Preparation Method of Ultra-High Pressure Compacted Lithium Iron Phosphate Cathode Material and Lithium Ion Battery

Abstract

The present invention discloses a preparation method of a lithium iron phosphate cathode material with ultra-high pressure compaction and a lithium ion battery, relating to the technical field of lithium iron phosphate cathode materials. In this application, a solvothermal method is used to react ferrous sulfate heptahydrate with a lithium phosphate hollow microsphere template to synthesize hollow lithium iron phosphate microspheres, and then directly continue the reaction on this basis to generate nanoscale lithium iron phosphate particles. At the same time as generating the nanoscale lithium iron phosphate particles, graphene is in-situ compounded, so that while the hollow lithium iron phosphate microspheres and the nanoscale lithium iron phosphate particles are more uniformly dispersed, graphene doping is completed. Using graphene as a conductive additive, sucrose is further used for surface carbon coating modification, thereby constructing an efficient conductive network in which graphene doping and carbon coating work together, solving the electron transport interface hindrance and point contact effect, so that electrons can be efficiently transported, and further improving the electrochemical performance of the material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate cathode materials, and specifically to a preparation method of a high tap density lithium iron phosphate cathode material and a lithium ion battery. Background Art

[0002] Currently, the methods for improving the tap density of lithium iron phosphate materials mainly include the following: 1. Increasing the primary particle size; 2. Reducing the carbon content; 3. Lowering the porosity; 4. Improving the coated carbon source; 5. Improving the particle size grading; 6. Improving the smoothness of the material surface, etc. Among them, the easiest option is to increase the primary particle size and then use smaller particles to fill the gaps between the large particles to achieve graded packing. However, the defects of this method are also obvious at present. That is, for the solid lithium iron phosphate particles with large particle sizes, compared with the nanosized particles, the lithium iron phosphate inside still has a long electron conduction and lithium transport distance, and it is difficult for the electrolyte to penetrate into the solid particles, which limits the performance. Therefore, the present application innovatively uses a method of compounding hollow microsphere lithium iron phosphate particles and lithium iron phosphate nanoparticles to achieve graded packing, without using metal doping for filling. This can not only avoid the problems of solid particles but also solve the problem of uneven dispersion of lithium iron phosphate caused by metal doping, and this method can also effectively improve the tap density. Summary of the Invention

[0003] The purpose of the present invention is to provide a preparation method of a high tap density lithium iron phosphate cathode material and a lithium ion battery to solve the problems existing in the prior art.

[0004] To solve the above technical problems, the present invention provides the following technical solutions:

[0005] A preparation method of a high tap density lithium iron phosphate cathode material, comprising the following preparation steps:

[0006] S1. According to a mass ratio of 1:45 - 50, add phosphoric acid with a mass fraction of 80 - 85% to a polyethylene glycol solution, stir evenly, and prepare solution A for standby; according to a mass ratio of 1:70 - 75, add lithium hydroxide to the polyethylene glycol solution, stir evenly, then add solution A which is 75 - 80 times the mass of lithium hydroxide, stir for 30 - 35 min, then vacuum filter, wash the filtered product with deionized water and absolute ethanol 3 - 5 times respectively, and vacuum dry at a temperature of 95 - 105 °C for 3 - 4 h to obtain a hollow microsphere template;

[0007] S2. According to the mass ratio of 4.8 - 5:1, add ferrous sulfate heptahydrate and hollow microsphere template into ethylene glycol with a mass 4.8 - 5.2 times that of ferrous sulfate heptahydrate. After stirring for 15 - 20 min, gradually heat up to 150 °C under an argon atmosphere. After reacting for 3 - 3.5 h, add L - ascorbic acid with a mass 0.1 - 0.08 times that of ferrous sulfate heptahydrate. After stirring evenly, add the precursor mixture and stir for 30 - 35 min. After the stirring is completed, heat up to 180 °C under an argon atmosphere and continue to react for 16 - 18 h. After the reaction is completed, cool to 25 - 35 °C, filter the lower - layer precipitate, and wash the filtered product with deionized water and absolute ethanol 3 - 5 times respectively. After washing, dry at a temperature of 45 - 50 °C for 24 - 26 h to obtain the composite; the precursor mixture includes the following preparation steps: Weigh lithium hydroxide monohydrate and phosphoric acid according to the mass ratio of 1:0.7 - 0.9. Add lithium hydroxide monohydrate into ethylene glycol with a mass 35 - 45 times that of lithium hydroxide monohydrate. After stirring and dissolving, add a graphene dispersion with a mass 1.1 - 1.3 times that of ethylene glycol, and stir evenly to prepare the precursor mixture for standby; among them, the molar ratio of ferrous sulfate heptahydrate, lithium hydroxide monohydrate, and phosphoric acid is 3.2 - 3.5:3:1;

[0008] S3. According to the mass ratio of 8 - 10:1, mix the composite and the carbon source. After mixing, stir for 1 - 1.5 h, and then dry at a temperature of 45 - 50 °C for 24 - 26 h. After drying, under an argon atmosphere, carry out two - step calcination treatment. After calcination, cool to 25 - 35 °C to obtain the lithium iron phosphate cathode material.

[0009] As an optimization, the graphene dispersion includes the following preparation steps: Add graphene oxide into ethylene glycol with a mass 900 - 950 times that of graphene oxide. After stirring evenly, perform ultrasonic dispersion for 30 - 40 min and then set aside.

[0010] As an optimization, the graphene oxide includes the following preparation steps: Weigh flake graphite and sodium nitrate according to a mass ratio of 1:0.5 - 0.6, add the flake graphite and sodium nitrate to concentrated sulfuric acid with a mass fraction of 98% which is 40 - 45 times the mass of the flake graphite, ultrasonicate for 30 - 35 min at a temperature of -5 - 0 °C. After ultrasonication, add potassium permanganate which is 2.8 - 3 times the mass of the flake graphite, then stir for 2 - 2.5 h at a temperature of 0 - 5 °C. Then, ultrasonicate for 30 - 35 min at a temperature of 30 - 35 °C, and then add deionized water which is 95 - 100 times the mass of the flake graphite. Then, stir for 30 - 35 min at a temperature of 90 - 95 °C. After that, add deionized water which is 55 - 60 times the mass of the flake graphite, let it stand for 20 - 30 min, then add hydrogen peroxide solution with a mass fraction of 30 - 35% which is 30 - 32 times the mass of the flake graphite, let it stand for 15 - 20 min, then add hydrochloric acid solution with a mass fraction of 10 - 15% which is 45 - 50 times the mass of the flake graphite. After standing and separating layers, take the lower layer precipitate, wash it with deionized water 3 - 5 times, and then centrifuge and separate to take the precipitate and wash it until neutral to obtain graphene oxide.

[0011] As an optimization, the two-step calcination treatment includes the following preparation steps: Calcinate at a temperature of 330 - 350 °C for 2 - 2.2 h, and then calcinate at a temperature of 690 - 700 °C for 4 - 10.2 h.

[0012] As an optimization, the heating and cooling rates during the two-step calcination treatment are both 5 °C / min.

[0013] As an optimization, the carbon source is sucrose.

[0014] A lithium iron phosphate cathode material prepared by the preparation method of the super-high-density lithium iron phosphate cathode material as described in any one of the above.

[0015] An application of the preparation method of the super-high-density lithium iron phosphate cathode material as described in any one of the above in a lithium-ion battery.

[0016] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0017] In this application, lithium phosphate hollow microsphere templates are first synthesized using phosphoric acid and lithium hydroxide as the phosphorus source and lithium source respectively. Then, a solvothermal method is used to react ferrous sulfate heptahydrate with the lithium phosphate hollow microsphere templates to synthesize hollow lithium iron phosphate microspheres. Subsequently, based on this, the reaction is directly continued to generate nanoscale lithium iron phosphate particles. At the same time as the nanoscale lithium iron phosphate particles are generated, graphene is in-situ composite, so that while the hollow lithium iron phosphate microspheres and the nanoscale lithium iron phosphate particles are more uniformly dispersed, graphene doping is completed. Using graphene as a conductive additive, sucrose is further used for surface carbon coating modification, thereby constructing an efficient conductive network in which graphene doping and carbon coating synergistically play a role, solving the problems of electron transport interface hindrance and point contact effect, so that electrons can be efficiently transported, and further improving the electrochemical performance of the material;

[0018] During the replacement of the hollow microsphere templates with ferrous sulfate heptahydrate, the reaction occurs from the surface layer of the particles and gradually reacts to the inside of the particles until it ends. Along with the exchange process of iron ions and lithium ions on the particles from the outside to the inside, the volume of the outer layer of the particles expands, and at the same time, the internal hollow microsphere templates are continuously consumed, and the particles gradually show a hollow structure. Finally, when the reaction is completed, a lithium iron phosphate microsphere material with a hollow structure is formed; this hollow structure can effectively avoid the long electron conduction and lithium ion transport distances inside the particles, and the electrolyte is easy to penetrate into the inside of the particles, which can fully exert its performance, reduce the side reactions between the electrode material and the electrolyte, and increase the packing density; after the replacement is completed, L-ascorbic acid is added to change the pH of the reaction materials to end the replacement reaction and promote the reaction to the coprecipitation process. Specific embodiments

[0019] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0020] Example 1

[0021] A1. Weigh natural flake graphite and sodium nitrate according to a mass ratio of 1:0.5. Add natural flake graphite and sodium nitrate into concentrated sulfuric acid with a mass fraction of 98% and 40 times the mass of natural flake graphite. Under the condition of -5°C, ultrasonicate for 30 min. After ultrasonication, add potassium permanganate with a mass 2.8 times that of natural flake graphite. Then, under the condition of 0°C, stir for 2 h. After that, under the condition of 30°C, ultrasonicate for 30 min, and then add deionized water with a mass 95 times that of natural flake graphite. Then, under the condition of 90°C, stir for 30 min. After that, add deionized water with a mass 55 times that of natural flake graphite again. Let it stand for 20 min, and then add hydrogen peroxide solution with a mass fraction of 30% and a mass 30 times that of natural flake graphite. Let it stand for 15 min, and then add hydrochloric acid solution with a mass fraction of 10% and a mass 45 times that of natural flake graphite. After standing and separating, take the lower-layer precipitate, wash it 3 times with deionized water, and then centrifuge and separate to take the precipitate and wash it until neutral to obtain graphene oxide; add graphene oxide into ethylene glycol with a mass 900 times that of graphene oxide, stir evenly, ultrasonically disperse for 30 min, and then set aside;

[0022] A2. Weigh lithium hydroxide monohydrate and phosphoric acid according to a mass ratio of 1:0.7. Add lithium hydroxide monohydrate into ethylene glycol with a mass 35 times that of lithium hydroxide monohydrate. After stirring and dissolving, add graphene oxide dispersion with a mass 1.1 times that of ethylene glycol, and stir evenly to prepare a precursor mixture for later use;

[0023] A3. According to a mass ratio of 1:45, add phosphoric acid with a mass fraction of 80% into polyethylene glycol solution, stir evenly to prepare solution A for later use; according to a mass ratio of 1:70, add lithium hydroxide into polyethylene glycol solution, stir evenly, and then add solution A with a mass 75 times that of lithium hydroxide. Stir for 30 min, then vacuum filter, and wash the filtered product 3 times with deionized water and anhydrous ethanol respectively. Then, under the condition of 95°C, vacuum dry for 3 h to obtain a hollow microsphere template;

[0024] A4. According to a mass ratio of 4.8:1, add ferrous sulfate heptahydrate and the hollow microsphere template into ethylene glycol with a mass 4.8 times that of ferrous sulfate heptahydrate. Stir for 15 min, then gradually heat up to 150°C under an argon atmosphere. After reacting for 3 h, add L-ascorbic acid with a mass 0.08 times that of ferrous sulfate heptahydrate, stir evenly, and then add the precursor mixture. Stir for 30 min. After stirring is completed, heat up to 180°C under an argon atmosphere and continue to react for 16 h. After the reaction is completed, cool to 25°C, filter the lower-layer precipitate, and wash the filtered product 3 times with deionized water and anhydrous ethanol respectively. After washing, dry at 45°C for 24 h to obtain a composite; the molar ratio among ferrous sulfate heptahydrate, lithium hydroxide monohydrate and phosphoric acid is 3.2:3:1;

[0025] A5. Mix the composite and sucrose in a mass ratio of 8:1, stir for 1 h after mixing, then dry at 45 °C for 24 h. After drying, perform two-step calcination treatment under an argon atmosphere. After calcination, cool to 25 °C to obtain the lithium iron phosphate cathode material. The two-step calcination treatment includes the following preparation steps: calcine at 330 °C for 2 h, and then calcine at 690 °C for 4 h. The heating and cooling rates during the two-step calcination treatment are both 5 °C / min.

[0026] Example 2

[0027] A1. Weigh natural graphite and sodium nitrate in a mass ratio of 1:0.55. Add natural graphite and sodium nitrate to concentrated sulfuric acid with a mass fraction of 98% and 42.5 times the mass of natural graphite. Under the condition of -2.5 °C, ultrasonicate for 32.5 min. After ultrasonication, add potassium permanganate 2.9 times the mass of natural graphite. Then, under the condition of 2.5 °C, stir for 2.25 h. After that, under the condition of 32.5 °C, ultrasonicate for 32.5 min, and then add deionized water 97.5 times the mass of natural graphite. Then, under the condition of 92.5 °C, stir for 32.5 min. After that, add deionized water 57.5 times the mass of natural graphite again. Let it stand for 25 min, then add hydrogen peroxide solution with a mass fraction of 32.5% 31 times the mass of natural graphite. Let it stand for 17.5 min, then add hydrochloric acid solution with a mass fraction of 12.5% 47.5 times the mass of natural graphite. After standing and separating, take the lower layer precipitate, wash it 4 times with deionized water, and then centrifuge and separate to wash the precipitate until it is neutral to obtain graphene oxide. Add the graphene oxide to ethylene glycol 925 times the mass of graphene oxide, stir evenly, ultrasonically disperse for 35 min, and set aside.

[0028] A2. Weigh lithium hydroxide monohydrate and phosphoric acid in a mass ratio of 1:0.8. Add lithium hydroxide monohydrate to ethylene glycol 40 times the mass of lithium hydroxide monohydrate. After stirring and dissolving, add the graphene oxide dispersion 1.2 times the mass of ethylene glycol, and stir evenly to prepare a precursor mixture for standby.

[0029] A3. Add phosphoric acid with a mass fraction of 82.5% to the polyethylene glycol solution in a mass ratio of 1:47.5, stir evenly to prepare solution A for standby. Add lithium hydroxide to the polyethylene glycol solution in a mass ratio of 1:72.5, stir evenly, then add solution A 77.5 times the mass of lithium hydroxide, stir for 32.5 min, then perform vacuum filtration, wash the filtration product 4 times with deionized water and anhydrous ethanol respectively, and then vacuum dry at 100 °C for 3.5 h to obtain the hollow microsphere template.

[0030] A4. According to a mass ratio of 4.9:1, ferrous sulfate heptahydrate and hollow microsphere templates are added to ethylene glycol with a mass 5 times that of ferrous sulfate heptahydrate. After stirring for 17.5 min, the temperature is gradually raised to 150 °C under an argon atmosphere. After reacting for 3.25 h, L-ascorbic acid with a mass 0.09 times that of ferrous sulfate heptahydrate is added. After stirring evenly, the precursor mixture is added and stirred for 32.5 min. After the stirring is completed, the temperature is raised to 180 °C under an argon atmosphere and the reaction continues for 17 h. After the reaction is completed and cooled to 30 °C, the lower-layer precipitate is filtered, and the filtered product is washed 4 times with deionized water and anhydrous ethanol respectively. After washing, it is dried at a temperature of 47.5 °C for 25 h to obtain a composite; the molar ratio of ferrous sulfate heptahydrate, lithium hydroxide monohydrate, and phosphoric acid is 3.35:3:1;

[0031] A5. According to a mass ratio of 9:1, the composite and sucrose are mixed. After mixing, they are stirred for 1.25 h, and then dried at a temperature of 47.5 °C for 25 h. After drying, under an argon atmosphere, two-step calcination treatment is carried out. After calcination is completed and cooled to 30 °C, a lithium iron phosphate cathode material is obtained; the two-step calcination treatment includes the following preparation steps: calcination is carried out at a temperature of 340 °C for 2.1 h, and then calcination is carried out at a temperature of 695 °C for 4.1 h; the heating and cooling rates during the two-step calcination treatment are both 5 °C / min.

[0032] Example 3

[0033] A1. Weigh flake graphite and sodium nitrate according to a mass ratio of 1:0.6. Add flake graphite and sodium nitrate to concentrated sulfuric acid with a mass fraction of 98% and a mass 45 times that of flake graphite. Under the condition of a temperature of 0 °C, ultrasonicate for 35 min. After ultrasonication, add potassium permanganate with a mass 3 times that of flake graphite. Then, under the condition of a temperature of 5 °C, stir for 2.5 h. Then, under the condition of a temperature of 35 °C, ultrasonicate for 35 min, and then add deionized water with a mass 100 times that of flake graphite. Then, under the condition of a temperature of 95 °C, stir for 35 min. Then, add deionized water with a mass 60 times that of flake graphite again. After standing for 30 min, add hydrogen peroxide solution with a mass fraction of 35% and a mass 32 times that of flake graphite. After standing for 20 min, add hydrochloric acid solution with a mass fraction of 15% and a mass 50 times that of flake graphite. After standing and separating, take the lower-layer precipitate, wash it 5 times with deionized water, and then centrifuge and separate to take the precipitate and wash it until it is neutral to obtain graphene oxide; add graphene oxide to ethylene glycol with a mass 950 times that of graphene oxide, stir evenly, ultrasonically disperse for 40 min, and then set aside;

[0034] A2. Weigh lithium hydroxide monohydrate and phosphoric acid according to a mass ratio of 1:0.9. Add lithium hydroxide monohydrate to ethylene glycol with a mass 45 times that of lithium hydroxide monohydrate. After stirring and dissolving, add the graphene oxide dispersion with a mass 1.3 times that of ethylene glycol, stir evenly, and prepare a precursor mixture for standby;

[0035] A3. Add phosphoric acid with a mass fraction of 85% to the polyethylene glycol solution at a mass ratio of 1:50, stir evenly, and prepare solution A for later use; add lithium hydroxide to the polyethylene glycol solution at a mass ratio of 1:75, stir evenly, then add solution A which is 80 times the mass of lithium hydroxide, stir for 35 min, then perform vacuum filtration, and wash the filtered product 5 times each with deionized water and absolute ethanol. Then, under the condition of a temperature of 105 °C, perform vacuum drying for 4 h to obtain a hollow microsphere template;

[0036] A4. Add ferrous sulfate heptahydrate and the hollow microsphere template to ethylene glycol which is 5.2 times the mass of ferrous sulfate heptahydrate at a mass ratio of 5:1, stir for 20 min, then gradually heat up to 150 °C under an argon atmosphere, react for 3.5 h, add L-ascorbic acid which is 0.1 times the mass of ferrous sulfate heptahydrate, stir evenly, then add the precursor mixture, stir for 35 min. After the stirring is completed, heat up to 180 °C under an argon atmosphere and continue to react for 18 h. After the reaction is completed, cool to 35 °C, filter the lower-layer precipitate, and wash the filtered product 5 times each with deionized water and absolute ethanol. After washing, dry at a temperature of 50 °C for 26 h to obtain a composite; the molar ratio among ferrous sulfate heptahydrate, lithium hydroxide monohydrate, and phosphoric acid is 3.5:3:1;

[0037] A5. Mix the composite and sucrose at a mass ratio of 10:1, stir for 1.5 h after mixing, then dry at a temperature of 50 °C for 26 h. After drying, perform two-step calcination treatment under an argon atmosphere. After the calcination is completed, cool to 35 °C to obtain the lithium iron phosphate cathode material; the two-step calcination treatment includes the following preparation steps: calcine at a temperature of 350 °C for 2.2 h, and then calcine at a temperature of 700 °C for 4.2 h; the heating and cooling rates during the two-step calcination treatment are both 5 °C / min.

[0038] Example 4

[0039] The difference from Example 2 is only in step A4: Add ferrous sulfate heptahydrate and the hollow microsphere template to ethylene glycol which is 5.2 times the mass of ferrous sulfate heptahydrate at a mass ratio of 5:1, stir for 20 min, then gradually heat up to 150 °C under an argon atmosphere, react for 3.5 h, then cool to 35 °C, filter the lower-layer precipitate, and wash the filtered product 5 times each with deionized water and absolute ethanol. After washing, dry at a temperature of 50 °C for 26 h to obtain a composite.

[0040] Example 5

[0041] The difference from Example 2 is only in step A4: Ferrous sulfate heptahydrate is added to ethylene glycol with a mass 5.2 times that of ferrous sulfate heptahydrate. After stirring for 20 min, the temperature is gradually raised to 150 °C under an argon atmosphere. After reacting for 3.5 h, L-ascorbic acid with a mass 0.1 times that of ferrous sulfate heptahydrate is added. After stirring evenly, the precursor mixture is added and stirred for 35 min. After the stirring is completed, the temperature is raised to 180 °C under an argon atmosphere and the reaction continues for 18 h. After the reaction ends, it is cooled to 35 °C, the lower-layer precipitate is filtered, and the filtered product is washed 5 times with deionized water and anhydrous ethanol respectively. After the washing is completed, it is dried at 50 °C for 26 h to obtain the composite; the molar ratio of ferrous sulfate heptahydrate, lithium hydroxide monohydrate, and phosphoric acid is 1:3:1;

[0042] Mechanical property and aging resistance property tests:

[0043] Pole piece property tests

[0044] Pole piece parameter measurement: Select the pole pieces after coating, pressing, 24 hours after pressing, and ultimate pressing treatment. Use an electronic balance to accurately weigh the weight of the pole pieces to obtain the pole piece surface density. Then, use a micrometer to measure the thickness of the pole pieces, and collect 10 thickness data for each pole piece.

[0045] Pole piece compaction density = pole piece surface density / (pole piece thickness - current collector thickness).

[0046] Performance evaluation under special charge and discharge regimes: In a Lanqi charging cabinet of model CT2001A, the battery performance is evaluated using special charge and discharge regimes. Due to the influence of thermodynamic factors, the discharge capacity of the battery may exceed 100%.

[0047] The specific evaluation method is as follows:

[0048] 5C rate discharge performance evaluation: First, charge the battery to 3.8 V at a constant current of 1C, and then discharge the battery to 2.0 V at a constant current of 5C to evaluate the performance of the battery during 5C rate discharge.

[0049] 2C rate charge performance evaluation: Charge the battery to 3.8 V at a constant current of 2C, and then discharge the battery to 2.0 V at a constant current of 1C to evaluate the performance of the battery during 2C rate charge.

[0050] 1C cycle performance test: Charge and discharge the battery 1000 times by charging at a constant current of 1C to 3.8 V and discharging at a constant current of 1C to 2.0 V, and calculate the capacity retention rate of the battery after 1000 cycles.

[0051] Table 1 Test results of mechanical properties and aging resistance properties

[0052] Specimen <![CDATA[The compaction density of the electrode sheet / (g / cm 3 ).]]> Discharge at 5C rate / % Charge at 2C rate / % Cycle performance / % Example 1 2.79 100 101 95 Example 2 2.72 98 100 98 Example 3 2.73 97 100 97 Example 4 2.33 87 89 88 Example 5 2.12 76 79 79

[0053] From the comparison of the experimental data of Examples 1 to 3 in Table 1, it can be found that the lithium iron phosphate particle electrode prepared by the present invention has an ultra-high tap density and a discharge cycle performance of more than 100%;

[0054] In Example 4, since only hollow microspheres were used as the lithium iron phosphate particles, there was a large porosity, resulting in poor final performance. In Example 5, nanoparticles were used as the filling main body, and its tap density was significantly reduced, resulting in a decline in the final performance.

[0055] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

Claims

1. A method for preparing an ultra-high-density lithium iron phosphate positive electrode material, characterized in that: The method comprises the following preparation steps: S1. According to a mass ratio of 1:45-50, add 80-85% phosphoric acid to the polyethylene glycol solution, stir evenly, and prepare solution A for standby; according to a mass ratio of 1:70-75, add lithium hydroxide to the polyethylene glycol solution, stir evenly, and then add solution A with a mass of 75-80 times of lithium hydroxide, stir for 30-35 minutes, vacuum filter, and wash the filtered product with deionized water and anhydrous ethanol for 3-5 times respectively, and then vacuum dry at a temperature of 95-105°C for 3-4 hours to obtain a hollow microsphere template; S2. Add ferrous sulfate heptahydrate and hollow microsphere template to ethylene glycol (4.8-5.2 times the mass of ferrous sulfate heptahydrate) in a mass ratio of 4.8-5:1, stir for 15-20 minutes, gradually heat to 150°C in an argon atmosphere, react for 3-3.5 hours, add L-ascorbic acid (0.1-0.08 times the mass of ferrous sulfate heptahydrate), stir evenly, add the precursor mixture, stir for 30-35 minutes, heat to 180°C in an argon atmosphere after stirring, continue to react for 16-18 hours, cool to 25-35°C after the reaction, filter the lower precipitate, and remove the filtered product with deionized water. The mixture is washed with water and anhydrous ethanol for 3 to 5 times respectively, and then dried at a temperature of 45 to 50°C for 24 to 26 hours to obtain a composite. The precursor mixture comprises the following preparation steps: lithium hydroxide monohydrate and phosphoric acid are weighed in a mass ratio of 1:0.7 to 0.9, and the lithium hydroxide monohydrate and phosphoric acid are added to ethylene glycol with a mass of 35 to 45 times the mass of lithium hydroxide monohydrate, and stirred to dissolve, and then a graphene dispersion with a mass of 1.1 to 1.3 times the mass of ethylene glycol is added, and stirred evenly to prepare a precursor mixture for standby use; wherein the molar ratio of ferrous sulfate heptahydrate, lithium hydroxide monohydrate and phosphoric acid is 3.2 to 3.5:3:1; S3, mixing the composite and the carbon source in a mass ratio of 8-10:1, stirring for 1-1.5 hours after mixing, and then drying at a temperature of 45-50°C for 24-26 hours. After drying, performing a two-step calcination treatment in an argon atmosphere, and cooling to 25-35°C after calcination to obtain a lithium iron phosphate positive electrode material; The carbon source is sucrose.

2. The method for preparing the ultra-high compacted lithium iron phosphate positive electrode material according to claim 1, characterized in that: The graphene dispersion comprises the following preparation steps: adding graphene oxide to ethylene glycol with a mass of 900 to 950 times that of graphene oxide, stirring evenly, and then ultrasonically dispersing for 30 to 40 minutes for standby use.

3. The method for preparing the ultra-high compacted lithium iron phosphate positive electrode material according to claim 2, characterized in that: The graphene oxide comprises the following preparation steps: weighing flake graphite and sodium nitrate in a mass ratio of 1:0.5-0.6, adding the flake graphite and sodium nitrate into 98% concentrated sulfuric acid with a mass fraction of 40-45 times the mass of the flake graphite, ultrasonicating for 30-35 minutes at a temperature of -5-0°C, adding potassium permanganate with a mass fraction of 2.8-3 times the mass of the flake graphite after the ultrasonication, stirring for 2-2.5 hours at a temperature of 0-5°C, ultrasonicating for 30-35 minutes at a temperature of 30-35°C, and then adding 95-10% of the mass of the flake graphite. 0 times of deionized water, then stirred for 30-35 minutes at a temperature of 90-95°C, then added 55-60 times of deionized water as the mass of the flake graphite, allowed to stand for 20-30 minutes, then added 30-32 times of the mass of the flake graphite with a mass fraction of 30-35% hydrogen peroxide, allowed to stand for 15-20 minutes, then added 45-50 times of the mass of the flake graphite with a mass fraction of 10-15% hydrochloric acid solution, allowed to stand for stratification, removed the lower precipitate, washed with deionized water for 3-5 times, centrifuged and washed the precipitate until neutral, to obtain graphene oxide.

4. The method for preparing the ultra-high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The two-step calcination treatment includes the following preparation steps: calcining at a temperature of 330-350° C. for 2-2.2 hours, and then calcining at a temperature of 690-700° C. for 4-10.2 hours.

5. The method for preparing the ultra-high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The heating and cooling rates during the two-step calcination treatment were both 5°C / min.

6. A lithium iron phosphate positive electrode material prepared by the method for preparing an ultra-high-density lithium iron phosphate positive electrode material as claimed in any one of claims 1 to 5.

7. Use of the method for preparing the ultra-high-density lithium iron phosphate positive electrode material as claimed in any one of claims 1 to 5 in a lithium-ion battery.