Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The application belongs to the technical field of secondary battery materials, and more particularly relates to a lithium iron phosphate positive electrode material and a preparation method thereof. The specific preparation steps of the application comprise the following steps: 20-30 parts of lithium iron phosphate powder, 200-220 parts of a carbon source mixed solution and 8-10 parts of a dopamine solution are taken according to weight fractions, the mixture is adjusted to weak alkalinity after mixing, and then water is removed by evaporation to obtain a mixed precursor; in the carbon source mixed solution, the following raw materials are included according to weight fractions: 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-fired in an inert atmosphere at a temperature of 360-380 DEG C, and then sintered at a temperature of 650-660 DEG C to obtain a sintered material; the water content of the sintered material is adjusted to 6-8% to obtain sintered material wet material; the sintered material wet material is transferred into a container, and trimethylaluminum vapor is introduced into the container, and then the container is subjected to a constant-temperature and constant-pressure reaction at a temperature of 140-150 DEG C and a pressure of 0.18-0.22 MPa, and then the material is discharged and calcined to obtain the lithium iron phosphate positive electrode material.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of secondary battery materials. More particularly, it relates to a lithium iron phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] Lithium iron phosphate is a typical material of polyanion olivine structure, and its theoretical specific capacity can reach 170 mAh / g, the charge-discharge platform can reach about 3.4 V, and the cycle stability is high. During the deintercalation process of lithium ions, the crystal structure almost has no significant change. Therefore, compared with other traditional lithium ion positive electrode materials, it has more reliable safety. At present, it has been widely studied and applied in electric vehicles.

[0003] The crystal structure of the lithium iron phosphate material contains FeO6 octahedron, LiO6 octahedron and PO4 tetrahedron. Among them, each FeO6 octahedron shares a side with two LiO6 octahedrons, and the PO4 tetrahedron shares a side with one FeO6 octahedron and shares two sides with the LiO6 octahedron. The P-O covalent bond has relatively high bond energy, which can fix the oxygen atom to a great extent to ensure the structural stability of LFP. The FeO6 octahedrons share corners with each other, and the PO4 tetrahedrons separate the FeO6 octahedrons, resulting in a discontinuous network of FeO6 octahedrons. Electrons can only be transmitted along the Fe-O-Fe path, so the conductivity of the lithium iron phosphate positive electrode material is very poor, only 10 -10 S / m. In addition, lithium ions are located on the a-c plane and are limited to move by the PO4 tetrahedron, and can only migrate back and forth in the one-dimensional channel parallel to the b axis, which restricts the migration rate of lithium ions, resulting in a low lithium ion diffusion coefficient of the LFP positive electrode material. SUMMARY

[0004] The technical problem to be solved by the present application is that the ion conductivity and electronic conductivity of the traditional lithium iron phosphate product are low, and the conventional carbon coating is difficult to achieve dense and uniform coating, and the long-term reliability problem. Based on the above problems, the present application provides a lithium iron phosphate positive electrode material and a preparation method thereof.

[0005] The purpose of the present application is to provide a lithium iron phosphate positive electrode material.

[0006] Another purpose of the present application is to provide a preparation method of the lithium iron phosphate positive electrode material.

[0007] The above purposes of the present application are achieved by the following technical solutions:

[0008] A preparation method of a lithium iron phosphate positive electrode material, the specific preparation steps comprising:

[0009] Take 20-30 parts of lithium iron phosphate powder, 200-220 parts of carbon source mixed solution, 8-10 parts of dopamine solution by weight, mix, adjust the pH to weak alkaline, then evaporate the water, get the mixed precursor;

[0010] The carbon source mixed solution includes the following raw materials by weight: 3-5 parts of glucose, 0.2-0.4 parts of water-soluble starch, 180-200 parts of water;

[0011] Put the mixed precursor in an inert atmosphere, pre-burn at a temperature of 360-380℃ for 60-80min, then continue to sinter at a temperature of 650-660℃ for 320-350min, cool down, discharge, get the sintered material;

[0012] Adjust the moisture content of the sintered material to 6-8%, get the sintered material wet material;

[0013] Put the sintered material wet material into a container, and introduce trimethylaluminum vapor into the container, at a temperature of 140-150℃ and a pressure of 0.18-0.22MPa, continue to heat and pressurize for 40-60min, then discharge and calcine, get the lithium iron phosphate positive electrode material.

[0014] The beneficial effects of the above technical solution are:

[0015] Firstly, glucose and water-soluble starch are used as the main carbon source, so due to the difference in the reactions that occur during dehydration and carbonization, the glucose is more easily dehydrated or carbonized, while the starch process is longer, thus a multi-level pore structure can be formed, which is beneficial to the improvement of ion conductivity, but the inventors found that during the rolling process of the product and the continuous cycling during the long-term use of the product, the multi-level pore structure collapses, resulting in a decrease in ion and electron conductivity as the product is rolled or the cycle life is extended;

[0016] Based on this, the above technical solution one aspect, by adding dopamine solution in the carbon source mixed solution, in weak alkaline environment, dopamine can occur on the surface of lithium iron phosphate powder oxidation self-polymerization reaction, and rely on its good adsorption capacity, glucose and starch molecules are adsorbed and fixed on the surface of lithium iron phosphate powder, thereby improving the uniformity in the carbonization process, especially, under the action of the two, the multi-level pore structure formed can be firmly adsorbed on the surface of lithium iron phosphate powder, avoiding the detachment between the coating and lithium iron phosphate powder in the rolling or circulation process; on the other hand, trimethyl aluminum vapor can diffuse and penetrate into the multi-level pore structure under the action of pressure and temperature during the pressure and temperature holding reaction, thereby contacting with water molecules in the pores to occur hydrolysis reaction, and finally forming aluminum oxide reinforcement points at the pore wall or pore defect through calcination, improving the overall strength of the pore structure, and avoiding the collapse of the pore in the processing and circulation process; in this way, by improving the bonding force between the coating and lithium iron phosphate powder and the stability of the pore structure, the above technical problems are solved.

[0017] Further, the D50 of the lithium iron phosphate powder is 200-500 nm.

[0018] Further, the carbon source mixed solution further comprises amino acids in an amount of 0.08-0.12 times the mass of the glucose.

[0019] Further, the amino acid is selected from any one of glycine, glutamic acid, aspartic acid, histidine, lysine, proline and arginine.

[0020] The molecular structure of amino acid contains both hydrophilic functional groups of amino and carboxyl, and also has a lipophilic carbon chain part, so it has a certain emulsifying effect, thereby facilitating the formation of more abundant pore structure in the sintering process, and more importantly, the N element in the molecular structure of amino acid, combined with the nitrogen element in the molecular structure of dopamine, can 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.

[0021] Further, the weak alkalinity is pH=7.2-7.8.

[0022] Further, the specific preparation steps further comprise:

[0023] The mixed precursor is slowly heated to 360-380℃ at a rate of 0.5-0.8℃ / min in a nitrogen atmosphere, and after holding for 60-80min, it is continuously heated to 650-660℃ at a rate of 4-6℃ / min, and after holding for 320-350min, it is cooled to room temperature, discharged, and the sintered material is obtained.

[0024] Since it is expected that a stable multi-level porous structure can be formed during the reaction process, a lower heating rate is used for pre-sintering during the sintering process, and in this process, the volatilization rate of small molecules such as water is controlled to avoid the rapid volatilization caused by the rapid heating rate, so that the surface coating layer is excessively loose under the action of the surface coating layer or the bonding force with the lithium iron phosphate powder is not firm.

[0025] Further, the calcination comprises:

[0026] After heating and calcining at a temperature of 350-500℃ for 1-3h in a nitrogen atmosphere, the furnace is cooled to room temperature.

[0027] Further, the concentration of the dopamine solution is 4-10g / L.

[0028] Further, the preparation steps of the lithium iron phosphate powder comprise:

[0029] LiOH·H2O, FeSO4·7H2O, H3PO4, and L-ascorbic acid with a mass fraction of 3-5% of FeSO4·7H2O are weighed according to the stoichiometric ratio Li:Fe:P= (3.02-3.06): (0.96-1.00): (0.98-1.00);

[0030] LiOH·H2O and water are mixed and dissolved to prepare a lithium hydroxide solution with a mass fraction of 10-12%, and then H3PO4 is added dropwise to obtain solution A;

[0031] FeSO4·7H2O and water are mixed and dissolved to prepare a ferrous sulfate solution with a mass fraction of 10-12%, and L-ascorbic acid is added to obtain solution B;

[0032] Solution B is slowly added to solution A at a rate of 5-8mL / min, and after the addition is completed, hydrothermal reaction is carried out at a temperature of 180-190℃ for 10-12h, and after cooling, centrifugal washing, drying, and dispersion and screening, the lithium iron phosphate powder is obtained.

[0033] A lithium iron phosphate positive electrode material is prepared by the above preparation method. DETAILED DESCRIPTION

[0034] The application will be further described in conjunction with specific examples, but the examples do not limit the application in any form. Unless otherwise specified, the reagents, methods and equipment used in the application are conventional reagents, methods and equipment in the technical field.

[0035] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0036] The specific meaning of hollow in the monodisperse hollow nanoparticles mentioned below is that, in the preparation process, by introducing bubbles through the use of emulsifiers and the like, the interior of the particles is hollow, which is distinguished from the completely solid state. Example 1

[0037] Preparation of lithium iron phosphate powder:

[0038] LiOH·H2O, FeSO4·7H2O, H3PO4, and L-ascorbic acid with a mass fraction of 3% of FeSO4·7H2O were weighed according to the stoichiometric ratio Li:Fe:P = 3.02:0.96:0.98;

[0039] LiOH·H2O and water were mixed and dissolved to prepare a lithium hydroxide solution with a mass fraction of 10%, and then H3PO4 was added dropwise to obtain solution A;

[0040] FeSO4·7H2O and water were mixed and dissolved to prepare a ferrous sulfate solution with a mass fraction of 10%, and L-ascorbic acid was added to obtain solution B;

[0041] Solution B was slowly added to solution A at a rate of 5 mL / min, and after the addition was completed, the hydrothermal reaction was carried out at a temperature of 180℃ for 10h. After cooling, centrifugal washing and drying, the powder material with a D50 of 200nm was sieved out, and the lithium iron phosphate powder was obtained;

[0042] According to the weight fraction, 20 parts of lithium iron phosphate powder, 200 parts of carbon source mixed solution, and 8 parts of dopamine solution were mixed, and the pH was adjusted to 7.2. Subsequently, water was removed by evaporation to obtain a mixed precursor;

[0043] The concentration of the dopamine solution is 4g / L;

[0044] The carbon source mixed solution includes the following raw materials by weight fraction: 3 parts of glucose, 0.2 parts of water-soluble starch, 180 parts of water, and amino acids with a mass of 0.08 times of the glucose;

[0045] The amino acid is selected from glycine;

[0046] The mixed precursor was slowly heated to 360℃ at a rate of 0.5℃ / min in a nitrogen atmosphere, and after 60min of temperature holding and pre-burning, it was quickly heated to 650℃ at a rate of 4℃ / min, and after 320min of temperature holding and sintering, it was cooled to room temperature with the furnace, and the sintered material was discharged to obtain a sintered material;

[0047] After the sintered material was laid flat, a water mist was sprayed to adjust the water content of the sintered material to 6% to obtain a sintered material wet material;

[0048] The sintering material wet material is transferred into a container, and trimethylaluminum vapor is introduced into the container, and after 40 min of continuous holding and pressure maintaining at a temperature of 140 DEG C and a pressure of 0.18 MPa, the material is discharged, and then after heating and calcining at a temperature of 350 DEG C for 1 h in a nitrogen atmosphere, the lithium iron phosphate positive electrode material is obtained after the furnace is cooled to room temperature. Example 2

[0049] Preparation of lithium iron phosphate powder:

[0050] According to the stoichiometric ratio of Li:Fe:P = 3.04:0.98:0.99, LiOH.H2O, FeSO4.7H2O, H3PO4, and L-ascorbic acid with a mass fraction of 4% of FeSO4.7H2O are weighed;

[0051] LiOH.H2O and water are mixed and dissolved to prepare a lithium hydroxide solution with a mass fraction of 11%, and then H3PO4 is added dropwise to obtain solution A;

[0052] FeSO4.7H2O and water are mixed and dissolved to prepare a ferrous sulfate solution with a mass fraction of 11%, and L-ascorbic acid is added to obtain solution B;

[0053] Solution B is slowly added to solution A at a rate of 6 mL / min, and after the addition is completed, hydrothermal reaction is carried out at a temperature of 185 DEG C for 11 h, and after cooling, centrifugal washing, drying, and dispersion, the powder material with a D50 of 300 nm is sieved to obtain the lithium iron phosphate powder;

[0054] According to weight parts, 25 parts of lithium iron phosphate powder, 210 parts of carbon source mixed solution, and 9 parts of dopamine solution are mixed, and then the pH is adjusted to 7.4, and then water is removed by evaporation to obtain a mixed precursor;

[0055] The concentration of the dopamine solution is 6 g / L;

[0056] The carbon source mixed solution includes the following raw materials by weight parts: 4 parts of glucose, 0.3 parts of water-soluble starch, 190 parts of water, and amino acids with a mass of 0.1 times of the glucose;

[0057] The amino acid is selected from glutamic acid;

[0058] The mixed precursor is slowly heated and warmed to 370 DEG C at a rate of 0.6 DEG C / min in a nitrogen atmosphere, and after holding and pre-baking for 70 min, it is continuously heated and warmed to 655 DEG C at a rate of 5 DEG C / min, and after holding and sintering for 330 min, the sintering material is discharged after the furnace is cooled to room temperature;

[0059] After the sintering material is laid flat, the water content of the sintering material is adjusted to 7% by spraying water mist to obtain a sintering material wet material;

[0060] The sintering material wet material is transferred into a container, and trimethyl aluminum vapor is introduced into the container, and after 50 min of continuous holding and pressure reaction at a temperature of 145 DEG C and a pressure of 0.2 MPa, the material is discharged, and then after heating and calcining at a temperature of 400 DEG C for 2 h in a nitrogen atmosphere, the material is cooled to room temperature in the furnace, and the lithium iron phosphate positive electrode material is obtained. Example 3

[0061] Preparation of lithium iron phosphate powder:

[0062] LiOH H2O, FeSO4 7H2O, H3PO4, and L-ascorbic acid with a mass fraction of 5% of FeSO4 7H2O are weighed according to the stoichiometric ratio of Li:Fe:P = 3.06:1.00:1.00;

[0063] LiOH H2O and water are mixed and dissolved to prepare a lithium hydroxide solution with a mass fraction of 12%, and then H3PO4 is added dropwise to obtain solution A;

[0064] FeSO4 7H2O and water are mixed and dissolved to prepare a ferrous sulfate solution with a mass fraction of 12%, and L-ascorbic acid is added to obtain solution B;

[0065] Solution B is slowly added to solution A at a rate of 8 mL / min, and after the addition is completed, hydrothermal reaction is carried out at a temperature of 190 DEG C for 12 h, and after cooling, centrifugal washing, drying, and dispersion, a powder material with a D50 of 500 nm is sieved, and the lithium iron phosphate powder is obtained;

[0066] According to weight parts, 30 parts of lithium iron phosphate powder, 220 parts of carbon source mixed solution, and 10 parts of dopamine solution are mixed, the pH is adjusted to 7.8, and then water is removed by evaporation to obtain a mixed precursor;

[0067] The concentration of the dopamine solution is 10 g / L;

[0068] The carbon source mixed solution includes the following raw materials by weight parts: 5 parts of glucose, 0.4 parts of water-soluble starch, 200 parts of water, and amino acids with a mass of 0.12 times of the mass of the glucose;

[0069] The amino acid is selected from aspartic acid;

[0070] The mixed precursor is slowly heated and warmed to 380 DEG C at a rate of 0.8 DEG C / min in a nitrogen atmosphere, and after holding and pre-burning for 80 min, it is continuously rapidly heated to 660 DEG C at a rate of 6 DEG C / min, and after sintering for 350 min, the sintering material is discharged after cooling to room temperature in the furnace.

[0071] After the sintering material is laid flat, the water content of the sintering material is adjusted to 8% by spraying water mist, and the sintering material wet material is obtained;

[0072] The sintering material wet material is transferred into a container, and trimethylaluminum vapor is introduced into the container. After 60 min of continuous heat preservation and pressure preservation reaction at a temperature of 150℃ and a pressure of 0.22 MPa, the material is discharged. Subsequently, the material is heated and calcined at a temperature of 500℃ for 3 h in a nitrogen atmosphere, and then cooled to room temperature in the furnace. Thus, the lithium iron phosphate positive electrode material is obtained. Example 4

[0073] Compared with Example 1, the difference is that no amino acid is added, and the other conditions remain unchanged. Example 5

[0074] Compared with Example 1, the difference is that:

[0075] The mixed precursor is slowly heated and raised to 360℃ at a rate of 4℃ / min in a nitrogen atmosphere, and then pre-fired for 60 min. Subsequently, the temperature is rapidly raised to 650℃ at a rate of 4℃ / min, and then sintered for 320 min. After cooling to room temperature in the furnace, the sintering material is discharged.

[0076] The other conditions remain unchanged.

[0077] Comparative Example 1

[0078] Compared with Example 1, the difference is that no trimethylaluminum is added, and the other conditions remain unchanged.

[0079] Comparative Example 2

[0080] Compared with Example 1, the difference is that the water content of the sintering material is not adjusted by spraying water mist, but the sintering material is directly reacted with trimethylaluminum vapor, and the other conditions remain unchanged.

[0081] Comparative Example 3

[0082] Compared with Example 1, the difference is that no dopamine is added, and the other conditions remain unchanged.

[0083] The products obtained in the examples and comparative examples are tested for performance, and the specific test methods and test results are as follows:

[0084] The electronic balance is used to weigh according to the mass ratio m(LFP):m(acetylene black):m(PVDF)=8:1:1. The LFP and acetylene black are placed in a blast drying oven, dried at 90℃ for 2 hours, and the PVDF is dissolved in NMP and magnetically stirred for 2 hours to mix uniformly. The dried LFP and acetylene black are ground in an agate mortar and sieved, and then added to the above-mentioned NMP solution with PVDF, stirred overnight until a uniform black slurry is formed. The slurry is coated on an aluminum foil using a 90μm face film of a four-sided film applicator, transferred to a blast drying oven, dried at 90℃ for 2 hours, then rolled at a pressure of 2MPa, and finally cut into sheets using a button cell punching machine to obtain electrode sheets.

[0085] The assembly of the battery is carried out in an Ar-filled vacuum glove box, and the assembled battery model is CR2016. The electrode sheet is used as a positive electrode, a lithium sheet is used as a counter electrode, 1mol / L LiPF6 / EC.DMC.EMC is used as an electrolyte, and the positive electrode shell, positive electrode material electrode sheet, separator (Celgard 2400), lithium sheet, gasket, spring, and negative electrode shell are sequentially placed and pressed into a button cell using a sheet press in the glove box, and then taken out after aging for 12 hours for testing.

[0086] The LAND CT2001A battery test system is used to test the specific discharge capacity of a certain cycle of the button cell, the charge-discharge performance under different rates, and the cycle performance after a certain number of cycles. The test voltage window is 2.5-4.2V, the rate is 0.1C, 3C, and 5C, the nominal specific capacity of LFP is set to 170mAh / g, and the capacity retention rate after 100 cycles under the corresponding rate conditions is tested.

[0087] ;

[0088] As can be seen from the test results in Table 1, the product obtained by the present application can have a more excellent capacity retention rate during high-rate charge-discharge, so that the performance of the product can be effectively maintained.

[0089] The above embodiments are preferred embodiments of the present application, but the embodiments of the present application are not limited by the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods, and are all included in the protection scope of the present application.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The specific preparation steps include: 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 by weight fraction, mixed, the pH is adjusted to weak alkaline, and then water is removed by evaporation to obtain a mixed precursor; The concentration of the dopamine solution is 4-10 g / L; The carbon source mixed solution includes the following raw materials by weight fraction: 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-fired in an inert atmosphere at a temperature of 360-380 ℃ for 60-80 min, then sintered at a temperature of 650-660 ℃ for 320-350 min, cooled, and discharged to obtain a sintered material; The water content of the sintered material is adjusted to 6-8% to obtain sintered material wet material; The sintered material wet material is transferred into a container, and trimethylaluminum vapor is introduced into the container, and the temperature is 140-150 ℃, the pressure is 0.18-0.22 MPa, and the reaction is carried out for 40-60 min, then the material is discharged and calcined to obtain the lithium iron phosphate positive electrode material.

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

3. The method for preparing a lithium iron phosphate cathode material according to claim 1, characterized in that, The carbon source mixed solution further 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 cathode 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 of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphorous pentoxide; heating the mixture to a temperature of 600-700°C for 6-8 hours; and cooling the mixture to room temperature. The weak alkaline is pH=7.2-7.

8.

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

7. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphorous pentoxide; heating the mixture to a temperature of 600-700°C for 6-8 hours; and cooling the mixture to room temperature. The calcination includes: Heating and calcining in a nitrogen atmosphere at a temperature of 350-500 ℃ for 1-3 h, and then cooling to room temperature in the furnace.

8. The method of claim 1, wherein the lithium iron phosphate cathode material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphorous pentoxide; heating the mixture to a temperature of 600-700°C for 6-8 hours; and cooling the mixture to room temperature. The preparation steps of the lithium iron phosphate powder include: LiOH·H2O, FeSO4·7H2O, H3PO4, and L-ascorbic acid in an amount of 3-5% of the mass of FeSO4·7H2O are weighed according to the stoichiometric ratio of Li:Fe:P= (3.02-3.06): (0.96-1.00): (0.98-1.00); LiOH·H2O and water are mixed and dissolved to prepare a lithium hydroxide solution with a mass fraction of 10-12%, and then H3PO4 is added dropwise to obtain solution A; FeSO4·7H2O and water are mixed and dissolved to prepare a ferrous sulfate solution with a mass fraction of 10-12%, and then L-ascorbic acid is added to obtain solution B; Solution B is slowly added to solution A at a rate of 5-8 mL / min, and then hydrothermal reaction is carried out at a temperature of 180-190 ℃ for 10-12 h, and then the mixture is cooled, centrifuged, dried, broken up, and sieved to obtain the lithium iron phosphate powder.

9. A lithium iron phosphate cathode material, characterized in that, Prepared by the preparation method of any one of claims 1-8.