Preparation method of spherical boron-doped lithium iron phosphate positive electrode material

Abstract

The invention relates to a spherical boron-doped lithium iron phosphate positive electrode material preparation method, which comprises: S1, weighing iron phosphate, a lithium source and a carbon source according to a certain ratio, mixing, dispersing in a pure water medium, and sanding to obtain a slurry with a certain particle size distribution; s2, weighing a certain amount of lithium fluoroborate, adding the lithium fluoroborate into the slurry, controlling the temperature and stirring time of the slurry to hydrolyze the slurry, and uniformly doping boride and lithium fluoride obtained by hydrolysis into the slurry; and S3, carrying out spray drying and high-temperature sintering on the doped slurry to obtain the spherical boron-doped lithium iron phosphate. According to the invention, a product boride slowly hydrolyzed from fluorinated lithium borate is used as a boron source, fluoride is used as a fluxing agent, uniform doping of the boron source and regulation and control of the morphology of lithium iron phosphate particles are realized, and finally, the morphology of the lithium iron phosphate positive electrode material particles is regularly spherical and shows excellent electrochemical performance.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery positive electrode materials, and in particular relates to a method for preparing a spherical boron-doped lithium iron phosphate positive electrode material. Background Art

[0002] Lithium iron phosphate has been widely favored as a lithium-ion battery electrode material in recent years due to its high safety, good cycle, and environmental friendliness. It has been widely used in various mobile power supply fields, especially in the large-scale power supply field and energy storage field required for electric vehicles. However, due to its own structural defects, lithium iron phosphate has poor electronic conductivity and low ion migration rate, which limits its large-scale application. In order to solve the above problems, researchers have tried a variety of modification methods, such as: 1) carbon coating to improve the electronic conductivity of the material and to a certain extent limit the growth of grains; 2) ion doping to create lattice defects, broaden ion diffusion channels and reduce lithium ion diffusion barriers; 3) morphology control, the special morphology can make the electrolyte fully infiltrate the electrode material, and the high sphericity particles contribute to high tap density, thereby improving the battery energy density.

[0003] However, the existing modification methods still have limitations in improving the performance of the final product, and the preparation process is cumbersome. Patent CN101388459 B controls the secondary growth of lithium iron phosphate particles by adding flux, thereby improving the true density, but this method has the problem of uneven physical mixing of the flux, which affects the consistency of particle size. Patent CN 109904423 A uses the hydrolysis product of lithium hexafluorophosphate to prepare fluoride-doped and lithium fluoride-coated lithium iron phosphate by high-temperature calcination, but because lithium hexafluorophosphate is extremely sensitive to water, it hydrolyzes in a short time after feeding, causing the problem of uneven distribution of dopants. Patent CN103219516 B adds a boron source to the lithium iron phosphate precursor and obtains the final product by high-temperature sintering, but the particle morphology consistency of the product obtained by this method is poor, and the rate performance needs to be improved. Summary of the invention

[0004] The present invention aims to provide a method for preparing a spherical boron-doped lithium iron phosphate positive electrode material in view of the shortcomings of the existing methods, control the hydrolysis rate of lithium fluoride borate salt, achieve uniform doping of its hydrolysis products boride and fluoride, and regulate the morphology of lithium iron phosphate particles, which specifically comprises the following steps: S1: Weigh iron phosphate, lithium source and carbon source in a certain proportion, mix and disperse them in a pure water medium, and grind them to obtain slurry A1 with a certain particle size; S2: Weigh a certain amount of lithium fluoride borate salt, add it to the slurry, control the slurry temperature and stirring time, and obtain a doping slurry B1; S3: The doped B1 slurry is spray-dried and sintered at high temperature to obtain spherical boron-doped lithium iron phosphate.

[0005] Preferably, the iron source and phosphorus source in S1 are iron phosphate, and the lithium source is at least one of lithium carbonate and lithium hydroxide; Preferably, the iron-to-phosphorus ratio of the ferric phosphate is 0.95-0.98; Preferably, the molar ratio of the iron source to the lithium source is 1: (1-1.05); Preferably, the carbon source in S1 is PEG4000, PEG6000 and glucose, and the amount of the carbon source is 3-10%.

[0006] Preferably, the lithium fluoride borate salt in S2 includes at least one of lithium difluoroborate, lithium tetrafluoroborate and lithium difluorooxalatoborate; Preferably, the molar ratio of the lithium fluoroborate salt to the iron source is (0.01-0.05):1.

[0007] Preferably, the temperature in S2 is controlled at -10 to 0°C, and the stirring time is 0.5 to 3 hours at 1 to 20 KPa.

[0008] More preferably, the controlled temperature in S2 is -10 to -5°C, and the stirring time is 1 to 2 hours at 10 to 20 KPa.

[0009] Under the above conditions, the lithium fluoride borate salt can be slowly hydrolyzed to produce a boride, which can be used as the boride to achieve boron doping.

[0010] Preferably, the sintering temperature in S3 is 600-800° C., and the sintering time is 4-9 hours.

[0011] The present invention has the following beneficial effects: 1) by controlling the slow hydrolysis characteristics of lithium fluoride borate salts in a certain water environment and temperature, it is beneficial to uniformly disperse the hydrolysis product in the precursor slurry; 2) the boride of the hydrolysis product is sintered at high temperature to obtain a boron-doped lithium iron phosphate material, which is beneficial to the improvement of electrochemical performance; 3) the lithium fluoride of the hydrolysis product is used as a flux, which is beneficial to the formation of high-sphericity lithium iron phosphate and improves the energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a scanning electron microscope image of the sample of Example 1 of the present invention.

[0013] Figure 2 This is a scanning electron microscope image of the sample of comparative example 1 of the present invention.

[0014] Figure 3 The charge and discharge curves of the samples of Example 1 and Comparative Example 1 at 3 C rate. DETAILED DESCRIPTION

[0015] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0016] In order to better understand the present invention, the content of the present invention is further described below in conjunction with embodiments and related drawings, but the content of the present invention is not limited to the following embodiments.

[0017] Example 1 Weigh 60 g of iron phosphate (Fe / P=0.96), 29.5 g of lithium carbonate, and 2.7 g of glucose, mix the three, and use water as the ball milling medium to obtain a particle size D 50 =~300 nm, slurry A1 with a solid content of 40%; controlling the slurry temperature at -5 ℃, adding 3.4 g of lithium difluorooxalatoborate to the slurry A1 under stirring conditions, stirring at -10 ℃ and 10KPa for hydrolysis reaction, and obtaining B-doped slurry B1 after 2 h of hydrolysis reaction; spray drying the slurry B1 to obtain a precursor C1; placing the precursor C1 in a tubular furnace, calcining at a temperature of 700 ℃ for 9 h, with an inert nitrogen atmosphere in the furnace, and cooling with the furnace to obtain a boron-doped lithium iron phosphate positive electrode material.

[0018] Example 2 Same as Example 1, except that the hydrolysis temperature of lithium difluorooxalatoborate is -20°C.

[0019] Example 3 Same as Example 1, except that the hydrolysis time of lithium difluorooxalatoborate is 1 h.

[0020] Example 4 Same as Example 1, except that the amount of lithium difluorooxalatoborate used is 1.1 g.

[0021] Example 5 Same as Example 1, except that lithium difluorooxalatoborate is replaced by lithium tetrafluoroborate in an amount of 2.2 g (the molar ratio of the boron source to the iron source remains unchanged).

[0022] Comparative Example 1 Weigh 60 g of iron phosphate (Fe / P=0.96), 29.5 g of lithium carbonate, and 2.7 g of glucose, mix the three, and use water as the ball milling medium to obtain a particle size D 50=~300 nm, slurry A2 with a solid content of 40%; controlling the slurry temperature at -5 °C, adding 3.4 g of lithium difluorooxalatoborate to the slurry A1 under stirring conditions, stirring at 20 °C and normal pressure for hydrolysis reaction, and obtaining the doped slurry B2 after 2 h of hydrolysis reaction; spray drying the slurry B2 to obtain the precursor C2; placing the precursor C2 in a tubular furnace, calcining at a temperature of 700 °C for 9 h, with an inert nitrogen atmosphere in the furnace, and cooling with the furnace to obtain a boron-doped lithium iron phosphate positive electrode material.

[0023] Comparative Example 2 Weigh 60 g of iron phosphate (Fe / P=0.96), 29.5 g of lithium carbonate, and 2.7 g of glucose, mix the three, and use water as the ball milling medium to obtain a particle size D 50 =~300 nm, slurry A3 with a solid content of 40%; controlling the slurry temperature at -5 °C, adding 3.4 g of lithium difluorooxalatoborate to the slurry A3 under stirring conditions, stirring at 10 °C and 20 KPa for hydrolysis reaction, and obtaining the doped slurry B3 after 2 hours of hydrolysis reaction; spray drying the slurry B3 to obtain the precursor C3; placing the precursor C3 in a tubular furnace, calcining at a temperature of 700 °C for 9 hours, with an inert nitrogen atmosphere in the furnace, and cooling with the furnace to obtain a boron-doped lithium iron phosphate positive electrode material.

[0024] Comparative Example 3 Weigh 60 g of iron phosphate (Fe / P=0.96), 29.5 g of lithium carbonate, and 2.7 g of glucose, mix the three, and use water as the ball milling medium to obtain a particle size D 50 =~300 nm, and a slurry A4 with a solid content of 40%; the slurry A4 is spray-dried to obtain a precursor C4; the precursor C4 is placed in a tubular furnace, the calcination temperature is 700 ° C, the time is 9 h, the furnace is an inert nitrogen atmosphere, and the furnace is cooled to obtain a lithium iron phosphate positive electrode material.

[0025] Figure 1 , Figure 2 They are scanning electron microscope images of the sample of Example 1 and the sample of Comparative Example 1. As can be seen from the comparison, the lithium iron phosphate particles prepared by the method of the present invention have good sphericity and high particle consistency.

[0026] Figure 3 The charge and discharge curves of the samples of Example 1 and Comparative Example 1 at 3C rate are shown in FIG. By comparison, it can be seen that boron-doped lithium iron phosphate increases the lithium ion migration rate and reduces the polarization voltage, thereby increasing the battery capacity.

[0027] Preparation of button cells The positive electrode material prepared in the embodiment and the comparative example was mixed with the conductive active material SP and the binder PVDF in a ratio of 90:5:5 to form a slurry, and then evenly coated on an aluminum foil to prepare a positive electrode sheet. The negative electrode was a high-purity lithium sheet, the separator was Celgard 2400, and the electrolyte was 1 mol / L LiPF 6 It was dissolved in a mixed solvent of EC and DMC (volume ratio of 1:1), assembled into a CR2032 button cell in a vacuum glove box, and then subjected to electrochemical testing.

[0028] Rate performance test The prepared button cell was charged at 0.1C in the voltage range of 2 to 3.75V at 25°C, and discharged at rates of 0.1C, 0.5C, 1C, 3C, and 5C, respectively, to obtain the discharge capacity of the battery at different rates. The results are shown in Table 1.

[0029] Table 1 Discharge capacity of each lithium iron phosphate sample at 0.1C, 0.5C, 1C, 3C, and 5C

[0030] In Table 1, the discharge specific capacity of Examples 1-5 at different rate performances indicates that the electrochemical performance of the boron-doped lithium iron phosphate obtained by the method of the present invention is improved. It can be seen from Examples 1-5 and Comparative Examples 1-3 that the type, hydrolysis temperature and time of lithium fluoride borate can affect the electrochemical performance of the final doped lithium iron phosphate, which is mainly attributed to the influence of temperature and time on the hydrolysis of lithium fluoride borate, resulting in the doping amount and uniformity, which ultimately affects the rate performance of lithium iron phosphate. Table 2 shows the results of three ICP tests on the B content of the samples of each example. From the average value of the three measurements, it can be found that the sample of Example 1 has the largest doping amount and the smallest deviation.

[0031] Table 2 B content test results

[0032] The tap density of the positive electrode materials obtained in the comparative example and each embodiment was measured by a JZ-1 powder tap density meter, and the results are shown in Table 3. As can be seen from Table 3, the tap density of the lithium iron phosphate obtained in each embodiment by the method of the present invention is improved to varying degrees, which helps to improve the volume energy density.

[0033] Table 3 Tap density test results

[0034] The above embodiments are only examples for clear explanation, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here, so the obvious changes or modifications derived are still within the protection scope of the invention.

Claims

1. A method for preparing a spherical boron-doped lithium iron phosphate positive electrode material, characterized in that: The specific steps include: S1: Weigh iron phosphate, lithium source and carbon source in a certain proportion, mix and disperse them in a pure water medium, and grind them to obtain slurry A1 with a certain particle size; S2: Weigh a certain amount of lithium fluoride borate salt, add it to slurry A1, adjust the slurry temperature and stirring time to control the hydrolysis rate, and dope the hydrolysis product into slurry B1; S3: The doping slurry B1 is spray-dried and sintered at high temperature to obtain spherical boron-doped lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that: The iron source and phosphorus source in S1 are iron phosphate, and the lithium source is at least one of lithium carbonate and lithium hydroxide.

3. The preparation method according to claim 1, characterized in that The iron-to-phosphorus ratio of the ferric phosphate is 0.95-0.98; the molar ratio of the iron source to the lithium source is 1: (1-1.05).

4. The preparation method according to claim 1, characterized in that: The carbon source in S1 is at least one of PEG4000, PEG6000 and glucose, and the amount of the carbon source is 3-10%.

5. The preparation method according to claim 1, characterized in that: The lithium fluoride borate salt in S2 includes at least one of lithium difluoroborate, lithium tetrafluoroborate and lithium difluorooxalatoborate; The molar ratio of the lithium fluoride borate salt to the iron source is (0.01-0.05):

1.

6. The preparation method according to claim 1, characterized in that: The control temperature in S2 is -10~0°C, and the stirring time is 0.5~3h at 1-20KPa.

7. The preparation method according to claim 6, characterized in that: The control temperature in S2 is -10~-5℃, and the stirring time is 1~2h at 10-20KPa.

8. The preparation method according to claim 1, characterized in that In the S3, the sintering temperature is 600-800°C, and the sintering time is 4-9h.

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