;

[0024] In some preferred embodiments, the amount of carbon source added is 3% to 10% of the mass of the intermediate added to the fluidized bed.

[0025] In some specific embodiments, the carbon source is uniformly sprayed into the fluidized bed through a nozzle at the bottom of the fluidized bed in the form of an aerosol, with a droplet size of 1 to 10 μm and a spraying rate of 2 to 5 L / h.

[0026] On the other hand, the carbon layer thickness of the lithium iron phosphate material prepared by the above preparation method is 2 to 10 nm, and the compaction density is ≥ 2.5 g / cm 3 , 0.1C first efficiency ≥ 97%, 1C cycle 500 times capacity retention rate ≥ 90%.

[0027] On the other hand, the high-density battery positive electrode material lithium iron phosphate prepared by the above-mentioned preparation method or the application of the above-mentioned high-density battery positive electrode material lithium iron phosphate in energy storage battery positive electrode materials.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The preparation method of the present invention is more simplified, and reduction and coating are completed in one step in a fluidized bed, thereby reducing the energy consumption of traditional multi-stage sintering by about 40%.

[0030] The performance of the lithium iron phosphate positive electrode material prepared by the preparation method of the present invention is greatly improved. The particle grading is achieved by controlling the size through the titanium gradient, thereby improving the compaction density. At the same time, the uniformity of the carbon layer coated by the method of the present invention is improved (thickness deviation ≤±2nm), the material's 0.1C first efficiency is ≥97%, and the capacity retention rate after 500 cycles at 1C is ≥90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a SEM image of the lithium iron phosphate prepared in Example 1 of the present invention.

[0032] Figure 2 This is a particle size distribution diagram of the lithium iron phosphate prepared in Example 1 of the present invention.

[0033] Figure 3 This is a TEM image of the lithium iron phosphate prepared in Example 1 of the present invention.

[0034] Figure 4 This is a SEM image of the lithium iron phosphate prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below in conjunction with specific embodiments, but these embodiments are not intended to limit the scope of protection of the present invention. Without departing from the scope of the present invention, minor changes to the process conditions of the preparation process should fall within the scope of protection of the present invention.

[0036] A method for preparing lithium iron phosphate as a positive electrode material for a high-density battery comprises the following steps:

[0037] Step 1: mixing a phosphorus source, an iron source, a lithium source, a titanium source, a dispersant and deionized water and grinding them until D50 is less than or equal to 800 nm (e.g., 750 nm, 700 nm, 650 nm, 600 nm, 550 nm, 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, etc.), and respectively obtaining a high titanium slurry and a low titanium slurry by controlling the amount of the titanium source added, spray drying the slurry with a spray dryer and granulating them to obtain a high titanium intermediate and a low titanium intermediate of 10 to 30 μm (e.g., 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 23 μm, 25 μm, 27 μm, 30 μm, etc.);

[0038] Step 2: Mix the high-titanium intermediate and the low-titanium intermediate in a certain proportion to obtain a lithium iron phosphate intermediate, which is then treated in a fluidized bed in two stages: reduction and carbon coating.

[0039] Reduction stage: using H2 / N2 mixed gas, wherein the volume proportion of H2 is 2% to 10% (for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, %, 10%, etc.), at 400 to 500°C (for example, 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, etc.) for 2 to 4h (for example, 2h, 2.5h, 3h, 3.5h, 4h, etc.) to reduce Fe 3+ Completely converted to Fe 2+ ;

[0040] Carbon coating stage: switching to a high-purity N2 atmosphere, spraying a carbon source such as glucose, sucrose, PVP in the form of an aerosol, coating at 650-800°C (e.g., 650°C, 680°C, 700°C, 730°C, 750°C, 780°C, 800°C, etc.) for 3-5h (e.g., 3h, 3.5h, 4h, 4.5h, 5h, etc.) to form a 2-10nm (e.g., 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, etc.) carbon layer; wherein the amount of carbon source added is 3% to 10% of the mass of the lithium iron phosphate intermediate, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.;

[0041] After the coating is completed, the solution is cooled to room temperature to obtain the finished product lithium iron phosphate.

[0042] In the present invention, the phosphorus source, iron source, lithium source, and titanium source in step 1 are not particularly limited, and commonly used phosphorus sources, iron sources, lithium sources, and titanium sources in the art can be used. For example, the phosphorus source is selected from one or more of iron phosphate, ammonium monohydrogen phosphate, and phosphoric acid, the iron source is selected from one or more of iron phosphate and iron oxide, and the lithium source is selected from one or both of lithium carbonate and lithium hydroxide.

[0043] In the present invention, the dispersant is selected from one or two of polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), N-methyl pyrrolidone (NMP) and 1-vinyl-2-pyrrolidone (NVP), and the titanium source is nano-titanium dioxide.

[0044] In the present invention, a lithium source, a phosphorus source and an iron source are added according to a stoichiometric ratio of Li:P:Fe=1.02-1.05:1:0.95-0.98, and the amount of the dispersant added is 0.5-3% of the total mass of the lithium source, the iron source and the phosphorus source, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc. The amount of the titanium source added to the high titanium slurry is 0.5-0% of the total mass of the lithium source, the iron source and the phosphorus source. 0.8wt%, such as 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, etc.; the amount of titanium source added in the low titanium slurry is 0.05-0.5wt% of the total mass of the lithium source, iron source and phosphorus source, such as 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, etc. Wherein, the amount of titanium source added is calculated as TiO2, and the titanium source content in the low titanium slurry is lower than the titanium source content in the high titanium slurry; preferably, the titanium source content in the low titanium slurry differs from the titanium source content in the high titanium slurry by more than 50%, that is, when the titanium source content in the low titanium slurry is a, the titanium source content in the high titanium slurry is preferably at least 1.5a.

[0045] In the present invention, a phosphorus source, an iron source, a lithium source, a titanium source and a dispersant are added to deionized water, and the solid content is controlled at 30% to 40%, for example, 32%, 35%, 38%, etc., and a sand mill is used to grind the slurry to a particle size D50 ≤ 800 nm (preferably D50 = 400 to 600 nm) so that the precursor particles are evenly dispersed.

[0046] In the present invention, the sand-milled high-titanium slurry and low-titanium slurry are spray-granulated separately, with the spray inlet temperature controlled at 180-220°C, for example, 190°C, 200°C, 210°C, and the outlet temperature controlled at 90-100°C, for example, 92°C, 95°C, 96°C, 98°C, to avoid local overheating leading to particle sintering. After spraying, spherical high-titanium intermediates and low-titanium intermediates with good fluidity and particle sizes of 10-30 μm are obtained.

[0047] In the present invention, in step 2, the high-titanium intermediate and the low-titanium intermediate are mixed in a ratio of 5:5 to 8:2, preferably 6:4 to 7:3 to obtain a lithium iron phosphate intermediate, and then the lithium iron phosphate intermediate is reduced and carbon-coated using a fluidized bed. The fluidized bed adopts a vibrating fluidized bed or a gas-solid circulating fluidized bed, the reaction temperature is 400 to 800°C, for example, 500°C, 600°C, 700°C, etc., and the reaction time is 4 to 10 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, etc., to ensure that the particles are fully fluidized and dead zones are avoided; multiple temperature control zones are set in the reactor to achieve precise control of the temperature gradient.

[0048] In the present invention, the reduction stage is carried out in a reducing atmosphere, for example, by introducing a H2 / N2 mixed gas, wherein the volume proportion of H2 is 2% to 10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, etc., and the reduction is carried out at 400 to 500°C for 2 to 4 hours; the gas flow rate is controlled at 0.5 to 1.5 m / s to ensure uniform mass transfer in the fluidized bed.

[0049] In the present invention, the carbon coating in step 2 is carried out in an inert atmosphere, for example, the reducing atmosphere in step 1 is switched to a high-purity N2 inert atmosphere, a carbon source is added, and the carbon coating is carried out at 650-800°C (e.g., 700°C, 750°C, etc.) for 3-5 hours. The carbon source is selected from one or more of glucose, sucrose, PEG, PVP or phenolic resin, and the addition amount is 3% to 10% of the mass of the lithium iron phosphate intermediate, for example, 4%, 5%, 6%, 7%, 8%, 9%, etc.

[0050] The carbon source is uniformly sprayed into the fluidized bed in the form of an aerosol through a nozzle at the bottom, with a droplet size of 1 to 10 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, etc., and a spraying rate of 2 to 5 L / h, such as 3 L / h, 3.5 L / h, 4 L / h, 4.5 L / h, 5 L / h, etc.

[0051] The carbon layer thickness of the lithium iron phosphate material prepared by the method of the present invention is 2 to 10 nm, and the compacted density is ≥ 2.5 g / cm 3 , 0.2C first efficiency ≥ 95%, 1C cycle 500 times capacity retention rate ≥ 90%, which is suitable for application in energy storage battery positive electrode materials.

[0052] The present invention is further explained below by more specific examples, but does not constitute any limitation.

[0053] Unless otherwise specified, the raw materials used in the following examples are commercially available conventional reagents and are of industrial purity.

[0054] Example 1

[0055] S1. Preparation of low-titanium intermediate: Weigh 1000 g of ferric phosphate, 251 g of lithium carbonate, 3 g of nano-titanium dioxide, and 20 g of dispersant (PEG-2000). Mix the raw materials with 2000 g of deionized water and sand-grind to 500 nm to obtain a low-titanium slurry. Dry the low-titanium slurry using a spray dryer (inlet temperature 200°C, outlet temperature 95°C) to obtain a low-titanium intermediate (D50 approximately 20 μm).

[0056] S2. Preparation of high-titanium intermediate: Weigh 1000g of raw materials: 1000g of ferric phosphate, 251g of lithium carbonate, 8g of nano-titanium dioxide, and 20g of dispersant (PEG-2000). Mix the raw materials with 2000g of deionized water and sand-mill to 500nm to obtain a high-titanium slurry. Dry the high-titanium slurry using a spray dryer (inlet temperature 200°C, outlet temperature 95°C) to obtain a high-titanium intermediate (D50 of approximately 20μm).

[0057] S3, fluidized bed treatment: Weigh 150g of low-titanium intermediate and 350g of high-titanium intermediate and mix them evenly (3:7), and then perform reduction treatment after mixing. Reduction uses H2 / N2 mixed gas (H2 volume proportion 5%), temperature 450℃, time 3h, gas flow rate 1.0m / s. After reduction, carbon coating is performed, weigh 25g of glucose and spray it in the form of aerosol (droplet size 5μm, rate 3L / h). High-purity N2 is used for carbon coating, the constant temperature section temperature is 750℃, and the constant temperature time is 5h.

[0058] S4. Testing: The lithium iron phosphate prepared in the above steps is characterized and electrochemical performance tested, wherein the electrical performance test methods all comply with the requirements of GB / T36276-2023, and the cycle capacity retention rate is tested under 1C rate conditions.

[0059] Example 2

[0060] In Example 2, only the preparation process of the low-titanium and high-titanium intermediates is different from that in Example 1, while the fluidized bed treatment process and the testing process are the same as those in Example 1.

[0061] S1. Preparation of low-titanium intermediate: Weigh 762.7 g of monoammonium phosphate, 513.5 g of red iron oxide, 251 g of lithium carbonate, 3 g of nano-titanium dioxide, and 10 g of dispersant (PVP-K30). Mix the raw materials with 2000 g of deionized water and sand-mill to 300 nm to obtain a low-titanium slurry. Dry the low-titanium slurry using a spray dryer (inlet temperature 200°C, outlet temperature 90°C) to obtain a low-titanium intermediate.

[0062] S2. Preparation of high-titanium intermediate: Weigh 762.7 g of monoammonium phosphate, 513.5 g of red iron oxide, 251 g of lithium carbonate, 8 g of nano-titanium dioxide, and 10 g of dispersant (PVP-K30). Mix the raw materials with 2000 g of deionized water and sand-mill to 300 nm to obtain a high-titanium slurry. Dry the high-titanium slurry using a spray dryer (inlet temperature 200°C, outlet temperature 90°C) to obtain a high-titanium intermediate.

[0063] Example 3

[0064] Example 3 Based on Example 1, the amount of titanium dioxide added in the low titanium intermediate preparation step in S1 was changed to 6g, and the dispersant was added to 5g. The amount of titanium dioxide added in the high titanium intermediate preparation step in S2 was changed to 6g and the dispersant was added to 5g. The amount of precursor in S3 was replaced with a mixture of 250g low titanium intermediate and 250g high titanium intermediate (5:5), and 40g of glucose was used for carbon coating. The remaining steps were the same as in Example 1.

[0065] Example 4

[0066] Example 3 Based on Example 1, the amount of lithium carbonate added in the low titanium intermediate preparation step in S1 was changed to 252.5g, the amount of titanium dioxide added was changed to 1g, and the dispersant was changed to 35g. The amount of lithium carbonate added in the high titanium intermediate preparation step in S2 was changed to 252.5g, the amount of titanium dioxide added was changed to 7g, and the dispersant was changed to 35g. The amount of precursor in S3 was replaced with a mixture of 100g of low titanium intermediate and 400g of high titanium intermediate (2:8). The remaining steps were the same as in Example 1.

[0067] Comparative Example 1

[0068] S1. Preparation of intermediate: Weigh 1000 g of ferric phosphate, 251 g of lithium carbonate, 5 g of nano-titanium dioxide, and 20 g of dispersant (PEG-2000). Mix the raw materials with 2000 g of deionized water and sand-grind to 500 nm to obtain a sand-grinded slurry. Dry the sand-grinded slurry using a spray dryer (inlet temperature 200°C, outlet temperature 95°C) to obtain an intermediate.

[0069] S2. Fluidized bed treatment: Weigh 500g of the above intermediate and perform reduction treatment. Reduction is performed using a H2 / N2 mixture (5% H2) at 450°C for 3 hours at a gas flow rate of 1.0 m / s. After reduction, carbon coating is performed by weighing 40g of glucose and spraying it in the form of an aerosol (droplet size 5μm, rate 3L / h). Carbon coating is performed using high-purity N2, with a constant temperature section at 750°C for 5 hours.

[0070] S3. Testing: The lithium iron phosphate prepared in the above steps is characterized and electrochemically tested, wherein the electrical performance test methods all comply with the requirements of GB / T36276-2023, and the cycle capacity retention rate is tested under 1C rate conditions.

[0071] Comparative Example 2

[0072] S1. Preparation of intermediate: Weigh 1000 g of ferric phosphate, 251 g of lithium carbonate, 5 g of nano-titanium dioxide, 80 g of a carbon source (glucose), and 20 g of a dispersant (PVP-K30). Mix the raw materials with 2000 g of deionized water and sand-mill to 500 nm to obtain a slurry. Dry the slurry using a spray dryer (inlet temperature 200°C, outlet temperature 95°C) to obtain the intermediate.

[0073] S2. Carbothermal reduction: Take 500 g of the above intermediate and perform carbothermal reduction in a box furnace (nitrogen atmosphere, constant temperature 780°C, constant temperature time 10 h)

[0074] S3. Testing: The lithium iron phosphate prepared in the above steps is characterized and electrochemically tested, wherein the electrical performance test methods all comply with the requirements of GB / T36276-2023, and the cycle capacity retention rate is tested under 1C rate conditions.

[0075] Comparative Example 3

[0076] Comparative Example 3 differs from Example 1 in that, in step S3, 40 g of glucose and the intermediate are mechanically mixed before reduction and carbon coating, the temperature program remains unchanged, but a nitrogen atmosphere is used throughout the process.

[0077] The characterization and electrical performance test results of the lithium iron phosphate of the above examples and comparative examples are shown in the following table, where D50 is the secondary particle size of the lithium iron phosphate after air flow grinding, and the carbon layer thickness difference is measured by SEM images.

[0078]

[0079]

[0080] It can be seen from the above table that titanium gradient grading is the key to improving compaction. Examples 1 and 3 both achieved 2.5 g / cm by different titanium content particle grading. 3 The above compaction, through Figure 1 SEM and Figure 3 The particle size distribution of Example 1 can be clearly observed to have large and small particle grading. However, Comparative Example 1 has a uniform titanium content, so the grading effect is poor. Figure 4 The SEM distribution of Comparative Example 1 clearly shows that there are only large particles, so the compacted density is less than 2.5 g / cm 3 .

[0081] By comparing Example 1 with Comparative Examples 2 and 3, it can be seen that fluidized bed spray coating of carbon is the key to improving the uniformity of the carbon layer. The carbon layer uniformity of traditional solid phase coating and premixed carbon source is poor. The difference in carbon layer thickness between Comparative Examples 2 and 3 is as high as 5nm, while that of Example 1 is only 1nm. Figure 2 It can be observed that the thickness difference of the carbon layer on the surface of the lithium iron phosphate in Example 1 is less than 1 nm.

[0082] In summary, the present invention achieves a high density of iron-lithium (≥2.5g / cm 3 ), which solves the problem of insufficient particle grading in traditional processes; achieves a long cycle life (1C cycle 500 times ≥ 90%), and the uniform carbon layer inhibits side reactions and improves structural stability; at the same time, the process is efficient and energy-saving, and the fluidized bed one-step method simplifies the process and reduces production costs.

Claims

1. A method for preparing lithium iron phosphate as a positive electrode material for a high-density battery, characterized in that: The following steps are involved: S1, mixing a phosphorus source, an iron source, a lithium source, a titanium source, a dispersant, and water into a slurry and grinding the mixture, respectively controlling the amount of the titanium source added to obtain a high-titanium slurry and a low-titanium slurry, spray drying the ground slurry, and granulating the mixture to obtain a high-titanium intermediate and a low-titanium intermediate; S2. Mixing the high-titanium intermediate and the low-titanium intermediate in proportion to obtain a lithium iron phosphate intermediate, and performing reduction and carbon coating in a fluidized bed to obtain a lithium iron phosphate material.

2. The preparation method according to claim 1, characterized in that In step S1, the amount of titanium source (calculated as TiO2) added to the high-titanium slurry is 0.5-0.8wt% of the total mass of the phosphorus source, iron source and lithium source, and the amount of titanium source (calculated as TiO2) added to the low-titanium slurry is 0.05-0.5wt% of the total mass of the phosphorus source, iron source and lithium source; wherein the titanium source content in the low-titanium slurry is lower than the titanium source content in the high-titanium slurry; Preferably, the titanium source content in the low titanium slurry differs from the titanium source content in the high titanium slurry by more than 50%; More preferably, the lithium source, phosphorus source and iron source are added according to a stoichiometric ratio of Li:P:Fe=1.02-1.05:1:0.95-0.98; Further preferably, the amount of the dispersant added is 0.5-3% of the total mass of the phosphorus source, the iron source and the lithium source.

3. The preparation method according to claim 1, characterized in that The particle size of the slurry after grinding in step S1 is D50≤800nm, preferably D50 is 400-600nm, the inlet temperature of the spray drying is 180-220°C, and the outlet temperature is 90-100°C; Preferably, a spray dryer is used for granulation, and the particle size of the obtained spherical high-titanium intermediate or low-titanium intermediate is 10 to 30 μm.

4. The preparation method according to claim 1, characterized in that The mixing mass ratio of the high-titanium intermediate to the low-titanium intermediate in step S2 is 5:5 to 8:2, preferably 6:4 to 7:

3.

5. The preparation method according to claim 1, characterized in that The fluidized bed in step S2 is a vibrating fluidized bed or a gas-solid circulating fluidized bed, the reaction temperature is 400-800° C., and the reaction time is 4-10 hours.

6. The preparation method according to claim 5, characterized in that The reduction and carbon coating in the fluidized bed in step S2 are carried out in two stages: The first stage: introducing H2 / N2 mixed gas, in which H2 accounts for 2% to 10% by volume, and reducing at 400-500℃ for 2-4h; The second stage: switch to high-purity N2 inert atmosphere, add carbon source and perform carbon coating at 650-800°C for 3-5h.

7. The preparation method according to claim 6, characterized in that The carbon source introduced into the carbon coating stage of the fluidized bed in step S2 is one or more of glucose, sucrose, polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP) or phenolic resin; Preferably, the amount of carbon source added is 3% to 10% of the mass of the lithium iron phosphate intermediate.

8. The preparation method according to claim 6 or 7, characterized in that The carbon source is uniformly sprayed in the form of aerosol through a nozzle at the bottom of the fluidized bed, with a droplet size of 1 to 10 μm and a spraying rate of 2 to 5 L / h.

9. Lithium iron phosphate, a high-density battery cathode material, prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The carbon layer thickness of the prepared lithium iron phosphate material is 2 to 10 nm, and the compaction density is ≥ 2.5 g / cm 3 , 0.1C first efficiency ≥ 97%, 1C cycle 500 times capacity retention rate ≥ 90%.

10. Use of the lithium iron phosphate, a positive electrode material for a high-density battery, prepared by the preparation method according to any one of claims 1 to 8, or the lithium iron phosphate, a positive electrode material for a high-density battery according to claim 9, in positive electrode materials for energy storage batteries.

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