Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention belongs to the technical field of secondary battery materials. More specifically, the invention relates to a lithium iron phosphate positive electrode material and a preparation method thereof. The preparation method comprises the following specific preparation steps: taking 20-30 parts by weight of lithium iron phosphate powder, 200-220 parts by weight of a carbon source mixed solution and 8-10 parts by weight of a dopamine solution, mixing, adjusting the pH value to alkalescence, and then evaporating to remove moisture to obtain a mixed precursor; the carbon source mixed solution is prepared from the following raw materials in parts by weight: 3 to 5 parts of glucose, 0.2 to 0.4 part of water-soluble starch and 180 to 200 parts of water; pre-sintering the mixed precursor in an inert atmosphere at the temperature of 360-380 DEG C, and continuously sintering at the temperature of 650-660 DEG C to obtain a sintered material; the water content of the sintered material is adjusted to be 6-8%, and a wet sintered material is obtained; and transferring the wet sintered material into a container, introducing trimethylaluminum steam into the container, carrying out a continuous heat and pressure maintaining reaction under the conditions that the temperature is 140-150 DEG C and the pressure is 0.18-0.22 MPa, discharging, and roasting to obtain the lithium iron phosphate positive electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary battery materials. More specifically, it relates to a lithium iron phosphate cathode material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate is a typical material with a polyanionic olivine structure. Its theoretical specific capacity can reach 170 mAh / g, the charge-discharge plateau can reach about 3.4 V, and it has high cycle stability. During the insertion and extraction of lithium ions, the crystal structure hardly changes significantly. Therefore, compared with other traditional lithium-ion cathode materials, it has more reliable safety. Currently, it has been widely studied and applied in electric vehicles.

[0003] The crystal structure of the lithium iron phosphate material contains FeO 6 octahedrons, LiO 6 octahedrons and PO 4 tetrahedrons. Among them, each FeO 6 octahedron shares an edge with two LiO 6 octahedrons, and the PO 4 tetrahedron shares an edge with one FeO 6 octahedron and shares two edges with the LiO 6 octahedron. The P-O covalent bond energy is relatively high, which can fix oxygen atoms to a large extent to ensure the structural stability of LFP. The FeO 6 octahedrons share corners with each other, and the PO 4 tetrahedrons separate the FeO 6 octahedrons, resulting in a discontinuous network of FeO 6 octahedrons. Electrons can only be transported along the Fe-O-Fe path. Therefore, the conductivity of the lithium iron phosphate cathode material is very poor, only 10 -10 S / m. In addition, lithium ions are located on the a-c plane and are restricted by the PO 4 tetrahedrons to move. They can only migrate back and forth in a one-dimensional channel parallel to the b-axis, which restricts the migration rate of lithium ions and results in a relatively low lithium ion diffusion coefficient of the LFP cathode material. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: aiming at the problems of low ionic conductivity and electronic conductivity of traditional lithium iron phosphate products, and the difficulty in achieving dense, uniform, and long-term reliable carbon coating by conventional methods. Based on the above problems, the present invention provides a lithium iron phosphate cathode material and a preparation method thereof.

[0005] The object of the present invention is to provide a lithium iron phosphate cathode material.

[0006] Another object of the present invention is to provide a preparation method of a lithium iron phosphate cathode material.

[0007] The above object of the present invention is achieved by the following technical solutions: A preparation method of a lithium iron phosphate cathode material, and the specific preparation steps include: By weight, take 20-30 parts of lithium iron phosphate powder, 200-220 parts of carbon source mixed solution, and 8-10 parts of dopamine solution. After mixing, adjust the pH to weak alkalinity, and then evaporate to remove water to obtain a mixed precursor; In the carbon source mixed solution, it includes the following raw materials by weight: 3-5 parts of glucose, 0.2-0.4 parts of water-soluble starch, and 180-200 parts of water; The mixed precursor is pre-calcined in an inert atmosphere at a temperature of 360-380°C for 60-80 minutes, and then sintered at a temperature of 650-660°C for 320-350 minutes, cooled, and discharged to obtain a sintered material; Adjust the water content of the sintered material to 6-8% to obtain a wet sintered material; Transfer the wet sintered material into a container, and introduce trimethylaluminum vapor into the container. At a temperature of 140-150°C and a pressure of 0.18-0.22 MPa, continuously maintain the temperature and pressure for reaction for 40-60 minutes, then discharge and calcine to obtain the lithium iron phosphate cathode material.

[0008] The beneficial effects of the above technical solutions: First, glucose and water-soluble starch are used in combination as the main carbon source. In this way, since glucose is a monosaccharide and starch is a polysaccharide, during the sintering process, due to different reactions during dehydration and carbonization, specifically, relatively speaking, glucose is more likely to dehydrate or carbonize, while the process of starch is longer. Therefore, a hierarchical pore structure can be formed, which is beneficial to the improvement of ionic conductivity. However, the inventor found that with the rolling of the product during processing and the continuous cycling during the long-term use of the product, this hierarchical pore structure will collapse, resulting in a decrease in ionic and electronic conductivity as the rolling processing or cycle life of the product extends; Based on this, on the one hand, in the above technical solution, by adding dopamine solution to the carbon source mixed solution, in a weakly alkaline environment, dopamine can undergo an oxidative self-polymerization reaction on the surface of lithium iron phosphate powder, and relying on its good adsorption ability, glucose and starch molecules are adsorbed and fixed on the surface of lithium iron phosphate powder, thereby improving the uniformity during its carbonization process. In particular, under its action, the hierarchical pore structure formed by the two can be firmly adsorbed on the surface of lithium iron phosphate powder, avoiding desorption between the coating layer and lithium iron phosphate powder during rolling or cycling; on the other hand, trimethylaluminum vapor can diffuse and penetrate into the hierarchical pore structure during the pressure-holding and heat-preserving reaction process under the action of pressure and temperature, so as to contact with the water molecules in the pores and undergo a hydrolysis reaction. Finally, after calcination, alumina reinforcement points are formed on the pore walls or pore defects, improving the overall strength of the pore structure and avoiding pore collapse during processing and cycling; thus, by simultaneously enhancing the bonding force between the coating layer and lithium iron phosphate powder and the stability of the pore structure, the above technical problems are solved.

[0009] Further, the D50 of the lithium iron phosphate powder is 200 - 500 nm.

[0010] Further, in the carbon source mixed solution, it also includes amino acids with a mass 0.08 - 0.12 times that of the glucose.

[0011] Further, the amino acids are selected from any one of glycine, glutamic acid, aspartic acid, histidine, lysine, proline, and arginine.

[0012] The molecular structure of amino acids contains both hydrophilic functional groups amino and carboxyl, and at the same time also has a lipophilic carbon chain part. Therefore, it has a certain emulsifying effect, which is conducive to forming a more abundant pore structure during the sintering process. More importantly, the N element in the amino acid molecular structure, combined with the nitrogen element in the dopamine molecular structure, can jointly form an N element-doped structure on the surface of lithium iron phosphate powder and in the pore structure, thereby improving the electronic conductivity and ionic conductivity.

[0013] Further, the weak alkalinity is pH = 7.2 - 7.8.

[0014] Further, the specific preparation steps also include: The mixed precursor is slowly heated and raised to 360 - 380 °C at a rate of 0.5 - 0.8 °C / min in a nitrogen atmosphere, pre-calcined for 60 - 80 min after heat preservation, then rapidly heated to 650 - 660 °C at a rate of 4 - 6 °C / min, sintered for 320 - 350 min after heat preservation, and then cooled to room temperature with the furnace, and the material is discharged to obtain the sintered material.

[0015] During the reaction process, it is desired to form a stable multi-level pore structure. Therefore, during the sintering process, a lower heating rate is first used for pre-sintering. During this process, the volatilization rate of small molecules such as water is controlled to avoid the rapid volatilization caused by too fast a heating rate, which may lead to excessive looseness of the surface coating layer or weak binding force with the lithium iron phosphate powder.

[0016] Further, the roasting includes: In a nitrogen atmosphere, at a temperature of 350 - 500 °C, after heating and roasting for 1 - 3 h, it is cooled to room temperature with the furnace.

[0017] Further, the concentration of the dopamine solution is 4 - 10 g / L.

[0018] Further, the preparation steps of the lithium iron phosphate powder include: According to the stoichiometric ratio of Li:Fe:P = (3.02 - 3.06):(0.96 - 1.00):(0.98 - 1.00), weigh LiOH·H 2 O, FeSO 4 ·7H 2 O, H 3 PO 4 , and L-ascorbic acid accounting for 3 - 5% of the mass of FeSO 4 ·7H 2 O; Mix and dissolve LiOH·H 2 O and water to prepare a lithium hydroxide solution with a mass fraction of 10 - 12%, and then dropwise add H 3 PO 4 to obtain solution A; Mix and dissolve FeSO 4 ·7H 2 O and water to prepare a ferrous sulfate solution with a mass fraction of 10 - 12%, and add L-ascorbic acid to obtain solution B; Slowly drop solution B into solution A at a rate of 5 - 8 mL / min. After the dropping is completed, carry out a hydrothermal reaction at a temperature of 180 - 190 °C for 10 - 12 h. After cooling, centrifuge, wash, dry, break up, and screen to obtain the lithium iron phosphate powder.

[0019] A lithium iron phosphate cathode material is prepared by the above preparation method. Specific Embodiments

[0020] The following specific examples are used to further illustrate the present invention, but the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the technical field.

[0021] Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0022] In the monodisperse hollow nanoparticles mentioned below, the specific meaning of "hollow" is that during the preparation process, by using emulsifiers and other means, air bubbles are introduced to make the interior of the particles hollow, so as to distinguish from the completely solid state. Example 1

[0023] Preparation of lithium iron phosphate powder: Weigh LiOH·H 2 O, FeSO 4 ·7H 2 O, H 3 PO 4 , and L-ascorbic acid accounting for 3% of the mass of FeSO 4 ·7H 2 O; Mix LiOH·H 2 O and water to dissolve and prepare a lithium hydroxide solution with a mass fraction of 10%, and then dropwise add H 3 PO 4 to obtain solution A; Mix FeSO 4 ·7H 2 O and water to dissolve and prepare a ferrous sulfate solution with a mass fraction of 10%, and add L-ascorbic acid to obtain solution B; Slowly drop solution B into solution A at a rate of 5 mL / min. After the dropping is completed, carry out a hydrothermal reaction at 180 °C for 10 h. After cooling, centrifuge, wash, dry, break up, and screen out a powder material with a D50 of 200 nm to obtain lithium iron phosphate powder; By weight, take 20 parts of lithium iron phosphate powder, 200 parts of carbon source mixed solution, and 8 parts of dopamine solution. After mixing, adjust the pH to 7.2, and then evaporate to remove water to obtain a mixed precursor; The concentration of the dopamine solution is 4 g / L; In the carbon source mixed solution, it includes the following raw materials by weight: 3 parts of glucose, 0.2 part of water-soluble starch, 180 parts of water, and an amino acid with a mass 0.08 times that of the glucose; The amino acid is selected from glycine; Heat the mixed precursor in a nitrogen atmosphere and slowly heat it up to 360 °C at a rate of 0.5 °C / min. After holding and pre-sintering for 60 min, continue to rapidly heat it up to 650 °C at a rate of 4 °C / min. After holding and sintering for 320 min, cool it down to room temperature with the furnace and discharge to obtain a sintered material; After spreading the sintered material flat, the water content of the sintered material was adjusted to 6% by spraying water mist to obtain the wet sintered material; Transfer the wet sintered material into a container, and introduce trimethylaluminum vapor into the container. Under the conditions of a temperature of 140 °C and a pressure of 0.18 MPa, keep the temperature and pressure constant for 40 min, then discharge the material. Subsequently, in a nitrogen atmosphere, heat and calcine at a temperature of 350 °C for 1 h, and then cool to room temperature with the furnace to obtain the lithium iron phosphate cathode material. Example 2

[0024] Preparation of lithium iron phosphate powder: Weigh LiOH·H 2 O, FeSO 4 ·7H 2 O, H 3 PO 4 , and L-ascorbic acid which is 4% of the mass of FeSO 4 ·7H 2 O; Mix and dissolve LiOH·H 2 O and water to prepare a lithium hydroxide solution with a mass fraction of 11%, and then dropwise add H 3 PO 4 to obtain solution A; Mix and dissolve FeSO 4 ·7H 2 O and water to prepare a ferrous sulfate solution with a mass fraction of 11%, and add L-ascorbic acid to obtain solution B; Slowly drop solution B into solution A at a rate of 6 mL / min. After the dropping is completed, carry out a hydrothermal reaction at a temperature of 185 °C for 11 h. After cooling, centrifuge, wash, dry, break up, and screen out the powder material with a D50 of 300 nm to obtain the lithium iron phosphate powder; By weight, take 25 parts of lithium iron phosphate powder, 210 parts of carbon source mixed solution, and 9 parts of dopamine solution. After mixing, adjust the pH to 7.4, and then evaporate to remove the water to obtain the mixed precursor; The concentration of the dopamine solution is 6 g / L; In the carbon source mixed solution, it includes the following raw materials by weight: 4 parts of glucose, 0.3 part of water-soluble starch, 190 parts of water, and amino acid which is 0.1 times the mass of the glucose; The amino acid is selected from glutamic acid; The mixed precursor was slowly heated in a nitrogen atmosphere at a rate of 0.6 °C / min to 370 °C, pre-sintered for 70 min while maintaining the temperature, then rapidly heated to 655 °C at a rate of 5 °C / min, sintered for 330 min while maintaining the temperature, and then cooled to room temperature in the furnace. After discharging, the sintered material was obtained; After spreading out the sintered material, the water content of the sintered material was adjusted to 7% by spraying water mist to obtain the wet sintered material; The wet sintered material was transferred into a container, and trimethylaluminum vapor was introduced into the container. The reaction was carried out at a temperature of 145 °C and a pressure of 0.2 MPa for 50 min while maintaining the temperature and pressure. After discharging, the material was then heated and calcined in a nitrogen atmosphere at a temperature of 400 °C for 2 h, and then cooled to room temperature in the furnace to obtain the lithium iron phosphate cathode material. Example 3

[0025] Preparation of lithium iron phosphate powder: According to the stoichiometric ratio of Li:Fe:P = 3.06:1.00:1.00, LiOH·H 2 O, FeSO 4 ·7H 2 O, H 3 PO 4 , and L-ascorbic acid accounting for 5% of the mass of FeSO 4 ·7H 2 O were weighed; LiOH·H 2 O and water were mixed and dissolved to prepare a lithium hydroxide solution with a mass fraction of 12%. Then, H 3 PO 4 was added dropwise to obtain solution A; FeSO 4 ·7H 2 O and water were mixed and dissolved to prepare a ferrous sulfate solution with a mass fraction of 12%, and L-ascorbic acid was added to obtain solution B; Solution B was slowly added dropwise to solution A at a rate of 8 mL / min. After the addition was completed, the hydrothermal reaction was carried out at a temperature of 190 °C for 12 h. After cooling, the product was centrifuged, washed, dried, dispersed, and the powder material with a D50 of 500 nm was screened out to obtain the lithium iron phosphate powder; By weight, 30 parts of lithium iron phosphate powder, 220 parts of carbon source mixed solution, and 10 parts of dopamine solution were taken, mixed, and the pH was adjusted to 7.8. Then, the water was evaporated to obtain the mixed precursor; The concentration of the dopamine solution was 10 g / L; In the carbon source mixed solution, the raw materials included the following parts by weight: 5 parts of glucose, 0.4 part of water-soluble starch, 200 parts of water, and amino acids 0.12 times the mass of the glucose; The amino acid is selected from aspartic acid; The mixed precursor is slowly heated in a nitrogen atmosphere at a rate of 0.8 °C / min to 380 °C, pre-sintered at a constant temperature for 80 min, then rapidly heated at a rate of 6 °C / min to 660 °C, sintered at a constant temperature for 350 min, cooled to room temperature in the furnace, and discharged to obtain a sintered material; After the sintered material is laid flat, the water content of the sintered material is adjusted to 8% by spraying water mist to obtain a wet sintered material; The wet sintered material is transferred to a container, trimethylaluminum vapor is introduced into the container, and the reaction is continuously carried out at a temperature of 150 °C and a pressure of 0.22 MPa for 60 min, then discharged. Subsequently, in a nitrogen atmosphere, it is heated and calcined at a temperature of 500 °C for 3 h, and then cooled to room temperature in the furnace to obtain the lithium iron phosphate cathode material. Example 4

[0026] Compared with Example 1, the difference in this example is that no amino acid is added, and the other conditions remain unchanged. Example 5

[0027] Compared with Example 1, the difference in this example is that: The mixed precursor is slowly heated in a nitrogen atmosphere at a rate of 4 °C / min to 360 °C, pre-sintered at a constant temperature for 60 min, then rapidly heated at a rate of 4 °C / min to 650 °C, sintered at a constant temperature for 320 min, cooled to room temperature in the furnace, and discharged to obtain a sintered material; The other conditions remain unchanged.

[0028] Comparative Example 1 Compared with Example 1, the difference in this comparative example is that no trimethylaluminum is added, and the other conditions remain unchanged.

[0029] Comparative Example 2 Compared with Example 1, the difference in this comparative example is that instead of adjusting the water content of the sintered material by spraying water mist, the sintered material is directly reacted with trimethylaluminum vapor, and the other conditions remain unchanged.

[0030] Comparative Example 3 Compared with Example 1, the difference in this comparative example is that no dopamine is added, and the other conditions remain unchanged.

[0031] The products obtained in the examples and comparative examples are subjected to performance tests. The specific test methods and test results are as follows: Weigh according to the mass ratio of m(LFP):m(acetylene black):m(PVDF)= 8:1:1 using an electronic balance. Place LFP and acetylene black in a blast drying oven and dry at 90°C for 2 hours. At the same time, dissolve PVDF in NMP and stir magnetically for 2 hours until evenly mixed. Grind the dried LFP and acetylene black in an agate mortar and sieve them, then add them to the above-mentioned NMP solution containing PVDF, and stir overnight until a uniform black slurry is formed. Use the 90μm side of a four-sided slicer to scrape the film, coat the slurry on the aluminum foil, transfer it to the blast drying oven, take it out after drying at 90°C for 2 hours, then roll press it under a pressure of 2MPa, and finally slice it using a button battery punching machine to obtain the electrode sheet.

[0032] The assembly of the battery is carried out in a vacuum glove box filled with Ar. The model of the assembled battery is CR2016. Use the electrode sheet as the positive electrode, the lithium sheet as the counter electrode, 1mol / L LiPF6 / EC.DMC.EMC as the electrolyte, and place the positive electrode shell, positive electrode material electrode sheet, separator (Celgard 2400), lithium sheet, gasket, spring sheet, and negative electrode shell in sequence, and use a tablet press to make a button battery, and age it in the glove box for 12 hours and then take it out for testing.

[0033] Use a LAND CT2001A battery test system to test the discharge specific capacity of a certain cycle of the button battery, the charge and discharge performance at different rates, and the cycle performance after a certain number of cycles. The test voltage window is 2.5 - 4.2V, the rates are 0.1C, 3C, 5C, and the nominal specific capacity of LFP is set to 170mAh / g; respectively test and obtain the capacity retention rate after 100 cycles under the corresponding rate conditions.

[0034] ; It can be seen from the test results in Table 1 that the product obtained by the present invention can have a more excellent capacity retention rate during high-rate charge and discharge, so that the performance of the product can be effectively maintained.

[0035] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The specific preparation steps include: By weight, 20-30 parts of lithium iron phosphate powder, 200-220 parts of carbon source mixed solution, and 8-10 parts of dopamine solution are taken, mixed, and the pH is adjusted to weak alkalinity, and then the water is evaporated to obtain a mixed precursor; The carbon source mixed solution includes the following raw materials in parts by weight: 3-5 parts of glucose, 0.2-0.4 parts of water-soluble starch, and 180-200 parts of water; The mixed precursor is pre-sintered in an inert atmosphere at a temperature of 360-380°C for 60-80 minutes, and then sintered at a temperature of 650-660°C for 320-350 minutes, cooled, and discharged to obtain a sintered material; Adjust the moisture content of the sintering material to 6-8% to obtain wet sintering material; The wet sintering material is transferred into a container, and trimethylaluminum vapor is introduced into the container. Under the conditions of temperature of 140-150°C and pressure of 0.18-0.22MPa, the reaction is continued for 40-60 minutes under the conditions of heat preservation and pressure maintenance, and then the material is discharged and calcined to obtain lithium iron phosphate positive electrode material.

2. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The D50 of the lithium iron phosphate powder is 200-500nm.

3. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The carbon source mixed solution also includes amino acids in an amount of 0.08-0.12 times the mass of the glucose.

4. The method for preparing a lithium iron phosphate positive electrode material according to claim 3, characterized in that: The amino acid is selected from any one of glycine, glutamic acid, aspartic acid, histidine, lysine, proline and arginine.

5. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The weak alkalinity is pH=7.2-7.

8.

6. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The specific preparation steps also include: The mixed precursor is slowly heated to 360-380°C at a rate of 0.5-0.8°C / min in a nitrogen atmosphere, pre-sintered for 60-80 minutes, and then rapidly heated to 650-660°C at a rate of 4-6°C / min. After sintering for 320-350 minutes, the mixed precursor is cooled to room temperature in the furnace and discharged to obtain a sintered material.

7. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The roasting comprises: In a nitrogen atmosphere, at a temperature of 350-500° C., the mixture is heated and calcined for 1-3 hours, and then cooled to room temperature in the furnace.

8. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The concentration of the dopamine solution is 4-10 g / L.

9. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The steps of preparing the lithium iron phosphate powder include: According to the stoichiometric ratio of Li:Fe:P=(3.02-3.06):(0.96-1.00):(0.98-1.00), weigh LiOH·H2O, FeSO4·7H2O, H3PO4, and L-ascorbic acid with a mass percentage of 3-5% of FeSO4·7H2O; Mix and dissolve LiOH·H2O and water to prepare a lithium hydroxide solution with a mass fraction of 10-12%, and then dropwise add H3PO4 to obtain solution A; Mix and dissolve FeSO4·7H2O and water to prepare a ferrous sulfate solution with a mass fraction of 10-12%, and add L-ascorbic acid to obtain solution B; Solution B is slowly added dropwise to solution A at a rate of 5-8 mL / min. After the addition is completed, the solution is hydrothermally reacted at 180-190°C for 10-12 hours. After cooling, the solution is centrifuged, washed, dried, broken up, and sieved to obtain lithium iron phosphate powder.

10. A lithium iron phosphate positive electrode material, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 9.