Fluorine-containing lithium iron phosphate composite material, preparation method and application thereof

Abstract

The application discloses a fluorine-containing lithium iron phosphate composite material and a preparation method and application thereof. The lithium iron phosphate anode material prepared by using a hydrothermal method has the characteristics of adjustable morphology and uniform particle size, and further makes the electrochemical performance of the material excellent. The doping of cations can improve the ion conduction rate and electronic conductivity, and the doping of anions can improve the electrochemical performance of the lithium iron phosphate, especially the rate discharge performance. The co-doping of anions and cations is beneficial to preparing the anode material with more excellent electrochemical performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium batteries, and particularly relates to a fluorine-containing lithium iron phosphate composite material and a preparation method and application thereof. BACKGROUND

[0002] The lithium iron phosphate battery refers to a lithium ion battery using lithium iron phosphate as a positive electrode material. This type of battery is characterized by not containing precious metal elements (such as cobalt, etc.). In actual use, the lithium iron phosphate battery has the advantages of high temperature resistance, strong safety and stability, low price, and better cycle performance. The raw material price is low, and the content of phosphorus and iron in the earth's resources is rich, so there will be no supply problem.

[0003] A Chinese invention application with the application number CN201310316696.4 discloses a nano lithium zirconate modified lithium iron phosphate composite material and a preparation method thereof. The structural formula of the composite material is Li x Fe y PO4·zLi2ZrO3 / C, which is composed of lithium iron phosphate, nano lithium zirconate and organic carbon. The application is characterized by being prepared by one of the following methods: (1) a precursor mixed solution containing nano lithium zirconate is prepared by hydrothermal reaction, an organic carbon source is added after drying, and then mixed and calcined at high temperature in an inert atmosphere; (2) a precursor mixed solution is prepared by hydrothermal reaction, dried and calcined at low temperature in an inert atmosphere, then an organic carbon source and nano lithium zirconate are added, mixed and calcined at high temperature in an inert atmosphere.

[0004] A Chinese invention application with the application number CN202111629862.7 discloses a titanium and zirconium co-doped carbon-coated lithium iron phosphate material, a preparation method and application thereof. The chemical expression of the material is Li 1-y Zr y Fe 1-x Ti x PO4 / C, wherein titanium is doped into the Fe site, zirconium is doped into the Li site, and 0.001≤x≤0.05, 0.001≤y≤0.02; the preparation method is to mix phosphoric acid iron, lithium carbonate, carbon source, titanium source and zirconium source in a liquid medium, ball mill and sand mill the mixed material to a certain slurry particle size, then use spray drying technology for granulation, and finally sinter the dried spray material in an atmosphere furnace. The material is applied as a positive electrode material in a lithium ion battery.

[0005] However, the above-mentioned existing preparation method of lithium iron phosphate has not developed a high-performance product, and the morphology of the precursor directly determines the performance of the final product. Among the many preparation methods, the liquid phase method has great advantages in particle uniformity and molecular level dispersion compared with the solid phase reaction method. The liquid phase method mainly includes hydrothermal method and co-precipitation method, and the lithium iron phosphate precursor and lithium iron phosphate prepared by the liquid phase method have the advantages of small particle size, uniform dispersion, and low energy consumption. However, the lithium iron phosphate particles prepared by the hydrothermal method are small, but the agglomeration is serious. The material prepared by the co-precipitation method has poor performance due to particle agglomeration. SUMMARY

[0006] The present application aims at the defects and improvement needs of the prior art, and proposes a preparation method of fluorine-containing lithium iron phosphate composite material. The preparation method of the present application adopts a hydrothermal method, and by adjusting the hydrothermal time and temperature and selecting a suitable surfactant, the morphology of the precursor can be controlled in real time, and the agglomeration phenomenon can be improved. The doping of cations can improve the ion conduction rate and electronic conductivity, and the doping of anions can improve the electrochemical performance of lithium iron phosphate, especially the rate discharge performance. The co-doping of anions and cations is beneficial to the preparation of a positive electrode material with more excellent electrochemical performance.

[0007] A preparation method of fluorine-containing lithium iron phosphate composite material, the general formula of the fluorine-containing lithium iron phosphate composite material is Li (1-x) Zr x FePO (4-y) F y / C, wherein 0.001≤x≤0.02, 0.001≤y≤0.05. The preparation method comprises the following steps:

[0008] (1) Dissolving a phosphorus source and an iron source in water, adding a surfactant, mixing uniformly, and then performing a hydrothermal reaction to obtain a lithium iron phosphate precursor;

[0009] (2) Mixing the lithium iron phosphate precursor obtained in step (1) with a lithium source, a fluorine source, and a zirconium source in water to obtain a mixture;

[0010] (3) Adding a carbon source to the mixture obtained in step (2), mixing and ball milling, and then performing calcination to obtain a product Li (1-x) Zr x FePO (4-y) F y / C.

[0011] Preferably, the surfactant is at least one of the following: hexadecyl trimethyl ammonium bromide, polyethylene glycol, polyvinyl pyrrolidone, iron- and lithium-containing polyvinyl pyrrolidone surfactant,

[0012] The preparation method of the polyvinylpyrrolidone surfactant containing iron and lithium comprises the following steps: dispersing N-vinylpyrrolidone, vinylferrocene and lithium acrylate three reaction monomers in a solvent water, and performing a free radical polymerization reaction under the action of an initiator to obtain the polyvinylpyrrolidone surfactant containing iron and lithium.

[0013] The mass ratio of the N-vinylpyrrolidone, the vinylferrocene and the lithium acrylate is 1000-15000:1:1.25-2.5. The initiator is preferably a t-butyl hydroperoxide-ammonium sulfite redox system initiator, wherein the t-butyl hydroperoxide can be first added to the system, and then the ammonium sulfite is added dropwise to the reaction system to initiate the reaction. The addition amount of the t-butyl hydroperoxide is 3.3-5 g per 100 g of N-vinylpyrrolidone. After the reaction monomers and the t-butyl hydroperoxide are dispersed in the water, the pH is adjusted to 1.2, and then the ammonium sulfite solution is added dropwise. The use amount of the ammonium sulfite is 0.67-1.25 g per 100 g of N-vinylpyrrolidone. Before the ammonium sulfite solution is added dropwise, inert gas protection is performed, and the temperature is increased to 50℃; after the ammonium sulfite solution is added dropwise, the temperature is increased to 60-70℃, and the reaction is performed for 20-50 minutes, and then the temperature is increased to 80-90℃, and the reaction is performed for 1-4 hours. After the reaction is completed, the product is cooled and spray dried.

[0014] The polyvinylpyrrolidone surfactant containing iron and lithium can increase the compatibility of the surfactant with the lithium iron phosphate material, improve the full mixing effect of the iron source, the phosphorus source, the lithium carbonate, the fluorine source and the zirconium source, and obtain the fluorine-zirconium co-doped lithium iron phosphate which is uniformly dispersed.

[0015] Preferably, the phosphorus source is at least one of phosphoric acid and dihydrogen phosphate;

[0016] The iron source is at least one of iron oxide, iron chloride, iron nitrate and hydrates thereof;

[0017] The lithium source is at least one of lithium carbonate, lithium hydroxide and lithium nitrate;

[0018] The fluorine source is at least one of lithium fluoride, ammonium fluoride, hydrogen fluoride and iron fluoride;

[0019] The zirconium source is at least one of zirconium dioxide, zirconium hydroxide, zirconium phosphate, hydrogen zirconium phosphate and hydrates thereof.

[0020] Preferably, the phosphorus source is NH4H2PO4, and the iron source is Fe(NO3)3 or its hydrate. Because NH4H2PO4 reacts with Fe(NO3)3 to form ferric phosphate and ammonium nitrate, the ammonium nitrate is a strong acid weak base salt, and the pH is low, so the pH needs to be adjusted to an appropriate size, such as using ammonia to adjust the pH. Too low pH will affect the performance of the final product. Preferably, in step (1), after the phosphorus source and the iron source are dissolved in water, the pH is adjusted to 1.2, and then the surfactant is added.

[0021] Preferably, in step (1), the molar ratio of the phosphorus source to the iron source is 1-1.04:0.96-1. The phosphorus source is slightly more than the iron source, which can avoid oxidation of iron.

[0022] Preferably, in step (1), the molar ratio of the surfactant to the iron source is 0.01-0.013:0.96-1; in step (2), the molar ratio of the ferric phosphate precursor to the lithium source, the fluorine source, and the zirconium source is 1:1.01-1.06:0.001-0.05:0.001-0.02. The lithium source is slightly more than the iron in the ferric phosphate precursor, which can avoid oxidation of iron. Therefore, there will be a small amount of basic lithium mixed in the final product.

[0023] Preferably, in step (3), the carbon source is at least one of the following: glucose, sucrose, and starch; and the amount of the carbon source added is 1%-3% of the mass of the mixture obtained in step (2).

[0024] Preferably, in step (1), the hydrothermal reaction temperature is 80-120°C, and the time is 6-10h; in step (3), the calcination temperature is 500-700°C, and the calcination time is 6h.

[0025] The reaction temperature and reaction time of the FePO4 precursor prepared by the hydrothermal method restricts the particle growth and particle morphology, wherein the hydrothermal temperature affects the nucleation and growth of the micro-particles in the solution, and the hydrothermal reaction time has a great influence on the growth process of the crystal and the final particle morphology. Through the experiment, it can be known that when the temperature is 100 DEG C, the particle morphology is good and the particle size is consistent when the hydrothermal reaction is 8 hours. With the increase of the temperature, the solubility of the material becomes larger, the supersaturation degree is reduced, and the viscosity of the solution is reduced. When the temperature is increased, the reaction rate is accelerated, the primary crystal nucleus formed in the solution is increased, and the particle size is reduced. However, when the temperature exceeds a certain critical value, due to the reduction of the size, the surface energy of the particle becomes larger, and the excessive crystal grains will cause the particle agglomeration. When the temperature is low, the saturation degree of the solution is large, the reaction rate is slow, and the nucleation speed is also slow. The hydrothermal reaction time has a great influence on the growth process of the crystal and the final particle morphology. If the reaction time is too short, the crystal may not complete the nucleation and growth process, and the particle structure may be relatively disordered. If the reaction time is too long, the particle will become large. The large particle will affect the electrochemical performance of the material, so the hydrothermal reaction time and temperature are crucial to the electrochemical performance of the synthesized material.

[0026] The application further provides the fluorine-containing lithium iron phosphate composite material prepared by the preparation method.

[0027] The application further provides application of the fluorine-containing lithium iron phosphate composite material as a positive electrode material in preparation of a lithium ion battery.

[0028] The application has the following beneficial effects:

[0029] (1) The lithium iron phosphate positive electrode material prepared by the hydrothermal method has the characteristics of adjustable morphology and uniform particle size, and the electrochemical performance of the material is excellent.

[0030] (2) The doping of cations can improve the ion conduction rate and electronic conductivity, the doping of anions can improve the electrochemical performance of the lithium iron phosphate, especially the rate discharge performance. The co-doping of anions and cations is beneficial to preparation of a positive electrode material with more excellent electrochemical performance. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The LFP rate charge-discharge curve of the positive electrode material prepared for Example 1 is shown in the following figure.

[0032] Figure 2 The retention rate graph of the positive electrode material prepared for Example 1 after 50 cycles at 0.2C is shown in the following figure. DETAILED DESCRIPTION

[0033] The evaluation method of the specific embodiment of the application is as follows:

[0034] The positive electrode material performance evaluation adopts CR2032 type button half battery for experimental research, and the preparation process of the positive electrode sheet is as follows:

[0035] (1) The positive electrode uses modified lithium iron phosphate, acetylene black and binder PVDF (polyvinylidene fluoride), and the mass ratio is 80:10:10. Add to the agate mortar, grind thoroughly until mixed evenly. Add appropriate amount of N-methyl pyrrolidone to the above mixture drop by drop, and grind again to form a uniform slurry with a certain viscosity. Use a scraper to coat on a dry carbon-coated aluminum foil. First, dry it in a forced air drying oven at 70°C for 3h, then dry it in a vacuum environment at 120°C for 12h. Take out the electrode sheet when the temperature drops below 50°C. Use a roller press to compact the electrode sheet, and then cut it into a circular positive electrode sheet with a diameter of 12mm using a punching machine.

[0036] (2) Select lithium sheet as negative electrode, 1.0mol / L LiPF6 / (EC+EMC+DMC) as electrolyte (wherein LiPF6 is lithium hexafluorophosphate, EC is ethylene carbonate, EMC is ethyl methyl carbonate, DMC is dimethyl carbonate, and the volume ratio of EC, EMC and DMC is 1:1:1), and Celgard2400 polypropylene microporous membrane as separator. All materials are placed in a glove box filled with high-purity argon (H2O <0.1ppm, O2<0.1ppm) to assemble CR2032 button cell, and sealed with a sealing machine. Finally, the assembled CR2032 button cell is activated at room temperature for 12h for standby.

[0037] Example 1

[0038] (1) First, select NH4H2PO4 and Fe(NO3)3·9H2O each 1 mol as phosphorus source and iron source, respectively, and dissolve them in 4L deionized water in turn, and use a magnetic stirrer to stir until dissolved uniformly. Adjust the pH of the solution to 1.2 with ammonia water, and continue stirring for 20min. Dissolve 0.012mol of surfactant cetyltrimethylammonium bromide (CTAB) in the mixed solution of iron source and phosphorus source, mix thoroughly. Transfer the solution to a polytetrafluoroethylene-lined high-pressure reactor, and perform water bath reaction at 100°C for 8h under nitrogen atmosphere. Finally, the hydrothermal product is separated by centrifuge. Wash with deionized water and anhydrous ethanol for 3 times, and dry to obtain iron phosphate (FePO4) precursor.

[0039] (2) Mix the FePO4 precursor obtained in step (1) with Li2CO3 (lithium source), NH4F (fluorine source) and ZrO2 (zirconium source) in deionized water in a certain proportion, and the molar ratio of FePO4 precursor, lithium source, fluorine source and zirconium source is 1:1.01:0.05:0.001. Filter and separate the precipitate, and dry to obtain a mixture.

[0040] (3) In the mixture of step (2), glucose is added to the mixture of step (2) in an amount of 2wt%, and then calcination is carried out under a nitrogen atmosphere at 700°C for 6 hours to obtain a product Li (1-x) Zr x FePO (4-y) F y / C, wherein x = 0.001, y = 0.05, i.e. the product is Li 0.999 Zr 0.001 FePO 3.95 F 0.05 / C.

[0041] The obtained product is assembled into a CR2032 type button half-cell as a positive electrode material for performance testing, and the discharge capacity is 160.2 mAh / g at 0.1C Figure 1 ) and the retention rate is still 99.2% after 50 cycles at 0.2C Figure 2 ).

[0042] Example 2

[0043] The rest is consistent with Example 1, the difference is that the molar ratio of phosphorus source, iron source and surfactant is 1.01:0.99:0.01, the molar ratio of FePO4 precursor:Li2CO3:NH4F:Zr(OH)4 is 1:1.06:0.001:0.02, and the hydrothermal reaction temperature and time is: oil bath heating at 80°C for 10h. The surfactant is polyethylene glycol (PEG 2000), the carbon source added is sucrose, the amount of sucrose added is 1wt% of the mixture of step (2), and the calcination temperature is 600°C, and the calcination time is 11h.

[0044] The product prepared in this example is Li (1-x) Zr x FePO (4-y) F y / C, wherein x = 0.02, y = 0.001, i.e. the product is Li 0.98 Zr 0.02 FePO 3.999 F 0.001 / C, as a positive electrode material for performance testing, the first charge capacity is 157.8 mAh / g, and the retention rate is still 98.25% after 50 cycles at 0.2C, and D50 = 2.35 μm.

[0045] Example 3

[0046] The rest is consistent with example 1, the difference is that the molar ratio of phosphorus source, iron source and surfactant is 1.04:0.96:0.013, the molar ratio of FePO4 precursor:Li2CO3:NH4F:Zr(HPO4)2·H2O is 1:1.03:0.025:0.01. The hydrothermal reaction temperature and time is: oil bath heating at 120℃ for 6h. The surfactant is polyvinylpyrrolidone (PVP), the carbon source for coating is starch, the amount of starch added is 3wt% of the mixture in step (2), the calcination temperature is 500℃, and the calcination time is 16h.

[0047] The product prepared in this example is Li (1-x) Zr x FePO (4-y) F y / C, wherein x=0.01, y=0.025, i.e. the product is Li 0.99 Zr 0.01 FePO 3.975 F 0.025 / C, as a positive electrode material, the first charge capacity is 156.5mAh / g, and the retention rate after 50 cycles at 0.2C is still 97.43%, D50=2.2μm.

[0048] Example 4

[0049] In this example, an iron and lithium containing polyvinylpyrrolidone surfactant is prepared, instead of cetyltrimethylammonium bromide (CTAB) in example 1, the specific steps are as follows:

[0050] In a sealed reaction kettle, 1500g of deionized water, 350g of N-vinylpyrrolidone, 0.2g of vinyl ferrocene, 0.25g of lithium acrylate and 15g of tert-butyl hydroperoxide are added, and then ammonia water is added to adjust the pH to 1.2; 3.5g of ammonium sulfite is dissolved in 25g of water; nitrogen is introduced and the temperature is raised to 50℃, and the ammonium sulfite solution is added to the three-necked flask through a peristaltic pump, the dropping time is 10 minutes, and the temperature is raised to 65℃ and stabilized for 35 minutes; the temperature is raised to 85℃ and stabilized for 2.5 hours, then cooled and spray dried to obtain an iron and lithium containing polyvinylpyrrolidone surfactant.

[0051] The rest of the operation steps and the amount of raw materials remain consistent with example 1.

[0052] The product prepared in this example is Li 0.999 Zr 0.001 FePO 3.95 F 0.05 / C, as the positive electrode material, the discharge capacity was 166.4 mAh / g at 0.1 C, and the retention rate was still 99.6% after 50 cycles at 0.2 C, and D50=2.36 μm.

[0053] Example 5

[0054] In this embodiment, an iron and lithium-containing polypyrrolidone surfactant was prepared, instead of the polyethylene glycol (PEG) in Example 2, and the specific steps were as follows:

[0055] In a closed reaction kettle, 1000 g of deionized water, 300 g of N-vinyl pyrrolidone, 0.02 g of vinyl ferrocene, 0.05 g of lithium acrylate, and 10 g of tert-butyl hydrogen peroxide were added, and ammonia was added to adjust the pH to 1.2; 2 g of ammonium sulfite was dissolved in 20 g of water; nitrogen was introduced to heat to 50°C, and the ammonium sulfite solution was added to a three-necked flask by a peristaltic pump, the dropping time was 10 minutes, the temperature was raised to 60°C and stabilized for 20 minutes; the temperature was raised to 80°C and stabilized for 1 hour, and then the product was cooled and spray dried to obtain the iron and lithium-containing polypyrrolidone surfactant.

[0056] The remaining operation steps and the amount of the materials were consistent with those of Example 1.

[0057] The product prepared in this embodiment was Li 0.98 Zr 0.02 FePO 3.999 F 0.001 / C, as the positive electrode material, the discharge capacity was 166.4 mAh / g at 0.1 C, and the retention rate was still 99.6% after 50 cycles at 0.2 C, and D50=2.36 μm.

[0058] Example 6

[0059] In this embodiment, an iron and lithium-containing polypyrrolidone surfactant was prepared, instead of the polyethylene glycol (PEG) in Example 2, and the specific steps were as follows:

[0060] In a closed reaction kettle, 1000 g of deionized water, 300 g of N-vinyl pyrrolidone, 0.02 g of vinyl ferrocene, 0.05 g of lithium acrylate, and 10 g of tert-butyl hydrogen peroxide were added, and ammonia was added to adjust the pH to 1.2; 2 g of ammonium sulfite was dissolved in 20 g of water; nitrogen was introduced to heat to 50°C, and the ammonium sulfite solution was added to a three-necked flask by a peristaltic pump, the dropping time was 10 minutes, the temperature was raised to 60°C and stabilized for 20 minutes; the temperature was raised to 80°C and stabilized for 1 hour, and then the product was cooled and spray dried to obtain the iron and lithium-containing polypyrrolidone surfactant.

[0061] The remaining operation steps and the amount of the materials were consistent with those of Example 1.

[0062] The product prepared in this example is Li 0.99 Zr 0.01 FePO 3.975 F 0.025 / C, and the performance test was carried out as a positive electrode material, the first charge capacity was 160.1 mAh / g, and the retention rate was still 98.58% after 50 cycles at 0.2 C, D50 = 2.41 μm.

Claims

1. A method for producing a fluorine-containing lithium iron phosphate composite material, characterized by, The general formula of the lithium iron phosphate composite material containing fluorine is Li (1-x) Zr x FePO (4-y) F y / C, wherein 0.001≤x≤0.02, 0.001≤y≤0.05, and the preparation method comprises the following steps: (1) dissolving a phosphorus source and an iron source in water, adding a surfactant, mixing uniformly, and then performing a hydrothermal reaction to obtain an iron phosphate precursor; The surfactant is an iron-lithium-containing polypyrrolidone surfactant, wherein the preparation method of the iron-lithium-containing polypyrrolidone surfactant comprises the following steps: dispersing N-vinyl pyrrolidone, vinyl ferrocene and lithium acrylate three reaction monomers in solvent water, and performing a free radical polymerization reaction under the action of an initiator to obtain the iron-lithium-containing polypyrrolidone surfactant; wherein the mass ratio of N-vinyl pyrrolidone, vinyl ferrocene and lithium acrylate is 1000-15000:1:1.25-2.5; In step (1), the temperature of the hydrothermal reaction is 80-120°C, and the time is 6-10h; (2) mixing the iron phosphate precursor obtained in step (1) with a lithium source, a fluorine source and a zirconium source in water to obtain a mixture; (3) adding a carbon source to the mixture obtained in step (2), mixing and ball-milling, and then calcining to obtain a product Li (1-x) Zr x FePO (4-y) F y / C.

2. The preparation method of the fluorinated lithium iron phosphate composite material according to claim 1, characterized in that, The phosphorus source is at least one of phosphoric acid and ammonium dihydrogen phosphate; The iron source is at least one of iron oxide, iron chloride, iron nitrate and hydrates thereof; The lithium source is at least one of lithium carbonate, lithium hydroxide and lithium nitrate; The fluorine source is at least one of lithium fluoride, ammonium fluoride, hydrogen fluoride and iron fluoride; The zirconium source is at least one of zirconium dioxide, zirconium hydroxide, zirconium phosphate, hydrogen zirconium phosphate and hydrates thereof.

3. The method for preparing the fluorinated lithium iron phosphate composite material according to claim 1, characterized in that, In step (1), the molar ratio of the phosphorus source to the iron source is 1-1.04:0.96-1.

4. The preparation method of the fluorinated lithium iron phosphate composite material according to claim 1, characterized in that, In step (1), after the phosphorus source and the iron source are dissolved in water, the pH is adjusted to 1.2, and then the surfactant is added.

5. The method of claim 1, wherein the lithium fluorophosphate iron composite material is prepared by the steps of: mixing a lithium compound, a fluorine compound, and an iron compound; and heating the mixture at a temperature of 600 to 800°C for 1 to 10 hours in an inert gas atmosphere. In step (1), the molar ratio of the surfactant to the iron source is 0.01-0.013:0.96-1; In step (2), the molar ratio of the iron phosphate precursor to the lithium source, the fluorine source and the zirconium source is 1:1.01-1.06:0.001-0.05:0.001-0.

02.

6. The method of claim 1, wherein the lithium fluorophosphate iron composite material is prepared by the steps of: mixing a lithium compound, a fluorine compound, and an iron compound; and heating the mixture at a temperature of 600 to 800°C for 1 to 10 hours in an inert gas atmosphere. In step (3), the carbon source is at least one of the following: glucose, sucrose, starch; The amount of the carbon source added is 1%-3% of the mass of the mixture obtained in step (2).

7. The method for preparing the fluorinated lithium iron phosphate composite material according to claim 1, characterized in that, In step (3), the temperature of calcination is 500-700°C, and the calcination time is 6-16h.

8. The fluorine-containing lithium iron phosphate composite material prepared by the preparation method of any one of claims 1-7.

9. The application of the fluorine-containing lithium iron phosphate composite material of claim 8 as a positive electrode material in the preparation of a lithium ion battery.

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