Preparation method of high-rate and high-sphericity lithium iron phosphate positive electrode material

Abstract

The application provides a preparation method of a high-rate and high-sphericity lithium iron phosphate positive electrode material, in which iron phosphate and a lithium source are weighed according to a certain proportion and dispersed in pure water, a slurry A is obtained after mixing and sand milling; under stirring, a small amount of a fluorine source is added into the slurry A to obtain a lithium fluoride doped slurry B; under stirring, an organic carbon source is added into the slurry B to obtain an electrostatic spinning stock solution C, and a high-rate and high-sphericity lithium iron phosphate composite positive electrode material is obtained after drying and high-temperature calcination. The lithium fluoride doped stock solution is synthesized through in-situ reaction of the fluorine source and the lithium source, and the uniformly doped precursor is prepared through electrostatic spinning; the generated lithium fluoride acts as a fluxing agent to promote the formation of the high-sphericity lithium iron phosphate in the sintering process; meanwhile, the doping of the fluorine ion weakens the interaction between Li-O, which is beneficial to the extraction / insertion of lithium ions in the bulk phase during charging and discharging, and the lithium fluoride is rich in the conductive carbon layer, thereby improving the migration efficiency of lithium ions at the positive electrode / electrolyte interface.

Description

TECHNICAL FIELD

[0001] The application belongs to the preparation method of lithium ion secondary battery cathode material, and particularly relates to a preparation method of high-rate and high-spherical lithium iron phosphate composite cathode material. BACKGROUND

[0002] With the rapid development of renewable energy industries such as solar energy, wind energy, wave energy and tidal energy, effective energy storage technology has become the focus of research and development in recent years. Since the successful commercial application in the early 1990s, lithium ion batteries have developed rapidly. Lithium iron phosphate material has attracted widespread attention in the market due to its low price, non-toxicity, environmental friendliness, high safety performance, long service life and other advantages, especially in the field of electric vehicles and electrochemical energy storage. However, lithium iron phosphate material still has disadvantages such as poor electronic / ionic conductivity, low tap density, poor large current density charge-discharge capacity, etc., which hinders the application of lithium iron phosphate battery in high energy density and high rate performance demand.

[0003] One of the main ways to achieve high energy density is to improve the tap density of lithium iron phosphate powder. By reducing the carbon content, increasing the calcination temperature and other ways to make the primary particles grow, and then improve the powder compaction. However, due to the growth of particles, the electrochemical performance will deteriorate sharply, and the discharge capacity and coulombic efficiency will decrease. In terms of improving high-rate performance, ion doping and nano-grinding to reduce particle size can improve the rate performance of the material to some extent, but at the same time, it will reduce the tap density of the material, so that lithium iron phosphate cannot meet the demand of high specific energy battery.

[0004] Patent CN 109904423A discloses a preparation method of fluorine ion doped and lithium fluoride coated lithium iron phosphate positive electrode material, characterized in that: lithium hexafluorophosphate and lithium iron phosphate mixture are put into deionized water, stirred, dried, and then calcined at 300-400 DEG C in a muffle furnace to obtain the final product. This method improves the lithium ion conduction rate to some extent, but the fluorine ion doping degree and the uniformity of lithium fluoride coated lithium iron phosphate need to be improved, and the energy density improvement is limited. Patent CN 101388459B discloses a preparation method of lithium iron phosphate composite positive electrode material, characterized in that: lithium iron phosphate obtained by high temperature solid phase synthesis is heated and secondarily granulated with fluxing agent NaPO3F and LiPO3F. This method improves the tap density of lithium iron phosphate, but does not improve the rate performance. Patent CN 104743537A discloses a preparation method of high rate lithium iron phosphate / carbon composite positive electrode material, characterized in that: ferrous sulfate with a sheet-like structure is used to prepare ferrous phosphate, and then lithium iron phosphate / carbon composite positive electrode material with a sheet-like structure is obtained. The positive electrode material obtained by this method has the characteristics of large first charge and discharge specific capacity, long cycle life and excellent rate performance, but the compaction density is not high due to the sheet-like structure.

[0005] In summary, the existing preparation methods of lithium iron phosphate cannot improve the compaction density and rate performance of the material at the same time, so that the material cannot have good electrochemical performance and high energy density at room temperature and low temperature; therefore, it is of great significance to provide a preparation method of lithium iron phosphate positive electrode material with compaction and rate performance for the research and development of lithium ion battery. SUMMARY

[0006] The present application provides a preparation method of high rate and high sphericity lithium iron phosphate positive electrode material to overcome the shortcomings of the prior art.

[0007] A preparation method of high rate and high sphericity lithium iron phosphate positive electrode material, characterized by the following specific steps:

[0008] (1) A certain proportion of iron phosphate and lithium source is weighed and dispersed in pure water, and after mixing and sand milling, slurry A is obtained;

[0009] (2) Under stirring conditions, a small amount of fluorine source is added to slurry A, and after a period of reaction, lithium fluoride doped slurry B is obtained;

[0010] (3) Under stirring conditions, an organic carbon source is added to the above slurry B to prepare electrospinning stock solution C, and after drying and high temperature calcination, high rate and high sphericity lithium iron phosphate composite positive electrode material is obtained.

[0011] Further, in step (1), the iron-phosphorus molar ratio of the iron phosphate is 0.95-0.99.

[0012] Further, in step (1), the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium chloride, and the molar ratio of the lithium source to the iron source is 1.02-1.1.

[0013] Further, in step (1), the slurry A has a primary particle size D 50 =250-500 nm and a solid content of 30-50%.

[0014] Further, in step (2), the fluorine source is one or more of hydrofluoric acid, ammonium fluoride and iron fluoride.

[0015] Further, in step (2), the amount of the fluorine source is in a molar ratio of 0.01-0.05:1 of fluorine atoms to lithium atoms.

[0016] Further, in step (2), the stirring reaction time is 0.5-5 h, and the reaction type is neutralization, double decomposition.

[0017] In step (2), the particle size is controlled by ball milling during the reaction process, and the ball milling is performed at a dynamic frequency of 30-45 Hz for 10-30 min to ensure that the particle size is 80-150 nm, and further preferably 80-100 nm.

[0018] The dynamic frequency refers to ball milling at 30-35 Hz for 3-5 min, further ball milling at 40-45 Hz for 3-5 min, and then ball milling at 30-35 Hz for 3-5 min.

[0019] Further, in step (3), the organic carbon source is one or more of PEG2000, PEG4000, PEG6000 and PEG8000, and the amount is 2-4 wt% based on the carbon content of the final product.

[0020] Further, in step (3), the electrospinning parameters are as follows: the spinning voltage is 3-20 KV, and the control solution feeding speed is 0.1-20 mL / h.

[0021] Further, in step (3), the drying temperature is 60-100 ℃.

[0022] Further, in step (3), the protective atmosphere is at least one of nitrogen, argon and hydrogen.

[0023] Further, in step (3), the electrostatic spinning stock solution C calcination temperature is 500-750 ℃; the electrostatic spinning stock solution C calcination holding time is 6-12 h.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The lithium source and the fluorine source are reacted by neutralization and / or metathesis to obtain lithium fluoride, which is uniformly dispersed in the sand-milled slurry, and then a precursor is prepared by electrostatic spinning, and a lithium iron phosphate composite material uniformly doped with fluorine ions / lithium fluoride is obtained by sintering.

[0026] 2. In the sintering process, the lithium fluoride acts as a fluxing agent, which reduces the calcination temperature and improves the sphericity of lithium iron phosphate, and is beneficial to the formation of small-particle-size and highly spherical lithium iron phosphate.

[0027] 3. The fluorine ion doping weakens the Li-O bonding effect, which is beneficial to the deintercalation of lithium ions in the charging and discharging process, and the lithium fluoride coating can increase the lithium ion migration rate, thereby improving the rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a scanning electron microscope graph of the sample of Example 1 of the present application.

[0029] Figure 2 It is a scanning electron microscope graph of the sample of Comparative Example 1 of the present application.

[0030] Figure 3 It is a discharge curve of a half-cell based on the lithium iron phosphate composite material obtained in Example 1 of the present application at different rates (0.1, 1, 3, 5 and 10 C). DETAILED DESCRIPTION

[0031] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects more clear and explicit, the following combines embodiments, and the present application is further described in detail. However, the present application can be realized in many different forms, and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terms used in the specification of the present application herein are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0033] Example 1

[0034] 15 g of iron phosphate (Fe / P=0.97) and 7.6 g of lithium carbonate were weighed, water was used as the ball milling medium, and the particle size D 50~270 nm, solid content 38% of slurry A1;

[0035] Under stirring condition, 0.75 g of hydrofluoric acid (50 wt%) was added into slurry A1, after 0.5 h reaction, 2.1 g of PEG6000 was added to continue stirring for 0.5 h to obtain lithium fluoride doped slurry B1; during the reaction, ball milling was used for slurry, the ball milling was carried out at 30 Hz - 35 Hz for 5 min, further at 40-45 Hz for 5 min, and then at 30 Hz - 35 Hz for 5 min to control the particle size to be between 80-100 nm;

[0036] Slurry B1 was obtained by electrospinning and drying to obtain precursor C1; the electrospinning parameters were as follows: the spinning voltage was 10 KV; the control solution feeding speed was 10 mL / h, and the drying temperature was 80°C;

[0037] The precursor C1 was placed in a tube furnace, the calcination temperature was 600°C, the time was 8 h, the furnace was in inert nitrogen atmosphere, and the furnace was cooled to obtain lithium fluoride / carbon positive electrode material of fluorine doped lithium iron phosphate.

[0038] Example 2

[0039] The same as example 1, except that the fluorine source was ammonium fluoride.

[0040] Example 3

[0041] The same as example 1, except that the carbon source was PEG2000.

[0042] Example 4

[0043] The same as example 1, except that the amount of hydrofluoric acid was adjusted to 0.375 g.

[0044] Comparative example 1

[0045] 15 g of iron phosphate (Fe / P=0.97) and 7.6 g of lithium carbonate were weighed, water was used as the ball milling medium, and the particle size D 50 ~270 nm, solid content 38% of slurry A1;

[0046] Under stirring condition, 2.1 g of PEG6000 was added to continue stirring for 0.5 h to obtain slurry B11; during the stirring, ball milling was used for slurry, the ball milling was carried out at 30 Hz - 35 Hz for 5 min, further at 40-45 Hz for 5 min, and then at 30 Hz - 35 Hz for 5 min to control the particle size to be between 80-100 nm;

[0047] The slurry B11 is obtained by electrospinning and drying, the electrospinning parameters are as follows: the spinning voltage is 10KV; the control solution feeding speed is 10 mL / h, and the drying temperature is 80°C.

[0048] The precursor C11 is placed in a tube furnace, the calcination temperature is 600°C, the time is 8 h, the furnace is in inert nitrogen atmosphere, and the furnace cooling is performed to obtain the lithium fluoride / carbon positive electrode material of fluorine-doped lithium iron phosphate.

[0049] Comparative Example 2

[0050] 15 g of iron phosphate (Fe / P = 0.97) and 7.6 g of lithium carbonate are weighed, water is used as the ball milling medium to obtain a slurry A22 with a particle size D 50 =~270nm and a solid content of 38%;

[0051] Under stirring, 0.75 g of hydrofluoric acid (50 wt%) is added to the slurry A22, after 0.5 h of reaction, 2.1 g of PEG6000 is added to continue stirring for 0.5 h to obtain a lithium fluoride-doped slurry B22; during the reaction, ball milling is performed, the ball milling is performed at 40-45 Hz for 15 min;

[0052] The slurry B22 is obtained by electrospinning and drying, the electrospinning parameters are as follows: the spinning voltage is 10KV; the control solution feeding speed is 10 mL / h, and the drying temperature is 80°C.

[0053] The precursor C22 is placed in a tube furnace, the calcination temperature is 600°C, the time is 8 h, the furnace is in inert nitrogen atmosphere, and the furnace cooling is performed to obtain the lithium fluoride / carbon positive electrode material of fluorine-doped lithium iron phosphate.

[0054] Preparation of button cells

[0055] The positive electrode material prepared in the examples and comparative examples, the conductive active material SP, and the binder PVDF are mixed in a ratio of 90:5:5 to form a slurry, and then uniformly coated on an aluminum foil to prepare a positive electrode sheet, a high-purity lithium sheet is selected as the negative electrode, a Celgard 2400 is used as the separator, and an electrolyte is a mixed solvent of 1 mol / L LiPF6 dissolved in EC and DMC (volume ratio of 1:1), and then assembled into a CR2032 type button cell in a vacuum glove box, and then subjected to electrochemical test.

[0056] Rate performance test

[0057] The prepared button cell is discharged at 0.1C, 1C, 3C, 5C and 10C rates under 0.1C charging in the voltage range of 2-3.75V at 25°C to obtain the discharge capacity of the battery at different rates, and the results are shown in Table 1.

[0058] Table 1: Discharge capacity of each lithium iron phosphate material at 0.1C, 1C, 3C, 5C, 10C

[0059]

[0060] As can be seen from Table 1, the lithium iron phosphate prepared in Example 1 has good rate performance. Compared with Example 1, the rate performance of Example 2 is reduced due to the use of ammonium fluoride, and the main reason is that the replacement of the fluorine source leads to poor reaction doping effect; Example 3 uses PEG2000 as an organic carbon source, and due to the difference in solubility and viscosity in the medium, the electrochemical performance of the final lithium iron phosphate is reduced; Example 4 reduces the doping amount of the fluorine source, which leads to a decrease in effective doping, and in turn leads to a decrease in rate performance.

[0061] Comparative Example 1 prepared lithium iron phosphate without fluorine source has poor rate performance. At the same time, it can be seen from Figure 1 and Figure 2 that the lithium fluoride-doped precursor after high-temperature sintering obtains a spherical lithium iron phosphate composite material with higher sphericity. The results of Comparative Example 2 show that the difference in ball milling parameters in the ball milling and pulping process affects the rate performance of the final lithium iron phosphate, which is mainly due to the difference in particle size and morphology caused by ball milling.

[0062] The above examples are only for clearly illustrating the examples made, and are not a limitation on the embodiments. Based on the above description, other different forms of changes or variations can also be made by those skilled in the art, and it is not necessary or possible to exhaust all the embodiments, so the obvious changes or variations still fall within the protection scope of the present application.

Claims

1. A method for preparing a high-rate, high-sphericity lithium iron phosphate cathode material, characterized in that, The specific steps are as follows: (1) according to a certain proportion, the iron phosphate and lithium source are weighed and dispersed in pure water, and after mixing and sand grinding, a slurry A is obtained; (2) under stirring, a small amount of fluorine source is added to the slurry A, and after a period of reaction, a lithium fluoride doped slurry B is obtained; (3) under stirring, an organic carbon source is added to the above slurry B to prepare an electrospinning stock solution C, and a fibrous material is obtained by electrospinning, which is dried and calcined at high temperature to obtain a high rate and high sphericity lithium iron phosphate composite positive electrode material.

2. The method of claim 1, wherein: In step (1), the iron-phosphorus molar ratio of the iron phosphate is 0.95-0.99; The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium chloride, and the molar ratio of lithium source to iron source is 1.02-1.

1.

3. The method of claim 1, wherein: In step (1), the slurry A has a primary particle size D 50 = 250-500 nm and a solid content of 30-50%.

4. The method of claim 1, wherein: In step (2), the fluorine source is one or more of hydrofluoric acid, ammonium fluoride and iron fluoride; The amount of fluorine source is 0.01-0.05:1 of the molar ratio of fluorine atoms to lithium atoms; The stirring reaction time is 0.5-5 h.

5. The method of claim 1, wherein: In step (2), the particle size is controlled by ball milling during the reaction process, and the ball milling is carried out at a dynamic frequency of 30-45 Hz for 10-30 min to ensure that the particle size is 80-150 nm.

6. The method of claim 5, wherein: The particle size is between 80-100 nm.

7. The method of claim 5, wherein: The dynamic frequency refers to ball milling at 30 Hz-35 Hz for 3-5 min, then ball milling at 40-45 Hz for 3-5 min, and finally ball milling at 30 Hz-35 Hz for 3-5 min.

8. The method of claim 1, wherein: In step (3), the organic carbon source is one or more of PEG2000, PEG4000, PEG6000 and PEG8000, and the amount is 2-4 wt% of the carbon content of the final product.

9. The method of claim 1, wherein: In step (3), the electrospinning parameters are as follows: the spinning voltage is 3KV-20KV; the control solution feeding speed is 0.1-20 mL / h; and the drying temperature is 60-100 ℃.

10. The method of claim 1, wherein: In step (3), the high temperature calcination is carried out in a protective atmosphere, and the protective atmosphere is at least one of nitrogen and argon; The calcination temperature of the electrospinning stock solution C is 500-750 ℃; and the calcination holding time of the electrospinning stock solution C is 6-12 h.

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